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�6v�2v�32s2,3-Butanediol �21m �33s
    An organic substance, solid at room temperature, formula
CH3CH(OH)CH(OH)CH3. It is considered a possible source for 1,3-butadiene used
for the production of synthetic rubber and a source for a permanent-type
antifreeze. Other compounds readily made from 2,3-butanediol are methyl ethyl
ketone, methyl vinyl carbinol, and methyl vinyl ketone. The last two compounds
are used in the plastics industry.
    Preparation of 2,3-butanediol, also referred to as diol or 2,3-butylene
glycol, has been by fermentation on a pilot plant basis. It is the major
fermantation product of several species of bacteria found in the genera
Aerobacter, Bacillus, Serratia, and Pseudomonas. Most raw materials suitable
for the production of industrial alcohol are also satisfactory for the
manufacture of 2,3-butanediol. These fall into three main groups: (1) starchy
substrates, including corn, wheat, barley, sweet potatoes, and potatoes; (2)
sugars and sugar residues, such as beet and cane molasses; and (3) wood
hydrolyzates and pulp-mill residues, including waste sulfite liquor, although
the waste sulfite liquor presents some problems.
 �34s[George A. Ledingham /Francis Clark]
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�6v�2v�32sAardvark �21m �33s
    A nocturnal, burrowing, insectivorous mammal of the genus Orycteropus in
the order Tubulidentata. It averages 6 ft (1.8 m) in length and is covered
with short, sparse hair (see illustration).
    The aardvark is structurally modified for its diet of ants. The adult
lacks canines and incisors; the only teeth are 20 crushing or cheek teeth in a
series of 5 upper and 5 lower teeth on both sides of the jaw. These teeth,
which do not have roots or enamel, continually grow throughout the life of the
animal, have the appearance of tubelike structures of dentine, and are the
basis for the ordinal name Tubulidentata. The short, stout, powerful forelimbs
terminate in flat claws, which enable the aardvark to burrow rapidly into
termite mounds. The head is long and narrow, ending in a tubular snout. The
long tubular tongue extends up to 18 in. (45 cm) from the mouth.
    Some disagreement exists as to the number of species of Orycteropus. The
aardvark is edible and economically important for its thick hide which
resembles pigskin.


    See ANTEATER; DENTITION; MAMMALIA; TUBULIDENTATA.
 �34s[Charles B. Curtin]
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�6v�2v�32sAbaca   �21m �33s 
    One of the strongest of the hard fibers, commercially known as Manila 
hemp. Abaca is obtained from the leafstalks of a member of the banana family, 
Musa textilis. The plant resembles the fruiting banana, but is a bit shorter 
in stature, bears small inedible fruits, and has leaves that stand more erect 
than those of the banana, and that are slightly narrower, more pointed, and 5 
- 7 ft (1.5 - 2 m) long. The plant was domesticated long ago in the southern 
Philippines. See ZINGIBERALES. 
    Abaca prefers a warm climate with year-round rainfall, high humidity, and 
absence of strong winds. Soils must always be moist but the plant does not 
tolerate waterlogging. Abaca grows best on alluvial soils in the southern 
Philippines and northern Borneo below 1500 ft (450 m) elevation. The plant is 
best propagated by rootstalk suckers. There are about 75 varieties grown in 
the Philippines, grouped into seven categories, each of which varies slightly 
in height, length, and quality and yield of fiber. 
    The fiber ranges 6 - 14 ft (1.8 - 4.2 m) in strand length, is lustrous, 
and varies from white to dull yellow. As one of the longest and strongest 
plant fibers, resistant to fresh and salt water, abaca is favored for marine 
hawsers and other high-strength ropes. Abaca is also used in sackings, 
mattings, strong papers, and handicraft art goods. 
    Abaca is affected by several diseases, of which the chief are bunchy top, 
mosaic, and wilt. Bunchy top is caused by a virus spread by the banana aphid 
(Pentalonia nigronervosa). Mosaic is also caused by a virus spread by aphids 
(chiefly Rhopalosiphum nymphaeae and Aphis gossypii). Abaca wilt is caused by 
a soil or water-borne fungus, chiefly attacking plant roots. 
 �34s[Elton G. Nelson]
�33s 
  

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�6v�2v�32sAbacus �21m �33s
    The earliest known computing instrument, consisting of beads (counters)
strung on wires mounted in a frame (see illustration); technically the term
also covers the earlier counting tables and the grooved tablets of the Romans.
The abacus is still used in parts of the world.
    The Roman hand abacus was a bronze tablet with grooves in which small
spherical counters could slide, remarkably like the abacus which came into use
later in the Middle and Far East. Although this type of abacus is described in
the Chinese literature of the 2d century, it was not until the 13th century
that its use became widespread in China. The Chinese abacus had one more
counter in each section than the Roman hand abacus, and the counters were
mounted on wires in a frame rather than in grooves. It was introduced into
Japan in the 15th century, and by the 19th century a modified version had come
into wide use there. In China the abacus is called the suan-pan, and in Japan
the soroban. In the Middle East, still another version of the abacus is used,
which in the Soviet Union is called the schoty.
    By the 19th century the counting-table type of abacus had essentially gone
out of use in the Western world, and there the beads-on-shaft type had never
been widely used. Conversely, the 19th and 20th centuries, with their economic
development, brought an upsurge in the use of the latter type of abacus in the
Middle and Far East. By the end of World War II, abacus operation was included
in the curriculum of the fourth and upper grades in Japan, and licenses were
issued for proficiency in its use. Teachers prefer it over the pocket
electronic calculator. In the Soviet Union and China, as in Japan, the abacus
is still widely used. See CALCULATORS.

 �34s[Velma R. Huskey]
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�6v�2v�32sAbalone �21m �33s
    A gastropod mollusk comprising the single genus Haliotis, family
Haliotidae, also known as ear shell, ormer, or paua. The abalones are
cosmopolitan species of temperate and tropical seas. They are active
nocturnally and feed principally on algae.
    The shell is flattened, with an enlarged body whorl and reduced spire, and
looks like an ear (see illustration). Characteristically, the shell is
perforated by a series of small pores which allow for more direct elimination
of water from the mantle cavity. Exposed parts of the body are pigmented and
show a varying range of colors, such as black, green, or brown. The larvae are
pelagic and free-swimming among the plankton in coastal waters for about 2
days, after which they settle down and develop to the adult. See
ARCHAEOGASTROPODA; CIRCULATION; GASTROPODA; RESPIRATORY PIGMENTS
(INVERTEBRATE).

 �34s[Charles B. Curtin]
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�6v�2v�32sAbdomen   �21m �33s 
    A major body division of the vertebrate trunk lying posterior to the 
thorax; and in mammals, bounded anteriorly by the diaphragm and extending to 
the pelvis. The diaphragm separates the abdominal or peritoneal cavity from 
the pleural and pericardial cavities of the thorax. In all pulmonate 
vertebrates (possessing lungs or lunglike organs) other than mammals, the 
lungs lie in the same cavity with the abdominal viscera, and this cavity is 
known as the pleuroperitoneal cavity. 
    The large coelomic cavity that occupies the abdomen contains the viscera 
within the peritoneal sac. Connecting sheets of peritoneum from the body wall 
to the various organs form the mesenteries. Other folds of the peritoneum form 
the omenta. 
    The term abdomen is also applied to a similar major body division of 
arthropods and other animals. 
 �34s[Walter J. Bock]
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�6v�2v�32sAberration (astronomy)   �21m �33s 
    The apparent change in direction of a source of light caused by an 
observer's component of motion perpendicular to the impinging rays. 
    To visualize the effect, first imagine a stationary telescope 
(illustration a) aimed at a luminous source such as a star, with photons 
traveling concentrically down the tube to an image at the center of the focal 
plane. Next give the telescope a component of motion perpendicular to the 
incoming rays (illustration b). Photons passing the objective require a finite 
time to travel the length of the tube. During this time the telescope has 
moved a short distance, causing the photons to reach a spot on the focal plane 
displaced from the former image position. To return the image to the center, 
the telescope must be tilted in the direction of motion by an amount 
sufficient to ensure that the photons once again come concentrically down the 
tube in its frame of reference (illustration c). The necessary tilt angle 
ALPHA is given by tan ALPHA = vc, where v is the component of velocity 
perpendicular to the incoming light and c is the velocity of light. (An 
analogy illustrating aberration is the experience that, in order for the feet 
to remain dry while walking through vertically falling rain, it is necessary 
to tilt an umbrella substantially forward.) 
    This discovery provided the first direct physical confirmation of the 
Copernican theory. A second important application of aberration has been its 
clear-cut demonstration that, as is axiomatic to special relativity, light 
reaching the Earth has a velocity unaffected by the relative motion of the 
source toward or away from the Earth. LIGHT; PARALLAX (ASTRONOMY). 
    Figure caption: Demonstration of aberration. (a) Fixed telescope; photons
form image at center of focal plane. (b) Moving telescope; image is displaced
from center. (c) Tilted moving telescope is required to restore image to       
center.
 �34s[Harlan J. Smith]
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�6v�2v�32sAberration (optics)   �21m �33s 
    Deviation from perfect image formation. The deviation arising from the 
fact that light of different wavelengths follows different paths through an 
optical system is known as chromatic aberration. The chief monochromatic 
aberrations are spherical (aperture) aberrations, coma, astigmatism, curvature 
of field, and distortion. See ASTIGMATISM; CHROMATIC ABERRATION; COMA 
(OPTICS); CURVATURE OF FIELD; DISTORTION (OPTICS); GEOMETRICAL OPTICS; OPTICAL 
IMAGE; SPHERICAL ABERRATION. 
 �34s[Max J. Herzberger]
�33s 
  
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�6v�2v�32sAblastin   �21m �33s 
    An antibodylike substance, present in serum as a result of immunization, 
that inhibits the multiplication of an invading parasite. The best example is 
that of the serum reactions resulting from Trypanosoma lewisi infections in 
the rat. The ablastin appears a few days after infection, and the reproduction 
of adult forms of the trypanosome is progressively reduced. The ablastic 
activity is found in the globulin portion of the serum and can be transferred 
to normal animals. See ANTIBODY; GLOBULIN; SERUM. 
 �34s[Henry P. Treffers]
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�6v�2v�32sAbrasive   �21m �33s 
    A material of extreme hardness that is used to shape other materials by a 
grinding or abrading action. Abrasive materials may be used either as loose 
grains, as grinding wheels, or as coatings on cloth or paper. They may be 
formed into ceramic cutting tools that are used for machining metal in the 
same way that ordinary machine tools are used. Because of their superior 
hardness and refractory properties, they have advantages in speed of 
operation, depth of cut, and smoothness of finish. 
    Abrasive products are used for cleaning and machining all types of metal, 
for grinding and polishing glass, for grinding logs to paper pulp, for cutting 
metals, glass, and cement, and for manufacturing many miscellaneous products 
such as brake linings and nonslip floor tile. 
    The important natural abrasives are diamond, corundum, emery, garnet, 
feldspar, calcined clay, lime, chalk, and silica, SiO2, in its many forms - 
sandstone, sand, flint, and diatomite. 
    The synthetic abrasive materials are silicon carbide, aluminum oxide, 
titanium carbide, and boron carbide. The synthesis of diamond puts this 
material in the category of manufactured abrasives. 
 �34s[John F. McMahon]
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�6v�2v�32sAbscisic acid   �21m �33s 
    One of the five major plant hormones. It has a number of important 
functions in plant growth and development. The name abscisic acid (ABA) is 
derived from the ability of the substance to promote abscission. It is also a 
potent inhibitor of growth. In this capacity it helps to induce and prolong 
dormancy of buds. Abscisic acid is a powerful inhibitor of seed germination 
and has an important role in the closure of stomata. See ABSCISSION; DORMANCY. 
    Abscisic acid is distributed throughout the plant body but is found in 
highest concentrations in leafy tissues, fruits, and seeds. The principal site 
of ABA synthesis is the chloroplast. Synthesis of ABA increases markedly when 
the plant is under stress. 

                         *** CHEMICAL STRUCTURE ***

    The chemical structure of ABA is shown in the illustration. ABA is a 
member of the terpenoid family of chemicals, which includes a number of 
essential oils, insect hormones, steroids, gibberellins, carotenoids, and 
natural rubber. It is a weak organic acid, as are two other plant hormones, 
auxin and gibberellin. Natural ABA is dextrorotatory, hence the ``+'' in front 
of the name. See AUXIN; GIBBERELLIN; PLANT HORMONES. 
    Figure caption: Structural formula for +-abscisic acid.
 �34s[Fredrick T. Addicott]
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�6v�2v�32sAbscission �21m �33s
    The process whereby a plant sheds one of its parts. Leaves, flowers,
seeds, and fruits are parts commonly abscised. Almost any plant part, from
very small buds and bracts to branches several inches in diameter, may be
abscised by some species. However, other species, including many annual
plants, may show little abscission, especially of leaves.
    Abscission may be of value to the plant in several ways. It can be a
process of self-pruning, removing injured, diseased, or senescent parts. It
permits the dispersal of seeds and other reproductive structures. It
facilitates the recycling of mineral nutrients to the soil. It functions to
maintain homeostasis in the plant, keeping in balance leaves and roots, and
vegetative and reproductive parts.
    In most plants the process of abscission is restricted to an abscission
zone at the base of an organ (see illustration); here separation is brought
about by the disintegration of the walls of a special layer of cells, the
separation layer. The portion of the abscission zone which remains on the
plant commonly develops into a corky protective layer that becomes continuous
with the cork of the stem.
    Auxin applied experimentally to the distal (organ) side of an abscission
zone retards abscission, while auxin applied to the proximal (stem) side
accelerates abscission. The gibberellins are growth hormones which influence
abscission. When applied to young fruits or to leaves, they tend to promote
growth, delay maturation, and thereby indirectly prevent or delay abscission.
Abscisic acid has the ability to promote abscission and senescence and to
retard growth. Small amounts of ethylene have profound effects on the growth
of plants and can distort and reduce growth and promote senescence and
abscission. See ABSCISIC ACID; AUXIN; GIBBERELLIN.




 �34s[Fredrick T. Addicott]
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�6v�2v�32sAbsolute zero   �21m �33s 
    The lowest temperature on the scientific temperature scale. Gross matter, 
which is in complete thermal equilibrium with all of its subdivided parts, has 
a property called its temperature, and this is measured in kelvins (K). Like 
all other similar scalar quantities, the absolute temperature scale starts at 
zero and has arbitrary but convenient units going on up to high temperatures. 
A convenient and readily reproducible fixed point on this temperature scale is 
the triple point of pure water (an equilibrium mixture of ice, water, and 
water vapor) defined as 273.16 K. There are several convenient thermometers to 
measure the low temperature range, but none can perform the impossible task of 
indicating the absolute zero of temperature, 0 K, because that temperature 
cannot be reached. 
    The properties of matter at the absolute zero of temperature are always 
extrapolated from physical measurements made on them at available temperatures 
such as 1 K or even 0.01 K. Such studies have led to a statement of the third 
law of thermodynamics. The entropy of a system tends to a constant S0 as the 
temperature of the system is made to approach 0 K. One popular error is to 
regard the absolute zero of temperature as characterized by the complete 
absence of motion or of energy of the system. The atoms in a solid have 
considerable energy locked into the lowest allowed energy states of vibration, 
even at 0 K. See CHEMICAL THERMODYNAMICS; CRYOGENICS; ENTROPY; KINETIC THEORY 
OF MATTER; LOW-TEMPERATURE PHYSICS; TEMPERATURE. 
 �34s[Charles F. Squire]
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�6v�2v�32sAbsorption   �21m �33s 
    Either the taking up of matter in bulk by other matter, as in the 
dissolving of a gas by a liquid; or the taking up of energy from radiation by 
the medium through which the radiation is passing. In the first case, an 
absorption coefficient is defined as the amount of gas dissolved at standard 
conditions by 1 cm sup 3 of the solvent. Absorption in this sense is a volume 
effect: The absorbed substance permeates the whole of the absorber. In 
absorption of the second type, attenuation is produced which in many cases 
follows Lambert's law and adds to the effects of scattering if the latter is 
present. See ATTENUATION. 
    Absorption of electromagnetic radiation can occur in several ways. Light 
is absorbed by atoms of the medium through which it passes; in some cases 
selected frequencies from a heterochromatic source are strongly absorbed. 
Electromagnetic radiation can be absorbed by the photoelectric effect, where 
the light quantum is absorbed and an electron of the absorbing atom is 
ejected, and also by Compton scattering. Electron-positron pairs may be 
created by the absorption of a photon of sufficiently high energy. Photons can 
be absorbed by photoproduction of nuclear and subnuclear particles, analogous 
to the photoelectric effect. See COMPTON EFFECT; PHOTOEMISSION. 
    Sound waves are absorbed at suitable frequencies by particles suspended in 
the air, where the sound energy is transformed into vibrational energy of the 
absorbing particles. See SOUND ABSORPTION. 
    Absorption of energy from a beam of particles can occur by the ionization 
process, where an electron in the medium through which the beam passes is 
removed by the beam particles. The finite range of protons and ALPHA-particles 
in matter is a result of this process. In the case of low-energy electrons, 
scattering is as important as ionization, so that range is a less well-defined 
concept. Particles themselves may be absorbed from a beam. Low-energy 
positrons are quickly absorbed by annihilating with electrons in matter to 
yield two GAMMA-rays. 
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sAbstract algebra   �21m �33s 
    A term used synonymously with modern algebra and general algebra to 
describe the type of algebra which has been developed since the mid-1920s and 
has become a basic idiom of contemporary mathematics. In contrast with the 
earlier algebra, which was highly computational and was confined to the study 
of specific systems generally based on real and complex numbers, abstract 
algebra is conceptual and axiomatic and deals with systems which are arbitrary 
sets of elements of unspecified type, together with certain compositions 
satisfying prescribed lists of axioms. 
    A good insight into the difference between the older and the present 
approach can be obtained by comparing the older matrix theory with the more 
abstract linear algebra. Both deal with roughly the same portion of 
mathematics, the former a direct perspective which stresses calculations with 
matrices, while the latter from an axiomatic and geometric viewpoint which 
treats vector spaces and linear transformations as the basic notions, and 
matrices as secondary to these. See LINEAR ALGEBRA; MATRIX THEORY. 
    Abstract algebra deals with a number of important algebraic structures, 
such as groups, rings, and lattices. See GROUP THEORY; RING THEORY. 
 �34s[Nathan Jacobson]
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�6v�2v�32sAbstract data types   �21m �33s 
    Mathematical models which may be used to capture the essentials of a 
problem domain in order to translate it into a computer program. 
  
    Motivation.  It is generally understood that people join a line by 
standing at the end of the line and are serviced from its front. Stepping into 
the middle of a line violates the rules of line etiquette, as does servicing 
someone from the middle of the line. 
    To describe a particular line of people, it suffices to keep track of the 
first and last person as the line moves. Then, it will always be clear behind 
whom a new person must stand and who must be served next. Such a description 
of a line is an example of an abstract data type (ADT). It is a model for 
representing some sort of structure, with rules for making changes to that 
structure. The rules may be expressed as operators that perform specific 
functions.  
    In the field of computer programming, lines are usually called queues. For 
a person to enter a queue would be to perform an insertion; to leave the front 
of a queue would be a deletion. When the rules of line etiquette are broken, 
that is, an individual enters into or is serviced from the middle, the queue 
becomes a list. A list is a queue where insertions and deletions may be made 
at any point. A stack is a queue in which insertions and deletions are both 
made at the front. 
    There are many examples of abstract data types, but this formalism is 
rarely useful in everyday life. However, an ADT queue would be quite helpful 
in representing a customer line as a computer program. The queue could be used 
to express certain business operations in terms of a well-understood model for 
representing information. This would allow the programmer to simplify the 
problem to that of implementing the queue in a computer language and then 
using the queue to represent a business function. Techniques for implementing 
queues are readily available. 
    Abstract data types allow the use of previously discovered programming 
techniques. Another benefit of ADTs is that they allow a programmer to 
encapsulate the rules for altering a model within a set of procedures, and 
ignore these details when using the given ADT. In other words, the programmer 
concerned with modeling a business function could forget exactly how an 
insertion or deletion is made when procedures that perform these functions are 
used.  
  
    ADTs and data structures.  An ADT is indeed abstract, in that it must be 
translated into a programming language. This is done by using program data 
structures, which are formed from the primitive abstract data types supported 
directly by the given language. The programmer must also implement operators 
(for example, insert and delete in the queue example) that follow the rules 
for manipulating the ADT. In essence, then, ADTs are conceptualizations built 
of simple data structures and augmented with high-level operators. 
  
    Classification of data types.  Abstract data types may be grouped 
according to the ways they arrange information. First, linearly organized data 
types, such as queues, lists, and stacks, represent information as a 
one-dimensional series of elements. Second, nonlinear modeling techniques 
arrange this information so that one element may be logically followed by more 
than one other element. (This corresponds to a customer line where there is 
more than one person directly behind any specific person.) Examples of this 
category include trees, where each element may be logically followed by other 
elements, called its children; and graphs, which are generalized trees in 
which each element may have more than one parent. Third, an unordered data 
type, such as a set, assumes no specific order of the elements of the type. 
(This corresponds to customers waiting in a chaotic group.) These three data 
types are examples of models used to represent information in the fast-access, 
main memory of a computer. Other data types, such as files and data bases, are 
oriented toward mass storage devices like magnetic tapes and disks.  See  
DATA-BASE MANAGEMENT SYSTEMS; GRAPH THEORY; SET THEORY. 
  
    Primitive ADTs.  There are also a number of primitive abstract types that 
are typically implemented directly in a high-level programming language. These 
include integers and real numbers (with appropriate arithmetic operators), 
booleans (with appropriate logical operators), text strings (often a 
programming language provides no special operators for manipulating strings), 
and pointers. 
 �34s[Roger King]
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�6v�2v�32sAcantharia �21m �33s
    A subclass of Actinopodea. These marine protozoans are related to the
Radiolaria and possess a nonliving, organic capsular wall surrounding a
central mass of cytoplasm. The intracapsular cytoplasm is connected to the
extracapsular cytoplasm by fine cytoplasmic strands passing through pores in
the capsular wall. Skeletons are typically constructed of celestite (strontium
sulfate) instead of silica. The basic structural elements are 20 rods which
pass through the capsule to the center in regular arrangement (polar and
equatorial; see illustration). An equatorial rod forms an angle of 90DG with a
polar rod, and other groups are arranged with similar exactness. This type of
skeleton may be modified by addition of a latticework, apparently composed of
plates, each fused with a skeletal rod. Some genera show a double latticework,
concentric with the central capsule.
    Pseudopodia are more or less permanent. Zooxanthellae are of at least two
kinds: dinophyceae containing trichocysts in the cytoplasm; and a group of
algae characterized, among other things, by numerous discoidal plastids each
of which contains an interlamellar pyrenoid.
    Myonemes (myophrisks) are significant components of the hydrostatic
apparatus and apparently regulate the buoyancy of Acantharia by expanding or
contracting portions of the extracapsular cytoplasmic sheath.
    Although essentially pelagic, Acantharia may move vertically with the help
of their hydrostatic apparatus. Little is known about the ecology or
distribution of Acantharia. The Gulf Stream is rich in Acantharia in spring
and summer, but rather poor in other seasons.
    The taxonomy of Acantharia is under review, but according to one scheme
there are two orders: Acanthometrida, and Acanthophractida. See
ACANTHOMETRIDA; ACANTHOPHRACTIDA; ACTINOPODEA; PROTOZOA; SARCODINA;
SARCOMASTIGOPHORA.

 �34s[O. Roger Anderson]
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�6v�2v�32sAcanthocephala   �21m �33s 
    A distinct phylum of helminths, the adults of which are parasitic in the 
alimentary canal of vertebrates. They are commonly known as the spiny-headed 
worms. The phylum comprises the orders Archiacanthocephala, 
Palaeacanthocephala, and Eocanthocephala. Over 500 species have been described 
from all classes of vertebrates, although more species occur in fish than in 
birds and mammals and only a relatively few species are found in amphibians 
and reptiles. The geographical distribution of acanthocephalans is worldwide, 
but genera and species do not have a uniform distribution because some species 
are confined to limited geographic areas. Host specificity is well established 
in some species, whereas others exhibit a wide range of host tolerance. The 
same species never occurs normally, as an adult, in cold-blooded and 
warm-blooded definitive hosts. More species occur in fish than any other 
vertebrate; however, Acanthocephala have not been reported from elasmobranch 
fish. The fact that larval development occurs in arthropods gives support to 
the postulation that the ancestors of Acanthocephala were parasites of 
primitive arthropods during or before the Cambrian Period and became parasites 
of vertebrates as this group arose and utilized arthropods for food. See 
ARCHIACANTHOCEPHALA; EOCANTHOCEPHALA; PALAEACANTHOCEPHALA. 
    Adults of various species show great diversity in size, ranging in length 
from 0.04 in. (1 mm) in some species found in fish to over 16 in. (400 mm) in 
some mammalian species. 
    The body of both males and females has three subdivisions: the proboscis 
armed with hooks, spines, or both; an unspined neck; and the posterior trunk. 
The proboscis is the primary organ for attachment to the intestinal wall of 
the host. In most species the proboscis is capable of introversion into a 
saclike structure, the proboscis receptacle. The proboscis receptacle and neck 
can be retracted into the body cavity but without inversion. The body cavity, 
or pseudocoele, contains all the internal organs, the most conspicuous of 
which are the reproductive organs. There is no vestige of a digestive system 
in any stage of the life cycle. The reproductive organs of the male consist of 
a pair of testes and specialized cells, the cement glands. The products of the 
testes and cement glands are discharged through a penis. Female Acanthocephala 
are unique in that the ovary exists as a distinct organ only in the very early 
stages of development and later breaks up to form free-floating egg balls. The 
eggs are fertilized as they are released from the egg balls and are retained 
within the ligament sacs until embryonation is complete. The nervous system is 
composed of a chief ganglion or brain located within the proboscis receptacle. 
Two nerve trunks pass through the wall of the proboscis receptacle to 
innervate the trunk wall. Modified protonephridial organs are found closely 
adherent to the reproductive system, but in most species specialized excretory 
organs are completely lacking. 
 �34s[Donald V. Moore]
�33s 
  

A700000002363�254s�5v�1v5
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�6v�2v�32sAcanthodii �21m �33s
    A subclass of importance, including the earliest known jawed fishes or
gnathostomes, first appearing in the Lower Silurian and surviving until the
Lower Permian. They were usually small, less than 8 in. (20 cm)  in length,
though a few may have been as much as 100 in. (250 cm) long. The body was
fusiform, the mouth terminal or nearly so, the eyes large, and the nasal
capsules small. The tail was heterocercal; there were one or two dorsal fins,
and all fins except the caudal had a spine on the anterior edge (see
illustration). The scales had a square or rhombic crown and typically were
nonoverlapping; they grew by periodic additions all around, lacked a pulp
chamber, and were composed superficially of dentine or mesodentine and basally
of bone, often acellular. Acanthodii are best considered as a subclass of
Teleostomi, collateral with Osteichthyes, and can be classified as follows:

Subclass Acanthodii
    Order Climatiida
	Family Climatiidae
	Family Diplacanthidae
	?Family Gyracanthidae
    Order Ischnacanthida
	Family Ischnacanthidae
    Order Acanthodida
	Family Acanthodidae

See CHONDRICHTHYES; OSTEICHTHYES; TELEOSTOMI.


 �34s[Robert H. Denison]
�33s



A700000001257�254s�5v�1v6
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�6v�2v�32sAcanthometrida �21m �33s
    An order of Acantharia. These marine protozoans have a skeleton limited to
20 radially arranged rods which extend from the center, forming a
characteristic pattern in which angles are quite exact, as in Acanthometra
(see illustration). The cytoplasm surrounding the spines contains a conical
array of contractile microfilaments (myophrisks or myonemes) that act to
expand or contract the gelatinous sheath surrounding the cell, a phenomenon
that is apparently responsible for changes in the level of flotation. See
ACANTHARIA; ACTINOPODEA; PROTOZOA; SARCODINA; SARCOMASTIGOPHORA.


 �34s[O. Roger Anderson]
�33s



A700000001444�254s�5v�1v6
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�6v�2v�32sAcanthophractida   �21m �33s 
    An order of Acantharia. In this group of protozoans, skeletons typically 
include a latticework shell, although the characteristic skeletal rods are 
recognizable. The latticework may be spherical or ovoid, is fused with the 
skeletal rods, and is typically concentric with the central capsule. The body 
is usually covered with a single or double gelatinous sheath through which the 
skeletal rods emerge. Myonemes extend from the gelatinous sheath to each 
skeletal rod. These marine forms live mostly below depths of 150 - 200 ft (45 
- 60 m). The order includes Coleaspis, Diploconus, Dorotaspis, and many other 
genera. See ACANTHARIA; ACTINOPODEA; PROTOZOA: SARCODINA: SARCOMASTIGOPHORA. 
 �34s[Richard P. Hall]
�33s 
  
A700000001340�254s�5v�1v6
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�6v�2v�32sAcanthothoraci �21m �33s
    An order of class Placodermi, sometimes called Palaeacanthaspida or
Radotinida, known only from the Lower Devonian of Eurasia and arctic Canada.
The skull is long, rather narrow, and covered with dermal bones and sometimes
with a mosaic of small tesserae; it is characterized by the presence of two
pairs of paranuchal plates (see illustration). The orbits are lateral or
dorsolateral, and the nostrils dorsal and bounded in front by a premedian
plate, unknown in other placoderms except Antiarcha. The shoulder girdle is
short and high, and topped by a median dorsal plate that often has a high
spine. See ANTIARCHA; PLACODERMI.

 �34s[Robert H. Denison]
�33s



A700000005102�254s�5v�1v6
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�6v�2v�32sAcarina   �21m �33s 
    A subclass of Arachnida, the mites and ticks; also called Acari. All are 
small  (0.004 - 1.2 in. or 0.1 - 30 mm, most less than 0.8 in. or 2 mm in 
length), have lost most traces of external body segmentation, and have the 
mouthparts borne on a discrete body region, the gnathosoma. They are 
apparently most closely related to Opiliones and Ricinulei. 
    The chelicerae may be chelate or needlelike. Pedipalps are generally 
smaller than the walking legs, and are simple or sometimes weakly chelate. 
Legs may be modified by projections, enlarged claws, heavy spines, or 
bladelike rows of long setae for crawling, clinging to hosts, transferring 
spermatophores, or swimming. Mites frequently have one or two simple eyes 
(ocelli) anterolaterally on the idiosoma (occasionally one anteromedially as 
well). The ganglia of the central nervous system are coalesced into a single 
``brain'' lying around the esophagus. A simple dorsal heart is present in a 
few larger forms. 
    Mites are ubiquitous, occurring from oceanic trenches below 3000 ft (4000 
m) to over 20,800 ft (6300 m) in the Himalayas and suspended above 3300 ft 
(1000 m) in the atmosphere; there are species in Antarctica. Soil mites show 
the least specialized adaptations to habitat and are frequently 
well-sclerotized predators. Inhabitants of stored grains, cheese, and house 
dust are also relatively unspecialized. The dominant forms inhabiting mosses 
are heavily sclerotized beetle mites (Oribatei). Flowering plant associates 
include spider mites (Tetranychidae) and gall mites (Eriophyidae). Fungivores 
are usually weakly sclerotized inhabitants of moist or semiaquatic habitats, 
while the characteristic fresh-water mites include sluggish crawlers, rapid 
swimmers, and planktonic drifters. Mites associated with invertebrate animals 
include internal, external, and social parasites as well as inactive phoretic 
stages on Insecta, Crustacea, Myriapoda, Chelicerata, Mollusca, and Parazoa. A 
similar diversity of species are parasites or commensals of vertebrates. Some 
parasitic mites are disease vectors. 
    The widely accepted classification is: 
  
    Order Parasitiformes: ticks, other vertebrate parasites, free-living 
predators, and vestigially segmented forms 
    Order Acariformes: plant mites, velvet mites, chiggers, water mites, grain 
and cheese mites, house dust mites, mange mites, and beetle mites 
  
Over 30,000 species have been described, and it is estimated that as many as 
500,000 may exist. See ARACHNIDA. 
 �34s[David Barr]
�33s 
  
  

A700000001070�254s�5v�1v6
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�6v�2v�32sAccelerated reference frame   �21m �33s 
    A noninertial frame of reference. A point fixed in such a frame will be 
accelerated with respect to inertial frames and so must be held by a force. An 
observer in an accelerated frame will see accelerations that are not 
proportional to the impressed forces, since a force is required to keep the 
particle at rest. Newton's second law does not hold without modification. See 
FRAME OF REFERENCE; INERTIAL REFERENCE FRAME. 
 �34s[Bernard Goodman]
�33s 
  
A700000002123�254s�5v�1v7
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�6v�2v�32sAcceleration   �21m �33s 
    The time rate of change of velocity. Since velocity is a directed or 
vector quantity involving both magnitude and direction, a velocity may change 
by a change of magnitude (speed) or by a change of direction or both. It 
follows that acceleration is also a directed, or vector, quantity. If the 
magnitude of the velocity of a body changes from v1 ft/s to v2 ft/s in t, then 
the average acceleration a has a magnitude given by the equation below. To 
designate it fully the direction should be given, as well as the magnitude. 
Instantaneous acceleration is the limit of the ratio of the velocity change to 
the elapsed time as the time interval approaches zero. See VELOCITY. 
  
** a = velocity change/elapsed time = (v2-v1)/(t2-t1) = DELTA v/DELTA t ** 
  
    Accelerations are commonly expressed in feet per second per second 
(ft/s sup 2), meters per second per second (m/ssup2), or in any similar units. 
Whenever a body is acted upon by an unbalanced force, it will undergo 
acceleration.  
 �34s[Rogers D. Rusk]
�33s 
  

A700000004434�254s�5v�1v7
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�6v�2v�32sAccelerometer �21m �33s
    A mechanical or electromechanical instrument that measures acceleration.
The two general types of accelerometers measure either the components of
translational acceleration or angular acceleration.
    Most translational accelerometers fall into the category of seismic
instruments, which means the accelerations are not measured with respect to a
reference point. Of the two types of seismic instruments, one measures the
attainment of a predefined acceleration level, and the other measures
acceleration continuously. In one version of the first type of instrument, a
seismic mass is suspended from a bar made of brittle material which fails in
tension at a predetermined acceleration level.
    Continuously measuring seismic instruments are composed of a damped or an
undamped spring-supported seismic mass which is mounted by means of the spring
to a housing. The seismic mass is restrained to move along a predefined axis.
Also provided is some type of sensing device to measure acceleration.
    The type of sensing device used to measure the acceleration determines
whether the accelerometer is a mechanical or an electromechanical instrument.
One type of mechanical accelerometer consists of a liquid-damped cantilever
spring-mass system, a shaft attached to the mass, and a small mirror mounted
on the shaft. A light beam reflected by the mirror passes through a slit, and
its motion is recorded on moving photographic paper. The type of
electromechanical sensing device classifies the accelerometer as
variable-resistance, variable-inductance, piezoelectric, piezotransistor, or
servo type of instrument or transducer.
    There are several different types of angular accelerometers. In one type
the damping fluid serves as the seismic mass. Under angular acceleration the
fluid rotates relative to the housing and causes on two symmetrical vanes a
pressure which is a measure of the angular acceleration. Another type of
instrument has a fluid-damped symmetrical seismic mass in the form of a disk
which is so mounted that it rotates about the normal axis through its center
of gravity. The angular deflection of the disk, which is restrained by a
spring, is proportional to the angular acceleration.
 �34s[Roger C. Duffield; Teruo Ishihara]
�33s


A700000001221�254s�5v�1v7
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�6v�2v�32sAcceptor atom   �21m �33s 
    An impurity atom in a semiconductor which can accept or take up one or 
more electrons from the crystal and become negatively charged. An atom which 
substitutes for a regular atom of the material but has one less valence 
electron may be expected to be an acceptor atom. For example, atoms of 
aluminum, gallium, or indium are acceptors in germanium and silicon, and atoms 
of antimony and bismuth are acceptors in tellurium crystals. Acceptor atoms 
tend to increase the number of holes (positive charge carries) in the 
semiconductor. See DONOR ATOM. 
 �34s[Hsu Y. Fan]
�33s 
  

A700000001605�254s�5v�1v7
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�6v�2v�32sAcetals   �21m �33s 
    Stable ethers usually prepared by acid-catalyzedalcoholation of the 
carbonyl group of aldehydes. The Y-shaped general formula for these compounds 
is shown below. (R represents an organic radical.) Cyclic acetals are formed 

                        *** CHEMICAL STRUCTURE ***

from diols. Acetals may also be obtained through the Prins reaction of olefins 
with aldehydes, usually formaldehyde. The acetals are stable toward alkalies, 
but they hydrolyze in the presence of acids. Their thermal decomposition 
yields vinyl ethers. Few acetals have any large-scale industrial applications. 
Their hydrolytic stability to alkali commends the use of some in soaps as 
fragrances, and their solvency characteristics make them convenient solvents 
and plasticizers. See ALCOHOL; ALDEHYDE; GLYCOL; KETONE. 
 �34s[Frank Wagner]
�33s 
  

A700000001754�254s�5v�1v7
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�6v�2v�32sAcetate   �21m �33s 
    One of two types of compounds derived from acetic acid, HC2H3O2. One type 
is obtained by the reaction of acetic acid and bases to give salts containing 
the negative acetate ion, C2H3O2 NEGATIVE ION. The second type of compound is
an ester which is derived from acetic acid and an alcohol, for example, ethyl
acetate. See ACETYLATION. 
    Acetate is the official name (Federal Trade Commission) for the textile 
fiber produced from partially hydrolyzed cellulose acetate and once called 
acetate rayon. 
    All the metal acetates are water-soluble except silver acetate. The 
acetate ion is colorless; it may be identified by heating a sample with 
sulfuric acid to give the odor of acetic acid. The fruitlike odor of the ester 
formed when ethyl alcohol and a trace of sulfuric acid are heated with the 
sample is also characteristic. See ACETIC ACID; CARBOXYLIC ACID; ESTER; 
MANUFACTURED FIBER. 
 �34s[E. Eugene Weaver]
�33s 
  

A700000002533�254s�5v�1v7
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�6v�2v�32sAcetic acid   �21m �33s 
    A colorless, pungent liquid, CH3COOH, melting at 16.7 DG C and boiling at 
118.0 DG C. Acetic acid is the sour principle in vinegar. Concentrated acid is 
called glacial acetic acid because of its readiness to crystallize at cool 
temperatures.  
    Acetic acid is manufactured by three main routes: butane liquid-phase 
catalytic oxidation in acetic acid solvent, palladium - copper salt - 
catalyzed oxidation of ethylene in aqueous solution, and methanol 
carbonylation in the presence of rhodium catalyst. Large quantities of acetic 
acid are recovered in the manufacture of cellulose acetate and polyvinyl 
alcohol. Some acetic acid is produced in the oxidation of higher olefins, 
aromatic hydrocarbons, ketones, and alcohols. See OXIDATION PROCESS; WOOD 
CHEMICALS.  
    Pure acetic acid is completely miscible with water, ethanol, diethyl 
ether, and carbon tetrachloride, but is not soluble in carbon disulfide. In a 
water solution, acetic acid is a typical weakly ionized acid (Ka = 1.8 X 
10 sup -5). Acetic acid neutralizes many oxides and hydroxides, and decomposes 
carbonates to furnish acetate salts, which are used in textile dyeing and 
finishing, as pigments, and as pesticides; examples are verdigris, white lead, 
and paris green. See VINEGAR. 
 �34s[Frank Wagner]
�33s 
  

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�6v�2v�32sAcetone   �21m �33s 
    A chemical compound, CH3COCH3. A colorless liquid with an ethereal odor, 
it is the first member of the homologous series of aliphatic ketones. Its 
physical properties include boiling point 56.2 DG C (133.2 DG F), melting point 
-94.8 DG C (202.6 DG F), and specific gravity 0.791. 
    Acetone is used as a solvent for cellulose ethers, cellulose acetate, 
cellulose nitrate, and other cellulose esters. Cellulose acetate is spun from 
acetone solution. Lacquers, based on cellulose esters, are used in solution in 
mixed solvents including acetone. Acetylene is safely stored in cylinders 
under pressure by dissolving it in acetone, which is absorbed on inert 
material such as asbestos. It has a low toxicity. 
 �34s[David A. Shirley]
�33s 
  

A700000002141�254s�5v�1v8
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�6v�2v�32sAcetylation   �21m �33s 
    The introduction of an acetyl group into an organic compound containing    

                         *** CHEMICAL STRUCTURE ***

the alcoholic or phenolic hydroxyl (--OH) or the amino (--NH2) and substituted 
amino groups to yield esters or substituted amides, respectively. Acetylation 
can be carried out with acetic anhydride, acetyl chloride, or glacial acetic 
acid (in order of decreasing importance) and with or without an inert solvent, 
such as benzene or toluene. In general, acetylation reactions are exothermic. 
    The most important acetates formed by acetylation are cellulose acetate, 
vinyl acetate, and ethyl acetate. Other important acetylation reactions 
include production of pharmaceuticals such as aspirin, phenacetin, and 
derivatives; determination of the hydroxy acid content of fats and oils 
(acetyl number); preparation of acid anhydrides with acetic anhydride and an 
organic acid by an exchange reaction; and production of other chemical 
intermediates. See ACID ANHYDRIDE; AMINE. 
 �34s[Elbert H. Hadley]
�33s 
  

A700000004077�254s�5v�1v8
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�6v�2v�32sAcetylcholine   �21m �33s 
    A chemical compound which plays a key role in the series of reactions that 
control the permeability of some membranes to ions, a process of paramount 
importance in nerve excitability and bioelectricity. The action of 
acetylcholine controls ion permeability in all parts of the excitable 
membrane, in the axonal, pre- and postsynaptic membranes, and in muscle 
membranes. The acetylcholine cycle is formed by four proteins: acetylcholine 
esterase, acetylcholine receptor, acetylcholine storage protein, and choline 
acetyltransferase. The essential role of the acetylcholine cycle in conduction 
is well established. Potent inhibitors of acetylcholine esterase and 
acetylcholine receptor strongly affect and eventually block conduction. All 
conducting membranes (muscle, myelinated and unmyelinated nerve fibers) are 
protected by barriers such as myelin, lipid-rich Schwann cells, and connective 
tissue impervious to quaternary nitrogen derivatives, such as acetylcholine 
curare, and neostigmine. 
    The receptor and acetylcholine esterase proteins are present and 
functional in nerve terminal membrane. On stimulation, Na POSITIVE ions will 
enter the axon, and K POSITIVE ions flow into the cleft. Some 20,000,000 to 
40,000,000 ions per 1000 molecules of acetylcholine released from acetylcholine 
storage protein in the membrane move in each direction across the membrane. 
This high K POSITIVE concentration in the narrow synaptic cleft will decrease 
the resting-membrane potential to the threshold potential and release 
acetylcholine from storage protein in the postsynaptic membrane. The action of 
acetylcholine is limited to the junctions because conducting membranes are 
protected by barriers. Appearance of acetylcholine at synapses is observed 
only in the presence of eserine, which prevents the rapid removal by 
acetylcholine esterase. After degeneration of the nerve terminal, 
acetylcholine is released by direct stimulation of the muscle. See SYNAPTIC 
TRANSMISSION.  
 �34s[David Nachmansohn]
�33s 
  

A700000002613�254s�5v�1v8
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�6v�2v�32sAcetylene   �21m �33s 
    A colorless, aromatic gas that burns with a highly luminous flame, 
sublimes at -83.4 DG C/760 mmHg (-118 DG F/260 kilopascals) and is 
shock-sensitive at low temperatures and partial pressures above 20 - 30 pounds 
per square inch (138 - 207 kPa). Acetylene burns with oxygen to produce the 
highest achievable flame temperature, over 3300 DG C (6000 DG F), of any 
carbonaceous fuel. Oxyacetylene flames are of major importance in the welding 
and cutting of metals. See TORCH; WELDING AND CUTTING OF METALS. 
    Acetylene has the formula HC TRIPLE BOND CH. The triple bond undergoes 
addition reactions to produce ethylene and ethane derivatives. The hydrogen 
atoms of acetylene undergo replacement reactions. Aqueous solutions of copper 
and silver salts produce explosive metal acetylides. Reactive metals, for 
example sodium, on contact with acetylene form stable mono- and disodium salts 
with the generation of hydrogen. Formaldehyde reacts to produce 
1,4-butynediol, which can be hydrogenated to the commercially important 
1,4-butanediol. Acetylene reacts with carbon monoxide and alcohols in the 
presence of nickel carbonyl to form acrylate esters. 
    The major uses for acetylene are in the manufacture of 1,4-butanediol, 
vinyl chloride, acrylic acid and esters, and vinyl acetate. See ALKYNE. 
 �34s[Robert K. Barnes]
�33s 
  

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�6v�2v�32sAcetylide   �21m �33s 
    A derivative of acetylene formed by the replacement of one or both 
(carbides) of the hydrogens by a metal (although carbides such as SiC, B6C, 
and WC are not acetylides). Acetylides are prepared by the action of acetylene 
on active metals or metal compounds, or by the action of metals or metal 
compounds on carbon at high temperatures (electric furnace). Alkali and 
alkaline-earth acetylides are relatively stable, but most heavy-metal 
derivatives are thermodynamically unstable and may explode when dry. They are 
readily decomposed by dilute acid. Acetylides are salts of a very weak acid 
and therefore hydrolyze readily. The most important acetylide is calcium 
carbide, which is used to prepare acetylene. See ACETYLENE; CARBIDE. 
 �34s[Elbert H. Hadley]
�33s 
  

A700000001520�254s�5v�1v8
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�6v�2v�32sAcetyl phosphate   �21m �33s 
    The anhydride of acetic and phosphoric acids, with the formula below. The 
compound occurs in the intermediary metabolism of pyruvic acid and of pentose 
sugars by some bacteria. 

                        *** CHEMICAL STRUCTURE ***
                                      
    When acetyl phosphate acts as an acetylating agent in the presence of 
transacetylase, the acetyl group can be transferred reversibly to coenzyme A 
to yield phosphoric acid and acetyl coenzyme A. In the presence of a kinase, 
acetyl phosphate can phosphorylate adenosinediphosphate with the formation of 
acetic acid and of adenosinetriphospate. See ADENOSINEDIPHOSPHATE (ADP); 
ADENOSINETRIPHOSPHATE (ATP); BIOCHEMISTRY; CARBOHYDRATE METABOLISM; METABOLISM. 
 �34s[Michael Doudoroff]
�33s 
  

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�6v�2v�32sAchondroplasia   �21m �33s 
    A generalized deforming disease of the skeletal system, also known as 
chondrodystrophy fetalis. It is a hereditary disease and is thought to be 
transmitted as a simple mendelian dominant. Its effects usually appear within 
a few months after birth, and the deformity becomes progressively more severe 
as the individual matures. 
    The basic defect is an abnormality of the cartilaginous growth zones 
located at the ends of growing bones. There is retardation of growth of 
cartilage in the epiphysis and defective formation of new bone. The result is 
an abnormally short, thick, and misshapen bone. 
    Although the name implies complete absence of cartilage growth, such  
severe involvement is uncommon and results in early death or stillbirth. More 
commonly there is only partial retardation of cartilage growth, which may vary 
from slight, causing only moderately shortened stature, to severe, resulting 
in profound dwarfism. 
 �34s[W. Robert Adams]
�33s 
  
A700000007536�254s�5v�1v9
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�6v�2v�32sAcid and base �21m �33s
    Two interrelated classes of chemical compounds, the precise definitions of
which have varied considerably with the development of chemistry. These
changing definitions have led to frequent controversies, some of which are
still unresolved.
    Acids initially were defined only by their common properties as substances
which had a sour taste, dissolved many metals, and reacted with alkalies (or
bases) to form salts. For a time it was believed that a common constituent of
all acids was the element oxygen, but gradually it became clear that, if there
were an essential element, it was hydrogen, not oxygen. This concept of an
acid proved to be satisfactory for about 50 years.
    Bases initially were defined as those substances which reacted with acids
to form salts (they were the ``base'' of the salt). The alkalies, soda and
potash, were the best-known bases, but it soon became clear that there were
other bases, notably ammonia and the amines.
    When the concept of ionization of chemical compounds in water solution
became established, acids were defined as substances which ionized in aqueous
solution to give hydrogen ions, H POSITIVE, and bases were substances
which reacted to give hydroxide ions, OH NEGATIVE. These definitions are
sometimes known as the Arrhenius-Ostwald theory of acids and bases. Their use
makes it possible to discuss acid and base equilibria and also the strengths
of individual acids and bases.
    A powerful and wide-ranging protonic theory of acids and bases was
introduced by J. N. Broensted in 1923 and was rapidly accepted. Somewhat
similar ideas were advanced almost simultaneously by T. M. Lowry and the new
theory is occasionally called the Broensted-Lowry theory. The Broensted
definitions of acids and bases are: An acid is a species which can act as a
source of protons; a base is a species which can accept protons. Compared to
the water (Arrhenius) theory, this represents only a slight change in the
definition of an acid but a considerable extension of the term base. In
addition to hydroxide ion, the bases now include a wide variety of uncharged
species, such as ammonia and the amines, as well as numerous charged species,
such as the anions of weak acids. In fact, every acid can generate a base by
loss of a proton. Acids and bases which are related in this way are known as
conjugate acid-base pairs, and the table lists several examples.
    As the table shows, strengths of acids and bases are not independent. A
very strong Broensted acid implies a very weak conjugate base and vice versa.
A qualitative ordering of acid strength or base strength permits a rough
prediction of the extent to which an acid-base reaction will go. The rule is
that a strong acid and a strong base will react extensively with each other,
whereas a weak acid and a weak base will react together only very slightly.
    Studies of catalysis have played a large role in the acceptance of a set
of quite different definitions of acids and bases, those due to G. N. Lewis:
An acid is a substance which can accept an electron pair from a base; a base
is a substance which can donate an electron pair. Bases under the Lewis
definition are very similar to those defined by Broensted, but the Lewis
definition for acids is very much broader.
    Another comprehensive theory was proposed by M. Usanovich in 1939 and is
sometimes known as the positive-negative theory. Acids are defined as
substances which form salts with bases, give up cations, and add themselves to
anions and free electrons. Bases are similarly defined as substances which
give up anions or electrons and add themselves to cations. So far, this theory
has had little acceptance, quite possibly because the definitions are too
broad to be very useful. See BASE (CHEMISTRY); BUFFERS (CHEMISTRY); HYDROGEN
ION.

 �34s[Franklin A. Long; Richard H. Boyd]
�33s


A700000002743�254s�5v�1v9
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�6v�2v�32sAcid anhydride   �21m �33s 
    One of an important class of reactive organic compounds derived from acids 
via formal intermolecular dehydration; thus acetic acid, on loss of water, 
forms acetic anhydride. Anhydrides of straight-chain acids containing from 2 
to 12 carbon atoms are liquids with boiling points higher than those of the 
parent acids. They are relatively insoluble in cold water and are soluble in 
alcohol, ether, and other common organic solvents. The lower members are 
pungent, corrosive, and weakly lacrimatory. Anhydrides from acids with more 
than 12 carbon atoms and cyclic anhydrides from dicarboxylic acids are 
crystalline solids. 
    Because the direct intermolecular removal of water from organic acids is 
not practicable, anhydrides must be prepared by means of indirect processes. A 
general method involves interaction of an acid salt with an acid chloride. 
    Large quantities of anhydrides are used in the preparation of esters. 
Cellulose and acetic anhydride give cellulose acetate, used in acetate rayon 
and photographic film. The reaction of anhydrides with sodium peroxide forms 
peroxides (acetyl peroxide is violently explosive), used as catalysts for 
polymerization reactions and for addition of alkyl halides to alkenes. In 
Friedel-Crafts reactions, anhydrides react with aromatic compounds, forming 
ketones such as acetophenone. See ACID HALIDE; ESTER; FRIEDEL-CRAFTS REACTION. 
 �34s[Paul E. Fanta]
�33s 
  
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�6v�2v�32sAcid-base indicator �21m �33s
    A substance that reveals, through characteristic color changes, the degree
of acidity or basicity of solutions. Indicators are weak organic acids or
bases which exist in more than one structural form (tautomers) of which at
least one form is colored. Intense color is desirable so that very little
indicator is needed; the indicator itself will thus not affect the acidity of
the solution.
    Acid-base indicators are commonly employed to mark the end of an acid-base
titration or to measure the existing pH of a solution. Care must be used to
compare colors only within the indicator range. A color comparator may also be
used, employing standard color filters instead of buffer solutions.
    The indicator range is the pH interval of color change of the indicator. In
this range there is competition between indicator and added base for the
available protons; the color change, for example, yellow to red, is gradual
rather than instantaneous. Observers may, therefore, differ in selecting the
precise point of change.
    The table lists many of the common indicators, their ranges of pH and
color change, and pK values. See ACID AND BASE; HYDROGEN ION; TITRATION.

_FIELD034;-237[Allen L. Hanson]_FIELD034
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�6v�2v�32sAcid-fast stain   �21m �33s 
    A differential stain used in microbiology as one of the criteria in the 
identification of the species in the genus Mycobacterium and one species in 
the genus Nocardia. The stain technique is a measure of the resistance of 
stained bacteria to decolorization by acids. The air-dry smear is usually 
fixed by heating, and stained with a hot solution of carbol fuchsin. This is 
followed by rinsing with water and exposure to a solution of 3% by volume of 
concentrated hydrochloric acid in water or in 95% ethanol. The smear is then 
washed with water and counterstained with methylene blue. Acid-fast substrates 
appear red, and other substrates blue. Because acid-fast cells are also 
gram-positive, use of the alcohol hastens decolorization without altering the 
results. See ACTINOMYCETACEAE; GRAM'S STAIN. 
 �34s[Georges Knaysi]
�33s 
  

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�6v�2v�32sAcid halide   �21m �33s 
    One of a large group of organic substances possessing the halocarbonyl 
group, in which X stands for fluorine, chlorine, bromine, or iodine. The terms

                         *** CHEMICAL STRUCTURE *** 

acyl and aroyl halides refer to aliphatic or aromatic derivatives, 
respectively. 
    The great inherent reactivity of acid halides precludes their free 
existence in nature; all are made by synthetic processes. In general, acid 
halides have low melting and boiling points and show little tendency toward 
molecular association. With the exception of the formyl halides (which do not 
exist), the lower members are pungent, corrosive, lacrimatory liquids that 
fume in moist air. The higher members are low-melting solids. 
    Acid chlorides are prepared by replacement of carboxylic hydroxyl of 
organic acids by treatment with phosphorus trichloride, phosphorus 
pentachloride, or thionyl chloride. 
    Although acid bromides may be prepared by these methods, acid iodides are 
best prepared from the acid chloride treatment with either CaI2 or HI, and 
acid fluorides from the acid chloride by interaction with HF or antimony 
fluoride.  
    The reactivity of acid halides centers upon the halocarbonyl group, 
resulting in substitution of the halogen by appropriate structures. Thus with 
substances containing active hydrogen atoms (for example, water, primary and 
secondary alcohols, ammonia, and primary and secondary amines), hydrogen 
chloride is formed together with acids, esters, amides, and N-substituted 
amides, respectively. 
 �34s[Paul E. Fanta]
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�6v�2v�32sAcidolysis   �21m �33s 
    A chemical reaction involving the decomposition of a molecule with the 
addition of the elements of the acid. It is sometimes called acyl exchange. 
Acidolysis is most commonly an exchange reaction that is catalyzed by 
concentrated sulfuric acid, zinc chloride, or boron trifluoride. In many cases 
such reactions are reversible and thus will not go far toward completion 
unless one of the materials produced is removed as fast at it is formed by 
distillation or by reaction with a third component. Acidolysis is most 
commonly applied to the reaction of an organic acid and acetic anhydride to 
yield an acid anhydride and acetic acid. The acidolysis of esters and acyl 
halides is a convenient method for the preparation of other esters and acyl 
halides. It is also a useful reaction to convert neutral esters of dibasic 
acids to acid esters. The usual method for the preparation of acyl halides 
from an acid and phosphorus trichloride or thionyl chloride can also be 
considered to be acidolysis if these reagents are considered the acyl halides 
of phosphorous acid and sulfurous acid, respectively. See ALCOHOLYSIS; 
HYDROLYSIS.  
 �34s[Elbert H. Hadley]
�33s 
  
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�6v�2v�32sAcid rain   �21m �33s 
    Atmospheric precipitation with a pH below 5.6 - 5.7; such hydrogen ion 
concentrations are obtained when pure water is in solution equilibrium with 
aerial carbon dioxide at 77 DG F (25 DG C). Most researchers consider acid rain 
to be of significance when pH values are at least tenfold more acid than 5.7, 
that is, 4.7 and below. Alkaline rains are also found, particularly where 
limestone or alkaline dusts enter the atmosphere. Reports of acid rain are 
common throughout the Western world and in Japan, with pH values of 3.8 - 4.5 
becoming almost the norm. Snow, too, is now acid, as well as fogs and rime ice 
formations.  
    There are several natural sources of acid rain. Among the products of 
volcanic eruptions are sulfur compounds. The hot springs of Arkansas and the 
fumaroles in Yellowstone National Park contribute acidity to the air, and 
considerable amounts of oxides of nitrogen and sulfur are end products of the 
metabolism of several groups of bacteria. In spite of these natural air 
pollutants, the pH of glacial ice is close to 5.0, indicating that natural 
emissions of acid-forming compounds are not major sources of acid rain; 
anthropogenic sources are the primary contributors. 
    Burning of fossil fuels for heat and power is the major factor in the 
generation of oxides of nitrogen and sulfur which are convertible into nitric 
and sulfuric acids washed down in rain. Wood smoke, too, has significant 
acid-forming potential. Although gasoline contains only small amounts of 
nitrogen and sulfur, the modern high-compression engine can produce nitrogen 
oxides directly from the gaseous nitrogen that makes up about 75% of ordinary 
air. The combination of air compression in the cylinders, the igniting spark, 
and the catalytic action of cylinder metal converts N2 into NOx, which, in 
turn, can be converted into HNO3. Pulp and paper mills use acid bisulfites, 
sulfurous acid, and sodium sulfide to remove lignin from wood pulp, and the 
oxidized sulfur compounds are vented into the air. Mining and smelting 
operations involve the conversion of sulfides and other complexes of heavy 
metals into pure copper, cadmium, zinc, or iron, with the sulfur oxidized to 
SO2. Small but significant quantities of sulfur and nitrogen compounds reach 
the air from many industries. 
    All these sources have been active for more years than acid rain has been 
found. Modern technology is believed to have speeded up the process of 
acidification of precipitation. Starting about 1950, smokestacks were built to 
great heights in order to reduce local deposition of acid-forming substances 
in many steel towns, and communities with coal- and oil-burning 
power-generating plants. Thus, instead of SO2 and NOx or heavy metals 
dropping within 6 mi (10 km) of the source with only limited airborne time for 
conversion of oxides into acids, the acid-forming compounds are now carried 
into the upper atmosphere to be transported for distances known to exceed 600 
mi (1000 km). The development of electronic precipitators and filters to 
remove particulate matter as mandated by various national and local ordinances 
was also a factor, since fly ash is alkaline and potentially could neutralize 
acid-forming materials in the stacks. Catalytic converters in automobiles can 
oxidize sulfur and nitrogen compounds directly into acids. 
    The effects of acid rain on structures have been amply documented. 
Exfoliation of marble and limestone monuments and buildings and pitting of 
granitic stonework are common all over the industrial world. Biological 
consequences are also evident. Many remote, pristine lakes in northern Europe 
are virtually sterile, devoid of fish (particularly trout and salmon), and 
depauperate in invertebrates and most plants. A much more serious potential 
problem may exist in certain natural terrestrial ecosystems. Research suggests 
that the interaction of acid rains, fogs, and snows with anthromorphically 
caused additions of heavy metals (Pb, Cu, Zn) and aluminum can interfere with 
growth of many important microflora components of forested ecosystems.  See  
ECOSYSTEM; pH. 
 �34s[Richard M. Klein]
�33s 
  

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�6v�2v�32sAcipenseriformes �21m �33s
    An archaic order of actinopterygian fishes, represented by the sturgeons
and paddlefishes. The characters include a highly chondrified internal
skeleton; fins which are archaic and sharklike in appearance, with more than
one ray per pterygiophore; a caudal fin which is strongly heterocercal; scales
which are reduced and often form five lengthwise series of bony plates;
rostrum produced; and weak jaws (see illustration). Fleshy barbels are present
on the lower surface of the snout. The cartilaginous skeleton of the
acipenseriform fishes was long regarded as primitive, and was taken as
indicative of alliance with the Chondrichthyes. Research reveals, however,
that this is secondary, and the true relationship is with the typical bony
fishes.
    The Acipenseriformes include three families: the extinct Chondrosteidae;
the sturgeon family, Acipenseridae, with 4 Recent genera and about 23 species;
and the paddlefish family, Polyodontidae, consisting of two Recent species,
Psephurus gladius of China and Polyodon spathula of the eastern United States.
See ACTINOPTERYGII; OSTEICHTHYES; STURGEON.

 �34s[Reeve M. Bailey]
�33s



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�6v�2v�32sAckerman steering �21m �33s
    Differential gear or linkage that turns the two steered road wheels of a
self-propelled vehicle so that all wheels roll on circles with a common
center. If a vehicle is to turn without lateral skid of any wheel, the center
lines of all wheel axles must intersect, when extended, at every instant in a
common center about which the vehicle turns. This requirement is the Ackerman
principle of toe-out on turns. It is used universally on wheeled vehicles. For
straight, forward motion, the front wheels are substantially parallel, but as
the vehicle enters a curve the inner wheel turns more sharply than the outer
wheel. The extreme condition occurs when the vehicle is on a curve of its
turning radius.
    A common configuration that produces Ackerman steering inclines the
knuckle arms inwardly and rearwardly (see illustration). The angle of
inclination depends on wheelbase and tread of the vehicle. Wheelbase is the
distance from front to rear wheels, measured between centers of ground
contact. Tread is the distance between front or rear wheels, measured from
centers of ground contact. See FOUR-BAR LINKAGE.


 �34s[William K. Creson]
�33s



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�6v�2v�32sAcne   �21m �33s 
    A pleomorphic skin disease, commonly referring to acne vulgaris, 
characterized by blackheads, whiteheads, papules, pustules, and various-sized 
nodules and scars. The disease involves the sebaceous follicle characterized 
by large sebaceous glands and ducts and small hairs. See SEBACEOUS GLAND; SKIN. 
    Sebum, a complex lipid liberated by the breakdown of the sebaceous cells, 
is intimately associated with the development of acne. A perifolliculitis 
results following rupture of the follicular contents (sebum) into the dermis. 
This occurs secondary to obstruction of the opening of the sebaceous follicle 
by a whitehead or blackhead. The inflammatory response eventually heals with 
scar formation, the ultimate appearance of which is dependent on the depth and 
exuberance of the inflammatory response. 
    Other forms of acne are recognized, the most common being acne rosacea, 
which occurs in older persons. This is seen as reddened inflamed areas of the 
face, especially the nose and cheeks, and occurs in susceptible individuals 
following local irritation, faulty digestion, alcoholism, and other 
predisposing causes. 
 �34s[N. Karle Mottet]
�33s 
  

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�6v�2v�32sAcoela   �21m �33s 
    An order of marine Turbellaria, generally regarded as primitive, and 
lacking protonephridia, oviducts, yolk glands, permanent digestive cavity, and 
strictly delimited gonads. The nervous system is epidermal in the most 
primitive representatives, but it is generally submuscular and consists of 
three to six pairs of longitudinal strands without anterior concentrations 
recognizable as a brain. The anterior or median ventral mouth opens into a 
simple pharynx which lacks elaborate musculature and leads directly to the 
syncytial vacuolated mass of endoderm forming the spongy intestinal tissue and 
lacking any true lumen. 
    Eyes are often absent; when present they are of the usual paired 
turbellarian pigment-cup type, with one to a few pigment and retinal cells. 
The epidermis, typically, is uniformly ciliated, and locomotion is 
accomplished by means of rapid, smooth ciliary gliding or swimming. 
    The acoels (see illustration) are mostly small (one to several millimeters 
in length). They are virtually worldwide in distribution, living beneath 
stones, among algae, on the bottom mud, or interstitially, from the littoral 
zone to deeper waters. A few are pelagic (in tropical and subtropical zones), 
and others have become symbiotic, living entocommensally in various 
invertebrates but mainly in echinoderms. See COELENTERATA; PLATYHELMINTHES; 
TURBELLARIA.  
    Figure caption: Acoela: Childia spinosa.
 �34s[J. B. Jennings]
�33s 
  
  

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�6v�2v�32sAconchulinida   �21m �33s 
    An order of the subclass Filosia comprising a small group of naked amebas 
which form filamentous pseudopodia (filopodia). The genus Penardia, possibly 
the only valid one in the order, includes filopodia-forming amebas with 
variable shape, an outer zone of clear ectoplasm, and inner cytoplasm often 
containing many vacuoles and sometimes zoochlorellae (see illustration). 
Pseudopodia are sometimes extended primarily at the poles of the organism, but 
also may fuse laterally to form small sheets of clear cytoplasm. A single 
nucleus and one water-elimination vesicle are characteristic of these amebas. 
Size of described species ranges from less than 10 to about 400 micrometers. 
See FILOSIA; PROTOZOA; RHIZOPODEA; SARCODINA; SARCOMASTIGOPHORA. 
    Figure caption: Aconchulinida. (a) Penardia cometa. (b) P. mutabilis.
 �34s[Richard P. Hall]
�33s 
  
  

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�6v�2v�32sAcoustical holography �21m �33s
    A technique that utilizes the interference pattern formed when two beams
of sound intersect to form an image of an object placed in one of the sound
beams.
    The greatest interest in holography has resulted from its ability to
generate an extremely realistic three-dimensional visual image. However,
because the technique of holography involves the recording of a wave
interference pattern, any process involving waves, such as sound waves or
electromagnetic microwaves, can utilize the technique. Accordingly, numerous
acoustical holography experiments have been made, and they have shown promise
in the medical field (for locating cysts and tumors with x-ray-like acoustic
pictures), in underwater sound (for detecting underwater objects), and as a
supplement to present seismic procedures in locating offshore oil deposits.
See HOLOGRAPHY; INTERFERENCE OF WAVES.
    One procedure which permits the real-time viewing of acoustic holograms
uses two underwater quartz-crystal transducer sound sources (see
illustration). The sound waves of interest (those originating at the object
transducer) and the reference waves (those originating at the reference
transducer) generate an interference pattern at the surface of the liquid,
causing it to have extremely minute, stationary ripples (tiny ridges). These
mechanical deformations of the liquid surface correspond exactly to the
interference fringes of a hologram, so that immediate reconstruction of the
acoustic hologram image is accomplished by causing laser light to be
diffracted by these ridges as it is reflected off the liquid surface. This
technique differs from the earlier reflection or sonar techniques, such as the
procedure known as B-scan, in that it provides through-transmission images
comparable to the images formed when x-rays are transmitted through the
object. See DIFFRACTION.
    The synthetic aperture technique was first employed in radar. In it, a
radiator emits successive pulses of a highly coherent
(very-constant-frequency) signal. Because of this high coherence, the many
echoes that return can be processed as though a single line-radiator having a
length equal to the path had been used. The effective radiator length is thus
quite large, and this large ``synthetic'' aperture provides records having
extremely fine detail. This imaging technique has proved to be a very
important addition to radar technology, and accordingly, extensive
investigations have been undertaken to exploit its possibilities in various
acoustical holography fields, including sonar, the seismic technologies, and
the medical sonar technique called B-scan. See RADAR.
    Sonar technology employing acoustics for detecting underwater objects has
also benefited from acoustical holography developments. The hologram synthetic
aperture technique discussed above also appears to be valuable in sonar. See
SONAR.
    Geophysical prospecting often employs acoustic waves to appraise the
likelihood that oil or gas is present in the substructure. Here, too,
acoustical holography has proved useful, permitting special techniques (such
as partial filtering) to suppress, in the records, artifacts which would
otherwise obscure the desired information. See ACOUSTIC MICROSCOPE; SOUND;
ULTRASONICS.

 �34s[Winston E. Kock]
�33s



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�6v�2v�32sAcoustical image   �21m �33s 
    If a point source of sound is placed on one side of an extended reflecting 
surface, it may be considered to have an ``image'' at an equivalent distance 
on the other side of the surface along a perpendicular projection, analogous 
to the familiar optical image. The effects of reflecting surfaces on acoustic 
waves frequently can be predicted by the use of such images. A source in front 
of a very reflective wall will have an image of ``strength'' almost equal to 
that of the source, vibrating in phase with the source. To obtain the total 
effect at any point due to the combined action of a source and such a 
reflecting surface, the effects due to the source itself and another source of 
equal strength placed at the image point are added. If there is a second 
reflecting surface present, this so-called first-order image will have an 
image, called the second-order image of the source, at an equivalent distance 
on the other side of the second reflecting surface. Similarly, an image of the 
second-order image is said to be a third-order image, and so on. Although this 
method is precise only when the reflecting surface is nonabsorptive, it is 
often of considerable help in investigations of the action of sound waves in 
rooms. See ARCHITECTURAL ACOUSTICS; OPTICAL IMAGE. 
 �34s[Cyril M. Harris]
�33s 
  

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�6v�2v�32sAcoustic emission �21m �33s
    The phenomenon of transient elastic-wave generation due to a rapid release
of strain energy caused by a structural alteration in a solid material; also
known as stress wave emission. Generally these structural alterations are a
result of either internally or externally applied mechanical or thermal
stresses. Depending on the source mechanism, acoustic emission signals may
occur with frequencies ranging from several hertz up to tens of megahertz.
Often the observed acoustic emission signals are classified as one of two
types: burst and continuous (see illustration). The importance of acoustic
emission monitoring lies in the fact that proper detection and analysis of
acoustic emission signals can permit remote identification of source
mechanisms and the associated structural alteration of solid materials. This
information in turn can augment understanding of material behavior, can be
used as a quality-control method during materials processing and fabrication,
and as a nondestructive evaluation technique for assessing the structural
integrity of materials under service conditions. See NONDESTRUCTIVE TESTING;
SOUND.


 �34s[Robert E. Green, Jr.]
�33s



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�6v�2v�32sAcoustic filter �21m �33s
    A device for transmitting a given series of waves whose frequencies lie in
a preassigned frequency range, called the passband, and attenuating
frequencies which lie outside this range. The largest field of application of
such filters is in mufflers used on automobiles, exhaust engines, air ducts,
and so forth. Since in a muffler a steady or slowly varying stream of gas must
be permitted to pass while the more rapid vibrations constituting the
undesired noise must be suppressed, a low-pass acoustic filter is required in
this case. Such filters in the low-frequency range can be designed in direct
analogy to the design of electrical filters. The illustration shows the design
of a simple low-pass filter consisting of two short tubes placed in the ends
of a closed tank, with the equivalent electrical analogs and the type of
attenuation. Such so-called lumped impedance methods are applicable when the
length of the elements is less than an eighth of a wavelength. See ACOUSTIC
NOISE; ELECTRIC FILTER; MUFFLER.


 �34s[Warren P. Mason]
�33s



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�6v�2v�32sAcoustic impedance   �21m �33s 
    At a given surface, the complex ratio of effective sound pressure averaged 
over the surface to the effective flux (volume velocity or particle velocity 
multiplied by the surface area) through it. The unit is the N PRODUCT s/m
sup 5 (newton-second/meter sup 5), or the mks acoustic ohm. In the cgs system 
the unit is the dyn PRODUCT s/cm sup 5 (dyne-second/centimeter sup 5). See 
SOUND PRESSURE. 
    Specific acoustic impedance is the complex ratio of the effective sound 
pressure at a point to the effective particle velocity at a point. The unit is 
the N PRODUCT s/m sup 3, or the mks rayl. In the cgs system the unit is the 
dyn PRODUCT s/cm sup 3, or the rayl. The difference between specific 
acoustic impedance and acoustic impedance is in the specification of impedance 
at a point, as compared to the average over a surface. 
    Characteristic acoustic impedance is the ratio of effective sound pressure 
at a point to the particle velocity at that point in a free, progressive wave. 
This ratio is equal to the product of the density of the medium times the 
speed of sound in the medium. The characteristic impedance of a sound wave is 
analogous to the characteristic electrical impedance of an infinitely long, 
dissipationless transmission line. It is common in acoustical analyses to 
represent specific acoustic impedances in terms of their ratio to the 
characteristic impedance of air. 
    Acoustic impedance, being a complex quantity, can have real and imaginary 
components analogous to those in an electrical impedance. In applying this 
analogy, the real part of the acoustic impedance is termed acoustic 
resistance, and the imaginary part is termed acoustic reactance. See 
ELECTRICAL IMPEDANCE. 
 �34s[William J. Galloway]
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�6v�2v�32sAcoustic interferometer   �21m �33s 
    A device for measuring the velocity and attenuation of sound waves in a 
gas or liquid. It operates by sending an ultrasonic beam of sound waves 
through the medium to be measured in order to obtain a standing-wave system 
between the driving crystal and a reflector whose distance from the crystal 
can be varied by a screw system. An oscillator is attached to the crystal and 
tuned to the crystal's resonant frequency. The variation in the amplitude of 
current from the oscillator is measured as the distance between the crystal 
and the reflector is varied. At half-wavelength intervals, sharp dips occur in 
the received current, indicating that a high mechanical impedance is impressed 
on the end of the vibrating crystal by the standing-wave system between the 
crystal surface and the reflecting plate. By counting the number of half 
wavelengths occurring in a given displacement of the reflecting plate and by 
knowing accurately the frequency of vibration, it is possible to determine the 
velocity of propagation. See ULTRASONICS. 
 �34s[Warren P. Mason]
�33s 
  
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�6v�2v�32sAcoustic levitation   �21m �33s 
    The use of intense acoustic waves to suspend a body which is immersed in a 
fluid medium without obvious mechanical support. 
    Levitation can occur in the presence of fluid flow, including the 
back-and-forth fluid flow produced by the passage of an acoustic wave. Such 
acoustically generated forces are extremely small in common experience. But 
intense acoustic waves are nonlinear in their basic character and, therefore, 
may exert a net acoustic radiation pressure on an object sufficient to balance 
the gravitational force and thus levitate it. See ACOUSTIC RADIATION PRESSURE. 
    Applications of the levitation of objects in liquids have included 
measurement of the ultimate tensile strength of liquids, mechanical 
characterization of superheated and supercooled liquids, the measurement of 
properties of biological materials, and the study of shape oscillations and 
interfacial tension of levitated drops. See SOUND. 
 �34s[Robert E. Apfel]
�33s 
  
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�6v�2v�32sAcoustic microscope   �21m �33s 
    An instrument for monitoring elastic properties of microscopic scale. It 
has been used to study structural detail in integrated circuits, the grains 
and grain boundaries of complex materials, and intercellular detail in 
biology. It encompasses the domain that is now dominated by the optical 
microscope and provides a new dimension in microscopy. Acoustic radiation can 
be used to probe regions inaccessible to optical waves. See OPTICAL 
MICROSCOPE; SCANNING ACOUSTIC MICROSCOPE; SONOSCAN. 
 �34s[C. F. Quate]
�33s 
  
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�6v�2v�32sAcoustic mine   �21m �33s 
    A naval mine which is actuated by acoustic means. Such a mine generally 
rests on the bottom in fairly shallow water. It is set off by the noise 
emitted by its intended target as the target passes near by. Since the mine 
lies on the bottom, it can contain much more explosive charge than a buoyed 
mine (perhaps 500 - 1500 lb or 200 - 700 kg). Very often, the acoustic 
triggering mechanism is used in conjunction with another triggering device, 
such as a magnetic sensor. This makes defensive minesweeping more difficult, 
since the mine will not fire unless both an acoustic and a magnetic signal are 
received simultaneously. Minesweepers employ high-power underwater acoustic 
generators in attempting to fire such mines at safe distances. See ACOUSTIC 
TORPEDO.  
 �34s[Robert W. Morse]
�33s 
  
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�6v�2v�32sAcoustic noise   �21m �33s 
    Unwanted sound. Noise control is the process of obtaining an acceptable 
noise environment for people in different situations. Understanding noise and 
its control requires a knowledge of the major sources of noise, sound 
propagation, human response to noise, and the physics of methods of 
controlling noise. The continuing increase in noise levels from many different 
human activities in industrialized societies has led to the term noise 
pollution.  
    Noise as an unwanted by-product of an industrialized society affects not 
only the operators of machines and vehicles, but also other occupants of 
buildings in which machines are installed, passengers of vehicles, and most 
importantly the communities in which machines, factories, and vehicles are 
operated.  
 �34s[Malcolm J. Crocker]
�33s 
  
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�6v�2v�32sAcoustic radiation pressure   �21m �33s 
    The net pressure exerted on a surface or interface by an acoustic wave. 
One might presume that the back-and-forth oscillation of fluid caused by the 
passage of an acoustic wave will not exert any net force on an object, and 
this is true for sound waves normally encountered. Intense sound waves, 
however, can exert net forces in one direction of sufficient magnitude 
(proportional to the sound intensity) to balance gravitational forces and thus 
levitate an object in air. 
    Forces due to acoustic radiation pressure have been used to calibrate 
acoustic transmitters, to deform and break up liquids, to collect like 
objects, and to position objects in a sound field, sometimes levitating the 
sample so that independent studies of the object's properties can be 
performed. See ACOUSTIC LEVITATION; SOUND. 
 �34s[Robert E. Apfel]
�33s 
  
A700000002016�254s�5v�1v13
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1443328
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1572864
�6v�2v�32sAcoustic radiometer   �21m �33s 
    A device for measuring the intensity of sound waves in a gas or liquid. 
The oldest and simplest piece of apparatus for measuring sound intensity is 
the Rayleigh disk. This consists of a thin disk of radius r set at a 45 DG 
angle to the direction of the sound beam. Such a disk experiences a torque TAU 
with magnitude given by the equation below,  

                    *** PI = 4/3 RHOr sup 3 dXI/dt sup 2 ***

where RHO is the density of the medium and dXI/dt the particle velocity. This 
formula holds for wavelengths that are large compared with the diameter of the 
disk.  
    Other radiometers use the pressure of sound waves to deflect a spherical 
body. Since the radiation pressure is p = 2E, where E is the energy of the 
acoustic wave per unit area, a direct measurement of this pressure will 
determine the energy of a plane wave. See ACOUSTIC RADIATION PRESSURE; SOUND 
PRESSURE.  
 �34s[Warren P. Mason]
�33s 
  

A700000003136�254s�5v�1v14
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1572864
�6v�2v�32sAcoustic resonator   �21m �33s 
    A device consisting of a combination of elements having mass and 
compliance whose acoustical reactances cancel at a given frequency. Resonators 
are often used as a means of eliminating an undesirable frequency component in 
an acoustical system. In other instances resonators are used to produce an 
increase in the sound pressure in an acoustic field at a particular frequency. 
    Resonators are useful most often in the control of low-frequency sound. 
They are of particular value in reducing the noise from sources having 
constant frequency excitation. 
    Resonators have also found considerable application in architectural 
acoustics. It is often difficult to obtain adequate control of reverberation 
time at low frequencies in a large studio or auditorium using conventional 
acoustical materials. A number of designs for these spaces have included the 
construction of resonators behind walls or in the ceiling to obtain increased 
low-frequency absorption and thus provide more satisfactory reverberation 
characteristics. See ARCHITECTURAL ACOUSTICS. 
    The Helmholtz resonator (see illustration) is the simplest and most often 
utilized acoustical resonator. The unit consists of a straight tube of length 
l and cross-sectional area S, connected to a closed volume V. This combination 
is directly analogous to the simple series LC electrical circuit. See ACOUSTIC 
FILTER; ACOUSTIC IMPEDANCE; MUFFLER. 
    Figure caption: Helmholtz resonator. (a) Acoustical unit. (b) Electrical 
analog.
 �34s[William J. Galloway]
�33s 
  

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1691136
�6v�2v�32sAcoustics   �21m �33s 
    The science of sound, which in its most general form endeavors to describe 
and interpret the phenomena associated with motional disturbances from 
equilibrium of elastic media. An elastic medium is one such that if any part 
of it is displaced from its original position with respect to the rest, as for 
example by an impact, it will return to its original state when the disturbing 
influence is removed. Acoustics was originally limited to the human experience 
produced by the stimulation of the human ear by sound incident from the 
surrounding air. Modern acoustics, however, deals with all sorts of sounds 
which have no relation to the human ear, for example, seismological 
dusturbances and ultrasonics. 
    Basic acoustics may be divided into three branches, namely, production, 
transmission, and detection of sound. See ACOUSTIC NOISE; ARCHITECTURAL 
ACOUSTICS; ATMOSPHERIC ACOUSTICS; ELECTROACOUSTICS; HYPERSONICS; LOUDSPEAKER; 
MECHANICAL VIBRATION; MICROPHONE; PHYSIOLOGICAL ACOUSTICS; PSYCHOACOUSTICS; 
SOUND; SOUND RECORDING; SOUND-REINFORCEMENT SYSTEM; SOUND-REPRODUCING SYSTEMS; 
ULTRASONICS; UNDERWATER SOUND; VIBRATION; VIBRATION DAMPING; VIBRATION 
ISOLATION.  
 �34s[R. Bruce Lindsay]
�33s 
  
A700000004623�254s�5v�1v14
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�6v�2v�32sAcoustic signal processing   �21m �33s 
    A discipline that deals generally with the extraction of information from 
signals in the presence of acoustic noise. In the undersea, atmospheric, and 
solid earth environments both signals and noise are propagated as scalar 
pressure fields which are supported within the medium at audio frequencies 
ranging from a few hertz to several kilohertz. Because the transport 
mechanisms vary markedly and in a random way as a function of geometry and of 
time, the pressure fields are, at best, statistically described. The purpose 
of signal processing is to implement computational algorithms to produce 
statistics which can be used to make minimum risk decisions about the measured 
signal and noise parameters. 
    The detection, estimation, classification, and extraction of acoustic 
signals in a random, not necessarily homogeneous, medium are problems of 
importance in sonar and seismic technologies. In the acoustic environment, the 
signals, which contain the desired information as well as the noise (or 
undesired interference), have both spatial and temporal characteristics that 
are not, in general, appropriately described by simple ideal models. Hence, 
the treatment of the detection problem must include, explicitly, the physics 
of the medium, the geometry of the system, and the manner in which coupling to 
the medium is achieved. See ATMOSPHERIC ACOUSTICS; SEISMOLOGY; SONAR. 
    Acoustic signal processing is usually concerned with three basic systems. 
The active detection system is made up of a transmitter which ensonifies the 
medium, a target (if one is present), and a receiver situated either at the 
transmitter (the monostatic case) or at a different location (the bistatic 
case). The receiver must operate in the presence of reverberation noise 
(caused by the scattering of transmitted energy) as well as the usual ambient 
and system noise. 
    The passive detection system operates usually in the presence of ambient 
and system noise. In the passive operation (that is, surveillance) the system 
must detect and classify sources which radiate acoustic energy into the medium. 
    In a communication system the receiver must first acquire the transmitted 
information (detection) and then determine the waveform structure of the 
signal (extraction-estimation). 
 �34s[Daniel M. Viccione; W. A. Von Winkle]
�33s 
  
  

A700000001750�254s�5v�1v14
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�6v�2v�32sAcoustic torpedo   �21m �33s 
    A naval torpedo which locates its target by acoustic means. It is often 
called a homing torpedo. Except for the special method of finding its target, 
such a torpedo is similar to other types in size, warhead, and method of 
propulsion. Initially, it is aimed toward the general vicinity of the desired 
target; when sufficiently close for the acoustic detection system to acquire a 
fix on the target (perhaps a few hundred meters), the torpedo is steered the 
remaining distance by servomechanisms. The acoustic detector, which supplies 
target information for the guidance system, may be of two general types: 
passive, whereby the torpedo ``homes'' on sounds emitted by the target vessel, 
such as propeller noises; or active, in which the nose of the torpedo contains 
a small sonar device which reflects pulses of sound off the target. See 
ACOUSTIC MINE; SONAR; UNDERWATER SOUND. 
 �34s[Robert W. Morse]
�33s 
  
A700000005472�254s�5v�1v15
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1691136
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1903104
�6v�2v�32sAcoustooptics   �21m �33s 
    That field of scientific investigation dealing with the interaction of 
acoustical and optical energy. The interaction usually occurs in a medium 
capable of supporting the propagation of both forms of energy, that is, one 
which is reasonably transparent to both. The chief acoustooptic effect is the 
change of the optical index of refraction of the material under the influence 
of strain caused by the acoustic wave. As the pressure on a material changes, 
so does its density and thus its index of refraction. See LIGHT; WAVE MOTION. 
    Although most acoustooptic effects were described and observed before 
1935, their practical application awaited their successful union with lasers. 
Acoustooptical devices are being increasingly used in image scanners, 
recorders, and projectors, as well as in a large number of signal-processing 
devices and laser controllers. See LASER. 
    In the interaction of a light and a sound beam, where each is reasonably 
well collimated and monochromatic, and they cross each other at right angles 
in an acoustooptical material, let the diameter of the light beam be small 
compared to the wavelength of the sound. Under these conditions the light beam 
is alternately bent (refracted) back and forth from its original direction as 
the peaks and valleys of the acoustic pressure wave pass through the light 
beam. The acoustic wave acts as a time-varying prism for the light passing 
through it. This effect has been used in optical deflectors and control 
devices for lasers. As the frequency increases and the wavelength becomes 
comparable to the diameter, the acoustic disturbance appears as a time-varying 
lens, alternately converging and diverging the light beam. This effect has 
also been used to control certain laser systems. See REFRACTION OF WAVES. 
    When the sound wavelength has decreased well below the diameter of the 
light beam, the light now must interact simultaneously with a series of 
periodic variations in the index of refraction as the sound wave propagates 
through it. The light beam is diffracted by this ``acoustic diffraction 
grating'' into one or more additional light beams propagating in new 
directions angularly displaced from the initial direction. In this case, 
however, the ``grating'' is moving, and therefore causes a Doppler shift in 
the optical frequency of the diffracted light beams. See DIFFRACTION; DOPPLER 
EFFECT.  
    In addition to the effect of acoustic disturbances on optical beams, the 
opposite effect occurs. That is, two light beams can be made to interact in an 
acoustooptical material to produce a sound wave. This experiment has been made 
possible only by the availability of very-high-power, monochromatic laser 
beams.  
 �34s[Michael J. Brienza]
�33s 
  
A700000002536�254s�5v�1v16
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1903104
�6v�2v�32sAcquired immunological tolerance   �21m �33s 
    Failure of immunological responsiveness, brought about by exposure to 
antigen, in which antigen-sensitive cells become unable to initiate synthesis 
of a restricted range of antibodies. Through induced tolerance, animals can be 
prevented from rejecting grafts of foreign cells; rendered more sensitive to 
infection with pathogens; protected from experimentally induced autoimmune 
disease; and prevented from developing hypersensitivity to drugs. Tolerance 
derives its principal practical interest from possible future applications in 
surgery. It is hoped that permanent survival of transplanted organs will be 
obtained by inducing tolerance of donor antigen in the host, as can already be 
done in animal experiments. Other possible applications include treatment of 
allergy and autoimmune disease. See ANTIBODY; ANTIGEN; HYPERSENSITIVITY; 
IMMUNITY.  
    The terms immunological tolerance and immunological paralysis can be used 
interchangeably, although the former is applied mainly to tissue transplants 
and the latter only to purified antigens. Unresponsiveness to both types of 
antigen can be acquired in the same way, and probably involves the same 
cellular mechanism. See IMMUNOLOGY; IMMUNOPATHOLOGY; TRANSPLANTATION BIOLOGY. 
 �34s[N. A. Mitchison]
�33s 
  
A700000001075�254s�5v�1v16
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1903104
�6v�2v�32sAcrania   �21m �33s 
    A group name applied to the lower chordates - the tunicates of all kinds 
(Tunicata), and the lancelets (Cephalochordata). These animals have no cranium 
or vertebrae. The Acrania are contrasted with the Craniata or vertebrates - 
the cyclostomes, fishes, amphibians, birds, and mammals that have the 
characteristics mentioned. Some zoologists restrict the name Acrania to the 
lancelets or amphioxus (Branchiostoma). See CEPHALOCHORDATA; CHORDATA; 
CRANIATA; TUNICATA. 
 �34s[Walter J. Bock]
�33s 
  
A700000002363�254s�5v�1v16
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1903104
�6v�2v�32sAcrasida   �21m �33s 
    An order of Mycetozoia containing ``cellular'' slime-molds. The life cycle 
is unusual. As the bacterial food supply is exhausted in agar plate cultures 
of Dictyostelium discoideum, particular amebas (``evocators'') begin secreting 
acrasin, a substance which stimulates aggregation. The attracted amebas move 
in streams toward the evocator and begin secreting acrasin themselves, thus 
attracting more amebas. The amebas eventually become packed around the 
evocator to form a pseudoplasmodium, capable of sluggish locomotion. The 
pseudoplasm is later condensed into a slug, the name suggesting its form. 
Eventually, a reorganization produces an upright spherical sporangium on a 
stalk and base; in the complex process, groups of amebas migrate to final 
positions from different regions of the slug. A mature sporangium may contain 
100 - 100,000 cysts (spores), which are scattered when the wall is ruptured at 
maturity. A released spore hatches into an ameba, which begins another cycle. 
The order includes the genera Acrasis, Dictyostelium, Guttulina, 
Polyspondylium, and others. See MYCETOZOIA; PROTOZOA; RHIZOPODEA; SARCODINA; 
SARCOMASTIGOPHORA.  
 �34s[Richard P. Hall]
�33s 
  
A700000001033�254s�5v�1v16
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1903104
�6v�2v�32sAcrasiomycetes   �21m �33s 
    A class of microorganisms considered by some mycologists and many 
protozoologists to be related to protistans rather than to fungi because of 
their phagotrophism, mode of sporulation, biochemistry, and probable 
independent evolution from simpler euprotistans. There is also evidence that 
the cellular slime molds are polyphyletic in origin. The subclasses are 
Protostelidae, Acrasidae, and Dictostelidae.  See  MYXOMYCOTA. 
 �34s[Lindsay S. Olive]
�33s 
  
A700000001605�254s�5v�1v16
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1903104
�6v�2v�32sAcridine   �21m �33s 
    One of a group of organic heterocyclic compounds (also called 
9-azaanthracenes) containing benzene rings fused to the 2,3 and 5,6 positions 
of pyridine. Acridine, with the structure shown, is a typical member of the 
group. Several important dyes and medicinals are acridine derivatives. 

                        *** CHEMICAL STRUCTURE ***

    Acridine is a faintly yellow solid, mp 230-231 DG F (110 - 110.5 DG C), 
that shows marked fluorescence. The compound, as a tertiary amine, is a weak 
base (pKa 5.60 at 68 DG F or 20 DG C) forming simple and quaternary salts as 
well as Noxides. Acridine shows aromatic character. It is stable to heat, 
alkali, and acid, and it undergoes substitution reactions, such as bromination, 
nitration, and sulfonation. See DYE; HETEROCYCLIC COMPOUNDS. 
 �34s[Walter J. Gensler]
�33s 
  

A700000001471�254s�5v�1v16
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1903104
�6v�2v�32sAcrothoracica   �21m �33s 
    An order of the subclass Cirripedia. All members burrow into shells of 
mollusks and thoracican barnacles, echinoderm tests, polyzoans, dead coral, 
and limestone. The normal three pairs of cirriped mouthparts are present: 
mandibles, maxillules, and maxillae. Four to six pairs of cirri occur, often 
greatly reduced in size, the first pair close to the oral appendages and the 
remainder packed together at the posterior end of the body. The sexes are 
separate; dwarf males are found on the mantle or wall of the burrow of the 
female. Naupliar larval stages may be omitted, but a cypris larva always 
occurs in the life cycle. Three families, about eight genera, and 40 species 
are recognized. See CIRRIPEDIA. 
 �34s[H. G. Stubbings]
�33s 
  

A700000001513�254s�5v�1v16
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1903104
�6v�2v�32sAcrotretida   �21m �33s 
    An order of inarticulate brachiopods whose representatives are known in 
beds of all ages from Early Cambrian to the present time. 
    Within the order two suborders are recognized, the Acrotretidina and the 
Craniidina. Acrotretidina includes only species with shells of calcium 
phosphate. Cambrian and Early Ordovician representatives were typically small, 
commonly less than 0.08 in. (2 mm) in length, and biconvex or with a 
subconical pedicle valve. Craniidina probably developed from Acrotretidina in 
Early Ordovician times. Craniidines are not known to possess a pedicle; all 
forms are attached by cementing of part or all of the pedicle valve, a process 
seemingly effected by the periostracum. See BRACHIOPODA; INARTICULATA. 
 �34s[A. J. Rowell]
�33s 
  
A700000002133�254s�5v�1v16
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1903104
17
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2029056
�6v�2v�32sAcrylonitrile   �21m �33s 
    An explosive, poisonous, flammable liquid, boiling at 171 DG F (77.3 DG C), 
partly soluble in water. It may be regarded as vinyl cyanide, and its 
systematic name is 2-propenonitrile. Acrylonitrile is prepared by ammoxidation 
of propylene over various sorts of catalysts, chiefly metallic oxides. 
    Most of the acrylonitrile produced is consumed in the manufacture of 
acrylic and modacrylic fibers. Substantial quantities are used in 
acrylonitrile-butadiene-styrene (ABS) resins, in nitrile elastomers, and in 
the synthesis of adiponitrile by electrodimerization. Smaller amounts of 
acrylonitrile are used in cyanoethylation reactions, in the synthesis of 
drugs, dyestuffs, and pesticides, and as co-monomers with vinyl acetate, 
vinylpyridine, and similar monomers. 
    Acrylonitrile undergoes spontaneous polymerization, often with explosive 
force. It polymerizes violently in the presence of suitable alkaline 
substances. See CYANOETHYLATION; NITRILE; POLYMERIZATION. 
 �34s[Frank Wagner]
�33s 
  

A700000002762�254s�5v�1v17
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2029056
�6v�2v�32sActiniaria �21m �33s
    An order of the Zoantharia known as the sea anemones, which are the most
widely distributed of the anthozoans. Usually they are solitary animals which
live under the low-tide mark attached to some solid object by a basal
expansion or pedal disk. They feed on various prey such as copepods, mollusks,
annelids, crustaceans, and fish. The burrowing species lack a pedal disk and
bury their elongated bodies in the soft sediment of the oceans.
    The freely retractile, skeletonless polyp has a cylindrical body, with a
thick, tough, rough column wall often bearing rugae, verrucae, tubercles, or
suckers (see illustration). The body is often encrusted with sand grains,
pebbles, and other detritus. Some species have smooth, thin walls. Nematocysts
discharge a toxic substance; however, the human skin is seldom affected by
this. The colors of anemones vary with species and many variations occur even
among the same species. The tentacles increase in number regularly and are
arranged in several cycles. There are 6 primary, 6 secondary, 12 tertiary, 24
quaternary, and so forth in the hexamerous type. The musculature is the most
highly developed in the coelenterates.
    Most actinians are dioecious. Developing larvae pass through the Edwardsia
stage then the Halcampoides stage. Longitudinal fission frequently occurs as
well as budding. Sometimes new individuals result from laceration. See
COELENTERATA; ZOANTHARIA.

 �34s[Kenji Atoda]
�33s




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�6v�2v�32sActinide elements   �21m �33s 
    The series of elements beginning with actinium (atomic number 89) and 
including thorium, protactinium, uranium, and the transuranium elements 
through the element lawrencium (atomic number 103). These elements have a 
strong chemical resemblance to the lanthanide, or rare-earth, elements of 
atomic numbers 57 to 71. Their atomic numbers, names, and chemical symbols 
are: 89, actinium (Ac), the prototype element, sometimes not included as an 
actual member of the actinide series; 90, thorium (Th); 91, protactinium (Pa); 
92, uranium (U); 93, neptunium (Np); 94, plutonium (Pu); 95, americium (Am); 
96, curium (Cm); 97, berkelium (Bk); 98, californium (Cf); 99, einsteinium 
(Es); 100, fermium (Fm); 101, mendelevium (Md); 102, nobelium (No); 103, 
lawrencium (Lr). 
    Except for thorium and uranium, the actinide elements are not present in 
nature in appreciable quantities. The transuranium elements were discovered 
and investigated as a result of their synthesis in nuclear reactions. All are 
radioactive and except for thorium and uranium, weighable amounts must be 
handled with special precautions. 
    Most actinide elements have the following in common: trivalent cations 
which form complex ions and organic chelates; soluble sulfates, nitrates, 
halides, perchlorates, and sulfides; and acid-insoluble fluorides and 
oxalates. See ACTINIUM; LAWRENCIUM; PERIODIC TABLE; PROTACTINIUM; THORIUM; 
TRANSURANIUM ELEMENTS; URANIUM. 
 �34s[Glenn T. Seaborg]
�33s 
  
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2029056
�6v�2v�32sActinium   �21m �33s 
    A chemical element, Ac, atomic number 89, and atomic weight 227.0. 
Actinium was discovered by A. Debierne in 1899. Milligram quantities of the 
element are available by irradiation of radium in a nuclear reactor. 
Actinium-227 is a BETA-emitting element whose half-life is 22 years. Six other 
radioisotopes with half-lives ranging from 10 days to less than 1 minute have 
been identified. 
    The relationship of actinium to the element lanthanum, the prototype rare 
earth, is striking. In every case, the actinium compound can be prepared by 
the method used to form the corresponding lanthanum compound with which it is 
isomorphous in the solid, anhydrous state. See ACTINIDE ELEMENTS; LANTHANUM; 
NUCLEAR REACTION; RADIOACTIVITY. 
 �34s[Sherman Fried]
�33s 
  

A700000001477�254s�5v�1v18
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�6v�2v�32sActinomycetaceae   �21m �33s 
    A family of bacteria belonging to the order Actinomycetales. They produce 
elongated cells, usually filamentous in nature, with a tendency to the 
development of branches. Some of the cells frequently show swellings or 
clubbed or irregular shapes. Some species require oxygen and grow best at a 
high oxygen tension of 20% or more; others grow only in the complete absence 
of oxygen. Some are parasitic in humans and in animals. These organisms grow 
readily on artificial media and form well-developed colonies. Some of the 
organisms are colorless or white, whereas others are pigmented. The family is 
divided into genera: Actinomyces, Arachnia, Bifidobacterium, Bacterionema, and 
Rothia. See ACTINOMYCETALES. 
 �34s[Selman A. Waksman]
�33s 
  

A700000002433�254s�5v�1v18
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2343424
�6v�2v�32sActinomycetales   �21m �33s 
    An order of bacteria which produce filamentous cells or hyphae. These 
cells tend to develop branches, giving rise to a true branched mycelium. They 
are often spoken of as ``higher,'' filamentous, or ``moldlike'' bacteria. They 
occur abundantly in nature; some are pathogenic and others are saprophytic. 
Hyphae do not exceed 1.5 micrometers and are mostly about 1 micrometer or less 
in diameter. They may produce conidia, special spores, oidiospores, or 
sporangiospores.  
    Some species grow best at temperatures between 77 - 104 DG F 
(25 - 40 DG C); others grow best at temperatures above 122 DG F (50 DG C). 
Most forms are aerobic, although some are anaerobic. The aerobes require oxygen 
and grow best at a high oxygen tension of 20% or more, while the anaerobes can 
grow only in the complete absence of oxygen. They form a variety of both 
soluble and insoluble pigments.  
    The order Actinomycetales is divided into eight families: 
Actinomycetaceae, five genera; Mycobacteriaceae, one genus; Streptomycetaceae, 
four genera; Actinoplanaceae, ten genera; Frankiaceae, one genus; 
Dermatophilaceae, two genera; Nocardiaceae, two genera; and 
Micromonosporaceae, six genera.  
 �34s[Selman A. Waksman]
�33s 
  

A700000002212�254s�5v�1v18
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�6v�2v�32sActinomycosis   �21m �33s 
    An infectious disease of humans caused by Actinomyces israelii and 
Arachnia propionica. This disease is also seen in cattle, in which the 
etiologic agent is Actinomyces bovis. These microorganisms are normal 
inhabitants of the mucous membranes of the mouth and tonsillar crypts. 
    Cervicofacial actinomycosis is the most common form and is characterized 
by a swelling of the soft tissue, which becomes hard. As the disease process 
continues, abscess formation and multiple draining sinuses occur. The disease 
may progress to invade the bone of the lower jaw, causing osteomyelitis. 
Occasionally the organism may be aspirated into the lungs, resulting in 
pulmonary abscesses followed by invasion of the chest wall. Abdominal 
actinomycosis usually results from penetration of the intestinal mucosa. 
    Actinomyces israelii, Actinomyces bouvis, and Arachnia propionica are 
sensitive to most antibiotics. Penicillin is the drug of choice in treating 
this disease in humans. See ACTINOMYCETACEAE; ANTIBIOTIC; GRAM'S STAIN; 
MEDICAL MYCOLOGY. 
 �34s[Leanor D. Haley]
�33s 
  
A700000001174�254s�5v�1v18
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�6v�2v�32sActinomyxida   �21m �33s 
    An order of the protozoan class Myxosporidea (subphylum Cnidospora) 
characterized by the production of trivalved spores with three polar capsules 
and one to many sporoplasms. The spore membrane may be extended into 
anchor-shaped processes, which may have bifurcate tips. These parasites are 
found in the body cavity or in the intestinal lining of fresh-water annelids 
and in marine worms of the phylum Sipunculoidea. The life cycle for most 
species is not well known. See CNIDOSPORA; MYXOSPORIDEA; OLIGOCHAETA; PROTOZOA. 
 �34s[Ross F. Nigrelli]
�33s 
  
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�6v�2v�32sActinophage   �21m �33s 
    Any of a number of bacteriophages that infect and lyse bacteria of the 
order Actinomycetales. Actinophages of particular interest are those that 
include in their host range any organisms of the genus Streptomyces, the 
source of most of the therapeutically useful antibiotics. Contamination of a 
culture with a specific actinophage may result in lysis and destruction of the 
bacteria, which obviously halts antibiotic production. The problem of phage 
contamination is often solved by the isolation and use of a phage-resistant 
mutant of the antibiotic-producing bacterium. See BACTERIOPHAGE; 
STREPTOMYCETACEAE.  
 �34s[Lane Barksdale]
�33s 
  

A700000001436�254s�5v�1v18
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2343424
�6v�2v�32sActinophryida   �21m �33s 
    An order of Heliozoia. In these protozoans, a centroplast, a highly 
organized test, and a capsule are lacking (see illustration). The organisms 
may be uninucleate, as in Actinophrys, or multinucleate, as in Actinosphaerium 
(fresh water) and Camptonema (marine). The outer cytoplasm is usually highly 
vacuolated. In uninucleate genera the nucleus is approximately central. 
Axopodia are present, and their axial filaments end near the nucleus in 
uninucleate types. See ACTINOPODEA; HELIOZOIA; PROTOZOA; SARCODINA; 
SARCOMASTIGOPHORA.  
    Figure caption: Actinophryida: (a) Actinosphaerium elchorni.     
(b) Actinophrys pontica. (After R. P. Hall, Protozoology, Prentice-Hall, 1953)
 �34s[Richard P. Hall]
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�6v�2v�32sActinoplanaceae �21m �33s
    A family of bacteria of the order Actinomycetales, with well-developed
mycelium, and spores characteristically formed in sporangia. In many species
conidia are formed in moniliform chains, brushlike as in Penicillium, or in
clusters somewhat as in Micromonospora.
    Of the several genera described (see illustration), the commonest are
Actinoplanes, having spherical to subspherical planospores with a tuft of
flagella, Ampullariella, having rod-shaped planospores with polar flagella,
Spirillospora, having bent-rod to spiral planospores with polar or lateral
flagella, and Streptosporangium, having spherical to subspherical
nonflagellated and nonmotile spores.
    In the eighth edition of Bergey's Manual of Determinative Bacteriology
(1974), this family includes 10 genera: Actinoplanes, Spirillospora,
Streptosporangium, Amorphosporangium, Ampullariella, Pilimelia,
Planomonospora, Planobispora, Dactylosporangium, and Kitastoa. See
ACTINOMYCETALES; CILIA AND FLAGELLA.


 �34s[John N. Couch]
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�6v�2v�32sActinopodea   �21m �33s 
    A class of Sarcodina. Some of these protozoans have more or less permanent 
pseudopodia, composed of axial filaments surrounded by a cytoplasmic envelope 
(axopodia); others have delicate and often radially arranged filopodia 
(filamentous pseudopodia) or filoreticulopodia (filamentous branched 
pseudopodia) without axial filaments. Although some are stalked and sessile, 
most are floating types. There are four subclasses: Radiolaria, Acantharia, 
Heliozoia, and Proteomyxidia. See ACANTHARIA; HELIOZOIA; PROTEOMYXIDIA; 
RADIOLARIA; SARCODINA. 
 �34s[Richard P. Hall]
�33s 
  
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�6v�2v�32sActinopterygii   �21m �33s 
    The rayfin fishes, which form one of three subclasses of the Osteichthyes, 
or bony fishes. The terms Actinopteri, Teleostomi, and Palaeopterygii, plus 
Neopterygii of some authors, are equivalent to Actinopterygii. The rayfins are 
distinguished by the primitive presence of a single dorsal fin, the absence of 
an epichordal lobe of the caudal fin, and little extension of flesh and 
skeletal supporting elements into the paired fins. The rayfins lack internal 
nares, and the swim bladder, if present, usually rises from a dorsal 
out-pocketing from the gut or is free from it. The rayfins first appeared in 
the Lower Devonian; they experienced a tremendous differentiation in the 
Cretaceous and early Cenozoic, since which time they have been the dominant 
vertebrates of the seas and freshwaters. Approximately 97% of modern fishes 
are members of this subclass. See CROSSOPTERYGII; DIPNOI. 
    Three centralized structural types may be defined and have been ranked as 
infraclasses: Chondrostei, Holostei, and Teleostei. See CHONDROSTEI; HOLOSTEI; 
OSTEICHTHYES; TELEOSTEI. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sAction   �21m �33s 
    An integral quantity associated with each possible motion of a system of 
particles, of fields, or both. For a particle system the action is given by 
Eq. (1), where the qj's are generalized coordinates, the pj's their 
conjugate momenta, and the dq/dt j's are derivatives of the generalized 
coordinates with respect to time. See HAMILTON'S PRINCIPLE. 
  
     *** S = INTEGRATION SUMMATION (j = 1 TO N) pj(t) dq/dt j(t) dt ***   (1)
  
    The action permits the following formulation of the dynamical equations, 
which is similar to but more restrictive than Hamilton's principle: Of all 
possible motions with a given energy from the initial values qj to the final 
values qj', the dynamical motion is that for which the action is stationary 
(not always a minimum despite the name ``principle of least action''). 
    When the motion of a degree of freedom qj, pj is periodic, the 
separate integral shown in Eq. (2) holds. Taken over a period of the motion,
 
          *** INTEGRATION pj dq/dt j dt = INTEGRATION pj dqj ***          (2)

it is called the action variable Jj. Action variables are useful as adiabatic 
invariants (quantities insensitive to slow variations in the external 
parameters of the system). This property was exploited in early quantum theory 
by assigning to the action variables fixed ``quantized'' values, namely, 
integral multiples of Planck's constant. See QUANTUM MECHANICS. 
 �34s[Bernard Goodman]
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�6v�2v�32sActivated carbon   �21m �33s 
    A powdered, granular, or pelleted form of amorphous carbon characterized 
by very large surface area per unit volume because of an enormous number of 
fine pores. Activated carbon is capable of collecting gases, liquids, or 
dissolved substances on the surface of its pores. 
    Adsorption on activated carbon is selective, favoring nonpolar over polar 
substances. Compared with other commercial adsorbents, activated carbon has a 
broad spectrum of adsorptive activity, excellent physical and chemical 
stability, and ease of production from readily available, frequently waste 
materials. See ADSORPTION. 
    Almost any carbonaceous raw material can be used for the manufacture of 
activated carbon. Wood, peat, and lignite are commonly used for the 
decolorizing materials. Bone char made by calcining bones is used in large 
quantity for sugar refining. Nut shells (particularly coconut), coal, 
petroleum coke, and other residues in either granular, briqueted, or pelleted 
form are used for adsorbent products. 
    Activation is the process of treating the carbon to open an enormous 
number of pores in the 1.2- to 20-nanometer-diameter range (gas-adsorbent 
carbon) or up to 100-nm-diameter range (decolorizing carbons). After 
activation, the carbon has the large surface area (500 - 1500 m sup 2/g) 
responsible for the adsorption phenomena. Carbons that have not been subjected 
previously to high temperatures are easiest to activate. Selective oxidation 
of the base carbon with steam, carbon dioxide, flue gas, or air is one method 
of developing the pore structure. Other methods require the mixing of 
chemicals, such as metal chlorides (particularly zinc chloride) or sulfides or 
phosphates, potassium sulfide, potassium thiocyanate, or phosphoric acid, with 
the carbonaceous matter, followed by calcining and washing the residue. See 
CARBON: CHARCOAL. 
 �34s[H. Burnham Allport]
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�6v�2v�32sActivation analysis   �21m �33s 
    A technique in which a neutron, charged particle, or gamma photon is 
captured by a stable nuclide to produce a different, radioactive nuclide which 
is then measured. The technique is specific, highly sensitive, and applicable 
to almost every element in the periodic table. 
    In neutron activation analysis (NAA), the most widely used form of 
activation analysis, the sample to be analyzed is placed in a nuclear reactor 
where it is exposed to a flux of thermal neutrons. Some of these neutrons are 
captured by isotopes of elements in the sample; this results in the formation 
of a nuclide with the same atomic number, but with one more mass unit of 
weight. A prompt gamma ray is immediately emitted by the new nuclide. 
    Measurement of the induced radioactivities is the key to activation 
analysis. This is usually obtained from the gamma-ray spectra of the induced 
radionuclides. Gamma rays from radioactive isotopes have unique, discrete 
energies, and a device that converts such rays into electronic signals that 
can be amplified and displayed as a function of energy is a gamma-ray 
spectrometer. It consists of a detector [germanium doped with lithium, GeLi, 
or sodium iodide doped with thallium, NaI(Tl)] and associated electronics. 
    Activation analysis can also be performed with charged particles (protons 
or He sup 3 POSITIVE ions, for example), but because fluxes of such particles
are usually lower than reactor neutron fluxes and cross sections are much
smaller, charged-particle methods are usually reserved for special samples.
Charged particles penetrate only a short distance into samples, which is
another disadvantage. A variant called proton-induced x-ray emission (PIXE) has
been highly successful in analyzing air particulates on filters. 
    Activation analysis has been applied to a variety of samples. It is 
particularly useful for small (1 mg or less) samples, and one irradiation can 
provide information on 30 or more elements. Samples such as small amounts of 
pollutants, fly ash, very pure experimental alloys, and biological tissue have 
been successfully studied by neutron activation analysis. Of particular 
interest has been its use in forensic studies; paint, glass, tape, and other 
specimens of physical evidence have been assayed for legal purposes. In 
addition, the method has been used for authentication of art objects and 
paintings where only a small sample is available. See FORENSIC CHEMISTRY; 
NUCLEAR REACTION; PARTICLE DETECTOR; RADIOISOTOPE; TRACE ANALYSIS. 
 �34s[W. S. Lyon]
�33s 
  

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�6v�2v�32sActive galactic nuclei �21m �33s
    Cores of galaxies in which a substantial amount of energy is produced by
explosive or violent events. Galactic activity involves a rich diversity of
phenomena, including the production of unusual quantities of radiation at
almost all wavelengths in the electromagnetic spectrum, the emission of jets
of high-speed electrons, and the variability of the light from galactic nuclei
on very short time scales. Astrophysicists believe that supermassive objects,
possibly giant black holes, located at the cores of active galaxies are
responsible for all these phenomena.
    It is possible to recognize an active galaxy in a variety of ways. Radio
galaxies have a total radio power which exceeds the stellar power of a large
spiral galaxy. The most powerful active galaxies are the BL Lac objects. Still
more energetic than the BL Lac objects are the quasars - hyperactive relatives
of the active galaxies. Quasars are the most distant discrete objects in the
observable universe. See QUASARS.
    The working model favored by most astrophysicists is that a very massive
black hole, of 10 sup 6 to 10 sup 8 solar masses (1 solar mass = 2 X 10 sup 30
kg), is the central engine in a typical active galactic nucleus. Gas comes
from somewhere in the environment, falls toward the black hole, whirls around
it, and forms a nozzle near the poles of rotation of the accretion disk of
swirling, compressed material. Jets of high-speed electrons squirt out through
the nozzle at nearly the speed of light. These jets contain electrons that
produce synchrotron radiation, and when the synchrotron radiation interacts
with the surrounding materials, emission lines and large quantities of
infrared radiation can be produced if low-density gas and dust are there (see
illustration). Millions of light-years from the galactic nucleus, this jet is
finally stopped by the surrounding intergalactic medium and produces the huge
radio-emitting lobes of gas found on opposite sides of radio galaxies. See
BLACK HOLE.

 �34s[Harry L. Shipman]
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�6v�2v�32sActivity (thermodynamics)   �21m �33s 
    A function introduced by G. N. Lewis to aid in the thermodynamic treatment 
of real systems. Like the fugacity, the activity makes possible the 
correlation of changes in the chemical potential with changes in 
experimentally measurable quantities, such as concentrations or partial 
pressures, through relations formally equivalent to those holding for ideal 
systems. The activity concept retains its usefulness, however, for such cases 
as condensed phases of low volatility, for which fugacity determinations are 
impractical. See CHEMICAL THERMODYNAMICS; FREE ENERGY; FUGACITY; PHYSICAL 
CHEMISTRY.  
    The activity ai of a constituent i for a given state is defined as Eq. 
(1), where fi is the fugacity for the given state and fi sup 0 the fugacity 
of the constituent for a reference state, or standard state, at the same 

                           *** ai = fi/fi sup 0 ***                  (1)

temperature. This definition then requires that Eq. (2) hold, where MU i and 
MU i sup 0 are the values of the chemical potential for the given state and 

                     *** RT ln ai = MU i - MU i sup 0 ***            (2)

standard state, respectively. Because an activity is a relative quantity, a 
numerical value is meaningless unless the standard state involved is known. 
The standard state used may in principle be selected arbitrarily, but in 
practice, certain conventional choices are ordinarily made because a maximum 
of convenience can be obtained thereby. 
 �34s[Paul J. Bender]
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�6v�2v�32sAcylation   �21m �33s 
    The process whereby the acyl group is incorporated into a molecule by 
substitution. If the acetyl group is incorporated, the process is acetylation; 
if the benzoyl group is substituted, it is benzoylation. See ACETYLATION. 
    For the process to occur, the acylating agent must attack a molecule 
containing one or more active or easily replaceable hydrogen atoms. This 
limits the reaction, for all practical purposes, to simple or polyhydroxy 
alcohols, phenols, thiols, ammonia, primary and secondary amines, amino acids, 
esters of malonic acid, BETA-keto esters, and BETA-diketones. The common 
reagents are acid halides, preferably the chloride and occasionally the 
bromide, and acid anhydrides. For acetylation the highly reactive substance 
ketene may be used. A related reaction is the Friedel-Crafts acylation, in 
which the acyl group replaces the hydrogen atom of an aromatic nucleus. See 
FRIEDEL-CRAFTS REACTION. 
    The most important acylation reaction is acetylation, which is much used 
industrially. Acetylation of acetylene is accomplished catalytically by the 
addition of acetic acid to form vinyl acetate, which is an important monomer 
in the plastics industry. The reaction of cellulose with acetic anhydride in 
the presence of sulfuric acid gives cellulose triacetate, used in the 
manufacture of the fiber known as acetate rayon. The important analgesic 
aspirin, or acetylsalicylic acid, is prepared by the reaction of salicylic 
acid with acetic anhydride. See ACID ANHYDRIDE; ACID HALIDE; AMINE; AMINO 
ACIDS; ASPIRIN; CELLULOSE. 
 �34s[Paul E. Fanta]
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�6v�2v�32sAdamantane �21m �33s
    The C10H16 alicyclic hydrocarbon whose structure has the same arrangement
of carbon atoms as does the basic unit of the diamond lattice (see
illustration). Adamantane is composed of three interlocking, chair-form
cyclohexane rings. The molecular structure is highly symmetrical and compact.
All bond angles are found by x-ray crystallography to be nearly 109.5 DG, and
the bonds around all adjacent carbons are ideally staggered. These structural
features afford a high degree of stability to the adamantane molecule. The
highly symmetrical structure of adamantane also accounts for its extremely
high melting point 516 DG F (269 DG C), the highest known for aliphatic
hydrocarbons, and for the seemingly contradictory ease with which it sublimes.
Possibly the most interesting and significant uses of adamantane compounds are
found in the field of pharmacology. Several adamantane derivatives have been
found to exhibit antiviral activity, whereas the incorporation of an adamantane
moiety into many already useful drugs has been found to significantly enhance
their efficacy. See ALICYCLIC HYDROCARBON; ALIPHATIC HYDROCARBON.


 �34s[Paul E. Fanta]
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�6v�2v�32sAdaptation (biology)   �21m �33s 
    Any characteristic of or process in a living organism which contributes to 
or furthers the survival of an individual or of the species in a specialized 
lifestyle (niche) in a peculiar environment or habitat. The term is also used 
for the process by which an adaptation develops or is accomplished. Permanent, 
congenital, or inherited adaptations are generally believed to have arisen 
over long periods of time by means of natural selection of essentially random 
variations which are inherited. 
    The term acclimation or adjustment may be used to distinguish from 
evolutionary adaptations those changes which develop within the lifetime of an 
individual in response to changes in way of life or habitat, and which are not 
inherited as such, although their genetic basis may be. The ability to adjust 
or to become acclimated, possessed somewhat by every living organism, is one 
of the many adaptive features of life in general. 
 �34s[Bradley T. Scheer]
�33s 
  

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�6v�2v�32sAdaptive control   �21m �33s 
    A special type of nonlinear control system which can alter its parameters 
to adapt to a changing environment. The changes in environment can represent 
variations in process dynamics or changes in the characteristics of the 
disturbances. See NONLINEAR CONTROL THEORY. 
    A normal feedback control system can handle moderate variations in process 
dynamics. The presence of such variations is, in fact, one reason for 
introducing feedback. There are, however, many situations where the changes in 
process dynamics are so large that a constant linear feedback controller will 
not work satisfactorily. For example, the dynamics of a supersonic aircraft 
change drastically with Mach number and dynamic pressure, and a flight control 
system with constant parameters will not work well. See FLIGHT CONTROLS. 
    Adaptive control is also useful for industrial process control. Since 
delay and holdup times depend on production, it is desirable to retune the 
regulators when there is a change in production. Adaptive control can also be 
used to compensate for changes due to aging and wear. See PROCESS CONTROL. 
 �34s[Karl J. Astrom]
�33s 
  

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�6v�2v�32sAdaptive optics �21m �33s
    The science of optical systems that measure and correct wavefront
aberrations in real time, that is, simultaneous with the operation of the
system. This possibility of improving the performance of an optical system has
many applications for both passive imaging systems and laser systems. For
example, adaptive optics offers the possibility of removing the effect of
atmospheric turbulence on the images of objects viewed through the air; of
allowing diffraction-limited imaging of astronomical objects from the ground;
of compensating for the degrading effects of the atmosphere in transmitting
laser radiation; and of correcting for static or dynamic figure errors in
optical surfaces or aberrations in the gain medium of lasers. Active
correction and control of optical surfaces allows new methods of construction
of large optical telescopes, such as the phasing together of numerous smaller
elements or the effective use of thinner, less precise optical surfaces. See
LASER.
    To appreciate the usefulness of an adaptive optics system, it is useful to
look at the effect of the atmosphere upon the performance of any Earth-based
passive imaging system. Neglecting atmospheric effects, a perfect
1-m-diameter-aperture telescope operating at a wavelength of 0.5 micrometer
has a resolution capability that extends out to 10 cycles per arc-second.
However, for typical atmospheric conditions the resolution is limited to
about 1 cycle per arc-second, that is, the effective aperture diameter is
on the order of 0.1 m. Adaptive optics has the potential for making the
effects of the atmosphere nearly negligible for many situations. See
ASTRONOMICAL PHOTOGRAPHY; OPTICS.
    Adaptive optics systems consist of three basic components (see
illustration). All presently working techniques (other than nonlinear
phase-conjugation techniques) use a deformable mirror as the wavefront
corrector. The main difference between techniques is the wavefront sensor.
Three main adaptive optics techniques have been investigated: multidither
COAT (coherent optical adaptive techniques); sharpness-function maximization;
and phase-conjugate.
    The multidither COAT system is used in optical systems that transmit
laser radiation and must concentrate this radiation into as small an area
as possible. In the sharpness-function technique, the correction is dithered
to maximize a given function, such as the integral of the square of the
irradiance in the image plane or the amount of energy within a certain
region. In operating phase-conjugate systems, the wavefront to be corrected
is measured directly by using either a geometric or interferometric test.

 �34s[James C. Wyant]
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�6v�2v�32sAddison's disease   �21m �33s 
    A primary failure or insufficiency of the adrenal cortex to secrete the 
vital cortical hormones, many of which are necessary to life. Causes of 
adrenal insufficiency include adrenal surgery, infections, tumor formation or 
infiltration, and other conditions which produce a decrease or cessation of 
cortical hormone secretion. Occasionally no preceding factors can be 
demonstrated so the term idiopathic is used to describe these cases of 
Addisonism.  
    All four types of adrenocortical hormones are decreased or absent in 
Addison's disease. Serous fluid and electrolyte imbalances are typical and 
often accompanied by a peculiar tan skin pigmentation, nausea, vomiting, 
decreased blood pressure, weakness, weight loss, and other specific and 
constitutional changes. 
    Addisonian crisis, or acute insufficiency, may be triggered by a 
generalized infection or massive stress. In such a crisis rapid deterioration, 
shock, and death may ensue. See ADRENAL GLAND. 
 �34s[Edward G. Stuart/N. Karle Mottet]
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�6v�2v�32sAddition   �21m �33s 
    One of the four fundamental operations of arithmetic and algebra. The term 
addition is applied to many kinds of objects other than numbers. For example, 
two vectors x, y are added to produce a third vector z obtained from them by 
the ``parallelogram'' law, and two sets A, B are added to form a third set C 
consisting of all the elements of A and of B. See CALCULUS OF VECTORS. 
    As an operation on pairs of real or complex numbers, addition is 
associative, Eq. (1), and commutative, Eq. (2); and multiplication is 
  
                    *** a + (b + c) = (a + b) + c ***                       (1)
  
                          *** a + b = b + a ***                             (2)
  
                       *** a(b + c) = a X b + a X c ***                     (3)
  
distributive over addition, Eq. (3). There are important mathematical 
structures in which an addition operation is defined that lacks one or more of 
these properties. See ALGEBRA; DIVISION; MULTIPLICATION; NUMBER THEORY; 
SUBTRACTION.  
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sAdeno-associated satellite virus �21m �33s
    A group of defective viruses (see illustration) which seem unable to
reproduce without help from adenoviruses. Like adenovirus, the satellite
particle has a deoxyribonucleic acid (DNA) core. However, the defective
satellite, although dependent upon adenovirus for its growth, does not appear
to be structurally related to its ``helper.'' Its genetic content, as well as
its size, is much smaller than that of adenovirus, and its protein coat is
completely different from that of adenovirus.
    These particles contain single-stranded DNA and a protein coat with
icosahedral symmetry. A nucleic acid of the size found in satellite virus
contains enough genetic information (DNA) to code for several proteins of the
size normally present in viruses. Hence, the defectiveness of the satellite is
not due to insufficient amounts of DNA itself, but to the lack of the right
kind of genetic information to permit its independent replication.
    Four antigenic types of adenosatellite viruses have been recognized for
primates, and a number of others have been found which infect other
vertebrates. Types 1 - 3 naturally infect humans; type 4 infects monkeys. They
are not known to be pathogenic. The role of these defective viruses in humans
or animals has yet to be discovered. See ADENOVIRUS; ANIMAL VIRUS.



 �34s[Joseph L. Melnick]
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�6v�2v�32sAdenosinediphosphate (ADP)   �21m �33s 
    A coenzyme and an important intermediate in cellular metabolism as the 
partially dephosphorylated form of adenosinetriphosphate. The compound is 
5'-adenylic acid with an additional phosphate group attached through a 
pyrophosphate bond. ADP is produced from adenosinetriphosphate and reconverted 
to this compound in coupled reactions concerned with the energy metabolism of 
living systems. ADP is also produced from 5'-adenylic acid by the transfer of 
a phosphate group from adenosinetriphosphate in a reaction that is catalyzed 
by an enzyme, myokinase. See ADENOSINETRIPHOSPHATE (ATP); ADENYLIC ACID; 
METABOLISM.  
 �34s[Michael Doudoroff]
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�6v�2v�32sAdenosinetriphosphate (ATP)   �21m �33s 
    A coenzyme and one of the most important compounds in the metabolism of 
all organisms, since it serves as a coupling agent between different enzymatic 
reactions. Adenosinetriphosphate is adenosinediphosphate (ADP) with an 
additional phosphate group attached through a pyrophosphate linkage to the 
terminal phosphate group. ATP is a powerful donor of phosphate groups to 
suitable acceptors because of the pyrophosphate nature of the bonds between 
its three phosphate radicals. ATP serves as the immediate source of energy for 
the mechanical work performed by muscle. In its presence, the muscle protein 
actomyosin contracts with the formation of adenosinediphosphate and inorganic 
phosphate. ATP is also involved in the activation of amino acids, a necessary 
step in the synthesis of protein. 
    In metabolism, ATP is generated from adenosinediphosphate and inorganic 
phosphate mainly as a consequence of energy-yielding oxidation-reduction 
reactions. In respiration, ATP is generated during the transport of electrons 
from the substrate to oxygen via the cytochrome system. See 
ADENOSINEDIPHOSPHATE (ADP); ADENYLIC ACID; COENZYME; METABOLISM. 
 �34s[Michael Doudoroff]
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�6v�2v�32sAdeno-SV40 hybrid virus   �21m �33s 
    A type of defective virus particle in which part of the genetic material 
of papovavirus SV40 is encased within an adenovirus protein coat (capsid). 
Adenovirus progeny possesses properties different from those of the original 
parent adenovirus. It produces tumors in newborn hamsters; it replicates in 
monkey cell cultures; and although it does not produce infectious SV40, it 
causes monkey cells in culture to produce a new cellular antigen known to be 
specifically induced by SV40 - the SV40 tumor, or T, antigen. The new virus 
stock therefore behaves like a hybrid. It has proved to be a population of two 
distinct kinds of virus particles. One kind of particle is a true adenovirus. 
The other particle is the adeno-SV40 hybrid, which has an adenovirus coat, but 
whose genetic material appears to consist of defective adenovirus type 7 DNA, 
representing about 85% of the adenovirus genome, covalently linked to a 
portion (about 50%) of an SV40 genome. 
    In this virus population the particle carrying the SV40 genetic material 
has been termed PARA (particle aiding replication of adenovirus). The PARA is 
considered an unconditionally defective virus, since under no known conditions 
can it reproduce itself except in a cell coinfected with adenovirus. The 
adenovirus is considered conditionally defective, since it can reproduce 
independently in human cells but not in monkey cells. See ADENOVIRUS; ANIMAL 
VIRUS.  
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sAdenovirus   �21m �33s 
    A group of viral agents which cause febrile catarrhs and various other 
respiratory diseases. They have been associated with pharyngoconjunctival 
fever, acute respiratory disease, epidemic keratoconjunctivitis, and febrile 
pharyngitis in children. Although most of the illnesses caused by adenoviruses 
are respiratory, adenoviruses are frequently excreted in stools, and certain 
adenoviruses have been isolated from sewage. Distinct serotypes are known for 
simian, bovine, canine, murine, and avian species. See ANIMAL VIRUS; 
KERATOCONJUNCTIVITIS.  
    Infective virus particles, 70 nm in diameter, are icosahedrons with shells 
(capsids) composed of 252 subunits (capsomeres). No outer envelope is known. 
The genome is double-stranded deoxyribonucleic acid (DNA). Three major soluble 
antigens are separable from the infectious particle - a group-specific antigen 
common to all adenovirus types, a type-specific antigen unique for each type, 
and a toxinlike material which also possesses group specificity. 
    The known types of adenoviruses of humans total at least 33, and 
previously unrecognized types continue to be isolated. The serotypes are 
antigenically distinct in neutralization tests, but they share a 
complement-fixing antigen, which is probably a smaller soluble portion of the 
virus. Adenoviruses contain four complement-fixing antigens - A, B, C, and P. 
    The virus does not commonly produce acute disease in laboratory animals 
but is cytopathogenic in cultures of human tissue. Certain human adenovirus 
serotypes produce cancer when injected into newborn hamsters. Although 
complete, infective virus cannot be recovered from adenovirus-induced hamster 
tumors, a new antigen induced by the virus can be detected by complement 
fixation or immunofluorescence techniques. This antigen, called tumor or T 
antigen because of its association with tumor or transformed cells, can also 
be detected in the cytolytic cycle of the virus. 
    Live virus vaccines against type 4 and type 7 have been developed and used 
extensively in military populations. When both are administered 
simultaneously, vaccine recipients respond with neutralizing antibodies 
against both virus types. See ADENO-ASSOCIATED SATELLITE VIRUS; ADENO-SV40 
HYBRID VIRUS; ANTIGEN; COMPLEMENT-FIXATION TEST. 
 �34s[Joseph L. Melnick]
�33s 
  

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�6v�2v�32sAdenylic acid �21m �33s
    A generic term referring to a group of isomeric nucleotides. Adenylic
acid, the phosphoric acid ester of adenosine (9'-BETA-D-ribosidoadenine), is
also known as adenosinemonophosphate (AMP). The most common isomer is
5'-adenylic acid (5'-AMP or ``muscle adenylic acid''), in which the phosphate
group is attached to the fifth carbon atom of the ribose moiety (see
illustration).
    5'-Adenylic acid can be regarded as the parent compound of
adenosinediphosphate (ADP) and adenosinetriphosphate (ATP). By itself the
nucleotide functions as an allosteric effector in the regulation of
carbohydrate metabolism. It is also a constituent of several important
coenzymes, and in addition plays a role in the incorporation of amino acids
into proteins. See ADENOSINEDIPHOSPHATE (ADP); ADENOSINETRIPHOSPHATE (ATP);
CARBOHYDRATE METABOLISM; COENZYME.
    Other important isomers in this series are 3',5'-adenylic acid, also known
as cyclic AMP, 2'-adenylic acid, 3'-adenylic acid (``yeast adenylic acid''),
and 2',3'-adenylic acid. See CYCLIC ADENYLIC ACID; NUCLEIC ACID.

 �34s[G. Alan Robison]
�33s



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�6v�2v�32sAdhesive   �21m �33s 
    Substance capable of holding materials together by surface attachment. The 
term is general and includes, among other things, cement, glue, mucilage, and 
paste - names which are loosely used interchangeably. The oldest type of 
adhesives are those of natural origin, such as starch, asphalt, gelatinized 
collagen, vegetable proteins, plant saps and resins, rubber, and shellac; and 
most of these continue to find applications. Modified natural products, such 
as portland cement, dextrins, nitrated and acetylated celluloses or starches, 
and fatty amides, are widespread in industry. Wholly synthetic adhesives 
include the emulsion polymers such as polyvinyl acetate, polyvinyl alcohol, 
and elastomeric solution cements, and the epoxy, polyurethane, and polysulfide 
condensation adhesives. See MUCILAGE; STRUCTURAL ADHESIVE. 
 �34s[Frank Wagner]
�33s 
  

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�6v�2v�32sAdiabatic demagnetization   �21m �33s 
    The removal or diminution of a magnetic field applied to a magnetic 
substance when the latter has been thermally isolated from its surroundings. 
The process concerns paramagnetic substances almost exclusively, in which case 
a drop in temperature of the working substance is produced (magnetic cooling). 
Adiabatic demagnetization was employed from 1933 on to produce temperatures 
below those readily obtainable by using only liquid helium (that is, below 1 
K). See LIQUID HELIUM; PARAMAGNETISM. 
    Nuclear magnetic moments are one or two thousand times smaller than their 
ionic (that is, electronic) counterparts, and the characteristic temperature 
of their mutual interaction lies in the microkelvin rather than millikelvin 
region. Successful experiments in nuclear adiabatic demagnetization date from 
the mid-1950s. 
 �34s[Ralph P. Hudson]
�33s 
  

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�6v�2v�32sAdiabatic process   �21m �33s 
    A thermodynamic process in which the system undergoing the change 
exchanges no heat with its surroundings. Reversible adiabatic processes are 
also isentropic; that is, they take place with no change in entropy. See 
ENTROPY; ISENTROPIC PROCESS. 
    The events inside an engine cylinder are nearly adiabatic because the wide 
fluctuations in temperature take place rapidly compared to the speed with 
which the cylinder surfaces can conduct heat. Similarly, fluid flow through a 
nozzle may be so rapid that negligible exchange of heat between fluid and 
nozzle takes place. The compressions and rarefactions of a sound wave are 
rapid enough to be considered adiabatic. See THERMODYNAMIC PROCESSES. 
 �34s[Philip E. Bloomfield]
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�6v�2v�32sAdipose tissue   �21m �33s 
    A type of connective tissue which is specialized for the storage of lipids 
(fats). Adipose cells store lipids in the form of neutral triglycerines in 
intercellular droplets. Adipose tissue is composed largely of adipose cells, 
with some capillaries, and fibrous connective tissue. 
    White adipose tissue is the most common and represents stored food 
reserves and thermal and physical insulation. Layers of white adipose tissue 
below the skin, especially in aquatic mammals such as seals and whales, serve 
as heat barriers, preventing heat loss from the body. Adipose tissue also 
absorbs physical shock, acting like a cushion. The amount of white adipose 
tissue in an animal is controlled by sex hormones, metabolic hormones, diet, 
and physical activity. Many animals, especially hibernating and migratory 
animals, greatly increase their fat reserves in preparation for travel or 
hibernation.  
    Brown adipose tissue is mainly found in subscapular, interscapular, and 
mediastinal areas. It is associated with thermogenesis (heat production) in 
hibernating animals and newborn mammals and is metabolically much more active 
than white adipose tissue. It is thought that brown fat may be an endocrine 
gland. Although its function is unknown, it is thought to trigger arousal from 
hibernation and to regulate internal temperature in neonates. See CONNECTIVE 
TISSUE; LIPID. 
 �34s[Todd A. Georgi]
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�6v�2v�32sAdmittance   �21m �33s 
    The reciprocal of the impedance of an electric circuit. Admittance is 
expressed in the unit mho, coined from the inverse spelling of ohm, the unit 
of impedance. Admittance is used primarily in computations of parallel 
alternating-current circuits. By using admittance Y, current I can be 
expressed as I = EY, where E is the voltage across the impedance Z. See 
ALTERNATING CURRENT; CONDUCTANCE; SUSCEPTANCE. 
 �34s[Burtis L. Robertson]
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�6v�2v�32sAdrenal gland   �21m �33s 
    A gland of internal secretion in close proximity to the kidneys. It is 
always composed of two types of cellular elements with dissimilar embryonic 
origins, hormones, effects, and controls. One element secretes catecholamines 
- epinephrine (adrenaline) and norepinephrine (noradrenaline); the other 
secretes steroid hormones. The catecholamine-secreting cells are called 
chromaffin cells. The steroid-secreting cells are called adrenocortical cells. 
    The two types of tissue that compose the adrenal gland may occur as 
discrete, spatially separate bodies as in fishes; they may be intermixed as 
among amphibia and most reptiles and birds; or the adrenocortical tissue 
(cortex) may completely envelop the chromaffin tissue (medulla) as in mammals. 
    Norepinephrine and epinephrine are derivatives of catechol. The ratios of 
the two amines secreted is variable and species-specific. The catecholamines 
exert their effects by interacting with specific ALPHA-adrenergic or 
BETA-adrenergic receptors on their target cells. Norepinephrine exerts 
predominantly ALPHA-adrenergic action, while epinephrine exerts a 
predominantly BETA-adrenergic action, but overlap does occur. Excitation of 
BETA-adrenergic receptors causes stimulation of the heart, relaxation of 
branchioles, dilation of arterioles in skeletal muscle, and elevation of blood 
sugar levels. These effects are usually seen in response to epinephrine. 
Excitation of ALPHA-adrenergic receptors usually results in vasoconstriction 
of peripheral blood vessels, and contraction of the smooth muscle of the 
bladder and the sphincters of the gut. Epinephrine is effective in inducing 
these effects, but norepinephrine is the hormone usually involved even though 
it is somewhat less potent. See EPINEPHRINE. 
    The adrenal cortex elaborates many steroid hormones, some essential to 
life, which fall into four general categories: the mineralocorticoids (which 
affect mineral metabolism), the glucocorticoids (which affect glucose 
metabolism), the adrenal cortical androgens (which control secondary sex 
characteristics in males), and the estrogens (estrus-producing hormones). 
    Secretion of the glucocorticoids is controlled by adrenocorticotropin 
(ACTH), secreted by the anterior lobe of the pituitary. Secretion of the 
mineral corticoid is ultimately controlled by the kidney secretion of renin. 
See ANDROGEN; ESTROGEN; PITUITARY GLAND; STEROID. 
 �34s[Harry Lipner]
�33s 

    Malfunctions of the adrenal glands may be divided into two major 
categories, those of the medulla and those of the cortex. The principal 
disorders of the medulla are tumors. One rare type, the pheochromocytoma, is 
an actively secreting neoplasm which causes excessive production of the 
medullary hormones, epinephrine and norepinephrine. Other rare, nonsecreting 
tumors of the medulla are the ganglioneuroma and the highly malignant 
neuroblastoma of infancy and childhood. 
    Congenital defects of the adrenal glands are principally those related to 
the absence of glandular tissue or to the lack of its normal secretory ability 
(hypoplasia).  
    The adrenals are susceptible to certain infections, largely because they 
are so well vascularized. The resulting inflammations may produce acute 
reactions and crises, or in lesser cases, may eventually cause scarring and 
other degenerative changes with or without hormonal alteration. 
    There are many varieties of neoplasms of the adrenal cortex. They may be 
either primary or metastatic. Either type may cause alterations of adrenal 
hormone output, and the classification is often based on clinical findings and 
sometimes on histologic types. Major clinical categories include Cushing's 
syndrome, the adrenogenital syndrome, feminizing syndromes, 
hyperaldosteronism, and combinations of components of any or several of these. 
    The functional types, in which hormone output is altered, are more readily 
recognized because of the changes they effect. Nonfunctional tumors may be 
silent and therefore often offer a problem in diagnosis. The most common 
metastatic malignancies in the adrenals arise from bronchogenic carcinoma, 
although other carcinomas may also spread to these glands. See ADDISON'S 
DISEASE.  
 �34s[Edward G. Stuart/N. Karle Mottett]
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�6v�2v�32sAdsorption   �21m �33s 
    The property of an interface between two immiscible phases (solid, liquid, 
or vapor) to attract and concentrate components of either phase or both phases 
as an adsorbed interfacial film. Adsorption is a basic thermodynamic property 
of interfaces, resulting from a discontinuity in intermolecular or interatomic 
forces. It is also important in nearly all industrial processes and products. 
    Some definitions that describe adsorption are as follows: The adsorbent is 
the solid or liquid which adsorbs. The adsorbate is the solid, liquid, or gas 
which is adsorbed as molecules, atoms, or ions. Physical adsorption or 
physisorption is reversible adsorption by weak interactions only; no covalent 
bonds occur between the adsorbent and adsorbate; heats of physical adsorption 
are usually less than 15 - 20 kcal/mole (63 - 84 kilojoules/mole). Chemical 
adsorption or chemisorption is adsorption involving stronger interaction 
between adsorbate and adsorbent usually accompanied by rearrangement of atoms 
within or between adsorbates; reaction occurs between the surface of the 
adsorbent and the adsorbate; heats of chemisorption are usually in excess of 
20 - 30 kcal/mole (84 - 126 kilojoules/mole). 
    Nearly all vapors tend to adsorb onto inorganic solids at temperatures not 
too much above their boiling point. The intermolecular attractive forces which 
cause the physical adsorption of vapors are generally dominated by the London 
dispersion forces, an attraction caused by the perturbation of electron orbits 
by adjacent atoms. Another attractive force important in vapor adsorption is 
the interaction of electron-donor (basic) sites of vapor molecules with 
electron-acceptor (acidic) sites of adsorbents, or vice versa. These 
short-range attractions are much stronger than dipole interactions. Silica, an 
acidic adsorbent, adsorbs basic vapors (water, ammonia, and so forth) much 
more strongly than acidic vapors (chloroform, CO2, NO2, and so forth) 
regardless of the dipole moments. 
    The adsorption of water is dominated by hydrogen bonding, an 
intermolecular acid-base interaction onto neutral surfaces such as graphite or 
polyethylene, except for the acidic or basic sites provided by impurities on 
these neutral surfaces. 
    The strong interactions of chemisorption lead to surface compounds with 
various degrees of covalent bond character. The adsorbed layers are only one 
molecule thick because covalent bonds exist only between adjacent atoms. 
Chemisorption occurs on metals and semiconductors and on oxides and sulfides, 
but is most often observed on transition metals such as silver, nickel, 
cobalt, platinum, rhodium, and tungsten. Chemisorption is a necessary step in 
catalysis by these materials. See CHEMICAL DYNAMICS; INTERMOLECULAR FORCES. 
    Heterogeneous catalysis, in which gas or liquid reactants are specifically 
adsorbed to a dissimilar phase and chemically altered during their brief 
retention time, is basic to many industrial processes in the petrochemical, 
polymer, and chemical industries. 
    Purification by adsorption is perhaps the oldest known application; 
examples are wine and beer clarification, color removal in sugar processing, 
industrial wastewater treatment, and toxic gas adsorption in gas masks. 
    Adsorption is the basic phenomenon of chromatographic separations, which 
separate and concentrate components of mixtures according to strength of 
adsorption onto adsorbents in chromatographic columns. 
    Adsorption of surface-active substances is the key process in the use of 
soaps, detergents, emulsifiers, wetting agents, dyes, lubricants, and surface 
treatments. Other industries dependent on adsorption processes include 
agriculture, mining, petroleum recovery, papermaking, printing, and 
photography. See CATALYSIS; CHEMICAL SEPARATION TECHNIQUES; CHROMATOGRAPHY; 
RESPIRATOR.  
 �34s[Frederick M. Fowkes]
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�6v�2v�32sAegyptopithecus   �21m �33s 
    A primate that lived during Oligocene times, 30,000,000 years ago, in the 
Egyptian Fayum (see illustration), and is believed to be the common ancestor 
of humans and apes. The names Aegyptopithecus zeuxis and ``dawn ape'' were 
given by Elwyn Simons. 
    Aegyptopithecus had an estimated weight of 8 - 12 lb (3.6 - 5.4 kg). The 
cranial capacity was about 1.8 in. sup 3 (30 cm sup 3), larger than that of any
of its mammalian contemporaries, relative to body size. The teeth were equipped 
for a vegetarian diet; males had large, fanglike canines, whereas the canines 
of females were comparatively small. Studies of arm and leg bones lead to the 
conclusion that Aegyptopithecus was a tree dweller. See FOSSIL MAN; PRIMATES. 
 �34s[Sybil P. Parker]
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�6v�2v�32sAepyornithiformes   �21m �33s 
    The order of elephant birds, containing the single extinct family 
Aepyornithidae. Certain Oligocene and later fossil eggs from Africa are 
referred to this family, which is best represented in the Pleistocene of 
Madagascar. Aepyornis is the largest known bird, with an estimated body weight 
of 960 lb (438 kg), compared with about 220 lb (100 kg) for the ostrich. It 
had massive legs, in keeping with great body weight, and rudimentary wings. 
    Aepyornithiformes rather clearly belongs among the ratite orders of birds, 
but its relationships within this group are uncertain. The order is sometimes 
placed near the superficially similar moas (Dinornithiformes) of New Zealand, 
but there is considerable evidence, such as the very similar morphology of the 
skull, suggesting relationship to the geographically adjacent ostriches 
(Struthioniformes). See AVES; RATITES. 
 �34s[Kenneth C. Parkes]
�33s 
  

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�6v�2v�32sAerial photograph   �21m �33s 
    A photograph of a portion of the Earth's surface taken from an aircraft or 
from a satellite. Most often these photographs are taken sequentially and 
overlap each other, to give complete coverage of the area of interest. Thus 
they may be viewed stereoscopically to give a three-dimensional view of the 
Earth's surface. Although the camera in the aircraft may be pointed obliquely 
to the side of the line of flight, by far the most common type of aerial 
photograph is taken with the camera pointed vertically downward beneath the 
plane.  
    Aerial photographs have two main uses, the making of planimetric and 
topographic maps, and interpretation or data-gathering in a variety of 
specialized fields. All modern topographic (contour) maps are made from 
steroscopic aerial photographs, usually black-and-white panchromatic vertical 
photographs. The science of making accurate measurements and maps from aerial 
photographs is called photogrammetry. The geometric discrepancy between the 
distribution of features seen on the photograph and their actual distribution 
on the Earth's surface depends on several variables, the most important of 
which is the relief of the surface. The error becomes greater as relief 
increases. Photogrammetry deals with these errors and allows highly accurate 
and complete map production. Topographic maps are made from one or more 
stereoscopic pairs of aerial photographs by means of optical-mechanical 
plotting machines or mathematical methods (analytical photogrammetry). See 
CARTOGRAPHY; GEOPHYSICAL EXPLORATION; TOPOGRAPHIC SURVEYING AND MAPPING. 
    The other major use of aerial photographs is photo interpretation, 
utilizing all types of film. Photo interpretation is the attempt to extract 
information about the Earth's surface (and sometimes, subsurface) from aerial 
photographs. The systematic study of aerial photographs, particularly when 
they are viewed stereoscopically, gives specialists the ability to gather 
rapidly, record, map, and interpret a great deal of information. In addition, 
an aerial photograph is a record at a moment in time; photographs taken at 
intervals give an excellent record of the change with time in surface features 
of the Earth. 
 �34s[Laurence H. Lattman]
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�6v�2v�32sAerodynamic center   �21m �33s 
    A position in an airplane wing about which the wing coefficient of 
pitching moment is unaffected by wing coefficient of lift. Forces acting on a 
wing, tail, or other aerodynamic surface can be resolved into a lift and a 
drag acting at the aerodynamic center, and a pitching moment acting about the 
center (see illustration). The aerodynamic center of an airfoil section at 
subsonic speeds is usually close to the quarter chord. See AIRFOIL; CENTER OF 
GRAVITY; CENTER OF PRESSURE. 
    Figure caption: Aerodynamic forces on an airfoil section, showing        
aerodynamic center. (After C. D. Perkins and R. E. Hage, Airplane Performance
Stability and Control, John Wiley and Sons, Inc., 1949)
 �34s[Frank H. Rockett]
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�6v�2v�32sAerodynamic force   �21m �33s 
    The force between a body and the air caused by their relative motion, such 
as the wind blowing across the surface of the Earth. Such forces can be 
described in terms of the change in momentum of the air produced by its 
interaction with the body. Typical aerodynamic forces are those acting on an 
aircraft in subsonic flight. These forces are (1) the lift force, 
perpendicular to the flight path and concerned with the sustentation of an 
aircraft; (2) the drag force, or resistance acting to the rear along the 
flight path; and (3) the thrust from the power plant necessary to propel or 
sustain an aircraft. See SUBSONIC FLIGHT; WIND STRESS. 
 �34s[Joseph Bicknell]
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�6v�2v�32sAerodynamic resistance   �21m �33s 
    A retarding force that acts upon a body moving through a gaseous medium in 
a direction opposite to the motion of the body. It is also called aerodynamic 
drag. The aerodynamic resistance acting on an aircraft in flight is composed 
of four parts: (1) skin friction drag, which arises from the viscosity of the 
air; (2) pressure drag, which arises from the pressure field about the body; 
(3) wave drag, which arises from the compressible nature of air, and which 
occurs as flight speeds approach and exceed the speed of sound; and (4) 
induced drag, which is a resistance produced by the generation of lift. 
    The magnitude of skin friction drag depends upon the surface area of the 
airplane and the nature of the flow layer that is in immediate contact with 
it. If the boundary-layer flow is smooth and steady (laminar), the skin 
friction generated is much less than when this layer becomes turbulent. Unless 
special devices are employed, such as suction slots to remove the low momentum 
boundary-layer air before it becomes turbulent, the laminar flow will break 
down after it has passed over a sufficient length of surface. See 
BOUNDARY-LAYER FLOW. 
    If sufficient momentum is lost in the boundary layer, the main flow breaks 
away from the surface, forming a region of separation. Separation results in 
considerably lower surface pressures than would result in the ideal streamline 
case. As a consequence, suction pressure on the downstream portions of the 
body produces pressure drag. This drag is minimized by shaping the body 
geometrically to reduce the separation region to a minimum by giving it a 
streamlined shape. 
    As flight speed of the airplane increases, there comes a point where the 
flow passing over a surface reaches the local speed of sound. At this speed 
(Mach 1) pressure waves propagate through the air. This wave propagation forms 
shock patterns that interact with the boundary layer to cause premature 
separation and a sudden drag rise. As the speed continues to increase, shock 
patterns become much stronger until they become analogous to the surface waves 
created by a ship. Like such waves, at supersonic speeds the generation of 
these shock waves represents the major contribution to the drag of the 
vehicle. See AERODYNAMIC WAVE DRAG; TRANSONIC FLIGHT. 
    The final major type of aerodynamic resistance that must be considered 
arises from the fact that in creating lift the wing must impart a downward 
momentum to the air flowing past. This downward component of local velocity, 
termed downwash, rotates the resultant force of wing in such a manner that a 
force component is directed in the downstream direction. This component, which 
depends directly upon the magnitude of the lift force, is termed the induced 
drag.  
 �34s[David C. Hazen]
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�6v�2v�32sAerodynamics   �21m �33s 
    The branch of aeromechanics dealing with the properties and 
characteristics of, and the forces exerted by, air and other gases in motion. 
The field of aerodynamics includes the science of a gas itself in motion and 
the science of bodies immersed in a gas between which there exists a relative 
motion. See GAS DYNAMICS. 
    Aerodynamics is a broad field with numerous specializations and 
applications, some of which extend into apparently unrelated fields of science 
and engineering. Perhaps the most frequently practiced function of the 
aerodynamicist is the analysis of the forces and moments exerted on a solid 
body in motion through the air. 
    Of fundamental significance in the term aerodynamics is the prefix, aero, 
which refers to the air of the Earth's atmosphere. Until humans can achieve 
flight within the atmospheres of other planets, aerodynamic flight and the 
majority of practical considerations will be confined to the limits of the 
Earth's atmosphere as defined in aerodynamic terms. 
 �34s[William C. Walter]
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�6v�2v�32sAerodynamic wave drag   �21m �33s 
    The force retarding an airplane, especially in supersonic flight, as a 
consequence of the formation of shock waves. Although the physical laws 
governing flight at speeds in excess of the speed of sound are the same as 
those for subsonic flight, the nature of the flow about an airplane and, as a 
consequence, the various aerodynamic forces and moments acting on the vehicle 
at these higher speeds differ substantially from those at subsonic speeds. 
Basically, these variations result from the fact that at supersonic speeds the 
airplane moves faster than the disturbances of the air produced by the passage 
of the airplane. These disturbances are propagated at roughly the speed of 
sound and, as a result, primarily influence only a region behind the vehicle. 
    The primary effect of the change in the nature of the flow at supersonic 
speeds is a marked increase in the drag, resulting from the formation of shock 
waves about the configuration. These strong disturbances, which may extend for 
many kilometers from the airplane, cause significant energy losses in the air, 
the energy being drawn from the airplane. At supersonic flight speeds these 
waves are swept back obliquely, the angle of obliqueness decreasing with 
speed. See SHOCK WAVE; SUPERSONIC FLIGHT. 
    The shock waves are associated with outward diversions of the airflow by 
the various elements of the airplane. This diversion is caused by the leading 
and trailing edges of the wing and control surfaces, the nose and aft end of 
the fuselage, and other parts of the vehicle. Major proportions of these 
effects also result from the wing incidence required to provide lift. 
    The wave drag at the zero lift condition is reduced primarily by 
decreasing the thickness-chord ratios for the wings and control surfaces and 
by increasing the length-diameter ratios for the fuselage and bodies. Also, 
the leading edge of the wing and the nose of the fuselage are made relatively 
sharp. With such changes, the severity of the diversions of the flow by these 
elements is reduced, with a resulting reduction of the strength of the 
associated shock waves. The wave drag is further reduced by sweeping the wing 
back. Even with these improvements, aerodynamic efficiencies, or lift-drag 
ratios, for supersonic airplanes are substantially less than those for 
subsonic machines. 
 �34s[Richard T. Whitcomb]
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�6v�2v�32sAeroelasticity   �21m �33s 
    The branch of applied mechanics which deals with the interaction of 
aerodynamic, inertial, and structural forces. It is important in the design of 
airplanes, helicopters, missiles, suspension bridges, power lines, tall 
chimneys, and even stop signs. Variations on the term aeroelasticity have been 
coined to denote additional significant interactions. Aerothermoelasticity is 
concerned with effects of aerodynamic heating on aeroelastic behavior in 
high-speed flight. Aeroservoelasticity deals with the interaction of automatic 
controls and aeroelastic response and stability. In the field of 
hydroelasticity, a liquid rather than air generates the fluid forces. 
    The primary concerns of aeroelasticity include flying qualities (that is, 
stability and control), flutter, and structural loads arising from maneuvers 
and atmospheric turbulence. Methods of aeroelastic analysis differ according 
to the time dependence of the inertial and aerodynamic forces that are 
involved. For the analysis of flying qualities and maneuvering loads wherein 
the aerodynamic loads vary relatively slowly, quasi-static methods are 
applicable, although autopilot interaction could require more general methods. 
The remaining problems are dynamic, and methods of analysis differ according 
to whether the time dependence is arbitrary (that is, transient or random) or 
simply oscillatory in the steady state. 
    The redistribution of airloads caused by structural deformation will 
change the lifting effectiveness on the aerodynamic surfaces from that of a 
rigid vehicle. The simultaneous analysis of the equilibrium and compatibility 
among the external airloads, the internal structural and inertial loads, and 
the total flow disturbance, including the disturbance resulting from 
structural deformation, leads to a determination of the equilibrium 
aeroelastic state. If the airloads tend to increase the total flow 
disturbance, the lift effectiveness is increased; if the airloads decrease the 
total flow disturbance, the effectiveness decreases. 
    The airloads induced by means of a control-surface deflection also induce 
an aeroelastic loading of the entire system. Equilibrium is determined as in 
the analysis of load redistribution. Again, the effectiveness will differ from 
that of a rigid system, and may increase or decrease depending on the 
relationship between the net external loading and the deformation. 
    A self-excited vibration is possible if a disturbance to an aeroelastic 
system gives rise to unsteady aerodynamic loads such that the ensuing motion 
can be sustained. At the flutter speed a critical phasing between the motion 
and the loading permits extraction of an amount of energy from the airstream 
equal to that dissipated by internal damping during each cycle and thereby 
sustains a neutrally stable periodic motion. At lower speeds any disturbance 
will be damped, while at higher speeds, or at least in a range of higher 
speeds, disturbances will be amplified. See FLUTTER (AERONAUTICS). 
    Transient meteorological conditions such as wind shears, vertical drafts, 
mountain waves, and clear air or storm turbulence impose significant dynamic 
loads on aircraft. So does buffeting during flight at high angles of attack or 
at transonic speeds. The response of the aircraft determines the stresses in 
the structure and the comfort of the occupants. Aeroelastic behavior makes a 
condition of dynamic overstress possible; in many instances, the amplified 
stresses can be substantially higher than those that would occur if the 
structure were much stiffer. See CLEAR-AIR TURBULENCE (CAT); LOADS, DYNAMIC; 
TRANSONIC FLIGHT. 
 �34s[William P. Rodden]
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�6v�2v�32sAerology   �21m �33s 
    A largely obsolete term which was formerly almost synonymous with 
meteorology and aerography but was used mainly to denote the science and study 
of the upper atmosphere. Aerology was applied almost exclusively to United 
States naval meteorology. In 1917 the term aerography was adopted for naval 
meteorology. However, in 1922 the term aerology was substituted for 
aerography. Since that time U.S. Navy meteorologists have been designated as 
aerologists (officers) and aerographers (chief petty officers). Today, in the 
Navy, all are known as meteorologists, although terms such as aerographer's 
mate and chief aerographer survive. See METEOROLOGY; NAVAL METEOROLOGY. 
 �34s[Henry T. Harrison]
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�6v�2v�32sAeromechanics   �21m �33s 
    The science of air and other gases in motion or equilibrium. Aeromechanics 
has two branches, aerostatics and aerodynamics. Aeromechanics is a special 
case of the more general field of fluid mechanics, the science of fluids in 
motion or equilibrium. See FLUID MECHANICS. 
    Aerostatics deals with the equilibrium of air or other gases, and also 
with the equilibrium of bodies immersed in a gaseous medium. Aerodynamics is 
concerned with the properties and characteristics of, and the forces exerted 
by, air and other gases in motion. See AERODYNAMICS; AEROSTATICS. 
 �34s[William C. Walter]
�33s 
  

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�6v�2v�32sAeronautical engineering   �21m �33s 
    That branch of engineering which is concerned primarily with the special 
problems of flight. Historically, aeronautical engineering was an offshoot of 
mechanical engineering. However, as the technology of flight developed, 
distinctions were made in the curriculum of the aeronautical engineer. See 
AERODYNAMICS; AIRCRAFT; AIRCRAFT ENGINE; FLUID MECHANICS. 
    Since 1940 aeronautical engineering has drastically expanded. Flight 
speeds have increased from a few hundred miles per hour to satellite and space 
vehicle velocities. The common means of propulsion have changed from 
propellers to turboprops, turbojets, ramjets, and rockets. See JET PROPULSION; 
ROCKET; SPACE TECHNOLOGY. 
    The aeronautical engineer now works in fields which were once the 
exclusive domain of the physicist. Similarly, many jobs which were 
historically the exclusive domain of the aeronautical engineer are now being 
done by others who have acquired legitimate interests there. 
    In general, aeronautical engineering is no longer a definable domain. 
Rather, the aeronautical engineer with his or her broad training is often most 
usefully engaged in coordinating the many different disciplines which now 
enter into the engineering of flight. 
 �34s[John R. Sellars]
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�6v�2v�32sAeronautical meteorology   �21m �33s 
    That branch of meteorological study which deals with atmospheric effects 
on the operation of aircraft - heavier than air and lighter than air - 
rockets, missiles, and projectiles. Eight major effects are considered: low 
visibility at terminals, turbulence, clear-air turbulence, upper winds and 
temperature, jet stream, tropopause, lightning, and icing. See AEROLOGY; NAVAL 
METEOROLOGY.  
    Fog, snow, and rain prevent landings and takeoffs when horizontal 
visibility at the surface falls below regulatory minimum values for crewed 
aircraft. The minimum is generally 2400 ft (800 m) in the United States for 
airplanes equipped to operate under standard instrument conditions. Runway 
visual range (RVR), an operational concept of visibility, is rapidly 
supplanting the use of conventional meteorological visibility. It is the 
distance at which high-intensity lights may be observed down the instrument 
runway in the direction of landing or takeoff. 
    Atmospheric turbulence is defined as any deviation from the normal, 
steady, horizontal flow of air and occurs in two ways: sharp-edged gusts in 
the horizontal or vertical and large-scale up- and downdrafts which cause 
aircraft to sink or rise more gradually and for longer periods. 
 �34s[Henry T. Harrison]
�33s 
                                                                               
    The unexpected jolts which an aircraft may receive in clear air turbulence 
(CAT) add to the danger of this phenomenon, especially with respect to 
passenger safety and comfort. Forecasting of CAT has only been moderately 
successful in the past. Since clear-air turbulence is a small-scale 
phenomenon, the radiosonde network is insufficient in locating or predicting 
the precise locations of CAT zones. See CLEAR-AIR TURBULENCE (CAT). 
    Since aircraft are part of the air current in which they are embedded, 
they experience aiding or retarding effects determined by wind direction in 
relation to the course being flown. Careful planning of long-range flights is 
completed with the aid of prognostic upper-air charts. These make it possible 
to select flight tracks and cruising altitudes where more favorable winds and 
temperature will result in a lower elapsed flight time in spite of added 
ground distance. This planned procedure is known as minimal time track (least 
time track, pressure pattern) flying. 
    The jet stream is a meandering, shifting current of relatively swift 
windflow embedded in the general westerly circulation at upper levels. 
Sometimes girdling the globe at middle and subtropical latitudes where the 
strongest jets are found, this band of strong winds has great operational 
significance for aircraft flying at cruising levels of 4 - 10 mi (6 - 15 km). 
It is the cause of most serious flight-schedule delays and may result in 
unscheduled fuel stops. See JET STREAM. 
    The tropopause is the boundary between troposphere and stratosphere and is 
generally defined in terms of points in the vertical air-mass soundings where 
the temperature lapse rate becomes less than 5.7 DG F/mi (2 DG C/km). Its 
significance to aviation arises from its frequent association with mild forms 
of clear-air turbulence and change of vertical temperature gradient with 
altitude, which has considerable effect on turbine-engine performance. The 
tropopause also marks the vertical limit of clouds and storms; however, 
thunderstorms are known to punch through the surface to heights of 14.2 mi (23 
km) at times, and lenticular clouds of strong mountain waves have been 
observed at similar heights. See TROPOPAUSE. 
    Lightning strikes or static discharges, when experienced inside an 
aircraft, cause a blinding flash and usually a muffled  explosive sound 
audible above the roar of the engines. Atmospheric conditions favorable for 
strikes follow a consistent pattern: solid clouds or enough clouds for the 
aircraft to be intermittently on instruments, active precipitation of an icy 
character, and ambient air temperature close to 32 DG F (0 DG C). St. Elmo's 
fire, radio static, and choppy air often precede the strike. 
    Flight through a subcooled cloud, freezing rain, or wet snow commonly 
results in icing. Once one of the major hazards of flying, icing has been 
reduced to a minor weather factor by effective antiicing and deicing devices 
for the protection of critical components of aircraft. Icy or deeply 
snow-covered airport runways cause operational problems for aviation, but 
these factors have been reduced in importance through the use of modern 
snow-removal methods at major airports. 
 �34s[Henry T. Harrison]
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�6v�2v�32sAeronautics   �21m �33s 
    The principles underlying flight through the air. In the early years of 
flight, the term aeronautics referred to the art of operating aircraft of all 
sorts, and included the design of aircraft, which is now usually included as 
part of aeronautical engineering, and the actual flying of aircraft. However, 
as the term aeronautical engineering became more widespread, another word was 
sought to describe the basic scientific knowledge which underlies aeronautical 
engineering. Aeronautics was adopted to fit this need as being less 
restrictive than aerodynamics. Thus, for example, the theories of airflow 
about airfoils and wings may be categorized as aeronautics, although the 
application of those principles in developing a particular design is more 
precisely an engineering accomplishment. See AERONAUTICAL ENGINEERING; 
ASTRONAUTICS.  
 �34s[John R. Sellars]
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�6v�2v�32sAerospace medicine   �21m �33s 
    The special field of medicine that deals with humans in environments 
encountered beyond the surface of the Earth. It includes both aviation 
medicine and space medicine and is concerned with humans, their environment, 
and the vehicle in which they fly. Its objective is to ensure human health, 
safety, well-being, and effective performance through careful selection and 
training of flight personnel, protection from the unique flight environment 
and its physiological and psychological effects, and understanding of the 
flight vehicle and humans' interaction with it. 
    The environment encountered in air or space flight is very different from 
that on Earth. Consequently aerospace medicine must deal with the physics of 
the atmosphere and space and with the conditions and influences introduced by 
the flight vehicle. Aerospace medicine, environmental physiology, and 
psychology are therefore concerned with the effects of changes in barometric 
pressure, oxygen deprivation at reduced barometric pressure, atmospheric 
constituents, toxic substances, acceleration, weightlessness, noise, 
vibration, ionizing radiation, and thermal and other environmental stresses. 
    All of the environmental hazards encumbent in air travel are present in 
space flight, but space also has conditions that are unique. The near-vacuum 
of the space environment can be overcome through engineering design, but the 
deleterious effects of exposure to high-energy heavy-particle radiation (HZE) 
have been avoided only by the low Earth orbit and relatively short duration of 
space travel. Investigation of the biological response to HZE is needed prior 
to long-duration crewed space flight. The condition of near-weightlessness in 
space has produced significant physiological effects in humans, including 
motion sickness, bone and muscle loss, reduced red blood cell production, and 
cardiovascular deconditioning. Aerospace medicine is concerned with developing 
countermeasures, including physical, psychological, pharmacological, or 
nutritional means, to prevent or control these physiological effects. Within 
the confines of the spacecraft, life support devices must provide for the 
regeneration of oxygen, reclamation of water, and removal of wastes and toxic 
gaseous materials. See SPACE BIOLOGY; WEIGHTLESSNESS. 
 �34s[J. Woodland Hastings]
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�6v�2v�32sAerostatics   �21m �33s 
    The science of the equilibrium of gases and of solid bodies immersed in 
them when under the influence only of natural gravitational forces. 
Aerostatics is concerned with the balance between the weight of the gases and 
the weight of any object within them. Archimedes' law that an immersed body 
experiences a buoyancy force equal to the weight of the fluid displaced is the 
principal law of aerostatics, if the fluid is air, or of hydrostatics, if the 
fluid is water. Some phases of meteorology and the flight of balloons and 
dirigibles are based on aerostatics. In meteorology cloud and fog subsidence 
and simple pressure and temperature relations with altitude are predicted from 
aerostatic principles. See FLUID STATICS; HYDROSTATICS. 
 �34s[John R. Sellars]
�33s 
  

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�6v�2v�32sAerothermodynamics   �21m �33s 
    Flow of gases in which heat exchanges produce a significant effect on the 
flow. Traditionally, aerodynamics treats the flow of gases, usually air, in 
which the thermodynamic state is not far different from standard atmospheric 
conditions at sea level. In such a case the pressure, temperature, and density 
are related by the simple equation of state for a perfect gas; and the rest of 
the gas's properties, such as specific heat, viscosity, and thermal 
conductivity, are assumed constant. Because fluid properties of a gas depend 
upon its temperature and composition, analysis of flow systems in which 
temperatures are high or in which the composition of the gas varies (as it 
does at high velocities) requires simultaneous examination of thermal and 
dynamic phenomena. For instance, at hypersonic flight speed the characteristic 
temperature in the shock layer of a blunted body or in the boundary layer of a 
slender body is proportional to the square of the Mach number. These are 
aerothermodynamic phenomena. 
    Two problems of particular importance require aerothermodynamic 
considerations: combustion and high-speed flight. Chemical reactions sustained 
by combustion flow systems produce high temperatures and variable gas 
composition. Because of oxidation (combustion) and in some cases dissociation 
and ionization processes, these systems are sometimes described as 
aerothermochemical. In high-speed flight the kinetic energy used by a vehicle 
to overcome drag forces is converted into compression work on the surrounding 
gas and thereby raises the gas temperature. Temperature of the gas may become 
high enough to cause dissociation (at Mach number EQUAL TO OR GREATER THAN 7) 
and ionization (at Mach number EQUAL TO OR GREATER THAN 12); thus the gas 
becomes chemically active and electrically conducting. See HYPERSONIC FLIGHT; 
JET PROPULSION; ROCKET ENGINE. 
 �34s[Shih-Yuan Chen]
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�6v�2v�32sAflatoxin   �21m �33s 
    Any of a group of toxic compounds produced during growth of the common 
mold Aspergillus flavus on peanuts, cereal grains, meals, and other food or 
animal feedstuff. Aflatoxins have been responsible for acute poisoning of 
fish, poultry, and livestock fed moldy grain and peanut meal containing 
comparatively high levels of toxin. They can cause liver cancer in livestock, 
poultry, fish, or domestic animals when ingested in relatively low levels over 
long periods. In fact, aflatoxin B1 is the most potent liver carcinogen known. 
There is considerable indirect evidence that aflatoxins cause liver cancer in 
humans.  
 �34s[George E. Templeton]
�33s 
  

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�6v�2v�32sAfrica   �21m �33s 
    A continent of 11,700,000 mi sup 2 (30,300,000 km sup 2), Africa is the  
second largest continent, exceeded only by Eurasia. Despite its great area,  
Africa is a relatively simple landmass in terms of its climatic and vegetation-
al distributions, its geological structure, the shape of its outline, and the 
distribution of its surface features. In part this apparent simplicity is the 
result of its latitudinal extent, which is almost exactly bisected by the 
Equator, and in part the result of its long degradational history, inasmuch as 
the greater part of the continent has stood as a land mass of crystalline 
rocks since Precambrian times. Africa is the most stable portion of the 
southern, or Gondwana, part of a postulated original continent, Pangaea, from 
which the Southern Hemisphere continents separated in Cretaceous time. As a 
result of this remarkable tectonic stability, most of the continent has an 
elevated rolling plateau surface alternating between broad swells where the 
ancient rocks crop out and wide basins filled with continental sediments 
derived from the marginal swells. Because of the rolling surface and the 
central position of the Equator, Africa displays a notable symmetry in the 
distribution of climatic types and vegetational categories at equal latitudes 
north and south of the Equator. 
    The character of the climates of Africa depends more upon rainfall than 
upon temperature. Because every part of the continent receives strong 
insolation throughout the year, and no part of Africa is subjected to the 
invasion of cold continental air masses from higher latitudes, frost is 
unknown except in limited areas of high altitude or places so dry that 
nighttime radiation is very intense. It is at these same dry places that the 
very highest daytime temperatures occur. The temperature of 136.4 DG F (58 DG  
C) at Azizia, Tripoli, is cited by some authorities as the world's extreme. 
Nevertheless, temperatures are virtually everywhere within a range that 
assures vegetational growth throughout the year, and the amount, seasonal 
distribution, and reliability of rainfall are critical to the life of plants, 
and therefore broadly to the fauna, including humans. 
    The continent of Africa is dominated by three types of air masses: 
tropical maritime, which is warm and moist and has its sources over the oceans 
north and south of the Equator; tropical continental, which is warm and dry, 
one mass originating over the Sahara and the other over the Kalahari; and 
polar maritime, which is cool and moist and invades Africa in two areas, 
northwest in the Northern Hemisphere wintertime and southwest in the Southern 
Hemisphere wintertime. The air masses shift in position to conform in general 
with the shift in the apparent position of the Sun as the seasons pass during 
the year. See AIR MASS. 
    Africa is a continent much affected by aridity, which in some places 
endures throughout the year and in others is only seasonal. Contrary to the 
popular concept of an Africa teeming with vast dark forests of gigantic trees, 
the greater part is either scrubland, grassland, or desert. The true tropical 
rainforest is limited to the always rainy areas of humid equatorial climate in 
the central and western sections of the continent and to a narrow strip along 
the Guinea coast. All the tropical rainforests and the associated riverine 
forests and mangrove swamps together cover less than 10% of Africa. Many 
centuries of slashing and burning by the native inhabitants have reduced most 
of these forests to degenerate tropical bush. 
    Poleward from the tropical rainforest position in each hemisphere and also 
along the higher and drier eastern side of Africa there is a broad 
horseshoe-shaped band of semideciduous forest, often called dry forest or 
thorn forest, and of savanna or tall grass with scattered trees and bushes. 
These vegetation combinations reflect the alternation of precipitation seasons 
between the wet growing season during ``high sun'' and the drought during 
``low sun.'' A full 40% of Africa experiences the annual changes. Another 45% 
of Africa is so poorly watered at all seasons that it supports only desert 
plants and dry savanna of short grasses. This dry zone runs broadly across 
northern Africa as the Sahara and its margins. It is present in the Southern 
Hemisphere in the same latitudes, but the continent is narrow there and the 
dry conditions are not so widely and severely developed. The remainder of 
Africa displays Mediterranean scrubforest vegetation at its northern and 
southern extremities, and some middle-latitude and alpine vegetation forms in 
the highlands of Ethiopia and on the upland areas of eastern and southern 
Africa.  
    Africa is sometimes called a plateau because in many places, particularly 
south of the Equator, the rolling surface of the continent is bounded by steep 
escarpments which face the sea and drop down to narrow coastal plains. The 
simple outline of Africa reflects the narrow continental shelf which borders 
it. Compared to other continents, Africa is a massive, rigid, and stable 
block, exhibiting folding and faulting in limited zones only, with the 
basement complex of crystalline rocks exposed over about one-third of its 
surface in marginal swells. 
    The surface of the African plateau drops from elevations of 4000 - 5000 ft 
(1200 - 1500 m) in the Southern Hemisphere to about 1000 ft (300 m) in the 
Sahara. In southern and central Africa there is evidence suggestive of three 
or four important erosional surfaces, which are presumed to have resulted from 
periodic uplift of the African block. Locally, the upthrust edges of the block 
may reach considerable heights, as in the Drakensberg of South Africa where 
elevations exceed 11,500 ft (3500 m). 
    The massiveness of the African continent and the simplicity of its outline 
are expressed in the paucity of islands associated with it. Excluding marshy 
deltaic islands, the Atlantic shore of Africa has a few islets jutting off 
points of land and the far more significant line of islands in the Gulf of 
Guinea which represent extensions of the volcanic peaks found in the Cameroon 
Mountains. The Indian Ocean coast also has only small close inshore deltaic 
and headland islands, and coral reefs. 
    The sole great island is Madagascar, which with an area of 250,000 mi sup 2 
(650,000 km sup 2) is one of the largest islands of the world. Separated from 
Africa by the Mozambique Strait and 250 mi (400 km) of open water, Madagascar 
is almost 1000 mi (1600 km) long. Lying athwart the southeast trade winds, 
Madagascar has orographic rainfall on its eastern high margin all of the year, 
whereas the west lies in a rain shadow. Madagascar's long separation from 
Africa is demonstrated by remarkable faunal and floral differences between the 
island and the mainland. 
    One-third of the African continent has no drainage to the surrounding 
oceans and seas, and most of the remainder is drained by only a few major 
rivers. Because so much of Africa is a plateau surface of swells and vast 
basins, most of the great rivers which reach the sea have long middle courses 
of remarkably low gradients between ungraded headwaters and the waterfalls and 
cataracts which mark their channels as they fall over the plateau rim to sea 
level. Although this fact has meant that African rivers were not easy ways of 
penetrating into the continent in the period of European exploration (nor are 
they paths of uninterrupted navigation today), the rivers do produce great 
amounts of energy so that Africa, despite its aridity, has an estimated 23% of 
the world's potential hydroelectric power. 
    The only perennial stream which enters the Mediterranean Sea from Africa 
is the Nile, whose 4000 mi (6400 km) makes it the longest of all African 
rivers. From the southern edge of the Sahara to its mouth in the Nile Delta, 
it is a dwindling river, without tributaries to replace water lost to 
percolation and evaporation and to withdrawal for irrigation. Within Egypt the 
Nile has been increasingly controlled over the last 2 centuries by engineering 
structures, culminating in 1971 in the Aswan High Dam which created Lake 
Nasser, over 300 mi (480 km) long and extending into the Republic of the 
Sudan. Above the Blue Nile junction the main stream is known as the White 
Nile. The plain between the two rivers is the Gezira. Upstream the White Nile 
is joined by the Bahr-el-Ghazel which feeds in water from the left bank. The 
Bahr-el-Jebel or upper White Nile has drained from the northeast corner of 
Lake Albert (Lake Mobutu) and the swamps of Lake Kyoga, and these waters have 
their source in Lake Victoria and the streams which feed it. 
    The rainy sub-Saharan lands west of the longitude of the Bight of Biafra 
have many rivers, most of them short, precipitous, and subject to large 
fluctuations in seasonal volumes. There are only three great river systems: 
The Senegal River makes a northwestern arc to reach the Atlantic north of Cape 
Verde and on the south margin of the Sahara. The Niger flows northeastward 
into ever drier climatic conditions and spreads its waters over a swampy 
``interior delta'' of its own making. Beyond the vicinity of Timbuktu the 
Niger swings abruptly southeastward, following the valley of a stream that 
captured the upper reaches, and after a total course of 2600 mi (4160 km) 
attains the Gulf of Guinea by way of a vast coastal delta. Within the great 
loop of the Niger lies the Volta system, which collects much of the waters of 
Ghana and lands to the north and west through stream integration. 
    The great river of tropical Africa is the Congo. Nearly 3000 mi (4800 km) 
long, its middle portion provides 1000 mi (1600 km) of low-gradient, navigable 
waters between Stanley Falls upstream and Stanley Pool downstream. From 
Stanley Pool to the Atlantic is a descent of over 800 ft (240 m) in rapids and 
waterfalls to form the premier site for waterpower development in Africa. 
Below the cataracts, the lower Congo has 80 mi (129 km) of waters navigable by 
ocean vessels. The upper Congo, or Lualaba, is navigable in stretches. 
    Because of its confused structure and topography, eastern Africa has 
developed no great integrated river system other than the Nile. The great 
lakes occupying troughs in the rift valley complex are minimally integrated. 
Lakes Albert, Edward, and George are connected to the Nile drainage. Lake Kivu 
drains to Lake Tanganyika and thence to the Congo system. Lake Malawi drains 
to the Shire River, which in turn enters the Zambezi and the Indian Ocean. 
Lake Rudolf and many smaller rift valley lakes have no outlets. 
 �34s[Hibberd V. B. Kline, Jr.]
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�6v�2v�32sAfterburner �21m �33s
    A device in a turbojet aircraft engine, between turbine and nozzle, which
improves thrust by the burning of additional fuel (see illustration). To
develop additional thrust for takeoff and climb and for periods of dash of
military aircraft, it is advantageous to augment the engine thrust. This is
done by afterburning, also called reheating, tail-pipe burning, or
postcombustion. The augmentation of thrust obtained by afterburning may be
well over 40% of the normal thrust and at supersonic flight can exceed 100% of
normal thrust. See TURBOJET.

 �34s[Benjamin Pinkel]
�33s



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�6v�2v�32sAgammaglobulinemia   �21m �33s 
    An immunologic deficiency state characterized by an increased 
susceptibility to infection. Diagnosis is based on the demonstration of low 
concentrations or absence of one of several immunoglobulins (gamma globulin), 
and a poor response of the immune system to infection or other antigenic 
challenge. Most of the significant agammaglobulinemias are the result of a 
decreased synthesis of the immune globulins because of a deficiency of plasma 
cell number or function. 
    One type of primary (hereditary) agammaglobulinemia is usually transmitted 
as an autosomal recessive and is termed alymphocytic agammaglobulinemia, 
characterized by a complete absence of lymphocytes. Affected infants cannot 
produce a humoral or a cell-mediated immune response and are severely 
predisposed to infections and usually die within the first year of life. 
Another type of agammaglobulinemia is one in which the individual is born 
without a thymus gland (Di George's syndrome). Symptoms include tetany and 
susceptibility to bacterial but not viral or fungal infections. Another 
hereditary type of agammaglobulinemia (Bruton's disease) is a sex-linked 
recessive disorder characterized by a deficiency of all types of 
immunoglobulins, reflecting a failure of the entire humoral antibody marrow 
system. In this disease the thymus may be normal, but the lymph nodes and 
spleen lack lymph cell follicles. See BLOOD; HUMAN GENETICS. 
    Treatment of deficiency disorders of the immunoglobulins has become 
possible. Infection is a problem, and treatment requires identification of the 
microorganism and use of specific chemotherapy. Low serum gamma globulin 
levels can be increased with concentrated human gamma globulin. See 
IMMUNOGLOBULIN.  
 �34s[N. Karle Mottet]
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�6v�2v�32sAgar   �21m �33s 
    A gelatinous product extracted from several species of red algae, 
particularly from Gelidium species and Gracilaria lichenoides (Ceylon moss) 
and species of Ahnfeltia and Pterocladia. Agar occurs in the natural state as 
a cell wall constituent of these plants. 
    Agar is prepared by boiling the algae in water, after which the product is 
cooled, purified, and dried. It is an amorphous, translucent material which is 
used as a gelling agent in culture media (chiefly for bacteria), 
confectionaries, and canned meat and as a sizing material. It is also used as 
food roughage, in laxative preparations, and in cosmetic creams and jellies. 
See CULTURE MEDIA. 
 �34s[Sybil P. Parker]
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�6v�2v�32sAgate   �21m �33s 
    A variety of chalcedonic quartz that is distinguished by the presence of 
color banding in curved or irregular patterns (see illustration). Most agate 
used for ornamental purposes is composed of two or more tones or intensities 
of brownish-red, often interlayered with white, but it is also commonly 
composed of various shades of gray and white. Since agate is relatively 
porous, it can be dyed permanently in red, green, blue, and a variety of other 
colors.  
    The term agate is also used with prefixes to describe certain types of 
chalcedony in which banding is not evident. Moss agate is a milky or almost 
transparent chalcedony containing dark inclusions in a dendritic pattern. Iris 
agate exhibits an iridescent color effect. Fortification, or landscape, agate 
is translucent and contains inclusions that give it an appearance reminiscent 
of familiar natural scenes. Banded agate is distinguished from onyx by the 
fact that its banding is curved or irregular, in contrast to the straight, 
parallel layers of onyx. The properties of agate are those of chalcedony: 
refractive indices of 1.535 and 1.539, a hardness of 6 1/2 to 7, and a specific 
gravity of about 2.60. See CHALCEDONY; GEM; QUARTZ. 
    Figure caption: Section of polished agate showing the characteristic        
banding. (Field Museum of Natural History, Chicago)
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sAgglutination reaction   �21m �33s 
    A clumping of a particulate suspension by a known or by a test reagent. 
The reactions are usually rapid and easily carried out, and in consequence, 
have wide application in diagnosis. Several types may be distinguished. In 
one, natural antigens (bacteria or red blood cells) clump when treated with 
their specific antibodies. This reaction may be used for the typing of blood 
for transfusion or the detection of antibodies to typhoid organisms in the 
sera of suspected carriers. The amount of antibody may be estimated by noting 
the serum dilution at which agglutination is just perceived. See ANTIBODY. 
    In a second type, a reagent antibody is added to a solution suspected of 
containing a soluble test antigen; if the antigen is present, the antibody is 
neutralized and the cell suspension subsequently added will not be 
agglutinated. The cell suspension has previously been sensitized with the 
soluble test antigen. This sensitive agglutination-inhibition test is widely 
used, for example, in the detection of urinary hormones for the determination 
of pregnancy. 
    In a third type, certain virus and red cell combinations are agglutinated 
in a nonspecific manner. However, the reaction may be inhibited, specifically, 
by antibody to the virus and this permits a high degree of control. This virus 
hemagglutination-inhibition test may be used to identify the virus antigen or 
to quantitate the antibody. See ANTIGEN; BLOOD GROUPS; VIRUS. 
 �34s[Henry P. Treffers]
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�6v�2v�32sAgglutinin   �21m �33s 
    A substance that will cause a clumping of particles such as bacteria or 
erythrocytes. Of major importance are the specific or immune agglutinins, 
which are antibodies that will agglutinate bacteria containing the 
corresponding antigens on their surfaces. Analogous reactions involve 
erythrocytes and their corresponding antibodies, the hemagglutinins. 
Hemagglutinins to a variety of erthyrocytes occur in many normal sera, and 
their amounts may be increased by immunization. The blood group isoagglutinins 
of humans and animals are important special cases which must be considered in 
all proposed blood transfusions lest transfusion reactions result. See 
AGGLUTINATION REACTION; ANTIBODY; BLOOD GROUPS; COMPLEMENT. 
 �34s[Henry P. Treffers]
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�6v�2v�32sAggression   �21m �33s 
    A form of behavior usually of a punitive, destructive nature and often 
caused by frustration. Two general views are held concerning it. One holds 
that aggression has an instinctual basis and that its expression and 
gratification is a biological imperative. Alternatively, it has been suggested 
that aggression is a reaction to frustration and that instigation to 
aggressive action may therefore be highly variable, and controllable. This 
reactive theory is more widely favored than the instinct hypothesis. However, 
frustration may be experienced which does not result in aggression, and 
aggression may occur for reasons other than frustration. See EMOTION. 
 �34s[O. Hobart Mowrer]
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�6v�2v�32sAgnatha   �21m �33s 
    The most primitive class of vertebrates, including the living Cyclostomata 
(hagfishes and lampreys), as well as extinct ostracoderms of the Ordovician, 
Silurian, and Devonian periods. They are distinguished by the fact that they 
have no true jaws. In the absence of jaws Agnatha have fed on minute food 
particles obtained by suction and trapped by filters or a mucous net, have 
become nibblers, or have developed a tongue set with horny teeth for rasping 
flesh, as in living Cyclostomata. The Agnatha are primitive also in their 
persistent notochord, in the absence of vertebrae, and often in the absence of 
paired fins. 
    The class Agnatha may be classified as follows: 
Subclass Diplorhina 
        Order: Heterostraci 
               Thelodonti (or Coelolepida) 
Subclass Monorhina 
    Superorder Hyperotreti 
        Order Myxinoidea 
    Superorder Hyperoartii 
        Order: Petromyzonida 
               Osteostraci 
               Anaspida 
Subclass Cyclostomata 
 �34s[Robert H. Denison]
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�6v�2v�32sAgnosia   �21m �33s 
    The loss of the ability to recognize objects or other constellations of 
stimuli. The essential feature of agnosia is the selectivity of the loss. It 
is confined to impressions received through one sensory channel and occurs 
despite relative preservation of sensitivity and intellect. The principal 
varieties of agnosia are visual, auditory, and tactile. Agnosia is caused by 
cerebral injury or disease. The critical loci of lesions in the various forms 
of this disorder have not been satisfactorily determined. 
    The traditional view of agnosia is that sensations of the affected sense 
modality, although adequate in themselves, fail to arouse the complex of 
memory images essential for the identification of the object. The failure is 
said to be due to destruction of associative centers or pathways. It is 
thought that the lesion should involve the cortex adjacent to the primary 
projection area, or subcortical tracts issuing from this area. This view of 
the nature of agnosia, and of the locus of lesion on which it depends, has not 
been universally accepted. See MEMORY; PERCEPTION. 
 �34s[Josephine Semmes]
�33s 
  

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�6v�2v�32sAgouti   �21m �33s 
    A large rodent that resembles the rabbit or hare in size and shape as well 
as in the elongated hindlegs, which make them well adapted for speed (see 
illustration). The agouti and the closely related smaller acouchi are 
inhabitants of clearings in forested areas of the Amazon region. Some range 
into Central America and as far as the Guianas. 
    Thirteen species of agouti have been described, the most common being 
Dasyprocta aguti. Some authorities are of the opinion that all of these are 
varieties or subspecies of D. aguti. The acouchi is represented by two 
species, the green acouchi (Myoprocta pratti) and the red acouchi (M. 
acouchy). See RODENTIA. 
    Figure caption: The agouti (Dasyprocta aguti), a rodent found in Mexico,
South America, and the West Indies.
 �34s[Charles B. Curtin]
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�6v�2v�32sAgricultural aircraft   �21m �33s 
    Aircraft adapted or designed for use in agriculture and forestry. 
Agricultural aircraft are indispensable farm machines in developed countries 
and are increasingly used in many developing countries as well. In addition to 
dispersing wet- and dry-type pesticides they perform seeding, fertilizing, 
crop-drying, and defoliating services. 
    Agricultural aircraft are a fundamental part of the food production 
system, but they do not necessarily offer an energy efficiency gain over other 
methods. Their desirability lies in the rapidity of their operation, in their 
ability to operate irrespective of field conditions, irrigation, and crop 
growth, and in their effectiveness as a pest control which is comparable to 
ground equipment. Aircraft use is limited by: size of fields; proximity to 
sensitive nontarget crops and human habitat; and biological effectiveness, 
primarily relating to crop plant coverage obtained by the aircraft application 
system.  
    Helicopter or rotary wing aircraft offer significant advantages in 
agricultural, forestry, and vector control work, due primarily to their 
maneuverability, low-forward-speed capability, and application precision. 
However, turbine engines add considerably to the already higher initial price 
of helicopters, and this, coupled with requirements of higher maintenance and 
greater pilot skill, has kept helicopter use at less than 10% of the total 
numbers and area covered by all aircraft. See HELICOPTER. 
 �34s[Norman B. Akesson; Wesley E. Yates]
�33s 
  

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�6v�2v�32sAgricultural chemistry   �21m �33s 
    The science of chemical compositions and changes involved in the 
production, protection, and use of crops and livestock. As a basic science, it 
embraces, in addition to test-tube chemistry, all the life processes through 
which food and fiber for humans and feed for animals are obtained. As an 
applied science or technology, it is directed toward control of those 
processes to increase yields, improve quality, and reduce costs. One important 
branch of it, chemurgy, is concerned chiefly with utilization of agricultural 
products as chemical raw materials. See CHEMURGY. 
    The goals of agricultural chemistry are to expand understanding of the 
causes and effects of biochemical reactions related to plant and animal 
growth, to reveal opportunities for controlling those reactions, and to 
develop chemical products that will provide the desired assistance or control. 
Agricultural chemistry is not a distinct discipline, but a common thread that 
ties together genetics, physiology, microbiology, entomology, and numerous 
other sciences that impinge on agriculture. See FERTILIZER; SOIL. 
    Chemical materials developed to assist in the production of food, feed, 
and fiber include scores of herbicides, insecticides, fungicides, and other 
pesticides, plant growth regulators, fertilizers, and animal feed supplements. 
Chief among these groups from the commercial point of view are manufactured 
fertilizers, synthetic pesticides (including herbicides), and supplements for 
feeds. The latter include both nutritional supplements (for example, minerals) 
and medicinal compounds for the prevention or control of disease. See 
AGRICULTURE; ANIMAL FEEDS; HERBICIDE; PESTICIDE. 
 �34s[Rodney N. Hader]
�33s 
  

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�6v�2v�32sAgricultural engineering   �21m �33s 
    The engineering discipline directly concerned with developing means for 
providing food and fiber. With the world population explosion and the desire 
for higher standards of living, the efficiency of food and fiber production 
must be continually increased. See AGRICULTURE. 
    Nearly all problems that agricultural engineers deal with involve 
biological materials, systems, and processes. Application of physical and 
engineering sciences to the solution of such problems is the distinguishing 
feature of agricultural engineering. 
    Mechanization of agricultural production has been responsible in large 
measure for the productivity record of agriculture. While great progress has 
been made in the application of mechanical power to the production and 
harvesting of field crops, many crops, such as fruits and vegetables, still 
require expensive and increasingly scarce hand labor, especially in harvesting 
operations.  
    One of the areas of great importance for the future is modification and 
control of the soil, water, and air environment of plants. As greater 
production is demanded from an ever-decreasing available land area, more and 
more attention is being given to modifying the natural environment to produce 
conditions which are more nearly ideal for plant growth. Controlled 
environment for animal production within confined housing systems is another 
area of activity of major concern to agricultural engineers. 
    The modern livestock farm can be likened to a factory with various imputs 
for poultry and dairy and meat animals. Mechanization of the modern farmstead 
has become a reality, and agricultural engineers have been largely 
responsible. Automation is now the goal that challenges agricultural engineers. 
    An important phase of agricultural engineering involves the management and 
beneficial use of soil and water resources. Agricultural engineers are 
involved in research to develop new techniques and design systems to control 
soil erosion and soil moisture for crop production, to prevent floods and 
water pollution, and to provide pure water supplies for human use. See SOIL 
CONSERVATION; WATER CONSERVATION. 
    Although much agricultural engineering work is involved with the 
development and design of power machinery for crop production, many 
engineering problems are encountered after the crops are harvested. 
Postharvest processing, such as drying of grains and forages, washing, 
grading, and storage of fruits and vegetables, including controlled-atmosphere 
systems, and ginning of cotton are typical of on-the-farm applications of 
agricultural engineering. 
 �34s[Orval C. French]
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�6v�2v�32sAgricultural machinery   �21m �33s 
    Mechanized systems of food and fiber production that extend from initial 
tillage of the soil through planting, cultural practices during the growing 
season, protection from pests, harvesting, conditioning, livestock feeding, 
and delivery for processing. The trend has been to larger self-propelled 
special-purpose machines, except for tillage where the trend has been to large 
four-, six-, or eight-wheel or crawler tractors used to tow high-capacity 
plows or disks, and also deep rippers used to loosen compacted soils. The use 
of hydraulic power has made possible highly specialized machinery to perform 
intricate operations wherever needed, because the fluid power is pumped 
through flexible hoses to provide linear or rotating motion with push-button 
control. The most sophisticated technology is used to increase the precision 
needed in modern agriculture. Lasers are used for laying out irrigation 
systems; microprocessors are used for sensing and controlling intricate 
operations, such as controlling feed mixtures for dairy cows and grading 
fruits and vegetables; electronic devices are used in the automation of many 
harvesters.  
    While many implements such as plows, disks, cultivators, and fertilizer 
spreaders are mounted on tractors, there are many that are too large and are 
trailed behind and are controlled and operated hydraulically. Some 
multipurpose machines are used where a high degree of precision is needed. The 
use of aircraft has revolutionized many farming operations. Rice is sown in 
flooded fields, and fertilizers and pesticides are applied from the air. 
Herbicides and insecticides are also applied by air on other crops, such as 
grains, vegetables, and tree fruits. 
 �34s[Michael O'Brien]
�33s 
  

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�6v�2v�32sAgricultural meteorology   �21m �33s 
    The study and application of meteorology and climatology (a branch of 
meteorology) to the specific problems of agriculture. Agriculture is the 
production of food and fiber in all its forms - crops for human and animal 
consumption, pasture and range for animal grazing, and crops (including trees) 
as raw materials for manufactured products. Hence, agricultural meteorology 
deals with farming, ranching, and forestry as well as with the transportation 
to the producer of substances required for production - water for irrigation, 
fertilizer, and agricultural chemicals - and transportation of the products to 
markets.  
    Agricultural meteorologists, or agrometeorologists, deal with a variety of 
problems in cooperation with many other types of specialists. These include 
prediction of effect of weather on crop yields, water use efficiency, 
weather-related diseases and pests, the length of the growing season, the 
timing and severity of frost, predominant speed and direction of wind, and 
problems associated with severe weather conditions. Some other areas of 
research and applications in agrometeorology are in turbulence and remote 
sensing. The Earth's surfaces (terrestrial and marine) exchange radiation and 
mass with the atmosphere and absorb momentum of the wind. Understanding the 
mechanisms of these exchanges is the function of micrometeorology. 
    Agricultural meteorologists have techniques for remotely sensing the 
condition of the land and the vegetation growing on it. Multispectral scanners 
on a number of National Oceanic and Atmospheric Administration (NOAA) and 
National Aeronautics and Space Administration (NASA) satellites and on 
aircraft provide data on the extent, density, and vigor of vegetation. 
Agricultural meteorologists have been especially active in conducting ground 
truth studies, which aid in the interpretation of remotely sensed information. 
 �34s[Norman J. Rosenberg]
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�6v�2v�32sAgriculture   �21m �33s 
    The art and science of crop and livestock production. In its broadest 
sense, agriculture comprises the entire range of technologies associated with 
the production of useful products from plants and animals, including soil 
cultivation, crop and livestock management, and the activities of processing 
and marketing. The term agri-business has been coined to include all the 
technologies that mesh in the total inputs and outputs of the farming sector. 
In this light, agriculture encompasses the whole range of economic activities 
involved in manufacturing and distributing the industrial inputs used in 
farming; the farm production of crops, animals, and animal products; the 
processing of these materials into finished products; and the provision of 
products at a time and place demanded by consumers. The sale of final product 
of the food and fiber industries in the United States represents about 
one-fourth of the gross national product and is about five times the value of 
farm-level production. 
    Many different factors influence the kind of agriculture practiced in a 
particular area. Among these factors are climate, soil, topography, nearness 
to markets, transportation facilities, land costs, and general economic level. 
    Climate, soil, water availability, and topography vary widely throughout 
the world. This variation brings about a wide range in agricultural production 
enterprises. Certain areas tend toward a specialized agriculture, whereas 
other areas engage in a more diversified agriculture. As new technology is 
introduced and adopted, environmental factors are less important in 
influencing agricultural production patterns. Continued growth in the world's 
population makes critical the continuing ability of agriculture to provide 
needed food and fiber. 
 �34s[Jules Janick]
�33s 
  

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�6v�2v�32sAgronomy   �21m �33s 
    The principles and procedures of field crop improvement, management, and 
production. Agronomy is also concerned with the improvement and management of 
special-purpose plants, such as turf grasses for home lawns, recreational 
areas, highway embankments, drainage ditches, and waterways. Soil 
characteristics, properties, uses, and conservation are part of agronomy, and 
management of soils for efficient production of specific crops is a major 
interest.  
    Regarding crops, primary consideration is given to relationship among crop 
plants as conditioned by their heredity, physiology, and ecology. Soil 
genesis, classification, and morphology; soil physical, chemical, and 
biological properties; and water relationships are of prime importance with 
respect to soils. 
 �34s[Walter I. Thomas]
�33s 
  

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�6v�2v�32sAileron �21m �33s
    The hinged rear portion of an aircraft wing, moved differentially on each
side of the aircraft to obtain lateral or roll control moments. The angular
settings of the ailerons are controlled by the human or automatic pilot
through the flight control system. Typical flap- and spoiler-type ailerons are
shown in the illustration. See FLIGHT CONTROLS.
    The operating principles of ailerons are the same as for all trailing-edge
hinged control devices. Deflection of an aileron changes the effective camber,
or airfoil curvature relative to the wing chord, of the entire wing forward of
the aileron. With the trailing edge deflected upward, reduced local flow
velocities are produced on the upper wing surface, and increased local flow
velocities are produced on the lower wing surface. By Bernoulli's law, this
results in a reduction of lift over the portion of the wing forward of the
aileron, and on the aileron itself. Conversely, trailing-edge down deflection
of a flap-type aileron increases the lift in the same areas. Ailerons are
located as close as possible to the wing tips, to maximize rolling moment by
increasing the moment arm of the force due to the change in wing lift. In the
case of flap-type ailerons, when the trailing edge is raised on one wing, say
the left, the trailing edge of the aileron on the opposite or right wing is
lowered by about the same amount. The decrease in lift on the left wing is
accompanied by a lift increase on the right wing. While the net wing lift
remains about the same, a rolling moment or torque about the aircraft's
fore-and-aft axis develops in a left, or counterclockwise, direction as seen
by the pilot. See BERNOULLI'S THEOREM.
    Flap-type ailerons are replaced or supplemented by spoiler-type ailerons
for a variety of reasons. Spoiler ailerons are usually installed forward of
the landing flaps on commercial jet transports, in order to supplement aileron
effectiveness during landing approaches, when the landing flaps are extended.
Greatly reduced takeoff and landing speeds can be obtained by devoting the
trailing edge of the entire wing to high-lift flaps. This is made possible by
substituting spoilers for flap-type ailerons.

 �34s[Malcolm J. Abzug]
�33s



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�6v�2v�32sAir   �21m �33s 
    A predominantly mechanical mixture of a variety of individual gases 
enveloping the terrestrial globe to form the Earth's atmosphere. In this sense 
air is one of the three basic components, air, water, and land (atmosphere, 
hydrosphere, and lithosphere), that interblend to form the life zone at the 
face of the Earth. See ATMOSPHERE. 
 �34s[Charles V. Crittenden]
�33s 
  
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�6v�2v�32sAir Almanac   �21m �33s 
    A periodical publication of astronomical statistics useful to, and 
designed primarily for, air navigation. The U.S. Naval Observatory and H.M. 
Nautical Almanac Office of Great Britain collaborate on this joint publication 
available in the United States from the Superintendent of Documents. Several 
other countries also publish air almanacs. Although designed for air 
navigators, the Air Almanac is used increasingly by mariners, who accept its 
reduced precision because of its greater convenience compared with the 
somewhat similar Nautical Almanac. See CELESTIAL NAVIGATION; NAUTICAL ALMANAC. 
    The Air Almanac is arranged with two ``daily pages'' (front and back) for 
each day giving ephemeridal information of the Sun, first point of Aries, 
three planets, and the Moon at 10-min intervals of Greenwich mean time (GMT) 
and other useful information. 
 �34s[Alton B Moody]
�33s 
  
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�6v�2v�32sAir armament �21m �33s
    A term which includes all the equipment through which a combat airplane
can release destructive power on a target. Guns, bombs, and guided aircraft
rockets (air-to-air missiles) are examples of the armament employed by the
U.S. Air Force. Airborne devices used to aim and direct guns, bombs, and
rockets are also included in this definition. Steady advances in armament and
aircraft have made it possible for fighter-bombers like the F-105 Thunderchief
to carry as much destructive power as the heavy bombers of World War II (see
illustration).
    Guns mounted in combat airplanes are designed in response to the classical
problems of weight and drag. Since aerodynamic drag seriously hinders the
operational efficiency of aircraft, aircraft armament must be not only as
light as possible but also streamlined in order to offer the least wind
resistance. Reliability at extreme altitudes and in frigid temperatures, as
well as during the acceleration loads of high-g maneuvering, is another
considerations. Also, aircraft guns must have a high rate of fire because of
the extremely short span of time during which targets may be attacked in
aerial combat. See AERODYNAMIC WAVE DRAG.
    Bombs, the symbol of air armament, represent the ultimate in destructive
power. Contrasted with guns and rockets, bombs constitute a more efficient
method of destroying a target since, for a given installed weight, they can
release more energy than their rivals. Chemical-based bombs are classified
explosive, general-demolition, armor-piercing, and fragmentation types.
    Devices for aiming guns, bombs, and rockets are called fire-control
mechanisms, and include gunsights, rocketsights, and bombsights. Because of
the high speed of a combat aircraft, the short time available for acquisition
of data and computation of aiming points, and the lack of precise information
about targets, the aerial fire-control problem is extremely difficult. Radar
is used, as are optical means, to determine exact location and range.
    Rockets and missiles used by combat aircraft employ warheads similar to
cannon shells, that is, armor-piercing, incendiary, and explosive. They are
particularly suited for air armament, where immediate aerodynamic stability is
achieved by launching them from a moving platform into a high relative wind.
They impart no recoil forces to the airplane structure and can carry a
relatively large explosive load, because the warhead is not subjected to the
extreme forces of acceleration encountered by gun-launched shells.




 �34s[Daniel P. Griffin; Jeffrey J. Clarke]
�33s



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�6v�2v�32sAirborne radar �21m �33s
    Radar equipment carried by aircraft to provide services such as navigation
by pilotage, determination of drift, mapping of weather fronts, and
determination of ground speed; reconnaissance-system, target-detection,
terrain-following, and bombing and gunlaying services provide assistance for
tactical missions. See RADAR.
    To use radar for pilotage, the presentation on the cathoderay tube (plan
position indicator, or PPI) is compared with a chart as in Fig. 1. This method
is especially effective when applied to well-mapped terrain that includes
bodies of water (bays, rivers, or harbors). See PILOTAGE.
    To determine drift, a transparent disk engraved with parallel lines is
mounted on the face of the PPI tube. The disk is then rotated against a scale
calibrated in degrees of drift (Fig. 2). The center of the disk coincides with
the center of the cathode-ray pattern. A trace on the face of the tube
indicates the position of the antenna when it is pointing directly along the
fore-and-aft axis of the aircraft. To measure drift, the operator selects a
prominent spot on the PPI and rotates the disk until the spot appears to move
along or parallel to the lines on the disk. The angle between the center line
on the disk and the trace representing the fore-and-aft axis of the aircraft
then represents the drift angle. By timing the movement of the spot, ground
speed may be estimated.
    Airborne radar has been used for determining drift through the Doppler
principle. Because of the aircraft movement, the frequency of the reflected
signal returning to the aircraft from the ground differs from the frequency of
transmission. This difference is proportional to the frequency of the original
transmission, twice the aircraft speed, and the cosine of the angle between
the direction of aircraft motion and a line to the reflecting point.
    One of the most valuable uses of airborne radar is for weather detection.
This capability makes for safer and smoother navigation of thunderstorm and
precipitation areas. By using radar, a pilot may avoid turbulence associated
with these areas. See RADAR METEOROLOGY.
    Reconnaissance systems, also called ground-mapping systems and
side-looking radar, provide data on both fixed and moving ground targets for
military reconnaisance purposes. These radars differ from other types in that
they use fixed antennas located on both sides of the aircraft fuselage. With
this configuration, an antenna system is provided which has an aperture (about
5 ft or 1.5 m) that is much wider than can be provided by any parabolic
antenna which would be practical to mount on an aircraft. The wider aperture
provides definition of the ground targets that could not otherwise be
distinguished. This performance, however, is possible only because of the
novel scanning system. Instead of rotating the antenna to scan, the radar beam
moves in space because the aircraft carrying the antenna moves.
    Terrain following is provided by airborne radar which enables attack
aircraft to remain hidden in terrain and thus avoid detection by enemy
defenses. Flight must be made below the peaks of the terrain, but there is a
limit to how low a level the pilot's own capability may be safely applied.
Airborne radar provides range, altitude, and other information that assists
the pilot in deciding the proper maneuver which will attain optimal flight.
    Airborne radar provides the service which is most essential to the
performance of the military aircraft's mission - the detection of targets to
be destroyed. These targets are of two general types - strategic and tactical.
Strategic targets are those which, when destroyed, inhibit the enemy's
capability to wage war. Tactical targets are the weapons and weapon systems
which the enemy employs against the opponent.
    With a quasi-visual radar PPI presentation of the target area below, a
computer is all that is needed to solve the bombing problem. This problem
involves the ballistics of the bomb, the range and azimuth offset, and the
range closing rate.






 �34s[Peter C. Sandretto]
�33s


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�6v�2v�32sAir brake   �21m �33s 
    An energy-conversion mechanism used to retard, stop, or hold a vehicle or, 
generally, any moving element, the activating force being applied by a 
difference in air pressure. With an air brake only slight effort by the 
operator quickly applies full braking force. 
    An air brake performs the energy conversion like other brakes by friction. 
The feature that distinguishes an air brake is that the friction-producing 
device, such as disk or drum, is applied by air in contrast to mechanical, 
hydraulic, or electrical means. In a particular use the choice between an air 
brake and any other type of brake depends in part on the availability of an 
air supply and on the method of brake control. 
    Because air is compressible, air brakes inherently accommodate to wear, 
such as in brake shoes. Also, because the force with which brakes are applied 
depends on the area of an air-driven diaphragm as well as on air pressure, 
large forces can be developed from moderate pressures. Air pressure can be 
accurately regulated and can also be controlled by other forces besides the 
operator's hand or foot; consequently air brakes can be made to respond to 
static and dynamic forces that influence the efficiency of brake operation. 
    In a typical air brake system an air compressor takes in and compresses 
air for use by the brakes and usually for other air-operated components of the 
vehicle. The compressed air is stored in an air brake reservoir until needed. 
By means of a foot- or hand-operated brake valve the operator controls 
application of the brake (see illustration). A brake rod from the cylinder 
diaphragm drives the friction element against the moving surface to provide 
the braking action. When the operator releases the brake valve, an auxiliary 
reservoir recharges to full pressure through the triple valve. Also, after the 
braking action is no longer required, a quick release valve assures rapid 
discharge of air from the brake cylinder. 
    Figure caption: Diagram of air brake. When operator actuates brake valve,
pressure to triple valve is reduced. Triple valve then admits compressed air
from auxiliary reservoir to brake cylinder, actuating brake rod.
 �34s[Frank H. Rockett]
�33s 
  
  

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�6v�2v�32sAir-capacitor dosimeter   �21m �33s 
    A type of dosimeter most often about the size and shape of a fountain pen 
and provided with a pen clip; hence, they are sometimes referred to as pen 
meters or pocket meters. The outer container is usually made of plastic, and 
the inside chamber wall and central collecting electrode are made of 
conducting plastic or of aluminum coated with graphite. This inside chamber 
serves as an air capacitor or ion chamber. The central electrode is insulated 
from the outer cylinder wall by polyethylene or some other suitable insulating 
material. An electric charge can be placed on the central electrode by 
removing an end cap and applying a voltage of 100 - 200 V between the 
electrode and the chamber wall. 
    Such dosimeters, when charged, are essentially air capacitors, and the 
amount of discharge during use is proportional to the absorbed dose of x- or 
GAMMA-radiation received. The usual dosimeter is relatively energy-independent 
for radiation from 300 keV to 2 MeV but often reads high by a factor of 2 to 3 
at about 100 keV. 
 �34s[Karl Z. Morgan]
�33s 
  
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�6v�2v�32sAir conditioning   �21m �33s 
    The maintenance of certain aspects of the environment within a defined 
space to facilitate the intended function of that space. Environmental 
conditions generally encompassed by the term air conditioning include air 
temperature and motion, radiant heat energy level, moisture level, and 
concentration of various pollutants, including dust, germs, and gases. Comfort 
air conditioning refers to control of spaces inhabited by people to promote 
their comfort, health, or productivity. Process air conditioning systems are 
designed to facilitate the functioning of a production, manufacturing, or 
operational activity. 
    A comfort air conditioning system is designed to help maintain human body 
temperature at its normal level without undue stress and to provide an 
atmosphere which is healthy to breathe. The heat-dissipating factors of 
temperature, humidity, and air motion must be considered simultaneously. 
Within limits, the same amount of comfort (or, more objectively, of 
heat-dissipating ability) is the result of a combination of these factors in a 
three-dimensional continuum. 
    In practice most air-conditioning systems for offices and institutions, 
schools, and other light-work spaces are designed to maintain a temperature at 
75 PLUS/MINUS DG F (24 PLUS/MINUS DG C) year-round with relative humidity 
maintained at 50 PLUS/MINUS % by dehumidification in summer. In winter relative 
humidities within the conditioned spaces are usually much lower, on the order 
of 20-30%, as a result of heating relatively dry outside air, and the 
impracticability of maintaining higher humidities which would cause excessive 
condensation on cold surfaces, such as window panes. Both conditions 
correspond closely to comfort requirements; hence, a single year-round 
thermostat setting is feasible. 
    Engineering of an air-conditioning system starts with selection of design 
conditions; air temperature and relative humidity are principal factors. Next, 
loads on the system are calculated. Finally, equipment is selected and sized 
to perform the indicated functions and to carry the estimated loads. Each 
space is analyzed separately. A cooling load will exist when the sum of heat 
released within the space and transmitted to the space is greater than the 
loss of heat from the space. A heating load occurs when the heat generated 
within the space is less than that lost from it. Similar considerations apply 
to moisture. 
    Heat generated within the space consists of body heat, heat from all 
electrical appliances and lights, and heat from other  sources such as gas 
cooking stoves and industrial ovens. Heat is transmitted through all parts of 
the space envelope, which includes walls, floor, ceiling, and windows. Whether 
heat enters or leaves the space depends upon whether the outside surfaces are 
warmer or cooler than the inside surfaces. The rate at which heat is conducted 
through the space envelope is a function of the temperature difference across 
the envelope and the thermal conductance of the envelope. Conductances, which 
depend on materials of construction and their thicknesses along the path of 
heat transmission, are a large factor in walls and ceilings exposed to the 
outdoors in cold winters and hot summers. In these cases insulation is added 
to decrease the overall conductance of the envelope. 
    Solar heat loads are an especially important part of load calculation 
because they represent a large percentage of heat gain through walls and 
roofs, but are very difficult to estimate because solar irradiation is 
constantly changing. 
    Humidity as a load on an air-conditioning system is treated by the 
engineer in terms of its latent heat, that is, the heat required to condense 
or evaporate the moisture, approximately 2.3 x 10 sup 6 J/kg (1000 Btu/lb) of 
moisture. As with heat, moisture travels through the space envelope, and its 
rate of transfer is calculated as a function of the difference in vapor 
pressure across the space envelope and the permeability of the envelope 
construction. To decrease permeability where vapor pressure differential is 
large, vapor barriers (relatively impermeable membranes) are incorporated in 
the envelope construction. 
    A complete air-conditioning system is capable of adding and removing heat 
and moisture and of filtering dust and odorants from the space or space it 
serves. Systems that heat, humidify, and filter only, for control of comfort 
in winter, are called winter air-conditioning systems; those that cool, 
dehumidify, and filter only are called summer air-conditioning systems, 
provided they are fitted with proper controls to maintain design levels of 
temperature, relative humidity, and air purity. 
    Air-conditioning systems are either unitary or built-up. The window or 
through-the-wall air conditioner is an example of a unitary summer 
air-conditioning system; the entire system is housed in a single package which 
contains heat removal, dehumidification, and filtration capabilities. When an 
electric heater is built into it with suitable controls, it functions as a 
year-round air-conditioning system. 
    Built up or field-erected systems are composed of factory-built 
subassemblies interconnected by means such as piping, wiring, and ducting 
during final assembly on the building site. Most large buildings are so 
conditioned.  
    There are three principal types of central air-conditioning systems: 
all-air, all-water, and air-water. In the all-air system all return air is 
processed in a central air-handling apparatus. In the all-water system the 
principal thermal load is carried by chilled and hot water generated in a 
central facility and piped to coils in each space; room air then passes over 
the coils. In an air-water system both treated air and hot or chilled water 
are supplied to units in each space. See AIR COOLING; AIR FILTER. 
 �34s[Richard L. Koral]
�33s 
  

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�6v�2v�32sAir cooling �21m �33s
    Lowering of air temperature for comfort, process control, or food
preservation. Air and water vapor occur together in the atmosphere. The
mixture is commonly cooled by direct convective heat transfer of its internal
energy (sensible heat) to a surface or medium at lower temperature. In the
most compact arrangement, transfer is through a finned (extended surface)
coil, metallic and thin, inside of which is circulating either chilled water,
antifreeze solution, brine, or boiling refrigerant. The fluid acts as the heat
receiver. Heat transfer can also be directly to a wetted surface, such as
water droplets in an air washer or a wet pad in an evaporative cooler. See AIR
CONDITIONING; HEAT TRANSFER.
    For evaporative cooling, nonsaturated air is mixed with water. Some of the
sensible heat transfers from the air to the evaporating water. The heat then
returns to the airstream as latent heat of water vapor. The technique is
employed for air cooling of machines where higher humidities can be tolerated;
for cooling of industrial areas where high humidities are required, as in
textile mills; and for comfort cooling in hot, dry climates, where partial
saturation results in cool air at relatively low humidity. See HUMIDITY.
    In the evaporative cooler the air is constantly changed and the water is
recirculated, except for that portion which has evaporated and which must be
made up. Water temperature remains at the adiabatic saturation (wet-bulb)
temperature. If water temperature is controlled, as by refrigeration, the
leaving air temperature can be controlled within wide limits. Entering warm,
moist air can be cooled below its dew point so that, although it leaves close
to saturation, it leaves with less moisture per unit volume of air than when
it entered. An apparatus to accomplish this is called an air washer. It is
used in many industrial and comfort air-conditioning systems, and performs the
added functions of cleansing the airstream of dust and of gases that dissolve
in water, and in winter, through the addition of heat to the water, of warming
and humidifying the air.
    The most important form of air cooling is by finned coils, inside of which
circulates a cold fluid or cold, boiling refrigerant (see illustration). The
latter is called a direct-expansion (DX) coil. In most applications the finned
surfaces become wet as condensation occurs simultaneously with sensible
cooling. Usually, the required amount of dehumidification determines the
temperature at which the surface is maintained and, where this results in air
that is colder than required, the air is reheated to the proper temperature.
Droplets of condensate are entrained in the airstream, removed by a suitable
filter (eliminator), collected in a drain pan, and wasted.
    Well water is available for air cooling in much of the world. Temperature
of water from wells 30 to 60 ft (10 to 20 m) deep is approximately the average
year-round air temperature in the locality of the well. For installations that
operate only occasionally, such as some churches and meeting halls, water
recirculated and cooled over ice offers an economical means for space cooling.
Where electric power is readily available, the cooling function of the ice is
performed by a mechanical refrigerator.

 �34s[Richard L. Koral]
�33s



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�6v�2v�32sAircraft   �21m �33s 
    Any vehicle which carries one or more persons and which navigates through 
the air. The two main classifications of aircraft are lighter-than-air and 
heavier-than-air. The term lighter-than-air is applied to all aircraft which 
sustain their weight by displacing an equal weight of air, for example, blimps 
and dirigibles. Heavier-than-air craft are supported by giving the surrounding 
air a momentum in the downward direction equal to the weight of the aircraft. 
See AIRPLANE; CONVERTIPLANE; HELICOPTER. 
 �34s[Richard G. Bowman]
�33s 
  
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�6v�2v�32sAircraft compass system   �21m �33s 
    Compass systems are used on aircraft for navigation without reference to 
ground-based signals. Magnetic compasses use the Earth's magnetic field as the 
reference; celestial systems use the stars and Sun as the reference; inertial 
systems use the changes of position and time as references. 
    The commonest system is the magnetic compass, which at present is found on 
all aircraft, even when more sophisticated systems are present. Dual magnets 
are mounted on a float, along with the compass card, to provide the pilot with 
the direction in which the aircraft is headed. See MAGNETIC COMPASS. 
    Celestial systems constantly track the stars or Sun by precise 
photodetector-optical systems, and by reference to time and the vertical 
provide a continuous indication of direction (heading) and position (latitude 
and longitude). 
    Inertial systems provide heading and position by the use of high-precision 
gyroscopes and accelerometers whose signal changes are integrated with respect 
to time. A lightweight aircraft computer that requires a minimum of service is 
a critical component of the system. See CELESTIAL NAVIGATION; INERTIAL 
GUIDANCE SYSTEM; RADIO COMPASS; STAR TRACKER. 
 �34s[James W. Angus]
�33s 
  
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�6v�2v�32sAircraft engine   �21m �33s 
    A component of an aircraft that develops either shaft horsepower or thrust 
and incorporates design features most advantageous for aircraft propulsion. An 
engine developing shaft horsepower requires an additional means to convert 
this power to useful thrust for aircraft, such as a propeller, a fan, or a 
helicopter rotor. It is common practice in this case to designate the unit 
developing shaft horsepower as the aircraft engine, and the combination of 
engine and propeller, for example, as an aircraft power plant. In case thrust 
is developed directly as in a turbojet engine, the terms engine and power 
plant are used interchangeably. 
    Air-breathing types of aircraft engines use oxygen from the atmosphere to 
combine chemically with fuel carried in the vehicle, providing the energy for 
propulsion, in contrast to rocket types in which both the fuel and oxidizer 
are carried in the aircraft. See INTERNAL COMBUSTION ENGINE; JET PROPULSION; 
PROPELLER (AIRCRAFT); RECIPROCATING AIRCRAFT ENGINE; ROCKET ENGINE; TURBINE 
PROPULSION.  
 �34s[Ronald Hazen]
�33s 
  
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�6v�2v�32sAircraft fuel   �21m �33s 
    The source of energy required for the propulsion of airborne vehicles. 
Aircraft fuel is burned with ambient air and is thereby distinct from rocket 
propellants, which carry both fuel and oxidant. An important criterion for 
aircraft fuel is that its energy density, or heat of combustion per unit of 
weight, be high. This allows reasonable expenditures of fuel during takeoff, 
efficient performance in flight, and long range of flight duration. 
    There are two general types of aircraft fuels in conventional use; 
gasolines for reciprocating (piston) engines, and kerosenelike fuels (called 
jet fuels) for turbine engines. Because of anticipated limitations in the 
supply of these crude-oil derived fuels, alternative fuels are being 
considered for future aircraft. 
    Piston engine fuels, or aviation gasolines, are special blends of gasoline 
stocks and additives that produce a high-performance fuel that is graded by 
its antiknock quality. The gasoline blending stocks are virgin (uncracked) 
naphtha, alkylate, and catalytically cracked gasoline. In general, the 
chemical composition of aviation gasoline can be approximated as 
Cx H 1.9x, where the number of carbon atoms x is between 4 and 
approximately 10. Tetraethyllead (Tel) is a common additive used in 
concentrations of up to 4 ml/gal (1.057 ml/liter) of fuel to increase the 
antiknock quality of the fuel. See GASOLINE; NAPHTHA; TETRAETHYLLEAD. 
    Turbine engine fuels are distillate hydrocarbon fuels, like kerosenes, 
used to operate turbojet, turbofan, and turboshaft engines. While all piston 
engine fuels have the same volatility but differ in combustion 
characteristics, jet fuels differ primarily in volatility; differences in 
their combustion qualities are minor. Turbine fuel contains aromatic 
hydrocarbons; limits are placed on this content owing to concerns about smoke 
and coke formation. An increasingly important requirement is to provide a fuel 
that is stable at relatively high temperatures. 
    Many forecasts of world crude production indicate a peak or plateau in 
the oil production rate before 2010; hence, the availability of crude-based 
fuels for aircraft is not certain. Alternative fuels made from coal, oil shale,
or solar or nuclear energy plus a suitable raw material are being considered. 
 �34s[Jon B. Pangborn]
�33s 
  

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�6v�2v�32sAircraft instrumentation   �21m �33s 
    A coordinated group of instruments that provide the flight crew with 
information about the aircraft and its subsystems. These instruments provide 
flight data, navigation, power plant performance, and aircraft auxiliary 
equipment operating information to the flight crew, air-traffic controllers, 
and maintenance personnel. While not considered as instrumentation, 
communication equipment is, however, directly concerned with the 
instrumentation and overall indirect control of the aircraft. 
    Situation information on the operating environment, such as weather 
reports and traffic advisories, has become a necessity for effective flight 
planning and decision making. The prolific growth and multiplicity of 
instruments in the modern cockpit and the growing need for knowledge about the 
aircraft's situation are leading to the introduction of computers and advanced 
electronic displays as a means for the pilot to better organize and assimilate 
this body of information. 
    Instrumentation complexity and accuracy are dictated by the aircraft's 
performance capabilities and the conditions under which it is intended to 
operate. Light aircraft may carry only a minimum set of instruments: an 
airspeed indicator, an altimeter, an engine tachometer and oil pressure gage, 
a fuel quantity indicator, and a magnetic compass. These instruments allow 
operation by a pilotage technique. See ALTIMETER; AIRSPEED INDICATOR; 
PILOTAGE; RATE-OF-CLIMB INDICATOR. 
    Operation under low visibility and under Instrument Flight Rules (IFR) 
requires this same information in a more precise form and also requires 
attitude and navigation data. An attitude-director indicator (ADI) presents an 
artificial horizon, bank angle, and turn coordination data for attitude 
control without external visual reference. The attitude-director indicator may 
contain a vertical gyro within the indicator, or a gyro may be remotely 
located as a part of a flight director or navigational system. Flying through 
a large speed range at a variety of altitudes is simplified if the indicated 
airspeed is corrected to true airspeed for navigation purposes and the Mach 
number (M) is also shown on the ADI for flight control and performance 
purposes. Rate-of-climb is provided by an instantaneous vertical-speed 
indicator (IVSI). Heading data are provided by a directional gyro or data 
derived from an inertial reference system. See GYROSCOPE; INERTIAL GUIDANCE 
SYSTEM.  
    Navigational aids include: very-high-frequency omnidirectional radio 
ranges (VOR) that transmit azimuth information for navigation at specified 
Earth locations; distance-measuring equipment (DME) that indicates the 
distance to radio aids on or near airports or to VORs; automatic direction 
finders (ADF) that give the bearing of other radio stations (generally 
low-frequency); low-range radio altimeters (LRRA) which by radar determine the 
height of the aircraft above the terrain at low altitudes; and instrument 
landing systems (ILS) that show vertical and lateral deviation from a 
radio-generated glide-path signal for landing at appropriately equipped 
runways. Some inertial navigation systems include special-purpose computers 
that provide precise Earth latitude and longitude, ground speed, course, and 
heading. See AIR-TRAFFIC CONTROL; AUTOPILOT; DIRECTION-FINDING EQUIPMENT; 
DISTANCE-MEASURING EQUIPMENT; ELECTRONIC NAVIGATION SYSTEMS; INSTRUMENT 
LANDING SYSTEM (ILS); RADIO RANGE. 
    Engines require specific instruments to indicate limits and efficiency of 
operation. For reciprocating engines, instruments may display intake and 
exhaust manifold pressures, cylinder head and oil temperature, oil pressure, 
and engine speed. For jet engines, instruments display engine pressure ratio 
(EPR), exhaust gas temperature (EGT), engine rotor speed, oil temperature and 
pressure, and fuel flow. Vibration monitors on both types of engines indicate 
unbalance and potential trouble. 
    Depending on the complexity of the aircraft and the facilities that are 
provided, there is also an assortment of instruments and controls for the 
auxiliary systems. 
    Electronic technology developments include: ring laser gyros, strap-down 
inertial reference systems, microprocessor digital computers, color 
cathode-ray tubes (CRT), liquid crystal displays (LCD), light-emitting diodes 
(LED), and digital data buses. Application of this technology allows a new era 
of system integration and situation information on the aircraft flight deck 
and instrument panels. Commercial jet transports developed during the 1980s 
will use digital electronics to improve safety, performance, economics, and 
passenger service. The concept of an integrated flight management system (FMS) 
includes automatic flight control, electronic flight instrument displays, 
communications, navigation, guidance, performance management, and crew 
alerting to satisfy the requirements of the current and future air-traffic and 
energy-intensive environment. 
    Effective flight management is closely tied to providing accurate and 
timely information to the pilot. The nature of the pilot's various tasks 
determines the general types of data which must be available. The key is to 
provide these data in a form best suited for use. If the pilot is not required 
to accomplish extensive mental processing before information can be used, then 
more information can be presented and less effort, fewer errors, and lower 
training requirements can be expected. Computer-generated displays offer 
significant advances in this direction. 
    The electronic horizontal-situation indicator (EHSI) provides an 
integrated multicolor map display of the airplane's position, plus a color 
weather radar (WXR) display. The scale for the radar and map can be selected 
by the pilots. 
 �34s[Bernard C. Hainline]
�33s 
  
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�6v�2v�32sAircraft low-approach systems   �21m �33s 
    Means for furnishing guidance in the vertical and horizontal planes to 
aircraft during descent from an initial approach altitude to a point near the 
ground. Completion of a landing requires guidance to touchdown by means other 
than approach systems. 
    Generically these systems are of two types - the fixed beam and the radar. 
The fixed-beam system generates a radio field that varies with the position in 
the space which surrounds the ground-located or shipborne equipment but not 
with time. The field is received on equipment in the aircraft. The radar 
system, commonly called the GCA (ground-controlled-approach) system, generates 
beams which vary with time and which are detected on the ground after being 
reflected from the using aircraft. 
    The fixed-beam systems have been called ILS (instrument landing systems). 
This appellation conveys an impression of performance which the system is 
incapable of providing safely. See ELECTRONIC NAVIGATION SYSTEMS; 
GROUND-CONTROLLED-APPROACH (GCA) SYSTEM; INSTRUMENT LANDING SYSTEM (ILS); 
PRECISION APPROACH RADAR (PAR). 
 �34s[Peter C. Sandretto]
�33s 
  
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�6v�2v�32sAircraft propulsion �21m �33s
    Flying machines obtain their propulsion by the rearward acceleration of
matter. This is an application of Newton's third law: For every action there
is an equal and opposite reaction.
    In propeller-driven aircraft, the propulsive medium is the ambient air
which is accelerated to the rear by the action of the propeller. The
acceleration of the air that passes through the engine provides only a
secondary contribution to the thrust.
    In the case of turbojet and ramjet engines, the ambient air is again the
propulsive medium, but the thrust is obtained by the acceleration of the air
as it passes through the engine. After being compressed and heated in the
engine, this air is ejected rearward from the engine at a greater velocity
than it had when it entered. See JET PROPULSION.
    Rockets carry their own propulsive medium. The propellants are burned at
high pressure in a combustion chamber and are ejected rearward to produce
thrust. See ROCKET ENGINE.
    In every case, the thrust provided is equal to the mass of propulsive
medium per second multiplied by the increase in its velocity produced by the
propulsive device. This is substantially Newton's second law. See FLUID FLOW.
    The airplane lift-drag ratio L/D is a primary factor that determines the
thrust required from the propulsion system to fly a  given airplane. To
sustain flight, the airplane lift must be equal to airplane gross weight, and
the engine thrust must be equal to the airplane drag. The higher the lift-drag
ratio, the more efficient is the airplane. A sharp reduction in L/D occurs
with increase in flight Mach number in the vicinity of a Mach number of unity,
and this is reflected in a sharp increase in the thrust required for flight.
Flight Mach number is the ratio of the airplane speed to the speed of sound in
the ambient atmosphere. At standard sea-level conditions, the speed of sound
is 773 mi/h (346 m/s).
    The competition among nations and among commercial airlines has created a
continuing demand for increased flight speed. The reduction in aircraft L/D
that accompanies an increase in speed (see illustration) requires an increase
in engine thrust for an airplane of a given gross weight. For a given engine
specific weight, an increase in required engine thrust results in an increase
in engine weight and hence a reduction in fuel load and payload that an
airplane of a given gross weight can carry. If the engine weight becomes so
large that no fuel can be carried by the airplane, the airplane has zero
flight range regardless of the efficiency of the engine. At some speed before
this point is reached, it becomes advantageous to shift to an engine type that
has a lower specific engine weight even at the cost of an increased specific
fuel consumption.
    At low subsonic flight speeds, the piston-type reciprocating engine,
because of its low specific fuel consumption, provides the best airplane
performance in terms of payload and flight range. As flight speed increases,
specific weight of reciprocating engines increases because of falling
propeller efficiency. This effect, coupled with reduction in L/D which
accompanies increase in flight speed, results in the weight of reciprocating
engines becoming excessive at a flight speed of about 400 mi/h (180 m/s). At
about this speed it is advantageous to shift to the lighter-weight turboprops
even if the efficiency of the latter is poorer. At about 550 mi/h (245 m/s) it
is advantageous to shift from the turboprop to the lighter but less efficient
turbojet. Intermediate between the turboprop and turbojet in the spectrum of
flight speeds is the turbofan. See RECIPROCATING AIRCRAFT ENGINE; TURBOFAN;
TURBOJET; TURBOPROP.



 �34s[Benjamin Pinkel]
�33s


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�6v�2v�32sAircraft rudder �21m �33s
    The hinged rear portion of an aircraft vertical stabilizing surface, used
to obtain directional or yaw control moments. The angular setting of the
rudder is controlled by the human or automatic pilot through the
flight-control system. A typical rudder control surface includes aerodynamic
balance and tab features, as shown in the illustration. The principles of
operation of rudder control surfaces and flutter prevention methods are
identical with those for the elevator and for aileron control surfaces.
All-moving vertical stabilizers replace rudders on some supersonic aircraft.

 �34s[Malcolm J. Abzug]
�33s



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�6v�2v�32sAircraft stability   �21m �33s 
    The tendency of forces and moments to be set up that restore an aircraft 
to its equilibrium position if it is disturbed from this position. Stability 
is a necessary condition for successful flight. For an airplane to be in 
equilibrium, the sum of the forces acting at its center of gravity must equal 
zero, as must the sum of the moments. 
    To examine the condition of stability, suppose that the equilibrium of the 
airplane is suddenly disturbed in a manner that causes it to accelerate. This 
means that its lift coefficient decreases. A stable airplane would immediately 
be subjected to a nose-up moment which would tend to increase the lift 
coefficient and reduce the speed. Similarly, a reduction in the speed of a 
stable airplane should produce a nose-down moment. 
    Stability is achieved by balancing the force and moment contributions of 
various portions of the airplane so that the summation is stable. Because the 
balance is achieved for moments about the center of gravity, the location of 
this point is important. Its position depends upon the load condition and can 
change in flight as fuel is consumed or as disposable load is dropped. The 
airplane must be designed so that it is stable throughout the entire range of 
center-of-gravity locations encountered in service. 
 �34s[David C. Hazen]
�33s 
  
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�6v�2v�32sAir-cushion vehicle �21m �33s
    A transportation device supported by low-pressure, low-velocity air. It is
also known as a ground-effect machine (GEM). Configurations, from which
practical applications are developing, grew out of aeronautical experience.
While an aircraft is operating close to the ground, as during takeoff or
landing, reaction between high-velocity downwash from its wing or rotor and
the ground serves to augment lift. By depending predominantly on this
reaction, and by designing the underside of the vehicle to retain the
delivered air, the vehicle need not be restricted to operation over a prepared
roadbed; it can travel equally well over water, marsh, or relatively level
land (see illustration).
    The air cushion is maintained by several techniques. Some or all of the
ground-effect support may be produced by a ram wing operated at high speed
near the ground. Support may be produced, as in a helicopter, by a fan that
delivers the needed volume of downdraft air against the ground-induced back
pressure. Air may be injected at the periphery of the machine in a downward
and inward direction. This annular jet creates an air-momentum curtain that
reduces escape of supporting air. An apron around the lower portion of the
vehicle can form a plenum chamber. For a vehicle that skims over the water,
the apron may be fins or thin keels that project into the water, effectively
capturing the air as a bubble below the vehicle. For an over-the-ground
vehicle the apron may be a flexible skirt of abrasion-resistant plastic. In
contrast to an aircraft, the air-cushion vehicle can hover for a protracted
period in one location. See ANTIFRICTION BEARING; DUCTED FAN; HELICOPTER.


 �34s[Frank H. Rockett]
�33s



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�6v�2v�32sAir-entraining portland cement   �21m �33s 
    Portland cement containing a substance that entrains many minute air 
bubbles in the concrete in which it is used. Air-entrained concrete can also 
be made by adding such substances at the concrete mixer. Before hardening, 
air-entrained concrete is more plastic and homogeneous than ordinary concrete. 
When set, it offers considerable resistance to freezing, thawing, and the 
deteriorating effects of deicing chemicals. See CONCRETE; PORTLAND CEMENT. 
 �34s[J. H. Walker]
�33s 
  
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�6v�2v�32sAir filter   �21m �33s 
    A component of most systems in which air is used for industrial processes, 
for ventilation, or for comfort air conditioning. The function of an air 
filter is to reduce the concentration of solid particles in the airstream to a 
level that can be tolerated by the process or space occupancy purpose. See 
AIR; AIR CONDITIONING; VENTILATION. 
    Solid particles in the airstream range in size from 0.01 micrometer to 
objects that can be caught by ordinary fly screens, such as lint, feathers, 
and insects. The particles generally include soot, ash, soil, lint, and smoke, 
but may include almost any organic or inorganic material, even bacteria and 
mold spores. This wide variety of airborne contaminants, added to the 
diversity of systems in which air filters are used, makes it impossible to 
have one type that is best for all applications. 
    Three basic types of air filters are in common use: viscous impingement, 
dry, and electronic. The principles employed by these filters in removing 
airborne solids are viscous impingement, interception, impaction, diffusion, 
and electrostatic precipitation. Some filters utilize only one of these 
principles; others employ combinations. A fourth method, inertial separation, 
is finding increasing use as a result of the construction boom throughout most 
of the Middle East. See DRY-AIR FILTER; ELECTRONIC AIR CLEANER; INERTIAL 
SEPARATOR; VISCOUS-IMPINGEMENT AIR FILTER. 
 �34s[Morton A. Bell]
�33s 
  
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�6v�2v�32sAirfoil �21m �33s
    A body of such shape that the force exerted on it by its motion through a
fluid has a larger component normal to the direction of the motion than along
the direction of motion. Examples are the wing of an airplane; a tail surface
of an airplane; a blade of a propeller, windmill, or helicopter rotor; and a
blade of an axial-flow compressor or turbine. The term is sometimes used for
a certain cross section of an airfoil. The closed curve defining the external
shape of the cross section of an airfoil is called an airfoil profile, airfoil
section, airfoil shape, or wing section. Knowledge of the geometry of airfoil
profiles is necessary in designing airfoil structures. Both analytical and
experimental studies are used in predicting the flow of fluid past an airfoil
and therefore in determining the fluid dynamic characteristics of the airfoil.
This discussion deals largely with airplane wings, although most of it is
applicable to the other examples.
    The root chord of a wing cr (see illustration) is the chord of the cross
section at the center of the wing, or at the vertical plane of symmetry of the
airplane. The tip chord ct is the chord of the cross section at the tip of the
wing. If the tip is rounded, the rounding should be ignored in determining the
tip section. For one of the wing halves, the semispan s is the distance between
the root chord and tip chord. For the whole wing the span b is twice the
semispan. The leading edge of a wing is the locus of the leading edge points
of the cross sections, and the trailing edge of a wing is the locus of the
trailing edge points of the cross sections.
    Also important in describing the planform shape of a wing are the
sweepback angles. The angle LAMBDA k is the angle between a line in the
basic plane perpendicular to the root chord and a line through the points on
the chords of the cross sections, which are kc back from the leading edge.
The three most used sweepback angles are LAMBDA 0, the sweepback of the
leading edge; LAMBDA 1/4, the sweepback of the quarterchord line; and
LAMBDA 1, the sweepback of the trailing edge. See SUPERCRITICAL WING;
SUPERSONIC FLIGHT; WING.
    In general, the chord of a cross section is a function of spanwise
location. The geometric mean chord of a wing OVERBAR c is equal to S/b where
S is the area of the wing. The aspect ratio of a wing is defined by the
equation below. From the formula OVERBAR c = S/b, it is seen that aspect

			*** A = b (sup 2)/S ***

ratio can also be given by A = b/OVERBAR c. The aspect ratio is very important
in airplane performance, as it affects most of the performance characteristics.
    As in the two-dimensional case, the component of the fluid dynamic force
normal to the direction of motion is called lift, and the component along the
direction of motion is called drag. It is desirable to design the airfoil so
that the lift is as large as possible relative to the drag, that is, for a
high lift-drag ratio.
    Because the wing is terminated at the wing tips, the flow, especially near
the tips, is modified from the two-dimensional pattern. For a wing creating
lift the pressure on the lower surface is greater than that on the upper
surface, and therefore the air flows up around the tips from the lower side
to the upper. Since meanwhile the wing has moved forward, this air continues
to move, forming a vortex. This is known as the induced flow and represents
the chief difference between the two- and three-dimensional cases.
See SUBSONIC FLIGHT.
    Of considerable use are the concepts of mean aerodynamic center and the
aerodynamic mean chord of a wing. Consider a wing of general nature, swept,
twisted, and tapered. There exists an untwisted rectangular wing with the same
area which will have the same lift and moment as the original wing, provided
that its fore-and-aft location is properly chosen and that its chord is the
right length. The chord length and the fore-and-aft position of the rectangular
wing define respectively the aerodynamic mean chord and the mean aerodynamic
center of the original wing. The value of the concept is that the analysis is
simpler for an untwisted rectangular wing than for a swept, twisted, and
tapered wing. See IDEAL AERODYNAMICS.

 �34s[Frank McL. Mallett]
�33s



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�6v�2v�32sAirframe   �21m �33s 
    The structure that supports the loads acting upon the airplane. The loads 
consist of aerodynamic forces exerted by the air flowing around the airplane; 
hydrodynamic forces, in the case of hydroplanes and flying boats, produced by 
the water in which the floats or the hull are partially submerged; the weight 
of the airplane and of the passengers, crew, and stores it is carrying; the 
dynamic loads caused by accelerations; and the ground loads which appear when 
the plane is resting or rolling on the ground. The structure must be strong 
enough to withstand these loads without failure and stiff enough to preclude 
unduly large deformations; when such deformations appear, they can interfere 
with the proper functioning of the airplane and can lead to oscillations 
called flutter which are capable of destroying the structure. See 
AEROELASTICITY; FLUTTER (AERONAUTICS). 
    Until the end of World War I, all successful planes were built as 
externally braced monoplanes or biplanes. The externally braced fighters of 
World War I usually had two spars that supported wooden truss ribs in the 
shape of the wing section which were covered with canvas. This structure is 
light in weight, but produces much drag because the wires are exposed to the 
airflow and the canvas covering cannot maintain the exact aerodynamic shape. 
In addition, in the biplane the interaction between the flows around the two 
wings is the source of more drag (induced drag). 
    The cantilever wing was first used successfully by Hugo Junkers and 
Anthony Fokker in Germany during World War I. Junkers had found in his wind 
tunnel that the minimum drag of a thick wing was not much larger than that of 
the thin biplane wings in general use at that time. Unfortunately this 
conclusion was correct only at the low speeds of the Aachen tunnel; it was 
incorrect at the much higher speeds (or Reynolds numbers) of actual flight. 
Hence in later years designers came to prefer wing sections whose maximum 
thickness was about 12% of the chord length (which is the distance from the 
leading edge of the wing to the trailing edge) rather than those with the 20 
to 25% proposed by Junkers or the 3 or 4% common in biplanes of World War I. 
See REYNOLDS NUMBER; WIND TUNNEL. 
    The predecessor of the modern transport airplane, the six-passenger 
Lockheed Vega, designed in 1927, had a plywood-covered high wing and a wood 
monocoque fuselage (monos means single, and coque means shell). The fuselage 
of the relatively thick-walled wood Vega was so stable that it needed few 
internal stiffeners. But the much thinner walls (with thicknesses as low as 
0.016 in. or 0.4 mm) of the aluminum alloy fuselages of the planes that 
followed had to be stiffened to keep them from buckling. The early solution of 
this design problem, mainly the Reissner-Junkers method of stiffening the skin 
by corrugating it, was no longer acceptable when the flying speed of the 
planes increased above 150 mi/h (241 km/h), as the corrugations increased the 
drag inadmissibly. For this reason many ingenious arrangements of the 
longitudinal and circumferential stiffeners (stringers and rings) were devised 
in the late 1920s and early 1930s. 
    The fuselage of the Vultee VI-A is stiffened only with a set of rings; 
this arrangement is not favored today because it requires a relatively thick 
(and heavy) wall to ensure structural stability. However, the fuselage 
structure of the Lockheed Electra, incorporating both rings and stiffeners, is 
still in use. See FUSELAGE. 
    The wing of the Vultee has two spars built of wood, as was usual in the 
earlier wings. The Lockheed wing has one main shear web or spar and an 
auxiliary shear web or spar. On the upper surface of the wing, Lockheed 
stiffened the skin in the neighborhood of the main shear web by riveting to it 
on the inside a corrugated sheet to prevent the skin from buckling. 
    The aluminum-alloy-reinforced monocoque construction described here has 
been in use since the early 1930s essentially unchanged. But shortly before 
World War II, the De Havilland company proved that efficient monocoque 
structures can also be built with a sandwich skin. The sandwich consists of 
two thin but strong faces kept apart by a thick but weak core. Since the core 
material is light, the sandwich is light in weight; and since the total 
sandwich thickness is substantial, the wall is stiff and is not likely to 
buckle.  
    The most recent development in airplane structures is the use of composite 
materials. A composite consists of fibers of very small diameter and very high 
strength embedded in a matrix of lower strength. Among artificial fibers, thin 
glass fibers of 10 sup 4-in. (0.0025-mm) diameter have a failing stress of 
about 500,000 pounds per square inch (3500 megapascals). When such fibers are 
spun into a thread, woven into a fabric, and combined with epoxy resin, the 
average failing stress is reduced to about 55,000 psi (380 MPa), which is the 
failing stress of the older aluminumalloys. However, the glass fiber - epoxy 
fabric is lighter than the aluminum alloy, as its specific density is 2.0 
(against 2.8 for the aluminum alloy). 
    The great advantage of glass fiber textile is its suitability for the 
construction of surfaces of double curvature. Thus glass fiber can be used in 
the manufacture of beautifully streamlined fuselages and wings. Yet the 
construction has a serious drawback: the material has a low stiffness as 
expressed by its elastic modulus. Because of the low modulus value, glass 
fiber composite wings are very flexible and prone to failure through 
aeroelastic instability. 
    Two kinds of fibers of very high strength and rigidity are boron and 
carbon (or graphite). The main drawback of these materials is their high 
price. When first produced industrially, boron-epoxy cost as much as gold of 
the same weight. At first, graphite-epoxy was even more expensive, but its 
price dropped more rapidly than that of boron-epoxy. Besides its lower price, 
graphite has the advantage over boron that its minimum fiber diameter, 0.0002 
in. (0.005 mm), makes it more flexible than boron, whose minimum fiber 
diameter is 0.004 in. (0.1 mm). For these reasons, graphite-epoxy composites 
are used increasingly in aircraft construction, where high strength and low 
weight are of extreme importance. See COMPOSITE MATERIAL. 
 �34s[Nicholas J. Hoff]
�33s 
  

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�6v�2v�32sAirglow   �21m �33s 
    Weak, nonthermal emissions from the atmosphere above 60 km, other than 
aurora. Subdivisions are dayglow, twilight glow, and nightglow. The energy of 
the nightglow comes from reactions of atomic oxygen between 70 and 100 km, and 
from ionic recombination around 300 km. Many of the emissions are the direct 
result of sunlight, scattered by the appropriate species; others are produced 
by electrons ejected by solar photons, or by enhanced nightglow processes. The 
twilight period, besides showing the transition between day and night, allows 
observation of some dayglow emissions from the ground, with much less 
interference from the overwhelming foreground light of the day sky. See ALKALI 
EMISSIONS.  
 �34s[Donald M. Hunten]
�33s 
  
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�6v�2v�32sAir heater   �21m �33s 
    A component of a steam-generating unit that absorbs heat from the products 
of combustion after they have passed through the steam-generating and 
superheating sections. Heat recovered from the gas is recycled to the furnace 
by the combustion air and is absorbed in the steam-generating unit, with a 
resultant gain in overall thermal efficiency. Use of preheated combustion air 
also accelerates ignition and promotes rapid burning of the fuel. Air heaters 
often are used in conjunction with economizers, because the temperature of the 
inlet air is less than that of the feedwater to the economizer, and in this 
way it is possible to reduce further the temperature of flue gas before it is 
discharged to the stack. See BOILER ECONOMIZER. 
 �34s[George W. Kessler]
�33s 
  
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�6v�2v�32sAir mass   �21m �33s 
    A term applied in meteorology to an extensive body of the atmosphere which 
approximates horizontal homogeneity in its weather characteristics. An air 
mass may be followed on the weather map as an entity in its day-to-day 
movement in the general circulation of the atmosphere. The expressions air 
mass analysis and frontal analysis are applied to the analysis of weather maps 
in terms of the prevailing air masses and of the zones of transition and 
interaction (fronts) which separate them. 
    The relative horizontal homogeneity of an air mass stands in contrast to 
the sharp horizontal changes in a frontal zone. The horizontal extent of 
important air masses is reckoned in millions of square miles. In the vertical 
dimension an air mass extends at most to the top of the troposphere, and 
frequently is restricted to the lower half or less of the troposphere. The 
frontal zones between air masses usually slope in such a manner that the 
colder air mass underlies the warmer as a wedge. In the vertical direction the 
properties of an air mass, specifically its content of heat and moisture, may 
vary between a high degree of stratification and one of homogeneity produced 
by vertical mixing. See FRONT; METEOROLOGY; WEATHER MAP. 
    The thermodynamic properties of air mass determine not only the general 
character of the weather in the extensive area covered by the air mass but 
also, to some extent, the severity of the weather activity in the frontal zone 
of interaction between air masses. Those properties which determine the 
primary weather characteristics of an air mass are defined by the vertical 
distribution of the two elements, water vapor and heat (temperature). The 
vertical distribution of water vapor determines the presence or absence of 
condensation forms and, if present, the elevation and thickness of fog or 
cloud layers. The vertical distribution of temperature determines the relative 
warmth or coldness of the air mass and, more importantly, the vertical 
gradient of temperatures, known as the lapse rate. The lapse rate determines 
the stability or instability of the air mass for thermal convection and, 
consequently, the stratiform or convective cellular structure of the cloud 
forms and precipitation. 
 �34s[Harold C. Weber]
�33s 
  
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�6v�2v�32sAir navigation   �21m �33s 
    The process of directing and monitoring the progress of an aircraft 
between selected geographic points or with respect to some predetermined plan 
from the moment that it leaves the loading dock to the time that it takes off, 
attains operating altitude, and reaches its point of disembarkation in safety. 
Air navigation thus introduces the need for guidance, control, and navigation 
in the third dimension. The underlying methodology employed is dead reckoning, 
which in its simplest form is the process of following a planned compass 
direction at an assumed speed for a specified interval of time. Its 
fundamental tools are the compass, airspeed meter, altimeter, vertical speed 
indicator, outside air temperature sensor, clock, chart, and some reference 
independent of the aircraft, as, for example, a radio navigation aid, by which 
to verify position. See DEAD RECKONING. 
    All serious air navigation begins with a carefully constructed flight 
plan, in which the pilot identifies the desired ground track or course, the 
distance to be flown, the values of magnetic variation and drift due to wind 
that are assumed to affect the progress of the aircraft, and the compass 
heading to be steered to make good the desired course. The flight plan 
includes selection of a flying altitude which is consistent with the 
performance capability of the aircraft, consideration of the forecast 
meteorological conditions, and the evaluation of the known or expected hazards 
to flight, that is, high ground, restricted areas, and obstructions. 
    The principal air work required of the pilot-navigator is to periodically 
confirm aircraft position and rate of progress with respect to flight plan and 
clearance. Each operating region within domestic and international airspace 
has published performance criteria to be met with respect to allowable 
deviation from assigned track, altitude, and speed, and these are rigidly 
enforced for the protection of all users of the airspace. Of particular 
interest to the flight crew is the validity of estimated time of arrival (ETA) 
and fuel required to reach destination. The task of confirming progress 
involves use of dead-reckoning (DR) equipment and procedures and an externally 
referenced position determination aid. 
    The dead-reckoning function on long-distance overwater flights may be 
performed either manually or semi-automatically, depending upon installed 
equipment. Regardless of equipment employed, the dead-reckoning process is 
periodically subjected to validation through measurement of position by one or 
more of the external aids to navigation. These aids can include the pilotage 
fix, the celestial fix, position derived from one of a variety of radio aids, 
or position determined from use of a hyperbolic navigation aid. Navigation 
satellites are becoming an important source of position data as well. See 
CELESTIAL NAVIGATION; ELECTRONIC NAVIGATION SYSTEMS; PILOTAGE; SATELLITE 
NAVIGATION SYSTEMS. 
    High ground, artificial obstacles to flight, other aircraft in the sky, 
turbulence, and icing are avoided through preflight planning, special rules of 
the road, control of traffic imposed by specially trained ground-based 
personnel, and detection of potentially dangerous cloud forms by use of radar 
and special navigation procedures. See AERONAUTICAL METEOROLOGY; AIR-TRAFFIC 
CONTROL.  
    Operations on the airport surface are under the supervision of ground 
control. Departure from the ramp and during taxi operations to the duty runway 
is monitored by this agency. Any assistance in identifying turning points, 
taxiways, and run-up areas that can be obtained from ground control though the 
pilot's standard airport charts should be sufficient to assist in orientation. 
Takeoff and the initial stages of climbout are supervised by departure 
control, a radar-equipped facility, which assists the pilot from start of 
takeoff roll until the aircraft has joined the en route system. At small 
airports, or during visual flight rules (VFR) operations, the pilot will 
generally work with the tower operator in lieu of departure control. The 
approach and landing phases of flight are the reciprocals of the departure. 
The departure control facility provides the link between the en route 
air-traffic control and the airfield until the aircraft is on the ground. At 
that point the pilot is directed to work with airport ground control as at the 
start of the flight. 
 �34s[William L. Polhemus]
�33s 
  
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�6v�2v�32sAirplane   �21m �33s 
    A heavier-than-air vehicle designed to use the pressures created by its 
motion through the air to lift and transport useful loads. To achieve 
practical, controllable flight, an airplane must consist of a source of thrust 
for propulsion, a geometric arrangement to produce lift, and a control system 
capable of maneuvering the vehicle within prescribed limits. Further, to be 
satisfactory, the vehicle should display stable characteristics, so that if it 
is disturbed from an equilibrium condition, forces and moments are created 
which return it to its original condition without necessitating corrective 
action on the part of the pilot. Efficient design will minimize the 
aerodynamic drag, thereby reducing the propulsive thrust required for a given 
flight condition, and will maximize the lifting capability per pound of 
airframe and engine weight, thereby increasing the useful, or transportable, 
load. See AERODYNAMIC RESISTANCE; AIRCRAFT PROPULSION; AIRCRAFT STABILITY; 
AIRFRAME; FLIGHT CONTROLS. 
 �34s[David C. Hazen]
�33s 
  
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�6v�2v�32sAir pocket   �21m �33s 
    A concept developed by the early aviators to describe an abrupt loss of 
altitude. When an airplane encounters a downdraft, the effect is a sudden 
reduction in the angle of attack of the wings relative to the air. Because the 
airplane momentarily has less lift than its weight, it drops toward the earth. 
The acceleration downward allows it to gather a fall velocity that changes the 
angle of attack back to normal when fall velocity of the airplane equals 
velocity of the downdraft. With small airplanes the sensation is much what one 
would imagine to be produced by flying across a pocket devoid of air. 
 �34s[Victor L. Streeter]
�33s 
  
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�6v�2v�32sAir pollution   �21m �33s 
    Alteration of the atmosphere by the introduction of natural and artificial 
particulate contaminants. Most artificial impurities are injected into the 
atmosphere at or near the Earth's surface. The atmosphere cleanses itself of 
these quickly, for the most part. This occurs because in the troposphere 
temperature decreases rapidly with increasing altitude, resulting in rapid 
vertical mixing; the rainfall sometimes associated with these conditions also 
assists in removing the impurities. See ATMOSPHERE; TROPOSPHERE. 
    In the stratosphere, either temperature is constant or it increases with 
altitude, a condition that characterizes the entire stratosphere as a 
permanent inversion layer. As a result, vertical mixing in the stratosphere 
(and hence, self-cleansing) occurs much more slowly than that in the 
troposphere. Contaminants introduced at a particular altitude remain near that 
altitude for periods as long as several years. It is a matter of concern that 
while the turbulent troposphere cleanses itself quickly, the relatively 
stagnant stratosphere does not. Nonetheless, the pollutants injected into the 
troposphere and stratosphere have impact on humans and the habitable 
environment. See STRATOSPHERE; TROPOPAUSE. 
    All airborne particulate matter, liquid and solid, and contaminant gases 
exist in the atmosphere in variable amounts. Typical natural contaminants are 
salt particles from the oceans or dust and gases from active volcanoes; 
typical artificial contaminants are waste smokes and gases formed by 
industrial, municipal, household, and automotive processes, and aircraft and 
rocket combustion processes. Another postulated important source of artificial 
contaminants is certain fluorocarbon compounds (gases) used widely as 
refrigerants, as propellants for aerosol products, and for other applications. 
Pollens, spores, rusts, and smuts are natural aerosols augmented artificially 
by humans' land-use practices. See SMOG; SMOKE. 
    Dispersion of pollution is dependent on atmospheric conditions. Winds 
transport and diffuse contaminants; rain may wash them to the surface; and 
under cloudless skies, solar radiation may induce important photochemical 
reactions.  
    Wind direction, speed, and turbulence influence atmospheric pollution. 
Wind direction determines the area into which the pollution is carried. 
Dilution of contaminants from a source is directly proportional, other factors 
being constant, to wind speed, which also determines the intensity of 
mechanical turbulence produced as the wind flows over and around surface 
objects, such as trees and buildings. Other factors are eddy diffusion by wind 
turbulence, the primary mixing agency in the troposphere, and thermal 
turbulence, which occurs in an unstable layer of air. Precipitation, fog, and 
solar radiation exert secondary meteorological influences. Natural ventilation 
in the atmosphere is best when the winds are strong and turbulent so that 
mixing is good, and when the volume in which mixing occurs is large so that 
dilution of pollution is rapid. As cities have grown in size, air pollution 
has become more widespread. It has become necessary to think of whole urban 
complexes as large area sources of pollution. The rate of natural ventilation 
of an urban area is dependent on two quantities: the wind speed and the mixing 
volume over the city. Active mixing upward is often limited by a stable layer, 
perhaps even a very stable inversion layer, aloft. The upward extent of this 
region of active mixing, known as the mixing height, determines the magnitude 
of the mixing volume of the city. 
    Pollution is removed from the atmosphere in such ways as washout, 
rain-out, gravitational settling, and turbulent impaction. Washout is the 
process by which contaminants are washed out of the atmosphere by raindrops as 
they fall through the contaminants; in rain-out the contaminants unite with 
cloud droplets, which may later grow into precipitation. 
    Gravitational settling is significant mainly for large particles, those 
having a diameter greater than 20 micrometers. Agglomeration of finer 
particles may result in larger ones which settle out by gravitation. Fine 
particles may also impact on surfaces by centrifugal action in very small 
turbulent eddies. Gases may be converted to particulates, as by photochemical 
action of sunlight, which may then be removed by settling or impaction. 
    The rate of natural cleansing may be slower than the rate of injection of 
pollutants into the atmosphere, in which case pollution may increase on a 
global scale. There is evidence that the concentration of atmospheric carbon 
dioxide has been increasing slowly since the beginning of the century because 
of combustion of fossil fuels. The tropospheric burden of very small particles 
and of Freon gas may also be increasing. 
    Pollution of the stratosphere with nitrogen oxide causes reduction of 
stratospheric ozone. Ozone reduction in the stratosphere has been linked to 
biological effects such as skin cancer in two steps: (1) reduced ozone in the 
stratosphere causes an increase in uv radiation reaching the Earth's surface, 
and (2) increased uv radiation enhances the normal biological effects of 
natural uv radiation. 
    Pollution of the stratosphere also may involve a climate chain of 
cause-and-effect relations by which aircraft engine effluents, notably sulfur 
dioxide (SO2), and to a lesser degree water vapor (H2O) and nitrogen oxides 
(NOx), affect climatic variables such as temperature, wind, and rainfall. 
    If enough particles larger than 0.1 MU m in diameter were added to the 
stratosphere, they could alter the radiative heat transfer of the Earth-Sun 
system, and thereby influence climate. Particles of this size are produced by 
several constituents of engine emissions, in particular those of SO2. 
    Among the many effects of tropospheric pollution are damage to human 
health, animal health, and vegetation, as well as corrosion of materials. 
Tropospheric pollution may also affect weather in a number of ways. Heavy 
precipitation at Laporte, Indiana, is attributed to a substantial source of 
air pollution there, and similar but less pronounced effects have been 
observed elsewhere. Industrial smoke reduces visibility and also ultraviolet 
radiation from the Sun, and polluted fogs are more dense and more persistent 
than natural fogs occurring under similar conditions. See ACID RAIN. 
    Four main methods of air-pollution control are prevention of pollution by 
improved equipment design and smokeless fuels; collection of contaminants at 
the source; containment of pollutants whose noxious characteristics may 
decrease with time, such as radioactive contaminants from nuclear power 
plants, or containment with destruction or conversion of the offending 
substances; and dispersion in industrial processes which can be varied to take 
advantage of the periods when dispersion conditions are so good that 
contaminants may be distributed very widely in such small concentrations that 
they inconvenience no one. See AIR-POLLUTION CONTROL. 
 �34s[A. J. Grobecker; S. C. Coronit; E. Wendell Hewson]
�33s 
  

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�6v�2v�32sAir-pollution control   �21m �33s 
    Air-pollution control suggests in its simplest form a background knowledge 
concerning desirable criteria for clear air, the ability to relate air quality 
to levels of emissions, the development of emission limits or other control 
standards, the means to measure emissions and air quality, and the 
availability of practical techniques to reduce air-pollutant emissions. In 
actuality there are many complex relationships among the parameters listed 
above, and the air-pollution professional has become more concerned with 
scientific management of the air resource than with simple preventive measures. 
    While increasing attention has been directed to process alterations to 
reduce air-pollutant emissions, great reliance is still placed upon physical 
removal processes. Because of the significant contribution of vehicular 
emissions to carbon monoxide, nitrogen oxides, and hydrocarbons in the 
atmosphere, it is convenient to consider stationary sources apart from 
vehicular or mobile sources. 
    Air-cleaning devices for particulates from stationary sources are 
essentially based upon some form of aerodynamic capture such as inertial 
impaction, direct interception, and diffusion. Electrical and thermal forces 
are utilized in some equipment. 
    Other than settling chambers for large particles, cyclone-type collectors 
are the most common of the inertial particle collector class. Cyclones are 
often used as primary collectors because of their relatively low efficiency 
(50 - 90% is usual). Some small-diameter high-efficiency cyclones are utilized. 
    Fabric filters are in very extensive use and involve both direct 
interception and diffusional processes so that very high efficiencies can be 
obtained for small as well as large particles. Some efforts have been made to 
enhance collection efficiencies through electrically charging the entering 
particles. Fabric filters operate with collection efficiencies up to 99.9+%. 
See AIR FILTER. 
    Wet scrubbers for particulates operate by contacting the particles in the 
gas stream with a liquid. In principle the particles are incorporated in a 
liquid bath or in liquid particles which are much larger and therefore more 
easily collected. The foam scrubber is a modification of a wet scrubber in 
which the particle-laden gas is passed through a foam generator where the gas 
and particles are enclosed by small bubbles of foam. 
    Electrostatic precipitation is the third major technique for particulate 
collection. In general, particles in a gas stream are charged by a 
high-voltage discharge electrode and collected at collection plates of 
opposite polarity. Electrical precipitators are capable of high efficiencies 
of 99+% under optimum conditions, but performance is still difficult to 
predict in new situations. See DUST AND MIST COLLECTION; GAS ABSORPTION 
OPERATIONS.  
    Incineration of refuse introduces problems of air pollution that are quite 
different from those of fuel-fired combustion. The refuse material is not 
homogeneous, and has a wide variation in fuel value ranging from 600 to 6500 
Btu/lb (1.4 to 15.1 MJ/kg) of refuse as fired. Increasingly stringent 
air-pollution control regulations have made it difficult to operate refuse 
incinerators without emission-control equipment. Because so much attention is 
being directed toward the use of refuse-derived fuel for steam and power 
generation, the search for pollution controls in this areas has become 
especially important. 
    Most controls on mobile sources are limited to motor vehicles. Both 
evaporative emissions from fuel systems and crankcases and exhaust 
contaminants are regulated by the U.S. Environmental Protection Agency. Carbon 
monoxide and hydrocarbons are controlled by oxidation catalysts and by special 
fuel-injection and combustion-chamber designs. Nitrogen oxides are controlled 
by spark timing, by exhaust gas recirculation, and by reduction catalysts in 
the exhaust. See CATALYTIC CONVERTER. 
    The general approach to air-pollution control in the United States is 
directed toward managing the air resource. The first step is to develop 
air-quality standards which represent desirable or at least acceptable levels 
of air quality. Following implementation, plans are developed which include 
conventional regulatory approaches, such as emission standards and limitation 
of impurities or contaminating substances in fuels or raw materials. Permit 
processes are also used to determine whether, in critical locations, sources 
can be established at all without supplemental means of control. For very 
large industries of national importance, the Federal government has adopted a 
number of emission standards which are applied nationwide. See AIR POLLUTION. 
 �34s[Robert J. Bryan]
�33s 
  

A700000010447�254s�5v�1v47
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�6v�2v�32sAirport engineering   �21m �33s 
    The planning, design, construction, and operation and maintenance of 
facilities providing for the landing and takeoff, loading and unloading, 
servicing, maintenance, and storage of aircraft. Inasmuch as an airport may be 
considered a self-contained community, the subject of airport engineering 
requires the technical knowledge and diversified skills of many branches of 
the engineering profession. See AIRCRAFT. 
    To determine the location of an airport, whether or not it should be small 
or large, what facilities are required, and the stages of airport development 
involve extensive study and planning. The culmination of the master plan study 
is expressed in a report and an airport layout plan, which shows in graphic 
form the existing and planned facilities, airport land uses, and anticipated 
impact of the airport on the surrounding environment. 
    The foremost consideration in the design of an airport is the selection of 
a suitable construction site. Where possible, several potential sites are 
considered. Adequate size is required to allow for ultimate development 
requirements, including land acquisition for protection of runway approaches 
and for buffer zones. 
    Following site selection, a master plan layout is prepared, showing the 
relative location of runways, taxiways, aprons, control tower, hangars, and 
revenue-producing facilities. The ultimate development is often subdivided 
into suitable stages of construction, such that the completion of each stage 
will provide a complete and usable facility adequate to meet the growing needs 
of the area to be served. 
    Runway lengths are based on the planned category of the airfield and the 
types of aircraft anticipated. Taxiways usually parallel each runway. They 
connect at each runway end and at high-speed turnouts at two or more 
intermediate points. These permit aircraft to exit from the runway as soon as 
possible after touchdown, thus minimizing headway required between landing 
aircraft. See RUNWAY. 
    The grading and drainage of an airfield area is important and expensive; 
however, careful design can achieve major economies. The slope of the runway 
center line is restricted, and transverse slopes off runways and taxiways must 
be relatively flat to minimize damage in the event an aircraft performs 
erratically. Drainage ditches and headwalls are not constructed within the 
landing strip limits nor immediately off runway ends. The grades in turfed 
areas slope away from paved surfaces toward drainage structures which conduct 
the water into underground pipes and discharge it clear of the landing area. 
See SURFACE WATER. 
    Airport pavements are designed to support aircraft wheel loads and to 
provide an even, nonskid, lasting surface, free from loose particles. They 
should require a minimum of maintenance and repair. A trend in runway surfaces 
is to provide grooves in the pavement to prevent hydroplaning of aircraft 
during landing. The two basic pavement classifications are rigid and flexible. 
Rigid pavements are generally composed of rectangular slabs of portland cement 
concrete interlocked by keys or steel dowels and supported on a well-compacted 
thickness of granular subbase material. The flexible type consists of 
bituminous surface courses on compacted granular base and subbase layers. The 
type selected for a particular site will depend on material and equipment 
availability, cost, existing soil conditions, and operational considerations. 
See PAVEMENT. 
    To protect the major investment in a modern airport, a vigorous program of 
operations and maintenance of the airfield and facilities is essential. 
Systematic preventative maintenance of the airfield pavements, drainage 
system, lighting, and navigational aids pays handsome dividends not only in 
reduction of major repair costs, but also in ensuring that the airfield is in 
operation for the maximum amount of time. The airport terminal, hangars, cargo 
buildings, maintenance facilities, fueling system, and many other related 
facilities and support systems also need the same type of preventative 
maintenance program to ensure their availability and satisfactory performance 
for their intended function. See AVIATION. 
 �34s[Michael Haratunian]
�33s 
  

A700000003215�254s�5v�1v48
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�6v�2v�32sAirport surface detection equipment   �21m �33s 
    Equipment used for the observation of the position of aircraft and other 
vehicles moving on the surface of the airport. This equipment is also known as 
the airport surface movement indicator (ASMI). See AIRPORT ENGINEERING. 
    With the increase in the density of aircraft movements at the world's 
major airports, it became evident that control of traffic on the airport 
surface is as necessary to air safety as is the control in the airspace above 
the airport. There must be a capability of detecting all vehicles that are on 
the surface of the airfield, including tractors and trucks. A number of 
different developments were tried, but it was soon recognized that only 
high-performance radar could fulfill most of the requirements. See AIR-TRAFFIC 
CONTROL.  
    Modern ASMI equipment operates in the Q band (36 to 46 gigahertz). The 
range of these radars is from 200 yd (183 m) to 2.5 mi (4.0 km) from the 
antenna. At a distance of 1000 yd  (914 m), the radar is capable of 
discriminating by 30 ft (9.1 m)  or less in range and 36 ft (11.0 m) or less 
in azimuth. With this radar it is readily possible to discern the outline of 
runways and taxiways and detect small vehicles out to distances of 1.5 mi (2.4 
km). The lift-off point of the aircraft is easily seen. Some difficult 
problems in the design of radars of this type are precipitation and high 
snowbanks. These problems have been reduced by the use of circular 
polarization and moving-target indicator (MTI) techniques. See ELECTRONIC 
NAVIGATION SYSTEMS; RADAR. 
 �34s[Peter C. Sandretto]
�33s 
  

A700000012330�254s�5v�1v39
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�6v�2v�32sAir pressure   �21m �33s 
    The force per unit area that the air exerts on any surface in contact with 
it, arising from the collisions of the air molecules with the surface. It is 
equal and opposite to the pressure of the surface against the air, which for 
atmospheric air in normal motion approximately balances the weight of the 
atmosphere above, about 15 pounds per square inch (psi) at sea level. It is 
the same in all directions and is the force that balances the weight of the 
column of mercury in the Torricellian barometer, commonly used for its 
measurement. See BAROMETER. 
    The basic units of pressure, which are the ones mainly used in 
meteorology, are based on the bar, which is defined as equal to 1,000,000 
dynes/cm sup 2. One bar equals 1000 millibars (mb) equals 100 centibars (cb). 
    Also widely used in practice are units based on the height of the mercury 
barometer under standard conditions, expressed commonly in millimeters or in 
inches. The standard atmosphere (760 mmHg) is also used as a unit, mainly in 
engineering where large pressures are encountered. The following equivalents 
show the conversions between the commonly used units of pressure, where 
(mmHg)n and (in. Hg)n denote the millimeter and inch of mercury, 
respectively, under standard (normal) conditions, and where (kg)n and 
(lb)n denote the weight of a standard kilogram and pound mass, 
respectively, under standard gravity. 
  
              1 mb = 1000 dynes/cm sup 2 = 0.750062 (mmHG)n
                   = 0.0295300 (in. Hg)n
             1 atm = 1013.250 mb = 760 (mmHg)n
                   = 29.9213 (in. Hg)n = 14.6959 (lb)n/in. sup 2
                   = 1.03323 (kg)n/cm sup 2
         1 (mmHg)n = 1 torr = 1.333224 mb
                   = 0.03937008 (in. Hg)
       1 (in. Hg)n = 33.8639 mb = 25.4 (mmHg)n
  
    Because of the almost exact balancing of the weight of the overlying 
atmosphere by the air pressure, the latter decreases with height. A standard 
equation is used in practice to calculate the vertical distribution of 
pressure with height above sea level. The temperature distribution in a 
standard atmosphere, based on mean values in middle latitudes, has been 
defined by international agreement. The use of the standard atmosphere yields 
a definite relation between pressure and height. This relation is used in all 
altimeters which are basically barometers of the aneroid type. The difference 
between the height estimated from the pressure and the actual height is often 
considerable; but since the same standard relationship is used in all 
altimeters, the difference is the same for all altimeters at the same 
location, and so causes no difficulty in determining the relative position of 
aircraft. Mountains, however, have a fixed height, and accidents have been 
caused by the difference between the actual and standard atmosphere. See 
ALTIMETER.  
    In addition to the large variation with height, atmospheric pressure 
varies in the horizontal and with time. The variations of air pressure at sea 
level, estimated in the case of observations over land by correcting for the 
height of the ground surface, are routinely plotted on a map and analyzed, 
resulting in the familiar weather map representation with its isobars showing 
highs and lows. The movement of the main features of the sea-level pressure 
distribution, typically from west to east, produces characteristic 
fluctuations of the pressure at a fixed point, varying by a few percent within 
a few days. Smaller-scale variations of sea-level pressure, too small to 
appear on the ordinary weather map, are also present. These are associated 
with various forms of atmospheric motion, such as small-scale wave motion and 
turbulence. Relatively large variations are found in and near thunderstorms, 
the most intense being the low-pressure region in a tornado. The pressure drop 
within a tornado can be a large fraction of an atmosphere, and is the 
principal cause of the explosion of buildings over which a tornado passes. See 
ISOBAR (METEOROLOGY); WEATHER MAP. 
    It is a general rule that in middle latitudes at localities below 1000 m 
(3280 ft) in height above sea level, the air pressure on the continents tends 
to be slightly higher in winter than in spring, summer, and autumn; whereas at 
considerably greater heights on the continents and on the ocean surface, the 
reverse is true. 
    The practical importance of air pressure lies in its relation to the wind 
and weather. It is because of these relationships that pressure is a basic 
parameter in weather forecasting, as is evident from its appearance on the 
ordinary weather map. 
    The large-scale variations of pressure at sea level shown on a weather map 
are associated with characteristic patterns of vertical motion of the air, 
which in turn affect the weather. Descent of air in a high heats the air and 
dries it by adiabatic compression, giving clear skies, while the ascent of air 
in a low cools it and causes it to condense and produce cloudy and rainy 
weather. These processes at low levels, accompanied by others at higher 
levels, usually combine to justify the clear-cloudy-rainy marking on the 
household barometer. 
 �34s[Raymond J. Deland]
�33s 
  

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�6v�2v�32sAir register �21m �33s
    A device attached to an air-distributing duct for the purpose of
discharging air into the space to be heated or cooled. These openings are
referred to as registers, diffusers, supply outlets, or grills (see
illustration). By common acceptance, a register is an opening provided with
means for discharging the air in a confined jet, whereas a diffuser is an
outlet which discharges the air in a spreading jet. Both registers and
diffusers may be placed at a number of locations in a room, including the
floor, baseboard, low on the sidewall, window sill, high on the sidewall, or
ceiling.
    For heating, the preferred location is in the floor, at the baseboard, or
at the low sidewall of the outside wall, preferably under a window. For
cooling, the preferred location is high on the inside wall or the ceiling. For
(Titus Manufacturing Corp.) in homes, a compromise location is the floor,
baseboard, or low sidewall at the exposed wall, especially if adequate air
velocity in an upward direction is provided at the supply outlet.


 �34s[Seichi Konzo]
�33s


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�6v�2v�32sAirship �21m �33s
    A propelled and steered aerial vehicle, dependent on the displacement of
air by a lighter gas for lift. An airship, or dirigible balloon, is composed
primarily of a streamlined hull, usually a prolate ellipsoid which contains
one or more gas cells, fixed and movable tail surfaces for control, a car or
cabin for the crew or passengers, and a propulsion system.
    Two fundamentally different designs have been successfully used for past
airships, the nonrigid and the rigid. A third type, the semirigid, is
essentially a variant of the nonrigid type, differing by the addition of rigid
keel.
    A typical nonrigid airship, or blimp, consists of a flexible envelope,
usually fabric, filled with lifting gas that is slightly pressurized. Internal
air compartments (ballonets) expand and contract to maintain the pressure in
the envelope as atmospheric pressure and temperature vary. Ballonet volume is
controlled by ducting air from the prop wash or by electric blowers. The
weights of the car structure, propulsion system, and other concentrated loads
are supported by catenary systems attached to the envelope.
    The nonrigid airships are historically significant for two reasons. First,
a nonrigid airship was the first aircraft of any type to achieve controllable
flight, in the 1850s. Second, nonrigid airships were the last type to be used
on an extensive operational basis; the U.S. Navy decommissioned the last of
its nonrigid airship fleet in the early 1960s.
    The structure of a rigid airship (see illustration) was usually an
aluminum ring-and-girder frame. An outer covering was attached to the frame to
provide a suitable aerodynamic surface. Several gas cells were arrayed
longitudinally within the frame. These cells were free to expand and contract,
thereby allowing for pressure and temperature variations. Thus, despite their
nearly identical outward appearance, rigid and nonrigid airships were
significantly different in their construction and operation.
    Many modern airship vehicle concepts have been proposed. The fully buoyant
conventional concepts are modern versions of the classical, fully buoyant,
rigid, and nonrigid airship concepts. Fully buoyant means all the lift is
provided by displacement. These airships would make extensive use of modern
aircraft structural materials, propulsion systems, control systems, and
electronics.
    Other concepts include partially buoyant, or hybrid, designs in which the
buoyant lift is substantially augmented by aerodynamic or propulsive lift.
Thus, these vehicles are partly heavier than air and partly lighter than air.
Some hybrids are short-takeoff and landing (STOL) vehicles, and others have
vertical-takeoff and landing (VTOL) capability. An important advantage of the
hybrids is that they promise to alleviate the costly ground-handling
requirement of past airship designs. See SHORT TAKEOFF AND LANDING (STOL);
VERTICAL TAKEOFF AND LANDING (VTOL).
    Airships are once again being seriously considered for many civil and
military applications. An airship's natural attributes compared with other
vehicles draw attention to short-range transportation and to missions
requiring high loiter time, that is, patrol and surveillance applications. See
BALLOON.

 �34s[Mark D. Ardema]
�33s



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�6v�2v�32sAirspeed indicator   �21m �33s 
    A device that computes and displays speed of an aircraft relative to the 
air mass in which the aircraft is flying. The commonest type is the indicated 
airspeed meter, which measures differential pressure between the total ram 
pressure from the Pitot system and the total static pressure; it then converts 
this difference into units of speed (mph or knots) under standard conditions. 
Although the indicated values are incorrect above zero altitude, the 
relationship to the aircraft handling remains essentially unchanged, thus 
providing a measure of the flyability of the aircraft. 
    True airspeed indicators are similar but include a more complex mechanism 
that also senses both the absolute pressure and temperature, and compensates 
for the change of density of the air mass, thus obtaining true airspeed. This 
indication is of value in computing course information. 
    For those aircraft that reach higher speeds (transonic and supersonic), 
the ratio of the actual speed to the local speed of sound is used. Devices 
that compute this value are known as Machmeters. See MACH NUMBER; PITOT TUBE. 
 �34s[James W. Angus]
�33s 
  
A700000007212�254s�5v�1v40
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�6v�2v�32sAir temperature   �21m �33s 
    The temperature of the atmosphere represents the average kinetic energy of 
the molecular motion in a small region, defined in terms of a standard or 
calibrated thermometer in thermal equilibrium with the air. Many different 
types of thermometer are used for the measurement of air temperature, the most 
common depending on the expansion of mercury with temperature, the variation 
of electrical resistance with temperature, or the thermoelectric effect 
(thermocouple). See TEMPERATURE; THERMOMETER. 
    The temperature of a given small mass of air varies with time because of 
heat added or subtracted from it, and also because of work done during changes 
of volume. 
    The rate at which the temperature changes at a particular point, that is, 
as measured by a fixed thermometer, depends on the movement of air as well as 
physical processes such as absorption and emission of radiation, heat 
conduction, and changes of phase of water involving latent heat of 
condensation and freezing. The large changes of air temperature from day to 
day are mainly due to the horizontal movement of air, bringing relatively cold 
or warm air masses to a particular point, as the large-scale pressure-wind 
systems move across the weather map. See AIR MASS; AIR PRESSURE. 
    Temperatures near the surface are read at one or more fixed times daily, 
and the day's extremes are obtained from special maximum and minimum 
thermometers, or from the trace (thermogram) of a continuously recording 
instrument (thermograph). The average of these two extremes, technically the 
midrange, is considered in the United States to be the day's average 
temperature. The true daily mean, obtained from a thermogram, is closely 
approximated by the mean of 24 hourly readings, but may differ from the 
midrange by 1 or 2 DG F (0.5 or 1 DG C), on the average. In many countries 
temperatures are read daily at three or four fixed times, chosen so that their 
weighted mean closely approximates the true daily mean. 
    Averages of daily maximum and minimum temperature for a single month for 
many years give mean daily maximum and minimum temperatures for that month. 
The average of these values is the mean monthly temperature, while their 
difference is the mean daily range for that month. Monthly means, averaged 
through the year, give the mean annual temperature; the mean annual range is 
the difference between the hottest and coldest mean monthly values. The 
hottest and coldest temperatures in a month are the monthly extremes; their 
averages over a period of years give the mean monthly maximum and minimum 
(used extensively in Canada), while the absolute extremes for the month (or 
year) are the hottest and coldest temperatures ever observed. The interdiurnal 
range or variability for a month is the average of the successive differences, 
regardless of sign, in daily temperatures. 
    Over the oceans the mean daily, interdiurnal, and annual ranges are 
slight, because water absorbs the insolation and distributes the heat through 
a thick layer. In tropical regions the interdiurnal and annual ranges over the 
land are small also, because the annual variation in insolation is relatively 
small. The daily range also is small in humid tropical regions, but may be 
large (up to 40 DG F or 22 DG C) in deserts. Interdiurnal and annual ranges 
increase generally with latitude, and also with distance from the ocean; the 
mean annual range defines continentality. The daily range depends on aridity, 
altitude, and noon Sun elevation. See ATMOSPHERE; INSOLATION; RADIATION. 
 �34s[Raymond J. Deland]
�33s 
  

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�6v�2v�32sAir-traffic control   �21m �33s 
    A service which promotes the safe and expeditious movement of aircraft 
operating in the air or on an airport surface. Air-traffic control (ATC) is a 
component of the aviation system which has evolved to move goods and people 
through airspace to desired destinations with reliability, efficiency, and 
minimum infringement on individual intents. The objective of ground-based ATC 
is to provide pilots of aircraft with information and control services that 
will enable them to use the airspace efficiently and to conduct operations 
with maximum safety. Increases in the amount of air traffic using the limited 
airspace and airports at heavily congested metropolitan terminal areas have 
profoundly affected traffic control. The challenge to ATC in such an 
environment is to match system capacity to system demand without compromising 
safety.  
    Delays increase with traffic demand when capacity is exceeded. Under 
saturation demand conditions, delays can be reduced by flow control procedures 
and additional capacity. Among the possibilities for attaining increased 
capacity are additional runways at existing airports, additional airports, 
procedures and equipment that allow operation under conditions of lower 
visibility, rules to provide segregation of aircraft by speed characteristics, 
automated techniques to communicate clearances between pilots and controllers, 
and systems which permit aircraft to operate safely though using reduced 
spacings in the final approach area. See AIRPORT SURFACE DETECTION EQUIPMENT. 
    Two cardinal rules of flight that apply to the ATC system are instrument 
flight rules (IFR) and visual flight rules (VFR). IFR governs all flights 
operating in weather conditions that prevent pilots from seeing well enough to 
avoid each other; IFR flight prevails when weather conditions fall below the 
minimum ceiling and visibility prescribed for flight under VFR. All flights 
receiving ATC separation service in en route airspace must be conducted under 
IFR rules regardless of weather conditions. All aircraft operating above 
18,000 ft (5486 m), where aircraft velocities preclude the pilot from 
providing visual separation and detecting possible collisions with sufficient 
lead time to perform an evasive maneuver, must operate within the ATC system. 
    Two broad categorizations of ATC procedures are terminal and en route. 
Terminal operations pertain to departures and arrivals at airports. En route 
procedures cover that portion of the flight between the departure and arrival 
terminals.  
    Technical devices and equipment of four broad classes are required in 
order to fly safely and in compliance with the general rules and procedures: 
navigation systems, communication systems, surveillance systems, and data 
systems.  
    To fly within designated airspace, the pilot uses navigational instruments 
to ascertain aircraft position in three dimensions and to conduct the aircraft 
over any chosen route from terminal to terminal. The facility used most 
commonly over the continental United States is known as the 
very-high-frequency omnirange combined with distance-measuring equipment 
(VORTAC). It includes a ground radio-transmitting station that provides 360 
course radials from the antenna source. The airborne portion of this facility 
is a receiver and display device, which indicates the magnetic bearing to a 
particular VOR facility. 
    Radio voice communication is the means whereby the centralized 
ground-based ATC entity can learn the position of all aircraft within its 
jurisdiction, and the means by which safe-flight instructions are transmitted 
rapidly and unambiguously to all aircraft concerned. Radar is used as a 
substitute for voice position reporting where available. Digital data-link 
systems are proposed for future use to increase system capacity and accuracy 
and to provide relief from voice frequency congestion. See RADIO-SPECTRUM 
ALLOCATIONS.  
    Surveillance of aircraft in en route and terminal airspace is provided by 
radar systems. Each primary radar has an ATC radar beacon system (ATCRBS) 
associated with it which interrogates transponders in equipped aircraft to 
determine identity code and pressure height data. See RADAR. 
    The ATCRBS is to be expanded to include a discrete mode of interrogation 
in which each aircraft transponder is called by its discrete identity code. 
This combined system is called the discrete address beacon system (DABS). The 
system includes the capability of exchanging data-link messages with the 
aircraft during surveillance interrogation periods. 
    The ground-based ATC system assimilates pertinent flight information, such 
as present position, course, speed, and identification, for each aircraft in 
its jurisdiction and determines safe, nonconflicting flight procedures for all 
these aircraft. Computer-based systems which perform computations and drive 
digital displays have replaced combinations of radar and written information. 
    ATC and flight advisory services are provided by the ATC command center 
and associated national flow control facility; air route traffic control 
centers (ARTCCs); terminal control facilities; and flight service stations. 
Statistical and real-time information on active and proposed flights, severe 
weather information, and current and forecast major-airport acceptance-rate 
information is processed in the national flow control computer facility 
located at the ARTOC in Jacksonville, Florida. Projected traffic and delay 
information is forwarded to the ATC command center in Washington, D.C., for 
dissemination to major airports. 
    The ARTCC facilities control all traffic operating above a pressure 
altitude of 18,000 ft or 5486 m. Traffic operating below this altitude may 
receive control service by filing an IFR flight plan. Terminal control service 
is provided by the ARTCCs to airports that do not have terminal control 
facilities and are not satellite airports receiving service from another 
terminal control facility. The ARTCCs located within the coterminous 48 states 
have been automated, and automation has been undertaken at centers in Alaska, 
Hawaii, and Puerto Rico. 
    Navigation in the terminal area is provided by the VOR in conjunction with 
DME (VORTAC). When aircraft are about 20 - 40 mi (32 - 64 km) from the runway, 
the air-traffic controllers define the sequence of aircraft arrivals and 
determine the course, altitude, and speed changes needed to achieve the 
desired interaircraft spacings on the final approach course to the runway. The 
final approach guidance is provided by the instrument landing system (ILS) 
which provides both horizontal and vertical path guidance to the pilot. When 
development is complete, this service is to be furnished by the microwave 
landing system (MLS). See INSTRUMENT LANDING SYSTEM (ILS). 
    Control of the aircraft in the terminal area is based primarily on radar 
position data obtained from the ATCRBS. Many of the major terminals are 
equipped with an automated radar terminal system (ARTS III) offering 
automation capabilities equivalent to the en route system. Additional radar 
terminals are being equipped with a limited automation capability (ARTS II). 
 �34s[Neal A. Blake]
�33s 
  

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�6v�2v�32sAir transportation   �21m �33s 
    The movement of passengers and cargo by aircraft such as airplanes and 
helicopters. Within 50 years air transportation has become the primary means 
of common-carrier traveling. Greatest efficiency and value are obtained when 
long distances are traveled, high-value payloads are moved, immediate needs 
must be met, or surface terrain prevents easy movement or significantly raises 
transport costs. Although the time and cost efficiencies obtained decrease as 
distance traveled is reduced, air transport is often worthwhile even for 
relatively short distances. Air transportation also provides a communication 
link, which is sometimes vital, between the different groups of people being 
served.  
 �34s[Dal V. Maddalon]
�33s 
  
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�6v�2v�32sAir-velocity measurement   �21m �33s 
    The measurement of the rate of displacement of air or gas at a specific 
location, for example, wind speed, air velocity in the test section of a wind 
tunnel, airspeed of an aircraft, or air velocity produced by a fan or blower 
in air conditioning. Velocity indicates the magnitude and direction of the 
flow rate; the latter is usually measured separately. To obtain the total flow 
velocity as required in pipes or conduits, integration over the cross section 
is required, or another method of measurement is used. See AIRSPEED INDICATOR; 
FLOW MEASUREMENT. 
    Three primary methods are used in measuring air velocity. These methods 
and the instruments which make use of each are listed in the table. See 
ANEMOMETER; BRIDLED PRESSURE PLATE; KATA THERMOMETER; PITOT TUBE; SHIELDED 
THERMOCOUPLE; VENTURI TUBE. 
    Table title: Methods of measuring air velocity.
 �34s[Herbert Fox]
�33s 
  

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�6v�2v�32sAir waves, upper synoptic   �21m �33s 
    Wavelike oscillations in the pattern of wind flow aloft, usually with 
reference to the stronger portion of the westerly current. The flow is 
anticyclonically curved in the vicinity of a ridge line in the wave pattern, 
and is cyclonically curved in the vicinity of a rough line. 
    Any given hemispheric upper flow pattern may be represented by the 
superposition of sinusoidal waves of various lengths in the general westerly 
flow. Analysis of a typical pattern discloses the presence of prominent long 
waves, of which there are three or four around the hemisphere, and of 
distinctly evident short waves, of about half the length of the long waves. 
    Typically, each short wave trough and ridge is associated with a 
particular cyclone and anticyclone, respectively, in the lower troposphere. 
These circulations produce the rapid day-to-day weather changes which are 
characteristic of the climate of the middle latitudes. The long waves aloft do 
not generally correspond to a single feature of the circulation pattern at low 
levels. By virtue of their position and amplitude, they can exert an indirect 
influence on the character of the weather over a given region for a period of 
the order of weeks. See ATMOSPHERE; JET STREAM; STORM; VORTEX; WEATHER 
FORECASTING AND PREDICTION; WIND. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sAistopoda   �21m �33s 
     An order of amphibians in the subclass Lepospondyli mainly represented by 
three Upper Carboniferous genera - Dolichosoma, Ophiderpeton, and 
Phlegethontia. The limbs are reduced or lost, and there is an elongate 
snakelike body with up to 200 vertebrae; peculiarly forked single-headed ribs 
are a diagnostic feature. The slender skull is very different in construction 
from that in other older groups. Undescribed remains have been found in very 
early Carboniferous rocks which appear to belong to this group, thus ranging 
aistopods among the oldest of all known amphibians. See AMPHIBIA; LEPOSPONDYLI. 
 �34s[Alfred S. Romer]
�33s 
  
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�6v�2v�32sAlanine   �21m �33s 
    An amino acid. The biosynthesis of alanine starts with pyruvic acid, by 
transamination or reductive amination. During metabolic degradation, alanine 

                         *** CHEMICAL STRUCTURE ***

is deaminated to pyruvic acid. See AMINO ACIDS; CARBOHYDRATE METABOLISM. 
 �34s[Edward A. Adelberg]
�33s 
  

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�6v�2v�32sAlarm systems   �21m �33s 
    Systems which operate a warning device after the occurrence of an abnormal 
or dangerous condition. Alarm systems are used to signal undesirable or 
dangerous situations such as the presence of an intruder or the existence of a 
runaway condition in a petroleum refinery process. In factories, these systems 
signal such things as excessive process temperature or pressure or rapid 
change in a process condition. 
    Alarm systems are usually open-loop control systems. A basic alarm system 
contains two essential components: an alarm detector and an alarm indicator. 
Frequently, they are remote-control systems, that is, the detector is located 
remotely from the indicator. See OPEN-LOOP CONTROL SYSTEM; REMOTE-CONTROL 
SYSTEM.  
 �34s[Donald L. Turrell]
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�6v�2v�32sAlbedo �21m �33s
    A term used to describe the reflecting properties of surfaces. White
surfaces have albedos close to 1; black surfaces have albedos close to 0.
    Two types of albedos are in common use: the bond albedo determines the
energy balance of a planet and is defined as the fraction of the total
incident solar energy that the planet reflects back to space. The ``normal
albedo'' of a surface, more properly called the normal reflectance, is a
measure of the relative brightness of the surface when viewed and illuminated
vertically. Such measurements are referred to as a ``perfectly white Lambert
surface'' - a surface which absorbs no light, and scatters the incident energy
isotropically - and are usually approximated by magnesium oxide (MgO),
magnesium carbonate (MgCO3), or some other bright powder. See PHOTOMETRY;
PLANET.
    Bond albedos for solar system objects range from 0.76 for cloud-shrouded
Venus to values as low as 0.01 - 0.02 for some asteroids and satellites. The
value for Earth is 0.33. The bond albedo is defined over all wavelengths, and
its value therefore depends on the spectrum of the incident radiation.
    Normal reflectances of some common materials are listed in the table. The
normal reflectances of many materials are strongly wavelength-dependent, a
fact which is commonly used in planetary science to infer the composition of
surfaces remotely. While the bond albedo cannot exceed unity, the normal
reflectance of a surface can if the material is more backscattering at
opposition than the reference surface.

 �34s[Joseph Veverka]
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�6v�2v�32sAlbite   �21m �33s 
    The sodium feldspar end member NaAlSi3O8, a common rock-forming mineral 
component with crystal structure, morphology, and thermochemistry dictated by 
the degree of Al-Si order in the three-dimensional tetrahedral framework 
structure. The most ordered triclinic albite occurs frequently as a late-stage 
mineral associated with gem pocket - bearing lithium-rich pegmatites as the 
platy variety called cleavelandite. Albite and anorthite are components in the 
complex plagioclase series, NaAlSi3O8 - CaAl2Si2O8. Albite breaks down into 
jadeite (NaAlSi2O6) and quartz (SiO2) at high pressure. See FELDSPAR; IGNEOUS 
ROCKS; PEGMATITE. 
 �34s[Paul B. Moore]
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�6v�2v�32sAlbumin   �21m �33s 
    A globular protein characterized by its solubility in water and in 50% 
saturated aqueous ammonium sulfate. Albumins are present in mammalian tissues, 
bacteria, molds, and plants, and in some foods. Serum albumin, which contains 
584 amino acid residues, is the most abundant protein in human serum. This 
protein is responsible for about 80% of the total osmotic regulation in blood, 
and it transports fatty acids between adipose tissues. 
    Another important albumin, ovalbumin, is found in egg white. This protein 
is about two-thirds the size of serum albumin, and it contains sugar residues 
in addition to amino acid residues (that is, it is a glycoprotein). See 
GLOBULAR PROTEIN; PROTEIN. 
 �34s[James M. Manning]
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�6v�2v�32sAlcohol �21m �33s
    A member of a class of organic compounds composed of carbon, hydrogen, and
oxygen. They can be considered as hydroxyl derivatives of hydrocarbons
produced by the replacement of one or more hydrogens by one or more hydroxyl
(--OH) groups.
    Alcohols may be mono-, di-, tri-, or polyhydric, depending upon the number
of hydroxyl groups they possess. They are classified as primary (RCH2OH),
secondary (R2CHOH), or tertiary (R3COH), depending on the number of hydrogen
atoms attached to the carbon atom bearing the hydroxyl group. Alcohols can
also be characterized by the molecular configuration of the hydrocarbon
portion (aliphatic, cyclic, heterocyclic, or unsaturated). There are two
systems in use for alcohol nomenclature: the common naming system and the
IUPAC (International Union of Pure and Applied Chemistry) naming system. The
common name is sometimes associated with the natural source of the alcohol or
with the hydrocarbon portion (for example, methyl alcohol, ethyl alcohol). The
IUPAC method is a systematic procedure and follows agreed-upon rules. Examples
of these two systems are given in the table.

 �34s[Joseph Veverka]
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�6v�2v�32sAlcoholic beverages   �21m �33s 
    Potable beverages obtained by distilling an alcohol-containing liquid and 
further treating the distillate to obtain a beverage of specific character. 
  
    Brandies.  Brandy is a spirit obtained from the distillation of wine or a 
fermented fruit juice, usually after aging of the wine in wooden casks. Cognac 
is a brandy distilled from wines made of grapes grown within the legal limits 
of Charente and Charente Inferieure Departments, the Cognac region of France. 
Armagnac is brandy made in the Department of Gers, southeast of Bordeaux. Both 
types of brandy are aged for many years in oak before bottling. Brandies 
distilled from grape pomace of the wine press are called eau de vie de marc in 
France and grappa in Italy; Spanish brandies are usually distilled from sherry 
wines. Whereas in Europe pot stills are most common, continuous stills are 
preferred in California, where most American brandies are produced. Apple 
brandy, called applejack in the United States and calvados in France, is 
distilled from completely fermented apple juice and aged in oak barrels for 5 
- 10 years. Other fruits from which brandy is made include black, wild 
cherries (kirsch or kirschwasser), plums (slivovitz from Hungary or Rumania, 
and quetsch or mirabelle from France), blackberries, and apricots. When stone 
fruits are used, some of the stones are broken or crushed and a small amount 
of the oil is distilled over with the spirit, giving the brandy a more or less 
pronounced bitter almond flavor. 
  
    Whiskeys.  These distilled spirits are made by distilling fermented grain 
mashes and aging the distillate in wood, usually oak. 
    Scotch whisky is made primarily from barley. The barley is converted to 
malt by allowing it to sprout after which it is dried in a kiln over a peat 
fire. The malt absorbs some of the smoke aroma which is carried over later 
with the spirit distilled from it. The drying and roasting of the malt is to a 
large extent responsible for the flavor of the whisky. After the malt is made 
into a mash, it is fermented, distilled, and aged in oak casks. A Scotch 
blended whisky may contain a certain percentage of grain whisky besides malt 
whisky.  
    Irish whiskey is made from malt, unmalted barley, and other grains such as 
wheat, rye, and oats. The malt is not smoke-cured as in Scotland, and as a 
result the flavor of the resulting whiskey is different from that of Scotch 
whisky.  
    Many types of whiskey are made in the United States. Depending on the 
principal source of grain, whiskeys are classified as bourbon (corn or grain), 
rye whiskey (rye), and others. Blended whiskeys are differentiated from 
straight whiskeys by a certain content of neutral spirits. American whiskeys 
are aged for a number of years in new charred white-oak barrels. In all types 
of whiskeys discussed above, the type and quality of the water used in the 
plant is an important factor in the quality of the finished product. 
  
    Gins.  These consist essentially of a pure grade of alcohol which has been 
flavored with an extract of the juniper berry as the chief flavoring agent. 
The flavor may be imparted by distillation of herbs (distilled gin) or by the 
addition of essential oils (a compounded gin). There are two principal types 
of gin: English, or London dry, gin and Dutch gin (jenever). 
    English gin is made from pure neutral spirits, which are redistilled in 
the presence of juniper berries and small amounts of other ingredients such as 
coriander seed, cardamon seed, orange peel, anise seed, cassia bark, and 
fennel. English gin is not aged. 
    Dutch gin is made of malt. The flavoring ingredients are mixed directly in 
the mash, after which it is distilled at a rather low proof. Dutch gins are 
heavier in body and contain more fusel alcohols and other volatile compounds 
(congenerics) besides ethyl alcohol than does English gin. Some types of Dutch 
gin are aged. 
  
    Rum.  The alcoholic distillate from fermented sugar cane juice or molasses 
is known as rum. Cuban rum is light-bodied and light-colored. The middle 
fraction of the distillation, known as aguardiente, is used for making rum. 
The aguardiente is aged in uncharred oak barrels; it is then decolorized, 
filtered, and supplied with some caramel to give it the proper color. 
Occasionally some fruit aroma is added. It is then aged for several more 
years. Jamaica rum is heavier-bodied and contains more congenerics (fusel 
alcohol, esters, and aldehydes) then Cuban rum. Arak is a rum that comes from 
the island of Java. It is a dry, highly aromatic rum. A natural fermentation 
of the molasses by various species of yeast also contributes to the flavor of 
this drink. 
  
    Vodka.  A product originally produced in Russia but now popular in many 
countries, vodka is usually made from wheat. It is highly rectified during 
distillation and thus is a very pure neutral spirit without a pronounced 
taste. It is not aged. 
  
    Pulque and tequila.  Pulque and tequila are of Mexican origin. A sweet sap 
obtained from agave is collected and fermented by natural process and is then 
called pulque. Distilled pulque is tequila. 
  
    Liqueurs.  Liqueurs or cordials are alcoholic beverages prepared by 
combining a spirit, usually brandy, with certain flavorings and sugar. In 
fruit liqueurs, the color and flavor are obtained by an infusion process using 
the specified fruit and spirit. Sugar is added after the extraction is 
complete. Plant liqueurs are made by maceration of plant leaves, seeds, or 
roots with spirit and then distilling the product. Sugar and coloring matter 
are added after distillation. 
 �34s[Emil M. Mrak; Herman J. Phaff]
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�6v�2v�32sAlcoholism   �21m �33s 
    The continuous or excessive use of alcohol with associated pathologic 
results. The type of interaction of alcohol with the biological system and the 
frequency of interaction combine to project the image commonly referred to as 
alcoholism.  
    Alcohol is absorbed most rapidly from solutions in the order of 15 - 30% 
(30 - 60 proof) and less rapidly from beverages containing below 10% and over 
30%. At the lower concentrations this is to be expected since the rate of 
absorption depends on the concentration gradient across the mucosal surface. 
At the higher concentrations ethanol abolishes the rhythmic opening of the 
pylorus, thus preventing the passage to the intestine, where absorption is 
faster. The presence of foods in the stomach is also known to delay gastric 
emptying and thus to slow absorption. Once in the bloodstream, alcohol is 
distributed evenly in all tissues, according to their water content. 
    Alcohol can act as a stimulant at lower doses and as a depressant at 
higher doses. Even at very low doses alcohol can impair the sensitivity to 
odors and taste. Also, low doses are known to alter motor coordination and 
time and space perception, important aspects of car driving. Pain sensitivity 
is diminished with moderate doses. Doses in the order of 0.6 g/kg (about 35 oz 
of 100-proof per person of 150 lb) are normally lethal by depression of 
respiratory activity. Alcohol is known to release feelings of self-criticism 
and to inhibit fear and anxiety. 
    Continuation of alcohol ingestion produces adaptive changes which act by 
opposing the effects of alcohol in the brain. This leads to nervous-tissue 
tolerance. In this new state larger doses of alcohol are required to produce 
effects similar to those obtained in the initial phases of drinking. 
Discontinuation of alcohol use leaves an unbalanced biological state which 
appears to be responsible for the withdrawal syndrome. Characteristic of this 
state are a marked hyperexcitability, hallucinations, insomnia, sleep 
fragmentation, and moderate to extreme tremor as seen in delirium tremens. The 
person is now physically dependent on alcohol. 
    Since alcohol has a large caloric value, a person drinking 25 oz (750 ml) 
of a 100-proof beverage per day ingests about 2000 cal (8400 kilojoules), or 
70 - 80% of his total caloric intake as alcohol. However, since alcoholic 
beverages are normally devoid of essential food factors such as proteins, 
vitamins, and trace metals, malnutrition is very common in alcoholics. 
Further, it has been shown that heavy alcohol consumption reduces, both 
directly and indirectly, the capacity of the intestine to absorb various 
nutrients such as the vitamins B1 and B12 and some essential amino acids, thus 
accentuating the malnutrition state. Encephalopathies, beriberi, pellagra, 
anemia, scurvy, and other hypovitaminoses are very often observed in 
alcoholics. The encephalopathies are among the most serious complications 
associated with alcoholism. Among them Wernicke-Korsakoff's encephalopathy is 
most common. See MALNUTRITION. 
    It was formerly believed that only about 1 out of 10 alcoholics developed 
cirrhosis. However, it has been shown that the production of cirrhosis is 
correlated with the amount of ethanol ingested multiplied by the period 
(years) of ingestion. Thus, an alcoholic that has consumed 200 g of ethanol 
(18 oz of 100-proof beverage) daily for 20 years has a 50% chance of 
developing cirrhosis. See CIRRHOSIS. 
 �34s[Yedy Israel]
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�6v�2v�32sAlcoholysis   �21m �33s 
    A chemical reaction involving the decomposition of a molecule with the 
addition of the elements of an alcohol. The reaction is most commonly an 
exchange reaction. The term is frequently applied to the reaction between an 
alcohol and an ester to produce a different alcohol and a different ester 
(transesterification). In such cases, higher-molecular-weight alcohols readily 
replace those of lower molecular weight; tertiary alcohols are more readily 
replaced than primary alcohols. See ALCOHOL. 
    The term alcoholysis also applies to reactions involving alcohols and a 
number of other classes of organic compounds, such as nitriles, acid 
anhydrides, alkyl halides, acyl halides, BETA-keto esters, lactones, and 
amides. See ACIDOLYSIS; HYDROLYSIS; TRANSESTERIFICATION. 
 �34s[Elbert H. Hadley]
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�6v�2v�32sAlcyonacea �21m �33s
    An order of the subclass Alcyonaria. Alcyonacea, the soft corals (see
illustration), are littoral anthozoans, which form massive or dendriform
colonies with yellowish, brown, or olive colors. Most attach to some solid
substratum; however, some remain free in sandy or muddy places. The only
skeletal structures are small, elongated, spindle-shaped or rodlike, warty
sclerites which are scattered over the mesoglea. The colony is supple and
leathery. The polyp body is embedded in the coenenchyme, from which retractile
anthocodia protrude in Alcyonium. In Xenia and Heteroxenia, anthocodia are
nonretractile. The polyp base is protected by many sclerites and is termed a
calyx. See ALCYONARIA; COELENTERATA.


 �34s[Kenji Atoda]
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�6v�2v�32sAlcyonaria   �21m �33s 
    A subclass of the Anthozoa. These coelenterates are colonial; most are 
sedentary and littoral, and some live at great depths. The ordinary polyp, 
autozooid, has eight motile, contractile, hollow, pinnately branched 
tentacles. Eight complete mesenteries are present, of which the dorsal or 
asulcal pair is the most developed and bears the largest, heavily ciliated 
ectodermal filaments. The remaining mesenteries bear endodermal filaments 
having many glandular cells. Longitudinal retractor muscles are strongly 
developed on their ventral or sulcal surfaces. The stomodeum or pharynx, with 
a single siphonoglyph in its sulcal edge, is lined by a ciliated glandular 
epithelium. The gastrovascular cavity of each polyp is interconnected by 
complex canal systems or solenia. These permeate the colonial spiculiferous 
mesoglea or coenenchyme. The skeleton is formed by a deposition of horny 
material secreted by ectoderm cells, or by ectodermal scleroblasts which 
secrete calcareous spicules or sclerites of various shapes. The musculatures 
are not particularly well developed. 
    The oral end of the polyp is termed the anthocodium. The basal portion, or 
anthostele, is embedded in coenenchyme containing numerous ameboid cells and 
scleroblasts.  
    The Alcyonaria are characterized by a strongly developed siphonoglyph, 
which serves to circulate water in a colony. Alcyonaria are either dioecious 
or monoecious. The gonads ripen in the endodermal mesenteric filaments. The 
daughter polyp buds asexually from the solenial system or from a parent's body 
wall. Pennatulacea and some other forms are dimorphic. The siphonozooid is 
smaller than the autozooid. It lacks tentacles or the tentacles are 
rudimentary and usually sterile. See ANTHOZOA; COELENTERATA. 
 �34s[Kenji Atoda]
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�6v�2v�32sAldebaran   �21m �33s 
    Alpha Tauri, a red giant star of spectral type K5, temperature 3600 K, and 
luminosity about 400 times that of the Sun. Aldebaran is at a distance of 20 
parsecs (6.2 X 10 sup 14 km); it is large enough to show a barely detectable 
disk, 0".23 of arc, with the Mount Wilson (California Institute of 
Technology/Palomar Observatory) stellar interferometer. See ASTRONOMICAL 
PHOTOGRAPHY; STAR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sAldehyde �21m �33s
    One of a class of organic chemical compounds represented by the general
formula RCHO. Formaldehyde, the simplest aldehyde, has the formula HCHO, where
R is hydrogen. For all other aldehydes, R is a hydrocarbon radical which may
be substituted with other groups such as halogen or hydroxyl (see table).
Because of their high chemical reactivity, aldehydes are important
intermediates for the manufacture of resins, plasticizers, solvents, dyes, and
pharmaceuticals.
    At room temperature formaldehyde is a colorless gas. The other
low-molecular-weight aldehydes are colorless liquids having characteristic,
somewhat acrid odors. The unsaturated aldehydes acrolein and crotonaldehyde
are powerful lacrimators. The important reactions of aldehydes include
oxidation, reduction, aldol condensation, Cannizzaro reaction, and reactions
with compounds containing nitrogen.
    Because of the importance of aldehydes as chemical intermediates, many
industrial and laboratory syntheses have been developed. The more important of
these methods include catalytic dehydrogenation of primary alcohols, oxidation
of primary alcohols, oxidation of olefins, and hydroformylation of olefins.
See FORMALDEHYDE.

 �34s[Paul E. Fanta]
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�6v�2v�32sAldosterone �21m �33s
    The steroid hormone found in the biologically active amorphous fraction
that remains after separation of the various crystalline steroid substances,
such as cortisol and corticosterone, from adrenal extracts. In solution,
aldosterone exists as an equilibrium mixture of aldo and lactol forms (see
illustration).

			 *** CHEMICAL STRUCTURE ***

    The chief function of aldosterone is the regulation of electrolyte
metabolism. Aldosterone is the most potent of the hormones which are concerned
in this type of metabolism; in adrenalectomized dogs, minute doses on the
order of 1 - 2 micrograms per kilogram per day can maintain the electrolyte
balance of the animals and keep them alive. See ADRENAL GLAND; HORMONE;
STEROID.


 �34s[Choh Hao Li]
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�6v�2v�32sAlfalfa   �21m �33s 
    The world's most valuable forage legume, Medicago sativa, known also as 
lucerne. It is grown for hay, pasture, and silage. Valued highly as livestock 
feed, alfalfa hay cut at a late bud or early bloom stage (and cured properly) 
contains 17 - 24% protein and is a good source of certain vitamins and 
minerals. See ROSALES. 
    Alfalfa is a herbaceous perennial legume characterized by a deep taproot, 
which shows a varying degree of branching. Erect or semierect stems, bearing 
an abundance of leaves, grow to a height of 2 - 3 ft (0.6 - 0.9 m). The number 
of stems arising from a single woody crown may vary from just a few to 50 or 
more. New stems develop when older ones are mature or have been cut or grazed. 
    Flowers are borne on axillary racemes which vary greatly in size and 
number of flowers. Flower color is predominantly purple, or bluish-purple, but 
white, cream, yellow, green, lavender, and reddish-purple occur. 
    The fruit is a legume, or pod, usually spirally coiled in M. sativa, but 
crescent-shaped or straight in M. falcata. Seeds are small and the color 
varies from yellow to brown. 
    Reproduction in alfalfa is mainly by cross-fertilization. Pollination is 
effected largely by bees. Seed production is favored by bright sunny days, 
cool nights, an abundance of bees, and dry weather for harvesting. 
    Alfalfa is widely adapted to temperate and subtropical climates and soils. 
It is grown from 40 DG S in Argentina and New Zealand to 60 DG N in Canada, 
Sweden, and the Soviet Union. It is not well adapted to humid tropical 
conditions.  
    Deep fertile loams are best and good drainage is essential. The water 
requirement for sustained high yields is great, and moisture deficiency often 
is a serious limiting factor in areas dependent on natural rainfall. 
    More than 80 improved cultivars of alfalfa are recognized as eligible for 
seed certification in the United States. These trace to three basic stocks: M. 
sativa, purple-flowered, narrow-crowned, and erect; M. falcata, 
yellow-flowered, somewhat prostrate, with deep-set crown and branching roots; 
and an intermediate form, often called variegated, derived from crossing M. 
sativa and M. falcata. 
 �34s[Clarence H. Hanson]
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�6v�2v�32sAlgae   �21m �33s 
    A general name (meaning sea wrack) given to the chlorophyll-bearing 
organisms in the subkingdom Thallobionta (Thallophyta) of the plant kingdom. 
They are predominantly aquatic, in both fresh and marine waters, ranging in 
size from unicells no more than 0.8 micrometer in diameter to the huge 
seaweeds, 200 - 300 ft (60 - 90 m) or more in length. Fossil forms are 
recognized as early as the Precambrian. There is a great range in form 
including one-celled motile and nonmotile genera, motile and nonmotile 
colonies, branched and unbranched filaments, coenocytic (multinucleate) 
siphons, complexes of filaments and siphons, and massive parenchymatous thalli. 
    The algae comprise nine divisions of the plant kingdom: Chlorophyta, or
green algae; Cyanophyta, or blue-green algae (considered by some to be a class,
Cyanophyceae, of the division Schizophyta); Euglenophyta, or Euglenida;
Chrysophyta, or yellow-green algae; Cryptophyta (considered by some to be a
class, Cryptophyceae, of the Pyrrhophyta); Pyrrhophyta, or dinoflagellates;
Chloromonadophyta, or chloromonads (considered by some to be a class,
Chloromonadophyceae, of the Chrysophyta); Phaeophyta, or brown algae; and
Rhodophyta, or red algae. These divisions are recognized and differentiated by
pigment composition; chemical nature of the food reserve; flagellation (form,
number, and insertion of flagella); and the chemical and physical nature of the
cell wall. See CHLOROPHYTA; CHRYSOPHYTA; CYANOPHYCEAE; EUGLENOPHYTA;
PHAEOPHYTA; PYRRHOPHYTA; RHODOPHYTA. 
    Algae may be planktonic (phytoplankton) or benthic, attached to the bottom 
or to substrates, sometimes at great depths. A few are attached to the surface 
film (neuston). Although predominantly aquatic, many fresh-water algae adjust 
to soil and to moist surfaces of rocks and trees. Further, a few become 
endophytic or epiphytic on higher plants, especially in the tropics. In these 
situations the algae may be parasitic and produce pathological conditions in 
the host plant. 
    Some green or blue-green algae may become commensal with, or parasitized 
by, different species of fungi to produce different ``species'' of lichens. In 
the sea many algae, especially Rhodophyta, are epiphytic or parasitic on other 
larger algal forms which sometimes are close relatives. There are thermophilic 
species of fresh-water algae (Cyanophyta) existing in hot springs and about 
geysers in temperatures up to 185.4 DG F (85.2 DG C), and by contrast a 
surprising number occur in snow at high altitudes and in glaciers (red and 
yellow snow; black ice). A few species occur as endozoophytes within the 
cells of small animals (protozoa, sponge, and Hydra) or in the digestive 
tract of mammals; some are epizoic (externally attached) on skin, hair, or 
scales. Marine algae show distinct zonations along coasts and in the 
marginal waters of continental shelves. Littoral algae (Fucus, Ulva, and
Chondrus) occupy the intertidal zone. In the supralittoral zone are forms 
especially adapted for existing during intermittent moist and long dry 
periods (the splash zone), whereas the infralittoral is characterized by 
those genera which must live submersed, occurring below the lowest low-tide 
level and on out into deeper waters. See ECOLOGICAL INTERACTIONS; LICHENES; 
PHYTOPLANKTON. 
    Three general types of reproduction are used: vegetative, which includes 
cell division and fragmentation of the thallus; asexual, by motile (zoospores) 
or nonmotile spores (akinetes, aplanospores); and sexual, by gametes which may 
be isogamous (undifferentiated with respect to maleness and femaleness), 
anisogamous (with a degree of differentiation), heterogamous (differentiated 
as egg and sperm), or oogamous. 
    A chief, indirect economic importance of the algae is the role that 
microscopic, planktonic forms play in the food chain of aquatic animals and 
birds and the relationship of these to humans. Further, algal photosynthetic 
activity carried on throughout the oceans and lakes produces a prodigious 
amount of organic matter that eventually finds its way into the cycling of 
elements in nature. Many larger algae are used as food directly, whereas 
substances contained in seaweeds are commercially important (iodine, potash, 
agar-agar, alginic acid, and carrageenin). Pharmaceutical products, creams, 
emulsifiers, and clearing agents are a few of the many uses of algal 
substances. Diatomite (fossil diatom deposits) is used for filters, insulating 
material, hollow-tile brick, and scouring powders. Seaweeds are used as 
ingredients of cattle and chicken feed and as soil fertilizers. Mass culture 
of the freshwater Chlorella yields a nutritive flour, and this genus also 
plays a role in space-flight projects as an oxygenator. Sewage oxidation ponds 
employ algae in the process of digestion. Planktonic blue-green algae are 
negatively important economically because of the deadly toxins they produce 
that frequently kill cattle, sheep, and birds which drink from heavily 
infested waters. The same (and other) algae are also costly because of their 
ability to ruin drinking water and to spoil lakeside recreational sites. 
 �34s[Gerald W. Prescott]
�33s 

    The remains of seaweeds of former geologic periods are found in rocks. The 
majority represent calcareous algae although, rarely, impressions or carbon 
films of noncalcareous types are found. 
    Today, as in the past, the calcareous algae are important contributors to 
the building of reefs. Many times during the geologic past, calcareous algae 
developed locally in such abundance as to form or help to form beds of 
limestone, bioherms, or reefs. 
    Only a small percentage of the algae ever developed the ability to secrete 
or to deposit calcium carbonate. Those making good fossils have belonged to 
three major groups: (1) the Rhodophyta, or red algae, represented by the 
families Corallinaceae, Solenoporaceae, and Gymnocodiaceae; (2) the 
Chlorophyta, or green algae, represented by the families Dasycladaceae and 
Codiaceae; and (3) Bacillariophyceae, or diatoms. In addition to these three 
groups, there is another large group of fossil algae known as the 
Stromatolites.  
    Some Stromatolites are the oldest known fossils. They occur in rocks of 
all ages from Precambrian to Recent. At various times and places they have 
been important limestone builders, especially in the late Proterozoic, during 
the Cambrian, and during the Early Ordovician. 
    The Solenoporaceae are known from the Late Cambrian to the Late 
Cretaceous. Coralline algae range in age from Late Jurassic to Recent, 
although most of the present-day genera appeared in the Late Cretaceous or 
early Tertiary. Dasycladaceae appeared in the Late Cambrian and extend to 
Recent, while Codiaceae are known from the Cambrian to the present. See 
CORALLINE ALGAE; CRYPTOZOON; STROMATOLITE. 
 �34s[J. Harlan Johnson]
�33s 
  

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�6v�2v�32sAlgebra   �21m �33s 
    Classical algebra is a generalization of arithmetic, made possible by the 
use of symbols, usually letters such as x, y, a, b, ALPHA, and BETA, for 
unknown numbers. The principal instrument for solving problems is the 
equation. The answers sought are denoted by letters, and statements in the 
form of equations are deduced about the quantities in a problem. To solve one 
or more equations, the value or values must be found which, when substituted 
for the unknowns, make the equation or equations true. Equations are said to 
be satisfied by such values. Most of the great developments in algebra were 
inspired and guided by the search for methods of solving equations, which is 
the central problem in algebra. 
    The extension of the number system from the whole numbers and fractions of 
arithmetic to a complete algebraic number system was influenced by the need 
for numbers to satisfy equations. Thus, the equations x sup 2 - 2 = 0 and 
x sup 2 + 1 = 0 have no solutions among whole numbers and fractions. Even the 
extended number system does not provide solutions for all equations. And even 
though solutions can be shown to exist, they may be impossible to find. For 
example, there is no general method for finding the solutions of all 
fifth-degree equations.  
    Algebra is also concerned with inequalities, such as x sup 2 + y sup 2 
> 2xy; with arrangements, such as the number of ways a set of elements can be 
chosen from a larger set (combinations), or the number of ways a set can be 
ordered (permutations); with the study of special sets of numbers such as the 
set of coefficients in the expansion of a binomial like (a + b) sup n; and with 
manipulations of elements such as matrices, which have properties similar to, 
but different from, those of numbers. In modern algebra the relationship to 
arithmetic seems more remote than in classical algebra, and the elements are 
often combined by rules at variance with those which stemmed from the 
arithmetic of experience. 
    The operations of arithmetic - addition, subtraction, multiplication, 
division, raising to powers, and extracting roots - are the algebraic 
operations. Applications of these operations to symbols for numbers, known and 
unknown, produce various kinds of algebraic expressions. A term is formed by 
multiplication: 4x sup 2 y. In a term the known multiplier of the literal part 
is the coefficient. The degree of the term is the sum of the exponents of 
unknowns. The degree of 4x sup 2 y is 3. By multiplication and addition (which 
includes subtraction because of the rules for combining signed numbers), 
polynomials, or rational integral expressions, can be constructed. Polynomials 
are classified according to the number of terms into monomials, binomials, 
trinomials, and other multinomials. In polynomials the coefficients can be any 
fixed numbers: 
  
** 5x sup 2 y - 6y sup 3 + 2/3x     3x sup 4 - SQUARE ROOT [2x sup 2] + 5/3 **
  
    A fractional expression permits division by unknowns: (x sup 2 + 1)/
(x + 1). An expression is irrational if it denotes root extraction of an 
expression involving an unknown. 
    Whereas the operations in arithmetic are derived from practical 
experience, their generalizations in algebra accentuate the necessity of 
axioms. These axioms govern all operations. 
    The positive and negative whole numbers, zero, and ratios of whole numbers 
constitute the rational number system. Addition, multiplication, and division 
are the rational operations. Root extraction is not a rational operation since 
its results are not usually rational numbers. For example, there is no 
rational number whose square is 2, nor any rational number whose square is -1. 
Two extensions of the number system provide irrational numbers and imaginary 
numbers.  

1. A rational number is a number which can be expressed as the ratio of two 
integers, a/b, with b NOT EQUAL TO 0. (An integer is a rational number; for 
example, 2 = 2/1.) 
2. An irrational number is a number which is not rational but which can be 
approximated as closely as desired by rational numbers. (Thus SQUARE ROOT [2] 
is not rational, but for any EPSILON > 0 there are rational numbers ar such 
that the difference between ar and SQUARE ROOT [2] is less than EPSILON.) 
3. The rational numbers and the irrational numbers make up the real number 
system.  
4. A pure imaginary number is a number whose square is negative. 
5. A complex number is the sum of a real number and a pure imaginary number. 

    A pure imaginary number can be expressed as a product b SQUARE ROOT [-1],
where b is a real number. The symbol i, replacing SQUARE ROOT [-1], is the 
imaginary unit; hence all pure imaginary numbers have the form bi, and all 
complex numbers have the form a + bi, with a and b both real. The complex 
numbers include the real numbers (when b = 0) and the pure imaginary numbers 
(when a = 0). The term ``imaginary'' is applied to any complex number a + bi, 
with b NOT EQUAL TO 0. The complex number system is sufficient for the needs of 
classical algebra. In modern algebra the number concept is more abstract. 
    The operation of repeated multiplication of like numbers is denoted by 
exponents and the results are called powers. Thus 2 X 2 X 2 X 2 = 2 sup 4 = 16; 
the product of the four 2s is denoted by the exponent 4 applied to the base 2, 
and 16 is the fourth power of 2. 
    The inverse operation to forming a power is that of extracting a root and 
is expressed by a radical sign. For example, 

             *** SQUARE ROOT [4] = 2     CUBE ROOT [8] = 2 *** 

In N ROOT [a], a is the radicand and n is the index. 
    An equation is a statement that two expressions are equal. The statement 
may be false, for example, x + 2 = x + 1. It may be true for some values of 
the unknown or unknowns but not for all possible values. Such an equation is a 
conditional equation. To solve it is to find the values for which it holds 
true. For example, 3x - 12 = 0 is true for x = 4 only. Or a statement of 
equality may be true for all values of the letters for which the expressions 
have meaning. For example, 3x - 2 - 2x + 6 = x + 4. Such an equation is called 
an identity. 
    Solving conditional equations is the central problem of algebra. 
Identities are used for transforming and simplifying expressions, mainly to 
facilitate solving equations. Operations on algebraic expressions, including 
the factoring of polynomials and the combining of fractions and radicals, 
produce identities that make possible the reduction of conditional equations 
to manageable form. See ANALYTIC GEOMETRY; BINOMIAL THEOREM; EQUATIONS, THEORY 
OF.  
 �34s[Hollis R. Cooley]
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�6v�2v�32sAlgebraic geometry   �21m �33s 
    A study of solutions of systems of polynomial equations fj(x) = 0 in 
several variables xi, thought of as coordinates of a point x = (x1, 
x2, . . ., xn) in n-dimensional space. Classical studies emphasize properties 
of an irreducible algebraic plane curve f(x1, x2) = 0 (f an irreducible 
polynomial), such as singular and multiple points and genus. In modern studies, 
the coefficients in the polynomials fj are chosen from an arbitrary field k, 
and solutions x are sought in an algebraically closed field K containing k. 
The set X of all points x at which all the fj vanish is called an algebraic 
closed set, or variety. If X is not empty, then the set of all polynomials 
f(x) that vanish on X form an ideal with a finite basis in the ring of 
polynomials. Modern algebraic geometry studies these ideals and corresponding 
varieties. See POLYNOMIAL SYSTEMS OF EQUATIONS; RING THEORY. 
 �34s[James S. Frame]
�33s 
  

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�6v�2v�32sAlgorithm   �21m �33s 
    A precise formulation of a method for doing something. In computers, an 
algorithm is usually a collection of procedural steps or instructions 
organized and designed so that computer processing results in the solution of 
a specific problem. 
    Algorithms play an important role in computers. It has been suggested that 
science may be defined as knowledge which is understood well enough to be 
taught to a computer. In this view, the concept of an algorithm or computer 
program furnishes an extremely useful test for the depth of knowledge about 
any particular subject, and the process of going from an art to a science 
involves learning how to construct an algorithm. 
    Algorithms are employed to accomplish specific tasks using data and 
instructions when applying computers. The task may be well definable in either 
mathematical or nonmathematical terms; it may be either logical or heuristic, 
and either simple or complex; and it may be either computational or 
data-processable, or involve sensing and control. In any case, the task must 
be definable. Then an algorithm can be devised and specified for a computer to 
perform the task. 
    Algorithms are further characterized by several properties. Either the 
data set over which the algorithm will operate or the process of how the 
computer is to get access to the data must be specifiable. The process 
required to be performed can be defined with a finite set of operations or 
actions together with a unique starting point. The sequence of steps, the 
tree, the list, or the network describing the process is mappable. This, 
however, does not imply that the path through these steps is known, since in 
some cases the data or prior process steps will dictate the actual path. That 
is, for classical computers it must be possible to ``program'' the algorithm. 
The algorithm process must terminate, either with the task completed or with 
some kind of indication that the task (problem) is unsolvable. 
    Algorithms define the method of operation for performing a task. However, 
for each task to be performed, there usually are many different mappable 
methods for the computer to execute it - but they are not of equal 
desirability. From the above definitions and characterizations it follows that 
any computer program that does its intended task is also an algorithm. But to 
be practical an algorithm must perform its task within the time and memory 
capacity constraints of the system.  See  COMPUTER; DIGITAL COMPUTER. 
 �34s[Earl C. Joseph]
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�6v�2v�32sAlicyclic hydrocarbon �21m �33s
    Organic compounds containing only carbon and hydrogen atoms joined to form
one or more rings (see illustration). Synonyms are cycloaliphatic compounds,
cycloalkanes, or cycloparaffins. Unsaturated alicyclic hydrocarbons are known
as cycloolefins, cycloalkenes, or cycloalkynes, depending on the type of
unsaturation.

			 *** CHEMICAL STRUCTURE ***

    A number of alicyclic hydrocarbons have industrially important uses.
Cyclopropane is an anesthetic. Cyclopentadiene is used in the preparation of
the persistent insecticide chlordane. Cyclohexane is the key intermediate in
the preparation of nylon-6 from cyclohexanone and caprolactam. Adipic acid,
one of the constituents of nylon-66, can be obtained by the oxidation of
cyclohexane. The naturally occurring terpenes, limonene and ALPHA-pinene, are
used in flavors and perfumes and as starting materials in the preparation of
synthetic camphor, which is a cyclic ketone, and racemic menthol, which is a
cyclic alcohol.
    Alicyclic hydrocarbons occur widely in nature. Cyclopentanes and
cyclohexanes are found in petroleum and are called naphthenes. An important
group of alicyclic hydrocarbons are the cyclic terpenes, such as limonene,
derived from citrus fruits, and ALPHA-pinene, the principal constituent of
turpentine, which is the volatile portion of the gum that exudes from
incisions in trunks of living pine trees.
    The alicyclic hydrocarbons boil 18 - 36 DG F (10 - 20 DG C) higher than the
corresponding aliphatic hydrocarbons. A comparison of the relationship between
the chain length and physical properties of alkanes and cycloalkanes suggests
that in rings containing up to 14 carbon atoms, the molecule is roughly
spherical in shape, while in the higher rings the sides become parallel and
the properties approach those of the straight-chain hydrocarbons. Alicyclic
hydrocarbons in general exhibit a stability toward heat and chemical attack
comparable to that of the corresponding open-chain compounds. Only in
cyclopropanes, and in cyclobutanes to a lesser extent, is there any marked
ring instability.
    Alicyclic hydrocarbons are generally synthesized by three methods: (1)
ring expansions, (2) ring closures, and (3) ring contractions, each followed
by removal of any existing functional group. See ADAMANTANE; ALIPHATIC
HYDROCARBON; ALKENE; AROMATIC HYDROCARBON; HYDROCARBON; TROPOLONE.

 �34s[Paul E. Fanta]
�33s


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�6v�2v�32sAlidade   �21m �33s 
    An instrument for topographic surveying and mapping by the plane-table 
method; or the part of the transit which rotates with the telescope about the 
horizontal circle; or any sighting device of pointer employed for angular 
measurement. The surveying alidade has a telescope, with attached graduated 
vertical circle, mounted on a flat base that can be moved about the plane 
table. The base embodies a straightedge which itself may be a graduated scale 
for readily plotting the point being sighted (see illustration). 
    The stadia technique is usually applied in measuring distances and 
elevation differences between the plane-table point and observed points. The 
direction to any point is established by sighting the point through the 
telescope. A direction line is then drawn along the straightedge, and its 
distance is laid off with the scale See PLANE TABLE; STADIA; SURVEYING; 
TRANSIT (ENGINEERING). 
    Figure caption: An alidade. (Kern Instruments, Inc.)
 �34s[B. Austin Barry]
�33s 
  

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�6v�2v�32sAliphatic hydrocarbon �21m �33s
    One of a group of hydrocarbons in which the carbon atoms are joined in
open chains. Two major classes are the saturated and the unsaturated, the
latter including several homologous series depending on the number of double
and triple bonds present.
    The systematic names adopted by the International Union of Pure and
Applied Chemistry for the aliphatic hydrocarbons are formed by adding a suffix
(-ane, -ene, -yne, -adiene, and so forth) indicating the type of compound to a
prefix indicating the number of carbon atoms present. The first four prefixes
are meth-, eth-, prop-, and but-. The succeeding prefixes are derived from the
Greek or Latin word for the number: pent-, hex-, hept-, oct-, and so forth
(see table).
    The saturated compounds, which are known as paraffin hydrocarbons, or more
systematically, as alkanes, fit the empirical formula CnH2n+2. The
members having four or more carbon atoms exist in straight-chain (or normal)
and branched-chain isomers. Not counting optical isomers, there are 2 butanes
with different carbon skeletons, 3 pentanes, 5 hexanes, 9 heptanes, 18
octanes, and 35 nonanes, all of which are known.
    Compounds containing one double bond and having the formula CnH2n
are olefins (or alkenes). The first two members of the series, ethylene and
propene, both exist in one form only. The next higher homolog, C4H8, has two
straight-chain isomers (1-butene and 2-butene) and one branched-chain isomer
(isobutylene or methylpropene, sometimes incorrectly called isobutene). There
are two geometrical isomers of 2-butene, namely, cis-2-butene and
trans-2-butene. As the length of the chain increases in both the
straight-chain and the branched-chain olefins, the number of isomers formed by
a change in the position of the double bond increases rapidly, as shown in the
figure. See MOLECULAR ISOMERISM.

			*** CHEMICAL STRUCTURE ***

    There are two homologous series of aliphatic hydrocarbons having the
formula CnH2n-2: (1) acetylenes (or alkynes), which contain one triple
bond, and (2) diolefins (or alkadienes), which contain two double bonds.
Aliphatic hydrocarbons having three double bonds per molecule are termed
triolefins or alkatrienes; those containing a larger number of double bonds
are named in analogous fashion. Similarly, the open-chain hydrocarbons
containing two triple bonds have the generic names alkadiynes, while those
groups of hydrocarbons containing both double and triple bonds are identified
by names such as alkenynes, alkadienynes, or alkenediynes. See ALICYCLIC
HYDROCARBON; ALKANE; HYDROCARBON.


 �34s[Louis Schmerling]
�33s



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�6v�2v�32sAlismatales   �21m �33s 
    A small order of flowering plants, division Magnoliophyta (Angiospermae), 
which gives its name to the subclass Alismatidae of the class Liliopsida 
(monocotyledons). It consists of three families (Alismataceae, Butomaceae, and 
Limnocharitaceae) and less than a hundred species. They are aquatic and 
semiaquatic herbs with a well-developed, biseriate perianth that is usually 
differentiated into three sepals and three petals, and with a gynoecium of 
several or many, more or less separate carpels. Each flower is usually 
subtended by a bract. Butomus umbellatus (flowering rush) and species of 
Sagittaria (arrowhead, family Alismataceae) of this order are sometimes 
cultivated as ornamentals. See LILIOPSIDA; MAGNOLIOPHYTA. 
    The Alismatales and some related orders have often been treated as a 
single order Helobiae or Helobiales, embracing most of what is here treated as 
the subclass Alismatidae. See ALISMATIDAE; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sAlismatidae   �21m �33s 
    A relatively primitive subclass of the class Liliopsida (monocotyledons) 
of the division Magnoliophyta (Angiospermae), the flowering plants, consisting 
of 4 orders, 14 families, and scarcely 500 species. Typically they are aquatic 
or semiaquatic, with apocarpous flowers and nonendospermous seeds. They have 
trinucleate pollen, and the stomates usually have two subsidiary cells. The 
orders Alismatales, Hydrocharitales, and Najadales are closely related among 
themselves and have often been treated as a single order, Helobiae or 
Helobiales. See ALISMATALES; HYDROCHARITALES; LILIOPSIDA; MAGNOLIOPHYTA; 
NAJADALES; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sAlkali   �21m �33s 
    Broadly, any compound having highly basic properties, that is, a compound 
that readily ionizes in aqueous solution to yield hydroxyl ions so that the pH 
is high (above 7) or so that litmus paper changes from red to blue. 
Specifically, an alkali is a hydroxide of one of the alkali metals, although 
an ammonium salt may also be referred to as an alkali. Alkalies are caustic in 
that they produce a relatively high concentration of OH ions in solution. 
Alkalies are soluble in water, tarnish in air, and in concentrated form are 
corrosive to the touch. 
    Alkalies are used commercially in soap manufacture, soluble oils, cutting 
compounds, cleaning solutions, and aluminum etching. In commerce, alkali 
usually refers to sodium carbonate in the form of soda ash, either from 
natural deposits or manufactured from salt and either sulfuric acid or 
ammonium bicarbonate. Other alkali products include lye, potash, caustic soda, 
potassium hydroxide, water glass, and bicarbonate of soda. See ACID AND BASE; 
ALKALI METALS: ALKALINE-EARTH METALS; HYDROXIDE; SODIUM; SOIL; WATER SOFTENING. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sAlkali emissions   �21m �33s 
    The alkali metals lithium, potassium, and especially sodium can be 
observed in twilight by the scattering of their resonance lines. Analysis has 
revealed that the free atoms are concentrated in a layer 50 - 60 mi (90 - 100 
km) above the Earth's surface. The existence of free atoms can be explained by 
reactions of the type NaO+O->Na+O2. Thus, the alkali layer coincides with the 
region where oxygen is dissociated. The most likely source of the material may 
be vaporized meteoric material, but for sodium and perhaps potassium a 
contribution may also be made by marine salt particles which have been lifted 
to mesopheric heights. See MESOPHERE. 
    The sodium abundance (integrated density) has a marked seasonal variation 
which is also a function of latitude, reversing in the Southern Hemisphere. 
Sodium dayglow emission has been observed both from the ground and from 
rockets. The sodium distribution is often observed to be remarkably sharp, 
with scale height much less than that of the atmosphere. Natural lithium 
emission can usually be observed by very sensitive equipment, but many 
artificial enhancements have been measured as well. The behaviors of lithium 
and sodium are strikingly different, and it seems likely that the lithium 
enhancements are related to meteoric influx; perhaps certain dust showers are 
responsible. Or it may be that lithium atoms condense almost irreversibly on 
the dust particles, whereas the sodium is easily reevaporated. Readily 
observed with sensitive equipment, potassium emission seems to have about the 
same intensity at high and low latitudes. See ATMOSPHERE; ATMOSPHERIC 
CHEMISTRY.  
 �34s[Donald M. Hunten]
�33s 
  

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�6v�2v�32sAlkali metals   �21m �33s 
    The elements of group Ia in the periodic table (lithium, sodium, 
potassium, rubidium, cesium, francium). Of the alkali metals, lithium differs 
most from the rest of the group, and tends to resemble the alkaline-earth 
metals (group IIa of the periodic table) in many ways. In this respect lithium 
behaves as do many other elements that are the first members of groups in the 
periodic table; these tend to resemble the elements in the group to the right 
rather than those in the same group. Francium, the heaviest of the 
alkali-metal elements, has no stable isotopes and exists only in radioactive 
form.  
    In general, the alkali metals are soft, low-melting, reactive metals. This 
reactivity accounts for the fact that they are never found uncombined in 
nature but are always in chemical combination with other elements. This 
reactivity also accounts for the fact that they have no utility as structural 
metals (with the possible exception of lithium in alloys) and that they are 
used as chemical reactants in industry rather than as metals in the usual 
sense. The reactivity in the alkali-metal series increases in general with 
increase in atomic weight from lithium to cesium. See CESIUM; ELECTROCHEMICAL 
SERIES; FRANCIUM; LITHIUM; PERIODIC TABLE; POTASSIUM; RUBIDIUM; SODIUM. 
 �34s[Marshall Sittig]
�33s 
  
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�6v�2v�32sAlkaline-earth metals   �21m �33s 
    Usually calcium, strontium, and barium, the heaviest members of group IIa 
of the periodic table (excepting radium). Other members of the group are 
beryllium, magnesium, and radium, sometimes included among the alkaline-earth 
metals. Beryllium resembles aluminum more than any other element, and 
magnesium behaves more like zinc and cadmium. The gap between beryllium and 
magnesium and the remainder of the elements of group IIa makes it desirable to 
discuss these elements separately. Radium is often treated separately because 
of its radioactivity. 
    The alkaline earths form a closely related group of highly metallic 
elements in which there is a regular gradation of properties. The metals, none 
of which occurs free in nature, are all harder than potassium or sodium, 
softer than magnesium or beryllium, and about as hard as lead. The metals are 
somewhat brittle, but are malleable, extrudable, and machinable. They conduct 
electricity well; the specific conductivity of calcium is 45% of that of 
silver. The oxidation potentials of the triad are as great as those of the 
alkali metals. 
    The elements and their compounds find important industrial uses in 
low-melting alloys, deoxidizers, and drying agents and as cheap sources of 
alkalinity. See BARIUM; BERYLLIUM; CALCIUM; MAGNESIUM; PERIODIC TABLE; RADIUM; 
STRONTIUM.  
 �34s[Reed F. Riley]
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�6v�2v�32sAlkaloid   �21m �33s 
    One of a group of nitrogenous bases of plant origin, such as nicotine, 
cocaine, and morphine. Most of the alkaloids show marked physiological 
activity, and crude extracts of various alkaloid-bearing plants have been used 
since antiquity because of their curative or poisonous effects. Well over 1000 
alkaloids have since been isolated or have been shown to be present in some 97 
families of plants. Alkaloid-bearing plants have been found in virtually every 
habitat in which vascular plants grow. 
    Alkaloids are found most frequently in the higher seed-bearing plants, and 
especially in dicotyledons. As more plants are examined, more families are 
included in the alkaloid-bearing group, for new genera containing alkaloids 
are reported regularly. The elaboration of alkaloids in plants is not 
localized but appears to be a characteristic of all organs, including the 
seeds. However, not all organs of any one species must have alkaloids. When 
plants elaborate more than one alkaloid, their ratio in the plant is not 
necessarily the same in all stages of growth. Some alkaloids are virtually 
absent in the young plant, but increase to isolable amounts as the plant 
approaches maturity. Furthermore, cultural and climatic conditions exert an 
effect on the alkaloid content of plants. 
    The nomenclature of the individual alkaloids has not been systematized, 
and they draw their names from a variety of sources. A great many important 
alkaloids have received names derived from those of plants, such as 
papaverine, quinine, and berberine. A few are named from their physiological 
action, such as morphine and emetine. Some are named from their physical 
characteristics, such as hygrine. Only one, pelletierine, has been named for 
an alkaloid chemist. 
    The alkaloids are usually crystalline, colorless substances; only a few 
are liquid. Colored alkaloids are rare. Almost all the known 
nitrogen-containing ring systems, both saturated and unsaturated, are 
encountered among the alkaloids. The molecules vary widely in complexity, 
ranging from the relatively simple systems such as nicotine to the very 
complex polycyclic structures of morphine and strychnine. 
    Many alkaloids have great value in medical practice because of specific 
pharmacological actions. The isolation of pure individual alkaloids on a large 
scale is sometimes prohibitively expensive. Furthermore, the synthesis of many 
of the more useful alkaloids is frequently impractical. Therefore considerable 
effort has been spent on the synthesis of related substitute compounds. In the 
treatment of malaria, quinine has been largely replaced by synthetic 
compounds. Synthetic procaine and similar drugs have supplanted naturally 
occurring cocaine in local anesthesia. See ATROPINE; COCAINE; MORPHINE; 
NICOTINE; QUININE; STRYCHNINE. 
 �34s[S. Morris Kupchan]
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�6v�2v�32sAlkane �21m �33s
    A member of the series of saturated aliphatic hydrocarbons having the
empirical formula CnH2n+2. The members of this series are also called
paraffinic hydrocarbons or simply paraffins. Alkanes are relatively unreactive
compared to unsaturated and aromatic hydrocarbons, and are little used in
laboratory synthesis. On the other hand, reactions of alkanes are carried out
on a huge scale industrially to convert the hydrocarbons found in natural gas
and petroleum to the products needed in modern life.
    The alkanes are usually named in accordance with the rules of the
International Union of Pure and Applied Chemistry (IUPAC) which adopted the
ending ``-ane'' for saturated hydrocarbons. Branched-chain paraffins are named
as derivatives of the longest straight chain in the compound, the location and
name of the alkyl radicals in the branches being indicated as prefixes. The
carbon atoms in the longest, or parent, chain are numbered so that the
positions of the branches have the lower of the two possible values. By this
system the two isomeric heptanes having a single branch containing a single
carbon atom are named 2-methylhexane and 3-methylhexane, as shown in formulas
I - III.

			 CH3CH2CH2CH2CH2CH2CH3
			  Heptane or n-heptane	       (I)

			  CH3CHCH2CH2CH2CH3
			     |
			     CH3
			     2-Methylhexane
			     or isoheptane	      (II)

			   CH3CH2CHCH2CH2CH3
				 |
				 CH3
			     3-Methylhexane
			     (no trivial name)	     (III)

    In an alternative, trivial system of naming alkanes, the straight-chain
compound is designated with the prefix ``n-'' (an abreviation for normal)
and the isomer having a methyl branch on the second carbon atom is
given the prefix ``iso-'' (for instance, isobutane, isopentane, isohexane). A
selected list of n-alkanes is shown in the table. In this connection it should
be noted that, although the name isooctane correctly designates
2-methylheptane, it should be avoided because of the unfortunate use of the
misnomer ``isooctane'' in the petroleum industry to represent
2,2,4-trimethylpentane.
    The prefix ``neo-'' indicates the gem-dimethyl isomer (two methyl branches
on the same carbon atom), but can be used unambiguously only in naming
neopentane (dimethylpropane) and neohexane (2,2-dimethylbutane).
    For convenience in discussion, the symbol R is used to represent the alkyl
group, and X represents a halogen. Thus RH is the general representation of
any alkane, and RX is an alkyl halide.
    Alkanes containing fewer than 5 carbon atoms are gases at room temperature
and atmospheric pressure. The pentanes through the hexadecanes are mostly
liquids ( with some exceptions). Straight-chain alkanes having more than 16
carbon atoms are waxy solids. The boiling points and the melting points of the
normal paraffins increase with increasing molecular weight, and the boiling
point decreases with increased branching within a family of isomeric alkanes.
The alkanes have lower refractive indices and densities than do the other
types of hydrocarbons having the same number of carbon atoms.
    The alkanes are soluble in many organic solvents such as alcohol and
ether. They are practically insoluble in water.
    Natural gas and petroleum contain literally hundreds of alkanes, both
straight-chain and branched, and a number of pure alkanes can be isolated from
petroleum by careful fractionation. The number of possible isomers increases
rapidly with the number of carbon atoms. Beyond the nonanes and decanes,
relatively few branched isomers have been isolated from natural sources or
prepared synthetically and characterized, since there has been no particular
reason for doing so.
    A number of laboratory procedures are available for the preparation of
those alkanes which are not obtainable in pure form from natural sources. The
classical Wurtz reaction often gives poor yields and is of limited utility.
The Kolbe synthesis, consisting of the anodic oxidation of a carboxylic acid,
is useful for the synthesis of symmetrical alkanes having an even number of
carbon atoms. Branched-chain alkanes may be synthesized by hydrogenation of
the corresponding alkene. See KOLBE HYDROCARBON SYNTHESIS. The petroleum
industry has been primarily responsible for the development of reactions for
the conversion of the hydrocarbons naturally present in petroleum to other,
more useful alkanes. These include alkylation, isomerization, and
Fischer-Tropsch synthesis. See FISCHER-TROPSCH PROCESS.
    The alkanes, particularly the straight-chain isomers, are not readily
affected at mild temperature conditions by acids and oxidizing agents, such as
sulfuric acid, nitric acid, and potassium permanganate. Because they are
saturated hydrocarbons, they react chiefly by substitution of other atoms or
groups of atoms for hydrogen atoms. They also undergo reactions involving
scission, or splitting, of carbon-carbon or carbon-hydrogen bonds. See
ALIPHATIC HYDROCARBON; ALKYLATION; ALKYLATION (PETROLEUM); CRACKING; ETHANE;
HALOGENATION; HYDROGENATION; ISOMERIZATION; METHANE; NITRATION; OCTANE;
PETROLEUM PROCESSING; PROPANE.

 �34s[Paul E. Fanta]
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�6v�2v�32sAlkene   �21m �33s 
    One of the class of acyclic hydrocarbons containing one or more 
carbon-to-carbon double bonds. Alkenes (also called olefins) and alkynes (also 
called acetylenes) together constitute the family of organic compounds called 
unsaturated hydrocarbons, since they contain less than the number of hydrogens 
found in the corresponding saturated compound, alkane. When the double bond is 
present in a nonaromatic ring (alicyclic hydrocarbon), the compound is termed 
a cycloalkene. Hydrocarbons containing more than one double bond are termed 
dienes, trienes, and so forth, or collectively, polyenes. See ALICYCLIC 
HYDROCARBON; ALKANE; ALKYNE. 
    In naming alkenes by the system of the International Union of Pure and 
Applied Chemistry (IUPAC), the longest chain containing the double bond is 
identified. The presence of the double bond is indicated by changing the 
``-ane'' ending of the alkane having the same number of carbon atoms to 
``-ene,'' and the position of the double bond is indicated by a prefixed 
number. Examples are given in the table, with common or nonsystematic names 
which are still frequently used given in parentheses. 
    The lower alkenes and dienes which have up to five carbon atoms are gases 
at room temperature and pressure. Higher alkenes are colorless liquids or 
solids. Like other hydrocarbons, alkenes are insoluble in water. Liquid 
alkenes have specific gravities well below 1.0. Alkenes may undergo 
polymerization, cyclization, and addition reactions. A major share of 
structural and elastic polymers are based on homopolymers or copolymers of 
alkenes and dienes. Alkenes and dienes cyclize readily under various 
conditions.  
    Addition reactions of alkenes are among the most important in the entire 
field of organic chemistry. Industrially, high-octane gasoline is made by the 
acid-catalyzed alkylation of the three- and four-carbon alkenes. A variety of 
alkylated aromatics are made by the alkylation of benzene with olefins. 
    The commercially important alkenes are produced on a large scale in the 
petroleum industry by thermal or catalytic cracking processes. In the 
laboratory, most methods for the preparation of alkenes involve some type of 
elimination reac Alkenes and dienes (common name given in parentheses). are 
removed with concomitant formation of the carbon-carbon double bond.  See 
ALIPHATIC HYDROCARBON; ALKYLATION; ALKYLATION (PETROLEUM); CRACKING; 
HALOGENATION; HYDROGENATION. 
    Table title: Alkenes and dienes (common name given in parentheses).    
 �34s[Paul E. Fanta]
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�6v�2v�32sAlkylation   �21m �33s 
    A chemical process in which an alkyl radical (CnH2n+1) is introduced 
into an organic compound by substitution or addition. By the use of alkylating 
agents, such as olefins, alcohols, or alkyl halides, alkyl groups are bonded 
together or to other compounds through carbon, oxygen, nitrogen, sulfur, or 
metals. Though the general terms addition and substitution cover all 
alkylation processes, the detailed reactions can be quite dissimilar. Some 
reactions classified as alkylations are the Williamson ether synthesis, the 
Friedel-Crafts reaction, and the Wurtz reaction. 
    Many important chemicals are made by alkylation, some being used directly, 
others as intermediates, for the manufacture of other chemical compounds. 
Ethylbenzene, for example, is dehydrogenated to make styrene, important in 
rubber and plastic manufacture. Ethyl ether is the most widely known 
anesthetic. Tetraethyllead is an important antiknock additive for gasoline. 
Many other applications could be mentioned, including some of the synthetic 
detergents (the alkylaryl sulfonates). See ALKYLATION (PETROLEUM); 
FRIEDEL-CRAFTS REACTION; ORGANIC REACTION MECHANISM; PETROLEUM PROCESSING; 
SUBSTITUTION REACTION. 
 �34s[Harold C. Ries]
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�6v�2v�32sAlkylation (petroleum) �21m �33s
    As applied to the petroleum refining industry, the reaction of
isoparaffins with olefins to produce higher-boiling isoparaffinic compounds.
Specifically, C3, C4, and C5 olefins are reacted with isobutane to form
alkylates which are important components of fuels for internal combustion
engines and jet aircraft engines. The desirable properties of alkylate include
high octane number, excellent stability, high heat of combustion, and low
vapor pressure. The illustration shows the alkylation process in a typical
plant, including the reaction, isoparaffin separation and recycling, and
product fractionation. Cooling is employed because the reaction is highly
exothermic.
    The effort to change the fuel used in automobiles in the United States to
gasoline without the tetraethyllead antiknock additive may change the
application of the alkylation process. Although the unleaded octane number of
alkylate is higher than that of many other gasoline blending stocks, the
refining industry has depended upon the high susceptibility of alkylates to
the lead additive to raise the octane number of the final gasoline product.
    Successful energy savings in the process have been concentrated at the
isobutane fractionation step. A less pure theoretical separation here results
in significant operating cost reductions with little influence on product
quality. See ALKANE; ALKYLATION; ISOMERIZATION; PETROLEUM PROCESSING.

 �34s[Robert S. Newsham]
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�6v�2v�32sAlkyne   �21m �33s 
    One of a group of organic compounds containing a carbon-to-carbon 
triple-bond linkage (--C TRIPLE BOND C--). They are termed acetylenes or 
alkynes. While exhibiting many of the characteristics of alkenes as regards 
unsaturation, the acetylenes have many unique properties. Since the bonding in 
alkyne molecules is linear, R--C TRIPLE BOND C--R, cis-trans isomerism is not 
possible. In the simplest alkyne, acetylene (HC TRIPLE BOND CH), or in 
monosubstituted acetylenes, the hydrogen attached to triply bonded carbon is 
acidic to such a degree that it is replaceable with metals such as sodium. 
Structural formulas of several alkynes are as follows: 

               CH TRIPLE BOND CH 
               Ethyne or acetylene  

               CH TRIPLE BOND CH2--CH2--CH3
               1-Butyne

               CH TRIPLE BOND C--CH3
               Propyne

               CH3--C TRIPLE BOND C--CH3
               2-Butyne

    General methods for the preparation of alkynes depend on 
dehydrohalogenation of ALPHA, BETA-dihaloparaffins, conversion of aldehydes or 
ketones to dihaloparaffins with subsequent dehydrohalogenation, and alkylation 
of metallic acetylides with alkyl halides in liquid ammonia. Grignard reagents 
of 1-alkynes behave similarly. 
    The reactions of triply bonded carbon compounds are in general similar to 
those of compounds containing ethylenic bonds. Addition reactions proceed in 
two stages to form first a vinyl compound or substituted ethylene, and second 
the substituted paraffin. In the presence of catalytic quantities of 
alkoxides, acetylene adds to alcohols and phenols to give vinyl ethers. 
Reactions of this type, in which addition takes place by replacement of 
hydrogen with a vinyl group, are termed vinylation. Another form of addition 
reaction known as ethynylation involves the addition of acetylene to 
unsaturated compounds. A general reaction of alkynes involves the addition of 
carbon monoxide and water, or other compounds having an active hydrogen, in 
the presence of nickel carbonyl. 
    Polymerization of alkynes may yield acyclic, aromatic, or alicyclic 
derivatives. In the presence of cuprous chloride, acetylene dimerizes to vinyl 
acetylene, which adds hydrogen chloride to give chloroprene 
(2-chloro-1, 3-butadiene). Polymerization of chloroprene in the presence of 
free radical initiators gives the commercially important synthetic rubber, 
neoprene. Thermal polymerization of acetylene yields benzene as the major 
product and also a wide variety of polynuclear aromatic compounds. Of great 
theoretical and practical importance is the polymerization of acetylene to the 
cyclic tetramer, cyclooctatetraene. See ACETYLENE; ALKENE. 
 �34s[Charles A. Cohen/Paul E. Fan]
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�6v�2v�32sAllanite   �21m �33s 
    A mineral, also known as orthite, distinguished from other members of the 
epidote group of silicates by a relatively high content of rare earths, 
chiefly cerium, lanthanum, and yttrium. Chemical composition is 
(Ca,Ce,La,Y)2 (Al,Fe)3 Si3O12(OH). Other lanthanide elements are generally 
present, as is Be in some varieties. Small amounts of thorium and uranium are 
often present, and the mineral then may be metamict. Allanite is monoclinic in 
crystallization. The color is black or brownish-black, and the specific 
gravity ranges from 3.4 to 4.2. See CERIUM; EPIDOTE; METAMICT STATE; 
RADIOACTIVE MINERALS; RARE-EARTH ELEMENTS. 
 �34s[Clifford Frondel]
�33s 
  

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�6v�2v�32sAllantois   �21m �33s 
    A fluid-filled sac- or sausagelike, extraembryonic membrane lying between 
the outer chorion and the inner amnion and yolk sac of the embryos of 
reptiles, birds, and mammals. It is composed of an inner layer of endoderm 
cells, continuous with the endoderm of the embryonic gut, or digestive tract, 
and an outer layer of mesoderm, continuous with the splanchnic mesoderm of the 
embryo. It arises as an outpouching of the ventral floor of the hindgut. The 
allantois remains connected to the hindgut by a narrower allantoic stalk which 
runs through the umbilical cord. 
    The allantois eventually fuses with the overlying chorion to form the 
compound chorioallantois, which lies just below the shell membranes in 
reptiles and birds. The chorioallantois is supplied with an extensive network 
of blood vessels and serves as an important respiratory and excretory organ 
for gaseous interchange. The allantoic cavity also serves as a reservoir for 
kidney wastes in some mammals, in reptiles, and in birds. In the latter two 
groups the allantois assists in absorption of albumin. In some mammals, 
including humans, the allantois is vestigial and may regress, yet the 
homologous blood vessels persist as the important umbilical arteries and veins 
connecting the embryo with the placenta. See FETAL MEMBRANE; GERM LAYERS. 
 �34s[Nelson T. Spratt, Jr.]
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�6v�2v�32sAllele   �21m �33s 
    Any of the different forms of a gene occupying a particular locus on a 
chromosome. Alleles are symbolized by one or more letters, often with 
appropriate superscripts; symbols for dominant alleles begin with capital 
letters. Thus, w+, w, and w sup cho represent alleles which respectively 
cause the eye color of Drosophila to be red, white, or chocolate. 
    The allele most commonly found in a species is called the wild-type 
allele, designated by the superscript +. A diploid individual may carry two 
identical or two different alleles of a gene. With a combination of a wild 
type and a mutant allele, the organism often has a normal phenotype. The 
wild-type allele is then said to be dominant and the mutant allele recessive. 
Sometimes mutant alleles are dominant or partially dominant. 
    Mutant alleles differ from wild type in one or more nucleotide residues. 
New alleles arise by substitution, addition, deletion, or rearrangement of 
nucleotides in the wild-type sequence. An average gene contains several 
hundred nucleotides, and alleles may carry mutational alterations at different 
sites (heteroalleles) or the same site (isoalleles). In cells containing 
allelic mutations at nonidentical sites, recombination occasionally gives rise 
to wild-type alleles. Recombination between alleles can be used to analyze the 
fine structure of a gene. See MUTATION. 
    The function of an allele may be examined at the level of its primary 
product or through its effect on the organism's phenotype. Geneticists study 
interactions between different alleles in order to gain information about gene 
activity. Alleles that lack gene activity are called amorphic, while those 
with reduced levels of activity are called hypomorphic. See GENE ACTION. 
    In microorganisms, functionally related genes are often situated on the 
chromosome in clusters called operons, the genes of which function in a 
coordinated fashion. Certain mutations of one gene can also block the 
expression of other genes in the operon. These mutant alleles fail to 
complement mutations in other genes, which must be considered nonallelic. 
Clustering of functionally related genes also occurs in higher organisms, 
where such loci have been described as complex loci. Lack of complementation 
between apparently different genes has been called pseudoallelism. The 
distinction between allelism and pseudoallelism is not always easy to make. 
See GENETICS; OPERON. 
 �34s[O. Siddiqi]
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�6v�2v�32sAllelopathy   �21m �33s 
    The harmful effect of one plant or microorganism on another caused by the 
release of secondary metabolic products into the environment. The effect is 
usually visible as a partial or complete inhibition of growth or reduction of 
seed germination. The donor plant releases biochemical toxins into the soil, 
water, or atmosphere, from which they are absorbed by receptor plants and act 
as toxins. Such inhibitions can influence species composition of a plant 
community, ecological succession, and plant productivity. The extent of these 
influences varies with strength of the allelopathic reaction. If the 
inhibition is not severe, the effect may be only a slight loss in 
productivity; stronger reactions may eliminate a species from the plant 
association.  
    Inhibition of one species by another is direct allelopathy. Indirect 
allelopathy is the inhibition of an intermediate organism - often a bacterium, 
alga, or fungus - upon which the inhibited plant is dependent for nutrients or 
water. Autoallelopathy is the inhibition of a species by self-produced toxins. 
    Allelopathy must be distinguished from competition, which involves the 
removal or reduction of some vital factor - nutrient, moisture, light, space - 
from the environment by competing species. It must also be distinguished from 
pollution, which is a general accumulation from a variety of sources of 
organic or inorganic substances deleterious to many forms of life. Allelopathy 
is more specific inhibition caused by the metabolic by-products of a 
particular plant species under conditions which do not necessarily involve 
crowding, competition, or stagnation. 
 �34s[Isabel F. Ahlgren]
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�6v�2v�32sAlligator �21m �33s
    Large aquatic reptile of the family Alligatoridae. Common usage generally
restricts the name to the two living species of the genus Alligator. The
American alligator (A. mississippiensis) ranges throughout the southeastern
United States from coastal North Carolina (southeastern Virginia in historical
times) to the Rio Grande in Texas, and north into southeastern Oklahoma and
southern Arkansas (see illustration).
    The second species is the Chinese alligator (A. sinensis), restricted to
the region of the Yangtze River valley in China, where it inhabits burrows in
the floodplains and riverbanks. It is also an endangered species and is now
protected in China.
    The American alligator is by far the larger of the two species, reaching a
length in excess of 15 ft (4.5 m). The average length of A. sinensis is 4 - 5
ft (1.2 - 1.5 m). See REPTILIA.

 �34s[Howard W. Campbell]
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�6v�2v�32sAlloeocoela   �21m �33s 
    An order of the Turbellaria, mainly marine but with some fresh-water and 
terrestrial species. They possess a well-defined gut, with a simple, variable 
(modified doliiform) or plicate pharynx and a diverticulated intestine. The 
nervous system has three or four pairs of longitudinal nerves, with connecting 
commissures. The ovary is paired. The testes are follicular, and a 
well-defined penis papilla is present. There is a pair of protonephridia, 
often with two or three main branches to each and with a number of 
nephridiopores. Anterior ciliated pits or grooves are common, eyes and 
statocysts are less so, but present in some. 
    Alloeocoels are generally 0.04 - 0.40 in. (1 - 10 mm) in length and 
cylindrical in shape, but sometimes plump or elongate. They are generally 
grayish white to brown, but may be more brightly colored due to gut contents 
or yellow to orange lipid reserves. They occur in the littoral zone among 
algae, on sandy or muddy bottoms, and occasionally in clear rock pools. See 
TURBELLARIA.  
 �34s[J. B. Jennings]
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�6v�2v�32sAllosteric enzyme   �21m �33s 
    Any one of the special bacterial enzymes involved in regulatory functions. 
End-product inhibition is a bacterial control mechanism whereby the end 
product of a biosynthetic pathway can react with the first enzyme of the 
pathway and prevent its activity. This end-product inhibition is a device 
through which a cell conserves its economy by shutting off the synthesis of 
building blocks when too many are present. 
    A model to explain the action of allosteric enzymes suggests that the 
enzyme molecule is a complex consisting of identical subunits, each of which 
has a site for binding the substrate and another for binding the regulatory 
substance. These subunits interact in such a way that two conformational forms 
may develop. One form is in a relaxed condition (R state) and has affinity for 
the substrate and activator; the other form is in a constrained or taut 
condition (T state) and has affinity for the inhibitor. The forms exist in a 
state of equilibrium, but this balance can be readily tipped by binding one of 
the reactants. The substrate and activator are bound by the relaxed form; when 
this happens, the balance is tipped in favor of that state. Conversely, the 
inhibitor will throw the balance toward the constrained state. The balance is 
thus tipped one way or the other, depending on the relative concentrations of 
substrate and inhibitor. Since the two states require subunit interaction for 
their maintenance, it can be seen why dissociation of the subunits leads to a 
simple monomeric enzyme which no longer exhibits allosteric effects. The model 
also shows how the binding sites may interact in either a cooperative or 
antagonistic manner. See ENZYME. 
 �34s[Joseph S. Gots]
�33s 
  
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�6v�2v�32sAllotheria   �21m �33s 
    One of the four subclasses of Mammalia, containing a single order, the 
Multituberculata. The Allotheria first appeared in the Late Jurassic and 
survived well into the Cenozoic, a period of at least 100,000,000 years. 
Fossils are known from North America, Europe, and Asia. 
    The diagnostic features of the subclass are in the dentition. There is a 
pair of enlarged incisors above and below; reduced lateral incisors may 
persist in the upper jaw. Canines are absent, leaving a diastema between 
incisors and cheek teeth. Premolars are variable and often reduced. The lower 
molars have five or more cusps in two parallel longitudinal rows, and the 
upper molars have two or three parallel rows of cusps; hence the molars are 
multituberculate. See DENTITION; MAMMALIA; MULTITUBERCULATA. 
 �34s[D. Dwight Davis/Frederick S. Szalay]
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�6v�2v�32sAllowance   �21m �33s 
    An intentional difference in sizes of two mating parts. With running or 
sliding fits, allowance is a clearance, usually for a film of oil. In this 
sense, allowance is the space ``allowed'' for motion between parts. 
    With force or shrink fits allowance is an interference of metal; that is, 
a portion of metal in one part tends to occupy the same space as the adjacent 
portion of metal in the mating part. In this sense, allowance is the 
interference ``allowed'' to produce pressure between parts. See FORCE FIT; 
LOCATION FIT; PRESS FIT; RUNNING FIT; SHRINK FIT; TOLERANCE. 
 �34s[Paul H. Black]
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�6v�2v�32sAlloy   �21m �33s 
    A metal product containing two or more elements (1) as a solid solution, 
(2) as an intermetallic compound, or (3) as a mixture of metallic phases. 
Alloys are frequently described on the basis of their technical applications. 
They may also be categorized and described on the basis of compositional 
groups. For example, see BERYLLIUM ALLOYS; COPPER ALLOYS; IRON ALLOYS. 
    Except for native copper and gold, the first metals of technological 
importance were alloys. Bronze, an alloy of copper and tin, is appreciably 
harder than copper. This quality made bronze so important an alloy that it 
left a permanent imprint on the civilization of several millennia ago now 
known as the Bronze Age. Today the tens of thousands of alloys involve almost 
every metallic element of the periodic table. 
    Alloys are used because they have specific properties or production 
characteristics that are more attractive than those of the pure, elemental 
metals. For example, some alloys possess high strength; others have low 
melting points; others are refractory with high melting temperatures; some are 
especially resistant to corrosion; and others have desirable magnetic, 
thermal, or electrical properties. These characteristics arise from both the 
internal and the electronic structure of the alloy. An alloy is usually harder 
than a pure metal and may have a much lower conductivity. 
    Bearing alloys are used for metals that encounter sliding contact under 
pressure with another surface; the steel of a rotating shaft is a common 
example. Most bearing alloys contain particles of a hard intermetallic 
compound that resist wear. These particles, however, are embedded in a matrix 
of softer material which adjusts to the hard particles so that the shaft is 
uniformly loaded over the total surface. The most familiar bearing alloy is 
babbitt. Bearings made by powder metallurgy techniques are widely used because 
they permit the combination of materials which are incompatible as liquids, 
for example, bronze and graphite, and also permit controlled porosity within 
the bearings so that they can be saturated with oil before being used, the 
so-called oilless bearings. See ANTIFRICTION BEARING; WEAR. 
    Certain alloys resist corrosion because they are noble metals. Among these 
alloys are the precious-metal alloys. Other alloys resist corrosion because a 
protective film develops on the metal surface. This passive film is an oxide 
which separates the metal from the corrosive environment. Stainless steels and 
aluminum alloys exemplify metals with this type of protection. The bronzes, 
alloys of copper and tin, also may be considered to be corrosion-resisting. 
See CORROSION; STAINLESS STEEL. 
    Dental alloys contain precious metals. Amalgams are predominantly 
silver-mercury alloys, but they may contain minor amounts of tin, copper, and 
zinc for hardening purposes. Liquid mercury is added to a powder of a 
precursor alloy of the other metals. After being compacted, the mercury 
diffuses into the silver-base metal to give a completely solid alloy. 
Gold-base dental alloys are preferred over pure gold because gold is 
relatively soft. The most common dental gold alloy contains gold, silver, and 
copper. For higher strengths and hardnesses, palladium and platinum are added, 
and the copper and silver are increased so that the gold content drops. 
Vitallium and other corrosion-resistant alloys are used for bridgework and 
special applications. See SILVER ALLOYS. 
    Die-casting alloys have melting temperatures low enough so that in the 
liquid form they can be injected under pressure into steel dies. Such castings 
are used for automotive parts and for office and household appliances which 
have moderately complex shapes. Most die castings are made from zinc-base or 
aluminum-base alloys. Magnesium-base alloys also find some application when 
weight reduction is paramount. Low-melting alloys of lead and tin are not 
common because they lack the necessary strength for the above applications. 
See METAL CASTING. 
    In certain alloy systems a liquid of a fixed composition freezes to form a 
mixture of two basically different solids or phases. An alloy that undergoes 
this type of solidification process is called a eutectic alloy. A homogeneous 
liquid of this composition on slow cooling freezes to form a mixture of 
particles of nearly pure copper embedded in a matrix (background) of nearly 
pure silver. 
    The advantageous mechanical properties inherent in composite materials 
have been known for many years. Attention is being given to eutectic alloys as 
they are basically natural composite materials. See EUTECTICS; METAL MATRIX 
COMPOSITE.  
    Fusible alloys generally have melting temperatures below that of tin 
(449 DG F or 232 DG C), and in some cases as low as 122 DG F (50 DG C). Using 
eutectic compositions of metals such as lead, cadmium, bismuth, tin, antimony, 
and indium achieves these low melting temperatures. These alloys are used for 
many purposes, for example, in fusible elements in automatic sprinklers, 
forming and stretching dies, filler for thin-walled tubing that is being bent, 
and anchoring dies, punches, and parts being machined. 
    High-temperature alloys have high strengths at high temperatures. In 
addition to having strength, these alloys must resist oxidation by fuel-air 
mixtures and by steam vapor. At temperatures up to about 1380 DG F (750 DG C), 
the austenitic stainless steels serve well. An additional 180 DG F (100 DG C) 
may be realized if the steels also contain 3% molybdenum. Both nickel-base and 
cobalt-base alloys, commonly categorized as superalloys, may serve useful 
functions up to 2010 DG F (1100 DG C). Nichrome, a nickel-base alloy containing 
chromium and iron, is a fairly simple superalloy. More sophisticated alloys 
invariably contain five, six, or more components; for example, an alloy called 
Rene-41 contains Cr, Al, Ti, Co, Mo, Fe, C, B, and Ni. Other alloys are 
equally complex. A group of materials called cermets, which are mixtures of 
metals and compounds such as oxides and carbides, have high strength at high 
temperatures, and although their ductility is low, they have been found to be 
usable. One of the better-known cermets consists of a mixture of titanium 
carbide and nickel, the nickel acting as a binder or cement for the carbide. 
See CERMET. 
    Metals are bonded by three principal procedures: welding, brazing, and 
soldering. Welded joints melt the contact region of the adjacent metal; thus 
the filler material is chosen to approximate the composition of the parts 
being joined. Brazing and soldering alloys are chosen to provide filler metal 
with an appreciably lower melting point than that of the joined parts. 
Typically, brazing alloys melt above 750 DG F (400 DG C), whereas solders melt
at lower temperatures. See BRAZING; SOLDERING; WELDING AND CUTTING OF METALS. 
    Aluminum and magnesium, with densities of 2.7 and 1.75 g/cm sup 3, 
respectively, are the bases for most of the light-metal alloys. Titanium (4.5 
g/cm sup 3) may also be regarded as a light-metal alloy if comparisons are made 
with metals such as steel and copper. Aluminum and magnesium must be hardened 
to receive extensive application. Age-hardening processes are used for this 
purpose. See ALUMINUM; MAGNESIUM. 
    Low-expansion alloys include Invar, the dimensions of which do not vary 
over the atmospheric temperature range, and Kovar, which is widely used 
because its expansion is low enough to match that of glass. See THERMAL 
EXPANSION.  
    Soft and hard magnetic materials involve two distinct categories of 
alloys. The former consists of materials used for magnetic cores of 
transformers and motors, and must be magnetized and demagnetized easily. For 
alternating-current applications, silicon-ferrite is commonly used. This is an 
alloy of iron containing as much as 5% silicon. Permalloy and some comparable 
cobalt-base alloys are used in the communications industry. Ceramic ferrites, 
although not strictly alloys, are widely used in high-frequency applications 
because of their low electrical conductivity and negligible induced-energy 
losses in the magnetic field. Permanent or hard magnets may be made from 
steels which are mechanically hardened, either by deformation or by quenching. 
The Alnicos are also widely used for magnets. Since these alloys cannot be 
forged, they must be produced in the form of castings. The newest hard magnets 
are being produced from alloys of cobalt and the rare-earth type of metals. 
See ALNICO; MAGNETIC MATERIALS. 
    In addition to their use in coins and jewelry, precious metals such as 
silver, gold, and the heavier platinum metals are used extensively in 
electrical devices in which contact resistances must remain low, in catalytic 
applications to aid chemical reactions, and in temperature-measuring devices 
such as resistance thermometers and thermocouples. The unit of alloy impurity 
is commonly expressed in karats, where each karat is a 1/24 part. The most 
common precious-metal alloy is sterling silver (92.5% Ag, with the remainder 
being unspecified, but usually copper). The copper is very beneficial in that 
it makes the alloy harder and stronger than pure silver. 
    Metallic implants demand extreme corrosion resistance because body fluids 
contain nearly 1% NaCl, along with minor amounts of other salts, with which 
the metal will be in contact for indefinitely long periods of time. Type 316 
stainless steels resist pitting corrosion but are subject to crevice 
corrosion. Vitallium and other cobalt-base alloys have orthopedic 
applications. Titanium alloys gained wide usage in Europe during the early 
1970s for pacemakers and for retaining devices in artificial heart valves. 
While excellent for corrosion resistance, this alloy is subject to mechanical 
wear; therefore, it is not satisfactory in hip-joint prostheses and 
applications with similar frictional contacts. See PROSTHESIS. 
    Shape memory alloys have a very interesting and desirable property. In a 
typical case, a metallic object of a given shape is cooled from a given 
temperature T1 to a lower temperature T2 where it is deformed so as to change 
its shape. Upon reheating from T2 to T1, the shape change accomplished at T2 
is recovered so that the object returns to its original configuration. This 
thermoelastic property of the shape memory alloys is associated with the fact 
that they undergo a martensitic phase transformation (that is, a reversible 
change in crystal structure that does not involve diffusion) when they are 
cooled or heated between T1 and T2. Shape memory alloys are capable of being 
employed in a number of useful applications. One example is for thermostats; 
another is for couplings on hydraulic lines or electrical circuits. 
    Superconducting alloys, with zero resistivity, are of great interest in 
the design of certain fusion reactors which require very large magnetic fields 
to contain the plasma in a closed system. The advantage of the use of a 
material with a resistivity approaching zero is obvious. However, two 
significant problems are involved in the use of superconducting alloys in 
large electromagnetics: the critical temperature, and the fact that above a 
certain critical current density the superconducting materials tend to become 
normal conductors with a finite resistance. Serious materials problems still 
have to be solved before these materials can be used successfully. See 
SUPERCONDUCTIVITY.  
    Thermocouple alloys include Chromel and Alumel. These two alloys together 
form the widely used Chromel-Alumel thermocouple, which can measure 
temperatures up to 2200 DG F (1204 DG C). Another common thermocouple alloy, 
constantan, is used to form iron-constantan and copper-constantan couples, 
employed at lower temperatures. See STEEL; THERMOCOUPLE. 
 �34s[Lawrence H. Van Vlack/Robert E. Reed-Hill]
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�6v�2v�32sAllspice   �21m �33s 
    The dried, unripe fruits of a small, tropical, evergreen tree, Pimenta 
officinalis, of the myrtle family (Myrtaceae). This species is a native of the 
West Indies and parts of Central and South America. The spice, alone or in 
mixtures, is much used in sausages, pickles, sauces, and soups. The extracted 
oil is used for flavoring and in perfumery. Allspice is so named because its 
flavor resembles that of a combination of cloves, cinnamon, and nutmeg. See 
MYRTALES; SPICE AND FLAVORING. 
 �34s[Perry D. Strausbaugh/Earl L. Core]
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�6v�2v�32sAlmanac   �21m �33s 
    A book that contains astronomical or meteorological data arranged 
according to the days, weeks, and months of a given year, and often includes a 
miscellany of other information. This article is restricted to astronomical 
and navigational almanacs. 
    The Astronomical Almanac is published annually by the U.S. Naval 
Observatory and the Royal Greenwich Observatory. It contains ephemerides, 
which are tabulations at regular time intervals of the positions of the Sun, 
Moon, planets, satellites, and some minor planets. It also contains mean 
places of stars, quasars, pulsars, galaxies, and radio sources and the time 
for certain astronomical phenomena. This volume contains the fundamental 
astronomical data needed by astronomers, geodesists, navigators, surveyors, 
and space scientists.  See  ASTRONOMICAL COORDINATE SYSTEMS; EPHEMERIS. 
    The Nautical Almanac provides the data necessary for celestial navigation 
at sea. The Air Almanac, published semiannually, contains the data necessary 
for celestial navigation in the air, but it can also be used at sea.  See  
CELESTIAL NAVIGATION. 
    The annual Almanac for Computers provides power series and Chebyshev 
coefficients, which permit the calculations of the data contained in the 
Astronomical, Nautical, and Air Almanacs for any specific time. 
 �34s[P. K. Seidelmann]
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�6v�2v�32sAlmond   �21m �33s 
    A small deciduous tree, Prunus amygdalus (also known as P. dulcis or 
Amygdalus communis), closely related to the peach and other stone fruits and 
grown widely for its edible seeds. Almonds are of two general types: the 
bitter type is a source of prussic acid and flavoring extracts, and the sweet 
type has various foods uses. Almond kernels contain approximately 50% fat or 
oil, 20% protein, 20% carbohydrate, and a variety of minerals and vitamins. 
See ROSALES. 
    In the United States, commercial production is limited to California. 
Spain is the second leading producer, but the amount produced is about 
one-half that of the United States. Italy has historically been a leading 
producer, primarily from the Bari and Sicily areas, but production has 
declined sharply. 
    Almonds are used in a variety of products. Some are roasted whole and 
salted to be used as snacks. Others are blanched (the skin is removed) by 
steam and subjected to slicing, dicing, or halving. These may be roasted, and 
go into products such as candy bars, bakery products, ice cream, and almond 
paste, among many other uses. 
 �34s[Dale E. Kester]
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�6v�2v�32sAlnico   �21m �33s 
    A name given to a series of alloys used as permanent magnets; these alloys 
contain iron, cobalt, nickel, aluminum, and copper, and sometimes titanium and 
niobium. Alnico 5 has been the most commonly used material of the series; it 
has a coercive force of about 600 oersteds (48 kiloamperes per meter) and a 
residual induction of about 12,500 gauss (1.25 teslas). See FERROMAGNETISM; 
MAGNETIC MATERIALS. 
 �34s[Richard M. Bozorth]
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�6v�2v�32sAlpaca   �21m �33s 
    A member of the camel family, Camelidae, which belongs to the mammalian 
order Artiodactyla, the even-toed ungulates. The alpaca (Lama pacos) is found 
at elevations above 12,000 ft (3600 m) along the shores of Lake Titicaca on 
the boundaries of Peru and Bolivia. 
    The alpaca's neck and head are elongate, and the upper lip has a deep 
cleft. The long, slender legs terminate in two toes; the feet are digitigrade, 
that is, the animals walk on the toes and not on the entire foot or the tip of 
the digits. The long, fine repellent hair, or wool, ranges in color from black 
to white and is highly prized for manufacturing cloth, particularly the white 
wool. Like many breeds of domesticated animals, the alpaca has been bred to 
produce pure strains for the wool. Although the alpaca is raised chiefly for 
its wool, its flesh is edible and palatable. See ARTIODACTYLA; CAMEL; LLAMA; 
MAMMALIA; NATURAL FIBER. 
 �34s[Charles B. Curtin]
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�6v�2v�32sAlpha Centauri   �21m �33s 
    The brightest star in the southern constellation Centaurus (visible only 
south of geographic latitude +30 DG). Alpha Centauri is one of a triple system, 
and is Earth's nearest neighbor in space, at a distance of 1.33 parsecs. 
    The visual binary of 80-year period contains main-sequence stars of types 
G2 and K0 separated by about 24 astronomical units, that is, by about the 
diameter of the solar system. The brighter of the visual binary is almost a 
twin of the Sun in luminosity, temperature, and mass. There is a faint 
companion, Proxima Centauri, separated from the other two by about 10,000 
astronomical units. See STAR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sAlpha rays �21m �33s
    Helium nuclei which have been ejected at high velocity from atomic nuclei
as products of radioactive decay or as products of induced nuclear reactions.
Helium nuclei which have been accelerated to high velocities for use as
bombarding particles in nuclear reactions also may be called ALPHA-rays or
ALPHA-particles. The helium nucleus has a charge of +2e, that is, twice the
magnitude of the charge e of a proton or of an electron, and a mass of 4.00015
atomic mass units. The velocity of ejection of the ALPHA-ray varies from one
radioactive substance to another, but is usually in the domain of 1 to 2 x
10 sup 9 cm/s. Many radioactive substances emit ALPHA-rays in two or more
discrete energy groups, usually in the domain of 4 - 6 MeV, but occasionally
as low as 2 MeV or as high as 10 MeV. The other radiations emitted by
radioactive atoms are beta rays (negative or positive electrons) and gamma
rays (high-frequency electromagnetic radiation). For the theory of radioactive
decay see RADIOACTIVITY. For ALPHA-particle detection see PARTICLE DETECTOR.
See also BETA RAYS; GAMMA RAYS; NUCLEAR REACTION; NUCLEAR SPECTRA.
    As an ALPHA-ray passes through matter, it ionizes many of the atoms along
its path, thereby gradually loses its kinetic energy, and is stopped
(absorbed). For example, a 5-MeV ALPHA-ray is stopped after traversing 1.4 in.
(3.5 cm) of air at 1 atm (10 sup 2 kilopascals) pressure and 59 DG F (15 DG C)
and engaging in enough collisions with atomic electrons along its path to
produce about 150,000 ion pairs.
    Along the path of an ALPHA-ray in normal air, some 2000 - 6000 ion
pairs/mm are produced, depending upon the velocity of the ALPHA-ray at the
point under consideration. The number of ion pairs per unit path length is
called the specific ionization. The illustration shows the specific ionization
produced by an ALPHA-ray in air for various values of the residual range, that
is, at various distances measured back from the end of its range.
    The mean range R of a monoenergetic group of ALPHA-rays cannot be derived
wholly from theory but must rest on the results of many careful measurements
plus interpolation and extrapolation based on theory.


 �34s[Robley D. Evans]
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�6v�2v�32sAlternating current �21m �33s
    Electric current that reverses direction periodically, usually many times
per second. Electrical energy is ordinarily generated by a public or a private
utility organization and provided to a customer, whether industrial or
domestic, as alternating current.
    One complete period, with current flow first in one direction and then in
the other, is called a cycle, and 60 cycles per second (60 hertz) is the
customary frequency of alternation in the United States and in all of North
America. In Europe and in many other parts of the world, 50 Hz is the standard
frequency. On aircraft a higher frequency, often 400 Hz, is used to make
possible lighter electrical machines.
    When the term alternating current is used as an adjective, it is commonly
abbreviated to ac, as in ac motor. Similarly, direct current as an adjective
is abbreviated dc.
    The voltage of an alternating current can be changed by a transformer.
This simple, inexpensive, static device permits generation of electric power
at moderate voltage, efficient transmission for many miles at high voltage,
and distribution and consumption at a conveniently low voltage. With direct
(unidirectional) current it is not possible to use a transformer to change
voltage. On a few power lines, electric energy is transmitted for great
distances as direct current, but the electric energy is generated as
alternating current, transformed to a high voltage, then rectified to direct
current and transmitted, then changed back to alternating current by an
inverter, to be transformed down to a lower voltage for distribution and use.
    In addition to permitting efficient transmission of energy, alternating
current provides advantages in the design of generators and motors, and for
some purposes gives better operating characteristics. Certain devices
involving chokes and transformers could be operated only with difficulty, if
at all, on direct current. Also, the operation of large switches (called
circuit breakers) is facilitated because the instantaneous value of
alternating current automatically becomes zero twice in each cycle and an
opening circuit breaker need not interrupt the current but only prevent
current from starting again after its instant of zero value.
    Alternating current is shown diagrammatically in Fig. 1. In this diagram
it is assumed that the current is alternating sinusoidally; that is, the
current i is described by the following equation where Im is the maximum
instantaneous current, f is the frequency in cycles per second (hertz), and t
is the time in seconds. See SINE WAVE.

		      *** i = (I sub m) sin 2 PI ft ***

    A sinusoidal form of current, or voltage, is usually approximated on
practical power systems because the sinusoidal form results in less expensive
construction and greater efficiency of operation of electric generators,
transformers, motors, and other machines.
    A useful measure of alternating current is found in the ability of the
current to do work, and the amount of current is correspondingly defined as
the square root of the average of the square of instantaneous current, the
average being taken over an integer number of cycles. This value is known as
the root-mean-square (rms) or effective current. It is measured in amperes. It
is a useful measure for current of any frequency. The rms value of direct
current is identical with its dc value. The rms value of sinusoidally
alternating current is Im / SQUARE ROOT [2], where Im is the maximum
instantaneous current. See Fig. 1 and the equation. Other useful quantities
are the phase difference SIGMA PHI between voltage and current and the power
factor. See PHASE (PERIODIC PHENOMENA); POWER FACTOR.
    The phase angle and power factor of voltage and current in a circuit that
supplies a load are determined by the load. Thus a load of pure resistance, as
an electric heater, has unity power factor. An inductive load, such as an
induction motor, has a power factor less than 1 and the current lags behind
the applied voltage. A capacitive load, such as a bank of capacitors, also has
a power factor less than 1, but the current leads the voltage, and the phase
angle SIGMA PHI is a negative angle.
    Three-phase systems are commonly used for generation, transmission, and
distribution of electric power. A customer may be supplied with three-phase
power, particularly if a large amount of power is used or the use of
three-phase loads is desired. Small domestic customers are usually supplied
with single-phase power. A three-phase system is essentially the same as three
ordinary single-phase systems, with the three voltages of the three
single-phase systems out of phase with each other by one-third of a cycle (120
degrees), as shown in Fig. 2. The three-phase system is balanced if the
maximum voltage in each of the three phases is equal, and if the three phase
angles are equal, 1/3 cycle each as shown. It is only necessary to have three
wires for a three-phase system (a, b, and c of Fig. 3) plus a fourth wire n to
serve as a common return or neutral conductor. On some systems the earth is
used as the common or neutral conductor.
    Each phase of a three-phase system carries current and conveys power and
energy. If the three loads on the three phases of the three-phase system are
equal and the voltages are balanced, then the currents are balanced also. The
sum of the three currents is then zero at every instant. This means that
current in the common conductor (n of Fig. 3) is always zero, and that
conductor could theoretically be omitted entirely. In practice, the three
currents are not usually exactly balanced, and either of two situations
obtains. Either the common neutral wire n is used, in which case it carries
little current (and may be of high resistance compared to the other three line
wires), or else the common neutral wire n is not used, only three line wires
being installed, and the three phase currents are thereby forced to add to
zero even though this requirement results in some inbalance of phase voltages
at the load.
    The total instantaneous power from generator to load is constant (does not
vary with time) in a balanced, sinusoidal, three-phase system. This results in
smoother operation and less vibration of motors and other ac devices. In
addition, three-phase motors and generators are more economical than
single-phase machines.
    AC circuits are also used to convey information. An information circuit,
such as telephone, radio, or control, employs varying voltage, current,
waveform, frequency, and phase. Efficiency is often low, the chief requirement
being to convey accurate information even though little of the transmitted
power reaches the receiving end. For further consideration of the transmission
of information see RADIO; TELEPHONE; WAVEFORM.
    An ideal power circuit should provide the customer with electric energy
always available at unchanging voltage of constant waveform and frequency, the
amount of current being determined by the customer's load. High efficiency is
greatly desired. See CAPACITANCE; CIRCUIT (ELECTRICITY); ELECTRIC CURRENT;
ELECTRIC FILTER; ELECTRICAL IMPEDANCE; ELECTRICAL RESISTANCE; INDUCTANCE;
JOULE'S LAW; OHM'S LAW; RESONANCE (ALTERNATING-CURRENT CIRCUITS).



 �34s[Hugh H. Skilling]
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�6v�2v�32sAlternating-current generator   �21m �33s 
    A machine which converts mechanical power into alternating-current 
electric power. The most common type, sometimes called an alternator, is the 
synchronous generator, so named because its operating speed is proportional to 
system frequency. Another type is the induction generator, the speed of which 
varies somewhat with load for constant output frequency. See INDUCTION 
GENERATOR; SYNCHRONOUS GENERATOR. 
 �34s[Leon T. Rosenberg]
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�6v�2v�32sAlternating-current motor �21m �33s
    An electric rotating machine which converts alternating-current electric
energy to mechanical energy; one of two general classifications of electric
motor. Because ac power is widely available, ac motors are commonly used. They
are made in sizes from a few watts to thousands of horsepower. See
DIRECT-CURRENT MOTOR.
    Each type of ac motor has special properties. These motors are generally
classified by application, construction, principle of operation, or operating
characteristics. The most common type of ac motor is the induction motor.
Current is induced in a rotor as its conductors cut lines of magnetic flux
created by currents in a stator. Where constant speed is essential, a
synchronous motor is used. It runs at a fixed speed in synchronism with the
frequency of the power supply. For operation from either ac or dc power, the
series motor has its field winding connected in series with the armature
winding through a commutator and brush arrangement, as in a dc series motor.
This universal motor has high starting torque. The repulsion motor is also a
commutator motor, but the brushes and commutator are short-circuited and not
connected in series with the stator. See INDUCTION MOTOR; REPULSION MOTOR;
SYNCHRONOUS MOTOR; UNIVERSAL MOTOR.
    The table gives a comparison of available sizes and characteristics of
some common ac motors. For general principles see ELECTRIC ROTATING MACHINERY;
MOTOR; WINDINGS IN ELECTRIC MACHINERY.
_FIELD034;-237[Albert F. Puchstein]_FIELD034

    The rotor speed of the synchronous motor is a direct function of the
number of stator and rotor field poles and the frequency of the ac applied to
the stator. Since the number of rotor poles of the polyphase synchronous motor
is not easily modified, the change of frequency method is the only way to
control synchronous speed of the motor. The speed of the polyphase induction
motor is an asynchronous speed varied by one or more of the following methods:
(1) by changing the applied frequency to the stator (same method as for a
synchronous motor, already discussed); (2) by controlling the rotor slip by
means of rheostatic rotor resistance control (used for wound-rotor induction
motors); (3) by changing the number of poles of both stator and rotor; and (4)
by ``foreign voltage'' control, obtained by conductively or inductively
introducing applied voltages of the proper frequency to the rotor. A number of
electromechanical or purely mechanical methods also serve to provide speed
control of ac motors.
    The principal method of speed control used for fractional hp single-phase
induction-type, shaded-pole, reluctance, series, and universal motors is the
method of primary line-voltage control. It involves a reduction in line
voltage applied to the stator winding (of the induction type) or to the
armature of series and universal motors. In the former this produces a
reduction of torque and an increase in rotor slip. In the latter it is simply
a means of controlling speed by armature voltage control or field flux control
or both. The reduction in line voltage is usually accomplished by any one of
five methods: autotransformer control, series reactance control, tapped
main-winding control, saturable reactor (or magnetic amplifier) control, and
silicon-controlled rectifier feedback control.
    The development of the thyristor, or silicon-controlled rectifier (SCR),
has created unlimited possibilities for control of virtually all types of
motors (single-phase, dc, and polyphase). SCRs in sizes up to 1600 amperes
(rms) with voltage ratings up to 1600 volts are available. See RECTIFIER;
SEMICONDUCTOR RECTIFIER.

 �34s[Irving L. Kosow]
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�6v�2v�32sAlternator   �21m �33s 
    A machine that generates an alternating voltage when its rotating portion 
is driven by a motor, engine, or other means. It is thus a means for 
converting mechanical power into electric power. The frequency of the 
generated alternating voltage is exactly proportional to the speed at which 
the alternator is driven. An alternator generates one cycle of alternating 
current each time one of its coils passes a pair of magnetic poles in the 
field structure. See ALTERNATING-CURRENT GENERATOR. 
 �34s[John Markus]
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�6v�2v�32sAltimeter   �21m �33s 
    Any device which measures the height of an aircraft. The two chief types 
are the pressure altimeter, which measures distance above sea level, and the 
radio altimeter, which measures distance above the ground. See PRESSURE 
ALTIMETER; RADIO ALTIMETER. 
 �34s[James W. Angus]
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�6v�2v�32sAlum   �21m �33s 
    A colorless to white crystalline substance which occurs naturally as the 
mineral kalunite and is a constituent of the mineral alunite. Alum is produced 
as aluminum sulfate by treating bauxite with sulfuric acid to yield alum cake 
or by treating the bauxite with caustic soda to yield papermaker's alum. Other 
industrial alums are potash alum, ammonium alum, sodium alum, and chrome alum 
(potassium chromium sulfate). Major uses of alum are as an astringent, 
styptic, and emetic. For water purification alum is dissolved: it then 
crystallizes out into positively charged crystals that attract negatively 
charged organic impurities to form an aggregate sufficiently heavy to settle 
out. Alum is also used in sizing paper, dyeing fabrics, and tanning leather. 
With sodium bicarbonate it is used in baking powder and in some fire 
extinguishers. See ALUMINUM; COLLOID. 
 �34s[Frank H. Rockett]
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�6v�2v�32sAluminate   �21m �33s 
    A negative ion usually given the formula AlO2 NEGATIVE ION and derived from
aluminum hydroxide. Aluminum hydroxide is an amphoteric substance and thus can
react with a strong base, such as sodium hydroxide, to form sodium aluminate.
It also reacts with acids to form aluminum salts. Solutions of aluminates are
strongly basic and the reaction with sodium hydroxide is easily reversed, even
by weak acids, to form the insoluble aluminum hydroxide. This is the basis for
the commercial use of sodium aluminate in the clarification of water. See
ALUMINUM; WATER TREATMENT. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sAluminum   �21m �33s 
    A metallic chemical element, symbol Al, atomic number 13, atomic weight 
26.98154, in group IIIA of the periodic system. Pure aluminum is soft and 
lacks strength, but it can be alloyed with other elements to increase strength 
and impart a number of useful properties. Alloys of aluminum are light, 
strong, and readily formable by many metalworking processes; they can be 
easily joined, cast, or machined, and accept a wide variety of finishes. 
Because of its many desirable physical, chemical, and metallurgical 
properties, aluminum has become the most widely used nonferrous metal. 
    Aluminum is the most abundant metallic element on the Earth and Moon but 
is never found free in nature. The element is widely distributed in plants, 
and nearly all rocks, particularly igneous rocks, contain aluminum in the form 
of aluminum silicate minerals. When these minerals go into solution, depending 
upon the chemical conditions, aluminum can be precipitated out of the solution 
as clay minerals or aluminum hydroxides, or both. Under such conditions 
bauxites are formed. Bauxites serve as principal raw materials for aluminum 
production. See BAUXITE; CLAY MINERALS; IGNEOUS ROCKS; WEATHERING PROCESSES. 
    Aluminum is a silvery metal having a density of 1.56 oz/in. sup 3 at 
68 DG F (2.70 g/cm sup 3 at 20 DG C). Naturally occurring aluminum consists 
of a single isotope, (27 OVER 13) Al. Aluminum crystallizes in the
face-centered cubic structure with edge of the unit lattice cube of 4.0495
angstroms (0.40495 nanometer). Aluminum is known for its high electrical and
thermal conductivities and its high reflectivity. 
    The electronic configuration of the element is 1s sup 2 2s sup 2 2p sup 6 
3s sup 2 3p sup 1. Aluminum exhibits a valence of +3 in all compounds, with the
exception of a few high-temperature monovalent and divalent gaseous species. 
    Aluminum is stable in air and resistant to corrosion by sea water and many 
aqueous solutions and other chemical agents. This is due to protection of the 
metal by a tough, impervious film of oxide. At a purity greater than 99.95%, 
aluminum resists attack by most acids but dissolves in aqua regia. Its oxide 
film dissolves in alkaline solutions, and corrosion is rapid. See CORROSION. 
    Aluminum is amphoteric and can react with mineral acids to form soluble 
salts and to evolve hydrogen. 
    Molten aluminum can react explosively with water. The molten metal should 
not be allowed to contact damp tools or containers. 
    At high temperatures aluminum reduces many compounds containing oxygen, 
particularly metal oxides. These reactions are used in the manufacture of 
certain metals and alloys. 
    Applications in building and construction represent the largest single 
market of the aluminum industry. Millions of homes use aluminum doors, siding, 
windows, screening, and downspouts and gutters. Aluminum is also a major 
industrial building product. Transportation is the second largest market. Many 
commercial and military aircraft have become virtually all-aluminum. In 
automobiles, aluminum is apparent in interior and exterior trim, grilles, 
wheels, air conditioners, automatic transmissions, and some radiators, engine 
blocks, and body panels. Aluminum is also found in rapid-transit car bodies, 
rail cars, forged truck wheels, cargo containers, and in highway signs, 
divider rails, and lighting standards. In aerospace, aluminum is found in 
aircraft engines, frames, skins, landing gear, and interiors, often making up 
80% of a plane's weight. The food packaging industry is a fast-growing market. 
    In electrical applications, aluminum wire and cable are major products. 
Aluminum appears in the home as cooking utensils, cooking foil, hardware, 
tools, portable appliances, air conditioners, freezers, and refrigerators, and 
in sporting equipment such as skis, ball bats, and tennis rackets. 
    There are hundreds of chemical uses of aluminum and aluminum compounds. 
Aluminum powder is used in paints, rocket fuels, and explosives, and as a 
chemical reductant. See ALUMINUM ALLOYS; ALUMINUM METALLURGY. 
 �34s[Allen S. Russell]
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�6v�2v�32sAluminum alloys   �21m �33s 
    Substances formed by the addition of one or more elements, usually metals, 
to aluminum. The principal alloying elements in aluminum-base alloys are 
magnesium, silicon, copper, zinc, and manganese. In wrought products, which 
constitute the greatest use of aluminum, the alloys are identified by 
four-digit numbers of the form NXXX, where the value of N denotes the alloy 
type and the principal alloying element(s) as follows: 1 (Al; at least 99% 
aluminum by weight), 2 (Cu), 3 (Mn), 4 (Si), 5 (Mg), 6 (Mg + Si), 7 (Zn), 8 
(other). See ALUMINUM. 
    Iron and silicon are commonly present as impurities in aluminum alloys, 
although the amounts may be controlled to achieve specific mechanical or 
physical properties. Minor amounts of other elements, such as Cr, Zr, V, Pb, 
and Bi, are added to specific alloys for special purposes. Titanium additions 
are frequently employed to produce a refined cast structure. 
    Aluminum-base alloys are generally prepared by making the alloying 
additions to molten aluminum, forming a liquid solution. As the alloy freezes, 
phase separation occurs to satisfy phase equilibria requirements and the 
decrease in solubility as the temperature is lowered. The resultant solidified 
structure consists of grains of aluminum-rich solid solution and crystals of 
intermetallic compounds. See EUTECTICS. 
    A decrease in solubility with falling temperature also provides the basis 
for heat treatment of solid aluminum alloys. In this operation, the alloy is 
held for some time at a high temperature to promote dissolution of soluble 
phases and homogenization of the alloy by diffusion processes. The limiting 
temperature is the melting point of the lowest melting phase present. The time 
required depends both on temperature and on the distances over which diffusion 
must occur to achieve the desired degree of homogenization. The solution heat 
treatment is followed by a quenching operation in which the article is rapidly 
cooled, for example, by plunging it into cold or hot water or by the use of an 
air blast. See HEAT TREATMENT (METALLURGY). 
    Casting alloys are significant users of secondary metal (recovered from 
scrap for reuse). Thus, casting alloys usually contain minor amounts of a 
variety of elements; these do no harm as long as they are kept within certain 
limits. The use of secondary metal is also of increasing importance in wrought 
alloy manufacturing as producers take steps to reduce the energy required in 
producing fabricated aluminum products. See ALLOY. 
 �34s[Allen S. Russell]
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�6v�2v�32sAluminum metallurgy   �21m �33s 
    The extraction of aluminum from ore and its subsequent purification and 
processing. The most widely used technology for producing aluminum involves 
two steps: extraction and purification of alumina from ores, and electrolysis 
of the oxide after it has been dissolved in fused cryolite. Bauxite is by far 
the most used raw material. 
    In the alumina extraction process, bauxite is crushed and ground, then 
digested at elevated temperature and pressure in a strong solution of caustic 
soda. The gibbsite, boehmite, or diaspore in the bauxites reacts with the 
caustic soda to form soluble sodium aluminate. The residue, known as red mud, 
contains the insoluble impurities. The red mud is separated from the solution. 
After cooling, the solution is supersaturated with respect to alumina. It is 
seeded with recycled synthetic gibbsite (alumina trihydrate) and agitated. A 
large part of the alumina in solution thus crystallizes out as gibbsite. For 
metal production, the product is calcined at temperatures up to 2370 DG F 
(1300 DG C) to produce alumina containing about 0.3 - 0.8% soda, less than 0.1% 
iron oxide plus silica, and trace amounts of other oxides. 
    In a smelter, alumina is dissolved in cells (pots) - rectangular steel 
shells lined with carbon - containing a molten electrolyte (bath) consisting 
mostly of cryolite. The bath usually contains 2 - 8% alumina. Carbon anodes 
are hung from above the cells with their lower ends extending to within about 
1.5 in. (3.8 cm) of the molten metal, which forms a layer under the molten 
bath. The heat required to keep the bath molten is supplied by the electrical 
resistance of the bath as current passes through it. 
    The passage of direct current through the electrolyte decomposes the 
dissolved alumina. Metal is deposited on the cathode, and oxygen on the 
gradually consumed anode. The smelting process is continuous. Alumina is 
added, anodes are replaced, and molten aluminum is periodically siphoned off 
without interrupting current to the cells. 
    Molten aluminum is siphoned from the smelting cells into large crucibles. 
From there the metal may be poured directly into molds to produce foundry 
ingot, or transferred to holding furnaces for further refining or alloying 
with other metals to form fabricating ingot. As it comes from the cell, 
primary aluminum averages about 99.8% purity. See ALUMINUM; ELECTROMETALLURGY. 
    In plants not adjacent to a smelter, it is necessary to remelt charges, 
usually consisting of mill scrap or returned scrap with enough primary metal 
to provide composition control. Scrap recycling has grown through the years to 
considerable economic importance and provides a valuable source of aluminum at 
a much lower energy expenditure than for primary metal. 
    Although many types of casting methods for ingot have been used 
historically, most ingot for fabricating is cast by the DC (direct chill) 
process or some modification thereof. The DC process consists of pouring the 
metal into a short mold. The base of the mold is a separate platform that is 
lowered gradually into a pit while the metal is solidifying. The frozen outer 
shell of metal retains the still-liquid portion of the metal when the shell is 
past the mold wall. Water is used to cool the mold, and by impingement on the 
ingot shell also cools the ingot as it is lowered. By sawing the ingot while 
casting is progressing, the process can be made truly continuous. See METAL 
CASTING.  
    Aluminum alloys are fabricated by all the methods known to metalworking 
and are commercially available in the form of castings; in wrought forms 
produced by rolling, extruding, forging, or drawing; in compacted and sintered 
shapes made from powder; and in other monolithic forms made by a wide variety 
of processes. In addition, pieces produced separately can be assembled by 
common joining procedures to build up complex shapes and structures. See METAL 
FORMING.  
 �34s[Allen S. Russell]
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�6v�2v�32sAlunite   �21m �33s 
    A mineral of composition KAl3 (SO4)2 (OH)6. Alunite occurs in white to gray 
rhombohedral crystals or in fine-grained, compact masses. Alunite is produced 
by sulfurous vapors on acid volcanic rocks and also by sulfated meteoric 
waters affecting aluminous rocks. Alunite is used as a source of potash or for 
making alum. Alum has been manufactured from the well-known alunite deposits 
at Tolfa, near Civita Vecchia, Italy, since the mid-15th century. In the 
United States alunite is widespread in the West. See ALUM. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sAmalgam   �21m �33s 
    An alloy of mercury. Practically all metals will form alloys or amalgams 
with mercury, with the notable exception of iron. Amalgams are used as dental 
materials, in the concentration of gold and silver from their ores, and as 
electrodes in various industrial and laboratory electrolytic processes. 
Amalgams used in dental work require the following composition: silver, 65% 
minimum; copper, 6% maximum; zinc, 2% maximum; and tin, 25% minimum. These 
amalgams are prepared by the dentist as needed, and harden within 3 - 5 min, 
but may be shaped by carving for 15 min or so. See ALLOY; GOLD; MERCURY 
(ELEMENT); SILVER. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sAmateur radio   �21m �33s 
    Two-way radio communications by private individuals as a leisure-time 
activity. Amateur, or ``ham,'' radio is defined in international treaty as a 
``service of self-training, intercommunications, and technical investigations 
carried on by amateurs; that is, by duly authorized persons interested in 
radio technique solely with a personal aim and without pecuniary interest.'' 
See RADIO. 
    There are about 750,000 amateur operators worldwide, and 275,000 in the 
United States. Licenses in the United States are issued by the Federal 
Communications Commission in five classes - novice, technician, general, 
advanced, and amateur extra. Knowledge of the international Morse code and of 
radio theory and regulations is required for a license of any class. With each 
succeeding license, greater privileges are granted. 
    The amateur radio operators' organization is the American Radio Relay 
League, a nonprofit, nonstock association of radio amateurs in North America. 
The League serves as the voice for amateurs in regulatory matters; it presents 
new technical developments and conducts operating contests and other 
activities to increase the hams' enjoyment of, and skill in, the hobby; it 
organizes networks to handle messages and coordinates emergency communications 
training. The League is also headquarters for the International Amateur Radio 
Union, composed of some 85 similar national radio societies. 
 �34s[Perry F. Williams]
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�6v�2v�32sAmber   �21m �33s 
    Most commonly, a generic name for all fossil resins, although it has been 
restricted by some to refer only to succinite, the mineralogical species of 
fossil resin making up most of the Baltic Coast deposits. Resins generally are 
complex mixtures of mono-, sesqui-, di-, and triterpenoids; however, some 
resins contain aromatic phenols. Among the plants, primarily trees, that 
produce copious amounts of resin that may fossilize to become amber, 
two-thirds are tropical. 
    Although ambers occur throughout the world in deposits from Carboniferous 
to Pleistocene in age, they have been reported most commonly from Cretaceous 
and Tertiary strata and often are associated with coal. Amber may contain 
beautifully preserved insects, spiders, flowers, leaves, and even small 
animals. The most extensively studied deposits are those from the Baltic 
Coast, Alaska, Canada, Burma, Dominican Republic, and Mexico. 
    When amber is used for jewelry, it usually is transparent yellow, 
reddish-brown, or ``amber'' color. Translucent or semitranslucent amber is 
used for pipe stems, decorating small boxes, and a variety of ornamental 
purposes. The specific gravity varies from 1.05 to 1.10, and hardness from 1 
to 3 on Mohs scale. 
    At one time, chemical studies of amber were mineralogically oriented 
because the purpose was to describe and classify amber as a semiprecious gem. 
However, phytochemical studies comparing fossil and present-day resins are 
providing information regarding the botanical origins of ambers. 
    The predominantly tropical or subtropical occurrence of amber-producing 
plants through geologic time has led to evolutionary studies of the natural 
purpose of resins and their possible defensive role for trees against injury 
and disease inflicted by the high diversity of insects and fungi in tropical 
environments. See GEM; MINERALOGY; RESIN. 
 �34s[Jean H. Langenheim]
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�6v�2v�32sAmbergris   �21m �33s 
    A fatty substance formed in the intestinal tract of the sperm whale 
(Physeter catodon). Ambergris contains acids, alkaloids, and a fatty 
substance, its main constituent, called ambrein. Although fresh ambergris is 
soft and black and has an offensive odor, it hardens into pleasantly fragrant 
gray or yellow masses when exposed to the air, sun, and sea. Being lighter 
than water, it is found in lumps floating on tropical seas or cast up on the 
shores. It is also gathered directly from the abdomens of dead or captured 
whales. Collecting grounds for ambergris are principally on the shores of 
China, Japan, Africa, the Americas, tropical islands, and the Bahamas. 
    Ambergris is valued in the manufacture of perfumes. The ambergris is 
ground and used in the form of a tincture, dissolved in a dilute solution of 
alcohol, which when added to perfume acts as a fixative, increasing the 
duration of the fragrance while adding its own sweet, earthy scent. 
 �34s[Sybil P. Parker]
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�6v�2v�32sAmblygonite   �21m �33s 
    A mineral consisting of lithium, sodium, and aluminum phosphate 
(Li,Na)Al(PO4)(F,OH); generally Li is in excess of Na. Amblygonite 
crystallizes in the triclinic system. Crystals are short and prismatic. Colors 
range from white through shades of gray, yellow, green, and brown. 
    Amblygonite, found at many places in the world, occurs mostly in granitic 
pegmatites. It is mined in the Black Hills of South Dakota, in southwestern 
Africa, and elsewhere for its lithium content. 
 �34s[Wayne R. Lowell]
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�6v�2v�32sAmblypygi   �21m �33s 
    An order of arachnids, the tailless whip scorpions, comprising about 50 
species of flattened, crablike forms from the tropical and warm temperate 
regions of the world. The somber red or brownish species vary considerably in 
size, from 0.16 - 1.80 in. (4 to 45 mm), the largest being Acanthophrynus 
coronatus of California and Mexico. The pedipalps are long raptorial organs 
set with many sharp spines that grasp and crush insect prey. The first pair of 
legs is modified into very long, lashlike whips which are used as sensory 
feelers. No  tail is present on the abdomen. The amblypygids are harmless, 
nocturnal types that live under stones, in rock fissures and caves, and 
frequently in houses. No venom or repellent glands are present. See ARACHNIDA. 
 �34s[Willis J. Gertsch]
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�6v�2v�32sAmeba �21m �33s
    The name for a number of species of acellular or unicellular animals of
the order Amoebida, which constitutes one of the 11 orders of the class
Rhizopodea. Members of this order are naked protozoans; that is, they lack a
shell known as a test (see illustration). Characteristically they move by
means of protoplasmic waves or protoplasmic extrusions called pseudopods.
Amebas are asymmetrical because they do not possess a constant or definite
shape. These organisms occur in a variety of habitats. There are fresh-water,
marine, and terrestrial species as well as a number which are commensals and
parasites (see table).
    Many species of ameba are uninucleate, whereas others are multinucleate.
The type of nucleus is important in differentiating between commensal and
parasitic forms. Binary fission, an asexual method of reproduction, is common
among the amebas. Under adverse environmental conditions, some amebas encyst
or form a heavy cell wall around themselves, and during this period active
multiplication of the species occurs. Among the parasitic amebas, this is the
infective stage for transmission from one host to another. See AMEBIASIS;
AMOEBIDA; RHIZOPODEA.



 �34s[Charles B. Curtin]
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�6v�2v�32sAmebiasis   �21m �33s 
    A disease of humans caused by the pathogenic ameba Entamoeba histolytica. 
It is characterized by an acute phase (amebic dysentery) and by other clinical 
pathological manifestations in the intestinal tract and in other organs. 
Entamoeba histolytica has two stages in its life cycle: the resistant and 
quiescent cyst form and the actively motile, tissue-invading trophozoite. The 
cysts, discharged in the feces of the sick and the carriers, survive for long 
periods outside the body. When taken up in contaminated water or with 
vegetables, they reach the intestinal tract, and the active amebas emerge to 
divide and to invade the tissues. By their lytic enzymes and active movements, 
they penetrate the intestinal mucosa to cause typical ulcerations which may 
lead to perforation of the intestinal wall. The amebas may be carried by the 
bloodstream to other sites and organs, especially to the liver, to cause 
dangerous abcesses. On rare occasion they may reach the brain to initiate 
severe, even fatal complications. 
    Though amebiasis is cosmopolitan in its distribution, it is in the 
tropical and subtropical climates that it commonly manifests itself in its 
severe and acute form and in the liver complications. The diagnosis of an 
amebic infection is made by microscopic demonstration of the causative E. 
histolytica in the feces of the patient or from liver abcess. A number of 
drugs have been employed in the treatment of amebiasis. Iodides and bismuth 
compounds are used, as well as the broad-spectrum antibiotics. Combined 
treatment with both groups has given the best results. For liver 
complications, emetine injections and chloroquine by mouth are effective. 
    Epidemic outbreaks have shown that faulty sewage systems are responsible 
for contamination of drinking water. Great attention should therefore be given 
to all city water systems and their regular inspection and supervision. Food 
handlers in hotels and camps should be examined before employment. Vegetables 
should be properly cleaned before serving. Tourists going abroad to tropical 
countries should be advised of the danger of intestinal disorders and the 
methods of their prevention. See MEDICAL PARASITOLOGY. 
 �34s[Meir Yoeli]
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�6v�2v�32sAmericium   �21m �33s 
    A chemical element, symbol Am, atomic number 95. The isotope 
sup 241 Am is an alpha emitter with a half-life of 433 years. Other 
isotopes of americium range in mass from 232 to 247, but only the isotopes of 
mass 241 and 243 are important. The isotope sup 241 Am is routinely 
separated from ``old'' plutonium and sold for a variety of industrial uses, 
such as 59-keV gamma sources and as a component in neutron sources. The 
longer-lived sup243 Am (half-life 7400 years) is a precursor in 
sup 244 Cm production. 
    In its most prominent aqueous oxidation state, 3+, americium closely 
resembles the tripositive rare earths. The formal analogy to the rare earths 
is also marked in anhydrous compounds of both tripositive and tetrapositive 
americium. Americium is different in that it is possible to oxidize Am sup 3+ 
to both the 5+ and 6+ states. 
    Americium metal has a vapor pressure markedly higher than that of its 
neighboring elements and can be purified by distillation. The metal is 
nonmagnetic and superconducting at 0.79 K. Under high pressure the metal has 
been compressed to 80% of its room-temperature volume and displays the 
ALPHA-uranium structure. See ACTINIDE ELEMENTS; BERKELIUM; CURIUM; NUCLEAR 
REACTION; TRANSURANIUM ELEMENTS. 
 �34s[Robert A. Penneman]
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�6v�2v�32sAmethyst   �21m �33s 
    The transparent purple to violet variety of the mineral quartz. Amethyst 
is rare in the deep colors that characterize fine quality. It is usually 
colored unevenly and is often heated slightly in an effort to distribute the 
color more evenly. Heating at higher temperatures usually changes it to yellow 
or brown (rarely green), and further heating removes all color. The principal 
sources are Brazil, Arizona, Uruguay, and the Soviet Union. See GEM; QUARTZ. 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sAmide   �21m �33s 
    A derivative of a carboxylic acid with general formula RCONH2, where R is 
hydrogen or an alkyl or aryl radical. Amides are divided into subclasses, 
depending on the number of substituents on nitrogen. The simple, or primary, 
amides are considered to be derivatives formed by replacement of the 
carboxylic hydroxyl group by the amino group, NH2. They are named by dropping 
the ``-ic acid'' or ``-oic acid'' from the name of the parent carboxylic acid 
and replacing it with the suffix ``amide.'' In the secondary and tertiary 
amides, one or both hydrogens are replaced by other groups. The presence of 
such groups is designated by the prefix capital N (for nitrogen). 
    Except for formamide, all simple amides are relatively low-melting solids, 
stable, and weakly acidic. They are strongly associated through hydrogen 
bonding, and hence soluble in hydroxylic solvents, such as water and alcohol. 
Because of ease of formation and sharp melting points, amides are frequently 
used for the identification of organic acids and, conversely, for the 
identification of amines. 
    Commercial preparation of amides involves thermal dehydration of ammonium 
salts of carboxylic acids. Thus, slow pyrolysis of ammonium acetate forms 
water and acetamide. N,N-dimethylacetamide may be similarly prepared from 
dimethylammonium acetate. 
    Amides are important chemical intermediates since they can be hydrolyzed 
to acids, dehydrated to nitriles, and degraded to amines containing one less 
carbon atom by the Hofmann reaction. In pharmacology, acetophenetidin is a 
popular analgesic. However, the most important commercial application of 
amides is in the preparation of polyamide resins, also called nylons. See ACID 
ANHYDRIDE; POLYAMIDE RESINS. 
 �34s[Paul E. Fanta]
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�6v�2v�32sAmiiformes   �21m �33s 
    An order of Actinopterygian fishes, also known as the Halecomorphi. The 
characters include abbreviate heterocercal tail, in some almost symmetrical; 
usually fusiform body; median fin rays arranged one per pterygiophore; scales 
with a ganoine surface but typically thin; no spiracle; vascularized swim 
bladder with a duct; reduced maxilla which is free from the preopercle 
posteriorly; an orbit which is bordered below and behind by a series of 
enlarged bones; and, in Recent species, an enlarged gular plate and elongate 
dorsal fin. 
    The order, which appeared first in the Triassic, includes six families, of 
which only the Amiidae survived into the Cenozoic. The single Recent species, 
Amia calva, inhabits sluggish freshwaters of eastern North America. See 
ACTINOPTERYGII; BOWFIN; OSTEICHTHYES. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sAmination �21m �33s
    The preparation of amines. Several processes are employed for the
preparation of amines. Amines can be prepared by reducing a compound that
already has a carbon-nitrogen bond, such as a nitro, nitroso, azoxy, or azo
compound. They can also be prepared by treating compounds containing (1) a
labile group (SINGLE BOND Cl, SINGLE BOND OH, SINGLE BOND SO3H), (2) a carbonyl
(> C DOUBLE BOND O) group, or (3) a highly reactive structure, such as oxirane,
with ammonia. The preparation of amines from compounds containing a C-N bond
is termed amination by reduction; the production of amines by reaction with
ammonia or a mixture of ammonia and hydrogen is termed ammonolysis or
hydroammonolysis. See AMINE.
 �34s[P. H. Groggins/Robert S. Kapner]
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�6v�2v�32sAmine   �21m �33s 
    A member of a group of organic compounds which can be considered as 
derived from ammonia by replacement of one or more hydrogens by organic 
radicals. Generally amines are bases of widely varying strengths, but a few 
which are actually acidic are known. 
    Amines constitute one of the most important classes of organic compounds. 
The lone pair of electrons on the amine nitrogen enables amines to participate 
in a large variety of reactions as a base or a nucleophile. Amines play 
prominent roles in biochemical systems; they are widely distributed in nature 
in the form of amino acids, alkaloids, and vitamins. Many complex amines have 
pronounced physiological activity, for example, epinephrine (adrenalin), 
thiamin or vitamin B1, and Novocaine. The odor of decaying fish is due to 
simple amines produced by bacterial action. Amines are used to manufacture 
many medicinal chemicals, such as sulfa drugs and anesthetics. The important 
synthetic fiber nylon is an amine derivative. 
    Amines are classified according to the number of hydrogens of ammonia 
which are replaced by radicals. Replacement of one hydrogen results in a 
primary amine (RNH2), replacement of two hydrogens results in a secondary 
amine (R2NH), and replacement of all three hydrogens results in a tertiary 
amine (R3N). The substituent groups (R) may be alkyl, aryl, or aralkyl. 
Another group of amines are those in which the nitrogen forms part of a ring 
(heterocyclic amines). Examples of such compounds are nicotine, which is 
obtained commercially from tobacco for use as an insecticide, and serotonin, 
which plays a key role as a chemical mediator in the central nervous system. 
    Many aromatic and heterocyclic amines are known by trivial names, and 
derivatives are named as substitution products of the parent amine. Thus, 
C6H5NH2 is aniline and C6H5NHC2H5 is N-ethylaniline. See ANILINE. 
    According to the Broensted-Lowry theory of acids and bases, amines are 
basic because they accept protons from acids. Stable salts suitable for the 
identification of amines are in general formed only with strong acids, such as 
hydrochloric, sulfuric, oxalic, chloroplatinic, or picric. 
    Commercial preparation of aliphatic amines can be accomplished by direct 
alkylation of ammonia or by catalytic alkylation of amines with alcohols at 
elevated temperatures. Reduction of various nitrogen functions carrying the 
nitrogen in a higher state of oxidation also leads to amines. Such functions 
are nitro, oximino, nitroso, and cyano. For the preparation of pure primary 
amines, Gabriel's synthesis and Hofmann's hypohalite reaction are preferred 
methods. The Bucherer reaction is satisfactory for the preparation of 
polynuclear primary aromatic amines. See AMINO ACIDS. 
 �34s[Paul E. Fanta]
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�6v�2v�32sAmino acids �21m �33s
    Organic compounds possessing one or more basic amino groups and one or
more acidic carboxyl groups. Of the more than 80 amino acids which have been
found in living organisms, about 20 serve as the building blocks for the
proteins. The amino acids are characterized physically by the following: (1)
the pK1, or the dissociation constant of the various titratable groups; (2)
the isoelectric point, or pH at which a dipolar ion does not migrate in an
electric field; (3) the optical rotation, or the rotation imparted to a beam
of plane-polarized light (frequently the D line of the sodium spectrum)
passing through 1 decimeter of a solution of 100 g in 100 milliliters; (4)
solubility.
    All the amino acids of proteins, and most of the others which occur
naturally, are ALPHA-amino acids, meaning that an amino group (- NH2)
and a carboxyl group (- COOH) are attached to the same carbon atom.
This carbon (the ALPHA carbon, being adjacent to the carboxyl group) also
carries a hydrogen atom; its fourth valence is satisfied by any of a wide
variety of substituent groups. In the simplest amino acids, glycine, the
substituent group is a hydrogen atom. In all other amino acids, it is an
organic radical; for example, in alanine it is a methyl group (- CH3),
while in glutamic acid it is an aliphatic chain terminating in a second carboxyl
group (-- CH2 DOUBLE BOND CH - COOH). Chemically, the amino
acids can be considered as falling roughly into nine categories based on the
nature of R (see table).
    Amino acids occur in living tissues principally in the conjugated form.
Most conjugated amino acids are peptides, in which the amino group of one
amino acid is linked to the carboxyl group of another. This type of linkage is
known as a peptide bond; a molecule of water is split out when a peptide bond
is formed, and a molecule of water must be added when a peptide bond is broken.
    Since each amino acid possesses both an amino group and a carboxyl group,
the acids are capable of linking together to form chains of various lengths,
called polypeptides. Proteins are polypeptides ranging in size from about 50
to many thousand amino acid residues.
    Although most of the conjugated amino acids in nature are proteins,
numerous smaller conjugates occur naturally, many with important biological
activity. The line between large peptides and small proteins is difficult to
draw, with insulin (molecular weight=7000; 50 amino acids) usually being
considered a small protein and adrenocorticotropic hormone (molecular
weight=5000; 39 amino acids) being considered a large peptide. In addition to
their role as hormones, peptides often occur in coenzymes, bacterial capsules,
fungal toxins, and antibiotics. See ANTIBIOTIC; COENZYME.
    Free amino acids are found in living cells, as well as in the body fluids
of higher animals, in amounts which vary according to the tissue and to the
amino acid. The amino acids which play key roles in the incorporation and
transfer of ammonia, such as glutamic acid, aspartic acid, and their amides,
are often present in relatively high amounts, but the concentrations of the
other amino acids of proteins are extremely low, ranging from a fraction of a
milligram to several milligrams per 100 g wet weight of tissue. In view of the
fact that amino acid and protein synthesis goes on constantly in most of these
tissues, the presence of free amino acids in only trace amounts points to the
existence of extraordinarily efficient regulation mechanisms.
    In addition to the amino acids of protein, a variety of other free amino
acids occurs naturally. Some of these are metabolic products of the amino
acids of proteins; for example, GAMMA-aminobutyric acid occurs as the
decarboxylation product of glutamic acid. Others, such as homoserine and
ornithine, are biosynthetic precursors of the amino acids of protein.
    At ordinary temperatures, the amino acids are white crystalline solids;
when heated to high temperatures, they decompose rather than melt. They are
stable in aqueous solution, and with few exceptions can be heated as high as
120 DG C for short periods without decomposition, even in acid or alkaline
solution. Thus, the hydrolysis of proteins can be carried out under such
conditions with the complete recovery of most of the constituent free amino
acids.
    Since all of the amino acids except glycine possess a center of asymmetry
at the ALPHA carbon atom, they can exist in either of two optically active,
mirror-image forms or enantiomorphs. All of the common amino acids of proteins
appear to have the same configuration about the ALPHA carbon: this
configuration is symbolized by the prefix L-. The opposite, generally
unnatural, form is given the prefix D-. Some amino acids, such as isoleucine,
threonine, and hydroxyproline, have a second center of asymmetry and can exist
in four stereoisomeric forms. The prefix allo- is used to indicate one of the
two alternative configurations at the second asymmetric center; thus,
isoleucine, for example, can exist in the L, L-allo, D, and D-allo forms. See
STEREOCHEMISTRY.
    Another important general property of all amino acids is their ionic
state. The basic amino group can bind a proton from solution and become a
cation; the acidic carboxyl group can release a proton into solution and
become an anion. At the isoelectric point (the pH at which the molecule has no
net charge), amino acids exist as dipolar ions or zwitterions, while in strong
acid solution, the carboxyl group exists in the undissociated form, and the
molecule becomes a cation.
    Since most amino acids occur in conjugated form, their isolation usually
requires their prior release in free form by acid or alkaline hydrolysis.
Hydrolysates of proteins or other polypeptides, or crude extracts of plant,
animal, or microbial materials, serve as the starting point for the isolation
in pure form of single amino acids. Chromatography has been the method of
choice for amino acid isolation ever since its first application in 1941. See
CHROMATOGRAPHY.
    Since amino acids, as precursors of proteins, are essential to all
organisms, all cells must be able to synthesize those they cannot obtain from
their environment. The biosynthesis of amino acids is relatively complicated
and usually requires at least three enzymatic steps. The actual metabolic
pathways by which amino acids are synthesized are generally found to be the
same in all living cells investigated, whether microbial or animal.
Biosynthetic mechanisms thus appear to have developed soon after the origin of
life and to have remained unchanged throughout the divergent evolution of
modern organisms. There is only one quantitatively important reaction by which
organic nitrogen enters the amino groups of amino acids: the reductive
amination of ALPHA-ketoglutaric acid to glutamic acid by the enzyme glutamic
acid dehydrogenase. All other amino acids are formed either by transamination
(transfer of an amino group, ultimately from glutamic acid) or by a
modification of an existing amino acid.
    Most organisms are capable of degrading some amino acids, and metabolic
pathways leading to degradation to CO2 and H2O are known for each of the
common amino acids. These pathways are detailed in the articles on the
individual amino acids.
    The nutritional requirement for the amino acids of protein can vary from
zero, in the case of an organism which synthesizes them all, to the complete
list, in the case of an organism in which all the biosynthetic pathways are
blocked. There are 8 or 10 amino acids required by certain mammals; most
plants synthesize all of their amino acids, while microorganisms vary from
types which synthesize all, to others (such as certain lactic acid bacteria)
which require as many as 18 different amino acids. See NUTRITION; PROTEIN.


 �34s[Edward A. Adelberg; Paul T. Magee]
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�6v�2v�32sAmino sugar �21m �33s
    A sugar in which a nonglycosidic hydroxyl group is replaced by an amino or
substituted amino group (see illustration). The most abundantly occurring
amino sugar in nature is D-glucosamine (2-amino-2-deoxy-D-glucose). This amino
sugar is a constituent of glycoproteins, which include the mucins from saliva
and from egg albumin. It most frequently occurs as chitin. D-Glucosamine also
occurs together with neutral sugars in many bacterial polysaccharides. Other
types of amino sugars have been recognized in antibiotics. Uridine diphosphate
N-acetyl-D-glucosamine occurs in baker's yeast and in a higher plant (mung
bean). See MONOSACCHARIDE.

 �34s[William Z. Hassid]
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�6v�2v�32sAmmeter   �21m �33s 
    An electrical instrument for the measurement of electric current. In the 
usual indicating ammeter, an electromechanical system causes a pointer to 
traverse a calibrated scale, the pointer position indicating the value of the 
current. Several kinds of ammeter mechanism are used for different kinds of 
current, different applications, and differing degrees of accuracy. See 
ELECTRODYNAMIC AMMETER; PERMANENT-MAGNET MOVABLE-COIL AMMETER; POLARIZED-VANE 
AMMETER; SOFT-IRON AMMETER; THERMAL AMMETER. 
 �34s[John H. Miller/Jesse Beams]
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�6v�2v�32sAmmine   �21m �33s 
    One of a group of complex compounds formed by the coordination of ammonia 
molecules with metal ions and, in a few instances, such as calcium, strontium, 
and barium, with metal atoms. Although ammines are formally analogous to many 
salt hydrates, the general characteristics of the group of ammines differ 
considerably from those of the hydrates. For example, hydrated Co(III) salts 
are strong oxidizing agents whereas Co(II) ammines are strong reducing agents. 
The ammines of principal interest are those of the transition metals and of 
the zinc family, but even here there is wide variation in stability or rate of 
decomposition. Ammines are prepared by treating aqueous solutions of the metal 
salt with ammonia or, in some instances, by the action of dry gaseous or 
liquid ammonia on the anhydrous salt. See AMMONIA; COORDINATION CHEMISTRY. 
 �34s[Harry H. Sisler]
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�6v�2v�32sAmmonia   �21m �33s 
    The most familiar compound composed of the elements nitrogen and hydrogen, 
NH3. It is formed as a result of the decomposition of most nitrogenous organic 
material, and its presence is indicated by its pungent and irritating odor. 
    Ammonia has a wide range of industrial and agricultural applications. 
Examples of its use are the production of nitric acid and ammonium salts, 
particularly the sulfate, nitrate, carbonate, and chloride, and the synthesis 
of hundreds of organic compounds including many drugs, plastics, and dyes. Its 
dilute aqueous solution finds use as a household cleansing agent. Anhydrous 
ammonia and ammonium salts are used as fertilizers, and anhydrous ammonia also 
serves as a refrigerant, because of its high heat of vaporization and relative 
ease of liquefaction. See FERTILIZER. 
    The physical properties of ammonia are analogous to those of water and 
hydrogen fluoride in that the physical constants are abnormal with respect to 
those of the binary hydrogen compounds of the other members of the respective 
periodic families. These abnormalities may be related to the association of 
molecules through intermolecular hydrogen bonding. Ammonia is highly mobile in 
the liquid state and has a high thermal coefficient of expansion. 
    Most of the chemical reactions of ammonia may be classified under three 
chief groups: (1) addition reactions, commonly called ammonation; (2) 
substitution reactions, commonly called ammonolysis; and (3) 
oxidation-reduction reactions. 
    Ammonation reactions include those in which ammonia molecules add to other 
molecules or ions. Most familiar of the ammonation reactions is the reaction 
with water to form ammonium hydroxide. The strong tendency of water and 
ammonia to combine is evidenced by the very high solubility of ammonia in 
water. Ammonia reacts readily with strong acids to form ammonium salts. 
Ammonium salts of weak acids in the solid state dissociate readily into 
ammonia and the free acid. Ammonation occurs with a variety of molecules 
capable of acting as electron acceptors (Lewis acids), such as sulfur 
trioxide, sulfur dioxide, silicon tetrafluoride, and boron trifluoride. 
Included among ammonation reactions is the formation of complexes (called 
ammines) with many metal ions, particularly transition metal ions. Ammonolytic 
reactions include reactions of ammonia in which an amide group (SINGLE BOND 
NH2), an imide group (DOUBLE BOND NH), or a nitride group (TRIPLE BOND N) 
replaces one or more atoms or groups in the reacting molecule. 
    Oxidation-reduction reactions may be subdivided into those which involve a 
change in the oxidation state of the nitrogen atom and those in which 
elemental hydrogen is liberated. An example of the first group is the 
catalytic oxidation of ammonia in air to form nitric oxide. In the absence of 
a catalyst, ammonia burns in oxygen to yield nitrogen. Another example is the 
reduction with ammonia of hot metal oxides such as cupric oxide. 
    The physical and chemical properties of liquid ammonia make it appropriate 
for use as a solvent in certain types of chemical reactions. The solvent 
properties of liquid ammonia are, in many ways, qualitatively intermediate 
between those of water and of ethyl alcohol. This is particularly true with 
respect to dielectric constant; therefore, ammonia is generally superior to 
ethyl alcohol as a solvent for ionic substances but is inferior to water in 
this respect. On the other hand, ammonia is generally a better solvent for 
covalent substances than is water. 
    The Haber-Bosch synthesis is the major source of industrial ammonia. In a 
typical process, water gas (CO, H2, CO2) mixed with nitrogen is passed through 
a scrubber cooler to remove dust and undecomposed material. The CO2 and CO are 
removed by a CO2 purifier and ammoniacal cuprous solution, respectively. The 
remaining H2 and N2 gases are passed over a catalyst at high pressures (up to 
1000 atm or 100 megapascals) and high temperatures (approx. 1300 DG F or 
700 DG C). Other industrial sources of ammonia include its formation as a 
by-product of the destructive distillation of coal, and its synthesis through 
the cyanamide process. In the laboratory, ammonia is usually formed by its 
displacement from ammonium salts (either dry or in solution) by strong bases. 
Another source is the hydrolysis of metal nitrides. See AMIDE; NITROGEN. 
 �34s[Harry H. Sisler]
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�6v�2v�32sAmmonia maser clock   �21m �33s 
    A type of atomic clock which employs maser oscillation of ammonia 
molecules in a microwave cavity. One pair of energy levels in the ground state 
of the ammonia molecule is separated by 23,800 megahertz (the inversion line). 
It is possible to focus ammonia molecules in the upper state into a cavity 
resonant at the inversion frequency by means of inhomogeneous electric fields. 
If the cavity losses are low enough and a large enough number of molecules per 
unit time enter, and if the cavity is long enough so that the interaction time 
is sufficient, maser oscillator action will occur. Unfortunately the 
interaction time is not very long, so that the line width is broad and the 
clock is not very accurate. The stability for very short times is good, 
however, since it depends on the power level and frequency of the maser, both 
of which are relatively high. See ATOMIC CLOCK; MASER. 
 �34s[Leonard S. Cutler]
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�6v�2v�32sAmmonium salt �21m �33s
    A product of a reaction between ammonia, NH3, and various acids. The
general reaction for formation is NH3 + HX YIELDS NH2X. Examples of ammonium
salts are ammonium chloride, NH4Cl, ammonium nitrate, NH4NO3, ammonium sulfate,
(NH4)2SO4, and ammonium carbonate, (NH4)2CO3. These compounds are addition
products of ammonia and the acid. For this reason, their formulas are sometimes
written as [H(NH3)]X. 
    All ammonium salts decompose into ammonia and the acid when heated. Their
stability, however, varies according to the nature of the acid. Salts of weak
acids decompose at lower temperatures than do salts of strong acids.
    Ammonium chloride is made by absorbing ammonia in hydrochloric acid. This
salt, sometimes called sal ammoniac, is used in galvanizing iron, in textile
dyeing, and in manufacturing dry cell batteries.
    Ammonium nitrate is prepared from ammonia and nitric acid. It is used as a
source of nitrous oxide, N2O, or laughing gas, and in the manufacture of
explosives. A mixture of ammonium nitrate and trinitrotoluene is known as
amatol.
    Ammonium sulfate, obtained from ammonia and sulfuric acid, is prepared
commercially by passing ammonia and carbon dioxide, CO2, into a suspension of
finely ground calcium sulfate, CaSO4. Large quantities are also produced as a
by-product of coke ovens and coal-gas works. The chief use of ammonium sulfate
is as a fertilizer.
    Ammonium carbonate may be prepared by bringing ammonia and carbon dioxide
together in aqueous solution. It is also obtained by heating a mixture of
ammonium sulfate and a fine suspension of calcium carbonate. See AMMONIA;
FERTILIZER; HYDROLYSIS.
 �34s[Francis J. Johnston]
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�6v�2v�32sAmmonoidea   �21m �33s 
    An order of the tetrabranchiate cephalopods possibly derived from the 
Nautiloidea. They first appeared in the Ordovician, reached their greatest 
period from the Devonian to the Jurassic, and disappeared at the end of the 
Cretaceous. The soft parts are unknown; the shells are coiled, have wrinkled 
septa, more or less highly sculptured exteriors, and complex sutures. It is 
the largest order of the Cephalopoda. Their morphology and structure are so 
varied that they are important index fossils. See CEPHALOPODA; INDEX FOSSIL; 
NAUTILOIDEA.  
 �34s[Gilbert L. Voss]
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�6v�2v�32sAmnesia   �21m �33s 
    The pathological loss or impairment of memory. Amnesia may be psychogenic 
with no detectable organic basis, or it may reflect a gross disturbance of 
cerebral function. 
    Psychogenic amnesia varies in degree from the mild lapses of memory to 
fugue states, in which the subject has no memory for extended periods. Current 
psychiatric opinion holds that these disorders are the result of emotional 
conflicts which the amnesia temporarily resolves. Thus the psychiatrist views 
amnesia as a defense mechanism. 
    Amnesias associated with cerebral dysfunction may arise from almost any 
form of injury or disease involving the brain. In cerebral dysfunction of 
sudden onset, as in acute trauma to the head or in epileptic attacks, there 
may be amnesia for immediately preceding events (retrograde amnesia), as well 
as for events subsequent to the apparent restoration of full consciousness 
(anterograde amnesia). With recovery, the period of complete memory loss may 
diminish in both directions, sometimes leaving ``islands'' of memory 
surrounded by periods of residual memory loss. Amnesia associated with chronic 
cerebral dysfunction is characterized by continuous difficulty in remembering 
recent events, whereas early memories are relatively intact. See MEMORY. 
 �34s[Josephine Semmes]
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�6v�2v�32sAmnion   �21m �33s 
    A thin, cellular, extraembryonic membrane forming a closed sac which 
surrounds the embryo in all reptiles, birds, and mammals and is present only 
in these forms; hence the collective term amniotes is applied to these 
animals. The amnion contains a serous fluid in which the embryo is immersed. 
See AMNIOTA; FETAL MEMBRANE. 
    Typically, the amnion wall is a tough, transparent, nerve-free, and 
nonvascular membrane consisting of two layers of cells: an inner, 
single-cell-thick layer of ectodermal epithelium, continuous with that 
covering the body of the embryo as the outer layer of its skin, and an outer 
covering of mesodermal, connective, and specialized smooth muscular tissue, 
also continuous with the mesodermal germ layer of the embryo. 
    The major function of the amnion and its fluid is to protect the delicate 
embryo. Thus, developmental stages of terrestrial animals are provided with 
the same type of cushioning against mechanical shock as is provided by the 
water environment of aquatic forms. See GERM LAYERS. 
 �34s[Nelson T. Spratt, Jr.]
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�6v�2v�32sAmniota   �21m �33s 
    A collective term for the classes Reptilia, Aves, and Mammalia of the 
subphylum Vertebrata. Members are characterized by having a series of 
specialized protective extraembryonic membranes during development. Three of 
the membranes - amnion, chorion or serosa, and allantois - occur only in this 
group, but a fourth, the yolk sac, is sometimes present and is found in many 
anamniotes. The presence of the extraembryonic membranes makes it possible for 
the embryonic development of the amniotes to take place out of the water. In 
the most primitive forms the early stages of development take place inside a 
shell-covered egg that is deposited on land. See ALLANTOIS; AMNION; ANAMNIA; 
CHORION; YOLK SAC. 
 �34s[Jay M. Savage]
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�6v�2v�32sAmoebida   �21m �33s 
    An order of Lobosia without protective coverings (tests). Pellicles may be 
thin, as in Amoeba proteus, or thicker and less flexible, as in Thecamoeba 
verrucosa. Pellicular folds may develop during locomotion, particularly in 
species with thick pellicles. Both flagellate and ameboid stages occur in 
certain soil amebas. 
    Locomotion of Amoebida varies somewhat from genus to genus. In some cases 
movement involves protoplasmic flow of the body as a whole, without typical 
pseudopodia. There may be several ridged indeterminate pseudopodia, into one 
of which the organism appears to flow in locomotion. In some species 
determinate pseudopodia never become large enough to direct locomotion. In 
some cases the form of the pseudopodia may vary in a single species. 
    Amebas, normally phagotrophic, usually contain food vacuoles. Certain 
species contain crystals of apparently differing chemical nature. Other 
inclusions are globules of different sizes, mitochondria, and stored food 
reserves. In addition, bacteria or algae may occur in the cytoplasm, changing 
the color to a gray or green. Nuclei range in number from one to several 
hundred, as in Chaos carolinensis. The giant amebas are visible without a 
microscope.  
    Those species found in the digestive tract of invertebrates and 
vertebrates include relatively harmless species and a few pathogens, such as 
Entamoeba histolytica of humans and E. invadens of reptiles. Entamoeba coli 
does not invade human tissues. Also limited to the lumen of the colon are 
Endolimax nana, Iodamoeba buetschlii, and Dientamoeba fragilis. These four are 
relatively harmless although sometimes associated with digestive disturbances. 
Uncooked vegetables from soil fertilized with human feces are a potential 
source of infection. Standard methods of water purification seem reasonably 
protective, but it is difficult to control spread by food handlers. See 
AMEBIASIS; LOBOSIA; PROTOZOA; SARCODINA; SARCOMASTIGOPHORA. 
 �34s[Richard P. Hall]
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�6v�2v�32sAmorphous solid   �21m �33s 
    A rigid material whose structure lacks crystalline periodicity; that is, 
the pattern of its constituent atoms or molecules does not repeat periodically 
in three dimensions. In the present terminology amorphous and noncrystalline 
are synonymous. A solid is distinguished from its other amorphous counterparts 
(liquids and gases) by its viscosity: a material is considered solid (rigid) 
if its shear viscosity exceeds 10 sup 14.6 poise (10 sup 13.6 Pa PRODUCT s). 
See CRYSTAL; SOLID-STATE PHYSICS; VISCOSITY.  
    Oxide glasses, generally the silicates, are the most familiar amorphous 
solids. However, as a state of matter, amorphous solids are much more 
widespread than just the oxide glasses. There are both organic (for example, 
polyethylene and some hard candies) and inorganic (for example, the silicates) 
amorphous solids. Glasses can be prepared which span a broad range of physical 
properties. Dielectrics (for example, SiO2) have very low electrical 
conductivity and are optically transparent, hard, and brittle. Semiconductors 
(for example, As2SeTe2) have intermediate electrical conductivities and are 
optically opaque and brittle. Metallic glasses have high electrical and 
thermal conductivities, have metallic luster, and are ductile and strong. See 
METALLIC GLASSES. 
    The obvious uses for amorphous solids are as window glass, container 
glass, and the glassy polymers (plastics). Less widely recognized but 
nevertheless established technological uses include the dielectrics and 
protective coatings used in integrated circuits, and the active element in 
photocopying by xerography, which depends for its action upon photoconduction 
in an amorphous semiconductor. In optical communications a highly transparent 
dielectric glass in the form of a fiber is used as the transmission medium. 
See OPTICAL COMMUNICATIONS. 
    It is the changes in short-range order (on the scale of a localized 
electron), rather than the loss of long-range order alone, that have a 
profound effect on the properties of amorphous semiconductors. For example, 
the difference in resistivity between the crystalline and amorphous states for 
dielectrics and metals is always less than an order of magnitude and is 
generally less than a factor of 3. For semiconductors, however, resistivity 
changes of 10 orders of magnitude between the crystalline and amorphous states 
are not uncommon, and accompanying changes in optical properties can also be 
large.  
    One class of amorphous semiconductors is the glassy chalcogenides, which 
contain one (or more) of the chalcogens sulfur, selenium, or tellurium as 
major constituents. These materials have application in switching and memory 
devices. Another group is the tetrahedrally bonded amorphous solids, such as 
amorphous silicon and germanium. These materials cannot be formed by quenching 
from the melt (that is, as glasses) but must be prepared by one of the 
deposition techniques mentioned above. 
    When amorphous silicon (or germanium) is prepared by evaporation, not all 
bonding requirements are satisfied, so a large number of dangling bonds are 
introduced into the material. These dangling bonds create states deep in the 
gap which limit the transport properties. The number of dangling bonds can be 
reduced by a thermal anneal below the crystallization temperature, but the 
number cannot be reduced sufficiently to permit doping. See GLASS SWITCH; 
SEMICONDUCTOR.  
 �34s[Brian G. Bagley]
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�6v�2v�32sAmpere's law   �21m �33s 
    A law of electromagnetism which expresses the contribution of a current 
element of length dl to the magnetic induction (flux density) B at a point 
near the current. Ampere's law, sometimes called Laplace's law, was derived by 
A. M. Ampere after a series of experiments during 1820-1825. 
    Whenever an electric charge is in motion, there is a magnetic field 
associated with that motion. The flow of charges through a conductor sets up a 
magnetic field in the surrounding region. Any current may be considered to be 
broken up into infinitesimal elements of length dl, and each such element 
contributes to the magnetic induction at every point in the neighborhood. The 
contribution dB of the element is found to depend upon the current I, the 
length dl of the element, the distance r of the point P from the current 
element, and the angle THETA between the current element and the line joining 
the element of the point P (see illustration). Ampere's law expresses the 
manner of the dependence by Eq. (1). The field near a current may be 
calculated by finding the vector sum of the contributions of all the various 
elements that make up the current. 
  
                  *** dB = k(I dl sin THETA) / r sup 2 ***              (1)
  
    The proportionality factor k depends upon the units used in Eq. (1) and 
upon the properties of the medium surrounding the current. In the SI system, 
the factor k is assigned a value of 10 sup -7 weber/ampere-meters when the 
current is in empty space. As in other equations associated with electric and 
magnetic fields, for example Coulomb's law, it is convenient to replace k by a 
new factor MU 0 related to k as in Eq. (2). This substitution removes the 

                           *** MU 0 = 4 PI k ***                        (2)

factor 4 PI from many derived equations in which it would otherwise appear. 
With this substitution Ampere's law becomes Eq. (3). The factor MU 0 is called 
the permeability of empty space. See ELECTRICAL UNITS AND STANDARDS; MAGNETIC 
  
  
             *** dB = MU 0/4 PI (I dl sin THETA) / r sup 2 ***          (3)
  
    The direction of dB at each point may be described in terms of a 
right-hand rule. If the current element is grasped by the right hand with the 
thumb pointing in the direction of the current, the fingers encircle the 
current in the direction of the magnetic induction. 
    Figure caption: Graphic representation of Ampere's law.
 �34s[Kenneth V. Manning]
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�6v�2v�32sAmphetamine   �21m �33s 
    A stimulating drug that affects the brain and the body in a variety of 
ways; also known by the trade name Benzedrine. Chemically, amphetamine is a 
racemic mixture of the l and d isomers of ALPHA-methyl-BETA-phenethylamine. 
The l isomer has more pronounced effects on the body, while the d isomer 
(commonly referred to as Dexedrine) has a greater effect on the brain. On the 
whole, the pharmacological effects of an amphetamine are to produce an 
increase in blood pressure, a relaxation of bronchial smooth muscle, a 
constriction of the blood vessels supplying the skin and mucous membranes, and 
a variety of alterations in behavior. 
    The ability of amphetamine to contract blood vessels in the mucous 
membranes and to relax smooth muscles in the lung make it an efficacious nasal 
decongestant. In a like manner, the pronounced excitatory effects of 
amphetamine on the brain have led to several other medical uses. For example, 
its ability to cause hyperventilation has resulted in its use as an analeptic. 
The arousing and insomnia-producing effects have been exploited to treat 
narcolepsy, a disease characterized by an inability to stay awake. 
Furthermore, the ability of amphetamine to cause a loss of appetite has 
promoted its use in diet programs as an anorexic; however, tolerance to this 
effect limits its usefulness. Finally, the enhanced sense of well-being and 
mild euphoria that are seen after taking amphetamine have led to its use in 
certain forms of psychiatric depression. One important use of amphetamine is 
in the treatment of hyperkinetic children, although the paradox of how a 
stimulant can do this is not understood. 
    The typical side effects on the body include dry mouth, heart rhythm 
alterations (palpitations and arrhythmias), hypertension, stomach cramps, and 
decreased urinary frequency. The central nervous system side effects include 
dizziness, dysphoria, headache, tremor, restlessness, insomnia, decreased 
appetite, increased aggressiveness, anxiety, and paranoid panic states. 
Extreme overdosage can result in convulsions, cerebral hemorrhaging, coma, and 
death.  
    Serious abuse occurs in some individuals who take the drug over long 
periods. As with other drugs of abuse (such as morphine, alcohol, or 
barbiturates), tolerance will occur after repeated dosing. Thus, whereas in 
the past one pill might be sufficient to induce the appropriate high, it 
becomes necessary to increase that amount in the future to achieve the same 
feeling. Over time, the users may become ``psychologically dependent'' on 
amphetamine. Chronic use of large amounts of amphetamine can have severe 
effects on personality. It has been shown that large doses of amphetamine can 
induce a behavioral state in humans that is nearly indistinguishable from 
paranoid schizophrenia, but can be reversed upon cessation of the drug. The 
question of whether or not physical dependence occurs with amphetamine is 
unresolved. It is known that after long-term treatment with amphetamine, 
removal of the drug results in feelings of anxiety, stomach cramps, headache, 
and extreme fatigue. However, the mild severity of these symptoms makes in 
unclear if this is a true withdrawal response. See ALCOHOLISM; DRUG ADDICTION; 
NARCOTIC.  
 �34s[Richard E. Chipkin]
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�6v�2v�32sAmphibia   �21m �33s 
    One of the four classes composing the superclass Tetrapoda of the 
subphylum Vertebrata. The living amphibians number approximately 2460 species, 
and are classified in three orders; the Anura or Salientia (frogs and toads, 
slightly less than 2000 species); Urodela or Caudata (salamanders, 300 
species); and Apoda or Gymnophiona (caecilians, about 160 species). The orders 
in the subclasses Labyrinthodontia and Lepospondyli were in the geologic past 
and are now extinct. A classification scheme for the Amphibia follows. 

Class Amphibia 
    Subclass Labyrinthodontia 
        Order Ichthyostegalia 
        Order Temnospondyli 
        Order Anthracosauria 
    Subclass Lepospondyli 
        Order Nectridea 
        Order Aistopoda 
        Order Microsauria 
    Subclass Lissamphibia 
        Order Anura 
        Order Urodela 
        Order Apoda 

    A typical amphibian is characterized by a moist, glandular skin, the 
possession of gills at some point in its life history, four limbs, and an egg 
lacking the embryonic membrane called the amnion. See AMNION; ANAMNIA. 
    Amphibians are among the so-called cold-blooded animals; that is, the 
temperature of the body of an amphibian is not regulated internally to a high 
level as is that of mammals and birds, but fluctuates with that of the 
environment. An animal such as an amphibian that burns none of its food energy 
in keeping warm is able to get along on much less food than a bird or mammal 
of similar size. This advantage is offset by the inability of amphibians to be 
active under cold conditions that do not inhibit a warm-blooded animal. Thus 
the far northern and southern parts of the world which support large 
populations of birds and mammals are almost devoid of amphibian life. See 
THERMOREGULATION.  
    The amphibians mark a significant point in the evolution of the 
vertebrates, the transition from aquatic to terrestrial life. As animals 
neither divorced from the water nor fully at home on land, they suffer from 
their intermediate mode of life. Reptiles, and later mammals, came to dominate 
the land, and fishes the waters, leaving the amphibians of today as a 
relatively unimportant but nevertheless highly interesting group of 
vertebrates. Amphibian history can be divided into two nearly completely 
separable portions: an older chapter, from Devonian to Triassic, in which the 
fossils belong to entirely extinct orders, and a later chapter, treating the 
development of the modern anurans and urodeles. See TETRAPODA. 
 �34s[Richard G. Zweifel]
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�6v�2v�32sAmphibole   �21m �33s 
    A large group of common rock-forming inosilicate (metasilicate) minerals. 
The amphiboles exhibit a wide range of compositional variation, as indicated 
by the generalized formulas 

   *** (Na,Ca)2-3 (Mg,Fe sup 2+, Fe sup 3+,Al)5 (Si,Al)8 O22 (OH,O,F)2 ***     
   ***            (Mg,Fe sup 2+,Fe sup 3+,Al)7 (Si,Al)8 O22 (OH,O,F)2  ***
  
The amphiboles represent a complex series of solid solutions between a variety
of idealized end members. The species names of these end members ae usually
the best known; the more important ones are listed below. See SILICATE
MINERALS.

        Orthorhombic amphiboles
Anthophyllite             (Mg,Fe sup 2+)7 Si8 O22 (OH)2
Gedrite                   (Mg,Fe sup 2+)5 Al2 (Si6 Al2) O22 (OH)2

        Monoclinic amphiboles
Cummingtonite             (Fe sup 2+,Mg)7 Si8 O22 (OH)2
Tremolite                 Ca2 Mg5 Si8 O22 (OH)2
Tschermakite              Ca2 Mg3 Al2 (Si6 Al2) (OH)2
Edenite                   NaCa2 Mg5 (Si7 Al) O22 (OH)2
Glaucophane               Na2 (Mg3Al2) Si8 O22 (OH)2
Riebeckite                Na2 (Fe3 sup 2+ Fe2 sup 3+) Si8 O22 (OH)2
Arfvedsonite              Na3 Fe4 sup 2+ Fe sup 3+ Si8 O22 (OH)2
Eckermannite              Na3 Mg4 (Fe sup 3+, Al) Si8 O22 (OH)2

    Each of the listed calcium (Ca) amphiboles has a ferrous iron equivalent 
and would be indicated by the prefix ferro-, for example, ferrotremolite. 
Also, some ferric iron and oxygen in the place of the OH groups can occur in 
the ferroamphiboles and is indicated by the prefix ferri-, for example, 
ferritremolite; the term oxyhornblende is commonly applied to the 
ferriamphiboles. A high content of fluorine is indicated by the prefix 
fluoro-, for example, fluorocummingtonite. For the Na-Ca amphiboles, solid 
solution between any of the end members is possible so that a single crystal 
could be composed of contributions from some 10 different end members. The 
general term hornblende is usually applied to these solid solutions of the 
Na-Ca amphiboles. See ANTHOPHYLLITE; CUMMINGTONITE; HORNBLENDE. 
    In detail, the magnesium-rich amphiboles are generally light colored 
(white, gray, light green) with the color darkening to dark green, dark brown, 
or black with increasing iron content. The presence of sodium is often 
indicated by a bluish color, especially in thin sections. The Mg-rich 
amphiboles commonly develop long needle-like or fibrous crystals, and there is 
a rough correlation such that, as the iron and aluminum content increases, the 
individual crystals become progressively more short and stubby. 
    The amphiboles as a group are present as minor constituents in many 
volcanic, igneous, and metamorphic rocks and are thus able to form over most 
of the temperature range observed in the Earth's crust. Amphiboles are present 
as major constituents in many metamorphic schists and gneisses. The amphiboles 
can be altered to a variety of decomposition products with talc, antigorite, 
chlorite, and epidote the most commonly observed. See AMPHIBOLITE; ASBESTOS; 
GLAUCOPHANE; JADE; TREMOLITE. 
 �34s[George W. DeVore]
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�6v�2v�32sAmphibolite   �21m �33s 
    A class of metamorphic rocks with one of the amphibole minerals as the 
dominant constituent. Most of the amphibolites are dark green to black 
crystalline rocks that occur as extensive layers widely distributed in 
mountain belts and deeply eroded shield areas of the continental crust. 
Amphibolite is the main country rock that has been intruded by the large 
granite masses found in most mountain ranges, with small and large masses of 
amphibolite present also as inclusions in granites. See AMPHIBOLE. 
    Amphibolites are the products of regional metamorphism and crustal 
deformation of older materials of appropriate composition. The features of the 
original rock are obliterated; thus it is difficult and sometimes impossible 
to determine the premetamorphic rock. Apparent differences in the formation of 
the bulk composition are used to classify amphibolites as ortho or para. 
Compositional relations between the minor elements titanium, chromium, and 
nickel have been used to distinguish the ortho from para amphibolites in some 
occurrences. See METAMORPHIC ROCKS. 
 �34s[George W. DeVore]
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�6v�2v�32sAmphidiscophora �21m �33s
    A subclass of sponges of the class Hexactinellida, in which the
parenchymal microscleres are birotulates and never hexasters. The parenchymal
megascleres  are always free and unconnected, never fusing to form a rigid
network. The sponges are never firmly fixed to a hard substratum, but are
anchored to the bottom sediments by a basal tuft or tufts of spicules (see
illustration). The recognized orders are Amphidiscosa and Hemidiscosa. Fossils
of entire amphidiscophoran sponges are rare. See AMPHIDISCOSA; HEMIDISCOSA;
HEXACTINELLIDA.


 �34s[Willard D. Hartman]
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�6v�2v�32sAmphidiscosa   �21m �33s 
    An order of the subclass Amphidiscophora in the class Hexactinellida. 
These sponges are distinguished from the order Hemidiscosa in that the 
birotulates are amphidiscs. Examples of this order are Pheronema, Monorhaphis, 
and Hyalonema. The record of fossil Amphidiscosa is poor, but goes back to the 
Carboniferous Uralonema. See AMPHIDISCOPHORA; HEMIDISCOSA. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sAmphilinidea   �21m �33s 
    An order of tapeworms of the subclass Cestodaria. These worms have a 
protrusible proboscis and frontal glands at the anterior end. No holdfast 
organ is evident. All members of the order inhabit the coelom of sturgeon and 
other fish. The only life history which is completely known is that of 
Amphilina. The 10-hooked embryos leave the parental uterus through a pore, and 
if upon escaping into the water they are eaten by an amphipod crustacean, they 
undergo further development to the procercoid larva. When the parasitized 
amphipod is eaten by a sturgeon, the larval worm enters the coelom of the fish 
and develops to sexual maturity. See CESTODARIA. 
 �34s[Clark P. Read]
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�6v�2v�32sAmphipoda   �21m �33s 
    An order of crustaceans in the subclass Malacostraca, which lack a 
carapace, bear unstalked eyes, and respire by thoracic branchiae, or gills. 
The abdomen usually bears three pairs of biramous swimmerets (pleopods), three 
pairs of rather rigid uropods, and a telson which may be lobed or entire. The 
body is usually flattened laterally, and the pereiopods (walking legs) are 
elongated so that walking is difficult. The maxillipeds lack epipodites. The 
sexes are separate, but reproductive and copulatory organs are very simple. 
The eggs are extruded by the female into a ventral brood pouch composed of 
setose lamellae attached to the medial bases of the legs. The young hatch as 
miniature adults, growing usually to a length of 0.12 - 0.18 in. (3 - 12 mm), 
and in exceptional cases to 5.6 in. (140 mm). See ISOPODA. 
    Four suborders are known, the Gammaridea, Hyperiidea, Caprellidea, and 
Ingolfiellidea. Amphipods are very abundant in the oceans, being represented 
by 3200 species. More than 600 other species occur in streams, lakes, and 
subterranean waters and in terrestrial leaf molds and mosses. Many are 
excellent swimmers. Nonpelagic species of the suborders Gammaridea and 
Caprellidea live on aquatic bottoms, plants, and epifaunal growths. Predation 
by amphipods is occasional. Their mouthparts are well adapted for chewing: 
they either eat aquatic plants, debris, and detritus or swallow mud containing 
food particles. 
    Marine species are important food for various stages of many commercial 
fishes. Hyperiids are the principal food of seals at certain seasons, and also 
of balaenoid whales at times. One gammaridean genus, Chelura, is a minor wood 
borer, associated with the isopod Limnoria. 
    A few fossil species are known in Tertiary amber deposits. See AMBER; 
CAPRELLIDEA; GAMMARIDEA; HYPERIIDEA; MALACOSTRACA. 
 �34s[J. Laurens Barnard]
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�6v�2v�32sAmphotericin B   �21m �33s 
    An antifungal antibiotic suitable for systemic therapy of deep mycotic 
infections as well as of superficial infections. It is produced by 
fermentation with a strain of Streptomyces nodosus. See MEDICAL MYCOLOGY; 
STREPTOMYCETACEAE.  
    In human therapy, desirable therapeutic activity in systemic infections is 
best shown when the drug is administered intravenously. Oral dosage is 
apparently not absorbed rapidly enough by humans for the best therapeutic 
effect in any systemic infections except with very sensitive yeastlike fungi. 
    In therapy of deep mycoses a more soluble mixture of amphotericin B with 
sodium deoxycholate is dissolved in dextrose solution and infused 
intravenously. Correct dosages give therapeutic results in many cases of 
disseminated mycotic infections, including coccidioidomycosis, cryptococcosis, 
moniliasis, and histoplasmosis. See ANTIBIOTIC; COCCIDIOIDOMYCOSIS; 
CRYPTOCOCCOSIS; HISTOPLASMOSIS. 
 �34s[Ralph E. Bennett]
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�6v�2v�32sAmplifier   �21m �33s 
    A device capable of increasing the magnitude or power level of a physical 
quantity that is varying with time, without distorting the wave shape of the 
quantity. The great majority of amplifiers are electronic and depend upon 
transistors or vacuum tubes for their operation. A small number of electronic 
amplifiers are magnetic amplifiers, while others take the form of rotating 
electrical machinery, such as the Amplidyne. A common nonelectrical amplifier 
is the hydraulic actuator, which is an amplifier of mechanical forces. See 
DIFFERENTIAL AMPLIFIER; DIRECT-CURRENT MOTOR; HYDRAULIC ACTUATOR; MAGNETIC 
AMPLIFIER; OPERATIONAL AMPLIFIER; TRANSISTOR AMPLIFIER; VACUUM-TUBE AMPLIFIER. 
    Amplifiers of either the transistor or tube variety may be classified in
any one of several ways. One is by means of the coupling circuitry, such as
an RC-coupled (resistance-capacitance-coupled) amplifier. Other categories 
include classification by purpose, such as hi-fi (high-fidelity audio 
amplifier), and mode of operation, such as class A. Some of the more common 
methods of classification follow with a brief description of the operating 
properties.
    A great deal of information about an amplifier is conveyed by merely
stating the interstage coupling method used. The RC-coupled amplifier is so
named because of the coupling capacitor from the load resistor of one stage
to the input of the following stage. The transformer-coupled amplifier is so
named because two stages are coupled through a transformer. The direct-coupled 
amplifier, usually called a dc amplifier, allows the amplification of dc as
well as ac signals. It has special circuitry, either in the coupling network or
in the type of amplifier stages, which eliminates the need for the coupling
capacitor. 
    An important feature of some amplifiers is their ability to amplify only
those signals lying within a certain band of frequencies and to reject signals
outside this band. Other amplifiers are designed to amplify an extremely wide
range of frequencies. A tuned amplifier is one which is designed to amplify
signals in a frequency band that is centered on a chosen carrier frequency and
not to amplify signals which lie outside this band. Tuned amplifiers form an
important part of radio and television receivers in the rf and i-f amplifier
sections. An untuned amplifier is one in which the coupling circuitry is not a
tuned circuit. The audio amplifier and the direct-coupled amplifier are the
most important of this type. 
    The operating modes of vacuum tubes are prescribed by the designations 
class A, class B, and class C, with certain divisions among the classes. The
distinction between the various classes is determined for a sinusoidal signal
voltage applied to the input. The position of the quiescent point and the
extent of the characteristic that is being used determine the method of 
operation.
    Amplifiers may be classified by the use for which they are designed. Audio-
frequency (af) amplifiers are intended to operate over the general range of
about 20 - 20,000 Hz. Radio-frequency (rf) amplifiers are used to amplify     
signals in the range of about 100 - 1,000,000 kHz. They are usually used as    
the first amplifier stage in selective radio receivers, including automobile
receivers. Because of the additional cost they are usually not used in the
average home receiver designed for local reception. Intermediate-frequency
(i-f) amplifiers are used in radio, radar, and television receivers. The
majority of broadcast radio receivers employ i-f amplifiers operating at 445
kHz. In frequency-modulated (FM) receivers an i-f of 10.7 MHz is used.
Television receivers use 26- or 46-MHz i-f frequencies. 
    Since, in most applications, one stage of amplification does not provide
enough gain, two or more stages are connected together (cascaded) to provide
the required gain. For example, many radio receivers have two i-f amplifiers,
and the more sensitive receivers have three stages. Similarily, there may
be two stages of amplification, and possibly a preamplifier, preceding the
power amplifier in an audio amplifier.
    The gain of an amplifier is defined as the ratio of the output voltage
to the input voltage for a sinusoidal input voltage. Under the usual method
for analyzing ac circuits, the gain is a complex number indicating the 
magnitude of the gain and the phase angle by which the output voltage lags
the input voltage in time. Since the gain is a function of frequency, because
of the reactive circuit elements, the figure for the gain should be accompanied
by the frequency at which it was determined.
    An amplifier cannot exactly reproduce the waveform or frequency spectrum
of its input signal. The amplifier's ability to faithfully reproduce a signal
is measured by either its frequency response or its time response. Special
techniques are used to compensate for decreases in gain both above and below
the limits of the mid-band region. An amplifier may be called upon to increase
the size of a signal whose wave shape is that of a narrow pulse. The           
definition for pulse rise time is the time required for the pulse to increase
from 10% to its maximum value (assuming no overshoot) to 90% of its maximum
value. The rise time of the pulse is a function of the frequency response of
the amplifier; it is inversely proportioned to bandwidth. 
    There are a number of undesirable conditions that may occur in amplifiers,
produced by improper circuit design and by inherent limitations in the physical
operation of the devices. In general, good circuit design can reduce all of the
undesirable conditions, including those caused by physical limitations, to the
point where they are not noticeable. If the output signal from an amplifier is
not an exact replica of the input signal, distortion has occurred. In theory it
is impossible for an amplifier to avoid introducing distortion. On the other
hand, amplifiers have been designed in which the distortion is extremely small.
The noise encountered in amplifiers may, in general, be classified into thermal
noise and shot noise. Thermal noise is caused by the random motion of electrons
inside resistors, conductors, tubes, and transistors. Shot noise is the name
given to the noise generated in transistors or vacuum tubes by the random
emission of holes or electrons from the emitter or cathode. Degeneration is the
loss of gain through unintentional negative feedback. Degeneration can be
reduced to a negligible value by having a bypass capacitor with sufficiently
small reactance at the signal frequency. Regeneration is an increase in gain
through unintentional positive feedback. In its worst form, the amplifier
becomes an oscillator. 
_FIELD034;-237[Christos C. Halkias; Charles L. Alley]_FIELD034
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�6v�2v�32sAmplitude modulation   �21m �33s 
    The process or result of the process whereby the amplitude of a carrier 
wave is changed in accordance with a modulating wave. This broad definition  
includes applications using sinusoidal carriers, pulse carriers, or any other 
form of carrier, the amplitude factor of which changes in accordance with the 
modulating wave in any unique manner. See MODULATION. 
    Practical examples of amplitude modulation (AM) include AM radio 
broadcasting, single-sideband transmission systems, vestigial-sideband 
systems, frequency-division multiplexing, time-division multiplexing, 
phase-discrimination multiplexing, and reduced-carrier systems. See 
MULTIPLEXING; SINGLE SIDEBAND; VESTIGIAL-SIDEBAND MODULATION. 
    Amplitude modulation is also defined in a more restrictive sense to mean 
modulation in which the amplitude factor of a sine-wave carrier is linearly 
proportional to the modulating wave. AM radio broadcasting is a familiar 
example. At the radio transmitter the modulating wave is the audio-frequency 
program signal to be communicated; the modulated wave that is broadcast is a 
radio-frequency, amplitude-modulated sinusoid. See AMPLITUDE-MODULATION RADIO. 
    In AM the modulated wave is composed of the transmitted carrier, which 
conveys no information, plus the upper and lower sidebands, which (assuming 
the carrier frequency exceeds twice the top audio frequency) convey identical 
and therefore mutually redundant information. J. R. Carson in 1915 was the 
first to recognize that, under these conditions and assuming adequate 
knowledge of the carrier, either sideband alone would uniquely define the 
message. This eventually led to the development of single-sideband (SSB) and 
vestigial-sideband (VSB) modulation. Apart from a scale factor, the spectrum 
of the upper sideband and lower sideband is the spectrum of the modulating 
wave displaced, respectively, without and with inversion by an amount equal to 
the carrier frequency. See AMPLITUDE-MODULATION DETECTOR; AMPLITUDE MODULATOR; 
FREQUENCY MODULATION. 
 �34s[Harold S. Black]
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�6v�2v�32sAmplitude-modulation detector �21m �33s
    A device for the recovery of the modulating signal from an
amplitude-modulated carrier. A detector, like a modulator, must employ a
nonlinear device and therefore usually includes either a diode or a nonlinear
amplifier. Detectors are sometimes known as demodulators. Several types of
detector circuits are discussed in this article. See MODULATION.
    A simple diode detector circuit (Fig. 1) was frequently used as an entire
receiver in the early days of radio; it was known as a crystal set. Its
radio-frequency (rf) input signal, which was obtained by attaching an antenna
and a ground to the input terminals, induced a voltage in a secondary coil L2
by magnetic induction. Capacitor Csubt was adjusted to resonate with
inductance L2 at the desired input frequency. Because of the high resonant
impedance of the tuned circuit, a voltage was induced in a narrow band of
frequencies about the resonant frequency to a high enough level for
satisfactory operation of the diode. The desired radio station was thus
selected. See RADIO.
    Modern diode detectors operate similarly. The average value of the
modulated input voltage is zero because the negative half cycles have the same
amplitude as the positive half cycles. Only high, or radio, frequencies,
consisting of the carrier plus sideband frequencies, are present in this input
signal; the original modulating frequency appears as the modulation envelope.
The diode or rectifier recovers this envelope by allowing current to flow
primarily in only one direction, thereby eliminating either the positive or
negative half cycles (Fig. 2). Average current through the diode is not zero,
but has a component which varies with the amplitude of the input signal and
thus contains the original modulating frequencies. Theoretically, nonlinearity
produces sum and difference frequencies, as well as harmonics, of the input
frequencies. The difference between the carrier frequency and a sideband
frequency is the modulating frequency that produced the sideband frequency
during modulation. Thus amplitude modulation and detection are basically the
same process. See RECTIFIER.
    Another important detector is the square-law detector. This device has an
output current proportional to the square of the input voltage. Almost all
diodes and electronic amplifying devices have this type of characteristic,
basically, and linear operation is obtained only by careful selection of the
combination of operating point, signal level, driving source resistance, and
load resistance. The square-law detector distorts or changes the waveform of
the modulation envelope.  This distortion is sometimes desirable, as in
single-sideband transmission, where one sideband has been eliminated.
Single-frequency modulation then produces a nonsinusoidal modulation envelope,
but the square-law detector produces a sinusoidal or single-frequency output.
In other words, as can be shown mathematically, a square-law detector is
necessary for distortionless single-sideband transmission.
    A high-gain narrow-band radio receiver can be achieved with a single
amplifying detector by providing positive or regenerative feedback from the
output to the input. This arrangement is known as a regenerative detector. The
feedback adjustment is critical and distortion is high because the feedback
must maintain operation on the verge of oscillation.
    A four-quadrant multiplier, available as an integrated circuit, may be
used as a demodulator or detector. A filter in the output of the multiplier
will eliminate high frequencies and leave only the modulating frequency. This
type of detector is known as a synchronous detector because the original
carrier, without modulation, must be reinserted into the detector.




 �34s[Charles L. Alley]
�33s


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�6v�2v�32sAmplitude-modulation radio   �21m �33s 
    Radio communication employing amplitude modulation of a radio-frequency 
carrier wave as the means of conveying the desired intelligence. In amplitude 
modulation the amplitude of the carrier wave is made to vary corresponding to 
the fluctuations of a sound wave, television image, or other information to be 
conveyed. See AMPLITUDE MODULATION; RADIO. 
    Amplitude modulation (AM), the oldest and simplest form of modulation, is 
widely used for radio services. The most familiar of these is broadcasting; 
others include radiotelephony and radiotelegraphy, television picture 
transmission, and navigational aids. 
    European and Asian countries use low frequencies in the range 150 - 255 
kilohertz (kHz) for some broadcast services. An advantage of these frequencies 
is stable and relatively low-attenuation wave propagation. When not limited by 
atmospheric noise, large areas may be served by one station. In the United 
States these frequencies are reserved for navigational systems and so are not 
available for broadcasting. 
    The frequencies in the range from 535 to 1605  kHz are reserved all over 
the world for AM (standard) broadcasting. In the Western Hemisphere this band 
is divided into channels at 10-kHz intervals, certain channels being 
designated as clear, regional, and local, according to the licensed coverage 
and class of service. European medium-frequency (mf) broadcasting channels are 
assigned at 9-kHz intervals. 
    Small bands of high frequencies between 3000 and 5000 kHz are used in 
tropical areas of high atmospheric noise for regional broadcasting. This takes 
advantage of the lower atmospheric noise at these frequencies and permits 
service under conditions where medium frequencies have only severely limited 
coverage.  
    The first radiotelephony was by means of amplitude modulation, and its use 
has continued with increasing importance. Radiotelephony refers to two-way 
voice communication. Amplitude modulation and a modified form called 
single-sideband are used almost exclusively for radiotelephony on frequencies 
below 30 megahertz (MHz). Above 30 MHz, frequency or phase modulation is used 
almost exclusively, a notable exception being 118 - 132 MHz, where amplitude 
modulation is used for all two-way vhf radiotelephony in aviation operations. 
    The least expensive method known for communicating by telephony over 
distances longer than a few tens of miles is by using the high frequencies of 
3 - 30 MHz. Furthermore, since radio is the only way to communicate with ships 
and aircraft, hf AM radiotelephony has remained essential to these operations, 
except for short distances that can be covered from land stations using the 
very high frequencies. 
    Single-sideband (SSB) hf telephony is a modified form of amplitude 
modulation in which only one of the modulation sidebands is transmitted. In 
some systems the carrier is transmitted at a low level to act as a pilot 
frequency for the regeneration of a replacement carrier at the receiver. Since 
1933 most transoceanic and intercontinental telephony has been by 
single-sideband reduced-carrier radio transmission on frequencies between 4000 
and 27,000 kHz. In time, SSB will gradually displace AM radiotelephony to 
reduce serious interference due to overcrowding of the radio spectrum. 
    Amplitude-modulated radio has a dominant role in guidance and position 
location, especially in aviation, which is almost wholly under radio guidance. 
    Amplitude modulation is used everywhere for the broadcasting of the 
picture (video) portion of television. In England, France, and a few other 
places amplitude modulation is also used for the sound channel associated with 
the television picture, but frequency modulation is more commonly used for 
sound.  
    Countries of the Western Hemisphere, Japan, Philippines, Thailand, and 
Iran broadcast television video in an emission band of 4.25 MHz; the English 
video bandwidth is 3 MHz; the French system, 10 MHz. The rest of continental 
Europe (except the Soviet Union) use a bandwidth of 5.25 MHz. The carrier 
frequencies employed are between 40 and 216 MHz, and 470 to 890 MHz. A channel 
allocation includes the spectrum needed for both sound and picture. Japan and 
Australia also use 88 - 108 MHz for television broadcasting. 
 �34s[Edmund A. Laport]
�33s 
  

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�6v�2v�32sAmplitude modulator   �21m �33s 
    A device for amplitude-modulating a carrier signal. The carrier is usually 
the radio frequency (rf) in a communications system, but it may also be a 
carrier signal in a multichannel cable communication system, a telemetering 
system, a control system, or a data-collecting system. The modulator is 
usually a vacuum tube or semiconductor amplifier, the output power of which 
controls the output level of another amplifier which modulates the carrier. 
See AMPLITUDE MODULATION; MODULATION. 
    The goal of a modulator is to vary the amplitude of a carrier in 
proportion to the modulating voltage. Departure from this linear relation 
results in modulation distortion. If the carrier being modulated is at a high 
power level, such as the rf in the final stage of a radio transmitter, the 
power output capability and efficiency of the modulator and the efficiency of 
the carrier amplifier become important. 
    The process of amplitude modulation produces new frequencies known as side 
frequencies or sidebands; they are the sum and difference of the carrier and 
modulating frequencies. Because new frequencies are produced only in nonlinear 
devices, the carrier amplifier in which modulation is to take place must be 
nonlinear, insofar as the carrier signal is concerned, and must therefore be 
either a class B or class C amplifier. 
    The most frequently used method of amplitude modulation employed in radio 
communications is known as plate or collector modulation. As this name 
implies, the modulator varies the plate or collector voltage in order to 
accomplish the modulation. 
    Low-level amplitude modulation can be produced by applying the modulating 
voltage to one of the grids of a vacuum tube or to the base of a transistor rf 
or carrier amplifier. Modulation is accomplished by varying the bias of the 
grid or base. Low-level modulation can be accomplished by a four-quadrant 
multiplier. The output of the multiplier modulator contains neither the 
modulating frequency nor the carrier frequency but only the sidebands. This 
type of modulator is known as a suppressed carrier or balanced modulator. A 
four-quadrant multiplier which will handle either positive or negative 
voltages on both inputs is available as an inexpensive integrated circuit. 
Regular amplitude modulation, without carrier suppression, may be produced by 
adding an offset bias voltage to the modulating input of the four-quadrant 
multiplier.  
 �34s[Charles L. Alley]
�33s 
  

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�6v�2v�32sAmplitude (wave motion)   �21m �33s 
    The maximum magnitude (value without regard to sign) of the disturbance of 
a wave. The term ``disturbance'' refers to that property of a wave which 
perturbs or alters its surroundings. It may mean, for example, the 
displacement of mechanical waves, the pressure variations of a sound wave, or 
the electric or magnetic field of light waves. Sometimes in older texts the 
word amplitude is used for the disturbance itself; in that case, amplitude as 
meant there is called peak amplitude. This is no longer common usage. 
    If the medium which a wave disturbs dissipates the wave by some nonlinear 
behavior or other means, then the amplitude will, in general, depend upon 
position. See DISPLACEMENT (MECHANICS); LIGHT; SOUND; WAVE MOTION. 
 �34s[S. A. Williams]
�33s 
  
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�6v�2v�32sAmygdule �21m �33s
    A mineral filling (cast) of a vesicle (gas cavity) in congealed lava. A
rock containing an amygdule is called amygdaloid, or amygdaloidal lava. Gases
dissolved in magma (molten rock) at depth exsolve and form bubbles as
lower-pressure regions are reached. When the magma is chilled upon eruption,
these gas bubbles are preserved in the lava as open cavities or vesicles (see
illustration). In pahoehoe lava, the vesicles typically are spheroidal, but
they may be flattened owing to flowage. Vesicles in aa and block lava tend to
be more irregular.
    Amygdules generally consist of mineral matter deposited by fluids of
extraneous origin moving through the vesicular lava after final consolidation.
These depositing agents may be hot gases and hydrous solutions rising in
volcanic areas, hydrothermal mineralization fluids unrelated to volcanism, or
cold solutions in which the water is of meteoric (atmospheric) origin. The
material deposited may be derived from remote sources or by alteration of
ambient rocks. See MAGMA; VOLCANO.




 �34s[Gordon A. Macdonald/Robert I. Tilling]
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�6v�2v�32sAmylase   �21m �33s 
    An enzyme which breaks down (hydrolyzes) starch, the reserve carbohydrate 
in plants, and glycogen, the reserve carbohydrate in animals, into reducing 
fermentable sugars, mainly maltose, and reducing nonfermentable or slowly 
fermentable dextrins. Amylases are classified as saccharifying (BETA-amylase) 
and as dextrinizing (ALPHA-amylases). See CARBOHYDRATE; ENZYME; GLYCOGEN; 
MALTOSE.  
    In animals the highest concentrations of amylase are found in the saliva 
and in the pancreas. Salivary amylase is also known as ptyalin and is found in 
humans, the ape, pig, guinea pig, squirrel, mouse, and rat. Pig pancreas is 
rich in amylase, whereas cattle, sheep, and dog pancreases have lower 
concentrations.  
    Amylase is also used as a diastase in industry. It is used (1) in brewing 
and fermentation industries for the conversion of starch to fermentable 
sugars, (2) in the textile industry for designing textiles, (3) in the laundry 
industry in a mixture with protease and lipase to launder clothes, (4) in the 
paper industry for sizing, and (5) in the food industry for preparation of 
sweet syrups, to increase diastase content of flour, for modification of food 
for infants, and for the removal of starch in jelly production. 
 �34s[Daniel N. Lapedes]
�33s 
  

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�6v�2v�32sAnaerobic infections   �21m �33s 
    Infections caused by anaerobic bacteria (organisms that are intolerant of 
oxygen). Many such infections are mixed, involving more than one anaerobe and 
often aerobic or facultative bacteria as well. Anaerobes are prevalent 
throughout the body as indigenous flora, and essentially all anaerobic 
infections arise endogenously. 
    Anaerobic gram-negative bacilli (Bacteroides, Fusobacterium) and anaerobic 
gram-positive cocci (Peptococcus, Peptostreptococcus) are the most common 
anaerobic pathogens. Clostridium (spore formers) may cause serious infection. 
The prime pathogen among gram-positive nonsporulating anaerobic bacilli is 
Actinomyces.  
    In terms of frequency of occurrence, the pleuropulmonary, intraabdominal, 
and obstetric-gynecologic infections are most important. Foul order is 
definitive evidence of involvement of anaerobes in an infection, but absence 
of such odor does not exclude the possibility. Other important clues to 
anaerobic infection, though not specific, are location near a mucosal surface, 
bite infection, tissue necrosis, gas formation, prior aminoglycoside therapy, 
``sulfur granules'' (actinomycosis), certain distinctive clinical 
presentations (such as gas gangrene), septic thrombophlebitis, unique 
morphology of certain anaerobic organisms on smear, and failure to grow 
organisms aerobically from specimens. 
    Therapy includes surgery and antimicrobial agents. Bacteroides fragilis is 
the most resistant anaerobe; only chloramphenicol, clindamycin, and 
metronidazole are essentially active against all strains. Most other anaerobes 
are susceptible to penicillin G. See ANTIBIOTIC; GANGRENE; INFECTION. 
 �34s[Sydney M. Finegold]
�33s 
  
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�6v�2v�32sAnalcime   �21m �33s 
    A mineral with a framework structure in which all the aluminosilicate 
tetrahedral vertices are linked, thus allying it to the feldspars, 
feldspathoids, and zeolites. Its formula is Na(H2O)[AlSi2O6]; in this sense it 
is a tectosilicate. 
    The analcime structure type includes several other mineral species. These 
include high-temperature leucite, pollucite, and wairakite. Crystals are most 
frequently trapezohedra, and rarely the mineral is massive granular. Hardness 
is 5 - 5 1/2 on Mohs scale; specific gravity is 2.27. 
    Analcime most frequently occurs as a low-temperature mineral in vesicular 
cavities in basalts, where it is associated with zeolites (particularly 
natrolite), datolite, prehnite, and calcite. Small grains are frequent 
constituents of sedimentary rocks and muds in oceanic basins associated with 
volcanic sources. See FELDSPAR; FELDSPATHOID; LEUCITE ROCK; ZEOLITE. 
 �34s[Paul B. Moore]
�33s 
  

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�6v�2v�32sAnalgesic   �21m �33s 
    Any of a group of drugs of diverse chemical structure and physiological 
effects which are commonly used for the relief of pain. To qualify as an 
analgesic a drug must selectively reduce or abolish pain without causing 
impairment of consciousness, mental confusion, incoordination or paralysis, or 
other derangements of the nervous system. 
    One class of analgesics are the narcotic alkaloids. The oldest and 
best-known analgesics are opium, and its most active alkaloid, morphine. Apart 
from morphine there are also other naturally occurring alkaloids derived from 
opium; the best known of these is codeine. The narcotic analgesics relieve 
cough and suppress respiration. They abolish painful spasms of muscles and 
stimulate smooth muscles. If taken for weeks or months, however, the recipient 
needs and tolerates larger and larger doses to obtain the desired effects. In 
other words, the individual becomes habituated. If the drug is then stopped 
for any reason, very disagreeable withdrawal or abstinence effects are 
experienced. There is then severe pain, sweating, salivation, 
hyperventilation, restlessness, and confusion. Thus the narcotic alkaloids are 
all potentially addictive, and the addicted person will go to extreme measures 
to obtain the narcotic and avoid the withdrawal symptoms. See DRUG ADDICTION; 
MORPHINE; NARCOTIC; OPIATES. 
    Because of the strong addictive properties of morphine and related 
compounds, chemists have synthesized other drugs of similar chemical 
structure, in the hope of securing analgesia without addiction. This effort 
has been only partially successful. Methadone, a drug given to addicts as a 
substitute for morphine, is an effective analgesic when taken orally and is 
less addictive than morphine. Meperidine (Demerol) is a strong synthetic 
analgesic but definitely addictive. Other synthetic analgesics are oxycodone 
(Percodan), levorphanol (levodromoran), propoxyphene (Darvon), and pentazocine 
(Talwin). The last two of this series cause little or no addiction but, 
unfortunately, are not strong analgesics. 
    Another class of analgesic drugs, which are nonnarcotic (nonaddictive), 
are the salicylates, the most familiar being acetylsalicylic acid (aspirin), 
and salicylatelike drugs such as phenylbutazone (Butazolidine), indomethacin 
(Indocin), acetaminophen, and phenacetin. These drugs are most effective in 
relieving skeletal pain due to inflammation (such as arthritis). Their 
analgesic properties are not nearly as strong as those of morphine and the 
synthetic opioids. These drugs also have many other effects, such as reducing 
fever (antipyrexia) and preventing platelet agglutination. They are the most 
commonly used of all analgesic medications and are often combined with 
caffeine or a barbiturate sedative under a variety of trade names and sold for 
the relief of headache, backache, and so forth. See ASPIRIN; CENTRAL NERVOUS 
SYSTEM; PAIN. 
 �34s[Raymond D. Adams]
�33s 
  
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�6v�2v�32sAnalog computer �21m �33s
    A computer or computational device in which the problem variables are
represented as continuous, varying physical quantities. An analog computer
implements a model of the system being studied. The physical form of the
analog may be functionally similar to that of the system, but more often the
analogy is based solely upon the mathematical equivalence of the
interdependence of the computer variables and the variables in the physical
system. See SIMULATION.

    Types.  Analog computers can be classified in two ways: in accordance with
their use, and in accordance with the type of components used to assemble
them. In terms of use, there are general-purpose and special-purpose analog
computers. The general-purpose computer is designed so that it can be
programmed to solve many kinds of problems or permit the development of
different simulated models as needed. The special-purpose computer has a fixed
program with a few or no permitted adjustments; it is generally built into or
appended to the physical system it serves.
    In terms of the components with which computers are made, there are
mechanical, hydraulic, pneumatic, optical, electrical, and electronic analog
computers serving in various applications. Because of its ease of programming,
flexibility of operation, and repeatable results, the general-purpose
electronic analog computer has come into wide use. The flexibility of the
electronic analog computer has allowed it to be augmented with interface
channels to the electronic digital computer, so that during the 1960s a third
type of general computer, the hybrid computer, came into being.
    Because of the practical difficulties of scaling, interconnecting, and
operating an electronic analog computer, the equivalent of its powerful
problem-solving capability has been sought and realized in other forms. Since
1965 many digital computer programs have been developed which essentially
duplicate the functions of analog computers via digital algorithms. Digital
equivalents to the electronic analog computer are free from scaling
requirements and are convenient to run; on the other hand, no interaction
within the dynamic response is possible, the solution speed is slower by a
factor of a hundred times or more, and little or no on-line model building and
exploring is possible.
    Another type of analog computer is the digital multiprocessor analog
system, in which the relatively slow speeds of sequential digital increment
calculations have been radically boosted through parallel processing. In this
type of analog computer it is possible to retain the programming convenience
and data storage of the digital computer while approximating the speed,
interaction potential, and parallel computations of the traditional electronic
analogs. The computer typically utilizes several specially designed high-speed
processors for the numerical integration functions, the data (or variable)
memory distributions, the arithmetic functions, and the decision (logic and
control) function.

    Description and uses.  The typical modern general-purpose analog computer
consists of a console containing a collection of operational amplifiers;
computing elements, such as summing networks, integrator networks,
attenuators, multipliers, and function generators; logic and interface units;
control circuits; power supplies; a patch bay; and various meters and display
devices. The patch bay is arranged to bring input and output terminals of all
programmable devices to one location, where they can be conveniently
interconnected by various patch cords and plugs to meet the requirements of a
given problem. Prewired problem boards can be exchanged at the patch bay in a
few seconds and new coefficients set up typically in less than a half hour.
Extensive automatic electronic patching systems have been developed to permit
fast setup, as well as remote and time-shared operation.
    The analog computer basically represents an instrumentation of calculus,
in that it is designed to solve ordinary differential equations. This
capability lends itself to the implementation of simulated models of dynamic
systems. The computer operates by generating voltages that behave like the
physical or mathematical variables in the system under study. Each variable is
represented as a continuously varying (or steady) voltage signal at the output
of a programmed computational unit. Specific to the analog computer is the
fact that individual circuits are used for each feature or equation being
represented, so that all variables are generated simultaneously. Thus the
analog computer is a parallel computer in which the configuration of the
computational units allows direct interactions of the computed variables at
all times during the solution of a problem.
91aElnora2-23-82

    Components.  Manipulations of the signals (voltages) in the analog
computer are based upon the fundamental electrical properties associated with
circuit components. The interrelation of voltages for representing
mathematical functions is derived by combining currents at circuit nodes or
junctions. See KIRCHHOFF'S LAWS OF ELECTRIC CIRCUITS; OHM'S LAW.
    The simplest arrangement of components for executing addition would be to
impress the voltages to be added across individual resistors. The resistors
would then be joined to allow the currents to combine and to develop the
output voltage across a final resistor. Use of this simple configuration of
elements for computation is impractical because of the undesirable interaction
between inputs. A change in one input signal (voltage) causes a change in the
current that flows through the input resistor; this changes the voltage at the
input resistor junction, and the change secondarily causes a different current
to flow in the other input resistors. The situation gets more interactive when
another computing circuit is attached so that part of the summing current
flows away from the summing junction. This interaction effect also prevents
exact computing. If, in some way, each voltage to be summed could be made
independent of the other voltages connected to the summing junction, and if
the required current fed to other circuits could be obtained without loading
the summing junction, then precise computation would be possible.
    The electronic analog computer satisfies these needs by using high-gain
(high-amplification) dc operational amplifiers. A symbol to represent a dc
direct-coupled amplifier is shown in Fig. 1. According to convention, the
rounded side represents the input to the amplifier, and the pointed end
represents the amplifier output. A common reference level or ground exists
between the amplifier input and output, and all voltages are measured with
respect to it. The ground reference line is understood and is usually omitted
from the symbol. The signal input network (consisting of summing resistors)
connects to the inverting (or negative) amplifier input terminal. The
noninverting (or positive) amplifier input terminal is normally connected to
the reference ground. Generally the inverting input is called the summing
junction (SJ) of the amplifier. Internal design of the amplifier is such that,
if the signal at the summing junction is positive with respect to ground, the
amplifier output voltage is negative with respect to ground. The amplifier has
an open-loop voltage gain of -A; therefore an input voltage of Es results in
an output voltage of -AEs. Gain of a commercial computing amplifier is
typically 10 sup 8; thus, an input voltage of less than 1 microvolt can produce
several volts at the amplifier output. See AMPLIFIER; DIRECT-COUPLED AMPLIFIER.
    Because the operational amplifier thus inverts the signal (changes its
sign or polarity), it lends itself to the use of negative feedback, whereby a
portion of the output signal is returned to the input. This arrangement has
the effect of lowering the net gain of the amplifier signal channel and of
improving overall signal-to-noise ratio and increasing computational accuracy.
    Circuit operation (Fig. 2) can be viewed in the following manner. A dc
voltage Ein applied to input resistor Rin produces a summing junction voltage
Es. The voltage is amplified and appears at the amplifier output as voltage
Eo, (equal to -AEs, where A is voltage gain of the amplifier). Part of output
voltage Eo returns through feedback resistor Rf to the summing junction.
Because the returned or feedback voltage is of opposite (negative) polarity to
the initial voltage at the summing junction, it tends to reduce the magnitude
of Es, resulting in an overall input-output relationship that may be expressed
as in the equation. In fact, the summing junction voltage Es is so small that
it is considered to be at practically zero, a condition called virtual ground.

 (E sub o)/(E sub in) = -A/(A + 1) = -1/(1 + [1/A])
 APPROXIMATELY EQUALS -1   A > 10 sup 8

    To illustrate how the operational amplifier serves the needs of the
computing network, consider the currents that flow and the corresponding
algebraic expressions (Fig. 3). The operational amplifier is designed to have
high input impedance (high resistance to the flow of current into or out of
its input terminal); consequently the amplifier input current Is can then be
considered to be practically zero. The resulting current equation states that
input current Iin is equal to feedback current If. Since the amplifier has a
very high gain, the summing junction voltage is virtually zero. Voltage drop
across Rin is thus equal to Ein; voltage drop across Rf is Eo. The equation in
the box is the fundamental relationship for the amplifier. As long as the
amplifier has such a high gain and requires a negligible current from the
summing junction, the amplifier input and output voltages are related by the
ratio of the two resistors and are thus not affected by the actual electronic
construction of the amplifier. If several input resistors are connected to the
same summing junction and voltages are applied to them (Fig. 4), then because
the summing junction remains at practically zero potential, none of the inputs
will interfere with the other inputs. Thus all inputs will exert independent
and additive effects on the output.
    The input and feedback elements that can be used with an operational
amplifier are not restricted to resistors alone. A reactive feedback component
such as a capacitor may also be used, resulting in mathematical operations
analogous to integration with respect to time. An operational amplifier with a
feedback capacitor will generate an output voltage that is the time integral
of its input voltage. See CALCULUS; CAPACITANCE.
    In nature nearly all systems or processes exhibit nonlinear features in
the form of constraints, discontinuities, and variable gains. If an analog
computer is to be programmed and operated as a realistic system model, it must
include devices with programmable nonlinear characteristics. The most
versatile computing element for this service is the diode function generator.
See SEMICONDUCTOR DIODE.
    Diodes, in addition to being used to implement simulations of unique
nonlinearities such as limits, backlash, and deadzones, can be combined with a
network of resistors to produce particular nonlinear functions. A diode
function generator (DFG) uses the one-way conducting property of diodes to
selectively alter the resultant gain of an operational amplifier in accordance
with a desired function of the input voltage. The DFG can be arranged to
operate on either positive or negative input signals by choosing the
orientation of diodes and reference voltage polarity. Further, by utilizing
fully adjustable potentiometers for breakpoint and slope resistances, a
variable diode function generator (VDFG) is established, permitting the
programming of most analytic, arbitrary, and empirical functions, including
functions with one or more inflections.
    The principal device for multiplication of one variable by another (to
yield a product of variables) is the electronic quarter-square multiplier.
This device achieves the multiplication function to frequencies of several
thousand hertz by implementing an algebraic identity. The squaring operations
are accomplished with high accuracy by two DFG circuits.
    An electronic comparator - analog switch combination provides the analog
computer with decision-making and preprogrammed automatic signal-rerouting
capabilities. These are nonlinear functions preset to take place at some
particular point in a calculation run and to cause a change in coefficients or
signals, that is, a branching in mathematical procedure or a change in the
control of the run.

    Programming.  To solve a problem using an analog computer, the problem
solver goes through a procedure of general analysis, data preparation, analog
circuit development, and patchboard programming. Test runs of subprograms may
also be made to examine partial-system dynamic responses before eventually
running the full program to derive specific and final answers. The
problem-solving procedure typically involves eight major steps, listed below:
    1. The problem under study is described with a set of mathematical
equations or, when that is not possible, the system configuration and the
interrelations of component influences are defined in block-diagram form, with
each block described in terms of black-box input-output relationships.
    2. Where necessary, the description of the system (equations or system
block diagram) is rearranged in a form that may better suit the capabilities
of the computer, that is, avoiding duplications or excessive numbers of
computational units, or avoiding algebraic (nonintegrational) loops.
    3. The assembled information is used to sketch out an analog circuit
diagram which shows in detail how the computer could be programmed to handle
the problem and achieve the objectives of the study.
    4. System variables and parameters are then scaled to fall within the
operational ranges of the computer. This may require revisions of the analog
circuit diagram and choice of computational units.
    5. The finalized circuit arrangement is patched on the computer problem
board.
    6. Numerical values are set up on the attenuators, the initial conditions
of the entire system model established, and test values checked.
    7. The computer is run to solve the equations or simulate the black boxes
so that the resultant values or system responses can be obtained. This gives
the initial answers and the ``feel'' for the system.
    8. Multiple runs are made to check the responses for specific sets of
parameters and to explore the influences of problem (system) changes, as well
as the behavior which results when the system configuration is driven with
different forcing functions.
    The complementary nature of the input and feedback networks of the
operational amplifier gives it one of its most flexible properties. If, for
example, a squaring module in an amplifier input circuit is moved to the
amplifier feedback circuit, the amplifier will compute the square root. In
general, when a function is exchanged between the input and feedback positions
of an operational amplifier, the inverse function is obtained.
    Because of its fundamental ability to perform continuous integration, the
analog computer is particularly well suited to solve ordinary differential
equations, typically arising in the problems of calculus. Because of this
ability, the analog computer has at times been referred to as the electronic
differential analyzer. Significantly, the analog computer integrator will
respond correctly to any simple or complex signal waveform presented at its
input; it is not restricted to applications involving conventional classes of
functions defined by analytic calculus.

    Operation.	After a program has been connected up on the patchboard in
accordance with the foregoing programming procedures, the problem patchboard
inserted in the patch bay, and the attenuators adjusted to their proper
values, the analog computer is ready for a solution run. The mode of operation
of the computer is governed by integrator controls. The input to an integrator
amplifier is connected to one of four possible points: potset (PS) for
adjusting potentiometers, in which the integrator is effectively
short-circuited; the reset or initial condition (IC) mode, in which the
capacitor is charged up to the initial value EsubIsubC; hold (HD), which is a
stop and freeze mode mainly for readout purposes; and operate (OP) mode, in
which the intended continuous integration with respect to time takes place.
The switching between these modes is done with high-speed electronic analog
signal switches (older computers use relay contacts) controlled from the
computer console or via logic command signals connected on the patchboard. At
the beginning of a run, the computer is put in the reset or initial condition
mode, energizing a bus circuit so that the integrators assume their initial
conditions. Next, the computer is switched to the operate mode, removing the
initial condition inputs and connecting the integrand inputs to the
integrators. The generated and connected input signals are now integrated by
the integrator according to its programmed time scale.
    In operating the computer, the user must be prepared to deal with many
practical circumstances and to diagnose the troubles or errors that arise from
the analog computer program. Mistakes in circuit patching, incorrect
attenuator settings, wrong reference polarities, misapplication of ranges of
computer units and display devices, and inappropriate time scale factor
assignments may occur, as well as a host of details associated with the
electronic properties of the computer. A smooth-running program, however,
returns an extremely large amount of problem insight and experience, well
worth the effort required to get the problem running.

      Hybrid computers.The accuracy of the calculations on a  digital computer
can often be increased through double precision techniques and more precise
algorithms, but at the expense of extended solution time, due to the
computer's serial nature of operation. Also, the more computational steps
there are to be done, the longer the digital computer will take to do them. On
the other hand, the basic solution speed is very rapid on the analog computer
because of its parallel nature, but increasing problem complexity demands
larger computer size. Thus, for the analog computer the time remains the same
regardless of the complexity of the problem, but the size of the computer
required grows with the problem.
    Interaction between the user and the computer during the course of any
calculation, with the ability to vary parameters during computer runs, is a
highly desirable and insight-generating part of computer usage. This hands-on
interaction with the computed responses is simple to achieve with analog
computers. For digital computers, interaction usually takes place through a
computer keyboard terminal, between runs, or in an on-line stop-go mode. An
often-utilized system combines the speed and interaction possibilities of an
analog computer with the accuracy and programming flexibility of a digital
computer. This combination is specifically designed into the hybrid computer.
    In a modern analog-hybrid console, the mode switches in the integrators
are interfaced with the digital computer to permit fast iterations of dynamic
runs under digital computer control. Data flow in many ways and formats
between the analog computer with its fast, parallel circuits and the digital
computer with its sequential, logic-controlled programs. Special high-speed
analog-to-digital and digital-to-analog converters translate between the
continuous signal representations of variables in the analog domain and the
numerical representations of the digital computer. Control and logic signals
are more directly compatible and require only level and timing compatibility.
See ANALOG-TO-DIGITAL CONVERTER; BOOLEAN ALGEBRA; DIGITAL-TO-ANALOG CONVERTER.
    The programming of hybrid models is a more complex challenge than
described above, requiring the user to consider the parallel action of the
analog computer interlaced with the step-by-step computations progression in
the digital computer. For example, in simulating the mission of a space
vehicle, the capsule control dynamics will typically be handled on the analog
computer in continuous form, but interfaced with the digital computer, where
the navigational trajectory is calculated. See COMPUTER.











 �34s[Per A. Holst]
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�6v�2v�32sAnalog states   �21m �33s 
    Certain states belonging to neighboring nuclear isobars and possessing 
identical structure except for the transformation of one or more neutrons into 
the same number of protons. Hundreds of examples of analog states (also known 
as isobaric analog states) have been observed throughout the periodic table, 
and their existence is considered to be a fundamental property of nuclear 
structure. See ISOBAR (ATOMIC PHYSICS); NUCLEAR STRUCTURE. 
    Starting with a nucleus of N number of neutrons and Z number of protons in 
a particular state of energy, angular momentum, parity, and so forth, an 
analog state may be constructed in the neighboring isobar with N-1 neutrons 
and Z + 1 protons by replacing a neutron with a proton in the same orbit and 
then coupling the proton to the remaining nucleons in exactly the same fashion 
as the original neutron. Under the assumption of charge independence of 
nuclear forces, the only difference between a state and its analog should 
arise from the increase in the Coulomb interactions attributed to the 
increased number of protons in the nucleus. Since the total energy of a state 
has a Coulomb contribution, the most outstanding difference between analog 
states is their Coulomb energy. When this difference is taken into account, 
then the total energies of corresponding analog states become nearly equal. 
Another, smaller effect which must also be considered is the neutron-proton 
mass difference of 0.782 MeV. However, even then, all properties of analog 
states do not become precisely identical because there remain subtle 
differences in the behavior of neutrons and protons due to electromagnetic 
effects.  
    Until about 1961, it was thought that analog states were a property only 
of light-mass nuclei with mass number A less than about 40. However, in a 
number of experiments not specifically designed to search for analog states, 
dramatic evidence was found for their existence throughout the periodic table 
up to the heaviest known nuclei. The discovery of analog states in heavy 
nuclei has provided a powerful tool for studying nuclear structure that was 
previously unforeseen. First, it has enabled the study of Coulomb energies to 
be extended from low-mass nuclei to the heaviest nuclei; and second, because 
of their high excitation energy, analog states normally are able to decay by 
particle emission, unlike their corresponding parent states which usually 
decay only by GAMMA-ray emission to the ground state. By measuring the 
properties of the particles that are emitted from the analog state, unique 
information is derived about the parent state which often cannot be obtained 
in any other fashion. See RADIOACTIVITY. 
 �34s[Nelson Stein]
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�6v�2v�32sAnalog-to-digital converter �21m �33s
    A device for converting the information contained in the value or
magnitude of some characteristic of an input signal, compared to a standard or
reference, to information in the form of discrete states of a signal, usually
with numerical values assigned to the various combinations of discrete states
of the signal.
    Analog-to-digital (A/D) converters are used to transform analog
information, such as audio signals or measurements of physical variables (for
example, temperature, force, or shaft rotation) into a form suitable for
digital handling, which might involve any of these operations: (1) processing
by a computer or by logic circuits, including arithmetical operations,
comparison, sorting, ordering, and code conversion, (2) storage until ready
for further handling, (3) display in numerical or graphical form, and (4)
transmission.
    If a wide-range analog signal can be converted, with adequate frequency,
to an appropriate number of two-level digits, or bits, the digital
representation of the signal can be transmitted through a noisy medium without
relative degradation of the fine structure of the original signal. See
COMPUTER GRAPHICS; DATA COMMUNICATIONS; DIGITAL COMPUTER.
    Conversion involves quantizing and encoding. Quantizing means partitioning
the analog signal range into a number of discrete quanta and determining to
which quantum the input signal belongs. Encoding means assigning a unique
digital code to each quantum and determining the code that corresponds to the
input signal. The most common system is binary, in which there are 2 sup n
quanta (where n is some whole number), numbered consecutively; the code is a
set of n physical two-valued levels or bits (1 or 0) corresponding to the
binary number associated with the signal quantum.
    The illustration shows a typical three-bit binary representation of a
range of input signals, partitioned into eight quanta. For example, a signal
in the vicinity of 3/8 full scale (between 5/16 and 7/16) will be coded 011
(binary 3). See NUMBER SYSTEMS.


 �34s[Daniel H. Sheingold]
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�6v�2v�32sAnalysis of variance   �21m �33s 
    Total variation in experimental data is partitioned into components 
assignable to specific sources by the analysis of variance. This statistical 
technique is applicable to data for which (1) effects of sources are additive, 
(2) uncontrolled or unexplained experimental variations (which are grouped as 
experimental errors) are independent of other sources of variation, (3) 
variance of experimental errors is homogeneous, and (4) experimental errors 
follow a normal distribution. When data depart from these assumptions, one 
must exercise extreme care in interpreting the results of an analysis of 
variance. Statistical tests indicate the contribution of the components to the 
observed variation. See STATISTICS. 
 �34s[Robert L. Brickley]
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�6v�2v�32sAnalytical chemistry   �21m �33s 
    The science of chemical characterization and measurement. Qualitative 
analysis is concerned with the description of chemical composition in terms of 
elements, compounds, or structural units, whereas quantitative analysis is 
concerned with the measurement of amount. 
    Instrumental methods, which might better be termed physicochemical 
methods, make up the vast bulk of both qualitative and quantitative analysis. 
They include measurements of purely physical characteristics, as well as the 
use of physical measurements to follow the course of chemical reactions. In 
the latter category are titrations in which the end points are detected by the 
measurement of electrical quantities such as electrode potential, conductance, 
current, impedance, and capacitance; optical quantities such as transmittance, 
absorbance, emittance, scattering, and reflectance; or other physical 
quantities such as temperature, density, and surface tension. See TITRATION; 
TURBIDIMETRIC ANALYSIS; VOLUMETRIC ANALYSIS. 
    The scope of analytical chemistry may involve the spatial distribution of 
elements or compounds within a sample, the distinction between different 
crystalline forms of a given element or compound, the distinction between 
different chemical forms (such as the oxidation states of an element), the 
distinction between a component on the surface or in the interior of 
particles, and so forth. To permit these more detailed questions to be 
answered, as well as to improve the speed, accuracy, sensitivity, and 
selectivity of traditional analysis, a large variety of physical measurements 
are used. These include methods based on spectroscopic, electrochemical, 
chromatographic, chemical, and nuclear principles. 
    The spectroscopic area includes the measurement of emission, absorption, 
reflection, and scattering phenomena in various regions of the spectrum 
ranging from gamma rays and x-rays through the ultraviolet, visible, infrared, 
and microwave. Coupled with these measurements are a wide variety of sample 
preparation and excitation techniques. Thus the introduction of a sample as 
either a solid or a solution into a flame, arc, spark, or plasma may be the 
source of excitation for the measurement of atomic emission, atomic 
absorption, or atomic fluorescence spectra. 
    Molecular spectroscopy involves lower-energy forms of excitation such as 
ultraviolet, visible, or infrared radiation. X-rays are used through emission 
of characteristic radiation, absorption, or diffraction. Gamma-ray 
spectroscopy, used to sort out and measure gamma rays according to energy, is 
usually coupled with nuclear techniques such as neutron activation analysis or 
photon activation analysis. See ACTIVATION ANALYSIS; GAMMA-RAY DETECTORS; 
MOLECULAR BEAMS; X-RAY DIFFRACTION. 
    Mass spectrometry is an important method for both inorganic and organic 
analysis. In the inorganic field, spark-source mass spectrometry permits the 
detection and estimation of some 70 elements. Secondary-ion mass spectrometry 
(SIMS) involves the bombardment of a surface with an ion beam, causing the 
ejection of ions characteristic of the sample. Ion-probe mass spectrometry 
permits the scanning of a sample to reveal the point-by-point elemental 
composition of a surface; by gradual erosion of the surface, it determines the 
composition in the third dimension as well. In the organic field, important 
structural information is gleaned through high-resolution mass spectrometry, 
in which molecular fragments are identified through fractional mass 
differences in atoms of the same principal mass number. See MASS SPECTROMETRY; 
OPTICAL METHODS OF CHEMICAL ANALYSIS; SECONDARY ION MASS SPECTROMETRY (SIMS). 
    More subtle nuclear phenomena are employed in nuclear magnetic resonance 
(NMR) spectroscopy, in which the chemical environment of atoms possessing 
nuclear spins can be probed through the interaction between the nuclear spin 
and an external magnetic field. See NUCLEAR MAGNETIC RESONANCE (NMR). 
    Several forms of electron spectroscopy are especially useful for surface 
analysis. The scanning electron microscope (SEM) involves a finely collimated 
electron beam which sweeps across the surface to produce an image. At the same 
time, the surface atoms are excited to emit characteristic x-rays which are 
sorted out according to their energy to reveal point-by-point elemental 
composition. The same basic technique is used in the electron microprobe. 
Low-energy electron diffraction (LEED) involves the observation of the 
diffraction pattern of a reflected electron beam to give structural 
information about the arrangement of surface atoms. 
    Techniques used to determine compositional and structural information by 
measuring the energies of electrons emitted from surface atoms include 
ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron 
spectroscopy (XPS). The latter technique is sometimes called electron 
spectroscopy for chemical analysis (ESCA). A related technique, Auger 
spectroscopy, involves the bombardment of a surface with an electron beam and 
the observation of the energies of electrons secondarily emitted from surface 
atoms. See ELECTRON DIFFRACTION; ELECTRON PARAMAGNETIC RESONANCE (EPR) 
SPECTROSCOPY; ELECTRON SPECTROSCOPY. 
    Methods based on electrochemical principles began with the detection of 
titration end points through the measurement of electrical quantities. 
Coulometry utilizes the faraday (96,500 coulombs) as the basis for 
quantitative measurement, either via a titration with electrochemically 
generated reagent or via quantitative electrolysis. Potentiometry, the 
measurement of electrode potentials, is used with various ion-selective 
electrodes. Various forms of electroanalysis are carried out by applying an 
electrical signal to an indicator electrode and observing a readout signal. 
These methods are named according to the measurements; they include cyclic 
voltammetry, chronopotentiometry, chronoamperometry, and chronocoulometry. 
Polarography, a special form of diffusion-controlled electrolysis with the 
dropping mercury electrode, is useful for a wide variety of inorganic and 
organic analytical applications. See COULOMETRIC ANALYSIS; ELECTROCHEMISTRY; 
ION-SELECTIVE MEMBRANES AND ELECTRODES; POLAROGRAPHIC ANALYSIS. 
    Chromatographic methods are based on the separation of mixtures by a phase 
distribution using multiple stages. The stages are not discrete physical 
entities but are functionally achieved by using a mobile phase, either liquid 
or gaseous, flowing past a stationary phase, either solid or liquid (in the 
form of a film on a solid support). In column chromatography the stationary 
phase is contained in a cylindrical column, whereas in thin-layer 
chromatography a flat layer is used. In gas chromatography the mobile phase is 
gaseous, while in liquid chromatography it is liquid. Gel permeation 
chromatography is a special form of liquid chromatography in which large  
molecules move faster than small ones because they fail to permeate the 
structure of the stationary phase. See CHEMICAL SEPARATION TECHNIQUES; 
CHROMATOGRAPHY; GAS CHROMATOGRAPHY; GEL PERMEATION CHROMATOGRAPHY; QUALITATIVE 
CHEMICAL ANALYSIS; QUANTITATIVE CHEMICAL ANALYSIS. 
 �34s[Herbert A. Laitinen]
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�6v�2v�32sAnalytic function   �21m �33s 
    A complex-valued function f of a complex variable defined on some region 
OMEGA in the complex plane is analytic or holomorphic in OMEGA if for each 
point a in OMEGA the values of the function at all points z sufficiently near 
a are given by power the series expansion below. This power series must then 
be the Taylor series expansion of the function f at the point a and must 
converge in at least the largest circle centered at a and contained in OMEGA. 
An analytic function in region OMEGA is necessarily a continuous function in 
OMEGA; moreover, as a function of the two real variables (x,y), an analytic 
function has partial derivatives of all orders. 
  
    *** f(z) = SUMMATION (n = 0 TO INFINITY) (a sub n) (z - a) (sup n) *** 
  
    There are a number of alternative characterizations of analytic functions, 
all of which are useful. The most common and perhaps the most surprising 
characterization, which is often taken as the definition of an analytic 
function, is that a function f is analytic in OMEGA precisely when the limit 
given by the expression below exists at all points a in OMEGA. This limit is 
the complex analog of the ordinary derivative and is therefore called the 
complex derivative and written f'(a). See CALCULUS. 
  
              *** LIMIT (z TO a) [f(z) - f(a)] / z - a *** 
  
    An analytic function f must satisfy the Cauchy-Riemann equation PARTIAL 
DIFFERENTIAL f/PARTIAL DIFFERENTIAL x + i PARTIAL DIFFERENTIAL f/PARTIAL 
DIFFERENTIAL y = 0. Conversely if f is a complex-valued function defined in a 
region OMEGA of the complex plane, and if f has continuous first partial 
derivatives with respect to the real coordinates x and y, and if they satisfy 
the Cauchy-Riemann equation at all points of OMEGA, then f is analytic 
in OMEGA.
    If f is an analytic function in a region OMEGA of the complex plane, then
it follows from the Cauchy-Riemann equation that, INTEGRAL (sub GAMMA) fdz = 0 
for any closed path GAMMA homologous to zero in OMEGA. Conversely Morera's 
theorem asserts that if f is a continuous complex-valued function in OMEGA and 
if INTEGRAL (sub GAMMA) fdz = 0 for all closed paths GAMMA homologous to zero 
in OMEGA, then f is necessarily analytic in OMEGA.
    By analyzing the condition imposed by the Cauchy-Riemann equation it 
follows that if f'(a) IS NOT EQUAL TO 0, if GAMMA (sub 1) and GAMMA (sub 2) are 
two differentiable paths through the point a, and if the tangent vectors to 
GAMMA (sub 1) and GAMMA (sub 2) at the point a are at an angle THETA apart, 
then the tangent vectors to the paths f [GAMMA (sub 1)] and f [GAMMA (sub 2)] 
at the point f(a) are also at an angle THETA apart. The mapping f thus
preserves angles at a, and is therefore said to be conformal at a. Conversely
any continuously differentiable conformal mapping is an analytic function with
a nonzero complex derivative, thus providing a very geometric characterization
of analytic functions. See CAUCHY INTEGRAL FORMULA; CONFORMAL MAPPING; ISOLATED
SINGULARITY; PARTIAL DIFFERENTIATION. 
 �34s[Robert C. Gunning]
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�6v�2v�32sAnalytic geometry �21m �33s
    A branch of mathematics in which algebra is applied to the study of
geometry; the subject is also called cartesian geometry. The basis for an
algebraic treatment of geometry is provided by the existence of a one-to-one
correspondence between the elements, ``points'' of a directed line g, and the
elements, ``numbers,'' that form the set of all real numbers. Such a
correspondence establishes a coordinate system on g, and the number
corresponding to a point of g is called its coordinate. The point O of g with
coordinate zero is the origin of the coordinate system. A coordinate system on
g is cartesian provided that for each point P of g, its coordinate is the
directed distance BAR OP.  Then all points of g on one side of O have positive
coordinates (forming the positive half of g) and all points on the other side
have negative coordinates.  The point with coordinate 1 is called the unit
point.	Since the relation BAR OP + BAR PQ = BAR OQ is clearly valid for each
two points P, Q of directed line g, then BAR PQ = BAR OQ - BAR OP = q - p, where
p and q are the coordinates of P and Q, respectively.

    Choose any two intersecting lines g1, g2 of the plane, with cartesian
coordinate systems selected on each so that the intersection point has
coordinate O in each system. To each point P of the plane an ordered pair of
numbers (x,y) is attached as coordinates, where x is the coordinate of the
point of intersection of g1 with the line through P parallel to g2 and y is
the coordinate of the point of intersection of g2 with the line through P
parallel to g1. The lines g1 and g2 are called the x axis and y axis,
respectively. It is usually convenient to take the same scale on each axis;
that is, the segments joining the unit points on the two axes to the origin
are congruent. The notation P(x,y) denotes a point P with coordinates (x,y).
If OMEGA denotes the angle made by the positive halves of the two axes, and d
the distance of points P1(x1,y1), P2(x2,y2), application of the law of cosines
yields
d = [(x1 - x2)(sup 2) + (y1 - y2)(sup 2)+ 2(x1 - x2)(y1 - y2) cos OMEGA] 1/2
Since cos 90 DG = 0, this important formula is simplified by taking the axes
mutually perpendicular. Though it is occasionally useful to employ oblique
axes (OMEGA = 90 DG), the simplifications resulting from a rectangular
cartesian coordinate system make it the usual choice. Such a cartesian
coordinate system is assumed in what follows. Thus, the distance d of
P1(x1,y1), P2(x2,y2) is given by d = [(x1 - x2)(sup 2) + (y1 - y2)(sup 2)]1/2.
    The correspondence between the geometric entity ``point'' and the
arithmetic entity ``pair of real numbers,'' upon which plane analytic geometry
is based, results in associating with each geometric locus one or more
equations that are satisfied by the coordinates of all those (and only those)
points forming the locus (equations of the locus), and in associating with
each system of equations in the variables x, y the figure (graph of the
equations) whose points are determined by the pairs of numbers satisfying the
equations. Thus the algebraic method of studying geometry is balanced by a
geometric interpretation of algebra. A central problem in analytic geometry is
that of finding equations of certain important figures among curves and
surfaces. See CURVE FITTING.
    By use of the formula for the distance of two points and the definition of
a circle, an equation for a circle with center C(x0,y0) and radius r (r =] 0)
is found to be (x - x0)(sup 2) + (y - y0)(sup 2) = r(sup 2).
    Much of plane analytic geometry deals with a class of curves which (from
the way in which they were first studied) are known as conic sections or
conics. A conic is the locus of a point P that moves so that its distance from
a fixed point F (the focus) is in a constant positive ratio e (the
eccentricity) to its distance from a fixed line (the directrix) not through F.
Let (c,0) be the coordinates of F, c > 0, and take the y axis as the
directrix. Then P(x,y) satisfies the equation [(x - c)(sup 2)+y(sup 2)]1/2 = ex;
(1 - e[sup 2])x(sup 2) + y(sup 2) - 2cx + c(sup 2) = 0, and it is easily seen
that each point whose coordinates satisfy this equation is on the conic. Hence
each conic is represented by a second-degree equation in the cartesian
coordinates (x,y). A conic is called a parabola, ellipse, or hyperbola
accordingly as e = 1, e < 1, e > 1, respectively. See CONIC SECTION.
    Let cartesian coordinate systems be established on each of three pairwise
mutually perpendicular lines of 3-space that intersect in O, the common origin
of the systems. Suppose equal scales and call the lines the x axis, y axis,
and z axis. To each point P of space an ordered triple (x,y,z) of real numbers
is attached as rectangular cartesian coordinates, where x is the coordinate of
the foot of the perpendicular from P to the x axis, and y and z are similarly
defined. Thus every point of space has unique coordinates, and each ordered
triple of real numbers is the coordinates of a point of space. If d denotes the
distance of two points P1(x1,y1,z1), P2(x2,y2,z2), then d = [(x1 - x2)(sup 2)
+ (y1 - y2)(sup 2) + (z1 - z2)(sup 2)]1/2.
    It follows from the definition of a sphere and the formula for distance of
two points that (x - a)sup2 + (y - b)(sup 2) + (z - c)(sup 2) = r(sup 2) is an
equation for the sphere with center (a,b,c) and radius r, and by completing the
squares of the x, y, and z terms in the equation x(sup 2) + y(sup 2) + z(sup 2)
+ 2Dx + 2Ey +  2Fz + G = 0, it is seen that the locus of such an equation is a
sphere with positive or zero radius, or there is no (real) locus.
    Any equation in just two of the three coordinates is an equation of a
cylinder whose elements are parallel to the axis of the missing variable. Thus
the locus in 3-space of x(sup 2) + y(sup 2) = r(sup 2) is a (right) circular
cylinder whose elements are parallel to the z axis and which intersects the xy
plane in the circle x(sup 2) + y(sup 2) = r(sup )2, z = 0.
    Any equation f(x,y,z) = 0, with f(x,y,z) homogeneous in x, y, z (for
example, 4xy - xz + yz = 0, x(sup 3) - xy(sup 2) + z(sup 3) = 0) has a cone with
vertex O as locus.
    A surface of revolution is obtained by rotating a plane curve C about a
line g of its plane. If f(x,y) = 0, z = 0 are equations of C, and g is the x
axis, the resulting surface of revolution has equation f(x, SQUARE ROOT OF
[y(sup 2) + x(sup 2)]) = 0.  Thus the surface generated by revolving the circle
x2 + (y - b)(sup 2) = a(sup 2), z = 0, about the x axis (the torus or anchor
ring, if b >  a) has the equation x(sup 2) + ([SQUARE ROOT OF y(sub 2) +
z(sup 2)] - b)(sup 2) = a(sup 2).  A  quadric surface is the locus of points
whose coordinates satisfy an equation of  the form Ax(sup 2 + By(sup 2) +
Cz(sup 2) + Dxy + Exz + Fyz + Gx + Hy + Jz + K = 0,
where at least one coefficient of a second-degree term is not zero.  Some
surfaces obtained by rotating conics about a line belong to this class, for
example, spheres, prolate and oblate spheroids (given by rotating an ellipse
about its major and minor axes, respectively), hyperboloids and paraboloids
resulting from rotations of hyperbolas and parabolas about their axes of
symmetry, and right circular cones and cylinders.  Cylinders with conics for
directrix curves are also members.
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sAnamnia   �21m �33s 
    Those vertebrate animals, sometimes called Anamniota, which lack an amnion 
in development. In early classifications the vertebrates were commonly 
separated on this basis, and the expressions Amniota and Anamniota are still 
useful in grouping higher and lower vertebrates. Anamnia as a group name 
includes the Recent classes Agnatha, Chondrichthyes, Osteichthyes, and 
Amphibia and, by presumption, the class Placodermi, which is known only from 
fossils. See AMNION; AMNIOTA; AMPHIBIA; PISCES (ZOOLOGY). 
 �34s[Reeve M. Bailey]
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�6v�2v�32sAnaphylaxis   �21m �33s 
    An allergic reaction of the immediate hypersensitive type. This can be a 
local reaction in the skin, if antigen is applied there, or a systemic acute 
shock syndrome. Both generally begin within seconds after exposure to the 
appropriate allergen. Anaphylaxis in humans may be of either type. 
    Sensitization resulting in anaphylaxis and other hypersensitive states 
requires exposure to an allergen. The induction period for sensitization is 
usually 7 - 10 days; this figure is also applicable to the induction period 
for antibodies. 
    The antibodies involved in causing anaphylactic reactions share the 
property of attaching themselves to cell membranes of the sensitive subject 
and combining with antigen in that locale. Certain cells (mast cells) to which 
attachment is made contain substances (histamine; ``slow-reacting substance,'' 
SRS-A; bradykinin; serotonin) which cause contraction of smooth muscle and 
dilatation of small blood vessels. An eosinophile-attracting substance (ECF-A) 
and heparin are also released. The antibody-antigen reaction at these cell 
membranes releases these substances without injury to the cells; these 
mediators then reveal their activities. See HISTAMINE; SEROTONIN. 
    Useful drugs for the treatment of immediate hypersensitive states are 
epinephrine, norepinephrine, isoproterinol, certain prostaglandins, histamine 
and disodium cromoglycate, which act to stabilize mast cell granules. The 
antihistamine drugs act to block attachment of histamine to receptors on 
target cells. 
    The Schultz-Dale reaction provides a method for demonstrating anaphylactic 
hypersensitivity outside the body. A strip of tissue containing smooth muscle 
is removed from an animal, suspended in a bath containing a physiologically 
balanced salt solution, and supplied with oxygen. The tissues commonly used 
for this purpose are uterine, intestinal, or arterial. With tissue taken from 
a sensitized animal, addition of the proper allergen to the bath causes 
contraction of the smooth muscle. The strip may be removed from a normal 
animal and sensitized by the addition of antibodies to the bath fluid. Again, 
addition of the appropriate allergen will cause contraction. The test may be 
useful for studying antigenic relationships of allergenic substances, or for 
demonstrating antibodies which are too weakly reactive to reveal themselves in 
test tube reactions. See CELLULAR IMMUNOLOGY; HYPERSENSITIVITY; IMMUNOLOGY. 
 �34s[Sidney Raffel]
�33s 
  
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�6v�2v�32sAnaplasia   �21m �33s 
    The appearance of cells with an embryonic, immature, or undifferentiated 
structure at a site where under normal conditions only more mature and 
differentiated cells are  found. Anaplastic cells usually are capable of more 
rapid growth than normal cells. They appear to have larger, more irregular, 
darker-staining nuclei than their normal counterparts. There are often several 
nucleoli and bizarre mitoses in anaplastic cells. The cytoplasm often loses 
its distinguishing characteristics, for example, the cross striations in 
muscle. Anaplastic cells are believed by some pathologists to be derived from 
adult, differentiated cells; it appears more likely that they are derived from 
the less differentiated intermitotic elements in tissues and under normal 
conditions would mature as normal cells. Assessment of the degree of anaplasia 
of a tumor is important because this property usually correlates well with the 
malignancy of the tumor. See ONCOLOGY. 
 �34s[Wilson F. Powell/N. Karle Mottet]
�33s 
  

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�6v�2v�32sAnapsida   �21m �33s 
    A subclass of reptiles characterized by a roofed temporal region in which 
there are no temporal fenestrations. Chelonia (turtles), with living 
representatives, and the extinct Cotylosauria are the two major subdivisions 
of this subclass. The Mesosauria, an extinct group of aquatic reptiles from 
the early Permian, have been included, but the assignment is far from certain. 
    Among the cotylosaurs are found the most primitive known reptiles, which 
date from the Early Pennsylvanian. These forms and their immediate descendants 
flourished in the Permian and Triassic periods of the late Paleozoic and early 
Mesozoic eras, respectively. Turtles first appear as fossils in Triassic rocks 
and are well represented in the fossil record from that time to the present. 
Throughout their history anapsids have, for the most part, inhabited areas 
close to water, with some exceptions among the turtles, and many of them have 
been semiaquatic in habitat and adaptations. See CHELONIA; COTYLOSAURIA; 
MESOSAURIA; REPTILIA. 
 �34s[Everett C. Olson]
�33s 
  
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�6v�2v�32sAnaspida �21m �33s
    An extinct order of fresh- or brackish-water Agnatha, known from the Upper
Silurian of Europe and Canada and from the Upper Devonian of Canada. The
Middle Silurian Scottish Jamoytius is thought by some to be an anaspid.
    Members of this group are small, not exceeding 10 in. (25 cm) in length,
and typically have a slender, fusiform body covered with small scales and a
rounded, jawless, terminal or subterminal mouth (see illustration). Long
paired fins are present, at least in some genera, and the tail is unusual in
having the muscular lobe turned downward. They were probably active, nectonic
swimmers, adapted for feeding on minute particles.
    A relationship to Osteostraci and Petromyzonida is indicated by the single
dorsal nostril lying in front of the pineal eye and between the large paired
eyes and by the paired rows of circular gill openings; all three orders are
grouped in the superorder Hyperoartii. Anaspida are possibly ancestral to
living lampreys. See AGNATHA; HYPEROARTII; MONORHINA; OSTEOSTRACI;
PETROMYZONIDA.


 �34s[Robert H. Denison]
�33s


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�6v�2v�32sAnaspidacea   �21m �33s 
    An order of the crustacean superorder Syncarida, the most primitive of the 
Eumalacostraca. The families Anaspididae and Koonungidae have living 
representatives, while the Clarkecarididae are true fossils. The families 
Gampsonychidae and Palaeocarididae, formerly included in this order, now are 
assigned to a new order, Palaeocaridacea. The Anaspidacea arose during the 
Paleozoic Era, and the recent species are now restricted as living fossils to 
fresh-water habitats in Tasmania and South Australia (not New Zealand). They 
exhibit a great adaptability to special habitats. The first thoracic somite is 
incorporated with cephalic tagmata. 
 �34s[Hans Jakobi]
�33s 
  
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�6v�2v�32sAnatomy, regional   �21m �33s 
    The detailed study of the anatomy of a part or region of the body of an 
animal, most commonly applied to regional human anatomy. This is in contrast, 
but supplementary, to study of organ systems where all the structures 
pertaining to the system are studied in their continuity. 
    There are many methods of dividing the body into regions for study, and 
one such means of classification is shown in the illustration. This system 
includes only externally visible areas; other systems would include special 
internal regions as well. 
    The head, trunk, and extremities are the principal regions. Subdivision of 
these can be carried out, as illustrated, so that many distinct areas or 
especially vital regions are indicated. There is no end to the dividing and 
subdividing a specialist may do to make the task eventually less difficult. 
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sAndalusite   �21m �33s 
    A nesosilicate mineral, composition Al2SiO5, crystallizing in the 
orthorhombic system. It occurs commonly in large, nearly square prismatic 
crystals. There is poor prismatic cleavage; the luster is vitreous and the 
color red, reddish-brown, olive-green, or bluish. Transparent crystals may 
show strong dichroism, appearing red in one direction and green in another in 
transmitted light. The specific gravity is 3.1 - 3.2; hardness is 7 1/2 on Mohs 
scale, but may be less on the surface because of alteration. See SILICATE 
MINERALS.  
    Andalusite is abundant in the White Mountains near Laws, California, where 
for many years it was mined for manufacture of spark plugs and other highly 
refractive porcelain. Variety chiastolite, in crystals largely altered to 
mica, is found in Lancaster and Sterling, Massachusetts. Water-worn pebbles of 
gem quality are found at Minas Gerais, Brazil. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  

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�6v�2v�32sAndesine   �21m �33s 
    A plagioclase feldspar with a composition ranging from Ab70An30 to 
Ab50An50, where Ab = NaAlSi3O8 and An = CaAl2Si2O8. In the high-temperature 
state, andesine has albite-type structure. In the course of cooling, natural 
material develops a peculiar structural state which, investigated by x-rays, 
shows reflections that indicate the beginning of an exsolution process, 
sometimes accompanied by a beautiful variously colored luster (labradorizing). 
If Fe2O3 is present as thin flakes and oriented parallel to certain 
structurally defined planes, such andesine is called aventurine or sunstone. 
See FELDSPAR; GEM; IGNEOUS ROCKS. 
 �34s[Fritz H. Laves]
�33s 
  

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�6v�2v�32sAndesite   �21m �33s 
    An abundant volcanic rock prevalent in island arcs such as the Aleutians 
and in ranges of mountains on some continental margins such as the Andes. Most 
andesite is erupted explosively from steep-sided volcanoes. Andesite has been 
erupted from most of the world's 400 active volcanoes, including some of the 
most spectacular mountains (Fuji, Mayon, Shasta, Kliuchevskaya). Four of the 
United States' national parks encompass andesitic volcanoes (Lassen, Crater 
Lake, Rainier, Katmai). See VOLCANO. 
    Andesite is a rock with a groundmass of submillimeter-sized crystals. The 
principal mineral is plagioclase. Pyroxenes, iron oxides, tridymite, 
cristobalite, and residual glass generally constitute about 40% of the rock. 
Amphibole or olivine may constitute up to a few percent. Most andesites have 
53 - 62 wt % of SiO2 and 15 - 20 wt % of Al2O3. 
    The chemical and mineralogical composition of andesite varies in space and 
time in a broadly regular way, but with many exceptions. Andesites on 
continental margins (such as the Alaskan peninsula) are more silicic, in 
general, than andesites on oceanic islands (such as the Aleutian Islands). 
Andesites near the ocean and above shallow zones of earthquakes tend to be 
poorer in alkalies and richer in lime than those farther inland associated 
with deeper seismicity. Volcanoes formed in the early stage of development of 
island arcs contain the most iron-rich andesites. 
    Andesite is similar to continental crust in chemical composition. It is a 
principal rock type formed at continental margins and along island arcs. The 
arcs may eventually become attached to the margins of continents, therefore 
the andesite on island arcs is largely new continental material. Most old 
andesite now seen on continents probably is also similarly derived. Therefore, 
continents probably develop by additions of andesite. See BASALT; IGNEOUS 
ROCKS; OCEANIC ISLANDS; PLATE TECTONICS. 
 �34s[Alfred T. Anderson, Jr.]
�33s 
  

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�6v�2v�32sAndreaeales �21m �33s
    An order of the mosses composed of one family, the Andreaeaceae. The
family has two genera, Andreaea (see illustration), with about 122 species,
and Neuroloma, which is monotypic.
    The glossy, dark-brown or reddish-brown to blackish plants grow in
cushions or tufts in the mountains on noncalcareous rock (on soil in arctic
regions) and are small, commonly 0.4 - 0.6 in. (10 - 15 mm) in height, 0.8 -
2.8 in. (2 - 7 cm) in some species. Dry plants are brittle. The slender stems
are dichotomous, with fascicles of branches and basal rhizoids.
    The crowded leaves are small, thick, and opaque; in some species they are
smooth, in others papillose. The costa is present in some species and absent
in others. Only very young leaves appear green. The calyptra is delicate and
membranaceous and envelops the sporophyte until the latter nears maturity.
With age the calyptra becomes lacerate at the base. The antheridia and
archegonia are terminal on the same plant but on different branches, the
former on a long stalk and the latter on a short one. See ANDREAEOBRYA;
BRYOPSIDA.


 �34s[Winona H. Welch]
�33s


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�6v�2v�32sAndreaeobrya   �21m �33s 
    A subclass of the Bryopsida (Musci) which contains the single order 
Andreaeales. The characteristics are the same as for the order Andreaeales. 
These plants have some characteristics in common with both Sphagnobrya and 
Eubrya, as well as with the Marchantiopsida (Hepaticae). See ANDREAEALES; 
BRYOPSIDA; EUBRYA; MARCHANTIOPSIDA; SPHAGNOBRYA. 
 �34s[Winona H. Welch]
�33s 
  
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�6v�2v�32sAndrogen   �21m �33s 
    One of a class of steroid hormones. Androgens play a major role in the 
development and maintenance of masculine secondary sexual characters, for 
example, the seminal vesicle and prostate gland of the male mammal, and the 
comb, wattles, and spur of the male fowl. They also influence certain other 
secondary sexual characters, such as hair growth pattern and voice quality in 
humans. In the fowl, they affect the pattern and seasonal coloration of its 
feathers, as well as crowing. In addition, androgens affect nitrogen 
metabolism (anabolic). Androgens are produced in the testis, ovary, adrenal, 
and most likely, in the placenta. A small portion of the androgen is from 
corticoids, or adrenal cortex steroids, and from other C21 steroids, such as 
progesterone. See HORMONE; OVARY; PROGESTERONE; STEROID; TESTIS. 
 �34s[Ralph I. Dorfman]
�33s 
  

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�6v�2v�32sAndrogenesis   �21m �33s 
    The development of an egg under the influence of the sperm nucleus, 
without the participation of the egg nucleus. Spontaneous androgenesis occurs 
as a rare event in nature. It is easily induced experimentally by the 
following methods: (1) elimination of the egg nucleus from the fertilized egg 
surgically, with microneedles and pipets; (2) destruction of the egg nucleus 
in the unfertilized egg by radiation; or (3) by the prolonged refrigeration of 
freshly fertilized eggs. 
    Androgenetic development is usually haploid, since the sperm nucleus 
contains a single, haploid set of chromosomes and genes. Most haploid embryos, 
in frogs and salamanders at least, become abnormal and retarded at an early 
stage. See GYNOGENESIS; MEROGONY. 
 �34s[Gerhard Fankhauser]
�33s 
  
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�6v�2v�32sAndromeda Galaxy   �21m �33s 
    The spiral galaxy of type Sb nearest to the Milky Way system. This galaxy 
is a member of a small cluster of galaxies known as the local group. This 
group contains also the Milky Way system, the Triangulum Nebula (M33), the 
Large and Small Magellanic Clouds, NGC 6822, and several faint dwarf 
elliptical galaxies. 
    The Andromeda Galaxy M31 or NGC 224 is particularly important because it 
is close enough for its stellar and other content to be studied in great 
detail. The approximate distance to M31 is known from the apparent 
luminosities of the cepheid variable stars. The period-luminosity relation for 
cepheids, combined with the apparent brightness of these stars, gives a 
distance of about 2,500,000 light-years. All studies show that M31 is typical 
of other regular spiral galaxies of the Sb class. See CEPHEIDS; GALAXY, 
EXTERNAL.  
 �34s[Allan Sandage]
�33s 
  
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�6v�2v�32sAnechoic chamber   �21m �33s 
    A room in which essentially an acoustic free field exists. It is sometimes 
referred to as a free-field or dead room. The word anechoic is derived from 
the Greek, meaning ``without echo.'' However, a room that is free from echoes 
(in the usual sense of the word) is not necessarily an anechoic room; in 
addition, there must be no significant reflections from the boundaries of the 
room.  
    Free-field conditions can be approximated when the absorption by the 
boundaries of the room approach 100%. To reduce sound reflected by the 
boundaries to a minimum, the absorption coefficient must be high and the 
surface areas of the boundaries should be large. 
    The absorptive material usually installed in such rooms consists of glass 
fibers or mineral wool held together with a suitable binder. In order to 
achieve large surface area, parallel blankets of absorptive material are 
sometimes used to provide a wall construction of considerable thickness. Other 
types of wall construction include long wedges of absorptive material. 
 �34s[Cyril M. Harris]
�33s 
  
  
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�6v�2v�32sAnemia   �21m �33s 
    A reduction in the total quantity of hemoglobin or of red blood cells in 
the circulation. As a matter of convenience and practicality, measures of 
concentration, rather than the total quantity, are usually employed. However, 
there are circumstances when, because the plasma volume is less than normal, 
the concentration of hemoglobin in the circulation will be normal even though 
the total quantity of hemoglobin, or the red cell mass, is less than normal. 
In such cases, anemia would be masked. Conversely, in instances where the 
plasma volume is expanded, the concentration of hemoglobin may be low, even 
though the total mass of hemoglobin and of red cells is not low. Those cases 
are examples of spurious or dilutional anemia, which is not really anemia at 
all. An example is the ``anemia'' reported in many women during the third 
trimester of pregnancy when, in fact, the total red cell mass has been 
increased from the nonpregnant state. See BLOOD; HEMOGLOBIN. 
    There is a normal range over which values of hemoglobin and red cells are 
distributed among healthy individuals. Values which are less than 2.5 or 3 
standard deviations below the mean are indicative of anemia. The mean values 
are greater for adult males than for females and are greater in adults than in 
children. Higher altitude of residence results in higher mean values in 
healthy individuals. The three measures of concentration most often employed 
are the hemoglobin, the red cell count, and the hematocrit. 
    The proliferative response to anemia may be insufficient if the production 
of erythropoietin by the kidneys is defective, or the marrow stem cells 
themselves are damaged or diminished in number. Ineffective erythropoiesis 
results when red cell precursors in the marrow fail to mature and survive for 
delivery to the circulation. Abnormal nuclear maturation results from 
malabsorption of vitamin B12 or of folic acid and from competitive effects of 
analogs which affect purine or pyrimidine metabolism. Defective synthesis of 
hemoglobin impairs cytoplasmic maturation. The majority of cases are due to 
deficiency of body stores of iron and to abnormal release of iron from 
reticuloendothelial stores. The former occurs in iron-deficiency anemia, and 
the latter in the anemia of chronic inflammatory diseases. Acute blood loss 
reduces the total blood volume and produces symptoms of weakness, dizziness, 
thirst, faintness, and shock, in that order according to increasing magnitude 
of blood loss. Chronic blood loss results in iron-deficiency anemia. 
    Hemolysis, the accelerated destruction of red cells, also induces a 
proliferative response from the marrow. However, it differs from hemorrhage 
because red cells are lost without plasma, and it thus affects the measures of 
concentration at the outset. 
    Pallor, weakness, and fatigue are common to all anemias. They may not be 
noticed until anemia is advanced, if it is of gradual onset and there has been 
time for cardiovascular and biochemical adaptation. Faint jaundice in the 
scleras is a feature of hemolytic anemia, whereas in anemia due to lack of 
vitamin B12, glossitis and neuropathy occur and may be noted. 
    The administration of iron will resolve iron-deficiency anemia. In the 
absence of proved deficiency, the administration of iron, vitamin B12, and 
folic acid are not of value to the anemic individual. Further, the 
administration of iron, especially by injection, may be harmful. Prednisone 
and other adrenal corticosteroids are helpful in hemolytic anemias associated 
with autoantibodies. Hereditary disorders are generally not amenable to 
therapy, except for those hemolytic diseases which may benefit after 
splenectomy. In all other cases, the treatment of the anemia is achieved by 
treating the underlying disease, such as hypothyroidism, rheumatoid arthritis, 
or leukemia. Blood transfusions are reserved for acute blood loss, or in 
chronic anemia if there are signs of inadequate cardiovascular or pulmonary 
compensation and an underlying cause cannot be found or treated. See 
HEMATOLOGIC DISORDERS. 
 �34s[Arthur Haut]
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�6v�2v�32sAnemometer   �21m �33s 
    A device which measures the magnitude of air velocity. Anemometers are 
commonly known as the devices which measure wind magnitude, but they are also 
used to measure the rate of flow of air or of other gases in other 
applications, for example, in wind tunnels and on aircraft. The most common 
types are the cup, vane, and hot-wire anemometers. See AIR-VELOCITY 
MEASUREMENT; CUP ANEMOMETER; HOT-WIRE ANEMOMETER; VANE ANEMOMETER. 
 �34s[Herman Feshbach]
�33s 
  
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�6v�2v�32sAnesthetic   �21m �33s 
    A drug used to induce a state of insensibility to pain. There are two 
types of anesthetics, general and local. 
    General anesthetics are most often administered in gaseous form by 
inhalation, and they lead to a state of complete insensibility or 
unconsciousness. If the dosage is too large, the induced coma is followed by 
death. Formerly, nitrous oxide, ether, and chloroform were the most important 
anesthetics; but of these, only nitrous oxide (with oxygen) continues to be 
employed, and this only in the inductive phase of anesthesia. Ethylene and 
cyclopropane have replaced ether and chloroform, since these newer drugs were 
found to be more easily controlled and safer. Unfortunately, cyclopropane is 
highly explosive. The most widely used anesthetic today is halothane, in 
conjunction with nitrous oxide and oxygen. 
    Local anesthetics are drugs that locally block the conduction of nerve 
impulses. They may be injected into the skin and subcutaneous tissues, where 
they paralyze nerves and nerve endings. Within a few minutes the region in and 
around the injection site becomes insensitive to pain and other sensations. 
Dental nerves are similarly paralyzed for the extraction and filling of teeth. 
If the anesthetic is injected into the lumbar subarachnoid space, it blocks 
conduction in the spinal roots, and the lower parts of the body are rendered 
anesthetic. This is called spinal anesthesia. 
    Cocaine was the first drug found to have local anesthetic properties. A 
search for synthetic substitutes has resulted in the discovery of other drugs 
such as procaine, lidocaine, and many others, which are now widely used. See 
COCAINE.  
    Acupuncture is another method employed for the purpose of causing 
insensibility to pain. It involves the insertion of needles into the skin at 
strategic sites. Nothing is injected. Its physiological basis is not known. 
See CENTRAL NERVOUS SYSTEM. 
 �34s[Raymond D. Adams]
�33s 
  
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�6v�2v�32sAneurysm   �21m �33s 
    A localized abnormal dilatation of an artery due to a weakening of the 
vessel wall. Aneurysms are classified as true and false. The wall of a true 
aneurysm is composed of some layer of the original vessel, while a false 
aneurysm is a blood-filled space communicating with an artery through an 
abnormal opening. Among the causes of aneurysms are arteriosclerosis, 
syphilis, degenerative changes and cystic necrosis of the media of vessels, 
trauma, bacterial infections, arteritis, and congenital deformities. 
    Arteriosclerotic and syphilitic aneurysms are the most common types and 
involve principally the thoracic and abdominal aorta. A less common type of 
aneurysm is the dissecting aneurysm of the aorta, in which blood enters the 
wall of the vessel, separating it into two layers, thus creating a new 
channel. Perforation through the outer layer of the aorta with hemorrhage and 
sudden death is the usual outcome. Congenital ``berry'' aneurysms (small 
spherical dilations, usually 0.4 - 0.6 in. (1 - 1.5 cm) in diameter) occur in 
vessels of the brain and are probably developmental in origin, related to 
defects in the muscular coat of the arteries at their angles of bifurcation. 
Rupture of these aneurysms accounts for the majority of spontaneous, 
nontraumatic subarachnoid (intracranial) hemorrhages. See HEMORRHAGE; 
THROMBOSIS.  
 �34s[Frank A. Carone]
�33s 
  
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�6v�2v�32sAngina pectoris   �21m �33s 
    A clinical complex characterized by various degrees of typical chest pain 
which occurs in sudden attacks. The chest pain may be accompanied by other 
symptoms, notably pain or discomfort of the arms, shoulders, or other sites. 
These symptoms are most often induced by some physical or emotional stress and 
subside promptly, in most cases, with rest or appropriate therapy. 
    The most common occurrence preceding an attack of angina is any change 
which may cause a decrease of blood supply to the heart muscle or sudden extra 
demands on the heart so that there is a relative inadequacy of blood. 
Treatment is usually quite effective, and the prognosis is favorable, 
particularly if the patient develops the proper attitude toward the disorder. 
See HEART DISORDERS. 
 �34s[Edward G. Stuart/N. Karle Mottet]
�33s 
  

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�6v�2v�32sAngle �21m �33s
    The word angle is commonly used with three different meanings: geometric,
arithmetic, and algebraic. Also, there is a distinction made between angles
formed by lines (rays) and angles formed by planes.
    An angle (in the geometric sense of the term) is a geometric figure formed
by two rays that have a common vertex (Fig. 1). The two rays are called the
sides of the angle. Two angles are equal if and only if they are congruent. A
straight angle is one whose sides lie in the same straight line but do not
coincide. All straight angles are equal. Three rays having a common vertex
form three angles. If one of these angles is a straight angle, the other two
angles are called supplementary. If two angles that are supplementary are also
equal, each is said to be a right angle. The lines forming the two sides of a
right angle are said to be perpendicular or orthogonal or normal to each
other. All right angles are equal. If two lines intersect, the pairs of
adjacent angles are supplementary and the pairs of vertical angles (opposite
angles) are equal.
    Just as two points A and B determine a geometric figure [AB] called a line
segment, a denominate number BAR AB called its length, and an algebraic signed
quantity ARROW AB called its directed length, so two rays UP ARROW O(A) and
UP ARROW O(B) emanating from a common vertex O determine not only a geometric
figure ANGLE AOB, but also both an arithmetic quantity ARC ANGLE AOB and an
algebraic signed quantity ARROW ANGLE AOB, which measure the figure. The word
angle, in the second sense, means the number of angular units, commonly
between 0DG and 180DG, assigned as a measure of the sector formed, or of the
circular arc intercepted, by the two rays. This is denoted by ARC ANGLE AOB.
    In the third sense, the word angle is used to denote a signed measure
ARROW ANGLE AOB, which may be positive, zero, or negative, without limit in
size, and which, if UP ARROW O(A), UP ARROW O(B), UP ARROW O(C) are any three
coplanar rays with common vertex, satisfies the equations

		  ARROW ANGLE AOB = - ARROW ANGLE BOA

	  ARROW ANGLE AOB + ARROW ANGLE BOC = ARROW ANGLE AOC

In this third sense an angle is not determined by its sides alone. An order
must be assigned to the sides so that one is the initial side and the other
the terminal side; a positive direction of rotation must be assigned in the
plane of the angle; and an integer n, positive, negative, or zero, must be
assigned such that the angle is greater than, or equal to, n complete
revolutions of four right angles but less than n + 1 complete revolutions of
four right angles. In mathematical texts the usual method of assigning the
positive direction of rotation in a sensed plane is to define it as the
counterclockwise direction (opposite to that of the hands of a clock) when
viewed from the positive side of the plane (Fig. 2).
 �34s[James S. Frame]
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�6v�2v�32sAngle modulation   �21m �33s 
    Modulation in which the angle (entire argument) of a sinusoidal carrier is 
the parameter changed by the modulating wave. This variation in angle may be 
related to the modulating wave in any predetermined unique manner. Frequency 
and phase modulation are particular forms of angle modulation. Often the term 
frequency modulation is used to connote angle modulation. See FREQUENCY 
MODULATION; MODULATION; PHASE MODULATION. 
 �34s[Harold S. Black]
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�6v�2v�32sAnglesite   �21m �33s 
    A mineral with the chemical composition PbSO4. Anglesite occurs in white 
or gray, orthorhombic, tabular or prismatic crystals or compact masses. It is 
a common secondary mineral, usually formed by the oxidation of galena. 
Fracture is conchoidal and luster is adamantine. Hardness is 2.5 - 3 on Mohs 
scale and specific gravity is 6.38. The mineral does not occur in large enough 
quantity to be mined as an ore of lead, and is therefore of no particular 
commercial value. Fine exceptional crystals of anglesite have been found 
throughout the world. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sAnguilliformes �21m �33s
    A large order of actinopterygian fishes containing the true eels. This
group, also known as the Apodes, now includes the former order
Saccopharyngiformes or Lyomeri (gulpers or gulper eels). The Anguilliformes
have a ribbonlike, larval stage in development. The chief characters of the
Anguilliformes include a pectoral girdle which, when present, is free from the
head and suspended from the vertebral column; no symplectic, mesocoracoid, and
posttemporal bones; pectoral fin present or absent; absence of a pelvic fin
and girdle in Recent forms; no fin spines; elongate body with numerous
vertebrae; scales present or absent; paired orbitosphenoids; and restricted
gill apertures.
    The typical eels (suborder Anguilloidei) have a swim bladder with a duct,
a small opercle, and 6 - 22 branchiostegals, and the premaxilla, mesethmoid,
and lateral ethmoids are fused into a tooth-bearing ethmopremaxillary block
that separates the maxillae. Eels, which date from the Upper Cretaceous, are
classified in about 20 Recent families and 110 genera, and there are several
hundred species. See EEL.
    The gulpers (suborder Saccopharyngoidei) have degenerative adaptations,
including loss of swim bladder, opercle, branchiostegal rays, caudal fin,
scales, and ribs. The tremendous mouth is greatly modified (see illustration),
the eyes are tiny and placed far forward, the pharynx is enormously
distensible, and the tail is slender and tapering. These are rare oceanic
fishes, classified in three families, three genera, and nine species. Their
fossil history is unknown. See ACTINOPTERYGII; OSTEICHTHYES; TELEOSTEI.


 �34s[Reeve M. Bailey]
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�6v�2v�32sAngular acceleration   �21m �33s 
    A vector quantity representing the rate of change of angular velocity of a 
body experiencing rotational motion. If, for example, at an instant t sub 1, a 
rigid body is rotating about an axis with an angular velocity OMEGA sub 1, and 
at a later time t sub 2 it has an angular velocity OMEGA sub 2, the average 
angular acceleration OVERBAR ALPHA is given by the equation below expressed in 

      ** OVERBAR ALPHA = (OMEGA sub 2 - OMEGA sub 1) / 
                   [(t sub 2) - (t sub 1)] = DELTA OMEGA / DELTA t **    

radians per second per second. The instantaneous angular acceleration is given 
by OVERBAR ALPHA = d OMEGA dt. See ACCELERATION; ROTATIONAL MOTION. 
 �34s[Carl E. Howe/R. J. Stephenson]
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�6v�2v�32sAngular frequency �21m �33s
    A measure of the rate of oscillation of sinusoidally varying phenomena. If
a particle is moving with constant speed v on a circular path of radius r, as
indicated in the figure, it will require a time T = 2 PI r/v to complete one
cycle and the frequency of the motion is f = 1/T. The angular frequency OMEGA
is then defined to be the number of radians traversed per unit time. Since
there are 2 PI radians per cycle, OMEGA = 2 PI/T = 2 PI f.
    The position of the particle in the diagram can be written in component
form as x = r cos THETA, y = r sin THETA. If the particle is arbitrarily taken
to have been at the point p of the figure at the zero of time, t = 0, then
THETA = OMEGA t and the position of the particle is given by x = r cos OMEGA t,
y = r sin OMEGA t. These latter equations, indicating the sinusoidal or
harmonic character of the position components, also illustrate the primary
role of the angular frequency. See FREQUENCY (WAVE MOTION); HARMONIC MOTION.

 �34s[K. L. Kliewer]
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�6v�2v�32sAngular momentum   �21m �33s 
    In classical physics, angular momentum is the moment of momentum and is, 
conceptually, the momentum associated with rotation. A particle of mass m 
moving with velocity v and having position vector r with respect to a fixed 
coordinate origin is said to have an angular momentum L about this origin 
given by the equation below, where p is the linear momentum. See CALCULUS OF 
VECTORS; MOMENTUM. 

                       *** L = m(r MULT v) = r MULT p ***

    Angular momentum - like linear momentum - is an additive vectorial quantity.
The angular momentum of a system of particles is obtained by summing the 
angular momenta of the individual particles.
    The importance of angular momentum in classical mechanics derives from the 
fact that it is conserved (remains constant in time) if no external torques 
are applied. Angular momentum conservation and rotation are of key importance, 
for example, in the structure, shape, and evolution of celestial bodies of all 
kinds. There is a close connection between conservation laws and invariance 
(symmetry) transformations (Noether's theorem). To be precise, if Lagrange's 
equations are invariant for spatial (temporal) translations, then linear 
momentum (respectively energy) is conserved. Similarly, if Lagrange's 
equations are invariant to rotations, then angular momentum is conserved. This 
deep connection between symmetry and conservation laws is not valid for 
Newton's laws. The requirement that Lagrange's equations apply constitutes a 
restriction on the forces. See CONSERVATION OF MOMENTUM; LAGRANGE'S EQUATIONS; 
SYMMETRY LAWS (PHYSICS). 
    This connection between symmetry and conservation laws is valid in quantum 
physics, since quantum mechanics is based on hamiltonian (and hence 
lagrangian) equations. See QUANTUM MECHANICS. 
 �34s[L. C. Biedenharn]
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�6v�2v�32sAnharmonic oscillator   �21m �33s 
    A system that oscillates with a periodic motion that is not simple 
harmonic. An oscillator is anharmonic if the restoring force opposing a 
displacement from the position of equilibrium is a nonlinear function of the 
displacement. The free motion of such an oscillator may be a complicated 
function of time. It is periodic, but with a period that depends on the 
amplitude. A damping nonlinearity in the velocity can also give rise to 
anharmonicity in an oscillator. 
    Many oscillators that are approximately harmonic for sufficiently small 
amplitudes become anharmonic for larger motions. The thermal expansion of 
solids is attributed to the nonlinear forces between atoms. Small atomic 
vibrations (at low temperatures) are harmonic, and are centered about the 
static equilibrium distance. As the temperature rises, the amplitude of atomic 
oscillations increases, and the oscillations become anharmonic. One 
consequence is that the resulting motions are unsymmetric, and the mean 
separation between atoms increases beyond the static equilibrium distance. See 
HARMONIC OSCILLATOR; LATTICE VIBRATIONS; THERMAL EXPANSION. 
 �34s[Joseph M. Keller]
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�6v�2v�32sAnhydrite   �21m �33s 
    A mineral with the chemical composition CaSO4. Anhydrite occurs commonly 
in white and grayish granular masses, rarely in large, orthorhombic crystals. 
Fracture is uneven and luster is pearly to vitreous. Hardness is 3 - 3.5 on 
Mohs scale and specific gravity is 2.98. Anhydrite is an important 
rock-forming mineral and occurs in association with gypsum, limestone, 
dolomite, and salt beds. Under natural conditions, anhydrite hydrates slowly, 
but readily, to gypsum. It is not used as widely as gypsum. Anhydrite is of 
worldwide distribution. Large deposits occur in the Carlsbad district, Eddy 
County, New Mexico, and in salt-dome areas in Texas and Louisiana. See GYPSUM; 
SALINE EVAPORITE. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sAniline   �21m �33s 
    An aromatic primary amine used in the manufacture of urethane polymers 
(50%), rubber chemicals (antioxidants, accelerators), agricultural chemicals 
(herbicides, insecticides, fungicides), and dyes, in that order of volume. Its 
properties are: boiling point 363 DG F (184 DG C); melting point -21 DG F 
(-6 DG C); density 1.0215; refractive index 15863; solubility about 3 g/100 g 
of water; basic dissociation constant Kb = 3.8 X 10 sup -10. Aniline is 
prepared by the catalytic reduction of nitrobenzene in the presence of 
hydrogen. Aniline readily undergoes the reactions characteristic of primary 
aromatic amines. In addition, it may be oxidized with reagents such as hydrogen 
peroxide, chromic acid, or sodium chlorate. Reduction by hydrogen can take 
place in the presence of a suitable catalyst. The reaction of aniline and 
phosgene will produce carbonic acid derivatives. See AMINE; AROMATIC 
HYDROCARBON. 
 �34s[Leallyn B. Clapp]
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�6v�2v�32sAnimal feeds   �21m �33s 
    Substances suitable for the nutrition of animals, mainly livestock, 
poultry, and pets. 
    In formulating livestock and poultry diets, animal nutritionists select 
from a variety of bulk feed ingredients including forages, feed grains, 
protein concentrates, by-product feeds, and vitamin-mineral supplements. These 
are formulated into complete rations in modern feed manufacturing plants to 
provide a balance of energy, protein, vitamins, and minerals to meet the 
nutrient requirements of various classes of livestock and poultry. Drugs and 
other additives may also be needed to meet specific needs. 
    In a broad sense, forages consist of grasses and legumes fed to ruminants 
(cattle, sheep, goats) and horses. Because of symbiotic microbes (bacteria and 
protozoa) in the digestive tract of these animals, they are uniquely equipped 
to utilize nutrients in forages; thus they do not compete with humans for the 
same food source. 
    Pasture and rangeland areas are covered with forage that is harvested by 
the grazing animal. Grazing quality forage has the advantages of providing 
more digestible protein per acre than most other feeds, eliminating harvesting 
and feeding costs, and reducing soil erosion that results from the action of 
water and wind. 
    Because of variable climatic conditions, forage species do not grow at a 
uniform rate throughout the growing season, and most farms produce more forage 
than can be consumed by the livestock. This surplus forage is conserved as hay 
or silage and either fed or sold at later periods. Hay is dried forage which 
has been cut and sun-cured in the field to about 10 - 20% moisture and then 
baled or otherwise packaged. It is commonly fed to ruminants and horses when 
grazing is inadequate or when the animals are confined. Silage is fermented 
forage which is produced by the action of acetic and lactic acid - producing 
anaerobic bacteria in closed structures called silos. Preserving forage by 
dehydration is relatively new compared to hay-making or ensiling. Mechanically 
dehydrated forages usually do not compete with hay or silage as roughages for 
ruminants. They are used primarily as supplements in swine and poultry rations 
and pet foods. 
    Feed grains are any of several grains commonly used as livestock and 
poultry feeds; they include corn, barley, oats, and grain sorghum (milo). 
Although wheat is second only to corn as a cereal grain in the United States, 
wheat is not usually fed to livestock. However, when properly used, wheat is a 
satisfactory feed for all classes of livestock. Accordingly, when the price of 
feed grains is high and surplus wheat is available at low price, considerable 
amounts may be fed to animals. See WHEAT. 
    Protein supplements are feeds which contain more than 20% protein or 
protein equivalent. They are primarily added to rations containing feed grains 
or forages which are low in protein. High-protein meals can be made from 
soybean, cottonseed, sunflower seed, safflower seed, rapeseed, peanut, 
linseed, and coconut (copra). These meals are by-products of the 
oilseed-crushing industry; they vary in feeding value (that is, protein 
quantity and quality), depending upon the amount of hull or seed coat 
remaining in the meal, conditions of time and temperature during extraction, 
and constituent amino acids of the protein. Protein supplements of animal 
origin primarily consist of inedible products derived from meat packing or 
rendering plants, marine sources, poultry processing plants, and dairies. In 
general, such products are high in protein and of excellent quality, being 
well balanced in amino acids and providing vitamins and minerals. Cattle and 
other ruminants are able to derive a portion of their dietary protein from 
nonprotein nitrogen via microbial protein synthesis in the rumen. Amino acids, 
amides, ammonium salts, and other nonprotein nitrogen sources provide nitrogen 
from which symbiotic rumen bacteria synthesize microbial protein. This 
microbial protein is then digested by the ruminant animal in the abomasum and 
gastrointestinal tract. 
    The absence in the diet of one or more of the required vitamins may 
prevent the animal from growing or reproducing, or may cause deficiency 
diseases. If the deficiency is severe, the animals may die. Farm animals fed 
diets containing high-quality green forage usually receive an adequate supply 
of most of the required vitamins. To ensure that rations contain sufficient 
vitamins to meet dietary requirements, manufacturers often fortify feed 
formulations with one or more vitamins. See VITAMIN. 
    Animals, like plants, require minerals to grow, and if deficiencies occur, 
livestock become unthrifty, lose weight, or exhibit other deficiency symptoms. 
Minerals serve many vital functions. To ensure an adequate supply of minerals, 
feed manufacturers often add mineral supplements. Farm animals may also be 
permitted free access to a mineral mixture. If this is allowed, care should be 
taken, particularly with trace minerals, to see that the animals do not 
consume too much, or toxicity may result. 
    Feed additives and implants are nonnutritive substances which enhance the 
utilization of a feed or productive performance of the animal. It is estimated 
that in the United States 75% of the finishing cattle, finishing lambs, and 
growing-finishing pigs receive feed additives or implants. Use of these 
products and guidelines for administration and withdrawal of approved products 
are regulated and prescribed by the U.S. Food and Drug Administration (FDA). 
Also, feed-control officials in individual states may regulate use of these 
substances for livestock marketed intrastate. Prior to receiving approval for 
marketing a product, the manufacturer must prove its safety and efficacy 
through a rigorous testing program. Antibiotics, hormones, and hormonelike 
substances are used. Approval of the use of some has been withdrawn because of 
undesirable effects on humans consuming the animal products as food. See 
NUTRITION.  
 �34s[Donald Burdick]
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�6v�2v�32sAnimal growth   �21m �33s 
    Growth may be most simply defined as increase in mass or dimensions of an 
organism with time. It is one of the basic characteristics of living things 
and represents the visible result of a complex and interrelated series of 
metabolic and developmental events. One of the unique features of biological 
growth is that the organism changes in size and shape, and to some extent in 
chemical composition, although it still retains its integrity and its 
individuality. This is true because growth fundamentally involves synthesis by 
the organism of more materials like itself. 
    Growth occurs by two main processes: increase in the number of cells and 
increase in the size of cells. Living cells, removed from the body and placed 
in an appropriate culture medium, display growth in its most uncomplicated 
form. They grow by synthesizing new protoplasm, dividing into smaller cells, 
and then repeating the process over and over as long as essential nutrients 
are supplied and accumulation of deleterious waste products is prevented. 
    In the intact animal, growth begins at, or soon after, the initiation of 
development. It involves cell division and synthesis of new protoplasm from 
raw material, either contained in the egg or derived from the environment. 
However, the process is much more complex than the growth of cells in tissue 
culture. Except in early embryonic development, growth is normally never 
dissociated from such processes as differentiation (diversification of cell 
structure and function) and morphogenesis (change in the form and pattern of 
the embryo). In later development, growth by cell enlargement is the 
predominant process. See ANIMAL MORPHOGENESIS; CELL DIVISION; DIFFERENTIATION; 
EMBRYOLOGY; FERTILIZATION; MITOSIS; TISSUE CULTURE. 
    The vast majority of animals, including most protozoa, require already 
elaborated organic molecules in order to synthesize new living material during 
growth. For animals in general, the chemical requirements for growth are (1) 
inorganic substances, (2) organic substances, especially certain amino acids 
and fatty acids, and (3) accessory factors or vitamins. 
    In both invertebrates and vertebrates, silicon, calcium, magnesium, carbon 
as carbonate, and phosphorus as phosphate are extremely important. Salts of 
sodium, potassium, calcium, and magnesium are essential components of body 
fluids. They contribute to the osmotic properties of the body fluids and 
provide a milieu in which cells, tissues, and organs may function properly. 
See OSMOREGULATORY MECHANISMS. 
    The organic compounds used by animals as raw materials for growth are 
carbohydrates, proteins, and fats, or their breakdown products. Carbohydrate 
apparently is not essential; laboratory animals can grow in the complete 
absence of carbohydrate. Fat, as such, is probably also nonessential, although 
some animals need certain unsaturated fatty acids to sustain growth. 
    Proteins represent the chief organic constituent of living tissues and are 
the most important raw material for growth. During growth, the proteins stored 
in the egg or provided as food from outside are digested into their 
constituent amino acids. These are then resynthesized into the substance of 
the living cells. The proteins synthesized by the organism have specific 
characteristics which depend upon the kinds and numbers of amino acids they 
contain; therefore not all proteins are equally capable of supporting growth. 
Proteins which fail to induce growth are deficient in one or more essential 
amino acids (amino acids needed by the organism but which it cannot 
synthesize).  
    In addition to the materials used to synthesize the bulk of the 
protoplasmic system, animal organisms also require certain accessory 
substances in order to grow. Some of these are now known as vitamins. See 
VITAMIN.  
    The growth of vertebrates is influenced by a specific growth hormone 
produced by the anterior lobe of the pituitary gland. Secretions from the 
thyroid gland, the adrenal cortex, and, in some instances, the gonads also 
affect growth, but perhaps less directly. Growth in various arthropods is 
controlled by hormone action. Molting, an essential prelude to growth in 
insects and crustaceans, is under hormonal control. See ENDOCRINE SYSTEM 
(INVERTEBRATE); ENDOCRINE SYSTEM (VERTEBRATE). 
    Relative growth is also referred to as heterogony or heterauxesis. Growth 
determines not only the size of the animal but also its shape and form. As 
long as an animal grows at the same rate along all its dimensions, it will not 
change in bodily proportions. However, when the growth rate in certain 
directions is different from that along others, or when one or more parts of 
the growing organism develop more rapidly or more slowly than others, 
progressive changes in form result. 
    The form changes produced by alterations in body proportion are well 
illustrated in human development. At the second month of fetal life, the head 
and neck account for almost one-half the total volume of the fetus; at birth 
the relative size of the head is only 32% of the body; at maturity it is 10%. 
Conversely, at the same three stages, the legs comprise, respectively, 2%, 
16%, and 29% of the total body volume. During this developmental span the 
relative size of the trunk remains constant at approximately 50% of body 
volume. Obviously, the growth of the head during prenatal development after 
the second fetal month is relatively smaller and that of the legs relatively 
larger than growth of the body as a whole. 
    There are many factors involved in the regulation of growth, some 
intrinsic, some environmental. One important factor is the histological 
differentiation of the cells of the organism; as differentiation proceeds, the 
rate of growth declines. Heredity is important in determining the limit and 
the rate of growth. Tall parents give rise to tall children, and the offspring 
of toy terriers and mastiffs grow to characteristic size. Hormonal influences 
also affect growth, as do such environmental factors as nutritive level, 
temperature, and degree of crowding. In most organisms, growth ceases at 
maturity, but in some, growth continues throughout life. Complete cessation of 
growth when the adult stage has been reached occurs mainly in terrestrial 
animals.  
 �34s[Edgar J. Boell]
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�6v�2v�32sAnimal kingdom   �21m �33s 
    One of the two generally accepted major divisions of organisms which live 
or have lived on the Earth. More than 1,000,000 species of animals are known, 
and these are divided into various groups (taxa), large and small, according 
to their relationships and complexity of organization. 
    Most members of the animal kingdom can be distinguished from those of the 
plant kingdom by their greater motility, by the more constant form and 
structure of their bodies, by the placement of the organs, which are internal 
rather than external, and by the nature of the tissue cells, which are 
enclosed in delicate membranes rather than rigid walls of cellulose. See CELL 
(BIOLOGY); CELL WALLS (PLANT). 
    Animals subsist on complex organic materials, derived directly or 
indirectly from plants, which are chemically altered and metabolized to 
provide energy for their functions and materials for growth. These processes 
require oxygen, and the end products are largely carbon dioxide, water, and 
nitrogenous wastes. Most animals possess a nervous system and exhibit a rapid 
response to stimuli. However, forms exist which are intermediate between 
plants and animals in many of these features; for example, certain one-celled 
organisms such as Euglena move about and take food like an animal but contain 
chlorophyll like a plant. For these reasons, some biologists recognize up to 
five kingdoms: Monera (bacteria, blue-green algae); Protista (protozoa, 
chrysophytes); Fungi (slime molds, true fungi); Plantae (algae and higher 
plants); and Animalia (multicellular animals). Disagreement still exists among 
zoologists on the limits and relationships of some phyla. See ANIMAL 
SYSTEMATICS; PLANT KINGDOM; TAXONOMIC CATEGORIES. 
 �34s[Walter J. Bock]
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�6v�2v�32sAnimal morphogenesis   �21m �33s 
    The development of form and pattern in animals. Most animals begin their 
lives as a single-celled product of the fusion of an egg and a sperm. As the 
result of repeated cellular divisions, growth, and cellular specialization, 
this fertilized egg, or zygote, becomes a complicated organism. The number, 
shape, and arrangement of the cells and the arrangement of their structural 
products determine the form of the organism. See CELL LINEAGE. 
    The major question is how the specialized cells arise in the proper 
pattern time after time during normal development. The zygote contains a 
number of codes within its DNA (deoxyribonucleic acid). These codes can be 
translated into the different proteins which are synthesized by the cells. It 
is necessary that only one or a few of the special proteins appear in any one 
cell; chaos would result if the proteins appeared in the wrong places. For 
example, the cells of the lens of the eye contain special transparent proteins 
known as crystallins. If these cells produced opaque materials, vision would 
be impaired. See DEOXYRIBONUCLEIC ACID (DNA); GENE ACTION; GENETIC CODE. 
104A 
    The axes along which cells will communicate during specialization are 
established in early development. Anterior and posterior poles of an egg are 
usually established in the ovary. In the eggs of frogs, the dorsoventral axis 
is determined by the orientation of the egg when it is laid. The middorsal 
line is the center of a field in which cells will communicate during 
differentiation. At first and for many hours of development, only quantitative 
differences are detectable along axes of amphibian eggs. The rates of all 
activities, such as cellular division, oxygen consumption, and production of 
RNA (ribonucleic acid), are most rapid in the anterior and dorsal regions and 
grade off toward the posterior and ventral regions. 
    Qualitative differentiation begins during gastrulation when the mesoderm 
invaginates and comes to lie between the ectoderm and endoderm. As the 
mesoderm glides under the ectoderm, the ectodermal cells are activated. There 
are many pathways of differentiation open to them, and what they will form 
depends upon their position with respect to their neighbors. Most cells of the 
early gastrula are still labile and, when transplanted, can develop in 
conformity with their new environment. After a day or so the dividing cells 
begin to differentiate along the anteroposterior and dorsoventral axes. Those 
in the most anterior position develop more rapidly and become parts of the 
head, and those in the most dorsal position become parts of the back. Thus, a 
polarity is established which is focused in the anterior dorsal region. See 
EMBRYONIC DIFFERENTIATION. 
    As cells differentiate, their surfaces also become different and cellular 
affinities begin to play a role in the development of pattern. Differences in 
cellular affinity become apparent as early as the gastrula stage when the 
embryo is becoming a three-layered sphere. When ectodermal and endodermal 
cells are removed and cultured together, they segregate, each associating only 
with its own kind. Mesodermal cells, forerunners of the connective and other 
tissues, make contact with each other but also make tight junctions with 
ectodermal and endodermal cells. As differentiation proceeds, more and more 
different cell surfaces arise. 
    If two partially developed organs are disaggregated and the dissociated 
cells are mixed in culture dishes, the dissociated cells move like amebas, 
coming to rest only when they meet their own kind or a few other kinds with 
which they can associate. Completely jumbled differentiating kidney tubule 
cells and precartilage cells segregate and do not come to rest until the 
kidney cells have made kidney tubules and solid groups of cartilage cells have 
started to form a cartilaginous matrix. The shape of the group of cells 
depends on surface patterns of individual cells. 
    Like differentiating cells, structural molecules secreted by some cells 
can associate in only one way, depending upon atomic quality and spacing. 
Under the proper conditions, molecules of collagen join to form long fibers 
which, in turn, can associate with other fibers in forming tendons and 
ligaments. Under other suitable conditions, a fibrous substrate produced by 
fibroblasts serves as a surface upon which another kind of tissue can grow. 
Under still other conditions, a meshwork of fibers is formed into which other 
cells will secrete a more rigid matrix, as in the case of cartilage and bone. 
While molecular forces are important in the origin of tissues, part of the 
form of the structural tissues depends upon mechanical forces. Sheets of bone, 
for example, form in the directions which most resist applied pressures. 
    The form of an organism also depends upon the size of the parts. Normal 
tissues and organs grow rapidly and then reach an equilibrium, in which there 
is only enough growth to compensate for losses. For example, if a large part 
of an adult mammalian liver is cut away, the remainder grows rapidly until the 
total mass is reconstituted. See ANIMAL GROWTH; EMBRYOLOGY. 
 �34s[S. Meryl Rose]
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�6v�2v�32sAnimal symmetry �21m �33s
    Animal symmetry relates the organization of parts in animal bodies to the
geometrical design that each type suggests. Spherical symmetry is exhibited by
some protozoans, such as the Heliozoia and Radiolaria: the body is spherical
with its parts concentrically around, or radiating from, a central point.
Radial symmetry is exemplified by the echinoderms and most coelenterates: the
body is structually a cylinder having a central axis named the longitudinal,
anteroposterior, or oral-aboral axis (see illustration). Any plane though this
axis divides the animal into like halves. Often several planes, from the axis
outward, can divide the body into a number of like portions, or antimeres. The
ctenophores and many sea anemones and corals possess biradial symmetry,
basically radial but with some parts arranged on one plane through the central
axis. The great majority of animals have bilateral, or two-sided, symmetry, in
which a median or sagittal plane divides the body into equivalent right and
left halves, each a mirror image of the other.



 �34s[Tracy I. Storer]
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�6v�2v�32sAnimal systematics   �21m �33s 
    The science of animal classification. The term had its origin in the 
systems of classification developed by the early naturalists, such as the 
Systema Naturae (1735) of C. Linnaeus. Animal taxonomy, derived from a word 
proposed (1813) originally by A. de Candolle for the theory of plant 
classification, is now generally used interchangeably with animal systematics. 
The task of naming, characterizing, and cataloging the tremendous number of 
species has required the cooperative effort of systematists from all parts of 
the world during the last 200 years, and the job is far from complete, 
especially in the less conspicuous groups of invertebrates. The more important 
task of arranging the animals in a hierarchial system of taxa which is 
designed to reflect their evolutionary relationships has challenged some of 
the greatest zoologists and has required the use of comparative data derived 
from many basic fields, including morphology, physiology, ethology, ecology, 
and genetics. The complex superstructure of classification, into which all 
newly discovered forms must be made to fit, is being constantly modified and 
perfected as new knowledge permits. See ANIMAL KINGDOM. 
    The written record of the study of animals, as with most knowledge, traces 
back to such early Greek scholars as Hippocrates and Democritus. However, 
Aristotle (384 - 322 B.C.) was probably the first to organize the knowledge of 
animals and to characterize their bodily structures and habits in a 
comparative manner. His terminology reveals that he recognized animal groups 
which correspond to many modern phyla and classes and, at a lower level, he 
utilized the term genos to explain a group concept often equivalent to the 
modern higher categories, and the term eidos to designate the individual 
animal form. 
  
    Descriptive phase.  For nearly 2000 years these rudiments of an 
Aristotelian classification were little improved upon, although the knowledge 
of animal kinds was greatly expanded by the descriptive works of the 
Renaissance zoologists. It remained for the Swedish naturalist C. Linnaeus 
(1707 - 1778) and his immediate predecessors to crystallize emerging concepts, 
methods, and techniques into a workable system which permitted zoological 
classification to proceed in an orderly manner and animal systematics to 
develop as a science. In this system the key position of the species was 
recognized, a binominal system for their designation was provided, and a 
framework of classification was set up in which they could be arranged 
according to degrees of similarity and differences. This ushered in the 
initial or descriptive phase of animal systematics sometimes referred to as 
alpha taxonomy. See SPECIES CONCEPT; TAXONOMIC CATEGORIES; ZOOLOGICAL 
NOMENCLATURE.  
  
    Phylogenetic phase.  The second major step in the development of the 
science of animal systematics was the concept of organic evolution, which was 
developed independently by the traveling field naturalists. C. R. Darwin (1809 
- 1882) and A. R. Wallace (1823 - 1913) and brought to a dramatic focus by the 
publication of Darwin's Origin of Species (1859). This ushered in a period of 
phylogenetic speculation, in which much of the interest of taxonomists shifted 
from the naming of species to attempts to understand the significance of 
higher categories and interpret their relationships. 
    During this period, Ernst Haeckel introduced (1886) the method of 
representing phylogeny (evolutionary history) by means of treelike diagrams. 
Modifications of his method are still used by animal systematists to express 
degrees of presumed relationship in various groups. This level of systematic 
studies is sometimes referred to as beta taxonomy. See PHYLOGENY. 
  
    Species problem.  The third stage in the history of animal systematics 
involved a renewal of interest in the species, with the growth of the genetic 
approach to problems of speciation and more widespread recognition of the 
significance of variation. This has been termed the new systematics or gamma 
taxonomy. Emphasis on the population aspect of the species problem led 
systematists to give more attention to the geographical distribution of 
populations within the species, to the ecological requirements of species and 
smaller populations, and to their genetic composition. The application of this 
dynamic concept of species to the samples (specimens) of the practicing animal 
systematist resulted in a sounder and more useful classification, which has 
permitted more widespread biological generalizations. Renewed interest in 
evolutionary, morphology, and macroevolutionary mechanisms, combined with the 
findings of the new systematics, has led to a revival of work on the 
theoretical foundations of classification and on the practical methods of 
formulating actual classifications for animal groups.  See SPECIATION. 
  
    Higher classification.  While intensive research continues on the nature 
of species and species formation, renewed interest has developed in the 
systematics of the higher categories. This is especially true in the 
philosophical and basic methodological approaches to classifications. The 
sources of evidence for classification are comparative studies of observable 
features of recent and fossil organisms and inferences derived from these 
studies. Taxa (taxon, singular) or groups of organisms are presumed to be 
monophyletic (evolving from a common ancestor), and as a general rule, the 
degree of phenotypical similarity between organisms is closely correlated with 
the degree of their phylogenetic relationship and time of common origin. The 
more similar organisms are, the closer is their relationship and the more 
recent is the time of their common ancestry. However, many complicating 
factors, such as different evolution rates, extinction of lineages, 
convergence, parallelism, and independent origin of similar features, obscure 
these relationships and render systematic work more an art than an exact 
science.  
    The several methods of systematic studies of the higher categories can be 
grouped under three headings: the phenetic, the cladistic (phylogenetic 
systematics), and the classical evolutionary approaches. All these approaches 
are based upon comparative studies of organisms, but differ in basic 
assumptions and philosophies. 
    The phenetic approach is based upon the strict assessment of the degree of 
similarity between organisms in contrast to the strict genealogical or 
cladistic method. The basic assumption is that a classification may be based 
on a comparison of phenotypical characters, with the degree of relationship 
being correlated absolutely with the degree of phenotypical similarity; no 
other inferences or assumptions are used. It is believed that the problems of 
convergence and other obscuring factors are overcome by using large numbers of 
features in the comparison. The family of methods called numerical taxonomy 
utilizes the evidence based on a series of hypotheses concerning first, the 
relationships of gene loci to phenotypic characters, that is, observable 
characters; and second, the sampling of information on these gene loci 
indirectly by sampling the characters of the phenotype. 
    In sharp contrast to the phenetic method, which is based solely upon 
analysis of the degree of similarity between organisms, the cladistic 
(phylogenetic) flatly rejects similarity as a guide to relationships on the 
grounds that it is too subjective. W. Hennig, the major proponent of the 
cladistic method, argues that the only objective approach to classification is 
on the basis of the forks or branching points between phyletic lineages. In 
this approach the notion of relationship is restricted to the concept of forks 
in much the same way as a strict genealogical relationship is in a family. It 
is argued that basically all branches are dichotomies, and that at least one 
change in one character occurs at a branch in one of the two lineages, and at 
least one character change occurs between two forks in a lineage. The two 
lineages at a branching point represent sister groups, and by definition every 
taxon has one sister group. Characters are either plesiomorphs (the original 
character found in the ancestral form) or apomorphs (the derived character 
found in one of the descendant sister groups). Every monophyletic taxon 
includes all species that have descended from the stem species of the taxon 
(the stem species is, by definition, a member of this taxon), and is 
characterized by the possession by all members of a series of autapomorphous 
features (those restricted to that taxon). 
    Both the phenetic approach and the cladistic approach are logical systems 
as far as they go, but both suffer from the same defect, namely, that they 
include only one of the two factors that characterize evolutionary 
relationships between living organisms. These factors are the branching points 
in the phylogenetic lineages (emphasized strictly by the cladistic school) and 
the degree of evolutionary modification between groups, or conversely, the 
degree of similarity between taxa (emphasized strictly by the phenetic 
school). Because the evolutionary method includes both factors in the analyses 
of relationships between organisms, it cannot claim or hope to be as simple as 
either the phenetic or the cladistic methods. Moreover, it is not possible to 
formulate simple rules for evolutionary systematics, as is the case for the 
other approaches. However, the evolutionary method has been and still is the 
prevailing approach by far to classification of living organisms since Darwin. 
It is used by most taxonomists and is the basis for most classificatory 
systems used by biologists. Both the phenetic and cladistic methods have been 
developed and advocated strongly since 1945, partly as a reaction to the very 
real difficulties in applying the evolutionary approach to the exceedingly 
complex tangle of relationships between living organisms. 
  
    Importance of systematics.  Taxonomy is not only the most elementary part 
of zoology, since animals cannot be discussed or treated in a scientific way 
until some taxonomy has been achieved, but at the same time it is the most 
inclusive, since in its various aspects and branches it gathers together, 
utilizes, summarizes, and implements everything that is known about animals, 
whether morphological, physiological, psychological, or ecological. 
    Not only is sound systematics essential to progress in most phases of the 
basic biological sciences, but it also contributes materially to the applied 
sciences, in particular, medicine, public health, agriculture, conservation, 
and natural-resource management. The naming and identification of animal 
species has provided a filing system for the vast amount of information 
accumulated about injurious forms. The natural classification has given a 
means for making predictions and generalizations as to the probable habits, 
distribution, future importance, and means of control of newly discovered 
pests, as well as leads and clues as to the endemic source of possible 
parasites and predators, which might be sought to help suppress populations of 
introduced pests, especially insects. Applied taxonomy is also the basis of 
plant quarantine enforcement, in which accurate identification may prevent 
losses of millions of dollars to farmers, and the basis of agricultural pest 
control, in which erroneous identification may invalidate costly control 
procedures or upset the natural balance of populations in an economically 
disastrous manner. Careful systematic studies have led also to the solution of 
such public health problems as the cause of the uneven distribution of malaria 
in Europe and to the concept of species sanitation, a practice which has 
greatly accelerated the suppression of some arthropod-borne diseases. See 
PLANT TAXONOMY. 
 �34s[Walter J. Bock]
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�6v�2v�32sAnimal virus   �21m �33s 
    Viruses are the smallest infectious agents, 20 - 300 nanometers in 
diameter, containing as their genome (genetic material) a molecule of either 
RNA (ribonucleic acid) or DNA (deoxyribonucleic acid). The complete infective 
virus particle is called the virion. In some instances (adenoviruses, 
papovaviruses, picornaviruses) it may be identical with the nucleocapsid, that 
is, the capsid together with the enclosed nucleic acid core. More complex 
virions (herpesviruses, myxoviruses) include the nucleocapsid plus a 
surrounding envelope. The capsid is the symmetric protein shell which encloses 
the genome. Often empty capsids are by-products of the viral replicative 
cycle. The basic building blocks of the capsid are structural units; these may 
be individual polypeptides. Morphological units known as capsomeres are seen 
in the electron microscope on the surfaces of isometric virus particles, that 
is, those with cubic symmetry. These units represent clusters of polypeptides 
which, when completely assembled, form the capsid. 
    Viruses are able to propagate only within living cells. The viral genome 
causes the normal cellular metabolism to be diverted into supplying the raw 
materials, energy, and synthetic machinery for producing more viral nucleic 
acid. It codes also for a viral protein shell (capsid), which is necessary for 
the cell-to-cell transfer of the complete virus particle (virion). Viruses 
outside living cells are inert particles which neither respire as do bacteria 
nor possess enzymes customarily associated with metabolic activity. See 
NUCLEIC ACID. 
    Viruses are generally destroyed by heating at 60 DG C for 30 min and can 
be preserved by freezing at low temperatures and, in some instances, by 
lyophilization. See LYOPHILIZATION. 
    Like other microbiological organisms, viruses also mutate. Examples of 
virus mutations may be seen in nature in the evolution of vaccinia virus and 
in the almost yearly appearance of new variants of influenza virus. See 
INFLUENZA; MUTATION. 
    Meaningful genetic studies with animal viruses have been possible only 
since the early 1970s. Two factors have made these studies possible. The first 
was the development of the plaque assay for quantitation of virus infectivity. 
The second factor was the development of stable genetic markers, which, 
ideally, should be easily recognized and should result from single mutations. 
Some commonly used markers are specific virus-induced antigens, drug 
resistance, host range, inability to grow at elevated temperatures, and plaque 
size. Virus mutants that possess these markers are obtained either after 
spontaneous mutation or, more frequently, after treatment of the virus 
population with a mutagen. The use of conditional-lethal mutants provides a 
powerful tool for the study of the genetics and molecular biology of viruses. 
Conditional-lethal mutants are mutants which are lethal (in that no infectious 
virus is produced) under one set of conditions - termed nonpermissive 
conditions - but which yield normal infectious progeny under other conditions 
- termed permissive conditions. 
    Viral multiplication begins with the attachment of the intact virus 
particle to the host cell. Most of its nucleic acid but little of its protein 
then enters the cell. The cell nucleic acid disintegrates as a part of the 
infectious process. The biochemical events in the infected cell are determined 
by the viral nucleic acid, which not only directs the production of new viral 
nucleic acid, but also (in separate biochemical steps) governs the synthesis 
of new viral protein. Complete virus particles are then assembled within the 
cell, in some cases leaving almost as soon as they are manufactured, but in 
other cases accumulating and leaving when the cell dies and disintegrates. 
During viral replication the capsid sometimes encloses host nucleic acid 
rather than viral nucleic acid. Such particles, pseudovirions, look like 
ordinary virus particles when observed with the electron microscope, but they 
do not replicate as do ordinary virus particles. They contain the wrong 
nucleic acid. 
    Viruses may be transmitted by (1) direct transmission from person to 
person by contact, in which droplet or aerosol infections may play the major 
role (for example, influenza, measles, and smallpox); (2) transmission by 
means of the alimentary tract as a result of intimate association with 
carrier, food, and drink (for example, enterovirus infections and infectious 
hepatitis); (3) transmission by means of dust (for example, Q fever); (4) 
transmission by bite (for example, rabies); (5) transmission from lower 
vertebrate to humans by contact or excretions (for example, Q fever and 
psittacosis); and (6) transmission by means of an arthropod vector (for 
example, yellow fever, dengue, and equine, St. Louis, and Russian 
spring-summer encephalitis). 
    In vertebrates the invasion of most viruses evokes a violent reaction, 
usually of short duration. The result is decisive. Either the host succumbs or 
it lives through the production of antibodies that neutralize or kill the 
virus. Regardless of the outcome, the sojourn of the active virus is usually 
short (although latent virus states may occur, as in herpes, adenovirus, and 
cytomegalovirus infections). In arthropod vectors of a virus, the relationship 
is usually quite different. The viruses may produce little or no ill effect, 
and remain active in the arthropod throughout the host's natural life. Thus 
arthropods, in contrast to vertebrates, act as permanent hosts and reservoirs. 
In tick-borne infections the virus may be transmitted from the adult arthropod 
to its offspring by means of the egg (transovarian passage); thus the cycle 
may continue with or without intervention of a vertebrate host. See 
ADENOVIRUS; ARBOVIRAL ENCEPHALITIDES; CHICKENPOX AND SHINGLES; COMMON COLD; 
COXSACKIEVIRUS; CYTOMEGALOVIRUS INFECTION; ECHOVIRUS; ENTEROVIRUS; EQUINE 
ENCEPHALITIS; EXANTHEM SUBITUM; FOOT-AND-MOUTH DISEASE; HERPES; INFECTIOUS 
MONONUCLEOSIS; INTERFERON; KERATOCONJUNCTIVITIS; MEASLES; MUMPS; MYXOVIRUS; 
NEWCASTLE DISEASE; PARAINFLUENZA VIRUS; PARAMYXOVIRUS; PHLEBOTOMUS FEVER; 
PICORNAVIRUS; RABIES; RHINOVIRUS; SMALLPOX; TICK FEVER; VIRUS; VIRUS 
INTERFERENCE; WEST NILE FEVER. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sAnise   �21m �33s 
    One of the earlist aromatics mentioned in literature. The plant, 
Pimpinella anisum (Umbelliferae), is an annual herb about 2 ft (0.6 m) tall 
and a native of the Mediterranean region. It is cultivated extensively in 
Europe, Asia Minor, India, and parts of South America. The small fruits are 
used for flavoring cakes, curries, pastry, and candy. The distilled oil is 
used in medicine, soaps, perfumery, and cosmetics. See SPICE AND FLAVORING; 
UMBELLALES.  
 �34s[Perry D. Strausbaugh/Earl L. Core]
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�6v�2v�32sAnisomyaria   �21m �33s 
    An order containing seven superfamilies of marine and brackish bivalves 
with byssal attachment at some time in their geological history (the Ostreacea 
are exceptional, with the lower valve attached by byssal cement at settlement, 
and with no record of ancestral attachment by byssal threads). The Anisomyaria 
possess some primitive features and some  secondarily simple features: the 
hinge is edentulous or with inconspicuous teeth (except Spondylus); there are 
no siphons; minimal points of fusion occur between the left and right mantle 
lobes; the Mytilacea, Pteriacea, Pectinacea, and Anomiacea have filibranch 
ctenidia. Specialized features include simultaneous hermaphroditism in some 
Pectinacea, sex reversal and incubation of larvae in some Ostreacea, and 
protandrous hermaphroditism in others. Few features are common to all 
Anisomyaria, and the systematic status is debatable. Studies suggest addition 
of the isomyarian Arcacea and Limopsacea, and placing in an order 
Pteriomorpha, subclass Lamellibranchia, class Bivalvia. See BIVALVIA; MOLLUSCA. 
 �34s[Richard D. Purchon]
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�6v�2v�32sAnisotropy (physics)   �21m �33s 
    The quality of variation of a physical property with the direction in a 
body along which the property is measured. For example, the resistivity of 
certain single crystals measured with the electric field along a particular 
crystallographic direction may be higher than along directions perpendicular 
to it. Thus such crystals are anisotropic with respect to resistivity. See 
ELASTICITY; ISOTROPY (PHYSICS). 
 �34s[David Turnbull]
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�6v�2v�32sAnkerite   �21m �33s 
    The carbonate mineral Ca(Fe,Mg)(CO3)2, also commonly containing some 
manganese. The mineral has hexagonal (rhombohedral) symmetry and has the 
cation-ordered structure of dolomite. The name is applied only to those 
species in which at least 20% of the magnesium positions are occupied by iron 
or manganese; species containing less iron are termed ferroan dolomites. The 
pure compound, CaFe(CO3)2, has never been found in nature and has never been 
synthesized as an ordered compound. See DOLOMITE. 
    Ankerite is commonly white to light brown, its specific gravity is about 
3, and its hardness is about 4 on Mohs scale. See CARBONATE MINERALS. 
 �34s[Alan M. Gaines]
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�6v�2v�32sAnkle   �21m �33s 
    The part of the leg consisting of the ankle joint, the tarsal (ankle) 
bones, and related structures. The joint is formed by two bones of the leg, 
the tibia and fibula, which articulate with the talus (astragalus), one of the 
seven tarsal bones. Another prominent tarsal bone is the calcaneus, or heel 
bone, which bears much of the weight in walking. The inner and outer 
prominences commonly known as ``ankle bones'' are the overhanging portions of 
the tibia and fibula; each prominence is called a malleolus. See LEG (ANATOMY). 
    The tarsal bones are held in place, yet allowed movement, by strong 
elastic ligaments, as well as by the shapes of the bony surfaces. The ankle 
joint is a synovial, modified hinge joint, to which movement is imparted by 
numerous muscles of the leg and foot. See JOINT (ANATOMY). 
 �34s[Thomas S. Parsons]
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�6v�2v�32sAnnelida   �21m �33s 
    The phylum comprising the multisegmented, invertebrate wormlike animals, 
of which the most numerous are the marine bristle worms and the most familiar 
the terrestrial earthworms. The Annelida (meaning little annuli or rings) 
includes the Polychaeta (meaning many setae); the earthworms and freshwater 
worms, or Oligochaeta (meaning few setae); the marine and freshwater Hirudinea 
or leeches; and two other marine classes having affinities with the 
Polychaeta: The Archiannelida (meaning primitive annelids) are small 
heteromorphic marine worms, and the Myzostomaria (meaning sucker mouth) are 
parasites of crinoid echinoderms. See ARCHIANNELIDA; HIRUDINEA; MYZOSTOMARIA; 
OLIGOCHAETA; POLYCHAETA. 
    These five groups share few common characters and little resemblance 
except that most have a wormlike body. Typically they are bilaterally 
symmetrical, lack a skeleton, and have a short to long linear body divided 
into rings or segments (metameres), which are separated from one another by 
transverse walls or septa. The mouth is an anteroventral or anterior vent at 
the forward end of the alimentary tract, and the anus is posterodorsal or 
posterior at the hind end of the gut. 
    The metameres may be similar throughout, resulting in an annulated 
cylinder, as in earthworms and Lumbrineris. More frequently the successive 
segments are dissimilar, resulting in regions modified for particular 
functions. Each segment may be simple or uniannular, corresponding to a 
metamere, or it may be divided or multiannulate. Segments may have lateral 
fleshy outgrowths called parapodia (meaning side feet), armed with special 
secreted bristles or rods, called setae and acicula; they provide protection 
and aid in locomotion. Setae are lacking in Hirudinea and some polychaetes. 
The body is covered by a thin to thick epithelium which is never shed. 
    Annelids have sense organs of many kinds. Most conspicuous are those on 
the anterior, or head end, and on parapodia. Eyes which function as 
photoreceptors may be variously developed, simple to complex, small to large, 
absent to numerous, and obscure to bizarre. Structurally they are simple 
light-absorbing pigment spots, usually paired on the head, but also on body 
segments or the anal end. Feelers or tactoreceptors are frequently on the 
prostomium as antennae; on anterior segments as long filiform or thick fleshy 
structures called tentacles; and on parapodia as cirri, papillae, scales, or 
tactile hairs. Cilia in bands or clusters or in grooves occur in specific 
patterns. The most conspicuous receptors are the large caruncles (nuchal 
organs) of amphinomid polychaetes, as simple or  complex fleshy, folded, 
paired organs surrounding the cephalic structures. The nuchal organs of some 
syllids and phyllodocids, as well as the ciliated ridges of errantiate and 
sedentary polychaetes, illustrate the diversity and importance of these 
structures.  
    Light production is known in many annelids. Some oligochaetes shed 
luminous mucus through the mouth or anus. Many polychaetes have special 
photocytes which luminesce. Some Chaetopterus colonies shed bright mucus from 
the palpi and parapodia. Polycirrus among the terebellids is highly 
luminescent. Pelagic spawning stages of Odontosyllis (a syllid) and Tharyx 
(cirratulid) flash bright light when disturbed. Some scale worms (polynoids) 
have light-producing cells on the undersides of elytra. See BIOLUMINESCENCE. 
    The alimentary tract is a straight or sinuous tube, consisting of mouth, 
pharynx, esophagus, stomach, gut, and pygidium or anus. Its several parts may 
be further differentiated and named for structure and function. The mouth may 
be a simple anterior (oligochaetes) or anteroventral pore (many polychaetes) 
provided with highly complex organs or accessory parts. The upper lip is 
formed by the prostomium, and the lower lip by the ventrum of the first 
segment. The mouth is followed by the buccal cavity which may be modified as a 
proboscis or saclike eversible pouch. The buccal cavity may be followed by a 
short to long, muscularized, eversible proboscis which captures and breaks up 
or compresses food particles. Its inner walls may be fortified by papillae or 
hard gnaths or jaws. A short esophagus leads to the muscular stomach or 
digestive region, which is followed by the gut. The proctodaeum, or region 
preceding the anus, expels the wastes as fecal pellets of characteristic form. 
    The nervous system consists of a dorsal, bilaterally symmetrical, 
ganglionic mass or brain within or behind the preoral region. The brain is 
connected to the ventral cord through the circumesophageal connectives which 
extend about the oral cavity. The ventral cord may be single or paired, nearly 
smooth or nodular, or ganglionated according to the segmental pattern of the 
body. The brain sends out lateral branches to the eyes, palpi, antennae, or 
other structures; the ventral cord has lateral branches to all fleshy parts 
which receive stimuli. 
    The circulatory system consists of dorsal and ventral longitudinal, median 
vessels located above and below the alimentary tract. Lateral branches extend 
to all parts of the body. Pulsating or propelling contractile portions, 
sometimes called hearts, are in the anterior dorsal vessel or also at 
intervals in the ventral vessels. In oligochaetes these are segmental vessels  
of varying number connecting the dorsal and ventral vessels and surrounding 
some portion of the alimentary tract. The contained blood is red through the 
presence of a hemoglobinlike substance, which may be in corpuscles or granular 
in the fluid, or it is green, through the presence of chlorocruorin. These 
colors when diluted are yellow or colorless, as in many small annelids. Some 
annelids lack a closed circulatory system so that blood and coelomic fluids 
mix freely, resulting in a hemocoel; it may be partial or complete. Many 
annelids have a special organ called a cardiac or heart body surrounding the 
pulsating vessel and sometimes visible as a thick brown or red body of spongy 
tissue; its function is to dispose of circulatory wastes. 
    The excretory system functions to dispose of wastes in the coelom. The 
special organs for the function are called nephridia. Nephridia may be few in 
number and conspicuous, or limited to anterior segments, as in many sedentary 
families, or may be segmental, a pair to a segment, or dispersed and irregular. 
    The muscular system consists of an outer circular and an inner 
longitudinal system of muscles, each varying in extent and density according 
to species. In addition, an oblique series, between outer and inner layers, is 
well developed in annelids performing complex lateral movements. 
    The ability to replace lost parts is highly developed in annelids. Most 
frequent is the replacement of tail, parapodia, and setae. The anterior end 
may be replaced, provided the break is postpharyngeal. Least differentiated 
species regenerate more completely than highly modified ones. Younger 
individuals repair more easily or quickly than older ones. Fragmentation may 
result in as many new individuals as there are pieces. The presence of a part 
of the ventral nerve cord is necessary to develop a new head. See REGENERATION 
(BIOLOGY).  
    Depending on the species, reproduction may be sexual, asexual, or both. 
Sexual reproduction may be dioecious, in which male and female are similar, or 
rarely dissimilar. Individuals may be hermaphroditic, both male and female, 
but with cross fertilization. Some annelids are protandric hermaphrodites, in 
which the sexual stages alternate. 
    Fossil annelids, or segmented worms, are relatively rare, yet sufficiently 
common to indicate that this large and varied group of invertebrates has been 
active and abundant for more than 500,000,000 years. 
 �34s[Olga Hartman]
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�6v�2v�32sAnopla   �21m �33s 
    A class of the phylum Rhynchocoela which is divided into the orders 
Palaeonemertini and Heteronemertini. The simple tubular proboscis lacks 
stylets and resembles the body wall in structure. The mouth is posterior to 
the brain. The nervous system lies either immediately below the epidermis or 
among the musculature of the body wall. The vascular system is well developed. 
Anoplan rhynchocoelans are common on rocky shores, living beneath stones or 
interstitially in shell debris or coarser sands. See ENOPLA; HETERONEMERTINI; 
PALAEONEMERTINI; RHYNCHOCOELA. 
 �34s[J. B. Jennings]
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�6v�2v�32sAnoplura �21m �33s
    A small group of insects usually considered to constitute an order. They
are commonly known as the sucking lice. All are parasites living in the
covering hair of mammals. About 250 species are now known, and these comprise
probably about half of the species in the world, but the group is known in
detail to few persons.
    These lice are distinguishable from the Mallophaga by their mouthparts and
manner of feeding. The mouthparts consist of three very slender stylets which
form a tube. When at rest, they are retracted in a pocket which lies just
behind the mouth. The antennae are usually five-segmented, rarely
three-segmented. The thoracic segments are always very closely fused and lack
wings. The claw is one-segmented, and on at least one pair of legs this claw
is enlarged and can be folded into a process from the tibia to grasp a hair of
the host. In some species two and in others all of the legs are thus modified.
The ovipositor is very much reduced. It consists of but little more than two
flaps which are able to close around a hair. Eyes are commonly lacking, and in
those species in which they do occur, they are reduced to a pair of simple
lenses or two light-receptive spots. All the species are quite small; the
largest scarcely exceeds 0.2 in. (5 mm) in length and the smallest does not
attain 0.04 in. (1 mm).
    Feeding is accomplished by thrusting forth the tube formed by the stylets,
piercing the skin of the host, and sucking blood by a pump in the throat of
the insect. This habit of sucking blood gives the Anoplura a special
importance. They take up any disease-producing organisms in the host's blood
and may transfer these organisms to another individual. Thus, they transfer
the organisms which cause epidemic typhus, relapsing fever, and trench fever.
The transfer of a louse, from one host to another, can occur only when the
hosts are in close bodily contact, as in the nest or at the time of mating. In
the case of humans, the lice may become detached and enter the clothing or the
bedding. In times of social disturbance, such as war or when people are
crowded together as formerly occurred on ships, or in jails or slums, the
opportunities for an exchange of these parasites are enormously increased. The
close restriction of each species of lice to its special host is sufficient to
account for the fact that diseases are not transferred from one species of
mammal to another.
    There are two species of lice which occur as common ectoparasites upon
humans. One of them is Pediculus humanus, the head and body louse which
transmits the diseases mentioned above (see illustration). The other species
is Phthirius pubis, known as the crab louse. It transmits no known disease.
The lice of the Old World monkeys are referred to another genus, Pedicinus.
See INSECTA; LOUSE; MALLOPHAGA.


 �34s[Gordon F. Ferris/Dwight M. DeLong]
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�6v�2v�32sAnorthoclase   �21m �33s 
    The name usually given to alkali feldspars which have a chemical 
composition ranging from Or40Ab60 to Or10Ab90 PLUS/MINUS up to approximately 
20 mole % An (Or, Ab, An = KAlSi3O8, NaAlSi3O8, CaAl2Si2O8) and which deviate 
in one way or another from monoclinic symmetry tending toward triclinic 
symmetry. When found in nature, they usually do not consist of a single phase 
but are composed of two or more kinds of K- and Na-rich domains mostly of 
submicroscopic size. In addition, they are frequently polysynthetically 
twinned after either or both of the albite and pericline laws. It appears that 
they originally grew as the monoclinic monalbite phase, inverting and unmixing 
in the course of cooling during geological times. They are typically found in 
lavas or high-temperature rocks. See FELDSPAR; IGNEOUS ROCKS. 
 �34s[Fritz H. Laves]
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�6v�2v�32sAnorthosite   �21m �33s 
    A rock composed of 90 mole % or more of plagioclase feldspar. Strictly, 
the rock is composed entirely of crystals discernible with the eye, but some 
finely crystalline examples from the Moon have been called anorthosite or 
anorthositic breccia. Scientists have been fascinated with anorthosites 
because they are spectacular rocks (dark varieties are quarried and polished 
for ornamental use); valuable deposits of iron and titanium ore are associated 
with anorthosites; and the massif anorthosites appear to have been produced 
during a unique episode of ancient Earth history (about 1-2 X 10 sup 9 years 
ago).  
    Pure anorthosite has less than 10% of dark minerals - generally some 
combination of pyroxene, olivine, and oxides of iron and titanium; amphibole 
and biotite are rare, as are the light minerals apatite, zircon, scapolite, 
and calcite. Rocks with less than 90% but more than 78% of plagioclase are 
modified anorthosites (such as gabbroic anorthosite), and rocks with 78 - 65% 
of plagioclase are anorthositic (such as anorthositic gabbro). See GABBRO. 
    The structure, texture, and mineralogy vary with type of occurrence. One 
type of occurrence is as layers (up to several meters thick) interstratified 
with layers rich in pyroxene or olivine. The second type of occurrence is the 
massifs type and can have an area up to 11,600 mi sup 2 (30,000 km sup 2). 
Commonly, the massifs are domical in shape and weakly layered. Possibly  there 
is a third group of anorthosite occurrences: extremely ancient bodies of 
layered rock in which the layers of anorthosite contain calcium-rich 
plagioclase and the adjacent layers are rich in chromite and amphibole in 
addition to pyroxene. There are only a few examples of these apparently 
igneous complexes, in Greenland, southern Africa, and India. However, they 
appear to be terrestrial counterparts of lunar anorthosites. 
    By comparison with terrestrial occurrences, most lunar anorthosites are 
very fine grained, although one rock has crystals up to a centimeter long. 
Much of the fine grain size results from comminution by meteorite impact, and 
some of it probably results from rapid crystallization of impact melts. See 
ANDESINE; IGNEOUS ROCKS; LABRADORITE; METAMORPHISM; MOON. 
 �34s[Alfred T. Anderson, Jr.]
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�6v�2v�32sAnostraca �21m �33s
    An order of the subclass Branchiopoda. Commonly called fairy shrimps, they
lack a carapace; hence the name, which means ``without carapace.'' About 175
species have been described. The trunk contains 20 or more segments, of which
the anterior 11 to 19 bear appendages (see illustration). These are followed
by two genital somites and six limbless somites plus a telson. The head
carries the five pairs of appendages typical of crustacea: antennules,
antennae, mandibles, maxillules, and maxillae. In the male the antennae are
modified into claspers. Anostraca feed on the small particles which are
strained from the water by the setose appendages and concentrated in a median
ventral groove which leads to the mouth. The body is covered by a thin but
remarkably impermeable exoskeleton which is molted every 4 to 6 days
throughout the adult life of several months. Along with efficient excretory
glands, this exoskeleton enables Artemia to inhabit very salty waters.
    In bisexual strains, fertilization is necessary to trigger embryonic
development, and copulation occurs after every female molt. However, isolated
parthenogenetic types occur, especially in Europe and Asia, in which females
give rise to females, and chromosomal polyploidy is common. In favorable
environments, nauplius larvae hatch within the ovisac. In unfavorable
situations, a copious secretion from shell glands provides a protective cyst
wall which helps the embryo to survive years of desiccation if necessary. See
BRANCHIOPODA.


 �34s[D. S. Grosch]
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�6v�2v�32sAnoxia   �21m �33s 
    The failure of oxygen to gain access to, or to be utilized by, the body. 
Although the term anoxia is commonly used, a more precise term, hypoxia, is 
more often applicable because there is seldom a complete oxygen defect. 
    Oxygen deprivation may result from interference with some stage of the 
inspiration, lung diffusion, blood transport, cellular absorption, and final 
utilization by enzyme systems. A defect at any one or more of these major 
stages quite often induces a decreased ability of other related mechanisms to 
survive. This is seen most dramatically in any form of anoxia in which the 
brain is deprived of the necessary oxygen for more than a few minutes. Nerve 
cell degeneration begins quickly, and although the original cause of anoxia is 
removed, damage to the respiratory centers prevents resumption of breathing. 
See RESPIRATORY SYSTEM. 
    The term anoxia is used by many authorities to indicate an oxygen 
deficiency at the tissue level, and failure of cellular respiration may be 
designated histotoxic anoxia. There are other terms employed to differentiate 
the type of oxygen deficiency or the stage in the total respiratory process 
where defects occur. 
 �34s[Edward G. Stuart/N. Karle Mottet]
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�6v�2v�32sAnoxic zones   �21m �33s 
    Oxygen-depleted regions in marine environments. The dynamic steady state 
between oxygen supply and consumption determines the oxygen concentration. In 
regions where the rate of consumption equals the rate of supply, sea water 
becomes devoid of oxygen and thus anoxic. In the open ocean, the only large 
regions which approach anoxic conditions are between 165 and 3300 ft (50 and 
1000 m) deep in the equatorial Pacific and between 330 and 3300 ft (100 and 
1000 m) in the northern Arabian Sea and the Bay of Bengal in the Indian Ocean. 
The Pacific region consists of vast tongues extending from Central America and 
Peru nearly to the middle of the ocean in some places. In parts of this zone, 
oxygen concentrations become very low, 15 MU mol/liter (atmospheric saturation 
is 200 - 300 MU mol/liter). Pore waters of marine sediments are sometimes 
anoxic a short distance below the sediment-water interface. The degree of 
oxygen consumption in sediment pore waters depends upon the amount of organic 
matter reaching the sediments and the rate of bioturbation (mixing of the 
surface sediment by benthic animals). In shallow regions (continental shelf 
and slope), pore waters are anoxic immediately below the sediment-water 
interface; in relatively rapid sedimentation-rate areas of the deep sea, the 
pore waters are usually anoxic within a few centimeters of the interface; and 
in pore waters of slowly accumulating deep-sea sediments, oxygen may never 
become totally depleted. See MARINE SEDIMENTS. 
    Restricted basins (areas where water becomes temporarily trapped) are 
often either permanently or intermittently anoxic. Classic examples are the 
Black Sea, the Carioca Trench off the coast of Venezuela, and fiords which 
occupy the Norwegian and British Columbia coasts. Lakes which receive a large 
amount of nutrient inflow (either from natural or human-produced sources) are 
often anoxic during the period of summer stratification. See BLACK SEA; FIORD. 
    The chemistry of many elements dissolved in sea water (particularly the 
trace elements) is vastly changed by the presence or absence of oxygen. Since 
large areas of the ocean water mass are in contact with oxygen-depleted pore 
waters, the potential exists for anoxic conditions to have a marked effect on 
the chemistry of the sea. See SEA WATER; SEA-WATER FERTILITY. 
 �34s[Steven R. Emerson]
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�6v�2v�32sAnseriformes   �21m �33s 
    An order of birds comprising two living families, the screamers 
(Anhimidae) of South America and the cosmopolitan waterfowl (Anatidae), and 
one family (Paranyrocidae) erected to include a Miocene species from South 
Dakota. The number of valid genera and species of waterfowl is a subject of 
continuing debate among ornithologists; approximately 45 genera and 150 
species are usually recognized. Birds of this order are of great economic 
importance.  
    The three species of screamers (Anhimidae) are superficially quite unlike 
waterfowl. They have long, unwebbed toes, long legs, and a chickenlike bill. 
The bend of the wing is armed with powerful bony spurs. The inner surface of 
the skin is highly pneumatic and appears to be covered with small air bubbles. 
Nevertheless, there are several anatomical features which link the screamers 
and the waterfowl. 
    The most primitive of the waterfowl is the semipalmated goose (Anseranas 
semipalmata) of Australia, now placed in its own subfamily. It approaches the 
Anhimidae in several anatomical characters. The toes are only partially 
webbed, and the flight feathers of the wing are molted gradually instead of 
simultaneously as in all other waterfowl. 
    The ducks, geese, and swans (subfamily Anatinae) have in common a thin 
layer of skin covering the bill, which is tipped with a horny nail; webbed 
toes; a relatively long neck; and the simultaneous molting of wing quills, 
which results in a flightless period. See AVES; WATERFOWL. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sAnt   �21m �33s 
    All ants are classified in a single family, Formicidae; this reflects 
their limited structural variation. They are thought to have evolved from a 
wasplike ectoparasite of soil insects. Australia contains many primitive 
species, including the bulldog ants, which are large and fierce and live in 
small colonies (subfamily Myrmicinae). A common tropical group (Ponerinae) has 
evolved specialists in group raiding for soil insects, especially preying upon 
the ubiquitous termites. Also tropical is a set of predacious species 
(Dorylinae) whose colonies can contain a million workers, but only one queen; 
as well as hunting in groups, the whole colony roams nomadically through the 
forests. Other subfamilies are no longer exclusively predacious: in the 
Formicinae, which include the most conspicuous temperate region ants, 
plant-sucking bugs are used as a source of honeydew which, though mainly 
sugary, contains some soluble proteins. Wood ants (Formica species) pile dead 
vegetation into huge mound nests inside which they can retain their body heat. 
From the nests they establish permanent trackways to aphid-bearing trees, on 
which they also capture many insect larvae, including those injurious to 
forest trees. Each tree is part of a large territory which is defended against 
neighboring colonies. Yellow ants (Lasius flavus) culture aphids of many sorts 
on roots belowground and apparently use their carcasses as meat; in Europe 
they build soil mounds in old hillside grassland. In the tropics the 
leaf-nesting ants (Oecophylla species), which use larval silk to bind leaves 
together into a nest, culture plant-bugs in trees as well as preying upon a 
variety of insects; they can be used to protect fruit bushes from harmful 
insects.  
    In subfamily Myrmicinae, though there are many simple, mundane species 
with mixed diets, some specialists have almost given up predation. Thus grain 
collectors store and eat seeds; any seeds that germinate are put out and, 
mixed with rubbish, may start new plants nearer home (surely agriculture in 
its infancy). Then there are the leafcutting ants (tribe Attini) of Central 
America that collect vegetation and feed it to a species of fungus whose 
special bodies they then eat. These ants secrete a battery of chemicals for 
stopping the growth of weed species. Ants of another subfamily 
(Pseudomyrmicinae) have close mutualistic relations with plants: the plants 
supply special hollow galls, stems, or swollen thorns and offer nourishing 
tissues inside; the ants live and feed in these cavities and in exchange 
protect the plant from phytophagous insects and other enemies. 
    All female ants are social insects (the males exist only briefly to 
provide sperm) that live in dense clusters in nests made to protect against 
weather and enemies and to provide a work surface on which to rear their 
young. There are two types of female: small, wingless ones (called workers) 
that construct and defend the nest as well as collect and prepare food for the 
almost helpless larvae; and large, winged ones (called queens) that fly off to 
copulate, disperse, and start new nests, and which, after breaking off their 
wings, not only lay most of the eggs (usually all the female eggs and often 
the male ones too) but stimulate and organize worker activity. Workers usually 
have ovaries but no sperm sac, and the few eggs each lays are unfertilized; 
though haploid, they can produce males by parthenogenesis, for sex is 
determined by a haplo-diploid mechanism as in most insects of this order 
(Hymenoptera). See HYMENOPTERA; SOCIAL INSECTS. 
 �34s[M. V. Brian]
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�6v�2v�32sAntarctica   �21m �33s 
    A snow- and ice-covered continent roughly centered on the South Pole and 
surrounded by an ocean consisting of the southern parts of the Atlantic, 
Pacific, and Indian oceans. It is the fifth largest continent, having an area 
of 14,000,000 km sup 2 or 5,400,000 mi sup 2, which is 1 1/2 times the size of 
the United States. Antarctica is the coldest and highest of all the continents, 
and there are no permanent inhabitants. For millions of years snow and ice 
have built up on the land, burying all but 1% of it. 
    The continent holds 24,000,000 km sup 3 or 5,750,000 mi sup 3 of ice, which 
is 90% of all the ice on Earth. The mean height of the ice surface is 2000 m 
(6700 ft) above sea level, giving Antarctica almost three times the mean 
height of the other continents. The South Pole is 2900 m (9500 ft) above sea 
level. About 10% of the area is made up of ice shelves, which are ice sheets 
floating on the sea and commonly 300 m (1000 ft) thick, but in places are over 
1900 m (6000 ft) thick. The remainder of the ice sheet rests on rock. If all 
this ice were to  melt, an unlikely event which would take thousands of years, 
world sea levels would rise some 60 m (200 ft). 
    As in glaciers, the ice of this huge sheet is moving slowly downhill at 
speeds varying from a few meters or so to about 900 m (3000 ft) per year. When 
it reaches the coast, it breaks off to create the tabular icebergs which are 
characteristic of the Antarctic waters. See ANTARCTIC OCEAN. 
    Antarctica is divided into two parts by one of the world's great mountain 
chains, the Transantarctic Mountains. Radioactive dating indicates that parts 
of East Antarctica are at least 1,800,000,000 years old. Similarities of rock 
types and sequences in other continents indicate that at one time it was part 
of a single great landmass known as Gondwanaland, including what is now the 
Indian peninsula, Australia, South America, and Africa. 
    Antarctica is the coldest region on Earth, the interior being colder than 
Siberia in winter and colder than the North Pole. At the South Pole there are 
6 months (March 21 to September 21) when the Sun remains below the horizon. 
The length of the period of continuous darkness in winter and continuous 
daylight in summer decreases with distance from the pole, so that many coastal 
points have only a few weeks of each. There are considerable differences of 
climate between coast and interior. The average temperature at the South Pole 
is -51 DG C (-60 DG F), while at Anvers Island off the Antarctic Peninsula it 
is -3 DG C (26 DG F). There is very little precipitation and what there is 
falls as snow; the high plateau that covers most of Antarctica is the world's 
largest desert. See POLAR METEOROLOGY. 
    Algae, mosses, and lichens are the characteristic plants of Antarctica; 
only two genera of flowering plants are known. There are no vertebrate land 
animals; the largest creature living entirely on land is an insect 0.12 in. (3 
mm) long. Four species of seal and four species of flightless bird, the 
penguins, breed on or near the shores of Antarctica, though they probably 
spend more than half their lives at sea. Apart from the penguins, only 10 
species of birds breed on the coast of Antarctica at higher latitudes than the 
Antarctic Peninsula. Evidently because of the lack of food, few birds stray 
inland, though skuas and snow petrels have been seen far inland during the 
summer months. See ALGAE; BRYOPSIDA; LICHENES; OCEANIC BIRDS. 
    Antarctica was first sighted in 1820 by American and British sealing 
vessels and by a Russian expedition. But it was not  known to be a continent 
until discoveries made in the period 1838 - 1843. Extensive land explorations 
were made by several expeditions during the first 35 years of the 20th 
century. Since World War II voyages of discovery and great journeys have given 
way to scientific research on a semipermanent footing. The International 
Geophysical Year 1957 - 1958 provided a great stimulus to research in 
meteorology, glaciology, auroral physics, geomagnetism, ionospherics, 
seismology, and gravimetry. The Antarctic Treaty of 1959, ratified by 16 
countries, encourages cooperation and ensures freedom of scientific 
investigation in any part of the continent. 
 �34s[Charles W. Swithinbank]
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�6v�2v�32sAntarctic Circle   �21m �33s 
    The parallel of latitude approximately 66 1/2 DG (66 DG .55) south of the 
Equator and 23 1/2 DG from the South Pole; the same angular distance from the 
Equator as the inclination of the Earth's axis from the plane of the ecliptic. 
It is the parallel of latitude to which the Sun's rays will extend beyond the 
South Pole on December 22 (the Northern Hemisphere winter solstice), when that 
pole is inclined 23 1/2 DG toward the Sun. On that day the Sun will be above 
the horizon for 24 h at the Antarctic Circle. On this same day the Sun's rays 
at noon will just reach the horizon at the Arctic Circle, 66 1/2 DG north. 
    Although the longest period of continuous sunshine at the Antarctic Circle 
is 24 h, the long days preceding and following the solstice, and the long 
periods of twilight, allow a season of about 5 months of continuous daylight. 
The highest altitude of the noon Sun will be 47 DG above the horizon on 
December 22. See MATHEMATICAL GEOGRAPHY; SOLSTICE. 
 �34s[Van H. English]
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�6v�2v�32sAntarctic Ocean �21m �33s
    The three oceans of the Earth, in order of decreasing area are the
Pacific, the Atlantic, and the Indian. In the Northern Hemisphere and most of
the Southern Hemisphere, the oceans are connected only by narrow, shallow
passages which prohibit significant interactions. Such isolation would in time
lead to each of the oceans having differing characteristics were it not for a
major interconnecting artery farther south. This passage exists between the
southern shores of the Australian, South American, and African continents and
the coastline of Antarctica. It is a broad, deep, circumpolar ocean belt and
is essential in equalizing the characteristics of the three oceans and in
maintaining the deep and bottom layers of these oceans as an aerobic, cold
environment. Because of the importance of this ocean area and its obvious
homogeneity in climate (due to its subpolar geographic position), it is
convenient to name it the Antarctic Ocean. This region is not officially
recognized as a separate ocean, mainly because of its lack of a northern
boundary in the classical sense, that is, a complete (or nearly so) land mass.
However, from studying its sea-water characteristics, it becomes possible to
set a northern oceanographic limit as the Antarctic Convergence or Subtropical
Convergence.
    The major flow is the Antarctic Circumpolar Current, or West Wind Drift
(see illustration). Along the Antarctic coast is the westward-flowing East
Wind Drift. The strongest currents are in the vicinity of the polar front zone
and restricted passages such as the Drake Passage and deep breaks in the
meridionally oriented submarine ridge systems. Though the magnitude of the
current is small, there is little attenuation of flow with depth, which
results in the great volume transport of the Antarctic Circumpolar Current. It
is estimated that over 7 X 10 sup 9 ft sup 3/s (2 X 10 sup 8 m sup 3/s) of
water goes through the Drake Passage, about three times the flow of the Gulf
Stream. See OCEAN CURRENTS.
    The extreme cold of the polar regions causes an extensive ice field to
form over the southern regions of the Antarctic Ocean. The extent of the ice
is seasonal in that during the October-to-March period the area decreases, and
it increases during the remaining months. Satellite photographs reveal that
the sea ice field is not uniform, but has many large ice-free regions, called
polynyas. The sea ice plays an important role in the heat balance since it
reflects much more solar radiation (and therefore heat) back into space than
would be the case for a water surface. See HEAT BALANCE, TERRESTRIAL
ATMOSPHERIC; ICEBERG; SEA ICE.
    The composition of the sediments is influenced by two factors: the high
biological productivity in the nutrient-rich euphotic zone, and the input of
glacial debris carried seaward by icebergs. The siliceous ooze (diatoms and
radiolaria) is found roughly between the polar front zone and the limit of
the pack ice. To the north, the siliceous ooze is replaced by calcareous ooze
and red clay in deeper basins. Within the pack ice field, sediments are of
glacial marine type.


_FIELD034;-237[Arnold L. Gordon]_FIELD034
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�6v�2v�32sAntares   �21m �33s 
    Alpha Scorpii, a bright, red supergiant of spectral type M1, temperature 
about 3000 K. Antares is variable in light and radial velocity. Antares has a 
close blue companion of spectral type B5, at a distance of 300 AU 
(astronomical units). The hot star is involved in nebulosity with an unusual 
spectrum. The emission lines are of forbidden Fe II and of Si II, without 
trace of hydrogen, which is normally strong in all other emission nebulae. It 
is possible that solid interstellar or circumstellar matter is being 
evaporated near the hot star. This type of circumstellar nebula has been found 
in many red supergiants, not in the normal spectral region but in the 
very-far-infrared. Apparently the strong stellar winds, or mass loss, from the 
red supergiants result in emission-line fluorescence of matter in the 
immediate surroundings at a rate so large that the nebula then becomes 
visible. See STAR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sAnteater �21m �33s
    A name associated with several animals in five different orders of the
three major groups of living mammals (see table). They are so named because
they are insectivorous, having a diet of ants and termites. The animal most
frequently associated with this name is the ground-living Myrmecophaga
tridactyla, the giant anteater, a member of the family Myrmecophagidae in the
order Edentata (see illustration). This family has three other species,
Tamandua longicaudata, T. tetradactyla, and Cyclopes didactylus, all of which
are arboreal.
    All four species are restricted to the tropical regions of South and
Central America. M. tridactyla prefers the grasslands and more open forested
areas of these regions. The animal is about 6 ft (1.8 m) long including the
tail length, which measures about 2 ft (0.6 m). It is toothless, and the head
extends into a long, tubular snout with a small mouth opening. The tongue is
long, protrusible, and covered with a viscous mucous material which entraps
the insects. The front feet have greatly enlarged claws used for tearing into
ant and termite mounds and as defensive weapons. The body is covered with long
hair, and in Cyclopes the tail is prehensile. Usually a single young is
produced by the female, which she may carry on her back until it is quite
large. See AARDVARK; EDENTATA; MARSUPIALIA; MONOTREMATA; PHOLIDOTA;
TUBULIDENTATA.



 �34s[Charles B. Curtin]
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�6v�2v�32sAntelope   �21m �33s 
    The name given to a group of hollow-horned, hoofed ruminants of the order 
Artiodactyla which are strictly confined to various areas of Africa and Asia. 
They are assigned to the subfamily Antilopinae in the family Bovidae and 
include 91 species in 31 genera. 
    In general, the third and fourth toes are well developed, while the second 
and fifth toes are reduced or absent. The males and usually the females have a 
pair of unbranched horns, which differ considerably in appearance among the 
various species of antelopes. These animals vary in size from the royal 
antelope (Neotragus pygmaeus), which is about 10 - 12 in. (25 - 30 cm) high at 
the shoulder, to the giant eland (Taurotragus derbianus), which measures about 
6 ft (1.8 m) at the shoulder and weighs about 1 ton (0.9 metric ton). See 
ARTIODACTYLA; MAMMALIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sAntenna (electromagnetism) �21m �33s
    A portion of a radio system especially designed to couple electromagnetic
energy between free space and transmission lines. At a radio transmitter the
antenna couples energy from the transmitter circuits into free space, thus
serving to radiate the transmitter's output as radio waves. At the radio
receiver the antenna couples energy from the radio waves into the receiver's
circuits. The same antenna structure functions equally well for transmission
or reception. However, in describing antenna operation, it is often more
direct to describe the radiation action, with the understanding that reception
is the reciprocal. See RECIPROCITY PRINCIPLE.
    Despite this reciprocity, antennas used for transmission may differ
significantly from those used for reception because of the power levels at
which they operate. An antenna for transmitting to a major portion of the
world or to a space vehicle must handle large powers; the antenna for
receiving from radar reflections or from communication satellites handles
negligible power. For these reasons antennas are classified by use; they may
also be classified by operating frequency. Antennas for very low frequencies
may be a mile long, whereas antennas for very high frequencies may be only a
few feet long. More basically, if antenna size is measured in units of the
wavelength of the frequency at which the antenna operates, structures that
differ greatly in size can be described in similar terms.
    For an antenna to radiate, it must contain charges that oscillate. Such a
charge is the electron shown in Fig. 1. Because  no other nearby charges
oscillate to counteract the effect of the electron, its field extends
infinitely into space. This field contains two components: an electric field E
and a magnetic field H. Energy stored in the electric field when the electron
is stationary equals energy in the magnetic field when the electron is moving
most rapidly. Basically, the changing electric field induces an adjacent
magnetic field, and the changing magnetic field induces an adjacent electric
field. The result of this continuing energy exchange is radiation.
    Figure 1 is an elementary graphical representation of the radiating
electric field while a single electron oscillates on a short, thin conductor.
At the top of the figure, one line of the electric field terminates at the
electron. Successive drawings down the figure show this field line as the
electron oscillates on the conductor. Motion of the electron displaces the
line. This displacement travels along the field line, moving in the figure to
the right, with the velocity of light. After the electron has completed one
cycle of oscillation on its conductor, the initial displacement has traveled
one wavelength LAMBDA from the conductor.
    More rigorously, the oscillating electron has an induction field which
decreases rapidly with distance. Energy in this field flows alternately away
from and toward the electron and never escapes into space. The magnetic
component of this field is a maximum when the electric component is zero. In
the radiation field, which extends indefinitely into space, energy flows only
outward at all instants of time and at all locations in space; magnetic and
electric components of this field pass through zero together.
    Along with the oscillating negative charge is a positive charge. If all
oscillating charges on a short, thin conductor, called a doublet, are
considered, the total electromagnetic field is found to contain terms that
decrease with distance in the ratios l/r sup 3 and l/r sup 2 in the induction
field and l/r in the radiation field. The l/r decrease in radiation field as
it spreads out into space from the doublet is due to the greater area of the
sphere that bounds it. See ELECTROMAGNETIC RADIATION; ELECTROMAGNETIC WAVE
TRANSMISSION; RADIO-WAVE PROPAGATION.
    The elemental radiator can be either a short electric device or a small
current device. If infinitesimally small, their far fields are as given in
Fig. 2.
    As Fig. 2 shows, a small electric or magnetic dipole radiates no energy
along its axis, the contour of constant energy being a toroid. This contour in
space, called the radiation pattern, is usually the most basic requirement of
an antenna. The purpose of a transmitting antenna is to direct power into a
specified region; whereas the purpose of a receiving antenna is to accept
signals from a specified direction. In the case of a vehicle, such as an
automobile with a car radio, the receiving antenna needs a nondirectional
pattern so that it can accept signals from variously located stations, and
from any one station, as the automobile moves. The antenna of a broadcast
station may be directional; for example, a station in a coastal city would
have an antenna that concentrated most of the power over the populated land.
The antenna for transmission to or from a communication satellite should have
a narrow radiation pattern directed toward the satellite for efficient
operation, preferably radiating essentially zero power in other directions to
avoid interference. See DIRECTIVITY; NAVIGATION; RADAR; RADIO BROADCASTING.
    The plane of the electric field depends on the direction in which the
electric charge moves on the antenna. The electric field is in a plane
orthogonal to the axis of a magnetic dipole (Fig. 2). This dependence of the
plane of the radiated electromagnetic wave on the orientation and type of
antenna is termed polarization. By combining fields from electric and magnetic
dipoles that have a common center, the radiated field can be elliptically
polarized; by control of the contribution from each dipole, any ellipticity
from plane polarization to circular polarization can be produced. A receiving
antenna requires the same polarization as the wave that it is to intercept.
See POLARIZATION OF WAVES.
    The input impedance to an antenna is the ratio at its terminals, where the
transmission line and transmitter or receiver are connected, of voltage to
current. If the antenna is tuned to resonance at the operating frequency, the
input impedance will be a pure resistance; otherwise it will also have a
reactance component.
    Bandwidth of an antenna is related to its input impedance characteristics.
Bandwidth may be limited by pattern shape, polarization characteristics, and
impedance performance. Bandwidth is critically dependent on the value of Q;
hence the larger the amount of stored reactive energy relative to radiated
resistive energy, the less will be the bandwidth. See Q (ELECTRICITY).
    There are two principal approaches to frequency-independent antennas. The
first is to shape the antenna so that it can  be specified entirely by angles;
hence when dimensions are expressed in wavelengths, they are the same at every
frequency. Planar and conical equiangular spiral antennas adhere to this
principle. The second principle depends upon complementary shapes so that if,
for example, an antenna that is cut from a flat conducting sheet is exactly
the same shape as the part removed, its input impedance will be
frequency-independent. Many log-periodic antennas employ this second
principle, and before the structure shape changes very much, when measured in
wavelengths, the structure repeats itself. By combining periodicity and angle
concepts, antenna structures of very large bandwidths become feasible.
    Antennas whose mechanical dimensions are short compared to their operating
wavelengths are usually characterized by low radiation resistance and large
reactance. This combination results in a high Q and consequently a narrow
bandwidth. Current distribution on a short conductor is sinusoidal with zero
current at the free end, but because the conductor is so short electrically,
typically less than 30 DG of a sine wave, current distribution will be
essentially linear. By end loading to give a constant current distribution,
the radiation resistance is increased by four times, thus greatly improving
the efficiency but not noticeably altering the pattern.
    Dipoles and monopoles illustrate resonant antennas which have an
approximate sinusoidal current distribution and a pure resistance at their
input terminals. However, when the ratio of diameter to length is small, the
input impedance varies rapidly and precludes their use as broadband antennas.
The impedance bandwidth limitation can be mitigated by increasing the diameter
to form a cylinder or by using conical conductors. Long-wire antennas, or
traveling-wave antennas, are usually one or more wavelengths long and are
untuned or nonresonant.
    When they are to be used at short wavelengths, antennas can be built as
horns, mirrors, or lenses. Such antennas use conductors and dielectrics as
surfaces or solids.
    By using reflectors one can achieve high gain, modify patterns, and
eliminate backward radiation. A low-gain dipole, a slot, or a horn, called the
primary aperture, radiates toward a larger reflector called the secondary
aperture. The large reflector further shapes the radiated wave to produce the
desired pattern. See REFLECTION OF ELECTROMAGNETIC RADIATION.
    A beam can be formed in limited space by a two-reflector system. The
commonest two-reflector antenna, the Cassegrain system, consists of a large
paraboloidal reflector. It is illuminated by a hyperbolic reflector, which in
turn is illuminated by the primary feed (Fig. 3).
    An array of antennas is an arrangement of several individual antennas so
spaced and phased that their individual contributions add in the preferred
direction and cancel in other directions. One practical objective is to
increase the signal-to-noise ratio in the desired direction. Another objective
may be to protect the service area of other radio stations, such as broadcast
stations.
 �34s[Carl E. Smith]
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�6v�2v�32sAnthocerotae   �21m �33s 
    A small class of the plant division Bryophyta. Anthocerotae is made up of 
only a few genera of plants commonly called hornworts, or horned liverworts. 
Some species are perennial, others annual; a few are seasonal. Anthoceros is 
the best-known genus. 
    All of the plants are small thalloid types found in very moist, shady 
habitats. Some have a tendency toward dichotomous (forked) branching. There is 
little internal differentiation of tissue in the gametophyte, or 
gamete-producing generation. Each cell usually contains a single large 
chloroplast and one large pyrenoid. In a number of hornworts, colonies of the 
alga Nostoc have been found growing in the intercellular cavities, and these 
may be involved in the fixation of free nitrogen. Numerous smooth-walled 
rhizoids (rootlike structures) differentiate on the ventral (under) surface. 
The sex organs, antheridia (male) and archegonia (female), are usually borne 
on the same thallus, but they may be produced on separate gametophytes. A 
number of eggs on the same thallus are usually fertilized by sperms, resulting 
in the development of numerous sporophytes. Each sporophyte develops into a 
slender, cylindrical capsule, slightly tapering at the apex. The presence of 
chloroplasts permits the sporophyte to be partially independent of the 
gametophyte for food, but still dependent on it for water and minerals. See 
BRYOPHYTA; MARCHANTIOPSIDA; TRACHEOPHYTA. 
 �34s[Paul A. Vestal]
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�6v�2v�32sAnthocyanin �21m �33s
    Any of the intensely colored plant pigments responsible for most scarlet,
crimson, purple, mauve, and blue colors in higher plants. Unlike the
chlorophylls and carotenoids, which are lipid-soluble chloroplast pigments,
the anthocyanins are sap-soluble and are located in the cell vacuole.
Chemically, they are a class of flavonoids and are particularly closely
related, both structurally and biosynthetically, to the flavonols. Their value
to the plant lies in the contrasting colors they provide in flower and fruit,
against the green background of the leaf, to attract insects and animals for
purposes of pollination and seed dispersal. See CAROTENOID; CHLOROPHYLL;
FLAVONOID.
    Anthocyanins are glycosides, and on acid hydrolysis yield sugars and
colored aglycones called anthocyanidins. The commonest aglycone is
3,5,7,3',4'-pentahydroxyflavylium, or cyanidin (see illustration), so called
after the cornflower (Centaurea cyanus), from which it was first isolated. The
other 15 known anthocyanidins all have the same basic structure as cyanidin
and differ only in having fewer or more hydroxyl groups or in being
O-methylated. Common anthocyanidins are pelargonidin, peonidin, delphinidin,
petunidin, and malvidin. The sugar present in anthocyanins may be glucose,
galactose, rhamnose, xylose, or arabinose, a combination of two or three of
these monosaccharides, or a combina tions of a monosaccharide with an aromatic
hydroxy acid. See GLYCOSIDE.
    Anthocyanin colors are most prominent and widespread in flowers in the
red, mauve, and blue shades of most cultivated plants, but they are also very
effective in fruits such as the plum, raspberry, blackberry, and strawberry.
In the leaf, anthocyanin is masked by chlorophyll, except where the pigment
concentration is very high, as in the red cabbage or red Coleus. Anthocyanin
can be present in many other plant tissues, for example, in bean seed, anemone
pollen, banana bract, pomegranate juice, potato tuber, and radish root.
Anthocyanins frequently occur in plants as mixtures, and a series of related
glycosides can often be detected in a single species. Anthocyanins are all but
universal in higher plants. They are also present in ferns, pigmenting the
juvenile fronds with orange or red coloration, and in a few mosses, notably
red sphagnum, but they are not known among the fungi or bacteria.
    The primary function of anthocyanins is undoubtedly as flower pigments
and, together with scent and the presence of nectar, they attract insect and
animal pollinators to plants. The purpose of anthocyanin pigmentation in leaf
tissue is more obscure. While permanent leaf coloration due to anthocyanin is
not usual, transient anthocyanin synthesis in either young or autumnal leaves
is quite common.
    Anthocyanins, although they are intensely colored, have never been widely
used in commerce for dyeing cloth. Their practical value can only be measured
in esthetic terms. Anthocyanin color in foods and in drinks such as red wine
is certainly a stimulus to the appetite, and much effort is expended by food
scientists in retaining these colors during the canning and processing of
fruits and vegetables.
 �34s[Jeffrey B. Harborne]
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�6v�2v�32sAnthophyllite   �21m �33s 
    The name given to the orthorhombic amphiboles. The anthophyllite 
amphiboles form a limited solid-solution series between the magnesian end 
member Mg7 Si8 O22 (OH)2 and an approximately 50 mole % ferric iron member. 
Minerals with more iron than this apparently form the monoclinic mineral 
cummingtonite, thereby indicating a limited tolerance of the orthorhombic form 
for iron. Anthophyllite can accept various amounts of aluminum up to the 
approximate composition (Mg,Fe)5 Al2 (Al2 Si6) O22 (OH)2. Depending on the iron 
content, it can be white, gray, green, or brown, but usually is colorless in 
thin sections. Anthophyllite commonly occurs in fibrous masses, sometimes as 
asbestiform masses and in prismatic needles. See AMPHIBOLE. 
    Anthophyllite is a metamorphic mineral occurring in many schists and 
gneisses in association with such minerals as tremolite, chlorite, cordierite, 
garnet, talc, serpentine, spinel, and quartz. Asbestiform varieties are 
sometimes used as asbestos. See CUMMINGTONITE; TREMOLITE. 
 �34s[George W. DeVore]
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�6v�2v�32sAnthozoa   �21m �33s 
    A class of the phylum Coelenterata. These organisms are marine, solitary 
or colonial, and exclusively polypoid coelenterates with no traces of a 
medusoid stage. Most anthozoans live attached to some firm object of the shore 
or on the sea bottom; some embed in the soft sediment. Anthozoans have a 
cylindrical body with an oral disk, mouth, stomodeum, hollow tentacles, 
endodermal gonad, and cellular mesoglea. The gastrovascular cavity is 
partitioned longitudinally into radial compartments by endodermal mesenteries 
or septa whose free edges, particularly, thicken and differentiate into 
mesenteric or septal filaments. The nervous system is a diffuse network of 
scattered nerve cells over the ectoderm and the endoderm. No localized sense 
organs are present. 
    Both sexual and asexual reproduction occurs. The germ cells are derived 
from the endoderm, and fertilization occurs either in the female 
gastrovascular cavity or in the sea. The zygote develops into either a 
ciliated swimming larva, the planula, or a young polyp. 
    The class Anthozoa includes the soft, horny, stony, and black corals, the 
sea pens, and sea anemones. The horny corals include the sea fans, sea whips, 
and sea feathers. The Anthozoa may be classified as listed below. Separate 
articles appear on each group. 

Class Anthozoa 
    Subclass Alcyonaria (Octocorallia) 
        Order: Stolonifera 
               Telestacea 
               Coenothecalia 
               Alcyonacea 
               Gorgonacea 
               Pennatulacea 
    Subclass Zoantharia (Hexacorallia) 
        Order: Actiniaria 
               Scleractinia (Madreporaria) 
               Zoanthidea 
               Antipatharia 
               Ceriantharia 
               Rugosa 
               Tabulata 

    All anthozoans are marine and most are sedentary, except the free-swimming 
larval stages, while actinians, cerianthids, and pennatulans are somewhat 
movable. They are widely distributed over the world, extending from the Arctic 
to the Antarctic; however, they predominate in the tropic and subtropic areas 
of the Indo-Pacific Ocean. Actinians also inhabit colder water areas from 
which deep-sea species of gorgonians, pennatulans, and scleractinians have 
been collected. 
    Anthozoans seldom tolerate desiccation or heavy sedimentation. They are so 
sensitive to reduced salinity that they usually do not live near coastal areas 
where there is river drainage. Tropical corals are able to endure high 
temperatures and are adversely affected by low temperatures. Therefore, coral 
reefs are commonly located in tropic and subtropic regions. See COELENTERATA; 
HYDROZOA; SCYPHOZOA. 
 �34s[Kenji Atoda]
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�6v�2v�32sAnthracene   �21m �33s 
    A colorless crystalline hydrocarbon, C14H10, which melts at 421.2 DG F 
(216.2 DG C) and boils at 644 DG F (340 DG C). When pure, anthracene shows a 
blue-violet fluorescence. It is obtained from coal tar, which usually contains 
about 1% of the hydrocarbon. 
    Anthracene has had little commercial importance since a direct method for 
the synthesis of anthraquinone from benzene derivatives was discovered. 
Anthracene has been used as a light-screening additive for plastics and finds 
some application in the manufacture of pesticides. See AROMATIC HYDROCARBON; 
POLYNUCLEAR HYDROCARBON. 
 �34s[Charles K. Bradsher]
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�6v�2v�32sAnthracosauria   �21m �33s 
    An order of Carboniferous and Permian labyrinthodont amphibians much less 
common than their temnospondyl relatives but important as including the 
ancestors of reptiles. In contrast with temnospondyls, the pleurocentra are 
retained and developed in the vertebral column, the tabular bones of the skull 
roof are in contact with the parietals, and the cheek is primitively but 
loosely attached to the skull roof. See AMPHIBIA; LABYRINTHODONTIA; 
TEMNOSPONDYLI.  
 �34s[Alfred S. Romer]
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�6v�2v�32sAnthraquinone pigments   �21m �33s 
    Coloring materials which occur in plants, fungi, lichens, and insects. 
About 50 different derivatives of the parent compound, anthraquinone, have 
been isolated. Several of the pigments have been used as dyes; others have 
been utilized as cathartic drugs. The best-known anthraquinone dye is 
alizarin, a natural dye known to the ancient Egyptians and Persians. It occurs 
in the root of the madder (Rubia tinctorum), native to Asia. Another natural 
anthraquinone pigment is carminic acid. It is the dyeing principle of the 
cochineal dye, obtained from the dried bodies of the female of the insect 
Coccus cacti. This and other natural anthraquinone dyes have largely given way 
to better and cheaper synthetic dyes. 
    Emodin is an anthraquinone pigment with strong cathartic properties and is 
an active principle of cascara, senna, aloe, and rhubarb. See DYE. 
 �34s[S. Morris Kupchan]
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�6v�2v�32sAnthrax   �21m �33s 
    An acute infectious disease, primarily of animals from which humans may be 
secondarily infected, caused by Bacillus anthracis. In animals the disease, 
known as splenic fever, occurs when spores of B. anthracis are eaten with 
forage. Humans contract the disease by contact with infected animals or animal 
products, such as bone meal, meat, hide, and fur. 
    The most susceptible animals are cattle, sheep, pigs, horses, and goats. 
Usually septicemia, or blood poisoning, occurs. The effects vary from a sudden 
apoplectic attack to a subacute but eventually fatal illness manifesting 
fever, an enlarged spleen, and frequently intestinal disturbances. Sometimes 
local manifestations, which are less often fatal, occur. For example, in 
cattle and horses circumscribed cutaneous carbuncles may appear, and in swine 
similar lesions are commonly found in the throat. 
    The disease in humans occurs almost exclusively from contact with animals 
or animal products. It takes three main forms: malignant pustule, pulmonary 
anthrax, and intestinal anthrax. 
    Malignant pustule (cutaneous anthrax), the commonest form, results from 
contamination of the skin. Pulmonary anthrax (wool-sorter's disease) is caused 
by the inhalation of dust containing spores. Intestinal anthrax may follow the 
eating of infected food. The two latter types of anthrax are rare but almost 
invariably fatal. Treatment is difficult because of the short period of time 
between onset of symptoms and death. 
    Live spores of attenuated virulence form an effective vaccine for cattle 
and other animals. A cell-free protective vaccine has been produced for use in 
humans. The disease is diagnosed by microscopic identification of bacteria in 
the blood and by the Ascoli thermoprecipitin test. In the Ascoli test, a 
precipitate forms when a boiled saline extract of infected tissue is added to 
a suitable immune serum. See INFECTIOUS-DISEASE TRANSMISSION; MEDICAL 
BACTERIOLOGY.  
 �34s[Harry Smith]
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�6v�2v�32sAnthropology   �21m �33s 
    The observation, measurement, and explanation of human variability in time 
and space. This includes both biological variability and contemporary behavior 
in the human species, and these studies are closely allied with the fields of 
archeology and linguistics as well. Studies range from rigorously scientific 
approaches, as in work on the demography, physiology, and ecology of 
traditional hunter-gatherers, to more humanistic research on topics such as 
symbolism and belief in contemporary religious systems. See ARCHEOLOGY. 
    Research has been emphasizing the integration of these different 
subfields. Anthropologists increasingly are asking questions about the 
adaptiveness of various kinds of human behavior in different societies and are 
approaching these questions in a holistic manner. New subfields such as 
medical anthropology routinely examine attitudes toward illness and medical 
practices in non-Western societies with respect to both the social and 
biological factors involved. See PHYSICAL ANTHROPOLOGY. 
    Most anthropologists are primarily concerned with empirical studies of 
particular human societies, although there are increasing tendencies to look 
for generalizing principles that will allow improved predictions of behavior. 
 �34s[Richard A. Gould]
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�6v�2v�32sAnthropometry   �21m �33s 
    The measurement of the human body; a basic technique of physical 
anthropology. Measurement of continuously varying, quantitative traits finds 
application in studies of evolution, race, genetics, growth and aging, 
physiology, disease, behavior, constitution (individual anthropology), and 
equipment design - in fact, in virtually all areas of human biology. See 
ANTHROPOSCOPY; PHYSICAL ANTHROPOLOGY. 
    The two most important body measurements are height and weight. A ratio of 
the two, usually in the form height / (CUBE ROOT [3] weight), measures 
stockiness-leanness of build, which relates to race, geography, physiological 
function, and disease. The purpose of an anthropometric study will determine 
what measurements are required beyond height and weight - body lengths and 
diameters for studies of race, growth, and machine design; girths for clothing 
sizing; and skinfolds for nutrition and body composition. Stature generally 
correlates with body and limb lengths, and weight with body breadths, depths, 
girths, and skinfolds. 
    Anthropometry has practical as well as scientific applications. The 
soldier's physique has always been of concern to military authorities 
responsible for recruiting and supplying armies. European armies provided 
information on long-term trends in physique within a country, as well as from 
one country to another. Military anthropometry in the United States continued 
with large-scale studies on demobilized soldiers of World Wars I and II and 
with Air Force research during and since World War II by students of E. 
Hooton. Military anthropometry has led to industrial anthropometry. Physical 
anthropologists are currently employed by aircraft and automobile 
manufacturers and are consulted by designers of other equipment for human use. 
See HUMAN-FACTORS ENGINEERING. 
    With a shift in emphasis from describing to explaining human variation, 
new problems and methods have appeared, chiefly interest in body composition 
and in constitution or anthropology of the individual. Partitioning the body 
into percentages of water, fat, bone, and muscle helps to establish standards 
for normal growth and aging, to estimate nutritional status, and to follow the 
course of medical and surgical treatment. Skinfolds, a pure measure of 
subcutaneous fat, correlate well with total body fat, determined by measuring 
specific gravity. Bone density is estimated from x-rays, by comparison with 
wedges of known density. 
    Constitutional anthropology, in which the individual person rather than 
the group is the focus of study, has had a long history dating back to 
Hippocrates, the father of medicine, about 430 B.C. Relating physique to 
behavior, physiology, and disease requires more refined assessment of physique 
than the stocky-lean continuum provides, but at the same time minute details 
must be avoided. The anthroposcopic classifications of Hippocrates and Ernst 
Kretschmer, scaled and made more objective by William Sheldon, have indeed 
proved to be associated with behavior and disease. 
 �34s[Albert Damon]
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�6v�2v�32sAnthroposcopy   �21m �33s 
    The description of physical variation in humans by inspection rather than 
by measurement, as employed in anthropometry. Details of form are rated 
against standards which can be either subjective or objective. The trend has 
been to replace subjective with objective ratings, which is easier for some 
physical features than for others. See ANTHROPOMETRY. 
    Artificial ceramic and fiber standards of skin, hair, and eye color have 
been virtually supplanted by photometric devices identifying spectral 
wavelengths. Skin color, for example, can now be partitioned and individuals 
or groups compared in respect to amounts of melanin, hemoglobin, and carotene. 
Hair pigment can likewise be described and compared with respect to copper, 
iron, and traces of other minerals, in addition to melanin. 
    Other physical measurements, such as hair diameter, period of rotation of 
the hair shaft, and crystallography, are replacing earlier classification of 
hair color as a (subjectively estimated) mixture of melanin and red-gold 
pigment; of hair texture as fine, medium, or coarse, and of hair form as 
straight, low wave, deep wave, curly, or spiral. 
    Subjective ratings still have a place in field studies. Ratings  have not 
been replaced for the whole category of specific details such as forehead 
slope, ear slant, nasal root depression, nasal tip thickness, and the like. 
The trend here too is toward objectification by photography, but subjectivity 
remains. The usual standard has been the adult North European male. The usual 
scale varies from two categories (present and absent) or three categories 
(small, medium, and large) up to five or six (absent, ssm [subsubmedium], sm 
[submedium], + [medium], ++ [marked], +++ [very marked]). Clearly one 
observer's + may be another's sm or ++, and even the same observer's standards 
may change from one series to another or over time. As long as comparisons are 
confined to a single observer and a single series, these problems are minimal. 
The difficulty comes when broader comparison is attempted. See PHYSICAL 
ANTHROPOLOGY.  
 �34s[Albert Damon]
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�6v�2v�32sAnthuridea �21m �33s
    A suborder of the Isopoda. These crustaceans are characterized by slender,
elongate, subcylindrical bodies, and by the fact that the outer branch of the
paired tail appendage (uropod) arches over the base of the terminal abdominal
segment, the telson (see illustration). The uropods of anthurideans attach
laterally to the abdomen and together with the telson form a caudal fan.
    Marked sexual dimorphism is shown in the first pair of antennae which, in
males of many species in both sections, develop brushlike whorls of setae on
the flagellum. Female anthurideans carry the developing young beneath the
thorax in a brood pouch, formed by overlapping plates originating from the
bases of several pairs of legs. See SEXUAL DIMORPHISM.
    Anthurideans are mostly marine, but some live in brackish or freshwater
habitats. Some are facultative ectoparasites of fishes. See ISOPODA.
 �34s[Milton A. Miller]
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�6v�2v�32sAntiarcha �21m �33s
    An order of highly specialized Placodermi, characterized by heavy armor
consisting of a short head shield and a very long, flat-bottomed trunk shield
(see illustration). Most antiarchs are small or moderate in size, the largest
reaching a length of about 48 in. (120 cm). The eyes and nasal openings occupy
an orbital fenestra on top of the skull; in front of them is a premedian
plate, unknown in most other placoderms. The mouth is ventral. There is an
articulation between the dermal bones of the head and trunk shields, but
contrary to the condition in Arthrodira, the condyles are on the head shield,
and the glenoid fossae on the trunk shield. The trunk shield has both anterior
and posterior median dorsal plates. There are peculiar pectoral appendages,
completely covered by small plates, usually jointed near midlength, and
articulating in axial fossae on the sides of the trunk shield; they were
possibly derived from the pectoral fins of more typical placoderms. The body
behind the shield is scaled or naked, has one or two dorsal fins, and a
heterocercal caudal fin; pelvic fins are possibly represented by ``frills''
behind the shield of Bothriolepsis. See ARTHRODIRA.
    Antiarcha were predominantly freshwater stream and lake dwellers, though a
few lived in the sea. They were benthic forms, propelling themselves along the
bottom by their pectoral appendages, and surely feeding on the bottom. See
PLACODERMI.
 �34s[Robert H. Denison]
�33s


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�6v�2v�32sAntibacterial agents   �21m �33s 
    Synthetic or natural compounds which inhibit the growth or division of 
bacteria. This article considers gaseous and dye agents. For other 
antibacterial compounds, see ANTIBIOTIC; ANTISEPTIC; SULFA DRUGS. 
    Gaseous agents serve to disinfect rooms, bedding and other bulky 
materials, surgical instruments, and valuable heat-labile materials such as 
viral vaccines which cannot be filter-sterilized. Most widely used is ethylene 
oxide. Ethylene oxide, boiling point 51.4 DG F (10.8 DG C), is commercially 
available as nonexplosive mixtures with CO2 or fluorinated hydrocarbons. 
Special sterilizing chambers are commercially available. Like other potent 
alkylating agents, it is poisonous and presumably mutagenic. Ethylene oxide 
has the advantages of quick removal and of not seriously lessening the 
nutritive value of microbiological culture media. Good ventilation must attend
its use. BETA-Propiolactone, boiling point 311 DG F (155 DG C), is 4000 times
more effective than ethylene oxide and 50,000 times more effective than CH3Br 
(used for fumigating grain elevators). It works best at high humidities, is 
active even below 50 DG F (10 DG C), and presumably is hazardous to humans. 
Volatile preservatives protect nutrient solutions against microbial action and 
also preserve samples of sea water for later analysis, especially for vitamins. 
The preservative is removed on steam sterilization of the culture medium. They 
probably act as slow alkylating agents and are unsuitable for solutions of 
compounds having free SH (sulfhydryl) groups. 
 �34s[Seymour H. Hutner]
�33s 

    Dye agents are chiefly pyronine (mercurochrome), acridine (neutral 
acriflavine), and triphenylmethane or rosaniline (gentian violet) dyes. 
Historically, the discovery of their ability to prevent selectively the growth 
of and to kill particular kinds of bacteria led to the modern era of search 
for chemotherapeutic drugs. Except as skin disinfectants, their clinical use 
has been rapidly displaced by more specific and more effective antibiotics and 
synthetic drugs with less toxicity for animal tissues. In diagnostic 
laboratory practice, dyes are among the most valuable substances in 
bacteriological media for the selective isolation and differentiation of pure 
culture growth of bacteria from natural sources. See CHEMOTHERAPY; ELECTIVE 
CULTURE.  
 �34s[Carl Lamanna]
�33s 
  

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�6v�2v�32sAntibiotic   �21m �33s 
    A chemical substance, produced by microorganisms, that has the capacity in 
dilute solutions to inhibit the growth of, and even to destroy, bacteria and 
other microorganisms. 
    The first antibiotic to receive widespread attention was penicillin, which 
was developed to meet the need for effective anti-infectious agents during 
World War II. The action of these substances had been recognized for 
centuries. Even in 1942 the full importance of antibiotic substances in nature 
was not recognized. Although penicillin had been discovered in 1928 by 
Alexander Fleming, the scope of the sulfonamides as anti-infectious agents had 
not been defined at that time. Gramicidin and tyrothricin had not been 
discovered, and the fact that antimicrobial agents of natural origin could 
exert an effect against systemic infections in animals and humans had not been 
demonstrated. It was not until the pressure of war stimulated research on 
agents suitable for the control of infectious diseases that Howard Florey and 
his associates succeeded in demonstrating the potentialities of penicillin 
therapy. But only after the discovery of streptomycin in 1944 and subsequent 
observations on the action of bacitracin, neomycin, polymyxin, viomycin, 
chloramphenicol, chlor- and oxytetracycline, and other antibiotics that were 
soon isolated did the full import of the antibiotics become apparent. 
    Penicillin is produced by strains of Penicillium notatum and P. 
chrysogenum. Most of the other antibiotics in clinical use are produced by 
actinomycetes (streptomycetes), a group of organisms that produce a wide 
variety of such substances. Antibiotics are also produced by bacteria and by 
fungi.  
    In general, microorganisms may be subdivided into yeasts, molds (fungi), 
streptomycetes, bacteria, rickettsiae, and viruses. A large proportion of the 
infectious diseases of animals and humans is caused by three of these groups, 
that is, by bacteria, rickettsiae, and viruses. It is against the bacteria and 
rickettsiae that the antibiotics are most effective. See MEDICAL BACTERIOLOGY; 
MEDICAL MYCOLOGY; RICKETTSIOSES; VIRUS. 
    Certain antibiotics, for example, penicillin and erythromycin, are 
primarily effective against gram-positive organisms, while others, such as 
streptomycin, are also effective against gram-negative and acid-fast 
organisms. Some antibiotics, such as chloramphenicol and tetracycline, are 
effective against both gram-positive and gram-negative bacterial species; 
these are spoken of as broad-spectrum antibiotics because of their more 
extensive antimicrobial spectrum. It is the broad-spectrum group of 
antibiotics that has activity against certain of the rickettsiae and large 
viruses. Several antibiotics are active against pathogenic fungi. Among them 
may be mentioned amphotericin B, nystatin, griseofulvin, and saramycetin. 
Griseofulvin is effective against fungi causing skin diseases, whereas the 
others are active against systemic fungal infections. No antibiotic has so far 
been found that is effective against small viruses, such as the virus which 
causes poliomyelitis. 
    The pioneering observation in 1952 on the antitumor activity of 
actinomycin sparked an intensive search for antitumor antibiotics. Several 
were found that received limited use against certain forms of cancer. Among 
these antibiotics are duanomycin, mitomycin C, chromomycin A3, and sarkomycin. 
Unfortunately, the antitumor antibiotics are significantly toxic to the 
patient, so that great caution must be taken with their use. 
    Antibiotics are bacteriostatic or bactericidal in action; that is, they 
either inhibit growth of susceptible microorganisms or actually destroy them. 
The extent to which they may exert bacteriostatic or bactericidal effects 
depends upon the concentration of antibiotic present, the number of 
microorganisms against which it must exert its effect, and various 
environmental conditions. The exact mechanism by which many antibiotics 
inhibit microbial growth is not known. Many antibiotics interfere with vital 
processes of the bacterium, such as protein synthesis, nucleic acid synthesis, 
cell wall formation, and maintenance of the integrity of the cell membrane. 
    To be an effective anti-infectious agent, an antibiotic must be capable of 
exerting its growth inhibitory action in the host against the specific 
microorganism causing infection without simultaneously damaging to any 
significant degree the tissues or organs of the host. It must be capable of 
reaching the site at which the infecting microorganism resides within the 
body, and it must reach that site in sufficient concentration and remain there 
for a sufficient period of time to permit it to exert its effects. 
    Antibiotics may be administered parenterally (that is, by injection), 
orally, or topically. When administered parenterally or orally, they must be 
absorbed into the body and transported by the blood and extracellular fluids 
to the site of the infecting organisms. When administered topically, such 
absorption is rarely possible, and the antibiotics then exert their effect 
only against those organisms present at the site of application. 
 �34s[Edward Meyers]
�33s 
  
  
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�6v�2v�32sAntibody   �21m �33s 
    A protein found principally in blood serum and characterized by a specific 
reactivity with the corresponding antigen. Antibodies are important in 
resistance against disease, in allergy, and in blood transfusions, and can be 
utilized in laboratory tests for the detection of antigens or the estimation 
of the immune status. Antibodies are normally absent at birth unless derived 
passively from the mother through the placenta or colostrum. In time, certain 
antibodies appear in response to environmental antigens, although the inducers 
have not been determined for all normal antibodies, particularly the human 
blood-group isoagglutinins. Antibodies are also induced by artificial 
immunization with vaccines or following natural infections. The resulting 
antibody level declines over a period of months, but rapidly increases 
following renewed contact with specific antigen, even after a lapse of years. 
This is known as an anamnestic or booster response. See ANTIGEN; BLOOD GROUPS; 
HYPERSENSITIVITY; ISOANTIGEN. 
    Three principal groups (IgG, IgM, IgA) and two minor groups (IgD, IgE) of 
antibodies are recognized. These all form part of the wider classification of 
gamma globulins (immunoglobulins). See IMMUNOGLOBULIN. 
 �34s[Margaret J. Polley]
�33s 
  
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�6v�2v�32sAnticline �21m �33s
    A fold in layered rocks in which the strata are inclined down and away
from the axes. The simplest anticlines (see illustration) are symmetrical, but
in more highly deformed regions they may be asymmetrical, overturned, or
recumbent. Most anticlines are elongate with axes that plunge toward the
extremities of the fold, but some have no distinct trend; the latter are
called domes. Generally, the stratigraphically older rocks are found toward
the center of curvature of an anticline, but in more complex structures these
simple relations need not hold. Under such circumstances, it is sometimes
convenient to recognize two types of anticlines. Stratigraphic anticlines are
those folds, regardless of their observed forms, that are inferred from
stratigraphic information to have been anticlines originally. Structural
anticlines are those that have forms of anticlines, regardless of their
original form. See SYNCLINE.
 �34s[Philip H. Osberg]
�33s



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�6v�2v�32sAntiferromagnetism �21m �33s
    A property possessed by some metals, alloys, and salts of transition
elements in which the atomic magnetic moments, at sufficiently low
temperatures, form an ordered array which alternates or spirals so as to give
no net total moment in zero applied magnetic field. The most direct way of
detecting such arrangements is by means of neutron diffraction. See NEUTRON
DIFFRACTION.
    The transition temperature below which the spontaneous antiparallel
magnetic ordering takes place is called the Neel temperature. A plot of the
magnetic susceptibility of a typical antiferromagnetic powder sample versus
temperature is shown in the illustration. Below the Neel point, which is
characterized by the sharp kink in the susceptibility, the spontaneous
ordering opposes the normal tendency of the magnetic moments to align parallel
to the applied field. Above the Neel point, the substance is paramagnetic, and
the susceptibility CHI obeys the Curie-Weiss law, as in Eq. (1), with a
negative paramagnetic Curie temperature -THETA. The Neel temperature is similar
to the Curie temperature in ferromagnetism. See CURIE TEMPERATURE; CURIE-WEISS
LAW; MAGNETIC SUSCEPTIBILITY.

			 *** x = C / (T + THETA) ***			 (1)

    The cooperative transition that characterizes antiferromagnetism is
thought to result from an interaction energy U of the form given in Eq. (2),

		    *** U = -2 SUMMATION Jij S MULT Sj ***		 (2)

where Si and Sj are the spin angular momentum vectors associated with the
magnetic moments of neighbor atoms i and j, and Jij is an interaction constant.
If all Jij are positive, the lowest energy is achieved with all Si and Sj
parallel, that is, coupled ferromagnetically. Negative Jij between nearest-
neighbor pairs (i,j) maylead to simple antiparallel arrays; if the distant
neighbors also have sizable negative Jij, a spiral array may have lowest total
energy. The interaction constant in Eq. (2) probably arises from superexchange
coupling. This is an effective coupling between magnetic spins which is
indirectly routed via nonmagnetic atoms in salts and probably via conduction
electrons in metals. See FERROMAGNETISM; HELIMAGNETISM.
    The magnetic moments are known to have preferred direction. Anisotropic
effects come from magnetic dipole forces and also from spin-orbit coupling
combined with superexchange. Some nearly antiparallel arrays such as Fe2O3,
show a slight bending (called canting) and exhibit weak ferromagnetism. The
anisotropy affects the susceptibility of powder samples and is of extreme
importance in antiferromagnetic resonance. See MAGNETIC RESONANCE.
 �34s[Elihu Abrahams; Frederic Keffer]
�33s


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�6v�2v�32sAntifreeze mixture   �21m �33s 
    An antifreeze is a substance which is added to a liquid to lower its 
freezing point. Common usage limits the term to materials added to water in 
the cooling systems of internal combustion engines. However, it can also 
include mixtures such as refrigeration brines, snow-melting and deicing 
agents, aqueous hydraulic fluids, and heat-transfer fluids. 
    The major criteria for satisfactory antifreeze substances are sufficient 
miscibility with the solvent to lower freezing point; boiling point somewhat 
above maximum operating temperatures of the system in which it is used; low 
viscosity; high degree of chemical stability; low electrical conductivity; 
good heat-transfer properties; and inertness toward materials of construction 
in the system used. 
    The principal automotive antifreeze material is ethylene glycol. 
Methoxypropanol, methyl and isopropyl alcohols, and diethylene and propylene 
glycols are also used to a limited extent. 
    Because of the numerous metals in an automotive cooling system (cast iron, 
steel, brass, copper, solder, aluminum), commercial antifreeze mixtures also 
contain corrosion inhibitors. Frequently, an antifoam agent is also added. 
Electrolytes (inorganic salts), although highly effective, are undesirable for 
most uses because of their corrosivity and electrical conductivity. Most 
windshield wiper fluids are solutions of ethanol, methanol, and isopropyl 
alcohol. Brines for refrigeration of food or making ice are solutions of 
calcium chloride or propylene glycol. The water content of these solutions is 
determined by the lowest temperature encountered. See ENGINE COOLING; 
INHIBITOR (CHEMISTRY); REFRIGERATION; SOLUTION. 
 �34s[Edward F. Harford]
�33s 
  
THETA 
THETA 
  
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�6v�2v�32sAntifriction bearing �21m �33s
    A machine element that permits free motion between moving and fixed parts.
Antifrictional bearings are essential to mechanized equipment; they hold or
guide moving machine parts and minimize friction and wear.
    In its simplest form, a bearing consists of a cylindrical shaft, called a
journal, and a mating hole, serving as the bearing proper. Ancient bearings
were made of such materials as wood, stone, leather, or bone, and later of
metal. It soon became apparent for this type of bearing that a lubricant would
reduce both friction and wear and prolong the useful life of the bearing.
Petroleum oils and greases are generally used for lubricants, sometimes
containing soap and solid lubricants such as graphite or molybdenum disulfide,
talc, and similar substances.

    Materials.	The greatest single advance in the development of improved
bearing materials took place in 1839, when I. Babbitt obtained a United States
patent for a bearing metal with a special alloy. This alloy, largely tin,
contained small amounts of antimony, copper, and lead. This and similar
materials have made excellent bearings. They have a silvery appearance and are
generally described as white metals or as Babbitt metals.
    Wooden bearings are still used for limited applications in light-duty
machinery and are frequently made of hard maple which has been impregnated
with a neutral oil. Wooden bearings made of lignum vitae, the hardest and
densest of all woods, are still used.
    Some of the most successful heavy-duty bearing metals are now made of
several distinct compositions combined in one bearing. This approach is based
on the widely accepted theory of friction, which is that the best possible
bearing material would be one which is fairly hard and resistant but which has
an overlay of a soft metal that is easily deformed. Figure 1 shows bearings in
which graphite, carbon, plastic, and rubber have been incorporated into a
number of designs illustrating some of the material combinations that are
presently available.
    Rubber has proved to be a surprisingly good bearing material, especially
under circumstances in which abrasives may be present in the lubricant. The
rubber used is a tough resilient compound similar in texture to that in an
automobile tire. Cast iron is one of the oldest bearing materials. It is still
used where the duty is relatively light.
    Porous metal bearings are frequently used when plain metal bearings are
impractical because of lack of space or inaccessibility for lubrication. These
bearings have voids of 16 - 36% of the volume of the bearing. These voids are
filled with a lubricant by a vacuum technique. During operation they supply a
limited amount of lubricant to the sliding surface between the journal and the
bearing. In general, these bearings are satisfactory for light loads and
moderate speeds.

    Lubricants.  The method of supplying the lubricant and the quantity of
lubricant which is fed to the bearing by the supplying device will often be
the greatest factor in establishing performance characteristics of the
bearing. For example, if no lubricant is present, the journal and bearing will
rub against each other in the dry state. Both friction and wear will be
relatively high. The coefficient of friction of a steel shaft rubbing in a
bronze bearing, for example, may be about 0.3 for the dry state. If lubricant
is present even in small quantities, the surfaces become contaminated by this
material whether it be an oil or a fat, and depending upon its chemical
composition the coefficient of friction may be reduced to about 0.1. Now if an
abundance of lubricant is fed to the bearing so that there is an excess
flowing out of the bearing, it is possible to develop a self-generating
pressure film in the clearance space as indicated in Fig. 2. These pressures
can be sufficient to sustain a considerable load and to keep the rubbing
surfaces of the bearing separated.
    The types of oiling devices that usually result in insufficient feed to
generate a complete fluid film are, for example, oil cans, drop-feed oilers,
waste-packed bearings, and wick and felt feeders. Oiling schemes that provide
an abundance of lubrication are oil rings, bath lubrication, and forced-feed
circulating supply systems. The coefficient of friction for a bearing with a
complete fluid film may be as low as 0.001.

    Fluid-film hydrodynamic bearings.  If the bearing surfaces can be kept
separated, the lubricant no longer needs an oiliness agent. As a consequence,
many extreme applications are presently found in which fluid-film bearings
operate with lubricants consisting of water, highly corrosive acids, molten
metals, gasoline, steam, liquid refrigerants, mercury, gases, and so on. The
self-generation of pressure in such a bearing takes place no matter what
lubricant is used, but the maximum pressure that is generated depends upon the
viscosity of the lubricant. Thus, for example, the maximum load-carrying
capacity of a gas-lubricated bearing is much lower than that of a
liquid-lubricated bearing. The ratio of capacities is in direct proportion to
the viscosity. Gas is the only presently known lubricant that can be used for
operation at extreme temperatures. Because the viscosity of gas is so low, the
friction generated in the bearing is correspondingly of a very low order. Thus
gas-lubricated machines can be operated at extremely high speeds because there
is no serious problem in keeping the bearings cool.
    The self-generating pressure principle is applied equally as well to
thrust bearings as it is to journal bearings. The tilting-pad type of thrust
bearing (Fig. 3a) excels in low friction and in reliability. A typical
commercial thrust bearing (Fig. 3b) is made up of many tilting pads located in
a circular position. One of the largest is on a hydraulic turbine at the Grand
Coulee Dam. There, a bearing 96 in. (2.4 m) in diameter carries a load of
2,150,000 lb (9,560,000 newtons) with a coefficient of friction of about
0.0009.

    Fluid-film hydrostatic bearings.  Sleeve bearings of the self-generating
pressure type, after being brought up to speed, operate with a high degree of
efficiency and reliability. However, when the rotational speed of the journal
is too low to maintain a complete fluid film, or when starting, stopping, or
reversing, the oil film is ruptured, friction increases, and wear of the
bearing accelerates. This condition can be eliminated by introducing
high-pressure oil to the area between the bottom of the journal and the
bearing itself, as shown schematically in Fig. 4. If the pressure and quantity
of flow are in the correct proportions, the shaft will be raised and supported
by an oil film whether it is rotating or not. Friction drag may drop to
one-tenth of its original value or even less, and in certain kinds of heavy
rotational equipment in which available torque is low, this may mean the
difference between starting and not starting. This type of lubrication is
called hydrostatic lubrication and, as applied to a journal bearing in the
manner indicated, it is called an oil lift. Hydrostatic lubrication in the
form of a step bearing has also been used on various machines to carry thrust.
    Large structures have been floated successfully on hydrostatic-type
bearings. For example, the Hale 200-in. (5-m) telescope on Palomar Mountain
(California Institute of Technology/Palomar Observatory) weighs about
1,000,000 lb (450,000 kg); yet the coefficient of friction for the entire
supporting system, because of the hydrostatic-type bearing, is less than
0.000004. The power required is extremely small and a 1/12-hp (62-W) clock
motor rotates the telescope while observations are being made.

    Rolling-element bearings.  Everyday experiences demonstrate that rolling
resistance is much less than sliding resistance. This principle is used in the
rolling-element bearing which has found wide use. In the development of the
automobile, ball and roller bearings were found to be ideal for many
applications, and today they are widely used in almost every kind of machinery.
    These bearings are characterized by balls or cylinders confined between
outer and inner rings. The balls or rollers are usually spaced uniformly by a
cage or separator. The rolling elements are the most important because they
transmit the loads from the moving parts of the machine to the stationary
supports. Balls are uniformly spherical, but the rollers may be straight
cylinders, or they may be barrel- or cone-shaped or of other forms, depending
upon the purpose of the design. The rings, called the races, supply smooth,
hard, accurate surfaces for the balls or rollers to roll on. Some types of
ball and roller bearings are made without separators. In other types there is
only the inner or the outer ring, and the rollers operate directly upon a
suitably hardened and ground shaft or housing. Figure 5 shows a typical
deep-grooved ball bearing, with the parts that are generally used.
    These bearings may be classified by function into three groups: radial,
thrust, and angular-contact bearings. Radial bearings are designed principally
to carry a load in a direction perpendicular to the axis of rotation. However,
some radial bearings, such as the deep-grooved bearings shown in Fig. 5, are
also capable of carrying a thrust load, that is, a load parallel to the axis
of rotation and tending to push the shaft in the axial direction. Some
bearings, however, are designed to carry only thrust loads. Angular-contact
bearings are especially designed and manufactured to carry heavy thrust loads
and also radial loads.
    A unique feature of rolling-element bearings is that their useful life is
not determined by wear but by fatigue of the operating surfaces under the
repeated stresses of normal use. Fatigue failure, which occurs as a
progressive flaking or pitting of the surfaces of the races and rolling
elements, is accepted as the basic reason for the termination of the useful
life of such a bearing.
 �34s[Dudley D. Fuller]
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�6v�2v�32sAntigen   �21m �33s 
    A substance that initiates and mediates the formation of the corresponding 
immune body, termed antibody. Antigens can also react with formed antibodies. 
Antigen-antibody reactions serve as host defenses against microorganisms and 
other foreign bodies, or are used in laboratory tests for detecting the 
presence of either antigen or antibody. See ANTIBODY. 
    With a few exceptions, such as the autoantigens and the isoantigens of the 
blood groups, antigens produce antibody only in species other than the ones 
from which they are derived. All complete proteins are antigenic, as are many 
bacterial and other polysaccharides, some nucleic acids, and some lipids. 
Antigenicity may be modified or abolished by chemical treatments, including 
degradation or enzymatic digestion; it may be notably increased by the 
incorporation of antigen into oils or other adjuvants. See BLOOD GROUPS; 
ISOANTIGEN.  
    Bacteria, viruses, protozoans, and other microorganisms are important 
sources of antigens. These may be proteins or polysaccharides derived from the 
outer surfaces of the cell (capsular antigens), from the cell interior (the 
somatic or O antigens), or from the flagella (the flagellar or H antigens). 
Gram-negative bacteria also contain the Vi antigen, believed by some 
investigators to be associated with virulence, and the Boivin antigens, which 
contain polysaccharide, lipid, and protein. See VIRULENCE. 
    Other antigens are either excreted by the cell or are released into the 
medium during cell death and disruption; these include many enzymes and 
toxins, of which diphtheria, tetanus, and botulinus toxins are important 
examples. The presence of antibody to one of these constituent antigens in 
human or animal sera is presumptive evidence of past or present contact with 
specific microorganisms, and this finds application in clinical diagnosis and 
epidemiological surveys. See BOTULISM; DIPHTHERIA; TETANUS. 
    Microbial antigens, prepared to induce protective antibodies, are termed 
vaccines. They may consist of either attenuated living or killed whole cells, 
or extracts of these. Since whole microorganisms are complex structures, 
vaccines may contain 10 or more distinct antigens, of which generally not more 
than one or two engender a protective antibody. Examples of these are smallpox 
vaccine, a living attenuated virus; typhoid vaccine, killed bacterial cells; 
and diphtheria toxoid, detoxified culture fluid. Several independent vaccines 
may be mixed to give a combined vaccine, and thus reduce the number of 
injections necessary for immunization, but such mixing can result in a lesser 
response to each component of the mixture. See VACCINATION. 
    Haptens, or haptenes, are partial antigens that can inhibit reactions 
between complete antigens and corresponding antibodies. Some haptens can react 
visibly with antibodies formed to complete antigens, but the haptens cannot 
themselves engender the formation of new antibodies. Haptens are usually 
simple molecules which are small in size, such as degradation products of 
complete antigens. Some haptens may be converted to complete antigens by 
combination with a protein 
carrier. 
    Allergens are antigens that induce allergic states in humans or animals. 
Examples are preparations from poison ivy, cottonseed, or horse dander, or 
simple chemicals such as formaldehyde or picryl chloride. See BIOLOGICALS; 
HETEROPHILE ANTIGEN; HYPERSENSITIVITY; IMMUNITY; IMMUNOLOGY. 
 �34s[Margaret J. Polley]
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�6v�2v�32sAntihistamine   �21m �33s 
    Any drug that combats the effects of alleged histamine release following 
tissue damage by blocking the receptor site at which histamine has its action. 
They may also act by preventing antigen-antibody reactions and by preventing 
the release of histamine from mast cells. The antihistaminics are not potent 
antagonists and fail completely in preventing some of the actions of 
histamine. They are more potent in preventing the actions of histamines than 
in reversing these actions once they develop. These drugs may be effective in 
certain allergies and hypersensitivity reactions but give only symptomatic 
relief since the reaction will continue, even though suppressed, as long as 
the inciting agent is present. See HISTAMINE; HYPERSENSITIVITY. 
    Undesirable side effects of the antihistamines vary considerably from one 
compound to another and according to individual patients as well. Drowsiness, 
dizziness, dryness of the mouth, nausea, gastrointestinal disturbances, and 
muscular weakness are not uncommon. 
    Some antihistamines may be applied locally in solutions, spray, or cream 
form while others are taken orally or by injection. 
    When properly used on an individual drug and patient basis, the 
antihistamines frequently are dramatically effective. However, individual 
reactions and idiosyncrasies and the variable nature of the disorders treated 
militate against their indiscriminate or unregulated use. 
 �34s[N. Karle Mottet]
�33s 
  

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�6v�2v�32sAntiknock agent   �21m �33s 
    A substance added to most finished motor and aviation gasolines, and some 
other hydrocarbon fuels, to increase the resistance of the fuel to knock in 
spark-ignited engines. 
    The antiknock quality of a fuel is determined primarily by its hydrocarbon 
composition, but this quality can be increased by the addition of antiknock 
agents. These may be organic chemicals such as aromatic amines, or 
metalloorganic compounds such as tetraethyllead, tetramethyllead, the mixed 
methylethyl lead alkyls, or methyl cyclopentadienyl manganese tricarbonyl. 
    The lead alkyls are many times as effective as aromatic amines. The 
manganese compound, methylcyclopentadienyl manganese tricarbonyl, is also 
being used as an antiknock agent, principally in the manufacture of lead-free 
gasolines for vehicles equipped with catalytic converters. A chelate of the 
rare earth cerium has been patented as an antiknock but has not been developed 
for commercial use. 
    All metallic antiknock agents vary widely in their effectiveness in 
different fuels, depending upon constitution of the fuel and the presence of 
antagonists, such as sulfur compounds. Antiknock agents are believed to 
function chemically in the engine by decomposing thermally to metals and 
oxides which interrupt oxidative chain reactions leading to autoignition. See 
COMBUSTION KNOCK; GASOLINE; INTERNAL COMBUSTION ENGINE; OCTANE NUMBER; 
ORGANOMETALLIC COMPOUND; TETRAETHYLLEAD. 
 �34s[Harold J. Gibson]
�33s 
                                                                               
  
  
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�6v�2v�32sAntimatter   �21m �33s 
    Substance consisting of atoms which are charge conjugates of atoms found 
in ordinary matter. Since physicists have demonstrated the existence of the 
positron, the antiproton, and the antineutron, which are respectively the 
antiparticles (charge-conjugate particles) of the electron, the proton, and 
the neutron, it is clear that at least in principle it is possible to have the 
charge conjugate of any atom. Such an atom would be formed in perfect analogy 
to the ordinary atom, with each particle replaced by its charge conjugate. 
Such atoms would constitute antimatter. 
    An atom of matter and its counterpart of antimatter, if brought in 
contact, would annihilate each other, giving rise to PI-mesons and other 
particles; but all of these particles created would transform within 
microseconds into GAMMA-rays, neutrinos, and electrons and their antiparticles. 
    Antimatter out of contact with ordinary matter would be stable, and there 
has been speculation about the presence in the cosmos of antiworlds, in which 
antimatter is prevalent. See ANTINEUTRON; ANTIPROTON; ELEMENTARY PARTICLE; 
POSITRON; SYMMETRY LAWS (PHYSICS). 
 �34s[Joseph Lach]
�33s 
  

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�6v�2v�32sAntimetabolite   �21m �33s 
    An enzyme inhibitor that is structurally related to the substrate and 
which may form a complex with the enzyme, analogous to the normal 
enzyme-substrate complex. Products of enzymic reactions have long been known 
to inhibit the reaction to a greater extent than would be expected by mass 
action alone, and frequently analogs of the substrate competitively inhibit an 
enzyme.  
    Antimetabolite is a term which has been given to analogs of metabolites 
that inhibit the utilization of their corresponding metabolite. While most 
antimetabolites are competitive inhibitors, some appear to react with the 
enzyme in a manner which results in a noncompetitive inhibition. 
Antimetabolites are frequently termed metabolite antagonists. Structural 
analogs of most of the known metabolites have been prepared and found to exert 
inhibitory effects on the utilization of their corresponding metabolites in 
many biochemical systems; for example, analog antagonists of amino acids, 
purines, pyrimidines, vitamins, hormones, and other metabolites have been 
prepared.  
    Metabolite antagonists have shown some promise, in a few instances, as 
chemotherapeutic agents. Specificity of the sulfonamides, for example, 
probably results from the fact that in the host a particular coenzyme is 
synthesized from folic acid but not from p-aminobenzoic acid, whereas in the 
infective organism this coenzyme is synthesized from p-aminobenzoic acid but 
not from folic acid. Another application of chemotherapy has been the attempt 
to produce antitumor agents which will inhibit growing cells without damage to 
nonproliferating cells. 
    Inhibitions, involving competition of an analog with a metabolite for an 
enzyme site, can be used to demonstrate metabolic pathways in living organisms 
through analysis of the effect of reversing agents other than the metabolite. 
See ENZYME INHIBITION; SULFA DRUGS. 
 �34s[William Shive]
�33s 
  
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�6v�2v�32sAntimicrobial agents   �21m �33s 
    A chemical compound biosynthetically or synthetically produced, which 
either destroys or usefully suppresses the growth or metabolism of a variety 
of microscopic or submicroscopic forms of life. On the basis of their primary 
activity, they are more specifically called antibacterial, antifungal, 
antiprotozoal, antiparasitic, or antiviral agents. Antibacterials which 
destroy are bactericides or germicides; those which merely suppress growth are 
bacteriostatic agents. See ANTIBACTERIAL AGENTS; FUNGICIDE. 
    Of the thousands of antimicrobial agents, only a small number are safe 
chemotherapeutic agents, effective in controlling infectious diseases in 
plants, animals, and humans. A much larger number are used in almost every 
phase of human activity: in agriculture, food preservation, and water, skin, 
and air disinfection. A compilation of some common uses for antimicrobials is 
shown in the table. 
    The most important antimicrobial discovery of all time, that of the 
chemotherapeutic value of penicillin, was made after hope of developing its 
clinical usefulness had been abandoned. The subsequent establishment of 
penicillin as a nontoxic drug with 200 times the activity of sulfanilamide 
opened the flood gates of antibiotic research. In the next 20 years, more than 
a score of new and useful microbially produced antimicrobials entered daily 
use.  
    New synthetic antimicrobials are found today, as in Ehrlich's day, by 
synthesis of a wide variety of compounds, followed by broad screening against 
many microorganisms. Biosynthetic antimicrobials, although first found in 
bacteria, fungi, and plants, are discovered primarily in actinomycetes. See 
ACTINOMYCETACEAE; ANTIBIOTIC; PENICILLIN. 
    Table title: Common antimicrobial agents and their uses.
 �34s[George M. Savage]
�33s 
  

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�6v�2v�32sAntimonate   �21m �33s 
    The negative radical in salts derived from antimony pentoxide, Sb4O10, and 
bases. Most antimonates contain the [Sb(OH)6] NEGATIVE ion in spite of the fact
that they were called pyro- or meta-antimonates by analogy with phosphates and
arsenates. There is very little similarity between the phosphates and
antimonates. The hypothetical acid, HSb(OH)6, is not isolated; instead, a
gelatinous precipitate of hydrated antimony pentoxide is obtained when antimony
solutions are acidified. Sodium salt, NaSb(OH)6, is one of the very few
insoluble sodium salts and can be used as a test for the sodium ion. See
ANTIMONY; ARSENATE; PHOSPHATE. 
 �34s[E. Eugene Weaver]
�33s 
  

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�6v�2v�32sAntimony   �21m �33s 
    A chemical element, symbol Sb, atomic number 51. Antimony is not a 
naturally abundant element; it is occasionally found native, often in 
isomorphous mixture with arsenic, as allemonite. The symbol Sb is derived from 
the Latin name stibium. 
    The element is dimorphic, existing as a yellow, metastable form composed 
of Sb4 molecules, as in antimony vapor and the structural unit in yellow 
antimony; and a gray, metallic form, which crystallizes with a layered 
rhombohedral structure. Antimony differs from normal metals in having a lower 
electrical conductivity as a solid than as a liquid (as does its congenor, 
bismuth). Metallic antimony is quite brittle, bluish-white with a typical 
metallic luster, but a flaky appearance. Although stable in air at normal 
temperatures, it burns brilliantly when heated, with the formation of a white 
smoke of Sb2O3. Vaporization of the metal gives molecules of Sb4O6, which 
break down to Sb2O3 above the transition temperature. 
    Antimony occurs in nature mainly as Sb2S3 (stibnite, antimonite); Sb2O3 
(valentinite) occurs as a decomposition product of stibnite. Antimony is 
commonly found in ores of copper, silver, and lead. The metal antimonides NiSb 
(breithaupite), NiSbS (ullmannite), and Ag2Sb (dicrasite) also are found 
naturally; there are numerous thioantimonates such as Ag3SbS3 (pyrargyrite). 
    Antimony is produced either by roasting the sulfide with iron, or by 
roasting the sulfide and reducing the sublimate of Sb4O6 thus produced with 
carbon; high-purity antimony is produced by electrolytic refining. 
    Commercial-grade antimony is used in many alloys (1 - 20%), especially 
lead alloys, which are much harder and mechanically stronger than pure lead; 
batteries, cable sheathing, antifriction bearings, and type metal consume 
almost half of all the antimony produced. The valuable property of Sn-Sb-Pb 
alloys, that they expand on cooling from the melt, thus enabling the 
production of sharp castings, makes them especially useful as type metal. 
 �34s[John L. T. Waugh]
�33s 
                                                                               
  
  

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�6v�2v�32sAntineutron   �21m �33s 
    The antiparticle (charge-conjugate particle) of the neutron. All 
electromagnetic properties of a particle are inverted in sign in the charge 
conjugate; thus the antineutron has a magnetic moment equal and opposite to 
that of the neutron. Mass, spin, and the decay constant for BETA-decay are 
identical for the antineutron and the neutron. Both are fermions of spin 1/2. 
See ELEMENTARY PARTICLE; NEUTRON. 
 �34s[Emilio G. Segre]
�33s 
  
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�6v�2v�32sAntioxidant   �21m �33s 
    An inhibitor which is effective in preventing oxidation by molecular 
oxygen (autoxidation). Such inhibitors have great commercial significance in 
the preservation of food and food products and in the prevention of 
deterioration of petroleum products, rubber, and plastics. 
    Autoxidations are free-radical chain reactions characterized by the 
interaction of the radicals with oxygen to yield peroxy radicals, organic 
peroxides, and a broad spectrum of stable oxygenated products. Autoxidation 
chains are often long, so that a single initiating event may produce many 
stable product molecules. Thus only a very small amount of an effective 
antioxidant need be employed for the protection of a large quantity of a 
substrate. The role of the antioxidant is to provide an alternate path for 
oxidation which does not involve the substrate. The antioxidant is destroyed 
in the process and thus does not function indefinitely. 
    The major types of antioxidants in use are the phenols, the aromatic 
amines, sulfur compounds, and a variety of naturally occurring materials. The 
latter find particular use in the protection of foods and cosmetics from 
oxidation. Naturally occurring antioxidants include raw seed oils, wheat germ 
oil, tocopherols, and gums. The activity of the last-named category may often 
be increased by the use of synergists. These are substances which have little 
or no activity alone but which enhance the activity of stronger antioxidants. 
Some effective synergists are phosphoric, citric, and ascorbic acids. 
    The wide variety of antioxidants available is necessitated the extreme 
range of conditions under which protection from oxidation is required. For 
example, an antioxidant which can delay the development of rancidity in stored 
butter will seldom prove to be suitable for the protection of hot lubricating 
oil in the crankcase of an automobile. See CATALYSIS; FREE RADICAL; INHIBITOR 
(CHEMISTRY).  
 �34s[Lee R. Mahoney]
�33s 
  
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�6v�2v�32sAntipatharia �21m �33s
    An order of the subclass Zoantharia. These animals are the black or horny
corals which live in rather deep tropical and subtropical waters and usually
form regular or irregularly branching plant-like colonies, often 6.6 or 9.9 ft
(2 or 3 m) in height, with thorny, solid lamellar, horny axial skeletons (see
illustration). Stichopathes forms an unbranching wirelike colony.
    The polyp or zooid has six unbranched, nonretractile tentacles with a
warty surface due to the presence of nematocysts. Six primary, complete,
bilaterally arranged mesenteries occur, of which only two lateral ones bear
filaments and gonads. Adjacent zooids are united by a coenenchyme, but their
gastrovascular cavities have no connection. The musculature is the most weakly
developed in the anthozoans.
    The polyps are dioecious. Schizopathidae are dimorphic; the gastrozooid
has a mouth and two tentacles, while the gonozooid, the only fertile polyp,
lacks a mouth. See COELENTERATA; ZOANTHARIA.
 �34s[Kenji Atoda]
�33s



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�6v�2v�32sAntiproton   �21m �33s 
    The antiparticle (charge-conjugate particle) of the proton. P. A. M. 
Dirac's equation of relativistic quantum theory for the electron admits 
solutions which describe a particle of mass identical to that of the electron 
but of opposite charge; this is the positron. A generalization of the 
relativistic equations for fermions of spin 1/2 leads to the prediction that 
there should be a particle of mass identical to that of the proton but of 
opposite charge; this is the antiproton. It was first experimentally 
demonstrated in 1955. See ELEMENTARY PARTICLE; RELATIVISTIC QUANTUM THEORY. 
 �34s[Emilio G. Segre]
�33s 
  
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�6v�2v�32sAntiresonance   �21m �33s 
    The condition for which the impedance of a given electric, acoustic, or 
dynamic system is very high, approaching infinity. In an electric circuit 
consisting of a capacitor and a coil in parallel, antiresonance occurs when 
the alternating-current line voltage and the resultant current are in phase. 
Under these conditions the line current is very small because of the high 
impedance of the parallel circuit at antiresonance. See RESONANCE 
(ALTERNATING-CURRENT CIRCUITS). 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sAntiseptic   �21m �33s 
    A drug used to destroy or prevent the growth of infectious microorganisms 
on or in the human or animal body, that is, on living tissue. Many chemical 
substances have been employed as antiseptics, such as certain halogens 
(iodine), mercurial compounds, essential oils, silver compounds, alcohols, 
bisphenols, and quaternary ammonium compounds. See HALOGEN ELEMENTS. 
    Iodine is the most important of the halogens used as an antiseptic. 
Tincture of iodine (iodine in an alcohol solution) has been employed widely as 
a preoperative antiseptic and in first aid. Tincture of iodine is germicidal 
by laboratory test in 0.02% concentration, but 2.0% solutions are usually 
employed in surgery and first aid. 
    Compounds of mercury were used to prevent infection before the germ theory 
of disease was established. Because of their high toxicity and severe caustic 
action, such inorganic mercurials as mercuric chloride, mercuric oxycyanide, 
and potassium mercuric iodide have been largely replaced by certain organic 
mercury compounds. Organic mercurial compounds are far less toxic and are 
nonirritating in concentrated solutions. They are highly bacteriostatic, and 
in concentrated solutions germicidal as well. They are also nonspecific in 
antimicrobial activity. 
    Essential oils have been defined as odoriferous oily substances obtained 
from such natural sources as plants by steam distillation. Essential oils in 
alcoholic solutions also were early employed in place of the carbolic acid 
solution of Lister, and because of the toxic and corrosive action of mercury 
bichloride, they also replaced this compound. Alcoholic solution of essential 
oils was first developed in 1881 and was admitted as liquor antisepticus to 
the U.S. Pharmacopoeia in 1900 and to the National Formulary IV in 1916. 
Alcoholic solutions of essential oils as represented by liquor antisepticus 
have proved effective in a wide variety of clinical applications and in first 
aid.  
    Silver compounds have been widely used for a variety of purposes. Because 
of the bland nature of most of these compounds, they have been successfully 
used in the eyes, nose, throat, urethral tract, and other organs. The most 
widely used silver compounds are silver nitrate, ammoniacal silver nitrate 
solution, silver picrate, and certain colloidal silver preparations such as 
strong protein silver and mild silver protein. These are effective germicides 
of low tissue toxicity and are not counteracted by organic matter. 
    Such compounds as ethyl alcohol and isopropyl alcohol are germicidal 
rather than bacteriostatic and are effective against the vegetative forms of 
bacteria and virus, but do not kill spores. Ethyl alcohol in 62.5 - 70% 
solution is most commonly used, being widely employed for disinfecting the 
skin before hypodermic injections and other skin punctures. Isopropyl alcohol 
is equal, if not superior, to ethyl alcohol and is widely used for degerming 
the skin and for disinfecting oral thermometers. Alcohols are also widely used 
in other antiseptic preparations, in which they serve to lower the surface 
tension and to promote spreading and penetration. 
    Bisphenol compounds such as dichlorophene and tetrachlorophene are 
essentially bacteriostatic agents and are weaker as germicides. They have 
proved quite effective as skin-degerming agents, when used in soaps and other 
detergents, and as mildew-preventing formulations. The halogenated form, such 
as dichlorophene, tetrachlorophene, hexachlorophene, and bithionol, is most 
commonly employed. When used repeatedly on the skin, as in soaps and 
detergents, bisphenols have a tendency to remain for long periods, thus 
reducing skin bacteria to a significant degree. For this purpose they are 
especially useful in preoperative hand washing. 
    Quaternary ammonium compounds have high germicidal activity. Although they 
are more properly classified as surface-active disinfectants, some of them are 
employed in certain antiseptic formulations, for instance, Zephiran, 
especially suited for use on the skin, and Cepacol, for mucous surfaces. 
Nontoxic and nonirritating, they may be used in place of alcohol after 
preoperative scrub-up. See ANTIMICROBIAL AGENTS; BIOASSAY; FUNGICIDE. 
 �34s[George F. Reddish/Francis Clark]
�33s 
  

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�6v�2v�32sAntitoxin   �21m �33s 
    An antibody that will combine with and generally neutralize a particular 
toxin. When the manifestations of a disease are caused primarily by a 
microbial toxin, the corresponding antitoxin, if available in time, may have a 
pronounced prophylactic or curative effect. Apart from this, the other 
properties of an antitoxin are those of the antibody family (IgG, IgA, IgM) to 
which it belongs. See ANTIBODY; BIOLOGICALS; IMMUNOGLOBULIN. 
    Antitoxins have been developed for nearly all microbial toxins. 
Diphtheria, tetanus, botulinus, gas gangrene, and scarlatinal toxins are 
important examples. Antitoxins may be formed in humans as a result of the 
disease or the carrier state, or following vaccination with toxoids, and these 
may confer active immunity. The status of this can be evaluated through skin 
tests, or by titration of the serum antitoxon level. See BOTULISM; DIPHTHERIA; 
GANGRENE; IMMUNITY; SKIN TEST; TETANUS; TOXIN-ANTITOXIN REACTION. 
 �34s[Henry P. Treffers]
�33s 
  
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�6v�2v�32sAnt lion �21m �33s
    Any insect of the family Myrmeleontidae in the order Neuroptera. Myrmeleon
immaculatus is a common representative (see illustration). These insects occur
more commonly in dry areas, such as the South and Southwest, than in other
sections of the United States; however, they are not limited to these locales.
The larvae construct conical pits by using their heads, which are flat and
shovel-shaped. The pits are about 1 in. (2.5 cm) across, and the larvae remain
at the bottom as passive predators feeding upon ants and other insects which
fall into the pit. A pair of sickle-shaped mandibles, which are sharp, smooth,
and curved at the tip, extend from the head of the larvae. There is no mouth.
The prey that falls into the pit is grasped by the mandibles. These structures
are similar to a hypodermic needle, through which the ant lion can pump its
digestive enzymes into the victim. Digestion occurs outside the animal and,
since all undigestible material remains outside the larva, solid elimination
does not take place; actually, the hindgut of the ant lion is closed. The
larval stage may extend from 1 to 3 years, and then pupation occurs with the
production of a silky cocoon. See NEUROPTERA.


 �34s[Charles B. Curtin]
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�6v�2v�32sAnura �21m �33s
    One of the three living orders (sometimes called Salientia) of the class
Amphibia which includes the frogs and toads. About 2400 species of frogs are
known. Only the frozen polar regions and remote oceanic islands are without
native frogs, and 80% of the species live in the tropics.
    Frogs are short-bodied animals with a large mouth and protruding eyes. The
externally visible part of the ear, absent in some forms, is the round, smooth
tympanum situated on the side of the head behind the eye. There are five
digits on the hindfeet and four on the front. Teeth may be present on the
upper jaw and the vomerine bones of the roof of the mouth, but are found on
the lower jaw of only one species. Often teeth are totally lacking, as in
toads of the genera Bufo and Rhinophrynus. The short vertebral column consists
of from 6 to 10 vertebrae, usually 9, and the elongate coccyx. The sacral
vertebra precedes the coccyx and bears more or less enlarged lateral processes
with which the pelvic girdle articulates. A characteristic feature of frogs is
the fusion of the bones in the lower arm and lower leg, so that a single bone,
the radioulna in the arm and the tibiofibula in the leg, occupies the position
of two in most other tetrapods.
    The one character of frogs that comes to the attention of most persons,
including many who may never see a frog, is the voice. Most frogs have voices
and use them in a variety of ways. In the breeding season great numbers of
male frogs may congregate in favorable sites and call, each species giving its
own characteristic vocalization. Because no two species breeding at the same
time and place have identical calls, it is assumed that the call is important
in aiding individuals to find the proper mate. In some species it appears that
the female is active in selecting the mate and may be responding to the mating
call, but the call may not act in exactly the same way in other species. The
mating call is given with the mouth closed. Air is shunted back and forth
between the lungs and the mouth, so frogs can call even though submerged. Many
species possess one or two vocal sacs, which are expansible pockets of skin
beneath the chin or behind the jaws. The sacs, (Fig. 1), which may be inflated
to a volume as great as that of the frog itself, serve as resonators.
    Other noises made by frogs include the so-called fright scream given with
the mouth open, and the warning chirp, which evidently serve as a sex
recognition signal when one male contacts another. Some calls evidently serve
as territorial signals.
    Breeding and development typically take place in the following manner. The
male grasps the female about the body with the forelegs, a procedure called
amplexus, and fertilizes the eggs externally as they are extruded. The number
of eggs may be quite large (up to 20,000 in the bullfrog or 25,000 in a common
toad) or may be as few as one in a frog of the West Indies. The larva, called
a tadpole, is at first limbless and has external gills and a muscular tail
with dorsal and ventral fins (Fig. 2). At hatching there is no mouth opening
present, but one soon forms that develops a horny beak and several rows of
labial teeth not at all like the true teeth of the adult frog. Shortly after
the tadpole hatches, the gills become enclosed within chambers and are no
longer visible externally. Except for the gradual development of the
hindlimbs, no additional external changes take place as the tadpole grows
until the time for metamorphosis. The anterior limbs, which have been forming
hidden in the gill chambers, break through the covering skin as metamorphosis
begins. The tail dwindles in size as it is absorbed, while the mouth assumes
the shape of that of the adult frog. Many other changes are taking place
internally, including shortening of the intestine and adapting it to the
carnivorous diet of the adult frog.
    All frogs are carnivorous. The kind of food seems to depend largely upon
the size of the frog, whose capacious mouth permits somewhat astonishing feats
of swallowing. A large bullfrog, for example, may snap up low-flying bats,
ducklings, snakes, and turtles. Insects and other invertebrates form the bulk
of the diet of most frogs. The tongue, moistened by a sticky secretion from
the intermaxillary gland in the roof of the mouth, is used to catch smaller
prey, while larger items of food may bring the front limbs into play. When
swallowing, a frog will usually depress the eyeballs into the head to aid in
forcing the food down the pharynx. In contrast to transformed frogs, most
tadpoles are vegetarian and feed on algae. A few are largely carnivorous or
sometimes cannibalistic, and even vegetarian species will scavenge for dead
animal matter.
    The habitats of frogs are as various as the places where fresh water
accumulates. Lakes and streams are tenanted year-round by many species, and
others migrate to these places in the breeding season. Any permanent source of
water in the desert is likely to support a population of one or more species,
and when rainstorms occur, the air around a temporary pool may be filled with
mating calls for a few nights, while the frogs take advantage of the water for
breeding. As often as not, the pool goes dry before the tadpoles metamorphose,
and the adult frogs retreat underground to await another rain. Moist tropical
regions provide an abundance of habitats little known to temperate regions,
such as the air plants (bromeliads) that hold water and so provide a moist
home and breeding site for frogs that may never leave the trees.
    Although the majority of frogs fall into fairly well-defined familial
categories, the arrangement of the families into subordinal groups by
different authorities is not consistent, and there is controversy about the
relationships of some smaller groups. See FROG.
 �34s[Richard G. Zweifel]
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�6v�2v�32sAorta   �21m �33s 
    The main vessel of the systemic arterial circulation arising from the left 
ventricle of the heart; it is divided into three parts for convenience only. 
The first portion, the ascending aorta, passes upward under the pulmonary 
artery; the coronary arteries arise at the base of the ascending aorta behind 
the aortic valves. The second part, or aortic arch, curves over the hilum of 
the left lung, giving off the innominate, left carotid, and left subclavian 
arteries, which supply the neck, head, and forelimbs. The third portion, or 
descending aorta, continues downward in the thorax on the left side of the 
vertebral column to the diaphragm, giving off small arteries to the bronchi, 
esophagus, and other adjacent tissues. Below the diaphragm this vessel, known 
as the abdominal aorta, descends to the level of the fourth lumbar vertabra 
where it bifurcates into the two common iliac arteries supplying the hindlimbs. 
    In the abdomen the major branches of the aorta include the single celiac, 
superior mesenteric and inferior mesenteric, and the paired renal and internal 
spermatic (male) or ovarian (female) arteries. In addition, many small 
branches go to other organs and to the body wall. See ARTERY; CIRCULATORY 
SYSTEM.  
 �34s[Walter J. Bock]
�33s 
  

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�6v�2v�32sApatite   �21m �33s 
    The most abundant and widespread of the phosphate minerals, crystallizing 
in the hexagonal system. The apatite structure type includes no less than 10 
mineral species and has the general formula X5(YO4)3Z, where X is usually 
Ca sup 2 POSITIVE or Pb sup 2 POSITIVE, Y is P sup 5 POSITIVE or As sup 5 
POSITIVE, and Z is F NEGATIVE, Cl NEGATIVE, or (OH) NEGATIVE. The apatite 
series takes X = Ca, whereas the pyromorphite series includes those members 
with X = Pb. Three end members form a complete solid-solution series involving 
the halide and hydroxyl anions, and these are fluorapatite, Ca5(PO4)3F; 
chlorapatite, Ca5(PO4)3Cl; and hydroxyapatite, Ca5(PO4)3(OH). Thus, the 
general series can be written Ca5(PO4)3(F,Cl,OH), the fluoride member being 
the most frequent and often simply called apatite. 
    The apatite isosmorphous series of minerals occurs as grains, blebs, or 
short to long hexagonal prisms terminated by pyramids, dipyramids, and the 
basal pinacoid. The minerals are transparent to opaque, and can be 
asparagus-green (asparagus stone), grayish-green, greenish-yellow, gray, 
brown, brownish-red, and more rarely violet, pink, or colorless. Apatites are 
brittle, with hardness 5 on Mohs scale, and specific gravity 3.1 - 3.2; they 
are also botryoidal, fibrous, and earthy. 
    Apatite occurs in nearly every rock type as an accessory mineral. It often 
crystallizes in regional and contact metamorphic rocks, especially in 
limestone and associated with chondrodite and phlogopite. It is very common in 
basic to ultrabasic rocks; enormous masses occur associated with 
nepheline-syenites in the Kola Peninsula, Soviet Union, and constitute 
valuable ores which also contain rare-earth elements. Large beds of oolitic, 
pulverulent, and compact fine-grained carbonate-apatites occur as phosphate 
rock, phosphorites, or collophanes. Extensive deposits of this kind occur in 
the United States in Montana and Florida and in North Africa. The material is 
mined for fertilizer and for the manufacture of elemental phosphorus. See 
FERTILIZER; PHOSPHORUS; PYROMORPHITE.  
 �34s[Paul B. Moore]
�33s 
  

A700000001155�254s�5v�1v118
\IMG\CI1.IMG
16846848
�6v�2v�32sAphanite   �21m �33s 
    An igneous rock in which the constituents cannot be distinguished by the 
unaided eye. Aphanitic material may be minutely crystalline, or it may be 
glassy. Most lavas solidify so rapidly that coarse mineral grains cannot form; 
thus, volcanic rocks are mostly aphanites. Under conditions of slow cooling, 
however, such as exist deep below the Earth's surface, molten rock material 
(magma) solidifies to coarse-grained plutonic rocks. Such visibly crystalline 
rocks are called phanerites. See IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
�33s 
  
A700000004534�254s�5v�1v118
\IMG\CI1.IMG
16846848
�6v�2v�32sAphasia   �21m �33s 
    Impairment in the use of spoken or written language caused by injury to 
the brain, which cannot be accounted for by paralysis or incoordination of the 
articulatory organs, impairment of hearing or vision, impaired level of 
consciousness, or impaired motivation to communicate. The language zone in the 
brain includes the portion of the frontal, temporal, and parietal lobes 
surrounding the sylvian fissure and structures deep to these areas. In 
right-handed persons, with few exceptions, only injury in the left cerebral 
hemisphere produces aphasia. Lateralization of language function is variable 
in left-handers, and they are at greater risk for becoming aphasic from a 
lesion in either hemisphere. 
    Distinctive recurring patterns of deficit are associated with particular 
lesion sites within the language zone. These patterns may entail selective 
impairment of articulation, ability to retrieve concept names, or syntactic 
organization. Other dissociations affect principally the auditory 
comprehension of speech, the repetition of speech, or the recognition of 
written words. The erroneous production of unintended words in speech 
(paraphasia), oral reading (paralexia), or writing (paragraphia) is a feature 
of some forms of aphasia. 
    Mixed forms of aphasia, caused by multiple lesions or lesions spanning 
anterior and posterior portions of the speech zone, are quite common, and 
massive destruction of the entire language area results in a global aphasia. 
Further, individual variations in behavioral manifestations of similar lesions 
have set limits on the strict assignment of function to structures within the 
language area. 
    Preadolescent children suffering aphasia after unilateral injury usually 
recover rapidly, presumably by virtue of the capacity of the right cerebral 
hemisphere early in life to acquire the language functions originally mediated 
by the left hemisphere. Capacity for recovery of function decreases during 
later adolescence and young adulthood. 
    Complete recovery in adults after a severe injury is much less common, and 
severe aphasia may persist unchanged for the duration of the person's life. 
Many patients are aided by remedial language training, while others continue 
severely impaired. See AGNOSIA; MEMORY. 
 �34s[Harold Goodglass]
�33s 
  
A700000001006�254s�5v�1v118
\IMG\CI1.IMG
16846848
�6v�2v�32sAphelenchoidea   �21m �33s 
    A superfamily of small, oral stylet bearing, predominantly plant- and 
insect-associated nematodes. These nematodes range from free-living species of 
saprophagous, fungivorous, and predacious habits, to obligate parasites of 
plants, commensals, and ecto- and endoparasites of insects. Although primarily 
found in terrestrial environments, some species occur in marine habitats. See 
NEMATA; PARASITOLOGY. 
 �34s[Bruce Hopper]
�33s 
  

A700000001107�254s�5v�1v118
\IMG\CI1.IMG
16846848
�6v�2v�32sAphelion   �21m �33s 
    In astronomy, that point at one extremity of the major axis of an 
elliptical orbit about the Sun where the orbiting body is farthest from the 
Sun. The Earth passes aphelion on or near July 4, referred to as the time of 
aphelion passage. Because the orbit of the Earth is nearly a circle, the Earth 
is then only some 1.6 X 10 sup 6 mi (2.5 X 10 sup 6 km) farther from the Sun 
than at its mean distance of 9.30 X 10 sup 8 mi (149.6 X 10 sup 7 km). See 
ORBITAL MOTION.  
 �34s[Raynor L. Duncombe]
�33s 
  

A700000003023�254s�5v�1v118
\IMG\CI1.IMG
16846848
119
\IMG\CI1.IMG
16972800
�6v�2v�32sAphid �21m �33s
    One of a group of mostly soft-bodied plant-feeding insects of the suborder
Homoptera, superfamily Aphidoidea. The worldwide fauna of over 4000 species is
most abundant in north temperate regions. Aphids feed on phloem sap from
vascular plants, tapping it through a feeding tube formed from modified
mandibles and maxillae called stylets. In so doing they may transmit viruses
from plant to plant, spreading serious disease in crops such as potatoes,
cereals, sugarbeet, and citrus. Plants sometimes react to aphid feeding by
forming galls in which the aphids live protected from drought and enemies. The
so-called Chinese gall is valued in commerce for its high tannin content.
    Aphids have evolved complex life styles to exploit the changing growth
phases of plants. Many divide their yearly cycle by flying between a primary
host, on which sexual forms mate and lay winter eggs, and a secondary host,
where only parthenogenetic females multiply. Only one generation of males and
sexual oviparous females occurs each year, usually in autumn. Most
parthenogenetic females are also viviparous, and reproduce very rapidly under
favorable conditions. Viviparae are winged or wingless (see illustration).
Development of young aphids can be switched toward either wingedness or
sexuality by outside factors, such as crowding, decreasing temperature, or
shortening days. Aphids in the tropics often remain wholly parthenogenetic.
 �34s[Henry L. G. Stroyan]
�33s



A700000004767�254s�5v�1v119
\IMG\CI1.IMG
16972800
�6v�2v�32sApical meristem   �21m �33s 
    Permanently embryonic tissue involved in cell division at the apices of 
roots and stems, and forming dynamic regions of growth. These apical 
meristems, usually consisting of small, densely cytoplasmic cells, become 
established during embryo development. Thereafter they divide to produce the 
primary plant body of root and shoot. Below the apical meristems, tissue 
differentiation begins; the protoderm gives rise to the epidermal system, the 
procambium to the primary vascular system, and the ground meristem to the pith 
and cortex. 
    Leaf primordia are formed on the flanks of the shoot apex, in an 
arrangement or phyllotaxis characteristic of the species. The leaf primordium 
grows apically at first, but in angiosperms apical growth is short-lived and 
is followed by intercalary growth; in fern leaf primordia, apical growth is 
prolonged. Marginal meristems divide to form the leaf blade or lamina; an 
adaxial meristem may contribute to the thickness of the leaf. In most 
angiosperms, in addition to leaf primordia the apical meristem, or its 
derivatives, gives rise to bud primordia. These usually occupy axillary 
positions (that is, above the junction of the leaf with the stem), and are 
exogenous (that is, originate superficially), sometimes developing from 
detached meristems, pockets of meristematic tissue separated from the apical 
meristem by more mature, differentiated tissue. 
    Adventitious buds are formed on root (as in raspberry) or leaf (as in 
Begonia) cuttings, and may be exogenous or endogenous in origin. 
    Primary thickening meristems bring about growth in thickness of many 
monocotyledons, and a few dicotyledons which have little secondary growth. 
They consist of files of elongated cells on the ``shoulders'' of the shoot 
tip, usually below the leaf primordia; by their division, they add to the 
width of the shoot tip and sometimes raise the periphery of the apical region 
so that the apex itself lies in a depression. 
    Under appropriate environmental stimuli (such as daylength), vegetative 
shoot apices may be induced to develop as apices or inflorescences or 
individual flowers. Inflorescence apices may increase markedly in size, become 
more steeply domed, and finally form floret primordia. Floral apices give rise 
to a sequence of primordia of floral organs (sepals, petals, stamens, 
carpels), many of which originate in a manner similar to leaf primordia. 
 �34s[Elizabeth G. Cutter]
�33s 
  

A700000002425�254s�5v�1v119
\IMG\CI1.IMG
16972800
�6v�2v�32sAplacophora �21m �33s
    A class, also known as solenogastres, of vermiform mollusks ubiquitous in
the deep oceanic basins and trenches to 30,000 ft (9000 m) and common on the
continental shelf and slope regions of the world, where they burrow through or
creep upon the mud or wrap around alcyonarian corals. Most Aplacophora are
less than 10.4 in (10 mm) in length.
    There are two distinct taxa: the subclass Chaetodermomorpha
(= Caudofoveata; see illustration); and the subclass Neomeniomorpha
(= Ventroplicida; Solenogastres).
    Several features are common to all Aplacophora. The ladderlike nervous
system consists of paired lateral and ventral cords with many cross-
commissures, a buccal ring, and cerebral and suprarectal ganglia. The
coelom is restricted to coelomoducts, a spacious pericardium, and gonads that
uniquely connect to the pericardium. There is a mantle cavity into which empty
the anus and coelomoducts. Other molluscan characters present, but not common
to all families, are a radula and its supports; paired gills in the mantle
cavity; a mucus-secreting vestigial foot used in creeping; and a style sac
containing a mucoid rod which turns against a chitinous gastric shield. See
MOLLUSCA.
 �34s[Amelie H. Scheltema]
�33s



A700000001702�254s�5v�1v119
\IMG\CI1.IMG
16972800
120
\IMG\CI1.IMG
17103360
�6v�2v�32sAplite   �21m �33s 
    A fine-grained, sugary-textured rock, generally of granitic composition; 
also any body composed of such rock. This light-colored rock consists chiefly 
of quartz, microcline, or orthoclase perthite and sodic plagioclase, with 
small amounts of muscovite, biotite, or hornblende and traces of tourmaline, 
garnet, fluorite, and topaz. Much quartz and potash feldspar may be 
micrographically intergrown in cuneiform fashion. See GRANITE; IGNEOUS ROCKS. 
    Aplites may form dikes, veins, or stringers, generally not more than a few 
feet thick, with sharp or gradational walls. Some show banding parallel to 
their margins. Aplites usually occur within bodies of granite and more rarely 
in the country rock surrounding granite. They are commonly associated with 
pegmatites and may cut or be cut by pegmatites. See PEGMATITE. 
 �34s[Carleton A. Chapman]
�33s 
  
A700000001027�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sApodacea   �21m �33s 
    A subclass of the Holothuroidea. The tentacles are simple (undivided) or 
pinnate. The tube feet are markedly reduced or, more usually, not present. 
There are no retractor muscles; hence the anterior part of the body cannot be 
retracted as an introvert. The skeletal deposits in the skin may include 
anchors and anchor plates. There are two orders, Apodida and Molpadida. See 
APODIDA; ECHINODERMATA; HOLOTHUROIDEA; MOLPADIDA. 
 �34s[Howard B. Fell]
�33s 
  
A700000001433�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sApodida �21m �33s
    An order of Apodacea. The usually worm-shaped or snakelike cylindrical
body (see illustration) has calcareous deposits that often include
wheel-shaped spicules. Respiratory trees and anal papillae are lacking. The
Apodida are widely distributed in most shallow seas, where they hide under
coral or stones, or are buried in mud or sand. Most species are tropical, and
the group is most varied on the archipelagoes of the Indo-Pacific.
    Three families may be defined: Synaptidae, with anchors in the skin;
Chiridotidae, with six-spoked wheels which may or may not have sigmoid
spiculae; and Myriotrochidae, with wheels which have eight or more spokes. See
APODACEA; ECHINODERMATA; HOLOTHUROIDEA.
 �34s[Howard B. Fell]
�33s


A700000001243�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sApodiformes   �21m �33s 
    An order of birds conventionally considered to contain the two suborders 
Apodi, the swifts, and Trochili, the hummingbirds. The relationship of these 
dissimilar-appearing birds had been deduced from wing structure and other 
anatomical features. Further study, however, suggests that these resemblances 
are convergent, and the Apodiformes thus an unnatural group. The swifts are 
probably related, if distantly, to the goatsuckers (Caprimulgiformes); 
relationships of the hummingbirds are less clear, partly because of their 
extreme specialization. See AVES. 
 �34s[Kenneth C. Parkes]
�33s 
  
A700000000576�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sApogee   �21m �33s 
    The position most distant from Earth in the orbit of a satellite, as in 
the orbit of the Moon or of an artificial satellite. The Moon at apogee is 
5 1/2% further from Earth than at its mean distance; that is, its orbital 
eccentricity is 0.055. See APHELION; MOON; PERIGEE. 
 �34s[Gerard P. Kuiper]
�33s 
  

A700000002006�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sApophyllite   �21m �33s 
    A hydrous calcium-potassium silicate containing fluorine. The composition 
is variable but approximates to KFCa4(Si2O5)4 PRODUCT 8H2O. It resembles the 
zeolites, with which it is sometimes classified, but differs from most 
zeolites in having no aluminum. It exfoliates (swells) when heated, losing 
water, and is named from this characteristic; the water can be reabsorbed. It 
is essentially white, with a vitreous luster, but may show shades of green, 
yellow, or red. The symmetry is tetragonal, and the crystal structure contains 
sheets of linked SiO4 groups; this accounts for the perfect basal cleavage of 
the mineral. It occurs as a secondary mineral in cavities in basic igneous 
rocks, commonly in association with zeolites. The specific gravity is about 
2.3 - 2.4, the hardness 4.5 - 5 on Mohs scale, the mean refractive index about 
1.535, and the birefringence 0.002. See SILICATE MINERALS; ZEOLITE. 
 �34s[George W. Brindley]
�33s 
  

A700000001530�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sApoplexy   �21m �33s 
    An acute vascular lesion of the brain, also called stroke. This may be the 
result of hemorrhage from, thrombosis in, or embolism to a cerebral vessel. 
Cerebral hemorrhage can result from a rupture of the vessel wall, as from a 
defect in the wall such as a congenital aneurysm, or it may be secondary to 
death of cerebral tissue as occurs with an infarct. 
    With cerebral hemorrhage or death of cerebral tissue there is loss of 
function of the regions involved. One such complication is paralysis of the 
muscles of the extremities on the side opposite the lesion. Since the brain is 
enclosed in a nondistensible box, the skull, the increase in size due to the 
addition of the blood raises intracranial pressure.  See  EMBOLISM; HEMORRHAGE. 
 �34s[Romeo A. Vidone]
�33s 
  
A700000001214�254s�5v�1v120
\IMG\CI1.IMG
17103360
�6v�2v�32sAporidea   �21m �33s 
    An order of tapeworms of uncertain composition and affinities, found in 
anseriform birds. The scolex may lack suckers and have only a simple rostellum 
with hooks, or it may have four large suckers and a complex glandular 
rostellum with small hooks. The small cylindroid body lacks segmentation, 
although some species have an internal serial arrangement of reproductive 
organs. Lack of reproductive ducts and openings to the outside prevents cross 
fertilization between strobilae. The life cycle is unknown. See CESTODA; 
TAPEWORM DISEASE. 
 �34s[Reino S. Freeman]
�33s 
  
A700000001235�254s�5v�1v120
\IMG\CI1.IMG
17103360
121
\IMG\CI121.IMG
0
�6v�2v�32sApostomatida �21m �33s
    A group of ciliates comprising an order of the Holotrichia. The majority
occur as commensals on marine crustaceans. Their life histories may become
exceedingly complicated, and they appear to bear a direct relationship to the
molting cycles of their hosts. Apostomes are particularly characterized by the
presence of a unique rosette in the vicinity of an inconspicuous mouth opening
and the possession of only a small number of ciliary rows wound around the
body in a spiral fashion (see illustration). See CILIOPHORA; HOLOTRICHIA;
PROTOZOA.
 �34s[John O. Corliss]
�33s



A700000002310�254s�5v�1v121
\IMG\CI121.IMG
0
�6v�2v�32sAppendicitis   �21m �33s 
    An inflammation of the vermiform appendix. Inflammation may result from 
bacterial infection following obstruction of the lumen by impacted feces or 
other causes. The normal bacterial inhabitants of the gastrointestinal tract 
gain access to the wall of the organ either through penetration of the mucosa 
or by way of vascular or lymphatic spread. Acute swelling, congestion, and 
edema ensue if no natural or surgical relief occurs. This may be followed by 
actual necrosis and gangrene, resulting in a ruptured or perforated appendix 
and the development of peritonitis, abscesses, or fistulas. See APPENDIX 
(ANATOMY); PERITONITIS. 
    Occasionally, appendicitis is associated with the presence of worms or 
other parasites or with inflammatory changes in neighboring tissues. 
    Chronic appendicitis, as an active low-grade infection, is rare, but 
scarred and obliterated appendixes are seen quite often. This condition may be 
the result of previous inflammations or age changes, because the appendix 
normally loses its lymphoid tissue with an increase in connective-tissue 
components.  
 �34s[Edward G. Stuart; N. Karle Mottet]
�33s 
  

A700000000521�254s�5v�1v1982
\IMG\CI1921.IMG
2512896
1983
\IMG\CI1921.IMG
2579456
1984
\IMG\CI1921.IMG
2648576
1985
\IMG\CI1921.IMG
2713088
1986
\IMG\CI1921.IMG
2777088
1987
\IMG\CI1921.IMG
2842624
1988
\IMG\CI1921.IMG
2909696
�6v�2v�32sAPPENDIX Classification of living organisms �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000342�254s�5v�1v1952
\IMG\CI1921.IMG
611840
1953
\IMG\CI1921.IMG
659968
1954
\IMG\CI1921.IMG
708096
�6v�2v�32sAPPENDIX Conversion factors for measurement systems �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000225�254s�5v�1v1980
\IMG\CI1921.IMG
2437632
�6v�2v�32sAPPENDIX Elementary particles �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000317�254s�5v�1v1959
\IMG\CI1921.IMG
972288
1960
\IMG\CI1921.IMG
1022464
1961
\IMG\CI1921.IMG
1047040
�6v�2v�32sAPPENDIX Fundamental constants �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000216�254s�5v�1v1954
\IMG\CI1921.IMG
708096
�6v�2v�32sAPPENDIX Greek alphabet �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000315�254s�5v�1v1956
\IMG\CI1921.IMG
827392
1957
\IMG\CI1921.IMG
875008
1958
\IMG\CI1921.IMG
923136
�6v�2v�32sAPPENDIX Mathematical notation �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000236�254s�5v�1v1956
\IMG\CI1921.IMG
827392
�6v�2v�32sAPPENDIX Mathematical signs and symbols �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000236�254s�5v�1v1955
\IMG\CI1921.IMG
761856
�6v�2v�32sAPPENDIX Periodic table of the elements �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000235�254s�5v�1v1981
\IMG\CI1921.IMG
2459648
�6v�2v�32sAPPENDIX Schematic electronic symbols �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000376�254s�5v�1v1962
\IMG\CI1921.IMG
1084928
1963
\IMG\CI1921.IMG
1170432
1964
\IMG\CI1921.IMG
1259520
1965
\IMG\CI1921.IMG
1347072
�6v�2v�32sAPPENDIX Scientific and technical organizations �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000342�254s�5v�1v1977
\IMG\CI1921.IMG
2176512
1978
\IMG\CI1921.IMG
2265600
1979
\IMG\CI1921.IMG
2347008
�6v�2v�32sAPPENDIX Semiconductor symbols and abbreviations �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000673�254s�5v�1v1966
\IMG\CI1921.IMG
1381376
1967
\IMG\CI1921.IMG
1454080
1968
\IMG\CI1921.IMG
1531904
1969
\IMG\CI1921.IMG
1607680
1970
\IMG\CI1921.IMG
1682944
1971
\IMG\CI1921.IMG
1759232
1972
\IMG\CI1921.IMG
1832960
1973
\IMG\CI1921.IMG
1906688
1974
\IMG\CI1921.IMG
1980416
1975
\IMG\CI1921.IMG
2055680
1976
\IMG\CI1921.IMG
2131456
�6v�2v�32sAPPENDIX Specialized abbreviation �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000260�254s�5v�1v1954
\IMG\CI1921.IMG
708096
�6v�2v�32sAPPENDIX Symbols and atomic numbers for chemical elements �21m �33s














�1v                          SEE FIGURE.�2v 
A700000000273�254s�5v�1v1950
\IMG\CI1921.IMG
109568
1951
\IMG\CI1921.IMG
237056
�6v�2v�32sAPPENDIX The International System, or SI �21m �33s














�1v                          SEE FIGURE.�2v 
A700000002024�254s�5v�1v121
\IMG\CI121.IMG
0
�6v�2v�32sAppendix (anatomy)   �21m �33s 
    A narrow, elongated tube closed at one end, extending from the cecum, a 
blind pocket off the first part of the large intestine. It is found in only a 
few mammals. The size and detailed structure of the appendix vary markedly, 
depending on the species and the age of the individual. Most reptiles, birds, 
and mammals have a single or a paired cecum at the anterior end of the large 
intestine, but it is quite rare that this cecum has a thinner projection or 
true appendix. 
    In humans the appendix is about 3 in. (7.5 cm) long, but it varies greatly 
in both size and its specific location in the lower right quarter of the 
abdomen. The exact function of the human appendix is unknown, and it is 
considered to be a remnant of a portion of the digestive tract which was once 
more functional and is now in the process of evolutionary regression. The 
appendixes presumably function in digestion in forms with larger ones. 
 �34s[Thomas S. Parsons]
�33s 
  
A700000011047�254s�5v�1v121
\IMG\CI121.IMG
0
�6v�2v�32sApple   �21m �33s 
    Apples (genus Malus) belong to the family Rosaceae. There are about 30 
Malus species in the North Temperate Zone. The fruits of most species are 
edible. More apples are consumed than any other temperate-zone tree fruit. 
Apples are eaten fresh, processed into jellies or preserves, cooked in pies 
and pastries, or made into sauces. Apple juice is drunk fresh, or at various 
stages of fermentation as cider, applejack, or brandy. Apple cider vinegar is 
popular for use in salads and in many processed foods. 
    The ``European'' cultivated apple is now thought to have been derived 
principally from M. pumila, a Eurasian species which occurs naturally from the 
Balkans eastward to the Tien Shan of central Asia. In the wild, some forms of 
M. pumila approach present cultivars in size and quality. Another Asian 
species, M. sylvestris, whose range extends into western Europe, grows side by 
side with M. pumila and hybridizes with it in the Caucasus Mountains. Thus M. 
sylvestris probably also had some genetic input into the cultivated apple. 
    Wild apples, mainly the edible M. baccata, grow so thickly east of Lake 
Baikal in Siberia that the region is called Yablonovy Khrebet (``Apple 
Mountains'').  
    The success of the Delicious and Golden Delicious cultivars may be laid to 
the demand for better-quality fresh fruit. Both originated on farms as chance 
seedlings near the end of the 19th century. In spite of the dominance of such 
``chance'' cultivars, cultivars such as Cortland and Idared that were produced 
by scientific breeding have begun to achieve prominence. Most of the apple 
breeding programs under way earlier this century have ceased, but the few 
remaining ones are now introducing some exciting new cultivars. Many of the 
new cultivars are of excellent quality and in addition are resistant to the 
most damaging diseases, such as scab, rust, and fire blight. These cultivars 
promise to revolutionize apple growing in the future. See BREEDING (PLANT). 
    The apple is probably the most widely distributed fruit crop in the world, 
although it ranks behind grapes, bananas, and oranges in total production. 
There are substantial apple-growing areas on all temperate-zone continents; 
the United States, Italy, France, and West Germany are leading producers. 
    Apples can be grown as far north as 60 DG latitude in the maritime climate 
of northwestern Europe. In North America, apple culture does not extend much 
above 50 DG north latitude. Away from the coasts, the buffering effects of the 
Great Lakes on temperature extremes facilitate heavy apple production in New 
York, Michigan, and Ontario. Hardier cultivars are continuing to be developed 
for use in colder regions. Apples can be grown in the tropics at higher 
elevations where sufficient chilling to break dormancy is available. Cultivars 
with lower chilling needs are being developed. 
    The principal apple-growing regions in North America are, in order of 
importance: the Northwest (Washington, Oregon, Idaho, and British Columbia), 
the Northeast (New York, New Jersey, New England, Ontario, Quebec, and Nova 
Scotia), the Cumberland-Shenandoah area (Pennsylvania, Maryland, Virginia, 
West Virginia, and North Carolina), Michigan, California, the Ohio Basin 
(Ohio, Indiana, and Illinois), and Colorado. 
    Apples may be sold fresh immediately, or after a period of storage, or 
they may be processed into less perishable products such as canned, frozen, or 
dried slices or chunks for baking, applesauce, apple juice or cider, and 
vinegar.  
    Fresh apples may be sold locally at roadside stands or at farmers' 
markets, or sold in wholesale quantities to supermarkets. Often, large 
quantities of fruit are traded by cooperatives or independent buyers for sale 
in distant markets. Thousands of tons of apples are shipped annually from the 
state of Washington to the Midwest and the East Coast. Apples can be stored 
for long periods at low temperatures under controlled atmosphere (CA storage), 
so that fresh fruits are now available year round. 
    There is a large international trade in apples, particularly in Europe 
between the large producing countries of France and Italy and the 
net-importing countries of West Germany, Britain, and Scandinavia, and from 
the United States to Canada. Apples grown in the Southern Hemisphere 
(Australia, New Zealand, South Africa, and Argentina) are shipped in large 
quantities to western Europe during the northern winter. See FRUIT, TREE; 
ROSALES.  
 �34s[H. S. Aldwinckle; Steven V. Beer]
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�6v�2v�32sApplications satellites   �21m �33s 
    Artificial satellites which contribute directly to the well-being of 
society by providing various services. Satellites were put to practical use 
almost as soon as the ability to orbit the Earth was attained. The first 
utilization involved the vantage point of space for observation of the Earth, 
and the application was in the field of meteorology. Almost as quickly, the 
unprecedented global communications capability of the satellite was 
demonstrated, and probably this application has reached the highest state of 
development. Other areas have rapidly evolved which may be even more 
important, a situation particularly true of satellites designed to accomplish 
earth resources surveys. More specialized areas have also received attention, 
for example, satellite geodesy. See COMMUNICATIONS SATELLITE; METEOROLOGICAL 
SATELLITES; REMOTE SENSING; SATELLITE (SPACECRAFT). 
 �34s[S. I. Rasool]
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�6v�2v�32sApraxia   �21m �33s 
    An inability to perform certain acts despite the absence of complete 
paralysis of the muscles involved or severe impairment of kinesthesis. 
Complicated skilled acts are most affected. However, even simple movements, 
such as protrusion of the tongue or deviation of the eyes, may be impossible 
except in contexts where they are highly automatic. Apraxia is caused by brain 
damage; although there has been much speculation about the locus of the 
causative lesion, the evidence remains inconclusive. 
    Several forms of apraxia are usually distinguished. The lowest-order 
apraxia is called limb-kinetic or motor; the intermediate form, ideokinetic or 
ideomotor (apraxia in the strictest sense); and the highest form, ideational. 
Limb-kinetic apraxia refers to a loss of coordination usually affecting one 
upper limb only. Gross movements may be performed fairly well, whereas fine 
individual movements of the fingers are lost. Ideokinetic apraxia presents the 
classic feature of this disorder, namely, a dependence of the ability to 
perform given acts on the evocative situation. For example, the patient cannot 
make the military salute on command or in imitation, but can raise a hand to 
the head to ward off a blow. Ideational apraxia is supposedly a loss of the 
ideational plan governing a series of acts directed toward a goal. The patient 
may perform the component acts correctly, but the order or the objects 
utilized may be inappropriate. 
 �34s[Josephine Semmes]
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�6v�2v�32sApricot �21m �33s
    The stone fruit Prunus armeniaca native to China, from which it has been
distributed across southern Europe to the United States. Although the tree is
slightly hardier than the peach, it blossoms early and is susceptible to
damage from spring frost and winter killing. Therefore culture is limited to
frost-protected spots and to regions where winter temperatures do not fall
below -10 to -15 DG F (-23 to -26 DG C). The fruit is nearly smooth, yellow or
orange, and excellent for eating raw as well as canned and dried (see
illustration). See ROSALES.
    Of the total commercial production in North America, California produces
90%, followed by Washington and Utah. Of this, more than 90% is processed (75%
canned). Cultivated varieties are Moorpark, Blenheim, Royal, Tilton,
Newcastle, and Wiggins. The Japanese apricot (P. mume) is a favorite
ornamental and fruit tree in China and Japan, but it is little known in the
United States. See FRUIT, TREE.
 �34s[Harold B. Tukey]
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�6v�2v�32sApsides   �21m �33s 
    In astronomy, the two points in an elliptical orbit that are closest to, 
and farthest from, the primary body about with the secondary revolves. In the 
orbit of a planet or comet about the Sun, the apsides are, respectively, 
perihelion and aphelion. In the orbit of the Moon, the apsides are called 
perigee and apogee, while in the orbit of a satellite of Jupiter, these points 
are referred to as perijove and apojove. The major axis of an elliptic orbit 
is referred to as the line of apsides. See ORBITAL MOTION. 
 �34s[Raynor L. Duncombe]
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�6v�2v�32sApsidospondyli   �21m �33s 
    A term coined to include, as a subclass, amphibians in which the vertebral 
centra are formed, in contrast with the lepospondylous type, from 
cartilaginous ``arches.'' The major apsidospondyls are the members of the 
important extinct superorder Labyrinthodontia. The ancestors of the modern 
frogs and toads (Anura) may have had vertebrae of this type. See AMPHIBIA; 
ANURA; LABYRINTHODONTIA; LEPOSPONDYLI. 
 �34s[Alfred S. Romer]
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�6v�2v�32sApterygiformes   �21m �33s 
    An order of birds containing the single family Apterygidae, the kiwis of 
New Zealand. There are three living species, all similar in form, of which the 
largest and best known is the common kiwi (Apteryx australis). 
    Although nocturnal, kiwis have small eyes and limited eyesight (unlike 
such birds as owls, nightjars, and night herons). They hunt for worms and 
other invertebrates by probing with the long bill, which differs from that of 
other birds in having the nostrils situated near the tip. Although an 
olfactory sense is well developed in a few species of birds, probably none 
except kiwis rely almost wholly on the sense of smell in foraging. The wings 
of kiwis are vestigial, and the legs are relatively short but powerful. 
Nesting and daytime resting take place in underground burrows. The kiwi egg, 
the largest known in comparison with the parent, weighs as much as one-quarter 
of the weight of an adult female. See AVES; DINORNITHIFORMES; RATITES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sApterygota   �21m �33s 
    A subclass of the Insecta characterized by being primitively wingless. 
Young Apterygota do not change greatly in their metamorphosis to adults, other 
than for changes in the proportions of structures, and in developing sexual 
maturity. Although their relationships to each other are not clearly 
understood, four orders are included in this subclass: Protura, Collembola, 
Diplura, and Thysanura. See INSECTA; PTERYGOTA. 
 �34s[Harold B. Maynard]
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�6v�2v�32sAquaculture �21m �33s
    The cultivation of fresh-water and marine species (the latter type is
often referred to as mariculture). The concept of aquaculture is not new,
having begun in China about 4000 years ago, and the practice of fish culture
there has been handed down through the generations. China probably has the
highest concentration of aquaculture operations in a single country today. Yet
the science of aquaculture, as a whole, is far behind that of its terrestrial
counterpart, agriculture. Aquaculture, though still partly art, is being
transformed into a modern, multidisciplinary technology.
    Aquacultural ventures occur worldwide. China grows macroalgae (seaweeds)
and carp. Japan cultures a wide range of marine organisms, including
yellowtail, sea bream, salmonids, tuna, penaeid shrimp, oysters, scallops,
abalone, and algae. The Soviet Union concentrates on the culture of fish such
as sturgeon, salmon, and carp. North America grows catfish, trout, salmon,
oysters, and penaeid shrimp. Europe cultures flatfish, trout, oysters,
mussels, and eels. Presently, plant aquaculture is almost exclusively
restricted to Japan, China, and Korea, where the national diets include
substantial amounts of macroalgae.
    The worldwide practice of aquaculture runs the gamut from low-technology
extensive methods to highly intensive systems. At one extreme, extensive
aquaculture can be little more than contained stock replenishment, using
natural bodies of water such as coastal embayments, where few if any
alterations of the environment are made. Such culture usually requires a low
degree of management and low investment and operating costs; it generally
results in low yields per unit area. At the other extreme, intensive
aquaculture, animals are grown in systems such as tanks and raceways, where
the support parameters are carefully controlled and dependence on the natural
environment is minimal. Such systems require a high degree of management and
usually involve substantial investment and operating costs, resulting in high
yields per unit area. Obviously, many aquacultural operations fall somewhere
between the two extremes of extensive and intensive, using aspects of both
according to the site, species, and financial constraints.
    A unique combination of highly intensive and extensive aquaculture occurs
in ocean ranching, as commonly employed with anadromous fish (which return
from the ocean to rivers at the time of spawning). The two most notable
examples are the ranching of salmon and sturgeon. In both instances, highly
sophisticated hatchery systems are used to rear young fish, which are then
released to forage and grow in their natural environment. The animals are
harvested upon return to their native rivers.
    While extremely extensive aquacultural operations are continuing, as they
have been for generations, in the lesser-developed areas of the world, the
more industrialized nations - where land and water resources are limited - are
turning increasingly to intensive aquaculture. Intensive aquaculture brings
with it high energy costs, necessitating the design of energy-efficient
systems. As this trend continues, aquaculture will shift more to a year-round,
mass-production industry using the least amount of land and water possible.
With this change to high technology and dense culturing, considerable
knowledge and manipulation of the life cycles and requirements of each species
are necessary. Specifically, industrialized aquaculture has mandated the
development of reproductive control, hatchery technology, feeds technology,
disease control, and systems engineering.
    Regardless of the type of system used, aquacultural products are marketed
as are fisheries products, except for some advantages. For one, fisheries
products often must be transported on boats and may experience spoilage;
whereas cultured products, which are land-based, can be delivered fresh to the
various nearby markets. Also, intensively cultured products through genetic
selection can result in a more desirable food than those caught in the wild,
with uniform size and improved taste resulting from controlled feeding and
rearing in pollution-free water.
    Aquaculture is following the lead of the development of agriculture,
beginning with hunting, moving through stock containment and low-technology
farming, and finally arriving at high-density, controlled culture. As the
methodology improves, the modern techniques of aquaculture - and of
agriculture - will have to be transferred to the lesser-industrialized areas
of the world in an effort to offset the predicted food shortages in the coming
years. See AGRICULTURE; MARINE FISHERIES.
 �34s[Wallis H. Clark, Jr. ;Ann B. McGuire]
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�6v�2v�32sAqua regia   �21m �33s 
    A mixture of one part by volume of concentrated nitric acid and three 
parts of concentrated hydrochloric acid. Aqua regia was so named by the 
alchemists because of its ability to dissolve platinum and gold. Either acid 
alone will not dissolve these noble metals. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sAquarius �21m �33s
    The Water Bearer, in astronomy, a large zodiacal constellation visible in
both summer and autumn. Aquarius is the eleventh sign of the zodiac. To
ancients, the constellation resembled a man pouring a stream of water from a
jar. Four stars, NU, ETA, ZETA, PI, and GAMMA, arranged like a Y, form the head
of the Water Bearer (see illustration). The stream of water flows into the
Fish's Mouth (Fomalhaut) in the constellation Pisces Austrinus (Southern Fish).
Fomalhaut, bright and solitary in this part of the sky, is one of the
relatively important navigational stars. From earliest time this constellation
has been associated with water, probably because the Sun is seen in Aquarius
during the rainy season of February. See CONSTELLATION. 

 �34s[Ching-Sung Yu]
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�6v�2v�32sAquifer   �21m �33s 
    A subsurface zone that yields economically important amounts of water to 
wells. The term is synonymous with water-bearing formation. An aquifer may be 
porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. 
    Aquifers are important reservoirs storing large amounts of water 
relatively free from evaporation loss or pollution. If the annual withdrawal 
from an aquifer regularly exceeds the replenishment from rainfall or seepage 
from streams, the water stored in the aquifer will be depleted. This mining of 
groundwater results in increased pumping costs and sometimes pollution from 
sea water or adjacent saline aquifers. Lowering the piezometric pressure in an 
unconsolidated artesian aquifer by overpumping may cause the aquifer and 
confining layers of silt or clay to be compressed under the weight of the 
overlying material. The resulting subsidence of the ground surface may ause 
structural damage to buildings, altered drainage paths, increased flooding, 
damage to wells, and other problems. See ARTESIAN SYSTEMS; GROUNDWATER 
HYDROLOGY.  
 �34s[Ray K. Linsley]
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�6v�2v�32sArabinose �21m �33s
    A plant product which is usually a constituent of polysaccharides such as
various gums, hemicelluloses, pectic substances, and some glycosides. It
exists in these products mostly in the L form (see illustration), and usually
possesses the furanose modification when it is present in combined form as a
constituent of an oligosaccharide, polysaccharide, or glycoside. See
CARBOHYDRATE; GUM; HEMICELLULOSE; MONOSACCHARIDE; POLYSACCHARIDE.

 �34s[William Z. Hassid]
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�6v�2v�32sArachnida �21m �33s
    The familiar modern spiders, scorpions, daddy longlegs, ticks, mites, and
others, besides a large number of fossil species, are included in this class.
The prosoma is commonly called the cephalothorax, and the opisthosoma is
calledthe abdomen. However, as in the scorpions, the abdomen may be
differentiated into two parts, and in some forms the whole body is in one
piece. The six pairs of appendages on the cephalothorax are the chelicerae,
the pedipalps, and four pairs of walking legs. The pedipalps may be leglike,
but they often bear large pincers, or chelae. In male spiders they are
modified for reproductive purposes. If abdominal appendages are present, they
are much reduced and serve for silk spinning. Like the other chelicerates, the
arachnids have no jaws. The stomach usually has branching tubular diverticula,
in the walls of which the fully digested food is absorbed. The respiratory
organs are both book lungs and tracheae. The genital openings of each sex are
ventral on the base of the abdomen.
    The class Arachnida includes the orders Palpigradida, Solpugida,
Scorpionida, Uropygi, Amblypygi, Ricinuleida, Pseudoscorpionida (or
Chelonethida), Phalangida, Araneida (spiders), and Acarina (mites and ticks).
See separate articles on each order. See also ARTHROPODA.
 �34s[Jerome G. Rozen, Jr.]
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�6v�2v�32sAraeoscelidia   �21m �33s 
    A provisional order of extinct reptiles (subclass Euryapsida) comprising a 
group of Permian and Mesozoic forms that are difficult to place in the system 
because their cranial structure is insufficiently known. Neither is it certain 
that the genera included in the order actually are closely related. As 
tentatively constituted, the group is highly varied, including delicately 
built animals of lizardlike appearance and improbable-looking forms with 
bizarre specializations. Their limb skeletons indicate that they inhabited 
continental, in some cases paralic, environments, and their remains are found 
in both continental and marginal marine deposits. 
    In most forms the forelimbs are significantly smaller than the hindlimbs, 
and both hand and foot is of primitive construction. The terminal phalanges 
were covered with horny claws. See EURYAPSIDA; REPTILIA. 
 �34s[Rainer Zangerl]
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�6v�2v�32sAragonite   �21m �33s 
    A mineral species of calcium carbonate; other cations such as strontium 
and lead may substitute for some of the calcium. Aragonite is much less common 
than calcite, to which it is frequently or partially transformed. When pure, 
aragonite is white, but with impurities may assume gray, blue, green, or pink 
tints. The hardness on Mohs scale is 3.5 - 4. Specific gravity is 2.947. 
    Aragonite frequently grows metastably in near-surface secondary deposits 
in cavities, such as in limestone caverns. It also occurs in precipitates from 
hot springs, and it forms biogenically in pearls and in the hard parts 
(shells) of some fossils, pelecypods, and gastropods. 
    Aragonite is found in many localities throughout the world. In the United 
States, fine crystals associated with calcite and cerussite are found in the 
Magdalena district, Socorro County, New Mexico. See CALCITE; CARBONATE 
MINERALS.  
 �34s[Robert I. Harker]
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�6v�2v�32sArales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Arecidae of the class Liliopsida (monocotyledons). It consists of 
two families, the Araceae, with about 1800 species, and the Lemnaceae, with 
only about 30. 
    The Araceae are herbs (seldom woody climbers) with ordinary roots, stems, 
and leaves (often broad and net-veined) and with the vessels confined to the 
roots. They have numerous tiny flowers grouped together in a small to very 
large spadix. They are commonest in forested tropical and subtropical regions. 
Anthurium (elephant ear), Arisaema (jack-in-the-pulpit), Dieffenbachia (dumb 
cane), Monstera, and Philodendron are some well-known members of the Araceae. 
    The Lemnaceae, or duckweeds, are small, free-floating, thalloid aquatics 
that are generally conceded to be derived from the Araceae. Their flowers, 
seldom seen, are much reduced and form a miniature spadix. Pistia (water 
lettuce), a free-floating aquatic (but not thalloid) aroid is seen as pointing 
the way toward Spirodela, the least reduced genus of the Lemnaceae. See 
ARECIDAE; LILIOPSIDA; MAGNOLIOPHYTA; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
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�6v�2v�32sAraneida   �21m �33s 
    A natural order of the class Arachnida, also called Araneae, commonly 
known as the spiders. These animals are widespread over most of the land areas 
of the world and are well adapted to many different habitats. They are known 
to be one of the oldest of all groups of arthropods, and their remains are 
known from the Devonian and Carboniferous geological deposits. Through 
successive geological periods spiders have become adapted to use insects as 
their chief source of food. On the other hand, certain insects consume the 
eggs of spiders, others parasitize the eggs, and still others capture adults 
and place them in their nests for food for their young. 
    Spiders have only two subdivisions of the body, the cephalothorax and the 
abdomen, joined by a slender pedicel. All parts of the body are covered by 
chitinous plates which often extend into curious outgrowths, such as spines, 
horns, and tubercles. Only simple paired eyes, ocelli, are present, with the 
number varying from eight, the most common number, to none in a few species 
inhabiting lightless habitats. 
    The first of six pairs of appendages are termed the chelicerae or jaws. 
The second pair of appendages is the six-segmented pedipalps, simple organs of 
touch and manipulation in the female, but curiously and often complexly 
modified in males for use in copulation. The four pairs of thoracic legs 
consist of seven segments each. Just in front of the genital opening in most 
females there is a more or less specific and often elaborately formed plate, 
the epigynum. This organ is of importance in the reproductive activities of 
the female. In most of the true spiders internal breathing tubules occur with 
ventral openings posterior to the genital apertures. Distinctive paired 
ventral spinnerets occur near the posterior end of the abdomen. In certain 
families a sievelike plate lies immediately anterior to the foremost 
spinnerets. From this plate, the cribellum, a special kind of banded silk is 
extruded and used in conjunction with a comb on the fourth legs. The 
spinnerets and cribellum are directly associated with several types of 
abdominal glands and responsible for the production of the different types of 
silk characteristic of these animals. 
    Silk produced by spiders is a scleroprotein which is fine, light, elastic, 
and strong. It is used industrially only in the making of cross hairs in 
optical instruments. This use is diminishing as metal filaments and etched 
glass come into more common usage. In addition to the attractive orb webs, 
these animals also construct sheet webs, funnel webs, tube webs, and reticular 
webs. The spider's reliance upon silk extends its use to the making of egg 
cocoons, sperm webs by males, molting sheets, gossamer threads for ballooning, 
attachment disks, lining for burrows, hinges for trap doors, binding for 
captives, retreats, and drag lines. 
    No general agreement exists at present among araneologists concerning the 
classification and exact arrangement of the families of spiders. A. 
Petrunkevitch (1939) recognized 5 suborders: Liphistiomorphae containing only 
2 families with a primitively segmented abdomen, and restricted to regions in 
the Eastern Hemisphere; Mygalomorphae with 8 families; Hypochilomorphae with a 
single relict family native to the southern Appalachian region; 
Dipneumonomorphae with 48 families; and finally the Apneumonomorphae with 3 
families, which lack the book lungs and represent the most highly modified 
members of the order. There is now a tendency to increase the number of 
recognized families. See ARACHNIDA. 
 �34s[Arthur M. Chickering]
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�6v�2v�32sArbacioida   �21m �33s 
    An order of Echinacea with a stirodont lantern, arbacioid ambulacral 
plates, partly sculptured test, imperforate noncrenulate tubercles, solid 
radioles, and the anus guarded by four or five periproctal plates. The order 
arose in the Jurassic, possibly from the Hemicidaroida. The type genus Arbacia 
is found on all Atlantic coasts, as well as in western North and South 
America. See ECHINACEA; ECHINODERMATA; ECHINOIDEA; HEMICIDAROIDA. 
 �34s[Howard B. Fell]
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�6v�2v�32sArboretum   �21m �33s 
    An area set aside for the cultivation of trees and shrubs for educational 
and scientific purposes. An arboretum differs from a botanical garden in 
emphasizing woody plants, whereas a botanical garden includes investigation of 
the growth and development of herbaceous plants as well as trees and shrubs. 
The largest of the arboretums in the United States is the Arnold Arboretum of 
Harvard University, founded in 1872. See BOTANICAL GARDENS. 
 �34s[Earl L. Core]
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�6v�2v�32sArborvitae �21m �33s
    A plant, sometimes called the tree of life, belonging to the genus Thuja
of the order Pinales (Coniferales). It is characterized by flattened
branchlets with two types of scalelike leaves. At the edges of the branchlets
the leaves may be keeled or rounded; on the upper and lower surfaces they are
flat, and often have resin glands. The cones, about 1/2 in. (1.25 cm) long,
have the scales attached to a central axis. See PINALES; RESIN; SECRETORY
STRUCTURES (PLANT).
    The tree is valued both for its wood and as an ornamental. T.
occidentalis, of the eastern United States, is known as the northern white
cedar. It occurs in moist or swampy soil from Nova Scotia to Manitoba and in
adjacent areas of the United States, and extends south in the Appalachians to
North Carolina and Tennessee. Other important species include the giant
arborvitae (T. plicata); oriental arborvitae (T. orientalis); and Japanese
arborvitae (T. standishii). Among the horticultural forms are the dwarf
pendulous and juvenile varieties. See FOREST AND FORESTRY; TREE.
 �34s[Arthur H. Graves /Kenneth P. Davis ]
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�6v�2v�32sArboviral encephalitides   �21m �33s 
    A number of diseases, such as St. Louis, Japanese B, and equine 
encephalitis, which are caused by arthropod-borne viruses (abbreviated as 
``arboviruses''). In their most severe human forms, the diseases invade the 
central nervous system and produce brain damage, with mental confusion, 
convulsions, and coma; death or serious aftereffects are frequent in severe 
cases. Inapparent infections are common. See EQUINE ENCEPHALITIS. 
    The illness begins 4 - 21 days after the bite of the infected vector, with 
sudden onset of fever, headache, chills, nausea, and generalized pains; marked 
drowsiness and neck rigidity may ensue. Signs and symptoms are often diphasic, 
the second phase being more severe and involving mental confusion, speech 
difficulties, convulsions, and coma. Mortality case rates vary in different 
diseases and epidemics. Serious aftereffects such as mental and sensory 
defects, epilepsy, and paralysis occur. 
    All arboviruses causing encephalitis require an infected arthropod vector 
for transmission. The usual natural hosts are mammals or birds, with humans 
accidentally infected in the cycle. Ticks may serve not only as vectors but 
also as a natural reservoir, because the virus may pass transovarially to 
offspring; in other instances birds or mammals are the probable reservoir. In 
highly endemic regions many persons have antibodies, chiefly from inapparent 
infections. Each virus is limited to definite geographic areas. The 
population's immunity to its local virus may cross immunize against closely 
related viruses. 
    There is no proved specific treatment. In animals, hyperimmune serum given 
early may prevent death. Killed virus vaccines have been used in animals and 
in persons occupationally subjected to high risk; little success has yet been 
achieved in large field trials in humans. In general, however, control of 
these diseases continues to be chiefly dependent upon elimination of the 
arthropod vector. See ANIMAL VIRUS. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sArc discharge �21m �33s
    A type of electrical conduction in gases characterized by high current
density and low potential drop. It is closely related to the glow discharge
but has a much lower potential drop in the cathode region, as well as greater
current density. See GLOW DISCHARGE.
    There are many arc devices, and they operate under a wide range of
conditions. For example, an arc discharge may be sustained at either high
pressure (of the order of atmospheres) or low pressure. The cathode may or may
not be heated from an external source. Furthermore, the applied potential
difference may be either direct or alternating. Numerous applications have
been made of such devices, some having large commercial value. A few of these
applications are illuminating devices, high-current rectifiers, high-current
switches, welding devices, and ion sources for nuclear accelerators and
thermonuclear devices.
    Although an arc may be initiated in several ways, it is instructive to
consider the transition from a cold-cathode glow discharge (see illustration).
In the normal condition, the glow partially covers the cathode, and the
voltage drop remains nearly constant as the current is increased. In this
condition, the ionization is produced primarily by electron impact. As the
current is increased, the cathode glow spreads out, eventually covering the
cathode completely. Further increase in current can now be obtained only by an
increase in the potential drop across the discharge. The cathode temperature
increases in this process. This is the abnormal glow region. As the cathode
temperature is increased further, thermionic emission becomes an important
factor. At this point the discharge characteristic may acquire a negative
slope; that is, a further increase in current may increase the cathode
temperature, and the resulting thermionic emission, enough actually to reduce
the potential drop. Unless the external resistance is sufficiently great to
make the overall resistance positive, the discharge will change suddenly to
the arc mode. Typical values in this region are a potential drop of a few tens
of volts and a current ranging from amperes to thousands of amperes. See
ELECTRICAL CONDUCTION IN GASES.
    There are three geometrical regions in an arc. These are the cathode fall,
the anode rise, and the arc body. The main body of the arc (arc body) is
characterized by secondary ionization. This is predominantly a temperature
effect. The electrons produce very little ionization by impact, because the
electron energy is generally low. However, energy can be imparted to the gas
molecules in the form of atomic and molecular excitation. With the many
intermolecular collisions that occur, this energy is readily degraded to
thermal energy of the gas. Thus a high temperature may be achieved and
ionization occur by virtue of intermolecular collisions. See ARC HEATING; ARC
LAMP.

 �34s[Glenn H. Miller]
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�6v�2v�32sArcellinida   �21m �33s 
    An order of Lobosia. Characteristically, the shell (test) of these 
protozoans has a well-defined aperture through which slender fingerlike 
pseudopodia (lobopodia) extend. The test often has an inner chitinous layer 
and an outer wall composed of secreted siliceous elements or collected sand 
grains or the like cemented together by a secretion. Most Arcellinida are 
uninucleate, but Arcella is binucleate. In reproduction certain species 
prefabricate their tests to some extent. During growth the organism may 
assemble materials by secretion or ingestion for a new test. During fission 
such materials pass through the aperture into the extruded protoplasm, where 
they are moved to the surface and cemented into the outer wall of the new 
test. The old test is kept by the sister organism. Cyst walls may be formed 
inside the test; the aperture is often plugged with debris before encystment. 
The order includes Arcella, Centropyxis, Difflugia, and many others. See 
LOBOSIA; PROTOZOA; SARCODINA; SARCOMASTIGOPHORA. 
 �34s[Richard P. Hall]
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�6v�2v�32sArch �21m �33s
    A structure, usually curved, that when subjected to vertical loads causes
its two end supports to develop reactions with inwardly directed horizontal
components. The designations of the various parts of an arch are given in Fig.
1. The commonest uses for an arch are as a bridge, supporting a roadway,
railroad track, or footpath, and as part of a building, where it provides a
large open space unobstructed by columns. Although arches are usually built of
steel, reinforced concrete, or timber, the Saguenay River Bridge is an
all-aluminum arch spanning 290 ft (88 m) from center to center of skewbacks.
    On the basis of structural behavior, arches are classified as fixed
(hingeless), single-hinged, two-hinged, or three-hinged (Fig. 2). An arch is
considered to be fixed when rotation is prevented at its supports. Reinforced
concrete ribs are almost always fixed. For long-span steel structures only
fixed solid-rib arches are used. The Rainbow Bridge at Niagara Falls, with a
span of 950 ft (290 m), is the longest fixed steel arch in the world. The
Henry Hudson Bridge in New York City is a fixed arch of 800-ft (244-m) span.
Because of its greater stiffness, the fixed arch is better suited for long
spans than hinged arches.
    Concrete is relatively weak in tension and shear but strong in compression
and is therefore ideal for arch construction. Figure 1 is a sketch of an
open-spandrel concrete fixed-arch bridge. Precast reinforced concrete arches
of the three-hinged type have been used in buildings for spans up to 160 ft
(49 m).

    Steel arches are solid-rib or braced-rib arches. Solid-rib arches usually
have two hinges but may be hingeless. The braced-rib arch has a system of
diagonal bracing replacing the solid web of the solid-rib arch. The world's
longest arch spans are both two-hinged arches of the braced-rib type: the
Sidney Harbor Bridge in Australia and the Bayonne Bridge at Bayonne, New
Jersey, which have spans of 1650 and 1652 ft (502.9 and 503.5 m),
respectively. The spandrel-braced arch is essentially a deck truss with a
curved lower chord, the truss being capable of developing horizontal thrust at
each support. This type of arch is generally constructed with two or three
hinges because of the difficulty of adequately anchoring the skewbacks.
    Wood arches may be of the solid-rib or braced-rib type. Solid-rib arches
are of laminated construction and can be shaped to almost any required form.
Arches are usually built up of nominal 1- or 2-in. (2.5- or 5-cm) material
because bending on individual laminations is more readily accomplished.
Because of ease in fabrication and erection, most solid-rib arches are of the
three-hinged type. This type has been used for spans of more than 200 ft (60
m). The lamella arch has been widely used to provide wide clear spans for
gymnasiums and auditoriums. The wood lamella arch is more widely used than its
counterpart in steel. The characteristic diamond pattern of lamella
construction provides a unique and pleasing appearance. See BRIDGE; BUILDINGS;
TRUSS.

 �34s[Charles N. Gaylord]
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�6v�2v�32sArchaeocyatha �21m �33s
    An extinct group of Early and Middle Cambrian marine animals, considered
by some to be an extinct class of sponge and by others an extinct phylum. They
are among the oldest known animals to have a mineralized skeleton - in their
case, one made of calcium carbonate. Archaeocyathids lived in great abundance
on shallow sea bottoms and were able to form reefs by attaching to hard
substrates with their holdfasts. These reefs are the oldest constructed by
animals.
    Most archaeocyathids are shaped like a cup or inverted cone, although
flat, saucerlike, annulated, and tubular forms are also encountered (see
illustration). The cup of archaeocyathids averages 0.04 - 0.10 in. (10 - 25
mm) in diameter and 3.2 - 6.0 in. (80 - 150 mm) in height, but specimens with
diameters as large as 24 in. (600 mm) have been reported. Although most
archaeocyathids are solitary, colonial forms (grouped cups) are occasionally
found.
    Because archaeocyathids are among the oldest fossils of multicelled
animals, their position in the phyletic system is crucial to understanding the
evolution of animals. Yet there is little agreement concerning the phyletic
affiliations of this problematic group. Three alternatives are considered the
most likely: (1) they are an extinct phylum more primitive than the Porifera
(sponges) but more organized than the Protozoa; (2) they are an extinct phylum
of multicelled animals on the same organizational level as the Porifera but
arising from a different ancestral stock; or (3) they are an extinct class of
sponges. For convenience the following classification may be adopted:
    Subkingdom Parazoa
    Phylum Archaeocyatha
	Class Regulares - inner wall and radial skeletal elements develop
	  earlier than dissepiments
	Class Irregulares - dissepiments appear earlier than radial skeletal
	  elements or inner wall
    Because of uncertainties about the classification of archaeocyathids and
the nature of their soft tissue, little is known about the functioning of
these enigmatic animals. They were bottom-dwelling marine animals, preferring
water depths of 33 - 100 ft (10 - 30 m), although estimates of their depth
range extend to 330 ft (100 m). They seem to have been confined to shallow
environments with substantial water movement because of their dependence on an
external flow to propel water through their bodies. In archaeocyathid reef
communities, algae and archaeocyathids interacted in much the same way as
algae and coral do today. Archaeocyathids formed a skeletal framework which
was bound and stabilized by blue-green and red algae. However, unlike modern
reef communities, the reefs formed by archaeocyathids were apparently quite
tolerant of terrigenous sediments. See PARAZOA; PORIFERA.

 �34s[William L. Balsam]
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�6v�2v�32sArchaeogastropoda �21m �33s
    The most primitive order of the subclass Prosobranchia of the class
Gastropoda (snails) of the phylum Mollusca. The members retain greatest
evidence of the effects of torsion and coiling, which together represent the
basic characteristics of the Gastropoda.
    Despite their primitive character, the Archaeogastropoda display a wide
range of adaptive radiation. The shell is primitively top-shaped, with a
rounded, sometimes calcareous, operculum which closes the opening when the
animal withdraws. However, the limpet form, with the shell as a widely open
cone and the animal permanently attached by a suckerlike foot, has evolved at
least three times. Some of the limpets are among the most numerous gastropods
of the seashore. For structural reasons, archaeogastropods are generally
confined to hard substrates and, with the exception of one freshwater group
whose affinities are dubious, to the sea.
    The Archaeogastropoda are divided into well-defined groups. In the
Zeugobranchia the shell is coiled (Fig. 1), with openings for expulsion of the
water current produced in the midline between the paired, often somewhat
asymmetrical gills. The flattened, somewhat coiled Haliotis (abalone or ormer)
is a widely distributed and highly successful type of a symmetric limpet. In
the keyhole limpets (Fissurellidae) the shell is secondarily symmetrical; the
gills have regained paired symmetry and expel water through a single apical or
marginal opening. The Patellacea include the most common and most enormously
successful limpets. They are divided into those with a single gill
(Acmaeidae); those in which the gill is replaced by a fringe of secondary
growths (Patellidae); and the rarer species from below tidal levels which are
without gills (Lepetidae). The Trochacea are coiled ``top shells'' with a
single gill and no shell aperture (Fig. 2). Although they are included in the
Archaeogastropoda in most classifications, the Neritacea should probably
constitute a separate order. See ABALONE; GASTROPODA; LIMPET; MOLLUSCA;
PROSOBRANCHIA.

 �34s[C. Maurice Yonge]
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�6v�2v�32sArchaeopteridales   �21m �33s 
    An order of spore-bearing plants with woody trunks and simple leaves borne 
on determinate branch systems that resemble bipinnate fern fronds. The 
best-known genus is Archaeopteris, which is sometimes used as an index fossil 
of the Upper Devonian. Archaeopteris has become one of the best known of 
Devonian plants. It is no longer classified as a fern, but has been assigned 
to a new class, the Progymnospermopsida, which is intermediate between ferns 
and gymnosperms. The Archaeopteridales also include Actinopodium, Svalbardia, 
and Siderella. See INDEX FOSSIL; PINOPSIDA; POLYPODIOPSIDA; PTEROPSIDA. 
 �34s[Chester A. Arnold]
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�6v�2v�32sArchaeornithes   �21m �33s 
    One of two subclasses of the class Aves, containing the single order 
Archaeopterygiformes, the most primitive category of birds. It was established 
exclusively for the few specimens of the oldest known fossil bird, 
Archaeopteryx, from the Late Jurassic Solnhofen limestones of Bavaria. Only 
six specimens are assigned to the Archaeornithes. 
    Archaeopteryx was pigeon-sized, with a long tail skeleton and unbirdlike 
jaws with sharp teeth. The tail and forelimbs bore large ``flight''-like 
feathers, but except for the presence of a wishbone (furcula), the forelimb 
and shoulder skeleton lacks all the specialized flight structures that are 
found in modern birds. This indicates that flight powers were limited, perhaps 
even nonexistent. Other evidence suggests that Archaeopteryx may not have been 
arboreal, but instead was a cursorial ground dweller. The skeletal anatomy is 
not birdlike, but rather is almost identical to that of the small carnivorous 
dinosaurs (Theropoda). This dinosaurlike anatomy is considered by some as 
clear evidence that Archaeopteryx and later birds descended directly from a 
small theropod dinosaur, rather than from pseudosuchian thecodonts, as has 
long been believed. See AVES; NEORNITHES; SAURISCHIA. 
 �34s[John H. Ostrom]
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�6v�2v�32sArc heating   �21m �33s 
    The heating of a material by the heat energy from an electric arc. The 
electric arc is a component of an electric circuit, like a resistor, but with 
its own peculiar characteristics. Its resistance decreases as its current 
increases and vice versa. The electric arc is characterized also by its high 
temperature and high concentration of heat energy. Millions of watts are 
controllably dissipated in rather confined space, producing temperatures 
measured in thousands of degrees Celsius. At these temperatures most materials 
in the Earth's crust melt quickly. The molten materials can be processed under 
an oxidizing, neutral, or reducing slag or combination of slags to produce a 
variety of finished materials which are used for many purposes. See ARC 
DISCHARGE; ELECTROCHEMICAL PROCESS. 
    The electric arc is the heating element in a number of heating, melting, 
and smelting appliances classified as direct-arc furnaces, submerged-arc 
furnaces, indirect-arc furnaces, air-arc furnaces, arc welders, arc-machining 
tools, and arc-boring machines. See ARC WELDING; FURNACE. 
    In iron and steel foundries, in melt shops of mini-steel and large steel 
mills, and in some nonferrous melt shops, direct-arc furnaces are used for 
their ability to bring the burden quickly to pour temperature. The direct 
arcing from electrodes to burden gives the direct-arc furnace its name. In the 
iron foundry, the direct-arc furnace is used as a batch melter, a duplexer, or 
continuous melter for making gray iron, malleable iron, nodular iron, and 
alloy iron castings of closely controlled composition within a wide range of 
any metallurgical formulation. In the steel foundry and in steel mills, the 
direct-arc furnace isused almost exclusively as the batch melter. In 
nonferrous melt shops, the direct-arc furnace is commonly used for either a 
batch melter or continuous melter for metals, such as nickel, copper, and 
cobalt for making billets, cakes, ingots, and castings. The direct-arc furnace 
is used also in vacuum melting of various metals to improve purity and 
physical properties. 
    Arcs may be completely submerged under the charge or in the molten bath 
under the charge. Furnaces operated in this manner are generally smelting 
furnaces with fixed shells, into which ore granules are fed. The bath is 
tapped at one, two, or three levels as required for the product, slag, and 
impurities. Many ores are smelted in the submerged-arc furnace, and the 
products are numerous. 
    To use only the radiant heat from an arc, the furnace has a 
refractory-lined cylindrical shell with its axis horizontal. The shell is 
mounted on rollers so that it may roll or rock through something less than one 
revolution. Two electrodes, one from each end, enter the cylinder along the 
axis and touch and draw an arc inside the furnace. The heat of the arc is 
radiated to the refractories. The burden in the furnace, as it rocks back and 
forth, ``washes'' the heat off the refractories. This indirect heating of the 
burden gives the indirect-arc furnace its name. It is particularly applicable 
to melting low-temperature metals and alloys, such as brass, where the 
difference in the melting points of the burden and the refractory is enough to 
maintain a high rate of heat transfer, and where the heat of the arc is spread 
before reaching the burden to prevent large losses of volatile elements, such 
as zinc, in the burden. 
    An entirely different application of the arc furnace is to power wind 
tunnels. The furnace employs a cylindrical shell and a pair of electrodes 
arranged similarly to those in the indirect-arc furnace; however, the shell is 
fixed and mounted usually with its axis vertical. There is no refractory; 
instead, a whorl of water surrounds the arc. Air (or other gas) is blown or 
pumped through the space between water and arc. The air is superheated to 
20,000 K and expanded to emerge at supersonic speeds. 
 �34s[Charles W. Vokac]
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�6v�2v�32sArcheological chemistry   �21m �33s 
    The application of chemical theories and experimental procedures, 
especially the procedures of analytical chemistry, to the solution of problems 
in archeology. Ordinary analytical procedures may often be applied without 
modification to the examination of objects and materials from excavations, but 
frequently some modifications are necessary. Microanalytical methods are 
especially useful since the amount of material available for examination is 
often very small. Chemical microscopy is also an important general technique 
for the examination of ancient objects and materials. 
    For the examination of valuable objects, which cannot be altered in any 
way, even to the extent of taking very small samples for analysis, 
nondestructive methods must be used. Neutron activation analysis is especially 
useful for the nondestructive testing of metal, glass, and pottery objects. 
X-ray fluorescence analysis is especially useful for estimating the chemical 
composition of the surfaces of objects. 
    The most frequent application of analytical chemistry to archeology is the 
exact identification of materials found in excavations so that they may be 
accurately described in archeological reports. Copper is sometimes confused 
with bronze, bronze with brass, lime mortar with gypsum plaster, solid 
bitumens with vegetable resins, and so on for other materials that are 
superficially similar. The problem of making correct identifications of 
ancient materials from excavations may be very different from identifying the 
same materials in a fresh condition. In identifying a material found in an 
excavation, it is usually much less important to determine what it is now than 
to establish what it was originally, before it underwent extensive chemical 
change. Though the original nature of inorganic materials may usually be 
determined with little difficulty, it is otherwise with organic materials, for 
these are likely to undergo complicated chemical changes during long burial in 
the ground. 
    Interpretations of archeological significance that may arise from the 
exact identification of a particular material may be more important than its 
identification for the purpose of exact description. Also, adequate chemical 
analyses of chronologically ordered specimens of materials from a given site 
or group of sites may yield significant information about technological or 
economic changes in an ancient civilization. 
    During long burial in the ground, a great variety of chemical reactions 
occur between the material of most objects and the surrounding soil and 
groundwater. To explain fully the appearance of objects found in excavations, 
to deduce with confidence their probable original appearance, or to decide on 
proper methods of restoration, an understanding of the nature and course of 
these chemical reactions is required. The appearance and condition of objects 
of the same material and of the same age taken from different excavation sites 
may be very different. The deterioration of buried objects is largely the 
result of chemical change, and it is therefore reasonable to expect that the 
restoration of such objects should usually be best brought about by chemical 
treatment. This principle has been applied with much success to the 
restoration of metal objects by various electrolytic reduction procedures. 
Even when objects are of such a nature that they undergo little chemical 
deterioration during burial, chemical treatment may be required for the 
effective and safe removal of the foreign matter that encrusts them. 
    Whether ancient objects are restored by chemical means or not, problems 
concerning their preservation for exhibition or study often arise that are 
best solved by the application of chemical principles. For example, the 
corrosion of an ancient metal object may continue at an appreciable rate in 
ordinary air unless proper precautions are taken. In addition, certain 
problems not directly connected with ancient objects, yet important to the 
archeologist, may sometimes be solved by a physical application of products 
made available by modern chemistry. The preservation of paper impressions of 
stone inscriptions is an example. See ACTIVATION ANALYSIS; ANALYTICAL 
CHEMISTRY; ARCHEOLOGY; CHEMICAL DATING; CHEMICAL MICROSCOPY; NONDESTRUCTIVE 
TESTING.  
 �34s[Earle R. Caley]
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�6v�2v�32sArcheology   �21m �33s 
    The scientific study of past material culture. The initial objective of 
archeology is to construct cultural chronologies, attempting to order past 
material culture into meaningful temporal segments. The intermediate objective 
is to breathe life into these chronologies by reconstructing past lifeways. 
The ultimate objective of contemporary archeology is to determine the cultural 
processes that underlie human behavior, both past and present. 
    The material culture of the past is of infinite variety. The scientific 
study of this evidence is such a broad task that there is no such thing as any 
single archeological method, although over the past century archeologists have 
evolved what can be termed an overall archeological approach. By constant 
confirmation, the archeologist often attempts to establish synchronism with 
what has already been established historically. 
    Archeologists use a number of types in order to categorize similar 
artifacts. Most common is the temporal type, a principle similar to the index 
fossil concept used by the geologist. A temporal type can be any kind of 
archeological artifact or feature, but ideally it is some object of common use 
in which the form is subject to change, due to either the whim of fashion or 
technological improvement. One example is the simple flint arrowhead with side 
barbs and central tang. It is typical of the British Bronze Age and was not in 
fashion earlier or later. Ceramic types have been established by archeologists 
working around the world, and a thoroughly tested ceramic chronology is 
invaluable as a temporal ordering device, no matter where the archeologist is 
working. The nature of the artifact employed as a temporal type is irrelevant, 
and its use may not even be known. Archeologists also establish other kinds of 
types. Functional types attempt to group artifacts on the basis of known or 
presumed functions. Technological types, divisions which reflect the mode of 
manufacture, are particularly helpful when studying stone tool manufacture. 
    The concept of culture is used in two different ways by contemporary 
archeologists. When dealing with cultural chronologies, the archeologist most 
commonly uses a modal or shared view of culture. It is this normative 
collection of shared ideas which cause artifacts to change in systematic ways 
through time, and temporal types can be established on the basis of this 
shared culture. When attempting to reconstruct lifeways, however, the 
archeologist can no longer rely on the shared aspects of culture. When 
transcending temporal associations, contemporary archeologists tend to view 
culture systematically, as people's extrasomatic (that is, learned) method of 
dealing with the social and cultural environment. 
    The principles of stratigraphy are applied to archeology in terms of the 
law of superposition, which states that, all else being equal, older deposits 
will tend to be buried beneath younger ones. Mere stratigraphic equivalence, 
however, does not necessarily indicate contemporaneity, as there can be 
misleading mixtures of successive occupational debris on one surface. 
Archeologists must therefore study the processes of cultural deposition in 
order to recognize the difference between intact and disturbed strata. 
    Contemporary excavation must be conducted with a plan, a firm research 
design that attempts to provide answers to definite questions. Archeology is 
one of the few sciences which destroys its own data in the process of 
generating them. Archeologists must therefore be extremely careful to make the 
appropriate observations at the time of excavation. 
    The task of deciphering meaning from past material culture is so complex 
that the archeologist is often required to borrow from allied disciplines in 
the physical and natural sciences, including geology, climatology, 
paleobotany, paleontology, mineralogy, physics, chemistry, and anthropology. 
The archeologist must have some understanding of all these sciences to extract 
from sites and materials every possible piece of information which may lead to 
a better understanding of prehistory. The archeologist must be able to record 
and publish every minor fact for the benefit of colleagues and successors, 
because the writing of prehistory requires the synthesis of all archeological 
discovery and interpretation. See ARCHEOLOGICAL CHEMISTRY; PHYSICAL 
ANTHROPOLOGY.  
 �34s[David Hurst Thomas]
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�6v�2v�32sArcheozoic   �21m �33s 
    A term for the earlier interval in the twofold subdivision of Precambrian 
time. It has been more or less synonymous with Archean, which has been more 
widely used, especially outside the United States. Most students of the 
Precambrian find it very difficult to apply the old twofold subdivision, 
however, and many have a strong tendency to discard it and to build a new time 
scale more in keeping with the numerous events of Precambrian time. See 
PRECAMBRIAN.  
 �34s[J. Paul Fitzsimmons]
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�6v�2v�32sArchiacanthocephala   �21m �33s 
    An order of the phylum Acanthocephala. The adult worms are parasitic in 
terrestrial vertebrates. Some common archiacanthocephalans are Oncicola canis, 
Moniliformis moniliformis, and Macracanthorhynchus hirudinaceus. 
    Oncicola canis is a short plump acanthocephalan, primarily parasitic in 
dogs and other Canidae. It occurs also in cats. The body of the adult, 0.24 - 
0.56 in. (6 - 14 mm) long, is short and heavy with irregular cross furrows. 
The globular proboscis has six spiral rows of six hooks each. The arthropod 
intermediate host is unknown. Cystacanths have been found in armadillos and in 
the esophageal walls of turkeys, which indicates a transport host in the life 
cycle.  
    Moniliformis moniliformis is an elongate acanthocephalan which is 
parasitic in house rats. The females are 4 - 12 in. (10 - 30 cm) long, whereas 
the males measure 2.4 - 5.2 in. (6 - 13 cm). The body of both sexes exhibits 
conspicuous pseudosegmentation except on the extremities. The proboscis is 
cylindrical with 12 - 15 rows of 10 or 11 hooks each. Cockroaches (Periplaneta 
americana) serve as the intermediate host; however, in Europe a beetle, Blaps 
mucronata, is the intermediate host. Occasionally infections have been found 
in humans. 
    Macracanthorhynchus hirudinaceus is the giant thorn-headed worm of hogs 
and is probably the best known of all acanthocephalans because of its 
cosmopolitan distribution. Females measure 10 - 24 in. (25 - 60 cm) in length 
and the males 2 - 4 in. (5 - 10 cm). The worms are pinkish with a transversely 
wrinkled body which tapers from a rather broad anterior end to a slender 
posterior end. The proboscis is globular with six spiral rows of six hooks 
each. At least 25 species of scarabaeid beetle larvae have been reported as 
intermediate hosts. In addition to their occurrence in the domestic pig, 
adults have been reported to occur in squirrels, chipmunks, moles, and 
occasionally humans. This acanthocephalan is of considerable economic 
importance to the hog-raising industry. See ACANTHOCEPHALA. 
 �34s[Donald V. Moore]
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�6v�2v�32sArchiannelida �21m �33s
    A name applied to a small group of unrelated annelids, probably not
primitive as the name implies. Some resemble existing families of polychaetes
(see illustration). Most live in marine or brackish water and occur in
intertidal and estuarine habitats. They are characterized by their vermiform
body and small sizes. Lengths range from less than 0.04 in. (1 mm) to 0.4 in.
(10 mm), rarely to 4 in. (100 mm). Number of segments varies from five to
many, or segmentation is obscure. Parapodia with setae may be altogether
lacking, and the epithelium may be ciliated. Cephalic and anal structures may
be modified as holdfast organs to maintain existence in turbulent, intertidal
zones and shifting sands. See POLYCHAETA.
    Three families with about 82 species in 15 genera are recognized. The
Nerillidae have well-developed parapodia and setae; they are known for 20
species in 7 genera, all but one limited to Europe and western Africa; Nerilla
is worldwide. The Protodrilidae are known through 47 species in 5 genera; the
best known are Polygordius, Protodrilus, and Saccocirrus, which are recorded
from cosmopolitan areas. The Dinophilidae are known through 17 species in 3
genera, of which the best known is Dinophilus, with 6 or 7 species. See
ANNELIDA.

 �34s[Olga Hartman]
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�6v�2v�32sArchichlamydeae   �21m �33s 
    An artificial group of flowering plants, recognized in the Englerian 
system of classification, consisting of those families of dicotyledons 
(Magnoliopsida) that lack petals or have the petals separate from each other, 
in contrast to the Metachlamydeae, which have the petals united into a 
sympetalous corolla. See MAGNOLIOPSIDA; METACHLAMYDEAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sArchigregarinida   �21m �33s 
    An order of the protozoan subclass Gregarinia, class Telosporea, subphylum 
Sporozoa. All gregarines are parasites of the digestive tract and body cavity 
of invertebrates or lower chordates; their large, mature trophozoites 
(vegetative stages) live outside the host's cells. The Archigregarinida are 
primitive gregarines and live in marine worms (annelids and lower chordates - 
enteropneustids, sipunculids, and ascidians). Their life cycle includes sexual 
and asexual phases and involves three periods of schizogony (multiple 
fission). There are only 28 named species in five genera. The most important 
genus is Selenidium, which has 24 species. Its members occur in the intestine 
of marine polychaete annelids, sipunculids, enteropneustids, and ascidians. 
See GREGARINIA; PROTOZOA; SPOROZOA; TELOSPOREA. 
 �34s[Norman D. Levine]
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�6v�2v�32sArchimedes' principle   �21m �33s 
    A body immersed in static fluid is acted upon by a vertical force equal to 
the weight of fluid displaced, and a body floating in the fluid displaces its 
own weight of fluid. For example, a balloon ascends because it displaces a 
volume of air which weighs more than the weight of the balloon. The principle 
can be proved by determining the difference in vertical components of fluid 
force acting on the lower and upper curved surfaces of the body. This force, 
called the buoyant force, acts vertically upward through the centroid of the 
displaced volume of fluid. See BUOYANCY; FLUID STATICS; HYDROSTATICS. 
 �34s[Victor L. Streeter]
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�6v�2v�32sArchitectural acoustics �21m �33s
    The science of planning and building an enclosure to ensure the most
advantageous flow of properly diffused sound to all listeners. The goal of
architectural acoustics is to design a structure in such a manner that it will
contribute to speech intelligibility and to the esthetic qualities of music in
an environment free from external noise. The primary problems are the
selection of the site; the arrangement of the rooms within the building; the
design of the shape and size of each room; the selection and placement of
absorptive and reflective materials to provide optimum conditions for the
growth, decay, and steady-state distribution of sound in the room; and
provisions for adequate noise control. See ACOUSTIC NOISE.
    Sound fields in enclosures.  The boundaries of an enclosure alter the
distribution of sound emanating from a source by confining the energy that in
the open would proceed outward into space. For example, if a source of sound
that produces the same sound level at all frequencies at a given point in the
open air is placed in a room, the sound pressure at the same distance from the
source no longer will be constant with frequency but will be much higher at
the resonant frequencies of the enclosure. The resonant frequencies of a room
usually are not distributed uniformly with respect to frequency. In some
frequency regions the modes ``pile up,'' thereby affecting the acoustic
response of the room. The distribution of the normal modes of vibration of a
room is one of the considerations that determine the optimum proportions of
small rooms whose acoustical characteristics are of importance, such as
broadcasting studios. See MODE OF VIBRATION.
    Wave theory can be used to calculate the transient and steady-state
behavior of sound in rectangular rooms and in a few other regular shapes
having special distributions of absorptive materials. However, the formulas
become so complicated in practical problems in which nonuniform boundaries and
many modes are involved that it is not feasible to present a complete set of
practical design formulas based on physical acoustics. However, theory can be
useful in providing a qualitative explanation of acoustical phenomena, even in
complex situations. For example, when a source of sound is turned off in an
enclosure, the decay of sound generally does not take place uniformly;
instead, the sound pressure fluctuates as it decreases. These fluctuations may
be explained by physical acoustics in the following way: The source sets into
vibration the normal modes of the room, exciting to the greatest extent modes
that have resonant frequencies nearest the frequencies of the source. Each
mode has a different sound pressure distribution. When the source is turned
off, the sound pressure of each excited mode will decay at its own rate. In so
doing, the decaying modes interfere with each other and thus produce
fluctuations in pressure. See REVERBERATION.
    In contrast to physical acoustics, the methods of geometrical, or ray,
acoustics assume that sound travels as rays, in a manner similar to that of
light rays. Upon reflection from a wall, part of the acoustic energy is
absorbed and part reflected; the angle of the reflected ray is equal to the
angle of the incident ray if the surface is large compared with the
wavelength. See REFLECTION OF SOUND.
    Sound is said to be perfectly diffuse in a room if its pressure is
everywhere the same and if at all points in the room it is equally probable
that waves are traveling in every direction. It is impossible (and undesirable
from the standpoint of both performers and listeners) to obtain complete
diffusion.
    Sound absorption.  Sound is absorbed by any mechanism which converts
incident sound waves into other forms of energy and ultimately to heat. All
materials used in building construction absorb some sound, but those
especially designed to have relatively high absorption are popularly known as
acoustical materials. Most manufactured acoustical materials depend largely on
their porosity for their acoustic absorption. The sound waves are converted
into heat by propagation through the interstices of the material, and also by
vibration of the small fibers of the material. Another important mechanism of
absorption occurs when sound waves force a panel into motion; the resulting
flexual vibration converts a fraction of the incident sound energy into heat.
    When sound waves strike a surface, a fraction of the incident energy is
absorbed and a fraction is reflected. The sound absorption coefficient of a
surface is the ratio of the energy absorbed by the surface to the energy
incident on the surface.
    A totally absorbing surface having an area of S square feet is said to
have an absorption of S sabins. Thus the sabin, sometimes called a square-foot
unit of absorption, is the absorption equivalent of 1 ft sup 2 (0.093 m sup 2)
of material having an absorption coefficient of unity. Similarly, a totally
absorbing surface having an area of S' square meters is said to have an
absorption of S' metric sabins. (1 sabin = 0.093 metric sabin.)
    A quantity which describes the acoustical properties of a material and
which is more fundamental than the absorption coefficient is its acoustic
impedance, defined as the complex ratio of sound pressure to the corresponding
particle velocity at the surface. See ACOUSTIC IMPEDANCE; SOUND ABSORPTION.

    Transmission through partitions.  The fraction of airborne incident sound
energy transmitted through a partition is called its transmission coefficient
TAU. In rating the noise-insulating value of partitions, windows, and doors,
it is convenient to employ a logarithmic quantity, transmission loss TL, which
is equal to the number of decibels by which sound energy incident on a
partition is reduced as a result of transmission through it. The two
quantities are related by the equation below.

			   TL = 10 log (1/TAU) dB

    Sound transmission class (STC) is a single number representation of the
sound insulation performance of a partition in the frequency range from 125 to
4000 Hz. In this method of rating, the STC value is determined by comparing a
curve of the measured transmission loss versus frequency with a set of
standard curves.
    Airborne sound is transmitted through a so-called rigid partition by
forcing the partition into vibration; then the vibrating partition becomes a
secondary source and radiates sound to the side opposite the original source.
Over a large portion of the audible range, such a partition approximates a
mass-controlled system, so that one might expect that its transmission loss
would increase by 6 dB each time the weight of the partition is doubled. In
most partitions the increase is usually 4 - 5 dB. A compound-wall
construction, for example, a double-leaf wall, can yield relatively high sound
insulation with relatively low mass per unit wall area.

    Acoustical design of rooms.  The floor plan and the shape of the ceiling
and walls of a room are important factors in determining its acoustical
properties. The optimum length-width ratio is not a fixed number, but varies
with the size and shape of the seating area; it also depends on whether a
sound-amplification system is used. For most rooms, length-width ratios
between 2:1 and 1.2:1 have been found satisfactory.
    In order to bring a large audience as close as possible to the stage of an
auditorium, it is usually advantageous to design a floor plan with diverging
side walls. Reflections from the ceiling and from these walls can aid in the
establishment of a higher sound level at the rear of the auditorium. These
reflections must be carefully controlled because the reflected sound always
travels a greater distance than the sound that reaches the listener by a
direct path. If the difference in these path lengths is greater than 65 ft (20
m), the time delay in the arrival of the reflected sound is sufficient to
enable the listener to hear it as a separate sound, that is, as an echo. Path
length differences of 50 - 65 ft (15 - 20 m) produce a blurring quality. The
geometrical law of reflection can be used to determine the proper angle for
the ceiling and side-wall reflecting surfaces so that they will guide sound to
seats where the sound level is not adequate. Some parts of the side walls in
very large auditoriums may cause echoes. In such instances these surfaces
should not be smooth and reflective but should either be ``acoustically
rough'' to diffuse the sound or be covered with highly absorptive material.
See ECHO.
    Flutter echoes (echoes in rapid succession) frequently occur in rooms
having smooth, hard, parallel walls. They can be avoided by the use of
diffusing elements on the walls, diverging or tilted walls, or splays. As
little as 5/8 in. (16 mm) splay to the running foot will prevent flutter.
 �34s[Cyril M. Harris]
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�6v�2v�32sArchitectural engineering   �21m �33s 
    That branch of engineering dealing primarily with the properties and 
characteristics of building materials and components, and with the design of 
structural systems for buildings rather than the structural design of heavy 
construction, such as bridges, dams, and highways. The term is also used to 
indicate a field embracing all of the engineering aspects of building design, 
including mechanical and electrical equipment, architectural acoustics, 
illumination, air conditioning, and layout for usage, in addition to the 
historic primary interest in design of the building structure. When used in 
this broader sense, architectural engineering connotes building structures as 
its specialty, since the growing complexity of the other fields generally 
requires their engineering design to be accomplished by specialists trained in 
the pertinent branches of engineering. However, the coordination of the 
structural, mechanical, and electrical systems with each other and with the 
architectural scheme is often the responsibility of the architectural 
engineer. See ARCHITECTURAL ACOUSTICS; BUILDINGS. 
 �34s[Harold D. Hauf]
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�6v�2v�32sArchosauria �21m �33s
    A subclass of reptiles more easily defined by listing its five component
orders, four of them extinct, than by its anatomical characters. The earliest,
most primitive archosaurs were the Thecodontia, from which arose independently
the Saurischia (``lizard-hipped'' dinosaurs), the Ornithischia
(``bird-hipped'' dinosaurs), the Pterosauria (flying reptiles, including the
pterodactyls), and the Crocodilia (including the living crocodiles,
alligators, caimans, and gavials). See CROCODILIA; ORNITHISCHIA; PTEROSAURIA;
SAURISCHIA; THECODONTIA.
    Many Archosauria are certainly bipeds. They have two pairs of openings
(fenestrae) in the temporal region of the skull (diapsid condition) and never
lose the bony arcades around those openings. Typically there are also one or
more antorbital openings in front of the orbit, a character which is virtually
confined to this subclass (see illustration).
    The earliest archosaurs are Lower Triassic, with a single species in the
uppermost Permian. The Archosauria were undoubtedly a highly successful group,
with locomotor systems, sense organs, and probably physiological adaptations
far superior to those of any other reptiles. See REPTILIA.

 �34s[Alan J. Charig]
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�6v�2v�32sArc lamp   �21m �33s 
    A type of electric-discharge lamp in which an electric current flows 
between electrodes through a gas or vapor. In most arc lamps the light results 
from the luminescence of the gas; however, in the carbon arc lamp the light is 
produced by the incandescence of one or both electrodes. The color of the arc 
depends upon the electrode material and the surrounding atmosphere. Most lamps 
have a negative resistance characteristic so that the resistance decreases 
after the arc has been struck. Therefore some form of current-limiting device 
is required in the electric circuit. For other electric-discharge lamps see 
VAPOR LAMP. 
    The carbon arc lamp was the first practical commercial electric lighting 
device, but the use of arc lamps at present is limited. In many of its 
previous functions, the carbon arc lamp has been superseded by the 
high-intensity mercury vapor lamp. Arc lamps are now used to obtain high 
brightness from a concentrated light source, where large amounts of radiant 
energy are needed, and where spectral distribution is an ad-vantage. Typical 
uses of arc lamps are in projectors, searchlights, blue-printing, photography, 
therapeutics, and microscope lighting, and for special lighting in research. 
See CARBON ARC LAMP; FLAME ARC LAMP; METALLIC ELECTRODE ARC LAMP. 
 �34s[John O. Kraehenbuehl]
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�6v�2v�32sArctic and subarctic islands   �21m �33s 
    Defined primarily by climatic rather than latitudinal criteria, arctic 
islands are those in the Northern Hemisphere where the mean temperature of the 
warmest month does not exceed 50DGF (10DGC) and that of the coldest is not 
above 32DGF (0DGC). Subarctic islands are those in the Northern Hemisphere 
where the mean temperature of the warmest month is over 50DGF (10DGC) for less 
than 4 months and that of the coldest is less than 32DGF (0DGC). Such islands 
generally are in high latitudes. Distribution of land and sea masses, ocean 
currents, and atmospheric circulation greatly modifies the effect of latitude 
so that it is often misleading to use location relative to the Arctic Circle 
as a significant criterion of arctic or subarctic. The largest proportion by 
area of the islands lies in the Western Hemisphere, primarily in Greenland and 
in the Canadian Arctic Archipelago. Within this general description, 
individual islands vary considerably (see table). 
    Physiographically, the islands include all the varied major landforms 
found elsewhere in the world, from rugged mountains over 8000 ft (2500 m) 
high, through plateaus and hills, to level plains only recently emerged from 
the sea. All have been glaciated except Sakhalin and some of the islands in 
the Bering Sea sector. Removal of the weight of ice sheets and the resultant 
crustal rebound has exposed prominent marine beaches and wave-cut cliffs on 
many of the islands. These now commonly occur at elevations of over 300 ft 
(150 m) above sea level. 
    The general climatic pattern of these islands is set by their location 
relative to the two semipermanent centers of low pressure over the Aleutian 
Islands and over Iceland. Most of the precipitation is cyclonic in origin. 
Because they are marine areas, the islands receive more precipitation than 
they other wise would, yet even so this is very light for most of the arctic 
islands removed from the zone of cyclonic activity. Also, because they are 
marine areas, the islands, regions of low temperatures by definition, are not 
regions of extreme low temperatures. In general, the larger the island and the 
closer its proximity to a continental landmass, the higher are the summer 
temperatures and the lower its winter temperature. See POLAR METEOROLOGY. 
    The climatic differences between arctic and subarctic islands are 
reflected in their natural vegetation. The arctic islands are treeless. 
Natural vegetation consists of the tundra - mosses, sedges, lichens, grasses, 
and creeping shrubs. Bare gound is often exposed and in places plant growth 
may be lacking completely except for a few rock-encrusting lichens. In such 
places the ground surface may consist of frost-shattered rock fragments, tidal 
mud flats, boulder-strewn fell fields, or snow patches and ice. Permafrost 
(permanently frozen ground) occurs throughout the Arctic (and in parts of the 
subarctic) and is reflected in impeded drainage and patterned ground. See 
PERMAFROST; TUNDRA. 
    The natural vegetation of subarctic islands characteristically is the 
boreal forest or taiga, composed predominantly of conifers such as spruce, 
fir, pine, and larch with deciduous trees such as birch, aspen, and willow; 
the latter are especially common in regrowth of clearings in the forest. 
Impeded drainage because of permafrost or glaciation gives rise to numerous 
ponds and muskeg areas. A transitional type of vegetation, the forest-tundra, 
is recognized on some subarctic islands in sectors where smaller trees are 
widely spaced and abundant mosses cover the ground. See MUSKEG; TAIGA. 
    The typical soils of the subarctic islands are podzols - the grayish-white 
surface soil beneath the raw humus layer and highly acidic in nature. The 
tundra soils of the arctic islands really consist only of a dark-brown peaty 
surface layer over poorly defined thin horizons, and much of the ground cannot 
properly be termed soil. 
 �34s[William C. Wonders]
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�6v�2v�32sArctic Circle �21m �33s
    The parallel of latitude approximately 66 1/2 DG (66 DG 55) north of the
Equator, or 23 1/2 DG from the North Pole. The Arctic Circle has the same
angular distance from the Equator as the inclination of the Earth's axis from
the plane of the ecliptic. Thus, when the Earth in its orbit is at the Northern
Hemisphere summer solstice, June 21, the North Pole is tilted 23 1/2 DG toward
the Sun, the Sun's rays extend beyond the pole 23 1/2 DG to the Arctic Circle,
giving that parallel 24 h of sunlight. On this same date the Sun's rays at noon
will just reach the horizon at the Antarctic Circle, 66 1/2 DG south. The
highest altitude of the noon Sun at the Arctic Circle is on June 21, when it is
47 DG above the horizon.
    At the Arctic Circle the Sun remains above the horizon continuously only
24 h at the longest period. However, with twilight considered, it remains
daylight or twilight continuously for about 5 months. Twilight can be
considered to last until the Sun drops 18 DG below the horizon. See
MATHEMATICAL GEOGRAPHY; SOLSTICE.
 �34s[Van H. English]
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�6v�2v�32sArctic Ocean �21m �33s
    The north polar ocean lying between North America and Asia, extending over
about 386,000 mi sup 2 (10 sup 6 km sup 2). It is nearly completely covered by
6-9 ft (2 - 3 m) of ice in winter, and in summer it becomes substantially open
only at its peripheries. Its extent has been variably defined, but it is
oceanographically appropriate to consider it bounded on the south by a line
running from northern Greenland through Smith, Jones, and Lancaster sounds,
along northwestern Baffin Island to the Canadian mainland, thence to the Alaskan
coast, across Bering Strait, along the Siberian coast to Novaya Zemlya, across
to Franz Josef Land and Spitsbergen, and over to northern Greenland. This
definition omits the Barents, Norwegian, and Greenland seas and Baffin Bay,
which have a pronounced North Atlantic character.
    The central polar basin, somewhat triangular in shape, is surrounded by
continental shelves which are interrupted only by the deep passage running
through Fram Strait. The upper 650 ft (200 m) of the Arctic Ocean, referred to
as Surface Water or Arctic Water, is characterized by a significant density
stratification produced by the strong increase in salinity downward from the
surface. This density stratification is of considerable importance, for it
prevents a deep-reaching convection from developing within the Arctic Ocean
and also prevents the heat of the underlying warm Atlantic Water from reaching
the surface. The relatively low salinity at the surface is maintained against
the upward diffusion of salt by the addition of fresh water, principally
through river outflow. The upper 100 - 160 ft (30 - 50 m) of Surface Water
tends to be relatively uniform vertically in temperature and salinity. Except
for areas which become ice-free in summer, the water will be near the freezing
point. Currents in the upper waters tend to be relatively slow (4 in./s or 10
cm/s or less), and they are similar in both speed and direction to the ice
motion. The overall circulation in the upper waters has its ultimate cause in
the prevailing wind pattern over the Arctic Ocean.
    As in other oceans, the current at any instant can vary greatly from the
mean condition. The most spectacular example observed in the Arctic Ocean
occurs on an occasional basis in the Canadian Basin, consisting of a
high-speed current core. See OCEAN CURRENTS; SEA WATER.
    Below the Surface Water, the temperature increases to a maximum, which
over most of the region is about 40DGF (0.5DGC) and lies between 900 and 1500
ft (300 and 500 m). The salinity is nearly uniform, and since at low
temperatures the density of sea water depends almost solely on salinity, there
is virtually no density stratification beneath the upper waters. Significant
deviations from the stated temperature occur only in the southern Eurasian
Basin closest to Spitsbergen, for it is there that the warm and saline water
(called Atlantic Water) which maintains the temperature maximum throughout the
Arctic Ocean first enters. This water has its origin in the North Atlantic.
Once into the Arctic Ocean it sinks because of its high salinity and moves
eastward along the Eurasian continental slope. Beneath the Atlantic Water lies
cold, nearly uniform Bottom Water. These two water masses together constitute
over 90% of the volume of the Arctic Ocean. The Bottom Water is formed in the
Greenland Sea.
 �34s[Knut Aagaard]
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�6v�2v�32sArcturus �21m �33s
    Alpha Booetis, a red giant star of spectral type K2. Arcturus is one of
the apparently brightest stars in the sky, of apparent magnitude 0.2. Because
it is 11 parsecs (2.1 x 10 sup 1 sup 4 mi or 3.4 x 10 sup 1 sup 4 km) distant,
its absolute magnitude is -0.04, approximately 100 times brighter than the Sun,
with a radius 20 times as large.
    An interesting feature of Arcturus is its large space motion with respect
to the Sun; it belongs to the high-velocity, or population II, group. The
radial velocity is +56 mi/s (+90 km/s), and the observed proper motion in the
plane of the sky corresponds to 74 mi/s (120 km/s). W. W. Morgan recognized in
Arcturus the spectroscopic features that have since become generally
associated with population II. In particular, the bands of CN are weak
compared to normal K giants of this luminosity class.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sArc welding �21m �33s
    A welding process utilizing the concentrated heat of an electric arc to
join metal by fusion of the parent metal and the addition of metal to the
joint usually provided by a consumable electrode (see illustration). Electric
current for the welding arc may be either direct or alternating, depending
upon the material to be welded and the characteristics of the electrode used.
The current source may be a rotating generator, rectifier, or transformer and
must have transient and static volt-ampere characteristics designed for arc
stability and weld performance.
    There are three basic welding methods; manual, semiautomatic, and
automatic. Manual welding is the oldest method, and though its proportion of
the total welding market diminishes yearly, it is still the most common. Here
an operator takes an electrode, clamped in a handheld electrode holder,  and
manually guides the electrode along the joint as the weld is made. Usually the
electrode is consumable; as the tip is consumed, the operator manually adjusts
the position of the electrode to maintain a constant arc length.
    Semiautomatic welding is becoming the most popular welding method. The
electrode is usually a long length of small-diameter bare wire, usually in
coil form, which the welding operator manually positions and advances along
the weld joint. The consumable electrode is normally motor-driven at a
preselected speed through the nozzle of a hand-held welding gun or torch.
    Automatic welding is very similar to semiautomatic welding, except that
the electrode is automatically positioned and advanced along the prescribed
weld joint. Either the work may advance below the welding head or the
mechanized head may move along the weld joint.
    There are, in addition to the three basic welding methods, many welding
processes which may be common to one or more of these methods. A few of the
more common are described below.
    Carbon-electrode arc welding is in limited use for welding ferrous and
nonferrous metals. Normally, the arc is held between the carbon electrode and
the work. The carbon arc serves as a source of intense heat and simply fuses
the base materials together, or filler material may be added from a separate
source.
    Shielded metal arc welding is the most widely used arc-welding process. A
coated stick electrode is consumed during the welding operation, and therefore
provides its own filler metal. The electrode coating burns in the intense heat
of the arc and forms a blanket of gas and slag that completely shields the arc
and weld puddle from the atmosphere. Its use is generally confined to the
manual welding method.
    Submerged-melt arc welding uses a consumable bare metal wire as the
electrode, and a granular fusible flux over the work completely submerges the
arc. This process is particularly adapted to welding heavy work in the flat
position. High-quality welds are produced at greater speed with this method
because as much as five times greater current density is used. Automatic or
semiautomatic wire feed and control equipment is normally used for this
process.
    Tungsten - inert gas welding, often referred to as TIG welding, utilizes a
virtually nonconsumable electrode made of tungsten. Impurities, such as
thorium, are often purposely added to the tungsten electrode to improve its
emissivity for direct-current welding. The necessary arc shielding is provided
by a continuous stream of chemically inert gas, such as argon, helium, or
argon-helium mixtures, which flows axially along the tungsten electrode that
is mounted in a special welding torch. This process is used most often when
welding aluminum and some of the more exotic space-age materials. When filler
metal is desired, a separate filler rod is fed into the arc stream either
manually or mechanically. Since no flux is required, the weld joint is clean
and free of voids.
    Metal - inert gas welding, often referred to as MIG welding, saw its
greatest growth in the 1960s. It is similar to the TIG welding process, except
that a consumable metal electrode, usually wire in spool form, replaces the
nonconsumable tungsten electrode. This process is adaptable to either the
semiautomatic or the automatic method. In addition to the inert gases, carbon
dioxide has become increasingly common as a shielding means. See WELDING AND
CUTTING OF METALS.

 �34s[Emil F. Steinert]
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�6v�2v�32sArc (mathematics) �21m �33s
    Either of the two parts of a circle (considered as a curve) intercepted
between two distinct radii. If circle C has center O, radius r, and two radii
intersect C at points A, B, let THETA = ANGLE AOB denote the angle, not greater
than a straight angle, formed by these radii. When the length of arc AB = r,
the angle THETA is 1 radian (rad), and consequently there are 2 PI r/r =
2 PI rad in one complete revolution. It follows that 2 PI rad = 360 DG; that
is, 1 rad = 57 DG 17'45", to the nearest second. Radius r, angle THETA, and
corresponding arc length s satisfy the relation s = r PRODUCT THETA. See
CIRCLE; RADIAN MEASURE.
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sArea �21m �33s
    The superficial contents of a geometrical figure of two dimensions. The
area of any rectangle or square is the product of two adjacent sides, one of
which may be called the base and the other the altitude. In general, any line
segment that partially bounds a plane geometric figure may be called a base if
its line does not separate the figure, and a perpendicular drawn to the base
line from one of its points at greatest distance may be called the altitude.
Some area formulas are given in the table. See EUCLIDEAN GEOMETRY.

     TABLE CAPTION.  Area formulas

 �34s[James S. Frame]
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�6v�2v�32sAreal velocity   �21m �33s 
    The rate at which a line that joins a fixed point and a moving particle 
sweeps out a surface area is called the areal velocity with respect to the 
fixed point. In elliptical motion, if the origin is at one focus, the areal 
velocity is a constant. In astronomy, Kepler's law of areas expresses this 
characteristic.  
 �34s[Raynor L. Duncombe]
�33s 
  
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�6v�2v�32sArecales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), of 
the subclass Arecidae in the class Liliopsida (monocotyledons). The order 
consists of the single family Arecaceae (Palmae), the palms, with more than 
200 genera and about 3500 species, largely confined to tropical and 
subtropical regions. The order Arecales has also been called Palmales or 
Principes.  
    Most palms are trees with an unbranched trunk and a terminal crown of 
large leaves. The leaf has a blade, petiole, and sheath; the sheath is open or 
closed, but in any case it fully encircles the stem at the base. The blade as 
a whole is usually pinnately compound, less often palmately compound or merely 
lobed. It is pinnately or less often palmately veined, but the individual 
pinnae or segments have more or less parallel veins and are plicate between 
the veins. See ARECIDAE; CARNAUBA WAX; COCONUT; CYCLANTHALES; DATE; 
LILIOPSIDA; MAGNOLIOPHYTA; PALM; PLANT KINGDOM; VEGETABLE IVORY. 
 �34s[Arthur Cronquist]
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�6v�2v�32sArecidae   �21m �33s 
    A subclass of the class Liliopsida (monocotyledons) of the division 
Magnoliophyta (Angiospermae), the flowering plants, consisting of four orders 
(Arecales, Cyclanthales, Pandanales, and Arales), five families, and about 
6400 species. Except for the highly reduced family Lemnaceae (Arales), they 
have an inflorescence of usually numerous, small flowers, generally subtended 
by a prominent spathe and often aggregated into a spadix. Except in the 
Araceae (Arales), the endosperm seldom contains much starch. More than 80% of 
the species have broad, petiolate leaves that do not have the typical parallel 
venation commonly associated with monocotyle dons, and more than half of the 
species are arborescent (likewise an unusual character among the 
monocotyledons). See ARALES; ARECALES; CYCLANTHALES; LILIOPSIDA; 
MAGNOLIOPHYTA; PANDANALES; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sArenaceous rocks   �21m �33s 
    The arenaceous rocks (arenites) include all those clastic rocks whose 
particle sizes range from 0.8 to 0.0025 in. (2 to 1/16 mm), or if silt is 
included, to 1/256 mm. Some arenites are composed primarily of carbonate 
particles, in which case they are called calcarenites and grouped with the 
limestones. Some oolitic iron ores and glauconite beds are properly classified 
as arenites. But the vast majority of arenites are commonly called sandstones, 
and the two words are almost synonymous. See CALCARENITE; GRAYWACKE; OOLITE 
AND PISOLITE; ORTHOQUARTZITE; SANDSTONE; SEDIMENTARY ROCKS; SUBGRAYWACKE. 
 �34s[Raymond Siever]
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�6v�2v�32sArgentite   �21m �33s 
    A mineral having composition Ag2S. Argentite crystals are rare and are 
cubes or octahedrons. Argentite most commonly occurs in massive form or as 
coatings with a leadgray color. Its hardness is 2.5 (Mohs scale) and specific 
gravity 7.3; it is very sectile and is bright on the fresh surface but becomes 
dull black on exposure. It occurs in veins associated with other silver 
minerals and at some places is a major silver ore. Important localities are in 
Mexico, Peru, Chile, and Bolivia. In the United States it has been extensively 
mined in Nevada at the Comstock Lode and at Tonopah. See SILVER METALLURGY. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sArgillaceous rocks   �21m �33s 
    The argillaceous rocks (lutites) include shales, argillites, siltstones, 
and mudstones; they are clastic sediments whose constituent particles are less 
than 0.0025 in. or 1/16 mm (if siltstones are included) or less than 0.00015 
in. or 1/256 mm (if siltstones are excluded). They are the most abundant 
sedimentary rock type, varying according to different estimates from 44 to 56% 
of the total sedimentary rock column. Claystone is indurated clay, which 
consists dominantly of fine material of which at least a major proportion is 
clay mineral (hydrous aluminum silicates). Shale is a laminated or fissile 
claystone or siltstone, in general more consolidated than claystone. Mudstone 
is a claystone that is blocky and massive. The term argillite is used for 
rocks which are more indurated than claystone or shale but not metamorphosed 
to slate. All these argillaceous rocks are consolidated equivalents of muds, 
oozes, silts, and clays. Loess is a fine-grained, unconsolidated, wind-blown 
deposit. The term shale has been used by many authors generically to denote 
all of these types of rock. See ARGILLITE; BENTONITE; CLAY; CLAY MINERALS; 
LOESS; SEDIMENTARY ROCKS; SHALE. 
 �34s[Raymond Siever]
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�6v�2v�32sArgillite   �21m �33s 
    A nonfissile argillaceous rock that seems to have formed as the result of 
incipient metamorphism. The degree of induration is greater than that of 
shales, and the rock is dense and hard. The term has been used to designate 
argillaceous rocks in low-grade metamorphic belts that do not have the parting 
or cleavage of slates. Argillites are normally finely laminated, but the term 
has also been applied to dense, hard, massive, nonbedded argillaceous rocks. 
Argillites seem to have a gross mineral and chemical composition similar to 
that of shales. The argillites have no fissility, for which there is no ready 
explanation. They may represent conditions under which original settling did 
not result in preferred orientation of platy minerals, or in which there has 
been no reorientation due to compaction and pressure. Alternatively, the clay 
mineral crystal habit in the argillites may be rodshaped and fibrous rather 
than platy. See ARGILLACEOUS ROCKS;  SEDIMENTARY ROCKS; SHALE. 
 �34s[Raymond Siever]
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�6v�2v�32sArginine �21m �33s
    An amino acid. Arginine forms a red color when treated with sodium
hypochlorite and ALPHA-naphthol (Sakaguchi reaction). The amino acid's special
functions are: (1) Arginine is an intermediate in the process of urea
formation from ammonia and carbon dioxide, by virtue of its position in the
urea cycle. (2) Arginine is probably the source of the guanidine groups of
streptomycin.
    The biosynthesis of arginine starts with glutamic acid and proceeds by way
of ornithine and citrulline. The metabolic degradation begins with the
hydrolysis of arginine to ornithine and urea. Ornithine can be deaminated to
glutamic acid semialdehyde. This compound is oxidized to glutamic acid, which
is catabolized via ALPHA-ketoglutaric acid and the Krebs cycle. See AMINO
ACIDS; KREBS CYCLE.
 �34s[Edward A. Adelberg]
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�6v�2v�32sArgon �21m �33s
    A chemical element, Ar, atomic number 18, and atomic weight 39.948. Argon
is the third member of group 0 in the periodic table. The gaseous elements in
this group are called the noble, inert, or rare gases, although argon is not
actually rare. The Earth's atmosphere is the only natural argon source;
however, traces of this gas are found in minerals and meteorites. Argon
constitutes 0.934% by volume of the Earth's atmosphere. Of this argon, 99.6%
is the argon-40 isotope; the remainder is argon-36 and argon-38. There is good
evidence that all the argon-40 in the air was produced by the radioactive
decay of the radioisotope potassium-40. See INERT GASES.
    Argon is colorless, odorless, and tasteless. The element is a gas under
ordinary conditions, but it can be liquefied and solidified readily. Some
salient properties of the gas are listed in the table. Argon does not form any
chemical compounds in the ordinary sense of the word, although it does form
some weakly bonded clathrate compounds with water, hydroquinone, and phenol.
There is one atom in each molecule of gaseous argon.
    The oldest large-scale use for argon is in filling electric light bulbs.
Welding and cutting metal consumes the largest amount of argon. Metallurgical
processing constitutes the most rapidly growing application. Argon and
argon-krypton mixtures are used, along with a little mercury vapor, to fill
fluorescent lamps. Argon mixed with a little neon is used to fill luminous
electric-discharge tubes employed in advertising signs (similar to neon signs)
when a blue or green color is desired instead of the red color of neon. Argon
is also used in gas-filled thyratrons, Geiger-Mueller radiation counters,
ionization chambers which measure cosmic radiation, and electron tubes of
various kinds. Argon atmospheres are used in dry boxes during manipulation of
very reactive chemicals in the laboratory and in sealed-package shipments of
such materials.
    Most argon is produced in air-separation plants. Air is liquefied and
subjected to fractional distillation. Because the boiling point of argon is
between that of nitrogen and oxygen, an argon-rich mixture can be taken from a
tray near the center of the upper distillation column. The argon-rich mixture
is further distilled and then warmed and catalytically burned with hydrogen to
remove oxygen. A final distillation removes hydrogen and nitrogen, yielding a
very-high-purity argon containing only a few parts per million of impurities.

     TABLE CAPTION.  Properties of argon

 �34s[Arthur W. Francis]
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�6v�2v�32sArhynchobdellae �21m �33s
    An order of the class Hirudinea (the leeches) which do not have an
eversible proboscis, but frequently have three jaws armed with sharp teeth.
The blood of these annelids contains hemogloblin. They may be divided into the
Gnathobdellae, with jaws, and the Pharyngobdellae, without jaws.
    Gnathobdellae have bodies which are oval in cross section and have a
conspicuous posterior sucker. The anterior sucker does not project beyond the
sides of the body but forms a deep cup on the underside of the head in which
the jaws can work to make their incision in the host. This group contains most
of the important bloodsucking leech parasites of humans and other warm-blooded
animals. The land leeches are members of this group. They occur in great
numbers on vegetation in swamp and jungle areas and attach themselves to
passing warm-blooded animals.
    Pharyngobdellae are specialized for carnivorous diets, and in many cases
have completely lost the jaws. They have a strong muscular pharynx which
extends nearly half the length of the body. Erpobdella (see illustration) is
common in lakes and streams in the Northern Hemisphere, while Trocheta tends
to leave the water and forage in moist soil. See HIRUDINEA; RHYNCHOBDELLAE.

 �34s[Kenneth H. Mann]
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�6v�2v�32sAries �21m �33s
    The Ram, in astronomy, a zodiacal and autumnal constellation (see
illustration). Among the 12 zodiacal constellations, Aries was considered as
the first, because about 2000 years ago when the zodiacal constellations were
organized, the Sun was in Aries where it crossed the Equator at vernal
equinox. Today, because of the precession of the equinoxes, this reference
point has moved into the constellation Pisces. However, Aries remains the
first sign of the zodiac. See CONSTELLATION.

 �34s[Ching-Sung Yu]
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�6v�2v�32sAristolochiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Magnoliidae of the class Magnoliopsida (dicotyledons). It 
contains only the family Aristolochiaceae, with seven genera and about 600 
species, most of them in tropical and subtropical regions. Within its subclass 
the order is marked by the presence of ethereal oil cells, by its 
uniaperturate or nonaperturate pollen, and especially by its strongly 
perigynous to epigynous flowers, usually with united carpels, that typically 
lack petals and have the sepals joined into a highly irregular, corolloid 
calyx. Many of the species are climbing vines. Aristolochia (birthwort or 
Dutchman's pipe) and Asarum are well-known genera of the order. See FLOWER; 
MAGNOLIIDAE; MAGNOLIOPHYTA; MAGNOLIOPSIDA; POLLEN; RAFFLESIALES. 
 �34s[Arthur Cronquist]
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�6v�2v�32sArkose   �21m �33s 
    An arenaceous rock that contains a high proportion of feldspar in addition 
to quartz and other detrital minerals. Arkose is also known as feldspathic 
sandstone. Although there is no universal agreement, many geologists consider 
a minimum of 25% feldspar a requisite for calling sandstone an arkose. Other 
geologists accept a lower value. Arkoses may contain a high proportion of 
other nonquartz detritus, such as igneous and metamorphic rock fragments, 
micas, amphiboles, and pyroxenes. Frequently the accessory heavy mineral suite 
consists of a variety of species. 
    Sedimentary structures of arkoses are similar in kind to those of the 
orthoquartzites. Cross-bedding, the major feature, may be displayed on a huge 
scale, some cross-bedded units being many feet thick. Arkoses are associated 
with a variety of clastic rocks, dominantly conglomerates, and reddish-colored 
shales. Arkoses also are found with basic lava flows. Most arkoses are found 
in geosynclinal areas, but the thin, reworked, granite-wash arkoses can be 
found on stable continental platforms. See GEOSYNCLINE. 
    The granite-wash arkoses appear to have formed as the result of a 
transgression of the sea over a land area underlain by granite. The fragmented 
granite in the soil and mantle rock is incorporated in the basal sediment. In 
some areas the original granite is changed so slightly that the arkose is 
called recomposed granite and may be almost indistinguishable from the 
original granite. Since high relief and climatic extremes generally are 
associated with orogenic movements, arkoses are usually interpreted as 
sediments that result from tectonically active regions. See ARENACEOUS ROCKS; 
FELDSPAR; GRAYWACKE; ORTHOQUARTZITE; SANDSTONE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sArmadillo �21m �33s
    The name for 21 species of mammals of the order Edentata, a group
characterized by the lack of enamel on their teeth. They are indigenous to the
New World, especially South America.
    Armadillos range in size from the lesser pichiciego or fairy armadillo
(Chlamyphorus truncatus), in which the adult is about 5 in. (7.5 cm) long, to
the giant armadillo (Priodontes giganteus), which is about 4 ft (1.2 m) in
length. The body is covered with horny dermal scales that replace the hair
common to most mammals and overlay bony plates. These structures fuse to form
rigid shields covering the anterior and posterior ends of the animal, whereas
in the midregion they form jointed bands allowing a certain amount of
flexibility. The giant armadillo has about 100 teeth, more than any other land
mammal. The snout is long, and the tongue is cylindrical and viscous to assist
in capturing food. The toes are clawed and are used by the animal to dig into
ant and termite colonies for food, as well as for burrowing. When disturbed,
many species roll into a ball or wedge themselves into a burrow opening.
    The nine-banded armadillo (Dasypus novemcinctus; see illustration) is the
best-known species and ranges from South America to the southwestern and
southern United States. It is the only edentate which inhabits the United
States, ranging from the Rio Grande area to Oklahoma and eastward along the
coast to Louisiana. The nine-banded armadillo has been studied because of its
unusual life cycle. Four young are born in a den or chamber at the end of the
burrow. The young are always of the same sex and are identical quadruplets.
See DENTITION; EDENTATA; SCALE (ZOOLOGY).

 �34s[Charles B. Curtin]
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�6v�2v�32sArmature   �21m �33s 
    That part of an electric rotating machine which includes the main 
current-carrying winding. The armature winding is the winding in which the 
electromotive force (emf) produced by magnetic flux rotation is induced. In 
electric motors this emf is known as the counterelectromotive force. 
    On machines with commutators, the armature is normally the rotating 
member. On most ac machines, the armature is the stationary member and is 
called the stator. The core of the armature is generally constructed of steel 
or soft iron to provide a good magnetic path, and is usually laminated to 
reduce eddy currents. The armature windings are placed in slots on the surface 
of the core. On machines with commutators, the armature winding is connected 
to the commutator bars. On ac machines with stationary armatures, the armature 
winding is connected directly to the line. See COMMUTATOR; CORE LOSS; WINDINGS 
IN ELECTRIC MACHINERY. 
 �34s[Arthur R. Eckels]
�33s 
  
  
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�6v�2v�32sArmature reaction   �21m �33s 
    The reaction of the magnetic field produced by the armature current on the 
magnetic field produced by the field current. This causes a distorted 
flux-density distribution in the air gap. Due to the saturation of the 
armature teeth, the flux density is decreased by a greater amount under one 
pole tip than it is increased under the other, and therefore the 
cross-magnetizing armature reaction produces a demagnetizing effect, and the 
generated voltage or counter-emf will be reduced when the armature is loaded. 
In a generator this degrades the voltage regulation. In a motor it tends to 
increase the speed and may cause instability. 
    For machines subject to heavy overloads, rapidly changing loads, or 
operation with a weak mainfield, the resultant flux-distribution distortion by 
excessive cross-magnetizing armature reaction will cause nonuniform 
distribution of voltage between commutator segments, and may result in 
flashover between commutator segments. A poleface (or compensating) winding, 
embedded in slots in the pole face and excited by armature current, is 
provided to neutralize the cross-magnetizing effect under the pole faces. See 
ELECTRIC ROTATING MACHINERY. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sArmy armament �21m �33s
    The equipment of a military unit. Army armament covers a broad range of
weaponry, from hand-held, small-caliber infantry weapons through sophisticated
aircraft armament to heavy, self-propelled, large-caliber guns and howitzers.
Many weapons have families of ammunition encompassing, for example, high
explosives (HE); antipersonnel, armor-penetrating, cannon-launched guided
projectiles; and nuclear rounds. Armament also includes numerous devices such
as grenades, grenade launchers, mines, and missiles intended to neutralize
enemy defenses.
    The United States has joined with several of its allies in cooperative
efforts to design, develop, produce, and deploy armament under the auspices of
rationalization, standardization, and interoperability (RSI) agreements.
Rationalization involves increasing the effectiveness of alliance forces
through the development of more efficient use of the defense resources
committed to the alliance. Standardization means adopting common or compatible
technical, logistical, and tactical procedures while developing a new system
of interchangeable supplies, components, weapons, and equipment.
Interoperability is the ability of allied forces to provide and accept
services from each other. Thus, RSI promotes an efficient strategy that
reduces defense expenditures while increasing technological advances, and
nourishes cooperation in deployment of new weapon systems with the advantage
of unifying the alliance defense network.
    Army armament is usually divided into categories such as infantry,
aircraft armament, artillery, armor, and miscellaneous defense devices.
    U.S. Army infantry has several systems. The Tow is a heavy antitank weapon
capable of accurate, effective fire against armored vehicles and hard targets.
It is a tube-launched, optically tracked, wire-guided missile, launched from a
crew-served weapon from either the ground or a suitable vehicle (land or air).
The Dragon is a missile weapon system capable of destroying armor vehicles and
field fortification. The Dragon is a medium-range antitank/assault weapon,
light enough to be carried and deployed by one person. Useful range extends to
3300 ft (1000 m). The missile is optically tracked, with signals from the
tracker being transmitted to the missile by means of wires. The Viper is a
rocket system intended to replace the M72A2 (LAW) as the Army's light antitank
weapon. It is a light-tactical-round, human-portable, shoulder-launch rocket
intended for use against tanks and other hard targets. The rocket is unguided
after launch, and follows a free ballistic trajectory to the target. The M16A1
rifle (see illustration) is a lightweight, air-cooled, gas-operated weapon
which uses a 20- or 30-round magazine. It can be fired from the shoulder or
hip in the semi- or full-automatic mode at a cycle rate of 700 to 940 rounds
per minute. The M203 40-mm grenade launcher is a lightweight, compact,
breech-loading, pump-action, single-shot, manually operated weapon used with
the M16A1 rifle. The squad automatic weapon is a one-person automatic weapon
capable of delivering a large rate of automatic, lethal, accurate, and
sustained fire to 3300 ft (1000 m). The weapon will supplement and reinforce
firepower of other weapons in the rifle squad fire team. The squad automatic
weapon will eventually replace the M16A1 in the automatic rifle mission, and
may replace one or more of the M60 machine guns in the rifle platoon.
    Through the use of Air Force tactical support and Army aviation support,
the Armed Forces employ air/ground operations systems (AGOS). Close air
support is used to include air attacks from all services against hostile
targets which are in close proximity to friendly forces. The Air Force A-10
aircraft, used to sustain close air support, employs a 30-mm GAU-8
seven-barrel gatling gun. The intended mission for GAU-8 is antiarmor. The
GAU-8 family consists of three types of cartridges: armor-piercing incendiary
(API), high-explosive incendiary (HEI), and target practice (TP). The advanced
attack helicopter requires an area weapon subsystem designed to optimize the
capability of the aircraft to carry out the Army's area fire mission in
support of ground operations. The area weapon system for the AH-64 consists of
the following major components: 30-mm chain gun, turret, ammunition storage,
and transfer mechanism and control system.
    Mine warfare is an integral element in modern land combat, as conflicts
since the mid-1960s have clearly demonstrated. Modern tactics and strategies
have threatened the effectiveness of conventional mines; therefore, the U.S.
Army Armament Research and Development Command (ARRADCOM), in association with
the Air Force and Navy, has developed FASCAM (family of scatterable mines).
FASCAM provides Allied forces with antiarmor and antipersonnel mines with
quick-strike emplacement capabilities through air, artillery, and
special-purpose ground vehicle delivery techniques, as well as remotely
activated and deactivated weapons to counter the enemy's growing strength and
technology. Mine fields have the greatest effectiveness when used in
conjunction with direct and indirect firepower; therefore, they can be
emplaced in advance as an integral element in a well-prepared defense to
block, delay, divert, or destroy attackers. During offensive and counterattack
operations, FASCAM mine fields can be emplaced to deny escape routes to the
enemy.

    The field artillery (FA) mission is to destroy, neutralize, or suppress
the enemy by cannon, rocket, and missile fire. Field artillery has a target
acquisition capability, a responsive and accurate gunnery team, a variety of
weapons and ammunition, plus a command and control element. The weapons and
ammunition element of field artillery include high- and low-angle cannon
fires, rocket and missile fires, scatterable mine delivery, plus appropriate
ammunition selection to provide desirable effects on the target. Artillery
armament includes both towed (T) and self-propelled (SP) weapons. A variety of
field artillery ammunition is available including high-explosive (HE),
high-explosive antitank (HEAT), smoke, illumination, white phosphorus (WP),
chemical, nuclear, beehive (flechette), and the improved conventional
munitions (ICM), both antipersonnel and dual-purpose. Various fusing options
such as point-detonating, base-detonating, concrete-piercing, variable-time,
and mechanical-time/superquick increase munition lethality. The Honest John,
Lance, and strategic weapon missile system augment field artillery capability.
    Armor constitutes a basic combat branch of the U.S. Army equipped with
tank-mounting artillery and other weapons, and other supporting units and
vehicles. Armor includes tank units, armored infantry, armored artillery, and
armored engineers. The M60A3 tank is a full-track-laying, heavily armored,
combat vehicle. The main weapon is a 105-mm high-velocity cannon mounted to a
rotatable turret. The gun is used as an armor-penetrating antimateriel weapon,
but can be used against personnel and soft targets. A 7.62-mm machine gun
coaxially mounted with the main gun is used as the primary antipersonnel
weapon. A .50-caliber machine gun is mounted in the cupola, serving as an
antipersonnel and soft target weapon for use in defense against
low-performance aircraft.

 �34s[Micheal E. P. Davis ;Michael R. Mulvihill]
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�6v�2v�32sArm (anatomy) �21m �33s
    The upper, or superior, limb, consisting of the upper arm and forearm. Its
bony framework (see illustration) consists of the humerus in the upper arm and
the two forearm bones, the radius (outer) and ulna (inner). The ulna hinges
with the humerus at the elbow, and the radius rotates on the lower (distal)
end of the ulna, allowing the wrist to turn.
    Two major muscle groups are found in each portion, the extensors and the
flexors. Those of the upper arm act principally on the forearm; those of the
forearm act on the wrist, hand, and fingers. Total arm movements are made by
means of shoulder and trunk muscles.

 �34s[Thomas S. Parsons]
�33s


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�6v�2v�32sAromatic   �21m �33s 
    A term which describes those organic compounds having physical and 
chemical properties resembling those of benzene. This quality, aromaticity, is 
reflected in the stability of these compounds. Although the carbons in 
aromatic compounds are bonded to three atoms rather than four, aromatic 
compounds do not behave chemically like unsaturated compounds. Their stability 
is considered to be the result of a resonating electron structure in which all 
carbon-to-carbon bonds have the same length. Aromatic compounds show a marked 
tendency to undergo substitution rather than addition reactions. 
    There are four main classes of aromatic compounds: benzenoid aromatic 
compounds, containing one benzene ring; polynuclear benzenoid, containing two 
or more fused benzene rings; nonbenzenoid, containing planar cyclic carbon 
rings other than benzene; and those heterocyclic compounds whose properties 
resemble benzene. 
    Aromatic compounds owe their name to their original derivation from 
fragrant natural compounds. See AROMATIC HYDROCARBON; BENZENE; RESONANCE 
(MOLECULAR STRUCTURE). 
 �34s[U.S. Bureau of Reclamation]
�33s 
  
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�6v�2v�32sAromatic hydrocarbon   �21m �33s 
    An organic compound with a chemistry similar to that of benzene. Usually 
aromatic hydrocarbons have structures that contain at least one benzene ring. 
Their properties are so different from those of other hydrocarbons that they 
are considered to be a separate class of compounds, the arenes. Several 
important aromatic hydrocarbons are benzene, naphthalene, toluene, styrene, 
biphenyl, and anthracene. Other species usually classed as aromatic 
hydrocarbons are aromatic ions, for example, the cyclopentadienyl ion; 
benzologs of benzene such as naphthalene; and a nonbenzenoid polycyclic 
aromatic hydrocarbon, azulene. 
    Aromatic hydrocarbons share the aromatic character of benzene and its 
derivatives in that they exhibit diminished unsaturation, ease of formation, 
and stability of the nucleus. Benzene's structure has been the focus of 
attention because the concept of aromatic character has developed in terms of 
benzene and its derivatives. While the Kekule structure showed a ring 
consisting of six carbons with three alternate double bonds, the modern view 
is that all six bonds of benzene are alike and that all are intermediate in 
nature between double and single bonds. See RESONANCE (MOLECULAR STRUCTURE). 
    Aromatic hydrocarbons, and particularly polycyclic aromatic hydrocarbons, 
have the ability to form stable molecular complexes with some polynitro 
compounds. Some of these molecular complexes have proved of value in the 
isolation and identification of aromatic derivatives. Among the most useful 
nitro compounds for this are picric acid, trinitrobenzene, and 
trinitrofluorenone. See ANTHRACENE; BENZENE; CARBORANE; HYDROCARBON; 
NAPHTHALENE; POLYNUCLEAR HYDROCARBON.  
 �34s[Charles K. Bradsher]
�33s 
  
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�6v�2v�32sAromatization �21m �33s
    The conversion of any nonaromatic hydrocarbon structures, especially those
found in petroleum, to aromatic hydrocarbons. There are numerous routes and
means to accomplish this transformation, the simplest and most important of
which are: direct dehydrogenation of naphthenes to aromatics;
dehydroisomerization of naphthenes to aromatics; dehydrocyclization of
aliphatics to aromatics; and high-temperature condensation of hydrocarbons to
aromatics. See AROMATIC HYDROCARBON.
    Beginning in 1948, reforming of naphthas, with catalysts comprising small
amounts of platinum on an acidified alumina support, has provided an important
means of aromatization that has rapidly displaced earlier processes. It is a
major process for making benzene, toluene, and other aromatics from petroleum
sources. See PETROLEUM PROCESSING.
 �34s[Bernard S. Greensfelder /Mott Souders]
�33s

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�6v�2v�32sArrowroot starch �21m �33s
    A food product derived from different plant species. Rhizomes of Maranta
arundinacea of tropical America supply the West Indian arrowroot. Florida
arrowroot is the flour made from the rhizomes of the cycad Zamia floridana,
the Seminole bread plant. Queensland arrowroot is obtained from the edible
rhizomes of Canna edulis. The tubers of Curcuma angustifolia yield the East
Indian arrowroot. Arrowroot starch is of no importance in the industries, but
because it is very easily digested, it is valued highly as a food for infants
and invalids.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sArsenate �21m �33s
    A negative ion having the formula AsO 3 NEGATIVE. Arsenates are derived
from orthoarsenic acid, H3AsO4. Arsenates are quite similar to the phosphates
because the term arsenates also includes metaarsenates and pyroarsenates, as
well as three series of salts from orthoarsenic acid.
    The various arsenates are also very similar to the phosphates in their
solubilities and crystal form. The alkali metal salts are the only soluble
tertiary orthoarsenates. Lead and calcium tertiary salts, Pb3(AsO4)2 and
Ca3(AsO4)2, are among those arsenates used in insecticides, all of which are
poisonous to humans.
    Because of their similarities it is difficult to distinguish between the
PO4 (sup 3 NEG) and AsO4 (sup 3 NEG) ion. However, silver nitrate gives a
chocolate-brown precipitate, Ag3AsO4, with arsenate, whereas the corresponding
phosphate is yellow. See ARSENIC; ARSENITE; PHOSPHATE.
 �34s[E. Eugene Weaver]
�33s

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�6v�2v�32sArsenic �21m �33s
    A chemical element, symbol As, atomic number 33. Arsenic is found widely
distributed in nature (approximately 5 x 10 sup -4 % of the Earth's crust). It
is one of the 22 known elements composed of only one stable nuclide, (75 over
33) As; the atomic weight is 74.92158. There are 17 other radioactive arsenic
nuclides known.
    There are three polymorphic modifications of arsenic. The yellow cubic
ALPHA-form is made by condensing the vapor at very low temperatures. The black
BETA-polymorph is isostructural with black phosphorus. Both these
modifications revert to the stable GAMMA-form, gray or metallic, rhombohedral
arsenic, on heating or exposure to light. The metallic form is a moderately
good thermal and electric conductor and is brittle, easily fractured, and of
low ductility.
    Arsenic is found native as the mineral scherbenkobalt, but generally
occurs among surface rocks combined with sulfur or metals such as Mn, Fe, Co,
Ni, Ag, or Sn. The prinicipal arsenic mineral is FeAsS (arsenopyrite,
mispickel); other metal arsenide ores are FeAs2 (loellingite), NiAs
(nicolite), CoAsS (cobalt glance), NiAsS (gersdorffite), and CoAs2 (smaltite).
Naturally occurring arsenates and thioarsenates are common, and most sulfide
ores contain arsenic. As4S4 (realgar) and As4S6 (orpiment) are the most
important sulfur-containing minerals. The oxide, arsenolite, As4O6, is found
as the product of the weathering of other arsenical minerals, and is also
recovered from flue dusts collected during the extraction of Ni, Cu, and Sn
from their ores; it also results when the arsenides of Fe, Co, or Ni are
roasted in air or oxygen. The element may be obtained by roasting FeAsS or
FeAs2 in the absence of air or by reduction of As4O6 with carbon, when As4 may
be sublimed away.
    Elemental arsenic has few uses. It is one of the few minerals available in
99.9999+ % purity, which is largely used in the laser material GaAs and as a
doping agent in the manufacture of various solid-state devices. Arsenic oxide
is used in glass manufacture. The arsenic sulfides are used as pigments and in
pyrotechnics. Dihydrogen arsenate is used in medicine, as are several other
arsenic compounds. Most of the medicinal uses of arsenic compounds depend on
their toxic nature. See ANTIMONY; PHOSPHORUS.
 �34s[John L. T. Waugh]
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�6v�2v�32sArsenic intoxication   �21m �33s 
    Arsenic is an element that is not essential for human nutrition but is 
normally found in trace amounts in human tissues (less than 0.10 ppm). It has 
been used therapeutically in small doses since ancient Grecian civilization 
for a variety of disorders, ranging from skin rashes and anemia to cancer. 
During the 19th century, organic arsenicals, such as arsphenamine, were used 
in the treatment of syphilis. In higher doses arsenic compounds are extremely 
poisonous and have been used for herbicides, insecticides, rodenticides, and 
homicides.  
    The biological effects of arsenicals vary because of differences in 
chemical form, solubility, dose, rate of exposure, and route of intake. In 
general, arsenic compounds are protoplasmic poisons that block cell and tissue 
respiration. Arsenicals also paralyze smooth muscle, disrupt the continuity of 
capillary endothelium, causing many small hemorrhages, and block cell division. 
    Arsenic hydride (AsH3, arsine) is a highly toxic gaseous form. Its 
inhalation even in small amounts is the commonest form of industrial arsenic 
poisoning. Acute poisonings are usually characterized by profound shock due to 
dilatation of the peripheral blood vessels and extreme reduction in blood 
flow. These compounds also attack the central nervous system, paralyzing the 
central control of the cardiovascular system. 
    The symptoms in acute poisonings are severe abdominal pain and vomiting, 
usually shortly after ingestion, and a feeling of giddiness, headache, extreme 
general weakness, and diarrhea. Those individuals who survive longer may 
develop an exfoliation of the skin and aches and pains in the extremities. The 
lesions produced by chronic poisoning are much more subtle and involve the 
gastrointestinal tract, nervous system, and skin. Depending on the dose, the 
lesions may be similar to those in acute poisoning, although less severe. 
    Arsenic has been identified as a carcinogenic element. With very prolonged 
low-dose exposure, cancers of the skin or internal organs may develop. The 
skin cancers begin as a series of focal overgrowths of the skin cells called 
hyperkeratoses at multiple sites on the body, particularly on the palms of the 
hands and soles of the feet. After many years of growth some of these 
keratotic lesions will evolve into cancers. The commonest skin sites are the 
urogenital organs, mouth, esophagus, and respiratory tract. Internal cancers 
often involve the endocrine organs. See ARSENIC; METALS, PATHOLOGY OF; 
ONCOLOGY.  
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sArsenite �21m �33s
    A negative ion having the formula AsO (sup 3 NEGATIVE). Arsenites are
derived from arsenious acid. Arsenious acid is the name given to aqueous
solutions of As4O6 although the pure acid has never been isolated. These
solutions are weakly acidic and can be neutralized by bases to give a variety of
ortho- and meta-arsenites. Paris green, Cu2(C2H3O2)(AsO3), is used as an
insecticide and Scheele's green, CuHAsO3, is used as a pigment. See ARSENATE;
ARSENIC.
 �34s[E. Eugene Weaver]
�33s

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�6v�2v�32sArsenopyrite   �21m �33s 
    A mineral having composition FeAsS and crystallizing in the monoclinic 
system. Crystals have pseudo-orthorhombic symmetry because of twinning. The 
Mohs hardness is 5.5 - 6.0, and the specific gravity is 6.0. The luster is 
metallic and the color silver-white. Arsenopyrite is the most widespread 
arsenic-bearing mineral. It is commonly found in veins containing gold (Lead, 
South Dakota; Deloro, Ontario), tin or tungsten minerals (Bolivia; Cornwall, 
England), or nickel-cobalt-silver minerals (Cobalt, Ontario; Freiberg, 
Germany). See ARSENIC. 
 �34s[Lawrence Grossman]
�33s 
  
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�6v�2v�32sArteriosclerosis   �21m �33s 
    The name given a group of degenerative diseases of arteries characterized 
by thickening and hardening of their walls. The group includes three types of 
lesions: (1) atherosclerosis involves the aorta and its major branches; (2) 
medial sclerosis involves the muscular arteries of the legs; and (3) 
arteriolosclerosis involves the small branches of the arterial tree, called 
the arterioles. 
    Atherosclerosis is by far the most common and important form of 
arteriosclerosis, and the two terms are often used interchangeably. Some 
degree of atherosclerosis is found at autopsy in persons of all ages, even 
children, but it increases in severity and extent with advancing years. It is 
more common in males than in females, especially females before the age of 
menopause. After menopause the sex incidence and severity are about equal. 
Atheromas (anatomic lesions) begin with the lining layer of the aorta or its 
branches and subsequently extend into the middle layer of these vessels. As 
the lesions enlarge, a plaque forms, and fibrin clots are deposited on the 
surface, diminishing the diameter of the lumen of the vessel. 
    Atherosclerosis has global distribution and now occurs in virtually 
epidemic proportions in the industrialized nations of Western civilization. 
There is an extremely high prevalence and severity of the lesions in the 
Northern European countries, the United States, Australia, and New Zealand. 
There is a relatively low occurrence in economically deprived areas such as 
India, Africa, and South America. Factors associated with the high incidence 
and severity of atheromas include a high total caloric intake, high fat 
intake, sedentary living, aggressive personality, emotional stress, and 
cigarette smoking. Hypertension (high blood pressure) does not induce 
arteriosclerosis, but it augments its development and accelerates the progress 
of the disease if it is present. Although virtually ubiquitous, 
atherosclerosis fortunately does not cause serious disease symptoms in most 
individuals. However, in the United States over 60% of the deaths each year 
are due to cardiovascular disease produced by atherosclerosis and its 
sequelae. Although the vast majority of cases of atherosclerosis appear to be 
principally caused by environmental factors, there are some specific genetic 
defects associated with the genesis of the process. Primary diabetes mellitus 
patients develop severe arteriosclerosis at an earlier age than nondiabetic 
individuals do. Familial hypercholesterolemia, homocystinuria, and 
hypothyroidism are other examples of metabolic defects associated with 
arteriosclerosis.  
    Medial sclerosis is an uncommon type of arteriosclerotic lesion of little 
clinical significance. It affects the arteries of the arms, legs, and genital 
tracts of both sexes equally. It rarely occurs under the age of 50, and its 
cause is obscure. The disorder is characterized by ringlike calcifications 
within the media (middle layer) of affected vessels. The endothelial lining of 
the vessel is not altered, and the size of the lumen not impinged upon. 
Therefore, occlusion of the vessel seldom occurs. 
    Arteriolosclerosis is an increased generalized thickening of the walls of 
arterioles related to hypertension. The change is often most prominent in the 
kidneys, although other internal organs may be similarly affected. Patients 
with diabetes mellitus have an increased incidence of the lesion. 
 �34s[N. Karle Mottet]
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�6v�2v�32sArteritis   �21m �33s 
    A general term denoting an inflammatory process in arterial walls; it may 
be due to various bacterial infections, syphilis, allergic or hypersensitivity 
states, exceptionally elevated blood pressure, or unknown factors. Arteries 
situated in regions of localized bacterial infection are frequently involved 
in the inflammatory process. Such vessels may become thrombosed or weakened to 
the point of rupture with resulting hemmorhage. Syphilitic arteritis occurs 
primarily in the proximal or thoracic portion of the aorta. Allergic or 
hypersensitivity arteritis may be due to drugs or unknown factors. This type 
of arteritis includes such diseases as periarteritis nodosa, temporal 
arteritis, lupus erythematosus disseminata, and rheumatic arteritis. This 
group shows alterations in the vessel wall characterized by inflammation, 
necrosis, and often thrombosis. Thrombosis with occlusion is a common 
complication which results in regions of infarction or necrosis of organ 
tissue due to lack of blood supply. Aneurysms often form in the weakened 
arteries and frequently rupture, causing fatal hemorrhage. See ANEURYSM; 
HEMORRHAGE; LUPUS ERYTHEMATOSUS; SYPHILIS; THROMBOSIS. 
 �34s[Frank A. Carone]
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�6v�2v�32sArtery   �21m �33s 
    Any of the vascular tubes which carry blood away from the heart. The 
arteries may be divided into four or five categories, depending upon their 
diameter and proportions of different tissues present in their walls. All 
arteries have a smooth endothelial lining, which is quite thin in children but 
gradually thickens with advancing age. See ARTERIOSCLEROSIS. 
    The largest arteries - the aorta and the pulmonary with their major 
branches - have an ill-defined zone of elastic fibers around the endothelium 
and its supporting tissue. The middle coat, or tunica media, of these arteries 
is composed of some smooth muscle, but the predominant tissue is elastic 
fibers, giving these elastic arteries a yellowish hue in cross section. The 
outer coat, or adventitia, is a connective tissue containing nerve, 
collagenous and elastic fibers, and small nutrient blood vessels, the vasa 
vasorum.  
    The medium-sized arteries, also known as muscular arteries, have a 
red-grayish hue in cross section because of the preponderance of smooth 
muscles in the tunica media. The muscular layer is bounded internally and 
externally by an elastic membrane. The outer layer, the adventitia, contains a 
plentiful supply of nerves from the autonomic system that innervate the smooth 
muscles.  
    The smaller arteries and arterioles are marked by their small size and 
progressive loss of first the inner and then the outer elastic membrane, and 
finally the muscular layer in the smallest vessels. The smallest arterioles, 
found just before the capillaries, are about 20 micrometers or less in 
diameter, and consist only of the universal endothelium and a thin layer of 
smooth muscles. These metaarterioles are still richly innervated, as they are 
important in the final regulation of blood to the capillary beds. See 
CAPILLARY (ANATOMY). 
    The terminal ramification of arteries is also connected to veins by direct 
arteriovenous anastomoses in many parts of the body. These anastomoses contain 
a well-innervated layer of smooth muscles by which the diameter of their lumen 
may be regulated. They apparently serve to regulate local blood flow and 
pressure by allowing blood to bypass the capillary beds and flow directly into 
the veins. See AORTA; BLOOD VESSELS; CIRCULATORY SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sArtesian systems �21m �33s
    Groundwater conditions formed by water-bearing rocks (aquifers) in which
the water is confined above and below by impermeable beds. Because the water
table in the intake area of an artesian system is higher than the top of the
aquifer in its artesian portion, the water is under sufficient head to cause
it to rise in a well above the top of the aquifer. Many of the systems have
sufficient head to cause the water to overflow at the surface, at least where
the land surface is relatively low. Flowing artesian wells were extremely
important during the early days of the development of groundwater from drilled
wells, because there was no need for pumping. Their importance has diminished
with the decline of head that has occurred in many artesian systems and with
the development of efficient pumps and cheap power with which to operate the
pumps. See GROUNDWATER HYDROLOGY.
 �34s[Albert N. Sayre /Ray K. Linsley]
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�6v�2v�32sArthritis   �21m �33s 
    A group of diseases affecting joints of their component tissues. Several 
types of arthritis are recognized, and these can be divided into four groups 
by their clinical course or pathologic appearance: inflammatory arthritis, 
degenerative joint disease, nonarticular rheumatism, and miscellaneous 
arthritis.  
    Inflammatory arthritis is a group of diseases characterized by 
inflammation of joint fluid and of synovial tissue and other supporting 
tissues surrounding diarthrodial and fibrocartilaginous joints. Connective 
tissue diseases, crystal deposition diseases, infectious arthritis, and 
spondyloarthropathies are examples of inflammatory arthritis. Connective 
tissue diseases are a group of acute and chronic diseases characterized by 
involvement of joints, connective tissue, serosal membranes, and small blood 
vessels. These diseases are divided into acquired disorders (rheumatoid 
arthritis, systemic lupus erythematosus, scleroderma, polymyositis, 
vasculitis) and rare hereditary diseases (Ehlers-Danlos syndrome). Rheumatoid 
arthritis is the most common variety of inflammatory arthritis occurring in 
younger and middle-aged persons and is characterized by uninfected 
inflammation of the synovium, frequently associated with extraarticular 
manifestations. The systemic disease follows a variable but slowly progressive 
course, marked by spontaneous remissions and exacerbations. The finger joints 
are often first involved symmetrically, with the hands, wrists, feet, and 
other smaller joints later becoming affected. Joint destruction is common with 
advancing disease, so that the incidence of disability is high. There are 
three groups of crystal deposition disease based on the type of crystal 
involvement: monosodium urate (gout), calcium pyrophosphate (pseudogout), and 
hydroxyapatite (calcific tendonitis). Gout, the metabolic disorder of uric 
acid breakdown, has long been associated with a violent arthritis, 
particularly of the toes. Infectious arthritis is an inflammatory joint 
disease caused by the invasion of synovial membrane by living microorganisms, 
including gonorrheal, streptococcal, and staphylococcal bacteria. These 
usually result from a generalized infection but may appear following local 
spread or after trauma. The outcome varies considerably with the patient's age 
and resistance, adequacy of the treatment, and the nature of the organism. 
Another group of inflammatory arthritis characterized by involvement of the 
axial (central) skeleton is spondyloarthropathy. Ankylosing spondylitis, 
Reiter's syndrome, spondylitis associated with inflammatory bowel diseases, 
and psoriasis are examples of the spondyloarthropathies. See GOUT; LUPUS 
ERYTHEMATOSUS.  
    Degenerative joint disease (osteoarthritis) is a ubiquitous joint disease 
characterized pathologically by deterioration of articular cartilage and new 
bone formation in the subchondral areas. The disease is thought to be inherent 
in the aging process, being very common in older persons. In addition to 
aging, a variety of other factors are involved in pathogenesis such as joint 
injury, dysplasia, and other forms of underlying arthritis. Degenerative joint 
disease is marked by a progressive stiffness, loss of function, and 
destruction of the larger, weight-bearing joints of the body. With advancing 
age, the continued slow damage causes increasing disability. 
    The nonarticular rheumatism group includes tendonitis, bursitis, 
tenosynovitis, fibrositis, and psychogenic rheumatism. Calcific tendonitis and 
bursitis usually follow local trauma, muscle strain, or excessive exercise and 
can result from deposition of hydroapatite crystals (crystal deposition 
disease). Fibrositis is defined as a complex of musculoskeletal complaints 
frequently involving neck, shoulder girdle, and extremities. Fibrositis and 
psychogenic rheumatism both are thought to have a psychic origin. See 
RHEUMATISM.  
    Miscellaneous forms of arthritis include those systemic diseases of 
unknown or quite different etiology which may produce arthritis or joint 
destruction. There are neurologic, blood, and endocrine examples of these 
unusual rheumatic diseases. 
    These and less common forms of arthritis affect an estimated 11,000,000 
persons in the United States. With an increasing survival time in the 
population, arthritis constitutes one of the greatest medical, social, and 
economic problems in existence.   
 �34s[Robert Searles]
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�6v�2v�32sArthrodira �21m �33s
    The best-known, most varied, and most successful order of Placodermi,
found in freshwater and marine sediments of Devonian and lowest Carboniferous
age in all continents except South America. The most primitive Arthrodira are
the Actinolepidae of Lower and Middle Devonian age. Similar to them are the
Phlyctaeniidae, from which arose a number of families of intermediate
advancement, particularly characteristic of the Middle Devonian but surviving
into the Upper Devonian. The best known of these are the Coccosteidae (see
illustration); others are the Homosteidae and the Brachydeiridae. One family
which reached high specialization early is the Heterosteidae of the Middle
Devonian. The most specialized families, grouped in the suborder Pachyosteina,
are mostly of Upper Devonian age. Included here are the large, predacious
Dinichthyidae; the huge, flat-bodied, bottom-dwell ing Titanichthyidae; the
large-eyed Selenosteidae; and the Mylostomatidae, with jaws adapted for
crushing hard-shelled invertebrates. See PLACODERMI.

     FIGURE CAPTION.  Coccosteus, a small arthrodire (about 16 in. or 40 cm
long) of the Middle and Late Devonian.

 �34s[Robert H. Denison]
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147ATerri 3-2-82

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�6v�2v�32sArthropoda   �21m �33s 
    The largest phylum in the animal kingdom, comprising about 75% of all 
animals that have been described. The estimated number of arthropod species is 
between 1,000,000 and 2,000,000. Of this number the class Insecta alone 
contains 750,000 - 1,000,000 species. Arthropods vary in size from the 
microscopic mites to the giant decapod crustaceans, such as the Japanese crab 
with an appendage span of 5 ft (1.5 m) or more. This phylum includes the 
well-known insects, spiders, ticks, and crustaceans, as well as many smaller 
groups, some of which are known as fossils. 
    The adult arthropod typically has a body composed of a series of ringlike 
segments, muscularly movable on each other. The integument is sclerotized by 
the formation of hardening substances in the cuticle, and the segmental limbs 
are many-jointed. These characteristics, taken together, distinguish the 
arthropods from all other animals. Young stages may be quite different from 
the adults, and some parasitic species differ very radically from their 
relatives.  
    In their evolution the arthropods have diverged structurally into three 
major groups, the Trilobitomorpha, Chelicerata, and Mandibulata. A 
classification scheme of the Arthropoda follows: 
Phylum Arthropoda 
    Subphylum Trilobitomorpha (including Trilobita) 
    Subphylum Chelicerata 
        Class: Merostomata 
               Pycnogonida 
               Arachnida 
    Subphylum Mandibulata 
        Class: Crustacea 
               Pauropoda 
               Symphyla 
               Diplopoda 
               Chilopoda 
               Insecta (Hexapoda) 
    Body segmentation, or metamerism, is the most fundamental character of the 
arthropods, but it is shared by the annelid worms, so there can be little 
doubt that these two groups of animals are related through some remote, 
simple, segmented, wormlike ancestor. Modern arthropods possess segmental 
limbs, highly modified and more specialized than those of any other animals, 
that serve for walking, feeding, grasping, swimming, silkspinning, egg laying, 
and sperm transfer. The body segments, corresponding to specialized sets of 
appendages, tend to be consolidated or united in groups, or tagmata, forming 
differentiated body regions, such as head, thorax, and abdomen. 
    The arthropods have all the internal organs essential to any complex 
animal. An alimentary canal extends either straight or coiled from the 
subapical ventral mouth to the terminal anus. The nervous system includes a 
brain and a subesophageal ganglion in the head, united by connectives around 
the stomodeum, and a ventral nerve cord of interconnected ganglia. A usually 
tubular pulsatory heart lies along the dorsal side of the body and keeps the 
blood in circulation. In some arthropods arteries distribute the blood from 
the heart; in others it is discharged from the anterior end of the tube 
directly into the body cavity. The blood reenters the heart through openings 
located along its sides. Aquatic arthropods breathe by means of gills. Most 
terrestrial species have either flat air pouches or tubular tracheae opening 
from the outside surface; some have both. A few small, soft-bodied forms 
respire through the skin. Excretory organs open either at the bases of some of 
the appendages or into the alimentary canal. Most arthropods have separate 
sexes, but some are hermaphroditic, and parthenogenesis is of common 
occurrence. The genital openings differ in position in different groups and 
are not always on the same body segment in the two sexes. See CHELICERATA; 
MANDIBULATA; TRILOBITOMORPHA. 
 �34s[Jerome G. Rozen, Jr.]
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�6v�2v�32sArthus reaction   �21m �33s 
    An allergic reaction of the immediate hypersensitive type. The classical 
Arthus reaction in a sensitized rabbit injected with the appropriate allergen 
is a localized skin reaction characterized by swelling (edema) and 
inflammation. This reaction differs from other immediate hypersensitive states 
because larger quantities of circulating antibody (and antigen ) are required 
for its elicitation, and because on passive transfer the recipient becomes 
immediately reactive, without the incubation period required in passive 
transfer of anaphylaxis or atopy. The Arthus reaction becomes visible within 1 
- 2 h after injection of the antigen, reaches its peak in 8 - 12 h, and may 
persist for 24 h. If the reaction is sufficiently severe, local damage to 
blood vessels may lead to necrosis due to deficient blood supply; this degree 
of damage is never seen in the hive, which a local anaphylactic reaction. The 
Arthus reaction also requires the participation of complement, components of 
which call forth the leukocytes that make up part of the inflammatory and 
necrotizing reaction. The sequestration of complexes of antibody, antigen, and 
complement in the basement membranes of small vessels, such as those of the 
kidney glomerulus, can also cause systemic disease such as glomerulonephritis 
and periarteritis nodosa. See ANAPHYLAXIS; ATOPIC ALLERGY; HYPERSENSITIVITY; 
IMMUNOLOGY; IMMUNOPATHOLOGY.  
 �34s[Sidney Raffel]
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�6v�2v�32sArctic and subarctic islands �21m �33s
    Defined primarily by climatic rather than latitudinal criteria, arctic
islands are those in the Northern Hemisphere where the mean temperature of the
warmest month does not exceed 50 DG F (10 DG C) and that of the coldest is not
above 32 DG F (0 DG C). Subarctic islands are those in the Northern Hemisphere
where the mean temperature of the warmest month is over 50 DG F (10 DG C) for
less than 4 months and that of the coldest is less than 32 DG F (0 DG C). Such
islands generally are in high latitudes. Distribution of land and sea masses,
ocean currents, and atmospheric circulation greatly modifies the effect of
latitude so that it is often misleading to use location relative to the Arctic
Circle as a significant criterion of arctic or subarctic. The largest
proportion by area of the islands lies in the Western Hemisphere, primarily in
Greenland and in the Canadian Arctic Archipelago. Within this general
description, individual islands vary considerably (see table).
    Physiographically, the islands include all the varied major landforms
found elsewhere in the world, from rugged mountains over 8000 ft (2500 m)
high, through plateaus and hills, to level plains only recently emerged from
the sea. All have been glaciated except Sakhalin and some of the islands in
the Bering Sea sector. Removal of the weight of ice sheets and the resultant
crustal rebound has exposed prominent marine beaches and wave-cut cliffs on
many of the islands. These now commonly occur at elevations of over 300 ft
(150 m) above sea level.
    The general climatic pattern of these islands is set by their location
relative to the two semipermanent centers of low pressure over the Aleutian
Islands and over Iceland. Most of the precipitation is cyclonic in origin.
Because they are marine areas, the islands receive more precipitation than
they other wise would, yet even so this is very light for most of the arctic
islands removed from the zone of cyclonic activity. Also, because they are
marine areas, the islands, regions of low temperatures by definition, are not
regions of extreme low temperatures. In general, the larger the island and the
closer its proximity to a continental landmass, the higher are the summer
temperatures and the lower its winter temperature. See POLAR METEOROLOGY.
    The climatic differences between arctic and subarctic islands are
reflected in their natural vegetation. The arctic islands are treeless.
Natural vegetation consists of the tundra - mosses, sedges, lichens, grasses,
and creeping shrubs. Bare gound is often exposed and in places plant growth
may be lacking completely except for a few rock-encrusting lichens. In such
places the ground surface may consist of frost-shattered rock fragments, tidal
mud flats, boulder-strewn fell fields, or snow patches and ice. Permafrost
(permanently frozen ground) occurs throughout the Arctic (and in parts of the
subarctic) and is reflected in impeded drainage and patterned ground. See
PERMAFROST; TUNDRA.
    The natural vegetation of subarctic islands characteristically is the
boreal forest or taiga, composed predominantly of conifers such as spruce,
fir, pine, and larch with deciduous trees such as birch, aspen, and willow;
the latter are especially common in regrowth of clearings in the forest.
Impeded drainage because of permafrost or glaciation gives rise to numerous
ponds and muskeg areas. A transitional type of vegetation, the forest-tundra,
is recognized on some subarctic islands in sectors where smaller trees are
widely spaced and abundant mosses cover the ground. See MUSKEG; TAIGA.
    The typical soils of the subarctic islands are podzols - the grayish-white
surface soil beneath the raw humus layer and highly acidic in nature. The
tundra soils of the arctic islands really consist only of a dark-brown peaty
surface layer over poorly defined thin horizons, and much of the ground cannot
properly be termed soil.

     TABLE CAPTION.  Size of larger arctic and subarctic islands*

 �34s[William C. Wonders]
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�6v�2v�32sArtichoke   �21m �33s 
    Cynara scolymus, a herbaceous perennial plant, in the family Compositae; 
also called globe artichoke. Its origin is in the Mediterranean region. 
Artichoke requires a mild winter and cool summer with fog and little bright 
sunshine. It is a delicacy in Europe, Africa, and North and South America. 
Artichoke is also a medicinal plant; it is rich in the cynarin and 
ortho-phinol constituents. In the United States, artichokes are grown in the 
Pacific Coast area between south San Francisco and Los Angeles, mainly 
Monterey County. 
    The marketable portion of the plant, the so-called bud, is actually the 
immature flower head, made up of numerous closely overlaid bracts or  scales 
(see illustration). The edible portion consists of the tender bases of the 
bracts, the young flowers, and the receptacle or fleshy base upon which the 
flowers are borne. The bud can be various shapes, from round to oblong to 
flat, and the color can be light green to dark green, often with purple or red. 
 �34s[Aly M. Ibrahim]
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�6v�2v�32sArticulata (Brachiopoda) �21m �33s
    A class of the Brachiopoda, which includes about 92% of the known genera
of the phylum. It is presently divided into six orders: Orthida,
Terebratulida, Strophomenida, Pentamerida, Rhynchonellida, and Spiriferida. A
seventh order, the Kutorginida, is currently unplaced in either the Articulata
or the Inarticulata. Although there is no universal agreement on the
phylogenetic relationships of the articulate orders, there is little doubt
that the Orthida are the oldest stock, appearing in the Early Cambrian, and
that ultimately these are the ancestors of all articulate brachiopods. See
ORTHIDA; PENTAMERIDA; RHYNCHONELLIDA; SPIRIFERIDA; STROPHOMENIDA;
TEREBRATULIDA.
    One of the characteristic features of the class is its shell
microstructure. The basic pattern, found in the earliest orthides and typical
of the remainder of the class, with the exception of the strophomenidines, is
in two distinct layers of calcite. A thin outer, primary layer secreted
extracellularly by a narrow band of columnar epithelium at the mantle margins
is underlain by a thicker development of a secondary layer of fibrous calcite,
secreted by the remainder of the mantle. The fibers in the latter layer are
deposited intracellularly and are bounded by cytoplasmic sheaths in continuity
with the cell walls. Articulation is typically present in the class. A pair of
teeth are developed as dorsally directed protuberances, articulating with
sockets in the brachial valve. These structures may be replaced by a row of
denticles, as in some strophomenidines, or lost completely as in the majority
of the productidines. Even when articulation by teeth and sockets is absent,
the musculature develops to allow rotation about a hinge axis. The valves are
closed by contraction of the adductor muscles, opening being affected by
contraction of the diductor muscles. Other muscles are typically present which
control movement of the shell relative to the pedicle. See BRACHIOPODA.
 �34s[A. J. Rowell]
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�6v�2v�32sArticulata (Echinodermata) �21m �33s
    The only surviving subclass of the Crinoidea, differentiated during
Triassic times. The calyx is dicyclic, but considerable reduction of the
infrabasals and basals may occur. The uniserial arms bear pinnules and usually
branch, and the arm retains its movable articulation with the radial plate,
despite the incorporation of the lower brachial ossicles into the calyx.
Extant stalked forms with nodal rings of cirri (Metacrinus) are included in
the order Isocrinida (see illustration). They do not tolerate turbulent waters
and live at depths below current action, although they do inhabit shallow
water when the conditions are suitable. The feather stars, of the order
Comatulida, discard the stem when young, and thereafter remain free, either as
swimming animals or as creeping benthic forms. They prefer shallow, clear
water, rich in nutrients, and therefore abound on tropical coasts and in polar
seas rather than in temperate waters. Four other orders have been defined. See
CRINOIDEA; ECHINODERMATA.

 �34s[Howard B. Fell]
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�6v�2v�32sArtificial intelligence   �21m �33s 
    The subfield of computer science concerned with understanding the nature 
of intelligent action and constructing computer systems capable of such 
action. It embodies the dual motives of furthering basic scientific 
understanding and making computers more sophisticated in the service of 
humanity.  
    Many activities involve intelligent action - problem solving, perception, 
learning, symbolic activity, creativity, language, and so forth - and therein 
lies an immense diversity of phenomena. Scientific concern for these phenomena 
is shared by many fields, for example, psychology, linguistics, and philosophy 
of mind, in addition to artificial intelligence. This implies no 
contradiction, the phenomena being rich enough to support many starting points 
and diverse perspectives. The starting point for artificial intelligence is 
the capability of the computer to manipulate symbolic expressions that can 
represent all manner of things, including the programs of action of the 
computer itself. 
    The approach of artificial intelligence is largely experimental, although 
it contains some general principles and small patches of mathematical theory. 
The unit of experimental investigation is the computer program. New programs 
are created as probes to explore and test new ideas about how intelligent 
action might be attained. 
    The foundations of artificial intelligence are divided into 
representation, problem-solving methods, architecture, and knowledge. The four 
together are necessary ingredients of any intelligent agent. To work on a 
task, a computer must have an internal representation in its memory, for 
example, the symbolic description of a room for a moving robot, or a set of 
algebraic expressions for a program integrating mathematical functions. The 
representation also includes all the basic programs for testing and measuring 
the structure, plus all the programs for transforming the structure into 
another one in ways appropriate to the task. The representation used for a 
task can make an immense amount of difference, turning a problem from 
impossible to trivial. 
    Given the representation of a task, a method must be adopted that has some 
chance of accomplishing the task. Artificial intelligence has gradually built 
up a stock of relevant problem-solving methods (the so-called weak methods) 
that apply extremely generally: generate-and-test (a sequence of candidates is 
generated, each being tested for solutionhood); heuristic search (sequences of 
operations are tried to construct a path from an initial situation to the 
desired one); hill climbing (a measure of progress is used to guide each 
step); means-ends analysis (the difference between the desired situation and 
the present one is used to select the next step); operator subgoaling (the 
inability to take the desired next step leads to a subgoal of making the step 
feasible); planning by abstraction (the task is simplified, solved, and the 
solution used as a guide); and matching (the present situation is represented 
as a schema to be mapped into the desired situation by putting the two in 
correspondence). An important feature of all the weak methods is that they 
involve the process of search. 
    A general intelligent agent has multiple means for representing tasks and 
dealing with them. Also required is an operating frame within which to select 
and carry out these activities. Often called the executive or control 
structure, it is best viewed as a total architecture (as in computer 
architecture), that is, a machine that provides data structures, operations on 
those data structures, memory for holding data structures, accessing 
operations for retrieving data structures from memory, a programming language 
for expressing integrated patterns of conditional operations, and an 
interpreter for carrying out programs. Any digital computer provides an 
architecture, as does any programming language. Architectures are not all 
equivalent, and the scientific question is what architecture is appropriate 
for a general intelligent agent. 
    In artificial intelligence, this question has taken the form of 
determining what language is good for programming artificial intelligence 
systems. Although the question is seemingly about research tools, in reality 
its investigation has been a search for the properties that make intelligence 
possible. The main development has been that of list-processing languages, 
which embody a general homogeneous and flexible notion of symbols and symbolic 
structure. LISP, one of the early list-processing languages, has evolved until 
it functions as the common language for artificial intelligence programs. 
    The basic paradigm of intelligent action is that of search through a space 
(called the problem space) for a goal situation. The search is combinatorial, 
each step offering several possibilities, leading to a cascading of 
possibilities in a branching tree. What keeps the search under control is 
knowledge, which tells how to choose or narrow the options at each step. Thus 
the fourth fundamental ingredient is how to represent knowledge in the memory 
of the system so it can be brought to bear on the search when relevant. 
    A general intelligent system will have immense amounts of knowledge. This 
implies a major problem, that of discovering the relevant knowledge as the 
solution attempt progresses. This is the second problem of search that 
inevitably attends an intelligent system. Unlike the combinatorial explosion 
characteristic of the problem space search, this one involves a fixed, though 
large, data base, whose structure can be carefully tailored by the 
architecture to make the search efficient. This problem of encoding and access 
constitutes the final ingredient of an intelligent system. 
    Three examples will provide some flavor of research in artifical 
intelligence: (1) All the ingredients of an artificial intelligence program 
can be seen in game programs. A chess program must have a representation of 
the current chess position. Besides the bits that describe the position, there 
must be procedures for analyzing the position and for making the moves to 
create new legal positions. The basic method used for chess is to generate the 
moves and test their worth. The test is a form of heuristic search that 
explores the consequences of a candidate move many potential moves into the 
future. This search is combinatorial and much too big to accomplish without 
the aid of knowledge. The main technique for bringing knowledge to bear is the 
evaluation function, which can be applied to a position to estimate directly 
its chance of being on a winning path. Many features of the position are 
calculated, each representing a bit of chess knowledge (for instance, queens 
are better than pawns). Such value functions are heuristic, only approximating 
a correct analysis. Evaluation turns the search into hill climbing. Not all 
the knowledge stems from the evaluation functions; many subtleties of the 
search allow great improvement. (2) Perception is the formation, from a 
sensory signal, of an internal representation suitable for intelligent 
processing. Though there are many types of sensory signals, computer 
perception has focused on vision and speech. Perception might seem to be 
distinct from intelligence, since it involves incident time-varying continuous 
energy distributions prior to interpretation in symbolic terms. However, all 
the same ingredients occur: representation, search, architecture, and 
knowledge. Thus, perception by computers is a part of artificial intelligence. 
(3) Extensive experience enables humans to exhibit expert performance in many 
tasks, even though no firm scientific or calculational base exists and the 
knowledge does not exist in any explicit form. So-called expert artificial 
intelligence programs attempt to exhibit equivalent performance by acquiring 
and incorporating the same knowledge that the human expert has. An example is 
MYCIN, developed at Stanford University, which makes judgments on the 
diagnosis of bacterial infection in patients and proposes courses of therapy 
with antibiotics. 
 �34s[Allen Newell]
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�6v�2v�32sArtificial voice   �21m �33s 
    A device, consisting of a small loudspeaker in a baffle, whose shape is 
proportioned to simulate the acoustical constants of the human head. It is 
used as a sound source in measuring the characteristics of microphones 
intended for use in close proximity to the lips of a talker. Since these types 
of microphone (often referred to as close-talking microphone) are used in the 
near sound field produced by a person's mouth, it is necessary when measuring 
their performance to utilize a sound source which approximates the 
characteristics of the human sound generator. See MICROPHONE. 
    Ideally, an artificial voice should be capable of reproducing speech 
signals or frequencies without distortion over the range of intensities 
produced by the human voice. Also, the distribution of the sound field about 
the source should be the same as that of the human source. Finally, when a 
microphone is placed in the near sound field of the artificial voice, reaction 
on the output of the voice and the distortion of the field should be the same 
as that produced by the same microphone placed in front of a talker's lips. 
Two types of artificial voice appear to have gained general acceptance: a 
hemispherical artificial voice and a voice simulator. 
    The hemispherical artificial voice consists of a loudspeaker driver unit 
acoustically coupled to a hemispherical baffle. The throat of the artificial 
voice is acoustically damped to provide a smooth response in the frequency 
pass band of interest. The voice simulator, an electronic dummy, is a complete 
replica of the head and torso of the average United States astronaut. The head 
consists of a rigid skull of polyester-impregnated fiber glass covered with 
Plastisol flesh which simulates the acoustical and mechanical properties of 
real flesh. Besides the artificial voice, the head also contains two 
artificial ears. The device is mounted on a base which contains amplifiers and 
power supplies. 
 �34s[Michel Copel]
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�6v�2v�32sArtiodactyla �21m �33s
    An order of mammals which were derived from late Paleocene condylarths and
which gradually became one of the most important orders of terrestrial
herbivores. By the later Tertiary artiodactyls had forced the previously
dominant perissodactyls into a role of secondary importance as herbivores.
Living representatives of the artiodactyls include such well-known animals as
pigs, peccaries, hippopotamuses, camels, llamas, deer and allies, giraffes,
okapi, pronghorn ``antelope,'' bison, buffaloes, antelopes, gazelles, cattle,
goats, sheep, and many others. The order also includes numerous extinct
species. See PERISSODACTYLA.
    The ordinal name, Artiodactyla, refers to the fact that most modern and
fossil species of the order have an even number of toes. The most
characteristic feature of all artiodactyls is that the main axes of the limbs
pass between the third and fourth toes, which are strong and usually equal in
size (Fig. 1). The first of the five toes is completely reduced, and the
second and fifth (the outer toes) when present (lost in advanced forms) are
smaller than the third and fourth toes. Probably the most important structural
innovation which was responsible for the successful radiation of the
artiodactyls is the diagnostic astragalus, the ankle bone. The surface of the
astragalus on which the tibia articulates is deeply grooved, restricting
lateral motion of the hindfoot, whereas the pulley surface below the
astragalus which articulates with the navicular and cuboid (these two bones
fuse in several advanced forms), allow swift, free antero-posterior motion
with an ability for springing off the ground.
    In the living ruminants the stomach is subdivided into three or four
chambers, whereas in the more primitive pigs and hippopotamuses the stomach is
less advanced, that is, not specialized for rumination. Integumentary glands
are common in artiodactyls. Many of these glands are odoriferous and are found
on the abdominal and inguinal regions, the back, feet, and tail, and in front
of the eyes. As expected, with such variety of scent glands used for marking,
artiodactyls have a very keen sense of smell. Thus a great part of artiodactyl
biology, from mating and territory marking to predator detection, depends on
scent recognition. The essentially terrestrial artiodactyls (no known species
is completely arboreal or aquatic) are, as a rule, gregarious. See SCENT
GLAND; TERRITORIALITY.
    A classification scheme for the Artiodactyla is listed below.
Order Artiodactyla
    Suborder Palaeodonta
	Superfamily Dichobunoidea
	Family Diacodectidae
	Family Dichobunidae
	Family Homacodontidae
	Family Leptochoeridae
	Family Achaenodontidae
	Superfamily Entelodontoidae
	Family Entelodontidae
	Family Choeropotamidae
	Family Cebochoeridae
	Superfamily Suoidae
	Family Suidae
	Family Tayassuidae
	Superfamily Anthracotherioidea
	Family Anthracotheriidae
	Family Hippopotamidae
    Suborder Ruminantia
	Infraorder Tylopoda
	Superfamily Merycoidodontoidea
	Family Merycoidodontidae
	Family Agriochoeridae
	Superfamily Cainotheriodea
	Family Cainotheriidae
	Superfamily Anaplotherioidea
	Family Anaplotheriidae
	Family Xiphodontidae
	Superfamily Amphimerycoidea
	Family Amphimerycidae
	Superfamily Cameloidea
	Family Oromericidae
	Family Camelidae
	Infraorder Pecora
	Superfamily Traguloidea
	Family Gelocidae
	Family Protoceratidae
	Family Hypertragulidae
	Family Tragulidae
	Superfamily Cervoidea
	Family Palaeomerycidae
	Family Cervidae
	Family Giraffidae
	Superfamily Bovoidea
	Family Antilocapridae
	Family Bovidae

    Palaeodonta.  Of the archaic palaeodonts the dichobunoids are the
primitive Eocene and Oligocene families, composed of small- to medium-sized
forms with tritubercular to quadritubercular bunodont upper teeth. The
entelodontoids, particularly the Oligocene-Miocene entelodonts, are remarkable
for their huge skulls, both absolutely and in proportion to the body, enlarged
incisors, and large flanges on the zygoma and mandible. These ``giant pigs,''
although not true swine, probably paralleled the pigs in their mode of life.
    Of the suoids several forms of pigs and peccaries survive. The exclusively
New World peccaries can be traced back to the earliest Oligocene, whereas the
pigs, an exclusively Old World assemblage, have a fossil record going back to
middle Oligocene times. Both pigs and peccaries are gregarious and have an
omnivorous diet, which includes anything from grubs and snails to roots, rats,
and snakes. See PIG.
    Of the anthracotherioids the ubiquitous, long-snouted anthracotheres were
once abundant from the middle Eocene into the Pleistocene. The medium- to
large-sized anthracotheres ranged throughout Europe, Asia, and Africa, and a
few entered North America. Judged by their probable descendants, the living
hippopotamuses, anthracotheres also were probably amphibious. Hippopotamuses
are now restricted to African rivers and lakes, but during the Pleistocene
they were widespread in the Old World tropics. See HIPPOPOTAMUS.


    Tylopod ruminants.	Of the ruminant tylopods the merycoidodontoids
(oreodonts and allies) were an exceptionally successful North American group.
After oreodonts appeared in the early Oligocene, about 35,000,000 years ago,
they became exceptionally abundant, probably outnumbering all other mammals of
equivalent size throughout the Oligocene and Miocene. Oreodonts survived well
into the Pliocene.
    The anaplotheres, xiphodontids, and amphimericids are a late Eocene to
middle Oligocene assemblage of primitive ruminants confined to Europe. The
European cainotheres, rabbit-sized ruminants, were convergently similar in
many features of the skull to the hares and rabbits (Lagomorpha). See
LAGOMORPHA.
    Camels and llamas are the only survivors of these once abundant tylopods.
The Camelidae are known as early as the Eocene. The more modern camels,
appearing first in the Pliocene, lost the characteristic ungulate hooves,
evolving expanded, distal phalanges embedded in the cutaneous pad which formed
the sole of the foot. The late Eocene - early Oligocene oromerycids are very
camellike, but more primitive in the construction of their limbs and teeth.
See ALPACA; CAMEL; LLAMA.

    Pecoran ruminants.	The Pecora, represented by such living forms as deer,
gazelles, and sheep, are very successful ruminants with a reduced ulna and
usually with antlers (deer), horns (bovids), or deciduous horns
(antilocaprids).
    Of the traguloids, the most primitive pecorans with large canines, only
the chevrotains (Tragulidae) survive. Chevrotainlike hypertragulids were
common during the late Eocene in central Asia and throughout the Oligocene and
Miocene in North America. The Protoceratidae are spectacular derivatives of
the hypertragulids. Some protoceratids have developed a grotesque forked horn
on the tip of the snout, as well as an elongated snout. See CHEVROTAIN.
    The Cervidae (deer) and Giraffidae (giraffe, okapi, and relatives) evolved
from a palaeomerycid ancestry sometime during early Miocene or possibly late
Oligocene. The varied species of deer range in size from the rabbit-sized
Chinese water deer to the giant extinct Irish elk. Unlike bovid horns, deer
antlers are supported by permanent bony pedicles and are normally shed and
regrown annually. Giraffids are browsers, paralleling the deer in the Old
World tropics, but never as successfully. The living giraffe and okapi have
large eyes and ears, long thin lips, long extensible tongues, and bony
prominences (``horns''), covered by unmodified skin, on the skull (Fig. 2).
The Sivatherium giraffids of the Pleistocene in Africa and southern Eurasia
grew to be giants, and the males developed a variety of large, apparently
permanent giraffelike ``horns.'' See DEER; GIRAFFE.
    Of the bovids the pronghorns (Antilocapridae) appeared in the Miocene. The
Antilocapridae is now restricted to North America; only one species, the
pronghorn ``antelope,'' survives. Several fossil antilocaprids had strongly
branching horns, which were probably permanent, and shed only the horny cover,
as does the modern prongbuck. The bovids are the most abundant of the late
Tertiary and Recent artiodactyls, and also of ungulates in general. Because
bovids are mainly grassland grazers of warm climates, the once abundant
species and genera of Europe and Siberia were forced to retreat southward or
became extinct as a result of Pleistocene glaciations. A few species, such as
the musk-ox, bison, mountain sheep, and mountain ``goat,'' adapted to the cold
northern climates of the Pleistocene and migrated through the Bering land
bridge into the New World. See ANTELOPE; BISON; BUFFALO; GOAT; MOOSE; MUSK-OX;
PRONGHORN; REINDEER; SHEEP.

 �34s[Frederick S. Szalay]
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�6v�2v�32sAsbestos   �21m �33s 
    General name for the useful, fibrous varieties of six rock-forming 
minerals. The value of asbestos derives from its strength as a reinforcing 
agent, its relative chemical inertness, and the incombustible nature of 
certain asbestos-containing products. Originally the term applied only to 
amphibole varieties, but it now applies to all types used in commerce. Over 
95% of production is fibrous serpentine. 
    The fibrous amphiboles that are termed asbestos are generally more heat- 
and acid-resistant than serpentine asbestos, though not as strong, 
alkali-resistant, or available. These fibrous amphiboles include amosite, a 
trade name for a complex iron-magnesium silicate (mostly 
cummingtonite-grunerite), and crocidolite, a sodium-iron hydrous silicate 
variety of riebeckite. Of minor importance are tremolite asbestos, 
anthophyllite abestos, and actinolite asbestos, a variety of tremolite 
containing 15 - 20% ferrous iron. Fibrous serpentine (chrysotile) is usually 
found associated with altered massive serpentine bodies in olivine-rich 
ultrabasic rocks, such as dunite and peridotite. Chrysotile is sometimes found 
in serpentinized dolomitic limestone in association with diabase sills. The 
more frequent occurrences in either type of host are of the cross-fiber 
orientation, in which fibers lie perpendicular to the vein walls. 
    Long fibers are spun into yarn and woven into cloth, either with or 
without auxiliary fiber material, such as cotton, glass wool, or copper wire. 
Brake linings, heavy packings and gaskets, electrical insulating materials, 
and protective clothing require the best fibers. Medium and short fibers are 
used to make asbestos shingles, sheet siding, pipe, floor tile, less critical 
packings and gaskets, paper, binders for other heat insulators, and fillers 
for asphalts, plastics, paints, and greases. 
    A significant area of asbestos research for the last few years has been 
concerned with the controversial health aspects of the fibers. Prolonged 
exposure to high concentrations of airborne asbestos fibers has been 
demonstrated to be injurious to human health. However, there is disagreement 
as to the possible hazard from ingestion of asbestos particles. The 
Environmental Protection Agency (EPA), Occupational Safety and Health 
Administration (OSHA), and Mine Safety and Health Administration (MSHA) are 
Federal agencies which conduct the enforcement programs in the United States. 
In their published regulations, the term ``asbestos'' includes only the 
fibrous form of one serpentine group and five amphibolegroup minerals. In late 
1976, the U.S. Bureau of Mines established a Particulate Mineralogy Unit at 
College Park, Maryland, to provide technical information on particulate 
mineralogy, especially in reference to asbestos; to develop a solid scientific 
basis for research into particle-related pollution problems; and to facilitate 
decision making by regulatory bodies. 
 �34s[Robert A. Clifton]
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�6v�2v�32sAscariasis   �21m �33s 
    The infection of humans by Ascaris lumbricoides, a nematode inhabiting the 
small intestine. The worms may vary in length from about 4 in. (10 cm) in 
severe infections to over 12 in. (30 cm) in light ones. The severity of the 
infection may be related to the presence of a larger number of the smaller 
worms. Large females lay up to 200,000 eggs daily; thus it is easy to diagnose 
the condition by routine microscopic examination of feces. 
    Ascariasis may produce toxic, allergic, and mechanical symptoms or damage. 
The most serious forms are intestinal occlusion or perforation with resulting 
peritonitis and invasion of the common bile duct, pancreatic duct, or 
appendix. Death is often the outcome. Chemotherapy is based on oral 
administration of piperazine compounds. The use of Hetrazan is firmly 
contraindicated because of  possible mechanical complications. See 
ASCARIDOIDEA; FILARIASIS; MEDICAL PARASITOLOGY; NEMATA; PINWORM INFECTION. 
 �34s[Jose F. Maldonado]
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�6v�2v�32sAscaridida   �21m �33s 
    A major parasitic group of the class Nematoda constituting an order of the 
subclass Phasmidia (Secernentea). It includes about 200 genera. Some species 
are scarcely visible; one attains a length of slightly more than 1 ft. (0.3 
m). Hosts include vertebrates of all classes, insects, and some other 
arthropods. The classification of the Nematoda and of this group is not fixed. 
Those who recognize this order divide it into two suborders, Ascaridina or 
Ascaridata and Oxyurina or Oxyurata. These are equivalent to the Ascaroidea or 
Ascaridoidea and Oxyuroidea, respectively, in the prior systems of 
classification that are most familiar to medical and veterinary 
parasitologists. See ASCARIDINA; NEMATA; OXYURINA. 
    The majority of the species have a direct life cycle. However, many 
species have an indirect life cycle in which they require one kind of host 
(intermediate or secondary) for their early parasitic development  and another 
kind (primary or definitive) for their development to sexual maturity. 
    The adults parasitize the stomach and small or large intestine. The larval 
forms of some species with a direct life cycle are not confined to the 
digestive tract. They invade many other parts of the body of both definitive 
and ``abnormal'' hosts. These worms are cosmopolitan in their distribution and 
occur wherever the terrestrial or aquatic animals that serve as hosts live. 
See ASCARIDOIDEA; OXYUROIDEA. 
 �34s[John T. Lucker]
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�6v�2v�32sAscaridina   �21m �33s 
    A major group of the Nematoda, also known as Ascaridata. They are one of 
two suborders of Ascaridida. In one modern system of classification, it has 
essentially the composition and characteristics Ascaridoidea. In another, it 
also contains certain groups (Heterakidae, Cosmocercidae, and Kathlaniidae), 
which the first system assigns to the suborder Oxyurina. See ASCARIDIDA; 
ASCARIDOIDEA; NEMATA; OXYURINA. 
 �34s[John T. Lucker]
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�6v�2v�32sAscaridoidea   �21m �33s 
    A large, prevalent group of parasitic roundworms, also known as the 
Ascaroidea, of the class Nematoda. These roundworms are mostly medium to large 
thick-bodied Ascaridida. For convenience, this group is treated here as a 
superfamily and includes the families Ascarididae and Anisakidae, or 
Heterocheilidae. About 65 genera have been placed in it. Definitive hosts 
include terrestrial and aquatic mammals, birds, reptiles, and fishes. The 
adults usually occur in the stomach or small intestine; the parasitic larval 
stages of many species occur, either temporarily or indefinitely, in other 
parts of the host's  body. 
    Ascaris lumbricoides, the large roundworm of humans, has a life cycle 
similar to that of A. suum. Infestations are very common in tropical and 
temperate regions where sanitation is poor or lacking. These worms often are 
as big as an ordinary pencil. 
    Ascaris suum, the pig ascarid, usually is considered distinct from A. 
lumbricoides, mainly on the basis of biological criteria of speciation. The 
eggs of this worm pass with the pig's manure onto the ground where they 
develop until each contains an infective larva. Such eggs hatch in the gut of 
a pig that swallows them. The liberated larvae penetrate into the intestinal 
wall, enter the bloodstream, break out of capillaries into the air spaces of 
the lungs, and via the trachea reenter the gut where they develop into adults. 
Lung damage from the larval invasion may lead to a fatal pneumonia in young 
pigs.  
    Ascaridia galli, the large intestinal roundworm of chickens, can cause 
intestinal blockage and death, if the infestation is heavy; lesser 
infestations stunt growth and interfere with good bone development. The life 
cycle resembles that of Ascaris suum, except that migration of the larvae in 
the host is restricted to entry into, and emergence from, the intestinal wall. 
Turkeys also are subject to infestation. 
    Toxocara canis, one of the common ascarids of dogs, is very injurious to 
young puppies; heavy infestations can cause death. Its life cycle is like that 
of Ascaris suum, but infection can also occur prenatally, or if a dog eats a 
rat that has encap sulated T. canis larvae in its tissues. See ASCARIDIDA; 
NEMATA.  
 �34s[John T. Lucker]
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�6v�2v�32sAscidiacea   �21m �33s 
    A class of Tunicata which occurs as solitary zooids or, by a process of 
asexual budding, develops into col onies. Zooids vary in length from about 0.1 
to 10 in. Individuals or colonies are invested by a protective covering, the 
tunic or test, made of polysaccharide material structurally close to 
cellulose. Beneath the test is the body wall or mantle. Each zooid has two 
apertures; inhalant (oral) and exhalant (atrial). Water currents, created by 
cilia on the margins of stigmata in the pharyngeal wall, draw water into the 
branchial sac, where it is filtered and passed out through the exhalant 
aperture. Digestive enzymes are secreted into the stomach, and a pyloric 
gland, of unknown function, enters at the junction of stomach and intestine. 
Gonads are hermaphroditic, and may be situated in the loop of the intestine or 
in the mantle wall. 
    Three orders of ascidian are recognized. Ascidians occur throughout the 
seas of the world and at all depths, including the abyssal region. Most 
species feed on minute particulate matter, but a few are carnivores and engulf 
small zooplankters. See TUNICATA. 
 �34s[Ivan Goodbody]
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�6v�2v�32sAscolichenes   �21m �33s 
    A major class of the Lichens characterized by the production of an ascus. 
This class comprises the orders Pseudosphaeriales (lichenized), Hysteriales 
(lichenized), Pyrenulales, Lecanorales, and Caliciales. The asci are very 
similar to those of the nonlichenized Ascomycetes. There are three types of 
ascocarps: apothecia in Lecanorales and Caliciales, perithecia in Pyrenulales, 
and hysterothecia in the Hysteriales (lichenized). There are also ascolocular 
ascocarps in the Hysteriales (lichenized) and Pseudosphaeriales (lichenized). 
Lichen ascocarps are persistent and produce viable spores for many years. 
Pycnidia (spermagonia) and microconidia are found in all groups. A 
characteristic feature of the Ascolichenes is the frequent occurrence of 
isidia, soredia, and other unique means of vegetative reproduction. There are 
between 16,000 and 18,000 species distributed in the 5 orders, with more than 
60 families and 250 genera. See ASCOMYCOTINA; CALICIALES; HYSTERIALES 
(LICHENIZED); LECANORALES; PSEUDOSPHAERIALES (LICHENIZED); PYRENULALES. 
 �34s[Mason E. Hale, Jr.]
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�6v�2v�32sAscomycotina   �21m �33s 
    A subdivision of the Eumycota, known as sac fungi. These fungi are better 
known under the older name Ascomycetes, which persists as their common name. 
The distinguishing feature of these fungi is the ascus, a specialized 
sporangium in which reduction division occurs and in which ascospores are 
delimited by free-cell formation. 
    The vegetative body may be unicellular (yeasts) or composed of filaments 
with cross walls. Nonsexual reproduction is by budding, fission, or conidia 
(unicellular or multicellular spores borne exposed on conidiophores). In 
yeastlike forms, sexual reproduction involves fusion of vegetative cells or 
ascospores which function as gametes. In higher forms, an antheridium, a 
spermatium, or a conidium fuses with a specialized female sexual organ 
(ascogonium), and a binucleate stage intervenes between pairing of nuclei in 
the ascogonium and nuclear fusion in the ascus. Lower Ascomycotina lack 
fruiting bodies (ascocarps); higher Ascomycetes form asci in ascocarps. See 
YEAST.  
    The Ascomycotina are important as plant and human pathogens, as food 
(truffles, morels), in baking, brewing, and wine making (yeasts), in cheese 
making, in enzyme production, and as sources of such drugs as penicillin and 
ergot. See ASPERGILLOSIS; PENICILLIN. 
 �34s[Robert M. Page]
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�6v�2v�32sAscorbic acid   �21m �33s 
    A vitamin also known as vitamin C. It is a white crystalline compound that 
is highly soluble in water. The compound has the structural formula shown 
below. The stability of ascorbic acid decreases with increases in temperature 
and pH. This destruction by oxidation is a serious problem in that a 
considerable quantity of the vitamin C content of foods is lost during 
processing, storage, and preparation. 
    While vitamin C is widespread in plant materials, it is found sparingly in 
animal tissues. Of all the animals studied, only the guinea pig, the red 
vented bulbul bird, the fruit-eating bat, and the primates, including humans, 
require a dietary source of vitamin C. The other species studied are capable 
of synthesizing the vitamin in such tissues as liver and kidneys. 
    Vitamin C - deficient animals suffer from defects in their mesenchymal 
tissues. Their ability to manufacture collagen, dentine, and osteoid, the 
intercellular cement substances, is impaired. People with scurvy lose weight 
and are easily fatigued. Their bones are fragile, and their joints sore and 
swollen. Their gums are swollen and bloody, and in advanced stages, their 
teeth fall out. They also develop internal and subcutaneous hemorrhages. See 
SCURVY.  
    There has been great difficulty in establishing the human requirements for 
vitamin C. The recommended dietary allowances of the Food and Nutrition Board 
of the National Research Council are 30 mg per day for 1- to 3-month infants, 
80 mg per day for growing boys and girls, and 100 mg per day for pregnant and 
lactating women. Many nutritionists believe that the human intake of ascorbic 
acid should be many times more than that intake level which produces 
deficiency symptoms. See VITAMIN. 
 �34s[Stanley N. Gershoff]
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�6v�2v�32sAscothoracica �21m �33s
    An order of the subclass Cirripedia. Mem bers are ecto- or endoparasites
of coelenterates and echinoderms. The body is inclosed in a voluminous saclike
mantle up to 20 mm long, whereas the body is only about one-fourth of this
length (see illustration). Ascothoracica are not attached permanently to the
host by the antennular region, but the antennules may be modified as a
clasping organ. Up to six pairs of thoracic appendages are present, reduced in
number and development in the endoparasites. The mouthparts are modified for
piercing and sucking. Unlike all other Cirripedia, Ascothoracica retain a more
or less segmented abdomen in the adult. There are no cement glands present.
Diverticula of the alimentary canal extend into the mantle. See CIRRIPEDIA.

     FIGURE CAPTION.  Ascothorax ophioctenis, a parasite in the bursae of
brittle stars.

 �34s[H. G. Stubbings]
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�6v�2v�32sAsellota �21m �33s
    A suborder of the Isopoda containing aquatic species of considerable
morphologic and ecologic diversity. These crustaceans are usually divided into
three major groups, the Paraselloidea, Aselloidea, and Stenetrioidea.
Asellotes such as Asellus (see illustration) are found in freshwater streams;
Caecidotea, in subterranean water; Caeioniropsis, in the interstices of marine
sands as commensals with other isopods such as Caecijaera and Jaera and in
shallow waters of the seas; and Macrostylus, in the greatest depth of the
seas. See CRUSTACEA; ISOPODA.

     FIGURE CAPTION.  Asellus communis.

 �34s[Robert J. Menzies]
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�6v�2v�32sAsh �21m �33s
    A genus, Fraxinus, of deciduous trees of the olive family Oleaceae, order
Scrophulariales, which have opposite, pinnate leaflets, except in one species,
F. anomala, which has only a single leaflet. There are about 65 species in the
Northern Hemisphere. This tree occurs in America south to Mexico, in Asia
south to Java, and in Europe. See SCROPHULARIALES.
    The white ash (F. americana), of the eastern United States, has stalked
leaflets, rusty-colored winter buds, and an erect trunk that is valuable for
lumber. The wood is light, strong, but flexible, and is used for oars,
baseball bats, furniture, motor vehicle parts, boxes, baskets, and crates. The
black ash (F. nigra) grows in wet soils in the northeastern United States and
Canada and has sessile leaflets and friable outer bark. The wood of black ash
is used for the same purposes as that of white ash. The red ash (F.
pennsylvanica), also of the eastern United States and adjacent Canada, has
pubescent (hairy) twigs and leafstalks. The uses of the wood of this species
are also similar to those of white ash. Some species of ash are ornamental
trees, such as the flowering ash (F. ornus) with gray winter buds and white
flowers, and the European ash (F. excelsior) with black buds and sessile
leaflets. See FOREST AND FORESTRY; TREE.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sAsia �21m �33s
    The largest of the world's continents. With its peninsular extension,
commonly called the continent of Europe, it is the major portion of the broad
east-west extent of the Northern Hemisphere land masses. In many ways Asia is
more a cultural concept than a physical entity. There is no logical physical
separation between Asia and Europe, and even Africa is separated from Asia
merely by the width of the Suez Canal. For convenience, however, the Eurasian
land mass is considered to be divided by the Ural Mountains into Europe in the
west and Asia in the east. Thus restricted, Asia has an area of about
17,700,000 mi sup 2 (45,800,000 km sup 2), about one-third of the land area of
the Earth. In the north, Siberia reaches past the 80th latitude. Southward,
India and Sri Lanka (Ceylon) reach nearer than 10 DG N of the Equator, while the
Indonesian islands extend more than 10 DG S of the Equator. The continental
heart of Asia is more than 2000 mi (3200 km) from the nearest ocean. See
CONTINENT; EUROPE.

    Topography.  In the topographic framework of Asia, the great mountain
systems are the most impressive features. From the central knot of the mighty
Pamirs and Kopet Dagh in the heart of the continent originate chains radiating
in several directions. In the Peter the First Range there are such heights as
Communism Peak, 24,584 ft (7493 m) and Lenin Peak, 23,377 ft (7125 m), above
sea level. Running westward through Afghanistan is the Hindu Kush, reaching
elevations over 20,000 ft (6100 m). The mountain trendline continues, after a
jog northwestward, in the Elburz of northern Iran and thence in the Armenian
highlands and the Caucasus, each with elevations reaching 18,000 ft (5500 m),
decreasing thereafter to the Pontus and Taurus ranges of northern and southern
Turkey. In western and southern Iran are the massive Zagros and Makran ranges.
    Southeastward from the Pamir knot run the three most imposing mountain
chains on Earth: the Karakorum, which continues the line of the Hindu Kush
eastward in an arc convex to the north; the Himalaya in an arc convex to the
south; and the shorter Trans-Himalaya, or Nyen-chen Tangla, north of the
Himalaya, with higher average elevations but peaks of lesser height. In all of
these, the average elevations exceed 4 mi (6400 m), with several scores of
peaks reaching a height in excess of 25,000 ft (7600 m) above sea level.
Everest, 29,141 ft (8882 m), and Kinchinjunga, 28,146 ft (8579 m), lie in the
Himalaya, while the peak designated as K2, 28,250 ft (8611 m), rises in the
Karakorum.
    In eastern Tibet the Himalaya and Nyen-chen Tangla bend sharply toward the
south, and the former is cut through by the gorge of the Brahmaputra River.
From the bend zone, great ridges divided by deep gorges run south to form the
Burma-China frontiers and the mountain backbones of the Malay peninsula and
Vietnam. The Nan-ling system of south China diverges eastward to divide the
Yang-tzu (Yangtze) from the Hsi (Si) drainage.
    From the western Himalaya, the 11,000-ft (3400-m) Sulaiman Range runs
south and, together with the Kirthar Range, divides West Pakistan from
Afghanistan.
    Beginning at heights over 20,000 ft (6100 m) and branching off from the
Karakorum south of Kashgar, the Kuen-lun Mountains run eastward across western
China. Genetically they form the longest mountain system of China. With their
eastward extensions in the 12,000 ft (3700 m) Ch'in-ling and the lesser
Ta-pieh mountains and Huai-yang hills, they reach almost to the Pacific.
Together with the northeastward arc of the Altyn Tagh and the Nan Shan
branching from it, the Kuen-lun forms the northern wall of the Tibetan
plateau. Near the eastern end of the Kuen-lun proper lie the Amne Machin
Mountains, with peaks up to 25,000 ft (7600 m) in elevation.
    Northeastward of the Pamir knot runs the east-west oriented T'ien Shan,
over 1000 mi (1600 km) long and maintaining heights of 18,000 - 20,000 ft
(5500 - 6100 m) over much of its length. Roughly parallel and trending east
and west is a series of great ranges to its north, with mutual connections in
the west. These include the Altai-Sayan, the Tannu Ola, and the Kentei, which
form natural boundaries for Outer Mongolia. They continue the systems of young
mountains crossing central Asia; farther northeast, they extend further in the
Stanovoi Mountains of Eastern Siberia.
    The Asian plateaus are in various stages of erosion and thus present a
great variety of landscapes. The Tibetan plateau is a prime example. The
western half, because of little rainfall, exhibits a rolling topography with
relatively slight local relief except where mountain chains cross it; it is a
land of internal drainage basins. Average elevations are over 16,000 ft (4900
m). The eastern half is humid or subhumid and is cut by numerous rivers,
producing deep canyons and great ridges. In contrast to this is the Mongolian
plateau. This plateau consists mostly of vast, rather level plains 3000 - 5000
ft (900 - 1500 m) high, surmounted in places by mountains, and containing
broad, shallow basins divided by land swells of low elevation.
    Other major topographic units of Asia are blocs of hill lands. Most of
southern China and much of southeastern Asia comprise hills which may be
roughly defined as slope lands with local relief under 1000 - 1500 ft (300 -
450 m) although in absolute elevation they may rise many thousands of feet
above sea level. Hilly lands are found to predominate in the northern part of
the Indian peninsula and along both flanks of the Indian plateau, where they
are called ghats. In southern India are the Nilgiri and Cardomom hills, rising
to mountainous el evations of 8000 ft (2400 m). Many parts of different
plateaus have hilly regions where erosion has produced uneven local relief, as
in the Shan or North Vietnam plateau. Hills are prominent features of
southwestern Asia, including eastern Mediterranean regions, such as Israel,
Syria, and Lebanon.
    The most significant topographic units of Asia are the great alluvial
plains and river deltas. The gross drainage pattern of Asia is radial; the
rivers flow from the highlands in the heart of the continent and run outward
in all directions. Only in the south, east, and north sectors of the continent
do the rivers reach the sea. Flowing into the peripheral seas of the Pacific
are such mighty rivers as the Mekong, the Hsi, the Yang-tzu, the Huai, the
Yellow, and the Amur, each building large, heavily populated plains and, with
the exception of the Amur, densely settled deltas. The Yellow Plain (North
China Plain), with some 125,000 mi sup 2 (324,000 km sup 2) of area, and the
Yangtzu Plain, with about 75,000 mi sup 2 (194,000 km sup 2), are among the most
extensive alluvial plains of the Earth. In the shallow South China, East
China, and Yellow seas, the deltas of the first five rivers mentioned above
are pushing steadily seaward.
    Important sectors of Asia, containing some 200,000,000 people, are
completely insular. The most important are the Japanese, Philippine, and
Indonesian islands and Taiwan. Almost all of Asia's islands lie in great
volcanic arcs bounding large seas off the continent's Pacific coast. At least
160 active volcanoes are found here and in Kamchatka. Few islands lie along
the Asiatic coasts of the Indian Ocean, although the Sunda chain of Indonesia
has perhaps more of a claim to Indian Ocean frontage than to Pacific frontage.
Sri Lanka is the only significant island in the northern part of the Indian
Ocean west of Sumatra. In the Persian Gulf off the north coast of Arabia lies
the small island Bahrein.
    Few islands lie off the alluviated coastlands of northern Siberia. Some
moderately large ones are included in the barren and rocky Severnaya Zemlya
group, the New Siberian Islands, and Wrangel Island. The Commander Islands and
Karaginski Island lie in the Bering Sea only a short distance from the
Aleutians.

    Climates.  Continentality is a strong factor in Asia's climates. Eastward
extratropical cyclonic drift, typhoons and westward tropical cyclonic drift,
and contrast in rainfall patterns bring regional diversity to the climates of
Asia. Describing the climatic patterns very grossly, at least four
characteristic types of Asian climatic systems may be distinguished: (1) the
monsoonal system of eastern Asia, (2) the monsoonal system of southern Asia,
(3) the cyclonic and convectional storm systems of cen tral and northern Asia,
and (4) the winter rainfall - summer drought system of southwestern Asia.
    Fundamental to the whole problem, however, are the vastness of the
unbroken land mass and the long latitudinal stretch from the polar realm to
south of the Equator. These are responsible for the great temperature and
humidity extremes that occur. The greatest ranges of temperatures in the world
have been recorded in interior Asia. Continentality, therefore, is the
outstanding feature of climates of interior Asia. In coastal and insular areas
of east Asia, however, winds moving over the warm, northward-flowing Japan
Current and the western Pacific waters moderate the coastland and island
climates. See CONTINENTALITY (METEOROLOGY); MARINE INFLUENCE ON WEATHER AND
CLIMATE; MONSOON.
    The driest portions of Asia include the vast areas of southern Mongolia,
Hsin-chiang, Soviet Central Asia, and southwestern Asia. Except for small,
favored mountain areas, most of this region from the Gobi to the Red Sea gets
less than 10 in. (25 cm) of precipitation per year. With the exception of
southern Arabia, which is subtropical desert, these are mid-latitude desert
and dry steppe regions. Favored with higher rainfall are the Yemen Mountains
and the coastal mountains of Turkey, together with Lebanon, Syria, and
northern Israel. The highlands of Armenia and the Elburz of Iran are favored
also with more abundant rainfall, which may range from 25 to 50 in. (64-127
cm) or more per year.
    The northeastern Siberian mountains and the Arctic coastal lands also
receive meager rainfall, less than 8 in. (20 cm), but are not dry because
evaporation is low and the water table is high. Most of Siberia has permafrost
below a few feet of surface soil, so that rainwater does not filter far down
into the earth. Between the arid belt of central Asia and the northeast
Siberian low-precipitation zone, the annual rainfall ranges be tween 10 and 18
in. (25 and 45 cm).
    In eastern Asia the precipitation increases in a southeasterly direction
from interior Asia to the coast. The annual maximum seldom exceeds 80 in. (203
cm) in the wetter southeast coastal regions, whereas this drops to less than
30 in. (76 cm) in the North China Plain and less than 15 in. (38 cm) at the
Great Wall. In some mountainous parts of Japan and Taiwan, the yearly average
may be more than 100 in. (254 cm).
    In the Indian subcontinent rainfall is heaviest along the western plateau
fringe and in East Bengal, where it may average over 100 in. (254 cm) per
year. The interior of the peninsula is relatively dry. Northwestern India and
Pakistan share the drought of southwestern Asia. With the exception of the
extreme north, Ceylon generally has abundant rainfall.
    Southeastern Asia has the heaviest rainfall of the entire Asiatic region.
The mainland mountains facing the southwest summer monsoon crossing the Bay of
Bengal, and parts of the Vietnamese and Laotian cordilleras facing the
humidified northeast winter monsoons of eastern Asia, regularly get average
rainfalls of 120 - 150 in. (305 - 381 cm) or even more. Equally heavy
rainfalls occur in the southwestern half of Sumatra, southwestern Java, the
northwestern half of Borneo, and the Pacific fringe of the Philippine Islands.
With a few small exceptions, southeastern Asia has no areas that are subject
to severe drought.

    Vegetation.  Asia's vegetation belts and zones follow, in general, the
climatic patterns from desert lands through tropical to Arctic margins.
    A wide belt of tundra made irregular by topography occupies the entire
Arctic lowland of Siberia with widths varying from 250 to 500 mi (400 to 800
km) north and south. It is widest in the extreme northeast and it extends
southward and inland with higher elevations. The frozen subsoil permits the
growth of little more than mosses, lichens, dwarfed trees, and scrub. See
PERMAFROST; TUNDRA.
    The largest unbroken expanse of forest in the world is the Siberian taiga,
a dominantly coniferous forest of larches, spruce, fir, and pines, with such
deciduous trees as birch and aspen occurring intermixed with the conifers or
taking over as a secondary growth in burnt-over areas. The width of this belt
in Siberia is more than 1000 mi (1600 km) and it stretches about 4000 mi (6400
km) from the Sea of Okhotsk to the Urals. See TAIGA.
    Various admixtures of coniferous and deciduous trees compose the
vegetation of mid-latitude mixed forests. In the west Siberian plain there is
a narrow zone of mixed taiga and deciduous forests including oaks, maples,
ash, and lindens. This zone, with a width of 50 - 100 mi (80 - 160 km), lies
somewhat south of the parallel of 60 DG N and fades into the steppelands that
form the great spring-wheat region of Siberia. Mixed mid-latitude deciduous
and coniferous forest areas of a similar type occupy most of Korea, the
northern half of Honshu in Japan, and the hill lands surrounding the Yellow
Plain, as well as the Ch'in-ling Mountains. In southern Asia these forests are
found chiefly in a narrow belt of mountain land in the outer ranges of the
Himalaya. The remaining areas of these mixed forests run from the Elburz
Mountains through the Armenian highlands and the Black Sea fringe of Turkey to
the Aegean coast, and in southwestern Asia in the Elburz of northern Iran.
    From the mixed and deciduous forests of the west Siberian plain southward,
an increasingly dry steppeland is encountered. It extends for 400 - 500 mi
(640 - 800 km) in a belt about 1000 mi (1600 km) long between the Urals and
the Altai-Sayan and associated uplands. The northern half of this belt with
its higher annual precipitation of 12 - 16 in. (30 - 40 cm) is the
agricultural heart of the plain. The southern part gradually changes to desert
steppe and then to desert along about the 50th parallel. Eastward of Lake
Baikal a broadened steppe zone occupies the Trans-Baikal region extending
southward to the Gobi Desert of southern Mongolia and eastward to the Great
Hsing-an Mountains, where the zone, about 200 mi (320 km) wide, runs southward
in Inner Mongolia. The steppe zone in Inner Mongolia widens with the
increasing moisture south of the Great Wall to include most of China's loess
plateau. Grasses also form the natural vegetation of the Manchurian plain,
with tall grass in the eastern portion thinning out to short-grass steppe in
the Hsing-an Mountain flanks. The Gobi Desert is flanked by steppe lands to
its north, east, and south, as well as by mountain steppe zones in the eastern
Altai and eastern T'ien Shan.
    Mixed evergreen forests appear to be limited mostly to interior southern
China and to Japan from the Kwanto Plain southward, South of the Yang-tzu
Valley, this forest type extends from the coast at Shanghai to the gorge lands
of eastern Tibet. In Asia the characteristic trees of the mixed forest include
broad-leafed evergreen trees such as banyans and camphor, and coniferous trees
such as pines, cedars, and cypresses, as well as varieties of bamboo.
    Tropical and subtropical rainforest is restricted to warm or hot regions
of southern and southeastern Asia which get ample rainfall the year round or
get so much rain during a large part of the year that a high groundwater table
is maintained during the short dry season. The subtropical sectors are found
along the southeastern China coast, in Taiwan, and in northern Burma; they
merge with the tropical rainforest farther south, where rainfall and
temperature increase. See RAINFOREST.
    Monsoon tropical deciduous forests comprise the tropical parts of Asia
which have a moderately high rainfall but a long dry season (usually in the
low-sun period or winter). These forests consist mostly of mixed species, but
sometimes a single species becomes dominant as a result of selection from
frequent burnings.
    A large region of savanna grassland surrounds the Thar Desert of
northwestern India and occupies most of the Indus Valley, the Punjab, and the
Kathiawar peninsula. Much of the drier interior peninsular Deccan of India
also has this as a natural vegetation. Other Asian regions with similar cover
are found in Yemen and the region in southeastern Arabia from Oman as far
westward as the Qatar peninsula; and similar vegetation extends over the Korat
plateau of Thailand, lower Thailand west of Bangkok, southern Cambodia, and
small areas in interior Borneo and the Philippines. See SAVANNA.
    Immense areas of central and southwestern Asia have little or no
vegetative cover, and bare rock alternates with sand veneering. In places
shifting sand dunes are formed. Although the deserts are not necessarily
lifeless, the vegetation is so widely spaced that much bare ground is exposed.
The tropical desert areas generally receive their meager rainfall in
torrential downpours on rare occasions. After such rains numerous herbs may
spring to life and flower, while the bunch grass here and there may become
green for a short season.
 �34s[Herold J. Wiens]
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�6v�2v�32sAsparagine �21m �33s
    An amino acid. Asparagine forms a brown color with ninhydrin. The amide
group of asparagine presumably serves as a storage site for nitrogen,
especially in plants. Although it is known that asparagine is formed
biosynthetically from aspartic acid, it is not certain whether the mechanism
involves amide transfer from glutamine, transamination, or ammonia
incorporation. Asparagine is deamidated to aspartic acid, which is further
catabolized by way of oxaloacetic acid and the Krebs cycle. See AMINO ACIDS;
ASPARTIC ACID; KREBS CYCLE; TRANSAMINATION. 
 �34s[Edward A. Adleberg]
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�6v�2v�32sAsparagus   �21m �33s 
    A dioecious perennial monocot (Asparagus officinalis) of Mediterranean 
origin belonging to the plant order Liliales. Asparagus is grown for its young 
shoots or spears, which are canned, frozen, or cooked fresh as a vegetable. 
These aerial stems arise from rhizomes (underground stems). The rhizomes and 
the fleshy and fibrous roots constitute the massive underground part of the 
plant. Blanched or white asparagus is grown by ridging soil over the rows and 
cutting the spears beneath the soil surface. Chemical weed control is commonly 
used.  
    Commercial production is limited to areas where crowns will have a dormant 
period of 3 - 5 months each year. Dormancy in the northern states is induced 
by low temperatures and in California by withholding irrigation. California, 
New Jersey, and Washington are important asparagus-producing states. See 
LILIALES.  
 �34s[H. John Carew]
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�6v�2v�32sAspartic acid   �21m �33s 
    An amino acid. The functions are as follows: (1) It is a key intermediate 
in many transamination reactions and is a product of ammonia incorporation in 
plants and some microorganisms. (2) It functions as a source (from the amino 
group) of one of the ring nitrogens of purines and of the amino group of 
adenylic acid. Aspartic acid also reacts with carbamyl phosphate to form 
carbamyl aspartate, a precursor of pyrimidines. (3) Aspartic acid can be 
amidated to form asparagine; it is also the biosynthetic precursor of 
methionine, threonine, isoleucine, and, in bacteria, of lysine. See AMINO 
ACIDS; ISOLEUCINE; METHIONINE; PYRIMIDINE; THREONINE. 
    Aspartic acid is formed, biosynthetically, by transamination of 
oxaloacetate or by the addition of ammonia to fumaric acid. The metabolic 
degradation pathway is by deamination to oxaloacetate, or by decarboxylation. 
Two decarboxylases are known, one forming ALPHA-alanine, the other 
BETA-alanine.  
 �34s[Edward A. Adelberg]
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�6v�2v�32sAspect ratio �21m �33s
    As originally conceived, the ratio of the span of a wing or airfoil to the
chord of a wing, where the span is the maximum cross-stream dimension and the
chord is the dimension in the streamwise direction, as illustrated. This
definition is unambiguous only in the case of a rectangular wing.
    Because early wings were usually nearly rectangular, no confusion resulted
from the original definition. Later, when wings were tapered or had complex
planforms, another definition became necessary. It was desirable that the new
and more general definition correspond to the old definition for the special
case of the rectangular wing. The more general definition of geometrical
aspect ratio which is now universally used is given in the equation below,
where A is the aspect ratio and

			     A = b (sup 2)/S

b and S are defined in the diagram. Because S is equal to bc for a wing of
rectangular planform, the definition of aspect ratio given in the equation
corresponds to the original idea of the ratio of the span to the chord for a
rectangular wing.

		       aspect ratio=(b/c)=(b (sup 2)/S)


[Albert E. Von Doenhoff / Raymond L Bisplinghoff]

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�6v�2v�32sAspergillosis   �21m �33s 
    A fungus disease of humans caused by several species of Aspergillus, 
particularly A. fumigatus, A. flavus, and A. terreus. Invasive pulmonary and 
systemic aspergillosis are considered opportunistic diseases, developing in 
patients whose primary disease may be leukemia, Hodgkin's disease, or a 
lymphoma. Aspergillosis may develop in patients receiving immunosuppressive 
drugs for tissue transplantation or cytotoxic and antimitotic drugs. See 
OPPORTUNISTIC INFECTIONS. 
    Aspergillus species are common airborne contaminants in the laboratory as 
well as in the general environment; thus, any materials inoculated on 
laboratory media yielding an Aspergillus species must be interpreted 
carefully. Even repeated isolation of an Aspergillus species cannot be 
considered conclusive proof that aspergillosis exists. Immunodiffusion tests 
are available and should be used as an additional tool in attempting to 
diagnose aspergillosis. Treatment of severe forms of aspergillosis requires 
use of amphotericin B. See AMPHOTERICIN B; MEDICAL MYCOLOGY; MONILIALES. 
 �34s[Leanor D. Haley]
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�6v�2v�32sAsphalt and asphaltite �21m �33s
    Varieties of naturally occurring bitumen. Asphalt is also produced as a
petroleum by-product. Both substances are black and largely soluble in carbon
disulfide. Asphalts are of valuable consistency, ranging from a highly viscous
fluid to a solid, whereas asphaltites are all solid. Asphalts fuse readily,
but asphaltites fuse only with difficulty. Asphalts may, moreover, occur with
or without appreciable percentages of mineral matter, but asphaltites usually
have little or no associated mineral matter. See BITUMEN; GILSONITE;
IMPSONITE; WURTZILITE.
    Many asphalts occur as viscous impregnations in sandstones, siltstones,
and limestones. Most such deposits are thought to be petroleum reservoirs from
which volatile constituents have been stripped by exposure of the rock.
Relatively pure asphalt occurs in Kern, San Luis Obispo, and Santa Barbara
counties, California. Occurrences of asphalt are also known in Kentucky and
Oklahoma. Although asphalt seeps have long been known in France, Greece,
Russia, Cuba, and other countries, the best known and largest are those of
Venezuela and Trinidad.
    The asphaltites (gilsonite, grahamite, and glance pitch) were probably
derived from a saline lacustrine sapropel and owe their variable properties to
differences in environment of deposition. These substances occur on a large
scale in the Uinta Basin of northeastern Utah, where they are derived from
upper Eocene Green River sediments, most of which are oil shales high in
carbonate content. See OIL SHALE; SAPROPEL.
[Irving A. Breger]
    Asphalt is derived from petroleum in commercial quantities by removal of
volatile components. It is an inexpensive construction material used primarily
as a cementing and waterproofing agent. See PETROLEUM PRODUCTS.
    Asphalt is composed of hydrocarbons and heterocyclic compounds containing
nitrogen, sulfur, and oxygen; its components vary in molecular weight from
about 400 to 5000. It is thermoplastic and viscoelastic; at high temperatures
or over long loading times it behaves as a viscous fluid, while at low
temperatures or short loading times it behaves as an elastic body.
    The three distinct types of asphalt made from petroleum residues are
straight-run, air-blown, and cracked. Straight-run asphalt, characterized by a
nearly viscous flow, is used in the construction of pavement surfaces for
roads and airport runways. Air-blown asphalt is resilient and has a viscosity
that is less susceptible to temperature change than that of straight-run
asphalt. It is used mainly for roofing, pipe coating, paints, underbody
coatings, and paper laminates. Cracked asphalt, with limited applications such
as dust laying or as an insulation board saturant, has a nearly viscous flow,
and its viscosity is more susceptible to temperature change than straight-run
asphalt.
 �34s[Thomas K. Miles]
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�6v�2v�32sAspidochirotacea   �21m �33s 
    A subclass of Holothuroidea in which there are 10 - 30  shield-shaped 
tentacles, and tube feet. There are no pharyngeal retractor muscles; hence the 
anterior part of the body cannot be retracted as an introvert. Bilateral 
symmetry of the body is always evident and is sometimes quite conspicuous. See 
ASPIDOCHIROTIDA; ECHINODERMATA; ELASIPODIDA; HOLOTHUROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sAspidochirotida   �21m �33s 
    An order of Aspidochirotacea in which there are respiratory trees and the 
mesentery of the posterior loop of the intestine is attached in the right 
ventral interradius. The dorsal tube feet are often converted into tactile 
warts. Members of the order live on the sea floor, feeding on organic 
detritus, which they spoon into the mouth with the shield-shaped tentacles. 
Three families of Aspidochirotida have been defined: Synallactidae, which are 
mainly deep-sea forms that lack tentacle ampullae; Holothuriidae, which are 
shallow-water forms, common on reefs and in tropical lagoons; and 
Stichopodidae, which occur in most shallow seas. See ASPIDOCHIROTACEA; 
ECHINODERMATA; HOLOTHUROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sAspidogastrea   �21m �33s 
    A group of entoparasites considered to be a subclass or order of the 
Trematoda. They have strongly developed ventral holdfasts. Two families, 
Aspidogastridae and Stichocotylidae, are recognized. Aspidogastridae, which 
are the commonest, occur in various cavities of mollusks and in digestive 
tracts of fishes and turtles. Elongate Stichocotylidae occur in the digestive 
tracts of skates. Little is known of the physiology of aspidogastreids, but 
they appear less host-specific than other trematodes. See DIGENEA; TREMATODA.  
 �34s[William J. Hargis, Jr.]
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�6v�2v�32sAspidorhynchiformes �21m �33s
    A small order of specialized holostean fishes which are first recorded
from Middle Jurassic deposits of Europe, and probably had a worldwide
distribution in the warm seas of the Cretaceous Period. The order contains one
family, Aspidorhynchidae, and two genera: Aspidorhynchus and Belonostomus.
These fishes, some of which reached a length of over 3 ft (0.9 m), are
characterized by a ganoid scale covering with much deepened scales along the
flank, by an elongate fusiform body and head with long slender snout, and by
an externally symmetrical tail. All the fins are small and fringing fulcra are
reduced or absent. The dorsal and anal fins are positioned opposite one
another far back on the body, and the pelvic fins are inserted closer to the
anal than to the pectorals (see illustration). See HOLOSTEI.

    In the body form, fin position, and elongated snout, the
aspidorhynchiforms resemble some of the living teleostean Exocoetoidei
(needlefishes and sauries). It seems likely that these two widely separated
and unrelated groups of fishes shared a similar mode of life, being predacious
open-water forms that utilized their long snouts and strong swimming ability
in capturing prey. See OSTEICHTHYES: TELEOSTEI.

     FIGURE CAPTION.  Aspidorhynchus acutirostris, Upper Jurassic, Bavaria.

 �34s[Ted M. Cavender]
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�6v�2v�32sAspirin �21m �33s
    The trade name for acetylsalicyclic acid. Aspirin is a white, crystalline,
weakly acidic substance, with melting point 137 DG C. It is the most commonly
used analgesic and antipyretic against neuralgia, influenza, colds, arthritis,
headache, and fever. It is dispensed as tablets or powder in capsules, often
mixed with sodium bicarbonate. Soluble aspirin is calcium acetylsalicylate.
    Insignificantly affected by digestive juices, aspirin undergoes hydrolysis
in the intestinal tract (alkaline) to liberate salicylic acid. The latter is
readily absorbed through the intestinal mem brane and is responsible for the
physiological response.
 �34s[Evans B. Reid]
�33s
    Aspirin is a superior analgesic, but its effect on inflammation or fever
is about the same as, and certainly not more than, that of sodium salicylate.
Interest in aspirin has increased, in part because of the discovery that it
modifies the synthesis of prostaglandins. Prostaglandins participate in a
variety of biological reactions and are synthesized on demand and released; in
fact, synthesis probably equals release. Aspirin and other peripheral
analgesics inhibit prostaglandin synthesis, and it appears that the
effectiveness of peripheral analgesic drugs and their ability to inhibit
prostaglandin synthetase parallel one another.
    Intolerance to aspirin is not uncommon. It tends to develop in middle age,
showing a classical pattern: it occurs as a ``triad'' that includes formation
of nasal polyps and intermittent (but occasionally progressive) bronchial
asthma; some patients have urticaria. Death rarely ensues, because patients
rapidly become aware of their intolerance. See ANALGESIC.
 �34s[Max Samter]
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�6v�2v�32sAstatine �21m �33s
    A chemical element, At, atomic number 85. Astatine is the heaviest of the
halogen groups, filling the place immediately below iodine in group VII of the
periodic table. Astatine is a highly unstable element existing only in
short-lived radioactive forms. About 25 isotopes have been prepared by nuclear
reactions of artificial transmutation. The longest lived of these is
(sup 210) 0 At, which decays with a half-life of only 8.3 h. It is unlikely that
a stable or long-lived form will be found in nature or prepared artificially.
The most important isotope, used for tracer studies, is (sup 211) At. Astatine
exists in nature in uranium minerals, but only in the form of trace amounts of
short-lived isotopes, continuously replenished by the slow decay of uranium.
The total amount of astatine in the Earth's crust is less than 1 oz (28 g).
    In aqueous solution, astatine resembles iodine except for differences
attributable to the fact that astatine solutions are of necessity extremely
dilute. Like the halogen iodine, when astatine exists as a free element in
solution, it is extracted by benzene. The element in solution is reduced by
agents such as sulfur dioxide and is oxidized by bromine. It is more
electropositive than the other halogens. It has oxidation states with
coprecipitation characteristics similar to those of the iodide ion, free
iodine, and the iodate ion. Powerful oxidizing agents produce an astatate ion,
but not a perastatate ion. The free state is most readily obtained and is
characterized by high volatility and high extractability into organic
solvents. See HALOGEN ELEMENTS.
 �34s[Earl K. Hyde]
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�6v�2v�32sAsterales �21m �33s
    An order of flowering plants, division Magnoliophyta (Angiospermae). The
order consists of only the single, very large family Compositae (Asteraceae),
with about 19,000 species, occurring in nearly all parts of the world but most
abundant and conspicuous in areas which are not densely forested.
    Most members of the order are herbaceous, but some, such as the sagebrush
(Artemisia tridentata), are shrubs, and a few of the less familiar tropical
species are trees. Many well-known garden ornamentals, such as aster (see
illustration), chrysanthemum, dahlia, daisy, sunflower, and zinnia, belong to
the Asterales. A few garden vegetables (lettuce and artichoke) and some common
weeds (dandelion, thistle, and ragweed) also belong to the order. See
ARTICHOKE; ASTERIDAE; LETTUCE; MAGNOLIOPHYTA; MAGNOLIOPSIDA; PLANT KINGDOM;
SUNFLOWER.

     FIGURE CAPTION.  New England aster (Aster novae-angliae), a characteristic
member of the order Asterales. The numerous apparent petals around margin of
flower head are corollas of individual flowers, called ray flowers in contrast
to disk flowers which occupy the center of head.

 �34s[Arthur Cronquist]
�33s

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�6v�2v�32sAstereognosis   �21m �33s 
    The term which is used in clinical neurology to mean loss of recognition 
of objects by touch in the presence of recognition through another sense, 
usually vision. 
    Astereognosis, in the broad usage, may be secondary to gross somatosensory 
defects caused by lesion of the afferent nerve pathways. It may also occur 
without gross defects of pressure, pain, and thermal sensitivity, but in 
association with other sensory changes, such as impaired localization of 
touch, diminished tactile acuity (two-point discrimination), and defective 
postural sense. Concomitant difficulty in discriminating the size, weight, 
texture, and shape of objects by palpation is usually encountered. See 
PERCEPTION; SENSATION. 
    Astereognosis, in narrow usage, is synonymous with tactile agnosia, 
sometimes called pure astereognosis. Agnosia literally means lack of 
knowledge, and is used to indicate a loss which is intellectual in nature yet 
restricted to one sense modality. Failure to identify objects and their 
function is thought to be based on a disturbance of associative memory. This 
view implies a theory of brain function which considers associative activities 
as based on processes distinct from those involved in sensation or movement, 
separately localized, and hence selectively vulnerable to brain injury or 
disease. See AGNOSIA; MEMORY. 
 �34s[Josephine Semmes]
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�6v�2v�32sAsteridae   �21m �33s 
    A large subclass of the class Magnoliopsida (dicotyledons) of the division 
Magnoliophyta (Angiospermae), the flowering plants, consisting of nine orders, 
43 families, and more than 50,000 species. The largest orders of the group are 
the Asterales (about 19,000 species), Scrophulariales (about 10,000 species), 
Lamiales (about 7800 species), and Rubiales (about 6500 species). Other orders 
are the Gentianales, Polemoniales, Plantaginales, Campanulales, and 
Dipsacales. See individual articles on each order. See MAGNOLIOPHYTA; 
MAGNOLIOPSIDA; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
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�6v�2v�32sAsteroid �21m �33s
    One of the many thousands of small planets (minor planets) revolving
around the Sun, mainly between the orbits of Mars and Jupiter. Newly
discovered asteroids are assigned a catalog number and name (such as 433 Eros)
only after they are observed often enough to compute an accurate orbit. There
are over 2200 cataloged asteroids. See PLANET.
    The vast majority of asteroids have semi-major axes (mean distances to the
Sun; symbolized a) between 2.2 and 3.2 astronomical units (1 AU = distance
from Earth to the Sun = 1.496 x 10 sup 8 km = 9.30 x 10 sup 7 mi). However,
several small asteroids orbit between Earth and Mars, and two groups, the
Trojan asteroids, orbit at Jupiter's distance from the Sun.
    Two new techniques were used in the 1970s to measure the diameters and
albedos of over 130 asteroids: the polarimetric technique, based on an
empirical correlation between the albedos of materials and how they polarize
light: and the radiometric technique, which measures diameters by comparing
the brightness of reflected visible sunlight from an asteroid with the
brightness of the asteroid's emitted thermal radiation in the infrared. See
ALBEDO.
    Asteroids are much darker, hence larger, than had been assumed before.
There are about 35 asteroids larger than 200 km in diameter; about 75% of them
are soot-black (geometric albedos of 3 - 5%). Asteroids are much more numerous
at smaller sizes, generally following a size distribution characteristic of
fragmentation processes, as one would expect if the asteroids were smashing
into each other. Indeed, there are so many large asteroids confined in the
volume of the asteroid belt that collisions sufficient to destroy all but the
larger asteroids occur every few billion years, and much more often for
smaller ones. Thus the asteroids are mainly collisional fragments, grinding
themselves down to dust.
    Spectra of sunlight reflected from asteroids have shapes, including
absorption bands, characteristic of different rock-forming minerals. Combined
with the albedo data from polarimetry and radiometry, the spectra of surfaces
of over 270 asteroids show that about 90% fall into two compositional groups:
the C-type, having very dark, neutral-colored surfaces of probable
carbonaceous composition like the primitive carbonaceous chondritic
meteorites; and the S-type, having moderate-albedo, reddish surfaces
containing pyroxene and olivine silicates, probably mixed with metallic iron,
similar to stony-iron meteorites or ordinary chondrites. Only a few asteroids
have spectra clearly indicating a composition like ordinary chondritic
meteorites, the most common meteorites to fall on Earth. Since current
cosmogonical models for the origin of planets involve accretion from myriads
of asteroidlike planetesimals, it is likely that asteroids are a remnant of
the planetesimals that failed to accrete into a planet between Mars and
Jupiter. Probably bombardment of the asteroid zone by large planetesimals
scattered from massive, nearby Jupiter increased the relative velocities of
asteroids to the present value of 5 km/s (3.1 mi/s) so that asteroids fragment
rather than accrete when they meet each other. Instead of forming a planet,
the asteroids have been smashing each other to bits, and those seen now are
mostly fragments, except for the ``lucky few'' that have escaped catastrophic
collisions so far.
    Evidently some asteroids of primitive, nonvolatile solar composition were
heated within the first few hundred million years after the origin of the
solar system, perhaps by the solar wind or extinct radionuclides, and they
melted. Iron sank to their centers, forming strong stony-iron cores while
basaltic lavas floated to their surfaces, producing minor planets like Vesta.
The numerous collisions fragmented away the outer crusts and mantles of most
of these bodies, with Vesta a prime, intact exception. While the unmelted,
weak, C-type asteroids may have been depleted by a large factor by the
collisions, most of the strong stony-iron cores of the melted proto-asteroids
have survived, perhaps as the S-type asteroids observed today. The asteroids
still collide and fragment, occasionally spraying the inner solar system with
chips that produce craters or fall as meteorites.
 �34s[Clark R. Chapman]
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�6v�2v�32sAsteroidea �21m �33s
    A subclass of the class Stelleroidea in the subphylum Asterozoa; asteroids
are known as starfish. The arms are not sharply demarcated from the rest of
the body. The ambulacral ossicles never fuse to form vertebrae. The tube feet
are locomotor organs, usually suctorial, emerging from an open ambulacral
groove. The dominant growth gradients are such as to cause the skeletal
ossicles to lie in longitudinal rows known as series (for example,
adambulacral series, ventrolateral series, and inferomarginal series). A
representative asteroid is shown in the illustration.
    The 1700 or so known fossil and recent species of Asteroidea are now
grouped into five orders: Platyasterida, Paxillosida, Valvatida, Spinulosida,
and Forcipulatida. Asteroids range in size from about 0.4 to 40 in./(10 mm to
1 m) across. Many starfish are brightly colored and attractive animals, but
some are dowdy and cryptic. Like most echinoderms, starfish have a lifespan of
about 5 years.
    Several species of starfish are regarded as pests, particularly in oyster,
mussel, and scallop fisheries. Beyond this they are of little significance,
with the notable exception of the coral predator Acanthaster planci, which
occurs in the Indo-Pacific. When present in large numbers, it may kill
extensive areas of reef-building corals. However, large aggregations of this
species are occasional phenomena, and coral killed by it is capable of
regenerating more quickly than was hitherto supposed.

    Starfish inhabit all types of bottom throughout the world's seas and
oceans. Some burrow in sand and mud, and others live on rocks and coral reefs,
where they are at their most diverse. Most families show well-defined
bathymetrical preferences, but there are exceptions, for instance, the
offshore fauna of New Zealand. There are 12 families which occur at depths
greater than 2 mi (3.2 km), and several genera extend below 4 mi (6.4 km).
    The outer surface is coated with a thin layer of ciliated epithelium.
Below this lies connective and muscle tissue which forms the body wall and in
which the skeletal ossicles are embedded, forming the test. Some of the
ossicles, notably the spines, protrude to the exterior. The arms can be slowly
moved by the body wall musculature. Although 5 arms are common, up to 12 occur
in a number of genera, and more than 12 may occur.
    Pedicellariae occur in some starfish. They are small seizing organs used
to grip intruders and, according to one report, even to catch food. They
probably assist in protecting the delicate epithelium from predators and
sediments. The well-developed water-vascular system follows the pattern for
the phylum, with the following distinctive features. With the exception of
some Paleozoic genera, the madreporite is always on the upper (aboral) side.
There is usually only one madreporite situated interradially. The tube feet
may be peglike and suckerless or columnar and suckered. The nervous system
also follows the pattern for the phylum. Starfish cannot see, but they clearly
detect changes in light intensity such as those caused by shadows. They have
photosensitive eyespots located at the tip of each arm. The terminal tube feet
of each ambulacrum appear to be especially sensitive to water-borne chemicals
which may be emitted by prey species, and they are thus used in food detection.
    Starfish are voracious feeders. Small food, such as amphipods and young
mollusks, may be engulfed whole. Acanthaster everts part of its stomach out of
the mouth and wraps it over coral polyps, which are thus digested outside the
predator's body. Some of the Forcipulatida, for example, Asterias, can open
large clam and other bivalve shells by applying a continuous force which
overcomes the molluscan adductor muscles. The mouth is in the middle of the
lower (oral) surface of the disk and leads through a short esophagus to the
cardiac stomach. Above the cardiac stomach lies the pyloric stomach, with as
many branches as there are arms. Within each arm, the branch forks to give two
blind digestive ceca. In most species, a short intestine leads upward to the
rectum and anus. This is generally situated near the middle of the upper
(aboral) surface. In those starfish which lack an anus, the feces are extruded
through the mouth.
    Asteroids are generally mature and able to reproduce at 1 year. They
normally continue to grow for about 4 years, with growth taking place when
food is abundant. The sexes are usually separate, but in a few genera, such as
Fromia, hermaphrodites occur, the sex changing with age. Regeneration of lost
or damaged parts is a characteristic of most genera. In Linckia this is most
marked, and a whole new individual can be regenerated from a fragment of an
arm. See ECHINODERMATA.

     FIGURE CAPTION. A representative asteroid (Astrostole scabra).

 �34s[Andrew C. Campbell]
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�6v�2v�32sAsterozoa   �21m �33s 
    One of four known subphyla of echinoderms, characterized by a star-shaped 
body and radially divergent axes of symmetry, which are frequently five in 
number. Asterozoans first appeared in the Upper Cambrian and had pinnate arms 
resembling those of present-day crinoids. 
    The Asterozoa form a relatively uniform assemblage and are all placed in a 
single class, the Stelleroidea, whose characters therefore coincide with those 
of the subphylum itself. The tube feet are restricted to the lower surface of 
the body, and the radial water vessels run along the undersides of the body 
external to the ambulacral ossicles. The madreporite lies in the integument. 
    There are three subclasses: Somasteroidea, composed of generalized forms 
with crinoidlike features, ranging from the lower Paleozoic onward; 
Asteroidea, the starfish or sea stars; and Ophiuroidea, the brittle stars. See 
ASTEROIDEA; ECHINODERMATA; OPHIUROIDEA; SOMASTEROIDEA. 
 �34s[Andrew C. Campbell]
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�6v�2v�32sAsthenosphere   �21m �33s 
    A region in the interior of the Earth characterized by less mechanical 
strength and less resistance to deforming stresses than regions above 
(lithosphere) and below (mesosphere). This zonation is in contrast to the more 
familiar subdivision of the interior into crust, mantle, and core, based on 
discontinuities in seismic-wave velocities. The asthenosphere lies within the 
seismically defined upper mantle. See LITHOSPHERE. 
    The asthenosphere, because of its ability to flow, has figured prominently 
in geodynamic theories on the causes of vertical motions observed at the 
Earth's surface, such as postglacial rebound. Likewise, the asthenosphere 
plays a prominent role in models of the large horizontal movements of the 
lithosphere as observed in continental drift and plate tectonics. The 
asthenosphere is envisioned as the lubricating layer over which the plates 
glide. See CONTINENTAL DRIFT; ISOSTASY; PLATE TECTONICS. 
    The asthenosphere is thought to be that range of depth where the actual 
earth temperature is closest to the temperature of incipient melting of mantle 
rocks. In some places the temperature may actually reach the rock-melting 
temperature and produce magma, thus giving the asthenosphere another important 
role in geodynamics - as the source region of many types of igneous rocks. 
    One consequence of the ability of the asthenosphere to relieve stress 
through creep and flow is that it appears to be largely earthquake-free. Those 
intermediate- and deep-focus earthquakes that do occur in the depth range of 
the asthenosphere are nearly all found in regions where rigid lithosphere has 
been subducted (recycled) into the Earth's interior and has penetrated the 
asthenosphere at subduction zones. Deep earthquake activity appears to occur 
within the subducted lithosphere and not in the surrounding asthenosphere. See 
EARTHQUAKE.  
 �34s[Henry N. Pollack]
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�6v�2v�32sAsthma   �21m �33s 
    A common disease of the bronchioles that is marked by wheezing, 
breathlessness, and sometimes cough and expectoration of mucus. Typical asthma 
is intermittent or paroxysmal, but a small proportion of patients suffer 
continuously if not treated. If severe enough, asthma may become resistant to 
conventional treatment, and hospitalization is necessary (status asthmaticus). 
    Asthma may begin at any age, but most cases develop before age 60 and 
about half before age 10. For some reason childhood asthma is more common in 
boys than girls. Asthma is a heterogeneous disease, with one common feature: 
obstruction to air flow which is due to hyperreactive bronchioles. This 
obstruction may be produced or aggravated by numerous factors. Some are 
nonspecific, such as dust, tobacco smoke, polluted air, or psychological 
stress, and may affect most people with asthma. 
    In about one-third of all cases, including most children, the obstruction 
has been established to be an allergic reaction. People with allergic asthma 
have acquired specific sensitivity to common inhaled substances that are 
ordinarily innocuous, including house dust, animal danders, fungus spores, and 
pollens. Certain foods may also provoke asthma, especially in early childhood. 
These inhaled substances and foods are called allergens. 
    Chronic asthma is often associated with or initiated by bronchopulmonary 
or sinus infections. Most adult-onset asthma is of this type. Besides 
infections, other nonallergic factors that may produce or aggravate asthma in 
some patients, but not others, are exercise, certain occupational dusts, and a 
few drugs, especially aspirin. Some asthmatic patients, for unknown reasons, 
become extremely intolerant of aspirin, which has produced fatal reactions. 
The problem usually develops in adults, especially those with a history of 
sinus disease and nasal polyps, but occasionally appears in children with 
allergic asthma as well. An intolerant person may also have trouble with other 
drugs that are similar to aspirin in their action. 
    Whatever the type, asthma has typical anatomic features. All cases suffer 
from spasm of the smooth muscle of the bronchioles and from swelling of the 
mucous lining. More severe and chronic asthma may be associated with 
accumulation of inflammatory cells, basement membrane thickening, mucous gland 
enlargement, and filling of the airway with thick mucus. During asthma 
attacks, air can enter the lungs more readily than it can leave, so the lungs 
become hyperinflated. 
    Asthma can rarely be cured, but almost always can be controlled by a 
variety of methods. The environment should be altered or the patient relocated 
to minimize exposure to allergens and irritants. Basic drugs are the 
bronchodilators: theophylline and modern relatives of adrenaline. For more 
severe cases, treatment with a glucocorticoid (steroid) drug may be necessary. 
Immunotherapy (desensitization) is an appropriate way to treat patients with 
allergic asthma who are unable to avoid allergens that are responsible for 
their symptoms. Psychotherapy may be necessary for the occasional depressed 
patient, but is not generally helpful. See RESPIRATORY SYSTEM DISORDERS. 
 �34s[Paul P. Van Arsdel, Jr.]
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�6v�2v�32sAstigmatism   �21m �33s 
    The optical aberration which occurs because a wave surface in general has 
double curvature. Even for a small circular stop, the rays from an object 
point do not come to a point focus but intersect a set of image planes in a 
set of ellipses, the diameters of which are proportional to the distances of 
the two foci from the image plane under consideration. Such an error exists 
even on the axis in systems which are not rotation symmetric, such as 
cylindrical and toric lenses and the astigmatic eye. In systems with rotation 
symmetry, it exists in general for the rays from an off-axis point going 
through a small pupil. 
    Astigmatism on the axis is a common error in the human eye arising from 
the fact that the refracting surfaces, especially the cornea, can have 
different powers in different meridians. It can be corrected by a spectacle 
lens in which at least one surface has different curvatures in different 
planes through the lens axis. The lens may be a cylinder, a torus, or a 
surface of second order with double symmetry. See ABERRATION (OPTICS); VISION. 
 �34s[Max J. Herzberger]
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�6v�2v�32sAstomatida   �21m �33s 
    An order of protozoans, subclass Holotrichia, in which all species are 
mouthless. All species are parasitic in other animals, typically oligochaete 
annelids. Many astomatids possess an elaborate holdfast organelle. Anoplophyra 
is a typical example. See CILIOPHORA; HOLOTRICHIA; OLIGOCHAETA; PROTOZOA. 
 �34s[John O. Corliss]
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�6v�2v�32sAstrapotheria �21m �33s
    A relatively small group of extinct South American ungulates, ranging from
the late Paleocene to the late Miocene. They are customarily divided into two
suborders: the late Paleocene - Eocene Trigonostylopoidea, and the early
Eocene - late Miocene Astrapotheroidea.
    One of the most spectacular and advanced members of the order was
Astrapotherium (see illustration). This animal, known from the late Oligocene
to the late Miocene, averaged 9 - 10 ft (2.7 - 3 m) in length, although some
other forms grew even larger. The anterior part of the skull was striking with
the huge, persistently growing, curving canines, whose function is
unexplained. The retracted, chopped-off appearance of the snout region
strongly suggests that this animal had a moderately large trunk. The front
legs were somewhat more strongly constructed than the hind ones, and this
makes the habits of this animal a puzzle. See CONDYLARTHRA; MAMMALIA.

     FIGURE CAPTION.  Astrapotherium magnum skeleton.

 �34s[Frederick S. Szalay]
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�6v�2v�32sAstrometry �21m �33s
    Measurement of space-time relations of celestial objects, primarily as
observed on the celestial sphere, that is, perpendicular to the line of sight.
    Over the past two centuries the positions of celestial objects have been
measured visually with small, stable telescopes designed to measure meridian
altitudes with high precision. These transit or meridian-circle telescopes and
their variations, in combination with sidereal clocks, are the instrumental
backbone of classical knowledge of the positions of celestial objects on the
celestial sphere.
    While the positions of Sun, Moon, and planets are thus easily obtained,
only stars brighter than about the tenth magnitude can be observed in this
fashion, because of the limited size of the instruments. Photographs, derived
from standardized small telescopes in different locations, have yielded data
which includes stars down to the thirteenth magnitude. The positions of stars
down to the eighteenth magnitude and fainter are obtained photographically by
specially designed powerful telescopes of short focal ratio. Among the most
important results of the photographic method is the wealth of newly discovered
faint stars of large proper motions, all of which have proved to be
comparatively nearby. See ASTRONOMICAL PHOTOGRAPHY.
    A special branch of astrometry is long-focus photographic astrometry.
Accurate stellar positions referred to faint, distant reference stars are
measured by means of refractors of focal lengths of 8 m or more, commonly of
large apertures of 40 cm or more; that is, with focal ratios up to f/20.
Originally, long-focus photographic astrometry was developed for the specific
problem of stellar parallax, and as a result, a great body of information on
the distances of stars now exists. The method has been extended to other
astrometric problems requiring high precision, such as the orbital motion of
double stars, including the discovery and measurement of unseen companions
from the perturbations on visible primary stars. See PARALLAX (ASTRONOMY).
 �34s[Peter Van deKamp]
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    The field of radio astrometry is achieving very precise positions (0.005
arc-second) of celestial radio sources. The technique utilizes two radio
telescopes configured as an interferometer. One method employs two radio
telescopes, situated a few kilometers apart, linked by cables to a receiving
system that instantly measures the difference in phase between the radio
signals arriving at the telescope. Another technique, very-long-baseline
interferometry (VLBI), utilizes a very accurate timing system, usually atomic
clocks, to record the signals received at the end of a baseline several
thousand kilometers apart.
 �34s[Benny L. Klock]
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�6v�2v�32sAstronautical engineering   �21m �33s 
    The engineering aspects of flight and navigation in space, also known as 
astronautics. Astronautical engineering deals with vehicles, instruments, and 
other equipment used in space, but not with the sociological or economic 
aspects of space flight, except as they influence the equipment. 
    There is a lack of parallelism between astronautic and aeronautic vehicle 
terminology. An aircraft is a self-contained vehicle, having within its 
structure essentially all the equipment required to transport its payload from 
one place to another. A spacecraft, in the more restricted sense, is the 
container for the payload. Sometimes the word is used to denote the container 
and payload. Most spacecraft, to date, have had either very limited propulsion 
or none at all. Since enormous speeds are the hallmark of all astronautic 
missions, unpowered spacecraft require a ``booster,'' or ``launch vehicle,'' 
usually a rocket many times as large as the spacecraft. The weight of the 
spacecraft, in fact, seldom exceeds 5% of the total launch vehicle weight. 
    It is extremely expensive to put a pound of payload into Earth orbit. Thus 
designers have been justified in going to great lengths to convert a pound of 
structure into a pound of payload. Great improvement appears possible in this 
respect; only the cost of the propellant seems to be irreducible. In view of 
the high cost of space operations, it is especially important that space 
vehicles operate long enough to successfully fulfill their missions. A severe 
reliability requirement is thus imposed upon vehicles and equipment intended 
for missions, such as journeys to the planets, which may require up to a year 
or more to accomplish. For complex equipment in space vehicles, operating 
lifetimes of this order of magnitude are difficult to attain. The requirements 
for high reliability and low weight add tremendously to the cost of the 
payloads themselves, to the extent that their cost approaches that of the 
launch vehicle. All space missions through 1975 used expendable launch 
vehicles. The space shuttle, a reusable launch vehicle, is expected to reduce 
the costs of Earth-to-orbit transportation. See SPACE SHUTTLE. 
    Gravity is a dominating influence in the design of space launch vehicles. 
Despite the fact that the pull of gravity extends to infinity, it is 
nonetheless possible to escape permanently from the Earth's gravity in the 
sense of never being drawn back to the ground. The key is speed. Circular 
velocity is the minimum at which a space vehicle can remain permanently above 
the Earth. At low altitudes, this velocity is about 25,000 (ft/s; 7.9 km/s). 
As the speed is increased above the circular velocity, the path of a vehicle 
becomes a larger circle or an elongated ellipse. When the speed reaches 37,000 
ft/s, or about 7 mi/s (11.2 km/s), the path becomes a parabola and the vehicle 
will travel along one of the legs to infinity without further propulsion. See 
ESCAPE VELOCITY; SATELLITE (SPACECRAFT). 
    These velocities are tremendous by any previous standard. To reach them, a 
vehicle must carry the corresponding amount of energy in the form of 
propellant.  
    Even with the most energetic propellants and the lightest structures, it 
has not yet been possible to reach orbital velocity with a single rocket. To 
overcome this seemingly insurmountable obstacle, one rocket is carried as the 
payload of a larger one. When the larger burns out, the second is ignited and 
adds its velocity to that of the first. This is known as the step-rocket or 
staging technique. For lunar and planetary missions, lightweight vehicles, 
powerful propellants, and many stages are used. The lunar orbit rendezvous 
method required a total of six stages to take the Apollo astronauts to the 
Moon and back. See ROCKET STAGING; SPACECRAFT PROPULSION. 
    Although propulsion is the key to space flight, other elements are 
essential and present numerous new problems. One such element is guidance and 
control. For the ascent phase of space vehicle flight, guidance systems 
similar to those used for ballistic missiles are employed. Another control 
requirement of many types of space vehicles is that of maintaining the desired 
vehicle attitude over long periods of time. Displacement gyroscopes, even 
excellent ones with very low drift rates, cannot provide an accurate reference 
for days or weeks. Such devices must be corrected frequently by an external 
reference.  
    At least two such references are available: sources of electromagnetic 
radiation, and the gravitational gradient. The first might be used by such 
devices as a Sun seeker, a star tracker, or a horizon scanner. In the vicinity 
of the Earth (or any large celestial body) the difference in the pull of 
gravity between points on the craft having different distances from the Earth 
can be usefully employed. 
    Reaction wheels or other devices capable of storing angular momentum may 
be used to provide the torque to effect or maintain a given orientation. Such 
devices are very efficient, both from a weight and an energy standpoint, where 
disturbing torques on the spacecraft are small, random, and long continued. At 
the opposite end of the torque spectrum, torques that are large and 
uncompensating, rocket engines are the most suitable. 
    Vehicle and payload equipment require electric power. For small amounts of 
energy, chemical sources, such as batteries or chemically fueled generators, 
may be used. A great deal more energy can be obtained from a nuclear reactor. 
Energy also comes continuously from the Sun but at a fairly low density at 
Earth's distance. 
    Communications equipment comprises an essential item of nearly all space 
vehicles. This equipment is designed for light weight, low power consumption, 
and, usually, long life. 
    Although a large percentage of the problems of space flight are associated 
with the vehicles, it would be a mistake to assume that these constitute even 
a major fraction of the total operating system. Indeed, the cost of overcoming 
the Earth's gravity is so great that any portion of the total operation which 
can be performed on the ground should be done there. The supporting ground 
equipment consists of the preparation and launching equipment, and the 
tracking, communications, and payload-oriented equipment for turning the 
received data into usable form. For missions which involve return of space 
vehicles or booster rockets, recovery equipment may also be required. See 
LAUNCH cOMPLEX; SPACECRAFT GROUND INSTRUMENTATION. 
    In their interaction with the terrestrial and atmospheric environment 
during reentry, space vehicles resemble ballistic missiles. However, although 
ballistic reentry techniques have been proved successful, the use of winged 
vehicles also has certain attractive aspects. There is a basic difference in 
these two methods in respect to the way atmospheric heat is handled. The 
ballistic approach absorbs the heat in the reentry body or rejects it back to 
the air by mass transfer. The winged vehicle dissipates the heat by radiation. 
Considerable research has been done on compromise reentry vehicles, such as 
the lifting body approach. The orbital stage of the shuttle is a winged craft 
designed to land like an airplane. It utilizes a combination of techniques to 
overcome the reentry heating problems: lift, temperature-resistant materials, 
and local ablative cooling. See ATMOSPHERIC ENTRY. 
    Astronautical engineering must contend with the unique environment of 
space outside the Earth's atmosphere. Although gravity is present in space, 
whenever a vehicle is coasting unpropelled, the shell and everything in it are 
acted on equally by gravity and therefore appear weightless. Fluids do not 
flow naturally, but must be confined and extruded. Liquids exposed to the 
vacuum of space evaporate or freeze. External transfer of heat takes place 
only by radiation. Metals exposed to the ultraviolet rays of the Sun emit 
electrons. Small particles of cosmic dust strike external surfaces at 
fantastic velocities and gradually erode them. Cosmic radiation creates a 
spectrum of secondary radiation that may reach levels damaging to equipment or 
personnel.  
 �34s[Robert C. Truax]
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�6v�2v�32sAstronautics   �21m �33s 
    The application of scientific principles and engineering techniques to 
flight in space. Astronautics deals with space vehicles in the sense that 
aeronautics deals with aircraft. The distinguishing feature between 
astronautics and aeronautics is the extent to which the vehicles are 
influenced by the Earth's atmosphere. Astronautics encompasses both crewed and 
uncrewed missions. See AERONAUTICS: ASTRONAUTICAL ENGINEERING: SPACE 
TECHNOLOGY.  
    The subject matter of astronautics is flight in regions where a vehicle 
overcomes gravitational attraction and controls its course by reactive 
propulsion. Aeronautics concerns flight in regions where a vehicle resists 
gravitational attraction and controls its course by aeromechanical forces. The 
distinction is convenient but not clear cut. Rockets, by their reaction, 
assist airplanes to take off. Space vehicles may glide back to Earth. See 
INTERPLANETARY PROPULSION: ROCKET ENGINE. 
 �34s[Robert C. Truax]
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�6v�2v�32sAstronomical coordinate systems �21m �33s
    Systems of spherical coordinates serving to locate astronomical objects on
the celestial sphere, which is the sphere of indeterminate radius, with its
center at the observer, on the inside-surface of which astronomical objects
may be imagined to be projected. The celestial sphere has no physical
existence; it is a concept convenient for specifying directions in space.
    The system of astronomical spherical coordinates is analogous to the
system of latitudes and longitudes on the Earth, the most important difference
being that the Earth is viewed from the outside and the celestial sphere is
viewed from the inside. Every such system contains two diametrically opposite
poles, a north pole and a south pole, the north pole being the one that is
visible from the North Pole of the Earth. There is a primary circle, which is
a great circle of the sphere, so placed that each pole is 90 DG from it. The
latitudinal coordinate of an object is its angular distance measured northward
or southward from the primary circle. Small circles parallel to the primary
circle are called parallels. The longitudinal coordinate of an object is the
angular distance measured along the primary circle from a specified origin to
the point of intersection with a great circle passing through the object and
the two poles: the one of the two intersections that is nearest the object is
used. The great circles passing through the poles and perpendicular to the
primary circle are sometimes called secondary circles. See LATITUDE AND
LONGITUDE.
    The astronomical system most used for locating objects is the equatorial
system (see illustration). It is the geometrical extension of the terrestrial
system. The poles are the intersections of the Earth's axis of rotation with
the celestial sphere, and the primary circle is called the celestial equator,
or simply equator. The latitudinal coordinate is called declination, and the
parallels are parallels of declination. The longitudinal coordinate is called
right ascension. The origin is the vernal equinox, which is the point where
the Sun crosses the equator about March 21.

    For some purposes it is convenient to modify the equatorial system by
using an origin for the longitudinal coordinate that is fixed with respect to
the observer. The origin is then the intersection of the celestial equator
with the observer's meridian, and the corresponding coordinate is called the
hour angle. Another system much used for theoretical studies of planetary
motion is the ecliptic system. The primary circle is the ecliptic which,
except for very small deviations, is the path traced out by the Sun on the
celestial sphere during a tropical year. A system used for studies of stellar
motions is the galactic system. The primary circle is the plane of the galaxy
and is called the galactic equator. The coordinates are called galactic
latitude and galactic longitude. The horizon system is fixed relative to the
observer. The poles are the zenith, which is the point directly overhead, and
the nadir. The primary circle is the horizon. The latitudinal coordinate is
called altitude if it is measured upward from the horizon, and zenith distance
if it is measured downward from the zenith. The longitudinal coordinate is
called azimuth. It may be measured in several ways, the commonest origin being
the north point of the horizon.

     FIGURE CAPTION.  Equatorial astronomical coordinate system.

 �34s[Gerald M. Clemence]
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�6v�2v�32sAstronomical observatory   �21m �33s 
    A building or group of buildings which houses optical telescopes or the 
electronics required for radio telescopes or space telescopes, and the 
astronomers studying the observations. There are over 300 active astronomical 
observatories, with over 50 of them in the United States. This omits hundreds 
of amateur telescopes, five or six Earth-orbiting space observatories 
controlled by radio and computers operated by astronomers on the ground, and 
at least one high-flying airplane equipped with telescopes. The optical 
telescopes used at these observatories vary widely in size, from the huge 
200-in. (5.1-m.) mirror reflector and the 48-in (1.2-m) Schmidt at Palomar 
Mountain in California, used mostly to study faint stars, nebulae, and 
galaxies, to the 10-in. (0.25)-m) lens telescopes at Zurich, Switzerland, used 
to study sunspots. Radio telescopes vary from the 1000-ft (305-m) bowl at 
Arecibo, Puerto Rico, through the 300-ft (91-m) dish at the National Radio 
Astronomy Observatory in West Virginia, to arrays of much smaller dishes. The 
telescopes in orbit outside the Earth's atmosphere are smaller and designed 
for special purposes, although the Space Telescope, to be put in orbit by NASA 
in 1985, has an aperture of 95 in. (2.4 m). 
  
    The typical optical observatory has a telescope in a building with a 
hemispherical, rotatable dome in which there is a slot up one side with a 
cover that can be pulled aside from within, so that the telescope can ``see 
out'' (see illustration). Starting about 1890, astronomers took photographs, 
using the telescope as a large camera, so the observatories have been equipped 
with darkrooms where photographic plates can be developed, plate files where 
they are stored, and measuring engines which are used to measure star images 
on the plates accurately. After 1910 or so, spectrographs were attached to the 
telescopes to spread the light of a star into a spectrum from red to blue. 
Photographs of these spectra (spectrograms) are measured on a one-dimensional 
measuring engine - a microscope mounted on a long, accurate screw. Since the 
mid-1930s, electronic detectors have been used on optical telescopes to 
measure starlight accurately. The electronics in large observatories have 
become increasingly complex. Since radio and space telescopes are all 
electronic, modern observatories require electronic technicians and computers. 
See ASTRONOMICAL PHOTOGRAPHY; ASTRONOMICAL SPECTROSCOPY; OPTICAL TELESCOPE; 
RADIO ASTRONOMY; RADIO TELESCOPE; SCHMIDT CAMERA; TELESCOPE. 
 �34s[Thornton Page]
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�6v�2v�32sAstronomical photography   �21m �33s 
    The application of the photographic process to astronomy. Over the past 
century, photographic observations have made profound contributions to 
astronomy and astrophysics; and for much of this period, photographs were the 
best light detectors available for a wide range of applications. Since the 
mid-1970s, a variety of image detectors utilizing photocathodes or silicon 
diode arrays have offered detective performance several times better than the 
best photographic materials. However, it is clear that many astronomers will 
continue to use photographic techniques for some time to come and for a number 
of reasons, including low initial cost, relative ease of use, and tradition. 
In particular, for applications where coverage of a wide field is required, 
photography is still the detector of choice, because emulsions can be 
manufactured in much larger formats than electronic detectors. 
    Nearly all astronomical photography is done with special spectroscopic 
plates, which are available in several spectral sensitizations. These 
materials come in several classes of grain sizes. The ultimate accuracy with 
which photographic photometry, the determination of the amount of liquid that 
exposes each picture elements, can be carried out is set by the uncertainties 
due to graininess. For long exposure times there is a serious breakdown in the 
reciprocity law that would predict that the photographic sensitivity should 
depend only on the product of the exposing intensity I and the exposure time 
t. The loss of sensitivity at low light levels and long exposure times is 
often called low-intensity reciprocity failure (LIRF). During the 1970s there 
were major advances in understanding the sources of low-intensity reciprocity 
failure and in developing various treatments for its reduction. Most of these 
hypersensitization techniques involve preexposure treatments such as 
evacuation, baking, bathing, or hypering with hydrogen gas. Many of the 
procedures work best if the exposures are made in a controlled atmosphere by 
using special cassettes. Since at least five different mechanisms are 
involved, it is often beneficial to combine treatments. 
    Even when they are optimally hypersensitized, astronomical plates have a 
detective quantum efficiency of only about 2 or 3%. Photocathodes, on the 
other hand, can have quantum efficiencies as high as 25% or more. A stack of 
image tubes is often used at the focus of a spectrograph or direct camera, 
with the image from the final phosphor screen transferred optically onto a 
film or plate for recording a long exposure. The disadvantages of intensified 
photography are the distortions, nonuniformities, blemishes, and ion flashes 
that plague many tubes. Moreover, the tubes must be cooled to reduce the dark 
current for long exposures. Perhaps the biggest disadvantage for survey work 
is that the largest photocathodes are rarely 4 in. (100 mm) in diameter. 
Astronomers have also developed special image tubes which use a nuclear track 
emulsion to record directly the electrons produced at the photocathode. 
_Enormous effort has gone into the development of detector systems which give 
out the image as an electrical signal which can be digitized either for 
immediate computer processing or magnetic storage for later analysis. These 
systems divide roughly into two classes. In the first class, a photocathode 
device such as an image intensifier chain is used to detect the incoming 
light, while an electronic device such as a television tube or diode array is 
used to read out the intensifier image. In the second class, silicon diode 
arrays are illuminated directly. 
    While the development of new electronic imaging hardware has improved the 
sensitivity of the big telescopes dramatically, perhaps the greatest advances 
in astronomical imagery, both photographic and electronic, have come in the 
area of image processing. Not only are modern high-precision microphotometers 
commonly available for digitizing photographic images, but elaborate computer 
schemes for processing and analyzing the images have been developed at several 
centers.  
 �34s[David W. Latham]
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�6v�2v�32sAstronomical reference frame �21m �33s
    A frame of reference that is fixed relative to celestial objects. The most
convenient one for observational purposes is, of course, a geocentric system.
The Earth's axis of rotation is used as a coordinate axis. A second reference
direction is chosen as the intersection of the Earth's equatorial plane and
the plane of the Earth's orbit (the ecliptic). The basic time unit is the
Earth's rotation period, the sidereal day. The positions and times of distant
astronomical events are determined indirectly. See ASTRONOMICAL COORDINATE
SYSTEMS.
    Reference frames are regarded as more basic as more phenomena appear
regular with respect to them. Thus the planetary motions are described
naturally in an ecliptic coordinate system centered on the Sun, and the
motions of stars as viewed from the Earth appear regular in a coordinate
system with its origin at the center of the galaxy. See FRAME OF REFERENCE.
 �34s[Bernard Goodman]
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�6v�2v�32sAstronomical spectroscopy   �21m �33s 
    The use of spectroscopy as a technique for obtaining observational data on 
the compositions, physical conditions, and velocities of astronomical objects. 
Astronomical applications of optical spectroscopy from ground-based 
observatories cover the electromagnetic spectrum from the infrared through the 
visible to the near-ultraviolet. The advent of space-age techniques - 
high-altitude balloons, rockets, and satellites - has extended spectroscopic 
observations through the far-infrared to the domain of radio astronomy, and 
through the far-ultraviolet to the region of x-rays and gamma rays. See 
GAMMA-RAY ASTRONOMY; RADIO ASTRONOMY; X-RAY ASTRONOMY. 
    In optical spectroscopy a spectrograph is attached to a telescope, which 
serves as a light collector. If the telescope is of the reflector type, one 
distinguishes between prime-focus, newtonian, Cassegrain, and coude 
spectrographs, depending on the focus at which they are used. The coude focus 
is preferred for high-dispersion work. The image of the celestial body being 
studied is focused on the spectrograph slit by the telescope objective. The 
light from the slit passes through the collimator, which is a lens or mirror. 
The parallel rays of light are dispersed by glass or quartz prisms or, as in 
most modern spectrographs, by a diffraction grating. The grating is usually 
``blazed'' by shaping its grooves in such a fashion that most of the dispersed 
light is cast into a chosen order of the spectrum. See DIFFRACTION GRATING; 
OPTICAL TELESCOPE; SPECTROSCOPY. 
    Until the 1970s, spectra were recorded photographically. When high 
photographic speeds are required, Schmidt cameras are often used. If only a 
small field (2 - 3 cm) in the focal plane suffices, an image tube can be used. 
The thus intensified image can be registered on a photographic plate or can be 
scanned electronically, as in the Robinson-Wampler image tube scanner. 
    Fourier transform spectroscopy, used particularly in infrared work, 
employs a concept entirely different from the spectrographs described. Instead 
of being dispersed in a spectrograph, the light of a wide band of wavelengths 
is passed through a Michelson interferometer with variable spacing of its two 
apertures. The resulting interferogram, which is an electronic record of the 
interference signal produced by the interferometer as the separation of the 
apertures is varied, is converted into a record of intensity versus wavelength 
by a high-speed computer. See INTERFEROMETRY. 
 �34s[L. H. Allen, Jr.]
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�6v�2v�32sAstronomical transit instrument   �21m �33s 
    A telescopic instrument adapted to the observation of the passage, or 
transit, of an astronomical object across the meridian of the observer. The 
astronomical transit instrument is the classic instrument of positional 
astronomy, the study of the positions and motions of astronomical objects and 
the related determination of positions by observation of these astronomical 
bodies from the Earth (the specific categories of astronomy concerned with 
these investigations are astrometry and celestial mechanics). The chief 
variants of the classic design include the meridian circle, the vertical 
circle, the horizontal transit circle, the broken or prism transit, and the 
photographic zenith tube. 
    The modern transit instrument has a telescopic objective with a diameter 
of 6 - 10 in. (15 - 25 cm) and a focal length of 72 - 90 in. (180 - 230 cm). 
The instrument consists of a telescope mounted on a single fixed horizontal 
axis of rotation. The horizontal axis has a central hollow cube (sometimes a 
sphere) and two conical semiaxes ending in cylindrical pivots. The objective 
and eyepiece halves of the telescope are also fastened to the cube of the 
instrument, perpendicular to the horizontal axis. Rotation of the instrument 
in its bearings, or wyes, permits the optical axis to sweep only in the plane 
of the meridian. An accurate clock is the essential ancillary scale by which 
the transits of the astronomical objects are observed.  See  ASTRONOMICAL 
COORDINATE SYSTEMS. 
  
    The astronomical transit instrument has three interrelated uses. (1) From 
a known position on the Earth, observations of transits of stars lead to the 
determination of their right ascension with respect to the astronomical 
coordinate system. (2) The determination of corrections to the clock may be 
made by the observation of stars of known position with an instrument situated 
at a known longitude. (3) Finally, with a knowledge of the positions of the 
stars observed and of Greenwich time, the longitude of the observer can be 
computed from observations of the time of transit of a star.  See  OPTICAL 
TELESCOPE.  
 �34s[Benny L. Klock]
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�6v�2v�32sAstronomical unit   �21m �33s 
    The basic unit of length in the solar system. The astronomical unit (AU) 
is used to a limited extent for interstellar distances as well through the 
definition of the parsec (1 pc = 206,265 AU). It is nearly equal to the mean 
distance a between the center of mass of the Sun and the center of mass of the 
Earth-Moon system (a = 1.000 000 23 AU), and for that reason it is often 
convenient to think of it as the mean distance between the Sun and Earth. See 
PARSEC.  
    The most accurate determination of the length of the astronomical unit in 
physical units, such as meters, is obtained from phase-modulated 
continuous-wave (CW) radio signals beamed to other planets. The round-trip 
travel times of the signals are determined by cross-correlating the returned 
signal from the planet with the transmitted signal, and as a result, planetary 
distances are measured directly. CW signals returned from the two Viking 
orbiters and two landers on the surface of Mars have made possible a 
determination of the astronomical unit to an accuracy of 100 m (330 ft). With 
an adopted value of 299,792.458 kms for the speed of electromagnetic 
propagation in vacuum, the value of the astronomical unit from the Viking data 
is 149,597,870.7 km. See DOPPLER EFFECT; PARALLAX (ASTRONOMY).  
 �34s[John D. Anderson]
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�6v�2v�32sAstronomy   �21m �33s 
    The science concerned with the observation and interpretation of the 
radiation received in the vicinity of the Earth from the component parts of 
the universe. For the greater part of the history of humanity, the radiation 
was confined to the visual portion of the electromagnetic spectrum, and the 
eye was the sole receiver. The invention of the telescope greatly enhanced the 
power of visual observation of celestial objects, revealing heretofore hidden 
details on the Sun, Moon, and planets and disclosing the existence of hordes 
of stars never before seen by the human eye. The increased light-gathering 
power of the telescope made possible greatly refined observations, which went 
hand in hand with growing philosophical and theoretical considerations of the 
nature of the solar system and the universe. These advantages in turn led to 
the adoption of the heliocentric view of the solar system and the rise of 
celestial mechanics, which flowered under I. Newton and his followers and gave 
a detailed physical picture of the motion of celestial objects under 
gravitational forces.  
    The telescope made possible the recognition of stars as distant suns 
through the measurement of their parallaxes and proper motions. Study of the 
physical nature of the stars became possible when the telescope was 
subsequently used in conjunction with auxiliary instruments such as the 
photometer and the spectrograph, designed to measure and analyze the radiation 
from the stars. The introduction of the photographic plate was the significant 
step that made possible physical astronomy, or the application of modern 
physics to astronomy astrophysics. See ASTROPHYSICS;PHOTOMETRY; SPECTROGRAPHY; 
STAR; SUN; TELESCOPE. 
    The limitation of astronomical observations to the visual portion of the 
electromagnetic spectrum, imposed by the human eye, was eliminated by the 
extension of the spectral sensitivity of astronomical instruments and 
recording devices, including the photographic plate, to the infrared and 
ultraviolet portions of the spectrum. The most dramatic advance was the 
development of the radio telescope, which has made radio astronomy a major 
branch of astronomy. 
    The observation of the total electromagnetic spectrum of celestial sources 
came with the lifting of the opaque atmospheric veil by the use of 
high-altitude balloons, rockets, and craft orbiting well above the Earth's 
atmosphere. Space probes to the Moon and nearer planets placed astronomical 
equipment in their vicinity and even upon them. Rockets, high-altitude 
balloons, and satellites have spawned new branches of astronomy - x-ray and 
gamma-ray astronomy. The existence of sources of x-rays and gamma rays on the 
celestial sphere has been revealed in a manner reminiscent of the first 
detection of discrete radio sources. See CELESTIAL SPHERE; GAMMA-RAY 
ASTRONOMY; X-RAY ASTRONOMY; X-RAY STAR; X-RAY TELESCOPE. 
    The theoretical and philosophical aspects of astronomy, once concerned 
with the central position of the human race in a relatively tiny universe, are 
now concerned with the problems of cosmology and cosmogony. Interpretation of 
the observed red shift of distant galaxies as evidence for the expansion of 
the universe is theoretically supported by relativistic cosmology, which 
presents a picture of the universe in which every galaxy appears to be a 
center from which all other galaxies are receding with speeds proportional to 
their distances from that given galaxy. See COSMOLOGY; GALAXY; GALAXY, 
EXTERNAL; INFRARED ASTRONOMY; PULSAR; REDSHIFT; RELATIVITY. 
 �34s[J. Allen Hynek]
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�6v�2v�32sAstrophysics   �21m �33s 
    The application of modern physics to the problems of astronomy. 
Astrophysics embraces much of the activity of present-day astronomy; 
specifically excluded are the study of the motions of the planets and 
satellites (celestial mechanics), the measurement of positions and motions 
(astrometry), and usually also the structure and dynamics of the Galaxy and 
other galaxies, although such topics as density wave theory of spiral arms are 
sometimes considered to be part of astrophysics. See ASTRONOMY; GALAXY. 
    From an operational point of view, astrophysics differs from terrestrial 
physics in that it is primarily an observational subject, although certain 
special problems lend themselves to experimental treatment. The basic problem 
is often the measurement of the quantity and quality, that is, the energy 
distribution and polarization, of the electromagnetic radiation of the Sun, 
stars, planets, and nebulae, and the study of their spectra. 
    The field of experimental astrophysics involves measurement of shapes and 
strengths of spectral lines emitted under controlled conditions of temperature 
and pressure. Techniques involve the use of shock tubes, atomic beams, 
controlled arcs, beam foils, laser excitation of discrete levels, and so 
forth. Also involved are measurements of cross sections for atomic and 
molecular collisional excitation and ionization, low-energy nuclear 
transformations, and even synthesis of molecular fragments that may be found 
in interstellar space. 
    Theoretical astrophysics constitutes a branch of theoretical physics and 
embraces many subjects that usually are considered the province of the latter. 
For purposes of discussion, astrophysics may be divided somewhat as follows, 
according to the types of celestial bodies that are involved. 
    Solar physics includes all phenomena connected with the Sun and overlaps 
with geophysics in the consideration of solar-terrestrial relationships, for 
example, the connection between solar activity and auroras, magnetic storms, 
and sudden ionospheric disturbances. See SUN. 
    The physics of the solar system includes the nature of planetary 
atmospheres and interiors and the chemical and physical constitution of 
comets, meteors, and small particles such as those that constitute the 
zodiacal cloud. In addition, much attention is paid to interplanetary plasmas 
(the solar wind) and magnetic fields, and to the magnetosphere of the Earth 
and Jupiter. See COMET; COSMOCHEMISTRY; MAGNETOSPHERE; METEOR; METEORITE; 
PLANETARY PHYSICS; SOLAR WIND. 
    The study of stellar atmospheres, including certain aspects of the solar 
atmosphere, constitutes an important and active field of astrophysics. Much 
effort is devoted to gaseous nebulae and the interstellar medium. The former 
includes examples of both thermal excitation, as in Orion and planetary 
nebulae, and nonthermal excitation, as in the Crab Nebula. The interstellar 
medium pervades the galactic plasma and extends beyond it as the galactic 
halo. Stellar structure and evolution trace the development of a star from its 
condensation from the interstellar medium to its eventual fate as a white 
dwarf, neutron star, or black hole. See BINARY STAR; BLACK HOLE; INTERSTELLAR 
MATTER; NEBULA; NOVA; STELLAR EVOLUTION; SUPERNOVA; WHITE DWARF STAR. 
    Radio astronomy data and observations secured from above the Earth's 
atmosphere have greatly stimulated the development of high-energy 
astrophysics, particularly the study of the acceleration of charged particles 
to high energies in supernova remnants, pulsars, and quasistellar sources. 
Much attention is paid to galaxies, both normal and exotic. See GALAXY, 
EXTERNAL; PULSAR; QUASARS. 
    Verifications of general relativity are astronomical or astrophysical. In 
addition to classical tests such as deflection of light by the Sun or advance 
of the perihelion of Mercury, predictions of dissipation of energy by gravity 
waves have been verified by observations of a pulsar binary. General 
relativity has to be applied in interpretation of black holes, which are 
difficult to observe but are believed to occur in certain binary star systems 
and nuclei of galaxies. It is also used in constructing large-scale models of 
the physical universe, in cosmology, and in describing the first moments of 
the universe following the ``big bang.'' See COSMOLOGY; RELATIVITY. 
 �34s[Lawrence H. Allen]
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�6v�2v�32sAsymmetric synthesis   �21m �33s 
    The chemical synthesis of a pure enantiomer or of an enantiomorphic 
mixture in which one enantiomer predominates, without the use of resolution. 
Absolute asymmetric synthesis is achieved by providing dissymmetric conditions 
by purely physical means such as circularly polarized light or, more commonly, 
by an optically active reagent. Where a new asymmetric center is produced, the 
process may be called asymmetric induction, and the selective destruction of 
one member of an enantiomorphic pair may be called asymmetric decomposition. 
    Few absolute asymmetric syntheses have been accomplished experimentally, 
although the natural occurrence of optically active substances presupposes 
significant numbers of such syntheses early in the evolutionary process. 
Asymmetric synthesis generally involves introduction of a new asymmetric 
center into a pure optically active compound; since the original substance is 
not symmetrical, the probability of producing exactly equal quantities of the 
new diastereoisomers (epimers) is small. Asymmetric synthesis in the 
laboratory serves principally for the determination of configurations and for 
study of reaction mechanisms rather than practical synthesis of optically 
active products. See CONFORMATIONAL ANALYSIS; OPTICAL ACTIVITY; RACEMIZATION; 
STEREOCHEMISTRY.  
 �34s[Wyman R. Vaughan]
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�6v�2v�32sAsymptote �21m �33s
    A line that is a limit of lines tangent to a curve as the contact points
of those tangents approach infinity along the curve. Thus, an asymptote of a
curve is an ordinary line (that is, not the ``line at inifinity'') that is
tangent to a curve at the points in which the curve intersects the line at
infinity (see illustration).

     FIGURE CAPTION.  Asymptotes of a hyperbola.

 �34s[Leonard M. Blumenthal]
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�6v�2v�32sAtelostomata   �21m �33s 
    A superorder of the Echinoidea, subclass Euechinoidea, characterized by 
having a rigid, exocyclic test, and lacking a lantern, or jaw, apparatus. The 
included orders are the Holasteroida and Spatangoida. See ECHINODERMATA; 
ECHINOIDEA; EUECHINOIDEA; HOLASTEROIDA; SPATANGOIDA.  
 �34s[Howard B. Fell]
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�6v�2v�32sAthalamida �21m �33s
    An order of Granuloreticulosia in which the naked amebas form branched,
threadlike interconnected pseudopodia (reticulopodia; see illustration).
Species are known from fresh, salt, and brackish water. General
characteristics are difficult to select. Heterogeneity may extend even to a
genus; described species of Biomyxa differ appreciably in morphology and
exhibit, for example, uninucleate and multinucleate condition in different
species (although these may represent young and mature stages in life cycles).
In addition to Biomyxa, the genera Arachnula, Gymnophrys, and Pontomyxa have
been assigned to this order. See GRANULORETICULOSIA; PROTOZOA; RHIZOPODEA;
SARCODINA; SARCOMASTIGOPHORA.

     FIGURE CAPTION.  Biomyxa vagans.

 �34s[Richard P. Hall]
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�6v�2v�32sAthecanephria   �21m �33s 
    An order of Pogonophora, a group of elongate, tentaculated, tube-dwelling, 
sedentary, nonparasitic marine worms lacking a digestive system. The coelom in 
the tentacular region is sac-shaped, with the two ducts set far apart from one 
another, and their excretory (osmoregulatory) sections lie close to the 
lateral cephalic blood vessels. The Athecanephria contain two families: 
Siboglinidae and Oligobrachiidae, with two and five genera respectively. See 
POGONOPHORA; THECANEPHRIA. 
 �34s[Edward B. Cutler]
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�6v�2v�32sAtheriniformes �21m �33s
    An order of actinopterygian fishes that includes the flyingfishes,
needlefishes, killifishes, silversides, and their allies.  Atheriniform fishes
date from the Upper Cretaceous. Modern forms are grouped in 3 suborders, 16
families, about 170 genera, and probably 700 - 800 species. They are chiefly
inhabitants of fresh, brackish, and oceanic surface waters of the tropics and
subtropics, but some enter the temperate zones, both in fresh water and the
sea. They are circumtropical in distribution, abounding in fresh waters of
Africa and the Americas and in all warm seas. None lives in deep oceanic
waters.
    Atheriniformes are mostly small, although some needle-fishes reach 5 ft
(1.5 m) or more in length; most have a single dorsal fin of soft rays, but the
silversides have a short anterior spinous dorsal; the pectoral fin is placed
high on the side; the pelvic fin, if present, is abdominal, subabdominal, or
thoracic and has six (occasionally seven) or fewer rays, one of which may be a
spine; scales are usually cycloid, sometimes ctenoid, and rarely absent. The
most familiar families include flyingfishes (see illustration) and halfbeaks
(Exocoetidae), the former with stiffened, elongated rays on the paired fins
that permit gliding in air; kilifishes (Cyprinodontidae); live-bearers
(Poeciliidae), among them guppies, swordtails, platyfishes, mollies, and
mosquito fishes; and silversides (Atherinidae), that abound on marine beaches
and reefs. Viviparity is more frequent and better understood in Atheriniformes
than in any other order of bony fishes, occurring in no fewer than eight
families. See ACTINOPTERYGII; FLYINGFISH; OSTEICHTHYES; TELEOSTEI.

     FIGURE CAPTION.  California flyingfish (Cypselurus californicus).

 �34s[Reeve M. Bailey]
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�6v�2v�32sAtlantic Ocean �21m �33s
    The large body of sea water separating the continents of North and South
America in the west from Europe and Africa in the east and extending south
from the Arctic Ocean to the continent of Antarctica. The Atlantic is the
second largest ocean water body and in area covers nearly one-fifth of the
Earth's surface. The two major divisions, North and South Atlantic oceans,
have the Equator as the common boundary. The North Atlantic, because of
projecting land areas and island arcs, has numerous subdivisions. These
include three large mediterranean-type seas, the Mediterranean Sea, the Gulf
of Mexico plus Caribbean Sea, and the Arctic Ocean; two small
mediterranean-type seas, the Baltic Sea and Hudson Bay; and four marginal
seas, the North Sea, English Channel, Irish Sea, and Gulf of St. Lawrence.
Parts of the Atlantic are given special names but lack precise boundaries,
such as the Bahama Sea, Irminger Sea, Labrador Sea, Sargasso Sea, and Gulf of
Guinea.
    The mean depth of the Atlantic Ocean is 12,960 ft (3868 m), and its volume
is 76,300,000 mi sup 3 (318,000,000 km sup 3). Broad shelves with depths less
than 660 ft (200 m) are found in the region of the North Sea and the British
Isles, on the Grand Banks of Newfoundland, and off the coasts of northeastern
South America and Patagonia. The Mid-Atlantic Ridge, which extends from the
Arctic Ocean to 55 DG S, is less than 9800 ft (3000 m) beneath the surface and
is characterized by a pronounced relief. It separates the east and west Atlantic
troughs, both of which have relatively uniform relief.
    Three marked east-west ridges - the Greenland-Scotland Ridge in the North
Atlantic and the Walvis and Rio Grande Ridges in the South Atlantic - and
several less-conspicuous east-west rises separate the two Atlantic troughs
into a series of basins including the West European, Canary, and Angola in the
eastern Atlantic and the North American, Brazilian, and Argentine basins in
the western Atlantic.
    Islands in the Atlantic are predominantly of volcanic origin. The Bermudas
are the northermost coral reefs, rising from an old submarine volcanic cone.
Some islands, such as the British Isles, are continental in character. See
OCEANIC ISLANDS; WEST INDIES.
    The primary circulation of surface winds over the Atlantic Ocean is
characterized by a zonal distribution pattern oriented in an east-west
direction. The greatest storm frequency, more than 30% in winter, is in the
zone of the prevailing westerlies. Air temperatures also follow a zonal
pattern of distribution. They are lower in the South Atlantic than in the
North Atlantic, and lower in the tropics and subtropics over the eastern
Atlantic than they are in the same latitudes over the western Atlantic.
Maximum precipitation occurs in the doldrum zone (80 in./year or 2000
mm/year). Precipitation also is relatively great in the zone of westerlies but
is low in the trade-wind zones.
    Sea ice is formed in the northernmost and southernmost parts of the
Atlantic Ocean. From these areas drift ice moves equatorward into neighboring
regions where it becomes a hazard to sea traffic and limits fishing. Many
icebergs drift southward into the sea lanes of the North Atlantic. Most of
these have their origin in the valley glaciers of western Greenland. Icebergs
generally drift south of the Grand Banks, and some are known to have drifted
southeast of Bermuda. In the South Atlantic large, tabular icebergs separate
from the Antarctic ice shelf and drift northward. See ICEBERG; SEA ICE.
    Surface currents in the Atlantic Ocean flow in much the same direction as
the prevailing surface winds (see illustration). Deflections from these
directions are caused by the bottom topography and the latitude or increased
effect of Coriolis forces. The fairly constant flow of the North and South
Equatorial currents is sustained largely by the trade winds. As a result, warm
water is piled up along the poleward borders of these currents and on the
western sides of the Atlantic Ocean. See ANTARCTIC OCEAN; ARCTIC OCEAN;
CARIBBEAN SEA; GULF OF MEXICO; GULF STREAM; OCEAN CURRENTS.
    The surface water in certain areas takes on a particularly high density in
winter under the influence of climatic conditions. These water masses sink to
a depth where the surrounding waters have a corresponding density and then
spread out at that level. At the same time they are constantly mixing with the
surrounding waters. In this way a multistoried stratification arises. Compared
with that of the Indian and Pacific oceans, the deep circulation in the
Atlantic Ocean is very vigorous, and the deeper water is therefore rich in
oxygen. The abundance of nutrients permits a greater rate of organic
production where the nutrient-rich waters nearly reach the surface, as in the
Antarctic waters. See SEAWATER; SEA-WATER FERTILITY.
    The semidiurnal tidal form predominates in the Atlantic Ocean. The mean
tidal range is about 3.3 ft (1 m) in the open ocean, but it decreases to 6.3
in. (16 cm) off Rio Grande do Sul in southern Brazil and to 3.5 in. (9 cm) off
Puerto Rico. Tidal ranges increase beyond broad shelves under favorable
physical conditions. The tides of the mediterranean and marginal seas are
cooscillations of the tides of the Atlantic Ocean. See TIDE.
    The Atlantic Ocean, especially the North Atlantic, is by far the most
important bearer of the world's sea traffic. Favorable trends include
increased transportation capacities for handling bulk goods, regular weather
observations for the safety of air and sea traffic by weather ships in
selected positions, and the observation and reporting of drifting icebergs by
the International Ice Patrol. Communication facilities, including telegraph
and telephone cables and radio stations, have been improved and increased in
number.

     FIGURE CAPTION.  Currents of the Atlantic Ocean.

 �34s[Gunter O. Dietrich]
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�6v�2v�32sAtmosphere �21m �33s
    The gaseous envelope surrounding a celestial body. The terrestrial
atmosphere, by its composition, control of temperature, and shielding effect
from harmful wavelengths of solar radiation, makes possible life as known on
Earth. The atmosphere, which is retained on the Earth by gravitational
attraction and in a large measure rotates with it, is a system whose chemical
and physical properties and fields of motion constitute the subject matter of
meteorology. The changing atmospheric conditions which affect the environment,
particularly temperature, wind, humidity, cloudiness, and precipitation,
constitute weather, and the synthesis of these conditions over a period
determines the climate at any place. See CLIMATOLOGY; METEOROLOGY; WEATHER.
    The average atmospheric pressure at the Earth's surface is about 1013
millibars (101.3 kilopascals) and the density 1.2 CTR DOT kg m sup -3, and these
vary by only a few percent over the globe. They both decrease rapidly and
roughly exponentially with height, and at several earth radii the density can
be said to have fallen to that of interplanetary space.
    The atmosphere is thought to have developed as the result of chemical and
photochemical processes combined with differential escape rates from the
Earth's gravitational field. Chemical abundances in the atmosphere, therefore,
are not directly related to cosmic abundances; in particular, the atmosphere
is highly oxidized and contains very little hydrogen.
    The atmosphere, apart from its highly variable water-vapor content in the
troposphere, its liquid droplets and solid matter in suspension, and its
variable ozone content in the stratosphere, is well mixed and constant in
composition up to about 62 mi (100 km). This region is termed the homosphere.
At higher levels where there is little mixing, diffusive separation tends to
take place, with the lighter elements becoming progressively more dominant
with height. Moreover, in this region oxygen and the minor constituents, such
as carbon dioxide and water vapor, are dissociated by solar ultraviolet
radiation. At about 186 mi (300 km) atomic oxygen probably becomes the most
important constituent, until about 800 km where helium and hydrogen in turn
predominate. This region of highly variable composition is termed the
heteorosphere. Ionization of the various constituents, also due to absorption
of ultraviolet solar radiation, becomes a major factor above about 37 mi (60
km), the base of the ionosphere, which is of major importance to radio
communications. At levels above 370 - 500 mi (600 - 800 km), collisions
between atmospheric particles become so infrequent that some traveling outward
may escape from the atmosphere. This region is termed the exosphere. Table 1
gives a summary of the composition of the atmosphere. See AIR POLLUTION.

    A convenient division of the atmosphere is by spherical shells, spheres,
each characterized by the way its temperature varies in the vertical, and with
tops denoted by pauses, as in the illustration. The troposphere includes the
layer closest to the Earth and is the seat of all the important weather
phenomena affecting the environment. The general circulation of the
troposphere includes wind systems on all scales - prevailing winds, monsoons,
long waves, anticyclones and depressions, fronts, hurricanes, thunderstorms,
and shower clouds - each system being associated with characteristic weather
patterns.
    Other divisions of the atmosphere are the stratosphere, mesosphere, and
thermosphere. The stratosphere, extending from about 6 - 10 mi to about 31 mi
(10 - 16 km to 50 km), is generally very stable, with no weather in the
popular sense. The mesosphere, extending from 34 mi (55 km) to about 50 mi (80
km), contains the mesopause, a region where noctilucent clouds are sometimes
formed, and most of the so-called D layer, a region of low ionization. The
thermosphere, extending from 80 km to the edge of the atmosphere, receives
energy directly from solar radiation and exhibits phenomena related to solar
activity, such as the aurora. See AIRGLOW; AURORA; CLOUD PHYSICS; MESOSPHERE;
NOCTILUCENT CLOUDS; STRATOSPHERE; WEATHER FORECASTING AND PREDICTION.
 �34s[R. J. Murgatroyd]
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    The chemical composition and total weight of the atmosphere have varied
through time. Today the atmosphere contains a small number of major components
(nitrogen, oxygen, argon) and a very large number of minor components
(including carbon dioxide, helium, krypton, xenon, hydrogen, and oxides of
nitrogen). Each component exerts a pressure, which is essentially constant for
some components at sea level and which is variable in time and space for other
components. At present, gases like ammonia, NH3, methane, CH4, and hydrogen
can exist in the atmosphere as trace components only, because they are
unstable in the presence of the large quantities of oxygen. Conversely, oxygen
could not have been a major component of an atmosphere in which ammonia,
methane, and hydrogen were abundant.
    Free oxygen is somewhat of an anomaly on the Earth. Rocks more than a few
feet below the Earth's surface are out of equilibrium with free oxygen and are
oxidized in contact with the atmosphere. There are two predominant theories
concerning the origin of oxygen in the Earth's atmosphere. The first proposes
that atmospheric oxygen has been produced through geologic time by the
continuing effect of photosynthesis, during which carbon is effectively
separated from oxygen in carbon dioxide. It can be shown that nearly all of
the oxygen produced by photosynthesis during a given period of time is lost by
plant decay, but that the small amount not lost could account for the present
rather large quantity of atmospheric oxygen.
    The second theory proposes an alternative way of producing free oxygen.
Ultraviolet light from the Sun decomposes water molecules in the upper
atmosphere. Most of these recombine, but there is a finite possibility that a
given hydrogen atom will manage to escape from the Earth's atmosphere before
recombination has taken place. Oxygen atoms, being 16 times as heavy as
hydrogen atoms, escape very much more slowly or not at all. The decomposition
of water vapor followed by hydrogen escape is therefore a distinctly plausible
manner of generating free atmospheric oxygen. A strongly limiting factor for
oxygen production by this mechanism is the formation of ozone, O3, as a
by-product of the photodissociation of water. Because it absorbs ultraviolet
light very readily, ozone tends to form a screen preventing it from reaching
the lower levels of the atmosphere, where water vapor is abundant. The oxygen
content of the atmosphere today is therefore only slightly influenced by
hydrogen loss from the upper atmosphere; rather, it depends almost exclusively
on the operation of a feedback system which links the rate of oxygen
production during photosynthesis to the rate of oxygen use by weathering and
the decay of organic matter. See ATMOSPHERIC OZONE.
    Interpretations based on paleontological and geochemical evidence seem to
lead quite naturally to the threefold division in Table 2 of the history of
the Earth's atmosphere. During the first stage, very shortly after the
accretion of the Earth, the atmosphere may have been quite reducing. Reduced
gases issuing from volcanoes probably would have given rise to an atmosphere
consisting of methane with minor quantities of hydrogen, nitrogen, and
ammonia. After metallic iron was removed from the upper mantle, the oxidation
state of volcanic gases probably approached its present value, methane was
replaced by carbon dioxide, and ammonia was converted to nitrogen. In the
atmosphere during this (its second) stage, nitrogen was its dominant
component, and carbon dioxide and argon were its most important minor
constituents.
    The third stage opened when the rate of oxygen production by
photosynthesis became sufficiently great so that oxygen became more than a
trace component of the atmosphere. The transition from stage 2 to stage 3 may
well have occurred about 1.8 x 10 sup 9 years ago. Since the opening of stage 3,
the oxygen pressure has climbed to its present value by a path which was
probably simple but which could have been complex in detail. During this
period the nitrogen, neon, argon, krypton, and xenon pressures have also
gradually climbed to their present value, while the helium and CO2 pressure
have remained reasonably constant, the former suspended between the rate of
input and the rate of escape, the latter controlled in large part by reactions
which involve carbonate and silicate minerals.









 �34s[Heinrich D. Holland]
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�6v�2v�32sAtmospheric acoustics �21m �33s
    The science of sound in the atmosphere. The term is usually reserved for
situations where departures of the atmosphere from an ideal homogeneous medium
affect propagation. Atmospheric acoustics is concerned with sound outdoors
rather than indoors. Infrasound, as well as sound of audible frequencies, is
within the scope of the subject.
    Audible sound and infrasound with wave periods somewhat shorter than 30 s
propagate through the atmosphere primarily along rays. A ray is the trajectory
(Fig. 1a) of a point that, relative to a person moving with the ambient wind,
appears to move with speed c (the speed of sound relative to air) normal to a
wavefront. Relative to a stationary coordinate system, the ray velocity
dxray/dt is increased by the wind velocity, such that Eq. (1) results, where n
is the unit vector normal to a wavefront and v0 is the equilibrium fluid
velocity. If c and v0 vary with position, as is typical in the actual
atmosphere, then the directions of the normal vector n and of the ray velocity
dx/dt will in general change along the path; the ray will be bent, or
refracted. The general behavior or ray in the atmosphere can be deduced from
ray-tracing equations that are derived from Eq. (1). SeeREFRACTION OF WAVES.

			     *****EQUATION***** 			     (1)

    General features of long-range ray propagation can be explained
qualitatively from a graph of effective sound velocity ceff, sound speed plus
wind component in plane of propagation, versus height (Fig. 2). Rays can be
trapped in either a low- or an upper-atmosphere sound channel where ceff has a
minimum. The greatest height attainable by a ray is its upper turning point,
which is also the height for which ceff is maximum along the ray path. If a
path has a lower turning point, then at that altitude ceff is the same as at
the upper turning point. If there is some altitude at which ceff is larger
than at the ground, then it is possible for a ray proceeding obliquely upward
from a source at an intermediate altitude to bend downward such that it
eventually reaches the ground.
    The magnitude and direction of upper atmosphere winds largely determine
whether ceff in the stratosphere can exceed its value at the ground. Since
winds in the altitude range of 20 - 80 km reverse direction in spring and fall
at temperate latitudes, long-range propagation east to west or west to east is
markedly different in summer from that in winter. Local meteorological
conditions such as inversions (temperature increasing with height near the
ground) can also cause a bending back to the ground of rays initially
proceeding obliquely upward.
    A spectacular consequence of a ceff profile that initially decreases with
height, reaches a minimum, then increases to a value higher than that at the
ground, is the existence of abnormal zones of audibility. Points at
intermediate horizontal distances may receive no sound, while audible sound is
received at greater distances.
    Refraction can cause the existence of shadow zones within which no ray
passes. Since wind velocity near the ground increases with height, shadow
zones are frequently found upwind of a source.
    Dissipation of acoustic energy in the atmosphere is caused by viscosity,
thermal conduction, and molecular relaxation. The last arises because
fluctuations in apparent molecular vibrational temperatures lag in phase the
fluctuations in translational temperatures. The vibrational temperatures of
significance are those characterizing the relative populations of O2 and N2
molecules in ground and first-excited vibrational quantum states. Since
collisions with H2O molecules are much more likely to induce vibrational state
changes than are collisions with other O2 or N2 molecules, the sound
attenuation varies markedly with absolute humidity.


    Larger-scale turbulence in the atmosphere causes the effective wave speed
to fluctuate from point to point, so a nominally smooth wavefront develops
ripples. Consequently, the amplitude of the sound at a distant point will
fluctuate with time and with small displacements in position.
    Infrasound waves with periods longer than a minute are strongly affected
by the Earth's gravity. The dominant features of some observed disturbances at
stratospheric and ionospheric heights can be interpreted in terms of the model
of planar sound waves in an isothermal atmosphere. In this model, there is a
forbidden range of angular frequencies OMEGA in which sound waves cannot
propagate, namely OMEGAsubb<OMEGA<OMEGAsuba, where OMEGAsuba and OMEGAsubb are
given by Eqs. (2). (The wave period corresponding to OMEGAsubb is of the order
of 5 to 10 min in the lower atmosphere.) Possible planar waves are divided
into ordinary acoustic waves (OMEGA>OMEGAsuba) and acoustic-gravity waves
(OMEGA<OMEGAsubb).

			     *****EQUATION***** 			   (221)

			     *****EQUATION***** 			   (221)

    Acoustic-gravity waves can be channeled in the lower atmosphere and along
the ground such that they can carry infrasonic signals over large horizontal
distances. Thus, for example, pressure waves generated by large explosions and
traveling at slightly less than the sound speed at the ground can be detected
at distances greater than halfway around the globe by microbarograph
instrumentation. See SOUND.

     FIGURE CAPTION.  Ray acoustics. (a) Definition of a ray path for a moving
medium. (b) Ray tube formed by adjacent rays in an inhomogeneous medium.

     FIGURE CAPTION.  Model atmospheric profiles of effective sound speed
c sub eff versus height for propagation east to west in northeastern United
States.

 �34s[Allan D. Pierce]
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�6v�2v�32sAtmospheric electricity   �21m �33s 
    The electrical processes constantly taking place in the lower atmosphere. 
This activity is of two kinds, the intense local electrification accompanying 
storms, and the much weaker fair-weather electrical activity over the entire 
globe, which is produced by the many electrified storms continuously in 
progress over the Earth. The mechanisms by which storms generate electric 
charge are unknown, and the role of atmospheric electricity in meteorology has 
not been determined. 
    Almost all precipitation-producing storms throughout the year are 
accompanied by energetic electrical activity. The most intense of these are 
the thunderstorms, in which the electrification attains values sufficient to 
produce lightning. Electrical measurements show that most other storms, even 
though they do not give lightning, are also quite strongly electrified. The 
electric fields of thunderstorms cause three currents to flow, each of a few 
amperes: lightning, point discharge from the ground beneath, and conduction in 
the surrounding air. Because the external field and conductivity are greatest 
over the top of the cloud, most of the conduction current flows to the 
ionosphere, the upper, highly conductive layer of the atmosphere. See 
THUNDERSTORM.  
    Fair-weather measurements, irrespective of place and time, show the 
invariable presence of a weak negative electric field caused by the estimated 
several thousand electrified storms continually in progress. Together these 
storms cause a 2000-A current from the earth to the ionosphere that raises the 
ionosphere to a positive potential of about 300,000 V with respect to the 
earth. This potential difference is sufficient to cause a return flow of 
positive charge to the earth by conduction through the intervening lower 
atmosphere equal and opposite to the thunderstorm supply current. The 
fair-weather field is simply the voltage drop produced by the flow of this 
current through the atmosphere. Because the electrical resistance of the 
atmosphere decreases with altitude, the field is greatest near the Earth's 
surface and gradually decreases with altitude until it vanishes at the 
ionosphere.  
    No importance is presently attached to fair-weather atmospheric 
electricity except that according to some theories it is responsible for the 
initiation of the thunderstorm electrification process. See CLOUD PHYSICS; 
LIGHTNING; SFERICS; STORM DETECTION; TORNADO. 
 �34s[Bernard Vonnegut]
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�6v�2v�32sAtmospheric entry   �21m �33s 
    The entry of a space vehicle into the atmosphere of a planet. Principal 
problems during entry are deceleration and heating, the severity of which is 
governed primarily by the type of vehicle and its entry velocity. Entry 
vehicles are either ballistic or lifting. A ballistic vehicle is acted upon 
only by gravity and aerodynamic drag forces, while a lifting vehicle also 
makes use of aerodynamic forces which permit maneuvering. Two types of natural 
entry bodies are recognized: those coming from interplanetary space 
(meteoroids and comets), and those thought to come from the Moon (termed 
tektites).  
    The maximum deceleration that is caused by aerodynamic forces is 
determined primarily by the velocity and angle of entry of the vehicle and the 
density of the atmosphere; it is almost independent of the vehicle size, 
weight, and shape, although the altitude at which maximum deceleration occurs 
is not. Maximum deceleration increases rapidly with increasing entry angle. 
Fairly steep entry angles are required for ballistic vehicles at greater than 
satellite speed to reduce the centrifugal force that tends to make the vehicle 
skip out of the atmosphere. Therefore ballistic vehicles experience high 
deceleration during entry. For crewed vehicles, 10 g is presently considered 
to be the maximum practical deceleration. The required entry angles are so 
shallow that aerodynamic lift is necessary to balance the centrifugal force 
and assure capture within the atmosphere. 
    Since the large kinetic energy of entry vehicles is converted to heat by 
shock waves and air friction as the vehicle passes through the atmosphere, the 
air surrounding the vehicle is heated to a temperature of many thousands of 
degrees. Careful vehicle design can result in more than 99% of this energy 
remaining in the air. The three means for heat shielding on entry vehicles are 
heat sinks, mass injection, and reradiation. 
    The most widely used method of heat shielding is mass injection through 
natural ablation. A vehicle is coated with an  ablative material, frequently a 
fiber-impregnated plastic; large amounts of heat are absorbed as the ablator 
is heated and vaporized. In addition, the ablation vapors fend off the hot 
air, thereby tending to greatly reduce the convective heating. 
 �34s[Michael E. Tauber]
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�6v�2v�32sAtmospheric ozone �21m �33s
    Ozone is found in trace quantities throughout the atmosphere, the largest
concentrations being located in a layer in the lower stratosphere between the
altitudes of 9 and 18 mi (15 and 30 km). This ozone results almost entirely
from the dissociation of molecular oxygen by solar ultraviolet radiation in
the upper atmosphere.
    Although present in only trace quantities, atmospheric ozone plays a
critical role for the biosphere by absorbing the ultraviolet radiation with
wavelength LAMBDA between 2400 and 3200 angstroms (240 and 320 nanometers),
which would otherwise be transmitted to the Earth's surface. This radiation is
lethal to simple unicellular organisms (algae, bacteria, protozoa) and to the
surface cells of higher plants and animals. It also damages the genetic
material of cells (DNA) and is responsible for sunburn in human skin. In
addition, the incidence of skin cancer has been statistically correlated with
the observed surface intensities of the ultraviolet wavelengths between 2900
and 3200 A (290 and 320 nm), which are not totally absorbed by the ozone
layer. See BIOSPHERE.
    Ozone also plays an important role in photochemical smog and in heating
the upper atmosphere by absorbing solar ultraviolet and visible radiation
(LAMBDA < 7100 A or 710 nm) and thermal infrared radiation (LAMBDA CONGRUENT
9.6 micrometers). As a consequence, the temperature increases steadily from
about 220 K at the tropopause (8-16 km altitude) to about 280 K at the
stratopause (50 km altitude). This ozone heating provides the major energy
source for driving the circulation of the upper stratosphere and mesosphere.
    There has also been concern that industrial production of the
chlorofluoromethanes CFCl3 (Freon 11) and CF2Cl2 (Freon 12) may be
significantly altering the natural chlorine cycle in the atmosphere. These
very inert species are principally used as refrigerants, as blowing agents for
plastic foams, and as aerosol-can propellants. Once they are released into the
atmosphere, their only presently recognized removal mechanism involves
photodissociation in the stratosphere. Chlorine atoms released in such
reactions can catalytically destroy ozone by conversion to chlorine oxide
radicals.
    A number of other potential ozone-destroying processes with anthropogenic
origins have also been identified. The global measurements of ozone which are
required to ascertain the reality of all possible long-term ozone depletions
have become possible from satellites. See AIR POLLUTION; OZONE.
 �34s[Ronald G. Prinn]
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�6v�2v�32sAtoll   �21m �33s 
    An annular coral reef, with or without small islets, that surrounds a 
lagoon without projecting land area. Most atolls are isolated reefs rising 
from the deep sea, and vary considerably in size. Small rings, usually without 
islets, may be less than a mile in diameter, but many atolls have a diameter 
of about 20 mi (32 km) and bear numerous islets. 
    The reefs of the atoll ring are flat, pavementlike areas, large parts of 
which, particularly along the seaward margin, may be exposed at times of low 
tide. The reefs vary in width from narrow ribbons to broad bulging areas more 
than a mile (1.6 km) across. The structures form a most effective baffle that 
robs the incoming waves of much of their destructive power, and at the same 
time brings a constant supply of refreshing sea water with oxygen, food, and 
nutrient salts to wide expanses of the reef. 
    Atolls, like other types of coral reefs, require strong light and warm 
waters and are limited in the existing seas to tropical and near-tropical 
latitudes. A large percentage of the world's atolls are contained in an area 
known as the former Darwin Rise that covers much of the central and 
southwestern Pacific. Atolls are also numerous in parts of the Indian Ocean 
and a number are found, mostly on continental shelves, in the Caribbean area. 
See OCEANIC ISLANDS; REEF. 
 �34s[Harry S. Ladd]
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�6v�2v�32sAtom �21m �33s
    The individual structure which constitutes the basic unit of any chemical
element. This structure, consisting of a positively charged nucleus surrounded
by a number of electrons of total negative charge equal to the positive charge
on the nucleus, is essentially identical for all atoms of any one element. The
nuclear charge, measured in units of the electronic charge, is called the
atomic number and specifies the element. See ATOMIC STRUCTURE AND SPECTRA;
ISOTOPE; NUCLEAR STRUCTURE.
 �34s[Francis A. Jenkins /William Watson]
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�6v�2v�32sAtomic-beam clock �21m �33s
    A type of atomic clock which makes use of the atomic-beam magnetic
resonance technique. In this method a beam of atoms traverses an evacuated
space, in which the atoms are irradiated with the electromagnetic energy and
subsequently detected. Deflection magnets are arranged so that only those
atoms that undergo a transition (caused by the excitation being at their
resonance frequency) are deflected to the detector. Beam clocks generally use
cesium-133 and have line widths of the order of 1 part in 10 sup 7 to 1 part in
10 sup 8. Thallium has the advantages over cesium of reduced sensitivity to
magnetic fields and a higher frequency (21,300 megahertz as compared to 9192
MHz) but is much harder to detect and deflect. See ATOMIC BEAMS; ATOMIC CLOCK;
MAGNETIC RESONANCE.
 �34s[Leonard S. Cutler]
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�6v�2v�32sAtomic beams   �21m �33s 
    Unidirectional streams of neutral atoms passing through a vacuum. These 
atoms are virtually free from the influence of neighboring atoms but may be 
subjected to electric and magnetic fields so that their properties may be 
studied. The method of atomic beams yields extremely accurate spectroscopic 
data about the energy levels of atoms, and hence detailed information about 
the interaction of electrons in the atom with each other and with the atomic 
nucleus, as well as information about the interaction of all components of the 
atom with external fields. See MOLECULAR BEAMS. 
 �34s[Polykarp Kusch]
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�6v�2v�32sAtomic bomb �21m �33s
    A device for suddenly producing an explosively rapid neutron chain
reaction in a fissile material such as (sup 235)U or (sup 239)Pu. In a wider
sense, it is any explosive device which  derives its energy from nuclear
reactions, including not only the foregoing fission bomb but also a
fusion bomb, which gets its energy largely from reactions of heavy hydrogen
(hydrogen bomb) or other light nuclei, and a fission-fusion bomb which derives
its energy in comparable amounts from fission and fusion. Because an atomic bomb
derives its energy from nuclear reactions, it is more properly called a
nuclear bomb. See CHAIN REACTION (PHYSICS); HYDROGEN BOMB; NUCLEAR FISSION;
NUCLEAR FUSION; NUCLEAR REACTION.
    The first fission bombs released energies of the order of 20 kilotons (1
kiloton is 1000 tons of equivalent chemical high explosive calculated at a
conventional energy release of 1000 cal/g; 1 kiloton = 4.18 x 10 sup 12 joules),
but fractional kiloton yields have since been obtained, overlapping
the energy of conventional explosives. On the upper end the yield of fission
bombs overlaps that of fusion bombs. Yields of energetic fusion bombs in
excess of 100 megatons (4.18 x 10 sup 17 J) have been reported.
 �34s[John A. Wheeler]
�33s
    The energy is communicated by mechanical shock and radiative transport to
the surrounding water, earth, or air, ionizing it out to a radius which in the
case of explosions in air is known as the fireball radius (150 yd or 135 m
about 1 s after the explosion of a conventional or 20-kiloton atomic bomb).
Energy goes out from such a fireball into the surrounding relatively
transparent air in not very different orders of magnitude in the form of a
shock wave and in the form of heat radiation that may continue for a number of
seconds. See NUCLEAR EXPLOSION; RADIOACTIVE FALLOUT.
 �34s[John A. Wheeler]
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�6v�2v�32sAtomic clock   �21m �33s 
    An electronic clock whose frequency is supplied or governed by the natural 
resonance frequencies of atoms or molecules of suitable substances. These 
frequencies are generally in the microwave range from about 1400 to 40,000 
megahertz in the clocks presently used or under development. Some work is also 
being done in the optical and infrared frequency ranges. Atomic clocks are the 
most precise of all clocks; some of those presently available have errors 
sufficiently small that they would gain or lose less than 1 s in 30,000 years. 
The present internationally accepted unit of time, the atomic second, is based 
on an atomic clock and is defined as that time interval during which occur 
exactly 9,192,631,770 cycles of the hyperfine resonance frequency of the 
ground state of the cesium-133 atom. See ATOMIC TIME. 
    Some uses that require the precision of these clocks are: (1) navigation 
systems such as loran C and Omega, in which position is determined by the 
relative times of arrival of radio pulses of electromagnetic energy emitted at 
precisely known times from several transmitters at different locations; (2) 
deepspace communications and Doppler navigation, both of which require 
extremely stable frequencies; (3) tests of relativitly theory by comparison of 
rates of clocks carried on aircraft, rockets, or satellites with that of 
clocks on the ground. See CLOCK PARADOX; ELECTRONIC NAVIGATION SYSTEMS; LORAN; 
OMEGA; RELATIVITY. 
    The clocks presently in use or under development employ: (1) hyperfine 
structure (splitting of the spectral lines due to the  nuclear magnetic 
moment) in the ground state of atoms such as cesium, rubidium, hydrogen, or 
thallium, or in ions such as singly ionized mercury, barium, or magnesium; (2) 
a molecular line such as the inversion line of ammonia; or (3) a narrow 
optical or infrared resonance in an atom, molecule, or ion. They may be 
further classed as passive or active. In a passive clock the atoms or 
molecules are irradiated with electromagnetic energy close in frequency to 
their resonance, and some method is provided to sense the difference in 
frequency and to control the source frequency so as to reduce the error toward 
zero. The source is usually a stable quartz oscillator at a frequency of a few 
megahertz followed by a frequency multiplier. Active clocks generate their own 
signal directly from the atoms or molecules, using the principle of 
oscillation by stimulated emission of radiation. The microwave frequency 
signal so generated is low in power, so an oscillator and frequency multiplier 
chain is usually synchronized to it. In both types some form of frequency 
synthesis (frequency multiplication by a rational fraction) is necessary to 
provide an output at a cardinal frequency, such as 1.0 MHz, suitable for clock 
use. Frequency synthesis into the infrared or optical range is very difficult 
and requires complex equipment. See AMMONIA MASER CLOCK; ATOMIC-BEAM CLOCK; 
HYDROGEN MASER CLOCK; RUBIDIUM-GAS CELL CLOCK. 
 �34s[Leonard S. Cutler]
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�6v�2v�32sAtomic constants �21m �33s
    That group of physical constants which play a fundamental role in the
basic theories of physics. These include the speed of light in vacuum, c; the
magnitude of the charge on the electron, e, which is the fundamental unit of
electric charge; the mass of the electron, m sub e; Planck's constant, h; and
the fine-structure constant, ALPHA.
    These five quantities typify the different origins of the fundamental
constants: c and h are examples of quantities which appear naturally in the
mathematical formulation of certain physical theories - Einstein's theories of
relativity, and quantum theory, respectively; e and m sub e are examples of
quantities which characterize the elementary particles of which all matter is
constituted; and ALPHA, the fundamental constant of quantum electrodynamics
(QED), is an example of quantities which are combinations of other fundamental
constants, but are actually constants in their own right since the same
combination always appears together in the basic equations of physics.
    Reliable numerical values for the fundamental physical constants are
required for two main reasons. First, they are necessary if quantitative
predictions from physical theory are to be obtained. Second, and even more
important, the self-consistency of the basic theories of physics can be
critically tested by a careful intercomparison of the numerical values of
fundamental constants obtained from experiments in the different fields of
physics. In general, the accuracy of fundamental constants determinations has
continually improved over the years. Whereas in the past, 100 ppm (0.01%) and
even 1000 ppm (0.1%) measurements were commonplace, today 0.01 ppm and better
determinations are not unusual (ppm = parts per million).
    Complex relationships can exist among groups of constants and conversion
factors, and a particular constant may be determined either directly by
measurement or indirectly by an appropriate combination of other directly
measured constants. If the direct and indirect values have comparable
accuracy, then both must be taken into account in order to arrive at a best
value for that quantity. (By best value is meant that value believed to be
closest to the true but unknown value.) Generally, each of the several routes
which can be followed to a particular constant, both direct and indirect, will
give a slightly different numerical value. Such a situation may be
satisfactorily handled by the mathematical method known as least-squares. This
technique provides a self-consistent procedure for calculating best
``compromise'' values of the constants from all of the available data. It
automatically takes into account all possible routes and determines a single
final value for each constant being calculated. It does this by weighting the
different routes according to their relative uncertainties. The appropriate
weights follow from the uncertainties assigned the individual measurements
constituting the original set of data.
    The most recent least-squares study was that of E. R. Cohen and B. N.
Taylor in 1973. Recommended values of selected constants from this adjustment
are given in the table and those still in general use.

     TABLE CAPTION.  Recommended values of selected fundamental constants as
taken from the 1973 least-squares adjustment of Cohen and Taylor

 �34s[Barry N. Taylor]
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�6v�2v�32sAtomic energy   �21m �33s 
    The energy released in the rearrangement of the particles making up the 
nucleus of an atom, popularly referred to as atomic energy, but preferably 
called nuclear energy. Atomic properly refers to phenomena involving the 
orbital electrons of the atom, but not involving any transformation of the 
nucleus. See NUCLEAR POWER. 
 �34s[James A. Lane]
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�6v�2v�32sAtomic mass unit �21m �33s
    An arbitrarily defined unit in terms of which the masses of individual
atoms are expressed. One atomic mass unit is defined as exactly 1/12 of the
mass of an atom of the nuclide (sup 12)C, the predominant isotope of carbon.
The unit, also known as the dalton, is often abbreviated amu, and is
designated by the symbol u. The relative atomic mass of a chemical element is
the average mass of its atoms expressed in atomic mass units. See ATOMIC
WEIGHT; RELATIVE ATOMIC MASS.
 �34s[Jonathan F. Weil]
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�6v�2v�32sAtomic nucleus �21m �33s
    The central region of an atom. Atoms are composed of negatively charged
electrons, positively charged protons, and electrically neutral neutrons. The
protons and neutrons (collectively known as nucleons) are located in a small
central region known as the nucleus. The electrons move in orbits which are
large in comparison with the dimensions of the nucleus itself. Protons and
neutrons possess approximately equal masses, each roughly 1840 times that of
an electron. The number of nucleons in a nucleus is given by the mass number A
and the number of protons by the atomic number Z. Nuclear radii r are given
approximately by r = 1.4 x 10- sup 13 A (sup 1/3) cm. See NUCLEAR STRUCTURE.
 �34s[Henry E. Duckworth]
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�6v�2v�32sAtomic number �21m �33s
    The number of elementary positive charges (protons) contained within the
nucleus of an atom. It is denoted by the letter Z. For an electrically neutral
atom, the number of planetary electrons is also given by the atomic number.
Atoms with the same Z (isotopes) belong to the same element. The lightest
element, hydrogen, has Z = 1. The heaviest naturally occurring element,
uranium, has Z = 92. All elements up to and including Z = 106 either occur in
nature or have been created artificially. When specifically written, the
atomic number is usually placed before and below the elemental symbol, for
example, sub 1 H, sub 92 U. See MASS NUMBER; RADIOACTIVITY.
 �34s[Henry E. Duckworth]
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�6v�2v�32sAtomic physics   �21m �33s 
    The study of the structure of the atom, its dynamical properties, 
including energy states, and its interactions with particles and fields. These 
are almost completely determined by the laws of quantum mechanics, with very 
refined corrections required by quantum electrodynamics. Despite the enormous 
complexity of most atomic systems, in which each electron interacts with both 
the nucleus and all the other orbiting electrons, the wavelike nature of 
particles, combined with the Pauli exclusion principle, results in an 
amazingly orderly array of atomic properties. These are systematized by the 
Mendeleev periodic table. In addition to their classification by chemical 
activity and atomic weight, the various elements of this table are 
characterized by a wide variety of observable properties. These include 
electron affinity, polarizability, angular momentum, multiple electric 
moments, and magnetism. See ATOMIC WEIGHT; PERIODIC TABLE; QUANTUM 
ELECTRODYNAMICS; QUANTUM MECHANICS. 
    Each atomic element, normally found in its ground state (that is, with its 
electron configuration corresponding to the lowest state of total energy), can 
also exist in an infinite number of excited states. These are also ordered in 
accordance with relatively simple hierarchies determined by the laws of 
quantum mechanics. The most characteristic signature of these various excited 
states is the radiation emitted or absorbed when the atom undergoes a 
transition from one state to another. The systemization and classification of 
atomic energy levels (spectroscopy) has played a central role in developing an 
understanding of atomic structure. 
 �34s[Benjamin Bederson]
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�6v�2v�32sAtomic spectroscopy   �21m �33s 
    Analytical atomic spectroscopy utilizing the emission, absorption, or 
fluorescence of light at discrete wavelengths by atoms in a vaporized sample 
for the determination of the elemental composition of the sample. The sample, 
usually an aqueous solution, is introduced into a high-temperature device, 
such as an electrical discharge, flame, or furnace, where it is dissociated 
into its component atoms. 
    In atomic emission spectroscopy the sample atoms absorb energy by 
collisions with high-kinetic-energy atoms and molecules in the hot gas, 
promoting electrons to higher energy levels within the atom (excitation). A 
fraction of these excited atoms then return to their normal or ``ground'' 
state by emission of electromagnetic radiation at discrete energies (and, 
thus, discrete wavelengths) corresponding to the energy differences between 
the electronic energy levels involved. 
    A typical emission spectrometer consists of the following: the energy 
medium for vaporization, atomization, and excitation of the material to be 
analyzed; a dispersive unit which isolates the various wavelengths of light 
emitted; and a measurement or detection device. Three-component systems of 
this type are widely used to determine what elements are present in a variety 
of materials (qualitative analysis) and how much of each element is present 
(quantitative analysis). The latter relies on the fact that the intensity of 
light emitted at a particular wavelength is proportional to the concentration 
of atoms which are capable of emitting at the wavelength. 
    Just as excited atoms can emit radiation, ground-state atoms can absorb 
radiation of the appropriate wavelength at these same discrete energies, and 
this reverse of the emission process forms the basis of atomic absorption 
spectroscopy. In its usual configuration, an atomic absorption spectrometer 
consists of four basic components: (1) a light source, emitting the spectrum 
of the desired analyte element; (2) a sample atomization cell, such as a flame 
or graphite tube furnace; (3) a monochromator, to isolate the desired source 
emission line; and (4) a detector/readout system, to allow measurement of the 
change in source line intensity by sample atom absorption. 
    As an analytical technique for trace-element determinations, atomic 
absorption spectrometry has several important advantages: it has high 
sensitivity for many elements, and it is simple, rapid, relatively low-cost, 
and highly specific (subject to few interferences). The principal limitations 
of the technique are: a separate line source is needed for each element to be 
determined, and the technique has limited dynamic range (for a given 
determination all samples must be within about a one-decade concentration 
interval).  
    A third technique, atomic fluorescence spectroscopy, is basically a 
combination of the first two: the sample atoms first absorb radiation from an 
external source, becoming excited atoms, and the reradiation (fluorescence) is 
measured.  
    Atomic fluorescence spectrometry utilizes the same basic components as 
atomic absorption in a different optical arrangement. The light source(s) is 
placed at right angles to the optical axis. The fluorescence intensity is 
directly proportional to the source intensity (over the atom's absorption line 
width), and so intense sources such as simple electrodeless discharge lamps 
are used. Tunable dye lasers show considerable promise as sources because of 
their extremely high intensity. 
    Atomic fluorescence spectrometry combines some of the advantages of 
emission and absorption and has a few of its own. It has the simplicity, 
speed, cost, and specificity advantages of atomic absorption, the wide dynamic 
range and adaptability to multielement determinations of atomic emission, and 
higher demonstrated and potential sensitivity for many elements. Its principal 
limitation is scattering of source radiation (common to fluorescence 
techniques), and so caution must be used in the design of atomization systems 
to eliminate particulate matter from the optical path. 
 �34s[Claude Veillon]
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�6v�2v�32sAtomic structure and spectra �21m �33s
    The idea that matter is subdivided into discrete and further indivisible
building blocks called atoms dates back to the Greek philosopher Democritus,
whose teachings of the 5th century B.C. are commonly accepted as the earliest
authenticated ones concerning what has come to be called atomism by students
of Greek philosophy. The weaving of the philosophical thread of atomism into
the analytical fabric of physics began in the late 18th and the 19th
centuries. Robert Boyle is generally credited with introducing the concept of
chemical elements, the irreducible units of which are now recognized as
individual atoms of a given element. In the early 19th century John Dalton
developed his atomic theory, which postulated that matter consists of
indivisible atoms as the irreducible units of Boyle's elements, that each atom
of a given element has identical attributes, that differences among elements
are due to fundamental differences among their constituent atoms, that
chemical reactions proceed by simple rearrangement of indestructible atoms,
and that chemical compounds consist of molecules which are reasonably stable
aggregates of such indestructible atoms. See CHEMISTRY.
    The work of J. J. Thomson in 1897 clearly demonstrated that atoms are
electromagnetically constituted and that from them can be extracted
fundamental material units bearing electric charge that are now called
electrons. The electrons of an atom account for a negligible fraction of its
mass. By virtue of overall electrical neutrality of every atom, the mass must
therefore reside in a compensating, positively charged atomic component of
equal charge magnitude but vastly greater mass. See ELECTRON.
    Thomson's work was followed by the demonstration by Ernest Rutherford in
1911 that nearly all the mass and all of the positive electric charge of an
atom are concentrated in a small nuclear core approximately 10,000 times
smaller in extent than an atomic diameter. Niels Bohr in 1913 and others
carried out some remarkably successful attempts to build solar system models
of atoms containing planetary pointlike electrons orbiting around a positive
core through mutual electrical attraction (though only certain ``quantized''
orbits were ``permitted''). These models were ultimately superseded by
nonparticulate-matter wave quantum theories of both electrons and atomic
nuclei. See QUANTUM MECHANICS.
    The modern picture of condensed matter (such as solid crystals) consists
of an aggregate of atoms or molecules which respond to each other's proximity
through attractive electrical interactions at separation distances of the
order of 1 atomic diameter (approximately 10sup-10 m) and repulsive electrical
interactions at much smaller distances.  These interactions are mediated by the
electrons, which are in some sense shared and exchanged by all atoms of a
particular sample, and serve as a kind of interatomic glue which binds the
mutually repulsive, heavy, positively charged atomic cores together.  See
SOLID-STATE PHYSICS.
    The hydrogen atom is the simplest atom, and its spectrum (or pattern of
light frequencies emitted) is also the simplest. The regularity of its
spectrum had defied explanation until Bohr solved it with three postulates,
these representing a model which is useful, but quite insufficient, for
understanding the atom.
    Postulate 1: The force that holds the electron to the nucleus is the
Coulomb force between electrically charged bodies.
    Postulate 2: Only certain stable, nonradiating orbits for the electron's
motion are possible, those for which the angular momentum is an integral
multiple of h/2pi (Bohr's quantum condition on the orbital angular momentum).
Each stable orbit represents a discrete energy state.
    Postulate 3: Emission or absorption of light occurs when the electron
makes a transition from one stable orbit to another, and the frequency nu of
the light is such that the difference in the orbital energies equals hnu (A.
Einstein's frequency condition for the photon, the quantum of light).
    Here the concept of angular momentum, a continuous measure of rotational
motion in classical physics, has been asserted to have a discrete quantum
behavior, so that its quantized size is related to Planck's constant h, a
universal constant of nature. See ANGULAR MOMENTUM.
    Modern quantum mechanics has provided justification for Bohr's quantum
condition on the orbital angular momentum. It has also shown that the concept
of definite orbits cannot be retained except in the limiting case of very
large orbits. In this limit, the frequency, intensity, and polarization can be
accurately calculated by applying the classical laws of electrodynamics to the
radiation from the orbiting electron. This fact illustrates Bohr's
correspondence principle, according to which the quantum results must agree
with the classical ones for large dimensions. The deviation from classical
theory that occurs when the orbits are smaller than the limiting case is such
that one may no longer picture an accurately defined orbit. Bohr's other
hypotheses are still valid.
    According to Bohr's theory, the energies of the hydrogen atom are
quantized (that is, can take on only certain discrete values). These energies
can be calculated from the electron orbits permitted by the quantized orbital
angular momentum. The orbit may be circular or elliptical, so only the
circular orbit is considered here for simplicity. Let the electron, of mass m
and electric charge -e, describe a circular orbit of radius r around a nucleus
of charge +e and of infinite mass. With the electron velocity v, the angular
momentum is mvr, and the second postulate becomes Eq. (1). The integer n is
called the principal quantum number. The possible energies of the nonradiating
states of the atom are given by Eq. (2). Here epsilon(sub 0) is the permittivity
of  free space, a constant included in order to give the correct units according
to the mks statement of Coulomb's law.

		    mvr = n(h/2pi)     (n = 1,2,3,...)			     (1)


E = -{me[sup 4]/[8(epsilon[sub 0])(sup 2) h(sup 2)]}(1/n[sup 2])	     (2)

    The same equation for the hydrogen atom's energy levels, except for some
small but significant corrections, is obtained from the solution of the
Schroedinger equation for the hydrogen atom. See SCHROEDINGER'S WAVE EQUATION.
    The frequencies of electromagnetic radiation or light emitted or absorbed
in transitions are given by Eq. (3) where E' and E'' are the

				nu = (E' - E'')/h                            (3)

energies of the initial and final states of the atom.
Spectroscopists usually express their measurements in wavelength lambda or in
wave number sigma in order to obtain numbers of a convenient size. The wave
number of a transition is shown in Eq. (4). If T = -E/hc, then Eq. (5)
results. Here T is called the spectral term.

			sigma = nu/c = E'/hc - E''/hc                        (4)


			      sigma = T'' - T'                               (5)

    The allowed terms for hydrogen, from Eq. (2), are given by Eq. (6). The
quantity R is the important Rydberg constant. Its value, which has been
accurately measured by laser spectroscopy, is related to the values of other
well-known atomic constants, as shown in Eq. (6). See RYDBERG CONSTANT.
									     (6)
T = [me(sup 4)/8(epsilon[sub  0])(sup 2) ch(sup 3)](1/n[sup 2]) = R/n(sup 2)

    The effect of finite nuclear mass must be considered, since the nucleus
does not actually remain at rest at the center of the atom. Instead, the
electron and nucleus revolve about their common center of mass. This effect
can be accurately accounted for and requires a small change in the value of
the effective mass m in Eq. (6).
    In addition to the circular orbits already described, elliptical ones are
also consistent with the requirement that the angular momentum be quantized.
A. Sommerfeld showed that for each value of n there is a family of n permitted
elliptical orbits, all having the same major axis but with different
eccentricities. Illustration a shows, for example, the Bohr-Sommerfeld orbits
for n = 3. The orbits are labeled s, p, and d, indicating values of the
azimuthal quantum number l = 0, 1, and 2. This number determines the shape of
the orbit, since the ratio of the major to the minor axis is found to be n/(l
+ 1). To a first approximation, the energies of all orbits of the same n are
equal. In the case of the highly eccentric orbits, however, there is a slight
lowering of the energy due to precession of the orbit (illustration b).
According to Einstein's theory of relativity, the mass increases somewhat in
the inner part of the orbit, because of greater velocity. The velocity
increase is greater as the eccentricity is greater, so the orbits of higher
eccentricity have their energies lowered more. The quantity l is called the
orbital angular momentum quantum number or the azimuthal quantum number. See
RELATIVITY.
    In attempting to extend Bohr's model to atoms with more than one electron,
it is logical to compare the experimentally observed terms of the alkali
atoms, which contain only a single electron outside closed shells, with those
of hydrogen. A definite similarity is found but with the striking difference
that all terms with l > 0 are double. This fact was interpreted by S. A.
Goudsmit and G. E. Uhlenbeck as due to the presence of an additional angular
momentum of 1/2 (h/2pi) attributed to the electron spinning about its axis.
The spin quantum number of the electron is s = 1/2.
    The relativistic quantum mechanics developed by P. A. M. Dirac provided
the theoretical basis for this experimental observation.
    Implicit in much of the following discussion is W. Pauli's exclusion
principle, first enunciated in 1925, which when applied to atoms may be stated
as follows: no more than one electron in a multielectron atom can possess
precisely the same quantum numbers. In an independent, hydrogenic electron
approximation to multielectron atoms, there are 2n(sup 2) possible independent
choices of the principal (n), orbital (l), and magnetic (msubl, msubs) quantum
numbers available for electrons belonging to a given n, and no more. Here
msubl and msubs refer to the quantized projections of l and s along some
chosen direction. The organization of atomic electrons into shells of
increasing radius (the Bohr radius scales as n[sup 2]) follows from this
principle. See EXCLUSION PRINCIPLE.
    The energy of interaction of the electron's spin with its orbital angular
momentum is known as spin-orbit coupling. A charge in motion through either
``pure'' electric or ``pure'' magnetic fields, that is, through fields
perceived as ``pure'' in a static laboratory, actually experiences a
combination of electric and magnetic fields, if viewed in the frame of
reference of a moving observer with respect to whom the charge is momentarily
at rest. For example, moving charges are well known to be deflected by
magnetic fields. But in the rest frame of such a charge, there is no motion,
and any acceleration of a charge must be due to the presence of a pure
electric field from the point of view of an observer analyzing the motion in
that reference frame. See RELATIVISTIC ELECTRODYNAMICS.
    A spinning electron can crudely be pictured as a spinning ball of charge,
imitating a circulating electric current. This circulating current gives rise
to a magnetic field distribution very similar to that of a small bar magnet,
with north and south magnetic poles symmetrically distributed along the spin
axis above and below the spin equator. This representative bar magnet can
interact with external magnetic fields, one source of which is the magnetic
field experienced by an electron in its rest frame, owing to its orbital
motion through the electric field established by the central nucleus of an
atom. In multielectron atoms, there can be additional, though generally
weaker, interactions arising from the magnetic interactions of each electron
with its neighbors, as all are moving with respect to each other, and all have
spin. The strength of the bar magnet equivalent to each electron spin, and its
direction in space are characterized by a quantity called the magnetic moment,
which also is quantized essentially because the spin itself is quantized.
Studies of the effect of an external magnetic field on the stages of atoms
show that the magnetic moment associated with the electron spin is equal in
magnitude to a unit called the Bohr magneton.
    The energy of the interaction between the electron's magnetic moment and
the magnetic field generated by its orbital motion is usually a small
correction to the spectral term, and depends on the angle between the magnetic
moment and the magnetic field, or equivalently, between the spin angular
momentum vector and the orbital angular momentum vector (a vector
perpendicular to the orbital plane whose magnitude is the size of the orbital
angular momentum). Since quantum theory requires that the quantum number j of
the electron's total angular momentum shall take values differing by integers,
while l is always an integer, there are only two possible orientations for s
relative to l: s must be either parallel or antiparallel to l.
    For the case of a single electron outside the nucleus, the Dirac theory
gives Eq. (7) for the spin-orbit correction to the spectral terms. Here alpha
= esup2/(2epsilon0 hc) APPROXIMATELY EQUAL 1/137 is called the fine structure
constant.

DELTA T = [(R alpha(sup 2) z[sup 4])/n(sup 3)]{[j(j + 1) - l(l + 1) -
	  s(s + 1)]/l(2l + 1)(l + 1)					   (7)

    In atoms having more than one electron, this fine structure becomes what
is called the multiplet structure. The doublets in the alkali spectra, for
example, are due to spin-orbit coupling; Eq. (7), with suitable modifications,
can be applied.
    When more than one electron is present in the atom, there are various ways
in which the spins and orbital angular momenta can interact. Each spin may
couple to its own orbit, as in the one-electron case; other possibilities are
orbit - other orbit, spin-spin, and so on. The most common interaction in the
light atoms, called LS coupling or Russell-Saunders coupling, is described

	      {(l1,l2,l3,. . .) (s1,s2,s3,. . .)} = {L,S} = J		     (8)

schematically in Eq.  (8).  This notation indicates that the lsubi are coupled
strongly together to form a resultant L, representing the total orbital angular
momentum.  The ssubi are coupled strongly together to form a resultant S, the
total spin angular momentum.  The weakest coupling is that between L and S to
form J, the total angular momentum of the electron system of the atom in this
state.
    Coupling of the LS type is generally applicable to the low-energy states
of the lighter atoms. The next commonest type is called jj coupling,

{(l1,s1) (l2,s2) (l3,s3) . . .} = {j(sub v), j(sub 2), j(sub 3),. . .} = J   (9)

represented in Eq.  (9).  Each electron has its spin coupled to its own orbital
angular momentum to form a j(sub i) for that electron.	The various j(sub i) are
then  more weakly coupled together to give J.  This type of coupling is seldom
strictly observed.  In the heavier atoms it is common to find a condition
intermediate between LS and jj coupling; then either the LS or jj notation may
be used to describe the levels, because the number of levels for a given
electron configuration is independent of the coupling scheme.
    Most atomic nuclei also possess spin, but rotate about 2000 times slower
than electrons because their mass is on the order of 2000 or more times
greater than that of electrons. Because of this, very weak nuclear magnetic
fields, analogous to the electronic ones that produce fine structure in
spectral lines, further split atomic energy levels. Consequently, spectral
lines arising from them are split according to the relative orientations, and
hence energies of interaction, of the nuclear magnetic moments with the
electronic ones. The resulting pattern of energy levels and corresponding
spectral-line components is referred to as hyperfine structure. See NUCLEAR
MOMENTS.
    The enormous capabilities of tunable lasers have allowed obervations which
were impossible previously. For example, high-resolution saturation
spectroscopy, utilizing a saturating beam and a probe beam from the same
laser, has been used to measure the hyperfine structure of the sodium
resonance lines (called the D1 and D2 lines). The smallest separation resolved
was less than 0.001 cm(sup -1), which was far less than the Doppler width of the
lines. See DOPPLER EFFECT; HYPERFINE STRUCTURE; LASER SPECTROSCOPY.
    Nuclear properties also affect atomic spectra through the isotope shift.
This is the result of the difference in nuclear masses of two isotopes, which
results in a slight change in the Rydberg constant. There is also sometimes a
distortion of the nucleus.
    It would be misleading to think that the most probable fate of excited
atomic electrons consists of transitions to lower orbits, accompanied by
photon emission. In fact, for at least the first third of the periodic table,
the preferred decay mode of most excited atomic systems in most states of
excitation and ionization is the electron emission process first observed by
P. Auger in 1925 and named after him. For example, a singly charged neon ion
lacking a ls electron is more than 50 times as likely to decay by electron
emission as by photon emission. In the process, an outer atomic electron
descends to fill an inner vacancy, while another is ejected from the atom to
conserve both total energy and momentum in the atom. The ejection usually
arises because of the interelectron Coulomb repulsion. See AUGER EFFECT.
 �34s[Ivan A. Sellin]
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�6v�2v�32sAtomic time �21m �33s
    Time based on quantum transitions. According to the quantum theory, an
atom or a molecule may exist in discrete states, each of which has a distinct
energy level. Energy will be absorbed or radiated when the atom jumps between
two states. The frequency of the electromagnetic radiation emitted or absorbed
is related to the difference in energy: f = (E1 - E2)/h. Here f is the
frequency, E1 is the initial energy, E2 is the final energy, and h is Planck's
constant. For a selected transition of a given atom, the frequency emitted
will remain constant. In contrast, the frequency of oscillation of a pendulum
or of a quartz crystal depends upon factors such as size, which may change.
See CLOCK; QUARTZ CLOCK.
    The transition first used to obtain very high precision, in 1955, is the
one between the two hyperfine levels of cesium-133 in the ground state. The
frequency 9,192,631,770 hertz (Hz) for cesium was determined in 1958. It was
adopted in 1967 by the General Conference of Weights and Measures to define
the unit of time, the second, in the International System of Units (SI).
    A scale of International Atomic Time, or TAI (abbreviation in French), is
formed from the operation of a number of cesium-beam atomic clocks, located at
various observatories and laboratories about the world. See ATOMIC CLOCK.
 �34s[William Markowitz]
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	 ALPHA

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�6v�2v�32sAtomic weight �21m �33s
    A number assigned to each chemical element which specifies the average
mass of its atoms. Since an element may consist of two or more isotopes having
atoms which differ in mass, the atomic weight of such an element depends on
the relative proportions of its isotopes. The isotopic composition of all
elements as they occur in nature, except those that are the products of
natural radioactivity, is practically constant. The atomic weight refers to
this natural mixture.
    A unit called relative nuclidic mass, defined as 1/12 the mass of
carbon-12, was introduced in 1960. It is designated by the symbol u; thus,
(sup 12)C = 12 u. The table of relative atomic weights is now based on the
atomic mass of (sup 12)C = 12. See RELATIVE ATOMIC MASS.
    There are four precision methods of determining atomic weights, all of
which have comparable accuracy in the most favorable cases: (1) chemical
combining weights, (2) limiting gas densities, (3) mass spectrometer
measurements, and (4) nuclear reaction energies. The first two give results on
the chemical scale, and the last two on the physical scale.
 �34s[Francis A. Jenkins /William Watson]
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�6v�2v�32sAtomization   �21m �33s 
    The mechanical subdivision of a bulk liquid. Other terms are also applied 
to this process, although the terminology is not standardized. Spraying 
usually implies the production of coarse drops (100 - 1000 micrometers in 
diameter). Sprinkling suggests very coarse drops (greater than 1000 MUm). 
Misting is often applied to the production of fine drops (10 - 100 MUm), and 
nebulizing to the production of very fine drops (under 10 MUm), usually used 
in inhalation aerosol therapy. Atomization may also be used to subdivide 
meltable solids, such as certain metals. See DUST AND MIST COLLECTION; 
PARTICLE PROPERTIES. 
    Subdivision of a liquid (or a solid) may be desired (1) to distribute 
materials throughout an area or space; (2) to expose a large surface for 
increased rates of heat or mass transfer; or (3) to provide desired flow, 
packing, optical, insulation, deposition, or other properties. The atomization 
process may be utilized in such operations as spray drying, evaporation, 
humidification, gas scrubbing, chemical reaction, combustion, fire fighting, 
spray painting, powder metallurgy, and application of insecticides, 
herbicides, fungicides, and defoliants. It may also occur inadvertently 
whenever liquids are handled to produce undesired droplets, for example, 
entrainment from flash evaporators. 
    The various techniques by which liquids can be atomized are distinguished 
either by the geometry of the atomizing device or by the source of the 
external motivating force employed. Hydraulic, pneumatic, and rotary are the 
mechanical techniques most widely used in industry, agriculture, and domestic 
applications. Vibrational and electrostatic techniques are largely in a 
developmental stage. Ultrasonic nebulizers have been marketed. Much of the 
interest in electrostatic atomization has been in the application of ion 
propulsion to rockets in outer space. Explosive techniques have been used in 
military applications, for example, chemical agent dissemination. 
Film-bursting and gravitational techniques are prevalent in the natural 
environment, but not normally capable of atomizing liquids at high rates. 
    Under normal operating conditions (30 - 200 psi or 200 - 1400 
kilopascals), hydraulic nozzles produce relatively coarse drops, 100 - 3000 
MUm in diameter, the finest ones being produced by small swirl nozzles, 
wherein pressure is converted into high relative jet velocity. Hydraulic 
nozzles are exemplified by garden hose nozzles, insecticide spray nozzles, and 
nozzles in humidification and scrubbing towers. Pneumatic atomizers normally 
use compressed air (30 - 100 psi or 200 - 700 kPa) and produce drops in the 5 
- 100-MUm diameter range. They are used in spray painting and fine misting 
applications, scrubbers or reactors (venturi atomizers), and aircraft 
application of insecticides. Rotary atomizers (spinning disks) are basically 
hydraulic atomizers, in which the pump and nozzle are combined, and normally 
produce drops in the 30 - 300-MUm diameter range. They are widely used in 
spray drying because of their ability to handle viscous liquids or slurries. 
Most nebulizers are either pneumatic or ultrasonic in nature and may rely 
heavily on evaporation to produce the finest drops. In electrostatic spray 
painting, pneumatic or hydraulic atomization is usually employed, the 
electrical field merely acting to guide the charged drops to the deposition 
surface.  
    In the absence of any disruptive influence, surface tension acts to pull a 
liquid into spherical form, that is, a form with minimum surface energy. An 
atomization process involves a disruption of this consolidating influence of 
surface tension by the action of other internal or external forces or 
pseudoforces, such as inertia. Instabilities can result that will permit the 
bulk liquid to break up into smaller units. Any shear stresses set up within 
the liquid as the result of liquid viscosity will resist a change in system 
geometry and hence will exert a stabilizing influence, that is, attenuate the 
disruption process. On the other hand, external shear stresses in the ambient 
medium may aid the disruption process by applying an external distorting force 
to the bulk liquid. 
    The practical application of atomization processes requires that droplets 
be produced at some predetermined rate, that is, liquid must be supplied at 
some finite rate and continuously converted into droplets. The kinetics of all 
such atomization processes involves the sequential steps listed below, 
although some steps may be negligible or absent under certain circumstances. 
    1. The extension of a bulk liquid into sheets, jets, films, or streams by 
accelerating the liquid in some prescribed manner (as through a nozzle or off 
a rotating disk). 
    2. The initiation of small disturbances at the liquid surface in the form 
of local ripples, protuberances, or waves. 
    3. The formation of short ligaments on the liquid surface, as the result 
of fluid pressure or shear forces. 
    4. The collapse of the ligaments into drops, as the result of surface 
tension.  
    5. The further breakup of the drops as they move through the gaseous 
medium by the action of fluid pressure or shear forces. 
 �34s[Charles E. Lapple]
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�6v�2v�32sAtopic allergy   �21m �33s 
    A type of immediate hypersensitivity which occurs in humans as the result 
of spontaneous sensitization, usually by inhaled or ingested allergens. 
Examples of atopic allergy are asthma, hay fever, hives, infantile eczema, and 
gastrointestinal disturbances. Atopic hypersensitivities differ from the 
anaphylactic state in these respects: (1) The atopic allergies may be acquired 
through exposure to minute quantities of antigens, as suggested by the fact 
that inhalation of hard-shelled grains of pollen is sufficient to induce the 
state and subsequent reactions. (2) The antibodies associated with human 
atopic allergy constitute a special class of molecules, one of the five 
families of immunoglobulin molecules known to exist in human blood. This 
special class is termed IgE (immunoglobulin E). See ANAPHYLAXIS; ASTHMA; 
IMMUNOGLOBULIN.  
    The only convenient method available for demonstrating such antibodies is 
the Prausnitz-Kuestner reaction. This is simply a specialized instance of 
passive transfer of hypersensitivity, carried out in human beings. Serum from 
a sensitive individual is injected intradermally into a normal subject. 
Twenty-four hours later, injection of the appropriate allergen into the same 
site results in a local hive. See ANTIBODY; HYPERSENSITIVITY; IMMUNOLOGY. 
 �34s[Sidney Raffel]
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�6v�2v�32sAtrophy   �21m �33s 
    The diminution in the size of a cell, tissue, or organ which was once 
fully developed and of normal size. Almost all examples of atrophy can be 
attributed to an alteration in the metabolism of the affected cells, usually 
through interference with cellular nutrition. It is postulated that increased 
intracellular activity of proteolytic enzymes is the fundamental cause of the 
loss of cell substance. 
    Physiologic atrophy affects certain organs in all individuals as a part of 
the normal aging process. An example is the atrophy of the thymus and lymphoid 
tissue that occurs at the time of puberty. In advanced age all organs undergo 
some atrophy which may in part be physiologic. 
    Pathologic atrophy refers to a decrease in the size of tissues or organs 
which is outside the range of normal variability. It is categorized on the 
basis of apparent clinical cause as disuse atrophy, vascular atrophy, pressure 
atrophy, exhaustion atrophy, and endocrine atrophy. Disuse atrophy occurs as a 
result of inactivity or diminished action of a tissue or organ, for example, 
the muscle atrophy which occurs secondary to immobilization by a plaster cast 
or secondary to denervation in poliomyelitis. Vascular atrophy occurs when 
blood supply to an organ or tissue becomes reduced below a critical level. 
This causes organs to undergo progressive loss of substance. Pressure atrophy 
results from persistent pressure upon an organ or tissue. It also occurs at a 
cellular level because of deposition of abnormal substances within tissues. 
Exhaustion atrophy develops in endocrine organs in conditions of prolonged 
overwork of the organ. Endocrine atrophy occurs in organs which are dependent 
upon endocrine stimulation for the maintenance of their normal structure and 
function. Destruction of the pituitary gland with resultant loss of hormone 
production leads to atrophy of the endocrine glands which rely upon pituitary 
hormones for their action. 
 �34s[N. Karle Mottet]
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�6v�2v�32sAtropine �21m �33s
    An alkaloid with the formula C17H23NO3; in pharmacy it is sometimes known
as dl-hyoscyamine. It occurs in minute amounts in the leaves of Atropa
belladonna, A. betica, Datura stramonium, D. innoxia, and D. sanguinea, as
well as many related plants. It melts at 237 - 241 DG F (114 - 116 DG C) and is
poorly soluble in water. The nitrate and sulfate are used in medicine instead
of the free base. Atropine is used clinically as a mydriatic (pupil dilator).
It is also administered in small doses before general anesthesia to lessen
oral and air-passage secretions. See ALKALOID.
 �34s[Frank Wagner]
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�6v�2v�32sAttenuation �21m �33s
    The reduction in level of a quantity as a function of a characteristic
parameter. Usually the quantity is an intensity I, or energy per unit area,
and the parameter is the distance from the source x. However, the attenuation
of voltage in suitable electrical networks by attenuators should also be
mentioned. In the more usual case, the reduction in level follows Lambert's
(or Bouguer's) law: I(x) = l0(sub O)e(sup -ALPHAx), where x is the parameter
in question, frequently a distance. The coefficient ALPHA is called the
attenuation or extinction coefficient.
    For unconfined radiation from a point source, the attenuation is inversely
proportional to the square of the distance from the source. The attenuation of
an electromagnetic wave in a wave guide or a sound wave in a pipe does not
follow this inverse-square law. The scattering of radiation and its absorption
(that is, loss of energy) are both attenuation processes. See ABSORPTION;
INVERSE-SQUARE LAW; POINT SOURCE; SCATTERING OF ELECTROMAGNETIC RADIATION.
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sAttenuator   �21m �33s 
    An arrangement of fixed and variable resistors mounted in a compact 
package, used to reduce the strength of a radio- or audio-frequency signal by 
a desired amount without causing appreciable distortion. The attenuator is so 
designed that its impedance matches that of the circuit in which it is 
connected, regardless of the amount of attenuation introduced. This 
characteristic is achieved by properly proportioning the resistance values of 
the series and shunt elements.  
 �34s[John Markus]
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�6v�2v�32sAudio amplifier   �21m �33s 
    An electronic circuit for amplification of signals within, and in some 
cases above, the audible range. The term may mean either the complete 
amplifier, consisting of the voltage-amplifier and power-amplifier stages, or 
it may mean just one stage. In the usual case, the term implies that the 
amplifier is not capable of amplifying a direct-current signal; such an 
amplifier would more likely be called a direct-coupled or dc amplifier. See 
DIRECT-COUPLED AMPLIFIER. 
    The function of an audio amplifier is to amplify a weak electrical signal, 
such as from a microphone or a phono pickup, to a level capable of driving a 
loudspeaker at the desired output sound level. The sound power level may be 
small in such applications, as in a hearing aid. Audio amplifiers may be 
constructed with vacuum tubes, but current technology favors transistors and 
integrated circuits. See AMPLIFIER. 
 �34s[Harold F. Klock]
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�6v�2v�32sAudio delayer   �21m �33s 
    A device for introducing delay in the audio signal in a sound-reproducing 
system. The loudspeaker-pipe-microphone combination, the magnetic tape 
recorder-reproducer, and the digital delayer are the most common. See 
SOUND-REPRODUCING SYSTEMS. 
    The loudspeaker-pipe-microphone system consists of an equalizer, power 
amplifier, loudspeaker, pipe, microphone, and additional amplifiers. Delay is 
introduced by causing sound waves to travel along a pipe from the loudspeaker 
to the microphone. There is attenuation in the pipe, which increases with 
frequency. Amplitude compensation with respect to frequency is introduced in 
the input to the power amplifier to compensate for the frequency 
discrimination due to attenuation. See FREQUENCY-RESPONSE EQUALIZATION. 
    The magnetic tape recorder-reproducer system consists of an audio magnetic 
tape recorder and reproducer with multiple spaced heads, and a magnetic tape 
belt which introduces delay by carrying the recorded signal a suitable 
distance from the record to the reproduce heads. The system provides a simple 
means for introducing delay; however, the objection is that the tape belt 
wears out after a few hours and must be replaced. See MAGNETIC RECORDING. 
    The first element of an audio delay system employing a digital delay is a 
low-pass filter which eliminates all audio components above 12,000 Hz. A 
sampling switch operates at a  frequency of 30,000 Hz. The sampled signal is 
converted into the proper digital format by the analog-to-digital converter 
and fed into the digital shift register serving as a memory. The shift 
register elements are connected so that each element transfers its digital 
word to the next stage and reads the word from the previous stage during the 
sampling interval. In this way each word is entered at the input of the memory 
and moves down the line relatively slowly. The output of the shift register is 
fed to the digital-to-analog converter and then to the sampling switch. The 
output of the sampling switch is smoothed by the low-pass filter. The audio 
signal output, is the same as the audio wave input but with a delay. 
    One of the major applications for audio delayers is in sound-reinforcement 
systems. Audio delay of up to 500 milliseconds can be obtained with any of the 
three systems described above. A signal-to-noise ratio of 60 dB is attainable. 
See SOUND-REINFORCEMENT SYSTEM. 
 �34s[Harry F. Olson]
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�6v�2v�32sAudiometry �21m �33s
    The quantitative assessment of individual hearing, either normal or
defective. Three types of audiometric tests are used: pure tone, speech, and
bone conduction tests. Such tests may serve various purposes, such as
investigation of auditory fatigue under noise conditions, human engineering
study of hearing aids and communication devices, screening of individuals with
defective hearing, and diagnosis and treatment of defective hearing. In all of
these situations, individual hearing is measured relative to defined standards
of normal hearing.
    The pure-tone audiometer is the instrument used most widely in individual
hearing measurement. It is composed of an oscillator, an amplifier, and an
attenuator to control sound intensity. For speech tests of hearing, word lists
called articulation tests are reproduced on records or tape recorders.
Measurements of detectability or intelligibility can be made by adjusting the
intensity of the test words. To make bone conduction tests, sound vibrations
from the audiometer activate a vibrator located on the forehead or mastoid
bone.
    Scientific advance in audiometry demands careful control of all
environmental sound. Two types of rooms especially constructed for research
and measurement of hearing are the random diffusion, or reverberation, chamber
and the anechoic room. In the reverberation chamber, sounds are randomly
reflected from heavy nonparallel walls, floor, and ceiling surfaces. In the
anechoic room, the fiber glass wedges absorb all but a small percent of the
sound.
    The measurement of hearing loss for pure tones in defective hearing is
represented by the audiogram (see illustration). Sounds of different
frequencies are presented separately to each ear of the individual, and the
intensity levels of the absolute thresholds for each frequency are determined.
The absolute threshold is the lowest intensity which can be detected by the
individual who is being tested.
    In clinical audiometry the status of hearing is expressed in terms of
hearing loss at each of the different frequency levels. In the audiogram the
normal audibility curve, representing absolute thresholds at all frequencies
for the normal ear, is represented as a straight line of zero decibels. Amount
of hearing loss is then designated as a decibel value below normal audibility.
The audiogram in the illustration reveals a hearing loss for tones above 500
Hz. Automatic audiometers are now in use which enable individuals to plot an
audiogram for themselves.
    Articulation tests are speech perception or speech hearing tests used to
assess hearing and loss of hearing for speech. The threshold of
intelligibility for speech is defined as the intensity level at which 50% of
the words, nonsense syllables, or sentences used in the articulation test are
correctly identified. The hearing loss for speech is determined by computing
the difference in decibels between the individual intelligibility threshold
and the normal threshold for that particular speech test. Discrimination loss
for speech represents the difference between the maximum articulation score at
a high intensity level (100 dB), expressed in percent of units identified, and
a score of 100%. The measure of discrimination loss is important in
distinguishing between conduction loss and nerve deafness.
    Bone conduction audiograms are compared with air conduction audiograms in
order to analyze the nature of deafness. Losses in bone conduction hearing
generally give evidence of nerve deafness, as contrasted to middle-ear or
conduction deafness. See DEAFNESS; EAR; HEARING (HUMAN).

     FIGURE CAPTION.  Audiogram for determining the audibility curve for
pure-tone hearing loss at various frequency levels.

 �34s[Karl U. Smith]
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�6v�2v�32sAuger effect   �21m �33s 
    An internal photoelectric process in an atom in which, for example, 
instead of the emission of a single characteristic x-ray from the filling of a 
vacancy in the K shell of electrons by an electron from a higher shell, an 
additional electron from the L, M, . . . shell is emitted with a kinetic 
energy equal to the energy of this x-ray minus the binding energy of this 
ejected electron. Such an electron is called an Auger electron. If the K-shell 
vacancy is in an atom of high atomic number Z, the return to the normal state 
may involve the emission of several x-rays and Auger electrons. That fraction 
of the vacancies in any electron shell that is filled by this Auger process is 
called the Auger yield, just as the fraction filled with accompanying x-ray 
emission is the fluorescence yield. The Auger effect is sometimes called 
autoionization. See X-RAY FLUORESCENCE ANALYSIS. 
 �34s[William W. Watson]
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�6v�2v�32sAugite �21m �33s
    A group of monoclinic calcic pyroxenes which have the general chemical
formula (Ca,Mg,Fe)(Mg,Fe)Si2O6, in which calcium is the dominant cation in the
first cation position. Monoclinic pyroxene with substantial iron or magnesium
in place of calcium is called pigeonite, and has a different crystal structure
from augite. Augite is generally considered a combination of the four end
members diopside (CaMgSi2O6), hedenbergite (CaFe (sup 2 POSITIVE) Si2O6),
enstatite (Mg2Si2O6), and ferrosilite (Fe2 (sup 2 POSITIVE) Si2O6), but it
almost always has substantial aluminum and minor to substantial amounts of
sodium, ferric iron, chromium, and titanium.
    Augite occurs in both igneous and metamorphic rocks. It is nearly
universal in basalts and gabbros, and occurs somewhat less frequently in less
mafic igneous rocks. Magnesium-rich augite is a characteristic mineral in many
ultramafic rocks and in rocks of the Earth's mantle. Augite and pigeonite are
also rather common constituents of lunar basalts and basaltic meteorites. See
DIOPSIDE; ECLOGITE; ENSTATITE; PIGEONITE; PYROXENE.
 �34s[Robert J. Tracy]
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�6v�2v�32sAurora   �21m �33s 
    A large-scale electrical dischage process which surrounds the Earth. The 
discharge is powered by the so-called solar wind - magnetosphere generator. A 
curtainlike luminous feature of the aurora is produced by upper atmospheric 
atoms and molecules which emit their own characteristic light after colliding 
with the current-carrying electrons. The most common light of the aurora (the 
greenish-white light) comes from excited oxygen atoms. Excited and ionized 
molecular nitrogen adds several band emissions. Since the discharge current 
flows into and out from the annular region, the aurora appears as a 
ring-shaped luminousity (called the auroral arc) surrounding both the northern 
and southern magnetic poles (see illustration). See ATMOSPHERIC ELECTRICITY. 
    The aurora becomes active during geomagnetic storms which occur often 
about 40 h after an intense solar flare. This is because the efficiency of the 
solar wind - magnetosphere generator becomes high and variable when the solar 
wind becomes gusty after a solar flare. During a great magnetic storm, the 
auroral oval expands from its usual latitude of about 67DG to 50DG or a little 
less. It is on such an occasion when the aurora can be seen widely across the 
continental United States. See ATMOSPHERE; ATOMIC STRUCTURE AND SPECTRA; 
MAGNETOSPHERE; PLASMA PHYSICS; SOLAR WIND. 
 �34s[S. -I. Akasofu]
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�6v�2v�32sAustralia �21m �33s
    An island continent in the Southern Hemisphere with a total area of
2,941,526 mi sup 2 (7,618,552 km sup 2). It is bounded on the west by the Indian
Ocean and on the east by the Pacific Ocean and the Tasman Sea. Numerous small
and several large islands lie off the coast, including Tasmania and New
Zealand. Australia is generally of remarkably low elevation and moderate
relief. Three-fourths of the land mass lies between 600 and 1500 ft (180 and
450 m) in the form of a huge plateau. A cross section from east to west shows
first a narrow belt of coastal plain, then the steep escarpments of the
eastern face of the Great Dividing Range, stretching 1200 mi (1900 km) from
the north of Queensland to the south of Victoria. The descent on the western
slope of the Dividing Range is gradual until often elevation in the inland
basins is below sea level, rising gradually again across the great plateau
until the low ranges of western Australia fringing the plateau are reached,
and beyond these lies another coastal plain. With the exception of the Gulf of
Carpentaria and Cape York peninsula in the north and the Great Australian
Bight in the south, there are few striking features in the configuration of
the coast. Australia may conveniently be divided into three great structural
and landform regions.
    The region called East Australian Highlands consists of a narrow plain
extending north and south along the eastern coast. Flanking the plain are the
series of ranges and tablelands making up the Great Dividing Range. The East
Australian Highlands is the best-watered region in Australia, and some of the
river systems are of considerable size. On the flanks of the East Australian
Highlands are Australia's principal coal deposits - in the vicinity of Sydney
and Newcastle and in the Bowen and Ipswich fields in Queensland. Petroliferous
basins at Surat (Roma), flanking the divide in Queensland and off the coast of
Victoria in Bass Strait, are Australia's most promising deposits of petroleum
and natural gas.
    The region known as the Interior Lowland Basins comprises a region of
sedimentary rocks that occupy one-third of the continent between the western
slope of the eastern highlands and the inner eastern margin of the ancient
shield which forms the Western Plateau. Little land is over 500 ft (150 m),
and some is below sea level. The rivers of the Murray-Darling Basin, draining
the western slopes of the Great Dividing Range, have a marked seasonal
variation in flow but never dry up in the lower reaches. South Australia's
shallow lakes are more often dry expanses of encrusted white salt than bodies
of water - the result of low rainfall and high evaporation. In most parts of
the region water from deep artesian wells is available.
    The region known as the Western Plateau is the largest area, occupying
almost three-fifths of the continent, and is a great shield of ancient rocks
standing 750 - 1500 ft (225-450 m) high. Much of it is buried in desert sand,
and only a few ridges of ancient mountains (such as the Macdonnel and Musgrave
ranges) break the monotony of the plateau surface. Only in the southwestern
corner of the continent and along the northwestern coast is rainfall
sufficient to support a sclerophyll forest of eucalypts and a monsoon
woodland, respectively. In the north, coastal rivers are of considerable size
but change from flooded torrents after rains to a succession of water holes in
dry seasons.
    Tasmania is a small mountainous island lying 150 mi (240 km) southeast of
Australia across Bass Strait, with a total area of 26,383 mi sup 2 (68,332
km sup 2). The island is structurally similar to the East Australian Highlands.
The dominant feature is the central plateau, falling from a general level of
3500 ft (1070 m) in the northwest toward the southeast. A dense eucalypt
forest covers most of the island except along the wetter west coast, where
beech forest predominates. The rivers have short, rapid courses with little
seasonal variation in flow. See NEW ZEALAND.
 �34s[Kenneth B. Cumberland]
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�6v�2v�32sAustralopithecus �21m �33s
    An extinct genus of the family Hominidae. The existence of this hominid
was first demonstrated through the recovery in 1924 of an infant's skull at
Taung, northern Cape Province, South Africa. R. A. Dart identified the
specimen as humanlike and named it Australopithecus africanus. Subsequently,
other sites in South Africa yielded further, abundant fossils of this species,
as well as numerous remains of a larger, more robust form, usually termed A.
robustus.
    The genus Australopithecus is characterized by relatively to absolutely
small body size, adaptation of the lower limb for upright posture and bipedal
gait, enlarged brain in comparison with the brain of anthropoid apes, large
facial skeleton with small, non-ape-like anterior teeth (incisors and
canines), and large to massive posterior teeth (premolars and molars). The
postural and locomotor adaptations and the distinctive anterior dentition
clearly ally Australopithecus with the family Hominidae rather than with the
Pongidae (apes), as was once suggested. However, the relationship of
Australopithecus to the genus Homo has long been debated.
    The various discoveries in Africa demonstrate the substantial antiquity of
the family Hominidae and of the genus Australopithecus. The family Hominidae
now appears to have a long evolutionary history, extending back about 7 to 8 x
10 sup 6 years. However, the fossil record is still too scanty to gain any idea
of the skeletal anatomy and adaptation of those very ancient hominids. At
least three, and perhaps more, species of Australopithecus are known between 3
or 4 and 1 x 10 sup 6 years ago. The small species A. africanus appears to be
the oldest, but its evolutionary relationships to the geologically younger
robust species A. boisei and A. robustus still remain unclear. A. africanus,
or a similar species, is reasonably regarded as a likely ancestor of Homo, but
the time and place of origin of that genus are still unresolved. See FOSSIL
MAN.
 �34s[F. Clark Howell]
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�6v�2v�32sAuthigenic minerals �21m �33s
    Minerals that are formed in place within sediments or sedimentary rocks
during or long after  their deposition. Their origin distinguishes them from
detrital (allogenic) minerals, which are formed elsewhere and transported to
the site of deposition, and from biogenic minerals, which compose the hard
parts of organisms or are indirectly precipitated by organisms at the site of
deposition. Authigenic minerals are among the principal products of diagenetic
processes and are frequently the only tangible evidence of these processes.
See DIAGENESIS.
    Authigenic minerals can occur in almost all types of sedimentary rock and
can range in abundance from a trace to virtually the total rock. The more
common authigenic minerals are shown in the table. Probably the most abundant
are calcite and dolomite, the principal minerals composing limestones and
dolostones, respectively. The detrital minerals composing sandstones are often
cemented by authigenic quartz, calcite, dolomite, or the iron oxides hematite
or goethite. Authigenic pyrite and marcasite are frequent constituents of
shales and mudstones. Recent marine sediments often contain one or more of the
zeolite minerals which have formed from the devitrification of volcanic glass.

[I. Edgar Odom]
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�6v�2v�32sAutoclave   �21m �33s 
    A pressure vessel for heating and usually agitating mixtures of gases and 
liquids or liquids and solids. Small-batch autoclaves are used in laboratories 
and for sterilizing and cooking; large ones in the chemical and petroleum 
industries for carrying out chemical reactions. The contents may also flow in 
and out continuously. 
    Heat may be supplied by steam or hot oil in a jacket or by pipe coils or 
electric heaters inside. The walls as well as the contents may be heated. 
Temperatures above a few hundred degrees Fahrenheit may reduce the strength of 
the wall unless it is of a suitable alloy. At higher temperatures it is 
preferable to design the autoclave with a cold wall. This is done by heating a 
space within the vessel cavity with an internal electric heater or with the 
incoming fluid stream, insulating this space from the inner wall, and cooling 
the wall with fluid passed through channels in it. 
    Agitation is provided in various ways. The contents may be stirred by a 
propeller driven by a shaft which passes through a packing gland, or the 
entire vessel may be rocked or given a linear reciprocating motion. Agitation 
in continuous-flow autoclaves may also be provided by baffles inside the 
vessel.  
    In batch autoclaves the cover or closure design depends on the use and 
conditions of operation. If the vessel is seldom opened, a suitable gasket and 
bolts may suffice. Frequent and ready access requires quick-opening and 
self-sealing designs. Sight glasses are windows in the wall to view the 
contents. Breakage has resulted in costly and fatal accidents. A design in 
which the packing exerts radial pressure on the glass disk increases safety. 
See STERILIZATION. 
 �34s[Edward W. Comings]
�33s 
  
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�6v�2v�32sAutogiro �21m �33s
    A type of aircraft which utilizes a rotating wing (rotor) to provide lift.
An autogiro is similar to a helicopter, but uses a conventional
engine-propeller combination in addition to the rotor to pull the vehicle
through the air like a fixed-wing aircraft (see illustration).
    Unlike the helicopter, the autogiro rotor maintains its speed of rotation
in the air because of the aerodynamic forces acting upon the rotor blades, and
is without direct mechanical drive from the engine (autorotation). All the
power required to maintain flight is supplied through the propeller of the
autogiro at the front (tractor type) or rear (pusher type) of the fuselage.
The rotor is thereby pulled through the air and creates a lifting force.
    The autogiro has several inherent advantages. It is capable of takeoff and
landing in a shorter distance than the fixed-wing airplane, and is capable of
level controlled flight at extremely slow speed, on the order of 20 mi/h (30
km/h). It is mechanically simpler than the helicopter. An autogiro, unlike an
airplane, cannot be stalled. Among the disadvantages of the vehicle is that
its speed performance is limited as compared to an airplane, although it is
capable of flying slightly faster than the helicopter. It cannot hover as can
a helicopter. See HELICOPTER.

     FIGURE CAPTION.  In-flight loaded dynamic components of a typical
 autogiro.

 �34s[Leonard Goland]
�33s

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�6v�2v�32sAutomata theory �21m �33s
    In its most general sense, automata theory is concerned with ways in which
inputs to a system combine with current states of a system to produce outputs
from the system. The subject matter can be divided into (1) abstract theory,
(2) structural theory, and (3) self-organizing and adaptive systems.
Abstract-theory encompasses general definitions, Turing machines, and language
theory. The structural category is concerned more with practical machine
construction and includes the propositional calculus, switching algebra (or
Boolean algebra), and information theory. Self-organizing systems include
neural nets, artificial intelligence, game theory, and pattern recognition.
See ARTIFICIAL INTELLIGENCE; GAME THEORY; INFORMATION THEORY; NEURAL MODELING;
PATTERN RECOGNITION; PROPOSITIONAL CALCULUS; TURING MACHINE.
    An automaton can be characterized by three sets and two functions: the set
A of all input signals, the set S of internal states of a system subjected to
the inputs, and a set Z of all output signals (see illustration). At any given
time the set of internal states S is determined from both a combination of the
set of possible states and the set of possible input terms, so that A x S maps
into S. Similarly, the output functions, or members of the set Z, are
determined from system states and inputs, so that A x S maps into Z.

     FIGURE CAPTION.  Representation of an automaton.

 �34s[Norman B. Reilly]
�33s




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�6v�2v�32sAutomatic frequency control (AFC)   �21m �33s 
    The automatic control of the intermediate frequency in a radio, 
television, or radar receiver, to correct for variations of the frequency of 
the transmitted carrier or the local oscillator. In high-fidelity broadcast 
receivers, AFC keeps distortion due to detuning to a low figure. In the 
reception of long-haul telegraph signals, AFC reduces the error rate due to 
signal pulse distortion or interference from lower-intensity signals in the 
same frequency band. 
    AFC techniques are varied but are mainly of two types. One uses a 
discriminator to furnish a voltage whose magnitude and polarity are determined 
by the frequency change. This voltage is used to adjust the frequency of the 
local oscillator of the receiver, thereby keeping the intermediate frequency 
constant. The other uses two-polarity pulse accumulation which furnishes a dc 
potential proportional to frequency error. 
 �34s[Walter Lyons]
�33s 
  
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�6v�2v�32sAutomatic gain control (AGC) �21m �33s
    The automatic maintenance of a nearly constant output level of an
amplifying circuit by adjusting the amplification in inverse proportion to the
input field strength, also called automatic volume control (AVC). Almost all
radio receivers in use employ AGC. In broadcast receivers, AGC makes it
possible to receive incoming signals of widely varying strength, yet have the
sound remain at nearly the same volume. In communications receivers a type of
AGC circuit called a squelch circuit is used to prevent noise during periods
of no transmission, such as in the reception of on-off keying, frequency-shift
keying (FSK), and phone. AGC is also useful in accelerating the switching
action between receivers in diversity connection. See RADIO RECEIVER.
    AGC action depends on the characteristic, possessed by most electronic
tubes and transistors, of adjustment of gain by the variation of the applied
bias voltage. If the dc voltage applied to the control grid of a vacuum tube
is made more negative, the amplification of that stage will be reduced.
    In most broadcast receivers the AGC voltage is taken from the detector.
This dc voltage, proportional to the average level of the carrier, adjusts the
gain of the radio-frequency (rf) and intermediate-frequency (i-f) amplifiers
and the converter, as shown in the illustration. AGC tends to keep the input
signal to the audio-frequency (af) amplifier constant despite variations in rf
signal strength. There are several modifications of this basic circuit.
    AGC circuits are also used in dictation recording equipment, public
address systems, and similar equipment where a constant output level is
desirable. See AMPLIFIER.

     FIGURE CAPTION.  Block diagram of broadcast receiver using AGC.

 �34s[Walter Lyons]
�33s


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�6v�2v�32sAutomatic horizon   �21m �33s 
    A device that provides the pilot with symbols representative of the 
attitude of an aircraft relative to an artificial horizon. In an automatic or 
artificial horizon a vertical gyro moves a horizon bar relative to a fixed 
aircraft index. Motion of the bar simulates changes in pitch or roll of the 
aircraft. As the pilot views the indicator, the horizon is seen relative to 
the aircraft as the horizon would appear if the pilot could see it through the 
windscreen. See GYROSCOPE. 
 �34s[James W. Angus]
�33s 
  
  
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�6v�2v�32sAutomatic landing system   �21m �33s 
    The means for guiding and controlling aircraft from an initial approach 
altitude to a point where safe contact is made with the landing surface. Such 
systems differ from low-approach systems in three major respects: (1) They 
furnish not only guidance but control of the aircraft as well. (2) They 
furnish information on the aircraft's position with respect to the terrain 
below it, and the rate at which the landing surface is being approached. (3) 
They do not require the pilot to assume manual control near the ground. 
    Two automatic landing systems have been developed. One, a radar-beam type, 
detects the position and rate of change in position of the landing aircraft by 
means of a radar beam emitted from a ground derived-control complex. The 
other, a fixed-beam type, derives position and rate of change in position by 
instrumentation within the landing aircraft, but it makes use of instrument 
landing system (ILS) type equipment on the ground. In the aircraft are 
accelerometers (which may be part of an inertial navigation system) and a 
radio altimeter. Essential to both systems is an autopilot in the aircraft, 
commanded by a computer on the ground in the radar-beam system and by a 
computer in the aircraft for the fixed-beam system. See INSTRUMENT LANDING 
SYSTEM (ILS). 
 �34s[John L. Loeb]
�33s 
  
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�6v�2v�32sAutomatic sprinkler system �21m �33s
    A safety system used to prevent the spread of fires. An automatic
sprinkler system is usually installed inside a building, although sprinklers
may also be installed outside for protection against exposure fires.
    Common automatic sprinkler systems are wet pipe, dry pipe, preaction, and
deluge. In a wet pipe system, the pipes contain water and are connected to an
ample water supply. Individual sprinkler heads are normally closed and are
preset to open, usually by the melting of an alloy insert, when the local
temperature reaches between 135 and 165 DG F (57 and 74 DG C) for areas having
normal atmospheric temperatures. Thus, the sprinklers discharge only in the
area of a fire and in adjacent areas where hot fumes collect.
    In a dry pipe system, the sprinkler pipes contain air under pressure. The
opening of a sprinkler allows the air to escape and water to enter the system
and to discharge from the sprinkler. Dry pipe systems are used in unheated
buildings such as warehouses.
    In preaction and in deluge systems, separate heat-responsive devices in
the same area as the sprinklers admit water to the normally dry sprinkler
piping when a fire occurs. The preaction system provides warning before the
sprinkler heads operate. In the deluge system, all sprinklers are always open
so that water discharges promptly to prevent spread of fire in a hazardous
area. See FIRE EXTINGUISHER.
 �34s[Frank H. Rockett]
�33s

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�6v�2v�32sAutomation �21m �33s
    A coined word having no precise, generally accepted technical meaning but
widely used to imply the concept, development, or use of highly automatic
machinery or control systems. See CONTROL SYSTEMS; SERVOMECHANISM. Common
concepts of automation are: production systems so integrated that materials
move through the required operations with little or no human assistance;
control systems that automatically maintain machinery performance within
desired limits; machinery that mathematically manipulates information (usually
in the form of numbers), storing, selecting, presenting, and recording input
data or internally generated data as required; any machinery or equipment (or
changes in product design, materials, or processing technique) that reduces
the labor content in a product or service.
    Seventeen levels of mechanization have been identified. These levels can
be arranged in a chart (see illustration) that reveals a distinct evolution.
This evolution explains why the effect of increasing automaticity generally is
to lower the skill requirements of the operating work force rather than to
raise them. It is true, of course, that the increased complexity of automatic
machinery and control may increase the amount of maintenance personnel and the
degree of maintenance skill required to the point that the increase of skilled
personnel in the total work force becomes significant. New types of jobs
requiring higher skill on the part of operators also are occasionally created.
    By using levels of mechanization as a vertical scale, a mechanization
profile can be constructed for any system by plotting the level of each
required activity in sequence. Mechanization profiles of so-called automated
plants reveal that the spans of mechanization are quite short, that the levels
of mechanization fluctuate widely, that very little self-correcting action
(levels 12 - 17) is found in most systems, and that automatic operations are
most frequently achieved by mechanization on levels 5 - 8. Often growth in the
span is more significant than an increase in the level of mechanization in
achieving automatic operations.

     FIGURE CAPTION.  Levels of mechanization and their relationship to power
and control sources.

 �34s[James R. Bright]
�33s


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�6v�2v�32sAutomobile   �21m �33s 
    A generic term for a self-propelled, trackless personal or public carrier 
which encompasses passenger cars, recreational vehicles, taxis, and buses used 
to transport people in cities, on highways, or across country. Passenger cars 
are available in several body styles, with optional components, and they are 
made in various sizes. The sedan, in two- or four-door models, seats five or 
six passengers. The hardtop, a metal-roof model that replaced the convertible, 
offers some of the all-weather advantages of the sedan. The station wagon, a 
utility vehicle with two or four doors for occupants and a rear door for 
loading commodities, provides comfortable seating for up to nine passengers. 
The van is a type of commercial vehicle that has been adapted for personal 
use. Variations of these body types have been developed for special purposes. 
See AUTOMOTIVE BODY; AUTOMOTIVE BRAKE; AUTOMOTIVE CHASSIS; AUTOMOTIVE ENGINE; 
AUTOMOTIVE FRAME; AUTOMOTIVE STEERING; AUTOMOTIVE TRANSMISSION; AUTOMOTIVE 
VEHICLE; BUS. 
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive air conditioning   �21m �33s 
    A system for maintaining comfort of driver and occupants of automobiles, 
buses, and trucks, limited to air cooling, air heating, and ventilation. 
Humidification and filtration are not incorporated, but dehumidification is 
available when refrigeration is employed. Systems built into automobiles by 
the manufacturers provide a minimum quantity of fresh air to the cab when the 
vehicle is in motion. The cab is thereby pressurized at 1 - 2 in. (2.5 - 5 cm) 
of water column, and it exhausts by exfiltration through cracks around doors 
and other body leaks. 
    Warm air is provided by a coil in the supply airstream; the coil is heated 
by hot water from the engine cooling system. Cool air is provided by a 
refrigeration system which consists of a refrigerant compressor, belt-driven 
by the vehicle engine through a magnetic clutch; a condenser whose 
configuration is somewhat like the vehicle's radiator and whose location is 
ahead of the radiator; and a thermostatic expansion valve to meter refrigerant 
to an evaporator, usually located ahead of the auto's fire wall and across 
which cab air is recirculated, cooled, and returned to the cab. See AIR 
CONDITIONING; REFRIGERATION. 
 �34s[Richard L. Koral]
�33s 
  
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�6v�2v�32sAutomotive alternator   �21m �33s 
    An alternating-current generator used in automotive vehicles to supply 
electric current to the vehicle electric system. The typical automotive 
alternator is a three-phase, Y-wound unit with a six-diode rectifier. Current 
is supplied to the rotor (rotating field) through slip rings. When the vehicle 
is inoperative, the field is disconnected from the battery, either by opening 
the ignition switch or by opening field relay contacts. The six-diode 
rectifier changes the alternating current to the direct current required by 
the vehicle electric system. See ALTERNATING-CURRENT GENERATOR; ALTERNATOR. 
AUTOMOTIVE VOLTAGE REGULATOR. 
 �34s[William H. Crouse]
�33s 
  
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�6v�2v�32sAutomotive body   �21m �33s 
    An enclosure mounted on and attached to the frame of an automotive 
vehicle. The passenger car body is fabricated from sheet metal, glass, 
interior trim, and upholstery materials for the convenience and protection of 
the occupants from the weather and from annoyances such as dirt, dust, noise, 
and smoke. 
    All-steel body shells are assembled from numerous stampings in automated 
welding bucks with a minimum of mechanical fasteners in order to reduce noise 
accountable to moving parts. Roof, quarter panels, doors, deck lid, and hood 
undergo similar welding operations in fabrication and are then cleaned and 
undercoated prior to internal and external painting. Body interior, doors, 
deck lid, and hood are sprayed with sound-deadening materials before door 
glass, interior trim, upholstery, and floor and deck carpeting are installed; 
backup lights, windshield, and similar items are added during the final 
assembly process. 
    Unitized bodies are a departure from conventional body-frame construction 
in that shorter side rails are used for a frame that is integrated with the 
body and floor pan; in conventional construction, full-length standard side 
rails with fore-and-aft extensions beyond the body shell proper are used. 
    Commercial bodies are customized to their job to a much greater degree 
than passenger cars are, with the type of vehicle ranging from light pickup 
trucks, panel trucks, and vans to huge bus and tractor units for tankers and 
trailers, many having refrigeration, temperature-control equipment, and so on 
for critical commodities. See AUTOMOTIVE VEHICLE. 
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive brake   �21m �33s 
    A system with mechanical and hydraulic components to slow or stop a 
vehicle by means of the action of friction upon wheel drums, disks, or drive 
shafts.  
    In an automotive power brake assembly, movement of the brake pedal 
operates a valve which allows one side of a piston to be exposed to 
atmospheric pressure, while engine-intake-manifold vacuum is applied to the 
other side. The difference in pressure causes the piston to move. This 
movement is transmitted to the piston in the brake master cylinder, causing 
the brakes to be applied. 
    In an air-brake system, an engine-driven air compressor provides the 
pressure to the braking system. Operation of the brake pedal admits 
high-pressure air to brake chambers at each wheel. A diaphragm or piston in 
the brake chambers is displaced, forcing the brake shoe against the brake 
drum. See AIR BRAKE. 
    Brakes can also be applied by electromagnets located at each wheel. The 
driver operates a controller that connects the electromagnet to the vehicle 
battery. The electromagnet is attracted to disks on the rotating wheels, 
causing the electromagnet to shift through a limited arc. This movement 
actuates the brake shoes. Further movement of the controller allows more 
current to flow in the electromagnets, producing stronger magnetic fields and 
greater braking action. See ELECTROMAGNETIC BRAKE. 
 �34s[William H. Crouse]
�33s 
    Conventional drum-type service brakes have two steel shoes, faced with 
heat- and wear-resistant lining, for each wheel, which are forced against the 
inner brake drum surfaces when pedal pressure is applied by the driver. A dual 
master cylinder operates the hydraulic systems for front and rear brakes 
separately to assure braking action in the event that one fails. Power brakes, 
which are standard in some cars and optional in others, reduce braking effort 
by means of intake manifold vacuum or atmospheric pressure. Parking brakes are 
independent of service brakes, but they may be integral with wheel brakes, or 
used on the drive shaft. Parking brakes are set on rear wheels or the drive 
shaft by the foot pedal and are released manually. See FRICTION BRAKE. 
    Disk-type brakes are less subject to water fading and more resistant to 
heat fading due to high speed or repeated stops than drum brakes are, and they 
are capable of straight-line stops; that is, hard braking effort does not 
cause them to pull to the right or left. They are available in pressure-plate 
and fixed- or floating-caliper types. Because high hydraulic operating 
pressure is necessary for disk brakes, power-assist units must be added. See 
BRAKE.  
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive chassis   �21m �33s 
    A bare vehicle (minus body) with frame, power plant, power train, brakes, 
and wheels, as well as cooling, fuel, exhaust, electrical, lubrication, 
steering, and suspension systems assembled and operational. For unitized 
bodies, the chassis components are attached directly to frame members which 
are integrated with the body shell and which are adapted to serve the 
functions of conventional frames. In heavy load-bearing commercial vehicles 
the frames are retained as the foundation for chassis assembly and body 
attachment. See AUTOMOTIVE BODY; AUTOMOTIVE FRAME; AUTOMOTIVE VEHICLE. 
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive engine   �21m �33s 
    An integral major component of and source of power for automotive 
vehicles. Several types of engines are available for passenger and commercial 
vehicles, but most vehicles are driven by reciprocating internal combustion 
gasoline engines. Many large commercial vehicles, as well as some American and 
imported passenger cars, use diesel engines. 
    Domestic passenger car engines evolved from an assortment of electrics and 
steamers, designed with two or four cycles. Present four-, six-, and 
eight-cylinder engines are four-cycle types of inline, V-block, slant, or 
horizontally opposed ``pancake'' configurations. Formerly popular in-line 
eights and V-12 and V-16 powerplants were discontinued when higher compression 
ratios were developed. Redesign of six- and eight-cylinder engines provided 
equivalent horsepower, with incidental reduction in weight. 
    As vehicles grew in size and type, and greater road speed was demanded, 
engines tended to increase in size and weight and had to improve in 
performance. Present cylinder blocks and heads are gray iron castings, as are 
numerous other engine parts. Weight reduction has been achieved by decreasing 
cylinder wall sections and substituting stampings for castings, and plastics 
for metal parts. 
    Higher engine compression ratios produced greater combustion temperatures, 
and ignition knocks which could be eliminated only with higher-octane fuel, 
made possible by introduction of gasoline additives. Such developments created 
a new family of high-performance engines. This developmental trend was 
modified by the Clean Air Act of 1970, which mandated specific emission 
controls and use of nonleaded fuel, and encouraged manufacturers to get more 
miles per gallon. See INTERNAL COMBUSTION ENGINE. 
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive frame   �21m �33s 
    The basic structure of an automotive vehicle. Frames are composites of 
heavy steel stampings which include fabricated side rails, cross members, and 
mounting brackets assembled into a foundation for the major components and 
bodies of passenger and commercial vehicles. 
    Side rails have either channel or box sections, with a swept perimeter 
conforming to body contours. Heavy cross members support the engine in the 
front and the loads carried in the rear; additional cross members tie the side 
rails together through their length; bumpers are attached at both ends. Butyl 
rubber body mounts insulate occupants from noises or vibrations coming from 
the power plant and power train. See AUTOMOTIVE BODY. 
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive steering �21m �33s
    Mechanical means by which a driver controls the course of an automotive
vehicle. A typical system (Fig. 1) consists of a manually operated steering
wheel in the driver's compartment connected by a steering column to the
steering gear attached to a connecting rod (pitman arm) from which linkages
attach to steerable road wheels mounted at the front of the vehicle. Clockwise
rotation of the steering wheel actuates the steering system to turn the
vehicle to the right; rotation counterclockwise turns the vehicle to the left.
In manual steering the driver provides all the force necessary to turn the
steered road wheels; in power steering an auxiliary mechanism assists the
driver by supplying part of the steering force. See POWER STEERING.
    A steering system provides four essential features: (1) sufficient
mechanical advantage for a driver to steer the vehicle without excessive
physical effort, (2) steering of road wheels without hindering their free
movement on the spring suspensions, (3) differential turning of steered wheels
so that both roll on circles with a common center, and (4) tendency for
steered wheels to return to the straight forward position when the steering
wheel is released. See ACKERMAN STEERING.
    To roll smoothly when the vehicle is directed into the desired course, the
steerable wheels are mounted at slight angles to the normal (Fig. 2). This
tilt is called camber and is positive if outward. The wheels are given slight
positive camber (usually less than a degree) initially so that, when the
vehicle is loaded, the wheels become almost vertical.
    The kingpin, or the axis of the balls on ball suspension, is tilted
inward. This kingpin inclination causes the chassis to rise when the wheel
turns. Thus the weight of the car tends to keep the wheel turned straight
ahead or to return it to that position after a turn. This self-return of a
steering system to straight-line travel is called recovery.
    Camber and kingpin inclination together are called the included angle. If
the apex of this angle lies on the road surface, road resistance on the tire
and forward push at the kingpin are along the line of roll. If the apex is not
on the road, the tire is forced to toe in or out. To take up play in the front
wheel supports, the front wheels are purposely toed in slightly so that the
planes of the wheels intersect ahead of the vehicle. Road resistance then
forces them to roll parallel and takes up any play in the steering system.
    The inclination of the kingpin or its equivalent forward or backward is
called caster. Caster is positive if the kingpin inclines backward, negative
if it inclines forward, and zero if the kingpin is vertical as viewed along
the axis of the front wheels. Positive caster aids in directional stability
and recovery.

     FIGURE CAPTION.  Components of a typical steering system.

     FIGURE CAPTION.  Front left wheel. (a) Viewed from driver's position.
(b) Viewed from inside hood. Angles exaggerated.

 �34s[William K. Creson]
�33s



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�6v�2v�32sAutomotive suspension �21m �33s
    The springs and related parts intermediate between the wheels and frame of
an automotive vehicle that support the car body and frame on the wheels and
absorb road shock caused by passage of the wheels over irregularities. The
four types of spring have been used: coil, leaf, torsion bar, and air. See
SPRING (MACHINES).
    A typical automotive suspension arrangement uses coil springs at the front
wheels and leaf springs at the rear. Weight of the vehicle applies an initial
compression to the springs. When the wheels encounter irregularities in the
road, the springs further compress or expand to absorb most of the shock. The
suspension at the rear wheels is relatively simple in most cars. It is
considerably more complex at the front because the front wheels must swing
from side to side in steering. This calls for multiple-point attachments that
permit the wheel to move up and down and still be swung away from straight
ahead for steering. Shock absorbers are used at each wheel to restrain spring
movement and prevent prolonged spring oscillations. See SHOCK ABSORBER.
 �34s[William H. Crouse]
�33s
    In an air suspension system, suspension units on each wheel contain a
cushion of air which supports the sprung weight of the vehicle while
maintaining road clearance, alignment of wheels with chassis, and protection
of car and passengers from road shock. As in other types of suspension
systems, shock absorbers are attached to the frame and suspension unit for
additional passenger comfort and prevention of roll on curves and severe shock
at all speeds for better steering control and safety.
 �34s[Paul A. Obrist]
�33s
    In a torsion-bar system the spring is a bar or rod stressed in torsion
about its long axis. In a typical torsion-bar system the bar is rigidly fixed
to the car frame at the rear end and is free to rotate at its front end, near
which the lower wheel-control arm is splined to the bar. The longitudinal
stiffness of the bar aids steering and directional stability. See TORSION BAR.
 �34s[Neil MacCoull]
�33s

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�6v�2v�32sAutomotive transmission   �21m �33s 
    The device for providing different gear or drive ratios between the engine 
and drive wheels of an automotive vehicle. A principal function of the 
transmission is to enable the vehicle to accelerate from rest up to a maximum 
speed through a wide speed range while the engine operates within its most 
effective range. The transmission in most applications is placed in the 
vehicle power train between the engine and the propeller shaft. Power passes 
through the transmission and propeller shaft and is delivered to the 
differential and drive axles. See AXLE; DIFFERENTIAL. 
    There are two general types of transmission in passenger cars: manual and 
automatic. Because transmissions in trucks and off-the-road vehicles come in 
such a variety of designs and number of speeds, only passenger car units will 
be described here. 
    Manual transmissions employ a layshaft or countershaft gearing in 
conjunction with a clutch. This mechanism disengages the engine from the 
transmission while the sliding of gears in the transmission is controlled 
manually. Manual transmissions are mainly of the three-speed type with 
synchronizers in all forward speeds. This type carries a clutch slidably 
splined on the main shaft to obtain third-speed engagement by shifting to the 
left, and second speed by shifting to the right. The clutch at the right, also 
slidably splined on the main shaft, obtains first speed when shifted to the 
left and reverse when shifted to the right. The two clutches are actuated by 
forks mechanically connected to the hand shift lever, which may be located on 
the steering column or on the floor directly over the transmission. Gear 
ratios are in the order of 1.5:1 for second speed, 2.5:1 for first speed, and 
2.7:1 for reverse. Four-speed manual-shift transmissions have enjoyed 
popularity, particularly in sports cars. Generally they are synchronized in 
all forward speeds. Reverse generally involves sliding a gear on the main 
shaft into mesh with the reverse idle by a separate lever. Gear ratios are in 
the order of 1.28:1 for third speed, 1.64:1 for second speed, 2.2:1 for first 
speed, and 2.25:1 for reverse. See CLUTCH. 
    In 1974, the first five-speed manual transmission for automobiles to be 
mass-produced in the United States was introduced. It has overdrive in fifth 
speed with its ratio either 0.8:1 or 0.84:1. One gear set, for cars with 
engines under 175 lbf-ft (237 N-m) torque, has ratios of 3.41:1 in first 
speed, 2.08 in second, 1.40 in third, 1:1 in fourth (direct drive), and 0.8:1 
in fifth or overdrive. Reverse is 3.36:1. The other gear set, covering torque 
up to about 205 lbf-ft (278 N-m) has ratios of 3.10 in first, 1.89 in second, 
1.27 in third, 1:1 in fourth, and 0.84:1 in fifth. Reverse is 3.06:1. 
    In American passenger cars, automatic transmissions generally employ a 
three-element hydrodynamic torque converter with planetary gear sets of two or 
three speeds. The torque converter provides a smooth start with a torque 
(turning effort) multiplication within itself that is continuously variable, 
depending upon the degree of engine input. The torque may rise as high as 
2.0:1 or 2.4:1 at stall speed (full engine torque, the driven member of 
converter, or turbine, stationary). As car speed increases, the maximum 
converter torque ratio available decreases until it becomes 1:1 with little 
slippage. Converters provide an additional advantage in isolating the 
transmission gear set from torsional vibrations of the engine, which generally 
make special dampers necessary when engine clutches are used. 
 �34s[Forest R. McFarland]
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�6v�2v�32sAutomotive vehicle   �21m �33s 
    A trackless, self-propelled vehicle for land transportation of people or 
commodities or for moving materials. Passenger and commercial vehicles have 
comparable components but separate identities, primarily because of their 
different functions. Passenger vehicles are built for the comfort and 
protection of occupants, whereas commercial vehicles are designed more 
ruggedly to withstand daily abuse and are available in an infinite variety of 
forms. See AUTOMOBILE; BUS; EARTHMOVER; TRACTOR. 
    Passenger vehicles include automobiles, recreation vehicles and 
all-terrain vehicles such as campers and motor homes with sleeping 
accommodations, motor bikes, motorcycles, scooters, trail bikes, dune buggies, 
and snowmobiles. Included in the broad range of body types and configurations 
of commercial vehicles are ambulances, airport limousines, taxicabs, buses, 
vans, tankers, light- and heavy-duty highway and industrial trucks, tractors, 
and self-propelled farm equipment, off-highway construction and excavation 
units, and various military vehicles. 
    A typical motor vehicle begins to operate when the ignition switch is 
turned on. Battery current is released so that the starter motor can crank the 
engine, as the carburetor delivers an explosive fuel mixture to the engine 
cylinders. Simultaneously, the ignition system sends high-tension current to 
the spark plugs to ignite the fuel. Connecting rods transmit energy from the 
pistons to the crankshaft, while the camshaft (geared to the crankshaft) times 
intake or exhaust valve operation and firing intervals for spark plugs. Power 
impulses from each piston rotate the crankshaft, which is a source of motion 
for the vehicle. Power plant energy is transmitted to the power train, 
consisting of the clutch, transmission, drive shaft, and rear axle. In front- 
or rear-drive vehicles the power train is known as a transaxle when its 
elements are located in the front or rear. Engine output to the wheels, 
controlled by the driver, is governed by the power train and the accelerator 
for required speed and direction. See AUTOMOTIVE ENGINE; AUTOMOTIVE 
TRANSMISSION; AXLE; DIFFERENTIAL. 
    In addition to the major components (power plant and power train), 
automotive vehicles have several operating systems vital to proper 
functioning. The fuel system includes the fuel tank, fuel lines leading to the 
fuel pump, filter, and carburetor, with an air cleaner mounted on the 
carburetor to supply adequate clean air. 
    Conventional carburetors and intake manifolds have been replaced in some 
automobiles by fuel-injection systems that have a metering computer which 
controls fuel flow to each cylinder. This system produces better combustion 
and less fuel waste than conventional systems, which distribute fuel to all 
cylinders. The distribution is based on air and engine temperatures and 
manifold pressure and speed required, with automatic controls for all 
operating conditions. See CARBURETOR; FUEL INJECTION. 
    There are three basic sources of pollution from exhaust systems: fuel tank 
vapor, crankcase fumes, and exhaust fumes. Although conventional intake and 
exhaust systems remain in use, emission-control equipment and no-lead gasoline 
are now required in new vehicles. Positive crankcase ventilation (PCV) uses 
engine suction to draw crankcase and fuel tank fumes, as well as additional 
quantities of air, from the air cleaner into the system. Normal fuel and air 
mixtures for these added combustibles are controlled by the PCV valve; a vapor 
separator returns liquid to the fuel tank, or vapor to the PCV unit. The 
exhaust manifold has an air-injection system driven by an air pump belted to 
the crankshaft, to create afterburn of unexploded hot engine exhaust gas for 
more complete combustion. A catalytic converter inserted in the exhaust pipe 
line ahead of the muffler breaks down noxious nitrogen oxides, hydrocarbons, 
and carbon monoxide into harmless or inert matter. 
    At the heart of the whole electrical system are the ignition switch, 
battery, alternator, voltage regulator, and battery discharge sensor; without 
them none of the ancillary circuits would function. The ignition switch 
controls a half-dozen circuits which use these components; the starting, 
charging, and ignition circuits, lighting system, accessories, and warning 
sensors that keep the driver informed of changes in electrical, mechanical, 
fuel, oil, and other operating conditions that require correction. See 
AUTOMOTIVE ALTERNATOR; AUTOMOTIVE VOLTAGE REGULATOR; BATTERY; IGNITION SYSTEM. 
    Power plants are cooled by water or a solution of water and permanent 
antifreeze for year-round protection against boiling over in summer and 
freezing in winter. The system includes a water pump and fan driven by the 
engine, water jackets around cylinders in the engine head and block, a 
radiator to dissipate heat from the coolant, and a thermostat to control 
operating temperatures. Also included in cooling systems are vehicle heaters 
for passenger comfort, as well as a transmission oil cooling element. See 
ENGINE COOLING. 
    Several types of steering systems are used for manual or hydraulic 
power-assist control when the vehicle is in motion, with provisions for manual 
operation in the event of failure of the power unit. The introduction of power 
steering reduced the manual effort needed, and subsequently, variable-ratio 
steering has produced faster vehicle response when full right or left turns 
are negotiated. See AUTOMOTIVE STEERING; POWER STEERING. 
    Suspension systems are the means by which passenger and vehicle are 
protected from road shock due to irregular street and highway surfaces. Shock 
absorbers are designed for air or hydraulic operation, or for either type with 
auxiliary spring-loaded units for heavy-duty usage in conjunction with 
suspension systems. Mounting methods may be axle to frame or suspension unit 
to frame, or within the suspension unit itself. See AUTOMOTIVE SUSPENSION; 
SHOCK ABSORBER. 
    Conventional four-wheel drum brake systems have progressed to disk brakes; 
some imports are equipped with four-wheel disk brakes.  See AUTOMOTIVE BRAKE; 
BRAKE.  
    Two lubricating systems are used in passenger and commercial vehicles to 
reduce friction and wear of moving engine and chassis parts. The engine 
lubrication system includes an oil pan and sump for oil supply and return, oil 
pump, and filter; oil level is measured by a dipstick and checked periodically 
for replenishment. Lubricant is also applied to the fittings of strategic 
working parts of the chassis at specified points and at prescribed service 
intervals for satisfactory vehicle operation. 
 �34s[Paul A. Obrist]
�33s 
  
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�6v�2v�32sAutomotive voltage regulator   �21m �33s 
    A device in the automotive electrical system to prevent generator or 
alternator overvoltage. There are two general types: one for direct-current 
and another for alternating-current generators. 
    The direct-current voltage regulator contains a set of spring-loaded 
contacts and a winding shunted across the generator. When the generated 
voltage exceeds the preset voltage of the regulator, the magnetic flux is 
sufficient to open the contacts, thereby inserting resistance into the 
generator field. The generator voltage is reduced, the contacts close, and the 
cycle is repeated. Included with the voltage regulator are a current regulator 
and a cutout relay. The current regulator prevents generator overload. The 
cutout relay opens the circuit when the generator is inoperative to prevent 
battery discharge through the generator. 
    The alternating-current voltage regulator has two sets of contacts, lower 
and upper. At intermediate speeds, the lower contacts open and close, 
inserting and removing the resistance in the alternator field to provide 
voltage-limiting action. At higher speeds, the resistance is not sufficient to 
hold the voltage down. It increases slightly and the upper contacts begin to 
make and break. When they close, the alternator field is grounded by the 
contacts. With both ends of the field grounded, additional regulation is 
achieved. The alternator also has a field relay which connects the alternator 
field to the battery when the ignition switch is closed and disconnects it 
when the ignition switch is opened to shut off the engine. This keeps the 
battery from discharging through the alternator field. Many automotive 
alternators have integral transistorized voltage regulators which do not use 
vibrating contacts and do not require adjustment. They prevent excessive 
alternator voltage by inserting and removing field resistance. See AUTOMOTIVE 
ALTERNATOR; GENERATOR. 
 �34s[William H. Crouse]
�33s 
  
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�6v�2v�32sAutonomic nervous system   �21m �33s 
    The part of the nervous system which innervates smooth, cardiac, and some 
striated muscle and glands and viscera; also known as the visceral or 
involuntary division. The thoracolumbar (sympathetic) and craniosacral 
(parasympathetic) systems comprise the autonomic system. 
    The autonomic system is a pure motor system consisting of efferent 
neurons. The autonomic system influences the rate of metabolism, muscle tone 
of the viscera, blood flow, and other aspects of general homeostasis. The 
craniosacral division and thoracolumbar division are generally antagonistic in 
their effects. The actions of the autonomic division are duplicated by various 
hormones, such as epinephrine, and drugs, such as acetylcholine. See 
HOMEOSTASIS; NERVOUS SYSTEM (INVERTEBRATE); NERVOUS SYSTEM (VERTEBRATE); 
PARASYMPATHETIC NERVOUS SYSTEM; SYMPATHETIC NERVOUS SYSTEM. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sAutopilot �21m �33s
    An automatic means for steering an aircraft or other vehicle. An autopilot
replaces the human pilot in guided missiles and drone aircraft. In piloted
aircraft the autopilot is used to relieve the pilot during cruise, to effect
automatic approaches to landing fields, and to control rapid maneuvers, as in
tracking a dodging target. For helicopters and vertical takeoff and landing
aircraft, the autopilot can provide automatic hovering and transition to
forward flight, and may be necessary to provide stable control. Autopilots are
also used to steer ships, submarines, torpedoes, and spacecraft, and the
automatic control of automobiles is in the developmental stage. This article
discusses autopilots as used in aircraft and space vehicles.
    Elements of an aircraft autopilot are shown in the illustration. Steering
commands may come from a radio receiver, autonavigator, fire-control, or other
guidance system, or they may be set by the pilot. The computer compares actual
aircraft motion (obtained as signals from the gyroscopes, airspeed indicator,
altimeter, and other sensors) with steering commands. Errors between aircraft
motion and steering commands are corrected by control-surface servos, which
deflect the rudder, ailerons, and elevators to alter the aerodynamic forces on
the aircraft and make it roll, pitch, and yaw as necessary to eliminate the
errors. See FLIGHT CONTROLS; GUIDANCE SYSTEMS.
    The oldest form of autopilot simply maintained an airplane in straight and
level flight. This is called cruise control. Cruise-control autopilots
commonly consist of a heading system and an altitude-control system, in which
heading and altitude commands are manually set by the pilot. Guided missiles,
drones, and interceptor aircraft can be controlled from remote stations by
means of radio signals. Automatic approach to a landing field may be
accomplished either by ground command or by furnishing a strong radio beam
down which the airplane can be flown. In homing missile or fighter attacks on
other aircraft, radar or infrared tracking-error signals from the guidance
section feed directly into tight autopilot rate loops to steer the vehicle.
    Modern supersonic piloted aircraft are sometimes aerodynamically unstable.
They are made flyable by retaining the autopilot rate loops even during manual
operation. In such application the autopilot is an essential part of the
machine, whereas in the earlier applications described above, it is an assist
or substitute for the person in the humans machine system. Hovering helicopter
or vertical takeoff and landing (VTOL) aircraft are particularly challenging
to control because they are not inherently stable. To assist the pilot,
automatic stabilization equipment may be provided during pilot-operated
control. In addition, both hover and transition from hover to forward flight
may be controlled automatically by using airborne radar sensors to measure
vehicle location with respect to the ground. Control is effected by changing
blade angles appropriately, as in helicopters; by using gas reaction jets; or
by controlling thrust direction, as in deflected-thrust VTOL craft. For
piloted aircraft, a mandatory autopilot design feature is that autopilot
failure must not endanger life or equipment; that is, the design must be
fail-safe.

     FIGURE CAPTION.  Autopilot system block diagram.

 �34s[Robert H. Cannon, Jr.]
�33s


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�6v�2v�32sAutoradiography   �21m �33s 
    A photographic technique for detecting radioactive substances in almost 
any object; also known as radioautography. A photographic emulsion is placed 
in direct contact with the object to be tested and is left for several hours, 
days, or weeks, depending on the suspected concentration of the radioactive 
material. The film is then developed, fixed, and washed as in normal 
photographic processes. At sites where the film was close enough to the 
radioactive substance it appears dark because of the presence of silver 
grains. When the number of grains is insufficient to darken the film to the 
unaided eye, the film may be examined with the aid of a microscope. 
    Autoradiograms (the picture obtained by autoradiography) can be made with 
many objects which can be brought in contact with photographic film. The 
resolution, that is, the precision with which the radioactive substances can 
be located, is good only when the specimen is flat and in close contact with 
the film or photographic plate. Whole plants, animals, rocks, or any other 
objects may be used, but the flat, cut surface of the object provides better 
contact. However, if the radioactivity is distributed throughout the object 
being tested, a rather distinct picture results. 
    The high-resolution autoradiogram is a powerful tool in detecting and 
measuring small amounts of radioactivity that could not be detected in any 
other way. Most radioactive isotopes produce radiations that penetrate the 
film deep enough to produce somewhat out-of-focus autoradiograms, but tritium 
(radiohydrogen) is an exception. Its very low-energy BETA-particles are 
stopped at the surface of the film. Therefore, the grains appear very close to 
the radioactive site. It has been possible to label chromosomes with a 
selective label, tritium-thymidine. When they reproduce, each labeled 
chromosome can be shown to yield one labeled and one unlabeled daughter 
chromosome. See RADIOGRAPHY. 
 �34s[J. Herbert Taylor]
�33s 
  
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�6v�2v�32sAutotomy   �21m �33s 
    A process by which appendages of crabs and other crustaceans and tails of 
some salamanders and lizards may be cast off under stress. The break, in 
crustaceans, is at a definite site with a preformed internal partition that 
restricts loss of essential body fluids. In the salamanders and lizards, the 
center of each tail vertebra is unossified and breaks easily. Autotomy is a 
protective mechanism. When grasped by an enemy, the part fractures off and the 
victim escapes. Severed parts usually regenerate. In crustaceans, the 
replacement increases in size at each successive molt. With salamanders and 
lizards, repair growth is continuous. 
 �34s[Tracy I. Storer]
�33s 
  
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�6v�2v�32sAutotransformer �21m �33s
    A special form of transformer having one winding, a portion of which is
common to both the primary and the secondary circuits. The current in the
high-voltage circuit flows through the series and common windings (see
illustration).
    The current in the low-voltage circuit flows through the common winding
and adds vectorially to the current in the high-voltage circuit to give the
common winding current. Thus, an electrical connection exists between
high-voltage and low-voltage windings. Because of this sharing of parts of the
winding, an autotransformer having the same kilovolt-ampere (kVA) output
rating is generally smaller in weight and dimensions than a two-winding
transformer. One possible disadvantage of autotransformers is that the
windings are not insulated from each other and that the autotransformer
provides no isolation of the primary and secondary circuits.
    Autotransformers of large sizes are used for interconnecting high-voltage
power systems.They are used in small sizes for intermittent-duty starting of
motors. For this use the motor is connected for a short time to the common
winding voltage, and then connected to the full line voltage. Small,
variable-ratio autotransformers are used in testing and as components of other
apparatus.

     FIGURE CAPTION.  Typical autotransformer circuit.

 �34s[J. R. Sutherland]
�33s


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�6v�2v�32sAuxin   �21m �33s 
    The generic term originally referring to substances capable of promoting 
plant growth in a manner similiar to that of the native growth hormone. The 
currently accepted definition is that of an organic compound which promotes 
growth along the longitudinal plant axis when applied in low concentrations to 
shoots freed as far as practical from endogenous growth-promoting substances. 
Auxins may, and generally do, influence other plant systems, but the effect on 
plant growth is critical to characterizing an organic compound, synthetic or 
natural, as an auxin. See PLANT GROWTH. 
    Because of the minute amounts of auxin present in plants, chemical 
identifications of auxins have been difficult. It was not until 1946 that 
indole-3-acetic acid (IAA), now considered the most widely occurring auxin, 
was proved by good chemical characterization techniques to occur in higher 
plants. It has been isolated from fruits, seeds, pollen, root tips, young 
leaves, and buds in the flowering plants and from both fungi and bacteria. In 
general, plant organs, just prior to and during cellular expansion, are good 
sources of auxin, particularly in actively growing buds and developing ovules 
in flowers. Many synthetic auxins have been produced as well. 
    Starting with the first practical use of plant-regulating substances, the 
stimulation of rooting, the importance of auxins for plant production has 
steadily increased. In the United States alone over 35 growth-regulating 
chemicals have been approved for over 100 different agricultural uses. A large 
portion of the 35 chemicals are either auxins, chemicals that are converted to 
auxins by certain plants, auxin antagonists, or chemicals that interact with 
auxins in modifying growth. The largest use of auxins has been as herbicides, 
particularly 2,4-D (2,4-dichlorophenoxyacetic acid) and 2,4,5-T 
(2,4,5-trichlorophenoxyacetic acid). Many of the other uses are based on the 
correlative action of auxin on tissue differentiation: rooting of cuttings; 
organ abscission, both promotion and inhibition; fruit development; induction 
of flowering; and alteration of fruiting patterns. See PLANT HORMONES. 
 �34s[Peter B. Kaufman]
�33s 
  
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�6v�2v�32sAvalanche �21m �33s
    In general, a large mass of snow, ice, rock, earth, or mud in rapid motion
down a slope or over a precipice. In the English language, the term avalanche
is reserved almost exclusively for snow avalanche. Minimal requirements for
the occurrence of an avalanche are snow and an inclined surface, usually a
mountainside. Most avalanches occur on slopes between 30 and 45 DG.
    Two basic types of avalanches are recognized according to snow cover
conditions at the point of origin. A loose-snow avalanche originates at a
point and propagates downhill by successively dislodging increasing numbers of
poorly cohering snow grains, typically gaining width as movement continues
downslope. This type of avalanche commonly involves only those snow layers
near the surface. The mechanism is analogous to dry sand. The second type, the
slab avalanche, occurs when a distinct cohesive snow layer breaks away as a
unit and slides because it is poorly anchored to the snow or ground below. A
clearly defined gliding surface as well as a lubricating layer may be
identifiable at the base of the slab, but the meteorological conditions which
create these layers are complex.
    In the case of the loose avalanche, release mechanisms are primarily
controlled by the angle of repose, while slab releases involve complex
strength-stress problems. A release may occur simply as a result of the
overloading of a slope during a single snowstorm and involve only snow which
accumulated during that specific storm, or it may result from a sequence of
meteorological events and involve snow layers comprising numerous
precipitation episodes. Most large snow slides are believed to be caused by an
unstable layer of ice grains that develop deep in mountain snow. Called depth
hoar by students of avalanche dynamics, these crystals owe their formation to
heat from earth and rock which are buried by the snow, and which in late
autumn are warmer than the surrounding air. Snow nearest the ground vaporizes,
causing growth of angular ice grains that exhibit poor bonding qualities.
Gravity combining with the weakness of the depth hoar crystals loosens the
upper stable layers. Once the stable layers begin to slide, the depth hoar
acts in a manner similar to ball bearings to speed the descent of the slide.
    Where snow avalanches constitute a hazard, that is, where they directly
threaten human activities, various defense methods have evolved. Attempts are
made to prevent the avalanche from occurring by artificial supporting
structures or reforestation in the zone of origin. The direct impact of an
avalanche can be avoided by construction of diversion structures, dams, sheds,
or tunnels. Hazardous zones may be temporarily evacuated while avalanches are
released artificially, most commonly by explosives. Finally, attempts are made
to predict the occurrence of avalanches by studying relationships between
meteorological and snow cover factors.
 �34s[Richard L. Armstrong]
�33s

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�6v�2v�32sAves   �21m �33s 
    The class of birds, warm-blooded, egg-laying vertebrates primarily adapted 
for flying. All known flightless birds, both living and extinct, are believed 
to have evolved from flying ancestors. Birds are unique in possessing 
feathers, the epidermal structures that form most of their external covering. 
See BIRD; FEATHER. 
    About 9000 species of birds are known, with few yet to be discovered. 
Although birds have been much studied, there is still substantial disagreement 
about their classification at almost all hierarchal levels. A classification 
scheme of the birds is given below, and separate articles appear on each group. 
Class Aves 
    Subclass Archaeornithes 
        Order Archaeopterygiformes 
    Subclass Neornithes 
        Superorder Odontognathae 
        Order Hesperornithiformes 
        Superorder Neognathae 
        Order: Struthioniformes 
               Aepyornithiformes 
               Dinornithiformes 
               Casuariiformes 
               Apterygiformes 
               Rheiformes 
               Tinamiformes 
               Gaviiformes 
               Podicipediformes 
               Procellariiformes 
               Sphenisciformes 
               Pelecaniformes 
               Ciconiiformes 
               Anseriformes 
               Falconiformes 
               Galliformes 
               Gruiformes 
               Diatrymiformes 
               Charadriiformes 
               Ichthyornithiformes 
               Columbiformes 
               Psittaciformes 
               Cuculiformes 
               Strigiformes 
               Caprimulgiformes 
               Apodiformes 
               Coliiformes 
               Trogoniformes 
               Coraciiformes 
               Piciformes 
               Passeriformes 
                
    The first six orders of Neognathae are collectively referred to as the 
ratites. See RATITES. 
 �34s[Kenneth C. Parkes]
�33s 
    Birds are derived from archosaurian reptiles. They most likely evolved 
from pseudosuchians of the order Thecodontia, although some workers believe 
that the ancestry of birds is to be found within theropod dinosaurs of the 
order Saurischia. At present there are no fossil forms known that bridge the 
large gap in time between the Triassic psuedosuchians and the oldest fossil 
bird, Archaeopteryx, of Upper Jurassic deposits in Germany. 
 �34s[David W. Steadman]
�33s 
  
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�6v�2v�32sAvian leukosis   �21m �33s 
    A complex consisting of a group of viruses that cause several diseases in 
fowl. Some viruses in the group cause more than one neoplastic disease. For 
example, the BAI strain A usually causes myeloblastic leukemia in chickens, 
but it can also cause visceral lymphomatosis (lymphosarcoma), osteopetrosis 
(marble bone or big leg), and kidney tumors. Other viruses of the complex 
induce erythroblastosis, endothelioma, fibrosarcoma, and ovarian carcinoma. 
Visceral lymphomatosis is of particular economic importance to the poultry 
industry since it is highly contagious and spreads rapidly in crowded domestic 
flocks.  
    These viruses are spherical particles with a diameter of about 100 
nanometers. The inner region, or core, of the particle consists primarily of 
ribonucleic acid (RNA) and protein. The core is surrounded by an outer 
flexible envelope or membrane which contains lipid and protein. The viruses 
are released from cells by budding from the cell membrane. See ANIMAL VIRUS; 
TUMOR VIRUSES. 
 �34s[William Scher]
�33s 
  
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�6v�2v�32sAviation   �21m �33s 
    A general term including the science and technology of flight through the 
air. Aviation also applies to the mode of travel provided by aircraft as 
carriers of passengers and cargo, and as such is part of the total 
transportation system. Aviation also describes the employment of aircraft in 
such fields as military aviation. The world of the airplane, including the 
people who manufacture, market, and repair aircraft or who work in allied 
industries, is frequently spoken of as aviation. See AIRPLANE; MILITARY 
AIRCRAFT.  
    Aviation is broadly grouped into three classes: general aviation, air 
transport aviation, and military aviation. General aviation comprises all 
aviation not included in military or air-transport aviation. Military aviation 
includes all forms of aviation in military activities, and air-transport 
aviation is primarily the operation of commercial airlines essentially as a 
public utility for the movement of persons and commodities. See GENERAL 
AVIATION.  
 �34s[Leslie A. Bryan]
�33s 
  
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�6v�2v�32sAvocado   �21m �33s 
    A tropical and subtropical fruit tree, Persea americana, in the Lauraceae 
family. It originated in Central America or adjoining regions of North or 
South America. It has now spread to much of the near-tropical world. 
    The species is divided into three horticultural races with differing 
commercial qualities. The so-called West Indian race is least tolerant of 
cold, the Mexican most tolerant, and the Guatemalan intermediate. This same 
gradation is found in salt tolerance (West Indian highest) and oil content 
(West Indian lowest). But in some other respects the West Indian race is 
intermediate (skin thickness), or one of the races is different from the other 
two (the West Indian fruit is less tolerant of cold storage; the Mexican has 
smaller fruit with a unique aniselike odor; the Guatemalan has a smaller seed 
ratio, and takes twice as long to mature - 14 months or more in California). 
    Mexico is the world's leading producer, followed by Brazil and California, 
then Colombia and Venezuela, countries of eastern South America, Central 
America, Caribbean Islands, Florida, Philippines, and Zaire (central West 
Africa). South Africa and Israel have important export industries, primarily 
to Europe. Many other countries have begun development. The California 
industry is expanding rapidly, and the avocado has become one of the state's 
leading fruit crops. See FRUIT; FRUIT, TREE. 
 �34s[Berthold O. Bergh]
�33s 
  
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�6v�2v�32sAvogadro number �21m �33s
    The number of molecules N sub 0 in one mole of a substance, 6.02 x 10 sup
23. Equal numbers of moles of all substances contain the same numbers of
molecules. For a perfect gas, the molar volume contains a number of molecules
equal to the Avogadro number. See MOLE.
    Determinations of the Avogadro number by a variety of independent methods
give results that agree very closely. The earliest estimates of its value were
made during the latter half of the 19th century from the kinetic theory of
gases. A modern method that yields the most accurate value of N sub O is
obtained from x-ray measurements and density data. The most recent measurements
give 6.02210 x 10 sup 23 N sub 0.
 �34s[Thomas C. Waddington]
�33s

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�6v�2v�32sAvogadro's law   �21m �33s 
    The principle that equal volumes of all gases and vapors, under the same 
conditions of temperature and pressure, contain identical number of molecules; 
also known as Avogadro's hypothesis. From Avogadro's law the converse follows 
that equal numbers of molecules of any gases under identical conditions occupy 
equal volumes. Therefore, under identical physical conditions the 
gram-molecular weights of all gases occupy equal volumes. See GAS; KINETIC 
THEORY OF MATTER. 
 �34s[Thomas C. Waddington]
�33s 
  
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�6v�2v�32sAxinellida �21m �33s
    An order of sponges of the class Demospongiae, subclass Tetractinomorpha,
with monactinal or diactinal megascleres or both arranged in plumose tracts
bearing more or less spongin (see illustration). The axial skeleton may be
dense, with an abundance of spongin especially near the base of the sponge.
Axinellidan sponges vary in form from branching or massive to cup-shaped or
lamellate. Most occur in shallower waters down to 330 ft (100 m), but a few
descend to abyssal depths of at least 14,500 ft (4400 m). See DEMOSPONGIAE.

     FIGURE CAPTION.  Axinella polycapella, a representative axinelline
sponge.

 �34s[Willard D. Hartman]
�33s


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�6v�2v�32sAxle   �21m �33s 
    A supporting member that carries a wheel. An axle may rotate with the 
wheel to transmit power to or from it, as does the rear axle of an automobile, 
or it may allow the wheel to rotate freely, as does the axle of a trailer. 
    Rear axles in American cars are usually of the unsprung type in which the 
rear-axle housing carries the right and left rear-axle shafts and the wheels 
are mounted at the outer end of each shaft. Rear axles in many European and 
some English cars are of the sprung type, in which the rear-axle gearing 
housing at the center is mounted to the frame or body, while the wheels and 
axle shaft outer ends are attached by linkage to the frame, body, or central 
gear housing. Each type of rear axle has its own characteristics and is 
selected for suitability to the road conditions found in the countries where 
the car is to be sold. The unsprung axle is usually simpler and cheaper, 
permits easier alignment of the rear wheels, and provides an easier design to 
reduce lowering of the rear end of the car when accelerating or to reduce 
raising of the rear end when braking. The sprung rear axle permits reducing 
the unsprung weight at the rear wheels, generally considered an advantage on 
rough roads. It also permits lowering the tunnel in the floor of the car. See 
AUTOMOTIVE TRANSMISSION; WHEEL AND AXLE.  
 �34s[Forest R. McFarland]
�33s 
  
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�6v�2v�32sAxonolaimoidea �21m �33s
    A superfamily of free-living nematodes with outstretched ovaries, with
amphids that are spiral to cryptospiral or variants thereof, and with an
elongate, conoid, cuticularized stoma, which is usually armed anteriorly with
six eversible odontia. The odontia are thought to function during feeding by
scraping minute foodstuffs (bacteria, diatoms) from various substrates.
Species of this superfamily inhabit marine and brackish-water environments.
See NEMATA.
 �34s[Bruce Hopper]
�33s

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�6v�2v�32sAye-aye �21m �33s
    A rare prosimian primate indigenous to eastern Madagascar. A single living
species, Daubentonia madagascariensis, makes up the family Daubentoniidae (see
illustration). The aye-aye is a nocturnal, arboreal animal. A single young is
produced in early spring in a special nest, which is constructed by the
female. The hindtoes are opposable, and the fingers are quite long and
slender, especially the middle one. Using its middle finger or its sharp
incisors, the aye-aye digs insect larvae out of tree bark or extracts the
contents of sugarcane.
    The phylogenetic relationship of this species is obscure; however, the
general consensus is that this animal is an aberrant or divergent form of a
lemuroid ancestral stock. See MAMMALIA; PRIMATES.

     FIGURE CAPTION.  The aye-aye, Daubentonia madagascariensis.

 �34s[Charles B. Curtin]
�33s

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�6v�2v�32sAzeotropic mixture   �21m �33s 
    A solution of two or more liquids, the composition of which does not 
change upon distillation. The composition of the liquid phase at the boiling 
point is identical to that of the vapor in equilibrium with it, and such 
mixtures or azeotropes form constant-boiling solutions. The exact composition 
of the azeotrope changes if the boiling point is altered by a change in the 
external pressure. A solution of two components which form an azeotrope may be 
separated by distillation into one pure component and the azeotrope, but not 
into two pure components. See DISTILLATION: SOLUTION. 
 �34s[Francis J. Johnston]
�33s 
  
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�6v�2v�32sAzide �21m �33s
    One of several types of compounds containing the  - N3 group and derived
from hydrazoic acid, HN3. The organic azides are represented by two groups of
compounds, the alkyl and aryl azides, RN3, and the acid azides, RCON3. The
inorganic azides are important commercially because of the use of lead azide
in priming compositions and initial detonating agents. The alkali and
alkaline-earth metal azides are watersoluble; hence sodium azide, NaN3, is
usually the starting material for the production of lead azide or other
desired salts. See NITROGEN.
 �34s[E. Eugene Weaver]
�33s



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�6v�2v�32sAzimuth �21m �33s
    An angle measured in the plane of the observer's horizon from one of
several arbitrary departure points; also the angle at the observer's zenith
between the local meridian and the vertical circle passing through the object
being observed. See ASTRONOMICAL COORDINATE SYSTEMS: CELESTIAL NAVIGATION.
    In astronomical and surveying practice, azimuth has been referenced to
each of the four cardinal points of the horizon: north, east, south, and west.
In current navigation practice, however, azimuth is measured from the north
point of the horzon eastward from 0 to 360 DG. The horizon angle measured from
0 DG at the north or south horizon point eastward or westward through 90 or
180 DG is connoted azimuth angle, in contrast to azimuth.
 �34s[Raynor L. Duncombe]
�33s

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�6v�2v�32sAzole �21m �33s
    A suffix designating organic compounds with a five-membered N-heterocycle
containing two double bonds. Formally the term may be, but seldom is, applied
to pyrrole. In practice, azole is reserved for diunsaturated rings containing
two or more heteroatoms, one of which is nitrogen. Familiar azole systems are
given in the table. See HETEROCYCLIC COMPOUNDS; IMIDAZOLE; OXAZOLE; PYRAZOLE;
THIAZOLE.

     TABLE CAPTION.  Azole systems

 �34s[Walter J. Gensler]
�33s

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�6v�2v�32sAzotobacteraceae   �21m �33s 
    A family of aerobic gram-negative bacteria capable of vigorous 
assimilation of elemental nitrogen. 
    The genus Azotobacter has big, bluntly rod-shaped, oval, or spherical 
cells approximately 2 - 6 micrometers in diameter, which are usually motile by 
means of lateral or polar flagella. Some species form thick-walled resting 
cells (cysts). The type species A. chroococcum is commonly found in soils of 
neutral and alkaline reaction, especially fertile, cultivated soils. 
    A group of related organisms, often regarded as a separate genus, is 
Beijerinckia. They differ from Azotobacter in the smaller size of their 
rod-shaped cells with characteristic polar fat inclusions, slower growth rate, 
formation of tenacious slime, and much greater tolerance of acid reaction 
(growth between pH approximately 3 - 9, optimum at pH 4 - 6). Nitrogen 
fixation is at least as efficient as in Azotobacter, but a narrower range of 
carbon sources is utilized. The Beijerinckia are largely confined to tropical 
and subtropical soils. 
    The genus Azomonas, found in soil and water, has large ovoid cells that 
are motile with polar or peritrichous flagella. The type species is A. agilis. 
    A fourth genus, Derxia, has been found in soil from India but seems to be 
rare. A single species, D. gummosa, somewhat resembles Beijerinckia, but 
differs in its long rod-shaped cells which lack the typical polar bodies, and 
in its lesser acid tolerance (growth ceases at pH below 5). Nitrogen is fixed 
with the same efficiency as in Azotobacter and Beijerinckia. 
 �34s[Hans L. Jensen ;Manley Mandel]
�33s 
  
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�6v�2v�32sAzurite �21m �33s
    A basic carbonate of copper with the chemical formula Cu3(OH)2(CO3)2.
Azurite is normally associated with copper ores and often occurs with
malachite. Azurite is monoclinic. It may be massive or may occur in tabular,
prismatic, or equant crystals. Invariably blue, azurite was originally used
extensively as a pigment. Hardness is 3.5 - 4 (Mohs scale) and specific
gravity is 3.8. Notable localities for azurite are at Tsumeb, Southwest
Africa, and Bisbee, Arizona See COPPER.
 �34s[Robert I. Harker]
�33s

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�6v�2v�32sBabbitt metal   �21m �33s 
    A white alloy of tin, lead, antimony, and copper used as an antifriction 
metal in sleeves and bearings where high pressures need not be sustained and 
where high temperatures are not likely. The alloy is a moderately soft matrix 
carrying cubic crystals of a hard compound of either copper or lead with the 
antimony. Wear soon relieves the softer matrix, exposing the hard crystals 
which carry the load. Their small exposed area decreases friction, and the 
undercut matrix permits lubricant to circulate between crystals. See 
ANTIFRICTION BEARING; TIN ALLOYS. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sBabinet's principle �21m �33s
    A principle which states that the diffraction patterns produced by
complementary screens are identical. Two screens are said to be complementary
when the opaque parts of one correspond to the transparent parts of the other,
and vice versa. Babinet's principle is not very useful in dealing with Fresnel
diffraction. In Fraunhofer diffraction the pattern due to a disk is the same
as that due to a circular hole of the same size. See DIFFRACTION.
 �34s[Francis A. Jenkins /William W. Watson]
�33s

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�6v�2v�32sBacillaceae   �21m �33s 
    The family of the rod-shaped, endospore-forming bacteria. Species in this 
family range from organisms used in retting flax to those pathogenic for 
animals and humans, such as anthrax. The special features of the group are 
largely associated with spore production. The spore is formed within the 
vegetative cell and is therefore of the type known as an endospore. Its 
outstanding property is a capacity to tolerate conditions which destroy the 
vegetative stage. The two principal genera of this family are Bacillus and 
Clostridium. Other genera include Sporolactobacillus, Desulfotomaculum, and 
Sporosarcina.  
    All Bacillaceae appear to have the same mode of life. A large reserve of 
dormant spores occurs in nature, principally in soil. If a spore is induced to 
germinate by the presence of food material, rapid multiplication may occur. 
When the food supply is exhausted, spores may again be formed, and they join 
the natural reserve. The short multiplication phase becomes possible in the 
refuse of plant and  animal life, in foods, in the bowel of the animal in the 
case of clostridia, and in the tissues of animals in the case of pathogenic 
species. See BLACKLEG; BOTULISM; TETANUS. 
 �34s[T. Gibson]
�33s 
  
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�6v�2v�32sBacillary dysentery   �21m �33s 
    An infectious disease of humans and primates which affects the large bowel 
and is caused by members of the genus Shigella. The organisms enter the body 
via the fecal-oral route (for example, from unclean hands) or through fecally 
contaminated water or food, and penetrate the intestinal mucosa only. The 
incubation time varies from 1 to 7 days. The characteristic symptoms are 
watery, watery-mucoid, or bloody diarrhea accompanied by fever, dehydration, 
abdominal cramps, and prostration. Most cases, however, are ``subclinical'' or 
mild. Short-term convalescent carriers are an important reservoir; long-term 
carriers are rare. The main diagnostic tool is the stool culture. See SHIGELLA. 
    Effective treatment has to rely on the results of sensitivity testing of 
the strains involved, since some of them have acquired resistance to 
sulfonamides, ampicillin, and tetracycline. Prevention efforts must stress 
proper sanitation, cleanliness in handling food and water, and the use of 
handwashing facilities. See MEDICAL BACTERIOLOGY; SALMONELLA. 
 �34s[Alexander W. C. von Graeveni]
�33s 
  
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�6v�2v�32sBacitracin   �21m �33s 
    An antibiotic produced by Bacillus licheniformis (B. subtilis, strain 
Tracy). Purified bacitracin is a white to light-brown powder, soluble in water 
and a variety of aqueous alcohols, including n-butanol and sec-butanol. It is 
active against a wide variety of gram-positive bacteria and only a few 
gram-negative ones. The use of bacitracin is almost entirely limited to 
topical (including local injection and oral dosage) application because of the 
toxicity shown after systemic administration. It is therapeutically effective 
in the local treatment of a wide variety of infections such as furuncles, 
abscesses, infected wounds, carbuncles, and impetigo caused by staphylococci, 
streptococci, and certain other gram-positive bacteria. 
 �34s[Ralph E. Bennett]
�33s 
  
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�6v�2v�32sBackground count   �21m �33s 
    The number of counts that must be subtracted from an observed number of 
counts in an experiment where atomic or nuclear particles coming from a source 
are being enumerated. The background count may be due to cosmic rays; to 
natural radioactivity in the air, in the walls of the room, or in the counter 
itself; or to the scattered radiation in the vicinity of a source of nuclear 
particles, such as an accelerator or a nuclear reactor. See RADIOACTIVITY. 
 �34s[William B. Fretter]
�33s 
  
A700000001213�254s�5v�1v189
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�6v�2v�32sBacking wind   �21m �33s 
    A wind which changes direction in a counterclockwise sense, for example, a 
south wind changing to an east wind or an east wind changing to northerly. At 
a particular place, the wind gradually backs with time when a cyclone passes 
eastward on a path south of the observer's location. The wind 
characteristically backs with increasing height, on the west side of an 
extratropical cyclone. The meaning, in terms of changes of cardinal 
directions, is reversed in the South Hemisphere. This term is opposite in 
sense to veering wind.  
 �34s[Chester W. Newton]
�33s 
  
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�6v�2v�32sBackward-wave tube �21m �33s
    A type of microwave traveling-wave tube in which energy on a slow-wave
circuit flows opposite in direction to the travel of electrons in a beam.
Chief characteristics of backward-wave tubes are regenerative feedback
produced by interaction of circuit and beam, and a wide range of electrical
tuning, easily produced by changing the beam voltage. Such tubes are useful as
voltage-tuned oscillators for signal generators, as power sources for quick
tuning transmitters, and as local oscillators in receivers for systems that
have quick tuning transmitters. If backward-wave tubes are operated as
regenerative amplifiers, they are useful as narrowband amplifiers in
wide-range rapidly tuned receivers. See TRAVELING-WAVE TUBE.
    An O-type backward-wave oscillator (or O-carcinotron) may be similar in
appearance to a forward-wave traveling-wave tube. An electron gun produces an
electron beam; this beam is focused longitudinally throughout the length of
the tube. A slow-wave circuit interacts with the beam, and at the end of the
tube a collector terminates the beam (Fig. 1).
    An M-type backward-wave oscillator (or M-carcinotron) is similar in
principle to the O-type, except that focusing and interaction are through
magnetic fields, as in magnetrons. In the M-type oscillator (Fig. 2), a
transverse magnetic field and a static radial electric field between sole and
backward-wave circuit structure confine the beam to the interaction space.
Either the voltage connected to the sole or to the slow-wave circuit tunes the
frequency. See MAGNETRON.
    When either type tube is operated with currents below the
start-oscillation value, narrow-band regenerative amplification is obtained.
Amplifier frequency is electrically tunable over a wide range by change of
beam voltage. See MICROWAVETUBE.








 �34s[James W. Gewartowski]
�33s



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�6v�2v�32sBacteria �21m �33s
    Extremely small - usually 0.3 - 2.0 micrometers in diameter - and
relatively simple microorganisms possessing the prokaryotic type of cell
construction. Although traditionally classified within the fungi as
``Schizomycetes,'' they show no phylogenetic affinities with the fungi, which
are eukaryotic organisms. The only group that is clearly related to the
bacteria are the blue-green algae. See MICROORGANISMS.
    Bacteria are found almost everywhere, being abundant in soil, water, and
the alimentary tracts of animals. Each kind of bacterium is fitted
physiologically to survive in one of the innumerable habitats created by
various combinations of space, food, moisture, light, air, temperature,
inhibitory substances, and accompanying organisms. Dried but often still
living bacteria can be carried into the air. One of the few locations in which
bacteria are not usually found is within the cells of other healthy organisms,
though even this is subject to exceptions, as there are many bacteria that do
live intracellularly in a number of eukaryotic organisms.
    As in higher forms of life, each bacterial cell arises either by division
of a preexisting cell with similar characteristics, or through combination of
elements from two such cells in a sexual process. See REPRODUCTION (ANIMAL).
    Descriptions of bacteria are preferably based on the studies of pure
cultures. Pure cultures are sometimes called axenic, a term denoting that all
cells had a common origin in begin descendants of the same cell, without
implying exact similarity in all characteristics. Pure cultures can be
obtained by selecting single cells, but indirect methods achieving the same
result are more common. These involve separating the bacteria from each other
and keeping their progeny separate. This is done on a streak plate. Another
method is to inoculate the suspension of mixed bacteria into melted nutrient
medium, which is cooled to just above the solidification temperature in order
not to kill the inoculated cells; the medium is then solidified. See AXENIC
CULTURE; PURE CULTURE.
    The morphology, that is, the shape, size, arrangement, and internal
structures, of bacteria can be distinguished microscopically and provides the
basis for classifying the bacteria into major groups. Stains are used to
visualize bacterial structures otherwise not seen, and the stain reaction with
Gram's stain provides a characteristic used in classifying bacteria. See
ENDOSPORES (BACTERIA); STAIN (MICROBIOLOGY).
    The submicroscopic differences that distinguish many bacterial genera and
species are due to structures such as enzymes and genes that cannot be seen.
The nature of these structures is determined by studying the metabolic
activities of the bacteria. Data are accumulated on the temperatures and
oxygen conditions under which the bacteria grow, their response in
fermentation tests, their pathogenicity, and their serological reactions.
There are also methods for determining directly the similarity in
deoxyribonucleic acids between different bacteria. See FERMENTATION; PATHOGEN;
SEROLOGY.

    Interrelationships with other organisms may be close and may involve
particular species. Examples are the parasitic association of many bacteria
with plant and animal hosts, and the mutualistic association of
nitrogen-fixing bacteria with leguminous plants, of cellulolytic bacteria with
grazing animals, and of luminous bacteria with certain deepsea fishes.
 �34s[Robert E. Hungate]
�33s
    The study of genetics in bacteria, as in all forms of life, requires the
occurrence of readily detectable variations in the properties that
characterize a particular type of species. The rapid growth of bacteria (in
some cases the number will double in 20 min) and the large populations
obtained (as many as 10 sup 9 organisms per cubic centimeter) allow for a wide
fluidity in the number of such variations and for detection of any rare change
that may occur. These changes, called mutations, are the result of alterations
in genes. Depending upon the gene involved, the spontaneous rate of mutation
may be as low as one affected bacterium per 10 sup 9 organisms per doubling time
or as frequent as one per 10,000. This rate may be increased 100 to 1000 times
by the use of mutagenic agents, such as ultraviolet light, x-rays, or a
variety of chemicals (nitrogen mustard, acridine dyes, nitrous acid, and
purine or pyrimidine analogs). See MUTATION.
    The genetic characteristics that are most commonly studied are those that
occur frequently and are easily detected or selected for. Bacteria produce
characteristic colonies when plated on the surface of a solid nutrient agar
medium. Morphologic variants may be detected readily by looking for variations
in size, shape, color, or texture of such colonies. Biochemical mutants are
those in which the change can be related to a specific chemical substance such
as a protein, an enzyme, or a unique functional ingredient of the cell. This
may show as a difference in the ability to ferment a certain sugar; to survive
the inhibitory action of antibiotics, ultraviolet light, or even bacterial
viruses; to produce infections in experimental animals; or to produce immunity
substances. Mutations that have proved extremely useful to the bacterial
geneticists are those which do not allow growth of the bacteria unless a
specific growth factor is added to the medium. These are the nutritional or
auxotrophic mutants, and they have been of special value in helping to unravel
the complex biosynthetic mechanisms of bacteria.
    Sexlike mechanisms have been demonstrated in an a number of bacterial
species by showing that genes can be transferred from one bacterial cell to
another. Three main types of genetic transfer have been described:
transformation, transduction, and conjugation. Transformation involves the
transfer of genetic material in the form of virtually pure, highly polymerized
DNA. The DNA is extracted from the donor cells or released from them by lysis
(illustration a). In transduction, small fragments of the bacterial chromosome
are transferred by viral vectors, bacteriophages, which pick up the genes in
the course of their development in the donor hosts. The host is destroyed by
the viral infection; the viruses are liberated and then may be used ot infect
recipient cells, but this time under conditions favoring the survival of the
virus-infected cells (illustration b).
    Conjugation is the most highly developed form of sexuality in bacteria. It
involves a direct union between donor and recipient bacteria. A cytoplasmic
bridge is formed, and a considerable part of the single chromosome is
transferred (illustration c). See BACTERIOPHAGE; DEOXYRIBONUCLEIC ACID (DNA);
TRANSDUCTION (BACTERIA); TRANSFORMATION (BACTERIA)
[Joseph S. Gots]





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�6v�2v�32sBacteriology   �21m �33s 
    The science and study of bacteria, and hence a specialized branch of 
microbiology. It deals with the nature and properties of the bacteria as 
living entities, their morphology and developmental history, ecology, 
physiology and biochemistry, genetics, and classification. See BACTERIA; 
MICROBIOLOGY.  
    The major subjects that have consecutively occupied the forefront of 
bacteriological research have been the origin of bacteria, the constancy or 
variability of their properties, their role as causative agents of disease and 
of spoilage of foods, their significance in the cycle of matter, their 
classification, and their physiological, biochemical, and genetic features.  
 �34s[Cornelis B. Van Niel]
�33s 
  
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�6v�2v�32sBacteriophage �21m �33s
    Any of the viruses that infect bacterial cells. They are discrete
particles with dimensions from about 20 nanometers to about 200 nm. A given
bacterial virus can infect only one or a few related species of bacteria;
these constitute its host range. Bacteriophages consist of two essential
components: nucleic acid, in which genetic information is encoded (this may be
either ribonucleic acid or deoxyribonnucleic acid), and a protein coat
(capsid), which serves as a protective shell containing the nucleic acid and
is involved in the efficiency of infection and the host range of the virus.
    The description of a bacterial virus involves a study of its shape and
dimensions by electron microscopy (see illustration), its host range, the
serological properties of its capsid, the kind of nucleic acid it contains,
and the characters of the plaques it forms on a given host. Both the nucleic
acid and the capsid proteins are specific to the individual virus; in the case
of the capsid proteins this specificity is the basis for serological
identification of the virus.
    The most striking form of phage infection is that in which all of the
infected bacteria are destroyed in the process of the formation of new phage
particles. This results in the clearing of a turbid liquid culture as the
infected cells lyse. When lysis occurs in cells fixed as a lawn of bacteria
growing on a solid medium, it produces holes, or areas of clearing, called
plaques. These represent colonies of bacteriophage. The size and other
properties of the plaque vary with individual viruses and host cells. See
ACTINOPHAGE; COLIPHAGE; LYSOGENY; LYTIC INFECTION; VIRUS.



 �34s[Lane Barksdale]
�33s

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�6v�2v�32sBacteroidaceae   �21m �33s 
    A family of gram-negative anaerobic bacteria. They are obligate parasites 
normally occurring in the intestinal tracts and mucous membranes of animals 
and humans. They are sometimes pathogenic, causing calf diphtheria, contagious 
polyarthritis in mice, and ratbite fever and Haverhill fever in humans. The 
microorganisms are gram-negative, rod-shaped, obligatorily anaerobic or 
microaerophilic (need oxygen, but grow best at, or may even require, a reduced 
oxygen tension of less than 10%), nonsporing, and usually nonmotile. In the 
eighth edition of Bergey's Manual of Determinative Bacteriology (1974), the 
family Bacteroidaceae is not assigned to an order and is included in Part 9, 
gram-negative anaerobic bacteria. The family is divided into three genera, 
Bacteroides, Fusobacterium, and Leptotrichia. 
 �34s[E. A. Freundt]
�33s 
  
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�6v�2v�32sBadger �21m �33s
    The name for a number of species of heavily built omnivorous mammals
assigned to the subfamily Melinae of the weasel family, Mustelidae. There are
eight species in six genera (see table). Taxidea taxus, the American badger is
the only representative in North America. It tends to be more carnivorous with
a diet consisting of small rodents, rabbits, prairie dogs, and ground
squirrels in addition to vegetation. The American badger is more frequently
found in open terrain than is the Eurasian species, which prefers wooded
regions. Badgers live in burrows, called sets.
    Badgers are essentially nocturnal animals which have nonretractile claws
on each of the five digits. Anal scent glands are present. The badgers walk on
their feet and toes, and therefore are plantigrade. They have 38 teeth.
Although little is known about the breeding behavior of the animal, the usual
litter is three or four, and the gestation period for the Eurasian badger is
known to be 7 months. In the colder regions of their range, badgers hibernate
for varying periods from October on, with the exception of Meles meles, which
is active during the winter. See CARNIVORA; HIBERNATION.



 �34s[Charles B. Curtin]
�33s

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�6v�2v�32sBaking powder   �21m �33s 
    A yeast substitute made of baking soda (sodium bicarbonate) plus cream of 
tartar (potassium acid tartrate), tartaric acid, and alum (in modern baking 
powders, usually anhydrous sodium aluminum sulfate), or monocalcium phosphate, 
or any combination of these acids so formulated as to release carbon dioxide 
from the baking soda when moistened. Baking powder causes breadstuffs to rise 
without the fermentation effects of yeast. Commercial baking powders contain 
some inert materials, such as starch or flour, to slow down the chemical 
reactions between the active ingredients.  
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sBalance �21m �33s
    An instrument for measuring weight, or mass. There are many types of
balances: spring-type scales, single-lever-deflection types, precise
two-lever-arm designs, torsion designs, substitution designs, and complete
electronic weighing systems. The application, whether for commercial or
scientific usage, determines the type of balance. See MASS; WEIGHT.
    The two-pan balance, in its crudest form, consists of a rigid bar (lever),
two pans suspended from each end, and a pivotal point in the center of the bar
(Fig. 1). The center of gravity (cg) of the beam system is directly beneath
the point of rotation. This design keeps the beam in a horizontal position
when unloaded. If, however, a small sample (unknown mass) is placed on one of
the pans, the beam will deflect to a specific angle that is proportional to
mass. This angle of deflection can be expressed in mass units and read from a
graduated scale by means of a pointer. If the unknown mass is so great that
the pointer goes off the scale, weights (known standards) are placed in the
second pan to bring the balance beam back into a state of equilibrium. Fine
differences between the known weights and unknown mass are shown as beam
deflection and are read on a graduated scale by means of a pointer. The
unknown mass equals the sum of the known weights plus the mass units read from
the graduated scale.
    The substitution balance is a mechanical balance consisting of one pan,
two knife-edges, and one lever arm to compare the known and the unknown mass.
A counterweight is rigidly affixed to the beam. Calibrated weights are built
into the system and are suspended above the pan. When the unknown sample is
placed on the pan, known calibrated weights are removed from the beam until
the system equilibrates. In effect, the weights are substituted, giving the
relationship between the known and the unknown. Small differences are read as
a result of angular deflection of the beam. A simplified schematic of this
design is shown in Fig. 2.
    Balance manufacturers produce mechanical balances employing the
substitution principle in two classifications, top-loading and analytical
balances. Analytical balances have a limited capacity but offer better
precision and readability than the top-loading counterparts. The top loaders
are faster, more rugged, and easier to operate. However, they sacrifice
precision and thus readability.
    In an electronic weighing system, determination of mass is based on the
principle of electronically controlled electrical force compensation. A
scanner monitors displacement of the pan support. This modulates the variable
gain amplifier, which generates a current that is proportional to the weight
of the pan. The current flows through a coil and causes the pan support,
located in the force field of the magnet, to return to its zero position. From
the signal generated, automatic tare determines the mass value and subtracts
any previous stored tare value. The difference is displayed on a digital
readout and is also available for further processing at digital output. See
WEIGHT MEASUREMENT.





 �34s[Paul E. Lepard]
�33s



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�6v�2v�32sBalanomorpha �21m �33s
    A suborder of the Thoracica. This group includes the common acorn
barnacles. The mantle secretes a highly calcified test forming the bilaterally
symmetrical conical shell, consisting typically of anterior rostrum, posterior
carina, and one to three pairs of lateral plates. The broad basis is
membranous or calcified. The shell (mantle) opening is closed by paired
valves, each formed of two interlocking plates. These animals are
hermaphroditic, and cross-fertilization is normal. Dwarf males have been
recorded.
    Balanomorph barnacles are found on almost every possible site for
attachment, inanimate or living. They are most abundant intertidally and in
shallow water. Some species occur at great depths. A few have become an
economic problem as fouling organisms on ship hulls and in marine
installations. See BARNACLE; THORACICA. 			   [H.G.St.]
 �34s[H. G. Stubbings]
�33s

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�6v�2v�32sBalantidiasis   �21m �33s 
    An intestinal infection of humans caused by the ciliated protozoan 
Balantidium coli. The natural reservoir of the parasite are pigs, in whom B. 
coli causes no clinical signs of disease. The parasite exists in two forms: 
the resistantcyst and the actively motile trophozoite. Infection occurs by 
contaminative routes of people in close contact with infective fecal material. 
The trophozoites, which penetrate the mucosa of the large intestine, cause 
erosions and ulceration and subsequently the clinical signs of diarrhea and 
Balantidium dysentery. The tetracycline antibiotics have been found to cure 
this disorder. Prevention is based on personal hygiene and sanitary measures. 
See HETEROTRICHIDA. 
 �34s[Meir Yoeli]
�33s 
  
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�6v�2v�32sBall-and-race-type pulverizer �21m �33s
    A grinding machine in which balls rotate under pressure to crush
materials, such as coal, to a fine consistency. The material is usually fed
through a chute to the inside of a ring of closely spaced balls. In most
designs the upper spring-loaded race applies pressure to the balls, and the
lower race rotates and grinds the coarse material between it and the balls
(see illustration). Two or more rings of balls can be cascaded in one machine
to obtain greater capacity or output. Counterrotating top and bottom rings
also are used to increase pulverizer capacity. Such pulverizers are compact
and the power required per ton of material ground is relatively low. See
CRUSHING AND PULVERIZING.





 �34s[George W. Kessler]
�33s


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�6v�2v�32sBallast resistor   �21m �33s 
    A resistor that has the property of increasing in resistance as current 
through it increases and decreasing in resistance as current decreases. Thus 
it tends to maintain a constant current through it, despite variations in 
applied voltage or changes in the rest of the circuit. See RESISTOR. 
    The ballast action is obtained by using resistive material that increases 
in resistance as temperature increases. Any increase in current then causes an 
increase in temperature, which results in an increase in resistance and 
reduces the current. Ballast resistors are usually mounted in an evacuated 
envelope to reduce heat radiation; they are therefore also called ballast 
tubes.  
    Ballast resistors have been used to compensate for variations in line 
voltage or to compensate for negative volt-ampere characteristics of other 
devices, such as fluorescent lamps and other vapor lamps. See VAPOR LAMP; 
VOLTAGE REGULATOR.  
 �34s[Arthur A. Welch]
�33s 
  
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�6v�2v�32sBallistic missile   �21m �33s 
    A vehicle capable of guiding and propelling itself in a direction and to a 
velocity such that it will then follow a ballistic trajectory to a desired 
point. This trajectory is the nearly elliptic path determined by the momentum 
gained by self-propulsion, by gravitational acceleration toward the center of 
mass of Earth, and by the deceleration of aerodynamic drag. 
    The ballistic missile differs from the guided missile in that the latter 
applies either aerodynamic or propulsive force, under the direction of its 
guidance system, to control its trajectory during all or most of its flight. 
Because aerodynamic forces on a missile in the Earth's atmosphere are much 
less predictable than the errors of a guidance system, the guided-missile type 
of design is more commonly chosen for applications that call for a trajectory 
either entirely or primarily in the Earth's atmosphere. Because aerodynamic 
forces are essentially absent outside the Earth's atmosphere and because 
gravitational forces can be predicted with a smaller error than guidance 
system errors, the ballistic missile type of design is more commonly chosen 
for applications in which a major portion of the trajectory is exoatmospheric. 
See GUIDED MISSILE; MISSILE. 
    The characteristics of the ballistic missile fit the requirements of a 
military weapon to destroy a target located at a known point on the surface of 
the Earth at ranges of hundreds or thousands of miles. Hence the primary 
application of ballistic missile principles has been to military weapons, the 
most widely known of which are the intercontinental ballistic missiles and the 
submarine-launched ballistic missiles. 
 �34s[Levering Smith]
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�6v�2v�32sBallistic range �21m �33s
    A long, instrumented enclosure wherein tests of gun-launched projectiles
are conducted. Ballistic ranges were originally used for study of projectile
flight characteristics such as the rate of velocity loss and the dispersion of
trajectories, that is, the imperfect following of the bore sight line of the
gun. Ballistic ranges are now used for the measurement of aerodynamic
characteristics of projectiles and of scale models of vehicles intended for
flight through the atmosphere, including missiles, aircraft, and space-flight
capsules.
    Ballistic ranges may vary in length from less than 10 ft (3 m), for
limited investigations of small arms projectiles, to greater than 1000 ft (300
m), for detailed studies of projectiles from large-bore guns. Instrumentation
may be as simple as a series of sheets of paper hung normal to the flight path
(yaw cards) in which the projectile, by punching its outline, gives evidence
of its vertical and lateral position and its angular orientation at the
instant of penetration. The more refined instruments required in modern
practice are usually photographic. A spark of extremely short duration (from 1
to 0.1 microsecond) is discharged when the projectile reaches a position
between the spark and a piece of film (Fig. 1). The resulting shadow
photograph (shadowgraph) records in projection two components of the linear
position of the projectile and one component of angular orientation. Two
orthogonal pictures, as in the simple system shown, completely define the
position in space of the model. A number of shadowgraph stations placed in
sequence along the flight path will define, as a function of distance flown,
the history of position and attitude of the projectile in flight. Corollary
timing apparatus is used to measure the precise times, frequently accurate
within microseconds, at which the pictures are recorded.
    The shadowgraph pictures further aid aerodynamic studies by making visible
certain details of the airflow about the projectile (Fig. 2). Refraction, or
bending, of light rays by the variable-air-density field about the model makes
visible the shock waves, turbulence, boundary layers, and boundaries of
expansion fans. The flow visualization is most valuable for the study of
aerodynamics.
    In modern practice the velocity performance of ballistic ranges has
sometimes been enhanced by placing the instrumented range within the test
section of a supersonic wind tunnel, which provides a high-velocity airflow
opposite in direction to the model's flight direction. The earliest facility of
this type used a conventional supersonic wind tunnel with a very long test
section, driven by a high-pressure reservoir of air at room temperature. Later
devices have been driven by high-temperature air generated in a shock tube. See
WIND TUNNEL.							     
[Alvin Seiff]




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�6v�2v�32sBallistics   �21m �33s 
    That branch of applied physics which deals with the motion of projectiles 
and the conditions governing that motion. Commonly called the science of 
shooting, it is, for practical purposes, subdivided into exterior and interior 
ballistics. Exterior ballistics begins at the instant the projectile leaves 
the muzzle of the gun barrel; interior ballistics, logically, deals with the 
events preceding this instant, that is, the events inside the gun barrel. See 
EXTERIOR BALLISTICS; INTERIOR BALLISTICS. 
 �34s[Willy Ley /David Wollersheim]
�33s 
  
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�6v�2v�32sBalloon   �21m �33s 
    A nonporous envelope of thin material filled with a lifting gas and 
capable of lifting any surrounding material and usually a suspended payload 
into the atmosphere. A balloon which is supported chiefly by buoyancy imparted 
by the surrounding air is often referred to as an aerostat. The balloon rises 
because of a differential displacement of air according to Archimedes' 
principle, which states that the total upward bouyant force is equal to the 
weight of the air displaced. The upper practical limit for useful ballooning 
is approximately 34 mi (55 km). Beyond this altitude, the exponential nature 
of the atmosphere would require balloons of enormous size and delicately thin 
skin. A record altitude of 32.2 mi (51.8 km) has been recorded. See 
ARCHIMEDES' PRINCIPLE. 
    Balloons have been configured in many geometrical shapes, but the most 
common are spheres, oblate spheroids, and aerodynamic configurations. The 
materials used in the manufacture of the balloon envelope have been paper, 
rubber, fabric, and various plastics. Several types of lifting gases have been 
used to inflate balloons, but the most common in use are helium, hydrogen, and 
heated air. 
    The many types of balloons in use fall into twin main categories: 
extensible (expandable) and nonextensible. There are three methods of balloon 
operation: free balloons which are released into the atmosphere, tethered or 
moored balloons, and powered or controlled free balloons. The various types of 
balloons in use are the hot-air balloon, meteorological balloon, zero-pressure 
balloon, superpressure balloon, tethered balloon, and powered balloon 
(airship). See AIRSHIP; METEOROLOGICAL BALLOON; SUPERPRESSURE BALLOON; 
TETHERED BALLOON; ZERO-PRESSURE BALLOON. 
 �34s[Walter R. Nagel]
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�6v�2v�32sBalsa �21m �33s
    A fast-growing tree, Ochroma lagopus, widely distributed in tropical
America, especially in Ecuador. The leaves are simple, angled, or lobed, and
the flowers are large and yellowish-white or brownish, and they are terminal
on the branches. See MALVALES.
    With plenty of room for growth in a rich, well-drained soil at low
elevations, the wood is very light and soft. However, under adverse
conditions, the wood is heavier. Culture is important, for if the trees are
injured only slightly, the wood develops a hard and fibrous texture, thereby
losing its commercial value. To secure a uniform product the trees must be
grown in plantations.
    The wood decays easily in contact with the soil and is subject to sap
stain if not promptly dried. Seasoned lumber absorbs water quickly, but this
can be largely overcome by waterproofing.
    Balsa owes most of its present commercial applications to its insulating
properties. Balsa also has sound-deadening qualities, and is also used under
heavy machinery to prevent transmission of vibrations. The heartwood of balsa
is pale brown or reddish, whereas the sapwood is nearly white, often with a
yellowish or pinkish hue. Luster is usually rather high, and the wood is
odorless and tasteless.
 �34s[Arthur H. Graves /Kenneth P.Davis]
�33s

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�6v�2v�32sBaltic Sea �21m �33s
     A semienclosed brackish sea located in a humic zone, with a positive water
balance relative to the adjacent ocean (the North Sea and the North Atlantic).
The Baltic in its present form is a relatively young postglacial sea which
developed from the melting Fenno-Scandian glaciers about 14,000 years ago.  It
is connected to the North Sea by the Great Belt (70% of the water exchange),
the Oresund (20% of the water exchange), and the Little Belt.  The total area
of the Baltic is 147, (381,705 km sup 2), its total volume 4,982 mi sup 3
(20,764 km sup 3), and its average depth 181 ft (55.2 m).  The greatest depth
is 1506 ft (459 m), in the Landsort Deep.  The topography of the Baltic is
characterized by a sequence by sills and by two large gulfs, the Gulf of
Bothnia (40,000 mi sup 2 or 104,000 km sup 2) and the Gulf of Finland (11,400
mi sup 2 or 29,500 km sup 2).  More than 200 rivers discharge an averate of
104 mi sup 3 (433 km sup 3) area of 636,900 mi sup 2 (1,649,550 km sup 2).  The
largest river is the Newa, with 18.5% of the total fresh-water discharge.  From
December to May, the northern and eastern parts of the Baltic are frequently
covered with ice.  On the average, the area is of maximum ice coverage is
83,000 mi sup 2 (214,000 km sup 2).  The mean maximum surface-water temperature
in summer is between 59 and 63 DG F (15 and 17 DG C).
    As the Baltic stretches from the boreal to the arctic continental climatic
zone, there are large differences between summer and winter temperature in the
surface waters, ranging from about 68 to 30 DG F (20 to -1 DG C 68 to 30 DG F)
in the Western Baltic and 57 and 31.7 DG F (14 to -0.2 DG C 57 and 31.7 DG F)
in the Gulf of Bothnia and the Gulf of Finland. Usually one or more sharp
thermoclines develop April through June between 15 and 80 ft (5 and 25 m),
separating the brackish winter water from the surface layer. The salt content
of the Baltic waters is characterized by two major water bodies: the brackish
surface water and the more saline deep water. Total average annual outflow of
brackish surface water from the Baltic is 226 mi sup 3 (942 km sup 3), and the
salt balance of the Baltic is maintained by an average annual inflow of 113 mi
sup 3 (471 km sup 3). The surface currents of the Baltic are dominated by a
general counterclockwise movement and by local and regional wind-driven
circulations.
    The flora and fauna of the Baltic are those of a typical brackish-water
community, with considerably reduced numbers of species compared to an oceanic
community. The productivity is relatively low compared to other shelf seas.
Other than fish the only major resources that have been exploited are sand and
gravel in the Western Baltic. It is believed that the deeper layer under the
Gotland Basin contains mineral oil, but so far only exploratory drilling has
been carried out in near-coastal regions. Limited amounts of mineral oil have
also been located in the Gulf of Kiel.
 �34s[K. Grasshoff]
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�6v�2v�32sBamboo �21m �33s
    The common name of various perennial, ornamental grasses (Gramineae).
There are five genera with approximately 280 species. They have a wide
distribution, but occur mainly in tropical and subtropical parts of Asia,
Africa, and America, extending from sea level to an elevation of 15,000 ft
(4600 m). Their greatest development occurs in the monsoon regions of Asia.
Most plants are woody; a few are herbaceous or climbing. The economic uses of
bamboo are numerous and varied. The seeds and young shoots are used as food,
and the leaves make excellent fodder for cattle. In varying sizes, the stems
are used for pipes, timber, masts, bows, furniture, bridges, cooking vessels,
buckets, wickerwork, paper pulp, cordage, and weaving. Entire houses are made
of bamboo stems. Certain bamboos have been naturalized in California,
Louisiana, and Florida. See CYPERALES.
 �34s[Perry D. Strausbaugh /Earl L.Core]
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�6v�2v�32sBanana �21m �33s
    A large tropical plant of the family Musaceae; also its edible fruit,
which occurs in hanging clusters, is usually yellow when ripe, and is about 6
- 8 in. (15 - 20 cm) long. The banana of commerce (Musa sapientum), believed
to have originated in the Asian tropics, was one of the earliest cultivated
fruits. For commercial production the plant requires a tropical climate within
the temperature range 50 - 105 DG F (10 - 40 DG C) and a constant supply of
moisture by rainfall or irrigation.
    The plant portion above the ground is a false stem (pseudostem) consisting
of several concentrically formed leaves, from the center of which develops the
inflorescence stalk. The rhizome or true stem is underground. Near the tip of
the flower stalk are several groups of sterile male flowers subtended by
brilliant purple bracts. The lower female flower clusters on the same stalk
give rise to the fruit and contain aborted stamens (male organs). The single
fruits are called fingers, a single group of 8 - 12 fingers is termed a hand,
and the several (6 - 18) hands of the whole inflorescence make up the stem.
    The fruit bunch requires 75 - 150 days to mature and must be removed from
the plant to ripen properly. Chilled banana fruits do not soften normally;
hence for best edibility the fruit is kept well ventilated at room
temperature. Banana fruits of commerce set without pollination, by
parthenocarpy, and hence are seedless. When mature, most varieties are yellow,
although fine red-skinned types are well known. There are several hundred
varieties grown throughout the world. The Cavendish banana (M. nana, variety
Valery) is becoming important in the American tropics. The more starchy
bananas, known as plantains, must be cooked before they can be eaten. See
FRUIT; FRUIT, TREE; ZINGIBERALES.
 �34s[Charles A. Schroeder]
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�6v�2v�32sBand spectrum �21m �33s
    A spectrum consisting of groups or bands of closely spaced lines. Band
spectra are characteristic of molecular gases or chemical compounds. When the
light emitted or absorbed by molecules is viewed through a spectroscope with
small dispersion, the spectrum appears to consist of very wide asymmetrical
lines called bands. These bands usually have a maximum intensity near one
edge, called a band head, and a gradually decreasing intensity on the other
side. In some band systems the intensity shading is toward shorter waves, in
others toward longer waves. Each band system consists of a series of nearly
equally spaced bands called progressions; corresponding bands of different
progressions form groups called sequences.
    When spectroscopes with adequate dispersion and resolving power are used,
it is seen that most of the bands obtained from gaseous molecules actually
consist of a very large number of lines whose spacing and relative
intensities, if unresolved, explain the appearance of bands of continua. For
the quantum mechanical explanations of the details of band spectra see
MOLECULAR STRUCTURE AND SPECTRA.
 �34s[William F. Meggers /William Watson]
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�6v�2v�32sBand theory of solids �21m �33s
    A quantum-mechanical theory of the motion of electrons in solids which
predicts certain restricted ranges, or bands, for the electron energies.
    If the atoms of a solid are separated from each other to such a distance
that they do not interact, the energy levels of the electrons will then be
those characteristic of the individual free atoms, and thus many electrons
will have the same energy. As the distance between atoms is decreased, the
electrons in the outer shells begin to interact, thus altering their energy
and broadening the sharp energy level out into a range of possible energy
levels called a band. One would expect the process of band formation to be
well advanced for the outer, or valence, electrons at the observed interatomic
distances in solids. Once the atomic levels have spread into bands, the
valence electrons are not confined to individual atoms, but may jump from atom
to atom with an ease that increases with the increasing width of the band.
    Although energy bands exist in all solids, the term energy band is usually
used in reference only to ordered substances, that is, those having
well-defined crystal lattices. In such a case, an electron energy state can be
classified according to its crystal momentum p or its electron wave vector k =
p/PLANCK'S CONSTANT (where PLANCK'S CONSTANT is Planck's constant h divided by
2 PI). If the electrons were free, the energy of an electron whose wave vector
is k would be as shown in the equation below, where E0 is the  energy of the
lowest state of a valence electron and m0 is the electron mass. In a crystal,
however, the electrons are not free because of the effect of the crystal
binding and the forces exerted on them by the atoms; consequently, the relation
E(k) between energy and wave vector is more complicated. The statement of this
relationship constitutes the description of an energy band.

   *** E(k) = E (sub 0) + PLANCK CONSTANT (sup 2) k (sup 2) / 2m (sub 0) ***

    The bands of possible electron energy levels in a solid are called allowed
energy bands. There are also bands of energy levels which it is impossible for
an electron to have in a given crystal. Such bands are called forbidden bands,
or gaps. The allowed energy bands sometimes overlap and sometimes are
separated by forbidden bands. The presence of a forbidden band immediately
above the occupied allowed states (such as the region A to B in the
illustration) is the principal difference in the electronic structures of a
semiconductor or insulator and a metal. In the first two substances there is a
gap between the valence band or normally occupied states and the conduction
band, which is normally unoccupied. In a metal there is no gap between
occupied and unoccupied states. The presence of a gap means that the electrons
cannot easily be accelerated into higher energy states by an applied electric
field. Thus, the substance cannot carry a current unless electrons are excited
across the gap by thermal or optical means.
    Under external influences, such as irradiation, electrons can make
transitions between states in the same band or in different bands. The
interaction between the electrons and the vibrations of the crystal lattice
can scatter the electrons in a given band with a substantial change in the
electron momentum, but only a slight change in energy. This scattering is one
of the principal causes of the electrical resistivity of metals. See
ELECTRICAL RESISTIVITY.
    An external electromagnetic field (for example, visible light) can cause
transitions between different bands. Here momentum must be conserved. Because
the momentum of a photon hv/c (where v is the frequency of the light and c its
velocity) is quite small, the momentum of the electron before and after
collision is nearly the same. Such a transition is called vertical in
reference to an energy band diagram. Conservation of energy must also hold in
the transition, so absorption of light is possible only if there is an
unoccupied state of energy hv available at the same k as the initial state.
These transitions are responsible for much of the absorption of visible and
near-infrared light by semiconductors.



 �34s[Joseph Callaway]
�33s


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�6v�2v�32sBandwidth requirements (communications) �21m �33s
    The difference between the limiting frequencies of a frequency band
containing the useful components of a signal is called the bandwidth,
expressed in the unit of frequency, hertz (Hz; the unit was formerly cycles
per second cps). Each communication system requires some optimum bandwidth for
the satisfactory transmission of information.
    In the case of simple wire telephony, where the speech sound waves cause
variation of current in the line, the bandwidth required is the same as the
desired bandwidth of the signal to be reproduced. In wire telegraphy, the
bandwidth required depends on the signaling speed used.
    Radio communications bandwidth requirements are characterized by reference
(1) to the bandwidth of the transmitted signal (emission bandwidth) or (2) to
the frequency response of the electrical circuits through which the signal
passes, including transmitter and receiver. Emission bandwidth is usually
stated in terms of the frequency band which contains a specified portion of
the total signal power.
    The following list shows the approximate bandwidth necessary for several
kinds of communications systems.
    System				Bandwidth, Hz
Telegraphy
  100 words/min 			170-400
Telephony				  3,000
High-speed data
  transmission
  1000 bits/s			    2,000-3,000
AM radiotelephony
  Commercial				  6,000
  Broadcasting			   8,000-20,000
FM radiotelephony
  Commercial				 36,000
  Broadcasting				180,000
Radiotelegraphy
  Frequency shift			    600
AM facsimile				  5,000
FM facsimile				 25,000
Television, U.S.
  Broadcasting			      6,000,000
  Radio relay			     25,000,000


    When the signal to be transmitted causes the variation of the amplitude,
phase, frequency, or pulsing of a carrier wave, the process is known as
modulation. Bandwidth requirements then depend on the frequency stability of
the carrier wave, the character of the modulation process, and the bandwidth
of the modulating signal.
    For further information on specific systems see FACSIMILE; RADIO;
TELEGRAPHY; TELEPHONY; TELEVISION.
 �34s[Edward W. Allen, Jr.]
�33s

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�6v�2v�32sBarbiturates   �21m �33s 
    A group of drugs widely used for the suppression of anxiety, the induction 
of sleep, and the control of seizures. Some of them, when injected 
intravenously, produce a general anesthesia. See ANESTHETIC. 
    Barbituric acid, from which the various barbiturate congeners come, is a 
malonyl urea. Following the synthesis of this compound, a dozen or more 
closely related compounds were synthesized by adding or substituting various 
radicals to the general formula. The names of many of them have become 
familiar; examples are Phenobarbital, Meberal, Seconal, Nembutal, Amytal, and 
Pentothal.  
    These drugs act by suppressing the excitability of all tissues; but all 
tissues are not equally sensitive. Low dosages induce drowsiness, and high 
dosages coma and death. The spread between the therapeutic and fatal doses 
varies with the different barbiturates. 
    The prolonged use of these drugs results in habituation; and insomnia, 
agitation, confusional psychosis, and seizures may occur within 24 to 36 hours 
of withdrawal. Overdose is one of the commonest means of suicide in Western 
countries, and life can be saved only by admission to a hospital where 
respiration can be maintained and cerebral anoxia prevented. See DRUG 
ADDICTION.  
 �34s[Raymond D. Adams]
�33s 
  
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�6v�2v�32sBarite   �21m �33s 
    An orthorhombic mineral with chemical composition BaSO4. It posesses one 
perfect cleavage, and two good cleavages, as do the isostructural minerals. 
The mineral has a specific gravity of approximately 4.5, and is relatively 
soft, approximately 3 on Mohs scale. The color ranges through white to 
yellowish, gray, pale blue, or brown, and a thin section is colorless. 
    Barite is often an accessory mineral in hydrothermal vein systems, but 
frequently occurs as concretions or cavity fillings in sedimentary limestones. 
It also occurs as a residual product of limestone weathering and in hot spring 
deposits. It occasionally occurs as extensive beds in evaporite deposits. 
Occurrences of barite are extensive. It is found as a vein mineral associated 
with zinc and lead ores in Derbyshire, England. Large deposits occur at 
Andalusia, Spain. Commercial residual deposits occur in limestones throughout 
the Appalachian states such as Georgia, Tennessee, and Kentucky. It also 
occurred in substantial amounts in the galena ore deposits in Wisconsin and 
Missouri.  
    Since barite is dense and relatively soft, its principal use is as a 
weighting agent in rotary well-drilling fluids. It is the major ore of barium 
salts, used in glass manufacture, as a filler in paint, and, owing to the 
presence of a heavy metal and inertness, as an absorber of radiation in x-ray 
examination of the gastrointestinal tract. 
 �34s[Paul B. Moore]
�33s 
  
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�6v�2v�32sBarium �21m �33s
    A chemical element, Ba, with atomic number 56 and atomic weight of 137.34.
Barium is eighteenth in abundance in the Earth's crust, where it is found to
the extent of 0.04%, making it intermediate in amount between calcium and
strontium, the other alkaline-earth metals. Barium compounds are obtained from
the mining and conversion of two barium minerals. Barite, barium sulfate, is
the principal ore and contains 65.79% barium oxide. Witherite, sometimes
called heavy spar, is barium carbonate and is 72% barium oxide. See BARITE;
WITHERITE.
    The metal was first isolated by Sir Humphry Davy in 1808 by electrolysis.
Industrially, only small amounts are prepared by aluminum reduction of barium
oxide in large retorts. These are used in barium-nickel alloys for spark-plug
wire (the barium increases the emissivity of the alloy) and in frary metal,
which is an alloy of lead, barium, and calcium used in place of babbitt metal
because it can be cast.
    The metal reacts with water more readily than do strontium and calcium,
but less readily than sodium; it oxidizes quickly in air to form a surface
film that inhibits further reaction, but in moist air it may inflame. The
metal is sufficiently active chemically to react with most nonmetals. Freshly
cut pieces have a lustrous gray-white appearance, and the metal is both
ductile and malleable. The physical properties of the elementary form are
given in the table.
    For the manufacture of barium compounds, soft (easily crushable) barite is
preferred, but crystalline varieties may be used. Crude barite is crushed and
then mixed with pulverized coal. The mixture is roasted in a rotary reduction
furnace, and the barium sulfate is thus reduced to barium sulfide or black
ash. Black ash is roughly 70% barium sulfide and is treated with hot water to
make a solution used as the starting material for the manufacture of many
compounds.
    Lithopone, a white powder consisting of 20% barium sulfate, 30% zinc
sulfide, and less than 3% zinc oxide, is widely used as a pigment in white
paints. Blanc fixe is used in the manufacture of brilliant coloring compounds.
It is the best grade of barium sulfate for paint pigments. Because of the
large absorption of x-rays by barium, the sulfate is used to coat the
alimentary tract for x-ray photographs in order to increase the contrast.
Barium carbonate is useful in the ceramic industry to prevent efflorescence on
claywares. It is used also as a pottery glaze, in optical glass, and in rat
poisons. Barium chloride is used in purifying salt brines, in chlorine and
sodium hydroxide manufacture, as a flux for magnesium alloys, as a water
softener in boiler compounds, and in medicinal preparations. Barium nitrate,
or the so-called baryta saltpeter, finds use in pyrotechnics and signal flares
(to produce a green color), and to a small extent in medicinal preparations.
Barium oxide, known as baryta or calcined baryta, finds use both as an
industrial drying agent and in the case-hardening of steels. Barium peroxide
is sometimes used as a bleaching agent. Barium chromate, lemon chrome or
chrome yellow, is used in yellow pigments and safety matches. Barium chlorate
finds use in the manufacture of pyrotechnics. Barium acetate and cyanide are
used industrially as a chemical reagent and in metallurgy, respectively.



 �34s[Reed F. Riley]
�33s

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�6v�2v�32sBarium titanate �21m �33s
    A dielectric crystalline material which exhibits the anomalous properties
typical for ferroelectricity. On lowering the temperature, cubic barium
titanate, BaTiO3, transforms to a tetragonal structure at about 130 DG C
(Curie temperature), where the crystal spontaneously acquires an electric
polarization (a permanent electric dipole moment). As the polarization can in
general point in any one of the originally cubic crystal axes, the crystal
consists, in the ferroelectric state, of regions (domains) which differ in the
direction of the spontaneous polarization. An applied electric field can align
the polarization in the whole crystal and can reverse the polarization
direction whereby electric hysteresis occurs.
    Barium titanate has a high dielectric constant, typically 2000 for a
ceramic sample at room temperature. Thus, capacitors of very small dimensions
can be produced for electronics applications. Barium titanate ceramics, when
suitably doped with a trivalent oxide such as lanthanum oxide, for example,
exhibit a variation of the electrical conductivity by a factor up to 10 sup 6 in
the vicinity of the Curie temperature. This effect can be used for sensitive
temperature-control devices.
    Ceramics of barium titanate and, in particular, of a mixed lead titanate
zirconate can be poled by an electric field and retain a substantial remanent
polarization. These materials then have a strong piezoelectric effect. In
addition, they are mechanically rugged and rather insensitive to temperature
and humidity. Therefore, these ceramics find wide applications in devices such
as microphones, ultrasonic and underwater transducers, and spark generators.
See DIELECTRIC CONSTANT; PIEZOELECTRICITY.
 �34s[H. Graenicher]
�33s

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�6v�2v�32sBark   �21m �33s 
    A word generally referring to the surface region of a stem or a root. 
Sometimes part or all of the bark is called rind. Occasionally the word bark 
is used as a substitute for periderm or for cork only. Most commonly, however, 
it refers to all tissues external to the cambium. If this definition is 
applied to stems having only primary tissues, it includes phloem, cortex, and 
epidermis; the bark of roots of corresponding age would contain cortex and 
epidermis. In the more general usage the term bark is restricted to woody 
plants with secondary growth. 
    In most roots with secondary growth, the bark consists of phloem and 
periderm since the cortex and epidermis are sloughed off with the first cork 
formation in the pericycle that is, beneath the cortex. In stems, the first 
cork cambium may be formed in any living tissue outside the vascular cambium, 
and the young bark may include any or all of the cortex in addition to the 
phloem and periderm. The region composed of the successive layers of periderm 
and the enclosed dead tissues is called outer bark. The outer bark composed of 
dead phloem alternating with bands of cork is called, technically, rhytidome. 
Both stems and roots may have rhytidome. The inner bark is living, and 
consists of phloem only. See CORTEX (PLANT); EPIDERMIS (PLANT); PARENCHYMA; 
PERICYCLE; PERIDERM; PHLOEM; ROOT; STEM. 
 �34s[H. Weston Blaser]
�33s 
  
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�6v�2v�32sBarkhausen effect �21m �33s
    An effect, due to discontinuities in size or orientation of magnetic
domains as a body of ferromagnetic material is magnetized, whereby the
magnetization proceeds in a series of minute jumps. See FERROMAGNETISM;
MAGNETIZATION.
    Ferromagnetic materials are characterized by the presence of microscopic
domains of some 10 sup 1 sup 2 to 10 sup 1 sup 5 atoms within which the
magnetic moments of the spinning electrons are all parallel. In an unmagnetized
specimen, there is random orientation of the various domains. When a magnetic
field is applied to the specimen, the domains turn into an orientation parallel
to the field, or if parallel to the field, the domains increase in size. During
the steep part of the magnetization curve, whole domains suddenly change in
size or orientation, giving a discontinuous increase in magnetization. If the
specimen being magnetized is within a coil connected to an amplifier and
loudspeaker, the sudden changes give rise to a series of clicks or, when
there is a rapid change, a hissing sound. This is called the Barkhausen
effect; it is an important piece of evidence in support of a domain theory of
magnetism.
 �34s[Kenneth V. Manning]
�33s

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�6v�2v�32sBarley �21m �33s
    A cereal grass plant belonging to the family Gramineae and genus Hordeum.
Most of the modern cultivated barleys are H. vulgare (six-rowed) or H.
distichum (two-rowed; see illustration). All cultivated barleys are annuals,
are naturally self-pollinated, and have seven pairs of chromosomes.
    Barley is grown in nearly all cultivated areas of the temperate parts of
the world, and is an important crop in Europe, North and South America, North
Africa, much of Asia, and Australia. Barley is the most dependable cereal crop
where drought, summer frost, and alkali soils are encountered. In the United
States, barley is grown to some extent in 49 states, with the most important
production areas in North Dakota, Montana, and California. Principal uses of
barley grain are as livestock feed, for the production of barley malt, and as
human food. It is also used in some areas for hay and silage.
    In the cultivated varieties, a wide diversity of morphological,
physiological, and anatomical types are known. There are spring, facultative,
and winter growth habits; hulled and naked grain; awned, awnless, and hooded
lemmas; black, purple, and white kernels; and also a wide range of plant
heights, spike densities, and resistances to a wide range of diseases and
insects. There are in excess of 150 cultivars presently commercially grown in
the United States and Canada alone, and many additional cultivars are grown in
other parts of the world.
    Barley can be grown in a variety of rotations with other crops common to a
particular region. In areas with long growing seasons, it is possible to
double-crop (growing two crops in 12 consecutive months on the same land) with
summer crops such as soybeans or cotton. Barley is ideally suited to this
purpose because it is usually the earliest maturing of the small grains. See
CYPERALES; GRAIN CROPS.
 �34s[David A. Reid]
�33s
    The term pot barley is applied to the food product which results after the
husk has been removed by coarse grinding. Pearled barley is obtained by a more
involved, dry-milling process.
    For malting purposes, the brewing industries in the United States use
different types of barley from those desired throughout the remainder of the
world. Outside the United States the demand is for large-kerneled barley of
the two-row type which is high in starch and low in nitrogen content. In North
America the demand is for the six-row, small-kernel type with higher nitrogen
and enzyme content.
    There are two systems of malting; flour malting and pneumatic or drum
malting. The malting process involves the germination of the barley in a
controlled state, followed by drying under conditions that retain enzymatic
activity. Industrial value of malt depends on its starch-splitting and
protein-degrading enzymes. Brewers' malts are used to produce beer and ale
and, to a lesser extent, for malt syrups and evaporated malt extracts.
Distillers' malts are employed for starch conversion in the production of
distilled beverages and industrial alcohol and for converting starch for the
sizing of paper and textiles.




 �34s[John A. Shellenberger]
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�6v�2v�32sBarnacle   �21m �33s 
    The name popularly applied to two types of Crustacea, subclass Cirripedia, 
the goose barnacles and acorn barnacles. Both were formerly of importance to 
seafarers (and acorn barnacles are still so) because of their habit of 
adhering to ships hulls. Four orders of barnacles are recognized, of which one 
only, the Thoracica, fits the popular conception of barnacles. See CIRRIPEDIA; 
THORACICA.  
 �34s[H. G. Stubbings]
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�6v�2v�32sBaroclinic field   �21m �33s 
    A distribution of atmospheric pressure and mass such that the specific 
volume, or density, of air is a function not solely of pressure. When the 
field is baroclinic, solenoids are present, there is a gradient of air 
temperature on a surface of constant pressure, and there is a vertical shear 
of the geostrophic wind. Significant development of cyclonic and anticyclonic 
wind circulations typically occurs only in strongly baroclinic fields. Fronts 
represent baroclinic fields which are locally very intense. See FRONT; 
GEOSTROPHIC WIND; SOLENOID (METEOROLOGY); STORM; WIND. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sBarometer �21m �33s
    An absolute pressure gage specifically designed to measure atmospheric
pressure. This instrument is a type of manometer with one leg at zero pressure
abolute. See MANOMETER.
    The common meterological barometer (see illustration) is a liquid-column
gage filled with mercury. The top of the column is sealed, and the bottom is
open and submerged below the surface of a reservoir of mercury. The
atmospheric pressure on the reservoir keeps the mercury at a height
proportional to that pressure. An adjustable scale, with a vernier scale,
allows a reading of column height. Aneroid barometers using metallic diaphragm
elements are usually less accurate, though often more sensitive, devices, and
not only indicate pressure but may be used to record. See PRESSURE MEASUREMENT.

     FIGURE CAPTION.  Mercury barometer.

 �34s[John H. Zifcak]
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�6v�2v�32sBarotropic field   �21m �33s 
    A distribution of atmospheric pressure and mass such that the specific 
volume, or density, of air is a function solely of pressure. When the field is 
barotropic, there are no solenoids, air temperature is constant on a surface 
of constant pressure, and there is no vertical shear of the geostrophic wind. 
Significant cyclonic and anticyclonic circulations typically do not develop in 
barotropic fields. Considerable success has been achieved, paradoxically, in 
prediction of the flow pattern at middle-tropospheric elevations by methods 
which are strictly applicable only to barotropic fields, despite the fact that 
the field in this region is definitely not barotropic. See BAROCLINIC FIELD; 
GEOSTROPHIC WIND; SOLENOID (METEOROLOGY); WEATHER FORECASTING AND PREDICTION; 
WIND.  
 �34s[Frederick Sanders]
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�6v�2v�32sBarracuda   �21m �33s 
    The name for about 20 species of fish found in warm seas throughout the 
world. All species belong to the genus Sphyraena and are members of the order 
Perciformes. The larger species, such as the great barracuda (Sphyraena 
barracuda), are solitary and not found in schools. Generally, all species are 
good food fish. 
    The teeth, which occur both in the jaws and on the roof of the large 
mouth, are strong and pointed, resembling canine teeth. The jaws are long, 
with the lower projecting beyond the upper, and the upper incapable of being 
protracted. The head is large and pointed, and the body is compressed and 
long, somewhat like the pike. The body is covered with cycloid scales, and the 
lateral line is well developed. Many of these fish weigh over 100 lb (45 kg) 
and reach a length of 10 ft (3 m). See PERCIFORMES. 
 �34s[Charles B. Curtin]
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mpgs. 1056-1058 
7:30 
  
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�6v�2v�32sBarretter �21m �33s
    A fine wire, having a high temperature coefficient of resistance, which is
used as a power-absorbing device in bolometric methods. Its change of
resistance is a measure of the power dissipated within it, and it is used
either to measure the power directly or to indicate when a comparison dc power
is equal to an unknown radio-frequency (rf) power. The overall length of
typical barretters (see illustration) is about 1 in. (2.5 cm). See BOLOMETER.




 �34s[Donald B. Sinclair]
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�6v�2v�32sBarrier islands   �21m �33s 
    Elongate, narrow accumulations of sediment which have formed in the 
shallow coastal zone and are separated from the mainland by some combination 
of coastal bays and marshes. They are typically several times longer than 
their width and are interrupted by tidal inlets. Although their origin has 
been widely discussed, at least three possibilities exist: longshore spit 
development and subsequent cutting of inlets; drowning of old coastal ridges; 
and upward shoaling of subtidal sediment accumulations. All three may have 
occurred; however, the last seems most likely and most prevalent. 
    Barrier islands must be considered in terms of the adjacent and closely 
related environments within the coastal system. Beginning offshore and 
proceeding landward, the sequence of environments crossed is shoreface, beach, 
dunes, back-island flats or marsh, coastal bay, marsh, and mainland. The 
barrier island proper consists of the beach, dunes, and back-island flats or 
marsh; however, of the remaining environments, at least the shoreface is 
closely integrated with the barrier island in terms of morphology, processes, 
and sediments. See DUNE. 
    A variety of physical processes exists along the coast. These processes 
act to shape and maintain the barrier-island system and also to enable the 
barrier to migrate landward as sea level continues to rise. The most important 
process in the barrier-island system is the waves, which also give rise to 
longshore currents. Waves and longshore currents dominate the outer portion of 
the barrier system, whereas tidal currents are dominant landward of the 
barrier, although small waves may also be present. Tidal currents are most 
prominent in and adjacent to the inlets. On the supratidal portion of the 
barrier island, the wind is the most dominant physical process. See NEARSHORE 
SEDIMENTARY PROCESSES. 
    Barrier-island sands represent one of the best sources of oil and gas, 
with the tight organic-rich source rocks being in the form of the bay and 
shelf muds and the barrier itself being the reservoir rock. These elongate 
sand bodies have been sought by exploration geologists for decades. The 
Tertiary sequences of the Texas Gulf coasts are an example of such barrier 
systems which have been very productive. See COASTAL LANDFORMS. 
 �34s[Richard A. Davis, Jr.]
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�6v�2v�32sBaryon �21m �33s
    The generic name for any hadronic particle with baryon number B=+1. By far
the most common baryons are the proton and neutron, the two states of the
nucleon doublet N = (p,n), whose intrinsic properties are listed in Table 1.
The baryon number of any particular state may be deduced from its production
or decay processes, or both, since the total baryon number is conserved (with
possible rare exceptions discussed below) and B = 0 holds for all mesons and
leptons. See LEPTON; MESON; NEUTRON; NUCLEON; PROTON.
     The scientific view of the hadrons changed greatly during the 1970s.  It is
now generally accepted that they are composite, consisting of spin 1/2 quarks
(q), corresponding antiquarks (q), and some number of gluons, the last being the
quanta of the intermediate field which binds the quarks and antiquarks to form
hadrons.  B = + 1/3 holds for a quark q, B = -1/3 for an antiquark q, while B =
0 holds for a gluon.  Thus, a baryon consists of three ("valence") quarks,
together with some number of quark-antiquark (qq) pairs (called the
quark-antiquark sea) and of gluons.  The known quarks are listed in Table 2.
They must be assigned fractional charge values, relative to the proton charge.
See  GLUONS; HADRON; QUARKS.
     This quark theory of the hadrons has been proposed in a quite specific
form, known as quantum chromodynamics (QCD).  It is a gauge theory based on a
symmetry hypothesized for the hadronic interactions of the quarks, which says
that these interactions are invariant with respect to an SU[3](sub c) group of
unitary transformations with modulus unity acting in an abstract
three-dimensional space known (whimsically) as color space.  Each quark type
then has three color states, usually labeled by the suffixes r = red, g = green,
and b = blue, corresponding to the three axes of this space.  The gauge particle
of this symmetry theory is the gluon, a neutral vector particle coupled
universally with the currents of color, just as the photon, the gauge particle
of quantum electrodynamics (QED), is coupled universally with the
electromagnetic current.  However, whereas the photon has no charge, the gluon
has [3(sup 2) - 1] = 8 color components, so that it is a color octet.
Consequently, there is a gluon contribution to the color currents, and so the
gluon field must interact with itself, introducing a nonlinearity into quantum
chromodynamics which has no parallel in quantum electrodynamics.  This
nonlinearity has important implications for quantum chromodynamics, leading to
its asymptotic freedom, the property that the coupling of gluon to the color
current approaches zero at short distances, which is essential for even
qualitative agreement between quantum chromodynamics predictions and the
empirical data on high-energy collision processes.  See  COLOR (QUANTUM
MECHANICS); QUANTUM CHROMODYNAMICS; QUANTUM ELECTRODYNAMICS; SYMMETRY LAWS
(PHYSICS).
    An important element in quantum chromodynamics is the confinement dogma,
the assertion that only color singlet states have finite energy. This
assertion implies that neither a quark nor a gluon can exist in a free state,
since the former is a color triplet and the latter a color octet. Zero mass is
expected for a gauge particle (as is the case for the photon), hence for the
gluon, and a mass of order 10 MeV is given for the (u,d) quarks in Table 2,
but these values refer to the masses effective within hadronic states. In
accord with this dogma, no observations of free gluons or quarks have yet been
confirmed. Many theoreticians anticipate that this dogma will be deducible
from quantum chromodynamics itself but, despite some favorable indications for
this expectation, no rigorous proof that the dogma follows from quantum
chromodynamics has been given.
    Quantitative predictions of the properties of baryonic states are
currently made using a simplified quark-quark (qq) potential with the
following features: (1) an attractive long-range potential, increasing with
separation to ensure confinement, and (2) a spin-dependent potential
representing one-gluon exchange, effective at small separation, where the
regime of asymptotic freedom holds and perturbation theory is valid. Such
predictions have had a great deal of success.
    The quark content of the nucleons is given by Eqs. (1).

		     p = (uud)	    n = (udd)				     (1)

    The replacement of a d quark in the nucleon by an s quark produces a
baryon state with spin parity 1/2(sup +) and strangeness number s = - 1, the
latter being given by [n(s) - n(s)] where n(q) denotes the number of quarks of
type q in the system considered. The states thus reached have the quark
structures of Eq. (2), thus the isotriplet SIGMA and isosinglet LAMBDA states
are obtained. If two of the d quarks in Eq. (1) are replaced by s quarks, the
isodoublet XI states of Eq. (3) are obtained. The s quark has I=0, being
unaffected by the SU(2)TAU transformations in the (u,d) space. Baryonic states
with s = 0 are collectively termed hyperons. See HYPERON; STRANGE PARTICLES.

  [SIGMA(sup +), SIGMA(sup 0), SIGMA(sup -)] = (uus, (ud + du)
		    s/SQUARE ROOT OF 2,dds)				   (2a)

		 LAMBDA = (ud - du) s/SQUARE ROOT 2			   (2b)


		 [XI(sup 0),XI(sup -)] = (uss,dss)			     (3)

    These eight baryon states [p, n, SIGMA(sup +), SIGMA(sup 0), SIGMA(sup -),
LAMBDA, XI(sup 0), XI(sup -)] all have the spin parity 1/2(sup +) and the same
internal wave functions.  They are arrayed in Fig.  1, where the symmetry of
their relationship is made evident; for this, it is helpful to use the quantum
number Y = (B + s), named hypercharge.
    The mass difference DELTA m = (m(s) - m(u,d)) is quite large, and so
SU(3)TAU symmetry is much more strongly violated than SU(2)TAU symmetry. This
is most apparent in the baryon mass values, which vary widely over the octet;
the leading variation is that proportional to the strangeness s, which counts
the s-quark content of each baryon. The approximately 75-MeV difference
between the mean SIGMA mass m(SIGMA) and m(LAMBDA) has a more subtle origin,
but is well accounted for on the basis of the quark-quark (qq) potential from
quantum chromodynamics, described above. The small mass-differences within
each isospin multiplet are belived due to electromagnetic effects.
    The production and reaction processes observed for these baryons are
governed by flavor selection rules. For hadronic processes (but not for weak
processes) these selection rules are simply Eq. (4) for each flavor f = B, Q,
s, c, . . .; that is, each type of quark is independent of the others, and
quarks do not change type. Quantum chromodynamics provides a ready
understanding of these rules.

			    n(f) = 0					     (4)

    The baryon-baryon interactions are of particular interest. Between
nucleons, this interaction gives rise to the existence of atomic nuclei, and
it has been particularly well studied, both empirically and theoretically, for
the NN system. For large separations [greater than 0.8 x 10(sup -15) m], the
NN force is due to the exchange of pions and of other known mesons with masses
less than about 1 GeV; for small separations [less than 0.4 x 10(sup -15) m],
a strong short-range repulsion is observed, possibly arising from the
suppressive effects of the Pauli principle for quarks when the quark
structures of the two nucleons overlap. At low energies, the outstanding
feature of the NN interaction is its strong noncentral tensor component, which
is due to one-pion exchange and is a direct consequence of the pseudoscalar
nature of the pion. It also has strong spin-orbit interactions, observed in NN
interactions at higher energy and of much importance for the shell structure
of nuclei.
    Many particle-unstable baryon states, with lifetimes in the range
10(sup -22) to 10(sup -23) s, have become established. All of the resonances
now established are consistent with the limitations of the three-quark model.
    This qqq model for baryons is far from complete. It provides a good
mnemonic for baryon spectroscopy, and even works well quantitatively, but it
does not include any account of the quark sea or of multigluon systems, even
for the ground states. Other kinds of excitation are also possible; for
example, if s = + 1 resonances are established, they might correspond to
multiquark states with the structure (qqqqq), and there might also exist
excited states where the excitations are gluonic. However, there is no data
calling for such excitations at present.
    Further baryon states can be formed by replacing one or more of the u,d
and s quarks of the states discussed above by a c quark. If the s and c quarks
both had the same mass as the (u,d) quarks, the states formed would correspond
to an SU(4)f	symmetry. This is emphasized by Fig. 2, which shows the
extensions of the 1/2(sup +) baryon octet (Fig. 2a) and the 3/2(sup +) baryon
decuplet (Fig. 2b) obtained in this way. The lowest plane of each part of the
figure consists of the charmless baryon states; those in Fig. 2a are in accord
with the octet of Fig. 1.
    In reality, the c-quark mass is so large that little quantitative detail
of SU(4)(sub f)symmetry can survive in the physical situation, and the structure
of Fig. 2 has value mainly for general comprehension and for the counting of
states. The lowest calculated levels are given in Table 3; the states are
labeled as for SU(3)(sub f), the suffix c meaning that the s quark has been
replaced by a c quark (which increases the charge by +1). With these mass
values, only the isosinglet LAMBDA(sub c)(sup +)(2260) will be semistable, and
its weak decay will be due to the dominant c-quark transition c -> s.
    The existence of a fifth quark b was demonstrated in 1978 in work with the
PETRA electron-positron storage ring at the DESY laboratory, Hamburg, and
theoreticians anticipate the existence of a sixth quark, to be named t, whose
mass must exceed 19 GeV in order to account for the lack of evidence for this
quark even at the top energy of PETRA. These quarks will add two further
additive quantum numbers with conservation laws of the form of Eq. (4). Of
course, there could well exist still further, heavier quarks at present
unsuspected. Baryon states built of these heavier quarks, together with the
lighter quarks, will certainly exist and will be observed in the course of
time; no candidates to be identified with LAMBDAsubb have yet been brought
forward. See PARTICLE ACCELERATOR.
    For every baryonic state mentioned above, there will exist an antibaryon
state with opposite flavor quantum numbers, in particular with B = -1. The
antiproton p was first identified in 1954, and antiproton beams have been
available at high-energy proton accelerators for many years. Antibaryon decay
processes correspond directly to those for the corresponding baryons. They
have the same lifetimes, within measurement accuracy.
    In grand unified theories (GUT), which attempt to account for both quarks
and leptons, together with their strong, electromagnetic, and weak
interactions, transitions q -> l generally exist, at some level, since such
theories assign leptons and quarks to common multiplets. This violation of
baryon conservation opens the possibility that the lightest baryon, the
proton, might not be absolutely stable but undergoes decay processes such as p
-> e+PI(sup 0) at a very low rate. Empirically, the proton is known to have a
half-life exceeding 6 x 10(sup 35) s. The simplest grand unified theories
under discussion do predict a total proton decay rate of order 10(sup -39
PLUS/MINUS 2) s(sup-1), while other theories may predict smaller decay rates,
even zero.  Since proton decay offers the possibility of discriminating between
various grand unified theories, the detection of proton decay, or at least the
improvement of the present empirical limits on its rate, is an important subject
of investigation.  See ELEMENTARY PARTICLE; FUNDAMENTAL INTERACTION.
 �34s[Richard H. Dalitz]
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�6v�2v�32sBasalt �21m �33s
    An igneous rock characterized by small grain size (less than about 0.2 in.
or 5 mm) and approximately equal proportions of calcium-rich plagioclase
feldspar and calcium-rich pyroxene, with less than about 20% by volume of
other minerals. Olivine, calcium-poor pyroxene, and iron-titanium oxide
minerals are the most prevalent other minerals. Most basalts are dark gray or
black, but some are light gray. Various structures and textures of basalts are
useful in inferring both their igneous origin and their environment of
emplacement. Basalts are the predominant surficial igneous rocks on the Earth,
Moon, and probably other bodies in the solar system. Several
chemical-mineralogical types of basalts are recognized. The nature of basaltic
rocks provides helpful clues about the composition and temperature within the
Earth and Moon. The magnetic properties of basalts are responsible in large
part for present knowledge of the past behavior of the Earth's magnetic field
and of the rate of seafloor spreading. Some meteorites are basaltic rocks.
They differ significantly from lunar basalts and appear to have originated
elsewhere in the solar system at a time close to the initial condensation of
the solar nebula. See IGNEOUS ROCKS; METEORITE.
    Basalt flows out of fissures and cylindrical vents. Repeated or continued
extrusion of basalt from cylindrical vents generally builds up a volcano of
accumulated lava and tephra around the vent. Fissure eruptions commonly do not
build volcanoes, but small cones of tephra may accumulate along the fissures.
See VOLCANO; VOLCANOLOGY.
    Basalts display a variety of structures mostly related to their
environments of consolidation. On land, basalt flows form pahoehoe, aa, and
block lava, while under water, pillow lava is formed. Basalt also occurs as
pumice and bombs. Commonly, basaltic pumice is called scoria to distinguish it
from the lighter-colored, more siliceous rhyolitic pumice.
    The mineralogy and texture of basalts vary with cooling history and with
chemical composition. As basalt crystallizes, both the minerals and the
residual melt change in composition because of differences between the
composition of the melt and the crystals forming from it. In basalts, because
of the rapid cooling, there is little chance for crystals to react with the
residual melt after the crystals have formed. Completely solid basalts
generally preserve a record of their crystallization in the zoned crystals and
residual glass.
    Most basalts contain minor amounts of chromite, magnetite, ilmenite,
apatite, and sulfides in addition to the minerals mentioned above. Magnetite
in basalts contains a history of the strength and orientation of the Earth's
magnetic field at the time of cooling. Therefore, although magnetite is minor
in amount, it is probably the most important mineral in terrestrial basalts,
because it enables earth scientists to infer both the magnetic history of the
Earth and the rate of the production of basaltic ocean floor at the oceanic
ridges.
    Basalts occur in all four major tectonic environments: ridges in the sea
floor, islands in ocean basins, island arcs and mountainous continental
margins, and interiors of continents. The principal environment is the deep
sea floor. Significant differences exist in the composition of basalt which
relate to different tectonic environments (see table). Chemical analyses of
basalts are now used instead of, or together with, the textural criteria as a
basis of classification. Geologists customarily recast the chemical analysis
into a set of ideal minerals according to a set of rules. The result is called
the norm of the rock. In general there is rather close correspondence between
the normative minerals and the observed minerals in basaltic rocks. See
PETROLOGY.
    Basalt is considered the most widespread material available at the surface
of the Earth which is derived from the mantle. Study of the regional
distribution of various basalts helps earth scientists formulate some details
regarding the tectonic processes affecting their generation. After these
factors are taken into account, the differences remain which may be understood
in terms of the compositionally inhomogeneous mantle. Some inhomogeneities may
relate to earlier episodes of mountain building and possibly to primal
planetary differentiation. See MAGMA: PETROGRAPHIC PROVINCE.

     TABLE CAPTION.  Chemical compositions of basalts, in weight percent.

 �34s[Alfred T. Anderson, Jr.]
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�6v�2v�32sBasement rock �21m �33s
    The more resistant, generally crystalline rock beneath layers or irregular
deposits of younger, relatively deformed sedimentary rock. The term is
frequently equated with ``basement complex,'' but the latter term should be
used only when the composition and structure are truly complex. In much of the
world basement rock consists of Precambrian igneous and metamorphic rocks of
various types, but locally the basement rock beneath layers of sedimentary
strata may be Paleozoic, Mesozoic, or even Cenozoic. It may lie at great
depths, beneath hundreds or thousands of feet of sediments, or it may occur at
the surface, where it has been exposed by faulting or erosion or both (see
illustration). See PRECAMBRIAN.





 �34s[J. Paul Fitzsimmons]
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�6v�2v�32sBase (chemistry) �21m �33s
    Any chemical species, ionic or molecular, capable of accepting or
receiving a proton (hydrogen ion) from another substance. The other substance
acts as an acid in giving up the proton. A substance may act as a base, then,
only in the presence of an acid. The greater the tendency to accept a proton,
the stronger the base. The hydroxyl ion acts as a strong base. Water acts as a
weaker base in the reaction H2O + H sup + EQUILIBRIUM H3O sup +. This broader
definition replaces the older concept of a base as a substance yielding
hydroxyl ions in solution. A more general concept of a base is that of a
substance which donates a pair of electrons to form a coordinate bond. See
ACID AND BASE.
 �34s[Francis J. Johnston]
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�6v�2v�32sBasidiolichenes   �21m �33s 
    A class of the Lichenes. This is a very small and unusual group 
characterized by the production of basidia rather than asci. The plant body is 
usually bracket-shaped and up to 2 in. (5 cm) broad. Identified as a fungus, 
it would fall in the order Agaricales near the family Thelephoraceae. 
Basidiolichens are distinguished from bracket fungi by an internal layer of 
green symbiotic algae. The algae provide food for the fungus and enable it to 
live autonomously. Therefore, basidiolichens occur commonly on soil and rocks 
where normal bracket fungi could hardly survive. The plant bodies are 
persistent and live for many years; none of them are parasitic or saprophytic 
on other living plants. There are two major genera, Cora and Dictyonema, with 
about 20 species, all restricted to tropical regions throughout the world. See 
LICHENES.  
 �34s[Mason E. Hale, Jr.]
�33s 
  
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�6v�2v�32sBasidiomycotina   �21m �33s 
    A division of the fungi; commonly known as Basidiomycetes, the class name 
used in older classifications. The Basidiomycetes are important as food 
(mushrooms), as causal agents of plant diseases (rusts, smuts, heart rots of 
trees), and as causes of dry rots of timbers. The characteristic structure of 
the Basidiomycetes is the basidium, a specialized sporangium which is the site 
of nuclear fusion and reduction division. After this division usually four 
basidiospores are formed in outpocketings in such a way that they appear to be 
borne on the basidium. There are four classes: Teliomycetes, 
Phragmobasidiomycetes, Hymenomycetes, and Gasteromycetes. See GASTEROMYCETES; 
HYMENOMYCETES; MUSHROOM; PHRAGMOBASIDIOMYCETES; TELIOMYCETES. 
 �34s[Robert M. Page]
�33s 
  
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�6v�2v�32sBasin   �21m �33s 
    A low-lying area, wholly or largely surrounded by higher land. Basins vary 
from small, nearly enclosed valleys to extensive mountain-rimmed depressions, 
such as the Bighorn Basin of north-central Wyoming and the larger Tarim basin 
of central Asia. 
    The term is also applied (sometimes as drainage basin) to the entire area 
drained by a given stream and its tributaries. An example of this usage is the 
Mississippi Basin, occupying most of the United States between the Rocky 
Mountains and the Appalachians, or the Amazon Basin, an area of similar size 
in northern South America. Depressions that drain entirely inward, without 
outlet to the sea, are basins of interior drainage. 
    A geologic basin is an area in which the rock strata are inclined downward 
from all sides toward the center. Because of erosion and deposition, the 
surface may not show the form of a depression. 
 �34s[Edwin H. Hammond]
�33s 
  
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�6v�2v�32sBasommatophora   �21m �33s 
    A superorder of the molluscan subclass Pulmonata containing about 2500 
species that are grouped into 11 - 15 families. Only a few members of the 
family Ellobiidae are terrestrial, with the other species today being tidal to 
supratidal or estuarine in habitat. The families Siphonariidae and 
Trimusculidae are marine limpets with caplike shells. The families Otinidae 
and Amphibolidae also are marine taxa. The remaining families inhabit a great 
variety of fresh-water situations and are quite varied in shell structure and 
shape. Because of this great variation in habitat and form, it is difficult to 
find structures that are common to all taxa and thus diagnostic of the group, 
which indeed may not have a common origin. The most easily observable 
characters are having the eyespots normally located at the base of two slender 
tentacles that are neither contractile nor retractile, and usually having two 
external genital orifices. Most features of the anatomy are shared with other 
pulmonate superorders, or are specializations related to major changes in 
habitat or body form. See PULMONATA. 
 �34s[G. Alan Solem]
�33s 
  
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�6v�2v�32sBass   �21m �33s 
    The name for a number of fishes assigned to two families in the order 
Perciformes. Both marine and fresh-water species are included under this 
common name, and all are highly prized as game fish as well as for food. 
Members of the family Centrarchidae are commonly referred to as the 
fresh-water or black basses, while the Serranidae are designated as the sea 
basses. See PERCIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sBasswood �21m �33s
    A member of the linden family in the order Malvales. One species, known as
the American linden (Tilia americana), is a timber tree of the northeastern
quarter of the United States and the adjacent area of Canada. Tilia is also an
ornamental tree. T. europea, or lime tree of Europe, is often cultivated along
the streets. The lindens are also important as bee trees. The leaves are
heart-shaped, coarsely toothed, long, pointed, and alternate. All species of
Tilia can be recognized by the peculiar winter buds, which have a large outer
scale that gives the bud a humped, asymmetrical appearance, and by the small,
spherical, nutlike fruits borne in clusters.
    The wood of basswood, also known as whitewood, is white and soft and is
used for boxes, venetian blinds, millwork, furniture, and woodenware. There
are about 30 species in the temperate regions of the Northern Hemisphere in
North America south to Mexico, but none in the western part. In Asia lindens
grow south to central China and southern Japan. See MALVALES.
 �34s[Arthur H. Graves /Kenneth P.Davis]
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�6v�2v�32sBat �21m �33s
    The common name for all mammals included in the order Chiroptera. Nearly
850 species are currently recognized; they are grouped in 17 families (see
table). They can be combined on the basis of a variety of characters,
including size and feeding modifications, into two general groups, recognized
as suborders: Megachiroptera, mostly large fruit and flower feeders, with
about 150 species; and Microchiroptera, chiefly small insect eaters, with
about 700 species. Bats are worldwide in distribution, except for the polar
regions and a few isolated oceanic islands.
    Small to medium in size, bats are the only mammals that are capable of
true flight, with forelimbs highly modified for this function. The bones of
the manus, or hand, and the digits are elongate and covered by a membrane;
this membrane connects to the body and extends from the forelimb to some
position on the lower leg or foot (according to the species). The first digit,
or thumb, usually retains a claw, and a membrane runs from its base to the
neck. In many species, the tail, if present, is partly or wholly enclosed by
the interfemoral membrane, which extends between the small hindlimbs, each of
which has five toes that terminate in strong curved claws. The hindlimbs are
used by most species for hanging from perches head down while at rest. Bats
are principally nocturnal in their activities, and they rest during the
daylight hours.
    Most bats of the cooler portions of the Northern Hemisphere hibernate
during the winter months. Some bats tend to roost singly in a hibernaculum,
others in large aggregations. Species may differ markedly in their
requirements as regards temperature, humidity, air currents, and other
environmental factors. See HIBERNATION; THERMOREGULATION.
    Bats are one of several different groups of animals that utilize reflected
sound for orientation (echolocation). The sense of hearing is indispensable in
bat orientation, and bats which cannot transmit sound become disoriented. All
Microchiroptera emit high-frequency shortwave sounds, which are used in
orientation and often in pursuit of insect prey.
    Like other mammals, bats may act as reservoir hosts or as transmitters of
pathogens. Most importantly, they can transmit rabies, which may affect not
only true vampire bats but also insectivorous species. Some fruit bats can be
pests in orchards. On the other hand, bats consume vast quantities of insects,
and some are important in seed dispersal and pollination. Bats and their
roosts (particularly nursery colonies and hibernacula) should in general be
protected, especially since some species have shown alarming declines in
numbers in recent years. See CHIROPTERA.



 �34s[Karl F. Koopman]
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�6v�2v�32sBatales   �21m �33s 
    A small order of flowering plants, division Magnoliophyta (Angiospermae), 
in the subclass Caryophyllidae of the class Magnoliopsida (dicotyledons). It 
consists of a single family, with only one genus (Batis), of two shrubby, 
maritime species. The order is somewhat doubtfully placed in the subclass 
Caryophyllidae, where it is marked by its anatropous ovules, straight embryo, 
binucleate pollen, four-locular ovary, much reduced flowers borne in unisexual 
catkins, and the absence of anthocyanins, betalains, endosperm, and perisperm. 
See ANTHOCYANIN; CARYOPHILLIDAE; FLOWER; MAGNOLIOPHYTA; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sBatholith �21m �33s
    A geologic name for a body of igneous rock of moderate or large size [40
mi sup 2 (100 km sup 2) or more in cross-sectional diameter] emplaced at great
or intermediate depth in the Earth's crust. Batholiths are commonly discordant
with (cut across) the enclosing rocks, but on a grand scale most of the large
batholiths are elongated parallel to the dominant structure of the enclosing
crustal rocks. The shape of batholiths in depth is a point of debate. One
school argues that batholiths continue downward with increasing size or at
least with no reduction in size. The other school argues that large batholiths
are composites of many smaller units, each of which maintains a nearly
constant cross section, thus suggesting variation in shape with downward
development. See PLUTON.
 �34s[James A. Noble]
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�6v�2v�32sBathynellacea �21m �33s
    An order of syncarid crustaceans found in subterranean waters in central
Europe and England. The body is elongate and segmented. Organs and limbs are
primitive. Each of the thoracic limbs has two epipodites which function as
gills. The thorax and abdomen have a similar form (see illustration). These
animals have no metamorphosis. See SYNCARIDA.



 �34s[Hans Jakobi]
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�6v�2v�32sBatoidea �21m �33s
    Skates and rays, constituting one of the two Recent orders of the subclass
Elasmobranchii. The order is also known as Rajiformes. The skates and rays
differ from the sharks (order Selachii) in having ventral gill slits, the edge
of the pectoral fin attached to the side of the head anterior to the gill
clefts, and the upper margin of the orbit not free from the eyeball (see
illustration).
    Rays have been classified in 5 suborders, 16 families, 47 genera, and 300
- 350 species. They occur in all oceans, and most are sluggish bottom
inhabitants, although the mantas commonly swim at the surface. A few,
especially stingrays and sawfishes, penetrate estuaries of tropical rivers or
even live far upstream. There are some bizarre skates that live at depths of
as much as 1500 fathoms (2700 m); however, none of these is luminescent. The
torpedoes have a pair of enlarged electric organs located lateral to the eyes
on the expansive pectoral fins; these can deliver a strong and temporarily
disabling shock to bathers. See ELECTRIC ORGAN (BIOLOGY).
    Internal fertilization is practiced by all rays. Most rays are
ovoviviparous, but skates deposit eggs that are protected by horny cases. Rays
are carnivorous, feeding on a wide variety of marine worms, mollusks,
crustaceans, and other invertebrates, as well as small fishes. See
ELASMOBRANCHII.




 �34s[Reeve M. Bailey]
�33s


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�6v�2v�32sBatrachoidiformes �21m �33s
    The toadfishes, which make up an order (or suborder) of actinopterygian
fishes. This group is also known as the Haplodoci. The first vertebra of
Batrachoidiformes is rigidly fused with the broad, flattened cranium; the
short posttemporal is not forked and is suturally attached to the skull. There
are no epiotics, intercalars, or ribs, and the four or five pectoral radials
are elongate. There are only three gill arches, and the gill openings are
reduced in size. The pelvic fins are on the throat; the spinous dorsal fin is
short, and the soft dorsal and anal fins are long (see illustration).
    The single family, Batrachoididae, known from Miocene to Recent time,
includes 10 genera and about 35 species. Most live in tropical and temperate
oceanic shore waters or in moderate depths, but a few ascend tropical rivers.
Some species are luminescent; some have hollow opercular and dorsal spines
associated with venom glands. Toadfishes have powerfully muscled jaws and
strong teeth and are predacious. See ACTINOPTERYGII; BIOLUMINESCENCE;
OSTEICHTHYES.




 �34s[Reeve M. Bailey]
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�6v�2v�32sBattery   �21m �33s 
    A device which transforms chemical energy into electric energy. The term 
is usually applied to a group of two or more electric cells connected together 
electrically. In common usage the term battery is also applied to a single 
cell, such as a flashlight battery. 
    There are in general two types of batteries, primary batteries and 
secondary storage or accumulator batteries. Primary types, although sometimes 
consisting of the same plate-active materials as secondary types, are 
constructed so that only one continuous or intermittent discharge can be 
obtained. Secondary types are constructed so that they may be recharged, 
following a partial or complete discharge, by the flow of direct current 
through them in a direction opposite to the current flow or discharge. By 
recharging after discharge, a higher state of oxidation is created at the 
positive plate or electrode and a lower state at the negative plate, returning 
the plates to approximately their original charged condition. 
    Primary and secondary cells may be constructed from several materials. For 
the more important of these types see PRIMARY BATTERY; STORAGE BATTERY. For 
other sources of electric energy known as batteries or cells see FUEL CELL; 
NUCLEAR BATTERY; SOLAR CELL. 
 �34s[Harold C. Riggs]
�33s 
  
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�6v�2v�32sBauxite   �21m �33s 
    A rock name for weathering products composed largely of hydrous aluminum 
oxides and aluminous laterite. Bauxite, the ore of aluminum, is a mixture and 
not a mineral species. See ALUMINUM. 
    Bauxite occurs in a variety of structures, textures, and colors. The color 
depends on the content of iron oxides and ranges from white to dark red or 
brown. Commonly bauxite is pisolitic and concretionary, but it may be 
fine-grained, dense, or vermicular. 
    Commercial bauxite deposits consist largely of gibbsite, Al(OH)3, or 
boehmite, AlO(OH), or mixtures of these minerals. The chief impurities are 
clay minerals and iron oxides, and bauxite may grade into clay or into 
ferruginous laterite. Some bauxite derived from basaltic rocks in India and 
Hawaii contains large amounts of titania. 
    C. S. Fox in 1927 suggested two main groups of bauxite deposits: laterite 
type and terra rossa type. All deposits are not easily placed in this 
classification, but bauxite formed more or less in place from crystalline 
rocks is of the first type, whereas the deposits that overlie limestone or 
dolomite commonly are referred to as the terra rossa type. 
    The world production of bauxite has steadily increased. The United States 
at one time produced one-fourth of the total bauxite used. However, other 
countries have begun to supply most of the world's needs. Imports are mainly 
from Jamaica, Australia, and Guinea. Ferruginous bauxite has been explored in 
Oregon and Hawaii. 
 �34s[Samuel S. Goldich]
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�6v�2v�32sBdelloidea   �21m �33s 
    A class (formerly order) of the phylum Rotifera. Bdelloid rotifers have a 
very characteristic appearance; the agile elongate body consists of several 
(typically 16) false segments or annuli which do not correspond to the true 
segmentation of the body. Of these, the shorter and smaller head, neck, and 
foot segments are telescopically retractile into the larger and longer trunk 
joints. In typical species the corona has the characteristic form of two 
trochal disks, raised on pedicels, and a single cingulum of smaller cilia. The 
corona can be completely withdrawn, to reveal the true anterior end of the 
body. The mouth is large and funnel-shaped, and leads via a ciliated buccal 
tube into the mastax (a gizzardlike structure) whose ramate trophi are in 
constant motion. The intestine is syncytial, often colored brown or red by its 
contents, and digestion appears to be extracellular, rapid, and occurring in 
an alkaline medium. No males are known, and reproduction is believed to be 
parthenogenetic. A few genera are viviparous, but most are oviparous. 
    The Bdelloidea are typically bottom dwellers and crawl over the substratum 
leech-fashion. They are the most common inhabitants of standing fresh waters 
throughout the world, occurring in the smallest pools, in the littoral zones 
of large lakes, and also in mosses, liverworts, and lichens, some of which may 
be wetted only intermittently. See ROTIFERA. 
 �34s[J. B. Jennings]
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�6v�2v�32sBdellonemertini   �21m �33s 
    An order of the class Enopla in the phylum Rhynchocoela. The proboscis is 
unarmed. The alimentary system comprises the mouth, papillate foregut, sinuous 
intestine without diverticula, and anus. Members are characterized by the 
presence of a posterior adhesive disk and dorsoventral flattening of the body, 
which lacks eyes, cephalic slits, and cerebral organs. The order contains the 
single genus Malacobdella, with three species ectocommensal in the mantle 
cavity of marine bivalves and one in the pulmonary sac of a fresh-water 
gastropod. See ENOPLA; RHYNCHOCOELA. 
 �34s[J. B. Jennings]
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�6v�2v�32sBeam   �21m �33s 
    A structural member extending between two reaction points and supporting 
loads such as floors, walls, and other types of surfacing is a simple beam. 
The loads are generally placed to produce a bending action about a principal 
axis of the beam cross section. When the structural member extends over 
several reaction points, it is referred to as a continuous beam. See LOADS, 
TRANSVERSE.  
 �34s[John B. Scalzi]
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�6v�2v�32sBeam column   �21m �33s 
    A structural member subjected to the combined action of axial and 
transverse loads which produce compression and bending moments in the member. 
Both compression and bending moments may also be produced by eccentrically 
applied axial loads. Beam columns exist in frame structures. See COLUMN. 
 �34s[John B. Scalzi]
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�6v�2v�32sBeam-foil spectroscopy   �21m �33s 
    A method of determining the energies and mean lives of excited electronic 
levels in monatomic ions and in atoms. Beams of particles (ions) of any 
element are energized in a particle accelerator and then sent in vacuum 
through a thin foil, usually of carbon. The particles emerge from the foil in 
various stages of ionization. Numerous energy levels are excited in each of 
those stages of ionization. The spontaneous loss of the energy of excitation 
takes place as the beam moves downstream from the foil. That loss is detected 
by means of the electromagnetic radiation or the electrons which the ions 
emit. See PARTICLE ACCELERATOR. 
    The mean life of a level provides sensitive information about the quantum 
nature of the level itself and the mechanism whereby it connects, by either 
the absorption or emission of light, to some other level. In beam-foil 
spectroscopy, this mean life is found from observations on the intensity of a 
particular spectral line as a function of the separation of the emitter and 
the exciter foil. Beam-foil spectroscopy is presently the only known way of 
measuring mean lives for highly ionized systems. 
    The degree of ionization which can be achieved depends on the atomic 
number of the ions and their energy. Early work was restricted to a few 
elements at the low end of the periodic table and to energies of a few million 
electronvolts. Subsequently, more than 60 of the 94 naturally occurring 
elements have been used, at energies extending to nearly 900 MeV. See 
SPECTROSCOPY.  
 �34s[Stanley Bashkin]
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�6v�2v�32sBean   �21m �33s 
    Any of several leguminous plants, or their seeds, long utilized as food by 
humans or livestock. Some 14 genera of the legume family contain species 
producing seeds termed ``beans'' which are useful to humans. Twenty-eight 
species in 7 genera produce beans of commercial importance, which implies that 
the bean can be found in trade at the village level or up to and including 
transoceanic commerce. 
    The principal Asiatic beans include the edible soybeans, Glycine sp., and 
several species of the genus Vigna, such as the cowpea and mung, grams, rice, 
and adzuki beans. The broad bean (Vicia faba) is found in Europe, the Middle 
East, and Mediterranean region, including the North African fringe. Farther 
south in Africa occur Phaseolus beans, of the vulgaris (common bean) and 
coccineus (scarlet runner) species. Some Phaseolus beans occur in Europe also. 
The cowpea, used as a dry bean, is also found abundantly in Nigeria. See 
COWPEA; SOYBEAN. 
    In the Americas, the Phaseolus beans, P. vulgaris and P. lunatus (lima 
bean), are the principal edible beans, although the blackeye cowpea, mung 
bean, and chick pea or garbanzo (Cicer arietinum) are grown to some extent. P. 
coccineus is often grown in higher elevations in Central and South America, as 
is Vicia faba. The tepary bean (P. acutifolius) is found in the drier 
southwestern United States and northern Mexico. See ROSALES. 
    Bean plants may be either bush or vining types, with white, yellow, red, 
or purple flowers. The seed itself is the most differentiating characteristic 
of bean plants. It may be white, yellow, black, red, tan, cream-colored, or 
mottled, and range in weight from 0.0044 to over 0.025 (125 to over 700 mg) 
per seed. Seeds are grown in straight or curved pods (fruit), with 2 - 3 seeds 
per pod in Glycine to 18 - 20 in some Vigna. 
    Beans are consumed as food in several forms. Lima beans and snap beans are 
used as fresh vegetables, or they may be processed by canning or freezing. 
Limas are also used as a dry bean. Mung beans are utilized as sprouts. Usage 
of dry beans (P. vulgaris) for food is highly dependent upon seed size, shape, 
color, and flavor characteristics, and is often associated with particular 
social or ethnic groups. Popular usage includes soups, mixed-bean salads, pork 
and beans, beans boiled with meat or other vegetables or cereals, baked beans, 
precooked beans, and powder, and, in the Peruvian Andes, parched or roasted 
beans. Because of the presence of heat-labile growth retardants, beans must be 
thoroughly cooked before being used as food. Newer developments are in the 
areas of cooking time, the manufacture of precooked dry powders that can be 
readily rehydrated to make a palatable soup, and new research impetus to 
identify the flatulence factors in dry beans. Since the food grain legumes are 
an important protein source in human diets, intensive efforts are being made 
in research and development. See LEGUME. 
 �34s[M. W. Adams]
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�6v�2v�32sBear �21m �33s
    The name for a number of species of carnivorous mammals in the family
Ursidae (see table). Bears are large and heavy bodied, with short, strong legs
and short tails. They are completely plantigrade, walking with their
metatarsal and metacarpal regions touching the ground, but are able to stand
upright. The foot has five toes which terminate in nonretractile claws. The
toes are separate with no membranes between them. The foot soles of the polar
bear are covered with hair, lacking in other species, which aids in walking on
ice. The eyes of bears are relatively small and vision is rather poor;
however, hearing and smell are acute. The rounded ears are small, and the
muzzle is elongate.
    Depending upon the species, the female produces one to three young per
litter; gestation varies from 180 to 250 days. The small, blind, toothless
offspring weigh less than 1 lb (0.5 kg). The female cares for the cubs since
the male usually does not remain with her.
    Bears originated in the Northern Hemisphere and are now widely distributed
throughout the world. The brown bear (Ursus arctos) is the commonest species,
in North America as well as in other areas of the world. The grizzly bear is
an example of a species that is on the verge of extinction. It is found in
national park areas, Alaska, and the Yukon region. The spectacled bear
(Tremarctos ornatus) is the only native bear in the Southern Hemisphere, where
it is found in the forested foothills of the western slopes of the Andes. The
Himalayan bear (Selenarctos thibetanus) occurs in China, Japan, Iran, and the
Himalayas. The black bear (Ursus americanus) is still numerous across North
America in forested areas. It is hunted for its fur and as a game animal in
many parts of the United States. The sun bear (Helarctos malayanus) is found
in the tropical forested areas of southern Asia, and thus differs from other
bears in its habitat preference. It is the smallest of all bears. The polar
bear (Thalarctos maritimus) occurs in the polar regions of the Northern
Hemisphere. See CARNIVORA; HIBERNATION.




 �34s[Charles B. Curtin]
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�6v�2v�32sBeat   �21m �33s 
    A variation in the intensity of a composite wave which is formed from two 
distinct waves with different frequencies. Beats were first observed in sound 
waves, such as those produced by two tuning forks with different frequencies. 
Beats also can be produced by other waves. They can occur in the motion of two 
pendulums of different lengths and have been observed among the 
different-frequency phonons in a crystal lattice. 
    One important application of beat phenomena is to use one object with an 
accurately known frequency to determine the unknown frequency of another such 
object. The beat-frequency or heterodyne oscillator also operates by producing 
beats from two frequencies. See OSCILLATOR. 
 �34s[Brian DeFacio]
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�6v�2v�32sBeaver   �21m �33s 
    The common name for two different and unrelated species of rodents - 
Aplodontia rufa, the mountain beaver, and Castor canadensis, the common or 
true beaver. 
    The mountain beaver is the only living species of the family Aplodontidae. 
It is a medium-sized animal and resembles a muskrat. The eyes are small as are 
the external ears, an adaptation to its fossorial (digging) habits. The tail 
is a short stump, the body is short and stout, and the limbs are short and 
terminate in broad feet with five toes and long claws. The mountain beaver is 
a vegetarian. Although not strictly nocturnal, it is more active at night. The 
animals form colonies with a single family inhabiting a series of runways. The 
mountain beaver is found along the Pacific Coast and ranges from British 
Columbia, Canada, to California. 
    The common beaver, a member of the family Castoridae, is a large rodent 
(weighing up to 50 lb or 23 kg). It occurs across Europe, North America from 
Labrador southward, and Asia, and is a valuable fur-bearing animal. The tail 
is broad and spatulate and is used, together with the webbed hindfeet, for 
swimming. The beaver is aquatic and builds its lodge in water by using mud, 
sticks, and branches interwoven. These beavers are vegetarians. Mating occurs 
in February, and the average litter of four young is born in May. These 
beavers appear to be monogamous and mate for life. See RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sBedbug �21m �33s
    The name for the approximately 36 recognized species of insects of the
family Cimicidae in the order Hemiptera. In the bedbugs true wings are absent;
the forewings are represented by two scalelike pads. The mouth parts are
modified for piercing and sucking. Bedbugs are small, oval-shaped,
reddish-brown insects with a thin, flat body covered with a tough integument.
The legs are short; compound eyes are present, and ocelli, or simple eyes, are
lacking. These insects are common household pests, having a cosmopolitan
distribution. Bedbugs are ectoparasites which feed upon warm-blooded animals,
including humans, bats, and birds. Two species attack humans: Cimex
lectularius and C. rotundatus. Cimex lectularius (see illustration) is the
common species of the temperate and subtropical regions, while the second
species is restricted to tropical regions of Asia and Africa. See HEMIPTERA.




 �34s[Charles B. Curtin]
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�6v�2v�32sBee   �21m �33s 
    Insects of the superfamily Apoidea in the order Hymenoptera. There are 
some 3000 species of bees divided into 19 families. Present-day bees subsist 
almost entirely on pollen as a source of protein, and on the sugar in nectar 
as an energy source. They are now obligately dependent on flowers, although a 
few species can sometimes feed on ``honeydew'' secretions from aphids and 
scale insects. In turn, many plants have become obligately dependent on bees 
for pollination. See HYMENOPTERA. 
    Most of the world's bee species are solitary. That is, a female builds a 
nest cell, provisions the cell with a nectar-pollen mixture, lays an egg on 
this food, and seals the nest cell, allowing the larva to develop and emerge 
as much as a year later. In primitively social bees such as some of the sweat 
bees (family Halictidae) and bumblebees (family Bombidae), an overwintered 
female (queen) that has mated the previous fall emerges from her underground 
hibernacula in the spring and attempts to start her own colony. A colony 
consists initially of sterile female offspring, who are her workers. Near the 
end of the colony cycle, in late summer or fall, the workers aid the queen in 
producing large numbers of sexuals, the drones and new queens. Colonies 
generally consist of several dozen and up to several hundred individuals at 
the height of the colony cycle in the summer. The workers, drones, and old 
queens all die off in the fall, and the new queens disperse and hibernate. 
    In the highly social bees, the honeybee (Apis mellifera) and stingless 
(Trigona) species, as many as 50,000 and 150,000 individuals, respectively, 
may be present in the colonies. In these bees the old queen lives several 
years, and the colonies are perennial rather than annual. The queens are 
nearly exclusively concerned with egg laying, and the workers are produced not 
only during times of food availability in the field, but also before major 
times of flowering by relying on large stores of pollen and honey. These bees 
reproduce by swarming, when the old queen leaves the hive accompanied by about 
half of the workers, which help her to initiate a new colony. See SOCIAL 
INSECTS.  
    The highly social organization of honeybees and stingless bees is 
orchestrated by scent, sound, and ``dance'' signals. The queen emits a 
chemical (pheromone) which inhibits the workers from developing and laying 
eggs. Absence or low concentrations of the queen pheromone cause the workers 
to rear a new queen. The workers also produce pheromones that act as signals 
to other workers. Honeybees indicate the direction, distance, and quality of 
rewarding food sources to hive mates by a symbolic dance language. Honeybees 
also buzz during the dance, which may serve to alert potential followers that 
then attempt to ``read'' the dancer's message. Stingless bees also alert hive 
mates by buzzing. The hive mates follow the scout bee (that has discovered the 
food) out of the hive; the scout then deposits a trail of scent droplets onto 
vegetation that aid the recruit to find the food. Bumblebees do not seem to 
have any way of communicating food found by successful hive mates. 
    By visiting flowers, bees serve as agents of cross-pollination. They are 
of inestimable importance in the pollination of crops and in pollination of 
the natural flora. Their activity is thus vital, not only directly for the 
human food supply, but also for land, water, and animal resources. See 
BEEKEEPING; POLLINATION. 
 �34s[Bernd Heinrich]
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�6v�2v�32sBeech �21m �33s
    A genus, Fagus, of deciduous trees of the beech family Fagaceae, order
Fagales. They can best be distinguished by their long (often more than 1 in.
or 2.5 cm), slender, scaly winter buds; their thin, gray bark, smooth even in
old trees; and their simple, toothed, ovate or ovate-oblong, deciduous leaves.
    The American beech (F. grandifolia) is native in the United States east of
the Mississippi River and in the lower Mississippi Valley. The hard, strong
wood is used for furniture, handles, woodenware, cooperage, and veneer. The
small, edible, three-sided nuts, called beechnuts, are valuable food for
wildlife. The European beech (F. sylvatica) is more popular as an ornamental
tree than the American species. Its leaves are smaller, with 5 - 9 pairs of
primary veins compared with 9 - 14 pairs in the American beech. The leaf teeth
are also shorter. Important ornamental varieties are F. sylvatica purpurea,
the copper or purple beech; var. incisa, the cut-leaved or fern-leaved beech;
and F. pendula, the weeping European beech. See FAGALES.
 �34s[Arthur H. Graves /Kenneth P.Davis]
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�6v�2v�32sBeekeeping   �21m �33s 
    The management and maintenance of colonies of honeybees. Although the 
commonly known honeybee species is native to Europe and Africa only, humans 
have transported them to other continents, and in most places they have 
flourished. The natural home for a honeybee colony is a hollow tree, log, or 
cave. European strains of the honeybee build a nest only in locations which 
are dry and protected from the wind and sunlight. African bees are less 
selective and may nest in hollowed-out termite mounds, rock piles, and 
locations which are less well protected. See BEE. 
    The honey which beekeepers harvest is made from nectar, a sweet sap or 
sugar syrup produced by special glands in flowers, collected from both wild 
and cultivated plants. Nectar, the honeybees' source of sugar or carbohydrate, 
and pollen, their source of protein and fat, make up their entire diet. Nectar 
contains 50 - 90% water, 10 - 50% sugar (predominantly sucrose), and 1 - 4% 
aromatic substances, coloring material, and minerals. To transform nectar into 
honey, bees reduce its moisture content, so that the final honey produced 
contains between 14 and 19% water, and also add two enzymes which they produce 
in their bodies. 
    Scientific beekeeping started in 1851 when an American, L. L. Langstroth, 
discovered bee space and the movable frame hive. Bee space is the open space 
which is about 0.4 in. (1 cm) wide and maintained around and between the combs 
in any hive or natural nest and in which the bees walk. If this space is 
smaller or larger than 0.4 in. (1 cm), the bees will join the combs. When the 
combs are stuck together, the hive is not movable, and it is not possible for 
beekeepers to manipulate a colony or to examine a brood nest. It was found, in 
1857, that bees could be forced to build a straight comb in a wooden frame by 
giving them a piece of wax, called foundation, on which the bases of the cells 
were already embossed. Bees use these bases to build honeycomb, the cells of 
which are used for both rearing brood and for storing honey. When a hive of 
bees is given a frame of foundation, they are forced to build the comb where 
the beekeeper wants it and not where they might otherwise be inclined to build 
it. Another discovery, made in 1865, was that honey can be removed from the 
comb by placing a comb full of honey in a centrifugal force machine, called an 
extractor. If the beekeeper can return an intact comb to a hive after removing 
the honey from it, the bees are saved the time and trouble of building a new 
comb, and the honey harvest is increased. The next discovery, in 1873, was the 
modern smoker. When bees are smoked, they engorge with honey and become 
gentle. Without smoke to calm a hive, normal manipulation of the frames would 
not be possible. By 1880, honey, which had once been a scarce commodity, 
became abundant. 
    All beehives used in the industry today are made of wooden boxes with 
removable frames of comb. Beekeepers have standardized their equipment so that 
parts will be interchangeable within an apiary and from one commercial 
operation to another. To be successful in commercial beekeeping, beekeepers 
must locate in those areas where nectar-producing plants abound. The 
best-known honey in the United States is clover honey. Alfalfa and oranges are 
also good nectar-producing plants and are major sources of honey. Bees collect 
nectar from hundreds of kinds of flowers, and thus there are a great variety 
of honey flavors. Thousands of colonies of honeybees are rented each year by 
growers of crops needing cross pollination. As the plants come into bloom, the 
beekeeper moves the bees, usually at night when all the bees are in the hive, 
and places them in groups in groves, orchards, and fields. While beekeepers 
make most of their living producing honey, the real importance of their bees 
in the agricultural economy is as cross pollinators. Without cross pollination 
the abundance and variety of food and flowers would not exist. Commercial 
beekeepers feel they need to own 500 to 2000 colonies to make a living, 
depending on how they market their honey. Some beekeepers sell their honey on 
the wholesale market only, while others pack their honey themselves and devote 
a great deal of time to sales. 
 �34s[Roger A. Morse]
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�6v�2v�32sBeet   �21m �33s 
    The red or garden beet (Beta vulgaris), a cool-season biennial of 
Mediterranean origin belonging to the plant order Caryophyllales 
(Centrospermales). This beet is grown primarily for its fleshy root, but also 
for its leaves, both of which are cooked fresh or canned as a vegetable 
Detroit Dark Red strains predominate. Cool weather favors high yields of dark 
red roots. Wisconsin, New York, and Texas are important beet producing states. 
See CARYOPHYLLALES; SUGARBEET. 
 �34s[H. John Carew]
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�6v�2v�32sBeetle   �21m �33s 
    A vernacular equivalent of the scientific name Coleoptera. The latter is 
an order of insects distinguished by the combination of a complete 
metamorphosis and biting mouthparts with the transformation of the front wings 
into hard protective elytra, under which the hindwings are folded in repose. 
Beetle larvae are commonly known as grubs. The beetles include numerous 
economically important species. A feature of the order is that adults are 
often long-lived and actively feeding, so that they may be as injurious as the 
larvae. See COLEOPTERA. 
 �34s[Roy A. Crowson]
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�6v�2v�32sBeggiatoales   �21m �33s 
    Formerly recognized as an order of filamentous or unicellular bacteria in 
the class Schizomycetes. The sulfur bacteria in this order are widely 
distributed in habitats where sulfur and hydrogen sulfide are present, for 
example, swamps, sewage, and sulfur mines. The microorganisms move by gliding 
on a substrate, a method of locomotion also found in many Cyanophyceae 
(blue-green algae), from which the Beggiatoales differ in their lack of 
photosynthetic pigments. 
    In Bergey's Manual of Determinative Bacteriology (1957), the Beggiatoales 
were divided into the Beggiatoaceae, Vitreoscillaceae, Leucotrichaceae, and 
Achromatiaceae. In the 8th edition of Bergey's Manual (1974), Beggiatoaceae, 
Leucotrichaceae, and Achromatiaceae are redefined and included with seven 
other families as Gliding Bacteria. See SCHIZOMYCETES. 
    Although many members of the Beggiatoales are able to oxidize hydrogen 
sulfide and deposit droplets of amorphous sulfur in their cells, the 
Vitreoscillaceae are not able to do this. There is a certain similarity, 
morphologically and in the type of locomotion, between members of the 
Vitreoscillaceae and filamentous Myxobacteria. Further investigations, for 
example, in cytology, might determine whether or not there is a true 
relationship. See MYXOBACTERALES. 
 �34s[Ernest G. Pringsheim]
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�6v�2v�32sBehavioral psychophysics   �21m �33s 
    A branch of psychology concerned primarily with the measurement of the 
sensory capacities of normal intact animals. In principle, the methods are 
similar to those of human psychophysics: The animal is ``instructed'' to 
observe a stimulus and to ``report'' what is observed. The stimulus situation, 
coupled with the past history and present physiological state of the animal, 
provides the necessary instructions; the animal's behavioral response 
constitutes the report. See PSYCHOLOGY, PHYSIOLOGICAL AND EXPERIMENTAL. 
    The fundamental measures used are based on the quality, intensity, and 
spatial and temporal extent of physical stimuli required to elicit some 
standard response, and include (1) least detectable stimulus intensities 
(absolute thresholds), (2) least detectable stimulus differences (difference 
thresholds), and (3) values of sensible stimuli which are not discriminable 
(points of apparent equality). Sets of such measures, along appropriate 
dimensions of stimuli, map the range and degree of the animal's sensitivity to 
those stimuli. 
    The stimulus-response patterns utilized in these measures may be either 
innate or learned. Stereotyped unlearned behavior (such as a taxis, 
orientation of the whole animal in a directional field of stimulation) is 
particularly useful in the study of lower animals which cannot readily be 
taught special responses to stimuli. But the use of unlearned behavior is not 
confined to lower forms; unconditioned reflexes (such as constriction of the 
pupil in response to light) are useful indices of the effectiveness of stimuli 
acting on the sense organs of higher animals, including humans. See TAXIS. 
    Such unconditioned responses, even though innately under the control of 
rather specific stimuli, may be brought under the control of other stimuli by 
classical (Pavlovian) conditioning, and then used in measuring the animal's 
sensitivity to these new stimuli. For example, salivary secretion may be 
conditioned to occur in direct response to a sound by pairing the sound with 
presentation of food. See CONDITIONED REFLEX. 
    More complex learned discriminative behavior utilizes responses which are 
instrumental in obtaining some reward. For example, the subject may be 
required to open one of two doors or to press a lever, in response to some 
stimulus, to obtain food. Such instrumental behavior may be conditioned to 
occur in response to almost any sensible stimulus, simply by reinforcing (that 
is, rewarding) the response only when that stimulus is present. Once such a 
discriminative response is learned, experiments may be undertaken which are 
comparable to human psychophysical experiments in their method, scope, and 
results. See PSYCHOPHYSICAL METHODS. 
 �34s[Floyd Ratliff]
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�6v�2v�32sBehavioral toxicology   �21m �33s 
    The study of behavioral abnormalities induced by exogenous agents such as 
drugs, chemicals in the general environment, and chemicals encountered in the 
workplace. Just as some substances are hazardous to the skin or liver, some 
are hazardous to the function of the nervous system. In the case of permanent 
effects, changes in sensation, mood, intellectual function, or motor 
coordination would obviously be undesirable, but even transient alterations of 
behavior are considered toxic in some situations. For example, operating room 
personnel accidentally exposed to very small doses of anesthetic do not 
exhibit altered performance on an intelligence test, a dexterity test, or a 
vigilance task. However, significant decrements in performance occur in 
recognizing and recording visual displays, detecting changes in audiovisual 
displays, and recalling series of digits. 
    By comparing the behavior of exposed subjects and control subjects, 
behavioral toxicologists seek to identify agents capable of altering behavior 
and to determine the level of exposure at which undesirable effects occur. 
When the agent under study is one in common use, and there is no evidence of 
its being hazardous to health, experiments may be carried out on human 
volunteers, or comparisons may be made from epidemiologic data. More 
frequently, safety considerations dictate the use of laboratory animals in 
toxicology research. 
    Perhaps the best-known example of toxicity in humans is methyl mercury 
poisoning (Minimata disease), which occurred in epidemic proportions in a 
Japanese coastal town where the inhabitants ate fish contaminated with mercury 
from industrial pollution. Although mercury affects a variety of behaviors, 
the most obvious symptoms are tremors and involuntary movements. 
    A different set of functional problems is exemplified by the effects of 
ethyl alcohol, a single agent with direct and indirect, short- and long-term 
consequences. The short-term, low-dose effects of alcohol include sensory 
disturbances, motor problems, and difficulties with processing information. 
Neurologically, alcohol is usually described as a central nervous system 
depressant which is general, in the sense that it disrupts many functions. 
    In some individuals, large quantities of alcohol consumed over a long 
period lead to permanent damage to the nervous system. Behaviorally, 
individuals with Korsakoff's syndrome exhibit severe memory deficits. 
Anatomically, their brains are found to have degenerative changes in the 
thalamus. This syndrome is not just an extension of the short-term effects. In 
fact, it is thought to arise from alcohol-induced malnutrition rather than as 
a direct effect of alcohol itself. 
    Lasting injuries to the nervous system have been reported to occur in 
children exposed to alcohol before birth. The behavioral problems associated 
with fetal alcohol syndrome do not appear to be related to either Korsakoff's 
syndrome or the immediate effects of alcohol. Rather, they constitute a third 
set of effects, including learning deficits, problems with inhibiting 
inappropriate behavior, and fine motor dysfunction, along with some visible 
physical abnormalities. While malnutrition may play a role in this congenital 
syndrome, the mechanism and locus of damage are not known. See ALCOHOLISM; 
CONGENITAL ANOMALIES; FETAL ALCOHOL SYNDROME. 
    When toxicity is considered only in terms of direct risk to survival, 
behavioral toxicity may seem to be of minor importance. However, survival is 
not the only criterion of good health. In a complex society that places heavy 
demands on an individual's educability, alertness, and emotional stability, 
even small deviations in behavior are potentially hazardous. Severe 
disabilities, as in the gross motor malfunctions of Minimata disease, have 
drawn attention to behavioral toxicology. Such incidents represent failures of 
control of toxic substances. Successes are difficult to measure, for they can 
be seen only in reduction of risk - the ultimate goal of toxicology. See 
TOXICOLOGY.  
 �34s[Patricia M. Rodier]
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�6v�2v�32sBejel   �21m �33s 
    An infectious disease of humans caused by a spirochete. It is similar to 
syphilis in character and occurs among seminomadic peoples of the Middle East. 
It is usually acquired in childhood, either by direct contact or through 
common drinking utensils. In the latter case, the initial lesion is on the 
lips. In the generalized stage, lesions are particularly common about 
mucocutaneous junctions. Bone lesions are common. See SYPHILIS. 
 �34s[Thomas B. Turner]
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�6v�2v�32sBel �21m �33s
    A logarithmic unit expressing the ratios of power, voltage, current, or
sound intensity. The number of bels separating two power readings is the
logarithm to the base 10 of their ratio (for example, two powers differ by 1
bel when their actual ratio is 10:1), while the number of bels separating two
current readings or the sound pressures of an acoustical signal is twice the
logarithm of the ratio of the currents.
    It is convenient in acoustics to express sound intensity in logarithmic
units because of the wide range of pressures to which the ear is sensitive.
The strength of a sound is usually specified as the square root of the mean of
the squares of the instantaneous pressures measured over a period of time. The
sound intensity is proportional to the square of the sound pressure. The
measure of the level in bels of a sound is log (I/I sub R), where I is the
intensity of the sound and I sub R is a specified reference intensity. A smaller
unit called the decibel, equal to 1/10 bel, is more commonly used. See
DECIBEL; NEPER; SOUND INTENSITY.
 �34s[Karl D. Kryter]
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�6v�2v�32sBelemnoidea �21m �33s
    An order of extinct dibranchiate cephalopods. These mollusks ranged from
the Upper Mississippian through the Cretaceous. The oldest known
representative is highly specialized, although the belemnoids are considered
to be the most primitive of the dibranchiates. Belemnoids have a chambered
shell or phragmocone which fits into a guard or rostrum, the apical portion of
the shell (see illustration). Belemnites is an important index fossil. See
CEPHALOPODA; DECAPODA (MOLLUSCA); DIBRANCHIA: INDEX FOSSIL.




 �34s[Charles B. Curtin]
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�6v�2v�32sBell   �21m �33s 
    A hollow metallic cylinder closed at one end and flared at the other. A 
bell is used as a fixed-pitch musical instrument or a signaling device, 
frequently in clocks. A bell is set vibrating by a clapper or tongue which 
strikes the lip. The bell has two tones, the strike note or key, and the hum 
tone, which is a major sixth below the strike note. Both tones are rich in 
harmonics. The finer bells are made for carillons. Small carillons have two 
full chromatic octaves and large ones have four or more chromatic octaves, the 
bell with the lowest tone being called the bourdon. All bells of a carillon 
are shaped for homogeneity of timbre. 
    Bell metal is approximately four parts copper and one part tin, although 
zinc, lead, or silver may be used. The shape of a bell - its curves and full 
trumpet mouth - evolved from experience. A bell can be pitched slightly. The 
tone is raised by grinding the outer surface, effectively decreasing the 
diameter, or lowered by grinding the inner surface. Too much tuning of a 
well-shaped bell is apt to degrade its voice or timbre. 
 �34s[Frank H. Rockett]
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�6v�2v�32sBelladonna �21m �33s
    The drug and also the plant known as the deadly nightshade, Atropa
belladonna, which belongs to the nightshade family (Solanaceae). This is a
coarse, perennial herb native to the Mediterranean regions of Europe and Asia
Minor, but now grown extensively in the United States, Europe, and India.
During the blooming period, the leaves, flowering tops, and roots are
collected and dried for use. The plant contains several important medicinal
alkaloids, the chief one being atropine, which is much used to dilate the
pupil of the eye. See ATROPINE; POLEMONIALES.
 �34s[Perry D. Strausbaugh /Earl L.Core]
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�6v�2v�32sBelt drive �21m �33s
    The lowest-cost means for transmitting power between shafts that are not
necessarily parallel. Belts run smoothly and quietly, and they cushion motor
and bearings against load fluctuations. Belts typically are not as strong or
durable as gears or chains. However, improvements in belt construction and
materials are making it possible to use belts where formerly only chains or
gears would do.
    Advantages of belt drive are: They are simple. They are economical.
Parallel shafts are not required. Overload and jam protection are provided.
Noise and vibration are damped out. Machinery life is prolonged because load
fluctuations are cushioned (shock-absorbed). They are lubrication-free. They
require only low maintenance. They are highly efficient (90 - 98%, usually
95%). Some misalignment is tolerable. They are very economical when shafts are
separated by large distances. Clutch action may be obtained by relieving belt
tension. Variable speeds may be economically obtained by step or tapered
pulleys.
    Disadvantages include: The angular-velocity ratio is not necessarily
constant or equal to the ratio of pulley diameters, because of belt slip and
stretch. Heat buildup occurs. Speed is limited to usually 7000 feet per minute
(35 meters per second). Power transmission is limited to 370 kilowatts (500
horsepower). Operating temperatures are usually restricted to -31 to 185 DG F
(-35 to 85 DG C). Some adjustment of center distance or use of an idler pulley
is necessary for wear and stretch compensation. A means of disassembly must be
provided to install endless belts.
    There are four general types of belts: flat belts, V-belts, film belts,
and timing belts. Each has its own special characteristics, limitations,
advantages, and special-purpose variations for different applications.
    Flat belts, in the form of leather belting, served as the basic belt drive
from the beginning of the Industrial Revolution. They can transmit large
amounts of power at high speeds. Flat belts find their widest application
where high-speed motion, rather than power, is the main concern. Flat belts
are very useful where large center distances and small pulleys are involved.
They can engage pulleys on both inside and outside surfaces, and both endless
and jointed construction are available.
    V-belts are the basic power-transmission belt, providing the best
combination of traction, operating speed, bearing load, and service life. The
belts are typically endless, with a trapezoidal cross section which runs in a
pulley with a V-shaped groove. The wedging action of the belt in the pulley
groove allows V-belts to transmit higher torque at less width and tension than
flat belts. V-belts are far superior to flat belts at small center distances
and high reduction ratios. V-belts require larger pulleys than flat belts
because of their greater thickness. Several individual belts running on the
same pulley in separate grooves are often used when the power to be
transmitted exceeds that of a single belt. These are called multiple-belt
drives.

    Film belts are often classified as a variety of flat belt, but actually
they are a separate type. Consisting of a very thin strip of material, usually
plastic but sometimes rubber, their widest application is in business
machines, tape recorders, and other light-duty service.
    Timing belts have evenly spaced teeth on their bottom side which mesh with
grooves cut on the periphery of the pulleys to produce a positive, no-slip,
constant-speed drive. They are often used to replace chains or gears, reducing
noise and avoiding the lubrication bath or oiling system requirement. They
have also found widespread application in miniature timing applications.
Timing belts, known also as synchronous or cogged belts, require the least
tension of all belt drives and are among the most efficient.
    The most common belt-pulley arrangement, by far, is the open belt drive
(Fig. 1). Here both shafts are parallel and rotate in the same direction. The
cross-belt drive of Fig. 2 shows parallel shafts rotating in opposite
directions. Timing and standard V-belts are not suitable for cross-belt drives
because the pulleys contact both the inside and outside belt surfaces.
    Industrial belts are usually reinforced rubber or leather, the rubber type
being predominant. Nonreinforced types, other than leather, are limited to
light-duty applications.
    Belts probably fail by fatigue more often than by abrasion. The fatigue is
caused by the cyclic stress applied to the belt as it bends around the
pulleys. Belt failure is accelerated when the following conditions are
present: high belt tension; excessive slippage; adverse environmental
conditions; and momentary overloads caused by shock, vibration, or belt
slapping. See CHAIN DRIVE: GEAR DRIVE; PULLEY.





 �34s[Arthur Erdman ;Raymond Giese]
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�6v�2v�32sBending moment �21m �33s
    The algebraic sum of all moments that are on one side of a cross section
of a structural member. For example, in the illustration diagraming of a floor
joist, a load on the joist tends to bend it at any particular cross section of
the joist. At section A-A the bending moment is the reactive upward force of
the left wall on the joist times the distance from wall to section A-A. At
section B-B the bending moment is the upward reactive force of the left wall
times the distance from wall to B-B plus (in the algebraic sense) the moment
produced by the downward load acting at its distance from section B-B. The
lower portion of the diagram shows the resultant bending moment at each cross
section of the joist. A bending moment that bends a beam convex downward is
positive, and that bends a beam convex upward is negative. See LOADS,
TRANSVERSE; SHEAR.




 �34s[Frank H. Rockett]
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�6v�2v�32sBentonite �21m �33s
    The term first applied to a particular, highly colloidal plastic clay
found near Fort Benton in the Cretaceous beds of Wyoming. This clay swells to
several times its original volume when placed in water and forms thixotropic
gels when small amounts are added to water. Later investigations showed that
this clay was formed by the alteration of volcanic ash in place; thus, the
term bentonite was redefined by geologists to limit it to highly colloidal and
plastic clay materials composed largely of montmorillonite clay minerals, and
produced by the alteration of volcanic ash in place. The term has been used
commercially for any plastic, colloidal, and swelling clays without reference
to a particular mode of origin. See CLAY; GEL; MONTMORILLONITE.
    Bentonites have been found in almost all countries and in rocks of a wide
variety of ages. They appear to be most abundant in rocks of Cretaceous age
and younger. In the United States, bentonites are mined extensively in
Wyoming, Arizona, and Mississippi. England, Germany, Yugoslavia, the Soviet
Union, Algeria, Japan, and Argentina also produce large tonnages of bentonite.
Many bentonites are of great commercial value. They are used in decolorizing
oils, in bonding molding sands, in the manufacture of catalysts, in the
preparation of oil well drilling muds, and in numerous other relatively minor
ways. The properties of a particular bentonite determine its economic use. See
CLAY.
 �34s[Ralph E. Grim ;Floyd M. Wahl]
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�6v�2v�32sBenzene �21m �33s
    A colorless, liquid, inflammable, aromatic hydrocarbon of chemical formula
C6H6 which boils at (80.1 DG C) (1762 DG F) and freezes at (5.4 - 5.5 DG C)
(41.7 - 41.9 DG F). In the older American and British technical literature
benzene is designated by the German name benzol. In current usage the term
benzol is commonly reserved for the less pure grades of benzene.
    Benzene is used as a solvent and particularly in Europe as a constituent
of motor fuel. In the United States the largest uses of benzene are for the
manufacture of styrene and phenol. Other important outlets are in the
production of dodecylbenzene, aniline, maleic anhydride, chlorinated benzenes
(used in making DDT and as moth flakes), and benzene hexachloride, an
insecticide.
    The six carbon atoms of benzene, each with a hydrogen atom attached, are
arranged symmetrically in a plane, forming a regular hexagon. The hexagon
symbol, commonly used to represent the structural formula for benzene, implies
the presence of a carbon atom at each of the six angles and, unless
substituents are attached, a hydrogen at each carbon atom. Whereas the three
double bonds usually included in the formula are convenient in accounting for
the addition reactions of benzene, present evidence is that all the
carbon-to-carbon bonds are identical.
    Nearly all commercial benzene is a product of petroleum technology. The
gasoline fractions obtained by reforming or steam cracking of feedstocks from
petroleum contain benzene and toluene which can be separated economically.
Benzene may also be produced by the dealkylation of toluene.
    Benzene is a toxic substance, and prolonged exposure to concentrations in
excess of 35 - 100 parts per million in air may lead to symptoms ranging from
nausea and excess fatigue to anemia and leukopenia. See AROMATIC HYDROCARBON.
 �34s[Charles K. Bradsher]
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�6v�2v�32sBenzoic acid �21m �33s
    The aromatic carboxylic acid with formula C6H5COOH. Benzoic acid melts at
122.4 DG C (252.3 DG F), boils at 250 DG C (482 DG F), is slightly soluble in
water, and is relatively soluble in alcohol and ether. It is found free and
combined (gum benzoin; many berries) in nature. The acid of commerce is
prepared by hydrolysis of benzotrichloride, C6H5CCl3; by monodecarboxylation of
phthalic acid; and by oxidation of toluene. 
    Various derivatives of benzoic acid have applications as miticides, as
contrast media in urological and cholecystographic examinations, and in the
manufacture of pharmaceuticals. Benzoic acid is also used in curing tobacco;
as a preservative (sodium benzoate) of foods (juices and catsup); in dyes; as
a mordant in cloth printing (calico); in many esters (benzoates); and as a
reference standard in volumetric analysis. See BENZOATE; CARBOXYLIC ACID.
 �34s[Evans B. Reid]
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�6v�2v�32sBeriberi   �21m �33s 
    A disorder resulting from a deficiency of vitamin B1 (thiamine), due 
usually to idiosyncrasies of diet or excessive cooking or processing of food. 
Vitamin B1 is a coenzyme involved in decarboxylation of alpha-keto acids 
(pyruvic and alpha-ketoglutaric acid) and is important in normal carbohydrate 
metabolism. It also acts as a factor in maintaining the integrity of the 
normal conductivity of the nerve fiber for transmission of neural impulses. 
See CARBOHYDRATE METABOLISM; THIAMINE. 
    Beriberi is characterized by neurologic symptoms, such as tingling of the 
extremities and muscular weaknesses. Actual inflammation of the nerve fibers 
occurs and this neuritis, in turn, leads to other alterations in reflexes, in 
coordination, and in sensory perception, as well as the appearance of 
occasional psychic states. 
    When beriberi occurs suddenly, the symptomatology frequently centers about 
the heart. Derangement of rhythm, decreased blood pressure, the appearance of 
valvular murmurs, and electrocardiogram changes are typical. These may lead, 
sometimes precipitously, to cardiac failure and death. In many of these 
cardiac cases resulting from beriberi, there is associated digestive upset and 
irregularity and sometimes a paralysis of the diaphragm. A progressive edema 
of the legs which finally involves the entire body is a common occurrence in 
severe cases. 
 �34s[N. Karle Mottet]
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�6v�2v�32sBering Sea �21m �33s
    A water body north of the Pacific Ocean, 875,000 mi sup 2 (2,268,000 km sup
2) in area, bounded by Siberia, Alaska, and the Aleutian Islands. The Bering
Sea is a biologically productive area, with large populations of marine birds
and mammals. An active pollock fishery and a developing bottom-fish industry
are evidence of its rich biological resources. 
    The Bering Sea consists of a large, deep basin in the southwest portion,
where depths as great as 9900 ft (3000 m) are encountered. To the north and
east, an extremely wide, shallow continental shelf extends north to the Bering
Strait. The two major regions are separated by a shelf break, the position of
which coincides with the southernmost extent of sea ice in a cold season. Ice
is a prominent feature of the Bering Sea shelf during the cold months. Coastal
ice begins to form in late October, and by February coastal ice is found in
the Aleutians. The sea ice may extend as far south as 58 DG N. Thus, the ice
edge in the eastern Bering Sea advances and retreats seasonally over a
distance as great as 600 mi (1000 km). Ice-free conditions can be expected
throughout the entire region by early July. See SEA ICE.
    The main water connections with the Pacific are in the west of the
Aleutian Islands, the 6600-ft-deep (2000-m) pass between Attu and
Komandorskiye Islands and the 14,000-ft-deep (4400-m) pass between the
Komandorskiyes and Kamchatka. Aleutian passes also serve to exchange water.
The Bering Sea connection with the Arctic Ocean (Chukchi Sea) is the Bering
Strait, 53 mi (85 km) wide and 30 mi (45 m) deep.
    Tides in the Bering Sea are semidiurnal, with a strong diurnal inequality
typical of North Pacific tides. Three water masses are associated with Bering
Sea water - Western Subarctic, Bering Sea, and the Alaskan Stream. The general
circulation of the Bering Sea is counterclockwise, with many small eddies
superimposed on the large-scale pattern. The currents in the Bering Sea are
generally a few centimeters per second except along the continental slope, the
coast of Kamchatka, and in certain eddies, where somewhat higher values have
been found. See OCEAN CURRENTS; TIDE.
 �34s[Vera Alexander]
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�6v�2v�32sBerkelium �21m �33s
    Element number 97, symbol Bk, the eighth member of the actinide series of
elements. In this series the 5f electron shell is being filled, just as the 4f
shell is being filled in the lanthanide (rare-earth) elements. These two
series of elements are very similar in their chemical properties, and
berkelium, aside from small differences in ionic radius, is especially similar
to its homolog terbium. See RARE-EARTH ELEMENTS; TERBIUM.
    Berkelium does not occur in the Earth's crust because it has no stable
isotopes. It must be prepared by means of nuclear reactions using more
abundant target elements. These reactions usually involve bombardments with
charged particles, irradiations with neutrons from high-flux reactors, or
production in a thermonuclear device.
    Berkelium metal is chemically reactive, exists in two crystal
modifications, and melts at 986 DG C (1806 DG F). Berkelium was discovered in
1949 by S. G. Thompson. A. Ghiorso, and G. T. Seaborg at the University of
California in Berkeley and was named in honor of that city. Nine isotopes of
berkelium are known, ranging in mass from 243 to 251 and in half-life from 1
hour to 1380 years. The most easily produced isotope is sup 2 sup 4 sup 9 Bk,
which undergoes beta decay with a half-life of 314 days and is therefore a
valuable source for the preparation of the isotope sup 2 sup 4 sup 9 Cf. The
berkelium isotope with the longest half-life is sup 2 sup 4 sup 7 Bk (1380
years), but it is difficult to produce in sufficient amounts to be applied to
the studies of berkelium chemistry. See ACTINIDE ELEMENTS; TRANSURANIUM
ELEMENTS.
 �34s[Glenn T. Seaborg]
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�6v�2v�32sBermuda grass   �21m �33s 
    A long-lived perennial, Cynodon dactylon, native to the Old World. Bermuda 
grass is now common in all tropical and subtropical regions and was introduced 
to southern United States in colonial days. 
    Bermuda grass is propagated by surface runners, underground rootstocks, 
and seed. Common Bermuda grass grows 6 - 8 in. (15 - 20 cm) tall; it has 
short, flat leaves and somewhat wiry stems, and flowers are borne on slender 
spikes, three to six in a cluster. The grass grows on any soil that is fertile 
and not too wet, but does best on heavier soils. It requires warm weather for 
growth, and therefore is adapted to the southern half of the United States. It 
endures hot dry periods well and thrives under irrigation. Although formerly 
regarded as a pest in row crops, Bermuda grass is now grown as a valuable 
forage plant. See CYPERALES; GRASS CROPS. 
 �34s[Howard B. Sprague]
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�6v�2v�32sBernoulli's theorem   �21m �33s 
    The relation between fluid pressure p and fluid velocity v along a 
streamline in the steady flow of an incompressible, inviscid fluid. 
Bernoulli's theorem may be written as the equation below, where H is a 
constant along a  streamline, p is the mass density of the fluid, g is 
acceleration due to gravity, and z is vertical height. This equation is 

                 *** p/RHO + gz + (v sup 2)/2 = H ***

essentially an expression of conservation of energy because p/RHO is the 
pressure energy per unit mass of the fluid, gz is the potential energy per 
unit mass due to gravity, and (v sup 2)/2 is the kinetic energy per unit 
mass. It can also be regarded as an integral of momentum. See EULER'S 
MOMENTUM THEOREM. 
 �34s[Arthur E. Bryson, Jr.]
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�6v�2v�32sBeroida   �21m �33s 
    The only order of the class Nuda in the phylum Ctenophora. There is no 
trace of tentacles in any stage of development. The adults are bell-shaped and 
have a large mouth and pharynx. Meridional canals have many lateral branches. 
The polar fields are provided with papilliform processes. A common example is 
Beroee. See CTENOPHORA; NUDA. 
 �34s[Taku Komai]
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�6v�2v�32sBeryciformes �21m �33s
    An order of somewhat intermediate position among actinopterygian fishes.
Beryciformes, or Berycomorphi, have fin spines (see illustration) and ctenoid
scales, an upper jaw that is bordered by the premaxillae, and a ductless swim
bladder and lack a mesocoracoid.
    Beryciformes have a rich fossil history extending back to the Upper
Cretaceous. Recent representatives are classified in 3 suborders, 12 families,
some 30 genera, and about 135 species. They are cosmopolitan in tropical and
temperature seas; many are restricted to deep water but some are pelagic and
others live in shore waters. Most familiar of the beryciforms are the
squirrelfishes and soldierfishes, family Holocentridae, nocturnal animals
found in shallow tropical and subtropical reefs. Most are reddish in color.
See ACTINOPTERYGH; OSTEICHTHYES; TELEOSTEI.



 �34s[Reeve M. Bailey]
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�6v�2v�32sBeryl   �21m �33s 
    An aluminum beryllosilicate mineral, Al2[Be3-Si6O18], crystallizing in the 
hexagonal system. The hardness is 8 on Mohs scale, specific gravity 2.8, 
luster vitreous, and color bluish-green, light yellow, and less frequently 
green, pink, golden yellow, and less frequently green, pink, golden yellow, or 
colorless. It is easily confused with apatite. The rare scandium analog, 
Sc2[Be3Si6O18], is called bazzite. 
    Beryl and its gem varieties aquamarine (pale blue), goshenite (colorless), 
and morganite (pink) most frequently occur in granite pegmatite druses along 
with quartz, feldspar, lepidolite, topaz, and tourmaline, where the beryllium 
ions have concentrated in residual pegmatitic fluids. Beryl crystals up to 
many tons in weight have been recovered, and the mineral, constitutes an 
important ore of beryllium. Beryl occurs more rarely in mica schists and 
bituminous limestones (sometimes as emerald, the green gem variety of great 
value) and nepheline-syenites. Important occurrences include certain 
pegmatites in New England and California and pegmatites in Brazil, Madagascar, 
and Siberia. Emeralds occur in certain mica schists in the Urals and in 
bituminous limestones in Colombia. See BERYLLIUM; EMERALD. 
 �34s[Paul B. Moore]
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�6v�2v�32sBeryllium �21m �33s
    A chemical element, Be, atomic number 4, with an atomic weight of 9.0122.
Beryllium, a rare metal, is one of the lightest structural metals, having a
density about one-third that of aluminum. Some of the important physical and
chemical properties of beryllium are given in the table. Beryllium has a
number of unusual and even unique properties.
    The largest volume uses of beryllium metal are in the manufacture of
beryllium-copper alloys and in the development of beryllium-containing
moderator and reflector materials for nuclear reactors. Addition of 2%
beryllium to copper forms a nonmagnetic alloy which is six times stronger than
copper. These beryllium-copper alloys find numerous applications in industry
as nonsparking tools, as critical moving parts in aircraft engines, and in the
key components of precision instruments, mechanical computers, electrical
relays, and camera shutters. Beryllium-copper hammers, wrenches, and other
tools are employed in petroleum refineries and other plants in which a spark
from steel against steel might lead to an explosion or fire. See
ALKALINE-EARTH METALS; BERYLLIUM ALLOYS.
    Beryllium has found many special uses in nuclear energy because it is one
of the most efficient materials for slowing down the speed of neutrons and
acting as a neutron reflector. Consequently, much beryllium is used in the
construction of nuclear reactors as a moderator and as a support or alloy with
the fuel elements. See NUCLEAR REACTOR.
    The following list shows some of the principal compounds of beryllium.
Many of the compounds listed are useful as intermediates in the processes for
the preparation of ceramics, beryllium oxide, and beryllium metal. Other
compounds are useful in analysis and organic synthesis.

Acetylacetonate 			Fluoride, BeF2
Ammonium beryllium			Hydroxide
  fluoride				Nitrate,
Aurintricarboxylate			  Be(NO3)2 CTR DOT 4H2O
Basic acetate				Nitride, Be3N2
  BeO CTR DOT Be3(CH3COO)6		Oxide, BeO
Basic beryllium 			Perchlorate,
  carbonate				  Be(ClO4)2 CTR DOT 4H2O
Beryllate, Be2 sup 2 sup -		Plutonium-beryllium,
Beryllium ammonium			  PuBe13
  phosphate				Salicylate
Bromide, BeBr2				Silicates (emerald)
Carbide, Be2C				Sulfate, BeSO4 CTR DOT 4H2O
Chloride, BeCl2 			Uranium-beryllium,
Dimethyl, Be(CH3)2			  UBe13

    Beryllium is surprisingly rare for a light element, constituting about
0.005% of the Earth's crust. It is about thirty-second in order of abundance,
occurring in concentrations approximating those of cesium, scandium, and
arsenic. Actually, the abundances of beryllium and its neighbors, lithium and
boron, are about 10- sup 5 times those of the next heavier elements, carbon,
nitrogen, and oxygen. At least 50 different beryllium bearing minerals are
known, but in only about 30 is beryllium regular constituent. The only
beryllium-bearing mineral of industrial importance is beryl. Bertrandite
substitutes for beryl as a domestic source in the United States. See BERYL.
 �34s[Jack Schubert]
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    Metallurgy	Beryllium metallurgy involves two processes for the extraction
of beryllium oxide or hydroxide from beryl ore. There are also two methods
employed to reduce BeO to beryllium metal. The two extraction methods are
based on dissolving beryl as either a fluoride or a sulfate. Reduction is
accomplished thermally by means of magnesium with beryllium fluoride, and
electrolytically with beryllium chloride. Over 90% of the metal is made from
the thermal process.
    Vacuum-cast beryllium ingots either are produced as raw material for the
powder-metal process, which accounts for over 90% of the material being used,
or are cast directly for fabrication. Machined parts are usually made from
powder-metal products, while ingot beryllium is usually rolled or otherwise
worked. See BERYLLIUM.
    Beryllium can be welded, soldered, brazed, or plastically bonded, with
brazing and plastic bonding, as well as mechanical fasteners, being the usual
production methods. Surface protection of finished beryllium surfaces can be
provided by anodizing, plating, optical polishing, or using conversion
coatings. Chemical machining and chemical milling are used to provide
patterned and low-damage surfaces.
    Beryllium has had a long history of use in atomic energy as neutron
sources, reflectors, and moderators in thermal and intermediate reactors.
Applications of beryllium to aerospace usually provides weight saving up to
60% of that of a competing material on the same design basis. Extensive use of
beryllium in reentry structures depends on high-temperature strength and
thermal capacity. The metal's transparency to radiation (as used in x-ray
windows) also is important to missile structures, which must withstand
electromagnetic pulse conditions created by antimissile tactics. Finally,
ability to machine to high tolerance coupled with dimensional stability has
created almost exclusive employment of beryllium in inertial guidance
navigation and control devices.
 �34s[Wallace W. Beaver /W. Dean Trautman]
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�6v�2v�32sBeryllium alloys �21m �33s
    Dilute alloys of base metals which contain a few percent of beryllium in a
precipitation-hardening system. Although beryllium has some solid solubility
in copper, silver, gold, nickel, cobalt, platinum, palladium, and iron and
forms precipitation-hardening alloys with these metals, the copper-beryllium
system and, to a considerably lesser degree, the nickel-beryllium alloys are
the only ones used commercially. See BERYLLIUM.
    In addition to these precipitation-hardening systems, small amounts of
beryllium are used in alloys of the dispersion type wherein there is little
solid solubility (Al and Mg). Various amounts of beryllium combine with most
elements to form intermetallic compounds. Development of beryllium-rich alloys
has been chiefly confined to the ductile matrix Be-Al, Be-Cu solid solution
alloy with up to 4% Cu, and dispersed-phase-type alloys having relatively
small amounts of compounds (0.25 - 6%), chiefly as BeO or intermetallics, for
dimensional stability, elevated temperature strength, and elastic limit
control. See ALLOY.
    Primary applications of beryllium-copper alloys are found in the
electronics, automotive, appliance, instrument, and temperature-control
industries for electric current - carrying springs, diaphragms, electrical
switch blades, and other devices. Applications in structural aerospace and
nuclear fields are submarine repeater cable housings for transoceanic cable
systems, wind tunnel throats, liners for magnetohydrodynamic generators for
gas ionization, and scavenger tanks for propane-Freon bubble chambers in
high-energy physics research.
    Beryllium intermetallic compounds (beryllides) have high strength at high
temperature, good thermal conductivity, high specific heat, and good oxidation
resistance. Beryllides are formed with actinide and rare metals, as well as
with the transition metals. They are of interest to the nuclear field, to
power generation, and to aerospace applications. Evaluation of the
intermetallics as refractory coatings, reactor hardware, fuel elements,
turbine buckets, and high-temperature bearings has been carried out.
 �34s[Wallace W. Beaver /W. Dean Trautman]
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�6v�2v�32sBeryllium diseases �21m �33s
    The widespread use of beryllium in industry has been associated with
numerous instances of illness, often severe and fatal. Although toxic exposure
to beryllium or its salts may affect the entire body, the lung has been found
to be the most common target. Inhalation of toxic material in high
concentration may result in a nonspecific acute form of the disease. The
chronic form affects only a small proportion of workers and may occur after
much less intense exposure; it develops after a latent period ranging from
months to many years, and its effects range from mild respiratory distress to
complete incapacitation and death. The chronic disease may also affect
individuals indirectly exposed within the plant or living in its vicinity.
Various clinical and pathological features suggest that immune mechanisms, as
well as direct toxic effects, are involved in the pathogenesis of the chronic
disease.
    The acute form due to massive exposure may result in a fulminating
pneumonia appearing within 72 hours; exposure to lower doses produce a more
insidious illness developing over a period of about 2 weeks. Most patients
with acute beryllium pneumonia recover within 1 - 6 months, but death may
occur following intense overexposure as a result of severe pulmonary edema.
    Dermatitis and conjunctivitis may occur after skin or eyes have been
exposed to such soluble acid salts as beryllium sulfate or chloride.
Accidental introduction of a crystal of such a compound into a crack in the
skin may produce a chronic ulcer which resists healing until the crystal has
been extruded or removed. Similar chronic ulcerations (``lamp lesions'') were
observed following accidental cuts on broken fluorescent lamps during the
period when beryllium phosphors were still being used.
    After a latent period varying from a few months to more than 20 years
following exposure, the chronic form of the disease may develop. Only a
comparatively small proportion of exposed workers are affected, and most of
these give no history of prior acute disease. Onset is usually insidious, with
gradually developing difficulty in breathing (especially after exertion),
cough, fatigue, malaise, loss of appetite, and often marked loss of weight.
Remissions and exacerbations are common. At least one-third of the known cases
have died of their disease after periods ranging from a few months to more
than 15 years.See BERYLLUIM.
 �34s[David G. Freiman]
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�6v�2v�32sBessel functions �21m �33s
    The solutions of Bessel's differential equation, Eq. (1). Bessel
functions, also called cylinder func

z(sup 2) d(sup 2)/dz(sup 2) + zdy/dz + (z(sup 2) - v(sup 2))y = 0	     (1)

tions, are examples of special functions which are introduced by a
differential equation. Bessel functions are of great interest in purely
mathematical concepts and in mathematical physics. They constitute additional
functions which, like the elementary functions z(sup n), sin z, e(sup z), can
be used to express physical phenomena.
    Applications of Bessel functions are found in such representative problems
as heat conduction or diffusion in circular cylinders, oscillatory motion of a
sphere in a viscous fluid, oscillations of a stretched circular membrane,
diffraction of waves by a circular cylinder of infinite length or by a sphere,
acoustic or electromagnetic oscillations in a circular cylinder of finite
length or in a sphere, electromagnetic wave propagation in the waveguides of
circular cross section, in coaxial cables, or along straight wires, and in
skin effect in conducting wires of circular cross section. In these problems
Bessel functions are used to represent such quantities as the temperature, the
concentration, the displacements, the electric and magnetic field strengths,
and the current density as function of space coordinates. The Bessel functions
enter into all these problems because boundary values on circles
(two-dimensional problems), on circular cylinders, or on spheres are
prescribed, and the solutions of the respective problems are sought either
inside or outside the boundary, or both.
    The independent variable z in Bessel's differential equation may in
applications assume real or complex values. The parameter v is, in general,
also complex. Its value is called the order of the Bessel function. Since
there are two linearly independent solutions of a linear differential equation
of second order, there are two independent solutions of Bessel's differential
equation. They cannot be expressed in finite form in terms of elementary
functions such as z(sup n), sin z, or e(sup z) unless the parameter is one-half
of an odd integer. They can, however, be expressed as power series with an
exception for integer values of v. The function defined by Eq. (2) is
designated

J(sub v) (z) = (z/2)(sup v) SUMMATION (l=  to INFINITY) [(-z(sup 2))/4](sup l)/
			[l/GAMMA (nu + l + 1)]				     (2)

as Bessel's function of the first kind, or simply the Bessel function of order
v. GAMMA (nu + l + 1) is the gamma function. The infinite series in Eq. (2)
converges absolutely for all finite values, real or complex, of z. Along with
J(sub v)(z), there is a second solution J(sub -v)(z). It is linearly independent
of J(sub v)(z) unless v is an integer n. In this case J(sub -n)(z) = (-1)(sup n)
J(sub n)(z). See GAMMA FUNCTION.
    The Bessel function of the second kind, also called the Neumann function,
is defined by Eq. (3).

  Y(sub nu)(z) = {cos [nu pi J(sub nu)(z)] - J(sub -nu)(z)}/(sin nu pi)      (3)

    If v = n, this expression is indeterminate, and the limit of the right
member is to be taken. There are two Bessel functions of the third kind,
designated as first and second Hankel functions. They are defined as Eqs. (4).

     H(sub nu)(sup 1)(z) = J(sub nu)(z) + iY(sub nu)(z) 		     (4)

     H(sub nu)(sup 2)(z) = J(sub nu)(z) - iY(sub nu)(z)

    Figures 1 and 2 give the Bessel and Neuman function for for v = 0,1,2 and
positive values of z.

 �34s[Josef Meixner]
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�6v�2v�32sBeta rays �21m �33s
    Fast, charged particles emitted from certain radioactive nuclei. These
particles, which are all identical except for the sign of their charge, are
classified as positrons (+) and negatrons (-). The latter class is identical
with atomic electrons. The kinetic energies range from zero up to 3 - 5 MeV.
For the theory of BETA-decay see RADIOACTIVITY. See also ALPHA RAYS; ELECTRON;
POSITRON.
    Beta rays, often designated BETA-rays or BETA-particles, interact strongly
with matter: The particles are generally completely stopped in passing through
0.4 in./(1 cm) of solid material. If various thicknesses of some material are
placed across a beam of BETA-rays, the number of penetrating particles is
found to decrease gradually to zero as the thickness increases. There is a
definite thickness of absorber which is just sufficient to stop all the
BETA-particles. This quantity, which is called the range of the BETA-ray, is
usually expressed as the product px (g/cm sup 2) of density and thickness, since
then this ``range'' will be approximately independent of the material.
    The passage of BETA-rays through matter is macroscopically observable as
heat, due to the kinetic energy dissipated. This radiation also may promote
certain chemical reactions and cause structural changes in materials, for
instance, the discoloring of glass.
    The track of a fast electron, as observed in a photographic emulsion, does
not follow a straight line until the particle is stopped, but zigzags in a
random way because of collisions with the much heavier nuclei. A BETA-particle
loses its energy by excitation and ionization of atoms and at higher energy
also by emitting bremsstrahlung, or brake radiation, when scattered. See
BREMSSTRAHLUNG.
    Apart from the stopping process, which proceeds similarly, negatrons and
positrons behave differently in matter. Usually after it has been stopped, the
positron combines with an atomic negatron and both vanish (annihilate). The
two electron masses appear in the form of the electromagnetic energy of two
photons, each of energy 0.511 MeV, which are emitted in opposite directions in
order to conserve momentum. Before annihilation the positron-negatron pairs
form bound systems, called positronium atoms. See POSITRONIUM.
    Beta detectors may function so as to produce visible tracks of the
particles, as in photographic emulsions and cloud chambers, or may give a signal
at the advent of each individual particle.  See PARTICLE DETECTOR.
 �34s[Josef Meixner]
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�6v�2v�32sBeta-ray spectometer �21m �33s
    An instrument for measuring the energy or momentum distribution of
BETA-particles. The energy of an electron is in principle inferred from the
way it is influenced by a magnetic field. If a charged particle is moving
perpendicularly to the flux of a uniform magnetic field, its orbit is bent
into a circle of radius p. The momentum is found to be proportional to the
product BETA p of magnetic flux density and radius. See ELECTRON MOTION IN
VACUUM.
    A simple and generally used spectrometer is the semicircular type, shown
schematically in the illustration. From the electrons that leave the source in
all directions, a thin bundle (represented by an angular spread 2 SIGMA PHI in
the illustration) is selected by the entrance slit E to improve resolution.
Particles of a definite energy move in congruent circles and are brought to a
focus after going through an angle of 180 DG. Particles of different energies
come to a focus at different distances. The particles may be detected by means
of a photographic plate along the focal line x, which permits the simultaneous
recording of a large energy range. The angular spread 2 SIGMA PHI allowed by the
finite slit width introduces a spread DELTA x in the position at which the
particle strikes the photographic plate. Alternatively a Geiger counter may be
located at a fixed distance from the source and the field strength varied to
focus one energy after another on a slit in front of the counter.	[G.B.]





 �34s[Gunnar Backstrom]
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�6v�2v�32sBetatron �21m �33s
    A device for accelerating charged particles in an orbit by means of the
electric field E from a slowly changing magnetic flux PHI. The electric field
is given by E = (1/2 PIr sub 0) dPHI/dt (in SI or mks units), where r sub 0 is
the orbit radius. The name was chosen because the method was first applied to
electrons. In the usual betatron both the accelerating core flux and a guiding
magnetic field rise with similar time dependence, with the result that the orbit
is circular. However, the orbit can have a changing radius as acceleration
progresses. For the long path (usually more than 60mi or/100 km), variations
of axial and radial magnetic field components provide focusing forces, while
space charge and space current forces due to the particle beam itself also
contribute to the resulting betatron oscillations about the equilibrium orbit.
In many other instances of particle beams, the term betatron oscillation is
used for the particle oscillations about a beam's path.
    Collective effects from self-fields of the beam have been found important
and helpful in injecting. Circulating currents of about 3 amperes are
contained in the numerous industrial and therapeutic betatrons, although the
average currents are below 10-sup7 A. See PARTICLE ACCELERATOR.
 �34s[Donald W. Kerst]
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�6v�2v�32sBetelgeuse �21m �33s
    Alpha Orionis, a very bright, cool, red supergiant star, spectral type M2,
temperature near 3000 K. Typical of the low-temperature end of the supergiant
sequence, Betelgeuse varies in light and velocity in periods of about 6 years.
Because of high luminosity and low temperature, it has an enormous radius
(about as big as the orbit of Mars), 300 times that of the Sun. The angular
diameter was measured by the stellar interferometer on Mount Wilson and found
to be variable, 0DOUBLE PRIME.034 - 0DOUBLE PRIME.047 of arc - the largest
among the half-dozen stars with measurable disks. The star's mean density is
only 5 x 10 sup- sup 7 g/cm sup 3. Partly because of low surface gravity, the
atmosphere is unstable and shows outward streaming of matter at low
temperature and density. See STAR.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sBetel nut �21m �33s
    The dried, ripe seed of the palm tree Areca catechu (Palmae), a native of
Ceylon and Malaya. The nuts, slightly larger than a chestnut, have a faint
odor when broken open and a somewhat acrid taste. Betel nuts are chewed by the
inhabitants together with the leaves of the betel pepper, Piper betle
(Piperaceae), and lime. The mixture discolors and eventually destroys the
teeth. Frequently, habitual chewers of the nut are toothless by the age of 25.
The seeds contain a narcotic that produces some stimulation and a sense of
wellbeing. See NARCOTIC; PALM; PIPERALES.
 �34s[Perry D. Strausbaugh /Earl L.C.]
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�6v�2v�32sBevel gears �21m �33s
    Where shafts intersect, bevel gears transmit the motion. Such gears may be
used only to change the shaft axis direction or to change speed as well as
direction. Two bevel gears with equal numbers of teeth and running together
with their shaft axes intersecting at 90 DG are called miter gears. Several
forms of bevel gears are in use, including straight-tooth, spiral, and skewed
bevel gears (see illustration).
    External bevel gears have pitch angles less than 90 DG. Internal bevel
gears have pitch angles greater than 90 DG, hence their pitch cones are
inverted. A crown gear is one having a pitch angle of 90 DG. Thus its pitch
surface is a plane, and the crown gear corresponds in this respect to a rack
in spur gearing.

    Straight bevel gears.  The simplest form of bevel gear has straight teeth
which, if extended inward, would come together at the intersection of the
shaft axes. Speeds of the shafts of bevel gears (velocity ratio) are inversely
proportional to the numbers of teeth on the gears or to the sines of the pitch
angles, but not to the formative number of teeth. Use of straight bevel gears
is limited to low-speed operations, ordinarily below 1000 surface feet per
minute or, in the case of small gears, 1000 revolutions per minute.

    Spiral bevel gears.  To provide a gradual engagement, as contrasted to the
full line engagement of straight bevel gears, the teeth of spiral bevel gears
are curved and oblique. Theoretically the curve is a spiral but, to facilitate
manufacture, the curve is actually a circular arc which, within the tooth face
width, closely approximates a spiral. This tooth inclination brings more teeth
in contact at any one time than with an equivalent straight-tooth bevel gear.
The result is smoother and quieter operation, particularly at high speeds, and
greater load-carrying ability than with straight bevel gears of the same size.
Spiral bevel gears are used in sewing machines, motion picture equipment,
machine tools, and other applications where quiet, smooth operation is
essential.

    Zero bevel gears. A special form of bevel gear has curved teeth with a
zero-degree spiral angle. Thus the teeth are not oblique as is the case with
spiral bevel gears. Rather, the teeth lie in the same general direction as those
of an equivalent straight-tooth bevel gear, and so the gears are usually used
in the same types of drives as the straight-tooth gear.

    Hypoid gears. To connect nonparallel, nonintersecting shafts, usually at
right angles, hypoid gears are used. The shape of the tooth is similar to that
of the spiral bevel gear and gives progressive contact across the tooth. In
operation these gears run even more smoothly and quietly than spiral bevel
gears. To maintain line contact of the teeth with the offset shaft, the pitch
surface of the hypoid gear is a hyperboloid of revolution rather than a cone as
in bevel gears. The operating smoothness of hypoid gears, along with the lower
body lines made possible by the offset pinion shaft, has made them extremely
popular for automotive use. 						     
[J.R.Z.] 

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�6v�2v�32sBicosoecida �21m �33s
    An order of Zoomastigophorea (Protozoa). They are colorless, free-living
cells, each with two flagella, one of which is used for attaching the organism
to its exoskeleton (lorica). Although the attachment is normally from the
front end, the flagellum emerges alongside the primary or vibrating one (see
illustration). The anterior end in many species (Codomonas annulata,
Poteriodendron petiolatum, and Stephanocodon stellatum) is ameboid, or at
times appears to be formed into a lip which can turn in and engulf a bacterium.
    These Bicosoecida are common in fresh water and often seen attached to
desmids or other algae. Bicosocca mediterranea is common at times in salt
water, where assemblages of it are found on diatoms. The order has very few
genera and species. See PROTOZOA; ZOOMASTIGOPHOREA.



 �34s[James B. Lackey]
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�6v�2v�32sBig bang theory �21m �33s
    The theory that the universe began in a state of compression to infinite
density and has been expanding since some particular instant that marked the
origin of the universe. The big bang is the most generally accepted
cosmological theory.
    Two observations are at the base of observational big bang cosmology.
First, the universe is expanding uniformly, with objects at greater distances
receding at a greater velocity. Second, the Earth is bathed in an isotropic
glow of radiation that has the characteristics expected from the remnant of a
hot pri meval fireball. See HUBBLE CONSTANT.
    Tracing the expansion of the universe back in time, it is found that the
universe was compressed to infinite density ap proximately 1.3 - 2 x
10 sup 1 sup 0 years ago. According to the leading theories of cosmology, at
that time a ``big bang'' began the universe. It was the origin of time and
space. From the time of the big bang, the universe has been expanding.
    In 1917 Einstein found a solution to his own set of equations from his
general theory of relativity that predicted the nature of the universe. His
universe, though, was unstable: it could only be expanding or contracting.
This seemed unsatisfactory at the time, for the expansion had not yet been
discovered, so Einstein arbitrarily introduced a special term - the
cosmological constant - into his equations to make the universe static. The
need for the cosmological constant disappeared with Hubble's discovery of the
expansion.
    In 1922 A. Friedmann worked out solutions that are at the basis of the
cosmological models that are now generally accepted. G. Lemaitre in 1927
independently found similar solutions. Lemaitre's solutions indicated that the
original ``cosmic egg'' from which the universe was expanding was hot and
dense. This is the origin of the current view that the universe began from a
hot big bang. Friedmann's solution did not involve the cosmological constant
and Lemaitre's did, but otherwise the solutions were similar.
    Modern theoretical work has been able to trace the universe back to the
first instants in time after the big bang. At the earliest instants, the
universe may have been filled with exotic elementary particles. Individual
quarks may also have been present. By 1 microsecond after the big bang, the
exotic particles and the quarks had been incorporated in other fundamental
particles. See ELEMENTARY PARTICLE; QUARKS.
    It is not definitely known why there is an apparent excess of matter over
antimatter, though attempts in elementary particle physics to unify the
electromagnetic, the week, and the strong forces show promise in explaining
the origin of the matter-antimatter asymmetry. The asymmetry seems to have
arisen before the first millisecond after the big bang. See ANTIMATTER;
FUNDAMENTAL INTERACTIONS.
    By 5 s after the big bang, the temperature had cooled to 10 sup 9 K, and
only electrons, positrons, neutrinos, antineutrinos, and photons were
important. A few protons and neutrons were mixed in, and they grew relatively
more important as the temperature continued to drop. The universe was so dense
that photons traveled only a short way before being reabsorbed. By the time 4
min had gone by, elements had started to form.
    After about a million years, when the universe cooled to 3000 K and the
density dropped sufficiently, the protons and electrons suddenly combined to
make hydrogen atoms, a process called recombination. Since hydrogen's spectrum
absorbs preferentially at the wavelengths of sets of spectral lines rather
than continuously across the spectrum, and since there were no longer free
electrons to interact with photons, the universe became transparent at that
instant. The average path traveled by a photon - its mean free path - became
very large. The blackbody spectrum of the gas at the time of recombination was
thus released and has been traveling through space ever since. As the universe
expands, this spectrum retains its blackbody shape though its characteristic
temperature drops. See HEAT RADIATION.
    As the early universe cooled, the temperatures became sufficiently low for
element formation to begin. By about 100 s, deuterium (one proton plus one
neutron) formed. When joined by another neutron to form tritium, the amalgam
soon decayed to form an isotope of helium. Ordinary helium, with still another
neutron, also resulted.
    Big bang nucleosynthesis, although at first thought to be a method of
forming all the elements, founders for the heavy elements at mass numbers 5
and 8. Isotopes of these mass numbers are too unstable to form heavier
elements quickly enough. The gap is bridged only in stars. Thus the lightest
elements were formed as a direct result of the big bang, while the heavier
elements as well as additional quantities of most of the lighter elements were
formed later in stars or supernovae. See NUCLEOSYNTHESIS.
    The two basic possibilities for the future of the universe are that the
universe will continue to expand forever, or that it will cease its expansion
and begin to contract. It can be shown that the case where the universe will
expand forever corresponds to an infinte universe. The term applied is the
open universe. The case where the universe will begin to contract corresponds
to a finite universe. The term applied is the closed universe. Several lines
of evidence agree that the universe is open, including studies of differential
velocities of nearby galaxies due to density perturbations, the cosmic
abundance of deuterium, and the absence of hot intergalactic gas. The possible
existence of neutrino mass is a piece of evidence favoring the opposite
conclusion. See COSMOLOGY.
 �34s[Jay M. Pasachoff]
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�6v�2v�32sBile acid �21m �33s
    A steroid acid produced in the liver. The bile acids lower surface tension
and promote emulsification of fat to aid in digestion, and absorption of fats
from the intestine. The bile acids have a five-carbon side chain and occur in
the liver in combination, through peptide linkage, with glycine and taurine,
forming glycocholic acid and taurocholic acid, respectively (see
illustration). Other bile acids are cholic, deoxycholic, lithocholic, and
chenodeoxycholic acids. See LIVER; STEROID; STEROL.



 �34s[Ralph I. Dorfman]
�33s


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�6v�2v�32sBilirubin   �21m �33s 
    The predominant orange pigment of bile. It is the major metabolic 
breakdown product of heme, the prosthetic group of hemoglobin in red blood 
cells, and other chromoproteins such as myoglobin, cytochrome, and catalase. 
The breakdown of hemoglobin from the old red cells takes place at a rapid rate 
in the reticuloendothelial cells of the liver, spleen, and bone marrow. The 
steps in this breakdown process include denaturation and removal of the 
protein globin, oxidation and opening of the tetrapyrrole ring, and the 
removal of iron to form the green pigment biliverdin, which is then reduced to 
bilirubin by the addition of hydrogen. The formed bilirubin is transported to 
the liver, probably bound to albumin, where it is conjugated into 
water-soluble mono- and diglucuronides and to a lesser extent with sulfate. 
See LIVER. 
    In mammalian bile essentially all of the bilirubin is present as a 
glucuronide conjugate. Bilirubin glucuronide is passed through the liver cells 
into the bile caniculi and then into the intestine. The bacterial flora 
further reduces the bilirubin to colorless urobilinogen. Most of the 
urobilinogen is either reduced to stercobilinogen or oxidized to urobilin. 
These two compounds are then converted to stercobilin, which is ex creted in 
the feces and gives the stool its brown color. See HEMOGLOBIN. 
 �34s[Morton K. Schwartz]
�33s 
  
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�6v�2v�32sBinary number system   �21m �33s 
    A system in which numbers are represented as linear combinations of powers 
of 2. Below is shown the relationship between the binary system and the 
decimal system. 
  
     3 = 1 x 2sup1 + 1s= 11 _ 7 = 1 x 2sup2 + 1 x 2sup1 + 1s= 111 _17 = 1 x 
2sup4 + 0 x 2sup3 + 0 x 2sup2 
  
    The binary system has come into importance because of its value in 
computer applications. See NUMBER SYSTEMS. 
 �34s[Charles D. Oviatt]
�33s 
  
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�6v�2v�32sBinary star �21m �33s
    Two stars held together by their mutual gravitational attraction in a
permanent (or at least long-term) association. The term binary system would
actually be more appropriate. The two components of the binary system revolve
in close elliptical (or circular) orbits around their common center of
gravity, which carries them through space so that they also have a common
proper motion. Usually treated together with binary stars are multiple
systems, consisting of several stars.
    The formation of binary stars must be a very common process related to
stellar formation itself. According to L. Lucy, the bulk of binary systems are
probably formed by successive fragmentations of protostars.
    Binary stars play an important role in astrophysics for three reasons: (1)
They are a very common phenomenon. It is more likely for a given star to be a
member of a binary or multiple system than to be single. (2) Binary systems
reveal much more information about stellar properties than do single stars. In
particular, masses of stars can be measured only by their gravitational
attraction on nearby bodies. The radii, luminosities, and effective
temperatures of stars in binary systems can also be obtained, and the mutual
relations of these quantities can then be studied. (3) In binary stars where
the components interact strongly, new phenomena and new types of stars occur
which otherwise would not exist: novae, cataclysmic variables, and binary
x-ray sources are examples. See HERTZSPRUNG-RUSSELL DIAGRAM; MASS-LUMINOSITY
RELATION.
    Separations vary over an enormous range from one binary system to another.
The upper limit depends critically on the density of stars in space. The lower
limit on separations is given by the physical dimensions of the component
stars.

    Observational techniques.  With the enormous range of separations and
periods, it is not surprising that different observational techniques must be
used. (1) Systems of large separations are often observable directly through a
telescope as two separate stars. They are then called visual binary stars. (2)
For a closer pair, the orbital velocities may be sufficiently large to be
detectable by a periodic shift of spectral lines caused by the Doppler effect.
These objects are called spectroscopic binaries. (3) When the two stars are so
close to each other that their dimensions are not negligible compared to their
separation, they can eclipse each other, provided that the orbital plane is
suitably oriented. These objects are then easily detected photometrically from
the periodic changes of the light received from the system, and are called
eclipsing binaries or photometric binary stars. See DOPPLER EFFECT.

    Evolution. Stars spend most of their lifetime on the main sequence,
generating energy by conversion of hydrogen into helium in their cores. When
the hydrogen in the core is exhausted, the stars expand. Single stars
encounter no problem in this expansion, but the gravitational field of a
nearby companion sets a definite upper limit to the volume available for
expansion to a binary star component; it also considerably distorts the shape
of the expanded star. Systems in which evolution is substantially affected by
the duplicity are called close binary systems. When the expanding component
reaches the limiting surface, the evolution of both stars is completely
changed. See STELLAR EVOLUTION.
    The sequence of shapes through which an expanding component passes is
shown in the illustration. The curves drawn are cross sections of three-
dimensional equipotential surfaces, for a system, with stars of masses M and
M', viewed edge-on. When a star is still small with respect to the dimensions
of the system, it is very nearly spherical. As it expands, its shape becomes
that of a distorted ellipsoid more and more extended toward the companion,
until the star reaches the critical surface marked with a heavy line in the
illustration. The next equipotential surface already surrounds both stars. A
star that has reached its critical surface will lose mass in the vicinity of
the point L1 (the first lagrangian point) where the effective gravity is zero,
and a gas stream will flow from there toward the other star. The stream either
impacts directly on the companion or leads to the formation of an accretion
disk surrounding it. In either case, the star gains (accretes) mass either
directly from the stream or through the disk, in which viscosity makes some
particles fall on the star.

    X-ray binaries. Since 1970, x-ray satellites have discovered a number of
strong sources of x-rays clearly associated with binary systems: the x-ray
source undergoes eclipses or the optical counterpart is a binary, or both.
The picture which has been developed from study of these systems is as
follows. The optically visible star is unstable and loses mass, either by
stellar wind or by Roche lobe overflow. The stream flows to the compact star
and falls into a very deep "potential well," since the compact star is
extremely small. Usually the accretion occurs through a viscous disk, but
eventually always a large fraction of the potential energy of the particles
is released as radiation. The hot gas reaches temperatures of order 10 sup 8 -
10 sup 9 DG F (50 - 500 X 10 sup 6 DG C) and radiates mostly in x-rays. Some
of the x-ray binary sources are among the most powerful energy emitters in
the Galaxy. The regular rapid pulses of x-rays are believed to be generated
by a rapid rotation of the magnetized neutron star. See PULSAR; X-RAY STAR.
    The neutron stars found in binary x-ray sources are thought to be remnants
of supernova explosions that in the past terminated the normal stellar life-
time of their parental stars. It is also believed that very massive stars
leave black holes as the remnants after their supernova explosion, rather than
neutron stars. See BLACK HOLE.

    Cataclysmic variables. Probably all novae are close binary systems. Novae
are a special class (conspicuous because of a very high increase in light in
an outburst) of cataclysmic variables. Other classes include recurrent novae
and dwarf novae. All these objects appear to be binaries of a basically
similar nature: a cooler object, usually a red dwarf, is unstable and
transfers mass to a hotter and much more compact object, a degenerate white
dwarf or a similar body. A viscous disk surrounding the accreting star plays
an important role. Variable rates of mass transfer, variable viscosity, and
continual interaction between the star, the disk, and the stream lead to
instabilities and eruptive events. In the more violent cases, thermonuclear
reactions at the surface of the accreting star are believed to be the source
of the energy for a nova outburst. See NOVA; VARIABLE STAR.


_FIELD034;-237[Mirek Plavec]_FIELD034
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�6v�2v�32sBinaural sound   �21m �33s 
    A sound-reproducing system in which sound is recorded or transmitted by 
using two microphones mounted at the ears of a dummy human head. To preserve 
the binaural effect, the sound must be monitored by a listener wearing a set 
of earphones identically spaced. In an ideal binaural transmission system, 
both the amplitude and phase of the sound waves incident on the dummy's ears 
are duplicated at the listener's ears. 
    Binaural systems have been made so perfect that the listener is unable to 
distinguish the monitored sound from the real sound. For example, when a 
person walks around the dummy head, the listener, upon hearing the footsteps, 
has the compelling illusion of someone walking around him. No other sound 
system thus far devised can even approximate such an effect. 
    Binaural sound-reproducing systems may never become popular because of the 
inconvenience of having to wear a set of earphones during the rendition of a 
recording to obtain the true binaural effect. See SOUND-REPRODUCING SYSTEMS. 
 �34s[Harry F. Olson]
�33s 
  
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�6v�2v�32sBinoculars �21m �33s
    Optical instruments designed for use with both eyes to give enhanced views
of distant objects. The distinguishing performance feature of binoculars,
compared to monocular instruments, is the depth perception obtainable.
    When an object is viewed with both eyes, the distance of approximately 25
in. (6.5 cm) between the eyes requires each to view the object from different
angles. To bring the images onto the fovea centralis of each eye, the eyes
rotate inward until the lines of sight converge at the object. This
convergence is the signal to the brain which stimulates the sense of depth,
and its magnitude is the parameter from which judgment of depth is made. The
minimum difference between convergence angles which the average adult can
detect is about 30 seconds of arc, which is approximately the value
corresponding to an object at 500 yd (450 m). The convergence angles for more
distant objects are less, and therefore objects beyond 500 yd cannot be
resolved stereoscopically.
    With binoculars this range can be greatly extended. Binoculars employing
prism-erecting systems (see illustration) can be so constructed that the
distance between objectives is twice that between the eyepieces. Hence, if the
convergence angle at an object subtended by the unaided eyes is ALPHA, the
angle subtended by the binocular objectives is 2ALPHA. This angle in the real
field of the binocular becomes 2MALPHA in the apparent field, M being the
magnification of the binocular. Thus the minimum covergence angle in the real
field becomes 30 seconds/2M, and the maximum range at which objects can be
resolved stereoscopically, using, for example, a seven-power binocular, is
extended to about 7000 yd (6500 m).



 �34s[Harold E. Rosenberger]
�33s


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�6v�2v�32sBinomial theorem �21m �33s
    One of the most important algebraic identities, with many applications in
a variety of fields. The binomial theorem, discovered by Isaac Newton,
expresses the result of multiplying a binomial by itself any number of times:

(a + b)(sup n) = a(sup n) + c(sub 1)a(sup n-1)b + c(sup 2)a(sup n-2)b(sup 3)

		+ . . . + csubr a(sup n-r)b(sup r) + . . . b(sup n)

where the coefficients c1, c2, c3, . . ., c(sub r) . . . are

	    c1 = n/1
	    c2 = n(n - 1)/(1 x 2)
	    c3 = n(n - 1)(n - 2)/(1 x 2 x 3)
	    c(sub r) = [n(n - 1)(n - 2) . . . (n - r + 1)]/(1 x 2 x 3 . . . r)

The standard notation for these coefficients is

	c1 = (n over 1), c2 = (n over 2), . . . , c(sub r) = (n over r)

Here (n over r) is the coefficient of the term containing b(sup r) in the
expansion of (a + b)(sup n). It is a fraction with the numerator and denominator
each containing r factors; those in the denominator begin with 1 and increase
by 1; those in the numerator begin with n and decrease by 1. It is easily shown
that

		     (n over r) = [n over (n - r)]

    Under suitable conditions the binomial formula is valid when n is not a
positive integer. In this case the formula does not terminate, but generates
an infinite series.
    Much of the utility of the binomial theorem stems from the	properties of
the coefficients. In particular, the coefficient (n over r) gives the number of
combinations of n distinct objects taken  r at a time. The set of coefficients
for any value of n forms a distribution that has fundamental importance in the
study of probability and statistics. See ALGEBRA; DISTRIBUTION (PROBABILITY);
SERIES.
 �34s[Hollis R. Cooley]
�33s

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�6v�2v�32sBioacoustics   �21m �33s 
    Bioacoustics in its broadest sense treats of the role of sound in the 
behavior of living things. Hearing and speech of human beings constitute a 
large segment of this discipline. Yet the acoustical properties of the 
biological media making up living systems of all kinds wholly aside from the 
ear and vocal mechanisms have considerable importance. The use of sound 
(particularly at high frequency) in the diagnosis and treatment of disease has 
necessitated the acquisition of precise knowledge of the acoustical properties 
of animal tissue.  See  ACOUSTIC MICROSCOPE; ACOUSTICAL HOLOGRAPHY; HEARING 
(HUMAN); PHONORECEPTION; PHYSIOLOGICAL ACOUSTICS; PSYCHOACOUSTICS. 
    Since all mammals and many other animal species can generate and detect 
sound, the study of the mechanisms responsible for these activities and the 
way they are used in animal behavior has become of increasing importance. 
Bats, whales, and porpoises emit sounds which on striking obstacles are 
reflected; the reflected sound can be detected by the emitting animal and used 
in the location of the obstacles. This is known as echolocation or animal 
sonar. It is widely used by the animals in question to avoid obstacles in 
their motion, to locate prey, and to communicate with other animals of the 
same species. 
 �34s[R. Bruce Lindsay]
�33s 
  
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�6v�2v�32sBioassay �21m �33s
    A method for quantitatively determining the concentration of a substance
(or class of substances) by its effect on the growth of a suitable animal,
plant, or microorganism under controlled conditions. The substance estimated
may be stimulatory or essential for growth, such as a vitamin or an amino
acid, or it may be inhibitory to growth, for example, the antibiotic
penicillin. In its application to the estimation of vitamins, amino acids, and
trace elements, the method is an outgrowth of studies of the nutritional
requirements of animals and microorganisms. Such studies have revealed that
nutritional requirements of exacting microorganisms are similar to those of
higher animals. 				       [B.M.G.;E.E.S.]
 �34s[Beverly M. Guirard ;Esmond E. Snell]
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�6v�2v�32sBiochemistry �21m �33s
    The study of the substances and chemical processes which occur in living
organisms. It includes the identification and quantitative determination of
the substances, studies of their structure, determination of how they are
synthesized and degraded in organisms, and elucidation of their role in the
operation of the organism. Some processes of particular interest are the
conversion of foods to energy, respiration, the synthesis of nucleic acids and
proteins, and the regulation of the chemical activities of cells and organisms.
    Carbohydrates are a class of substances which includes simple sugars such
as glucose, large polysaccharides such as cellulose, and starch. Carbohydrates
are an important source of energy and structural materials in organisms. See
CARBOHYDRATE; CELLULOSE.
    Proteins are intimately involved in all life processes. Some function as
biological catalysts called enzymes, while others may act as carriers of
material, as in the case of hemoglobin, which transports oxygen in many
animals. Some proteins act as structural material, for instance, as portions
of cellular membranes and as the major component of hair, horn, skin, and
feathers. Contractile proteins are involved in muscular activity and cell
division. See AMINO ACIDS; ENZYME; MUSCLE PROTEINS; PROTEIN.
    There are two classes of nucleic acids, deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). The sequential array of the nucleotides which compose
these large molecules constitutes the form in which genetic information is
stored and transferred. See DEOXYRIBONUCLEIC ACID (DNA); NUCLEIC ACID;
RIBONUCLEIC ACID (RNA).
    Lipids are a diverse group of ``greasy'' substances which are soluble in
organic solvents but insoluble in water. Cellular membranes are composed
mainly of a class of lipids called phospholipids. The most common lipids are
the fats. Some other lipids are cholesterol and steroid hormones. See LIPID.
    A number of minerals are essential to bodily functions, and trace metals
have been found to be constituents of certain enzymes. Vitamins and hormones
are substances which are biologically effective in small amounts. See CALCIUM
METABOLISM; COENZYME; ENDOCRINE SYSTEM (VERTEBRATE); HEMOGLOBIN; HORMONE;
PHOSPHATE METABOLISM; VITAMIN.
    All of these substances are involved in a variety of biochemical
processes. Investigations have sought to explain: the chemical steps involved
in the biological breakdown of sugars, fats, and amino acids; the relationship
of enzyme structure and catalytic activity; genetics in terms of molecular
biology; the biochemical bases of disease; the regulation of biological
processes; selective permeability, active transport of materials, and transfer
of information across biological membranes; and photosynthesis and nitrogen
fixation. See BIOLOGICAL OXIDATION; CARBOHYDRATE METABOLISM; CELL MEMBRANES;
DISEASE; GENETIC CODE; LIFE, ORIGIN OF; LIPID METABOLISM; MOLECULAR BIOLOGY;
NITROGEN CYCLE; PHOTOSYNTHESIS; PROTEIN METABOLISM.
    The development of biochemistry closely parallels developments in methods
of separately and quantitatively measuring small amounts of substances and of
determining the structures of substances. See ACTIVATION ANALYSIS;
AUTORADIOGRAPHY; CHROMATOGRAPHY; GAS CHROMATOGRAPHY; HISTORADIOGRAPHY; MASS
SPECTROMETRY; RADIOCHEMISTRY; SPECTROPHOTOMETRIC ANALYSIS; SPECTROSCOPY;
ULTRACENTRIFUGE; X-RAY CRYSTALLOGRAPHY.
 �34s[A. S. L. Hu]
�33s

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�6v�2v�32sBioelectric model �21m �33s
    A conceptual model explaining electrical phenomena of living cells and
tissues in terms of physical and structural principles. The phenomena include
the excitation and electrical responses of nerves and muscles and the
discharge of electricity at relatively high voltage by electric fishes such as
the electric eel, Torpedo. See ELECTRIC ORGAN (BIOLOGY).
    The first successful model of this type was proposed for nerves by J.
Bernstein, on the basis of W. Nernst's theory of diffusion potentials and on
two facts about nerves. First, when one end of an excised nerve is crushed or
treated with toxic agents, a potential difference develops between the damaged
end and the undamaged portion, the damaged end being negative. Second, the
magnitude of this ``demarcation potential'' depends on the concentration of
KCl in the medium bathing the nerve. Bernstein proposed that the demarcation
potential is a membrane-diffusion potential dependent on a difference in KCl
concentration between the interior and exterior of the undamaged nerve; in the
damaged portion of the nerve, this concentration difference is abolished.
    Neuron theory, and the theory of electrochemical equilibrium developed by
Willard Gibbs and applied to membrane systems by F. G. Donnan, led to a
modification of the expression of Bernstein's hypothesis. The nerve axon was
seen in neuron theory as a cylinder surrounded by a membrane separating the
axoplasm or protoplasm of the nerve cell or neuron from the external saline
medium. The potential measured on the whole nerve was seen as the expression
of potentials in individual axons. The axon membrane was assumed to be
permeable only to potassium ion (K sup +) and impermeable to the other major
ions of the medium and axoplasm. The demarcation potential results from damage
to the membrane, with resultant increase in permeability to other ions. The
potential difference across the undamaged membrane is seen as a potassium
equilibrium potential, dependent upon the concentrations of K sup + in the
external medium and in the axoplasm. Calculations on this basis accurately
predict observed changes in potential for given changes in external K sup +
concentration. See BIOPOTENTIALS AND ELECTROPHYSIOLOGY; DONNAN EQUILIBRIUM;
NEURON.
    A. L. Hodgkin and A. F. Huxley developed a new model of the axon membrane:
The ion pump is regarded as nonelectrogenic; it is assumed that Na sup + is
exchanged for K sup + on a 1:1 basis, and hence no net transfer of charge, or
current, is produced by the pump. The electromotive force across the neuron
membrane remains, as in the Bernstein model, a diffusion potential resulting
from differential distribution of K sup +. Excitation, normally initiated by
partial depolarization of the membrane, results in a sudden increase in
permeability to Na sup +. The resulting inflow of Na sup +, driven by the
electric potential and by the difference in concentration of Na sup +, carries
enough positive charge across the membrane to reverse the membrane potential
locally, and this is accompanied by a delayed increase in permeability to K sup
+. Electrical and concentration forces now drive K sup + out of the axon,
carrying positive charge which restores the membrane potential to its former
resting level. During the period of recovery, the normal ion balance, locally
disturbed, is restored by the ion pump without change in potential. The action
potentials of muscle fibers can also be accounted for by minor modifications
of the theory.
    The classical picture of the plasma membrane of a cell is that of a lipoid
bilayer, a layer of fatty materials two molecules thick. This model is
consistent with a large body of evidence, but other evidence has supported the
mosaic concept, accord ing to which the lipoid bilagen is interrupted by
pores, iobide may contain molecules of protein and fixed electric charges. The
ion pump which exchanges Na sup + for K sup + utilizing metabolic energy has
been J identified with a form of the enzyme adenosinetriphosphatase (ATPase).
 �34s[Bradley T. Scheer]
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�6v�2v�32sBioelectromagnetics �21m �33s
    The study of the interactions of electromagnetic energy (usually referring
to frequencies below those of visible light) with biological systems. This
includes both experimental and theoretical approaches to describing and
explaining biological effects. Diagnostic and therapeutic uses of
electromagnetic fields are also included in bioelectromagnetics.
    The induction of cataracts is commonly associated with exposure of animals
to intense microwave fields. Although heating the lens of the eye with
electromagnetic energy can cause cataracts, the threshold for cataract
production is so high that, if the whole animal were exposed to the
cataractogenic level of radiation, it would usually die before cataracts were
produced.
    In 1961 it was reported that people can ``hear'' pulsed mi crowaves at
very low averaged power densities (50 microwatts/cm sup 2 ). It is now generally
accepted that the perceived sound is caused by elastic-stress waves which are
created by rapid thermal expansion of the tissue that is absorbing microwaves.
Most biological effects of microwaves can be explained by the response of the
animal to the conversion of electromagnetic energy into thermal energy within
the animal. However, a few experiments yield results that are not readily
explained by changes of temperature.
    There are reports that microwave irradiation at very low intensities can
affect behavior, the central nervous system, and the immune system, but many
of these reports are controversial. Animals exposed to more intense
electromagnetic fields that produce increases in body temperature of
1.8 DG F(1 DG C) or higher (thermal load equal to one to two times the animal's
basal metabolic rate) demonstrate modification of trained behaviors and
exhibit changes in neuroendocrine levels. Exposure of small animals to weaker
fields has been shown to produce some changes in the functioning of the
central nervous system and the immune system.
    In an industrial society, the effects of stationary electric and magnetic
fields, and of extremely low-frequency fields are important because of the
ubiquitous nature of electricity. Both strong and weak magnetic fields are
being studied - the former to determine if they are hazardous to those who
work near cyclotrons, magnetohydrodynamic devices, or isotope-separation
facilities; the latter to understand how magnetic fields interact with
biological systems, and to appreciate the ecological import. Some bacteria
swim northward in stationary magnetic fields as weak as 0.1 g (the Earth's
magnetic field is about 0.5 g at its surface). These bacteria contain iron
organized into crystals of magnetite. Magnetite is also found in the brains
and harderian glands of birds that can use local variations in the Earth's
magnetic field for navigation and orientation.
    In medicine, the therapeutic heating of tissue, diathermy, has been used
for many years. Shortwave diathermy provides deeper, more uniform heating than
does diathermy at higher frequencies. High-intensity radio-frequency fields
have been used to produce hyperthermia in cancer patients. The development of
bone tissue (osteogenesis) can be stimulated electrically either with implanted
electrodes or by inductive coupling through the skin. This therapy has been
used successfully to join fractures that have not healed by other means.
Electromagnetic fields were first used for medical diagnosis in 1926 when the
electrical resistance across the chest cavity was used to diagnose pulmonary
edema. Another diagnostic use of electromagnetic radiation is based on the fact
that the spectrum of radiation emitted by any object depends on its temperature
(blackbody radiation). Because a tumor is often at a higher temperature than the
surrounding tissue, its spectrum of emitted radiation is different. A technique
of diagnosing tumors by the radiaiton they emit (radiometry) is being evaluated.
Internally generated fields associated with nerve activity (EEG) and with muscle
activity (ECG, MCG) are used to monitor normal body functions. There may be
other uses of electric currents or fields in growth differentiation or
development which have not yet been explored. See ELECTROMAGNETIC RADIATION.
[E.P.]

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�6v�2v�32sBiogeochemical cycles �21m �33s
    The more or less circular paths of chemical elements passing back and
forth between organisms and environment. The rate at which vital elements
become available to biological components of the ecosystem is more important
in determining primary and secondary productivity than flow of solar energy.
If an essential element or compound is in short supply in terms of potential
growth, the substance may be said to be a limiting factor. The productivity of
an entire ecosystem is sometimes limited by one material available in least
amount in terms of need. Thus water limits the desert ecosystem, and nitrogen
or phosphorus often limits ocean ecosystems. However, nature has considerable
powers of adaptation and compensation. In many environments, species and
varieties have evolved which have low requirements for scarce materials. Also,
the amount of one substance which, in itself, may not be limiting often
greatly affects the requirement for another substance which is approaching a
critical minimum. Consequently, it is usually necessary to consider the
interaction of the essential materials if the limiting factors operating in a
given situation are to be determined. See BIOLOGICAL PRODUCTIVITY; ECOSYSTEM.
    Dissolved salts essential to life may be conveniently termed biogenic
salts or nutrients. They may be divided into two groups, the macronutrients
and the micronutrients. The macronutrients include elements and their
compounds needed in relatively large quantities, for example, carbon,
hydrogen, oxygen, nitrogen, potassium, calcium, phosphorus, and magnesium. The
micronutrients include those elements and their compounds necessary for the
operation of living systems but which are required only in minute amounts. At
least 10 micronutrients are known to be necessary for primary production:
iron, manganese, copper, zinc, boron, sodium, molybdenum, chlorine, vanadium,
and cobalt. Several others such as iodine are essential for certain
heterotrophs.
    From the standpoint of the biosphere as a whole, biogeochemical cycles
fall into two groups: the gaseous-type cycles, as illustrated by the nitrogen
cycle; and the sedimentary-type cycles involving movement of the more
earthbound elements. Cycles of oxygen, carbon, and water resemble the cycle of
nitrogen, in that a large gaseous pool is important in the continuous flow
between inorganic and organic states. In the nitrogen cycle, the nitrogen of
protoplasm is broken down from organic to inorganic form by a series of
decomposer bacteria. The nitrogen ends up as nitrate or other form usable by
green plants in the synthesis of new organic matter. The air is the great
reservoir and safety valve of the system. Nitrogen is continually entering the
air and continually returning to the cycle. See NITROGEN CYCLE.
    Most biogenic substances are more earthbound than nitrogen, and their
cycles follow the pattern of erosion, sedimentation, mountain building, and
volcanic activity. Biological activity on land and in the upper layers of
water results in local cycles from which there is usually a continual loss
downhill and replacement from uphill runoff and from solid matter moving in
the air as dust, that is, natural fallout. Humans often disrupt the
sedimentary cycles by tending to increase the downhill movement. The
phosphorus cycle is a good example. Phosphorus is relatively rare in the
surface materials of the Earth in terms of biological demand. At present more
phosphorus is apparently escaping to the deep ocean sediments (where it is
unavailable to producers) than is being replaced by natural processes. For the
time being, the considerable reserves of underground phosphate rock can be
mined to make up some of this loss, but eventually a means may have to be
found to recover phosphorus from the sea.
    Biogeochemical cycles involve elements essential to life. The nonessential
elements pass back and forth between organisms and environment and many of
them are involved in the general sedimentary cycle. Although they have no
known value to the organism, many of these elements become concentrated in
tissues, apparently because of similarity to specific vital elements. The
ecologist would have little interest in most of the nonessential elements were
it not for the fact that atomic bombs and nuclear power operations produce
radioactive isotopes of some of these elements, which then find their way into
the environment and into food chains. Even a rare element, in the form of a
radioactive isotope, can be of biological concern because a very small amount
of material from a geochemical standpoint can have marked biological effects.
Thus the cycling of such things as strontium, cesium, cerium, ruthenium, and
many others is of great concern. See BIOSPHERE.
 �34s[Eugene P. Odum]
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�6v�2v�32sBiogeography �21m �33s
    The science concerned with distribution of life on the Earth. Plants and
animals are irregularly distributed, both on the continents and in the oceans.
Some areas have a great abundance and variety of life forms, whereas others
are relatively sterile. Biogeographic studies are concerned with learning the
manner in which living organisms are arranged on the Earth and the causative
factors of this arrangement. All animals are dependent in the final analysis
on plants to supply food. Therefore, the distribution of plant life is the
basic component of biogeography. Animal life is the secondary or dependent
component.
    Plant geography is concerned with the climatic conditions and the supply
of nutrient substances on which plants feed. Stratification of vegetation is
controlled largely by conditions of temperature and moisture, which vary from
the flat lowlands to the mountaintop. The nature of the underlying rock, from
which plant nutrients are derived by weathering, may strongly influence the
growth of plants, but the effect is less striking than that of climate. See
TERRESTRIAL ECOSYSTEM.
    Animal life in turn is stratified in conformance with the zonation of
plants. There are typical animals of the desert, the foothills, the forest
zone, and the alpine crest. Each of these major communities of plants and
animals may be called a life zone, a vegetation zone, a biome, or a climax
formation according to various systems of classification. All of these
communities are part of the temperate region of the world. To the north is the
boreal region, which extends generally from the North Pole to the latitude of
southern Canada, and south of the temperate region is the tropical region,
extending from the Equator roughly to central Mexico. The Southern Hemisphere
may be similarly divided into biotic regions, and these in turn into
vegetation zones or communities.
     The distribution of plants on the surface of the Earth is constantly
changing with shifts in climate, with emergence, submergence, and movement of
land masses, and with such surface disturbances a volcanic activity or geologic
erosion.  As environmental conditions change, floras tend to migrate and in so
doing some species become extinct and new species evolve.  Certain  floristic
communities have moved long distances in geologic time.  The study of plant
geography must consider the historic background of plant communities as well as
problems of ecologic adaptation to existing conditions.  See  PALEOBOTANY;
POPULATION DISPERSAL.
     Terrestrial animal populations are distributed largely in accordance with
vegetation patterns.  The distribution of individual species of animals often
conforms rather closely to the distribution of particular vegetation zones.
Some animals make seasonal use of different habitats by migrating.  Thus many
species of migratory birds nest in northern boreal zones in summer but return
southward to temperate or even tropical habitats in winter.  On a more local
scale, some species of both birds and mammals migrate vertically in mountainous
terrain, that is, upward in summer and downward in winter.  In geologic time,
whole faunas have moved up or down the continental land masses in response to
major climatic changes or to shifting of the continents.  Animal life in the
oceans is distributed as irregularly as on land.  The temperature of the water
and the concentration of mineral nutrients varies with air temperature, ocean
currents, and especially with points of upwelling of enriched waters from the
ocean depths.  Areas rich in oceanic life are situated as a rule in cold
currents and along continental shelves.  See  MARINE ECOSYSTEM.
     The distribution of human populations on the Earth bears a close
relationship to the distribution of natural plant and animal life.  Throughout
history, humans have thrived best in areas that are highly productive of
foodstuffs.  By and large the most favorable habitats for humans are situated in
the temperate regions of the world, where climates are moderate and soils are
rich.
     Unlike most other animals, humans have altered their environment
considerably.  Through blind destruction of agricultural soils and exploitation
of other resources, many past civilizations have become impoverished or have
even destroyed themselves.  In the last analysis, humans are still dependent
upon many natural features of the environment.	See  HUMAN ECOLOGY;
ZOOGEOGRAPHY.
     Perhaps the outstanding aspect of human biogeography today is the explosive
increase of population since 1900.  Modern medicine has reduced the normal death
rate among people and technological improvements have intensified the processes
of resource extraction, permitting, for the moment at least, more people than
ever before to live upon the Earth.
 �34s[A. Starker Leopold]
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�6v�2v�32sBioherm   �21m �33s 
    A lenslike to moundlike structure of strictly organic origin. This term 
involves two concepts: shape and organic internal composition. 
    The term shape denotes original topographic relief above the sea floor as 
well as a three-dimensional quality: crudely conical (sugar loaf - shaped) or 
ellipsoidal (bread loaf - shaped). Such forms are massive or unbedded, their 
upbuilding resulting from the very rapid rate of accretion of organic 
carbonate once it starts in a favorable locality. There are size limitations: 
bioherms a meter or so in diameter are known, and some rose 300 ft (100 m) or 
more above the horizontal strata of the sea floor. 
    The second concept, organic internal composition, not only embraces 
sessile, bottom-dwelling organisms forming frame-building reefy bondstone but 
also includes piles of organically derived debris replete with organisms which 
encrust it and cement it in place. Even inorganic cement precipitated from 
marine and meteoric water is known to play a role in the buildup of massive, 
moundlike structures. When the internal material is coarse and identifiable, 
no problem is encountered in applying the term bioherm in its original sense. 
In some buildups, however, an appreciable amount of lime mud is present, and 
relatively few organisms are identifiable which could have secreted, bound, 
encrusted, or trapped carbonate mud (as in some early Carboniferous mounds). 
In such cases, problems arise in applying that part of the definition based on 
internal composition. In fact, as a field term, bioherm can hardly ever be 
completely diagnostic because careful petrographic study is commonly necessary 
for details of internal composition to be ascertained. 
    Bioherms may occur on shelves (where they are normally lens-shaped) or in 
shallow basins, often at the basin margin. In the latter position, they have 
been called reef knolls or pinna cle reefs. See BIOSTROME; REEF; STROMATOLITE. 
 �34s[James L. Wilson]
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�6v�2v�32sBioinorganic chemistry �21m �33s
    The borderline field between biochemistry and inorganic chemistry. This
field, which has also been called inorganic biochemistry, generally involves
the application of the principles of inorganic chemistry to biochemical
problems. Traditionally biochemistry has been considered ``organic,'' and in
fact much of biochemistry involves organic, or carbon, chemistry. It has been
known for a long time, however, that living organisms require many other
elements, some such as calcium in large quantities, and others such as copper
in trace amounts. Most of these elements are metals, though some are
nonmetallic. These ``inorganic'' elements are extremely important in the
regulation of many biological phenomena, and it is for that reason that
inorganic chemistry and biochemistry have intermingled.
    The major achievements of bioinorganic chemistry have consisted of the
elucidation of the structures of metal-containing biochemical compounds, and
the correlation of structure and function of these compounds. Generally they
are complexes of metal ions with organic ligands, and therefore coordination
compounds. Much of bioinorganic chemistry can be considered an application of
coordination chemistry to biological problems. See COORDINATION CHEMISTRY;
ION; ORGANOMETALLIC COMPOUND.
    Some of the coordination compounds found in living organisms are of
relatively low molecular weight; that is, they contain relatively few atoms in
comparison to the very large molecules common to living cells. Examples of
these small molecules are siderochromes (iron-accumulating and -transporting
molecules found in bacteria and fungi), ionophores (substances important in
the transport of alkali and alkaline-earth metal ions across biological
barriers), porphyrins, chlorophyll, and vitamin B sub 1 sub 2. See CHLOROPHYLL;
COENZYME; ENZYME; PHOTOSYNTHESIS; PORPHYRIN; VITAMIN B sub 1 sub 2.
    The large molecules common to living cells, such as proteins, nucleic
acids, and lipids, also bind to metal ions, and in many instances the metal
ions are important in the biological functioning of the molecules. They
include metalloproteins (metal complexes of proteins) and metal enzymes
(metalloproteins in the form of enzymes). See CATALYSIS; COMPLEX COMPOUNDS.
    Many of the bioinorganic substances are biological catalysts, transport
agents, and regulators. Metal ions are involved in many biological phenomena
such as kidney function, muscle contraction, cell division, and nerve impulse
transmission, but little is understood about the structures of the molecules
to which the metal ions are bound during these processes.
    Metal ions are important to the structure not only of individual
biological molecules, but also of cell particulates that are molecular
aggregates. They are constituents of chromatin, the genetic material in the
cell nucleus that is composed of nucleic acids and proteins. Metal ions
influence the structure of ribosomes, the cellular particles on which protein
synthesis occurs. The structures of these and other cellular particles are
strongly affected by metal ions, and unfavorable concentrations of metal ions
can distort these structures. Calcium is very important in the structure of
bones and teeth. Although metal ions in limited concentration are required for
the proper functioning of many biological processes, excesses of metal ions or
the presence of undesirable metal ions can produce effects which are
deleterious. See METALS, PATHOLOGY OF.
    Metal complexing agents have been useful for the elimination of toxic
metals from the body and in the treatment of diseases characterized by an
excess of certain metal ions. Examples of such diseases are Wilson's disease,
which is associated with the accumulation of copper, and hemochromatosis,
which results from an overload of iron. The toxic effects of metal ions point
to relationships between bioinorganic chemistry, toxicology, and studies on
environmental effects on the aging process, which is accompanied by changes in
the concentration of metal ions in tissues. The biological requirement of
inorganic elements, as well as the detrimental effects of excesses of these
elements, suggests a relationship to nutrition.
    Not all of bioinorganic chemistry is intended to lead to an understanding
of the participation of inorganic elements in biological phenomena. Sometimes
chemists make use of the reactions of metal ions or metal complexes with large
biological molecules to determine the structures of these molecules. The
structure of many large biological molecules is elucidated by x-ray
diffraction studies that require the formation of metal complexes with the
biomacromolecules by a method called isomorphic replacement. See BIOCHEMISTRY;
GENETIC CODE; INORGANIC CHEMISTRY; ORGANIC CHEMISTRY.
 �34s[Gunther L. Eichhorn]
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�6v�2v�32sBiological equilibrium   �21m �33s 
    A lifeless object is said to be in equilibrium, or in a state of balance, 
when all forces acting upon it cancel. This results in a state of rest. In an 
actively moving animal, internal as well as external forces have to be 
considered, and the maintenance of a balanced attitude in a body consisting of 
a number of parts that are loosely connected by movable joints is a complex 
matter.  
    The maintenance of equilibrium is relatively easy in limbless animals, 
such as worms, snails, or starfish, which rest and move on an extensive 
surface of support. The tactile contact between the normal creeping surface 
and the substrate furnishes sensory information on the animal's orientation 
with respect to the plumb line. When such an animal is turned on its back, the 
lack of contact pressure on the creeping surface and the tactile stimulation 
of the back initiate muscular movements which return the animal to its normal 
position. This is known as the righting reflex. The accompanying change in the 
direction of illumination may also contribute to efficient righting. 
    Free-swimming and flying animals are often in a precariously poised state 
of equilibrium, so that the normal attitude can be maintained only by the 
continuous operation of corrective equilibrating mechanisms. The same applies 
to a somewhat lesser degree to long-legged quadrupeds, such as many mammals, 
and to bipeds, such as birds and some primates (including humans). In these 
animals the center of gravity lies some distance above the area of support 
which, in some cases, such as in humans, can be relatively small. 
    The weight of the body is supported by what are known as antigravity 
muscles, and the coordination of their activity is controlled by sensory 
information from a number of sources: (1) the eyes; (2) static and dynamic 
mechanoreceptors incorporated in the various types of statocysts in 
invertebrate animals and in the vestibular organ or labyrinth of the 
vertebrates; (3) proprioceptor organs such as muscle spindles, Golgi endings 
in tendons, Pacinian corpuscles and similar encapsulated endings associated 
with tendons and joints, and other pressure receptors in supporting surfaces 
(for example, the soles of feet); and (4) sensory endings in the viscera, 
capable of being differentially stimulated by changes in the direction of 
visceral pull on mesenteries, and other structures. All these receptor organs 
supply the central nervous system with information on the orientation of head, 
trunk, and limbs in the gravitational field. 
 �34s[Otto E. Lowenstein]
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�6v�2v�32sBiological productivity   �21m �33s 
    The amount and rate of production which occur in a given ecosystem over a 
given time period. It may apply to a single organism, a population, or entire 
communities and ecosystems. See BIOMASS. 
    Productivity is best considered both in terms of dry matter produced (net 
production) and in terms of the thermodynamic cost of producing it (gross 
production, being respiration and heat losses plus net production). The 
following definitions are useful in calculating production: (1) Gross primary 
production (GPP) is the total energy fixed by photosynthesis. (2) Net primary 
production (NPP) is the gross production less losses due to plant respiration. 
(3) Secondary production is production by heterotrophs (animals, 
microorganisms), which feed on plant products or other heterotrophs. Note that 
human-associated terms, such as harvest or yield, refer to just that fraction 
of production of use to the plant harvester or animal husbander. Thus yield of 
a grain crop refers solely to the harvested seed portion of the aboveground 
net primary production. All energy units are presented in kilojoules (kJ); the 
outmoded, but frequently used, kilocalorie of nutritionists is calculated by 
kJ/0.2388. See ECOSYSTEM; FOOD CHAIN; PHOTOSYNTHESIS.  
 �34s[David C. Coleman]
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�6v�2v�32sBiologicals �21m �33s
    Biological products used to induce immunity to various infectious diseases
or noxious substances of biological origin. The term is therefore much more
restrictive than the words themselves imply, because it is usually limited to
immune serums, antitoxins, vaccines, and toxoids that have the effect of
providing protective substances of the same general nature that a person
develops naturally from having survived an infectious disease or having
experienced repeated contact with a biological poison. See IMMUNITY.
    One major class of biologicals includes the animal and human immune
serums. All animals, including humans, develop protective substances
(antibodies) in their blood plasma during recovery from many (but not all)
infectious diseases or following the injection of toxins or killed bacteria
and viruses. These antibodies usually are found in the gamma globulin fraction
of the plasma and are specific in the sense that they react with and
neutralize only substances identical or closely similar to those that caused
them to be formed. See SERUM.
    Antibody-containing serum from another animal is useful in the treatment,
modification, or prevention of certain diseases of humans when it is given by
injection. The use of these preformed ``borrowed'' antibodies is called
passive immunization, to distinguish it from active immunization, in which
people develop their own antibodies. Serums which contain antibodies active
chiefly in destroying the infecting virus or bacterium are usually called
antiserums or immune serums; those containing antibodies capable of
neutralizing the secreted toxins of bacteria are called antitoxins. Immune
serums have been prepared to neutralize the venoms of certain poisonous snakes
and black widow spiders; they are called antivenins.
    Products used to produce active immunity constitute the other large class
of biological products. They contain the actual toxins, viruses, or bacteria
that cause disease, but they are modified in a manner to make them safe to
administer. Because the body does not distinguish between the natural toxin or
infectious agent and the same material when properly modified, immunity is
produced in response to injections of these materials in a manner very similar
to that which occurs during the natural disease. Vaccines are suspensions of
the killed or attenuated (weakened) bacteria or viruses or fractions thereof.
Toxoids are solutions of the chemically altered specific bacterial toxins
which cause the major damage produced by bacterial infections. Biological
products producing active immunity are usually named to indicate the disease
they immunize against and the kind of substance they contain: thus typhoid
vaccine, diphtheria toxoid, tetanus toxoid, measles vaccine, mumps vaccine,
and poliomyelitis vaccine.
    Because it is frequently expedient to immunize people simultaneously
against several diseases, various immunizing agents have been combined so that
they can be given in a single series of two or three doses. The combination of
a large number of different immunizing agents within a single immunization
dosage is an exceedingly complex problem, because dosage volume must be kept
small and the various agents must be compatible.
    In addition to products for active and passive immunization, a few other
substances of human or animal origin are also classed (mostly for regulatory
convenience) as biological products. One group of these is normal human blood
plasma and its derivatives. Whole plasma and serum albumin are used to combat
hemorrhage and shock, and other fractions are used in the treatment of
hemophilia and for their clotting properties. The use of anti-Rh sub 0 (D) gamma
globulin in Rh-negative women at parturition inhibits Rh sensitization and
erythroblastosis fetalis in subsequent offspring. See BLOOD GROUPS;
ERYTHROBLASTOSIS FETALIS.
    Another group of biological products consists of a miscellany of reagents
used in the diagnosis of infectious diseases. These include immune serums for
the serological typing and identification of pathogenic bacteria and viruses,
and various antigens for the detection of antibodies in patients' serums as a
means of assisting in diagnosis. In addition to these substances used in the
laboratory diagnosis of disease, certain extracts of bacteria and fungi have
been prepared for injection into the skin to detect allergic hypersensitivity.
These skin tests also assist in the diagnosis of various infectious diseases
such as tuberculosis and brucellosis. See SKIN TEST.
    Allergenic products are administered to humans for the diagnosis,
prevention, or treatment of allergies to various plant and animal substances.
They usually are suspensions of the allergenic substance (pollens, feathers,
hairs, or danders) in 50% glycerin. See IMMUNOLOGY.
 �34s[Lee F. Schuchardt]
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�6v�2v�32sBiological specificity �21m �33s
    Although all living cells contain hundreds of potentially reactive
chemical substances, their metabolic and developmental reactions do not occur
at random, but follow orderly patterns that give rise to the unique
characteristics of the individual and of its species. The guiding principle is
termed biological specificity; examples abound in all areas of biology.
    Specificity may originate, first, at a chemical level. A most important
example is the structural correspondence (presumably in an inverse,
complementary sense) between an enzyme and the substrates whose reaction it
catalyzes. Analogous relations govern the specific reactions between antigens
and antibodies. Again, the specificity of fertilization of an egg by sperm or
pollen is universally important in preserving the unique characteristics of
animal and plant species, although occasional hybrids can be produced, as
exceptions. See ANTIGEN-ANTIBODY REACTION; ENZYME; FERTILIZATION; SPECIATION.
    In addition to chemical determinants, the specificity of reproduction may
also be governed by anatomical or behavioral characteristics of the plant or
animal which minimize cross matings between separate species. The relations
between two or more widely divergent species may also be precisely ordered, as
in parasitism. Some associations may not be unfavorable to either partner. The
fertilizations of Smyrna figs, yuccas, and certain orchids require particular
insects for pollen transfer. The association of algae and fungi in lichens is
also a nonrandom one. See LICHENES; PARASITOLOGY.
 �34s[Henry P. Treffers]
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�6v�2v�32sBiology   �21m �33s 
    A division of the natural sciences dealing with life. Biology is the 
science of living organisms, in contrast to the physical sciences, which are 
concerned with inanimate matter. Biology is the broad general field of 
knowledge concerned with the study of all aspects of living organisms which 
can be approached by the methods of natural science. Because the field is so 
broad in scope, it has been divided into two main areas, botany and zoology, 
each with a large number of specialized branches. No one worker can know the 
entire field of biology thoroughly. Specialized subdivisions of biology 
pertaining to both plants and animals include taxonomy, morphology, 
embryology, physiology, cytology, genetics, evolution, and ecology. The term 
biology embraces those principles of widest application to the origin, growth 
and development, structure, function, evolution, and distribution of plants 
and amimals. See BOTANY; CYTOLOGY; ECOLOGY; EMBRYOLOGY; GENETICS; 
MICROBIOLOGY; PHYSIOLOGY; TAXONOMY; ZOOLOGY. 
 �34s[Charles B. Curtin]
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�6v�2v�32sBioluminescence   �21m �33s 
    The emission of light by living organisms, due to an energy-yielding 
chemical reaction in which a specific biochemical substance, called luciferin, 
undergoes oxidation, catalyzed by a specific enzyme called luciferase. There 
are many specific luciferins and luciferases which are chemically different, 
each involved in some different living luminescent organism. 
    Bioluminescence does not come from, or depend on, light absorbed by the 
organism. It is a type of chemiluminescent reaction, in which chemical energy 
is transformed into light energy. Such reactions represent an unusual kind of 
chemical reaction, but are not peculiar to living organisms. Bioluminescence 
is a special type of chemiluminescence because the chemicals are synthesized 
by living cells and because they are catalyzed by an enzyme, a unique kind of 
biological catalyst. Since bioluminescence is a type of chemiluminescence, it 
is not necessary to have a live organism to obtain light emission. The simple 
preservation of the chemicals involved will suffice. This can be done in some 
cases by rapidly drying the organism under mild conditions. See 
CHEMILUMINESCENCE.  
    There is a great diversity of organisms which have the ability to give off 
light, and they do it in many different ways and for many different reasons. 
In the plant kingdom only certain mushrooms and some marine algae and bacteria 
are luminous. The bacteria and mushrooms emit continuously, whereas the algae, 
which are dinoflagellates (the same group that causes ``red tides'' in the 
ocean), give a brief flash when disturbed. Among the animals are many luminous 
representatives, ranging from the simplest protozoans to fish. Of the 
approximately 80 different classes of animals, about 30 have light-emitting 
members. These are mostly marine forms, and include sponges and various 
jellyfish and related animals, such as comb jellies (sea walnuts). Most of the 
luminescent annelid worms are marine polychaetes, but a few common terrestrial 
earthworms possess luminous cells located in the body cavity. Many kinds of 
mollusks emit light, including clams, snails, and octopuses. Among arthropods 
are many luminescent organisms, such as crustaceans (shrimp), millipedes, 
centipedes, and numerous insects, notably the lampyrid beetles, or fireflies. 
There are also luminous representations among the brittle stars, acorn worms, 
and tunicates. 
    Virtually all of the known or proposed ways in which bioluminescence 
functions can be classed under three major rubrics: assisting in predation, 
aiding in escaping predators, and communicating. The first two are 
interspecific, involving interactions between different species, and the last 
is intraspecific, that is, involving different individuals of one species. 
    It seems clear that, unlike many key biochemical molecular species - the 
hemes, the cytochromes, the histones, or the nucleic acids - luminescent 
systems have originated and evolved independently more than once. The elegance 
of the display of luminescence is matched by its diversity - diversity in 
form, diversity in mode of display, diversity in the chemistry, and diversity 
in function. In fact, the impressive number of different species possessing 
bioluminescence, and the various uses for it, is an object lesson on and 
testimonial to the evolutionary process. 
 �34s[J. Woodland Hastings]
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�6v�2v�32sBiomass �21m �33s
    The mass of living material present in an organism or organisms. Biomass
may include some nonliving material, such as the hair and feathers of
vertebrates or the heartwood of trees. The intent of the term is to provide a
useful quantitative measure of the organism and not to make a sharp
distinction between life and nonlife. It is typically referred to or
calculated as dry weight. In a model, biomass can be a state variable, into
and out of which several flows occur. For example, a heterotrophic organism,
whether it is a bacterium, bird, or buffalo, ingests food or nutrients and in
turn gives off respiratory gases (carbon dioxide) and waste products, and (at
times) makes new body tissue (see illustration). Growth is an increase, and
starvation a decrease, in biomass. See BIOLOGICAL PRODUCTIVITY.
    Due to extensive losses of energy at each step in a food chain, going from
primary producers on up the chain, there is a change in the amount of energy
flowing to new biomass at each step in the chain. The change in amount of
standing crop at each step is often represented as a horizontal bar. With the
size of the bar usually decreasing as one ascends the food chain, the shape of
the histograms assumes a pyramidal form. This leads to the term biomass
pyramid. The terminology has gone out of favor, since there are many instances
where one finds small standing crops (as with oceanic phytoplankton) and high
turnover rates, so that one can describe inverted pyramids as well. See FOOD
CHAIN.




 �34s[David C. Coleman]
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�6v�2v�32sBiome   �21m �33s 
    A complex biotic community covering a large geographic area and 
characterized by the distinctive life forms of important climax species of 
plants and animals. A life form is the common morphological features that 
characterize a group of organisms. On the land, a biome is identified by the 
life form of the dominant climax plants, as well as by the distinctive types 
of vegetation and landscape in which they grow. In the ocean, the life forms 
of the predominant animals serve as the criterion. The biome incorporates all 
seral (successional) as well as climax stages of the community, and its 
distribution is controlled by climate or, in the ocean, by other physical 
factors of the environment. Principal terrestrial biomes are the temperate 
deciduous forests, coniferous forest, woodland, chaparral, tundra, grassland, 
desert, tropical savanna, and tropical broad-leaved forest. Principal marine 
biomes are the pelagic, the barnacle - gastropod - brown algae, the sea urchin 
- large snail, the bivalve-annelid, the coral reef, and the abyssalbenthic. 
    Terrestrial biomes are divisible into plant associations, which are 
distinguished by distinctive combinations of climax dominant species. For 
instance, the temperate deciduous forest biome includes the beech-maple, 
oak-hickory, and other associations. Both terrestrial and marine biomes are 
divisible into animal biociations. A biociation is distinguished by the 
distinctiveness of the predominant animal species. See COMMUNITY; ECOLOGY. 
 �34s[S. Charles Kendeigh]
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�6v�2v�32sBiomechanics �21m �33s
    The study of the mechanics of living things. It demands knowledge both of
biology and of the various branches of physics and engineering which comprise
mechanics.
    A major branch of biomechanics is concerned with the properties of
biological materials. Particular attention has been paid to bone and blood,
because of their medical importance. The aims are to measure the properties
and then to explain them in molecular terms. Proteins and polysaccharides are
high polymers, like rubber and plastics, and biologists studying their
properties have made extensive use of theories and techniques developed by
rubber and plastics technologists. Most skeletal materials are composite:
bone, for instance, consists of tiny crystals of a calcium salt embedded in a
mass of fibers of collagen (the material of which tendons are made). Bone is
stronger than tendon and has a higher Young's modulus; that is, it is stiffer
(see illustration). That a mass of tiny, brittle crystals strengthens bone may
seem strange; their effect has not been fully explained, but it seems to be
comparable to the effect of carbon particles on rubber. Soft rubber is
converted to the much stiffer, stronger material of which motor tires are made
by incorporating carbon particles in it. See YOUNG'S MODULUS.
    Other biomedical studies are concerned with surface tension. It is well
known that the pressure is greater inside a bubble than immediately outside
it, because of surface tension in the wall of the bubble, and that the smaller
the bubble, the greater the pressure. The inside of a lung consists largely of
very tiny pockets known as alveoli. If the moisture on the walls of the
alveoli had a surface tension similar to that of water or of blood plasma, it
would have a substantial effect on the pressure in the lung. As one breathed
out and the alveoli got smaller, the pressure needed to resist the surface
tension and keep the alveoli open would increase, and the alveoli would tend
to collapse completely. A strong effort would be needed to inflate them for
the next breath. This does not happen, because there is a film of
surface-active material on the moist surfaces which makes the surface tension
fall to a very low value during expiration. Infants with the respiratory
distress syndrome, a dangerous ailment of newborn babies, do not have enough
of this material, and are liable to die of exhaustion from the effort of
breathing. See LUNG; SURFACE TENSION; SURFACE-ACTIVE AGENT.
    Aerodynamics, a branch of aeromechanics, has been used in a wide variety
of biological investigations, including the study of the falling speeds of
winged seeds and the flight of hummingbirds. See AERODYNAMICS; BIOPHYSICS.





 �34s[R. McNeill Alexander]
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�6v�2v�32sBiomedical engineering   �21m �33s 
    An engineering discipline whose objective is to apply engineering science 
and technology to devise practical solutions to medical problems. In the 
broadest sense, biomedical engineering embraces all aspects of the application 
of technology to living systems. It is an emerging field, and the disciplinary 
boundaries of biomedical engineering are as yet not well defined. It is 
distinct from, but closely related to, space medicine, sports medicine, 
nuclear medicine, biochemistry, and biophysics. See AEROSPACE MEDICINE; 
BIOCHEMISTRY; BIOPHYSICS; NUCLEAR MEDICINE. 
    Biomedical engineering has had a major impact on all aspects of medicine 
including: monitoring of a patient's condition and diagnosis of disorders; 
surgery or treatment to correct or alleviate disorders; rehabilitation to help 
the disabled patient to participate more fully in society; and fundamental 
research to gain understanding of the nature and working of living systems. 
    The first task for a medical practitioner is to determine the current 
state of health of the patient and to identify or diagnose a malfunction when 
it occurs. Modern medicine has at its command a number of technological 
devices to facilitate this task, including those of radiology, ultrasound 
imaging, electrocardiography, and electroencephalography. See CARDIAC 
ELECTROPHYSIOLOGY; ELECTROENCEPHALOGRAPHY; RADIOLOGY; ULTRASONICS. 
    Biomedical engineering has provided an ever-growing armory of surgical 
tools and treatment techniques, including ultrasound focusing, iontophoresis, 
the vibratory saw, and functional electrical stimulation. In addition, 
biomedical engineering provides an array of implantable devices and artificial 
organs to assist or replace defective or malfunctioning natural organs. In 
this area the major problem is biocompatibility. Despite near-insurmountable 
difficulties, many implantable devices are in common use. The crippling 
degeneration of the joints which is characteristic of arthritis is commonly 
treated by replacing the malfunctioning joint by an artificial joint. The 
development of an implantable pump to replace a failing heart is a major focus 
of research in biomedical engineering. Internal artificial pacemakers which 
control the rate at which the heart beats are commonly used. See PROSTHESIS. 
    The major goal of rehabilitation engineering is to make the disabled 
person as independent as possible by providing assistive devices to alleviate 
the handicap and compensate for the disability. The toughest problem in 
rehabilitation engineering is to design the human/machine interface, the means 
whereby the patient and the assistive device interact with one another. The 
interface between a person and a machine is treated as two communication 
pathways: the forward pathway which includes all ways the user commands the 
machine, and the feedback pathway which includes all ways the user observes 
the machine. The interface must be designed such that the interactions between 
the patient and the assistive device will be intimate and unobtrusive. 
Communication must be achieved without placing further burdens on the 
patient's limited sensory or motor capabilities, and without drawing unwelcome 
attention to the disability. 
    The greatest single impact which biomedical engineering has had on 
medicine and biology is in the understanding of the underlying phenomena. This 
is largely due to the fresh perspective which the engineering discipline 
brings to medical problems. The applicable fundamentals of engineering can be 
broadly categorized into: (1) observation of fundamental phenomena, making 
quantitative measurements whenever possible; (2) characterization of phenomena 
and development of the simplest workable description or mathematical model of 
the phenomena; (3) quantitative prediction of the consequences of a specific 
action; (4) design, the creative integration of 1, 2, and 3 to yield new tools 
and techniques to accomplish a particular purpose. Of these, the development 
of models shows the value of the engineer's practical-mindedness most clearly. 
A model is a partial description abstracted from the basic observations. 
    An engineering background allows the biomedical engineer not only to 
provide new interpretations of existing data but to devise the correct 
instrumentation or experimental procedures to obtain the critical data 
required for the solution of a particular problem. For example, from the 
engineer's functional standpoint osteoarthritis is the degeneration of a 
bearing (the interface between two parts in relative motion). The biomedical 
engineer can draw on the large body of prior knowledge concerning the stresses 
on a bearing and the modes of failure of a bearing to postulate the causes of 
osteoarthritis and suggest possible ways to alleviate the condition. 
Furthermore, engineering experience indicates that the single most informative 
piece of data about the bearing will be the distribution of pressure (force 
per unit area) across its surface. Some of the most promising research into 
the cause of osteoarthritis is being performed by biomedical engineers whose 
primary objective is to measure the pressure distribution across a living 
joint.  
 �34s[Neville Hogan]
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�6v�2v�32sBiophysics   �21m �33s 
    A hybrid science involving the overlap of physics, chemistry, and biology. 
A dominant aspect is the use of the ideas and methods of physics and chemistry 
to study and explain the structures of living organisms and the mechanisms of 
life processes. The recognition of biophysics as a separate field is 
relatively recent, having been brought about, in part, by the invention of 
physical tools such as the electron microscope, the ultracentrifuge, and the 
electronic amplifier, which greatly facilitate biophysical research. These 
tools are peculiarly adapted to the study of problems of great current 
importance to medicine, problems related to virus diseases, cancer, heart 
disease, and the like. 
    There are several areas within biophysics, the major ones being molecular 
biophysics, radiation biophysics, physiological biophysics, and mathematical 
or theoretical biophysics. 
    Molecular biophysics has to do with the study of large molecules and 
particles of comparable size which play important roles in biology. The most 
important physical tools for such research are the electron microscope, the 
ultracentrifuge, and the x-ray diffraction camera. See ELECTRON MICROSCOPE; 
MOLECULAR BIOLOGY; ULTRACENTRIFUGE; X-RAY DIFFRACTION. 
    Radiation biophysics consists of the study of the response of organisms to 
ionizing radiations, such as ALPHA-, BETA-, GAMMA-, and x-rays, and to 
ultraviolet light. The biological responses are death of cells and tissues, if 
not of whole organisms, and mutation, either somatic or genetic. 
    Physiological biophysics, called by some classical biophysics, is 
concerned with the use of physical mechanisms to explain the behavior and the 
functioning of living organisms or parts of living organisms and with the 
response of living organisms to physical forces. 
    Mathematical and theoretical biophysics deals primarily with the attempt 
to explain the behavior of living organisms on the basis of mathematics and 
physical theory. Biological processes are being examined in terms of 
thermodynamics, hydrodynamics, and statistical mechanics. Mathematical models 
are being investigated to see how closely they simulate biological processes. 
See BIOMECHANICS; BIOPOTENTIALS AND ELECTROPHYSIOLOGY; BONE (BIOPHYSICS); 
MATHEMATICAL BIOLOGY; MICROMANIPULATION; MICROSCOPE; MUSCLE (BIOPHYSICS); 
MUSCULAR SYSTEM; OXIMETRY; SKELETAL SYSTEM; SPACE BIOLOGY; SYMBALLOPHONE; 
THERMOREGULATION; THERMOTHERAPY. 
 �34s[Max A. Lauffer]
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�6v�2v�32sBiopotentials and electrophysiology �21m �33s
    Biopotentials are voltage differences which are measured between points in
living cells, tissues, and organisms. Electrophysiology is the science
concerned with determining the basic mechanisms underlying the generation of
such potentials, and the laws governing the operation of systems contributing
to and involving bioelectrical phenomena. Electrophysiology encompasses both
the origin and the functional significance of these potentials and their
experimental and natural perturbation by a variety of means. Among these,
electrical stimulation of cells and tissues has been a long-standing and
valuable aspect of electrophysiological methodology.
    The potential difference measured with appropriate electrodes between the
interior cytoplasm and the exterior aqueous medium of the living cell is
generally called the membrane or resting potential (E sub R sub P). This
potential difference is usually in the order of several tens of millivolts and
is relatively constant or steady. The range of E sub R sub P values in various
striated muscle cells of animals from insects through amphibia to mammals is
about -50 to -100 mV (negative taken inside with respect to outside). Nerve
cells show similar range of E sub R sub P values in such diverse species as
squid, cuttlefish, crabs, lobsters, frogs, cats, and humans. Even in the
coenocytic marine algae, Valonia, Halicystis, and Nitella, potentials up to
about -140 mV are measured between the cell sap and the outside medium.
    Steady potential differences also can be measured across various tissues,
such as the frog skin and the gastric mucosa of the stomach, and often have
been shown to exist between cells. These potentials are often of complex
cellular origins, but it is thought that their origin is predicted from the
resting potentials of the cells making up the tissues, or involve mechanisms
similar in kind if not in detail to those described here. See BIOPHYSICS; CELL
(BIOLOGY); CELL MEMBRANES; ELECTRIC ORGAN (BIOLOGY); MUSCLE.
    Another type of bioelectric potential is the action potential, a transient
change in the cellular transsurface potential which is propagated along the
surface of the cell and its processes. Although it has been investigated most
extensively in nerve and muscle cells, the action potential probably occurs in
some form in all cells; it is even seen in certain plant cells such as
Nitella, but its function there is unknown. It is the propagated signal or
unit of information by which communication is achieved between receptors
(sense organs) and effectors (muscles, gland cells) via the central nervous
system, between one part of the nervous system and another, and between the
nervous system and other organs. It travels along the surface of muscle cells
and leads to contraction. It is initiated by a decrease in the resting
potential. See NERVOUS SYSTEM. (VERTEBRATE).
 �34s[William J. Adelman, Jr.]
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�6v�2v�32sBiorheology   �21m �33s 
    The study of the deformation and flow of biological materials. The 
application of the principles of rheology to biological systems is a natural 
consequence of the significant effects which rheological parameters have on 
physiological functions. Of the many flow systems in the body, there are 
several in which fluid behavior is not that of a simple liquid such as water. 
These require more thorough study. The most thoroughly studied biological 
fluid is blood. Other fluids of interest are mucus and synovial fluid. 
Characterizing the parameters of natural materials is important not only for 
the understanding of physiological function in the healthy and diseased state 
but also for the design of synthetic replacements for diseased or traumatized 
parts of the body, particularly artificial joints and synovial fluid. See 
RHEOLOGY.  
 �34s[Henry L. Gabelnick]
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�6v�2v�32sBiosphere �21m �33s
    The thin film of living organisms and their environments at the surface of
the Earth. Included in the biosphere are all environments capable of
sustaining life above, on, and beneath the Earth's surface as well as in the
oceans. Consequently, the biosphere includes virtually the entire hydrosphere
and portions of the atmosphere and outer lithosphere.See ATMOSPHERE;
HYDROSPHERE; LITHOSPHERE.
    Neither the upper nor lower limits of the biosphere are sharp. A variety
of organisms inhabit the ocean depths. Spores of microorganisms can be carried
to considerable heights in the atmosphere, but these are resting stages that
are not actively metabolizing. Evidence exists for the presence of bacteria in
oil reservoirs at depths of about 7000ft (2000 m) within the Earth. These are
extreme limits to the biosphere; most of the mass of living matter is within
the upper 300 ft (100 m) of the lithosphere and hydrosphere, although there
are places even within this zone that are too dry or too cold to support much
life. Most of the biosphere is within the zone which is reached by sunlight
and where liquid water exists.
    The biosphere is characterized by the interrelationship of living things
and their environments. Communities are interacting systems of organisms tied
to their environments by the transfer of energy and matter. Such a coupling of
living organisms and the nonliving matter with which they interact defines an
ecosystem. An ecosystem may range in size from a small pond to a tropical
forest to the entire biosphere. See ECOSYSTEM.
    The major metabolic processes occurring within the biosphere are
photosynthesis and respiration. Green plants, through the process of
photosynthesis, form organic compounds composed essentially of C, H, O, and N
from CO2, H2O, and nutrients, with O2 being released as a by-product. The O2
and organic compounds are partially reconverted into CO2 and H2O through
respiration by plants and animals. Driving this cycle is energy from the Sun;
respiration releases the Sun's energy which has been stored by photosynthesis.
See PHOTORESPIRATION; PHOTOSYNTHESIS.
    The fundamental types of organisms engaged in these activities are:
producers, or green plants that manufacture their own food through the process
of photosynthesis and respire part of it; consumers, or animals that feed on
plants directly or on other animals by ingestion of organic matter; and
decomposers, or bacteria that break down organic substances to inorganic
products. See BIOLOGICAL PRODUCTIVITY; ECOLOGICAL INTERACTIONS; FOOD CHAIN.
    Over 60 elements are reported to occur in organisms. Of these, over 30
play a vital role in cellular processes. On an atomic basis hydrogen, the
lightest element, constitutes almost 50% of the content of organisms in the
biosphere, with carbon and oxygen combined making up the other half. Taken
together, these three elements make up over 99% of all atoms in the biosphere.
Other important constituents such as nitrogen, phosphorus, and sulfur total
only about 3 atoms per 1000.
    The carbon cycle (CO2-HCO sup - sub 3 CaCO3 system) is one of the most
important systems affected by organisms because of its effect on the pH balance
in the ocean, the content of CO2 in the atmosphere, and the production of CaCO3
sediment. Despite the dominance of the biosphere in the production of CaCO3,
the atmosphere and hydrosphere contain important components of the carbonate
system. The inorganic carbon cycle is also an integral part of the larger
carbon cycle of the Earth and, in fact, most of the carbon in the cycle is
stored in the limestones and dolomites initially precipitated for the most
part by organisms. The nitrogen cycle is considerably complex, and portions of
it are only incompletely known. While almost all the carbon is contained in
the Earth's crust, a significant portion of the nitrogen is stored in the
atmosphere, largely as N2 but also as reactive nitrogen oxides and ammonia.
See NITROGEN CYCLE; NITROGEN FIXATION.
    Organisms, through their metabolic activities, continually influence the
processes and reactions occurring in the environments of the Earth's surface.
Microorganisms, because of their large numbers, biochemical versatility, large
surface-to-volume ratio, and rapid growth rate, are the most active agents in
these influences. Consequently, numerous reactions affecting the cycling of
elements through the biosphere are frequently controlled by microbial
processes. Organisms also indirectly influence the chemical behavior of an
element or species by altering the concentration of other ions or elements
such as H sup + or O2, which in turn affects equilibrium concentrations of the
substance of interest. Mineral surfaces may acquire organic coatings from the
dissolved organic matter, a process which may affect the rate and extent of
equilibration of the mineral with the water.
    Human beings, of course, are part of the biosphere. Some of their
activities have an adverse impact on many ecosystems and on themselves by the
addition of toxic or harmful substances to the outer lithosphere, hydrosphere,
and atmosphere. Many of these materials are eventually incorporated into or
otherwise affect the biosphere. The major types of environmental pollutants
are sewage, trace metals, petroleum hydrocarbons, synthetic organic compounds,
and gaseous emissions. See AIR POLLUTION; ATMOSPHERE; HUMAN ECOLOGY;
HYDROSPHERE; LITHOSPHERE; WATER POLLUTION.
 �34s[Roy Middleton]
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�6v�2v�32sBiostrome �21m �33s
    An evenly bedded and generally horizontally layered stratum composed
mostly of organic remains, normally considered to be those of sedentary
organisms which lived, died, and were buried essentially in place.
    The criterion of formation by in-place growth of organisms is subject to
some interpretation. Crinoidal limestones obviously resulted from the
accumulation of decayed pieces of millions of these stalked echinoderms, often
with accompanying detritus of associated fenestrate bryozoans. Usually it is
impossible to ascertain whether such debris dropped vertically a few
centimeters or meters through the water column as the organisms died and
collapsed (an essentially in-place deposit) or whether the layers of crinoidal
grainstone were piled mechanically by currents. The same is true of coquinas
of many other thin-shelled calcareous tests, such as those of brachiopods,
bryozoans, and trilobites.
    Biostromal layers need not have been horizontally deposited when they
occur as flanking beds around organic buildups. Dips of up to 25 or 30 DG are
possible here. Such biostromes are probably veneers of sessile organisms which
lived somewhat above the realm of deposition and were buried as sediment
cascaded down the flank of a mound.
 �34s[James L. Wilson]
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�6v�2v�32sBiotelemetry   �21m �33s 
    The use of telemetry techniques on living things, telemetry being metering 
(or measuring) at a distance. As the word is generally used, the link between 
the subject and the measurer is understood to be wireless radio waves. In 
cases where radio waves are ineffective, sound waves, light, magnetic 
radiations, or electricity in wires is used. See TELEMETERING. 
    The purpose in using biotelemetry is to be able to measure something on, 
in, or about a living animal or plant that cannot be measured in any other 
way. Biotelemetry is also a method of tracking animals, as they wander about 
their home range or as they migrate, that does not necessitate visual contact 
with the animals. While astronauts and cosmonauts are in space, they need to 
have several body functions monitored to help them perform their job in 
safety. Body temperature, breathing rate, heart rate, blood pressure, and 
alertness are routinely telemetered, first from the space suit to the 
spacecraft, and then to the ground, where a doctor is on hand to diagnose 
changes and to anticipate troubles. 
    Biotelemetry is also used in medicine in the study and treatment of 
diseases in humans and in patient care. Usually a radio link is used so that 
the patient is not tied down by wires. 
    A biotelemetry system has two main parts: a transmitter carried on, or in, 
the animal or plant subject, and a receiver to pick up the transmitted 
information. Sound energy is transmitted by a loudspeaker and received by a 
microphone; light is transmitted by a lamp and received by a photoelectric 
cell; and magnetic fields are both transmitted and received by coils of wire. 
A transducer is needed by both the transmitter and the receiver to convert the 
energy carrying the information to a suitable form. The transmitting 
transducer must convert electrical energy to a flow of energy which can travel 
through space. The receiving transducer must accept energy from space and 
convert it to a current flow in a wire in the receiver. In the case of radio, 
both the transmitting and the receiving transducer are antennas. The simplest 
biotelemetry transmitters are those used for tracking. 
    Sound and ultrasonic waves travel much farther than radio waves 
underwater. For this reason, tagging of seals and fish for tracking purposes 
is done with small sonar transmitters, or pingers, fastened to the outside of 
the animal or fed to it. One of the problems in using sound transmitters on 
aquatic animals is that many of these animals can hear well up into the 
ultrasonic range. Carrying such a transmitter may therefore alter the animal's 
behavior because it can hear its own transmitter. 
    For extremely short-range work in laboratory animals of all kinds, it is 
sometimes desirable to use low-frequency magnetic field transmitters. 
Transmissions at low frequencies will pass through soil, trees, or the body of 
an animal or fish without being appreciably affected. What is perhaps more 
important, low-frequency transmissions may not affect the animal as much as 
the higher frequencies. 
 �34s[John H. Busser]
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�6v�2v�32sBiotin   �21m �33s 
    A vitamin, widespread in nature. It is only sparingly soluble in water; it 
is stable in boiling water solutions, but can be destroyed by oxidizing 
agents, acids, and alkalies. Under some conditions, it can be destroyed by 
oxidation in the presence of rancid fats. Biotin's occurrence in nature is so 
widespread that it is difficult to prepare a natural deficient diet. Biotin 
deficiency in animals is associate with dermatitis, loss of hair, muscle 
incoordination and paralysis, and reproductive disturbances. Biotin deficiency 
produced in humans by feeding large amounts of egg white resulted in 
dermatitis, nausea, depression, muscle pains, anemia, and a large increase in 
serum cholesterol. See COENZYME. 
 �34s[Stanley N. Gershoff]
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�6v�2v�32sBiotite �21m �33s
    The most abundant and widely distributed species of the mica group of
minerals, also called black mica or magnesium-iron mica. Its chemical
composition is variable:

  K sub 2 [Fe(II),Mg] sub 6-4 [Fe(III),Al,Ti] sub 0-2 (Si sub 6-5,Al sub
     2-3)O sub 20-22(OH,F) sub 4-2

The color ranges from black to dark brown or green. Unaltered basal cleavage
sheets (easy and perfect) are both flexible and elastic. The hardness is 2.5-3
(Mohs scale) and specific gravity 2.8-3.2. Biotite itself is of no commercial
importance, but under hydrothermal conditions or when exposed to groundwater
action it transforms to vermiculite, which expands (exfoliates) upon heating
and, has widespread application as lightweight concrete and plaster aggregate,
insulation, plant-growing medium, and lubrication. Biotite grades into
phlogopite, and no sharp division exists between the two. In a general way,
biotite represents the iron-rich half of the biotite-phlogopite series.
    Biotite, an important rock-forming mineral, may be an essential
constituent of both intrusive and extrusive igneous rocks, ranging in
composition from gabbro to granite. It is especially common in granite,
granodiorite, and syenite. In porphyritic extrusive rocks biotite appears
chiefly as phenocrysts, very rarely in the matrix. Many pegmatites also
contain biotite in large amounts in coarse flakes or rough crystals. See MICA;
SILICATE MINERALS.
 �34s[E. William Heinrich]
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�6v�2v�32sBiot-Savart law   �21m �33s 
    A law of physics which states that the magnetic flux density (magnetic 
induction) near a long, straight conductor is directly proportional to the 
current in the conductor and inversely proportional to the distance from the 
conductor. The field near a straight conductor can be found by application of 
Ampere's law. The magnetic flux density near a long, straight conductor is at 
every point perpendicular to the plane determined by the point and the line of 
the conductor. Therefore, the lines of induction are circles with their 
centers at the conductor. Furthermore, each line of induction is a closed 
line. This observation concerning flux about a straight conductor may be 
generalized to include lines of induction due to a conductor of any shape by 
the statement that every line of induction forms a closed path. See AMPERE'S 
LAW; MAGNETIC INDUCTION. 
 �34s[Kenneth V. Manning]
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�6v�2v�32sBiphenyl �21m �33s
    A colorless aromatic hydrocarbon, with the formula C12H10 (melting point
70.0 DG C or 158 DG F, boiling point 255.9 DG C or 492.6 DG F, and density
1.9896g/cm sup 3 or 1.1501oz/in sup 3), also called diphenyl or phenylbenzene,
which crystalizes as small plates. Although small amounts are present in coal
tar, the biphenyl of commerce is obtained by the pyrolysis of benzene at about
750 DG C or as a by-product in the thermal dealkylation of toluene. Biphenyl is
flammable, flash point (open cup) 124 DG C (255 DEG F).
    The high thermal stability and low vapor pressure of biphenyl make it a
valuable heat-transfer medium, which can be used without high-pressure
equipment. In certain applications it is chlorinated or alkylated to produce
agents in some ways superior to the parent hydrocarbon. Unfortunately,
polychlorinated biphenyls (PCBs), despite their much prized thermal and
chemical stability, are definitely hazardous to health if ingested and have
been listed by the Occupational Safety and Health Administration as potential
carcinogens. See AROMATIC HYDROCARBON; POLYNUCLEAR HYDROCARBON.
 �34s[Charles K. Bradsher]
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�6v�2v�32sBirch �21m �33s
    A deciduous tree of the genus Betula which is distributed over much of
North America, in Asia south to the Himalaya, and in Europe. About 40 species
are known. The birches comprise the family Betulaceae in the order Fagales.
The sweet birch, B. lenta, the yellow birch, B. alleghaniensis, and the paper
birch, B. papyrifera, are all important timber trees of eastern United States.
The yellow and the paper species extend into Canada. The gray birch, B.
populifolia, is a smaller tree of the extreme northeastern United States and
adjacent Canada. Both sweet (black) and yellow birches can be recognized by
the wintergreen taste of the bark of the young twigs. A flavoring similar to
oil of wintergreen is obtained from the bark of the sweet birch. The paper and
gray birches can be easily identified by their white bark. The bark of the
paper birch peels off in thin papery sheets, a characteristic not true of the
gray birch. The river birch, B. nigra, is a less common tree of wet soils and
banks of streams and is important as an ornamental and for erosion control.
The hard, strong wood of the yellow and the sweet birches is used for
furniture, boxes, baskets, crates, and woodenware. The European birches, B.
pubescens and B. pendula, are the counterparts of the paper and gray birches
in the United States. European birches are also cultivated in America. See
FAGALES.
 �34s[Arthur H. Graves /Kenneth P.Davis]
�33s

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�6v�2v�32sBird   �21m �33s 
    Any of about 9022 extant species of warm-blooded vertebrates which make up 
the class Aves. They are distributed throughout the world and range from the 
Arctic to the Antarctic. 
    Birds have certain features, particularly the horny bill and feathers, 
which distinguish them from other vertebrates. Feathers are specialized 
epidermal (skin) derivatives, as are many vertebrate structures, such as the 
hair of mammals, and are nonliving when fully developed. Although most species 
are aerial, a number of species are incapable of flight. 
    One group of mainly giant flightless birds are known collectively as the 
ratites, or running birds. This group represents a monophyletic assemblage, 
including the ostrich, emu, rhea, kiwi, and tinamous, most of which are large 
and have lost their ability of flight; they are characterized by the flat 
keelless sternum. Tinamous can still fly and have a strongly keeled sternum. 
Many other flightless birds, for example, dodos, rails, coots, and cormorants, 
are known, some of which also have a flat sternum while others, such as the 
penguins, still possess a keeled sternum. The bones in most birds are light 
and not heavy, massive structures found in terrestrial vertebrates and some 
flightless birds. 
    The fossil record for birds is poorly known, but is better known for some 
groups of water and swamp birds. Flying reptiles, the pterosaurs, are not 
closely related to birds and evolved flight independently. Archaeopteryx, the 
earliest known bird, is an excellent example of an intermediate link. It 
possessed feathers and a wing, but the hand bones are not fused. However, 
Archaeopteryx retained a long tail, teeth, and a reptilian skull. See AVES; 
BIRDS, UPLAND GAME; BIRDS OF PREY; FEATHER. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sBirds of prey �21m �33s
    Birds of prey belong to two unrelated orders: the Falconiformes, which
includes the eagles, falcons, hawks, vultures, and buzzards; and the
Strigiformes, or owls (see illustration). There are a number of species that
are economically important as predators upon mice, rats, and other pests, as
well as on game and poultry. Many of these birds are nocturnal while others
are diurnal. Members of both orders show similarities in the shape of the bill
and have strong claws, adaptations for their mode of life; otherwise, they are
dissimilar. Rather dramatic decreases in populations of predatory birds have
not been the result of destruction of natural habitats, but are related to the
incorporation of chlorinated hydrocarbons into the ecological systems.
    Diurnal species have long, powerful wings for sustained flight and are
usually large, fierce, and strong, with a keen sense of vision. The toes on
the short, strong legs end in pointed talons. Normally, the birds prey upon
warm-blooded vertebrates, but some species are carrion feeders while others
prefer fish or amphibians. These birds are monogamous and usually nest in
inaccessible areas. The young are helpless for a considerable period of time
after hatching.
    New World vultures, members of the family Cathartidae, are voiceless
species. They include the California condor (Gym nogyps californianus), the
largest North American bird, which is on the verge of extinction, the Andean
condor (Vultur gryphus), the king vulture (Sarcorhamphus papa), and the turkey
buzzard or turkey vulture (Cathartes aura).
    Old World vultures are large predatory birds, such as kites, harriers,
true hawks, and eagles, which are classified as subfamilies in the family
Accipitridae. Included are the bald or American eagle (Haliaeetus
leucocephalus), the golden eagle (Aquila chrysaeetos), and the true hawks,
exemplified by the goshawk (Accipiter gentilis) and the European sparrowhawk
(A. nisus). Other hawks, grouped together in the genus Buteo, include the
red-tailed hawk (B. jamaicensis), the broad-winged hawk (B. platyperus), and
the rough-legged hawk (B. lagopus).
    The family Pandionidae includes a single species, Pandion haliaeetus, the
osprey or fish hawk. This bird lives near water and feeds on fish.

    Falcons are included in the family Falconidae. They usually nest in
forested areas and are considered to be the most specialized birds of prey
because they destroy large numbers of animal pests, but do not overkill. Birds
that have been captured and trained for hunting include the peregrine falcon
(Falco peregrinus) and the gyrfalcon (F. rusticolus).
    The owls are frequently referred to as the nocturnal birds of prey. Unlike
the diurnal species, owls are silent fliers, mainly because they have soft
feathers that extend down to the toes. They differ from most birds in that the
eyes are located in a frontal position rather than on the lateral side of the
head. Their keen sense of hearing has led many observers to believe that some
species hunt by sound. Some of the more common species of owls are Tyto alba,
the barn owl; Asio otus, the long-eared owl; Bubo virginianus, the great
horned owl; Otus asio, the screech owl; Nyctea scandiaca, the snowy owl; and
Speotyto cunicularia, the burrowing owl. See AVES; FALCONIFORMES; STRIGIFORMES.




 �34s[Charles B. Curtin]
�33s

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�6v�2v�32sBirds, upland game   �21m �33s 
    A general category for a large number of birds that are hunted for sport, 
the majority being members of the order Galliformes. Included are the 
pheasants, quails, grouse, partridges, guinea fowls, turkeys, and the less 
commonly known brush turkeys and curassows. 
    All members of the Galliformes have small heads and thick- set bodies. The 
short wings are not adapted to sustained flight, but the long sturdy legs make 
them good runners; most species are terrestrial and nonmigratory. That they 
are graminivorous (seedeaters) is apparent by the stout curved bill and strong 
claws used for scratching soil in search of seeds. Sexual dimorphism is well 
developed in this group with differences in size, plumage, comb, and wattle. 
The males are usually polygamous, fierce, and aggressive; the females build 
nests and rear the young alone; and the young are precocial, that is, they are 
capable of leaving the nest soon after hatching and feeding by themselves, but 
are usually brooded and cared for by the female for several weeks.See SEXUAL 
DIMORPHISM.  
    Natural habitats of these birds have been diminishing for years as a 
result of human encroachment for a variety of reasons. Extensive efforts have 
been made to maintain breeding populations in nature. Upland game birds are 
extensively hunted, and legal restrictions, such as limits on quantity and a 
season for hunting, have been imposed in most states. Many state game and 
fisheries commissions raise and release young in selected areas or have 
farmers incubate eggs with the eventual release of young in selected areas. 
    The family Meleagrididae includes two species of wild turkeys found in 
North and Central America. The common turkey (Meleagris gallopavo), from which 
the domestic turkey has been bred, is found along the eastern and southern 
coasts of the United States from Canada to Mexico, and the ocellated turkey 
(Agriocharis ocellata) is found in Central America and Mexico. A 
characteristic feature of these birds is the bare head and neck. 
    The family Phasianidae contains the quails, partridges, peafowls, and true 
pheasants. The American quail constitute a distinct subfamily unique to the 
New World, and no other members of the family are native there. Typically, 
members of the Phasianidae are ground feeders. The partridges belong to two 
genera, Alectoris and Perdix, of which the most typical representatives are 
the common partridge (P. perdix) and the red-legged partridge (A. rufa). These 
birds are commonly found in open country living in groups. European quail 
differ from partridges in having a shorter beak and feathers around the eyes 
and in being less social. They are among the few species of game birds that 
migrate, forming flocks only for this activity. The bobwhite (Colinus 
virginianus) is the commonest American quail, found in the southern United 
States where it is often called a partridge. The principal game bird is 
probably the ring-necked pheasant (Phasianus colchicus), a species that has 
been introduced into the United States, where it is now common and has partly 
displaced the bobwhite. Other species include Reeve's pheasant (Syrmaticus 
reevesii), the silver pheasant (Lophura nycthemera), and the golden pheasant 
(Crysolophus pictus). 
    The Tetraonidae include 18 recognized species of ptarmigans and grouse. 
Members of this family are restricted to the Northern Hemisphere and have a 
circumpolar distribution. The rock ptarmigan (Lagopus mutus) and the willow 
ptarmigan (L. lagopus) are found throughout the colder regions of the Northern 
Hemisphere. Another ptarmigan, the red grouse (L. scoticus), is a species 
believed by some to be an aberrant form of the willow ptarmigan. A fourth 
species, the white-tailed ptarmigan, is found in the Rocky Mountains. The 
largest grouse is the capercaille or wood grouse (Tetrao urogallus) with a 
wing span up to 5 ft (1.5 m). Though once widespread in central Europe, 
hunting and disease have reduced this species to a more restricted area in 
some parts of France, Scandinavia, and the Soviet Union. The ruffed grouse 
(Bonasa umbellus) is a resident of mixed and deciduous forest areas in Canada 
and the United States. 
    The guinea fowl (Numididae) are common in Africa and adjacent islands, 
where they live in large groups in forest clearings and feed on insects or 
plants, depending on the season. The common guinea fowl (Numida meleagris) has 
been domesticated for over 2000 years. 
    The little-known megapodes (Megapodiidae; brush turkeys), native to 
Australia, are also known as incubator birds, mound builders, and thermometer 
birds. Curassows (Cracidae) and their relatives range from Texas to Argentina. 
See AVES; BIRD; GALLIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sBirefringence �21m �33s
    The splitting which a wavefront experiences when a wave disturbance is
propagated in an anisotropic material; also called double refraction. In
anisotropic substances the velocity of a wave is a function of displacement
direction. Although the term birefringence could apply to transverse elastic
waves, it is usually applied only to electromagnetic waves.
    In birefringent materials either the separation between neighboring atomic
structural units is different in different directions, or the bonds tying such
units together have different characteristics in different directions. Many
crystalline materi als, such as calcite, quartz, and topaz, are birefringent.
Diamonds, on the other hand, are isotropic and have no special effect on
polarized light of different orientations. Plastics composed of long-chain
molecules become anisotropic when stretched or compressed. Solutions of
long-chain molecules become birefringent when they flow. This first phenomenon
is called photoelasticity; the second, streaming birefringence. See CRYSTAL
OPTICS; PHOTOELASTICITY; POLARIZED LIGHT; REFRACTION OF WAVES.
 �34s[Bruce H. Billings]
�33s

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�6v�2v�32sBirthmark �21m �33s
    The common term for several types of skin lesions which are present at
birth or appear sometime later. Two major types are recognized, the cellular
and the vascular nevi. Both are abnormal, benign growths of some tissue
component native to an area and result from faulty development. Most are
pigmented, and the color depends upon the amount of pigment or blood present
and the nearness to the surface of the skin. Pigmented nevi, or common moles,
vary from natural flesh color through a range of increasing pigmentation to
dark brown or black. See MOLE (MEDICINE).
    Moles range in size from a few millimeters to several centimeters; they
may be flat or elevated, smooth or rough, and other features can be present.
Rarely does a tendency toward malignant change occur, so that moles which
deepen in color and show increases in size or other changes should be
evaluated promptly. The common hairy brown mole is least prone to show
malignant changes, especially if present on the legs from childhood.
    Vascular birthmarks are either developmental abnormalities of blood
vessels or benign neoplasms which arise postnatally. Capillary hemangiomas
consist of local overgrowths of small blood vessels. Port wine stains are
forms of this lesion which appear as large plaquelike areas of red to blue
discoloration on the face or trunk. A strawberry hemangioma is typically a
soft, raised nodule of varying size which often regresses spontaneously in
childhood. These may be congenital or may appear during the first few weeks of
life. Cavernous hemangiomas consist of large dilated vessels which frequently
have smaller vessels within them. Although they occur as birthmarks on the
skin, they are also found in deeper tissues, such as the liver and brain. See
ONCOLOGY; TUMOR.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sBismuth �21m �33s
    The metallic element, Bi, of atomic number 83 and atomic weight 208.980
belonging in the periodic table to group Va. Bismuth is the most metallic
element in this group in both physical and chemical properties. The only
stable isotope is that of mass 209. It is estimated that the Earth's crust
contains about 0.00002% bismuth. It occurs in nature as the free metal and in
ores. The principal ore deposits are in South America. However, the primary
source of bismuth in the United states is as a by-product in refining of
copper and lead ores.
    The main use of bismuth is in the manufacture of low-melting alloys which
are used in fusible elements in automatic sprinklers, special solders, safety
plugs in compressed gas cyl-linders, and automatic shutoffs for gas and
electric water-heating systems. Some bismuth alloys, which expand on freezing,
are used in castings and in type metal. Another important use of bismuth is in
the manufacture of pharmaceutical compounds.
    Bismuth is a gray-white, lustrous, hard, brittle, coarsely crystalline
metal. It is one of the few metals which expand on solidification. The thermal
conductivity of bismuth is lower than that of any metal, with the exception of
mercury. The table cites the chief physical and mechanical properties of
bismuth. Bismuth is inert in dry air at room temperature, although it oxidizes
slightly in moist air. It rapidly forms an oxide film at temperatures above
its melting point, and it burns at red heat, forming the yellow oxide, Bi2O3.
The metal combines directly with halogens and with sulfur, selenium, and
tellurium; however, it does not combine directly with nitrogen or phosphorus.
Bismuth is not attacked at ordinary temperatures by air-free water, but it is
slowly oxidized at red heat by water vapor.
    Almost all compounds of bismuth contain trivalent bismuth. However,
bismuth can occasionally be pentavalent or monovalent. Sodium bismuthate and
bismuth pentafluoride are perhaps the most important compounds of Bi(V). The
former is a powerful oxidizing agent, and the latter a useful fluorinating
agent for organic compounds.



 �34s[Samuel J. Yosim]
�33s

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�6v�2v�32sBison �21m �33s
    The name for two species of the Bovidae in the mammalian order
Artiodactyla, found in North American and Europe. The European bison (Bison
bonasus) is commonly known as the wisent. The American species (B. bison),
shown in the illustration, is often called buffalo but should not be confused
with the African buffalo.
    The European bison was originally abundant in the forested areas of
western Europe during the late Cenozoic Era. The wisent is a browsing,
woodland animal which congregates in relatively small herds. It is almost
extinct in its natural range and, although a few herds exist, it is known
mainly from a few hundred specimens preserved in zoological gardens and zoos.
An intensive effort has been made to preserve the species by breeding and
maintaining this animal in captivity.
    Although there are differences, the wisent is closely allied to the
American species. The wisent has a small head carried in a high position and a
short mane, and is more graceful and less massive than the North American
species. The hump is less noticeable, the legs are longer, the horns are more
slender, and the body is not so shaggy.
    Enormous herds of the American bison existed on the Plains area of North
American in western Canada and the western United States during the 19th
century. It is estimated that there are still about 20,000 bison in Canada and
the United States on preserves and national parks. These bovines are massive,
with the males attaining a length of 9 ft (2.7 m), a height of 6 ft (1.8 m) at
the shoulder, and weight up to 3000 lb (1350 kg). They are herbivorous, and
migrated originally when the grass or forage became scarce. The large head is
held in a low position, and behind the neck region is a characteristic hump.
The forequarters are shaggy with the hair more pronounced in the male than the
female. The senses of smell and hearing are well developed, while vision is
poor. Both the male and female have horns which are small and set far apart.
See ARTIODACTYLA; BUFFALO; MAMMALIA.



 �34s[Charles B. Curtin]
�33s


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�6v�2v�32sBit   �21m �33s 
    In the pure binary numeration system, either of the digits 0 or 1. The 
term may be thought of as a contraction of binary digit. See NUMBER SYSTEMS. 
    In a binary notation, bits are used as two different characters. For 
example, in the American National Standard Code for Information Interchange 
(ASCII), a seven-bit coded character set, the seven bits 1000111 represent the 
letter G. 
    Bit is widely used as a synonym for binary element, a constituent element 
of data that takes either of two values or states (on-off, yes-no, zero-one, 
and so forth). The brains of animals and registers, memories, and other 
storage devices of digital computers and electronic calculators store bits 
(binary elements) as the smallest unit of information. 
    A byte is a string of bits (binary elements) operated on or treated as a 
unit and usually shorter than a word. An eight-bit byte comprises eight bits. 
A word is a string of bits (binary elements) that consists of two or more 
bytes. The terms halfword, fullword, and doubleword are also used. A computer 
word is a word stored in one computer location, usually 16, 32, 36, 48, or 64 
bits in length, depending on the design of the computer. In microcomputers or 
in particular applications, a nybble is a string of bits (binary elements) 
operated as a unit, larger than a bit, and smaller than a byte. See DIGITAL 
COMPUTER.  
    In information theory, bit is a synonym for the new, preferred term 
shannon, a unit of measure of information equal to the decision content of a 
set of two mutually exclusive events. For example, the decision content of a 
character set of eight characters equals three shannons (the logarithm of 8 to 
base 2). A shannon also is called an information content binary unit. A 
hartley is the information content decimal unit. See INFORMATION THEORY. 
 �34s[Helmut E. Thiess]
�33s 
  
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�6v�2v�32sBitumen   �21m �33s 
    A term used to designate naturally occurring or pyrolytically obtained 
substances of dark to black color consisting almost entirely of carbon and 
hydrogen with very little oxygen, nitrogen, and sulfur. Bitumen may be of 
variable hardness and volatility, ranging from crude oil to asphaltites, and 
is largely soluble in carbon disulfide. See ASPHALT AND ASPHALTITE. 
 �34s[Irving A. Breger]
�33s 
  
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�6v�2v�32sBivalvia   �21m �33s 
    One of the two largest classes in the phylum Mollusca. The class is 
sometimes termed Pelecypoda or Acephala, or known by the name of its largest 
subclass, Lamellibranchia. All bivalves are aquatic, living at all depths in 
the sea and also in brackish and fresh waters. Although the number of living 
bivalve species (possibly 30,000) is second to the class Gastropoda (over 
74,000), the total biomass of marine bivalves is much greater, and certain 
bivalve species are the most abundant marine benthic animals. The most 
primitive bivalves are all infaunal, burrowing into soft substrates, but many 
families are epifaunal, attached to rocks. 
    Bivalvia are basically bilaterally symmetrical and laterally compressed, 
with the mantle lobes and thus the shell which they secrete entirely enclosing 
the body. The head is only the site of the mouth and is without the jaws or 
radula of other mollusks, but is flanked by pairs of extended lips forming 
ciliated labial palps. The ciliated molluscan gills (ctenidia) are enlarged, 
fully occupying the extensive mantle cavity. The shell consists of two valves 
with a connecting ligament. It is formed of three layers. The ligament is 
formed of the same three layers similarly produced, but is not calcified. The 
body is attached to the shell by one or more pairs of pedal retractors with 
sometimes an additional pair of pedal protractors. The foot is extended by 
blood pressure, then dilated terminally to form an anchor on which the animal 
is drawn forward when the retractors contract. A few bivalves, notably 
scallops and file shells, can swim by jet propulsion, water being suddenly 
ejected from between the flattened valves. 
    Bivalves may be hermaphroditic or have separate sexes. Many species show 
sex change, usually from male to female (protandric hermaphroditism); a few, 
notably certain oysters, show alternating sexuality. 
    Various bivalves are economically important. Some, notably oysters, 
mussels, hard and soft shell clams, and scallops, are widely eaten, many being 
cultivated. Others are a major food of important food fishes, such as cod, 
halibut, and plaice. Marine and fresh-water mussels, along with other attached 
bivalves, may be serious fouling agents, while shipworms (Teredinidae) do 
serious damage to wooden vessels and harbor works. 
 �34s[C. Maurice Yonge]
�33s 
    The fossil record of the Bivalvia can be traced to the Lower Cambrian 
Fordilla, a small, laterally compressed shell with a simple, rounded profile. 
Fordilla did not become abundant until the Ordovician Period, several million 
years later. However, throughout the Paleozoic Era, these bivalves remained 
second to bivalves of the phylum Brachiopoda in diversity and abundance in 
most aquatic environments except the ocean margins and fresh-water 
environments. During the Mesozoic Era, the Bivalvia gradually took over as the 
dominant marine bivalves. The transition from Mesozoic to Cenozoic time saw 
the extinction of the rudists and the gradual replacement of many ancient 
superfamilies with a great variety of newer superfamilies. However, many 
ancient superfamilies have maintained a high taxonomic diversity to the 
present day. See LAMELLIBRANCHIA; MOLLUSCA; PROTOBRANCHIA; SEPTIBRANCHIA; 
SHIPWORM  
 �34s[J. G. Carter]
�33s 
  
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�6v�2v�32sBlackberry �21m �33s
    Those species of the genus Rubus, plant order Rosales, having fruits that
persist on the receptacles. The plants are upright or trailing shrubs, usually
thorny, with perennial roots and biennial canes (stems). Horticulturally,
these fruits are classified as upright and trailing blackberries and
black-fruited and red-fruited dewberries, the latter being closely related to
the raspberry, with which there are a few hybrids. See FRUIT.
    Blackberries are grown in home gardens over most of the United States,
except for the coldest parts of the Great Plains area and the hottest part of
the South. Commercial production is extensive in many states. The upright
types are grown in the North, whereas the more tender dewberries and trailing
blackberries are grown in the South and along the Pacific Coast. States
usually leading in blackberry production are Oregon, Texas, California,
Washington, Michigan, Arkansas, Oklahoma, Alabama, and North Carolina.
Although the fruit is sold fresh or canned for dessert purposes and is made
into jelly or jam, quick-freezing is the largest outlet for the crop. See
ROSALES.
 �34s[J. Harold Clarke]
�33s

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�6v�2v�32sBlackbody   �21m �33s 
    An ideal energy radiator, which at any specified temperature emits in each 
part of the electromagnetic spectrum the maximum energy obtainable per unit 
time from any radiator due to its temperature alone. A blackbody also absorbs 
all the energy which falls upon it. The radiation properties of real radiators 
are limited by two extreme cases - a radiator which reflects all incident 
radiation, and a radiator which absorbs all incident radiation. Neither case 
is completely realized in nature. Carbon and soot are examples of radiators 
which, for practical purposes, absorb all radiation. Both appear black to the 
eye at room temperature, hence the name blackbody. Often a blackbody is also 
referred to as a totol absorber. See HEAT RADIATION. 
 �34s[Heinz G. Sell ;Peter J. Walsh]
�33s 
  
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�6v�2v�32sBlack dwarf �21m �33s
    A star that cannot generate thermonuclear energy. Not necessarily
invisible, it may radiate its internal heat. None has ever been observed, but
as-yet-invisible companions of nearby faint stars have been suspected of
having masses within the probable range. Isolated very low masses cannot form
by gravitational contraction from interstellar gas and dust. Thermonuclear
energy is released if the core temperatures (proportional to the mass of a
self-gravitating gas sphere) exceed 2 x 10 sup 6 K (3.6 x 10 sup 6| DG F). Thus,
black dwarf masses should be between 0.007 and 0.07 solar mass.
 �34s[Jesse L. Greenstein]
�33s

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�6v�2v�32sBlack hole �21m �33s
    One of the end points of gravitational collapse, in which the collapsing
matter fades from view, leaving only a center of gravitational attraction
behind. General relativity predicts that, if a star of more than about 3 solar
masses has completely burned its nuclear fuel, it should collapse to a
configuration known as a black hole. The resulting object is independent of
the properties of the matter that produced it and can be completely described
by stating its mass and spin. The most striking feature of this object is the
existence of a surface, called the horizon, which completely encloses the
collapsed matter. The horizon is an ideal one-way membrane; that is, particles
and light can go inward through the surface, but none can go outward. As a
result, the object is dark, that is, black, and hides from view a finite
region of space (a hole). See GRAVITATIONAL COLLAPSE; RELATIVITY.
    The possible formation of black holes depends critically on what other end
points of stellar evolution are possible. There can always be chunks of cold
matter which are stable, but their mass must be considerably less than that of
the Sun. For masses on the order of a solar mass, only two stable
configurations are known for cold, evolved matter. The first, the white dwarf,
is supported against gravitational collapse by the same quantum forces that
keep atoms from collapsing. However, these forces cannot support a star which
has a mass in excess of 1.2 solar masses, this limiting value being known as
the Chandrasekhar limit. The second stable configuration, the neutron star, is
supported against gravitational collapse by the same forces that keep the
nucleus of an atom from collapsing. There is also a maximum mass for a neutron
star, estimated to be between 1 and 3 solar masses, the uncertainty being due
to the poor knowledge of nuclear forces at high densities. It would appear
from the theory that if a collapsing star of over 3 solar masses does not
eject matter, it has no choice but to become a black hole. Since there are
many stars of over 3 solar masses, and more massive stars evolve more quickly,
a considerable number of black holes should have formed.
    Because black holes themselves are dark, and therefore essentially
unobservable, their existence must be inferred from their effect on other
matter. Thus a theoretical model must be constructed to explain the
observational data and to show that the most reasonable explanation is that a
black hole is responsible for the observed effects. Such is the case with the
binary x-ray star system Cygnus X-1. There are a number of binary x-ray
systems known. The model which best explains the data is one in which a fairly
normal star is in mutual orbit about a very compact object. Because these two
are so close, mass flows from the star onto an accreting disk about the
compact object, as shown in the illustration. As the mass in the disk spirals
inward, it heats up by frictional forces. Because the central body is so
compact, the matter heats to a temperature at which thermal x-rays are
produced. The only compact objects known that could accomplish this are
neutron stars and black holes. The existence of very short time bursts of
radiation also points to an object of small diameter, that is, compact. In
some of these binary x-ray systems, there is also a regular pulsed component
to the x-rays, indicating a rotating neutron star (by reasoning similar to
that given for pulsars). In these systems, the compact object could not be a
black hole because that would imply a more complicated structure than a black
hole would allow. In other systems, however, there are only irregular
pulsations or fluctuations; they are candidates for possible black holes. In
one of these systems, the Cygnus X-1, a rough mass determination has been
made, with the result that the compact object must be more massive than 5
solar masses. According to theory, there is no compact system with this mass
other than a black hole. It is in this sense that it can be said that black
holes have been observed. See BINARY STAR; X-RAY ASTRONOMY; X-RAY STAR.



 �34s[Philip C. Peters]
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�6v�2v�32sBlackleg   �21m �33s 
    An acute, usually fatal, disease of cattle and occasionally of sheep, 
goats, and swine. The infection is caused by Clostridium chauvoei (C. feseri), 
a strictly anaerobic, spore-forming bacillus of the soil. The disease is also 
called symptomatic anthrax or quarter-evil. The characteristic lesions in the 
natural infection consist of crepitant swellings in involved muscles, which at 
necropsy are dark red, dark brown, or blue black. Artificial immunization is 
possible; animals surviving an attack of blackleg are permanently immune to 
recurrence of the disease. See IMMUNITY. 
 �34s[Leland S. McClung]
�33s 
  
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�6v�2v�32sBlack pepper �21m �33s
    One of the oldest and most important of the spices. It is the dried,
unripe fruit of a weak climbing vine, Piper nigrum, a member of the pepper
family (Piperaceae), and a native of India or Indomalaysia. The fruits are
small one-seeded berries which, in ripening, undergo a color change from green
to red to yellow. When in the red stage, they are picked, sorted, and dried.
The dry, wrinkled berries (peppercorns) are ground to make the familiar black
pepper of commerce. White pepper is obtained by grinding the seed separately
from the surrounding pulp. See PIPERALES.
 �34s[Perry D. Strausbaugh /Earl L.Core]
�33s

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�6v�2v�32sBlack Sea �21m �33s
    A body of water that with the Azov, Caspian, and Aral seas forms a series
of bodies of water extending far into the Eurasian landmass. As the distance
from the Mediterranean increases, these seas (or lakes) freshen and the
composition of their salts departs more widely from that of the oceans. The
Black Sea is tenuously connected with the world ocean through the Bosporus,
the Sea of Marmara, the Dardanelles, and the Mediterranean Sea.
    Runoff and precipitation on the Black Sea exceed losses by evaporation,
causing a net outward flow through the surface layers of the Bosporus and a
compensating inflow of Mediterranean water along the bottom. The upper layers
of the Black Sea are so diluted that the density increases rapidly with depth,
preventing effective vertical mixing. Because of this and the shallowness of
the Bosporus (130 to 300 ft or 40 to 90 m), the deep waters of the Black Sea
are stagnant.
    The surface currents form two counterclockwise gyres with complicated
countercurrents between them, and a current sets outward through the Bosporus.
The Bosporus undercurrent introduces water denser than that in the Black Sea,
but this inflow is so small that it would take 2500 years for it completely to
replace the water below a depth of 100 ft (30 m). The tides of the Black Sea
are semidiurnal and of small amplitude: about 3.5 in (9 cm) along the
central-eastern and central-western coasts and only 0.8 to 1.2 in (2 to 3 cm)
on the Crimean coast.
    The Black Sea is distinctly less saline than the open ocean, but its salt
has nearly the same composition as that of ocean salt, indicating that the
Black Sea is a part of the ocean system, not a saline lake. River inflow
lowers the salinities in the northwestern part of the sea, which allows the
formation of ice as much as 70 mi (112 km) offshore in exceptionally severe
winters. Surface temperatures vary from the freezing point in winter to over
72 DG F (22 DG C) in summer, but at depths greater than 480 - 660ft (150 -
200 m), the temperature is remarkably constant at around 47 DG F (8.75 DG C).
    The Black Sea has fewer species of plants and animals than the
Mediterranean, but more than the Azov, Caspian, or Aral seas. The 1500 animal
species are mainly a depleted Mediterranean fauna with a few relict Caspian
types and, near the Danube delta, a few fresh-water forms. There are around
170 fish species, of which over 100 are Mediterranean immigrants; 20% of the
fishes are of commercial use. Among the most important of these are mackerel,
bonito, anchovies, herring, mullets, carp, and sturgeon. See MEDITERRANEAN
SEA; SEA WATER.
 �34s[Francis A. Richards]
�33s

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�6v�2v�32sBlack shale   �21m �33s 
    A dark mud rock rich in organic carbon. Black shales are typically very 
fine-grained and contain pyrite, phosphate, and abnormally large amounts of 
heavy metals. They commonly display excellent fissility and well-preserved 
planktonic and nektonic faunas and plant debris. Benthic fossils are rare or 
absent. Some black shales are sources of hydrocarbons. See SHALE. 
    Black shales are enigmatic deposits. Although the large organic carbon 
content (3 - 15%) must have required reducing conditions of deposition, there 
are few unambiguous indicators of the specific environment, most especially of 
the depth of water. See MARINE SEDIMENTS; SEDIMENTATION (GEOLOGY). 
    Black shales are typically well laminated on a scale of millimeters. 
Laminae are produced by variations in the supply of sediment, such as seasonal 
alternations of clay and planktonic algae. Delicate laminae can be preserved 
only in the total absence of benthic life, for burrowing animals disrupt 
lamination, producing bioturbated texture. Hence it is possible to 
differentiate between totally anaerobic conditions of deposition and 
marginally oxygenated (dysaerobic) conditions by recognizing laminated or 
bioturbated fabrics in a shale. See FACIES (GEOLOGY). 
 �34s[Charles W. Byers]
�33s 
  
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�6v�2v�32sBlasting �21m �33s
    The process of loosening or fragmenting solid materials such as rock,
masonry, or hard clays through the use of explosives to permit their
excavation. Blasting consists of drilling and cleaning blast holes, loading
the holes with a suitable explosive and igniter or detonator, stemming the
hole (tamping sand or clay over the charge to help prevent dissipation of its
explosive force), and firing or setting off the charge.
    The most common explosives are ANFO (ammonium nitrate - fuel oil), water
gels, and dynamite. Black powder, the original commercial explosive, is almost
never used today. About 80% of all blasting is done with ANFO because of its
low cost. Dynamite and water gels are used for special situations.
    Dynamite and some water gels are detonated with blasting caps or
detonating cord, while other water gels and ANFO require a high-explosive
primer detonated with a cap or detonating cord. Electrical blasting caps are
detonated with blasting machines, either hand-driven or battery-powered, or
from power lines through special switches. Modern nonelectrical systems reduce
the posibility of premature detonation because they are not affected by stray
electric currents. Explosives in most blasts involving more than a few holes
are detonated in sequence to provide better rock breakage and reduce vibration
or shock in the earth that could cause damage. The sequential delays between
holes are provided by delay blasting caps, delay devices in detonating cord
lines, sequential blasting machines with electronic delay circuits, or a
combination of these methods.
    The blast holes near an exposed face in construction or quarrying are
detonated first to move rock out of the way of material which will be moved by
explosives further from the face. In a tunnel, holes near the center of the
face are detonated first to provide space for the rock nearer the edges to
move into. See CONSTRUCTION METHODS.
 �34s[William Diegel]
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�6v�2v�32sBlastoidea �21m �33s
    A class of extinct Pelmatozoa in the subphylum Crinozoa, which arose in
the Ordovician and flourished during Carboniferous times; they did not survive
the Permian. The symmetrical bud-shaped theca comprised 13 rigid plates
arranged in three horizontal rings of three basals, five radials, and five
deltoids. There was often a jointed aboral stem. The ambulacral grooves were
carried on five lancet plates lying in notches in the radial and deltoid
plates. Series of lateral plates margined the lancet plates on either side,
and each lateral plate supported a brachiole (see illustration). Each
brachiole carried a branch of the ambulacral groove. Underneath the ambulacra
the radial and deltoid plates were thrown into vertical pleats whose folds
hung into the coelom. The reverse folds on the upper side opened to the
exterior via pores. See CRINOIDEA; CRINOZOA; ECHINODERMATA; PELMATOZOA.



 �34s[Howard B. Fell]
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�6v�2v�32sBlastomycosis �21m �33s
    A fungus disease caused by Blastomyces dermatitidis. It was formerly
thought that this disease was limited to the United States and Canada, but
there have been reports of several cases in Africa.
    There are two recognized forms of blastomycosis: cutaneous and systemic.
In most instances, the cutaneous form is a clinical manifestation of
dissemination from some primary site, probably the respiratory tract. However,
a large percentage of cases with cutaneous lesions fail to reveal any lesions
in the lungs. Systemic blastomycosis may involve the lungs, with dissemination
to the viscera, central nervous system, or bones, as well as the skin.
Treatment of blastomycosis requires the use of amphotericin B. Another drug,
2-hydroxystilbamidine, may be used if the disease is not severe. See
AMPHOTERICIN B; ASCOMYCOTINA; MEDICAL MYCOLOGY.
 �34s[Leanor D. Haley]
�33s

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�6v�2v�32sBlastulation   �21m �33s 
    The formation of a segmentation cavity or blastocoele within a mass of 
cleaving blastomeres and rearrangement of blastomeres around this cavity in 
such a way as to form the type of definitive blastula characteristic of each 
species. The blastocoele originates as an intercellular space which sometimes 
arises as early as the four- or eight-cell stage. Thus blastulation is 
initiated during early cleavage stages, and formation of the definitive 
blastula is thought to terminate cleavage and to initiate gastrulation. 
Initially the diameter of the blastula is no greater than that of the 
activated egg; subsequently it increases. See CLEAVAGE (EMBRYOLOGY); 
GASTRULATION.  
    The blastula is usually a hollow sphere. Its wall may vary from one to 
several cells in thickness. In eggs which contain considerable amounts of yolk 
the blastocoele may be eccentric in position, that is, shifted toward the 
animal pole. The animal portion of its wall is always completely divided into 
relatively small cells, whereas the vegetative portion tends to be composed of 
relatively large cells and may be incompletely cellulated in certain species. 
The blastocoele contains a gelatinous or jellylike fluid, which originates in 
part as a secretion by the blastomeres and in part by passage of water through 
the blastomeres or intercellular material, or both, into the blastocoele. 
    The wall of the blastula is a mosaic of cellular areas, each of which will 
normally produce a certain structure during subsequent development. In other 
words, each area of cells in the wall of the blastula has a certain 
prospective fate which will be realized in normal development. 
 �34s[Ray L. Watterson]
�33s 
  
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�6v�2v�32sBleaching   �21m �33s 
    The process in which natural coloring matter is removed from a fiber to 
make it white. The process may be used on fiber, yarn, or fabric. Prior to the 
bleaching of a fabric, preliminary purification processes should be used. 
These processes remove applied encrustants (desizing) and natural encrustants 
(scouring or boil-off) so that the bleaching agent may act uniformly on the 
material free of impediment. See DESIZING; SCOURING. 
    Bleaching is also classified as a purification process and varies with the 
content of the substrate or fibrous content of the material. It should not be 
confused with the stripping process, which is the removal of applied color. 
    The fabric off the loom is called gray, grey, or greige goods to 
distinguish it from the partially or completely finished fabric. 
    The three most prominent commercial bleaching processes are the peroxide, 
the chlorine, and the chlorite, in that order. In home bleaching the 
predominant bleach is the chlorine bleach, followed by the use of peroxygen 
compounds such as persulfates and perborates. The latter two are recommended 
for minimum care and permanent-press fabrics to preclude the yellowing of the 
whites and the stripping of colored fabrics, which are always potentials when 
chlorine bleaches are used. 
    Optical bleaching uses organic compounds which are capable of absorbing 
waves shorter than visual waves and emitting waves within the visible range. 
The most notable of the short waves absorbed is the ultraviolet light present 
in daylight. These compounds absorb ultraviolet light which is less than 400 
micrometers (0.02 in.) and emit blue light which is in the visible range of 
400 - 700 micrometers (0.02 - 0.03 in.). This emitted blue light will 
counteract the yellow on the fabric surface and by the subtractive theory of 
color will produce a white. These agents are also characterized as brighteners 
and are extensively used in household detergents. 
    There are several bleaching systems which are classified as combination 
bleaches because two different bleaching agents are used together or in 
tandem. These have been projected to lower the cost, lower the degradation, or 
shorten the cycle as well as to preclude equipment damage in certain cases. 
See NATURAL FIBER; OXIDIZING AGENT; TEXTILE CHEMISTRY. 
 �34s[John J. McDonald]
�33s 
  
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�6v�2v�32sBlimp �21m �33s
    A name originally applied to nonrigid airships and usually implying small
size. Early blimps contained about 100,000 ft sup 3 (2800 m sup 3) and were only
a fraction of the size of the rigid airships of that period. The nonrigid
pressure-type airship, however, did not stay small, but each succeeding model
built was substantially larger than its predecessor until ships with volumes
of approximately 1,500,000 ft sup 3 (42,000 m sup 3) were operational. With the
advent of these larger sizes, the name blimp tended to be replaced by the more
general term airship. However, the principles of construction of these ships
are basically the same regardless of the size. See AIRSHIP.
    The fabric of the main envelope or pressure hull is usually made up of two
or three plies of cloth impregnated with an elastomer; at least one of these
plies is placed in a bias direction with respect to the others. This tends to
make the fabric more resistant to shear loads and results in a stabilized
structure. The materials used for the envelope must be lightweight, extremely
strong, and resistant to gas diffusion. The airship envelope is a symmetrical
airfoil in the form of a body of revolution, circular in cross section and
elliptical along its longitudinal axis.
    Inside the envelope are catenary curtains (see illustration) that support
the weight of the car by distributing the loads imposed by it into the
envelope fabric. This suspension system consists of cable systems attached to
the car which terminate in fabric curtains, which in turn are cemented and
sewed to the envelope proper. The envelope also contains one or more air
cells, called ballonets, fastened to the bottom or sides of the envelope,
which are used to maintain the required pressure in the envelope without
adding or valving of gas as the ship ascends or descends.
    The only commercial airships operating in the United States are the
Goodyear Tire and Rubber Co. advertising blimps of about 150,000 ft sup 3 (4200
m sup 3). Fundamentally, they are communications platforms used for public
service where airship characteristics are superior to other forms of flight.



 �34s[Robert S. Ross]
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�6v�2v�32sBlindness �21m �33s
    Strictly speaking, the absence of vision. However, this definition would
exclude persons who, though capable of experiencing some visual stimulation,
have little or no useful vision and are therefore functionally blind.
    Visual impairment implies a defect of the visual system. Visual disability
implies a deficit in visually dependent performance. If blindness is defined
in terms of visual impairment, those classified as blind are heterogeneous
with respect to visual ability. If it is defined in terms of visual ability,
those classified as blind are heterogeneous with respect to visual impairment,
but it is visual disability that affects daily living.
    The principal causes of blindness are accidents, cataracts, glaucoma, and
atrophy of the optic nerve. The onset of blindness may occur at any age.
However, with the nationwide increase in the quality of health care in the
United States, there has been a rapid increase in the number of persons who
live long enough to be blinded by the visual disorders that affect older
persons.
    When blind persons perform the same tasks as sighted persons, they are
often handicapped. Developments in the technology of electronics and
computers, such as reading machines, speech calculators, and braille computer
terminals, have made possible the construction of a variety of aids which are
reducing the severity of this handicap. See EYE DISORDERS; VISION.
 �34s[Emerson Foulke]
�33s

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�6v�2v�32sBloch theorem   �21m �33s 
    The theorem which states that the wave function of an electron moving in a 
periodic potential (such as in a crystal lattice) has the form of a plane 
wave, modulated by a function which has the same periodicity as the potential. 
See BAND THEORY OF SOLIDS. 
 �34s[Joseph Callaway]
�33s 
  
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�6v�2v�32sBlock and tackle �21m �33s
    Combination of a rope or other flexible material and independently
rotating frictionless pulleys; the pulleys are grooved or flat wheels used to
change the direction of motion or application of force of the flexible member
(rope or chain) that runs on the pulleys (see illustration). The block and
tackle is used where a large multiplication of the applied forces is
desirable. Examples are: lifting weights, sliding heavy machinery into
position, and tightening fences. See PULLEY; SIMPLE MACHINE.




 �34s[Richard M. Phelan]
�33s

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�6v�2v�32sBlock diagram �21m �33s
    A diagram consisting of unidirectional, operational blocks that represent
the transfer function of the variables of interest in a system. The importance
of the cause-and-effect relationship of a transfer function is expressed in
diagrammatic form by a block diagram.
    A block diagram representation of cause-and-effect relationships can be
used in engineering and management science. An example of a two-block process
is shown in the illustration. The first process G acts on the input X to yield
an output Y. The output Y of the first block becomes the input to the second
block, H. For the total system, X is the input and C is the output. In order
to represent a system with several variables, an interconnection of blocks is
utilized. See CONTROL SYSTEMS; SYSTEMS ENGINEERING.

     FIGURE CAPTION.  Block and tackle. (a) Actual view. (b) Schematic. A block
diagram with two blocks.

 �34s[Richard C. Dorf]
�33s


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�6v�2v�32sBlocking oscillator   �21m �33s 
    A relaxation oscillator that generates a short-time-duration pulse using a 
single transistor or vacuum tube and associated circuitry. The input and 
output are coupled together by a transformer in a regenerative feedback 
arrangement, with the output current being periodically interrupted because of 
the regenerative action which drives the input far into the reverse-bias or 
cutoff region. The duration of the period between pulses, which coincides with 
the period during which the output current is blocked, is dependent upon the 
value of the RC time constant in the input circuit. The blocking oscillator 
may be made to operate in either a monostable or astable fashion. When 
operated as a monostable device, considerable control can be exercised over 
the shape and duration of the pulse, making the circuit useful as a pulse 
generator. See PULSE GENERATOR; RELAXATION OSCILLATOR. 
    The blocking oscillator may be synchronized with external pulses occurring 
at a rate slightly faster than its own natural frequency in the same manner as 
multivibrators. It may be used in the same manner as a frequency divider. See 
MULTIVIBRATOR; SCALING CIRCUIT. 
 �34s[Glenn M. Glasford]
�33s 
  
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�6v�2v�32sBlood �21m �33s
    The fluid that circulates in the blood vessels of the body. Blood consists
of plasma and cells floating within it. The cells are derived from
extravascular sites and then enter the circulatory system. They frequently
leave the blood vessels to enter the extravascular spaces, where some of them
may be transformed into connective tissue cells. The fluid part of the blood
is in equilibrium with the tissue fluids of the body. The circulating blood
carries nutrients and oxygen to the body cells, and is thus an important means
of maintaining the homeostasis of the body. It carries hormones from their
sites of origin throughout the body, and is thus the transmitter of the
chemical integrators of the body. Blood plasma also circulates immune bodies
and contains several of the components essential for the formation of blood
clots. Finally, blood transports waste products to excretory organs for
elimination from the body. Because of its basic composition (cells surrounded
by a matrix), development, and ability to modify into other forms of
connective tissues, blood can be regarded as a special form of connective
tissue. See CONNECTIVE TISSUE.

    Formed elements.  The cells of the blood include the red blood cells and
the white blood cells. In all vertebrates, except nearly all mammals, the red
blood cells or corpuscles contain a nucleus and cytoplasm rich in hemoglobin.
In nearly all mammals the nucleus has been extruded during the developmental
stages.
    In normal adult men the blood contains about 5,000,000 red blood
corpuscles or erythrocytes per cubic millimeter; in  normal adult women, about
4,500,000. Human erythrocytes are about 8 micrometers in diameter and about 2
MUm at their thickest and have a biconcave shape. They contain hemoglobin,
which imparts to them their color, and possess an envelope. When circulating
in the blood vessels, the red blood cells are not evenly dispersed. In the
capillaries the erythrocytes are often distorted. In certain conditions they
may be densely aggregated. This is known as a sludge. The erythrocytes respond
to changes in osmotic pressure of the surrounding fluid by swelling in
hypotonic fluids and by shrinking irregularly in hypertonic fluids. Shrunken
red blood cells are referred to as crenated cells. The average life of the
mature red blood cells is surprisingly long, having a span of about 120 days.
See HEMATOLOGIC DISORDERS; HEMOGLOBIN.
    In humans the white blood cells in the blood are fewer in number. There
are about 5000 - 9000/mm sup 3. In general, there are two varieties, agranular
and granular. The agranular cells include the small, medium, and large
lymphocytes and the monocytes (see illustration). The small lymphocytes are
spherical, about the diameter of erythrocytes or a little larger, and
constitute about 20 - 25% of the white blood cells. The medium and large
lymphocytes are relatively scarce. In all lymphocytes the nucleus occupies
nearly the whole volume of the cell, and the cytoplasm which surrounds it
forms a thin shell. The typical monocyte is commonly as large as a large
lymphocyte (12 micrometers), and constitutes 3 - 8% of the white blood cells.
The nucleus is relatively small, eccentric, and oval or kidney-shaped. The
cytoplasm is relatively larger in volume than that in lymphocytes.
    The granular leukocytes are of three varieties: neutrophil, eosinophil,
and basophil. Their structure varies somewhat in different species, and the
following applies to those of humans. The neutrophils make up 65 - 75% of the
leukocytes. They are about as large as monocytes with a highly variable
nucleus, consisting of three to five lobes joined together by threads of
chromatin. The cytoplasm contains numerous minute granules which stain with
neutral dyes and eosin. The eosinophils (also called acidophils) are about the
same size as the neutrophils but are less numerous, constituting about 1% of
the leukocytes. The nucleus commonly contains but two lobes joined by a thin
thread of chromatin. The granules which fill the cytoplasm are larger than
those of the neutrophils and stain with acid dyes. The basophils are about the
same size as the other granular leukocytes. The nucleus may appear elongated
or with one or more constrictions. The granules are moderately large, stain
with basic dyes, and are water-soluble.
    The functions of the leukocytes while they are circulating in the blood
are not known. However, when they leave the blood vessels and enter the
connective tissue, they constitute an important part of the defense mechanism
and of the repair mechanism. Many of the cells are actively phagocytic and
engulf debris and bacteria. Lymphocytes are of two major kinds, T cells and B
cells. They are involved in the formation of antibodies and in cellular
immunity.
    The blood platelets are small spindle-shaped or rodlike bodies about 3 MUm
long and occur in large numbers in circulating blood. In suitably stained
specimens they consist of a granular central portion (chromomere) embedded in
a homogeneous matrix (hyalomere). They change their shape rapidly on contact
with injured vessels or foreign surfaces and take part in clot formation. The
platelets are not to be regarded as cells and are thought to be cytoplasmic
bits broken off from their cells of origin in bone marrow, the megakaryocytes.
 �34s[Isidore Gersh]
�33s


    Plasma.  Plasma is the residual fluid of blood left after removal of the
cellular elements. Serum is the fluid which is obtained after blood has been
allowed to clot and the clot has been removed. Serum and plasma differ only in
their content of fibrinogen and several minor components which are in large
part removed in the clotting process. See SERUM.
    The major constituents of plasma and serum are proteins. The total protein
concentration of human serum is approximately 7 g/ml, and most other mammals
show similar levels. By various methods it can be demonstrated that serum
protein is a heterogeneous mixture of a large number of constituents. Only a
few are present in higher concentrations, the majority being present in trace
amounts. More than 60 protein components have been identified and
characterized. Albumin makes up more than one-half of the total plasma
proteins and has a molecular weight of 69,000. Because of its relatively small
molecular size and its high concentration, albumin contributes to 75 - 80% of
the colloid osmotic pressure of plasma. The immunoglobulins, which represent
approximately onesixth of the total protein, largely constitute the
GAMMA-globulin fraction. The immunoglobulins are antibodies circulating in the
blood, and therefore are also called humoral antibodies. They are of great
importance in the organism's defense against infectious agents, as well as
other foreign substances. See IMMUNOGLOBULIN.
    In addition to the proteins, many other important classes of compounds
circulate in the blood plasma. Most of these are smaller molecules which
diffuse freely through cell membranes and are, therefore, more similarly
distributed throughout all the fluids of the body and not as characteristic
for plasma or serum as the proteins. In terms of their concentration and their
function, the electrolytes are most important. They are the primary factors in
the regulation of the osmotic pressure of plasma, and contribute also to the
control of the pH. The chief cations are sodium, potassium, calcium, and
magnesium. The chief anions are chloride, bicarbonate, phosphate, sulfate, and
organic acids. The circulating blood also contains the many small compounds
which are transported to the sites of synthesis of larger molecules in which
they are incorporated, or which are shifted as products of metabolic breakdown
to the sites of their excretion from the body.
 �34s[Hartwig Cleve]
�33s

    Coagulation.  When mammalian blood is shed, it congeals rapidly into a
gelatinous clot of enmeshed fibrin threads which trap blood cells and serum.
Modern theories envision a succession of reactions leading to the formation of
insoluble fibrin from a soluble precursor, fibrinogen (factor I). Blood also
clots when it touches glass or other negatively charged surfaces, through
reactions described as the intrinsic pathway. Several of the steps in this
process are dependent upon the presence in blood of calcium ions and of
phospholipids, the latter derived principally from blood platelets. The
coagulation of blood can also be induced by certain snake venoms which either
promote the formation of thrombin or clot fibrinogen directly, accounting in
part for their toxicity.
    Platelets, besides furnishing phospholipids for the clotting process, help
to stanch the flow of blood from injured blood vessels by accumulating at the
point of injury, forming a plug. Platelets participate in the phenomenon of
clot retraction, in which the blood clot shrinks, expelling liquid serum.
Although the function of retraction is unknown, individuals in whom this
process is impaired have a bleeding tendency.
    Hereditary deficiencies of the function of each of the protein-clotting
factors have been described, notably classic hemophilia and Christmas disease,
which are disorders of males and clinically indistinguishable. The various
hereditary functional deficiencies are associated with a bleeding tendency
with one inexplicable exception. Acquired deficiencies of clotting factors,
sometimes of great complexity, are also recognized. Therapy for bleeding due
to deficiencies of clotting factors often includes the transfusion of blood
plasma or fractions of plasma rich in particular substances the patient may
lack. See HUMAN GENETICS.
    Clinical tests of the coagulability of the blood include (1) determination
of the clotting time, that is, the time elapsing until shed blood clots; (2)
the prothrombin time, the time elapsing until plasma clots in the presence of
tissue thromboplastin (and therefore a measure of the extrinsic pathway of
clotting); (3) the partial thromboplastin time, the time elapsing until plasma
clots in the presence of crude phospholipid (and therefore a measure of the
intrinsic pathway of clotting); (4) the enumeration of platelets; and (5)
crude quantification of clot retraction and of the various plasma
protein-clotting factors.
    Heparin, a polysaccharide - sulfuric acid complex found particularly in
the liver and lungs, impairs coagulation; its presence in normal blood is
disputed. Both coumarin and heparin are used clinically to impede coagulation
in thrombotic states, including thrombophlebitis and coronary heart diease.
See FIBRINOGEN.



 �34s[Oscar D. Ratnoff]
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�6v�2v�32sBlood groups �21m �33s
    The blood of humans and higher animals has been classified according to
the red blood cell antigens. About 200 different blood groups have been
identified as part of the surface structure of human red blood cells. Each of
these is a unique serologic and biochemical entity, and is one member of a
complex genetic polymorphism. Each of the blood groups is identified by its
ability to react, selectively and specifically, with a corresponding antibody.
In many cases the pattern of reactions with various antibodies not only
identifies the blood group, but also places it within the genetic confines of
one of the 19 known blood group systems. The blood groups have medical, legal,
and anthropologic importance, and are valuable genetic markers in linkage
studies. See POLYMORPHISM (GENETICS).
    Every blood group is a definite and specific structure that is a part of
one of the many different blood group antigens. The inheritance of each
antigen is genetically controlled, and data support the concept of one gene
for each antigen, with the understanding that each of these antigens is a
complex serologic (and biochemical) entity. Every blood group antigen has at
least several different sites, each of which reacts with a specific blood
group antibody. About 200 of these sites have been identified, and each is a
separate blood group.
    Antibodies directed against blood group antigens are of two types, immune
and naturally occurring. Naturally occurring antibodies, anti-A and anti-B,
may be found normally in any person who lacks the corresponding antigen (see
table). In all probability, these antibodies are formed after contact with
immunogens related to, but not identical with, the blood group antigens. Most
blood group antibodies are true immune antibodies and do not occur in plasma
until after actual immunization with blood group antigen itself. The
immunizing stimulus is almost always transfusion or pregnancy.
    Blood group systems.  The inheritance pattern of many of the 200 known
blood groups is well enough understood to place them in one of the clearly
established blood group systems. Genetic control for each system is at a
single locus, and in most cases the number of allelic genes for each system is
greater than two. Since an individual inherits one gene for every blood group
system from each parent, the individual's blood group phenotypes are the sum
of genetic information from both parents. Almost all blood groups are examples
of codominance, and genes are equally well expressed when present in
homozygous or heterozygous states. All loci, except for Xg sup a, are on
autosomal chromosomes.
    The four common blood types in the ABO system are A, B, AB, and O. ABO
antigens are found not only on red cells but also in soluble form in body
fluids of about 80% of the population. Antibodies reacting with the A or B
blood factors are always found in the plasma of persons who lack the antigens,
and are the most important of the naturally occurring antibodies. No antibody
that identifies the product of the O gene has been reported.
    The inherent complexity of the Rh system has been magnified far out of
proportion by a long-standing controversy over the proper genetic
interpretation of the data and the choice of terms to use in describing the Rh
blood groups. All Rh antigenic determinants are transmitted by a segment on
one pair of autosomal chromosomes. At least 36 alleles are known, of which
only 8 are common and can be considered basic. Each of these alleles
determines an Rh antigen on the red cell. There is almost no biochemical or
structural information about the Rh antigens. Of the 30 Rh blood groups now
defined, Rh sub 0, or D, is the one of greatest clinical importance, and its
presence or absence is the criterion by which bloods are classified as Rh
positive or Rh negative. This is because Rh sub 0 (D) is the most immunogenic of
the Rh blood groups. As a result, individuals who are Rh sub 0 (D) negative
should, except in emergencies, be transfused with donor blood that is also Rh
sub 0 (D) negative.
    The Lewis blood group system is unique in several ways and, although of
relatively little clinical importance, is of great scientific interest. Lewis
differs from ABO, and from all other groups, in that the Lewis characteristics
of erythrocytes are passively acquired and are not an integral part of the
cell membrane.
    Xg sup a is the only blood group gene not carried on one of the autosomal
chromosomes. The frequency of Xg (a+) females is higher than that of Xg (a+)
males. In addition, careful family studies have shown without doubt that the
gene for the Xg sup a blood group is on the X chromosome. Only one antibody in
the Xg sup a system has been found so that, at present, but one allele, Xg sup
a, and two phenotypes, Xg (a+) and Xg (a-), can be identified. The Xg sup a
system is of little importance in transfusion, but, because of the localization
of the gene, is of great genetic interest.
    Details of other blood group systems are beyond the scope of this article.
Some, such as Kell, Duffy, and Kidd, are relatively important to
transfusionists, since antibodies to these groups are more frequent and
transfusion reactions from incompatibility a distinct hazard.

    Significance.  Study of blood group factors has made transfusion a safe
practice, identified the cause of erythroblastosis fetalis, provided a method
for paternity exclusion, and contributed to ethnology, genetics, and medicine.
    In most cases only the ABO and Rh sub 0 blood groups of donor and recipient
are determined before transfusion. The prime consideration in selecting blood
for infusion is to avoid the transfusion reaction that will occur when blood
containing any of the bloood groups is given to a recipient who has developed
antibody against that group. Since the antibodies in the ABO system are
naturally occurring and always present when the corresponding antigen is
lacking from the cells, a transfusion reaction is inevitable if ABO
incompatible blood is given. For this reason it is essential to do careful ABO
typing on both donor and recipient and to select compatible blood for
transfusion. Group O erythrocytes, lacking both A and B antigens, can be used
in recipients of other ABO groups without fear of a reaction from ABO
incompatibility. In a similar fashion, persons of group AB can be safely
transfused with cells of any ABO group. A second consideration in selecting
blood for transfusion is to prevent, if possible, the recipient from forming
antibodies. Since most blood groups are not strongly immunogenic, it is
current practice to disregard all of them except Rh sub 0(D).
    Erythroblastosis fetalis is a hemolytic disease of newborn infants that
can occur as a result of blood group incompatibility between mother and fetus.
In the classifical form, it occurs when an Rh-negative mother carries an
Rh-positive fetus. During pregnancy Rh-positive erythrocytes from the baby
cross the placenta and immunize the Rh-negative mother, so that she produces
anti-Rh. Her antibody, in turn crosses the placenta and enters fetal
circulation, where it reacts with and hemolyzes fetal red cells. In severe
cases this leads to intrauterine death; less severely affected infants require
treatment by exchange transfusion at birth. Intrauterine transfusion of unborn
infants has been used with good results. See ERYTHROBLASTOSIS FETALIS.
    The inheritance pattern of most blood groups is such that blood group
tests of mother, child, and putative father can, at times, exclude paternity.
No blood factor can occur in a child unless it is present in at least one of
his parents. If both mother and putative father lack a factor that is present
on the child's cells, it can be assumed that one of the two is not the real
parent. Since maternity is usually not at issue, such findings are interpreted
as an exclusion of paternity. Maternity exclusion can also be determined, if
indicated, as when it is suspected that babies have been mixed up in a
nursery. At present, ABO, Rh, and MN blood group systems are accepted by most
courts in paternity trials.
    The frequency of certain blood groups in patients with various diseases
has been studied at length. Many data have been gathered, and almost every
disease has been associated with some blood group. Most of these correlations
do not stand close statistical scrutiny, but a few have been so well studied
and so commonly found that there is little doubt that they are real. These
include the increased incidence of group O in persons with peptic ulcer
disease and the increased incidence of group A in cancer of the stomach or
pernicious anemia. The mechanism for these relationships is unknown. See
BLOOD.



 �34s[Joseph R. Bove]
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�6v�2v�32sBlood-plate hemolysis   �21m �33s 
    The destruction of red blood cells in an agar medium, such as beef heart 
infusion agar with rabbit's blood, contained in a petri dish. The cells are 
destroyed by hemolysin, a toxin produced by such bacteria as streptococcus and 
staphylococcus. The types of destruction or hemolysis are given Greek letters 
and usually apply to streptococcus and its classification. Green discoloration 
(alpha) around a streptococcal colony, or the halo of clearness (beta) 
contrast with the deep-pink background of growth medium. Absence of change in 
the red cells and consequent absence of discoloration around the colony 
represent the gamma type. See LANCEFIELD DIFFERENTIATION SCHEME; 
STAPHYLOCOCCUS; STREPTOCOCCUS. 
 �34s[Paul L. Boisvert]
�33s 
  
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�6v�2v�32sBlood vessels   �21m �33s 
    Tubular channels for blood transport, of which there are three principal 
types: arteries, capillaries, and  veins. Only the larger arteries and veins 
in the body bear distinct names. Arteries carry blood away from the heart 
through a system of successively smaller vessels. Capillaries are the smallest 
but most extensive blood vessels, forming a network everywhere in the body 
tissues. Veins carry blood from the capillary beds back to the heart through 
increasingly larger vessels. In certain locations blood vessels are modified 
for particular functions, as the sinusoids of the liver and the spleen and the 
choroid plexuses of the brain ventricles. See ARTERY; CAPILLARY (ANATOMY); 
CIRCULATORY SYSTEM; LYMPHATIC SYSTEM; VEIN. 
 �34s[Walter J. Bock]
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�6v�2v�32sBlowout coil �21m �33s
    A coil that produces a magnetic field in an electrical switching device
for the purpose of lengthening and extinguishing an electric arc formed as the
contacts of the switching device part to interrupt the current. The magnetic
field produced by the coil is approximately perpendicular to the arc. The
interaction between the arc and current and the magnetic field produces a
force driving the arc in the direction perpendicular to both the magnetic flux
and the arc current (see illustration).




 �34s[Thomas H. Lee]
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�6v�2v�32sBlowpipe   �21m �33s 
    In glass blowing, a long straight tube on which molten glass is gathered 
and worked, partly by blowing into the tube. The blowpipe is spun to shape the 
glass object further by centrifugal force, or by a tool, in which case the 
blowpipe acts as a spindle for turning. 
    In analytical chemistry, a blowpipe is a small, tapered, and frequently 
curved tube that directs a jet, usually of air, into a flame to concentrate 
the flame onto an unknown substance. Coloration of the flame and other 
characteristic reactions in the reducing and in the oxidizing portions of the 
flame created by the blowpipe aid in identifying the substance. Such a 
blowpipe may be blown directly by the analyst, from a bellows, or from a 
pressurized line. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sBlueberry   �21m �33s 
    Several species of the genus Vaccinium, plant order Ericales, ranging from 
low-growing, almost prostrate plants to vigorous shrubs reaching a height of 
12 - 15 ft (3.7 - 4.6 m). The fruit, a berry, is usually black and covered 
with bluish bloom, generally occurring in clusters, and has numerous small 
seeds, a characteristic that distinguishes the blueberry from the huckleberry, 
which has 10 rather large, gritty seeds. Although there are blueberry species 
on other continents, all cultivated varieties in the United States are 
American in origin. 
    The dryland blueberry (V. ashei) is adapted to relatively dry soils and 
has been brought under cultivation in Florida and Georgia. In the Northeast 
the lowbush blueberry (V. lamarckii) grows wild over thousands of acres of dry 
hillsides, where it is harvested commercially, especially in Maine, but also 
in other New England states, Michigan, Minnesota, and a few others. In the 
Northwest fruit of the evergreen blueberry (V. ovatum) is harvested in the 
wild, and large tonnages of the leafy twigs are shipped for use as florists' 
greens. The highbush blueberry, represented by V. australe and V. corymbosum, 
provides most of the cultivated plants. The highbush blueberry is found in 
swampy land, usually on hummocks. 
    The fruit is sold fresh, canned, and frozen. The blueberry is becoming 
increasingly popular in home gardens, although its cultural requirements are 
rather exacting. For garden culture, mulching with sawdust or other organic 
matter is desirable be cause the roots are shallow. See ERICALES. 
 �34s[J. Harold Clarke]
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mpgs. 1244-1246 
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�6v�2v�32sBluefish   �21m �33s 
    A predatory and voracious species of fish that ranges throughout the 
tropical and temperate seas of the world, except for the eastern and central 
Pacific areas. A single species, Pomatomus saltatrix, makes up the family 
Pomatomidae. This fish, also known as the skipjack, is bluish-gray with an 
average length of 3 ft (0.9 m) and weight of about 5 lb (2.3 kg). The mouth is 
large with sharp, strong teeth. The bluefish form schools and migrate north 
along the Atlantic coast, following schools of smaller fish upon which they 
prey. The bluefish continue to kill and destroy their prey even after feeding. 
About June they reach the New England coast, where the young can be found in 
estuaries and bays. See PERCIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sBluegrass �21m �33s
    A common name applied to several species but particularly to Kentucky
bluegrass (Poa pratensis) and to Canada bluegrass (P. compressa). Kentucky
bluegrass is widely grown in temperate humid regions, doing best on fertile
soils that are loamy or heavy in texture. Canada bluegrass is more abundant on
less fertile soils in cooler regions. Both species are perennial, form sod by
underground creeping stems, and have extensive root systems which are finely
branched and comparatively shallow.
    These grasses are utilized primarily for pasture and lawns. Bluegrasses
respond to fertilizers, especially those supplying nitrogen, and the herbage
generally is high in the mineral elements, particularly phosphorus. Growth
occurs from early spring to late fall, except for near-dormancy during the
hot, dry summer period. Kentucky bluegrass is the most important species in
permanent pastures, recreation areas, and lawns in the northern half of the
United States, where moisture is adequate from rainfall or irrigation.
However, in pastures grown in rotation with crops in humid regions, it has
been largely replaced by orchard grass and bromegrass. Several varieties have
been developed specifically for turf use with special emphasis on disease
resistance. See BROMEGRASS; CYPERALES.
 �34s[Howard B. Sprague]
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�6v�2v�32sBlueschist metamorphism �21m �33s
    Regional metamorphism with the highest pressures and lowest temperatures,
commonly above 5 kilobars (500 megapascal) and below 752 DG F (400 DG C).
Metamorphic rocks of the relatively uncommon blueschist facies contain
assemblages of minerals that record these high pressures and low temperatures.
The name ``blueschist'' derives from the fact that at this metamorphic grade,
rocks of ordinary basaltic composition are often bluish because they contain the
sodium-bearing blue amphiboles glaucophane or crossite rather than the
calcium-bearing green or black amphiboles actinolite or hornblende, which are
developed in the more common greenschist- or amphibolite-facies metamorphism.
    Blueschist metamorphism developed almost exclusively in young Mesozoic and
Cenozoic mountain belts, primarily during the last 5 to 10% of geologic time.
During most of geologic time, high-pressure metamorphism has been dominated by
high-temperature amphibolite-facies metamorphism, as well as by very
high-temperature granulite-facies metamorphism, the latter being uncommon
during the last 5 to 10% of geologic time. These phenomena are generally
interpreted as being in some way an effect of the cooling of the Earth.
    A variety of mineral assemblages are characteristic of blueschist-facies
metamorphism, depending upon the chemical composition of the rock and the
actual temperature and pressure within the field of blueschist metamorphism.
Detailed consideration of these assemblages and their mineral composition
allows the determination of the physical conditions of metamorphism.
    Blueschist metamorphic rocks are found almost exclusively in the young
mountain belts of the circum-Pacific and Alpine-Himalayan chains. The rocks
are usually metamorphosed oceanic sediments and basaltic oceanic crust.
Previously continental rocks rarely exhibit blueschist metamorphism. The
tectonic mechanism for blueschist metamorphism must move the rocks to depths
of more than 6 to 12 mi (10 to 20 km) while maintaining relatively cool
temperatures (390 - 750 DG F or 200 - 400 DG C). These temperatures are much
cooler than for continental crust at those depths.
    What is not well understood is how the blueschist metamorphic rocks return
to the surface; it is clear that the mechanism is not simple uplift and
erosion of 12-18 mi (20 - 30 km) of the Earth's crust. Blueschist metamorphic
rocks are usually in immediate fault contact with much less metamorphosed or
unmetamorphosed sediments, indicating that they have been tectonically
displaced relative to their surroundings since metamorphism. See METAMORPHIC
ROCKS; METAMORPHISM.
 �34s[John Suppe]
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�6v�2v�32sBluestem grass   �21m �33s 
    The common generic name often applied to the genera Andropogon, 
Dichanthium, Bothriochloa, and Schizachyrium in the grass tribe Andropogoneae. 
They are adapted to environments with high light intensities and high 
temperatures during the growing season. The bluestems are medium to tall, 
warm-season, perennial grasses. The two most important forage species are big 
bluestem (A. gerardi) and little bluestem (S. scoparius). 
    Big bluestem is a tall (usually 3 to 6 ft or 1 to 2 m), deep-rooted grass 
with strong rhizomes; it occurs throughout the continental United States 
(except in the extreme western states), in southern Canada, and in northern 
Mexico. One of the most palatable of all grasses when it is actively growing, 
its nutritional value declines sharply with maturity. Because of its high 
palatability and tall growth habit, big bluestem has been reduced or 
eliminated by heavy grazing over much of its range. 
    Little bluestem is a medium-height bunchgrass (usually 1 to 2 ft or 30 to 
60 cm, but taller in the south) which occurs in the same geographic area as 
big bluestem but extends farther north in the prairie provinces of Canada to 
the southern Yukon. It is an excellent grass for grazing and hay when actively 
growing, but nutritional value declines rapidly with maturity. This is the 
grass that provides the major aspect of the Flint Hills of Kansas and the 
Osage Hills of Oklahoma. See CYPERALES; GRASS CROPS. 
 �34s[James K. Lewis]
�33s 
  
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�6v�2v�32sBoa   �21m �33s 
    Name given to reptiles which are members of the family Boidae in the order 
Squamata. There are about 51 species of boas, none of which is poisonous. 
These snakes, along with the closely related pythons, are among the largest 
and are the most primitive of living snakes. They kill their prey by 
constriction. All species are viviparous, producing living young. Most of the 
species are found in the New World, being most numerous in the West Indies and 
Central and South America, although some species are found in Asia and 
northern and eastern Africa.See REPTILIA; SQUAMATA. 
    Constrictor constrictor, the boa constrictor, is one of the best-known 
species, living in tropical America from Mexico to Argentina and in the West 
Indies. It attains a length of 12 ft (3.7 m), is not dangerous to humans, and 
is easily reared in captivity. 
    The largest member of the boa family and largest living snake is the giant 
anaconda (Eunectes murinus); 25 ft (7.6 m) is the longest authenticated size. 
It is arboreal as well as aquatic, in that it may await its prey in trees 
lining a stream and then drag the captured mammal or bird into the water to 
kill and devour it. 
    An arboreal species, the greenish-yellow emerald boa (Boa canina), preys 
on birds and monkeys in forested areas of South America. It grows to about 6 
ft (1.8 m) in length. An inhabitant of the arid desert regions of Asia Minor 
is the sand boa (Eryx jaculus), which preys upon small animals. 
 �34s[Charles B. Curtin]
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�6v�2v�32sBoat propulsion �21m �33s
    The action of propelling a boat through water. A boat machinery plant
consists principally of a propul sion engine, propulsor (propeller or jet
pump), and drive-line components. The engines are almost exclusively of the
familiar internal combustion types: gasoline, diesel, or gas turbine. The
gasoline engine has traditionally dominated the pleasure-boat field, while the
diesel is favored for commercial and military craft. The gas turbine is
comparatively rare and is found only in applications where high power from
machinery of small weight and volume is essential.
    Auxiliary items, such as bilge pumps, domestic water pumps and heaters,
and electric generators and switchboards typically are found in the machinery
plants of the larger boats. The generator (or alternator) is often driven by a
belt from the propulsion engine, as in automotive practice, but is sometimes
driven by a separate engine.
    The marine gasoline engine appears in inboard and outboard forms. The
outboard engine is a unit assembly of engine, propeller, and vertical drive
shaft that is usually clamped to the boat transom. It is traditionally the
power plant for the smallest motor boats, or an auxiliary for rowboats and
sailboats. The inboard form is almost exclusively adapted from one of the
mass-produced automotive engines. Inboard engines are compact, light, lower in
first cost than any competitor, and familiar to untrained users, and so they
predominate among pleasure boats and the smaller fishing boats.
    The diesel engine is generally higher in first cost than the gasoline
engine and is somewhat heavier for the same power, but it consumes less fuel.
Fuel savings make the diesel attractive if the engine is to be used more than
a few hundred hours a year or if the boat must have a long cruising range. Its
principal market is thus in commercial and military craft, but some of the
more compact models are used in pleasure craft.
    The gas turbine engine is comparatively light and compact, making it
attractive for high-speed, high-power boats. Its disadvantages are high first
cost, high fuel consumption, and large exhaust and intake ducts. The last
factor is due to its high rate of air consumption; the need for large volumes
of air also makes it difficult, in a small vessel, to keep spray from being
drawn into the engine, with consequent fouling and corrosion. See DIESEL
ENGINE; GAS TURBINE; INTERNAL COMBUSTION ENGINE; MARINE ENGINE; MARINE
MACHINERY.
 �34s[John B. Woodward]
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�6v�2v�32sBog �21m �33s
    Plant communities which develop and continue to grow in areas with
permantly waterlogged peat substrates. Synonyms such as mire, quagmire, and
moor have been used to describe this community. See PEAT.
    Ecologists have described numerous types of bogs from different parts of
the world, and various systems of nomenclature and classification have been
adopted. The principal morphological distinction between bog types is
concerned with the source of their water: from drainage (rheophilous) or
direct precipitation (ombrophilous).
    The major features of a bog are the peat substratum and its waterlogged
condition, with the water table frequently lying just below, at, or above the
peat surface. The peat itself is composed of partially decayed organic matter,
originating chiefly from the plants. The reason for the accumulation of this
organic matter is the very slow rate of breakdown, because of the lack of
oxidative microorganisms in such waterlogged conditions. This adaptive process
operates in geographical areas of low temperature and high rainfall.
    Bogs are being utilized for an increasing range of purposes. Reclamation
for agriculture and forestry and the extraction of peat are accelerating
rapidly in some parts of the world. In some regions bogs contribute toward
maintaining the hydrological stability of the landscape because they hold
large quantities of water which is released slowly. Thus bogs are very
important for water conservation and in controlling floods.

 �34s[Tom Pritchard]
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�6v�2v�32sBoiler �21m �33s
    A pressurized system in which water is vaporized to steam, the desired end
product, by heat transferred from a source of higher temperature, usually the
products of combustion from burning fuels. Steam thus generated may be used
directly as a heating medium, or as the working fluid in a prime mover to
convert thermal energy to mechanical work, which in turn may be converted to
electrical energy. Although other fluids are sometimes used for these
purposes, water is by far the most common because of its economy and suitable
thermodynamic characteristics.
    The physical sizes of boilers range from small portable or shop-assembled
units to installations comparable to a multistory 200-ft-high (60-m) building
equipped, typically, with a furnace which can burn coal at a rate of 6
tons/min (90 kg/s). Boilers operate at positive pressures and offer the
hazardous potential of explosions. Pressure parts must be strong enough to
withstand the generated steam pressure and must be maintained at acceptable
temperatures, by transfer of heat to the fluid, to prevent loss of strength
from overheating or destructive oxidation of the construction materials.
    The overall functioning of steam-generating equipment is governed by
thermodynamic properties of the working fluid. By the simple addition of heat
to water in a closed vessel, vapor is formed which has greater specific volume
than the liquid, and can develop increase of pressure to the critical value of
3208 psia (22.1 megapascals absolute pressure). If the generated steam is
discharged at a controlled rate, commensurate with the rate of heat addition,
the pressure in the vessel can be maintained at any desired value, and thus be
held within the limits of safety of the construction. See STEAM.
    Addition of heat to steam, after its generation, is accompanied by
increase of temperature above the saturation value. The higher heat content,
or enthalpy, of superheated steam permits it to develop a higher percentage of
useful work by expansion through the prime mover, with a resultant gain in
efficiency of the power-generating cycle. See SUPERHEATER.
    If the steam-generating system is maintained at pressures above the
critical, by means of a highpressure feedwater pump, water is converted to a
vapor phase of high density equal to that of the water, without the formation
of bubbles. Further heat addition causes superheating, with corresponding
increase in temperature and enthalpy. The most advanced developments in
steam-generating equipment have led to units operating above critical
pressure, for example, 3600-5000 psi (25-34 MPa). Superheated steam
temperature has advanced from 500 PLUS/MINUS DG F (260 PLUS/MINUS DG C) to the
present practical limits of 1050 - 1100 DG F (566 - 593 DG C). See MARINE
ENGINEERING; NUCLEAR POWER; STEAM-GENERATING UNIT.
 �34s[Theodore Baumeister]
�33s

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�6v�2v�32sBoiler economizer   �21m �33s 
    A component of a steam-generating unit that absorbs heat from the products 
of combustion after they have passed through the steam-generating and 
superheating sections. The name, accepted through common usage, is indicative 
of savings in the fuel required to generate steam. 
    An economizer is a forced-flow, once-through, convection heat-transfer 
device to which feedwater is supplied at a pressure above that in the 
steam-generating section and at a rate corresponding to the steam output of 
the unit. The economizer is in effect a feedwater heater, receiving water from 
the boiler feed pump and delivering it at a higher temperature to the steam 
generator or boiler. Economizers are used instead of additional 
steam-generating surface because the feedwater, and consequently the 
heat-receiving surface, is at a temperature below that corresponding to the 
saturated steam temperature; thus, the economizer further lowers the flue gas 
temperature for additional heat recovery. See FEEDWATER; THERMODYNAMIC CYCLE. 
    Generally, steel tubes, or steel tubes fitted with externally extended 
surface, are used for the heat-absorbing section of the economizer; usually, 
the economizer is coordinated with the steam-generating section and placed 
within the setting of the unit. See AIR HEATER; BOILER; STEAM-GENERATING UNIT.  
 �34s[George W. Kessler]
�33s 
  
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�6v�2v�32sBoiling point �21m �33s
    The temperature at which the transition from the liquid to the gaseous
phase occurs. For pure substances at a fixed pressure, the boiling or
vaporization process occurs at a single temperature; as heat is added, the
temperature remains constant until all the liquid has boiled.
    The normal boiling point is defined as the boiling point at a total
applied pressure of 1 atm (101.325 kilopascals), that is, the temperature at
which the vapor pressure of the liquid equals 1 atm. The boiling point
increases with applied pressure. For substances boiling in the region of room
temperature, the rate of change of boiling point with temperature is
approximately 0.04 DG /mmHg or 0.3 DG /kPa (where the pressure is approximately
1 atm). 
    The boiling point cannot be raised indefinitely, however. As the pressure
is increased, the density of the gas phase increases until it finally becomes
indistinguishable from the liquid phase with which it is in equilibrium; this
is the critical temperature, above which no distinct liquid phase exists.
Helium has the lowest normal boiling point (4.2 K) of any substance, and
tungsten carbide has one of the highest (6300 K). See CRITICAL CONSTANT;
DISTILLATION; LIQUID; PHASE EQUILIBRIUM; VAPOR PRESSURE.
 �34s[Robert L. Scott]
�33s

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�6v�2v�32sBolometer   �21m �33s 
    A device for detecting and measuring small amounts of thermal radiation. 
The bolometer is a simple electric circuit, the essential element of which is 
a slab of material with an electrical property, most often resistance, that 
changes with temperature. Typical operation involves absorption of radiant 
energy by the slab, producing a rise in the slab's temperature and thereby a 
change in its resistance. The electric circuit converts the resistance change 
to a voltage change, which then can be amplified and observed by various, 
usually conventional, instruments. 
    Although bolometers are useful in studying a variety of systems where 
detection of small amounts of heat is important, their primary application 
remains as the instrument of choice for measuring weak radiation signals in 
the infrared and far infrared, that is, at wavelengths from about 1 to 2000 
micrometers, from stars and interstellar material. See BARRETTER; INFRARED 
RADIATION; RADIOMETRY; THERMISTOR. 
 �34s[William E. Keller]
�33s 
  
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�6v�2v�32sBolt �21m �33s
    A rod, usually of metal, with a head at one end and a screw thread on the
other. A bolt is used to fasten objects  together. A bolt is passed through
clearance holes in two or more parts, a nut is engaged on the threaded end,
and the parts are drawn together. Bolts can be obtained in a variety of forms,
each suited for a specific application (see illustration). Standard hexagon
wrench-head bolts and nuts are available in three degrees of finish:
Unfinished bolts are finished only on the thread, semifinished bolts are
machined under the head, and finished bolts are fully machined on all
surfaces. The two standard weights for bolts are regular, for general use, and
heavy, for applications where a greater bearing surface is needed. For the
same nominal size, a heavy bolt or nut has larger head and nut dimensions than
one belonging to the regular series.
    Wrench-head bolts are specified by giving the diameter, number of threads
per inch, series, class of thread, length, finish, and type of head. Machine
bolts are used in the automotive, aircraft, and machinery fields, where a snug
full-bodied fit is needed. Stove bolts are used on electrical equipment and
household appliances, either in tapped holes or with nuts. The carriage bolt
is a round-head type with nut; it is used as a through bolt. Special bolts are
elevator, plow, step, strut, tire, T-head, and track bolts. Plow bolts are
employed on agricultural equipment. See NUT (ENGINEERING); SCREW FASTENER.





 �34s[Warren J. Luzadder]
�33s


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�6v�2v�32sBolted joint �21m �33s
    The assembly of two or more parts by a threaded bolt and nut or by a screw
that passes through one member and threads into another (see illustration). A
bolted joint can be disassembled more readily than welded or riveted joints.
See RIVETED JOINT.
    Structural members, such as I beams, H beams, angles, and plates, may be
joined by bolting. When properly tightened, such joints are as strong and
reliable as riveted ones. For joints of this type, high-strength alloy-steel
bolts and nuts tightened with impact wrenches are generally used. No locking
feature is necessary for properly selected and tightened bolts.
    The strength of a bolted assembly depends to a great extent on the initial
loading and stress placed on the bolt by the assembly torque. Devices such as
torque wrenches are used to determine tension as a function of the torque
applied to the assembly of nut and bolt or cap screw and machine part despite
the uncertain coefficient of friction between nut, bolt, and assembled
members. A better measure is to gage the elongation of the bolt as the
tightening torque is applied. Bolt and nut assemblies often include special
lock washers to prevent accidental loosening of the fastening by vibration.
Where critical loads are to be carried and safety is paramount, bolts or
screws smaller than 1/2 in. (1.25 cm) in diameter are dangerous to use because
it is easy to overload the bolt or screw in assembly and actually twist it off
with standard wrench sizes. See JOINT (STRUCTURES); SCREW FASTENER; STRUCTURAL
CONNECTIONS; WELDED JOINT.




 �34s[L. Sigfred Linderoth, Jr.]
�33s

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�6v�2v�32sBoltzmann constant �21m �33s
    A constant occurring in practically all statistical formulas and having a
numerical value of 1.3807 x 10(sup -23) joule/K. It is represented by the
letter k. If the temperature T is measured from absolute zero, the quantity kT
has the dimensions of an energy and is usually called the thermal energy. At
300 K (room temperature) kT = 0.0259 electronvolt.

				 PV = RT				   (1a)


				 PV = NkT				   (1b)

    The value of the Boltzmann constant may be determined from the ideal gas
law. For 1 mole of an ideal gas Eq. (1a) holds, where P is the pressure, V the
volume, and R the universal gas constant. The value of R, 8.31 J/K mole, may
be obtained from equation-of-state data. Statistical mechanics yields for the
gas law Eq. (1b). Here N, the number of molecules in 1 mole, is called
Avogadro's number and is equal to 6.02 x 10(sup 23) molecules/mole. Hence,
comparing Eqs. (1a) and (1b), one obtains Eq. (2).

		       k = R/N = 1.3807 x 10(sup -23) J/K		    (2)

    Almost any relation derived on the basis of the partition function or the
Bose-Einstein, Fermi-Dirac, or Boltzmann distribution contains the Boltzmann
constant. See AVOGADRO NUMBER; BOLTZMANN STATISTICS; BOSE-EINSTEIN STATISTICS;
FERMI-DIRAC STATISTICS; KINETIC THEORY OF MATTER; STATISTI CAL MECHANICS.
 �34s[Max Dresden]
�33s

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�6v�2v�32sBoltzmann statistics �21m �33s
    To describe a system consisting of a large number of particles in a
physically useful manner, recourse must be had to so-called statistical
procedures. If the mechanical laws operating in the system are those of
classical mechanics, and if the system is sufficiently dilute, the resulting
statistical treatment is referred to as Boltzmann or classical statistics.
(Dilute in this instance means that the total volume available is much larger
than the proper volume of the particles.) A gas is a typical example: The
molecules interacting according to the laws of classical mechanics are the
constituents of the system, and the pressure, temperature, and other
parameters are the overall entities which determine the macroscopic behavior
of the gas. In a case of this kind it is neither possible nor desirable to
solve the complicated equations of motion of the molecules; one is not
interested in the position and velocity of every molecule at any time. The
purpose of the statistical description is to extract from the mechanical
description just those features relevant for the determination of the
macroscopic properties and to omit others.
    The basic notion in the statistical description is that of a distribution
function. Suppose a system of N molecules is contained in a volume V. The
molecules are moving around, colliding with the walls and with each other.
Construct the following geometrical representation of the mechanical system.
Introduce a six-dimensional space (usually called the mu space), three of its
coordinate axes being the spatial coordinates of the vessel x, y, z, and the
other three indicating cartesian velocity components v(sub x), v(sub y),
v(sub z). A molecule at a given time, having a specified position and velocity,
may be represented by a point in this six-dimensional space. The state of the
gas, a system of N molecules, may be represented by a cloud of N points in this
space. In the course of time, this cloud of N points moves through the mu
space.
    Note that the mu space is actually finite; the coordinates x, y, z of the
molecules' position are bounded by the finite size of the container, and the
velocities are bounded by the total energy of the system.  Imagine now that the
space is divided into a large number of small cells, of sizes w(sub 1), .  .  .
, w(sub i), .  .  .  .	A certain specification of the state of the gas is
obtained if, at a given time t, the numbers n(sub 1)(t), .  .  .  , n(sub i)(t),
.  .  .  of molecules in the cells 1, .  .  .  , i, .  .  .  are given.  To
apply statistical methods, one must choose the cells such that on the one hand a
cell size w is small compared to the macroscopic dimensions of the system, while
on the other hand w must be large enough to allow a large number of molecules in
one cell.  If the cells are thus chosen, the numbers nsubi (t), the occupation
numbers, will be slowly changing functions of time.  The distribution functions
f(sub i)(t) are defined by Eq.	(1).

	       n(sub i)(t) = f(sub i)(t)w(sub i)			  (1)

    The distribution function f(sub i) describes the state of the gas, and
f(sub i) of course varies from cell to cell.  Since a cell i is characterized by
a given velocity range and position range, and since for appropriately chosen
cells f should vary smoothly from cell to cell, f is often considered as a
continuous function of the variables x, y, z, v(sub x), v(sub y), v(sub z).  The
cell size w then can be written as dxdydzdv(sub x)dv(sub y)dv(sub z).

    Since a cell i determines both a position and a velocity range, one may
associate an energy epsilonsubi with a cell. This is the energy a single
molecule possesses when it has a representative point in cell i. This assumes
that, apart from instantaneous collisions, molecules exert no forces on each
other. If this were not the case, the energy of a molecule would be determined
by the positions of all other molecules.

(A[sub ij])(sup kl) = n(sub i)n(sub j)(a[sub ij])(sup kl)		  (2)

    Most of the physically interesting quantities follow from a knowledge of
the distribution function; the main problem in Boltzmann statistics is to find
out what this function is.  It is clear that n(sub i)(t) changes in the course
of time for three reasons:  (1) Molecules located at the position of cell i
change their positions and hence move out of cell i; (2) molecules under the
influence of outside forces change their velocities and again leave the cell i;
and (3) collisions between the molecules will generally cause a (discontinuous)
change of the occupation numbers of the cells.	Whereas the effect of (1) and
(2) on the distribution function follows directly from the mechanics of the
system, a separate assumption is needed to obtain the effect of collisions on
the distribution function.  This assumption, the collision-number assumption,
asserts that the number of collisions per unit time, of type (i,j) -> (k,l)
(molecules from cells i and j collide to produce molecules of different
velocities which belong to cells k and l), called (A[sub ij])(sup kl), is given
by Eq.	(2).  Here (a[sub ij])(sup kl) depends on the collision configuration
and on the size and kind of the molecules but not on the occupation numbers.
Gains and losses of the molecules in, say, cell i can now be observed.	If the
three factors causing gains and losses are combined, the Boltzmann transport
equation, written as Eq.  (3), is obtained.  Here DELTA(sub x) f(sub i) is the
gradient of f with respect to the positions, DELTA(sub v)f(sub i) refers
similarly to the velocities, and BOLD X(sub i) is the outside force per unit
mass at cell i.  This nonlinear equation determines the temporal evolution of
the distribution function.  Exact solutions are difficult to obtain.  Yet Eq.
(3) forms the basis for the kinetic discussion of most transport processes.
There is one remarkable general consequence, which follows from Eq.  (3).  If
one defines H(t) as in Eq.  (4), one finds by straight manipulation from Eqs.
(3) and (4) that Eq.  (5) holds.  Hence H is a function which in the course of
time always decreases.	This result is known as the H theorem.	The special
distribution which is characterized by Eq.  (6) has the property that collisions
do not change the distribution in the course of time; it is an equilibrium or
stationary distribution.

CURLY Df(sub i)/CURLY Dt + (BOLD v(sub i) DOT DELTA(sub x)f[sub i])
   + (BOLD X DOT DELTA[sub v] f[sub i])
   = SUMMATION (in terms of j,k,l)[a(sub ij)(sup kl)w(sub j)(f(sub k)f(sub l) -
     f(sub i)f(sub j)]							     (3)


    H(t) = SUMMATION (in terms of i) n(sub i) ln f(sub i)		     (4)


		dH/dt < or = 0
		dH/dt = 0 if f(sub i)f(sub j) = f(sub k) f(sub l)	     (5)



	     f(sub i) f(sub j) = f(sub k) f(sub l)			     (6)

    The form of the equilibrium distribution may be determined from Eq. (6),
with the help of conservation laws. For a gas which as a whole is at rest, it
may be shown that the only solution to functional Eq. (6) is given by Eqs.
(7a) or (7b). Here A and beta are parameters, not determined by Eq. (6), and U
is the potential energy at the point x, y, z. Equations (7a) and (7b) are the
Maxwell-Boltzmann distribution. Actually A and beta can be determined from the
fact that the number of particles and the energy of the system are specified.

		   f(sub i) = Ae(sup -beta epsilon)			    (7a)


	f(BOLD x,BOLD v) = Ae[sup (-1/2)beta mv(sup 2) - beta U]	    (7b)

    The indiscriminate use of the collision-number assumption leads, via the H
theorem, to paradoxical results. The basic conflict stems from the
irreversible results that appear to emerge as a consequence of a large number
of reversible fundamental processes. A careful treatment of the explicit and
hidden probability assumptions is the key to the understanding of the apparent
conflict. The equilibrium distribution may be thought of as the most probable
state of a system. If a system is not in equilibrium, it will most likely (but
not certainly) go there; if it is in equilibrium, it will most likely (but not
certainly) stay there. By using such probability statements, it may be shown
that the paradoxes and conflicts may indeed be removed. A consequence of the
probabilistic character of statistics is that the entities computed also
possess this characteristic. For example, one cannot really speak definitively
of the number of molecules hitting a section of the wall per second, but only
about the probability that a given number will hit the wall, or about the
average number hitting. In the same vein, the amount of momentum transferred
to a unit area of the wall by the molecules per second (this, in fact, is
precisely the pressure) is also to be understood as an average. This in
particular means that the pressure is a fluctuating entity. The fluctuations
in pressure may be demonstrated by observing the motion of a mirror, suspended
by a fiber, in a gas. On the average, as many gas molecules will hit the back
as the front of the mirror, so that the average displacement will indeed be
zero. However, it is easy to imagine a situation where more momentum is
transferred in one direction than in another, resulting in a deflection of the
mirror. From the knowledge of the distribution function the probabilities for
such occurrences may indeed be computed; the calculated and observed behavior
agree very well. This clearly demonstrates the essentially statistical
character of the pressure. See BROWNIAN MOVEMENT.
 �34s[Max Dresden]
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�6v�2v�32sBomb calorimeter �21m �33s
    This calorimeter is widely used in determining the heats of combustion of
organic compounds and has been developed to yield measurements reproducible to
within PLUS/MINUS 0.01%. Its operation is fairly typical of calorimetric
processes carried out near room temperature. See CALORIMETRY.
    Some of the essential features of the apparatus are illustrated in the
figure. The bomb, a strong-walled metal container of 0.3 - 0.4-liter capacity,
is constructed of a corrosion-resistant alloy. It is fitted with a screw head,
which contains a valve for introducing oxygen to a pressure of about 30 atm
(3.0 megapascals). The weighed sample, liquid or solid, is placed in the
combustion crucible and ignited by electrical connections which pass through
the head.
    The bomb is immersed in about 2.5 liters of water in the calorimeter
proper, a metal container which is equipped with a platinum resistance
thermometer, a stirrer, and a tightly fitting cover to prevent evaporation of
its water. It rests on small ivory pegs within the jacket, the two being
separated by a 1-cm air space.
    This bomb calorimeter is calibrated either electrically or, more
frequently, by burning a sample of standard benzoic acid, supplied by the U.S.
National Bureau of Standards. After the heat capacity of the calorimeter has
been evaluated, the unknown heat value can be computed from the measured
temperature rise. The result so obtained, corrected to 1 atm (101.3
kilopascals) pressure and computed for 1 mole of the substance, represents
DELTAE, the heat of combustion at constant volume. The corresponding heat
effect at constant pressure can be derived by the relation DELTAH = DELTAE +
PDELTAV.



 �34s[Bruno J. Zwolinski]
�33s


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�6v�2v�32sBond angle and distance   �21m �33s 
    In molecules two atoms are said to be bonded to each other when there are 
strong attractive forces between them. These two atoms constitute a chemical 
bond. One important characteristic of a chemical bond is the separation 
(called the bond length or distance) of the two nuclei of the atoms involved. 
A given atom may be involved in more than one chemical bond, and another 
important characteristic is the angle between bonds sharing a common atom. It 
is also often important to know the angle between two bonds which are 
connected by a third bond. This angle is called the dihedral or torsional 
angle. See CHEMICAL BONDING. 
    Bond angles and lengths determine the shape and size of a molecule, and 
thus determine many physical properties. For example, molecules which are 
spherical in shape will pack in a crystal in a different manner from long, 
rodlike molecules, and will likely have a different density and crystal 
structure. Similarly, the molar volumes in the liquid are likely to be 
different, as well as the diffusion coefficients. Another example is the 
observation that the melting points for normal alkanes with an even number of 
carbons behave differently from those with an odd number. This arises because 
of their different shapes and the way in which they interact in the solid. 
    Bond angles and distances are functions of the type of chemical bonding 
present and give information about the electronic structure of the molecule. 
For example, bond lengths shorten considerably in going from a single bond to 
a triple bond, and are thus dependent on the hybridization present. See 
CHEMICAL STRUCTURES. 
    Since the strength of a bond increases with its multiplicity, there is an 
inverse relation between the strength of a bond and its length. Related to the 
strength of a bond is its stretching-force constant, and in fact general 
relations exist between the stretching-force constant for a bond and its 
length.  
    Dimensions and structure are exceedingly important in biological 
molecules. For example, the functioning of a molecule such as deoxyribonucleic 
acid depends very precisely on the structure of the individual, small 
components which make up this very large molecule. Another example is the 
great specificity of enzymes, which is believed to be associated with the 
exact shape and size of the enzyme. See HYDROGEN BOND.  
 �34s[Victor W. Laurie]
�33s 
  
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�6v�2v�32sBonding   �21m �33s 
    The act of connecting the various structural metal parts of a metal 
enclosure or vehicle (as in an aircraft or automobile) so that these parts 
form a continuous electrical unit. Bonding serves to minimize or eliminate 
interference, such as that caused by ignition systems. It also prevents 
buildup of static electricity on one part of the structure, which can, by 
subsequent discharge to other parts, cause static interference. Bonding is 
achieved by bolting the parts together in such a way as to achieve good 
electrical contact or by connecting them with heavy copper cables or straps. 
    Bonding also refers to the fastening together of two pieces by means of 
adhesives, as in anchoring the copper foil of printed wiring to an insulating 
baseboard. See ADHESIVE.  
 �34s[John Markus]
�33s 
  
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�6v�2v�32sBone   �21m �33s 
    The hard connective tissue which (along with cartilage) forms the skeleton 
of vertebrates. See SKELETAL SYSTEM. 
    Bone consists of collagen fibers which are organized in layers and upon 
which mineral salts, especially calcium phosphate and carbonates, are 
impregnated. Other organic materials, consisting of blood, bone cells 
(osteocytes), and cartilage, are also present. See CONNECTIVE TISSUE. 
    The outer portion of most bones is formed of a hard shelllike compacted 
layer; the inner portions are composed of spongy bone. In long bones of adults 
the compact outer bone forms the long, hollow shaft surrounding a medullary 
cavity containing either hematopoietic bone marrow or fatty tissue. The spongy 
parts of these long bones are present in the ends of the bones where the 
forces on the bone are distributed over a broad area. 
    Some bones are formed by transformation of a cartilaginous precursor, 
while other bones develop directly by deposit of mineral salts in fibrous 
tissue. The time of ossification in the skeleton follows a definite pattern, 
providing there is no abnormality of formation. 
    Aside from furnishing the main support for the body, the bones serve as 
levers for muscular action and motion of the body. The movement of each bone 
depends on the structure of the articular surface and associated ligaments. 
See MUSCULAR SYSTEM. 
    The closely adherent fibrous covering of bones, the periosteum, is 
osteogenic, that is, bone-producing, especially in younger individuals not yet 
having attained full growth. The osteogenic properties of the periosteum are 
vital in the formation of new bone after injury, as in the repair of fractures. 
    Bone is not an inert substance as commonly believed but participates as a 
dynamic and powerful factor in general body metabolism, particularly with 
regard to calcium and other inorganic ions. The influence of vitamin D, 
pituitary growth hormone, estrogens, and thyroid and parathyroid hormones can 
readily be seen in many diseases in which these important regulating 
substances are altered. Moreover, the structure and shape of bones are 
modified rapidly with changes in the mechanical forces acting upon them. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sBooetes �21m �33s
    The Bear Driver, in astronomy, a northern and summer constellation.
Booetes is one of the earliest recorded constellations. Arcturus, an orange,
first-magnitude navigational star, dominates the constellation. Five prominent
stars in this constellation form a pentagon, shaped much like an elongated
kite, with Arcturus at the junction of the tail. Booetes is conventionally
pictured as a driver of the Bear (Ursa Major) nearby. In a more recent version
he is seen seated, smoking a pipe, his feet dangling and with one hand holding
the leash of the Hunting Dogs (Canes Venatici). See CONSTELLATION.
 �34s[Chin-Sung Yu]
�33s

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�6v�2v�32sBook manufacture   �21m �33s 
    A series of professional operations including editing, designing, 
typography, platemaking, printing, and binding to transform a manuscript and 
its related illustrations into book form. 
    Books follow a distinctive technique in assembling and binding pages in 
sequential order and in fitting and sealing the pages into a separate cover. 
Those designed for relatively permanent usage are sealed into rigid or 
semirigid covers (called edition, case, or hardback binding), and those 
designed for less permanent usage are sealed into paper covers (called soft or 
paperback binding). 
    The basic cell of a book's construction is called a signature or section 
and is composed of multiples of four pages. The folding of signatures printed 
on a web press is integrated with the printing units. In sheet-fed printing, 
folding is separate from presswork and is performed on special machines 
located in a book bindery. 
    Folding a sheet of paper once produces 4 pages, and each additional fold 
doubles the number of pages. A sheet folded three times yields a 16-page 
signature, the first page being the lowest numbered (a page number is called a 
folio). All right-hand pages are odd numbered, and left-hand pages are even 
numbered. A book of 160 pages has ten 16-page signatures, the first being 
folioed 1 - 16 and the last 145 - 160. 
    Signatures, assembled in sequence, are mechanically held together along 
one folded edge, called the binding edge or spine, by continuous thread sewing 
through the fold of each signature; or by thread stitching through the side of 
the book from front to back; or by wire stapling; or by cutting off the 
binding-edge folds and applying a coating of strong flexible adhesive, 
sometimes adding an additional locking device of wide mesh textiles or stretch 
papers.  
    Purely decorative features include colorful bits of textiles added to the 
head and tail of the spine (called headbands) and tinted or gilded edges. One 
spectacular technical breakthrough was the mechanical gilding of book edges; 
for centuries this had been a closely guarded handicraft process. 
    The book cover serves to protect the pages against disintegration, to 
announce the title of the book, and to stimulate the visual interest of the 
prospective buyer. The degree of usage and permanency influences the selection 
of raw materials for a book cover (as in reference sets and text and library 
books), and visual appeal is influenced by the book's price and marketability. 
Permanent-type covers are made on special equipment in which the cover 
material and pulp-boards are assembled and joined with special adhesives. In 
embellishing the cover, the designer has numerous choices of special equipment 
and processes, such as flat ink, pigment or metal-foil stampings, embossing, 
silk-screening, and multicolor printing. 
    Preceding the final assembly of the book into its cover (called ``casing 
in''), strong four-page front and back endsheets and one or more kinds of 
hinges (of textiles or paper) have been applied to strengthen the interlocking 
between book and cover (called ``lining up''). The book also has acquired a 
concave and convex edge and a pair of ridges or joints along the binding edge 
(called ``backing''). These features are added by special equipment and are 
specifically engineered to securely lock the book into its cover and to 
transmit the strains of opening the book throughout the entire body instead of 
only to the first and last pages. 
    For shelf display, a book is often wrapped in a colorful, eye-catching 
jacket (called dust wrapper), or in a transparent synthetic if the cover is 
highly decorative. 
    Books reach their markets packaged in many different ways. Single copies 
are packaged and mailed directly to the consumer; sets are assembled into 
cartons, one set per carton; textbooks and other single titles are 
bulk-cartoned or skid-packed and shipped to central warehouses. See PRINTING.  
 �34s[Edward J. Triebe]
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�6v�2v�32sBoolean algebra �21m �33s
    A branch of mathematics that was first developed systematically, because
of its applications to logic, by the English mathematician George Boole,
around 1850. Closely related are its applications to sets and probability.
Boolean algebra also underlies the theory of relations. A modern engineering
application is to computer circuit design. See DIGITAL COMPUTER; SWITCHING
THEORY.
    Most basic is the use of Boolean algebra to describe combinations of the
subsets of a given set I of elements; its basic  operations are those of
taking the intersection or common part S INTERSECTION T of two such subsets S
and T, their union or sum S UNION T, and the complement S' of any one such
subset S. These operations satisfy many laws, including those shown in Eqs.
(1), (2), and (3).

    S INTERSECTION S = S    S INTERSECTION T = T INTERSECTION S
  S INTERSECTION (T INTERSECTION U) = (S INTERSECTION T) INTERSECTION U      (1)

		   S UNION S = S UNION T = T UNION S
	       S UNION (T UNION U) = (S UNION T) UNION U		     (2)

S INTERSECTION (T UNION U) = (S INTERSECTION T) UNION (S INTERSECTION U)
S UNION (T INTERSECTION U) =
	(S UNION T) INTERSECTION (S UNION U)				     (3)

    If O denotes the empty set, and I is the set of all elements being
considered, then the laws set forth in Eq. (4) are also fundamental. Since
these laws are fundamentals, all other algebraic laws of subset combination
can be deduced from them.

		      O INTERSECTION S = O
		      O UNION S = O
		      I UNION S = S= S
		      I UNION S = I
		      S INTERSECTION S' = O
		      S UNION S' = I                                         (4)

    In applying Boolean algebra to logic, Boole observed that combinations of
properties under the common logical connectives and, or, and not also satisfy
the laws specified above. These laws also hold for propositions or assertions,
when combined by the same logical connectives. See LOGIC CIRCUITS.
    Boole stressed the analogies between Boolean algebra and ordinary algebra.
If S INTERSECTION T is regarded as playing the role of st in ordinary algebra,
S UNION T that of s + t, O of 0, I of 1, and S' as corresponding to 1 - s, the
laws listed above illustrate many such analogies. However, as first clearly
shown by Marshall Stone, the proper analogy is somewhat different.
Specifically, the proper Boolean analog of s + t is (S' INTERSECTION T) UNION
(S INTERSECTION T'), so that the ordinary analog of S UNION T is s + t - st.
Using Stone's analogy, Boolean algebra refers to Boolean rings in which s(sup 2)
= s, a condition implying s + s = 0. See RING THEORY; SET THEORY.
    In 1941 M. H. A. Newman developed a remarkable generalization which
included Boolean algebras and Boolean rings. This generalization is based on
the laws shown in Eqs. (5) and (6). From these assumptions, the idempotent,
commutative, and associative laws (1) and (2) can be deduced.

		 a(b + c) = ab + ac(a + b)c = ac + bc			     (5)

			      a1 = 1
			      a + 0 = 0 + a = a
			      aa' = 0
			      a + a' = 1                                     (6)

    Such studies lead naturally to the concept of an abstract Boolean algebra,
defined as a collection of symbols combined by operations satisfying the
identities listed in formulas (1) to (4). Ordinarily, the phrase Boolean
algebra refers to such an abstract Boolean algebra.
    The class of finite (abstract) Boolean algebras is easily described. Each
such algebra has, for some nonnegative integer n, exactly 2(sup n) elements and
is algebraically equivalent (isomorphic) to the algebra of all subsets of the
set of numbers 1, . . . , n, under the operations of intersection, union, and
complement.
    The theory of infinite Boolean algebras is much deeper; it indirectly
involves the whole theory of sets. One important result is Stone's
representation theorem. Let a field of sets be defined as any family of
subsets of a given set I, which contains with any two sets S and T their
intersection S INTERSECTION T, union S UNION T, and complements S', T'.
Considered abstractly, any such field of sets obviously defines a Boolean
algebra. Stone's theorem asserts that, conversely, any finite or infinite
abstract Boolean algebra is isomorphic to a suitable field of sets. His proof
is based on the concepts of ideal and prime ideal, concepts which have been
intensively studied for their own sake.
 �34s[Garrett Birkhoff]
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�6v�2v�32sBoracite �21m �33s
    A borate mineral with chemical composition Mg3B7O13Cl. It occurs in
Germany, England, and the United States, usually in bedded sedimentary
deposits of anhydrite, gypsum, and halite, and in potash deposits of oceanic
type. The chemical composition of natural boracites varies, with Fe sup 2 sup +
or Mn sup 2 sup + replacing part of the Mg sup 2 sup + to yield ferroan boracite
or manganoan boracite. See BORATE MINERALS.
    The hardness is 7 - 7J on Mohs scale, and specific gravity is 2.91 - 2.97
for colorless crystals and 2.97 - 3.10 for green and ferroan types. Luster is
vitreous, inclining toward adamantine. Boracite is colorless to white,
inclining to gray, yellow, and green, and rarely pink (manganoan); its streak
is white; and it is transparent to translucent. It is strongly piezoelectric
and pyroelectric and does not cleave.
 �34s[Charles L. Christ]
�33s

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�6v�2v�32sBorane �21m �33s
    One of a class of binary compounds of boron and hydrogen, often referred
to as boron hydrides. The term borane is sometimes used to denote substances
which may be considered to be derivatives of the boron-hydrogen compounds,
such as boron trichloride (BCl3) and diiododecaborane (B10H12I2).
    The simplest borane is diborane (B2H6); other boranes of increasingly
higher molecular weight are known, one of the least volatile of which is an
apparently polymeric solid of composition (BH) sub x. Certain boranes, such as
BH3 and B3H7, are not known as such, but can be prepared in the form of
adducts with electron-donor molecules.
    The most spectacular projected large-scale use of the boranes and their
derivatives is in the field of high-energy fuels for jet planes and rockets.
The thermal decomposition of diborane (6) [B2H6] has been used to produce
coatings of pure elementary boron for neutron-detecting devices and for
applications requiring hard, corrosion-resistant surfaces. Boranes can also be
used as vulcanizing agents for natural and synthetic rubbers, and are
especially effective in the preparation of silicone rubbers.
    The molecular structures possessed by the boranes are exhibited by no
other class of substances. Because of the lack of sufficient electrons for the
formation of the requisite number of covalent bonds, normal covalently bonded
structures of the hydrocarbon type are not possible. The boranes are sometimes
referred to as electron-deficient substances. In no case are the simple chain
and ring configurations of carbon chemistry encountered in the more complex
boranes. Instead, the boron atoms are situated at the corners of polyhedrons.
An example of such a structure is that of pentaborane (9) [B5H9], shown in the
illustration. Boron nomenclature uses a prefix to designate the number of
boron atoms in the molecule and a numeral suffix in parentheses to indicate
the number of hydrogen atoms.
    As a class, the boranes are quite reactive substances and are generally
decomposed, at times explosively, on contact with air. Their reactivities with
air and water decrease with increasing molecular weight. Because boranes react
readily with air, laboratory investigations are almost invariably carried out
in all-glass vacuum apparatus or in inert-atmosphere dry boxes. With the
possible exception of decaborane (14) [B10H14], the known boranes are not
indefinitely stable at room temperature. They decompose more or less rapidly
to yield elementary hydrogen and boranes richer in boron.
    The known derivatives of the boranes (other than BH3) are relatively few
in number. Several halo, alkyl, and amino boranes have been reported but, in
general, these have not been extensively characterized. See BORON; CARBORANE;
METAL HYDRIDE.
 �34s[Thomas Wartik]
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�6v�2v�32sBorate   �21m �33s 
    A generic term referring to salts related to boric oxide, B2O3, or 
commonly to only the salts of orthoboric acid, H3BO3. Only orthoboric acid is 
important in its free state. Some of the others are known only in the form of 
salts.  
    A dilute solution of any sodium borate contains mainly H2BO3- and BO2- 
ions. The borates are used in manufacture of borosilicate glasses, ceramic 
glazes, vitreous enamels, detergents and water-softening agents, flameproofing 
materials, preservatives, and fluxes. Minerals related to borax are the main 
source of borates. Research has produced boron nitride, an abrasive harder 
than diamond, and boron hydrides, which are used as rocket fuels. See BORATE 
MINERALS; BORON. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sBorate minerals �21m �33s
    A large and complex group of naturally occurring crystalline solids in
which boron occurs in chemical combination with oxygen; hence the term borate
(see table). With the exception of avogadrite, (K,Cs)BF4, and ferrucite,
NaBF4, all known boron minerals are borates. Borate minerals may contain
silicon, phosphorus, or arsenic. These, specifically, are the borosilicates,
the borophosphates, and the boroarsenates. Of these three groups, the first is
by far the largest. See BORON.
    The average amount of boron in the Earth's crust is estimated as 3 ppm.
Fortunately, large quantities are concentrated in deposits of hydrated borate
minerals that are easily worked, and boron is not, like some minor elements,
so completely dispersed that it is hard to find and use. Deposits are found in
the United States (Mojave Desert), Turkey, Soviet Union (Inder district),
Tibet, Italy, Germany, Argentina, Bolivia, Canada, China, India, Iran, New
Zealand, New Guinea, and Syria. The minerals of chief commercial importance
are borax, Na2B4O7 CTR DOT 10H2O, and kernite, Na2B4O7 CTR DOT 4H2O. Large
quantities of these minerals are mined at the Kramer Deposit, Boron, CA. Borax
is extracted in large amounts from brine at Searles Lake, California. See
BORATE; KERNITE.



 �34s[Charles L. Christ]
�33s

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�6v�2v�32sBorda mouthpiece �21m �33s
    A reentrant tube in a hydraulic reservoir. The contraction coefficient of
a Borda mouthpiece can be calculated more simply than for other discharge
openings.
    As a liquid accelerates toward a discharge opening, the streamlines
converge (see illustration). This convergence may cause the issuing jet to
contract to a smaller cross section than that of the opening, the region of
the jet of least cross section being called the vena contracta. The ratio of
vena contracta area to opening area is the contraction coefficient. The
frictional forces in the vicinity of a simple opening alter the accelerations
in the liquid, but with a Borda mouthpiece, friction is so reduced near the
opening that the contraction coefficient can be calculated from hydrostatic
considerations alone with close agreement to measurements. The coefficient
thus calculated is 0.5. See METERING ORIFICE; VENTURI TUBE.



 �34s[Frank H. Rockett]
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�6v�2v�32sBoride   �21m �33s 
    A binary compound of boron and a metal, formed by heating a mixture of the 
two elements. Boron does not conform to the usual rules of valence in forming 
these compounds. The borides, along with the carbides and the nitrides, are 
called interstitial compounds. These compounds have crystal structures and 
properties very similar to those of the original metal; hence, it is suggested 
that the boron is located in the cavities of the metallic lattice. 
    The borides are very hard and refractory and show high electrical 
conductivity. Typical borides are Ni2B, NiB2, MnB, MnB2, and Cu3B2. Boron 
nitride is an abrasive harder than diamond. See ABRASIVE; BORON; SOLID-STATE 
CHEMISTRY.  
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sBoring   �21m �33s 
    A machining operation that increases the size of an existing hole in a 
workpiece. The usual purpose of boring is to machine a hole to a desired 
diameter while obtaining required accuracy and finish. In metalworking, boring 
is performed after making a circular hole in the piece by some method such as 
coring, drilling, burning, punching, or trepanning. See DRILLING MACHINE. 
    Boring machines may be listed in two categories, horizontal and vertical. 
The former type holds the workpiece stationary on a movable table. A rotating 
spindle, holding the cutting tool, can be fed into the work from a vertically 
adjustable head. On vertical machines the work is revolved on a circular table 
while tools can be fed in for boring as well as facing operations. See 
MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sBoring bivalves   �21m �33s 
    A variety of marine bivalve mollusks which penetrate solid substrata. They 
represent seven families and vary in the extent to which they are specialized, 
in the type of substrata they utilize, and in their method of boring. 
    Unlike other bivalves which use their shells as abrasive ``tools,'' date 
mussels (Lithophaga, Mytilidae) penetrate calcareous rocks, corals, and shells 
by chemical means, possibly a weak carbonic acid. Larvae of Botula (another 
small, elongate mytilid), which settle in crevices of soft rock, corals, or 
wood, abrade their way into the substrate by continued movement of their 
shells and siphons. Rupellaria and Petricola (Petricolidae) are nonspecialized 
borers in peat, firm mud, and soft rock. Platyodon (Myidae), closely related 
to the soft-shelled clam, press their valves against the walls of the burrow 
by engorging the mantle and then abrade the soft rock by continued movement of 
their unspecialized valves. Gastrochaena and Spengleria (Gastrochaenidae) are 
specialized for boring by having a closed mantle cavity, a large pedal gape, 
and a truncate foot, allowing them to press the foot and shell against the 
burrow wall. The Pholadacea (Pholadidae and Teredinidae) are worldwide in 
distribution. The family Pholadidae (common name: piddocks) is composed of 17 
genera, of which 5 are restricted to wood. All species of Teredinidae are 
obligate wood, nut, or plant-stem borers. The major difference between these 
two families is the presence of accessory plates in the pholads and pallets in 
the teredinids. See MOLLUSCA; SHIPWORM. 
 �34s[Ruth D. Turner]
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�6v�2v�32sBoring sponges   �21m �33s 
    Sponges that excavate galleries in mollusk shells, corals, limestone, and 
other calcareous matter. Boring sponges are known in three families of the 
class Demospongiae; they are also called burrowing sponges. All species of the 
family Clionidae, order Hadromerida, excavate burrows in calcium carbonate 
substrata, as do some species of the genera Anthosigmella and Spheciospongia 
(family Spirastrellidae, order Hadromerida) and all species of Siphonodictyon 
(family Adociidae, order Haplosclerida). 
    Water currents are maintained through the tissues of these boring sponges 
exactly as in other Demospongiae. They do not obtain food from their hosts 
when the calcareous matter into which they burrow happens to be the shell or 
skeleton of a living organism. Instead, boring sponges feed on bacteria, 
flagellates, and other kinds of particulate organic matter. The burrowing 
process is similar in all genera and involves the excavation of numerous 
minute particles of calcium carbonate (20 - 70 micrometers in diameter) that 
pass out by way of the exhalant canals and oscula. In coral reef environments, 
up to 30 - 40% of fine sediment-particles may be derived from the activities 
of burrowing sponges in low-energy environments; lesser proportions (5 - 22%) 
characterize sediments in high-energy environments. 
    Clionids are regarded as a nuisance in oyster beds because their 
excavations weaken the shells of these mollusks and thus hinder processing in 
canning factories. Boring sponges are also economically important when they 
attack limestone breakwaters. 
    Clionid boring sponges are found in all seas, chiefly in tidal and shallow 
waters. A few species reach a depth of at most 3000 ft (1000 m.). Their 
excavations are known in fossil mollusk shells at least as far back as the 
Devonian Period. Burrowing species of the genera Anthosigmella, 
Spheciospongia, and Siphonodictyon are found on coral reefs at depths of more 
than 200 ft (60 m.). See DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
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�6v�2v�32sBorneo �21m �33s
    An island in Sundanaland made up of Kalimantan, Sarawak, Brunei, and
Sabah. Borneo has an area of 282,000 mi sup 2 (730,000 km sup 2) and is the
third largest island of the world. Mount Kinabalu, 13,445 ft (4098 m), in Sabah
is the highest mountain on the island.
    Crossed by the Equator, Borneo has a tropical climate with little range in
temperature, coastal stations varying between 77 and 79 DG F (25 and 26 DG C)
average monthly temperatures throughout the year. Annual rainfall in the
lowlands varies from 90 to 130 in. (2.3 to 3.3 m). This is evenly distributed
throughout the year without a distinct wet or dry season, except in the north
and southeast, where there is a slight tendency toward a short dry season. A
tropical evergreen forest covers large areas of Borneo, but there are also
large areas of savanna grass and scrub trees, a result of past farming and
fires of human origin. See EAST INDIES.
 �34s[M. G. Audley-Charles]
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�6v�2v�32sBornite   �21m �33s 
    A sulfide of composition Cu5FeS4, specific gravity 5.07, and hardness 3 
(Mohs scale), commonly occurring as a primary mineral in many copper ore 
deposits. Crystals are rare; bornite is usually massive or granular. The 
metallic and brassy color of a fresh surface rapidly tarnishes upon exposure 
to air to a characteristic iridescent purple, giving rise to the name 
``peacock ore.'' Though of lesser importance as an ore than chalcocite or 
chalcopyrite, masses of bornite have been mined in Chile, Peru, Bolivia, and 
Mexico and in the United States in Arizona and Montana. See CHALCOCITE: 
CHALCOPYRITE: COPPER. 
 �34s[Bernhardt J. Wuensch]
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�6v�2v�32sBoron �21m �33s
    A chemical element, B, atomic number 5, atomic weight 10,811, in group
IIIa of the periodic table. It has three valence electrons and is nonmetallic
in behavior. It is classified as a metalloid and is the only nonmetallic
element which has fewer than four electrons in its outer shell. The free
element is prepared in crystalline or amorphous form. The crystalline form is
an extremely hard, brittle solid. It is of jet-black to silvery-gray color
with a metallic luster. One form of crystalline boron is bright red. The
amorphous form is less dense than the crystalline and is a dark-brown to black
powder. In the naturally occurring compounds, boron exists as a mixture of two
stable isotopes with atomic weights of 10 and 11.
    Many properties of boron have not been sufficiently established
experimentally as a result of the questionable purity of some sources of
boron, as well as of the variations in the methods and temperatures of
preparation. A summary of the physical properties is shown in the table.
    Boron and boron compounds have numerous uses in many fields, although
elemental boron is employed chiefly in the metal industry. Its extreme
reactivity at high temperatures, particularly with oxygen and nitrogen, makes
it a suitable metallurgical degasifying agent. It is used to refine the grain
of aluminum castings and to facilitate the heat treatment of malleable iron.
Boron considerably increases the high-temperature strength characteristics of
alloy steels. Elemental boron is used in the atomic reactor and in
high-temperature technologies. The physical properties tht make boron attractive
as a construction material in missile and rocket technology are its low density,
extreme hardness, high melting point, and remarkable tensile strength in
filament form. When boron fibers are used in an epoxy (or other plastic) carrier
material or matrix, the resulting composite is stronger and stiffer than steel
and 25% light than aluminum. Refined borax, Na2B4O7 CTR DOT 10H20, is an
important ingredient of a variety of detergents, soaps, water-softening
compounds, laundry starches, adhesives, toilet preparations, cosmetics, talcum
powder, and glazed paper. It is also used in fireproofing, disinfecting of fruit
and lumber, weed control, and insecticides, as well as in the manufacture of
leather, paper, and plastics.
    Boron makes up 0.001% of the Earth's crust. It is never found in the
uncombined or elementary state in nature. Besides being present to the extent
of a few parts per million in sea water, it occurs as a trace element in most
soils and is an essential constituent of several rock-forming silicate
minerals, such as tourmaline and datolite. The presence of boron in extremely
small amounts seems to be necessary in nearly all forms of plant life, but in
larger concentrations, it becomes quite toxic to vegetation. Only in a very
limited number of localities are high concentrations of boron or large
deposits of boron minerals to be found in nature; the more important of these
seem to be primarily of volcanic origin. See BORATE MINERALS.
 �34s[F. H. May /V. V. Levasheff]
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�6v�2v�32sBoron polymers �21m �33s
    Inorganic polymers of boron compounds. Hexagonal boron nitride, (BN) sub n,
is structurally related to graphite. Like graphite, it has lubricating
properties, reflecting the relationship between molecular structure and
physical properties, but unlike graphite, it is an electrical insulator.
Molybdenum disulfide, (MoS2) sub n, with a similar and related structure, is
also a solid lubricant. Both graphite and hexagonal boron nitride can be
readily machined. Outstanding properties of the latter include high thermal
and chemical stability and good dielectric properties. Crucibles and such
items as nuts and bolts can be made from this material.  See  BORON; GRAPHITE;
INORGANIC POLYMER.
 �34s[Robert A. Shaw]
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�6v�2v�32sBose-Einstein statistics �21m �33s
    The statistical description of quantum mechanical systems in which there
is no restriction on the way in which particles can be distributed over the
individual energy levels. This description applies when the system has a
symmetric wave function. This in turn has to be the case when the particles
described are of integer spin.
    Suppose one describes a system by giving the number of particles nsubi in
an energy state epsilon(sub i), where the n(sub i) are called occupation numbers
and the index i labels the various states. The energy level epsilonsubi is of
finite width, being really a range of energies comprising, say, gsubi
individual (nondegenerate) quantum levels. If any arrangement of particles
over individual energy levels is allowed, one obtains for the probability of a
specific distribution Eq. (1a). In Boltzmann statistics, this same probability
would be written as Eq. (1b). See BOLTZMANN STATISTICS.
								     (EQ. 1a)
(EQ.
W = PI CTR DOT (in terms of i) (n[sub i] + g[sub i] - 1)!/n!(g[sub i] - 1)!

								     (Eq. 1b)
W = PI CTR DOT (in terms of i) g(sub i)(sup ni)/n(sub i)

    The equilibrium state is defined as the most probable state of the system.
To obtain it, one must maximize Eq. (1a) under the conditions given by Eqs.
(2a) and (2b), which express the
fact that the total number of particles N and the total energy E are fixed.

		  SUMMATION n(sub i) = N				(2a)

	     SUMMATION epsilon(sub i) n(sub i) = E			(2b)

One finds for the most probable distribution that Eq. (3) holds. Here, A and
beta are parameters to be determined
from Eqs. (2a) and (2b); actually, beta = 1/kT, where k is the Boltzmann
constant and T is the absolute temperature.

n(sub i) = (g[sub i])/[(1/A)(e[sup beta epsilon(sub i)] + 1)]		    (3)

    An interesting and important result emerges when Eq. (3) is applied to a
gas of photons, that is, a large number of photons in an enclosure. (Since
photons have integer spin, this is legitimate.) The formula for the energy
density (energy per unit volume) in a given frequency range is found to be the
celebrated Planck radiation formula for blackbody radiation. Thus, blackbody
radiation must be considered as a photon gas, with the photons satisfying
Bose-Einstein statistics. See HEAT RADIATION.
    If an ideal Bose gas, consisting of a fixed number of material particles,
is compressed beyond a certain point, some of the particles will condense in a
zero state, where they do not contribute to the density or the pressure. If
the volume is decreased, this curious condensation phenomenon results,
yielding the zero state which has the paradoxical properties of not
contributing to the pressure, volume, or density. There is now considerable
evidence that many of the superfluid properties exhibited by liquid helium are
in fact manifestations of an Einstein condensation. See FERMI-DIRAC
STATISTICS; NONRELATIVISTIC QUANTUM THEORY; QUANTUM STATISTICS; STATISTICAL
MECHANICS.
 �34s[Max Dresden]
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�6v�2v�32sBotanical gardens   �21m �33s 
    A garden for the culture of plants collected chiefly for scientific and 
educational purposes. Such a garden is more properly called a botanical 
institution, in which the outdoor garden is but one portion of an organization 
including the greenhouse, the herbarium, the library, and the research 
laboratory. See HERBARIUM. 
    It was only in modern Europe, after the foundation of the great medieval 
universities, that botanical gardens for educational purposes began to be 
established in connection with the schools. The oldest gardens are those in 
Padua (established 1533) and at Pisa (1543). The botanical garden of the 
University of Leiden was begun in 1587, and the first greenhouse is said to 
have been constructed there in 1599. The Royal Botanical Gardens at Kew, 
England, were officially opened in 1841. This institution came to be known as 
the botanical capital of the world. 
    The first of the great tropical gardens was founded at Calcutta in 1787. 
The original name, Royal Botanic Garden, was changed in 1947 to Indian Botanic 
Garden. Another great tropical garden, the Jardin Botanico of Rio de Janeiro, 
was founded in 1808. The great tropical botanical garden of Buitenzorg 
(Bogor), Java, which originated in 1817, has an area of 205 acres (83 
hectares) with an additional 150 acres (61 hectares) in the Mountain Garden. 
    The first great garden of the United States was founded by Henry Shaw at 
St. Louis in 1859, and is now known as the Missouri Botanical Garden. The New 
York Botanical Garden was chartered in 1891 and the Brooklyn Botanic Garden in 
1910. The Jardin Botanique of Montreal, the leading garden of Canada, was 
opened in 1936. See ARBORETUM. 
 �34s[Earl L. Core]
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�6v�2v�32sBotany   �21m �33s 
    That branch of biological science which embraces the study of plants and 
plant life. Botanical studies may range from microscopic observations of the 
smallest and obscurest plants to the study of the trees of the forest. One 
botanist may be interested mainly in the relationships among plants and in 
their geographic distribution, whereas another may be primarily concerned with 
structure or with the study of the life processes taking place in plants. 
    Botany may be divided by subject matter into several specialties, such as 
plant anatomy, plant chemistry, plant cytology, plant ecology (including 
autecology and synecology), plant embryology, plant genetics, plant 
morphology, plant physiology, plant taxonomy, ethnobotany, and paleobotany. It 
may also be divided according to the group of plants being studied; for 
example, agrostology, the study of grasses; algology (phycology), the study of 
algae; bryology, the study of mosses; mycology, the study of fungi; and 
pteridology, the study of ferns. Bacteriology and virology are also parts of 
botany in a broad sense. Furthermore, a number of agricultural subjects have 
botany as their foundation. Among these are agronomy, floriculture, forestry, 
horticulture, landscape architecture, and plant breeding. See AGRICULTURE; 
AGRONOMY; BACTERIOLOGY; BREEDING (PLANT); CYTOLOGY; ECOLOGY; FLORICULTURE; 
GENETICS; LANDSCAPE ARCHITECTURE; PALEOBOTANY; PLANT ANATOMY; PLANT GROWTH; 
PLANT MORPHOGENESIS; PLANT PATHOLOGY; PLANT PHYSIOLOGY; PLANT TAXONOMY. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sBothriocidaroida �21m �33s
    An order of Perischoechinoidea in which the ambulacra comprise two columns
of plates, the interambulacra one column, and the madreporite is placed
radially. Bothriocidaris (see illustration), the only known genus, occurs in
the Upper Ordovician of Estonia, and is the oldest recognized echinoid. See
ECHINODERMATA; ECHINOIDEA; PERISCHOECHINOIDEA.



 �34s[Howard B. Fell]
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�6v�2v�32sBotulism   �21m �33s 
    A type of food poisoning due to intoxication by the extremely potent 
exotoxin produced by the anaerobic bacteria Clostridium botulinum and C. 
parabotulinum. Immunization against botulism is generally not considered 
necessary since the disease is comparatively rare. Prevention of botulism is 
best accomplished by safe procedures of food preservation. 
    The disease in humans most commonly follows ingestion of unheated, 
improperly processed food, with pH above 4.5, in which growth and toxin 
production has occurred. It is characterized by respiratory failure. In 
humans, an acute gastroenteritis may or may not precede the principal symptoms 
of extreme weakness, dry mucosa, loss of accommodation, ocular muscle 
paralysis, and difficulty in swallowing. These symptoms commonly occur within 
48 h after the ingestion of the toxic food. See TOXIN. 
 �34s[Leland S. McClung]
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�6v�2v�32sBoundary-layer flow �21m �33s
    The flow of that portion of a viscous fluid which is in the neighborhood
of a body in contact with the fluid and in motion relative to the fluid.
Wherever a viscous fluid flows past a boundary, the layers of the fluid
nearest the boundary are subjected to shearing forces, which cause the
velocity of these layers to be reduced. As the boundary is approached, the
velocity continuously decreases until, immediately at the boundary, the fluid
particles are at rest relative to the body. This region of retarded velocity
is called the boundary layer, and a graph of the variation of velocity with
distance from the wall or boundary describes a boundary-layer profile (see
illustration). The primary effects of the viscosity of the fluid are
concentrated in this boundary layer, whereas in the outer or free-stream flow
the viscous forces are negligible. Because the friction forces due to the
viscosity of the fluid are restricted to the boundary layer, this region is
also often called the friction layer. See FLUID FLOW; NAVIER-STOKES EQUATIONS;
VISCOUS FLOW.
    The velocity of the fluid within the boundary layer approaches the local
free-stream velocity asymptotically as the distance from the wall is
increased; thus the actual thickness of the boundary layer is difficult to
determine. An arbitrary boundary-layer thickness DELTA is usually defined as
that distance from the wall or boundary where the velocity of the flow in the
boundary layer u reaches 99% of the local free-stream velocity U (see
illustration). The thicknesses of boundary layers surrounding vehicles moving
through the air range from less than 0.1 in. (2.5 mm) near the front of a
high-speed aircraft to more than 10 ft (3 m) at the rear of a dirigible or an
airship.
    The viscous shearing force created within the boundary layer constitutes a
major portion of the resistance or drag experienced by an airplane in flight
or by fluids passing through pipes and channels; this shearing force opposes
the relative motion of any viscous fluid past a bounding or immersed surface.
The boundary-layer thickness generally increases with the extent of the
surface over which the fluid has passed. In the flow of fluid through pipes,
the boundary layer formed at the walls thickens until the pipe is entirely
filled with boundary-layer flow. When such a condition is reached, fully
developed pipe flow is said to exist.

    In flowing over a surface of changing contour, the velocity and pressure
of a fluid vary. As the flow velocity increases, the pressure must decrease.
In the free stream, where there are negligible losses to viscosity, the flow
is always able to proceed into regions of increasing pressure at the expense
of its velocity, the opposing pressure forces being balanced by the forces
that result from the change in momentum of the flow. However, within the
boundary layer, where the momentum of the fluid has been reduced in overcoming
viscous forces, the remaining momentum may be insufficient to allow the flow
to proceed into regions of increasing pressure. At the position on the body
where this condition exists, the boundary layer no longer continues to follow
the contour of the boundary but instead leaves, or separates from, the
surface. This phenomenon is known as boundary-layer separation. Beyond the
point of separation, between the surface and the separated boundary layer, a
region of reversed or upstream flow exists. This reversed flow and the
separated boundary layer subsequently coalesce to form a wake of low-velocity
fluid behind the body. The separation of the flow in the boundary layer limits
the lifting capabilities of wings and causes the stalling of airplanes. The
efficiencies of fluid machines and mechanisms, such as turbines, compressors,
pumps, and propellers, are decreased by flow separation originating in the
boundary layer. See AIRPLANE; BERNOULLI'S THEOREM.
    The flow in the boundary layer formed on a body exists in one of two
characteristic states: laminar or turbulent. If the flow is laminar, strata or
laminae of fluid pass smoothly across the surface, whereas if it is turbulent,
the flow is characterized by a rapid churning or mixing between layers of flow
having different velocities. The phenomenon of transition from laminar to
turbulent flow may occur within the boundary layer after the flow has passed
over a surface for some distance.
    The natural development of a boundary layer is affected by the contour and
surface roughness of the body on which it is formed. Therefore, by the
reduction of surface roughness and the choice of surface contours, some amount
of control may be exerted upon the development of the boundary layer.
Attention to surface roughness, waviness, and continuity is particularly
important in the preservation of laminar boundary layers. The overall contour
of the body is also an important consideration if laminar boundary-layer flow
is to be maintained. Many laminar airfoil sections have been developed for
high-speed aircraft. In fact, the term streamlining has come to describe the
technique of designing shapes upon which the boundary-layer growth is
minimized. The body shape also has a large influence upon the separation of
the flow, and, to give low resistance or drag, shapes must be designed which
prevent or delay boundary-layer separation. See STREAMLINING.
    Because the amount of control available by purely geometric means is
limited, additional methods for influencing the development of the boundary
layer have evolved. These methods in general are intended either to remove the
low-momentum flow from the boundary layer or to restore the lost momentum. The
method of controlling the boundary layer by the removal of the retarded flow,
called suction boundary-layer control, may be applied both to maintain laminar
flow and to prevent boundary-layer separation. By sucking away the flow in the
lower regions of the boundary layer through slots or perforations in the
surface, the development of the boundary layer can be readily influenced.



 �34s[Jeffrey J. Clarke]
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�6v�2v�32sBourdon-spring pressure gage �21m �33s
    A mechanical pressure-measuring instrument employing as its sensing
element a curved or twisted metallic tube, flattened in cross section. One end
of the tube is closed, and the fluid pressure to be measured is applied
through the other end. As the pressure is increased, the tube becomes more
nearly circular in cross section and tends to straighten. The motion of the
free (closed) end of the tube is a measure of the internal pressure.
    Pressure-measuring instruments using bourdon tubes are used for pressure
ranges from 0 - 10 lb/in. sup 2 gage (psig) or 0 - 70 kilopascals gage, to 0 -
100,000 psig or 0 - 700 megapascals gage, as well as for vacuum. The accuracy
may be from 0.1 to 2.0% of full-scale reading, depending on the materials,
design, and precision of components.
    The bourdon pressure element may be made in any one of a number of shapes.
The commonest forms are the C tube (see illustration), the spiral, and the
helical element. The bourdon gage is the most popular industrial
pressure-measuring instrument because of its relatively low cost, reasonable
accuracy, and durability. See PRESSURE MEASUREMENT.




 �34s[Heinz G. Sell ;Peter J. Walsh]
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�6v�2v�32sBowfin �21m �33s
    A fish, Amia calva, also known as the mudfish, dogfish, and grindle, that
is the only living species of the family Amiidae and of the order Amiiformes
(see illustration). This line of fishes originated during the early Mesozoic
and became a dominant group during the Jurassic and Cretaceous, when it was
represented by many species.
    The single extant species has features which are characteristic of extinct
primitive fishes, and it is frequently referred to as a living fossil. The
group is intermediate between the cartilaginous and bony fishes since the
skeletal system is partly cartilaginous, the head shows bony dermal plates,
and the scales are cycloid. This fish has a well-developed air bladder which
functions as a lung. The lung allows the bowfin to survive out of water for
long periods of time, up to a full day.
    The bowfin ranges from the Great Lakes through the Mississippi Valley to
Florida. This fish is inedible and preys upon game fish. See ACTINOPTERYGII;
AMIIFORMES.



 �34s[Charles B. Curtin]
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�6v�2v�32sBoyle's law   �21m �33s 
    A law of gases which states that at constant temperature the volume of a 
gas varies inversely with its pressure. This law, formulated by Robert Boyle 
(1627 - 1691), can also be stated thus: The product of the volume of a gas 
times the pressure exerted on it is a constant at a fixed temperature. The 
relation is approximately true for most gases, but is not followed at high 
pressure. The phenomenon was discovered independently by Edme Mariotte about 
1650 and is known in Europe as Mariotte's law. See GAS; KINETIC THEORY OF 
MATTER.  
 �34s[Frank H. Rockett]
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�6v�2v�32sBrachiopoda �21m �33s
    A phylum of solitary, exclusively marine, coelomate, bivalved animals,
with both valves symmetrical about a median longitudinal plane. They are
typically attached to the substrate by a posteriorly located fleshy stalk or
pedicle. Anteriorly, a relatively large mantle cavity is always developed
between the valves, and the filamentous feeding organ, or lophophore, is
suspended in it, projecting forward from the anterior body wall (see
illustration).
    There are two clearly defined groups within the phylum, a division that is
particularly marked if only Recent animals are considered. These two groups
are regarded as classes; several names have been given to them, but
Inarticulata and Articulata are the most widely used and are based on one of
the most readily observed differences between them, the presence or absence of
articulation between the two valves of the shell. Among the Articulata the
valves are typically hinged together by a pair of teeth with complementary
sockets in the opposing valve; these hinge teeth are lacking in the
Inarticulata, whose valves are held together only by the soft tissue of the
living animal. The phylum is currently classified into the following groups:
Class Inarticulata
    Order: Lingulida
	   Acrotretida
	   Obolellida
	   Paterinida
Class Incertae Sedis
    Order Kutorginida
Class Articulata
    Order: Orthida
	   Strophomenida
	   Pentamerida
	   Rhynchonellida
	   Spiriferida
	   Terebratulida
    The pedicle is the only organ protruding outside the valves, while the
remainder of the animal is enclosed in the space between them. This space is
divided into two unequal parts, a smaller posteriorly located body cavity and
an anterior mantle cavity. The two mantles approach each other and ultimately
fuse along the posterior margin of articulate brachiopods; in contrast, the
mantles are invariably discrete in the inarticulates and are separated by a
strip of body wall.
    The body cavity contains the musculature; the alimentary canal; the
nephridia, which are paired excretory organs also functioning as gonoducts;
the reproductive organs; and primitive circulatory and nervous systems. Except
for the openings through the nephridia, the body cavity is enclosed, but the
mantle cavity communicates freely with the sea when the valves are opened. The
lophophore is suspended from the anterior body wall within the mantle cavity
and is always symmetrically disposed about the median plane. The lophophore
consists of a variably disposed, ciliated, filament-bearing tube, with the
ciliary beat producing an ordered flow of water within the cavity, flowing
across the filaments. The latter trap food particles which are carried along a
groove in the lophophore to the medially situated mouth.
    All modern brachiopods are marine, and there is little doubt from the
fossil record that brachiopods have always been confined to the sea. Recent
brachiopods occur most commonly beneath the relatively shallow waters of the
continental shelves, which seems to have been the most favored environment,
but the bathymetric range of the phylum is large. A few modern species live
intertidally and, at the other extreme, a limited number have been dredged
from depths of over 16,000 ft (5000 m).
    The majority of brachiopods form part of the sessile benthos and are
attached by their pedicle during postlarval life. Glottidia and Lingula are
exceptional in being infaunal and making burrows. A commoner modification
involves loss of the pedicle, either complete suppression or atrophy early in
the life history of the individual. Such forms either lie free on the sea
floor, are attached by cementation of part or all of the pedicle valve, or are
anchored by spines. The geographic distribution and geological setting of some
fossil species suggest that they may have been epiplanktonic, attached to
floating weed, but such a mode of life is unknown in modern faunas. See
ARTICULATA (BRACHIOPODA): INARTICULATA.




 �34s[A. J. Rowell]
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�6v�2v�32sBradyodonti   �21m �33s 
    An order, perhaps composite in nature, of Paleozoic cartilaginous fishes 
(Chondrichthyes), presumably derived from primitive sharks. They are mainly 
represented by dentitions, which appear to have been adapted to the eating of 
mollusks and other shelled invertebrates; the body (seldom preserved) appears 
to have been broad and flattened, with large pectoral fins, as in the later 
skates and rays of similar habits. The first bradyodonts appeared at the end 
of the Devonian; they flourished during the Carboniferous, but declined and 
vanished by the end of the Permian, concomitantly with a reduction in the 
invertebrate fauna at that time. There are four families: Cochliodontidae, 
Petalodontidae, Psammodontidae, and Copodontidae. See CHIMAERIFORMES: 
CHONDRICHTHYES: ELASMOBRANCHII. 
 �34s[Alfred S. Romer]
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�6v�2v�32sBrain �21m �33s
    That portion of the central nervous system enclosed within the skull. It
is composed of the brainstem, or segmental portion of the brain, and the
suprasegmental apparatus, comprising a posterior cerebellum and anterior
paired cerebral hemispheres (see illustration). See CENTRAL NERVOUS SYSTEM.
    The telencephalon is a part of the brain that includes the olfactory bulbs
(and paraolfactory bulbs) and cerebral hemispheres. Whereas the olfactory and
paraolfactory bulbs are very similarly organized in all vertebrates, the
cerebral hemispheres manifest greater diversity of structure among vertebrates
than any other part of the brain. The olfactory and paraolfactory bulbs
receive and process information from the chemosense organs of the nasal region
and transmit this information to the cerebral hemispheres. Not surprisingly,
the size of this portion of the telencephalon is proportional to the size and
importance of these chemosenses in a given animal. The cerebral hemispheres
play an integrative function in all vertebrates. The cerebrum is largest in
the vertebrates with the greatest integrative ability, such as humans and
whales.
    The diencephalon is the anteriormost part of the brainstem. In all
vertebrates the diencephalon can be divided into four regions; from dorsal to
ventral, these are the epithalamus, dorsal thalamus, ventral thalamus, and
hypothalamus. The epithalamus is always relatively small, and its functions
poorly understood. It is best developed in vertebrates such as lizards that
have pineal or parietal median eyes. The dorsal thalamus is largest in
vertebrates with the largest cerebral hemispheres. It is a complex of nuclei
relaying information from the spinal cord and other parts of the brain to the
cerebral hemispheres. The ventral thalamus is relatively inconspicuous in all
vertebrates and functions in motor activity arising from deep portions of the
cerebral hemispheres. The hypothalamus, to which the pituitary gland attaches,
is a major integrating center for endocrine and visceral activities.
    The mesencephalon (midbrain) is divided into a dorsal tectum and a ventral
tegmentum. The tectum has large visual and auditory centers and a small
somatic sensory center. These sensory centers are integrative in function and
send already processed information to both the cerebral hemispheres and other
portions of the central nervous system. The tegmentum contains nervous
pathways going to and from the forebrain and important motor centers such as
the red nucleus.
    The dorsal portion of the metencephalon forms the suprasegmental
cerebellum. The cerebellum, although it does not direct motor activity, is the
all-important organ for coordination of motor activity and maintenance of
equilibrium. It is smallest in vertebrates without paired appendages. It is
large and well developed in mammals, birds, and active fishes such as sharks.
The ventral portion of the metencephalon contains nerve fiber tracts traveling
through it, and the central nuclei of several cranial nerves.
    Combined with the ventral portion of the metencephalon, the myelencephalon
is called the medulla oblongata. In addition to fiber tracts to and from the
spinal cord, the medulla oblongata contains the centers for respiration and
arousal ``reticular'' centers. It also has the central nuclei for most of the
cranial nerves.
    The entire brain has a lumen filled with cerebrospinal fluid, and
dilations of the lumen are called ventricles. The size, shape, and number of
these ventricles vary greatly among vertebrates. See MENINGES; NERVOUS SYSTEM
(VERTEBRATE).



 �34s[Douglas B. Webster]
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�6v�2v�32sBrake   �21m �33s 
    A machine element for applying a force to a moving surface to slow it down 
or bring it to rest in a controlled manner. Brakes are used in cars, trains, 
airplanes, elevators, and other machines. Most brakes are of a frictional type 
in which a fixed surface is brought into contact with a moving part that is to 
be slowed or stopped. Brakes in general connect a moving and a stationary body. 
    The limitations on the applications of brakes are similar to those of 
clutches, except that the service conditions are more severe because the 
entire energy is absorbed by slippage which is converted to heat that must be 
dissipated. The important thing is the rate at which energy is absorbed and 
heat dissipated. With frictional brakes, if the temperature of the brake 
becomes too high, the result is a lowering of the friction force, called 
fading.  
    There are also electrical and hydrodynamic brakes. The electrical type may 
be electromagnetic, eddy-current, hysteresis, or magnetic-particle. The 
hydrodynamic type works somewhat like a fluid coupling with one element 
stationary. Another type of brake, used on electric trains, is the 
regenerative brake. The dynamo on this machine can be used as a motor or a 
generator. As a generator it brakes the train and stores the generated 
electricity in an accumulator. Another type of brake is the air brakes (flaps) 
on an airplane. See AIR BRAKE; DYNAMO; ELECTROMAGNETIC BRAKE; FRICTION BRAKE.  
 �34s[Harry P. Hale]
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�6v�2v�32sBranch circuit �21m �33s
    The portion of an electric wiring system that follows the final circuit
overcurrent protector and that terminates at the utilization device (lighting
fixture, motor, or heater) or a plug receptacle for the connection of portable
lamps and appliances.
    Branch circuits serving more than one outlet or load are limited by the
National Electrical Code to three types:
    1. Circuits of 15 or 20 amperes may serve lights and appliances; the
rating of one portable appliance may not exceed 80% of the circuit capacity;
the total rating of fixed appliances may not exceed 50% of circuit capacity if
lights or portable appliances are also supplied.
    2. Circuits of 30 A may serve fixed lighting units with heavy-duty
lampholders in other than dwellings or appliances in any occupancy.
    3. Circuits of 40 or 50 A may serve fixed lighting with heavy-duty
lampholders in other than dwellings, fixed cooking appliances, or infrared
heating units.
 �34s[William T. Stuart ;Joseph F.McPartland]
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�6v�2v�32sBranchiopoda   �21m �33s 
    A subclass of crustaceans which are of small or moderate size. There are 
five orders: Anostraca, Lipostraca, Notostraca, Conchostraca, and Cladocera. 
See separate articles on each order. 
    The trunk region consists of 5 - 44 body rings or segments. The number is 
usually constant but sometimes varies within a genus or even a species. The 
posterior part of the body is usually without legs. A carapace is present in 
most species, in the form of a bivalved shell or a dorsal shield. Most species 
are detritus and plankton feeders. All of them live in fresh water, except for 
a few Cladocera. See CRUSTACEA. 
 �34s[Folke Linder]
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�6v�2v�32sBranchiura �21m �33s
    A subclass of the Crustacea known as the fish lice. They are ectoparasites
of fresh-water and marine fish. Argulus (see illustration) is a common genus.
They are a small homogeneous group, less than 100 species, with worldwide
distribution, and very much alike in appearance. The disklike head and thorax
(cephalothorax) is strongly flattened and bears a small unsegmented abdomen
bilobed at its distal end. Larger species may exceed 1 in. (25 mm) in length.
    The appendages, all on the cephalothorax, consist of two pairs of
antennae, the mouthparts which are one pair of mandibles and two pairs of
maxillae, and four pairs of swimming legs. The wafer-thin cephalothorax with
its appendages appressed to the underside permits fish lice to flatten
themselves against a fish's skin. The resulting highly streamlined contour
serves to minimize the considerable force of frictional drag found in a dense
medium like water.
    Other adaptations for holding fast to a mucus-covered swimming fish
include numerous spinules and strategically placed hooks on the underside of
the body and utilization of specialized portions of the body as suction cups.
Despite these elaborate specializations, fish lice never lose the ability to
abandon a host and swim. At least some of this activity is associated with
mating and spawning.
    Branchiurans feed on tissue fluids, especially blood. The mouth, located
at the end of a movable proboscis, is applied to the host's skin and the
rasping mandibles make the necessary wound. Fish lice usually are no threat to
fish populations. In restricted areas such as hatchery ponds, however, an
infestation can increase to levels that bring about high fish mortality.
Treatment may require such drastic measures as drainage and cleaning before
restocking can be successful. See CRUSTACEA.

     FIGURE CAPTION.  Argulus japonicus.

 �34s[Abraham Fleminger]
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�6v�2v�32sBrass   �21m �33s 
    An alloy of copper and zinc. In manufacture, lump zinc is added to molten 
copper, and the mixture is poured into either castings ready for use or into 
billets for further working by rolling, extruding, forging, or similar 
process. Brasses containing 75 - 85% copper are red-gold and malleable; those 
containing 60 - 70% are yellow and also malleable; and those containing 50% or 
less copper are white, brittle, and not malleable. Alpha brass contains up to 
36% zinc; beta brass contains nearly equal proportions of copper and zinc. 
Specific brasses are designated as follows: gilding (95% copper: 5% zinc), red 
(85:15), low (80:20), and admiralty (70:29, with balance of tin). Naval brass 
is 59 - 62% copper with about 1% tin, less than that of lead and iron, and the 
remainder zinc. The nickel silvers contain 55 - 70% copper and the balance 
nickel. Leaded brass is used for castings. 
    Brass stains in moist air; however, the oxide so formed is sufficiently 
continuous and adherent to retard further oxidation. When brass is required to 
remain bright, it is either washed in nitric acid and then coated with clear 
lacquer, or it is regularly polished and waxed. 
    Brass is widely used in cartridge cases, plumbing fixtures, valves and 
pipes, screws, clocks, and musical instruments. See ALLOY; COPPER ALLOYS; ZINC. 
 �34s[Frank H. Rockett]
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�6v�2v�32sBrassin   �21m �33s 
    A lipid complex from rape pollen that is biologically active on a wide 
range of crop plants. Plants produce hormones which control their growth and 
behavior. Synthetically prepared growth-regulating substances have been used 
for many years to supplement endogenous hormones, and they change to some 
extent the behavior of crop plants to suit the needs of humans. But hormones 
produced by crop plants themselves so far have not been used in place of 
synthetic substances for this purpose, mainly because they occur in extremely 
small amounts and are difficult to extract. Tons of plant material are 
required to produce a few milligrams of hormones. However, some hormones made 
by plants eventually may be useful in plant control because sources have been 
discovered from which relatively large amounts of endogenous regulators can be 
obtained. Brassins, for example, are hormones that may eventually be useful; 
they have been discovered in pollen of rape (Brassica napus), a widely grown 
crop plant. Furthermore, endogenous hormones induce some responses that cannot 
be obtained with synthetic growth-regulating substances. Also of importance 
for food crops is the fact that these hormones, which are present in plants 
consumed by humans and animals, are relatively nontoxic substances. See PLANT 
GROWTH; PLANT HORMONES. 
 �34s[J. F. Worley ;N. Mandava]
�33s 
  
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�6v�2v�32sBrayton cycle   �21m �33s 
    A thermodynamic cycle (also variously called the Joule or complete 
expansion diesel cycle) consisting of two constant-pressure (isobaric) 
processes interspersed with two reversible adiabatic (isentropic) processes. 
    The thermal efficiency for a given gas, air, is solely a function of the 
ratio of compression. This is also the case with the Otto cycle. For the 
diesel cycle with incomplete expansion, the thermal efficiency is lower.  
    The Brayton cycle, with its high inherent thermal efficiency, requires the 
maximum volume of gas flow for a given power output. The Otto and diesel 
cycles require much lower gas flow rates, but have the disadvantage of higher 
peak pressures and temperatures. These conflicting elements led to many 
designs, all attempting to achieve practical compromises. With the development 
of fluid acceleration devices for the compression and expansion of gases, the 
Brayton cycle found mechanisms which could economically handle the large 
volumes of working fluid. This is perfected in the gas turbine power plant. 
See GAS TURBINE; THERMODYNAMIC CYCLE. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sBrazil nut �21m �33s
    A large broad-leafed evergreen tree, Bertholettia excelsa, that grows wild
in the forests of the Amazon valley of Brazil and Bolivia. The fruit is a
spherical capsule weighing 2 - 4 lb (1 - 2 kg) which, when mature, consists of
an outer hard indehiscent husk enclosing an inner hard-shelled container or
pod filled with about 20 rather triangular seeds or nuts.
    Although there are a few plantations in Brazil, almost the entire
production is gathered from wild trees. The nuts are mostly exported to
Europe, Canada, and the United States. About one-fourth of the crop is shelled
in Brazil before export.
    Brazil nuts have a high oil and protein content and require careful
handling and refrigeration to prevent spoilage. The nuts are used in
confectionery, baked goods, and nut mixtures. See LECYTHIDALES.
 �34s[L.H.MacDonald]
�33s

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�6v�2v�32sBrazing �21m �33s
    A method of joining metals, and other materials, by applying heat and a
brazing filler metal. The filler metals used have melting temperatures above
840 DG F (450 DG C), but below the melting temperature of the metals or
materials being joined. They flow by capillary action into the gap between the
base metals or materials and join them by creating a metallurgical bond between
them, at the molecular level. The process is similar to soldering, but differs
in that the filler metal is of greater strength and has a higher melting
temperature.
    When properly designed, a brazed joint will yield a very high degree of
serviceability under concentrated stress, vibration, and temperature loads. It
can be said that in a properly designed brazement, any failure will occur in
the base metal, not in the joint. There are many design variables to be
considered. First among them is the mechanical configuration of the parts to
be joined, and the joint area itself. All brazements can be categorized as
having one of two basic joint designs: the lap joint (see illustration) or the
butt joint. Others are adaptations of these two.
    Design considerations should include the informed selection of the base
and filler metals. In addition to the basic mechanical requirements, the base
metals used in the brazement must retain the integrity of their physical
properties throughout the heat of the brazing cycle. No universal filler metal
that will satisfy all design requirements is possible, but there are many
types available, ranging from pure metals such as copper, gold, or silver to
complex alloys of aluminum, gold, nickel, magnesium, cobalt, silver, and
palladium.
    There are 11 basic brazing processes. In torch brazing, heat is applied by
flame, from some type of torch, directly to the base metal. A mineral flux is
normally used. The brazing filler metal may be preplaced in the joint, or
face-fed into the joint. In induction brazing, brazing temperatures are
developed in the parts to be brazed by placing them in or near a source of
high-frequency ac electricity. Flux and preplaced filler metals are normally
employed. Resistance brazing employs electrodes, which are arranged so that
the joint forms a part of an electric circuit. Heat is developed by the
resistance of the parts to the flow of the electric current. In dip brazing,
the brazing filler metal is preplaced in or at the joint, and the assembly is
immersed in a bath of molten salt or flux until the brazing temperature is
achieved. In a variation of this process, the assembly is prefluxed and dipped
into a bath of molten brazing filler metal. Infrared brazing is a process in
which high-intensity quartz lamps are directed on the metals to be joined.
    Furnace brazing is a widely used technique, especially useful where the
parts to be brazed are machined or formed to their final dimensions, or
constitute a complex assembly that has already been lightly joined or
fixtured. The atmosphere within a brazing furnace is usually controlled, which
permits a great deal of flexibility. An important advantage is that potential
distortion of metal, created by heating and cooling, can be predicted and
controlled and thereby minimized or eliminated. Also the capacity for
automation is facilitated in the furnace brazing process.
    Diffusion brazing, unlike furnace brazing, is defined not by the method of
heating but rather by the degree of mutual filler-metal solution and diffusion
with the base metal resulting from the temperature used and the time interval
at heat. In diffusion brazing, temperature, time, in some cases pressure, and
selection of base and filler materials are so controlled that the filler metal
is partially or totally diffused into the base metal. The joint properties
then closely approach those of the base metal.
    Other, less used processes include arc brazing, block brazing, flow
brazing, and twin carbon arc brazing.
    The use of brazing as a means of joining metals of similar and dissimilar
properties will continue to expand, partly because brazing processes are
easily automated. Many familiar products, such as bicycles, faucets,
automobile radiators, and air-conditioning equipment, are manufactured through
using the brazing process extensively. See SOLDERING; WELDING AND CUTTING OF
METALS.






 �34s[Robert L. Peaslee]
�33s


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�6v�2v�32sBreadboarding   �21m �33s 
    Assembling an electronic circuit in the most convenient manner on a board 
or other flat surface, without regard for final locations of components, to 
prove the feasibility of the circuit and to facilitate changes when necessary. 
Standard breadboards for experimental work are made with mounting holes and 
terminals closely spaced at regular intervals, so that parts can be mounted 
and connected without drilling additional holes. 
    Printed-circuit boards having similar patterns of punched holes, with 
various combinations of holes connected together by printed wiring on each 
side, are often used for breadboarding when the final version is to be a 
printed circuit. See CIRCUIT (ELECTRONICS); PRINTED CIRCUIT. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sBreadfruit   �21m �33s 
    The multiple fruit of an Indo-Malaysian tree, Artocarpus altilis, of the 
mulberry family (Moraceae), now cultivated in tropical lowlands around the 
world. The fruits vary considerably in size and are often borne in small 
clusters. Breadfruit is a wholesome food for both humans and animals, although 
it has a high carbohydrate content. It is eaten fresh or baked, boiled, 
roasted, fried, or ground up and made into bread. There are many varieties, 
both with and without seeds. See URTICALES. 
 �34s[Earl L. Core]
�33s 
  
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�6v�2v�32sBreakdown potential   �21m �33s 
    That potential difference in a gaseous discharge at which a sudden 
increase in its electrical conductivity takes place. An alternative term is 
sparking potential. See ELECTRIC SPARK. 
    In general, the breakdown potential is a complicated function which 
depends on such factors as electrode material, both macroscopic and 
microscopic electrode shape, chemical surface states of the electrodes, 
constitution and pressure of the gas, temperature, electrode separation 
distance, and time-dependence of applied potential difference. 
    In certain cases the nature of the influence of one of the above factors 
may be readily determined. The presence of sharp points at the electrodes, and 
particularly at the cathode, produces a marked lowering of the breakdown 
potential. This is because the strong electric field around such a point 
results in intense ionization once an initial electron enters the region.  
 �34s[Glenn H. Miller]
�33s 
  
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�6v�2v�32sBreakwater   �21m �33s 
    A structure to protect against wave action of the seas and to create 
comparatively calm water behind for (1) harbors where adjacent land 
projections and nearby islands are inadequate to give protection, (2) coastal 
lands subject to damage by the sea, and (3) offshore structures. 
    The usual breakwater is a mound of stone rubble dumped on the ocean or 
harbor bed and rising to a height above mean sea level. Sometimes it is capped 
on the top and sea face with variously shaped concrete blocks. Other common 
forms of breakwaters are vertical-faced structures built of solid blocks of 
masonry or concrete, precast-concrete cellular walls floated into place and 
filled with earth or stone or steel-sheet-piling cells filled with stone 
(common on the Great Lakes). Masonry and concrete walls are placed on low 
rubble mounds or on the sea bed if firm. Scouring of the sea bed in front may 
require a stone protection bed. See COASTAL ENGINEERING; HARBOR. 
 �34s[Edward J. Quirin]
�33s 
  
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�6v�2v�32sBreast �21m �33s
    The human mammary gland, usually well developed in the adult female but
rudimentary in the male. Each adult female breast contains 15 - 20 separate,
branching glands that radiate from the nipple. During lactation their
secretions are discharged through separate openings at the base of the nipple.
    In the female, hormonal changes in adolescence cause enlargement of breast
tissue, but much of this is connective tissue although some glandular buds
form. With the advent of full menstruation ovarian estrogenic hormones
influence breast development. If pregnancy ensues, the glandular tissue
reaches full development and full lactation begins shortly after birth. After
cessation of lactation the breasts regress considerably and once again reflect
cyclic regulation. See LACTATION.
 �34s[Walter J. Bock]
�33s
    Breast disorders may result from congenital or developmental
abnormalities, inflammations, hormonal imbalances, and, most important, from
tumor formation.
    Congenital defects are usually unimportant except for their psychic or
cosmetic implications. Supernumerary nipples and breasts or accessory breast
tissue are common examples.
    Inflammations are not encountered frequently and usually result from a
staphylococcal or streptococcal invasion incurred during lactation. A special
form of inflammation may result from fat necrosis. Although any age is
susceptible, older women show a slightly higher incidence of fat necrosis, the
commonest cause of which is injury from trauma. See STAPHYLOCOCCUS;
STREPTOCOCCUS; SYPHILIS; TUBERCULOSIS.
    Hormonal imbalances are believed to be responsible for the variants of the
commonest nontumorous breast disorder of women, cystic hyperplasia. The
changes are thought to result from exaggeration or distortion of the normal
cyclic alterations induced during the menstrual interval. Although a wide
range of clinical and pathologic variation is commonplace, three major types
or tendencies prevail. The first, called fibrosis or mastodynia, is marked by
an increase of connective tissue in the breast, without a proportionate
increase in glandular epithelium. The second, cystic disease, is characterized
by an increase in the glandular and connective tissues in local areas, with a
tendency toward formation of cysts varying in size. The third major type is
adenosis, in which glandular hyperplasia is predominant. Each major form of
cystic hyperplasia has its own clinical characteristics, ages of highest
incidence, and distribution. Each is important because the breast masses which
occur require differentiation from benign and malignant tumors. These lesions
also have been found to predispose to the subsequent development of carcinoma.
    Breast cancer is the most significant lesion of the female breast,
accounting for 25,000 - 30,000 deaths in the United States each year. It
rarely occurs before the age of 25, but its incidence increases each year
thereafter, with a sharper climb noted about the time of menopause. Early
breast cancer may appear as a small, firm mass which is nontender and freely
movable. Diagnosis at this time carries a more favorable prognosis than later,
when immobility, nipple retraction, lymph node involvement, and other signs of
extension or spread are noted. Paget's disease of the nipple is a special form
of breast cancer, in which there are early skin changes about the nipple. See
CANCER (MEDICINE); HORMONE; ONCOLOGY.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sBreccia   �21m �33s 
    A rock composed of fragments, similar to a conglomerate in distribution of 
sizes of the materials (coarser than 0.08 in. or 2 mm in diameter). Breccias 
differ from conglomerates in the sharp, angular nature of the fragments. The 
sedimentary breccias are to be distinguished from volcanic breccias formed by 
the agglomeration of volcanic materials and tectonic breccias formed by 
fracturing and disruption due to faulting. Breccias derived from erosion of 
cliffs or scarps with little or no transportation are called talus breccia or 
scree. The intraformational conglomerates are frequently called breccias if 
the pebbles are angular. See TALUS. 
    A type of breccia that has received increased attention is a soft rock 
deformation product. Such breccias are similar both to flat shale pebble 
conglomerates and to fault breccias. They develop as a result of gravitational 
slumping or sliding down a slope where a soft shale bed is interbedded with 
more competent beds, such as sandstone or siltstone. Very large pieces of the 
shale bed may become detached and deformed and even squeezed up through 
overlying layers, giving a picture of badly distorted and in some cases, 
almost unrecognizable bedding. A characteristic of these breccias is that they 
are intraformational; that is, higher beds carry across the deformed zone 
without disturbance. See CONGLOMERATE; GRAVEL; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
�33s 
  
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�6v�2v�32sBreeding (animal)   �21m �33s 
    The application of genetic principles to improving heredity for 
economically important traits in domestic animals. Examples are improvement of 
milk production in dairy cattle, meatiness in pigs, feed requirements or 
growth rate in beef cattle, and egg production in chickens. Selection permits 
the best parents to leave more offspring in the next generation than poorer 
parents. The science of animal breeding is based on genetic principles; 
however, the current application of these principles relies heavily on 
statistical knowledge. Measurement of performance of individuals, such as 
growth rate or milk production, is essential. Modern computers allow the 
application of complicated statistical procedures to large volumes of 
performance data to choose the best individuals as parents. This allows 
selection to be efficient. 
    Selection is the primary tool for generating directed hereditary changes 
in animals. It may be concentrated on one characteristic, may be directed 
independently on several traits, or may be conducted on an index or total 
score which includes information on several important traits. In general, the 
second and third methods are preferable to the first when several important 
and heritable traits need attention. 
    Heritability is a key parameter in making decisions in selection. It is 
the fraction of the total variation in a trait that is due to genetic 
differences. Managing animals to equalize environmental influences on them, or 
statistically adjusting for environmental differences between animals, is 
necessary to choose accurately those with the best inheritance for various 
traits.  
    The improvement achieved by selection is directly related to the accuracy 
with which the breeding values of the subjects can be recognized. Accuracy, in 
turn, depends upon the heritabilities of the traits and upon whether they can 
be measured directly upon the subjects for selection (mass selection), upon 
their parents (pedigree selection), upon their brothers and sisters (family 
selection), or upon their progeny (progeny testing). For traits of medium 
heritability, the following sources of information are approximately equally 
accurate for predicting breeding values of subjects: (1) one record measured 
on the subject; (2) one record on each ancestor for three previous 
generations; (3) one record each on five brothers or sisters where there is no 
environmental correlation between family members; and (4) one record each on 
five progeny having no environmental correlations, each from a different mate. 
    Artificial insemination is an effective tool for increasing the intensity 
of selection, especially in males. It allows one male to breed many more 
females than with natural mating. It has been most widely applied in dairy 
cattle, where carefully planned young sire sampling programs are being 
combined with thoroughly conducted progeny testing schemes to identify 
genetically superior sires. See AGRICULTURAL SCIENCE (ANIMAL); GENETICS. 
 �34s[A. E. Freeman]
�33s 
  
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�6v�2v�32sBreeding (plant)   �21m �33s 
    The application of genetic principles to the improvement of cultivated 
plants. New varieties of cultivated plants can result only from genetic 
reorganization that gives rise to improvements over the existing varieties in 
particular characteristics or in combinations of characteristics. In 
consequence, plant breeding can be regarded as a branch of applied genetics, 
but it also makes use of the knowledge and techniques of many aspects of plant 
science, especially physiology and pathology. Related disciplines, like 
biochemistry and entomology, are also important, and the application of 
mathematical statistics in the design and analysis of experiments is 
essential. See GENETICS. 
    The cornerstone of all plant breeding is selection, or the picking out of 
plants with the best combinations of agricultural and quality characteristics 
from populations of plants with a variety of genetic constitutions. Seeds from 
the selected plants are used to produce the next generation, from which a 
further cycle of selection may be carried out if there are still differences. 
Conventional breeding is divided into three categories on the basis of ways in 
which the species are propagated. First come the species that set seeds by 
self-pollination; that is, fertilization usually follows the germination of 
pollen on the stigmas of the same plant on which it was produced. The second 
category of species sets seeds by cross-pollination; that is, fertilization 
usually follows the germination of pollen on the stigmas of different plants 
from those on which it was produced. The third category comprises the species 
that are asexually propagated; that is, the commercial crop results from 
planting vegetative parts or by grafting. The procedures used in breeding 
differ according to the pattern of propagation of the species. Several 
innovative techniques have been explored to enhance the scope, speed, and 
efficiency of producing new, superior cultivars. Advances have been made in 
extending conventional sexual crossing procedures by laboratory culture of 
plant organs and tissues and by somatic hybridization through protoplast 
fusion.  
    The essential attribute of self-pollinating crop species, such as wheat, 
barley, oats, and many edible legumes, is that, once they are genetically 
pure, varieties can be maintained without change for many generations. When 
improvement of an existing variety is desired, it is necessary to produce 
genetic variation among which selection can be practiced. This is achieved by 
artificially hybridizing between parental varieties that may contrast with 
each other in possessing different desirable attributes. This system is known 
as pedigree breeding, and it is the method most commonly employed, and can be 
varied in several ways. 
    Another form of breeding often employed with self-pollinating species 
involves backcrossing. This is used when an existing variety is broadly 
satisfactory but lacks one useful and simply inherited trait that is to be 
found in some other variety. Hybrids are made between the two varieties, and 
the first hybrid generation is crossed, or backcrossed, with the broadly 
satisfactory variety which is known as the recurrent parent. Backcrossing has 
been exceedingly useful in practice and has been extensively employed in 
adding resistance to diseases, such as rust, smut, or mildew, to established 
and acceptable varieties of oats, wheat, and barley. See PLANT PATHOLOGY. 
    Natural populations of cross-pollinating species are characterized by 
extreme genetic diversity. No seed parent is truebreeding, first because it 
was itself derived from a fertilization in which genetically different parents 
participated, and second because of the genetic diversity of the pollen it 
will have received. In dealing with cultivated plants with this breeding 
structure, the essential concern in seed production is to employ systems in 
which hybrid vigor is exploited, the range of variation in the crop is 
diminished, and only parents likely to give rise to superior offspring are 
retained.  
    Plant breeders have made use either of inbreeding followed by 
hybridization or of some form of recurrent selection. During inbreeding 
programs normally cross-pollinated species, such as corn, are compelled to 
self-pollinate by artificial means. Inbreeding is continued for a number of 
generations until genetically pure, true-breeding, and uniform inbred lines 
are produced. During the production of the inbred lines, rigorous selection is 
practiced for general vigor and yield and disease resistance, as well as for 
other important characteristics. To estimate the value of inbred lines as the 
parents of hybrids, it is necessary to make tests of their combining ability. 
The test that is used depends upon the crop and on the ease with which 
controlled cross-pollination can be effected. 
    Breeding procedures designated as recurrent selection are coming into 
limited use with open-pollinated species. In theory, this method visualizes a 
controlled approach to homozygosity, with selection and evaluation in each 
cycle to permit the desired stepwise changes in gene frequency. Experimental 
evaluation of the procedure indicates that it has real possibilities. Four 
types of recurrent selection have been suggested: on the basis of phenotype, 
for general combining ability, for specific combining ability, and reciprocal 
selection. The methods are similar in the procedures involved, but vary in the 
type of tester parent chosen, and therefore in the efficiency with which 
different types of gene action (additive and nonadditive) are measured. 
    A very few asexually propagated crop species are sexually sterile, like 
the banana, but the majority have some sexual fertility. The cultivated forms 
of such species are usually of widely mixed parentage, and when propagated by 
seed, following sexual reproduction, the offspring are very variable and 
rarely retain the beneficial combination of characters that contributes to the 
success of their parents. Varieties of asexually propagated crops consist of 
large assemblages of genetically identical plants, and there are only two ways 
of introducing new and improved varieties. The first is by sexual 
reproduction, and the second is by the isolation of sports or somatic 
mutations.  
    Special breeding techniques include the following. (1) When cytoplasmic 
male sterility is utilized, the male sterile lines are pollinated by other, 
so-called restorer, lines. Using crosses between male sterile lines and 
restorer lines, hybrid seed supplies can easily be obtained on a large scale. 
(2) Polyploidy, which can be induced artificially by colchicine treatment and 
in other ways, produces hybrids that are sterile and thus bear fruit without 
seeds - obviously a desirable attribute. (3) Multilinear varieties, made up of 
genetically similar plants containing several lines that differ in having 
genetically different forms of disease resistance, are advantageous because if 
the resistance of one of the constituent lines breaks down, production will be 
maintained by the remaining resistant lines. (4) Means have been discovered of 
producing essentially normal plants (sporophytes) of several crop species with 
the haploid chromosome number. The controlled production of such haploids, 
followed by doubling of the chromosome number using colchicine, enables the 
immediate fixation of the first products of segregation and recombination. 
Thus there is a very rapid return to complete homozygosity and homogeneity 
from hybrids of self-pollinated species. Alternatively, the procedure can give 
rise to the equivalents of inbred lines in outbreeding species. 
 �34s[Ralph Riley]
�33s 
    Plants are also being bred from new sexual crosses, to obtain better 
disease resistance and crop yield; from somatic cells, to select new 
disease-resistant cultivars; from somatic hybridization, through protoplast 
fusion; from selection by genetic complementation, through the use of albino 
mutants; from cytoplasmic hybridization, which involves the transfer of the 
male sterility trait; from intergeneric hybridization; through the use of 
recombinant DNA and the transformation process; and through germ-plasm 
preservation.  
 �34s[Oluf L. Gamborg]
�33s 
  
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�6v�2v�32sBremsstrahlung �21m �33s
    The electromagnetic radiation emitted by electrons when they pass through
matter. Charged particles radiate when accelerated, and in this case the
electric fields of the atomic nuclei provide the force which accelerates the
electrons. The continuous spectrum of x-rays from an x-ray tube is that of the
bremsstrahlung; in addition, there is a characteristic x-ray spectrum due to
excitation of the target atoms by the incident electron beam. The major energy
loss of highenergy (relativistic) electrons (energy =s 10 MeV, depending
somewhat upon material) occurs from the emission of bremsstrahlung, and this
is the source of the GAMMA-rays in a high-energy cosmic-ray shower. See COSMIC
RAYS.
 �34s[Charles Goebel]
�33s

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�6v�2v�32sBrick �21m �33s
    A building material made usually from clay, molded as a rectangular block,
and baked or burned in a kiln. An American building brick measures about 2H x
3L x 8 in. (57 x 95 x 203 mm); an English brick is about 2K x 4I x 9 in. (67 x
111 x 229 mm). The red color is due to iron oxides in the finished brick.
Unselected brick as it comes from the kiln is common brick; brick selected for
uniformity is face brick.
    Bricks may also be made of other materials. The clay may be mixed with
sand or lime, the brick being pressed and steamed. Bricks are commonly
fashioned as insulating material, as refractory material termed firebrick, or
for structural use. Adobe brick is molded of earth or clay mixed with straw,
then hardened by the sun. See REFRACTORY.
 �34s[Frank H. Rockett]
�33s

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�6v�2v�32sBridge �21m �33s
    A structure built to provide ready passage over natural or artificial
obstacles, or under another passageway. Bridges serve highways, railways,
canals, aqueducts, utility pipelines, and pedestrian walkways. In many
jurisdictions, bridges are defined as those structures spanning an arbitrary
minimum distance, generally about 10 - 20 ft (3 - 6 m); shorter structures are
classified as culverts or tunnels. In addition, natural formations eroded into
bridgelike form are often called bridges. This article covers only bridges
providing conventional transportation passageways.
    Bridges generally are considered to be composed of three separate parts:
substructure, superstructure, and deck. The substructure or foundation of a
bridge consists of the piers and abutments which carry the superimposed load
of the superstructure to the underlying soil or rock. The superstructure is
that portion of a bridge or trestle lying above the piers and abutments. The
deck or flooring is supported on the bridge superstructure; it carries and is
in direct contact with the traffic for which passage is provided.
    Bridges are classified in several ways. Thus, according to the use they
serve, they may be termed railway, highway, canal, aqueduct, utility pipeline,
or pedestrian bridges. If they are classified by the materials of which they
are constructed (principally the superstructure), they are called steel,
concrete, timber, stone, or aluminum bridges. Deck bridges carry the deck on
the very top of the superstructure. Through bridges carry the deck within the
superstructure. The type of structural action is denoted by the application of
terms such as truss, arch, suspension, stringer or girder, stayed-girder,
composite construction, hybrid girder, continuous, cantilever, or orthotropic
(steel deck plate).
    The two most general classifications are the fixed and the movable. In the
former, the horizontal and vertical alignment of the bridge are permanent; in
the latter, either the horizontal or vertical alignment is such that it can be
readily changed to permit the passage beneath the bridge of traffic. Movable
bridges are sometimes called drawbridges in an anachronistic reference to an
obsolete type of movable bridge spanning the moats of castles.
    A singular type of bridge is the floating or pontoon bridge, which can be
a movable bridge if it is designed so that a portion of it can be moved to
permit the passage of water traffic.
    The term trestle is used to describe a series of short spans supported by
braced towers, and the term viaduct is used to describe a high structure of
short spans, often of arch construction.

    Fixed bridges.  This type of construction is selected when the vertical
clearance provided beneath the bridge exceeds the clearance required by the
traffic it spans. For very short spans, construction may be a solid slab or a
number of beams; for longer spans, the choice may be girders or trusses. Still
longer spans may dictate the use of arch construction, and if the spans are
even longer, stayed-girder bridges are used. Suspension bridges are used for
the longest spans.
    Beam bridges consist of a series of beams, usually of rolled steel,
supporting the roadway directly on their top flanges. The beams are placed
parallel to traffic and extend from abutment to abutment. Plate-girder bridges
are used for longer spans than can be practically traversed with a beam
bridge. In its simplest form, the plate girder consists of two flange plates
welded to a web plate, the whole having the shape of an I. Box-girder bridges
have steel girders fabricated by welding four plates into a box section. A
conventional floor beam and stringer can be used on box-girder bridges, but
the more economical arrangement is to widen the top flange plate of the box so
that it serves as the deck. When this is done, the plate is stiffened to
desired rigidity by closely spaced bar stiffeners or by corrugated or
honeycomb-type plates. These stiffened decks, which double as the top flange
of the box girders, are termed orthotropic. The wearing surface on such
bridges is usually a relatively thin layer of asphalt.
    Truss bridges, consisting of members vertically arranged in a triangular
pattern, can be used when the crossing is too long to be spanned economically
by simple plate girders. Where there is sufficient clearance underneath the
bridge, the deck bridge is more economical than the through bridge because the
trusses can be placed closer together, reducing the span of the floor beams.
    The continuous bridge is a structure supported at three or more points and
capable of resisting bending and shearing forces at all sections throughout
its length. The bending forces in the center of the span are reduced by the
bending forces acting oppositely at the piers. Trusses, plate girders, and box
girders can be made continuous. The advantages of a continuous bridge over a
simple-span bridge (that is, one that does not extend beyond its two supports)
are economy of material, convenience of erection (without need for falsework),
and increased rigidity under traffic. The disadvantages are its sensitivity to
relative change in the levels of supporting piers, the difficulty of
constructing the bridge to make it function as it is supposed to, and the
occurrence of large movements at one location due to thermal changes.
    The cantilever bridge consists of two spans projecting toward each other
and joined at their ends by a suspended simple span. The projecting spans are
known as cantilever arms, and these, plus the suspended span, constitute the
main span. The cantilever arms also extend back to shore, and the section from
shore to the piers offshore is termed the anchor span. Trusses, plate girders,
and box girders can be built as cantilever bridges. The chief advantages of
the cantilever design are the saving in material and ease of erection of the
main span. The cable-stayed bridge, a modification of the cantilever bridge
which has come into modern use, resembles a suspension bridge. It consists of
girders or trusses cantilevering both ways from a central tower and supported
by inclined cables attached to the tower at the top or sometimes at several
levels.
    The suspension bridge is a structure consisting of either a roadway or a
truss suspended from two cables which pass over two towers and are anchored by
backstays to a firm foundation. If the roadway is attached directly to the
cables by suspenders, the structure lacks rigidity, with the result that wind
loads and moving live loads distort the cables and produce a wave motion on
the roadway. When the roadway is supported by a truss which is hung from the
cable, the structure is called a stiffened suspension bridge. The stiffening
truss distributes the concentrated live loads over a considerable length of
the cable.
    Since the development of the prestressing method, bridges of almost every
type are being constructed of concrete. Prior to the advent of prestressing,
these bridges were of three types: (1) arches, which were built in either
short or long spans; (2) slab bridges of quite short spans, which were simply
reinforced concrete slabs extending from abutment to abutment; and (3) deck
girder bridges, consisting of concrete slab built integrally with a series of
concrete girders placed parallel to traffic. The advent of prestressed
concrete greatly extended the utility and economy of concrete for bridges,
particularly by making the hollow box-girder type practicable. See PRESTRESSED
CONCRETE.

    Movable bridges.  Modern movable bridges are either bascule, vertical
lift, or swing; with few exceptions, they span waterways. They are said to be
closed when set for the traffic they carry, and open when set to permit
traffic to pass through the waterway they cross. Bascule and swing bridges
provide unlimited vertical clearance in the open position. The vertical
clearance of a lift bridge is limited by its design.
    The bascule bridge consists primarily of a cantilever span, which may be
either a truss or a plate girder, extending across the channel. Bascule
bridges rotate about a horizontal axis parallel with the waterway. The portion
of the bridge on the land side of the axis, carrying a counterweight to ease
the mechanical effort of moving the bridge, drops downward, while the forward
part of the leaf opens up over the channel much like the action of a
playground seesaw. Bascule bridges may be either single-leaf, where rotation
of the entire leaf over the waterway is about one axis on one side of the
waterway, or double-leaf, where the leaves over the waterway rotate about two
axes on opposite sides of the waterway.
    The vertical-lift bridge has a span similar to that of a fixed bridge and
is lifted by steel ropes running over large sheaves at the tops of its towers
to the counterweights, which fall as the lift span rises and rise as it falls.
If the bridge is operated by machinery on each tower, it is known as a tower
drive. If it is driven by machinery located on the lift span, it is known as a
span drive.
    Swing bridges revolve about a vertical axis on a pier, called the pivot
pier, in the waterway. There are three general classes of swing bridges: the
rim-bearing, the center-bearing, and the combined rim-bearing and
center-bearing. Rim-bearing bridges are supported on circular girder drums on
rollers, center-bearing on a single large bearing at the center of rotation.

    Vibration.	Bridges are generally considered to be statically loaded
structures under their own dead load, with dynamic loadings from the live load
and from the wind. Vibrations of bridges or individual components of a bridge
occur when a resonant frequency of the bridge or a component is excited by one
or more of the applied dynamic loadings. Excitation from live loads is more
apt to happen on bridges carrying rail traffic than on those carrying highway
traffic because of the uniform spacing of railroad cars. Excitation from wind
is caused by the repeated formation of eddies or vortices as the wind travels
past nonstreamlined members. Light lateral bracing on many spans will vibrate
under the passage of live load. See VIBRATION.

    Substructure.  Bridge substructure consists of those elements that support
the trusses, girders, stringers, floor beams, and decks of the bridge
superstructure. Piers and abutments are the primary bridge substructure
elements. Other types of substructure, such as skewbacks for arch bridges,
pile bents for trestles, and various forms of support wall, are also commonly
used for specific applications.
    Both piers and abutments are generally supported on either spread footings
or pile footings. A spread footing is usually a concrete pad large enough in
area to transmit all superimposed loads and forces directly to the soil on
which it is founded. A pile footing is usually a large concrete block
supported on piles, so that the superimposed substructure loads and forces are
transmitted to the support piles through the footing. The footing piles are
used primarily to transmit loads through soil formations having poor
supporting properties into or onto formations that are capable of supporting
the loads. See FOUNDATIONS.
 �34s[Egbert R. Hardesty; Henry W. Fischer; R.W. Christie; Bernard Habet]
�33s


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2961920
�6v�2v�32sBridge circuit �21m �33s
    An electric network composed of four impedances forming a diamond-shaped
network. These impedances, shown in the illustration as Z sub 1, Z sub 2,
Z sub 3, and Z sub 4, may be combinations of resistors, capacitors, and
inductors. Bridge circuits perform numerous functions, depending upon the type
of circuit elements used in the arms of the bridge. For example, the bridge
circuit can be used to determine the value of an unknown resistance. This is
done by making the arm impedances resistances with one of them the unknown. The
load impedance Z sub L can be that of a measuring instrument, such as a
galvanometer. The current through Z sub L, and the voltage across it, will be
zero when Z sub 1|Z sub 4 =Z sub 2|Z sub 3. With the proper combination of
resistors and capacitors in the bridge arms, the voltage across Z sub L will
be zero for some frequency. Thus, the bridge acts as a frequency-measuring
device. See FREQUENCY MEASUREMENT; WHEATSTONE BRIDGE.
 �34s[Harold F. Klock]
�33s

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2961920
�6v�2v�32sBridled pressure plate �21m �33s
    An instrument that is useful in measuring air velocities in gusts, since
it has a greater speed of response than do rotating elements. The velocity
pressure on the plate exposed to the wind is balanced by the force of a
spring. The deflection is measured by an inductance-type transducer. The
signal from the transducer is amplified and transmitted to a recorder. The
indication is dependent upon the air density. Simple mechanical designs are
also used to measure low air velocities. See AIR-VELOCITY MEASUREMENT.
 �34s[Herbert Fox]
�33s


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2961920
�6v�2v�32sBrillouin zone �21m �33s
    In the propagation of any type of wave motion through a crystal lattice,
the frequency is a periodic function of wave vector k. This function may be
complicated by being multivalued; that is, it may have more than one branch.
Discontinuities may also occur. In order to simplify the treatment of wave
motion in a crystal, a zone in k-space is defined which forms the fundamental
periodic region, such that the frequency or energy for a k outside this region
may be determined from one of those in it. This region is known as the
Brillouin zone (sometimes called the first or the central Brillouin zone). It
is usually possible to restrict attention to k values inside the zone.
Discontinuities occur only on the boundaries. If the zone is repeated
indefinitely, all k-space will be filled. Sometimes it is also convenient to
define larger figures with similar properties which are combinations of the
first zone and portions of those formed by replication. These are referred to
as higher Brillouin zones.
    The central Brillouin zone for a particular solid type is a solid which
has the same volume as the primitive unit cell in reciprocal space, that is,
the space of the reciprocal lattice vectors, and is of such a shape as to be
invariant under as many as possible of the symmetry operations of the crystal.
See CRYSTAL STRUCTURE; CRYSTALLOGRAPHY.
 �34s[Joseph Callaway]
�33s

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2961920
�6v�2v�32sBrittleness   �21m �33s 
    That characteristic of a material that is manifested by sudden or abrupt 
failure without appreciable prior ductile or plastic deformation. A brittle 
fracture occurs on a cleavage plane which has a crystalline appearance at 
failure because each crystal tends to fracture on a single plane. On the other 
hand, a shear fracture has a fibrous appearance because of the sliding of the 
fracture surfaces over each other. Brittle failures are caused by high tensile 
stresses, high carbon content, rapid rate of loading, and the presence of 
notches. Materials such as glass, cast iron, and concrete are examples of 
brittle materials. 
 �34s[John B. Scalzi]
�33s 
  
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2961920
�6v�2v�32sBroaching   �21m �33s 
    The machine shaping of metal by pushing or pulling a multiple-tooth, 
barlike tool across a surface or through an existing hole in a workpiece. The 
peripheries or cutting edges of the broach teeth are shaped to give a desired 
surface or contour, whereas cutting action results from the fact that each 
tooth is progressively higher or projects further than the preceding one. 
    Broaching machines are classed as either horizontals or verticals, 
depending on the mounting plane of the broach. Most horizontal machines are of 
the pull type, while vertical models are made to use pull-up, pull-down, push, 
or surface broaches. See MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
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2961920
�6v�2v�32sBroccoli   �21m �33s 
    A cool-season biennial crucifer, Brassica oleracea var. italica, of 
Mediterranean origin, belonging to the plant order Papaverales. Broccoli is 
grown for its thick branching lower stalks which terminate in clusters of 
loose green flower buds. Stalks and buds are cooked as a vegetable or may be 
processed in either canned or frozen form. California and Texas are important 
broccoli-producing states. See PAPAVERALES. 
 �34s[H. John Carew]
�33s 
  
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2961920
�6v�2v�32sBromate �21m �33s
    A negative ion having the formula BrO3- and derived from bromic acid,
HBrO3. Bromic acid exists only in aqueous solution. The solid bromates are
stable and can be prepared by passing bromine into a warm solution of sodium
carbonate.
    Since bromine has the oxidation state of 5+ in bromates and these bromates
are strong oxidizing agents, these compounds are useful in analytical
chemistry. Commercially, they are used in permanent-wave preparations and in
flour treatment. See BROMINE.
 �34s[E. Eugene Weaver]
�33s

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2961920
�6v�2v�32sBromegrass   �21m �33s 
    A common name designating a number of grasses found in the North Temperate 
Zone that produce highly palatable and nutritious forage. Of these, smooth 
bromegrass (Bromus inermis) is the most important. Although first widely used 
in the eastern Great Plains and western Corn Belt regions, improved strains 
are now grown extensively for hay and rotation pastures north of the 
Mason-Dixon line, from the Plains to the Atlantic. Smooth bromegrass is a 
long-lived perennial, spreads by underground creeping stems, and is fairly 
deep rooted and drought-tolerant. Top growth is used for hay or pasture. 
Regional strains are available for Canada and the northern two-thirds of the 
United States. See CYPERALES; GRASS CROPS. 
 �34s[Howard B. Sprague]
�33s 
  
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3077120
�6v�2v�32sBromeliales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae) in the 
subclass Commelinidae of the class Liliopsida (monocotyledons). It consists of 
the single fam-ily Bromeliaceae, with nearly 50 genera and more than 1700 
species, occurring chiefly in tropical and subtropical America. They are 
firm-leaved, terrestrial xerophytes, or very often epiphytes, with six stamens 
and regular or somewhat irregular flowers that usually have septal nectaries 
and an inferior ovary. Spanish moss (Tillandsia) and the cultivated pineapple 
(Ananas) are familiar members of the Bromeliales, and many others attract 
attention as unusual houseplants. See LILIOPSIDA; MAGNOLIOPHYTA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sBromide   �21m �33s 
    A compound which is derived from hydrobromic acid, HBr, and contains the 
bromine atom in the 1- oxidation state. The properties of bromides are 
intermediate between those of chlorides and iodides. The bromide ion is a 
better reducing agent and forms less stable complexes than chloride. See 
CHLORIDE.  
    The solubilities of bromides are very similar to those of chlorides, and 
PbBr2, CuBr, AgBr, and Hg2Br2 are the common insoluble salts. The bromides are 
usually produced from bromine, which in turn is obtained from salt brines or 
sea water. 
    Bromides are used in medicine as sedatives in treatment of nervous 
disorders. Silver bromide is used in photographic films and paper. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sBromine �21m �33s
    A chemical element, Br, atomic number 35, atomic weight 79.909, which
normally exists as Br2, a dark-red, low-boiling but high-density liquid of
intensely irritating odor. This is the only nonmetallic element that is liquid
at normal temperature and pressure. Bromine is very reactive chemically; one
of the halogen group of elements, it has properties intermediate between those
of chlorine and iodine. See HALOGEN ELEMENTS.
    The most stable valence states of bromine in its salts are -1 and +5,
although +1, +3, and +7 are known. Within wide limits of temperature and
pressure, molecules of the liquid and vapor are diatomic, Br2, with a formula
weight of 159.818. There are two stable isotopes (sup 7 sup 9 Br and sup 8 sup
1 Br) that occur naturally in nearly equal proportion, so that the atomic weight
is 79.909. A number of radioisotopes are also known.
    The solubility of bromine in water at 20 DG C (68 DG F) is 3.38 g/100 g
(3.38 oz/100 oz) solution, but its solubility is increased tremendously in the
presence of its salts and in hydrobromic acid. The ability of this inorganic
element to dissolve in organic solvents is of considerable importance in its
reactions. The table summarizes the physical properties of bromine.
    Although it is estimated that from 10 sup 1 sup 5 to 10 sup 1 sup 6 tons of
bromine are contained in the Earth's crust, the element is widely distributed
and found only in low concentrations in the form of its salts. The bulk of the
recoverable bromine, however, is found in the hydrosphere. Sea water contains
an average of 65 parts per million (ppm) of bromine. The other major sources
of bromine in the United States are underground brines and salt lakes, with
commercial production in Michigan, Arkansas, and California.
    While many inorganic bromides have found industrial use, the organic
bromides have even wider application. Because of the ease of reaction of
bromine with organic compounds and the ease of its subsequent removal or
replacement, organic bromides have been much studied and used as chemical
intermediates. In addition, any of the bromine reactions are so clean-cut that
they can be used for the study of reaction mechanisms without complication of
side reactions. The ability of bromine to add into unusual places on organic
molecules has added to its value as a research tool.
    Bromine and its compounds have found acceptance as disinfection and
sanitizing agents in swimming pools and potable water. Certain
bromine-containing compounds are safer to use than the analogous chlorine
compounds due to certain persistent residuals found in the chlorine-containing
materials. Other bromine chemicals are used as a working fluid in gages, as
hydraulic fluids, as chemical intermediates in the manufacture of organic dyes,
in storage batteries, and in explosion-suppressant and fire-extinguishing
systems. Bromine compounds, because of their density, also find use in the
gradation of coal and other minerals where separations are effected by density
gradients. The versatility of bromine compounds is illustrated by the
commercial use of over 100 compounds that contain bromine.
    Bromine is almost instantaneously injurious to the skin, and it is
difficult to remove quickly enough to prevent a painful burn that heals slowly.
Bromine vapor is extremely toxic, but its odor gives good warning; it is
difficult to remain in an area of sufficient concentration to be permanently
damaging. Bromine can be handled safely, but the recommendations of the
manufacturers should be respected.		       [Randy C. Stauffer]



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�6v�2v�32sBronchial disorders   �21m �33s 
    A heterogeneous group of diseases of the conducting airways (bronchi and 
bronchioles) of the lung. 
    Acute bronchitis is an inflammation of the bronchi that can be caused by 
irritant gases, bacteria, viruses, and rarely fungi. In the initial stages of 
the disease there is edema and excessive mucus production. This results in a 
decreased air flow into the gas exchange units (alveoli) of the lung. In 
general, acute bronchitis is most commonly seen in debilitated and terminally 
ill patients and is frequently accompanied by bronchopneumonia. Treatment is 
aimed at the underlying disease and eliminating the causative agent. 
    Acute bronchiolitis is an inflammation of the small airways of the lung 
(bronchioles) usually caused by a variety of viruses, and characteristically 
occurs in infants and young children. Because of the location of the 
inflammation, it may be a life-threatening disease. The condition is usually 
treated with vaporization of water and antibiotics. 
    Chronic bronchitis is a chronic, widespread, frequently disabling disease 
which in part is defined clinically by coughing and excessive mucus production 
for at least 3 months per year for 2 consecutive years. It commonly is 
associated with emphysema and is a significant cause of morbidity and 
mortality. The disease is most commonly seen in industrialized large cities. 
The condition is causally associated with air pollution and, probably more 
importantly, cigarette smoking. Chronic bronchitis is at least partially 
reversible if the causitive agents are removed. 
    Bronchial asthma, a disease of uncertain etiology, is characterized by 
episodes of incomplete bronchial obstruction (bronchospasm). Asthma is 
frequently associated with eczema, hay fever, and allergic reactions to a wide 
variety of substances. If the bronchospasm can be directly related to an 
allergic reaction to a specific substance, the condition is referred to as 
extrinsic asthma. In the majority of asthmatics no specific allergen can be 
demonstrated, and in this instance the disease is called intrinsic asthma. 
Clinically, persons with bronchial asthma have episodic attacks of difficult 
breathing, with wheezing and prolongation of the expiratory phase of 
respiration. The attacks usually last several hours and are followed by 
coughing, with expulsion of the mucous plugs. Occasionally the asthmatic 
attacks are unremitting, a condition referred to as status asthmaticus. 
Bronchial asthma alone rarely causes death. The disease is treated with 
various drugs such as bronchodilators, mucolytic agents, antihistamines, and 
antibiotics.  
    Bronchiectasis is a permanent, abnormal dilatation of bronchi resulting 
from severe inflammatory injury to their walls. Rarely, congenitally malformed 
bronchi may provide the basis for secondary damage. In most cases, however, 
there is a history of severe bronchitis or  bronchopneumonia, particularly 
during childhood. In some instances the disease may follow an inconspicuous 
clinical course and thus remain undiagnosed. 
    Cystic fibrosis, also called mucoviscidosis, not only affects the pancreas 
but is a systemic disease involving the bronchial glands as well. Viscid 
mucous secretion in the bronchi makes them prone to severe and often 
intractable bronchitis and bronchiectasis. The infection invariably extends 
into the lungs. 
    Bronchial adenoma is a low-grade malignant or potentially malignant tumor 
of bronchi that appears in relatively young or middle-age adults. In contrast 
with bronchogenic carcinoma, bronchial adenoma grows slowly but does 
infiltrate and can metastasize, that is, transfer the disease to a distant 
focus. Surgical removal results in complete cure in a substantial number of 
cases. See RESPIRATORY SYSTEM DISORDERS. 
 �34s[Samuel P. Hammar]
�33s 
  
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�6v�2v�32sBronchus �21m �33s
    At its lower end, the trachea branches into two primary bronchi which
connect to each lung. Within the lung, each primary bronchus divides again and
again, with the resulting bronchioles forming progressively smaller ducts
which finally terminate in the alveoli, where gas exchange occurs. The
structure of the large bronchi is similar to that of the trachea, but the
smaller bronchioles lack the cartilaginous rings. See LUNG; RESPIRATORY SYSTEM.
 �34s[Walter J. Block]
�33s

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3233280
�6v�2v�32sBronze   �21m �33s 
    Usually an alloy of copper and tin. Bronze is used in bearings, bushings, 
gears, valves, and other fittings both for water and steam. 
    The properties of bronze depend on its composition and working. Lead, 
zinc, silver, and other metals are added for special-purpose bronzes. Tin 
bronze, including statuary bronze, contains 2 - 20% tin; bell metal 15 - 25%; 
and speculum metal up to 33%. Gun metal contains 8 - 10% tin plus 2 - 4% zinc. 
Phosphor bronze is tin bronze hardened and strengthened with traces of 
phosphorus; it is used for fine tubing, wire springs, and machine parts. Lead 
bronze may contain up to 30% lead; it is used for cast parts such as 
low-pressure valves and fittings. Manganese bronze with 0.5 - 5% manganese 
plus other metals, but often no tin, has high strength. Aluminum bronze also 
contains no tin; its mechanical properties are superior to those of tin 
bronze, but it is difficult to cast. Silicon bronze, with up to 3% silicon, 
casts well and can be worked hot or cold by rolling, forging, and similar 
methods. Beryllium bronze (also called beryllium copper) has about 2% 
beryllium and no tin. The alloy is hard and strong and can be further hardened 
and strengthened by precipitation hardening; it is one of the few copper 
alloys that responds to heat treatment, approaching three times the strength 
of structural steel. See ALLOY; COPPER; TIN. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sBrownian movement   �21m �33s 
    The irregular motion of a body arising from the thermal motion of the 
molecules of the material in which the body is immersed. Such a body will of 
course suffer many collisions with the molecules, which will impart energy and 
momentum to it. Because, however, there will be fluctuations in the magnitude 
and direction of the average momentum transferred, the motion of the body will 
appear irregular and erratic. 
    In principle, this motion exists for any foreign body suspended in gases, 
liquids, or solids. To observe it, one needs first of all a macroscopically 
visible body; however, the mass of the body cannot be too large. For a large 
mass, the velocity becomes small. See KINETIC THEORY OF MATTER. 
 �34s[Max Dresden]
�33s 
  
  
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�6v�2v�32sBrucellaceae   �21m �33s 
    The family Brucellaceae traditionally was composed of several genera of 
gram-negative, aerobic bacteria that are pathogenic for humans and animals. 
They are ovoid in shape and similar in size (0.1 - 0.5 micrometer by 0.3 - 1.5 
micrometers), and exhibit pleomorphism (variable shape and size) in artificial 
culture. Many of these organisms are intracellular and, therefore, are 
resistant to antibiotic therapy. Even though they share this array of 
characteristics, they are not closely related genetically as determined by 
guanine-plus-cytosine base ratios and DNA hybridization. Accordingly, this 
family was deleted from the classification scheme in the eighth edition of 
Bergey's Manual (1974), and its member genera of Pasteurella, Brucella, 
Moraxella, Bordetella, Haemophilus, Actinobacillus, Calymmatobacterium, and 
Noguchia were placed either into other families or into general groups of 
gram-negative bacteria of uncertain affiliation. 
 �34s[Margaret E. Meyer]
�33s 
  
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�6v�2v�32sBrucellosis   �21m �33s 
    An infectious disease of animals and humans caused by bacteria of the 
genus Brucella. The organisms are B. melitensis, found in sheep and goats, B. 
abortus in cattle, and B. suis in swine. Brucellosis occurs throughout the 
world and is the most important of the nearly 100 diseases of animals 
transmissible to human beings. 
    The brucellae localize in the reproductive organs and mammary glands of 
animals. This makes the disease of serious economic concern to livestock 
producers and dairy farmers because it causes abortions and reduces the 
production of milk. The disease in cattle is known as Bang's disease, or 
contagious abortion. 
    Brucellosis in humans, also known as undulant fever or Malta fever, is 
primarily an occupational disease in which infection through the skin is 
acquired by contact with the tissues of freshly killed animals, or with the 
contaminated environment of the animals. The disease is also contracted by 
drinking unpasteurized milk and by eating fresh cheese prepared from 
unpasteurized milk. The acute illness is characterized by chills, fever, 
sweats, and weakness. Chronic disease results in weakness and mental 
depression. The usual course of the disease is less than 3 months, but 
antibiotic therapy reduces the length of the illness. 
    Human brucellosis can be eliminated only by eradicating the disease in 
animals. The disease is controlled in cattle by eliminating infected animals. 
Successful vaccination of cattle against the disease has been achieved with a 
live brucella culture having stable, attenuated virulence. An effective 
vaccine has been developed for the protection of sheep and goats, but not for 
swine.  
 �34s[Wesley W. Spink]
�33s 
  
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�6v�2v�32sBrucite �21m �33s
    A magnesium hydroxide mineral, Mg(OH)2, crystallizing in the trigonal
system. It is a member of the important Cd(OH)2 structure type, consisting of
hexagonal close-packed oxygen atoms with alternate octahedral layers occupied
by Mg. The ``brucite layer'' is an important structural component in the clay,
mica, and chlorite mineral groups. Brucite occurs as tabular crystals and as
elongated fibers (as the variety nemalite), hardness 2J (Mohs scale), color
white to greenish, and specific gravity 2.4. Fe sup 2 sup + and Mn sup 2 sup +
commonly substitute for Mg sup 2 sup +.
    Brucite often occurs in a low-temperature vein paragenesis, usually with
serpentine and accessory magnesite. It is also derived by the action of water
on periclase, MgO, which results from the thermal metamorphism of dolomites
and limestones. Carbonate rocks rich in periclase and brucite are called
predazzites. See DOLOMITE; MAGNESITE; SERPENTINE.
 �34s[Paul B. Moore]
�33s

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�6v�2v�32sBrush discharge   �21m �33s 
    A particular form of corona discharge characterized by strongly ionized 
streamers. It generally occurs at a field slightly less than that required for 
complete breakdown. The conditions for a brush discharge are as follows: The 
pressure should be about atmospheric; one or both of the electrodes should 
have a radius of curvature small compared to the gap separation; and the gap 
itself should be large, at least of the order of a few centimeters. The 
ionized streamers take on a treelike form. The light originates from 
recombination and radiative transitions from excited states. See BREAKDOWN 
POTENTIAL; CORONA DISCHARGE. 
 �34s[Glenn H. Miller]
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�6v�2v�32sBrush-shifting motor �21m �33s
    A polyphase induction motor whose speed can be controlled similarly to
that of a dc shunt motor by use of a regulating winding, a commutator, and a
brush-shifting device. Such motors are used for knitting and spinning
machines, paper mills, and other industrial services that require controlled
variable-speed drive. The primary winding on the rotor is supplied from the
line through slip rings. The stator windings are the secondary windings (S sub
1, S sub 2, S sub 3), and the third winding, also in the rotor, is an adjusting
winding provided with a commutator (see illustration). Voltages collected from
the commutator are fed into the secondary circuit. Brushes (1 - 3, 4 - 6) are
mounted on separate movable yokes. Each set of brushes can be moved as a
group. Thus both the spacing between sets of brushes and the angular position
of the brushes are adjustable. Brush spacing determines the magnitude of the
voltage applied to the secondary. When brush sets are so adjusted that pairs
of brushes are in contact with the same commutator segment, the secondary is
short-circuited and no voltage is supplied. Under these conditions the motor
behaves as an ordinary induction motor. The speed can be reduced by separating
the brushes so that secondary current produces a negative torque. The machine
can be operated above synchronism by interchanging the position of the
brushes, so the voltage collected is in a direction to produce a positive
torque. The motor can be reversed by reversing two of the leads supplying the
primary. See INDUCTION MOTOR.



 �34s[Irving L. Kosow]
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�6v�2v�32sBrussels sprouts �21m �33s
    A cool-season biennial crucifer (Brassica oleracea var. gemmifera), which
is of northern European origin and belongs to the plant order Capparales. The
plant is grown for its small headlike buds formed in the axils of the leaves
along the plant stem (see illustration). These buds are eaten as a cooked
vegetable. Popular varieties (cultivars) are Half Dwarf and Catskill; however,
hybrid varieties are increasingly planted. California and New York are
important producing states. See CAPPARALES.




 �34s[H. John Carew]
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�6v�2v�32sBryophyta �21m �33s
    A relatively small phylum of the plant kingdom including the mosses,
Bryopsida (Musci); the liverworts, Marchantiopsida (Hepaticae); and the
hornworts, or horned liverworts (Anthocerotae). These plants are found growing
on soil, rocks, and upon other plants throughout the world wherever there is
sufficient moisture. As individuals these plants are rather inconspicuous, but
when closely associated in patches or beds, they present a striking appearance
(see illustration). In the tropics many of them live as epiphytes, and in the
cold tundra regions vast acreages may be covered by a single species.
    Although they lack the true roots, stems, and leaves characteristic of the
higher vascular plants, many bryophytes have structures which superficially
resemble these organs. Rootlike structures known as rhizoids function in
anchorage and absorption. Since the stemlike and leaflike structures contain
chlorophyll and manufacture food, the plants are independently
self-sufficient. In structure these are the simplest land plants. Being
without specialized conducting and supporting tissues (xylem and phloem), they
have remained small. See CHLOROPHYLL; LEAF; PHLOEM; PHOTOSYNTHESIS; ROOT;
STEM; XYLEM.
    The life cycle of bryophytes consists of a regular alternation of
generations in which the gametophyte, or gamete-producing generation,
alternates with the sporophyte, or spore-producing generation. The
gametophyte, though small, is green and independent (self-sustaining). The
sporophyte is always attached to the gametophyte and is never entirely
independent, although in a few species the sporophyte contains green plastids
(chloroplasts) and can manufacture a limited amount of food. In all cases the
two generations are entirely different in both appearance and structure.
Neither generation produces true flowers, fruits, and seeds. Bryophytes are
not considered to be ancestral to any other group. See ANTHOCEROTAE;
BRYOPSIDA; MARCHANTIOPSIDA; REPRODUCTION (PLANT); TRACHEOPHYTA.



 �34s[Paul A. Vestal]
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�6v�2v�32sBryopsida   �21m �33s 
    A class of small green plants with some 600 genera and 14,000 species 
commonly called mosses which make up the largest and most highly developed 
group of the division Bryophyta. They are worldwide in distribution, occurring 
in nearly all damp habitats except the ocean. They are abundant on soil, 
rocks, and other plants in moist woodlands and may form conspicuous carpets of 
vegetation in arctic and alpine regions. The class is commonly divided into 
three groups: the peat mosses, the granite mosses, and the true mosses. See 
BRYOPHYTA.  
    The peat mosses contain only a single genus, Sphagnum, which nevertheless 
has numerous species. In size these are large mosses, being perennial and 
having practically indefinite growth. The are markedly social, forming 
extensive masses on boggy soils, often being the dominant plant covering. This 
habit of growth is responsible for the name sphagnum bog. Peat moss is used as 
a fuel, as a mulch, and for packing the roots of ornamental seedlings. Brazil 
is the chief center of commercial distribution of peat moss. See BOG. 
    The granite mosses are represented by only two genera, both having a 
rather limited distribution. They are restricted to arctic regions or 
mountaintops, where they are found as blackish masses on siliceous rocks. The 
general structure resembles that of the true mosses, but in several details 
they are like the peat mosses. 
    The true mosses are the largest group of the Bryopsida and include all the 
common genera. Despite their abundance and wide distribution, they display 
great uniformity of structure and have a common life history. They differ from 
the other Bryopsida in having a filamentous protonema, a midrib in their 
leaflike structures, no pseudopodium, and no differentiation of tissue within 
the capsule. 
    Whereas the peat mosses and the granite mosses are relatively limited in 
geographical distribution, the true mosses are found from the polar regions to 
the tropics. They may grow on soil or rocks, in bogs, marshes, ponds or 
streams, or as epiphytes on trees or leaves. Most of them live in moderately 
moist habitats. Many are pioneer species, colonizing any opening in the 
vegetation. In this respect they are of considerable importance in 
forestalling or stopping erosion. 
 �34s[Paul A. Vestal]
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�6v�2v�32sBryozoa �21m �33s
    A phylum of sessile aquatic invertebrates (also called Polyzoa) which form
colonies of zooids. Each zooid, in its basic form, has a lophophore of
ciliated tentacles situated distally on an introvert, a looped gut with the
mouth inside the lophophore and the anus outside, a coelomic body cavity, and
(commonly) a protective exoskeleton. The colonies are variable in size and
habit. Some are known as lace corals and others as sea mats, but the only
general name is bryozoans (sea mosses).
    The colony may be minute, of not more than a single feeding zooid and its
immediate buds, or substantial, forming masses 3 ft (1 m) in circumference,
festoons 1.6 ft (0.5 m) in length, or patches 2.7 ft sup 2 (0.25 m sup 2) in
area. Commonly the colonies form incrustations not more than a few square
centimeters in area, small twiggy bushes up to about 1.2 in. (3 cm) in height,
or soft masses up to about 0.3 ft (0.1 m) in the largest dimension. In many
colonies much of the bulk consists of the zooid exoskeletons which may persist
long after the death of the organism and account for the abundance of
fossilized bryozoan remains.
    Many bryozoans display polymorphism, having certain zooids adapted in
particular ways to perform specialized functions, such as protection, cleaning
the surface, anchoring the colony, or sheltering the embryo. The evolution of
nonfeeding polymorphs is dependent upon some form of intercommunication
between zooids.
    Bryozoa is the name of a phylum for which Ectoprocta is generally regarded
as a synonym, these names being used by zoologists according to personal
preference. Entoprocta (synonym Callyssozoa) is likewise regarded as an
independent phylum. A minority regard Ectoprocta and Entoprocta as subphyla
within the Bryozoa, while others maintain Ectoprocta and Entoprocta as phyla
but link them under Bryozoa as a name of convenience. See ENTOPROCTA.
    The phylum contains some 20,000 described species, one-fifth of them
living. These are distributed among three classes and a somewhat variable
number of orders:

    Phylum Bryozoa
    Class Phylactolaemata
    Order Plumatellida
    Class Gymnolaemata
    Order Ctenostomata
    Order Cheilostomata
    Class Stenolaemata
    Order Cyclostomata
    Order Cystoporata
    Order Trepostomata
    Order Cryptostomata
    Order Rhabdomesonata
    Order Fenestrata

Some authorities regard the last two as suborders of an enlarged
Cryptostomata. See GYMNOLAEMATA; PHYLACTOLAEMATA; STENOLAEMATA.
    Fresh-water bryozoans are present on submerged tree roots and aquatic
plants in most lakes, ponds, and rivers, especially in clear water of alkaline
pH. Most other bryozoans are marine, although some gymnolaemates inhabit
brackish water. They are common in the sea, ranging from the middle shore to a
depth of over 26,000 ft (8000 m), and are maximally abundant in waters of the
continental shelf. Most attach to firm substrata, so that their distribution
is primarily determined by the availability of support. Mud is unfavorable and
so is sand unless well provided with stone, dead shells, hydroids, or large
foraminiferans.
    Colony form in bryozoans is to some extent related to habitat. Encrusting
and bushy flexible species are adapted to wave exposure; brittle twiglike and
foliaceous species are found deeper; some erect branching species tolerate
sediment deposition. One group of tiny discoid species lives on sand in warm
seas, and in one genus the colonies are so small that they live actually among
the sand grains; a few species live anchored in mud. A number of stolonate
ctenostomes bore into the substance of mollusk shells; other species are
associated only with hermit crabs, and a few are commensal with shrimps or
polychaete worms.
    Bryozoans have few serious predators. Nudibranch mollusks and pycnogonids
(sea spiders) specialize in feeding on zooids but are rarely destructive of
entire colonies. Loxosomatids (Entoprocta) and a hydroid (Zanclea) are common
commensals.
    Life spans vary. Small algal dwellers complete their life cycle in a few
months. Many species survive a year but have two overlapping generations;
others are perennial, with one known to survive for 12 years.
    Bryozoans may be a nuisance in colonizing ship hulls and the insides of
water pipes, and one species has caused severe dermatitis in fishers. Recently
some delicate kinds have been used in costume jewelry, and green-dyed clumps
of dried Bugula are often sold as ``everlasting plants.''
 �34s[John S. Ryland]
�33s
    Fossil Bryozoa have a long geological history from early in the Ordovician
Period (500,000,000 years ago) to the Recent. Individual colonies range in
size from a few millimeters to several meters in height. A vast array of
diverse colony forms record the heterogeneity of this phylum. There are about
1100 known fossil genera and about 15,000 known fossil species.
Representatives of the marine orders that secreted calcareous skeletons
(Cryptostomata, Cyclostomata, Cystoporata, Fenestrata, Rhabdomesonata,
Trepostomata, and Cheilostomata) commonly are abundant in sedimentary rocks
deposited in various environments of marine basins where sessile benthic
animals flourished. The calcareous skeletons are generally calcite, but some
are aragonite or a mixture of calcite and aragonite. The Ctenostomata with
only soft membranous or gelatinous colonies are preserved as impressions,
excavations, or borings generally in marine shells. The fresh-water
Phylactolaemata with soft gelatinous colonies are mostly not preserved except
for statoblasts (dormant reproductive bodies) that are reported from
Quaternary (2,000,000 years ago) rocks.
 �34s[June R. P. Ross]
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�6v�2v�32sBubble chamber �21m �33s
    A particle detector used in elementary particle physics research. Charged
particles passing through liquid contained in the chamber leave visible tracks
along their paths. These tracks are then photographed for later study. Each
curved track is actually a string of small bubbles marking the path of an
elementary particle. Bubble chambers vary in size, in the liquid used, and in
how fast photographs can be taken. They are particularly useful because they
allow direct visualization of all the charged particles involved in a
high-energy interaction, and they have been used to discover many new
particles, such as the rho (RHO) and omega minus (OMEGA-). See ELEMENTARY
PARTICLE.
    The bubble chamber was invented by Donald Glaser in 1952. It was known
that a pure liquid in a clean glass container can be raised above its boiling
temperature without its starting to boil spontaneously. The boiling must be
triggered by something. For example, a broken piece of glass dropped into a
superheated liquid can make boiling violently erupt. As an electrically
charged particle passes through any material (such as a liquid), it interacts
with electrons in the material, leaving atoms ionized. Glaser discovered that
in a superheated liquid this ionization energy is enough to trigger boiling
along the path of the particle. The boiling bubbles grow rapidly, and so a
photograph is quickly taken before the resolution of the individual tracks is
destroyed. In order to make the bubble chamber sensitive to the arrival of
another charged particle, the boiling must be stopped, and the liquid must be
superheated again.
    Interest is focused on the nuclear interactions that a beam particle has
with the particles in the liquid. Since the interactions occur with particles
making up the liquid, the choice of liquid is important to the experimenter.
Liquid hydrogen is used to study proton interactions because hydrogen is a
simple atom made up of a single proton and electron. Likewise deuterium, with
one neutron and one proton, is used to study neutron  and proton interactions.
Liquid hydrogen and deuterium boil at a very low temperature; thus chambers
using these liquids operate at about 49 DG F (27 K) above absolute zero. See
PARTICLE DETECTOR.
 �34s[Donovan A. Ljung]
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�6v�2v�32sBuckeye �21m �33s
    A genus, Aesculus, of deciduous trees or shrubs belonging to the plant
order Sapindales, Buckeyes grow in North America, southeast Europe, and
eastern Asia to India. The distinctive features are opposite, palmately
compound leaves and a large fruit having a firm outer coat and containing
usually one large seed with a conspicuous hilum.
    The Ohio buckeye (A. glabra) is found mainly in the Ohio valley and in the
southern Appalachians. It can be recognized by the glabrous winter buds,
prickly fruits, and compounds leaves having five leaflets. Another important
species, the yellow buckeye (A. octandra), is native in the Central states,
has five leaflets and smooth buds, but differs in its smooth, larger fruit.
The horse chestnut (A. hippocastanum), which usually has seven leaflets and
resinous buds, is a native of the Balkan Peninsula. It is planted throughout
the United States and is a beautiful ornamental tree bearing cone-shaped
flower clusters in early summer.
    The seeds of all species contain a bitter and narcotic principle. The wood
of the native tree species is used for furniture, boxes, crates, baskets, and
artificial legs. See SAPINDALES.
 �34s[Arthur H. Graves /Kenneth P.Davis]
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�6v�2v�32sBuckwheat   �21m �33s 
    A herbaceous, erect annual, the dry seed or grain of which is used as a 
source of food and feed. It is not a true cereal and is one of the very few 
plants other than those of the Gramineae family, used for their starchy seed, 
which is processed as a meal or flour. Buckwheat belongs to the Polygonaceae 
family. Species of buckwheat that have been commercially grown are Fagopyrum 
sagittatum (= F. esculentum), F. emarginatum, and F. tataricum. 
    The plant grows to a height of 2 - 5 ft (0.6 - 1.5 m) with many broad 
heart-shaped leaves. It produces a single main stem which usually bears 
several branches and is grooved, succulent, and smooth except for nodes. 
Buckwheat is an indeterminate species in response to photoperiod and produces 
flowers and fruits (so-called seeds) until the beginning of frost. See 
POLYGONALES.  
 �34s[H. G. Marshall]
�33s 
    Buckwheat milling is similar to wheat milling in principle. The grain is 
cleaned and crushed between a series of steel rolls, and the flour and feed 
by-products are separated by sieving. Buckwheat flour is used almost 
exclusively in pancakes or as one of the ingredients in prepared pancake 
flour. Sometimes buckwheat flour is milled for groats. Two types of food 
products are made from buckwheat groats, roasted kernels and a farina, or 
granular product, used in soup. 
 �34s[John A. Shellenberger]
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�6v�2v�32sBud �21m �33s
    An embryonic shoot containing the growing stem tip surrounded by young
leaves or flowers or both, and the whole frequently enclosed by special
protective leaves, the bud scales.
    The bud at the apex of the stem is called a terminal bud (illustration a).
Any bud that develops on the side of a stem is a lateral bud. The lateral bud
borne in the axil (angle between base of leaf and stem) of a leaf is the
axillary bud (illustration a and d). It develops concurrently with the leaf
which subtends it, but usually such buds do not unfold and grow rapidly until
the next season. Because of the inhibitory influence of the apical or other
buds, many axillary buds never develop actively or may not do so for many
years. These are known as latent or dormant buds. Above or beside the axillary
buds, some plants regularly produce additional buds called accessory, or
supernumerary, buds. Accessory buds which occur above the axillary bud are
called superposed buds (illustration c), and those beside it collateral buds
(illustration d). Under certain conditions, such as removal of terminal and
axillary buds, other buds may arise at almost any point on the stem, or even
on roots or leaves. Such buds are known as adventitious buds. See PLANT GROWTH.
    Buds that give rise to flowers only are termed flower buds, or in some
cases, fruit buds. If a bud grows into a leafy shoot, it is called a leaf bud,
or more accurately, a branch bud. A bud which contains both young leaves and
flowers is called a mixed bud.
    Buds of herbaceous plants and of some woody plants are covered by
rudimentary foliage leaves only. Such buds are called naked buds. In most
woody plants, however, the buds are covered with modified protective leaves in
the form of scales. These buds are called scaly buds or winter buds. In the
different species of plants, the bud scales differ markedly. They may be
covered with hairs or with water-repellent secretions of resin, gum, or wax.
Ordinarily when a bud opens, the scales fall off, leaving characteristic
markings on the stem (bud scale scars). See LEAF.




 �34s[Nelle Ammons]
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�6v�2v�32sBudding   �21m �33s 
    A vegetative method of propagation in which a single bud (often a bud 
sport or bud mutation) serves as a scion and is grafted laterally upon a 
stock. Bud grafting may be performed throughout the growing season when the 
bark of the stock is soft and loose, but summer or early fall is usually the 
most satisfactory time. Success depends upon joining the vascular cambia of 
the stock and scion. This requires genetic compatibility of stock and scion, 
pressure to hold the scion firmly against the stock, and protection from 
desiccation. See GRAFTING OF PLANTS; REPRODUCTION (PLANT). 
 �34s[James E. Gunckel]
�33s 
  
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�6v�2v�32sBudding and appendaged bacteria   �21m �33s 
    The vernacular name given to Part 4 in the organization of bacteria 
according to the eighth edition of Bergey's Manual of Determinative 
Bacteriology (1974). Typical bacteria divide asexually by binary transverse 
fission. Some bacteria, that is, many of the bacteria considered in this 
section, divide by budding which is analogous to budding in yeasts. In this 
process the daughter bud is formed as a small outgrowth on the surface of the 
mother cell. It enlarges and eventually separates from the mother cell. In 
contrast to binary transverse fission, this process involves the formation of 
the wall layers of the daughter cell (that is, bud) largely from newly 
synthesized material, with the mother cell retaining essentially all of the 
components of the cell wall of the original cell. 
    The appendaged representatives in this group have either stalks or 
prosthecae extending from the surface of the organism. Stalks are excreted or 
extracellular appendages, whereas prosthecae are part of the cell, in that 
they are enclosed by the cell wall of the bacterium that has them. 
    Most of the bacteria in this group are widely distributed in soil and in 
fresh waters and marine waters. With rare exception they are all free-living 
microbes whose major role in nature is the decomposition of the nonliving 
organic remains of plants, animals, and other microorganisms. 
    This group is a heterogeneous collection of organisms containing some 
types that are at best only distantly related to others. The most common 
genera are: 
    Group I, budding bacteria with buds formed on tip of prosthecae. Included 
here are the genera Hyphomicrobium, Hyphomonas, and Rhodomicrobium. 
    Group II, prosthecate bacteria whose progeny are not produced on tip of 
prostheca. Included here are the genera Caulobacter, Ancalomicrobium, and 
Prosthecomicrobium.  
    Group III, budding bacteria which do not have prosthecae; includes the 
genus Pasteuria. 
    Group IV includes the stalked bacteria, represented by the genus 
Gallionella.  
 �34s[James T. Staley]
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�6v�2v�32sBuerger's disease   �21m �33s 
    A progressive, chronic inflammation of arteries, veins, and nerves of 
young to middle-aged men; also known as thromboangiitis obliterans. Buerger's 
disease is marked by a segmental involvement of larger arteries by 
inflammation, fibrosis, and a tendency toward formation of thromboses. 
    The disease is relatively rare and is usually found in men of 25 - 40 
years of age, although a few women and younger males have been affected. A 
familial susceptibility is sometimes present and the disease shows an 
increased incidence among Jews. 
    Although the specific etiologic agent is unknown, almost all patients with 
Buerger's disease can cause an increase in their symptoms by the excessive use 
of tobacco or exposure to cold. Conversely, many of those afflicted show 
beneficial effects or retardation of the process when these two 
symptom-increasing factors are removed. 
    Vessels of the lower extremity are usually first affected and the clinical 
course is marked by muscle pain, aching, and tenderness brought on by exertion 
and relieved by rest. These symptoms may be accompanied or followed by sensory 
changes or sensitivity to extremes of temperature. In advanced cases 
discoloration of the skin, atrophy of the tissues, and even gangrene result 
from the progressive occlusion of blood vessels. 
    Progressive invalidism, hastened by the necessity of amputations of 
affected parts, is commonplace. Death may follow after a long period of 
illness or may occur during the frequent complications. Although peripheral 
vessels are most often damaged, the arteries of certain organs may be 
similarly involved, in which cases the prognosis is much less favorable. See 
CARDIOVASCULAR SYSTEM. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sBuffalo �21m �33s
    The name for members of the family Bovidae in the mammalian order
Artiodactyla. The buffalo is an Old World species and resembles the oxen in
general appearance. The North American bison is often called a buffalo, but is
not related to the true buffalo.
    The Asiatic buffalo (Bubalus bubalis), known as the Indian or water
buffalo and also as the carabao, is found as a domestic animal in the Balkans,
Asia Minor, and Egypt. These buffalo exist in the wild state in southern Asia
and Borneo, where they are considered to be ferocious and dangerous. Water
buffalo are stocky, heavy-built animals. They have very short hair and short,
splayed horns. Like all buffalo, they have a liking for marshes, where they
wallow and become caked with mud that affords protection against insects.
    Two other Asiatic species related to, but smaller than, the water buffalo
are the tamarau (Anoa mindorensis), which is indigenous to the Philippines,
and the still smaller anoa (A. depressicornis), or wild dwarf buffalo, found
in the Celebes.
    The African buffalo, classed in the genus Syncerus, was very numerous
until the turn of the century, when the infectious disease rinderpest caused
many deaths. They are still abundant though widely hunted. There are several
varieties of African buffalo, and it is thought that all may be subspecies of
S. caffer, the Cape buffalo. They live in the open country of central,
eastern, and southern Africa. Except for its size, this animal is difficult to
distinguish from the rare dwarf or forest buffalo (S. caffer nanus). It lives
in marshy, forested areas of western Africa, where it is known as the bush
cow. See ARTIODACTYLA; BISON; MAMMALIA.
 �34s[Charles B. Curtin]
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�6v�2v�32sBuffers (chemistry)   �21m �33s 
    A solution selected or prepared to minimize changes in hydrogen ion 
concentration which would otherwise tend to occur as a result of a chemical 
reaction. In general, chemical buffers are systems which, once constituted, 
tend to resist further change due to external influences. Thus it is possible, 
for example, to make buffers resistant to changes in temperature, pressure, 
volume, redox potential, or acidity. The commonest buffer in chemical solution 
systems is the acid-base buffer. 
    Chemical reactions known or suspected to be dependent on the acidity of 
the solution, as well as on other variables, are frequently studied by 
measurements in comixture with an appropriate buffer. For example, it may be 
desirable to investigate how the rate of a chemical reaction depends upon the 
hydrogen ion activity (pH). This is accomplished by measurements in several 
buffer systems, each of which provides a nearly constant, different pH. 
Alternatively, it may be desirable to measure the effects of other variables 
on a pH-sensitive system, by stabilizing the pH at a convenient value with a 
particular buffer. See PH. 
    Buffer action depends upon the fact that, if two or more reactions coexist 
in a solution, then the chemical potential of any species is common to all 
reactions in which it takes part, and may be defined by specification of the 
chemical potentials of all other species in any one of the reactions. To be 
effective, a buffer must be able to respond to an increase as well as a 
decrease of the species to be buffered. In order to do so, it is necessary 
that the proton transfer step of the buffer be reversible with respect to the 
species involved, in the reaction to be buffered. In aqueous solution the 
proton transfer between most acids, their conjugate bases, and water, is so 
rapid and reversible that the dominant direct source of protons for a chemical 
reaction is H3Osupp, the hydronium ion. 
    Buffers are particularly effective in water, because of the unusual 
properties of water as a solvent. Its high dielectric constant tends to 
promote the existence of formally charged ions (ionization). Because it has 
both an acidic (H) and a basic (O) group, it may form bonds with ionic species 
leading to an organized sheath of solvent surrounding an ion (solvation). 
Water also tends to self-ionize to form its own conjugate acid-base system. 
See ACID AND BASE; ACID-BASE INDICATOR; IONIC EQUILIBRIUM; SOLVATION. 
 �34s[A. M. Hartley]
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�6v�2v�32sBuffing   �21m �33s 
    The smoothing and brightening of a surface by an abrasive compound pressed 
against the work by means of a soft wheel or belt. The abrasive is a fine 
powder or flour mixed with tallow or wax to form a smooth composition or 
paste. This is applied as required to the buffing wheel or belt, which is made 
of a pliable material, such as soft leather, linen, muslin, or felt. Buffing 
is accomplished on the same types of machines as polishing and frequently both 
types of wheels are included on one machine. See POLISHING. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sBuhrstone mill �21m �33s
    A mill for grinding or pulverizing, in which a flat siliceous rock,
generally of cellular quart, rotates against a stationary stone of the same
material. Grooves in the stones facilitate the movement of the material.
Fineness of the product is controlled by the pressure between the stones and
by the grinding speed. A finely ground product is achieved by slowly rotating
the stone at a high pressure against the materials and its mate (see
illustration). See CRUSHING AND PULVERIZING.





 �34s[George W. Kessler]
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�6v�2v�32sBuildings �21m �33s
    Fixed structures for human occupancy and use. The space and structure must
be planned to produce the environment and facilities required for the purpose
of the building. Items such as light, heat, air conditioning, acoustics, and
color must be given careful consideration.
    Many governments, states, professional organizations, and municipalities
have adopted building codes to establish minimum load capacities and to ensure
safety to the occupants under normal use and under common hazards like fire
and windstorms. Buildings are usually classified according to occupancy and
use as follows: (1) public (auditoriums, churches, theaters), (2)
institutional (hospitals, schools, penitentiaries), (3) residential (hotels,
apartments, studios), (4) business (stores, factories, office buildings), and
(5) storage (garages, warehouses).
    The loads that a building supports are called dead load, which includes
the weight of walls, columns, partitions, floors, and roof; and live load,
which includes the weight of the physical equipment and occupants of the
building. Snow load on the roof is considered a live load. Because there is
little likelihood of applying the full specified live load all over large
areas, codes permit a reduction in the basic design live load when areas
exceed a certain minimum. A live load reduction is therefore allowed for
supporting members such as girders, columns, piers, and foundations. An
allowance for the dynamic effect of all moving loads, such as elevator
machinery and cranes, is usually specified as a fixed percentage of the load.
    The evaluation of pressures exerted by wind on a building is complex, and
code requirements for the wind forces are only rough approximations. In
earthquake areas the building code provides for special seismic design
requirements. On the basis of predicted earthquake shocks, all components of a
building must be designed to take into account the lateral forces that might
be induced by such ground motion.
    The roof and the floors of the buildings are generally supported by one of
the three common framing systems: (1) bearing wall, (2) skeletal framing
composed of beams and columns or slabs and columns, and (3) special long-span
roof systems. In bearing-wall construction, walls support the ends of the
floor and roof beams and slabs in addition to enclosing partitioning space.
The bearing wall can be made of bricks, concrete blocks, cast-in-place
reinforced concrete, or precast concrete panels. The skeletal frame
construction can be either of steel framing or of reinforced concrete.
Skeletal frames of reinforced concrete normally have monolithic rigid
connections between beams and columns. However, steel skeletal frame
construction can be simple, rigid, or semirigid. Long-span construction is
used for auditoriums, hangars, gymnasiums, and other structures requiring a
wide unobstructed floor area that cannot be economically spanned with standard
steel beams or typical reinforced concrete or solid-timber girders. The
appropriate forms of construction are plate girders, prestressed girders,
trusses, arches, rigid frames, domes, cantilever suspension frames, space
frames, and suspended cables. ARCH; CANTILEVER; ROOF CONSTRUCTION; TRUSS; See
WALL CONSTRUCTION.
    It is usually economical to support heavy industrial buildings with a
steel framework consisting of a series of trusses supported on columns. Such
industrial buildings often attain huge dimensions. Spectacular achievements
have been made in the design and construction of ultra-high-rise buildings.
Reinforced concrete has been economically and competitively used in a number
of structures for both residential and commercial purposes. The newer
high-rise buildings, ranging from 50 to 110 stories are the result of
relatively recent innovations and development of new structural systems.
    Greater height entails increased column and beam sizes to make buildings
more rigid so that under wind load they will not sway beyond an acceptable
limit. Excessive lateral sway may cause serious recurring damage to
partitions, ceilings, and other architectural details. In addition, excessive
sway may cause discomfort to the occupants of the building because of their
perception of such motion. New structural systems of reinforced concrete, as
well as steel, take full advantage of the inherent potential stiffness of the
total building and therefore do not require additional stiffening to limit the
sway.
 �34s[Fazlur R. Khan]
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�6v�2v�32sBulk-handling machines �21m �33s
    A diversified group of materials-handling machines specialized in design
and construction for handling unpackaged, divided materials.
    Solid, free-flowing materials are said to be in bulk. The handling of
these materials requires that the machinery both support their weight and
confine them either to a desired path of travel for continuous conveyance or
within a container for handling in discrete loads. Wet or sticky materials may
also be handled successfully by some of the same machines used for bulk
materials. Characteristics of materials that affect the selection of equipment
for bulk handling include (1) the size of component particles, (2)
flowability, (3) abrasiveness, (4) corrosiveness, (5) sensitivity to
contamination, and (6) general conditions such as dampness, structure, or the
presence of dust or noxious fumes.
    Equipment that transports material continuously in a horizontal, inclined,
or vertical direction in a predetermined path is a form of conveyor. The many
different means used to convey bulk materials include gravity, belt, apron,
bucket, skip hoist, flight or screw, dragline, vibrating or oscillating, and
pneumatic conveyors. Wheel or roller conveyors cannot handle bulk materials.
    Gravity chutes are the only unpowered conveyors used for bulk material.
They permit only a downward movement of material.
    Belt conveyors of many varieties move bulk materials. Fabric belt
conveyors have essentially the same operating components as those used for
package service; however, these components are constructed more ruggedly to
stand up under the more rigorous conditions imposed by carrying coal, gravel,
chemicals, and other similar heavy bulk materials. Belts may also be made of
such materials as rubber, metal, or open wire. Their advantages include low
power requirements, high capacities, simplicity, and dependable operation.
    An apron conveyor is a form of belt conveyor, but differs in that the
carrying surface is constructed of a series of metal aprons or pans pivotally
linked together to make a continuous loop. This type of conveyor is suitable
for handling large quantities of bulk material under severe service
conditions. Apron conveyors are most suitable for heavy, abrasive, or lumpy
materials.
    Bucket conveyors are constructed of a series of buckets attached to one or
two strands of chain or in some instances to a belt. These conveyors are most
suitable for operating on a steep incline or vertical path, sometimes being
referred to as elevating conveyors. This type of conveyor is most ideal for
bulk materials such as sand or coal.
    Flight conveyors employ the use of flights, or bars attached to single or
double strands of chain. The bars drag or push the material within an enclosed
duct or trough. These are frequently referred to as drag conveyors. This type
of conveyor is commonly used for moving bulk material such as coal or metal
chips from machine tools.
    Spiral or screw conveyors rotate upon a single shaft to which are attached
flights in the form of a helical screw. When the screw turns within a
stationary trough or casing, the material advances. These conveyors are used
primarily for bulk materials of fine and moderate sizes, and can move material
on horizontal, inclined, or vertical planes.
    Vibrating or oscillating conveyors employ the use of a pan or trough bed,
attached to a vibrator or oscillating mechanism, designed to move forward
slowly and draw back quickly. The inertia of the material keeps the load from
being carried back so that it is automatically placed in a more advanced
position on the carrying surface.
    Pneumatic, or air, conveyors employ air as the propelling media to move
materials. One implementation of this principle is the movement within an air
duct of cylindrical carriers, into which are placed currency, mail, and small
parts for movement from one point to discharge at one of several points by use
of diverters. Pneumatic pipe conveyors are widely used in industry, where they
move granular materials, fine to moderate size, in original bulk form without
need of internal carriers.
    Power cranes and shovels perform many operations moving bulk materials in
discrete loads. When functioning as cranes and fitted with the many
below-the-hook devices available, they are used on construction jobs and in
and around industrial plants. Such fittings as magnets, buckets, grabs,
skullcrackers, and pile drivers enable cranes to handle many products. The
machines of the convertible, full-revolving type are mounted on crawlers,
trucks, or wheels. Specialized front-end operating equipment is required for
clamshell, dragline, lifting-crane, pile-driver, shovel, and hoe operations.
Specialized equipment for mechanized pit mining has been developed. Power
cranes, shovels, and scoops are actively engaged in strip mines, quarries, and
other earth-moving operations. See CONVEYING MACHINES; CONVEYOR; ELEVATING
MACHINES; HOISTING MACHINES; INDUSTRIAL TRUCKS; MATERIALS-HANDLING MACHINES;
MONORAIL.
 �34s[Arthur M. Perrin]
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�6v�2v�32sBulkhead   �21m �33s 
    A structure primarily used to retain earth fill or protect embankments 
along the shore. Bulkheads sometimes also serve as wharves. They are usually 
used on comparatively sheltered areas and therefore require less strength 
against sea action than sea walls. Sheet piling of timber, steel, or concrete 
is the commonest form of construction, with tiebacks to ``deadmen'' buried in 
the earth. Pile-supported platforms behind sheet piling help to relieve the 
earth pressure against the sheeting and platforms on the water side help to 
reduce the unsupported height of the sheeting. Masonry or concrete gravity 
wall construction and stone-filled crib construction are also used. See 
COASTAL ENGINEERING; SEA WALL. 
 �34s[Edward J. Quirin]
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�6v�2v�32sBulldozer   �21m �33s 
    An assembly consisting of a reinforced, curved steel plate or blade and a 
frame which is attached to the front of a wheeled or crawler tractor for the 
purpose of pushing dirt, rock, or other materials. 
    The blade may be straight, angling, or U-shaped. The U blades and straight 
blades mount perpendicular to the longitudinal axis of the tractor and push 
material straight ahead; the U blade, because of its shape, can contain and 
move more material than the straight blade. The angle blade can be set so that 
either of its ends is ahead of the other; thus it is useful in windrowing 
material or backfilling trenches. Blades are controlled either by cables or 
hydraulic units, although the latter are far commoner. The straight and U 
blades can be raised and lowered, worked with one side lower than the other, 
and tipped forward or backward. The angle blade cannot be tipped. See 
EARTHMOVER.  
 �34s[Edward M. Young]
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�6v�2v�32sBuoy   �21m �33s 
    An anchored or moored floating object, other than a lightship, intended as 
an aid to navigation. Buoys are the most numerous of all artificial aids to 
navigation.  
    Buoys are intended to serve as daymarks. Some buoys, particularly those at 
turning points in channels, are provided with lights for location and 
identification at night. Some buoys are equipped with apparatus for providing 
distinctive sounds at intervals so they can be used as aids to navigation in 
fog and darkness. Some buoys are equipped with radio beacons, and some have 
reflectors to make them more conspicuous to radar. 
    Two general systems of buoyage are in use. In the cardinal system buoys 
are assigned shape, color, and number in accordance with their locations 
relative to the nearest obstruction. In the lateral system, used in a modified 
form in United States waters, buoys are assigned shape, color, and number in 
accordance with their locations relative to navigable waters. See PILOTING. 
 �34s[Alton B Moody]
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�6v�2v�32sBuoyancy �21m �33s
    The resultant vertical force exerted on a body by a static fluid in which
it is submerged or floating. The buoyant force F sub B acts vertically upward,
in opposition to the gravitational force that causes it. Its magnitude is
equal to the weight of fluid displaced, and its line of action is through the
centroid of the displaced volume, which is known as the center of buoyancy.
See AEROSTATICS; FLUID STATICS; HYDROSTATICS.
    By weighing an object when it is suspended in two different fluids of
known specific weight, the volume and weight of the solid may be determined.
See ARCHIMEDES' PRINCIPLE.
    A body floating on a static fluid has vertical stability. A small upward
displacement decreases the volume of fluid displaced, hence decreasing the
buoyant force and leaving an unbalanced force tending to return the body to
its original position. Similarly, a small downward displacement results in a
greater buoyant force, which causes an unbalanced upward force.
    A body has rotational stability when a small angular displacement sets up
a restoring couple that tends to return the body to its original position.
When the center of gravity of the floating body is lower than its center of
buoyancy, it will always have rotational stability. Many a floating body, such
as a ship, has its center of gravity above its center of buoyancy. Whether
such an object is rotationally stable depends upon the shape of the body.
 �34s[Victor L. Streeter]
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�6v�2v�32sBuret   �21m �33s 
    An instrument used to deliver variable volumes of a liquid. Burets are 
usually made from uniform-bore glass tubing in capacities of 5 - 100 ml, the 
commonest being 50 ml. The uniform portion is graduated in small divisions, so 
that volumes of liquid accurate to a small fraction of a milliliter may be 
measured by reading the volumes in the buret before and after sample has been 
withdrawn. Most burets are equipped with a stopcock to control the flow of 
liquid. See TITRATION; VOLUMETRIC ANALYSIS. 
 �34s[Clark E. Bricker]
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�6v�2v�32sBurgess Shale   �21m �33s 
    The Burgess Shale is part of a clay and silt sequence that accumulated 
beneath a colossal algal reef. The Burgess Shale itself was deposited at the 
reef base, and various lines of evidence demonstrate that the fauna (and 
flora) were transferred from a relatively oxygenated environment into deeper 
anaerobic water in a series of turbidites. See TURBIDITE. 
    The Burgess Shale fauna, occuring in a single locality near Field in 
southern British Columbia, is probably the finest assemblage of soft-bodied 
fossils known. It is Mid-Cambrian and thus approximately 530,000,000 years 
old. The fauna affords a unique glimpse into the state of metazoan evolution 
comparatively shortly after the major adaptive radiations of the late 
Precambrian and Early Cambrian. 
    The importance of the Burgess Shale fauna goes beyond contributing to 
knowledge of a wide variety of animals otherwise unknown from the fossil 
record. It must be asked to what extent the Burgess Shale fauna represents a 
typical Cambrian marine community with ``normal'' fossil faunas, that is, hard 
parts only, representing impoverished assemblages decimated by preservational 
factors such as decay. No firm answer can yet be given, but the scattered 
occurrence of soft-bodied fossils in other Cambrian rocks strongly suggests 
that the Burgess Shale is unique only in terms of preservation. See CAMBRIAN; 
FOSSIL; PALEONTOLOGY. 
 �34s[Simon Conway Morris]
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�6v�2v�32sBurn �21m �33s
    An injury to tissues caused by heat, chemicals, electricity, or
irradiation effects.
    The commonest type of burn is that due to thermal injury, in which some
portion of the body surface is exposed to either moist or dry heat of
sufficient temperature to cause local and systemic reactions. Clinically, the
extent of such a burn is often expressed as first degree, second degree, and
so forth. Different systems of classification exist.
    First-degree burns result in some redness and swelling of the injured
part, without necrosis of any tissue or the formation of blisters. Healing is
completed in a few days without scarring.
    Second-degree burns show a variable destruction of parts of the epidermis
so that blistering occurs. Healing by regeneration in such superficial burns
does not necessitate skin grafting, unless secondary infections ensue; no
scarring results.
    Third-degree burns are marked by complete destruction of the epidermis of
a region, including the necrosis of accessory skin structures like hair and
sweat glands. A brownish-black eschar marks the destroyed tissue. This is
sloughed off and that defect becomes filled with granulation tissue that later
consolidates and changes to form a dense, thick scar. Complications may occur
without adequate care, and grafting is not unusual, sometimes being required
because of contracture of the scar tissue.
    In fourth-degree burns, tissue is destroyed to the level of or below the
deep fascia lying beneath the subcutaneous fat and connective tissue of the
body. Muscle, bone, deeper nerves, and even organs may be injured or destroyed
by this severe degree of burn. Healing is usually a slow, involved process,
requiring much reparative and reconstructive work by surgical specialists.
    Electrical burns result from the amount of heat incident to the flow of a
certain amount of electricity through the resistance offered by tissues. From
a practical standpoint, most of the resistance offered to the passage of an
electric current is that of the skin and the interface between the skin and
the external conductor. Therefore, most electrothermal injuries are limited to
the skin and immediately subjacent tissues, although deep penetration may
follow large voltages.
    Most chemical burns result from the action of corrosive agents which
destroy tissues at the point of contact. Exposure of the skin, eyes, and
gastrointestinal tract are commonest.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sBursa   �21m �33s 
    A simple sac or cavity with smooth walls and containing a clear, slightly 
sticky fluid interposed between two moving surfaces of the body to reduce 
friction. Subcutaneous bursae are found where the skin stretches around the 
greater curvature of a joint, as in the elbow or knee, and considerable 
chafing may occur; they may be single or multiple sacs. These bursae may 
enlarge as a result of continuous excessive irritation, as in housemaid's knee 
or miner's elbow. See BURSITIS; JOINT (ANATOMY) DISORDERS. 
    Synovial bursae are small closed sacs of fibrous tissue continuous with 
the joint cavity of a diarthrosis. They are lined with a complex membrane that 
secretes a clear lubricating fluid, serving to reduce friction between the 
opposing surfaces of the articulation. See JOINT (ANATOMY). 
    Bursae may exist in the form of elongated sheaths surrounding tendons or 
ligaments, where these moving bands are in contact with another structure, 
such as a bone, muscle, or another tendon or ligament. Tendon sheaths are 
especially common where tendons bend around the ends of two bones at an 
articulation. See MUSCULAR SYSTEM; SKELETAL SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sBursitis �21m �33s
    Any inflammation of a bursa. Inflammatory changes produce acute or chronic
swelling, an increase in the fluid contents, and variable degrees of pain and
tenderness. Although most bursitis is nonspecific, it may follow the invasion
of certain organisms or may result from trauma or physical stress. In most
instances the fluid aspirated from an inflamed bursa is sterile, thereby
ruling against an infectious etiology. In chronic bursitis the wall becomes
thickened, shaggy, and irregular, with calcium deposits commonly present. See
BURSA; CONNECTIVE TISSUE.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sBus   �21m �33s 
    A wheeled, self-propelled public carrier available in various capacities 
and sizes accommodating from 8 to more than 60 people for mass transit of 
passengers on school, local, intercity, or interstate routes. As an automotive 
vehicle, the bus retains many passenger car and truck components. 
    Basic bus design keeps gross weight at a minimum for economic operation 
and makes maximum utilization of space for entry, aisle, and exit areas; in 
the United States, seating and safety devices must comply with Federal 
regulations. Heavy frames perform the same functions as those in other 
commercial vehicles for load-bearing capacity, rigidity, and resistance to 
impact. See AUTOMOTIVE FRAME. 
    The chassis assembly includes all the components and systems essential to 
operation, usually the fundamental package upon which bus bodies built by 
other manufacturers are mounted. Body styles, passenger capacity, power plant, 
and equipment are available in great variety. The type of accommodation ranges 
from three or four bench seats, in suburban units, to units equipped with air 
conditioning, sanitary facilities, and conveniences for passengers traveling 
intercity or transcontinental routes. Gasoline and diesel engines are the 
principal sources of motive power. See AUTOMOTIVE VEHICLE. 
 �34s[Paul A. Obrist]
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�6v�2v�32sBus-bar   �21m �33s 
    An electric conductor that serves as a common connection between load 
circuits and the source of electric power of one polarity in direct-current 
systems or of one phase in alternating-current systems. Two or more bus-bars 
which serve all polarities or phases are collectively called a bus. 
    Bus-bars must be designed to carry the continuous current without 
overheating. The highest continuous current can be in the order of hundreds of 
thousands of amperes. Bus-bars must also be designed to withstand the 
mechanical forces caused by short-circuit currents. Special metallic shields 
are designed to enclose the bus, thus minimizing the effect of short-circuit 
forces. Such a bus is known as an isolated phase bus. See WIRING. 
 �34s[Thomas H. Lee]
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�6v�2v�32sBushing   �21m �33s 
    A removable metal lining, usually in the form of a bearing to carry a 
shaft. Generally a bushing is a small bearing in the form of a cylinder and is 
made of soft metal or graphite-filled sintered material. Bushings are also 
used as cylindrical liners for holes to preserve the dimensional requirements, 
such as in the guide bearings in jigs and fixtures for drilling holes in 
machine parts. 
 �34s[James J. Ryan]
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�6v�2v�32sBusiness communications system �21m �33s
    An automatic switching system usually located on the customer's premises
to interconnect terminal equipment, such as telephone sets served by the
system, and to connect terminal equipment to the nationwide telecommunications
network via interfaces with network facilities. It is also called a telephone
private branch exchange (PBX), a term that is used especially for the older
systems. A business communications system (BCS) frequently includes
private-line terminals to provide direct (nonswitched) connections to other
business communications systems, terminals to provide connections to dedicated
switching networks (enhanced private switched communications service and
common control switching arrangement), and special service terminals such as
wide-area telephone service and foreign exchange lines. Business
communications systems also provide a wide variety of user features designed
to aid the business customer and to meet unique needs of special customers,
such as hotels and hospitals.
    The architecture of the modern business communications system strongly
resembles that of the general-purpose computer. The control of the system is
provided by an integrated-circuit processor. A generic set of operating
instructions used to control call-processing sequences is stored in the system
memory. Additional memory is provided for a specific set of instructions
related to the particular installation and the features to be provided for
that installation.
    The type of switching network is dependent generally on the size of the
business communications system. Larger business communications systems often
use time division switching, in which voice signals are first converted to a
digital format. Individual calls are then separated from one another on a
timing basis. Smaller business communications systems usually use space
division switching, in which individual calls are separated by discrete paths
through the switching network.	See  INTEGRATED CIRCUITS; MICROPROCESSOR;
SWITCHING SYSTEMS (COMMUNICATIONS).
    The evolution of the modern business communications system has prompted a
rapid transition from older-technology private branch exchanges. These include
manual private branch exchanges in which all switching functions were
performed by an attendant. Older-technology dial private branch exchanges
included step-by-step systems in which a series of switches were directly
controlled and calls advanced by the station user's dial. Other dial systems
used common control technology space-division-switching networks. In these
systems, overall system control was exercised by the high-speed common control
using either electromechanical techniques or wired logic electronic
technology.  See  TELEPHONE SERVICE.
 �34s[Wolfram Schiavoni]
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�6v�2v�32sButanol   �21m �33s 
    Any one of the four isomeric alcohols, liquids at room temperature, having 
the formula C4H9OH and molecular weight 74.12. The butanols are colorless, 
toxic, flammable materials which are soluble in most organic liquids; their 
solubilities in water depend upon their respective structures. The chemical 
properties of the butanols depend on the position of the hydroxyl group in the 
molecule. n-Butyl alcohol and isobutyl alcohol are primary alcohols and, 
therefore, may be oxidized to the respective aldehydes or carboxylic acids, 
whereas sec-butyl alcohol oxidizes to methyl ethyl ketone; tert-butyl alcohol 
cannot be oxidized without degradation. 
    The major use of the butanols is as chemical intermediates, although 
significant quantities are used directly as solvents. Important derivatives 
include plasticizers and acetate ester solvents for plastics and paints. See 
ALOCHOL.  
 �34s[John W. Lynn]
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�6v�2v�32sButter   �21m �33s 
    A food fat product made exclusively from milk or cream or both, with or 
without common salt and added coloring. Butter contains not less than 80% milk 
fat as well as 1 - 3% salt, 1% milk solids, and water. It is the second most 
important food fat. See MILK. 
    On the farm, processing is carried out by natural ripening of the cream, 
churning, and manually working to the desired consistency. Country butter is 
more variable in flavor then is creamery butter. 
    At the creamery, cream having a fat content of 30 - 40% is skimmed from 
milk by centrifuging. Pasteurization and spraying, or stripping, under vacuum 
remove undesirable flavors and odOrs and reduce action of microorganisms. 
Butter may be made from sweet or ripened cream. Ripening is carried out at 50 
- 70 - DGF (10 - 21.1DGC) by introduction of a starter prepared from a 
bacterial culture and pasteurized milk. After ripening overnight, color is 
added. In the batch process sweet or ripened cream is agitated in large rotary 
churns; this action converts the emulsion from water in oil to an oil-in-water 
type and coalesces the fat globules. Churns have internal baffles which lift 
and drop the contents to provide agitation and splashing. After churning to 
the desired consistency, the buttermilk is drawn off and the butterfat quickly 
washed with water. The butter is then worked between grooved rolls or with 
augurs which impart a kneading action. Salt is added during working, and the 
moisture content is adjusted. After it is worked, the butter is packaged or 
formed into prints and wrapped. Butter oil is made by heating butter to break 
the emulsion and settling or centrifuging to separate the milk serum from the 
fat. Moisture is reduced to a low level by drying, the butterfat content being 
over 99%. It is usually canned, is much more stable than butter, and does not 
require refrigeration. Ghee, a common food fat in India, is produced from 
boiled buffalo milk. It manufacture is similar to that of butter oil. It can 
be kept for months, or years, without refrigeration but has a more intense 
flavor than butter or butter oil. 
 �34s[Frank G. Dollear]
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�6v�2v�32sButterfly   �21m �33s 
    Insects which make up one of the two divisions of the order Lepidoptera. 
All butterflies have flattened, scalelike hairs, which cover the wings as well 
as the body and obscure the venation that is important in species 
identification. Butterflies have club-shaped antennae, which terminate in a 
knob, and mouth parts which are modified and adapted to their feeding habits. 
The mandibles, absent or vestigial in the adults, are well developed in the 
larvae since they are principally plant feeders. These insects undergo a 
complete metamorphosis, and most species produce a single generation each 
year. Generally, the pupal stage is the overwintering stage, although some 
overwinter as the egg or larval stage. Butterflies have a worldwide 
distribution, with the larger, more colorful species occurring in tropical 
regions. They are an interesting group biologically, since the phenomena of 
mimicry, protective coloration, warning coloration, and other protective 
adaptations can be studied. See CATERPILLAR; LEPIDOPTERA; MOTH; PROTECTIVE 
COLORATION.  
 �34s[Charles B. Curtin]
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�6v�2v�32sBuxbaumiales   �21m �33s 
    An order of mosses composed of three genera of very small plants, 
scattered or clustered on soil, rock, or rotten wood. The protonema is 
persistent. The stems are very short (about 0.04 in. or 1 mm high) or absent. 
The leaves are few and small. The seta varies from almost none to (0.8 in. or 
2 cm) high. The capsule in this order is quite distinctive and very large in 
comparison with the gametophores. It is asymmetrical, oblique, somewhat 
ventricose, flattened above, narrowed to a small mouth, resembling a bug on a 
stick or a grain of wheat. See BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sBytownite   �21m �33s 
    A member of the plagioclase feldspar solid-solution series with a 
composition ranging from Ab30An70 to Ab10An90 (Ab = NaAlSi3O8 and An = 
CaAl2Si2O8). Bytownite is very abundant in basic igneous rocks where it is the 
first plagioclase to crystallize under plutonic conditions, it forms the cores 
of zoned plagioclase phenocrysts in basaltic volcanics, and it sometimes 
occurs in anorthosites. See FELDSPAR. 
 �34s[Lawrence Grossman]
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�6v�2v�32sCabbage �21m �33s
    A hardy, cool-season crucifer (Brassica oleracea var. capitata) of
Medetirranean origin and belonging to the plant order Capparales. Cabbage is
grown for its head of overlapping leaves (see illustration), which are
generally eaten raw in salads, cooked fresh, or processed into sauerkraut.
Because it normally produces seed the second year, cabbage is considered to be
a biennial by most authorities. Others regard it a perennial because it will
remain vegetative unless subjected to cold weather.
    Chinese cabbage is a related annual of Asiatic origin. Two species are
grown in the United States, pe-tsai (B. pekinensis) and pakchoi (B.
chinensis). See ORIENTAL VEGETABLES.
    Cabbage varieties (cultivars) are generally classified according to season
of maturity, leaf surface (smooth, savoyed, or wrinkled), head shape
    FIGURE CAPTION.  Cabbage (brassica oleracea var. capitata), cultivar
Golden Acre 84. (Joseph Harris Co., Inc. Rochester, New York).
(flattened, round, or pointed), and color (green or red). Round,
smooth-leaved, green heads are commonest. Varieties differ in their resistance
to disease and in the tendency for heads to crack or split in the field.
    Texas and Florida are important winter crop producing states; Georgia,
Mississippi, and North Carolina produce large acreages in the spring; and New
York, North Carolina, and Wisconsin are important for the summer and fall
crops. New York and Wisconsin are the important kraut cabbage states. See
CAPPARALES.




 �34s[H. John Carew]
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�6v�2v�32sCacao �21m �33s
    Theobroma cacao, a small tropical tree (see illustration) that is
cultivated for the almond-shaped seeds which are used to make chocolate. The
species is native to the rainforest of the Amazon basin, and two regions of
distribution in pre-Columbian times are recognized. The crop was first
cultivated in Central America and northern South America, the varieties found
there being known as Criollos. The second region comprises the Amazon and
Orinoco basins, where the cacao populations are known as Amazonian Forastero.
The second type is more commonly cultivated, particularly in Brazil, Ivory
Coast, Ghana, and Nigeria.
    The produce is generally exported in the form of dry beans. The farmers'
production is purchased by dealers and exported by registered exporters or
government marketing boards. Sales are effected through contracts or futures
markets, principally in New York and London. The market distinguishes between
bulk and fine cocoas. The latter have preferred flavor or other
characteristics and receive a price premium. See COCOA POWDER AND CHOCOLATE;
THEALES.

 �34s[Paulo De T. Alvim]
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�6v�2v�32sCachexia �21m �33s
    Weight loss, weakness, and wasting of the body encountered in certain
diseases or in terminal illnesses. Cachexia is most often associated with some
malignancy and appears to be the actual cause of death in many cases. Such
death, at a specific time, is difficult to explain since often there are no
obvious specific events which affect vital organs. See ONCOLOGY.
    Cachexia predisposes to other disorders, principally infections. There is
little resistance to such processes and quite often the emaciated patient
dies, apparently as the result of a minor infection which would ordinarily not
even be incapacitating. See INFECTION.
    Extensive irradiation may also cause a cachexia. The symptoms may appear
at some variable time after exposure, sometimes not becoming pronounced for
several months. Recovery may occur or a chronic state of illness may ensue.
Usually death follows a gradual deterioration. See RADIATION BIOLOGY.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sCactus �21m �33s
    The common name for any member of the cactus family (Cactaceae). There are
120 genera with perhaps 1700 species, nearly all indigenous to America. The
cacti are among the most extremely drought-resistant plants, and consequently
they thrive in very arid regions. The group is characterized by a fleshy
habit, presence of spines and bristles, and large, brightly colored, solitary
flowers. There is a great variety of body shapes and patterns, and many of the
species are grown as ornamentals or oddities. A few have edible fruits. The
saguaro (Cereus giganteus) of Arizona and Sonora is the largest of the cacti,
attaining a height of 70 ft (21 m). See CARYOPHYLLALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCaddis fly   �21m �33s 
    The name for over 3600 species of insects which constitute the order 
Trichoptera. The adults are mothlike insects (see illustration), small to 
medium size, dull brown in color, and weak fliers. With one exception, 
Enoicyla, the immature stages are aquatic. The larvae are unusual in that many 
species construct a silklike case in which they live. These portable cases are 
of various shapes and are usually strengthened by impregnated material such as 
small stones, sand grains, bits of snail shells, and vegetation of various 
types. Other species live in small, silk, netlike structures where several 
individuals reside. All larvae spin a cocoon before pupation. The larvae live 
in almost all free-flowing streams and rivers in North America. Species that 
weave nets may play an important role in keeping the water free of pollutants 
by filtering out detritus and organic materials. See TRICHOPTERA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCadmium �21m �33s
    A relatively rare chemical element, symbol Cd, atomic number 48, closely
related to zinc, with which it is usually associated in nature. It is a
silvery-white ductile metal with a faint bluish tinge. It is softer and more
malleable than zinc, but slightly harder than tin. It has an atomic weight of
112.40 and a specific gravity of 8.65 at 20 DG C (68 DG F). Its melting point
of 321 DG C (610 DG F) and boiling point of 765 DG C (1410 DG F) are lower than
those of zinc. There are eight naturally occurring stable isotopes, and eleven
artificial unstable radio isotopes have been reported. Cadmium is the middle
member of group IIb (zinc, cadmium, and mercury) in the periodic table, and
its chemical properties generally are intermediate between zinc and mercury.
The cadmium ion is displaced by zinc metal in acidic sulfate solutions.
Cadmium is bivalent in all its stable compounds, and its ion is colorless. See
TIN; ZINC.
    Cadmium does not occur uncombined in nature, and the one true cadmium
mineral, greenockite (cadmium sulfide), is not a commercial source of the
metal. Almost all of the cadmium produced is obtained as a by-product of the
smelting and refining of zinc ores, which usually contain 0.2 - 0.4% cadmium.
The United States, Canada, Mexico, Australia, Belgium-Luxembourg, and the
Republic of Korea are principal sources, although not all are producers.
    At one time an important commercial use of cadmium was as an
electrodeposited coating on iron and steel for corrosion protection.
Nickel-cadmium batteries are the second-largest application, with pigment and
chemical uses third. Sizable amounts are used in low-melting-point alloys,
similar to Wood's metal, and in automatic fire sprinklers, and relatively
smaller uses are in brazing alloys, solders, and bearings. Cadmium compounds
are used as stabilizers in plastics and the production of cadmium phosphors.
Because of its great neutron-absorbing capacity, especially the isotope 113,
cadmium is used in control rods and shielding for nuclear reactors. See ALLOY;
CADMIUM DISEASE; CADMIUM METALLURGY.
 �34s[Wilbur Hague]
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�6v�2v�32sCadmium disease �21m �33s
    An illness caused by exposure to cadmium or its compounds and solutions.
Cadmium is very slowly absorbed from the gastrointestinal tract and is
eliminated in both urine and feces. In healthy human adults the cadmium levels
are low, except for the kidneys, which are uniquely high in cadmium. With
increased age, the tissue cadmium level progressively increases.
    Ingestion of 15 ppm of cadmium in food, or about 15 times the normal
amount, can produce mild symptoms of poisoning. Food poisonings from
contamination of food and drink by cadmium-plated containers have been
recorded frequently. However, the principal human risk is from industrial
exposure, namely, inhalation of cadmium fumes. Cadmium is primarily a
respiratory poison and has higher lethal potential than most other metals,
with a mortality rate of 15% in poisoning cases.
    Mild cadmium intoxication causes smarting of the eyes, dryness and
irritation of the throat, tightness in the chest, and headache. With increased
exposure, the respiratory distress increases with uncontrollable cough and
gastrointestinal pain, nausea, vomiting, and diarrhea.
    Tissue reaction to the inhalation of cadmium fumes results in the initial
deposition of cadmium in the lungs. From there the metal is widely distributed
throughout the body and accumulates in the liver, kidneys, and spleen. The
high concentrations may remain in the tissues for many years after the
cessation of exposures. Excretion in the urine is extremely low. Another
consequence of chronic cadmium exposure is an increased brittleness of bone,
to an extent that the mere act of coughing may produce fractures of the ribs.
See CADMIUM.
 �34s[N. Karle Mottet]
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�6v�2v�32sCadmium metallurgy   �21m �33s 
    Most cadmium occurs in solid solution in the zinc sulfide mineral called 
sphalerite. Although cadmium may be recovered from some lead and copper ores, 
it is associated with the zinc which is also found in these ores. Since 
cadmium is entirely a by-product metal, the supply available is closely 
aligned with zinc production, averaging about 0.4% of zinc production. 
    All cadmium recovery processes involve the dissolution of cadmium-bearing 
feed material, followed by various purification and cadmium displacement 
steps. Methods of processing can be grouped conveniently into two basic 
categories, electrolytic and electromotive. In the former case, cadmium is 
recovered by electrolyzing purified solutions; in the latter case, cadmium in 
the form of a metallic sponge is displaced from purified solutions by a less 
noble metal, zinc being used in every known commercial application, and the 
sponge is melted or distilled, or both. 
    Major end uses for cadmium are in corrosion-resistant plating and in 
cadmium compounds for use as pigments in paints, ceramics, and plastics. 
Cadmium is also used in alloys, plastic stabilizers, batteries, and television 
picture tube phosphors. In view of the changing world energy situation, uses 
for cadmium are anticipated in applications such as solar energy cells and 
energy storage systems. See CADMIUM. 
 �34s[Robert E. Lund]
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�6v�2v�32sCaffeine �21m �33s
    An alkaloid, formerly synthesized by methylation of theobromine isolated
from cacao, but now recovered from the solvents used in the manufacture of
decaffeinated coffee. It is widely used in medicine as a stimulant for the
central nervous system and as a diuretic. It occurs naturally in tea, coffee,
and yerba mate, and small amounts are found in cola nuts and cacao. Caffeine
crystallizes into long, white needlelike crystals that slowly lose their water
of hydration to give a white solid that melts at 235 - 237.2 DG C (455 -
459.0 DG F). It sublimes without decomposition at lower temperatures. Caffeine
has an intensely bitter taste, though it is neutral to litmus. See ALKALOID.
 �34s[Frank Wagner]
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�6v�2v�32sCaiman   �21m �33s 
    A large aquatic reptile of the family Alligatoridae. There are five 
species of caimans, once often sold as ``baby alligators.'' Caimans differ 
from alligators in technical details of their internal anatomy and scalation. 
    The black caiman (Melanosuchus niger) of the Amazon Basin of South America 
strongly resembles the American alligator in superficial appearance and may 
reach a length approaching 16 ft (4.8 m). The spectacled caiman (Caiman 
crocodilus), with its several subspecies, is the one sold in the pet trade. It 
is the most widely distributed of the caimans, ranging from southwestern 
Mexico to Paraguay and Bolivia. One subspecies, C. c. yacare, is recognized as 
endangered. Spectacled caimans reach a length of 8 ft (2.4 m) and can be 
distinguished from alligators by the presence of a ridge across the snout 
between their eyes, the nose bridge of the ``spectacles.'' 
    The broad-nosed caiman (C. latirostris) occurs from southern Brazil to 
Paraguay and northern Argentina and reaches a length of 9 ft (2.7 m; see 
illustration). The two species of smooth-fronted caiman, Paleosuchus 
trigonatus and P. palpebrosus, are the smallest of the family, reaching a 
maximum length of only 7 ft (2.1 m). They occur over most of northern South 
America, south to Peru, Bolivia, and southeastern Brazil. See ALLIGATOR. 

      FIGURE CAPTION.   Broad-nosed caiman (Caiman latirostris).
                           
    Alligators are generally distinguished from crocodiles by their broader, 
rounded, and more massive snout, the arrangement of their teeth, and other 
technical anatomical details. The teeth are conical and sharp, equivalent in 
shape (homodont), and replaced throughout life. Moreover, they possess a 
functionally four-chambered heart, a secondary palate, a septum or 
``diaphragm'' that separates the lung and peritoneal regions, a compressed 
tail, webbed feet, and other adaptations for an aquatic existence. See 
CROCODILIA; REPTILIA. 
 �34s[Howard W. Campbell]
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�6v�2v�32sCaisson foundation �21m �33s
    A permanent substructure that, while being sunk into position, permits
excavation to proceed inside and also provides protection for the workers
against water pressure and collapse of soil. The term caisson covers a wide
range of foundation structures. Caissons may be open, pneumatic, or floating
type; deep or shallow; large or small; and of circular, square, or rectangular
cross section. The walls may consist of timber, temporary or permanent steel
shells, or thin or massive concrete. Large caissons are used as foundations
for bridge piers, deep-water wharves, and other structures. Small caissons are
used singly or in groups to carry such loads as building columns. Caissons are
used where they provide the most feasible method of passing obstructions,
where soil cannot otherwise be kept out of the bottom, or where cofferdams
cannot be used. See BRIDGE; PILE FOUNDATION.
    The bottom rim of the caisson is called the cutting edge (see
illustration). The edge is sharp or narrow and is made of, or faced with,
structural steel. The narrowness of the edge facilitates removal of ground
under the shell and reduces the resistance of the soil to descent of the
caisson.
    An open caisson is a shaft open at both ends. It is used in dry ground or
in moderate amounts of water. A pneumatic caisson is like a box or cylinder in
shape; but the top is closed and thus compressed air can be forced inside to
keep water and soil from entering the bottom of the shaft. A pneumatic caisson
is used where the soil cannot be excavated through open shafts or where soil
conditions are such that the upward pressure must be balanced. A floating or
box caisson consists of an open box with sides and closed bottom, but no top.
It is usually built on shore and floated to the site where it is weighted and
lowered onto a bed previously prepared by divers. See FOUNDATIONS.
 �34s[Robert D. Chellis]
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�6v�2v�32sCalamine �21m �33s
    A term that may refer to either a zinc mineral, Zn4Si2O7(OH)2 CTR DOT 2O,
which is also known as hemimorphite, or to zinc oxide, ZnO, which is used in
medicinal or pharmaceutical products and in cosmetics. See HEMIMORPHITE.
 �34s[E. Eugene Weaver]
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�6v�2v�32sCalamitales �21m �33s
    Extinct group of reedlike plants of the late Paleozoic which propagated by
spores and belongs to the Sphenopsida (Equisetophyta). The plants were of
shrub or tree size, and most species developed wood (secondary xylem). Stems
and branches were characterized by a jointed appearance. Cones, branches, and
roots were attached only on the nodes (joints), and leaves appeared as whorls
on the nodes of stems and branches. The stems had a hollow pith which was
often filled with sediment after the death of the plant, forming the commonly
found pith casts. Calamites were a common element of floras in the
coal-producing swamps and in nonswamp environments of Carboniferous
(Mississippian and Pennsylvanian) and Permian time. Calamites are relatively
similar to the living genus Equisetum (scouring rush, horsetail) in several
aspects of anatomy and appearance (see illustration). See EQUISETOPHYTA;
SPHENOPSIDA.




 �34s[Hermann W. Pfefferkorn]
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�6v�2v�32sCalanoida �21m �33s
    An order of Copepoda that includes the larger and more abundant of the
pelagic species. Some authorities consider the Calanoida an order of the
subclass Copepoda. In the food cycles of the sea these copepods are the most
important group of marine animals because of their overwhelming numbers,
ubiquitous distribution, and position at the base of the animal food chain.
See COPEPODA.
    The anterior part of the body is cylindrical with five or six segments,
and much broader than the posterior part. The first antennae are not for
locomotion, but are stabilizers and sinking retarders, and also have an
olfactory function. The second antennae and mandibular palps are biramous and
create water currents for feeding and slow movement. The five pairs of
swimming legs are biramous, but the last pair is sometimes reduced or absent
in the female, and the male's right fifth leg may be modified for grasping the
female (see illustration).
    Nearly all calanoids are planktonic and, as a group, occur in all parts of
the oceans from the surface to abyssal depths. The geographic and bathymetric
ranges of many species are, however, restricted by the nature of water
currents and the chemical and physical conditions of the water. In their
southernmost range the northern species are found at greater depths.




 �34s[Harry C. Yeatman]
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�6v�2v�32sCalcarea   �21m �33s 
    A class of the phylum Porifera, including sponges with a skeleton composed 
of spicules of calcium carbonate. Calcarea vary from radially symmetrical 
vase-shaped species to colonies made up of a reticulum of thin tubes to 
irregular massive forms. Calcareous sponges are mostly of small size and 
inhabit the shallow waters of all seas, from tidal areas to depths of 600 ft 
(200 m), with a few species extending down to at least 12,000 ft (4000 m). 
    Primitive calcareous sponges with an ascon grade of construction consist 
of colonies of upright tubes with unfolded walls made up of an outer epidermis 
of pinacocytes and an inner lining of choanocytes. Between these layers of 
cells is a stratum of mesoglea containing amebocytes and spicules. Cells 
called porocytes, each perforated by a tubular canal, pierce the walls at 
intervals and allow water to enter the central cavity or spongocoel. Water 
leaves by way of a terminal osculum. 
    A somewhat more complicated structure is seen in calcareous sponges of the 
sycon grade of construction. Syconoid sponges are usually individual 
vase-shaped forms with a thick wall enclosing a large central spongocoel 
opening out through a terminal osculum. In the simplest forms the wall is 
pushed out at intervals into fingerlike projections, called radial canals, in 
which the choanocytes are localized. Water enters the radial canals directly 
through pores without the intervention of special inhalant canals. In most 
syconoid species, however, a dermal membrane made up of pinacocytes and 
mesenchyme forms a cortex of greater or less thickness which joins the outer 
ends of the radial canals. Pores or ostia pierce the dermis and open into 
inhalant canals which are simply the spaces between the radial canals in some 
cases.  
    The leuconoid grade of construction has probably evolved independently 
among the several lines of calcareous sponges. In those with a syconoid 
ancestry, the radial canals subdivide into many small flagellated chambers 
which arise as outpocketings of the radial canal wall. 
 �34s[Willard D. Hartman]
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    Fossils clearly referable to the class Calcarea do not appear before the 
Carboniferous, later than any other class of sponges. The dominant Calcarea 
preserved from the late Paleozoic (Carboniferous and Permian periods) are the 
Sphinctozoa. In the Permian probable Calcinea with the pharetronid type of 
skeleton first appear. Both the Sphinctozoa and the pharetronids become highly 
varied and abundant in the Permian and Triassic periods. Calcareous sponges 
remained fairly abundant in shallow water and reefy deposits until the end of 
the Cretaceous. After that time the Sphinctozoa became extinct and the 
pharetronids became gradually less abundant. See PORIFERA. 
 �34s[Robert M. Finks]
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�6v�2v�32sCalcarenite   �21m �33s 
    A mechanically deposited (clastic) limestone in which the constituent 
particles of carbonate are sand-grain size. If the grains are larger than 0.08 
in. (2 mm), the rock is a calcirudite. If the grains are smaller than 0.0025 
in (6.25 mm), it is a calcilutite. The calcarenites are normally cemented by 
clear calcite. The detrital carbonate grains are fossil fragments, broken 
fragments of calcilutite, oolites, and fecal pellets. Where oolites are 
dominant, the rock is called an oolite or oolitic limestone. 
    The fine-grained equivalent of calcarenite, calcilutite, is called 
lithographic limestone if it is homogeneous, dense, and very-fine-grained and 
breaks with conchoidal or subconchoidal fracture; the name is derived from the 
use of the rock for lithography. See LIMESTONE. 
 �34s[Raymond Siever]
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�6v�2v�32sCalcaronea   �21m �33s 
    A subclass of sponges of the class Calcarea, in which the larvae are 
amphiblastulae. This subclass may be divided into the orders Leucosoleniida 
and Sycettida. The triradiate spicules in species of this subclass 
characteristically have one ray longer than the other two. Monaxonid spicules 
are usually present. The canal system is asconoid, syconoid, or leuconoid. The 
oldest known fossil member of this group preserved entire is Leucandra 
walfordi, of Jurassic age. See CALCAREA; LEUCOSOLENIIDA; SYCETTIDA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sCalcinea   �21m �33s 
    A subclass of sponges of the class Calcarea, in which the larvae are 
blastulae. Most species possess at least some triradiate spicules which have 
equal rays and equal angles between the rays. Monaxonid spicules are present 
or absent. The canal system is leuconoid. Included in this subclass are two 
orders: the Clathrinida and Leucettida. See CALCAREA; CLATHRINIDA; LEUCETTIDA. 
 �34s[Willard D. Hartman]
�33s 
    This group has a rich fossil record based on the assignment of the 
pharetronid sponges to the Calcinea. It begins somewhat modestly in the 
Permian Period, expands in number and variety through the Cretaceous Period, 
and then slowly contracts to their present obscurity. Some 70 genera of 
pharetronids are known from the Cretaceous, and about one-seventh as many 
today. The fossil record of nonpharetronid Calcinea is practically nonexistent. 
 �34s[Robert M. Finks]
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�6v�2v�32sCalcite �21m �33s
    A mineral with the chemical formula CaCo3; one of the most common and
widespread minerals in the Earth's crust. Calcite may be found in a great
variety of sedimentary, metamorphic, and igneous rocks. It is also an
important rock-forming mineral and is the sole major constituent in
limestones, marbles, and many carbonatites. Calcite in such rocks is the main
source of the world's quicklime and hydrated, or slaked, lime. It is also
widely used as a metallurgical flux to a scavenge siliceous impurities by
forming a slag in smelting furnaces. It provides the essential calcium oxide
component in common glasses and cement. Limestones and marbles of lower purity
may find uses as dimension stone, soil conditioners, industrial acid
neutralizers, and aggregate in concrete and road building. Calcite in
transparent well-formed crystals is used in certain optical instruments. See
CRYSTAL OPTICS; GLASS AND GLASS PRODUCTS; LIME (INDUSTRY); LIMESTONE; STONE
AND STONE PRODUCTS.
    When pure, calcite is either colorless or white, but impurities can
introduce a wide variety of colors: blues, pinks, yellow-browns, greens, and
grays have all been reported. Hardness is 3 on Mohs scale. The specific
gravity of pure calcite is 2.7102 PLUS/MINUS 0.0002 at 68 DG F (20 DG C).
Calcite has a very low solubility in pure water (less than 0.001% at 77 DG F
or 25 DG C), but the solubility increases considerably with CO2 added, as in
natural systems from the atmosphere, when more bicarbonate ions and carbonic
acid are formed. The solubility is also increased by falling temperature and
rising total pressure. Shallow warm seas are supersaturated with calcite,
while enormous quantities of calcite are dissolved in the unsaturated deep
oceans.
See CALCIUM; CARBONATE MINERALS.
 �34s[Robert I. Harker]
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�6v�2v�32sCalcium �21m �33s
    A chemical element, Ca, of atomic number 20, fifth among elements and
third among metals in abundance in the Earth's crust. Calcium compounds make
up 3.64% of the Earth's crust. The physical properties of calcium metal are
given in the table. The metal is trimorphous and is harder than sodium, but
softer than aluminum. Like beryllium and aluminum, but unlike the alkali
metals, it will not cause burns on the skin. It is less reactive chemically
than the alkali metals and the other alkaline-earth metals.
    Occurrence of calcium is very widespread; it is found in every major land
area of the world. This element is essential to plant and animal life, and is
present in bones, teeth, eggshell, coral, and many soils. Calcium chloride is
present in sea water to the extent of 0.15%. See CALCIUM CARBONATE; CARBONATE
MINERALS.
    Calcium metal is prepared industrially by the electrolysis of molten
calcium chloride. Calcium chloride is obtained either by treatment of a
carbonate ore with hydrochloric acid or as a waste product from the Solvay
carbonate process. The pure metal may be machined in a lathe, threaded, sawed,
extruded, drawn into wire, pressed, and hammered into plates. See
ELECTROMETALLURGY.
    In air, calcium forms a thin film of oxide and nitride, which protects it
from further attack. At elevated temperatures, it burns in air to form largely
the nitride. The commercially produced metal reacts easily with water and
acids, yielding hydrogen that contains noticeable amounts of ammonia and
hydrocarbons as impurities.
    The metal is employed as an alloying agent for aluminum-bearing metal, as
an aid in removing bismuth from lead, and as a controller for graphitic carbon
in cast iron. It is also used as a deoxidizer in the manufacture of many
steels, as a reducing agent in preparation of such metals as chromium,
thorium, zirconium, and uranium, and as a separating material for gaseous
mixtures of nitrogen and argon.
    Calcium oxide CaO, is made by the thermal decomposition of carbonate
minerals in tall kilns using a continuous-feed process. The oxide is used in
high-intensity arc lights (limelights) because of its unusual spectral
features and as an industrial dehydrating agent. The metallurgical industry
makes wide use of the oxide during the reduction of ferrous alloys.
    Calcium hydroxide, Ca(OH)2, is used in many applications where hydroxide
ion is needed. During the slaking process for producing calcium hydroxide, the
volume of the slaked lime Ca(OH)2 produced expands to twice that of quicklime
(CaO), and because of this, it can be used for the splitting of rock or wood.
Slaked lime is an excellent absorbent for carbon dioxide to produce the very
insoluble carbonate. See LIME (INDUSTRY).
    Calcium slicide, CaSi, an electric-furnace product made from lime, silica,
and a carbonaceous reducing agent, is useful as a steel deoxidizer. Calcium
carbide, CaC2, is produced by heating a mixture of lime and carbon to 5432 DG F
(3000 DG C) in an electric furnace. The compound is an acetylide which yields
acetylene upon hydrolysis. Acetylene is the starting material for a great number
of chemicals important in the organic chemicals industry.
    Pure calcium carbonate exists in two crystalline forms: calcite, the
hexagonal form, which possesses the property of birefringence, and aragonite,
the rhombohedral form. Naturally occurring carbonates are the most abundant of
the calcium minerals. Iceland spar and calcite are essentially pure carbonate
forms, whereas marble is a somewhat impure and much more compact variety
which, because it may be given a high polish, is much in demand as a
construction stone. Although calcium carbonate is quite insoluble in water, it
has considerable solubility in water containing dissolved carbon dioxide,
because in these solutions it dissolves to form the bicarbonate. This fact
accounts for cave formation in which limestone deposits have been leached away
by the acidic ground waters. See CARBONATE MINERALS.
    The halides of calcium include the phosphorescent fluoride, which is the
most widely distributed calcium compound and which has important applications
in spectroscopy. Calcium chloride has in the anhydrous form important
deliquescent properties which make it useful as an industrial drying agent and
as a dust quieter on roads. Calcium chloride hypochlorite (bleaching powder)
is produced industrially by passing chlorine into slaked lime, and has been
used as a bleaching agent and a water purifier. See BLEACHING; CHLORINE.
    Calcium sulfate dihydrate is the mineral gypsum. It constitutes the major
portion of portland cement, and has been used to help reduce soil alkalinity.
A hemihydrate of calcium sulfate, produced by heating gypsum at elevated
temperatures, is sold under the commercial name plaster of paris. See PLASTER
OF PARIS.
    Calcium is an invariable constituent of all plants because it is essential
for their growth. It is contained both as a structural constituent and as a
physiological ion. Calcium is found in all animals in the soft tissues, in
tissue fluid, and in the skeletal structures. The bones of vertebrates contain
calcium as calcium fluoride, as calcium carbonate, and as calcium phosphate.
See CALCIUM METABOLISM.



 �34s[Reed F. Riley]
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�6v�2v�32sCalcium metabolism   �21m �33s 
    Calcium, an essential inorganic element needed by all animals and animal 
cells, subserves four types of functions in mammals: (1) as an integral part 
of bone mineral; (2) as an integral part of biomembranes; (3) as the link 
between excitation and contraction in all forms of muscle, and similarly as 
the link between excitation and secretion in both exocrine and endocrine 
glands; and (4) as a regulator, activator, or inhibitor of key enzymes. See 
BIOELECTRIC MODEL; BLOOD; BONE; MUSCLE (BIOPHYSICS). 
    Because of the critical importance of calcium in many of these fundamental 
processes, its concentration in plasma is closely regulated by the organism. 
However, this regulation is complex because over 90% of the calcium in the 
body resides in bone. It is further complicated by an intimate relationship 
with phosphate metabolism. The two needs of ensuring a sufficient supply of 
both calcium and phosphates are met by the interrelated actions of the 
parathyroid hormone, thyrocalcitonin, and vitamin D. 
    A fall in blood calcium, hypocalcemia, leads to tetany, the spontaneous 
firing of nerves with resultant continuous, uncontrolled muscular contraction. 
A rise in blood calcium, hypercalcemia, leads to calcification of the kidney 
with eventual uremia. See KIDNEY; PHOSPHATE METABOLISM; PARATHYROID HORMONE; 
SKELETAL SYSTEM; THYROCALCITONIN; THYROID HORMONE; VITAMIN D. 
 �34s[Howard Rasmussen]
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�6v�2v�32sCalculators �21m �33s
    Portable or desktop devices, primarily electronic, that are used to
perform arithmetic and other numerical processing operations at the direction
of an operator or a stored program. Early calculators were exclusively
mechanical. Although some printing 10-key mechanical calculators are used in
offices and small businesses, mechanical calculators have been almost
completely replaced by quieter, more efficient, and more capable electronic
units.
    Inexpensive hand-held calculators employing large-scale integrated
circuits as processing elements are in wide use. Inexpensive arithmetic units
are available, while more expensive models provide a wide range of complex
operations. The rapid evolution of the electronic calculator has been made
possible by technological improvements in integrated circuits. An integrated
circuit with an area of less than 0.5 in.(sup 2) [3 cm(sup 2)] can hold all the
circuit elements needed to implement the algorithms and the control and timing
functions of a calculator. Forty or more algorithms may be held in memory, to
be retrieved and used when requested by depression of a function key. A
hand-held calculator may use a single integrated circuit to interpret
key-switch closures, carry out the requested operation, and multiplex the
individual digits of the result to the display. See INTEGRATED CIRCUITS.
    Any calculator can be considered a computer in that it contains in
nonvolatile memory a fixed set of algorithms for execution of the processing
operations in its repertoire. These algorithms are available whenever power is
applied to the unit. Programmable calculators are able to accept and act upon
a higher level of programming that directs the units through sequences of
processing steps without operator intervention other than operand entry.
Programming may be done by the operator or, in advance, by the manufacturer,
and storage of the program may be volatile or nonvolatile. Programming
relieves the drudgery of repeated keystroke entry when a repetitive
calculation must be made.
    Programming by the operator on the unit to be used for the calculation is
known as keystroke programming. The desired sequence of operations is entered
into calculator memory by depression of keys as though a calculation were
being made, but with the unit in program mode. Each keystroke enters an
instruction into the semiconductor memory of the unit. When the unit is
returned to the normal run mode, the program sequence can be initiated by the
operator through depression of a special function key on the keyboard.
    The semiconductor storage in a calculator is usually volatile - program
information is lost when power is removed. Some units can accept magnetic tape
cartridges or miniaturized magnetic cards, from which the volatile memory can
be loaded and to which the memory contents can be written. The magnetic medium
provides a means of saving and reentering a program written by the keystroke
method. Manufacturers also offer preprogrammed tapes and cards for specialized
calculations in a wide range of fields.
    Several programmable calculators accept plug-in elements such as memory
extender modules, read-only memory modules for specialized computations,
magnetic card readers, and printers (see illustration). Programmer assistance
in the form of prompting is provided in an alphanumeric display - a display
capable of showing letters as well as numbers. A calculator with these
capabilities could be likened to a numerical processing computer with
peripheral devices. As the range of calculator expansion elements increases
and calculation capabilities increase, the differences between calculators and
computers will become even less distinct. See COMPUTER; DIGITAL COMPUTER.
 �34s[W. W. Moyer]
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�6v�2v�32sCalculus �21m �33s
    The branch of mathematics dealing with two fundamental operations,
differentiation and integration, which are carried out on functions. The
subject, as tradtionally developed in college textbooks, is partly an
elementary development of the purely theoretical aspects of these operations
and their interrelation, partly a development of rules and formulas for
applying calculus to the standard functions which arise in algebra and
trigonometry (with exponentials and logarithms included), and partly a
collection of applications to problems of geometry, physics, chemistry,
engineering, economics, and perhaps a few other subjects.
    The fundamental concept of differential calculus is that of the derivative
of a function of one variable. The classical physical prototype of this
concept is that of instantaneous velocity, which is the derivative of distance
as a function of time. The derivative also has a highly significant
geometrical realization which depends upon the graphical representation of a
function is retangular coordinates (x,y). If y is a differentiable function of
x, perhaps as x increases from x sub 1 to x sub 2, the graph of the function is
a continuous curve with exactly one y for each x, and at each point the curve
has a tangent line which is not parallel to the y axis. If PHI is the angle,
measured counterclockwise, from the positive x direction to the tangent (see
Fig. 1), then tan PHI is equal to the derivative of y with respect to x. (This
is on the supposition that the same unit of length is used along the two
axes.) This tan PHI is also called the slope of the curve.
    The standard notation for the derivative of y with respect to x is dy/dx.
If the functional notation y = f(x) is used, the derivative is often denoted
by f'(x). See DIFFERENTIATION; LAW OF THE MEAN.
    If f is a function defined on the finite interval from x sub 1 to x sub 2
inclusive, the definite integral of f from x sub 1 to x sub 2, denoted by

	     INTEGRATION (x sub 1 to x sub 2) f(x) dx

is defined by applying to f a rather intricate process which entails the
consideration of what are called approximating sums. When the function f is
subjected to certain restrictions, this process culminates in the
determination of a number as the limit of the approximating sums, and this
number is called the definite integral of f from x sub 1 to x sub 2. The
integral is not defined unless the approximating sums do converge to a
well-defined limit. A sufficient condition that this be so is that the
function f be continuous.
    There is a geometrical representation of the process of defining the
definite integral, and it furnishes a plausible argument for the convergence
of the approximating sums to a limit. Divide the interval from x sub 1 to x sub
2 into a finite number N of not necessarily equal parts. Let the lengths of
these parts be h sub 1, h sub 2, . . . h sub N, and let t sub k be the value of
x in the kth part (see Fig. 2). Then the expression 

     f(t sub 1)h sub 1 + f(t sub 2)h sub 2 + . . . + f(t sub N)h sub N

is called an approximating sum. In Fig. 2, where the function is continuous
and the function values are all positive, each term f(t sub k)h sub k in the
approximating sum is equal to the area of a certain shaded rectangle, and the
whole sum is an approximation of the area between the graph of the function
and x axis, from x sub 1 to x sub 2 inclusive. The limiting process is carried
on by the increasing N and making the largest of the h sub k's approach 0. It is
then intuitively clear that the definite integral is the number which represents
the exact area between the x axis and the graph. This geometrical interpretation
of the integral is the basis of an important application of integral calculus,
to the calculation of areas.
    It would be tedious and difficult in practice to compute definite
integrals by actually working out the limits of approximating sums. It is
therefore fortunate that by purely mathematical reasoning it is possible to
demonstrate a theorem which links derivatives and integrals and makes it
possible, in many important instances, to compute definite integrals by an
easier procedure. See INTEGRATION.
    The two fundamental theorems of calculus are as follows:
    1. For the calculation of

	INTEGRATION (x sub 1 to x sub 2) f(x) dx

find, if possible, a function F with continuous derivative F' such that F'(x)
= f(x) when x1 </= x </= x2.  The Eq. (1)

     INTEGRATION (x sub 1 to x sub 2) f(x) dx = F(x sub 2) - F(x sub 1)     (1)

can be written. This is one of the two central theorems.
    2. Suppose f is continuous, and consider the function F defined by Eq. (2).

	  F(x) = INTEGRATION (x sub 1 to x sub 2) f(t) dt		   (2)

Then F has a derivative given by F'(x) = f(x).


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�6v�2v�32sCalculus of tensors �21m �33s
    The systematic study of tensors which led to an extension and
generalization of vectors, begun in 1900 by two Italian mathematicians, G.
Ricci and T. Levi-Civita, following G. F. B. Riemann's proposal concerning a
generalization of euclidean geometry. The principal aim of the tensor calculus
(absolute differential calculus) is to construct relationships which are
generally covariant in the sense that these relationships or laws remain valid
in all coordinate systems. The differential equations for the geodesics in a
Riemannian space are covariant expressions; they yield a description of the
geodesics which is valid for all coordinate systems. On the other hand,
Newton's equations of motion require a preferred coordinate system for their
description, namely, one for which force is proportional to acceleration (an
inertial frame of reference). Thus Albert Einstein was led to a study of
Reimannian geometry and the tensor calculus in order to construct the general
theory of relativity. See CALCULUS OF VECTORS; RIEMANNIAN GEOMETRY.
 �34s[Harry Lass]
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�6v�2v�32sCalculus of variations �21m �33s
    An extension of the part of differential calculus which deals with maxima
and minima of functions of a single variable.  The functions of the calculus of
variations depend in an essential way upon infinitely many independent
variables.  Classically these functions are usually integrals whose integrand
depends on a function whose specification by any finite number of parameters is
impossible.  For example, let R be a smooth bounded region of a space of m
variables, x1, x2, .  .  ., x(sub m), let y be any function of some smooth class
on R and its boundary into real numbers or into n-tuples of real numbers and
taking specified values on the boundary, and let f(x,y,p) be a smooth function
of 2m + 1 variables x1, x2, .  .  ., x(sub m), y, p1, p2, .  .	., p(sub m).
Then the integral, shown in the equation, is a function on the space of
functions y to the real numbers, and this space of functions is infinitely
dimensional unless excessive restrictions are placed on it.  Here y(sub x)
denotes the derivatives dy/dx.

      J = INTEGRATION . . . INTEGRATION (in terms of R) f(x,y, y[sub x])

    The calculus of variations studies such functions and their maxima and
minima. The limitation of the competing functions is made realistically, and
with sufficient restrictions it is possible to arrive at a rewarding theory;
these restrictions do not always include the fixed boundary conditions stated
above.
    Principal applications may be to physical systems involving flexible
components or time-dependent orbits; equilibrium positions or orbits may be
determined by minimizing energy or action integrals. The problems are of
mathematical interest because of intrinsic difficulties (largely related to
lack of topological compactness of bounded regions in spaces of infinitely
many dimensions) and possibly because more progress with difficult non-linear
problems has been made here than elsewhere. See DIFFERENTIATION; INTEGRATION.
 �34s[Charles B. Tompkins]
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�6v�2v�32sCalculus of vectors �21m �33s
    In its simplest form, a vector is a directed line segment. Physical
quantities, such as velocity, acceleration, force, and displacement, are
vector quantities, or simply vectors, because they can be represented by
directed line segments. Vector analysis is a tool of the mathematical
physicist, because many physical laws can be expressed in vector form.
    Two vectors BOLD a and BOLD b are added according to the parallelogram law
(Fig. 1).
    An equivalent definition is as follows:  From the end point of BOLD a, a
vector is constructed parallel to BOLD b, of the same magnitude and direction as
BOLDb.	The vector from the origin of BOLD a to the end point of BOLD b yields
the vector sum BOLD s = BOLD a + BOLD b (Fig.  2).  Any number of vectors can be
added by this rule.  Given a vector BOLD a, a class of vectors can be formed
which are parallel to a but of different magnitudes.  If x is a real number, the
vector x(BOLD a) is defined to be parallel to BOLD a of magnitude |x| times that
of BOLD a.  For x > 0, the two vectors BOLD a and x(BOLD a) have the same sense
of direction, whereas for x < 0 the vector x(BOLD a) is in a reverse direction
from that of BOLD a.  The vector BOLD -a is the negative of the vector BOLD a,
such that BOLD a + (BOD -a) = 0, with 0 designated as the zero vector (a vector
with zero magnitude).  Subtraction of two vectors is defined by Eq.  (1).

		     BOLD a - BOLD b = BOLD b + (-BOLD b)		    (1)

    The cartesian coordinate frame of analytic geometry is very useful for
yielding a description of a vector (Fig. 3). The unit vectors BOLD i, BOLD j,
BOLD k lie parallel to the positive x, y, and z axes, respectively.  Any vector
can be written as a linear combination of BOLD i, BOLD j, BOLD k.  From Fig. 
3, it is noted that Eq.  (2) holds.  Furthermore, the scalars 

	BOLD a = a(sub x)BOLD i + a(sub y)BOLD j + a(sub z)BOLD k	     (2)

a(sub x), a(sub y), a(sub z) are simply the projections of BOLD a on the x, y,
and z axes, repectively, and are designated as the components of BOLD a.  Thus,
a(sub x) is the x

alpha BOLD a + beta BOLD b =
(alpha a[sub x] + beta b[sub x])BOLD i + (alpha a[sub y] + beta b[sub y])BOLD j
  + (alpha a[sub z] + beta b[sub z])BOLD k				     (3)

component of BOLD a, and so on. If the vector BOLD b is described by BOLD b =
b(sub x) BOLD i + b(sub y) BOLD j + b(sub z) BOLD d, then Eq. (3) holds.
    From two vectors BOLD a and BOLD b, a scalar quantity is formed from the
definition in Eq. (4), where theta is the angle between the two

       BOLD a DOT BOLD b = |BOLD a| DOT |BOLD b|cos theta		    (4)

vectors when drawn from a common origin (Fig. 4).
    In three-dimensional space, a vector can be formed from two vectors BOLD a
and BOLD b in the following manner if they are nonparallel.  Let BOLD a and BOLD
b have a common origin defining a plane, and let BOLD c be that vector
perpendicular to this plane of magnitude |BOLD c| = |BOLD a| |BOLD b| sin theta.
If BOLD a is rotated into BOLD b through the angle theta, a right-hand screw
will advance in the direction of BOLD c (Fig.  5).  Thus Eq.  (5) can be written

BOLD c = BOLD a CROSS PRODUCT BOLD b = |BOLD a| |BOLD b| sin theta BOLD n   (5)

    The distributive law can be shown to hold for the vector product so that
Eq. (6) holds.

(BOLD a + BOLD b) CROSS PRODUCT (BOLD c + BOLD d)
   = BOLD a CROSS PRODUCT BOLD c + BOLD a CROSS PRODUCT BOLD d
   + BOLD b CROSS PRODUCT BOLD c + BOLD b CROSS PRODUCT BOLD d		    (6)

It follows from

     BOLD i CROSS PRODUCT BOLD i = BOLD j CROSS PRODUCT BOLD j
		= BOLD k CROSS PRODUCT BOLD k = 0
   BOLD i CROSS PRODUCT BOLD j = BOLD k, BOLD j CROSS PRODUCT BOLD k
		 = BOLD i, BOLD k CROSS PRODUCT BOLD i = BOLD j

that for

BOLD a = a(sub x) BOLD i + a(sub y) BOLD j + a(sub z) BOLD k
BOLD b = b(sub x) BOLD i + b(sub y) BOLD j + b(sub z) BOLD k

Eq. (7) holds. The expression in Eq. (7) is to be expanded by the ordinary rules
governing determinants.

BOLD a CROSS PRODUCT BOLD b =
	|BOLD i 	 BOLD j 	  BOLD k
	 a(sub x)	 a(sub y)	  a(sub z)
	 b(sub x)	 b(sub y)	  b(sub)z|			    (7)

    It is easy to verify that a reflection of the space coordinates given by
x' = -x, y' = -y, z' = -z, reverses the sign of the components of a vector.
Under a space reflection, however, the components of the vector BOLD	A CROSS
PRODUCT BOLD b do not change sign.  This is seen from Eq.  (7), for if a(sub x),
a(sub y), a(sub z) are replaced by -a(sub x), -a(sub y), -a(sub z), and if b(sub
x), b(sub y), b(sub z) are replaced by -b(sub x), -b(sub y), -b(sub z), the
components of BOLD a CROSS PRODUCT BOLD b remain invariant.  Hence, BOLD a CROSS
PRODUCT BOLD b is not a true vector and therefore is given the title
pseudovector.  In electricity theory the magnetic field vector BOLD B is a
pseudovector, whereas the electric field vector is a true vector provided the
electric charge is a true scalar under a reflection of axes.  There are three
differentiation processes that are of conceptual value in the study of vectors:
the gradient of a scalar, the divergence of a vector, and the curl of a vector.
From the scalar phi(x,y,z), one can form the three partial derivatives

CURLY D phi/CURLY Dx, CURLY D phi/CURLY Dy, CURLYD phi/CURLY D z, from which the
vector in Eq.  (8)

{(gradient of phi) over (grad phi) over (del phi IDENTICAL TO DEL phi)}
	= (CURLY D phi/CURLY Dx)BOLD i + (CURLY D phi/CURLY Dy)BOLD j
	+ (CURLY D phi/CURLY Dz)BOLD k					      (8)

can be formed. The vector, grad PHI, has two important properties. Grad phi is
a vector field normal to the surface phi(x,y,z) = constant at every point of
the surface. Moreover, grad phi yields that unique direction such that phi
increases at its greatest rate.
    The del operator defined by Eq. (9) plays an important role in the
development of the differential vector calculus. For the vector field of Eq.
(10), the divergence of BOLD v is defined by Eq.  (11).  The divergence of a
vector is a scalar.

	 DEL = BOLDi (CURLY D/CURLY Dx) + BOLD j (CURLY D/CURLY Dy)
		+ BOLD k(CURLY D/CURLY Dz)				    (9)

      BOLD v = v1(x,y,z)BOLDi + v2(x,y,z)BOLD j + v3(x,y,z)BOLD k	    (10)

 div BOLD v = DEL DOT BOLD v = CURLY Dv1/CURLY Dx + CURLY Dv2/CURLY Dy +
			   CURLY Dv3/CURLY Dz				   (11)

    By use of the del operator one obtains quite formally Eq. (12). The vector

DEL CROSS PRODUCT BOLD v =
   |BOLD i	      BOLD j		 BOLD k
    CURLY D/CURLY Dx  CURLY D/CURLY Dy	 CURLY D/CURLY Dz
    v1		       v2		 v3|

	= (CURLY Dv3/CURLY Dy - CURLY Dv2/CURLY Dz)BOLDi
	+ (CURLY Dv1/CURLY Dz - CURLY Dv3/CURLY Dx)BOLDj
	+ (CURLY Dv2/CURLY Dx - CURLY Dv1/CURLY Dy)BOLDk		    (12)

DEL CROSS PRODUCT BOLD v is called the curl of BOLD v (curl BOLD v). Under
a reflection of space coordinates, CURLY Dv3/CURLY Dy --> CURLY D(-v3)/CURLY
D(-y) = CURLY Dv3/CURLY Dy, and so on, so that the curl of BOLD v is a
pseudovector.  If a closed surface is decomposed into a large number of small
surfaces, a vector field normal to the surface can be constructed, each normal
element being represented by d BOLD sigma.  The magnitude of d BOLD sigma is the
area of the surface element dS, d BOLD sigma = BOLD N dS.  If f is a vector
field defined at every point of the surface, then

       DOUBLE INTEGRATION (in terms of S) BOLD f DOT d BOLD sigma	   (13)

notation (13) represents the total flux of BOLD f through the
surface S. The elements d BOLD sigma point outward from the interior of S.

  DOUBLE INTEGRATION (in terms of S) BOLD f DOT d BOLD sigma =
      TRIPLE INTEGRATION (in terms of R) (DEL DOT BOLD f) d tau 	   (14)

    The divergence theorem of Gauss states that Eq.  (14) is true, where R is
the region enclosed by S, d tau a volume element of R.	See GAUSS' THEOREM.

    The line integral of a vector field is described as follows: Let GAMMA be
a space curve, and let BOLD t be the unit vector field tangent to GAMMA at every
point of GAMMA in progressing from A to B, the initial and end points of the
trajectory GAMMA.

	     INTEGRATION (A to B) (BOLD f DOT BOLD t) ds		   (15)

   The scalar integral in notation (15) is called the line integral of BOLD f
along GAMMA, with arc length s measured along GAMMA.  If BOLD f is a force
field, notation (15) represents the work performed by the force field if a unit
test particle is taken from A to B along GAMMA.  The value of the integral of
notation (15) will generally depend on the path from A to B.  However, if BOLD f
= DEL phi, then

	     INTEGRATION (A to B) (BOLD f DOT BOLD t) ds =
	     INTEGRATION (A to B) DEL phi DOT d BOLD v =
	     INTEGRATION (A to B) d phi =
	     phi (B) - phi (A)

and the line integral is independent of the path of integration.

LINE INTEGRAL (in terms of LAMBDA) BOLD f DOT BOLD r =
DOUBLE INTEGRATION (in terms of S) (DEL CROSS PRODUCT BOLD f) DOT d BOLD sigma
									    (16)

    The theorem of Stokes states that Eq. (16) holds, where I is the boundary
of the open surface S. See STOKES' THEOREM.
 �34s[Harry Lass]
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�6v�2v�32sCalendar �21m �33s
    A list, usually in the form of a table, showing the correspondence between
days of the week and days of the month; also, a list of special observances
with their dates, especially those that fall on different dates in different
years, as a church calendar; also, a set of rules that serves to attach to any
day a specific number or name or combination of number and name.
    The calendars that have been used at various times and places are numerous
and diverse. Most of them represent attempts to divide the synodic month or
the tropical year into numbered days. According to whether the emphasis is on
month or year, calendars are called lunar or solar.
    The calendar used for civil purposes throughout the world, known in
Western countries as the Gregorian calendar, was established by Pope Gregory
XIII, who decreed that the day following Thursday, October 4, 1582, should be
Friday, Oct. 15, 1582, and that thereafter centennial years (1600, 1700, and
so on) should be leap years only when divisible by 400 (1600, 2000, and so
on), other years being leap years when divisible by four, as previously. The
effect of the reform was to restore the vernal equinox (beginning of spring,
which governs the observance of Easter) to March 21, and to reduce the number
of days in 400 calendar years by 3, making the average number of days in the
calendar year 365.2425, instead of 365.25 as formerly, and bringing it into
closer agreement with the tropical year, which has 365.24220 days.
    The Gregorian calendar is not perfectly adjusted to the tropical year, the
error accumulating to 1 day after about 3300 years. Numerous proposals for
calendar reform have been made with the object of reducing the error, and
there have also been many proposals for altering the succession of days for
the purpose of bringing greater regularity into the calendar.
 �34s[Gerald M. Clemence]
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�6v�2v�32sCalibration   �21m �33s 
    The process of determining, by measurement or by comparison with a 
standard, the correct value of each scale reading on a meter or the correct 
value of each setting of a control knob. In a radio receiver, for example, 
calibation would mean adjusting the tuned circuits in the oscillator to make 
the readings of the tuning dial correspond exactly to the frequencies of the 
incoming signals. 
    With measuring instruments, calibration generally involves adjusting the 
values of internal components so that the indication is correct at a specified 
number of points on the indicating scale and approximately correct between 
these points. With highly accurate instruments, calibration involves the 
preparation of a graph or table that gives the exact value corresponding to 
each line on the indicating scale. See INSTRUMENTATION. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sCaliche   �21m �33s 
    A term used to describe the crude soda niter of the Chilean nirate 
deposits. It is a variable admixture of salts, including, in addition to the 
soda niter, bloedite, anhydrite, gypsum, polyhalite, halite, glauberite, 
darapskite, and minor amounts of various iodate, chromate, and borate 
minerals. See BORATE MINERALS; SODA NITER. 
    The term caliche has several other meanings. It is commonly used in Mexico 
and the southwestern United States to designate gravel, sand, or desert debris 
cemented by porous, calcium carbonate. Other uses of the term include a thin 
layer of clayey soil capping auriferous veins; whitish clay in the selvage of 
veins; a white mineral vein recently discovered; and in placer mining, a bank 
composed of clay, sand, and gravel. 
 �34s[George Switzer]
�33s 
  
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�6v�2v�32sCaliciales   �21m �33s 
    An order of the Ascolichenes. This order is characterized by an unusual 
apothecium. The hymenial layer originates normally, but by the time the spores 
are mature, the asci and paraphyses have partially disintegrated into a mass 
of spores and hymenial tissues known as a mazaedium. The two families, 
Caliciaceae and Roesleriaceae, are so close that some species in either one 
may or may not lichenize symbiotic algae. The family Cypheliaceae is more 
typically crustose, with sessile apothecia and a more fully developed thallus. 
The Sphaerophoraceae are fruticose but the thallus is solid. The apothecia are 
open or enclosed in a spherical chamber at the tips of branches. The largest 
genus, Sphaerophorus, is widespread in boreal zones and mountains of both 
hemispheres. See ASCOLICHENES. 
 �34s[Mason E. Hale, Jr.]
�33s 
  
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�6v�2v�32sCalifornium �21m �33s
    A chemical element, Cf, atomic number 98, the ninth member of the actinide
series of elements. Its discovery and production have been based upon
artificial nuclear transmutation of radioactive isotopes of lighter elements.
All isotopes of californium are radioactive, with half-lives ranging from a
minute to about 1000 years. Because of its nuclear instability, californium
does not exist in the Earth's crust. See ACTINIDE ELEMENTS; BERKELIUM;
RADIOACTIVITY.
    The chemical properties are similar to those observed for other 3+ actinide
elements: a water-soluble nitrate, sulfate, chloride, and perchlorate.
Californium is precipitated as the fluoride, oxalate, or hydroxide.
Ion-exchange chromatography can be used for the isolation and identification of
californium in the presence of other actinide elements. Californium metal is
quite volatile and can be distilled at temperatures of the order of 1100-1200
DEG C (2010-2190 DEG F). It is chemically reactive and appears to exist in
three different crystalline modifications between room temperature and its
melting point, 900 DEG C (1600 DEG F).
    The most easily produced isotope for many purposes is sup 252 Cf, which is
obtained in gram quantities in nuclear reactors and has a half-life of 2.6
years. It decays partially by spontaneous fission, and has been very useful for
the study of fission. It has also had an important influence on the development
of counters and electronic systems with applications not only in nuclear physics
but in medical research as well. See TRANSURANIUM ELEMENTS.
[Glenn T. Seaborg]
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�6v�2v�32sCaligoida   �21m �33s 
    A suborder of the Copepoda, which are external parasites widely 
distributed on fish. Their parasitic mode of existence is accompanied by a 
modification of their normal structure. The carapace is distinct in many 
cases, but other segments are often fused. The mouth is a sucking type and 
possesses styliform mandibles adapted for piercing the host's skin. Swimmerets 
exhibit various modifications, such as a loss of setae, fusion or reduction of 
joints, loss of the endopodite, or reduction in size or number of the limbs. 
The suborder comprises numerous genera and species, contained in 11 families 
(see table). See COPEPODA: CRUSTACEA. 
 �34s[Sueo M. Shiino]
�33s 
  
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�6v�2v�32sCaliper �21m �33s
    An instrument with two legs used for measuring linear dimensions. Calipers
may be fixed, adjustable, or movable. Fixed calipers are used in routine
inspection of standard products; adjustable calipers are used similarly but
can be reset to slightly different dimensions if necessary. Movable calipers
can be set to match the distance being measured. The legs may pivot about a
rivet or screw in a firm-joint pair of calipers; they may pivot about a pin,
being held against the pin by a spring and set in position by a knurled nut on
a threaded rod; or the legs may slide either directly (caliper rule) or along
a screw (micrometer caliper) relative to each other. See MICROMETER.
    The legs may be shaped to facilitate measuring outside dimensions, inside
dimensions, surface dimensions as between points on a plate, or from a surface
into a hole as in a keyway (see illustration). See GAGES.





 �34s[Frank H. Rockett]
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�6v�2v�32sCalomel   �21m �33s 
    Mercury(I) chloride, Hg2Cl2, a covalent compound which is insoluble in 
water. The substance sublimes when heated. The formula weight is 472.086 and 
the specific gravity is 7.16 at 20 DG C (68 DG F). The material is a white, 
impalpable powder consisting of fine tetragonal crystals. 
    Calomel is used in preparing insecticides and medicines. It is well known 
in the laboratory as the constituent of the calomel reference electrodes which 
are commonly used in conjunction with a glass electrode to measure pH. See 
CALOMEL ELECTRODE; MERCURY (ELEMENT). 
 �34s[E. Eugene Weaver]
�33s 
  

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�6v�2v�32sCalomel electrode �21m �33s
    An electrode used widely as a reference electrode of known potential, for
example, in electrometric measurements of acidity and alkalinity, in corrosion
studies, and in the measurements of the potentials of other electrodes (see
illustration). The calomel electrode consists of mercury covered with a layer
of paste of mercury (I) chloride in an aqueous solution of potassium chloride.
A small amount of mercury is dispersed in the paste of mercury(I) chloride. It
is made in three well-known types: the saturated, the normal, and the
one-tenth normal. These designations refer to the concentrations of the
potassium chloride solutions used in the preparation. The first type is more
widely used than the other two because of the ease with which it may be
assembled; no analysis of the concentration is required. Crystals of potassium
chloride are placed above the layer of mercury(I) chloride in the saturated
type. The calomel electrode is commonly referred to as a half-cell because it
includes an aqueous phase of potassium chloride as one arm of the cell to be
used in the electric circuit. Frequently, calomel electrodes are used in
assemblies such as pH meters. See ELECTRODE POTENTIAL; HYDROGEN ION; SILVER
CHLORIDE ELECTRODE.
 �34s[Walter J. Hamer]
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�6v�2v�32sCalorimetry   �21m �33s 
    The measurement of the quantity of heat involved in various processes, 
such as chemical reactions, changes of state, and formation of solutions, or 
in the determination of the heat capacities of substances. The fundamental 
unit for these heat measurements is the joule, but many workers prefer to use 
the calorie, defined as 4.184 absolute joules. 
    A calorimeter is an apparatus for measuring these heat quantities. Since 
the problem involves various kinds of processes and experimental work over a 
range of temperatures, calorimeters have been developed in great variety. 
    Nonisothermal calorimeters yield evaluations of the heat quantities by the 
temperature changes they produce. Many of these use a liquid as a heat 
reservoir, since uniformity of temperature thus can be realized easily by 
proper stirring, and the temperature change for the system can be evaluated 
readily. In contrast, the aneroid type depends on the use of a metal of high 
thermal conductivity, such as copper, to promote rapid equalization of 
temperature.  
    Various isothermal calorimeters also have been developed in which the heat 
reservoir contains a liquid in equilibrium with its crystalline solid at the 
melting point or with its vapor at the boiling point. While the isothermal 
calorimeter is simple, and can yield excellent data, its disadvantage is that 
it can be operated only at the equilibrium temperature of the two-phase system 
involved.  
    All these types of calorimetric apparatus contain (1) the calorimeter 
proper, or heat reservoir for measuring the amount of heat in the process 
under study, and (2) a jacket which can be used to control thermal exchange 
between the calorimeter and its environment. In many cases this jacket is a 
hollow vessel which may be filled with a liquid, such as water, that can be 
stirred and accurately regulated in temperature. In the aneroid type the 
jacket usually is a hollow, cylindrical block of metal with a top and bottom, 
electrical heating coils, and a suitable thermometer system. 
  
    The measurement of temperature is an important factor of calorimetry which 
has been greatly refined since 1920. Today, resistance thermometers and 
thermocouples are the instruments most used in high-grade precision 
measurements. See BOMB CALORIMETER; CHEMICAL THERMODYNAMICS; TEMPERATURE 
MEASUREMENT; THERMOMETER. 
 �34s[Bruno J. Zwolinski]
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�6v�2v�32sCambrian �21m �33s
    An interval of time in Earth history (Cambrian Period) and its rock record
(Cambrian System). The Cambrian Period spanned about 100,000,000 years and
began with the first appearance of marine animals with mineralized (calcium
carbonate, calcium phosphate) shells. The Cambrian System includes many
different kinds of marine sandstones, shales, limestones, dolomites, and
volcanics. There is very little provable record of nonmarine Cambrian
environments. Because of the difficulty of finding rocks that can be dated
radiometrically in association with rocks that can be dated empirically by
fossils, the age in years of most Cambrian deposits is only approximate. The
best present estimates suggest that Cambrian time began about 570,000,000
years ago and ended about 475,000,000 years ago. It is one of the longest
Phanerozoic periods. See PALEOZOIC.
    For most practical purposes, rocks of Cambrian age are recognized by their
content of distinctive fossils. On the basis of the successive changes in the
evolutionary record of Cambrian life that have been worked out during the past
century, the Cambrian System has been divided globally into Lower, Middle, and
Upper series, each of which has been further divided on each continent into
stages, each stage consisting of several zones. Precise intercontinental
correlation of series and stage boundaries and of zones is still difficult.
    The record preserved in rocks indicates that essentially all Cambrian
plants and animals lived in the sea. The few places where terrestrial
sediments have been preserved suggest that the land was barren of major plant
life, and there are no known records of Cambrian insects or of terrestrial
vertebrate animals of any kind. The most abundant remains of organisms in
Cambrian rocks are trilobites. They are present in almost every fossiliferous
Cambrian deposit and are the principal tools used to describe divisions of
Cambrian time and to correlate Cambrian rocks. The next most abundant Cambrian
fossils are brachiopods. These bivalved animals lived on the sediment surface
or on the surfaces of other organisms. Limestones of Early Cambrian age may
contain large reeflike structures formed by an association of algae and an
extinct phylum of invertebrates called Archaeocyatha. The Cambrian record of
mollusks and echinoderms is characterized by many strange-looking forms.
Representatives of these phyla, such as cephalopods, clams, and true crinoids,
are rare in Cambrian rocks. Snails, however, are found throughout the Cambrian
system.
    The Cambrian world can be resolved into at least four major continents.
These were (1) North America, minus a narrow belt along the eastern coast from
eastern Newfoundland to northern Georgia which is most closely related to
Cambrian Gondwana; (2) Baltica, consisting of present-day northern Europe
north of France and west of the Ural Mountains, but excluding most of Scotland
and northern Ireland, which are remains of marginal Cambrian North America;
(3) Gondwana, a giant continent whose present-day fragments are Africa, South
America, India, Australia and Antarctica, parts of southern Europe, the Middle
East, and southeast Asia; (4) Siberia, including much of the northeastern
quarter of Asia. Unfortunately, there is not enough reliable information to
accurately locate these continents relative to one another on the Cambrian
globe. See CONTINENTS; PLATE TECTONICS.
 �34s[Allison R. Palmer]
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�6v�2v�32sCamel   �21m �33s 
    The name given to two species of mammals which are members of the family 
Camelidae in the order Artiodactyla. These are the Bactrian camel (Camelus 
bactrianus) and the Arabian or dromedary camel (C. dromedarius). Both species 
are domesticated, but a few wild herds of Bactrian camels  are still in 
existence in the Gobi desert. The legs of these animals are long and slender 
and terminate in two toes. The neck and head are elongate, and there is a 
cleft upper lip. The period of gestation is about 1 year and the female breeds 
every second year, producing one young (colt). 
    The Bactrian camel is stronger and more heavily built than the dromedary 
and is more suitable as a pack animal. There are two humps of fatty tissue, 
one over the shoulders and the other atop the hindquarters. This animal is 
economically important as it provides milk, meat, and leather for the nomads 
in central Asia. 
    The Arabian camel is taller than the Bactrian and has a single hump of 
fatty tissue, which can be used as a food reserve. There are two varieties of 
this species found in the desert. One is the baggage camel, used as a beast of 
burden. The other type is the more slightly built racing camel. The species is 
well-suited to desert life with its broad feet adapted to walking on sand, its 
ability to close its nostrils completely, and its double row of interlocking 
eyelashes.  
    These two species have a most important physiological adaptation in their 
ability to conserve water. Camels do not store water but conserve it, since 
the body is well insulated by fur and has a temperature range of over 12 DG F 
(7 DG C) before it perspires sufficiently to prevent a further rise. The camel 
can lose over 40% of its body water without fear of dehydration. However, 
although able to survive for long periods without water, it may drink as much 
as 15 gal (57 liters) when water is available. See ARTIODACTYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCamel's hair   �21m �33s 
    A fine hair known to the American consumer chiefly in the form of 
high-quality coat fabrics. This textile fiber is obtained from the two-humped 
Bactrian camel. Camel's-hair fabrics are ideal for comfort, particularly when 
used for overcoating, as they are especially warm and light in weight. Camel's 
hair is characterized by strength, luster, and smoothness. The best quality is 
expensive. It is often mixed with wool to improve the quality of the wool 
fabric. The price of such a mixed cloth is much less than that of a 100% 
camel's-hair fabric. See CAMEL; NATURAL FIBER; WOOL. 
 �34s[M. David Potter]
�33s 
  
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�6v�2v�32sCameo   �21m �33s 
    A type of carved gemstone in which the background is cut away to leave the 
subject in relief. Often cameos are cut from stones in which the coloring is 
layered, resulting in a figure of one color and a background of another. The 
term cameo, when used without qualification, is usually reserved for those cut 
from a gem mineral, although they are known also as stone cameos. The commonly 
encountered cameo cut from shell is properly called a shell cameo. 
    Most cameos are cut from onyx or agate, but many other varieties of 
quartz, such as tiger's-eye, bloodstone, sard, carnelian, and amethyst, are 
used; other materials used include beryl, malachite, hematite, labradorite, 
and moonstone. See GEM; INTAGLIO (GEMOLOGY). 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sCamera �21m �33s
    The basic instrument of photography: in its simplest form, a light-tight
box with a lens at one end, which forms an image of the subject on
light-sensitive film at the other end. A pinhole may be substituted for the
lens, but forms an unsharp image. Nearly all modern cameras contain a glass or
plastic lens to focus a sharp image of the subject onto the film; a shutter to
control the length of time the film is exposed, normally a fraction of a
second; a viewfinder, to show the subject being photographed; and a mechanism
to hold the film and change film between exposures. Many cameras include a
lens aperture control, to regulate image brightness on the film. Advanced
cameras may accept a variety of lenses for different photographic effects.
Many accessories are commonly used: flash units, exposure meters to measure
light, filters to control color rendition, tripods, and close-up focusing
attachments.
    Cameras designed for still photography include the following types: box
and other simple cameras, 35-mm, reflex, view, instant picture, panoramic,
underwater, aerial, stereo, medical, and document-copying or microfilm. Some
of these types overlap. There are also motion picture cameras, both
conventional and high-speed, and television cameras - which are similar to
photographic cameras but which use a light-sensitive electronic device instead
of film. See CINEMATOGRAPHY; PHOTOGRAPHY.
    Box and simple cameras usually have no adjustments, making them
inexpensive to manufacture and simple to use. The lens focus is fixed, so that
reasonably sharp pictures can be taken at any distance beyond a few feet. Lens
aperture and shutter speed are factory-set to give correct exposure outdoors
in sunshine and indoors with flash.
    The 35-mm cameras take perforated film 35 mm wide (about 1 3/8 in.), in
cassettes holding 20 or 36 exposures. The normal image size is 24 x 36 mm (1 x
1 1/2 in.), though some cameras make images 18 x 24 mm 2/3 x 1 in. or 24 mm
square. The 35-mm cameras can be very compact, and range from simple,
inexpensive models to complex camera systems with a great variety of lenses,
attachments, and accessories. The two principal kinds of 35-mm cameras are the
rangefinder type and the reflex type.

(a)
     (b)
    In reflex cameras the light rays that pass through the camera lens are
reflected from a mirror onto a horizontal ground-glass screen, where they form
an image (illustration a). The mirror is at a precise 45 DG angle, and the
distance from mirror to film is exactly equal to the distance from mirror to
ground glass, so the image on the ground glass is the same size, focus, and so
on, as the image on the film. Since the mirror prevents light from reaching
the film, it must be hinged to flip out of the way when the exposure is made.
It is also necessary to have the lens shutter and aperture fully open at
first, to provide a bright image for viewing and focusing; next, they must
close, to shield the film, when the mirror flips up; then they reopen for the
set exposure. Alternatively, cameras can be designed with two identical lenses
(illustration b). The bottom lens actually takes the picture: the top one is
used only as a viewfinder, together with a fixed mirror and a ground glass.
The two lenses are coupled so that focusing the image on the ground glass also
makes it sharp on the film.
    Originally, view cameras were the large cameras used for taking landscapes
or views. In present usage the name describes cameras with a range of camera
movements: swinging and shifting the front of the camera relative to the back.
Other cameras always keep the front parallel to the back, and the center of
the lens always aligned with the center of the film. But for special control
of perspective and focus, swinging and shifting the camera front and lens
independently from the back, and from the film, makes possible photographs
which could not be taken any other way. To permit these movements, the view
camera body is a flexible bellows, attached to front and rear panels which can
swivel on a rigid base.
    Polaroid-Land cameras, originally introduced in 1948, were the first
cameras to produce a finished print directly. Jellylike  developing chemicals,
stored in pods attached to the film, quickly process each picture as it is
removed from the camera. The photographer can immediately evaluate the print,
make any desired changes, and take another.
    Panoramic or wide-field cameras are useful for photography of groups of
people or in confined spaces, and for taking ultra-wide-angle views. One type
uses a wide-angle lens and an elongated film format. Another uses a swinging
lens design; some swinging-lens cameras can even photograph a full 360 DG view.
    Underwater cameras follow two general designs. Some are specialized models
designed specifically for underwater operation; others are conventional
cameras enclosed in waterproof, pressure-resistant housings, with flexible
linkages to the camera controls.
    Cameras for aerial photography may take photographs as large as 9 x 9 in.
(22.5 x 22.5 cm), for maximum sharpness and detail. The pictures overlap by a
set amount, so that two photographs can be studied together in a stereo
viewer, which gives them the appearance of three dimensions and permits
mapping the height of terrain and buildings. Several cameras photographing
simultaneously, with special films and color filters, can make spectral
analysis photographs to reveal pollution, mineral deposits, and the condition
of crops and plant life. Multispectral satellite cameras have provided new
information about the Earth's resources. See AERIAL PHOTOGRAPH; APPLICATIONS
SATELLITES; REMOTE SENSING.
    Stereo cameras use two lenses to make two photographs side by side. When
properly viewed, the photographs can recreate the feeling of three-dimensional
depth in the original scene. Stereo cameras have had periods of popularity in
amateur photography and are also used in certain areas of technical and
scientific photography.
    Special medical cameras can actually photograph inside the body, by means
of instruments inserted into the body. A flexible bundle of fiber optics
projects light down the probe and relays the image back to the camera.




 �34s[Jerome P. O'Neill, Jr.]
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�6v�2v�32sCamerata �21m �33s
    An extinct subclass of stalked Crinoidea comprising about 140 Paleozoic
genera, ranging from the Ordovician to Permian. The calyx was invested by a
rigid boxlike theca, composed of numerous small polygonal plates, together
with some of the lower brachial ossicles and some other small interradial and
interbrachial plates; these plates were all firmly sutured together (see
illustration). The uniserial or biserial arms branched several times. The stem
was sometimes unanchored and in such cases served as a prehensile limb. See
CRINOIDEA; ECHINODERMATA.
 �34s[Howard B. Fell]
�33s



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�6v�2v�32sCam mechanism �21m �33s
    A mechanical linkage whose purpose is to produce, by means of a contoured
cam surface, a prescribed motion of the output link of the linkage, called the
follower. Cam and follower are a higher pair. See LINKAGE (MECHANISM); SLIDING
PAIR.
    A familiar application of a cam mechanism is in the opening and closing of
valves in an automotive engine. The cam rotates with the cam shaft, usually at
constant angular velocity, while the follower moves up and down as controlled
by the cam surface. A cam is sometimes made in the form of a translating cam.
Other cam mechanisms, employed in elementary mechanical analog computers, are
simple memory devices, in which the position of the cam (input) determines the
position of the follower (output or readout).
    Although many requisite motions in machinery are accomplished by use of
pin-jointed mechanisms, such as four-bar linkages, a cam mechanism frequently
is the only practical solution to the problem of converting the available
input, usually rotating or reciprocating, to a desired output, which may be an
exceedingly complex motion. No other mechanism is as versatile and as
straightforward in design. However, a cam may be difficult and costly to
manufacture, and it is often noisy and susceptible to wear, fatigue, and
vibration.
    Cams are used in many machines. They are numerous in automatic packaging,
shoemaking, typesetting machines, and the like, but are often found as well in
machine tools, reciprocating engines, and compressors. They are occasionally
used in rotating machinery.
    Cams are classified as translating, disk, plate, cylindrical, or drum (see
illustration). The link having the contoured surface that prescribes the
motion of the follower is called the cam. Cams are usually made of steel,
often hardened to resist wear and, for high-speed application, precisely
ground.
    The output link, which is maintained in contact with the cam surface, is
the follower. Followers are classified by their shape as roller, flat face,
and spherical face. Followers are also described by the nature of their
constraints, for example, radial, in which motion is reciprocating along a
radius from the cam's axis of rotation; offset, in which motion is
reciprocating along a line that does not intersect the axis of rotation
(illustration b); and oscillating, or pivoted (illustration a).
Three-dimensional cam-and-follower systems are coming into more fre-quent use,
where the follower may travel over a lumpy surface.
 �34s[Donald P. Abbott]
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�6v�2v�32sCampanulales   �21m �33s 
    An order of flowering plants, division Magnoliophyta in the subclass 
Asteridae of the class Magnoliopsida (dicotyledons). It consists of 6 families 
and about 2500 species. The order is distinguished in this subclass by its 
chiefly herbaceous habit, alternate leaves, inferior ovary, and stamens which 
are free from the corolla or attached at the base of the tube. About 2000 of 
the species belong to the single family Campanulaceae. Several familiar 
ornamentals, including the Canterbury bell (Campanula medium) and the cardinal 
flower (Lobelia cardinalis), belong to the Campanulaceae. See ASTERIDAE; 
MAGNOLIOPSIDA; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sCamphor   �21m �33s 
    A bicyclic, saturated terpene ketone, formula C10H16O. It exists in the 
optically active dextro and levo forms, and as the racemic mixture of the two 
forms. All of these melt within a degree or so of 178 DG C (352 DG F). The 
principal form is dextro-camphor, which occurs in the wood and leaves of the 
camphor tree (Cinnamomum camphora). Camphor is also synthesized commercially 
on a large scale from pinene which yields mainly the racemic variety. See 
CAMPHOR TREE. 
    Camphor has a characteristic odor; it crystallizes in thin plates and 
sublimes readily at ordinary temperatures. It has a wide use in liniments and 
as a mild rubefacient, analgesic, and antipruritic. It has a local action on 
the gastrointestinal tract, producing a feeling of warmth and comfort in the 
stomach. It is also used in photographic film and as a plasticizer in the 
manufacture of plastics. See PINENE; TURPENTINE. 
 �34s[Everett L. Saul]
�33s 
  
  

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�6v�2v�32sCamphor tree �21m �33s
    The plant Cinnamomum camphora, a member of the laurel family (Lauraceae)
and a native of China, Japan, and Formosa. The tree is dense-topped and has
shiny, dark, evergreen leaves. All parts of the tree contain camphor, an
essential oil which is obtained from the finely ground wood and leaves by
distillation with steam. See CAMPHOR; MAGNOLIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sCanal �21m �33s
    An artificial open channel usually used to convey water or vessels from
one point to another. Canals are generally classified according to use as
irrigation, power, flood-control, drainage, or navigation canals or channels.
All but the last type are regarded as water conveyance canals.
    Canals may be lined or unlined. Linings may consist of plain or reinforced
concrete, cement mortar, asphalt, brick, stone, buried synthetic membranes, or
compacted earth materials. Linings serve to reduce water losses by seepage or
percolation through pervious foundations or embankments and to lessen the cost
of weed control. Concrete and other hard-surface linings also permit higher
water velocities and, therefore, steeper gradients and smaller cross sections,
which may reduce costs and the amount of right-of-way required.
    Navigation canals are artificial inland waterways for boats, barges, or
ships. A canalized river is one that has been made navigable by construction
of one or more weirs or overflow dams to impound river flow, thereby providing
navigable depths. Locks may be built in navigation canals and canalized rivers
to enable vessels to move to higher or lower water levels. A lock is a chamber
equipped with gates at both upstream and downstream ends. Water impounded in
the chamber is used to raise or lower a vessel from one elevation to another.
The lock chamber is filled and emptied by means of filling and emptying valves
and a culvert system usually located in the walls and bottom of the lock. See
IRRIGATION (AGRICULTURE); TRANSPORTATION ENGINEERING; WATER SUPPLY ENGINEERING.
 �34s[Corp of Engineers; U.S. Bureau of Reclamation]
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�6v�2v�32sCancer (constellation)   �21m �33s 
    The Crab, in astronomy, a winter constellation and the faintest of the 
zodiacal groups. Cancer, the fourth sign of the zodiac, is important because 
during early times it marked the northernmost limit of the ecliptic, when the 
zodiacal system was adopted. The Tropic of Cancer takes its name from this 
constellation. Four faint stars form a rough Y outline, which is suggestive of 
a crab. In the center of Cancer is a hazy object. This is a magnificent 
cluster of faint stars called Praesepe (the Beehive) or the Manger. See 
CONSTELLATION.  
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sCancer (medicine)   �21m �33s 
    The common name for a malignant neoplasm or tumor. Neoplasms are new 
growths and can be divided into benign and malignant types, although in 
certain instances the distinction is unclear. The most important 
differentiating feature between a benign and malignant neoplasm is that a 
malignant tumor will invade surrounding structures and metastasize (spread) to 
distant sites, whereas a benign tumor will not. Other characteristics used to 
differentiate between benign and malignant growths include: malignancies are 
composed of highly atypical cells; malignancies tend to show more rapid growth 
than benign neoplasms and are composed, in part, of cells showing abnormal 
mitotic activity, for example, tripolar mitosis; malignant tumors tend to grow 
progressively without self-limitation. See MITOSIS. 
    Malignant neoplasms which arise from cells of mesenchymal origin (that is, 
bone, muscle, connective tissue, and so forth) are called sarcomas. Those 
which develop from epithelial cells and tissues such as skin, mucosal 
membrane, and glandular tissues are termed carcinomas. Carcinomas usually 
initially metastasize via lymphatic channels, whereas sarcomas spread to 
distant organs through the bloodstream. 
    The cause of most types of cancer is unknown. However, there can be little 
remaining doubt that cigarette smoking is directly related to cancer of the 
lung, oral cavity, larynx, and urinary bladder. Likewise, the combination of 
other substances such as ethyl alcohol or asbestos and cigarette smoking  is 
causally related to other types of cancer. Abundant epidemiologic and 
experimental data have shown a direct cause-and-effect relationship between 
various types of industrial exposure and certain neoplasms. Several studies 
have shown that postmenopausal women taking estrogens have a much higher 
incidence of cancer of the lining of the uterine cavity. Some malignant tumors 
seem to be genetically determined or related to chromosomal abnormalities. 
    The physical changes that cancer produces in the body vary considerably 
depending on the type of tumor, location, rate of growth, and whether it has 
metastasized. The American Cancer Society has widely publicized cancer's seven 
warning signals: (1) change in bowel or bladder habits; (2) a sore that does 
not heal; (3) unusual bleeding or discharge; (4) thickening or lump in the 
breast or elsewhere; (5) indigestion or difficulty in swallowing; (6) obvious 
changes in a wart or a mole; and (7) nagging cough or hoarseness. 
    The progression, or lack thereof, of a given cancer is also highly 
variable and depends on the type of neoplasm and its response to treatment. 
Treatment modalities include surgery, chemotherapy, radiation therapy, 
hormonal manipulation, and immunotherapy. Often a combination of the various 
modalities will be used. See CHEMOTHERAPY; ONCOLOGY; TUMOR. 
 �34s[Samuel P. Hammar]
�33s 
  
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�6v�2v�32sCancrinite �21m �33s
    A family of minerals, related to the scapolite family, characteristically
occurring in basic rocks such as nepheline syenites and sodalite syenites.
Cancrinite is hexagonal. Four-, six-, and twelve-membered aluminosilicate
rings can be discerned in the structure. Large anions such as [SO4] sup 2 sup
- and [CO3] sup 2 sup - occur in the hexagonal channels of the structure. Four
members of the cancrinite family are cancrinite, vishnevite, hauyne, and
afghanite. The cancrinite member is white, yellow, greenish, or reddish. It has
perfect prismatic cleavage, hardness is 5 - 6 on Mohs scale, and the specific
gravity is 2.45. Localities include the Fen area, southern Norway; the Kola
Peninsula, Soviet Union; Bancroft, Ontario, Canada; and Litchfield, Maine,
U.S.A. See NEPHELINE SYENITE; SILICATE MINERALS.
 �34s[Paul B. Moore]
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�6v�2v�32sCandidiasis   �21m �33s 
    A fungus infection of humans caused by several species of yeasts of the 
genus Candida. The disease may involve the mucous membranes, skin, lungs, 
central nervous system, and viscera. When the mucous membranes are affected, 
the disease is known as thrush. The cutaneous form of candidiasis is seen on 
those areas of the skin where moisture and irritation are common. Amphotericin 
B and 5-fluorocytosine are used in the treatment of systemic candidiasis. 
Nystatin is used in the treatment of lesions of the skin and mucous membranes. 
See MEDICAL MYCOLOGY; YEAST. 
 �34s[Leanor D. Haley]
�33s 
  
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�6v�2v�32sCandlepower   �21m �33s 
    Luminous intensity expressed in candelas. The term refers only to the 
intensity in a particular direction and by itself does not give an indication 
of the total light emitted. The candlepower in a given direction from a light 
source is equal to the illumination in footcandles falling on a surface  
normal to that direction, multiplied by the square of the distance from the 
light source in feet. The candlepower is also equal to the illumination of 
metercandles (lux) multiplied by the square of the distance in meters. 
    The apparent candlepower is the candlepower of a point source which will 
produce the same illumination at a given distance as produced by a given light 
source.  
    The mean horizontal candlepower is the average candlepower of a light 
source in the horizontal plane passing through the luminous center of the 
light source. 
    The mean spherical candlepower is the average candlepower in all 
directions from a light source as a center. Since there is a total solid angle 
of 4PI (steradians) emanating from a point, the mean spherical candlepower is 
equal to the total luminous flux (in lumens) of a light source divided by 4PI 
(steradians). See LUMINOUS INTENSITY; PHOTOMETRY. 
 �34s[Russell C. Putnam]
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�6v�2v�32sCanonical transformations   �21m �33s 
    Transformations among the coordinates and momenta describing the state of 
a classical dynamical system which leave the canonical or Hamiltonian form of 
the equations of motion unchanged. See HAMILTON'S EQUATIONS OF MOTION; 
HAMILTON'S PRINCIPLE. 
 �34s[Philip M. Stehle]
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�6v�2v�32sCantaloupe   �21m �33s 
    In the United States the name applied to muskmelon cultivars belonging to 
Cucumis melo var. reticulatus of the family Cucurbitaceae. However, this is a 
misnomer, and the name cantaloupe should be restricted to cultivars of C. melo 
var. cantalupensis. The fruits of this group are rough and scaly, with deep 
vein tracts and a hard rind. Cultivars of the variety cantalupensis are grown 
in Europe and Asia, but seldom in the United States. 
    The fruits grown in the United States are round to oval; the surface is 
netted and has shallow vein tracts. At maturity the skin color changes from 
dark green or gray to light gray or yellow. The flesh is usually 
salmon-colored, but it may vary from green to deep salmon-orange. When mature 
the melon is sweet, averages 6 - 8% sugar, and has a distinct aroma and 
flavor. The flesh is high in potassium and vitamin C, and, when deep orange, 
rich in vitamin A. See MUSKMELON; VIOLALES. 
 �34s[Oscar A. Lorenz]
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�6v�2v�32sCantilever �21m �33s
    A beam supported at one end and supporting a load along its length or at
its free end, the upper portion of the beam, if horizontal, being everywhere
in tension and the lower portion being everywhere in compression (see
illustration). Familiar examples are the symmetrical paired cantilevers of a
seesaw or teeterboard, the beam in a chemical balance, and the unsymmetrical
cantilever in the overhang of a roof. Cantilever construction permits spanning
wide distances without obstructing supports. See BEAN.




 �34s[Frank H. Rockett]
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�6v�2v�32sCanvas   �21m �33s 
    A fabric woven originally from hemp, now also from flax, jute, cotton, or 
a blend of fibers. In its stronger, coarser forms, canvas is used for tents, 
awnings, sails (sailcloth), webbing and belting, sport shoes, luggage, and 
carrying bags. When waterproofed by tarring, canvas can be made into 
tarpaulins for protecting materials. Finer grades are used as tapestry and 
embroidery bases (art canvas) and for painting in oils (artist's canvas). 
Canvas duck is a lightweight cotton or linen cloth used in making trousers, 
although the term is occasionally used for heavier canvases as well. See 
COTTON; FLAX: HEMP; JUTE. 
 �34s[Frank H. Rockett]
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�6v�2v�32sCapacitance   �21m �33s 
    The ratio of the charge q on one of the plates of a capacitor (there being 
an equal and opposite charge on the other plate) to the potential difference v 
between the plates; that is, capacitance (formerly called capacity) is C=q/v. 
    In general, a capacitor, often called a condenser, consists of two metal 
plates insulated from each other by a dielectric. The capacitance of a 
capacitor depends on the geometry of the plates and the kind of dielectric 
used, since these factors determine the charge which can be put on the plates 
by a unit potential difference existing between the plates. 
    In an ideal capacitor, no conduction current flows between the plates. A 
real capacitor of good quality is the circuit equivalent of an ideal capacitor 
with a very high resistance in parallel or, in alternating-current (ac) 
circuits, of an ideal capacitor with a low resistance in series. See 
CAPACITOR; DIELECTRICS. 
 �34s[Ralph P. Winch]
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�6v�2v�32sCapacitance box   �21m �33s 
    An assembly of capacitors and switches which permits adjustments of the 
capacitance existing at the terminals in nominally uniform steps from a 
minimum value near zero to the maximum which exists when all the capacitors 
are connected in parallel. Several arrangements are possible; the most common 
are those in which each decade consists of four capacitors in the ratios 
1:2:2:5 or 1:2:3:4. The switch connects successively the combinations of these 
four capacitors necessary to provide increasing capacitance at the terminals 
in uniform steps. The term capacitance decade box is sometimes used. See 
CAPACITOR.  
 �34s[F. Ralph Kotter]
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�6v�2v�32sCapacitor   �21m �33s 
    A device for introducing capacitance into a circuit. In general, a 
capacitor consists of two metal plates insulated from each other by a 
dielectric. The capacitance of a capacitor depends primarily upon the shape 
and size of the capacitor and upon the relative dielectric constant of the 
medium between the plates. In vacuum, and approximately so in air and most 
gases, k, the dielectric constant, is unity. For other dielectrics, k ranges 
from one to several hundred. See CAPACITANCE; DIELECTRIC CONSTANT. 
    One classification of capacitors comes from the physical state of their 
dielectrics. The dielectric may be a gas (or vacuum), a liquid, a solid, or a 
combination of these. Each of these classifications may be subdivided 
according to the specific dielectric used. 
    Capacitors are also classified as fixed, adjustable, or variable. The 
capacitance of fixed capacitors remains unchanged, except for small variations 
(caused by temperature fluctuations or vibration, for example); the 
capacitance of adjustable capacitors may be set at any one of several discrete 
values; and the capacitance of variable capacitors may be adjusted 
continuously and set at any value between minimum and maximum limits fixed by 
construction. Trimmer capacitors are relatively small variable capacitors used 
in parallel with larger variable or fixed capacitors to permit exact 
adjustment of the capacitance of the parallel combination. 
    Made in both the fixed and variable types, air, gas, and vacuum capacitors 
are constructed with flat parallel metallic plates (or cylindrical concentric 
metallic plates) with air, gas, or vacuum as the dielectric between plates. 
Alternate plates are connected, with one or both sets supported by means of a 
solid insulating material such as glass, quartz, ceramic, or plastic. The 
purpose of a high vacuum, or a gas under pressure, is to increase the voltage 
breakdown value for a given plate spacing. Air, gas, and vacuum capacitors are 
used in high-frequency circuits. Fixed and variable air capacitors of special 
design are used as standards in electrical measurements. 
    Solid-dielectric capacitors use one of several dielectrics such as a 
ceramic, mica, glass, or plastic film. Alternate plates of metal, or metallic 
foil, are stacked with the dielectric, or the dielectric may be metal-plated 
on both sides. 
    The plate assemblies of liquid-dielectric capacitors are mounted in a tank 
filled with a suitable oil or liquid dielectric. External connection is 
through insulators or bushings, and provision is made for expansion and 
contraction of the liquid during temperature changes. They have high capacity 
and low losses. 
    A large capacitance-to-volume ratio and a low cost per microfarad of 
capacitance are chief advantages of electrolytic capacitors. These use 
aluminum or tantalum plates on which an oxide film forms and acts as the 
dielectric. The characteristics of the oxide film depend on the polarity of 
the applied voltage. 
 �34s[Wilson S. Pritchett]
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�6v�2v�32sCapillary viscometer �21m �33s
    The simplest and oldest device for measuring the rheological properties of
liquids. In this instrument, the sample is usually allowed to flow by gravity
through a cylindrical capillary, and the transit time is measured. Flow
through the capillary is described by Poiseuille's law, Eq. (1), where Q =
volumetric flow rate, DELTA p = pressure drop through the capillary, R =
internal radius of capillary, and L = length of capillary.

    Q = (PI DELTA P (R sup 4))/(8 L (sub n))		  (1)

    The shear stress at the wall is TAU sub w = (DELTApR)/2L, and for newtonian
liquids the shear rate at the wall is D GAMMA(sub OMEGA)/Dt=(4Q)/(PI(R sup 3)).
Although some precautions must be taken to either minimize or correct for
errors caused by kinetic and viscous energy entrance and exit effects,
variation in the head during gravity flow, heat effects, various wall effects,
and so on, the capillary viscometer offers the advantages of a
straightforward, inexpensive technique for measuring viscosities. For non-
newtonian liquids, the variation in shear rate within the capillary becomes
a problem with respect to calculatin shear rate.  Models such as a Bingham
plastic, whcih has a yield stress ana an apparent viscosity given by Eq. (2)
or a power law mode Tau = KAPPA (d GAMMA(sup n))/dt, where k is a constant and
n is an empirically determined value, must be incorporated into theoretical
equations in order to calculate shear rate.  Furthermore, to study the degree
of non-newtonian behavior, the overall average shear rate must be changed by
using varying capillary sizes or by using pumps to get greater pressure drops.
Rheological properties of materials with time-dependency of the same order of
magnitude as the transit time cannot be measured with this type of viscometer.

  *** N (sub apparent) = (TAU - Tau sub yield)/(d GAMMA/dt)***	(2)


 �34s[Henry L. Gabelnick]
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�6v�2v�32sCapillary wave   �21m �33s 
    Capillary waves, or ripples, occur at the interface between two fluids, in 
which the principal restoring force is controlled by surface tension. Ripples 
generated by wind at the interface between air and water on oceans and lakes 
are of importance to the friction of air flowing over water, and to the 
reflection and scattering of electromagnetic and sound waves. See SURFACE 
TENSION.  
    In nature, ripples are observed to grow rapidly when the wind blows and to 
die away rapidly when the wind stops. When the water surface is contaminated, 
as by an oil film or other surface-active agent, ripples are damped still more 
rapidly because the contaminated surface acts as an inextensible film against 
which the water motions due to the ripples must rub. For moderate winds, the 
increased damping of ripples almost completely inhibits their growth; the 
surface appears smooth and is called a slick. 
 �34s[Charles S. Cox]
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�6v�2v�32sCapillary (anatomy)   �21m �33s 
    The smallest vessel of both the circulatory and lymphatic systems. 
Lymphatic capillaries are slightly larger in diameter than blood capillaries. 
The capillary wall is composed of a single layer of flat endothelial cells. 
The inside diameter of a blood capillary is about 8 micrometers, the size of a 
red blood cell, but varies slightly in different tissues and organs. Extensive 
capillary beds result from anastomoses of branching capillaries. These 
capillary networks are denser in those tissues possessing a high metabolic 
activity.  
    Capillaries are the exchange areas of the circulatory system, where 
passage of materials to and from these vessels and body tissues occurs under 
the influence of hydrostatic pressure, osmotic pressure, and other factors 
which affect permeability and rates of diffusion. See CIRCULATORY SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sCapparales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). It 
consists of 5 families and nearly 4000 species. 
    The families Cruciferae (Brassicaceae), with about 3000 species, and 
Capparaceae (often spelled Capparidaceae), with about 800 species, form the 
core of the order. These two families have alternate, usually compound or 
dissected leaves, and hypogynous flowers with mostly four sepals, four petals, 
centrifugal stamens, and (at least apparently) two carpels which form a 
compound ovary with more or less distinctly parietal placentation. Nearly all 
the Cruciferae and many of the Capparaceae have specialized myrosin cells, 
which are chiefly but not entirely restricted to this order. Myrosin is an 
enzyme involved in the formation of mustard oil. Several common vegetables, 
including cabbage, radish, rutabaga, turnip, water cress, and the various 
kinds of mustards, belong to the Cruciferae. A number of familiar ornamentals, 
including candytuft (Iberis), stock (Matthiola), and sweet alyssum (Lobularia 
maritima) in the Cruciferae, spider flower (Cleome spinosa) in the 
Capparaceae, and mignonette (Reseda) in the Resedaceae belong to the 
Capparales. See BROCCOLI; BRUSSELS SPROUTS; CABBAGE; CAULIFLOWER; COLLARD; 
CRESS; HORSERADISH; KALE; KOHLRABI; MAGNOLIOPSIDA; MUSTARD; RADISH; RAPE; 
RUTABAGA; TURNIP. 
 �34s[Arthur Cronquist]
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�6v�2v�32sCaprellidea   �21m �33s 
    A common crustacean suborder in marine and estuarine environments, 
belonging to the superorder Peracarida, order Amphipoda. As peracarids, female 
Caprellidea have a ventral brood pouch and eggs develop directly (that is, 
there are no planktonic larvae); as amphipods, Caprellidea have the second and 
third thoracic appendages formed into enlarged subchelate claws (gnathopods). 
The abdomen and abdominal appendages of caprellideans are reduced; thus 
caprellideans are restricted to a clinging and crawling life-style. Almost all 
caprellideans use other organisms as substrata. Family distinctions within the 
Caprellidea are being reevaluated, but the classical divisions are the 
families Caprellidae and Cyamidae (whale lice). The ecology of both families 
is poorly understood. See AMPHIPODA; CRUSTACEA. 
 �34s[Edsel A. Caine]
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�6v�2v�32sCapricornus �21m �33s
    The Sea Goat, in astronomy, an inconspicuous zodiacal constellation in the
southern sky lying between Aquarius and Sagittarius. Capricornus is the tenth
sign of the zodiac. The constellation has been described from the earliest
times as a goat, or as a figure that is part goat with the tail of a fish (see
illustration). The Tropic of Capricorn originates from this constellation,
which marked the southern limit of the ecliptic in ancient times. See
CONSTELLATION.






 �34s[Ching-Sung Yu]
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�6v�2v�32sCaprimulgiformes   �21m �33s 
    An order containing five families of nocturnal or crepuscular birds. The 
largest and best-known family is the Caprimulgidae, the nightjars or 
goatsuckers, found virtually throughout the tropical and temperate areas of 
the world. All of the approximately 70 species are colored for good 
concealment when at rest during the day, but many have contrasting white 
patches in wing or tail, presumably used in signaling behavior. These are 
highly vocal birds; several, such as the North American whippoorwill 
(Caprimulgus vociferus), are named for their call notes. A hibernationlike 
torpidity, previously unknown in birds, was first demonstrated in the poorwill 
(Phalaenoptilus nuttallii), and is now known to occur in other nightjars as 
well as in some hummingbirds. 
    Larger relatives are the neotropical potoos (Nyctibiidae) and the 
heavy-billed frogmouths (Podargidae) of Asia and Australia. The small 
Australo-Papuan owlet-nightjars (Aegothelidae) suggest a link between this 
order and the owls. The oilbird or guacharo (Steatornis caripensis), sole 
member of the South American family Steatornithidae, nests in caves, often in 
total darkness. It navigates by echolocation. See AVES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sCarat   �21m �33s 
    The unit of weight now used for all gemstones except pearls. It is also 
called the metric carat (m.c.). By international agreement, the carat weight 
is set at 200 milligrams. Pearls are weighed in grains, a unit of weight equal 
to 50 mg, or 1/4  carat. The application of the term carat as a unit of weight 
must not be confused with the term karat used to indicate fineness or purity 
of the gold in which gems are mounted. See GEM.  
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sCaraway �21m �33s
    An important spice from the fruits of the perennial herb Carum carvi, of
the family Umbelliferae. A native of Europe and western Asia, it is now
cultivated in many temperate areas of both hemispheres. The small, brown,
slightly curved fruits are used in perfumery, cookery, confectionery, in
medicine, and for flavoring beverages. See UMBELLALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCarbide   �21m �33s 
    A binary compound of carbon with an element less electronegative than 
carbon. Carbon-hydrogen compounds are excluded. Effectively then, carbides are 
composed of metal-carbon compounds if boron and silicon are included among the 
normal metals. Essentially no volatile compounds (except AlC or its dimer) are 
known, because decomposition sets in at higher temperatures before 
volatilization of the carbide as such. 
    Most carbides can be prepared by heating a mixture of the powdered metal 
and carbon, usually to high temperatures, but not necessarily as high as the 
melting point. Generally, the same result is possible by heating a mixture of 
the oxide of the metal with carbon (CaO + 3C->CaC2 + CO). Some may be prepared 
by passing a hydrocarbon vapor over the hot metal in the form of an 
electrically heated filament. See CARBON. 
 �34s[Russell K. Edwards]
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�6v�2v�32sCarbohydrate   �21m �33s 
    A term applied to a group of substances which include the sugars, 
starches, and cellulose, along with many other related substances. This group 
of compounds plays a vitally important part in the lives of plants and 
animals, both as structural elements and in the maintenance of functional 
activity. Plants are unique in that they alone in nature have the power to 
synthesize carbohydrates from carbon dioxide and water in the presence of the 
green plant chlorophyll through the energy derived from sunlight, by the 
process of photosynthesis. This process is responsible not only for the 
existence of plants but for the maintenance of animal life as well, since 
animals obtain their entire food supply directly or indirectly from the 
carbohydrates of plants. See CARBOHYDRATE METABOLISM; PHOTOSYNTHESIS. 
    Ths term carbohydrate originated in the belief that naturally occurring 
compounds of this class, for example, D-glucose (C6H12O6), sucrose 
(C12H22O11), and cellulose (C6H10O5) sub n, could be represented formally as 
hydrates of carbon, that is, C sub x(H2O) sub y. Later it became evident that
this definition for carbohydrates was not a satisfactory one. New substances
were discovered whose properties clearly indicated that they had the 
characteristics of sugars and belonged in the carbohydrate class, but which 
nevertheless showed a deviation from the required hydrogen-to-oxygen ratio. 
Examples of these are the important deoxy sugars, D-deoxyribose, L-fucose, and 
L-rhamnose, the uronic acids, and such compounds as ascorbic acid (vitamin C). 
The retention of the term carbohydrate is therefore a matter of convenience 
rather than of exact definition. A carbohydrate is usually defined as either a 
polyhydroxy aldehyde (aldose) or ketone (ketose), or as a substance which 
yields one of these compounds on hydrolysis. However, included within this 
class of compounds are substances also containing nitrogen and sulfur. See 
DEOXYRIBOSE; L-FUCOSE; L-RHAMNOSE. 
    The properties of many carbohydrates differ enormously from one substance 
to another. The sugars, such as D-glucose qlor sucrose, are easily soluble, 
sweet-tasting, and crystalline; the starches are colloidal and paste-forming; 
and cellulose is com-pletely insoluble. Yet chemical analysis shows that they 
have a common basis; the starches and cellulose may be degraded by different 
methods to the same crystalline sugar, D-glucose. 
    The carbohydrates usually are classified into three main groups according 
to complexity: monosaccharides, oligosaccharides, and polysaccharides. 
Monosaccharides are simple sugars that consist of a single carbohydrate unit 
which cannot be hydrolyzed into simpler substances. These are characterized, 
according to their length of carbon chain, as trioses (C3H6O3), tetroses 
(C4H8O4), pentoses (C5H10O5), hexoses (C6H12O6), heptoses (C7H14O7), and so 
on. Oligosaccharides are compound sugars that are condensation products of two 
to five molecules of simple sugars and are subclassified into disaccharides, 
trisaccharides, tetrasaccharides, and pentasaccharides, according to the 
number of monosaccharide molecules yielded upon hydrolysis. Polysaccharides 
comprise a heterogeneous group of compounds which represent large aggregates 
of monosaccharide units, joined through glycosidic bonds. They are tasteless, 
nonreducing, amorphous substances that yield a large and indefinite number of 
monosaccharide units on hydrolysis. Their molecular weight is usually very 
high, and many of them, like starch or glycogen, have molecular weights of 
several million. They form colloidal solutions, but some polysaccharides, of 
which cellulose is an example, are completely insoluble in water. On account 
of their heterogeneity they are difficult to classify. See MONOSACCHARIDE; 
OLIGOSACCHARIDE; POLYSACCHARIDE. 
    The sugars are also classified into two general groups, the reducing and 
nonreducing. The reducing sugars are distinguished by the fact that because of 
their free, or potentially free, aldehyde or ketone groups they possess the 
property of readily reducing alkaline solutions of many metallic salts, such 
as those of copper, silver, bismuth, mercury, and iron. The most widely used 
reagent for this purpose is Fehling's solution. The reducing sugars constitute 
by far the larger group. The monosaccharides and many of their derivatives 
reduce Fehling's solution. Most of the disaccharides, including maltose, 
lactose, and the rarer sugars cellobiose, gentiobiose, melibiose, and 
turanose, are also reducing sugars. The best-known nonreducing sugar is the 
disaccharide sucrose. Among other non re ducing sugars are the disaccharide 
trehalose, the trisaccharides raffinose and melezitose, the tetrasaccharide 
stachyose, and the pentasaccharide verbascose. 
    The sugars consist of chains of carbon atoms which are united to one 
another at a tetrahedral angle of 109DG28'. A carbon atom to which are 
attached four different groups is called asymmetric. A sugar, or any other 
compound containing one or more asymmetric carbon atoms, possesses optical 
activity; that is, it rotates the plane of polarized light to the right or 
left. See OPTICAL ACTIVITY. 
 �34s[William Z. Hassid]
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�6v�2v�32sCarbohydrate metabolism �21m �33s
    The fields of biochemistry and physiology deal with the breakdown and
synthesis of simple sugars, oligosaccharides, and polysaccharides, and with
the transport of sugars across cell membranes and tissues. The breakdown or
dissimilation of simple sugars, particularly glucose, is one of the principal
sources of energy for living organisms. The dissimilation may be anaerobic, as
in fermentations, or aerobic, that is, respiratory. In both types of
metabolism, the breakdown is accompanied by the formation of energy-rich
bonds, chiefly the pyrophosphate bond of the coenzyme adenosinetriphosphate
(ATP), which serves as a coupling agent between different metabolic processes.
In higher animals, glucose is the carbohydrate constituent of blood, which
carries it to the tissues of the body. In higher plants, the disaccharide
sucrose is often stored and transported by the tissues. Certain
polysaccharides, especially starch and glycogen, ars stored as endogenous food
reserves in the cells of plants, animals, and microorganisms. Others, such as
cellulose, chitin, and bacterial polysaccharides, serve as structural
components of cell walls. As constituents of plant and animal tissues, various
carbohydrates become available to those organisms which depend on other living
or dead organisms for their source of nutrients. Hence, all naturally
occurring carbohydrates can be dissimilated by some animals or microorganisms.
See ADENOSINETRIPHOSPHATE (ATP); CARBOHYDRATE; CELLULOSE; CHITIN; GLYCOGEN;
STARCH.
    Certain carbohydrates cannot be used as nutrients by humans. For example,
cellulose cannot be digested by humans or other mammals and is a useful food
only for those, such as the ruminants, that harbor cellulose-decomposing
microorganisms in their digestive tracts. The principal dietary carbohydrates
available to humans are the simple sugars glucose and fructose, the
disaccharides sucrose and lactose, and the polysaccharides glycogen and
starch. Lactose is the carbohydrate constituent of milk and hence one of the
main sources of food during infancy. The disaccharides and polysaccharides
that cannot be absorbed directly from the intestine are first digested or
hydrolyzed by enzymes (glycosidases) secreted into the alimentary canal.
    The simple sugars reach the intestine or are produced there through the
digestion of oligosaccharides. They are absorbed by the intestinal mucosa and
transported across the tissue into the bloodstream. This process involves the
accumulation of sugar against a concentration gradient and requires active
metabolism of the mucosal tissue as a source of energy. The sugars are
absorbed from the blood by the liver and are stored there as glycogen. The
liver glycogen serves as a constant source of glucose in the bloodstream. The
mechanisms of transport of sugars across cell membranes and tissues are not
yet understood, but they appear to be highly specific for different sugars and
to depend on enzymelike components of the cell.
    The degradation of monosaccharides may follow one of several types of
metabolic pathways. In the phosphorylative pathways, the sugar is first
converted to a phosphate ester. The phosphorylated sugar is then split into
smaller units, either before or after oxidation. In the nonphosphorylative
pathways, the sugar is usually oxidized to the corresponding aldonic acid.
This may subsequently be broken down either with or without phosphorylation of
the intermediate products. Among the principal intermediates in carbohydrate
metabolism are glyceraldehyde-3-phosphate and pyruvic acid. The end products
of metabolism depend on the organism and, to some extent, on the environmental
conditions. Besides cell material, the products may include carbon dioxide
(CO2), alcohols, organic acids, and hydrogen gas. In the so-called complete
oxidations, CO2 is the only excreted end product. In incomplete oxidations,
characteristic of the vinegar bacteria and of certain fungi, oxidized end
products such as gluconic, ketogluconic, citric, or fumaric acids may
accumulate. Organic end products are invariably found in fermentations. See
FERMENTATION.
    The principal phosphorylative pathway involved in fermentations is known
as the glycolytic, hexose diphosphate, or Embden-Meyerhof pathway (see
illustration). This sequence of reactions is the basis of the lactic acid
fermentation of mammalian muscle and of the alcoholic fermentation of yeast.
The metabolism of simple sugars other than glucose usually involves the
conversion of the sugar to one of the intermediates of the phosphorylative
pathways described for glucose metabolism.
    The oxidative or respiratory metabolism of sugars differs in several
respects from fermentative dissimilation. First, the oxidative steps, that is,
the reoxidation of DPNH, are linked to the reduction of molecular oxygen.
Second, the pyruvic acid produced through glycolytic or other mechanisms is
further oxidized, usually to CO2. Third, in most aerobic organisms,
alternative pathways either supplement or completely replace the glycolytic
sequence of reactions for the oxidation of sugars. Where pyruvic acid appears
as a metabolic intermediate, it is generally oxidatively decarboxylated to
yield CO2 and the two-carbon acetyl fragment which combines with coenzyme A.
The acetyl group is then further oxidized via the Krebs cycle. See KREBS CYCLE.
    The dissimilation and biosynthesis of the oligosaccharides and
polysaccharides are affected through the enzymatic cleavage or formation of
glycosidic bonds between simple monosaccharide constituents of the complex
carbohydrates. The principal types of enzyme which split or synthesize
glycosidic bonds are the hydrolases or glycosidases, phosphorylases, and
transglycosylases. The enzymes are generally highly specific with respect to
the glycosidic portion, or moiety, and the type of linkage of the substrates
which they attack. Lactases, for example, combine the BETA-galactosyl portion
of the disaccharide lactose with hydroxyl ion of water to yield galactose,
liberating glucose. Maltases can hydrolyze maltose to two molecules of
glucose. Sucrases or invertases are enzymes which hydrolyze sucrose to yield
glucose and fructose. Amylases or diastases are enzymes which hydrolyze






 �34s[Michael Doudoroff]
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�6v�2v�32sCarbomycin   �21m �33s 
    A macrolide antibiotic isolated from fermentation broths of strains of 
Streptomyces halstedii, S. hygroscopicus, and perhaps other species. Macrolide 
antibiotics are basic antibiotics characterized by a macrocyclic ring 
structure. Not infrequently, small amounts of a second closely related 
antibiotic, carbomycin B, occur in the broth. Carbomycin is known commercially 
as Magnamycin. 
    The antibacterial spectrum of carbomycin is very similar to that of 
penicillin; for example, it is principally active against gram-positive 
bacteria. Unlike penicillin, carbomycin is also active against various 
rickettsiae. See ANTIBIOTIC 
 �34s[Fred W. Tanner, Jr.]
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�6v�2v�32sCarbon �21m �33s
    A chemical element, C, with an atomic number of 6 and an atomic weight of
12.01115. Carbon is unique in chemistry because it forms a vast number of
compounds, larger than the sum total of all other elements combined. By far
the largest group of these compounds are those composed of carbon and
hydrogen. It has been estimated that there are at least 1,000,000 known
organic compounds, and this number is increasing rapidly each year. Although
the classification is not rigorous, carbon forms another series of compounds,
classified as inorganic, comprising a much smaller number than the organic
compounds. See ORGANIC CHEMISTRY.
    Elemental carbon exists in two well-defined crystalline allotropic forms,
diamond and graphite. Other forms, which are poorly developed in
crystallinity, are charcoal, coke, and carbon black. Chemically pure carbon is
prepared by the thermal decomposition of sugar (sucrose) in the absence of
air. The physical and chemical properties of carbon are dependent on the
crystal structure of the element. The density varies from 2.25 g/cm sup 3 (1.30
oz/in. sup 3) for graphite to 3.51 g/cm sup 3 (2.03 oz/in. sup 3) for diamond.
For graphite, the melting point is 3500 DG C (6332 DG F) and the extrapolated
boiling point is 4830 DG C (8726 DG F). Elemental carbon is a fairly inert
substance. It is insoluble in water, dilute acids and bases, and organic
solvents. At elevated temperatures, it combines with oxygen to form carbon
monoxide or carbon dioxide. With hot oxidizing agents, such as nitric acid and
potassium nitrate, mellitic acid, C6(CO2H)6, is obtained. Of the halogens, only
fluorine reacts with elemental carbon. A number of metals combine with the
element at elevated temperatures to form carbides.
    Carbon forms three gaseous compounds with oxygen: carbon monoxide, CO;
carbon dioxide, CO2; and carbon suboxide, C3O2. The first two oxides are the
more important from an industrial standpoint. Carbon forms compounds with the
halogens which have the general formula CX4, where X is fluorine, chlorine,
bromine, or iodine. At room temperature, carbon tetrafluoride is a gas, carbon
tetrachloride is a liquid, and the other two compounds are solids. Mixed
carbon tetrahalides are also known. Perhaps the most important of them is
dichlorodifluoromethane, CCl2F2, commonly called Freon. See CARBON DIOXIDE;
CARBON TETRACHLORIDE; FREON; HALOGENATED HYDROCARBON.
    Carbon and its compounds are found widely distributed in nature. It is
estimated that carbon makes up 0.032% of the Earth's crust. Free carbon is
found in large deposits as coal, an amorphous form of the element which
contains additional complex carbon-hydrogen-nitrogen compounds. Pure
crystalline carbon is found as graphite and as diamonds.
    Extensive amounts of carbon are found in the form of its compounds. In the
atmosphere, carbon is present in amounts of up to 0.03% by volume as carbon
dioxide. Various minerals such as limestone, dolomite, marble, and chalk all
contain carbon in the form of carbonate. All plant and animal life is composed
of complex organic compounds containing carbon combined with hydrogen, oxygen,
nitrogen, and other elements. The remains of past plant and animal life are
found as deposits of petroleum, asphalt, and bitumen. Deposits of natural gas
contain compounds that are composed of carbon and hydrogen. See CARBONATE
MINERALS.
    The free element has many uses, ranging from ornamental applications of
the diamond in jewelry to the black-colored pigment of carbon black in
automobile tires and printing inks. Another form of carbon, graphite, is used
for high-temperature crucibles, arc-light and dry-cell electrodes, lead
pencils, and as a lubricant. Charcoal, an amorphous form of carbon, is used as
an absorbent for gases and as a decolorizing agent. See CARBON BLACK;
CHARCOAL; DIAMOND; GRAPHITE.
    The compounds of carbon find many uses. Carbon dioxide is used for the
carbonation of beverages, for fire extinguishers, and in the solid state as a
refrigerant. Carbon monoxide finds use as a reducing agent for many
metallurgical processes. Carbon tetrachloride and carbon disulfide are
important solvents for industrial uses. Freon is used in refrigeration
devices. Calcium carbide is used to prepare acetylene, which is used for the
welding and cutting of metals as well as for the preparation of other organic
compounds. Other metal carbides find important uses as refractories and metal
cutters.
 �34s[E. Eugene Weaver]
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�6v�2v�32sCarbon arc lamp   �21m �33s 
    An arc lamp whose electrodes are pure carbon. The lamp is either open, 
enclosed, or an intensified arc with incandescence at the electrodes and some 
light from the luminescence of the arc. The open-arc form of the carbon arc is 
obsolete. In the enclosed type, a restricted supply of air slows the electrode 
consumption and secures a life approximately 100 h. The intensified type uses 
a small electrode, giving a higher intensity output that is more white in 
color and more steady than either the open or enclosed types. The electrode 
life is approximately 70 h. In the high-intensity arc the current may reach 
values of 125 - 150 A with positive volt-ampere characteristics, and these may 
operate directly from the line without a ballast. The brightness of the 
high-intensity arc is higher and depends markedly on current. Although carbon 
arc lamps are built for alternating-current operation, direct current produces 
a steadier and more satisfactory operation. See ARC LAMP. 
 �34s[John O. Kraehenbuehl]
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�6v�2v�32sCarbonate �21m �33s
    The CO2-/3 ion derived from carbonic acid, H2CO3, which is a solution of
carbon dioxide in water. Because there are two replaceable hydrogen ions in
carbonic acid, the salts obtained by neutralizing only one hydrogen are called
bicarbonates, for example, sodium bicarbonate, NaHCO3.
    Carbonates can be converted into bicarbonates by adding excess carbon
dioxide. The reaction can be reversed by heating. Bicarbonates in general are
more soluble than carbonates. All the carbonates except those of the alkali
metals are insoluble.
 �34s[E. Eugene Weaver]
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�6v�2v�32sCarbonate minerals �21m �33s
    Mineral species containing the carbonate ion as the fundamental anionic
unit. The carbonate minerals can be classified as (1) anhydrous normal
carbonates, (2) hydrated normal carbonates, (3) acid carbonates
(bicarbonates), and (4) compound carbonates containing hydroxide, halide, or
other anions in addition to the carbonate. See CARBONATE.
    Most of the common carbonate minerals belong to group (1), and can be
further classified according to their structures. The rhombohedral carbonates
are typified by calcite, CaCO3, and by dolomite, CaMg(CO3)2. The other
structural type within this group is that of aragonite, which has orthorhombic
symmetry. See ANKERITE; ARAGONITE; CALCITE; CERUSSITE; DOLOMITE; HUNTITE;
KUTNAHORITE; MAGNESITE; RHODOCHROSITE; SIDERITE; SMITHSONITE; STRONTIANITE;
WITHERITE.
    The minerals in groups (2) and (3) all decompose at relatively low
temperatures and therefore occur only in sedimentary deposits (typically
evaporites) and as low-temperature hy-drothermal alteration products. The only
common mineral in these groups is trona, Na3H(CO3)2 CTR DOT 2H2O. See SALINE
EVAPORATE.
mpgs. 1166-1168
8:45
    Similarly, the group (4) minerals are relatively rare and are
characteristically low-temperature hydrothermal alteration products. The
commonest members of this group are malachite, Cu2CO3(OH)2, and azurite,
Cu3(CO3)2(OH)2, which are often found in copper ore deposits. See AZURITE;
MALACHITE.
    Important occurrences of carbonates include ultrabasic igneous rocks such
as carbonatites and serpentinites, and metamorphosed carbonate sediments,
which may recrystallize to form marble. The major occurrences of carbonates,
however, are in sedimentary deposits as limestone and dolomite rock. See
DOLOMITE ROCK; LIMESTONE; MARBLE; SEDIMENTARY ROCKS.
 �34s[Alan M. Gaines]
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�6v�2v�32sCarbonatites   �21m �33s 
    Carbonate-rich rocks of magmatic derivation or descent. Carbonatites are 
important scientifically because their study provides information (1) on the 
origin of their associated subsilicic alkalic rocks (nepheline syenites and so 
forth), (2) on the origin of low-silicate or nonsilicate magmas, (3) on 
explosive vulcanism, (4) on the origin of kimberlites and subcrustal 
processes, and (5) on the tectonics of cratonic areas. Their economic 
significance also is immense. In addition to being the major source of niobium 
(chiefly as pyrochlore), some carbonatites yield large amounts of rare-earth 
elements and others are sources or potential sources for phosphate (apatite), 
barite, thorium, vermiculite, magnetite, titanium, and carbonate rock. One of 
the world's largest copper deposits (at Palabora, South Africa) is in a 
carbonatite. The chief carbonate species are calcite, dolomite, and ankerite. 
Most of the features that characterize silicate igneous rocks also typify 
carbonatites. There are both intrusive and extrusive types. See IGNEOUS ROCKS. 
 �34s[E. William Heinrich]
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�6v�2v�32sCarbon black   �21m �33s 
    An amorphous form of carbon produced commercially by thermal or oxidative 
decomposition of hydrocarbons. It is used principally in rubber goods, 
pigments, and printer's ink. It is not an inert filler but enhances and 
reinforces various properties of rubber. 
    Manufacturing processes may be classed as contact, furnace, or thermal. In 
the channel (contact) process, natural gas is burned with insufficient air for 
complete combustion. The smoky flame from individual burners impinges on a 
cool channel iron, and carbon black deposited on the channel is removed by a 
scraper blade. In the furnace process, the hydrocarbon and air are fed into a 
reactor. Combustion of part of the hydrocarbon raises the temperature to 2000 
- 3000 DG F (1100 - 1650 DG C), causing decomposition of the unburned
hydrocarbon to carbon black. A water spray quickly cools the hot reaction
products, and the finely divided black is recovered by cyclones and bag
filters. In the thermal process, natural gas is decomposed to carbon and
hydrogen by heated refractories.  
 �34s[Carl J. Helmers]
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�6v�2v�32sCarbon dioxide   �21m �33s 
    A colorless, odorless, tasteless gas, formula CO2, about 1.5 times as 
heavy as air. Under normal conditions, it is stable, inert, and nontoxic. The 
decay (slow oxidation) of all organic materials produces CO2. Fresh air 
contains approximately 0.033% CO2 by volume. In the respiratory action 
(breathing) of all animals and humans, CO2 is exhaled. 
    Carbon dioxide gas may be liquefied or solidified. Solid CO2 is known as 
dry ice. Carbon dioxide is obtained commercially from four sources: gas wells, 
fermentation, combustion of carbonaceous fuels, and as a by-product of 
chemical processing. Applications include use as a refrigerant, in either 
solid or liquid form, inerting medium, chemical reactant, neutralizing agent 
for alkalies, and pressurizing agent. 
    Most CO2 is obtained as a by-product from steam-hydrocarbon reformers used 
in the production of ammonia, gasoline, and other chemicals; other sources 
include fermentation, deep gas wells, and direct production from carbonaceous 
fuels. Whatever the source, the crude CO2 (containing at least 90% CO2) is 
compressed in either two or three stages, cooled, purified, condensed to the 
liquid phase, and placed in insulated storage vessels. Carbon dioxide is 
distributed in three ways; in high-pressure uninsulated steel cylinders; as a 
low-pressure liquid in insulated truck trailers or rail tank cars; and as dry 
ice in insulated boxes, trucks, or boxcars. 
 �34s[J. S. Lindsey]
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�6v�2v�32sCarboniferous �21m �33s
    A division of later Paleozoic rocks, economically important because of the
fossil fuel (coal) deposits they contain. The Carboniferous represents a 65 x
10 sup 6 year time span that began 345 x 10 sup 6 years ago and ended 280 x 10
sup 6 years ago. The name Carboniferous was first applied to the coal-bearing
rocks in England: it is now widely used in Europe, Asia, northern Africa,
northern South America, and Greenland. In North America the term Carboniferous
is not widely used; instead, the time interval and representative rock record
are subdivided into two major units, the earlier Mississippian period (345 to
310 x 10 sup 6 years ago) and the later Pennsylvanian period (310 to 280 x 10
sup 6 years ago). In southern Africa, southern South America, and Antarctica
(the areas referred to as Gondwanaland), the late Paleozoic and early Mesozoic
Gondwana System include Carboniferous age rocks. See MISSISSIPPIAN; PALEOZOIC;
PALEOZOIC; PENNSYLVANIAN.
    Although no type locality was designated, the Pennine up-lands of
Yorkshire and Derbyshire, England, were intended to be used as the typical
area of the Carboniferous system of rocks and period of time. They are limited
below (earlier) by the Devonian and above (later) by the Permian. The
Carboniferous rocks are separated from the overlying and underlying systems by
unconformities in many places; however, where there are no physical
interruptions, the positions of the lower and upper limits have been based on
a comparison of fossil faunas and floras with those from the type Devonian
(southwestern England) and the type Permian (European Soviet Union). A
widespread and distinct unconformity separates the Lower Carboniferous
(Mississippian) rocks from the Upper Carboniferous (Pennsylvanian) rocks
throughout much of North America. Recognition of this unconformity, separating
the largely non-coal-bearing Mississippian from the coal-bearing Pennsylvanian
strata, led to the early disuse of the term Carboniferous in North America.
Although Carboniferous rocks are widespread on all continents, they are absent
or poorly known in areas of much older rocks, such as the Canadian or African
Shield, or in areas where most of the rocks are made up of younger Mesozoic
and Cenozoic sediments and volcanics. See DEVONIAN; PERMIAN.
    Theories of global tectonics and continental drift propose that at the
beginning of Carboniferous time there were three major continental masses
moving toward each other; they eventually came together at the end of the
Carboniferous to form the supercontinent Pangaea. These three continents were
North America - Greenland - western Europe; eastern Europe northern and
eastern Asia; and Gondwanaland (South America, Africa, India, Antarctica, and
Australia). North America - western Europe was separated from northern Africa
and eastern Europe by a narrow proto-Atlantic, and eastern Europe - northern
Asia was separated from northern Africa and Gondwanaland by an eastern
widening sea called Tethys. It has been proposed that Gondwanaland was in the
region of the South Pole during the Carboniferous and early Permian time. See
CONTENTAL DRIFT; CONTINENTS; PLATE TECTONICS.
    During the Carboniferous the broad epeieric seas supported an abundant
diversity of invertebrates, including brachiopods, cephalopods, pelecypods
(clams), gastropods (snails), crinoids, corals, and bryozoa. Among the
microfossils, fusulinids, ostracods, and conodonts were abundant and diverse.
Trilobites  were present, but nearing the end of their long history. Corals
formed reefs, and calcareous algae formed reeflike mounds in southern Kansas
and western and central Texas. Although insect fossils are uncommon, insects
were highly diverse and included large dragonflies and giant cockroaches.
Among vertebrates there were large sharks, adapted to fresh and marine waters,
and amphibians. During the later Carboniferous the first reptiles appeared.
    Fossil plants include large trees in widespread swampy, coastal forests
throughout many continents. There were also primitive gymnosperms, related to
conifers of today, which seem to have grown in somewhat less swampy areas and
drier climates. The first seed-bearing plants (pteridosperms), with foliage
resembling ferns, appeared during the Middle Carboniferous. See PALEOBOTANY;
PALEONTOLOGY.
    The Upper Carboniferous (Pennsylvanian) is the world's leading source of
medium- and high-ranked coal. It is produced in the Appalachian coalfield, the
Illinois Basin, and eastern Mid-Continent in the United States, the Maritime
Provinces of eastern Canada, England, southern Wales, the Midland Valley of
Scotland, the Westphalian coal basin of western Europe, the Silurian coal
basin of Poland, the Donets Basin of the southern Soviet Union, and smaller
basins in France, Spain, Morocco, Algeria, Norway, and nothern China and
Korea. Petroleum is produced from Carboniferous rocks in many parts of the
central and western United States, western Canada, western Europe, and the
North Sea. Oil shales occur in Carboniferous rocks in Scotland. Refractory
clays (diaspore and kaolinite) are derived from Lower Pennsylvanian sediments
in Pennsylvania, Ohio, eastern Kentucky, northern and western Illinois,
eastern Missouri, and the Missouri Ozarks.
 �34s[Wayne A. Pryor]
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�6v�2v�32sCarbon-nitrogen-oxygen cycles �21m �33s
    A group of nuclear reactions that involve capture of protons by carbon,
nitrogen, and oxygen nuclei. These cycles are believed to be the source of
energy in main-sequence stars which are more massive than the Sun. Completion
of any one of the cycles results in consumption of four protons, synthesis of
one helium nucleus (sup 4 He) and two neutrinos, and 26.73 MeV of energy. This
energy E reflects the difference in mass m between the four protons and the
helium nucleus and is equal to the mass difference times the square of the
velocity of light c, as is known from Einstein's statement of mass-energy
equivalence, E = mc sup 2. Because the nuclear fuel consumed in these processes
is hydrogen, they are referred to as hydrogen burning by means of the
carbon-nitrogen-oxygen (CNO) cycles.
 �34s[Georgeanne R. Caughlan]
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�6v�2v�32sCarbon star   �21m �33s 
    Any of a class of stars with an apparently high abundance ratio of carbon 
to hydrogen. In normal stars this ratio also prevails, oxygen is more abundant 
than carbon, and carbon and nitrogen are about equal. In carbon stars, 
however, carbon is more abundant than oxygen. The majority of the carbon-rich 
stars are low-temperature red giants of the C class, showing bands of C2, CN, 
and CH (the older designation of this is R or N). A few hot carbon-rich stars 
are also known. The importance of the C/O ratio lies in the theory of the 
origin of chemical elements, in which it is shown that carbon is easily 
synthesized at temperatures found in cores of evolving red giants. See STAR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sCarbon tetrachloride   �21m �33s 
    A colorless, dense liquid (specific gravity 1.595 at 20 DG C), formula CCl. 
It is manufactured by treating carbon disulfide with dry chlorine gas in the 
presence of a catalyst and is widely used in dry cleaning; as a solvent for 
waxes, oils, and greases; in fire extinguishers, particularly for oil and 
electrical fires; and in the manufacture of Freon refrigerants. See 
HALOGENATED HYDROCARBON. 
 �34s[Frank H. Rockett]
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�6v�2v�32sCarborane �21m �33s
    A generic term for a class of chemical compounds composed of boron,
carbon, and hydrogen. The term is also the common name for an important member
of this class of compounds, C2B10H12. Carborane derivatives have been used to
develop new types of polymers which withstand high temperatures. In addition,
carboranes have been reacted with transition metals to produce new types of
complexes, some of which exhibit catalytic activity.
    The carboranes and the boranes have structures which can  be rationalized
on the basis of a knowledge of triangulated polyhedrons, or fragments of
triangulated polyhedrons. Triangulated polyhedrons with 4 to 12 vertexes are
especially important in understanding the structures of boranes and
carboranes. Three polyhedrons, those with 4, 6, and 12 vertexes, are very
symmetrical, and their vertexes are all geometrically equivalent. (see
illustration). The structures of most boranes and carboranes are derived by
placing a B-H unit or a C-H unit at the vertexes of the polyhedrons. For
example, the borane anion B12H12 sup 2- has the icosahedral structure with a B-H
unit at each of the 12 vertexes. A carborane exists when a CH unit replaces a
B-H unit at a vertex. Two known icosahedral carboranes are CB10H12- and
C2B10H12, in which one and two C-H units reside at the vertexes of the
icosahedron. There is only a single isomer of CB10H12-, but all three possible
carbon atom in the three-dimensional skeleton of the structure. If all the
carbon atoms are in side groups, replacing a hydrogen atom in a borane, the
compound is not a carborane. For example, C2H5B12H11 sup 2- is a derivative of
B12H12 sup 2- in which a hydrogen atom has been replaced by an ethyl group; it
is thus not a carborane even though it is composed of boron, carbon, and
hydrogen. See BORANE.






 �34s[Patrick A. Wegner]
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�6v�2v�32sCarboxylic acid   �21m �33s 
    One of a large family of organic substances widely distributed in nature, 
and characterized by the presence of one or more carboxyl groups ( - COOH). 
These groups typically yield protons in aqueous solution. In the type formula, 
R(CXY) sub nCOOH, symbols R, X, and Y can be hydrogen, saturated or unsaturated 
groups, carboxyl, alicyclic, or aromatic groups, halogens, or other 
substituents, and n may  vary from zero (formic acid, HCOOH) to more than 100, 
provided that the normal carbon covalence of four is maintained. 
    Physical and chemical properties of carboxylic acids are represented, 
grossly, by the resultant of the various chemical groupings present in the 
molecule. A short-chain aliphatic acid, wherein the carboxyl is dominant, is a 
pungent, corrosive, water-soluble liquid of abnormally high boiling point 
(because of molecular association), with specific gravity close to 1 (higher 
for formic and acetic acids). With increasing molecular weight, the 
hydrocarbon grouping overbalances the carboxyl; sharpness of odor diminishes, 
boiling and melting points rise, the specific gravity falls toward that of the 
parent hydrocarbon, and the water solubility decreases. Thus the typical 
high-molecular-weight saturated acid is a bland, waxlike solid. 
    Acids are used in large quantities in the production of esters, acid 
halides, acid amides, and acid anhydrides. They find wide use in the 
manufacture of soaps and detergents, in thickening lubricating greases 
(stearate soaps), in modifying rigidity in plastics, in compounding buffing 
bricks and abrasives, and in the manufacture of crayons, dictaphone cylinders, 
and phonograph records. The solvent action of acids finds use in manufacture 
of carbon paper, inks, and in the compounding of synthetic and natural rubber. 
Because of the stability of saturated fatty acids toward oxidation, these are 
often used as solvents for carrying out oxidation reactions upon sensitive 
compounds.  
 �34s[Evans B. Reid]
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�6v�2v�32sCarbuncle   �21m �33s 
    An infection of the subcutaneous tissues in which multiple sinuses are 
created by tissue destruction caused by the spreading microorganism. The 
commonest agent responsible for carbuncles is Staphylococcus aureus, although 
other microorganisms are sometimes responsible. See STAPHYLOCOCCUS. 
    Most carbuncles begin with the spread of infection from an initial site in 
a hair follicle or other superficial structure. The invading organisms 
multiply and penetrate into the deeper regions of the dermis, subcutaneous 
tissues, and sometimes even below the deep fascia of the body layers. Although 
the boil is typically a single pocket of pus, the carbuncle differs by being 
multiloculated and usually deeper in its location. Quite often tissue 
penetration produces several superficial openings into the carbuncle, all of 
which may drain pus. 
    Regional tenderness, pain, and swelling are usual. In older or debilitated 
persons there is a greater chance of such infections becoming blood-borne, 
with resultant bacteremia, spread of abscesses to distant sites, and severe 
damage to such organs as the kidney, brain, and lungs. 
 �34s[Edward G. Stuart /N. Karle Mo]
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�6v�2v�32sCarburetor   �21m �33s 
    The device that controls the power output and fuel feed of internal 
combustion spark-ignition engines generally used for automotive, aircraft, and 
auxiliary services. Its duties include control of the engine power by the air 
throttle; metering, delivery, and mixing of fuel in the airstream; and 
graduating the fuel-air ratio according to engine requirements in starting, 
idling, and load and altitude changes. 
    A simple updraft carburetor illustrates basic carburetor action (see 
illustration). Intake air charge, at full or reduced atmospheric pressure as 
controlled by the throttle, is drawn into the cylinder by the downward motion 
of the piston to mix with the unscavenged exhaust remaining in the combustion 
chamber from the previous explosion. The total air-charge weight per unit time 
is approximately proportional to the square root of its pressure drop in the 
carburetor venturi throat (with a square root correction, direct for change in 
air pressure, and inverse for change in air absolute temperature). Also, each 
individual cylinder air-charge volume follows the intake pressure minus about 
one-sixth the exhaust pressure, so that the intake manifold pressure and 
temperature are often taken as an approximate indication of the engine torque; 
this factor times the engine speed is similarly taken as a measure of the 
engine power output. Any given fixed part-throttle opening gives higher intake 
manifold pressure, increased individual-cylinder air charge, and greater 
engine torque, as the engine speed is decreased. 
    The fuel is usually metered through a calibrated metering orifice at a 
differential pressure derived from the pressure drop in a venturi throat in 
the intake air passage. For normal steady speed, the limits of consistent 
ignition lie between 0.060 and 0.083 fuel-air by weight. Richer mixtures (more 
fuel) give higher power, apparently cooler combustion, and less detonation. 
The weaker mixtures give better fuel economy. A narrowly controlled rich 
mixture is required for idling because low charge densities and relatively 
high contamination with residual exhaust make combustion conditions 
unfavorable. Individual adjustments are required both for the idling fuel feed 
and for the throttle closure stop to keep the engine firing at minimum speed. 
    There are two reasons for condensation of atmospheric moisture and 
formation of ice in the intake system. (1) At partly closed throttle, the 
adiabatic pressure drop across the throttle orifice generates a temperature 

     FIGURE CAPTION.  Elements which basically determine air and fuel charges
received by an engine through the carburetor.

drop. When atmospheric humidity is high, ice sometimes partly clogs the 
throttle orifice, requiring further throttle opening to keep the engine 
running for a short period after starting and before the engine heat has had 
time to build up. (2) As fuel evaporates, ice tends to form wherever fuel 
spray impinges upon an unheated surface. The problem has been largely 
eliminated either by use of the injection carburetor, which delivers fuel only 
in the warm parts of the intake system, or by direct injection into the engine 
cylinders. See INTERNAL COMBUSTION ENGINES. 
 �34s[Frank C. Mock /J. A. Bolt]
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�6v�2v�32sCardamon �21m �33s
    The plant Elettaria cardamomum (Zingiberaceae), a perennial herb that is a
native of India. The small, light-colored seeds, borne in capsules, have a
delicate flavor. They are used in curries, cakes, pickles, and in general
cooking, as well as in medicine. See ZINGIBERALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCardinal points �21m �33s
    The four intersections of the horizon with the meridian and with the prime
vertical circle, or simply prime vertical, the intersections with the meridian
being designated north and south, and the intersections with the prime
vertical being designated east and west (see illustration). The cardinal
points are 90 DG apart; they lie in a plane with each other and correspond to
the cardinal regions of the heavens. The four intermediate points, northeast,
southeast, northwest, and southwest, are the collateral points.



 �34s[Frank H. Rockett]
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�6v�2v�32sCardioid �21m �33s
    A heart-shaped curve generated by a point of a circle that rolls (without
slipping) on a fixed circle of the same diameter. In point-wise construction
of the curve, let O be a fixed point of a circle C of diameter a, and Q a
variable point of C. Lay off distance a along the secant OQ, in both
directions from Q. The locus of the two points thus obtained is a cardioid
(see illustration). If a rectangular coordinate system is chosen with O for
origin initially and y axis tangent to C at O, the cardioid has equation
(x (sup 2) + y (sup 2) - ax)sup 2 = a (sup 2)(x (sup 2) + y (sup 2)).
The equation in polar coordinates is p = a(1 + cos THETA).
Its area is 3/2PIa(sup 2), or six times the area of C, and its length is 8a.
 See ANALYTIC GEOMETRY.




 �34s[Leonard M. Blumenthal]
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�6v�2v�32sCardiovascular system   �21m �33s 
    Those structures, such as the heart, or pumping mechanism, and the 
arteries, veins, and capillaries, which provide channels for the flow of 
blood. The cardiovascular system is sometimes called the blood-vascular 
system. The circulatory system includes both the cardiovascular and lymphatic 
systems; the latter consists of lymph channels (lymphatics), nodes, and fluid 
lymph which finally empties into the bloodstream. See ARTERY; BLOOD; CAPILLARY 
(ANATOMY); HEART (VERTEBRATE); HEMATOPOIESIS; LYMPHATIC SYSTEM; VEIN. 
 �34s[Charles K. Weichert]
�33s 
    Circulatory physiology describes the structure and operation of the 
circulation in living animals, and enquires as to how or why the circulatory 
system may have evolved. The circulatory system in all vertebrates has 
multiple functions, but all functions are involved in regulating the internal 
environment of the animal (promoting homeostasis). In all vertebrates the 
circulatory system consists of a central pump, the heart, which drives a 
liquid transport medium, the blood, continuously around a closed system of 
tubes, the vascular system. The arterial portion of this system is divided 
into larger elastic and smaller resistance vessels (arterioles) which 
distribute blood to specialized regions or organs where transfer of nutrients, 
oxygen, or waste products takes place across the walls of a fine network of 
microscopic capillaries. Blood from the capillaries passes through the venules 
(small venous vessels) into the main vein and returns to the heart. The 
arterioles, venules, and capillaries make up the microcirculation, which is 
arguably the most important functional role of the vertebrate circulatory 
system from a functional point of view. 
    Cardiovascular system disorders are those disorders which involve the 
arteries, veins, and lymphatics. See ARTERITIS; ARTERIOSCLEROSIS; 
LYMPHADENITIS; LYMPHANGITIS; PHLEBITIS; VARICOSE VEINS. 
 �34s[David R. Jones]
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�6v�2v�32sCaribbean Sea   �21m �33s 
    A deep ocean basin roughly 1700 mi (2800 km) long and up to 930 mi (1500 
km) wide, bordered by Central and South America and the West Indies island  
chain. Together with the Gulf of Mexico, it is sometimes called the American 
Mediterranean Sea. Two major basins, the large Caribbean Basin and the smaller 
and deeper Cayman Basin, are separated by the Jamaica Rise. The mild and 
equable climate of the Caribbean Sea may be marred in the summer and early 
autumn by occasional hurricanes. 
    The basic circulation is from east to west with North Equatorial Current 
water entering through passages between the eastern islands and exiting 
through the Yucatan Channel to the Gulf of Mexico, and eventually through the 
Florida Straits  to become an important portion of the Gulf Stream system. See 
ATLANTIC OCEAN; GULF OF MEXICO; OCEAN CURRENTS. 
 �34s[William G. Metcalf]
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�6v�2v�32sCarnauba wax   �21m �33s 
    Product exuded from the leaves of the wax palm, Copernicia cerifera, a 
native of Brazil and other regions in tropical South America. It is the 
hardest, highest-melting natural wax and is used in making candles, shoe 
polish, high-luster wax, varnishes, phonograph records, and surface coating of 
automobiles. See PALM; WAX, ANIMAL AND VEGETABLE. 
 �34s[Earl L. Core]
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�6v�2v�32sCarnivora   �21m �33s 
    One of the larger orders of placental mammals, including fossil and living 
dogs, raccoons, pandas, bears, weasels, skunks, badgers, otters, mongooses, 
civets, cats, hyenas, seals, walruses, and many extinct groups organized into 
12 families, with about 112 living genera and more than twice as many extinct 
genera. The subdivision of the order into three superfamilies has long been 
practiced and the following groups seem appropriate: Miacoidea, Canoidea, and 
Feloidea. The primary adaptation in this order was for predation on other 
vertebrates and invertebrates. A few carnivorans (for example, bear and panda) 
have secondarily become largely or entirely herbivorous, but even then the 
ancestral adaptations for predation are still clearly evident in the structure 
of the teeth and jaws. The Carnivora have been highly successful animals since 
their first appearance in the early Paleocene. 
    Structural adaptations involve the teeth and jaws. The dentition is 
sharply divided into three functional units. The incisors act as a tool for 
nipping and delicate prehension, and the large, interlocking upper and lower 
canines for heavy piercing and tearing during the killing of prey. The cheek 
teeth are divided into premolars (for heavy prehension) and molars (for 
slicing and grinding), which may be variously modified depending on the 
specific adaptation, but there is a constant tendency for the last (fourth) 
upper premolar and the first lower molar to enlarge and form longtitudinal 
opposed shearing blades (the carnassials). In all carnivorans the jaw 
articulation is arranged in such a manner that movement is limited to vertical 
hinge motions and transverse sliding. The temporal muscle dominates the jaw 
musculature, forming at least one-half of the total mass of the jaw muscles. 
mpgs. 1193-1194 
10:48 
    The earliest fossil records are early Paleocene, but the earliest 
well-represented material comes from the middle Paleocene of North America. 
During the Paleocene and Eocene the stem-carnivorans or miacoids underwent 
considerable diversification in both the Old and New World. At the end of 
Eocene and beginning of Oligocene time throughout the Northern Hemisphere, a 
dramatic change took place within the Carnivora; this was the appearance of 
primitive representatives of modern carnivoran families. See BADGER; BEAR; 
CAT; CIVET; COATI; DOGS AND ALLIES; FERRET; FISHER; HYENA; MAMMALIA; MARTEN; 
MINK; MONGOOSE; OTTER; PANDA; RACCOON; SEALS AND ALLIES; SKUNK; WEASEL; 
WOLVERINE.  
 �34s[Richard H. Tedford]
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�6v�2v�32sCarnot cycle �21m �33s
    A hypothetical thermodynamic cycle used as a standard of comparison for
actual cycles. The Carnot cycle shows that, even under ideal conditions, a
heat engine cannot convert all the heat energy supplied to it into mechanical
energy; some of the heat energy must be rejected.
    In a Carnot cycle, an engine accepts heat energy from a high-temperature
source, or hot body, converts part of the received energy into mechanical (or
electrical) work, and rejects the remainder to a low-temperature sink, or cold
body. The greater the temperature difference between the source and sink, the
greater the efficiency of the heat engine.
    The Carnot cycle (see illustration) consists first of an isentropic
compression, then an isothermal heat addition, followed by an isentropic
expansion, and concludes with an isothermal heat rejection process. In short,
the processes are compression, addition of heat, expansion, and rejection of
heat, all in a qualified and definite manner. The net effect of the cycle is
that heat is added at a constant high temperature, somewhat less heat is
rejected at a constant low temperature, and the algebraic sum of these heat
quantities is equal to the work done by the cycle.
    A Carnot cycle consists entirely of reversible processes; thus it can
theoretically operate to withdraw heat from a cold body and to discharge that
heat to a hot body. To do so, the cycle requires work input from its
surroundings. The heat equivalent of this work input is also discharged to the
hot body. Just as the Carnot cycle provides the highest efficiency for a power
cycle operating between two fixed temperatures, so does the reversed Carnot
cycle provide the best coefficient of performance for a device pumping heat
from a low temperature to a higher one. See HEAT PUMP; REFRIGERATION CYCLE.
    Good as the ideal Carnot cycle may be, there are serious difficulties that
emerge when one wishes to make an actual Carnot engine. The necessarily high
peak pressures and temperatures limit the practical thermal efficiency that an
actual engine can achieve. Although the Carnot cycle is independent of the
working substance, and hence is applicable to a vapor cycle, the difficulty of
efficiently compressing a vapor-liquid mixture renders the cycle impractical.
See POWER PLANT; THERMODYNAMIC CYCLE; THERMODYNAMIC PRINCIPLES.




 �34s[Theodore Baumeister]
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�6v�2v�32sCarnotite   �21m �33s 
    A mineral which is a hydrous vanadate of potassium and uranium, 
K2(UO2)2(VO4)2 CTR DOT nH2O. The water content varies at ordinary temperatures 
from one to three molecules. Carnotite generally occurs as a powder or as a 
slightly coherent microcrystalline aggregate. Color ranges from bright yellow 
to lemon- and greenish-yellow. 
    In the United States the principal region of carnotite mineralization is 
the Colorado Plateau and adjoining districts of Utah, New Mexico, and Arizona. 
Carnotite is found also in Wyoming and in Carbon County, Pennsylvania. 
Deposits are located at Radium Hill near Olary, Australia, and in Katanga 
(Zaine). Carnotite is the chief source of uranium in the United States. It is 
also a source of radium and vanadium. See RADIOACTIVE MINERALS; URANIUM; 
VANADIUM.  
 �34s[Wayne R. Lowell]
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�6v�2v�32sCarotenoid   �21m �33s 
    Any of a class of yellow, orange, red, and purple pigments that are widely 
distributed in nature. Carotenoids are generally fat-soluble unless they are 
complexed with proteins. In plants, carotenoids are usually located in 
quantity in the grana of chloroplasts in the form of carotenoprotein 
complexes. Carotenoprotein complexes give blue, green, purple, red, or other 
colors to crustaceans, echinoderms, nudibranch mollusks, and other 
invertebrate animals. Some coral coelenterates exhibit purple, pink, orange, 
or other colors due to carotenoids in the calcareous skeletal material. Cooked 
or denatured lobster, crab, and shrimp show the modified colors of their 
carotenoproteins.  
    The general structure of carotenoids is that of aliphatic and 
aliphatic-alicyclic polyenes, with a few aromatic-type polyenes. Most 
carotenoid pigments are tetraterpenes with a 40-carbon (C40) skeleton. More 
than 300 carotenoids of known structure are recognized, and the number is 
still on the rise. See TERPENE. 
    There are several biochemical functions in which the role of carotenoids 
is well understood. These include carotenoids in the photosynthetic apparatus 
of green plants, algae, and photosynthetic bacteria, where carotenoids 
function as a blue light-harvesting pigment (antenna or accessory pigment) for 
photosynthesis. Thus carotenoids make it possible for photosynthetic organisms 
more fully to utilize the solar energy in the visible spectral region. See 
CHLOROPHYLL; PHOTOSYNTHESIS. 
  
    Another function of carotenoids is to protect biological systems such as 
the photosynthetic apparatus from photodynamic damage. This is done by 
quenching the powerful photodynamic oxidizing agent, singlet oxygen, produced 
as an undesirable by-product of the exposure of pigmented organisms to light. 
    Perhaps the most important industrial application of carotenoids is in 
safe coloration of foods, as exemplified in the coloring and fortification of 
margarine and poultry feedstuff. 
 �34s[Pill-Soon Song ;Tae Young Lee]
�33s 
  
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�6v�2v�32sCarotid body   �21m �33s 
    A special sensory organ (glomus caroticum) which is located in the angle 
between the bifurcation of the common carotid artery into the external and 
internal carotid arteries. The carotid body is a bilateral ovoid structure 
which in humans measures approximately 0.2 by 0.3 in. (5 by 7 mm) and is 
usually embedded within the outer connective tissue layer of the adjacent 
common carotid artery. The carotid bodies are termed chemoreceptors because 
they closely monitor the oxygen and carbon dioxide (CO2) content of the blood. 
See CHEMORECEPTION. 
 �34s[John Hansen]
�33s 
  
A700000001416�254s�5v�1v301
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8990208
�6v�2v�32sCarp   �21m �33s 
    The common name for a number of cypriniform fishes of the family 
Cyprinidae. The carp (Cyprinus carpio) is closely related to the goldfish 
(Crassius auratus). The fish originated in China, where for centuries it was 
raised for food. It was imported into the United States from Europe, where it 
also has been raised for years as a source of food. 
    The carp has pharyngeal teeth and a suckerlike mouth. A very long dorsal 
fin is preceded by a strong spine. The swim bladder of the carp is associated 
with a group of small bones at the anterior end, the Weberian ossicles. This 
structural modification enables the carp to perceive sound quite well. See 
CYPRINIFORMS.  
 �34s[Charles B. Curtin]
�33s 
  
A700000001361�254s�5v�1v301
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�6v�2v�32sCarpoids �21m �33s
    An assemblage of enigmatic, rare, Paleozoic echinoderms formerly grouped
together in one class (called Carpoidea), but now viewed as comprising three
distinct classes which together constitute the subphylum Homalozoa. Like all
echinoderms, carpoids had skeletal plates, each of which was made up of a
single crystal of calcite; they differ from all other echinoderms in showing
no trace of radial symmetry. Bilaterally arranged plates roofed the dorsal
side to form a carapace, and similar plates formed a plastron to sheathe the
concave ventral side (see illustration). Carpoids range from Early Cambrian to
Devonian. See ECHINODERMATA; HOMALOZOA.




 �34s[Howard B. Fell]
�33s


A700000001135�254s�5v�1v301
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�6v�2v�32sCarrier   �21m �33s 
    Any wave to which modulation is subsequently applied. Commonly encountered 
examples are a sinusoidal wave, a recurrent series of pulses, and a direct 
current or voltage. See AMPLITUDE MODULATION; ANGLE MODULATION; FREQUENCY 
MODULATION; PHASE MODULATION. For a definition of pulse carrier see PULSE 
MODULATION; for a definition of subcarrier see MODULATION. 
    It is not always necessary or even desirable to transmit the carrier wave; 
when not transmitted, it is termed a suppressed carrier. 
 �34s[Harold S. Black]
�33s 
  
A700000001152�254s�5v�1v301
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�6v�2v�32sCarrion's disease   �21m �33s 
    A specific infection, also known as bartonellosis, caused by the 
microorganism Bartonella bacilliformis. The disease presents two clinical 
types, severe (Oroyafever) and benign (verruga peruana). The disease is 
endemic to certain regions of South America - Peru, Ecuador, and Colombia - in 
narrow river valleys and canyons of high altitudes; its distribution parallels 
the prevalence of certain species of blood-sucking sand flies (Phlebotomus 
noguchi and P. verrucarum) which transmit the infection. 
 �34s[Meir Yoeli]
�33s 
  
A700000001152�254s�5v�1v301
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�6v�2v�32sCarrot   �21m �33s 
    A biennial umbellifer (Daucus carota) of Asiatic and Mediterranean origin 
belonging to the plant order Umbellales. The carrot is grown for its edible 
roots which are eaten raw or cooked. Varieties are classified according to 
length of root (long or short or stump-rooted) and use (fresh market or 
processing). Popular varieties for fresh market are Imperator and Gold Pak; 
for processing, Red Cored Chantenay and Royal Chantenay. Texas, California, 
and Arizona are important producing states. See UMBELLALES. 
 �34s[H. John Carew]
�33s 
  
A700000002661�254s�5v�1v301
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�6v�2v�32sCartilage   �21m �33s 
    A specialized kind of connective tissue which is bluish, translucent, and 
hard but yielding. While it serves as the model for most bones during 
development, it persists in adults in certain limited regions in three forms: 
hyaline, or glassy cartilage; elastic cartilage; and fibrocartilage. 
    Hyaline cartilage is the type found at the ventral ends of ribs, in 
joints, and in the walls of the larger respiratory passages. The cartilage 
cells (chondrocytes) occupy about one-third the volume of the cartilage. The 
remainder is occupied by the matrix, which separates the cells and is thought 
by many to contain collagen fibrils. As there are no blood vessels in 
cartilage, exchanges with blood take place over larger distances than in other 
forms of connective tissue. Hyaline cartilage calcifies during middle age. 
    Elastic cartilage occurs in mammals in the external ear, the walls of the 
external auditory and Eustachian tubes, and the epiglottis. It is yellower, 
more opaque, and more flexible than hyaline cartilage and is rich in elastic 
fibers.  
    Fibrocartilage occurs in intervertebral disks, in the symphysis pubis, and 
in certain tendons near their attachment to bones. It is rich in dense, 
closely apposed bundles of collagen fibers, and the cells are disposed in 
columns between the fibers. See CONNECTIVE TISSUE. 
 �34s[Isidore Gersh]
�33s 
  
A700000001305�254s�5v�1v301
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�6v�2v�32sCartography   �21m �33s 
    The making of maps and charts for the purpose of visualizing spatial 
distributions over various areas of the face of the Earth. A map can be 
defined as a conventionalized picture, on a much reduced scale, of the Earth's 
surface as seen from above. Most maps are laid out on a parallel and meridian 
system, and have lettering added for identification. Some kinds of maps have 
long been designated charts, such as aerial and marine navigation charts. A 
most realistic Earth representation, the globe, is less used than flat maps 
because of its bulk. See GLOBE (EARTH); MAP DESIGN. 
 �34s[Erwin Raisz /Wesley Calef]
�33s 
  
  
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�6v�2v�32sCaryophyllales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
subclass Caryophyllidae, class Magnoliopsida (dicotyledons). It consists of 11 
families and about 10,000 species. The names Centrospermae and Centrospermales 
have often been applied to this order. 
    The four largest families are the Aizoaceae (about 2500 species), 
Cactaceae (about 2000 species), Caryophyllaceae (about 2000 species), and 
Chenopodiaceae (about 1500 species). The Aizoaceae are unarmed 
leaf-succulents, chiefly of Africa. The Cactaceae are American 
stem-succulents, mostly spiny and with much reduced leaves. The 
Caryophyllaceae lack betalains but are otherwise typical of the order. The 
Chenopodiaceae have reduced, mostly greenish flowers. 
    Familiar garden plants belonging to the Caryophyllales include Beet (Beta 
Vulgaria) and spinach (Spinacia oleracea), both of the Chenopodiaceae; sweel 
william (Dianthus barbatus) and carnation (D. caryophyllus), of the 
Caryophyllaceae; rose moss (Portulaca grandiflora), of the Portulacaceae; and 
the four-o'clock (Mirabilis jalapa), of the Nyctaginaceae. See BEET; 
CARYOPHYLLIDAE; SPINACH; SUGARBEET. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001255�254s�5v�1v302
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�6v�2v�32sCaryophyllidae   �21m �33s 
    A relatively small subclass of the class Magnoliopsida (dicotyledons) of 
the division Magnoliophyta (Angiospermae), the flowering plants, consisting of 
4 orders, 14 families, and about 11,000 species. Most of these plants contain 
betalain pigments instead of anthocyanins, and the seeds very often have a 
perisperm. Most of the families and species of the subclass belong to the 
order Caryophyllales. The other orders (Batales, Plumbaginales, and 
Polygonales) have only a single family each. See BATALES; CARYOPHYLLALES; 
MAGNOLIOPSIDA; PLUMBAGINALES; POLYGONALES. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001736�254s�5v�1v302
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�6v�2v�32sCascode amplifier �21m �33s
    A transistor amplifier consisting of a common-emitter stage in series with
a common-base stage. The input resistance and current gain are nominally equal
to the corresponding values for a single common-emitter stage (see
illustration). The output resistance is approximately equal to the high output
resistance of the common-base stage. The reverse open-circuit amplification
parameter h sub r is very much smaller for the cascode connection than for a
single common-emitter stage. The small value of h sub r makes this circuit
particularly useful in turned amplifier design where the collector load is
replaced with a tuned circuit.
RECEIVER.

Q2 = transistors; I1, I2 = currents; R sub s = input (source) resistance;
R sub L = output (load) resistance; V sub s = source voltage; V sub l = V1
= input voltage; V' = intermediate voltage; V sub o = V2 = output voltage.






 �34s[Christos C. Halkias]
�33s



A700000001436�254s�5v�1v302
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�6v�2v�32sCasein   �21m �33s 
    The principal protein of milk. It is present in milk as calcium caseinate 
and amounts of 80% of the milk protein. Casein has had certain industrial 
uses, such as in the manufacture of brush bristles, buttons, billiard balls, 
jewelry, paper, glue, paint, sodium caseinate, and protein hydrolyzates. The 
competition from synthetic plastics reduced the demand for casein for some of 
these uses. See MILK. 
    Casein is made by either of two methods: coagulation by rennet or 
precipitation by acid. The rennet method is used for producing cassein for the 
plastics industry. The acid precipitation method is used for casein production 
for other commercial uses, including the food industry. 
 �34s[Paul H. Tracy]
�33s 
  
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�6v�2v�32sCashew �21m �33s
    a medium-sized, spreading evergreen tree (Anacardium occidentale) native
to Brazil, but now grown widely in the tropics for its edible nuts and the
resinous oil contained in the shells. The fruit consists of a fleshy, red or
yellow, pear-shaped receptacle, termed the apple, at the distal end of which
is borne a hard-shelled, kidney-shaped ovary or nut. Although cashew trees are
spread throughout the tropics, commercial production is centered in India,
which handles 90% of the world trade.
    Cashew nut kernels are eaten as nuts and used extensively in the
confectionery and baking trade. The cashew shell liquid is a valuable
by-product, containing 90% anacardic acid and 10% cardol, and is used in the
varnish and plastic industries.
    The cashew apples are too astringent for eating without being processed,
but when processed may be used for jams, chutney, pickles, and wine. See
SAPINDALES.
 �34s[Lawrence H. MacDaniels]
�33s

A700000001102�254s�5v�1v302
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�6v�2v�32sCashmere   �21m �33s 
    The natural fiber obtained from the Cashmere goat, native to the Himalayan 
region of China and India. The fleece of this goat has long, straight, coarse 
outer hair of little value; but the small quantity of underhair, or down, is 
made into luxuriously soft woollike yarns with a characteristic highly napped 
finish. Cashmere is a much finer fiber than mohair or wool fiber obtained from 
sheep. However, it is not as durable as wool. See MOHAIR; NATURAL FIBER; WOOL. 
 �34s[M. David Potter]
�33s 
  
A700000002676�254s�5v�1v302
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�6v�2v�32sCassava �21m �33s
    The plant Manihot esculenta (Euphorbiaceae), also called manioc. It is one
of the 10 most important food plants, and the most important starchy root or
tuber of the tropics. It originated in Central or South America, possibly
Brazil, and was domesticated and widely distributed well before the time of
Columbus. Subsequent distribution has established cassava as a major crop in
eastern and western Africa, in India, and in Indonesia.
    The cassava plant is a slightly woody, perennial shrub. The leaves are
deeply palmately lobed; the flowers are inconspicuous, and the prominent
capsules are three-seeded and explosive at maturity. The roots (see
illustration) are enlarged by the deposition of starch and constitute the
principal source of food from the plant. The leaves are also eaten (after
cooking), and are noteworthy for their high protein content.
    The chief use of cassava is as a boiled vegetable. It is also a source of
flour, called farinha in Brazil and gari in western Africa, and of toasted
starch granules, the familiar tapioca. In spite of its popularity, however,
cassava root is a poor food. Its protein content is extremely low, and its
consumption as a staple food is associated with the protein deficiency disease
kwashiorkor. See GERANIALES.

     FIGURE CAPTION.  Tuberous roots from a single cassava plant.

 �34s[Franklin W. Martin]
�33s


A700000001453�254s�5v�1v303
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�6v�2v�32sCassiduloida �21m �33s
    An order of exocyclic Euechinoidea which possess five similar ambulacra
which form petal-shaped areas (phyllodes) around the mouth. The inner-most
interambulacral plate between each pair of adjoining phyllodes is swollen and
is termed a bourrelet. The five bourrelets and five phyllodes thus form a
flower/shaped pattern around the mouth (see illustration). All extant forms
live in shallow or moderately deep tropical seas, partly buried in sand. The
oldest-known cassiduloids occur in Jurassic strata. The order was most
abundant during the Eocene Epoch, after which a gradual decline occurred. In
the latest revision of the Cassiduloida, 10 families are recognized. See
ECHINODERMATA; EUECHINOIDEA.




 �34s[Howard B. Fell]
�33s



A700000000732�254s�5v�1v303
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�6v�2v�32sCassiopeia �21m �33s
    A prominent northern circumpolar constellation as seen from the middle
latitudes. The five main bright second- and third-magnitude stars of
Cassiopeia form the rather distorted W or M by which the constellation is
usually idetified (see illustration). Cassiopeia and the Big Dipper lie across
the opposite sides of the North Celestial Pole. See CONSTELLATION; URSA MAJOR.
 �34s[Ching-Sung Yu]
�33s



A700000001504�254s�5v�1v303
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�6v�2v�32sCassiterite   �21m �33s 
    A mineral having the composition SnO2. It is the principal ore of tin. 
Cassiterite is usually massive granular, but may be in radiating fibrous 
aggregates with reniform shapes (wood tin). The hardness is 6 - 7 (Mohs 
scale), and the specific gravity is 6.8 - 7.1 (unusually high for a 
nonmetallic mineral). The luster is adamantine to submetallic. Pure tin oxide 
is white, but cassiterite is usually yellow, brown, or black because of the 
presence of iron. 
    Cassiterite is most abundantly found as stream tin (rolled pebbles in 
placer deposits). The world's supply comes mostly from placer or residual 
deposits in the Malay Peninsula, Indonesia, Zaire,and Nigeria. It is also 
mined in Bolivia. See TIN.  
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
A700000002237�254s�5v�1v303
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�6v�2v�32sCast iron   �21m �33s 
    A generic term describing a family of iron alloys containing 1.8 - 4.5% 
carbon. Cast iron usually is made into specified shapes, called castings, for 
direct use or for processing by machining, heat treating, or assembly. In 
special cases it may be forged or rolled moderately. Generally, it is 
unsuitable for drawing into rods or wire, although to a limited extent it has 
been continuously cast into rods and shapes from a liquid bath or swaged from 
bars into smaller-dimensional units. Silicon usually is present in amounts up 
to 3%, but special compositions are made containing up to 6% (Silal) and up to 
12% (Duriron). Cast iron of the above composition range is often made into 
blocks or rough shapes and called pig iron. It is an intermediate form of cast 
iron used for remelting into iron castings. Cast iron may be purchased in 
several commercial grades called gray iron, chilled iron, mottled iron, white 
iron, malleable iron, ductile iron, spheroidal graphite iron, nodular iron, 
and austenitic cast iron. See ALLOY; CASTING; IRON ALLOYS; WROUGHT IRON. 
 �34s[James S. Vanick]
�33s 
  
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�6v�2v�32sCastor plant   �21m �33s 
    A plant, Ricinus communis, belonging to the spurge family (Euphorbiaceae). 
Castor seeds are poisonous and also contain allergens. 
    Current distribution is in the warmer regions of the world, the plant 
often growing in waste places. The castor oil plant has been of utilitarian 
value since antiquity. Oil from the seeds is among the world's oldest nonfood 
products in commerce. Castor oil contains about 85% ricinoleic acid, used in 
making industrial products such as alkyde resins for surface coatings, blown 
oil used in plasticizers, cracked oil for production of synthetic perfumes, 
nylon, sebacic acid, synthetic detergents, drying oils, and special 
lubricating oils. 
    Production in the United States, mostly in west Texas, peaked in 1968 but 
declined to nil because of larger economic return from food and fiber crops. 
Brazil is the world's largest producer and exporter of castor seed and oil. 
 �34s[Leroy H. Zimmerman]
�33s 
  
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�6v�2v�32sCasuariiformes   �21m �33s 
    An order of large flightless birds of Australia and New Guinea. There are 
two families, the cassowaries (Casuariidae) with six living species and the 
emus (Dromiceidae) with one living species. All are somewhat ostrichlike in 
appearance, the common emu (Dromiceius novaehollandiae) being the second 
largest of living birds. Unlike the ostriches and the rheas, cassowaries and 
emus have the wings reduced to mere rudiments, which, in the cassowaries only, 
bear long spinelike modified quills. The legs are exceedingly powerful and are 
used in defense; cassowaries have, in addition, a long sharp claw on the inner 
toe. Cassowaries differ further from emus in having featherless, often wattled 
necks and bony casques on the head. They are forest birds of New Guinea and 
adjacent islands and northernmost Australia, while emus inhabit Australian 
grasslands. See RATITES. 
 �34s[Kenneth C. Parkes]
�33s 
  
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�6v�2v�32sCasuarinales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of a single family (Casuarinaceae) and genus (Casuarina), with about 
50 species. Native to the southwestern Pacific region, especially Australia, 
they are sometimes called Australian pine. They are trees with much reduced 
flowers and green twigs that bear whorls of scalelike, much reduced leaves. 
Some species are grown as street trees in tropical and subtropical regions. 
See HAMAMELIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sCat   �21m �33s 
    The term used to describe any member of the mammalian family Felidae. More 
commonly, the term is restricted to the domestic cat and those felids that 
resemble it in size, shape, and habits. Those that are larger in size are 
referred to as the big cats. All members of the cat family have a round head, 
are digitigrade (walk on their toes), have retractile claws (with the 
exception of the cheetah), and have 30 teeth. All species are carnivorous. 
    The origin of domestic cats is unknown, but it is established that they 
have been associated with humans for many centuries. Domestic mixed-breed cats 
are generally characterized by long, thin tails, straight ears, and short hair 
of a variety of colorations. The pure breeds, however, are notable exceptions 
to this general description. There is a long-haired race, apparently developed 
in Persia, which is represented by the Persian and the Angora. The Abyssinian 
breed, in contrast to the common domestic cat, is characterized by being ruddy 
brown in color and having a longer face and ears. There are at least two 
tailless breeds, of which the best known is the Manx from the Isle of Man. The 
other tailless variety is found in Japan. One of the most popular breeds is 
the Siamese cat. Its eyes are deep blue, it has a long, kinky tail, and its 
fur is short and cream to buff in color. 
    Wildcats include cats such as the lynx, bobcat, serval, ocelot, puma, 
leopard, lion, tiger, jaguar, and cheetah. In addition there are a few lesser 
known forms that are of interest. Among these are the Scottish wildcat and the 
caracal. See CARNIVORA. 
 �34s[Charles B. Curtin]
�33s 
mpgs. 1202-1203 
1:19 
  
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�6v�2v�32sCatalan numbers �21m �33s
    A series of numbers given by the equation below. They count the ways to
insert parentheses in a string of n terms so that their product may be
unambiguously carried out by multiplying two quantities at a time. Thus c4 =
5, since abcde may be computed by (ab)(cd), ((ab)c)d, (a(bc))d, a((bc)d), or
a(b(cd)). The Catalan numbers also enumerate the ways to place nonintersecting
diameters in a convex polygon with n + 1 vertices so that only triangular
regions result. Further, csubn gives the number of ways to draw in succession
all the balls from a container which originally contains n - 1 white balls and
n - 1 black balls, so that at every stage there are at least as many black
balls as white balls remaining in the container. Catalan numbers also appear
in the theoretical measurement of preferences.


		c(sub n) = (1/n) [2n -2) over (n-1)]

See COMBINATORIAL THEORY;ENUMERATION OF ARRANGEMENTS.

 �34s[Thomas Brylawski]
�33s



A700000003374�254s�5v�1v304
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�6v�2v�32sCatalysis �21m �33s
    The phenomenon in which a relatively small amount of foreign material
called a catalyst, augments the rate of a chemical reaction without itself
being consumed. A catalyst is material, and not light or heat. It augments a
rate. The term negative catalyst is obsolete. See ANTIOXIDANT; INHIBITOR
(CHEMISTRY).
    If the reaction A + B -> D occurs very slowly but is catalyzed by some
catalyst, Cat, the addition of Cat must open new channels for the reaction. In
the very simple case shown below, the two propagation processes, which are


A + Cat -> ACat & ACat + B -> D + Cat	 Chain propogation


	       A + B -> D	  Overall reaction


fast compared to the uncatalyzed reaction, A + B -> D, provide the new channel
for reaction. The catalyst reacts in the first step, but is regenerated in the
second step to commence a new cycle. A catalytic reaction is thus a kind of
chain reaction.
    If a reaction is in chemical equilibrium under some fixed conditions, the
addition of a catalyst cannot change the position of equilibrium without
violating the second law of thermodynamics. Therefore, if a catalyst augments
the rate of A + B -> D, it must also augment the reverse rate, D -> A + B.
    Catalysis is conventionally divided into three categories: homogeneous,
heterogeneous, and enzyme. Heterogeneous catalysis plays a dominant role in
chemical processes in  the petroleum, petrochemical, and chemical industries.
Homogeneous catalysis is important in the petro chemical and chemical
industries. Enzyme catalysis plays a key role in all metabolic processes and
in some industries, such as the fermentation industry. See ENZYME;
HETEROGENEOUS CATALYSIS; HOMOGENEOUS CATALYSIS.
 �34s[Robert L. Burwell]
�33s


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�6v�2v�32sCatalytic converter �21m �33s
    An aftertreatment device used for pollutant removal from automotive
exhaust. Since the 1975 model year, increasingly stringent government
regulations for the allowable emission levels of carbon monoxide (CO),
hydrocarbons (HC), and oxides of nitrogen (NO sub x) have resulted in the use
of catalytic converters on most passenger vehicles sold in the United States.
The task of the catalytic converter is to promote chemical reactions for the
conversion of these pollutants to carbon dioxide, water, and nitrogen.
    For automotive exhaust applications, the pollutant removal reactions are
the oxidation of carbon monoxide and hydrocarbons and the reduction of
nitrogen oxides. Metals are the catalytic agents most often employed for this
task. Small quantities of these metals, when present in a highly dispersed
form (often as individual atoms), provide sites upon which the reactant
molecules may interact and the reaction proceed.
    Two types of catalyst systems, oxidation and three-way, are found in
automotive applications. Oxidation catalysts remove only CO and HC, leaving
NO sub x unchanged. Platinum and palladium are generally used as the active
metals in oxidation catalysts. Three-way catalysts are capable of removing all
three pollutants simultaneously, provided that the catalyst is maintained in a
``chemically correct'' environment that is neither overly oxidizing or
reducing. In both oxidation and three-way catalyst systems, the production of
undesirable reaction products, such as sulfates and ammonia, must be avoided.
    Maintaining effective catalytic function over long periods of vehicle
operation is often a major problem. Catalytic activity will deteriorate due to
two causes, poisoning of the active sites by contaminants, such as lead and
phosphorus, and exposure to excessively high temperatures. To achieve
efficient emission control, it is thus paramount that catalyst-equipped
vehicles be operated only with lead-free fuel and that proper engine
maintenance procedures be followed. See AUTOMOTIVE ENGINE; CATALYSIS.
 �34s[Norman Otto]
�33s

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�6v�2v�32sCatalytic cracking �21m �33s
    The major cracking process used throughout most of the world oil industry
for the production of high-octane quality gasoline by the conversion of
intermediate- and high-boiling petroleum distillates to lower-molecular-weight
products. Oil heated to within the lower range of thermal cracking
temperatures (850 - 1025 DG F; 454 - 551 DG C) reacts in the presence of an
acidic inorganic catalyst under low pressures (10 - 35 pounds per square inch
gage or 70 - 240 kilopascals, gage pressure). Gasoline of much higher octane
number is obtained than from thermal cracking, a principal reason for the
widespread adoption of catalytic cracking. All nonvolatile carbonaceous
materials are deposited on the catalyst as coke and are burned off during
catalyst regeneration. See THERMAL CRACKING.
    Although the primary objective of catalytic cracking is the production of
maximum yields of gasoline concordant with efficient operation of the process,
large amounts of normally gaseous hydrocarbons are produced at the same time.
The gaseous hydrocarbons include propylene and butylenes, which are in great
demand for chemical manufacture. The other chief product is the material
boiling above gasoline, designated as catalytically cracked gas oil. It
contains hydrocarbons relatively resistant to further cracking, particularly
polycyclic aromatics. The lighter portion may be used directly or blended with
straight-run and thermally cracked distillates of the same boiling range for
use as diesel and heating oils. Part of the heavier portion is recycled with
fresh feedstock to obtain additional conversion to lighter products. The
remainder is withdrawn for blending with residual oils to reduce the viscosity
of heavy fuel, or else subjected to a final step of thermal cracking.
    With the emphasis on conservation of petroleum resources, catalytic
cracking has assumed a more important role. The heavier products from a
refinery could be replaced by coal-, shale-, or tar-sand-derived products.
Thus, there is a need for greater conversion of petroleum to gasoline with
emphasis on catalytic cracking as the cheapest major conversion process. See
CRACKING.
 �34s[Edward C. Luckenbach]
�33s


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�6v�2v�32sCatalytic reforming �21m �33s
    A process used for upgrading gasoline by improving its antiknock
characteristics (increasing the octane number). It is also widely used for the
production of aromatic hydrocarbons for the petrochemical industry.
    The process utilizes a supported platinum catalyst and involves a number
of different reactions. These reactions convert, or reform, the
low-octane-number feed components, such as the paraffins, to components with
increased octane number. The most important reactions in catalytic reforming
are those leading to the formation of aromatic hydrocarbons because these are
high-octane components as well as valuable petrochemical intermediates.
Aromatics are formed by dehydrogenation of six-membered ring cycloparaffins,
rearrangement and dehydrogenation of five-membered ring cycloparaffins, and
cyclization (dehydrocyclization) of paraffins.
    Liquefied petroleum gas (LPG) consists of propane and butanes and is
usually derived from natural gas. In locations where there is no natural gas
and LPG is more important than gasoline, or in refineries that require
additional isobutane for alkylation, naphtha can be converted to LPG
(primarily isobutane) by catalytic reforming. Under suitable conditions it is
possible to convert 40% of the naphtha to LPG, the by-product being
high-octane gasoline. See AROMATIC HYDROCARBON; AROMATIZATION;
DEHYDROGENATION; HYDROCRACKING; LIQUEFIED PETROLEUM GAS (LPG); OCTANE NUMBER;
PETROLEUM PROCESSING.
 �34s[Ernest L. Pollitzer ;Vladimir Haensel]
�33s

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�6v�2v�32sCataract   �21m �33s 
    Any clouding or opacity of the crystalline lens of the eye. Cataracts vary 
markedly in degree of density and may be due to many causes, but the majority 
are associated with aging. Cataracts are the single leading cause of blindness 
in the world. 
    Senile cataract occurs with aging and is by far the most common type. 
Progressively blurred vision is the only symptom. There are a number of other 
varieties (congenital, metabolic, secondary, or traumatic) and causes (such as 
a reaction to certain drugs or irradiation). Cataracts may also be associated 
with systemic diseases such as hypoparathyroidism, myotonic dystrophy, atopic 
dermatitis, galactosemia, and Lowe's, Werner's, and Down's syndromes. 
    The treatment for cataracts, when they are sufficiently advanced to impair 
the vision, is surgical removal. When a cataract is surgically removed, the 
crystalline lens of the eye is removed much as a lens would be removed from a 
camera. In order to restore normal vision after surgery, any of a number of 
methods may be followed: cataract glasses may be prescribed; contact lenses 
may be fitted; or intraocular lenses made of the plastic 
polymethylmethacrylate can be inserted into the eye to replace the cataractous 
lens. See EYE (VERTEBRATE). 
 �34s[Jack Hartstein]
�33s 
  
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�6v�2v�32sCatechol   �21m �33s 
    A naturally occurring dihydric phenol, also known as pyrocatechol, 
pyrocatechuic acid, and 1,2-dihydroxybenzene, with the empirical formula 
C6H6O2. Catechol is a colorless crystalline substance (melting point 105 DG C
or 221 DG F) which dissolves readily in water. It resembles phenol in being a 
severe irritant to skin and in possessing systemic toxicity. 
    Although catechol and its derivatives are widely distributed among plants, 
the substance is produced synthetically for use in the manufacture of 
antioxidants (stabilizers) for rubber and plastics, and of a feed additive for 
poultry. Catechol also is used in medicinal and photographic preparations. See 
PHENOL.  
 �34s[Martin Stiles]
�33s 
  

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�6v�2v�32sCatenary �21m �33s
    The curve formed by an ideal heavy uniform string hanging freely from two
points of support. The lowest point A (see illustration) is the vertex. The
portion AP is in equilibrium under the horizontal tension H at A, the tension
F directed along the tangent at P, and the weight W of AP. If the weight of
the string is w per unit length and s is the arc AP, W = ws; and from the
force triangle, tan PSI = ws/H = s/c, where c = H/w, is called the parameter
of the catenary. Thus the catenary has the differential equation (1).

			    *** dy/dx = s/c *** 			   (1)

    The horizontal line at a distance c below the vertex A is the directrix of
the catenary. With the x axis as directrix and the y axis through the vertex,
the integration of Eq. (1) yields Eqs. (2) for the ordinate and arc of the
catenary. All catenaries are geometrically similar to the hyperbolic cosine
curve, y = cosh x.

	       *** y = c cosh x/c	s = c sinh x/c ***		   (2)

    The surface generated by revolving a catenary about its directrix is a
minimal surface, the catenoid. The catenary is an extremal for the problem of
finding a curve joining two given points so that the surface generated by
revolving it about a given line has minimum area. See HYPERBOLIC FUNCTION.

 �34s[Louis Brand]
�33s



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�6v�2v�32sCaterpillar   �21m �33s 
    A term applied to the larval stage of a butterfly or moth and to some 
scorpion flies and sawflies. In insects that undergo a complete metamorphosis 
during development, a sequence of stages occurs. As the eggs hatch, a 
vermiform (wormlike) larval form is released that differs considerably in 
morphology from the other stages, and several distinct types are recognized. 
    The caterpillar is an eruciform larva, which is one that has a 
well-developed head, a cylindrical form, thoracic legs and abdominal prolegs. 
Most of these larvae are phytophagous. Many caterpillars are cryptically 
colored and blend in with their environment, with some forms closely 
resembling twigs. Some larvae are naked, that is, they do not have hairs, 
while others are covered with hairs or spines. After a period of activity the 
caterpillar (larva) goes into a quiescent stage, the pupa, from which will 
emerge the adult. See BUTTERFLY; LEPIDOPTERA; MOTH. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sCatfish   �21m �33s 
    The name for a number of fishes which constitute the suborder Siluroidei 
in the order Cypriniformes. There are about 11 families which include mostly 
fresh-water species of wide distribution. 
    The bodies of these fishes do not have scales but may be naked, have 
plates, or be covered with spines. The common name is derived from the 
presence of barbels around the mouth. Some species live at great depths and 
these are usually blind and lack pigmentation. 
    Catfish are valuable food fish. Common species include the blue catfish, 
found in the Mississippi drainage, and the European catfish. Some smaller 
South American species are parasitic and suck blood from the gill filaments of 
larger fish. See CYPRINIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sCathartics   �21m �33s 
    Agents used to produce evacuation of the bowel. They are too frequently 
used and should probably be administered only after possible organic causes of 
constipation have been considered. Adding fiber to the diet softens stool, 
increases daily fecal weight, decreases intestinal transit time, and is often 
an effective remedy for mild constipation. Bulk can be added medicinally with 
hydrophilic preparations of psyllium seed, or naturally by ingesting 
high-fiber foods. 
    Nonfiber laxatives may be classified in many ways. One useful 
classification considers them as secretagogues, lubricants, or osmotic 
cathartics. Secretagogues, such as castor oil, inhibit net transport of water 
and electrolytes by the intestinal mucosa. Lubricants, such as mineral oil, 
reduce friction between the stool and the intestinal mucosa. Osmotic 
cathartics, such as phosphates of magnesium, are poorly absorbed salts that 
preempt water in the gut lumen because isotonicity must be maintained on both 
sides of the mucosal membrane. 
 �34s[Robert L. Protell]
�33s 
  
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�6v�2v�32sCathode rays �21m �33s
    The name given to the electrons originating at the cathodes of gaseous
discharge devices. The term has now been extended to include low-pressure
devices such as cathode-ray tubes. Furthermore, cathode rays are now used to
designate electron beams originating from thermionic cathodes, whereas the
term was formerly applied only to cold-cathode devices. See CATHODE-RAY TUBE.
 �34s[Glenn H. Miller]
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�6v�2v�32sCathode-ray tube �21m �33s
    An electron tube in which a beam of electrons can be focused to a small
area and varied in position and intensity on a surface. In common usage, the
term cathode-ray tube is usually reserved for devices in which the surface
referred to is cathodoluminescent, that is, luminescent under electron
bombardment. In these tubes the output information is presented in the form of
a pattern of light. The character of this pattern is related to, and
controlled by, one or more electrical signals applied to the cathode-ray tube
as input information.
    The most familiar form of the cathode-ray tube is the television picture
tube, found in home television receivers. Cathode-ray tubes are also used in
large quantities in measuring instruments such as oscilloscopes, which have
become indispensable in laboratories devoted to experimental studies in the
physical and biological sciences and in engineering. The oscilloscope has also
been adopted as a vital tool in the fields of automobile, aircraft, and
television-receiver maintenance and repair. For navigation the cathode-ray
tube is the output device of radars. Large users of cathode-ray tubes are the
military services, which employ these devices to display data to guide the
employment of weapons systems. Increasingly, cathode-ray tubes are finding use
in input/output terminals of digital computers.
    The cathode-ray tube is also used in the general field of printed
communications, both as an adjunct to, and as a competitor of, traditional
means of preparing ``hard copy.'' In the first instance, cathode-ray tubes
expose photographic masters that, in turn, are used to prepare offset printing
plates. Where only a single copy is required, photosensitive paper is exposed
by a cathode-ray tube to deliver printed copy in the business offices from an
electronic communication link. See ELECTRONIC DISPLAY; OSCILLOSCOPE; STORAGE
TUBE; TELEVISION RECEIVER.
    The basic elements of a cathode-ray tube are the envelope, electron gun,
and phosphor screen. The envelope serves as the vacuum enclosure, the
substrate for the phosphor screen, and the support for the electron gun. The
envelope of a cathode-ray tube is typically funnel-shaped; its small opening
is stopped by the stem. This stem is essentially a disk of glass through which
the metallic leads which support and carry applied voltages to the several
elements of the gun are sealed. The large end of the funnel is closed by a
glass faceplate on the inside of which the phosphor screen is deposited. Some
typical cathode-ray tubes are shown in Fig. 1.
    The electron gun produces, controls, focuses, and deflects an electron
beam. In some cases one or more of the functions enumerated above are
performed by components not included in the gun or even in the cathode-ray
tube itself. These components are invariably electromagnetic-field-producing
devices; a common example is a magnetic solenoid used for beam focusing. A
typical electron gun used in a cathode-ray tube for oscillographic
applications (Fig. 2) is functionally self-contained because no external
components are required to perform beam-intensity control, focusing, or
deflection. The phosphor screen converts electrical energy to visible
radiation. Materials known as phosphors are said to be luminescent. A phosphor
which is excited to luminescence by electron bombardment is described as
cathodoluminescent. In the case of phosphors for cathode-ray tubes,
luminescence invariably persists after cessation of excitation. See
CATHODOLUMINESCENCE; LUMINESCENCE.
    In cathode-ray tubes built for specialized applications, unique tube
characteristics often require distinctive features. Cathode-ray tubes
employing multiple independent electron guns are often required for use in
oscilloscopes for the simultaneous display of several concurrent signals.
Tubes having two guns are common; some have been produced with as many as ten
guns. For color television, three guns, one to excite each of the three
primary color phosphors, are built into the tube.
    For computer input/output terminals, the information to be displayed is in
the form of alphanumeric messages. There are a wide variety of electronic
techniques available for generating alphanumeric characters. In many instances
these techniques require as a display device a cathode-ray tube of
conventional design, with due attention to proper choice of electron gun and
phosphor screen. However, in some techniques, the combined requirements for
high speed and low power consumption require a cathode-ray tube with dual
deflection systems.
    A character-producing tube is used in some alphanumeric display systems.
It employs an electron gun of essentially conventional design to
``illuminate,'' one at a time, characters etched in a thin metal stencil
mounted in the envelope neck perpendicular to the tube axis. The electron beam
issuing from the stencil, having assumed a cross section corresponding to an
appropriate position on the phosphor screen, on which a luminous image of the
selected character then appears.
    In the simplest printing systems, the image on the face of a conventional
cathode-ray tube, usually designed to have a very small luminous spot and an
optically flat faceplate, is optically projected onto the photosensitive
medium page by page. Higher optical coupling efficiency, important where speed
of printing must be maximum, can be obtained if the cathode-ray tube faceplate
is made of an array of optical fibers, fused together so as to be
vacuum-tight. Such a device permits contact printing on the photographic
medium. Frequently, to reduce device cost, the fiber optics faceplate takes
the shape of a narrow rectangle and the printing is done line by line by
moving the paper continuously past the faceplate. Cathode-ray tubes are being
made in which the fiber optics assembly is replaced by a window of glass or
mica several thousandths of an inch thick. Printing cathode-ray tubes are
available in which fine wires, suitably insulated from each other, are brought
through the faceplate. These wires conduct charges deposited by the electron
beam directly to charge-sensitive paper, which is then passed through a
suitable toner material to develop the final printed image. See OPTICAL
FIBERS; PRINTING.
    Television-type color cathode-ray tubes are generally not used in
specialized display systems because the dot or line structure does not permit
an image of sufficiently high resolution. This limitation is overcome in a
color cathode-ray tube of the penetration type. In this device two or more
layers of phosphor, each emitting radiance of a different color, are deposited
on the inside of the faceplate. If the electron gun is operated at an
appropriate overall voltage, only the phosphor layer closest to the gun is
excited. At higher voltages a further layer absorbs most of the beam energy
and thus its characteristic color dominates the display. By controlling the
beam accelerating voltage, therefore, the display can include a range of
colors. See COLOR TELEVISION.
 �34s[M. Duffield Harsh]
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�6v�2v�32sCathodoluminescence �21m �33s
    A luminescence resulting from the bombardment of a substance with an
electron beam. The major application of cathodoluminescence is in cathode-ray
oscilloscopes and television picture tubes. In these devices a thin layer of
luminescent powder is evenly deposited on the transparent glass face plate of
a cathode-ray tube. The electron beam originating in the cathode, after
undergoing acceleration, focusing, and deflection by various electrodes in the
tube, impinges on the luminescent coating. The resulting emission is normally
observed through the glass face plate of the tube, that is, from the
unbombarded side of the coating. Depending on the type of signal information
to be displayed on the tube, phosphors or phosphor blends of various emission
colors and persistences are used. See LUMINESCENCE; PHOTOLUMINESCENCE.
 �34s[Clifford C. Klick ;James H. Schulman]
�33s

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�6v�2v�32sCat scratch disease   �21m �33s 
    A benign systemic illness in humans characterized by malaise, occasional 
fever, and a granulomatous lymphadenitis, which may resemble neoplastic 
disease. Since only a portion of the cases have a history of contact with 
cats, the term benign lymphoreticulosis may have some justification. The 
disease is not uncommon, and most cases occur in winter months. Sporadic cases 
are observed more often in children; occasionally small household outbreaks 
arise when cats scratch or bite family members. 
    A few days after a cat scratch has occurred, a persistent scabbed ulcer or 
papule appears as the primary skin lesion. About 1 - 3 weeks later the 
regional lymph nodes on one or both sides become enlarged. Fever, malaise, and 
symptoms of infection usually develop. The ocular form consists of 
conjunctivitis, with enlargement of the lymph nodes around the ear. 
Meningoencephalitis, an inflammation of the brain and the membranes covering 
it, may complicate the disease. Though benign and self-limiting, the disease 
may last from 2 weeks to 2 years. 
 �34s[Karl F. Meyer]
�33s 
  
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�6v�2v�32sCauchy integral formula �21m �33s
    An integral representation formula due to A. Cauchy which plays a
fundamental role in complex analysis and is traditionally the principal tool
used in proving the equivalence of some of the alternative characterizations
of analytic functions. The formula asserts that if gamma is the smooth
boundary curve of a finite region OMEGA in the complex plane and if f is
analytic in an open neighborhood of the closed region OMEGA UNION gamma (the
union of OMEGA and gamma), then the equation below is valid. This shows, for
example, that the values of the analytic function f in OMEGA are completely
determined by the values of that function on the boundary of OMEGA. See
ANALYTIC FUNCTION.
 �34s[Robert C. Gunning]
�33s

(1/2pii) INTEGRATION {[f(xi) dxi]/(xi -z)} = f(z) where z is a member of OMEGA
	 or = 0 whenever z is not a member of OMEGA UNION gamma

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�6v�2v�32sCauliflower   �21m �33s 
    A cool-season biennial crucifer (Brassica oleracea var. botrytis) of 
Mediterranean origin. Cauliflower belongs to the plant order Capparales. It is 
grown for its white head or curd, a tight mass of flower stalks, which 
terminates the main stem. Cauliflower is commonly cooked fresh as a vegetable; 
to a lesser extent, it is frozen or pickled and consumed as a relish. 
California and New York are important cauliflower-producing states. See 
CAPPARALES.  
 �34s[H. John Carew]
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�6v�2v�32sCausality   �21m �33s 
    In classical mechanics causality has been taken to mean that all the 
dynamical variables of a system can be precisely measured and their evolution 
in time is strictly determined by the forces. Therefore, classically, one can 
repeatedly prepare a system in the same specific initial state with arbitrary 
precision and then measure the same final state after a given lapse in time. 
    In quantum mechanics there is also a rigorous law of causality. However, 
there are important differences. All the variables cannot be completely 
specified: there are inherent uncertainties specified by the Heisenberg 
uncertainty relation, which is completely built into the structure of quantum 
mechanics. Hence, one prepares an initial state and specifies a wave function 
whose (absolute) square is interpreted as a probability distribution. The law 
of causality in quantum mechanics states that for an isolated system the 
initial state completely determines, by the dynamics of the Schroedinger 
equation, the future state, which again leads to a probability distribution. 
To understand this physically, consider the scattering of protons off a target 
nucleus. Quantum mechanics can predict only an average distribution for the 
scattering process: thus it is the average of a large number of measured 
events that one accurately compares with theory. One also notes that in accord 
with the uncertainty principle there will always be some spread in momentum 
for the initial proton states. Furthermore, the detection of the protons 
disturbs the system (again because of the uncertainty principle), destroying 
the exact relation between the ware function before and after the measurement. 
See QUANTUM MECHANICS. 
    Causality has also been used in reference to the principle that an event 
cannot precede its cause. Consider, for example, a packet of light (or 
electromagnetic radiation) having a sharp wave front incident on an obstacle. 
After the incident wave reaches the obstacle or scattering center (located at 
the origin), scattered light moves out in all directions. Since light travels 
with the finite velocity, causality requires that no scattered light be 
observed at any point until there is sufficient time for the incident light to 
reach that point. This requirement places important restrictions on the 
behavior of the scattering processes as a function of frequency. 
 �34s[Gordon L. Shaw]
�33s 
  
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�6v�2v�32sCave   �21m �33s 
    A natural cavity in rock beneath the surface of the earth. Most caves have 
been formed in calcareous rock (limestone and dolomite) by the dissolving 
action of groundwater circulating along the partings in the rocks. Such 
partings include existing joints and bedding (stratification) planes. Joints 
commonly occur in sets as semiparallel and nearly vertical planes, the sets 
intersecting each other at large angles. Bedding planes usually are 
horizontal. The intersection of these planes accounts for the 
three-dimensional pattern observed in caves, but in most caves the horizontal 
extent greatly exceeds vertical development. See JOINT (GEOLOGY). 
    Caves of no great length may be formed by mechanical erosion of waves 
against coastal cliffs and by weathering attack on cliffs of jointed rock. 
Lava flows may, on cooling, develop a stiff upper crust over still molten rock 
which, continuing to flow after supply at the source has ceased, may leave 
long caves. See WEATHERING PROCESSES. 
 �34s[J. Harlen Bretz]
�33s 
  
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�6v�2v�32sCavies   �21m �33s 
    Rodents comprising the family Caviidae, which includes the guinea pig, 
rock cavies, mountain cavies, capybara, salt-desert cavy, and mara. All 
members of the group are indigenous to South America and comprise 15 species 
in six genera. Cavies have either rounded bodies with large heads and short 
ears and limbs, or rabbitlike bodies with long limbs and moderately long ears. 
    The guinea pig (Cavia aperea) originated in Peru, where there is still a 
wild stock. The domestic form (C. porcellus) has been produced by selective 
breeding and is a valuable laboratory animal with a life-span of 3 - 5 years. 
Closely related to the guinea pig is the mara or Patagonian cavy (Dolichotis 
patagonum) which resembles a large hare. The salt-desert cavy (Pediolagus 
salinicola) is a smaller species found in the salt deserts of southern 
Argentina. The largest of all rodents is the capybara or carpincho 
(Hydrochoerus hydrochaeris) which grows to the size of a small pig. It is 
essentially an aquatic animal which lives in small groups along the banks of 
lakes and streams in tropical South America. See RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCavitation   �21m �33s 
    The formation of vapor- or gas-filled cavities in liquids by mechanical 
forces. If understood in this broad sense, cavitation includes the familiar 
phenomenon of bubble formation when water is brought to a boil and the 
effervescence of champagne wines and carbonated soft drinks. In engineering 
terminology, the term cavitation is used in a narrower sense, namely, to 
describe the formation of vapor-filled cavities in the interior or on the 
solid boundaries of vaporizable liquids in motion when the pressure is reduced 
to a critical value without change in ambient temperature. Cavitation in the 
engineering sense occurs at suitable combinations of low pressure and high 
speed in pipelines; in hydraulic machines such as turbines, pumps, and 
propellers; on submerged hydrofoils; behind blunt submerged bodies; and in the 
cores of vortices. This type of cavitation has great practical significance 
because it restricts the speed at which hydraulic machines may be operated 
and, when severe, lowers efficiency, produces noise and vibrations, and causes 
rapid erosion of the boundary surfaces, even though these surfaces consist of 
concrete, cast iron, bronze, or other hard and normally durable material. 
 �34s[K. E. Schoenherr]
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�6v�2v�32sCavity resonator   �21m �33s 
    An enclosure with conducting walls surrounding a dielectric medium, 
capable of resonating with electromagnetic waves. It is used mainly as a 
reservoir of electromagnetic oscillations at microwave or ultra-high 
frequencies. The conducting walls are usually made of metal such as copper or 
silver, while the dielectric medium is usually air or vacuum. The enclosure 
has at least one aperture for coupling the electromagnetic energy between the 
inside and outside of the cavity. 
    For an electromagnetic wave to exist inside a cavity, the electric and 
magnetic field intensities must be solutions of Maxwell's equations, and at 
the same time they must satisfy the boundary conditions on the inside surfaces 
of the cavity walls. For all practical purposes it can be assumed that the 
wall conductivity is so high that the tangential component of the electric 
field vanishes everywhere along the inside walls. This restriction requires 
that only waves of certain discrete frequencies be allowed in the cavity. 
Thus, when the frequency of an exciting wave is varied, the cavity does not 
extract or contain any energy from the wave unless the exciting frequency 
happens to be one of the allowed values. When the latter condition holds, then 
the cavity will resonate strongly with the exciting wave by absorbing a part 
or all of the incoming energy. The cavity is thus acting as a resonator. 
    Coaxial cavity resonators are often used as wavemeters, particularly for 
lower than microwave frequencies. Since cavity resonators are essentially 
frequency-selective circuit elements for microwaves, they are very useful as 
components in a microwave filter network. In a radar set, a resonant cavity 
can be used in a so-called TR box, where a gas-filled cavity breaks down to a 
short-circuit impedance to protect the receiver from damage when the 
high-powered transmitter starts working. In high-energy linear accelerators, 
resonant cavities are used to accelerate charged particles in intense 
high-frequency electric fields. See KLYSTRON; PARTICLE ACCELERATOR; RADAR. 
 �34s[C. K. Jen]
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�6v�2v�32sCayley-Klein parameters �21m �33s
    A set of four complex numbers used to specify the orientation of a body,
or equivalently, the rotation R which produces that orientation, starting from
some reference orientation. They can be expressed in terms of the Euler angles
psi, theta, and phi, as in the equations below.

		alpha = cos (theta/2) e sup[-i(psi-phi)/2]

	       beta = -i sin (theta/2) e sup[-i(psi-phi)/2]

	      gamma = -i sin (theta/2) e sup[-i(psi-phi)/2]

		delta = cos (theta/2) e sup[i(psi-phi)/2]

    Although these parameters have been used to simplify somewhat the
mathematics of spinning top motion, their main use is in quantum mechanics.
There they are related to the Pauli spin matrices and represent the change in
the spin state of an electron or other particle of half-integer spin under the
space rotation R(psi,theta,phi). See MATRIX THEORY; SPIN (QUANTUM MECHANICS).
 �34s[Bernard Goodman]
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�6v�2v�32sCaytoniales   �21m �33s 
    A group of Mesozoic plants. The remains consist of palmately compound 
leaves with 3 - 6 lanceolate leaflets previously known as Sagenopteris 
phillipsi, pinnately branched microsporophylls (named Caytonanthus arberi) 
that bore winged pollen in four-chambered microsporangia, and fruit-bearing 
inflorescences, Caytonia (with two species), which bore a dozen or more 
globular, short-stalked fruits in subopposite rows. The Caytoniales appear 
related to the pteridosperms. They range from the Triassic to the Cretaceous. 
See PALEOBOTANY. 
 �34s[Chester A. Arnold]
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�6v�2v�32sCedar �21m �33s
    Any of a large number of evergreen trees having fragrant wood of great
durability. Arborvitae is sometimes called northern white cedar. See
ARBORVITAE.
    Chamaecyparis thyoides, the southern white cedar, grows only in swamps
near the eastern coast of North America, where it is also known as Atlantic
white cedar. The wood is soft, fragrant, and durable in the soil and is used
for boxes, crates, small boats, tanks, woodenware, poles, and shingles. The
Port Orford cedar (C. lawsoniana), also known as Lawson cypress, is native to
southwestern Oregon and northwestern California. It is the principal wood for
storage battery separators, but is also used for venetian blinds and
construction purposes. Alaska cedar (C. nootkatensis) is found from Oregon to
Alaska. The wood is used for interior finish, cabinetwork, small boats, and
furniture. It is also grown as an ornamental tree. Incense cedar (Libocedrus
decurrens) is found from Oregon to western Nevada and Lower California.
Incense cedar is one of the chief woods for pencils, and is also used for
venetian blinds, rough construction, and fence posts and as an ornamental and
shade tree.
    Eastern red cedar (Juniperus virginiana) is distributed over the eastern
United States and adjacent Canada. The very fragrant wood is durable in the
soil and is used for fence posts, chests, wardrobes, flooring, and pencils.
Cedarwood oil is used in medicine and perfumes. Cedar of Lebanon (Cedrus
libani) and Atlas cedar (C. atlantica) resemble the larch, but the leaves are
evergreen and the cones are much larger and erect on the branches. The cedar
of Lebanon is a native of Asia Minor. See LARCH.
    The cigarbox cedar (Cedrela odorata), also known as the West Indian cedar,
belongs to the mahogany family, is a broad-leaved tree with pinnate, deciduous
leaves, and is related to the Ailanthus and sumac. The wood is very durable
and fragrant and is valued in the West Indies for the manufacture of cabinets,
furniture, and canoes. See PINALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sCelastrales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 10 families and about 2300 species, with the families Celastraceae 
(about 850 species), Aquifoliaceae (about 450 species), Icacinaceae (about 400 
species), and Hippocrateaceae (about 300 species) forming the core of the 
group. The order is marked by its simple leaves and regular flowers, varying 
from hypogynous (those with the perianth and stamens attached directly to the 
receptacle, beneath the ovary) to perigynous (those with the perianth and 
stamens united at the base into a hypanthium distinct from the ovary) and with 
a single set of stamens which alternate with the petals. Nearly all of the 
species are woody plants. Various species of holly (Ilex, family 
Aquifoliaceae) and Euonymus (Celastraceae) are often cultivated. See FLOWER; 
HOLLY; ROSIDAE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sCelebes   �21m �33s 
    An irregular or K-shaped island now called Sulawesi, with a 3000-mi 
(4800-km) coast and an area of approximately 72,000 misup2 (186,000 kmsup2). 
Celebes, part of Wallacea, is a rugged island, with little land under 600 ft 
(180 m) in elevation; in fact, most of the land is above 1500 ft (450 m). A 
peak of the Quarles Mountains rises to 11,296 ft (3443 m). Numerous lakes 
occupy depressions along the major faults thought to be graben. One of these, 
Lake Poso, is 5000 ft (1500 m) deep. Although at present there are no 
important mines, indications of nickel, gold, iron, and petroleum have been 
found. Hot unvarying temperatures, heavy rainfall, and high relative humidity 
result in a heavy cover of tropical rainforest. See EAST INDIES. 
 �34s[M. G. Audley-Charles]
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�6v�2v�32sCelery   �21m �33s 
    A biennial umbellifer (Apium graveolens var. dulce) of Mediterranean 
origin and belonging to the plant order Umbellales. Celery is grown for its 
petioles or leafstalks, which are most commonly eaten as a salad but 
occasionally cooked as a vegetable. California, Florida, and Michigan are 
important producing states. See UMBELLALES. 
 �34s[H. John Carew]
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�6v�2v�32sCelestial navigation �21m �33s
    Navigation with the aid of celestial bodies, primarily for determination
of position when landmarks are not available. In celestial navigation,
position is not determined relative to the objects observed, as in navigation
by piloting, but in relation to the points on the Earth having certain
celestial bodies directly overhead.
    Celestial bodies are also used for determination of horizontal direction
on the Earth and for regulating time, which is of primary importance in
celestial navigation because of the changing positions of celestial bodies in
the sky as the Earth rotates daily on its axis.
    The navigator, concerned less with the actual motions of celestial bodies
than with their apparent motions as viewed from the Earth, pictures the
heavens as a hollow celestial sphere of infinite radius, with the Earth at its
center and the various celestial bodies on its inner surface. The navigator
visualizes this sphere as rotating on its axis once in about 23 h 56 min - one
sidereal day.
    Several systems of coordinates are available to identify points on the
celestial sphere. The celestial equator system of coordinates is an extension
of the equatorial system commonly used on the Earth. The navigator also uses
the horizon system of coordinates, in which the primary great circle is the
horizon of the observer. See ASTRONOMICAL COORDINATE SYSTEMS.
    Position determination in celestial navigation is primarily a matter of
converting one set of coordinates to the other. This is done by solution of a
spherical triangle called the navigational triangle.
    The concept of the spherical navigational triangle is graphically shown in
the illustration, a diagram on the plane of the celestial meridian. The
celestial meridian passes through the zenith of the observer, and is therefore
a vertical circle of the horizon system. Elements of both systems are shown in
this illustration, indicating that an approximate solution can be made
graphically.
    The vertices of the navigational triangle are the elevated pole (P sub n),
the zenith (Z), and the celestial body (M). The angles at the vertices are,
respectively, the meridian angle (t), the azimuth angle (Z), and the
parallactic angle (X). The sides of the triangle are the codeclination of the
zenith or the colatitude (colat) of the observer, the coaltitude or zenith
distance (z) of the body, and the codeclination or polar distance (p) of the
body.
    A navigational triangle is solved, usually by computation, and compared
with an observed altitude to obtain a line of position by a procedure known as
sight reduction. With the emergence of electronic computers and hand-held
calculators, sight reduction has been performed increasingly with limited use
or elimination of tables.
    To establish a celestial line of position, the navigator observes the
altitude of a celestial body, noting the time of observation. Observation is
made by a sextant, so named because early instruments had an arc of one-sixth
of a circle. By means of the double reflecting principle, the altitude of the
body is double the amount of arc used. The marine sextant uses the visible
horizon as the horizontal reference. An air sextant has an artificial, built-in
horizontal reference based upon a bubble or occasionally a pendulum or
gyroscope. The sextant altitude, however measured, is subject to certain
errors, for which corrections are applied. See SEXTANT.
    Time is repeatedly mentioned as an important element of a celestial
observation because the Earth rotates at the approximate rate of 1 minute of
arc each 4 sec of time. An error of 1 sec in the timing of an observation
might introduce an error in the line of position of as much as one-quarter of
a mile. Time directly affects longitude determination, but not latitude. The
long search for a method of ascertaining longitude at sea was finally solved
two centuries ago by the invention of the marine chronometer, a timepiece with
a nearly steady rate. See NAVIGATION; PILOTING.
 �34s[Alton B Moody]
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�6v�2v�32sCelestial sphere �21m �33s
    A hypothetical sphere of infinite radius with its center at the observer.
Because all stars are so far from an Earth-bound observer that they can be
located solely by their direction, it is convenient to consider them as
located on the inside of a spherical surface centered on the observer. Because
the radius of Earth is negligible, it is frequently convenient to consider
Earth, rather than the terrestrial observer, centered in the celestial sphere.
Stars are designated by their location on this sphere, and planets and other
heavenly bodies are located against this background. See ASTRONOMICAL
COORDINATE SYSTEMS.
 �34s[Frank H. Rockett]
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�6v�2v�32sCelestite   �21m �33s 
    A mineral with the chemical composition SrSO4. Celestite occurs commonly 
in colorless to sky-blue, orthorhombic, tabular crystals. Fracture is uneven 
and luster is vitreous. Hardness is 3 - 3.5 on Mohs scale and specific gravity 
is 3.97. It fuses readily to a white pearl. The strontium present in celestite 
imparts a characteristic crimson color to the flame. 
    Celestite occurs in association with gypsum, anhydrite, salt beds, 
limestone, and dolomite. Large crystals are found in vugs or cavities of 
limestone. It is deposited directly from sea water, by groundwater, or from 
hydrothermal solutions. Celestite is the major source of strontium. Although 
celestite deposits occur in Arizona and California, domestic production of 
celestite has been small and sporadic. Much of the strontium demand is 
satisfied by imported ores from England and Mexico. See STRONTIUM. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sCell (biology)�21m�33s
    One of the microscopic structural units of which living organisms, both 
plant and animal, are built. Some simple organisms consist of one free-living 
cell, whereas more complex plants and animals contain from hundreds to 
billions of cells. A typical cell consists of two major compartments: a 
nucleus, containing the hereditary material in the form of chromosomes, and a 
surrounding mass of cytoplasm, which includes a variety of smaller components. 
See CYTOPLASM. 
    Cells are characterized by certain universal structural features. Almost 
all cells possess an external limiting membrane, a nucleus (with chromosomes), 
mitochondria, ribosomes, and a system of internal cytoplasmic membranes. In 
addition, other components are widely distributed, such as lysosomes, 
centrioles, cilia, microtubules, and the walls and chloroplasts of plant cells. 
    The external cell membrane is a very delicate structure less than 10 
nanometers in thickness. It is seen in the electron microscope as a 
three-layered sheet. Other membranes inside the cell display a similar 
structure, and the name unit membrane has been applied to all these. The cell 
membrane regulates the exchange of materials in and out of the cell, and it 
plays an essential role in nervous transmission, contraction, and interaction 
of cells with one another. See CELL MEMBRANES. 
    Within the cytoplasm is a system of membranes which  constitute the 
endoplasmic reticulum and part of the Golgi zone. The endoplasmic reticulum 
consists of a series of closed sacs. frequently flattened and stacked in 
multiple layers. Connections between the sacs establish a complex pattern of 
membranes and spaces. The outside of the sacs is usually studded with 
ribosomes. The ribosomes are the site of protein synthesis in the cells, but 
their relationship to the membranes of the endoplasmic reticulum is not clear. 
See ENDOPLASMIC RETICULUM; RIBOSOMES. 
    The Golgi zone also contains thin flattened sacs somewhat like those of 
the endoplasmic reticulum. However, the sacs of the Golgi zone are smaller and 
more tightly packed, and lack ribosomes on their surfaces. In addition, this 
part of the cell regularly contains droplets or granules of various sizes. The 
exact function of the Golgi zone has been much debated, but it is known that 
products which are later secreted from the cell receive some kind of 
processing or packaging in the Golgi zone. See GOLGI APPARATUS. 
    A major role in cell function is played by the mitochondria, which are the 
respiratory centers of the cell. Mitochondria appear as minute rods or spheres 
in the cytoplasm, often numbering in the thousands in a single cell. They are 
surrounded by two unit membranes, the innermost of which sends finger- or 
sheetlike projections into the interior. Various respiratory enzymes form an 
integral part of the structure of the membranes. See MITOCHONDRIA. 
    The nuclear envelope is another membranous component of the cell. It was 
known as the nuclear membrane until the electron microscope showed that it is, 
in fact, two closely spaced membranes completely surrounding the nucleus. It 
is pierced by numerous octagonal pores, which connect the cytoplasm with the 
interior of the nucleus. Within the nucleus are one or more chromosomes, the 
threadlike structures which control inheritance. Chromosomes are exceedingly 
long and thin when stretched out completely. In addition to the chromosomes, 
the nucleus contains one or more nucleoli, which appear as prominent spherical 
bodies in nondividing cells. Each nucleolus is attached to a chromosome at a 
point known as the nucleolus organizer. See CHROMOSOME; MITOSIS. 
    Among the widespread but not universal cell components are cell walls and 
chloroplasts, characteristic of plant cells. The majority of plant cells 
secrete one or more rigid walls outside the cell membrane. These walls contain 
cellulose and other strengthening materials and may be extremely thick, as in 
the woody parts of stems. They are responsible for the characteristically 
rigid form of plants, as well as for the elasticity and toughness of their 
tissues. Chloroplasts contain the green pigment chlorophyll and are typical, 
therefore, of the green plants. They absorb light energy and use it to carry 
out chemical reactions by the process of photosynthesis. See CELL PLASTIDS; 
CELL WALLS (PLANT); PLANT CELL. 
    Many cells contain membrane-bounded vesicles of indefinite form known as 
lysosomes. These harbor a number of hydrolytic enzymes, one of which, acid 
phosphatase, is characteristic of this organelle. Lysosomes appear to play a 
digestive role in the cell. See LYSOSOME. 
    Several cytoplasmic components of widespread occurrence contain thin rods 
or tubes which are known as microtubules. They may occur free or in bundles in 
the cytoplasm, where they evidently help to maintain cell shape. The mitotic 
spindle contains microtubules, which may be concerned with the orderly 
movements of the chromosomes. See MICROTUBULES. 
�34s[Joseph G. Gall]�33s

  
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�6v�2v�32sCell biology   �21m �33s 
    The name applied to that division of biology which deals with the 
structure, behavior, growth, and reproduction of cells and the function and 
chemistry of cell components. Cytology is therefore the branch of science that 
explores the properties of living matter, or protoplasm. See CELL (BIOLOGY); 
CYTOCHEMISTRY; MOLECULAR BIOLOGY. 
 �34s[Keith R. Porter]
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�6v�2v�32sCell division   �21m �33s 
    A process by which living cells reproduce their kind. In the larger view, 
biological increase must be described as an increase in numbers of cells. The 
only cells which normally will not divide are found in differentiated tissues 
of multicellular organisms. Cell division includes nuclear division, which 
occurs most commonly by mitosis, and cytokinesis (fission of cytoplasm). See 
CYTOKINESIS; MEIOSIS; MITOSIS. 
    Overall growth of biological systems may be described in terms of the 
cellular growth division cycle. Living systems are composed of cells which 
have the capacity to synthesize new molecules similar to those already 
present. As a result, cells grow in mass, but the capacity of an individual 
cell to grow is limited. Normally, although there are exceptions, a cell can 
only double its mass before growth ceases. Division refreshes the capacity for 
growth, because the daughter cells begin to grow immediately after division. 
They normally cannot divide again until they have grown again. Division makes 
growth possible, growth makes division possible, and thus biological systems 
increase by stages of doubling and halving. 
    Genetic conservation, in this cycle, depends on the duplication of the 
genetic material before division, followed by its exact qualitative and 
quantitative distribution to the daughter cells. The distribution is 
accomplished by mitosis in all plant and animal cells, and in many protozoa, 
molds, and other microorganisms. Multiplication of cells always involves 
multiplication of nuclei. On the other hand, there are cases in which cells do 
grow without division. In these, nuclear divisions parallel the growth, but 
the division of the cell body is omitted. 
 �34s[Daniel Mazia]
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�6v�2v�32sCell lineage   �21m �33s 
    A type of embryological study in which the history of individual 
blastomeres is traced to their ultimate differentiation into tissues and 
organs. The lineage of blastomers can be readily traced only in certain 
invertebrate animals, chiefly polyclad flatworms, annelids, mollusks, and 
ascidians, where the cleavage of the egg follows a rigidly stereotyped 
pattern. Individual cells are recognizable by differences in size, shape, 
position, composition, and time of origin. See CLEAVAGE (EMBRYOLOGY); FATE 
MAPS (EMBRYOLOGY); INVERTEBRATE EMBRYOLOGY; OVUM. 
 �34s[Anthony C. Clement]
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�6v�2v�32sCell membranes �21m �33s
    Thin layers, only a few molecules thick, consisting mainly of lipids and
proteins. They cover the surface of all cells and are then called plasmalemma,
plasma membrane, or cell membrane in the narrower sense. Other membranes
similar in composition and appearance to plasma membranes are found in the
cytoplasm of most cells, where they create a number of different compartments.
Most cell organelles are bounded by such membranes; some of them, for example,
mitochondria and chloroplasts, are again subdivided into compartments by
internal membranes.
    The plasma membrane controls exchange of matter between the interior of
the cell and the surrounding medium and enables the cell to maintain
concentrations of water, small ions, and larger molecules different from, and
within limits independent of, the environment. Plasma membranes are almost
impermeable to some ions and molecules; other ions and molecules can diffuse
freely through the membrane; and some are transported actively against
concentration or electric gradients. Closely related to the permeability
properties are the functions of the plasma membrane in excitation and impulse
conduction. Bulk transport of fluid (pinocytosis) or solids (phagocytosis)
across the cell border occurs through enfoldings of the membrane that close
form vesicles which are detached and move into the cytoplasm. The reverse of
this mechanism is used for the secretion of cell products. Membranes also
carry enzymes and integrated enzyme systems, especially systems that function
as energy transducers. Plasma membranes also carry so-called receptors, which
are able to bind specific compounds such as hormones or more complex
structures such as virus particles. See BIOELECTRIC MODEL; ENDOCYTOSIS;
PHAGOCYTOSIS.
    Investigations into the molecular structure of cell membranes have for a
long time been dominated by the Danielli model, which describes membranes as
consisting of a bimolecular layer of lipids 4.0 - 5.0 nm wide covered with
layers of protein on both surfaces to give a total width of 7.5 - 10.0 nm. In
the bilayer the lipid molecules are oriented with their hydrophilic groups
facing outward and the hydrocarbon chains facing each other to form the
hydrophobic core (see illustration). This model may still correctly describe
the structure of some membranes, for example, the myelin sheath of nerve
fibers, but it must be modified for most other membranes. Electron microscopy,
x-ray diffraction, and a variety of spectroscopic techniques have confirmed
the existence of a lipid bilayer as the structural backbone of membranes but
also show that a large part of the protein is present in the hydrophobic core
of the membrane.
    The lipids in the bilayer can exist in two states, an essentially solid or
gellike state and a liquidlike state which is disordered and in which the
lipid molecules have a high degree of mobility in the plane of the bilayer.
Membranes in living cells appear to exist usually in the liquidlike state but
are often close to, or even in, the transition to the gellike state. The
transition from the liquidlike to the gellike state of the lipids can be
induced through changes in many physical parameters, for example, by lowering
the temperature or changing pH and other ion concentrations in the surrounding
medium. The transition in natural membranes is usually broad and depends not
only on the lipid composition but also on the proteins present.
    The thickness of cell membranes cannot be resolved in the light
microscope. In electron micrographs of sectioned cells or tissues they appear
as triple-layered structures 7.5 - 10 nm wide, consisting of two outer dense
bands and a central lighter region. This has been termed the unit membrane,
because it is the characteristic image of nearly all cell membranes.
    Little is known about the formation of new membranes in cells. Most of the
direct evidence indicates that membranes grow by insertion of new molecules or
molecular complexes into preexisting membranes. It is not known whether areas
of undifferentiated membranes are first formed, to which specific functional
components are added later, or whether a fully functional membrane piece is
assembled and inserted. The Golgi apparatus has been specifically implicated
in the synthesis of new cell membranes. The reconstitution of membranelike
structures from disaggregated cellular membranes suggests that formation of
new membranes without the requirement of preexisting membrane is also a
possibility. See CELL WALLS (PLANT); PLANT CELL.
 �34s[Walther Stoeckenius]
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�6v�2v�32sCellobiose �21m �33s
    A disaccharide, with the structure shown, usually prepared from cellulose,
in which molecule it can be viewed as a repeating unit. It is composed of a
BETA-D-glucopyranosyl group joined by 1->4 glycosidic linkages to D-glucose.
Its proper scientific name is 4-O-BETA-D-glucopyranosyl-D-glucose. The sugar
is rarely found free in nature, but its occurrence in pine needles has been
reported. See CELLULOSE.
 �34s[Roy L. Whistler ;James R. Dan]
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�6v�2v�32sCellophane   �21m �33s 
    A transparent polycrystalline film produced by regeneration of cellulose 
in the form of wood pulp, cotton fiber, or another natural source of 
cellulose. Cellulose nitrate is frequently used as a coating to enhance 
imperviousness to moisture and receptivity to printing ink. 
    Cellophane is available as film (thickness less than 0.0098 in. or 0.25mm) 
or sheeting (thickness greater than 0.0098). It exhibits excellent 
transparency, gloss, stiffness, tensile strength, and flexibility, and 
transmits a high percentage of ultraviolet radiation. Used chiefly in food 
packaging, it is rapidly being replaced by polypropylene film. However, it has 
applications in high performance packaging where stiffness and abrasion 
resistance are primary concerns. Cellophane is considered the most easily 
processed film in large-scale printing, converting, and in-line packaging 
operations. See CELLULOSE. 
 �34s[Frank H. Rockett]
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�6v�2v�32sCell plastids   �21m �33s 
    Specialized structures found in the cytoplasm of plant cells, diverse in 
distribution, size, shape, composition, structure, function, and mode of 
development. A number of different types are recognized. Chloroplasts occur in 
the green parts of plants and are responsible for the green coloration, for 
they contain the chlorophyll pigments. These pigments, along with certain 
others, absorb the light energy that drives the processes of photosynthesis, 
by which sugars, starch, and other organic materials are synthesized. 
Amyloplasts, nearly or entirely colorless, are packed with starch grains and 
occur in cells of storage tissue. Proteoplasts are less common and contain 
crystalline, fibrillar, or amorphous masses of protein, sometimes along with 
starch grains. In chromoplasts the green pigment is masked or replaced by 
others, notably carotenoids, as in the cells of carrot roots and many flowers 
and fruits. See CAROTENOID; CHLOROPHYLL. 
    All types of plastids have one structural feature in common, a double 
envelope consisting of two concentric sheets of membrane. The outer of these 
is in contact with the cytoplasmic ground substance; the inner with the 
plastid matrix, or stroma. They are separated by a narrow space of about 10 
nanometers.  
    Another system of membranes generally occupies the main body of the 
plastid. This internal membrane system is especially well developed in 
chloroplasts, where the unit of construction is known as a thylakoid. In its 
simplest form this is a sac such as would be obtained if a balloon-shaped, 
membrane-limited sphere were to be flattened until the internal space was not 
much thicker than the membrane itself. It is usual, however, for thylakoids to 
be lobed, branched, or fenestrated. 
    The surface area of thylakoids is very large in relation to the volume of 
the chloroplast. This is functionally significant, for chlorophyll molecules 
and other components of the light-reaction systems of photosynthesis are 
associated with these membranes. A chloroplast, however, is much more than a 
device for carrying out photosynthesis. It can use light energy for uptake and 
exchange of ions and to drive conformational changes. The stroma contains the 
elements of a protein-synthesizing system - as much deoxyribonucleic acid 
(DNA) as a small bacterium, various types of ribonucleic acid (RNA), 
distinctive ribosomes, and polyribosomes. There is evidence to indicate that 
much of the protein synthesis of a leaf takes place within the chloroplasts. 
See PHOTOSYNTHESIS; PROTEIN; RIBOSOMES. 
    One of the most challenging problems in cell biology concerns the autonomy 
of organelles, such as the plastids. Chloroplasts, for instance, have their 
own DNA, DNA-polymerase, RNA-polymerase; can make proteins; and, 
significantly, can mutate. All this suggests a measure of independence. It is 
known, however, that some nuclear genes can influence the production of 
molecules that are normally found only in chloroplasts, so their autonomy 
cannot be complete. It remains to be seen whether they control and regulate 
their own morphogenetic processes. See CYTOPLASMIC INHERITANCE. 
 �34s[Brian E. S. Gunning]
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�6v�2v�32sCellular immunology   �21m �33s 
    The study of the cells of the lymphoid organs, which are the main agents 
of immune reactions in all the vertebrates. The central cell of immunology is 
the lymphocyte. Two types of lymphocytes are derived from stem cells in the 
yolk sac, fetal liver, and bone marrow. T lymphocytes undergo maturation and 
proliferation in the thymus before entering the ``pool'' of peripheral 
immunocompetent lymphocytes. B lymphocytes mature and proliferate in the 
marrow or fetal liver or (in birds) in a special organ, the bursa of 
Fabricius. Both cells, when mature, enter the bloodstream. 
    The two lymphocyte populations, once they enter the peripheral pool, have 
entirely different roles. The B cell, when adequately stimulated, becomes an 
antibody-producing and secreting cell. The T cell, on the other hand, becomes 
the immune effector cell of cell-mediated immunity. Some T cells also play an 
important role in cooperation, helping B lymphocytes to respond to antigen, 
and in suppression of excessive or unwanted responses. There are thus several 
subpopulations of T cells with different functional attributes. 
    Advances in knowledge of this complex field have depended on development 
of new materials and techniques. The most important of these has been the use 
of inbred lines of mice, in which cells and tissues can be freely moved from 
one animal to another genetically identical recipient. By transfer of 
different purified cell populations, one can analyze their role in immunity. 
Also, genetic differences between inbred lines has permitted identification of 
immune response genes and chromosomal mapping of other significant elements of 
the immune response. 
    Immunologists are in the middle of a period of rapid application of the 
new techniques and new insights to problems of human medicine. A great 
challenge facing researchers in this field is the use of immunologic methods 
to alleviate the large range of autoimmune diseases, to permit free grafting 
of tissues and organs as needed, and to enhance immunity to tumors. See 
IMMUNOLOGY.  
 �34s[Byron H. Waksman]
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�6v�2v�32sCellulose �21m �33s
    The main polysaccharide in living plants, empirical formula (C6H10O5) sub n.
It forms the skeletal structure of the cell wall, hence the name cellulose;
wood normally contains 40 - 50% cellulose. It occurs together with
polysaccharides and hemicelluloses derived from other sugars. In the woody
parts of plants, cellulose is intimately mixed, and sometimes covalently
linked, with lignin. Cellulose is a polymer of BETA-D-glucopyranosyl units
which are linked together, with elimination of water, to form chains of 2000 -
4000 units, as shown in the figure. See CELL WALLS (PLANT).

    Cellulose of high chemical purity for research purposes is prepared from
cotton. This cellulose is almost entirely free from other components and is
one of the purest readily available polysaccharides. High-molecular-weight
cellulose is used for making cellophane, viscose, and acetate rayon. Both
high-molecular-weight and lower-molecular-weight celluloses may be used for
making other derivatives.
    Commercial cellulose may be obtained from cotton linters, but is produced
mainly from woods, particularly pine and spruce, by pulping procedures
designed to remove lignin and other noncellulosic constituents. In addition to
mechanical and semimechanical processes which separate wood fibers for use in
paper manufacture, there are three chemical pulping processes in wide
commercial use, the kraft, soda, and sulfite processes. See CARBOHYDRATE;
CELLOBIOSE; HEMICELLULOSE; PAPER AND PAPER PRODUCTS; POLYSACCHARIDE.
 �34s[Roy L. Whistler]
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�6v�2v�32sCell walls (plant)   �21m �33s 
    The cell wall is the layer of material secreted by the plant cell outside 
its plasma membrane. With some exceptions among the algae and most of the 
fungi, plants more complex than prokaryotes (bacteria and blue-green algae) 
have cell walls that are generally very similar in chemical composition, 
organization, and development. From an engineering point of view, the plant 
cell wall can be considered a nonliving system, although its role in the 
economy of the organism and of the cell is basic to any understanding of the 
living system. The wall serves as the first point of entry of materials into 
cells, functions in the movement of water throughout the plant, and is one of 
the major mechanical strengthening factors. In addition, the wall must be 
sufficiently flexible and plastic to withstand mechanical stresses while still 
permitting the growth of the cell. See CELL MEMBRANES. 
    Cellulose forms the skeletal structure of the cell wall. Like starch, it 
is basically a polymer of glucose, a six-carbon monosaccharide, although the 
biosynthetic route for cellulose synthesis differs from that of starch. Each 
chain of cellulose may be as long as 8000 to 12,000 glucose monomers, or up to 
4 micrometers long. These are arranged linearly, with no side branching. 
Cellulose chains are aggregated into bundles of approximately 40 chains each, 
the cellulose micelles, which are held together by hydrogen bonds. The 
micelles themselves are embedded in a matrix of other polysaccharides, the 
hemicelluloses. Micelles, in bundles of variable number, are bound together 
into the cellulose microfibril, a unit sufficiently large to be seen under the 
electron microscope; these, in turn, are bound together into macrofibrils 
which are observable under the light microscope. See CELLULOSE. 
 �34s[Richard M. Klein]
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�6v�2v�32sCement   �21m �33s 
    Any substance that acts as a bonding agent for materials. In construction 
and engineering the word almost always means hydraulic cement. For information 
about other cementing materials see ADHESIVE; ASPHALT AND ASPHALTITE; CLAY; 
EPOXY RESIN; GLUE; MUCILAGE. 
    Hydraulic cements are produced by burning an intimate mixture of finely 
divided calcareous and argillaceous materials and grinding the resulting 
clinker to a fine powder, usually with gypsum to retard the set. The calcining 
process produces calcium silicates and calcium aluminates that can react 
chemically with water to form a hard, stonelike mass. When mixed with sand, 
coarse aggregate, and water, these cements produce mortars and concretes. See 
CONCRETE; MORTAR; PORTLAND CEMENT. 
 �34s[J. H. Walker]
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�6v�2v�32sCenozoic   �21m �33s 
    The youngest of the eras or major subdivisions of geologic time, extending 
from the end of the Mesozoic Era to the present, or Holocene. The term 
Cenozoic is also applied to all the rocks formed during this time and all the 
remains of past life which they contain. Although no unit term, such as era, 
has in the past been used for fossiliferous rock groups of era magnitude, the 
term erathem has been proposed for such major time-rock units. 
    Rocks formed during Cenozoic time were deposited mechanically or 
chemically upon many kinds of older rocks, or in volcanic form extruded to 
rest also upon older rocks or intruded into or below such older rocks. From 
time to time these Cenozoic rocks in certain areas of concentrated deposition 
have been elevated, locally broken, and widely crumpled by stresses operating 
in the crust of the Earth to produce great mountain ranges such as those 
around the entire rim of the Pacific Ocean (the Andes, the West Coast ranges 
of North America, those of Kamchatka, Japan, the East Indies, New Zealand) and 
the Himalayan chain. Geologically, these are the youngest mountains. 
    Distribution of land and sea was different at different times during the 
Cenozoic. Although this was a continuous process of change, with local 
exceptions to the general trend, three great expansions of the oceans over the 
continental shelf (with several lesser fluctuations) were interspersed between 
four extended intervals of great land emergence, uplift, and increased surface 
relief, all prior to the attainment of geologically modern times. 
Historically, recognition of subdivisions within the Cenozoic first arrived at 
a threefold classification (Eocene, Miocene, Pliocene), with intervening gaps 
filled in only subsequently (Paleocene, Oligocene, ``Mio-Pliocene,'' 
Pleistocene, Holocene). 
    Modern landforms and the present configuration of the continental land 
masses have come into being largely from dynamic interplay of deposition, 
erosion, mountain-making (orogeny and epeirogeny), and volcanism during 
Cenozoic time. Eventually continental glaciation became widespread, during a 
great Ice Age (Pleistocene) that finally began to wane some 25,000 years ago 
(beginning of the Holocene Epoch). 
    The entire Cenozoic Era lasted on the order of 70,000,000 years. 
Customarily, it is formally divided into two larger subdivisions, or periods, 
of unequal length: the older, or Tertiary, period, and the younger, or 
Quaternary. See QUATERNARY; TERTIARY. 
 �34s[Richard M. Klein]
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�6v�2v�32sCentaurus   �21m �33s 
    The Centaur, in astronomy; one of the most magnificent of the southern 
constellations. Two first-magnitude navigational stars, Alpha and Beta 
Centauri, mark the right and left front feet, respectively, of the centaur. 
The former is Rigil Kentaurus, or simply Rigil Kent, and the latter Hadar. 
Rigil Kent is the third brightest star in the whole sky, fainter than only 
Sirius and Canopus. The line joining Rigil Kent and Hadar points to the 
constellation Crux (Southern Cross). Thus they are also called the Southern 
Pointers, in contradistinction to the northern pointers of the Big Dipper. See 
CONSTELLATION; URSA MAJOR. 
 �34s[Ching-Sung Yu]
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�6v�2v�32sCenter of gravity   �21m �33s 
    A fixed point in a material body through which the resultant force of 
gravitational attraction acts. The resultant of all forces or attractions 
produced by the Earth's gravity on a body constitutes its weight. This weight 
is considered to be concentrated at the center of gravity in mechanical 
studies of a rigid body. The location of the center of gravity for a body 
remains fixed in relation to the body regardless of the orientation of the 
body. If supported at its center of gravity, a body would remain balanced in 
its initial position. See GRAVITY; RESULTANT OF FORCES. 
 �34s[Nelson S. Fisk]
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�6v�2v�32sCenter of mass   �21m �33s 
    That point of a material body or system of bodies which moves as though 
the system's total mass existed at the point and all external forces were 
applied at the point. The Earth-Moon system moves in the Sun's gravitational 
field as though both masses were located at a center of mass some 3000 mi 
(4700 km) from the Earth's geometric center. The function of the center-of-mass 
concept is to permit analysis of the motion of an entire system as distin-
guished from that of its individual parts. 
    Consider a system of mass M composed of n bodies with masses m1, m2, 
. . . , mn and radius vectors r1, r2, . . . , rn measured from some common 
reference point. Define a point with radius vector R, such that Eq. (1) holds. 
  
              *** MR = SUMMATION (j) (m sub j) (r sub j) ***              (1)
  
Then, it is possible to derive Eq. (2), an expression of Newton's second law,
  
                 *** [d (sup 2) R] / dt (sup 2) = F/M ***                 (2)
  
which states that the center of mass at R moves as though it possessed the 
total mass of the system and were acted upon by the total external force. 
    A simplification of the description of collisions can be obtained by using 
a coordinate system which moves with the velocity of the center of mass before 
collision. See COLLISION (PHYSICS); RIGID-BODY DYNAMICS. 
 �34s[John P. Hagen]
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�6v�2v�32sCenter of pressure   �21m �33s 
    A point on a plane surface through which the resultant force due to 
pressure passes. Such a surface can be supported by a single mounting fixture 
at its center of pressure if no other forces act. For example, a water gate in 
a dam can be supported by a single shaft at its center of pressure. See 
RESULTANT OF FORCES. 
 �34s[Nelson S. Fisk]
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�6v�2v�32sCentral force   �21m �33s 
    A force whose line of action is always directed toward a fixed point. The 
central force may attract or repel. The point toward or from which the force 
acts is called the center of force. If the central force attracts a material 
particle, the path of the particle is a curve concave toward the center of 
force; if the central force repels the particle, its orbit is convex to the 
center of force. Undisturbed orbital motion under the influence of a central 
force satisfies Kepler's law of areas. 
 �34s[Raynor L. Duncombe]
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�6v�2v�32sCentral heating and cooling   �21m �33s 
    The use of a single heating or cooling plant to serve a group of 
buildings, facilities, or even a complete community through a system of 
distribution pipework that feeds each structure or facility. Central heating 
plants are basically of two types: steam or hot-water. The latter type uses 
high-temperature hot water under pressure and has become the more usual 
because of its considerable advantages. Steam systems are only used today 
where there is a specific requirement for high-pressure steam. Central cooling 
plants utilize a central refrigeration plant with a chilled water distribution 
system serving the air-conditioning systems in each building or facility. 
    Advantages of a central heating or cooling plant over individual ones for 
each building or facility in a group include reduced labor cost, lower energy 
cost, less space requirement, and simpler maintenance. Central cooling plants, 
using conventional, electrically driven refrigeration compressors, have the 
advantage of utilizing bulk electric supply, at voltages as high as 13.5 kV, 
at wholesale rates. Additionally, their flexible load factor, resulting from 
load divergency in the various buildings served, results in major operating 
economies.  
    The disadvantages of a central heating plant concern mainly the 
maintenance of the distribution system where steam is used. Corrosion of the 
condensate water return lines shortens their life, and the steam drainage 
traps need particular attention. These disadvantages do not occur with 
high-temperature hot-water installations. See AIR CONDITIONING; BOILER; 
COMFORT HEATING; REFRIGERATION; STEAM HEATING; WARM-AIR HEATING SYSTEM. 
 �34s[John K. M. Pryke]
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�6v�2v�32sCentral nervous system   �21m �33s 
    That portion of the nervous system composed of the brain and spinal cord. 
The brain is enclosed in the skull, and the spinal cord within the spinal 
canal of the vertebral column. The brain and spinal cord are intimately 
covered by membranes called meninges and bathed in an extracellular fluid 
called cerebrospinal fluid. Approximately 90% of the cells of the central 
nervous system are glial cells which support, both physically and 
metabolically, the other cells, which are the nerve cells or neurons. See 
MENINGES; NEURON. 
    Functionally similar groups of neurons are clustered together in so-called 
nuclei of the central nervous system. When groups of neurons are organized in 
layers (called laminae) on the outer surface of the brain, the group is called 
a cortex, such as the cerebral cortex and cerebellar cortex. The long 
processes (axons) of neurons course in the central nervous system in 
functional groups called tracts. Since many of the axons have a layer of shiny 
fat (myelin) surrounding them, they appear white and are called the white 
matter of the central nervous system. The nuclei and cortex of the central 
nervous system have little myelin in them, appear gray, and are called the 
gray matter of the central nervous system. See BRAIN; NERVOUS SYSTEM 
(VERTEBRATE); SPINAL CORD. 
 �34s[Douglas B. Webster]
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�6v�2v�32sCentrifugal force �21m �33s
    A fictitious or pseudo outward force on a particle rotating about an axis
which by Newton's third law is equal and opposite to the centripetal force.
Like all such action-reaction pairs of forces, they are equal and opposite but
do not act on the same body and so do not cancel each other. Consider a mass M
tied by a string of length R to a pin at the center of a smooth horizontal
table and whirling around the pin with an angular velocity of OMEGA radians
per second. The mass rotates in a circular path because of the centripetal
force FsubC = MOMEGAsup2R which is exerted on the mass by the string. The
reaction force exerted by the rotating mass M, the so-called centrifugal
force, is MOMEGAsup2R in a direction away from the center of rotation. See
CENTRIPETAL FORCE.
    From another point of view, consider an experimenter in a windowless,
circular laboratory that is rotating smoothly about a centrally located
vertical axis. No object remains at rest on a smooth surface; all such objects
move outward toward the wall of the laboratory as though an outward,
centrifugal force were acting. To the experimenter partaking in the rotation,
in a rotating frame of reference, the centrifugal force is real. An outside
observer would realize that the inward force which the experimenter in the
rotating laboratory must exert to keep the object at rest does not keep it at
rest, but furnishes the centripetal force required to keep the object moving
in a circular path. The concept of an outward, centrifugal force explains the
action of a centrifuge. See CENTRIFUGATION.
 �34s[Carl E. Howe /R. J. Stephenson]
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�6v�2v�32sCentrifugal pump   �21m �33s 
    A machine for moving fluid by accelerating it radially outward. More fluid 
is moved by centrifugal pumps than by all other types combined. Centrifugal 
pumps consist basically of one or more rotating impellers in a stationary 
casing which guides the fluid to and from the impeller or from one impeller to 
the next in the case of multistage pumps. Impellers may be single suction or 
double suction. Additional essential parts of all centrifugal pumps are (1) 
wearing surfaces or rings, which make a close-clearance running joint between 
the impeller and the casing to minimize the backflow of fluid from the 
discharge to the suction; (2) the shaft, which supports and drives the 
impeller; and (3) the stuffing box or seal, which prevents leakage between 
shaft and casing. 
    The rotating impeller imparts pressure and kinetic energy to the fluid 
pumped. A collection chamber in the casing converts much of the kinetic energy 
into head or pressure energy before the fluid leaves the pump. A free passage 
exists at all times through the impeller between the discharge and inlet side 
of the pump. Rotation of the impeller is required to prevent backflow or 
draining of fluid from the pump. Because of this, only special forms of 
centrifugal pumps are self-priming. Most types must be filled with liquid, or 
primed, before they are started. 
    Every centrifugal pump has its characteristic curve, which is the relation 
between capacity or rate of flow and pressure or head against which it will 
pump. At zero pressure-difference, maximum capacity is obtained, but without 
useful work. As resistance to flow external to the pump increases, capacity 
decreases until, at a high pressure, flow ceases entirely. This is called 
shut-off head and again no useful work is done. Between these extremes, 
capacity and head vary in a fixed relationship at constant rpm. When the 
required head exceeds that practical for a single-stage pump, several stages 
are employed. Multistage pumps range from two-stage pumps to pumps built with 
as many as 20 or 30 stages for high lifts from relatively small-diameter 
wells. See PUMP; PUMPING MACHINERY. 
 �34s[Elliott F. Wright]
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�6v�2v�32sCentrifugation   �21m �33s 
    A mechanical method of separating immiscible liquids or solids from 
liquids by the application of centrifugal force. This force can be very great, 
and separations which proceed slowly by gravity can be speeded up enormously 
in centrifugal equipment. See CENTRIFUGAL FORCE. 
    Centrifugal force is generated inside stationary equipment by introducing 
a high-velocity fluid stream tangentially into a cylindrical-conical chamber, 
forming a vortex of considerable intensity. Cyclone separators based on this 
principle remove liquid drops or solid particles from gases, down to 1 or 2 
MUm in diameter. Smaller units, called liquid cyclones, separate solid 
particles from liquids. The high velocity required at the inlet of a liquid 
cyclone is obtained with standard pumps. Much higher centrifugal forces than 
in stationary equipment are generated in rotating equipment (mechanically 
driven bowls or baskets, usually of metal, turning inside a stationary 
casing). Rotating a cylinder at high speed induces a considerable tensile 
stress in the cylinder wall. This limits the centrifugal force which can be 
generated in a unit of a given size and material of construction. Very high 
forces, therefore, can be developed only in very small centrifuges. 
    There are two major types of centrifuges: sedimenters and filters. A 
sedimenting centrifuge contains a solid-wall cylinder or cone rotating about a 
horizontal or vertical axis. An annular layer of liquid, of fixed thickness, 
is held against the wall by centrifugal force; because this force is so large 
compared with that of gravity, the liquid surface is essentially parallel with 
the axis of rotation regardless of the orientation of the unit. Heavy phases 
``sink'' outwardly from the center, and less dense phases ``rise'' inwardly. 
Heavy solid particles collect on the wall and must be periodically or 
continuously removed. 
    A filtering centrifuge operates on the same principle as the spinner in a 
household washing machine. The basket wall is perforated and lined with a 
filter medium such as a cloth or a fine screen; liquid passes through the 
wall, impelled by centrifugal force, leaving behind a cake of solids on the 
filter medium. The filtration rate increases with the centrifugal force and 
with the permeability of the solid cake. Some compressible solids do not 
filter well in a centrifuge because the particles deform under centrifugal 
force and the permeability of the cake is greatly reduced. The amount of 
liquid adhering to the solids after they have been spun also depends on the 
centrifugal force applied; in general, it is substantially less than in the 
cake from other types of filtration devices. See MECHANICAL SEPARATION 
TECHNIQUES.  
 �34s[Julian C. Smith]
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�6v�2v�32sCentrifuge microscope �21m �33s
    An instrument which makes possible the magnification and observation of
living cells or small organisms while they are being centrifuged. The
fundamental optical principle involved is a mirror or prism system. The image
of the material magnified by the objective, which rotates near the periphery
of the centrifuge head, is brought to the axis of rotation where it is
observed with a stationary ocular (see illustration). In this respect it
differs radically from the optical method used to observe or photograph the
boundary of sedimenting protein molecules in an ultracentrifuge, where only a
uniform distribution of material in one dimension (that along a radius of
rotation) can be photographed. The combination of a centrifuge with the
appropriate parts of a microscope provides for a perfect two-dimensional image
of the material under observation. High dry objectives (43x) can be used, and
clarity of the image is independent of speed of rotation. The instrument can
be used to observe stretching of living cells and movement of granules and
other cell structures in a centrifugal field, thereby giving information on
viscosity and surface forces of the cell.
    It is not necessary to use stroboscopic illumination with the instrument.
If the image of an incandescent lamp filament parallel to a radius of rotation
is focused on the object to be examined, the image will be sufficiently clear
over most of the microscope field.
    For mounting material for all types of centrifuge microscope, a special
slide is used, designed so that centrifugal force throws the organisms in
position in approximately the focus of the objective. Final focusing is
carried out by moving the ocular or an accessory lens between the ocular and
central prisms. See MICROSCOPE.
 �34s[E. Newton Harvey /Douglas A. Marsland]
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�6v�2v�32sCentriole �21m �33s
    A cellular organelle of great morphologic complexity, found in metazoans,
protozoans, and many lower plants. It is usually near the nucleus in the
interphase cell and at the poles of the spindle during mitosis. In
differentiating cells, centrioles may give rise to or function as basal
bodies, which are necessary to induce the formation of cilia or flagella. See
CILIA AND FLAGELLA.
    The mature centriole is a short, hollow cylinder containing nine groups of
three microtubules in a squirrel-cage arrangement (illustration a). The
centriole is about 300 - 500 nm in length and about 150 nanometers in
diameter. Each microtubule is about 23 - 27 nanometers in diameter; its lumen
is about 10 nm in diameter (illustration b). The wall of a microtubule
comprises 13 chains of globular subunits, consisting of a protein called
tubulin, and arranged cylindrically down the length of the tubule. Each
subunit is 4.5 - 5.0 nm in diameter and 120,000 in molecular weight. See
GLOBULAR PROTEIN.
    The cellular location of the centriole varies. In certain epithelial cells
a pair of centrioles, at right angles to each other and partially surrounded
by an extensive Golgi complex, is found near the nucleus. In epithelial cells
lining cavities, the centrioles are found just under the membrane of each
cell's free surface. In white blood cells the centrioles are found at the
center of the cell, surrounded by Golgi cisternae. The location of centrioles
also changes with the cell cycle. When the cell is ready to divide, the paired
centrioles migrate to opposite sides of the nucleus. Once the cell has
divided, a pair of centrioles can again be seen near the nucleus. See MITOSIS.
    During cell division the positions of the centrioles are related to the
plane of cleavage. The orientation of the spindle is established by the
migration of the centrioles at the start of mitosis. The cell divides in a
plane perpendicular to the main axis of the spindle. It has also been shown
that once centrioles function as basal bodies, they induce the formation of
cilia and flagella. They are also involved in the formation of the specialized
cilia which develop into the rods of the retina and into the specialized
processes of other sensory organs.
    The centrioles for the first mitotic division of the ovum after
fertilization are usually derived from the sperm midpiece, which usually
contains two centrioles. Once in the cell, centrioles seem to have morphologic
continuity, for they are present at all stages of the mitotic cycle. However,
there are instances in which a mature centriole cannot be found in an early
developmental stage, but appears at later stages. Centrioles are notably
absent from the cells of higher plants.
 �34s[Ellen R. Dirksen]
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(a)
    (b)

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�6v�2v�32sCentripetal force   �21m �33s 
    The inward force required to keep a particle or an object moving in a 
circular path. It can be shown that a particle moving in a circular path has 
an acceleration toward the center of the circle along a radius. See 
ACCELERATION.  
    This radial acceleration, called the centripetal acceleration, is such 
that, if a particle has a linear or tangential velocity v when moving in a 
circular path of radius R. The centripetal acceleration is (v sup 2)/R. If the 
particle undergoing the centripetal acceleration has a mass M, then by 
Newton's second law of motion the centripetal force F sub c in the direction of 
the acceleration. This is expressed by the equation below, where OMEGA is the 
constant angular velocity and is equal to v/R. From Newton's laws of motion it 

          *** F sub c = [Mv (sup 2)] / R = MR OMEGA (sup 2) *** 

follows that the natural motion of an object is one with constant speed in a 
straight line, and that a force is necessary if the object is to depart from 
this type of motion. Whenever an object moves in a curve, a centripetal force 
is necessary. In circular motion the tangential speed is constant but is 
changing direction at the constant rate of OMEGA, so the centripetal force 
along the radius is the only force involved. 
 �34s[R. J. Stephenson]
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�6v�2v�32sCentrode   �21m �33s 
    The path traced by the instantaneous center of a plane figure when it 
undergoes plane motion. If a plane rigid body is constrained to move in its 
own plane but is otherwise free to undergo an arbitrary translational and 
rotational motion, it is found that at any instant there exists a point, 
called the instantaneous center, about which the body is rotating. The path 
that this instantaneous center traces out in space as the motion unfolds is 
called the space centrode. The path that it would trace out in a coordinate 
system which is rigidly attached to the body is called the body centrode. The 
motion may therefore be specified, when the two centrodes are given, by 
allowing one curve to roll without slipping along the other. See FOUR-BAR 
LINKAGE; RIGID-BODY DYNAMICS. 
 �34s[H. C. Casey, Jr. /author unknown]
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�6v�2v�32sCentrohelida   �21m �33s 
    An order of the Heliozoia. There is no central capsule. The usually 
siliceous skeletal elements are scales and spines secreted by nearly all 
species. Pseudopodia may be either axopodia or filopodia. A centroplast is 
typical and the nucleus is consequently eccentric. The order includes 
Acanthocystis, Heterophrys, Actinolophus, and other genera. A few unusual 
types are stalked sessile organisms. See HELIOZOIA. 
 �34s[Richard P. Hall]
�33s 
  
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�6v�2v�32sCentroids (mathematics) �21m �33s
    Points positioned identically with the centers of gravity of corresponding
homogeneous thin plates or thin wires. Centroids are involved in the analysis
of certain problems of mechanics, for example, the phenomenon of bending.
    The centroid of plane area A is point C (see illustration). Coordinates
BARx and BARy of C as referred to the indicated X and Y coordinate axes are

		  BARx = (INTEGRATION x dA)/(INTEGRATION dA)		     (1)

		  BARy = (INTEGRATION y dA)/(INTEGRATION dx)


given by Eq. (1). Similarly, the centroidal coordinates of plane curve L are
given by Eq. (2). In these

		  BARx = (INTEGRATION x dL)/(INTEGRATION dL)

		  BARy = (INTEGRATION y dL)/(INTEGRATION dL)		     (2)

equations BARx and BARy are the coordinate locations of infinitesimal area
element dA and infinitesimal line element dL, respectively.
 �34s[Nelson S. Fisk]
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�6v�2v�32sCentrosome   �21m �33s 
    A poorly staining, optically structureless spherical region which 
surrounds the centriole, and which may vary in radius from one cell type to 
another. While it is generally free of other cytoplasmic constituents or 
organelles, this is not obligatory, and ribosomes, membrane-bounded vesicles, 
or even mitochondria are sometimes found in what is operationally termed the 
centrosome, encroaching on the centriole itself. The lack of a precise 
physical definition of the centrosome within the cell precludes any but the 
most general characterization of it. Apparently it, or cellular constituents 
within it, are involved in the fabrication of the mitotic spindle at prophase. 
See CENTRIOLE; MITOSIS. 
 �34s[Elliott Robbins]
�33s 
  
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�6v�2v�32sCephalobaenida   �21m �33s 
    One of two orders in the class Pentastomida of the phylum Arthropoda. This 
order includes primitive pentastomids with six-legged larvae. The hooks are 
simple, lacking a fulcrum, and disposed in trapeziform pattern, with the 
anterior pair internal to the posterior pair. The mouth is anterior to the 
hooks. There are two families: Raillietiellidae, which contains two genera 
(Raillietiella and Cephalobaena); and Reighardiidae, which contains the single 
genus Reighardia with one species. See PENTASTOMIDA. 
 �34s[Horace W. Stunkard]
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�6v�2v�32sCephalocarida   �21m �33s 
    A group of minute marine crustaceans of great interest to students of 
arthropod evolution because its members seem to possess many primitive 
characters and to show relationships not only to various other groups of 
Crustacea but perhaps also to the trilobites. Their closest apparent relatives 
are primitive members of other subclasses, such as Notostraca and Lipostraca 
(including Devonian Lepidocaris) within the Branchiopoda, and Leptostraca 
within the Malacostraca. Nine species are recognized, placed in four genera. 
They have been found in flocculent surface deposits of mud or silty sand, from 
the intertidal zone down to depths of 4900 ft (1500 m), on the shores of all 
continents except Europe. Population densities up to an average of 16 
individuals/ftsup2 (177 individuals/msup2) have been recorded. See 
BRANCHIOPODA; LEPTOSTRACA; TRILOBITA. 
    The best-known species is Hutchinsoniella macracantha, from the east coast 
of North and South America, about 0.12 - 0.16 in. (3 - 4 mm) long, with a 
shovel-shaped head and a slender, very flexible body which is not covered by a 
carapace. Other species are very similar. Among the features that appear to be 
primitive, most notable is the pronounced serial homology seen in the trunk 
limbs, the trunk musculature, the ventral nerve cord, and the heart. Other 
features in the anatomy of Hutchinsoniella that seem likely to be primitive 
are the uniramous, multisegmented antennules, the biramous antennae with both 
rami multisegmented, the large, flattened pleura on all the limb-bearing 
somites, and the telson freely articulated with the trunk and bearing a caudal 
furca. In the larval development, there is a primitive ``nauplius'' stage, 
followed by stages showing an unusually regular addition of new somites and 
limbs.  
    Hutchinsoniella is a nonselective deposit feeder, subsisting on the 
organic matter present in its habitat. Its feeding mechanism may well be like 
that which was used by trilobites and early crustaceans. The animal cannot 
swim but creeps through the flocculent mud on the bottom, and as it does so, 
lightweight particles of detritus are stirred up by the antennae and by distal 
claws on the trunk-limb endopods. See CRUSTACEA. 
 �34s[Jean H. Langenheim]
�33s 
  
  
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�6v�2v�32sCephalochordata   �21m �33s 
    A subphylum of the phylum Chordata comprising the lancelets, including 
Branchiostoma (amphioxus). They are also known as the Leptocardii. Lancelets 
are small fishlike animals, not exceeding 80 mm in length. They burrow in sand 
on the ocean bed or in estuaries in tropical and temperate regions throughout 
the world. Only two genera are recognized: Branchiostoma (23 species) and 
Asymmetron (6 species). 
    The structure of the lancelet is based on the same fundamental plan as all 
other chordates, but there is neither head nor paired fins. The skeletal rod 
of the back, or notochord, extends the entire length of the body. The animal 
is thus pointed at both ends and lanceolate in form, hence the name lancelet. 
    The adults burrow in rather coarse sand or shell gravel and usually lie 
with the mouth open at the surface of the sand. Large numbers of lancelets 
congregate in small areas for spawning. The egg develops into a larva which at 
first is bottom-living, but later becomes planktonic. Metamorphosis takes 
place after about 11 - 12 weeks of life. The young adult sinks to the bottom 
and swims actively until a suitable sand in which it can burrow and remain 
undisturbed is found. 
    The importance of lancelets lies chiefly in their being one of the most 
primitive chordates. However, lancelets are eaten by the Chinese. See CHORDATA. 
 �34s[J. E. Webb]
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�6v�2v�32sCephalopoda   �21m �33s 
    A group of exclusively marine animals constituting the most advanced class 
of the Mollusca. In Paleozoic and Mesozoic times representatives of the class 
were numerous, far exceeding the present species, and were represented by the 
nautiloids, ammonoids, and belemnoids. Today the class contains only a single 
nautiloid, Nautilus, the octopuses, the squids, and the cuttlefishes. The 
class contains about 150 genera and 650 species of living forms, ranging in 
size from the pygmy squid (Idiosepius pygmaeus), with a total length of .67 
in. (17 mm), to the giant squid (Architeuthis princeps), over 50 ft (15 m) in 
length and the largest known invertebrate. 
    The cephalopods show great variation in form and structure and their 
systematics is incompletely known. Several systems of classification are in 
use. The older classification of Tetrabranchia and Dibranchia is common in 
textbooks; among specialists a later system, with various modifications, is 
more favored. These classificatory systems follow. 
Older system 
Class Cephalopoda 
    Subclass Tetrabranchia 
        Order Nautiloidea 
        Order Ammonoidea 
    Subclass Dibranchia 
        Order Belemnoidea 
        Order Decapoda 
        Suborder Myopsida 
        Suborder Oegopsida 
        Order Vampyromorpha 
        Order Octopoda 
        Suborder Cirromorpha 
        Suborder Incirrata 
Later system 
Subclass Nautiloidea 
Subclass Ammonoidea 
Subclass Coleoidea 
    Order Belemnoidea 
    Order Sepioidea 
    Order Teuthoidea 
        Suborder Myopsida 
        Suborder Oegopsida 
    Order Vampyromorpha 
    Order Octopoda 
        Suborder Cirromorpha 
        Suborder Incirrata 
    The cephalopods differ greatly from the rest of the Mollusca. The 
molluscan organ of locomotion, the foot, has moved forward and surrounds the 
mouth and is subdivided into eight or more appendages which sometimes are 
contractile, and in all but Nautilus are provided with suckers. As a result of 
the forward movement of the foot, the digestive tract is U-shaped and the 
viscera form a hump contained within a fleshy mantle in which the gills are 
also suspended. Fins are found in widely divergent forms, but the main means 
of locomotion is the funnel, found beneath the head. During rapid movement, 
water enters the mantle cavity and is violently ejected through the funnel, 
the animal moving posteriorly from the force of the jet. 
    Cephalopods feed voraciously upon crustaceans and larval and adult fish, 
and cannibalistically upon their own kind. Food is obtained by darting out the 
tentacles or arms and seizing the prey with suckers or, in some squids, with 
the clawlike hooks. 
    Cephalopods are numerous in the sea, particularly in the bathypelagic 
regions, where they occur in vast shoals. They form the most important part of 
the diet of the sperm whales and are the sole food of many of the smaller 
toothed whales, as well as a major item of food for the larger pelagic fishes. 
They are eaten by humans and fished for in many parts of the world. They may 
represent one of the largest nearly untouched food resources in the sea. See 
AMMONOIDEA; DECAPODA (MOLLUSCA); DIBRANCHIA; NAUTILOIDEA; OCTOPODA; OCTOPUS; 
SQUID; TETRABRANCHIA. 
 �34s[Gilbert L. Voss]
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�6v�2v�32sCepheids   �21m �33s 
    A class of brightness-variable stars whose prototype is the star Delta 
Cephei in the constellation Cepheus. While both bluer and redder stars also 
vary in their intrinsic light, the properties of these BETA Cephei, ZZ Ceti, 
RV Tauri, and Mira variables are much less understood than the yellow-color 
Cepheids. These yellow stars are known to be pulsating in radius by as much as 
10% or more. Their light variations are due to their changing surface 
temperature. Larger yellow stars are intrinsically brighter because they have 
more surface area, and they have larger pulsation periods because they have a 
larger radius. See STAR; VARIABLE STAR. 
    The interest in these stars is twofold: If their intrinsic brightnesses 
can be inferred from their pulsation period, the brightnesses can be used as 
indicators of their distance from the Earth. The observed period and a 
calibrated period-luminosity relation is used to give an intrinsic brightness. 
The observed distance-dependent apparent brightness then gives the actual 
distance. The second, and more current, interest in Cepheids is that their 
pulsation properties reveal their masses and internal structure, which help in 
understanding how stars age. Thus, Cepheids and the related classes of yellow 
pulsating stars have been extremely useful in mapping the scale of the 
universe and in probing the details of stellar interiors. See STELLAR 
EVOLUTION.  
 �34s[Arthur N. Cox]
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�6v�2v�32sCeractinomorpha   �21m �33s 
    A subclass of Demospongiae. Among the Ceractinomorpha, the genus 
Halisarca, lacking skeletal elements, is a primitive form. The larva of 
Halisarca is a diploblastula or parenchymella with an outer layer of 
flagellated cells and an inner mass of presumptive ectomesenchymal cells. The 
outer flagellated cells lose their flagella, migrate into the interior, and 
later differentiate into choanocytes. Other cell types characteristic of the 
adult sponge differentiate, and inhalant canals begin to form. 
    In form, ceractinomorph sponges vary from encrustations, thin or massive, 
to lobate and upright branching colonies. The shallow-water species tend to be 
more plastic in form than deep-water species, which usually exhibit little 
intraspecific variation in shape. See DEMOSPONGIAE; PORIFERA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sCeramics   �21m �33s 
    The application of the findings of science and engineering to the 
production of useful products from the nonmetallic, inorganic materials. 
Ceramics are materials which cover a great range in both applications and 
time. In general, ceramics are hard, brittle, electrical and thermal 
insulators, require high-temperature processing, and are formed from powders. 
The major divisions of ceramic technology are similar in processing and in the 
properties of the materials. However, differences in applications and 
differences in the behavior of materials during processing require that 
diverse techniques be used. See CERMET; COMPOSITE MATERIALS; SINTERING. 
    It is convenient to divide ceramic products into two groups: those, such 
as pottery and brick, which are shaped or formed before high-temperature 
treatment, and those, such as glass and cement, which are shaped afterward. 
Only the first group is discussed here. See CEMENT; GLASS AND GLASS PRODUCTS; 
MORTAR; PLASTER. 
    Structural clay is one of the oldest branches of ceramics and includes 
building brick, sewer pipe, and decorative ceramic block for walls. To form 
these products from raw clays, use is made of the plastic forming technique 
known as extrusion. Extrusion is carried out by forcing a stiff plastic mass 
through an opening or die in the form of the desired cross section; the 
continuous ribbon which emerges is cut to the desired lengths. Drying is 
carried out in conditions of controlled humidity and temperature which prevent 
the ware from cracking or warping. The dried material is hard and can be 
broken with hand pressure. After drying, the material is heat-treated to a 
temperature where the clay is broken up into less complicated molecular 
structural units. The resulting material is now held together by chemical 
bonds between glass and oxide compounds, resulting in a hard, brittle material 
which is resistant to corrosion. This process is known as vitrification. See 
CLAY.  
    There are two major divisions of whiteware: art ware and consumer ware 
(tableware, portable lamps, sanitary ware, and so on). To form these products, 
powders of clay, potter's flint, and feldspar are used. The clay, when 
sufficiently wet, imparts plasticity or workability to the body. The forming 
methods used for consumer whiteware production are slip casting and jiggering. 
Jiggering is a mechanization of the forming process of throwing clay by hand 
as done by the potter. For large items such as sanitary ware, artware, and 
portable lamps, slip casting is most often used. Slip casting of clays is done 
by pouring a water suspension of the body (a slip) into a plaster of paris 
mold of the desired shape. The porous plaster mold withdraws water from the 
slip, which results in a buildup of a layer of solid clay next to the mold. 
Once the materials have been formed into the desired shape, drying and firing 
are carried out. See POTTERY. 
    Properties that make ceramic products desirable in electrical applications 
are high resistivity, high dielectric strength, low dielectric loss factor, 
high dielectric constant, and controllable magnetic properties. The ceramic 
products used in the electrical industry include porcelains, glasses, 
steatites, cordierites, titanates, zirconates, carbides, oxides, and ferrites. 
Ceramic products are used in magnets, electronic tubes, condensers, resistors, 
transformers, amplifiers, memory devices, transducers, capacitors, and 
insulators. See FERRITE; PORCELAIN. 
    Ceramics known as refractories are products which thermally insulate the 
furnaces that produce steel, aluminum, and other metals. They also insulate 
the furnaces that produce the steam for the generation of electricity, as well 
as insulate fireplaces in the homes. The manufacture of refractories is one of 
the key industries in the United States. See REFRACTORY. 
 �34s[Garland E. Scott]
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�6v�2v�32sCerargyrite   �21m �33s 
    A mineral with composition AgCl. Its structure is that of the isometric 
NaCl type, but well-formed cubic crystals are rare. The hardness is 2J on Mohs 
scale and specific gravity 5.5. Cerargyrite is colorless to pearl-gray but 
darkens to violet-brown on exposure to light. It is perfectly sectile and can 
be cut with a knife like horn; hence the name horn silver. Bromyrite, AgBr, is 
physically indistinguishable from cerargyrite and the two minerals form a 
complete series. Both minerals are secondary ores of silver and occur in the 
oxidized zone of silver deposits. See SILVER. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sCercaria   �21m �33s 
    The larval generation which terminates the development of a digenetic 
trematode in the intermediate host, a mollusk or rarely an annelid. It becomes 
the adult fluke after entering the definitive host, a vertebrate. Access to 
the vertebrate may be either direct or indirect by entering a second 
intermediate, or vector, host which is ingested by the vertebrate. The 
intermediate host is usually aquatic and the cercaria has a tail used in 
swimming when it escapes from that host, but the tail may be inactive, 
reduced, or absent and the larva unable to swim. When the tail is absent, the 
organism is called a cercariaeum. Larvae with unknown life cycles are commonly 
described and named as species such as Cercaria micrura or Cercariaeum helicis. 
    The cercaria is important in taxonomy because that stage of closely 
related trematodes usually is less variable than the adult, and thus indicates 
kinships that may be masked in the adult by adaptations to parasitism of the 
vertebrate host. See DIGENEA; TREMATODA. 
 �34s[Raymond M. Cable]
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�6v�2v�32sCereal   �21m �33s 
    Any member of the grass family (Gramineae) which produced edible grains 
usable as food by humans and livestock. Common cereals are rice, wheat, 
barley, oats, maize (corn), sorghum, rye, and certain millets, with corn, 
rice, and wheat being the most important. Developed by scientists, triticale 
is a new cereal derived from crossing wheat and rye and then doubling the 
number of chromosomes in the hybrid. Occasionally, grains from other grasses 
(for example, teff) are used for food. Cereals provide more food for human 
consumption than any other crops. 
    Four general groups of foods are prepared from the cereal grains. (1) 
Baked products, made from flour or meal, include breads, pastries, pancakes, 
cookies, and cakes. (2) Milled grain products, made by removing the bran and 
usually the germ (or embryo of the seed), include polished rice, farina, wheat 
flour, cornmeal, hominy, corn grits, pearled barley, semolina (for macaroni 
products), prepared breakfast cereals, and soup, gravy, and other thickenings. 
(3) Beverages such as beer and whiskey, made from fermented grain products 
(distilled or undistilled) and from boiled, roasted grains. (4) Whole-grain 
products include rolled oats, brown rice, popcorn, shredded and puffed grains, 
and breakfast foods. 
    All cereal grains have high energy value, mainly from the starch fraction 
but also from the fat and protein. In general, the cereals are low in protein 
content, although oats and certain millets are exceptions. See GRAIN CROPS; 
GRASS CROPS. 
 �34s[Louis P. Reitz]
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�6v�2v�32sCerebral palsy �21m �33s
    In the broadest sense, any nonprogressive motor disorder caused by brain
damage incurred during development. Although in common use by both laypeople
and physicians, the term cerebral palsy is nonspecific and includes many kinds
of brain damage from various causes. In fact, few authorities agree on the
disorders to be included.
    Nonprogressive motor disorders, such as spasticity, paralysis,
convulsions, muscular spasms, abnormal reflexes, and delayed or abnormal
development of speech, movement, and behavior patterns, are characteristic.
Intelligence is not noticeably altered in over two-thirds of the cases.
    Possible prenatal causes include certain hereditary and congenital
defects, as well as some maternal diseases, such as German measles (rubella)
in early pregnancy. Toxemias of pregnancy, abnormal or difficult labor, and
insufficient oxygen before, during, or after birth are precipitating causes of
brain damage. See PREGNANCY DISORDERS; RUBELLA.
    Birth injuries are a major cause of cerebral palsy. The infant head is
subjected to tremendous stresses during labor, especially in a first
pregnancy, which may cause anoxia, edema, hemorrhage, rupture of brain tissue,
or other mechanical effects which, in turn, cause degeneration or death of
nervous tissue. No regeneration of such tissue takes place and even if nervous
tissue damage is slight, there is still the chance that a scar may form and
cause later difficulty. Skillful obstetrical techniques have greatly reduced
the incidence and severity of damage in many forms of cerebral palsy.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sCerenkov radiation �21m �33s
    Light emitted by a high-speed charged particle when the particle passes
through a transparent, nonconducting, solid material at a speed greater than
the speed of light in the material. The blue glow observed in the water of a
nuclear reactor, close to the active fuel elements, is radiation of this kind.
The emission of Cerenkov radiation is analogous to the emission of a shock
wave by a projectile moving faster than sound, since in both cases the
velocity of the object passing through the medium exceeds the velocity of the
resulting wave disturbance in the medium.
    Particle detectors which utilize Cerenkov radiation are called Cerenkov
counters. They are important in the detection of particles with speeds
approaching that of light, such as those produced in large accelerators and in
cosmic rays, and are used with photomultiplier tubes to amplify the Cerenkov
radiation. These counters can emit pulses with widths of SIMILAR  10- sup 1 sup
0 second and are therefore useful in time-of-flight measurements when very short
times must be measured. They can also give direct information on the velocity
of the passing particle. See PARTICLE DETECTOR.
 �34s[William B. Fretter]
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�6v�2v�32sCeres �21m �33s
    The first asteroid discovered. It was found serendipitously by G. Piazzi
on January 1, 1801. With a diameter of approximately 600 mi (1000 km), Ceres
is the largest asteroid but not the brightest since it reflects only 6% of the
visual light it receives. Ceres's mass of 1.17 x 10 sup 2 sup 1 kg (that is, 2 x
10- sup 4 that of the Earth) contains approximately 40% of the asteroid belt's
total mass. See ASTEROID.
 �34s[Edward F. Tedesco]
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�6v�2v�32sCeriantharia   �21m �33s 
    An order of the Zoantharia, typified by Cerianthus, which lives in sandy 
marine substrata (illustration a and b). The animal is enclosed in a sheath 
formed by mucus secreted from gland cells of the column ectoderm, in which 
discharged nematocysts, sand grains, and other foreign objects are embedded. 
    The polyp is a muscular, skeletonless, elongated, cylindrical body with a 
smooth wall. Long, slender, unbranching, freely retractile tentacles are 
arranged in two cycles and consist of smaller labial and larger marginal ones. 
See ZOANTHARIA. 
 �34s[Kenji Atoda]
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�6v�2v�32sCerium �21m �33s
    A chemical element, Ce, atomic number 58, atomic weight 140.12. It is the
most abundant metallic element of the rare-earth group in the periodic table.
The naturally occurring element is made up of the isotopes sup 1 sup 3 sup 6Ce,
sup 1 sup 3 sup 8Ce, sup 1 sup 4 sup 0Ce, and sup 1 sup 4 sup 2Ce. A radioactive
ALPHA-emitter, Ce sup 1 sup 4 sup 2 has a half-life of 5 x 10 sup 1 sup 5 years.
Cerium occurs mixed with other rare earths in many minerals, particularly
onazite and blastnasite, and is found among the products of the fission of
uranium, thorium, and plutonium.
    Although the common valence of cerium is 3, it also forms a series of
quadrivalent compounds and is the only rare earth which occurs as a
quadrivalent ion in aqueous solution. Although it can be separated from the
other rare earths in high purity by ion-exchange methods, it is usually
separated chemically by taking advantage of its quadrivalent state. See
RARE-EARTH ELEMENTS.
 �34s[Frank H. Spedding]
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�6v�2v�32sCermet   �21m �33s 
    A group of composite materials consisting of an intimate mixture of 
ceramic and metallic components. Cermets can be fabricated by mixing the 
finely divided components in the form of powders or fibers, compacting the 
components under pressure, and sintering the compact to produce physical 
properties not found solely in either of the components. Cermets can also be 
fabricated by internal oxidation of dilute solutions of a base metal and a 
more noble metal. When heated under oxidizing conditions, the oxygen diffuses 
into the alloy to form a base metal oxide in a matrix of the more noble metal. 
See COMPOSITE MATERIAL; CORROSION; POWDER METALLURGY; SINTERING. 
    The combination of metallic and ceramic components can result in cermets 
characterized by increased strength and hardness, higher temperature 
resistance, improved wear resistance, and better resistance to corrosion, each 
characteristic depending on the variables involved in composition and 
processing. Friction parts as well as cutting and drilling tools have been 
successfully made from cermets for many years. Certain nuclear reactor fuel 
elements, such as dispersion-type elements, are also made as cermets. See 
CERAMICS.  
 �34s[Henry H. Hausner]
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�6v�2v�32sCerussite   �21m �33s 
    The mineral form of lead carbonate, PbCO3. Cerussite is common as a 
secondary mineral associated with lead ores. In the United States it occurs 
mostly in the central and far western regions. Cerussite is white when pure 
but is sometimes darkened by impurities. Hardness is 3H on Mohs scale and 
specific gravity is 6.5. Crystals may be tabular, elongated, or arranged in 
clusters. See CARBONATE MINERALS. 
 �34s[Robert I. Harker]
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�6v�2v�32sCesium   �21m �33s 
    A chemical element, Cs, with an atomic number of 55 and an atomic weight 
of 132.905, the heaviest of the alkali metals in group la of the periodic 
table (except for francium, the radioactive member of the alkali metal 
family). Cesium is a soft, light, very low-melting metal. It is the most 
reactive of the alkali metals and indeed is the most electropositive and the 
most reactive of all the elements. 
    Cesium reacts vigorously with oxygen to form a mixture of oxides. In moist 
air, the heat of oxidation may be sufficient to melt and ignite the metal. 
Cesium does not appear to react with nitrogen to form a nitride, but does 
react with hydrogen at high temperatures to form a fairly stable hydride. 
Cesium reacts violently with water and even with ice at temperatures as low as 
-116DGC (-177DGF). Cesium reacts with the halogens, ammonia, and carbon 
monoxide. In general, cesium undergoes some of the same type of reactions with 
organic compounds as do the other alkali metals, but it is much more reactive. 
See SODIUM. 
    The physical properties of cesium metal are summarized in the table. 
    Cesium is not very abundant in the Earth's crust, there being only 7 parts 
per million (ppm) present. Like lithium and rubidium, cesium is found as a 
constituent of complex minerals and not in relatively pure halide form as are 
sodium and potassium. Indeed, lithium, rubidium, and cesium frequently occur 
together in lepidolite ores, such as those from Rhodesia. 
    Cesium metal is used in photoelectric cells, spectrographic instruments, 
scintillation counters, radio tubes, military infrared signaling lamps, and 
various optical and detecting devices. Cesium compounds are used in glass and 
ceramic production, as absorbents in carbon dioxide purification plants, as 
components of getters in radio tubes, and in microchemistry. Cesium salts have 
been used medicinally as antishock agents after administration of arsenic 
drugs. The isotope cesium-137 is supplanting cobalt-60 in the treatment of 
cancer. See ALKALI METALS. 
 �34s[Marshall Sittig]
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�6v�2v�32sCestida   �21m �33s 
    An order of ribbon-shaped ctenophores having a very short tentacular axis 
and an elongated pharyngeal axis. The subtentacular ribs consist of a few 
plates, while the subpharyngeal extend over the whole aboral margin of the 
body. The primary tentacles have been replaced by secondary tentacles. Cestum 
veneris is a common example. See CTENOPHORA; TENTACULATA. 
 �34s[Taku Komai]
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�6v�2v�32sCestoda   �21m �33s 
    A subclass of tapeworms including most of the members of the members of 
the class Cestoidea. All species are endoparasites of vertebrates, living in 
the intestine or related ducts. 
    Like other members of the class, the cestodes have no digestive tract or 
mouth. Nutrition presumably occurs by absorption of food through the body 
surface. The body is usually very elongated and tapelike and frequently 
divided into segments, or proglottids, with replication of the hermaphroditic 
reproductive systems. In a few species there is duplication of both male and 
female organs within a single segment. The anterior end is usually modified 
into a holdfast organ, the scolex. Since a digestive tract is completely 
absent, the scolex is of solid construction, typically highly muscular with 
sucking depressions and hooks (see illustration). 
  
    The worms require carbohydrate for growth and reproduction, and this 
requirement is satisfied only from the host ingesta. On the other hand, 
nitrogenous nutrients and many micronutrients may be obtained from the body 
stores of the host; deleting such materials from the host's diet has no 
appreciable effect on the worms. 
    Most authorities agree that the tapeworms are ancient parasites, probably 
evolving as parasites of the earliest fishes. It seems probable that the 
tapeworms did not evolve from trematodes or other present-day groups of 
parasitic flatworms. Their ancestry may be directly derived from the acoele or 
rhabdocoele turbellarians and represents a line of evolution which is 
completely independent of other parasitic flatworms. These relationships 
remain obscure in the absence of any fossil record. See CESTOIDEA; TURBELLARIA. 
 �34s[Clark P. Read]
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�6v�2v�32sCestodaria   �21m �33s 
    A subclass of worms belonging to the class Cestoidea. Only a few species 
are known. All are endoparasites of primitive fishes. The subclass is usually 
divided into two orders, Amphilinidea and Gyrocotylidea. These worms differ 
from the other Cestoidea in being unsegmented, in not having the anterior end 
modified as a holdfast organ, and in frequently occurring as parasites of the 
coelomic cavity rather than the digestive tract. Some species have the 
posterior end modified into a holdfast organ. The animals are hermaphroditic 
and sexual reproduction occurs in the vertebrate host. See AMPHILINIDEA; 
CESTOIDEA; GYROCOTYLIDEA. 
 �34s[Clark P. Read]
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�6v�2v�32sCestoidea   �21m �33s 
    A class of the phylum Platyhelminthes commonly referred to as tapeworms. 
All members are endoparasites, usually in the digestive tract of vertebrates. 
The class has been subdivided as follows: 
Class Cestoidea 
    Subclass Cestodaria 
        Order: Amphilinidea 
               Gyrocotylidea 
    Subclass Cestoda 
        Order: Proteocephaloidea 
               Tetraphyllidea 
               Lecanicephaloidea 
               Trypanorhyncha 
               Diphyllidea 
               Pseudophyllidea 
               Cyclophyllidea 
               Nippotaeniidea 
Separate articles appear on each group listed in the classification. 
    In size the tapeworms range from less than 0.04 in. (1.0 mm) to several 
meters in length. The class is differentially characterized by the presence of 
a cuticle rather than a cellular epidermis and by the total absence of a mouth 
and digestive tract. In most species of the class the body is divided into 
proglottids, each proglottid containing one or two hermaphroditic reproductive 
systems. The anterior end is usually modified into a holdfast organ, bearing 
suckers or sucking grooves, and frequently armed with hooks. Early embryonic 
development occurs in the parental body, usually in a uterus, to a 
hook-bearing stage, the oncosphere. The oncosphere leaves the parental body 
through a uterine pore or by liberation of the terminal segment from the main 
body of the worm. Further development of the worm always occurs within the 
body of a host, most often an invertebrate, which commonly ingests the larval 
form.  
    Food materials are presumed to be absorbed through the external surface. 
It has been shown that tapeworms have catalytic mechanisms for the active 
transport into the body of simple sugars and amino acids. The chemical 
composition of these worms is unique in that polysaccharide and fat represent 
a much larger proportion of the dry weight than does protein. See 
PLATYHELMINTHES.  
 �34s[Clark P. Read]
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�6v�2v�32sCetacea   �21m �33s 
    An order of aquatic mammals including about 80 living species, of which 
the larger ones are called whales and the smaller ones dolphins or porpoises. 
Cetaceans live in all oceans and in several fresh-water systems. They are 
remarkably adapted to aquatic life and die within a few hours or days on land. 
All have a smooth, streamlined body; the forelimbs are flattened and rigid 
from shoulder to tip, without external fingers; the hindlimbs are absent, 
though represented by vestiges in the embryo or adult; the tail is flattened 
horizontally. The external ears are minute pits. The skin is hairless, though 
some fetal cetaceans have hairs on the snout and some adults have sensory 
bristles along the body. All cetaceans have a thick layer of blubber 
(subcutaneous fat) which serves as heat insulation and as food reserve. 
    So far as known, all cetaceans copulate belly to belly and quickly, near 
the surface of the sea. The gestation period is 9 - 16 months. A single, large 
precocious calf is born tail first, and it rises immediately to the surface 
for air. The nursing period may extend for 2 years (sperm whale). Cetacean 
milk is rich, being up to 50% fat, and low in sugars. Cetaceans are generally 
long-lived.  
    Specialization of cetaceans for sensing their environment has led to 
extinction of smell, reduction of vision, and extreme development of hearing. 
Vibrations serve for communication and echolocation (sonar). While the 
intelligence of cetaceans is debatable, by most standards (gross weight of 
brain, weight of brain in relation to cord, ready response to training, social 
interaction, and playfulness), the species which have been studied in 
captivity rate nearer the primates than do the dog, cat, rat, and horse. The 
food of cetaceans is exclusively flesh. 
    The ancestors of the cetaceans are completely unknown. The earliest 
fossils are from middle Eocene beds and are clearly whales. Separate lines of 
evolution have given rise to three suborders: Archaeoceti, Mysticeti, and 
Odontoceti. See DOLPHIN; MAMMALIA; PORPOISE; WHALE. 
 �34s[Victor B. Scheffer]
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�6v�2v�32sCetane number   �21m �33s 
    A number that indicates the ability of a diesel engine fuel to ignite 
quickly after being injected into the cylinder. In high-speed diesel engines, 
a fuel with a long ignition delay tends to produce rough operation. See 
COMBUSTION CHAMBER; DIESEL FUEL. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sCetomimiformes   �21m �33s 
    An order of oceanic, mostly deep-water fishes that are structurally 
diverse and rare; most of the 41 species are known from one or a few 
specimens. Thus, their scientific study has been hindered, the anatomy is 
imperfectly known, and the relationships are in dispute. Five of the 10 
families and 11 of the 21 genera currently placed in the order have been 
described since World War II. There is no fossil record. 
    Cetomimiforms are soft-rayed fishes. In most the mouth is large. Many are 
naked but a few have scales that are thin and deciduous or form an irregular 
mosaic; a few have the skin spinulose. Pelvic fins may be abdominal, thoracic, 
or jugular in position or, commonly, absent. In most forms the single dorsal 
and anal fins are placed rather well back and are opposed; an adipose fin is 
present in only one species. 
    Because of the diversity, the five currently recognized suborders may be 
mentioned separately. Best known are the Cetomimoidei (or Cetunculi) 
whalefishes, a group of 3 families and 15 rare species of small, red or black 
deep-sea fishes with whale-shaped bodies and enormous mouths; they are 
bioluminescent. The Ateleopoidei (or Chondrobrachii) consist of 1 family, 3 
genera, and 11 species of elongate fishes in which the long anal fin is 
continuous with the caudal and there is no dorsal fin. The Mirapinnatoidei (or 
Miripinnati) are tiny, perhaps larval fishes, all recently described. Three 
families, 4 genera, and 5 species are included. The Giganturoidei, with 2 
families, 3 genera, and 6 species, are small mesopelagic fishes with large 
mouths and strong teeth; some have telescopic eyes. The Megalomycteroidei, or 
mosaic-scaled fishes, consist of 1 family, 4 genera, and 4 rare species of 
small, elongate deep-sea fishes with degenerate eyes and irregularly disposed 
scales. See ACTINOPTERYGII; OSTEICHTHYES; TELEOSTEI. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sCetyl alcohol   �21m �33s 
    A straight-chain primary aliphatic alcohol of 16 carbon atoms found free 
in feces, with the formula CH3(CH2)14CH2OH. Its ester with palmitic acid 
comprises 90% of spermaceti, the wax from marine animals. It is also present 
in esterified form in wool fat. See ALCOHOL; WAX, ANIMAL AND VEGETABLE. 
 �34s[Herbert E. Carter ;Roy H. Gig]
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�6v�2v�32sChabazite �21m �33s
    A mineral belonging to the zeolite family of silicates. Hardness is 4-5
on Mohs scale; specific gravity is 2.05-2.15. The luster is vitreous and the
color usually white but may be yellow, red, or pink. See ZEOLITE.
    Chabazite is essentially a hydrous calcium sodium aluminum silicate,
(CaNa2)[Al2Si4O12] CTR DOT 6H20. The ratio of calcium to sodium may vary, and
potassium is usually present in small amounts. Chabazite has been the most
used natural zeolite for cation exchange and gas sorption, but synthetic
zeolites are now widely employed.
    Chabazite is a secondary mineral usually found lining cavities in basalt
and related rocks. Notable localities are the Giant's Causeway, Ireland; Faeroe
Islands; Trentino, Italy; and Oberstein, Germany. In the United States it is
found at West Paterson and Bergen Hill, New Jersey; and Table Mountain, Oregon.
See SILICATE MINERALS.					      [C.Fr.; C.S.Hu.]
 �34s[Clifford Frondel ;Cornelius S. Hurlbut]
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�6v�2v�32sChaetognatha   �21m �33s 
    A phylum of abundant planktonic arrowworms. Their bodies are tubular and 
transparent, and divided into three portions: head, trunk, and tail. The head 
possesses one or two rows of minute teeth anterior to the mouth and usually 7 
- 10 larger chaetae, or seizing jaws, on each side of the head. One or two 
pairs of lateral fins and a caudal fin are present. 
    Nine genera and about 42 species are recognized by some specialists. Most 
species belong to the genus Sagitta, which can be recognized by the presence 
of two pairs of teeth and two pairs of lateral fins. 
    Chaetognaths are cosmopolitan forms which live not only at the surface but 
also at great depths; however, no one species is found in all latitudes and at 
all depths. One of the Arctic species, Eukrohnia hamata, may extend to the 
Antarctic by way of deep water across the tropics. A few species are neritic 
and are not found normally beyond the continental shelf. Their food consists 
principally of copepods and other small planktonic crustaceans; however, they 
are very predacious and will even eat small fish larvae and other chaetognaths 
on rare occasions. 
    Studies have shown them to be useful as indicator organisms. Certain 
species appear to be associated with characteristic types or masses of water, 
and when this water is displaced into an adjacent water mass, the chaetognaths 
may be used as temporary evidence for such displacement. 
 �34s[E. Lowe Pierce]
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�6v�2v�32sChaetonotoidea   �21m �33s 
    An order of the class Gastrotricha. These animals usually have two 
adhesive tubes connected with the distinctive paired, posterior ``tail forks'' 
or ``toes.'' Paired protonephridia are present. Parthenogenetic females 
deposit cuticular-covered, smooth or spinous eggs, and direct development 
occurs. Chaetonotus, Ichthydium, Lepidodermella, and Polymerurus are 
well-known genera (see illustration), with over 200 species, only about 20 of 
which have been reported from the United States. See GASTROTRICHA.  
 �34s[Charles E. Packard]
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�6v�2v�32sChagas' disease   �21m �33s 
    An acute and chronic disease, also known as South American 
trypanosomiasis, caused by the pathogenic hemoflagellate Trypanosoma 
(Schizotrypanum) cruzi and transmitted to humans by bloodsucking triatoma 
bugs. The infection originates in wild animals (armadillo and opossum) as a 
zoonosis.  
    The acute form of Chagas' disease occurs predominantly in young children 
and is characterized by fever, lymph node swelling, spleen and liver 
enlargement, and facial edema. Involvement of the brain occurs often, and 
convulsions may sometimes end in grave permanent mental damage or even in 
death. Although the chronic form may be mild, myocarditis due to parasite 
development in the heart muscles and irreversible tissue damage have not 
infrequently been described, as well as megalocolon complication. 
    There is no generally effective treatment. Preventive measures should be 
applied against the vectors of the disease. See TRYPANOSOMATIDAE. 
 �34s[Meir Yoeli]
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�6v�2v�32sChain drive   �21m �33s 
    A flexible device of connected links used to transmit power. A drive 
consists of an endless chain which meshes with sprockets located on the shaft 
of a driving source, such as an electric motor/reducer and a driven source, 
such as the head shaft of a belt conveyor. 
    The roller chain (see illustration) meets the demands of heavy-duty oil 
well drilling equipment, high-production agricultural machinery, construction 
machinery, and similar equipment. It also meets the precise timing 
requirements of lighter-duty equipment such as printing, packaging, and 
vending equipment. 
    Another type of chain, the engineering steel chain, is usually identified 
by the offset/cranked link sidebar design. Generally, larger pitch sizes as 
compared to the roller chain and higher-strength chains characterize such 
chains. A third group used for chain drives is the inverted tooth/silent 
chains. A familiar application is their use as automotive timing chains in 
automobile engines. 
    The use of chains for power transmission rather than another device, such 
as V-belts or a direct coupling to the power source, is usually based on the 
cost effectiveness and economy of chains and sprockets. Chains and sprockets 
offer the following advantages: large speed ratios; sufficient elasticity to 
absorb reasonable shocks; a constant speed ratio between the driving and 
driven shaft; long life without excessive maintenance; mechanical 
understandability regarding installation and functionality; coupling and 
uncoupling with simple tools; and a simple means to get power from its source 
to the location where needed. See BELT DRIVE. 
 �34s[Victor D. Petershack]
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�6v�2v�32sChain reaction (chemistry) �21m �33s
    A chemical reaction in which many molecules undergo chemical reaction
after one molecule becomes activated. In ordinary chemical reactions, every
molecule that reacts must first become activated by collision with other
rapidly moving molecules. The number of these violent collisions per second is
so small that the reaction is slow. After a chain reaction is started, it is
not necessary to wait for more collisions with activated molecules to
accelerate the reaction because the reaction now proceeds spontaneously.
    A typical chain reaction is the photochemical reaction between hydrogen
and chlorine as described by the following reactions.

		  CI sub 2 + light -> Cl + Cl

		     Cl + H sub 2 -> HCl + H

		     H + Cl sub 2 -> HCl + H

		     Cl + H sub 2 -> HCl + H

    The light absorbed by a chlorine molecule dissociates the molecule into
chlorine atoms; these in turn react rapidly with hydrogen molecules to give
hydrogen chloride and hydrogen atoms. The hydrogen atoms react with chlorine
molecules to give hydrogen chloride and chlorine atoms. The chlorine atoms
react further with hydrogen and continue the chain until some other reaction
uses up the free atoms of chlorine or hydrogen. The chain-stopping reaction
may be the reaction between two chlorine atoms to give chlorine molecules, or
between two hydrogen atoms to give hydrogen molecules. Again the atoms may
collide with the walls of the containing vessel, or they may react with some
impurity which is present in the vessel only as a trace.
    Certain oxidations in the gas phase are known to be chain reactions. The
carbon knock which occurs at times in internal combustion engines is caused by
a too-rapid combustion rate caused by chain reactions. This chain reaction is
reduced by adding tetraethyllead, which acts as an inhibitor.
    The polymerization of styrene to give polystyrene and the polymerization
of other organic materials to give industrial plastics involve chain
reactions. The spoilage of foods, the precipitation of insoluble gums in
gasoline, and the deterioration of certain plastics in sunlight involve chain
reactions, which can be minimized with inhibitors. See CHEMICAL DYNAMICS;
PHOTOCHEMISTRY.
 �34s[Farrington Daniels]
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�6v�2v�32sChain reaction (physics) �21m �33s
    A succession of generation after generation of acts of division (called
fission) of certain heavy nuclei. The fission process releases about 200 MeV
(3.2 x 10- sup 4 erg = 3.2 x 10- sup 1 sup 1 joule) in the form of energetic
particles including two or three neutrons. Some of the neutrons from one
generation are captured by fissile species (sup 2 sup 3 sup 3U, sup 2 sup 3 sup
5U, sup 2 sup 3 sup 9Pu) to cause the fissions of the next generations. The
process is employed in nuclear reactors and nuclear explosive devices. See
NUCLEAR FISSION.
 �34s[Norman C. Rasmussen]
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�6v�2v�32sChalcanthite   �21m �33s 
    A mineral with the chemical composition CuSO4PRODUCT 5H2O. Chalcanthite 
commonly occurs in blue to greenish-blue triclinic crystals or in massive 
fibrous veins or stalactites. Fracture is conchoidal and luster is vitreous. 
Hardness is 2.5 on Mohs scale and specific gravity is 2.28. It has a 
nauseating taste and is readily soluble in water. It dehydrates in dry air to 
a greenish-white powder. Although deposits of commercial size occur in arid 
areas, chalcanthite is generally not an important source of copper ore. Its 
occurrence is widespread in the western United States. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sChalcedony �21m �33s
    A fine-grained fibrous variety of quartz, silicon dioxide. The individual
fibers that compose the mineral aggregate usually are visible only under the
microscope. Subvarieties of chalcedony recognized on the basis of color
differences, some valued since ancient times as semiprecious gem materials,
include carnelian (translucent, deep flesh red to clear red in color), sard
(orange-brown to reddish-brown), and chrysoprase (apple green). See GEM;
QUARTZ.
    Chalcedony occurs as crusts with a rounded, mammillary, or botryoidal
surface and as a major constituent of nodular and bedded cherts. The hardness
is 6.5-7 on Mohs scale. The specific gravity is 2.57-2.64.
    Crusts of chalcedony generally are composed of fairly distinct layers
concentric to the surface. Agate is a common and important type of chalcedony
in which successive layers differ markedly in color and degree of translucency.
In the most common kind of agate the layers are curved and concentric to the
shape of the cavity in which the material formed. See AGATE.
 �34s[Raymond Siever]
�33s

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�6v�2v�32sChalcocite �21m �33s
    A mineral having composition Cu2S and crystallizing in the orthorhombic
system (below 217DGF or 103DGC). Crystals are rare and small, usually with
hexagonal outline because of twinning. Most commonly, the mineral is
fine-grained and massive with a metallic luster and a lead-gray color which
tarnishes to dull black on exposure. The Mohs hardness is 2.5-3, and the
density 5.5-5.8. Chalcocite is an important copper ore found at Miami,
Morenci, and Bisbee, Arizona, Butte, Montana, Kennecott, Alaska, and Tsumeb,
South-West Africa.
 �34s[Lawrence Grossman]
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�6v�2v�32sChalcogenide glasses   �21m �33s 
    Amorphous cross-linked inorganic polymers. Probably the best known is 
arsenic sulfide, (As2S3)subn, which can be used for infrared transparent 
windows. Threshold and memory switching are also interesting properties of 
these glasses. Ultraphosphate glasses resemble glassy organic plastics and can 
be processed by the same methods, such as extrusion and injection molding. 
They are used for antifouling surfaces for marine applications and in the 
manufacture of nonmisting spectacle lenses.  See  AMORPHOUS SOLIDS; INORGANIC 
POLYMER.  
 �34s[Robert A. Shaw]
�33s 
  
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�6v�2v�32sChalcopyrite �21m �33s
    A mineral having composition CuFeS2.Crystals are usually small and
resemble tetrahedra. Chalcopyrite is usually massive with a metallic luster,
brass-yellow color, and sometimes an iridescent tarnish. The Mohs hardness is
3.5-4.0, and the density 4.1-4.3. Chalcopyrite is a so-called fool's gold, but
is brittle while gold is sectile. Pyrite, the most widespread fool's gold, is
harder than chalcopyrite. See PYRITE.
    Chalcopyrite is the most widespread primary copper ore mineral. It is
commonly found in veins (Braden mine, Chile; Cornwall, England; Butte, Montana;
Freiberg, Saxony; Tasmania; Rio Tinto, Spain). Chalcopyrite is also found in
contact metamorphic deposits in limestone (Bisbee, Arizona) and as sedimentary
deposits (Mansfield, Germany). See COPPER.
[Lawrence Grossman]

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�6v�2v�32sChalk   �21m �33s 
    A variety of limestone formed from pelagic, or floating, organisms that is 
very fine-grained, porous, and friable. It is white or very light-colored and 
consists almost entirely of calcite. The rock is made up of the calcite shells 
of microorganisms partially cemented by structureless calcite. The most 
conspicuous are the Foraminiferida, Globigerina and Textularia. See LIMESTONE. 
    The best-known chalks are those of the Cretaceous, exposed in cliffs on 
both sides of the English Channel. The Selma (Cretaceous) chalk of Alabama, 
Mississippi, and Tennessee and the Niobrara chalk of the same age in Nebraska 
are well-known deposits in the United States. Chalk is used for cements, 
powders (as soft abrasives and polishers), crayons, and fertilizers. 
 �34s[Raymond Siever]
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�6v�2v�32sChameleon   �21m �33s 
    The name for about 80 species of small-to-medium-sized lizards that make 
up the family Chamaeleontidae and occur mainly in Africa and Madagascar. The 
American chameleons (Anolis) belong to a different family of lizards, the 
Iguanidae. See IGUANIDS. 
    Chamaeleo chamaeleon is the most common species and is a typical example 
of the group. Its body is flattened from side to side; it has a long, 
prehensile tail; and both the forelimbs and hindlimbs have two digits that 
oppose the other three. These feet and the tail make the chameleon well 
adapted for its arboreal habitat. The eyes are large and can move 
independently of each other in all directions. The tongue is also prehensile, 
being extensible for a great distance, about the length of the animal itself, 
and is a highly efficient organ for capturing insects. The head is triangular 
in profile and has a pointed crest. Chameleons are noted for their ability to 
change color. Color changes appear to be related to environmental temperatures 
as well as other external stimuli. 
    The chameleon is oviparous. The female digs out a hollow in the ground for 
a nest where several dozen eggs are laid and then covered with soil. The 
period of incubation varies inversely with the temperature and may be as short 
as 4 months or as long as 10 months. Parental care of the young has not been 
observed. See CHROMATOPHORE; LIZARD; REPTILIA; SQUAMATA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sChanneling in solids   �21m �33s 
    The steering of energetic charged particles by the atomic rows or atomic 
planes of a crystalline solid. Solid materials are generally made up of small 
crystals, and in some cases, materials can be found or made wherein the entire 
specimen is one single crystal. Within a crystal, the atoms are arranged in a 
lattice, and various lattice configurations have been observed and classified. 
In the lattice, the atoms are arranged in orderly rows and columns so that, 
viewed in some directions, the lattice appears dense, and viewed in other 
directions, called channels, the lattice has a more transparent appearance. 
There are planar channels as well as axial channels. Planar channels are the 
spaces between sheets of atoms. When the atoms in each sheet are along the 
line of sight, an axial channel will appear. See CRYSTAL STRUCTURE; 
CRYSTALLOGRAPHY.  
    The phenomenon called channeling occurs when swift particles such as 
ALPHA-particles, protons, electrons, or heavy ions from an accelerator move in 
a crystal along one of the channel directions. When this occurs, the particle 
tends to be gently deflected away from the atoms by a series of 
very-small-angle scatterings; in this way the particle tends to stay in the 
open spaces of the crystal as it passes through. Such a channeled particle 
will lose less energy than it would if it moved along some nonchannel path. 
Since channeled particles have very few close encounters with the atoms, 
nuclear reactions between particle and atom are almost completely suppressed; 
the same statement applies to excitation and ionization of inner-shell 
electrons on the atom, since these processes require close collisions as well. 
    Channeling has been used as a tool to investigate the properties of 
solids. For example, when small amounts of a foreign substance exist in a 
crystal (as in the ion-implantation doping of semiconductor devices), 
channeling experiments can be used to determine which locations within the 
crystal are occupied by these added atoms. Another example pertains to the 
transverse oscillations a channeled particle makes as it passes through a 
crystal. In this case the exact details of the trajectories of channeled 
particles provide information about the electrostatic forces acting upon the 
particle, and these forces relate to the details of the charges distributed 
through the crystal. See CRYSTAL DEFECTS; ION IMPLANTATION. 
 �34s[C. D. Moak]
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�6v�2v�32sChaparral �21m �33s
    A vegetation formation characterized by woody plants of low stature (3.3 -
10 ft or 1-3 m tall), impenetrable because of tough, rigid, interlacing
branches, with small, simple, waxy, evergreen, thick leaves. The term refers to
evergreen oak, Spanish chapparo, and therefore is uniquely southwestern North
American. This type of vegetation has its center in California and occurs
continuously over wide areas of mountainous to sloping topography. The Old
World Mediterranean equivalent is called maquis or macchie, with nomenclatural
and ecological variants in the countries from Spain to the Balkans.
Physiognomically similar vegetation occurs also in South Africa, Chile, and
southwestern Australia in areas of Mediterranean climates, that is, with very
warm, dry summers and maximum precipitation during the cool season. The floras
of these five areas with Mediterranean climates are altogether different. See
MAQUIS.
    The characteristic species of the true chaparral of California include
Adenostema fasciculatum, Ceanothus cuneatus, Quercus dumosa, Heteromeles
arbutifolia, Rhamnus californica, R. crocea, and Cercocarpus betuloides, plus a
host of endemic species of Arctostaphylos and Ceanothus and other Californian
endemics, both shrubby and herbaceous. These plants determine the formation's
physiognomy. It is a dense, uniform-appearing, evergreen, shrubby cover with
sclerophyllous leaves and deep-penetrating roots.
    Ecologically, chaparral occurs in a climate which is hot and dry in summer,
cool but not much below freezing in winter, with little or no snow, and with
excessive winter precipitation that leaches the soil of nutrients. The need for
water and its supply are exactly out of phase.
    Chaparral soils are generally rocky, often shallow, or of extreme chemistry
such as those derived from serpentine, and are always low in fertility. In the
very precipitous southern Californian mountains, soil erosion rates may be
0.039 in. (1mm) per year over large watershed areas.
 �34s[Jack Major]
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�6v�2v�32sCharacteristic curve   �21m �33s 
    A curve that shows the relationship between two changing values. A typical 
characteristic curve is that which shows how the changes in the control-grid 
voltage of an electron tube affect the anode current. When three variables are 
involved, a family of characteristic curves is frequently drawn, with each 
curve representing one value of the third variable. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sCharacter recognition   �21m �33s 
    The technology of using machines to automatically identify human-readable 
symbols, most often alphanumeric characters, and then to express their 
identities in machine-readable codes. This operation of transforming numbers 
and letters into a form directly suitable for electronic data processing is an 
important method of introducing information into computing systems. See 
DATA-PROCESSING SYSTEMS. 
    Character recognition machines, sometimes called character readers, print 
readers, scanners, or reading machines, automatically convert printed 
alphanumeric characters or symbols into a machine-readable code at high 
speeds. The output of the character recognition machine may be temporarily 
stored on magnetic disks or magnetic tape. Alternatively, the recognition 
system may be operated on line with the data processor. The first commercial 
application of character recognition was in the banking industry, which 
adopted magnetic ink character recognition (MICR). In most applications, the 
printed or typed characters are sensed optically; this process is called 
optical character recognition (OCR). 
    Optical mark reading (OMR) refers to the simpler technology of optically 
sensing marks. The information being read is encoded as a series of marks such 
as lines or filled-in boxes on a test answer sheet, or as some special pattern 
such as the Universal Product Code (UPC). The UPC uses a predetermined and 
reproducible standard pattern which corresponds to each number in a 10-digit 
code. A relatively simple optical scanner detects the pattern and decodes the 
number.  
    Manual alternatives to automatic character recognition equipment for data 
entry include keypunch, key tape, and key disk systems, in which the data are 
entered through a keyboard and are recorded on punched cards, magnetic tape, 
or rotating magnetic disks, respectively. Limited-vocabulary isolated-word 
voice recognition equipment is available for data entry. However, voice input 
is also a slow process and would not be competitive with OCR in situations 
where the data already exist in printed or typed form. 
    Of all the above application areas, OCR is the single most important 
commercial application of pattern recognition technology. The technologies 
used in OCR systems include optical, electronic, mechanical, and computer 
techniques. In general an OCR system has the following functional systems: 
input; transport; scanner; preprocessor; feature extraction and classification 
logic; output. 
    The classes of OCR applications range from highly controlled stylized 
fonts to unconstrained multifont typewritten, printed, and handwritten 
characters encountered by postal address readers. Important elements in rating 
the performance of an OCR are the error rate, the throughput, the number of 
fonts capable of being recognized, and the cost, size, and reliability of the 
machine. The error rate has two components: the undetected substitution rate 
and the reject rate, that is, the proportion of characters not classified into 
any character class. 
    Stylized characters are designed to make automatic recognition easier. One 
widely used special font is the OCR-A or ISO-A (Fig. 1), which was the first 
character set standardized by the International Standards Organization. This 
font is used extensively on turnaround documents such as utility bills and 
gasoline credit card slips. A common example of stylized font character 
recognition is the magnetic ink character recognition (MICR) system which is 
used on bank checks. This special font, adopted as a standard by the American 
Banking Association, is called E-13B (Fig. 2). 
    Single-stylized-font OCR wands, that is, handheld readers, are being used 
more and more in inventory and point-of-sale applications. User-programmable 
hand-held wands in which a microprocessor can be programmed through the wand 
by reading special codes in the stylized character set have been introduced. 
    At the opposite end of the scale from stylized font characters is the 
variability of character fonts used in postal address readers which sort mail 
in large cities in the United States. With the additional OCR and bar code 
readers that are to be installed and use of a nine-digit business zip code and 
special incentives for large-volume business mailers, this equipment should 
improve service for large-volume mailers. The reading of hand-printed zip 
codes has been successfully accomplished in Japan by requiring that the 
numerals be carefully written in boxes preprinted on a standard location on 
the envelope. 
    Many data-entry and word-processing applications are aided by the 
automatic reading of material typed in one of the various popular typewriter 
fonts. Performance figures similar to those for stylized font characters may 
be obtained with certain typewriter fonts when typed on a specially aligned 
typewriter using carbon film ribbon, high-quality paper, and well-specified 
format conventions. In general, with standard typefaces, error rates are 
likely to be one or two orders of magnitude higher, depending on the typeface 
and print quality. 
    A number of manufacturers offer OCRs for constrained hand-printed 
numerics, and a few office readers for constrained alphanumeric hand-printed 
characters. State payroll tax forms, driver's license applications, magazine 
subscription renewal forms, and other short forms with boxes and directions on 
how to print the characters give evidence of the business use of constrained 
hand-printed OCR. The recognition performance depends, among other things, on 
the number of writers, the number of different character classes, and the 
training of the writers. 
    The increasing use of microprocessor technology is aimed at developing 
low-cost, decentralized OCR devices which can handle a wide variety of fonts 
and formats. Combinations of word-processing and OCR equipment, and OCR and 
facsimile store-and-forward communications are likely to lead to many new 
applications for OCR. See COMPUTER; DIGITAL COMPUTER; MICROPROCESSOR; WORD 
PROCESSING.  
 �34s[Laveen N. Kanal]
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�6v�2v�32sCharadriiformes   �21m �33s 
    An order of birds of worldwide distribution, most members of which live 
near water. Sixteen living families are usually recognized, but some 
authorities combine certain of these. The order contains large, well-known and 
widespread families as well as small families of limited distribution. Among 
the latter are the pantropical jacanas (Jacanidae); the partridgelike seed 
snipe (Thinocoridae) of southern South America, mostly in the high Andes: the 
white sheathbills (Chionididae) of subantarctic islands; and the unique crab 
plover (Dromadidae) of the shores of the Indian Ocean. Nesting in both the 
Arctic and Antarctic regions and ranging the seas between during the 
nonbreeding season are the skuas and jaegers (Stercorariidae). In warmer 
regions of the Old World is the family Glareolidae, which includes both the 
ploverlike coursers and the swallowlike pratincoles. The thick-knees or stone 
curlews (Burhinidae) inhabit dry grassy or stony areas in both the Old and New 
worlds.  
    In two small families, the painted snipe (Rostratulidae) and phalaropes 
(Phalaropodidae), the roles of the sexes are reversed, males being less 
brightly colored than females and responsible for care of eggs and young. See 
SEXUAL DIMORPHISM. 
    Members of the large, worldwide plover and sandpiper families 
(Charadriidae and Scolopacidae) have been popular game birds in many 
countries, but only the common snipe (Gallinago gallinago) and American 
woodcock (Philohela minor) are now hunted legally in North America. 
    The Alcidae, a family predominantly of northern coasts, contains such 
groups as the murres, guillemots, puffins, auklets, and auks, including the 
extinct flightless great auk (Piguinus impennis) of the North Atlantic. See 
AVES.  
 �34s[Kenneth C. Parkes]
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�6v�2v�32sCharales   �21m �33s 
    The single order of the class Charophyceae in the division Chlorophyta. 
These plants, commonly known as the stoneworts, are of macroscopic size and 
have erect, stemlike axes with nodes and internodes. Whorls of branches 
(``leaves'') arise from the nodes. Two tribes, Nitelleae and Chareae, with 
seven extant genera and a number of fossil representatives, make up the single 
family Characeae. See CHAROPHYCEAE; CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sCharcoal   �21m �33s 
    A porpous solid product containing 85 - 98% carbon produced by heating 
carbonaceous materials such as cellulose, wood, peat, and coals of bituminous 
or lower rank at 930 - 1100DGF (500 - 600DGC) in the absence of air. 
    Chars or charcoals from cellulose or wood are soft and friable. They are 
used chiefly for decolorizing solutions of sugar and other foodstuffs and for 
removing objectionable tastes and odors from water. Chars from nutshells and 
coal are dense, hard carbons. They are used in gas masks and in chemical 
manufacturing for many mixture separations. Another use is for the tertiary 
treatment of waste water. Residual organic matter is adsorbed effectively to 
improve the water quality. See ADSORPTION; CARBON. 
 �34s[Joseph H. Field]
�33s 
  
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�6v�2v�32sCharge-coupled devices   �21m �33s 
    Semiconductor devices in which minority charge is stored in a spatially 
defined depletion region (potential well) at the surface of a semiconductor, 
and is moved about the surface by transferring this charge to similar adjacent 
wells. The formation of the potential well is controlled by the manipulation 
of voltage applied to surface electrodes. Since a potential well represents a 
nonequilibrium state, it will fill with minority charge from normal thermal 
generation. Thus a charge-coupled device (CCD) must be continuously clocked or 
refreshed to maintain its usefulness. In general, the potential wells are 
strung together as shift registers. Charge is injected or generated at various 
input ports and then transferred to an output detector. By appropriate design 
to minimize the dispersive effects associated with the charge-transfer 
process, well-defined charge packets can be moved over relatively long 
distances through thousands of transfers. 
    There are several methods of controlling the charge motion, all of which 
rely upon providing a lower potential for the charge in the desired direction. 
When an electrode is placed in proximity to a semiconductor surface, the 
electrode's potential can control the near-surface potential within the 
semiconductor. The basis for this control is the same as for metal oxide 
semiconductor (MOS) transistor action. If closely spaced electrodes are at 
different voltages, they will form potential wells of different depths. Free 
charge will move from the region of higher potential to the one of lower 
potential.  
    An important property of a charge-coupled device is its ability to 
transfer almost all of the charge from one well to the next. Without this 
feature, charge packets would be quickly distorted and lose their identity. 
This ability to transfer charge is measured as transfer efficiency, which must 
be very good in order for the structure to be useful in long registers. Values 
greater than 99.9% per transfer are not uncommon. This means that only 10% of 
the original charge is lost after 100 transfers. 
    A second important property of a charge-coupled device register is its 
lifetime. When the surface electrode is clocked high, the potential within the 
semiconductor also increases. Majority charge is swept away, leaving behind a 
depletion layer. If the potential is taken sufficiently high, the surface goes 
into deep depletion until an inversion layer is formed and adequate minority 
charge collected to satisfy the field requirements. The time it takes for 
minority charge to fill the well is the measure of well lifetime. The major 
sources of unwanted charge are: thermal diffusion of substrate minority charge 
to the edge of the depletion region, where it is collected in the well; 
electronhole pair generation within the depletion region; and the emission of 
minority charge by traps. Surface-channel charge-coupled devices usually have 
better lifetime, since surface-state trap emission is suppressed and the 
depletion regions are usually smaller. 
    Once adequate transfer efficiency and sufficient lifetime have been 
achieved, input and output ports must be established. The port structure of a 
charge-coupled device depends upon its application. Analog registers require 
linear inputs and outputs with good dynamic range. Digital registers require 
precisely metered and detected ones and zeros. Imaging devices use 
photoemission at the register sites as input with analog-output amplifers. See 
COMPUTER STORAGE TECHNOLOGY; INTEGRATED CIRCUITS; SEMICONDUCTOR. 
 �34s[Melvin R. George]
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�6v�2v�32sCharge-density wave �21m �33s
    The ground state of a metal in which the conduction-electron charge
density is sinusoidally modulated in space. The periodicity of this extra
modulation is unrelated to the lattice periodicity. Instead it is determined
by the dimensions of the conduction-electron Fermi surface in momentum space.
    Coulomb interactions between electrons are usually the cause of a
charge-density wave instability. Two such contributions are the exchange
energy, an effect of the Pauli exclusion principle, and the correlation
energy, an effect of electron-electron scattering. Both energies are reduced
in a charge-density wave structure. However, the classical, electrostatic
energy opposes the instability and will suppress it unless the positive-ion
lattice intervenes. See EXCLUSION PRINCIPLE.
    A wavelike displacement of this lattice will generate a positive-ion
charge density that tends to cancel the electronic charge modulation of the
charge-density wave. Typical values of the displacement amplitude are A APPROX
EQUAL 10- sup 1 sup 2-10- sup 1 sup 1 m. The ion-ion interactions must be small
in order to permit this. Consequently charge-density waves are more likely to
occur in metals with small elastic moduli. See BAND THEORY OF SOLIDS; CRYSTAL
STRUCTURE; SPIN-DENSITY WAVE. 
 �34s[Albert W. Overhauser]
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�6v�2v�32sCharles' law   �21m �33s 
    A thermodynamic law, also known as Gay-Lussac's law, which states that at 
constant pressure the volume of a fixed mass or quantity of gas varies 
directly with the absolute temperature. Conversely, at constant volume the gas 
pressure varies directly with the absolute temperature. See GAS. 
 �34s[Frank H. Rockett]
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�6v�2v�32sCharm   �21m �33s 
    Ordinary atoms of matter consist of a nucleus composed of neutrons and 
protons and surrounded by electrons. Over the years, however, a host of other 
particles with unexpected properties have been found, associated with both 
electrons (leptons) and protons (hadrons). The hadrons number in the hundreds, 
and can be explained as composites of more fundamental constituents, called 
quarks. The originally simple situation of having an up quark (u) and a down 
quark (d) has evolved as several more varieties or flavors have had to be 
added. These are the strange quark (s) with the additional property or quantum 
number of strangeness to account for the unexpected characteristics of a 
family of strange particles; the charm quark (c) possessing charm and no 
strangeness, to explain the discovery of the J/PSI particles, massive states 
three times heavier than the proton; and a fifth quark (b) to explain the 
existence of the even more massive upsilon (UPSILON) particles. See HADRON; J 
PARTICLE; QUARKS. 
    The members of the family of particles associated with charm fall into two 
classes: those with hidden charm, where the states are a combination of charm 
and anticharm quarks (cc), charmonium; and those where the charm property is 
clearly evident, such as the Dsupp (c*EQN*) meson and LAMBDA*EQN* (cud) 
baryon. Although reasonable progress has been made in the study of charmed 
states, only a handful of states has been observed, and much work remains to 
be done. See ELEMENTARY PARTICLE. 
 �34s[Nicholas P. Samios]
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�6v�2v�32sCharophyceae   �21m �33s 
    One of the two classes of the Chlorophyta with one order, Charales. Two 
tribes, Nitelleae and Chareae, with seven extant genera, including Nitella and 
Chara, constitute the order. This group is known as the stoneworts because 
most species of Chara are lime-encrusted. See CHARALES; CHLOROPHYCEAE. 
    The plants have rhizoidal extensions into the substrate and a vertical 
axis with nodes and internodes. Whorls of short branches bearing sex organs 
occur at the nodes. An apical cell conducts terminal growth. The axis and 
``leaves'' of Chara are usually enclosed by overlaying, elongate, fluting or 
corticating cells. Plants usually grow in mats and may form extensive bottom 
meadows in lakes and slow-flowing streams. They are important in the food 
chain of aquatic animals and birds. See CHAROPHYTA; CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
�33s 
  
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�6v�2v�32sCharophyta   �21m �33s 
    A group of small aquatic plants that have been limited to fresh- and 
brackish-water environments since the Early Devonian Period. The charophytes 
live entirely submerged in quiet or slowly moving bodies of alkaline to mildly 
acidic water. Their calcareous remains are widely distributed in nonmarine 
shales and limestones, and the group is useful in stratigraphic subdivision 
and paleoecological interpretation of these strata. Modern forms are a source 
of food for wildfowl. 
    Charophyta are generally classified with the green algae, but their 
reproductive organs, antheridia and sporophydia (unfertilized oogonia), are so 
specialized that some systematists consider them a separate group. 
Classification of fossil forms is based chiefly on the gyrogonites (see 
illustration), the small calcified portions of the sporophydia. See ALGAE; 
CHARALES; PALEOBOTANY. 
 �34s[Raymond E. Peck]
�33s 
  
  
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�6v�2v�32sCheese   �21m �33s 
    A product of milk, selectively concentrated from major milk components. It 
is generally rich in flavor and contains high-quality nutrients. There are 
many varieties of cheese, all produced in the following general manner. Raw or 
pasteurized milk is clotted by acid, rennet, or both. The curd is cut and 
shaped into the special form of the cheese with or without pressing. Salt is 
added, or the cheese is brined after pressing. 
    Acid is produced during manufacture of cheese by fermentation of the milk 
sugar, lactose. This fermentation is initiated by the addition of a culture of 
specially selected acid bacteria (starter culture) to the milk. Acid 
production in cheese curd retards growth of bacteria that cause undesirable 
fermentations in cheese. Moreover, it favors the expulsion of the whey and the 
fusion of the curd particles. Fresh cheese (cottage or cream cheese) does not 
require any ripening, and it is sold soon after it is made. Other varieties of 
cheese are cured or ripened to obtain the desirable consistency, flavor, or 
aroma. The flavor and aroma of cheese are obtained by a partial breakdown of 
mild proteins and fat by the action of microbial, milk, and rennet enzymes. In 
hard varieties (Cheddar, Gouda, Edam, Emmentaler or Swiss, and provolone) this 
is done by the microorganisms in the interior of cheese; in semisoft or soft 
types (Limburger, Camembert, and Roquefort) by the organisms on, or in contact 
with, the surface of cheese. 
    Process cheese is produced from cheeses of different ages by blending. The 
mixture, melted with the aid of emulsifying salts (citrates and phosphates), 
is packed in sealed containers (tins, paperboard, foil, or plastic). Few 
bacteria other than spore-formers survive the heat treatment. No substantial 
growth of flora occurs in well-preserved process cheese, but spoilage by 
anaerobic spore-formers may occur. 
 �34s[Norman Olson]
�33s 
    Key materials for cheesemaking include fresh or precultured milk, 
cultures, milk-coagulating enzyme preparations, special microorganisms, salt, 
and beta carotene or annatto color. The amounts used and the manner in which 
these materials are applied strongly influence the cheese character. Cheese 
may be made from the milk of the cow, sheep, goat, water buffalo, and other 
mammals, but the milk of the cow is most widely used despite some limitations. 
Sheep milk and water buffalo milk generally give more flavor to the cheeses, 
and the color is uniformly white because of a lack of carotene in such 
mammalian milks, but they are more expensive to make into cheese. 
    Two major classes of cheeses exist, fresh and ripened. Fresh cheeses are 
simpler to make than ripened, are more perishable, and do not develop as 
intense flavors, but give a mild acid, slightly aromatic flavor and soft, 
smooth texture. Three basic groups characterize fresh cheese types: group I - 
ricotta and Broccio; group II - cottage, Neufchatel, and cream; and group III 
- mozzarella. Most ripened cheeses are contained in one of six basic groups. 
The characteristic cheeses in each group include: group I - Cheddar and 
Monterey; group II - Swiss (Emmentaler) and Gruyere; group III - Edam and 
Gouda; group IV - Muenster, brick, and Limburger; group V - provolone; and 
group VI - Camembert, Brie, and bleu. 
    Processed cheese is made from natural types. Nearly any natural cheese can 
be processed, except that for blue cheese technical difficulties cause 
blackening of the blue mold due to the high heat. Processed cheese is produced 
by grinding selected lots of natural cheese and adding cream, color, salt, and 
emulsifying agents. Processed cheese foods and spreads are made similarly, but 
include more water and less protein, fat, and other solids. See MILK. 
 �34s[Frank V. Kosikowski]
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�6v�2v�32sCheilostomata   �21m �33s 
    An order of ectoproct bryozoans in the class Gymnolaemata. Cheilostomes 
possess delicate colonies composed of loosely grouped, highly complex, short, 
uncurved, box- or vase-shaped zooecia, with solid, porous, chitinous, or 
calcareous walls, and with apertures closed by lidlike opercula. Cheilostome 
colonies may be encrusting threadlike networks; thin encrusting sheets; 
tabular, nodular, or globular masses; or erect tuftlike, twiglike, frondlike, 
or trellislike growths. These colonies are not divisible into distinct 
endozone and exozone regions. Although all lack stolons and monticules or 
maculae, and some have rhizoids or small coelomic chambers, most cheilostome 
colonies bear ovicells (brood chambers) developed at the distal ends of the 
normal autozooecia (feeding zooids). Many cheilostome colonies possess either 
one or both of two types of heterozooids, avicularia, and vibracula. See 
BRYOZOA; CTENOSTOMATA; GYMNOLAEMATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sChelation �21m �33s
    A chemical reaction or process involving chelate ring formation and
characterized by multiple coordinate bonding between two or more of the
electron-pair-donor groups of a multidentate ligand and an
electron-pair-acceptor metal ion. The multidentate ligand is usually called a
chelating agent, and the product is known as a metal chelate compound or metal
chelate complex. Metal chelate chemistry is a subdivision of coordination
chemistry and is characterized by the special properties resulting from the
utilization of ligands possessing bridged donor groups, two or more of which
coordinate simultaneously to a metal ion. See COORDINATION CHEMISTRY.
    Many of the functional groups of both synthetic and naturally occurring
organic compounds can form coordinate bonds to metal ions, producing
metal-organic complexes or chelates, many of which are biologically active.
Thus chelate compounds are frequently found in an interdisciplinary field of
science called bioinorganic chemistry. The biological significance of chelates
is demonstrated by the large number of biologically important compounds that
are either metal chelates or chelating agents. Included in this group are the
alpha amino acids, peptides, proteins, enzymes, porphyrins (such as
hemoglobin), corrins (such as vitamin B12), catechols, hydroxypolycarboxylic
acids (such as citric acid), ascorbic acid (vitamin C), polyphosphates,
nucleosides and other genetic compounds, pyridoxal phosphate (vitamin B6), and
sugars. The ubiquitous green plant pigment, chlorophyll, is a magnesium
chelate of a tetradentate ligand formed from a modified porphin compound, and
similarly the oxygen transport heme of red blood cells contains an Fe(II)
chelate. See BIOINORGANIC CHEMISTRY; ORGANOMETALLIC COMPOUNDS.
    The ability of chelating agents to reduce the chemical activity of metal
ions has found extensive application in many areas of science and industry.
Ethylenediaminetetraacetic acid (EDTA), a hexadentate chelating agent has been
employed commercially for water softening, boiler scale removal, industrial
cleaning, soil metal micronutrient transport, and food preservation.
Nitrilotriacetic acid (NTA) is a tetradentate chelating agent which, because
of lower cost, has taken over some of the commercial applications of EDTA.
Many commercially important dyes and pigments, such as copper phthalocyanines,
are chelate compounds. Humic and fulvic acids are plant degradation products
in lake and sea-water sediments that have been suggested as important
chelating agents which regulate metal-ion balance in natural waters. By virtue
of its abundance, low toxicity, low cost, and good chelating tendencies for
metal ions that produce water hardness, the tripolyphosphate ion (as its
sodium salt) is used in large quantities as a builder in synthetic detergents.
Both synthetic ion exchangers and the mineral zeolites are chelating
ion-exchange resins which are used in analytical and water-softening
applications. As final examples, less conventional chelating agents are the
multidentate, cyclic ligands, termed collectively crown ethers, which are
particularly suited for the complexation of the alkali and alkaline-earth
metals. See COMPLEX COMPOUNDS; CROWN ETHERS AND CRYPTANDS;
ETHYLENEDIAMINETETRAACETIC ACID.
 �34s[Alice E. Moore ;R. J. Motekai]
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�6v�2v�32sChelicerata   �21m �33s 
    A subphylum of the phylum Arthropoda which includes the classes 
Merostomata, Pycnogonida, and Arachnida. The first pair of ventral appendages, 
the chelicerae, is modified as pincers. The body usually consists of the 
cephalothorax and abdomen, except in the Acarina in which these body parts are 
fused. The cephalothorax is unsegmented in all but the solpugids or sun 
spiders. Four pairs of legs are common to most members of this group. 
Respiratory structures include book lungs, gills, and tracheae. The sexes are 
usually separate; there is a single sex opening located anteriorly on the 
abdomen. Fertilization is usually internal and most species lay eggs. 
Chelicerates are chiefly terrestrial and predacious. See ARACHNIDA; 
ARTHROPODA; MEROSTOMATA; PYCNOGONIDA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sChelonia   �21m �33s 
    An order of the Reptilia, subclass Anapsida, including the turtles, 
terrapins, and tortoises. This order is also known as the Testudinata. The 
group first appeared in the Triassic and its representatives are among the 
commonest fossils from that time on. Members of the order are most frequently 
found in fresh-water streams, lakes, and ponds or in marshy areas. However, a 
number of strictly terrestrial species are known and several are marine. 
Turtles occur on all the major continents and continental islands in tropic 
and temperate regions. The marine forms are basically tropic in distribution 
but some individuals stray into temperate waters. See ANAPSIDA; REPTILIA. 
376 A 
                The living turtles are usually divided into two major groups, 
the suborders Pleurodira and Cryptodira, based upon the structures of the head 
and neck. Both lines are thought to be evolved from primitive forms placed in 
the suborder Amphichelydia (Triassic to Eocene). This latter group differs 
most markedly from recent species in having elongate neural spines on the neck 
vertebrae so that the neck is not retractible into the shell. The table shows 
the principal groups and distribution of the approximately 225 living species 
of the order. See TURTLE. 
    The Chelonia differ from most other vertebrates in possessing a hard bony 
shell which encompasses and protects the body. The shell is made up of a 
dorsal portion, the carapace, and a ventral segment, the plastron, connected 
by soft ligamentous tissue or a bony bridge. The rigid bony shell of turtles 
imposes a basic body plan subject to relatively little variation. Turtles are 
always recognizable as such and the principal obvious differences between them 
are in the shell shape, limbs, head, and neck. 
    The majority of turtles have limbs more or less adapted to aquatic or 
semiaquatic life with moderate to well-developed palmate webbed feet. However, 
in strictly terrestrial forms such as the tortoises, the limbs are elephantine 
with the weight of the body being borne on the flattened soles of the feet. 
    Associated with the tendency of most turtles toward a life in or near 
water is the auditory apparatus. Even though an eardrum is present, hearing is 
adapted to picking up sounds transmitted through the water or substratum. 
However, there is evidence that airborne sound can be heard. Vision is also 
important, and turtles have color vision. Sounds are produced by many species 
through explusion of air through the glottis. Respiration is by means of the 
rather rigid lungs, which are inflated and deflated by a series of special 
muscles. Several species with aquatic habits have special vacularized areas in 
the mouth or cloaca which act as auxiliary respiratory devices and make 
possible gaseous exchanges when the turtle is submerged in water. Most forms 
emit highly pungent oily substances from specialized cloacal glands, and these 
secretions probably are useful as recognition cues in the water. Most of the 
species are gregarious and diurnal, and territoriality is unknown in the order. 
    All turtles lay shelled eggs which are buried in sand or soil in areas 
where females congregate. The shell is calcareous in most forms, but the 
marine turtles have leathery shells. Eggs are rather numerous, as many as 200 
being laid by a single individual in some species. Incubation takes 60 - 90 
days, and the little turtle cuts its way out of the shell with a small horny 
egg caruncle at the end of the snout. 
    Turtles feed on all types of organisms. Aquatic species may eat algae, 
higher plants, mollusks, crustaceans, insects, or fishes; terrestrial forms 
are similarly catholic in tastes. Most species are omnivores, but some have 
very specialized diets. The edges and internal surfaces of the horny beaks of 
these reptiles are frequently denticulate and modified to form specialized 
mechanisms adapted to handle particular food items. 
 �34s[Jay M. Savage]
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�6v�2v�32sChemical bonding   �21m �33s 
    The fundamental fact of chemistry is that elements, such as oxygen, 
hydrogen, carbon, and iron, can combine to form compounds, such as water, 
methane, and iron oxide, with properties completely different from those of 
their components. The description of this in terms of atomic theory is that 
atoms of the elements attach themselves to each other to form molecules. This 
property that atoms possess, of joining together to form molecules, is known 
as chemical bonding. 
    Atoms contain electric charges - a small, positively charged nucleus 
surrounded by a cloud of moving, negatively charged electrons. All chemical 
bonding is caused by the mutual attractions and repulsions of these electric 
charges.  
    Chemical bonds are very strong. To break one bond in each molecule in a 
gram mole of material will typically require an energy of many tens of 
kilocalories per mole. Most of the energy used by man is chemical energy, 
derived from changing chemical bonds in food or fuel. 
    Ionic bonding is the simplest type of bonding, in which one or more 
electrons are transferred completely from one atom to another, thus converting 
the neutral atoms into electrically charged ions. These ions are approximately 
spherical in shape and attract one another because of their opposite charges. 
The ions are drawn together until their spherical electron clouds sufficiently 
interpenetrate and repel one another to balance the force of attraction. 
Molecules can consist of two or more such ions. Many inorganic crystals can be 
considered as giant molecules made up of ions. Thus common salt, sodium 
chloride, consists of a lattice of Nasupp ions (positive sodium) each 
surrounded by six Cl- ions (negative chlorine) and vice versa. See IONIC 
CRYSTALS.  
    Covalent bonding is another limiting type of chemical bonding. Here each 
atom of a bonded pair contributes one electron to form a pair of electrons 
which move in such a manner as to increase the density of electric charge in 
the space between the two atoms. The negative charge in the region between the 
two atoms attracts the two positive nuclei. This type of bond is also called 
an electron pair bond because its essential feature is the formation of a pair 
of electrons which spend much time in the region between the two atoms. These 
two electrons have their spins pointing in opposite directions; in other 
words, they are paired. 
    Metallic bonding is a third type of chemical bonding, which is exemplified 
in the common metals. There are several ways of looking at this bonding, but 
perhaps the simplest is to consider the crystal as consisting of positive ions 
of the metallic element immersed in a sea of electrons. The attraction of the 
positive ions for the electrons holds the crystal together. Some of the 
electrons are free to move about the whole crystal of the metal, and this is 
what makes the metal an electrical conductor. 
    Although many crystals are giant molecules whose atoms are completely 
linked together by strong ionic or covalent bonds, others consist of discrete 
molecules, strongly bonded internally, but held to each other by much weaker 
forces. Most organic crystals are soft and have low melting points. Their 
discrete molecules are held together by what are called van der Waals or 
dispersion forces. Although these forces are electrical in nature, they are 
too weak to be considered as true chemical bonding. See CHEMICAL STRUCTURES; 
INTERMOLECULAR FORCES; MOLECULAR STRUCTURE AND SPECTRA; QUANTUM CHEMISTRY; 
VALENCE.  
 �34s[E. Bright Wilson, Jr.]
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�6v�2v�32sChemical compounds   �21m �33s 
    Substances composed of two or more elements which do not vary in 
composition from sample to sample, and which have fixed and definite physical 
properties, such as density and refractive index. The elements in compounds 
cannot be separated by simple physical or mechanical means, but only by 
chemical treatment. When compounds are formed from their elements, heat is 
generated or absorbed. These properties distinguish them from mixtures. See 
ELEMENTS (CHEMISTRY); MIXTURE. 
    Most chemical compounds are formed in fixed and definite proportions by 
weight from their elements, and they obey the laws of chemical combination. 
However, there are a number of solid compounds, known as nonstoichiometric 
compounds, that exhibit departures from the law of definite proportions. See 
DEFINITE COMPOSITION, LAW OF; EQUIVALENT WEIGHT; INORGANIC CHEMISTRY; MULTIPLE 
PROPORTIONS, LAW OF NONSTOICHIOMETRIC COMPOUNDS. 
 �34s[Thomas C. Waddington]
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�6v�2v�32sChemical conversion �21m �33s
    The term for the chemical change from reactants to products of a chemical
industrial process. Preferred for decades in the petroleum industry, the term
is being used to an increasing extent for the entire chemical process
industries in place of unit processes, since it is a more exact definition of
industrial chemical reactions. The following are important chemical
conversions:
	Alkylation			Calcination
	Amination			Combustion
	Ammonolysis			Condensation
	Aromatization			Dehydration
	Diazotization and coupling	Isomerization
	Esterification (sulfation)	Neutralizaion
	Fermentation			Nitration
	Halogenation			Oxidation
	Hydroformylation (oxo)		Polymerization
	Hydrogenation,			Pyrolysis, cracking
	  hydrogenolysis		Reduction
	Hydrolysis, hydration		Sulfation
	Ion exchange			Sulfonation
All these terms refer to the classes of chemical changes, each embracing many
individual reactions or chemical conversions into which chemical industrial
processes can be divided. For additional details see the separate articles on
individual chemical conversions (unit processes). See CHEMICAL ENGINEERING;
CHEMICAL PROCESS INDUSTRY; UNIT OPERATIONS.
 �34s[Joseph A. Brink, Jr.]
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�6v�2v�32sChemical dating   �21m �33s 
    Determination of the relative or absolute age of minerals and of ancient 
objects and materials by measurement of their chemical composition. By 
inference this method of age determination may be extended to the dating of 
geological formations and to the dating of geological and human events. Most 
of the observational methods for dating geological formations and events, such 
as that based on the order and thickness of rock strata, are reliable only for 
obtaining relative or sequence dates. This is true, also, of analogous methods 
of dating artifacts and human events, for instance, the method based on the 
depth at which objects are buried in the soil. Most of the chemical methods, 
however, yield approximate absolute dates, involve fewer assumptions, and are 
less likely to result in large errors. They are exceeded in accuracy only by 
the method of tree-ring counting. Unfortunately, these methods are applicable 
only to the direct dating of a few minerals and a few kinds of ancient objects 
and materials. See ARCHEOLOGICAL CHEMISTRY; DENDROCHRONOLOGY. 
    Almost all chemical dating methods are based on measurements of: (1) the 
proportion of some stable element known to vary in a systematic way; (2) the 
degree of epimerization or racemization of an amino acid in residual organic 
matter; (3) the ratio of the weight of a decomposition product of 
radioactivity to the weight of original radioactive element or elements still 
present; or (4) the residual activity of a radioactive isotope. See 
RADIOCARBON DATING; ROCK AGE DETERMINATION. 
    The oldest and least reliable method is based on the increase in the 
fluorine content of bones with increase in time elapsed since they were buried 
in the soil. This increase is due to contact of the bones with groundwater 
containing fluoride ions and the consequent partial replacement of the 
phosphate of the bones with fluoride. This method must be used with great 
caution, and is usually applicable only where all conditions appear to have 
been fairly constant over long periods, as in caves. 
    A much superior method for dating ancient bones is based on the 
measurement of the degree of epimerization or racemization of one of the amino 
acids they often contain. The amino acids in living bone are, with the 
exception of glycine, mostly in the form of the L-isomers of these acids. On 
death of the organism, these isomers change slowly and progressively, at a 
rate governed by temperature, to the D-isomers of the acids, except for 
L-isoleucine which changes by epimerization to the nonprotein amino acid 
D-alloisoleucine. See AMINO ACIDS; GEOCHRONOMETRY; RACEMIZATION. 
    Another method, limited in its application to certain historical times and 
to certain regions, is based on systematic differences in the composition of 
the metal of ancient coins of certain classes. The Greek bronze coins struck 
from about the middle of the 4th century B.C. to about the end of the 1st 
century A.D. constitute one such class. In these coins the proportion of tin 
progressively decreases with time, a decrease which is accompanied by a 
progressive and relatively greater increase in the proportion of lead. A 
traditional mint practice of melting worn coins with additional lead to 
produce metal for new coins is believed to be the chief cause of this 
systematic change in composition. The additional lead makes up for the copper 
and tin lost during the wearing of the coins. The proportion of tin and of 
lead, or the ratio of the two determined by conventional analytical methods, 
is the index of time of manufacture. 
 �34s[Earle R. Caley]
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�6v�2v�32sChemical dynamics   �21m �33s 
    That branch of physical chemistry which seeks to explain time-dependent 
phenomena, such as energy transfer and chemical reaction, in terms of the 
detailed motion of the nuclei and electrons which constitute the system. 
  
    Reaction kinetics.  Although the ultimate state of a chemical system is 
specified by thermodynamics, the time required to reach that equilibrium state 
is highly variable. As a consequence, determining the rate of chemical 
reactions has proved to be important for practical reasons. For example, 
diamonds are thermodynamically unstable with respect to graphite, but the rate 
of transformation of diamonds to graphite is negligible. Rate studies have 
also yielded fundamental information about the details of the nuclear 
rearrangements which constitute the chemical reaction. See CHEMICAL 
EQUILIBRIUM.  
    Traditional chemical kinetic investigations of the reaction between 
species X and Y to form Z and W, reaction (1), sought a rate of the form given 
in Eq. (2), where d[Z]/dt is the rate of appearance of product Z, f is some 

                           *** X + Y -> Z + W ***                        (1)

               *** d[Z] / dt = kf([X], [Y], [Z], [W]) ***                (2)

function of concentrations of X, Y, Z, and W which are themselves functions of 
time, and k is the rate constant. Chemical reactions are incredibly diverse, 
and often the function f is quite complicated, even for seemingly simple 
reactions such as that in which hydrogen and bromine combine directly to form 
hydrogen bromide. These are actually complex reactions which proceed through a 
sequence of simpler reactions, called elementary reactions. For reaction (3d), 
the sequence of elementary reactions is a chain mechanism known to involve a 
series of steps, reactions (3a) - (3c). This sequence of elementary reactions 
was formerly known as the reaction mechanism, but in the chemical dynamical 
sense the word mechanism is reserved to mean the detailed motion of the nuclei 
during a collision. 
  
                        *** Br sub 2 -> 2Br ***                          (3a) 
  
                    *** Br + H sub 2 -> HBr + H ***                      (3b) 
  
                    *** H + Br sub 2 -> HBr + Br ***                     (3c)  
  
              *** H sub 2 + Br sub 2 -> 2HBr ***                         (3d) 
  
    An elementary reaction is considered to occur exactly as written. Reaction 
(3b) is assumed to occur when a bromine atom hits a hydrogen molecule. The 
products of the collision are a hydrogen bromide molecule and a hydrogen atom. 
On the other hand, the overall reaction is a sequence of these elementary 
steps and on a molecular basis does not occur as reaction (3d) is written. 
With few exceptions, the rate law for an elementary reaction A + B -> C + D is 
given by d[C]/dt = k[A][B]. The order (sum of the exponents of the 
concentrations) is two, which is expected if the reaction is bimolecular 
(requires only species A to collide with species B). The rate constant k for 
such a reaction depends very strongly on temperature. 
    In some instances, particularly for decompositions AB -> A + B, the 
elementary reaction step is first-order, Eq. (4), which means that the 

                *** d[A] / dt = d[B] / dt = k[AB] ***                    (4)  

reaction is unimolecular. The species AB does not spontaneously dissociate; it 
must first be given some critical amount of energy, usually through            
collisions, to form an excited species AB*. It is the species AB* which 
decomposes unimolecularly. 
  
    Molecular dynamics.  In principle, it is possible to prepare two reagents 
in specific quantum states and to determine the quantum-state distribution of 
the products. In practice, this is much too difficult, and experiments have 
been limited to preparing one reagent or to determining some aspect of the 
product distribution. This approach yields data concerning the gross aspects 
of the dynamics rather than the fine details. 
    Molecular-beam and luminescence techniques have played a major role in the 
development of chemical dynamics. Since these techniques largely complement 
each other, they are illustrated by discussing the results of a single 
bimolecular reaction, the formation of deuterium chloride (DCl), reaction (5). 
  
                        *** D + Cl2 -> DCl + Cl ***                      (5)
  
    For infrared chemiluminescence measurements deuterium atoms are made by 
dissociating D sub 2 gas in an electrical discharge and injecting them into an 
observation cell. The reagents mix and react inside a vessel with walls at
77 K (-321 DG F), which freeze out species hitting the wall. The pressure is 
kept low to minimize vibrationally deactivating collisions. Infrared (IR) 
emission is analyzed with an IR spectrometer. By analyzing the spectrum, it is 
possible to determine which vibration-rotation states are emitting and, as a 
consequence, which vibration-rotation states are formed in the reaction. See 
CHEMILUMINESCENCE.  
    Molecules can be isolated in molecular beams, and collisions between these 
isolated molecules can be observed by crossing two tenuous molecular beams in 
a region of otherwise high vacuum. Measurements are made of the scattered 
product intensity and speed at various scattering angles. The speed of the 
product is high, and corresponds to about half of the reaction exoergicity 
appearing in translational recoil of the products, with the balance appearing 
in vibration and rotation of the DCl consistent with the chemiluminescence 
results. See MOLECULAR BEAMS. 
    Unimolecular reactions of collisionally activated reaction complexes have 
been studied using both crossed-beam and chemiluminescence techniques. The 
availability of lasers opened new vistas in chemical dynamics, not only 
because lasers facilitate conventional measurements, as in detecting reaction 
products by laser-induced fluorescence, but also because they have uncovered 
new phenomena. Absorption of photons by molecules has been known for many 
years, and it has become apparent that under the right circumstances a 
molecule can absorb not one photon, but so many as to cause the molecule to 
dissociate or even or ionize. 
 �34s[Philip R. Brooks]
�33s 
  
    Theoretical methods.  The goal of chemical dynamics is to understand 
kinetic phenomena from the basic laws of molecular mechanics, and it is thus a 
field which sees close interplay between experimental and theoretical research. 
    An important question regarding the dynamics of chemical reactions has to 
do with the product energy distribution in exothermic reactions. For example, 
because the HF molecule is more strongly bound than the H sub 2 molecule, 
reaction (6) releases a considerable amount of energy. The two possible paths 

                      *** F + H sub 2 -> HF + H ***                      (6)

for this energy release to follow are into translation, that is, with HF and 
H speeding away from each other, or into vibrational motion of HF. 
    In this case it is vibration, and this has rather dramatic consequences: 
the reaction creates a population inversion among the vibrational energy 
levels of HF - that is, the higher vibrational levels have more population 
than the lower levels - and the emission of infrared light from these excited 
vibrational levels can be made to form a chemical laser. 
    The rates of most chemical reactions are increased if they are given more 
energy. In macroscopic kinetics this corresponds to increasing the 
temperature, and most reactions are faster at higher temperatures. It seems 
reasonable, though, that some types of energy will be more effective in 
accelerating the reaction than others. The general rule of thumb is that 
vibrational energy is more effective for endothermic reactions (those for 
which the new molecule is less stable than the original molecule), while 
translational energy is most effective for exothermic reactions. 
    Many different theoretical models and methods have been useful in 
understanding and analyzing kinetic phenomena. Probably the single most useful 
approach has been the calculation of classical trajectories. Assuming that the 
potential energy function or a reasonable approximation is known for the three 
atoms in reaction (6), for example, it is possible by use of electronic 
computers to calculate the classical motion of the three atoms. It is thus an 
easy matter to give the initial molecule more or less vibrational or 
translational energy, and then compute the probability of reaction. Similarly, 
the final molecule and atom can be studied to see where the energy appears, 
that is, as translation or as vibration. 
    It is thus a relatively straightforward matter theoretically to answer the 
questions and to see whether or not mode-specific excitation leads to 
significantly different chemistry than simply increasing the temperature under 
bulk conditions. See INORGANIC PHOTOCHEMISTRY; PHOTOCHEMISTRY. 
 �34s[William H. Miller]
�33s 
  
    Relaxation methods.  Since about 1950, considerable use has been made of 
perturbation techniques to measure rates and determine mechanisms of rapid 
chemical reactions. These methods provide measurements of chemical reaction 
rates by displacing equilibria. In situations where the reaction of interest 
occurs in a system at equilibrium, perturbation techniques called relaxation 
methods have been found most effective for determining reaction rate constants. 
    A chemical system at equilibrium is one in which the rate of a forward 
reaction is exactly balanced by the rate of the corresponding back reaction. 
Examples are chemical reactions occurring in liquid solutions, such as the 
familiar equilibrium in pure water, shown in reaction (7). The essence of any 
  
       *** H2O EQUILBRIUM (REACTION CONDITIONS = kb, kf)
                         H POSITIVE (aq) + OH NEGATIVE (aq) ***          (7)
  
of the relaxation methods is the perturbation of a chemical equilibrium (by a 
small change in temperature, pressure, electric-field intensity, or solvent 
composition) in so sudden a fashion that the chemical system, in seeking to 
reachieve equilibrium, is forced by the comparative slowness of the chemical 
reactions to lag behind the perturbation. 
    The temperature jump (T-jump) relaxation method has proved to be very 
versatile. The choice of the particular means of effecting the temperature 
perturbation is dictated only by the requirement that the temperature rise 
somewhat more rapidly than the time constant of the chemical reaction to be 
explored, so that a tedious deconvolution can be avoided. 
    In a situation, such as reaction (7), in which electrically neutral 
reactant species dissociate into oppositely charged ions, an especially 
sensitive tool for measuring rate constants of forward and backward reactions 
is the electric-field jump (E-jump) technique with conductometric detection. 
In a strong electric field (of the order of [4 X (10 sup 6)] V (m sup -1), a 
weak acid in solution is caused to dissociate to a greater degree than it would 
in the absence of the electric field. With a sensitive, high-voltage Wheatstone 
bridge, the exponential increase with time in the concentration of ions 
following a precipitous increase in electronic-field strength is readily 
detected. The measured relaxation time (TAU) is clearly that corresponding to 
the highelectric-field environment, but since the rate constants for these 
reactions differ little in and out of the electric field, no serious problem 
is posed. 
    Two other relaxation methods more widely used than the E-jump technique 
are pressure jump (P-jump) and ultrasonic absorption. Each relies for its 
effectiveness on a volume change, DELTA (V sup 0), occurring in an aqueous 
sample equilibrium undergoing kinetic investigation. As electrically neutral, 
weak electrolyte solute species dissociate into ions in aqueous solution, there 
is an increase in the number of solvent molecules drawn into a highly ordered 
solvation sheath. The higher the charge density of the ion, the more water 
molecule dipoles are bound and the greater the change in V sup 0 as reactants 
become products. Unlike the T-jump and E-jump relaxation methods, which 
usually employ step function perturbations, the ultrasonic absorption 
techniques are continuous-wave experiments in which the sample chemical 
equilibrium absorbs a measurable amount of the sound wave's energy when the 
frequency of the sound wave (f) and the relaxation time of the chemical 
equilibrium bear the relation to one another given by Eq. (8). See ULTRASONICS. 
  
                       *** TAU sup -1 = 2 PI f ***                        (8)
     
    The typical pressure-jump (P-jump) experiment is one in which a liquid 
sample under about 200 atmospheres (20 megapascals) pressure is suddenly 
brought to atmospheric pressure by the bursting of a metal membrane in the 
sample cell autoclave. Relaxation times measured spectrophotometrically or 
conductometrically are thus accessible if TAU > 100 microseconds. This 
technique has proven particularly useful in the elucidation of micellar 
systems of great interest for catalysis and for petroleum recovery from 
apparently depleted oil fields. 
    The continuous- and stopped-flow techniques antedate somewhat the 
relaxation techniques described above, and have the sometimes important 
advantage of permitting kinetic measurements in chemical systems far from 
equilibrium. The stopped-flow experiment is one in which two different liquids 
in separate syringes are mixed rapidly in a tangential jet mixing chamber and 
then the rapid flow of mixed reactants is almost immediately brought to a halt 
in a spectrophotometric, conductometric, or calorimetric observation chamber. 
See CHEMICAL THERMODYNAMICS. 
 �34s[Edward M. Eyring]
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�6v�2v�32sChemical ecology   �21m �33s 
    Chemical ecology is concerned with the interaction of organisms mediated 
by the chemicals they produce. In 1959 scientists coined the word pheromone to 
designate any substance produced by an organism that serves to induce a 
behavioral or developmental response in an individual of the same species. 
Pheromones include sex attractants, alarm substances, trail substances, trail 
markers, and aggregation-inducing compounds. Animals and plants produce other 
substances as well, which some behaviorists call allomones and kairomones, 
that transmit information to individuals of different species. 
    Allomones are chemicals produced by an organism which induce in a member 
of another species a behavioral or physiological reaction favorable to the 
emitter. These chemicals are involved in a variety of mutualistic and 
antagonistic relationships. The scents of flowers are mutualistic allomones; 
insects pollinate plants from which they obtain nectar in return. Venoms and 
repellents, which organisms use in defense against predators, are antagonistic 
allomones.  
    Kairomones are chemical substances that benefit the recipient rather than 
the emitter. These compounds elicit positive responses of predators to their 
prey, herbivores to their food plants, and parasites to their hosts. 
    The development and use of synthetic insecticides during this century has 
saved millions of lives from lethal insect-borne diseases and has helped 
farmers to feed a larger part of the world's population. At the same time, 
however, the widespread and often indiscriminate use of toxic chemicals to 
control agricultural and other pests has created serious problems. Continued 
use of these toxins has led to the development of insects resistant to 
insecticides, and insecticide residues have disrupted nontarget ecological 
systems.  
    The trend in pest control has shifted toward ecological pest management, 
usually referred to as integrated control. The goal is not to eradicate a pest 
but to maintain the pest population below levels at which it causes economic 
damage or health hazards (insectistasis). Integrated control employs one or a 
variety of techniques appropriate for a particular species and situation, and 
among these methods is the use of pheromones. 
    Now that the chemical structure of many pheromones has been determined and 
the compounds synthesized, a number of them have been used in integrated 
control programs. For example, sex and food attractants of certain moths and 
beetles that destroy stored food products have been used for the early 
detection of infestations in granaries, mills, and warehouses, and to prevent 
the increase of pest populations by mass trapping. The use of these pheromone 
traps minimizes the need for treatment with insecticides. Sex pheromones have 
been used to disrupt sexual communication among moths and to prevent mating. 
In this method the sex pheromones are evaporated from many sources to confuse 
the males and prevent their locating conspecific females. The technique has 
shown promise in control of the plum fruit moth, codling moth, larch bud moth, 
and grape moth. 
    The odorous secretions of mammals have been of interest to perfume makers 
for ages, but little is known about their role in animal communication, and 
the chemical structure of the compounds is largely unknown. Studies indicate 
that mammals do not give an automatic, standardized response to their odor 
signals in the way that insects react to their pheromones, and the term 
semiochemicals (chemical or odor signals) has been suggested for mammalian 
scents.  
    In mammals, only the male pheromone of the domestic pig has been applied 
to practical purpose. The saliva and sweat glands of the sexually aroused boar 
contain two steroids which induce the mating stance in estrous sows. The 
compounds have been used to facilitate the artificial insemination of pigs. 
See INSECT CONTROL, BIOLOGICAL; SOCIAL INSECTS. 
 �34s[Louis M. Roth]
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�6v�2v�32sChemical energy   �21m �33s 
    In most chemical reactions, heat is either taken in or given out. By the 
law of conservation of energy, the increase or decrease in heat energy must be 
accompanied by a corresponding decrease or increase in some other form of 
energy. This other form is the chemical energy of the compounds involved in 
the reaction. The rearrangement of the atoms in the reacting compounds to 
produce new compounds causes a change in chemical energy. This change in 
chemical energy is equal numerically and of opposite sign to the heat change 
accompanying the reaction. 
    Most of the world's available power comes from the combustion of coal or 
of petroleum hydrocarbons. The chemical energy released as heat when a 
specified weight, often 1 g, of a fuel is burned is called the calorific value 
of the fuel. 
    It is not possible to measure an absolute value for the chemical energy of 
a compound; only changes in chemical energy can be measured. It is therefore 
necessary to make some arbitrary assumption as a starting point. One such 
assumption would be to take the chemical energies of the free atoms as zero 
and measure the chemical energies of all elements and compounds relative to 
this standard. If this were done, all chemical energies would then be negative 
quantities. Heat is given out when all elements and compounds are formed from 
their atoms. Since chemical energy can be regarded as a form of potential 
energy, it is interesting that the formation of chemical bonds is always 
accompanied by a decrease of potential energy. The reason, in qualitative 
terms at least, lies in the quantum theory. When chemical bonds are formed 
between atoms, electrons are shared between the two atoms, and this electron 
sharing produces a lowering of the potential energies of the shared electrons, 
and thus a lowering of the potential energy of the molecule relative to the 
free atoms. 
    Although it would be more fundamental to take the chemical energies of the 
separated atoms as zero, in practice it is more convenient to take a more 
arbitrary starting point and to assume that the chemical energies of the 
elements are zero. For precision, the standard states of the elements at 25DGC 
(77DGF) and 1 atm (101.325 kilopascals) pressure are chosen. Thus, in the case 
of carbon the chemical energy of graphite, not diamond, is said to be zero at 
25DGC and 1 atm. Diamond then has a definite enthalpy value. See CHEMICAL 
THERMODYNAMICS; ENERGY SOURCES; ENTHALPY. 
 �34s[Thomas C. Waddington]
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�6v�2v�32sChemical engineering   �21m �33s 
    The branch of engineering serving those industries that chemically convert 
basic raw materials into a variety of products. Starting with ores, salt, 
sulfur, limestone, coal, natural gas, petroleum, air, water, and so forth, 
these industries, by chemical processing techniques, produce widely 
diversified products, such as aluminum, magnesium, and titanium metals; 
refined petroleum fractions, such as fuels and solvents; synthetic 
fertilizers; synthetic fibers, resins, and plastics; antibiotics; wood pulp 
and paper; and petrochemicals. See CHEMICAL CONVERSION; CHEMICAL PROCESS 
INDUSTRY.  
    To bring about the conversion of raw materials into such a broad spectrum 
of products, the chemical engineer starts with basic scientific data on 
chemical reactions discovered by research chemists. Working first on a small 
laboratory scale, the engineer develops engineering data for various steps in 
the process of changing the starting material into a product. Before 
completing this small-scale investigation, the next step may involve actual 
operation of various parts of the process in mini- or pilot-scale equipment, 
simulating what may happen in the commercial-scale process. See PILOT 
PRODUCTION.  
    In carrying out these development functions, the chemical engineer relies 
on previous training in scientific and engineering fundamentals and in 
advanced mathematics to plan the experimental programs and to analyze the 
resulting data. In many cases the engineer will evolve a mathematical model 
that interrelates all the process variables in mathematical terms. Then by 
using electronic computing equipment, the design engineer can attain the 
optimum process design. Ultimately, when the plant is built, an on-line 
computer and sophisticated instrumentation will be employed to control the 
commercial unit. See SYSTEMS ENGINEERING. 
    After completion of the plant, chemical engineering continues to play a 
vital role in the start-up and operation of the facility. Once in operation, 
the manufacturing unit becomes a continuing problem in optimization to achieve 
desired product output and specified quality at lowest cost in the face of 
constantly shifting variables of raw material supply, availability and quality 
of personnel, condition of plant equipment, and market conditions. 
    For their professional group activities, chemical engineers work to a 
large extent through the American Institute of Chemical Engineers. 
Headquartered in New York, the AIChE has more than 100 local sections 
throughout the United States. It also supports an extensive technical 
publishing program. 
 �34s[Calvin S. Cronan]
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�6v�2v�32sChemical equilibrium �21m �33s
    In a dynamic or kinetic sense, chemical equilibrium is a condition in
which a chemical reaction is occurring at equal rates in its forward and
reverse directions, so that the concentrations of the reacting substances do
not change with time. In a thermodynamic sense, it is the condition in which
there is no tendency for the composition of the system to change; no change
can occur in the system without the expenditure of some form of work upon it.
From the viewpoint of statistical mechanics, the equilibrium state places the
system in a condition of maximum freedom (or minimum restraint) compatible
with the energy, volume, and composition of the system. The statistical
approach has been merged with thermodynamics into a field called statistical
thermodynamics; this merger has been of immense value for its intellectual
stimulus, as well as for its practical contributions to the study of
equilibria. See CHEMICAL THERMODYNAMICS; STATISTICAL MECHANICS.
    Of the three viewpoints, the thermodynamic approach is by far the most
powerful and fruitful in treating the quantitative relationships between the
position of equilibrium and the factors which govern it. Since thermodynamics
is concerned with relationships among observable properties, such as
temperature, pressure, concentration, heat, and work, the relationships
possess general validity, independent of theories of molecular behavior.

    Chemical potential.  Thermodynamics attributes to each chemical substance
a property called the chemical potential, which may be thought of as the
tendency of the substance to enter into chemical (or physical) change.
Although the chemical potential of a substance cannot be directly measured
(except on a relative basis), differences in chemical potential are
measurable. (The units are those of energy per mole.)
    The importance of the chemical potential lies in its relation to the
affinity or driving force of a chemical reaction. Consider general reaction
(1). Let mu(sub A) be the chemical potential per mole

			   aA = bB <--> gG + hH 			   (1)

of substance A, mu(sub B) be the chemical potential per mole of B, and so on.
Then, according to one of the fundamental principles of thermodynamics (the
second law), the reaction will be spontaneous when the total chemical
potential of the reactants is greater than that of the products. Thus, for
spontaneous change (naturally occurring processes) notation (2) applies.

[g mu(sub G) + h mu(sub H)] - [a mu(sub A) + b mu(sub B)] < 0		  (2)

When equilibrium is reached, the total chemical potentials of products and
reactants become equal; thus Eq. (3) holds at

[g mu(sub G) + h mu(sub H)] - [a mu(sub A) + b mu(sub B)] = 0		  (3)

equilibrium. The difference in chemical potentials in Eqs. (2) and (3) is
called the driving force or affinity of the process or reaction; naturally, it
is zero when the chemical system is in chemical equilibrium.
    For reactions at constant temperature and pressure (the usual restraints
in a chemical laboratory), the difference in chemical potentials becomes equal
to the free energy change DELTA G for the process in Eq. (4). The decrease in
free energy represents the maximum net work obtainable from the process. When
no more work is obtainable, the system is at equilibrium. Conversely, if the
value of DELTAG for a process is positive, some useful work will have to be
expended upon the process, or reaction, in order to make it proceed; the
process cannot proceed naturally or spontaneously. (The term spontaneously as
used here implies only that a process can occur. It does not imply that the
reaction will be rapid or instantaneous. Thus, the reaction between hydrogen
and oxygen is a spontaneous process in the sense of the term as used here,
even though a mixture of hydrogen and oxygen can remain unchanged for years
unless ignited or exposed to a catalyst.)

DELTA G = [g mu(sub G) + h mu(sub H)] - [a mu(sub A) + b mu(sub B)]	  (4)

    Since by definition a catalyst remains unchanged chemically through a
reaction, its chemical potential does not appear in Eqs. (2), (3), and (4). A
catalyst, therefore, can contribute nothing to the driving force of a
reaction, nor can it, in consequence, alter the position of the chemical
equilibrium in a system. See CATALYSIS.
    In addition to furnishing a criterion for the equilibrium state of a
chemical system, the thermodynamic method goes much further. In many cases, it
yields a relation between the change in chemical potentials (or change in free
energy) and the equilibrium concentrations of the substances involved in the
reaction. To do this, the chemical potential must be expressed as a function
of concentration (and other properties of the substance). See CONCENTRATION
SCALES.

    Activity and standard states.  It is often convenient to utilize the
product fx, called the activity of the substance and defined by a = fx. The
activity may be looked upon as an effective concentration of the substance,
measured in the same units as the concentration x with which it is associated.
The standard state of the substance is then defined as the state of unit
activity (where a = 1) and is characterized by the standard chemical potential
mu(sup 0). Clearly, the terms mu(sup 0), f, and x are not independent; the
choice of  the activity scale serves to fix the standard state. For example, for
an  aqueous solution of hydrochloric acid, the standard state for the solute
(HCl) would be an (hypothetical) ideal 1 molar (or molal) solution, and for the
solvent (H2O) the standard state would be pure water (mole fraction = 1). The
reference state would be an infinitely dilute solution; here the activity
coefficients would be unity for both solute and solvent. For the vapor of HCl
above the solution, the standard state would be the ideal gaseous state at 1
atm (101.325 kilopascals) partial pressure; the reference state would be a
state of zero pressure. (For gases, the term fugacity is used instead of
activity.)
    It should be noted that the reference state is a limiting state which in
many cases can be reached only through an extrapolation from observed
behavior. See FUGACITY.

    Equilibrium constant.  If general reaction (1) occurs at constant
temperature T and pressure P when all of the substances involved are in their
standard states of unit activity, Eq. (4) would become Eq. (5). The quantity
DELTA G(sup 0) is known as the standard free energy change for the reaction at
that temperature and pressure for the chosen standard states. (Standard state
properties are commonly designated by a superscript, DELTA G(sup 0), mu(sup 0).
Since each of the standard chemical potentials mu(sup 0) is a unique property
determined by the temperature, pressure, standard state, and chemical identity
of the substance concerned, the standard free energy change DELTA G(sup 0) is a
constant (parameter) characteristic of the particular reaction for the chosen
temperature, pressure, and standard states.

DELTA G(sup 0) = [g mu(sub G)(sup 0) + h mu(sub H)(sup 0) - [a mu(sub A)(sup 0)
		 + b mu(sub B)(sup 0)					(5)

    When the system has come to chemical equilibrium at constant temperature
and pressure, DELTA G = 0, and DELTA G(sup 0) is given by Eq. (6), where the
value
	      DELTA G(sup 0) = -RT ln K(sup 0)


K(sup 0) = [a(sub G)(sup g)a(sub H)(sup h)]/[a(sub A)(sup a)a(sub B)(sup b)  (7)

of K(sub 0) is shown as Eq. (7), and the activities are the equilibrium values.
The ratio of the activities at equilibrium, K(sup 0), is called the equilibrium
constant or, more precisely, the thermodynamic equilibrium constant. (The
terms K(sup 0) and Q(sup 0) are written with superscripts to emphasize that they
represent ratios of activities.) The equilibrium constant is a characteristic
property of the reaction system, since it is determined uniquely in terms of
the standard free energy change. The term -DELTA G(sup 0) represents the maximum
net work which the reaction could make available when carried out at constant
temperature and pressure with the substances in their standard states.
    The kinetic concept of chemical equilibrium introduced by C. M. Guldberg
and P. Waage (1864) led to the formulation of the equilibrium constant in
terms of concentrations. Although the concept is correct in terms of the
dynamic picture of opposing reactions occurring at equal speeds, it has not
been successful in coping with the problems of activity coefficients.
Conversely, the thermodynamic approach yields no relationship between the
driving force of the reaction and the rate of approach to equilibrium. See
CHEMICAL DYNAMICS.
    The influence of temperature upon the chemical potentials, and hence upon
the equilibrium constant, is given by the Gibbs-Helmholtz equation, Eq. (8).
The derivative on the left represents the slope of the curve obtained when
values of ln K(sup 0) for a reaction, obtained at different temperatures but
always at the same pressure P, are plotted against temperature. The standard
heat of reaction DELTA H(sup 0) for the temperature T at which the slope is
measured is the heat effect which could also be observed by carrying out the
reaction involving the standard states in a calorimeter at the corresponding
temperature and pressure. See HEAT CAPACITY; THERMOCHEMISTRY.

[(d ln K(sup 0))/dT] (in terms of P) = (DELTA H(sup 0))/RT(sup 2)	     (8)


			     N2 + 3H2 <--> 2NH3 			     (9)


    Homogeneous equilibria.  These involve single-phase systems: gaseous,
liquid, and solid solutions. In most cases, solid solutions are so far from
ideal that equilibrium constants cannot be evaluated, and such systems are
treated in terms of the phase rule. A typical gas-phase equilibrium is the
ammonia synthesis shown in reaction (9). A typical liquid-phase equilibrium is
the dissociation of acetic acid in water, reaction (10.)

	     HC2H3O2 + H2O <--> H2O(sup +) + C2H3O2(sup -)		    (10)

    The solvent appears to be inert, since its chemical potential remains
practically unchanged over the useful concentration range. As a result of this
apparent inertness of the solvent, it is not possible to determine the extent
of hydration of any dissolved species from equilibrium studies. Thus, whether
the actual ion is H(sup +), H3O(sup +), or H9O4(sup +), it is the total
stoichiometric	concentration that is measured and used. See IONIC EQUILIBRIUM.

    Heterogeneous equilibria.  These are usually studied at constant pressure,
since at least one of the phases will be a solid or liquid. The imposed
pressure may be that of an equilibrium gaseous phase, or it may be an
externally controlled pressure.
    In describing such systems, the nature of each phase must be specified. In
the following example, the terms s, l, and g identify solid, liquid, and
gaseous phases, respectively. For solutions or mixtures, the composition is
needed, in addition to the temperature and pressure, to complete the
specification of the system. If not obvious, the identity of the solvent must
be given.

			       H2O(l) <--> H2O(g)			    (11)


  DELTA G(sup 0) = mu(sub g)(sup 0) - mu(sub l)(sup 0) = RT ln (p/N)	    (12)

    In the equilibrium shown as reaction (11), the relationship of Eq. (12)
holds. Here K(sup 0) = p/N, the ratio of the vapor pressure p to the liquid mole
fraction N. For pure water, the equilibrium constant is simply the standard
vapor pressure p(sup 0), and the Clausius-Clapeyron equation is just a special
case of the Gibbs-Helmholtz equation, Eq. (8). Now when a small amount of
solute is added, decreasing the mole fraction of solvent, the vapor pressure p
must be lowered to maintain equilibrium (Raoult's law). The effect of the
total applied pressure P upon the vapor pressure p of the liquid is given by
the Gibbs-Poynting equation, Eq. (13). Here V(sub l) and V(sub g) are the molar
volumes of liquid and vapor. The vapor pressure will increase as external
pressure is applied (activity increases with pressure). If the external
pressure is applied to a solution by a semipermeable membrane, an applied
pressure can be found which will restore the vapor pressure (or activity) of
the solvent to its standard state value. See OSMOSIS.

		    [dp/dT] (in terms of T) = Vl/Vg			(13)

    Other heterogeneous equilibria are solubility equilibria, reactions
involving two immiscible phases, and reactions involving condensed and
immiscible phases. See EXTRACTION; SOLUBILITY PRODUCT CONSTANT.
 �34s[Cecil E. Vanderzee]
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�6v�2v�32sChemical fuel   �21m �33s 
    The principal fuels used in internal combustion engines (automobiles, 
diesel, and turbojet) and in the furnaces of stationary power plants are 
organic fossil fuels. These fuels, and others derived from them by various 
refining and separation processes, are found in the earth in the solid (coal), 
liquid (petroleum), and gas (natural gas) phases. 
    Special fuels to improve the performance of combustion engines are 
obtained by synthetic chemical procedures. These special fuels serve to 
increase the  specific impulse of the engine or to increase the heat of 
combustion available to the engine per unit mass or per unit volume of the 
fuel. A special fuel which possesses a very high heat of combustion per unit 
mass is liquid hydrogen. It has been used along with liquid oxygen in rocket 
engines. Because of its low liquid density, liquid hydrogen is not too useful 
in systems requiring high heats of combustion per unit volume of fuel 
(``volume-limited'' systems). 
    A special fuel which produces high flame temperatures of the order of 
5000DGF is gaseous cyanogen. This is used with gaseous oxygen as the oxidizer. 
The liquid fuel hydrazine, and other hydrazine-based fuels, with the liquid 
oxidizer nitrogen tetroxide are used in many space-oriented rocket engines. 
The boron hydrides, such as diborane and pentaborane, are high-energy fuels 
which are used in advanced rocket engines. 
    For air-breathing propulsion engines (turbojets and ramjets), hydrocarbon 
fuels are most often used. For some applications, metal alkyl fuels which are 
pyrophoric (that is, ignite spontaneously in the presence of air), and even 
liquid hydrogen, are being used. See METAL-BASE FUEL. 
    Fuels which liberate heat in the absence of an oxidizer while decomposing 
either spontaneously or because of the presence of a catalyst are called 
monopropellants and have been used in rocket engines. Examples of these 
monopropellants are hydrogen peroxide and nitro-methane. 
    Liquid fuels and oxidizers are used in most large-thrust rocket engines. 
When thrust is not a consideration, solid-propellant fuels and oxidizers are 
frequently employed because of the lack of moving parts such as valves and 
pumps, and the consequent simplicity of this type of rocket engine. Solid 
fuels fall into two broad classes, double-base and composites. Double-base 
fuels are compounded of nitroglycerin (glycerol trinitrate) and 
nitrocellulose, with no separate oxidizer required. The double-base propellant 
is generally formed in a mold into the desired shape (called a grain) required 
for the rocket case. Composite propellants are made of a fuel and an oxidizer. 
The latter could be an inorganic perchlorate or a nitrate. Fuels for composite 
propellants are generally the asphalt-oil-type, thermosetting plastics or 
several types of synthetic rubber and gumlike substances. Metal particles such 
as boron, aluminum, and beryllium have been added to solid propellants to 
increase their heats of combustion and to eliminate certain types of 
combustion instability. See HYDROGEN PEROXIDE; LIQUID FUEL; ROCKET ENGINE. 
 �34s[Wallace Chinitz]
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�6v�2v�32sChemical laser �21m �33s
    A device in which laser action is obtained from the energy released in
some fast chemical reactions. Atoms or molecules produced during the reaction
are often in excited states. Under special circumstances there may be enough
atoms or molecules excited to some particular state for amplification to occur
by stimulated emission. Usually the reacting gases are mixed and then ignited
by ultraviolet light or fast electrons. Both continuous infrared output and
pulses up to several thousand joules of energy have been obtained in reactions
which produce excited hydrogen fluoride molecules. Pulsed laser action in the
ultraviolet (193 to 353 nm) has been obtained from excimer states of rare gas
monohalides (for example, KrF, XeF, KrCl, XeCl, XeBr).	See  LASER.
 �34s[Stephen F. Jacobs ;Arthur L. ]
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�6v�2v�32sChemical microscopy �21m �33s
    Application of the microscope to the solution of chemical problems. Such
problems encompass identification, correlation of structure and performance,
troubleshooting, and process control. Several axioms, based on experience,
bolster the chemical microscopist's confidence: (1) Apples, oranges, azaleas,
or robins are quickly and easily recognized; hence any tiny particles of
cement, abrasive, pollen, and so on, magnified microscopically to the size of
fruits, flowers, or birds, can also be quickly and easily recognized. (2) The
characteristic behavior of adhesives, lubricants, abrasives, and so on is
manifest and can be observed at microscopic sites. Such behavior is, then,
best investigated, explained, and improved by the chemical microscopist. (3)
Any large-scale process, be it baking a cake, cooking meat, solid-state
reactions at high temperatures (for example, reaction of sand and carbon to
form carborundum), electroplating, or extrusion of a synthetic fiber, can be
duplicated and studied on a microscope stage. See MICROSCOPE.
    The identification of small particles is of interest in air or water
pollution, clean-room control, general contamination problems, and
criminalistics. The particles may include minerals, fibers, pollens, protozoa,
and fly ash or any other industrial dust. Each has characteristic shape and
optical properties, and sizes and weights can be identified down to about 1
MUm (0.001 mm or 1/25,000 in.) and 1 pg (picogram, or 10- sup 1 sup 2g); about
10 sup 1 sup 5 such particles would weigh approximately 1 kg.
 �34s[Walter C. McCrone]
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�6v�2v�32sChemical process industry   �21m �33s 
    This term, abbreviated CPI, is now applied to what has long been known as 
the chemical and allied industries. It includes industries engaged in the 
manufacture of chemicals both inorganic and organic, petroleum, 
petrochemicals, rubber, plastics, fertilizers (agrichemicals), sugar and 
starch, paper, pulp, synthetic fibers, photography, pharmaceuticals, 
explosives, food additives and flavors, and dyes. See CHEMICAL ENGINEERING; 
PETROLEUM ENGINEERING. 
 �34s[Joseph A. Brink, Jr.]
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�6v�2v�32sChemical senses   �21m �33s 
    The senses of smell (olfaction) and taste and the so-called common 
chemical sense make up the three chemical senses. The sense cells of olfaction 
and taste are specialized receptor neural elements. The receptors of common 
chemical sense appear to be undifferentiated, free nerve endings. These are 
distributed throughout the moist mucous membranes of land-dwelling animals and 
over the skin of aquatic vertebrates. In order of thresholds, olfaction is the 
most sensitive, taste is intermediate, and common chemical sensitivity is 
least.  
    The chemical senses mediate the selection and acceptance of foodstuffs, 
the avoidance of irritants, and, in lower organisms especially, the detection 
of enemies and prey and the selection of mates. 
    The common chemical senses are considered to be sensitivity to mildly 
irritating chemicals like dilute solutions of alkali, acids, and salts. Such 
chemical sensitivity is distinct from tactile sensitivity and, in the mouth 
and nose, is distinct from taste and smell. See CHEMORECEPTION; OLFACTION; 
SENSATION; TASTE. 
 �34s[Carl Pfaffman]
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�6v�2v�32sChemical separation techniques   �21m �33s 
    A method used in chemistry to purify substances or to isolate them from 
other substances, for either preparative or analytical purposes. In industrial 
applications the ultimate goal is the isolation of a product of given purity, 
whereas in analysis the primary goal is the determination of the amount or 
concentration of that substance in a sample. There are three factors of 
importance to be considered in all separations: (1) the completeness of 
recovery of the substance being isolated, (2) the extent of separation from 
associated substances, and (3) the efficiency of the separation. 
    There are many types of separations based on a variety of properties of 
materials. Among the most commonly used properties are those involving 
solubility, volatility, adsorption, and electrical and magnetic effects, 
although others have been used to advantage. The most efficient separation 
will obviously be obtained under conditions for which the differences in 
properties between two substances undergoing separation are at a maximum. 
    The common aspect of all separation methods is the need for two phases. 
The desired substance will partition or distribute between the two phases in a 
definite manner, and the separation is completed by physically separating the 
two phases. The ratio of the concentrations of a substance in the two phases 
is called its partition or distribution coefficient. If two substances have 
very similar distribution coefficients, many successive steps may be required 
for a separation. The resulting process is called a fractionation. 
    Based on the nature of the second phase, the more commonly used methods of 
separation are classified as follows: 
    1. Methods involving a solid second phase include precipitation, 
electrodeposition, chromatography (adsorption), ion exchange, and 
crystallization.  
    2. The outstanding method involving a liquid second phase is solvent 
extraction, in which the original solution is placed in contact with another 
liquid phase immiscible with the first. 
    3. Methods involving a gaseous second phase include gas evolution, 
distillation, sublimation, and gas chromatography. Mixtures of volatile 
substances can often be separated by fractional distillation. See EXTRACTION; 
MASS-TRANSFER OPERATION. 
 �34s[George H. Morrison]
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�6v�2v�32sChemical structures �21m �33s
    Much of chemistry is explainable in terms of the structures of chemical
compounds. The understanding of the structures of chemical compounds hinges
very strongly on the understanding of the electronic configurations of the
elements. The union of atoms, and therefore the formation of compounds from
the elements, is associated with interactions among the extranuclear electrons
of the individual atoms. Electronic interactions among atoms may occur in
either of two ways: Electrons may be transferred from one atom to another, or
they may be shared by two (or more) atoms. The first type of interaction is
called electrovalence and results in the formation of electrically charged
monatomic ions. The second, covalence, leads to the formation of molecules and
complex ions. See CHEMICAL BONDING.

    Ions.  A number of relationships among the electronic structures of the
elements are important in determining the properties of ions.
    1. Only the outermost shell of electrons is of significance in compound
formation. This is commonly called the valence shell, and the electrons
contained within it are called the valence electrons.
    2. The maximum number of electrons encountered in the valence shell of any
single isolated atom is eight.
    3. In the case of transition elements, the electrons in the d subshell of
the penultimate (next outermost) shell are often of importance in compound
formation and are considered as essentially a part of the valence shell.
Consequently, rules 1 and 2 are rigorously true only for the representative
elements, not the transition elements.
    4. The number of valence electrons and their subshell assignments are
given by the arrangement of the elements in the long form of the periodic
table. The elements in any vertical family of the periodic table have the same
valence-electron configuration. See ATOMIC STRUCTURE AND SPECTRA; ELECTRON
CONFIGURATION; PERIODIC TABLE.
 �34s[Daryle H. Busch]
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			  M(g) -> M+(g) + e-				(1)

    Positive ions are formed by the removal of valence electrons, and negative
ions result from the addition of electrons to the neutral atom. The complete
analysis of the energy relationships determining the formation of compounds
composed of ions is complex; however, a few aspects of the problem are
directly related to the electronic structures of the isolated atoms. The
energy required for the removal of an electron from a neutral atom is measured
by the ionization potential and relates to reaction (1). A second ionization
potential indicates the amount of energy required to convert the unipositive
ion into the dipositive ion, and the successive removal of the remaining
electrons may similarly be related to ionization potentials. Typical positive
ions will be formed by the removal of all of the valence electrons of the
metallic elements of the ``a''-subgroups of the periodic table. The resulting
ions are called inert-gas-type ions because they have the electronic
configurations of the inert gases preceding the respective elements in the
periodic system. As a consequence of the increasing amounts of energy
(ionization potentials) required to remove successive electrons from ions,
monatomic positive ions that are highly charged do not exist among chemical
compounds under the usual conditions of temperature and pressure. The most
highly charged ions of any apparent significance are tripositive. A number of
the actinide elements form tetrapositive monatomic ions, Usup4supp,
Thsup4supp, and Pusup4supp. See IONIZATION POTENTIAL.
    In addition to positive ions of the inert-gas type, two other electronic
configurations recur with some regularity among chemical compounds. The
elements of groups Ib and IIb may lose their valence electrons, forming ions
having 18 electrons (instead of 8) in their outermost electron shells. These
ions are generally called pseudoinert-gas-type ions.

			 M(g) + e- -> M-(g)				    (2)

    The addition of an electron, as shown in reaction (2), to a neutral atom
with the resulting formation of a negative ion may be associated with the
evolution of energy (the electron affinity). It is generally true, however,
that the addition of a second electron will require the expenditure of energy.
This relationship militates against the formation of highly charged negative
ions. The known, monatomic negative ions are formed by the addition of the
number of electrons necessary for the atom to attain an inert gas electronic
configuration, and no monatomic negative ions are known to have a charge more
negative than 3-. See ELECTRONEGATIVITY.
 �34s[Ralph G. Pearson]
�33s

    Polyhedral structures.  In considering structures more complex than those
derived from simple monatomic ions, the next logical step is to consider
single polyhedral aggregates of atoms. At this point it is worth noting that
the term ``structure'' now takes on a much more quantitative meaning. In its
most precise sense structure is used to denote a knowledge of the bonding
distances and angles between atoms in chemical compounds and, in turn, the
geometrical arrangements which they form. These atomic arrangements and the
associated distances and angles serve uniquely as ``fingerprints'' of these
atoms' spatial configurations, and depend very much on understanding the
electronic configurations around atoms because the union of atoms to form
compounds occurs via electronic interactions of one atom with another through
their valence electrons. The chemical combination of neutral atoms to produce
uncharged species results in molecule formation, whereas the similar
combination of atoms or ions possessing a net charge results in the formation
of complex ions. A basic understanding of the species formed involves the
concept of the coordination polyhedron, which allows a simple classification
of the structures of many polyatomic molecules and ions. This type of
classification is particularly useful because it conveniently explains the
packing together of simple chemical molecules or ions in terms of highly
symmetrical polyhedra. There is an obvious connection between polyhedra and
the structures found in crystalline solids formed from them. Crystal formation
often involves the linking of convex polyhedra by the sharing of corners,
edges, or faces, ultimately forming space-filling assemblies in which all
faces of each polyhedron are in contact with faces of other polyhedra. The
most important simple polyhedrons are the tetrahedron, the trigonal bipyramid,
the octahedron, the pentagonal bipyramid, and the cube (Fig. 1). The most
commonly observed of these polyhedral configurations are the tetrahedron (four
faces) and the octahedron (six faces).

    The general properties of a coordination polyhedron are summarized in the
identification of a central atom, which in many cases resides at the center of
mass of the polyhedron, and the determination of the coordination number of
the central atom. The coordination number is equal to the number of groups
(atoms, ions) directly attached to the central atom. In the simplest cases,
the coordination number is also equal to the number of apexes in the
coordination polyhedron. It is therefore concluded that the tetrahedron may
occur among molecules and ions having a coordination number of 4; the trigonal
bipyramid is associated with a coordination number of 5; the octahedron, with
a coordination number of 6; and the pentagonal bipyramid, with a coordination
number of 7.
    Lower coordination numbers are associated with plane polygons rather than
polyhedrons, as shown in Fig. 2. The coordination number of 4 may be
associated with either a polygon (square plane) or a polyhedron (tetrahedron).
Similarly, alternative structures exist for the coordination numbers 5
(tetragonal pyramid), 6 (trigonal prism), 7 (irregular structures), and 8
(Archimedean antiprism, dodecahedron); however, these alternative structures
are rare in occurrence. See COORDINATION CHEMISTRY; MOLECULAR ORBITAL THEORY;
POLYHEDRON.

    The simplest correlative device which accurately summarizes a very large
number of such structures and enables the chemist to predict, with a good
chance of success, the geometric array of the atoms in a compound of known
composition is based on an extreme electrostatic model. This model, or theory,
represents the bonds in a purely formal way. The central atom is considered to
be a positive ion having a charge equal to its oxidation state. The groups
attached to the central atom (the ligands) are then treated either as negative
ions or as neutral dipolar molecules.
    A knowledge of the electronic structures of the elements, the sizes of the
ions, and their relationships within coordination polyhedrons permits
prediction of the geometric structures of binary compounds and complex ions
for given central atoms and ligands. Such a treatment may be applied to binary
halides and oxides and complex oxyanions and haloanions as well as most
polyatomic aggregates containing nontransition ions as central elements. In
addition, the oxyanions and molecular halides of the transition elements may
be treated similarly. Many of the hydrated and ammoniated metallic ions may
also be discussed in these terms. Among the particularly useful results of
these considerations is the prediction of which compounds have electron pairs
that might be shared with molecules or ions having the ability to expand their
coordination spheres. In this way the Lewis acids and bases are predicted from
their structures. See ACID AND BASE.
    The most significant limitation of such considerations is the complete
lack of concern with the detailed nature of the chemical bonds uniting the
central atom to its ligands in a coordination polyhedron. Ions of high charge
probably do not exist in chemical compounds, and physical properties of many
substances are not explainable in terms of simple electrostatic relationships.
The quantum-mechanical theories of valence provide the more detailed
descriptions of the manner of bonding required to overcome these limitations.
Some classes of compounds cannot be systematized, even from the standpoint of
their geometric structures, without the use of these more advanced concepts.
Two major theories exist for the application of the idea of the covalent bond
to the structures of chemical compounds. One of these is the so-called
valence-bond concept.
    According to the valence-bond theory, the principal requirements for the
formation of a covalent bond are a pair of electrons and suitably oriented
electron orbitals on each of the atoms being bonded. The geometry of the atoms
in the resulting coordination polyhedron is correlated with the orientation of
the orbitals on the central atom. The orbitals utilized depend on the energies
of the electrons in them. In general, the order of increasing energy of the
electron orbitals is (n - 1)d < ns < np < nd. It is concluded that a
nontransition atom having one valence electron will form a covalent bond
utilizing an s orbital. In those cases where an unshared pair of electrons may
be assigned to the ns orbital, as many as three equivalent bonds may be formed
by utilizing the three np orbitals of the central atom. See CRYSTAL FIELD
THEORY; VALENCE.
    With regard to the nature of doubly bonded compounds, another problem
arises when such structures are viewed from the standpoint of valence-bond
theory. In some species nonequivalent bonds are predicted, but this is not
found to be true experimentally so long as the similar atoms in the structure
are otherwise equivalent. To account for such facts as these, the concept of
resonance must be introduced. The resonance method of describing this
situation is to say that one of the pictorial structures is inadequate to
describe the substance, properly but that enough pictorial structures
(resonance structures) should be considered to permute the double bond about
all the equivalent bonds. The true structure is assumed to be something
intermediate to all the resonance structures and more stable than any of them
because it exists in preference to any one of them. See CONJUGATION AND
HYPERCONJUGATION; RESONANCE (MOLECULAR STRUCTURE).
    The valence-bond theory may be utilized to systematize structures
involving unshared pairs of electrons in a manner very similar to that of the
resonance method. According to this view, the hybridized orbitals of the
central atom determine a total polyhedral structure, but either bonds or
unshared electron pairs may make use of the hybridized orbitals.

    Condensed polyhedral structures.  In the preceding discussions of
polyatomic molecules and ions, attention has been confined to those species in
which single coordination polyhedrons effectively account for the geometric
structures. Understanding of these species is central to the problem of the
structures of chemical compounds, but does not provide an adequate basis for
the descriptions of the structures of all or even a majority of the known
compounds. Among the largest numbers of structures not covered by these
concepts are those in which two or more coordination polyhedrons are united to
form a more complex total structure. Large numbers of the compounds involving
such condensed polyhedrons fall naturally into the subject of solid-state
chemistry.
    The classic homologous series of compounds in organic chemistry provide
useful examples involving the condensation of polyhedrons containing the same
central element in the individual units. The series of saturated or aliphatic
hydrocarbons, having the general formula CsubnH2subn+2, contains a large
number of compounds extending from the lowest member, methane, CH4, to
polyethylene, a plastic of economic importance in which n is a very large
number. Two ways exist for the linking together of these tetrahedrons. This
gives rise to two molecular forms, both of which are stable, well-known
compounds. It is an essential part of these structures that each C -- C link
is linear (because it is merely a SIGMA-bond); however, when two carbon atoms
are linked to a third, the C -- C -- C angle is essentially determined by the
bond angle of the central carbon atom (that is, the other carbons may be
treated as ligands to the first). The other familiar homologous series of
organic chemistry differ from the saturated hydrocarbons in having at least
one unique coordination polyhedron of a different type. The olefins contain
two doubly bonded, or unsaturated, carbon atoms, whose polyhedral structures
are trigonal planar, and the remainder of the carbons are tetrahedral. As in
the case of the aliphatic hydrocarbons, the olefins exhibit an isomerism which
is associated with the branching of the chain structure. In addition, the
presence of two linked trigonal planar carbon atoms and the fact that the
polyhedrons cannot rotate about the double bond give rise to a different kind
of isomerism, called cistrans isomerism. See BOND ANGLE AND DISTANCE.
    The existence of a predicted isomerism provides one of the most important
confirmations of theories of chemical structure. In general, the polyhedral
view of molecular structure, as described here, has been thoroughly verified
by the discovery of the many types of predicted isomerism. The first really
convincing proof of the tetrahedral structures of saturated carbon atoms
involved optical isomerism. See MOLECULAR ISOMERISM; OPTICAL ACTIVITY.
    The property of an element to link to itself, which is so common for
carbon, is called catenation. Other elements exhibit this property to a lesser
extent. For example, nitrogen forms hydrazine, NH2 - NH2, the homolog next
higher than NH3; however, the remaining members of the series NsubnHsubn+2 are
unknown. Similarly, water, H2O, and hydrogen peroxide, HO - OH, constitute the
beginning of a homologous series of which only these two members are known.
    A second type of link is commonly involved in joining coordination
polyhedrons into more complex structures. This involves the interaction of
bridge-forming atoms to link other coordination polyhedrons instead of direct
union of the central atoms. The siloxanes provide examples of this behavior.
These interesting and useful substances are related to the silicates in
structure. The polyphosphates also involve the sharing of corners by PO4
tetrahedrons through oxygen bridges. See COMPLEX COMPOUNDS.
 �34s[Jack M. Williams]
�33s

    Electron-deficient structures.  A significant number of chemical compounds
are known in which the bonding cannot be accurately described by the
utilization of a pair of electrons for each bond and between contiguous atoms.
Rather, examples exist where the total number of valence-electron pairs is
less than the number of bonds. Such molecules are said to have
electron-deficient structures which are characterized by the formation of an
electron-pair bond between more than two atomic centers. An example is
diborane, B2H6, the simplest boron hydride. A total of only 12 valence
electrons (3 from each boron atom and 1 from each hydrogen atom) is available
to form what, under normal circumstances, would be 8 covalent bonds. The
bonding in this electron-deficient case can be understood by postulating the
existence of a three-center two-electron bond for each B--H--B bridge. That
is, the three atoms are joined together by a single electron pair. The
formation of such multicenter bonds in response to electron deficiency
frequently leads to molecular structures which are cyclic or cagelike. An
example involving metal-to-carbon bonds (an organometallic compound) is
trimethylaluminum. Electron deficiency leads to a dimeric structure with
bridging methyl groups. The bridges are joined by three-center two-electron
bonds. See BORANE; ORGANOMETALLIC COMPOUND.
 �34s[Tobin J. Marks]
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�6v�2v�32sChemical symbols and formulas �21m �33s
    A system of symbols and notation for the 106 chemical elements and the
combinations of these elements which form numerous chemical compounds. This
system consists of letters, numerals, and marks that are designed to denote
the chemical element, formula, or structure of the molecule or compound. These
symbols give a concise and instantly recognizable description of the element
or compound. In many cases, through the efforts of international conferences,
the symbols are recognized throughout the scientific world, and they greatly
simplify the universal language of chemistry.
    The presently known chemical elements have been given symbols, usually
derived from the name of the element. Examples of names and symbols are
chlorine, Cl; fluorine, F; beryllium, Be; aluminum, Al; oxygen, O; and carbon,
C. However, symbols for some elements are derived from Latin or other names
for the element. Examples are Au, gold (from aurum); Fe, iron (from ferrum);
Pb, lead (from plumbum); Na, sodium (from natrium); and K, potassium (from
kalium). The symbols consist of one or, more commonly, two letters. The first
letter is always a capital letter and is followed by a lowercase second letter.
    Simple diatomic molecules of a single element are designated by the symbol
for the element with a subscript 2, indicating that it contains 2 atoms. Thus
the hydrogen molecule is H2; the nitrogen molecule, N2; and the oxygen
molecule, O2. Polyatomic molecules of a single element are designated by the
symbol for the element with a numerical subscript corresponding to the number
of atoms in the molecule.
    Diatomic covalent molecules containing unlike elements are given a similar
designation. The formula for hydrogen chloride is HCl. The more
electropositive element is always designated first in the formula.
    For polyatomic covalent molecules containing unlike elements, numerical
subscripts are used to designate the number of atoms of each element present
in the molecule. An example is phosphorus(III) chloride, PCl3. Again, as in
diatomic molecules, the more electropositive element is placed first in the
formula.
    Ionic inorganic compounds are designated by a similar notation. The
positive ion is given first in the formula, followed by the negative ion;
subscripts are again used to denote the number of ions of each element present
in the compound. The formulas for several common compounds are sodium
chloride, NaCl; ammonium nitrate, NH4NO3; and aluminum sulfate, Al2(SO4)3.
    More complex inorganic compounds are designated in a similar manner. The
positive ion is given first, but may contain attached or coordinated groups,
and this is followed by the negative ion. Examples are hexammine cobalt(III)
chloride, [Co(NH3)6]Cl3, and dichlorobis(ethylenediamine)chromium(III)
nitrate, [Cr(en)2Cl2](NO3)3, where en is the abbreviation for ethylenediamine.
Hydrates of inorganic compounds, such as copper(II) sulfate pentahydrate, are
designated by the formula of the compound followed by the formula for water,
the number of water molecules being designated by a numerical prefix. Thus the
symbol for the above compound is CuSO4. 5H2O.
    Because there are many more organic than inorganic compounds, the
designation or notation for the first group becomes exceedingly complex. Many
different types of organic compounds are known.  The system of notation used
must distinguish between the various hydrocarbons as well as setting this
group of compounds apart from others such as alcohols, ethers, amines, esters,
and phenols.
    Organic compounds can be designated by several different methods. The
first is called the empirical formula method. An example is C4H10, butane. The
disadvantage of this system is that it neither describes the structure of the
particular molecule nor distinguishes between isomers. The second method
consists of the designation of a compound by its structural formula, for
example n-butane.

		  H  H	H  H
		  |  |	|  |
	       H--C--C--C--C--H
		  |  |	|  |
		  H  H	H  H
		   n-Butane

    There are many advantages of this method over the first. The exact
structure, providing it is known, is given in a schematic representation. This
removes any ambiguity concerning isomers or other structural variations.
    One of the disadvantages of the second method is that in many cases the
structures cannot be described on a single line of type. Special type is also
needed for the various ring systems and other complicating factors. To
overcome this disadvantage, a third method, the line-formula designation, is
used. Examples of this method are acetone, CH3COCH3; 3-chloropentane,
CH3CH2CHClCH2CH2;  melamine, NC(NH2)NC(NH2)NC(NH2); and malonic acid,
HOOCCH2COOH. However, this method is cumbersome when used with complicated
ring systems. See CHEMISTRY; COMPLEX COMPOUNDS; INORGANIC CHEMISTRY; ORGANIC
CHEMISTRY.
 �34s[William W. Watson]
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�6v�2v�32sChemical thermodynamics   �21m �33s 
    The application of thermodynamic principles to systems involving physical 
and chemical transformations in order to (1) develop quantitative 
relationships among the identifiable forms of energy and their conjugate 
variables, (2) establish the criteria for spontaneous change, for equilibrium, 
and for thermodynamic stability, and (3) provide the macroscopic base for the 
statistical-mechanical bridge to atomic and molecular properties. The 
thermodynamic principles applied are the conservation of energy as embodied in 
the first law of thermodynamics, the principle of internal entropy production 
as embodied in the second law of thermodynamics, and the principle of absolute 
entropy and its statistical thermodynamic formulation as embodied in the third 
law of thermodynamics. 
393 A 
    The basic goal of thermodynamics is to provide a description of a system 
of interest in order to investigate the nature and extent of changes in the 
state of that system as it undergoes spontaneous change toward equilibrium and 
interacts with its surroundings. This goal implicitly carries with it the 
concept that there are measurable properties of the system which can be used 
to adequately describe the state of the system and that the system is enclosed 
by a boundary or wall which separates the system and its surroundings. 
Properties that define the state of the system can be classified as extensive 
and intensive properties. Extensive properties are dependent upon the mass of 
the system, whereas intensive properties are not. Typical extensive properties 
are the energy, volume, and numbers of moles of each component in the system, 
while typical intensive properties are temperature, pressure, density, and the 
mole fractions or concentrations of the components. 
    The concept of a boundary enclosing the system and separating it from the 
surroundings requires specification of the nature of the boundary and of any 
constraints the boundary places upon the interaction of the system and its 
surroundings. Boundaries that restrain a system to a particular value of an 
extensive property are said to be restrictive with respect to that property. A 
boundary which restrains the system to a given volume is a fixed wall. A 
boundary which is restrictive to one component of a system but not to the 
other components is a semipermeable wall or membrane. A system whose 
boundaries are restrictive to energy and to mass or moles of components is 
said to be an isolated system. A system whose boundaries are restrictive only 
to mass or moles of components is a closed system, whereas an open system has 
nonrestrictive walls and hence can exchange energy, volume, and mass with its 
surroundings. Boundaries can be restrictive with respect to specific forms of 
energy, and two important types are those restrictive to thermal energy but 
not work (adiabatic walls) and those restrictive to work but not thermal 
energy (diathermal walls). 
    Changes in the state of the system can result from processes taking place 
within the system and from processes involving exchange of mass or energy with 
the surroundings. After a process is carried out, if it is possible to restore 
both the system and the surroundings to their original states, the process is 
said to be reversible; otherwise the process is irreversible. All naturally 
occurring spontaneous processes are irreversible. The first law defines the 
internal energy as a state function or property of the state of a system, and 
restricts the system and its surroundings to those processes which conserve 
energy. The second law establishes which of the permissible processes can 
occur spontaneously. 
    According to the first law of thermodynamics, the total energy E of a 
system is the sum of its kinetic energy T, its potential energy V, and its 
internal energy U, Eq. (1). If a system has constant mass and its center of 

                           *** E = T + V + U ***                         (1)

mass is moving with uniform velocity in a uniform potential, then changes in 
the total energy of the system DELTA E are equal to changes in its internal 
energy DELTA U. Chemical thermodynamics concentrates on the internal energy of 
the system, but kinetic and potential energy changes of the system as a whole 
can be important for chemical systems. The principle of conservation of energy 
requires that the change in the internal energy of a system be the result of 
energy transfer between the system and its surroundings. The internal energy U 
is a function of the set of extensive variables associated with the various 
forms of internal energy. Each form of internal energy is manifest by the 
product of an extensive variable and its conjugate intensive variable. 
    Thermal energy exchange or heat (that form of energy transferred as a 
result of temperature differences between a system and its surroundings) plays 
a central role in thermodynamics, and is singled out from the other forms of 
energy or work. This is expressed by Eq. (2), where DELTA q is the differential 

                     *** dU = DELTA q + DELTA w ***                      (2)

thermal energy (heat) absorbed by the system from the surroundings and DELTA w 
is the differential work performed on the system by the surroundings. It is 
convenient to write Eqs. (3), where T is temperature, S is the entropy, and 

                     *** DELTA q = TdS - DELTA a ***                    (3a)

                   *** dU = Tds + DELTA w - DELTA a ***                 (3b)

(-DELTA a) is a sum of the nonthermal differential work terms. The term DELTA a 
can be either zero or nonzero. If it is zero, the heat absorbed by the system 
is equal to TdS. In an adiabatic process DELTA q is zero and TdS = DELTA a, and 
hence if DELTA a is nonzero, it must correspond to an internally generated 
thermal energy. This is frequently referred to as the uncompensated heat of a 
process, since it does not result from the transfer of heat from the 
surroundings. See HEAT. 
    The heat capacity of a system is of particular importance in such 
thermochemical calculations. The heat capacity is the amount of thermal energy 
that can be absorbed by a system for a unit rise in temperature. This is 
defined by Eq. (4), where C (sub process) is the heat capacity of a system for 
a given type of process. See HEAT CAPACITY. 
  
                   *** DELTA q = C (sub process) dT ***                  (4)
  
    There are many possible and essentially equivalent statements of the 
second law of thermodynamics. It will suffice to state the empirical result 
that in all spontaneous processes the uncompensated heat DELTA a in Eqs. (3) is 
always positive.  Equation (3a) can be rewritten as Eq. (5), where the term 

                    *** dS = DELTA q/T + DELTA a/T ***                   (5)

DELTA q/T is the contribution to the entropy due to heat exchange with the 
surrounding [(d sub e) S], while DELTA a/T is the contribution to the entropy 
produced internally as a result of the interconversion of work terms 
[(d sub i) S]. The second law can then be summarized as Eqs. (6), where 

                  *** dS = (d sub e) S + (d sub i) S ***                (6a)

              *** (d sub i) S EQUAL TO OR GREATER THAN 0 ***            (6b)

(d sub i) S greater than zero applies to irreversible processes. When 
(d sub i) S = 0, that is, for a reversible process, Eq. (7) holds. This is the 

                    *** dS = DELTA q (sub rev) / T ***                   (7)

basic equation for establishing the thermodynamic temperature scale based upon 
the theoretical limits of reversible cycles. The requirement that 
(d sub i) S > 0 for spontaneous processes provides the criteria for examining 
the specific conditions for spontaneous paths, and the criteria for 
establishing the equilibrium state of a system. See CARNOT CYCLE; TEMPERATURE. 
    Many chemical systems can be considered closed systems in which a single 
parameter XI can be defined as a measure of the extent of the reaction or the 
degree of achievement of a process. If the reaction proceeds or the process 
advances spontaneously, entropy must be produced according to the second law 
and DELTA a must be positive. In terms of the advancement parameter XI, this 
uncompensated heat DELTA a can be given by Eq. (8), where UNDERLINED A is the 

           *** DELTA a = UNDERLINED A d XI = T (d sub i) S ***           (8)

affinity of the process or reaction. The affinity is related to internal 
entropy production by Eq. (9). The condition that the entropy production is 

*** UNDERLINED A = T (d sub i) S / d XI GREATER THAN OR EQUAL TO 0 ***   (9)

zero represents equilibrium, and hence UNDERLINED A = 0 is an equivalent 
condition for equilibrium in a closed system. For spontaneous processes, since 
the signs of UNDERLINED A and d XI must be the same, for positive UNDERLINED A 
the process must advance or go in a forward direction in the usual sense of 
chemical reactions or physical processes, while for negative UNDERLINED A the 
process must proceed in the reverse direction. 
    The affinity of a chemical reaction establishes the spontaneous direction 
of the reaction, and consequently methods for determining the affinity are 
important in thermochemical studies. The affinity is simply related to the 
stoichiometric coefficients of the reaction and the chemical potentials of the 
reactants and products in the reaction. 
    Classical equilibrium thermodynamics is primarily concerned with 
calculations for reversible processes, and deals with irreversibility in terms 
of inequalities. In the case of irreversible processes in systems slightly 
removed from equilibrium, the rate of internal entropy production 
(d sub i) S/dt is related to the fluxes Jsubi associated with thermal, 
concentration, or other differences in intensive parameters or potentials 
X sub i. This entropy production is then given by Eq. (10). The fluxes include 
heat conduction, diffusion, electric conduction, and other direct effects. 
  
        *** (d sub i) S / dt = SUMMATION (i) (J sub i)
                        (X sub i) GREATER THAN OR EQUAL TO 0 ***       (10)
  
    In addition, a flux of one type may be coupled to a potential difference 
of another type. For example, a thermal gradient can result in a mass flux 
(thermal diffusion), or a concentration gradient in any energy flux. Thermal 
conductivity, thermoosmosis, and thermoelectric effects are all coupled 
effects.  
    Far removed from equilibrium, thermodynamics must be formulated somewhat 
differently and more cautiously. The interplay of thermodynamic stability and 
kinetics can give rise to macroscopic structures with both temporal and 
spatial coherence called dissipative structures. Much theoretical effort is 
being directed to these studies because of their apparent relevance to 
biological structures, but it is still too early to assess how far-reaching 
these theories will be in the future. See THERMODYNAMIC PRINCIPLES; 
THERMODYNAMIC PROCESSES. 
 �34s[Robert A. Pierotti]
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�6v�2v�32sChemiluminescence   �21m �33s 
    The type of luminescence wherein a chemical reaction supplies the energy 
responsible for the emission of light (ultraviolet, visible, or infrared) in 
excess of that of a blackbody (thermal radiation) at the same temperature and 
within the same spectral range. Below 930DGF (500DGC), the emission of any 
light during a chemical reaction is a chemiluminescence. The blue inner cone 
of a bunsen burner or the Coleman gas lamp are examples. See BLACKBODY. 
    Many chemical reactions generate energy. Usually this exothermicity 
appears as heat; whereas, for a visible chemiluminescence to occur, one of the 
reaction products must be generated in an excited electronic state from which 
it can undergo deactivation by emission of a photon. Hence a chemiluminescent 
reaction can be regarded as the reverse of a photochemical reaction. 
    Only very exothermic, or ``exergonic,'' chemical processes can be expected 
to be chemiluminescent. Partly for this reason, most familiar examples of 
chemiluminescence involve oxygen and oxidation processes; the most efficient 
examples of these are the enzyme-mediated bioluminescences. See LUMINESCENCES. 
 �34s[Therese Wilson]
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�6v�2v�32sChemistry �21m �33s
    The study of the properties, composition, and structure of matter, the
changes in structure and composition which matter undergoes, and the
accompanying energy changes. The objective of the chemist is to aid in the
interpretation of the universe. Much progress has been made toward meeting
this objective because not only has the structure and composition of many of
the materials on the Earth been elucidated, but also those of the planets, the
satellites, the stars, and the materials of interstellar space.
    Matter can be classified in a number of different ways. One method is in
terms of the physical state - solid, liquid, and gas; but probably the most
useful method is in terms of composition - elements, compounds, and mixtures.
An element is a substance which cannot be broken down to simpler substances by
chemical reactions. It is also defined as a substance made of one kind of
building block (atom) only. There are only a few more than 100 elements known
in the entire universe. They can be classified into families or groups; and
this classification is called the periodic table. See ELEMENTS (CHEMISTRY);
PERIODIC TABLE.
    The early foundations of chemistry were based on two fundamental laws: the
law of conservation of matter and energy and the law of definite composition.
The establishment of these laws required much careful research on the
quantitative relationship of reactants and products in chemical processes and
also of the ratio of weights of the elements on compounds. After the law of
conservation of matter had been established, much research was done on the
combining weights of elements. These data not only led to the law of definite
composition (the composition of a pure substance is always perfectly
definite), but also to the atomic theory, to a system of atomic weights, and
to the methods of determining formulas of compounds.
    To explain several phenomena, including the law of definite composition,
which resulted from the observation that elements always combine in the same
ratio by weight to form a given compound, John Dalton proposed that matter is
made of particles called atoms, that reactions must take place between atoms
or groups of atoms, and that atoms of the same element are all alike, but that
they differ from atoms of another element. These assumptions are the basis of
the accepted modern atomic theory. They are also most useful in explaining
another law of chemical combination, called the law of multiple proportions,
which states that when two or more elements combine in more than one ratio,
they do so in the ratio of small whole numbers.
    Atoms are made up of three fundamental particles: protons and neutrons at
the heart or center of the atom (the nucleus), and electrons surrounding the
nucleus. All the atoms of the same element have the same atomic number
(positive charge on the nucleus). Elements whose atoms have the same number of
electrons in the outer shell have similar properties; this accounts for the
similar properties of the elements in each family of the periodic table. The
different atoms tend to react with each other by gaining or losing electrons
or sharing pairs of electrons. If the atoms combine by sharing pairs of
electrons, molecules are formed. If they combine by gaining and losing
electrons, ions are formed. Ions carry positive or negative charges. See
ATOMIC STRUCTURE AND SPECTRA; MOLECULAR STRUCTURE.
    Work done in the nucleus of atoms has indicated they contain a series of
energy levels similar to the electron levels on the outside of the atom. Work
is ongoing on the forces in the nucleus and the explanation of the many
fleeting, short-lived particles that have been identified in nuclear
reactions. See ELEMENTARY PARTICLE.
    Most of the elements are made of atoms of different atomic weights: such
atoms are called isotopes. More than 1000 isotopes, both stable and
radioactive, have been identified. Most of the radioactive isotopes of the
elements have been produced synthetically.
    Compounds are chemical combinations of elements. They are made of
molecules or ions. Several million compounds have been synthesized or have
been isolated and identified from products found in plant and animal life or
in the minerals of the Earth. The origin of the compounds in plant tissue is
the photosynthesis process which takes place in the green leaves. Animals then
use the plant tissue to build animal tissue. See PHOTOSYNTHESIS.
    The six types of chemical bonds that hold atoms to each other are
covalent, polar covalent, coordinate covalent, ionic, metallic, and hydrogen
bonds. The covalent bonds are formed by sharing pairs of electrons which spin
in opposite directions and hence form a strong bond between the atoms.
Examples of covalency are H2, H2O, NH3, HCl, and CH4. The covalent bond is
polar if the pair of electrons is located closer to one atom than the other as
the result of a difference in attraction for electrons. This gives one end of
the molecule a slight positive (+) charge and the other a slight negative (-)
charge. The water molecule contains polar covalent bonds; as a result, it has
good solvent action for electrovalent compounds. If both electrons are
supplied by one atom, the bond is coordinate covalent. This type of bond is
formed in acid-base reactions and in the formation of complexions, for
example, Cu(NH3)4 sup 2 sup +.
    The ionic bond is formed between ions. The metallic bond is present in all
metals and is believed to result from the ``sea'' of valence electrons free to
move through the metal lattice. The hydrogen bond is the result of the polar
covalent character of a covalent bond of hydrogen with another element, such
as fluorine, oxygen, or sulfur. The boiling point of water is much higher than
predicted because the water molecules are polymerized by the formation of
hydrogen bonds. It is also important in bonding large organic molecules to
each other if they have many OH groups along a chain of carbon atoms. This is
particularly true of many large molecules found in plant and animal tissue.
See CHEMICAL BONDING.
    Compounds are sometimes classified as electrolytes, those which dissociate
into ions in solution, and as nonelectrolytes, those which do not.
Electrolytes can be further classified as acids, bases, and salts. In water
solution, acids have a sour taste, change the colors of indicators, and
produce Hsupp ions in solution. Bases taste bitter or brackish, change the
colors of the indicators, and produce OH- ions in water. A salt is a product
of the reaction of an acid with a base. Generally, an acid is an ion or a
molecule that can accept a pair of electrons, that is, Hsupp and AlCl3. See
ACID AND BASE.
    Abbreviations or symbols are used to designate elements. The symbol of
oxygen is O, hydrogen H, helium He, copper Cu, sodium Na, radium Ra, plutonium
Pu, and so on. Groups of symbols called formulas are used to designate
compounds. The formula for water is H2O, for carbon dioxide CO2, for sulfuric
acid H2SO4, for grain alcohol C2H5OH. These symbols and formulas are used as
chemists' shorthand to indicate chemical reactions. For example,
Statement:
    Water decomposes to form hydrogen and oxygen.
Word equation:
	       Water -> hydrogen + oxygen

Symbol equation:
2H2O -> 2H2 + O2=cSee CHEMICAL SYMBOLS AND FORMULAS.
    The total mass of the neutrons and protons in the nucleus along with the
electrons gives the mass of the atom. Because the atoms are very tiny, their
masses likewise are very small.  The weight of an atom of carbon, for example,
is 2.0 x 10- sup 2 sup 3  (or 0.000 000 000 000 000 000 000 020) g. Because this
value is so small, a unit of weight known as the atomic weight unit (awu) is
often used to express the weights of atoms. It is defined as 1/12 the weight
of carbon atom. Carbon therefore weighs 12 awu and is the chemists' standard
of reference for atomic weights. See ATOMIC MASS UNIT; ATOMIC WEIGHT; RELATIVE
ATOMIC WEIGHT.
    The field of chemistry is a very large one. Some of the betterknown fields
are inorganic chemistry, organic chemistry, physical chemistry, analytical
chemistry, biological chemistry, pharmaceutical chemistry, nuclear chemistry,
industrial chemistry, colloidal chemistry, and electrochemistry.
    The chemical manufacturing industry is the major industry in the United
States in terms of value of production. Chemists perform a number of different
and important jobs. In the research laboratory, the chemists work on
fundamental research, such as synthesizing new compounds and studying the
properties of others. The importance of fundamental research is recognized
when it is realized that half of the jobs of the nation can be traced to the
research laboratory. In the analytical laboratory, chemists develop methods of
analysis of elements and of compounds. In industrial chemistry, one of the
important jobs is the synthesis or purification of chemicals on a large scale
for production purposes. See ANALYTICAL CHEMISTRY; BIOCHEMISTRY: CHEMICAL
ENGINEERING; INORGANIC CHEMISTRY; ORGANIC CHEMISTRY; PHYSICAL CHEMISTRY.
 �34s[Alfred B. Garrett]
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�6v�2v�32sChemolithotrophic bacteria �21m �33s
    Those bacteria capable of generating metabolically useful energy, that is,
adenosinetriphosphate (ATP), by the oxidation of inorganic compounds.
Molecular hydrogen, elemental sulfur, and salts of NH4 sup +, NO2-, Fe sup 2
sup +, Mn sup 2 sup +, S sup 2-, and S2O3 sup 2- can serve as exclusive energy
sources for the growth of specific bacteria.
    The respiration of NH4 sup +, NO2-, Fe sup 2 sup +, and Mn sup 2 sup +
occurs only aerobically, with oxygen as the terminal electron acceptor. Both
aerobic and anaerobic respiration of H2 and reduced sulfur compounds occur.
Nitrate, SO4 sup 2 sup +, and CO2 serve as electron acceptors for anaerobic
hydrogen utilization, and NO3- for anaerobic utilization of the sulfur
compounds. The intermediates in the oxidation of NH4 sup + and reduced sulfur
compounds, and the components of the electron transport systems, in these
oxidation processes are still under debate.
    Chemolithotrophic bacteria may be either autotrophic or heterotrophic.
There are two classes of autotrophic bacteria: obligate autotrophs growing
only in a completely mineral medium (autotrophy) by coupling energy from
ammonia oxidation, with the assimilation of CO2 as the only carbon source; and
facultative autotrophs, additionally capable of growing with organic energy
and carbon sources. Assimilation of CO2 is achieved primarily by the reductive
pentose (Calvin) cycle, as in green plants. However, chemolithotrophy need not
be coupled to CO2 assimilation. The best-known examples of chemolithotrophic
heterotrophs are the desulfovibrios and related sulfate-reducing bacteria.
These organisms can derive energy from the oxidation of molecular hydrogen and
couple this energy to the assimilation of carbon from organic compounds. They
cannot grow autotrophically with H2 and CO2.
    Chemolithotrophic bacteria are significant geochemical agents. The
bacteriological oxidations of ammonia to nitrite and nitrite to nitrate are
key steps in the completion of the  nitrogen cycle, upon which continued
organic productivity depends. The biological oxidations of reduced sulfur
compounds to sulfate are key processes in the sulfur cycle. The bacteria
oxidizing Fe sup 2 sup + are believed responsible for major deposits of iron
 oxide
ores. The sulfate-reducing bacteria are responsible for vast deposits of iron
sulfides and possibly other sulfide minerals. See NITROGEN CYCLE.
    A major chemolithotrophic process is that of methane production, which
occurs extensively in anaerobic marine and fresh-water sediments as well as in
regulated human activities such as sewage treatment. Methane production occurs
primarily, if not exclusively, by the anaerobic respiration of H2, with CO2 as
the electron acceptor.
 �34s[Sydney C. Rittenberg]
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�6v�2v�32sChemoreception   �21m �33s 
    The response of most organisms to a change in their chemical environment. 
Lower forms, such as protozoans, react to chemical stimuli as a consequence of 
a general property of irritability inherent in all living matter. A similar 
response to certain obnoxious chemicals is initiated by the small fine nerve 
endings commonly found in the cornea and mucous membranes of mammals. Such 
responses are associated with a common chemical sense. 
    Marine and aquatic invertebrates are highly chemosensitive. Higher 
invertebrates have discrete chemosensory organs. Small, nonvolatile organic 
molecules such as amines and amino acids are adequate stimuli for many 
invertebrate chemoreceptors. Mixtures of such substances interact 
synergistically and antagonistically to elicit feeding behavior in nature. 
Different combinations of chemicals are used to recognize different food 
types. Chemical stimuli also control complex behaviors such as mate 
recognition and host location between symbionts. 
 �34s[Barry W. Ache]
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    The chemical senses play an important role in the nutrition of all forms 
of animals. This importance is emphasized by the fact that specialized 
chemical sense organs evolved much earlier than the receptors of the other 
major senses. Specialized  taste receptors are responsive to water-soluble 
chemicals and are important in the monitoring of all food entering the oral 
cavity of many animals. Olfactory receptors respond to minute concentrations 
of odors. Odors are related to many forms of animal behavior. See CHEMICAL 
SENSES.  
    Specialized chemoreceptors are found within many mammals, and help 
maintain the internal environment of the organism. The respiratory center of 
the medulla responds to carbon dioxide of the blood and controls respiration 
and the oxygen content of the blood. Changes in the bodily constituents of an 
animal due to a disease may change the taste sensitivity. An animal may tend 
to compensate for its deficiency to a particular chemical by better detection 
and greater intake. Thus chemoreception plays an important role in all 
organisms, particularly in those functions involved with survival and 
reproduction. See CHEMICAL ECOLOGY; OLFACTION; SENSE ORGAN; TASTE. 
 �34s[Lloyd M. Beidler]
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�6v�2v�32sChemostat   �21m �33s 
    A principle for the continuous culture of a bacterial population in a 
steady state, achieved by restricting the concentration of a nutrilite in the 
medium. See CULTURE. 
    An apparatus (see illustration) for application of this principle includes 
a growth tube, provided with an inlet for the entrance of nutrient liquid and 
an overflow for the exit of bacterial suspension at a rate which keeps 
constant the volume of bacterial suspension in the tube. Typically air is 
bubbled through the growth tube for aeration and mixing. Nutrient liquid is 
fed to the growth tube at a constant rate, generally by an infusion pump. 
    The entering nutrient contains an excess of all the factors needed for the 
growth of the bacteria except one, the controlling growth factor, which is 
present at a selected concentration. Initially rate of growth of the bacteria 
exceeds rate of removal. The resulting rise in bacterial density (number of 
organisms per milliliter) causes a fall in the concentration of the 
controlling growth factor in the tube. Eventually the concentration falls so 
low as to cause a reduction in the specific bacterial growth rate. When the 
specific growth rate falls to the level of the dilution rate, a steady state 
is achieved in which the bacterial density remains constant and the 
concentration of the controlling growth factor in the growth tube has become 
independent of its concentration in the incoming nutrient. Because the 
concentration of all chemical substances in the growth tube remains constant, 
the bacterial population will grow for long periods of time in a constant 
physiological state. 
    The chemostat has been applied to problems in bacterial metabolism, 
mutations, evolution, and the kinetics of enzyme formation. It is possible to 
adapt the chemostat to the production of bacterial products. The chemostat 
method of continuous culture can be of great use in such fields of study as 
microbiology, physiology, genetics, and biochemistry. 
 �34s[Aaron Novick]
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�6v�2v�32sChemotaxonomy   �21m �33s 
    The use of plant chemistry in taxonomic studies. Plants produce many types 
of natural products in varying amounts, and quite often the biosynthetic 
pathways responsible for these compounds also differ from one taxonomic group 
to another. The distribution of these compounds and their biosynthetic 
pathways correspond well with existing taxonomic arrangements of plants based 
on more traditional criteria such as morphology. In some cases, chemical data 
have contradicted existing hypotheses which necessitates a reexamination of 
the problem or, more positively, have provided decisive information in 
situations where other forms of data are insufficiently discriminatory. 
    Modern chemotaxonomists often divide these natural plant products into two 
major classes: (1) micromolecules, that is, those compounds with a molecular 
weight of 1000 or less, such as alkaloids, terpenoids, amino acids, fatty 
acids, flavonoid pigments and other phenolic compounds, mustard oils, and 
simple carbohydrates; and (2) macromolecules, that is, those compounds (often 
polymers) with a molecular weight over 1000, including complex 
polysaccharides, proteins, and the basis of life itself, deoxyribonucleic acid 
(DNA).  
    In studies of existing taxonomic systems, a knowledge of plant chemistry 
can be a powerful adjunct tool, and in many cases can indicate possible 
evolutionary relationships between plants more precisely than other evidence 
alone can. Indeed, the potential of chemistry in plant taxonomy has finally 
been recognized, resulting in a close synergism between chemist and 
taxonomist, and in the establishment of the chemotaxonomist, a person trained 
in both fields. See PLANT TAXONOMY. 
 �34s[David E. Giannasi]
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�6v�2v�32sChemotherapy �21m �33s
    A term that implies treatment of the sick with chemical substances. It is
usually used, however, in a more restricted sense to refer to the treatment,
with chemical agents, of diseases caused by parasitic organisms and by cancer
cells. This is in essence the use of the term intended by the father of
chemotherapy, the German physician Paul Ehrlich. The substances used in
chemotherapy include those synthesized by chemists, such as sulfadiazine and
chloroguanide, and those which occur naturally, such as emetine and
penicillin. All of these substances are called chemotherapeutic drugs,
distinguishing them from substances such as antibodies which are produced by
the host as its defense against an invading parasite, protein, or other
antigenic substance.
    The essence of Ehrlich's concept of chemotherapy is specificity of action.
He considered a chemotherapeutic compound a chemical agent taken up by
parasites in quantities sufficient for lethal action while being tolerated by
the organism harboring the parasite. The closest to Ehrlich's ideal came in
the remarkable antibiotic penicillin. This drug has a very low inherent
toxicity for mammals, except in the sensitized individual, in whom it may
produce a wide range of allergic manifestations. Penicillin has the amazing
property of preventing the synthesis of the outer wall of certain bacterial
cells without influencing the development of mammalian cells. Such a
specificity implies that the drug can distinguish subtly between the
biochemistry of parasitic and host cells. This ability is decreased in virus
infections and in cancer, in which the metabolism of the parasite or neoplasm
is intimately associated with the metabolism of the host.
    Most of the clinically useful chemotherapeutic agents are not so highly
specific that their effects apply only to pathogenic organisms or even to a
single kind of microorganism. Thus, these drugs display spectra of activities
from the relatively narrow spectrum of penicillin (most specific) to the
so-called broad-spectrum antibiotics such as tetracycline, oxytetracycline,
and chlortetracycline. Broad-spectrum antibiotics inhibit the growth of
rickettsiae and large viruses as well as numerous types of bacteria. See
ANTIBIOTIC.
    The second great principle of chemotherapy fixation, led to the
formulation of Ehrlich's chemoreceptor, or side-chain, theory. This theory
puts forward the concept that the parasitic cell possesses chemical groupings
or side chains on its surface, for which the chemotherapeutic drug has an
affinity. Once the drug fixes itself to these side chains, the specific lethal
action on the parasite is initiated. Such a concept is used today, except that
Ehrlich's side chains are now called receptors. It has been a useful working
hypothesis which, with a certain extension into the area known as competitive
inhibition or metabolite antagonism, has led to the development of a number of
drugs useful in malaria, leukemia, and various lymphomata.
    The third basic principle proposed by Ehrlich is that a chemotherapeutic
drug, when injected into an infected host, must be distributed in effective
concentration to the site of the infection. This criterion must be met before
fixation to the parasite and the killing action can take place. One of the
great advantages of the modern chemotherapeutic approach is that it is
possible to attack an occult (hidden) infection by simply introducing the
chemotherapeutic agent either directly into the bloodstream or indirectly via
the oral route. However, problems emerge of accessibility of the drugs to the
site of infection.
    Whenever a chemotherapeutic agent is administered to an infected host, a
system of interrelationships is established involving three components: the
human host, the infecting parasite, and the chemotherapeutic drug. Each of
these may act upon, and react to, the other two. The drug itself exerts its
lethal action on the parasite, but at the same time produces certain toxic
effects upon the infected host. Each of these organisms reacts to the drug in
characteristic fashion. The parasite often possesses enzymes which can destroy
the drug (such as penicillinase, which destroys penicillin), and in addition a
small proportion of the population of parasites may have other mechanisms for
resisting the action of the drug. It is these drug-resistant organisms which
may, upon continued use of a drug, grow into a drug-resistant population when
all susceptible members of the original population have been destroyed by the
drug. The host disposes of the drug usually by first metabolically altering it
and then excreting it, chiefly via the kidneys. Finally, the host reacts to
the presence of the parasite independently of the presence of the drug by
marshaling its cellular and immunochemical defense forces to rid itself of the
parasite. It is probably the combined action of these forces and the drug
which ultimately eradicates the infection. One of the great obstacles in the
path of effective cancer chemotherapy may be related to the poorly developed
host defense mechanisms against invasion by the malignancy.
 �34s[Nicholas J. Giarman /Emanuel Grunberg]
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�6v�2v�32sChemurgy   �21m �33s 
    The application of science to extend to profitable utilization of 
agricultural products, especially for nonfood purposes. The term chemurgy 
describes a concept rather than a particular phase of exact science. 
Chemistry, biochemistry, physics, bacteriology, botany, genetics, mycology, 
engineering, and other disciplines are involved. Chemurgy looks to maximum 
exploitation of the vegetable kingdom, wherein resources are annually 
renewable, in contrast to the mineral kingdom, wherein resources do not 
reproduce. The concept originated as an approach to the problem of farm 
surpluses. It was assumed that, because effective food demand was limited, 
more farm products should become industrial raw materials. The more liberal 
definition today includes not only nonfood uses for farm-grown materials, but 
development of new crops, markets for presently unprofitable residues, and 
improved food forms. 
 �34s[Wheeler McMillen]
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�6v�2v�32sCherry   �21m �33s 
    The two principal cherries of commerce are the sweet cherry (Prunus avium) 
and the sour cherry (P. cerasus). Both are of ancient origin and seem to have 
come from the region between the Black and Caspian seas. Cherries of minor 
importance are the dwarf or western sand cherries (P. besseyi) of the plains 
region of North America; the Duke cherries, which are supposedly natural 
hybrids between the sweet and sour cherry; and the Padus cherries, which bear 
their small fruits in long clusters or racemes rather than in short fascicles. 
    Sweet cherries may be divided into two groups, firm-fleshed types known as 
Bigarreaus, represented by the Napoleon (also called the Royal Anne), and 
soft-fleshed types known as Hearts, represented by the Black Tartarian. Sour 
cherries may also be divided into two groups, clear-juice or Amarelle types, 
represented by the Montmorency, and colored-juice or Morello types, 
represented by the English Morello. 
    In North America the principal sweet commercial varieties are the Napoleon 
(white), Bing, Lambert, Van, Schmidt, and Windsor (dark). The principal 
commercial variety of sour cherry is the Montmorency. See FRUIT; ROSALES. 
 �34s[R. Paul Larsen]
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�6v�2v�32sChert   �21m �33s 
    A dense, microcrystalline or cryptocrystalline rock composed of free 
silica, either in the form of chalcedony and microcrystalline quartz or in the 
form of opaline silica of various kinds. The composition of cherts varies with 
age. The older cherts consist of microcrystalline quartz and chalcedony. 
Common nonsilica components of cherts are calcite and dolomite, detrital 
quartz, and clay minerals. Flint is synonymous with chert. The latter term is 
preferred for the rock, although the former is still used for artifacts, such 
as arrowheads. Other names that have been used for chert are silexite, 
hornstone, and phthanite. See CHALCEDONY. 
    Chert is found in two forms, as nodules in limestones and dolomites, and 
as bedded formations. The nodules are flattened parallel to the bedding and 
vary in size from a few inches to several feet in diameter. 
    Two theories have been suggested for the origin of chert. One is that of 
replacement origin of originally calcareous materials. The other calls for 
primary precipitation of silica from the water of the depositional 
environment. The relationship of volcanic activity to chert deposition is not 
wholly clear, for there are many deposits with no apparent link to volcanism; 
but in those deposits where there is a clear association, high dissolved 
silica concentrations are thought to have come from hydrolysis of volcanic 
glass. See SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sChestnut   �21m �33s 
    Any of several species of large, deciduous, nut-bearing trees native to 
the Northern Hemisphere. The nuts, usually three in a cluster, are enclosed in 
a spiny involucre or bur which opens at maturity. Each nut consists of a tough 
but flexible shell, the ovary wall, enclosing the cotyledons which are covered 
with the brown seed coats or integuments. Unlike most nuts, the stored food in 
the cotyledons is mostly starch instead of fat and protein. 
    The American chestnut (Castanea dentata) once was one of the most valuable 
forest trees in the northeastern United States. In 1904 the chestnut blight, 
caused by a fungus (Endothia parasitica), was recognized as present in the 
United States. It spread rapidly through the natural range of the chestnut and 
practically destroyed the species. The European or Italian chestnut (C. 
sativa) is a valuable tree in the Mediterranean region, particularly in the 
mountain villages of Italy. The nuts, which are larger than those of the 
American species, are an important export. However, chestnut production in 
Europe is declining because of the introduction and spread of the chestnut 
blight disease. The Chinese chestnut (C. mollissima) and the Japanese chestnut 
(C. crenata) are both resistant to the blight; they are being used in breeding 
programs in the United States to secure blight-resistant chestnut types that 
might replace the American chestnut. Selected varieties of the Chinese 
chestnut with palatable nuts are being grown as an orchard crop in the milder 
parts of the eastern United States, but the attempts to produce a good forest 
tree have not been successful to date. 
    For the most part, foreign production of chestnuts comes from wild forest 
trees or trees around farmsteads rather than formally planted orchards. There 
are some commercial plantings in the United States, but the total production 
is limited and the many problems of production, propagation, culture, 
processing, and storage remain troublesome. In the European countries and in 
parts of China and Japan where the chestnut is native, the nuts are an 
important staple food source made into flour for bread, roasted, boiled, and 
used in cookery. In the United States, which imports nuts mainly from Italy, 
the nuts are boiled and roasted and used in stuffing for poultry and other 
dishes. See FAGALES. 
 �34s[Laurence H. MacDaniels]
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�6v�2v�32sChevrotain   �21m �33s 
    Any of four species of mammals which constitute the family Tragulidae in 
the order Artiodactyla. These animals, also known as mouse deer, are the 
smallest ruminants, growing to a maximum height of 12 in. (30 cm) at the 
shoulder. The chevrotain lacks horns or antlers. There are two well-developed 
toes on the feet, and the upper canines of the male are elongate and protrude 
from the mouth as small tusks. The chevrotain is a shy animal which leads a 
solitary life except during the breeding season. After a gestation period of 
120 days, one or two young are born. The water chevrotain (Hyemoschus 
aquaticus) is found in west-central Africa along the banks of rivers in Sierra 
Leone through Cameroon to the Congo. The other species are all members of the 
genus Tragulus, and range through the forested areas of Sumatra into Borneo 
and Java. These are the Indian chevrotain (T. meminna), the larger Malay 
chevrotain (T. napu), and the lesser Malay chevrotain (T. javanicus). They are 
differentiated by the pattern of markings (stripes or spots) on their coats. 
Tragulus javanicus has a coat of uniform color. 
    The Eocene fossil traguloid, Archaeomeryx, which was unearthed in 
Mongolia, shows many general similarities to the modern chevrotains. The main 
line of evolutionary development of the traguloids occurred in Eurasia. See 
ARTIODACTYLA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sChickenpox and shingles   �21m �33s 
    Chickenpox is a mild, highly infectious viral disease characterized by 
vesicular rash, which occurs chiefly in epidemics among children. It is also 
known as varicella. Shingles (zoster), a sporadic, severe disease of adults, 
is characterized by inflammation of certain posterior nerve roots and ganglia, 
and is accompanied by skin vesicles limited to areas associated with the 
affected nerves. The viruses of chickenpox and shingles belong to the 
herpesvirus group. They are so similar morphologically and antigenically that 
many consider them identical. 
    In chickenpox, malaise and fever precede the rash, which erupts in 
successive crops; fever continues after eruption. In shingles, malaise and 
fever are followed by severe pain and eruption of vesicles in skin areas 
supplied by sensory nerves associated with the particular dorsal nerve roots 
and ganglia affected. 
    Shingles can be the source of epidemics of chickenpox in children, and 
adults exposed to chickenpox may develop shingles. Infections confer enduring 
immunity, but shingles may occur in persons who have had chickenpox in the 
past. This phenomenon may represent latent persistence of virus after 
chickenpox infection until trauma precipitates development of shingles; or 
treatment with immunosuppressants, as in organ transplants, may overwhelm the 
individual's immunity. See IMMUNITY. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sChicle �21m �33s
    A gummy exudate used in the manufacture of chewing gum. It is contained in
the bark of a tall evergreen tree, Achras zapota (Sapotaceae), a native of
Mexico and Central America. The latex is collected and carefully boiled to
remove excess moisture. When the water content is reduced to 33%, the chicle
is poured off and molded into blocks. The product is an amorphous, pale-pink
powder, insoluble in water, and forming a sticky paste when heated. In the
manufacture of chewing gum, the chicle is cleaned, filtered, and sterilized,
and various flavoring materials and sugar are added.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sChicory �21m �33s
    A perennial herb, Cichorium intybus (Compositae), with a long taproot, a
coarse branching stem, and a basal rosette of numerous leaves. Although the
plant is a native of Europe, it has become a common weed in the United States.
It is used as a salad plant or for greens. The roasted root is also used as an
adulterant of coffee. See CAMPANULALES.
 �34s[P. Dwight Sherman, Jr. /Earl L. Core]
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�6v�2v�32sChilopoda   �21m �33s 
    An order of the group Myriapoda commonly known as the centipedes. Like all 
myriapods, centipedes are ground dwellers whose spatial movements are 
restricted largely by a dependence upon high environmental moisture. Unlike 
the others, centipedes are evidently exclusively carnivorous and predatory, a 
way of life reflected in their great agility or fleetness and especially in 
the extraordinary modification of the first trunk appendages into a pair of 
usually massive raptorial pincers, the prehensors. Each prehensor contains a 
poison gland from which venom is conducted through a terminal claw into the 
victim's body. The poison immobilizes or kills arthropods and even some small 
vertebrates, but so far as is known it seems only harmlessly painful to human 
beings.  
    Excluding the last two or three trunk segments, each segment bears a 
single pair of functional legs. The genital ducts open at the posterior end of 
the body. Spiracles open laterally or dorsally; tracheal and circulatory 
systems are present and often elaborate. The antennae are unbranched; the 
eyes, when present, are either of the compound type or appear as simple 
ocelli. The mandibles have a partially free and movable gnathal portion. There 
are two pairs of maxillae. 
    In terms of families, genera, and individuals, the Diplopoda, or 
millipedes, are most abundant in the tropics, but this is not true of 
centipedes as a group, for whereas certain orders are basically tropical 
(Scutigeromorpha, Scolopendromorpha), one order, the Lithobiomorpha, is 
essentially temperate, and the Geophilomorpha is abundant in both zones. In 
general, centipedes seem less affected by temperature and more sensitive to 
environmental moisture than are millipedes. 
    The class is divisible into two subclasses, the Notostigmorphora and the 
Pleurostigmophora. The notostigmophorous centipedes comprise a single order, 
Scutigeromorpha. Its members, which are peculiar in embodying primitive as 
well as highly advanced characters, are signalized by possession of dorsal 
respiratory openings (stomata), of compound-type eyes, of long flagellate, 
multisegmental antennae, and of long thin legs with multisegmental tarsi. The 
order includes the common house centipedes, Scutigera coleoptrata, one of 
which is shown in the illustration. 
    The Pleurostigmophora, in contrast, have lateral respiratory openings, 
spiracles. They are reducible into four orders: Lithobiomorpha, 
Craterostigmomorpha, Scolopendromorpha, and Geophilomorpha. See MYRIAPODA. 
 �34s[Ralph E. Crabill, Jr.]
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�6v�2v�32sChimaeriformes   �21m �33s 
    The only order of the chondrichthyan subclass Holocephali. The 
chimaeriforms (ratfishes) are a distinctive group of marine bottom-feeding 
fishes that chiefly inhabit the deeper layers of the Atlantic and Pacific 
oceans. During the winter months some species move into coastal waters, and a 
few species may be restricted to this environment. 
    Like the elasmobranchs, the ratfishes have a cartilaginous skeleton 
(partly calcified), a urea retention mechanism, and clasper organs in the male 
for internal fertilization. Unlike the elasmobranchs, however, they have the 
upper jaw fused to the braincase, a complete hyoid arch that is not involved 
in jaw suspension, four gill slits opening into a common outer chamber covered 
by an opercular skin fold, a single branchial opening on each side of the 
head, no spiracle in the adult, dental plates rather than teeth, a persistent 
notochord surrounded by calcified centra, and a narrow, tapering tail. See 
ELASMOBRANCHII.  
    The main line of chimaeriform evolution apparently began with 
Mississippian to Permian forms called menaspoids. By the beginning of the 
Jurassic the menaspoids had been replaced by their presumed descendants, the 
myracanthoids. Modern ratfishes (see illustration) appeared during the Middle 
Jurassic. They are characterized by a laterally compressed head with a 
variably developed rostrum, which is frequently exotically shaped, and an 
ethmoidal canal. The dorsal fin spine is made up almost entirely of lamellar 
bone. The pectoral fins are dibasal and fairly large, and the caudal fin is 
tapered to almost whiplike proportions. Placoid scales cover the body, in 
contrast to the more complicated compound scales of the primitive 
chimaeriforms.  
    The living ratfishes are divided into three families: the Chimaeridae, the 
Rhinochimaeridae, and the Callorhinchidae, which can be readily distinguished 
by the shape of the rostrum. Some 28 species are recognized. Most of them feed 
on mollusks and crustaceans and occasionally on smaller fishes. See 
CHONDRICHTHYES.  
 �34s[Bobb Schaeffer]
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�6v�2v�32sChimera   �21m �33s 
    An individual animal or plant made up of cells derived from more than one 
zygote or otherwise genetically distinct. 
  
    Animals.  Although some chimeras do arise naturally, most are produced 
experimentally, either by mixing cells of very early embryos or by tissue 
grafting in late embryos or adults. Experimental chimeras have been used to 
study a number of biological questions, including the origin and fate of cell 
lineages during embryonic development, immunological self-tolerance, tumor 
susceptibility, and the nature of malignancy. 
    Two techniques used to form chimeras by mixing embryo cells are 
aggregation and injection. 
    Aggregation chimeras are produced by a technique that involves removing 
the zonae pellucidae from around 8 - 16 cell embryos of different strains of 
mice and pushing the morulae together so that the cells can aggregate. After a 
short period of laboratory culture, during which the aggregate develops into a 
single large blastocyst, the embryo is returned to a hormone-primed foster 
mother. Chimeric offspring are recognized in several ways. If derived from 
embryos of pigmented and albino strains, they may have stripes of pigmented 
skin and patches of pigment in the eye. Internal chimerism can be detected by 
use of chromosomal markers or genetically determined enzyme variants. Chimeras 
accept skin grafts from the two component strains, but reject grafts from 
third-party strains. 
    Injection chimeras are produced by a technique in which a blastocyst of 
the host mouse strain of mouse embryos is removed from its zona pellucida and 
held on a suction pipette. Cells of the donor strain are injected through a 
fine glass needle, either into the blastocoele cavity or into the center of 
the inner cell mass (the group of cells from which the fetus is derived). 
After a short period of culture, the blastocyst is returned to a foster mother. 
    Another kind of cell - the pluripotent stem cell of mouse teratocarcinomas 
- was found to give rise to normal tissues in adult chimeras after injection 
into the mouse blastocyst. Teratocarcinomas are tumors consisting of a 
disorganized mixture of adult and embryonic tissues. They develop 
spontaneously from germ cells in the gonads of certain mouse strains, or from 
cells in early embryos transplanted to ectopic sites. All the differentiated 
tissues in the tumor arise from pluripotent stem cells known as embryonal 
carcinoma (EC) cells. When embryonal carcinoma cells are injected into a 
genetically marked host blastocyst, they continue to divide and participate in 
normal development, and give rise to fully differentiated cells in all tissues 
of the adult, including skin, muscle, nerve, kidney, and blood. Embryonal 
carcinoma cells from several sources, including spontaneous and embryo-derived 
tumors and cultured lines selected to carry specific mutations or even human 
chromosomes, have contributed to normal chimeras. However, embryonal carcinoma 
cells from some other sources fail to integrate, but produce teratocarcinomas 
in the newborn animal or adult. The fact that certain embryonal carcinoma 
cells give rise to tumors when injected under the skin or into the body 
cavity, but behave normally in the blastocyst, has been used to support the 
idea that cancers can develop not only as a result of gene mutations but also 
as a result of disturbances in environmental factors controlling normal cell 
differentiation (epigenetic theory of cancer). 
    Animals that have accepted skin or organ grafts are technically chimeras. 
Radiation chimeras are produced when an animal is exposed to x-rays, so that 
blood-forming stem cells in the bone marrow are killed and then replaced by a 
bone marrow transplant from a genetically different animal. Lymphoid cells in 
the process of differentiating from stem cells in the donor marrow recognize 
the recipient as ``self'' and do not initiate an immune response against the 
host cells. See TRANSPLANTATION BIOLOGY. 
    Naturally occurring chimeras in humans are not rare and are most easily 
recognized when some cells are XX and others XY. Such individuals are usually 
hermaphrodite and probably result from fertilization of the egg by one sperm 
and the second polar body by another, with both diploid cells then 
contributing to the embryo (the small polar bodies normally degenerate). Blood 
chimeras are somewhat more common in animals such as cattle where the blood 
vessels in placentas of twins fuse, so that blood cells can pass from one 
developing fetus to the other. 
 �34s[Brigid Hogan]
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    Plants.  In modern botanical usage a chimera is a plant consisting of two 
or more genetically distinct kinds of cells. Chimeras can arise either by a 
mutation in a cell in some part of the plant where cells divide or by bringing 
together two different plants so that their cells multiply side by side to 
produce a single individual. They are studied not only because they are 
interesting freaks or ornamental, but also because they help in the 
understanding of many of the developmental features of plants that would 
otherwise be difficult to investigate. 
    The first type of chimera to be used in this way resulted from grafting. 
Occasionally a bud forms at the junction of the scion and stock incorporating 
cells from both, and it sometimes happens that the cells arrange themselves so 
that shoots derived from the bud will contain cells from both plants forever. 
See GRAFTING OF PLANTS. 
    Flowering plants have growing points (apical meristems) where the outer 
cells are arranged in layers parallel to the surface. This periclinal layering 
is due to the fact that the outer cells divide only anticlinally, that is, by 
walls perpendicular to the surface of the growing point. In many plants there 
are two such tunica layers and, because cell divisions are confined to the 
anticlinal planes, each layer remains discrete from the other and from the 
underlying nonlayered tissue called the corpus. The epidermis of leaves, 
stems, and petals is derived from the outer layer of the growing point. See 
APICAL MERISTEM. 
    With a periclinal chimera it is possible to trace into stems, leaves, and 
flowers which tissues are derived from each layer in the growing point. For 
leaves, this can also be done with variegated chimeras where the genetic 
difference between the cells rests in the plastids resulting from mutation 
whose effect is to prevent the synthesis of chlorophyll. Tracts of cells whose 
plastids lack this pigment appear white or yellow. A common form of variegated 
chimera has leaves with white margins and a green center (Fig. 1). The white 
margin is derived from the second layer of the tunica, and the green center is 
derived from inner cells of the growing point. The white leaf tissue overlies 
the green in the center of the leaf, but does not mask the green color. 
Chimeras with green leaf margins and white centers are usually due to a 
genetically green tunica proliferating abnormally at the leaf margin in an 
otherwise white leaf (Fig. 2). 
    Since the somatic mutation that initiates chimeras would normally occur in 
a single cell of a growing point or embryo, it often happens that it is 
propagated into a tract of mutant cells to form a sector of the plant. If the 
mutation resulted in a failure to form green pigment, the tract would be seen 
as a white stripe. Such chimeras are called sectorial, but they are normally 
unstable because there is no mechanism to isolate the mutant sector and, in 
the flux that occurs in a meristem of growing and dividing cells, one or other 
of the two sorts of cells takes over its self-perpetuating layer in the 
growing point. The sectorial chimera therefore becomes nonchimerical or else a 
periclinal chimera. 
    However, in one class of chimera an isolating mechanism can stabilize the 
sectorial arrangement. This propagates stripes of mutant tissue into the 
shoot, but because the tunica and corpus are discrete from each other, the 
plant is not fully sectored and is called a mericlinal chimera. Many chimeras 
of this type have a single tunica layer; those with green and white stripes in 
the leaves have the mutant cells in sectors of the corpus. They are always 
plants with leaves in two ranks, and consequently the lateral growth of the 
growing point occurs by cell expansion only in the plane connecting alternate 
leaves. This results in the longitudinal divisions of the corpus cells being 
confined to planes at right angles to the plane containing the leaves. A 
mutation in one cell therefore can result in a vertical sheet of mutant cells 
which, in the case of plastid defect, manifests itself as a white stripe in 
every future leaf. 
    The growing points of roots may also become chimerical, but in roots there 
is no mechanism to isolate genetically different tissues as there is in 
shoots, and so chimeras are unstable. 
    Since the general acceptance of the existence of organisms with 
genetically diverse cells, many cultivated plants have been found to be 
chimeras. Flecks of color often indicate the chimerical nature of such plants. 
Color changes in potato tubers occur similarly because the plants are 
periclinal chimeras. See SOMATIC CELL GENETICS. 
 �34s[F. A. L. Clowes]
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�6v�2v�32sChimney   �21m �33s 
    A vertical hollow structure of masonry, steel, or concrete, built to 
convey gaseous products of combustion from a building. Short, self-supporting 
steel chimneys are called stacks. A chimney should be high enough to furnish 
adequate draft and also to discharge the products of combustion without 
causing excessive local air pollution. For adequate draft, small industrial 
boilers and home heating systems depend entirely upon the enclosed column of 
hot gas. A draft is produced because the density of the hot gas inside is less 
than that of the air outside. The diameter, as well as the height, of a 
chimney determines the draft. Also, for adequate draft, a chimney in a 
building should extend at least 3 ft (0.9 m) above the point where it passes 
through the roof and at least 2 ft (0.6 m) higher than any ridge within 10 ft 
(3.0 m) of it. 
    Large power plants usually depend upon forced-draft fans and induced-draft 
fans to produce the draft necessary for operation, and the chimney is used 
only for removal of the flue gas. Most modern power plant chimneys are of 
reinforced concrete, and some reach to the 1000-ft (300-m) level and beyond, 
thus providing supplementary natural draft. 
 �34s[Waldo G. Bowman]
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�6v�2v�32sChimpanzee   �21m �33s 
    A Primate indigenous to Central West Africa. There are two species, the 
pigmy chimpanzee (Pan paniscus), also known as the bonobo, and the common 
chimpanzee (P. troglodytes). 
    These animals lack a tail, and their faces and ears are practically 
hairless. The brain, the dentition, and the relative length of the limbs 
resemble those of humans. The forelimbs are about the same length as the legs, 
and both the thumb and big toe are opposable. Adulthood for these animals is 
reached at 7 or 8 years and the life-span is about 50 years maximum. One baby 
is born after a gestation period of about 9 months. The time for menstrual 
cycle is about 4 weeks. See MAMMALIA; PRIMATES. 
    The chimpanzee is considered to be one of the most intelligent of the 
apes. It does well in captivity and provides both the biologist and 
psychologist an opportunity to make comparative studies of this animal and 
humans. In their native habitat chimpanzees are mainly vegetarians and feed 
upon fruits, nuts, and other vegetation, but some do eat termites and other 
animal food. In nature, they have been observed to make crude tools to probe 
termite nests. 
    Chimpanzees spend most of their time on the ground but are capable of 
moving rapidly through the trees. They ascend into the trees at night where 
they build a platform-type nest on which the entire family huddles together 
for warmth. Chimpanzees are social animals and move about in family groups 
consisting of a male and one to several females with the offspring. Mutual 
grooming is apparently an important aspect in social relationships. 
    From observations in nature, it has been deduced that two types of 
communication occur among chimpanzees, gesturing and calls. Intonations vary 
from low, soft sounds and grunts to screams. These sounds have been associated 
with definite activities such as greetings, attention to feeding areas, and 
approach of a new individual or a group or possible predators. 
 �34s[Charles B. Curtin]
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�6v�2v�32sChinchilla   �21m �33s 
    The name given to two species of rodents which, together with four species 
of viscachas, compose the family Chinchillidae. The two species of chinchilla 
are Chinchilla brevicaudata and C. lanigar. These animals resemble the 
squirrel in size and shape and are characterized by long, muscular hindlimbs, 
with elongate feet bearing four toes, and  short forelimbs. Blunt claws occur 
on the flexible fingers. Chinchillas are gregarious, nocturnally active 
animals and are found in arid, mountainous regions where they feed principally 
on vegetation. They often seek shelter in burrows or rock crevices, so that 
their capture is difficult. The female, which is larger than the male, bears 
one to six offspring twice each year after a gestation period of 105 - 111 
days.  
    These animals are native to several areas of South America and are widely 
bred on farms in North America and Europe for their fur, which is long, fine, 
and expensive. See RODENTIA; VISCACHA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sChinook   �21m �33s 
    A mild, dry, extremely turbulent westerly wind on the eastern slopes of 
the Rocky Mountains and closely adjoining plains. This wind effectively 
reduces a snow cover by melting or by sublimation. The chinook is a particular 
instance of a type of wind known as a foehn wind. Foehn winds, initially 
studied in the Alps, refer to relatively warm, rather dry currents descending 
the lee slope of any substantial mountain barrier. The dryness is produced by 
the condensation and precipitation of water from the air during its previous 
ascent of the windward slope of the mountain range. The warmth is attributable 
to the previous release of latent heat of condensation in the air mass and to 
the turbulent mixing of the surface air with the air of greater heat content 
aloft. See WIND; WIND STRESS. 
 �34s[Frederick Sanders]
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�6v�2v�32sChipmunk   �21m �33s 
    A member of the tribe Marmotini in the rodent family Sciuridae. There are 
18 species. The eastern chipmunk (Tamias striatus) is found in wooded areas of 
eastern Canada and the United States. The western species, although quite 
similar to the eastern form, are included in the separate genus Eutamias. 
    These rodents are intermediate between the squirrels and marmots, having 
lost the typical bushy tail, tufted ears, and silky fur of the squirrel. They 
are diurnal animals, active in collecting food such as nuts, grains, and 
seeds. They fill their large cheek pouches with gathered food to carry it to 
storage places for the winter. 
    The animals construct extensive burrows of several chambers at the bottom 
of a downward sloping entry tunnel, which is about 3 ft (1 m) long. The 
chambers, used for hoarding food and for nesting, are below the frost line. 
While chipmunks are not true hibernators, they tend to remain in their 
underground chambers during the winter months. In the early spring they emerge 
from the burrows and mating occurs. After a gestation period of 5 weeks six or 
more young are born, blind and helpless. See RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sChiroptera   �21m �33s 
    An order of mammals (bats) in which the front limbs are modified as wings, 
thus making the chiropterans the only truly flying mammals. Bats form the 
second largest order of living mammals (16 families, 171 genera, some 840 
species). They range from the limit of trees in the Northern Hemisphere to the 
southern tips of Africa, New Zealand, and South America, but most species are 
confined to the tropics. On many oceanic islands they are the only native land 
mammals.  
    The wing is formed by webs of skin running from the neck to the wrist 
(propatagium, or antebrachial membrane), between the greatly elongated second, 
third, fourth, and fifth fingers (chiropatagium), and from the arm and hand to 
the body (usually the side) and hindlegs (plagiopatagium). There is also 
usually a web between the hindlegs (uropatagium, or interfemoral membrane) in 
which the tail, if present, is usually embedded for at least part of its 
length (see illustration). The 16 living families may be briefly characterized 
as follows. 
    Pteropodidae are in general the most primitive of living bats and are 
placed in the suborder Megachiroptera, characterized by more primitive ears 
and shoulder joints. Most still retain a claw on the second digit (absent in 
all other bats, suborder Microchiroptera), and few have developed an 
echolocation (sonar) mechanism, found in all Microchiroptera. The teeth, 
however, are highly modified for eating fruit or nectar. The family, with 38 
genera and 149 species, is widely distributed in the tropics and subtropics of 
the Eastern Hemisphere. While some species are quite small, the family 
includes the largest of all bats, with wingspreads of up to 5J ft (1.65 m). 
    Rhinopomatidae is an insect-eating family, with one genus and two species, 
found chiefly in arid regions of northern Africa and southern Asia. These bats 
are characterized by long wirelike tails and rudimentary nose leaves. 
    Emballonuridae is an insectivorous family that includes 12 genera with 45 
species found in the tropics of both Eastern and Western hemispheres. Like the 
Pteropodidae, these bats have well-developed bony processes behind the eye 
sockets. The tail extends only partly across the uropatagium. 
    Noctilionidae, a tropical American family, is represented by two species 
in one genus, including a highly specialized fish-eating species. Fish are 
detected by echolocation and gaffed by the long clawed feet. 
    Nycteridae are insect eaters, with 1 genus and 12 species found in Africa 
and southern Asia. These bats have an extensive basin behind the nose, which 
is partly bridged over by flaps of skin, leaving a mere slit between them. 
    The four genera of the Megadermatidae (with five species) occur in 
tropical Africa, southeastern Asia and adjoining islands, and northern 
Australia. Some species are insect-eating; others feed on small vertebrates, 
including other bats. 
    Rhinolophidae are insect-eating bats that are widely distributed in the 
Eastern Hemisphere. They are remarkable for their extremely complex nose 
leaves. There are 11 genera represented by 129 species. 
    Phyllostomatidae includes 120 species (in 50 genera) of tropical and 
subtropical bats. They possess simple nose leaves and a tremendous variety in 
structure, reflecting an equal diversity in food habits. Primitively 
insect-eating, many have become fruit or nectar feeders, and a few are 
predators on small vertebrates, including other bats. 
    Desmodontidae are the true vampires, essentially confined to the mainland 
of tropical America. The teeth and tongue are highly modified for taking of 
blood. This family includes three genera with three species. 
    Natalidae (one genus and four species) are tropical American insect-eating 
species with large funnellike ears. 
    Furipteridae is an insect-eating family, confined to tropical South 
America, and represented by two species in two genera. The thumb is reduced in 
size and largely enclosed in the wing membrane. 
    Thyropteridae includes two species (in one genus), confined to the 
tropical American mainland. The large suction disks on the thumbs and hindfeet 
enable them to roost on the smooth inner surfaces of large rolled-up leaves. 
    Myzopodidae, with a single species, is confined to Madagascar, eats 
insects, and has suckers on the thumbs and feet rather like those of the 
Thyropteridae.  
    Vespertilionidae, a nearly cosmopolitan family with 279 species in 34 
genera, occur almost everywhere that bats occur.Almost all are insect-eating 
and a few catch fish. In all, the tail is long, extending to the edge of the 
uropatagium. Most have no special facial modifications. A few have very large 
ears or small simple nose leaves. In spite of its many species, the family 
shows little structural diversity. Echolocation, present in all families of 
bats except the Pteropodidae, is perhaps most highly developed here and is 
used for catching insects as well as avoiding obstacles. 
    Mystacinidae, with a single insect-eating species, is confined to New 
Zealand. It has a short tail, not reaching the edge of the uropatagium, and 
stout hindlegs and body. 
    Molossidae is widely distributed in the tropics and subtropics. Feeding on 
insects, they have long tails which extend beyond their uropatagia. The body 
and hindlegs are stoutly built. The family includes 81 species in 10 genera. 
    Bats have a poor fossil record, but have been distinct at least since the 
Eocene, some 50,000,000 years ago. See BAT; MAMMALIA. 
 �34s[Karl F. Koopman]
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�6v�2v�32sChitin   �21m �33s 
    A polysaccharide found abundantly in nature. Chitin forms the basis of the 
hard shells of crustaceans, such as the crab, lobster, and shrimp. The 
exoskeleton of insects is also chitinous, and the cell walls of certain fungi 
contain this substance. 
    Chitin is a long, unbranched molecule consisting entirely of 
N-acetyl-D-glucosamine units linked by BETA-1,4 bonds (see illustration). It 
may be contemplated as cellulose in which the hydroxyl groups on the second 
carbon are replaced with NHCOCH3 groups. Chitin is considered to be 
synthesized in nature by an enzyme which is capable of effecting a glycosyl 
transfer of the N-acetyl-D-glucosamine from 
uridinediphosphate-N-acetyl-D-glucosamine to a preformed chitodextrin 
acceptor, forming the polysaccharide. This stepwise enzymic transfer results 
in the production of the long chain of BETA-N-acetyl-D-glucosamine units, 
which is insoluble chitin. See CELLULOSE; OPTICAL ACTIVITY; POLYSACCHARIDE. 
 �34s[William Z. Hassid]
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�6v�2v�32sChiton   �21m �33s 
    A member of the class Polyplacophora in the phylum Mollusca. Chitons are 
also called loricates or coat-of-mail shells. All chitons are marine and, 
except for a few deep-sea forms, all live in the low intertidal and upper 
sublittoral, typically on wave-swept rocks. The flattened elliptical body 
bears eight articulated shell plates dorsally on the mantle and a ventral 
suctorial foot, features which are clearly adapted for life adhering to hard 
and uneven surfaces in the lower littoral zone (see illustration). Normally, 
chitons can resist the strongest surf action but, in the unlikely natural 
event of a chiton being washed off its rock (or if removed by a human 
collector), the articulated shell allows it to curl up to protect the soft 
foot and gills of its underside. 
    Chiton species living on both sides of the Atlantic are relatively small 
0.4 - 1.2 in. or (1 - 3 cm long) and unobtrusive. Only four genera have living 
species offshore and in deeper waters or (to 20,000 or 6000 m), ft but they 
may be related to the earliest fossil chitons. See MOLLUSCA; POLYPLACOPHORA. 
 �34s[W. D. Russell-Hunter]
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�6v�2v�32sChlamydial diseases   �21m �33s 
    Diseases caused by Chlamydia trachomatis, one of the most common human 
pathogens. There are 15 serotypes of C. trachomatis; four are associated with 
blinding or endemic trachoma. In many developing countries, trachoma (a 
serious chronic conjunctivitis which is considered the leading preventable 
cause of blindness) is hyperendemic. It is most severe in areas of low 
socioeconomic and poor sanitary conditions. 
    Three other serotypes cause lymphogranuloma venereum (LGV), a systemic 
infection characterized by inguinal lymphadenopathy in men. This disease is 
also found in some developing countries, but is relatively uncommon in 
industrialized societies. The remaining serotypes are commonly found in 
industrialized societies, where they cause nongonococcal urethritis (NGU) in 
men and cervicitis in women. See LYMPHOGRANULOMA VENEREUM; NONGONOCOCCAL 
URETHRITIS; TRACHOMA. 
    Many infections of the female genital tract may be transmitted to the 
newborn infant. Approximately 70% of infants passing through a 
Chlamydia-infected cervix will acquire a chlamydial infection. One-third to 
one-half of the exposed infants will develop a conjunctivitis called inclusion 
conjunctivitis of the newborn or inclusion blennorhea. The most serious 
consequence of chlamydial infection in the infant appears to be a 
characteristic pneumonia syndrome, which occurs in infants between 3 weeks and 
4 months of age and is characterized by an afebrile course, severe hacking 
cough, tachypnea, and elevated immunoglobulins. See INCLUSION BLENNORHEA. 
    Chlamydial diseases have been identified as major public health problems 
in the United States and other industrialized countries. Although treatment 
regimens with tetracyclines or erythromycin are readily available for 
symptomatic men, women, and infants, adequate methods for control of these 
infections have not been developed. 
 �34s[Julius Schachter]
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�6v�2v�32sChloramphenicol   �21m �33s 
    A broad-spectrum antibiotic, also known as Chloromycetin, used to treat 
serious infectious diseases caused by bacteria, rickettsiae, and certain large 
viruses.  
    Crystalline chloramphenicol has an empirical formula of C11H12Cl2N2O5, a 
molecular weight of 323, and a melting point of 150.5-151.5DGC. It is 
moderately soluble in water (2.5 milligrams/milliliter at 25DGC), neutral 
(nonionic but pH 5.5 in water), absorptive of ultraviolet in water solution, 
and optically  active. The biochemical method of manufacture involves aseptic 
aerobic fermentation of a nutrient broth culture medium seeded with 
Streptomyces venezuelae. 
    Chloramphenicol is used to treat many infectious diseases, notably typhoid 
fever and Hemophilus influenzae meningitis. It is widely used in systemic 
infections, including resistant staphylococcal infections, meningitis, 
septicemia, osteomyelitis, typhus and other rickettsial fevers, brucellosis, 
tularemia, relapsing fever, plague, pneumonia, whooping cough, psittacosis, 
endocarditis, peritonitis, and gastrointestinal, genitourinary, eye, ear, 
nose, throat, and skin infections (see articles on individual diseases). See 
ANTIBIOTIC.  
 �34s[John Ehrlich]
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�6v�2v�32sChlorate   �21m �33s 
    A negative ion having the formula ClO3-, and derived from chloric acid, 
HClO3. The chlorates are more stable than hypochlorites and chlorites but less 
stable, and thus better oxidizing agents, than bromates, iodates, and 
perchlorates. Chlorates are decomposed on heating to liberate oxygen. 
    Both potassium and sodium chlorates are prepared by electrolysis of a hot 
chloride solution. Chlorine liberated at the ande is allowed to react with 
OH-; hydrogen is liberated at the cathode. Sodium chlorate is used mainly in 
the production of perchlorates, but also as a weed killer; potassium chlorate 
is utilized in the manufacture of matches. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sChloride   �21m �33s 
    A compound which is derived from hydrochloric acid, HCl, and which 
contains the chlorine atom in the l-oxidation state. Compounds containing this 
group may be organic or inorganic. The inorganic chlorides can be divided into 
three classes: covalent chlorides, ionic chlorides, and complexes. The 
covalent chlorides can be metallic or nonmetallic and are recognizable because 
of their low boiling and melting points. 
    Of the chlorides, sodium chloride is most important commercially. It 
occurs widely in nature and is the starting point for all sodium, chlorine, 
and compounds containing these two elements. Calcium chloride is obtained as a 
by-product in the Solvay soda-ash process. The insoluble silver chloride is 
used to identify the chloride ion in the absence of bromide ion. Br-, and 
iodide ion, I-. See CHEMICAL BONDING; CHLORINE; HALIDE; HALOGENATED 
HYDROCARBON.  
 �34s[E. Eugene Weaver]
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�6v�2v�32sChlorine �21m �33s
    A chemical element, Cl, of atomic number 17 and atomic weight 35.453.
Chlorine exists as a greenish-yellow gas at ordinary temperatures and
pressures. It is second in reactivity only to fluorine among the halogen
elements, and hence is found free in nature only at the elevated temperatures
of volcanic gases. It is estimated that 0.045% of the Earth's crust is
chlorine. It combines with metals, nonmetals, and organic materials to form
hundreds of compounds. See HALOGEN ELEMENTS.

    Properties.  Naturally occurring chlorine is formed of stable isotopes of
mass 35 and 37; radioactive isotopes have been made artificially. The diatomic
gas has a molecular weight of 70.906. The boiling point of liquid chlorine
(golden-yellow in color) is -34.05DGC at 760 mm (101.325 kilopascals), and the
melting point of solid chlorine is -100.98DGC. The critical temperature is
144DGC; the critical pressure is 76.1 atm (7.71 megapascals); the critical
volume is 1.745 ml/g; and density at the critical point is 0.573 g/ml.
Thermodynamic properties include heat of sublimation at 7370 PLUS/MINUS 10
cal/mole at 0 K, heat of evaporation at 4878 PLUS/MINUS 4 cal/mole at
-34.05DGC, heat of fusion at 1531 cal/mole, heat capacity at 7.99 cal/mole at
1 atm (101.325 kilopascals) and 0DGC, and 8.2 at 100DGC.
    Chlorine is one of four closely related chemical elements which have been
called the halogen elements. Fluorine is more active chemically, and bromine
and iodine are less active. Chlorine replaces iodine and bromine from their
salts. It enters into substitution and addition reactions with both organic
and inorganic materials. Dry chlorine is somewhat inert, but moist chlorine
unites directly with most of the elements.

    Manufacture.  The first electrolytic process for chlorine production was
patented in 1851 by Charles Watt in Great Britain. In 1868 Henry Deacon
produced chlorine from hydrochloric acid and oxygen, at 400DGC (750DGF) with
copper chloride absorbed in pumice stone as a catalyst. Modern electrolytic
cells may be classified for the most part as diaphragm and mercury types. Both
make caustics (NaOH or KOH), chlorine, and hydrogen. The economics of the
chlor-alkali industry mainly involves the balanced marketing or internal use
of caustic and chlorine in the same proportions as obtained from the
electrolytic cell process.

    Inorganic compounds.  There are many inorganic chlorine compounds, some of
which are naturally occurring. Sodium chloride, NaCl, is used directly as
mined (rock salt), or as found on the surface, or as brine. It may also be
dissolved, purified, and reprecipitated for use in foods or when chemical
purity is required. Its main uses are in the production of soda ash and
chlorine products. Calcium chloride, CaCl2, is usually obtained from brines or
as a by-product of chemical processing. Its main uses are in road treatment,
coal treatment, concrete conditioning, and refrigeration. Wet chlorine reacts
with metals to form chlorides, most of which are soluble in water. It also
reacts with sulfur and phosphorus and with other halogens. See INTERHALOGEN
COMPOUNDS.
    Among the oxides of chlorine is chlorine dioxide, ClO2, a green gas that
has become increasingly important in commercial bleaching of cellulose, water
treatment, and waste treatment. Sodium and calcium hypochlorite solutions are
in general use for their oxidizing power or bleaching power. The use of sodium
chlorite permits controlled bleaching, and it is stable unless heated.
Acidified sodium chlorite bleaches are very popular for cellulose and textile
processing. Sodium chlorate is used as an intermediate in perchlorate
production. It is a weed killer, and is also used in dye preparation, textile
and fur processing, and metallurgical operations. Potassium chlorate, KClO3,
is used in match manufacture and for fireworks. Potassium perchlorate, KClO4,
and ammonium perchlorate, NH4ClO4, have become important as fuels for rockets
and jet propulsion and as explosives. See PERCHLORATE.
    The chlorides form a second important group of compounds. Hydrogen
chloride, HCl, is used as a strong acid and as a reducing agent. Aluminum
chloride, AlCl3, is used principally in anhydrous form. Hydrated and liquid
forms are also available, 50% of which are used in drug and cosmetic
production. Titanium tetrachloride, TiCl4, is used as a polymerization
catalyst and as a smoke screen agent and for sky writing. Phosphorus
trichloride, PCl3, and phosphorus oxychloride, POCl3, are important reagents
in organic chemical synthesis. Ferric chloride, FeCl3, is used for the
manufacture of salts, pigments, pharmaceuticals, and dyes and for
photoengraving, preparation of catalysts, and waste and sewage treatment. See
CHLORIDE.

    Organic compounds.	There are many organic chlorine compounds; many of
them are of the saturated aliphatic class. Methane may be chlorinated to a
variable extent. Methyl chloride, CH3Cl, is used in production of silicone,
tetramethyllead, and butyl rubber. Methylene chloride, CH2Cl2, has
applications in production of paint remover, plastics, solvent degreasing,
aerosol propellants, and other uses. Chloroform, CHCl3, is a colorless,
nonflammable, volatile liquid. Fluorocarbon products and resins consume most
of the chloroform made, leaving only a small quantity for dye, drug, and other
uses. Carbon tetrachloride, CCl4, is used in the production of
chlorofluorohydrocarbons, grain fumigation chemicals, and solvents. See
HALOGENATED HYDROCARBON.
    Tetrachloroethane, Cl2HCCHCl2, is used in the manufacture of the solvents
perchloroethylene and trichloroethylene. Ethylene dichloride, ClCH2CH2Cl, is
used in the production of vinyl chloride, ethylenediamine, and solvents. Ethyl
chloride, C2H5Cl, is used in the manufacture of tetraethyllead, but it also is
used in the production of ethyl cellulos, and for refrigerant, anesthetic, and
miscellaneous uses. Chloral, CCl3CHO, became important after World War II with
the introduction of newer insecticides.
    In addition, there are a number of important unsaturated aliphatic
compounds. Perchloroethylene, Cl2CDOUBLE BARCCl2, is used for dry cleaning,
metal degreasing, compounding, and miscellaneous purposes. Vinylidene
chloride, C2H2Cl2, polymerizes easily and hence is used to make copolymers,
such as Saran, Geon, and Velon. Vinyl chloride, C2H3Cl, also polymerizes to
make polymers, such as Koroseal, Vinylite, Geon, Tygon, Velon, and Saran.
    The aromatic chlorine compounds are also important. Chlorobenzene, C6H5Cl,
at one time used for phenol production, has become the major chemical for
phenol manufacture. Dichlorobenzene, C6H4Cl2, exists in two isomeric forms.
Moth repellents and deodorants are made of p-dichlorobenzene. Dye
intermediates and miscellaneous items are also produced from this substance.
Other aromatic chlorine compounds of interest are benzyl chloride, C6H5CH2Cl,
benzal chloride, C6H5CHCl2, and benzotrichloride, C6H5CCl3. These three
compounds are starting materials for the synthesis of pharmaceuticals
(benzedrine, demerol), and are used as constituents of perfumes and as
starting materials for the preparation of insecticides and insect repellents.
They are also used as lubricants and as intermediates in dye manufacture. The
chlorinated biphenyls find use in electrical insulation, hydraulic mediums,
lubricants, and as constituents of paints, varnishes, and plastics. Because
these compounds are chemically inert, they have been found to persist in the
environment and are being phased out of industrial uses. Dichlorophenol,
Cl2C6H3OH, is important in the production of another chlorine compound,
2,4-dichlorophenoxyacetic acid (2,4-D). The compound 2,4-D is widely used as a
weed killer. See HERBICIDE; INSECTICIDE; POLYCHLORINATED BIPHENYLS.
 �34s[Frederick W. Koerker ;Robert W. Belfit Jr.]
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�6v�2v�32sChlorite   �21m �33s 
    One of a group of layer silicate minerals, usually green in color, 
characterized by a perfect cleavage parallel to (001). The cleavage flakes are 
flexible but inelastic, with a luster varying from pearly or vitreous to dull 
and earthy. The hardness on the cleavage is about 2.5. The specific gravity of 
chlorite varies between 2.6 and 3.3 as a function of composition. 
    Chlorite is a common accessory mineral in low- to medium-grade regional 
metamorphic rocks and is the dominant mineral in chlorite schist. It can form 
by alteration of ferromagnesian minerals in igneous rocks and is found 
occasionally in pegmatites and vein deposits. It is a common constituent of 
altered basic rocks and of alteration zones around metallic ore bodies. 
Chlorite also can form by diagenetic processes in sedimentary rocks. See 
AUTHIGENIC MINERALS; CLAY MINERALS; DIAGENESIS; SILICATE MINERALS. 
 �34s[S. W. Bailey]
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�6v�2v�32sChloritoid   �21m �33s 
    A member of the brittle mica (clintonite) group, which are hydrous 
aluminum silicates resembling both micas and chlorites. Although they also 
possess a perfect basal cleavage, cleavage flakes of the clintonites are 
distinctly more brittle than those of the micas. The composition is 
[Fe(II),Mg][Al,Fe(III)](Si,Al)O5(OH)2. The variety ottrelite also contains 
manganese. Margarite, prehnite, and stilpnomelane are other members of the 
group. Chloritoid has both monoclinic and  triclinic modifications which may 
occur together. It forms rosettes, foliated aggregates, and plates. The color 
is gray to green, and crystals are weakly pleochroic. The hardness is 6.5, 
specific gravity 3.5. Inclusions of quartz and magnetite are common. 
Chloritoid is principally a metamorphic mineral in schists, phyllites, and 
quartzites, and is associated with quartz, muscovite, and chlorite. See MICA; 
SILICATE MINERALS. 
 �34s[E. William Heinrich]
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�6v�2v�32sChlorococcales   �21m �33s 
    A large and highly diverse order of green, mostly fresh-water algae in the 
class Chlorophyceae. These plants are unicellular or colonial. They are 
characterized by the failure to use customary cell division in vegetative 
reproduction. Rather, multiplication occurs by the cleavage of cell contents 
to form small replicas of the parent cell. Eventually these are liberated and 
enlarge to the adult size. The mother-cell wall sometimes persists as 
interconnecting strands or as swollen, enclosing membranes. 
    The order, based primarily on this method of reproduction, brings together 
a seemingly unrelated variety of genera which comprise eight families varying 
according to interpretation. The cells have parietal or axial chloroplasts of 
various forms such as plates, cups, or networks, or they may be stellate. See 
CHLOROPHYCEAE; PHYTOPLANKTON. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sChloroform   �21m �33s 
    A colorless, sweet-smelling, nonflammable liquid, of formula weight 119.39 
and formula CHCl3. Chloroform has a specific gravity of 1.489 at 20DGC 
(68DGF), and a boiling point of 61.2DGC (142DGF). It is insoluble in water but 
soluble in organic solvents. In the presence of ultraviolet light, or more 
slowly in the dark, chloroform tends to react with oxygen of the air to 
produce poisonous phosgene. Therefore, commercial chloroform contains 
inhibitors. It is a powerful anesthetic, but prolonged use causes metabolic 
disturbances and damage to the heart, liver, and kidney, so that it commonly 
has been replaced by less toxic anesthetics. It is widely used as an 
extractant and solvent, a chemical intermediate in the production of dyes and 
drugs, and in pharmaceuticals as an antispasmodic, sedative (particularly in 
cough medicines), analgesic liniment, and anthelmintic. See HALOGENATED 
HYDROCARBON.  
 �34s[Elbert H. Hadley]
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�6v�2v�32sChloromonadida   �21m �33s 
    An order of the class Phytamastigophorea also known as the Chloromonadina. 
These poorly known flagellates are grass-green or colorless, somewhat 
flattened, and have two equal flagella, one anterior, the other trailing. All 
known genera are free-swimming, although Reckertia and Thaumatomastix form 
pseudopodia. They vary in size from 30 to 100 micrometers. Chromatophores are 
small disks, stigmas are lacking, and fat is the storage product. Trichocysts 
are found in three genera. The nucleus is large, with nucleoli and chromosomes 
visible during interphase. One or two anterior vacuoles are present and a 
reservoir seems to be present in Trentonia and Gonyostomum. Life cycles are 
unknown, but longitudinal division occurs. The taxonomic position of the class 
is poorly defined. See PHYTAMASTIGOPHOREA; PROTOZOA. 
 �34s[James B. Lackey]
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�6v�2v�32sChlorophyceae   �21m �33s 
    One of the classes of the Chlorophyta. Twelve orders make up the group, 
which includes all of the green algae except Charophyceae. Unlike the latter, 
Chlorophyceae carry on cell division in practically any part of the plant. 
These are microscopic or macroscopic, chlorophyll-bearing organisms of both 
fresh and salt water. They range in form from unicellular to colonial or 
multicellular types which are either filamentous or, in marine forms, 
sheetlike and tubular. In fresh water these algae are known as pond silks or 
water moss. Minute, planktonic plants may develop as dense blooms and are 
sometimes involved in water spoilage. Although there are several pigments, 
chlorophyll predominates and starch accumulates as a food reserve. 
Reproduction may be vegetative, asexual, or sexual. The sex cells, gametes, 
may be isogamous, anisogamous, or heterogamous, with the gametes being formed 
in either unspecialized or specialized cells, the antheridia and oogonia. See 
CHAROPHYCEAE; CHLOROPHYLL; CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sChlorophyll �21m �33s
    The generic name for the intensely colored green pigments which are the
photoreceptors of light energy in photosynthesis. These pigments belong to the
tetrapyrrole family of organic compounds.
    Five closely related chlorophylls, designated a through e, occur in higher
plants and algae. The principal chlorophyll (Chl) is Chl a, found in all
oxygen-evolving organisms; photosynthetic bacteria, which do not evolve O2,
contain instead bacteriochlorophyll (Bchl). Higher plants and green algae
contain Chl b, the ratio of Chl b to Chl a being 1:3. Chlorophyll c (of two or
more types) is present in diatoms and brown algae. Chlorophyll d, isolated
from marine red algae, has not been shown to be present in the living cell in
large enough quantities to be observed in the absorption spectrum of these
algae. Chlorophyll e has been isolated from cultures of two algae, Tribonema
bombycinum and Vaucheria hamata. In higher plants the chlorophylls and the
above-mentioned pigments are contained in lipoprotein bodies, the plastids.
See CAROTENOID; CELL PLASTIDS; PHOTOSYNTHESIS.
    Chlorophyll molecules have three functions: They serve as antennae to
absorb light quanta; they transmit this energy from one chlorophyll to another
by a process of ``resonance transfer;'' and finally, this chlorophyll
molecule, in close association with enzymes, undergoes a chemical oxidation;
that is, an electron of high potential is ejected from the molecule; this
electron can then be made to do chemical work, that is, reduction of another
compound. In this way the energy of light quanta is converted into chemical
energy.
    The chlorophylls are cyclic tetrapyrroles in which four 5-membered pyrrole
rings join to form a giant macrocycle. Chlorophylls are members of the
porphyrin family, which plays important roles in respiratory pigments,
electron transport carriers, and oxidative enzymes. See PORPHYRIN.
    The structure of chlorophyll a is shown in the illustration. The
characteristic features of chlorophyll a are that it is a magnesium chelate of
a dihydroporphyrin with a cyclopentanone ring (V) and is esterified with
phytol. The alicyclic 5-membered ring V, which contains a keto CDOUBLE BARO
group at position 9, is unique to chlorophylls.
    The two major pigments of protoplasm, green chlorophyll and red heme, are
synthesized along the same biosynthetic pathway to protoporphyrin. Starting
from the small building blocks, glycine and succinic acid, they are converted
in a series of enzymic steps, identical in plants and animals, to
protoporphyrin. Here the pathway branches to form (1) a series of porphyrins
chelated with iron, as heme and related cytochrome pigments; and (2) a series
of porphyrins chelated with magnesium which are precursors of chlorophyll. See
HEMOGLOBIN.
    Chlorophylls reemit a fraction of the light energy they absorb as
fluorescence. Irrespective of the wavelength of the absorbed light, the
emitted fluorescence is always on the long-wavelength side of the lowest
energy absorption band, in the red or infrared region of the spectrum.
    The fluorescent properties of a particular chlorophyll are functions of
the structure of the molecule and its immediate environment. Thus, the
fluorescence spectrum of chlorophyll in the living plant is always shifted to
longer wavelengths relative to the fluorescence spectrum of a solution of the
same pigment. This red shift is characteristic of aggregated chlorophyll. See
FLUORESCENCE COMPOUNDS (PLANT).
 �34s[Govindjee ;George Papageorgio ;S. Granick]
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�6v�2v�32sChlorophyta   �21m �33s 
    A highly diversified plant division of about 20,000 species, known 
commonly as the green algae because  chlorophyll a and b are the predominating 
pigments. These pigments are contained within variously shaped cell bodies 
called chloroplasts. They occur as oval platelets, networks, ribbons, or bands 
or are stellate and radiate. Two carotenes and nine xanthophylls are also 
present. See CELL PLASTIDS. 
    The green algae show a great variety of body types, both microscopic and 
macroscopic, but not even in the most complex is there any high degree of 
tissue specialization. Essentially, the plant body is of two types: 
single-celled, or filamentous and many-celled (see illustration). In certain 
forms a red pigment, hematochrome, occurs, especially when plants are exposed 
to intense light on subaerial habitats such as tree trunks, rocky outcrops, 
and snow. In most species the chloroplast contains a proteinaceous body, a 
pyrenoid, around which starch collects as a food reserve in the form of grains 
or as a sheath. The cell wall possesses an inner layer of cellulose, and 
usually an outer layer of pectose. Beneath the cell wall are cytoplasm, 
bounded by a plasma membrane, and a well-organized nucleus, the chromosomes 
being recognizable during mitoses. Cells may be multinucleate (coenocytic) in 
some orders. See CAROTENOID; CHLOROPHYLL. 
415A 
    Some green algae are free-swimming and possess other protozoan 
characteristics. Indeed, some genera were once regarded as Protozoa even 
though they were chlorophyll-bearing. Motile plants or the swimming 
reproductive cells of those which are nonmotile are provided with two to eight 
(rarely one) whiplike organs, the flagella. Motile cells usually are provided 
with a red eyespot, which is light-sensitive. See PHYTAMASTIGOPHOREA. 
    Because Chlorophyta are similar to higher, green land plants in 
pigmentation, food reserve, wall chemistry, and type of flagellation, this 
group of algae is considered as their probable ancestors. 
    The chlorophyta are divided into two classes, the Chlorophyceae and 
Charophyceae. See separate articles on each group listed below. 
Division Chlorophyta 
    Class Chlorophyceae 
        Order: Volvocales 
               Tetrasporales 
               Ulotrichales 
               Ulvales 
               Schizogoniales 
               Cladophorales 
               Oedogoniales 
               Conjugales 
               Chlorococcales 
               Siphonales 
               Siphonocladales 
               Dasycladales 
    Class Charophyceae 
        Order Charales 
    Although the heterogeneous Chlorophyta are principally aquatic and 
submersed, and either benthic or planktonic, they may occur on soil, on land 
plants, in snow (red snow), or in ice (cryoconite). Algae, like vegetation on 
soil, are responsive to nutrients in water. Their distribution is determined, 
therefore, by water chemistry, by light intensity and periodicity, and to a 
lesser degree by temperature. Because the aquatic environment is more uniform 
than the land, species are more widely distributed. Yet there are patterns of 
distribution where variations in salt content, current, and temperature play 
determinative roles. There are tropical and polar species, an Atlantic 
association, a Pacific association, and others. In fresh-water habitats there 
are basic-water algal associations and acid-water affinities. See FRESH-WATER 
ECOSYSTEM; MARINE ECOSYSTEM. 
    Reproductive methods vary from order to order and often are generically 
characteristic. Although the Chlorophyta exhibit a scale of complexity in 
respect to both method of reproduction and specialization of reproductive 
organs, their reproduction falls into the general categories of vegetative, 
asexual, and sexual. 
    Vegetative reproduction involves cell division, especially in one-celled 
and colonial forms, or fragmentation of the plant, the parts being capable of 
regeneration. In asexual reproduction, specialized elements such as nonmotile 
spores and zoospores are frequently used and appear in a majority of families. 
These are capable of producing a new generation independently, that is, 
without a union of cells, and not only multiply but distribute the plant. 
    Sexual reproduction involves specialized cells, gametes, two of which must 
unite to produce a zygote, zygospore, or fertilized egg (oospore) to fulfill 
their function. Gametes may be formed in ordinary vegetative cells acting as 
gametangia or in specialized cells, the antheridia and oogonia. As in all 
other algae, sex organs are never composed of more than one cell. See 
REPRODUCTION (PLANT). 
    Green algae are important both directly and indirectly in human economy. 
Along with other kinds of algae they constitute the fundamental basis of the 
aquatic animal food chain, leading often to humans. Through their release of 
photosynthetic oxygen they also condition water advantageously for animal 
life. In this connection they are useful in oxidizing sewage and wastes. Of 
indirect importance is the relation of algae to various aspects of limnology, 
oceanography, and other sciences. See ALGAE; FOOD CHAIN. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sChloroplatinate �21m �33s
    One of a group of compounds containing the anion PtCl6 sup 2-, which is
derived from chloroplatinic acid. This acid is produced when platinum is
dissolved in aqua regia. Of the many salts known, the ammonium and potassium
chloroplatinates are the most useful because of their low solubility. The
formation of a precipitate is used as a qualitative test for the ammonium and
potassium ions. See AQUA REGIA; PLATINUM.
 �34s[E. Eugene Weaver]
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�6v�2v�32sChlortetracycline   �21m �33s 
    A broad-spectrum antibiotic. It was the first tetracycline antibiotic and 
is effective for many infections. See ANTIBIOTIC; TETRACYCLINE. 
    This crystalline yellow compound is produced by the soil microorganism 
Streptomyces aureofaciens. The original culture has been mutated in order to 
obtain a strain which produces satisfactory yields of chlortetracycline for 
commercial production. After several transfers in increasingly larger 
fermenters, the culture is finally grown for several days in 
20,000-gallon(75-msup3) tanks. 
    The antibiotic is used orally, parenterally, and topically. It is rapidly 
absorbed from the intestinal tract and is distributed into all body tissues. 
Chlortetracycline is widely used in both human and veterinary medicine for 
infections caused by sensitive organisms. It is also used as a feed supplement 
and as a food preservative. It promotes growth of poultry, hogs, and calves 
when used in their rations. At very low concentrations it increases the shelf 
life of perishable foods, such as meats. Commercially, it has been used for 
the preservation of poultry and fish. 
 �34s[Nestor Bohonos]
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�6v�2v�32sChoanoflagellida   �21m �33s 
    An order of the class Zoomastigophorea (Protozoa). They are principally 
small, single-celled or colonial, structurally simple, colorless flagellates 
distinguished by having the anterior end surrounded by a thin protoplasmic 
collar, within which is an ingestive area for particulate matter, for example, 
bacteria. A single flagellum arises from the center of this collar (see 
illustration). These organisms are widespread in fresh and salt water and are 
common in inshore waters. See PROTOZOA; ZOOMASTIGOPHOREA. 
 �34s[James B. Lackey]
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�6v�2v�32sChoke (electricity)   �21m �33s 
    An inductor used in a low-pass filter in which the useful output is a 
direct current, as in the filter of a power rectifier. At usual power 
frequencies a filter choke has an iron core, with an air gap to minimize 
variation of inductance with direct current. In a choke-input filter (a filter 
in which the first element is an inductance) some economy is sometimes 
afforded by designing the choke so that its inductance is allowed to reduce 
because of saturation at high currents. Such a choke is called a swinging 
choke. See ELECTRONIC POWER SUPPLY. 
 �34s[W. R. Le Page]
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�6v�2v�32sCholecystitis   �21m �33s 
    Inflammation of the gallbladder, a common disease in humans. It is nearly 
always associated with gallstones and is particularly common in obese 
middle-aged women. Most cases are thought to be the result of chemical 
irritation caused by excessively concentrated bile, which is in turn the 
result of partial or complete obstruction to the outflow of bile. The 
gallbladder which is irritated and inflamed in this fashion is susceptible to 
secondary bacterial infection, and this in turn may spread throughout the bile 
ducts into the liver tissue. Perforation may occur, leading to the development 
of peritonitis or to abnormal communications with the intestine. Prolonged or 
recurrent episodes of inflammation result in chronic cholecystitis, 
characterized by thickening and scarring of the wall, contraction, and 
impairment of normal function. See GALLSTONES; PERITONITIS. 
 �34s[Milton R. Hales]
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�6v�2v�32sCholesterol   �21m �33s 
    The chief sterol of vertebrates, with the structural formula. 
All vertebrate cells at one or another stage of development make cholesterol. 
In the adult, however, the chief sites for synthesis are the intestine, the 
liver, and the skin. Nervous tissue is particularly rich in cholesterol. It is 
a component of myelin, a complex lipoprotein that serves as the sheath of 
axons and behaves much like an insulator. Cholesterol in tissues and body 
fluids is always found in combination with proteins (lipoproteins). See 
MYELIN; STEROL. 
    In humans and other omnivores, and in the carnivores as well, there is a 
continued intake of cholesterol in food, although the sterol is inefficiently 
absorbed. Dietary cholesterol supplements biosynthesis. Plant sterols are not 
absorbed by these species and are absent from tissue sterols. Herbivores, can 
absorb cholesterol and are relatively defenseless against its accumulation in 
body tissue as a result of excess feeding. 
    There are two principal paths for removal of cholesterol from the body. 
The first is by excretion in bile and to a very much lesser degree through the 
wall of the small intestine. The biliary cholesterol mixes with dietary 
cholesterol in the intestine; a portion is reabsorbed via enterohepatic 
circulation and handled indistinguishably from dietary cholesterol. A portion 
that is not absorbed is metabolized by bacteria in the gastrointestinal tract 
to a variety of products, chief of which is coprostanol. This latter is not 
reabsorbed and is eliminated. The second path leads to production of bile 
acids, which differ with species; these acids are conjugated with compounds 
such as glycine and taurine and excreted in the bile. They serve as detergents 
to promote the emulsification of fat in the intestine and are in turn 
partially reabsorbed in an enterohepatic circulation, together with the 
products of fat digestion, and are reutilized. See LIPID METABOLISM; STEROID. 
    Cholesterol is a constituent of some gallstones and is also present in the 
arterial plaques that characterize arteriosclerosis. Many authorities consider 
elevated plasma cholesterol a causative factor in arteriosclerosis, although 
others consider its infiltration a consequence of primary damage to vessel 
walls. All agree that the two factors, a high level of plasma cholesterol and 
injury to the arterial wall, are intimately associated in formation of 
arterial plaques. Since arteriosclerosis is a major cause of death in humans, 
the importance of control of the level of cholesterol is clearly apparent. See 
ARTERIOSCLEROSIS.  
    Several disorders of cholesterol metabolism in humans are known, 
especially is the genetically determined elevation of plasma cholesterol known 
as essential hypercholesterolemia. It is characterized by high plasma 
cholesterol, with only mild elevation in plasma neutral fat. Other disorders 
may be associated with very high levels of fats and phospholipids. 
 �34s[Thomas F. Gallagher]
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�6v�2v�32sCholine �21m �33s
    A compound, trimethyl-BETA-hydroxyethylammonium hydroxide, used by the
animal organism as a precursor of acetylcholine and as a source of methyl
groups. It is a strongly basic hygroscopic substance with the formula

	       (CH sub 3)sub 3 TRIPLE BOND N-CH sub 2-CH sub 2 OH

    Choline deficiency in animals is associated with fatty livers, poor
growth, and renal lesions. It is a lipotropic agent. There is no direct
evidence of disease in humans due to choline deficiency, although there have
been suggestions that some of the liver, kidney, or pancreas pathology seen in
various nutritional deficiency states may be related to choline insufficiency.
Choline is found in acetylcholine, which is necessary for nerve impulse
propagation, and in phospholipids.
    Humans eat 50 - 600 mg of choline per day, but only excrete 2 - 4 mg.
Thus, conventional tests are of no value in studying choline requirements, and
no knowledge of human choline requirements exists. See ACETYLCHOLINE.
 �34s[Stanley N. Gershoff]
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�6v�2v�32sChondrichthyes   �21m �33s 
    A class of vertebrates comprising the cartilaginous, jawed fishes. The 
Chondrichthyes have traditionally included the subclasses Elasmobranchii 
(sharks, skates, and rays) and Holocephali (ratfishes). A classification 
scheme for the Chondrichthyes follows; for detailed information see separate 
articles on each group listed. 
Class Chondrichthyes 
    Subclass Elasmobranchii 
        Order: Cladoselachii 
               Pleurocanthodii 
               Selachii 
               Batoidea 
    Subclass Holocephali 
        Order Chimaeriformes 
    A group of Devonian armored fishes, the Placodermi, has usually been 
regarded as ancestral to the Chondrichthyes, but this derivation is not 
certain. Another group of primitive jawed fishes called acanthodians, which 
are considered by many as ancestral to the higher bony fishes, exhibit certain 
primitive elasmobranch-like features. In any case it is probable that the 
elasmobranchs and ratfishes arose independently of each other sometime during 
the Silurian or Early Devonian. See ACANTHODII; PLACODERMI. 
    The most distinctive feature shared by the elasmobranchs and ratfishes is 
the absence of true bone. In both groups the endoskeleton is cartilaginous; in 
some cases it may be extensively calcified. Because even calcified cartilage 
is rarely preserved, the fossil record of the Chondrichthyes is represented 
mainly by teeth and spines, with only occasional associated skeletons. See 
SKELETAL SYSTEM. 
    Other characteristics of the Chondrichthyes include placoid scales, 
clasper organs on the pelvic fins of males for internal fertilization, a 
urea-retention mechanism, and the absence of an air (swim) bladder. Both 
groups have primarily always been marine predators, although they have 
repeatedly invaded fresh water throughout their long history. The 
elasmobranchs have probably always fed as they do today, on other fishes as 
well as on soft and hard-bodied invertebrates. The ratfishes have most likely 
concentrated on invertebrates, although modern forms occasionally also feed on 
smaller fishes. See RAY; SCALE (ZOOLOGY); SHARK; SWIM BLADDER. 
 �34s[Bobb Schaeffer]
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�6v�2v�32sChondrostei   �21m �33s 
    The most archaic of three organizational levels (infraclasses) of the 
subclass Actinopterygii, or rayfin fishes. Chondrosteans of the order 
Palaeonisciformes appeared first in the Lower Devonian, and by Middle Devonian 
were abundant contemporaries of the early crossopterygians and lungfishes, 
representative of the other two subclasses of bony fishes. Palaeonisciformes 
were diversified and abundant in fresh and inshore marine waters of the late 
Paleozoic and the Triassic. In the Jurassic they gave rise to the forebears of 
the sturgeons (order Acipenseriformes), which persist in the modern fauna as 
highly specialized and in some ways degenerate chondrosteans. Even more like 
the palaeonisciforms than the sturgeons are the surviving African fishes of 
the order Polypteriformes, which are known only since the Eocene but are 
probably much older. See ACIPENSERIFORMES; PALAEONISCIFORMES; POLYPTERIFORMES. 
    Primitive chondrosteans were mostly small- or moderate-sized fishes with a 
heavy armor of rhomboidal, enameled scales. The heterocercal tail was preceded 
on the midline, above and below, by series of enlarged fulcral scales. There 
was usually a spiracle behind the eye. The head was heavily plated and 
relatively inflexible, with the maxilla firmly bound to the cheek bones. 
Feeding was accomplished largely by biting and gulping, aided by simple 
conical teeth. See ACTINOPTERYGII; SCALE (ZOOLOGY). 
 �34s[Reeve M. Bailey]
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�6v�2v�32sChonotrichida   �21m �33s 
    An order of the Holotrichia. This is a small group of curious, vase-shaped 
ciliates, commonly found as ectocommensals on marine crustaceans. They attach 
to their  hosts by a short, secreted stalk, and their bodies are practically 
devoid of ciliature. Spirochona (see illustration) is a common example of this 
order. See HOLOTRICHIA; PROTOZOA. 
 �34s[John O. Corliss]
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�6v�2v�32sChopping   �21m �33s 
    The act of interrupting an electric current, beam of light, or beam of 
infrared radiation at regular intervals. This can be accomplished mechanically 
by rotating fan blades in the path of a beam or by placing a vibrating mirror 
in the path of the beam to deflect it away from its intended source at regular 
intervals.  
    A current can be chopped with an electromagnetic vibrator having contacts 
on its moving armature. A current can also be chopped electronically by 
passing it through a multivibrator or other switching circuit. 
    Chopping is generally used to change a direct-current signal into an 
alternating-current signal that can more readily be amplified. See 
MULTIVIBRATOR; VIBRATOR. 
 �34s[John Markus]
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�6v�2v�32sChord   �21m �33s 
    A line segment whose end points are points of a circle (see illustration). 
A chord subtends two (conjugate) arcs of the circle. Equal chords of a circle 
subtend equal arcs, and of two unequal chords, the greater subtends the 
greater arc. The chord of maximum length passes through the center of the 
circle and is called the diameter. A diameter perpendicular to a chord bisects 
it. Equal chords of a circle are equally distant from the center of the 
circle, and conversely. An angle formed by two chords that intersect inside 
the circle is measured by half the sum of the intercepted arcs, and the 
product of the segments of one chord equals that of the other. See ARC 
(MATHEMATICS); CIRCLE. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sChordata   �21m �33s 
    The highest phylum in the animal kingdom, which includes the tunicates 
(Urochordata), the lancelets or amphioxus (Cephalochordata), and the 
vertebrates (Craniata), comprising the lampreys, sharks and rays, bony fish, 
amphibians, reptiles, birds, and mammals. Members of the first two groups, the 
lower chordates, are small and marine. The vertebrates are free-living, the 
aquatic ones are basically fresh-water types, and the members include animals 
of medium size, as well as the largest of all animals, and small ones such as 
mice, hummingbirds, frogs, and fish. See CEPHALOCHORDATA; CRANIATA; TUNICATA. 
    The typical chordata characteristics are the notochord, the dorsal, hollow 
nerve cord, and the pharyngeal slits. 
    The notochord appears in the embryo as a slender, flexible rod filled with 
gelatinous cells and surrounded by a tough fibrous sheath; it lies above the 
primitive gut. In lower chordates and the early groups of vertebrates, the 
notochord persists as the axial support for the body throughout life, but is 
surrounded and gradually replaced by segmental vertebrae in the higher fish. 
    The dorsal, hollow nerve cord flows from a specialized band of ectoderm 
along the middorsal surface of the early embryo by a folding together of two 
parallel ridges. The anterior end enlarges slightly in larval tunicates and 
somewhat more in lancelets, but enlarges greatly in the vertebrates to form 
the brain. Vertebrate evolution is characterized by continual enlargement of 
the brain. In the craniates or vertebrates, the brain is enclosed by the 
braincase or cranium, and the spinal cord by the neural arches of the 
vertebrae. See NERVOUS SYSTEM (VERTEBRATE). 
419a 
    Paired slits develop on the sides of the embryonic pharynx, a part of the 
digestive system, and are retained in all aquatic chordates. Pharyngeal slits 
originated as adaptations for filter feeding, but soon became the primary 
respiratory organ, as blood vessels line the fine filaments on the margins of 
each slit. Water passing over the gills serves for gas exchange in addition to 
the original filter feeding function, which is soon lost in the vertebrates. 
Internal gills are lost with the origin of tetrapods; larval and some adult 
amphibians possess external gills which are quite different structures. The 
pharyngeal slits in embryonic tetrapods close early in life, with the 
pharyngeal pouches becoming the site for glandular development, for example, 
the thyroid and tonsils. See RESPIRATORY SYSTEM. 
    The earliest known vertebrates were fishes of the Ordovician Period, when 
bone first appeared; thereafter vertebrates are common in the fossil records. 
Agnathan fish appeared in the Ordovician, and the earliest gnathostomes in the 
Silurian. The Devonian was a time of vast vertebrate radiation with the  
sharks and all major subgroups of bony fish and of amphibians (slightly later) 
appearing. Reptiles originated in the Carboniferous and radiated broadly 
throughout the Mesozoic Era. The first bird, Archaeopteryx, comes from the 
Jurassic, as do the archaic mammals. 
    The earliest vertebrates evolved in fresh water and were bottom dwellers 
with poor swimming abilities. A hard, dermal bony armor served as protection. 
Better-swimming pelagic groups evolved with gradual reduction of the heavy 
dermal scales and the origin of paired fins and girdles. Jaws and teeth 
evolved with feeding on large food particles. Although fish respire via their 
gills, lungs appeared early in vertebrate history as an accessory respiratory 
organ using air in stagnant waters. Kidneys serve as a water balance organ. 
The major line of fish evolution remained in fresh waters, which tetrapods 
left because of feeding and other advantages on land. Gills were lost and 
lungs took over respiration, fins became limbs, and kidneys took over 
nitrogenous waste elimination. Reproduction was still aquatic, with eggs and 
larval life in the waters. Reptiles were the first true terrestrial tetrapods 
with a water-impervious skin, specialized extraembryonic membranes, and a 
dry-shelled egg. Mammals and birds acquired a high sustained body temperature 
and an insulated body covering of hair or feathers, respectively. See 
VERTEBRATA.  
 �34s[Walter J. Bock]
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�6v�2v�32sChorion   �21m �33s 
    The outermost of the several extraembryonic membranes in amniotes 
(reptiles, birds, and mammals) enclosing the embryo and all of its other 
membranes. The chorion, or serosa, is composed of an outer layer of ectodermal 
cells and an inner layer of mesodermal cells, collectively the somatopleure. 
Both layers are continuous with the corresponding tissue of the embryo. The 
chorion arises in conjunction with the amnion, another membrane that forms the 
outer limb of the somatopleure which folds up over the embryo in reptiles, 
birds, and some mammals. The chorion is separated from the amnion and yolk sac 
by a fluid-filled space, the extraembryonic coelom, or body cavity. 
    In reptiles and birds the chorion fuses with another extraembryonic 
membrane, the allantois, to form the chorioallantois, which lies directly 
below the shell membranes. An extensive system of blood vessels develops in 
the mesoderm of this compound membrane which serves as the primary respiratory 
and excretory organ for gaseous interchanges. In all mammals above the 
marsupials, the chorion develops special fingerlike processes (chorionic 
villi) extending outward from its surface. To a varying degree in different 
species of mammals, the villous regions of the chorion come into more or less 
intimate contact with the uterine mucosa, or uterine lining, of the mother, 
thereby forming the various placental types. See ALLANTOIS; AMNIOTA; FETAL 
MEMBRANE.  
 �34s[Nelson T. Spratt, Jr.]
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�6v�2v�32sChoristida   �21m �33s 
    An order of sponges of the class Demospongiae, subclass Tetractinomorpha, 
in which at least some of the megascleres are tetraxons, usually triaenes. 
Monaxonid spicules may occur as well. Microsclere types are chiefly asters, 
streptasters, or sigmas. A well-developed cortex comprising an outer 
gelatinous layer and an inner fibrous layer is often present. 
    Many choristidan sponges are radially symmetrical; spherical or 
ellipsoidal shapes are common. Others are encrusting or massive; very few form 
branching colonies. Some species have basal tufts or mats of long, thin 
spicules which anchor them in mud or sand on the sea bottom (see 
illustration). Choristidans are common in tidal regions and shallow waters of 
all seas, and some species extend down to depths of at least 1200 ft (3500 m). 
See DEMOSPONGIAE; TETRACTINOMORPHA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sChromadoroidea   �21m �33s 
    A superfamily of small to moderate-sized free-living nematodes with 
spiral, transversely ellipsoidal, slitlike or reniform amphids which are 
located rather forward on the head, and a striated cuticle usually bearing 
internal ornamentation (dotlike or rod-shaped markings). Most species are 
marine, but members of the family Chromadoridae occur in soil and fresh water. 
A few species occur in hot springs and some are adapted to running water. The 
Chromadoridae are epigrowth feeders and are generally associated with algal 
substrates, while the Comesomatidae are nonselective deposit feeders and are 
found associated with relatively undisturbed, soft bottom sediments. See NEMAT. 
 �34s[Bruce Hopper]
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�6v�2v�32sChromate �21m �33s
    The CrO4 sup 2- ion derived from the unstable acid H2CrO4. Chromic oxide,
CrO3, gives, instead of the expected H2CrO4, condensed or polyacids such as
H2Cr2O7 and H2Cr3O10 in acid solutions. However, the solid chromates are
stable and are the most important salts of the series. The dichromates are
also well known. See DICHROMATE.
    The chromates are easily converted to dichromates by the addition of acid.
This is accompanied by a color change from yellow to orange. Most of the solid
chromate salts are also yellow, an exception being silver chromate, which is
red.
    The chromates of calcium, strontium, mercury (I), silver, barium, and lead
are insoluble and are used to identify the chromate ion. However, these salts
dissolve in strong acid to form dichromates.
    Chromates are good oxidizing agents. They are used in paints as pigments
and in the tanning of leather. See CHROMIUM; OXIDIZING AGENT; PIGMENT.
 �34s[E. Eugene Weaver]
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�6v�2v�32sChromatic aberration   �21m �33s 
    The type of error in an optical system in which the formation of a series 
of colored images occurs, even though only white light enters the system. 
Chromatic aberrations are caused by the fact that the refraction law 
determining the path of light through an optical system contains the 
refractive index, which is a function of wavelength. Thus the image position 
and the magnification of an optical system are not necessarily the same for 
all wavelengths, nor are the aberrations the same for all wavelengths. See 
ABERRATION (OPTICS); REFRACTION OF WAVES. 
 �34s[Max J. Herzberger]
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�6v�2v�32sChromatography   �21m �33s 
    A separation method whereby individual chemical compounds which were 
originally present in a mixture are resolved from each other by the selective 
process of distribution between two heterogeneous (immiscible) phases. The 
distribution of chemical species to be separated occurs in a dynamic process 
between the mobile phase and the stationary phase. The stationary phase is a 
dispersed medium, which usually has a relatively large surface area, through 
which the mobile phase is allowed to flow. The chemical nature of the 
stationary phase exercises the primary control over the separation process. 
The greater the affinity of a particular chemical compound (referred to as the 
solute) for the stationary medium, the longer it will be retained in the 
system. The mobile phase can be either gas or liquid; correspondingly, the 
methods are referred to as gas chromatography and liquid chromatography. See 
COLUMN CHROMATOGRAPHY; FLAT-BED CHROMATOGRAPHY; GAS CHROMATOGRAPHY; LIQUID 
CHROMATOGRAPHY.  
    There are four combinations of heterogeneous phase systems, which give 
rise to four different chromatographic methods: gas-solid, liquid-solid, 
gas-liquid, and liquid-liquid chromatography. In gas-solid and liquid-solid 
chromatography, sample molecules are caused to interact physically with the 
surface of a porous solid by means of a phenomenon called adsorption. Hence, 
these two methods are also generally referred to as adsorption chromatography. 
The adsorptive effect of the chromatographic medium for different solutes 
determines their rates of migration through the medium. In gas-liquid and 
liquid-liquid chromatography, the liquid stationary phase is held on the 
surface of an inert solid which serves merely as its support and, ideally, 
does not participate in the separation process. Primarily, then, the 
components of a mixture having different solubilities in the stationary phase 
separate by migrating at different rates. Since the partitioning of sample 
molecules between the two phases is the basis of these methods, they are also 
generally referred to as partition chromatography. The rate of migration of a 
solute can be related to its thermodynamic partition coefficient in a given 
two-phase system. See ADSORPTION. 
    The most important mode of operation in chromatography is elution. In 
elution chromatography, the sample (a mixture of components to be separated) 
is placed as a narrow concentration impulse at the beginning of the 
chromatographic medium. The mobile phase is then introduced, and the sample 
components migrate with it through the chromatographic bed. In gas 
chromatography, in which the mobile phase is most typically an inert inorganic 
gas, sample molecules are merely transported through the sorption medium. 
However, in liquid chromatography, the mobile fluid may have an appreciable 
``extraction'' effect on a sample molecule, causing it to migrate more slowly 
or more quickly; or, alternatively, the mobile-phase molecules can compete 
with the solute molecules for the available sites on the adsorbent (solid 
stationary phase). 
    Chromatography has been used primarily as a separation and isolation 
method. Unlike classical chemical separation methods (for example, 
precipitation or crystallization), chromatography is intended to separate many 
mixture components in a single-step procedure. The use of chromatography in 
general analytical procedures, in which the separation of mixture constituents 
can be followed by their direct identifications and quantitative measurements, 
has been dramatically increasing. Many analytical tasks cannot be adequately 
dealt with by any other available methods. Chromatographic methods can also be 
automated for routine analyses. See CHEMICAL SEPARATION TECHNIQUES. 
 �34s[Milos V. Novotny]
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�6v�2v�32sChromatophore �21m �33s
    A cell which contains pigment, especially in a movable form, located in
the skin or occasionally in deeper body tissues, and which is responsible for
many aspects of the coloration of organisms. Sometimes the word is used to
designate the large or complex colored plastids such as chromoplasts or
chloroplasts seen in certain plant cells. Usually, animal chromatophores are
single cells with specific pigments in their cytoplasm, which may be
concentrated in the main cell body or distributed into dendritic protrusions.
In cephalopod mollusks, however, chromatophores are miniature organs
consisting of a round, saclike pigment cell encircled by a radial arrangement
of muscle fibers. Colors and patterns of animals derive from the relative
distributions of chromatophores of various types, and color changes may be
relatively rapidly effected by the dispersion of pigments within the cells.
Depending on the nature and distribution of pigments, light and heat may be
absorbed or reflected, so that chromatophores also influence the penetration
of radiant energy into the deep body tissues.
    This discussion will include only those cells which are capable of
effecting color change by movement of their pigments. Also known as pigmentary
effectors, they may be found in polychaete worms and leeches, insects and
crustaceans, cephalopods, fishes, amphibians, and reptiles. Pigment cells
which produce structural color but are only indirectly (if at all) related to
color change, notably melanogenic cells in the epidermis, are especially
characteristic of birds and mammals, but are also found in other vertebrates.
Such cells are not usually called chromatophores. However, it should be noted
that ``pigmented cells,'' in the widest, all-inclusive sense, are extremely
widespread in many taxa. See INTEGUMENT.
    Several types of chromatophores are distinguished by their color. The most
common are the black or brown melanophores. Others are yellow, red, white, or
iridescent and are generally known as xanthophores, erythrophores,
leucophores, and iridophores, respectively. No lastingly blue chromatophores
are known, but a diffuse water-soluble blue pigment originating in
chromatophores of other color appears in the surrounding tissues in some
animals, including many crustaceans. In addition, blue, green, or other colors
occur as interference effects of light diffracted or refracted by the pigment
crystals or intervening films in iridophores or from iridophore colors
mingling with the colors of other cells.
    A partly different set of chromatophore types is predicated on the
chemical nature of their pigment. The predominant type characteristically, as
in vertebrates, contains melanin, a class of insoluble black and brown
pigments; this type accordingly retains the designation melanophore. Many
yellow and red chromatophores are characterized by fat soluble carotenoid
pigment (``lipochrome''); they have therefore sometimes been classed as
lipophores. Some others were distinguished as allophores because of their
different, not fat-soluble, red pigment. The term pterinophore has been
introduced for some which owe their yellow or deeper color to pterine, rather
than carotenoid, pigment. In the blackish chromatophores of many
invertebrates, the pigment has been found to be ommochrome, not melanin.
environmental conditions that affect the dispersion and concentration of
chromatophore pigment chiefly involve light, but in some cases other external
factors are decisive, either in themselves or by evoking a nervously excited
state.
    Induced by mechanical, visual, or other stimuli, which may originate from
an enemy or mate, nervous excitement evokes a striking concentration of dark
pigment, ``excitement pallor,'' in various reptiles and fishes. A darkening or
mottling change, however, is induced in some other reptiles and fishes.
    Little general effect on chromatophores results from humidity, but
walking-stick insects and frogs darken in moisture and become pale under drier
conditions.
    Temperature does not often determine chromatophore change when it is in
the normal range for vertebrates. But, as a rule, above 86 - 95DGF (30 -
35DGC), or near the extremes of warmth tolerable to the species, melanophore
pigment concentration occurs in the cold-blooded vertebrates that have been
tested.
    In some cases, light affects pigment movement without help from eyes or
structures of true visual function. In various crustaceans, light appears to
act directly on the chromatophores themselves, which in blinded animals
respond as independent effectors. Both white and darker pigments disperse when
brightly illuminated and become concentrated in total darkness. Similar
responses to light by melanophores have been observed in various fishes,
amphibians, and reptiles after blinding or in the absence of local nerves or
before embryonic development of means of visual mediation.
    The chief environmental factor influencing postlarval chromatophore
activity is light entering the eyes from the surroundings, or background, of
the animal. The responses to this background light depend primarily upon its
quantitative relation to the light entering the eyes from overhead or passing
to the retinal parts normally exposed to direct light from the sky or main
source (Figs. 1 and 2). Though far from universal, the ability to turn paler
or darker by such pigmentary responses and thus to conform visibly to the
shade of the background is widespread in the classes of
chromatophore-possessing animals that have eyes.
    A diurnal rhythm of pigment movements that persists for weeks or months
independently of the afore-discussed factors characterizes certain marine
isopod and decapod crustaceans and such widely representative vertebrates as
the lamprey, European minnow, frog, and chameleon. It is evinced particularly
under constant darkness; but it may be detected under constant illumination,
because by day it slows or reduces and in the night facilitates the
chromatophoral adaptation to a white background.
    Chromatophore control systems vary strikingly among different animals. In
crustaceans and insects, chromatophore responses to factors affecting them
indirectly depend entirely upon the blood's supply of hormones, secreted under
visual or other stimulation transmitted through the nervous system. See
NEUROSECRETION.
    Among vertebrates, both bloodborne melanophore hormones and nerve fibers
directly innervating the melanophores function prominently in certain
elasmobranch dogfishes and probably most bony fishes and reptiles, including
horned-toad lizards. Though some evidence of a direct nerve supply to
chromatophores has been reported for the lizard Anolis and for frogs and
toads, color changes in these animals and other amphibians are governed
predominantly by hormones. Generally, vertebrate melanophore nerves are
pigment-concentrating in action, but in some cases pigment-dispersing nerves
are also present. This is true of various bony fishes, including species with
partly hormonal systems as well as others with predominantly direct nervous
control.
    The vertebrate chromatophore hormones include one that is usually called
intermedin. It occurs in the intermediate or posterior lobe of the pituitary
gland in all species. Intermedin causes melanin dispersal in the many species
subject to its action.
    The term physiological color change designates any relatively transitory
color change that under visual or other stimulation results from pigmentary
effector activity, or redistribution of movable pigment already present in
existing chromatophores of an animal. In contrast, morphological color change
is any intrinsically longer-lasting quantitative change in pigmentation that
results from increase or decrease in the total number of chromatophores or
from synthesis or destruction of pigment in preexisting chromatophores or from
both together.
    Color change in certain animals has survival value as camouflage; color
adaptation to the background seems to contribute importantly to protection
from predators or to capture of prey. In addition, instances of physiological
and of morphological color change linked to breeding are found in most of the
major groups of chromatophore-possessing animals. Transitory color displays
are elements of male mating behavior in some reptiles, fishes, and
invertebrates. Bright nuptial coloring, distinctive of sex, develops for the
breeding season in many vertebrates, but chromatophores often serve a more
fundamental function as screens excluding or absorbing radiation. Thus they
can protect inner tissues against harmful illumination. In many reptiles they
evidently counteract loss of needed heat by increasing their absorbing area
and protect against too high temperature by decreasing that area and affording
more reflection. See PIGMENTATION; PROTECTIVE COLORATION.
 �34s[Paul F. Maderson]
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�6v�2v�32sChromite   �21m �33s 
    The only important ore mineral of chromium. Chromite crystallizes in the 
isometric system but crystals are rare and it usually is massive. The hardness 
is 5.5 (Mohs scale) and the specific gravity 4.6. The luster is submetallic 
and the color iron-black to brownish-black. Pure chromite with composition 
FeCr2O4 is rare, because magnesium usually substitutes for some ferrous iron, 
and aluminum and ferric iron substitute for chromium. Chromite is a common 
constituent of peridotites and serpentines and in some is present in large 
masses. Major producing countries are Turkey, the Union of South Africa, the 
Soviet Union, the Philippines, and Rhodesia. See CHROMIUM; PERIDOTITE; 
SERPENTINE.  
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sChromium �21m �33s
    Chemical element, Cr, atomic number 24, atomic weight 51.996, a
silver-white metal that is hard and brittle as normally encountered. However,
the bulk metal is relatively soft and ductile when unstressed and either
effectively scavenged or extremely pure. Its chief uses are production of
noncorrosive, high-strength, heat-resistant characteristics in alloys and as
an electroplated coating. Elemental chromium, as such, is not found in nature.
The most important mineral occurrence of chromium is in chromite. Of
geochemical interest is the fact that basalt found on the Moon contains 0.47
wt % Cr2O3, which is 3 - 20 times as much as that found in representative
terrestrial specimens. See CHROMITE.
    There are four naturally occurring isotopes of chromium, namely,
sup 5 sup 0 Cr, sup 5 sup 2 Cr, sup 5 sup 3 Cr, and sup 5 sup 4 Cr. Various
unstable isotopes can be produced by radiochemical reactions. The most important
is sup 5 sup 1Cr, which emits soft gamma rays and has a half-life of
approximately 27 days. Electroplated and polished chromium is bright bluish
white. Its reflecting power is 77% that of silver. Chromium has a density of
20DGC (68DGF) of 7.22 g/cm sup 3 (0.26 lb/in. sup 3) for a single crystal and
7.14 g/cm sup 3 (0.259 lb/in. sup 3) for a polycrystalline solid. Chromium has a
melting point of 1930 PLUS/MINUS 10DGC (3505 PLUS/MINUS 20DGF); its boiling
point is 2480DGC (4500DGF) at 760 mm Hg (101.325 kPa).
    Mechanical properties reflect the strength and workability of chromium
including, among others, its hardness, tensile strength, and forging, rolling,
and drawing characteristics. Chromium has relatively poor forging, rolling,
and drawing properties. However, when absolutely free of oxygen, hydrogen,
carbon, and nitrogen, it is relatively ductile and can be forged and drawn. It
is difficult to keep it free from these elements.
    Chromium forms three series of compounds with other elements; these can be
represented in terms of the chromium oxides: chromium, valence 2, CrO,
chromium(II) or chromous oxide; chromium, valence 3, Cr2O3, chromium(III) or
chromic oxide; and chromium, valence 6, CrO3, chromium(VI) or chromic acid
anhydride. Chromium is capable of forming compounds with other elements in
oxidation states of II, III, and VI. The table lists the typical compounds of
each valence state with oxygen.
    Peroxides, perchromic acid, and perchromates are also known. The halides
(chlorides, fluorides, bromides, and iodides) of chromium are fairly common
compounds of this metal. The chloride, for example, has been used to some
extent in the production of chromium metal by the hydrogen reduction of
chromous chloride, CrCl2.
 �34s[W. D. Wilkinson]
�33s

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159744
�6v�2v�32sChromium intoxication   �21m �33s 
    Chromium occurs in nature in several chemical forms and valency states. 
Only trivalent and hexavalent chromium are biologically significant. Trivalent 
chromium is an essential animal nutrient and is a necessary component for the 
metabolism of glucose and lipids. Chromium deficiency mimics diabetes mellitus 
and induces atherosclerotic plaques in rats. A deficiency in chromium 
increases the toxicity of lead. It has been estimated that the average human 
intake of chromium in all forms is about 30 - 100 micrograms per day. See 
ARTERIOSCLEROSIS; DIABETES; LEAD TOXICOLOGY. 
    Pathologic changes are associated with excessive intake of hexavalent 
chromium compounds. These compounds are irritating and corrosive when absorbed 
through the digestive tract, skin, or alveoli of the lungs. Body depots of 
chromium are principally the lungs, muscle, fat, and skin. Excretion of 
chromium is principally in the urine. 
    Because of the slow absorption of chromium, acute exposure does not 
produce generalized symptoms. Inhalation of dusts or mists of hexavalent 
chromium is irritating to the upper respiratory passages and causes sneezing, 
nasal discharge, and vascular congestion. Spasm of the airways (bronchospasm), 
resembling an asthmatic attack, may occur, and could possibly cause death if 
it is prolonged. Chromic intoxication with chromium produces local rather than 
generalized symptoms. Hexavalent chromium may cause skin irritation in the 
form of a rash by direct contact and by sensitization. The rash often 
disappears spontaneously. The formation of a chronic ulcer sometimes ensues. 
Chromium is a chemical carcinogen connected with long-term occupational 
exposures that produces bronchogenic carcinomas. See CHROMIUM; METALS, 
PATHOLOGY OF; ONCOLOGY. 
 �34s[N. Karle Mottet]
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�6v�2v�32sChromomycosis   �21m �33s 
    A cutaneous fungus infection of humans also known as chromoblastomycosis. 
It is usually seen on the legs, feet, and occasionally the arms. Three fungi 
are most frequently associated with this disease: Foncecaea pedrosoi, F. 
compactum, and Philophora verrucosa. The lesion begins as an itchy papule, 
slowly developing into a reddish, warty nodule which eventually becomes 
pedunculated. As the disease progresses, secondary nodules develop and often 
coalesce to give rise to a large lesion, cauliflowerlike in appearance. If the 
lesions are seen in an early stage of development, they may be removed by 
surgical excision. See MEDICAL MYCOLOGY. 
 �34s[Leanor D. Haley]
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�6v�2v�32sChromosome   �21m �33s 
    One of the cellular organelles which contains the factors of inheritance, 
or genes, arranged in linear order along its length. The genetic information 
in a chromosome resides in its deoxyribonucleic acid (DNA), and this linear 
component is associated laterally with basic proteins (histones) and acidic 
proteins to give complex, threadlike nucleoprotein structures. The chromosomes 
are located within, and give rise to, the nucleus in cells of all eukaryotes 
(plants and animals), but they become clearly visible only under the light 
microscope when they contract and the nuclear membrane disappears at the time 
of cell division (mitosis in the soma, meiosis in the gonads). At this time 
(see Fig. 1), they appear as compacted, bipartite, rodlike structures, each 
consisting of two identical parallel subunits called chromatids. With few 
exceptions each chromosome normally has a single primary constriction or waist 
at a fixed position along its length (the centromere or kinetochore), and this 
structure is involved in separating the two halves of the chromosome at the 
time of cell division. 
    New chromosomes originate only by duplication of preexisting ones, and 
this occurs in the interphase period prior to cell division. During the 
process of cell division the chromosomes contract, and the chromatids (sister 
half-chromosomes) separate to opposite poles of the dividing cell. After 
separation, chromatids swell and uncoil and give rise to a single daughter 
nucleus in each of the two daughter cells. Each chromosome in a newly formed 
daughter nucleus is therefore represented by a single chromatid which 
undergoes duplication prior to any succeeding cell division. See MEIOSIS; 
MITOSIS.  
  
    Size and number.  In the great majority of sexually reproducing organisms 
the fertilized egg and the cells derived from it contain two sets of 
chromosomes, one derived from the egg nucleus (maternal set) and the other 
from the sperm nucleus (paternal set). The gametes themselves contain a single 
set of haploid (n) number of chromosomes, so that the fertilized egg normally 
contains two sets or is diploid (2n). The minority of organisms, especially 
among plants, with more than two sets of chromosomes are referred to as 
polyploids (>2n). Thus, in the diploid organism the nucleus in each cell in 
the soma contains one copy of both the maternally and paternally derived 
chromosomes. The diploid set therefore contains pairs of chromosomes, and each 
species has a fixed number of pairs that make up its karyotype or genome. In 
general, and with the exception of the sex chromosomes, the members of each 
chromosome pair are identical in shape and size. However, the pairs within a 
karyotype may differ, and differences in chromosome size and shape are also 
evident between species. 
  
    The DNA that makes up the genetic information is present as a single 
threadlike molecule in each chromosome so that the bits of information, or 
genes, in each thread are physically linked together. Each chromosome 
therefore contains a defined group of genes collectively referred to as a 
linkage group. Each gene occupies a fixed and specific position within each 
linkage group, or along the chromosome; and homologous chromosomes, which make 
up a pair, share common genes. During meiotic cell division in the gonads, 
homologous chromosomes pair to form bivalent structures and may undergo an 
exchange of segments. Such reciprocal crossovers (chiasmata) result in a 
recombination of genes, as between paternally and maternally derived homologs. 
The number of linkage groups in an organism is thus the same as the number of 
chromosome pairs, that is, the haploid chromosome number. See DEOXYRIBONUCLEIC 
ACID (DNA); GENE; GENETIC CODE; LINKAGE (GENETICS); RECOMBINATION (GENETICS). 
  
    Sex chromosomes.  Genes responsible for or associated with sex 
determination may be located on a special chromosome pair. These are the sex 
chromosomes or allosomes; the remainder of the chromosomes are referred to as 
autosomes. Exchange or recombination between segments of chromosomes 
containing the sex-determing genes is prevented by the nonhomology of their 
DNAs. The limitation on recombination between allosomes results in their 
genetic isolation and separate evolution, which in many cases has led to a 
difference in their size. The larger partner normally carries genes that code 
for femaleness and is termed the X chromosome, and its smaller partner is the 
Y chromosome. The genes for maleness may be located in the Y chromosome, as in 
humans, or in the autosomes, as in the fruitfly. In many species in which 
maleness is determined by genes located in the autosomes, the Y chromosome may 
be absent. 
    An individual's sex is generally determined by the balance of sex genes, 
that is, by the ratio of the numbers of X to Y chromosomes, or  X chromosomes 
to autosomes. In mammals the female usually has two X chromosomes. In humans, 
with a normal complement of 46 chromosomes, a normal female has the 
constitution 46,XX, and all normal human eggs would have the constitution 
23,X. The male, being 46,XY, forms two kinds of sperm cells in equal numbers, 
those carrying an X chromosome (23,X) and those with a Y (23,Y). Random fusion 
of eggs and sperm at fertilization thus results in equal numbers of XX females 
and XY males. There are many variations on the basic XX:XY sex chromosome 
system.  
    Sex chromosomes, and particularly X chromosomes, not only contain genes 
concerned with the determination or development of sex, but contain large 
numbers of genes determining other characters. In humans, for instance, there 
are genes responsible for color vision, blood clotting, muscular dystrophy, 
and a host of other characteristics located on the X chromosome. Such genes 
form part of the linkage group on that chromosome, are associated with the 
inheritance of sex, and are referred to as sex-linked genes. See HUMAN 
GENETICS; SEX DETERMINATION; SEX-LINKED INHERITANCE. 
  
    Replication.  The exact replication of DNA and chromosomes is a basic 
necessity for the transmission of a complete and unaltered set of genes from 
parental to daughter cells in the soma and for the familial transmission of 
heritable traits necessary for the reproductive continuity of the species. The 
double-helical structure of DNA contains the potential for its exact 
duplication, since the two strands are complementary. During replication the 
strands separate, and each acts as a template on which is assembled a 
complementary strand built from free nucleotides. This results in the 
formation of two identical double helices, each containing one strand of 
parental origin (semiconservative replication). Replication takes place during 
a part of interphase. It is initiated at many hundreds of sites along the 
chromosome in a temporal pattern characteristic for a given chromosome. Thus 
some DNA replicates early and some, such as the DNA in heterochromatin, later. 
  
    Aberration.  An aberration is a change in the normal complement 
(karyotype) of chromosomes involving an alteration either in chromosome number 
(heteroploidy) or in chromosome structure (structural changes). 
    Alterations in chromosome number (heteropolidy) may involve loss, or gain, 
of entire sets of chromosomes (euploidy) or of one or more individual 
chromosomes (aneuploidy). There are various causes of heteroploidy, including: 
cell and nuclear fusion, a process that may be enhanced in animal cells by 
exposure to virus; successive nuclear divisions without intervening cell 
division (endomitosis); failure of spindle development and of chromatid 
separation at mitosis, which can be induced by exposure to drugs like 
colchicine; failure of meiosis, giving germ cells with a diploid chromosome 
number (1% of human sperm are diploid); double fertilization, that is, two 
sperms fertilizing one egg, or a fertilized egg fusing with a polar body. 
Polyploidy may occur in plant species and even in humans, but polyploid human 
embryos usually abort although some triploid fetuses may survive to birth. 
Aneuploidy results from failure of proper separation of chromosomes or 
chromatids at meiosis or mitosis. It occurs spontaneously, but with increased 
frequency after mutagen exposure, such as x-rays. 
  
    Structural changes within and between chromosomes result from damage to 
the DNA, leading to breakages across the chromatid or chromosome threads. 
Breaks may remain unjoined to give fragments which, if acentric (lacking the 
centromere), may be lost at cell division; alternatively, they may rejoin with 
other broken ends in the same or different chromosomes to give new structures 
and gene combinations (Fig. 2). Structural changes can be classified into 
intra- and interchromosomal events and, depending on the unit of breakage and 
exchange, into chromatid-type and chromosome-type aberrations. 
    Structural aberrations occur spontaneously, and in humans their incidence 
increases with age. However, their frequency can be very much increased on 
exposure to physical mutagens (x-rays, neutrons, ultraviolet light) or 
chemical mutagens/carcinogens (aflatoxin, burnt products of plant and animal 
origin, alkylating agents). Chromosomes may also be broken mechanically at 
cell division if they (abnormally) possess two or more centromeres. Structural 
rearrangements may be detected genetically, since the linear arrangement of 
certain genes will be altered, or in favorable cases by microscopic inspection 
of the chromosomes. See CARCINOGEN; MUTATION; RADIATION INJURY. 
 �34s[H. John Evans]
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�6v�2v�32sChromosphere �21m �33s
    A transparent tenuous layer of gas in the atmosphere of the Sun resting on
the photosphere. The chromosphere is between 2000 and 3000 mi (3000 and 5000
km) thick. It is a thermal buffer zone between the low temperature at the top
of the photosphere (7500DGF or 4400 K) and the high temperature of the corona
(12 - 4 x 10DGF or 1 - 2 x 10 sup 6 sup 0 K). The structure of the chromosphere
consists of a homogeneous atmosphere, in which are embedded forests of hairy
spikes known as spicules. The largest spicules extend 6000 mi (10,000 km) up
into the corona above the homogeneous layer. They are believed to carry the
energy that maintains the high coronal temperature. See SUN.
 �34s[John W. Evans]
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�6v�2v�32sChronograph   �21m �33s 
    A device for recording the epoch of an event. In its older form the 
astronomical chronograph includes a drum rotated at constant speed and a pen 
actuated by an electromagnet. A spiral line is traced on paper on the drum. 
Signals from a clock produce a set of time marks and signals made by an 
observer produce other marks, so that the epoch of the events observed can be 
determined.  
    Modern chronographs are of the digital printing type. In one form rotating 
type wheels carry numbers. In another form the time is accumulated 
electronically, and when a signal is given the output is printed by keys. See 
TIME-INTERVAL MEASUREMENT. 
 �34s[William Markowitz]
�33s 
  
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�6v�2v�32sChronometer   �21m �33s 
    A large, strongly built watch especially designed for precise timekeeping 
on ships at sea. The name is sometimes loosely applied to any fine watch. 
    The features that distinguish a chronometer from a watch are (1) a heavy 
balance wheel, the axis of which is kept always vertical; (2) a balance spring 
wound in cylindrical shape, instead of a nearly flat helix; (3) a special 
escapement; and (4) a fusee, by means of which the power of the mainspring is 
made to work through a lever arm of continuously changing length, being 
shortest when the spring is tightly wound and longest when it has run down, 
thus regulating the transmitted power so that it is approximately constant at 
all times. 
    Oceangoing ships during a voyage formerly relied completely on 
chronometers keeping Greenwich mean time as a means for determining longitude. 
The broadcasting of radio time signals that became widespread in the decade 
1920 - 1930 has made Greenwich mean time available to mariners at almost any 
time of day, and chronometers are no longer indispensable for determining 
longitude at sea. See CLOCK. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sChrysoberyl   �21m �33s 
    A mineral having composition BeAl2O4 and crystallizing in the orthorhombic 
system. The hardness is 8.5 (Mohs scale) and the specific gravity is 3.7 - 
3.8. The luster is vitreous and the color various shades of green, yellow, and 
brown. There are two gem varieties of chrysoberyl. Alexandrite, one of the 
most prized of gemstones, is an emerald green but in transmitted or in 
artificial light is red. Cat's eye, or cymophane, is a green chatoyant variety 
with an opalescent luster. When cut en cabochon, it is crossed by a narrow 
beam of light. This property results from minute tabular cavities that are 
arranged in parallel position. 
    Chrysoberyl is a rare mineral found most commonly in pegmatite dikes and 
occasionally in granitic rocks and mica schists. Gem material is found in 
stream gravels in Ceylon and Brazil. The alexandrite variety is found in the 
Ural Mountains. In the United States chrysoberyl is found in pegmatites in 
Maine, Connecticut, and Colorado. See BERYLLIUM; GEM. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sChrysocolla   �21m �33s 
    A silicate mineral, composition CuSiO3PRODUCT2H2O. Small acicular crystals 
have been observed, but it ordinarily occurs in impure cryptocrystalline 
crusts and masses with conchoidal fracture. The hardness varies from 2 to 4 on 
Mohs scale, and the specific gravity varies from 2.0 to 2.4. The luster is 
vitreous and it is normally green to greenish-blue, but may be brown to black 
when impure. Chrysocolla is a secondary mineral occurring in the oxidized 
zones of copper deposits, where it is associated with malachite, azurite, 
native copper, and cuprite. It is a minor ore of copper. See SILICATE 
MINERALS.  
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sChrysomonadida   �21m �33s 
    An order of the class Phytamastigophorea. Chrysomonads, also known as 
Chrysomonadina, are usually small flagellates. They are yellow to brown 
because of phycocrysin in the usual one or two chromatophores, but some lack 
chromatophores. Many species form diagnostic siliceous cysts. Palmelloid 
colonies (Hydrurus) consist of a tough, gelatinous matrix holding many 
nonflagellate cells. Starch is not formed, but fats are, and the refractive 
carbohydrate leucosin is common. The flagella are usually two, rarely three, 
and are subequal. Nuclei are small. See PHYTAMASTIGOPHOREA; SILICOFLAGELLATA. 
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sChrysophyta   �21m �33s 
    A division of plants often called the golden-brown algae because of their 
yellow-green or golden-brown color. The pigments localized in the plastids 
(chromatophores) are chlorophyll, carotenes, and xanthophyll. The predominance 
of the last two pigments is responsible for the color of these algae. Instead 
of forming starch, the cells of these algae produce and store the excess 
photosynthate (food) as droplets of oil and as granules of an insoluble 
carbohydrate called leucosin. See CAROTENOID; CHLOROPHYLL. 
    Structurally, these plants range from predominantly unicellular to 
colonial species, but in a few cases multicellular individuals occur which may 
be coenocytic. Usually the cell wall is composed of two overlapping halves, 
frequently impregnated with silica. Motile vegetative cells bear flagella 
which may be equal or unequal in length. For the most part reproduction is 
asexual by means of cell division, zoospores (swimming spores), or immobile 
spores.  
    Chrysophyta comprises three classes: the Xanthophyceae, or yellow-green 
algae; the Chrysophyceae, or golden-brown algae; and the Bacillariophyceae, or 
diatoms. These three classes are known to include more than 300 genera and 
6000 mainly microscopic species, three-fourths of which are fresh-water forms 
and one-fourth marine. 
    Yellow-green algae are predominantly fresh-water organisms but others may 
be found on moist terrestrial habitats, a few are marine, and some have been 
isolated from subterranean soil samples. The yellow-green algae are readily 
separated from the green (Chlorophyta) on the basis of pigmentation, lack of 
starch, unequal flagellations, and the overlapping construction of the cell 
walls in many genera. See CHLOROPHYTA. 
    Golden-brown algae are widely distributed in both fresh and salt water, 
but are usually not found in great abundance. Many are planktonic and are 
found only in cold water or during the colder portions of the year. The motile 
cells vary from those with a single flagellum to those with two flagella of 
either equal or unequal length. 
    Diatoms are the best-known and most numerous members of the Chrysophyta 
with over 5000 species. They are usually found wherever there is water and an 
adequate amount of light. All diatoms are unicellular, but some species are 
linked together to form chains or pseudocolonies; others may be enclosed in 
mucous sheaths and appear as small filamentous forms. They are divided into 
two orders on the basis of symmetry which is clearly shown by the pattern of 
markings on the valves: (1)Centrales, in which the form is often circular, and 
the markings on the valves are radial. The plastids are small and numerous. 
These organisms are nonmotile in vegetative condition and occur predominantly 
in the ocean. (2)Pennales, in which the form is elongate or boat-shaped and 
the markings of the valves are bilateral. In this order there are two 
plastids, which are large, frequently irregular, and embedded in the 
peripheral layer of the protoplasm. See ALGAE; DIATOM. 
 �34s[Paul A. Vestal]
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�6v�2v�32sCiconiiformes   �21m �33s 
    An order of predominantly long-legged, long-necked birds, including 
herons, storks, ibises, spoonbills, and their relatives. The flamingos 
(Phoenicopteridae) have been traditionally placed in the Ciconiiformes, but 
also show many strong resemblances to the waterfowl (Anseriformes); some 
authors place the flamingos in an intermediate order, Phoenicopteriformes. 
Only the flamingos have the toes fully webbed. Bill shape varies with food 
habits: spearing prey, probing in mud, or straining water or silt. Many 
species build tree nests but some nest on the ground; flamingo nests are 
mounds of mud. Colonial nesting is common in this order. The order is 
virtually cosmopolitan in distribution, and particularly well represented in 
tropical regions. 
    The true storks (Ciconiidae) differ from other members of the order in 
several anatomical and behavioral characters, resembling in these respects the 
New World vultures or condors (Cathartidae), and a relationship between these 
two families of superficially quite different birds has been suggested. See 
AVES.  
 �34s[Kenneth C. Parkes]
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�6v�2v�32sCidaroida   �21m �33s 
    An order of Perischoechinoidea in which the narrow ambulacra comprise two 
columns of simple plates, and each broad ambulacral plate carries only one 
primary tubercle. They are unique among echinoids as the only Paleozoic order 
surviving to the present day. They have remained prominent and beautiful 
elements in Indonesian and Australasian faunas. See ECHINOIDEA; 
PERISCHOECHINOIDEA.  
 �34s[Howard B. Fell]
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�6v�2v�32sCilia and flagella   �21m �33s 
    Centriole-based, motile cell extensions. These organelles are usually 
indistinguishable in fine structure as seen with the electron microscope, but 
quantitatively there are many (several hundred) cilia, and few or fewer 
(usually one or two) flagella, on one cell. Bacterial or prokaryotic flagella 
are entirely different organelles that are not considered in this article, 
which concerns only eukaryotic flagella. 
    Flagella move with undulatory motion in which successive bending waves 
progress along the length of the organelle, whereas cilia move with flexural 
motion consisting of a planar effective stroke, with the organelle extended 
perpendicular to the cell body, followed by a nonplanar curving recovery 
stroke, with the organelle pulled parallel to the cell body. Both organelles 
function to move water past the cell. Their action may bring food and oxygen 
into an animal, or it may propel the cell to a new environment. 
    The words cilium (eyelash) and flagellum (whip) are accurate descriptions 
of the appearance of these cell organelles when they are seen under the light 
microscope. Cilia are present on protozoa, such as Paramecium, and on metazoan 
cells of many different tissue types. A flagellum is present on human sperm; 
in fact, the sperm of most animals possess a flagellum, and, correspondingly, 
male gametes of many lower plants are flagellated. In addition, many ordinary 
types of cells of vertebrates, for example, from the thyroid, the kidney, or 
the pituitary gland, possess modified nonmotile derivatives that resemble 
cilia. See CILIOPHORA; EPITHELIUM; SPERM CELL. 
    Relative to cell size, both cilia and flagella are very long organelles. 
Flagella may be over 50 micrometers long. Certain compound cilia, such as the 
comb plates of ctenophores, are macroscopic structures, visible to the naked 
eye. Usually, however, cilia range from 10 to 15 MUm in length. 
    The electron microscope reveals that the cilium or flagellum is really an 
internal organelle since it is bounded by the cell membrane and enclosed at 
the tip. The main internal structure of the cilium is the axoneme. Under the 
electron microscope, a single axoneme appears to contain a fixed pattern of 
microtubules. The microtubules are not simple single units; rather, nine 
doublet microtubules are found on the periphery of the axoneme surrounding two 
central elements. This is the so-called 9 + 2 pattern (see illustration). Each 
peripheral doublet is composed of one complete and one partial microtubule. 
The microtubules are themselves composed of subunits arranged into microfibers 
or protofilaments. See MICROTUBULE. 
    At the base of the cilium or flagellum there is a basal body, or 
kinetosome, that is similar to, and sometimes derived from, a centriole. The 
basal body may have extensions of various sorts attached to it, notably a 
basal foot that indicates effective stroke direction and prominent striated 
rootlet fibers in many cilia. Ordinarily, the ciliary axoneme originates and 
grows in a membrane protrusion which forms just above the basal body, either 
at the cell surface or deeper inside the cytoplasm. The basal body remains 
attached to the cell membrane throughout morphogenesis by a structure that 
extends from the microtubules to the membrane, where it is seen as a ciliary 
necklace. See CENTRIOLE. 
 �34s[Peter Satir ;Ian Gibbons]
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�6v�2v�32sCiliatea   �21m �33s 
    The single class of the subphylum Ciliophora. This group has the 
characteristics of those defined for the subphylum. This protozoan class is 
divided into the subclasses Holotrichia, Peritrichia, Suctoria, and 
Spirotrichia. See CILIOPHORA; HOLOTRICHIA; PERITRICHIA; SPIROTRICHIA; SUCTORIA. 
 �34s[John O. Corliss]
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�6v�2v�32sCiliophora   �21m �33s 
    A subphylum of the Protozoa. The ciliates are a fairly homogeneous group 
of highly differentiated, unicellular organisms. Over 5000 species have been 
described, and many more surely exist but remain to be discovered. Typically, 
ciliates are larger than most other protozoans, ranging from 10 to 3000 
micrometers (about 1/2500 to 1/8 in.). Some larger species are easily visible 
to the naked eye. The majority of them are free-living forms, found abundantly 
in a variety of fresh- and salt-water habitats, although a few entire groups 
live in association with other organisms, generally as harmless ecto- or 
endocommensals. Their principal value to humans is as experimental animals in 
a host of biological investigations. 
    A classification scheme for the ciliates is given below. It is widely 
recognized as the most reasonable and useful classification currently 
available. Separate articles appear on each group listed. Subordinal divisions 
of some orders are commonly recognized by protozoologists. 
Subphylum Ciliophora 
    Class Ciliatea 
    Subclass Holotrichia 
        Order: Gymnostomatida 
               Trichostomatida 
               Chonotrichida 
               Apostomatida 
               Astomatida 
               Hymenostomatida 
               Thigmotrichida 
    Subclass Peritrichia 
        Orderd Peritrichica 
    Subclass Suctoria 
        Order Suctorida 
    Subclass Spirotrichia 
        Order: Heterotrichida 
               Oligotrichida 
               Tintinnida 
               Entodiniomorphida 
               Odontostomatida 
               Hypotrichida 
    The usual ciliate life cycle is fairly simple. An individual feeds and 
undergoes binary fission, and the resulting filial products repeat the 
process. Some commensal or parasitic forms have a more complicated life 
history. Some ciliates, including free-living species, have a cystic stage in 
their cycle. As in other kinds of Protozoa this stage often serves as a 
protective phase during adverse environmental conditions, such as desiccation 
or lack of food. It also may be important in distribution, and thus possibly 
in preservation, of the species. 
    Six major characteristics aid in distinguishing the Ciliophora from other 
protozoan groups. Not all of these are entirely unique, but when taken 
together they are definitely distinctive of ciliates: mouth, ciliation, 
infraciliature, nuclear apparatus, fission, and reproduction. 
    Most Ciliophora possess a true mouth or cytostome often associated with a 
buccal cavity containing compound ciliary organelles. However, some ciliates 
are completely astomatous, that is, mouthless. Nutrition is heterotrophic in 
ciliates.  
    The Ciliophora possess simple cilia or compound ciliary organelles, often 
in abundance, in at least one stage of their life cycle. Morphologically, 
cilia are relatively short and slender hairlike structures, whose 
ultrastructure is known, from electron microscope studies, to be composed of 
nine peripheral and two central fibrils. Membranes and membranelles are 
characteristically associated with the mouth or buccal areas and serve to 
bring food into the oral opening, although they sometimes aid in locomotion as 
well. See CILIA AND FLAGELLA. 
    Infraciliature is present, without exception, at a subpellicular level in 
the cortex. The infraciliature consists essentially of basal bodies, or 
kinetosomes, associated with cilia and ciliary organelles at their bases, plus 
certain more or less interconnecting fibrils. 
    Ciliophora possess two kinds of nuclei, and at least one of each is 
usually present. The smaller, or micronucleus, contains recognizable 
chromosomes and behaves much as the single nucleus in cells of metazoan 
organisms. The larger, or macronucleus, is considered indispensable in 
controlling metabolic functions, and is recognized as having genic control 
over all phenotypic characteristics of ciliates. 
    Ciliophora exhibit a type of binary fission commonly known as transverse 
division. In ciliates the splitting results in two filial organisms, the 
anterior or proter and the posterior or opisthe which, geometrically speaking, 
show homothety with respect to identical structures possessed by each. Thus, 
homothetogenic is both a broader and most exact descriptive term. 
    Ciliophora lack true sexual reproduction. Ciliates do not show syngamy, 
with fusion of free gametes. Processes such as conjugation are considered to 
be sexual phenomena, since meiosis and chromosome recombination are involved, 
but not sexual reproduction. In addition to conjugation, certain ciliates 
exhibit forms of sexual phenomena known as autogamy and cytogamy. See 
PROTOZOA; REPRODUCTION (ANIMAL). 
 �34s[John O. Corliss]
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�6v�2v�32sCinematography   �21m �33s 
    The process of producing the illusion of a moving picture. Cinematography 
includes two phases: the taking of the picture with a camera and the showing 
of the picture with a projector. The camera captures the action by taking a 
series of still pictures at regular intervals; the projector flashes these 
pictures on a screen at the same frequency, thus producing an image on the 
screen that appears to move. This illusion is possible only because of the 
persistence of vision of the human eye. The still pictures appear on the 
screen many times a second, and although the screen is dark equally as long as 
it is lighted by the projected image, they do not seem to be a series of 
pictures but appear to the viewer to be one continuous picture. 
  
    Cameras.  Still photographs are taken by motion-picture cameras (movie 
cameras) at the rate of 24/s, even faster at times. The photographs are sharp 
and clear, and if they were superimposed on one another would be found to be 
extremely uniform with respect to position. This last feature is essential in 
order to have the image appear steady on the screen. A motion-picture camera 
consists of five basic parts: the lens, the shutter, the gate, the film 
chamber, and the pull-down mechanism. 
    The lens on a motion-picture camera collects the light from the scene 
being photographed and focuses it on the photographic emulsion in the camera 
aperture. Lenses vary in focal length. Variable-focal-length lenses (zoom 
lenses) are used extensively not only for filming sport events but also in 
making feature films, documentaries, and television films. See TELEPHOTO LENS; 
ZOOM LENS. 
    The shutter in a motion-picture camera rotates and exposes the film 
according to its angle of opening. When the shutter is open to its widest 
angle, the film receives its maximum exposure. Many cameras have variable 
shutters; that is, the angle of opening can be changed while the camera is 
running.  
    The aperture called the gate is the passageway through which the film is 
channeled while it is being exposed. It consists of the aperture plate, which 
is in front of the film and masks the frame, or picture; the pressure plate, 
which is in back of the film and holds it firmly against the aperture plate; 
and the edge guides, which keep the film stable laterally. 
    The film chamber holds the unexposed film at one end and collects the 
exposed film at the other. Although some motion-picture cameras have an 
interior film chamber, it is usually a separate piece of equipment called the 
film magazine. 
    The part of the pull-down mechanism called the intermittent movement pulls 
the film down through the gate of the camera one frame at a time; in a 
conventional 35-mm camera, each frame is four perforations high. Some 
wide-screen cameras have frames five or six perforations high. The pull-down 
claw engages the perforations and pulls the film down into place to be 
exposed. At the bottom of its stroke, the claw remains stationary for a moment 
to position the film and then disengages itself and returns to the upper 
portion of the stroke to pull another frame into place. During the time the 
claw is returning to the top of its stroke, the film is stationary and the 
shutter opens and exposes the film. 
    When sound is being recorded, cameras must be absolutely quiet. If a 
camera is noisy, it must be put in a sound enclosure called a blimp. Most 
cameras have to be blimped, although the Mitchell BNC (Blimped News Camera) is 
self-blimped and is quiet enough to be used on a sound stage (see 
illustration). The Mitchell BNC, because of its quiet operation and its utmost 
reliability as well as its ease of operation, has been the standard sound 
camera the world over. It is used as a model of comparison of all 35-mm 
professional motion-picture cameras. Several other cameras have been built, 
however, that compare favorably with the classic Mitchell. These cameras are 
smaller and lighter than the Mitchell, thereby gaining much of their 
popularity. They are also reflex cameras, in which the scene being 
photographed can be viewed directly through the taking lens and does not 
require a side monitoring finder, which brings about a problem of parallax. 
Practically all cameras being manufactured today are reflex cameras, including 
the BNCR, the reflex version of the BNC. These smaller and lighter cameras 
have enabled production companies to achieve more mobility and to shoot many 
pictures on actual locations away from studios. 
    All sound is recorded on H-in. (6.35-mm) tape. Since there is no way to 
accurately control the speed of the tape through the machine, some sort of 
``sync'' pulse must be put on the tape in order to synchronize the recorded 
sound with the picture when they are put together in a ``married'' print. The 
sync pulse is usually an inaudible 1-volt, 60-Hz (in Europe, 50-Hz) pulse put 
on the tape at the time of recording. There are two types of sound tracks, 
optical and magnetic, on ``release prints,'' the prints seen by the audience. 
All 35-mm and practically all 16-mm release prints have optical sound tracks. 
A 16-mm film may have a magnetic sound track if only one or two prints are to 
be made, but this is seldom the case. An optical sound track is printed 
photographically on the film; a magnetic sound track is printed in the same 
manner as on a tape recorder, after the film is coated with an iron-oxide 
stripe in the area reserved for the sound track. 
    In order to give the camera more mobility and to move quickly from one 
position to another, 35-mm cameras are almost always used on a four-wheel 
vehicle, a ``crab dolly'' (see illustration). This dolly can be steered in the 
usual manner by turning the wheels for ``curve,'' and it can be put in a mode 
where all four wheels turn in the same direction so that it can go forward and 
backward and also ``crab'' across the stage sideways. A crane is a larger and 
more versatile version of the crab dolly. Available in a wide range of sizes, 
some cranes are slightly larger than the crab dolly; others extend to 30 ft (9 
m) in the air. 
  
    Film.  The film used in the standard motion-picture camera is 35 mm (1.378 
in.) wide and is perforated along both edges. There are 64 perforations (or 
sprocket holes) to each foot of film. Motion-picture film consists of a 
cellulose acetate base approximately 0.006 in. (0.0002 mm) thick and coated 
with a light-sensitive emulsion. In color film, several layers of emulsion are 
applied; each of three of the layers is sensitive to one of the three primary 
colors of light: blue, green, and red. 
    There are two different types of film: negative film, from which a print 
is made in order to see the original subject in its true likeness, and 
reversal film, in which a negative is first formed in the original film and 
from this a positive is formed in the same piece of film (for example, 
Kodachrome). In the negative film two pieces of film are necessary to get a 
picture that can be projected; in the reversal film only one piece of film is 
required, making it a much less expensive process. In 16 mm or Super 8, as 
well as regular 8, the reversal film is generally used because of the cost 
factor. In 16-mm professional work, the original, that is, the film used in 
the camera, is never used for projection. A ``work print'' is made in order to 
edit the film and the original is made to ``conform'' to the edited work 
print. The release prints are made from the ``conformed'' work print and then 
the sound is added. The original film is all-important and is kept in a vault 
at the laboratory; it is used only for making prints. 
  
    Projectors.  The projector in a modern motion-picture theater has four 
main assemblies: the optical sound head, which reproduces optical 
(photographically recorded) sound; the projector head, which projects the 
image onto the screen; the lamphouse, which furnishes the illumination for the 
picture; and the shutter. On some projectors, there is a second sound head for 
the reproduction of magnetic (magnetically recorded) sound. The release print 
of a picture, as contrasted with the negative and the original which are never 
projected in a theater, is usually mounted on a 2000-ft (600-m) reel. It is 
placed in the upper film magazine, and the film is threaded through the 
projector head, down through the optical sound head, and onto the takeup 
magazine. See OPTICAL PROJECTION SYSTEMS. 
  
    Wide-screen processes.  Until 1952 the motion-picture industry as a whole 
used film and equipment that was well standardized. Film was 35 mm wide and 
the camera had a four-perforation pull-down mechanism. These standards had 
been in effect since early in the 20th century. With the advent of sound in 
1926, standard camera speed became established at 24 frames/s and most sound 
tracks were reproduced optically. Beginning in 1952, however, many new 
processes were introduced which departed from these standards and utilized a 
multiplicity of film sizes, aspect ratios, and types of sound tracks. Some 
processes which continue to employ 35-mm film use a means of getting more 
picture on the film. Four-track, six-track, and even seven-track magnetic 
sound has become fairly common. Among the important wide-screen processes have 
been Cinerama, CinemaScope, Todd-AO, and Dimension 150. 
 �34s[Walter Gibbons-Fly]
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�6v�2v�32sCinnabar   �21m �33s 
    A mineral of composition HgS, crystallizing in the hexagonal system. 
Crystals are rare, usually of rhombohedral habit and often in penetration 
twins. Cinnabar most commonly occurs in fine, granular, massive form. It has 
perfect prismatic cleavage, a Mohs hardness of 2.0 - 2.5, and a density of 
8.09. It has either an adamantine luster and vermilion red color or a dull 
luster and brownish-red color. 
    Cinnabar is deposited from hydrothermal solutions in veins and as 
impregnations near recent volcanic rocks and hot springs. It is the principal 
ore of mercury. Notable occurrences are Almaden, Spain; Idria, Italy; near 
Belgrade, Yugoslavia; Kweichow and Hunan provinces, China; Soviet Turkistan; 
New Almaden and New Idria, California; Terlingua, Texas; and several 
localities in Utah, Nevada, New Mexico, Oregon, and Idaho. See MERCURY 
(ELEMENT).  
 �34s[Lawrence Grossman]
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�6v�2v�32sCinnamon �21m �33s
    An evergreen shrub or small tree, Cinnamomum zeylanicum, of the laurel
family (Lauraceae). A native of Ceylon, the plant is now in cultivation in
southern India, Burma, parts of Malaya, West Indies, and South America. The
bark is removed from suckers that grow up from the roots, dried, and packaged
for shipping. Cinnamon is a very important spice for flavoring foods. It is
used in confectionery, gums, incense, dentrifrices, and perfumes. Cinnamon oil
is used in medicine and as a source of cinnamon extract. See MAGNOLIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCircadian rhythms   �21m �33s 
    The daily periodic behavior of organisms is commonly governed by the light 
cycle. However, this cycle serves primarily as a synchronizer or time giver 
which works upon physiologically based rhythmic processes within the organism, 
adjusting the periodicity of these processes to the exact 24-h cycle of the 
environment. Other cyclic features of the environment, such as temperature 
variations, mechanical disturbances, or noise, may serve as time givers, 
either alone or in conjunction with a light cycle. With the experimental 
exclusion of such periodic synchronizing signals, the rhythms of the organisms 
may continue their characteristic form, but at rates (measured by frequency or 
period length) which differ slightly from the exact periodicity of the 
environment. These rhythms are termed circadian rhythms. 
    Without synchronizing cues, circadian rhythms drift out of phase with the 
environment so that an organism will perform rhythmic processes at 
progressively changing times of day. The actual period length of the circadian 
rhythm, while usually fixed in value for one individual under one set of 
experimental conditions, still varies from one individual to the next. The 
range of period lengths for various plants and animals is about 22 - 28 h, 
with the majority being fairly close to 24 h. Thus a group of organisms 
studied simultaneously under conditions which allow the expression of 
circadian rhythms will drift out of phase with one another as would a group of 
mechanical clocks each running at a slightly different speed. 
    The speed of ``biological clocks'' can be altered by changing the tissue 
temperature of the organism. Only a few chemicals are known to change the 
frequency of circadian rhythms. Alcohol slows the rhythm of bean (Phaseolus) 
leaf movements, and this rhythm, as well as a rhythm of swimming activity of 
the unicellular alga Euglena, is slowed by exposure to solutions of heavy 
water (D2O). Many of the powerful drugs which either inhibit or enhance 
metabolic activity are ineffective in altering the frequency of circadian 
rhythms, although some drugs can change the phase or abolish the measured 
expression of the rhythm. 
    The biochemical mechanisms of these rhythms are still unknown. The many 
rhythmic functions which have been measured appear to be expressions of the 
underlying timing process of the circadian rhythm without being directly 
involved in the determination of period length. There is limited evidence 
suggesting that more than one circadian rhythm may occur within a single 
organism.  See  BODY RHYTHM; PLANT GROWTH. 
 �34s[Kenneth S. Rawson]
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�6v�2v�32sCircle �21m �33s
    The curve that is the locus of points in a plane with equal distance
(radius) from a fixed point (center; see illustration). In elementary
mathematics, circle often refers to the finite portion of the plane bounded by
a curve (circumference) all points of which are equidistant from a fixed point
of the plane, that is, a circular disk. Circles are conic sections and are
defined analytically by certain second-degree equations in cartesian
coordinates. Electronic computers have calculated PI (the ratio of the length
of a circle to its diameter) to over 100,000 decimal places. The area of a
circle (circular disk) with radius r is PIr sup 2; the length (circumference)
is 2PIr. The area enclosed by a circle is greater than that bounded by any
other curve of the same length. See ANALYTIC GEOMETRY; CONIC SECTION.


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�6v�2v�32sCircuit breaker   �21m �33s 
    A device to open or close an electric power circuit either during normal 
power system operation or during abnormal conditions. A circuit breaker serves 
in the course of normal system operation to energize or deenergize loads. 
During abnormal conditions, when excessive current develops, a circuit breaker 
opens to protect equipment and surroundings from possible damage due to excess 
current. These abnormal currents are usually the result of short circuits 
created by lightning, accidents, deterioration of equipment, or sustained 
overloads.  
    Formerly, all circuit breakers were electromechanical devices. In these 
breakers a mechanism operates one or more pairs of contacts to make or break 
the circuit. The mechanism is powered either electromagnetically, 
pneumatically, or hydraulically. The contacts are located in a part termed the 
interrupter. When the contacts are parted, opening the metallic conductive 
circuit, an electric arc is created between the contacts. This arc is a 
high-temperature ionized gas with an electrical conductivity comparable to 
graphite. Thus the current continues to flow through the arc. The function of 
the interrupter is to extinguish the arc, completing circuit-breaking action. 
436A 
    In oil circuit breakers, the arc is drawn in oil. The intense heat of the 
arc decomposes the oil, generating high pressure that produces a fluid flow 
through the arc to carry energy away. At transmission voltages below 345 kV, 
oil breakers used to be popular. They are increasingly losing ground to 
gas-blast circuit breakers such as air-blast breakers and SF6 circuit breakers. 
    In air-blast circuit breakers, air is compressed to high pressures. When 
the contacts part, a blast valve is opened to discharge the high-pressure air 
to ambient, thus creating a very-high-velocity flow near the arc to dissipate 
the energy. In SF6 circuit breakers, the same principle is employed, with SF6 
as the medium instead of air. In the ``puffer'' SF6 breaker, the motion of the 
contacts compresses the gas and forces it to flow through an orifice into the 
neighborhood of the arc. Both types of SF6 breakers have been developed for 
ehv (extra high voltage) transmission systems. 
    Two other types of circuit breakers have been developed. The vacuum 
breaker, another electromechanical device, uses the rapid dielectric recovery 
and high dielectric strength of vacuum. A pair of contacts is hermetically 
sealed in a vacuum envelope. Actuating motion is transmitted through bellows 
to the movable contact. When the contacts are parted, an arc is produced and 
supported by metallic vapor boiled from the electrodes. Vapor particles expand 
into the vacuum and condense on solid surfaces. At a natural current zero the 
vapor particles disappear, and the arc is extinguished. Vacuum breakers of up 
to 242 kV have been built. 
    The other type of breaker uses a thyristor, a semiconductor device which 
in the off state prevents current from flowing but which can be turned on with 
a small electric current through a third electrode, the gate. At the natural 
current zero, conduction ceases, as it does in arc interrupters. This type of 
breaker does not require a mechanism. Semiconductor breakers have been built 
to carry continuous currents up to 10,000 A. 
 �34s[Thomas H. Lee]
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�6v�2v�32sCircuit testing (electricity) �21m �33s
    The testing of electric circuits to determine and locate any of the
following circuit conditions: (1) an open circuit, (2) a short circuit with
another conductor in the same circuit, (3) a ground, which is a short circuit
between a conductor and ground, (4) leakage (a high-resistance path across a
portion of the circuit, to another circuit, or to ground), and (5) a cross (a
short circuit or leakage between conductors of different circuits). Circuit
testing for complex systems often requires extensive automatic checkout gear
to determine the faults defined above as well as many quantities other than
resistance.
    In cable testing, the first step in fault location is to identify the
faulty conductor and type of fault. This is done with a continuity tester,
such as a battery and flashlight bulb or buzzer (Fig. 1), or an ohmmeter.
    Useful for locating faults in relatively low-resistance circuits, the
Murray loop is shown in Fig. 2 with a ground fault in the circuit under test.
A known ``good'' conductor is joined to the faulty conductor at a convenient
point beyond the fault but at a known distance from the test connection. One
terminal of the test battery is grounded. The resulting Wheatstone bridge is
then balanced by adjusting RB until a null is obtained, as indicated by the
detector in Fig. 2. Ratio (R sub A)/(R sub B) is then known. For a circuit
having a uniform ratio of resistance with length, circuit resistance is
directly proportional to circuit length. Therefore, the distance to the fault
is determined from the procedure given by Eqs. (1) - (3). From Eq. (3) and a
knowledge of total length l of the circuit, once ratio r has been measured, the
location of the fault x is determined. 

   *** (R sub C) VARIES AS l + (l - x)	     (R sub D) VARIES AS x ***	 (1)

  *** (R sub A)/(R sub B) = r = (R sub C)/(R sub D) = (2l - x)/x ***	 (2)

			   *** x = 2l/(r - 1) ***			 (3)

    The Varley loop test is similar to the Murray loop test except for the
inclusion of the adjustable resistance R. The Varley loop is used for fault
location in high-resistance circuits.
    An alternating-current capacitance bridge can be used for locating an open
circuit as shown in Fig. 3. One test terminal is connected to the open
conductor and the other terminal to a conductor of known continuity in the
cable. All conductors associated with the test are opened at a convenient
point beyond the fault but at a known distance from the test connection. An
audio oscillator supplies the voltage to the bridge, which is balanced by
adjusting R (sub B) for a null as detected by the earphones. Measured ratio
(R sub A)/(R sub B) equals the ratio of capacitances between the lines and the
grounded sheath. Because each capacitance is proportional to the length of line
connected to the bridge, the location of the open circuit can be determined
from Eq. (4).

      *** (R sub A)/(R sub B) = (C SUB C )/(C SUB D) ***	  (4)

See CIRCUIT ELECTRICITY.




 �34s[Charles E. Applegate]
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�6v�2v�32sCircuit (electricity)   �21m �33s 
    A general term referring to a system or part of a system of conducting 
parts and their interconnections through which an electric current is intended 
to flow. A circuit is made up of active and passive elements or parts and 
their interconnecting conducting paths. The active elements are the sources of 
electric energy for the circuit; they may be batteries, direct-current 
generators, or alternating-current generators. The passive elements are 
resistors, inductors, and capacitors. The electric circuit is described by a 
circuit diagram or map showing the active and passive elements and their 
connecting conducting paths. 
    Devices with an individual physical identity, such as amplifiers, 
transistors, loudspeakers, and generators, are often represented by equivalent 
circuits for purposes of analysis. These equivalent circuits are made up of 
the basic passive and active elements listed above. 
    Electric circuits are used to transmit power as in high-voltage power 
lines and transformers or in low-voltage distribution circuits in factories 
and homes; to convert energy from or to its electrical form as in motors, 
generators, microphones, loud-speakers, and lamps; to communicate information 
as in telephone, telegraph, radio, and television systems; to process and 
store data and make logical decisions as in computers; and to form systems for 
automatic control of equipment. 
  
    Electric circuit theory includes the study of all aspects of electric 
circuits, including analysis, design, and application. In electric circuit 
theory the fundamental quantities are the potential differences (voltages) in 
volts between various points, the electric currents in amperes flowing in the 
several paths, and the parameters in ohms or mhos which describe the passive 
elements. Other important circuit quantities such as power, energy, and time 
constants may be calculated from the fundamental variables. For a discussion 
of these parameters see ADMITTANCE; CONDUCTANCE; ELECTRICAL IMPEDANCE; 
ELECTRICAL RESISTANCE; REACTANCE; SUSCEPTANCE; TIME CONSTANT. 
    Electric circuit theory is often divided into special topics, either on 
the basis of how the voltages and currents in the circuit vary with time 
(direct-current, alternating-current, nonsinusoidal, digital, and transient 
circuit theory) or by the arrangement or configuration of the electric current 
paths (series circuits, parallel circuits, series-parallel circuits, networks, 
coupled circuits, open circuits, and short circuits). Circuit theory can also 
be divided into special topics according to the physical devices forming the 
circuit, or the application and use of the circuit (power, communication, 
electronic, solid-state, integrated, computer, and control circuits). See 
ALTERNATING-CURRENT CIRCUIT THEORY; CIRCUIT (ELECTRONICS); COUPLED CIRCUITS; 
DIRECT-CURRENT CIRCUIT THEORY; ELECTRIC TRANSIENTS; INTEGRATED CIRCUITS; 
MAGNETIC CIRCUITS; NEGATIVE-RESISTANCE CIRCUITS; OPEN CIRCUIT; PARALLEL 
CIRCUIT; SERIES CIRCUIT; SHORT CIRCUIT. 
 �34s[Clarence F. Goodheart]
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�6v�2v�32sCircuit (electronics)   �21m �33s 
    An electric circuit in which the equilibrium of electrons in some of the 
components is upset by a means other than an applied voltage. The means by 
which electron equilibrium is upset leads to further designation of electronic 
circuits. In circuits with thermionic tubes, electron equilibrium is upset by 
thermal emission. In solid-state circuits, electron equilibrium is upset by 
intentionally introduced irregularities of crystals. Electronic circuits are 
also classified by their functions. See AMPLIFIER; DETECTOR; DIGITAL COUNTER; 
ELECTRONIC POWER SUPPLY; MODULATOR; OSCILLATOR; RECTIFIER; SWEEP GENERATOR; 
SWITCHING CIRCUIT; WAVE-SHAPING CIRCUITS. 
    Electronic circuits can consist of discrete elements exclusively, coupled 
integrated circuits exclusively, or combinations of both kinds. Integrated 
circuits can consist of groups of lumped components such as resistors, 
capacitors, and active devices, or of exotic combinations of passive and 
active components intended to have properties not normally available in lumped 
discrete-element networks. See INTEGRATED CIRCUITS. 
    Electronic circuits find application in all branches of industry and in 
the home, both for entertainment equipment and increasingly for control. 
Because of their low power dissipation and fast response, they are excellent 
control circuits. Computers, communication systems, and navigation systems use 
many types of electronic circuits. 
 �34s[Keats A. Pullen, Jr.]
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�6v�2v�32sCirculation   �21m �33s 
    Those processes by which metabolic materials are transported from one 
region of an organism to another. Ultimately, the essential gases, nutrients, 
and waste products of metabolism are exchanged across cell membranes by 
diffusion. Diffusion is the movement of material, by random motion of 
molecules, from a region of high concentration to one of low concentration. 
The amount of material moved from one place to another depends on the 
difference in concentrations and on the distance between the two points. The 
greater the distance, the less movement of material per unit time for a given 
difference in concentration. Consequently, in all but the smallest animals, 
convection (or bulk circulation) of materials to the cell must be employed to 
supplement diffusion. 
    Protoplasmic movement aids diffusion at the intracellular level. In 
multicellular animals, however, either the external medium or extracellular 
body fluids, or both, are circulated. In sponges and coelenterates, water is 
pumped through definite body channels by muscular activity or, more often, by 
cilia or flagella on the cells lining the channels. 
    Coelenterates have a body wall derived from two cell layers; an outer 
ectoderm is separated from an inner endoderm by a noncellular gelatinous 
material (mesoglea). All higher animals have bodies consisting of three cell 
layers, with the ectoderm being separated from the endoderm by a cellular 
layer of mesoderm. The mesoderm proliferates and separates to develop a 
fluid-filled body cavity or coelom. The coelom separates the ectoderm 
(together with an outer layer of mesoderm) from the endoderm (which has an 
inner layer of mesoderm). Coelomic fluid is moved around by body movements or 
ciliary activity, but in larger animals this movement is usually inadequate to 
supply the metabolic requirements of the organs contained within the coelom. 
These needs are provided for by pumping a fluid, blood, to them through 
vessels, the blood vascular system. See BLOOD. 
    When the blood is in a separate compartment from the rest of the 
extracellular fluid, the vascular system is described as closed. The two 
principal components of such systems are hearts and blood vessels. In such a 
system, the blood is circulated by a pump, the heart, through special 
channels, blood vessels; it comes into close association with the tissues only 
in the capillaries, fine vessels with walls only one cell thick. In some 
tissues or regions, larger blood spaces may exist, called sinuses. A closed 
vascular system is found in most annelids (segmented worms and leeches), 
cephalopod mollusks (squids and octopods), holothurian echinoderms (sea 
cucumbers), and vertebrates. See BLOOD VESSELS; HEART (VERTEBRATE). 
    In vertebrates, a functional but anatomically closed connection exists 
between the extracellular spaces (between the cells) and the blood vascular 
system in the form of lymph channels. Lymph is derived from the noncellular 
component of blood (plasma), modified in its passage through the tissues, and 
is conducted to the veins by blind-ending lymphatic vessels, which are 
separate from blood vessels and coelomic space. See LYMPHATIC SYSTEM. 
    In most arthropods (crustaceans, insects), most mollusks (shellfish), and 
many ascidians (sea squirts), the extracellular spaces are confluent with the 
blood system. In these animals, blood is pumped through a limited network of 
vessels into a body cavity called a hemocoel. After bathing the tissues, blood 
(called hemolymph in these organisms) collects in sinuses and returns to the 
heart. This is the open vascular system. In animals with open circulatory 
systems, the coelom is much reduced. See HEART (INVERTEBRATE). 
 �34s[David R. Jones]
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�6v�2v�32sCirculation disorders   �21m �33s 
    The function of the circulatory system is to transport and distribute 
substances either used or produced by cells or both. Excluded are those 
materials that are discharged directly from sweat glands, digestive glands, 
and renal tubule cells. Included, however, are nutritive and metabolic 
substances, hormones, waste products, water, and heat. 
    Disturbances in the normal pattern of circulation can either result in or 
from disease conditions. An example is edema, which is an abnormal 
accumulation of fluid in the cells, tissue spaces, or cavities of the body. 
There are three main factors in the formation of generalized edema and a 
fourth which plays a role in the formation of local edema. They are the 
permeability of the capillary wall, the colloid osmotic pressure of the plasma 
proteins, and the hydrostatic pressure in the capillaries. The fourth factor, 
which is of importance in local edema formation, is lymphatic obstruction. 
    Increased permeability of the capillary walls plays an important role in 
the formation of inflammatory edema, the edema of severe infections, metabolic 
intoxications, asphyxia, anaphylactic reactions, secondary shock, and acute 
nephritis. See EDEMA. 
    A deficiency of circulating blood volume, both cellular elements and 
fluid, is called oligemia. This may be the result of an acute blood loss or it 
may be of a chronic nature, such as an anemia combined with dehydration. 
Anemia, or oligochromemia, is a deficiency of circulating red cell volume or, 
more specifically, hemoglobin content. It can result from a variety of causes. 
See ANEMIA; BLOOD. 
    Pancytopenia, or oligocythemia, is a deficiency of all circulating blood 
cellular elements. This is usually the result of a deficiency of the 
blood-forming tissue, the bone marrow. 
    The decrease of blood flow to an organ or tissue is known as ischemia. 
This can be sudden as when a vessel is ligated, when a thrombus or blood clot 
forms, or when an embolus comes to lodge in the vessel. A gradual occlusion 
can follow arteriosclerotic changes in the vessel wall. The effect of a sudden 
occlusion depends to a great extent on the collateral circulation to the organ 
or tissue involved. If an adequate collateral circulation comes to be 
established, the tissue survives; if not, it dies and an infarct results. See 
EMBOLUS; INFARCTION. 
    An excess of blood is referred to as plethora. This increase may be the 
result of an increase in the size or the number of red blood cells. The 
increase in red cell volume may be a polycythemia vera, or true polycythemia, 
which is a primary increase in the number of red blood cells with no regard to 
the needs of the organism. Polycythemia, or erythrocytosis, is usually a 
secondary increase in red cells following conditions of chronic hypoxia. 
Serous plethora is an increase in the watery part of the blood. 
    Hyperemia, or congestion, refers to an excess of blood within an organ or 
tissue. This condition may be localized or generalized. Active hyperemia, 
caused by an active dilatation of arterioles and capillaries, occurs under 
certain physiological conditions, such as in the muscle when there is an 
increased need for blood during exercise. It also occurs in pathological 
states such as inflammation. Passive hyperemia, a condition which results in 
an accumulation of blood in the venous system, may be generalized or localized 
and can result from any obstruction or hindrance to the outflow of blood from 
the venous circuit. Diseases of the lungs such as emphysema, fibrosis, or 
pulmonary hypertension of any origin can result in right ventricular failure 
and generalized congestion. Localized venous congestion results when a main 
vein from a region is occluded either by a thrombus or some extrinsic pressure 
such as a tumor or enlarged lymph nodes. 
    The escape of blood from within the vascular system is hemorrhage. This 
process can be the result of trauma to, or disease of, the vessel wall. The 
causes of hemorrhage other than trauma can be divided into three main groups. 
In the first group are those conditions in which there is a disease process 
affecting the vessel wall, such as arteriosclerosis or aneurysm formation. In 
the second group are conditions in which there is an acute process affecting 
the vessel wall such as in septicemia, poisoning by heavy metals, or even 
anoxia. The third group consists of those conditions in which there is a 
defect in the blood itself, which results in hemorrhage. Apoplexy, or stroke, 
is an acute vascular lesion of the brain. This can be the result of hemorrhage 
of, thrombosis in, or embolism to a cerebral vessel. See HEMORRHAGE. 
    Thrombosis is the formation of a thrombus, a solid body formed during life 
and composed of the elements of the blood: platelets, fibrin, red cells, and 
leukocytes. Thrombosis may occur on a vessel wall anywhere that the 
endothelium is damaged. However, because the platelets release 
thromboplastinogen, which activates the clotting mechanism, thrombosis and 
blood coagulation may occur together. See THROMBOSIS. 
    The sudden blocking of an artery or vein by a clot or other substance 
which has been brought to its place by the blood current is an embolism. The 
material carried in the circulation in this process is an embolus. Emboli may 
be composed of thrombi, fat, air, tumor cells, masses of bacteria or 
parasites, bone marrow, amniotic fluid, or atheromatous material from the 
vessel wall. See CARDIOVASCULAR SYSTEM DISORDERS; EMBOLISM. 
 �34s[Romeo A. Vidone]
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�6v�2v�32sCirculatory system   �21m �33s 
    The circulatory system of vertebrates actually includes two systems of 
ramifying tubes, the cardiovascular (blood-vascular) and lymphatic systems. 
The tubes provide the pathway for the circulating fluids which transport food, 
water, oxygen, and other necessary materials to the cells and aid in the 
removal of waste products. See BLOOD; CARDIOVASCULAR SYSTEM; LYMPHATIC SYSTEM. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCirque   �21m �33s 
    A cliffed rock basin, shaped like half a bowl, at the head of a mountain 
valley. Cirques may be shallow if glacier erosion is slight, but may be 
1600-2600 ft (500 - 800 m) from the top of the headwall to the cirque floor if 
erosion is deep. Cirques occur in glaciated mountains all over the world. Some 
cirques used repeatedly by glaciers over long periods of years are excavated 
profoundly. Many cirques contain small glaciers today that were inherited from 
a huge cirque cut by glaciers long ago. Well-formed cirques have steep rock 
walls and floors that slope down-valley or back toward the base of the 
headwall. Some cirques are cup-shaped rock basins holding rainwater; a lake 
formed in them is a tarn. If glacial deposits, such as till or a small moraine 
on the cirque floor, form a depression for a lake the lake is known as a 
moraine-dammed lake. 
 �34s[Sidney E. White]
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�6v�2v�32sCirrhosis �21m �33s
    Any one of several types of liver disease, all of which are characterized
by a real or apparent increased proportion of connective tissue in the organ.
Although the term cirrhosis refers to a yellowish or orange color, not all the
forms display such discoloration. Common usage has also diverted the term to
include scarring or fibrosis and, when used alone, to denote the most common
form, that of nutritional (also known as alcoholic, Laennec's, or portal)
cirrhosis. See LIVER.
    In the early stage of cirrhosis the liver is enlarged, loaded with fat,
and smooth. Progressive disease eventually leaves a typical nodular, shrunken
organ, which is largely scar tissue with an irregular pattern of regenerating
liver lobules. Hepatic insufficiency, associated disease, hepatic coma, and
death may follow a progressive course in a surprisingly high number of cases
which are free of severe symptoms until the terminal stage.
    Various classifications of cirrhosis are in use but none is universally
accepted, because etiologic factors are sometimes difficult to isolate or to
identify. Biliary obstruction and some forms of cardiac disease produce
biliary and cardiac cirrhosis, respectively. In addition, liver cell damage
from specific infections, chemicals, and other agents may lead to postnecrotic
cirrhosis, characterized by massive destruction of liver tissue. Syphilitic
cirrhosis is marked by characteristic lesions; other less common types are
also recognized.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sCirripedia   �21m �33s 
    A subclass of the Crustacea, permanently attached when adult, and called 
barnacles. The carapace forms a complete covering or mantle over the rest of 
the body and is usually strengthened by calcareous plates. The body within the 
mantle consists of a mouth region and thorax. The abdomen is usually 
vestigial. Typically the mouth appendages are paired mandibles with palps, 
maxillulae, and maxillae. The thorax bears six pairs of biramous appendages 
(cirri) composed of numerous segments, each with a considerable armament of 
setae. Compound eyes occur only in the larvae, there is no heart, and 
typically adults are hermaphroditic. The eggs are incubated in the mantle 
cavity and hatch into free-swimming nauplius larvae (illustration a). By 
successive molts a further five nauplius larvae follow. The sixth nauplius (or 
metanauplius) molts into an entirely different larval form, the cypris 
(illustration b), which is characteristic of all Cirripedia. The Cirripedia 
are divided into four orders: Thoracica, Acrothoracica, Ascothoracica, and 
Rhizocephala. Only the Thoracica, the goose barnacles, and acorn barnacles are 
at all conspicuous, and only the latter of any economic significance. See 
ACROTHORACICA; ASCOTHORACICA; BARNACLE; RHIZOCEPHALA; THORACICA. 
 �34s[H. G. Stubbings]
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�6v�2v�32sCitrate   �21m �33s 
    A salt or ester of citric acid. Citrates are formed by replacing the 
acidic hydrogens of citric acid by a metal or an organic radical. Many salts 
have commercial uses in pharmaceuticals (iron and calcium), cosmetics, and 
foods; as anticoagulants for blood, saline cathartics (sodium), and 
effervescent medicines (magnesium); in blueprinting (iron); as plasticizers in 
plastics; and as resins (simple esters). See CITRIC ACID; ESTER. 
 �34s[Elbert H. Hadley]
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�6v�2v�32sCitric acid �21m �33s
    An organic acid with formula C6H8O7PRODUCTH2O. Citric acid is present in
almost all natural foods and in the breast milk of nursing mothers. The
compound produced commercially by fermentation has a melting point of 153DGC
(307DGF).
    Citric acid has many applications in foods, pharmaceuticals, and industry.
It is used to enhance flavors, to increase preservative effectiveness, to
improve gel strength, and to conserve energy by reducing heat-processing
requirements in fruit and vegetable processing. In pharmaceuticals, citric
acid can contribute effervescence and improve flavor in certain medications.
It also acts to chelate metallic ions in dosage forms and thus lengthens shelf
life. In cosmetics and toiletries, citric acid is used in hair rinses,
permanent wave neutralizer solutions, low-pH shampoos, lotions, creams, and
toothpastes. Citric acid is nontoxic, noncorrosive, and biodegradable. It is
used in metal-cleaning applications and as an ingredient in laundry and
dishwashing products. In the textile industry it is used in production of
soil-release fabrics, carpeting, and dyestuffs. Citric acid is also used in
processing a number of minerals, in recovering sulfur from sulfur dioxide
emissions in waste gas, as an aid in recovery of secondary oil deposits, in
formulating adhesives, and in the concrete and cement industries.
 �34s[Jackson W. Foster /R. E. Kallio]
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�6v�2v�32sCitron   �21m �33s 
    Citrus medica, a species of true citrus. Commercially, citrons are grown 
almost exclusively in the Mediterranean area, principally in Italy, Sicily, 
Corsica, Greece, and Israel. The tree is evergreen, as are all citrus, and 
frost-tender. It is thorny, straggly, shrubby, and tends to be short-lived. 
    The fruit is scarcely edible fresh, having a very thick skin with little 
flesh and that lacking in juice. However, it is very fragrant and was valued 
in ancient times for its aroma and its fragrant peel oil, used in perfumes and 
as a moth repellent. It is grown commercially only as a source of candied peel 
for use in cakes and confections. The actual candying is usually done in the 
importing country, the citron peel being exported in brine. 
    Confusion sometimes arises due to the name ``citron'' also being used for 
a small, wild, inedible melon in the United States and for lemons (C. limon) 
in France. See FRUIT, TREE. 
 �34s[William Grierson]
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�6v�2v�32sCitronella �21m �33s
    A tropical grass, Cymbopogon nardus, from the leaves of which oil of
citronella is distilled. This essential oil is pale yellow, inexpensive, and
much used in cheap perfumes and soaps. It is perhaps best known as an insect
repellent. A large acreage is devoted to the cultivation of this grass in Java
and Ceylon. See CYPERALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCivet   �21m �33s 
    Any of 18 species of carnivores assigned to the family Viverridae. Also 
included in this family are genets, linsangs, and mongooses. See MONGOOSE. 
    Civets are small to medium size with a pointed muzzle, long head, slender 
body, and long, bushy tail. They have short limbs and nonretractile claws; 
they are digitigrade, that is, they walk on their toes. The Indian civet 
(Viverra zibetha) and the smaller African civet are two better-known species. 
    Civets are nocturnal and remain hidden in brush areas during the day. They 
have well-developed perianal glands, from which a scented substance used in 
perfumery is secreted. See CARNIVORA; SCENT GLAND. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCivil engineering   �21m �33s 
    The planning, design, construction, and management of all types of works 
and facilities, including buildings and structures, transportation facilities, 
water resource development projects, power generation plants, and other 
facilities for enhancement of the environment. These functions are generally 
categorized as public works, although the civil engineer also extensively 
serves industry and private clients. 
    The civil engineer plans, designs, and constructs residential, industrial, 
and commercial buildings ranging from warehouses to skyscrapers. Planning 
involves site selection, allocation of space to building functions, and 
provision for circulation of people and materials, all in compliance with 
zoning laws and building codes. Using steel and concrete as the main 
materials, the civil engineer designs a foundation suited to the geological 
conditions and loads to be accommodated, and then conceives a frame capable of 
resisting fire, wind forces, and seismic shocks to support the floors, walls, 
and roof. See BUILDINGS; STRUCTURES (ENGINEERING). 
    Civil engineering is involved in the creation of a broad spectrum of 
efficient and economical transportation facilities and terminals, including 
streets, highways, expressways, and parking facilities; rapid transit systems 
and subways; transmission systems and pipelines; waterways and harbors; 
airports; and the launch facilities for vehicles to outer space. See HIGHWAY 
ENGINEERING; TRAFFIC ENGINEERING; TRANSPORTATION ENGINEERING. 
    Bridges, tunnels, and other special structures appurtenant to highways and 
railroads are among the most challenging and spectacular projects of the civil 
engineer. The most creative skills are needed to meet all the parameters of 
load, topography, geology, social constraints, esthetics, and economy. See 
BRIDGE; TUNNEL. 
    Water transportation, the most economical method of all, calls upon the 
civil engineer for channel improvements, canals, locks, dams, ports, harbors, 
and navigational facilities. Works for controlling erosion by rivers and by 
ocean tides and currents are also important. See COASTAL ENGINEERING; HARBOR; 
RIVER ENGINEERING. 
    Air travel involves the civil engineer in airport planning, design, and 
construction. Site election, runway design, and terminal traffic control for 
planes, passengers, and baggage must meet demanding constraints of 
convenience, time-saving, safety, noise control, and esthetics. See AIRPORT 
ENGINEERING.  
    The vehicle assembly buildings, roadways, pads, and special strctures 
making up the rocket launching complex are demanding the most sophisticated 
planning, design, and construction techniques of the civil engineer. See 
LAUNCH COMPLEX. 
    The harnessing of rivers to control floods, produce hydroelectric power, 
and provide domestic, industrial, and agricultural water supplies is a major 
area of civil engineering. Groundwater resource development is also important. 
Efforts toward exploitation of ocean resources for minerals, aquatic life, and 
power will demand a wide spectrum of civil engineering technology. See DAM; 
HYDRAULICS; SANITARY ENGINEERING. 
    All large construction projects are planned, directed, and managed by the 
civil engineer, even though they may require major input by other engineering 
specialists and architects. Some of the most sophisticated civil engineering 
works are out of sight beneath the ground, such as complicated foundations and 
tunnels. Knowledge of geology and soil science is required to solve these 
problems. See ENGINEERING GEOLOGY; FOUNDATIONS; SOIL MECHANICS. 
 �34s[William H. Wisely]
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�6v�2v�32sCladding   �21m �33s 
    An old jewelry art, now employed on an industrial scale to add the 
desirable surface properties of an expensive metal to a low-cost or strong 
base metal. In the process a clad metal sheet is made by bonding or welding a 
thick facing to a slab of base metal; the composite plate is then rolled to 
the desired thickness. The relative thickness of the layers does not change 
during rolling. Cladding thickness is usually specified as a percentage of the 
total thickness, commonly 10%. 
    Gold-filled jewelry has long been made by this process: the surface is 
gold, the base metal bronze or brass with the cladding thickness usually 5%. 
The process is used to add corrosion resistance to steel and to add electrical 
or thermal conductivity, or good bearing properties, to strong metals. 
Corrosion-resistant pure aluminum is clad to a strong duralumin base, and many 
other combinations of metals are widely used in cladding; a development 
includes a technique for cladding titanium to steel for jet-engine parts. 
    Cladding supplies a combination of desired properties not found in any one 
metal. A base metal can be selected for cost or structural properties, and 
another metal added for surface protection or some special property such as 
electrical conductivity. Thickness of the cladding can be made much heavier 
and more durable than obtainable by electroplating. 
    Cladding can be added to both sides of a sheet or strip of base metal. 
Tubing can be supplied with a clad surface on inside or outside; round and 
rectangular wire can be clad similarly (see illustration). 
    For some forms of electrical contacts, the composite materials are bonded 
side by side, or silver is inset as a stripe on one side or along the edges. 
This construction can place solid silver just where it is needed to form an 
electrical contact with no waste of costly metal. 
    A related form of cladding is found in thermostatic bimetals in which 
equal thicknesses of low- and high-expansion metals are bonded together. With 
a change in temperature, differing expansion rates of the two metals cause the 
composite material to bend and thus operate valves in automobile cooling 
systems, or electrical contacts in room thermostats. 
    Clad wires with properly chosen proportions of materials of different 
thermal-expansion rates can match the thermal expansion of types of glass used 
for vacuum-tight seals for conductors in lamp bulbs and hermetically sealed 
enclosures.  
    In making parts from clad metal, the composite material can be bent, 
drawn, spun, or otherwise formed just the same as the base metal without 
breaking the bond. The maximum service temperature is limited by the melting 
point of the material at the juncture of the two metals. See ELECTROPLATING OF 
METALS; METAL COATINGS. 
 �34s[Robert W. Carson]
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�6v�2v�32sCladocera   �21m �33s 
    An order of small branchiopod crustaceans, commonly known as water fleas, 
found mostly in fresh water. Usually they possess a bivalve shell which covers 
the body and trunk limbs, but leaves the head and antennae free. The 
best-known species is Daphnia magna, which is bred in large numbers to provide 
living food for aquarium fishes. See BRANCHIOPODA. 
    Like other Crustacea, Cladocera have two pairs of antennae. Most Cladocera 
swim in a characteristically jerky way by beating the water with the second 
pair of antennae. The limbs of the trunk are usually enclosed in the valves of 
the shell. They are flattened, leaf-shaped organs fringed with setae, whose 
rhythmic movements create currents of water through the shell to bring in 
oxygen and particles of food. See FEEDING MECHANISMS (INVERTEBRATE). 
    There is a separate group of Cladocera in which there is no shell, apart 
from remnants forming the brood pouch, and the limbs are free and prehensile 
and used for capturing small planktonic animals, including other Cladocera. 
    There is a very wide geographical distribution of most species of 
Cladocera. For example, more species are common to the United States and the 
Soviet Union than are unique to either. Cladocera are comparatively feeble 
swimmers and are preyed upon by fishes and various freshwater invertebrates, 
including Hydra and some of the larger Protozoa. Anchistropus is remarkable, 
for not only is it the only known parasitic cladoceran, but the animal it 
parasitizes is Hydra, whose stinging cells are fatal to much larger Cladocera. 
See CRUSTACEA. 
 �34s[John P. Harding]
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�6v�2v�32sCladocopa   �21m �33s 
    A suborder of the order Myodocopida (subclass Ostracoda) which includes 
marine animals having a carapace that lacks a permanent aperture when the two 
valves are closed. This group is referred to also as Cladocopina. Both heart 
and eyes are absent, and the body has only two pairs of postoral appendages, 
the maxillae and thoracic legs. The body terminates in a pair of lamelliform 
furcae, provided with clawlike spines. The nearly circular carapace is 
slightly dimorphic, smaller in the male (see illustration). The suborder has a 
single family, the Polycopidae. These animals range from Norway to the Cape of 
Good Hope in the Atlantic Ocean and in the Mediterranean. See OSTRACODA. 
 �34s[Robert V. Kesling]
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�6v�2v�32sCladophorales   �21m �33s 
    An order of coarse, wiry filamentous algae in the class Chlorophyceae 
having a characteristic type of cell division. Some forms are unbranched, as 
in Chaetomorpha and Urospora, whereas others are bushily branched, like some 
species of Cladophora and the marine Spongomorpha. Most forms are attached and 
exhibit basal-distal differentiation with rhizoidlike holdfasts. Commonly, 
thalli become free-floating and Cladophora may form globular tangles known as 
Cladophora balls. Some species become symbiotic with sponges. The fresh-water 
species Basicladia occurs on mossback turtles. See CHLOROPHYCEAE. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sCladoselachii   �21m �33s 
    An order of extinct elasmobranch fishes including the oldest and most 
primitive of sharks. Best known is Cladoselache (see illustration), of which 
complete specimens, including even muscle and other soft tissues, have been 
obtained from shales in the region of Cleveland, Ohio. Primitive features are 
broad-based paired fins, amphistylic jaw support, and absence of claspers. See 
ELASMOBRANCHII.  
 �34s[Alfred S. Romer]
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�6v�2v�32sClamping circuit   �21m �33s 
    An electronic circuit that effectively functions as a switch to connect a 
signal point to a fixedreference voltage or current level, either at a 
specific time interval or at some prescribed amplitude level of the signal 
itself. The circuit is also called, simply, a clamp. Clamps are frequently 
used to reset the starting level of periodic waveforms, such as sweep 
generators, and to establish a direct-current (dc) level in a signal which may 
have lost its dc component because of capacitance coupling. They are also used 
in many frequency and voltage comparison circuits and in control circuits. 
    The basic elements of a clamping circuit are shown in the illustration. 
The clamp between terminals A and B is shown as a switch S in series with 
reference voltage VsubR, having a large resistance rsubR when the switch is 
open and a small resistance rsubF when the switch is closed. 
    If a clamp functions at specific time intervals controllable from separate 
voltage or current sources rather than from the signal itself, it is a keyed, 
or synchronous, clamp, and the sources, usually pulses, used to actuate the 
clamping action are called keying, or clamping, pulses. Such a keyed clamp may 
be unidirectional but more often it is a bidirectional clamp, defined as a 
clamp that functions at the prescribed time irrespective of the polarity of 
the signal source at the time the keying pulses are applied. A three-terminal, 
high-impedance device such as a field effect transistor (FET) may be used as a 
keyed clamp to eliminate the need for balanced keying pulses. 
    A dc restorer is a clamp circuit used to establish a dc reference level in 
a signal without modifying to any important degree the waveform of the signal 
itself. For this function to be performed, the signal must have a 
self-contained reference level which repeats at periodic intervals. A typical 
example of such a waveform is that comprising the video signal of the 
television system. See TELEVISION. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sClarification   �21m �33s 
    The removal of small amounts of fine, particulate solids from liquids. The 
purpose is almost invariably to improve the quality of the liquid, and the 
removed solids often are discarded. The particles removed by a clarifier may 
be as large as 100 micrometers or as small as 2 micrometers. Clarification is 
used in the manufacture of pharmaceuticals, beverages, and fiber and film 
polymers; in the reconditioning of electroplating solutions; in the recovery 
of dry-cleaning solvent; and for the purification of drinking water and waste 
water. The filters in the feed line and lubricating oil system of an internal 
combustion engine are clarifiers. 
    The methods of clarification include gravity sedimentation, centrifugal 
sedimentation, filtration, and magnetic separation. Clarification differs from 
other applications of these mechanical separation techniques by the low solid 
content of the suspension to be clarified (usually less than 0.2%) and the 
substantial completion of the particle removal. See FILTRATION; MAGNETIC 
SEPARATION METHODS; MECHANICAL SEPARATION TECHNIQUES; SEDIMENTATION 
(INDUSTRY); THICKENING. 
 �34s[Shelby A. Miller]
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�6v�2v�32sClassical field theory   �21m �33s 
    The study of distributions of energy and matter under circumstances where 
the discrete nature of the latter is unimportant; more commonly known as 
continuum physics or continuum mechanics. This will generally be the case for 
a system of a (usually extremely) large number of particles. In this category 
is the study of the flow of fluids, heat, and other forms of energy; 
electromagnetic currents and waves, including optical phenomena; and 
macroscopic theories of the elastic and plastic deformation of solids. Thus, 
most of applied physical science, such as strength of materials, hydraulics, 
heat transfer, and aerodynamics, is included. 
    The quantities involved in these theories, for example, mass and energy 
densities, fluid velocities, and electric field strengths in a material 
medium, may be regarded as local averages of corresponding microscopic 
variables.  
    Mathematically, field theories are characterized by equations having 
partial derivatives with respect to position and time. These equations may  
express the requirements of conservation of energy, or linear and angular 
momentum, and also the geometrical properties of the particular type of field. 
    So-called constitutive equations must be added to the general equations. 
These define ideal materials by restricting the field variables in some way. 
Simple examples are the assumptions of incompressible or nonviscous fluids, of 
perfect gases, or of magnetic materials without hysteresis. 
 �34s[Bernard Goodman]
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�6v�2v�32sClassical mechanics   �21m �33s 
    The science dealing with the description of the positions of objects in 
space under the action of forces as a function of time. Some of the laws of 
mechanics were recognized at least as early as the time of Archimedes (287? - 
212 B.C.). In 1638, Galileo stated some of the fundamental concepts of 
mechanics, and in 1687, Isaac Newton published his Principia, which presents 
the basic laws of motion, the law of gravitation, the theory of tides, and the 
theory of the solar system. This monumental work and the writings of J. 
D'Alembert, J. L. Lagrange, P. S. Laplace, and others in the 18th century are 
recognized as classic works in the field of mechanics. Jointly they serve as 
the base of the broad field of study known as classical mechanics, or 
Newtonian mechanics. This field does not encompass the more recent 
developments in mechanics, such as statistical, relativistic, or quantum 
mechanics.  
    In the broad sense, classical mechanics includes the study of motions of 
gases, liquids, and solids, but more commonly it is taken to refer only to 
solids. In the restricted reference to solids, classical mechanics is 
subdivided into statics, kinematics, and dynamics. Statics considers the 
action of forces that produce equilibrium or rest; kinematics deals with the 
description of motion without concern for the causes of motion; and dynamics 
involves the study of the motions of bodies under the actions of forces upon 
them. For some of the more important areas of classical mechanics see 
BALLISTICS; COLLISION (PHYSICS); DYNAMICS; ENERGY; FORCE; GRAVITATION; 
KINEMATICS; LAGRANGE'S EQUATIONS; MASS; MOTION; PRECESSION; RIGID-BODY 
DYNAMICS; STATICS; WORK. 
 �34s[Newell S. Gingrich]
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�6v�2v�32sClathrate compounds �21m �33s
    Well-defined addition compounds formed by inclusion of molecules in
cavities existing in crystal lattices or present in large molecules. The
constituents are bound in definite ratios, but these are not necessarily
integral. The components are not held together by primary valence forces, but
instead are the consequence of a tight fit which prevents the smaller partner,
the guest, from escaping from the cavity of the host. Consequently, the
geometry of the molecules is the decisive factor.
    Inclusion compounds can be subdivided into (1) lattice inclusion compounds
(inclusion within a lattice which, as such, is built up from smaller single
molecules); (2) molecular inclusion compounds (inclusion into larger ring
molecules with holes); and (3) inclusion compounds of macromolecules. The
best-known lattice inclusion compounds are the urea and thiourea channel
inclusion compounds, which are formed by mixing hydrocarbons, carboxylic
acids, or long-chain fatty alcohols with solutions of urea. Other
representatives of lattice inclusion compounds are the choleic acids, which
are adducts of deoxycholic acid with fatty acids, and other lipoic substances.
Some aromatic compounds form an open crystal lattice which can accommodate
smaller gas and solvent molecules (clathrates in the stricter sense of the
word). The gas hydrates are inclusion compounds of gases in a somewhat
expanded ice lattice. The gas or solvent molecules are inserted into definite
places within the ice lattice and are surrounded by water molecules on all
sides.
    Crown ether compounds are cyclic or polycyclic polyether compounds capable
of including another atom in the center of the ring. In this way, sodium or
potassium compounds can be solubilized in organic solvents. Similarly, a
series of ionophore antibiotics can complex inorganic cations. See CROWN
ETHERS AND CRYPTANDS.
    Some clay minerals are made up of distinct silicate layers. Between these
layers some free space may exist in the shape of channels. Smaller hydrocarbon
molecules can be accommodated reversibly within these channels. This
phenomenon is used in some technical separation processes for separating
hydrocarbons (molecular sieves). Furthermore, ion-exchange processes used for
water deionization are based on similar minerals. See CLAY MINERALS; MOLECULAR
SIEVE.
    Enzymes are believed to accommodate their substrates in active sites,
pockets, or clefts prior to the chemical reaction which then changes the
chemical structures of the substrates. These binding processes are identical
to those of low-molecular-weight inclusion compounds.
 �34s[Friedrich Cramer ;Wolfram Saenger]
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�6v�2v�32sClathrinida   �21m �33s 
    An order of sponges in the subclass Calcinea of the class Calcarea. These 
sponges have an asconoid structure and lack a true dermal membrane or cortex. 
The spongocoel is lined with choanocytes. This order contains the family 
Clathrinidae; Clathrina, Ascute, and Dendya are examples. See CALCAREA; 
CALCINEA.  
 �34s[Willard D. Hartman]
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�6v�2v�32sClavaxinellida   �21m �33s 
    An order of sponges of the class Demospongiae, subclass Tetractinomorpha, 
with monaxonid megascleres arranged in radial or plumose tracts. In the 
suborder Epipolasina, monactinal or diactinal megascleres are arranged in a 
radial pattern. Spongin is rare. 
    Clavaxinellidan sponges vary greatly in shape, from radailly symmetrical 
species which are spherical in shape to encrusting or massive species and 
upright branching types. They occur in tidal and shallow waters of all seas 
and extend down to depths of at least 18,000 ft (5500 m). Sponges comparable 
to existing clavaxinellidans occur scattered through the fossil record from 
Cambrian strata upward. See DEMOSPONGIAE; TETRACTINOMORPHA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sClay   �21m �33s 
    A term used both as a rock term and as a particle-size term. As a rock 
term, it is used for material that has been deposited as a sediment, has 
formed by hydrothermal action, or is the product of weathering. Chemical 
analyses of clays show them to be composed primarily of silica, alumina, and 
water. Iron, alkalies, and alkaline earths are also often present in 
appreciable quantities. As a rock term clay implies a natural, earthy, 
fine-grained material which develops plasticity when mixed with a limited 
amount of water. Plasticity is that property of the moistened material which 
allows deformation of the material when pressure is applied. In a plastic 
substance the deformed shape is maintained when the deforming force is removed. 
    As a particle-size term, clay refers to that fraction of an earthy 
material containing the smallest particles. In the mechanical analysis of 
sedimentary rocks and soils, material is normally divided into three size 
grades, or fractions, on the basis of particle size: sand, silt, and clay. The 
maximum size of the particles comprising the clay-size grade is defined 
differently by different observers. A large number of analyses have shown that 
the clay minerals tend to be concentrated in a size less than 2 micrometers. 
See CLAY MINERALS. 
    Some types of sedimentary rocks are substantially of the same composition 
as clay, but differ from clays in their textural characteristics. The 
expression clay material is often used for any fine-grained, natural, earthy, 
argillaceous material, when a more precise designation is not possible or 
desirable. Clay material includes clays, shales, and argillites. Soils would 
also be included if such materials contained an appreciable quantity of 
clay-size-grade material and were argillaceous. See ARGILLITE; SHALE. 
    The properties of clay materials are controlled by at least five major 
factors. These attributes which characterize a clay material are clay-mineral 
composition, non-clay-mineral composition, organic material, soluble salts and 
exchangeable ions, and texture. Generally the clay-mineral composition is the 
most important factor, and sometimes as little as 5% of a particular clay 
mineral may largely determine the properties of the whole clay. 
 �34s[Ralph E. Grim ;Floyd M. Wahl]
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    The commercial value of clays is related to their mineralogical and 
chemical composition, particularly the clay mineral constituents kaolinite, 
montmorillonite, illite, chlorite, and attapulgite. The presence of minor 
amounts of mineral or soluble salt impurities in clays can restrict their use. 
The more common mineral impurities are quartz, mica, carbonates, iron oxides 
and sulfides, and feldspar. In addition, many clays contain some organic 
material.  
  
    Kaolinitic clays.  Clays containing a preponderance of the clay mineral 
kaolinite are known as kaolinitic clays. Several commercial clays are composed 
predominantly of kaolinite. These are china clays, kaolines, ball clays, 
fireclays, and flint clays. The terms china clay and kaolin are used 
interchangeably in industry. See KAOLINITE. 
    China clays are high-grade white kaolins found in the southeastern United 
States, England, and many other countries. Many grades of kaolin are used in 
the manufacture of ceramics, paper, rubber, paint, plastics, insecticides, 
adhesives, catalysts, and ink. By far the largest consumer of white kaolins is 
the paper industry, which uses them to make paper products smoother, whiter, 
and more printable. The kaolin is used both as a filler in the sheet to 
enhance opacity and receptivity to ink and as a thin coating on the surface of 
the sheet to make it smoother and whiter for printing. See CERAMICS; 
PORCELAIN; POTTERY. 
    Ball clays are composed mainly of the mineral kaolinite but usually are 
much darker in color than kaolin. The term ball clay is used for a 
fine-grained, very plastic, refractory bond clay. Most ball clays contain 
minor amounts of organic material and the clay mineral montmorillonite, and 
are finer grained than china clays. This fineness, together with the 
montmorillonite and organic material, gives ball clays excellent plasticity 
and strength. For these reasons and because they fire to a light-cream color, 
ball clays are commonly used in whitewares and sanitary ware. 
    The term fireclay is used for clays that will withstand temperatures of 
773DGF (1500DGC) or higher. Such clays are composed primarily of the mineral 
kaolinite. Fireclays are generally light to dark gray in color, contain minor 
amounts of mineral impurities such as illite and quartz, and fire to a cream 
or buff color. Most fireclays are plastic, but some are nonplastic and very 
hard; these are known as flint clays. Fireclays are used primarily by the 
refractories industry. The foundry industry uses fireclay to bind sands into 
shapes in which metals can be cast. See REFRACTORY. 
  
    Diaspore clay.  This clay is composed of the minerals diaspore and 
kaolinite. Diaspore is a hydrated aluminum oxide with an Al2O3 content of 85% 
and a water content of 15%. Diaspore clay is used almost exclusively by the 
refractories industry in making refractory brick. However, after calcination, 
it is sometimes used as an abrasive material. 
  
    Mullite.  Mullite is a high-temperature conversion product of many 
aluminum silicate minerals, including kaolinite, pinite, topaz, dumortierite, 
pyrophyllite, sericite, andalusite, kyanite, and sillimanite. Mullite is used 
in refractories to produce materials of high strength and great 
refractoriness. Mullite does not spall, withstands the shock of heating and 
cooling exceptionally well, and is resistant to slag erosion. It is used in 
making spark plugs, laboratory crucibles, kiln furniture, saggars, and other 
special refractories. 
  
    Bentonites.  Those clays that are composed mainly of the clay mineral 
montmorillonite and are formed by the alteration of volcanic ash are known as 
bentonites. The term bentonite is a rock term, but in industrial usage it has 
become almost synonymous with swelling clay. Bentonites are used in many 
industries; the most important uses are as drilling muds and catalysts in the 
petroleum industry, as bonding clays in foundries, as bonding agents for 
taconite pellets, and as adsorbents in many industries. See BENTONITE; 
MONTMORILLONITE.  
    Drilling mud consists of bentonite, water, a weighting material such as 
barite, and various electrolytes. This mud is circulated in the hole at all 
times while the drilling is progressing. The functions of the drilling mud are 
to remove the drill cuttings from the drill hole, to form a filter cake on the 
wall of the drill hole to keep the drilling fluid in the hole and prevent 
water in the rock formations from entering, to cool and lubricate the bit, and 
to prevent blowouts. Bentonite, especially the sodium variety, imparts high 
viscosity and shear strength to the mud and also imparts a property called 
thixotropy. A thixotropic body acts as a fluid when agitated and as a thick 
gel when standing. 
    Bentonites are used in the foundry industry as bonding clays to bind sands 
into desired shapes in which metals can be cast. Only 3 - 5% bentonite is 
needed to bond the sand grains together. Because of their fine particle size 
and the nature of their water adsorption, bentonites give the mold a higher 
green, dry, and hot strength than does any other type of clay. 
    Bentonites are also used as bleaching clays to remove coloring matter from 
oils, and as clarifying agents for products such as wines and beers. Bentonite 
is used as an adsorbent for oils, insecticides, alkaloids, vitamins, proteins, 
and many other materials. Bentonite suspensions in water are widely used in 
industry, in addition to that suspension used as a drilling mud by the 
petroleum industry. 
  
    Attapulgite clays.  Attapulgite is a hydrated magnesium aluminum silicate 
with a needlelike shape. Each individual needle is exceedingly small, about 1 
micrometer in length and approximately 0.01 micrometer across. Attapulgite is 
used as a suspending agent, and gives high viscosity because of the 
interaction of the needles. Some commercial uses are as an oil well drilling 
fluid, in adhesives as a viscosity control, in oil base foundry sand binders, 
as thickeners in latex paints, in liquid suspension fertilizers, and as a 
suspending agent and thickener in pharmaceuticals. 
  
    Properties.  Most clays become plastic when mixed with varying proportions 
of water. Plasticity of a material can be defined as the ability of the 
material to undergo permanent deformation in any direction without rupture 
under a stress beyond that of elastic yielding. Clays range from those which 
are very plastic, called fat clay, to those which are barely plastic, called 
lean clay. The type of clay mineral, particle size and shape, organic matter, 
soluble salts, adsorbed ions, and the amount and type of nonclay minerals are 
all known to affect the plastic properties of a clay. 
    Green strength and dry strength properties are very important because most 
structural clay products are handled at least once and must be strong enough 
to maintain shape. Green strength is the strength of the clay material in the 
wet, plastic state. Dry strength is the strength of the clay after it has been 
dried.  
    Both drying and firing shrinkages are important properties of clay used 
for structural clay products. Shrinkage is the loss in volume of a clay when 
it dries or when it is fired. Drying shrinkage is high in most very plastic 
clays and tends to produce cracking and warping. It is low in sandy clays or 
clays of low plasticity and tends to produce a weak, porous body. Firing 
shrinkage depends on the volatile materials present, the types of crystalline 
phase changes that take place during firing, and the dehydration 
characteristics of the clay minerals. 
    The temperature range of vitrification, or glass formation, is a very 
important property in structural products. Vitrification is due to a process 
of gradual fusion in which some of the more easily melted constituents begin 
to produce an increasing amount of liquid which makes up the glassy bonding 
material in the final fired product. The degree of vitrification developed 
depends on the duration of firing as well as on the temperature attained. 
    Color is important in most structural clay products, particularly the 
maintenance of uniform color. The color of a product is influenced by the 
state of oxidation of iron, the state of division of the iron minerals, the 
firing temperature and degree of vitrification, the proportion of alumina, 
lime, and magnesia in the clay material, and the composition of the fire gases 
during the burning operation. 
 �34s[Haydn H. Murray]
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�6v�2v�32sClay minerals �21m �33s
    The clay minerals are essentially hydrous aluminum silicates. Alkalies or
alkaline earths are present as principal constituents in some of them. Also,
in some clay minerals magnesium or iron or both substitute wholly or in part
for the aluminum. The ultimate chemical constituents of the clay minerals vary
not only in amounts, but also in the way in which they are combined or are
present in various clay minerals. They are the major components of clay
materials and occur in extremely small particles which are essentially
crystalline and are limited in number. See CLAY.
    While a completely satisfactory classification of the clay minerals has not
yet been suggested, the following classification has proved to be a workable
one. A major subdivision into amorphous and crystalline groups is made even
though the amorphous components are relatively rare and of little importance.

I.  Amorphous
    Allophane group

II. Crystalline
    A.	Two-layer type (sheet structures composed of units of one layer of
    silica tetrahedrons and one layer of alumina octahedrons)
	1.  Equidimensional
	     Kaolinite group: kaolinite, nacrite, dickite, and so forth
	2.  Elongate
	     Halloysite group
    B.	Three-layer types (sheet structures composed of two layers of silica
	tetrahedrons and one central dioctahedral or trioctahedral layer)
	1.  Expanding structure
	    a. Equidimensional
		Montmorillonite group: montmorillonite, sauconite, and so forth
		Vermiculite
	    b. Elongate
		Montmorillonite group: nontronite, saponite, hectorite
	2.  Nonexpanding structure
	      Illite group
    C.	Regular mixed-layer types (ordered stacking of alternate structural
	types)
	  Chlorite group
    D.	Chain-structure types (similar to hornblende-chains of silica
	tetrahedrons linked together by octahedral groups of oxygens and
	hydroxyls containing Al and Mg atoms)
	  Attapulgite
	  Sepiolite
	  Palygorskite

    The three-layer minerals are divided into the expanding and nonexpanding
types in this classification. This subdivision is made even though there may be
a continuous gradation between the two types. The expanding attribute is a
readily determinable diagnostic property, since it imparts some unique physical
properties to the clay materials composed of these minerals. The expanding
materials are divided into equidimensional and elongate divisions, because
shape is a readily recognizable characteristic that probably reflects
structural attributes. See CHLORITE; HALLOYSITE; ILLITE; KAOLINITE;
MONTMORILLONITE; VERMICULITE.
    The atomic structures of the clay minerals consist of combinations of two
basic structural units. One unit consists of silica tetrahedrons, in which each
tetrahedron is made up of a silicon atom equidistant from four oxygen atoms or
hydroxyl ions arranged in the form of a tetrahedron with the silicon atom at
the center. The other structural unit consists of two sheets of closely packed
oxygen atoms or hydroxyl ions in which aluminum, iron, or magnesium atoms are
octahedrally coordinated. Because of the sheetlike nature of their structural
units, most of the clay minerals are flake-shaped. A few of the clay minerals
are tubular or elongate or both and some are fibrous.
    The varied properties of the clay minerals are important in that they
govern the economic use of clay materials.
    The ability of clay minerals to hold certain cations and anions which are
readily exchangeable for oterh cations and anions is most significant. This
property of ion exchange and the exchange reaction are of fundamental
importance in all fields in which clay materials are used. For example, in
soils the retention and availability of potash added in fertilizers depends on
cation exchange between the potash salt and the clay mineral in the soil. See
ION EXCHANGE; SOIL.
    Another important property of clay materials is their ability to hold
water. This water is of two types, low-temperature water and OH structural
water. The low-temperature water is driven off by heating to 36-62 DEG F
(100-150 DEG C). This is the water in pores, on the surfaces, and around the
edges of the minerals composing the material.
    When clay materials are heated, dehydration occurs. Thus there is a loss
of any water (adsorbed, interlayer, or structural OH water) held by the clay
minerals. The heating of clay materials also causes changes in the
clay-mineral structures. At relatively high temperatures these structural
changes facilitate the formation of new mineral phases. These structural
modifications are particularly important in the firing of clay materials.
    Another property of the clay minerals is their ability to react with
organic materials. Clays with a high adsorbing capacity are used in
decolorizing oils, while others provide catalysts in the cracking of organic
compounds.
    Many of the clay minerals are of a hydrothermal origin. Some hydrothermal
clay deposits are monomineralic, but most consist of a mixture of clay
minerals. The weathering of soils and of different rock types also leads to
the formation of clay minerals. The type of clay mineral formed, however,
depends upon a number of factors, such as  parent rock, climate, topography,
vegetation, and time. Clay minerals are abundant in sediments, both recent and
ancient. In some cases at least, the character of the clay minerals changes in
passing from one environment to another, such as from fresh water to marine,
so that the clay mineral composition reflects the geologic history of the
sediment. See MARINE SEDIMENTS; SEDIMENTARY ROCKS; SEDIMENTATION (GEOLOGY).
 �34s[Ralph E. Grim ;Floyd M. Wahl]
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�6v�2v�32sClear-air turbulence �21m �33s
    Turbulence above the boundary layer but not associated with cumulus
convection. The atmosphere is a fluid in turbulent motion. That turbulence of
a scale sensed by humans in aircraft is primarily associated with the boundary
layer within a kilometer or so of the Earth, where it is induced by the
surface roughness, or in regions of deep convection such as cumulus cloud
development or thunderstorms. However, aircraft occasionally encounter
turbulence when flying at altitudes well above the surface and far from
convective clouds. This phenomenon has been given the rather unsatisfactory
name of clear-air turbulence (CAT).
    What is primarily sensed in CAT by the human is vertical acceleration.
This acceleration will depend on the person's location in the plane, the speed
of flight relative to the air, and the response characteristics of the
airframe. A plane with a wing that generates aerodynamic lift more efficiently
or an airframe with less weight per unit wing area will respond more strongly
to a given gust magnitude.
    CAT is encountered in the atmosphere with a probability depending on
flight altitude, geographical location, season of the year, and meteorological
conditions. Given this variability and the small scale of the phenomenon, it
is difficult to establish reliable statistics on the frequency of its
occurrence. Although CAT may be encountered in unexpected meteorological
contexts, there are highly favored locations for its occurrence. One is in the
vicinity of the jet stream, particularly in ridges and troughs where the wind
direction is turning sharply. A second and even more common location of
occurrence is in the lee of a mountain range when a strong air flow is
distorted by being forced over the range. In this situation a gravity lee wave
is generated, which propagates to stratospheric heights. At various altitudes
and distances from the mountain, this wave may break, and as many as a dozen
or more CAT patches, light to severe, may be formed. Despite knowledge of
these favored meteorological areas, it is not possible to forecast with
confidence the precise location of a CAT patch. Warning forecasts for
substantial portions of routes are typically given to pilots when CAT
conditions prevail. See JET STREAM.
    Aside from the practical implications of CAT for air transport, this
phenomenon plays a role of undetermined magnitude in the dissipation of the
kinetic energy of the atmosphere. See UPPER ATMOSPHERE DYNAMICS.
 �34s[Morton G. Wurtele ;L. J. Ehernberger]
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�6v�2v�32sCleavage (embryology)   �21m �33s 
    The subdivision of eggs into cells called blastomeres. It occurs in eggs 
activated by fertilization or parthenogenetic agents. Cleavages follow one 
another so rapidly that there is little opportunity for daughter cells to grow 
before they divide again. Consequently the size of blastomeres diminishes 
progressively, although many times unequally, during cleavage. By contrast, 
the nucleus of each daughter cell enlarges following each cleavage with the 
result that the ratio of the volume of the nucleus to the volume of cytoplasm 
(the nucleoplasmic ratio) progressively increases. The cleavage period is said 
by some authorities to terminate when the nucleoplasmic ratios of various 
blastomeres attain values characteristic of adult tissues. Cells continue to 
divide thereafter, but each daughter cell then undergoes a period of growth 
prior to its division with the result that the nucleoplasmic ratio tends to 
remain approximately constant for each cell type following termination of 
cleavage. According to others, cleavage terminates with formation of the 
definitive blastula. Cleavage appears to be an essential step in development. 
See BLASTULATION. 
    Cleavage, however, does more than merely subdivide the substance of the 
egg quantitatively into smaller units, the blastomeres, which are then of such 
a size that they can readily undergo the subsequent events of blastulation, 
gastrulation, and interaction that are involved in formation of tissues and 
organs. Sooner or later cleavage segregates different cytoplasmic areas into 
different blastomeres, thus subdividing the substance of the egg 
qualitatively. These qualitative cytoplasmic differences among blastomeres are 
then sufficient to account for the initial establishment of different lines of 
differentiation in the progeny of different blastomeres, even though the 
genetic content of all blastomeres is identical. See CELL LINEAGE. 
 �34s[Ray L. Watterson]
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�6v�2v�32sCleft palate   �21m �33s 
    One of the most frequently encountered birth defects in the human, 
resulting from incomplete closure of the palate, or roof of the mouth, during 
early embryonic life. In general, clefts are more frequently encountered in 
white than in nonwhite races. Prior to the sixth week of development, the 
primitive oral and nasal cavities exist as a single large chamber. Beginning 
in the sixth week, lateral ridges appear on either side of this cavity and 
then grow toward the midline, meeting during the eighth week to complete 
formation of the palate. If the formation of these shelves is arrested prior 
to the eighth week, a defect of variable size will remain in the central part 
of the palate. As a result of this defect, the oral and nasal cavities are 
incompletely separated. The cleft may be complete, involving both the hard 
palate and the soft palate, or any degree of incomplete. Cleft palate is often 
associated with cleft lip (harelip), which also represents an arrest in 
development at about the same time in embryonic life. See HARELIP. 
    Of the two malformations cleft lip produces greater cosmetic deformity, 
while cleft palate produces more significant functional problems. With cleft 
palate feeding difficulties may be present because of frequent passage of food 
through the cleft into the nasal cavity. However, speech problems represent 
the major clinical complication of cleft palate, since an intact palate is 
essential to the formation of many normal speech sounds. In addition to 
difficulty in forming these sounds, the patient's voice tends to have an 
excessively nasal tone. Another problem often seen with cleft palate is ear 
infection, due in part to passage of food and fluid into the nose and then 
into the eustachian tubes, which communicate between the nose and middle ear 
via the pharynx. See EAR; SPEECH. 
 �34s[J. Bruce Beckwith]
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�6v�2v�32sClevis pin   �21m �33s 
    A fastener with a head at one end and a hole at the other used to join a 
clevis to a rod. A clevis is a yoke with a hole formed or attached at one end 
of a rod (see illustration). When an eye or hole of a second rod is aligned 
with the hole in the yoke, a clevis pin can be inserted to join the two. A 
cotter pin can then be inserted in the hole of the clevis pin to hold it in, 
yet the fastening is readily detachable. This joint is used for rods in 
tension where some flexibility is required. See COTTER PIN. 
 �34s[Paul H. Black]
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�6v�2v�32sClimatology �21m �33s
    That branch of meteorology concerned with climate, that is, with the
mean-physical state of the atmosphere together with its statistical variations
in both space and time, as reflected in the totality of weather behavior over
a period of many years. Climatology encompasses not only the description of
climate but also the physical origins and the wide-ranging practical
consequences of climate and of climatic change. Thus it impinges on a wide
range of other sciences, including solar system astronomy, oceanography,
geography, geology and geophysics, biology and medicine, agriculture,
engineering, economics, social and political science, and mathematical
statistics.
    Like meteorology, climatology is conveniently resolved into subdisciplines
in a way that recognizes a hierarchy of geographical scales of climatic
phenomena and their governing physics. Macroclimatology refers to the largest
(planetary) scale of regimes and phenomena; regional climatology to the scale
of continents and subcontinental areas; mesoclimatology to the scale of
individual physiographic features such as a mountain, lake, or urban area; and
microclimatology to the smallest scale, for example, a house lot or the
habitat of an insect. Climatology is also resolved in another way that
distinguishes between the theoretical, descriptive, and applied aspects of the
science: physical and dynamic climatology, concerned with the governing
physical laws; descriptive and synoptic climatology, concerned with
comparisons of climatic norms and anomalies, respectively, as concurrently
observed in different places; and applied climatology, concerned with the
practical utilization of climatological data in engineering design, operations
strategy, and activity planning. Other important subdisciplines are
climatography, concerned with the comprehensive documentation of climate by
means of data summaries, maps, and atlases; and statistical climatology,
concerned mostly with the estimation of climatological expectancies, risks of
extreme events, and probability distributions of climatic variables (including
joint distributions of combinations of variables) as needed to solve problems
in applied climatology.
    The climate of the Earth as a whole, including its geographical and
seasonal variations, is fundamentally prescribed by the disposition of solar
radiant energy that is intercepted by the atmosphere. This disposition depends
in part on astronomical factors and in part on terrestrial factors, such as
atmospheric composition, the distribution of land and sea, and the
physiographic nature of the land. At the mean Earth-Sun distance, solar energy
arrives in the vicinity of the Earth at the rate of approximately 1.95
g-cal/(cmsup2)(min), as measured outside the atmosphere perpendicular to the
incident rays. When reckoned per unit horizontal area, however, incoming solar
radiation (or insolation) is intercepted at different rates over different
parts of the Earth. The rotation of the Earth causes this rate to change
locally with the time of day. In addition, the shape of the Earth, together
with the seasonal changes in orbital distance and axial tilt of the Earth
relative to the Sun, causes solar radiation (per unit horizontal area and per
unit time) to vary with latitude and with the time of year. See CONTINENTALITY
(METEOROLOGY); GREENHOUSE EFFECT; INSOLATION; OCEAN-ATMOSPHERE RELATIONS;
TERRESTRIAL RADIATION.
    A wide variety of schemes are available for the definition of climatic
regions. These schemes fall into two classes, those intended to bring out one
or another aspect of relationships and processes within the atmosphere, and
those intended to bring out relationships between areal variations in climatic
conditions and corresponding variations in phenomena related to climate. It is
feasible and instructive to distinguish minor climatic regions, not on a
global scale, but within much smaller areas. Where such regions are defined
upon the lands, the topographic factor becomes very important. This is
especially true where the area is of the order of a few square miles or
less.See MICROMETEOROLOGY.

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�6v�2v�32sClinical microbiology   �21m �33s 
    The adaptation of microbiological techniques to the study of the 
etiological agents in infectious disease. Clinical microbiologists determine 
the nature of infectious disease and test the ability of various antibiotics 
to inhibit or kill the isolated microorganisms. A contemporary clinical 
microbiologist is also responsible for a wide range of microscopic and 
cultural studies in mycology, parasitology, and virology. The consultative 
skill of the clinical microbiologist is sought by many clinicians. The 
clinical microbiologist is often the most competent person available to 
determine the nature and extent of hospital-acquired infections, as well as 
public-health problems that affect both the hospital and the community. As 
clinical microbiology encompasses all aspects of infectious disease, the 
various specialties will be discussed individually.  See  ANTIBIOTIC; HOSPITAL 
INFECTIONS; MEDICAL BACTERIOLOGY; MEDICAL MYCOLOGY; MEDICAL PARASITOLOGY; 
VIROLOGY.  
    Clinical microbiology is a rapidly progressing laboratory science. During 
the 1970s, clinical microbiologists developed procedures which provide rapid 
information to the clinician on the causative agents of infectious disease. 
These mechanisms include direct microscopic evaluation of specimens by suing 
such powerful tools as electron and fluorescent microscopy. Other techniques 
involve the detection of soluble portions of microorganisms in the patient's 
body fluids. Detection of these soluable components can be done in a number of 
ways, including gas chromatography and immunochemical analysis. Many hospitals 
are using these immunochemical tests to quickly determine the nature of 
life-threatening diseases.  See  IMMUNOCHEMISTRY. 
    The clinical microbiology laboratory of the future may be one in which the 
identification of an infectious disease is made almost instantaneously by 
direct analysis of the patient's body fluid, and then a rapid computerized 
appraisal may be made of those antibiotics which might be effective in 
inhibiting or killing the microbial agents involved. 
 �34s[Richard C. Tilton]
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�6v�2v�32sClinometer   �21m �33s 
    A hand-held surveying device for measuring vertical angles; also called an 
Abney level. It consists of a sighting tube surmounted by a graduated vertical 
arc with an attached level bubble. A 45DG mirror inside the tube enables the 
observer to see the bubble at the same time that the observer sights a point 
or a graduated rod with a horizontal wire. Manipulation of the vertical arc 
brings the bubble to center in coincidence with the sighted point; the 
vertical angle is indicated on the arc (see illustration). 
    The clinometer is used mainly to determine slope angles for reduction of 
measured slope distance to horizontal distance. If set on a sloping surface 
(such as a street pavement), the clinometer can be used to give the angle of 
inclination of the surface. With the arc set at 0DG, it can be used as a hand 
level.  See  SURVEYING. 
 �34s[B. Austin Barry]
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�6v�2v�32sClipping circuit   �21m �33s 
    An electronic circuit that prevents transmission of any portion of an 
electrical signal exceeding a prescribed amplitude. The clipping circuit 
operates by effectively disconnecting the transmission path for the portion of 
the signal to be clipped. 
    A diode may be used as a switch to perform the clipping action. The 
direct-coupled circuit shown in the illustration represents a typical example. 
A time-varying signal voltage vsubs is applied to the input of the network 
containing a diode having terminals p and n. The diode functions as a very low 
resistance when p is positive and as a very high resistance when p is 
negative. Thus, if any portion of the waveform at p lies below the common 
reference or ground level, that portion will be removed, or clipped, and will 
not appear across output load resistance RsubL. Bidirectional clipping can be 
achieved by connecting two diodes. 
    Clipping action may be achieved by using bipolar or field-effect 
transistors. To do so the bias at the input is set at such a level that output 
current cannot flow during a portion of the amplitude excursion of the input 
voltage or current waveform. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sClock   �21m �33s 
    A device for indicating the passage of time. Most clocks contain a means 
for producing a regularly recurring action. This article describes only 
mechanical clocks.See ATOMIC CLOCK; QUARTZ CLOCK; WATCH. 
    The recurring action of a mechanical clock depends on the swing of a 
pendulum, or the oscillation of a balance wheel and balance spring or 
hairspring, or the vibration of a tuning fork, mechanisms capable of repeating 
their cyclic movements with great regularity. A counting mechanism, consisting 
of a gear train with calibrated dial and indicating hands, sometimes with a 
striking mechanism, marks the number of oscillations that have occurred, 
although the graduations are in seconds, minutes, and hours. A weight or 
spring ordinarily supplies power to operate the oscillating and the counting 
mechanisms. However, temperature changes, accelerations, automatic windings, 
or electricity may provide the power. 
    Usually an escapement transmits power from the counting mechanism to the 
oscillating mechanism. The accuracy of a clock depends primarily on the 
escapement. The illustration depicts an anchor or recoil escapement. Anchor A 
is connected loosely to the pendulum and swings about C. At some time after 
midswing, a tooth of the wheel escapes from pallet P1 or P2, giving the pallet 
a push to maintain oscillation. The other pallet then checks another tooth, 
the curve of the pallet forcing the wheel slightly backward.  The reaction 
helps to reverse the pendulum swing and to correct for circular error. 
 �34s[Robert d'E. Atkinson]
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�6v�2v�32sClock control systems   �21m �33s 
    Systems in which a timing device is used to generate the control function; 
also known as time-controlled systems. A common example of a clock control 
system is the household alarm clock. In this device the instant of time at 
which a buzzer sounds an alarm is controlled by a clock. 
    A clock control system contains two essential components, a timing device 
and a controlled process, or controlled device. The areas of application of 
such control systems range from the commonplace sounding of an alarm clock to 
the control of complex sequences of events in guided missiles and satellite 
launchers. Clock control systems are usually open-loop control systems, since 
the control action depends only on the system input, elapsed time, in this 
case. See OPEN-LOOP CONTROL SYSTEM. 
    There are many different types of timing devices. These devices may be 
divided into five functional classes: time switches, time-delay relays, 
interval timers, time-cycle controllers, and time-schedule controllers. See 
CONTROL SYSTEMS. 
 �34s[John G. Truxal]
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�6v�2v�32sClock paradox   �21m �33s 
    The paradox produced by the use of incorrect arguments, concepts, and 
premises in the relativistic twin clock problem in which two initially 
synchronized perfect clocks, A and B, are first separated and then reunited. 
It is the true logical contradiction inherent in the assertion that, when 
reunited, the two clocks, side by side, are slower than each other. Now, two 
clocks, side by side, cannot be both slower than each other; those who infer 
this paradox to be a conclusion have, in some way, introduced premises or 
concepts incompatible with Albert Einstein's theory of relativity. The clock 
paradox is not a consequence of this theory. When the relativistic twin clock 
problem is properly handled, no paradox occurs. 
    ``Time dilation'' and the twin clock problem must be distinguished. Time 
dilation is a symmetric artifact of the Lorentz transformations and the 
Einstein simultaneity convention. The twin clock effect, on the other hand, is 
an asymmetric physical phenomenon and the physical consequence of the fact 
that empty space has a metric structure. Time dilation asserts that clocks at 
rest in different inertial frames symmetrically observe each other to ``run 
slow.''  
    The twin clock effect is completely different from time dilation. The twin 
clock effect is the natural phenomenon that a clock having a past history of 
acceleration indicates a total elapsed time less than that indicated by an 
identical clock having no history of acceleration. It is a physical effect, 
and the amount by which the accelerated clock is found to be slow is the same 
regardless of the choice of inertial frame. The physical properties of empty 
space cause the twin clock effect. 
    A. H. Bucherer was the first (1909) to implicitly verify the twin clock 
effect when he experimentally found that high-speed electrons obey 
relativistic force laws rather than the classical Newtonian laws of motion. In 
1971 J. C. Hafele and R. E. Keating flew four automically stabilized clocks 
around the world and directly observed both the special and general 
relativistic twin clock effects. This experiment excluded theoretical attempts 
to ``explain'' relativistic twin clock effects solely in terms of energy 
conservation and the principle of equivalence. The Hafele-Keating experiment 
clearly indicates that both the special and general twin clock effects have as 
their common physical cause and source the metric structure of the physical 
field properties of empty space. See RELATIVITY; SPACE-TIME. 
 �34s[Richard E. Keating]
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�6v�2v�32sClonorchiasis   �21m �33s 
    An infection of humans and other fish-eating mammals caused by the 
trematode Opisthorchis (Clonorchis) sinensis, which is usually found in the 
bile ducts of the infected individuals. The condition is also called Chinese 
liver fluke disease. Southern China is the classic hyperendemic focus because 
of the habit there of eating raw fresh-water fish. Endemicity is also 
associated with the use of night soil as fertilizer. Cats and dogs act as 
reservoirs.  
    In the life cycle of the fluke, the minute eggs pass in feces and are 
ingested by aquatic snails, within which the larval cycle ensues. The larvae 
emerge and encyst in fish (Ctenopharyngodon idellus in particular). When the 
fish is eaten uncooked, the larvae excyst and arrive in the bile ducts, 
causing their progressive hypertrophy with resultant cholecystitis and 
hepatitis. See PARAGONI-MIASIS. 
 �34s[Jose F. Maldonado]
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�6v�2v�32sClosed-circuit television   �21m �33s 
    A video communication system in which the signal is transmitted from the 
point of origin only to those specific receivers that have access to it by 
previous arrangement. Closed-circuit television enables visual information to 
be transmitted, often in parallel with audio information, to a selected 
audience or between selected groups of people. This private nature of 
closed-circuit television distinguishes it from broadcast television, in which 
all receivers in a given area have access to the broadcast signals. See 
TELEVISION.  
    Closed-circuit television is used for a large number of different types of 
monitoring applications in which the remote viewing of an ongoing process or 
activity at a location provides economic, safety, or educational advantages. A 
growing number of educational institutions and corporations are also using 
closed-circuit television systems for lectures, presentations, and corporate 
training applications. Most of these applications consist of one-way 
television with two-way audio conference facilities to permit audience 
interaction. Because of nationwide and sometimes worldwide interest in certain 
sporting or entertainment events, one-way closed-circuit television systems 
are being used to permit a number of geographically dispersed audiences to 
view the event simultaneously. Two-way closed-circuit television systems are 
being used to permit groups of people in geographically separated locations to 
interact on both a visual and audio basis. 
    Community antenna television (CATV) or cable television systems are 
normally considered a type of closed-circuit television. As originally 
envisioned and implemented, these systems employ standard broadcast television 
signals which are received by a suitably located antenna and amplified and 
distributed through coaxial cables. This type of system can be used to provide 
reliable television reception in a community located beyond the useful range 
of broadcast television transmitters, or it can be used in an apartment 
building to avoid having each tenant install a separate antenna. 
    The range of signal formats and transmission means used in closed-circuit 
television is great. The signal formats are chosen to suit the particular use, 
and extend from high-resolution closed-circuit television applications, 
requiring bandwidths of 5 - 15 MHz, down to slow-scan television signals which 
require bandwidths as small as 2.5 kHz. The means of transmission range from 
broadband radio circuits to standard telephone message circuits. 
    Technological advances in terminal equipment have broadened the potential 
applications for closed-circuit television systems. Among these developments 
are cameras that are capable of operating under very low ambient light 
conditions, and color cameras that are portable and simple to use. 
Developments in video tape cassette recording, editing, and playback 
mechanisms have made the production of video program material easier and less 
expensive. Developments in video disk technology appear to have a great many 
potential applications, from mass entertainment to individual instruction in a 
variety of special fields. 
 �34s[Joseph J. Horzepa]
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�6v�2v�32sClosed-loop control system   �21m �33s 
    A system in which the value of some output quantity of the system is 
continuously or periodically measured and used to manipulate another quantity, 
an input to the system, in such a way as to make the first quantity or some 
other dependent output follow a desired pattern of values. At each instant the 
measured value is compared to the desired value of the measured quantity. The 
difference is used to change the value of an input to the sytem. Such systems 
are also called feedback control systems, because the measured value is fed 
back, to be used to manipulate an input quantity. See CONTROL SYSTEMS. 
 �34s[John A. Hrones]
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�6v�2v�32sCloud   �21m �33s 
    Suspensions of minute droplets or ice crystals produced by the 
condensation of water vapor (the ordinary atmospheric cloud). Other clouds, 
less commonly seen, are composed of smokes or dusts. See AIR POLLUTION; DUST 
STORM.  
    If water vapor is cooled sufficiently, it becomes saturated, that is, in 
equilibrium with a plane surface of liquid water (or ice) at the same 
temperature. Further cooling in the presence of such a surface causes 
condensation upon it. In the atmosphere, even in the apparent absence of any 
surfaces, there are invisible motes upon which the condensation proceeds at 
barely appreciable cooling beyond the state of saturation. Consequently, when 
atmospheric water vapor is chilled sufficiently, such motes, or condensation 
nuclei, swell into minute water droplets and form a visible cloud. 
    The World Meteorological Organization (WMO) uses a classification which 
divides clouds into low-level (base below about 1.2 mi or 2 km), middle-level 
(about 1.2 - 4 mi or 2 - 7 km), and high-level (4 - 8 mi or 7 - 14 km) forms 
within the middle latitudes. The names of the three basic forms of clouds are 
used in combination to define 10 main characteristic forms, or ``genera.'' 
     1. Cirrus are high white clouds with a silken or fibrous appearance. 
     2. Cumulus are detached dense clouds which rise in domes or towers from a 
level low base. 
     3. Stratus are extensive layers or flat patches of low clouds without 
detail.  
     4. Cirrostratus is cirrus so abundant as to fuse into a layer. 
     5. Cirrocumulus is formed of high clouds broken into a delicate wavy or 
dappled pattern. 
     6. Stratocumulus is a low-level layer cloud having a dappled, lumpy, or 
wavy structure. 
     7. Altocumulus is similar to stratocumulus but lies at intermediate 
levels.  
     8. Altostratus is a thick, extensive, layer cloud at intermediate levels. 
     9. Nimbostratus is a dark, widespread cloud with a low base from which 
prolonged rain or snow falls. 
    10. Cumulonimbus is a large cumulus which produces a rain or snow shower. 
See CLOUD PHYSICS. 
 �34s[Frank H. Ludlam]
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�6v�2v�32sCloud chamber   �21m �33s 
    A particle detector in which the path of a fast charged particle is made 
visible by the formation of liquid droplets on the ions left by the particle 
as it passes through the gas of the chamber. Cloud chambers are used in 
research in nuclear physics, and can give detailed information on the particle 
in addition to the simple fact that the particle passed through. They can be 
operated at pressures below atmospheric, near atmospheric, or up to 50 atm, 
(500 kilopascals), depending on the application. They vary in size from a few 
inches in diameter up to large walk-in chambers containing many metal plates. 
See PARTICLE DETECTOR. 
    Supersaturated vapor is vapor whose pressure exceeds the saturation vapor 
pressure at the temperature prevailing. It is unstable, and condensation 
occurs in the presence of suitable nuclei. The interior of a cloud chamber 
contains a mixture of a gas such as air, argon, or helium and a vapor such as 
water or alcohol. When such a mixture is made supersaturated in the vapor 
component by cooling of the gas, the vapor tends to condense preferentially on 
charged atoms in the gas. Since a fast-moving charged particle produces many 
such ions in its passage through the gas, condensation of the vapor leaves a 
trail of droplets to indicate the path of the particle. When properly 
illuminated, these droplets are visible and may be photographed. A permanent 
record of the path of the particle may thus be obtained for examination at a 
later time. 
    A cloud chamber can given information on the momentum of the particle if 
the chamber is placed in a magnetic field. The curvature of the track can be 
measured on the photograph, and the value of the curvature, together with the 
magnitude of the field, gives the momentum. Another physical quantity that may 
be determined in a cloud chamber is the velocity of the particle, since the 
ionization produced by the charged particle depends on its velocity in a known 
way.  
 �34s[William B. Fretter]
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�6v�2v�32sCloud physics �21m �33s
    The study of the physical and dynamical processes governing the structure
and development of clouds and the release from them of snow, rain, and hail
(collectively known as precipitation).
    The factors of prime importance are the motion of the air, its water-vapor
content, and the numbers and properties of the particles in the air which act
as centers of condensation and freezing. Because of the complexity of
atmospheric motions and the enormous variability in vapor and particle content
of the air, it seems impossible to construct a detailed, general theory of the
manner in which clouds and precipitation develop. However, calculations based
on the present conception of laws governing the growth and aggregation of
cloud particles and on simple models of air motion provide reasonable
explanations for the observed formation of precipitation in different kinds of
clouds.
    Clouds are formed by the lifting of damp air which cools by expansion
under continuously falling pressure. The relative humidity increases until the
air approaches saturation. Then condensation occurs on some of the wide
variety of aerosol particles present; these exist in concentrations ranging
from less than 1600 particles per cubic inch (100 particles per cubic
centimeter) in clean, maritime air to perhaps 1.6 x 10 sup 7 particles per cubic
inch (10 sup 6 particles per cubic inch) in the highly polluted air of an
industrial city. These droplets, usually present in concentrations of a few
hundreds per cubic centimeter, constitute a nonprecipitating water cloud.
Precipitation  release follows the production of a relatively few large
particles from a large population of much smaller ones, which may be achieved
in one of two ways: the coalescence process or the ice crystal process.
    Cloud droplets are seldom of uniform size. Droplets arise on nuclei of
various sizes and grow under slightly different conditions of temperature and
supersaturation in different parts of the cloud. A droplet appreciably larger
than average will fall faster than the smaller ones, and so will collide and
fuse (coalesce) with some of those which it overtakes.
    The second method of releasing precipitation can operate only if the cloud
top reaches elevations where temperatures are below -17DGF (0DGC) and the
droplets in the upper cloud regions become supercooled. At temperatures below
-38DGF (-40DGC) the droplets freeze automatically or spontaneously; at higher
temperatures they can feeze only if they are infected with special, minute
particles called ice nuclei. As the temperature falls below -17DGF (0DGC),
more and more ice nuclei become active, and ice crystals appear in increasing
numbers among the supercooled droplets. Such a mixture of supercooled droplets
and ice crystals is unstable. After several minutes the growing crystals will
acquire definite falling speeds, and several of them may become joined
together to form a snowflake. In falling into the warmer regions of the cloud,
however, the snowflake may melt and reach the ground as a raindrop.
    The deep, extensive, multilayer-cloud systems, from which precipitation of
a usually widespread, persistent character falls, are generally formed in
cyclonic depressions (lows) and near fronts. Although the structure of these
great raincloud systems, which are being explored by aircraft and radar, is
not yet well understood, radar signals from these clouds usually take a
characteristic form which has been clearly identified with the melting of
snowflakes.
    Precipitation from shower clouds and thunderstorms, whether in the form of
raindrops, pellets of soft hail, or true hailstones, is generally of greater
intensity and shorter duration than that from layer clouds and is usually
composed of larger particles. The clouds themselves are characterized by their
large vertical depth, strong vertical air currents, and high concentrations of
liquid water, all these factors favoring the rapid growth of precipitation
elements by accretion.
    The development of precipitation in convective clouds is accompanied by
electrical effects culminating in lightning. The mechanism by which the
electric charge dissipated in lightning flashes is generated and separated
within the thunderstorm has been debated for more than 200 years, but there is
still no universally accepted theory. However, the majority opinion holds that
lightning is closely associated with the appearance of the ice phase, and the
most promising theory suggests that the charge is produced by the rebound of
ice crystals or a small fraction of the cloud droplets that collide with the
falling hail pellets. See LIGHTNING.
    The various stages of the precipitation mechanisms raise a number of
interesting and fundamental problems in classical physics. Worthy of mention
are the supercooling and freezing of water; the nature, origin, and mode of
action of the ice nuclei; and the mechanism of ice-crystal growth which
produces the various snow crystal forms.
    The maximum degree to which a sample of water may be supercooled depends
on its purity, volume, and rate of cooling. The freezing temperatures of
waterdrops containing foreign particles vary linearly as the logarithm of the
droplet volumes for a constant rate of cooling. This relationship, which has
been established for drops varying between 10 micrometers and 1 centimeters in
diameter, characterizes the heterogeneous nucleation of waterdrops and is
probably a consequence of the fact that the ice-nucleating ability of
atmospheric aerosol increases logarithmically with decreasing temperature.
    Measurements made with large cloud chambers on aircraft indicate that the
most efficient nuclei, active at temperatures above 14DGF (-10DGC), are
present in concentrations of only about 10 in a cubic meter of air, but as the
temperature is lowered, the numbers of ice crystals increase logarithmically
to reach concentrations of about 1 per liter at -4DGF (-20DGC) and 100 per
liter at -22DGF (-30DGC). Since these measured concentrations of nuclei are
less than one-hundredth of the numbers that apparently are consumed in the
production of snow, it seems that there must exist processes by which the
original number of ice crystals are rapidly multiplied. Laboratory experiments
suggest the fragmentation of the delicate snow crystals and the ejection of
ice splinters from freezing droplets as probable mechanisms.
    The most likely source of atmospheric ice nuclei is provided by the soil
and mineral-dust particles carried aloft by the wind. Laboratory tests have
shown that, although most common minerals are relatively inactive, a number of
silicate minerals of the clay family produce ice crystals in a super-cooled
cloud at temperatures above -26DGF (-18DGC). A major constituent of some
clays, kaolinite, which is active below -21DGF (-9DGC), is probably the main
source of highly efficient nuclei.
    The fact that there may often be a deficiency of efficient ice nuclei in
the atmosphere has led to a search for artificial nuclei which might be
introduced into supercooled clouds in large numbers. In general, the most
effective ice-nucleating substances, both natural and artificial, are
hexagonal crystals in which spacings between adjacent rows of atoms differ
from those of ice by less than 16%. The detailed surface structure of the
nucleus, which is determined only in part by the crystal geometry, is of even
greater importance.
    Collection of snow crystals from clouds at different temperatures has
revealed their great variety of shape and form. This multiple change of habit
over such a small temperature range is remarkable and is thought to be
associated with the fact that water molecules apparently migrate between
neighboring faces on an ice crystal in a manner which is very sensitive to the
temperature. The temperature rather than the supersaturation of the
environment is primarily responsible for determining the basic shape of the
crystal, though the supersaturation governs the growth rates of the crystals,
the ratio of their linear dimensions, and the development of dendritic forms.
    The presence of either ice crystals or some comparatively large
waterdroplets (to initiate the coalescence mechanism) appears essential to the
natural release of precipitation. Rainmaking experiments are conducted on the
assumption that some clouds precipitate inefficiently, or not at all, because
they are deficient in natural nuclei; and that this deficiency can be remedied
by ``seeding'' the clouds artificially with dry ice or silver iodide to
produce ice crystals, or by introducing waterdroplets or large hygroscopic
nuclei.
    The science and technology of cloud modification is still in its infancy.
It may well be that further knowledge of cloud behavior and of natural
precipitation mechanisms will suggest new possibilities and improved
techniques which will lead to developments far beyond those which now seem
likely. See PRECIPATION (METEOROLOGY); WEATHER MODIFICATION.
 �34s[Basil J. Mason]
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�6v�2v�32sClove �21m �33s
    The unopened flower bud of a small, conical, symmetrical, evergreen tree,
Eugenia caryophyllata, of the myrtle family (Myrtaceae). The cloves are picked
by hand and dried. Cloves, one of the most important and useful spices, are
strongly aromatic and have a pungent flavor. They are used as a culinary spice
for flavoring pickles, ketchup, and sauces, in medicine, and for perfuming the
breath and the air in rooms. The essential oil distilled from cloves by water
or steam has even more uses. The chief clove-producing countries are Tanzania,
Indonesia, Mauritius, and the West Indies. See MYRTALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sClover   �21m �33s 
    A common name used loosely to designate the true clovers, sweet clovers, 
and other members of the plant family Leguminosa. 
  
    True clovers.  The true clovers are herbaceous annual or perennial plants 
of the genus Trifolium, order Rosales. There are approximately 250 species in 
the world. Collectively they represent the most important genus of forage 
legumes in agriculture. Most clovers are highly palatable and nutritious to 
livestock. The name clover is often applied to members of legume genera other 
than Trifolium. See ROSALES. 
    Clovers are used for hay, pasture, silage, and soil improvement. Certain 
kinds may be used for all purposes whereas others, because of their low 
growth, are best suited for grazing. All kinds, when well grown in thick 
stands, are good for soil improvement. Thoroughly inoculated plants add 50 - 
200 lb of nitrogen per acre (62 - 252 kg per hectare) when plowed under for 
soil improvement, the amount added depending on growth, thickness of stand, 
and length of growing season. See LEGUME FORAGES; NITROGEN CYCLE. 
    All the clover species of agricultural importance in the United States are 
introduced (exotic) plants. Some of the species most widely used are: red 
clover (T. pratense), alsike clover (T. hybridum), white clover (T. repens), 
crimson clover (T. incarnatum), subclover (T. subterraneum), strawberry clover 
(T. fragiferum), persian clover (T. resupinatum), and large hop clover (T. 
campestre, or procumbens). Other clovers of regional importance, mostly 
adapted to specific environmental conditions, are rose clover (T. hirtum), 
berseem clover (T. alexandrinum), ball clover (T. nigrescens), lappa clover 
(T. lappaceum), big-flower clover (T. michelianum), and arrowleaf clover (T. 
vesiculosum).  
    Red clover is composed of two forms, medium and mammoth, producing two and 
one hay cuts, respectively. The large purplish-red flower heads are round. An 
upright-growing perennial, red clover generally persists for 2 years in the 
northern United States, but behaves as a winter annual in the South. There are 
several varieties and strains of red clover such as Kenland, Pennscott, 
Lakeland, Dollard, and Chesapeake. These produce higher yields of forage and 
are more persistent than common red clover. 
    Alsike clover is an upright-growing species that behaves like a biennial. 
The growth pattern, seeding methods, mixtures, and uses are similar to those 
of red clover. The flower heads are much like those of white clover in shape 
and size, but are slightly more pinkish. Alsike clover is more tolerant of 
wet, poorly drained soils than red clover and occurs widely in mountain 
meadows of the West. 
    White clover, an inhabitant of lawns and closely grazed pastures, is the 
most important pasture legume in the humid  states. The flowers are generally 
white, but sometimes they are tinged with pink. There are three main types, 
large, intermediate, and small, with all gradations between. All types are 
nutritious and are relished by all classes of livestock and poultry. White 
clover is mainly grown with low-growing grasses, not being tolerant of the 
tall-growing kinds. For best growth of the clover, grass-clover mixtures 
should be grazed or cut frequently. 
    Crimson clover is used principally as a winter annual for pasture and as a 
soil-improving crop from the latitude of the Ohio River southward, and along 
the West Coast. Crimson clover is seeded alone, with small grains and grasses, 
or on grass turf. During the winter it may be grazed, although if it is too 
heavily grazed, regrowth is slow. The greatest return for soil improvement is 
obtained when the largest growth is plowed under. There are several varieties 
of crimson clover, including Dixie, Auburn, Autauga, Chief, Talladega, and 
many local strains. When used with Bermuda or other perennial summer-growing 
grasses, the fall growth of the grass must be closely grazed or clipped. 
    Subclover, a winter annual extensively used for grazing in the coastal 
sections of the Western states, is the basic pasture crop of the sheep and 
cattle industry of Australia. The Australian varieties Mount Barker, 
Tallarook, and Nangeela have proved to be best adapted to most conditions in 
the United States. Subclover appears to have considerable promise as a pasture 
legume under many conditions in the southern United States, but better adapted 
varieties are needed. 
  
    Sweet clover.  Sweet clover is the common name for all but one species of 
legumes of the genus Melilotus, order Rosales. The exception is sour clover 
(M. indica). There are approximately 20 species of sweet clover. Some of the 
biennial species have an annual form. Sweet clovers are native to the 
Mediterranean region and adjacent countries, but several are widely scattered 
throughout the world, generally by chance introduction. 
    Sweet clover is used as a field crop in regions of the United States and 
Canada where the rainfall is 17 in. (42 cm) or more during the growing season, 
where the soil is neutral, or where limestone and other needed minerals are 
applied. It is grown either alone or in rotations with small grains and corn, 
and is  used for grazing, soil improvement, and hay. Except for those of 
certain improved varieties, the plants are somewhat bitter because of the 
presence of coumarin. 
 �34s[Eugene A. Hollowell]
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�6v�2v�32sClupeiformes   �21m �33s 
    An order of teleost fishes of the subclass Actinopterygii that includes 
the herrings, sardines, anchovies, and their allies. The Recent clupeiforms 
are classified in 2 suborders, 3 (by some authorities as many as 6) families, 
71 genera, and about 300 species. 
    The Clupeiformes are generalized teleosts. They are mostly silvery and 
compressed. A distinctive feature is the extension of the cephalic canal 
system onto the operculum; the lateral line is undeveloped on the trunk except 
in Denticeps. They lack fin spines, adipose fin, and gular plate. The middle 
of the belly often bears one or a series of strong scutes. The pelvic fin is 
abdominal in position, free from the shoulder girdle, and the pectoral fin is 
placed low on the side. The cycloid scales are usually thin and loosely 
attached. The upper jaw is bordered by premaxillae and maxillae, but the 
dentition is usually feeble. 
    Clupeiform fishes have left a rich fossil record, especially from the 
Upper Cretaceous to the early Tertiary, but appeared first in the Upper 
Jurassic. Some herrings and a few anchovies live in lowland rivers and lakes, 
and others such as the shad and alewife enter rivers to reproduce; but the 
majority of clupeiforms occur in bays or shore waters of tropical, temperate, 
or even northern seas, where they commonly make up enormous schools. None 
inhabits deep water. Most feed on plankton or other minute organisms. Thus 
they are efficient converters from the base of the food chain to fish flesh. 
    Some of the great fisheries of the world are based on the conversion of 
plankton by clupeiform fishes: the tremendous anchovy fishery off western 
South America; the California fisheries for anchovy and (until depleted) 
Pacific sardine; the menhaden fisheries of the western Atlantic; and the 
herring and sardine fisheries of northern seas. The catch is processed 
variously into oil, fertilizer, or fish meal or is prepared directly for human 
consumption. See ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sClutch   �21m �33s 
    A machine element for the connection and disconnection of shafts in 
equipment drives. If both shafts to be connected can be stopped or made to 
move relatively slowly, a positive-type mechanical clutch may be used. If an 
initially stationary shaft is to be driven by a moving shaft, friction 
surfaces must be interposed to absorb the relative slippage until the speeds 
are the same. Likewise, friction slippage allows one shaft to stop after the 
clutch is released. 
    When positive connection of one shaft with another in a given position is 
needed, a positive clutch is used. This clutch is the simplest of all shaft 
connectors, sliding on a keyed shaft section or a splined portion and 
operating with a shift lever on a collar element. Because it does not slip, no 
heat is generated in this clutch. Interference of the interlocking portions 
prevents engagement at high speeds; at low speeds, if connection occurs, shock 
loads are transmitted to the shafting. Positive clutches may be of the square 
jaw type (Fig. 1) with two or more jaws of square section meshing together in 
the opposing clutches, or the spiral jaw type, a modification of the 
square-jaw clutch that permits more convenient engagement and provides a more 
gradual movement of the mating faces toward each other. 
    When the axial pressure of the clutch faces on each other serves to 
transmit torque instead of the mating shape of their parts, the clutch 
operates by friction. This friction clutch is usually placed between an engine 
and a load to be driven; when the friction surfaces of the clutch are engaged, 
the speed of the driven load gradually approaches that of the engine until the 
two speeds are the same. A friction clutch is necessary for connecting a 
rotating shaft of a machine to a stationary shaft so that it may be brought up 
to speed without shock and transmit torque for the development of useful work. 
The three common designs for friction clutches, combining axial and radial 
types, are cone clutches (Fig. 2), disk clutches, and rim clutches. In a cone 
clutch, the surfaces are sections of a pair of cones. The disk clutch consists 
essentially of one or more friction disks connected to a driven shaft by 
splines. A rim clutch has surface elements that apply pressure to the rim 
either externally or internally. 
    In the overrunning type of clutch, the driven shaft can run faster than 
the driving shaft. This action permits freewheeling as the driving shaft slows 
down or another source of power is applied. Effectively this is a friction 
pawl-and-ratchet drive, wherein balls or rollers become wedged between the 
sleeve and recessed pockets machined in the hub (Fig. 3). The clutch does not 
slip when the second shaft is driven, and is released automatically when the 
second shaft runs faster than the driver. The centrifugal clutch employs 
centrifugal force from the speed of rotation. This type of clutch is not 
normally used because it becomes unwieldy and unsafe with increasing size. 
Clutch action is also produced by hydraulic couplings, with a smoothness not 
possible with a mechanical clutch. Automatic transmissions in automobiles 
represent a fundamental use of hydraulic clutches. See FLUID COUPLING; TORQUE 
CONVERTER.  
    Magnetic coupling between conductors provides a basis for several types of 
clutches. The magnetic attraction between a current-carrying coil and a 
ferromagnetic clutch plate serves to actuate a disk-type clutch. Slippage in 
such a clutch produces heat that must be dissipated and wear that reduces the 
life of the clutch plate. Thus the electromagnetically controlled disk clutch 
is used to engage a load to its driving source. See BRAKE; COUPLING. 
 �34s[John E. Gibson ;James J. Ryan]
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�6v�2v�32sClypeasteroida   �21m �33s 
    An order of exocyclic Euechinoidea which have a monobasal apical system in 
which all the genital plates fuse together. The ambulacra are petaloid on the 
aboral side and wider than the interambulacra on the adoral side (see 
illustration). The order probably arose in the Late Cretaceous. Tertiary 
fossils are very common, and extant species range the coastlines of tropical 
and temperate seas. Relatively short time spans of some fossil species make 
them valuable as index forms in stratigraphy. Many species are gregarious, and 
occur in closely packed colonies of perhaps millions of individuals. There are 
16 families arranged in four suborders. See ECHINODERMATA; EUECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sCnidospora   �21m �33s 
    A subphylum of spore-producing Protozoa containing the two classes 
Myxosporidea and Microsporidea. Cnidospora are parasites of cells and tissues 
of invertebrates, fishes, a few amphibians, and turtles. See MICROSPORIDEA; 
MYXOSPORIDEA.  
    A distinctive structure of this subphylum is the spore, which contains one 
or more protoplasmic masses called sporoplasms. The spore also contains one or 
more filaments which lie coiled within the spore proper or within one or more 
polar capsules. 
    When the spore is ingested by a new host, the filaments are extruded and 
at the same time the sporoplasm is released. The sporoplasm, now called an 
amebula, reaches the specific site of infection directly through the 
intestinal wall or by way of the bloodstream. The amebula becomes a stage in 
the life cycle which feeds and grows at the expense of the host. Asexual 
reproduction by repeated binary fission, multiple fission, internal and 
external budding, or plasmotomy results in cells which develop into sporonts 
and eventually into sporoblasts, producing one or more spores. See PROTOZOA; 
SPOROZOA.  
 �34s[Ross F. Nigrelli]
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�6v�2v�32sCoal �21m �33s
    A brown to black combustible rock that had its origin in the accumulation
and physical and chemical alteration of vegetation. Original accumulations of
vegetation (mostly woody plants), usually in a swamp or moist environment
which reduced decay, resulted in formation of peat, the precursor of coal.
Peat is converted to coal after burial and subsequent geological processes,
including increased pressure and temperature, which progressively compressed
and indurated and otherwise altered the material through a series of coal
varieties to an extreme of graphite or graphitelike material. In American
terminology, the rank varieties of coal constituting the coalification series
consist of lignitic (including brown coal); subbituminous; bituminous
(including high-, medium-, and low-volatile); and anthracitic coal (including
semianthracite, anthracite, and metaanthracite or graphitic coal). See
GRAPHITE; LIGNITE; PEAT.
    The term coalification refers to the process of coal metamorphism brought
about by increasing weight of overlying sediments, by tectonic movements, by
an increase in temperatures resulting from depth of burial, or from close
approach to, or contact with, igneous intrusions or extrusions. Increase in
pressure affects principally the physical properties of the coal, that is,
hardness, strength, optical anisotropy, and porosity. Increase in temperature
acts chiefly to modify the chemical composition by increasing the carbon
content and decreasing the content of oxygen and hydrogen and increasing the
calorific value to a maximum in those coals having 15 - 30% volatile matter
(dry, mineral-matter-free).
    The classification and description of coal depend largely upon information
supplied by chemical analysis consisting of systematically standardized
procedures and the results of a number of empirical tests. Chemical analyses
are of two principal kinds: the elementary or ultimate analysis and the
proximate or commercial type of analysis. The ultimate analysis is limited to
the percent of hydrogen, carbon, oxygen, nitrogen, and sulfur, exclusive of
the mineral matter or ash. The proximate analysis, which is empirical in
character, is the prevailing form of analysis in North America. In this
analysis, percentage values are reported for volatile matter, fixed carbon,
moisture, and ash. Calorific and sulfur values are also determined, the latter
sometimes in terms of forms of sulfur such as pyritic, organic, and sulfate
sulfur. It is common practice to use qualifying terms denoting the basis of
reported analysis, such as ``as-received,'' ``moisture-free'' (mf),
``moisture-and-ash-free'' (maf), and dry-mineral-matter-free (dmmf) so-called
``pure coal,'' in presenting the results of proximate and ultimate analysis.
    Mineral matter is the inorganic material present in coal that is the
source of ash when coal is burned. It occurs predominantly as discrete mineral
phases, but also as chemical elements combined with organic matter and as
dissolved elements in the pore water. Mined coals commonly contain between 5
and 15% mineral matter, although some have greater and lesser amounts.
    The term rank refers to the stage of coalification reached in the course
of metamorphism. Classification of coal by rank is shown in the table. It is
fundamental in coal characterization and generally is based upon the chemical
composition, and upon proximate values based upon mineral-matter-free coal
obtained from standard face channel samples. The higher-rank coals are
classified with respect to fixed carbon on the dry basis, and the low-rank
coals according to calorific value on the moist basis. In Europe, coking
properties are a more common basis of classification than in North America.
    The calorific values of coal are expressed in British thermal units (Btu)
per pound in some English-speaking countries, and in calories per gram
generally elsewhere. Heat values, determined by a calorimeter, vary for coal
material from about 6300 Btu/lb (14.65 megajoules/kg) for California lignite
to about 16,300 Btu/lb (37.91 megajoules/kg) for some maf cannel coal. See
CALORIMETRY.
    The lithologic characteristics of coal concern the structural aspects of
the coal bed and texture as determined by the megascopic and microscopic
physical constitution of the coal itself. Structurally the coal bed (or seam)
is a geological stratum characterized by the same irregularities in thickness,
uniformity, and continuity as other strata of sedimentary origin. Coal beds
may consist of essentially uniform continuous strata or, like other
sedimentary deposits, may be made up of distinctly different bands or benches
of varying thickness. Like other sedimentary strata, coal beds may be
structurally disturbed by folding and faulting so that the originally
approximate horizontality of position is lost to the extent that beds may
become vertically oriented or even overturned.
    The texture of the coal itself is determined by the character, grain, and
distribution of its megascopic and microscopic components. In general, banded
coals composed of relatively coarse, highly lustrous vitrain lenses are
considered coarsely textured. As the thickness of the vitrain bands
progressively lessens, the texture becomes fine-banded and then microbanded,
with bright laminae composed of fine vitrainlike material. Coals displaying
homogeneous texture are regarded as nonbanded or canneloid coals. In general,
similar textural units are observed in bituminous and anthracite coals.
    Microscopic texture is determined by the physical composition of the
lithotypes, or of the anthraxylon and attritus, in terms of the microscopic
constituents. The microscopy and petrology of coal are concerned very largely
with these constituents.
    The uses for coal have varied widely with rank, but three general areas of
use may be distinguished. The major use is for combustion to generate
electricity and heat (electric utilities, industrial, domestic, and
railroads). A significant percentage of the world's coal with suitable
properites is used in the manufacture of metallurgical grade coke. Coal was
formerly used extensively to manufacture gas, and new research appears to be
leading to major developments for conversion of coal to liquid and gaseous
fuels to supplement supplies of oil and natural gas. Coal was an important
source of a wide range of chemicals before being largely supplanted by oil and
gas, but this possible renewed future use is similar to processes of
conversion to liquid and gaseous fuels. See COAL CHEMICALS; COAL GASIFICATION;
COAL LIQUEFACTION.
    Coal is found on every continent, in the islands of Oceania, and in the
West Indies. Most coal resources of the world are contained in strata from the
Carboniferous (Pennsylvanian-Mississippian of the United States) and younger
strata, although minor occurrences have been reported in Devonian and older
strata. Approximately 95% of the known coal resources and 85% of recoverable
reserves of the world lie in the Northern Hemisphere. Estimates of total coal
resources of the world are very large, but vary widely depending on
assumptions on which the estimates are based. Recoverable coal reserves of the
world based on current mining practice have been estimated to total over 8 x
10 sup 1 sup 1 tons.
 �34s[Jack A. Simon]
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�6v�2v�32sCoal balls   �21m �33s 
    Subspherical masses containing mineral matter and embedded plant material, 
found in coal seams and overlying beds of the late Paleozoic. The mineral 
content of the balls usually consists of calcium or magnesium carbonate, or 
sometimes iron pyrites. In some areas coal balls are found only in coal seams 
overlain by marine limestones. 
    All known deposits of coal balls in North America, Europe, and the British 
Isles are associated with Pennsylvanian and Permian beds of bituminous coal. 
They usually are present as rounded nodules or irregular masses at the surface 
of the bed or in the upper part of the coal seam. They vary from small 
pea-sized nodules to irregularly formed masses weighing tons. Some of the best 
specimens occur in stream beds in which coal seams have been exposed by 
erosion. See COAL PALEOBOTANY; CONCRETION. 
 �34s[Wilson N. Stewart]
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�6v�2v�32sCoal chemicals   �21m �33s 
    For about 100 years, chemicals obtained as by-products in the primary 
processing of coal to metallurgical coke have been the main source of aromatic 
compounds used as intermediates in the synthesis of dyes, drugs, antiseptics, 
and solvents. Although some aromatic hydrocarbons, such as toluene and xylene, 
are now obtained largely from petroleum refineries, the main source of others, 
such as benzene, naphthalene, anthracene, and phenanthrene, is still the 
by-product coke oven. Heterocyclic nitrogen compounds, such as pyridines and 
quinolines, are also obtained largely from coal tar. Although much phenol is 
produced by hydrolysis of monochlorobenzene and by decomposition of cumene 
hydroperoxide, much of the phenol, cresols, and xylenols are still obtained 
from coal tar. 
    Coke oven by-products are gas, light oil, and tar. Coke oven gas is a 
mixture of methane, carbon monoxide, hydrogen, small amounts of higher 
hydrocarbons, ammonia, and hydrogen sulfide. Most of the coke oven gas is used 
as fuel. Although several hundred chemical compounds have been isolated from 
coal tar, a relatively small number are present in appreciable amounts. These 
may be grouped as in the table. All the compounds in the table except the 
monomethylnaphthalenes are of some commercial importance. 
    The direct utilization of coal as a source of bulk organic chemicals has 
been the objective of much research and development. Oxidation of aqueous 
alkaline slurries of coal with oxygen under pressure yields a mixture of 
aromatic carboxylic acids. Because of the presence of nitrogen compounds and 
hydroxy acids, this mixture is difficult to refine. Hydrogenation of coal at 
elevated temperatures and pressures yields much larger amounts of tar acids 
and aromatic hydrocarbons of commercial importance than are obtained by 
carbonization. However, this operation is more costly than other sources of 
these chemicals. See COKE; DESTRUCTIVE DISTILLATION; PYROLYSIS. 
 �34s[Howard W. Wainwright]
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�6v�2v�32sCoal gasification   �21m �33s 
    The conversion of coal, coke, or char to gaseous products by reaction with 
air, hydrogen, oxygen, steam, carbon dioxide, or a mixture of these. Products 
consist of carbon dioxide, carbon monoxide, hydrogen, methane, and some other 
chemicals in a ratio dependent upon the particular reactants employed and the 
temperatures and pressures within the reactors, as well as upon the type of 
treatment which the gases from the gasifier undergo subsequent to their 
leaving the gasifier. Strictly speaking, reaction of coal, coke, or char with 
air or oxygen to produce heat plus carbon dioxide might be called 
gasification. However, that process is more properly classified as combustion, 
and thus is not included in this coal gasification summary. See COMBUSTION. 
    In most coal gasification processes which are available for use or under 
development, the reactions are endothermic. Air or oxygen is typically 
supplied to the gasifier to provide the necessary heat input. From the 
industrial user's viewpoint, the net result is to produce either low-Btu or 
intermediate-Btu gas. Each of these gas types has attracted industrial 
interest. The electric industry has principally investigated the production of 
clean, low-Btu gas from coal with the object of burning it in a combined-cycle 
power generation system. The natural gas industry has viewed intermediate-Btu 
gas production with strong interest since this same gasification product can 
also be upgraded to synthetic natural gas by using relatively conventional and 
proved gas-processing steps. Heavy industry has studied the feasibility of 
using both low- and intermediate-Btu gas for many applications, thereby 
freeing critical volumes of natural gas and also reducing imported oil 
consumption. Parts of the chemical industry have examined intermediate-Btu gas 
as a source of hydrogen and carbon monoxide to offset potentially reduced 
supplies of these synthetic chemical building blocks. Various projects have 
also been studied which would involve coproduction of SNG and intermediate-Btu 
gas for industrial or chemical use, electric power, coal-derived liquids, and 
various other products. See COAL; COKE; NATURAL GAS. 
 �34s[Richard H. McClelland]
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�6v�2v�32sCoal liquefaction   �21m �33s 
    The conversion of most types of coal, anthracite excepted, by direct or 
indirect technologies primarily to produce synthetic low-sulfur, low-ash 
liquid or solid fuels to replace some of the fuel coal used by electric 
utilities. Coal liquefaction can be utilized for making syncrude, a material 
suitable for use as a refinery feedstock and for petrochemical production. 
    Although the term liquefaction implies liquid products, several processes 
also produce gaseous and solid products. The advantage of coal liquefaction is 
that a wide range of liquid products, especially heavy fuels for electric 
utilities, distillate fuel oil, and gasoline, can be produced by varying the 
operating conditions of different processes. The atomic hydrogen:carbon (H:C) 
ratios of the products are equal or greater than the ratio in the feed coal, 
depending on the extent of hydrogenation used. Coal liquefaction requires less 
chemical transformation than gasification, and has an approximate 78% 
energy-conversion efficiency versus 60 - 70% for gasification. See COAL; COAL 
GASIFICATION.  
 �34s[Franklin D. Cooper]
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�6v�2v�32sCoal mining   �21m �33s 
    The technical and mechanical job of removing coal from the earth and 
preparing it for market. Coal, the most abundant and traditionally the most 
economical source of power in the world, is found in varying amounts 
throughout the globe. It lies in veins of various thickness and richness 
beneath the crust of the Earth, resting in giant subterranean sandwiches, 
shallow or deep, flat or pitched. Obtaining coal in sufficient quantities at a 
competitive cost and making it the proper grade for the market demand is, in 
its simplest terms, the science of coal mining. See COAL. 
    Of major importance in coal property development is the accumulation of 
seam information from borehole drilling. Chemical analysis of the cores will 
provide details concerning moisture, volatile material, fixed carbon, ash, 
sulfur, heat content, and fusion temperature of the ash. After preliminary 
analyses and correlations, the engineering details are planned. Production 
requirements and the extent of coal reserves generally determine the method of 
mining and the type and capacity of equipment to be used. 
    Coal is extracted from the earth by three basic methods: (1) underground, 
(2) strip, and (3) auger. Approximately two-thirds of America's coal comes 
from underground mining. The remaining one-third is mined from the surface, 
either by strip or auger mining. 
    The systems of underground coal mining generally in use are 
room-and-pillar, longwall, and, in a few places in Europe, hydraulic. In the 
room-and-pillar system, tunnels are carved into the seam, leaving pillars of 
coal for support. In longwall mining, widely spaced tunnels are driven, 
leaving large blocks of coal. Later, these blocks are completely extracted, 
allowing the roof material to collapse behind the coal face as it is removed. 
In hydraulic mining, a stream of water is directed against the coal face with 
sufficient pressure to dislodge the coal. The water also acts as a 
transporting medium. See UNDERGROUND MINING. 
    Where the coal seam lies close to the surface, strip mining is an 
efficient way to mine the coal. It is economical to remove the overburden of 
earth and rock that covers the coal seam. For this job, power shovels, 
draglines, or wheel excavators are used. A shovel excavates a ``furrow'' and 
casts the overburden parallel to the cut. Draglines sit on the bank above the 
coal seam and remove the overburden from the seam. Bucket-wheel machines 
excavate the material from above the coal seam. It then flows in a continuous 
stream via a transfer to the conveyor system, which in turn transports it to 
the discharge point. When the seam is exposed, the coal is loaded by smaller 
power shovels into trucks and hauled to preparation plants or loading bins. 
See STRIP MINING. 
    For coal seams which continue under rising land too thick for economical 
strip mining and where underground mining cannot burrow further to the surface 
because of the shallow and more treacherous roof conditions, the auger method 
was developed. The auger miner twists huge drills like carpenter's bits into a 
hillside coal seam, drawing out the coal to a conveyor which loads it into 
trucks. Auger recovery gives additional tonnage at minimal cost for equipment 
and labor and permits recovery of coal that might not be recovered by other 
methods.  
    After the coal is mined, it may be loaded directly into transportation 
facilities for the market if the foreign material in it is not excessive. 
However, coal from most seams requires preparation to provide a desired and 
uniform quality. A number of washing devices are used for cleaning coal, all 
of which operate on the basic principle of the difference of specific gravity 
between coal and foreign material. Once the coal is ready for market, it is 
loaded for shipment to the customer by railroad, water transportation, or in 
some cases belt conveyor systems. 
 �34s[James D. Reilly]
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�6v�2v�32sCoal paleobotany   �21m �33s 
    A special branch of the paleobotanical sciences concerned with the origin, 
composition, mode of occurrence, and significance of the fossil plant 
materials that occur in, or are associated with, coal seams. Information 
developed in this field of science provides knowledge useful to the biologist 
in attempting to describe the development of the plant world, aids the 
geologist in unraveling the complexities of coal measure stratigraphy in order 
to reconstruct the geography of past ages and to describe ancient climates, 
and has practical application in the coal, coke, and coal chemical industries. 
See PALEOBOTANY. 
    All coal seams consist of countless fragments of fossilized plant material 
admixed with varying percentages of mineral matter. The organic and inorganic 
materials initially accumulate in some type of swamp environment. In some 
instances, the fossilized plant fragments can be recognized as remnants of a 
plant of some particular family, genus, or species. When this is possible, 
information can be obtained on the vegetation extant at the time the source 
peat was formed, and such data aid greatly in reconstructing paleogeographies 
and paleoclimatic patterns. 
    Pollen grains and spores are more adequately preserved in coals than are 
most other plant parts. Recognition of this fact, coupled with an appreciation 
of the high degree to which these fossils are diagnostic of floral 
composition, has led to the rapid development of the paleobotanical subscience 
of palynology. Fruits, seeds, and identifiable woods also occur as coalified 
fossils. Occasionally, coal seams contain fossil-rich coal balls. Essentially 
all types of plant fossil, including entire leaves, cones, and seeds, are 
encountered in these discrete nodular masses. See COAL BALLS; FOSSIL SEEDS AND 
FRUITS; PALYNOLOGY. 
    Coal seams generally are composed of several superposed sedimentary 
layers, each having formed under somewhat different environmental conditions. 
The coal petrologist and paleobotanist recognize these layers because of their 
distinctive textural appearance and because each consists of a particular 
association of organic and inorganic materials. Accordingly, each coal seam 
usually contains several types of coal. These coal types, or lithotypes, 
possess characteristic suites of physical properties, and knowledge of these 
properties is very profitably employed in manipulating coal composition in 
coal preparation and beneficiation plants. 
 �34s[William Spackman]
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�6v�2v�32sCoal Sack   �21m �33s 
    An area in one of the brighter regions of the Southern Milky Way which to 
the naked eye appears entirely devoid of stars and hence dark with respect to 
the surrounding Milky Way region. Telescopic and spectroscopic observations 
reveal that the Coal Sack is a cloud of small, solid particles. The cloud not 
only abosrbs about two-thirds of the visible light passing through it but also 
strongly reddens the transmitted light. The Coal Sack is located just to the 
southeast of the Southern Cross. See INTERSTELLAR MATTER. 
 �34s[William Liller]
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�6v�2v�32sCoastal engineering   �21m �33s 
    A branch of civil engineering concerned with the planning, design, 
construction, and maintenance of works in the coastal zone. The purposes of 
these works include control of shoreline erosion; development of navigation 
channels and harbors; defense against flooding caused by storms, tides, and 
seismically generated waves (tsunamis); development of coastal recreation; and 
control of pollution in nearshore waters. Coastal engineering usually involves 
the construction of structures or the transport and possible stabilization of 
sand and other coastal sediments. 
    The successful coastal engineer must have a working knowledge of 
oceanography and meteorology, hydrodynamics, geomorphology and soil mechanics, 
statistics, and structural mechanics. Tools that support coastal engineering 
design include analytical theories of wave motion, wave-structure interaction, 
diffusion in a turbulent flow field, and so on; numerical and physical 
hydraulic models- basic experiments in wave and current flumes; and field 
measurements of basic processes such as beach profile response to wave attack, 
and the construction of works. Postconstruction monitoring efforts at coastal 
projects have also contributed greatly to improved design practice. 
    Coastal structures can be classified by the function they serve and by 
their structural features. Primary functional classes include seawalls, 
revetments, and bulkheads; groins; jetties; breakwaters; and a group of 
miscellaneous structures including piers, submerged pipelines, and various 
harbor and marina structures. 
    Seawalls, revetments, and bulkheads are structures constructed parallel or 
nearly parallel to the shoreline at the land-sea interface for the purpose of 
maintaining the shoreline in an advanced position and preventing further 
shoreline recession. Seawalls are usually massive and rigid, while a revetment 
is an armoring of the beach face with stone rip-rap or artificial units. A 
bulkhead acts primarily as a land-retaining structure and is found in a more 
protected environment such as a navigation channel or marina. See BULKHEAD; 
REVETMENT.  
    A groin is a structure built perpendicular to the shore and usually 
extending out through the surf zone under normal wave and surge-level 
conditions. It functions by trapping sand from the alongshore transport system 
to widen and protect a beach or by retaining artificially placed sand. See 
GROIN (ENGINEERING). 
    Jetties are structures built at the entrance to a river or tidal inlet to 
stabilize the entrance as well as to protect vessels navigating the entrance 
channel. See JETTY. 
    The primary purpose of a breakwater is to protect a shoreline or harbor 
anchorage area from wave attack. Breakwaters may be located completely 
offshore and oriented approximately parallel to shore, or they may be oblique 
and connected to the shore where they often take on some of the functions of a 
jetty. See BREAKWATER. 
 �34s[Robert M. Sorensen]
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�6v�2v�32sCoastal landforms   �21m �33s 
    The characteristic features and patterns of land in a coastal zone subject 
to processes of erosion and deposition. The interactions of various marine 
processes with the coast and with terrestrial processes produce a great 
variety of landforms along the coastal zone. Primary terrestrial processes 
that influence coastal landforms are those which carry sediment to the coast. 
These include river movement and mass movement by gravity, such as slumping 
and slides. Marine processes of waves and currents are predominant in 
controlling the formation of coastal morphology. Another indirectly important 
process is change in sea level. A passive yet quite important contributing 
factor in coastal morphology is the nature of the geology and geomorphology of 
the area adjacent to the coast. 
    Landforms associated with, and formed along, wide coastal plains are in 
striking contrast to those which are developed along high-relief, rocky 
coasts. A classification of coasts based on the modern concepts of global 
tectonics provides a good framework for consideration of coasts. There are 
essentially two broad types of plate margins: trailing-edge and leading-edge. 
Trailing-edge coasts are characterized by coastal plains and adjacent broad 
shelves with constructional landforms. The Atlantic coast of the United States 
south of New York is an example. The leading edge coasts are typically 
high-relief with bedrock at the coast due to structural deformation. The west 
coast of the United States is a good example. See PLATE TECTONICS. 
    As a rule, coastal plain coasts of trailing-edge plates present a greater 
diversity of depositional environments than highrelief leading-edge coasts. 
The primary reason is that submarine morphology generally mimics that of the 
adjacent subaerial coast. That is, trailing-edge plates show broad gently 
sloping continental shelves upon which sediment accumulates and is distributed 
by coastal processes. Leading-edge regions show high-relief submarine 
morphology and a general lack of a continental shelf, resulting in an absence 
of sites for accumulation of sediment. See COASTAL PLAIN. 
    Many coastal landform types show no particular association with a specific 
coastal type. Most, but not all, of these features are relatively small-scale, 
so-called second-order features. Reefs are probably the most extensive coastal 
landforms, and their presence has no relation to the type of coast, other than 
it must lack significant terrigenous runoff. Fiords, like reefs, are 
climate-related but may develop on a variety of coasts as long as relief is at 
least modest. These huge glacial excavations are the deepest of the coastal 
landforms. See FIORD; NEARSHORE SEDIMENTARY PROCESSES; REEF. 
 �34s[Richard A. Davis, Jr.]
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�6v�2v�32sCoastal plain   �21m �33s 
    A low-relief area which is bounded by the sea on one side and generally by 
highlands on the landward side. The continental shelf actually represents a 
drowned portion of the coastal plain and is geologically linked to the coastal 
plain. The development of coastal plains ranges widely throughout the world, 
with the Atlantic and Gulf coasts of the United States being among the most 
extensive. Development of coastal plains is associated with trailing-edge 
coasts in the context of global tectonics. These coasts are relatively stable 
and structurally simple. See CONTINENTAL SHELF AND SLOPE. 
    Existing coastal plains began their development in the Mesozoic Era and 
have continued to the present. Sediment accumulations are typically some 
combination of fluvial, deltaic, and marine shelf depositional environments. 
There is a general thickening of the stratigraphic section in the seaward 
direction. Down-to-the-coast normal faults and intrusion of salt domes 
represent the most widespread and significant structural features. Surface 
topography displays low-relief cuestas along the outcrop belt of the more 
resistant lithofacies. See COASTAL LANDFORMS; GEOMORPHOLOGY; SALT DOME. 
 �34s[Richard A. Davis, Jr.]
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�6v�2v�32sCoati   �21m �33s 
    The name for three species of carnivorous mammals assigned to the raccoon 
family Procyonidae. The common or ring-tailed coati (Nasua nasua) ranges 
through the forests of Central and South America, the mountain coati (Nasuella 
ovivacea) is found only in South America, while Nasua nelsoni is confined to 
Cozumel Island in Central America. 
    The coatis are characterized by their elongated snouts, body, and tail, 
which is held erect when they walk (see illustration). They are adept at 
climbing trees in search of birds and lizards, and their diet also includes 
fruit, insects, and larvae. Nasua nasua roams in bands of females and young 
during the day in search of food. The males are excluded from the band except 
during the brief breeding season, when a single male is allowed to join. After 
mating, the male leaves and the female builds an isolated nest in a tree. 
Here, following a gestation period of 11 weeks, a litter of about five young 
are born and are cared for by the female away from the group. After about 1 
month the female and her young rejoin the band. See CARNIVORA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCoaxial cable   �21m �33s 
    A two-conductor transmission line with an outer metal tube or braided 
shield concentric with and enclosing the center conductor. The inner conductor 
is supported by some form of dielectric insulation, solid, expanded plastic, 
or semisolid. Semisolid supports are polyethylene disks, helical tapes, or 
helically wrapped plastic strings. Beads, supporting pins, or periodically 
crimped plastic tubes are used in some designs. 
    The significant feature of the coaxial cable is that it is a shielded 
structure. The electromagnetic field associated with each coaxial unit is 
nominally confined to the space between the inner and outer conductors. Since 
alternating current concentrates on the inside of the outer conductor as the 
frequency of the current increases (skin effect), a coaxial unit is a 
self-shielded transmission line whose shielding improves at higher 
frequencies. Unshielded lines, such as the pairs of multipair cable, share the 
space for the electromagnetic fields. Thus, for equivalent transmission loss, 
pairs occupy less space than coaxials. The major use of the coaxial cable is 
for transmitting high-frequency broadband signals. Coaxial cables are rarely 
used at or near voice frequency since the shielding properties are poor and 
they are more expensive than twisted pairs having the same transmission loss. 
    Coaxial units are made in three general types for different applications: 
flexible, semirigid, or rigid. In general, the more rigid the unit, the more 
predictable and stable its electrical properties. Since loss on a transmission 
line increases approximately as the square of the frequency, low loss in a 
coaxial cable is important. Also, physical and electrical irregularities, 
especially if they are periodic, must be kept to a minimum, or the 
transmission loss of the coaxial will be very high at certain frequencies. See 
TRANSMISSION LINES. 
    One variation of the semirigid coaxial, insulated with polyethylene disks 
enclosed inside a copper tube, is widely used for transcontinental carrier 
transmission. The disk-insulated coaxial unit has very low loss and is now 
used in carrier systems in the United States, Japan, and Europe to transmit up 
to 10,800 two-way voice-frequency channels per pair of coaxial units. The 
illustration shows a 20-unit coaxial cable with 9 working coaxial pairs and 2 
standby coaxials, which automatically switch in if the electronics of the 
regular circuits fail. 
 �34s[John R. Apen]
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�6v�2v�32sCobalt   �21m �33s 
    A metallic chemical element, Co, with an atomic number of 27 and an atomic 
weight of 58.93. Cobalt is similar to iron and nickel in both its free and 
combined states. It is widely distributed in nature, making up about 0.001% of 
the igneous rocks of the Earth's crust, compared with 0.02% for nickel. It 
occurs in meteorites, stars, sea and fresh waters, soils, plants, animals, and 
in the manganese nodules found on the ocean floor. Traces of cobalt are found 
in many ores of iron, nickel, copper, silver, manganese, and zinc, but the 
commercially important cobalt minerals are the arsenides, oxides, and 
sulfides. Cobalt and its alloys are wear- and corrosion-resistant even at high 
temperatures. Important commercial applications are in making alloys for use 
at high temperatures, magnetic alloys, alloys for use in machine tools, 
glass-to-metal seals, and the dental and surgical alloy Vitallium. Cobalt is 
required in small amounts by plants and animals. The artificially produced 
radioactive isotope of cobalt, cobalt-60, is widely used in industry, 
research, and medicine. See RADIOISOTOPE (BIOLOGY). 
    Cobalt is ferromagnetic and resembles iron and nickel in hardness, tensile 
strength, machinability, thermal properties, and electrochemical behavior. The 
metal is unaffected by air or water under normal conditions, and is rapidly 
attacked by sulfuric acid, hydrochloric acid, and nitric acid, but only slowly 
by hydrofluoric acid, ammonium hydroxide, and sodium hydroxide. Cobalt 
exhibits variable valence and forms complex ions and colored compounds, as do 
all the transition elements. The table summarizes key properties. 
    Cobalt(II) chloride, nitrate, and sulfate are all formed by the 
interaction of the metal, oxide, hydroxide, or carbonate with the 
corresponding acid. There are three main oxides of cobalt: gray cobaltous 
oxide, CoO; black cobaltic oxide, Co2O3, formed by heating compounds at a low 
temperature in excess of air; cobaltosic oxide, Co3O4, the stable oxide, 
formed when salts are heated in air at temperatures which do not exceed 430DGF 
(850DGC). Most common cobalt salts are derivatives of cobalt(II), the higher 
valence state being encountered only in complex coordination compounds. An 
important, naturally occurring cobalt coordination compound is vitamin B12. 
Cobalt compounds have a variety of industrial applications, including their 
use as catalysts, and in agriculture for remedying cobalt deficiency in soil 
and natural vegetation. See TRANSITION ELEMENTS. 
 �34s[Roland S. Young]
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�6v�2v�32sCobaltite   �21m �33s 
    A mineral having composition (Co,Fe)AsS. Cobaltite is one of the chief 
ores of cobalt. It crystallizes in the isometric system, commonly in cubes or 
pyritohedrons, resembling crystals of pyrite. There is perfect cubic cleavage. 
The luster is metallic and the color silver-white but with a reddish tinge. 
The hardness is 5.5 (Mohs scale) and the specific gravity is 6.33. Notable 
occurrences of cobaltite are at Skutterud, Norway; Lunaberg, Sweden; 
Ravensthorpe, Australia; and Cobalt, Ontario, Canada. See COBALT. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sCobalt toxicity   �21m �33s 
    Cobalt in most naturally occurring foodstuffs is largely unabsorbed and is 
eliminated in the feces. Under natural circumstances cobalt does not produce a 
toxic syndrome in humans. It takes an unusual set of circumstances to produce 
cobalt toxicity. Intravenously injected, large doses of cobalt have been 
observed to cause paralysis and enteritis, and sometimes death. When cobalt is 
injected into the bloodstream, the amount distributed throughout the tissues 
is very small; higher concentrations are present in the pancreas, liver, 
spleen, kidney, and bone. Elimination of cobalt is rapid. 
    An unusual kind of cobalt toxicity is the ``beer drinker's syndrome.'' 
During the 1960s, some breweries in Nebraska and Quebec added cobalt to their 
beer to improve the stability of the foam. Subsequently, numerous fatalities 
occurred among people who consumed large quantities of beer. Following 
discontinuance of the cobalt additive, no new cases were reported. Whether the 
cobalt acted independently or synergistically with other constituents of beer 
to enhance its toxicity is not known. The principal lesion was found in the 
heart; autopsies revealed enlarged, flabby hearts that were more than twice 
normal size. See COBALT. 
 �34s[N. Karle Mottet]
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�6v�2v�32sCobra   �21m �33s 
    Any of several snakes that belong to the reptilian family Elapidae, which 
also includes kraits, mambas, and coral snakes. There are 10 species of cobra 
and all are included in the genus Naja. They are widely distributed throughout 
Africa and Asia. These snakes are oviparous, and the spectacled, or Indian, 
cobra (Naja naja) has been known to lay 20 eggs, of which half proved to be 
fertile. Food preferences include other reptiles, eggs, frogs, and small 
rodents. The characteristic hood, not pronounced in all species, is formed by 
a series of ribs, lying against the backbone, which can be extended laterally 
when the reptile is annoyed. The king cobra (N. hannah) is the largest and 
most dangerous of all poisonous snakes and will attack any animal, including 
humans, without hesitation. It may grow to a length of over 16 ft (4.8 m). See 
REPTILIA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sCoca   �21m �33s 
    Shrubs of the genus Erythroxylum in the coca family (Erythroxylaceae). The 
genus contains over 200 species, most in the New World tropics. Two species, 
E. coca and E. novogranatense, are cultivated for their content of cocaine and 
other alkaloids. Leaves of cultivated coca contain 14 alkaloids, chiefly 
cocaine, and significant amounts of vitamins and minerals. The cocaine content 
averages 0.5%, rarely exceeding 1.0%. 
    South American Indians ``chew'' coca as a mild stimulant by placing dried 
leaves in the mouth and masticating with various alkalies such as lime or 
plant ashes to promote the release of the alkaloids, which are swallowed. Coca 
is important in the folk medicine of Andean Indians, especially as a treatment 
for gastrointestinal disorders. It may regulate carbohydrate metabolism in a 
useful way and supplement an otherwise deficient diet. See COCAINE; COLA; 
LINALES.  
 �34s[Andrew T. Weil]
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�6v�2v�32sCocaine   �21m �33s 
    The principal alkaloid of coca leaves, a topical anesthetic and stimulant, 
and popular illicit drug. In 1884 C. Koller demonstrated cocaine's efficacy as 
an anesthetic in eye surgery, introducing the age of local anesthesia. For the 
next decade cocaine enjoyed the status of a wonder drug and panacea. It fell 
into disfavor with increasing reports of acute toxicity and long-term 
dependence. Today it is used as a topical anesthetic in the eye, nose, mouth, 
and throat; for injection anesthesia it has been replaced by synthetic drugs 
with fewer central nervous system effects. See COCA. 
    Cocaine increases heart rate and blood pressure and causes feelings of 
alertness and euphoria. It does not produce physical dependence, as alcohol 
and opiates do, but many people find it hard to use in a stable and moderate 
fashion if they have access to it in quantity. Although it is quite active 
orally, most users of illicit cocaine take it intranasally by snuffing; few 
inject it intravenously. Aside from local irritation of the nasal membranes, 
moderate users suffer few adverse effects. The soluble hydrochloride salt is 
the common form. Insoluble cocaine free base may be smoked, a practice that 
may be more harmful. See ALKALOID; DRUG ADDICTION. 
 �34s[Andrew T. Weil]
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�6v�2v�32sCoccidia �21m �33s
    A subclass of the class Telosporea. These protozoa are typically
intracellular parasites of epithelial tissues in both vertebrates and
invertebrates. The group is divided on the basis of life cycles into two
orders, the Protococcida (in which there is only sexual reproduction) and the
Eucoccida (in which there is both sexual and asexual reproduction). There are
only a few species in Protococcida, and all are parasites in marine
invertebrates. See EUCOCCIDA; PROTOCOCCIDA; TELOSPOREA.
 �34s[Elery R. Becker /Norman D. Levine]
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�6v�2v�32sCoccidioidomycosis   �21m �33s 
    An infectious disease of humans and animals caused by a fungus, 
Coccidioides immitis. This mycotic infection is endemic in the southwestern 
United States, and is known to occur in Mexico and Central and South America. 
    This disease has two forms most frequently recognized, primary pulmonary 
and disseminated. The primary pulmonary form is the one that most uninfected 
persons coming in contact with C. immitis acquire. More than 50% of these 
individuals do not develop any symptoms. When symptoms do occur, they are 
usually mild and are often diagnosed as a cold or influenza. This form of the 
disease, however, may be severe and can be manifested by a wide range of signs 
and symptoms which include fever, chills, cough, chest pain, and headache. 
Cavity formation may take place as well as pleural effusions. An allergic 
reaction may also be noted, characterized by the development of large, tender, 
reddish nodules on the legs. 
    Approximately one patient in 1000 with primary pulmonary 
coccidioidomycosis will have a dissemination of the organisms from the lungs 
to the skin, bones, viscera, and central nervous system. In this form of the 
disease, dissemination occurs in the Negro about 15 times and the Filipino 100 
times more frequently than in the Caucasian. 
    Cutaneous coccidioidomycosis in most instances is believed to be secondary 
to a pulmonary lesion. However, there are cases reported of a primary 
cutaneous form of this disease acquired by injury to the skin, followed by 
invasion of the fungus. In these instances the lesions are similar to the 
sporotrichotic chancre. 
 �34s[Leanor D. Haley]
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�6v�2v�32sCoccolithophorida   �21m �33s 
    A group of unicellular, biflagellate, golden-brown algae characterized by 
a covering of extremely small interlocking calcite plates called coccoliths. 
The Coccolithophorida are mainly classified by the shape of their coccoliths 
into two groups: the holococcoliths, with simple rhombic or hexagonal crystals 
arranged like a mosaic, and the heterococcoliths, with complex crystals 
arranged into boat, trumpet, basket, or collar-button shapes (see 
illustration). The Coccolithophorida are usually considered plants but possess 
also some animal characteristics. Botanists assign them to the class 
Haptophyceae of the phylum Chrysophyta, and zoologists to the class 
Phytamastigophorea of the phylum Protozoa. See PHYTAMASTIGOPHOREA; PROTOZOA. 
    Chiefly photosynthetic, although epiphytism, phagotrophism, and 
saprophytism have also been shown, the Coccolithophorida form a significant 
percentage of the nannoplankton of the tropic through subarctic-subantarctic 
waters of all oceans; a few brackish and fresh-water species also exist. 
Together with the diatoms they constitute the primary producers of the open 
ocean food chain. See DIATOM; PHYTOPLANKTON. 
    Coccoliths preserve well and have a fossil record dating back into the 
Jurassic, 180,000,000 years ago. Their long and involved evolutionary record 
makes them useful to geologists for dating ancient sediments. Because they 
live in surface waters, coccoliths are under direct climatic control. Thus 
many modern species with relatively long fossil records are excellent 
temperature indicators. See MICROPALEONTOLOGY. 
 �34s[Andrew McIntyre]
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�6v�2v�32sCockcroft-Walton accelerator   �21m �33s 
    An electrostatic particle accelerator characterized by the method used to 
obtain the dc accelerating voltage. This type of machine was used in 1932 by 
J. D. Cockcroft and E. Walton in studying the first nuclear reactions induced 
by an accelerated ion beam. Since that time, a number of variations have been 
developed and applied to a wide range of problems in both science and 
industry. In its most rudimentary form, a Cockcroft-Walton power supply 
contains a high-voltage transformer and a voltage multiplying circuit (see 
illustration).  
    The accelerator itself is basically an ion source which injects ions into 
an evacuated accelerating tube. The power supply voltage is connected across 
the tube. Either positive or negative ions or electrons may be accelerated 
depending on the nature of the ion source and the polarity of the accelerating 
voltage. Voltage gradients along the accelerating tube may be controlled by a 
resistive bleeder string or by connecting conducting sections of the 
accelerator tube to intermediate multiplier stages. See ION SOURCES. 
    The physical size of ordinary Cockcroft-Walton accelerators increases with 
operating voltage, because voltage gradients along the insulating support 
members and in the surrounding medium must be limited to avoid electrical 
breakdown (sparking). The size of an accelerator can be vastly reduced by 
housing it in a vessel filled with high-pressure insulating gas. The size can 
also be reduced if lower voltages are desired. Tiny accelerator systems have 
been devised for logging boreholes drilled for oil and mineral prospecting. 
These accelerators operate at 0.1 megavolt or less, but this is sufficient for 
deuterium ions to produce 14-MeV neutrons when they strike a tritium target. 
See PARTICLE ACCELERATOR. 
 �34s[J. C. Overley]
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�6v�2v�32sCockle   �21m �33s 
    Primarily a member of the bivalve family Cardiidae, with globose shells 
ornamented with radiating ribs and perhaps spines borne on some ribs 
posteriorly. The globular shape hinders rapid digging, and cockles occur 
superficially in sheltered sands where shell ornamentation prevents 
dislodgement by weak surf. Cockles occur intertidally and subtidally, in 
temperate and tropical seas. Cockles have a muscular L-shaped foot with which 
they can ``leap'' when lying upon the surface of the sand, and so avoid attack 
by starfish. Young cockles are preyed upon by shore crabs and oyster catchers. 
    Other applications of the name cockle include the bloody cockle, Anadara 
granosa, of western Malaysia, which resembles the western European Cardium 
edule in appearance, location in shallow water, and commercial exploitation; 
however, it is a filibranch in the family Arcidae. See BIVALVIA; MOLLUSCA. 
 �34s[Richard D. Purchon]
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�6v�2v�32sCockroach   �21m �33s 
    Any of a group of insects (approximately 2500 species) of the family 
Blattidae, order Dictyoptera. This group is among the most primitive of the 
living and oldest of the fossilized insects, being found in abundance in the 
Carboniferous age. In general, cockroaches have a flat oval body, a darkened 
cuticle, and a thoracic shield which extends dorsally over the head. A pair of 
sensory organs, the whiplike antennae, are located on the head, and a much 
shorter pair of sensory organs, the cerci, are located at the end of the 
abdomen. Although some species are wingless, most have two pairs of wings but 
seldom fly. Most species are nocturnal and primarily live in narrow crevices 
and under stones and debris. Cockroaches are omnivorous, and a few species 
have adapted to the human environment, becoming household pests since they can 
chew foodstuffs, clothing, paper, and even plastic insulation. They also can 
emit a highly disagreeable odor. 
    Females deposit egg cases almost anywhere; each case contains several eggs 
from which the young nymphs hatch and go through several molts as they grow. 
At the final molt a winged, sexually mature adult will emerge. The time from 
egg to adult varies from species to species but can be longer than a year. 
    Cockroaches have been claimed as vectors for numerous human diseases. 
There is little evidence to support this claim; however, there is evidence 
that they can produce allergenic reactions in humans. Because of their large 
size, cockroaches have proved to be very useful animals for scientific 
research on general problems of insect behavior, physiology, and biochemistry. 
 �34s[Charles R. Fourtner]
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�6v�2v�32sCoconut   �21m �33s 
    A large palm, Cocos nucifera, widely grown throughout the tropics and 
valuable for its fruit and fiber. Usually found near the seacoast, it requires 
high humidity, abundant rainfall, and mean annual temperature of about 85DGF 
(29DGC). Southern Florida, with mean temperature of 77DGF (25DGC), is at the 
limit of successful growth. 
    The fruit, 10 in. (25 cm) or more in length, is ovoid and obtusely 
triangular in cross section. The tough, fibrous outer husk encloses a 
spherical nut consisting of a hard, bony shell within which is a thin layer of 
fleshy meat or kernel. The meat is high in oil and protein and, when dried, is 
the copra of commerce. 
    Although many trees grow without special care, the crop lends itself to 
plantation culture with control of weeds, fertilization, and protection from 
diseases, insects, and animal pests. Palms begin to bear nuts the sixth year 
after planting and reach full bearing about the eighth year. Individual nuts 
mature about a year after blossoming and normally fall to the ground. 
    The oil from the dried coconut meats (copra) is widely used for margarine, 
soap, and industrial purposes. High-quality copra may be shredded for 
confectionery and the baking trade. The residue, after oil removal, is used 
for animal feed. Coconut husks are an important source of fiber called coir. 
Various grades of coir are used for ropes, mats and matting, and upholstery 
filling. See COIR. 
    The coco palm is the most useful of all tropical plants to the native 
population. An important source of food and drink, it also furnishes building 
material, thatch, hats, dishes, baskets, and many other useful items. See 
ARECALES.  
 �34s[Laurence H. MacDaniels]
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�6v�2v�32sCodfish   �21m �33s 
    Fish of a subfamily of the Gadidae in the order Gadiformes. Important 
commercially, these fish are found in cold waters, such as the Baltic and the 
North Atlantic. Gadus morhua is extensively fished off the Newfoundland banks 
and a circumpolar species, Boreogadus saida, the Arctic cod, is found around 
the ice pack during the summer. G. macrocephalus, a related species, occurs in 
the northern Pacific. 
    The cod is covered with cycloid scales, has a barbel under the chin, and 
has pelvic fins on the throat; there are two anal and three dorsal fins. 
Codfish average about 3 ft (0.9 m) in length and weigh 10-35 lb (4.5 - 15.7 
kg). They live at depths of 100 - 1500 ft (30 - 450 m), where they feed on 
mollusks, small fish, crustaceans, and worms. Spawning occurs from January on 
to spring. The livers are processed for cod liver oil, which is rich in 
vitamins, and the swim bladder is made into isinglass. See GADIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCoelacanthiformes   �21m �33s 
    An order of lobefin fishes, subclass Crossopterygii, which diverged from 
the order Osteolepiformes in Late Devonian times. They were common freshwater 
animals of the Carboniferous and Permian, but in the Triassic, although still 
numerous, coelacanths appeared in a marine environment. They persisted 
throughout the Cretaceous, after which their fossil record is a blank. A 
living coelacanth fish captured off the coast of East Africa in 1938 was 
improperly preserved, which seriously reduced the scientific value of this 
specimen. Continued search ultimately revealed a source of additional material 
in the Comores Islands north of Madagascar since 1952, additional specimens 
have been collected. 
    The coelacanths differ from the Osteolepiformes in having a rather short, 
stocky body ending in a three-lobed tail (see illustration) with the axial 
lobe prominent; maxillae are lacking; the dermal head skeleton is in large 
part chondrified, and there are no internal nares. See OSTEOLEPIFORMES. 
    The Coelacanthiformes include 5 families with 30 or more genera. Two small 
families died out before the end of the Pennsylvanian. The largest family, the 
Coelacanthidae, lived from Mississippian time to the Upper Cretaceous. The 
Laugiidae were all Mesozoic. The Latimeriidae are known only from the Recent 
Latimeria chalumnae. See CROSSOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sCoelenterata   �21m �33s 
    That group of the Radiata whose members typically bear tentacles and 
possess intrinsic nematocysts. The name Cnidaria is also used for this phylum 
and is preferred by some because the name Coelenterata, as first used, 
included the sponges (Porifera) and the comb jellies (Ctenophora), as well as 
the animals called coelenterates. See CTENOPHORA; PORIFERA. 
    The coelenterates are mainly marine organisms and are best known as 
jellyfish or medusae, sea anemones, corals, the Portuguese man-of-war, small 
polypoid forms called hydroids, and the freshwater hydras. Taken together, the 
phylum is dihv-isible into three classes as follows: (1) Hydrozoa, the 
hydroids, hydras, and hydrozoan or craspedote jellyfish (hydromedusae); (2) 
Scyphozoa, the acraspedote jellyfish; and (3) Anthozoa, the sea anemones, 
corals, sea fans, sea pens, and sea pansies. See ANTHOZOA; HYDROZOA; SCYPHOZOA. 
    It is convenient to recognize two basic body forms in this phylum, the 
polyp and the medusa, into which all coelenterates can be classified. The 
polyp and the medusa, however, have many features in common (Fig. 1). 
    The polyp is a radially, biradially, or radiobilaterally symmetrical 
individual having a longitudinal oral-aboral axis and usually sessile. The 
mouth is at the free end and is surrounded by one to many whorls or sets of 
tentacles which may be hollow or solid. The aboral end is commonly developed 
as an adhesive device for attachment and is conveniently referred to as a 
base. The central body cavity is the gastrovascular cavity, also called the 
enteron or coelenteron. 
  
    The medusa is a tetramerously or polymerously radial individual and is 
free-swimming. The body is usually bell- or bowl-shaped with the mouth 
suspended in the center of the underside of the bell on a stalk. Instead of 
directly surrounding the mouth as in the polyp, the tentacles are located the 
margin of the bell. The outer or aboral part of the bell is recognized as the 
exumbrella and the under or oral part as the subumbrella. The mouth leads to 
the central stomach which in turn gives rise to four or more radial canals. 
These radial canals run through the umbrella, on the subumbrellar side, and 
commonly lead to a ring canal at the margin which is continuous around the 
margin.  
    The unique and most distinctive feature of coelenterates is the possession 
of intracellular, independent effector organelles called nematocysts, but also 
known as stinging cells or nettle cells. A coiled thread tube in each cell may 
be rapidly everted under proper stimulation and used for food gathering and 
for defense against predators, intruders, or enemies (Fig. 2). Nematocysts are 
produced within cells, called cnidoblasts, and may be produced either in place 
or at a location some distance from their definitive site in the coelenterate. 
The morphologically simplest coelenterates, the Hydrozoa, have nematocysts 
limited to their outer epidermis whereas the more complex Scyphozoa and 
Anthozoa bear nematocysts in both the outer epidermis and inner gastrodermis. 
    The body systems of coelenterates may be divided into the following 
categories for the purpose of description: epithelial, nervous, muscular, 
mesogleal, skeletal, digestive, and reproductive. There is no analog of a 
circulatory system and the functions of respiration and excretion are carried 
out by each cell. 
    The outer epithelium or epidermis is in general a single layer of cells, 
varying from a flattened squamous nature to tall and columnar. The epidermis 
is usually ciliated in polyps so that the net effect of the ciliary beat is to 
move material away from the mouth toward the margin of the oral disk and 
toward the tips of the tentacles. Frequently, some of the epidermal cells have 
a dual function and the cell base is elongate and possesses contractile 
fibrils. Such cells are described as epitheliomuscular cells. The 
gastrodermis, or inner epithelial layer, is also abundantly ciliated. These 
cilia move food materials about within the gastric cavity and circulate the 
contained water. One of the prime functions of the gastrodermis is digestion, 
and essentially every cell of the gastrodermis is capable of ingesting small 
food particles. 
    The coelenterate nervous system can be defined as consisting basically of 
an unpolarized network of bipolar and multipolar neurons. The nervous system 
is subepithelial in location, both subepidermal and subgastrodermal, and may 
in some forms consist of two networks in only limited contact with one another 
and with each specialized either for rapid through conduction or for slower, 
more general spread of conduction. 
(a) 
     (b) 
      
    The muscular system varies considerably among coelenterates. In simple 
polyps there is an epidermal, longitudinal muscle sheath of epitheliomuscular 
cells and a gastrodermal, circular sheath. In medusae and in members of the 
classes Scyphozoa and Anthozoa, the musculature has become separated from the 
epithelial tissue as a subepithelial system. In many anthozoans a large 
sphincter muscle develops in the gastrodermal surface, at the top of the 
column. In medusae, where the primary swimming muscles are subumbrellar, large 
muscle bands, both circular and radial, develop. 
    The mesogleal system is represented by a layer between epidermis and 
gastrodermis which varies from the thin structureless cementing layer of 
hydrozoan polyps to the highly complex cellular, fibrous, gelatinous matrix of 
the scyphomedusans and anthozoans. Where the mesoglea is well developed, as in 
medusae and some anthozoans, it serves as a type of internal skeleton, against 
which muscles may act. 
    Two different types of skeleton occur in the phylum. The first is of an 
internal nature and is either the mesoglea against which the muscles operate 
or the contained hydroskeleton of most polyps. Of a quite different nature is 
the exoskeleton, seen in most hydrozoan polyps, some scyphozoan polyps, and 
corals, which supports and protects the organisms. Still another skeletal type 
known in the coelenterates is an axial skeleton composed of sclerified 
proteins. It is exceedingly tough and flexible and is characteristic of such 
anthozoans as gorgonians. 
    The digestive system in coelenterates is developed as a function of the 
gastrodermis. In hydrozoans, both polyps and medusae, glandular, 
enzyme-producing cells are abundant throughout the gastrodermis. These cells 
secrete proteolytic enzymes into the coelenteron which reduce food objects to 
a fine particulate state. In scyphozoans and anthozoans, although glandular 
cells are common throughout the gastrodermis, they tend to be most abundant 
along the free edges of the mesenteries of anthozoans and on the gastric 
tentacles of scyphozoans. These glandular cells are also the source of 
proteolytic enzymes which act in the extracellular environment of the 
coelenteron.  
    The phylum is characterized by its carnivorous diet, made possible first 
by the possession of nematocysts which make the predaceous habit successful. 
After food has been trapped, movements of the tentacles carry it to the mouth, 
where with the help of ciliary and muscular devices the food is moved to the 
coelenteron. Here extracellular proteases prepare the way for final 
intracellular digestion. No herbivorous coelenterates are known. 
    The reproductive system of coelenterates consists of specialized areas of 
epithelia, the gonads, which periodically appear and produce gametes. There 
are no ducts for the sex products or any accessory sexual structures. 
Fertilization usually occurs in the water surrounding the animal, although a 
few coelenterates have their eggs fertilized in place and may then brood their 
young.  
    The ability to regenerate lost parts is characteristic of coelenterates. 
Pieces cut from almost any part of polyps will in time grow into new polyps. 
The regenerative powers of medusae are much less well developed, and not only 
will the excised piece not develop but it may not even be replaced by the 
medusa. Gradients of regenerative ability in polyps exist with the ability for 
a piece to reconstitute a new whole organism decreasing from the mouth to the 
base. See REGENERATION (BIOLOGY). 
 �34s[Cadet Hand]
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    The Coelenterata have a long and impressive fossil record stretching from 
the present into the Precambrian, about 700,000,000 years ago. Thus, the known 
duration of this phylum equals or exceeds that of other animal phyla. Forms 
with skeletons, primarily the Conulata, Tabulata, Rugosa, and Scleractinia, 
have left fairly complete fossil records, whereas soft-bodied forms, such as 
most of the Hydrozoa, Scyphomedusae, and Alcyonaria, are represented by very 
few fossils. 
 �34s[Calvin H. Stevens]
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�6v�2v�32sCoelom   �21m �33s 
    The mesodermally lined body cavity of most animals above the flatworms and 
nonsegmented roundworms. Its manner of origin provides one basis for 
classifying the major higher groups. 
    Annelids, arthropods, and mollusks have a coelom which develops from solid 
mesodermal bands. Within the trochophore larva of annelids, a single pole cell 
proliferates two strips of mesoblast lying on either side of the ventral 
midline. These bands subdivide transversely into bilateral solid blocks, the 
somites. Each somite then splits internally to form a hollow vesicle, the 
cavity of which is the coelom. The somitic vesicles of each side expand until 
their surfaces become apposed above and below the gut. The mollusks also form 
bands of mesoderm from a single pole cell, but these bands do not segment. 
They split internally to form single right and left coelomic sacs, but the 
cavities are soon reduced and the surrounding mesoblast disperses as separate 
cells, many of which become muscle. In arthropods paired bands of mesoblast 
may proliferate from a posterior growth center or may separate inward from a 
blastoderm, a superficial layer of cells, on the ventral surface of the egg. 
These bands divide into linear series of somites which then hollow out. Their 
cavities represent the coelom. 
    Echinoderms and chordates constitute a second major group, characterized 
by the origin of the coelom from outpocketings of the primitive gut wall. In 
echinoderms one pair of bilateral pouches evaginates and separates from the 
archenteron or primitive digestive cavity. Each pouch constricts into three 
portions. The protochordates of the groups Hemichordata and Cephalochordata, 
exemplified by Balanoglossus and Branchiostoma, respectively, have three 
coelomic pouches formed by separate evaginations of the archenteral roof. In 
cephalochordates the head cavity divides into lateral halves. The left side 
communicates, by a pore, to an ectodermal pit called the wheel organ. The 
second pair of pouches forms the first pair of mesoblastic somites, and the 
third pouches subdivide transversely to give rise to the remainder of the 
linear series of somites. The upper or myotomic portion of each somite remains 
metameric and forms the segmental muscles. As it enlarges, the coelomic space 
is displaced ventrally and expands above and below the gut to form the 
perivisceral cavities and mesenteries, as described for annelids. In 
vertebrates the mesoderm arises as a solid sheet from surface cells that have 
been involuted through the blastopore. Lateral to the notochord, beginning at 
about the level of the ear, the mesoderm subdivides into three parts: (1) the 
somites; (2) the nephrotomic cord, temporarily segmented in lower vertebrates, 
which will form excretory organs and ducts; and (3) the unsegmented lateral 
plate. The coelom arises as a split within the lateral plate. See ANIMAL 
KINGDOM; GASTRULATION; INVERTEBRATE EMBRYOLOGY. 
 �34s[Howard L. Hamilton]
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�6v�2v�32sCoenopteridales   �21m �33s 
    True ferns which span the Late Devonian through Permian time between the 
recognizable beginnings of fernlike morphology and the earliest-appearing 
extant filicalean families (Gleicheniaceae, Osmundaceae). A true fern is a 
relatively advanced type of vascular land plant with distinct stem, fronds, 
roots, and foliar-borne annulate sporangia. Coenopterid ferns are mostly small 
and simple in contrast to late Paleozoic tree ferns of the Marattiales. 
    There are two major distinct groups. Zygopterid ferns are the most ancient 
and diverse, and apparently a dead-end evolutionary line; they differ the most 
from other ferns. Coenopterid ferns are usually placed in the 
Anachoropteridaceae and Botryopteridaceae, or in one of several extinct 
families assigned to the Filicales. Coenopterid ferns sensu stricto are 
probably ancestral to, and consequently form an imperceptible transition with, 
the Filicales. 
    The Zygopteridaceae appear in Late Devonian time, the Botryopteridaceae in 
Visean (Mississippian), the Anachoropteridaceae in the lower Westphalian A 
(Pennsylvanian), and all families extend into the Permian. See PALEOBOTANY. 
 �34s[Tom L. Phillips]
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�6v�2v�32sCoenothecalia   �21m �33s 
    An order of the class Alcyonaria. The Coenothecalia have no spicules but 
form colonies with a massive skeleton composed of fibrocrystalline aragonite 
fused into lamellae. The skeleton is perforated by both numerous wide 
cylindrical cavities occupied by the polyps, and narrow ones containing the 
solenial systems. The order includes a few genera, of which Heliopora, or the 
blue coral, is often found on coral reefs. See ALCYONARIA. 
 �34s[Kenji Atoda]
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�6v�2v�32sCoenurosis   �21m �33s 
    An infestation by a coenurus, the metacestode of Taenia (Multiceps) sp. 
The coenurus is most common in sheep, rabbits, and other herbivores. They 
become infested while browsing on vegetation contaminated with eggs of the 
parasite. Coenuri occurring in subcutaneous spaces and muscles cause less 
damage than those in the brain or spinal column. The latter type of 
infestation, found mainly in sheep, may cause the animal to stagger or circle, 
and such animals are said to be giddy, from which arises the term gid 
tapeworm. The infestation is rare in humans; cure requires surgical removal of 
the coenurus. See CYCLOPHYLLIDEA. 
 �34s[Reino S. Freeman]
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�6v�2v�32sCoenzyme   �21m �33s 
    An organic cofactor or prosthetic group (nonprotein portion of the enzyme) 
whose presence is required for the activity of many enzymes. In addition, many 
enzymes need metal ions, such as copper, manganese, and magnesium, for 
activation. The prosthetic groups attached to the protein of the enzyme (the 
apoenzyme) may be regarded as dissociable portions of conjugated proteins. The 
coenzymes usually contain vitamins as part of their structure. Neither the 
apoenzyme nor the coenzyme moieties can function singly, since dissociation of 
the two results in inactivation. In general, the coenzymes function as 
acceptors of electrons or functional groupings, such as the carboxyl groups in 
ALPHA-keto acids, which are removed from the substrate. Some of the well-known 
coenzymes are the pyridine nucleotides, thiamine pyrophosphate, flavin 
mononucleotide, hemin, the uridine phosphates, the phosporylated derivatives 
of adenylic acid, coenzyme A, and biotin. See ADENOSINEDIPHOSPHATE (ADP); 
ADENOSINETRIPHOSPHATE (ATP); ADENYLIC ACID; BIOLOGICAL OXIDATION; BIOTIN; 
CYTOCHROME; DIPHOSPHOPYRIDINE NUCLEOTIDE (DPN); ENZYME; HEMOGLOBIN; 
TRIPHOSPHOPYRIDINE NUCLEOTIDE (TPN); URIDINE DIPHOSPHOGLUCOSE (UDPG). 
 �34s[Mary B. McCann]
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�6v�2v�32sCoesite �21m �33s
    Naturally occurring coesite, a mineral of wide interest and the
high-pressure polymorph of SiO2, was first discovered and identified from
shocked Coconino sandstone of the Meteor Crater in Arizona in 1960. Since then
coesite has been identified from the Wabar (meteorite) Crater in Saudi Arabia,
from the Ries Crater in Bavaria in southern Germany, from the Lake Bosumtwi
Crater in Ashanti, Ghana, Africa, and from Lake Mien in Sweden. Coesite has
also been identified from some Thailand tektites which are considered to have
been formed also by an impact cratering process. The finding of natural
coesite elevates it as a true mineral species. Because it requires a unique
physical condition, extremely high pressure, for its formation, its occurrence
is diagnostic of a special natural phenomenon in this case, the hypervelocity
impact of a meteorite. See METEORITE.
    Synthetic coesite was first produced in the laboratory by L. Coes, Jr., as
a chemical compound at pressures of about 35 kilobars (3.5 x 10 sup 9 pascals)
in the temperature range of 246-404DGF (500 - 800DGC). Coesite has also been
found in synthetic diamonds and has been formed by transformation from alpha
quartz by the application of shearing stress. See DIAMOND; HIGH-PRESSURE
MINERAL SYNTHESIS.
    Coesite occurs in grains that are usually less than 0.0002 in. (5
micrometers) in size and are generally present in small amounts. The
properties of the mineral are known mainly from studies of synthesized
crystals. It is colorless with vitreous luster and has no cleavage. It has a
specific gravity of 2.915 PLUS/MINUS 0.015 and a hardness of about 8.
    Coesite has as yet no evident commercial use and therefore has no obvious
economic value. As a stepping-stone in scientific research, it serves in at
least two ways: where it occurs naturally, coesite is diagnostic of a past
history of high pressure; the occurrence of coesite from Meteor Crater clearly
suggests that shock as a process can transform a low-density ordinary
substance to one of high density and unique properties. Coesite has been found
in materials ejected from craters formed by the explosion of 500,000 tons
(450,000 metric tons) of TNT. Research on the occurrence of coesite from rocks
deformed by other energy sources, such as volcanic explosions and deep-seated
tectonic movement, is continuing. The study of coesite and craters may be
useful in understanding of the impact craters on the Earth as well as those on
the Moon.
 �34s[Edward C. T. Chao]
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�6v�2v�32sCoffee �21m �33s
    Arabica coffee is a shrub or small tree of the genus Coffea (Rubiaceae)
and a native of Abyssinia. There are 59 species, but only 4 are of commercial
importance. Nine-tenths of the world supply of coffee is obtained from the
Arabian species, C. arabica, of which there are about 15 varieties. Coffee is
strictly a tropical crop requiring a moderately cool climate that is moist but
not wet. The fruit, called a cherry, is a small spherical drupe containing two
seeds, the coffee beans. In the wet method the cherries are picked by hand,
then pulped and allowed to ferment, and thus the membranes are removed. In the
dry method cherries are harvested and allowed to dry, and the exocarp is
milled off. The coffee beans are then graded and packed in burlap bags.
Commercially, coffee is the most important caffeine beverage plant in the
world. See RUBIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s
    Coffee beverage is prepared by extraction of roasted and ground coffee
with hot water. This can be done in the home by the consumer, or by a coffee
manufacturer who prepares coffee extract in a concentrated form and removes
the water by spray-drying or freeze-drying.
    Green beans are packed and transported in hemp bags. On arrival at the
roasting plant, beans from many sources are blended to achieve a particular
flavor. The beans are cleaned both before and after roasting. Roasting is the
most important operation in coffee processing because it develops the flavor.
Systems now employ externally heated gases of combustion, mainly nitrogen,
carbon dioxide, and water vapor. In batch roasters, energy at relatively low
temperature, 800 - 900DGF (430 - 480DGC), develops fine flavor and a uniform
brown color in about 12 min. In continuous roasters, energy is transferred
even more efficiently. Temperature is reduced to about 500DGF (260DGC) and
roasting time to about 5 - 8 min. Light roasts are attained at bean
temperatures of 380 - 390DGF (193 - 199DGC); medium roasts, 400DGF (204DGC);
and dark roasts, 425 - 430DGF (218 - 221DGC). After roasting, and a second
cleaning operation called stoning, the beans are deposited in specially
designed bins which prevent segregation as the beans pass to the grinder.
Roasted and ground vacuum-packed coffee maintains freshness for at least 2
years.
    Soluble or instant coffee manufacturing utilizes about 20% of the green
beans imported. Soluble coffee accounts for about one-third of all coffee
consumed in the United States, and about 40% of this is freeze-dried. Coffee
extract is made by passing water at temperatures up to 350DGF (177DGC) through
coarsely ground coffee in a series of extracting columns. After filtration,
the extract can be concentrated in conventional evaporating equipment,
followed by spray-drying to remove most of the water. The spray-dried soluble
coffee can be processed further by agglomerating the particles with steam to
form a granular-appearing product. The extract also can be frozen and then
freeze-dried in a low-temperature vacuum system by sublimation from the ice
crystal structure to remove most of the water. Most manufacturers of soluble
coffees return aroma lost during processing to the package to provide the
aroma of a roasted and ground coffee. This requires protection in packaging,
and most soluble coffees are packed in a low-oxygen atmosphere by using an
inert gas such as carbon dioxide or nitrogen.
    Decaffeinated coffee, both regular and instant, is prepared from green
beans that have been treated before processing with low-boiling chlorinated
solvents to remove the caffeine. The green decaffeinated coffee than can be
processed in the manners described above for a regular or a soluble coffee.
 �34s[William P. Clinton]
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�6v�2v�32sCofferdam   �21m �33s 
    A temporary, wall-like structure to permit dewatering an area and 
constructing foundations, bridge piers, dams, dry docks, and like structures 
in the open air. A dewatered area can be completely surrounded by a cofferdam 
structure or by a combination of natural earth slopes and cofferdam structure. 
The type of construction is dependent upon the depth, soil conditions, 
fluctuations in the water level, availability of materials, working conditions 
desired inside the cofferdam, and whether located on land or in water (see 
illustration). An important consideration in the design of cofferdams is the 
hydraulic analysis of seepage conditions, and erosion of the bottom when in 
streams or rivers. 
    Where the cofferdam structure can be built on a layer of impervious soil 
(which prevents the passage of water), the area within the cofferdam can be 
completely sealed off. Where the soils are pervious, the flow of water into 
the cofferdam cannot be completely stopped economically, and the water must be 
pumped out periodically and sometimes continuously. 
    A nautical application of the term cofferdam is a watertight structure 
used for making repairs below the waterline of a vessel. The name also is 
applied to void tanks which protect the buoyancy of a vessel. 
 �34s[Edward J. Quirin]
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�6v�2v�32sCognition �21m �33s
    A rather loosely defined area that embraces the whole complex system of
human mental abilities focusing on perception, attention, learning and memory,
the comprehension and use of language, and reasoning. Traditionally,
psychologists have regarded cognition in terms of an information-processing
system in which a given stimulus is received by the organism and proceeds
through a sequence of processing stages until it either drops out of the
system or is deposited in long-term memory.
    When a stimulus has been perceived, say, a word or letter string, it
resides briefly in a preprocessing store called sensory memory, which
effectively prolongs its life even when the stimulus is no longer present.
Thus, like a fading afterimage, the information remains available for further
processing. Sensory memory is relatively unselective in its contents and has a
large capacity: most or all of the input information is recorded. At every
processing stage, attention is focused on those elements of the input that are
relevant for a given purpose.
    Short-term memory, unlike sensory memory, has a very small capacity. Only
some five to seven pieces of information can be maintained there, and new
items quickly displace the old ones. However, quite a bit of information can
be retained if it is organized into meaningful chunks - if the letters form
words and if the words combine associatively or into sentences.
    Short-term memory should more properly be considered the active,
conscious, working component of memory, where strict selection processes
determine the information relevant for long-term storage. Short-term memory
maintains information through rehearsal, reorganizes it in chunks, and relates
them to preexisting knowledge. This is what is meant by the encoding process.
    Long-term memory is the permanent memory store, the repository for all
personal experiences, general world knowledge, and procedures, goals, and
interests. Its capacity is virtually unlimited, and forgetting from long-term
memory is not so much a loss of information as a retrieval failure.
    An important branch of research in cognition has concerned itself with the
organization of long-term or semantic memory. Semantic memory is usually
defined as general knowledge, information that can be retrieved and used in a
variety of contexts. Episodic memory, on the other hand, refers to knowledge
of events, how and when something occurred and not so much what it was.
    The use of knowledge in reasoning and problem-solving tasks has been
another focus of work in cognition. Perceptual aspects appear to be important
in a variety of tasks, such as the ability to perceive patterns rather than
individual pieces on a chessboard, or to restructure a logical or mathematical
problem in such a way that some critical insight is gained. See INFORMATION
PROCESSING; INTELLIGENCE; MEMORY; PERCEPTION; PROBLEM SOLVING.
 �34s[Walter Kintsch ;Eileen Kintsch]
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�6v�2v�32sCoherence �21m �33s
    The attribute of two or more waves, or parts of a wave, whose relative
phase is constant during the resolving time of the observer. The concept has
been developed most extensively in optics, but is applicable to all wave
phenomena.
    Consider two waves, with the same mean angular frequency omega, given by
Eqs. (1) and (2). It is convenient, and no restriction, to choose both A and B
real. These expressions as they stand could describe de Broglie waves in
quantum mechanics. For real waves, such as components of the electric field in
light or radio beams, or the pressure oscillations in sound, it is necessary
to retain only the real parts of these and subsequent expressions. The
frequency spectrum is assumed to be narrow, in the sense that a Fourier
analysis of expressions (1) and (2) gives appreciable contributions only for
angular frequencies close to omega. This assumption means that, on the
average, delta(sub A)(t) and delta(sub B)(t) do not change much per period. See
ELECTROMAGNETIC RADIATION; QUANTUM MECHANICS; SOUND.

PSI(sub A)(x,t) = A exp i [k(omega)x--omega t--delta(sub A) (t)]	  (1)

PSI(sub B)(x,t) + B exp i [k(omega)x--omega t--delta(sub B) (t)]	  (2)

    Suppose that the waves are detected by an apparatus with resolving time T;
that is, T is the shortest interval between two events for which the events do
not seem to be simultaneous. For the human eye and ear, T is about 0.1 s,
while a fast electronic device might have a T of 10(sup -10) s. If the
relative phase delta(t), given by Eq. (3), does not, on the average, change
noticeably during T, then the waves are coherent. If during T there are
sufficient random fluctuations for all values of delta(t), modulus 2pi, to be
equally probable, then the waves are incoherent. If during T there are
noticeable random fluctuations in delta(t), but not enough to make the waves
completely incoherent, then the waves are partially coherent. These
distinctions are not useful unless T is specified. On the one hand, only waves
that have existed forever and that fill all of space can have absolutely fixed
frequency and phase. On the other hand, two independent sound waves in which
the phases change appreciably in 0.01 s would seem incoherent to the human
ear, but would seem highly coherent to a fast electronic device.

	 delta(t) = delta(sub B) (t) - delta(sub A) (t) 		   (3)

    The degree of coherence is related to the interference patterns that can
be observed when the two beams are combined. See INTERFERENCE OF WAVES.
    Coherence is also used to describe relations between phases within the
same beam. Suppose that a wave represented by Eq. (1) is passing a fixed
observer characterized by a resolving time T. The phase deltasubA may
fluctuate, perhaps because the source of the wave contains many independent
radiators. The coherence time DELTA t(sub W) of the wave is defined to be the
average time required for delta(sub A) (t) to fluctuate appreciably at the
position of the observer. If DELTA t(sub W) is much greater than T, the wave is
coherent; if DELTA t(subW) is of the order of T, the wave is partially coherent;
and if DELTA t(sub W) is much less than T, the wave is incoherent. These
concepts are very close to those developed above.
    Extended sources give partial coherence and produce interference fringes
with visibility V less than unity. A. A. Michelson exploited this fact with
his stellar interferometer, a modified double-slit arrangement with movable
mirrors that permit adjustment of the effective separation D' of the slits. It
can be shown that if the source is a uniform disk of angular diameter THETA,
then the smallest value of D' that gives zero V is 1.22 lambda/THETA. The same
approach has also been applied in radio astronomy. A different technique,
developed by R. Hanbury Brown and R. Q. Twiss, measures the correlation
between the intensities received by separated detectors with fast electronics.
See INTERFEROMETRY.
    Because they are highly coherent sources, lasers and masers provide very
large intensities per unit frequency. See LASER.
 �34s[Rolf G. Winter]
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�6v�2v�32sCohesion (physics)   �21m �33s 
    The tendency of particles of a condensed phase (a solid or liquid) to 
stick together, or the processes or forces that lead to this tendency. The 
cohesive energy is the work necessary to pull the body apart into its 
constituent particles. For example, the cohesive energy of a metal is the work 
necessary to break the metal apart into individual atoms in their ground 
state. The cohesive energy of a molecular solid such as solid carbon dioxide 
is the energy necessary to separate the solid into isolated carbon dioxide 
molecules.  
    The study of cohesion is the study of the competing processes that tend to 
establish an equilibrium separation between molecules or atoms in a condensed 
phase. In normal matter all such forces are the result either of the Coulomb 
interaction between positive and negative charges or of quantummechanical 
kinetic energy. The Coulomb interaction lowers the energy (increases the bond 
strength) and tends to pull atoms together. This tendency toward higher 
density and stronger bonding is opposed by the quantum-mechanical kinetic 
energy. As a quantum-mechanical particle such as an electron is confined to a 
smaller volume, its kinetic energy increases. The cohesion of matter is a 
delicate balance between these two effects. See COULOMB'S LAW; EXCLUSION 
PRINCIPLE; UNCERTAINTY PRINCIPLE. 
    The study of cohesion is the analog in physics of the study of chemical 
bonding and intermolecular forces in chemistry. Cohesion is the binding of a 
macroscopic condensed body, rather than the binding of a small collection of 
atoms or the mutual interactions of a diffuse gas of particles. See CHEMICAL 
BOND THEORY; CHEMICAL BONDING; INTERMOLECULAR FORCES. 
    There are a number of different classes of cohesion that can be 
distinguished by the nature of the balance between electrostatic and kinetic 
energies or by the spatial distribution of electronic charge. One such 
classification scheme distinguishes molecular, ionic, covalent, and metallic 
binding. The classes represent arbitrary distinctions among systems in which 
the microscopic physics of cohesion are quite similar. It is possible that any 
individual system will display aspects of several of these classes. 
    In a molecular crystal or liquid some of the atoms are strongly bound 
together in molecules, and these molecules are relatively weakly bound to the 
other molecules in a condensed phase. The intermolecular bond is usually due 
to the electrostatic dipole-dipole interaction. 
    In an ionic system, such as sodium chloride (NaCl), the constituent 
particles are ions with a net electronic charge, some positive and some 
negative. The Coulomb interaction between them leads to an attractive force. 
The system is kept from collapsing by an increase in kinetic energy due to the 
compression of charge as the system is compressed. 
    Covalently bound crystals are a third major class of crystals, in which 
the atoms are held together by directional bonds. A bond forms because atoms 
are able to share their electrons. The shared electrons are subject to the 
Coulomb attraction of more than one nucleus, and thus the system of electrons 
and nuclei is more tightly bound than the separated atoms. 
    The fourth class of cohesion is metallic bonding, which is characterized 
by relatively large cohesive energy and density, and by the fact that metals 
are good low-temperature electrical conductors in contrast to the other 
classes which are insulators. The concept of electron pair bonds is not 
applicable to these systems in which the coordination number, the number of 
atoms surrounding a given atom, is frequently much larger than the number of 
valence electrons, the electrons available for bonding. See BAND THEORY OF 
SOLIDS; SOLID-STATE CHEMISTRY; SOLID-STATE PHYSICS. 
 �34s[C. D. Gelatt, Jr.]
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�6v�2v�32sCoil   �21m �33s 
    One or more turns of wire used to introduce inductance into an electric 
circuit. At power line and audio frequencies a coil has a large number of 
turns of insulated wire wound close together on a form made of insulating 
material, with a closed iron core passing through the center of the coil. This 
is commonly called a choke and is used to pass direct current while offering 
high opposition to alternating current. 
    At higher frequencies a coil may have a powdered iron core or no core at 
all. The electrical size of a coil is called inductance and is expressed in 
henries or millihenries. In addition to the resistance of the wire, a coil 
offers an opposition to alternating current, called reactance, expressed in 
ohms. The reactance of a coil increases with frequency. See INDUCTOR; REACTOR 
(ELECTRICITY).  
 �34s[John Markus]
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�6v�2v�32sCoincidence amplifier   �21m �33s 
    An electronic circuit that amplifies only that portion of a signal present 
when another enabling or controlling signal is simultaneously applied. The 
controlling signal may be such that the amplifier functions during a specified 
time interval, called time selection, or such that it functions between 
specified voltage or current levels, called amplitude selection. For example, 
the amplifier in a transmission gate operates on the basis of time selection 
where a control voltage of fixed amplitude is applied during a specific time 
interval. This voltage is the enabling waveform; it allows the amplifier to 
function normally only during the time it is applied. See GATE CIRCUIT. 
    A linear amplifier used to amplify a section of a waveform above or below 
specified voltage or current limits, as determined by a control signal, is an 
example of a coincidence amplifier that operates by amplitude selection. 
Limiting and clipping circuits that incorporate the function of amplification 
may be considered coincidence amplifiers. See CLIPPING CIRCUIT; LIMITER 
CIRCUIT.  
 �34s[Glenn M. Glasford]
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�6v�2v�32sCoining   �21m �33s 
    A cold metalworking process in a press-type die. Coining is used to 
produce embossed parts, such as badges and medals, and for minting of coins. 
It is also used on portions of a blank or workpiece to form corners, 
indentations, or raised sections, frequently as part of a progressive die 
operation. The work is subjected to high pressure within the die cavity and 
thereby forced to flow plastically into the die details. The absence of 
overflow of excess metal from between the dies is characteristic of coining 
and is responsible for the fine detail achieved. See METAL FORMING. 
 �34s[Ralph L. Freeman]
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�6v�2v�32sCoir   �21m �33s 
    A natural fiber, also known as coco fiber, obtained from the husks of the 
coconut (Cocos nucifera). The outstanding characteristic of coir fiber is its 
resistance to rot. For example, a coir fiber doormat can stay out in the 
weather for years. Coir is only intermediate in strength among the vegetable 
fibers, but its elongation at break is greater than any of the bast or hard 
fibers. More than 95% of coir exports are from Sri Lanka and India; small 
amounts are exported from Thailand, Tanzania, Mexico, the Philippines, 
Malaysia, Kenya, and Trinidad and Tobago. 
    There are three main types of coir - yarn fiber, bristle fiber, and 
mattress fiber. Only the finest and longest fiber is suitable for spinning 
into yarn. It is obtained from the husks of unripe nuts and is the main cash 
crop rather than a by-product. The bristle fiber and most of the mattress 
fiber come from mature nuts and are by-products of copra production. Coir 
fiber ranges in color from light tan to dark brown. Bristles are sorted 
according to length and graded on the basis of length, stiffness, color, and 
cleanliness. Long, clean fibers and light color are desirable for any purpose, 
but stiffness is desirable in bristles and undesirable for yarn. 
    In developed countries, yarn fiber is used chiefly in mats and mattings; 
in the United States, for instance, its best-known use is in light-brown 
tufted doormats. In Asia it is used extensively for ropes and twines, and 
locally for hand-made bags. The principal outlet for bristle fiber has been in 
brush making, but the market has declined and most bristle fiber is now used 
in upholstery padding. Mattress fiber is used chiefly in innerspring 
mattresses, though it has found uses as an insulating material. See COCONUT: 
NATURAL FIBER. 
 �34s[Elton G. Nelson]
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�6v�2v�32sCoke   �21m �33s 
    A coherent, cellular, carbonaceous residue remaining from the dry 
(destructive) distillation of a coking coal. It contains carbon as its 
principal constituent, together with mineral matter and residual volatile 
matter. The residue obtained from the carbonization of a noncoking coal, such 
as subbituminous coal, lignite, or anthracite, is normally called a char. Coke 
is produced chiefly in chemical-recovery coke ovens, but a small amount is 
also produced in beehive or other types of nonrecovery ovens. 
    Coke is used predominantly as a fuel reductant in the blast furnace, in 
which it also serves to support the burden. As the fuel, it supplies the heat 
as well as the gases required for the reduction of the iron ore. It also finds 
use in other reduction processes, the foundry cupola, and house heating. 
    Coke is formed when coal is heated in the absence of air. During the 
heating in the range of 167 - 246DGF (350 - 500DGC), the coal softens and then 
fuses into a solid mass. The degree of softening attained during heating 
determines to a large extent the character of the coke produced. In order to 
produce coke having desired properties, two or more coals are blended before 
charging into the coke oven. In addition to the types of coals blended, the 
carbonizing conditions in the coke oven influence the characteristics of the 
coke produced. Oven temperature is the most important of these and has a 
significant effect on the size and the strength of the coke. In general, for a 
given coal, the size and shatter strength of the coke increase with decrease 
in carbonization temperature. 
    The important properties of coke that are of concern in metallurgical 
operations are its chemical composition, such as moisture, volatile-matter, 
ash, and sulfur contents, and its physical character, such as size, strength, 
and density. The moisture and the volatile-matter contents are a function of 
manner of oven operation and quenching, whereas ash and sulfur contents depend 
upon the composition of the coal charged. See CHARCOAL; COAL; DESTRUCTIVE 
DISTILLATION.  
 �34s[Michael Perch]
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�6v�2v�32sCoking (petroleum)   �21m �33s 
    A process for thermally converting the heavy residual bottoms of crude oil 
entirely to lower-boiling petroleum products and by-product petroleum coke. 
The heavy residual bottoms cannot be catalytically cracked, principally 
because the metals present are potent catalyst poisons. Traditionally, the 
residual bottoms have been blended with lighter stocks and marketed as 
low-quality heavy fuel oils. However, in the United States the demand for 
heavy fuel oil has remained nearly constant for a number of years, while 
demands for gasoline, heating oil, and other products, have increased 
substantially. To meet this shift in demand, refiners have resorted to deeper 
vacuum flashing, visbreaking (viscosity breaking), solvent deasphalting, and 
coking. The major products from coking are fuel gas, gasoline, gas oil, and 
petroleum coke. Generally the gasoline is of poor quality and must be further 
treated before use in premium fuels. The gas oil is commonly used as 
catalytic-cracking feed stock. See CRACKING; PETROLEUM PROCESSING. 
 �34s[J. F. Moser, Jr.]
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�6v�2v�32sCola   �21m �33s 
    A tree, Cola acuminata, of the sterculia family (Sterculiaceae) and a 
native of tropical Africa. Its fruit is a star-shaped follicle containing 
eight hard seeds, the cola nuts of commerce. These nuts are an important 
masticatory in many parts of tropical Africa. They have a caffeine content 
twice that of coffee. The nuts also contain an essential oil and a glucoside, 
kolanin, which is a heart stimulant. Cola nuts, in combination with an extract 
from coca, are used in the manufacture of the beverage Coca-Cola. Cola is now 
cultivated in Angola, Jamaica, Brazil, India, and other parts of tropical 
Asia. See COCA; MALVALES. 
 �34s[Perry D. Strausbaugh /Earl L.]
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�6v�2v�32sColchicine �21m �33s
    An alkaloid contained in the underground stem of the autumn crocus
(Colchicum autumnale) of the lily family (Liliaceae). In medicine the drug is
used principally in the treatment of gout. Colchicine has a specific influence
on the division of plant chromosomes, and has been used by geneticists and
plant breeders to bring about doubling of chromosomes, thus producing
tetraploid plants. Most colchicine comes from the Netherlands, Italy,
Yugoslavia, and Hungary. See ALKALOID; LILIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCold hardiness (plants)   �21m �33s 
    An adaptive capacity of plants enabling them to survive temperatures near 
and below the freezing point of water. Without this property there would be no 
forest or fruit trees growing in many regions of the north temperate zone. 
Furthermore, the limited level of frost hardiness which crops possess is a 
major limiting factor in the northward extension of agriculture. Even in 
cultivated areas, sporadic occurrences of unusually low temperatures may 
result in widespread crop damage with considerable economic loss. 
    The property of cold hardiness is genetically determined. In plants having 
the genetic capacity, the property is not always present but is elicited only 
following a conditioning period which is environmentally evoked. The condition 
of cold hardiness is one of tolerance of cells and tissues to actual freezing. 
Since plants do not usually generate large quantities of heat and lack 
thermoregulation, they assume the temperature of their surroundings. Also, 
they contain a high percentage of water and freeze solidly when temperatures 
drop appreciably below the freezing point. Unhardened plants are usually 
injured or killed under freezing conditions. However, no injury is incurred 
even when these plants are supercooled well below the freezing point, provided 
that ice crystallization has not been initiated. If plants or plant tissues 
whose cellular water has supercooled are quickly rewarmed before ice has 
formed, no injury will be detected. Also, any plants or plant tissues (such as 
seeds) which can endure desiccation can, in the desiccated condition, be 
immersed directly in liquid nitrogen without injury, since seeds contain 
virtually no free water capable of freezing. 
    The study of cold hardiness in temperate zone plants includes study of the 
cellular and environmental factors initiating the cellular phenomenon of cold 
hardiness, the nature of the injurious effects of ice formation, and the 
physiological and chemical adaptive mechanisms which have evolved in these 
plants to render them resistant to these effects. See DORMANCY; PLANT GROWTH; 
PLANT PHYSIOLOGY. 
 �34s[Dave Scott]
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�6v�2v�32sCold storage   �21m �33s 
    The storage of perishables at low temperatures, usually above freezing, by 
the use of refrigeration to increase the storage life. In general, the lower 
the temperature, the longer the storage life. If temperatures are maintained 
below the freezing point of the product stored, it is called freezer storage. 
Most fruits and many other products, however, are damaged by freezing and 
cannot be stored in freezer storage. A cold-storage plant is a large insulated 
building, with its attendant refrigeration equipment, for storage of 
commodities at low temperatures. Facilities are often included for 
quick-freezing fruits, vegetables, meats, and a variety of precooked foods and 
bakery products for the consumer convenience market. See REFRIGERATION. 
 �34s[Carl F. Kayan]
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�6v�2v�32sColeoidea   �21m �33s 
    A subclass of cephalopod mollusks listed in certain taxonomic schemes, in 
contrast to the subclasses Nautiloidea and Ammonoidea. This subclass includes 
all living cephalopods except Nautilus. One extinct order, the Belemnoidea, 
and four living orders, Sepioidea, Teuthoidea, Vampyromorpha, and Octopoda, 
make up the subclass. The coleoids were formerly known as the Dibranchiata 
because they have but two gills. The octopods have eight arms or tentacles, 
whereas the remaining groups formerly known as the Decapoda have ten arms. The 
Vampyromorpha have two retractile filaments which have been interpreted as 
being arms in addition to the eight regular arms. See AMMONOIDEA; BELEMNOIDEA; 
CEPHALOPADA; NAUTILOIDEA; OCTOPODA; SEPIOIDEA; TEUTHOIDEA; VAMPYROMORPHA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sColeoptera   �21m �33s 
    An order of Insecta (generally known as beetles) with a complete 
metamorphosis (with larva and pupa stages), the forewings forming hard 
protective elytra under which the hindwings are normally folded in repose, and 
the indirect flight muscles of the mesothorax having been lost. The head 
capsule is characterized by a firm ventral (gular) closure, the antennae are 
basically 11-segmented, and mouthparts are of the biting type with 4-segmented 
maxillary palpi. The prothorax is large and free. The abdomen has sternite I 
normally absent, sternite II usually membranous and hidden, and segment X 
vestigial, with cerci absent. The female has one pair of gonapophyses, on 
segment IX. There are four or six malpighian tubules, which are often 
cryptonephric. True labial glands are nearly always absent. 
  
    General biological features.  The order has well over 250,000 described 
species, more than any comparable group, showing very great diversity in size, 
form (see illustration), color, habits, and physiology. Beetles have been 
found almost everywhere on Earth (except as yet for the Antarctic mainland) 
where any insects are known, and species of the order exploit almost every 
habitat and type of food which is used by insects. Many species are 
economically important. 
    A common feature is for the exoskeleton to form an unusually hard and 
close-fitting suit of armor, correlated with a tendency for the adults to be 
less mobile but longer-lived than those of other orders of higher insects. It 
seems to be common in a number of groups for adults to survive and breed in 2 
or more successive years, a rare occurrence in other Endopterygota. Adult 
beetles tend to be better runners than most other Endopterygota, and some 
(such as Cicindelidae) are among the fastest of running insects. The flight of 
most beetles is not very ready or frequent, often requiring prior climbing up 
onto some eminence, and in many beetles occurs only once or twice in an 
average life. 
    Apart from a tendency for the sclerotized layers to be thicker, the 
cuticle of beetles differs little from that of other insects in constitution 
or ultrastructure. The black coloring which is prevalent in many groups of 
beetles is produced by deposition of melanin in the process of cuticle 
hardening. In many groups with adults active by day, the black is masked by a 
metallic structural color produced by interference in the surface cuticle 
layers. Pigmentary colors, mainly ommochromes, giving reddish to yellowish 
colors, occur in the cuticle of many species. 
    Special defenses against predation are common in species with long-lived 
adults, particularly the ground-living species of Carabidae, Staphylinidae, 
and Tenebrionidae. These groups commonly possess defensive glands, with 
reservoirs opening on or near the tip of the abdomen, secreting quinones, 
unsaturated acids, and similar toxic substances. In some cases, the secretion 
merely oozes onto the body surface, but in others may be expelled in a jet, as 
in the bombardier beetles (Brachinini). Other common defensive adaptations 
include cryptic and mimetic appearances. The most frequent behavioral 
adaptations are the drop-off reflex and appendage retraction, in which adults 
will react to visual or tactile stimuli by dropping off the plant foliage on 
which they occur, falling to the ground with retracted appendages, and lying 
there for some time before resuming activity. In death feigning, the 
appendages are tightly retracted, often into grooves of the cuticle, and held 
thus for some time. In such conditions, beetles often resemble seeds and are 
difficult for predators to perforate or grasp. 
  
    Sound production (stridulation) is widespread in beetles and may be 
produced by friction between almost any counterposed movable parts of the 
cuticle. Stridulatory organs may show sex dimorphism, and in some species may 
play an important role in the interrelations of the sexes. 
  
    Anatomy.  The head of beetles rarely has a marked posterior neck, and the 
antennal insertions are usually lateral, rather than dorsal as in most other 
Endopterygota, a feature possibly related to an original habit of creeping 
under bark. Antennal forms are very various, with the number of segments often 
reduced, but rarely increased, from the basic 11. Mandible forms are very 
diverse and related to types of food. 
    The prothorax commonly has lateral edges which separate the dorsal part 
from the ventral part of the tergum. The mesothorax is the smallest thoracic 
segment. Dorsally, its tergum is usually exposed as a small triangular 
sclerite between the elytral bases, and ventrally, its sternite may have a 
median pit receiving the tip of the prosternum. The form of the metathorax is 
affected by far-forward insertions of the hindwings an far-posterior ones of 
the hindlegs, so that the pleura are extremely oblique and the sternum usually 
long.  
    The tarsi of beetles show all gradations from the primitive five-segmented 
condition to total disappearance. The number of segments may differ between 
the legs of an individual or between the sexes of a species. 
    Most beetles have the entire abdomen covered dorsally by the elytra in 
repose, and ventrally only five or six sternites (of segments III to VII or 
VIII) exposed; segment IX is retracted inside VIII in repose. Tergites I-VI 
are usually soft and flexible, those of VII and VIII more or less sclerotized. 
A feature developed in several groups of Coleoptera, but particularly in 
Staphylinidae, is the abbreviation of the elytra to leaves part of the abdomen 
uncovered in repose, but usually still covering the folded wings. One 
advantage of this may be to give greater overall flexibility to the body. 
Perhaps surprisingly, many beetles with short elytra fly readily. 
    Internally, the gut varies greatly in length and detailed structure. The 
foregut commonly ends in a distended crop whose posterior part usually has 
some internal setae and may be developed into a complex proventriculus; the 
midgut may be partly or wholly covered with small papillae (regenerative 
crypts) and sometimes has anterior ceca. The four or six malpighian tubules 
open in various ways into the beginning of the hindgut and in many Polyphaga 
have their apices attached to it (cryptonephric). The central nervous system 
shows all degrees of concentration of the ventral chain, from having three 
thoracic and eight abdominal ganglia distinct to having all of these fused 
into a single mass. 
    The sense organs of beetles, adult or larval, are generally similar in 
type and function to those of other insects. For vision, some types are 
adapted for high visual acuity in bright light, others to high sensitivity in 
poor light, and many show light-dark adaptations of screening pigment. Color 
vision seems to be restricted to certain groups. In certain beetles, there is 
evidence of specialized infrared sensitivity. Antennae are mainly receptors 
for smell and touch, but detection of aerial vibrations (by Johnston's organ 
in the pedicel) may be important in some. The olfactory sensilla, which 
commonly are setae, are often concentrated on expanded apical segments, 
forming a club. Taste, or contact chemoreception, is usually mediated by 
sensilla with a single large apical opening. These sensilla are concentrated 
on the palpi and other mouthparts and are often found on tibiae or tarsi of 
the front legs. 
    Taste, or contact chemoreception, is usually mediated by sensilla with a 
single large apical opening. These sensilla are concentrated on the palpi and 
other mouthparts and are often found on tibiae or tarsi of the front legs. 
    Specialized sound receptors, other than Johnston's organ, have not been 
much studied in Coleoptera, though many species produce sounds and in some 
cases these play a significant part in courtship. A gravitational sense is 
evident in many burrowing beetles, which are able to sink accurately vertical 
burrows despite inclinations of the surface started from. A temperature sense 
is clearly present in some beetles and larvae, but little is known of the 
sensilla concerned. What might be called a time sense (otherwise known as a 
biological clock) clearly operates in many beetles that react specifically to 
changing daylight lengths marking the seasons in nontropical latitudes. 
  
    Food specialization.  The biting mouthparts of many adult and larval 
beetles are adaptable to many types of food, and fairly widely polyphagous 
habits are not uncommon in the order, though most species can be assigned to 
one or another of a few main food categories: fungivores, carnivores, 
herbivores, or detritivores. Included in the last category are species feeding 
on decaying animal or vegetable matter and on dung. Most parasites could be 
included with carnivores, but those myrmecophiles and termitophiles which are 
fed by their hosts, and those Meloidae which develop on the food stores of 
bees, form special categories. 
  
    Water beetles.  Beetles are unusual among the higher insects in that 
aquatic habits, where present, usually affect adults as well as larvae. The 
elytra may have been preadaptive to the invasion of water. Almost any type of 
fresh or brackish water body is liable to contain some types of water beetles, 
though very few species can live permanently in full marine salinities. Almost 
all aquatic beetles maintain an air reservoir, into which the second thoracic 
and abdominal spiracles open, under their elytra; and some, mainly small, 
species may have plastrons or physical gills. Aquatic larvae tend to have a 
last pair of abdominal spiracles that is large and effectively terminal, or 
tend to develop tracheal gills. Pupae are terrestrial in the large majority of 
water beetles, and eggs, when deposited underwater, are commonly laid in 
contact with airspaces in the stems of water plants, or in an air-filled egg 
cocoon. Most water beetles are confined to shallow waters, and many of them 
are ready colonists of temporary pools, through adult flight. 
  
    Special habitats and adaptations.  More or less unusual adaptations and 
modes of life are manifest in dung and carrion beetles, ambrosia beetles, cave 
and subterranean beetles, desert beetles, and luminous beetles. Dung and 
carrion beetles exploit sporadic and very temporary food resources in which 
competition (particularly with Diptera larvae) tends to be fierce. Many dung 
beetles avoid the difficulties by burying dung stores in underground cells 
where their larvae can develop safely, and some beetles (Necrophorinae) adopt 
a similar strategy for carrion. They may also develop phoretic and symbiotic 
relations with specific mites preying on fly eggs. 
    In ambrosia beetles, the adults usually excavate burrows in wood, carrying 
with them spores of special fungi, which proceed to develop along the walls of 
the burrows and are fed on by larvae developing from eggs laid there. This 
type of relation between beetles, fungi, and trees exists in various forms in 
a number of families and may be of ecological importance if the fungi 
concerned are liable to kill trees. 
    More than one family of beetles contains highly adapted types living 
exclusively in deeper cave systems (cavernicolous) or the deeper layers of the 
soil (hypogeous). Such species are usually flightless and eyeless, poorly 
pigmented, and slow-moving, and of very restricted geographical distribution. 
Another marginal habitat in which beetles are the principal insect group 
represented is the desert. Here, too, wings are usually lost, but the cuticle 
tends to be unusually thick, firm, and black. Also beetles may be found in hot 
springs. A few water beetles are adapted to underground (phreatic) waters, 
showing parallel features with the cavernicolous and hypogeous terrestrial 
ones.  
    More or less parasitic relations to animals of other groups have developed 
in a number of lines of Coleoptera. The closest parallels to 
Hymenoptera-Parasitica are to be seen in the families Stylopidae (Strepsiptera 
of many authors) and Rhipiphoridae. Stylopid larvae are exclusively 
endoparasites of other insects, while the adult females are apterous, often 
legless, and remain in the host's body, while males have large fanlike 
hindwings and the elytra reduced to halter-like structures. A considerable 
variety of Coleoptera develop normally in the nests of termites and social or 
solitary Hymenoptera-Aculeata, some being essentially detritivores or 
scavengers, but most feed either on the young or the food stores of their 
hosts. Ectoparasitism on birds or mammals is a rare development in beetles. 
    The beetles are notable in that some of them manifest the highest 
developments of bioluminescence known in nonmarine animals, the main groups 
concerned being Phengodidae, Lampyridae, and Elateridae-Pyrophorini, the 
glowworms and fireflies. In luminescent beetles, the phenomenon always seems 
to be manifest in the larvae and often in the pupae, but not always in the 
adults. In luminous adult beetles, there is often marked sex dimorphism, and a 
major function of the lights seems to be the mutual recognition of the sexes 
of a species. 
    Another possible function of adult luminosity, and the only seriously 
suggested for that of larvae, is as an aposematic signal. There is definite 
evidence that some adult fireflies are distasteful to some predators, and the 
luminous larvae of Phengodidae have dorsal glandular openings on the trunk 
segments that probably have a defensive function. See BIOLUMINESCENCE. 
  
    Phylogenetic history.  Coleoptera are older than the other major 
endopterygote orders. The earliest fossils showing distinctively beetle 
features were found rather before the middle of the Permian Period. By the 
later Permian, fossils indicate that beetles had become numerous and diverse, 
and during the Mesozoic Era they appear as a dominant group among insect 
fossils. Fossils in Triassic deposits have shown features indicative of all 
four modern suborders, and the Jurassic probably saw the establishment of all 
modern superfamilies. By early Cretaceous times, it is likely that all 
``good'' modern families had been established as separate lines. In the Baltic 
Amber fauna, of later Paleogene age (about 40,000,000 years ago), about half 
the fossil beetle genera appear to be extinct, and the other half have 
still-living representatives (often in remote parts of the world). Beetle 
fossils in Quaternary (Pleistocene) deposits are very largely of still-living 
species.  
  
    Geographical distribution.  Almost every type of continuous or 
discontinuous distribution pattern which is known in any animal group could be 
matched in some taxon of Coleoptera, and every significant zoogeographical 
region or area could be characterized by endemic taxa of beetles. In 
flightless taxa, distributional areas are generally more limited than those of 
comparable winged taxa. Distinct distributional categories can be seen in 
those small, readily flying groups (in Staphylinidae and Nitidulidae, for 
example) which are liable to form part of ``aerial plankton,'' and in those 
wood borers which may survive for extended periods in sea-drifted logs, both 
of which are liable to occur in oceanic islands beyond the ranges of most 
other beetle taxa. Climatic factors often seem to impose limits on the spread 
of beetle species (and sometimes of genera or families), and beetle remains in 
peats have been found to be sensitive indicators of climatic changes in 
glacial and postglacial times. See INSECTA. 
 �34s[Roy A. Crowson]
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�6v�2v�32sColiiformes   �21m �33s 
    An order of birds containing the six species of the single family 
Coliidae, the colies or mousebirds of Africa. The latter name is derived from 
the appearance of these birds, which are brownish or grayish, have long tails, 
and have a habit of scurrying along branches. The relationships of the 
Coliiformes are uncertain; some structural features and habits, such as use of 
the bill in climbing, suggest a distant affinity with the parrots. Colies are 
primarily frugivorous and travel in close flocks during the nonbreeding 
season. See AVES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sColiphage �21m �33s
    Any bacteriophage able to infect the bacterium Escherichia coli; many are
able to attack more than one strain of this organism. The T series of phage
(T1 - T7), propagated on a special culture of E. coli, strain B, have been
used in extensive studies, from which most of the knowledge of phages is
derived. See BACTERIOPHAGE; LYSOGENY.
 �34s[Philip B. Cowles]
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�6v�2v�32sColitis �21m �33s
    An inflammation of the large bowel, or colon. If the precise causative
agent remains unknown, that form of the disorder is called idiopathic colitis.
Idiopathic ulcerative colitis is common and may produce severe changes in the
bowel. Adults between 20 and 40 years of age are most affected and, at
present, multiple contributing factors are blamed for the disease. Small
hemorrhages in the lower part of the bowel usually mark the early stages.
These develop into tiny abscesses which increase in size, number, and
distribution. The symptoms of abdominal pain, bleeding, diarrhea, and those
symptoms related to specific complications tend to become chronic or to recur.
    Some agents may cause specific forms of colitis, which have very
characteristic pathological or clinical features. These include tuberculosis,
amebic and bacillary dysentery, typhoid fever, staphylococcal infections, and
mercury poisoning.
    Various brief, relatively benign cases of colitis occur frequently,
probably as a result of mild viral or bacterial infections, food poisoning or
intolerances, and emotional stresses. Colitis may also be secondary to other
disease or injury, notably as a local response to tumor formation. See
AMEBIASIS; BACILLARY DYSENTERY;  GASTROINTESTINAL TRACT DISORDERS;
STAPHYLOCOCCUS; TUBERCULOSIS.
 �34s[N. Karle Mottet]
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�6v�2v�32sCollagen   �21m �33s 
    The major fibrous protein of many animals. Located in the extracellular 
connective tissue, collagen is probably the most abundant animal protein in 
nature. It is estimated that collagen accounts for about 30% of the total 
human body protein. It is present in all types of multicellular animals, both 
invertebrate and vertebrate, although in insects and crustaceans chitin 
replaces collagen as the fibrous supporting matrix in the exoskeleton. 
Collagen constitutes the fibrillar component of the soft connective tissues 
(for example skin, ligament, and tendon) and is the major component of the 
organic matrix of the calcified tissues such as bone and dentine. Besides its 
structural significance, collagen plays an important role in such events as 
development and wound healing, and has been implicated in aging and in a 
number of disease processes. 
    At least four genetically distinct collagens have been described. The most 
familiar, type I, is composed of three ALPHA-polypeptide chains, each of about 
95,000 mol wt. Two chains are identical and are called ALPHA1(I); the other is 
called ALPHA2. Type I collagen forms the major portion of the collagen of both 
soft (skin, tendon, fascia, and so on) and hard (bone and dentine) connective 
tissue. Type II collagen is the major collagen of cartilage and is composed of 
three ALPHA1(II) chains. Type III colla gen is composed of three ALPHA1(III) 
chains and is found in blood vessels, wounds, and certain tumors and appears 
to be ele vated in the tissues of young animals. Reticulin fibers appear to be 
identified with type III collagen. Basement membrane collagens have been 
classified as type IV, and other collagen types have been identified. Collagen 
has a characteristic amino acid composition, containing 33% glycine and about 
25% pro line and hydroxyproline. The amino acid DELTA-hydroxy-L-lysine and the 
imino acid 4-hydroxy-L-proline are almost unique to collagen among the 
proteins of the vertebrates and inverte brates. 
    Collagen is recognized by its relative insolubility under mild conditions, 
by its resistance to most of the proteolytic enzymes of vertebrates, and by 
various histologic staining properties. Its appearance in the electron 
microscope is unique, and it has a characteristic banded or striated 
appearance.  
    Several heritable disorders of connective tissue have been elucidated at 
the molecular level. The basic defect has been determined in four types of the 
Ehlers-Danlos syndrome. People with this condition have ``human pretzel'' or 
``India rubber man'' characteristics, with hyperextensible skin, hypermobile 
joints, fragile tissues, and a bleeding diathesis. In three varieties of the 
syndrome an enzyme defect has been identified, and in the fourth variety it is 
suggested that the type III collagen is lacking. Several types of osteogenesis 
imperfecta have also been identified. The individuals exhibit fragile bones, 
clear or blue sclera, deafness, and loose ligaments. It appears that the bone 
collagen fails to mature, and there is evidence that the ratio of type I/type 
III collagen may be abnormal. See CONNECTIVE TISSUE; FIBROUS PROTEIN. 
 �34s[David A. Keith]
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�6v�2v�32sCollard   �21m �33s 
    A cool-season biennial crucifer, Brassica oleracea var. acephala, similar 
to nonheading cabbage. Collard is of Mediterranean origin and is grown for its 
rosette of leaves, which are cooked fresh as a vegetable. Important production 
centers are in the southern United States, where collards are an important 
nutritious green, especially during winter months. See CABBAGE; CAPPARALES; 
KALE.  
 �34s[H. John Carew]
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�6v�2v�32sCollembola   �21m �33s 
    An order of primitive insects, commonly called springtails, belonging to 
the subclass Apterygota. These tiny insects do not undergo a metamorphosis. 
They have six abdominal segments, some of which may be ankylosed to give an 
apparently smaller number (see illustration). Appended to the fourth abdominal 
segment in most of the species is a spring, or furcula. This consists of a 
basal piece and two apical parallel structures which end in hooks. Another 
pair of structures, united at the base, occur ventrally on the third abdominal 
segment. These bear teeth which engage with the furcula and hold it beneath 
the body. When released, the spring flies backward, catapulting the insect. 
    Collembola live in humid places, often in leaf mold. Some species are 
active at cool temperatures, hence another common name, snowflea, is used. The 
earliest known insect fossil, Rhyniella praecursor, is thought to belong to 
this order. See INSECTA. 
 �34s[Harold B. Maynard]
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�6v�2v�32sCollenchyma   �21m �33s 
    A primary, or early differentiated, supporting tissue of young shoot parts 
appearing while these parts are still elongating. It is located near the 
surface, usually just under the epidermis. When observed in transverse 
sections, it is characterized structurally by cell walls that are 
intermittently thickened, generally in the corners or places of juncture of 
three or more cells. Collenchyma is typically formed in the petioles and vein 
ribs of leaves, the elongating zone of young stems, and the pedicels of 
flowers. See CELL WALLS (PLANT). 
    As in parenchyma, the cells in collenchyma are living and may contain 
chloroplasts and starch grains. The cell wall of a collenchyma cell is its 
most striking feature structurally and functionally. It is composed of 
cellulose and pectic compounds plus a very high proportion of water. The 
cytoplasm is very rich in ribosomes and ribonucleic acids in the early stages 
of development. Another striking feature of collenchyma cell walls is their 
plasticity. They are capable of great elongation during the period of growth 
in length of the plant. The plasticity of collenchyma is associated with a 
tensile strength comparable to that shown by fibers of sclerenchyma. The 
combination of strength and plasticity makes the collenchyma effective as a 
strengthening tissue in developing stems and leaves having no other supporting 
tissue at that time. See CELLULOSE; EPIDERMIS (PLANT); PARENCHYMA; PECTIN. 
 �34s[Robert L. Hulbary]
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�6v�2v�32sCollision (physics)   �21m �33s 
    Any interaction between particles, aggregates of particles, or rigid 
bodies in which they come near enough to exert a mutual influence, generally 
with exchange of energy. The term collision, as used in physics, does not 
necessarily imply actual contact. 
    In classical mechanics, collision problems are concerned with the relation 
of the magnitudes and directions of the velocities of colliding bodies after 
collision to the velocity vectors of the bodies before collision. When the 
only forces on the colliding bodies are those exerted by the bodies 
themselves, the principle of conservation of momentum states that the total 
momentum of the system is unchanged in the collision process. This result is 
particularly useful when the forces between the colliding bodies act only 
during the instant of collision. The velocities can then change only during 
the collision process, which takes place in a short time interval. Under these 
conditions the forces can be treated as impulsive forces, the effects of which 
can be expressed in terms of an experimental parameter known as the 
coefficient of restitution. See CONSERVATION OF MOMENTUM; IMPACT. 
    The study of collisions of molecules, atoms, and nuclear particles is an 
important field of physics. Here the object is usually to obtain information 
about the forces acting between the particles. The velocities of the particles 
are measured before and after collision. Although quantum mechanics instead of 
classical mechanics should be used to describe the motion of the particles, 
many of the conclusions of classical collision theory are valid. See 
SCATTERING EXPERIMENTS (ATOMS AND MOLECULES); SCATTERING EXPERIMENTS (NUCLEI). 
    Collisions can be classed as elastic and inelastic. In an elastic 
collision, mechanical energy is conserved; that is, the total kinetic energy 
of the system of particles after collision equals the total kinetic energy 
before collision. For inelastic collisions, however, the total kinetic energy 
after collision is different from the initial total kinetic energy. 
    In classical mechanics the total mechanical energy after an inelastic 
collision is ordinarily less than the initial total mechanical energy, and the 
mechanical energy which is lost is converted into heat. However, an inelastic 
collision in which the total energy after collision is greater than the 
initial total energy sometimes can occur in classical mechanics. For example, 
a collision can cause an explosion which converts chemical energy into 
mechanical energy. In molecular, atomic, and nuclear systems, which are 
governed by quantum mechanics, the energy levels of the particles can be 
changed during collisions. Thus these inelastic collisions can involve either 
a gain or loss in mechanical energy. 
 �34s[Paul W. Schmidt]
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�6v�2v�32sColloid   �21m �33s 
    A system of which one phase is made up of particles having dimensions of 1 
- 1000 nanometers and is dispersed in a different phase. Some examples of 
colloidal systems are shown in the table. There are three important types of 
colloids: small solid particles having the same internal structure as the bulk 
solid phase (sulfur suspensions in water), aggregates formed from smaller 
molecules (soaps in water), and large molecules of colloidal size (proteins 
and high polymers). 
Insert Table Galley 2 
u1] 
u1] 
491a 
    The unique properties of colloids are due primarily to the large surface 
areas of the dispersed phase. One of the results of this large surface area is 
the adsorption of ions and other materials. The adsorbed ions impart an 
electric charge to the colloidal particles so that they repel each other. 
Lyophobic colloids exhibit little affinity between the dispersed phase and the 
solvent; lyophilic colloids are strongly associated with the solvent. See 
ADSORPTION.  
    Colloidal particles are so small that they cannot be seen directly in 
visible light even with an optical microscope. They can be detected by viewing 
them at an angle to the incident light path. The electron microscope furnishes 
a means by which the sizes and shapes of colloidal-size particles can be 
determined. See ELECTRON MICROSCOPE; TYNDALL EFFECT. 
    Because colloids are so small in size, they cannot be separated by 
filtration through paper or porous porcelain crucibles. They can be separated 
from solvent and dissolved substances by dialysis and ultrafiltration. 
Airborne colloidal particles can be separated by special techniques. See 
DIALYSIS; ELECTROSTATIC PRECIPITATOR; ULTRAFILTRATION. 
    Other more specialized aspects of colloidal properties and surface 
phenomena are discussed in other articles. See DONNAN EQUILIBRIUM; 
ELECTROPHORESIS; EMULSION; FOAM; GEL; MICELLE; SMOKE; ULTRACENTRIFUGE. 
 �34s[Quentin Van Winkle]
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�6v�2v�32sColon   �21m �33s 
    The portion of the intestine that runs from the cecum to the rectum; in 
some mammals, it may be separated from the small intestine by an ileocecal 
valve. It is also known as the large intestine. The colon is usually divided 
into ascending, transverse, and descending portions. In the human a fourth 
section, the sigmoid, is found. The colon is longer in herbivores and shorter 
in carnivores, and is about 4 to 6 ft (1.2 to 1.8 m) long in humans. No 
digestive enzymes are secreted in the colon. Much digestion (for example, all 
breakdown of cellulose) occurs by bacteria, of which Escherichia coli is the 
most common. Most of the fluid added to the food during digestion is 
reabsorbed into the body in the colon. All digestive action, water absorption, 
and so on, is completed before the food materials pass out of the colon into 
the rectum. See DIGESTIVE SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sColor   �21m �33s 
    That aspect of visual sensation enabling a human observer to distinguish 
differences between two structure-free fields of light having the same size, 
shape, and duration. Although luminance differences alone permit such 
discriminations to be made, the term color is usually restricted to a class of 
differences still perceived at equal luminance. These depend upon physical 
differences in the spectral compositions of the two fields, usually revealed 
to the observer as differences of hue or saturation. See COLOR VISION. 
    Color is usually presented to the individual by the surfaces of objects on 
which a more or less white light is falling. A red surface, for example, is 
one that absorbs most of the shortwave light and reflects the long-wave light 
to the eye. The surface colors are easily described by reference to the color 
solid shown in Fig. 1. The central axis defines the lightness or darkness of 
the surface as determined by its overall reflectance of white light, the 
lowest reflectance being called black and the highest, white. The 
circumference denotes hue, related primarily to the selective reflectance of 
the surface for particular wavelengths of light. The color solid is pointed at 
its top and bottom to represent the fact that as colors become whiter or 
blacker they lose hue. The distance from central axis to periphery indicates 
saturation, a characteristic that depends chiefly on the narrowness, or 
purity, of the band of wavelengths reflected by the surface. At the center of 
the figure is a medium-gray surface, one which exhibits a moderate amount of 
reflectance for all wavelengths of light. Colors that are opposite one another 
are called complementaries, for example, yellow and blue, red and blue-green, 
purple and green, and white and black. 
    Two complementaries, when added together in the proper proportions, can 
oppose or neutralize one another to produce a colorless white or gray. Various 
contrast effects also attest to the opposition of the complementaries. Staring 
at a bright-red surface against a gray background results in the appearance of 
a blue-green border around the red; this is an example of simultaneous 
contrast. Similarly, when a red light is turned off, there is frequently a 
negative afterimage that appears to have the opposite color, blue-green. 
    A set of primary colors can be chosen so that any other color can be 
produced from additive mixtures of the primaries in the proper proportions. 
Thus, red, green, and blue lights can be added together in various proportions 
to produce white, purple, yellow, or any of the various intermediate colors. 
Three-color printing, color photography, and color television are examples of 
the use of primaries to produce plausible imitations of colors of the original 
objects. See COLOR TELEVISION. 
    Colors lying along a continuum from white to black are known as the gray, 
or achromatic, colors. They have no particular hue, and are therefore 
represented by the central axis of the color diagram in Fig. 1. White is shown 
at the top of the diagram since it represents the high-brightness extreme of 
the series of achromatic colors. With respect to the surface of an object, the 
diffuse, uniform reflectance of all wavelengths characterizes this series from 
black through the grays to white in order of increasing reflectance. Whiteness 
is a relative term; white paper, paint, and snow reflect some 80% or more of 
the light of all visible wavelengths, while black surfaces typically reflect 
less than 10% of the light. 
    Gray is the term applied to all the intermediate colors in the series of 
achromatic colors. Gray may result from a mixture of two complementary colors, 
from a mixture of all primary colors, or from a fairly uniform mixture of 
lights of all wavelengths throughout the visible spectrum. Grayness is 
relative; a light gray is an achromatic color that is lighter than its 
surroundings, while a dark gray is so called because it is darker than its 
surroundings.  
    The opposite extreme from white in the series of achromatic colors is 
black. Blackness is a relative term applied to surfaces that uniformly absorb 
large percentages of light of all visible wavelengths. A black object in 
sunlight absorbs a large percentage of the light, but it may reflect a larger 
absolute quantity of light to the eye than does a white object in the shade. 
 �34s[Lorrin A. Riggs]
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    Colors are often specified in a two-dimensional chart known as the CIE 
chromaticity diagram, which shows the relations among tristimulus values 
independently of luminance. Such a diagram is shown in Fig. 2, in which the 
continuous locus of spectrum colors is represented by the outermost contour. 
All nonspectral colors are contained within an area defined by this boundary 
and a straight line running from red to violet. The diagram also shows 
discrimination data for 25 regions, which plot as ellipses represented at 10 
times their actual size. A discrimination unit is one-tenth the distance from 
the center of an ellipse to its perimeter. Predictive schemes for 
interpolation to other regions of the CIE diagram have been worked out. 
    If discrimination ellipses were all circles of equal size, then a 
discrimination unit would be represented by the same distance in any direction 
anywhere in the chart. Because this is dramatically untrue, other chromaticity 
diagrams have been developed as linear projections of the CIE chart. These 
represent discrimination units in a relatively more uniform way, but never 
perfectly so. 
    A chromaticity diagram has some very convenient properties. Chief among 
them is the fact that additive mixtures of colors plot along straight lines 
connecting the chromaticities of the colors being mixed. Although it is 
sometimes convenient to visualize colors in terms of the chromaticity chart, 
it is important to realize that this is not a psychological color diagram. 
Rather, the chromaticity diagram makes a statement about the results of 
metameric color matches, in the sense that a given point on the diagram 
represents the locus of all possible metamers plotting at chromaticity 
coordinates x, y. However, this does not specify the appearance of the color, 
which can be dramatically altered by preexposing the eye to colored lights 
(chromatic adaptation) or, in the complex scenes of real life, by other colors 
present in nearby or remote areas (color contrast and assimilation). 
Nevertheless, within limits, metamers whose color appearance is thereby 
changed continue to match. 
    For simple, directly fixated, and unstructured fields presented in an 
otherwise dark environment, there are consistent relations between the 
chromaticity coordinates of a color and the color sensations that are 
elicited. Therefore, regions of the chromaticity diagram are often denoted by 
color names, as shown Fig. 2. 
    Although the CIE system works rather well in practice, there are important 
limitations. Normal human observers do not agree exactly about their color 
matches, chiefly because of the differential absorption of light by inert 
pigments in front of the photoreceptors. Much larger individual differences 
exist for differential colorimetry, and the system is overall inappropriate 
for the 4% of the population (mostly males) whose color vision is abnormal. 
The system works only for an intermediate range of luminances, below which 
rods (the receptors of night vision) intrude, and above which the bleaching of 
visual photopigments significantly alters the absorption spectra of the cones. 
See COLORIMETRY. 
 �34s[Robert M. Boynton]
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�6v�2v�32sColor blindness   �21m �33s 
    A condition of faulty color vision. It appears to be the normal state of 
animals that are active only at night. It is also characteristic of human 
vision when the level of illumination is quite low or when objects are seen 
only at the periphery of the retina. In rare individuals, known as 
monochromats, there is total color blindness even at high light levels. Such 
persons are typically deficient or lacking in cone receptors, so that their 
form vision is also poor. 
    Dichromats are partially color-blind individuals whose vision appears to 
be based on two primaries rather than the normal three. Dichromatism occurs 
more often in men than in women because it is a sex-linked, recessive 
hereditary condition. One form of dichromatism is protanopia, in which there 
appears to be a lack of normal red-sensitive receptors. Red lights appear dim 
to protanopes and cannot be distinguished from dim yellow or green lights. A 
second form is deuteranopia, in which there is no marked reduction in the 
brightness of any color, but again there is a confusion of the colors normally 
described as red, yellow, and green. A third and much rarer form is 
tritanopia, which involves a confusion among the greens and blues. See HUMAN 
GENETICS.  
    Many so-called color-blind individuals might better be called color-weak. 
They are classified as anomalous trichromats because they have trichromatic 
vision of a sort, but fail to agree with normal subjects with respect to color 
matching or discrimination tests. 
    Color blindness is most commonly tested by the use of color plates in 
which various dots of color define a figure against a background of other 
dots. The normal eye readily distinguishes the figure, but the colors are so 
chosen that even the milder forms of color anomaly cause the figure to be 
indistinguishable from its background. 
 �34s[Lorrin A. Riggs]
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�6v�2v�32sColor centers   �21m �33s 
    Atomic and electronic defects of various types which produce optical 
absorption bands in otherwise transparent crystals such as the alkali halides, 
alkaline earth fluorides, or metal oxides. They are general phenomena found in 
a wide range of materials. Color centers are produced by gamma radiation or 
x-radiation, by addition of impurities or excess constituents, and sometimes 
through electrolysis. A well-known example is that of the F-center in alkali 
halides such as sodium chloride, NaCl. The designation F-center comes from the 
German word Farbe, which means color. F-centers in NaCl produce a band of 
optical absorption toward the blue end of the visible spectrum; thus the 
colored crystal appears yellow under transmitted light. On the other hand, KCl 
with F-centers appears magenta, and KBr appears blue. See CRYSTAL DEFECTS. 
    Color centers have been under investigation for many years. Theoretical 
studies guided by detailed experimental work have yielded a deep understanding 
of specific centers. The crystals in which color centers appear tend to be 
transparent to light and to microwaves. Consequently, experiments which can be 
carried out include optical spectroscopy, luminescence and Raman scattering, 
magnetic circular dichroism, magnetic resonance, and electromodulation. Color 
centers find practical application in radiation dosimeters; schemes have been 
proposed to use color centers in high-density memory devices; and tunable 
lasers have been made from crystals containing color centers. 
    The illustration shows the absorption bands due to color centers produced 
in potassium bromide by exposure of the crystal at the temperature of liquid 
nitrogen (81 K) to intense penetrating x-rays. Several prominent bands appear 
as a result of the irradiation. The F-band appears at 600 nanometers and the 
socalled V-bands appear in the ultraviolet. 
    Color bands such as the F-band and the V-band arise because of light 
absorption at defects dispersed throughout the lattice. This absorption is 
caused by electronic transitions at the centers. On the other hand, colloidal 
particles, each consisting of many atoms, dispersed through an optical medium 
also produce color bands. In this case, if the particles are large enough, the 
extinction of light is due to both light scattering and light absorption. 
Colloidal gold is responsible for the color of some types of ruby glass. 
Colloids may also form in alkali halide crystals - for example, during heat 
treatment of an additively colored crystal which contains an excess of alkali 
metal.  
    Atomically dispersed centers such as F-centers are part of the general 
phenomena of trapped electrons and holes in solids. The accepted model of the 
F-center is an electron trapped at a negative ion vacancy. Many other 
combinations of electrons, holes, and clusters of lattice vacancies have been 
used to explain the various absorption bands observed in ionic crystals. 
    Impurities can play an important role in color-center phenomena. Certain 
impurities in ionic crystals produce color bands characteristic of the foreign 
ion. For example, hydrogen can be incorporated into the alkali halides with 
resultant appearance of an absorption band (the U-band) in the ultraviolet. In 
this case, the U-centers interact with other defects. The rate at which 
F-centers are produced by x-irradiation is greatly increased by the 
incorporation of hydrogen, the U-centers being converted into F-centers with 
high efficiency. 
 �34s[Frederick C. Brown]
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�6v�2v�32sColor filter   �21m �33s 
    An optical element that partially absorbs incident light, often called a 
light filter. The absorption may be selective with respect to wavelength, in 
which case the color filter purifies the incident light in the sense of making 
it more nearly like that of a part of the pure spectrum. On the other hand, 
the absorption may be nonselective, in which case the color filter merely 
reduces the light flux without change in spectral composition. Color filters 
are used in photography, optical instruments, and illuminating devices to 
control the amount and spectral composition of light. 
    Color filters are made of glass for maximum permanence, of liquid 
solutions confined to cells with transparent faces for maximum flexibility of 
control, and of dyed gelatin or plastic for maximum convenience combined with 
excellent flexibility and often satisfactory permanence. Plastic color filters 
are supplied cemented between glass sheets for greater toughness than glass 
and greater scratch resistance than bare plastic. See ABSORPTION OF 
ELECTROMAGNETIC RADIATION; COLOR. 
 �34s[Deane B. Judd]
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�6v�2v�32sColorimetry �21m �33s
    Any technique by which an unknown color is evaluated in terms of known
colors. Colorimetry may be visual, photoelectric, or indirect by means of
spectrophotometry. These techniques are widely used in scientific studies
involving the appearance of objects and lights, but are of greatest importance
in the color specification of the raw materials and finished products of
industry.
    In visual colorimetry, the unknown color is presented beside a comparison
field into which may be introduced any one of a range of known colors from
which the operator chooses the one matching the unknown. To be generally
applicable, the comparison field must not only cover a sufficient color range
but must also be continuously adjustable in color.
    In indirect colorimetry, the light leaving the unknown specimen is split
into its component spectral parts by means of a prism or diffraction grating,
and the amount of each component part is separately measured by a photometer.
The quantity evaluated is spectral radiance of a light source, spectral
transmittance of a filter (glass, plastic, gelatin, or liquid), or spectral
reflectance of an opaque body. See SPECTROPHOTOMETRIC ANALYSIS.
    In photoelectric colorimetry, the light leaving the specimen is measured
separately by three photocells. The spectral sensitivity of these photocells
is adjusted, usually by color filters, to conform as closely as possible to
the three color-mixture functions for the average normal human eye (CIE
standard observer). The responses of the photocells give directly the amounts
of red, green, and blue primaries required to produce the color of the unknown
specimen for the kind of vision represented by the three photocells.
    If two objects have the same color because the light leaving one of them
toward the eye is spectrally identical to that leaving the other, any type of
colorimetry serves reliably to establish the fact of color match. If, however,
the two lights are spectrally dissimilar, they may still color-match for any
one observer; such pairs of lights are called metamers. Normal color vision
differs sufficiently from person to person so that a metameric color match for
one observer may be seriously mismatched for another. On this account, the
question of color match of spectrally dissimilar lights can be reliably
settled only by the indirect method which uses spectrophotometry combined with
a precisely defined standard observer. See COLOR; COLOR FILTER; COLOR VISION.
 �34s[Deane B. Judd /Jack L. Lambert]
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�6v�2v�32sColor photography   �21m �33s 
    The photographic reproduction of natural colors in terms of two or more 
spectral components of white light. Red, green, and blue are called additive 
primary colors because they add to form white light. Their complements are the 
subtractive primary colors, cyan, magenta, and yellow. Subtractive color 
processes produce color images in terms of cyan, magenta, and yellow dyes or 
pigments.  See  COLOR. 
  
    Additive color photography.  The screen plate system of additive 
photography uses a fine array of red, green, and blue filters in contact with 
a single panchromatic emulsion. Reversal processing produces a positive color 
transparency comprising an array of positive black-and-white image elements in 
register with the filter elements. 
  
    Subtractive color photography.  Many types of subtractive color negatives, 
color prints, and positive transparencies are made from multilayer films with 
separate red-, green-, and blue-sensitive emulsion layers. 
    In chromogenic or color development processes, dye images are formed by 
reactions between the oxidation product of a developing agent and color 
couplers contained either in the developing solution or within layers of the 
film. For positive color transparencies, exposed silver halide grains undergo 
nonchromogenic development, and the remaining silver halide is later developed 
with chromogenic developers to form positive dye images. Residual silver is 
removed by bleaching and fixing. 
    Kodachrome was the first commercially successful subtractive color 
transparency film. The Kodachrome process forms reversal dye images in 
successive color development steps, using soluble couplers. Following 
black-and-white development of exposed grains, the remaining red-sensitive 
silver halide is exposed to red light, then developed to form the positive 
cyan image. Next, the blue-sensitive layer is exposed to blue light and the 
yellow image developed, and finally the green-sensitive emulsion is fogged and 
the magenta image developed. 
    Simpler reversal processing is afforded by films with incorporated 
nondiffusing couplers that produce the three dye images in a single color 
development step. 
  
    One-step color photography.  The most dramatic change in the history of 
color photography was the transition to the single-structure, single-step 
Polacolor film and Polacolor process from the separate multiple-layer negative 
and positive and the multiple-step darkroom processing of each of these 
materials. In the Polarcolor process and later one-step, or instant, 
processes, color prints are produced under ambient conditions within moments 
after exposure of the film. The processing takes place rapidly, using reagent 
provided as part of the film unit.  See  PHOTOGRAPHY. 
 �34s[Vivian K. Walworth]
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�6v�2v�32sColor television   �21m �33s 
    Transmission and reception of transient visual images in full color. The 
technique used for color television broadcasting in the United States is 
compatible with monochrome, or black-and-white, television. That is, color 
television signals are sufficiently similar to monochrome signals so that they 
produce acceptable monochrome pictures on black-and-white receivers while 
producing full-color pictures on color receivers. Conversely, a standard 
monochrome signal produces a satisfactory monochrome image on a color 
receiver. Compatible color television is based on a combination of the 
primary-color principle, conventional television technology, and special 
encoding techniques for combining primary-color television signals in such a 
way that they can be transmitted through a single broadcast channel. 
    Color television images are formed by combining three primary-color 
images. The standard set which has proved to be most practical for color 
television consists of highly saturated red, green, and blue colors. The color 
kinescope, or picture tube, used to display the picture in a color television 
receiver can produce independent red, green, and blue images, usually in the 
form of tiny dots so closely intermingled that they cannot be resolved 
individually at the normal viewing distance. The three images are thus 
effectively fused by the visual system of the observer so that he sees only 
the combined, full-color image. See COLOR; COLOR VISION; PICTURE TUBE. 
    In the simplest type of color television system the three primary-color 
images in the tricolor picture tube are controlled directly by video signals 
developed by three separate pickup tubes, adjusted to respond to red, green, 
and blue light (see illustration). Close registration must be maintained among 
the three images so that the scanning beams in the three pickup tubes are 
always at the same relative points in their respective image areas. 
    Because of the great demand for television broadcast channels in the 
United States, it is not practical to utilize three separate channels for 
transmitting the red, green, and blue signals required to control a color 
kinescope. It is necessary, therefore, to encode the primary-color signals at 
the transmitter in such a way that they can be transmitted through a single 
television channel. The encoded color video signal contains three independent 
components, and is enough like a conventional monochrome signal to produce 
good images on monochrome receivers. Each color receiver contains decoder 
circuits (between the second detector and the kinescope) to retrieve from the 
color signal its red, green, and blue components. See TELEVISION; TELEVISION 
RECEIVER.  
 �34s[John W. Wentworth]
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�6v�2v�32sColor vision   �21m �33s 
    The ability to discriminate light on the basis of wavelength composition. 
It is found in humans, in other primates, and in certain species of birds, 
fishes, reptiles, and insects. These animals have visual receptors that 
respond differentially to the various wavelengths of visible light. Each type 
of receptor is especially sensitive to light of a particular wavelength 
composition. Evidence indicates that primates, including humans, possess three 
classes of cone photoreceptors that differ in the photopigments they contain 
and in their neural connections. In humans, two of these, the R and G cones, 
are sensitive to all wavelengths of the visible spectrum from 380 to 700 
nanometers. The B cones, whose sensitivity peaks at about 440 nm, are not 
appreciably excited by wavelengths longer than 540 nm. The perception of 
blueness and yellowness depends upon the level of excitation of B cones in 
relation to that of R and G cones. No two wavelengths of light can produce 
equal excitations in all three kinds of cones. It follows that, provided they 
are sufficiently different to be discriminable, no two wavelengths can give 
rise to identical sensations. See COLOR BLINDNESS; EYE (VERTEBRATE); VISION. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sColor (quantum mechanics)   �21m �33s 
    A term used to describe a hypothetical quantum number carried by the 
quarks which are thought to make up the strongly interacting elementary 
particles. It has nothing to do with the ordinary, visual use of the word 
color.  
    The quarks which are thought to make up the strongly interacting particles 
have a spin angular momentum of one-half unit of : (Planck's constant). 
According to a fundamental theorem of relativity combined with quantum 
mechanics, they must therefore obey Fermi-Dirac statistics and be subject to 
the Pauli exclusion principle. No two quarks within a particular system can 
have exactly the same quantum numbers. See EXCLUSION PRINCIPLE; FERMI-DIRAC 
STATISTICS.  
    However, in making up a baryon, it often seemed necessary to violate this 
principle. The OMEGA--particle, for example, is made of three strange quarks, 
and all three had to be in exactly the same state. O. W. Greenberg is 
responsible for the essential idea for the solution to this paradox. In 1964 
he suggested that each quark type (u, d, and s) comes in three varieties 
identical in all measurable qualities but different in an additional property, 
which has come to be known as color. The exclusion principle could then be 
satisfied and quarks could remain fermions, because the quarks in the baryon 
would not all have the same quantum numbers. They would differ in color even 
if they were the same in all other respects. See BARYON; ELEMENTARY PARTICLE; 
MESON; QUARKS. 
 �34s[Thomas Appelquist]
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�6v�2v�32sColumbiformes   �21m �33s 
    An order of birds usually considered to contain 3 families, of which the 
best known is that of the pigeons and doves (Columbidae), containing almost 
300 species. Pigeons are virtually cosmopolitan but are most abundant in 
tropical regions. Most are in shades of gray and brown, but certain tropical 
Old World species, notably among the fruit pigeons (subfamily Treroninae), are 
gaudy. Most pigeons lay white-shelled eggs, but a few produce lightly 
pigmented shells. The newly hatched young are at first fed a regurgitated 
glandular secretion of the crop called pigeon milk. Pigeons are one of the few 
groups of birds that can drink water by sucking instead of having to tilt the 
head back after each mouthful. 
    Humans have long been closely associated with members of this family. 
Almost all of the species are pursued as game, and two species have been 
domesticated, the rock dove or common pigeon (Columba livia) and the ring dove 
(Streptopelia risoria), both of Old World origin. The North American passenger 
pigeon (Ectopistes migratorius) was once perhaps the most abundant bird on the 
continent, but was extirpated by 1914. See AVES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sColumbite �21m �33s
    A mineral with composition (Fe,Mn)Nb2O6. Tantalum substitutes for niobium
in all proportions, and a complete series extends to tantalite (Fe,Mn)Ta2O6.
Columbite without some tantalum is rare; iron and manganese also vary
considerably in their relative amounts. It crystallizes in short prismatic
crystals. The hardness is 6 (Mohs scale), and the specific gravity is 5.2 for
pure columbite; both increase with increased amounts of tantalum. The luster
is submetallic and the color iron-black.
    Columbite is the chief ore mineral of niobium columbium and is found in
small amounts in many pegmatites throughout the world. See NIOBIUM; TANTALITE;
TANTALUM.
 �34s[Cornelius S. Hurbut Jr.]
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�6v�2v�32sColumn   �21m �33s 
    A slender compression member which fails by instability when the applied 
axial load reaches a critical value. Instability or buckling is said to occur 
when the system cannot attain an equilibrium condition and the load causes a 
deformation of an indeterminate amount. The collapse load is a function of the 
end restraints, the ratio of length to the least radius of gyration (L/r, the 
slenderness ratio), and the combined action of axial and lateral loads applied 
to the member. Columns are present in all types of structures, such as 
bridges, buildings machinery, cranes, and airplanes. Different materials may 
be used in the various structures, such as steel, concrete, wood, aluminum, 
and alloy steels of all types. 
    Column loads and moments are transferred by baseplates to footings and 
foundations. The plate must be large enough to distribute the load without 
excessive bearing stresses. The column is connected to the plate by welded or 
bolted connections. An approximate design procedure assumes uniform pressure 
and cantilever action of the plate projecting beyond the column boundaries. 
Anchorage is not required for axially loaded columns or when the resultant 
pressure due to moment and load falls within the mid-third of the base width. 
However, bolts are used to position the column. Where lateral forces, such as 
wind pressures, are involved, the resultant moment requires anchor bolts. 
 �34s[John B. Scalzi]
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�6v�2v�32sColumn chromatography   �21m �33s 
    A simple chromatographic procedure in which a sample dissolved in a small 
amount of solvent is placed on the top of a chromatographic column. In the 
course of the elution process, the different materials in the sample separate 
out on the column as discrete bands (see illustration). The chemical nature of 
both the sorbent and the eluting solvent exert primary control over the degree 
to which individual mixture components are retained on the column. Thus, 
nonpolar compounds travel more rapidly with the solvent than more polar 
mixture components do. The rule that ``like dissolves (or attracts) like,'' in 
chemical terms, is quite generally applicable to chromatography. In addition, 
some molecular entities may not be completely separated from each other. In 
order to achieve separation of components that are not resolved, either a 
different mobile phase or a different stationary phase, or both, should be 
tried.  
    The separated compounds can be further isolated in a pure state (if 
desired, for instance, for additional chemical studies) by drying the column 
content and subsequently isolating the individual zones mechanically and 
extracting with an appropriate solvent. Alternatively, the column can be 
excessively flushed with the mobile phase, and the separated zones recovered 
into test tubes inserted under the column. 
    Although both gas chromatography and modern liquid chromatography utilize 
these principles, the term column chromatography is universally reserved for 
the classical system, in which the mobile-phase flow is achieved by means of 
solvent gravity. In more modern systems, generally applicable to both gas and 
liquid chromatography, the sample is introduced into the controlled stream of 
mobile phase at the column top and continuous monitoring of the column 
effluent is achieved by a suitable detector, followed by a signal recording 
or, alternatively, further processing of recorded information. See GAROMA 
TOGRAPHY.  
 �34s[Milos V. Novotny]
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�6v�2v�32sComa (optics)   �21m �33s 
    The popular name for the asymmetric optical aberrations in the image of a 
point. Coma occurs for one of two reasons: (1) The rays from the object point 
form a symmetrical image, but its appearance is unsymmetrical because the 
diaphragm vignettes the rays in an unsymmetrical manner; (2) the rays from the 
object point form an unsymmetrical image in the absence of vignetting. 
    The first kind of asymmetry can be easily corrected by shifting the stop 
(diaphragm) in such a way that the central ray of the imaging bundle goes 
through its center. The stop in this position is sometimes called the natural 
stop. The second kind of asymmetry, however, is intrinsic in the design of the 
system. See ABERRATION (OPTICS). 
 �34s[Max J. Herzberger]
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�6v�2v�32sCombinatorial theory   �21m �33s 
    The branch of mathematics which studies arrangements of elements (usually 
a finite number) into sets under certain prescribed constraints. Problems 
combinatorialists attempt to solve include the enumeration problem (how many 
such arrangements are there?), the structure problem (what are the properties 
of these arrangements and how efficiently can associated calculations be 
made?), and, when the constraints become more subtle, the existence problem 
(is there such an arrangement?). See ENUMERATION OF ARRANGEMENTS; LATIN SQUARE. 
 �34s[Thomas Brylawski]
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�6v�2v�32sCombining volumes, law of   �21m �33s 
    The principle that when gases take part in chemical reactions the volumes 
of the reacting gases and those of the products, if gaseous, are in the ratio 
of small whole numbers, provided that all measurements are made at the same 
temperature and pressure. The law is illustrated by the following reactions: 
    1. One volume of chlorine and one volume of hydrogen combines to give two 
volumes of hydrogen chloride. 
    2. Two volumes of hydrogen and one volume of oxygen combine to give two 
volumes of steam. 
    3. One volume of ammonia and one volume of hydrogen chloride combine to 
give solid ammonium chloride. 
    4. One volume of oxygen when heated with solid carbon gives one volume of 
carbon dioxide. 
    The law of combining volumes is similar to the other gas laws in that it 
is strictly true only for an ideal gas, though most gases obey it closely at 
room temperatures and atmospheric pressure. Under high pressures used in many 
large-scale industrial operations, such as the manufacture of ammonia from 
hydrogen and nitrogen, the law ceases to be even approximately true. See GAS. 
 �34s[Thomas C. Waddington]
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�6v�2v�32sCombustion   �21m �33s 
    The burning of any substance, whether it be gaseous, liquid, or solid. In 
combustion, a fuel is oxidized, evolving heat and often light. The oxidizer 
need not be oxygen per se. The oxygen may be a part of a chemical compound, 
such as nitric acid (HNO3) or ammonium perchlorate (NH4ClO4), and become 
available to burn the fuel during a complex series of chemical steps. The 
oxidizer may even be a non-oxygen-containing material. Fluorine is such a 
substance. It combines with the fuel hydrogen, liberating light and heat. In 
the strictest sense, a single chemical substance can undergo combustion by 
decomposition, with emission of heat and light. Acetylene, ozone, and hydrogen 
peroxide are examples. The products of their decomposition are carbon and 
hydrogen for acetylene, oxygen for ozone, and water and oxygen for hydrogen 
peroxide.  
    The combustion of solids such as coal and wood occurs in stages. First, 
volatile matter is driven out of the solid by thermal decomposition of the 
fuel and burns in the air. At usual combustion temperatures, the burning of 
the hot, solid residue is controlled by the rate at which oxygen of the air 
diffuses to its surface. If the residue is cooled by radiation of heat, 
combustion ceases. 
    Liquid fuels do not burn as liquids but as vapors above the liquid 
surface. The heat evolved evaporates more liquid, and the vapor combines with 
the oxygen of the air. 
 �34s[Bernard Lewis]
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�6v�2v�32sCombustion chamber   �21m �33s 
    The space at the head end of an internal combustion engine cylinder where 
most of the combustion takes place. See COMBUSTION; INTERNAL COMBUSTION ENGINE. 
    In the spark-ignition engine, combustion is initiated in the mixture of 
fuel and air by an electrical discharge. The resulting reaction moves radially 
across the combustion space as a zone of active burning, known as the flame 
front. The velocity of the flame increases nearly in proportion to engine 
speed so that the distance the engine shaft turns during the burning process 
is not seriously affected by changes in speed. Occasionally a high burning 
rate, or too rapid change in burning rate, gives rise to unusual noise and 
vibration called engine roughness. Roughness may be reduced by using less 
squish or by shaping the combustion chamber to control the area of the flame 
front. A short burning time is helpful in eliminating knock because the last 
part of the charge is burned by the flame before it has time to ignite 
spontaneously. See COMBUSTION KNOCK; SPARK PLUG. 
    In compression-ignition (diesel) engines the fuel is injected late in the 
cycle into highly compressed air; mixing must take place quickly if the fuel 
is to find oxygen and burn while the piston remains near top center. Rapid 
mixing is particularly important in the smaller-sized engines, which are 
usually operated at higher rotative speeds. In simple combustion chambers, 
mixing can be improved by producing a general swirl in the cylinder so that 
the air is moving relative to the fuel spray during injection. 
    In some compression-ignition engines a considerable fraction of the 
cylinder air is forced into a small auxiliary chamber (prechamber) during the 
compression stroke. The fuel is injected into the swirl chamber where rapid 
mixing and combustion take place, and the burning mixture issues into the main 
chamber where any remaining fuel is burned. 
    In compression-ignition engines the injected fuel ignites from contact 
with the hot cylinder air after a short delay. There is no flame front travel 
to limit the combustion rate, and if mixing of fuel and air is too thorough by 
the end of the delay period, high rates of pressure rise result, and the 
operation of the engine is rough and noisy. To avoid this condition, the 
prechambers of most compression-ignition engines operate at high temperature 
so that the fuel ignites soon after injection begins. This reduces the amount 
of fuel present and the degree of mixing at the time ignition takes place. See 
DIESEL ENGINE. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sCombustion knock   �21m �33s 
    In spark-ignition engines, the sound and other effects associated with 
ignition and rapid combustion of the last part of the charge to burn, before 
the flame front reaches it. See COMBUSTION CHAMBER. 
    As the flame travels from the spark plug toward the far end of the 
combustion space, the gas behind the flame front, heated by combustion, 
expands and rapidly compresses the unburned mixture ahead of the flame. In 
this way the temperature of the unburned mixture is raised above its 
self-ignition temperature. Time-consuming chemical reactions must take place 
in this gas, however, before actual ignition takes place. If the flame front 
travels rapidly enough to consume the unburned mixture before these reactions 
are completed, normal combustion without knock takes place. If the reactions 
proceed two rapidly or if the flame front travels too slowly, the entire last 
part of the charge may burn almost instantaneously, producing a strong 
pressure wave, which is reflected back and forth across the combustion space 
at the speed of sound. The motion of the cylinder head and walls, under the 
action of this shock wave, causes the high-pitched sound known as combustion 
knock.  
    The results of combustion knock or engine detonation include undesirable 
noise and overheating or mechanical damage to engine parts such as pistons and 
spark plugs. Most of the power loss commonly attributed to combustion knock is 
probably an indirect effect, the result of overheated sparkplug electrodes, 
which ignite the mixture far in advance of the normal ignition spark. 
    A fuel of high octane number resists combustion knock principally because 
it has a longer self-ignition delay than other fuels under a given set of 
operating conditions. High-octane gasoline burns in the same manner and with 
the same flame velocity as low-octane gasoline. Combustion knock can thus be 
eliminated by using high-octane fuel, without loss of power or efficiency. The 
long delay of high-octane fuel results either from its natural chemical makeup 
or from the addition of an antiknock material. One of the most effective 
antiknock additives is tetraethyllead, Pb(C2H5)4, which is added to almost all 
gasoline sold for automobile or aircraft use. See ANTIKNOCK AGENTS; OCTANE 
NUMBER; TETRAETHYLLEAD. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sComet �21m �33s
    One of the major types of objects that move in closed orbits around the
Sun. Compared to the orbits of planets and asteroids, comet orbits are more
eccentric and have a much greater range of inclinations to the ecliptic (the
plane of the Earth's orbit). Physically, a comet is a small, solid body which
is approximately a kilometer in diameter, contains a high fraction of icy
substances, and shows a complex morphology, often including the production of
a tail, as it approaches the Sun. See ASTEROID; PLANET.
    About 10 comets are discovered or rediscovered each year. On the average,
one per year is a bright comet, visible to the unaided eye and generating much
interest among the public as well as among comet workers.
    Astronomers consider comets to be worthy of detailed study for several
reasons: (1) They are intrinsically interesting, involving a large range of
physical and chemical processes. (2) They are valuable tools for probing the
solar wind. (3) They are considered to be remnants of the solar system's
original material and, hence, prime objects to be studied for clues about the
nature of the solar system in the distant past.
    Comets are nebulous in appearance, and the tail is usually the most
visually striking feature. This tail can in some cases stretch along a
substantial arc in the sky. An example is given in the illustration, which
shows Comet West with tails some 30DG in length. Some fainter comets, however,
have little or no tail. The coma or head of a comet is seen as the ball of
light from which the tail or tails emanate. Within the coma, and not yet seen,
is the nucleus, the origin of the material in the tail and coma.

    The main components of a comet are the nucleus, the coma, the hydrogen
cloud, and the tails. Strong indirect evidence points to the existence of a
central nuclear body or nucleus from which all cometary material, both gas and
dust, originates. Unfortunately, no photograph of a nucleus exists. The coma
is observed as an essentially spherical cloud of gas and dust surrounding the
nucleus. The principal gaseous constituents are neutral molecules. As the gas
flows away from the nucleus, the dust particles are dragged along. In 1970,
observations of Comet Tago-Sato-Kosaka and Comet Bennett from orbiting
spacecraft showed that these comets were surrounded by a giant hydrogen cloud
extending to 6 x 10 sup 6 mi (10 sup 7 km), or a size larger than the Sun.
Photographs of bright comets generally show two distinct types of tails (see
illlustration): the dust tails and the plasma tails. They can exist separately
or together in the same comet.
    The plasma tails are generally straight and the plasma in these tails is
composed of electrons and molecular ions. The dust tails are usually curved
and the dust particles are probably silicate in composition.
    While not definitely proved, it seems likely that the population of
short-period comets can be produced from the population of the long-period
comets by their gravitational interaction with Jupiter. The evidence from
statistics of cometary orbits indicates an essentially spherical cloud of
comets with dimensions in the range 10 sup 1 sup 2 to 10 sup 1 sup 3 mi (10 sup
4 to 10 sup 5 AU). Gravitational perturbations from passing stars have several
effects on the cloud. They limit its size and tend to make the orbits random
(as observed). Most importantly, the perturbations continually send new comets
from the cloud into the inner solar system, where they are observed. The
current consensus on the origin of comets holds that they condensed from the
solar nebula at the same time as the formation of the Sun and planets. In
other words, although the details are sketchy, comets are probably a natural
by-product of the solar system's origin. They may be remnants of the formation
process, and their material may be little altered from the era of condensation
to the present time. See SOLAR SYSTEM.
 �34s[John C. Brandt]
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�6v�2v�32sComfort heating �21m �33s
    The maintenance of the temperature in a closed volume, such as a home,
office, or factory, at a comfortable level during periods of low outside
temperature. Two principal factors determine the amount of heat required to
maintain a comfortable inside temperature: the difference between inside and
outside temperatures and the ease with which heat can flow out through the
enclosure.
    The first step in planning a heating system is to estimate the heating
requirements. This involves calculating heat loss from the space, which in
turn depends upon the difference between outside and inside space temperatures
and upon the heat transfer coefficients of the surrounding structural members.
Outside and inside design temperatures are first selected. Ideally, a heating
system should maintain the desired inside temperature under the most severe
weather conditions. The design temperature selected depends upon the heat
capacity of the structure, amount of insulation, wind exposure, proportion of
heat loss due to infiltration or ventilation, nature and time of occupancy or
use of the space, difference between daily maximum and minimum temperatures,
and other factors. Usually the outside design temperature used is the median
of extreme temperatures. The selected inside design temperature depends upon
the use and occupancy of the space. Generally it is between 66 and 75DGF (19
and 24DGC).
    In localities where outdoor temperatures are often below 36DGF (2DGC), it
is advisable to provide means for adding moisture in heated spaces to improve
comfort. The colder the outside air is, the less moisture it can hold. When it
is heated to room temperature, the relative humidity in the space becomes low
enough to dry out nasal membranes, furniture, and other hygroscopic materials.
This results in discomfort as well as deterioration of physical products.
Various types of humidifiers are available. The most satisfactory type
provides for the evaporation of the water to take place on a mold-resistant
treated material which can be easily washed to get rid of the resultant
deposits.
    Good insulating material has air cells or several reflective surfaces. A
good vapor barrier should be used with or in addition to insulation, or
serious trouble may result. Air inside a space in which moisture has been
added from cooking, washing, drying, or humidifying has a much higher vapor
pressure than cold outdoor air. Therefore, moisture in vapor form passes from
the high vapor pressure space to the lower pressure space and will readily
pass through most building materials. When this moisture reaches a subfreezing
temperature in the structure, it may condense and freeze. When the structure
is later warmed, this moisture will thaw and soak the building material, which
may be harmful. Good vapor barriers include asphalt-impregnated paper, metal
foil, and some plastic-coated papers. Heat loss due to infiltration is the
most difficult item to estimate accurately and depends upon how well the house
is built. If a masonry or brick-veneer house is not well caulked or if the
windows are not tightly fitted and weather-stripped, this loss can be quite
large. See AIR REGISTER; CENTRAL HEATING AND COOLING; HOT WATER HEATING
SYSTEM; RADIATOR; STEAM HEATING; WARM AIR HEATING SYSTEM.
 �34s[Gayle B. Priester]
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�6v�2v�32sCommelinales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), which 
is included in the subclass Commelinidae in the class Liliopsida 
(monocotyledons). It consists of 4 families and nearly 1000 species, the bulk 
of which is in the families Commelinaceae (about 600 species) and Xyridaceae 
(about 270 species). The order is marked by its flowers that ordinarily have 
the perianth well differentiated into sepals and petals but do not have 
nectaries or nectar. The wandering Jew (species of Tradescantia and Zebrina in 
the Commelinaceae) is a familiar houseplant belonging to this order. See 
COMMELINIDAE.  
 �34s[Arthur Cronquist]
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�6v�2v�32sCommelinidae   �21m �33s 
    A subclass of the class Liliopsida (monocotyledons) of the division 
Magnoliophyta (Angiospermae), the flowering plants, consisting of 8 orders, 25 
families, and nearly 19,000 species. The orders include Commelinales, 
Eriocaulales, Restionales, Juncales, Cyperales, Typhales, Bromeliales, and 
Zingiberales. For further information see separate articles on each order. 
    These monocotyledons are syncarpous, (the carpels are united in a compound 
ovary) or pseudomonomerous (reduced to a single carpel from a syncarpous 
ancestry). The endosperm is usually starchy, and the perianth is either well 
differentiated into sepals and petals or more or less reduced and not 
petallike. Many of the families have well-developed vessels in all vegetative 
organs. Several of the orders of Commelinidae have often been treated as a 
single order Farinosae or Farinales. See LILIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sCommon cold   �21m �33s 
    A viral disease of humans, most frequently caused by the rhinoviruses. 
However, other viruses (including parainfluenza, influenza, and respiratory 
syncytial viruses and members of the adenovirus, enterovirus, and reovirus 
groups) also may cause common cold symptoms. See ANIMAL VIRUS. 
    The virus enters via the nose or mouth. After an incubation period of 2 - 
5 days, typical cold symptoms and histopathology of mucous membranes develop, 
and continue for 4 - 6 days. Experiments under controlled conditions have not 
shown that chilling, including the wearing of wet socks, produces the cold 
itself or increases susceptibility to the virus. Chilliness is an early 
symptom of the common cold. See ADENOVIRUS; ENTEROVIRUS; PARAINFLUENZA VIRUS; 
RHINOVIRUS.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sCommunication cables   �21m �33s 
    An assembly of from 6 to 4200 twisted wire pairs enclosed in a composite 
metal-plastic sheath. The major use of communications cables is in the 
telephone industry, to connect customers to their local switching centers 
(subscriber loops) and to interconnect local and long-distance switching 
centers (trunk plant). For telephone links, a pair of wires in an aerial or 
belowground cable transmits signaling and voice information between the 
subscriber and the switching center. The instrument most commonly connected to 
a cable is the telephone set, but teletypewriters, television cameras, and 
data sets all convert information into electrical signals that can be sent 
over cables. 
    Interference between circuits in multipair cables becomes more likely as 
larger bandwidths are transmitted. Very-high-frequency signals, such as 
television, high-speed data, or broadband carrier signals, are frequently 
transmitted over coaxial cables in which one conductor of the pair is a metal 
tube concentric with and enclosing the other. As higher frequencies are 
transmitted, the outer conductor becomes more effective in containing radiated 
electromagnetic energy and preventing one circuit in the cable from 
interfering with others. However, multipair cable has several advantages when 
compared with coaxial: a lower loss per circuit cross section; mechanical 
ruggedness; and rejection of interference from stray power currents. For these 
reasons, the twisted, balanced pair is the choice for transmission from 
telegraph frequencies up through a few megahertz. See COAXIAL CABLE; 
CROSSTALK; CONDUCTOR (ELECTRICITY); ELECTRICAL CONDUCTIVITY OF METALS. 
    Copper is the most commonly used electrical conductor, but aluminum is 
widely used in England and is becoming more prevalent in the United States. 
Metallic conductors are commonly insulated with paper pulp or plastics, 
although paper strip, helically or longitudinally wrapped around the wire, 
which was introduced in 1890, is still used. Sheaths protect the transmission 
properties of the cable pairs by providing mechanical protection, electrical 
protection from lightning-induced currents, and shielding. In paper-insulated 
cables sheaths also function to keep out moisture. See TELEPHONE SERVICE; 
TELEPHONE SYSTEMS CONSTRUCTION. 
    Communications cables or telephone cables are divided into four general 
classes. Local and exchange area cable is placed aerially or underground in 
ducts, or is directly buried. It is used to connect one switching office to 
another or to connect the switching office and the subscriber. It has pulp 
paper, plastic, or expanded plastic insulation, with either copper or aluminum 
conductors. Cables filled with petroleum jelly, that is, waterproofed, are 
rapidly supplanting unfilled cables in buried subscriber applications. 
    Toll cable is used to connect metropolitan areas and to bring toll 
circuits through cities to toll switching offices. It is generally designed to 
have superior transmission characteristics compared to exchange area designs. 
Frequently, this cable carries many voice or data channels multiplexed onto 
high-frequency carrier systems. Older versions still being manufactured have 
helically applied paper insulation. Newer versions have expanded plastic 
insulation. Sheaths are lead, lead-plastic composites, or special multilayer 
metal-plastic composites. Heavy-duty sheaths and elaborate gas-pressurization 
schemes are used to protect and monitor the integrity of the cable sheaths 
because of the large number of circuits. 
    Switchboard cable is intended to be used as equipment cabling in switching 
offices, business premises, and office buildings. Polyvinyl chloride (PVC) 
insulation has largely supplanted older insulations. 
    Examples of special cables are coaxial cables used for high-frequency 
carrier systems on heavy toll routes, video pairs for baseband video 
transmission, stub cables used to connect apparatus such as loading coils and 
repeater cases to main cables, terminating cables from the central office 
cable vault to the equipment, and building cables used to distribute telephone 
service within commercial buildings. Also included in this group are composite 
cables having various combinations of coaxials, video pairs, and regular 
pairs. See ELECTRIC DISTRIBUTION SYSTEMS; ELECTRIC POWER SYSTEMS. 
 �34s[John R. Apen]
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�6v�2v�32sCommunications satellite   �21m �33s 
    A spacecraft placed in orbit around the Earth to intercept and retransmit 
radio signals. The communications satellite may also amplify, sort, or route 
these signals. The communications function is similar to a ground microwave 
repeater facility but with greatly increased coverage. Whereas a ground 
repeater relays communications signals between two fixed locations, a 
communications satellite interconnects many locations, both fixed and mobile 
over a wide area. See MICROWAVE; SATELLITE (SPACECRAFT). 
    There are two types of communications satellites of practical interest: 
passive and active. There is no amplification capability in a passive 
satellite; it simply reflects incident signals. Passive communications 
satellite systems fall into two types: One uses large discrete structures, 
such as the Echo satellite; the other uses the West Ford approach, in which a 
large volume of space is filled with a large number of tiny passive 
satellites. The general disadvantage of passive systems is the limitation on 
message capacity resulting from the extremely low level of reradiated energy. 
    In all active satellite systems complete electronic equipment for 
processing and transmission of signals must be carried. Also, means must be 
provided for directing antennas and maintaining desired satellite orientation 
and orbital position. With on-board electronic processing capability, an 
active satellite can amplify, modify, and retransmit the signals as 
appropriate. Active satellites can also be considered as two basic types: the 
store-and-forward design and the real-time repeater. In the store-and-forward 
type an Earth station transmits a message to a satellite passing overhead. The 
message is stored in an on-board recorder and later played back through the 
satellite transmitter when the satellite passes over the appropriate Earth 
receiving stations. The only communications satellite now considered for 
practical use is the real-time repeater. Operation of this instantaneous 
repeater requires the transmitting and receiving stations to be simultaneously 
in view of the spacecraft. 
502 
    The first satellite with the primary purpose of communications was Score, 
launched December 18, 1958. It received messages from Earth stations in the 
United States and recorded them on tape for later transmission when the 
satellite was over the receiving stations. Echo 1, launched August 12, 1960, 
was the first artificial satellite whose primary purpose was to provide 
real-time relay of messages. It was a metallic-coated plastic balloon 100 ft 
(30 m) in diameter which could only reflect the signals directed toward it. 
The American Telephone and Telegraph Company developed and built Telstar, 
which was launched by NASA on July 10, 1962. It was the first active 
communications satellite with a real-time repeater. Telstar provided the first 
live television between the United States, Europe, Japan, and South America. 
On July 26, 1963, NASA launched the first successful Syncom into a synchronous 
orbit. This satellite proved that the synchronous equatorial orbit was 
achievable and that a satellite in this orbit was suitable for relaying 
telecommunication signals for uninterrupted service. 
    To exploit commercial utilization of communications satellites for 
international communications, an international consortium was formed in August 
1964. By 1979 more than 100 nations were members of this consortium 
(Intelsat). Early Bird, or Intelsat 1, was launched from Cape Kennedy (Cape 
Canaveral) on April 6, 1965. This satellite with its 240 two-way voice 
channels was designed to relay all types of communications traffic, including 
television between North America and Europe. Three successors to Early Bird 
were launched in 1967 to expand the commercial communications satellite 
system. These satellites, called Intelsat 2, have more powerful signals that 
enable them to operate with Earth stations in both the Northern and Southern 
hemispheres. The Intelsat 2 that extended full-time commercial service to the 
Pacific region was launched January 11, 1967. The development of the Intelsat 
3 satellite began in 1966, and the first was launched December 18, 1968. The 
Intelsat 4, the fourth-generation satellite, was first launched in 1971. The 
Intelsat 5, with a design capacity of 12,000 voice channels plus TV, went into 
service in 1980. 
    The Soviet Union also developed and launched communications satellites for 
its domestic use. The first launch was in April 1965, with inaugural service 
in November 1967. The satellite (Molniya) was placed in a highly elliptical 
orbit to provide maximum visibility over the Soviet Union with 8 - 10 h of 
continuous communication every 24 h. 
    The United States and Canada also have separate domestic systems 
(equatorial orbits) serving their own communication needs, while many 
countries lease satellite capacity from Intelsat to interconnect their Earth 
stations.  
    A separate maritime communications system, Marisat, provides reliable, 
high-quality, full-time communications connecting properly equipped ships and 
offshore equipment such as oil drilling platforms all over the globe with the 
world's telephone systems. 
 �34s[Louis B. Early]
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�6v�2v�32sCommunications systems protection   �21m �33s 
    The protection of wire communication systems equipment and services. This 
includes the electrical protection of lines and station equipment and 
inductive coordination, or the protection against interference from adjacent 
electric power lines and equipment. See ELECTRIC PROTECTIVE DEVICES. 
    Wire communications systems generally require protection against voltages 
and currents from lightning, from accidental energization by power lines and, 
under some conditions, from earth potentials from magnetic storms caused by 
sunspots. The basic methods of protection are: (1) to reduce the likelihood of 
energization through shielding or arranging the lines so they will be less 
subject to lightning strokes or contact with power wires, (2) to use adequate 
insulation so that physical contact may occur with power wires without 
energizing the communication plant, (3) to increase the insulation of the 
wires and the conductivity of cable sheaths to minimize damage from power 
contacts, and (4) to install protective devices that prevent damage to persons 
or equipment. 
    Protective devices for communications systems generally take the form of 
protector blocks and fuses. The protector blocks usually consist of two small 
carbon electrodes arranged to be separated by a few thousandths of an inch. 
These electrodes are paired together in a suitable mounting, so as to connect 
each set of protector blocks between the conductor to be protected and the 
grounded sheath of a cable or a grounding electrode at the equipment location. 
    To protect the user, the premises, and the communications equipment, 
telephone sets require protection when the lines serving them are subject to 
contact with power lines or lightning. The protector unit consists of carbon 
protector blocks connected between each wire and ground (usually the metallic 
water pipe serving the building). Sometimes a fuse on the incoming wire side 
of the protector blocks is also necessary. 
    Inductive coordination applies to measures which reduce the effects of 
potentials and currents induced in the communication plant by paralleling 
power facilities or power lines. When power and telephone lines on separate 
structures are parallel for long distances, a fault current to ground on the 
power circuit creates, in effect, a one-turn transformer primary. The induced 
voltages may be high enough to result in an electric-shock hazard to telephone 
personnel or interference to service on the communication plant. Remedial 
measures to reduce the induction or its effects include: (1) installing 
effectively grounded shield wires on either or both power and telephone 
plants, (2) using grounded metal-sheathed cable on either or both lines, (3) 
moving either line for greater separation to reduce the coupling between them, 
(4) using faster circuit breakers on the power line to reduce the duration of 
the fault, and (5) installing suitable devices in the communication plant to 
reduce the effect of the induced voltages on the services transmitted. See 
LIGHTNING AND SURGE PROTECTION. 
 �34s[J. J. Sheehan]
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�6v�2v�32sCommutation   �21m �33s 
    The process of current reversal in the armature windings of a 
direct-current (dc) electric rotating machine to provide dc at the brushes. 
See COMMUTATOR. 
    The basic process of commutation may be explained with the aid of the 
elementary motor diagram illustrated; N and S indicate the north and south 
magnetic poles; the force on conductor A is fsubA, and on conductor B is 
fsubB. With the rotor position shown at a, the battery will force current into 
conductor B and out of conductor A to produce a clockwise torque and rotation. 
The generated electromotive force (emf) in the conductors is opposite to the 
direction of current. With the rotor position of b, the conductors are not 
cutting lines of flux; no emf is induced in the coil; no tangential forces are 
produced in the coil; and the torque is zero. With the rotor position of c, 
the battery will force current into conductor A and out of conductor B. The 
torque is again clockwise; clockwise rotation will continue, and the generated 
emf will have a polarity opposite to the conductor current. Note that the coil 
current and the coil emf are reversed from a to c. The switching is 
accomplished by the commutator. Practical machines have many coils and many 
commutator segments on the armature. See DIRECT-CURRENT GENERATOR; 
DIRECT-CURRENT MOTOR; ELECTRICAL ROTATING MACHINERY. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sCommutator   �21m �33s 
    That part of a dc motor or generator which serves the dual function, in 
combination with brushes, of providing an electrical connection between the 
rotating armature winding and the stationary terminals, and of permitting the 
reversal of the current in the armature windings. For explanation of the 
necessity of this function see COMMUTATION. 
    A commutator (see illustration) is composed of copper bars assembled to 
form a drumlike cylinder which is concentric with the axis of rotation. 
Insulation, commonly mica, to provide exceptional mechanical and electrical 
stability, is placed between commutator bars and between the bars and the 
shaft. Conducting brushes commonly carbon, sufficient in size and number to 
carry the current, are spaced at intervals of 180 electrical degrees about the 
surface of the commutator and held in contact with the surface of the 
commutator by spring tension. See DIRECT-CURRENT GENERATOR; DIRECT-CURRENT 
MOTOR; EELECTRIC ROTATING MACHINERY. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sComparator   �21m �33s 
    A device used to inspect a gaged part for deviation from a specific 
dimension. The term comparator is also used to identify control-system and 
analog-computer devices which compare two information signals for such 
characteristics as simultaneity, size, direction, or rate of change. See 
ANALOG-TO-DIGITAL CONVERTER; COMPARATOR CIRCUIT; CONTROL SYSTEMS. 
    To check a gaged part, a comparator is usually preset to the basic 
critical dimension to be inspected, for example, to 1.505 in the dimension 
1.505 PLUS/MINUS 0.005 or 2.000 in 2.000 PLUS/MINUS 0.002. A master having 
this dimension is used for presetting the comparator. A comparator may also be 
set by two masters, one for the maximum limit and one for minimum limit. Two 
classes of comparator are in common use: one in which the comparison is made 
visually or optically, the other in which comparison is made by contact. The 
three common types of contact comparators are mechanical, electrical, and 
pneumatic. See INSPECTION AND TESTING. 
    A general-purpose comparator of the contact type usually consists of (1) a 
fixed contact point or surface; (2) an indicating contact or measuring head; 
and (3) a means for physically maintaining the distance between. This last 
means takes the form of a frame in a gage making external measurements, or the 
form of a plug in a gage making internal measurements. The indicator or meter, 
which records the results, may be an integral part of the measuring head or a 
separate unit. A general-purpose comparator can usually be set with gage 
blocks, as well as with a master. Movement in mechanical comparators is 
amplified usually by a rack, pinion, and pointer, or by a parallelogram 
arrangement. Movement in electrical comparators is amplified by several 
methods. The most popular method consists of a floating core in a solenoid 
attached to the contact point. See GAGES. 
    Pneumatic comparators sense the surface of the part by means of an 
airstream. A precision orifice is brought so close to the surface to be 
measured that the flow of air from the orifice is restricted. The closer the 
orifice comes to the surface, the greater the restriction; a measurement of 
this restriction in flow is, in effect, a measurement  of this distance. 
    Visual or optical comparators of the general-purpose type consist of (1) a 
light source, (2) a stage on which to place the part being inspected, (3) a 
series of lenses and mirrors for magnifying and projecting the shadow of the 
object, and (4) a screen on which the image is projected. The screen is 
usually a frosted glass. 
 �34s[Rush A. Bowman]
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�6v�2v�32sComparator circuit   �21m �33s 
    An electronic circuit that produces an output voltage or current whenever 
two input levels simultaneously satisfy predetermined amplitude requirements. 
A comparator circuit may be designed to respond to continuously varying 
(analog) or discrete (digital) signals, and its output may be in the form of 
signaling pulses which occur at the comparison point or in the form of 
discrete direct-current levels. 
    A linear comparator operates on continuous, or nondiscrete, waveforms. 
Most often one voltage, referred to as the reference voltage, is a variable dc 
or level-setting voltage and the other is a time-varying waveform. One common 
application of the comparator is in a linear time-delay circuit. Inputs 
consist of a sawtooth waveform of linearly increasing magnitude (ramp 
function) and a variable dc reference voltage. The reference voltage can be 
calibrated in units of time, as measured from the beginning of the sawtooth. A 
clipper, (usually called a pick-off diode for this application) and a 
coincidence amplifier, together with a resistance-capacitance (RC) 
differentiating circuit, can perform the function of comparator. See TIMEDELAY 
CIRCUITS.  
    Multivibrators can be used in several ways directly as comparators without 
need for the pick-off diodes; such comparators sense the required coincidence 
accurately and introduce little additional delay. These are called 
regenerative comparators. A simple type is the direct-coupled bistable 
circuit, sometimes known as the Schmitt circuit. 
    High-gain dc operational amplifiers (``op-amps'') operated in the 
nonfeedback mode are often used to perform the comparator function, and many 
such amplifiers are classified as comparators because they are specifically 
designed to meet the needs for accurate voltage comparison applications. Such 
``op-amps'' have two inputs, the output being inverting with respect to one 
and noninverting with respect to the other. 
    The term ``digital comparator'' has historically been used when the 
comparator circuit is specifically designed to respond to a combination of 
discrete level (digital) signals, for example, when one or more such input 
signals simultaneously reach the reference level which causes the change of 
state of the output. Among other applications, such comparators perform the 
function of the logic gate such as the AND, OR, NOR, and NAND functions. More 
often, however, the term ``digital comparator'' is used to describe an array 
of logic gates designed specifically to determine whether one binary number is 
less than or greater than another binary number. Such digital comparators are 
sometimes called magnitude comparators or binary comparators. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sComplement   �21m �33s 
    The complement system comprises a group of proteins in the blood serum and 
other body fluids that play an important role as mediators of immune and 
allergic reactions. When activated by antigen-antibody complexes, or by other 
agents, complement kills bacteria, fungi, protozoans, and cells of higher 
organisms by damaging the lipid bilayer of their membranes. Complement also 
destroys certain viruses or interferes with their infectivity. In addition, 
activation of complement generates permeability-enhancing and chemotactic 
factors, as well as ligands, which participate in a variety of immune and 
allergic processes. See COMPLEMENT-FIXATION TEST. 
 �34s[Manfred M. Mayer]
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�6v�2v�32sComplementation (genetics)   �21m �33s 
    The complementary action of different genetic factors. The term usually 
implies two homologous chromosomes or chromosome sets, each defective because 
of mutation and unable by itself to promote the normal development or 
metabolism of the organism, but able to do so jointly when brought together in 
the same cell. See CHROMOSOME; MUTATION. 
    S. Benzer proposed the term cistron for the unit within which mutants do 
not complement each other. The word gene is often used in the same sense. The 
usual biochemical function of a cistron, or gene, is to determine the 
structure of a specific polypeptide component of a protein. Full 
complementation between different genes is the rule except when, as sometimes 
in bacteria, the genes form part of a functionally coordinated complex 
(operon). Allelic mutants (mutants within one gene) show limited 
complementation in some cases, for example, when certain pairs of mutant 
polypeptides correct each other's defects through coaggregation in a complex 
protein. See GENETICS; OPERON. 
 �34s[J. R. S. Fincham]
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�6v�2v�32sComplement-fixation test   �21m �33s 
    A sensitive reaction used in serology for the detection of either antigen 
or antibody, as in the diagnosis of many bacterial, viral, and other diseases, 
including syphilis. It involves two stages: Stage I is the binding or fixation 
of complement if certain antigen-antibody reactions occur, and stage II is 
detection of residual unbound complement, if any, by its hemolytic action on 
the sensitized erythrocytes subsequently added. This is shown in the 
illustration.  
    In the first stage either the antigen or the antibody must be supplied as 
a reagent, with the other of the pair as the test unknown. Fresh guinea pig 
serum is normally used as a complement source. Sheep erythrocytes which are 
coated with their corresponding antibody (amboceptor or hemolysin) are used in 
the second stage. See COMPLEMENT. 
    The controls A, B, and C in the diagram demonstrate that sufficient 
complement is present to effect hemolysis of the sensitized indicator cells 
and that neither antigen or antibody added alone will interfere with this by 
binding complement. In the test system the combination of a suitable antigen 
and antibody in the presence of complement will bind the complement to the 
complex so that the complement becomes unavailable for the hemolysis of the 
indicator cells added in stage II. If either antigen or antibody is added as a 
reagent in stage I, then the presence of the other in the test unknown added 
can be detected through its ability to complete the antigen-antibody system. A 
lack of hemolysis denotes that the complement is bound. See IMMUNOLOGY; LYTIC 
REACTION.  
 �34s[Henry P. Treffers]
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�6v�2v�32sComplex compounds   �21m �33s 
    A group of chemical compounds in which a part of the molecular bonding is 
of the coordinate covalent type. For a discussion of the nature of the 
coordinate bond, and the stability and reactivity of complex compounds see 
CHELATION: COORDINATION CHEMISTRY. 
    Complex compounds contain a central atom or ion and a group of ions or 
molecules surrounding it. Many simple hydrates, such as MgCl2 X 6H2O, are best 
formulated as [Mg(H2O)6]Cl2 because it is known that the 6 molecules of water 
surround the central magnesium ion. Therefore, [Mg(H2O)6] sup 2 POSITIVE is a 
complex ion, and [Mg(H2O)6]Cl2 is a complex compound. The charge on this 
complex ion is +2, because this is the charge on the magnesium ion and the 
coordinated water molecules are neutral. However, if the coordinated groups 
are charged, then the charge on the complex is represented by the sum of the 
charge on the metal and that of the coordinated ions. See, for example, the 
progression of charges on the platinum(IV) complexes listed below. Thus the 
charge is +4 for [Pt(NH3)6] sup 4 POSITIVE because Pt is +4 and NH3 is neutral. 
But the chart is -2 for [PtCl6] sup 2 NEGATIVE because of the 6 Cl NEGATIVE, 
that is, +4 - 6 = -2. 

[Pt(NH3)6]Cl4  Hexaammineplatinum(IV)chloride 
[Pt(NH3)5Cl]Cl3  Chloropentaammineplatinum(IV) chloride 
[Pt(NH3)4Cl2]Cl2  Dichlorotetraammineplatinum(IV) chloride 
[Pt(NH3)3Cl3]Cl  Trichlorotriammineplatinum(IV) chloride 
[Pt(NH3)2Cl4]  Tetrachlorodiammineplatinum(IV) 
K[Pt(NH3)Cl5]  Potassium pentachloroammineplatinate(IV) 
K2[PtCl6]  Potassium hexachloroplatinate(IV) 

    Metal complexes exhibit various types of isomerism. In many ways, 
inorganic stereochemistry is similar to that observed with organic compounds. 
Geometrical isomers are common among the inert complexes of coordination 
numbers 4 and 6. See STEREOCHEMISTRY. 
    The synthesis of metal complexes containing only one kind of ligand 
generally involves just the reaction of the metal salt in aqueous solution 
with an excess of the ligand reagent, as shown below. 
  
       *** [Ni(H2O)6](NO3)2 + 6NH3 -> [Ni(NH3)6](NO3)2 + 6H2O ***
  
The desired complex salt can then be isolated by removal of water until it 
crystallizes, or by addition of a water-miscible organic solvent to cause it 
to separate. For the inert complexes (those slow to react), prolonged 
treatment at more drastic conditions is often necessary. The preparation of 
geometrical isomers is much more difficult, and in most cases, the approach 
used is rather empirical. Generally, reactions yield a mixture of cis-trans 
products, and these are separated on the basis of their differences in 
solubility. See AMMINE; HYDRATE.
 �34s[Fred Barker]
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�6v�2v�32sComplex numbers   �21m �33s 
    A natural and extremely useful extension of the familiar real numbers. 
They can be introduced formally as follows. Consider the two-dimensional real 
vector space consisting of all ordered pairs (a1,a2) of real numbers. 
Geometrically this space can be identified with the ordinary euclidean plane, 
viewing the real numbers a1,a2 as the coordinates of a point in the plane. 
    The addition of vectors is defined by (a1,a2) + (b1,b2) = (a1 + b1, a2 + 
b2) and is just the usual addition of vectors by the parallelogram law. The 
multiplication of a vector (a1,a2) by a real number c is defined by c(a1,a2) = 
(ca1,ca2), and is just the uniform dilation of the plane by the factor c. It 
may be asked whether it is possible to define a multiplication of one vector 
by another in such a manner that this multiplication is linear and satisfies 
the same formal rules as multiplication of real numbers. There does exist such 
a multiplication, given by the equation below, and it is essentially unique in 
the sense that any multiplication satisfying all the desired properties can be 
reduced to this equation by a suitable choice of coordinates in the plane. The 
plane with the ordinary addition and scalar multiplication of vectors and with 
the vector multiplication given above is the complex number system. 

         *** (a1,a2) MULT (b1,b2) = (a1b1 - a2b2, a1b2 + a2b1) ***  

    The above multiplication law can most easily be interpreted geometrically 
by introducing polar coordinates in the plane and writing (a1,a2) = (r cos 
THETA, r sin THETA), where r is the length of the vector (a1,a2), or the 
modulus of the complex number (a1,a2), defined by r sup 2 = a1 sup 2 + 
a2 sup 2, and THETA is the angle between the vector (a1,a2) and the first 
coordinate axis, or the argument of the complex number (a1,a2), defined by 
tan THETA = a2/a1. Then multiplication of complex numbers amounts to 
multiplying their moduli and adding their arguments (see illustration). See 
PLANE TRIGONOMETRY. 
    Introducing the basis vectors l = (1,0) and i = (0,1), any vector (x,y) 
can be written uniquely as the sum (x,y) = xl + yi. It follows readily from 
the multiplication law tha t l X l = l, l X i = i X l = i, and i X i = -l, so 
that in particular the vector l is the identity element for the multiplication 
of complex numbers. The complex number l can be identified with the ordinary 
real number 1, and reflecting this identification the notation can be 
simplified by writing a complex number z = (x,y) = xl + yi merely as 
z = x + iy. The component x is called the real part of the complex number z, 
and the component y is called the imaginary part. The modulus of the complex 
number z is denoted by |z|. See ANALYTIC FUNCTION. 
 �34s[Robert C. Gunning]
�33s 
  
  

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�6v�2v�32sComposite beam   �21m �33s 
    Composite beams are so called because they provide beam action of two 
materials joined together in such a way as to act as a unit. In civil 
engineering the term is most commonly given to the beam action developed by a 
concrete slab resting on a steel beam, usually an I or a wide flange shape, 
the slab and the beam being made to act together by shear connectors. See 
CONCRETE SLAB. 
    In noncomposite design the structural function of the concrete slab is 
only to span transversely across a series of parallel steel beams. In 
composite design the concrete slab performs its normal function and in 
addition increases to a considerable extent the capacity of the structure to 
resist bending in the direction of the steel beams. The unit stresses in the 
concrete slab are not added algebraically from the dual function the slab 
performs, because the flexural stresses are in mutually perpendicular 
directions. Thus the concrete slab thickness and area of reinforcing steel are 
not necessarily greater for this type of design than for the noncomposite. See 
STRESS AND STRAIN. 
    Shear connectors are devices that provide unified action between the 
concrete slab and the steel beam by resisting the horizontal shear developed 
in beam bending. They take the form of steel projections welded to the top 
flange of the steel beam and around which the concrete slab is poured. The 
connectors perform a necessary function in the development of composite beam 
action, for without them the two elements, the concrete slab and the steel 
beam, would act separately as two beams and the beam action would be analogous 
to two unconnected wooden planks placed one on top of the other and slipping 
on each other when bent under a transverse load. There are many types of shear 
connectors, but three commonly used types are stud bolts, a helical coil of 
steel rod, and short lengths of a standard channel shape. See SHEAR. 
    Composite beams are used both in building and highway bridge construction 
and can be used in any type of construction where it is appropriate to use a 
concrete slab resting on steel beams or stringers. Definite and considerable 
economy is achieved in composite construction when the spans are of sufficient 
length to offset the increased cost of fabricating the shear connectors with 
the saving from the lesser beam size required. 
 �34s[Henry L. Kinnier]
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�6v�2v�32sComposite material   �21m �33s 
    A material that results when two or more materials, each having its own, 
usually different characteristics, are combined in order to provide the 
composite with useful properties for specific applications. Each of the input 
materials should serve a specific function in the composite, which in turn 
should show distinctive new or improved characteristics. Advanced composites 
are a class of structural high-performance, fiber-reinforced plastic or 
metallic materials. 
    The development and use of composite materials goes back to antiquity. 
Combining two or more high- and low-carbon irons or steels by skillfully 
hand-forging them together into a samurai sword with a lightweight, tough 
blade which could keep its edge is a typical example. Modern technology 
applies them to a growing variety of functions from fishing poles to 
lightweight automobiles or aircraft, and from machine tool bits to the thermal 
protection systems for reentry from space. The expanding activity in 
composites is stimulated both by the demands of such applications and by the 
potential embodied in the outstanding properties which have been achieved in 
substances such as high-strength glass filaments or stiff, refractory graphite 
filaments. The challenge with such filaments, for example, is to combine them 
with a suitable supporting, load-transferring, environmental protective 
material such as an epoxy polymer or a graphitizable pitch to produce 
efficient structural or refractory composite materials. 
    Attempts are often made to differentiate composite materials from 
conventional materials on the basis of the heterogeneous or multiphase nature 
of the former, but many materials which are considered conventional by 
generally accepted usage, such as high-strength structural alloys, are also 
multiphase. Such familiar materials as concrete, plywood, or carbon black 
reinforced rubber may or may not be considered to be composites. 
 �34s[Harris M. Burte]
�33s 
    Advanced composites are rapidly emerging as the primary material for use 
in aircraft systems because they provide greater structural efficiency at 
lower weights than equivalent metallic structures. Advanced composites consist 
of basically two distinct materials, fibers and matrix, which are combined in 
a preimpregnated form and subsequently bonded under heat and pressure to 
achieve the properties of high strength and stiffness with low density. On a 
specific basis all the advanced composite materials are much stronger and 
stiffer than steel or aluminum. Other significant advantages of advanced 
composites are their resistance to fatigue and corrosion and their tendency to 
dampen vibrations more rapidly than less stiff materials. 
    The matrix material can be thermosetting resins, such as epoxies, 
polyesters, or polyimides, which usually cure (harden) in the presence of a 
catalyst, heat, or pressure. Or they can be of thermoplastic resins, such as 
polycarbonate or polysulfones that are also cured in the presence of heat and 
pressure but, unlike the thermosetting resins, can be softened and reshaped by 
heat. For metal matrixes the most commonly used materials are aluminum and 
titanium. Of all of these, expoxies are the matrix materials most commonly 
used for advanced composites. High-performance fibers available for such 
applications are boron, graphite, and the Aramid fiber known as Kevlar. See 
EPOXY RESIN; POLYMER; POLYMERIC COMPOSITE. 
    The use of advanced composites in military applications has progressed 
from the basic replacement of metal in selected secondary components to 
include primary structures (wing, fuselage, empennage) that are critical to 
flight safety. As a result, the F-14, F-15, F-16, F/A-18, and the AV8-B 
military aircraft use advanced composites in various degrees, ranging 3 - 20% 
of the structural weight. In addition to military applications, the commercial 
and general aviation industries use advanced composites because the weight 
savings and increased performance result in significant increases in payload 
or decreases in fuel consumption. 
    The technology developed by the aerospace industry provided a tremendous 
baseline and spin-off to nonaerospace applications. Golf shaft producers have 
manufactured graphite and boron shafts for clubs. Manufacturers also 
introduced composite tennis rackets, offering lighter weight at an increased 
stiffness. Other sports applications include fly rods and fishing rods, 
archery equipment, hang gliders, and skis. Substantial work in the medical 
field has led to use of advanced composites in external and implanted 
prosthetic devices, radiological equipment, and lightweight mechanical 
supports and equipment for orthopedic and handicapped patients. The automotive 
industry has initiated evaluations of graphite-reinforced composites as a 
possible means of reducing weight in automobiles. 
 �34s[Donald Stansbarger]
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�6v�2v�32sComposition board   �21m �33s 
    A wood product in which the grain structure of the original wood is 
drastically altered. Composition board may be divided into several types. When 
wood serves as the raw material for chemical processing, the resultant product 
may be insulation board, hardboard, or other pulp product. When the wood is 
broken down only by mechanical means, the resultant product is particle board. 
Because composition board can use waste products of established wood 
industries and because there is a need to find marketable uses for young 
trees, manufacture of composition board is one of the most rapidly developing 
portions of the wood industry. See PAPER AND PAPER PRODUCTS. 
    Fiberboard is produced from wood chips. Synthetic resin may be added as a 
binder before the board is formed. After the board is formed, it may be 
impregnated with drying oils and heated in a kiln until the oils are 
completely polymerized to produce tempered board. If insulating board is 
required instead of hardboard, the material is less compacted, the degree of 
compaction being described by the specific gravity of the finished board. 
    When formed from wood particles that retain their woody structure, the 
product is termed particle board. Properties of such boards depend on the size 
and orientation of the particles, which may be dimensioned flakes, 
random-sized shavings, or splinters. After the particular type of particles 
are produced, they are screened to remove fines and to return oversizes for 
further reduction. Graded particles are dried, mixed with synthetic adhesive 
and other additives such as preservatives, and delivered to the board-forming 
machine.  
    Development of adhesives specifically for composition boards is extending 
their utilitarian value, and variety of textures is increasing their esthetic 
appeal. See WOOD PRODUCTS. 
 �34s[Frank H. Rockett]
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�6v�2v�32sCompressible flow   �21m �33s 
    Flow in which the fluid density varies. In aerodynamic phenomena, when the 
flow velocity is large, it is necessary to consider that the fluid is 
compressible rather than to carry over from classical aerodynamics the 
assumption that the fluid has a constant density. Under this condition the 
speed of sound becomes an important factor. At relatively low speeds the 
changes of temperature and density of a fluid caused by the motion of a body 
in the fluid are almost negligible. However, if the body moves at high speed 
through the fluid, the motion can cause pronounced changes in density and 
temperature of the fluid. Hence, consideration of phenomena of this type 
involves not only classical fluid mechanics but thermodynamics as well. See 
AEROTHERMODYNAMICS.  
    The essential difference between an incompressible fluid and a 
compressible fluid is in the speed of sound. In an incompressible fluid the 
propagation of pressure change is essentially instantaneous; in a compressible 
fluid the propagation takes place with finite velocity. For example, if one 
strikes the surface of an incompressible fluid, the effect observed at great 
distances is, of course, less than at a smaller distance, but it reaches even 
infinite distance in essentially zero time. In a compressible fluid the effect 
propagates at finite velocity. A small disturbance propagates at the velocity 
of sound. 
 �34s[John E. Scott, Jr.]
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�6v�2v�32sCompression ratio   �21m �33s 
    In a cylinder, the piston displacement plus clearance volume, divided by 
the clearance volume. This is the nominal compression ratio determined by 
cylinder geometry alone. In practice, the actual compression ratio is 
appreciably less than the nominal value because the volumetric efficiency of 
an unsupercharged engine is less than 100%, partly because of late intake 
valve closing. In spark ignition engines the allowable compression ratio is 
limited by incipient knock at wide-open throttle. See COMBUSTION CHAMBER; 
INTERNAL COMBUSTION ENGINE; VOLUMETRIC EFFICIENCY. 
 �34s[Neil MacCoull]
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�6v�2v�32sCompressor   �21m �33s 
    Machines that increase the pressure of a gas or vapor, typically air, by 
increasing the gas density and delivering the fluid against the connected 
system resistance. The resistance may be on the suction side, as with vacuum 
pumps, or on the discharge side, as with air compressors. 
    Compressors are typically applied to the operation of pneumatic tools and 
rock drills, conveying systems, furnace blast systems, ventilation systems, 
and the inflation of automobile tires. Gas and vapor compressors are 
represented in the refrigeration, air conditioning, and heat pump fields, 
where they handle a wide variety of refrigerants near the condensation point, 
such as Freon, ammonia, carbon dioxide, and sulfur dioxide. Compressors are 
used in industrial process operations such as nitrogen fixation and gas 
liquefaction. They are used for repressuring and pumping on natural-gas 
transmission lines. 
    The reciprocating compressor is suited to the highest-pressure services. 
Rotary compressors are suitable for direct connection to high-speed drivers 
such as automotive engines and electric motors. 
    Free-compression devices include fans and blowers, of the centrifugal or 
axial-flow type, limited to pressure ratios so small that the change in 
density of the fluid on passage through the unit is negligible. 
    Jet compressors are free of moving parts. They are especially suitable for 
vacuum service when steam is used as the actuating jet for entrainment of the 
noncondensible gas. 
 �34s[T. Balderes]
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�6v�2v�32sCompton effect �21m �33s
    The increase in wavelength of electromagnetic radiation, observed mainly
in the x-ray and gamma-ray region, on being scattered by material objects.
This increase in wavelength is caused by the interaction of the radiation with
the weakly bound electrons in the matter in which the scattering takes place.
The Compton effect illustrates one of the most fundamental interactions
between radiation and matter and displays in a very graphic way the true
quantum nature of electromagnetic radiation. Together with the laws of atomic
spectra, the photoelectric effect, and pair production, the Compton effect has
provided the experimental basis for the quantum theory of electromagnetic
radiation. For information on these and related topics see ANGULAR MOMENTUM;
ATOMIC STRUCTURE AND SPECTRA: ELECTRON-POSITRON PAIR PRODUCTION; LIGHT;
NONRELATIVISTIC QUANTUM THEORY; PHOTOEMISSION: QUANTUM MECHANICS: UNCERTAINTY
PRINCIPLE.
    The Compton effect has played a significant role in several diverse
scientific areas. Compton scattering (often referred to as incoherent
scattering, in contrast to Thomson scattering or also Rayleigh scattering,
which are called coherent scattering) is important in nuclear engineering
(radiation shielding), experimental and theoretical nuclear physics, atomic
physics, plasma physics, x-ray crystallography, elementary particle physics,
and astrophysics, to mention some of these areas. In addition the Compton
effect provides an important research tool in some branches of medicine, in
molecular chemistry and solid-state physics, and in the use of high-energy
electron accelerators and charged-particle storage rings.
    Perhaps the greatest significance of the Compton effect is that it
demonstrates directly and clearly that in addition to its wave nature with
transverse oscillations, electromagnetic radiation has a particle nature and
that these particles, the photons, behave quite like material particles in
collisions with electrons. This discovery by A. H. Compton and P. Debye led to
the formulation of quantum mechanics by W. Heisenberg and E. Schroedinger and
provided the basis for the beginning of the theory of quantum electrodynamics,
the theory of the interactions of electrons with the electromagnetic field.
 �34s[E. Newton Harvey]
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�6v�2v�32sCompton wavelength �21m �33s
    A convenient unit of length that is characteristic of any particle. By
definition, the Compton wavelength LAMBDAsubC of a particle of rest mass m is
LAMBDAsubC = h/mc, where h is Planck's constant and c is the velocity of
light; this definition is analogous to that of the quantum-mechanical de
Broglie wavelength. See DE BROGLIE WAVELENGTH; PLANCK'S CONSTANT.
    The Compton wavelength of the electron is 2.42631 x 10- sup 1 sup 2 m; of
the proton 1.32141 x 10- sup 1 sup 5 m; of the muon 1.1734 x 10- sup 1 sup 4 m;
and of the pion 8.883 x 10- sup 1 sup 5 m. The last is, by definition, the range
of the strong nuclear force; in general, the Compton wavelength provides a
convenient scale length in any given quantum-mechanical situation. See
ELEMENTARY PARTICLE; QUANTUM FIELD THEORY.
    The so-called reduced Compton wavelength, LAMBDAsubC = LAMBDAsubC==/2PI,
is very frequently used instead of the Compton wavelength itself. See QUANTUM
MECHANICS.
 �34s[D. Allan Bromley]
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�6v�2v�32sComputer   �21m �33s 
    A device that receives, processes, and presents information. The two basic 
types of computers are analog and digital. Although generally not regarded as 
such, the most prevalent computer is the simple mechanical analog computer, in 
which gears, levers, ratchets, and pawls perform mathematical operations - for 
example, the speedometer and the watt-hour meter (used to measure accumulated 
electrical usage). The general public has become much more aware of the 
digital computer with the rapid proliferation of the hand-held calculator and 
a large variety of intelligent devices, ranging from typewriters to washing 
machines.  
    An analog computer uses inputs that are proportional to the instantaneous 
value of variable quantities, combines these inputs in a predetermined way, 
and produces outputs that are a continuously varying function of the inputs 
and the processing. These outputs are then displayed or connected to another 
device to cause action, as in the case of a speed governor or other control 
device. Small electronic analog computers are frequently used as components in 
control systems. If the analog computer is built solely for one purpose, it is 
termed a special-purpose electronic analog computer. General-purpose 
electronic analog computers are used by scientists and engineers for analyzing 
dynamic problems. See ANALOG COMPUTER. 
    In contrast, a digital computer uses symbolic representations of its 
variables. The arithmetic unit is constructed to follow the rules of one (or 
more) number systems. Further, the digital computer uses individual discrete 
states to represent the digits of the number system chosen. A digital computer 
works with a symbolic representation of variables; consequently, it can easily 
store and manipulate numbers, letters, or graphical information represented by 
a symbolic code. Through the use of the stored program, the digital computer 
achieves a degree of flexibility unequaled by any other computing or 
data-processing device. 
    The most prevalent special-purpose mechanical digital computers have been 
the supermarket cash register, the office adding machine, and the desk 
calculator. Each of these is being widely replaced by electronic devices 
allowing much greater increased speed. For example, in suitably equipped 
retail stores, the Universal Product Code (UPC) is scanned by a 
light-sensitive device, brining information about each product into the 
point-of-sale (POS) terminal that has replaced the mechanical cash register. 
    Most digital computers are occupied with performing applications related 
to bookkeeping, accounting, engineering design, or test data reduction. On the 
other hand, applications previously considered esoteric are beginning to lead 
to industrial applications, such as the use of robots on manufacturing 
assembly lines. Another area of major societal impact is the growing 
application of digital computers to word processing, including the entire 
concept of office automation. See ARTIFICIAL INTELLIGENCE; DATA-PROCESSING 
SYSTEMS; DIGITAL COMPUTER; ROBOTICS; WORD PROCESSING. 
 �34s[Bernard A. Galler]
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�6v�2v�32sComputer-aided design and manufacturing    �21m �33s 
        Application of the computer in combining design and manufacturing 
functions. Specifically, computer-aided design (CAD) refers to the use of 
computers to perform design calculations for determining an optimum shape and 
size for a variety of applications ranging from mechanical structures and tiny 
integrated circuits to maps of huge areas. Computer-aided manufacturing (CAM) 
employs computers to communicate the work instructions for automatic machinery 
in the handling and processing technology used to produce a workpiece. 
Computer-based automation is drastically changing the way things are made and 
profoundly changing the jobs of people who make them. Among the benefits of an 
integrated CAD/CAM system are increased productivity, a significant reduction 
in nonproductive time, improved product quality, and a payback potential 
obtained by lowering the cost per piece. This modern concept of manufacturing 
management has led to important advances in the design and production of 
components used in aerospace, automotive, electronics, and other industries 
throughout the world. 
 �34s[Herbert W. Yankee]
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�6v�2v�32sComputer graphics   �21m �33s 
    This term refers to the process of pictorial communication between humans 
and computers. The traditional (and still most widely used) means of inputting 
problems to computers and receiving answers has been the alphanumeric form - 
the letters, numbers, punctuation marks, and other symbols which can be 
handled by such input/output devices as punched-card and tape equipment, 
teletypewriters, and line printers. While any problem and its solution can be 
reduced to the alphanumeric form by suitable coding conventions, it is often 
an unnatural and time-consuming way for the person to think and work. 
    This situation has been changed by the introduction of output devices such 
as televisionlike cathode-ray-tube (CRT) displays and automatic mechanical 
plotting boards which operate under computer control and permit the computer 
to create almost any conceivable form of pictorial output. 
    The simplest forms of computer-controlled plotting boards or CRT displays 
are the point-plotting types. In these devices the computer specifies 
numerically the x and y coordinates of a point on the drawing surface or 
viewing screen, which causes an ink dot or spot of light to be made at the 
appropriate location. This mode of operation, while seemingly rudimentary, 
provides a complete drawing capability. Closely spaced dots plotted in a row 
merge into continuous-appearing lines, letters and numbers can be formed from 
arrays of dots similar to those used in football scoreboards, and so forth. 
The main disadvantages of point-plotting devices are that large amounts of 
data are required to produce a picture (20 or more binary digits for each 
point), which places a burden on the computer, and the plotting speed is 
limited.  
    Most display systems now include point, line, and symbol drawing modes. 
The latter two modes produce an entire picture element with a single command 
from the computer, and at a much higher plotting rate than if it were drawn 
with individual points. Line generators operate with specification either of 
both end points or of the length of line to be plotted relative to current 
beam position. In either case all of the intermediate points on the line are 
automatically filled in by the line generator of the display system - by rapid 
point plotting (incremental digital technique) or by continuously sweeping the 
beam (analog technique). Such graphic output devices are called cursive; that 
is, the computer draws narrow lines as if with a pencil, and produces line 
drawings. The ability of such display hardware to produce area-coverage tonal 
pictures is limited by the total length of line that can be drawn at a 
flicker-free rate. Interest in more pictorial output has led to the 
development of CRT computer displays borrowing television technology - raster 
displays, which include color capability. 
    Use of graphical output devices is rapidly increasing, particularly in 
engineering applications, and to many this is what is meant by computer 
graphics. However, graphical output alone is only half the story - graphical 
input is also desired in many situations. 
    Graphical input may be divided into two types: noninteractive and 
interactive. In the first, specialized off-line equipment, such as curve 
tracers and coordinate digitizers (human-guided or automatic), permits a 
sketch or drawing to be converted to the alphanumeric form on punched cards, 
punched tape, or magnetic tape. Such data can be used as input in batch 
computing, but will be subject to a slow error-correction process. 
    The second, or interactive, form of graphical input requires on-line 
access to the computer. In this case hand-held input devices, such as writing 
styli used with electronic tablets and light pens used with on-line CRT 
displays, permit the user to sketch the problem description, say a bridge 
truss to be analyzed, in an on-line interactive mode in which the computer 
acts as a drafting assistant with unusual powers. For example, since the 
computer can analyze each element of a sketch as it is entered and apply 
precoded rules and constraints such as ``all lines shall be either exactly 
vertical or exactly horizontal,'' it can instantly convert a rough freehand 
motion of the light pen into an accurate picture element. The computer can 
also establish connectivity between various picture elements so that the shape 
of the displayed object, say a rectangle, can be altered by simply indicating 
with the light pen that one of the sides should be moved. Since the computer 
knows which lines are connected, it can automatically adjust the lengths of 
the adjoining sides as any one side is moved. 
    The light pen, used for graphical input in conjunction with on-line CRT 
displays, consists of a photodetector mounted in a pen-shaped holder. When the 
pen is held over a displayed picture element, the pen senses the light created 
while the electron beam is writing that particular element, and sends a signal 
to the computer. Since the computer knows which element it is writing at any 
given time, the time of receipt of the signal from the pen immediately 
identifies to the computer which element in the picture the pen is pointing 
at, providing a convenient and natural method for the user to ``discuss'' a 
picture with the computer. Because the light pen is a passive device which can 
respond only to displayed information, it provides no information to the 
computer when held over a blank part of the screen. Thus, in order to draw 
with a light pen, it is necessary to perform a process known as pen tracking. 
This process is fairly effective for input drawing, but suffers from lack of 
precision and use of a substantial amount of computing time to perform the 
tracking function. 
    The effort to find a better input means than the light pen has produced a 
number of new devices, the most popular of which is the electronic tablet. A 
number of different electronic detection principles are used in making 
tablets, but the end result is the same: The circuitry produces a pair of 
digital x-y coordinate values corresponding continuously to the position of a 
writing stylus upon the surface of the tablet. The advantages of the tablet 
are that no computer time is required for operation as in pen tracking (except 
to sample the input from the tablet), and resolution equal to or exceeding 
that of CRT display is achieved. Tablets are considerably more costly than 
light pens, however. 
    To provide interaction between the tablet and the display, it is usual to 
display a cursor (a dot or tracking symbol) corresponding to the current 
position of the stylus on the writing surface. Other devices such as joy 
sticks, track balls, and slewing buttons are also used to steer cursors on the 
screen in many displays. These are all of relatively low cost and provide 
satisfactory pointing capability; however, the two-axis form of control that 
is characteristic of these devices is too awkward to permit drawing. 
 �34s[John E. Ward]
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�6v�2v�32sComputerized tomography   �21m �33s 
    An imaging technique in which the x-ray beam of a computerized tomography 
scanner passes through a human body or an object and is collected with an 
array of detectors; the beam is rotated to produce the equivalent of a 
``slice'' through the area of interest. The x-ray information collected during 
the rotation is then used by a computer to reconstruct the ``internal 
structures,'' and the resulting image is displayed on a television screen. 
This technique represents a noninvasive way of seeing internal structures, and 
has in many ways revolutionized diagnostic approaches. In the brain, for 
example, computerized tomography can readily locate tumors and hemorrhages, 
thereby providing immediate information for evaluating neurological 
emergencies. Equally important is the fact that, in a great majority of cases, 
computerized tomography offers an alternative approach to the more problematic 
invasive procedures such as pneumoencephalography and angiography. The 
reduction in scanning time made it possible to do whole-body scans; abdominal 
tumors, hemorrhages, and infections can be detected without the need of 
exploratory surgery. 
    Basically the scanner gantry is composed of an x-ray tube, an array of 
detectors opposite the tube, and a central aperture in which the person (or 
object) is placed. X-rays are generated in short bursts, usually lasting 2 - 3 
ms; the x-ray beam contains an ``invisible image'' of the internal structures. 
The role of the detectors is to collect this information, which is then fed 
into a computer. The computer reconstructs the image from the information 
collected by the detectors. In order to obtain enough information to calculate 
one image, the newer scanners can take as many as 90,000 readings (300 pulses 
and 300 detectors). See RADIOGRAPHY. 
 �34s[Nicole-Fr. Bolender]
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�6v�2v�32sComputer security �21m �33s
    The preservation of computing resources against abuse or unauthorized use,
especially the protection of data from accidental or deliberate damage,
disclosure, or modification. Safeguards are both technical and administrative.
Threats are both external (physical) and internal (logical) to the computer
system.
    Physical and logical security are two important parts of computer
security. Physical security measures, external to the hardware and software,
are intended to prevent, or to facilitate recovery from, physical disasters
such as fire, floods, riots, sabotage, and theft of physical resources.
Logical security mechanisms, internal to the computing system, are used to
protect against internal misuse of computing resources, such as the theft of
computing time and unauthorized access to data. Physical security includes the
control of physical access to computing resources. In order to make decisions
about who can have access to each terminal and each user account, as well as
to the computer room itself, the installation must have one or more mechanisms
for authenticating personal identification. The problems of logical security
are more specific to computing. Logical computer security has several facets,
including access controls, information flow controls, inference controls, and
communication controls.
    All modern multiuser operating systems have some access controls for the
sharing of resources, especially data. The objects protected by many systems
are files. A coarse granularity of protection allows access to all of a file
or none of it, and governs only a few access types, or operations on the
objects. The access matrix is the basis for many methods of representing
discretionary authorization information. The rows of the access matrix
represent subjects, or users, of the system. The columns correspond to
objects, or resources, of the system (mainly files).
    While access controls protect access to data, they do not solve the
problem of how to regulate what happens to the data after it is accessed. For
example, someone authorized to read a top-secret file can make a copy of it.
If access to the copy is not controlled as strictly as access to the original,
the security of the information in the original can be compromised through the
copy. Flow controls are concerned with labeling outputs of processes with
authorization information consistent with the protection given the inputs.
    The difficulty of verifying a piece of software as large as an operating
system had led to the concept of security kernels into which all
security-related functions have been concentrated. An operating system kernel
is a small, certifiably secure nucleus which is separate from the rest of the
operating system. Nonkernel operating system software can then be allowed to
run in a less protected environment.  See  OPERATING SYSTEMS.
    Statistical data bases are intended to provide statistical information
(for example, demographic data) about groups of people without allowing access
to data about specific individuals. However, it is often possible to infer
data about an individual, given the statistical results for a group and some
amount of prior knowledge about the individual. Theoretical results indicate
that very few statistical data bases can be made safe from all inference
without becoming too restrictive for intended uses. For very large statistical
data bases (for example, census data), it is effective to allow access only to
randomly selected sample subsets of data which summarize the statistical
characteristics of the full data base. Sometimes inference can be forestalled
by introducing noise, in the form of slight random variations, into the data
values.
    Cryptographic controls provide the means to encode data for storage or
transmission so that even if an unauthorized person sees the data it will
still be unreadable.  See  CRYPTOGRAPHY.
 �34s[H. Rex Hartson]
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�6v�2v�32sComputer storage technology �21m �33s
    The techniques, equipment, and organization for providing the memory
capability required by computers in order to store instructions and data for
processing at high electronic speeds. In early computer systems, memory
technology was very limited in speed and high in cost. Since the 1970s, the
advent of high-speed random-access memory (RAM) chips has significantly
reduced the cost of computer main memory by more than two orders of magnitude.
Chips are no larger than 1/4 in. (6 mm) square and contain all the essential
electronics to store thousands of bits of data or instructions. Traditionally,
computer storage has consisted of three or more types of memory hierarchy
storage devices (for example, magnetic core memories, disks, and magnetic tape
units).

    Memory hierarchy.  Memory hierarchy refers to the different types of
memory devices and equipment configured into an operational computer system to
provide the necessary attributes of storage capacity, speed, access time, and
cost to make a cost-effective, practical system. The fastest-access memory in
any hierarchy is the main memory in the computer. In most computers
manufactured after the late 1970s, RAM chips are used because of their high
speed and low cost. Magnetic core memories were the predominant main memory
technology in the 1960s and early 1970s prior to the RAM chip. The secondary
storage hierarchy usually consists of disks (both movable-arm and
head-per-track). Significant density improvements have been achieved in disk
technology so that disk capacity has doubled every 3 - 4 years. Between main
memory and secondary memory hierarchy levels, however, there has always been
what is called the memory gap. It is characterized by a wide gap in both
performance and cost. The last, or bottom, level (sometimes called the
tertiary level) of storage hierarchy is made up of magnetic tape transports
and mass storage tape systems.

    Memory organization.  The efficient combination of memory devices from the
various hierarchy levels must be integrated with the central processor and
input-output equipment, making this the real challenge to successful computer
design. The resulting system should operate at the speed of the fastest
element, provide the bulk of its capacity at the cost of its least expensive
element, and provide sufficiently short access time to retain these attributes
in its application environment. Another key ingredient to a successful
computer system is its operating system (that is, software) that allows the
user to execute jobs on the hardware efficiently. Operating systems are
available which achieve this objective to a reasonably high level of
optimization.
    The computer system hardware and the operating system software must work
integrally as one resource. In many computer systems, the manufacturer
provides what is called a virtual memory system. It gives each programmer
automatic access to the total capacity of the memory hierarchy without
specifically moving data up and down the hierarchy and to and from the central
processing unit. See DIGITAL COMPUTER PROGRAMMING; MULTIACCESS COMPUTER.

    Main memory.  There are two basic types of RAMs, static and dynamic. The
differences are significant. Dynamic RAMs are those which require their
contents to be refreshed periodically. They require supplementary circuits to
do the refreshing and to assure that conflicts do not occur between refreshing
and normal read/write operations. Even with those extra circuits, dynamic RAMs
still require fewer on-chip components per bit than do static RAMs (which do
not require refreshing). The fact that they require fewer components makes it
possible to achieve higher densities with dynamic RAMs than are possible with
static RAMs. These higher densities also lead to lower costs per bit. Static
RAMs are easier to design, and compete well in applications in which less
memory is to be provided, since their higher cost then becomes less important.
    There is another trade-off to be made with semiconductor random-access
memories in addition to the choice between static and dynamic types, namely
that between MOS and bipolar chips. Bipolar devices are faster, but have not
yet achieved the higher densities (and hence the lower costs) of MOS.
    Microcomputers have evolved their own special set of semiconductor memory
chips to suit their application needs. Whereas large, medium-size, and
``mini'' computers primarily use only RAMs that have read/write capability,
microcomputers have found significant use for read-only memory (ROM) chips and
programmable read-only memory (PROM) chips. Because microcomputers are
typically used in dedicated applications and not in general-purpose
programming installations, their data and program storage can be separately
allocated between RAMs and ROMs. ROMs are cheaper and provide protection,
since the contents are fixed or hard-wired. During microcomputer program
development, PROMs are used because they can be rewritten to modify the
contents. Typically, PROMs are cleared by putting the PROM chip under an
ultraviolet light to zero out its contents and then using a PROM programmer to
write in the new bit pattern for the modified program. After the program has
completed final test and acceptance, the final bit pattern can be put into
ROMs (using a chip mask) for making production units. There are also chip
devices called electrically alterable programmable read-only memories
(EPROMs); they have an electrical switch to allow one to rewrite new contents
into them.

    Memory gap technologies.  The wide gulf between fast, expensive main
memory devices and slow, inexpensive secondary memories can be seen in the
following memory gap performance/cost ranges: access time 1 microsecond - 30
milliseconds; capacity 10 sup 7 - 10 sup 9 bits; price per bit 0.1 - 0.001cents.
The memory gap for access time is bounded on one side by a 1-MUs typical
computer main memory cycle time and on the other by the 30-ms typical
disk-drive access time. Capacity is bounded by a reasonable main-frame memory
capacity of about 1 megabyte and by the typical disk capacity of 100
megabytes. The price is then determined by these existing technologies, that
is, 0.1cents per bit for RAMs and 0.001cents per bit for disks.
    The semiconductor RAM and disk technology continues to improve in
performance and price, narrowing the gap. It is unlikely, however, that the
proved technologies will spread far enough to completely close it.
Charge-coupled devices (CCDs) of 64K bits and 256K bits per chip have been
used integrally in a few commercial computer systems as bulk store between
main memory and secondary disks. Some disk replacement units have been
designed by using CCDs. Bubble memory units are on the slow-access-time side
of the memory gap. Bubble memories are most promising as low-capacity disk
replacements and for special applications. See CHARGE-COUPLED DEVICES;
MAGNETIC BUBBLE MEMORY.

    Secondary memories.  Secondary memories consist of disk units which vary
in capacity from the small 250-kilobyte floppy disks (used with
microcomputers), through 5-megabyte cartridge disks (used with minicomputers),
to large-capacity (up to 600-megabyte) disks used with mini, medium, and
large-scale computers. Disk memories are also characterized by access times in
the millisecond region (versus access time of 1 MUs to hundreds of nanoseconds
for RAMs).
    All the disk units have a movable arm which can be positioned to any
track. This device minimizes the cost of the unit, because there are less
read/write head electronics to be amortized over the large capacity, at the
expense of longer access times to position the arm. Disk memories store
information along tracks or cylinders traced in a circular fashion by a
transducer which is held in a fixed position during the write or read process.
The density of the data is referred to as the number of bits packed along the
track in the recording process.
    Many significant advances in the capabilities of commercially available
disk memory equipment were made during the late 1970s. One of the major trends
was toward larger-capacity disks from 100 megabytes to 600 megabytes per disk
drive. The majority of technology improvements to provide higher capacities,
lower costs, and better reliabilities resulted from what is known as the
Winchester technology. Prior to this technology, removable disk packs were
established as the most advanced technology for large-capacity disks. The
Winchester technology is characterized by nonremovable or sealed disk packs,
which inherently provide more reliability due to less handling and make more
stringent alignment tolerances practical. The extremely narrow Winchester
track has been achieved by a combination of factors. The Winchester head
weighs only 0.25 g and floats above the surface. The iron oxide particles on
the disk surface have also been magnetically oriented so that both the track
and the bits along the track are more precisely defined. Another improvement
which characterizes the new technology is a lubricated surface that allows the
lightweight head to rest on the disk during start and stop operations. The
in-contact start/stop capability of Winchester drives has largely eliminated
the ``head crash'' problems encountered with the earlier technologies.
    Central to the advance to increase the disk packing density was the
reduction in the flying height of the read/write head. Because flux spreads
with distance, reduced separation between the read/write head and the magnetic
surface will decrease the area occupied by an information bit. All movable
media memories utilize an air-bearing mechanism known as the slider or flying
head to space the transducer in close proximity to the relatively moving
media. Head-to-medium spacings, commonly up to 250 microinches (6 MUm) in
1965, were 50 MUin. (1.2 MUm) in 1975 and 20 MUin. (0.5 MUm) in 1980. See
TRANSDUCER.
    The head-per-track disk unit can make available many data tracks on the
disk without physically moving the read/write head and thus can provide faster
access times. The disk storage medium is nonremovable. These types of disks
are called swapping disks because different user programs can be more rapidly
exchanged than by using moving-arm disks. One prime application for
head-per-track disks is in the communications equipment portion of
data-processing systems.
    Removable storage cartridges containing single-disk packs are used
extensively with minicomputer systems. The price and performance features of
cartridge disks compete favorably with the more expensive, larger-capacity
disk pack storage equipment.
    Floppy disks have become the most widely used secondary memory for
microcomputer systems with random-access capability. Floppy disks derive their
name from the recording media itself, which is an oxide-coated flexible disk
(similar to, but more flexible than, plastic 45-rpm music records) enclosed
within a protective envelope. The envelope contains alignment holes for
spindle loading, head contact, sector/index detection, and write protect
detection. The capacity of this easily transportable media is 3 megabits laid
out on 77 tracks.
    Optical or video disks have the potential to provide very inexpensive
mass-produced read/write mass memory capability. These rotating devices are
used to provide read-only television storage on easily replicable disks. The
pressed disk has a spiral groove at the bottom of which are
submicrometer-sized depressions. These depressions are read out either through
a capacity pickup in a needle-in-the-groove or through a light-beam technique.
The storage capacity is equivalent to 10 sup 1 sup 0 bits per disk. See VIDEO
DISK RECORDING.

    Magnetic-tape units.  In magnetic-tape units the tape is held in intimate
contact with the fixed head while in motion,  allowing high-density recording.
The long access times to find user data on the tape are strictly due to the
fact that all intervening data have to be searched until the desired data have
been found. This, of course, is not true of rotating disk memories or RAM word
addressable main memories. The primary use of tape storage is for seldom-used
data files and as back-up storage for disk data files. Half-inch (12.7-mm)
tape has been the industry standard since it was first used commercially in
1953. Half-inch magnetic-tape drive transports are reel-to-reel recorders with
extremely high tape speeds (up to 200 in. or 5 m per second), and fast start,
stop, reverse, and rewind times (on the order of 1 ms).
    The most frequently used magnetic-tape memory devices for microcomputer
systems are cassette and cartridge tape units. Both provide very-low-cost
storage, although their access times are high and their overall throughout
performance is not as great as that of floppy disks. Cassette and cartridge
units both use quarter-inch-wide (6.35-mm) magnetic tape, but the packing
densities used vary considerably between the two types of units.

    Mass storage systems.  With the gradual acceptance of virtual memory and
sophisticated operating systems, a significant operational problem arose with
computer systems, particularly the larger-scale installations. The sheer
expense and attendant delays and errors of humans storing, mounting, and
demounting tape reels at the command of the operating system began to become a
problem. Mass-storage facilities are designed to alleviate this problem. All
such configurations mechanically extract from a bin, mount on some sort of
tape transport, and replace in a bin, following reading or writing, a reel of
magnetic tape. In practice, most users have been satisfied with tapes and
disks, and have not chosen the middle ground of mass-storage systems. See
COMPUTER; DATA-PROCESSING SYSTEMS; DIGITAL COMPUTER.
 �34s[Douglas J. Theis]
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�6v�2v�32sConcave grating   �21m �33s 
    A widely used form of reflection-type diffraction grating with which a 
spectrograph can be formed that has no auxiliary optical parts except a slit 
and a camera. Being ruled on a concave mirror, this type of grating can both 
collimate and focus the light that falls upon it. It is made by spacing 
straight grooves equally along the chord (rather than the arc) of a spherical 
or paraboloidal mirror surface. Light which passes through a slit and falls on 
such a grating is dispersed by it into spectra which are in focus on the 
Rowland circle, a circle drawn tangent to the face of the grating at its 
midpoint, having a diameter equal to the radius of curvature of the grating 
surface. See DIFFRACTION GRATING. 
 �34s[George R. Harrison]
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�6v�2v�32sConcentration scales   �21m �33s 
    Concentration is a very important property of mixtures, because it defines 
the quantitative relation of the components. In solutions the concentration is 
expressed as the mass, volume, or number of moles of solute present in 
proportion to the amount of solvent or of total solution. 
    The simplest scale to measure is percentage; hence it is often used for 
medicinal or household solutions. Weight percent is the number of parts of 
weight of solute per hundred parts of solution (total). For example, a 10% 
saline solution contains 10 g of salt in 90 g of water, that is, 100 g total 
weight. Gaseous mixtures, being difficult to weigh, are often expressed as 
volume percent. Thus, air is said to contain 78% nitrogen by volume. Solutions 
of liquids in liquids (say, alcohol in water) may also be expressed in volume 
percent.  
    To the chemist, the number of moles of solute is of more significance than 
the number of grams. The molarity (abbreviated M) is the number of moles of 
solute per liter of total solution. Thus, 12 M HC1 means that the solution 
contains 12 formula weights (12 x 36.5), or 438 g, of HCl/liter. (As used 
here, the mole is an amount of substance whose weight in grams is numerically 
the same as the molecular weight.) 
    Molality (abbreviated m) relates the number of moles of solute to the 
weight of solvent rather than to the volume of solution. This scale indicates 
the number of moles of solute/1000 g  of solvent. Thus 34.2 g of sucrose 
(C12H22O11, mol wt 342), if dissolved in 200 g of water, has the concentration 
of 0.5 mole of sucrose/1000 g of water, and hence is 0.5 m. 
    When it is important to know the reactive capacities of reagents, as in 
volumetric analysis, the normality scale is used. Normality (abbreviated N) is 
found by multiplying molarity by the number of active units in the formula. 
    In recent years many chemists have sought to avoid the molarity scale lest 
it imply that electrolyte solutes exist as molecules rather than ions. The 
formality scale (abbreviated F) represents formula weights per liter. Its 
values are identical with the molarities of un-ionized solutes. 
    Many properties of solutions (for example, vapor pressure of one 
component) are dependent on the ratio of the number of moles of solute to the 
number of moles of solvent, rather than on the ratios of respective volumes or 
masses. The mole fraction (abbreviated NsubA or XsubA for component A) is the 
ratio of the number of moles of solute to the total number of moles of all 
components. Thus for 16 g of methanol (0.5 mole) dissolved in 18 g of water (1 
mole), the mole fraction of methanol is 0.5/1.5, or 1/3; the mole percent is 
33.3. For gases the mole percent is identical with the volume percent. See 
GRAM-EQUIVALENT WEIGHT; GRAM-MOLECULAR WEIGHT; SOLUTION; TITRATION. 
 �34s[Allen L. Hanson]
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�6v�2v�32sConch   �21m �33s 
    The name given to many species of the Strombidae, a mainly tropical family 
of large, imposing, marine gastropods with massive shells. They are found on 
sand or mud bottoms in the Caribbean and Indo-Pacific, in shallow waters, from 
just below the littoral zone down to depths of 200 ft (60 m). In all strombids 
the inner nacreous (or mother-of-pearl) layer of the shell is heavily 
developed with the sheen of porcelain, and it is often colored a rich pink or 
orange.  
    Strombids are all herbivores, some species grazing specifically on 
epiphytic red seaweeds attached to larger brown algae and to turtle grass 
(Thalassia). The mantle cavity in strombids is in the typical mesogastropod 
condition, with a single pectinibranch gill and simplified asymmetric pallial 
and cardiac structures. The flesh of conchs is eaten throughout the Antilles 
and in several areas of the Pacific, and their shells have long been articles 
of commerce. See GASTROPODA; PROSOBRANCHIA; SNAIL; WHELK. 
 �34s[W. D. Russell-Hunter]
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�6v�2v�32sConchostraca   �21m �33s 
    An order of mussellike crustaceans of moderate size, belonging to the 
subclass Branchiopoda. The multisegmented, laterally compressed trunk 
terminates in a telson with two clawlike cercopods. A bivalved shell covers 
the entire animal (see illustration). The antennae are powerful swimming 
organs. The legs are foliaceous and of almost uniform structure, with one pair 
on each of the trunk segments. Among the many genera are Limnadia, 
Leptestheria, and the aberrant Limnetis. The group is allied to the Cladocera. 
See ANOSTRACA; BRANCHIOPODA; CLADOCERA. 
 �34s[Folke Linder]
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�6v�2v�32sConcrete   �21m �33s 
    A versatile engineering material consisting of a hydraulic cementing 
substance (usually portland cement), aggregate, water, and often controlled 
amounts of entrained air. 
    Concrete is initially a plastic, workable mixture which can be molded into 
a wide variety of shapes. Strength is developed in the hydration reaction 
between the cement and water. The products, mainly calcium silicates, calcium 
aluminate, and calcium hydroxide, are relatively insoluble and bind the 
aggregate in a hardened matrix. 
    Concrete is considerably stronger in compression than in tension. For 
structures required to carry only compressive loads, such as massive gravity 
dams and heavy foundations, no reinforcement is required, and the concrete is 
consequently called plain concrete. When the structure will involve tensile 
stresses, steel bars are embedded in the concrete, which is then called 
reinforced concrete. An extension of the reinforcement principle is 
prestressing, in which high-tensile-strength steel wires or wire strand 
embedded while in tension induces a precompression in the concrete so as to 
increase its resistance to tensile stress. See PRESTRESSED CONCRETE; 
REINFORCED CONCRETE. 
    Concrete has a wide variety of applications. In buildings it is used in 
footings, foundations, columns, beams, girders, wall slabs, roof units - in 
short, all important building elements. Concrete not only meets structural 
demands but also lends itself readily to architectural treatment. Other 
important applications of concrete are in road pavements, airport runways, and 
bridges. Normally, concrete for these structures is plain concrete, containing 
embedded steel only in order to resist the small volume changes caused by 
changes in temperature and moisture content. Concrete is widely used in 
conservation projects such as dams, irrigation canals, water diversion 
structures, sewage treatment plants, and water distribution pipelines. A 
considerable amount of concrete is used in the form of masonry units 
manufactured in equipment utilizing vibration and pressure to achieve 
compaction. Examples are building blocks and brick. See CEMENT; PORTLAND 
CEMENT.  
 �34s[Paul Klieger]
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�6v�2v�32sConcretion   �21m �33s 
    A loosely defined term used for a sedimentary mineral segregation that may 
range in size from inches to many feet. Concretions are usually distinguished 
from the sedimentary matrix enclosing them by a difference in mineralogy, 
color, hardness, and weathering characteristics. Some concretions show 
definite sharp boundaries with the matrix, while others have gradational 
boundaries. Most concretions are composed dominantly of calcium carbonate, 
with or without an admixture of various amounts of silt, clay, or organic 
material. Coal balls are calcareous concretions found in or immediately above 
coal beds. Concretions are normally spherical or ellipsoidal; some are 
flattened to disklike shapes. Frequently a concretion is dumbbell-shaped, 
indicating that two separate concretionary centers have grown together. See 
COAL BALLS; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sConcurrent processing   �21m �33s 
    The conceptually simultaneous execution of more than one sequential 
program on a computer or network of computers. The individual sequential 
programs are called processes. Two processes are considered concurrent only if 
they interact in some way. This interaction may range from cooperation (the 
exchange of information) to competition for scarce resources (such as the 
processor, the memory, or a printer). Concurrent processing is achieved on a 
single computer, that is, on a single processor, by interleaving the execution 
of the individual processes. 
    In comparison, distributed processing requires multiple processors for its 
implementation. According to this definition, all distributed programs are 
also concurrent programs, but the converse is not true.  See  DISTRIBUTED 
PROCESSING.  
    One form of concurrency allows multiple users to share one processor 
without having the user programs complicated by codes dealing with the sharing 
of resources, scheduling, and so forth. Different parts of a problem can be 
relegated to semi-independent processes that will communicate with one another 
only when necessary. However, the operating system causes an overhead in the 
use of the processor. Though distributed systems reduce contention for 
processors, they create contention for the communication network.  See  
MULTIACCESS COMPUTER. 
    Many programming languages provide the user with the ability to write 
concurrent programs. Some languages provide for statement-level (or 
procedure-level) concurrency, in which the user tells the system what portions 
of the code will be executed concurrently. In contrast, some data flow 
languages provide for operator-level concurrency. In such languages, the 
program is specified without reference to concurrency.  See  DATA FLOW 
SYSTEMS; PROGRAMMING LANGUAGES. 
    Interprocess communication invokes two classes of concurrent systems: 
synchronous and asynchronous. In a synchronous system, all processors work in 
lockstep. Asynchronous processes need only synchronize temporarily to exchange 
information or to request a service. Such synchronization and communication 
takes place in one of two ways: by reading and writing shared memory or by 
exchanging messages. 
 �34s[Brent T. Hailpern]
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�6v�2v�32sConcussion �21m �33s
    A state following injury in which there is temporary functional impairment
without physical evidence of damage to the impaired tissues. The term usually
refers to cerebral concussion produced by any type of trauma.
    From a clinical point of view cerebral concussion is produced by a head
injury which causes temporary unconsciousness but with complete recovery
within 24 h. This temporary alteration is believed to result from one of
several mechanisms. In all of these a sudden acceleration or deceleration
appears to be a prerequisite. The sudden movement is thought to cause an
unequal shifting of tissues of different specific gravities within the skull,
between skull and brain, or between different brain tissues. See BRAIN.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sCondensation reaction �21m �33s
    One of a class of chemical reactions involving a combination between
molecules or between parts of the same molecule. A relatively small molecule
such as water or alcohol is often eliminated in the process. The conversions
of oxygen to ozone and of diethyl pimelate to 2-carbethoxycyclohexanone are
examples, as seen in reactions (1) and (2).

			 CHEMICAL STRUCTURE				 (1)

			 CHEMICAL STRUCTURE				 (2)
    The term condensation reaction is used very loosely by chemists. Some
authors define the term to include all reactions which result in formation of
carbon-carbon bonds. General usage, however, does not classify all reactions
resulting in formation of carbon-carbon bonds as condensation reactions.
Furthermore, the reaction commonly referred to as condensation polymerization
involves formation of carbon-oxygen or carbon-nitrogen bonds. See
POLYMERIZATION.
 �34s[William B. Renfrow, Jr.]
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�6v�2v�32sCondenser (electricity)   �21m �33s 
    A device for introducing capacitance into a circuit. In modern usage, the 
word capacitor replaced the word condenser since nothing is really condensed 
in this device. Hence, for a discussion of this electric circuit component see 
CAPACITOR. See also CAPACITANCE. 
 �34s[Ralph P. Winch]
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�6v�2v�32sConditioned reflex   �21m �33s 
    A learned response performed by a trained animal to a signal that was 
previously associated with an event of consequence for that animal. 
Conditioned reflex (CR) was first used by the Russian physiologist I.  P. 
Pavlov to denote the criterion measure of a behavioral element of learning, 
that is, a new association between the signal and the consequential event, 
referred to as the conditioned stimulus (CS) and unconditioned stimulus (US), 
respectively. In Pavlov's classic experiment, the CS was a bell and the US was 
sour fluid delivered into the mouth of a dog restrained by a harness; CS was 
followed by US regardless of the dog's response. After training, the CR is 
manifested when the dog salivates to the sound of the bell. 
    Ideally certain conditions must be met to demonstrate the establishment of 
a CR according to Pavlov's classical conditioning method. Before conditioning, 
the bell CS should attract the dog's attention or elicit the orienting reflex 
(OR), but it should not elicit salivation, the response to be conditioned. 
That response should be specifically and reflexively elicited by the sour US, 
thus establishing its unlearned or unconditional status. After conditional 
pairings of the CS and the US, salivation is manifested prior to the delivery 
of the sour US. Salivation in response to the auditory CS is now a ``psychic 
secretion'' or the CR. 
    Pavlov's methods provide important guidelines for basic research upon 
brain mechanisms in learning and memory. Classical conditioning is considered 
a general biological or psychobiological phenomenon which promotes adaptive 
functioning in a wide variety of physiological systems in various phylogenetic 
settings.  
    The CR is the criterion of learning, but the nature and origin of the CR 
are still at issue. In his original experiment, Pavlov circumvented many 
ambiguities by his selection of an auditory CS and a salivary CR. His CR 
resembled the unconditioned reflex (UR), and was clearly differentiated from 
the dog's natural OR to the auditory CS. Early investigators were guided by 
the UR in their search for classically conditioned CRs. Although the UR may 
not be a sure guide in all situations, it is still a powerful analytical 
device.  
    Methodological complexities and conceptual confusions arise when an 
investigator selects a CS which, before training, has a natural tendency to 
elicit reactions similar to the CR or any reaction which is similar to that 
elicited by the chosen US. For example, before training, an electric shock US 
causes a startle UR in a subject; the soft sound of a bell elicits the OR but 
not the startle UR, and thus the bell seems to be a valid CS and the startle 
reaction a valid CR. When the bell CS and the electric shock US are paired, 
the bell quickly elicits the startle response, indicating that a new pathway 
between the auditory and startle mechanisms has been presumably established in 
the brain of the subject. But the loud clang of a bell will elicit the startle 
reflex without training in a second subject, indicating that connection with a 
low threshold of sensitivity may have existed in the brain of this subject 
before training. Presumably the loud clang was intense enough to activate this 
pathway, but the soft CS was not. Furthermore, administration of shock alone 
to a third subject sensitizes the subject to any sound, so that the subject 
starts at the soft sound of the bell CS, even though it has never been paired 
with the shock US. Because there has been no explicit CS-US pairing during 
training, the third subject is said to be pseudoconditioned. For the same 
reason, sensitization and pseudoconditioning are often called nonassociative 
effects.  
    Nonassociative effects cause some practical and theoretical problems in 
the interpretation of an apparent CR. Practically, the problems become 
formidable when the subject is a member of an exotic species, such as a marine 
invertebrate whose thresholds of sensitivity and response repertoires are 
unknown. Theoretically, nonassociative effects raise questions about the 
associative definition of learning, and perhaps any purely behavioral 
definition of learning. See LEARNING THEORIES. 
 �34s[John Garcia]
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�6v�2v�32sConductance �21m �33s
    For a series circuit the real, or in-phase, part of the complex
representation for the admittance Y of a circuit. This is shown in Eq. (1),
where B is the imaginary part, called the susceptance. Since Y = 1/Z = 1/(R +
jX), where R is the resistance and X is the total reactance X(sub L) - X(sub C),
Eq. (2) holds, and conductance G is given by Eq. (3). This is the general
expression for conductance and shows that conductance is a function involving
both resistance and reactance.

			     Y = G PLUS/MINUS jB			     (1)

	 Y - R/(R(sup 2) + X(sup 2)) - jX/(R(sup 2) + X(sup 2)) 	     (2)

			  G = R/(R(sup 2) + X(sup 2))			     (3)

    If reactance is negligible, as in a dc circuit, then G = R/R(sup 2) = 1/R,
or conductance is the reciprocal of resistance. See ADMITTANCE;
ALTERNATING-CURRENT CIRCUIT THEORY.
 �34s[Arthur A. Welch]
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�6v�2v�32sConduction band   �21m �33s 
    The electronic energy band of a crystalline solid which is partially 
occupied by electrons. The electrons in this energy band can increase their 
energies by going to higher energy levels within the band when an electric 
field is applied to accelerate them or when the temperature of the crystal is 
raised. These electrons are called conduction electrons, as distinct from the 
electrons in filled energy bands, which, as a whole, do not contribute to 
electrical and thermal conduction. In metallic conductors the conduction 
electrons correspond to the valence electrons (or a portion of the valence 
electrons) of the constituent atoms. In semiconductors and insulators at 
sufficiently low temperatures, the conduction band is empty of electrons. 
Conduction electrons come from thermal excitation of electrons from a lower 
energy band or from impurity atoms in the crystal. See BAND THEORY OF SOLIDS; 
ELECTRIC INSULATOR; ELECTRICAL CONDUCTIVITY OF METALS; SEMICONDUCTOR; VALENCE 
BAND.  
 �34s[Hsu Y. Fan]
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�6v�2v�32sConduction (electricity)   �21m �33s 
    Electrical conduction may be defined as the passage of electric charge. 
This can occur by a variety of processes. In metals the electric current is 
carried by free electrons. These are not bound to any particular atom and can 
wander throughout the metal. In semiconductors (germanium, silicon, and so on) 
there are a limited number of free electrons and also ``holes,'' which act as 
positive charges, available to carry current. Aqueous solutions of ionic 
crystals readily conduct electricity by means of the positive and negative 
ions present, for example, Nasupp and Cl- in an ordinary solution of sodium 
chloride. A strong electric field ionizes gas molecules, and thereby permits a 
flow of current through the gas in which the ions are the charge carriers. If 
sufficient ions are formed, there may be a spark. Electric current can flow 
across a vacuum, for example, in a vacuum tube. The charge carriers are 
electrons emitted by the filament. See ELECTRIC SPARK; ELECTRICAL CONDUCTION 
IN GASES; ELECTRICAL CONDUCTIVITY OF METALS; ELECTROLYTIC CONDUCTANCE; 
ELECTRON EMISSION; ELECTRON MOTION IN VACUUM; IONIC CRYSTALS; SEMICONDUCTOR; 
SUPERCONDUCTIVITY.  
 �34s[John W. Stewart]
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�6v�2v�32sConduction (heat)   �21m �33s 
    The flow of thermal energy through a substance from a higher- to a 
lower-temperature region. Heat conduction occurs by atomic or molecular 
interactions. Conduction is one of the three basic methods of heat transfer, 
the other two being convection and radiation. See CONVECTION (HEAT); HEAT 
RADIATION; HEAT TRANSFER. 
    Steady-state conduction is said to exist when the temperature at all 
locations in a substance is constant with time, as in the case of heat flow 
through a uniform wall. Examples of essentially pure transient or periodic 
heat conduction and simple or complex combinations of the two are encountered 
in the heat-treating of metals, air conditioning, food processing, and the 
pouring and curing of large concrete structures. Also, the daily and yearly 
temperature variations near the surface of the Earth can be predicted 
reasonably well by assuming a simple sinusoidal temperature variation at the 
surface and treating the Earth as a semi-infinite solid. The widespread 
importance of transient heat flow in particular has stimulated the development 
of a large variety of analytical solutions to many problems. The use of many 
of these has been facilitated by presentation in graphical form. 
    For an example of the conduction process, consider a gas such as nitrogen 
which normally consists of diatomic molecules. The temperature at any location 
can be interpreted as a quantitative specification of the mean kinetic and 
potential energy stored in the molecules or atoms at this location. This 
stored energy will be partly kinetic because of the random translational and 
rotational velocities of the molecules, partly potential because of internal 
vibrations, and partly ionic if the temperature (energy) level is high enough 
to cause dissociation. The flow of energy results from the random travel of 
high-temperature molecules into low-temperature regions and vice versa. In 
colliding with molecules in the low-temperature region, the high temperature 
molecules give up some of their energy. The reverse occurs in the 
high-temperature region. These processes take place almost instantaneously in 
infinitesimal distances, the result being a quasi-equilibrium state with 
energy transfer. The mechanism for energy flow in liquids and solids is 
similar to that in gases in principle, but different in detail. 
 �34s[Warren H. Giedt]
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�6v�2v�32sConductivity   �21m �33s 
    A measure of the ability of a material to conduct electric current. It is 
the reciprocal of resistivity. Conductivity is commonly expressed as siemens 
(mhos) per meter, since the unit of resistivity is the ohm-meter. The 
conductivity of metallic elements varies inversely with absolute temperature 
over the normal range of temperatures, but at temperatures approaching 
absolute zero the imperfections and impurities in the lattice structure of a 
material make the relationship more complicated. See ELECTRICAL CONDUCTIVITY 
OF METALS. 
    The conductivity associated with conduction electrons in a semiconductor 
is known as n-type conductivity; that associated with the holes in an inpurity 
semiconductor (equivalent to positive charges) is known as p-type 
conductivity. See CONDUCTION (ELECTRICITY); SEMICONDUCTOR. 
 �34s[John Markus]
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�6v�2v�32sConductor (electricity)   �21m �33s 
    Metal wires, cables, rods, tubes, and bus-bars used for the purpose of 
carrying electric current. (The most common forms are wires, cables, and 
bus-bars.) Although any metal assembly or structure can conduct electricity, 
the term conductor usually refers to the component parts of the 
current-carrying circuit or system. 
    Wires employed as electrical conductors are slender rods or filaments of 
metal, usually soft and flexible. They may be bare or covered by some form of 
flexible insulating material. They are usually circular in cross section; for 
special purposes they may be drawn in square, rectangular, ribbon, or other 
shapes. Conductors may be solid or stranded, that is, built up by a helical 
lay or assembly of smaller solid conductors. 
    Insulated stranded conductors in the larger sizes are called cables. 
Small, flexible, insulated cables are called cords. Assemblies of two or more 
insulated wires or cables within a common jacket or sheath are called 
multiconductor cables. 
    Bus-bars are rigid, solid conductors and are made in various shapes, 
including rectangular, rods, tubes, and hollow squares. Bus-bars may be 
applied as single conductors, one bus-bar per phase, or as multiple 
conductors, two or more bus-bars per phase. The individual conductors of a 
multiple-conductor installation are identical. 
    Most wires, cables, and bus-bars are made from either copper or aluminum. 
Copper, of all the metals except silver, offers the least resistance to the 
flow of electric current. Both copper and aluminum may be bent and formed 
readily and have good flexibility in small sizes and in stranded 
constructions. Aluminum, because of its higher resistance, has less 
current-carrying capacity than copper for a given cross-sectional area. 
However, its low cost and light weight (only 30% that of the same volume of 
copper) permit wide use of aluminum for bus-bars, transmission lines, and 
large insulated-cable installations. 
    For overhead transmission lines where superior strength is required, 
special conductor constructions are used. Typical of these are aluminum 
conductors, steel reinforced, a composite construction of electrical-grade 
aluminum strands surrounding a stranded steel core. Other constructions 
include stranded, high-strength aluminum alloy and a composite construction of 
aluminum strands around a stranded high-strength aluminum alloy core. 
    For extra-high-voltage transmission lines, conductor size is often 
established by corona performance rather than current-carrying capacity. Thus 
special ``expanded'' constructions are used to provide a large circumference 
without excessive weight. Typical constructions use helical lays of widely 
spaced aluminum strands around a stranded steel core. The space between the 
expanding strands is filled with paper twine, and outer layers of conventional 
aluminum strands are applied. 
 �34s[H. Wayne Beaty]
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�6v�2v�32sCondylarthra   �21m �33s 
    A mammalian order of extinct, primitive, hoofed herbivores with five-toed 
plantigrade to semidigitigrade feet. Condylarths are not far removed from 
carnivorous ancestors. The astragalus is carnivorelike and known skulls are 
carnivorelike in outline, although the dentition has been modified for an 
omnivorous or herbivorous diet. While the teeth are usually low-crowned and 
superficially piglike, one group possesses selenodont teeth. Condylarths are 
best represented in the early Cenozoic deposits of Europe and of North and 
South America, appearing first in the North American Cretaceous. The order is 
near the ancestry of all living hoofed mammals. Condylarthra is subdivided 
into seven families: Arctocyonidae, Hyopsodontidae, Phenacodontidae, 
Didolodontidae, Periptychidae, Meniscotheriidae, and Mesonychidae. See 
CARNIVORA.  
 �34s[Malcolm C. McKenna]
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�6v�2v�32sCone �21m �33s
    The solid of revolution obtained by revolving a right triangle about one
of its shorter sides is called a cone, or more precisely a right circular cone
(see illustration). More generally, the term cone is used in solid geometry to
describe a solid bounded by a plane and a portion of one nappe of a conical
surface. In analytic geometry, however, the term cone refers not to a solid
but to a conical surface. This is a surface generated by a straight line which
moves so that it always intersects a given plane curve, called the directrix,
and passes through a point, called the vertex, not in the plane of the
directrix. The generating line in each of its positions is called an element
of the cone. The vertex divides the surface into two parts, called nappes.
    If the elements of a cone make equal angles with a line through the
vertex, the cone is called a cone of revolution. Plane sections of a cone of
revolution are called conic sections.
    The volume of a solid cone is V = Bh/3, where B is the base area included
within the directrix, and h is the altitude (or height) measured from the
vertex to the plane of the base. The volume V and surface area S of a right
circular cone are V = pi r(sup 2)h/3 and S = 2 pi rl + 2 pi r(sup 2), where r
denotes the radius of the base and l = SQUARE ROOT [r(sup 2) + h(sup 2)] denotes
the slant height, measured from vertex to base along an element of the cone.
See CONIC SECTION; EUCLIDEAN GEOMETRY; SURFACE AND SOLID OF REVOLUTION. 
 �34s[James S. Frame]
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�6v�2v�32sConformal mapping �21m �33s
    A special operation in mathematics in which a point set in one coordinate
system is mapped or transformed into a corresponding point set in another
coordinate system.
    A mapping or transformation of a point set E in the xy plane is merely a
correspondence u = u(x,y), v = v(x,y) which makes each point of E correspond
to some point (u,v) of the uv plane. If distinct points (x,y) are transformed
into distinct points (u,v), the mapping is one-to-one. A mapping is conformal
if it is one-to-one and if small triangles are transformed into small
triangles approximately similar to the original triangles, the approximation
to similarity approaching perfection as the size of the triangles diminishes
indefinitely. Since any small figure can be cut up into triangles, it follows
that any small figure can be transformed into an approximately similar figure.
    The transformation defined by an analytic function whose derivative is
different from zero is conformal. It can also be shown that, conversely, a
conformal map must be defined by an analytic function with nonvanishing
derivative. The term conformal applies also to the transformation of any
surface onto another, such as the mapping of a portion of the Earth's surface
onto a portion of a plane. The Mercator and stereographic projections are
conformal in this same sense. Small regions are essentially unchanged in shape
but not in relative size. See ANALYTIC FUNCTIONS; MAP PROJECTIONS.
    When the map of a sector of the complex plane 0 < arg z < alpha is studied
under the transformation w = zsup2, it is seen that for small alpha the image
is the sector 0 < arg w < 2alpha, and that as alpha increases the image sector
also increases (see illustration). When alpha passes through the value pi, the
boundary of the image sector passes through the value 2pi and continues to
increase beyond 2pi, say, in a new sheet over the w plane, in the sense that
as new points z occur it is natural to continue to adjoin new points w. But
when the sector in the z plane reaches the angle 2pi, it abuts on values of z
already considered, so it is natural to ``heal'' the boundaries of the sector
of angle 4pi in the w plane by passing one sheet through another (say, along
the positive half of Ox) and joining the edges. There is thus formed a Riemann
surface over the w plane, which is useful for studying the behavior of z =
SQUARE ROOT [w] as a function of w with reference to the two branches of the
function, their continuity, and their passing (along various w curves)
continuously from one into the other.
    There exists more generally a Riemann surface of n sheets for the inverse
of any irreducible polynomial function w = a(sub 0)z(sup n) + a(sub 1)z(sup n-1)
+ .  |.  |.  + a(sub n).  Moreover, the exponential function w = e(sup z) =
e(sup x+iy) can be defined as e(sup x)(cos y + i sin y) for complex z, whose
analyticity for all z can be verified by the Cauchy-Riemann equations.	The
inverse function, given below, is infinitely many valued; its Riemann surface
consists of infinitely many sheets over the w plane, each connected (for
instance) along the positive half of Ox to its predecessor along one edge and to
its successor along the other.	Riemann surfaces may be defined abstractly also.
See TOPOLOGY.�34s[Joseph L.  Walsh]
�33s

	z = log w = log [r(cos theta + i sin theta)] = log r + i theta

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�6v�2v�32sConformational analysis   �21m �33s 
    The determination of the arrangement in space of the constituent atoms of 
a molecule that may rotate about a single bond. These rotations produce 
changes in conformation. Conformational analysis also includes the study of 
the chemical and physical differences between the conformations of a molecule. 
A particular conformation is chemically significant only when capable of 
finite existence. 
    Conformations may be treated in a manner analogous to the treatment of 
geometrical isomers, which are differentiated by the large energy barrier to 
rotation imposed by the carbon-carbon double bond. However, the barrier to 
rotation about a carbon-carbon single bond is usually considerably lower, and 
consequently different conformations are more readily interconvertible. 
    Ethane theoretically possesses an infinite number of conformations; but if 
the hydrogen atoms are not differentiated, there are but two significant 
conformations. Viewed along the axis of the carbon-carbon bond, one 
conformation has the hydrogen atoms of one methyl group exactly lined up 
behind those of the other methyl group, and the other has the hydrogens of one 
methyl group falling midway (60DG from the former position) between those of 
the other methyl group. The former is an eclipsed (hydrogens opposed to each 
other) conformation, and the latter a staggered conformation. The eclipsed 
conformation represents a free-energy maximum and the staggered conformation a 
free-energy minimum, presumably due to repulsions, or lack thereof, between 
the electrons of the carbon-hydrogen bonds. 
    If the ethane carbons each carry two substituents other than hydrogen, 
three staggered conformations are possible, and the most stable one is 
determined by the relative magnitudes of steric repulsions between the 
substituent groups on the central carbons. The staggered conformations for the 
(PLUS/MINUS) configurations (Ia), (Ib), and (Ic), and the meso configurations 
(IIa), (IIb), and (IIc) of stilbene dibromide are shown here. 
    The extension of conformational analysis to cyclic and polycyclic systems 
leads to many useful conclusions concerning the structure, configuration, and 
mode of reaction of such important natural products as the steroids, terpenes, 
and alkaloids. See STEREOCHEMISTRY. 
 �34s[Wyman R. Vaughan]
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�6v�2v�32sCongenital anomalies �21m �33s
    Abnormalities of any portion or organ of the human body that develop
during pregnancy. These structural changes are often called birth defects.
They may be minor deformities, such as supernumerary toes, which do not affect
the health of the individual, or they may be major defects of internal organs
that lead to serious physical disability or even death. Congenital anomalies
may be single or associated with other anomalies, or multiple defects that may
involve several organ systems. Birth defects may be inherited or produced by
harmful external factors; however, the cause of most anomalies is unknown.
    Certain common abnormalities show a significant regional or ethnic
difference. Thus anencephaly, a defective development of the brain, is
relatively uncommon in persons of African descent but is seen quite often in
India. Polydactyly, extra toes or fingers, is more common in Africans than in
other groups. In the United States, multiple congenital anomalies occur more
frequently in whites than in blacks.
    Congenital defects are found in a high proportion of abortions and
stillbirths and so are responsible for considerable pregnancy wastage. Defects
that cause death of the embryo, with spontaneous abortion, represent the
severe end of the biologic spectrum. Similar defects of a milder degree may
cause impairment that becomes manifest in childhood or adult life, while very
slight defects may cause no clinical manifestations. One mechanism that causes
syndromes of multiple malformations and also spontaneous abortions is gross
chromosomal abnormality. An abnormal chromosome constitution has been found in
approximately 25% of cases of spontaneous human abortion.
    While most abnormalities occur as often in males as in females, several
common ones display distinct sex differences. Pyloric stenosis, an abnormality
of the stomach outlet that causes severe vomiting in infants, is found five
times more frequently in boys than in girls. The common clubfoot abnormality
is twice as common in males, while anencephaly is twice as common in females.
Congenital dislocation of the hip occurs six times more often in females. By
studying twins with malformations, the genetic component can be further
identified. For cleft palate, congenital dislocation of the hip, clubfoot, and
pyloric stenosis, the monozygotic (single-egg) twin partner will have the
malformation 5 to 10 times more often than the dizygotic (two-egg) twin
partner. On the other hand, anencephaly is extremely rare in both identical
twins, indicating a minimal genetic contribution.
    Most defects of the neural tube are more common in older mothers or in
children born later in the birth order. This relationship of abnormality with
increasing maternal age is also well known for mongolism and the two other
autosomal trisomy syndromes. In congenital dislocation of the hip, the
opposite effect is true, with more cases occurring in the firstborn than would
be expected. A possible explanation for hip deformities developing in
firstborns is poor positioning of the fetus within the uterus during the last
trimester of pregnancy. See DOWN'S SYNDROME; TRISOMIC SYNDROME.
    Malformations of structures derived from the neural tube include severe
conditions like anencephaly, hydrocephaly (increased fluid in skull), spina
bifida, and meningocele (exteriorization of membranes). Approximately 50% of
all deaths due to malformations are caused by such neural defects. Congenital
heart disease includes many separate abnormalities, several of which are not
apparent at birth. Large defects in the septum separating the two sides of the
heart, however, may cause blood to be shunted from right to left, the
unoxygenated blood causing cyanosis (``blue baby''). Defects of the
esophagogastrointestinal tract may occur at any level. One common defect is
the tracheoesophageal fistula, a connection between the esophagus and major
airway, which may lead to aspiration with subsequent suffocation or pneumonia.
In imperforate anus, seen more often in males, the large intestine ends
blindly, leading to intestinal obstruction. Diaphragmatic hernia is caused by
incomplete formation of the diaphragm, permitting abdominal organs to enter
the thoracic cavity, with secondary pressure effects on the heart and lungs.
Harelip and cleft palate are defects which often coexist and which may vary in
severity from a slight dimple in the lip to extreme lack of fusion of palatal
and facial bones. Talipes (clubfeet) are common malpositions of the feet which
develop during the last trimester of pregnancy due to intrauterine mechanical
factors, as does congenital dislocation of the hip. Other common malformations
of the extremities include polydactyly, syndactyly (fusion of the fingers or
toes, ``webfeet''), and reduction deformities of various descriptions. Defects
of the genitourinary system encompass abnormalities of the external genital
organs which may confuse the assignment of the proper sex to the newborn
(intersex genitalia). If the infant has genital organs resembling those of the
opposite sex, it may suffer false sex assignment and be categorized as a
pseudohermaphrodite. Abnormalities of the urinary tract and kidneys may lead
to failure to produce adequate volumes of urine, to obstruction and
degeneration of the kidneys, or to other severe complications.
533A
    Syndromes are associations of two or more anomalies in one individual. The
finding of several abnormalities does not necessarily mean that they are
produced by the same etiologic mechanism; several anomalies may be present in
any person by coincidence. Furthermore, certain abnormalities may have diverse
etiologies. As an example of this principle, polydactyly may be caused by a
simple dominant mutant gene, may be a part of a malformation syndrome caused
by a single gene (Laurence-Moon-Biedl syndrome), may be a chromosomal
abnormality (D1 trisomy syndrome), or may be of complex (multifactorial)
genetic contribution. Most cases of polydactyly, however, are sporadic and of
unknown etiology.
    Another important principle of congenital abnormalities is the concept of
variable expressivity, where some cases of a certain condition have few
malformations while others have numerous ones. This principle is sometimes
referred to as the biologic spectrum of a disease; it can be applied to
syndromes with genetic causes and those produced by environmental factors. For
example, some women who received the tranquilizer thalidomide had infants with
severe limb deformities, while others, under almost identical conditions, bore
infants with few or no apparent anomalies. Some malformation syndromes are
characterized by the presence of few anomalies in the majority of cases, for
example, the XXX trisomy syndrome (an extra X chromosome). Other syndromes
typically have numerous anomalies, such as the XO syndrome (absence of a sex
chromosome), with a dozen or more anomalies in most cases. See TERATOLOGY.
 �34s[Stanley L. Inhorn]
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�6v�2v�32sConglomerate   �21m �33s 
    The consolidated equivalent of gravel. Conglomerates are aggregates of 
more or less rounded particles greater than 0.08 in (2 mm) in diameter. 
Frequently they are subdivided on the basis of size of particles into pebble 
(fine), cobble (medium), and boulder (coarse) conglomerates. The common 
admixture of sand-sized and gravel-sized particles in the same deposit leads 
to further subdivisions, into conglomerates (50% or more pebbles), sandy 
conglomerates (25 - 50% pebbles), and pebbly or conglomeratic sandstones (less 
than 25% pebbles). The pebbles of conglomerates are always somewhat rounded, 
giving evidence of abrasion during transportation; this distinguishes them 
from some tillites and from breccias, whose particles are sharp and angular 
(see illustration). 
    Conglomerates fall into two general classes: the well-sorted, matrix-poor 
conglomerates with homogeneous pebble lithology, and the poorly sorted, 
matrix-rich conglomerates with heterogeneous pebble lithology. The well-sorted 
class includes quartz-pebble, chert-pebble, and limestone-pebble conglomerates 
which tend to be distributed in thin, widespread sheets, normally interbedded 
with well-sorted, quartzose sandstones. The poorly sorted conglomerates 
include many different types, all related in having very large amounts of 
sandy or clayey matrix and pebbles of many different rock classes. The 
graywacke conglomerates are the outstanding representatives. All poorly sorted 
conglomerates tend to occur in fairly thick sequences, and some of them, 
typically the fanglomerates (conglomerates formed on alluvial fans) are 
wedge-shaped accumulations. See SEDIMENTARY ROCKS. 
    Special types of conglomerates, such as volcanic conglomerates and 
agglomerates and some intraformational conglomerates composed of shale pebbles 
or deformed limestone pebbles, do not seem to fall easily into either class. 
See BRECCIA; GRAVEL; GRAYWACKE; TILL. 
 �34s[Raymond Siever]
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520A 
  
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�6v�2v�32sConglutination   �21m �33s 
    A term used in serology to describe the completion or enhancement of an 
incomplete agglutinating system by the addition of certain substances. Some 
bacteria or erythrocyte suspensions do not exhibit the visible agglutination 
ordinarily expected after they have been coated with their specific antibodies 
and complement. Further addition of a conglutinating agent - normal bovine 
serum - however, initiates visible agglutination. Complement is an essential 
component of the system. The conglutination reagent is itself without effect 
in the absence of antibody and complement. Similar overall actions, such as 
agglutination of Rhsupp cells, occur when human serum, gelatin, or bovine 
serum albumin are added to the incomplete Rh antibodies found in some sera, 
but since this enhancing effect occurs also in the absence of complement, 
these reactions probably are to be distinguished in mechanism from that of the 
traditional conglutination. The conglutination reaction can be used for a 
variety of serological diagnostic reactions in bacterial and viral infections. 
See AGGLUTINATION REACTION; BLOOD GROUPS; COMPLEMENT; COMPLEMENT-FIXATION TEST. 
 �34s[Henry P. Treffers]
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�6v�2v�32sConiconchia   �21m �33s 
    A fossil group that has never seen general usage and probably should be 
abandoned. The hyoliths make up about half of the Coniconchia. The systematic 
position of Hyolitha continues to be troublesome. B. Runnegar and others 
suggest it constitutes an extinct phylum, whereas L. Mareck and E. L. 
Yochelson consider it as an extinct class of Mollusca. Studies of shell 
structure by J. Carter inferentially support assignment to the mollusks. Final 
systematic placement is still uncertain. It would appear that Tentaculitida 
and Dacryoconarida constitute classes within an extinct phylum. Possibly these 
fossils were lophophorate ``worms.'' See MOLLUSCA. 
 �34s[Ellis L. Yochelson]
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�6v�2v�32sConic section   �21m �33s 
    One of the class of curves in which a plane may cut a cone (surface) of 
revolution. The section is a parabola if the plane is parallel to an element 
of the cone, an ellipse or circle if the plane cuts all elements of one nappe 
(but does not go through the apex), and a hyperbola if the plane cuts elements 
of both nappes (for example, the plane parallel to the cone's axis of 
revolution) and does not go through the apex (see illustration). 
  
    After the advent of analytic geometry, the synthetic method used by the 
Greeks to develop the properties of conics gave way to algebraic procedures  
which, using other definitions, divorced these curves from their original 
relationship to cones and regarded them as graphs of second-degree equations 
in cartesian coordinates x,y. The third phase in the study of conic sections 
began with the development of projective geometry. See ANALYTIC GEOMETRY; 
CONE; ELLIPSE; HYPERBOLA; PARABOLA; PROJECTIVE GEOMETRY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sConjugales   �21m �33s 
    A strictly fresh-water order of the Chlorophyceae sharply distinguished by 
two characteristics: the lack of flagellated cells, either vegetative or 
reproductive, and the conjugation method of sexual reproduction. Another 
general feature is the large size of the chloroplast, which takes various 
forms - spiral ribbons, axial plates, stellate masses, or flanged axial 
bodies. The chloroplasts, usually few in number per cell, are rich  in 
chlorophyll and have conspicuous pyrenoids, though these are lacking in a few 
instances. See CELL PLASTIDS; CHLOROPHYLL. 
    The plants are either unicells or unbranched filaments without 
basal-distal differentiation. All are uninucleate. They are free-floating, 
although some forms may be incidentally adherent or loosely attached by 
adventitious rhizoids, as Spirogyra is on rocks. The cell wall has an inner 
layer of cellulose and an outer of pectic material, giving the plant a 
slippery feel. 
    There are three families, the Mesotaeniaceae, Desmidiaceae, and 
Zygnemataceae. The first two are mostly unicellular and known commonly as 
desmids; the last is entirely filamentous. See ALGAE; CHLOROPHYCEAE. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sConjugation (chemistry)   �21m �33s 
    An arrangement of bonds in a molecule such that a single bond lies 
directly between two multiple (that is, double or triple) bonds or between a 
multiple bond and a group containing a lone PI-electron, a PI-electron pair or 
quartet, or a PI-electron vacancy makes the molecule show unusual chemical 
behavior and physical properties. Such molecules and such multiple bonds are 
called conjugated. Simple examples are 1,3-butadiene (H2C|DOUBLE 
BAR|CH|--|CH|DOUBLE BAR|CH2) and allylamine (H2C|DOUBLE BAR|CH|--|NEH2). 
    The unusual properties of conjugated molecules can be understood 
theoretically, according to quantum mechanics, in terms of a bond structure in 
which, in the two examples mentioned, small proportions of H2C|--|CH|DOUBLE 
BAR|CH|--|CH2 or H2C-|--|CH|DOUBLE BAR|NsuppH2, respectively, ``resonate 
with'' the main bond structures given above. These examples are considered to 
exhibit sacrificial conjugation, since in the minor resonance structures there 
is one less bond, or else much energy is required to transfer a charge. See 
CHEMICAL BONDING; RESONANCE (MOLECULAR STRUCTURE). 
    Stronger resonance effects occur in conjugated molecules in which 
alternative structures with equal numbers of bonds can be written (isovalent 
conjugation), for example, in benzene (see illustration) or the allyl ion 
(H2C|DOUBLE BAR|CH|--|CsuppH2 and H2Csupp|--|CH|DOUBLE BAR|CH2) or radical 
H2C|DOUBLE BAR|CH|--|CH2 and H2C|--CH|DOUBLE BAR|CH2). Conjugation brings 
about energy stabilization, but much more so in isovalent than in sacrificial 
conjugation.  
 �34s[Robert S. Mulliken]
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�6v�2v�32sConjunctivitis   �21m �33s 
    An inflammation of the thin transparent membrane that covers the posterior 
surfaces of the eyelids and the anterior surface of the white of the eye. It 
is the most common eye disease in the Western Hemisphere and varies in 
severity from a mild redness (hyperemia) with tearing, such as hay-fever 
conjunctivitis, to a severe destrictive process. 
    Infectious agents that may cause conjunctivitis are bacteria, viruses, and 
rickettsia. Fungal infection is rare. Allergic conjunctivitis may take the 
form of a hay-fever conjunctivitis or a seasonal type (vernal conjunctivitis). 
Chemical inflammation of the conjunctiva may result from eye drops, drugs, 
acids, or other chemicals. Conjunctivitis may also be associated with systemic 
diseases, such as thyroid disease or gout. 
    The important signs of conjunctivitis include: redness, tearing, 
secretions, a drooping of the eyelids, and balloonlike swelling of the 
conjunctival tissue. Enlargement of the glands located immediately in front of 
the ear indicates a virus-type conjunctivitis. 
 �34s[Jack Hartstein]
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�6v�2v�32sConnecting rod   �21m �33s 
    A link in several kinds of mechanisms. Usually one end of a connecting rod 
is intended to follow a circular path, while the other end follows a path 
along a straight line or curve of large radius. The term is sometimes applied, 
however, to any straight link that transmits motion or power from one linkage 
to another within a mechanism. The illustration shows some conventional 
arrangements.  
    The connecting rod of the four-bar linkage, often called the coupler, has 
special significance. The motion of its plane can now be synthesized to 
furnish desired paths for points, or desired positions of the entire 
connecting-rod plane. The connecting rod is then not primarily used for 
transmission of force or motion from input to output crank, but the entire 
mechanism is employed to impart to the connecting-rod plane certain 
displacements, and sometimes velocities and accelerations. See FOUR-BAR 
LINKAGE; MECHANISM. 
 �34s[Douglas P. Adams]
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�6v�2v�32sConnective tissue   �21m �33s 
    One of the four primary tissues of the body. It differs from the other 
three tissues in that the extracellular components (fibers and intercellular 
substances) are abundant. It cannot be sharply delimited from the blood, whose 
cells may give rise to connective tissue cells, and whose plasma components 
continually interchange with and augment the ground substance of connective 
tissue. Bone and cartilage are special kinds of connective tissue. 
    The functions of connective tissues are varied. They are largely 
responsible for the cohesion of the body as an organism, of organs as 
functioning units, and of tissues as structural systems. The connective 
tissues are essential for the protection of the body both in the elaborate 
defense mechanisms against infection and in repair from chemical or physical 
injuries. Nutrition of nearly all cells of the body and the removal of their 
waste products are both mediated through the connective tissues. Connective 
tissues are important in the development and growth of many structures. 
Constituting the major environment of most cells, they are probably the major 
contributor to the homeostatic mechanisms of the body so far as salts and 
water are concerned. They act as the great storehouse for the body of salts 
and minerals, as well as of fat. The connective tissues determine in most 
cases the pigmentation of the body. Finally, the skeletal system (cartilage 
and bones) plus other kinds of connective tissue (tendons, ligaments, fasciae, 
and others) make motion possible. 
    The connective tissues consist of cells and extracellular or intercellular 
substance (see illustration). The cells include many varieties, of which the 
following are the most important: fibroblasts, macrophages (histiocytes), mast 
cells, plasma cells, melanocytes, and fat cells. Most of the cells of the 
connective tissue are developmentally related even in the adult; for example, 
fibroblasts may be developed from histiocytes or from undifferentiated 
mesenchymal cells. 
    The extracellular components of connective tissues may be fibrillar or 
nonfibrillar. The fibrillar components are reticular fibers, collagenous 
fibers, and elastic fibers. The nonfibrillar component of connective tissues 
appears amorphous with the light microscope and is the matrix in which cells 
and fibers are embedded. It consists of two groups of substances: (1) those 
probably derived from secretory activity of connective tissue cells including 
mucoproteins, protein-polysaccharide complexes, tropocollagen, and antibodies; 
and (2) those probably derived from the blood plasma, including albumin, 
globulins, inorganic and organic anions and cations, and water. In addition, 
the ground substance contains metabolites derived from, or destined for, the 
blood.  
    All the manifold varieties of connective tissue may contain all the cells 
and fibers discussed above in addition to ground substance. They differ from 
each other in the relative occurrence of one or another cell type, in the 
relative proportions of cells and fibers, in the preponderance and arrangement 
of one or another fiber, and in the relative amount and chemical composition 
of ground substance. They are classified as: 
    1. Irregularly arranged connective tissue - which may be loose 
(subcutaneous connective tissue) or dense (dermis). The dominant fiber type is 
collagen.  
    2. Regularly arranged connective tissue - primarily collagenous - with the 
fibers arranged in certain patterns depending on whether they occur in tendons 
or as membranes (dura mater, capsules, fasciae, aponeuroses, or ligaments). 
    3. Mucous connective tissue - ground substance especially prominent 
(umbilical cord). 
    4. Elastic connective tissue - predominance of elastic fibers or bands 
(ligamentum nuchae) or lamellae (aorta). 
    5. Reticular connective tissue - fibers mostly reticular, moderately rich 
in ground substance, frequently numerous undifferentiated mesenchymal cells. 
    6. Adipose connective tissue - yellow or brown fat cells constituting 
chief cell type, reticular fibers most numerous. 
    7. Pigment tissue - melanocytes numerous. 
    8. Cartilage - cells exclusively of one type, derived from mesenchymal 
cells.  
    9. Bone - cells are predominantly osteocytes, but also include 
fibroblasts, mesenchymal cells, endothelial cells, and osteoclasts. 
_See ADIPOSE TISSUE; BLOOD; BONE; CARTILAGE; COLLAGEN; HISTOLOGY; LIGAMENT; 
TENDON.  
 �34s[Isidore Gersh]
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�6v�2v�32sConodont   �21m �33s 
    A member of a group of extinct animals, the Conodonta, which were abundant 
in the seas from early in the Cambrian Period almost to the end of the 
Triassic Period. Conodonts are found as fossils in most marine sedimentary 
rocks, in which they are represented by tiny cone-, bar-, or platform-shaped 
skeletal elements (see illustration). These range from less than 0.04 in. (1 
mm) to somewhat more than 0.16 in. (4 mm) in size and, in unaltered form, are 
translucent and amber in color. They contain minor amounts of organic 
substances, but consist largely of phosphatic mineral matter which approaches 
the carbonate apatites in composition. 
    Most conodont elements are collected as discrete entities because the soft 
tissues of the conodont decayed rapidly upon the death of the animal. However, 
a few clusters of elements (called element clusters or natural assemblages) 
have been found on the surfaces of black-shale slabs. These elements are 
throught to be more or less undisturbed skeletons of individual conodonts. 
Analyses of the components of such clusters (and of others generated from 
large collections of discrete elements by various grouping techniques) reveal 
that a complete conodont skeleton may have consisted of 20 or  more separate 
elements, representing as many as seven different shape categories. 
    Most present-day students of conodonts base species on recurrent groups of 
differently shaped conodont elements (rather than on elements of different 
shape), and then group these species into genera and higher taxonomic 
categories on the basis of overall composition of the skeletal apparatuses. In 
this so-called multielement taxonomy careful attention must be paid not only 
to the composition of the element group but also to such characters as the 
shape, microscopic surface ornamentation, and internal structure of individual 
elements. More than 300 genera of conodonts have been named, but there is 
little agreement as to how they should be grouped together into families or 
larger taxonomic units that can be interpreted to be genetically interrelated. 
 �34s[Walter C. Sweet]
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�6v�2v�32sConservation of energy �21m �33s
    The principle of conservation of energy states that energy cannot be
created or destroyed, although it can be changed from one form to another.
Thus in any isolated or closed system, the sum of all forms of energy remains
constant. The energy of the system may be interconverted among many different
forms - mechanical, electrical, magnetic, thermal, chemical, nuclear, and so
on - and as time progresses, it tends to become less and less available; but
within the limits of small experimental uncertainty, no change in total amount
of energy has been observed in any situation in which it has been possible to
ensure that energy has not entered or left the system in the form of work or
heat. For a system that is both gaining and losing energy in the form of work
and heat, as is true of any machine in operation, the energy principle asserts
that the net gain of energy is equal to the total change of the system's
internal energy. See THERMODYNAMIC PRINCIPLES.
    There are many ways in which the principle of conservation of energy may
be stated, depending on the intended application. Of particular interest is
the special form of the principle known as the principle of conservation of
mechanical energy which states that the mechanial energy of any system of
bodies connected together in any way is conserved, provided that the system is
free of all frictional forces, including internal friction that could arise
during collisions of the bodies of the system.
    J. P. Joule and others demonstrated the equivalence of heat and work by
showing experimentally that for every definite amount of work done against
friction there always appears a definite quantity of heat. The experiments
usually were so arranged that the heat generated was absorbed by a given
quantity of water, and it was observed that a given expenditure of mechanical
energy always produced the same rise of temperature in the water. The
resulting numerical relation between quantities of mechanical energy and heat
is called the Joule equivalent, or mechanical equivalent of heat.
    In view of the principle of equivalence of mass and energy in the
restricted theory of relativity, the classical principle of conservation of
energy must be regarded as a special case of the principle of conservation of
mass-energy. However, this more general principle need be invoked only when
dealing with certain nuclear phenomena or when speeds comparable with the
speed of light (1.86 x 10 sup 5 mi/5 or 3 x 10 sup 8 m/s) are involved.
 �34s[Duane E. Roller /Leo Nedelsky]
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�6v�2v�32sConservation of mass   �21m �33s 
    The notion that mass, or matter, can be neither created nor destroyed. 
According to conservation of mass, reactions and interactions which change the 
properties of substances leave unchanged their total mass; for instance, when 
charcoal burns, the mass of all of the products of combustion, such as ashes, 
soot, and gases, equals the original mass of charcoal and the oxygen with 
which it reacted. 
    The special theory of relativity of Albert Einstein, which has been 
verified by experiment, has shown, however, that the mass of a body changes as 
the energy possessed by the body changes. Such changes in mass are too small 
to be detected except in subatomic phenomena. Furthermore, matter may be 
created, for instance, by the materialization of a photon (quantum of 
electromagnetic energy) into an electron-positron pair; or it may be 
destroyed, by the annihilation of this pair of elementary particles to produce 
a pair of photons. See ELECTRON-POSITRON PAIR PRODUCTION; RELATIVITY. 
 �34s[Leo Nedelsky]
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�6v�2v�32sConservation of momentum   �21m �33s 
    The principle that, when a system of masses is subject only to forces that 
masses of the system exert on one another, the total vector momentum of the 
system is constant. Since vector momentum is conserved, in problems involving 
more than one dimension the component of momentum in any direction will remain 
constant. The principle of conservation of momentum holds generally and is 
applicable in all fields of physics. In particular, momentum is conserved even 
if the particles of a system exert forces on one another or if the total 
mechanical energy is not conserved. Use of the principle of conservation of 
momentum is fundamental in the solution of collision problems. See COLLISION 
(PHYSICS); MOMENTUM. 
 �34s[Paul W. Schmidt]
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�6v�2v�32sConservation of resources   �21m �33s 
    Conservation is concerned with the utilization of resources - the rate, 
purpose, and efficiency of use. For treatment of individual resources see 
FISHERY CONSERVATION; FOREST MANAGEMENT AND ORGANIZATION; LAND-USE PLANNING; 
MINERAL RESOURCES CONSERVATION; RANGELAND CONSERVATION; SOIL CONSERVATION; 
WATER CONSERVATION; WILDLIFE CONSERVATION. 
    Universal natural resources are the land and soil, water, forests, 
grassland and other vegetation, fish and wildlife, rocks and minerals, and 
solar and other forms of energy. Some natural resources, such as metallic 
ores, coal, petroleum, and stone, are called fund or stock resources. They 
usually are referred to as nonrenewable natural resources because extraction 
from the stock depletes the usable quantity remaining and, even if some is 
being formed, the rate of formation is too slow  for practical meaning. Other 
natural resources, such as living organisms and their products and solar and 
atomic radiation, are called flow resources. They usually are referred to as 
renewable natural resources because they involve organic growth and 
reproduction or because they are relatively quickly recycled, or renewed in 
nature, as in the case of water in the hydrologic cycle and certain 
atmospheric phenomena. Some natural resources are difficult to fit into such a 
simple system. Soil, for example, is commonly thought of as a renewable, as 
erosion and nutrient depletion can in some cases be rather quickly corrected, 
but if the upper layers of the soil are removed or bedrock is exposed, renewal 
may take thousands of years. Water also is commonly renewable, but rapid 
extraction of water by wells from deep aquifers may be equivalent to mining 
minerals.  
    Human resources are of two types: the people themselves (their numbers, 
qualities, knowledge, and skills) and their culture (the tools and 
institutions of society). The three broad classes of resources correspond to 
the economic factors of production: land (natural resources), labor (personal 
human resources), and capital (cultural tools and institutions). The natural 
resources have meaning only as there is human ability to make use of them. 
    Conservation has received many definitions because it has many aspects. 
Conservation receives impetus from the social conscience aware of an 
obligation of future generations and is viewed differently according to one's 
social and economic philosophy. To some extent, the meaning of conservation 
changes with the time and place. It is understood differently when approached 
from the natural sciences and technologies, and when it is approched from the 
social sciences. 
    No definition of conservation exists that is satisfactory to all elements 
of the public and applicable to all resources. In its absence, an operational 
or functional definition can be arrived at by considering a series of 
conservation measures. 
    1. Preservation is the protection of nature from commercial exploitation 
to prolong its use for recreation, watershed protection, and scientific study. 
It is familiar in the establishment and protection of parks and reserves of 
many kinds. 
    2. Restoration, another widely familiar conservation measure, is 
essentially the correction of past willful and inadvertent abuses that have 
impaired the productivity of the resources base. This measure is familiar in 
modern soil and water conservation practices applied to agricultural land. 
    3. Beneficiation is the upgrading of the usefulness or quality of 
something, for instance, the utilization of ores that were formerly of 
uneconomic grade. Modern technology has provided many examples of this type of 
conservation.  
    4. Maximization includes all measures to avoid waste and increase the 
quantity and quality of production from resources. 
    5. Reutilization, in industry commonly called recycling, is the reuse of 
waste materials, as in the use of scrap iron in steel manufacture or of 
industrial water after it has been cleaned and cooled. 
    6. Substitution, an important conservation measure, has two aspects: the 
use of a common resource instead of a rare one when it serves the same end and 
the use of renewable rather than nonrenewable resources when conditions permit. 
    7. Allocation concerns the strategy of use - the best use of a resource. 
For many resources and products from them, the market price, as determined by 
supply and demand, establishes to what use a resource is put, but under 
certain  circumstances the general welfare may dictate usage and resources may 
be controlled by government through the use of quotas, rationing, or outright 
ownership.  
    8. Integration in resources management is a conservation measure because 
it maximizes over a period of time the sum of goods and services that can be 
had from a resource or a resource complex such as a river valley; this is 
preferable to maximizing certain benefits from a single resource at the 
expense of other benefits or other resources. This is one of the meanings of 
multiple use, and integration is a central objective of planning. 
    A generalized definition that fits many but not all meanings of 
conservation is ``the maximization over time of the next social benefits in 
goods and services from resources.'' 
    National resources policy in the United States is framed by acts of 
Congress and in the states by acts of legislatures. The language of specific 
acts, however, usually permits some freedom for administrative decisions and 
also permits different interpretations that must be settled in the courts of 
law. As the country has developed and conditions have changed, a sequence of 
laws has been passed to deal with the exigencies of natural resource 
conditions. Also, there has occurred some evolution of political philosophy to 
meet the changing conditions. In addition to legislative and court actions, 
some natural resources are subject to treaties and other international 
agreements.  
 �34s[Stanley A. Cain]
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�6v�2v�32sConstellation �21m �33s
    A name used in astronomy originally to designate any one of the star
groups that were imagined to form configurations in the sky. Now the term
constellation also refers to any one of the definite areas of the sky. The
ancients originated most of the constellations visible in northern latitudes.
Ptolemy, the Alexandrian astronomer, recorded 48 of them 2000 years ago, as
listed in the table; they are still used. The ancients living in northern
latitudes were unable to see the southern sky; hence it was not mapped. In
1603 J. Bayer charted the southern regions and named 13 new southern
constellations, and in 1690 Hevelius added nine more. Finally in 1763 N. L. de
Lacaille completed the list by adding 14 names to new constellations. These
constellations, together with Coma Berenices (Berenice's Hair) whose origin is
uncertain, make up a total of 88 groups, now recognized and universally
accepted.
    Constellation boundaries were irregular. When only bright stars were
considered, there was no ambiguity in locating them. But when fainter and
fainter stars were considered and new and more constellations were added, the
sky areas became so crowded that uncertainty and confusion naturally resulted
at the boundaries. To remedy the confusion and to secure a general agreement on
the boundaries, the International Astronomical Union in 1928 instituted a
much-needed reform by remapping all the constellations. Replacing the irregular
and uncertain outlines, the new boundaries now run only from north to south and
from east to west, though they zigzag considerably to contain as many stars as
possible in their former constellations. See ZODIAC.
[Ching-Sung Yu]

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�6v�2v�32sConstipation �21m �33s
    The passage of hard, dry stools. The term has come to indicate the
irregularity or absence of bowel movements for any period of time considered
abnormal by an individual. However, the character of the stool is generally of
more significance than the interval between bowel movements or the quantity of
feces evacuated. Atonic constipation is that form not accompanied by signs of
abdominal griping or distress, whereas hypertonic constipation is marked by
spasms, spasticity of the bowel, and abnormal activity of its movements.
    Common causes of constipation include faulty nutrition, inadequate water
intake, poor general health, and emotional stresses or tensions. The use and
abuse of laxatives is a common finding in cases of irregularity and bowel
complaints. Local disease of the colon such as inflammation, fistulas, tumors,
polyps, and obstructions may produce constipation, sometimes alternating with
diarrhea. Certain systemic diseases such as hyperthyroidism, circulatory
disturbances, stomach ailments, and infections may be accompanied by
constipation.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sConstraint   �21m �33s 
    A restriction on the natural degrees of freedom of a system. If n and m 
are the numbers of the natural and actual degrees of freedom, the difference n 
- m is the number of constraints. In principle n = 3N, where N is the number 
of particles, for example, atoms. In practice n is determined by the number of 
effectively rigid components. 
    A holonomic system is one in which the n original coordinates can be 
expressed in terms of m independent coordinates and possibly also the time. It 
is characterized by frictionless contacts and inextensible linkages. The new 
coordinates are called generalized coordinates. See LAGRANGE'S EQUATIONS. 
    Nonholonomic systems cannot be reduced to independent coordinates because 
the constraints are not on the n coordinate values themselves but on their 
possible changes. For example, an ice skate may point in all directions but at 
each position is must point along its path. See DGREES OF FREEDOM. 
 �34s[Bernard Goodman]
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�6v�2v�32sConstruction engineering   �21m �33s 
    A specialized branch of civil engineering concerned with the planning, 
execution, and control of construction operations for such projects as 
highways, buildings, dams, airports, and utility lines. Planning consists of 
scheduling the work to be done and selecting the most suitable construction 
methods and equipment for the project. Execution requires the timely 
mobilization of all drawings, layouts, and materials on the job to prevent  
delays to the work. Control consists of analyzing progress and cost to ensure 
that the project will be done on schedule and within the estimated cost. See 
CONSTRUCTION EQUIPMENT; CONSTRUCTION METHODS. 
 �34s[William Hershleder]
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�6v�2v�32sConstruction equipment   �21m �33s 
    A wide variety of relatively heavy machines which perform specific 
construction (or demolition) functions under power. The power plant is 
commonly an integral part of an individual machine, although in some cases it 
is contained in a separate prime mover, for example, a towed wagon or roller. 
It is customary to classify construction machines in accordance with their 
functions such as hoisting, excavating, hauling, grading, paving, drilling, or 
pile driving. The selection of a machine for a specific job is mainly a 
question of economics and depends primarily on the ability of the machine to 
complete the job efficiently, and secondarily on its availability. 
    Hoisting equipment is used to raise or lower materials from one elevation 
to another or to move them from one point to another over an obstruction. The 
main types of hoisting equipment are derricks, cableways, cranes, elevators, 
and conveyors. See BULK-HANDLING MACHINES; HOISTING MACHINES. 
    The two main types of derricks are the guy and the stiff-leg. The former 
has a mast that is held in a vertical position by guy wires and a boom that 
can rotate with the mast 360DG; the latter has the mast tied to two or more 
rigid structural members, with rotation of the boom limited by the position of 
these members. The derrick is practical only where little mobility is required. 
    The cableway is a combination hoist and tram system comprising a trolley 
that runs on main load cables stretched between two or more towers which may 
be stationary or tiltable. Its use in construction is generally restricted to 
dams and other special cases. 
    The construction elevator, like the passenger elevator, consists of a car 
or platform that operates within a structural framework and is raised and 
lowered by cables. It is limited to moving relatively light materials on jobs 
of small areas. 
    A crane is basically a fast-moving boom mounted on a frame containing the 
power supply and mechanisms for moving the boom and for raising and lowering 
the load-bearing cables that run through sheaves at the top of the boom. 
Mobile cranes are predominant, but tower cranes and climbing cranes have 
become popular throughout the United States and the world for high-rise 
building construction where mobility requirements are negligible. 
    Occasionally a conveyor is used to raise materials needed for construction 
of low buildings. This machine, which has a movable endless belt mounted on a 
frame, can carry only light loads such as bricks and cement sacks and is not 
practical for buildings of more than one or two floors. See CONVEYING MACHINES. 
    Excavating equipment is divided into two main classifications: standard 
land excavators and marine dredges, each of which has many variations. 
Standard land excavators are machines that dig earth and rock and place it in 
separate hauling units, as well as those that pick up and transport the 
materials. Among the former are power shovels, draglines, backhoes, cranes 
with a variety of buckets, front-end loaders, excavating belt loaders, 
trenchers, and the continuous bucket excavator. The second group includes such 
machines as bulldozers, scrapers of various types, and sometimes the front-end 
loader. Combination excavators and haulers include bulldozers, carrying 
scrapers, self-loading or elevating scrapers, and sometimes front-end loaders. 
    Hauling equipment is used to move excavated materials great distances. The 
most common conveyances are the self-propelled rubber-tired rear-dump trucks. 
Conveyors, while not commonly used on construction jobs for hauling earth and 
rock great distances, have been used to good advantage on large jobs where 
obstructions make impractical the passage of trucks. 
    Graders are high-bodied, wheeled vehicles that mount a leveling blade 
between the front and rear wheels. The principal use is for fine-grading 
relatively loose and level earth. Commonly considered a maintenance unit, the 
grader is still an important machine in highway construction. 
    Pavers place, smooth, and compact paving materials. They may be mounted on 
rubber tires, endless tracks, or flanged wheels. Asphalt pavers embody tamping 
pads that consolidate the material; concrete pavers use vibrators for the same 
purpose.  
    Drilling equipment is used for drilling wells and for blasting, grouting, 
and exploring. Drills are classified according to the way in which they 
penetrate rock, namely, percussion, rotary percussion, and rotary. 
    Among the most common specialized construction equipment are augers, 
compactors, and pile hammers. Augers are used for drilling wells, dewatering 
purposes, and cutting holes that can be filled with concrete for foundations. 
Compactors are machines designed solely to consolidate earth and paving 
materials to sustain loaders greater than those sustained in the uncompacted 
state. Actual compaction may be achieved by heavily loaded rubber tires or 
steel rollers, or by a vibratory action induced into the compacting units so 
that compaction is achieved by impact force rather than sheer weight. Pile 
hammers are machines used to install bearing piles for foundations, or sheet 
piles for cofferdams and retaining walls. Conventional hammers contain pistons 
that are raised and then either allowed to fall freely or to be driven 
downward to impart an impact to the pile. Vibratory hammers have been 
developed which vibrate piles into place. 
    Steam is seldom used as a source of power today except in some marine 
applications and rarely for pile driving. Gasoline and diesel engines of the 
piston type are the most common source of power for construction machines, 
with the diesel attaining an increasingly greater prominence for at least two 
major reasons. First, the economy of operation and maintenance of the diesel 
has come to far out weigh its greater initial cost. Second, manufacturers have 
been able to build lightweight economical diesel engines in very small sizes, 
making practical their application in small units. 
    Electricity is sometimes, though not often, used as a primary source of 
power for some construction machines. Electricity is making its appearance as 
a secondary source of power on several types of mobile construction machinery. 
A more common type of secondary power, however, is the hydraulic motor 
activated by a gasoline or diesel-powered hydraulic pump. 
 �34s[Edward M. Young]
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�6v�2v�32sConstruction methods   �21m �33s 
    The procedures and techniques utilized during construction. Construction 
operations are generally classified according to specialized fields. These 
include preparation of the project site, earthmoving, foundation treatment, 
steel erection, concrete placement, asphalt paving, and electrical and 
mechanical installations. Procedures for each of these fields are generally 
the same, even when applied to different projects, such as buildings, dams, or 
airports. However, the relative importance of each field is not the same in 
all cases. For a description of tunnel construction, which involves different 
procedures, see TUNNEL. 
 �34s[William Hershleder]
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�6v�2v�32sConsumer electronics   �21m �33s 
    A variety of consumer electronic products are available which simulate 
human characteristics of listening, talking, amusing, and educating. Among 
these microprocessor-controlled devices are hand-held language translators, 
programmable personal computers, educational talking toys, intelligent 
television receivers, and portable remote terminals that utilize the telephone 
system to gain access to various computer data bases. These electronic devices 
are made possible through electronic microminiaturization called large-scale 
integration (LSI). Large-scale integration allows more features to be 
implemented in compact, relatively inexpensive products that were bulky and 
expensive only a few years ago.  See  INTEGRATED CIRCUITS. 
    Portable language translators register keyboard entry of phrases in one 
language and speak the translated phrase in another. Each language for 
translation is preserved in memory modules called read-only-memory (ROM), 
which are basically tables storing corresponding words in both languages. 
    Hand-held computers have been developed that fit into a briefcase which 
has receptacles for acoustically coupling the computer with any telephone. A 
variety of information can thereby be accessed while the user is out of reach 
of data bases.  See  MICROCOMPUTER. 
    Two types of electronic games exist: those that measure users' dexterity 
and those that challenge their intelligence. Both types of games can be played 
alone against the computer or between two players. 
    Decoders are being developed that will enable text and graphics to be 
retrieved on the home television screen. The information is received by the 
television set as part of the regular broadcast but cannot be seen by the 
viewer unless a decoder is used. The decoder transforms information taken from 
the unseen (blanking) portion of the screen to the visible portion. 
    A more sophisticated system, called viewdata, is an outgrowth of the 
business community. Here the television again serves as the receiver of data 
and graphics and displays them when the decoder is activated; but the 
information reaches the set through the telephone. By modulating the voice 
line on the telephone, digital bit streams are sent through these circuits to 
the decoder. As a result, different data bases can be accessed by dialing 
their respective codes. In teletext a remote keypad is used to access the 
pages as they are broadcast. The switching process takes a few seconds and 
limits the viewer's participation in the service. In viewdata, on the other 
hand, the keypad is used as a two-way transmitter and can be used to engage in 
a two-way conversation with the system.  See  VIDEOTEXT AND TELETEXT. 
 �34s[Nicolas Mokhoff]
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�6v�2v�32sContact aureole   �21m �33s 
    A halo or zone of alteration surrounding a body of igneous rock (such as 
granite or granodiorite) and presumably formed by heat and emanations from the 
igneous mass. Aureoles are usually not more than a few thousand feet wide. 
They may be detected by mineralogical or textural changes in the surrounding 
(country) rocks, which become progressively more intense as the igneous 
contact is approached. The aureole, thus, represents a shell of changed or 
metamorphosed rock. See IGNEOUS ROCKS. 
    Aureoles are generally most pronounced where developed in previously 
unmetamorphosed or weakly metamorphosed rocks. They may not be detected where 
impressed upon intensely metamorphosed rocks. Shale and limestone generally 
show a more conspicuous aureole than does sandstone. The metamorphic effect 
around small igneous bodies is generally less extensive and intensive than 
that around large ones. The country rock against small dikes and sills may 
only be baked or indurated, whereas against stocks, large sheets, and 
laccoliths it is usually recrystallized. See PLUTON. 
    The inner boundary of the aureole (against the plutonic rock) may be sharp 
or transitional. Along some contacts abundant fragments (inclusions) of 
country rock appear to have been ripped off and enclosed in the granite, and 
apophyses (offshoots) of granite extend well into the country rock. Along 
other contacts, notably where granite comes against well-bedded or foliated 
rocks, migmatites (mixed rocks) have developed. See MIGMATITE. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sContact condenser   �21m �33s 
    A device in which a vapor is brought into direct contact with a cooling 
liquid and condensed by giving up its latent heat to the liquid. In almost all 
cases the cooling liquid is water, and the condensing vapor is steam. Contact 
condensers are classified as jet, barometric, and ejector condensers. In all 
three types the steam and cooling water are mixed in a condensing chamber and 
withdrawn together. Noncondensable gases are removed separately from the jet 
condenser, entrained in the cooling water of the ejector condenser, and 
removed either separately or entrained in the barometric condenser. The jet 
condenser requires a pump to remove the mixture of condensate and cooling 
water and a vacuum breaker to avoid accidental flooding. The barometric 
condenser is self-draining. The ejector condenser converts the energy of 
high-velocity injection water to pressure in order to discharge the water, 
condensate, and noncondensables at atmospheric pressure. See VAPOR CONDENSER. 
 �34s[Joseph F. Sebald]
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�6v�2v�32sContact potential difference   �21m �33s 
    The potential difference that exists across the space between two 
electrically connected metals; also, the potential difference between the bulk 
regions of a junction of two semiconductors. In illustration a consider two 
metals M1 and M2 with work functions PHI1 and PHI2, respectively. When the 
metals are brought in contact, the Fermi levels, that is, the levels 
corresponding to the most energetic electrons, must coincide, as in 
illustration b. Consequently, if PHI1 > PHI2, metal M1 will acquire a negative 
and metal M2 a positive surface charge at the contact area. Contact potentials 
must be taken into account in analyzing physical electronics experiments. See 
ELECTRONIC WORK FUNCTION. 
 �34s[Adrianus J. Dekker]
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�6v�2v�32sContinent �21m �33s
    A protuberance of the Earth's crustal shell with an area of several
million square miles and with sufficient elevation above neighboring
depressions (the ocean basins) so that much of it is above sea level. There
are six continents: Eurasia (Europe, China, and India are parts of this
largest continent), Africa, North America, South America, Australasia
(including Australia and New Guinea), and Antarctica.
    The term was originally applied to the most extensive continuous land
areas of the globe. The great majority of maps now in use imply that the
boundaries of continents are their shorelines. From the geological point of
view, however, the line of demarcation between a continent and an adjacent
ocean basin lies offshore, at distances ranging from a few to several hundred
miles, where the gentle slope of the continental shelf changes somewhat
abruptly to a steeper declivity. This change occurs at depths ranging from a
few to several hundred fathoms at different places around the periphery of
various continents. See CONTINENTAL MARGIN.
    All continents have similar structural features but display great variety
in detail. Each includes a basement complex (shield) of metamorphosed
sedimentary and volcanic rocks of Precambrian age, with associated igneous
rocks, mainly granite. Originally formed at considerable depths below the
surface, this shield was later exposed by extensive erosion, then largely
covered by sediments of Paleozoic, Mesozoic, and Cenozoic age, chiefly marine
limestones, shales, and sandstones. In at least one area on each continent,
these basement rocks are now at the surface (an example is the Canadian Shield
of North America). In some places they have disappeared beneath a sedimentary
platform occupying a large fraction of the area of each continent, such as the
area in the broad lowland drained by the Mississippi River in the United
States. In each continent there are long belts of mountains in which thick
masses of sedimentary rocks have been compressed into folds and broken by
faults. See BASEMENT ROCKS; OROGENY.
    Continents are the less dense, subaerially exposed portion of the large
and not-so-large plates that make up the Earth's lithosphere, or outer shell
of rigid rock material. As such, continents together with part of the ocean's
floor are intimately joined portions of the lithospheric plates. As plates rip
apart and migrate horizontally over the Earth's surface, so too do continents
rip apart and migrate, sometimes colliding with other continental segments
scores of millions of years later. Such continental collision and accretion
are believed to have occurred throughout most of the Earth's history.
    The continents, or more accurately, the protocontinents, began to form in
the final stages of the origin of the Earth. But because the precise processes
contributing to the origin of the Earth are speculative, the initial stages of
continent formation also remain enigmatic. It is known that the Earth accreted
from clouds and chunks of solar matter about 4.6 x 10 sup 9 years ago. What is
not known in detail is the actual rate, or kind of accretionary process
involved, or the precise composition of the particles. The Earth's accretion
is assumed to have occurred over a period of perhaps 0.25 to 1 x 10 sup 8
years. The crust of the primordial Earth, and especially the protocontinents,
evolved during the final stages of the Earth's origin, essentially as the last
stage of its accretionary growth. See EARTH, AGE OF.
    Studies of the nature, and the ages, of continental rocks clearly indicate
that the rate of continent formation has varied with time. The oldest
continental rocks discovered to date occur in Minnesota, in southwestern
Greenland, and in southern Rhodesia. In these places granitic rocks, as well as
metamorphosed sedimentary rocks formed in ancient seas, range from 3.5 to
almost 4 x 10 sup 9 years old. These ancient segments of the continents are
almost as old as the oldest definitively dated lunar crust.
    If the view that the Earth accreted inhomogeneously is correct, the early
fragments of the continents were probably quite large, conceivably covering the
entire Earth. If this conclusion is accepted, then the continents have
decreased in area but have greatly increased in thickness and become more
granitic as they have aged. Hence the more striking changes in the formation of
the contemporary continents are the shape, area, and distribution over the
Earth. Less dramatic but critical changes have occurred in the structure and
composition of the continents, and in their relationships to the Earth's other
crusts: the oceanic crust and the island arcs.
    Generally, the continents are coupled with large fragments of oceanic crust
and ride piggyback on the entire outer, cooler lithosphere. Great fragments, or
plates, of the lithosphere appear to jostle and move over the softer upper
mantle of the Earth. A crude analogy is that of the movements of great plates
of Arctic and Antarctic ice in response to the currents in the underlying
ocean. The most recent major episode of continental drift that has given the
continents their present form and distribution began about 2 x 10 sup 8 years
ago. Immediately prior to that time, the Americas, India, Australia, and
Anarctica were joined to Africa and Europe and formed one vast megacontinental
cluster sometimes called Pangea. The reconstruction of events involved in the
breakup of Pangea, and of one of its largest fragments, called Gondwana, which
resulted in the opening of new oceans, especially the Atlantic and Indian
oceans, has preoccupied earth scientists for decades (see illustration). See
CONTINENTAL DRIFT.
    Studies of the oceans have revealed many of the major features of the
Earth's crusts and their origins. One of the most imposing and significant
features of the sea floor is the "ridge and rise complex." This is a vast,
largely submarine mountain chain dotted with numerous volcanoes. The complex
is world-girdling, extending some 37,000,000 mi (60,000 km) largely through
the median parts of ocean basins. In the Pacific Ocean, however, one great
segment of the complex, the East Pacific Rise, converges with the western edge
of Central and North America. The ridge-rise complex is a tensional feature,
with great central rifts from which emerge floods of a unique basaltic magma.
These eruptions of basalt continuously add to the existing ocean crust, which
is spreading away from the ridge-rise complex as it forms. See MARINE GEOLOGY.
    The evidence for sea-floor spreading is severalfold. One indication of
spreading lies in the patterns of the stripelike masses of basalt episodically
welded to the crust. These basaltic stripes, which crudely parallel the ridges,
have frozen into them the orientation of the Earth's magnetic field extant at
the time of volcanic eruption. In places, the ocean floor is striped for about
600 mi (1000 km) or more from the zones of spreading and on both sides of this
zone. The result is a series of symmetrical magnetic patterns, indicative of
the rate of sea-floor spreading. Further corroboration of sea-floor spreading
and continental drift is found in the pattens of sediments formed on the sea
floor, and in the trench-island arc systems that almost encircle the Pacific,
offshore from the continents. See ROCK MAGNETISM.
    A tantalizing question is the fate of the striped ocean crust that is being
conveyed away from the ridge-rise spreading centers, presumably by convection
in the Earth's mantle. This process is a dynamic one that forces the great
platelike masses of the lithosphere, with its overlying continents and ocean
crust, against other plates. Patterns of plate movements, that is, plate
tectonics, can be defined by their pushing and frictional movements relative to
one another, because the movements of adjoining or colliding plates produce
most of the world's earthquakes. A world map of recorded earthquakes defines
the outlines and patterns of the great lithospheric plates active today
throughout the world. Plate outlines and relative motions are further defined
both by direction of slip during the earthquakes and by the relative
displacements of the magnetic stripes frozen into the basalts of the sea floor.
See PLATE TECTONICS.
    One of the most fascinating relationships that emerges is the appearance of
essentially all the world's recent mountain belts, the evolving island arcs,
and the great trenches of the sea floor at sites where moving lithospheric
plates are converging and either colliding or overriding one another. The
worldwide pattern is completely consistent with the patterns of rift and drift
of the continents and with the emergence of the Atlantic, Indian, and smaller
oceans. See OCEANIC ISLANDS; OROGENY.			      [A.E.J. Engel]

�34s[Donald Lee Johnson]
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�6v�2v�32sContinental drift �21m �33s
    The concept that the world's continents once formed part of a single mass
and have since drifted into their present positions. Although it was outlined
by Alfred Wegener in 1912, the idea was not particularly new. Paleontological
studies had already demonstrated such strong similarities between the flora
and fauna of the southern continents between 300,000,000 and 150,000,000 years
ago that a huge supercontinent, Gondwana, containing South America, Africa,
India, Australia, and Antarctica, had been proposed. However, Gondwana was
thought to be the southern continents linked by land bridges, rather than
contiguous units.
    Wegener's ideas were almost universely rejected in 1928; the fundamental
objection was the lack of a suitable mechanism. Almost simultaneously with the
temporary eclipse of Wegener's theory, Arthur Holmes was considering a
mechanism that is still widely accepted. Holmes conceived the idea of
convective currents within the Earth's mantle which were driven by the
radiogenic heat produced by radioactive minerals within the mantle. At that
time, Holmes's ideas, like those of Wegener, were largely ignored.
Nonetheless, several geologists, particularly those living in the Southern
Hemisphere, continued to believe the theory and accumulate more data in its
support.
    By the 1950s, convincing evidence had accumulated, with studies of the
magnetization of rocks, paleomagnetism, beginning to provide numerical
parameters on the past latitude and orientation of the continental blocks.
Early work in North America and Europe clearly indicated how these continents
had once been contiguous and had since separated. The discovery of the
midoceanic ridge system also provided further evidence for the geometric
matching of continental edges, but the discovery of magnetic anomalies parallel
to these ridges and their interpretation in terms of sea-floor spreading
finally led to almost universal acceptance of continental drift as a reality.
See PALEOMAGNETISM; PLATE TECTONICS; TECTONOPHYSICS.		 [D.H.T.]

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�6v�2v�32sContinentality (meteorology)   �21m �33s 
    The attributes of notably greater temperature ranges with associated 
weather characteristics that make inland and continental regions distinctly 
different from marine areas or areas bordering large bodies of water. 
    The reasons are associated with the greatly different thermal behavior of 
water and land surfaces. The soil surfaces of the continents can be heated or 
cooled comparatively rapidly because the thermal effects are limited to a 
shallow surface layer. The heat received at the ground warms the top layers, 
which consequently become hot in summer. In winter, by contrast, the surface 
becomes notably cold since there is little conduction of heat from subsurface 
depths. As a result of the responsiveness of the surface air temperature to 
the temperature of the underlying soil surface, the air in contact with the 
ground undergoes temperature fluctuations of large amplitude, lagging slightly 
behind the annual and the diurnal periods of insolation. 
    On the other hand, the water surfaces which make up the oceans are heated 
or cooled much less rapidly, partly because the thermal effects extend to 
greater depths and partly because much of the solar energy absorbed at the sea 
surface is utilized for evaporation rather than heating. As a result, the 
upper layers of the ocean are not heated as strongly as the land surfaces in 
summer or cooled as strongly by radiation in winter. It follows that large 
annual and diurnal ranges in temperature with hot summers and cold winters are 
characteristic of the continental interiors of middle and high latitudes. In 
contrast, the summers over the oceans are cool, while the winters are 
comparatively mild. See MARINE INFLUENCE ON WEATHER AND CLIMATE. 
 �34s[Woodrow C. Jacobs]
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�6v�2v�32sContinental margin   �21m �33s 
    The submerged fringes of a continents, comprising three characteristic 
zones: the continental shelf, the continental slope, and the continental rise. 
The continental shelf is the zone around the continent, extending from the 
low-water line to the depth at which there is a marked increase in slope to 
greater depth. The continental slope is the declivity from the edge of the 
shelf extending down to great ocean depths. The shelf and slope comprise the 
continental terrace, which is the submerged fringe of the continent, 
connecting the shoreline with the 2J-mi-deep (4-km) abyssal ocean floor. 
    The continental shelf is a comparatively featureless plain, with an 
average width of 45 mi (72 km), sloping gently seaward at about 10 ft/mi (1.9 
m/km). At a depth of about 70 fathoms (128 m) there generally is an abrupt 
increase in declivity called the shelf break, or the shelf edge. This break 
marks the limit of the shelf, the top of the continental slope, and the brink 
of the deep sea. For some purposes, especially legal, the 100-fathom (200-m) 
line is conventionally taken as the limit of the shelf. Characteristically, 
the shelves are thinly veneered with clastic sands, silts, and silty muds, 
which are patchily distributed. Geologically, the shelf is an extension of, 
and in unity with, the adjacent coastal plain. The position of the shoreline 
is geologically ephemeral, being subject to constant prograding and 
retrograding, so that its precise position at any particular time is not 
important.  
    The drowned edges of the low-density ``granitic'' or sialic continental 
masses are the continental slopes. The continental plateaus float like 
icebergs in the Earth's mantle with the slopes marking the transition between 
the low-density continents and the heavier oceanic segments on the Earth's 
crust. The continental slopes are the most imposing escarpments on the Earth. 
The slope is comparatively steep. Most slopes resemble a straight mountain 
front but are highly irregular in detail; in places they are deeply incised by 
submarine canyons, some of which cut deeply into the shelf. Usually the slope 
does not connect directly with the sea floor; instead there is a transitional 
area, the continental rise, or apron, built by the shredding of sediments from 
the continental block. See MARINE GEOLOGY: SUBMARINE CANYON. 
 �34s[Robert S. Dietz]
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�6v�2v�32sContinuous-wave radar   �21m �33s 
    A radar in which the transmitter output is uninterrupted, in contrast to 
pulse radar, where the output consists of short pulses. The chief disadvantage 
of a continuous-wave (CW) radar is that it is difficult to use a single 
antenna for both reception and transmission, because both the transmitter and 
receiver are on all the time. This difficulty has been overcome by means of 
the use of isolation circuitry, which gives the receiver protection from a 
transmitter output up to 200 watts. Among the advantages of CW radar is its 
ability to measure velocity with extreme accuracy by means of the Doppler 
shift in the frequency of the echo. See DOPPLER RADAR; RADAR. 
    In order to measure the range of targets, some form of frequency 
modulation of the CW output must be used. In one very effective form of 
modulation, the carrier frequency of the transmitted signal is varied at a 
uniform rate. Range is determined by comparing the frequency of the echo with 
that of the transmitter, the difference being proportional to the range of the 
target that produced the echo. Systems in which this is done are known as 
FM-CW radars. A modified form of FM-CW radar employs long, but not continuous, 
transmission. This might be regarded as the same as transmitting extremely 
long pulses on a frequency-modulated carrier. Systems of this type are 
referred to as pulse compression radars. See FREQUENCY MODULATION. 
    The principal use of CW radar is in short-range missile guidance. 
Typically the missile's course is tracked from the ground while the missile is 
simultaneously illuminated. CW radar, in some cases the same radar, can be 
used both for tracking and illumination, although it is more common for pulse 
Doppler radar to be used for tracking. 
    A variant of CW radar is used to illuminate persons under surveillance by 
techniques employing semiconductor tracer diodes. These devices are either 
secreted on the subject's person without his or her knowledge or else 
concealed in protected objects the subject has stolen. Despite the fact that 
pulsed X-band sources would provide the necessary high power levels more 
conveniently, the requirement to reduce clutter makes it essential to use CW 
power.  
 �34s[John M. Carroll]
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�6v�2v�32sContour   �21m �33s 
    The locus of points of equal elevation used in topographic mapping. 
Contour lines represent a uniform series of elevations, the difference in 
elevation between adjacent lines being the contour interval of the given map. 
Thus, contours represent the shape of terrain on the flat map surface (see 
illustration). Closely spaced contours indicate steep ground; sparseness or 
absence of contours indicates gentle slope or flat ground. Contours do not 
cross each other unless there is an overhang. See TOPOGRAPHIC SURVEYING AND 
MAPPING.  
 �34s[Robert H. Dodds]
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�6v�2v�32sControl charts   �21m �33s 
    Charts for the analysis and presentation of data obtained from production 
processes and research investigations. They are also used to display data in 
other commercial applications, such as labor turnover, office efficiency, 
costs, and accident rates. In the simplest charts, results are plotted in 
sequence over time, so that trends and changes may be identified and acted 
upon. In a more advanced form, limits calculated by statistical methods are 
placed on the chart. Provided that results stay within these limits, no action 
is taken, but if any results fall outside the limits, some action is 
indicated. Such statistical control charts are known as Shewhart charts. 
    Due to natural variations in raw materials and processes, it is rare to 
find any two manufactured units which are exactly alike. There is also a 
tendency for industrial processes to change over time (due to tool wear and 
changes in external conditions, for example). In order to detect such changes, 
quality-control personnel take samples from the process output and make 
measurements on important characteristics of the units in the sample. In 
isolation the results of individual samples are not very informative, but when 
plotted on a control chart they provide an effective cumulative picture of 
process behavior. Together with a history of changes made to the process, they 
provide an invaluable diagnostic tool to production supervision. 
    Even when a process is running satisfactorily there is still some 
variation in results about the target value. Such variations are due to a 
system of chance causes, and provided they can be tolerated in the finished 
product, it is unnecessary and usually uneconomical to try and reduce them. 
Under these conditions the process is said to be in a state of statistical 
control. However, changes which move the process average away from its target 
must be identified and dealt with. These changes are due to assignable causes, 
as distinct from chance causes. The object of Shewhart control charts is to 
provide rules for determining whether changes are more likely due to 
assignable causes or chance variation. 
    The statistical theory on which Shewhart charts are based depends on the 
assumption that the underlying data are normally distributed, and this must be 
kept in mind when interpreting charts which are constructed by other methods, 
such as average characteristics, fraction defective, or defects per unit. 
Misleading indications can result from the assumptions being violated, and 
therefore a thorough analysis of the data by skilled statisticians may be 
necessary. Custom-designed charts may be required in some cases. See CONTROL 
SYSTEMS; INDUSTRIAL COST CONTROL; INSPECTION AND TESTING; QUALITY CONTROL. 
 �34s[John A. Clements]
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�6v�2v�32sControlled rectifier   �21m �33s 
    A three-terminal semiconductor junction device with four regions of 
alternating conductivity type (p-n-p-n), also called a thyristor. This 
switching device has a characteristic such that, once it conducts, the voltage 
in the circuit in which it conducts must drop below a threshold before the 
controlled rectifier regains control. Such devices are useful as high-current 
switches and may be used to drive electromagnets and relays. 
    The principle of operation can be understood by referring to the 
illustration. The central junction is reverse-biased (positive collector, 
grounded emitter). The wide n region between collector and base regions 
prevents holes injected at the collector junction from reaching the 
collector-to-base barrier by diffusion. The junction between emitter and base 
is the emitter. When operated as a normal transistor, this device shows a 
rapid increase of current gain with collector current. This effect may be due 
to a field-induced increase of transport efficiency across the floating n 
region, or to increased avalanching in the high-field barrier region, or to 
increased injection efficiency at the two forward-biased junctions, or to a 
combination of these phenomena. 
    If this device is operated as a three-terminal device, the switching 
between the nonconducting and conducting states can be controlled by the base. 
See SEMICONDUCTOR RECTIFIER. 
 �34s[Lloyd P. Hunter]
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�6v�2v�32sControl systems   �21m �33s 
    Interconnections of components forming system configurations which will 
provide a desired system response as time progresses. The steering of an 
automobile is a familiar example. The driver observes the position of the car 
relative to the desired location and makes corrections by turning the steering 
wheel. The car responds by changing direction, and the driver attempts to 
decrease the error between the desired and actual course of travel. In this 
case, the controlled output is the automobile's direction of travel, and the 
control system includes the driver, the automobile, and the road surface. The 
control engineer attempts to design a steering control mechanism which will 
provide a desired response for the automobile's direction control. Different 
steering designs and automobile designs result in rapid responses, as in the 
case of sports cars, or relatively slow and comfortable responses, as in the 
case of large autos with power steering. 
  
    Open- and closed-loop control.  The basis for analysis of a control system 
is the foundation provided by linear system theory, which assumes a 
cause-effect relationship for the components of a system. A component or 
process to be controlled can be represented by a block. Each block possesses 
an input (cause) and output (effect). The input-output relation represents the 
cause-and-effect relationship of the process, which in turn represents a 
processing of the input signal to provide an output signal variable, often 
with power amplification. An open-loop control system utilizes a controller or 
control actuator in order to obtain the desired response (Fig. 1). 
    In contrast to an open-loop control system, a closed-loop control system 
utilizes an additional measure of the actual output in order to compare the 
actual output with the desired output response (Fig. 2). A standard definition 
of a feedback control system is a control system which tends to maintain a 
prescribed relationship of one system variable to another by comparing 
functions of these variables and using the difference as a means of control. 
In the case of the driver steering an automobile, the driver uses his or her 
sight to visually measure and compare the actual location of the car with the 
desired location. The driver then serves as the controller, turning the 
steering wheel. The process represents the dynamics of the steering mechanism 
and the automobile response. 
    A feedback control system often uses a function of a prescribed 
relationship between the output and reference input to control the process. 
Often, the difference between the output of the process under control and the 
reference input is amplified and used to control the process so that the 
difference is continually reduced. The feedback concept has been the 
foundation for control system analysis and design. 
  
  
  
    Applications for feedback systems.  Familiar control systems have the 
basic closed-loop configuration. For example, a refrigerator has a temperature 
setting for desired temperature, a thermostat to measure the actual 
temperature and the error, and a compressor motor for power amplification. 
Other examples in the home are the oven, furnace, and water heater. In 
industry, there are controls for speed, process temperature and pressure, 
position, thickness, composition, and quality, among many others. Feedback 
control concepts have also been applied to mass transportation, electric power 
systems, automatic warehousing and inventory control, automatic control of 
agricultural systems, biomedical experimentation and biological control 
systems, and social, economic, and political systems. See BIOMEDICAL 
ENGINEERING; ELECTRIC POWER SYSTEMS ENGINEERING; MATHEMATICAL BIOLOGY; 
RAILROAD CONTROL SYSTEMS; SYSTEMS ANALYSIS; SYSTEMS ENGINEERING. 
  
    Advantages of feedback control.  The addition of feedback to a control 
system results in several important advantages. A process, whatever its 
nature, is subject to a changing environment, aging, ignorance of the exact 
values of the process parameters, and other natural factors which affect a 
control process. In the open-loop system, all these errors and changes result 
in a changing and inaccurate output. However, a closed-loop system senses the 
change in the output due to the process changes and attempts to correct the 
output. The sensitivity of a control system to parameter variations is of 
prime importance. A primary advantage of a closed-loop feedback control system 
is its ability to reduce the system's sensitivity. 
    One of the most important characteristics of control systems is their 
transient response, which often must be adjusted until it is satisfactory. If 
an open-loop control system does not provide a satisfactory response, then the 
process must be replaced or modified. By contrast, a closed-loop system can 
often be adjusted to yield the desired response by adjusting the feedback loop 
parameters.  
    A second important effect of feedback in a control system is the control 
and partial elimination of the effect of disturbance signals. Many control 
systems are subject to extraneous disturbance signals which cause the system 
to provide an inaccurate output. Feedback systems have the beneficial aspect 
that the effect of distortion, noise, and unwanted disturbances can be 
effectively reduced. 
  
    Costs of feedback control.  While the addition of feedback to a control 
system results in the advantages outlined above, it is natural that these 
advantages have an attendant cost. The cost of feedback is first manifested in 
the increased number of components and the complexity of the system. The 
second cost of feedback is the loss of gain. Usually, there is open-loop gain 
to spare, and one is more than willing to trade it for increased control of 
the system response. Finally, a cost of feedback is the introduction of the 
possibility of instability. While the open-loop system is stable, the 
closed-loop system may not be always stable. 
  
    Stability of closed-loop systems.  The transient response of a feedback 
control system is of primary interest and must be investigated. A very 
important characteristic of the transient performance of a system is the 
stability of the system. A stable system is defined as a system with a bounded 
system response. That is, if the system is subjected to a bounded input or 
disturbance and the response is bounded in magnitude, the system is said to be 
stable.  
    The concept of stability can be illustrated by considering a right 
circular cone placed on a plane horizontal surface. If the cone is resting on 
its base and is tipped slightly, it returns to its original equilibrium 
position. This position and response is said to be stable. If the cone rests 
on its side and is displaced slightly, it rolls with no tendency to leave the 
position on its side. This position is designated as neutral stability. On the 
other hand, if the cone is placed on its tip and released, it falls onto its 
side. This position is said to be unstable. 
    The stability of a dynamic system is defined in a similar manner. The 
response to a displacement, or initial condition, will result in either a 
decreasing, neutral, or increasing response. 
  
    Design.  A feedback control system that provides an optimum performance 
without any necessary adjustments is rare indeed. Usually one finds it 
necessary to compromise among the many conflicting and demanding 
specifications and to adjust the system parameters to provide a suitable and 
acceptable performance when it is not possible to obtain all the desired 
optimum specifications. 
    It is often possible to adjust the system parameters in order to provide 
the desired system response. However, it is often not possible to simply 
adjust a system parameter and thus obtain the desired performance. Rather, the 
scheme or plan of the system must be reexamined, and a new design or plan must 
be obtained which results in a suitable system. Thus, the design of a control 
system is concerned with the arrangement, or the plan, of the system structure 
and the selection of suitable components and parameters. For example, if one 
desires a set of performance measures to be less than some specified values, 
one often encounters a conflicting set of requirements. If these two 
performance requirements cannot be relaxed, the system must be altered in some 
way. The alteration or adjustment of a control system, in order to make up for 
deficiencies and inadequacies and provide a suitable performance, is called 
compensation.  
    In redesigning a control system in order to alter the system response, an 
additional component or device is inserted within the structure of the 
feedback system to equalize or compensate for the performance deficiency. The 
compensating device may be an electric, mechanical, hydraulic, pneumatic, or 
other-type device or network, and is often called a compensator. 
  
    Digital computer systems.  The use of a digital computer as a compensator 
device has grown since 1970 as the price and reliability of digital computers 
have improved dramatically. 
    Within a computer control system, the digital computer receives and 
operates on signals in digital (numerical) form, as contrasted to continuous 
signals. The measurement data are converted from analog form to digital form 
by means of a converter. After the digital computer has processed the inputs, 
it provides an output in digital form. This output is then converted to analog 
form by a digital-to-analog converter. See ANALOG-TO-DIGITAL CONVERTER; 
DIGITAL-TO-ANALOG CONVERTER. 
    There are more than 1,000,000 control systems using a computer in 
industry, although the computer size and power may vary significantly. If only 
computer control systems of a relatively complex nature are considered, such 
as chemical process control or aircraft control, the number of computer 
control systems is approximately 100,000. Automatic handling equipment for 
home, school, and industry is particularly useful for hazardous, repetitious, 
dull, or simple tasks. Machines that automatically load and unload, cut, weld, 
or cast are used by industry in order to obtain accuracy, safety, economy, and 
productivity. Robots are programmable computers integrated with machines. They 
often substitute for human labor in specific repeated tasks. Some devices even 
have anthropomorphic mechanisms, including what might be recognized as 
mechanical arms, wrists, and hands. Robots may be used extensively in space 
exploration and assembly. They can be flexible, accurate aids on assembly 
lines. See ROBOTICS. 
 �34s[Richard C. Dorf]
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�6v�2v�32sConularida   �21m �33s 
    A small group of extinct invertebrates showing a fourfold symmetry and a 
narrow pyramidal shape; the cross section commonly is square. Specimens are 
chitinous and frequently impregnated with calcium phosphate. The four side 
walls are characteristically ornamented by numerous transverse lines. The 
posterior tip may bear an attachment disk or may be broken off and sealed with 
a septum. Internal structures are rare; the few known show fourfold symmetry. 
Conularida are regarded as a subclass of the Coelenterata because of the 
characteristic symmetry. See COELENTERATA. 
    Specimens are worldwide in occurrence and are known from rocks of Cambrian 
through Triassic age. They have been found in sandstone, shales, and 
limestones. About 20 genera and 150 species are known. Except for a few local 
occurrences, Conularida are rare fossils. 
 �34s[Ellis L. Yochelson]
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�6v�2v�32sConvection (heat)   �21m �33s 
    The transfer of thermal energy by actual physical movement from one 
location to another of a substance in which thermal energy is stored. A 
familiar example is the free or forced movement of warm air throughout a room 
to provide heating. Technically, convection denotes the nonradiant heat 
exchange between a surface and a fluid flowing over it. Although heat flow by 
conduction also occurs in this process, the controlling feature is the energy 
transfer by flow of the fluid - hence the name convection. Convection is one 
of the three basic methods of heat transfer, the other two being conduction 
and radiation. See CONDUCTION (HEAT); HEAT RADIATION; HEAT TRANSFER. 
    Natural convection is exemplified by the cooling of a vertical surface in 
a large quiescent body of air of temperature tINFINITY. The lower-density air 
next to a hot vertical surface moves upward because of the buoyant force of 
the higher-density cool air farther away from the surface. At any arbitrary 
vertical location x, the actual variation of velocity u with distance y from 
the surface will be similar to that in the illustration b, increasing from 
zero at the surface to a maximum, and then decreasing to zero as ambient 
surrounding conditions are reached. In contrast, the temperature t of the air 
decreases from the heated wall value tsubs to the surrounding air temperature. 
These temperature and velocity distributions are clearly interrelated, and the 
distances from the wall through which they exist are coincident because, when 
the temperature approaches that of the surrounding air, the density difference 
causing the upward flow approaches zero. 
    The region in which these velocity and temperature changes occur is called 
the boundary layer. Because velocity and temperature gradients both approach 
zero at the outer edge, there will be no heat flow out of the boundary layer 
by conduction or convection. See BOUNDARY-LAYER FLOW. 
    When air is blown across a heated surface, forced convection results. 
Although the natural convection forces are still present in this latter case, 
they are clearly negligible compared with the imposed forces. The process of 
energy transfer from the heated surface to the air is not, however, different 
from that described for natural convection. The major distinguishing feature 
is that the maximum fluid velocity is at the outer edge of the boundary layer. 
This difference in velocity profile and the higher velocities provide more 
fluid near the surface to carry along the heat conducted normal to the 
surface. Consequently, boundary layers are very thin. 
    Heat convection in turbulent flow is interpreted similarly to that in 
laminar flow. Rates of heat transfer are higher for comparable velocities, 
however, because the fluctuating velocity components of the fluid in a 
turbulent flow stream provide a macroscopic exchange mechanism which greatly 
increases the transport of energy normal to the main flow direction. Because 
of the complexity of this type of flow, most of the information regarding heat 
transfer has been obtained experimentally. See LAMINAR FLOW; TURBULENT FLOW. 
  
    Convection heat transfer which occurs during high-speed flight or 
high-velocity flow over a surface is known as aerodynamic heating. This 
heating effect results from the conversion of the kinetic energy of the fluid 
as it approaches a body to internal energy as it is slowed down next to the 
surface. In the case of a gas, its temperature increases, first, because of 
compression as it passes through a shock and approaches the stagnation region, 
and second, because of frictional dissipation of kinetic energy in the 
boundary layer along the surface. 
    The phenomena of condensation and boiling are important phase-change 
processes involving heat release or absorption. Because vapor and liquid 
movement are present, the energy transfer is basically by convection. Local 
and average heat-transfer coefficients are determined and used in the Newton 
cooling-law equation for calculating heat rates which include the effects of 
the latent heat of vaporization. 
 �34s[Warren H. Giedt]
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�6v�2v�32sConvertiplane   �21m �33s 
    An aircraft combining vertical takeoff and landing capabilities of the 
helicopter with forward-flight effectiveness and high-speed potentials of the 
airplane. In forward flight a convertiplane relies, at least partially, on the 
fixed wing, while for vertical takeoffs, landings, and hovering, a separate 
vertical thrust generator is provided. Between vertical and forward-flight 
regimes, the aircraft goes through a conversion. 
    Of many systems suitable as vertical-thrust generators, those applied to 
practical convertiplanes will probably be either of the direct type, in which 
combustion products are used directly for thrust generation (turbojets and 
rockets), or of the indirect type, in which all or a part of the available 
combustion energy is used to drive a mechanical system, functioning as an 
actuator disk to accelerate the ambient air. The actuator disks may be either 
free (helicopter rotors and propellers) or shrouded (shrouded propellers and 
ducted fans). 
    The principle of the convertiplane can be applied to many types of 
aircraft. The first practical attempts were directed toward rotary-wing 
aircraft. The presently used scheme consists of transferring as much lift as 
possible in forward flight from the rotor (which is either put into 
autorotation, or slowed down) to more efficient fixed wings, while a propeller 
(or propellers) provides the forward thrust. The MacDonnell XV-1 (Fig. 1) and 
the Fairey Rotodyne are examples of the pioneering efforts. 
    The so-called X-wing design represents another approach to the high-speed 
problem of helicopter-based convertiplanes. Here, the circulation-controlled, 
reaction-driven, four-bladed, rigid rotor is stopped in flight to form an 
X-shaped wing. Jet propulsion should assure high-subsonic-speed capabilities 
for this aerodynamically clean aircraft (Fig. 2). 
    In the fixed-wing, free-airscrew group, the original Bell XV-3 of the 
1950s paved the way for the development of a flight research aircraft with 373 
mi/h (600 km/h) speed capabilities - the NASA-Army-Bell XV-15, which in July 
1979 went through complete conversion (Fig. 3). 
    The Hawker Siddeley Harrier, with Bristol Siddeley Pegasus vectored-thrust 
turbofan engines, is a fighter in quantity production for the military forces 
(including the U.S. Marines) of several Western countries (Fig. 4), while, 
similar in concept, the Yakovlev single-seat, Carrier-based combat aircraft is 
in service in the Soviet Union. See ORNITHOPTER; VERTICAL TAKEOFF AND LANDING 
(VTOL).  
 �34s[Wieslaw Z. Seepniewski]
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�6v�2v�32sConveying machines   �21m �33s 
    A family of materials-handling machines designed to move individual 
articles of solid, free-flowing bulk materials over a fixed horizontal, 
inclined, declined, or vertical path of travel with continuous motion. The 
actuating medium may be the force of gravity, air, vibration, or 
power-operated components of the machine such as continuous belts, chains, or 
cables. Of the many varieties, some are used almost exclusively to convey bulk 
materials. The kinds described in this article fall largely in the category 
popularly referred to as package conveyors, and consist of gravity and powered 
belt, chain, and cable conveyors. See BULK-HANDLING MACHINES. 
    Gravity conveyors provide the most economical means for lowering articles 
and materials. Chutes depend upon sliding friction to control the rate of 
descent; wheel and roller conveyors use rolling friction for this purpose. 
    Gravity chutes may be made straight or curved and are fabricated from 
sheet metal or wood, the latter being sometimes covered with canvas to prevent 
slivering. The bed of the chute can be shaped to accommodate the products to 
be handled. In spiral chutes centrifugal force is the second controlling 
factor. Spirals with roller beds or wheels provide smooth descent of an 
article and tend to maintain the position of the article in its original 
starting position. Rollers may be constructed of metals, wood, or plastic and 
can be arranged in an optimum position, depending upon the articles to be 
carried.  
    To move loads on level or inclined paths, or declining paths that exceed 
the angle of sliding or rolling friction of the particular material to be 
conveyed, powered conveyors must be employed. There are various types of 
powered conveyors. Belt conveyors move loads on a level or inclined path by 
means of power-driven belts. Belt conveyors with rough-top belts make possible 
inclines up to 28DG; cleated belts are limited on degree of incline only by 
the position of the center of gravity of the conveyed item. 
    Live-roller conveyors move objects over series of rollers by the 
application of power to all or some of the rollers. The power-transmitting 
medium is usually belting or chain. 
    There are three basic types of chain conveyors: (1) those that support the 
product being conveyed, (2) those that carry actuating elements between the 
two chains, and (3) those that operate overhead or are set into the floor. In 
the sliding chain conveyor either plain links or links with such special 
attachments as lugs are made up into the conveyors to handle cases, cans, 
pipes, and other similar products. 
    Slats or apron conveyors are fitted with slats of wood or metal, either 
flat or in special shapes, between two power chains. The slats handle such 
freight as barrels, drums, and crates. Push-bar conveyors are a variation of 
the slat conveyor, in which two endless chains are cross-connected at 
intervals by pusher bars which propel the load along a stationary bed or 
trough of the conveyor. 
 �34s[Arthur M. Perrin]
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�6v�2v�32sConvulsive disorders   �21m �33s 
    Disorders characterized primarily by convulsions involving the whole body 
or part of it, often - though not always or necessarily== - ==associated with 
loss of consciousness. Convulsions are defined as episodes of involuntary, 
generally violent muscular contractions, rhythmically alternated with periods 
of relaxation. These are part of the symptomatology of a large number of 
systemic and neurological diseases in which the brain is affected by various 
pathological processes (inflammatory, tumoral, degenerative, traumatic, and so 
forth), or they can result from various types of intoxications. They are the 
most characteristic, though not exclusive, manifestation of epilepsy and are 
commonly referred to as epileptic seizures. On the other hand, their 
occurrence, especially if in the form of isolated episodes, does not justify 
the diagnosis of epilepsy. See EPILEPSY. 
 �34s[C. Ajmone Marsan]
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�6v�2v�32sCooling tower   �21m �33s 
    A tower- or buildinglike device in which atmospheric air (the heat 
receiver) circulates in direct or indirect contact with warmer water (the heat 
source) and the water is thereby cooled (see illustration). A cooling tower 
may serve as the heat sink in a conventional thermodynamic process, such as 
refrigeration or steam power generation, or it may be used in any process in 
which water is used as the vehicle for heat removal, and when it is convenient 
or desirable to make final heat rejection to atmospheric air. Water, acting as 
the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, is 
recirculated through the system, affording economical operation of the process. 
    Two basic types of cooling towers are commonly used. One transfers the 
heat from warmer water to cooler air mainly by an evaporation heat-transfer 
process and is known as the evaporative or wet cooling tower. Evaporative 
cooling towers are classified according to the means employed for producing 
air circulation through them: atmospheric, natural draft, and mechanical 
draft. The other transfers the heat from warmer water to cooler air by a 
sensible heat-transfer process and is known as the nonevaporative or dry 
cooling tower. Nonevaporative cooling towers are classified as air-cooled 
condensers and as air-cooled heat exchangers, and are further classified by 
the means used for producing air circulation through them. These two basic 
types are sometimes combined, with the two cooling processes generally used in 
parallel or separately, and are then known as wet-dry cooling towers. 
    Evaluation of cooling tower performance is based on cooling of a specified 
quantity of water through a given range and to a specified temperature 
approach to the wet-bulb or dry-bulb temperature for which the tower is 
designed. Because exact design conditions are rarely experienced in operation, 
estimated performance curves are frequently prepared for a specific 
installation, and provide a means for comparing the measured performance with 
design conditions. 
 �34s[Joseph F. Sebald]
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�6v�2v�32sCoordinate systems   �21m �33s 
    Schemes for locating points in a given space by means of numerical 
quantities specified with respect to some frame of reference. These quantities 
are the coordinates of a point. To each set of coordinates there corresponds 
just one point in any coordinate system, but there are useful coordinate 
systems in which to a given point there may correspond more than one set of 
coordinates.  
    A coordinate system is a mathematical language that is used to describe 
geometrical objects analytically; that is, if the coordinates of a set of 
points are known, their relationships and the properties of figures determined 
by them can be obtained by numerical calculations instead of by other 
descriptions. It is the province of analytic geometry, aided chiefly by 
calculus, to investigate the means for these calculations. 
    The most familiar spaces are the plane and the three-dimensional euclidean 
space. In the latter a point P is determined by three coordinates (x,y,z). The 
totality of points for which x has a fixed value constitutes a surface. The 
same is true for y and z so that through P there are three coordinate 
surfaces. The totality of points for which x and y are fixed is a curve and 
through each point there are three coordinate lines. If these lines are all 
straight, the system of coordinates is said to be rectilinear. If some or all 
of the coordinate lines are not straight, the system is curvilinear. If the 
angles between the coordinate lines at each point are right angles, the system 
is rectangular. 
    A cartesian coordinate system is one of the simplest and most useful 
systems of coordinates. It is constructed by choosing a point O designated as 
the origin. Through it three intersecting directed lines OX, OY, OZ, the 
coordinate axes, are constructed. The coordinates of a point P are x, the 
distance of P from the plane YOZ measured parallel to OX, and y and z, which 
are determined similarly (Fig. 1). Usually the three axes are taken to be 
mutually perpendicular, in which case the system is a rectangular cartesian 
one. Obviously a similar construction can be made in the plane, in which case 
a point has two coordinates (x,y). 
    A polar coordinate system is constructed in the plane by choosing a point 
O called the pole and through it a directed straight line, the initial line. A 
point P is located by specifying the directed distance OP and the angle 
through which the initial line must be turned to coincide with OP in position 
and direction. The coordinates of P are (r,THETA). The radius vector r is the 
directed line OP, and the vectorial angle THETA is the angle through which the 
initial line was turned, + if turned counterclockwise, - if clockwise. 
    Spherical coordinates are constructed in three-dimensional euclidean space 
by choosing a plane and in it constructing a polar coordinate system. At the 
pole O a polar axis OZ is constructed at right angles to the chosen plane. A 
point P, not on OZ, and OZ determine a plane. The spherical coordinates of P 
are then the directed distance OP denoted by P, the angle THETA through which 
the initial line is turned to lie in ZOP and the angle PHI = ZOP (Fig. 2). 
    Cylindrical coordinates are constructed by choosing a plane with a pole O, 
an initial line in it, and a polar axis OZ, as in spherical coordinates. A 
point P is projected onto the chosen plane. The cylindrical coordinates of P 
are (r,THETA,z) where r and THETA are the polar coordinates of Q and z = QP 
(Fig. 2). 
    By means of a system of equations the description of a geometrical object 
in one coordinate system may be translated into an equivalent description in 
another coordinate system. See ANALYTIC GEOMETRY; BARYCENTRIC CALCULUS; 
CALCULUS; CONFORMAL MAPPING; CURVE FITTING; SPHERICAL HARMONICS. 
 �34s[Morris S. Knebelman]
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�6v�2v�32sCoordination chemistry   �21m �33s 
    A field which, in its broadest usage, is acid-base chemistry as defined by 
G. N. Lewis. However, the term coordination chemistry is generally used to 
describe the chemistry of metals and metal ions in their interactions with 
other molecules or ions. For example, reactions (1) - (3) show acid-base-type 
reactions; the products formed are coordination ions or compounds, and this 
area of chemistry is known as coordination chemistry. 
  
                     *** Mg sup 2 + 6H2O -> Mg(H2O)6 sup 2 POSITIVE ***  (1)
  
                            *** Ni + 4CO -> Ni(CO)4 ***                  (2)
  
    *** Fe sup 2 POSITIVE + 6CN NEGATIVE -> Fe(CN)6 sup 4 NEGATIVE ***   (3)
  
    Thus, it follows that coordination compounds are compounds that contain a 
central atom or ion and a group of ions or molecules surrounding it. Such a 
compound tends to retain its identity, even in solution, although partial 
dissociation may occur. The charge on the coordinated species may be positive, 
zero, or negative, depending on the charges carried by the central atom and 
the coordinated groups. These groups are called ligands, and the total number 
of attachments to the central atom is called the coordination number. Other 
names commonly used for these compounds include complex compounds, complex 
ions, Werner complexes, coordinated complexes, chelate compounds, or simply 
complexes. See ACID AND BASE; CHELATION; COMPLEX COMPOUNDS. 
    Experimental observations as early as the middle of the 18th century 
reported the isolation of coordination compounds, but valence theory could not 
adequately account for such materials. The correct interpretation of these 
compounds was given by Alfred Werner in 1893. He introduced the concept of 
residual or secondary valence, and suggested that elements have this type of 
valence in addition to their normal or primary valence. Thus, platinum(IV) has 
a normal valence of 4 but a secondary valence or coordination number of 6. 
This then led to the formulation of PtCl4 X 6NH3 as [Pt(NH3)6] sup 4 POSITIVE, 
4Cl NEGATIVEand of PtCl4 X 5NH3 as [Pt(NH3)5Cl] sup 3 POSITIVE, 3Cl NEGATIVE. 
The compound with five ammonias has only three ionic chlorides, the fourth is 
inside the coordination sphere, and therefore is not readily precipitated upon 
the addition of silver ion. Although the exact nature of the coordinate bond 
between metal and ligand remains the subject of considerable discussion, it 
is agreed that the formulations of Werner are essentially correct. 
    Three theories have been used to explain the nature of the coordinate 
bond. These are the valence bond theory, the electrostatic theory, including 
crystal field corrections, and the molecular orbital theory. Currently, the 
theory used almost exclusively is the molecular orbital theory. The valence 
bond theory for metal complexes considers that the pair of electrons on the 
ligand enter the hybridized atomic orbitals of the metal and that the bond is 
either essentially covalent or essentially ionic.  Several of the properties 
of these substances can be explained on the basis of this theory. The 
electrostatic theory, plus the crystal field theory for the transition metals, 
assumes that the metal-ligand bond is caused by electrostatic interactions 
between point charges and dipoles and that there is no sharing of electrons. 
In addition to explaining the structure and magnetic properties, the crystal 
field theory affords an adequate interpretation of the visible spectra of 
metal complexes. The molecular orbital theory assumes that the electrons move 
in molecular orbitals which extend over all the nuclei of the metal-ligand 
system. In this manner, it serves to make use of both the valence bond theory 
and crystal field theory. The molecular orbital theory is therefore the best 
approximation to the nature of the coordinate bond because it is sufficiently 
flexible to permit both covalent and ionic bonding as well as the splitting of 
d orbitals into various energy levels. See CRYSTAL FIELD THEORY; MOLECULAR 
ORBITAL THEORY. 
    The stability of metal complexes depends both on the metal ion and the 
ligand. In general the stability of metal complexes increases if the central 
ion increases in charge, decreases in size, and increases in electron 
affinity. Several characteristics of the ligand are known to influence the 
stability of complexes: (1) basicity of the ligand, (2) the number of 
metal-chelate rings per ligand, (3) the size of the chelate ring, (4) steric 
effects, (5) resonance effects, and (6) the ligand atom. Since coordination 
compounds are formed as a result of acid-base reactions where the metal ion is 
the acid and the ligand is the base, it follows that generally the more basic 
ligand will tend to form the more stable complex. The size of the chelate ring 
is likewise an important factor. For saturated ligands such as 
ethylenediamine, five-membered rings are the most stable for chelates 
containing one or more double bonds. 
    Steric factors often have a very large effect on the stability of metal 
complexes. This is most frequently observed with ligands having a large group 
attached to the ligand atom or near it. Thus complexes, of the type shown in 
the illustration, with alkyl groups R in the position designated are much less 
stable than the parent complex where R = H. This results from the steric 
strain introduced by the size of the alkyl group on or adjacent to the ligand 
atom. In contrast to this, alkyl substitution at any other position results in 
the formation of more stable complexes because the ligand becomes more basic, 
and the bulky group is now removed from a position near the coordination site. 
    Finally, the ligand atom itself plays a significant role in controlling 
the stability of metal complexes. For most of the metal ions, the smallest 
ligand atom with the largest electron density will form the most stable 
complex.  
    Often the most stable complex is also the least reactive or most inert. 
Several factors, such as the electronic configuration of the central metal 
ion, its coordination number, and the extent of chelation, all have a marked 
effect on the rate of reaction of a given compound. See CHEMICAL BONDING; 
MAGNETOCHEMISTRY; SOLID-STATE CHEMISTRY; STEREOCHEMISTRY. 
    Figure caption: Structural formulas of metal complexes which are affected
by steric factors.
 �34s[Fred Basolo]
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�6v�2v�32sCoordination number   �21m �33s 
    The number of nearest neighbors of a point in a space lattice, of an atom 
or an ion in a structure, or of an anion or cation in a complex ion. The 
figure shows that the coordination number in the three cubic lattices P, I, 
and F is respectively 6, 8, and 12. In closely packed structures this number 
is 12. 
    In ionic structures each ion is surrounded by a number of ions of opposite 
sign. This coordination is respectively 8/8, 6/6, 4/4, 8/4 in the cesium 
chloride, sodium chloride, zinc sulfide, and calcium fluoride structures. See 
CRYSTAL.  
    The anions are, in nearly all cases, larger than the cations. For any 
given coordination the distribution of the anions is as regular as possible. 
Each cation is in the center of a coordination polyhedron. The structures can 
therefore be described in terms of the way in which the anion polyhedrons are 
packed. As an example, in the rock salt structure, each cation is in the 
center of an octahedron formed by six anions. Two neighboring octahedrons have 
a common edge. 
 �34s[Willy C. Dekeyser]
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�6v�2v�32sCopepoda �21m �33s
    A subclass of Crustacea. Most are free-living in aqueous environments,
though many are parasitic or symbiotic with other aquatic animals. The
free-living forms are the most abundant of all animals in the sea and directly
constitute the main food for vast numbers of fishes, many invertebrates, and
also at times for baleen whales.
    The structure of copepods, although similar in basic pattern, varies
greatly. In free-living forms the body is composed externally of a series of
small chitinous cylinders, or segments, some of which may be more or less
fused (see illustration). The head bears two pairs of antennae. The feeding
appendages are one pair of mandibles, two pairs of maxillae, and one pair of
maxillipeds. The thorax, consisting of six segments, bears the swimming feet.
The abdomen consists of one to five segments and bears no appendages except a
pair of terminal processes known as caudal rami. The sexes are separate and
sexual dimorphism is frequently evidenced. Free-living species usually range
in size from less than 0.2 in. to about 0.4 in. 5 mm to about 10 mm) in length.
    Seven orders (with over 6000 species) of copepods are usually recognized.
Three of these, Calanoida, Cyclopoida, and Harpacticoida, are wholly or
primarily free-living and because of overwhelming numbers are of greatest
general interest. The remaining orders (Notodelphoida, Monstrilloida,
Caligoida, and Lernaepodoida) are parasitic at least part of their lives.
Their numbers are relatively small, but they may be conspicuous and important
as ``fish lice,'' burrowing deep into the flesh or feeding externally upon the
skin and gills. See CALANOIDA; CALIGOIDA; CYCLOPOIDA; HARPACTICOIDA;
LERNAEPODOIDA; MONSTRILLOIDA; NOTODELPHOIDA.
    Copepod habitats are extremely varied, but the vast majority of species
live in the sea. They are mostly planktonic throughout their lives, some
occurring only in coastal waters while others live in the open sea. Many
frequent only the upper layers to about 200 - 300 m depth. Others are
bathypelagic at great depths. The harpacticoids, however, are primarily a
benthic, or bottom-dwelling, group, though many are truly planktonic.
    The role of copepods in the economy of the sea consists mainly of
converting plant into animal substance. Many species are adapted to graze
directly upon the chief synthesizers of organic material, the microscopic
diatoms and dinoflagellates that constitute the great pasturage of the sea.
Some species are predatory on other copepods, fish larvae, and other small
animals, such as protozoans. Commonly copepods constitute about 70% of the
zooplankton, and at times their numbers are so great as to impart a pinkish
color to the sea.
 �34s[Harry C. Yeatmen]
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�6v�2v�32sCopper �21m �33s
    A chemical element, Cu, atomic number 29, one of the transition elements
and an important nonferrous metal. Its usefulness is accounted for by its
combination of chemical, physical, electrical, and mechanical properties and
its fairly abundant supply. Copper is one of the first metals to have been
used by humans. See TRANSITION ELEMENTS.
    By far the greater part of the world's copper is obtained from the sulfide
ores chalcocite, covellite, chalcopyrite, bornite, and enargite. Oxidized ores
include cuprite, tenorite, malachite, azurite, chrysocolla, and brochantite.
Native copper, once widespread in the United States, is now mined in quantity
only in Michigan. The grade of ore used for copper production has been going
steadily downward as the richer ores have become exhausted and the demand for
copper has grown. There are vast amounts of copper in the ground, available
for future use if ores of still lower grades are utilized, and there is no
prospect of exhaustion for a long time to come.
    Copper is the first element in subgroup Ib of the periodic table, which
also includes the other coinage metals, silver and gold. The copper atom has
the electronic structure 1s sup 2 2s sup 2 2p sup 6 3s sup 2 3p sup 6 3d sup 1
sup 0 4s sup 1 . The low ionization potential of the 4s sup 1 electron results
in easy removal to give the copper(I), or cuprous ion, Cu sup +; and the
copper(II), or cupric ion, Cu sup 2 sup +, is readily formed by removal of one
electron from the 3d shell. The atomic weight of copper is 63.546. It has two
stable natural isotopes, sup 6 sup 3Cu and sup 6 sup 5Cu. There are also known
nine unstable (radioactive) isotopes. Copper is characterized by low chemical
activity. The element combines chemically in one of three valences. The most
common valence is 2+ (cupric), but 1+ (cuprous) is also frequent; the valence
3+ occurs in only a few unstable compounds.
    A comparatively heavy metal, pure solid copper has a density of 8.96
g/cm sup 3 at 20DGC. That of commercial copper varies with method of
manufacture, averaging 8.90 - 8.94. The melting point of copper is
1083.0PLUS/MINUS0.1DGC (1981.4PLUS/MINUS0.2DGF). Its normal boiling point is
2595DGC (4703DGF). Copper is nonmagnetic; or more precisely, it is slightly
paramagnetic. The thermal and electrical conductivities of copper are very
high. Copper is one of the strongest pure metals. It is moderately hard,
extremely tough, and wear resistant. The strength of copper is accompanied by
high ductility. The mechanical and electrical properties of a metal are
strongly dependent on physical condition, temperature, and grain size of the
metal. See METAL, MECHANICAL PROPERTIES OF.
    Of the hundreds of copper compounds, only a few are manufactured
industrially on a large scale. The most important is copper(II) sulfate
pentahydrate or blue vitriol, CuSO4 CTR DOT 5H2O. Others include bordeaux
mixture, 3Cu(OH)2CuSO4; paris green, a complex of copper metaarsenite and
acetate; cuprous cyanide, CuCN; cuprous oxide, Cu2O; cupric chloride, CuCl2;
cupric oxide, CuO; basic cupric carbonate; and copper naphthenate, the most
widely used agent for prevention of rotting in wood, fabric, rope, and fishing
nets. Leading uses of copper compounds are in agriculture, especially as
fungicides and insecticides; as pigments; in electroplating solutions; in
primary cells; as mordants in dyeing; and as catalysts. See COPPER ALLOYS;
COPPER INTOXICATION; COPPER METALLURGY.
 �34s[Allison Butts]
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�6v�2v�32sCopper alloys   �21m �33s 
    Solid solutions of one or more metals in copper. Many metals, although not 
all, alloy with copper to form solid solutions. Some insoluble metals and 
nonmetals are intentionally added to copper alloy to enhance certain 
characteristics. See SOLID SOLUTION. 
    Copper alloys form a group of materials of major commercial importance 
because they are characterized by such useful mechanical properties as high 
ductility and formability and excellent corrosion resistance. Copper alloys 
are easily joined by soldering and brazing. Like gold alloys, copper alloys 
have decorative red, pink, yellow, and white colors. Copper has the second 
highest electrical and thermal conductivity of any metal. All these factors 
make copper alloys suitable for a wide variety of products. 
    Copper and zinc melted together in various proportions produce one of the 
most useful groups of copper alloys, known as the brasses. Brasses containing 
5 - 40% zinc constitute the largest volume of copper alloys. One important 
alloy, cartridge brass (70% copper, 30% zinc), has innumerable uses, including 
cartridge cases, automotive radiator cores and tanks, lighting fixtures, 
eyelets, rivets, screws, springs, and plumbing products. 
    Lead is added to both copper and the brasses, forming an insoluble phase 
which improves machinability of the material. Free cutting brass (61% copper, 
3% lead, 36% zinc) is the most important alloy in the group. It is machined 
into parts on high-speed (10,000 rpm) automatic screw machines for a 
multiplicity of uses. Increased strength and corrosion resistance are obtained 
by adding up to 2% tin or aluminum to various brasses. 
    Alloys of copper, nickel, and zinc are called nickel silvers. Nickel is 
added to the copper-zinc alloys primarily because of its influence upon the 
color of the resulting alloys; color ranges from yellowish-white, to white 
with a yellowish tinge, to white. Because of their tarnish resistance, these 
alloys are used for table flatware, zippers, camera parts, costume jewelry, 
nameplates, and some electrical switch gear. 
    Three copper-base alloys containing 10%, 20%, and 30% nickel, with small 
amounts of manganese and iron added to enhance casting qualities and corrosion 
resistance, have gained commercial importance. These alloys are known as 
cupronickels and are well suited for application in industrial and marine 
installations as condenser and heat-exchanger tubing because of their high 
corrosion resistance and particular resistance to impingement attack. 
    Copper-tin alloys (3 - 10% tin), deoxidized with phosphorus, form an 
important group known as phosphorus bronzes. Tin increases strength, hardness, 
and corrosion resistance. These alloys are widely used for springs and screens 
in papermaking machines. See ALLOY; COPPER. 
 �34s[Ralph E. Ricksecker]
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�6v�2v�32sCopper intoxication   �21m �33s 
    Copper and its salts are remarkably nontoxic to mammalian tissues. It is 
necessary to ingest a large quantity of a soluble copper salt such as copper 
sulfate to produce intoxication. Nausea, vomiting, diarrhea, abdominal cramps, 
melana, coma, and death can ensue. The copper solution not only produces 
vascular congestion of the gastrointestinal tract but also acts as an irritant 
producing focal necrosis, microscopic thrombi in the capillaries of the 
intestinal wall, and liver necrosis. Death is usually a result of vascular 
collapse (shock). Ingestion of smaller amounts of copper salts dissolved from 
the walls of pots and vessels by the action of citrus juices seldom produces 
symptoms and does not represent a threat to life. The symptoms that occur are 
mild (nausea and vomiting) and are related to acute gastroenteritis. 
Inhalation of dust, fumes, or mists of copper salts can cause congestion and 
the irritation of the upper airway, but generalized symptoms do not ordinarily 
ensue.  
    Chronic copper poisoning does not represent a major health hazard. Chronic 
human toxicity is extremely rare except in those individuals who inherit a set 
of autosomal recessive genes producing an abnormal increase in body copper (a 
syndrome called hepatolenticular degeneration or Wilson's disease). A normal 
individual absorbs a scant amount of copper, sufficient to meet the body's 
essential needs, and excess copper is readily eliminated. Copper, in small 
amounts, is a trace element essential for the activity of such mammalian 
metalloenzymes as ceruloplasmin, cytochrome c oxidase, dopamine, and 
tyrosinase. See COPPER; METALS, PATHOLOGY OF. 
 �34s[N. Karle Mottet]
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�6v�2v�32sCopper loss   �21m �33s 
    The power loss due to the flow of current through copper conductors. When 
an electric current flows through a copper conductor (or any conductor), some 
energy is converted to heat. The heat, in turn, causes the operating 
temperature of the device to rise. This happens in transformers, generators, 
motors, relays, and transmission lines, and is a principal limitation on the 
conditions of operation of these devices. Excessive temperature rises lead to 
equipment failure. 
    Much research has been undertaken to reduce the loss in machines and thus 
to increase the output capability and efficiency for a given amount of 
conductor. For large machines and for transmission lines, the concept of 
superconductivity appears to hold much promise for substantial improvements in 
operating efficiencies. See SUPERCONDUCTING DEVICES. 
 �34s[Edwin C. Jones, Jr.]
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�6v�2v�32sCopper metallurgy   �21m �33s 
    There are many variations in the production processes, and improvements 
are constantly being made. Only the most common methods can be indicated here 
because of space limitations. 
    The ores are first concentrated, usually by flotation, to yield 
concentrates containing from 20 to over 40% copper. See ORE DRESSING. 
    Roasting has usually been the next step, but is omitted in plants treating 
the richer concentrates. Roasted concentrates are mixed with raw concentrates 
before smelting to obtain a sulfur content that will yield the desired grade 
of matte in the reverberatory furnace. 
    Smelting in blast furnaces has been replaced, except in a few instances, 
by reverberatory smelting. The reverberatory furnace has the advantage of 
being able to smelt finely divided flotation concentrates without sintering, 
and it is not dependent on coke as fuel. Whatever fuel is cheapest in the 
locality of the smelter can be used. The basis of the process is that 
reactions between copper oxide and iron sulfide in the molten charge form 
copper sulfide and iron oxide. With excess iron sulfide, the copper sulfide 
forms a molten solution called copper matte. The iron oxide formed in the 
reactions, together with that added as flux, unites with silica in the ore to 
form a slag. 
    The matte is taken to a converter where the sulfur in the matte is 
oxidized to SO2 and passes off in the gases, leaving copper in the converter. 
When cast, the copper forms cakes with a surface roughened and blistered by 
the escape of gases during freezing, and hence is called blister copper. It is 
usually 98 - 99% Cu. 
    The blister copper is next partly refined in a furnace and cast into 
anodes containing about 99.0 - 99.3% Cu. See PYROMETALLURGY, NONFERROUS. 
    The anodes are hung in electrolytic tanks, spaced alternately with 
cathodes, in an electrolyte of copper sulfate and free sulfuric acid, 
contaminated with soluble impurities. Insoluble impurities fall to the bottom 
of the tank. Soluble impurities enter the electrolyte and do not plate out at 
the cathode. Some of the impurities form insoluble compounds. Thus the copper 
is freed of nearly all impurities. The cathodes are usually remelted for 
casting into shapes such as wirebars, cakes, billets, or ingots. Some of the 
cathodes are sold without remelting. See ELECTROMETALLURGY. 
    Continuous casting of oxygen-free copper is practiced on a large scale, 
the copper being protected from air while molten. 
    Most copper ores contain gold and silver. The recovery of these and other 
metals as by-products is an important part of the metallurgy of copper. 
    A considerable amount of copper is produced by hydrometallurgy. The ore is 
leached without being concentrated, dilute sulfuric acid being the solvent 
most often used. The process is applied chiefly to oxidized ores. A further 
limitation of leaching is that the solvents employed do not dissolve gold or 
silver. See HYDROMETALLURGY. 
    In another proposed method of copper recovery, lowgrade ore in the ground 
would be shattered by a nuclear energy explosion, then leached in place with 
dilute sulfuric acid; the solution would next be pumped to the surface and the 
copper cemented out on scrap iron. See COPPER; METALLURGY. 
 �34s[Allison Butts]
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�6v�2v�32sCopulatory organ   �21m �33s 
    An organ utilized by certain male vertebrates for insemination, that is, 
to deposit spermatozoa directly into the female reproductive tract. In fish, 
internal fertilization is restricted to certain groups. The pelvic fin of male 
elasmobranchs and holocephalians is modified for the transmission of sperm and 
is known as the clasper or clasping organ. A pair of anterior claspers 
(illustration a) and a frontal clasper which protrudes from the head occur 
among holocephalians in addition to the modified pelvic clasper of 
elasmobranchs. The anal fin of teleosts, in which copulation occurs, may be 
elongated to form a gonopodium. 
    Hemipenes which can be everted during copulation are common to both snakes 
and lizards; however, these structures lack erectile tissue. Turtles and 
crocodiles possess a single penis with associated erectile tissue, the corpora 
cavernosa, which becomes distended with blood. A few species of birds have a 
penis; among these are the ostriches and anseriforms. The penis is present in 
all mammals. 
    The copulatory organ (illustration b - g) is variously modified 
morphologically. Among certain mammals such as rodents, bats, whales, some 
carnivores, and lower primates, a penis bone, the os priapi or baculum, 
occurs, increasing the rigidity of the penis. See PENIS. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCoquina   �21m �33s 
    A calcarenite or clastic limestone whose detrital particles are chiefly 
fossils, whole or fragmented. The term is most frequently used for an 
aggregate of large shells more or less cemented by calcite. If the rock 
consists of fine-sized shell debris, it is called a microcoquina. Some 
coquinas show little evidence of any transportation by currents. See 
CALCARENITE; LIMESTONE. 
 �34s[Rudolf Schmid]
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�6v�2v�32sCoraciiformes   �21m �33s 
    An order of birds currently considered to contain 10 families, varying in 
appearance from the tiny colorful todies (Todidae) of the West Indies to the 
cumbersome-looking hornbills (Bucerotidae) of the Old World tropics. Members 
of this order have the anterior toes more or less joined at the base 
(syndactyl). Most of the families are predominantly tropical and frequently 
brightly colored. The only worldwide family is Alcedinidae, the kingfishers. 
In spite of their name, many kingfishers may live far from water and feed on 
terrestrial vertebrates and invertebrates. The most famous of these is the 
Australian kookaburra, or laughing jackass, one of the largest members of the 
family. The Todidae and the motmots (Momotidae) are found exclusively in the 
New World, and two families, the ground rollers (Brachypteraciidae) and the 
cuckoo rollers (Leptosomatidae), are confined to Madagascar. The latter 
family, containing a single species, may be out of place in this order, since 
it differs sharply in several morphological characters. For example, its young 
are hatched with a dense coat of down, whereas in other Coraciiformes the 
young are either hatched naked or, as in the case of the widely distributed 
Old World hoopoes (Upupidae) and the strictly African wood hoopoes 
(Phoeniculidae), sparsely downy. The hornbills inhabit tropical Africa and 
Asia, while the bee-eaters (Meropidae) and rollers (Coraciidae) are found 
throughout much of the warmer part of the Old World, including southern 
Europe. See AVES; KINGFISHER. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sCorallimorpharia   �21m �33s 
    An order of the subclass Zoantharia or Hexacorallia. These sea anemones 
resemble corals in their weak musculature, capitate (spherically tipped) 
tentacles, and complex nematocysts. They do not possess a skeleton, however. 
The tentacles are arranged in radial rows, not cycles, on the disk, and the 
base has no musculature. The group occurs in shallow water from the temperate 
zone to the tropics. The species are solitary, although they may occur in 
large, asexually produced aggregations. Examples are Corallimorphus and 
Corynactis. See ANTHOZOA. 
 �34s[Cadet Hand]
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�6v�2v�32sCoralline algae   �21m �33s 
    Marine, benthic, calcareous red algae (Rhodophyta) belonging to the family 
Corallinaceae. Coralline algae constitute the largest family of the order 
Cryptonemiales and include a great diversity of forms, almost all of which are 
heavily calcified. Coralline algae have two distinctly different growth 
habits, one group containing crusts, nodules, and rigid-branched forms 
(subfamily Melobesioideae) and the other consisting of erect, articulated, or 
jointed forms (subfamily Corallinoideae). 
    Crustose corallines are generally attached, ranging from thin (a single 
cell layer thick) crusts about 0.04 in. (1 mm) across to massive, nodular 
developments 4 in. (10 cm) or more in diameter; also included are multiple 
crusts and rigid-branched forms. Articulated corallines are erect plants made 
up of calcified segments connected together by uncalcified joints. 
    Living coralline algae are a cosmopolitan, exclusively marine flora. As a 
group they range from tropical to polar seas and extend from intertidal zones 
to depths of 250 m. Temperature and light requirements are primary factors 
controlling the latitudinal and depth distribution of genera and species. 
    Coralline algae are important components in tropical reefs throughout 
Cenozoic time, both as frame builders and sediment producers. Prominent 
coralline algae of tropical reefs today are Neogoniolithon, Porolithon, and 
Lithoporella. See ALGAE; ATOLL; REEF. 
 �34s[John L. Wray]
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�6v�2v�32sCorbino disk   �21m �33s 
    A disk of conducting material with inner and outer concentric-ring 
electric contacts. When subjected to a magnetic field perpendicular to the 
plane of the disk, the current between the inner and outer ring contacts tends 
to spiral rather than flow straight out, thus introducing a considerable 
magnetoresistance. See CORBINOTRON; MAGNETORESISTANCE. 
 �34s[Lloyd P. Hunter]
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�6v�2v�32sCorbinotron   �21m �33s 
    The combination of a corbino disk, made of high-mobility semiconductor 
material, and a coil arranged to produce a magnetic field perpendicular to the 
disk. This device can be used as a switch in which the current in the coil 
controls the resistance of the disk and hence the radial current flowing in 
the disk. See CORBINO DISK. 
    If the coil is connected in series with the disk, the resulting 
two-terminal device exhibits rectification. If the coil is connected in 
parallel with the disk, the device exhibits a negative-resistance 
characteristic for one direction of current. 
 �34s[Lloyd P. Hunter]
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�6v�2v�32sCordaitales   �21m �33s 
    An extensive and for the most part natural grouping of forest trees of the 
late Paleozoic. With inclusion of certain presumed relatives, the 
stratigraphic range of the Cordaitales extends from the Upper Devonian to the 
Lower Triassic. The order may be divided into three families: Pityaceae, 
Cordaitaceae, and Poroxylaceae. The degree of relationship between the three 
families is quite uncertain, and the bulk of knowledge and significance of the 
group resides in the Cordaitaceae, abundant remains of which occur in 
Carboniferous sediments in most parts of the Earth. See PALEOBOTANY. 
 �34s[Elso S. Barghoorn]
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�6v�2v�32sCordierite   �21m �33s 
    An orthorhombic magnesium aluminosilicate mineral of composition 
Mg2[Al4Si5O18]. The crystal structure is related to beryl. Limited amounts of 
Fesup2supp may substitute for Mgsup2supp, and Fesup3supp for Alsup3supp. The 
hardness is 7 (Mohs scale); specific gravity 2.6; luster vitreous; cleavage 
poor; and color greenish-blue, lilac blue, or dark blue, often strongly 
pleochroic colorless to deep blue. Transparent pleochroic crystals are used as 
gem material. The disordered cordierite structure, Mg2[(Al,Si)9O18], is called 
indialite and is hexagonal, isotypic with beryl. Osumilite, 
KMg2Al3[(Al,Si)12O30]PRODUCTH2O, is also related but has a structure built of 
double six-membered rings of tetrahedrons, Cordierite, indialite, and 
osumilite are difficult to distinguish. Pale colored varieties are often 
misidentified as quartz, since these minerals have many physical properties in 
common. See BERYL; SILICATE MINERALS. 
    Cordierite possesses unusually low thermal expansion, and synthetic 
material has been applied to thermal-shock-resistant materials, such as 
insulators for spark plugs and low-expansion concrete. 
    Cordierite frequently occurs associated with thermally metamorphosed rocks 
derived from argillaceous sediments. It may occur in aluminous schists, 
gneisses, and granulites; though usually appearing in minor amounts, 
cordierite occurs at many localities throughout the world. 
 �34s[Paul B. Moore]
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�6v�2v�32sCordilleran belt   �21m �33s 
    A mountain belt or chain which is an assemblage of individual mountain 
ranges and associated plateaus and intermontane lowlands. A cordillera is 
usually of continental extent and linear trend; component elements may trend 
at angles to its length or be nonlinear. 
    The term cordillera is most frequently used in reference to the 
mountainous regions of western South and North America, which lie between the 
Pacific Ocean and interior lowlands to the east. Farther north, the extensive 
and geologically diverse mountain terrane of western North America is formally 
known as the Cordilleran belt or orogen. This belt includes such contrasting 
elements within the United States as the Sierra Nevada, Central Valley of 
California, Cascade Range, Basin and Range Province, Colorado Plateau, and 
Rocky Mountains. See MOUNTAIN SYSTEMS. 
    Cordilleras represent zones of intense deformation of the Earth's crust 
produced by the convergence and interaction of large, relatively stable areas 
known as plates. Mountain belts have been analyzed in terms of different modes 
of plate convergence. Cordilleran-type mountain belts, such as the North 
American Cordillera, are contrasted with collision-type belts, such as the 
Himalayas. The former develop during long-term convergence of an oceanic plate 
toward and beneath a continental plate, whereas the latter are produced by the 
convergence and collision of one continental plate with another or with an 
island arc. Characteristics of cordilleran-type mountain belts include their 
position along a continental margin, their widespread volcanic and plutonic 
igneous activity, and their tendency to be bordered on both sides by zones of 
low-angle thrust faulting directed away from the axis of the belt. See OROGENY. 
 �34s[Gregory A. Davis]
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�6v�2v�32sCore drilling   �21m �33s 
    The boring of a hole in the earth by drilling a circular groove and 
leaving a central core. It is usually done to obtain the core as a sample for 
visual examination and to test its physical and chemical properties. But core 
drilling is also done for making the hole itself whenever it is more 
economical to drill the annular groove and remove the core than to drill the 
entire cross-sectional area of the hole. 
    The greatest use for core drilling is in prospecting for mineral deposits. 
It is usually done with a diamond core drill, so called because it utilizes 
the extreme hardness of the diamond to cut the annular groove in rock. Diamond 
core drilling is also widely used in civil engineering work to determine the 
strength of the rocks that must support the tremendous weights of buildings, 
bridges, and dams. 
    In shaft sinking by the core-drill method, and in core drilling other 
large-diameter holes, chilled steel shot is used as a cutting medium instead 
of diamonds. Other cutting mediums sometimes used are hard alloy-steel teeth, 
sintered carbides, and corundum grit. See BORING AND DRILLING (MINERAL); 
PROSPECTING.  
 �34s[Frank C. Sturges]
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�6v�2v�32sCore loss   �21m �33s 
    The rate of energy conversion into heat in a magnetic material due to the 
presence of an alternating or pulsating magnetic field. It may be subdivided 
into two principal components, hysteresis loss and eddy-current loss. See EDDY 
CURRENT; MAGNETIC HYSTERESIS. 
    The energy consumed in magnetizing and demagnetizing magnetic material is 
called the hysteresis loss. It is proportional to the frequency and to the 
area inside the hysteresis loop for the material used. Most rotating machines 
are stacked with silicon steel laminations, which have low hysteresis losses. 
The cores of large units are sometimes built up with cold-reduced, 
grain-oriented, silicon iron punchings having exceptionally low hysteresis 
loss, as well as high permeability when magnetized along the direction of 
rolling. See MAGNETIZATION. 
    Induced currents flow within the magnetic material because of variations 
in the flux; this is called eddy-current loss. For 60-cycle rotating machines, 
core laminations of 0.014 - 0.018 in. (0.35 - 0.45 mm) are usually used to 
reduce this eddy-current loss. See ELECTRIC ROTATING MACHINERY. 
 �34s[Leon T. Rosenberg]
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�6v�2v�32sCoriander �21m �33s
    An ancient flavoring material. The plant, Coriandrum sativum
(Umbelliferae), is a strong-scented perennial herb. The small, spherical,
brown fruits have an unpleasant odor when fresh, but are pleasingly aromatic
when dried. They are used as a flavoring in cookery. The oil of coriander is
used in medicine and for flavoring beverages. See UMBELLALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCoriolis acceleration and force   �21m �33s 
    Two alternative concepts which pertain to the difference between the 
acceleration of motion as referred to and viewed from a coordinate system 
which does not partake of the Earth's rotation (absolute acceleration) and the 
acceleration of motion as referred to and viewed from a coordinate system 
fixed on the surface of the Earth (relative acceleration). Part of this 
difference is the centripetal acceleration of a fixed point on the Earth's 
surface toward the axis of rotation of the Earth. The sum of this acceleration 
and the true gravitational acceleration (which is directed toward the center 
of mass of the Earth) is equal to the apparent gravitational acceleration 
(which is observed to act normal to the ellipsoidal surface of the Earth). The 
remainder of the difference between the absolute and relative accelerations 
depends upon the motion relative to the Earth and is called the Coriolis 
acceleration.  
    If the vertical velocity component is small, as it is in the large-scale 
motions of the atmosphere, the horizontal component of the Coriolis 
acceleration is normal to the horizontal component of relative motion and is 
directed to the left of the direction of this component on the Northern 
Hemisphere and to the right on the Southern Hemisphere. The magnitude of the 
horizontal component of the Coriolis acceleration is then given by twice the 
product of the speed of the horizontal component of relative motion (or wind) 
times the angular speed of rotation of the Earth times the sine of the 
latitude. Though negligible in many physical problems, the Coriolis effect 
must be considered in meteorological problems relating to the dynamics of 
large-scale wind currents since its horizontal component is typically as large 
as any other horizontal force acting. 
    It is often most convenient to think in terms of an equality of the sum of 
the forces and the relative acceleration alone. In this instance the forces 
must include a fictitious force equal in magnitude to the Coriolis 
acceleration and opposed to it in direction. This fictitious force is the 
Coriolis force. See GEOSTROPHIC WIND; GRADIENT WIND. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sCorn   �21m �33s 
    Zea mays, a grain crop of the order Cyperales grown for its edible seeds 
(technically fruits). See CYPERALES. 
    Corn is a cross-pollinated plant, the staminate (male) and pistillate 
(female) inflorescences (flower clusters) are borne on separate parts of the 
same plant (see illustration). Plants of this type are called monoecious. The 
staminate inflorescence is the tassel; it produces pollen that is carried by 
the wind to the silks produced on the ears. 
  
    As a crop.  Corn occupies a larger area than any other grain crop in the 
United States, where 60% of the world production is grown. Production 
throughout the United States reaches its greatest concentration in the states 
of Iowa, Illinois, Indiana, Ohio, Nebraska, Minnesota, Wisconsin, Michigan, 
and Missouri. This area, called the Corn Belt, is characterized by moderately 
high temperature, fertile, well-drained soils, and normally adequate rainfall. 
The Corn Belt accounts for approximately 80% of corn production in the United 
States. Although corn is grown in the United States primarily for livestock 
feed, about 10% is used for the manufacture of starch, sugar, corn meal, 
breakfast cereals, oil, alcohol, and other specialized products. In many 
tropical countries, corn is used primarily for human consumption. 
    The origin of corn is still unsettled, but the most widely held hypothesis 
assumes that corn developed from its wild relative teosinte (Z. mexicana) 
through a combination of favorable mutations, recognized and selectively 
propagated by early humans. Corn migrated from its presumed center of origin 
in Mexico or Central America and was being cultivated by the Indians as far 
north as New England when the first European colonists arrived, whose survival 
was due largely to the use of corn as food. 
    Each form (botanical variety) of corn is conditioned by fairly few genetic 
differences, and each may exhibit the full range of differences in color, 
plant type, maturity, and so on, characteristic of the species. All types have 
the same number of chromosomes (10 pairs), and all may be intercrossed to 
produce fertile progeny. Dent corns (Z. m. indentata) are the most important 
in the United States. Sweet corn is grown more extensively in the United 
States than in any other country. It is eaten as fresh corn or canned or 
frozen. In other countries, flint (Z. m. indurata), dent, or flour (Z. m. 
amylaceae) corns may be eaten fresh, but at a much more mature stage than the 
sweet corn eaten in the United States. The commercial production of popcorn 
(Z. m. everta) is almost exclusively American. 
    The development of varieties and strains of corn made possible the 
extension of its culture under diverse soil and climatic conditions. However, 
modern research methods, used for its further improvement, led to the present 
widespread use of hybrid corn. Hybrid corn is the first generation of a cross 
involving inbred lines. Inbred lines are developed by controlled 
self-pollination. When continued for several generations, self-pollination 
leads to reduction in vigor but permits the isolation of types which are 
genetically pure or homozygous. Intense selection is practiced during the 
inbreeding phase to identify and maintain genotypes having the desired 
characteristics. Crosses involving any two unrelated lines will exhibit 
heterosis, that is, yields above the means of the two parents. See BREEDING 
(PLANT); HETEROSIS. 
    Planting dates depend upon temperature and soil conditions. Germination is 
very slow at soil temperatures of 50DGF (10DGC), and seedling growth is 
limited at temperatures of 60DGF (16DGC) or below. Because of these 
temperature relations, planting begins in Florida and southern Texas in early 
February and is completed in New York and New England by late May or early 
June. Planting rates are influenced by water supply, soil type, and fertility 
and by the maturity characteristics of the hybrid grown. With planting rates 
above 16,000 plants per acre (40,000 per hectare), drilling in rows 24 - 36 
in. (60 - 90 cm) apart has become common practice. The use of nitrogen 
fertilizer has increased greatly; lesser amounts of phosphorus and potash are 
applied as needed. 
    The corn plant grows vegetatively until about silking, after which all 
weight increase is in the form of grain. Almost the entire grain yield results 
from photosynthesis during the grain growth period, which runs from silking to 
maturity. Contrary to popular opinion, grain yields are highest under cool 
conditions, when the lengthened grain growth period more than compensates for 
the slower growth rate. 
    Relationships among solar radiation, temperature, growing-season length, 
soil moisture, day length, soil fertility, and corn genotype in producing 
grain yields are complex and not well understood. Attempts to study the system 
as a whole, using simulation models on digital computers, may add considerably 
to knowledge of the subject. 
 �34s[William G. Duncan]
�33s 
  
    Processing.  The high starch content and caloric value of corn have 
resulted in extensive production of animal feeds, human food, alcoholic 
beverages, and industrial products from the grain. Corn grain consists of 
three main parts: endosperm (horny and floury), germ, and hull, which differ 
in composition. The dry hard kernel of common dent corn is excellent for 
storage and handling but requires suitable processing for use. The whole grain 
may be ground, or alternatively, it may be degermed and dehulled during dry 
milling. It may also be wet-milled to further separate endosperm into starch 
and protein. 
    New World Indians discovered that cooking corn in solutions of lime or 
wood ashes (lye) rendered it soft and easier to grind or dehull. Lime-treated 
corn is used for preparation of tortillas and other popular Mexican-style 
foods, whereas lye-treated corn is widely sold as canned hominy. Modern steel 
mills are used to grind most corn today. 
    About 90% of the corn produced in the United States is used for animal 
feeds. Because of its nutritional inadequacy, the grain is generally 
supplemented with protein sources such as soybean meal and with minerals and 
vitamins.  
  
    The high unsaturated oil content of corn germ limits the shelf life and 
restricts the use of whole ground corn. Most dry-milled corn is produced by 
degermer processes that fractionate grain into various-size endosperm 
particles, bran, and germ. Endosperm particles, after separation according to 
size, are classified into grits, meal, and flour. Grits serve as breakfast 
porridge, in corn flakes, and as a brewing substrate. Meal is a constituent of 
corn bread or muffin recipes and snack items. Corn flour is used in pancakes, 
soup thickeners, and dusting flours. Corn flour may be cooked on heated rolls 
or in extruders to pregelatinize its starch for use in baby foods and in 
industrial binders and adhesives. See FAT AND OIL (FOOD). 
    Oil is expelled from the germ and refined. The germ fraction may also be 
highly purified and defatted by solvent extraction. This process yields 
additional oil and a germ flour food product. 
    Demand for purified starch and corn sugars has resulted in continued 
growth of the corn wet-milling industry in the United States. In wet milling, 
the cleaned corn is steeped in solutions of sulfurous acid in large tanks, 45 
h at 122DGF (50DGC). The extracted solids in the steep liquor are then 
concentrated. The corn is next passed through an attrition mill that tears the 
kernels apart. After washing and purification, starch and gluten, dewatered 
and dried, are the products. 
    Of the starch produced in the United States, 40% is used directly or 
chemically modified in food products and industrial applications, and the 
remainder is converted to corn syrup. High-fructose corn syrup is manufactured 
by treatment of the syrup solution with purified bacterial-derived isomerase 
enzyme, which converts half the glucose to fructose to yield a syrup with 
sweetness equivalent to sucrose. 
    Oil is recovered from clean dry germ and, after refinement, is sold as 
table oil. Valuable protein-rich feed by-products are also obtained, including 
defatted corn germ meal and 60% protein gluten meal. 
    For preparing alcoholic beverages and alcohol, whole ground corn is 
cooked, and the starch is hydrolyzed with malt enzymes to provide substrate 
for whiskey fermentation. Cooked grits treated with malt are used to produce 
beer by yeast fermentation. Corn sugar syrups are also widely used in brewing. 
Industrial ethyl alcohol may be distilled from the yeast fermentation broth of 
enzyme-treated cooked whole grain or wet-milling by-products. The need for 
gasoline has encouraged grain alcohol production for automotive fuel 
supplements. See ALCOHOLIC BEVERAGES; MALT BEVERAGE. 
 �34s[Joseph S. Wall]
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�6v�2v�32sCornales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 6 families and about 250 species. More than 200 of the species 
belong to only 2 of the families, the Rhizophoraceae (120) and the Cornaceae 
(90). Within its subclass the order is marked by its usually woody habit, 
simple leaves, flowers with the perianth attached to the surface of the ovary 
and appearing to grow from the top of it, usually well-developed endosperm, 
and fleshy fruits that remain closed at maturity (indehiscent). One of the 
common kinds of mangrove (Rhizophora mangle) and the various species of 
dogwood (Cornus) are well-known members of the order, as is the sour gum 
(Nyssa sylvatica, family Nyssaceae). See DOGWOOD; ROSIDAE; TUPELO. 
 �34s[Arthur Cronquist]
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�6v�2v�32sCorner reflector antenna   �21m �33s 
    A directional antenna consisting of the combination of a reflector 
comprising two conducting planes forming a dihedral angle and a driven 
radiator or dipole which usually is in the bisecting plane (see illustration). 
It is widely used both singly and in arrays, gives good gain in comparison 
with cost, and covers a relatively wide band of frequency. The distance S from 
the driven radiator D to edge E need not be critically chosen with respect to 
wave length. See ANTENNA (ELECTROMAGNETISM). 
 �34s[John C. Schelleng]
�33s 
  
  
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�6v�2v�32sCorona discharge   �21m �33s 
    A type of electrical conduction that generally occurs at or near 
atmospheric pressure in gases. A relatively strong electric field is needed. 
External manifestations are the emission of light and a hissing sound. The 
particular characteristics of the discharge are determined by the shape of the 
electrodes, the polarity, the size of the gap, and the gas or gas mixture. See 
ELECTRICAL CONDUCTION IN GASES. 
    In some cases corona discharge may be desirable and useful, whereas in 
others it is harmful and attempts are made to minimize it. The effect is used 
for voltage division and control in direct-current nuclear particle 
accelerators. On the other hand, the corona discharge that surrounds a 
high-potential power transmission line represents a power loss and limits the 
maximum potential which can be used. Because the power loss due to Joule 
heating decreases as the potential difference is increased, it is desirable to 
use the maximum possible voltage. 
    In the potential current characteristic, the corona region is found above 
the dark current region and is field-sustained. Near the upper end it goes 
into either a glow discharge or a brush discharge, depending on the pressure. 
Higher pressure favors the brush discharge. See BRUSH DISCHARGE; GLOW 
DISCHARGE.  
    For still higher potential difference, breakdown takes place and a 
continuously ionized path is formed. See ELECTRIC SPARK. 
 �34s[Glenn H. Miller]
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�6v�2v�32sCoronagraph �21m �33s
    A specialized astronomical telescope used for observation of the corona of
the uneclipsed Sun. This observation is difficult because the brightness of
the corona is only 10- sup 6 that of the Sun. The stray light surrounding the
solar image in a conventional telescope is about 100 times brighter, and its
glare drowns the corona completely. The coronagraph is designed to minimize
this glare by artificially eclipsing the solar image itself with an occulting
disk. Further, it suppresses the diffracted light from the edge of the primary
objective and the doubly reflected light from its center by forming an image
of the objective on an annular screen which occults these areas. In addition
to such matters of design, success depends on an objective which produces the
least possible scattered light. See SOLAR CORONA.
 �34s[John W. Evans]
�33s

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�6v�2v�32sCoronatae �21m �33s
    An order of the class Scyphozoa which includes  mainly abyssal species.
The exumbrella is divided by a circular or coronal furrow into two parts, an
upper central disk and a lower coronal part. The central disk is usually
domelike, but in Periphylla (see illustration) it often narrows toward the
top. The coronal part has gelatinous thickenings (pedalia) situated on the
radii running from the center of the umbrella to the tentacles and the sensory
organs. The pedalia are separated from one another by clefts, running down the
radii, that are half-way between each tentacle and rhopalium (a sensory
organ). Abyssal forms of this group are dark brown or reddish-purple. The
littoral species of Nausithoee is reported to show alternation of generations,
but the life history of abyssal forms remains unknown. See SCYPHOZOA.

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�6v�2v�32sCor pulmonale   �21m �33s 
    A cardiomyopathy consisting of a right ventricular hypertrophy caused by 
hypertension in the pulmonary blood vessels, as the result of primary diseases 
within the lungs or lung blood vessels. Almost any longstanding lung disease 
may lead to cor pulmonale, including chronic obstructive pulmonary diseases 
(chronic bronchitis, emphysema, pneumonconioses, interstitial fibrosis, and so 
on). Vascular obstructions of the pulmonary blood vessels  (thrombi or emboli 
within the pulmonary vessels) similarly produce hypertension within the 
pulmonary blood vessels and lead to cor pulmonale.  See  HEART; HEART 
DISORDERS.  
 �34s[N. Karle Mottet]
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�6v�2v�32sCortex (plant)   �21m �33s 
    The mass of primary tissue in roots and stems extending inward from the 
epidermis to the phloem. The cortex may consist of one or a combination of 
three major tissues: parenchyma, collenchyma, and sclerenchyma (see 
illustration); in roots the cortex almost always consists of parenchyma. The 
cortex is bounded, more or less distinctly, by the hypodermis (exodermis) on 
the periphery and by the endodermis on the inside. 
    Cortical parenchyma is composed of loosely arranged thin-walled living 
cells. Prominent intercellular spaces usually occur in this tissue. In stems 
the cells of the outer parenchyma may appear green due to the presence of 
chloroplasts in the cells. This green tissue is sometimes called chlorenchyma, 
and it is probable that photosynthesis occurs in it. 
    In some species the cells of the outer cortex are modified in aerial stems 
by an irregular deposition of hemicellulose as an additional wall substance, 
especially in the corners or angles of the cells. This tissue is called 
collenchyma, and the thickening of the cell walls gives mechanical support to 
the shoot. 
    Lignified fibers and sclereids, as well as other lignified cells, also 
occur in the cortex of many plants. The tissue composed of such hard-walled 
cells is often referred to as cortical sclerenchyma. 
    The cortex makes up a considerable proportion of the volume of the root, 
particularly in young roots, where it functions in the transport of water and 
ions from the epidermis to the vascular (xylem and phloem) tissues. In older 
roots it functions primarily as a storage tissue. 
    In additon to being supportive and protective, the cortex functions in the 
synthesis and localization of many chemical substances; it is one of the most 
fundamental storage tissues in the plant. The kinds of cortical cells 
specialized with regard to storage and synthesis are numerous. The 
localization of enzyme systems apparently results in a remarkable 
differentiation of functional cell types. 
    Because the living protoplasts of the cortex are so highly specialized, 
patterns and gradients of many substances occur within the cortex, including 
starch, tannins, glucosides, organic acids, crystals of many kinds, and 
alkaloids. Oil cavities, resin ducts, and laticifers (latex ducts) are also 
common in the mid-cortex of many plants. See COLLENCHYMA; ENDODERMIS; 
HYPODERMIS; PARENCHYMA; ROOT; SCLERENCHYMA; SECRETORY STRUCTURES (PLANT); STEM. 
 �34s[D. S. Van Fleet]
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�6v�2v�32sCorundum   �21m �33s 
    A mineral with the ideal composition Al2O3. It is one of a large group of 
isostructural compounds including hematite (Fe2O3) and ilmenite (FeTiO3), all 
of which crystallize in the hexagonal crystal system, trigonal subsystem. 
Corundum has the high hardness of 9 on Mohs scale and is therefore commonly 
used as an abrasive, either alone or in the form of the rock called emery, 
which consists principally of the minerals corundum and magnetite. Crystals 
occurring in igneous rocks usually have an elongated barrellike shape, while 
crystals from metamorphic rocks are generally tabular. The specific gravity is 
approximately 3.98. See HEMATITE; ILMENITE. 
    Pure corundum is transparent and colorless, but most specimens contain 
some transition elements substituting for aluminum, resulting in the presence 
of color. Substitution of chromium results in a deep red color; such red 
corundum is known as ruby. The term ``sapphire'' is used in both a restricted 
sense for the ``cornflower blue'' variety containing iron and titanium, and in 
a general sense for gem-quality corundums of any color other than red. Star 
ruby and star sapphire contain tiny needles of the mineral rutile. See RUBY; 
SAPPHIRE.  
    Corundum occurs as a rock-forming mineral in both metamorphic and igneous 
rocks, but only in those which are relatively poor in silica, and never in 
association with free silica. Igneous rocks which most commonly contain 
corundum include syenites, nepheline syenites, and syenite pegmatites. Both 
contact and regionally metamorphosed silica-poor rocks may contain corundum. 
See IGNEOUS ROCKS; METAMORPHIC ROCKS. 
 �34s[Donald R. Peacor]
�33s 
  
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�6v�2v�32sCoryneform bacteria   �21m �33s 
    A large and diverse group of non-spore-forming gram-positive rod-shaped 
aerobic bacteria which, at some time in their growth cycle on artificial 
media, yield cells with somewhat irregular outlines or predominant granules. 
These granules (polyphosphate) often have a metachromatic appearance; that is, 
they appear red when stained with a blue dye. For the sake of convenience, 
these organisms are classified together with the ``Actinomycetes and Related 
Organisms,'' which constitute Part 17 of the eighth edition of Bergey's 
Manual. The rationale for this arrangement, aside from the morphological 
resemblance to some of the bacteria included in the Actinomycetes, is the 
finding of chemical components in the cell walls of members of Corynebacterium 
which are also found in Mycobacterium and Nocardia. In addition, a type of 
branched lipid which has been found to be peculiar to Mycobacterium and to 
Nocardia has also been found in Corynebacterium. These mycolic acids and 
corynemycolic acids have the general structure shown here. 
    The coryneform group of bacteria loosely corresponds to the family 
Corynebacteriaceae, which was recognized in the sixth edition of Bergey's 
Manual and broadened in the seventh edition of that treatise. The group, as 
currently envisioned, includes the genera Corynebacterium, Arthrobacter, 
Cellulomonas, and Kurthia. In addition, a number of other genera are included 
as genera incertae sedis (of uncertain standing in the nomenclature); their 
representatives may most likely be assigned to Arthrobacter when adequate 
comparative studies of Brevibacterium and Microbacterium have been completed. 
 �34s[Manley Mandel]
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�6v�2v�32sCosmic background radiation �21m �33s
    A nearly uniform flux of microwave radiation that is believed to permeate
all of space. The discovery that ``empty'' space is filled with microwave
radiation has had a profound impact on understanding the nature and history of
the universe. Although the discovery of the radiation was made in 1965 by A.
Penzias and R. W. Wilson, it was only in the late 1970s that its origin as the
remnant fireball from the ``big bang'' became nearly universally accepted by
astrophysicists. The correct prediction of the entire spectrum by P. J. E.
Peebles, R. H. Dicke, R. G. Roll, and D. T. Wilkinson, after the observation
of the intensity at only one frequency, was one of the great triumphs of
cosmological theory.
    Cosmic microwave radiation is the strongest evidence available for a hot,
compressed early universe. In the theory of the big bang, as originally
detailed by G. Gamow, H. Alpher, and R. Hermann, the universe began with an
explosion about 15 x 10 sup 9 years ago. This was not an explosion of matter
into empty space, but an explosion of space itself. In the big bang theory the
entire early universe is assumed to be filled with dense, hot, glowing matter.
There is no region of space which is free of matter or radiation. The
explosion of space increases the volume of the matter and radiation, and thus
also reduces the density and the temperature. The initial temperature is so
high that, even after 500,000 years, the temperature of the universe is still
several thousand kelvins, the temperature of the surface of the present-day
Sun. The matter of the universe is in the form of a plasma of photons,
electrons, protons, and alpha particles (the nuclei of helium). About this
time the expansion of the universe causes the temperature to drop to the point
that the electrons and protons combine to form neutral hydrogen. The universe,
which was previously opaque to visible light, suddenly becomes transparent,
thus decoupling matter and radiation. This happens because the visible photons
are much less strongly absorbed by neutral hydrogen than by the plasma. From
that time until now the cosmic blackbody photons have been traveling virtually
unscattered, carrying information about the nature of the universe at the time
of the decoupling. See BIG BANG THEORY.
    To an observer moving with the plasma, the photons have a blackbody
spectrum with a characteristic temperature of a few thousand kelvins. Although
the glow from the plasma is in the visible region, the radiation is
red-shifted from the visible by a factor on the order of 1000 into the
microwave region, with a characteristic temperature of 3 K(-454DGF). When one
measures the radiation, one is really ``looking at'' the shell of matter which
last scattered the radiation. See HEAT RADIATION; RED-SHIFT.
    This microwave radiation is coming from the most distant region of space
ever observed, and was emitted earlier in time than any other cosmological
signal. The radiating shell of matter forms the spatial background in front of
which all other astrophysical objects, such as quasars, lie. Until methods are
devised to detect the neutrinos or gravity waves that were emitted earlier,
there will be no direct means of viewing beyond this background.
    In the late 1970s, in a series of experiments flown aboard a balloon
gondola above 99.9% of the Earth's atmosphere, P. L. Richards and D. Woody
observed the full spectrum of the radiation and verified that the shape was
essentially as predicted in the big bang theory.
    A small anisotropy in the radiation had been observed. The smooth
``cosine'' behavior of the data suggests that its cause is not cosmological
but local: the motion of the Earth relative to the radiation. From the maximum
amplitude of the anisotropy, which is about 10- sup 3 of the intensity of the 3
K signal, one can deduce that the velocity of the Earth is 10- sup 3 of the
speed of light, or about 300 km/s. See COSMOLOGY.
 �34s[Richard A. Mueller]
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�6v�2v�32sCosmic electrodynamics   �21m �33s 
    The science concerned with electromagnetic phenomena in ionized media 
encountered in interstellar space, in stars, and above the atmosphere. Because 
these ionized materials are excellent electrical conductors, they are strongly 
linked to magnetic-field lines; they can travel freely along but not across 
the field lines. Phenomena treated under cosmic electrodynamics include 
acceleration of charged particles to cosmic-ray energies; collisions between 
galaxies; the correlation of magnetic fields with galactic structure; sunspots 
and prominences; magnetic storms; aurora; and Van Allen radiation belts. See 
IONOSPHERE; MAGNETOHYDRODYNAMICS. 
 �34s[Rolf Landshoff]
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�6v�2v�32sCosmic rays �21m �33s
    Electrons and the nuclei of atoms - largely hydrogen - that impinge upon
Earth from all directions of space with nearly the speed of light. These
nuclei with relativistic speeds are often referred to as primary cosmic rays,
to distinguish them from the cascade of secondary particles generated by their
impact against air nuclei at the top of the terrestrial atmosphere. The
secondary particles shower down through the atmosphere and are found all the
way to the ground and below.
    The primary cosmic rays provide the only direct sample of matter from
outside the solar system. Measurement of their composition can aid in
understanding which aspects of the matter making up the solar system are
typical of the Galaxy as a whole and which may be so atypical as to yield
specific clues to the origin of the solar system. Cosmic rays are electrically
charged; hence they are deflected by the magnetic fields which are thought to
exist throughout the Galaxy, and may be used as probes to determine the nature
of these fields far from Earth. Outside the solar system the energy contained
in the cosmic rays is comparable to that of the magnetic field, so the cosmic
rays probably play a major role in determining the structure of the field.
Collisions between the cosmic rays and the nuclei of the atoms in the tenuous
gas which permeates the Galaxy change the cosmic-ray composition in a
measurable way and produce gamma rays which can be detected at Earth, giving
information on the distribution of this gas.

    Cosmic-ray detection.  All cosmic-ray detectors are sensitive only to
moving electrical charges. Neutral cosmic rays (neutrons, gamma rays, and
neutrinos) are studied by observing the charged particles produced in the
collision of the neutral primary with some type of target. At low energies the
ionization of the matter through which they pass is the principal means of
detection. A single measurement of the ionization produced by a particle is
usually not sufficient both to identify the particle and to determine its
energy. However, since the ionization itself represents a significant energy
loss to a low-energy particle, it is possible to design systems of detectors
which trace the rate at which the particle slows down and thus to obtain
unique identification and energy measurement.
    At energies above about 500 MeV/nucleon, almost all cosmic rays will
suffer a catastrophic nuclear interaction before they slow appreciably. An
ionization measurement is commonly combined with measurements of physical
effects which vary in a different way with mass, charge, and energy. Cerenkov
detectors and the deflection of the particles in the field of large
superconducting magnets or the magnetic field of the Earth itself provide the
best means of studying energies up to a few hundred GeV/nucleon. Detectors
employing the phenomenon of x-ray transition radiation promise to be useful
for measuring composition at energies up to a few thousand GeV/nucleon. See
CERENKOV RADIATION; SUPERCONDUCTING DEVICES.
    MS. Pgs. 669-67212:54
    Above about 10 sup 1 sup 2 eV, direct detection of individual particles is
no longer possible since they are so rare. Such particles are studied by
observing the large showers of secondaries they produce in Earth's atmosphere.
These showers are detected either by counting the particles which survive to
strike ground-level detectors or by looking at the flashes of light the
showers produce in the atmosphere with special telescopes and photomultiplier
tubes. See PARTICLE DETECTOR; PHOTOMUTIPLIER.

    Atmospheric cosmic rays.  The primary cosmic-ray particles coming into the
top of the terrestrial atmosphere make inelastic collisions with nuclei in the
atmosphere. When a high-energy nucleus collides with the nucleus of an air
atom, a number of things usually occur. Rapid deceleration of the incoming
nucleus leads to production of pions with positive, negative, or neutral
charge. A few protons and neutrons (in about equal proportions) may be knocked
out with energies a significant fraction of that of the incoming nucleus. They
are called knock-on protons and neutrons.
    All these protons, neutrons, and pions generated by collision of the
primary cosmic-ray nuclei with the nuclei of air atoms are the first stage in
the development of the secondary cosmic-ray particles observed inside the
atmosphere. Since several secondary particles are produced by each collision,
the total number of energetic particles of cosmic-ray origin will increase
with depth, even while the primary density is decreasing.
    The uncharged PI sup 0 -mesons decay into two gamma rays with a life of
SIMILAR 8 x 10- sup 1 sup 7 s. The two gamma rays, each produce a
positron-electron pair. Upon passing sufficiently close to the nucleus of an
air atom deeper in the atmosphere, the electrons and positrons convert their
energy into bremsstrahlung. The bremsstrahlung in turn create new
positron-electron pairs, and so on. This cascade process continues until the
energy of the initial PIsup0 has been dispersed into a shower of positrons,
electrons, and photons with insufficient individual energies (*1 MeV) to
continue the pair production. The electrons and photons of such showers are
referred to as the soft component of the atmospheric (secondary) cosmic rays.
See ELECTRON-POSITRON PAIR PRODUCTION.
    The PIPLUS/MINUS-mesons produced by the primary collisions have a life
about 2.5 x 10- sup 8 s before they decay into muons. Most low-energy
PIPLUS/MINUS decay into muons before they have time to undergo nuclear
interactions. The muons will not interact with nuclei, and are too massive
(207 electron masses) to produce bremsstrahlung. They can lose energy only by
the comparatively feeble process of ionizing an occasional air atom as they
progress downward through the atmosphere. Because of this ability to penetrate
matter, they are called the hard component.
    The high-energy nucleons - the knock-on protons and neutrons - produced by
the primary-particle collisions and a few pion collisions proceed on down into
the atmosphere. They produce nuclear interactions of the same kind as the
primary nuclei, though of course with diminished energies. This cascade
process constitutes the nucleonic component of the secondary cosmic rays.
    Solar modulation.  The cosmic-ray intensity is lower during the years of
high solar activity and sunspot number, which follow an 11-year cycle. This
effect has been extensively studied with ground-based and spacecraft
instruments.
    The primary cause of solar modulation is the solar wind, a highly ionized
gas (plasma) which boils off the solar corona and propagates radially from the
Sun at a velocity of about 250 mi/s (400 km/s). The wind is mostly hydrogen,
with typical density of 80 protons/in. sup 3 (5 protons/cm sup 3). This density
is too low for collisions with cosmic rays to be important. Rather, the high
conductivity of the medium traps part of the solar magnetic field and carries
it outward.
    In addition to the bulk sweeping action, another effect of great
importance occurs in the solar wind, adiabatic deceleration. Because the wind
is blowing out, only those particles which chance to move upstream fast enough
are able to reach Earth. However, because of the expansion of the wind,
particles interacting with it lose energy. Thus, particles observed at Earth
with 10 MeV/nucleon energy actually started out with several hundred
MeV/nucleon in nearby interstellar space, and those with only 100 - 200
MeV/nucleon initial energy probably never reach Earth at all.

    Composition of cosmic rays.  Nuclei ranging from protons to lead have been
identified in the cosmic radiation. The relative abundances of the elements
may be compared with the best estimate of the ``universal abundances''
obtained by combining measurements of solar spectra, lunar and terrestrial
rocks, meteorites, and so forth. Most obvious is the similarity between the
two distributions. However, a systematic deviation is quickly apparent: the
elements lithium-boron and scandium-manganese as well as most of the
odd-charged nuclei are vastly overabundant in the cosmic radiation. This
effect has a simple explanation: the cosmic rays travel great distances in the
galaxy and occasionally collide with atoms of interstellar gas - mostly
hydrogen and helium - and fragment. This fragmentation, or spallation as it is
called, produces lighter nuclei from heavier ones but does not change the
energy/nucleon very much. Thus the energy spectra of the secondaries are
similar to those of the primaries. Calculations involving reaction
probabilities determined by nuclear physicists show that the overabundances of
the secondary elements can be explained by assuming that cosmic rays pass
through an average of about 1 oz./in. sup 2 (5g/cm sup 2) of material on their
way to Earth.
    When spallation has been corrected for, differences between cosmic-ray
abundances and solar-system or universal abundances still remain. The most
important question is whether these differences are due to the cosmic rays
having come from a special kind of material (such as would be produced in a
supernova explosion), or simply to the fact that some atoms might be more
easily accelerated than others.
    Cosmic-ray electron measurements pose other problems of interpretation,
partly because electrons are nearly 2000 times lighter than protons, the next
lightest cosmic-ray component. Protons with kinetic energy above 1 GeV are
about 100 times as numerous as electrons above the same energy, with the
relative number of electrons decreasing slowly at higher energies. But it
takes about 2000 GeV to give a proton the same velocity as a 1-GeV electron.
Viewed in this way electrons are several thousand times more abundant than
protons. It is thus quite possible that cosmic electrons have a different
source entirely from the nuclei.

    Age.  Another important result which can be derived from detailed
knowledge of cosmic-ray isotopic composition is the ``age'' of cosmic
radiation. Certain isotopes are radioactive, such as sup 1 sup 0 Be with a
half-life of 1.6 x 10 sup 6 years. Since Be is produced entirely by spallation,
study of the relative abundance of sup 1 sup 0 Be to the other Be isotopes,
particularly as a function of energy to utilize the relativistic increase in
this lifetime, will yield a number related to the average time since the last
nuclear collision. Measurements show that sup 1 sup 0 Be is nearly absent at low
energies and yield an estimate of the age of the cosmic rays of approximately
10 sup 7 years. An implication of this result is that the cosmic rays propagate
in a region in space which has an average density of 1.5 - 3 atoms/in. sup 3
(0.1 - 0.2 atoms/cm sup 3). This is consistent with some astronomical
observations of the immediate solar neighborhood.

    Origin.  Although study of cosmic rays has yielded valuable insight into
the structure, operation, and history of the universe, their origin has not
been determined. The problem is not so much to devise processes which might
produce cosmic rays, but to decide which of many possible processes do in fact
produce them.
    It is thought that cosmic rays are produced by mechanisms operating within
galaxies and are confined almost entirely to the galaxy of their production,
trapped by the galactic magnetic field. The intensity in intergalactic space
would only be a few percent of the typical galactic intensity, and would be
the result of a slow leakage of the galactic particles out of the magnetic
trap.
 �34s[Paul Evenson]
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�6v�2v�32sCosmochemistry   �21m �33s 
    The science of the chemistry of the universe (excluding the Earth), that 
is, the abundance and distribution of elements, chemical compounds, and 
minerals; chemical processes, particularly in the formation of cosmic bodies; 
isotopic variations; radioactive transformations; and nuclear reactions, 
including those by which the elementary constituents were formed. 
Cosmochemistry is often functionally considered as a branch or extension of 
the older and more developed science of geochemistry. It is also closely 
related to astronomy. 
    Many techniques of geochemistry can be applied to extraterrestrial samples 
which can be obtained for analysis in the laboratory. The most important of 
the available samples are meteorites. For most of the bodies and objects of 
the universe, chemical information comes from an analysis and theoretical 
interpretation of the radiations received from them on Earth and by satellites 
above the atmosphere. Spectral analysis of the light (visible, ultraviolet, 
infrared) received from stars and luminous nebulae gives information on their 
elementary contents and on the few compounds which are stable in their exposed 
regions. Radio astronomy provides information on the interstellar medium, 
including a considerable number of molecular constituents, mainly in cool dust 
clouds. Very-high-temperature regions of stars, nebulae, and galaxies are 
studied in the short-wavelength ultraviolet and x-ray spectral regions by 
instruments outside the atmosphere, principally in artificial sa1ellites. See 
ASTRONOMICAL SPECTROSCOPY; RADIO ASTRONOMY; SATELLITE ASTRONOMY; ULTRAVIOLET 
ASTRONOMY; X-RAY ASTRONOMY. 
    Nonluminous bodies of the solar system can be investigated by their 
reflection spectra, that is, albedo as a function of wavelength. This yields 
information about atmospheres, clouds, and visible surfaces. Observations of 
the solar wind and energetic solar particles give chemical information about 
the Sun's outer layers, especially the corona. Cosmic rays bring similar 
information about more distant regions, although the curvature of their paths 
obscures the directions of their sources. See ALBEDO; COSMIC  RAYS; METEOR; 
SOLAR WIND. 
    The radioactive dating methods based on primary natural radionuclides 
which are useful in geochemistry are also applicable to cosmochemical samples 
in terrestrial laboratories. Methods based on short-lived extinct natural 
radionuclides are applicable to times very early in the solar system and thus 
are important in cosmochemistry, though not in geochemistry. Methods based on 
cosmic-ray-induced nuclides are especially useful for meteorites and 
lunar-surface materials, which have not been shielded by an atmosphere. 
 �34s[Truman P. Kohman]
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�6v�2v�32sCosmology   �21m �33s 
    The study of the large-scale structure and the evolution of the universe, 
and including, in its modern connotation, that part of cosmogony that deals 
with the origin of the universe and of the chemical elements. See UNIVERSE. 
  
    Structure of the universe.  Beginning about 1924, E. P. Hubble developed 
ways of estimating distances to remote galaxies, and also studied their 
distribution in space. Hubble concluded that on the large scale the 
distribution of galaxies is homogeneous and isotropic. Hubble's finding was 
the first observational evidence for the cosmological principle, which states 
that at any instant of cosmic time the universe is, on the large scale, the 
same as seen by all hypothetical observers; that is, it is the same everywhere 
(is isotropic and homogeneous). See GALAXY, EXTERNAL. 
    On the small scale, however, there is a tendency for galaxies to cluster. 
The Milky Way Galaxy, in fact, is a member of a system of about two dozen 
galaxies, most of them much smaller than the Milky Way, spread over a region 
of space with a linear diameter of about 3 X 10 sup 6 light-years (1 LY = 5.88
X 10 sup 12 mi = 9.46 X 10 sup 12 km); the system is called the Local Group.
The distribution of galaxy images in photographic surveys of the sky is
compatible with a statistical model of complete clustering. Occasional
clusters, the great clusters, contain thousands of member galaxies each, and
have radii of 5 X 10 sup 6 to 2 X 10 sup 7 LY. On the small scale, even the
clusters are clumped into larger aggregates, called superclusters, typical
superclusters are 1 - 3 X 10 sup 8 LY across. Observations suggest that the
superclusters are the end of the hierarchy. See SUPERCLUSTERS. 
    In summary, the structure of the universe is observed to be very lumpy on 
scales of up to a few hundred million light-years, but on larger scales 
appears to be the same everywhere. The cosmological principle, therefore, is a 
starting assumption for all theories of cosmology that are widely considered 
today.  
  
    Gravitation in the universe.  The theory of gravitation embodied in 
Einstein's general theory of relativity leads to field equations that, in 
their simplest form, do not permit a static stable universe even if it is 
infinite. In general relativity the field equations describe the curvature of 
space-time by matter; it is in curved space-time that all objects, material 
bodies as well as photons of light, move along unaccelerated paths 
(geodesics). Imbued with the idea that the universe should be static, 
Einstein, in his effort to apply the field equations to cosmology, modified 
them with the introduction of a term now called the cosmological constant. The 
cosmological constant, if positive, implies a repulsive force that is greater 
in proportion to distance. By choosing just the correct value for the 
constant, the force can be made to balance gravitation on the large scale, and 
permit a static universe. See GRAVITATION; RELATIVITY; SPACE-TIME. 
  
    Expanding universe.  In the 1920s it occurred to others that the universe 
need not necessarily be static, and that the field equations in their simplest 
form (zero cosmological constant) can apply perfectly well to a uniformly 
expanding or contracting universe. Indeed, observations that could test the 
hypothesis were begun about 1912 by V. M. Slipher of the spectra of the 
``nebulae'' that were later found to be galaxies. Slipher found the features 
in the spectra of the objects to be displaced from their normal wavelengths by 
the Doppler effect, indicating that the nebulae are moving away from the Earth 
at high speeds. See DOPPLER EFFECT; REDSHIFT. 
    Now, if the cosmological principle is applied to an expanding universe, 
the universe must expand uniformly; otherwise, irregularities would be 
generated that would make the universe inhomogeneous. In a uniform expansion, 
the separations of all pairs of objects must increase by the same factor in 
the same time, which means that widely separated objects, having further to 
move apart, must separate at a greater speed than objects close together, 
which have less far to move to increase their separations by the same relative 
amount. In other words, every observer, everywhere in an expanding universe, 
will see every other object moving away from him at a speed that is 
proportional to its distance; it is a necessary and unique condition for a 
uniform expansion. Of course, since all observers see the same effect, the 
fact that objects are moving away from a given observer at speeds proportional 
to their distances from him gives him no information about his location in the 
universe, nor of any ``center.'' In an infinite expanding universe, the 
concept of ``center'' has no meaning. 
    In 1931 Hubble and M. L. Humason jointly published a comparison of the 
distances and radial velocities of galaxies receding at speeds up to 15 times 
those of the most remote galaxies observed by Slipher. The velocities and 
distances were in direct proportion; the expansion of the universe had been 
firmly established. The observed relation between the velocities and distances 
of galaxies is now called the Hubble law. 
  
    Cosmological models.  Because the cosmological principle requires that the 
universe expand uniformly, its evolution can be described uniquely with a 
simple change in scale. Thus the actual distance r(t) between two remote 
objects in space at time t can be expressed as the product of a suitably 
normalized time-varying scale factor, usually denoted as R(t), and the fixed 
distance u between those objects at some arbitrarily chosen time. According to 
the Hubble law, the radial velocity Vr of a galaxy is given by the equation 
below, where the constant of proportionality, H, is called the Hubble 

                             *** Vr = Hr(t) ***

constant. The Hubble ``constant'' is not really constant over time, but has a 
value that changes as the universe evolves and the scale R(t) changes with 
time. Even if all galaxies maintained their present speeds, H would diminish 
as the scale grows. In fact, however, the mutual gravitation of the objects in 
the universe must slow its expansion, so that galaxies, as they recede from 
each other, are gradually reducing their relative speeds. A major goal of 
observational cosmology is to determine how strongly the gravitation of the 
universe decelerates its expansion. See HUBBLE CONSTANT. 
    In general relativity, photons and material objects flow unaccelerated 
along geodesics in space-time, and that space-time is curved by the presence 
of matter. In a random distribution of massive objects the curvature can be 
very complex, but for the universe at large, adoption of the cosmological 
principle results in an enormous simplification. Except for small-scale local 
effects, the curvature is the same everywhere. If the mean universal density 
of matter and energy is high enough, the curvature is positive, in which case 
the universe is unbounded but finite, for information can never be received 
from beyond a particular volume of space. Such a universe is said to be 
closed. If the universal density is below a critical value, however, the 
curvature is negative, and the universe is open and infinite. The dividing 
line between these two classes of space-time, requiring the critical density 
that does not quite close the universe, is flat space-time, in which the 
universe is infinite and the geometry is euclidean. 
    A closed universe is closed not only to radiation but to matter, for it 
implies that the geodesics of particles of matter and of photons are closed; 
such a universe will eventually cease expanding and start to contract. The 
contraction will continue until all of the matter and radiation approaches a 
singularity at the origin of a universal black hole - the ``big crunch,'' as 
J. A. Wheeler described it. 
    Alternatively, the open universe will expand forever into greater and 
greater infinities; neither radiation nor material objects will ever come 
together again. Closed and open universes are analogous to rockets fired from 
the surface of the Earth at respectively less than and greater than the 
velocity of escape. A rocket launched with greater than the escape velocity is 
always slowed by the Earth's gravitation, but it has enough energy that it 
never stops; similarly, in the open universe the mean density of matter and 
radiation is too low to provide enough gravitation to ever stop the expansion. 
    The flat universe has the critical density to barely stop the expansion, 
analogous to a rocket launched with precisely the escape velocity. It is 
mathematically equivalent to a model by Einstein later modified by the 
astronomer W. de Sitter, and so is also called the Einstein - de Sitter 
universe.  
    All general relativistic models of the universe with zero cosmological 
constant have a unique beginning, usually called the big bang, and a finite 
age. The maximum possible age for any of these models is that of the open 
universe with so low a mean density that gravitation can be ignored, so that 
the galaxies continue to recede at constant speed. In this case, the age of 
the expansion, is simply the reciprocal of the Hubble constant. With the range 
of current estimates for the Hubble constant, that maximum possible age lies 
in the range 1 - 2 X 10 sup 10 years. 
 �34s[George O. Abell]
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�6v�2v�32sCotter pin   �21m �33s 
    A cotter pin is a split pin (as illustrated) formed by doubling a piece of 
wire semicircular in cross section to form a loop at one end. After insertion 
in a hole or a slot in a nut and through a mating crosswise hole in a bolt, 
the ends of the pin are separated and bent to hold it in place. This fastener 
is widely used in locations where service inspection is difficult and where 
failure would be disastrous. Cotter pins are similarly used in many other 
applications to prevent relative rotation or sliding. See NUT (ENGINEERING). 
 �34s[Paul H. Black]
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�6v�2v�32sCotton   �21m �33s 
    A fiber obtained from the cotton plant Gossypium, of the order Malvales. 
It has been used for more than 3000 years, and is the most widely used natural 
fiber, because of its versatility and comparatively low cost to produce and to 
manufacture into finished products. Cotton traditionally has been used alone 
in textile products, but blends with artificial fibers are also important. 
Chemically, cotton is essentially pure cellulose. The natural waxes occurring 
on the original fiber surface act as a finish which facilitates spinning. See 
CELLULOSE; MALVALES; NATURAL FIBER; TEXTILE. 
    To mature, cotton requires about 180 days of continuously warm weather 
with adequate moisture and sunlight; frost is harmful and may kill the plant. 
The plants begin to bloom about 4 months after planting. When the flowers fall 
off, seed pods grow and become cotton capsules or bolls, which must be 
protected against the boll weevil or other insects. When fully grown, the 
bolls burst, exposing the fleecy white fiber. When the raw cotton is 
harvested, it contains seeds, leaf fragments, and dirt that must be removed 
before baling. Cotton seeds alone constitute approximately two-thirds of the 
weight of the picked cotton. 
    After harvesting, the fiber, or lint as it is then called, is separated 
from the seed by gins. The lint is compressed into bales, covered with jute 
bagging, and bound with metal bands. After baling, the lint is sampled, 
graded, and sent to cotton mills, where the lint is blended, cleaned, carded, 
and spun into yarns. In the oil mill processing industry, the cottonseed is 
separated into fuzz or linters, hulls, oil, and protein cake. Each of these 
products is converted to several subproducts. 
    The cotton plant is one of the world's major agricultural crops. It is 
grown in all countries lying within a wide band around the Earth. The limits 
of this band in the New World are at about 37DGN latitude and at about 32DGS 
latitude. In addition to the effects of latitude, suitability of climate for 
the growth of cotton is also regulated by elevation, wind belts, ocean 
currents, and mountain ranges. The regions of more intensive culture comprise 
the Cotton Belt of the United States, the northern valleys of Mexico, India, 
Pakistan, eastern China, central Asia, Australia, the Nile River valley, 
eastern Africa, South Africa, northeastern and southern Brazil, northern 
Argentina, and Peru. Within these regions and outlying districts, 44 countries 
report data on cotton production. 
 �34s[Elton G. Nelson]
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�6v�2v�32sCotton effect   �21m �33s 
    The characteristic wavelength dependence of the optical rotatory 
dispersion curve or the circular dichroism curve or both in the vicinity of an 
absorption band. 
    When an initially plane-polarized light wave traverses an optically active 
medium, two principal effects are manifested: a change from planar to elliptic 
polarization, and a rotation of the major axis of the ellipse through an angle 
relative to the initial direction of polarization. Both effects are wavelength 
dependent. The first effect is known as circular dichroism, and a plot of its 
wavelength (or frequency) dependence is referred to as a circular dichroism 
(CD) curve. The second effect is called optical rotation and, when plotted as 
a function of wavelength, is known as an optical rotatory dispersion (ORD) 
curve. In the vicinity of absorption bands, both curves take on characteristic 
shapes, and this behavior is known as the Cotton effect, which may be either 
positive or negative (see illustration). There is a Cotton effect associated 
with each absorption process, and hence a partial CD curve or partial ORD 
curve is associated with each particular absorption band or process. See 
POLARIZED LIGHT; ROTATORY DISPERSION. 
    The rotational strengths actually observed vary over quite a few orders of 
magnitude; this variation in magnitude is useful in stereochemical 
interpretation of molecular structure. 
 �34s[Albert Moscowitz]
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�6v�2v�32sCotton-Mouton effect �21m �33s
    A magnetooptic effect concerned with the double refraction of light in a
liquid when the liquid is placed in a transverse magnetic field. It is
analogous to the electrooptical Kerr effect and is observed in liquids with
complicated molecular structure. If the molecule has a magnetic moment, the
field tries to orient the molecule, but the thermal motion tends to oppose
this action. There is thus a degree of orientation, which is dependent on the
temperature. If the molecule itself is optically anisotropic, the liquid also
will be anisotropic and will exhibit double refraction.  See  KERR EFFECT.
    The Cotton-Mouton effect is observed chiefly in nitrobenzene and aromatic
organic liquids. Aliphatic compounds have a considerably smaller effect.  See
MAGNETOOPTICS.
 �34s[G. H. Dieke ;William W. Watson]
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�6v�2v�32sCotylosauria   �21m �33s 
    Primitive reptiles that form an order under the subclass Anapsida. They 
are called stem reptiles and include the ancestors of all of the more advanced 
Reptilia. They range in age from the earliest Pennsylvanian (Paleozoic) to the 
Late Triassic (Mesozoic). See ANAPSIDA. 
    Two distinct subclasses are recognized, Captorhinomorpha and 
Procolophonia. Most members are characterized by a completely roofed temporal 
region of the skull (see illustration), but a few, such as the millerosaurs, 
possess small temporal fenestrae. Most cotylosaurs have very broad neural 
arches; primitive members lack this feature. 
    The turtles (Chelonia) seemingly arose from among the procolophonids, and 
it is probable that the lepidosaurs, including the snakes and lizards, and the 
archosaurs, including the dinosaurs, crocodiles, and flying reptiles, took 
origin from among the Captorhinomorpha. See ARCHOSAURIA; CHELONIA; 
LEPIDOSAURIA; REPTILIA; SYNAPSIDA. 
 �34s[Everett C. Olson]
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�6v�2v�32sCoulomb excitation   �21m �33s 
    A process in which two atomic nuclei that approach each other undergo 
transitions to excited states that are caused by the long-range Coulomb forces 
acting between the nuclei. The process can occur even when the nuclei do not 
come sufficiently close to allow the strong short-range nuclear forces to act. 
    That nuclei can be excited by this process is not so important in itself. 
What has proved to be enormously fruitful is the fact that experimental 
results based on this process have provided accurate values for electrical or 
electromagnetic properties of nuclei. To take an easily visualized example, 
Coulomb excitation measurements were instrumental in showing that many nuclear 
species are not spherical but are, shaped like a football. See COULOMB'S LAW; 
NUCLEAR STRUCTURE. 
    Coulomb excitation can be measured either by the direct analysis of the 
energy spectrum of the scattered projectiles or by the analysis of the 
GAMMA-rays subsequently emitted by the excited nuclei. Both methods are used 
extensively.  
 �34s[Paul H. Stelson]
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�6v�2v�32sCoulomb's law �21m �33s
    For electrostatics, Coulomb's law states that the direct force F of point
charge q1 on point charge q2, when the charges are separated by a distance r, is
given by F = k0q1q2/r(sup 2), where k0 is a constant of proportionality whose
value depends on the units used for measuring F, q, and r.  It is the basic
quantitative law of electrostatics.  In the rationalized meter-kilogram-second
(mks) system of units, k0 = 1/4 pi epsilon0, where epsilon0 is called the
permittivity of empty space and has the value epsilon0 = (1/4pi x 8.98776 x
10[sup 9]) coulomb[sup 2]/newton-m[sup 2]APPROXIMATELY EQUAL TO 8.85 x 10(sup
-12) coulomb(sup 2)/newton-m(sup 2).  Thus, Coulomb's law in the rationalized
mks system is as in the equation below, where q1 and q2 are expressed in
coulombs, r is expressed in meters, and F is given in newtons.	See ELECTRICAL
UNITS AND STANDARDS.

		F = (1/4 pi epsilon0) (q1q2/r[sup 2])

    The direction of F is along the line of centers of the point charges q1
and q2, and is one of attraction if the charges are opposite in sign and one
of repulsion if the charges have the same sign. For a statement of Coulomb's
law as applied to point magnet poles see MAGNETOSTATICS.
    Experiments have shown that the exponent of r in the equation is very
accurately the number 2. Lord Rutherford's experiments, in which he scattered
alpha-particles by atomic nuclei, showed that the equation is valid for
charged particles of nuclear dimensions down to separations of about
10sup-12 cm. Nuclear experiments have shown that the forces between charged
particles do not obey the equation for separations smaller than this. See
ELECTROSTATICS.
 �34s[Ralph P. Winch]
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�6v�2v�32sCoulometer   �21m �33s 
    Electrolytic cell for the measurement of quantities of electricity by 
quantitative determination of chemical changes; sometimes known as a 
voltmeter. Coulometers are used for the determination of quantities of 
electricity in standardization procedures and some electroanalytical 
measurements.  
    The quantity of substance produced or consumed in a coulometer is 
generally determined by weighing, titration, or volume measurements. The 
deposition of silver, copper, or mercury, the electrolysis of water with 
hydrogen and oxygen evolution, and the oxidation of iodide to iodine are the 
electrode reactions generally selected for coulometers. In the silver 
coulometer the quantity of electricity is determined by weighing the amount of 
silver plated upon the platinum cup cathode during passage of the current. One 
coulomb of electricity will deposit 0.0011180 gram of silver. The passage of 1 
coulomb of electricity through the iodine coulometer will liberate 0.001315 
gram of iodine at the anode. 
 �34s[Paul Delahay]
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�6v�2v�32sCoulometric analysis   �21m �33s 
    An electroanalytical chemistry technique in which the amount of a 
substance is determined quantitatively by measuring the total amount of 
electricity required to deplete a solution of this substance. The total 
electricity or charge consumed is related to the concentration by Faraday's 
law. The faraday, F, is 96,487 coulombs per equivalent. 
    Coulometric analyses are classified as primary or secondary. Primary 
coulometric analyses are those in which the substance to be determined is 
electrolyzed directly. Secondary coulometric analyses are those in which an 
electrolytically generated intermediate reacts stoichiometrically with the 
substance to be determined. When the solution is completely depleted, the 
total charge used is related directly to the amount of starting material. 
Secondary coulometric processes are often referred to as coulometric 
titrations. However, any coulometric analysis is a titration with electrons 
and is therefore a coulometric titration. 
    A condition of 100% current efficiency must exist for a coulometric 
analysis; that is, all the current must be used either directly or indirectly 
to deplete the desired substance. For a coulometric analysis either the 
applied current or the applied potential can be controlled. The 
controlled-current method is seldom used in primary coulometric analysis, for, 
as the reaction proceeds, the electroactive substance is depleted and is 
unable to maintain the required current. The controlled-potential method, 
however, is frequently used for primary coulometric analyses. See COULOMETER; 
ELECTROCHEMICAL TECHNIQUES. 
 �34s[George W. O'Dom]
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�6v�2v�32sCountercurrent exchange (biology)   �21m �33s 
    Engineers have known for decades that efficient, almost complete heat or 
other exchange could be achieved between two fluids flowing in opposite 
directions in separate tubes. Such countercurrent systems have evolved 
numerous times in living organisms for all types of exchange function. They 
are most commonly found in the circulatory, respiratory, and excretory 
(kidney) systems, serving in heat, oxygen, and ion exchange. Biological 
countercurrent systems can be classified into two main types: downhill 
exchanges and hairpin multipliers. In both cases, the basic mechanism is the 
same - exchange of substance between fluids flowing in opposite directions - 
but the consequences are very different. 
    Downhill exchange systems are commonest in the circulatory system where 
their morphological structure is a rete (network) of closely oppressed sets of 
small arteries and veins. They are also found in gills of fish and in the 
minute air tubules of the avian lung. In downhill exchanges, fluids flow in 
opposite directions in separate tubes with the possibility of exchange, for 
example, heat flow or diffusion of oxygen, between them. The fluid entering 
one tube is warmest at that end, while that entering the second tube is 
coolest at the other end. Heat flows from higher to lower temperature. 
Although the temperature differential between the two fluids is small at any 
point along the length of the countercurrent system, almost all the heat 
contained in the warmer tube is transferred to the cooler tube. Exchange of 
heat or oxygen occurs by passive diffusion. Most of the heat that entered the 
countercurrent system at one end leaves the system at the same end. 
    Retia of blood vessels thus serve as thermal isolating mechanisms within 
the body. Downhill exchange systems in the gills of fish and in the air 
tubules of birds permit maximum exchange of oxygen from the environment into 
the blood. Blood in respiratory capillaries flows against the water or air 
current and thus can pick up most of the oxygen contained in the external 
fluid. The advantage of downhill exchangers is that they achieve greater 
efficiency without extra energy cost simply by arranging flow in a 
countercurrent rather than in a concurrent fashion. 
    Hairpin multiplier systems take their name from the structure of the 
tubes, which have a hairpin turn between the afferent (descending) and the 
efferent (ascending) limbs. Hairpin countercurrent systems are found in the 
nephron (the loop of Henle) of the kidney and in the capillary system of the 
gas gland in the swim bladder of many fish. In contrast to downhill systems, 
which operate by passive transport, hairpin multipliers must employ active 
transport of materials. These are always materials pumped out of the efferent 
limb of the system. See CIRCULATORY SYSTEM; KIDNEY; RESPIRATORY SYSTEM; SWIM 
BLADDER.  
 �34s[Walter J. Bock]
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�6v�2v�32sCountercurrent transfer operations   �21m �33s 
    Industrial processes in chemical engineering or laboratory operations in 
which heat or mass or both are transferred from one fluid to another, with the 
fluids moving continuously in very nearly steady state or constant manner and 
in opposite directions through the unit. Other geometrical arrangements for 
transfer operations are the parallel or concurrent flow, where the two fluids 
enter at the same end of the apparatus and flow in the same direction to the 
other end, and the cross-flow apparatus, where the two fluids flow at right 
angles to each other through the apparatus. 
    In heat transfer there can be almost complete transfer in countercurrent 
operation. The limit is reached when the temperature of the colder fluid 
becomes equal to that of the hotter fluid at some point in the apparatus. At 
this condition the heat transfer is zero between the two fluids. Most heat 
transfer equipment has a solid wall between the hot fluid and the cold fluid, 
so the fluids do not mix. Heat is transferred from the hot fluid through the 
wall into the cold fluid. Another type of equipment does use direct contact 
between the two fluids for example, the cooling towers used to remove heat 
from a circulating water stream. See COOLING TOWER; HEAT BALANCE; HEAT 
EXCHANGER; HEAT TRANSFER. 
    Mass transfer involves the changing compositions of mixtures, and is done 
usually by physical means. A material is transferred within a single phase 
from a region of high concentration to one of lower concentration by processes 
of molecular diffusion and eddy diffusion. In typical mass transfer processes, 
at least two phases are in direct contact in some state of dispersion, and 
mass (of one or more substances) is transferred from one phase across the 
interface into the second phase. Mass transfer takes place between two 
immiscible phases until equilibrium between the two phases is attained. In 
mass transfer there is seldom an equality of concentration in the two 
equilibrium phases. This means that a component may be transferred from a 
phase at low concentration (but at a concentration higher than that at 
equilibrium) to a second phase of greater concentration. The approach to 
equilibrium is controlled by diffusion transport across phase boundaries. 
    Although the two phases may be in concurrent flow or cross-flow, usual 
arrangements have the phases moving in countercurrent directions. The more 
dense phase enters near the top of a vertical cylinder and moves downward 
under the influence of gravity. The less dense phase enters near the bottom of 
the cylinder and moves upward under the influence of a small pressure 
gradient. See ADSORPTION; CHEMICAL SEPARATION TECHNIQUES; DIFFUSION IN GASES 
AND LIQUIDS; DISTILLATION; DRYING; ELECTROPHORESIS; EXTRACTION; LEACHING. 
 �34s[Frank J. Lockhart]
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�6v�2v�32sCounting circuit �21m �33s
    One of several types of functional circuits used in switching or digital
data-processing systems, such as dial telephone systems and digital computers.
The general function of a counting circuit is to receive and count repeated
current or voltage pulses, which represent information arriving from some
other circuit within the same switching system or from a source external to
the system. Counting circuits may be devised with a variety of switching
circuit devices, such as electromagnetic relays, vacuum tubes, and
transistors. See SWITCHING CIRCUIT; SWITCHING SYSTEMS (COMMUNICATIONS).
    The basic requirement of a counting circuit is to detect each incoming
pulse and to establish within itself a unique state for each successive
appearance of such a pulse. The number of pulses that have arrived can then be
indicated to another circuit by an output which reflects the particular state
of the counting circuit.
    The simplest counter is one that counts a single pulse. With
electromagnetic relays the counting of a single pulse requires at least two
relay actions, one to record the arrival of the input signal, and a second to
record its disappearance.
    A frequently used counting circuit configuration is a two-pulse, or binary,
counter which recycles or returns to its original state after the second pulse.
Several single-stage binary counters may be connected so that the output of one
counter stage acts as the input to the next counter stage. A two-stage circuit
of this type can count four pulses; a three-stage, eight pulses, and so on.
							  [J.Meszar]

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�6v�2v�32sCouple   �21m �33s 
    A system of two parallel forces of equal magnitude and opposite sense. 
Under a couple's action a rigid body tends only to rotate about a line normal 
to the couple's plane. This tendency reflects the vector properties of a 
couple.  
    The total force of a couple is zero. The total moment C of a couple is 
identical about any point. Accordingly, C is the moment of either force about 
a point on the other and is perpendicular to the couple's plane. See RESULTANT 
OF FORCES; STATICS. 
    The moment of a couple about a directed line is the component of its total 
moment in the line's direction. Couples are equivalent whose total moments are 
equal.  
 �34s[Nelson S. Fisk]
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�6v�2v�32sCoupled circuits   �21m �33s 
    Two or more electric circuits are said to be coupled if energy can 
transfer electrically or magnetically from one to another. If electric charge, 
or current, or rate of change of current in one circuit produces electromotive 
force or affects the voltage between nodes in another circuit, the two 
circuits are coupled. 
    Between coupled circuits there is mutual inductance, resistance, or 
capacitance, or some combination of these. The concept of a mutual parameter 
is based on the loop method of analysis. A mutual parameter can be one that 
carries two or more loop currents; such a network has conductive coupling 
because electricity can flow from one circuit to the other. See NETWORK THEORY. 
    Also, there can be purely inductive coupling, which appears if the 
magnetic field produced by current in one circuit links the other circuit. A 
two-winding transformer is an application of inductive coupling, with energy 
transferred through the magnetic field only. 
    It is also possible to have mutual capacitance, with energy transferred 
through the electric field only. Examples are the mutual capacitance between 
grid and plate circuits of a vacuum tube, or the capacitive interference 
between two transmission lines, as a power line and a telephone line, that run 
for a considerable distance side by side. 
    There are several ways to show the relative polarities of inductive 
coupling. Figure 1 shows two coils wound on the same core. Current flowing 
into the upper end of coil 1 would produce magnetic flux upward in the core, 
and so also would current flowing into the upper end of coil 2. For this 
reason the upper ends of the two coils are said to be corresponding ends. Dots 
are placed on a diagram at the corresponding ends of coupled coils. Such dots 
are shown in Fig. 1, though they are not needed in this figure. Dots are also 
shown in Fig. 2, where they give the only means of identifying corresponding 
ends of the coils shown. 
 �34s[Hugh H. Skilling]
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�6v�2v�32sCoupling   �21m �33s 
    The mechanical fastening that connects shafts together for power 
transmission, often to form long sections of shafting or to connect the shaft 
of a machine to an external drive. Couplings are classified with respect to 
the kind of alignment and the centerline position of the connecting shafts. 
Rigid couplings are used where the axes of the shafts are directly in line, 
flexible couplings where the axes may be at a slight angle and slightly 
displaced, and universal joint couplings for large angularity or large 
displacement.  
    A rigid coupling is suitable for shafts in close alignment or held in 
alignment without inducing destructive forces in shafts or bearings. For 
commercial shafting, the coupling may be a sleeve with the shafts pressed into 
each end, or it may be a clamping sleeve. The sleeve on each shaft end may 
have an external flange with bolt holes. Couplings for large power machines 
are bolted together to hold the shafts rigidly; hence the shafts must be 
accurately aligned before assembly. 
  
    Flexible coupling provides the mechanical connection for shafts at a 
slight angle and displaced somewhat, particularly where power machines may set 
up vibratory forces. Such couplings are used when alignment is difficult to 
obtain, when a nonrigid support makes alignment impossible to maintain, or if 
shaft position changes because of operation. Flexible couplings consist of 
symmetrical hubs on each end of the joined shafts with a flexible connecting 
member between. This member may take such forms as the floating gear, the 
embedded spring, the flexible pin, and the flexible disk. Flexible couplings 
not only provide for lateral and angular misalignment, but also reduce the 
transmission of shock loads and change the vibration characteristics and 
critical speeds of the shafts. Oldham's coupling is the original flexible 
coupling, and presents the basic principle from which most flexible couplings 
are derived. 
    A flexible coupling for connecting shafts that have appreciable permanent 
angularity is termed a universal joint. See CLUTCH; FLUID COUPLING; OLDHAM'S 
COUPLING; SHAFTING; UNIVERSAL JOINT. 
 �34s[James J. Ryan]
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�6v�2v�32sCovellite   �21m �33s 
    A mineral having composition CuS and crystallizing in the hexagonal 
system. It is usually massive or occurs in disseminations through other copper 
minerals. The luster is metalic and the color indigo blue. The hardness is 1.5 
(Mohs scale), the specific gravity 4.7. Covellite is a common though not 
abundant mineral in most copper deposits. It is a supergene mineral and is 
thus found in the zone of sulfide enrichment associated with other copper 
minerals, principally chalcocite, chalcopyrite, bornite, and enargite, and is 
derived by their alteration. See BORNITE; CHALCOCITE; CHALCOPYRITE; COPPER; 
ENARGITE.  
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sCover crops �21m �33s
    Crops grown for the express purpose of preventing and protecting a bare
soil surface. Many crops serve this purpose, and some give a high direct
economic return. Other crops of lower return value are mainly important as a
part of a system of good soil and water conservation and soil productivity
maintenance, and it is these that are ordinarily considered cover crops. Basic
benefits of cover crops are in the prevention of soil detachment by wind,
rainfall, and splash erosion and in the return of cover crop roots and tops to
replenish and add to the supply of soil organic matter.
    Cover crops can be readily grown in humid climates and in arid and
semiarid climates where irrigation provides sufficient water so that cover
crops do not rob the soil of needed moisture. Reseeded rangeland is an
outstanding example of the use of cover crops. Many special areas such as road
banks, ditch banks, and recreation areas need the protection of permanent
vegetative cover of high quality. A variety of grasses and legumes may be
used, depending upon soil and climatic conditions. See GRASS CROPS; RANGELAND
CONSERVATION; SOIL CONSERVATION.
 �34s[Paul J. Zwerman]
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�6v�2v�32sCowpea   �21m �33s 
    Vigna unguiculata ssp. unguiculata, a legume of such ancient cultivation 
that its country of origin, either in Asia or Africa, is uncertain. 
Large-scale production occurs mainly in the southern United States, Africa, 
the Mediterranean region, and Australia. Cowpeas are grown primarily in the 
United States as a vegetable crop and are also known as southern peas and 
blackeye peas, the latter referring to a group of varieties. There are at 
least 200 varieties in the United States, but only about 20 of these are of 
commercial and home garden importance. It is also grown as a hay and forage 
crop. In the tropics and subtropics, cowpeas provide food for millions of 
people and feed for a large number of livestock. See LEGUME. 
 �34s[Blake B. Brantley, Jr.]
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�6v�2v�32sCoxsackievirus   �21m �33s 
    A large subgroup of the enteroviruses, in the picornavirus group. The 
coxsackieviruses produce various human illnesses, including aseptic 
meningitis, herpangina, pleurodynia, and encephalomyocarditis of newborn 
infants. At least 23 antigenically distinct types in group A are now 
recognized, and 6 in group B. Several types (B1 - 6, A9) grow well in tissue 
culture. See ENTEROVIRUS; PICORNAVIRUS. 
    After incubation for 2 - 9 days, during which the virus multiplies in the 
enteric tract, clinical manifestations appear which vary widely. Diagnosis is 
by isolation of virus in tissue culture or in infant mice. Stools are the 
richest source of virus. Neutralizing and complement-fixing antibodies form 
during convalescence and are also useful in diagnosis. See ANTIBODY; 
COMPLEMENT-FIXATION TEST; NEUTRALIZING ANTIBODY. 
    The coxsackieviruses have worldwide distribution. Infections occur chiefly 
during summer and early fall, often in epidemic proportions. Spread of virus, 
like that of other enteroviruses, is associated with family contact and 
contacts among young children. See ANIMAL VIRUS. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sCrab   �21m �33s 
    The name applied to arthropods in sections Anomura and Brachyura of the 
reptantian suborder of the order Decapoda, class Crustacea. The term crab is 
also sometimes used for two species of sucking lice (order Anoplura) which 
prey upon humans. Phthirus pubis is the crab louse which inhabits the pubic 
region.  
    Section Anomura includes over 1400 species in 12 different families. 
Commonly called hermit, king, sand, or mole crabs, the anomurans have 
lobsterlike abdomens which bend beneath the cephalothorax in crablike manner, 
setting them apart in a somewhat ill-defined group. 
    Section Brachyura encompasses the so-called true crabs, with reduced 
abdomens folded snugly beneath the cephalothorax. More than 4500 species 
making up 28 families have been grouped into subsections. In addition to 
subsections Dromiacea, Gymnopleura, and Oxystomata, the Hapalocarcinidea has 
one family of the same name containing three genera of small crabs with 
elongated abdomens. Subsection Brachygnatha is the largest group of 
brachyurans and includes the most typical crabs. It has 20 families 
encompassing over 3700 species which occupy a variety of habitats, undergo 
different types of development, and exhibit contrasting patterns of behavior. 
See CRUSTACEA; DECAPODA (CRUSTACEA). 
 �34s[Marilyn S. Kerr]
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�6v�2v�32sCrabapple   �21m �33s 
    A fruit, represented commercially by such varieties as Martha, Hyslop, and 
Transcendent, comprising hybrids between Malus baccata (Siberian crabapple) 
and M. domestica (cultivated apple). 
    Trees are very hardy. At one time the fruit of the crabapple was esteemed 
because of its high pectin content and its usefulness in jam and jelly 
manufacture. With the introduction of commercial pectin preparations, demand 
for the crabapple declined sharply. Except for its use as a pickled product, 
there is little commercial interest in this fruit. See APPLE; FRUIT, TREE; 
PECTIN; ROSALES. 
 �34s[Harold B. Tukey]
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�6v�2v�32sCrab Nebula �21m �33s
    The Crab Nebula in Taurus (see illustration) is the most remarkable known
gaseous nebula. Observed optically, it consists of an amorphous mass that
radiates a continuous spectrum and is involved in a mesh of delicate filaments
which radiate a bright line spectrum characteristic of typical gaseous
nebulae. This nebula has been identified as the expanding remnant of a
supernova that appeared in A.D. 1054. Presumably it comprises only material
from the star not yet mixed with the interstellar medium. See SUPERNOVA.
    The Crab Nebula radiates strongly in the radio range, and the infrared,
ultraviolet, x-ray, and gamma-ray spectral regions. Furthermore, the radiation
of the central amorphous mass is strongly polarized, even in the x-ray region
- indicating that the source of the radiation must be the synchrotron emission
of electrons accelerated in a field on the order of 10- sup 8 tesla. Energy must
be supplied to these electrons at a rate equivalent to about 30,000 times the
solar power output. The source of energy for the Crab Nebula appears to be a
remarkable, rapidly spinning object known as a pulsar, or neutron star, which
is presumed to be the residue of the supernova that created the nebula. Its
period of variability (which is associated with its rotation period) is 0.033
s. Energy output variations are observed in the radio, infrared, optical, and
xray range. See NEBULA.
 �34s[Lawrence H. Allen]
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�6v�2v�32sCracking �21m �33s
    A process used in the petroleum industry to reduce the molecular weight of
hydrocarbons by breaking molecular bonds. Cracking is carried out by thermal,
catalytic, or hydrocracking methods. Increasing demand for gasoline and other
middle distillates relative to demand for heavier fractions makes cracking
processes important in balancing the supply of petroleum products.
    Thermal cracking depends on a free-radical mechanism to cause scission of
hydrocarbon carbon-carbon bonds and a reduction in molecular size, with the
formation of olefins, paraffins, and some aromatics. Side reactions such as
radical saturation and polymerization are controlled by regulating reaction
conditions. In catalytic cracking, carbonium ions are formed on a catalyst
surface, where bond scissions, isomerizations, hydrogen exchange, and so on,
yield lower olefins, isoparafins, isoolefins, and aromatics. Hydrocracking is
based on catalytic formation of hydrogen radicals to break carbon-carbon bonds
and saturate olefinic bonds. It converts intermediate- and high-boiling
distillates to middle distillates high in paraffins and low in cyclics and
olefins. SeeCATALYTIC CRACKING; HYDROCRACKING; THERMAL CRACKING.
 �34s[Earl L. Core]
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�6v�2v�32sCranberry   �21m �33s 
    The large-fruited American cranberry, Vaccinium macrocarpon, a member of 
the heath family, Ericaceae, is a native plant of open, acid peat bogs in 
northeastern North America. Well-tended cranberry bogs continue to produce 
annual crops for a century or more without replanting. Selections from the 
wild have been cultivated since the early 19th century. It is an evergreen 
perennial vine producing runners and upright branches with conspicuous 
terminal flower buds. 
    Commercial cranberry growing is confined to Massachusetts, New Jersey, 
Wisconsin, Washington, and Oregon and to several provinces in Canada. Most of 
the fruit destined for processing is harvested in flood waters, either by 
machines which pick and deliver the fruit into towed plastic boats or by water 
reels which detach the berries to float and be driven by wind  to shore where 
they are elevated into bulk trucks. Berries for juice manufacture are usually 
vine-ripened and deep-frozen for a month or more prior to thawing and 
extraction.  
    Good-quality fresh cranberries can be stored for several months, 
refrigerated, with very little loss to decay. Good-quality cranberries can be 
kept in deep-freeze storage for several years with only minor moisture loss. 
Frozen berries on thawing are soft and juicy, unlike the firm fresh berry, and 
must be utilized promptly. 
    The special requirements of the cranberry plant for low fertility, acid 
soil, and winter protection make it a poor choice for home garden cultivation. 
 �34s[Chester E. Cross]
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�6v�2v�32sCrane hoist   �21m �33s 
    A mobile construction machine built principally for lifting loads by means 
of cables. A crane hoist consists of three principal elements: (1) an 
undercarriage on which the unit moves, (2) a cab or house which envelops the 
main frame and which contains the power units and controls, and (3) a movable 
boom over which the cables run and to which miscellaneous working attachments 
are connected. 
    A crane is usually typed according to its undercarriage. The crawler crane 
is a self-propelled unit mounted on continuous tracks similar to those on army 
tanks. The truck crane is a unit consisting of a crane house and boom mounted 
on a truck chassis (see illustration). The wheeled crane is self-propelled and 
rides on a rubber-tired chassis that is an integral part of the unit. The rail 
or locomotive crane is mounted on a railroad flatcar or a special chassis with 
flanged wheels for use on standard- or special-gage tracks. The floating crane 
has a barge or scow for an undercarriage. It is used for water work and for 
work on waterfronts and generally lacks its own motive power, being towed from 
place to place by tugboats. See HOISTING MACHINES. 
 �34s[Edward M. Young]
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�6v�2v�32sCranial nerve   �21m �33s 
    Any peripheral nerve which has its central nervous system connection with 
the brain, as opposed to the spinal cord, and reaches the brain through a hole 
(foramen) in the skull. Nerve fibers are sensory if they carry information 
from the periphery to the brain, and motor if they carry information from the 
brain to the periphery. Sensory fibers are classified as (1) somatic sensory 
(ss) if they come from the skin or muscle sense organs, (2) visceral sensory 
(vs) if they come from the viscera, and (3) special sensory (sp.s) if they 
come from special sense organs such as the eye and ear. Motor fibers are 
classified as (1) somatic motor (sm) if they carry information to somatic 
striated muscles, (2) general visceral motor (gvm) if they carry information 
to glands, smooth muscle, or cardiac muscle, and (3) special visceral motor 
(svm) if they carry information to visceral striated muscle. A cranial nerve 
may have only one fiber type or several; cranial nerves with several fiber 
types are called mixed nerves. 
    In mammals, 12 pairs of cranial nerves, numbered I through XII, are 
usually described. However, mammals also have an anterior unnumbered nerve, 
the terminal nerve. Many vertebrates also have two pairs of lateral line 
nerves (unnumbered), and lack discrete nerves XI and XII. The table summarizes 
the cranial nerves of vertebrates. See BRAIN; NERVOUS SYSTEM (VERTEBRATE). 
 �34s[Douglas B. Webster]
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�6v�2v�32sCraniata   �21m �33s 
    The major subphylum of the phylum Chordata comprising the vertebrates, or 
the cyclostomes to mammals. The subphylum is also known as the Vertebrata. 
Their most distinctive characteristic is the cranium or braincase. A column of 
articulated vertebrae extends down the body from the cranium to the end of the 
tail in all forms except some cyclostomes. Other distinctive vertebrate 
features are bony tissue, a kidney with the nephron as the basic unit, a 
heart, red and white blood cells, a liver and a pancreas, specialized sense 
organs such as a distinctive eye and a lateral line organ plus ear, several 
unique endocrine organs such as the pituitary and thyroid, a true skin, and 
the neural crest - a unique by-product of neural tube formation that gives 
rise to a series of features from gill cartilages and pigment cells to the 
adrenal medulla. 
    The earliest vertebrates - agnathan fish - lacked jaws and teeth, paired 
fins, and a lung, all of which originated in the higher groups, the 
gnathostomes. Vertebrates originated in  fresh waters, with the kidney being a 
water balance organ, and heavy dermal bone being protection against larger 
predatory invertebrates. They were initially slow-moving bottom-dwelling 
forms, become free-swimming pelagic later, with several evolutionary lines 
reinvading the oceans. The main line of evolution leading to the tetrapods 
remained always in fresh waters. 
 �34s[Walter J. Bock]
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�6v�2v�32sCrank   �21m �33s 
    In a mechanical linkage or mechanism, a link that can turn about a center 
of rotation. The crank's center of rotation is in the pivot, usually the axis 
of a crankshaft, that connects the crank to an adjacent link. A crank is 
arranged for complete rotation (360DG) about its center; however, it may only 
oscillate or have intermittent motion. A bell crank is frequently used to 
change direction of motion in a linkage (see illustration).See LINKAGE 
(MECHANISM).  
 �34s[Douglas P. Adams]
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�6v�2v�32sCrayfish   �21m �33s 
    Fresh-water decapod crustaceans which are related to lobsters and 
classified with them in the section Astacura of the suborder Reptantia (see 
illustration); also called  crawfish or crawdads. They are generally cryptic 
and nocturnal, but other habits and habitats of crayfish vary. They live in 
streams, rivers, ponds, and lakes and demonstrate various extents of 
burrowing. Crayfish are scavengers and eat detritus and dead animals; they 
also eat each other, small fish, and probably any other small living animal 
they can catch; some eat algae and other forms of vegetation. Crayfish are 
eaten by some fishes, amphibians, reptiles, wading or diving birds, and 
mammals including humans. 
    Crayfish anatomy and physiological processes, including growth by molting, 
regeneration, and autotomy, are generally like those of their marine 
relatives. Crayfish undergo abbreviated development, having no larval stages 
characteristic of marine crustaceans. Hatchlings resemble adults except that 
they lack uropods, broad appendages of the terminal abdominal segment. Baby 
crayfish use their chelipeds to hang onto the mother's pleopods until they 
undergo the molt after which their uropods are formed and they swim off on 
their own. 
    The largest of freshwater crustaceans, long-lived and abundant in many 
locales, crayfish are integral components of their ecosystems. Astacus species 
have been products of commerce in Europe since before the Middle Ages. 
Crawfish farming, usually alternating a rice crop with a crawfish crop in what 
were originally rice ponds, has prospered in Louisiana and has been undertaken 
in several other states. In other areas, catches of natural populations are 
shipped to dealers in live fishing-bait, or are processed and frozen for 
export to European food markets. See CRUSTACEA; DECAPODA (CRUSTACEA); LOBSTER. 
 �34s[Marilyn S. Kerr]
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�6v�2v�32sCreeping flow �21m �33s
    Fluid flow in which the velocity of the flow is very small. For creeping
flow, the Reynolds number Rsube = Ud/v is small (less than unity). Here U is
the reference velocity, d is the reference length, and v is the kinematic
viscosity of the fluid. For low Reynolds number, the inertial force is
negligible and the nonlinear terms in Navier-Stokes equations are neglected.
The resultant flow is known as Stokes flow. See FLUID FLOW; NAVIER-STOKES
EQUATIONS; PARTICLE PROPERTIES; REYNOLDS NUMBER; VISCOSITY.
 �34s[Shih-I Pai]
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�6v�2v�32sCreep (materials) �21m �33s
    The time-dependent strain which occurs when solids are subjected to an
applied stress. See STRESS AND STRAIN.
    Materials exhibit different kinds of creep phenomena. For the most common
kind of creep response, following the loading strain, the creep rate is high
but decreases as the material deforms during the primary creep stage. At
sufficiently large strains, the material creeps at a constant rate. This is
called the secondary or steady-state creep stage. Ordinarily this is the most
important stage of creep since the time to failure is determined primarily by
the secondary creep rate. In the case of  tension creep, the secondary creep
stage is eventually interrupted by the onset of tertiary creep, which is
characterized by internal fracturing of the material, creep acceleration, and
finally failure. At low temperatures of applied stresses the time scale can be
thousands of years or longer while under different conditions the entire creep
process can occur in a matter of seconds. Another kind of creep response is
the sort of strain-time behavior observed when the applied stress is partially
or completely removed in the course of creep. This results in time-dependent
or anelastic strain recovery.
    Creep of materials often limits their use in engineering structures. The
centrifugal forces acting on turbine blades cause them to extend by creep. In
nuclear reactors the metal tubes that contain the fuel undergo creep in
response to the pressures and forces exerted on them. In these examples the
occurrence of creep is brought about by the need to operate these systems at
the highest possible temperatures. Creep also occurs in ordinary structures.
For example, when a bolt is tightened to fasten two parts together, the
tension stress in the bolt can be quite high. Over a long period of time the
bolt creeps, with the result that the force holdding the two parts together
dimishes. Another example of stress relaxation caused by creep is found in
prestressed concrete beams, which are held in compression by steel rods that
extend through them. Creep and stress relaxation in the steel rods eventually
leads to a reduction of the compression force acting in the beam, and this can
result in failure.
    There are many examples of creep that are not related to high technology.
The window panes in very old homes or cathedrals are often thicker at the
bottom than they are at the top due to creep under the force of gravity.
Similarly, when a glacier flows down the side of a mountain the ice undergoes
extensive creep deformation. See METAL, MECHANICAL PROPERTIES OF; PLASTICITY;
RHEOLOGY.						 [W.D.N.;J.C.G.]

below field info left in since it may be correct. 2 lines @ foot were deleted
LIF 10/17/86
 �34s[Gideon T. James]
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�6v�2v�32sCreosote   �21m �33s 
    Either of two products of tar distillation: commercial creosote is a toxic 
substance obtained by high-temperature carbonization of soft coal, and 
pharmaceutical creosote is an aromatic mixture obtained by redistillation of 
wood tar. 
    Commercial creosote is obtained from creosote oil, which amounts to about 
30 - 35% of coal tar. It contains numerous tar acids such as cresols, 
xylenols, naphthols, and higher homologs, as well as various tar bases such as 
picolines and collidines, and hydrocarbons. It is primarily used for wood 
treatment. Small amounts are consumed in the manufacture of lampblack, in 
disinfectant sprays for fruit trees, and as melting auxiliaries for pitch and 
bitumen.  
    Hardwood creosote is prepared from settled hardwood tar, chiefly derived 
from beechwood. It is the source of pharmaceutical-grade creosote. See COAL 
CHEMICALS; CRESOL; WOOD PRESERVATION. 
 �34s[Frank Wagner]
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�6v�2v�32sCresol   �21m �33s 
    Any one of the three methylphenols (see illustration). The name is also 
often used to describe a mixture of the three. At room temperature the ortho 
and para isomers are colorless solids when highly purified, whereas the meta 
isomer is a liquid, as is a mixture of the three. The cresols are moderately 
soluble in water and readily soluble in alcohol and ether. They are only 
slightly less acidic than phenol, and they also resemble the parent substance 
in toxicity. 
    Solutions of cresol have long been used as disinfectants. Large quantities 
are utilized in the manufacture of synthetic resins, antioxidants, 
surface-active agents, plasticizers, and antibacterial agents. The phosphate 
ester of cresol is used as an additive for fuel and lubricating oils. See 
PHENOL.  
 �34s[Martin Stiles]
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�6v�2v�32sCress   �21m �33s 
    A prostrate hardy perennial crucifer of European origin belonging to the 
plant order Capparales. Watercress (Nasturtium officinale) is generally grown 
in flooded soil beds and used for salads and garnishing. Virginia is an 
important producing state. Garden cress (Lepidium sativum) is a cool-season 
annual crucifer of western Asian origin grown for its flavorful leaves. Of 
lesser commercial importance is upland or spring cress (Barbarea verna), a 
biennial crucifer of European origin. See CAPPARALES. 
 �34s[H. John Carew]
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�6v�2v�32sCretaceous   �21m �33s 
    The latest system of rocks or period of the Mesozoic Era. It includes that 
part of the Earth's history from about 127,000,000 to 64,000,000 years ago - 
the time of the greatest flooding of the Earth's surface during the Mesozoic 
Era. It was also the time of extensive chalk deposition over much of the 
Northern Hemisphere.See MESOZOIC. 
    The Cretaceous marks the rapid rise and spread of deciduous trees over the 
landscape. Its close was marked by the withdrawal of the seas from the 
continents, and by the extinction of the dinosaurs, flying reptiles and huge 
marine reptiles, and the ammonites and certain other groups of marine mollusks 
that had been so conspicuous throughout the Mesozoic Era. 
    Much of the Earth's present land surface was submerged by shallow seas 
during the Cretaceous. Two seas covered much of Europe most of the time: A 
Northern sea extended from the British Isles eastward across Germany and 
Poland to Russia, and a southern sea, Tethys, covered most of the present 
Mediterranean Sea and the surrounding parts of southern Europe and 
northernmost Africa. 
    According to concepts of continental drift, at the beginning of the 
Cretaceous the Earth consisted of four closely spaced landmasses and a vast 
Pacific Ocean. The landmasses consisted of South America united to Africa, an 
India landmass lying to the east, North America united to Europe by way of 
Greenland, and Antarctica united to Australia. The northern landmass (North 
America - Eurasia) is known as Laurasia, and the rest of the landmasses are 
known collectively as Gondwana. See CONTINENT. 
    Two broad climatic belts were present in the Cretaceous: a southern 
Mediterranean or Tethyan belt and a northern or boreal belt. The Mediterranean 
belt, extending from Central America eastward across southern Europe and 
northern Africa to India and the East Indies, was characterized by much 
deposition of limestone and other calcareous rocks. Characteristic fossils 
include large thick-shelled gastropods, certain ammonites, and sessile 
bivalves known as rudistids which formed reefs. The boreal belt was 
characterized by an extinct group of squids known as belemnites, certain 
bivalves (such as aucellas), and numerous ammonites. Temperatures were warmer 
than at present and probably more uniform. Palms grew as far north as Alaska 
and Greenland. 
    The Cretaceous is generally divided into two epochs, Lower Cretaceous and 
Upper Cretaceous. These are subdivided into stages whose names derive from 
localities in western Europe, where the Cretaceous was first studied 
intensively (see table). 
 �34s[William A. Cobban]
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�6v�2v�32sCrinoidea   �21m �33s 
    A class of radially symmetrical Crinozoa in which the body is 
flower-shaped and either is carried on top of a long, anchored stem (see 
illustration), as in sea lilies, or is stemless and free to move about, as in 
comatulids or feather stars. The flowerlike appearance is the result of a 
circlet of arms that range like petals around the edge of a cup-shaped plated 
structure called the calyx. The skeleton of the calyx is first formed of two 
circlets of five plates each. The free-moving arms, usually five in number, 
occur around the upper margin of the calyx. Each of the five arms is usually 
divided into one or more branches, for a total of 10 to 30 branches. The stem 
is attached to the aboral pole of the calyx. It may be long and slender with a 
rootlike organ attaching it to the sea floor, or it may carry regular whorls 
of appendages, called cirri, arranged in groups of five at points on the stem 
termed nodes. The cirri on the distal part of the stem also serve to anchor 
the crinoid on the sea floor. 
    Crinoids are the only group of Crinozoa to have survived the Paleozoic 
Era, which ended 200,000,000 years ago, and  thus they are of great importance 
in the interpretation of the abundant fossil remains of other Crinozoa. 
    About 690 living species are known, and of these, 80 are stemmed forms. 
More than 5500 fossil species, representing about 800 genera, have been 
described. All living crinoids are referred to one subclass, the Articulata. 
The three other subclasses of crinoids, the Inadunata, Camerata, and 
Flexibilia, are known only from fossils. 
    No crinoids are of direct economic importance to humans because they are 
not used for food, nor do they appear to be eaten by other animals. None are 
venomous.  
    Crinoids occur in all seas except the Black and the Baltic. Shallow-water 
comatulids are most abundant and varied around tropical coral reefs. Many are 
nocturnal, coming out of crevices in the reefs to perch on coral heads at 
night to feed. The arms are arrayed in a fan-shaped filtration net to catch 
suspended food, which consists mainly of zooplankton. Crinoids prefer areas 
with moderate currents so that food can be carried through the feeding fan. 
All living stemmed crinoids inhabit water more than 330 ft (100 m) deep, where 
they and some comatulids are widespread. Sea lilies use the cirri on the lower 
end of the stem to anchor themselves on sea-floor sediments; and comatulids, 
which also have cirri on the centrodorsal plate, use them for temporary 
attachment. See ARTICULATA (ECHINODERMATA); CRINOZOA; ECHINODERMATA. 
 �34s[N. Gary Lane]
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�6v�2v�32sCrinozoa   �21m �33s 
    A subphylum of the Echinodermata including radially symmetrical 
echinoderms showing a partly meridional pattern of growth. This tends to 
produce an aboral cup-shaped theca and a partly radially divergent pattern of 
growth, forming appendages, called brachioles or arms and bearing ambulacral 
grooves. Included in the subphylum are the blastoids, cystoids, 
edrioblastoids, eocrinoids, parablastoids, and paracrinoids. See BLASTOIDEA; 
CRINOIDEA; CYSTOIDEA; ECHINODERMATA; EOCRINOIDEA; PARACRINOIDEA. 
  
  
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�6v�2v�32sCritical mass �21m �33s
    That amount of fissile material (sup 2 sup 3 sup 3U, sup 2 sup 3 sup 5U, or
sup 2 sup 3 sup 9Pu) which permits a self-sustaining chain reaction. The
critical mass is increased by the presence of neutron absorptive materials. It
is reduced by a moderator, such as graphite or heavy water, which slows down the
neutrons, inhibits their escape, and indirectly increases their chance to
produce fission. See CHAIN REACTION (PHYSICS); NUCLEAR FISSION; REACTOR
PHYSICS.
 �34s[John A. Wheeler]
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�6v�2v�32sCritical path method (CPM)   �21m �33s 
    A diagrammatic network-based technique, similar to the program evaluation 
and review technique (PERT), that is used as an aid in the systematic 
management of complex projects. The technique is useful in: organizing and 
planning; analyzing and comprehending; problem detecting and defining; 
alternative action simulating; improving (replanning); time and cost 
estimating; budgeting and scheduling; and coordinating and controlling. It has 
its greatest value in complex projects which involve many interrelated events, 
activities, and resources (time, money, equipment, and personnel) which can be 
allocated or assigned in a variety of ways to achieve a desired objective. It 
can be used to complete a multifaceted program faster or with better 
utilization of resources by reassignments, trade-offs, and judiciously using 
more or less assets for certain of the individual activities composing the 
overall project. See PERT. 
  
    Applications of CPM include: research and development programs; new 
product introductions; facilities planning and designs; plant layouts and 
relocations; construction projects; equipment installations and start-ups; 
major maintenance programs; medical and scientific researches; weapons systems 
developments; and other programs in which cost reduction, progress control, 
and time management are important. It is best suited to large, complex, 
one-time or first-time projects rather than to repetitive, routine jobs. 
    The illustration shows a critical path diagram constructed for a 
comparatively simple project, designing a personal home-use computer. The 
table lists the required events. The critical path is defined as the longest 
route from the first event in the project to the final outcome. This is the 
total time that the project can be expected to take. Analysis may shorten the 
critical path by identifying activities whose time can be shortened. A 
schedule is constructed showing the earliest and latest permissible start and 
completion dates of each activity. 
                             *****EQUATION*****The use of CPM adds another 
step to a project, and it requires continual updating and reanalysis as 
conditions change, but experience has shown that the effort can be a good 
investment in completing a project in less time, with less resources, with 
more control, and with a greater chance of on-time, within-budget completion. 
See INDUSTRIAL ENGINEERING. 
 �34s[Vincent M. Altamuro]
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�6v�2v�32sCrocco's equation �21m �33s
    A relationship between vorticity and entropy gradient for the steady flow
of an inviscid compressible fluid. Crocco's equation, given below, pertains to
isoener
getic flow, which is a common type of flow where the total energy, or
stagnation enthalpy, of the fluid per unit mass is constant throughout the
fluid. In the equation B is the fluid velocity vector, B = curl V is the
vorticity vector, T is the fluid temperature, and s is the entropy per unit
mass of the fluid. Thus entropy gradients can occur only at right angles to V
and B. An irrotational flow is one where B = 0 throughout the flow, and it
follows from Crocco's equation that it must also be an isentropic flow, that
is, one with constant entropy throughout. See FLUID-FLOW PRINCIPLES;
ISENTROPIC FLOW; LAPLACE'S IRROTATIONAL MOTION.
 �34s[Arthur E. Bryson, Jr.]
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			  v x OMEGA = - T grad s


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�6v�2v�32sCrocodile   �21m �33s 
    The common name used for 14 species of large reptiles included in the 
family Crocodylidae (order Crocodilia) which also includes the alligators, 
caimans, and the gharial (also known as the gavial). Like all crocodilians, 
the crocodiles are primarily distributed throughout the tropical regions of 
the world. Species occur in both saltwater and fresh-water habitats. 
Crocodiles are generally omnivorous, feeding on invertebrates, fish, other 
reptiles and amphibians, birds, and mammals - practically any animal they can 
overpower. A few, very narrow-snouted species are believed to subsist 
primarily on fish. Crocodiles are primarily aquatic and nocturnal, leaving the 
water only to bask by day or to build their nests. Some species construct 
burrows into the banks of rivers or lakes where they spend part of their time. 
    These animals are powerful predators with large teeth and strong jaws. 
Large adults of some species may exceed 20 ft (6 m) in length and are capable 
of overpowering and eating large grazing mammals even, occasionally, humans. 
The webbed feet, flattened tail, and placement of the nostrils, eyes, and ears 
on raised areas of the head are adaptations for an aquatic existence. The 
raised nostrils, eyes, and ears allow the animals to float almost completely 
submerged while still monitoring their environment. 
    Reproduction in crocodiles is the most elaborate of the reptiles. 
Courtship and mating occur in the water. The female digs a hole in the soil 
for the 30 or so eggs. Nests are often guarded by the female. The young may 
remain together as a pod with the female for a year or more. Hearing and vocal 
communication are well developed in the crocodiles, and a variety of bellows, 
snarls, and grunts are utilized in their elaborate social behavior. 
    Crocodiles are considered very valuable for the leather obtained from 
their hides, and all species are becoming very rare owing to hunting and to 
the loss of their habitats through land development for other uses. Most 
countries with native crocodile populations are implementing conservation 
measures, and international efforts are being made to regulate trade in 
crocodile products. See ALLIGATOR; CROCODILIA; REPTILIA. 
 �34s[Howard W. Campbell]
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�6v�2v�32sCrocodilia   �21m �33s 
    An order of the class Reptilia (subclass Archosauria) which is composed of 
large, voracious, aquatic species which include the alligators, caimans, 
crocodiles, and gavials. The group has a long fossil history from Late 
Triassic times and its members are the closest living relatives of the extinct 
dinosaurs and the birds. The 21 or 22 living species are found in tropic areas 
of Africa, Asia, Australia, and the Americas, one form, the salt-water 
crocodile (Crocodylus porosus), has traversed oceanic barriers from the East 
Indies as far east as the Fiji Islands. See ARCHOSAURIA. 
    The order is distinguished from other living reptiles in that it has two 
temporal foramina, an immovable quadrate, a bony secondary palate, no shell, a 
single median penis in males, socketed teeth, a four-chambered heart, and an 
oblique septum that completely separates the lung cavities from the peritoneal 
region. Certain of these unique features and other salient characteristics of 
the Crocodilia are intimately associated with their aquatic life. For example, 
there is a special pair of fleshy flaps at the posterior end of the mouth 
cavity which form a valvular mechanism which separates the mouth from the 
region where the air passage opens into the throat. This complex arrangement 
allows crocodilians to breathe even though most of the head is under water, or 
the mouth is open holding prey or full of water. 
    During the breeding season male crocodilians set up territories on land 
which they defend against intruders of the same species. Fertilization is 
internal and the hard-shelled eggs are deposited in excavations in the sand or 
in large nests of decaying vegetation, depending upon the species. 
    The living species are placed in two families and eight genera. The family 
Crocodylidae contains two subgroups: the true crocodiles, Crocodylinae, 
including the genera Crocodylus found in all tropic areas, Osteolaemus in 
central Africa, and the false gavial (Tomistoma) in Malaya and the East 
Indies; the alligators and caimans, Alligatorinae, including the genera 
Alligator of the southeastern United States and near Shanghai, China, the 
Caiman from Central and South America, and Melanosuchus and Paleosuchus of 
South America. The gavial (Gavialis gangeticus) of India and north Burma is 
the only living member of the family Gavialidae. Crocodiles differ most 
obviously from alligators and caimans in head shape and in the position of the 
teeth, although other technical details also separate them. See ALLIGATOR; 
CROCODILE; GAVIAL. 
    Crocodilians first appear in deposits of Late Triassic or possibly Early 
Jurassic age in North America and South Africa. They formerly were much more 
widely distributed in temperate latitude than today; abundant paleobotanical 
evidence confirms a warmer climate in mid-latitudes at this time. It seems 
probable that the restriction of crocodilians to the tropics was a direct 
result of cooling climate in the late Cenozoic. See REPTILIA. 
 �34s[Joseph T. Gregory]
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�6v�2v�32sCrocoite   �21m �33s 
    A mineral with the chemical composition PbCrO4. Crocoite occurs in yellow 
to orange or hyacinth red, monoclinic, prismatic crystals with adamantine to 
vitreous luster; it is also massive granular. Hardness is 2.5 - 3 on Mohs 
scale and specific gravity is 6.0. Streak, or color of the mineral powder, is 
orangish-yellow. It fuses easily. 
    Crocoite is a secondary mineral associated with other secondary minerals 
of lead such as pyromorphite and of zinc such as cerussite. It has been found 
in mines in California and Colorado. See LEAD. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sCro-Magnon man   �21m �33s 
    The name which refers to the earliest representatives of anatomically 
modern Homo sapiens in western and central Europe. These fossil humans, who 
are associated with the upper Paleolithic archeological period and date from 
about 35,000 to about 10,000 years ago, were physically within the ranges of 
variation of modern Europeans (see illustration). They differed only in their 
tendency to be ruggedly built, especially the males. 
  
    The name Cro-Magnon refers specifically to the remains of at least five 
individuals found in the Cro-Magnon rock shelter near Les Eyzies, Dordogne, 
France, in 1868 by E. Lartet and H. Christy. As these were the first 
well-known fossils of this period, their name has been employed for most 
European fossils of the same time period. Other notable sites which have 
yielded human remains of the Cro-Magnon type are Combe Capelle and La 
Madeleine in France, Grimaldi in Italy, and Mladec in Czechoslovakia. See 
FOSSIL MAN. 
 �34s[Erik Trinkaus]
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�6v�2v�32sCrop micrometeorology   �21m �33s 
    The science concerned with the interaction of crops and their immediate 
physical environment. Especially, it seeks to measure and explain net 
photosynthesis (photosynthesis minus respiration) and water use (transpiration 
plus evaporation from the soil) of crops as a function of meteorological, 
crop, and soil moisture conditions. These studies are complex because the 
intricate array of leaves, stems, and fruits modifies the local environment 
and because the processes of energy transfers and conversions are 
interrelated. As a basic science, crop micrometeorology is related to plant 
anatomy, plant physiology, meteorology, and hydrology. Expertise in radiation 
exchange theory, boundary-layer and diffusion processes, and turbulence theory 
is needed in basic crop micrometeorological studies. A practical goal is to 
provide improved plant designs and cropping patterns for light interception, 
for reducing infestations of diseases, pests, and weeds, and for increasing 
crop water-use efficiency. Shelter belts are modifications that have been used 
in arid or windy areas to protect crops and seedlings from a harsh 
environment. See AGRICULTURAL METEOROLOGY; AGRICULTURAL SCIENCE (PLANT); 
MICROMETEOROLOGY.  
 �34s[L. H. Allen, Jr.]
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�6v�2v�32sCrossopterygii   �21m �33s 
    A subclass of the bony fishes (class Osteichthyes), comprising the 
lobefins or choanate fishes. Crossopterygians have two separate dorsal fins, 
the first with the rays emerging flush with the body but the second, like the 
paired fins, with a fleshy pedicel from which the fin rays arise. The caudal 
fin was originally heterocercal but became symmetrical (diphycercal) in more 
advanced forms. 
    Crossopterygians appeared first in the Lower Devonian, and by mid-Devonian 
they were well separated from the contemporaneous lungfishes and 
actinopterygians. In body form early lobefins bore marked similarity to the 
lungfishes, but later these groups diverged. Two orders are distinguishable, 
the Osteolepiformes and the Coelacanthiformes. Crossopterygians gave rise, 
probably in Late Devonian, to the earliest amphibians, the progenitors of the 
other tetrapods. See COELACANTHIFORMES; OSTEICHTHYES; OSTEOLEPIFORMES. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sCrossover network   �21m �33s 
    A selective network used to divide the audio-frequency output of an 
amplifier into two or more bands of frequencies. The frequency separation is 
employed to feed two or more loudspeakers, each operating in a restricted 
frequency band and thereby operating more efficiently and with less 
distortion. Most crossover networks used in high-fidelity audio systems are 
designed for crossover frequencies somewhere between 400 and 2000 cycles. See 
LOUD-SPEAKER; SOUND-REPRODUCING SYSTEMS. 
 �34s[John Markus]
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�6v�2v�32sCrosstalk   �21m �33s 
    A term originated for the sound produced at a receiving terminal of one 
voice-frequency circuit by speech signals carried by one or more other such 
circuits. The term is used commonly for the induction of speech signals in one 
circuit from a neighboring circuit. It is also used for cross modulation. 
Crosstalk is unwanted; it impairs communication service to a degree determined 
by the tolerance of the receiving system or the listener, by the frequency of 
occurrence, and by the duration of high ratios of crosstalk to normal-speech 
signal intensity. If it causes loss of presumptive secrecy, the condition is 
intolerable. See DISTORTION (ELECTRONIC CIRCUITS). 
    Several forms of crosstalk are often distinguished. Intelligible crosstalk 
is sufficiently understandable under pertinent circuit and room noise 
conditions that meaningful information can be obtained by the more sensitive 
observers. Nonintelligible crosstalk cannot be understood regardless of its 
received volume, but because of its syllabic nature is more annoying 
subjectively than thermal-type noise. Near-end crosstalk is that type whose 
energy travels in the opposite direction to the speech signal in the 
disturbing circuit. Far-end crosstalk is that type whose energy travels in the 
same direction as the speech signal in the disturbing circuit. 
    With open-wire circuits, coupling control is effected most practically by 
transposing (interchanging) positions of the wires of each pair in accordance 
with a systematic pattern seeking to oppose the coupling of one interval by 
that of another. See TELEPHONE SERVICE. 
 �34s[A. Fiscarelli]
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�6v�2v�32sCrown ethers and cryptands   �21m �33s 
    The crown ethers are macrocyclic polyethers (structure I) which are large 
enough to encircle metal cations. The cryptands are macropolycyclic 
polyaza-polyethers, where the three-coordinate nitrogen atoms provide the 
vertices of a defined three-dimensional structure (II). These compounds 
possess cavities which can encapsulate or cryptate metal cations. The 
cryptands and crown ethers have the capability of forming stable and selective 
complexes with monoatomic or molecular ions and, as such, are potential 
receptor, carrier or catalyst molecules. See COMPLEX COMPOUNDS. 
    Crown ethers and cryptands have numerous applications in chemistry for 
solubilization of salts, anion activation, stabilization of unusual species, 
analytical techniques, and isotope separation. The ability of the crown ethers 
and crytands to complex metal ions selectively suggests possibilities as drugs 
for use in detoxification of organisms contaminated with heavy or radioactive 
metals.  
 �34s[Richard B. Sessions]
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�6v�2v�32sCrown gall   �21m �33s 
    A neoplastic disease of primarily woody plants, although the disease can 
be reproduced in species representing more than 90 plant families. The disease 
results from infection of wounds by the free-living soil bacterium 
Agrobacterium tumefaciens. At 1 or 2 weeks after infection, swellings and 
overgrowths take place in tissue surrounding the site of infection, and with 
time these tissues proliferate into large tumors (see illustration). If 
infection takes place around the main stem or trunk of woody hosts, continued 
tumor proliferation will cause girdling and may eventually kill the host. 
Crown gall is therefore economically important, particularly in nurseries 
where plant material for commercial use is propagated and disseminated. 
    Analyses made of crown gall genetic material (DNA) have revealed the 
presence of certain unique plasmidlike DNA and genetic products. This evidence 
has strengthened the hypothesis that the bacterial cells introduce part of 
their genetic information into plants. See PLANT PATHOLOGY. 
 �34s[Clarence I. Kado]
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�6v�2v�32sCrushing and pulverizing   �21m �33s 
    The reduction of materials such as stone, coal, or slag to a suitable size 
for their intended uses such as road building, concrete aggregate, or furnace 
firing. Reduction in size is accomplished by five principal methods: (1) 
crushing, a slow application of a large force; (2) impact, a rapid hard blow 
as by a hammer; (3) attrition, a rubbing or abrasion; (4) sudden release of 
internal pressure; and (5) ultrasonic forces. The last two methods are not in 
common use. 
    Crushing and pulverizing are processes in ore dressing needed to reduce 
valuable ores to the fine size at which the valueless gangue can be separated 
from the ore. These processes are also used to reduce cement rock to the fine 
powder required for burning, to reduce cement clinker to the very fine size of 
portland cement, to reduce coal to the size suitable for burning in pulverized 
form, and to prepare bulk materials for handling in many processes. See 
MATERIALS-HANDLING MACHINES. 
    Equipment suitable for crushing large lumps as they come from the quarry 
or mine cannot be used to pulverize to fine powder, so the operation is 
carried on in three or more stages called primary crushing, secondary 
crushing, and pulverizing. The three stages are characterized by the size of 
the feed material, the size of the output product, and the resulting reduction 
ratio of the material. The crushing-stage output may be screened for greater 
uniformity of product size. 
    There are four principal types of primary crushers. The Blake jaw crusher 
uses a double toggle to move the swinging jaw and is built in a variety of 
sizes from laboratory units to large sizes having a feed inlet 84 by 120 in. 
(213 by 305 cm). The Dodge jaw crusher uses a single toggle or eccentric and 
is generally built in smaller sizes. The Gates gyratory crusher has a cone or 
mantle that does not rotate but is moved eccentrically by the lower bearing 
sleeve. The Symons cone crusher also has a gyratory motion, but has a much 
flatter mantle or cone than does the gyratory crusher. The top bowl is 
spring-mounted. It is used as a primary or secondary crusher. 
    Secondary crushers include the single-roll crusher and the double-roll 
crusher which have teeth on the roll surface and are used mainly for coal. 
Smooth rolls without teeth are sometimes used for crushing ores and rocks. The 
hammer crusher is the type of secondary crusher most generally used for ore, 
rock, and coal. The reversible hammer mill can run alternately in either 
direction, thus wearing both sides of the hammers. 
    In open-circuit pulverizing, the material passes through the pulverizer 
once with no removal of fines or recirculation. In closed-circuit pulverizing, 
the material discharged from the pulverizer is passed through an external 
classifier where the finished product is removed and the oversize is returned 
to the pulverizer for further grinding. 
    Ball and tube mills, rod mills, hammer mills, and attrition mills are 
pulverizers operating by impact and attrition. In ball race and roller 
pulverizers, crushing and attrition are used. See BALL-AND-RACE-TYPE 
PULVERIZER; BUHRSTONE MILL; PEBBLE MILL; TUMBLING MILL. 
 �34s[Ralph M. Hardgrove]
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�6v�2v�32sCrustacea �21m �33s
    A class of animals with mandibles and jointed feet, in the subphylum
Mandibulata of the phylum Arthropoda. Most people are familiar with species of
Crustacea such as the shrimp, prawn, crab, or lobster. However, there are many
more with less common vernacular names such as the water fleas, beach fleas,
sand hoppers, fish lice, wood lice, sow bugs, pill bugs, barnacles, scuds,
slaters, and krill or whale food.
    The Crustacea are one of the most difficult animal groups to define
because of their great diversity of structure, habit, habitat, and
development. No one character or generalization will apply equally well to
all. The adults of some aberrant or highly specialized parasitic species are
not recognizable as crustaceans, and evidence of their kinship rests on a
knowledge of their life histories. Virtually every form of marine animal life
as well as a number of fresh-water animals have their crustacean parasites.
    In common with the two other classes of the Mandibulata, the Myriapoda and
the Hexapoda or Insecta, the Crustacea have segmented, chitin-encased bodies;
articulated appendages; mouthparts known as mandibles during some stage of
their life, however modified they may be for cutting, chewing, piercing,
sucking, or licking; and two pairs of accessory feeding organs, the maxillae.
In Crustacea the first pair are the maxilulae, and the second are the
maxillae. One or the other pair of maxillae is sometimes vestigial or may be
lacking. The Crustacea typically have two pairs of antennae: the first pair,
or antennules, and the second pair, the antennae proper. The latter, moreover,
are almost always functional at some stage of every crustacean's life,
although they disappear in the sedentary adult barnacles and in some lower
shrimplike forms. In some of the same forms the antennules may also become
reduced or vestigial.

    Taxonomy.  Although all questions regarding the subclasses of the
Crustacea have not been resolved, eight subclasses are recognized:
Cephalocarida, Branchiopoda, Ostracoda, Mystacocarida, Copepoda, Branchiura,
Cirripedia, and Malacostraca. See articles on each group.

    General morphology.  The true body segments, the somites or metameres, are
usually somewhat compressed or depressed. The somite's dorsum, or upper side,
is the tergum or tergite; the lower or ventral surface, the sternum or
sternite; and either lateral portion, the pleuron or epimeron. Between the
embryonic anterior or prostomial element and the telson at the posterior end,
the linear series of somites making up the body of a crustacean are more or
less distinctly organized into three regions, the head, thorax, and abdomen.
Where regional organization of the postcephalic somites is not clearly marked,
they collectively form the trunk. The somites are variously fused with one
another in diagnostic combinations in different groups of the Crustacea.

    The head region of the simplest type is found in the characteristic
crustacean larva, the nauplius. When the nauplius hatches, it has only three
pairs of appendages, the antennules, antennae, and mandibles, which correspond
to the three fused somites of the primary head of the lower Crustacea. In more
advanced forms, and at later stages, two additional somites bearing the
maxillules and maxillae become coalesced with this protocephalon to form the
most frequently encountered crustacean head, or cephacon. Thoracic somites
frequently are fused with the cephalon to form a cephalothorax. A dorsal shield
or carapace of variable extent arises from the dorsum of the third cephalic
somite and covers the cephalothorax. In some crustaceans it is in the form of a
bivalved shell; in the barnacles it is modified as a fleshy mantle reinforced by
calcareous plates. The carapace reaches its greatest development in the
malacostracan Decapoda.
    Body.  The chitinous cuticle covering the crustacean body is its external
skeleton (exoskeleton). The chitin is flexible at the joints, in foliaceous
appendages, and throughout the exoskeletons of many small and soft-bodied
species, but it is often thickened and stiff in others. It becomes calcified
in many species as a result of the deposition of lime salts, and figuratively
can be as hard as a rock in some crabs.
    The paired appendages are typically biramous and consist of two branches,
the endopodite and exopodite. Either the endopodite or exopodite or both may
be reduced or suppressed, or both may be coalesced as in the foliaceous limbs
of lower Crustacea. The endopodite is definitely segmented in the higher
Crustacea. The endopodites are variously modified to serve a variety of
functions and needs such as sensory perception, respiration, locomotion,
prehension and comminution of food, cleansing, defense, offense, reproduction,
and sex recognition and attraction. If retained in the adult, the exopodites
may remain leaf- or paddlelike, or become flagellated structures, facilitating
swimming or aiding respiration.
    The alimentary tract is a relatively straight tube that curves dorsally
from the mouth. In the alimentary tract three regions are recognizable, the
foregut, midgut, and hindgut. The anterior part of the foregut or stomodeum is
esophageal in nature, whereas the posterior and greater part is modified as a
stomach, gullet, or gizzard. The mechanical preparation of food occurs here
because this region (called the gastric mill) contains chitinous ridges or
``teeth'' and setae for the trituration and straining of food. The midgut or
mesenteron in most cases is provided with several ceca or diverticula which
produce digestive secretions and serve as organs for the absorption of food.
When these ceca are present in considerable number, they become organized into
a sizable gland, the hepato-pancreas or ``liver'' of higher Crustacea. The
hindgut or proctodeum is the rectal portion which terminates, with few
exceptions, in the anal opening on the underside of the telson.
    Most crustaceans have a heart perforated by openings which admit venous
blood from the pericardial sinus in which it is located. The heart may be
elongated and tubular and extend through the greater part of the body, but
generally it is a more compact organ. It pumps blood through an arterial
system or through connecting sinuses or lacunae within the body tissues. The
blood of the great majority of the crustaceans is bluish because it contains
the respiratory pigment hemocyanin. A few crustaceans have red blood as a
result of the presence of erythrocrurion. See RESPIRATORY PIGMENTS
(INVERTEBRATE).
    Crustacea have appropriately been described as water-breathing insects of
the sea, for the vast majority are aquatic arthropods. They take up oxygen by
means of gills, the general body surface, or special areas of it. Some of the
few species that have become more or less terrestrial in their habits have
developed modifications of their branchial apparatus which when sufficiently
moist enable them to breathe air. Some sow or pill bugs have special tracheal
developments in their abdominal appendages for the same purpose.
    The nervous system is distinctly arthropodan in character. A
supraesophageal ganglion somewhat larger than the other ganglia is considered
to be the brain. It is connected by circumesophageal commissures to a double
ventral nerve chord with segmentally arranged ganglia.
    The organs of special sense are the eyes, the antennules, and the
antennae, or feelers in a very real sense. The antennules have also sense of
taste of a kind, and organs of equilibration, the statocysts, in their basal
segments. Crustacea have setae variously located on their bodies which are
sensitive to various external stimuli.
    Those glands recognized as definite excretory organs are the maxillary and
antennal glands in the adult crustaceans. The antennal gland is best developed
in the Malacostraca. It is often called the green gland and opens externally
on the underside of the coxal segment of the antennal peduncle. Many
crustaceans, especially deepsea forms, have phosphorescent or luminous organs.
The barnacles and some other crustaceans have cement glands. This may also be
true of tube-building species which construct their homes of extraneous
materials. The most studied crustacean gland is the sinus gland of the
eyestalks. This gland is part of a neurosecretory system producing hormones
which control color change and pattern, molting cycle, oogenesis, and
development of eggs within the ovary. See ENDOCRINE SYSTEM (INVERTEBRATE);
NEUROSECRETION.

    Reproduction.  The sexes are separate in most Crustacea and usually can be
differentiated from each other by secondary sex characters. Chief among these
characters are the size and shape of the body, appendages, or both, and
placement of the genital apertures. Hermaphroditism is the rule in the sessile
Cirripedia (barnacles), in isolated cases in the lower crustaceans, and in
certain parasitic forms. Parthenogenesis (eggs developing and hatching without
prior fertilization) occurs frequently in some of the lower crustaceans such
as the Branchiopoda and Ostracoda, which have what might be called an
alternation of generations. The parthenogenetic generations (and there are
usually very many of them in succession) alternate with a generation produced
by fertilized eggs. See PARTHENOGENESIS.
    The eggs of most crustaceans are carried attached to the female until
hatched. Some females develop brood pouches in which the young are retained
for a time. A nutrient secretion which sustains the young until they are
released is produced in some species having a brood chamber. Penaeid shrimp
and a few of the lower Crustacea deposit their eggs in the medium in which
they live, in some cases attaching them to aquatic vegetation.

    Larvae.  The nauplius larva is characteristic of Crustacea. Its oval,
unsegmented body has three pairs of appendages indicative of three embryonic
somites. These are the antennules which are uniramus in the nauplius, the
biramous antennae, and mandibles. Besides subserving other functions such as
sensory and feeding, all three are the organs of locomotion of this
free-swimming larva. There is a single dorsal, median, naupliar eye composed
of three closely united, similar parts which form a relatively simple compound
eye and a frontal organ, probably sensory, evidenced externally by a very
minute pair of projections. The integument of the upper surface of the
nauplius body has the appearance of a dorsal shield, as in most crustaceans.
Typical as this first larval stage may be, it does not appear in all
crustaceans. It is common in the lower forms, but in many of the higher forms
it occurs during development in the egg, and the young are hatched as a
different and more advanced larva or, as in many Malacostraca, in a form
similar to the adult. Following the naupliar stage very remarkable larval
stages follow in some groups: the free-swimming cypris larvae of barnacles;
the phyllosomae of the spiny lobsters; the erichthus and alima larvae of the
mantis shrimps (Stomatopoda); and the zoeae and megalopa of crabs.

    Molting (ecdysis).	This process involves several steps: (1) preparation,
which includes some degree of resorption of the old cuticle; (2) the formation
of a new, temporarily soft and thin one within it; (3) the accumulation and
storing of lime salts in the hepato-pancreas or as lenticular deposits
(gastroliths), one of which occurs on either side of the stomach or gizzard in
certain forms such as crayfishes and lobsters. The preparatory period is less
involved in the thinly chitinous forms. The actual molt follows. The old shell
or cuticle splits at strategic and predetermined places, permitting the
crustacean within, already enclosed in new but still soft exoskeleton, to
withdraw. In the postmolt period the crustacean is quite helpless and, if
unsheltered, is at the mercy of its enemies. It is in the soft-shelled stage
immediately after molting that the East Coast blue crab (Callinectes) becomes
a much-sought seafood delicacy. Molting takes place quite frequently in the
larval stages when growth is rapid, but becomes less frequent as the animal
ages. Hormones from the sinus gland play an important role in both initiating
and inhibiting molting. See GASTROLITH.

    Autotomy and regeneration.	The mechanisms of autotomy and regeneration
are developed in the crustaceans to minimize injury or loss to an enemy. When
an appendage is broken, it is cast off or broken at the fracture or breaking
plane. This sacrifice often enables the victim to escape. Even more remarkable
is the fact that crustaceans, by voluntary muscular contraction, can part with
a limb which may be injured. See AUTOTOMY.
    Crustacea also have the ability to regenerate lost parts. Although the
regenerated parts are not always the same size as the original in the first
molt after injury, increase in size in successive molts soon restores a lost
limb to virtually its former appearance. See REGENERATION (BIOLOGY).

    Phylogeny.	The geologic history of crustaceans dates from the earliest
fossiliferous rock formations, the Cambrian, estimated to be about 350,000,000
years of age. Many lower forms of crustaceans have been discovered in those
strata, including some thought to be distantly related, morphologically, to
present-day Malacostraca, but the state of preservation of these early
fossils, especially of the appendages, leaves much to be desired. It is not
unlikely that because of their small size and fragility many crustaceans of
bygone ages never did become recognizably fossilized.

    Bionomics and economics.  Crustacea are ubiquitous. They live at almost
all depths and levels of the sea, in fresh waters at elevations up to 12,000
ft (3.7 km), in melted snow water, in the deepest of the sea's abysses more
than 6 mi (10 km) down, and in waters of 0DGC (32DGF) temperature. They live
on land, trees, and mountains, although most of these species must descend to
salt water areas again to spawn their young. Some live in strongly alkaline
waters and others in salt water which is at the saturation point, still others
in hot springs with temperatures of 55DGC (131DGF).
    All crustaceans of sufficient size are eaten by humans somewhere in the
world. Very few properly prepared have proved to be poisonous so far as is
known. Myriads of small fry are the sustenance of larger aquatic animals,
chiefly fish and whales.
    Most crustaceans are omnivorous, and essentially scavengers. Many are
filter feeders and screen particulate life, plankton, and organic detritus
from the waters in which they live; others are largely carnivorous, still
others vegetarian. Among the vegetarians are the grazers of the ocean meadows
which convert the microscopic plant life (diatoms) into flesh and food for
larger animals which in turn are harvested as food for humans. See ARTHROPODA.
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�6v�2v�32sCrux   �21m �33s 
    The Southern Cross, in astronomy, the most celebrated of the 
constellations of the far south. The four principal bright stars of the group 
ALPHA, GAMMA, BETA, and DELTA form the figure of a cross, giving the 
constellation its name (see illustration). The brightest star in Crux is Alpha 
Crucis at the foot of the cross, which has received the artificial name, 
Acrux. This is a navigational star. See CONSTELLATION; CYGNUS. 
 �34s[Ching-Sung Yu]
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�6v�2v�32sCryobiology   �21m �33s 
    The use of low-temperature environments in the study of living plants and 
animals. The principal effects of cold on living tissue are destruction of 
life and preservation of life at a reduced level of activity. Both of these 
effects are demonstrated in nature. Death by freezing is a relatively common 
occurrence in severe winter storms. Among cold-blooded animals winter weather 
usually results in a comalike sleep that may last for a considerable length of 
time.  
    The extreme cold of liquid nitrogen (boiling at -320DGF or -196DGC) can 
cause living tissue to be destroyed in a matter of seconds or to be preserved 
for an indefinite time, certainly for years and possibly for centuries, with 
essentially no detectable biochemical activity. The result achieved when heat 
is withdrawn from living tissue depends on processes occurring in the 
individual cells. Basic knowledge of the causes of cell death, especially 
during the process of freezing, and the discovery of methods which circumvent 
these causes have led to practical applications both for long-term storage of 
living cells or tissue (cryopreservation) and for calculated and selective 
destruction of tissue (cryosurgery). 
    The biochemical constituents of a cell are either dissolved or suspended 
in water. During the physical process of freezing, water tends to crystallize 
in pure form, while the dissolved or suspended materials concentrate in the 
remaining liquid. In the living cell, this process is quite destructive. In a 
relatively slow freezing process ice first begins to form in the fluid 
surrounding the cells, and the concentration of dissolved materials in the 
remaining liquid increases. The equilibrium between the fluid inside the cell 
and the fluid surrounding the cell is upset. A concentration gradient is 
established across the cell wall, and water moves out of the cell in response 
to the osmotic force. As freezing continues, the cell becomes quite 
dehydrated. Salts may concentrate to extremely high levels. In a similar 
manner the acid-base ratio of the solution may be altered during the 
concentration process. Dehydration can affect the gross organization of the 
cell and also the molecular relationships, some of which depend on the 
presence of water at particular sites. Cellular collapse resulting from loss 
of water may bring in contact intracellular components normally separated to 
prevent destructive interaction. Finally, as the ice crystals grow in size, 
the cell walls may be ruptured by the ice crystals themselves or by the high 
concentration gradients which are imposed upon them. 
  
    By speeding the freezing process to the point that temperature drop is 
measured in degrees per second, some of these destructive events can be 
modified. However, most of the destructive processes will prevail. To prevent 
dehydration, steps must be taken to stop the separation of water in the form 
of pure ice so that all of the cell fluids can solidify together. The chief 
tools used to accomplish this are agents that lower the freezing point of the 
water. Typical materials used for industrial antifreeze agents are alcohols or 
glycols (polyalcohols). Glycerol, a polyalcohol which is compatible with other 
biochemical materials in living cells, is frequently used in cell 
preservation. Besides the antifreeze additive, refrigeration procedures are 
designed to control the rate of decline in temperature to the freezing point, 
through the liquid-solid transition, and below, to very low temperatures. 
    In the case of preservation of sizable aggregates of cells in the form of 
whole organs, the problems are significantly increased. While there are signs 
of progress, whole-organ preservation is not yet a normally successful 
procedure.  
  
    Cryopreservation.  The earliest commercial application of cryopreservation 
was in the storage of animal sperm cells for use in artificial insemination. 
In the frozen state sperm may be transported great distances. This permits the 
spread of high-quality breeding stock to all parts of the Earth at much lower 
costs than would be involved in the shipment of an entire herd. 
    In an entirely similar manner the microorganisms used in cheese production 
can be frozen, stored, and transported without loss of lactic acid - producing 
activity. Pollen from various plants can be frozen for storage and transport. 
In plant-breeding experiments it is sometimes desired to crossbreed plants 
that do not grow in similar environments. In such instances the transport of 
frozen pollen can be utilized. 
    Among the most applications of cryo-preservation is the storage of whole 
blood or separated blood cells. Frozen storage permits the buildup of the 
large stocks of blood required in a major catastrophe. It is also useful in 
storing the less common types of blood. Frozen blood storage is also useful in 
long-term diagnostic studies. 
  
    Cryosurgery.  The causes of cellular destruction from freezing can be 
effected deliberately to destroy tissue as a surgical procedure. A technique 
for controlled destruction of tissue in the basal ganglia of the brain was 
developed for relief from tremor and rigidity in Parkinson's disease. 
    One of the significant advantages of cryosurgery is that the apparatus can 
be employed to cool the tissue to the extent that the normal or the aberrant 
function is suppressed; yet at this stage the procedure can be reversed 
without permanent effect. When the surgeon is completely satisfied that he has 
located the exact spot he wishes to destroy, the temperature can be lowered 
enough to produce irreversible destruction. This procedure is of particular 
assistance in neurosurgery. 
    A second major advantage of cryosurgery is that the advancing front of 
reduced temperatures tends to cause the removal of blood and the constriction 
of blood vessels in the affected area. This means that little or no bleeding 
results from cryosurgical procedures. 
    A third major advantage of cryosurgery is that cryosurgery equipment 
currently employs a freezing apparatus (which is about the size of a large 
knitting needle) that can be placed in contact with area to be destroyed with 
a minimum incision to expose the affected area. See CRYOGENIC ENGINEERING. 
 �34s[Arthur W. Francis]
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�6v�2v�32sCryogenic engineering �21m �33s
    A branch of engineering specializing in technical operations at very low
temperatures. Cryogenic engineers combine competence in the techniques of
low-temperature physics and chemistry with capability in conventional branches
of engineering, such as mechanical, chemical, electrical, and nuclear. For
discussions of other aspects of low-temperature technology see LIQUEFACTION OF
GASES; LOW-TEMPERATURE THERMOMETRY.
    An important military application of cryogenics has been the development
of oxygen-breathing apparatus for crews of high-altitude, long-range aircraft.
By storing oxygen as a dense liquid in lightweight, well-insulated containers
from which it can be vaporized as needed, significant weight reductions are
effected over the older method of high-pressure, low-density gas storage in
heavy steel cylinders. Cryogenic engineers have also been active in the
aerospace field where low-molecular-weight fuels are highly desirable. These,
however, are all cryogenic liquids.
    Gases employed in the chemical industry usually require extensive
pretreatment because their source is most often a gas mixture - either the
atmosphere, natural gas, or a by-product stream from some other chemical
operation. In consequence the components must first be separated, then
purified. Both these processes involve cryogenics. Depending upon the mixture,
one or more of the following low-temperature techniques must be used:
fractional distillation, scrubbing, and selective adsorption. The last
technique has proved particularly applicable for the separation and
purification of the noble, or inert, gases.
    Metallurgical applications for pure gases are increasing as new alloy
steels are created and as the production of hitherto rare or seldom-refined
metals is undertaken in order to meet the unusual demands of the nuclear and
the space age. Oxygen, nitrogen, argon, and hydrogen, all of which are
produced cryogenically, are the gases most commonly used.
    Although still in the developmental stage, the application of
superconductivity and associated cryogenic systems to electric power
generation and transmission appears promising. Development of superconducting
magnets to provide the magnetic fields needed for the generation of electric
power by magnetohydrodynamics (MHD) has been undertaken. Superconducting
magnets will also be needed for the magnetic confinement of plasmas in
thermonuclear (nuclear fusion) power generation. See ENERGY STORAGE;
MAGNETOHYDRODYNAMIC POWER GENERATOR; NUCLEAR FUSION; SUPERCONDUCTIVITY.
    Cryogenic engineering is applied to several modes of transportation.
Superconducting magnets are used in the development of magnetically levitated,
high-speed trains in which speeds of 300 mi/h (500 km/h) or more are
anticipated. The use of cryogenic (or liquid) hydrogen fuel appears promising
for the propulsion of jet aircraft and has also been investigated for use as a
fuel in both rail and automotive transportation.
    Several elementary superconducting circuits have been developed to produce
binary memory elements, switching devices, and multivibrators for high-speed
computers. The small space requirements, negligible power inputs, increased
speed, and high reliability of these units combine to promise significant
advantages over other types of computer components. See CRYOTRON.
    A high-speed vacuum pump has been developed for specialized applications,
capable of producing an extremely low ultimate vacuum and requiring less power
than conventional diffusion-type pumps. The principle employed is well known:
lowering the pressure within a gas-tight enclosure by freezing out condensible
gases on a cold surface. In the cryopump a low-temperature refrigeration unit
circulates helium at -487DGF (20 K) through coils in plate condensers upon
which the gas to be pumped condenses. See VACUUM PUMP.
    Gases whose critical temperatures lie below ambient temperature and which,
therefore, are not liquefiable by pressure alone have conventionally been
transported and stored in heavy-walled steel cylinders at high pressures. In
the case of hydrogen, for example, about 100 lb of steel must be used to
contain each pound of gas at high pressure. In contrast, current cryogenic
technology yields a corresponding weight ratio of only 3 for liquid
containment. Although this ratio is not quite so small for heavier gases, the
dead-weight savings are still sufficient to justify liquid storage of nitrogen
and oxygen in almost all large-scale industrial operations.
    Efforts have been undertaken to develop new and better superconductors
that have improved mechanical properties and that can carry higher currents at
higher temperatures. The development of Nb3Ge with a transition temperature of
over 23 K (-418DGF) has opened the possibility of operation of superconducting
devices using liquid hydrogen coolant. There has also been extensive research
into the low-temperature mechanical properties of structural materials,
including epoxy resins and composites.
    Low temperatures are finding increasing use in basic biological studies as
well as in practical medical procedures. See CRYOBIOLOGY.
    Large quantities of liquid nitrogen are employed in the preservation of
food by rapid freezing. This is carried out by direct immersion of the food
product in liquid nitrogen or by direct contact with a liquid-nitrogen spray
or cold vapors. See CRYOGENICS.
 �34s[K. D. Williamson ;F. J. Edeskuty]
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�6v�2v�32sCryogenics   �21m �33s 
    The science of producing and maintaining very low temperatures. From a 
practical standpoint, any given object may be cooled to and maintained at a 
low temperature most simply by placing it in thermal contact with a suitable 
liquefied gas held at a constant pressure. In principle, a liquefied gas can 
be used to provide constant bath temperatures from its triple point to its 
critical point. Bath temperature is varied by changing the pressure above the 
liquid, and, within the liquid range, any desired bath temperature can be 
maintained by removing precisely that amount of gas vaporized by heat leak 
into the bath liquid. In general, because heat leak cannot be excluded 
entirely, baths will eventually boil away unless replenished periodically. 
    Liquefied gases do not exist over the entire low-temperature range. The 
lowest temperature at which it is practical to use a liquid bath is about 0.3 
K (-459.1DGF), and gaps exist between 5 and 14 K (-451 and -434DGF) and 
between 44 and 55 K (-380 and -361DGF). If it is necessary to extend the 
accessible temperature range or realize certain ranges more conveniently, 
other methods of cold production are available. 
    Refrigerators can, in principle, be designed to produce and maintain a 
predetermined temperature at any point along the temperature scale. For 
refrigerators operating cyclically, the low temperature produced is not 
strictly constant because of the periodic withdrawal of heat from the 
low-temperature reservoir, but the temperature variation can be large or 
extremely small, depending on the cycle characteristics. See REFRIGERATION 
CYCLE.  
    Refrigerator cycles operating above about 1 K (-458DGF) generally involve 
compression and expansion of appropriately chosen gases. Because at lower 
temperatures it is impractical or impossible to use gases, liquid or solid 
working materials are employed along with different techniques to produce 
cooling. Adiabatic demagnetization of paramagnetic ions in solids, involving 
electron-spin paramagnets, is useful in the range 1 to 0.003 K (-458 to 
-459.665DGF), but could be applied at temperatures up to 300 K (80DGF) or more 
by appropriate choice of magnetic material. See ADIABATIC DEMAGNETIZATION. 
  
    The same basic principles of refrigeration may be used to explain gas 
liquefaction, the periodic temperature reduction of the refrigerator working 
substance, and the adiabatic demagnetization process. Cold production is 
related directly to the concept of temperature, which in turn is intimately 
connected with entropy S defined as INTEGRAL(C/T) dT, where C is the specific 
heat of the substance being cooled. Entropy is a measure of the intrinsic 
thermal disorder associated with a substance, and, at absolute zero, the 
entropy of every substance in internal equilibrium is zero. Entropy is 
increased either by the introduction of thermal energy or by appropriately 
varying some other parameter upon which S depends. A conjugate relation exists 
such that, provided all other parameters are held constant, a change in T 
results in a change of S of the same sign. The significance of S for low 
temperatures stems from the concept of idealized processes, called isentropic 
(or constant S), which result in the maximum possible decrease in T for a 
given spontaneous parametric change. Because an isentropic process must be 
both adiabatic (thermally isolated) and reversible, in practice it can be only 
approximated. Nevertheless, the isentropic process provides a criterion 
against which the efficiency of any actual process may be gaged. From the 
illustration, in which the variation of S and T for an idealized substance is 
shown at two different constant values of a thermodynamic parameter X such as 
pressure or magnetization, it is seen that an isentropic parameter change from 
A -> B produces the maximum reduction in T. (Spontaneous processes resulting 
in a decrease in S with a change in X are impossible.) See THERMODYNAMIC 
PRINCIPLES.  
  
    Once a temperature lower than ambient has been produced, it may be 
maintained indefinitely (provided the region itself contains no heat sources) 
if flow of heat to the cooled region can be prevented. Because no perfect 
thermal insulators exist, the problem resolves itself into minimizing the flow 
of heat to the refrigerated region. Devices designed to provide 
low-temperature environments in which operations may be carried out under 
controlled conditions are called cryostats. See CRYOGENIC ENGINEERING; 
LOW-TEMPERATURE PHYSICS. 
 �34s[F. J. Edeskuty ;K. D. Williams]
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�6v�2v�32sCryolite   �21m �33s 
    A mineral with chemical composition Na3AlF6. It crystallizes in the 
monoclinic system. Hardness is 2J on Mohs scale and the specific gravity is 
2.95. Crystals are usually snow-white but may be colorless and more rarely 
brownish, reddish, or even black. The mean refraction index is 1.338, 
approximately that of water, and thus fragments become invisible when immersed 
in water. 
    Cryolite was once used as a source of metallic sodium and aluminum, but 
now is used chiefly as a flux in the electrolytic process in the production of 
aluminum from bauxite. See ALUMINUM. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sCryotron   �21m �33s 
    A current-controlled switching device based on superconductivity for use 
primarily in computer circuits. The early version has been superseded by the 
tunneling cryotron, which consists basically of a Josephson junction. In its 
simplest form the device has two electrodes of a superconducting material (for 
example, lead) which are separated by an insulating film only about 10 atomic 
layers thick. Switching is accomplished by a magnetic field generated by 
sending a current through the control line on top of the junction. The 
tunneling cryotron is still in the research stage, but various computer 
circuits have already been realized. See SUPERCONDUCTING DEVICES; 
SUPERCONDUCTIVITY.  
 �34s[Peter Wolf]
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�6v�2v�32sCryptobiosis   �21m �33s 
    A state of life in which the metabolic rate of an organism is reduced to 
an imperceptible level. The several kinds of cryptobiosis (``hidden life'') 
include anhydrobiosis (life without water), cryobiosis (life at low 
temperatures), and anoxybiosis (life without oxygen). The most is known about 
anhydrobiosis.  
    States of anhydrobiosis occur in early developmental stages of various 
organisms, including seeds of plants, spores of bacteria and fungi, cysts of 
certain crustaceans, and larvae of certain insects; they occur in both 
developmental and adult stages of certain soil-dwelling micrometazoans 
(rotifers, tardigrades, and nematodes), mosses, lichens, and certain ferns. 
    A central question in the study of anhydrobiosis has been whether 
metabolism actually ceases. Available evidence strongly suggests that dry 
anhydrobiotes are ametabolic. In that case, a philosophical question 
immediately arises concerning the nature of life. This philosophical quandary 
can be avoided by applying the definition of life adopted by most students of 
anhydrobiosis: an organism is alive, provided its structural integrity is 
maintained. When that integrity is violated, it is dead. See METABOLISM. 
 �34s[John H. Crowe]
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�6v�2v�32sCryptococcales   �21m �33s 
    An order in the class Fungi Imperfecti created to accommodate the yeasts 
which do not form sexual spores. This group, also known as Torulopsidales, 
comprises the genera Cryptococcus, Rhodotorula, Torulopsis, Pityrosporum, 
Candida, Brettanomyces, Kloeckera, Trigonopsis, and Trichosporon. Some 
species, such as Torula utilis, are used in industrial fermentation to make 
food and fodder yeast; other species are pathogenic for humans and animals 
(Pityrosporum ovale is usually found associated with dandruff; Candida 
albicans causes thrush in infants; and Cryptococcus neoformans causes 
cryptococcosis). See CRYPTOCOCCOSIS. 
    Vegetative reproduction may be by budding, by fission, by formation of a 
pseudomycelium or true mycelium, or combinations thereof. The Cryptococcales 
are separated from the related order Moniliales by their lack of an aerial 
mycelium and aerial asexual reproductive structures. See FUNGI IMPERFECTI; 
MONILIALES; YEAST. 
 �34s[Herman J. Phaff]
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�6v�2v�32sCryptococcosis   �21m �33s 
    An infection of humans, primarily of the central nervous system, having as 
its etiological agent an encapsulated yeast, Cryptococcus neoformans. The 
skin, lungs, bones, and viscera may also be invaded by this 
organism.Cryptcoccus neoformans is widespread in nature and worldwide in 
distribution. Diagnosis of the disease is made by cultural and histological 
methods. Serologic tools are available for detection of antibodies and 
antigens. Treatment for this disease includes the use of amphotericin B and 
5-fluorocytosine. See AMPHOTERICIN B; MEDICAL MYCOLOGY; YEAST. 
 �34s[Leanor D. Haley]
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�6v�2v�32sCryptography �21m �33s
    The various methods for writing in secret code or cipher. As society
becomes increasingly dependent upon computers, the vast amounts of data
communicated, processed, and stored within computer systems and networks often
have to be protected, and cryptography is a means of achieving this
protection. It is the only practical method for protecting information
transmitted through accessible communication networks such as telephone lines,
satellites, or microwave systems. Cryptographic procedures can also be used
for message authentication, personal identification, and digital signature
verification for electronic funds transfer and credit card transactions. See
DATA-BASE MANAGEMENT SYSTEMS; DATA COMMUNICATIONS; DIGITAL COMPUTER;
ELECTRICAL COMMUNICATIONS.
    Cryptography helps resist decoding or deciphering by unauthorized
personnel; that is, messages (plaintext) transformed into cryptograms
(codetext or ciphertext) have to be able to withstand intense cryptanalysis.
Transformations can be done by using either code or cipher systems. Code
systems rely on code books to transform the plaintext words, phrases, and
sentences into ciphertext code groups. To prevent cryptanalysis, there must be
a great number of plaintext passages in the code book and the code group
equivalents must be kept secret, making it difficult to utilize code books in
electronic data-processing systems.
    Cipher systems are more versatile. Messages are transformed through the
use of two basic elements: a set of unchanging rules or steps called a
cryptographic algorithm, and a set of variable cryptographic keys. The
algorithm is composed of enciphering (E) and deciphering (D) procedures which
usually are identical or simply consist of the same steps performed in reverse
order, but which can be dissimilar. The keys, selected by the user, consist of
a sequence of numbers or characters. An enciphering key (Ke) is used to
encipher plaintext (X) into ciphertext (Y) as in Eq. (1), and a deciphering
key (Kd) is used to decipher ciphertext (Y) into plaintext (X) as in Eq. (2).

			    *** EKe(X) = Y ***				 (1)

		      *** DKd[EKE(X)] = DKd(Y) = X ***			 (2)

    Algorithms are of two types - conventional and public-key. The enciphering
and deciphering keys in a conventional algorithm either may be easily computed
from each other or may be identical (Ke = Kd = K, denoting EK (X) = Y for
encipherment and DK (Y) = X for decipherment). In a public-key algorithm,
one key (usually the enciphering key) is made public, and a different key
(usually the deciphering key) is kept private. In such an approach it must not
be possible to deduce the private key from the public key.
    When an algorithm is made public, for example, as a published encryption
standard, cryptographic security completely depends on protecting those
cryptographic keys specified as secret.

    Unbreakable ciphers.  Unbreakable ciphers are possible. But the key must
be randomly selected and used only once, and its length must be equal to or
greater than that of the plaintext to be enciphered. Therefore such long keys,
called one-time tapes, are not practical in data-processing applications. To
work well, a key must be of fixed length, relatively short, and capable of
being repeatedly used without compromising security. In theory, any algorithm
that uses such a finite key can be analyzed; in practice, the effort and
resources necessary to break the algorithm would be unjustified.

    Strong algorithms.	Fortunately, to achieve effective data security,
construction of an unbreakable algorithm is not necessary. However, the work
factor (a measure, under a given set of assumptions, of the requirements
necessary for a specific analysis or attack against a cryptographic algorithm)
required to break the algorithm must be sufficiently great. Included in the
set of assumptions is the type of information expected to be available for
cryptanalysis. For example, this could be ciphertext only; plaintext (not
chosen) and corresponding ciphertext; chosen plaintext and corresponding
ciphertext; or chosen ciphertext and corresponding recovered plaintext.
    A strong cryptographic algorithm must satisfy the following conditions:
(1) The algorithm's mathematical complexity prevents, for all practical
purposes, solution through analytical methods. (2) The cost or time necessary
to unravel the message or key is too great when mathematically less
complicated methods are used, because either too many computational steps are
involved (for example, in trying one key after another) or because too much
storage space is required (for example, in an analysis requiring data
accumulations such as dictionaries and statistical tables).
    To be strong, the algorithm must satisfy the above conditions even when
the analyst has the following advantages: (1) Relatively large amounts of
plaintext (specified by the analyst, if so desired) and corresponding
ciphertext are available. (2) Relatively large amounts of ciphertext
(specified by the analyst, if so desired) and corresponding recovered
plaintext are available. (3) All details of the algorithm are available to the
analyst; that is, cryptographic strength cannot depend on the algorithm
remaining secret. (4) Large high-speed computers are available for
cryptanalysis.

    Digital signatures.  Digital signatures authenticate messages by ensuring
that: the sender cannot later disavow messages; the receiver cannot forge
messages or signatures; and the receiver can prove to others that the contents
of a message are genuine and that the message originated with that particular
sender. The digital signature is a function of the message, a secret key or
keys possessed by the sender of the message, and sometimes data that are
nonsecret or that may become nonsecret as part of the procedure (such as a
secret key that is later made public).
    Digital signatures are more easily obtained with public-key than with
conventional algorithms. When a message is enciphered with a private key
(known only to the originator), anyone deciphering the message with the public
key can identify the originator. The latter cannot later deny having sent the
message. Receivers cannot forge messages and signatures, since they do not
possess the originator's private key.
    Since enciphering and deciphering keys are identical in a conventional
algorithm, digital signatures must be obtained in some other manner. One
method is to use a set of keys to produce the signature. Some of the keys are
made known to the receiver to permit signature verification, and the rest of
the keys are retained by the originator in order to prevent forgery.

    Data encryption standard.  Regardless of the application, a cryptographic
system must be based on a cryptographic algorithm of validated strength if it
is to be acceptable. The Data Encryption Standard (DES) is such a validated
conventional algorithm already in the public domain. This procedure enciphers
a 64-bit block of plaintext into a 64-bit block of ciphertext under the
control of a 56-bit key. The National Bureau of Standards accepted this
algorithm as a standard, and it became effective on July 15, 1977.

    Block ciphers.  A block cipher (Fig. 1) transforms a string of input bits
of fixed length (termed an input block) into a string of output bits of fixed
length (termed an output block). In a strong block cipher, the enciphering and
deciphering functions are such that every bit in the output block jointly
depends on every bit in the input block and on every bit in the key. This
property is termed intersymbol dependence.

    Stream ciphers: A stream cipher (Fig. 2) employs a bit-stream generator
to produce a stream of binary digits (0's and 1's) called a cryptographic bit
stream, which is then combined either with plaintext (via the LARGE OPEN BOX
operator) to produce ciphertext or with ciphertext (via the LARGE OPEN BOX
sup -1 operator) to recover plaintext.



 �34s[Carl H. Meyer; Stephen M. Matyas]
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�6v�2v�32sCryptomonadida   �21m �33s 
    An order of the class Phytamastigophorea, also known as the 
Cryptomonadina. Cryptomonads are considered to be protozoans by zoologists and 
algae by botanists. Most species occur in fresh water. They are olive-green, 
blue, red, brown, or colorless. All flagellated members have two sub-equal 
flagella inserted in a gullet opening through an obliquely truncate anterior 
end (see illustration). Although few in genera and species, Cryptomonadida 
form extensive marine and freshwater blooms. See PHYTAMASTIGOPHOREA: PROTOZOA. 
 �34s[James B. Lackey]
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�6v�2v�32sCryptostomata   �21m �33s 
    An extinct order of bryozoans in the class Stenolaemata. Cryptostome 
colonies have delicate, flattened branches of bifoliate construction in which 
the short, tubular, solid-walled, moderately complex zooecia are tightly 
packed back to back and open out onto the two opposite sides of the branches. 
See BRYOZOA; STENOLAEMATA. 
    Possibly evolving from early cystoporates or from an ancestor they shared 
with them, cryptostomes first appeared in the sea at the start of the Middle 
Ordovician. They became moderately common immediately, and remained so, until 
their extinction during latest Permian time. 
    Bifoliate cryptostomes are also termed ptilodictyoids or timanodictyoids. 
The order Cryptostomata was much more broadly defined in the past, and 
included the Fenestrata and Rhabdomesonata. Moreover, although formerly 
suggested as possibly ancestral to cheilostomes, the cryptostomes may have 
died out without modern descendants. See CHEILOSTOMATA; CYSTOPORATA; 
FENESTRATA; RHABDOMESONATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sCryptozoon   �21m �33s 
    A type of stromatolite; a sedimentary organic structure formed by 
blue-green algae, sedimentary particles, and fine biochemical calcareous 
material, resulting from the activity of the algae. The organisms develop 
rounded masses shaped like a cabbage or flattened cabbage which commonly range 
in size from 6 in. (15 cm) to more than 2 ft (60 cm) across. Structurally, 
they are composed of thin laminae, but they show little or no microstructure. 
    Cryptozoon colonies developed in such numbers as to be important builders 
of limestone (see illustration). They are found in all continents in rocks 
ranging in age from Precambrian to Permian. They were especially abundant 
during the late Proterozoic, Cambrian, and Early Ordovician. See ALGAE; 
STROMATOLITE.  
 �34s[J. Harlan Johnson]
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�6v�2v�32sCrystal �21m �33s
    This term, as used in science and technology, usually denotes a single
crystal. A single crystal is a solid throughout which the atoms or molecules
are arranged in a regularly repeating pattern. In electronics the term crystal
is usually restricted to mean a single crystal which is piezoelectric.
Examples of single crystals are most gems, piezoelectric quartz crystals used
in controlling the frequencies of radio transmitters, and single crystals of
galena (lead sulfide) used in crystal radios. See PIEZOELECTRIC CRYSTAL;
SINGLE CRYSTAL.
    Most crystalline solids are made up of millions of tiny single crystals
called grains and are said to be polycrystalline. These grains are oriented
randomly with respect to each other. Any single crystal, however, no matter
how large, is a single grain. Single crystals of metals many cubic centimeters
in volume are relatively easy to prepare in the laboratory. See
CRYSTALLOGRAPHY.
 �34s[Herman H. Hobbs]
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    Structure. Knowledge of the precise ways in which atoms and ions are
distributed in crystals is of prime importance in solid-state physics,
chemistry, metallurgy, mineralogy, geochemistry, and other fields.
Knowledge of crystal structure is and has been obtained mainly from x-ray
diffraction data, although electron diffraction and neutron diffraction have
become important tools in crystal analysis. See CRYSTALLOGRAPHY; ELECTRON
DIFFRACTION; IONIC CRYSTALS; NEUTRON DIFFRACTION; SOLID-STATE CHEMISTRY;
X-RAY CRYSTALLOGRAPHY; X-RAY DIFFRACTION.
    A three-dimensional, indefinitely extended array of points, each of which
is surrounded in an identical way by its neighbors, is known as a lattice or
space lattice. The space lattice of a crystal is the representation of the
periodicity with which matter is distributed in it. It is essential to
distinguish a lattice from a crystal structure; a crystal structure is formed
by associating with every lattice point an assembly of atoms identical in
composition, arrangement, and orientation. The space-lattice concept explains
the special geometric properties of crystal polyhedrons. It was originally
postulated that an elementary unit, having all the properties of the crystal,
should exist, or conversely that a crystal was built up by the juxtaposition
of such elementary units. If mathematical points forming the vertices of a
parallelepiped OABC (defined by three vectors OA, OB, OC) are considered (Fig.
1), a space lattice is obtained by translations parallel to and equal to OA,
OB, OC. The parallelepiped is called the unit cell. The vector r joining O to
any lattice point can be written as r = mOA + nOB + rOC, where m, n, and r are
integers.
    The Bravais lattices are the 14 different possible space lattices obtained
on the basis that two lattices are different when the environment of their
points is different. If considered as solids, the combination of symmetry
elements they exhibit can be determined. Seven point groups which are
respectively the most symmetrical of each system are found. Accordingly the
unit cells, and by extension the lattices, can be divided into seven groups
corresponding to the seven crystallographic systems. The external symmetry of a
crystal is due to the periodically repeated arrangement of its atoms or ions.
The Bravais lattices give the possible periodicities. A further step must
consist in finding the number of possible periodic arrangements, two such
arrangements being considered as different when they give rise to a different
symmetry.
    Two-dimensional structures are very well suited to illustrate this. In
Figs. 2 and 3 a numeral 9 is chosen as the object; it is fully asymmetric and
no false symmetry can therefore be introduced. It is multiplied by a periodic
array of mirror planes and fourfold axes. Several interpenetrating identical
lattices are formed. The two arrangements have the same Bravais lattice but
not the same symmetry. In three-dimensional space the problem is similar, but
other symmetry elements are considered.
    Screw axes combine the rotation of an ordinary symmetry axis with a
translation parallel to it and equal to a fraction of the unit distance in
this direction. If screw axes are present in crystals, it is clear that the
displacements involved are of the order of a few angstroms and that they
cannot be distinguished macroscopically from ordinary symmetry axes. The same
is true for glide mirror planes.
    Glide mirror planes combine the mirror image with a translation parallel
to the mirror plane over a distance which is half the unit distance in the
glide direction. These new symmetry elements must be taken into account when
the number of possible periodic arrangements is considered. A total of 230
arrangements or space groups is possible; of these, 32 can be distinguished
macroscopically.
    The forces which link the atoms together in metallic crystals are
nondirectional. This means that each atom tends to surround itself by as many
others as possible. This results in a dense packing, similar to that of
spheres of equal radius, and yields three distinct systems: close-packed
(face-centered) cubic, hexagonal close-packed, and body-centered cubic.
    Simple crystal structures are usually named after the compounds in which
they were first discovered (diamond or zinc sulfide, cesium chloride, sodium
chloride, and calcium fluoride). Many compounds of the type AsuppX-,
AsuppsuppX2- have such structures. They are highly symmetrical, the unit cell
is cubic, and the atoms or ions are disposed at the corners of the unit cell
and at points having coordinates which are combinations of 0, 1, 1/2, or 1/4.
    The sodium chloride structure is an arrangement in which each positive ion
is surrounded by six negative ions and vice versa. Systematic study of the
dimensions of the unit cells of compounds having this structure has revealed
that each ion can be assigned a definite radius and each ion tends to surround
itself by as many others as possible of the opposite sign because the binding
forces are nondirectional.
    In the cesium chloride structure each of the centers of the positive and
negative ions forms a primitive cubic lattice whose centers are mutually
shifted.
    In the diamond structure each atom is in the center of a tetrahedron
formed by its nearest neighbors. The 4-coordination follows from the
well-known bonds of the carbon atoms.
    The zinc blende structure, is basically similar to the diamond structure.
Each zinc atom is in the center of a tetrahedron formed by sulfur atoms (large
circles) and vice versa.
    In the calcium fluoride structure the unit cell is divided into eight
equal cubelets, and calcium ions are situated at corners and centers of the
faces of the cell. The fluorine ions are at the centers of the eight cubelets.
 �34s[Willy C. Dekeyser]
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    Growth. The growth of crystals, of which all crystalline solids are
composed generally occurs by means of the following sequence of processes: (1)
diffusion of the molecules of the crystallizing substance through the
surrounding environment (or solution) to the surface of the crystal, (2)
diffusion of these molecules over the surface of the crystal to special sites
on the surface, (3) incorporation of molecules into the crystal at these sites,
and (4) diffusion of the heat of crystallization away from the crystal surface.
The rate of crystal growth may be limited by any of these four steps. The
initial formation of the centers from which crystal growth proceeds is known as
nucleation. See CRYSTALLIZATION; NUCLEATION.
    During its growth into a fluid phase, a crystal often develops and maintains
a definite polyhedral form which may reflect the characteristic symmetry of the
molecular pattern of the crystal. The bounding faces of this form are those
which are perpendicular to the directions of slowest growth. How this comes
about is illustrated in Fig. 4, in which it is seen that the faces b, normal to
the slower-growing direction, become predominant.
    The molecular binding sites on the surface of a crystal can be of several
kinds. Thus a molecule must be more weakly bound on a perfectly developed plane
of molecules at the crystal surface than at a ledge formed by an incomplete
plane one molecule thick. Therefore, the binding of molecules in an island
monolayer on the crystal surface will be less per molecule than it would be
within a completed surface layer. The potential energy of a crystal is most
likely to be minimum in forms containing the fewest possible ledge sites.
According to the screw dislocation theory of F. C. Frank, growth is sustained
by indestructible surface ledges which result from the emergence of screw
dislocations in the crystal face.				     [D.T.]

    Defects. Departures of a crystalline solid from a regular array of atoms or
ions are referred to as defects. A "perfect" crystal of NaCl, for example,
would consist of alternating Na+ and Cl- ions on an infinite three-dimensional
simple cubic lattice, and a simple defect (a vacancy) would be a missing Na+ or
Cl- ion. There are many other kinds of possible defects, ranging from simple
and microscopic, such as the vacancy and other structures illustrated in Fig.
5, to complex and macroscopic, such as the inclusion of another material, or a
surface.
    Natural crystals always contain defects, often in abundance, due to the
uncontrolled conditions under which they were formed. The presence of defects
which affect the color can make these crystals valuable as gems, as in ruby
(Cr replacing a small fraction of the Al in Al2O3). Crystals prepared in the
laboratory will also always contain defects, although considerable control may
be exercised over their type, concentration, and distribution.
    The importance of defects depends upon the material, type of defect, and
properties which are being considered. Some properties, such as density and
elastic constants., are proportional to the concentration of defects, and so a
small defect concentration will have a very small effect on these. Other
properties, such as the color of an insulating crystal or the conductivity of a
semiconductor crystal, may be much more sensitive to the presence of small
numbers of defects. Indeed, while the term defect carries with it the
connotation of undesirable qualities, defects are responsible for many of the
important properties of materials, and much of solid-state physics and
chemistry and materials science involves the study and engineering of defects
so that solids will have desired properties. A defect-free silicon crystal would
be of little use in modern electronics; the use of silicon in devices is
dependent upon small concentrations of chemical impurities such as phosphorus
and arsenic which give it desired electronic properties.
    An important class of crystal defect is the chemical impurity. The simplest
case is the substitutional impurity, for example, a zinc atom in place of a
copper atom in metallic copper. Impurities may also be interstitial; that is,
they may be located where atoms atoms or ions normally do not exist. In metals,
impurities usually lead to an increase in the electrical resistivity.
Impurities in semiconductors are responsible for the important electrical
properties which lead to their widespread use. The energy levels associated
with impurities and other defects in nonmetals may also lead to optical
absorption in interesting regions of the spectrum.
    Even in a chemically pure crystal, structural defects will occur. These may
be simple or extended. One type of simple defect is the vacancy, but other types
exist (Fig. 5). The atom which left a normal site to create a vacancy may end up
in an interstitial position, a location not normally occupied. Or it may form a
bond with a normal atom in such a way that neither atom is on the normal site,
but the two are symmetrically displaced from it. This is called a split
interstitial. The name Frenkel defect is given to a vacancy-interstitial pair,
whereas an isolated vacancy is a Schottky defect.
    The simplest extended structural defect is the dislocation. An edge
dislocation is a line defect which may be thought of as the result of adding or
subtracting a half-plane of atoms. A screw dislocation is a line defect which
can be thought of as the result of cutting partway through the crystal and
displacing it parallel to the edge of the cut. Dislocations are of great
importance in determining the mechanical properties of crystals. A
dislocation-free crystal is resistant to shear, because atoms must be displaced
over high-potential-energy barriers from one equilibrium position to another.
It takes relatively little energy to move a dislocation (and thereby shear the
crystal), because the atoms at the dislocation are barely in stable equilibrium.
Such plastic deformation is known as slip. See PLASTIC DEFORMATION OF METAL.
    For both scientific and practical resons, much of the research on crystal
defects is directed toward the dynamic properties of defects under particular
conditions, or defect chemistry. Much of the motivation for this arises from the
often undesirable effects of external influences on material properties, and a
desire to minimize these effects. Examples of defect chemistry abound, including
one as familiar as the photographic process, in which incident photons cause
defect modifications in silver halides or other materials. Properties of
materials in nuclear reactors is another important case.
[W. Beall Fowler]

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�6v�2v�32sCrystal counter �21m �33s
    A detector of radiation in which the sensitive material is a
high-resistivity crystal. The crystals are diced into wafers 0.02 to 0.08 in
(0.5 to 2 mm) in thickness and 0.006 to 0.15 in sup 2 (4 to 100 mm sup 2) in
surface area. Various techniques are used to contact the larger surfaces. A
field of about 2.5 x 10 sup 4 V/in (10 sup 4 V/cm) is used to collect the charge
liberated by the radiation. Crystal and junction counters are very similar, but
differ fundamentally in the way the high-resistivity material is created. In a
junction counter, a reverse-biased junction creates a depletion region which
is the high-resistivity region. At the same time, the reverse bias gives high
charge-collection field. In the crystal counter, the high resistivity comes
from the basic crystalline material. Cadmium telluride (CdTe) and mercury
iodide (HgI2) are considered the most preferable crystals. See JUNCTION
DETECTOR.
    When a charged particle enters the crystal, it loses energy by creating
electron-hole pairs. A gamma ray entering the crystal interacts with an atom
of the crystal. Then, depending upon the interaction process - photoelectric.
Compton, or pair production - the gamma ray will transfer all or part of its
energy to an electron or to an electron-positron pair. These charged
particles, in turn, dissipate their energy by creating electron-hole pairs.
The number of electron-hole pairs is proportional to the energy deposited in
the crystal. See GAMMARAY DETECTORS; HOLES IN SOLIDS; PARTICLE DETECTOR;
SCINTILLATION
 �34s[James McKenzie]
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�6v�2v�32sCrystal field theory �21m �33s
    An essentially ionic approach to chemical bonding which is often used with
coordination compounds. These compounds consist of a central transition metal
ion that is surrounded by a regular array of coordinated atoms or ligands.
Accordingly, the ligands are assumed to be sources of negative charge which
perturb the energy levels of the central metal ion. In this respect the
ligands subject the metal ion to an electric field which is analogous to the
electric or crystal field produced by the regular distribution of nearest
neighbors within an ionic crystalline lattice. For example, the crystal field
produced by the Cl ion ligand in octahedral TiCl6 sup 3- is considered to be
similar to that produced by the octahedral array of the six Cl ions about each
Na ion in NaCl. The Na ion with its rare-gas configuration has an electronic
charge distribution which is spherically symmetric both within and without the
crystal field. The paramagnetic Ti(III) ion, which possesses one 3d electron
(d sup 1), has a spherically symmetric charge distribution only in the absence
of the crystal field produced by the ligands. The presence of the ligands
destroys the spherical symmetry and produces a more complex set of energy
levels within the central metal ion. The crystal field theory allows the
energy levels to be calculated and related to experimental observation. See
COORDINATION CHEMISTRY.
    To illustrate the results of a typical crystal field calculation, assume
that the single d electron in the Ti(III) ion will experience a coulombic
repulsion with each of the six nearest neighbor Cl ions which are taken as
point negative charges. This model for coulombic repulsion may be described
mathematically as the summation eqSIGMAr- sup 1, where e and q are the
electronic and ligand charges, respectively and r is the distance between the
electron and the ligand. The summation extends over all the ligands. A
detailed quantum mechanical calculation can then be made. Fortunately, it is
possible to arrive at identical results by a very qualitative procedure. This
method considers the spatial orientation of d orbitals with relation to the
ligands when both are viewed within the same coordinate system.
    Crystal field theory calculations have enabled inorganic chemists to
understand certain natural phenomena; for example, why some coordination
compounds containing a given metal ion exhibit full paramagnetism, while
others containing the metal in exactly the same formal oxidation state show
either a much weaker paramagnetism or none at all. A striking example of this
is provided by paramagnetic CoF6 sup 3-, which possesses six unpaired d
electrons and the diamagnetic Co(NH3)6 sup 3 sup +, in which none of the d
electrons are unpaired. Both contain Co(III).
    The many successes of crystal field theory in the interpretation of
phenomena associated with transition metal compounds should not lead one to
the conclusion that the bonding within these compounds can be truthfully
represented by a strictly ionic model. Crystal field theory is essentially a
very specialized form of the more complete molecular orbital theory, and some
aspects of transition metal behavior can only be interpreted in terms of
covalent bonding. See CHEMICAL BONDING; MOLECULAR ORBITAL THEORY.
 �34s[R. A. D. Wentworth]
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�6v�2v�32sCrystallization   �21m �33s 
    The formation of a solid from a solution, melt, vapor, or a different 
solid phase. Crystallization from solution is an important industrial 
operation because of the large number of materials marketed as crystalline 
particles. Fractional crystallization is one of the most widely used methods 
of separating and purifying chemicals. In fractional crystallization it is 
desired to separate several solutes present in the same solution. This is 
generally done by picking crystallization temperatures and solvents such that 
only one solute is supersaturated and crystallizes out. By changing 
conditions, other solutes may be crystallized subsequently. Repeated 
crystallizations are necessary to achieve desired purities when many 
inclusions are present or when the solid solubility of other solutes is 
significant. For a discussion of crystallization in glass, a supercooled melt, 
see GLASS. Polymer crystals obtained from solutions are used to study the 
properties of these crystals, while crystallization of polymer melts 
dramatically influences polymer properties; see POLYMER. For methods of 
preparing large crystals see SINGLE CRYSTAL. For crystallization from vapors 
see SUBLIMATION. For solubility and other relationships between solid and 
liquid phases see PHASE EQUILIBRIUM; SOLUTION. See also CRYSTAL; 
CRYSTALLOGRAPHY; NUCLEATION. 
 �34s[William R. Wilcox]
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�6v�2v�32sCrystallography �21m �33s
    The branch of science that deals with the geometric description of
crystals, their internal arrangement, and their properties. Long a part of
mineralogy, crystallography is now a vast, loosely defined area between
physics, chemistry, physical metallurgy, mineralogy, and even biology.
    The anisotropic nature of crystals results in a freely growing crystal,
bounded by flat faces. The following discussion presents a formalism for
describing crystals exactly in terms of these faces.
    Faces which are parallel to the same edge are said to form a zone; the
direction of the edge is the zone axis. The number and relative importance of
the faces bounding crystals of the same chemical compound can be very
different. The crystals are then said to have a different habit. Faces and
edges which are present as well as those which are geometrically possible must
therefore be taken into account, and in the following discussion the term face
is extended to any plane determined by two edges. An edge is, in the same
sense, the intersection of two faces.
    The angle between similar faces is constant for different-sized crystals
of the same substance. This means that a crystal grows by the parallel
displacement of its faces. Therefore, only the direction of a normal to a face
is of importance. As a consequence, the crystal is considered to be in the
center of a sphere (Fig. 1). The normals to its faces from the center cut the
sphere in points or poles whose position can be fixed by two coordinates
phi and rho (Fig. 2a). The constellation of faces is in this way replaced by a
constellation of points on a sphere. The zone axes are replaced by great
circles or zone circles; they are the intersections of the sphere with planes
passing through the center of the sphere, perpendicular to the zone axis. In
practice, phi and rho values are obtained by ``measuring'' the crystal;
two-circle goniometers are used for the purpose. All problems which can arise
are connected with angles between faces and between zone axes; they can in
principle be solved by spherical trigonometry once the phi and rho
values are known. Graphical constructions are, however, most helpful, and
gnomonic and stereographic projections are used in such constructions. These
projections are shown in Fig. 2. In stereographic projection, circles are
projected as circles, and the angles between them are not altered. In gnomonic
projection, great circles are projected as straight lines.

    Symmetry is a basic property of crystals. A crystal polyhedron (or better,
the constellation of the normals) can be brought into successive
indistinguishable positions by nontrivial operations. These are inversion,
rotation around an axis over a specific angle, reflection into a plane, or
combinations of these. It is, however, convenient to consider all operations
as rotations, and two types of axis can be considered: (1) a rotation or
symmetry axis involving a rotation over an angle of 360DG/n (n is the
multiplicity of the axis) and (2) an inversion axis which combines the
preceding operation with an inversion with respect to a point of the axis. It
is found that only multiplicities of 1, 2, 3, 4, and 6 occur.
    The seven groups obtained when only the dominant symmetry elements of the
classes are taken into account are called crystal systems. In order of
decreasing symmetry, they are called cubic, hexagonal, tetragonal, trigonal,
orthorhombic, monoclinic, and triclinic. Sometimes the hexagonal and trigonal
systems are considered as a single system.
    Miller indices are used for the analytical description of crystal faces,
and are shown in Fig. 3. A specially well-suited reference system used for
that purpose consists of three nonparallel possible edges of the crystal which
are chosen as axes and a possible plane of the crystal which intersects all
three axes respectively at A, B, C. It is defined by the three angles alpha,
beta, gamma and the ratio OA:OB:OC. Another plane, HKL, of the same crystal is
defined by the ratio OH:OK:OL. The lengths OH, OK, OL are respectively
measured with OA = a, OB = b, OC = c as units, that is, by the ratios a/OH,
b/OK, c/OL. In the equation below, h, k, l are rational numbers which are
small when the axes are well chosen (the law of rational indices). A different
unit is used along each axis to take the anisotropy of the crystal into
account. When, for instance, the temperature is raised, a, b, and c expand
differently, but the ratios a/OH, b/OK, c/OL remain constant. The ratios h:k:l
define the plane HKL; h, k, l are called its Miller indices, and by
convention, this is written (hkl), or (BAR h BAR k BAR l) when the indices are
negative numbers. See CRYSTAL.
 �34s[Willy C. Dekeyser]
�33s

			(a/OH)/(b/OK)/(c/OL) = h/k/l


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�6v�2v�32sCrystal optics �21m �33s
    The study of the propagation of light, and associated phenomena, in
crystalline solids. For a simple cubic crystal the atomic arrangement is such
that in each direction through the crystal the crystal presents the same
optical appearance. The atoms in anisotropic crystals are closer together in
some planes through the material than in others. In anisotropic crystals the
optical characteristics are different in different directions. In classical
physics the progress of an electromagnetic wave through a material involves
the periodic displacement of electrons. In anisotropic substances the forces
resisting these displacements depend on the displacement direction. Thus the
velocity of a light wave is different in different directions and for
different states of polarization. The absorption of the wave may also be
different in different directions. See DICHROISM; TRICHROISM.
    In an isotropic medium the light from a point source spreads out in a
spherical shell. The light from a point source embedded in an anisotropic
crystal spreads out in two wave surfaces, one of which travels at a faster
rate than the other. The polarization of the light varies from point to point
over each wave surface, and in any particular direction from the source the
polarization of the two surfaces is opposite. The characteristics of these
surfaces can be determined experimentally by making measurements on a given
crystal.
    In the most general case of a transparent anisotropic medium, the
dielectric constant is different along each of three orthogonal axes. This
means that when the light vector is oriented along each direction, the
velocity of light is different. One method for calculating the behavior of a
transparent anisotropic material is through the use of the index ellipsoid,
also called the reciprocal ellipsoid, optical indicatrix, or ellipsoid of wave
normals. This is the surface obtained by plotting the value of the refractive
index in each principal direction for a linearly polarized light vector lying
in that direction (see illustration). The different indices of refraction, or
wave velocities associated with a given propagation direction, are then given
by sections through the origin of the coordinates in which the index ellipsoid
is drawn. These sections are ellipses, and the major and minor axes of the
ellipse represent the fast and slow axes for light proceeding along the normal
to the plane of the ellipse. The length of the axes represents the refractive
indices for the fast and slow wave, respectively. The most asymmetric type of
ellipsoid has three unequal axes. It is a general rule in crystallography that
no property of a crystal will have less symmetry than the class in which the
crystal belongs.

    Accordingly, there are many crystals which, for example, have four- or
sixfold rotation symmetry about an axis, and for these the index ellipsoid
cannot have three unequal axes but is an ellipsoid of revolution. In such a
crystal, light will be propagated along this axis as though the crystal were
isotropic, and the velocity of propagation will be independent of the state of
polarization. The section of the index ellipsoid at right angles to this
direction is a circle. Such crystals are called uniaxial and the mathematics
of their optical behavior is relatively straightforward.
    In crystals of low symmetry the index ellipsoid has three unequal axes.
These crystals are termed biaxial and have two directions along which the wave
velocity is independent of the polarization direction. These correspond to the
two sections of the ellipsoid which are circular. See CRYSTALLOGRAPHY.
    The normal to a plane wavefront moves with the phase velocity. The Huygens
wavelet, which is the light moving out from a point disturbance, will
propagate with a ray velocity. Just as the index ellipsoid can be used to
compute the phase or wave velocity, so can a ray ellipsoid be used to
calculate the ray velocity. The length of the axes of this ellipsoid is given
by the velocity of the linearly polarized ray whose electric vector lies in
the axis direction. See PHASE VELOCITY.
    The refraction of a light ray on passing through the surface of an
anisotropic uniaxial crystal can be calculated with Huygens wavelets in the
same manner as in an isotropic material. For the ellipsoidal wavelet this
results in an optical behavior which is completely different from that
normally associated with refraction. The ray associated with this behavior is
termed the extraordinary ray. At a crystal surface where the optic axis is
inclined at an angle, a ray of unpolarized light incident normally on the
surface is split into two beams: the ordinary ray, which proceeds through the
surface without deviation; and the extraordinary ray, which is deviated by an
angle determined by a line drawn from the center of one of the Huygens
ellipsoidal wavelets to the point at which the ellipsoid is tangent to a line
parallel to the surface. The two beams are oppositely linearly polarized.
 �34s[Bruce H. Billings]
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�6v�2v�32sCrystal whiskers   �21m �33s 
    Single crystals that have grown in a filamentary form. Such filamentary 
growths have been known for centuries; however, great interest in them 
developed only after it was discovered at Bell Telephone Laboratories in the 
1950s that the strength exhibited by some whiskers in bend tests approaches 
that expected theoretically for perfect crystals. See SINGLE CRYSTAL. 
    Whiskers have been grown by spontaneous extrusion or out of a 
supersaturated medium, the medium being supersaturated by physical methods or 
by chemical reaction. In all instances whiskers apparently form from 
singularities in the medium. 
 �34s[David Turnbull]
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�6v�2v�32sCtenophora   �21m �33s 
    A phylum of exclusively marine organisms, formerly included in the 
jellyfish and polyps as coelenterates. These animals, the so-called comb 
jellies, possess a biradial symmetry of organization. They are characterized 
by having eight rows of comblike plates as the main locomotory structures. 
Most of these animals are pelagic, but a few genera are creeping. Many are 
transparent and colorless; others are faintly to brightly colored. These 
organisms are hermaphroditic. Development is biradially symmetrical, with a 
cydippid larval stage. Two classes and five orders constitute this phylum as 
follows:  
        Phylum Ctenophora 
        Class Tentaculata 
        Order: Cydippidea 
               Lobata 
               Cestida 
               Platyctenea 
        Class Nuda 
        Order Beroida 
    The body is gelatinous and extremely fragile; its form may be globular, 
pyriform, or bell- or helmet-shaped (see illustration). Some species resemble 
a ribbon. Their size ranges from about 0.1 in. to 20 in. (3 mm to 50 cm). Comb 
plates are the most characteristic structure of the ctenophores, possessed by 
all members of the phylum. Eight meridional comb-plate rows or ribs stretch 
from the aboral pole on the surface of the body. Each plate consists of a 
great number of very long related cilia. With the exception of Beroida, all 
ctenophores possess two highly extensile tentacles bearing many lateral 
branches (tentilla). Tentacles and tentilla are thickly covered with adhesive 
cells (colloblasts) which are exclusive to the phylum. When a prey collides 
with a tentillum, some colloblasts burst and their granules stick the prey to 
the helical threads. These threads apparently act as shock absorbers of the 
captured prey's movements rather than as springs projecting the head. 
  
    Fertilization usually occurs in sea water. Pelagic ctenophores are 
self-fertile, but cross-fertilization might also take place inside a swarm of 
ctenophores. The earliest larva formed in the development of all types of 
ctenophores is the cydippid, with a globular body, a pair of primary tentacles 
(or rudiments), and eight costae of comb plates. In the Lobata, Cestoida, and 
Platyctenea, the larva undergoes secondary changes in later development. 
    Almost all ctenophores are luminescent. This capacity, linked to a 
calcium-dependent photoprotein, is assumed by specialized cells underlying the 
comb plates. 
    The ctenophores feed on zooplankton. Prey are trapped either with the 
tentacles (Cydippidea, Cestida) or with oral extensions of the body (Lobata), 
or are engulfed directly through the widely opened mouth (Beroida). 
Extracellular digestion occurs first in the pharynx, where gland cells secrete 
enzymes. The partially digested material is then carried throughout the 
gastrovascular system, where it is phagocytized by digestive cells. 
    Ctenophores have high powers of regeneration. Asexual reproduction (in a 
few platyctenid genera) is by regeneration of an entire organism from a small 
piece of the adult body. 
    Ctenophores are themselves important plankton organisms. They are quite 
common and have a worldwide distribution in all seas, where they can appear in 
enormous swarms. Because of their voracity as predators of zooplankton, they 
can play an important role in plankton equilibrium and in fisheries. 
    The Ctenophora constitute a well-defined phylum. Among its orders, the 
Cydippidea are undoubtedly the most primitive, with a radiating evolution of 
the other orders. The ancestor of the phylum is possibly to be found among the 
Trachylina (Cnidaria, Hydrozoa). However, because of such forms as Coeloplana 
(Platyctenea), ctenophores have also been suspected of affinity with the 
polyclad Turbellaria. See POLYCLADIDA; TRACHYLINA. 
 �34s[Jean-Marie Franc]
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�6v�2v�32sCtenostomata   �21m �33s 
    An order of bryozoans in the class Gymnolaemata. Ctenostomes have 
inconspicuous delicate colonies made up of relatively isolated, simple, short, 
straight, vase-shaped zooecia with solid chitinous walls. See GYMNOLAEMATA. 
    Most ctenostome colonies are encrusting threadlike networks of stolons or 
proximal portions of autozooecia, with the distal portions of the autozooecia 
arising erect from the substrate at regular intervals. A few other ctenostome 
colonies are tuft-like or are gelatinous masses. Though sometimes having 
stolons or rhizoids, ctenostome colonies lack ovicells or het erozooids 
(avicularia, vibracula, and so on). The colonies cannot be divided into 
distinct endozone and exozone regions. 
    Each ctenostome zygote develops into only one larva. After attaching to a 
substrate, each larva metamorphoses into an ancestrular zooid. Additionally, 
the few fresh-water (but not marine) ctenostomes produce irregularly shaped 
resistant resting bodies (hibernacula) by asexual budding when unfavorable 
environmental conditions threaten. 
    First appearing early in the Early Ordovician, ctenostomes are the oldest 
known bryozoans and may possibly have been the stem group from which the other 
ectoproct groups evolved, the stenolaemates early in the Paleozoic and the 
cheilostomes and possibly phylactolaemates late in the Mesozoic. See BRYOZOA; 
CHEILOSTOMATA; PHYLACTOLAEMATA; STENOLAEMATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sCtenothrissiformes   �21m �33s 
    A small order of teleostean fishes that are of particular interest because
they seem to be on or near the main evolutionary line leading from the
generalized, soft-rayed salmoniforms to the spiny-rayed beryciforms and
perciforms, dominant groups among higher bony fishes. Ctenothrissiformes lack
fin spines but they share many characters with primitive beryciforms. The few
fishes confidently assigned to the Ctenothrissiformes, Ctenothrissus
(Ctenothrissidae; see illustration) and Aulolepis and Pateroperca
(Aulolepidae), are known as Upper Cretaceous marine fossils from Europe and
southwestern Asia. Two little-known genera of Recent oceanic fishes, Macristium
and Macristiella (Macristiidae), are assigned by some authors to the order but
some doubt attends this placement. See BERYCIFORMES; PERCIFORMES;
SALMONIFORMES. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sCube   �21m �33s 
    A parallelepiped whose six faces are all squares. The cube is one of the 
five regular solids known to the ancient Greeks. A cube has eight vertices and 
twelve edges. Each vertex is on three edges and three faces, each edge is on 
two vertices and two faces, and each plane is on four vertices and four edges. 
See PARALLELEPIPED; POLYHEDRON; REGULAR POLYTOPES. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sCubeb �21m �33s
    The dried, nearly ripe fruit (berries) of a climbing vine, Piper cubeba,
of the pepper family (Piperaceae). This species is native to eastern India and
Indomalaysia. The crushed berries are smoked, and the inhaled smoke produces a
soothing effect in certain respiratory ailments. Cubebs are used in medicine
as a stimulant, expectorant, and diuretic. Medicinal properties of cubeb are
due to the presence of a volatile oil which formerly was thought to stimulate
healing of mucous membranes. See PIPERALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCubomedusae   �21m �33s 
    An order of the class Scyphozoa. Their distribution is mostly tropical. 
The umbrella is cubic, and a pedalium, a gelatinous leaf-shaped body, is 
present at the base of each ridge of the exumbrella (see illustration). A 
well-developed tentacle, or a cluster of tentacles, arises from each pedalium. 
No connecting stage between polyp and medusa is known. See SCYPHOZOA. 
 �34s[Tohru Uchida]
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�6v�2v�32sCuckoo   �21m �33s 
    The name for about 130 species of the family Cuculidae, in the order 
Cuculiformes, which are primarily arboreal and widely distributed throughout 
the Old World. The arrangement of the toes is zygodactyl with two in front and 
two behind, an arrangement that is well suited for these essentially perching 
birds which fly from tree to tree. 
    There are about 50 species which are social parasites in their breeding 
habits, and they are widely represented in the Old World, ranging from 
northern Europe to Australia. Essentially, the parasitic species lay their 
eggs in the nests of other birds for incubation and for feeding of young after 
hatching. Egg mimicry makes these cuckoos well adapted to parasitism with the 
eggs being about the same size and color as those of the host species. The 
eggs of the hosts are removed from the nest by the cuckoo to make room for her 
own. The eggs of the cuckoo hatch before those of the host, or shortly after. 
The cuckoo nestlings attempt to push the host's eggs out of the nest, or to 
rid the nest of the nestlings. If they are unsuccessful in the latter, they 
beg for food with such intensity that they are fed first and more frequently. 
The female cuckoo occupies a definite territory during this season, even 
though she exhibits no interest in the offspring. See TERRITORIALITY. 
    In the nonparasitic species, both parents incubate the eggs and care for 
the young in a nest that is frequently constructed of twigs. The clutch is two 
to six eggs, and when hatched the young are more or less naked and are 
nidicolous; that is, they remain in the nest. See CUCULIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sCuculiformes   �21m �33s 
    The order of birds containing the cuckoos, family Cuculidae. The turacos 
(Musophagidae) are conventionally placed here also, but this African family is 
of uncertain affinities and some authors believe it to be related to the 
Galliformes.  
    Cuckoos are virtually cosmopolitan but are concentrated in warmer regions. 
Most are arboreal; terrestrial forms, typified by the roadrunners (Geococcyx) 
of western United States and middle America, have evolved independently in 
several areas. The European cuckoo (Cuculus canorus), inspiration for the 
cuckoo clock, is representative of a large group of parasitic species. Several 
groups of cuckoos are nonparasitic, including the coarse-feathered coucals 
(Centropus) of the Old World tropics, and the New World genus Coccyzus, which 
includes the familiar yellow-billed and black-billed cuckoos of North America. 
See AVES; CUCKOO; GALLIFORMES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sCucumber   �21m �33s 
    A warm-season annual cucurbit (Cucumis sativus) native to India and 
belonging to the plant order Campanulales. The cucumber is grown for its 
immature fleshy fruit which is used primarily for pickling and for slicing as 
a salad. The development of gynoecious (bearing only female flowers) and 
hybrid varieties (cultivars) with increased disease resistance is resulting in 
a continuing change in the varieties of both pickling and slicing cucumbers. 
Important cucumber-producing states are Florida and South Carolina for fresh 
market and Michigan and Wisconsin for pickling. See CAMPANULALES. 
 �34s[H. John Carew]
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�6v�2v�32sCulture   �21m �33s 
    A growth of living cells in a controlled artificial environment. It may be 
a culture of microorganisms, such as a bacterial culture, or one of animal or 
plant cells, such as a tissue culture. Cultures require appropriate sources of 
food and energy, provided by the culture medium, and a suitable physical 
environment. Tissue cultures can themselves become a culture medium for 
viruses, which grow only with live cells. 
    Cultures of only one kind of cells are known as pure cultures, as 
distinguished from mixed or contaminated cultures. Isolating microorganisms in 
pure culture for identification or study is a basic microbiologic procedure. 
Cultures are used commercially as ``starters'' in preparing several dairy 
products, in fermentation industries, in bread making, and in other instances 
in which seeding from a culture initiates further controlled massive growth to 
produce desired products, ranging from cottage cheese to antibiotics. See 
CULTURE MEDIA; EMBRYONATED EGG CULTURE; PURE CULTURE; TISSUE CULTURE. 
 �34s[Max S. Marshall]
�33s 
    Culture technique involves the procedures for isolation, cultivation, and 
manipulation of microorganisms, including viruses and rickettsia, and for 
propagation of plant and animal cells and tissues. A relatively minute number 
of cells, the inoculum, is introduced into the culture medium. The medium in a 
suitable vessel is protected by cotton plugs or loose-fitting covers with 
overlapping edges so as to allow diffusion of air, yet prevent access of 
contaminating organisms from the air or from unsterilized surfaces. The 
transfer, or inoculation, is usually done with the end of a flamed, then 
cooled, platinum wire. Sterile swabs may also be used and, in the case of 
liquid inoculum, sterile pipets. 
    The aqueous solution of nutrients may be left as a liquid medium or may be 
solidified by incorporation of a nutritionally inert substance, most commonly 
agar or silica gel. Special gas requirements may be provided in culture 
vessels closed to the atmosphere, as for anaerobic organisms. Inoculated 
vessels are held at a desired constant temperature in an incubator or water 
bath. Liquid culture media may be mechanically agitated during incubation. 
Maximal growth, visible as a turbidity or as masses of cells, is usually 
attained within a few days, although some organisms may require weeks to reach 
this stage. See CHEMOSTAT; INOCULATION. 
 �34s[J. W. Faller /R. E. Kallio]
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�6v�2v�32sCulture media �21m �33s
    Combinations of nutrients used for the growth of living cells in cultures.
Water is the chief constituent of most media. A medium may consist of natural
materials, such as enzymatic digests, extracts of yeast or beef, milk, potato
slices, or chick embryos. Artificial media are prepared by mixing various
ingredients according to particular formulas. A complex medium contains at
least one crude ingredient derived from a natural material, hence of unknown
chemical composition. A chemically defined, or synthetic, medium is one in
which the chemical structure and amount of each component are known.
    All media furnish minerals, sources of nitrogen, carbon, and energy, and
sometimes vitamins or other growth factors. An ingredient furnishing carbon -
usually the chief source - is called the substrate for the utilizing organism.
    A selective medium usually contains an individual organic compound as the
sole source of carbon, nitrogen, or sulfur for growth of an organism. An agar
medium of a given composition permits selection, through development of
colonies, of organisms capable of utilizing the specific sources of carbon,
energy, nitrogen, or sulfur available. A colony on such a medium is
prima-facie evidence that such organisms have been selected, but his must be
confirmed by restreaking (subculturing on agar medium) on a medium of the same
composition.
    Enrichment media are liquids of a given composition which permit
preferential emergence of certain organisms that initially may have made up a
relatively minute proportion of a mixed inoculum. This enrichment of the
population in the desired type expedites isolation by direct plating on the
corresponding selective solid medium.
    Indicator media are usually solid, and contain substances capable of
undergoing a color change in the vicinity of the colony which has effected a
particular chemical change, such as fermenting a certain sugar. Thus,
organisms with distinctive types of metabolism may be readily detected by the
color change in the medium, even when the surface of the medium contains many
other colonies. Indicators may be used in liquid media to characterize a pure
culture of organisms. See ELECTIVE CULTURE.
 �34s[Jackson W. Foster /R. E. Kallio]
�33s

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�6v�2v�32sCumacea   �21m �33s 
    An order of the class Crustacea, subclass Malacostraca. All have a 
well-developed carapace which is fused dorsally with at least the first three 
thoracic somites and which overhangs the sides to enclose a branchial cavity. 
The telson may be free or may appear lacking because it is fused with the last 
abdominal somite. Eyes are sessile and, except for a few genera, are coalesced 
in a single median organ or wholly wanting. There are eight pairs of thoracic 
appendages. There is no free larval stage. In the few species whose life 
history has been investigated there are five to seven molts after release from 
the brood pouch. There is always some sexual dimorphism in adults, but 
immature males resemble females until the later stages. 
    The order is rather uniform and classification is based on trivial 
characters. Seven families are recognized. Affinities are probably closer to 
the Mysidacea than to the Tanaidacea. A few fossils are now known. 
    Cumacea are found in all seas and in some estuaries and areas of brackish 
water, including the Caspian Sea. Most inhabit water shallower than 200 m, but 
many occur in great depths. They normally burrow in sand or mud with the front 
protruding. They feed on organic detritus or very small organisms and may 
themselves form part of the diet of various fishes. See MALACOSTRACA. 
 �34s[Norman S. Jones]
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�6v�2v�32sCummingtonite   �21m �33s 
    The name given to the calcium-poor monoclinic amphiboles that form a 
limited solid-solution series between the iron end member grunerite, 
Fe7Si8O22(OH)2, and a magnesian end member of approximately Mg5Fe2Si8O22(OH)2. 
  More magnesium-rich minerals apparently form the closely related 
orthorhombic form, anthophyllite. Cummingtonite is usually yellow-brown to 
brown and is slightly colored in thin sections. It forms elongate prismatic to 
fibrous crystals. Grunerite commonly occurs as brown fibrous crystals with a 
silky luster and is usually found associated with iron formation. 
    Cummingtonite is a metamorphic mineral that occurs in various schists, in 
contact metamorphic rocks, and in some ore deposits. It is found associated 
with such minerals as hornblende, biotite, garnet, plagioclase, quartz, and 
anthophyllite. See AMPHIBOLE; ANTHOPHYLLITE. 
 �34s[George W. DeVore]
�33s 
  
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�6v�2v�32sCup anemometer   �21m �33s 
    An instrument widely used to measure horizontal wind speed, independent of 
direction. The use of three hemispherical cups mounted on a vertical shaft is 
standard in the United States (see illustration). Commonly the cups rotate a 
worm gear which operates an electric contact every mile of wind, or fraction 
thereof. These contacts are recorded on a chart driven at constant speed. From 
the number of contacts in a selected time interval, the wind speed is deduced. 
Since the number of cup revolutions per mile varies with wind speed, 
particularly at low speeds, a correction must be made. 
 �34s[Herman Feshbach]
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�6v�2v�32sCuprite   �21m �33s 
    A mineral having composition Cu2O and crystallizing in the isometric 
system. Cuprite is commonly in crystals showing the cube, octahedron, and 
dodecahedron. It is various shades of red and a fine ruby red in transparent 
crystals which have a metallic to adamantine luster. The hardness is 3.5 - 4 
(Mohs scale) and the specific gravity is 6.1. 
    Cuprite is a widespread supergene copper ore. Fine crystals have been 
found at Cornwall, England, and Chessy, France. It has served as an ore in the 
Congo, Chile, Bolivia, and Australia. In the United States it has been found 
at Clifton, Morenci, Globe, and Bisbee, all in Arizona. See COPPER. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sCurie temperature   �21m �33s 
    The critical or ordering temperature for a ferromagnetic or a 
ferrimagnetic material. The Curie temperature Tc is the temperature below 
which there is a spontaneous magnetization M in the absence of an externally 
applied magnetic field, and above which the material is paramagnetic. In the 
disordered state above the Curie temperature, thermal energy overrides any 
interactions between the local magnetic moments of ions. Below the Curie 
temperature, these interactions are predominant and cause the local moments to 
order or align so that there is a net spontaneous magnetization. 
    In the ferromagnetic case, as temperature T increases from absolute zero, 
the spontaneous magnetization decreases from M0, its value at T = 0. At first 
this occurs gradually, then with increasing rapidity until the magnetization 
disappears at the Curie temperature. In ferrimagnetic materials the course of 
magnetization with temperature may be more complicated, but the spontaneous 
magnetization disappears at the Curie temperature. 
    In antiferromagnetic materials the corresponding ordering temperature is 
termed the Neel temperature. Below the Neel temperature the magnetic 
sublattices have a spontaneous magnetization, though the net magnetization of 
the material is zero. Above the Neel temperature the material is paramagnetic. 
See ANTIFERROMAGNETISM. 
    The ordering temperatures for magnetic materials vary widely. The ordering 
temperature for ferroelectrics is also termed the Curie temperature, below 
which the material shows a spontaneous electric moment. See FERROMAGNETISM; 
PYROELECTRICITY.  
 �34s[J. F. Dillon, Jr.]
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�6v�2v�32sCurie-Weiss law �21m �33s
    A relation between magnetic or electric susceptibilities and the absolute
temperature which is followed by ferromagnets, antiferromagnets, nonpolar
ferroelectrics and antiferroelectrics, and some paramagnets. The Curie-Weiss
law is usually written as the following equation, where CHI is the
susceptibility, C is a constant for each material,T is the absolute
temperature, and ;Gu is called the Curie temperature. Antiferromagnets and
antiferroelectrics have a negative Curie temperature. The Curie-Weiss law
refers to magnetic and electric behavior above the transition temperature of
the material in question. It is not always precisely followed, and it breaks
down in the region very close to the transition temperature. Often the
susceptibility will behave according to a Curie-Weiss law in different
temperature ranges with different values of C and THETA. See CURIE
TEMPERATURE; ELECTRIC SUSCEPTIBILITY; MAGNETIC SUSCEPTIBILITY.
 �34s[Elihu Abrahams;Frederic Keffer]
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			CHI = C/(T - THETA)

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�6v�2v�32sCurium �21m �33s
    A chemical element, Cm, in the actinide series, with an atomic number of
96. Curium does not exist in the terrestrial environment, but may be produced
artificially. The chemical properties of curium are so similar to those of the
typical rare earths that, if it were not for its radioactivity, it might
easily be mistaken for one of these elements. The known isotopes of curium
include mass numbers 238 - 250. The isotope sup 2 sup 4 sup 4 Cm is of
particular interest because of its potential use as a compact, thermoelectric
power source, through conversion to electrical power of the heat generated by
nuclear decay. 
    Metallic curium may be produced by the reduction of curium trifluoride
with barium vapor. The metal has a silvery luster, tarnishes in air, and has a
specific gravity of 13.5. The melting point has been determined as 2444
PLUS/MINUS 72DGF (1340+/-40DGC). The metal dissolves readily in common mineral
acids with the formation of the tripositive ion.
    A number of solid compounds of curium have been prepared and their
structures determined by x-ray diffraction. These include CmF4, CmF3. CmCl3,
CmBr3, CmI3, Cm2O3, and CmO2. Isostructural analogs of the compounds of curium
are observed in the lanthanide series of elements. See ACTINIDE ELEMENTS;
TRANSURANIUM ELEMENTS.
 �34s[Glenn T. Seaborg]
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�6v�2v�32sCurrant   �21m �33s 
    A shrubby, deciduous plant of the genus Ribes, order Rosales, related to 
the gooseberry. The fruits are berries, borne in clusters. The black currant 
(R. nigrum), popular in Europe, is not cultivated in the United States. Red 
currants are derived from five species, mostly European. There are also white 
currants which are variants of the red type. Only the red varieties are 
produced commercially. The greatest commercial production occurs in New York, 
Washington, and Michigan, but production has been declining in recent years. 
See GOOSEBERRY; ROSALES. 
 �34s[J. Harold Clarke]
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�6v�2v�32sCurrent balance   �21m �33s 
    An apparatus with which force is measured between current-carrying 
conductors (fixed and movable coils; see illustration). The purpose of the 
measurement is to assign the value of the ampere; therefore the relative 
locations of the conductors are prescribed so that the force between them can 
be computed from their geometry and the current they carry. Experimentally 
this force is measured in terms of the gravitational force on a known mass; 
thus the ampere may be said to be weighed. Three types of current balance have 
been used in ampere determinations. In two of these, the Rayleigh and the 
Ayrton-Jones balances, the fixed and movable coils are coaxial, and the motion 
is linear along the coil axis. In the third type, the Pellat balance, the axes 
of the fixed and movable coils are at right angles, and the movable coil turns 
about its center, the pivot point of the weighing balance. See CURRENT 
MEASUREMENT.  
 �34s[Forest K. Harris]
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�6v�2v�32sCurrent density   �21m �33s 
    A vector quantity equal in magnitude to the instantaneous rate of flow of 
electric charge, perpendicular to a chosen surface, per unit area per unit 
time. If a wire of cross-sectional area A carries a current I, the current 
density J is I/A. The units of J in the rationalized meter-kilogram-second 
system are amperes per square meter. 
 �34s[John W. Stewart]
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�6v�2v�32sCurrent measurement �21m �33s
    The measurement of the flow of electric current. The current balance is
tedious to operate, and is ordinarily used only by the national laboratories
of the various countries for the determination of the absolute ampere, whereby
simpler (but less accurate) current-measuring devices may be calibrated. In
the practical determination of current value the current-carrying wires are
formed into circular coils. The moving coil may move up or down and may rotate
in jeweled bearings against the counter torque of a spiral spring. If a
pointer is attached to the moving system rotating over an appropriately marked
scale, calibrated in terms of the absolute current determinations of an ampere
balance, the result is an indicating electrodynamometer ammeter. Such
instruments are widely used as laboratory standards for both direct- and
alternating-current measurements and for the calibration of secondary
instruments. See CURRENT BALANCE.
    Direct current may also be measured by placing the movable coil in the
magnetic field of a permanent magnet. This is the construction of the
conventional type of dc ammeter. See AMMETER.
    For ordinary switchboard or test indications of alternating current the
fixed-coil, moving-iron type of instrument is widely used. A fixed iron vane
and a similar vane on a staff are magnetized by the current in the coil, the
force between the vanes acting to rotate the staff against a spring as a
function of the rms value of the current.
    In the thermoammeter a short metal strip or wire is heated by the current
in question; its temperature rise is measured by a contacting thermocouple
connected, in turn, to a sensitive dc meter which deflects in proportion to
the temperature rise.
    When direct currents of over a fraction of 1 ampere are to be measured,
the bulk of the current is diverted around the instrument through a shunt or
bypass circuit. Large alternating currents, on the other hand, are connected
to meters rated at 5 A through current stepdown transformers, or current
transformers. Extremely heavy direct currents, above 10 sup 4 A, can be measured
with a transductor, essentially an ac transformer with the iron core
surrounding the dc bus.
 �34s[John H. Miller /Jack Bass]
�33s
    Small direct currents in low-resistance circuits can be measured with a
standard-resistance shunt and a potentiometer. In high-resistance circuits,
vacuum-tube devices are now usually preferred to electrometers. Currents as
low as 10- sup 1 sup 5 A can be measured with shunts of 10 sup 1 sup 1 or 10
sup 1 sup 2 ohms, and lower values can be measured by observing the rate at
which the input capacitance is charged by the flow of current. See ELECTROMETER.
 �34s[W. Norris Tuttle]
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�6v�2v�32sCurry �21m �33s
    A mixture of plant spices used with meats and fish and other seafoods,
each requiring its own specific combination. One recipe calls for a mixture of
turmeric, coriander, cinnamon, cumin, ginger, cardamon, cayenne pepper,
pimiento, black pepper, long pepper, cloves, and nutmeg.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sCurvature of field �21m �33s
    The optical aberration which occurs because the best image of a plane
object sometimes lies on a curved surface. An image formed on a flat screen
will then be subject to the error known as curvature of field. If RKAPPA
designates the curvature of the KAPPAth optical surface, the quantity shown in
the equation, where nKAPPA and n*EQN* are the refractive indices before and
after the KAPPAth surface, is called the Petzval curvature or Petzval sum of
the system.

	  R = SIGMAR sub K (1/n' sub K - 1/n sub K)

    When all the image-forming errors are corrected and the field is small,
the Petzval curvature gives the axial curvature of the image of a plane
object. The vanishing of R indicates that the Petzval condition is fulfilled.
    The discovery of the significance of the Petzval sum enabled photographic
lenses with plane fields to be constructed. These lenses were called
anastigmats to distinguish them from aplanats, in which the meridional and
sagittal fields were merely balanced against each other, one field having a
positive curvature and the other a negative of nearly equal size. See
ABERRATION (OPTICS); LENS (OPTICS).
 �34s[Max J. Herzberger]
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�6v�2v�32sCurve fitting �21m �33s
    A procedure in which the basic problem is to pass a curve through a set of
points, representing experimental data, in such a way that the curve shows as
well as possible the relationship between the two quantities plotted. It is
always possible to pass some smooth curve through all the points plotted, but
since there is assumed to be some experimental error present, such a procedure
would ordinarily not be desirable. See INTERPOLATION.
    The first task in curve fitting is to decide (1) how many degrees of
freedom (number of unspecified parameters, or independent variables) should be
allowed in fitting the points, and (2) what the general nature of the curve
should be. Since there is no known way of answering these questions in a
completely objective way, curve fitting remains something of an art. It is
clear, however, that one must make good use of any background knowledge of the
quantities plotted if he is to answer the above two questions. Thus, if one
knew that a discontinuity might occur at some value of the abscissa, one would
try to fit the points above and below that value by separate curves.
    Against this background knowledge of what the curve should be expected to
look like, one may observe the way the points fall on the paper. One may even
find it advantageous to make a few rough attempts to draw a reasonable curve
``through'' the points.
    A knowledge of the accuracy of the data is needed to help answer the
question of the number of degrees of freedom to be permitted in fitting the
data. If the data are very accurate, one may use as many degrees of freedom as
there are points. The curve can then be made to pass through all the points,
and it serves only the function of interpolation. At the opposite extreme when
the data are very rough, one may attempt to fit the data by a straight line
representing a linear relation, Eq. (1), between y and x. Using the above

				y = ax + b				  (1)

information and one's knowledge of the functions that have been found useful in
fitting various types of experimental curves, one selects a suitable function
and tries to determine the parameters left unspecified. At this point there are
certain techniques that have been worked out to choose the optimum value of the
parameters.
    One of the most general methods used for this purpose is the method of
least squares. In this method one chooses the parameters in such a way as to
minimize the sum, Eq. (2), where ysubi is the ordinate of ith point and

S = SUMMATION (i=1 to n) [y(sub i) - f(x[sub i])](sup 2)	      (2)

f(xsubi) the ordinate of the point on the curve having the same abscissa xsubi
as this point. See LEAST-SQUARES METHOD.
 �34s[Kaiser S. Kunz]
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�6v�2v�32sCutaneous sensation   �21m �33s 
    The sensory quality of skin. The skin consists of two main layers, the 
epidermis and the dermis. Sensory receptors in or beneath the skin are 
peripheral nerve-fiber endings that are differentially sensitive to one or 
more forms of energy. The sensory endings can be loosely categorized into 
three morphological groups: endings with expanded tips, such as Merkel's disks 
found at the base of the epidermis; encapsulated endings, such as Meissner's 
corpuscles (particularly plentiful in the dermal papillae), and other organs 
located in the dermis or subcutaneous tissue, such as Ruffini endings, 
Pacinian corpuscles, Golgi-Mazzoni corpuscles, and Krause's end bulb; and bare 
nerve endings that are found in all layers of the skin (some of these nerve 
endings are found near or around the base of hair follicles). 
    There is a remarkable relationship between the response specificities of 
cutaneous receptors and five primary qualities of cutaneous sensation, the 
latter commonly described as touch-pressure (mechanoreceptors), cold and 
warmth (thermoreceptors), pain, and itch. Each quality is served by a specific 
set of cutaneous peripheral nerve fibers. More complex sensations must result 
from an integration within the central nervous system of information from 
these sets of nerve fibers. Exploration of the skin surface with a rounded 
metal point reveals that there exist local sensory spots on the skin, 
stimulation of which evokes only one of the five qualities of sensation. Thus, 
there may be plotted maps of pressure, warm, cold, pain, or itch spots. See 
MECHANORECEPTORS; NOCICEPTORS; PAIN; SKIN; SOMESTHESIS; TEMPERATURE SENSES; 
TOUCH.  
 �34s[Robert LaMotte]
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�6v�2v�32sCyanamide   �21m �33s 
    A term used to refer to the free acid hydrogen cyanamide; H2NCN, or 
commonly to the calcium salt of this acid, calcium cyanamide, CaCN2. Calcium 
cyanamide is manufactured by the cyanamide process, in which nitrogen gas is 
passed through finely divided calcium carbide at a temperature of 1832DGF 
(1000DGC). Most calcium cyanamide is used in agriculture as a fertilizer; some 
is used as a weed killer, as a pesticide, and as a cotton defoliant. 
    Hydrogen cyanamide is prepared from calcium cyanamide by treating the salt 
with acid. It is used in the manufacture of dicyandiamide and thiourea. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sCyanate   �21m �33s 
    A compound containing the --OCN group and derived from cyanic acid, HOCN. 
Cyanates are isomeric with fulminates which have the same atoms arranged --ONC. 
    Cyanic acid may exist in two forms, HO--C TRIPLE BOND N and H--N DOUBLE 
BOND C DOUBLE BOND O, the latter of which is called isocyanic acid. The alkali 
and alkaline-earth metal cyanates contain an anion which may also be written 
in two forms: [N TRIPLE BOND C--O] NEGATIVE and [O DOUBLE BOND C DOUBLE BOND N] 
NEGATIVE. These salts are water-soluble. 
    The heavy metal cyanates are more covalent in nature and are water-
insoluble. The main use of cyanates is in the synthesis of organic compounds.  
    Ammonium cyanate rearranges easily to urea: NH4OCN EQUILIBRIUM NH2CONH2. 
See CYANIDE; THIOCYANATE. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sCyanide   �21m �33s 
    A compound containing the --|CN group, for example, potassium cyanide, 
KCN; calcium cyanide, Ca(CN)2; and hydrocyanic (or prussic) acid, HCN. 
Chemically, the simple inorganic cyanides resemble chlorides in many ways. 
Organic compounds containing this group are called nitriles, for example, 
acrylonitrile, CH2CHCN. See ACRYLONITRILE. 
    HCN is a weak acid. In the pure state, it is a highly volatile liquid, 
boiling at 26DGC. HCN and the cyanides are highly toxic to animals and humans. 
    The cyanide ion forms a variety of coordination complexes with 
transition-metal ions, a property responsible for several of the commercial 
uses of cyanides. The cyanide process T is the most widely used method for 
extracting gold and silver from the ores. In silver-plating, a smooth adherent 
deposit is obtained on a metal cathode when electrolysis is carried out in the 
presence of an excess of cyanide ion. See ELECTROPLATING OF METALS; GOLD 
METALLURGY; SILVER METALLURGY. 
    Ca(CN)2 is extensively used in pest control and as a fumigant in the 
storage of grain. In finely divided form, it reacts slowly with the moisture 
in the air to liberate HCN. 
    In case hardening of metals, an iron or steel article is immersed in a 
bath of molten sodium or potassium cyanide containing sodium chloride or 
carbonate. The cyanide decomposes at the surface, forming a deposit of carbon 
which combines with and penetrates the metal. See COORDINATION CHEMISTRY; 
FERRICYANIDE; FERROCYANIDE. 
 �34s[Francis J. Johnston]
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�6v�2v�32sCyanocarbon �21m �33s
    A derivative of hydrocarbon in which all of the hydrogen atoms are
replaced by the --|C|TRIPLE BOND|N group. However, the term cyanocarbon has
been applied to compounds which do not strictly follow the above definition:
tetracyanoethylene oxide, tetracyanothiophene, tetracyanofuran,
tetracyanopyrrole, tetracyanobenzoquinone, tetracyanoquinodimethane,
tetracyanodithiin, pentacyanopyridine, diazomalononitrile, and
diazotetracyanocyclopentadiene.
    Tetracyanoethylene (see formula), the simplest olefinic cyanocarbon, is a
colorless, thermally stable solid, having a melting range of
Tetracyanoethylene (198 - 200DGC), and readily forming stable complexes with
most aromatic systems. Cyanocarbon acids are among the strongest protonic
organic acids known and are usually isolated only as the cyanocarbon anion
salt.
 �34s[Norbert M Bikales]
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�6v�2v�32sCyanoethylation �21m �33s
    A chemical reaction involving the addition of acrylonitrile (CH2DOUBLE
BAR|CH|--|CN) to compounds carrying a reactive hydrogen, to introduce the
BETA-cyanoethyl grouping (--|CH2CH2CN). The reaction is represented by

	 ZH + CH sub 2 = CH - CN EQUILIBRIUM ZCH sub 2 CH sub 2 CN

    Cyanoethylation can be effected with HBr, HCl, HCN, and with compounds
containing the functional groups
 |     |     |
-AsH  -BH  -CH	 (the last when activated by adjacent electron-withdrawing
		  groups)

 |	   |	     |	   |
-NH  -OH  -PH  -SH  -SiH  -SnH
		     |	   |

    Because of its versatility, cyanoethylation has assumed great importance
in synthesis, as shown by the fact that over 1200 compounds have been used as
reactants. Introduction of the cyanoethyl group tends to increase hydrophobic
character and resistance to biological attack. Natural products, notably
cellulose, have been cyanoethylated to take advantage of these properties.
    The nature of the catalyst is used to classify substrates into three
groups:
(1) No catalyst:

 |     |    |		     |
-AsH  -BH  -NH (aliphatic)  -PH

 |
-SnH  HBr  HCl
 |

(2) Basic catalyst (for example, NaOH, NaOCH3, benzyltrimethylammonium
hydroxide):

 |    | 				    |
-CH  -NH (amide and certain heterocycles)  -OH	-PH
 |						 |

-P(O)H	-SH  HCN

(3) Acid catalyst (for example, acetic acid with or without a copper salt):

 |
-NH (aromatic)	-NH sub 2 (t-carbinamines)

    Yields in cyanoethylation are generally high. The reaction is strongly
exothermic and is usually carried out at moderate temperatures in a solvent
such as dioxane or t-butanol.
    Since the extent of cyanoethylation can be varied by choice of conditions,
a variety of products can be obtained from a given material. These may be
further modified by reactions of the cyanoethyl group, for example, to give
polyelectrolytes.
    Partially cyanoethylated cellulose, in which one-sixth of the hydroxyl
groups have reacted, is rot- and termite-proof; it has improved resistance to
degradation by heat and by acids, as well as altered dyeing characteristics
and greater hydrophobicity. Utility is in marine nets and ropes, electrical
insulation, industrial fabrics, structural wood, and dimensionally stable
paper.
    Cyanoethylation is also used in the preparation of amino acids,
chemotherapeutic agents, dyes, blowing agents, long-chain diamines,
special-purpose silicones, food stabilizers, and liquids to effect separations
by gas chromatography. See ACRYLONITRILE; TEXTILE CHEMISTRY.
 �34s[Norbert M. Bikales]
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�6v�2v�32sCyanogen   �21m �33s 
    A colorless, highly toxic gas having the molecular formula C2N2. 
Structurally, cyanogen is written N|TRIPLE BOND|C|--C|TRIPLE BOND|N. Cyanogen 
belongs to a class of compounds known as pseudohalogens, because of the 
similarity of their chemical behavior to that of the halogens. Liquid cyanogen 
boils at -21.17DGC (-6.11DGF) and freezes at -27.9DGC (-18.2DGF) at 1 atm 
(101.325 kilopascals). 
    Cyanogen reacts with hydrogen at elevated temperatures in a manner 
analogous to the halogens, forming hydrogen cyanide, HCN. With hydrogen 
sulfide, H2S, cyanogen forms thiocyanoformamide or dithiooxamide. Cyanogen 
burns in oxygen, producing one of the hottest flames known from a chemical 
reaction. It is considered to be a promising component of high-energy fuels. 
See CYANIDE. 
 �34s[Francis J. Johnston]
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�6v�2v�32sCyanophyceae   �21m �33s 
    The single class of the division Cyanophycota. Sometimes also known as 
Cyanophyta, Myxophyceae, or Schizophyceae, they are commonly called blue-green 
algae. There are about 1500 species of blue-green algae, which are arranged 
into orders, the Chroococcales, Pleurocapsales, Stigonematales, and Nostocales. 
    Blue-green algae occur free-floating in fresh and salt water and in moist 
subaerial habitats throughout the world. They are more abundant in fresh than 
in salt water, and more abundant near the surface than at depths of more than 
a few feet. Spray-dampened cliffs, dripping ledges, and temporary pools are 
likely to be especially rich in blue-green algae. They are also commonly 
present in and about hot springs, sometimes at temperatures not much below 
boiling.  
    Blue-green algae occur as single cells of microscopic size, as small cell 
colonies of various shapes, and as multicellular filaments which may form a 
tangled mat or a gelatinous ball. The individual cell consists of a 
prokaryotic protoplast surrounded by a cell wall, which in turn is usually 
covered by a gelatinous sheath composed of pectic compounds. The cell wall, 
like that of bacteria, consists chiefly of mucopolymers composed of 
glucosamine, amino acids, and muramic acid. Some genera also have cellulose in 
the cell wall, but the cellulose is not usually organized into the sort of 
fibrils found in other algae. Blue-green algae are completely without flagella 
or other obvious means of locomotion, but some of the filamentous ones move 
nevertheless. The whole filament may move slowly forward and backward, or the 
end of the filament may wave slowly back and forth, as in Oscillatoria. The 
mechanism of these movements is not yet clearly understood. No sexual 
processes are known in any blue-green algae. 
    The blue-green algae resemble the bacteria in the prokaryotic organization 
of their protoplasm. In general, prokaryotic organisms do not have a highly 
organized, vesicular nucleus in the cell, nor do they have complex cytoplasmic 
organelles such as mitochondria, dictyosomes, and membrane-bounded 
chloroplasts. Like other algae, the blue-green algae carry on photosynthesis 
and release oxygen as a by-product. In this respect they differ sharply from 
the bacteria. Those few bacteria which are photosynthetic carry on a more 
primitive type of photosynthesis which does not result in the release of 
oxygen. Phycobilin pigments play an essential (though accessory) role in the 
photosynthesis of blue-green algae. See BACTERIAL PHOTOSYNTHESIS; PHYCOBILIN. 
    All groups of algae are ultimate sources of food for fish and other 
aquatic animals, but the blue-green algae are less important than groups such 
as the diatoms and green algae. Like the eukaryotic photosynthetic organisms, 
the blue-green algae play an integral role in the balance of nature by using 
up carbon dioxide and contributing to the supply of atmospheric oxygen, which 
is necessary for human and other animal life. Some blue-green algae, like some 
bacteria, can also use atmospheric nitrogen in the formation of complex 
organic compounds, thus playing a critical role in the nitrogen cycle. 
Photosynthetic activity of many blue-green algae in water containing dissolved 
bicarbonates tends to cause the deposition of calcium carbonate on and about 
the cells, and this action by blue-green and other algae is believed to have 
been an important factor in the formation of many or most limestone deposits. 
Blue-green algae sometimes become so abundant in reservoirs and lakes as to 
color the water and make it offensive to the smell and taste, especially in 
water that has in effect been fertilized by inflow of detergents or sewage. 
See ALGAE; DIATOM; FOOD CHAIN; LIMESTONE; NITROGEN CYCLE; PHOTOSYNTHESIS; 
PHYTOPLANKTON.  
 �34s[Arthur Cronquist]
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�6v�2v�32sCycadales �21m �33s
    An ancient order of the plant division Pinophyta (gymnosperms), having a
few living representatives (living fossils). These plants are popularly but
incorrectly called palms. The tuberous or columnar stems bear a crown of
large, usually pinnate leaves. Cycas revoluta, which is the best-known cycad,
is called sago palm (see illustration). See PALM.
    The cycads appeared, along with the Bennettitales (Cycadeoidales), in the
Upper Carboniferous and made their greatest display in the Cretaceous. In the
Mesozoic the distribution was worldwide, but the living members of the order
are confined to the tropical and subtropical regions, where they occur in
scanty patches. Today, there is but 1 family with 9 genera and approximately
100 species. See CYCADEOIDALES; PINOPHYTA; PTERIODOSPERMAE.
 �34s[Perry D. Strausbaugh /Arthur Cronquest]
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�6v�2v�32sCycadeoidales   �21m �33s 
    An order of extinct plants that formed a conspicuous part of the landscape 
during the Triassic, Jurassic, and Cretaceous periods. The Cycadeoidales (or 
Bennettitales) had unbranched or sparsely branched trunks with a terminal 
crown of leaves (family Cycadeoidaceae; see illustration), or they were 
branched, at times profusely (family Williamsoniaceae). 
    No known relatives of the Cycadeoidales exist at the present time, 
although members of the order Cycadales show some resemblances to the extinct 
group. See CYCADALES; EMBRYOBIONTA. 
 �34s[Theodore Delevoryas]
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�6v�2v�32sCycadicae   �21m �33s 
    A subdivision of the plant division Pinophyta (gymnosperms), consisting of 
three classes, the Lyginopteridopsida (pteridosperms or seed ferns), 
Cycadopsida (cycads), and Bennettitopsida (cycadeoids). Only the Cycadopsida, 
with the single small order Cycadales, has any living species; the rest are 
represented by fossils chiefy from the Mesozoic and late Paleozoic eras. See 
PINOPHYTA.  
    The Cycadicae are gymnosperms with relatively large leaves that are often 
pinnately compound. The stems are stout and usually unbranched or with only a 
few relatively large branches. The seeds are borne on ordinary leaves (in many 
of the seed ferns) or on more or less strongly modified leaves (called 
megasporophylls) that are often aggregated into a simple strobilus (cone) at 
the end of the stem. See CYCADALES; CYCADEOIDALES; PTERIDOSPERMAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sCyclanthales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), 
sometimes called Synanthae or Synanthales, in the subclass Arecidae of the 
class Liliopsida (monocotyledons). The order consists of the single tropical 
American family Cyclanthaceae, with about 180 species. They are herbs or, 
seldom, more or less woody plants, with characteristic leaves that have a 
basal sheath, a petiole, and an expanded, usually bifid (cleft into two equal 
parts) blade which is often folded lengthwise (plicate) like that of a palm 
leaf. One of the species, Carludovicia palmata, is a principal source of fiber 
for panama hats. See ARECIDAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sCyclic adenylic acid   �21m �33s 
    A nucleotide, also known as adenosine 3',5'-monophosphate or cyclic AMP, 
with the structure shown. Widely distributed in nature, cyclic AMP may be 
regarded as a versatile regulatory agent which acts to control the rate of a 
number of cellular processes (see table). It has become especially important 
in endocrinology because of the evidence that many hormones and neurohormones 
produce a number of their effects by changing the concentration of cyclic AMP 
within the cells of their target tissues. 
    Cyclic AMP and the enzymes that produce and destroy it have been found in 
all nucleated animal cells that have been studied, as well as in bacteria and 
other unicellular organisms. The regulatory function of cyclic AMP seems to 
have emerged at a very early stage in the evolutionary process. However, 
cyclic AMP appears not to exist in green plants. A second cyclic nucleotide, 
cyclic guanylic acid (cyclic GMP), has been found to exist in most animal 
cells, but less is known about it than about cyclic AMP. Evidence suggests 
that both nucleotides may play important roles during growth and development 
and may also be involved in regulating the immune response. 
    Defects in the formation or action of cyclic AMP have been implicated in 
the etiology and pathogenesis of a number of disease processes, as for 
example, bronchial asthma and certain other allergic disorders. It is also 
known that abnormally high levels of cyclic AMP may be dangerous. For example, 
many of the metabolic abnormalities seen in diabetic patients are the result 
of an excess of glucagon over insulin, leading to excessive cyclic AMP 
formation in the liver and adipose tissue. See ADENYLIC ACID; CYCLIC 
NUCLEOTIDES; HORMONE. 
 �34s[G. Alan Robison]
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�6v�2v�32sCyclic nucleotides �21m �33s
    Mononucleotides in which the phosphate moiety is attached to both the
third and the fifth carbon atoms of the ribose component of the molecule, thus
forming a cyclic diester. They are more correctly referred to as
3',5'-mononucleotides, but usage of the adjective cyclic is very common. The
only cyclic nucleotides which have been unequivocally shown to exist in living
tissues are 3',5'-adenylic acid and 3',5'-guanylic acid, also known as cyclic
AMP and cyclic GMP, respectively. They are important because they influence a
large variety of essential biological processes, and sometimes the two
nucleotides may act in opposition to each other. A third cyclic nucleotide,
3',5'-cytidylic acid (cyclic CMP), has been reported to exist in some cells,
but its role is poorly understood. See ADENYLIC ACID; CYCLIC ADENYLIC ACID;
NUCLEIC ACID.
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�6v�2v�32sCyclocystoidea   �21m �33s 
    A class of small, disk-shaped echinozoans in which the lower surface of 
the body probably consisted of a suction cap for adhering to substrate, and 
the upper surface was covered by separate plates arranged in concentric rings. 
The mouth lay at the center of the upper surface, and the anus, also on the 
upper surface, lay some distance between the margin and the mouth. Little is 
known of the habits of cyclocystoids. They occur in rocks of middle Ordovician 
to middle Devonian age in Europe and North America. See ECHINODERMATA; 
ECHINOZOA.  
 �34s[Howard B. Fell]
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�6v�2v�32sCycloid   �21m �33s 
    A curve traced in the plane by a point on a circle that rolls, without 
slipping, on a line. If the line is the x axis of a rectangular coordinate 
system, at whose origin O the moving point P touches the axis, parametric 
equations of the cycloid are x = a(;Gu - sin ;Gu), y = a(1 - cos ;Gu), when a 
is the radius of the rolling circle, and the parameter ;Gu is the variable 
angle through which the circle rolls (see illustration). See ANALYTIC GEOMETRY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sCyclone �21m �33s
    An atmospheric circulation system in which the sense of rotation of the
wind about the local vertical is the same as that of the Earth's rotation.
Thus, a cyclone rotates clockwise in the Southern Hemisphere and
counterclockwise in the Northern Hemisphere. In meteorology the term cyclone
is reserved for circulation systems with horizontal dimensions of hundreds
(tropical cyclones) or thousands (extratropical cyclones) of kilometers. For
such systems the Coriolis force due to the Earth's rotation, which is directed
to the right of the flow in the Northern Hemisphere, and the pressure gradient
force, which is directed toward low pressure, are in opposite directions.
Thus, there must be a pressure minimum at the center of the cyclone, and
cyclones are sometimes simply called lows. See AIR PRESSURE; CORIOLIS
ACCELERATION AND FORCE..
    Extratropical cyclones are the common weather disturbances which travel
around the world from west to east in mid-latitudes. They are generally
associated with fronts, which are zones of rapid transition in temperature.
Extratropical cyclones arise due to the hydrodynamic instability of the
upper-level jet stream flow. See FRONT; JET STREAM.
    Tropical cyclones, by contrast, derive their energy from the release of
latent heat of condensation in precipitating cumulus clouds. Over the tropical
oceans, where moisture is plentiful, tropical cyclones can develop into
intense vortical storms (hurricanes and typhoons), which can have wind speeds
in excess of 200 mi/h (100m . s- sup 1). See HURRICANE; PRECIPITATION
(METEOROLOGY); STORM; WIND.
 �34s[James R. Holton]
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�6v�2v�32sCyclone furnace   �21m �33s 
    A water-cooled horizontal cylinder in which fuel (coal, gas, or oil) is 
fired and heat is released at extremely high rates. When firing coal, the 
crushed coal is introduced tangentially into the burner at the front end of 
the cyclone (see illustration). About 15% of the combustion air is used as 
primary and tertiary air to impart a whirling motion to the particles of coal. 
The whirling, or centrifugal, action on the fuel is further increased by the 
tangential admission of high-velocity secondary air into the cyclone. 
  
    The products of combustion are discharged through a water-cooled reentrant 
throat at the rear of the cyclone into the boiler furnace. Essentially, the 
fundamental difference between cyclone furnaces and pulverized coal - fired 
furnaces is the manner in which combustion takes place. In pulverized coal - 
fired furnaces, particles of coal move along with the gas stream; 
consequently, relatively large furnaces are required to complete the 
combustion of the suspended fuel. With cyclonic firing, the coal is held in 
the cyclone and the air is passed over the fuel. Thus, large quantities of 
fuel can be fired and combustion completed in a relatively small volume, and 
the boiler furnace is used to cool the products of combustion. See BOILER; 
STEAM-GENERATING FURNACE; STEAM-GENERATING UNIT. 
 �34s[George W. Kessler]
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�6v�2v�32sCyclophyllidea   �21m �33s 
    The order which includes most tapeworms that inhabit the gut of 
warm-blooded vertebrates. They are frequently referred to as the Taenioidea. 
Each worm has a head, or scolex, and a segmented body, called the strobila. 
The scolex typically has an apical rostellum, or muscular pad and hooks, and 
two pairs of lateral suckers. New segments are produced immediately posterior 
to the scolex, so that the oldest segment is at the hind end. As a segment is 
pushed further from the scolex, the male and female reproductive organs mature 
and open on the lateral margin. Ova, fertilized by sperm from the same or 
another segment, gather in a uterus. The ripe terminal segments containing 
infectious eggs are shed into gut contents of the host. 
    A number of tapeworms occur in humans and domestic animals, either as 
adults or immature stages, the metacestodes, but rarely in both stages. See 
COENUROSIS; CYSTICERCOSIS; ECHINOCOCCOSIS; PLATYHELMINTHES; TAPEWORM DISEASE. 
 �34s[Reino S. Freeman]
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�6v�2v�32sCyclopoida   �21m �33s 
    An order of small copepod Crustacea. The abundant fresh-water and 
salt-water species form an important link in the food chains of aquatic life, 
consuming tiny plants, animals, and detritus and, in turn, furnishing food for 
small fish, some large fish, and other organisms. Some species are important 
intermediate hosts for human parasites. 
    The front part of the body is oval and sharply separated from the tubular 
hind end, which bears two caudal rami with distinctly unequal setae (see 
illustration). Usually 10 body segments are present in the male, and 9 in the 
female because of fusion of two to form a genital segment. 
  
    Cyclopoids are intermediate hosts of the parasitic guinea worm 
(Dracunculus medinensis), and sometimes the fish tapeworm (Dibothriocephalus 
latus). Most fresh-water species live in shallow water, swimming from plant to 
plant, but salt-water species are generally water-treaders. Food is not 
secured by filtration, but is seized and eaten with the biting mouthparts. 
Many species have a world wide distribution. See COPEPODA. 
 �34s[Harry C. Yeatman]
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�6v�2v�32sCycloserine   �21m �33s 
    A broad-spectrum antibiotic produced during the growth of certain species 
of Streptomyces. It is effective in humans in the treatment of tuberculosis, 
including cases resistant to other drugs such as streptomycin, isoniazid, and 
p-aminosalicylic acid. It is also effective in urinary tract infections caused 
by gram-negative bacteria which are often resistant to other antibiotics. See 
PARA-AMINOSALICYLIC ACID; STREPTOMYCIN; TUBERCULOSIS. 
 �34s[Roger L. Harned]
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�6v�2v�32sCyclostomata (Bryozoa)   �21m �33s 
    An order of bryozoans in the class Stenolaemata. Cyclostomes tend to have 
delicate colonies composed of relatively isolated, loosely bundled, or tightly 
packed, comparatively simple, long, slender, and tubular zooecia, with thin, 
highly porous, calcareous walls. Cyclostome colonies most commonly are small 
and delicate, but some are moderately large and massive. They may be 
inconspicuous encrusting threadlike networks, thin encrusting sheets, nodular 
masses, or erect tuftlike, twiglike, or frondlike growths. Individual 
cyclostome zooecia are straight to slightly curved long tubes. The walls of 
adjacent zooecia may be either distinctly separate or fused together. 
    Exclusively marine, cyclostomes (occasionally termed Stenostomata) are 
known first from the Late Ordovician, when they may possibly have evolved from 
cystoporates (which were formerly included in the Cyclostomta). Rare and 
inconspicuous throughout the rest of the Paleozoic and Triassic, cyclostomes 
became moderately common in Jurassic and Cretaceous seas, but have since 
(including today) been less numerous. See BRYOZOA; CYSTOPORATA; STENOLAEMATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sCyclostomata (Chordata)   �21m �33s 
    The simplest and most primitive of living vertebrates. Cyclostomes, also 
known as Cyclostomi and Marsipobranchii, are the modern representatives of the 
class Agnatha. The Cyclostomata constitute a subclass that is descended at 
least in part (lampreys - Petromyzonida or Petromyzontiformes) from the 
subclass Monorhina (Cephalaspidomorphi). See MONORHINA. 
    Cyclostomes differ notably from all other Recent fishes. The absence of 
jaws and the presence of a single median nostril are distinctive. In addition, 
they have an uncalcified cartilaginous skeleton and lack true teeth and paired 
fins and girdles. 
    Cyclostomes are eellike in form and have tough, more or less slimy skin. 
They are oviparous and may be either bisexual or hermaphroditic. Development 
may involve a protracted larval stage (ammocoetes) followed by profound 
structural change at metamorphosis, as in lampreys, or it may be direct, as in 
hagfishes. See AGNATHA; MYXINOIDEA; PETROMYZONIDA. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sCyclothem   �21m �33s 
    A specific sequence of different kinds of rocks, one of which is generally 
coal. These sequences, each on the order of tens of feet in thickness, occur 
repeatedly, one above the other, in coal-bearing strata. Cyclothems have been 
recognized mainly in rocks of late Paleozoic age in the eastern United States 
but also have been described in late Mesozoic coal-bearing strata of the Rocky 
Mountain region. 
  
    Because there has been no real agreement about factual knowledge 
concerning cyclothems, there is even less about their origin. Some believe 
that the eroded surface represents an episode of uplift of the land, and the 
succeeding deposits a period of subsidence, resulting first in nonmarine 
deposition and then in marine deposition. Others believe that the land surface 
may have remained stable but that the sea level rose and fell in response to 
the growth and melting of continental ice sheets in the Southern Hemisphere. 
Some support for this notion is found in the late Paleozoic glacial deposits 
recognized in portions of South Africa and Australia. At the time when 
cyclothems were being formed, these now isolated areas are believed to have 
been joined in a single continent designated Gondwana. See SEDIMENTATION 
(GEOLOGY); STRATIGRAPHY. 
 �34s[John C. Ferm]
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�6v�2v�32sCydippidea   �21m �33s 
    An order of the Ctenophora which retains the cydippid state (resembles the 
cydippid larva) until the adult stage is reached in development. They are 
globular, pyriform, or cylindrical. They show a nearly round appearance in 
cross section across the main axis. They are often oblong, being elongate 
along the tentacular axis. The primary tentacles are well developed and have 
lateral branches. Representative genera are Hormiphora, Pleurobrachia, 
Callianira, Lampetia, and Euchlora. See CTENOPHORA; TENTACULATA. 
 �34s[Taku Komai]
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�6v�2v�32sCygnus   �21m �33s 
    The Swan, in astronomy, is a conspicuous northern summer constellation. 
The five major stars of the group, ALPHA, GAMMA, BETA, EPSILON, and DELTA, are 
arranged in the form of a cross (see illustration). Hence Cygnus is often 
called the Northern Cross, to distinguish it from the Southern Cross of the 
constellation Crux. The constellation is represented by a swan with widespread 
wings flying southward. The bright star Deneb is the tail of the swan; it lies 
at the head of the cross. Albireo, a beautiful double star of contrasting 
orange and blue colors, is the head of the swan. The whole constellation lies 
in, and parallel to, the path of the Milky Way. See CONSTELLATION; CRUX. 
 �34s[Ching-Sung Yu]
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�6v�2v�32sCylinder   �21m �33s 
    The solid of revolution obtained by revolving a rectangle about a side is 
called a cylinder, or more precisely a right circular cylinder. More generally 
the word cylinder is used in solid geometry to describe any solid bounded by 
two parallel planes and a portion of a closed cylindrical surface. In analytic 
geometry, however, the word cylinder refers not to a solid but to a 
cylindrical surface (see illustration). This is a surface generated by a 
straight line which moves so that it always intersects a given plane curve 
called the directrix, and remains parallel to a fixed line that intersects the 
plane of the directrix. Cylinders whose right sections are circles, ellipses, 
parabolas, or hyperbolas are called circular cylinders, elliptic cylinders, 
parabolic cylinders, or hyperbolic cylinders, respectively. All these are 
quadric cylinders. See EUCLIDEAN GEOMETRY. 
 �34s[James S. Frame]
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�6v�2v�32sCyperales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Commelinidae of the class Liliopsida (monocotyledons). The names 
Gluminflorae, Graminales, and Poales have also been used for this order. There 
are only two families, the Gramineae (Poaceae) with about 8000 species, and 
the Cyperaceae, with nearly 4000. The Cyperales are Commelinidae with reduced, 
mostly wind-pollinated or self-pollinated flowers that have a unilocular, two- 
or three-carpellate ovary bearing a single ovule. The flowers are arranged in 
characteristic spikes or spikelets representing reduced inflorescences. The 
perianth is represented only by a set of bristles or tiny scales, or is 
completely missing. See COMMELINIDAE. 
    The Cyperaceae, or sedge family, includes the bulrushes (Scirpus) and the 
papyrus (Cyperus papyrus) of Egypt, as well as the sedges (Carex). The 
Gramineae, or grass family, embraces all true grasses, including bamboos and 
the cereal grains such as wheat, maize, oats, and rye. The two families differ 
in a number of more or less consistent technical characters of the 
inflorescence, fruits, stems, and leaves. See BAMBOO; BARLEY; CEREAL; CORN; 
FLOWER; GRASS CROPS; MILLET; OATS; RICE; RYE; SORGHUM; SUGARCANE; TIMOTHY; 
WHEAT.  
 �34s[Arthur Cronquist]
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�6v�2v�32sCypress �21m �33s
    The true cypress (Cupressus), which is very close botanically to the
cedars (Chamaecyparis). All of the species of Cupressus in the United States
are western and are found from Oregon to Mexico. The Arizona cypress
(Cupressus arizonica) of the southwestern United States and the Monterey
cypress (Cupressus macrocarpa) of California are medium-sized trees and are
chiefly of ornamental value. The Italian cypress (Cupressus sempervirens) and
its varieties are handsome ornamentals, but usually do well only in the
southern parts of the United States. Other trees are also called cypress, such
as the Port Orford cedar (Chamaecyparis lawsoniana) known also as the Lawson
cypress, and the Alaska cedar (Chamaecyparis nootkatensis), known also as the
Nootka cypress or cedar.
    The bald cypress (Taxodium distichum) is an entirely different tree that
is found in the swamps of the South Atlantic and Gulf coastal plains and in
the lower Mississippi Valley. The soft needlelike leaves and short branches
are deciduous; hence, they drop off in winter and give the tree its common
name. Also known as the southern or red cypress, this tree yields a valuable
decay-resistant wood used principally for building construction, especially
for exposed parts or where a high degree of resistance to decay is required as
in ships, boats, greenhouses, railway cars, and railroad ties. See  CEDAR;
PINALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sCypriniformes �21m �33s
    An order of actinopterygian fishes which ranks second in size only to the
Perciformes. The order Cypriniformes (or Eventognathi) includes the characins,
minnows, and their allies. The Weberian apparatus of the Cypriniformes
involves only the first four vertebrae. The body is typically invested in
cycloid scales, although these are sometimes lost. In further contrast to
catfishes, the Cypriniformes have parietal, suboperculum, symplectic, and
intercalar bones; they lack a pectoral spine (present in most catfishes); and
they have intermuscular bones.
    The 8 families of the Cypriniformes, numbering about 3000 species, may be
arranged in 3 suborders: Characoidei, Gymnotoidei, and Cyprinoidei. The
suborder Characoidei includes the single family Characidae (split by some
authors into several smaller families) or characins. They are primarily
fresh-water fishes that abound in South America, where they have undergone an
extensive adaptive radiation resulting in perhaps 850 species. The Gymnotoidei
includes a single family, Gymnotidae, consisting of about 16 genera and perhaps
40 or 45 species that inhabit fresh waters of South and Central America. They
are eel-shaped fishes that seem to be capable of producing an electric shock.
The electric eel is the largest species. Cyprinoidei includes about 6 families
of primarily fresh-water fishes. The Cyprinidae, including minnows and carps,
are the largest family of fishes, with about 275 genera and more than 1500
species. The center of abundance and diversity of form of this group is in
southeastern Asia. The family is well represented in Africa, Europe, Asia, and
North America (about 230 species), but there are none in Central or South
America, Madagascar, Australia, or in that part of the East Indies lying east of
Lombok.
    In the mountainous regions of southeastern Asia are found 3 other small
families of the Cyprinoidei: the Homalopteridae, the Gastromyzontidae, with 16
genera and the Gyrinocheilidae. All are adapted to life in torrential streams.
See ACTINOPERYGII; CARP; CATFISH; EEL; MINNOW; OSTARIOPHYSI; SILURIFORMES.
[Reeve M. Bailey]

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�6v�2v�32sCyrtosoma   �21m �33s 
    A subphylum of the Mollusca comprising members of the classes 
Monoplacophora, Gastropoda, and Cephalopoda. Each class has, in turn, been the 
most diverse molluscan group. In the Ordovician the cephalopods succeeded the 
monoplacophorans as the most diverse molluscan class, and they held this 
position until the close of the Mesozoic. They were followed by the 
gastropods, which by then had diversified into a great variety of aquatic and 
terrestrial habitats. 
    Primitive members of the Cyrtosoma have, or had, a conical univalved 
shell, often twisted into a spiral. Although primitive cyrtosomes, including 
the only living shelled cephalopod, Nautilus, are shell-bearing animals, many 
advanced cyrtosomes have either lost or greatly reduced their shells, 
(examples include slugs, nudibranchs, squids, and octopuses). Consequently, it 
is not any particular shell form which unites all members of the Cyrtosoma; 
rather, it is a knowledge of their evolutionary history. See CEPHALOPODA; 
DIASOMA; GASTROPODA; MOLLUSCA; MONOPLACOPHORA. 
 �34s[Bruce Runnegar]
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�6v�2v�32sCysteine   �21m �33s 
    An amino acid with the formula shown. In microorganisms, cysteine probably 
is formed from serine and inorganic sulfate or sulfide. In animals, cysteine 
is formed by transsulfuration of serine with the homocysteine from dietary 
methionine. A variety of metabolic degradation pathways are known, of which 
the principal ones are the following: (1) degradation to pyruvate, hydrogen 
sulfide (H2S), and ammonia (NH2); (2) oxidation to cysteic acid and 
decarboxylation of this compound to taurine; (3) transamination (as 
cysteinesulfinic acid) with ALPHA-ketoglutarate to form BETA-sulfinylpyruvate, 
which breaks down spontaneously to pyruvate and sulfite. Cysteine furnishes 
the sulfhydryl group of coenzyme A and, in microorganisms, of homocysteine, 
the precursor of methionine. It is a constituent of the tripeptide coenzyme, 
glutathione, and of the amino acid cystine. See AMINO ACIDS; CYSTINE; 
METHIONINE; TRANSAMINATION. 
 �34s[Edward A. Adelberg]
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�6v�2v�32sCysticercosis   �21m �33s 
    The infestation by cysticerci of the genus Taenia. The cysticerci are 
rarely larger than a pea, and damage usually results from mechanical pressure 
on a variety of delicate tissues. The intermediate host becomes infested with 
eggs from the feces of the definitive host, which usually is a carnivore. 
    Cysticercus cellulosae is the old name for the metacestode of T. solium, 
the pork tapeworm. Humans acquire the adult worm by eating inadequately cooked 
pork containing the cysticerci. Not only do humans harbor the adult worm, but 
the much more dangerous cysticercus can develop in humans as well. The 
cysticercus has a predilection for nervous tissue, frequently in the brain or 
eye, causing serious pathology. Anthelmintics can eliminate the tapeworm, but 
surgical removal is the only known cure for cysticercosis. See CYCLOPHYLLIDEA; 
TAPEWORM DISEASE. 
 �34s[Reino S. Freeman]
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�6v�2v�32sCystine   �21m �33s 
    An amino acid with the formula shown. Cystine can be reversibly reduced to 
cysteine. It is formed biosynthetically from cysteine, by oxidation. In 
addition to the metabolic degradation pathway of cysteine, cystine may also be 
oxidized directly to cystine disulfoxide, which is decarboxylated and then 
further oxidized to taurine. See CYSTEINE. 
 �34s[Edward A. Adelberg]
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�6v�2v�32sCystitis   �21m �33s 
    Inflammation of a fluid-filled organ. As employed in clinical medicine, 
cystitis is assumed to refer to inflammation of the urinary bladder. When 
other viscera are affected, they are identified more accurately, for example, 
cholecystitis being inflammation of a gallbladder. Principal symptoms of 
urinary cystitis are pain and frequency of urination. The usual cause is 
bacterial infection, particularly by the coliform group of microorganisms. 
Cystitis often occurs in association with infection of the kidneys. 
 �34s[Paul B. Beeson]
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�6v�2v�32sCystoidea   �21m �33s 
    The Cystoidea were previously recognized as a formal class of extinct 
crinozoan echinoderms, but are now subdivided into two distinct classes: 
Rhombifera and Diploporita. The term cystoid is now used only in an informal 
sense to designate either of the two classes. See DIPLOPORITA; ECHINODERMATA; 
RHOMBIFERA.  
 �34s[Bruce N. Haugh]
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�6v�2v�32sCystoporata   �21m �33s 
    An extinct order of bryozoans in the class Stenolaemata. Cystoporates tend 
to have robust colonies composed of relatively simple, long, slender, and 
tubular zooecia, separated by blisterlike vesicles stacked upon one another. 
See BRYOZOA; STENOLAEMATA. 
    Cystoporate colonies range from small and delicate to large and massive; 
they can be thin encrusting sheets, tubular or nodular masses, or erect 
twiglike or frondlike growths. 
    The cystoporates may possibly have evolved from the earliest ctenostomes. 
Cystoporates first appeared late in the Early Ordovician, somewhat earlier 
than representatives of other stenolaemate orders; thus, the cystoporates may 
possibly have in turn given rise to those orders, although all these forms may 
simply share a common ancestor further back in time. Apparently marine, the 
cystoporates became moderately common by Mid-Ordovician time - contributing to 
building small reefs, as well as to level-bottom communities - and remained so 
until they died out in the latest Permian. See BRYOZOA; STENOLAEMATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sCytochalasin   �21m �33s 
    A class of lipophilic antibiotics produced by fungi. The cytochalasins 
elicit in animal and plant cells a puzzling diversity of membrane phenomena. 
There is evidence that numerous chemicals, including cytochalasins, interact 
directly with plasma membrane components, modulate activity of membrane-bound 
enzymes, and often produce changes in membrane structure. 
    In animals, cytochalasins inhibit cytokinesis, cell movement, and 
embryonic morphogenesis, as well as intracellular movement such as the 
transport of melanin granules. In addition, nuclear extrusion is induced; 
lymphocyte-mediated destruction of target cells is inhibited; and there is 
selective ``pulverization'' of certain chromosomes, which are converted to the 
unraveled, interphase form, while the other chromosomes in the cell remain in 
the condensed, metaphase form. 
    In plants, intracellular movements such as cytoplasmic streaming and 
chloroplast movements are inhibited. Cytochalasins also inhibit root growth 
and water uptake in onion seedlings; cells become more spherical in shape. 
    The great value of the cytochalasins as research tools is that they appear 
to achieve their reversible impact on cell behavior with a minimum of 
undesirable side effects such as inhibition of respiration or protein 
synthesis. Cytochalasin is extensively applied as a chemical ``scalpel'' to 
enucleate mammalian cells rapidly, precisely, and efficiently in studies of 
nuclear-cytoplasmic relations and in cell hybridization and nuclear transplant 
work. Another major application is in examining the consequences of arrested 
cytoplasmic movement. 
 �34s[Donovan Des S. Thomas]
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�6v�2v�32sCytochemistry   �21m �33s 
    A science concerned with the chemistry of cells and cell components, 
primarily with the location in cells of chemical constituents and enzymes. 
Cytochemical investigations fall into two main categories, visualization and 
biochemical analysis. 
    Visualizing methods involve the application of various microscopic or 
staining techniques to visualize and locate chemical constituents within 
cells. The techniques of tissue preparation and staining for this purpose are 
closely related to those used for histochemistry. In cytochemistry they must 
be selected and used in such a way as to achieve the greatest possible 
accuracy of localization, particularly when the investigator wishes to take 
advantage of the great resolving power of the electron microscope. See 
HISTOCHEMISTRY.  
    Biochemical methods make use of techniques whereby the outer membranes of 
cells in a tissue sample are mechanically ruptured, followed by centrifuging 
to separate the various cell constituents so released into different 
fractions. See CENTRI-FUGATION (BIOLOGY). 
 �34s[Stanley J. Holt]
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�6v�2v�32sCytochrome   �21m �33s 
    A complex protein occurring within the cells of a wide variety of animals 
and plants. An integral part (prosthetic group) of a cytochrome is a heme 
(iron tetrapyrrole) moiety. The heme imparts a color, generally red, to the 
cytochrome, as well as characteristic absorption bands which are used for the 
spectroscopic observation and identification of these intracellular 
hemoproteins. The iron atom of the heme may be reduced and oxidized by 
appropriate substances in the cell, thus constituting the function of the 
cytochromes as electron carriers and as essential components of biological 
oxidation systems. See BIOLOGICAL OXIDATION; MITOCHONDRIA. 
 �34s[Elmer H. Stotz]
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�6v�2v�32sCytokinesis   �21m �33s 
    The partitioning of cells, following nuclear division, or karyokinesis. As 
a result, each nucleus is contained in a portion of cytoplasm and is isolated 
from other nuclei by a cell membrane. Where cytokinesis does not take place 
following nuclear division, a syncytium results, that is, a cytoplasmic 
continuum containing more than one nucleus and correspondingly larger than the 
parent cell. Syncytial organisms and tissues are fairly common, but 
cytokinesis may be considered a normal terminal step in cell reproduction. 
Syncytia may vary in form from shapeless plasmodial masses, such as are seen 
in slime molds, to highly elongated cells containing a chain of nuclei, such 
as are found sometimes among the bacteria. See CELL DIVISION; MITOSIS. 
    The timing of cytokinesis with respect to the mitotic cycle is variable; 
the one rule that can be made is that it begins only after anaphase movement 
is completed. It may coincide with telophase changes in the chromosomes or may 
come later. The extreme case is that in which cytokinesis is delayed while 
many nuclear divisions proceed. The syncytium so produced may later be 
partitioned off into cells, usually providing a cell per nucleus. This delayed 
cytokinesis is observed in the growth of plant endosperm and in spore 
formation and gamete formation in many lower organisms. 
 �34s[Daniel Mazia]
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�6v�2v�32sCytokinins   �21m �33s 
    Plant hormones (phytohormones) that act together with other phytohormones, 
such as auxins, in regulating virtually every aspect of plant growth and 
development. Naturally occurring and synthetic compounds are operationally 
defined as cytokinins if they promote growth and cell division (hence the name 
cytokinin, from cytokinesis) of excised tissues from plants of tobacco, 
soybean, or carrot when these substances are added to a nutrient medium 
containing auxin, vitamins, mineral salts, and sucrose. Compounds active in 
these tissue culture bioassays have other effects on plants as well. See 
AUXIN; BIOASSAY. 
    One of the most important uses of cytokinins for both practical and 
experimental purposes involves its ability to regulate development and organ 
formation in tissue cultures. With tobacco callus cultures, for example, the 
formation of roots or shoots depends on the relative concentrations of auxin 
and cytokinin added to the medium. Practically speaking, this cytokinin effect 
enables agriculturalists to produce large numbers of genetically identical 
plants using tissue cultures. See TISSUE CULTURE. 
    Not only are cytokinins involved in the normal processes of plant 
development, but they also seem to be important as a cause of abnormal growth 
associated with certain plant diseases. Cytokinin effects also include 
promotion of seed germination, breaking of dormancy of some buds, stimulation 
of the mobilization of various metabolites to the point at which cytokinin has 
been applied, retardation of senescence, increased anthocyanin and flavanoid 
synthesis, and stimulation of the opening of stomates. See CROWN GALL; 
DORMANCY; PLANT HORMONES; SEED GERMINATION. 
 �34s[John Einset]
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�6v�2v�32sCytomegalovirus infection   �21m �33s 
    A viral illness also known as cytomegalic inclusion disease. The virus, a 
member of the herpesvirus group, produces enlargement of cells in the affected 
organs or in tissue cultures, as well as large, intranuclear inclusion bodies 
and sometimes intracytoplasmic inclusions. 
    In young infants clinical cases are characterized by jaundice, enlargement 
of liver and spleen, and blood and circulatory disturbances. Cytomegalovirus 
infection appears in its most severe, and sometimes fatal, form in infants 
under 2 years of age, who became infected before birth or when newborn. In the 
congenital infection the virus may persist even in the presence of antibody, 
an unusual situation similar to that found in congenital rubella infection. 
Mild or inapparent infection is common during late childhood and adolescence; 
it is recognized by the detection of the virus, which may be excreted in the 
urine for months. The virus may remain hidden for years, but its reappearance 
is brought on by immunosuppressive drugs, such as those given to patients 
receiving organ transplants. See ANTIBODY; TRANSPLANTATION BIOLOGY. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sCytoplasm   �21m �33s 
    That portion of living cells bordered externally by the plasma membrane 
(cell membrane) and internally by the nuclear envelope. In the terminology of 
classical cytology, the substance in living cells and in living organisms not 
compartmentalized into cells was called protoplasm. It was assumed at the time 
that the protoplasm of various cells was similar in structure and chemistry. 
Results of research on cell chemistry and ultrastructure after about 1960 
showed that each cell type had a recognizably different ``protoplasm.'' 
Primarily for that reason, the term protoplasm gradually fell into disuse in 
contemporary biology. The terms cytoplasm and nucleoplasm have been retained 
and are used descriptively; they are used almost synonymously with the terms 
cytosome (body of cytoplasm) and nucleus, respectively. 
    Many cells, especially the single-celled organisms or protistans, have 
regional cytoplasmic differentiation. The outer region is the cortex or 
ectoplasm, and the inner region is the endoplasm. In many cases the cortical 
layer is a gel made up of a meshwork of cytoskeletal fibers. 
    Cytoplasm contains mostly water, from 80 to 97% in different cells, except 
for spores and other inactive forms of living material, in which water may be 
present in lesser amounts. The dry mass of cells consists mainly of 
macromolecules: proteins, carbohydrates, nucleic acids, and lipids associated 
with membranes. The small molecules present in cells are mainly metabolites or 
metabolic intermediates. The principal ions other than the hydrogen and 
hydroxyl ions of water are the cations of potassium, sodium, magnesium, and 
calcium, and the anions chloride and bicarbonate. Many other elements are 
present in cytoplasm in smaller amounts. Iron is found in cytochrome pigments 
in mitochondria; magnesium is present in chlorophyll in chloroplasts; copper, 
zinc, iodine, bromine, and several other elements are present in trace 
quantities. See CYTOCHEMISTRY. 
    Sedimentation of cells by centrifugation shows that organelles and 
inclusions can be separated from the ground cytoplasm, the fluid phase of the 
cytoplasm in which they are suspended. The ground cytoplasm in turn has been 
shown to consist of a cytoskeletal network and the cytosol, the fluid in which 
the cytoskeleton is bathed. The cytoskeleton consists of several biopolymers 
of wide distribution in cells. Microtubules have been observed in electron 
micrographs of a vast number of different cell types. They consist of the 
protein tubulin, and are frequently covered by a fuzzy layer of 
microtubule-associated proteins. See CYTOSKELETON; MICROTUBULES. 
    In most cells the smaller particles exhibit Brownian motion due to thermal 
agitation. In some cells lacking extensive cytoskeletal structure, particles 
can be moved freely around the cell by Brownian motion. In others they are 
restricted by their surrounding cytoskeletal elements. Particles of various 
types may also undergo saltatory motions which carry them farther than 
Brownian motion possibly could. Such excursions result from the interaction of 
a particle with an element of the cytoskeleton such as one or more 
microtubules or microfilaments. See CELL (BIOLOGY); CELL MOVEMENTS. 
 �34s[Robert D. Allen]
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�6v�2v�32sCytoskeleton   �21m �33s 
    A system of filaments found in the cytoplasm of cells and responsible for 
the maintenance of and changes in cell shape, cell locomotion, movement of 
various elements in the cytoplasm, integration of the major cytoplasmic 
organelles, cell division, chromosome organization and movement, and the 
adhesion of a cell to a surface or to other cells. 
    Three major classes of filaments have been resolved on the basis of their 
diameter and cytoplasmic distribution: actin filaments (or microfilaments) 
each with an average diameter of 6 nanometers, microtubules with an average 
diameter of 25 nm, and intermediate filaments whose diameter of 10 nm is 
intermediate to that of the other two classes. The presence of this system of 
filaments in all cells, as well as their diversity in structure and 
cytoplasmic distribution, has been recognized only in the modern period of 
biology.  
    A technique that has greatly facilitated the visualization of these 
filaments, as well as the analysis of their chemical composition, is 
immunofluorescence applied to cells grown in tissue culture. See 
IMMUNOFLUORESCENCE.  
    Actin is the main structural component of actin filaments in all cell 
types, both muscle and nonmuscle. Actin filaments assume a variety of 
configurations depending on the type of cell and the state it is in. They 
extend a considerable distance through the cytoplasm in the form of bundles, 
also known as stress fibers since they are important in determining the 
elongated shape of the cell and in enabling the cell to adhere to the 
substrate and spread out on it. Actin filaments can exist in forms other than 
straight bundles. In rounded cells that do not adhere strongly to the 
substrate (such as dividing cells and cancer cells), the filaments form an 
amorphous meshwork that is quite distinct from the highly organized bundles. 
The two filamentous states, actin filament bundles and actin filament 
meshworks, are interconvertible polymeric states of the same molecule. Bundles 
give the cell its tensile strength, adhesive capability, and structural 
support, while meshworks provide elastic support and force for cell locomotion. 
    Microtubules are slender cylindrical structures that exhibit a cytoplasmic 
distribution distinct from actin filaments. Microtubules originate in 
structures that are closely associated with the outside surface of the nucleus 
known as centrioles. The major structural protein of these filaments is known 
as tubulin. Unlike the other two classes of filaments, microtubules are highly 
unstable structures and appear to be in a constant state of 
polymerization-depolymerization. See CENTRIOLE; MICROTUBULES; MITOSIS. 
    Intermediate filaments function as the true cytoskeleton. Unlike 
microtubules and actin filaments, intermediate filaments are very stable 
structures. They have a cytoplasmic distribution independent of actin 
filaments and microtubules. In the intact cell, they anchor the nucleus, 
positioning it within the cytoplasmic space. During mitosis, they form a 
filamentous cage around the mitotic spindle which holds the spindle in a fixed 
place during chromosome movement. 
 �34s[E. Lazarides]
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�6v�2v�32sDacite   �21m �33s 
    Aphanitic (very finely crystalline or glassy) rock of volcanic origin, 
composed chiefly of sodic plagioclase (oligoclase or andesine) and free silica 
(quartz or tridymite) with subordinate dark-colored (mafic) minerals (biotite, 
amphibole, or pyroxene). If alkali feldspar exceeds 5% of the total feldspar, 
the rock is a quartz latite. As quartz decreases in abundance, dacite passes 
into andesite. Thus, dacite is roughly intermediate between andesite and 
quartz latite. See ANDESITE. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sDactylochirotida   �21m �33s 
    An order of dendrochirotacean holothurians in which there are 8 - 30 
digitate or digitiform tentacles, the tentacles sometimes bifurcate. The body 
is enclosed in a test comprising imbricate plates. The calcareous ring is 
simple, lacking complex posterior processes. See DENDROCHIROTACEA; 
HOLOTHUROIDEA.  
    There are three extant families: Ypsilothuriidae, Vaneyellidae, and 
Rhopalodinidae. See ECHINODERMATA. 
 �34s[Howard B. Fell]
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�6v�2v�32sD'Alembert's paradox   �21m �33s 
    A theorem in fluid mechanics which states that no forces act on a body 
moving at constant velocity in a straight line through a large mass of 
incompressible, inviscid fluid which was initially at rest (or in uniform 
motion). This seemingly paradoxical theorem can be understood by first 
realizing that inviscid fluids do not exist. If such fluids did exist, there 
would be no internal physical mechanism for dissipating energy into heat; 
hence there could be no force acting on the body, because work would then be 
done on the fluid with no net increase of energy in the fluid. See FLUID FLOW. 
 �34s[Arthur E. Bryson, Jr.]
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�6v�2v�32sD'Alembert's principle   �21m �33s 
    The principle that the resultant of the external forces F and the kinetic 
reaction acting on a body equals zero. The kinetic reaction is defined as the 
negative of the product of the mass m and the acceleration a. The principle is 
therefore stated as F-ma=0. While D'Alembert's principle is merely another way 
of writing Newton's second law, it has the advantage of changing a problem in 
kinetics into a problem in statics. The techniques used in solving statics 
problems may then provide relatively simple solutions to some problems in 
dynamics; D'Alembert's principle is especially useful in problems involving 
constraints. See CONSTRAINT. 
 �34s[Paul W. Schmidt]
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�6v�2v�32sDalitz plot �21m �33s
    Pictorial representation in high-energy nuclear physics for data on the
distribution of certain three-particle configurations. Many
elementary-particle decay processes and high-energy nuclear reactions lead to
final states consisting of three particles (which may be denoted by a,b,c,
with mass values msuba, msubb, msubc). Well-known examples are provided by the
BAR K-meson decay processes, Eqs. (1) and (2), and by the BAR K- and
BAR K-meson reactions with hydrogen, given in Eqs. (3) and (4). For
definite total energy E (measured in the barycentric frame), these final
states have a continuous distribution of configurations, each specified by the
way this energy E is shared among the three particles. (The barycentric frame
is the reference frame in which the observer finds zero for the vector sum of
the momenta of all the particles of the system considered.) See ELEMENTARY
PARTICLE.

		     K+ -> pi+ + pi+ + pi-			 (1)

		     K+ -> pi(sup 0) + mu+ + nu-		       (2)

		     K+ + p -> K(sup 0) + pi+ + p			  (3)

		     K- + p -> LAMBDA + pi+ + pi-		    (4)

    If the three particles have kinetic energies T(sub a), T(sub b), and
T(sub c) (in the barycentric frame), Eq. (5) is obtained. As shown in the
illustration,  this energy sharing may be represented uniquely by a point F
within an equilateral triangle LMN of side 2Q/[SQUARE ROOT (3)], such that the
perpendiculars FA, FB, and FC to its sides are equal in magnitude to the
kinetic energies T(sub a), T(sub b), and T(sub c). The most important property
of this representation is that the area occupied within this triangle by any
set of configurations is directly proportional to their volume in phase space. 

T(sub a) + T(sub b) + T(sub c) =
E - m(sub a) c(sup 2) - m(sub b) c(sup 2) - m(sub c) c(sup 2) = Q      (5)

    Not all points F within the triangle LMN correspond to configurations
realizable physically, since the a,b,c energies must be consistent with zero
total momentum for the three-particle system. With nonrelativistic kinematics
and with equal masses m for a,b,c, the only allowed configurations are those
corresponding to points F lying within the circle inscribed within the
triangle, shown as (i) in the illustration. More generally, with relativistic
kinematics, the limiting boundary is distorted as illustrated by the boundary
curve (ii), drawn for the omega -> 3pi decay process, where the final masses
are equal.
 �34s[Richard H. Dalitz]
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�6v�2v�32sDallis grass   �21m �33s 
    A general term for a genus of grasses of which the most important species 
is the deeply rooted perennial Paspalum dilatatum. Dallis grass is widely used 
in the southern United States, mostly for pasture, and remains productive 
indefinitely if well managed. Dallis grass does best on fertile soils and 
responds to lime and fertilizer. On heavier soils it remains green throughout 
the winter unless checked by heavy frosts. Ergot-bearing seed heads are very 
toxic to livestock, and Dallis grass must be so managed as to prevent 
consumption of infected heads by livestock. See CYPERALES; ERGOT. 
 �34s[Howard B. Sprague]
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�6v�2v�32sDalton's law   �21m �33s 
    The total pressure that a mixture of gases exerts is equal to the sum of 
the separate pressures which each of the gases would exert if it alone 
occupied the whole volume. This law was observed by John Dalton (1766 - 1844) 
and is also called the law of additive pressures. It may be stated 
mathematically as psubm = psuba + psubb + psubc + PRODUCT PRODUCT PRODUCT , 
where psubm is the pressure produced by the mixture and psuba, psubb, psubc, 
PRODUCT PRODUCT PRODUCT are the partial pressures of the several gases in the 
mixture. The partial pressure is the pressure exerted by a constituent gas 
that occupies the whole volume occupied by the mixture at the same temperature 
and pressure in the absence of the other gases. See AVOGADRO'S LAW; GAS; 
THERMODYNAMIC PRINCIPLES. 
 �34s[George A. Hawkins]
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�6v�2v�32sDam   �21m �33s 
    A structure that bars or detains the flow of water in an open channel or 
watercourse. Dams are constructed for several principal purposes. Diversion 
dams divert water from a stream; navigation dams raise the level of a stream 
to increase the depth for navigation purposes; power dams raise the level of a 
stream to create or concentrate hydrostatic head for power purposes; and 
storage dams store water for municipal and industrial use, irrigation, flood 
control, river regulation, recreation, or power production. A dam serving two 
or more purposes is called a multiple-purpose dam. Dams are commonly 
classified by the material from which they are constructed, such as masonry, 
concrete, earth, rock, timber, and steel. Most dams are built either of 
concrete or of earth and rock. 
    Concrete dams may be typed as gravity, arch, or buttress type. Gravity 
dams depend on weight for stability against overturning and for resistance to 
sliding on their foundations. An arch dam may have a near-vertical face or, 
more usually, one that curves concave downstream. The dam acts as an arch to 
transmit most of the horizontal thrust from the water pressure against the 
upstream face of the dam to the abutments of the dam. Buttress dams depend on 
the weight of the structure and of the water on the dam to resist overturning 
and sliding. 
    Principal forces acting on a concrete dam are (1) vertical forces from 
weight of the structure and vertical component of water pressure against the 
upstream and downstream faces of the dam, (2) uplift pressures under the base 
of the structure, (3) horizontal forces from the horizontal component of the 
water pressure against the upstream and downstream faces of the dam, (4) 
forces from earthquake accelerations in regions subject to earthquakes, (5) 
temperature stresses, (6) pressures from silt deposits and earth fills against 
the structure, and (7) ice pressures. 
    The earth embankment is the most common dam. Earth dams may be built of 
rock, gravel, sand, silt, or clay in various combinations. Most earth dams are 
constructed with an inner impervious core, with upstream and downstream zones 
of more pervious materials, sometimes including rock zones. Earth dams limit 
the flow of water through the dam by use of fine-grained soils. Where 
possible, these soils are formed into a relatively impervious core. In some 
cases where pervious soils are scarce, the entire dam may be a homogeneous 
fill of relatively impervious soil. Downstream pervious drainage blankets are 
provided to collect seepage passing through, under, and around the abutments 
of the dam. 
    A spillway releases water in excess of storage capacity so that the dam 
and its foundation are protected against erosion and possible failure. All 
dams must have a spillway, except small ones where the runoff can be safely 
stored in the reservoir without danger of overtopping the dam. Ample spillway 
capacity is of particular importance for large earth dams, which would be 
destroyed or severely damaged by being overtopped. Failure of a large dam 
could result in severe hazards to life and property downstream. Spillways are 
of two general types: the overflow type, constructed as an integral part of 
the dam; or the channel type, located as an independent structure discharging 
through an open chute or tunnel. Either type may be equipped with gates to 
control the discharge. 
    Reservoir outlet works are used to regulate the release of water from the 
reservoir; they consist essentially of an intake and an outlet connected by a 
water passage, and are usually provided with gates. Outlet works usually have 
trashracks at the intake end to prevent clogging by debris. Bulkheads or stop 
logs are commonly provided to close the intakes so that the passages may be 
unwatered for inspection and maintenance. A stilling basin or other type of 
energy dissipator is usually provided at the outlet end. 
    A penstock is a pipe that conveys water from a forebay, reservoir, or 
other source to a turbine in a hydroelectric plant. It is usually made of 
steel, but reinforced concrete and woodstave pipe have also been used. 
Pressure rise, or water hammer, and speed regulation must be considered in the 
design of a penstock. 
    The foundation of a dam must support the structure under all operating 
conditions. For concrete dams, following removal of unsatisfactory materials 
to a sound foundation surface, imperfections such as adversely oriented rock 
joints, open bedding planes, localized soft seams, and faults lying on or 
beneath the foundation surface receive special treatment. In addition, the 
foundation of an earth dam must safely support the weight of the dam, limit 
seepage of stored water, and prevent transportation of dam or foundation 
material away or into open joints or seams in the rock by seepage. Earth-dam 
foundation treatment may include removal of excessively weak surface soils to 
prevent both potential sliding and excessive settlement of the dam, excavation 
of a cutoff trench to rock, and grouting of joints and seams in the bottom and 
downstream side of the cutoff trench. 
    Because failure of a dam may result in loss of life or property damage in 
the downstream area, it is essential that dams be inspected systematically 
both during construction and after completion. The design of dams should be 
reviewed to assure competency of the structure and its site, and inspections 
should be made during construction to ensure that the requirements of the 
design and specifications are incorporated in the structure. After completion 
and filling, inspections may vary from cursory surveillance during day-to-day 
operation of the project to regularly scheduled comprehensive inspections. The 
objective of such inspections is to detect symptoms of possible distress in 
the dam at the earliest time. Detection of any such symptoms of distress 
should be followed by an investigation of the causes, probable effects, and 
remedial measures required. Inspection of a dam and reservoir is particularly 
important following significant seismic events in the locality. Systematic 
monitoring of the instrumentation installed in dams is essential to the 
inspection program. 
 �34s[Jack R. Thompson]
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�6v�2v�32sDamping �21m �33s
    A term broadly used to denote either the dissipation of energy in, and the
consequent decay of, oscillations of all types or the extent of the
dissipation and decay. The energy losses arise from frictional (or analogous)
forces which are unavoidable in any system or from the radiation of energy to
space or to other systems. For sufficiently small oscillations, the analogous
forces are proportional to the velocity of the vibrating member and oppositely
directed thereto; the ratio of force to velocity is -R, the mechanical
resistance. For the role of damping in the case of forced oscillations, where
it is decisive for the frequency response, see FORCED OSCILLATION; RESONANCE
(ACOUSTICS AND MECHANICS). See also HARMONIC MOTION; MECHANICAL VIBRATION;
OSCILLATION; VIBRATION.
    An undamped system of mass m and stiffness s oscillates at an angular
frequency OMEGA0 = (s/m)1/2. The effect of a mechanical resistance R is
twofold: It produces a change in the frequency of oscillation, and it causes
the oscillations to decay with time. If u is one of the oscillating quantities
(displacement, velocity, acceleration) of amplitude A, then Eq. (1) holds in
the damped case, whereas in the undamped case Eq. (2) holds. The reciprocal
time 1/ALPHA in Eq. (1) may be called the damping constant.

		    u = Ae (sup -ALPHAt) cos OMEGA sub d t		(1)

		       u = A cos OMEGA sub O t				(2)


    The damped angular frequency OMEGAsubd in Eq. (1) is always less than
OMEGA0. According to Eq. (1), the amplitude of the oscillation decays
exponentially; the time required for the amplitude to decrease to the fraction
1/e of its initial value is equal to 1/ALPHA.
    A common measure of the damping is the logarithmic decrement DELTA,
defined as the natural logarithm of the ratio of two successive maxima of the
decaying sinusoid. If T is the period of the oscillation, then Eq. (3) holds.
Then 1/DELTA is the number of cycles required for the amplitude to decrease by
the factor 1/e in the same way that 1/ALPHA is the time required.

			   DELTA = ALPHAT				 (3)

    The Q of a system is a measure of damping usually defined from energy
considerations. The Q is PI times the ratio of peak energy stored to energy
dissipated per cycle and is equal to PI/DELTA.
    If ALPHA in Eq. (1) exceeds OMEGA0, then the system is not oscillatory and
is said to be overdamped. If the mass is displaced, it returns to its
equilibrium position without overshoot, and the return is slower as the ratio
ALPHA/OMEGA0 increases. If ALPHA = OMEGA0 (that is, Q = 1/2), the oscillator
is critically damped. In this case, the motion is again nonoscillatory, but
the return to equilibrium is faster than for any overdamped case.
 �34s[Martin Greenspan]
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�6v�2v�32sDark current   �21m �33s 
    An ambiguous term used in connection with both gaseous-discharge devices 
and photoelectric cells or tubes. In gaseous-conduction tubes it refers to the 
region of operation known as the Townsend discharge. When applied to 
photoelectric devices, the term applies to background current. This is current 
which may be present as the result of thermionic emission or other effects 
when there is no light incident on the photosensitive cathode. See ELECTRICAL 
CONDUCTION IN GASES; TOWNSEND DISCHARGE. 
 �34s[Glenn H. Miller]
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�6v�2v�32sDarter   �21m �33s 
    Any of 93 species of small, slender, fresh-water fishes of the family 
Percidae. They occur in a variety of habitats, including swift- and 
slow-moving streams and lakes and riffles with stony bottoms, and they are 
widely distributed throughout the various drainage systems of the United 
States.  
    Darters are typical bony fishes. There is a single external gill opening 
on each side which is covered with an opercle or bony flap. Pelvic fins are 
present, and the body is scaled and lacks an adipose fin. The body is 
semitranslucent in some species and most are brilliantly colored. These fishes 
are not of commercial value. See OSTEICHTHYES; PERCIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sDasheen �21m �33s
    Common name for the plant Colocasia esculenta, including the variety
antiquorum (taro). These plants are among the few edible members of the aroid
family (Araceae). Native to southeastern Asia and Malaysia, the plants supply
the people with their most important food. A main dish in the Polynesian menu
is poi, a thin, pasty gruel of taro starch, often fermented, which is
frequently formed into cakes for baking or toasting. The raw corms
(underground stems) are baked or boiled to eliminate an irritating substance
present in the cells. See ARALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sDasycladales   �21m �33s 
    An order of lime-encrusted marine algae in the Chlorophyta, sometimes 
included under the Siphonales. The thallus, of macroscopic size, is a complex 
of nonseptate, highly branched tubes which in aggregate produce an intricately 
constructed plant. They are plumose in Batophora, umbrellalike in 
Acetabularia, and saccate in Bornetella. In general, the architectural plan of 
the thallus is an axis from a rhizoidal base, with outturned branching 
elements which in many instances are swollen at the tips. 
    The Dasycladales are mostly tropical or subtropical. Because of their lime 
encrustation, they have fossilized readily and are known from the Lower 
Silurian. More genera are fossil than extant. Plants are coenocytic and the 
units contain numerous small, disklike or threadlike chloroplasts. See ALGAE; 
CHLOROPHYTA; SIPHONALES. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sData-base management systems �21m �33s
    Special data-processing systems or parts of data-processing systems which
are developed to aid in the storage, manipulation, reporting, and managing of
data. They may be considered as building blocks constructing a data-processing
system which also acts as a mechanism for the effective control of the data.
The data-base management system provides a means for management of data by
making available data to a large range of authorized users, while preserving
the integrity of the data and improving the overall system performance.
    It has been estimated that a medium-sized corporation may need to retain
10 sup 1 sup 0 characters of current important data in high-speed automated
storage. By comparison, a book of 300 pages, 40 lines per page, and 12 words
(each of about 6 characters) per line, contains about 10 sup 6 characters
(including spaces between words) and thus the 10 sup 1 sup 0-character ``data
base'' could be stored in 10 sup 4 books, which is indeed a good-sized library.
However, the organization of a library makes it relatively easy to find any
book, but imagine a library that had no catalog, no librarians and no one
putting the books back on the shelves. The cost of a library staff is similar
to the cost of organizing data in a corporate data base: it is a necessary
cost if data must be found and used effectively. Indeed the old systems,
without well organized data, suffered from the problem that people knew data
existed, but could not find it easily and became frustrated. One of the
reasons for the development of data-base management systems was to solve this
need.
    Data become truly available when a large cross section of potential users
know of their existence, when their location and format are known, and when
there is a mechanism for retrieving the data. If the mechanism to locate its
format is used by humans, the location is normally termed a directory, while
the format is stored in a dictionary. If the locating device is embedded in
the data-base system, the mechanism is termed a schema.
    A schema is therefore a machine readable version of the location and
format of the stored data. The difference between the schema and its
``instances'' or ``occurrences'' is shown in Fig. 1. This simple schema
contains information about a person: name, date of birth, and salary. The
representation is: name 10 characters in length, with blanks for shorter
names; date of birth as year, month, and day, with two decimal digits
representing each; and salary of up to $9999.99 per month. The ``model'' is
defined in the schema, while the data are recorded in each instance.
    In order for the data-base management system to retrieve or store data
about a person, it must consult the schema to determine its form as well as
its location in machine storage. This means that a data-base management system
with a ``query language'' interface can respond to a request like FOR PERSON,
FIND MONTHLY-SALARY, WHERE PERSON-NAME = ``SINGER,'' or FOR PERSON, FIND
PERSON-NAME AND DATE-OF-BIRTH, WHERE SALARY GREATER THAN 700.00
    This powerful capability of the DBMS enables a wide variety of potential
users to obtain information that would otherwise be unavailable. However, this
also creates some social problems, because access to the data may become too
easy. Typically, security may be enforced by insisting that users of the
data-base management system identify themselves both by name or employee
number and by some password.
    Because the data are often stored in a device with moving parts they can
be destroyed through mechanical or other failures. Data may also be lost
through user carelessness or program malfunction. To protect the integrity of
the data, several defensive steps may be taken. One major precaution is to
take dumps, that is, copies of the data. This action is taken at regular
intervals if the data are frequently changed. In order to be able to restore a
data base after it has been destroyed, it is merely necessary to get a copy of
the last dump and update it.
    The somewhat simplified architecture of a data-base management system and
its environment is shown in Fig. 2. It illustrates how a data request is
satisfied. For example, consider the request for person-name and date of birth
in the PERSON data base. An ad hoc user (that is, one who can query the system
without having recourse to a previously coded program) goes to an input/output
device with a keyboard and/or cathoderay-tube combination. The person types in
SIGNON JONES PASSWORD 51Z5QP, and then (after some response) AD HOC DBMS FOR
PERSON, FIND PERSON-NAME AND DATE-OF-BIRTH WHERE SALARY GREATER THAN 700.00.

    The following operations will be performed: (1) The word SIGNON is
transmitted by the checking program to the scheduler program which requests
the SIGNON program from its library. (2) The SIGNON program is run by the
scheduler program to check that Jones has given the correct password. (3) The
AD HOC program is called from the library and starts to look for more inputs.
(4) The word PERSON is taken by this program as the name of the schema. (5)
The scheduler program requests the data-base routine to load the PERSON schema
from storage. (6) The next sentence is now checked to see whether PERSON-NAME,
DATE-OF-BIRTH, and SALARY are all valid words in the PERSON schema. (7) A
search of the instances is made to determine which have a value of more than
700.00 in the SALARY item. In the above example, two people qualify. (8) The
names and dates of birth of these two people are displayed for the user. In
this example, the authority of the user (Jones) to view any element (for
example, date of birth) could be checked at step 6, if necessary.
    Different data-base management systems model the data in different ways.
The so-called relational system represents data as tables. No entry in the
table contains more than one elementary value (for example, it would be
impossible to have two salaries for BAKER). The hierarchical system allows
more structure. In this type of system, groupings of elements are connected by
parent-child relationships. Here, a particular person may have one or several
different educational categories (high school, bachelor's degree, and so
forth), as well as several job histories (office boy, clerk, manager,
president). The network system has more structure and complexity, because it
uses named relationships. Because these three types of systems are
structurally different, most current commercial data-base management systems
service only one type; future systems are expected to be able to allow the
users to choose the best system to suit their special needs. See
DATA-PROCESSING SYSTEMS; DIGITAL COMPUTER.
 �34s[Edgar H. Sibley]
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�6v�2v�32sData communications �21m �33s
    The function of electronically conveying digitally encoded information
from a source to a destination where the source and destination may be
displaced in space (distance) and time. Encoded information can represent
alphabetic, numeric, and graphic forms. Such information may originate as
keystrokes at a terminal, as images on a page, as motion of a hand-held
stylus, as a signal from some electrical or mechanical measurement device, or
as output from a computer. The data communications function includes
management and control of connections as well as the actual transfer of
information.
    Digitally encoded information typically takes the form of a series of 1s
and 0s called bits (contraction for binary digit). Groups of eight bits are
often referred to as bytes. Standard codes (such as ASCII and EBCDIC) have
been adopted to establish a correspondence between bytes (patterns of eight
bits) and alphabetic, numeric, and commonly used graphic and control
characters. These codes are used for the interchange of information among
equipments made by different manufacturers. See BIT.
    Data communications usually do not refer to the conveyance of voice and
video, although these too may be digitally encoded and then conveyed in
digital form just as data are conveyed. The increased use of computers in the
design and implementation of communications systems and the efficiencies
realized from the handling of digitally encoded information have markedly
broadened the range of data communications, and tend to mask old distinctions
between data communications and other forms of information transfer.
    Data communications has evolved from a function serving simple
terminal-to-terminal communications (telegraphy) to one dominated by
communications with and among computers. Terminal-to-computer communications
remains a sizable component of traffic, but the migration of intelligence into
terminal systems (an aspect of distributed processing) is causing terminals to
take on more of the communications characteristics of computers. Networks of
terminals and computers are emerging to serve the needs of business,
education, government, industry, medicine, recreation, and other forms of
human endeavor. These systems, both publicly and privately owned and operated,
manage and direct the movement of data between stations (terminals, computers,
and other sources of digitally encoded information) and across networks. Where
two stations are geographically separated, data communications manages the
transfer of information as a displacement in space. Where there is value in
delaying the transfer of information, data communications manages the transfer
also as a displacement in time. See COMPUTER; MULTIACCESS COMPUTER; TELEGRAPHY.
    Data communications applications fall into three principal categories:
transaction-oriented, message-oriented, and batch-oriented (see illustration).
Transaction, message, and batch-oriented applications exhibit different needs
in terms of total bytes transferred, delay, and special features.
Consequently, they impose different demands upon the functional components of
data communications: media conversion, communications processing, and
transmission.
    Transaction-oriented applications involve the transfer of small amounts of
data in the range of tens to hundreds of bytes and frequently require minimal
delay in the transfer process. Transaction-oriented applications are typically
bidirectional or conversational. Information is exchanged between source and
destination stations. Message-oriented applications involve data transfers in
the range of hundreds to a few thousand bytes or characters and are usually
unidirectional; information flows from source to destination. Message
applications frequently take advantage of time displacement (store and
forward) as well as space displacement, and they exhibit data transfer options
which go beyond the simple movement of information between two points.
Batch-oriented applications involve the transfer of thousands or even millions
of bytes of data and are usually point-to-point (place-to-place) and
computer-to-computer.
    Media conversion achieves the conversion of some input, such as a keyboard
keystroke, graphic material, a signal representing some measurement, and
information recorded on magnetic media such as tape or disk, into a digitally
encoded signal which may be transmitted. Likewise, it achieves the conversion
of a digitally encoded signal into similar forms of useful output.
    Communications processing provides the management and control in data
communications system. In the data terminal or media conversion system, it
must at least manage the protocol (rules) used for intelligibly exchanging
information with other stations including the administration of techniques
designed to detect errors in transmission. Depending on the application, it
may be necessary to perform code conversion and to multiplex or concentrate
inputs from several terminals into a single output stream.
    Transmission accomplishes the physical transfer of data. It includes the
provision of transmission channels, the signal conversions necessary to use
such channels, and any required switching among channels. Since it is
generally impractical to install privately owned cables beyond single
buildings or campus environments and because transmission distances of more
than a few miles are commonly of interest, the conventional approach is to use
the telephone network for transmission facilities. To support applications
where communications among multiple stations is required, data communications
systems offer switching beyond that available in the dial switching
capabilities of the telephone network. Circuit switching, packet switching,
and message switching are the common forms. See ELECTRICAL COMMUNICATIONS;
SWITCHING SYSTEMS (COMMUNICATIONS).
 �34s[Mark M. Rochkind]
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�6v�2v�32sData flow systems   �21m �33s 
    An alternative to conventional programming languages and architectures, in 
which values rather than value containers are dealt with, and all processing 
is achieved by applying functions to values to produce new values. These 
systems can realize large amounts of parallelism (present in many 
applications) and effectively utilize very-large-scale-integration (VLSI) 
technology.  
    Data flow systems use an underlying execution model which differs 
substantially from the conventional one. The model deals with values, not 
names of value containers. There is no notion of assigning different values to 
an object which is held in a global, updatable memory location. A statement 
such as X: = B + C in a data flow language is only syntactically similar to an 
assignment statement. The meaning of X: = B + C in data flow is to compute the 
value B + C and bind this value to the name X. Other operators can use this 
value by referring to the name X, and the statement has a precise mathematical 
meaning defining X. This definition remains constant within the scope in which 
the statement occurs. Languages with this property are sometimes referred to 
as single assignment languages. The second property of the model is that all 
processing is achieved by applying functions to values to produce new values. 
The inputs and results are clearly defined, and there are no side effects. 
Languages with this property are called applicative. Value-oriented, 
applicative languages do not impose any sequencing constraints in addition to 
the basic data dependencies present in the algorithm. Functions must wait for 
all input values to be computed, but the order in which the functions are 
evaluated does not affect the final results. There is no notion of a central 
controller which initiates one statement at a time sequentially. The model 
described above can be applied to languages and architectures. 
    The computation specified by a program in a data flow language can be 
represented as a data dependence graph, in which each node is a function and 
each arc carries a value. Very efficient execution of a data flow program can 
be achieved on a stored program computer which has the properties of the data 
flow model. The machine language for such a computer is a dependence graph 
rather than the conventional sequence of instructions. There is no program 
counter in a  data flow computer. Instead, a mechanism is provided to detect 
when an instruction is enabled, that is, when all required input values are 
present. Enabled instructions, together with input values and destination 
addresses (for the result), are sent to processing elements. Results are 
routed to destinations, which may enable other instructions. This mode of 
execution is called data-driven. 
    Data flow systems can overcome many of the disadvantages of conventional 
approaches. In principle, all the parallelism in the algorithm is exposed in 
the program and thus the programmer does not have to deal with parallelism 
explicitly.  
    Several important problems remain to be solved in data flow systems. The 
handling of complex data structures as values is inefficient, but there is no 
complete solution to this problem yet. Data flow computers tend to have long 
pipelines, and this causes degraded performance if the application does not 
have sufficient parallelism. Since the programmer does not have explicit 
control over memory, separate ``garbage collection'' mechanisms must be 
implemented. The space-time overheads of managing low levels of parallelism 
have not been quantified. Thus, though the parallelism is exposed to the 
hardware, it has not been demonstrated that it can be effectively realized.  
See  DIGITAL COMPUTER PROGRAMMING; PROGRAMMING LANGUAGES. 
 �34s[Tilak Agerwala]
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�6v�2v�32sData-processing management   �21m �33s 
    Managing the data-processing function, its people, and its equipment. This 
activity follows the well-recognized principles of planning, control, and 
operation. The basic prerequisites for data-processing management are 
therefore the same skills which are needed to manage any other enterprise. 
    Data processing is a continually evolving technology, and this fact 
differentiates data-processing management from many other managerial 
environments. Furthermore, data processing is most often a service function 
within an organization, and not an end product. Constant awareness of this 
aspect is essential to ensure successful discharge of data-processing 
managerial responsibilities. 
    Despite the apparently overwhelming and complex presence of computing 
equipment and its associated technology, the major and critical component in 
the data-processing function is people. To select, develop, and organize 
people for maximum effectiveness and efficiency should be the major concern.  
See  COMPUTER; DATA-PROCESSING SYSTEMS; DIGITAL COMPUTER. 
 �34s[Michael J. Samek]
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�6v�2v�32sData-processing systems   �21m �33s 
    Electronic, electromechanical, or mechanical machines for transforming 
information into suitable forms in accordance with procedures planned in 
advance. The term data-processing system is also applied to the scheme, or 
procedure, that prescribes the sequence of operations to be performed in 
processing the information. 
    Typical business applications of data-processing systems include record 
keeping, financial accounting and planning, processing personnel information 
(including payroll processing), sales analysis, inventory control, production 
scheduling, operations research, and market research. Data-processing systems 
are also used to process correspondence and other information in business 
offices. These systems are called word processors. See WORD PROCESSING. 
    In science and engineering, data-processing systems are used in data 
reduction, statistical analysis of experimental data, planning and design of 
engineering projects, and the display in tabular and graphic form of the 
results of research and development. Special-purpose computer systems, which 
also fall within the definition of data-processing systems, are used to 
monitor and control such things as chemical processes, machine tools, 
typesetting equipment, and power generation plants. See DATA REDUCTION. 
    Data-processing systems can be broadly classified as manual, 
semiautomatic, or automatic, depending on the degree of human effort required 
to control and execute the procedures. Automatic systems are usually built 
around electronic digital computers and termed electronic data-processing 
(EDP) systems. See DIGITAL COMPUTER. 
  
  
    Data-processing functions.  Virtually every data-processing system, 
regardless of the degree of automation, consists of six basic functions: 
recording, transmission, manipulation, reporting, storage, and retrieval - 
collectively termed the data-processing cycle. 
    Before information can be processed, it must be recorded in some form that 
is meaningful to a person or a machine. At one time the principal medium used 
to enter information into computers was the punched card. Subsequently 
keyboards linked to magnetic tapes (key to tape) or to magnetic disk (key to 
disk) systems became widely used. These recording systems are being replaced 
in turn by key entry devices that are linked directly to the computer via 
communication lines. 
    Once information has been recorded, it will usually need to be transmitted 
to another location for processing. Where fast response is desirable and 
economically feasible, the data can be transmitted at high speeds by means of 
telephone or telegraph circuits, microwave links, or satellite channels. See 
DATA COMMUNICATIONS. 
    Manipulation is the stage in which most of the actual processing is 
performed. The operations involved can range from simple to highly complex. 
The most common types of operations are classifying, sorting, calculating, and 
summarizing. More specifically, the manipulation stage frequently involves (1) 
arrangement of the information into a sequence that will facilitate further 
processing; (2) determination of the exact procedure to be followed in 
processing each item of information; (3) references to files containing data 
that must be associated with the current information; (4) arithmetic 
operations upon the current information or file data or both; and (5) updating 
(changing) of the file data to reflect the current information. 
    After information has been processed, it is usually necessary to report 
the results in a meaningful form. Reports are the people-oriented products of 
the data-processing activity. They should be timely, complete, understandable, 
and in a convenient format. 
    Some or all of the processed information will need to be stored for future 
reference or retrieval. Depending on the system, the information may be stored 
manually (as in ledgers or notebooks), electromechanically (as in punched 
cards or punched paper tape), or electronically (as in disk packs or magnetic 
tape).  
    The importance of the contents of storage and the availability of 
transmission facilities have resulted in many automatic data-processing 
systems providing immediate retrieval of stored information at work stations. 
Questions regarding the contents of storage can be posed at remote locations, 
and responses can be prepared and transmitted to these locations in a few 
seconds. This is called an on-line query facility.  
  
    Electronic data-processing (EDP) systems.  EDP systems take advantage of 
the great speed and versatility of  stored-program digital computers to 
process large volumes of data with little or no need for human intervention. 
There is no fundamental difference between the computers used for business 
data processing and those used for scientific calculations; in fact, a single 
computer is often used for both types of applications. Scientific computers, 
however, tend to emphasize high computational speeds, while computers used for 
business data processing often place primary emphasis on fast, flexible input 
and output equipment. 
    Every computer system has four basic functional parts: input equipment to 
permit data and instructions to be entered into the system; a storage unit to 
permit data and instructions to be stored until called for; a control unit, 
called the central processing unit (CPU), to interpret the stored instructions 
and direct their execution; and output equipment to permit the processed data 
to be removed from the system. The common types of input and output devices, 
their recording media, and their typical speeds are summarized in the table. 
Magnetic tape and disk can also be viewed as storage media that supplement the 
storage available in the computer itself. They have been termed auxiliary 
storage. See COMPUTER STORAGE TECHNOLOGY; DIGITAL COMPUTER PROGRAMMING. 
    Disk pack units. The disk pack has become the preeminent high-speed 
computer input/output medium. A disk pack typically consists of a stack of 
from 1 to 20 round stainless-steel plates mounted on a vertical spindle. Data 
are magnetically recorded on some or all of the surfaces of the disks. The 
disk pack can be conveniently mounted on a drive unit that contains magnetic 
read/write heads mounted on a comblike access mechanism that moves 
horizontally between the disks in  order to reach and record information on 
them. The key advantage of disk packs that accounts for their preeminence is 
their rapid-access capability; any record stored in a disk pack can be located 
and read into the computer in a fraction of a second, whereas it may take as 
long as several minutes to locate a particular record stored in a reel of 
magnetic tape. 
    One development in magnetic disk technology deserves special mention. 
Flexible plastic disks, called diskettes or floppy disks, are into widespread 
use for applications in which low cost and ease of use are more important than 
high speeds and large data-storage capacities. Most diskettes are 8 in. (20 
cm) in diameter, and may be housed in thin plastic envelopes; the envelope 
remains stationary while the drive unit spins the disk. 
    Magnetic tape units. Magnetic tape is an important input/output medium for 
EDP systems because it permits large quantities of information to be stored in 
a highly compact, economical, and easily erasable form. The tape is usually 
made of plastic with a magnetizable oxide coating on which data can be 
recorded in the form of magnetic spots. 
    As an alternative to high-performance magnetic tape units, many EDP 
systems include smaller, less costly tape transports in which the data are 
recorded on shorter lengths of narrow magnetic tape housed in conveniently 
interchangeable cartridges. The cartridge tape units have comparatively slow 
read/write speeds and small data-storage capacities; their offsetting 
advantages are economy and convenience of use. 
    Magnetic tape is widely used to provide back-up for disk units and to 
permit the exchange of large volumes of information by sending reels of tape 
from one place to another. For example, computer programs that are purchased 
are often delivered on one or more reels of magnetic tape. 
    Card reader and punch. The fact that most early computers replaced or 
augmented punched-card systems gave the punched card a strong head start as a 
computer input/output medium, and it is still widely used. The punched card is 
a highly flexible medium that has many advantages for use as a source document 
and as a storage medium for permanent records. In EDP systems its principal 
drawbacks are the difficulty of correcting errors in the punched data, the 
fixed upper limit on the amount of data a card can hold (usually 80 or 96 
characters), and the comparatively low speeds of even the fastest card readers 
and punches (see table). 
    Paper tape readers and punches. Paper tape can be punched and read by 
relatively simple, inexpensive equipment, making it practical to produce tape 
records as a direct by-product of the normal operations of many business 
machines, such as teletypewriters and cash registers. 
    Though generally called paper tape, the tape may be made of paper, 
plastic, metal, or laminated combinations of these materials. Data are 
recorded by punching round holes into the tape and read by sensing the holes 
either mechanically or photoelectrically. As with punched cards, the principal 
disadvantage of paper tape is the low speeds of the tape readers and punches 
available for use with computers. 
    Printers. Electromechanical printing devices are the primary means for 
making information processed by computers available to people. However, 
display units are overtaking printers in this regard. Computer printers can be 
broadly classified as either serial or line printers and as either impact or 
nonimpact printers. A serial unit prints one character at a time in the manner 
of a conventional typewriter, whereas a line printer prints a full line, 
usually consisting of 80 to 132 characters, at the same time. An impact 
printer uses direct mechanical force to produce character images on the paper, 
whereas nonimpact printers utilize various electrical or chemical processes to 
form the characters. 
    Matrix printers may be impact or nonimpact. The most successful are 
nonimpact and use thermal-chemical or electrostatic processes to create an 
image. Typically a 5-by-7 matrix of dots is used to print each character. The 
set of dots that is printed provides an image of the character. A matrix 
printer that is widely used, especially in offices using word processing, is 
the ink jet printer. 
    A laser can be used to write the printed output on the light-sensitive 
surface of a drum. The image on this surface is then transferred to paper. 
Speeds of tens of thousands of characters per second can be realized by this 
type of printer. See LASER. 
    Character reader. Optical character recognition (OCR) has been heralded as 
the solution to the great computer input bottleneck because it promises to 
reduce, and in some cases eliminate, the manual keystroke operations which are 
normally required in preparing the input data for machine processing. Machines 
capable of reading characters printed on paper documents by typewriters, cash 
registers, computer printers, and many other business machines are in use in 
applications such as utility billing and credit-card processing. Moreover, 
machines that can read hand-printed characters are on the market, and 
extensive development work is in progress on machines to recognize ordinary 
handwriting. The principal barriers to wider use of optical character readers 
have been reliability problems and comparatively high costs, and significant 
progress has been made in both areas. See CHARACTER RECOGNITION.  
    Optical recognition techniques can also be used effectively to read 
handmade pencil marks or printed bar codes. Optical mark readers and bar-code 
readers are considerably less costly than character readers and are widely 
used in applications such as test scoring, survey analysis, credit-card 
processing, and supermarket checkout. 
    Displays. Display units are widely used as ``electronic blackboards'' that 
provide rapid access to data stored in computer systems and facilitate 
human-machine communication. The great majority of current display units use 
cathode-ray tubes, but other technologies such as light-emitting diodes, 
liquid-crystal displays, and plasma (gaseous) displays are also used. The 
display units may be connected directly to the computer or located remotely 
and connected by a data communications link. The displayed information may 
consist of characters or lines on the face of a cathode-ray tube 6 - 22 in. 
(15 - 56 cm) in diameter. Most display units can also be used for input to the 
computer by a keyboard or handheld light pen which the operator focuses upon 
particular points on the screen. See ELECTRONIC DISPLAY. 
 �34s[Herbert Maisel]
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�6v�2v�32sData reduction   �21m �33s 
    The transformation of information, usually empirically or experimentally 
derived, into corrected, ordered, and simplified form. 
    The term data reduction generally refers to operations on either numerical 
or alphabetical information digitally represented, or to operations which 
yield digital information from empirical observations or instrument readings. 
In the latter case data reduction also implies conversion from analog to 
digital form either by human reading and digital symbolization or by 
mechanical means. See ANALOG-TO-DIGITAL CONVERTER; DIGITAL COMPUTER. 
    In applications where the raw data are already digital, data reduction may 
consist simply of such operations as editing, scaling, coding, sorting, 
collating, and tabular summarization. 
    More typically, the data reduction process is applied to readings or 
measurements involving random errors. These are the indeterminate errors 
inherent in the process of assigning values to observational quantities. In 
such cases, before data may be coded and summarized, the most probable value 
of a quantity must be determined. Provided the errors are normally 
distributed, the most probable (or central) value of a set of measurements is 
given by the arithmetic mean or, in the more general case, by the weighted 
mean. See STATISTICS. 
    Data reduction may also involve operations of smoothing and interpolation, 
because the results of observations and measurements are always given as a 
discrete set of numbers, while the phenomenon being studied may be continuous 
in nature. 
 �34s[Raymond J. Nelson]
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�6v�2v�32sDate   �21m �33s 
    The fruit of the evergreen date palm (Phoenix dactylifera). Date fruits 
are dry, semidry, or soft, depending on the moisture content at maturity of 
the particular variety. Soft types tend to spoil or sour more readily than dry 
types. Most varieties contain about 65% sugar as glucose or fructose. Palms 
are grown mainly in the semiarid and desert regions of Asia and Africa. 
Commercial data culture in the United States is limited to areas of high heat 
(100 - 120DGF; 38 - 49DGC) and low atmospheric humidity, such as the 
low-elevation deserts of California and a small part of Arizona. See ARECALES; 
FRUIT; FRUIT, TREE. 
 �34s[Charles A. Schroeder]
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�6v�2v�32sDating methods   �21m �33s 
    Methods and techniques used in archeology, biology, and geology to fix 
dates, assign periods of time, and determine age. The uranium-lead, 
rubidium-strontium, potassium-argon, and carbon-14 methods have yielded 
reliable results on suitable samples and have permitted the construction of 
absolute geologic history in many areas of the world. See GEOLOGICAL TIME 
SCALE; LEAD ISOTOPES (GEOCHEMISTRY); METEORITE; RADIOCARBON DATING; ROCK AGE 
DETERMINATION.  
    For descriptions of other methods and techniques used to establish the 
sequence of events, the succession of strata, and relative chronologies see 
ARCHEOLOGY; CHEMICAL DATING; DENDROCHRONOLOGY; FISSION TRACK DATING; 
GEOCHRONOMETRY; INDEX FOSSIL; ROCK MAGNETISM; STRATIGRAPHY; TREE-RING 
HYDROLOGY.  
 �34s[David E. Fogarty]
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�6v�2v�32sDatolite   �21m �33s 
    A mineral nesosilicate, composition CaBSiO4(OH), crystallizing in the 
monoclinic system. It usually occurs in crystals showing many faces and having 
an equidimensional habit. It may also be fine granular or compact and massive. 
Hardness is 5 - 5J on Mohs scale; specific gravity is 2.8 - 3.0. The luster is 
vitreous, the crystals colorless or white with a greenish tinge. Datolite is 
found in the Harz Mountains, Germany; Bologna, Italy; and Arendal, Norway. In 
the United States fine crystals have come from Westfield, Massachusetts; 
Bergen Hill, NewJersey; and various places in Connecticut. In Michigan, in the 
Lake Superior copper district, datolite occurs in fine-grained porcelainlike 
masses which may be coppery red because of inclusions of native copper. See 
SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sDawsoniales   �21m �33s 
    An order of mosses consisting of one family which is composed of one 
genus, occurring on soil in Australia, Tasmania, New Zealand, Mindanao, East 
Indies, New Guinea, and Borneo. The plants are rigid, mostly robust. The stems 
are erect from a rhizomelike base. The lower leaves are appressed, scalelike; 
the upper are crowded, rigid, erect-spreading, linear-lanceolate to 
linear-subulate from a sheathing base. The costa is excurrent in a toothed 
bristlelike point. The cucullate calyptra is hairy. The terminal sporophyte 
consists of a stout seta and an ovoid, small-mouthed capsule. The peristome is 
a large tuft of whitish or pale-brown, filiform, papillose bristlelike teeth. 
See BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sDay   �21m �33s 
    A unit of time equal to the period of rotation of Earth. Different sorts 
of day are distinguished, according to how the period of rotation is reckoned 
with respect to one or another direction in space. 
    The apparent solar day is the interval between any two successive meridian 
transits of the Sun. It varies through the year, reaching about 24 h 30 s of 
ordinary clock time in December and about 23 h 59 min 39 s in September. The 
mean solar day is the interval between any two successive meridian transits of 
an imagined point in the sky that moves along the celestial equator with a 
uniform motion equal to the average rate of motion of the Sun along the 
ecliptic. Ordinary clocks are regulated to advance 24 h during a mean solar 
day.  
    The sidereal day is the interval between any two successive meridian 
transits of the vernal equinox. Similarly, as for the solar day, a distinction 
is made between the apparent sidereal day and the mean sidereal day which, 
however, differ at most by a small fraction of a second. A mean sidereal day 
comprises 23 h 56 min 4.09054 s of a mean solar day. 
    The mean solar day and the sidereal day vary together in consequence of 
variations in the speed of rotation of Earth, which are of three sorts: 
seasonal, irregular, and secular. See TIME. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sDead reckoning   �21m �33s 
    A form of navigation that determines position of a craft by advancing a 
previous position to a new one on the basis of assumed distance and direction 
moved.  
    The parameters of dead reckoning are direction of motion and distance 
traveled. Several directions are involved in the motion of a craft, as shown 
exaggerated in the illustration. The intended direction of travel is called 
the course. The direction steered, which may differ somewhat because of 
anticipated offset due to wind or current, is also called course (or course 
steered) by the marine navigator, and heading by the air navigator. The 
direction actually made good between two points is called course made good. 
Air navigators often refer to this direction as the track, although this 
expression is used also to refer to the path followed or intended to be 
followed. The direction in which the craft is pointed at any moment is called 
heading by both air and marine navigators. 
    A compass is used to indicate direction. Distance is usually determined 
indirectly by measurement of speed and time, but it may be measured directly. 
    Air and land navigators, and some marine navigators, use the best estimate 
of direction and distance in their dead reckoning. Many marine navigators, 
however, prefer to use course steered and estimated speed through the water 
(without allowance for the effect of wind) for their dead reckoning; they 
consider positions determined by allowance for estimated effects of wind and 
current as estimated positions. 
    The uncertainty of dead-reckoning positions, however determined, increases 
with time and perhaps also with distance traveled. From time to time an 
independent determination of position is made by celestial navigation or 
piloting. When a reliable position, called a fix, is so obtained, a new dead 
reckoning is started from this point. 
    The average combined effect of wind, current, and other sources of error 
since the last fix can be determined by comparison of a fix with the position 
the craft would have occupied had there been no disturbing force. It is for 
this reason, in part, that many marine navigators prefer to use course steered 
and speed at which the vessel is propelled by its primary source of power for 
their dead reckoning. For convenience, they consider the entire offset effect 
as the result of current. The direction from the dead-reckoning position to a 
fix at the same time is called the set of the current. The distance between 
these two positions, divided by the time since the last fix, is called the 
drift (speed) of the current. Air navigators consider the entire offset effect 
as the result of wind, determined by comparison of a no-wind position, 
sometimes called an air position, with a fix at the same time. 
    Whatever other forms of navigation may be employed, dead reckoning is 
generally considered essential to safe navigation. It provides a continuous 
position which is valuable in the evaluation of all other navigational 
information received. It is always available in some form. 
    Aboard ship, distance or speed is measured by means of a log or by an 
engine revolution counter. The pitometer log, now in wide use, uses a 
pitot-static tube. The Forbes log uses a small rotor in a tube projecting 
below the bottom of the vessel. An electromagnetic log has a sensing element 
which produces a voltage directly proportional to speed through the water. 
    In aircraft, speed through the air is measured by means of an airspeed 
indicator or Mach meter. The latter provides an indication of speed in units 
of the speed of sound, which varies with density of the atmosphere. For 
measurement of air speed a pitot-static tube is generally  used with a 
suitable registering device. See AIR-VELOCITY MEASUREMENT. 
    Two sophisticated systems for performing the dead-reckoning process have 
been developed. One of these determines course and speed of an aircraft over 
the surface by measuring the Doppler shift of echoes of radio signals directed 
downward at an angle. Ships use sound signals in the same manner. The other 
system, employed both in aircraft and in ships, uses accelerometers and 
gyroscopes, with a good heading reference, to determine accelerations in 
various directions. These are converted by integration to components of speed 
and distance in each direction. 
    As speed of aircraft and volume of air traffic have both increased, and 
the need for following an assigned track to maintain established separation 
standards has developed, a need has arisen to reduce the pilot workload and 
decrease the time lag associated with the traditional dead-reckoning process. 
This need is being met by the development of light, compact, reliable digital 
computers suitable for use in aircraft. Errors can be reduced significantly by 
the use of a Doppler or inertial navigator, or a combination of both. See 
ELECTRONIC NAVIGATION SYSTEMS; PILOTING. 
 �34s[Alton B Moody]
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�6v�2v�32sDeafness   �21m �33s 
    Impairment of hearing, usually of severe degree, but commonly applied to 
any hearing impairment. The generic term hearing-impaired includes those with 
mild impairment, called hard of hearing, and those with severe impairment, 
called deaf. Deafness, both temporary and permanent, results from a broad 
range of etiologies. 
    Types of deafness are based on time of onset and on the nature of the 
impairment. Congenital deafness is present from birth, and may vary in 
severity from mild to profound. Adventitious deafness occurs at any time 
during a lifetime and may have a course of either gradual, progressive 
development or of sudden onset. The three broad categories based on the nature 
of the impairment are: conductive deafness, involving an impairment of the 
mechanism that conducts sound to the sense organ; sensorineural deafness, 
caused by an abnormality of the sense organ in the inner ear or the auditory 
nerve; and central deafness, which results from some injury or failure to 
function in the central nervous system. Combined conductive and sensorineural 
impairment is referred to as mixed deafness. Modern diagnostic tests are 
making a number of new distinctions possible within these broad types of 
deafness. See AUDIOMETRY; EAR. 
    Impedance of sound to the inner ear may be caused by congenital 
malformations, impaction by hardened earwax (cerumen), or infections in the 
external canal. 
    A very common cause of conductive deafness is otitis media, infection of 
the middle-ear cavity, that may begin with a common cold, allergy, or upper 
respiratory infection. It is the most common cause of conductive hearing 
impairment in children. Another major cause of conductive impairment is the 
hereditary disease otosclerosis. Portions of the bony capsule surrounding the 
inner ear, normally the hardest bone in the body, are decalcified and replaced 
by a spongy, softer bone that grows and then hardens, often fixing the foot 
plate of the stapes at the point where the middle-ear conductive mechanism 
joins the inner ear. The sound reaching the inner ear is greatly attenuated. 
This progressive disease begins in youth, and hearing loss is first noted at 
adolescence or in the early twenties. 
    Other forms of hereditary deafness are quite common in a population with 
severe congenital hearing impairment. Genetic causes are often suspected when 
no diagnosis can be made or the cause is otherwise unexplained, and especially 
when parents have a common ancestor. 
    Many drugs and chemicals are considered damaging to the inner ear, or 
ototoxic. Dihydrostreptomycin, kanamycin, and neomycin are definitely 
ototoxic, and both quinine and salicylates are highly suspect as causes of 
sensorineural deafness. 
    Acoustic trauma results from brief exposure to sudden explosive noises. A 
blast of noise, as from an explosion or gunshot, may permanently disorganize 
the organ of Corti, tearing pieces of it loose into the surrounding fluid of 
the inner ear. Exposure to any steady loud noise, such as that of factories, 
aircraft engines, or highly amplified music, usually results in tinnitus and 
short-term, high-frequency hearing loss referred to as temporary threshold 
shift. Eventually, with habitual exposure, permanent threshold shift will take 
place.  
    Deafness of old age, called presbycusis, is one of the most common causes 
of sensorineural deafness, particularly as life expectancy increases. Several 
aging processes contribute to presbycusis. Some changes take place in the 
middle ear with age, especially the loss of elasticity of connective tissue; 
some of the sensory cells of hearing are lost with age; or there may be a loss 
of neurons in the central nervous system, particularly with arteriosclerosis. 
    Prevention is the best treatment for deafness. Immunization for viral 
diseases, early medical intervention for upper-respiratory infections or 
earache, teaching children to blow their noses gently with both nostrils open, 
control of allergies, avoiding ototoxic drugs, and reduced exposure to loud 
noise are obvious prophylaxes. Routine hearing screening in pediatric offices, 
in well-baby clinics, and in schools, with appropriate medical follow-up, is 
also advisable. 
    Nonmedical aural rehabilitation for persons with deafness includes the 
judious selection of personal electroacoustic hearing aids, auditory training 
and training to use amplification, instruction in speech reading, and 
conservation of speech by consciously relating speech movements to their 
tactile and kinesthetic sensations. See HEARING (HUMAN). 
 �34s[Donald R. Calvert]
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�6v�2v�32sDeath   �21m �33s 
    Cessation of life functions. This can involve the whole organism (somatic 
death), individual organs (organ death), individual cells (cellular death), 
and individual parts of cells (organelle death). Although the next smaller 
level of organization, the macromolecules which make up the cell organelles, 
may also cease to function, their disintegration is ordinarily not spoken of 
as death. 
  
    Pathogenesis.  A normal animal, cell, or organelle exists in a normal 
state of homeostasis, or a normal ``steady state.'' Following a disturbance of 
the normal steady state, referred to as an ``injury,'' the living system can 
be assumed to undergo a change in steady-state level, that is, to exhibit an 
altered steady state (see illustration). Such altered steady states can be 
regarded as having increased, decreased, or unaltered levels of ability to 
maintain homeostasis. An altered steady state with increased homeostatic 
ability might include the hypertrophy of a muscle cell resulting from 
continued exercise; and altered steady state with decreased homeostatic 
ability is the atrophy or shrinking of cells that occurs following disuse or 
lack of stimulation by hormones or suitable nutrients. In the case of injuries 
that do not result in the death of the living system, the system may persist 
in one of the new or altered steady states for long periods up to many years; 
that is, although the system differs significantly from the normal steady 
state, it is able to continue its existence and to maintain homeostasis. On 
the other hand, certain severe injurious stimuli result in an altered state 
that ultimately passes a point beyond which recovery of homeostatic ability is 
impossible even if the injurious stimulus or situation is removed. This point 
may be considered as a point of no return or the point of death of the living 
system. Following this point, the system undergoes exponential decay, leading 
to complete equilibrium with its environment. This period of decay, which is 
characterized by reactions resulting in the breakdown of structural components 
of the system, is referred to as necrosis. See HOMEOSTASIS. 
    Somatic death. In the biological sense, somatic death refers to the 
cessation of characteristic life functions. In higher organisms it can be 
assumed to result from an interplay between the genetic background of the 
individual and the effects of the environment. Death is, in a sense, the price 
of differentiation; in another sense, protoplasm is immortal and all life 
springs from preexisting life. The undifferentiated amebas need never die, 
only divide. It would appear that death is inevitable for higher organisms 
such as mammals and, in a sense, life is a series of little deaths. 
    Developments in medicine and associated technology involving supportive 
therapy, resuscitation, hypothermia, extracorporeal circulation, and organ 
transplantation have reopened serious questions regarding the definition of 
when an individual is ``dead.'' The vagueness and inconsistency in previous 
legal and medical definitions have lead to serious dilemmas. These are of 
particular importance in the selection of donors for transplantation of vital 
organs such as the heart. 
    Medical techniques have shown that individuals can be sustained for 
prolonged periods without spontaneous respiratory or cardiac movements as well 
as without electrical evidence of central nervous system activity. These 
techniques have also achieved the resuscitation of individuals who would 
formerly have been considered dead and have clearly shown the survival of 
transplanted organs from cadavers. Just as organs may survive after the death 
of the organism, the organism, properly supported by mechanical or other 
means, can survive the death of organs, including the brain. 
    The following are changes that occur in the body after somatic death: 1. 
Cooling (algor mortis) of the body occurs postmortem. 2. Rigidity (rigor 
mortis) or stiffening of the musculature usually begins within 5 - 10 h and 
begins to disappear after 3 or 4 days. 3. Staining (livor mortis) occurs, that 
is, there is reddish-blue discoloration in the dependent portions of the body 
resulting from the gradual gravitational flow of unclotted blood. 4. Blood 
begins to clot shortly after death and sometimes prior to cessation of cardiac 
function. 5. Autolysis (necrosis) occurs in the cells of the body following 
somatic death. 6. Putrefaction, bacterial and enzymatic decomposition, begins 
shortly after death, caused by microorganisms which enter the body. 
  
    Organ death. Following somatic death, all organs do not die at the same 
rate. Length of survival can be prolonged by reducing the temperature of the 
organs to near 32 DGF (0DGC), which appreciably retards metabolic function and 
extends survival time. This survival of organs after the death of the host 
forms the basis for organ transplantation from cadavers. 
    Cellular death. Death and subsequent necrosis of cells form an important 
part of many reactions of cells to injury in disease states. The death and 
disintegration of cells within a living animal incite a vascular reaction 
called an inflammatory reaction on the part of the host. It is characterized 
by a series of vascular phenomena which include the migration of leukocytes 
(white blood cells) from the blood capillaries into the area of tissue damage 
with subsequent digestion and phagocytosis of the cellular debris resulting 
from cellular necrosis. 
 �34s[Benjamin F. Trump]
�33s 
  
    Issues.  The advancements of modern medicine have brought significant 
changes in the ways of life and death, and challenge former concepts and 
definitions of death. As the determination of death becomes increasingly 
complex in a technological society, the definition of death becomes 
correspondingly more complex. The meaning of death is a philosophical concept 
which lies outside the scientific point of view. Although dying is a process 
which may continue over a considerable period of time, for all practical 
purposes death occurs at that point where there is an irreversible loss of 
one's essential human characteristics. Such loss indicates the death of the 
organism as a whole, the death of a person. 
    An interdisciplinary committee at the Harvard Medical School examinined 
possible new criteria for determining death. In 1968 they cited four criteria 
as determinative of a permanently nonfunctioning brain: unreceptivity and 
unresponsivity to externally applied stimuli and inner need; absence of 
spontaneous movements of breathing; absence of reflexes other than spinal 
column reflexes, and fixed dilated pupils; and a flat electroencephalogram 
(EEG) to be used as a confirmatory but not mandatory test. 
    These tests were to be repeated after 24 h, and in the absence of 
hypothermia or depressant drugs, were to be considered a confirmation of brain 
death. It has since been conclusively demonstrated that when the flow of blood 
to the brain has been interrupted for approximately 15 min there is a uniform 
necrosis and liquefaction of the brain. This definition appears to be a major 
milestone in determining new criteria for making the determination that death 
has occurred. 
    With regard to the issue of organ transplantation, it is contrary to moral 
intuitions that a dying person might be prematurely defined as dead in order 
that someone else might harvest the organs. A moral obligation exists to treat 
the living as such until death occurs. To avoid a conflict in the role of the 
physician, it has become common practice that the physician for the recipient 
of an organ donation is not party to the judgment that the donor has died. The 
needs of a patient for a viable organ must not contaminate the judgment that 
another person has died. 
    The ability to define death with more precision is important in cases 
involving the comatose patient. It is generally accepted as both a moral and 
legal right that competent persons have the right to refuse medical treatment. 
Increasingly, dying patients claim a more active role in self-determination 
regarding medical treatment in the final stage of life. 
    One of the more radical issues is that of actively ending the life of a 
suffering person or helping another to die through a positive action. Some 
ethicists see no moral distinction between removing life-support systems or 
lifesaving medications which allow a patient to die and taking a direct action 
which causes a patient to die, since the intention and the consequence are the 
same in both cases. Others disagree, claiming that there is indeed a morally 
significant difference if a person dies of the underlying disease and is not 
made to endure still greater suffering by prolonging the dying process than if 
the person dies because another has caused the death through an intentional 
and fatal action. 
 �34s[Thomas R. McCormick]
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�6v�2v�32sDe Broglie wavelength �21m �33s
    The wavelength LAMBDA = h/p associated with a beam of particles (or with a
single particle) of momentum p; h = 6.61 x 10- sup 2 sup 7 erg-second is
Planck's constant. The same formula gives the momentum of an individual photon
associated with a light wave of wavelength LAMBDA. This formula, along with
the profound proposition that all matter has wavelike properties, was first
put forth by Louis de Broglie in 1924, and is fundamental to the modern theory
of matter and its interaction with electromagnetic radiation. See
NONRELATIVISTIC QUANTUM THEORY; QUANTUM MECHANICS.
 �34s[Edward Gerjuoy]
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�6v�2v�32sDecapoda (Crustacea) �21m �33s
    One of the more highly specialized orders of the class Crustacea. This
order includes the shrimps, lobsters, hermit crabs, and true crabs. The order
is so diverse that satisfactory definition is difficult, but a few characters
are common to nearly all decapods. The most obvious is a head shield, or
carapace, which covers and coalesces with all of the thoracic somites and
which overhangs the gills on each side. The first three of the eight pairs of
thoracic appendages are specialized as maxillipeds and closely associated with
the true mouthparts. The gills are usually well developed and arranged in
several series.
    Decapods vary in size from less than 1/2 in. (1.25 cm) in length to that
of the giant Japanese crab, the largest living arthropod, a spider crab which
may span more than 12 ft (3.6 m) between the tips of the outstretched claws.
Although most decapods are found in the sea, they are by no means restricted
to that habitat. Crayfishes are well-known inhabitants of fresh-water streams
and ponds, as are several kinds of shrimps and some true crabs. A number of
crabs and hermit crabs have become well adapted to a terrestrial existence far
from water; they return to the sea only seasonally to hatch their eggs. Many
crayfishes burrow in the ground, and one species of crab spends its entire
life in the tops of lofty trees.
    In a general way, the decapods may be divided into two groups: the
long-tailed and the short-tailed forms. The long-tailed species, such as the
shrimps and lobsters, have a more or less cylindrical, or laterally
compressed, carapace that often bears a head spine or rostrum. The large,
muscular abdomen permits the shrimp, or lobster, to dart quickly backward out
of danger, but the succulence of this tail makes the animals prey to numerous
enemies, including humans. In the short-tailed species, the crabs, the
carapace is often broadened and flattened dorsally and usually does not form a
rostrum. The much-reduced and feeble abdomen is tucked under the thorax, where
it serves the female as a brood pouch for eggs.

    Physiology.  The nervous system is somewhat variable. Although usually
fewer, there may be as many as 11 ganglia, 5 thoracic and 6 abdominal, in the
ventral nerve chain posterior to the subesophageal ganglion. The sense organs
include eyes, statocysts, olfactory filaments, and tactile setae. The eyes are
well developed in most decapods, but they may be reduced or entirely absent in
species living in the deep sea, in caves, or in burrows. Luminous organs of
various kinds occur in a number of deep-sea decapods, and some shrimps have
the ability to eject a luminous fluid.
    As in most crustaceans, the alimentary tract is a more or less straight
tube divided into three parts: the foregut, the midgut, and the hindgut. The
first and last have a chitinous, and sometimes calcified, lining. The foregut,
in most decapods, is dilated to form a stomach. Its anterior, or cardiac,
portion is at least partially lined with movably articulated, calcified
ossicles, controlled by a complex system of muscles called the gastric mill.
The posterior, or pyloric, part is provided with hairy ridges which combine to
form a straining apparatus. The midgut is variously furnished with blind
tubes, and a mass of minutely ramified tubules, known as the hepatopancreas or
liver, empties through one or more ducts into the anterior end of the midgut.
    The heart is short, polygonal, and situated under the posterior part of
the carapace. It has three or more pairs of venous ostia, and two pairs of
arteries lead from it, in addition to the median anterior and posterior
arteries. The blood collects in a venous sinus in the midventral line, passes
to the gills, and then back to the pericardial sinus surrounding the heart.
    In practically all decapods, respiration is accomplished by a series of
gills, attached to the lateral walls of the thoracic somites and to the basal
segments of the thoracic appendages. A continuous current of water is drawn
through the branchial chamber and over the gills by the fluttering motion of
the outer lobe of the second maxilla. The current usually moves forward, but
this direction may be periodically reversed in burrowing species. In those
crabs best adapted for life on land, the action of the gills is supplemented
by the lining membrane of the branchial chamber. This chamber is covered with
minute villi and is unusually well supplied with blood vessels, and thus
functions as a lung.
    The antennal gland is the chief excretory organ in all decapods. Maxillary
glands are often present in the larval stages, but they never persist in the
adult. The antennal gland is compact and more complex than in any other
crustacean group. It consists of three divisions: the saccule, which is
usually partitioned or ramose; the labyrinth, a spongy mass with a complex
system of canals; and the bladder with a duct leading to the exterior.
    The sexes are separate in most decapods, although protandric
hermaphroditism occurs in some shrimps. The testes most often lie partly in
the thorax and partly in the abdomen, and they are usually connected across
the midline. The ovaries resemble the testes in shape and position.

    Development.  In only one group of shrimps are all of the presumably
primitive larval stages represented. The first stage is a typical crustacean
nauplius with an unprotected oval body bearing a median eye and three pairs of
appendages. This stage is followed by a metanauplius in which four more
appendage rudiments appear. The third pair of the original appendages becomes
less of a swimming organ and more like a pair of primitive mandibles. In the
third stage, the protozoea, the seven pairs of appendages become more highly
developed, a carapace covers the anterior part of the body, the abdomen is
clearly formed though unsegmented at first, the rudiments of paired eyes
appear, and the heart is formed. In the next stage, the zoea, the eyes become
movable and the carapace develops a rostrum. All thoracic appendages are
present, at least in rudimentary form, and those of the abdomen appear,
especially the uropods. In the last larval stage, the mysis, the
well-developed thoracic appendages replace the antennae as the chief swimming
organs. The abdomen increases markedly in size and takes on a form similar to
that of the adult.

    Phylogeny.	No true decapod fossils are known with certainty from
Paleozoic deposits, but decapod shrimps are not uncommon in Triassic and,
especially, Jurassic rocks. Lobsterlike forms related to Recent deep-sea
species also appeared in the Triassic; true lobsters are known from the
Jurassic, and crayfishes from the Cretaceous. Crabs are well known as fossils,
with primitive groups appearing in the Jurassic; several more specialized
groups are found in the Cretaceous, and all of the major existing groups were
represented in the Tertiary.
    The evolution of the decapods has not been satisfactorily worked out and
may never be known completely. The presence of a nauplius larva in the
development of some shrimps suggests a link with the primitive crustaceans,
but it can hardly be doubted that the Decapoda, as a group, are highly
specialized. They are certainly closely related to the Euphausiacea, and
fusion of the two groups has been suggested by some authors.

    Classification. The classification of the 8500 living species of decapods
presents a difficult problem that has not yet been entirely solved. The chief
disparity seems to lie between the shrimps (Natantia) and all of the other
decapods (Reptantia), as follows:

  Order Decapoda
    Suborder Natantia
	Section Penaeidea
	Section Caridea
	Section Stenopodidea
    Suborder Reptantia
	Section Macrura
	Section Anomura
	Section Brachyura

    Penaeidea. The primitive section of the Decapoda, penaeideans, is
distinguished by the following characters: The pleura of the first abdominal
somite overlap those of the second; the  third legs are nearly always chelate,
but are no stouter than the first pair; the first pleopods of the male bear a
complicated flaplike appendix, the petasma; and the females have a
spermatophore receptacle, the thelycum, on the ventral surface of the
posterior thoracic somites. The best-known penaeideans, including most of the
commercially important shrimps or prawns of the warmer seas (Penaeidae), live
on muddy bottoms in shallow or moderate depths. However, they also occur both
in the deep sea and as pelagic organisms in the mid-depths. There are more
than 300 species of living penaeideans.
    Caridea. This is the largest and most diverse group of shrimps and prawns.
In this group, the pleura of the second abdominal somite overlap those of the
first; the third legs are never chelate; and there is no petasma or thelycum.
The sexes can usually be distinguished by the presence, in the male, of two
stylets located on the inner edge of the inner branch of the second pleopods.
Carideans are found in all parts of the sea, often in association with other
marine animals. Members of at least two families, the Atyidae and
Palaemonidae, are also widespread in fresh water. The edible shrimps and
prawns of northern Europe and of northwestern North America, of the genera
Crangon and Pandalus, belong to the Caridea, as do the large fresh-water
prawns of the tropical genus Macrobrachium. Most shrimps and prawns are
favorite foods of fishes, but some carideans maintain ``service stations'' for
fishes where they remove parasites from the outer skin, mouths, and gills of
the fish. More than 1500 species of Caridea are known.
    Stenopodidea. This is a small group of shrimps which superficially
resemble the Penaeidea. The third pereiopods are chelate but are much longer
and stouter than the first pair; the pleura of the second abdominal somite do
not overlap those of the first; and there is no petasma or thelycum. All of
the 20 or more living species are marine; some are closely associated with
other animals, such as sponges.
    Macrura. This long-tailed group of the Reptantia includes the deep-water
and fossil eryonids, the spiny lobsters, the true lobsters, and the mud
shrimps (Fig. 1). The abdomen is extended and bears a well-developed tail fan.
There are about 700 known living species.
    The Eryonidea are rather thin-shelled, blind inhabitants of the depths of
the sea. The carapace is flattened dorsally and considerably expanded
laterally. The first four, or all five, pairs of pereiopods are chelate, with
the first pair much elongated. There are about 40 living species; the group
was probably more numerous in ancient seas.
    The Scyllaridea include the spiny lobsters or langoustes (Palinuridae) and
the Spanish, or shovel-nosed, lobsters (Scyllaridae). They are heavily armored
like the true lobsters but are distinguished by the absence of a rostrum and
of chelae, except occasionally on the last pereiopod of the female. They are
abundant in shallow and moderate depths of warm and temperate seas, where they
are often of considerable commercial importance. There are about 85 Recent
species.
    The true lobsters and crayfishes, Nephropidea, also have a firm shell but
are also characterized by a rostrum and by chelae on the first three pairs of
pereiopods, with the first pair being noticeably larger than the others. Most
lobsters (Nephropidae) are found in cool seas or in the cool, off-shore waters
of the tropics. The most familiar lobsters (Homarus) are those found along the
Atlantic coasts of Europe and North America. The Norway lobster of Europe
(Nephrops) is also of some commercial importance but is smaller and less
meaty. The crayfishes (Cambaridae, Astacidae, Parastacidae) are widespread in
fresh waters of the temperate regions of all continents except Africa. They
are of commercial importance in southern Europe and Australia. More than 300
living species of lobsters and crayfishes are known, more than half of them
being from the fresh waters of the United States.
    The mud shrimps, Thalassinidea, are usually thin-shelled, burrowing
crustaceans with large, chelate or subchelate first pereiopods, and no chelae
on the third pereiopods. More than 250 living species are found in shallow and
deep seas throughout the world, especially in tropical and warm temperate
regions.
    Anomura. This intermediate, and possibly unnatural, group lies between the
Macrura and the Brachyura. It includes the galatheids, the porcelain crabs or
rock sliders, the hermit crabs, the king crabs, and the mole crabs or hippas.
In nearly all of these diverse crustaceans, the first, and sometimes the last,
pereiopods are chelate or subchelate. The abdomen is usually bent forward
ventrally or is asymmetrical, soft, and twisted. There are about 1300 Recent
species.
    Brachyura. These are the true crabs (Fig. 2). The abdomen is symmetrical,
without a tail fan, and bent under the thorax. The first pereiopods are always
chelate or subchelate. The true crabs are as numerous as all other decapods
combined, numbering nearly 4500 Recent species. They are divided into four
subsections.
    The Gymnopleura include about 30 species of primitive burrowing crabs;
their carapaces are more or less trapezoidal or elongate, the first pereiopods
are subchelate, and some or all of the remaining pereiopods are flattened and
expanded for burrowing.
    The Dromiacea is a primitive group of about 200 species. The first
pereiopods are chelate; the last pair is dorsal in position and modified for
holding objects, such as sponges, tunicates, and bivalve mollusk shells, over
the crab.
    In Oxystomata the first pair of pereiopods are chelate and the last pair
are either normal or modified as in the Dromiacea. The oxystomes include the
mask crabs (Dorippidae), the box crabs (Calappidae), and the purse crabs
(Leucosiidae). There are nearly 500 Recent species.

    Most of the crabs, more than 3700 living species, belong to the
Brachygnatha, in which the last pereiopods are rarely reduced or dorsal in
position. To this group belong the swimming crabs (Portunidae). Also worthy of
mention are freshwater crabs (Potamidae, Pseudothelphusidae, and
Trichodactylidae), found in tropical and some temperate regions, usually in
areas not inhabited by crayfishes. The crabs of the genus Cancer are large and
abundant enough to be of commercial importance in northern Europe and North
America. The rock, or Jonah, crabs of New England and the Dungeness crab of
the Pacific Coast are familiar edible crabs of this genus. The ubiquitous
little mud crabs (Xanthidae) are well known to every visitor to rocky shores.
One of them, the stone crab (Menippe), is highly esteemed by connoisseurs of
seafood. The pea crabs (Pinnotheridae) are often found in oysters and other
bivalve mollusks. The square-backed crabs (Grapsidae) are characteristic of
warm, marshy areas but are not restricted to that habitat; they mark the trend
toward the true land crabs (Gecarcinidae), the depredations and intrusions of
which are familiar to all who live in the tropics. The ghost crabs (Ocypode),
which scuttle almost unseen over sandy beaches, and the fiddler crabs (Uca) of
muddy shores, are closely related. The end of the list is reached with the
spider, or decorator, crabs (Majidae), slow-moving animals that often conceal
themselves by attaching seaweeds and various sessile animals to their
carapaces. One of them (Macrocheira) is the largest arthropod now alive. See
ARTHROPODA; CRAB; CRAYFISH; CRUSTACEA; LOBSTER; SHRIMP.
 �34s[Fenner A. Chace, Jr.]
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�6v�2v�32sDecapoda (Mollusca)   �21m �33s 
    An order of dibranchiate cephalopods, containing the squids and 
cuttlefishes. They are characterized by the possession of eight arms and two 
long, often retractile, tentacles. The cuttlefishes (Sepia) are confined to 
the Old World. Their shells are used in the manufacture of dentifrices and 
cosmetics, and artist's sepia is made from the ink. The true squids are 
slender, swift swimmers, and some are of economic importance. Two natural 
groups occur. The Myopsida, represented by Loligo, have the eye covered by the 
skin of the head and are mainly coastal species. The oceanic Oegopsida have 
the eyeball exposed to the water and the suckers may be modified into hooks. 
Both the giant Architeuthis and Illex belong to this group. See CEPHALOPODA; 
DIBRANCHIA.  
 �34s[Gilbert L. Voss]
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�6v�2v�32sDecca   �21m �33s 
    A hyperbolic navigation system which establishes a line of position from 
measurement of the phase difference between two continuous-wave signals. The 
intersection of the two lines of position from two pairs of transmitting 
stations establishes a navigational fix, or location. Charts surprinted with 
Decca hyperbolas of constant phase difference are used to show the navigator's 
position. Moving charts upon which a pen traces the track of the craft can be 
used to show position and provide a permanent record. 
    The two lines of position required for a fix can be provided by one 
``master'' and two ``slave'' transmitters. A third slave located at the apex 
of a triangle in which the master is centered is also used (see illustration). 
Since continuous waves are employed, each station must use a different radio 
frequency. See HYPERBOLIC NAVIGATION SYSTEM. 
 �34s[Alexander A. McKenzie]
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�6v�2v�32sDecibel   �21m �33s 
    A logarithmic unit used to express the magnitude of a change in level of 
power, voltage, current, or sound intensity. A decibel (dB) is 1/10 bel. 
    In acoustics a step of 1 bel is too large for most uses. It is therefore 
the practice to express sound intensity in decibels. The level of a sound of 
intensity I in decibels relative to a reference intensity IR is given by 
notation (1). Because sound intensity is proportional to the square of sound 
pressure P, the level in decibels is given by Eq. (2). The reference pressure 
is usually taken as 0.0002 dyne/cm sup 2 or 0.0002 microbar or 20 micropascals. 

   
                         *** 10 log 10 (I/IR) ***                        (1)
  
           *** 10 log 10 (P sup 2/PR sup 2) = 20 log 10 (P/PR) ***       (2)
 
    The neper is similar to the decibel but is based upon natural (napierian) 
logarithms. One neper is equal to 8.686 dB. See NEPER; VOLUME UNIT (VU). 
 �34s[Karl D. Kryter]
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�6v�2v�32sDeciduous plants   �21m �33s 
    Plants that regularly lose their leaves at the end of each growing season. 
Dropping of the leaves occurs at the inception of an unfavorable season 
characterized by either cold or drought or both. Most woody plants of 
temperate climates have the deciduous habit, and it may also occur in those of 
tropical regions having alternating wet and dry seasons. Many deciduous trees 
and shrubs of regions with cold winters become evergreen when grown in a warm 
climate. Conversely, such trees as magnolias, evergreen in warm areas, become 
deciduous when grown in colder climates. 
 �34s[Nelle Ammons]
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�6v�2v�32sDecimal number system   �21m �33s 
    The system, used in ordinary arithmetic, in which numbers are represented 
as linear combinations of powers of 10. The base 10 has as its origin the fact 
that humans have a total of 10 digits on both hands. Other than this, the 
decimal number system has little to recommend it over other systems. See 
NUMBER SYSTEMS. 
 �34s[Marvin Yelles]
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�6v�2v�32sDecision theory �21m �33s
    A broad spectrum of concepts and techniques which have been developed to
both describe and rationalize the process of decision making, that is, making
a choice among several possible alternatives. In the broadest sense, one can
differentiate between prescriptive decision theory, which formulates how
decisions should be made, and descriptive decision theory, which deals with
how people actually make decisions. Behavioral and social scientists and
psychologists are trying to discover more elaborate descriptive models of the
decision process in order to provide mathematicians, economists, strategists,
business managers, and others with more sophisticated prescriptive
decision-making procedures.
    The information necessary for a rational decision is usually summarized in
a decision matrix, as shown in the illustration. The alternate pure
(independent from each other) or mixed (combinations of pure strategies)
strategies (S1 to Sn) are listed in the first column. Across the top all
possible states of nature (N1 to Nm) which might affect the outcome of the
various strategies are enumerated. The probabilities of occurrence of various
states of nature (p1 to pm) must be estimated and listed with the states of
nature. Since these values will critically influence the decision, they should
be as reliable as possible. In value theory one distinguishes between the
subjective probability of occurrence which reflects the decision maker's
opinion on what the probability is and the objective probability which is
derivable mathematically or as a result of observations.
    Corresponding to each interaction of a strategy (Si) and a state of
nature (Nj), one determines a consequence or outcome (Oij) in terms
of some value or utility parameters. The outcome may sometimes be easily
computed, for example, the interest cost on a business loan, but quite often
must be based on insufficient data, past experience, or questionably valid
assumptions, such as would be the case in trying to assess the outcome of a
limited war. The generation of reliable outcome data can be expensive and
time-consuming, and a probability of accuracy is often associated with each
outcome. The question of how to assess the relative value or utility of mixed
parameters to present a single parameter for the outcome is still a paramount
question in decision and utility or value theory.
    The information in the decision matrix is used to generate the last column
which describes the overall expected outcome, or expected value (EV), or
expected utility (EU), or expected profit for each strategy. Depending upon
the confidence one places in the prediction of the probabilities of the states
of nature, the decision process is classified as one of three types.
    In the case of decision making under certainty (DMUC) the state of nature
which will interact with the strategies can be defined with sufficient
certainty so that its probability is close to 1. This case reduces the
decision matrix to one column showing the outcomes (profit, cost, and utility)
of each strategy with the state of nature. Decision theory indicates that,
assuming the alternate strategies are equal in all other respects, the one
which will maximize the utility (maximize profit or minimize cost) should be
selected. Since in many cases the dependent variable (outcome) is continuously
related to the independent variable (strategy), one can obtain relationships
which can be used to analytically find the strategy which will correspond to
maximum utility. The process is normally termed optimization, which in its
broadest interpretation refers to decision making under certainty. See
OPTIMIZATION.
    Decision making under risk (DMUR) implies that, although a number of
possible states of nature may interact with each strategy, the probability of
occurrence of each of these is known with some degree of certainty. The
expected value or utility for each strategy is obtained as a weighted average
sum of the individual possible outcomes or, in mathematical terms,

       *** (EV sub i) = SUMMATION (j = 1 TO m) (p sub j) (O sub ij) ***

The condition of dominance is usually used to eliminate all strategies which
are dominated. A dominant strategy has more desirable outcomes under any of
the possible states of nature than the dominated strategy.
    If the probabilities of the states of nature cannot be predicted, then the
decision has to be made under conditions of uncertainty (decison making under
uncertainty; DMUU). Decision theory provides a number of techniques to deal
with this situation. Which of these will work best in a given situation is
questionable, and their application depends primarily on the philosophical
outlook of the decision maker.
    1. Equal probability criterion: This is also known as the Laplace
criterion and assumes that all states of nature considered are equally likely
(that is, p sub j = 1/m). The decision matrix is then used just as in DMUR.
    2. Maxim criterion: This represents a pessimistic outlook in that it
prescribes the strategy which will maximize the minimum profit.
    3. Maximax criterion: The optimist will like this one since it chooses the
strategy which will maximize the maximum possible profit (the best of the best
outcomes in each row).
    4. Hurwicz criterion: A coefficient of optimism is defined as ALPHA = 1
corresponding to a complete optimist, and ALPHA = 0 corresponding to a
complete pessimist. The maximum possible profit for each strategy is then
multiplied by ALPHA and the minimum profit by (1 - ALPHA) to find the best
expected value.
    5. Regret criterion: Also known as the Savage principle, this criterion is
used to construct a regret matrix in which each outcome entry represents a
regret defined as the difference between the best possible outcome and the
given outcome. The matrix is then used as in DMUR with expected regret as the
decision-determining quantity. See GAME THEORY.


 �34s[Igor Paul]
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�6v�2v�32sDeclinometer   �21m �33s 
    A geomagnetic instrument for measuring magnetic declination, sometimes 
called variation of the compass, which is defined as the angle between true or 
geographic north and magnetic north - the direction of the magnetic meridian. 
The instrument is suited for determining the direction in which a perfect 
compass needle would point and for comparing this with the known true azimuth 
of a fixed reference point or a celestial object such as the Sun or the North 
Star.  
    The compass declinometer employs a thin compass needle 6 in. (15 cm) long, 
supported on a sapphire bearing and steel pivot of high quality. Peep sights 
serve for aligning the compass box on an azimuth mark. The accuracy attained 
is 2 - 3 minutes of arc in middle latitudes. 
    The transit declinometer, a surveyor's transit, built to exacting 
specifications with respect to freedom from traces of magnetic impurities and 
quality of the compass needle, has a 17-power telescope for sighting on a mark 
and for making solar and stellar observations to determine true directions. A 
microscope mounted on the side of the telescope permits a more accurate 
alignment of the instrument axis with the magnetic needle (see illustration). 
    For magnetic fieldwork of highest accuracy and for standard calibrating 
instruments in a magnetic observatory, the precision declinometer (sometimes 
called a magnetometer) employs a cylindrical permanent magnet suspended in a 
horizontal position by a fine gold ribbon and equipped either with a 
collimating lens and cross hair or with a mirror. The position of the magnet 
is observed with a suitable telescope and scale, which are integral parts of 
the equipment. The accuracy for an observatory declinometer is 0.1 - 0.2 
minute of arc. See MAGNETOMETER. 
 �34s[James H. Nelson]
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�6v�2v�32sDecomposition potential   �21m �33s 
    The electrode potential at which the electrolysis current begins to 
increase appreciably. Decomposition potentials are used as an approximate 
characteristic of industrial electrode processes. See ELECTROCHEMICAL PROCESS; 
ELECTROLYSIS.  
    Decomposition potentials are obtained by extrapolation of 
current-potential curves. Extrapolation is not precise because there is a 
progressive increase of current as the electrode potential is varied. The 
decomposition potential, for a given element, depends on the range of currents 
being considered. The cell voltage at which electrolysis becomes appreciable 
is approximately equal to the algebraic sum of the decomposition potentials of 
the reactions at the two electrodes and the ohmic drop, or voltage drop, in 
the electrolytic cell. The ohmic drop term is quite negligible for 
electrolytes with high conductance. For a discussion of current-potential 
curves see OVERVOLTAGE. See also ELECTROLYTIC CONDUCTANCE. 
 �34s[Paul Delahay]
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�6v�2v�32sDecompression illness   �21m �33s 
    Symptoms in humans which result from a sudden reduction in atmospheric 
pressure. It is also called dysbarism, caisson disease, the bends, and 
compressed-air illness. It is most commonly seen in two groups of subjects, 
(1) those who rapidly ascend in nonpressurized airplanes to altitudes in 
excess of 18,000 ft (5500 m), and (2) divers, scuba divers, sandhogs, and 
professional workers in hyperbaric chambers who work under increased ambient 
pressures and are decompressed. See DIVING. 
    This condition is caused by the formation of nitrogen bubbles in the 
tissues and blood vessels. These bubbles plug vessels and expand in tissue 
spaces producing the characteristic symptoms and signs of this illness. 
Helium, when present in high concentrations utilized by divers or found in 
some spacecraft, can also produce a similar condition. 
    The onset of symptoms may occur at any time from a few minutes to several 
hours after decompression. The most common manifestation is pain in the joints 
and muscles. However, skin, respiratory, and neurologic symptoms are not 
uncommon. The skin manifestations are itching, discoloration, and edema 
(swelling). Respiratory symptoms are coughing and dyspnea (difficulty in 
breathing). The neurologic symptoms vary from mild paresthesia (sensations of 
tingling, crawling, or burning of the skin) and weakness to total paralysis; 
loss of bladder and rectal sphincter control is common. The severe forms of 
this illness are followed by circulatory failure, paralysis, coma, and death. 
    Treatment is recompression, followed by gradual decompression to normal 
atmospheric pressure. Prognosis is generally good except in those subjects who 
show central nervous system damage. 
 �34s[McChesney Goodall]
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�6v�2v�32sDecontamination of radioactive materials   �21m �33s 
    The removal of radioactive contamination which is deposited on surfaces or 
may have spread throughout a work area. Personnel decontamination is also 
included.  
    Decontamination methods follow two broad avenues of attack, mechanical and 
chemical. Commonly used mechanical methods are vacuum cleaning, sand blasting, 
blasting with other abrasives, flame cleaning, scraping, ultrasonic radiation, 
and surface removal (for example, removal of concrete floors with an air 
hammer). The principal chemical methods of decontamination are water washing, 
steam cleaning, and scrubbing with detergents, acids, caustics, and solvents. 
    Another important method of handling contamination is to store the 
contaminated object, or temporarily abandon the contaminated space. This can 
be done when the use of the material or space is not necessary for a period of 
time and the half-life of the contaminant is relatively short. 
    Personnel decontamination methods differ from those used for materials 
primarily because of the possibilities of injury to the person being 
decontaminated. Soap and water (sequestrants and detergents) normally remove 
more than 99% of the contaminants. If it is necessary to remove the remainder, 
chemical methods which remove the outer layers of skin upon which the 
contamination has been deposited can be used. These chemicals - citric acid, 
potassium permanganate, and sodium bisulfite are examples - should be used 
with caution and preferably under medical supervision, because of the 
increased risk of injury to the skin surface. It is very difficult to remove 
radioactive material once it is fixed inside the body, and the ensuing hazard 
depends very little on the method of entry into the body. See MONITORING OF 
IONIZING RADIATION. 
 �34s[Karl Z. Morgan]
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�6v�2v�32sDeep inelastic collisions �21m �33s
    A type of nuclear reaction in which the two nuclei interact strongly while
their surfaces overlap. Deep inelastic collisions of one nucleus with another
represent a class of reaction mechanism with interaction times intermediate
between direct and compound nucleus processes. They often occur in collisions
of heavy nuclei at center-of-mass energies less than 5 MeV per nucleon above
the Coulomb barrier. During the brief encounter of the two nuclei, energy and
mass flow from one nucleus into the other. After a short time, corresponding
to the time it takes for the intermediate dinuclear complex to make a partial
rotation (10- sup 2 sup 2 to 5 x 10- sup 2 sup 1 s), the two nuclei separate
again. Of the characteristic features of these collisions, two of the most
important properties are the sizable amounts of energy dissipation and nucleon
(neutron and proton) exchange. See NUCLEAR REACTION.
 �34s[John R. Huizenga]
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�6v�2v�32sDeer   �21m �33s 
    The name for 41 species of even-toed ungulates (artiodactylids) that 
comprise the distinct and homogeneous family Cervidae. The males have antlers, 
which are bony outgrowths on their heads that do not contain horn, are 
generally branched, and are shed yearly. The feet have four toes and, as such, 
are less developed than those of other ruminants. The third and fourth digits 
are well developed, while the second and fifth are small and do not touch the 
ground.  
    Musk deer occur in the mountains of central Asia at high altitudes in 
Tibet, Korea, Mongolia, and Siberia. They are solitary animals and are active 
nocturnally. The male has a scent gland, known as a pod or musk gland, in the 
skin of the abdomen in front of the navel that secretes musk, used as a 
fixative in the perfume industry. 
    A number of species of deer that occur throughout Eurasia are called 
European deer. The red deer (Cervus elaphus), one of the best-known species, 
is still common throughout Europe and also is found in southwestern Asia and 
northwestern Africa. These deer are closely related to the wapiti of North 
America.  
    One of the best-known species of American deer is the white-tailed deer 
(Odocoileus virginianus), which is also known as the Virginia deer. This 
animal is found to range in North America and into South America as far as 
Peru, where it prefers woods and thickets to heavily forested areas. In the 
western states the mule deer (O. hemionus) may be distinguished from the 
white-tailed deer by the long ears, black-tipped tail, and greater tendency to 
occur in herds. These animals range from Alaska to New Mexico and prefer more 
open habitats. The wapiti or North American elk (Cervus canadensis) is more 
common in Canada than in the United States. The Key deer of the Florida Keys 
(O. virginianus clavium) is in danger of extinction because of the destruction 
of its habitat. See ARTIODACTYLA; MOOSE; REINDEER. 
 �34s[Charles B. Curtin]
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�6v�2v�32sDefinite composition, law of   �21m �33s 
    The law that a given chemical compound always contains the same elements 
in the same fixed proportions by weight. Thus, whatever its source, silver 
chloride always contains 100 g (3.53 oz) of silver to every 32.85 g (1.159 oz) 
of chlorine. If a compound is formed by the union of m atoms of one element, 
each weighing a, with n atoms of another element, each weighing b, the 
composition by weight of one molecule of the compound is in the ratio ma:nb. 
This must be the composition of any mass of the compound, provided that all 
atoms of the same kind have the same weight. It is now known that this is not 
usually the case but that the atoms of an element may consist of a number of 
isotopes, having different masses. However, as long as any sample of the 
element always contains the same relative proportions of the isotopes, the law 
still holds. Much more widespread and serious departures from the law of 
definite composition occur in a large variety of solid compounds (the 
nonstoichiometric compounds). See ATOMIC WEIGHT; ISOTOPE; NONSTOICHIOMETRIC 
COMPOUNDS; STOICHIOMETRY. 
 �34s[Thomas C. Waddington]
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�6v�2v�32sDefoliant and desiccant   �21m �33s 
    Defoliants are chemicals that cause leaves to drop from plants; 
defoliation facilitates harvesting. Desiccants are chemicals that kill leaves 
of plants; the leaves may either drop off or remain attached; in the 
harvesting process the leaves are usually shattered and blown away from the 
harvested material. Defoliants are desirable for use on cotton plants because 
dry leaves are difficult to remove from the cotton fibers. Desiccants are used 
on many seed crops to hasten harvest; the leaves are cleaned from the seed in 
harvesting. Defoliants and desiccants have also been used during war to 
destroy vegetation. 
 �34s[Alden S. Crafts]
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�6v�2v�32sDegaussing   �21m �33s 
    Neutralization of the magnetization of a ship by properly located and 
oriented current-carrying coils which produce a magnetic field of desired 
strength and direction. See DEMAGNETIZATION. 
    A steel ship has structural components of many different ferromagnetic 
characteristics. Many parts become magnetized during the construction, and 
retain that magnetization for a long time. Other parts are ``soft'' iron and 
do not retain a magnetized condition permanently but become magnetized by 
induction in the magnetic field of the Earth. The ship then has a magnetic 
field. This magnetization of the ship causes deviation of the magnetic compass 
and may trigger magnetic mines or other explosive devices when the ship passes 
near them. See MAGNETIZATION. 
    One method for neutralizing the magnetic field of the ship is to install 
coils in which currents are maintained to produce components of the field that 
will neutralize the field due to the magnetization of the ship. These coils 
are called degaussing coils. 
 �34s[Kenneth V. Manning]
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�6v�2v�32sDegeneracy (quantum mechanics)   �21m �33s 
    A term referring to the fact that two or more stationary states of the 
same quantum-mechanical system may have the same energy even though their wave 
functions are not the same. In this case the common energy level of the 
stationary states is degenerate. The statistical weight of the level is 
proportional to the order of degeneracy, that is, to the number of states with 
the same energy; this number is predicted from Schroedinger's equation. 
In quantum mechanics and in other branches of mathematical physics, the term 
degeneracy is employed also to characterize the eigenvalues of operators other 
than the energy operator. See EIGENVALUE (QUANTUM MECHANICS); NONRELATIVISTIC 
QUANTUM THEORY. 
 �34s[Edward Gerjuoy]
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�6v�2v�32sDegenerative cardiomyopathy   �21m �33s 
    Cardiomyopathy characterized by degenerative changes and fibrosis within 
the myocardium. This may occur as a secondary change associated with 
generalized disease processes. Metabolic disorders such as nutritional 
deficiencies (protein, calorie or vitamin deficiencies such as beriberi), 
anemia, endocrine disorders, hypersensitivity, lupus erythematosus, and 
amyloidosis may induce secondary cardiomyopathy. Primary cardiomyopathies are 
often caused by excessive alcohol consumption and toxic chemicals such as 
chloroform, carbon tetrachloride, arsenic, and phosphorus. Many of the 
degenerative cardiomyopathies are of unknown etiology. See HEART DISORDERS. 
 �34s[N. Karle Mottet]
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�6v�2v�32sDegree-day   �21m �33s 
    A unit used in estimating energy requirements for building heating and, to 
a lesser extent, for building cooling. It is applied to all fuels, district 
heating, and electric heating. Origin of the degree-day was based on studies 
of residential gas heating systems. These studies indicated that there existed 
a straight-line relation between gas used and the extent to which the daily 
mean outside temperature fell below 65 DG F (18 DG C). 
    The number of degree-days to be recorded on any given day is obtained by 
averaging the daily maximum and minimum outside temperatures to obtain the 
daily mean temperature. The daily mean so obtained is subtracted from 65 DG F 
and tabulated. Monthly and seasonal totals of degree-days obtained in this way 
are available from local weather bureaus. 
    A frequent use of degree-days for a specific building is to determine 
before fuel storage tanks run dry when fuel oil deliveries should be made. 
Number of Btu which the heating plant must furnish to a building in a given 
period of time is  

      *** Btu required = heat rate of building MULT 24 MULT degree days ***

where ``Btu required'' is the heat supplied by the heating system to maintain 
the desired inside temperature. ``Heat rate of building'' is the hourly 
building heat loss divided by the difference between inside and outside design 
temperatures. When the estimating procedure is applied to buildings with high 
levels of internal heat gains, as in a well-lighted office building, then 
degree-day data on other than a 65 DG F basis are required. See AIR 
CONDITIONING; COMFORT HEATING; PSYCHROMETRICS. 
 �34s[C. George Segeler]
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�6v�2v�32sDegree of freedom (mechanics)   �21m �33s 
    Any one of the number of independent ways in which the space configuration 
of a mechanical system may change. A material particle confined to a line in 
space can be displaced only along the line, and therefore has one degree of 
freedom. A particle confined to a surface can be displaced in two 
perpendicular directions and accordingly has two degrees of freedom. A 
particle free in physical space has three degrees of freedom corresponding to 
three possible perpendicular displacements. A system composed of two free 
particles has six degrees of freedom, and one composed of N free particles has 
3N degrees. Any requirement which diminishes by one the degrees of freedom of 
a system is called a holonomic constraint. See CONSTRAINT. 
 �34s[Russell A. Fisher]
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�6v�2v�32sDe Haas  -  van Alphen effect   �21m �33s 
    An oscillatory behavior of the magnetic moment of a pure metal crystal 
with changes in the applied magnetic field B, at very low temperatures. It is 
named after its discoverers W. J. de Haas and P. M. van Alphen. This effect 
has its origin in the Bohr-Sommerfeld quantization of the orbits of conduction 
electrons under the influence of the magnetic field. 
    A measurement of the temperature dependence of the oscillation amplitude 
permits a determination of the cyclotron frequency, or equivalently the 
electron mass. In a metal, as in a semiconductor, electrons behave with an 
effective mass m* rather than the free electron mass, and the de Haas - van 
Alphen effect thus allows a measurement of m* in metals. See KINETIC THEORY OF 
MATTER.  
    Application of the Bohr-Sommerfeld quantization rules shows that the 
``period'' of the de Haas - van Alphen oscillations (it is a period in 
magnetic field not time) is given by the equation below. Here ap is the 

      *** TRIANGLE (1/B) = (2 PI PLANCK'S CONSTANT e) / (a sub p) ***

area of the orbit in momentum  space of an electron at the Fermi level, that 
is, the cross section of the Fermi surface; e is the charge of the electron and 
PLANCK'S CONSTANT is Planck's constant divided by 2 PI. By studying the 
dependence of ap on the direction CIRCUMFLEX OVER B of the magnetic field, 
sufficient information can be obtained to construct the detailed shape of the 
Fermi surfaces of a metal. 
    Virtually all metals available in sufficient purity have been studied by 
the de Haas - van Alphen effect. The effect is also the most powerful probe of 
Fermi surface properties in alloys and intermetallic compounds. See BAND 
THEORY OF SOLIDS; FERMI SURFACE. 
 �34s[J. B. Ketterson]
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�6v�2v�32sDehumidifier   �21m �33s 
    Equipment designed to reduce the amount of water vapor in the atmosphere. 
There are three methods by which water vapor may be removed: (1) the use of 
sorbent materials, (2) cooling to the required dew point, and (3) compression 
with aftercooling. See DEW POINT. 
    Sorbents are materials which are hygroscopic to water vapor. Solid 
sorbents include silica gels, activated alumina, and aluminum bauxite. Liquid 
sorbents include halogen salts such as lithium chloride, lithium bromide, and 
calcium chloride, and organic liquids such as ethylene, diethylene, and 
triethylene glycols and glycol derivatives. 
    Solid sorbents may be used in static or dynamic dehumidifiers. Bags of 
solid sorbent materials within packages of machine tools, electronic 
equipment, and other valuable materials subject to moisture damage constitute 
static dehumidifiers. A dynamic dehumidifier for solid sorbent consists of a 
main circulating fan, one or more beds of sorbent material, reactivation air 
fan, heater, mechanism to change from dehumidifying to reactivation, and 
aftercooler.  
    The liquid-sorbent dehumidifier consists of a main circulating fan, 
sorbent-air contactor, sorbent pump, and reactivator including contactor, fan, 
heater, and cooler. This unit will control the effluent dew point at a 
constant level because dehumidification and reactivation are continuous 
operations with a small part of the sorbent constantly bled off from the main 
circulating system and reactivated to the concentration required for the 
desired effluent dew point. 
    A system employing the use of cooling for dehumidifying consists of a 
circulating fan and cooling coil. The cooling coil may use cold water obtained 
from wells or a refrigeration plant, or may be a direct-expansion 
refrigeration coil. In place of a coil, a spray washer may be used in which 
the air passes through two or more banks of sprays of cold water or brine, 
depending upon the dew-point temperature required. 
    Dehumidifying by compression and aftercooling is used when the reduction 
of water vapor in a compressed-air system is required. This is particularly 
important, for example, if the air is used for automatic control instruments 
or cleaning of delicate machined parts. The power required for compression 
systems is so high compared to power requirements for dehumidifying by either 
the sorbent or refrigeration method that the compression system is not an 
economical one if dehumidifying is the only end result required. See DRYING. 
 �34s[John Ehrlich]
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�6v�2v�32sDehydrogenation �21m �33s
    A reaction in which hydrogen is detached from a molecule. The reaction is
strongly endothermic, and therefore heat must be supplied to maintain the
reaction temperature. When the detached hydrogen is immediately oxidized, two
benefits accrue: (1) the conversion of reactants to products is increased
because the equilibrium concentration is shifted toward the products (law of
mass action); and (2) the added exothermic oxidation reaction supplies the
needed heat of reaction. This process is called oxidative dehydrogenation. On
the other hand, excess hydrogen is sometimes added to a dehydrogenation
reaction in order to diminish the complete breakup of the molecule into many
fragments.
    The primary types of dehydrogenation reactions are vapor-phase conversion
of primary alcohols to aldehydes, vapor
phase conversion of secondary alcohols to ketones, dehydrogenation of a side
chain, and catalytic reforming of naphthas and naphthenes in the presence of a
platinum catalyst. All four of these types of dehydrogenation reactions are of
major industrial importance. They account for the production of billions of
pounds of organic compounds that enter into the manufacture of lubricants,
explosives, plastics, plasticizers, and elastomers. See HYDROGENATION;
OXIDATION PROCESS.
 �34s[James W. Fulton]
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�6v�2v�32sDelayed neutron �21m �33s
    A neutron emitted spontaneously from a nucleus as a consequence of
excitation remaining from a preceding radioactive decay event. Analogously,
delayed emission of protons and alpha particles is also observed, but the
known delayed neutron emitters are more numerous, and some of them have
practical implications. In particular, they are of importance in the control
of nuclear chain reactors.
    In a (sup 235)U nuclear reactor, about 0.7% of the neutrons are delayed,
the others being prompt. In a conventional, moderated reactor, the prompt
neutrons are born, slowed down, and reabsorbed to produce the next fissions in a
cycling time of about 1 msec. (In a fast-neutron reactor, the time is much
shorter.) Consequently, if the reactor were to become overcritical (more
neutrons generated per millisecond than are absorbed or leak out), the chain
reaction would exponentiate or ``run away,'' and the reactor might overheat
itself and possibly cause a dangerous accident unless the control rods could
respond within a few milliseconds to correct the situation. The fortunate
presence of the delayed neutrons eases the situation, because so long as the
reactor operates within the margin of 0.7% (``delayed critical''), the control
rods can take as long as several seconds to respond, and thus the chain reaction
comes within the range of easy and leisurely control. See NEUTRON; NUCLEAR
FISSION; REACTOR PHYSICS.
 �34s[Arthur H. Snell]
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�6v�2v�32sDelay line   �21m �33s 
    A transmission line (as nearly dissipationless as possible) or an electric 
network approximation of it which, if terminated in its characteristic 
impedance, will reproduce at its output a waveform applied to its input 
terminals with little distortion but at a time delayed by an amount dependent 
upon the electrical length of the line. If the delay line is not terminated in 
its characteristic impedance, there is multiple reflection back and forth 
along the line. 
    Delay lines are used for establishing a time sequence for the occurrence 
of events. A delay line with a total length equal to the greatest time delay 
required in a system may be used as a basic element. Pulses occurring at 
intermediate times may be obtained from taps at various points along the line. 
A specific application is found in the synchronizing signal generator of the 
television system. Also the lumped-circuit delay line is an essential element 
of the wide-band distributed amplifier. 
    When a signal is digital in nature, or consists of a series of pulses, the 
series of pulses may be delayed by using a shift register, which might, for 
example, consist of a chain of cascaded type D flip-flops. If the register has 
n stages, the pulse series will appear at the output delayed by a time (n - 
1)T, where T is the periodicity of the pulses. The same function can be 
realized by using an array of charge-coupled devices (CCDs). See TRANSMISSION 
LINES.  
 �34s[Glenn M. Glasford]
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�6v�2v�32sDeliquescence   �21m �33s 
    The absorption of atmospheric water vapor by a crystalline solid until the 
crystal eventually dissolves into a saturated solution. This behavior is well 
known for certain salts such as hydrated calcium chloride, CaCl2PRODUCT 6H2O, 
and  zinc chloride, ZnCl2, but it is a property of all soluble salts in air of 
sufficiently high humidity. 
    Thermodynamically, the condition for deliquescence is that the partial 
pressure of the water vapor in the air exceed the vapor pressure (aqueous 
tension) of the water in the saturated solution of the salt. The speed at 
which the process takes place depends upon the rate of diffusion of water 
vapor into the crystal lattice, crystal size, and other factors. The process 
will stop when the water vapor in the atmosphere is depleted to the point at 
which its partial pressure equals that of the saturated solution. 
    Crystalline solids also may absorb water by increasing their water of 
hydration if the dissociation pressure of the hydrated species to be formed is 
less than the partial pressure of the water vapor. It is this process, not 
deliquescence, which is the opposite of efflorescence. 
    Deliquescent substances can be used to remove water vapor from air, 
although they have no special advantage over substances which merely add water 
of hydration and remain crystalline. See DESICCANT; DRYING; EFFLORESCENCE; 
VAPOR PRESSURE. 
 �34s[Robert L. Scott]
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�6v�2v�32sDelta   �21m �33s 
    A deposit of sediment at the mouth of a river or tidal inlet. It is also 
used for storm washovers of barrier islands and for sediment accumulations at 
the mouths of submarine canyons. See FLOODPLAIN. 
    The shape and internal structure of a delta depend on the nature and 
interaction of two forces: the sediment-carrying stream from a river, tidal 
inlet, or submarine canyon, and the current and wave action of the water body 
in which the delta is building. This interaction ranges from complete 
dominance of the sediment-carrying stream (still-water deltas) to complete 
dominance of currents and waves, resulting in redistribution of the sediment 
over a wide area (no deltas). This interaction has a large effect on the shape 
and structure of the delta body. 
    Most of the sediment carried into the basin is deposited when the 
inflowing stream decelerates. If there is little density contrast, this 
deceleration is sudden and most sediment is deposited near the mouth of the 
river. If the inflowing water is much lighter than the basin water, for 
example, fresh water flowing into a colder sea, the outflow spreads at the 
surface over a large distance away from the outlet. If the inflow is very 
dense, for instance, cold muddy water in a warm lake, it may form a density 
flow on or near the bottom, and the principal deposition may occur at great 
distance from the outlet. 
  
    Three principal components make up the bodies of most deltas in varying 
proportions: topset, foreset, and bottomset beds (see illustration). As 
defined for most deltas, the topset beds comprise the sediments formed on the 
subaerial delta: channel deposits, natural levees, floodplains, marshes, and 
swamp and bay sediments. The foreset beds are those formed in shallow water, 
mostly as a broad platform fronting the delta shore, and the bottomset beds 
are the deep-water deposits beyond the deltaic bulge. In marine deltas the 
fluviatile influence decreases and the marine influence increases from the 
topset to the bottomset beds. 
    In a different way, deltas can be viewed as being composed of three 
structural elements: (1) a framework of elongate coarse bodies (channels, 
river-mouth bars, levee deposits), which radiate from the apex to the 
distributary mouths (sand fingers); (2) a matrix of fine-grained floodplain, 
marsh, and bay sediments; and (3) a littoral zone, usually of beach and dune 
sands which result from sorting and longshore transport of river-mouth 
deposits by waves, currents, tides, and wind. The relative proportions of 
these components vary widely. The Mississippi delta consists almost entirely 
of framework and matrix; its rapid seaward growth is the result of deposition 
of river-mouth bars and extension of levees, and the areas in between are 
filled later with matrix. This gives the delta its characteristic bird-foot 
outline. A different makeup is presented by the Rhone delta, where the supply 
of coarse material at the distributary mouths is slow, and dispersal by wave 
action and longshore drift fairly efficient, so that nearly all material is 
evenly redistributed as a series of coastal bars and dunes across a large part 
of the delta front. This delta advances as a broad lobate front, while the 
present Mississippi delta grows at several localized and sharply defined 
points.  
    Despite difficult engineering problems, many cities, such as Calcutta, 
Shanghai, Venice, Alexandria (Egypt), and New Orleans, were constructed on 
deltas. These problems include shifting and extending shipping channels; lack 
of firm footing for construction except on levees; steady subsidence; poor 
drainage; and extensive flood danger. Moreover, in certain deltas the tendency 
of the main flow to shift away to entirely different areas, with resulting 
disappearance of the main channels for water traffic, is a constant problem 
that is difficult and costly to counter. See ESTUARINE OCEANOGRAPHY. 
 �34s[Tjeerd H. Van Andel]
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�6v�2v�32sDelta rays   �21m �33s 
    Energetic electrons ejected from atoms in matter by the passage of 
ionizing particles. In every primary ionizing collision between a charged 
particle and an atom, one or more electrons are ejected. Delta rays are, by 
definition, that small fraction of these emitted electrons having energies 
which are large compared to the ionization potential. The name is a 
traditional one - comparable to alpha rays, for energetic helium nuclei, and 
beta rays, for energetic electrons emitted in radioactive decays. See ALPHA 
RAYS; BETA RAYS; CHARGEDPARTICLE BEAMS; IONIZATION. 
    Delta rays are responsible for the ``hairy'' appearance of charged 
particle tracks when they are observed in cloud chambers or in photographic 
emulsions. In studies of super-high-energy particles in cosmic radiation and 
from the highest-energy accelerators, observation of the number of delta rays 
per centimeter of path length has been shown to lead to a reliable 
determination of the charge of the energetic particle. 
 �34s[D. Allan Bromley]
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�6v�2v�32sDeltatheridia   �21m �33s 
    An order of mammals that includes the dominant carnivores of the early 
Cenozoic. While all deltatheridians have been persistently primitive in most 
respects, such as a small brain and the pattern of carotid circulation, the 
teeth are specialized in diverse ways for eating invertebrates or vertebrates. 
In the carnivorous forms, the molars, not the last upper premolar and the 
first lower molar, become specialized slicing teeth. The paracone and metacone 
of the upper molars are close together or even fused, and the terminal 
phalanges are fissured for the insertion of the claws. See DENTITION. 
    The ancestral family, the Palaeoryctidae, was insectivorous and contained 
some of the most primitive of the known placental mammals. From this family 
evolved the Oxyaenidae and the Hyaenodontidae. These groups evolved a 
carnivorous habit separately from the order Carnivora, which contains all the 
terrestrial carnivores among recent mammals. See CARNIVORA. 
    The tenrecs and chrysochlorids of Africa may possibly be surviving 
insectivorous members of the Deltatheridia rather than belonging to the 
Insectivora. A few aberrant middle Cenozoic genera may also be 
deltatheridians. See INSECTIVORA; MAMMALIA. 
 �34s[Leigh Van Valen]
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�6v�2v�32sDelusion   �21m �33s 
    A conviction based upon faulty perceptions, feelings, and thinking. 
Delusions in adults are signs of mental abnormality and are accompanied by 
severe behavioral difficulties; yet, at times it is difficult to differentiate 
delusions from fixed, faulty, and rigid ideas, particularly if they are shared 
by groups. 
    Delusions seen in psychotic patients are usually classified according to 
content. The main groups are as follows: (1) persecutory or paranoid 
delusions; these may be systematized and have a logic of their own, as in 
paranoia, or may be unsystematic and incoherent; (2) delusions of influence 
related to paranoid delusions; one of the most frequent forms is the delusion 
of being hypnotized or under a spell; (3) delusions of grandeur in which the 
patient imagines he is an extraordinary person or that he has extraordinary 
powers; (4) self-depreciatory delusions occurring primarily in depressed 
patients; and (5) delusions of body changes and of unreality which may be a 
feeling of not being one's self, of having changed bodily, or of having gross 
abnormalities and diseases; these are also referred to as hypochondriacal 
delusions. See ABNORMAL BEHAVIOR; PARANOIA; PSYCHOSIS; SOMATIZATION. 
 �34s[Fredrick C. Redlich]
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�6v�2v�32sDemagnetization   �21m �33s 
    A process for reducing a magnetized material to a neutral state, that is, 
to a state when both the magnetic induction B and the magnetic field strength 
H are equal to zero. 
    The standard demagnetization process involves applying a magnetizing force 
above a critical value for the material used, and successively reversing H in 
direction as the value of H is steadily reduced to a value less than those to 
be used in subsequent experiments. If the final value of H is essentially 
zero, the specimen is demagnetized. 
    The adiabatic demagnetization of paramagnetic salts is a technique used to 
produce temperatures very near the absolute zero. See ADIABATIC 
DEMAGNETIZATION.  
 �34s[Kenneth V. Manning]
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�6v�2v�32sDematiaceae   �21m �33s 
    A family of fungi of the order Moniliales. Sporophores are not grouped in 
bundles or fascicles and are lacking in some genera. The hyphae are always 
dark and the spores hyaline or mostly dark. There are 200 genera and 2000 
species recognized. Genera in this family have arthrospores, blastospores, 
aleuriospores, meristem phialospores, or terminus phialospores. There are 400 
parasitic species, chiefly causing leaf spotting. See MONILIALES. 
 �34s[N. F. Buchwald]
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�6v�2v�32sDemeclocycline   �21m �33s 
    A broad-spectrum antibiotic formerly known as demethylchlortetracycline. 
The antibacterial activity of demeclocycline in the blood is markedly more 
prolonged than that of tetracycline, chlortetracycline, or oxytetracycline. 
See CHLORTETRACYCLINE; OXYTETRACYCLINE; TETRACYCLINE. 
    Demeclocycline has a wide range of clinical usefulness, corresponding 
generally to the range of usefulness of the other tetracyclines. It is rapidly 
absorbed from the intestinal tract and is distributed into all body tissues. 
The drug is used in the treatment of a wide range of diseases, including 
pneumonia, genitourinary infections, brucellosis, acute childhood infections, 
pustular dermatoses, gonorrhea, lymphogranuloma venereum, and granuloma 
inguinale. See ANTIBIOTIC. 
 �34s[J. R. D. McCormick]
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�6v�2v�32sDementia   �21m �33s 
    Any one of many diseases of the brain that cause intellectual 
deterioration (forgetfulness: these involve inability to reason clearly, to 
use words, and to perform arithmetic calculations) in an individual who has 
previously been mentally normal. These conditions, which are loosely called 
the dementias, take many forms and are increasingly frequent during the 
senium. In the later period of life, two diseases occur with notable frequency 
- Alzheimer's disease (called also senile dementia in the aged) and occlusions 
of many of the arteries of the brain (multi-infarct dementia). As much as a 
third of the patient population of psychiatric hospitals has these two types 
of diseases.  See  ARTERIOSCLEROSIS; SENESCENCE. 
 �34s[Raymond D. Adams]
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�6v�2v�32sDemodulator   �21m �33s 
    The stage in a radio, television, radar, or other receiver at which 
demodulation of the received signal takes place. Thus, in a tuned 
radio-frequency (rf) receiver the demodulator separates the audio-frequency 
(af) signal from the amplified incoming rf carrier signal. A demodulator is 
often called a detector. In a superheterodyne receiver the af signal is 
separated from the carrier signal at the second detector, because the 
converter or first detector merely serves to change the modulated rf carrier 
signal to a modulated intermediatefrequency (i-f) carrier signal. In a 
frequency-modulation receiver the demodulator converts carrier frequency 
changes into corresponding audio signals. In a color television receiver the 
demodulator extracts the color difference signals from the incoming modulated 
carrier signal. See DETECTOR. 
 �34s[John Markus]
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�6v�2v�32sDemospongiae   �21m �33s 
    A class of the phylum Porifera, including sponges with a skeleton of one- 
to four-rayed siliceous spicules or of spongin fibers or both. Several genera 
lack a skeleton. The Demospongiae constitute the most abundant and widely 
distributed group of sponges, occurring in the sea from the tidal zone down to 
abyssal depths (at least to 18000 ft or 5500 m). One family has invaded fresh 
water. The species vary in size from thin encrustations several centimeters in 
diameter to huge cake-shaped forms which may measure up to as much as 6.5 ft 
(2 m) in diameter. See PORIFERA. 
    Comparative studies of the embryology and early attached stages of sponges 
of the class Demospongiae suggest at least two evolutionary lines within this 
group: the subclass Ceractinomorpha and the subclass Tetractinomorpha. See 
CERACTINOMORPHA; TETRACTINOMORPHA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sDendroceratida   �21m �33s 
    A small order of sponges of the class Demospongiae. Members of this order 
either have a skeleton of spongin fibers or lack a skeleton. The spongin 
fibers, when present, are typically dendritic in form, seldom anastomosing to 
form a network, and arise from a basal plate of spongin adherent to the 
substratum. The fibers, which in most genera lack foreign inclusions such as 
sand grains, are made up of concentric layers of spongin, new layers 
apparently being added throughout the life of the sponge. The flagellated 
chambers are large and sac-shaped and open directly into the exhalant canals 
without the intervention of a special channel. Dendroceratid sponges occur 
chiefly in tidal and shallow coastal regions of all seas. See DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
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�6v�2v�32sDendrochirotacea   �21m �33s 
    A subclass of Holothuroidea. Retractor muscles are present that link the 
calcareous ring of the pharynx with the body wall. These muscles enable the 
anterior part of the body to be withdrawn as the so-called introvert. Tube 
feet and a respiratory tree are usually present. The madreporite lies freely 
suspended within the coelom. There are two included orders, Dactylochirotida 
and Dendrochirotida. See DACTYLOCHIROTIDA; DENDROCHIROTIDA; ECHINODERMATA; 
HOLOTHUROIDEA.  
 �34s[Howard B. Fell]
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�6v�2v�32sDendrochirotida   �21m �33s 
    An order of dendrochirotacean holothurians with 10 - 30 richly branched 
tentacles. There are six existing families. Three of these, the 
Placothuriidae, Paracucumidae, and Psolidae, are probably related to the 
Paleozoic Calclamnidae and have the body partly or completely invested in 
plates forming a test, analogous to that of other echinozoan classes. The 
remaining three families have fewer plates and consequently a rather naked 
skin. In this group a complex calcareous ring is found in the Phyllophoridae 
and Sclerodactylidae and a simplified ring in the Cucumaridae. The 
Dendrochirotida are believed to be the most archaic surviving echinozoan 
echinoderms. See DENDROCHIROTACEA; ECHINODERMATA; ECHINOZOA; EDRIOASTEROIDEA; 
HOLOTHUROIDEA.  
 �34s[Howard B. Fell]
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�6v�2v�32sDendrochronology   �21m �33s 
    The science of dating historical and environmental events by comparative 
studies of the annual variations in the width of growth rings of trees and 
wood specimens. 
    Dendrochronology developed by using the well-defined annual rings that are 
visible on stem cross sections of both coniferous and nonconiferous tree 
species growing in temperate and subpolar climates. Annual rings are formed by 
cell division in the cambial tissues located just inside the bark. When 
diameter growth begins in the spring, large, thin-walled cells are produced. 
These cells form the earlywood. Later in the growing season, the newly formed 
cells are smaller, more dense, and have thick walls. This part of the ring is 
called latewood. The ring boundary is formed by the contrasting structures of 
the dark-colored, dense latewood on the outside of one ring against the 
light-colored, low-density earlywood of the next. See STEM; XYLEM. 
    Tree growth is related to both genetic and environmental factors. Genetic 
factors determine the range of environmental conditions that an individual 
tree can tolerate, as well as its response to those factors. The environment 
supplies essential nutrients, moisture, solar radiation, and atmospheric 
gases. Growth is related to either an abundance or scarcity of any or all of 
these constituents. In most temperate regions, especially the American 
Southwest, precipitation is the limiting factor for diameter growth, hence the 
width of annual growth rings. Narrow rings are formed when moisture is scarce, 
and wide rings when it is abundant. Near the alpine and arctic timberlines, 
temperature is usually the controlling factor. 
    The principle of cross dating is basic to all dendrochronological work. 
The procedure of matching ring patterns between trees or wood specimens is 
used to identify and assign a calendar date to each annual ring. 
Unfortunately, all rings may not represent annual growth layers. In addition, 
many species inherently produce multiple rings and are of little use to 
dendrochronologists. However, those who become experienced in working with 
species in a given area often learn to recognize ring patterns that are 
associated with certain years, and can cross-date some specimens with relative 
ease. See TREE-RING HYDROLOGY. 
    Cross dating provides the basis for developing precisely dated ring 
sequences known as chronologies. Those developed for a specific location may 
be combined with chronologies from other sites, assuming that they cross-date, 
to form a master chronology for a region. A site or master chronology forms 
the base for dating older material, such as wood or charcoal, whose ages 
overlap those of the chronology. This is done by matching ring patterns of the 
unknown sample with those of the chronology. If cross dating is established, 
each ring of the unknown sample is assigned a calendar date. These records can 
then be incorporated in the master chronology and thus extend it farther back 
in time. 
    Dendrochronology has become a useful tool in many areas of research. As a 
result of intensive tree-ring research carried out in the Southwest, the 
chronological prehistory of the area is perhaps better known than in any part 
of the world. From its beginning in the American Southwest, 
dendrochronological dating has spread to other parts of the United States and 
countries throughout the world. 
    In the absence of instrumented records, tree-ring series can be used to 
determine past climatic patterns (dendroclimatology). The reconstruction 
procedure is complicated and requires the understanding and use of advanced 
computer techniques and statistical methods. These methods and techniques are 
still being refined and may provide even more precise information on climatic 
variation.  
    The same methods used for climatic reconstructions can be applied to 
hydrologic events such as streamflow - the amount of water flowing in a stream 
or river. Tree-ring chronologies can serve as a proxy data source to 
reconstruct past streamflows and to determine whether modern records are 
typical or represent periods of above- or below-normal flows (dendrohydrology). 
    Analysis of tree rings for trace elements can often provide a 
chronological record of pollution from industrial and other sources. Tree 
rings may also be used to date catastrophic events such as landslides, 
earthquakes, floods, volcanic eruptions, and other phenomena that cause an 
identifiable change in the growth rate. These are only a few of the 
interesting and unusual applications of tree rings. Other uses will 
undoubtedly be found as research continues. 
 �34s[William R. Boggess]
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�6v�2v�32sDendrology   �21m �33s 
    The division of forestry concerned with taxonomy of trees and other woody 
plants. Dendrology, called forest botany in some countries, usually is limited 
to taxonomy of trees but may also include shrubs and woody vines. This basic 
subject in the training of foresters teaches how trees are named 
(nomenclature), described (morphology), and grouped (classification); how to 
find the name of an unknown tree and recognize important forest species 
(identification); and where trees occur both by geographic ranges of species 
and by forest types (distribution). Forest stands of similar composition, 
appearance, and structure are grouped together into areas characterized by 
major forest types or formation, and are named from the predominant or 
characteristic species. See FOREST AND FORESTRY; FOREST RESOURCES; PLANT 
TAXONOMY.  
 �34s[Elbert L. Little, Jr.]
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�6v�2v�32sDengue fever   �21m �33s 
    An acute viral disease with fever, rash, prostration, and lymphadenopathy. 
Inapparent infections are frequent; complications or deaths are rare. The 
virus is a member of arbovirus group B. See ARBOVIRAL ENCEPHALITIDES. 
    The human infection cycle is from mosquito (Aedes aegypti) to human. 
Monkeys and A. albopictus possibly form the jungle reservoir. Outbreaks occur 
chiefly in Africa, India, the Far East, and also in Hawaii, the Philippine and 
Caribbean islands, and the eastern Mediterranean area. Destruction of A. 
aegypti in the American tropics may be eliminating dengue there. See ANIMAL 
VIRUS.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sDensity   �21m �33s 
    The mass per unit volume of a material. The term is applicable to mixtures 
and pure substances and to matter in the solid, liquid, gaseous, or plasma 
state. Common units of density are grams per cubic centimeter, and slugs or 
pounds per cubic foot. The specific gravity of a material is defined as the 
ratio of its density to the density of some standard material, such as water 
at a specified temperature, for example, 60DGF, or, for gases the basis may be 
air at standard temperature and pressure. Another related concept is weight 
density, which is defined as the weight of a unit volume of the material. See 
MASS; WEIGHT. 
 �34s[Leo Nedelsky]
�33s 
    The density of all substances depends on temperature; in the case of 
gases, on temperature and pressure. The temperature used as a base for 
determining or reporting values of density is not the same for all substances. 
For solids 32DGF (0DGC) is the preferred temperature; for many liquids the 
reference temperature is 60DGF (15.56DGC); and for gases 32DGF and a pressure 
of 29.921 in. (or 0DGC and 760 mm Hg/Hg of mercury or 101.325 kilopascals) are 
used for most scientific work and for tables of gas data. 
    In the case of a solid, if the sample is of regular shape, such as a cube 
or a cylinder, its volume may be determined by linear measurement. The mass of 
the sample is determined by weighing it on a suitable scale or balance; then 
this weight divided by the volume gives the density. Ordinarily the weighing 
is done in air, and the density value is the density in air, or apparent 
density. By adjusting for the buoyant effect of the air upon the weight of the 
sample (and also upon the weights if a balance is used), the real density is 
obtained.  
    A second procedure, applicable to irregular as well as regular shaped 
samples, is to weigh the sample in air and then to suspend it in a liquid of 
known density. The volume of the sample is equal to its loss of weight in the 
liquid divided by the density of the liquid. This is the method of hydrostatic 
weighing. Conversely, the density of a liquid may be determined by weighing in 
the liquid a weight of known mass and volume. 
    One method to determine the density of a gas is to completely evacuate a 
light but strong vessel of suitable size, the interior volume of which is 
known. The evacuated vessel is weighed, filled with a sample of the gas, and 
then weighed again. Of course the pressure and temperature of this sample of 
gas must be obtained. 
    A densitometer or gravitometer may be used to indicate and record the 
density or specific gravity of a flowing stream of a liquid or a gas. 
 �34s[Howard S. Bean]
�33s 
    The increasing demand for accurate values of the density of phases and 
systems has generated much effort toward improving the technology of the 
necessary measurements. Two methods, the oscillator or vibrator method and the 
magnetic method, have emerged which eliminate most of the past difficulties 
associated with rapid and accurate determinations on liquid systems. 
    In the oscillator method the density of a sample is related to the change 
in resonance frequency f of a laterally vibrating tube (made of special glass 
or quartz). This frequency is inversely proportional to the square root of the 
mass m of the tube and its contents. 
    The instruments utilizing the magnetic method are called magnetic 
densimeters. This densimeter is a device whereby a tiny (<0.4 in. or 10 
mmsup3) ferromagnetic cylinder, encased in a glass or plastic jacket, is held 
at a precise height within a medium by virtue of a solenoid controlled by a 
servo system in circuit with a height sensor. The jacket and ferromagnetic 
material constitute a buoy or float. 
 �34s[Donald W. Kupke]
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�6v�2v�32sDensity matrix �21m �33s
    A matrix which is constructed as the most general statistical description
of the states of a many-particle quantum-mechanical system. The state of a
quantum system is described by a normalized wave function PSI(x,t), (where x
stands for all coordinates of the system, and t for the time), which satisfies
the Schroedinger equation, Eq. (1), where H is the hamiltonian of the system,
and h is Planck's constant divided by 2pi. Furthermore, PSI(x,t) may be expanded
in terms of a complete orthonormal set {sigma(x)}, as in Eq. (2). Then, the
density matrix is defined by Eq. (3), and this density matrix describes a pure
state. Examples of pure states are a beam of polarized electrons and the photons
in a coherent beam emitted from a laser. See LASER; NONRELATIVISTIC QUANTUM
THEORY.

	   H PSI (x,t) = (h/i){[CURLY D PSI (x,t)]/CURLY D t}		     (1)

PSI (x,t) = SUMMATION (in terms of n) a(sub n)(t) sigma(sub n) (t)	     (2)

	     rho(sub mn)(t) = a(sub n) *(t)a(sub m) (t) 		     (3)

    In quantum statistics, one deals with an ensemble of N systems which have
the same hamiltonian. If the alphath member of the ensemble is in the state
PSIsupalpha in Eq. (4), the density matrix is defined as the ensemble average,
Eq. (5). In general, this density matrix describes a mixed state, for example, a
beam of unpolarized electrons or the photons emitted from an incoherent source
such as an incandescent lamp. The pure state is a special case of the mixed
state when all members of the emsemble are in the same state. See STATISTICAL
MECHANICS.
 �34s[Sam H. Liu]
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PSI(sup alpha) (x,t) = SUMMATION (in terms of n) a(sub n)((sup alpha))
			 (t)phi(sub n) (x)				    (4)


rho(sub mn) (t) = (1/N) SUMMATION [a(sub n)((sup alpha)) (t)] *a(sub m)
		  (sup alpha) (t)					     (5)

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�6v�2v�32sDentistry   �21m �33s 
    An autonomous branch of biomedical science that is concerned with the 
prevention, diagnosis, and treatment of diseases and abnormalities of the 
teeth, jaws, oral cavity, and adjacent structures. 
    Dental caries (tooth decay) is one of the most prevalent diseases 
affecting humans, and the greatest portion of the dentist's time and efforts 
is expended on treating dental decay and its consequences. In addition to 
caries, teeth can be damaged by trauma, erosion, and abrasion. Restorative 
dentistry encompasses efforts to conserve and restore decayed, defective, 
missing, and traumatically injured teeth. 
    Significant advances have been made in the practice of restorative 
dentistry. Development of high-speed, air-driven turbines combined with rotary 
cutting instruments fashioned from diamonds and ultrahard steel permits the 
rapid removal of tooth structure with little discomfort to the patient. In 
addition, many new materials for restorations and impression taking have 
become available. Especially important is the availability of composite resins 
which have sufficient strength to withstand biting and chewing pressures. 
    Seven branches of specialization are recognized by the American Dental 
Association: oral surgery, orthodontics, pedodontics, periodontics, 
prosthodontics, oral pathology, and public health dentistry. Other 
subspecialties such as oral medicine, dental radiology, and periodontal 
prosthetics exist but are not recognized. 
    Oral surgery treats diseases and abnormalities of the maxillofacial region 
by surgical means. Oral surgeons treat a wide variety of problems by removing 
teeth, reducing bone fractures, removing cysts, tumors, and growths, and 
correcting congenital anomalies and malformation of the structures of the 
maxillofacial region. 
    Orthodontists deal with abnormalities in tooth position and jaw 
relationships that result in facial disharmony and malfunction. The objective 
of orthodontic treatment is to establish normal occlusion and facial harmony. 
The teeth are repositioned and the jaws modified through the use of mechanical 
force applied with fixed or removable appliances. Successful treatment results 
in normal shape and expression of the mouth and lips, aids in enunciation and 
the sounding of words, and permits proper mastication. 
    Pedodontics is the branch of dentistry concerned with the detection, 
prevention, and treatment of oral and dental diseases and abnormalities in 
children. The deciduous or primary teeth are very small and have shapes which 
differ from those of adult teeth; special procedures and materials are 
required for their conservation and restoration. 
    Periodontics is the branch of dentistry devoted to the study, prevention, 
diagnosis, and treatment of diseases of the tissues supporting the teeth: 
gingiva (gum tissue), alveolar bone, periodontal ligament, and cementum. 
Periodontal diseases include gingivitis, periodontitis (sometimes called 
pyorrhea), primary and secondary occlusal traumatism, gingival hyperplasia, 
and periodontal atrophy. Several types of anaerobic gram-negative 
microorganisms are thought to be associated with chronic periodontitis. See 
ANAEROBIC INFECTIONS; PERIODONTAL DISEASE. 
    Prosthodontics is the branch of dentistry devoted to the construction and 
replacement of oral structures with artificial substitutes. The replacement of 
teeth and other oral structures is necessitated by congenital abnormalities, 
loss of teeth from disease or trauma, and destruction of teeth or jaws or 
other parts of the mouth by surgical management of neoplasms or trauma. 
    Oral pathology is concerned with the detection and diagnosis of the 
diseases of the teeth, oral cavity, and jaws, and also with the oral 
manifestations of systemic diseases. See PATHOLOGY. 
    Public health dentistry is defined as the science and art of preventing 
and controlling dental diseases and promoting dental health through organized 
community efforts. It comprises research, education, prevention, diagnosis, 
prescription, treatment of problems related to dentistry, and evaluation of 
community dental care. See MOUTH DISORDERS; TOOTH DISORDERS. 
 �34s[Roy C. Page]
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�6v�2v�32sDentition   �21m �33s 
    The arrangement, type, and number of teeth which are variously located in 
the oral or in the pharyngeal cavities, or in both. Teeth are found in areas 
where there is an underlying supporting structure of cartilage or bone and 
where stomodeal ectoderm is present. 
    Attachment of teeth is variable among vertebrates. They may be inserted in 
sockets in the jawbones (thecodont condition), fused to the edge of the bone 
proper (acrodont condition), or attached to the inner surface of the jawbone 
(pleurodont condition). In polyphyodont animals, teeth may be constantly 
replaced. The polyphyodont condition is characteristic of the fishes and the 
lower tetrapods in which the teeth are replaced as they are worn out. Most 
mammals have two sets of teeth during their lifetime, which is the diphyodont 
condition. These sets are the deciduous, or milk, teeth and the permanent 
dentition. Monophyodont dentition is the development of only one set of teeth. 
    Teeth may have a similar form in all regions where they occur (homodont), 
as distinct from those that are variable in shape (heterodont). The homodont 
condition is characteristic of nonmammalian vertebrates, although the anterior 
teeth may differ from those lying in the cheek region of the jaws. A 
heterodont dentition began to evolve in the reptilian ancestors of mammals, 
but became fully developed in the mammals. The mammalian heterodont condition 
is frequently described by a dental formula expressing both the number and 
kind of teeth in each half jaw, both upper and lower (see table). For humans 
the formula is I 2/2 C 1/1 Pm 2/2 M 3/3. This can be read for either the right 
or left side of the jaw; there are 2 upper and 2 lower incisors, 1 upper and 1 
lower canine, and 2 premolars and 3 molars in both the upper and lower jaws. 
According to this formula, humans have a total of 32 teeth, 8 located in each 
half of the upper jaw and 8 in each half of the lower jaw. See TOOTH. 
 �34s[Frederick S. Szalay]
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�6v�2v�32sDeoxyribonucleic acid (DNA) �21m �33s
    The main carrier of genetic information in all living organisms except
viruses that contain ribonucleic acid. Most of the deoxyribonucleic acid (DNA)
of the cell occurs in the nucleus as a component of the chromosomes. Small
amounts also occur in mitochondria, plastids, and other cytoplasmic organelles

    Structure and function.  The molecules of DNA are extremely long polymers,
built up of deoxyribonucleotide repeating units. Each unit comprises a sugar
(2-deoxyribose), phosphate, and a purine or pyrimidine base. The
deoxyribonucleotide units are linked together by the phosphate groups, joining
the 3' position of one sugar to the 5' position of the next. The alternate
sugar and phosphate residues form the backbone of the molecule, and the purine
and pyrimidine bases are attached to the backbone via the 1' position of the
deoxyribose. This sugar-phosphate backbone is the same in all DNA molecules;
what gives each DNA its individuality is the sequence of the purine and
pyrimidine bases. See DEOXYRIBOSE: PURINE: PYRIMIDINE.
    Most DNA molecules are double stranded, consisting of two DNA chains wound
round each other, the two chains running in opposite senses, or antiparallel
(Fig. 1). The structure is a right-handed helix in which the bases stack on
top of each other like a stack of coins (Fig. 2). The two chains are linked
together by a large number of weak bonds (hydrogen bonds) formed between
complementary bases. The complementary base pairs are adenine-thymine and
guanine-cytosine, that is, always a purine (adenine or guanine) with a
pyrimidine (cytosine or thymine). This base pairing is extremely important not
only in the structure of DNA but also in the performance of its two functions,
replication and transcription. The information carried in the DNA is encoded
in the sequence of purine and pyrimidine bases in the molecule, and
perpetuation of this information involves production of two exact copies of
each parental DNA molecule (replication). Much of this information is
expressed in the form of protein and the products of protein action.
Expression takes place in several stages: first, transcription of the DNA to
give rise to ribonucleic acid (RNA); second, RNA processing at both 3' and 5'
ends and by splicing within the molecule to remove intervening sequences;
third, translation of the resultant messenger RNA to protein. Each amino acid
in the protein chain is represented by a group of three bases in the RNA
derived from a group of three corresponding bases in the DNA. Some of the
information in DNA is expressed directly via transcription, as RNA, for
example, ribosomal RNA and transfer RNA. See GENETIC CODE; PROTEIN;
RIBONUCLEIC ACID (RNA); RIBOSOMES.



    Replication.  The structure of the DNA double helix immediately suggests
the mechanism by which replication might take place. The base sequence of
either strand defines the structure of the whole molecule. The mechanism of
DNA replication involves the separation of the complementary strands over a
short distance, followed by the synthesis of complementary strands on each of
the parental strands (Fig. 3). Errors in this process occur very rarely,
probably less than one per million bases incorporated. Once an error has
occurred, it may be corrected, but otherwise the change in the DNA is
perpetuated in all the progeny DNA that is derived from this altered molecule.
Errors may so alter the sequence of the protein coded for by the DNA that the
activity of the protein is lost or altered. A change in the base sequence of
the DNA constitutes a mutation. See MUTATION.


    Transcription.  The basic mechanism of transcription depends on base
pairing and is similar in a formal sense to that of replication, except that
only one of the strands of the DNA is copied and the copy differs from the
template. RNA differs from DNA in that it contains ribose rather than
deoxyribose and in that thymine is replaced by uracil as one of the two
pyrimidine bases. This latter change does not alter the basic pattern of base
pairing except that adenine now pairs with uracil. As in the case of DNA
synthesis the building blocks for RNA synthesis are the nucleoside 5'
triphosphates, each losing two of its phosphates in the process of being
attached to the growing end of the nucleic acid chain. Transcription occurs by
attachment of an enzyme (DNA-primed RNA polymerase) to specific sites on the
DNA at the beginning of operons. At these sites the strands of the DNA
separate locally, and one strand, which is rich in pyrimidines, is selected as
the template strand. Synthesis of the RNA then commences with the
complementary purine deoxyribonucleoside triphosphate and continues along the
DNA molecule, inserting complementary bases in the sequence dictated by the
template DNA strand, until reaching a stop signal at the end of an operon. The
strands of the DNA rejoin as soon as the wave of RNA synthesis has passed,
thus forming again the original structure.

    Variation.	In addition to the main chromosomal DNA, many cells contain
other DNA molecules. In bacteria these include plasmids, which are of
considerable importance since they carry genes for fertility and resistance to
drugs and antibiotics, and many other characters. The plasmids may replicate
independently or, in some cases, become integrated into the bacterial
chromosome. From here they can later be excised and return to their autonomous
state. The small size of plasmid DNAs has made it possible to examine their
structure in detail and to manipulate them in various ways. DNA sequences from
other sources can be inserted into such plasmids to form hybrid DNA molecules
capable of indefinite replication.
    Recombinant DNA technology uses bacteriophages and plasmids as vectors for
DNA from other sources. It makes use of the ability of restriction
endonucleases to cut DNA at specific sequences, ligases to join the fragments,
and DNA transformation to reinsert the hybrid molecules into recipient
organisms. This form of genetic engineering has great potential in the
elucidation of the structure and function of the DNA of higher organisms. The
practical advantages of being able to propagate specific regions of such DNAs
are also considerable. The many potential applications include insertion of
the genes for human proteins into plasmids and their production in bacteria.
The hormone insulin, used in treatment of diabetes, and the antiviral,
antitumor agent interferon may both be produced commercially in this way. See
GENETIC ENGINEERING; HORMONE; INTERFERON.
    Plasmid DNAs, many organelle DNAs, and the DNAs of some small viruses all
have circular structures. The importance of circular structure may simply be
that DNA strands without free ends are less subject to chemical or enzymic
degradation. Other viruses of animals and bacteria have unusual DNAs which are
single- rather than double-stranded. Unusual DNA compositions are also found
in some bacterial viruses, cytosine being completely replaced by methyl
cytosine or hydroxymethyl cytosine. Thymine may be replaced by uracil or
hydroxymethyl uracil. These alterations do not affect overall DNA structure
and base pairing in the double helix, but they may affect its stability. See
ANIMAL VIRUS; BACTERIOPHAGE; NUCLEIC ACID; VIRUS.
 �34s[Lionel Crawford]
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�6v�2v�32sDeoxyribose   �21m �33s 
    A nucleic acid (see illustration), also known as 2-D-deoxyribose, 
constituent of all animal, microbial, and plant cells. The deoxyribose is 
enzymically formed in living cells by reduction of ribonucleoside di- or 
triphosphate. The four deoxyribose nucleotides, containing adenine, guanine, 
cytosine, and thymine, are the major constituents of the deoxyribonucleic 
acids (DNA), which control the hereditary characteristics of every living 
organism. See DEOXYRIBONUCLEIC ACID (DNA); NUCLEIC ACID; RIBOSE. 
 �34s[William Z. Hassid]
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�6v�2v�32sDepolymerization �21m �33s
    The reversion of a polymer to a monomer. Depolymerization normally is not
employed as a distinct process in the petroleum industry, but may occur
incidentally during catalytic or thermal cracking operations if the feedstocks
contain polymers. Depolymerization by heat alone will proceed by a thermal
cracking mechanism. In both cases the product may contain substantial
quantities of polymer fragments differing from the original monomer See
CRACKING; POLYMERIZATION.
 �34s[Bernard Greensfelder/Mott Souders]
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�6v�2v�32sDepression (psychology) �21m �33s
     Along with mania, depression belongs to the general area of affective
disorders. There are 10 major models of depression, reflecting five dominant
schools of thought. For those in the psychoanalytic tradition, depression
represents the introjection of hostility resulting from the loss of an
ambivalently loved object, or a reaction to separation from a significant
object of attachment. More recent ego-psychology approaches focus on
helplessness, lowered self-esteem, and negative cognitive set. For the
behaviorist, depression is a set of maladaptive behavioral responses -
elicited by uncontrollable aversive stimuli or by loss of reinforcement - that
are additionally maintained by the rewards of the sick role. Each theoretical
framework has generated its own therapeutic modalities, ranging from
psychoanalysis to behavior therapy, from sociopolitical activism to
pharmacotherapy and electric convulsive therapy (ECT).
    The distinction between unhappiness and depressive illness is important
because almost all people experience mood swings in association with the
demands of everyday life. However, clinical depression involves sustained
states of deep dejection characterized by dysphoric mood (depressed, blue,
despondent, hopeless, discouraged, fearful, worried, irritable). Frequently
appearing symptoms include insomnia or hypersomnia, loss of energy, slowing
down of mental and physical activities, decreased libido, lowered self-esteem
and increased self-reproach, poor concentration, slow thinking, or confused
thoughts. Sometimes there are recurrent preoccupations with death or suicide.
    Of the different symptoms of depression, suicide and suicidal attempts are
the most alarming and must be considered in all serious depressions. An
increasing amount of psychiatric research is being done to try to determine
the high risk factors for suicide attempts and to develop ways to intervene
usefully.
    It is increasingly possible to treat and even prevent some, although
certainly not all, specialized forms of depression. Lithium for the treatment
and prophylaxis of manic-depressive disorders has made it possible for many
manic-depressives to avoid the frequent ups and downs that attend their
illness. Other drugs are being used for the treatment of different kinds of
depressive disorders, with reasonable success in properly selected patients.
For the milder forms of depression, researchers have acquired new knowledge
about the most effective psychotherapeutic or behavioral approaches that can
be used. See MANIC-DEPRESSIVE PSYCHOSIS.
 �34s[William. T. McKinney Jr.]
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�6v�2v�32sDermaptera   �21m �33s 
    An order of slender insects (commonly called earwigs) having incomplete 
metamorphosis, chewing mouthparts, short forewings, and cerci - the paired 
sensory appendages, on the last abdominal segment - that are shaped like 
forceps (illustration a). They have hard, shiny bodies and running legs. The 
forewings (elytra) extend only to the end of the metathorax; the hindwings are 
large and semicircular and at rest they fold up under the elytra (illustration 
b). Most species fly only rarely and many are apterous. 
    Earwigs are terrestrial and mainly nocturnal; they live in moist, shady 
regions and during the day hide under stones, logs, or the bark of trees. They 
are omnivorous feeders; a few are destructive to tender foliage and flowers, 
and others, mostly tropical species, are carnivorous. The order is a 
relatively small one, including about 1000 species; it is a primitive group 
extending back to the Jurassic Period. See INSECTA. 
 �34s[Frank M. Carpenter]
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�6v�2v�32sDermatitis �21m �33s
    An inflammation of the skin. Dermatitis can be caused by infection,
physical or chemical irritation, allergy, contact, and unknown agents.
Clinically, dermatitis is a skin disease that begins with vesicle formation
and may go on to a chronic, nonvesicular stage. Acute dermatitis is wet,
vesicular, and oozing, while chronic dermatitis is dry, scaling, and
lichenified.
    Contact dermatitis is an acute or chronic condition resulting from
irritation by, or sensitization to, some substance in the environment, for
example, poison ivy. Unfortunately, removal of the offending substance may not
ensure cessation of the dermatitis, which in many cases tends to persist and
spread in altered form. See HYPERSENSITIVITY.
    Atopic dermatitis or neurodermatitis may be a localized or a widespread
chronic inflammation characterized by an intense itching and resultant
thickening, or lichenification, of the skin. In many individuals there is a
preceding or concomitant record of nervous tension or emotional stress.
    Seborrheic dermatitis is a common disease of the scalp and other skin
regions in which there is a tendency for greasy scaling and variable degrees
of inflammation. Many factors are believed to be important in the etiology of
the disease, such as oily skin, nutritional or hormonal imbalances, and
certain emotional disturbances.
    Stasis dermatitis is a chronic disorder apparently resulting from poor
circulation and related conditions. It is found principally in older persons,
and the incidence is higher among people who suffer from varicose veins.
    Various forms of dermatitis represent scaling inflammations of
noninfectious, but often unknown, cause. The most common categories are
infantile, occupational, and senile eczemas. See ECZEMA; PSORIASIS.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sDermatophytosis   �21m �33s 
    An infection of the skin of humans and animals caused by fungi which live 
in the keratinized tissues but which are unable to invade the subcutaneous or 
deeper tissues. Classification of the infection is based upon its clinical 
appearance rather than upon the etiologic agents. For example, ringworm of the 
scalp may be caused by any one of several species of Microsporum or 
Trichophyton. Thus, fungus infection of the scalp and hair is tinea capitis; 
of the feet, tinea pedis. Fungi causing dermatophytoses are also included in 
the genes to Epidermophyton. 
    Some patients develop an allergy to the fungus which causes their 
dermatophytosis; this is manifested as an eruption secondary to the primary 
infection. The allergic reaction clears up as the primary lesions heal. See 
HYPERSENSITIVITY; MEDICAL MYCOLOGY. 
 �34s[Leanor D. Haley]
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�6v�2v�32sDermoptera �21m �33s
    An ancient order of somewhat primatelike herbivorous (plant-eating) and
frugivorous (fruit-eating) gliding mammals, now confined to the extreme
southeast corner of Asia and parts of the East Indies and the Philippines.
Colugos, or flying lemurs, as they are often called, are the only extant
members of the group. Modern dermopterans are usually placed in a single
genus, though two genera may exist.
    The patagium, a flap of skin used in gliding, extends from behing the ears
to the hands, then to the feet, and finally to the tip of the tail. The extent
of the patagium and the gliding ability, of course, are not known in fossil
forms, nor are certain features of the soft anatomy. Skeletal features not
not related to their gliding adaptations point to distant connections with
primitive primatelike insectivores that were no doubt living in southeastern
Asia as long ago as the Cretaceous. Colugos probably were restricted to
southeastern Asia by the contraction of the tropics that occurred during the
Oligocene and later Cenozoic. See EUTHERIA; INSECTIVORA; MAMMALIA.
[Malcolm C. Mckenna]
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�6v�2v�32sDerrick �21m �33s
    A hoisting machine consisting usually of a vertical mast, a slanted boom,
and associated tackle. Derricks have a wide variety of forms. The mast may be
no more than a base for the boom; it may be a tripod, an A-frame, a fixed
column, and so on. Fixed stays may guy it in place. The boom may be fixed, it
may pivot at the base of the mast, it may swing horizontally from near the top
of the mast, or it may be omitted. The derrick may be permanently fixed,
temporarily erected, or mobile on a cart or truck.
    Derricks are widely used in construction, in cargo handling, and in shops.
A manual or powered winch provides the lifting action by coiling in the
running tackle, the load swinging free as it rises. See CRANE HOIST; HOISTING
MACHINES.
 �34s[David O. Haynes /Frank H. Rockett]
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�6v�2v�32sDescriptive geometry �21m �33s
    A mathematical-graphical procedure for the visualization of structures and
their exact representation in drawings. After analysis of the structure, each
element is shown in the drawing in its exact geometrical relation to the other
elements. There are two basic methods of descriptive geometry: the projection
method and the direct method. The two methods differ as regards the attitude
of mind toward the structure and toward the drawing that represents the
structure.

    Projection method.	In the projection method the horizontal projection
plane H and the vertical projection plane V intersect in the line GL, which is
called the ground line (Fig. 1). These two projection planes divide space into
four quadrants, or angles, as numbered in Fig. 1. Point A, in the first
quadrant, is projected onto the horizontal plane at asubh by means of a
projection line perpendicular to the H plane, and onto the vertical plane at
asubv by means of a projection line perpendicular to the V plane. The
projections of the points B, C, and D in the other quadrants are located in a
similar manner. Right-angle projection, as described above, is called
orthographic projection.

    To represent horizontal and vertical projections on a flat sheet of paper,
the planes are conceived as being hinged along the ground line and brought
together by closing the second and fourth quadrants. Projections of A, B, C,
and D then appear in a single plane (Fig. 2). The H and V projections of a
point are always in the same perpendicular to the ground line.

    There are two general types of views, perspective and orthographic (Fig.
3). A perspective view of an object is observed from a fixed station point, or
point of view, by means of converging rays of light that meet at the eye of
the observer. An orthographic view of an object is observed in a chosen
direction by means of parallel rays of light.


    Direct method.  In the direct method the attention is focused on the
visualized structure or object. Each view of the object is obtained by looking
at the object in a definite direction. The view is orthographic. A view never
is considered as two-dimensional or as projected or drawn on a plane. This is
the way the engineer who makes and reads the drawing thinks of views.
    Orthographic views may be classified into three types: principal views,
auxiliary views, and oblique views. The object can be viewed from any
direction around three rings - horizontal, frontal, and profile (Fig. 4). The
rings represent three mutually perpendicular planes. The intersections of the
rings define three mutually perpendicular directions from which six principal
views are observed: front and rear, top and bottom, right and left sides. An
auxiliary view can be observed around any ring in a direction perpendicular to
one, and only one, of the directions in which principal views are observed.
All views other than principal or auxiliary views are oblique. In Fig. 4 a
single arrow, marked oblique, indicates one of the infinite number of
directions in which oblique views are observed. See DRAFTING; ENGINEERING
DRAWING.
 �34s[Albert S. Palmerlee /Charles J. Baer]
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�6v�2v�32sDesertification   �21m �33s 
    The spread of desertlike conditions in arid and semiarid areas, due to 
human influence or climatic change, particularly as a result of the 
interaction of naturally recurring drought with unwise land-use practices. 
Natural vegetation is being lost over increasingly larger areas because of 
overgrazing, especially in Africa and southwestern Asia. In many areas the 
deserts seem to be spreading, with an apparent speed of about one or more 
kilometers per year. 
    The world distribution of dry climates depends mainly on the subsidence 
associated with the subtropical high-pressure belts, which migrate poleward in 
summer and equatorward in winter. These migrations are connected with the 
atmospheric general circulation. There results a threefold structure in the 
arid zone: a Mediterranean fringe, with rains occuring only in winter; a 
desert core (in about 20 - 30DG latitude), with little or no rain; and a 
tropical fringe, with rains mainly in the high sun season. Throughout the arid 
zone, rainfall variability is high. Clear skies and low humidities in most 
regions give the dry climates very high solar radiation (averaging of 200 
W/msup2), which leads to high soil temperatures. The light color and high 
reflectivity of many dry surfaces cause large reflectional losses, and 
long-wave cooling is also severe. Hence net radiation incomes are relatively 
low.  
    Climatic variation takes place on many time scales. The world's deserts 
and semideserts are very old, although they  have shifted in latitude and 
varied in extent during geological history. The modern phase of climate began 
with a major change about 10,000 years before present, when a rapid warming 
trend removed most of the continental ice sheets. The Sahara and Indus valleys 
were at first moist, but since about 4000 years before present, when severe 
natural desiccation took place, aridity has been profound. 
    Recent climatic variations, such as the Sahelian drought, are natural in 
origin, and are not without precedent. Prolonged desiccation, lasting a decade 
and more, is common and often ends abruptly with excessive rainfall. This 
persistence suggests that feedback mechanisms may be operating, whereby 
drought feeds drought and rain feeds rain. 
    A common mechanism of desertification includes the following steps: (1) 
expansion and intensification of land use in marginal dry lands during wet 
years, including increased grazing, plowing, and cultivation of new lands, and 
wood collection around new camps or settlements; and (2) wind erosion during 
the next dry year, or water erosion during the next maximum rainstorm. 
     This desertification process has the following climatological 
implications: (1) Increased grazing during wet years tends to compact the soil 
near water holes, and increased livestock numbers cause pressure on perennial 
plants during dry seasons. The result is to expose surface soil to erosion by 
wind. Runoff may be increased, and so may albedo (that is, reflectivity of the 
ground with respect to solar radiation). (2) Increased cultivation during wet 
years greatly improves the chances of wind erosion of fine soil materials 
during the dry season, and possibly increases evapotranspiration (in the case 
of water-demanding crops). (3) Removal of wood increases direct solar heating 
and may considerably decrease evapotranspiration. (4) In the ensuing dry 
years, the acceleration of wind erosion by the above processes further reduces 
water storage capacity by removal of topsoil. The loss of some or all 
perennial plant cover lowers infiltration rates and hence the potential 
percolation. During subsequent rains, surface runoff is increased, with an 
attendant loss of water for subsequent use by shrubs, herbage, or crops. 
    More research is needed into the relationship between climate and the 
desertification process. Experimentation is needed that specifically examines 
the problems of the general circulation of the Earth's atmosphere, and even of 
the smaller-scale circulation, over the dry land areas of the subtropics. 
There is also a need for more detailed study of the physical climatology and 
bioclimatology of dry land surfaces, and particularly for a closer synthesis 
of climatology with geomorphology, soil science, hydrology, and ecology. See 
CLIMATE MODIFICATION; CLIMATIC CHANGE; DROUGHT. 
 �34s[Louis Berkofsky]
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�6v�2v�32sDesiccant   �21m �33s 
    A substance (adsorbant) used to withdraw moisture from other materials. 
Although the removal of large quantities of water is done by evaporation, 
aided by moving air currents and by elevated temperature, the last traces of 
moisture are often held very tightly and do not evaporate readily. 
Furthermore, evaporation ceases when the moisture content of the material is 
reduced to that of the drying-air current. For final drying, a desiccant is 
used. It may react with water chemically or retain water through capillarity 
of adsorption. The drying agent is placed directly into the gas or liquid to 
be dried; solid materials are placed in a desiccator, a closed vessel in which 
moisture diffuses to the desiccant through the dry desiccator atmosphere. A 
desiccant loses potency as it takes on water; often it can be renewed by 
heating. Desiccants which form hydrates can be selected to maintain certain 
levels of low humidity in a closed vessel. See ADSORPTION; DELIQUESCENCE; 
DRYING.  
    Among the more important types of solid desiccants are silica gel, 
activated alumina, anhydrous calcium sulfate, magnesium perchlorate, oxides 
(of barium and calcium), and activated carbon. 
 �34s[Allen L. Hanson]
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�6v�2v�32sDesign standards   �21m �33s 
    Specifications of materials, physical measurements, processes, performance 
of products, and characteristics of services rendered. Design standards may be 
established by individual manufacturers, trade associations, and national or 
international standards organizations. The general purpose is to realize 
operational and manufacturing economies, to increase the interchangeability of 
products, and to promote uniformity of definitions of product characteristics. 
    Individual firms often maintain extensive and detailed standards of parts 
that are available for use in their product designs. Usually the standards 
have the effect of restricting the variety of parts to certain sizes and 
materials. In this way the production lots required for inventory purposes are 
increased, and production economies may thereby be realized through the wider 
use of mass production. However, even if the larger quantities needed of the 
relatively few sizes do not in themselves lead to a cheaper manufacturing 
process, the costs of carrying inventory and setting up for production runs 
are reduced. A further development of this design approach may lead to the 
modulization of the entire product line, by reducing it to certain major 
subassemblies that are common to as many products as possible. Special jobs 
then typically require only a few added features, and cost savings may be 
realized.  
    A possible disadvantage is that the extensive use of general-purpose parts 
may jeopardize the space parts business, especially where outside 
manufacturers can skim off the market for the more commonly used and 
profitable spare parts once the original patents, if any, have expired, and 
then leave the more complex and slow-moving spares to the original 
manufacturer. However, it is precisely this aspect of standardization which is 
often welcomed by the users of the product. 
    Standardization also determines the nature of design practice. Especially 
when the specifications also give data on strength and performance as well as 
the usual dimensions, it is only necessary to compute loads approximately and 
then select the nearest standard sizes. Much design effort is thereby saved, 
especially on detail drawings, bills of material, and so forth. This approach 
also simplifies programming when computer-aided design is used. See 
COMPUTER-AIDED DESIGN AND MANUFACTURING. 
    Trade associations are the principal sources of American industrial 
standards. These involve standardization over an entire product line. In 
general, their scope is considerably less than that within firms with 
extensive standardization programs, but the technical and policy 
considerations in the two levels of standardization are quite similar. Trade 
standards are primarily concerned with specifying overall dimensions, so that 
products of different manufacturers may be used interchangeably; with 
performance, so that customers know what they are buying; and with certain 
design features, such as major materials, in order to assure proper function. 
In some cases, dimensional standards particularly must be related to standards 
in other industries; for instance, an American butter dish must accommodate 
the standard 4-oz (113.6-g) sticks in which butter is packed. Like national 
standards to which they are closely related, trade association standards 
should be established on the basis of as broad a consensus as possible within 
the industry. If standards were established such that any required burden of 
retooling and product change would fall in a discriminatory fashion upon only 
certain members of the industry, legal remedy would certainly be sought under 
the American antitrust laws. 
    The principal industrial countries have official agencies that approve, 
consolidate, and in some cases establish standards. Among them are the British 
Standards Institution (BSS), German Institute for Norms (DIN), and the 
American National Standards Institute (ANSI, formerly ASA). The national 
standardization agencies are members of a wide variety of international 
groupings and United Nations agencies. The principal ones are the 
International Organization of Standardization (ISO) and the International 
Electrotechnical Commission (IEC). These attempt to coordinate national 
activities and promote cooperation in the area of standardization. Several of 
the more than 50 organizations deal with weights and measures; others engage 
in transnational or international activities in the standardization of many 
products or cover specific regional issues and requirements. Some, like the 
European Economic Community (EEC), the International Telecommunications Union 
(ITU), the International Civil Aviation Organization (ICAO), or the 
administration of the General Agreement on Tariffs and Trade (GATT) mainly 
have other political, economic, and scientific concerns but must necessarily 
take note of standardization as part of their work. See ENGINEERING DESIGN; 
PHYSICAL MEASUREMENT; STANDARD. 
 �34s[John E. Ullmann]
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�6v�2v�32sDesizing   �21m �33s 
    A process that removes sizing agents (slasher compounds) which have been 
added to the warp yarns (lengthwise yarns) prior to weaving to protect them 
against the abrasive action of loom parts during the formation of gray fabric. 
Cellulosics are slashed primarily with starches which may be removed with 
enzymes of the malt or bacteriostatic types and are called amylolytic enzymes. 
Other effective agents are dilute mineral acids at high temperatures and 
sodium bromite in cold solution. Protein warps are sized or slashed or 
pretreated with proteinaceous agents such as gelatin, casein, albumin, or glue 
and require the use of proteolytic enzymes for their removal. Synthetics are 
slashed with water-soluble agents such as polyvinyl alcohol or sodium salts of 
polyacrylic acids which are removed by hot detergent solutions. Use of 
polyvinyl alcohols and carboxymethyl celluloses is increasing with cotton and 
other cellulosic fiber materials because of their biodegradable 
characteristics and as a means of decreasing stream pollution. Elimination of 
the desizing operation creates insurmountable problems in subsequent 
processes, including bleaching. See BLEACHING; TEXTILE CHEMISTRY. 
 �34s[John J. McDonald]
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�6v�2v�32sDesmodoroidea   �21m �33s 
    A superfamily of marine and brackish-water - inhabiting nematodes with an 
annulated, usually smooth cuticle which is reinforced at the head to form a 
``helmet,'' reflexed ovaries, and amphids in the form of a simple or multiple 
spiral. The stoma is generally provided with a well-developed dorsal tooth and 
is usually opposed by subventral dentition of variable development. The 
species are generally regarded as being epigrowth feeders, but some may be 
predacious. They are prominent in sublittoral soft-to-coarse bottom sediments. 
See NEMATA. 
 �34s[Bruce Hopper]
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�6v�2v�32sDesmoscolecoidea   �21m �33s 
    A small superfamily of free-living nematodes characterized by a ringed 
body, an armored head set off from the body, and hemispherical amphids. The 
species are small and plump and, with the exception of a few soil- and 
fresh-water - inhabiting species, are marine. Two families are recognized, 
Desmoscolecidae and Meyliidae. The Greeffiellidae, once contained in this 
superfamily, has been raised to superfamilial status and, together with the 
Desmoscolecoidea, constitutes the order Desmoscolecida. See NEMATA. 
 �34s[Bruce Hopper]
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�6v�2v�32sDesmostylia   �21m �33s 
    An extinct order of large, hippopotamuslike, amphibious, gravigrade, 
shellfish-eating mammals, that frequented shallow bays and coastal margins 
during the Oligocene and Miocene (see illustration). Their main distribution 
was northern trans-Pacific, but discoveries in Florida indicate a much broader 
distribution. There are two families, the Desmostylidae and the 
Paleoparadoxidae. These animals probably descended from an ancestral stock 
that also gave rise to the Proboscidea and Sirenia. See MAMMALIA; PROBOSCIDEA; 
SIRENIA.  
 �34s[Gideon T. James]
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�6v�2v�32sDesmothoracida   �21m �33s 
    An order of Heliozoia. The shell (test) of these protozoans is typically 
perforate and chitinous (often with appreciable siliceous impregnation, but a 
completely siliceous wall seems to be rare). The body is commonly spherical. 
Some species are normally floating (illustration a - c); others are stalked 
sessile forms (illustration d - e). There is no centroplast, and there is a 
more or less central nucleus. The pseudopodia are typically delicate 
filamentous types (filopodia), which may branch. Little is known about most of 
the described species. See HELIOZOIA; PROTOZOA. 
 �34s[Richard P. Hall]
�33s 
  
  
  
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�6v�2v�32sDestructive distillation �21m �33s
    The primary chemical processing of materials such as wood, coal, oil
shale, and some residual oils from refining of petroleum. It consists in
heating material in an inert atmosphere at a temperature high enough for
chemical decomposition. The principal products are (1) gases containing carbon
monoxide, hydrogen, hydrogen sulfide, and ammonia, (2) oils, and (3) water
solutions of organic acids, alcohols, and ammonium salts.
    Crude shale oil may be obtained by destructive distillation of
carboniferous shales. It may be subjected to a destructive or coking,
distillation to reduce its viscosity and increase its hydrogen content.
Residual oils from petroleum refinery operations are subjected to
coking-distillation to reduce the carbon content. The coke is used for the
manufacture of electrode carbon. The main product of the destructive
distillation of wood is 40 - 45% charcoal used in metallurgical processes in
which the low content of ash, sulfur, and phosphorus is important. See
CHARCOAL; COAL CHEMICALS; COKE; COKING (PETROLEUM); OIL SHALE; PYROLYSIS; WOOD
CHEMICALS.
 �34s[Henry H. Storch /Howard W. Wainwright]
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�6v�2v�32sDetector   �21m �33s 
    A device used to recover the original modulating signal from a modulated 
wave, also called a demodulator. In communications systems, and in some 
automatic control systems and data storage systems, the information to be 
transmitted is first impressed upon a periodic wave called the carrier; the 
carrier is then said to be modulated. After reception of the modulated 
carrier, the original modulating signal is recovered by the process of 
demodulation or detection. See MODULATION; MODULATOR. 
    The amplitude, frequency, or phase of a carrier may be changed in the 
modulation process. Therefore the process of detection and the practical 
circuits for accomplishing it differ in each case. Detection of an 
amplitude-modulated carrier is carried out by a rectifier. Detection of 
frequency- and phase-modulated signals is usually accomplished by a 
discriminator or a phase-locked oscillator. See AMPLITUDE MODULATOR; FREQUENCY 
MODULATOR; PHASE MODULATOR. 
    Detectors are also commonly used in various classes of measuring 
instruments to produce an indication of a signal or its relative magnitude. 
The indicating devices employed in impedance bridges or slotted lines 
illustrate such applications. 
    All detectors require use of nonlinear devices for recovering the desired 
signal from a modulated carrier. The simplest and most commonly used of these 
are semiconductor, or crystal, diodes. The nonlinear characteristics of 
transistors of multielement vacuum tubes are sometimes employed in special 
applications where simultaneous detection and amplification are most 
convenient. See AMPLITUDE-MODULATION DETECTOR; FREQUENCY-MODULATION DETECTOR; 
PHASE-MODULATION DETECTOR. 
 �34s[Charles L. Alley]
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�6v�2v�32sDetergent   �21m �33s 
    A substance used to enhance the cleansing action of water. A detergent is 
an emulsifier, which penetrates and breaks up the oil film that binds dirt 
particles, and a wetting agent, which helps them to float off. Emulsifier 
molecules have an oillike nonpolar portion which is drawn into the oil, and a 
polar group that is water-soluble; by bridging the oil-water interface, they 
break the oil into dispersible droplets (emulsion). As a surfactant, a 
detergent decreases the surface tension of water and helps it penetrate soil. 
    Soap, the sodium salt of long-chain acids, was the principal detergent 
until superseded in 1954 by synthetic detergents (syndets) which, unlike soap, 
do not form insoluble products with the calcium in hard water. Most syndets 
are of the anionic type, that is, sodium salts of alkyl sulfates or 
sulfonates. Alkyl benzene sulfonates (ABS) with branched carbon chains were 
found to persist in wastewater and have been replaced by linear alkyl benzene 
sulfonates (LAS), which are biodegradable by bacterial action. Anionic 
detergents are best for water-absorbing fibers such as cotton, wool, and silk. 
Nonionic detergents are polyethers made by combining ethylene oxide with a 
12-carbon lauryl alcohol. They are used for water-repelling ``permanent 
press'' fabrics, and their low-foaming property is desirable for automatic 
washers. Cationic syndets are quarternary base compound. They are more 
expensive, but some are germicidal; some are used as fabric softeners and as 
good metal cleaners. 
    Detergents must contain alkaline ``builders'' to bind dissolved metal ions 
and support emulsification. Sodium pyrophosphate or polyphosphate were 
preferred because of low cost and high cleansing effectiveness. However, when 
discharged with laundry wastewater, these compounds supply nutrient to 
phosphate-deficient lakes and streams and thus lead to eutrophication, and 
their use is now banned by law. Less harmful, but less effective, builders 
such as sodium carbonate are now widely used in detergents. See EUTROPHICATION. 
    Many additives are used in detergents to provide scent, brightening 
(usually through fluorescent action), or bleaching action. Biodegradability is 
essential for detergents; it ensures that components of detergents will be 
broken down by bacterial action before undesirable after-effects can occur. 
Nonbiodegradable detergents can prevent effective bacterial action in septic 
tanks and sewage treatment plants, and can cause undesirable persistent 
foaming in rivers. See SOAP. 
 �34s[Allen L. Hanson]
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�6v�2v�32sDeterminant �21m �33s
    The theory of determinants had its origin in the solution of linear
systems of equations. Determinants occur in virtually all branches of
mathematics and related fields. See LINEAR SYSTEMS OF EQUATIONS; MATRIX
THEORY; POLYNOMIAL SYSTEMS OF EQUATIONS.
    The concept of a determinant can best be explained with reference to a
matrix A.

		    A = (a11 a12 . . . a(sub 1n) over
Let			 a21 a22 . . . a(sub 2n) over
			 . . . . . . . . . . . . .
			 a(sub n1) a(sub n2) . . . a(sub nm))

be an n by n square array of numbers. Such an array is called an n by n square
matrix, and the numbers asubisubj in the array are called elements of the
matrix. The determinant of the matrix A, as indicated by Eq. (1), is a number
which is the value of a certain function of the elements of A.

		  |A| = |a11 a12     . . .  a(sub 1n)|
			|a21 a22     . . .	  a2n|
			|. . . . . . . . . . . .... .|
			|asubn1 asubn2 . . .a(sub nm)|			     (1)

    The determinant of Eq. (1) is called a determinant of order n, and it can
be defined by induction on n. For explanatory purposes, determinants of orders
n = 1, 2, and 3 are defined as follows: For n = 1, VERTICAL BAR a11 VERTICAL BAR
= a11.

For n = 2, |a11 a12|
	   |a21 a22| = a11|a22|-a12|a21| =a11a22 - a12a21

For n = 3, |a11 a12 a13|
	   |a21 a22 a23|
	   |a31 a32 a33|

= a11|a22 a23|
     |a32 a33| -a12|a21 a23| + a13|a21 a22|
		   |a31 a32|	  |a31 a32|

	 = a11 (a22a33 - a23a32) - a12(a21a33 - a23a31) + a13(a21a32 - a22a31)



The determinants of order 2 and 3 are defined in terms of determinants of
order 1 and 2, respectively. Suppose now that a determinant of order n - 1 has
been defined. In Eq. (1), denote by Asub1j, j = 1, 2, . . . , n, the
determinant of order n - 1 formed by deleting from Eq. (1) the first row and
the jth column. Equation (2) then, by definition, holds.

|A| = a11A11 - a12A12 + a13A13 + . . . + (-1)(sup n-2) a(sub 1n-1) A(sub 1n-1)
					     + (-1)subn-1 asub1n Asub1n     (2)

This definition is seen to agree with the definitions for determinants of
order 2 and 3 and, by the principle of mathematical induction, defines a
determinant of order n for every n. There are many other definitions of a
determinant of order n which can be proved to be equivalent to the definition
given here. See EQUATIONS, THEORY OF.
 �34s[Richard A. Bambach]
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�6v�2v�32sDetonator   �21m �33s 
    A device used to initiate the explosion of a high explosive. Detonators 
employ a very sensitive primary explosive which detonates readily when set 
afire by the primer. Primary explosives in common use are mercury fulminate 
and lead azide. In addition, the detonator or blasting cap usually contains a 
small charge of secondary explosive, such as PETN (pentaerythritol 
tetranitrate), to transmit a more powerful shock wave than the primary 
explosive along. An electric detonator is ignited by a fuse wire which serves 
to touch off the primer. The explosion of the detonator readily sets off a 
dynamite charge. See FUSE (EXPLOSIVE). 
 �34s[William E. Gordon]
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�6v�2v�32sDeuterium   �21m �33s 
    The isotope of the element hydrogen with atomic weight 2.0144 and symbols 
Hsup2 or D. The terrestrial natural abundance of deuterium is 1 part in 6700 
parts of ordinary hydrogen (protium). Small variations in natural sources are 
found as a result of fractionation by geological processes. 
    Deuterium is a gas (D2) at room temperature. It is prepared from heavy 
water, D2O, either by electrolysis or by reaction of D2O with metals such as 
zinc, iron, calcium, and uranium. It is also prepared directly by the 
fractional distillation of liquid hydrogen. 
    Deuterium is used mainly in the form of heavy water. In the uncombined 
state it finds uses as a research tool. Liquid deuterium is used in bubble 
chambers to study the reactions of elementary particles with the deuterium 
nucleus, the deuteron. Deuterons are frequently accelerated in cyclotrons to 
study their reactions with other nuclei and also to produce radioactive 
nuclides. Deuterium gas is used in the direct synthesis of organic compounds 
for tracer studies. See DEUTERON; HEAVY WATER; HYDROGEN; NUCLEAR FUSION; 
NUCLEAR REACTION; TRITIUM. 
 �34s[Jacob Bigeleisen]
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�6v�2v�32sDeuteron   �21m �33s 
    The nucleus of the atom of heavy hydrogen, Hsup2 (deuterium). The deuteron 
d is composed of a proton and a neutron; it is the simplest multinucleon 
nucleus. Its binding energy is 2.227 Mev; that is, this is the amount of 
energy which must be added to a deuteron for it to dissociate into a proton 
and a neutron. Deuterons are much used as projectiles in nuclear bombardment 
experiments. See DEUTERIUM; NUCLEAR REACTION. 
 �34s[Henry E. Duckworth]
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�6v�2v�32sDevelopmental biology   �21m �33s 
    The current and appropriate term for a large field of investigation that 
has taken shape by a coalescence of several paths of research. It was in the 
interdisciplinary spirit induced by the maturing of the biologically oriented 
natural sciences that a small group of experimentalists interested in organic 
development met for a symposium in 1939. Professionally, their fields included 
biochemistry, botany, cytology, embryology, genetics, mathematics, medical 
(cancer) research, microbiology, and zoology. They continued, as the Society 
of the Study of Development and Growth, to meet annually. In 1959 a journal 
entitled Developmental Biology was organized; it has become the official organ 
of the society, which a few years later changed its name to the Society for 
Developmental Biology. At present, there is also an International Society of 
Developmental Biologists. Individual investigators in the field work with all 
varieties of organism, at many levels, but in any case keep in mind the 
picture of cell function that is accumulating at the molecular level. A 
developmental view must also consider the life history of the organism at hand. 
     In multicellular animals the sexual cycle is simple in outline. An adult 
form, with a double (diploid) chromosome set, produces gametes by two 
specialized cell divisions (meiosis). Sperm and egg have single-set (haploid) 
chromosomes. Fertilization restores the diploid state, with added genetic 
variability. The zygote undergoes a developmental history, which may be direct 
or may be interrupted by one or more metamorphoses from a larval state. All 
cell divisions in the history will be mitotic, until germ cells are produced. 
See OOGENESIS; REPRODUCTION (ANIMAL); SPERMATOGENESIS. 
    In multicellular plants the haploid phase is prolonged. Spores, which may 
be of one type or may be distinguished as micro- or macrospores, are produced 
by meiotic divisions. The haploid spores undergo mitotic divisions to form 
gametophytes. In mosses the gametophyte is the principal, green plant. In seed 
plants the male and female gametophytes are very reduced in size; the female 
is sessile within a parental structure, to which the male pollen grain is 
transported. Sperm and egg differentiate in the haploid plants, then unite to 
develop into the plant as commonly known, which is the sporophyte generation. 
See REPRODUCTION (PLANT). 
  
    Embryogenesis.  Normally insemination follows release of the ovum, 
triggering activation of the egg. After the sperm enters the egg, male and 
female pronuclei reconstruct as they approach the center of the egg and unite, 
while the mitotic spindle assembles. The early development that follows the 
initial mitosis includes cleavage, leading to gastrulation, whereby three germ 
layers move to final position. The cells of the animal region remain external; 
vegetal cells become enclosed, as innermost entoderm and as mesoderm between 
the two layers. 
    Cleavage is a period of successive mitoses, subdividing the zygote into 
numerous cells, with almost no alteration of cytoplasmic localization or 
polarity, or change in mass or shape. The mitotic apparatus is the immediate 
mechanism, modified by the cytoplasmic structure, which may produce unequal 
divisions or restrict mitosis to one area, as in the case of the bird egg. 
Cleavage patterns differ among phyla and classes, morphologically and 
developmentally. In some groups, including vertebrates, isolated cells or 
parts of the cleaving zygote can form a complete organism, if both animal and 
vegetal cytoplasm are present in proportions related to the polar structure. 
By contrast, in phyla having so-called mosaic development, even the first 
cleavage is differential. See CLEAVAGE (EMBRYOLOGY). 
    In echinoderms, vetebrates, and related phyla, cleavage ceases as a 
fluid-filled hollow blastula stage is reached, with some hundreds of cells 
displaying rudimentary differentiation. The future ectoderm is a coherent 
epithelium. Gastrulation involves invagination of mesodermal and entodermal 
areas, while the ectoderm stretches to cover the entire structure, all without 
altering the original spherical shape. The involuting cells retain contact 
with one another and with the ectoderm by long pseudopodia if not by the 
entire surface. 
    Gastrulation is a critical period, known as a stage when incompatible 
hybrids die. Primitive organogenesis progresses from anterior to posterior. 
The dorsal ectoderm develops longitudinal folds that meet, pinching off a tube 
that will become the central nervous system; the entodermal layer, similarly, 
produces the tube that will line the digestive tract. Mesoderm, except for its 
anteriormost region, which remains relatively unorganized, forms the axial 
notochord (the foundation of the vertebral column), while adjacent mesoderm on 
either side assumes a segmental, repetitive pattern that will impose itself on 
the entire dorsal axis. The remaining mesoderm spreads laterally between 
ectoderm and entoderm, to meet ventrally, forming the heart and the body 
cavity. See GASTRULATION. 
    Cell differentiation, involved in these rearrangements, is at first 
reversible, becoming stabilized as the germ layers find their relative 
positions. Contacts between cells with different previous histories appear to 
be decisive factors in stabilization. Such contacts have been given the 
general term of embryonic induction; they are mutual, not one-way, reactions, 
eventually leading to a fixed cellular program not reversible by experimental 
methods so far employed. The primary induction in vertebrate development is 
that of the central nervous system by the dorsal mesoderm during gastrulation. 
In the absence of mesodermal lining, the ectoderm can retain only an 
undifferentiated epithelial structure. Different foreign molecules can 
regularly induce three recognizable regional levels of the nervous system: 
anteriormost (forebrain - nose - eye - midbrain); secondary (hindbrain - ear - 
spinal cord); and posterior (tail). Since these three levels, through 
gastrular movements, become underlain by structurally different mesodermal 
domains, regional differences of signal are not unexpected. See CELL LINEAGE. 
    After the original induction, cell differentiation progresses stepwise by 
new syntheses, or intensification of established ones. The process may depend 
on mitoses, migration, new contacts, and even cell fusion, as well as hormonal 
signals. Visible differentiation takes many forms, some well analyzed at 
molecular levels. Selective protein synthesis is basic; it may characterize 
cell types, such as red cells, keratin-producing epidermis, or, as an extreme 
example, the elaborately assembled fibers of striated muscle. Many cell types 
are characterized by elaboration and export of specific molecules or subunits: 
exocrine or endocrine secretions, matrix, or the various neurotransmitters and 
neurohormones that are continually being discovered. The plasma membrane is 
probably specifically structured in every cell type, or perhaps in each cell 
clone, as seems the case in the immune system. In neurons the plasma membrane 
not only is the functional messenger of impulses but, in response to a growth 
factor, is critical in production and guidance of the polarized outgrowing 
fibers that establish the incredibly complex structure of nervous tissue. See 
EMBRYONIC DIFFERENTIATION; EMBRYONIC INDUCTION. 
  
    Plant development.  In seed plants the preparatory functions for early 
development are not concentrated in a single oocyte cell. Meiosis is completed 
and a haploid gametophyte formed in a polarized capsule structure within a 
modified leaf; in flowering plants this is the pistil base, connected to the 
parental vascular tissue. Upon fertilization, the zygote commences syntheses; 
the cell soon divides. Cleavage of the zygote, unlike that in the animal, at 
first forms a linear cell series in the polar direction, later taking, 
successively, globular and bilateral shapes, while continually enlarging by 
mitosis. The growth pattern first emphasizes the storage structures of the 
embryo itself. Later a centrally located area of rapidly dividing cells 
indicates the future plant axis, with the shoot apex directed toward the 
nutritive pole, and the root tip and cap opposite. See PLANT GROWTH. 
    The plant axis is organized on the basis of cell and tissue 
differentiation in an elegant geometry pattern within a cylindrical structure. 
Growth is partly by addition, in layers, of axially aligned cells from 
actively dividing meristem apices. The walled, individually immovable cells, 
as they are distanced from the meristem, elongate, thus adding to axial 
growth. The cells differentiate, following a radial pattern of position within 
the cylinder. The basic pattern consists of an outer epidermis, an adjacent 
cortex of supporting or storage tissue, and a vascular ring, with or without a 
central core. These landmarks become variously elaborated according to 
species, region, and developmental stage. Radial growth derives from dividing 
tissue (cambium) organizing between the outer phloem and inner xylem layers of 
the vascular ring. 
    Much cell differentiation relates to wall structure. Tracheids and xylem 
tubes, phloem sieve tubes, sclerenchyma, epidermal walls with interspersed 
apertures such as lenticels and stomata, and depots of lignin and cork are 
examples. Final differentiation may result from death of the cell, as in xylem 
vessels. Parenchymal cells, distinguished by chloroplasts or starch grains in 
the cytoplasm, are typically thin-walled, with intercellular spaces. All 
living cells are interconnected by protoplasmic strands, passing through wall 
apertures.  
    It is not known to what extent differentiation is due to intercellular 
relations; the precision of the pattern suggests considerable local signaling. 
It is certain that translocation of growth substances plays an important role. 
Several molecular types have been identified. Three of these substances are 
organic acids: auxins, gibberellins, and abscisic acid. Another family of 
substances, the purine-related cytokinins are involved in seed germination and 
in releasing meristems from inhibition. Other transported substances, such as 
flower-inducing factors, have resisted isolation. See ABSCISIC ACID; AUXIN; 
CYTOKININ; GIBBERELLIN. 
    Developmental effects of growth substances often appear contradictory, 
probably because response of cultured cells is very sensitive to 
concentrations, and is also species-and stage-specific. Cytokinins and 
gibberellins are present in gametophyte tissues; abscisic acid is involved in 
the transfer of endosperm-stored carbohydrate to cotyledon tissue. See PLANT 
MORPHOGENESIS.  
  
    Cellular totipotency.  Not only is cell differentiation in vascular plants 
less diverse than in animals, it is reversible. Adventitious embryo formation 
is known in many species. Modern culture methods have demonstrated that many, 
perhaps all, living cells can be induced to dedifferentiate and to undergo 
embryo formation and even development to reproductive stages. Haploid embryos 
have been produced from isolated microspores, although, as in animal 
parthenogenesis, regulation to the diploid state is the rule. 
    Although differentiated animal cells may lose some specialized structures 
when cultured, nothing like a reversion to the zygote stage has been observed. 
In amphibians the nucleus of many types of differentiated cells can, if 
transplanted to oocyte cytoplasm, collaborate in the development of a 
reproductive adult, thus demonstrating a complete genome. It is known that in 
the oocytes of many animal phyla a cytoplasmic factor or factors, localized in 
the vegetal region, determine a germ cell lineage to be isolated until the 
gonad is organized. 
  
    Morphogenesis.  Although much is known about cellular aspects of organ and 
organism structure in both plants and animals, the molecular basis of these 
larger developmental problems is far from clear. In plants growth patterns 
appear to depend on mitosis and on axial and lateral distribution of growth 
substances. The suggestion, in geriatric investigation, that cells have 
genetically limited powers of mitosis might, if applicable to apical 
meristems, explain pattern and size limitations. A molecular counting device 
would be required; this might be an aspect of the biological clock problem 
that still remains to be solved. 
    In animals several levels of control must be understood. Anteroposterior 
polarity, originating in the oocyte as animal-vegetal differential, is 
transmitted, directionally unaltered, to blastula, gastrula, and embryo, as 
regards external tissue. In amphibian development a dorsoposterior pole is 
added in the zygote, recognizable by pigment migration in the cortex. 
Gastrulation movements follow these two axial directives, the mesoderm first 
migrating from posterior to anterior until the anterior pole is reached. At 
this point the anteroposterior direction takes over all layers, and the 
mesoderm spreads dorsoventrally. Regional differentiation must depend on 
establishment of subordinate centers; cell surfaces would be decisive in 
grouping, moving, or otherwise interacting to produce organ primordia. 
    In all cases of size or direction control, centers producing radiating or 
linear cellular gradients of communication appear to furnish a reasonable 
model. A spectacular case of reversal of the principal axis is found in the 
bilateral, elongated insect egg. In certain fly species, destruction of small 
bilateral areas of the anterior cytoplasm produces bicaudate adults: that is, 
the head and thorax are replaced by a reversed abdomen, mirror-imaging the 
original undamaged abdominal segments. It is difficult to interpret such 
organized reversal other than on a gradient pattern. 
    Specific size-controlling substances, with promotion or inhibition of 
mitosis as a target, have been identified or suspected in several cases of 
individual organs or tissues. These must be attributed to some 
species-specific spatial control. Something of the sort is strikingly 
illustrated in tetraploid amphibians, which, unlike plants, grow no larger 
than their diploid relatives. The large cell size, proportional to the 
enlarged nucleus, does not affect the size of functioning units; for example, 
the kidney tubules are no larger than normal, but are composed of fewer cells. 
 �34s[Dorothea Rudnick]
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�6v�2v�32sDevelopmental genetics   �21m �33s 
    A field that deals with problems of gene expression and its control on 
various levels from gene transcription to the phenotype. Studies in this field 
serve to identify: genes concerned with processes of development and 
differentiation; mechanisms of action of such genes and their control; and 
mechanisms of development. Genes involved in developmental processes include 
structural as well as regulatory genes of various types, for example, 
controlling and modifying genes. Mutations causing abnormalities of 
development and differentiation expose the existence of the respective normal 
genes, and thus serve as tools for the genes' identification. Gene analysis 
can be performed on both plants and animals. 
    Concepts and mechanisms pertaining to the fields of genetics and of 
developmental biology have guided analytical approaches to problems of 
developmental genetics. The genetic concepts are there of pleiotropy, 
phenocopies, penetrance, and expresivity. Two concepts of development that are 
of concern in developmental genetics are determination and inductive 
interaction. See GENE EXPRESSIVITY; PHENOCOPIES; PLEIOTROPISM. 
    The concept of determination in development refers to the progressive 
restriction of the multitude of developmental potentials of a cell to a single 
one serving the differentiation of one specific cell type only. After having 
undergone the process of determination, the cell is ``programmed'' to carry 
out its specific fate. This particular determined fate is perpetuated by cell 
hereditary. The development of vertebrates is characterized by inductive 
interactions between different cells and tissues; these interactions are 
responsible for progressive differentiation. Mutations in mice are known to 
interfere with induction processes, thus indicating a genetic basis for the 
responsible mechanisms. See DEVELOPMENTAL BIOLOGY. 
    Studies of developmental genetics involve a variety of systems, which 
include the following: 
    1. Studies of the developmental genetics of individual enzymes serve to 
identify genetic mechanisms of enzyme differentiation. The normal 
differentiation of an enzyme requires the interaction of structural genes, 
controlling genes, and regulatory genes of various types. These are involved 
in temporal as well as spatial aspects of enzyme differentiation, that is, 
time of expression of the enzyme and integration into subcellular compartments. 
    2. Mutations in the mouse provide particularly suitable model systems for 
studies of mammalian developmental genetics. The effects of such mutations 
have been described at any stage between fertilization and birth. The majority 
of developmental mutations act as embryonic lethals. Included among organs and 
tissue systems affected by developmental mutations are: pigment cells, blood 
cells, germ cells, kidneys, cartilage and bone, muscle, nervous system, and 
subcellular ultrastructure of liver and kidney cells. Mutations affecting 
enzyme and protein differentiation in prenatal stages include those causing 
specific liver and kidney enzyme deficiencies. 
    3. The T-locus is one of the most interesting and most thoroughly studied 
systems in mammalian developmental genetics. Several mutations in this complex 
are dominant mutations, whereas the larger portion is occupied by recessive 
alleles which seem to represent quantitative and qualitative changes of the 
respective chromosome region. The presence of T-locus mutations is expressed 
in effects on the tail. Embryos homozygous for certain recessive alleles die 
in early stages of development. The T-complex is a prime example of a genetic 
system involved in the regulation and control of early embryonic 
differentiation.  
    4. Lethal albino deletions in the mouse provide a system of deletion 
mutations affecting biochemical and morphogenetic differentiation and thus 
serving the analysis of the genetic control of differentiation. They include a 
series of overlapping deletions, all of which are lethal in homozygotes, 
either in early embryonic stages or perinatally, causing phenotypic 
abnormalities of ultrastructural morphogenesis of subcellular organelles, of 
embryonic membrane differentiation, of early cleavage stages of the zygote, 
and of the developmental pattern of certain proteins. 
    5. The genetic control of muscle differentiation has been studied with the 
help of a recessive mutation in the mouse. This mutation affects specifically 
the skeletal striated muscle of homozygotes, whereas cardiac and smooth muscle 
differentiate normally, thus demonstrating the existence of a separate genetic 
control of differentiation for one of the three types of muscle cells. 
    6. Several mutations in the mouse are known to affect fetal red blood cell 
differentiation and have thus served the analysis of the genetic control of 
blood development. Studies of development and regulation of gene expression 
find a particularly suitable system in mammalian hemoglobins and their 
development, both in humans and in mice. At least seven genes coding for 
hemoglobin polypeptide chains in humans and an approximately equal number in 
mice participate in the formation and the development of six different 
hemoglobins characteristically present at different stages between early 
embryonic and adult life. 
    7. A variety of mutations affect the nervous system in its morphological 
and functional development. They have served the identification of some of the 
normal processes involved in neural development and their genetic control. 
    8. The mammalian female has two X chromosomes. However, in any one adult 
female cell, only one X chromosome is active. The mechanism which governs and 
assures activity of a single X chromosome is unknown, but it may provide a 
model system for the study of activation and inactivation of genes in general. 
    9. Gene expression, gene control, and gene interaction during mammalian 
differentiation may be analyzed with the help of a powerful experimental 
system, that of experimental mouse chimeras. Various developmental processes 
and their genetic control may be studied with this method. In the case of 
lethal mutations it has been possible to ``rescue'' lethal homozygotes by 
fusing them with normal early embryos, with resulting survival of the fusion 
products. Mouse chimeras may also be obtained by injecting single cells from 
various embryonic donors into early mouse embryos. The injected cells become 
incorporated into the host embryo, providing opportunities for studies of 
differentiation. See CHIMERA; GENE; GENE ACTION; GENETICS; MUTATION. 
 �34s[Salome Gluecksohn-Waelsch]
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�6v�2v�32sDevonian   �21m �33s 
    The fourth period of the Paleozoic Era, encompassing an interval of 
geologic time between about 410,000,000 and 345,000,000 years before present, 
based on radiometric data. The Devonian System encompasses all sedimentary 
rocks deposited, and all igneous and metamorphic rocks formed, during the 
Devonian Period. It is conventional that recognition of Devonian time is 
determined by the definition of Devonian rocks. See PALEOZOIC. 
    The base of the Devonian System has been fixed, by international 
agreement, at an actual outcrop of sedimentary rocks in Czechoslovakia, where 
it corresponds to the base of the Monograptus uniformis graptolite zone. The 
top of the Devonian System, corresponding to the base of the Carboniferous 
System, has not yet been similarly fixed, but many stratigraphers use the base 
of the Siphonodella sulcata conodont zone for this boundary. 
    The Devonian is customarily divided into Lower, Middle, and Upper series 
and their corresponding epochs. These, in turn, have been divided into stages 
and their corresponding ages, of which several sets are in use (see 
illustration). Determination of time equivalency (correlation) and age dating 
in the Devonian are usually accomplished biostratigraphically, by utilizing 
zones based on the evolutionary successions in individual fossil groups, or on 
a composite zonal framework based on several fossil groups. See BIOCHRONOLOGY. 
    Devonian fossils are found to be distributed in three major realms. Within 
each realm there is taxonomic similarity, which indicates that there was 
reproductive interchange among members of the same phyletic groups, but 
between each two realms there are various degrees of taxonomic dissimilarity, 
which indicates that there were various degrees of reproductive isolation 
among members of the same phylogenetic groups. The largest realm covered 
Australia, Asia, Europe, western North America, and the Morocco-India fringe 
of Gondwana, and is termed the Old World Realm. The Appalachian Realm covered 
most of eastern North America and the Colombia-Venezuela-Amazon part of 
northern South American Gondwana, which was adjacent to Appalachian North 
America during the Devonian. The Malvinokaffric Realm covered central 
Gondwana, including southern South America, southern Africa, and Antarctica. 
  
    Devonian mountain building was particularly noticeable along the margins 
of Euramerica. The Acadian orogeny formed mountainous highlands accompanied by 
a chain of granitic intrusions from Nova Scotia to Pennsylvania during much of 
Devonian time. Simultaneously, along the Arctic margin of Canada, the 
Ellesmerian orogeny was producing folded mountains. In Late Devonian time, the 
Ellesmerian orogeny was joined by the Antler orogeny, which formed a narrow, 
incipient mountain belt along the ancient continental margin from northern 
British Columbia to Nevada. See OROGENY; PLATE TECTONICS. 
    Among the marine invertebrates, trilobites were much less abundant than 
during the Cambrian. The planktic members of the extinct graptolites died out 
during the Early Devonian, at about the same time as the pelagic ammonoid 
cephalopods first evolved. The brachiopods were at their greatest diversity. 
Lime-secreting corals and stromatoporoids were important and widespread in 
warm-water environments, and formed reefs during the Middle and Late Devonian. 
The extinct microfossil group known as conodonts was abundant, widespread, and 
rapidly evolving during the Devonian, so that conodont fossils are now 
regarded as the principal tools to be used for international correlation and 
relative age determination (see illustration). See BRACHIOPODA; CONODONT; 
GRAPTOLITHINA; MICROPALEONTOLOGY; STROMATOPOROIDEA; TRILOBITA. 
    Placoderm fish were successful predators in Devonian waters. Bony fishes 
or Osteichthyes were represented in the Devonian by the primitive 
acanthodians, but more modern groups of bony fishes appeared in the Early 
Devonian. Lobe-finned bony fishes, or sarcopterygians, include both the 
lungfish (Dipnoi) and crossopterygians in the Devonian. The oldest known 
amphibian, Ichthyostega, which evolved from the rhipidistian crossopterygians, 
occurs in strata thought to be high Upper Devonian. See CROSSOPTERYGII; 
DIPNOI; OSTEICHTHYES; PLACODERMI; SARCOPTERYGII. 
    Land plants began to flourish near the beginning of Devonian time. The 
latter gave rise in the Devonian to the Lycopsida (scale trees), Sphenopsida, 
and Pteropsida (true ferns) evolved in the Devonian; the pteropsids remain 
important in the world flora of today. See LYCOPODIOPHYTA; PALEOBOTANY; 
PSILOTOPHYTA; PTEROPSIDA; SPHENOPSIDA. 
 �34s[James G. Johnson /P. H. Hecke]
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�6v�2v�32sDew   �21m �33s 
    Drops or films of water formed by condensation of water on outside exposed 
surfaces, especially of vegetation. Hoarfrost is the corresponding phenomenon 
at temperatures below freezing. Dew forms on clear nights when there is 
cooling by radiation, provided exposed surfaces cool below the dew point of 
the air. Moisture then condenses on the exposed surfaces. If the ground is 
wet, moisture is evaporated, which adds to the supply and increases the 
deposit of dew on vegetation. See DEW POINT; HUMIDITY; VAPOR PRESSURE. 
 �34s[J. R. Fulks]
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�6v�2v�32sDewar flask �21m �33s
    A vessel having double walls, the space between being evacuated and the
surfaces facing the vacuum being heat reflective. It was invented in 1892 by
James Dewar as a container for liquid oxygen.
    Dewar's original flasks were made of glass with a coating of mirror
silver; this type is still used in the laboratory. But for shipment and
storage of liquid gases, metal vacuum vessels are used (see illustration).
Thermos Bottle is a trademark for a Dewar vessel for hot and cold foods.
 �34s[Howard W. Russell /George R. Harrison]
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�6v�2v�32sDewaxing of petroleum   �21m �33s 
    The process of separating hydrocarbons which solidify readily (waxes) from 
petroleum fractions. Removal of wax is usually necessary to produce 
lubricating oil which will remain fluid down to the lowest temperature of use. 
The wax removed may be purified further to produce commercial paraffin or 
microcrystalline waxes. 
    Most commercial dewaxing processes utilize solvent dilution, chilling to 
crystallize the wax, and filtration. Wax crystals are formed by chilling 
through the walls of scraped surface chillers, and wax is separated from the 
resultant wax-oil-solvent slurry by using fully enclosed rotary vacuum 
filters. In a process modification, most of the chilling is accomplished by 
multistage injection of very cold solvent into the waxy oil with vigorous 
agitation, resulting in more uniform and compact wax crystals which filter 
faster.  
    Complex dewaxing requires no refrigeration, but depends upon the formation 
of a solid urea-n-paraffin complex which is separated by filtration and then 
decomposed. The catalytic dewaxing process is based on selective 
hydro-cracking of the normal paraffins; it uses a molecular sieve-based 
catalyst in which the active hydro-cracking sites are accessible only to the 
paraffin molecules. See PETROLEUM; PETROLEUM PROCESSING; WAX. 
 �34s[Stephen F. Perry]
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�6v�2v�32sDew point   �21m �33s 
    The temperature at which air becomes saturated when cooled without 
addition of moisture or change of pressure. Any further cooling causes 
condensation; fog and dew are formed in this way. 
    Frost point is the corresponding temperature of saturation with respect to 
ice. At temperatures below freezing, both frost point and dew point may be 
defined because water is often liquid (especially in clouds) at temperatures 
well below freezing; at freezing (more exactly, at the triple point, +.01DGC) 
they are the same, but below freezing the frost point is higher. For example, 
if the dew point is -9DGC, the frost point is -8DGC. Both dew point and frost 
point are single-valued functions of vapor pressure. See DEW; EVAPORATION; 
FOG; HUMIDITY; VAPOR PRESSURE. 
 �34s[J. R. Fulks]
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�6v�2v�32sDextran   �21m �33s 
    A polyglucose biopolymer characterized by preponderance of ALPHA-1,6 
linkage, and generally produced by enzymes from certain strains of Leuconostoc 
or Streptococcus. While formerly its principal utility was as blood plasma 
substitute, dextran is also employed as packing material in column 
chromatography and as pharmaceutical agents; its average molecular weight 
determines usage to a great extent. Dextran's chemical and physical properties 
depend upon the strain of microorganism employed and the environmental 
conditions imposed upon the bacterium during growth, or the reaction 
conditions where an enzymatic method of dextran production is employed. 
Leuconostoc and Streptococcus species primarily convert sucrose to dextran and 
fructose. Acetobacter species convert dextrin to dextran. See DEXTRIN; 
POLYMERIZATION; POLYSACCHARIDE. 
 �34s[Henry M. Tsuchiya]
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�6v�2v�32sDextrin   �21m �33s 
    A polymer of D-glucose which is intermediate in complexity between starch 
and maltose. The dextrins are usually obtained by hydrolysis of starch with 
diastase (amylases). The higher dextrins resemble starch, while the lower 
dextrins more nearly resemble the sugars. Compared with the original starch, 
the dextrins produce less viscous solutions. They are soluble in water but 
insoluble in alcohol. Dextrins may be obtained from starch by controlled 
hydrolysis with acids. They are used commercially as adhesives. Tapioca, waxy 
maize, and sweet potato starch represent the best material for their 
manufacture. See GLUCOSE. 
 �34s[William Z. Hassid]
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�6v�2v�32sDiabase   �21m �33s 
    A fine-textured, dark-gray to black igneous rock composed mostly of 
plagioclase feldspar (labradorite) and pyroxene and exhibiting ophitic 
texture. It is commonly used for crushed stone. Its resistance to weathering 
and its general appearance make it a first-class material for monuments. 
Diabase is a common and extremely widespread rock type. It forms dikes, sills, 
sheets, and other small intrusive bodies. The Palisades of the Hudson, near 
New York City, are formed of a thick horizontal sheet of diabase. As defined, 
diabase is equivalent to the British term dolerite. The British term diabase 
is an altered diabase in the sense defined here. See IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sDiabetes   �21m �33s 
    A condition in which excessive amounts of some substance are excreted from 
the body. The term may refer to either of two unrelated diseases, diabetes 
mellitus and diabetes insipidus. 
  
    Diabetes mellitus.  This is a disease of abnormal carbohydrate metabolism 
in which the sugar glucose cannot enter the body's cells to be utilized and 
therefore remains in the blood in high concentrations. In diabetes mellitus 
the excess of sugar in the blood (hyperglycemia) leads to the excretion of 
sugar in the urine (glycosuria), one of the cardinal diagnostic signs of this 
disease. Glycosuria in turn causes the excretion of large amounts of urine 
(polyuria), which results in dehydration and intense thirst (polydipsia). 
Although blood glucose is high, glucose cannot enter the appetite-regulating 
cells of the hypothalamus, and hunger is therefore great, so that the diabetic 
person tends to eat constantly (polyphagia). But because glucose cannot enter 
and nourish the cells, body tissues are subjected to the equivalent of 
starvation conditions, and rapid weight loss occurs. See CARBOHYDRATE 
METABOLISM.  
    Diabetes mellitus appears in two varieties, each with its own cause: 
juvenile-onset diabetes, caused by a shortage of the pancreatic hormone 
insulin (whose chief function is to promote the entry of glucose into cells), 
and maturity-onset diabetes, in which insulin is available but the body's 
cells do not respond to it normally. Both juvenile-onset and maturity-onset 
diabetes tend to produce a number of debilitating and life-threatening 
complications despite the use of insulin or oral hypoglycemic agents. These 
include damage to the eyes and kidneys, premature atherosclerosis, and nervous 
system disturbances (collectively, diabetic neuropathy). It is primarily these 
complications, not the risk of ketoacidosis, that account for the heavy burden 
this disease inflicts on society, and make diabetes the third leading cause of 
death in the United States. See INSULIN; PANCREAS. 
    Three factors believed important in the causation of diabetes are 
heredity, virus infections, and immunological damage to the pancreas. Heredity 
does operate in determining one's risk of diabetes, but only as a predisposing 
factor, not an absolute determinant. The two types of diabetes mellitus have 
entirely different patterns of inheritance. Virus infection is strongly 
implicated as one causative factor in the beta-cell destruction that 
characterizes juvenile-onset diabetes, but it is not involved in 
maturity-onset diabetes. Autoimmune reactions, wherein the body's immune 
defense system attacks its own pancreatic tissue as though it were a foreign 
substance, are also suggested as a cause for the beta-cell destruction of 
juvenile-onset diabetes. See HUMAN GENETICS; IMMUNOPATHOLOGY. 
    Dietary management has always been part of diabetic therapy, including 
weight reduction for maturity-onset diabetes and carbohydrate restriction for 
both types of diabetes mellitus. Other dietary measures are now believed 
beneficial for maturity-onset diabetics. The inclusion of foods high in fiber 
content, for example, has been shown to reduce hyperglycemia, despite the fact 
that high-fiber foods are also high-carbohydrate foods. Also, foods with a 
high content of the mineral element chromium seem to enhance insulin's effect 
on body tissues; such foods include whole-grain products, mushrooms, bran, 
wheat germ, and brewer's yeast. 
    For juvenile-onset diabetics, the traditional diet is high in protein and 
quite restricted in carbohydrate, with only about 45% of the total daily 
caloric intake being carbohydrate-derived. There is a trend toward relaxing 
somewhat the carbohydrate restriction on diets for juvenile-onset diabetics, 
as evidence accumulates that diets containing even as much as 80% carbohydrate 
can be compatible with excellent control of diabetes, so long as the 
carbohydrate is complex (that is, whole grains, legumes, potatoes, bread) 
rather than refined (that is, sugars, candy, soft drinks, desserts, and so on). 
  
    Diabetes insipidus.  This is a condition in which a large amount of very 
dilute urine, sometimes as much as 6.5 gal (25 liters) per day, is excreted. 
Diabetes insipidus may be classified as primary, secondary, and nephrogenic. 
Both primary and secondary diabetes insipidus result from failure of the 
posterior lobe of the pituitary gland to secrete antidiuretic hormone (ADH), 
which encourages the reabsorption of water by kidney tubules. Both conditions 
can be treated by administration of ADH. In nephrogenic diabetes insipidus 
adequate ADH is present, but the kidney tubules are genetically defective and 
do not respond to its signal to reabsorb water from the urine. Nephrogenic 
diabetes insipidus is a sex-linked recessive disorder. All forms of diabetes 
insipidus are characterized by extreme polyuria and polydipsia, but polyphagia 
is not present. See PITUITARY GLAND. 
 �34s[Alice Chenault]
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�6v�2v�32sDiadematacea   �21m �33s 
    A superorder of Euechinoidea having a rigid or flexible test, perforate 
tubercles, aulodont lantern, and branchial slits. The group arose in the Late 
Triassic and differentiated into four stocks, the Diadematoida, Pedinoida, 
Echinothurioida, and Pygasteroida. See DIADEMATOIDA; ECHINOIDEA; 
ECHINOTHURIOIDA; PEDINOIDA; PYGASTEROIDA. 
 �34s[Howard B. Fell]
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�6v�2v�32sDiadematoida   �21m �33s 
    An order of Diadematacea with hollow primary radioles and diademoid 
ambulacral plates. The tubercles are normally crenulate and the anus remains 
within the apical system. Three families compose this order: Diadematidae, 
Micropygidae, and Aspidodiadematidae. See DIADEMATACEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sDiagenesis   �21m �33s 
    All the chemical, biochemical, and physical changes that sediments undergo 
from the time of deposition until the stage of metamorphism is reached. 
Diagenetic changes are gradational, with metamorphism at elevated temperatures 
or pressures, and with atmospheric weathering effects when sedimentary rocks 
become exposed at the surface. Sandstones, shales, and carbonate sediments are 
particularly susceptible to diagenetic modifications. See CARBONATE MINERALS; 
SANDSTONE; SHALE. 
    Diagenetic processes include purely physical ones that involve 
rearrangement of the sediments such as compaction, slumping, bioturbation by 
organisms, infiltration, and soft sediment deformation; biochemical or organic 
processes such as particle accretion, flocculation, boring, and decomposition; 
and physiochemical processes such as cementation, authigenesis (formation of 
new minerals), inversion, recrystallization, grain growth, replacement, and 
interstratal solution. Most of these processes involve a reduction in porosity 
and permeability, which are two important sediment properties in considering 
the migration of subsurface fluids and the accumulation of oil and gas and 
certain types of mineral deposits in subsurface rock units. However, one 
diagenetic process, interstratal solution, is of major importance in creating 
secondary porosity. See AUTHIGENIC MINERALS. 
  
    There have been attempts to group diagenetic processes into phases (or 
stages) in order to develop a comprehensive model for diagenetic evolution. 
The boundary limits of phases are a function of chemical as well as physical 
conditions and include pH, oxidation potential (Eh), ionic adsorption 
phenomena, temperature, pressure, depth of burial, and geologic time. In one 
model, three diagenetic phases are recognized: syndiagenesis, anadiagenesis, 
and epidiagenesis. Syndiagenesis includes sediment modifications that take 
place during and immediately following deposition. Anadiagenesis refers to the 
diagenesis processes that are characterized by expulsion and upward migration 
of connate water and other fluids, such as petroleum. Epidiagenesis includes 
those sediment-modifying processes that take place during and after uplift and 
emergence. See SEDIMENTARY ROCKS; SEDIMENTATION (GEOLOGY). 
 �34s[Frank G. Ethridge]
�33s 
  
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�6v�2v�32sDialysis   �21m �33s 
    A process of selective diffusion through a membrane by dissolved solutes 
in liquid solution. As dialysis is usually carried out, the membrane permits 
the diffusion of low-molecular-weight solutes (crystalloids) but prevents the 
passage of colloidal and high-molecular-weight solutes (macromolecules). 
Membranes suitable for this purpose include vegetable parchment, animal 
parchment, goldbeater's skin (peritoneal membranes of cattle), fish bladders, 
dialyzing cellophane (Visking sausage casing), and collodion (nitrocellulose 
deposited from alcohol-ether solution). 
    The solution is contained within such a membrane. The low-molecular-weight 
solutes are removed by placing pure solvent outside the membrane. This solvent 
is changed periodically or continuously until the concentration of diffusible 
solutes in the solution is reduced to near zero. The technique is used 
extensively in separating and purifying macromolecules of biological origin. 
See ELECTRODIALYSIS. 
 �34s[Quentin Van Winkle]
�33s 
  
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�6v�2v�32sDiamagnetism �21m �33s
    That branch of magnetism which treats of diamagnetic phenomena and of the
properties of diamagnetic bodies. Diamagnetism is a property exhibited by
substances with a negative magnetic susceptibility, that is, by substances
which magnetize in a direction opposite to that of an applied magnetic field.
A diamagnetic substance has a magnetic permeability less than 1, and is
repelled when placed near a magnet. The magnetization of diamagnetic
substances is associated with the currents induced on application of a
magnetic field. See MAGNETIC PERMEABILITY; MAGNETIC SUSCEPTIBILITY.
    Although all matter exhibits diamagnetism, only those substances in which
paramagnetism is absent are referred to as diamagnetic. This is because
paramagnetism, if present, usually predominates, and the gross magnetic
response of the material is paramagnetic. Important exceptions are the alkali
and alkaline earth metals. The condition for pure diamagnetism is that all
electronic spins be paired and all orbital moments either be zero or
effectively cancel one another. See ELECTRON SPIN; PARAMAGNETISM.
    As stated previously, the diamagnetic response of a substance is small;
only a very small fraction of the applied magnetic field is shielded from the
interior of the substance by the induced diamagnetic currents. There is one
case, however, in which the inducing field is completely shielded (except for
small surface effects). This is the perfect diamagnetism exhibited by
superconductors, and is known as the Meissner effect. See MEISSNER EFFECT;
SUPERCONDUCTIVITY.
 �34s[Elihu Abrahams ;Frederic Keffer]
�33s

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�6v�2v�32sDiameter   �21m �33s 
    Any chord of a circle that contains the center of the circle. It is also a 
chord of maximum length. (This property furnishes a basis for extending the 
concept to more general figures; for example, the diameter of any point set is 
the least upper bound of the distances of two points of the set.) See CIRCLE. 
 �34s[Leonard M. Blumenthal]
�33s 
  
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�6v�2v�32sDiamond �21m �33s
    A mineral composed entirely of the element carbon crystallized in the
isometric system. Gem diamonds have a density of 3.53, but the tough, black,
cokelike aggregates of microscopic crystals sold as carbons and known to the
layman as black diamond may have a density as low as 3.15. It is the hardest
known substance. Diamond slowly burns to carbon dioxide in oxygen at a
temperature as low as 1650DGF (900DGC) and slowly inverts to graphite at
temperatures as low as 1830DGF (1000DGC). Diamond has the highest thermal
conductivity of any known substance; at room temperature it is approximately
five times that of copper. The points of diamonds used as cutting tools do not
become hot owing to this very high thermal conductivity. All except a few
diamonds are nonconductors of electricity, but all are excellent heat
conductors, superior to iron and steel.

    Occurrence and recovery.  Diamonds crystallize directly from rock melts
rich in magnesium, at depths of 93 mi (150 km) or more in the Earth. The melts
from which they crystallize are essentially saturated in carbon dioxide gas at
exceedingly high pressures; temperatures in excess of 2550DGF (1400DGC) are
required. Thus rocks which contain diamonds as a natural component are samples
of the deep mantle of the Earth and worldwide are remarkably similar in
composition. Only garnet-bearing lherzolite and garnet-pyroxene eclogites are
known to contain diamond. See ECLOGITE.
    Diamond pipes are commonly called volcanic necks in the literature.
However, this terminology is incorrect, for there is no evidence that the
material in diamond pipes that has reached the surface is a liquid or is
related to volcanoes. They are more properly called gas-driven diatremes.
These dikes and pipes of diamond-bearing hydrated magnesium silicates form
primary diamond deposits. Productive dikes have been mined in Sierra Leone,
the Orange Free State, the Transvaal, the Kimberley district of Cape Province,
Tanzania, the Republic of the Congo, and the United States at Murfreesboro,
Arkansas. In addition, a substantial number of these pipes have been found in
Yakutsk, Siberia, and are now being actively mined. Several hundred dikes and
pipes of kimberlite that contained diamonds have also been found. Only a few
have been profitable to mine at depth. All pipes have an enriched surface
layer which has been developed by erosional processes. Initially the upper few
feet of many pipes may be profitably mined, although they may not warrant
mining at depth.
    Most diamonds have been produced from secondary (placer) deposits, called
alluvials. Most alluvial diamonds are recovered from modern gravels, all of
which are stream gravels except the beach gravels of the Atlantic Ocean.
Diamonds also have been found as irregular microcrystalline clumps up to a
millimeter or so in size and as tiny crystals in meteorites. These meteoritic
diamonds were formed from graphite nodules as the iron meteorites were
subjected to intense high pressures by the shock of the meteorite impacting on
the surface of the Earth.
    Of the world's output of diamonds, 95% comes from Africa. Most of the
production is by large mining companies, but a significant amount comes from
individual operations from stream-bed deposits in Sierra Leone, Ghana, and the
Union of South Africa. These small operations recover diamonds by a method
similar to the panning of gold from placers, taking advantage of the fact that
the density of diamonds is greater than that of most other minerals.
Concentrates of the heavy minerals recovered by panning are handpicked for
diamonds.

    Diamond cutting.  After recovery, diamonds are referred to as rough. Those
of gem quality are called cuttable rough, and all others are classed as
industrial rough. In cutting diamonds, the objective is to obtain the maximum
price for the finished gems, not the maximum weight of the finished stones.
The most important step in diamond cutting is the decision as to how the stone
will be cut.
    In subdividing an irregularly shaped crystal or eliminating flaws, the
stone may be either cleaved or sawed, but only in certain crystallographic
directions. There are four directions in which a diamond may be cleaved
parallel to any octahedron face, and nine directions in which it may be sawed
- three parallel to any cube face and six parallel to any dodecahedron face.
If the finished diamond is to be round or oval in shape, it goes to a person
who is known in the trade as a cutter. The process of putting on the facets is
known as polishing.
    The weight of the finished gems is 50 - 60% of the weight of the rough
stones if they are well-formed crystals with few flaws. The metric carat,
0.200 g, is the unit of weight by which both gem and industrial diamonds are
sold. A point is 1/100 of a carat and is used only in reference to gem
diamonds.

    Industrial diamonds.  Industrial diamonds vary from the better grades,
which are identical with inferior gems, to crushing bort, which is suitable
only for crushing to grit and powder sizes. The better qualities are made into
shaped diamond-cutting tools or wire-drawing dies. However, shaped diamond
tools have been supplanted in many of their former uses by sintered tungsten
carbide. For truing and shaping grinding wheels of alumina or silicon carbide,
diamond crystals, in the shapes in which they are mined, are used.
    Drill bort consists of crystals in their original form as mined which are
mounted in the end of a cylinder called a bit or crown. When the rotating
cylinder is forced against a rock surface, it wears its way into the rock. The
mining industry is the largest user of drill bort, although this method of
coring has been adapted to the testing of concrete and the foundations of
buildings, dams, and bridges.

    Synthetic and imitation diamonds. The synthesis of diamond has been
effected in various ways: by static crystallization from certain molten metals
or alloys at pressures upward of about 50 kilobars (5 gigapascals) and
temperatures over 2240 DEG F (1500 K); by shock conversion from graphite at
transient pressures of about 300 kilobars (30 GPa) and temperatures of about
1880 DEG F (1300 K); and by static conversion from graphite at pressures more
than about 5480 DEG F (3300 K). Synthetic diamonds are identical with natural
diamonds in fundamental properties but differ in those characteristics that
depend on the process of manufacture, such as impurities, size, and shape.
    Imitation diamonds are not made of carbon crystallized in its densest form.
Most imitation diamonds are yttrium-aluminum-garnet or strontium-titanate. The
distinction between synthetic gems and imitation gems is not always made.
Synthetic gems are chemically and crystallographically identical to the
natural stone, whereas imitation gems are made of some other component, such as
glass, or another crystal. See CARBON; GEM.			[C.Fr./G.C.K.]

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�6v�2v�32sDiapensiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
has only a single family, the Diapensiaceae, with about 18 species of herbs 
and dwarf shrubs in temperate and arctic regions of the Northern Hemisphere. 
Within its subclass the order is marked by sympetalous flowers (flowers with 
the petals joined to each other by their margins, at least toward the base, 
forming a basal tube, cup, or saucer), separate pollen grains, and ovules that 
have a single integument (unitegmic). Shortia and Galax genera are sometimes 
cultivated. See DILLENIIDAE; FLOWER. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sDiapiric structures   �21m �33s 
    Anticlines or domes in which a core of mobile rock has broken through the 
overlying strata. In the process of concentric folding, the rocks of the core 
occupy increasingly more limited space as the curvature of the fold becomes 
smaller. If the rocks of the core are extremely plastic, they may exert an 
upward pressure sufficient to cause expulsion of the core through the crest. 
Diapiric structures may also form as a result of unequal loading by the 
sediments overlying a plastic layer. See FOLD AND FOLD SYSTEMS; SALT DOME. 
 �34s[Philip H. Osberg]
�33s 
  
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�6v�2v�32sDiarrhea �21m �33s
    The passage of loose or watery stools, usually at more frequent than
normal intervals. Diarrhea is a symptom of many diseases and may be
accompanied by nausea, vomiting, griping, tenesmus, and other general or
specific indications of a disease.
    The more common specific disorders which may produce diarrhea include
intestinal infections, such as dysentery, cholera, typhoid fever, food
poisonings, and parasitic infestations; food sensitivities; drug and chemical
irritation; and vitamin deficiency states.
    Emotional and psychic disturbances frequently produce diarrhea and other
visceral derangements. The poorly understood entities of regional enteritis
and ulcerative colitis are perhaps related to these disturbances, as are other
psychosomatic disorders.
    Diarrhea is a common symptom in gastrointestinal obstruction or in
inflammations from local infections or tumor invasion. See BACILLARY
DYSENTERY; FOOD POISONING; MEDICAL PARASITOLOGY.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sDiastem   �21m �33s 
    A temporal break between adjacent geologic strata that represents 
nondeposition or local erosion but not a change in the general regimen of 
deposition (in contrast to unconformity). Diastems may be produced by the 
scouring action of shifting submarine currents which temporarily interrupt 
deposition on the continental shelf, or by a shifting river within the 
deposits of its floodplain. Or they may be produced simply by nondeposition in 
either environment of deposition where the absence of sediment reflects normal 
shifting of currents rather than an overall change in conditions. See MARINE 
SEDIMENTS; SEDIMENTATION (GEOLOGY); UNCONFORMITY. 
 �34s[John Rodgers]
�33s 
  
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�6v�2v�32sDiastereoisomer   �21m �33s 
    One of a pair of optical isomers which are not mirror images of each 
other. A given diastereoisomer (or diastereomer) may not be optically active, 
in which case it is an optically inactive meso form; or it may be optically 
active, in which case with its nonsuperimposable mirror image it constitutes 
an enantiomorphic pair of optical isomers. Thus mesotartaric acid is a 
diastereoisomer of both the (+) and (-) tartaric acids, and it can form two 
optically active monoesters, each of which is a diastereoisomer of monoesters 
of the (+) and (-) acids. See OPTICAL ACTIVITY. 
 �34s[Wyman R. Vaughan]
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�6v�2v�32sDiastrophism   �21m �33s 
    The process of deformation of the Earth's crust. The most spectacular and 
conspicuous products of diastrophism are the mountain systems within 
continents. These consist of regionally extensive belts of folded and buckled 
layers, igneous intrusions, volcanic outpourings, metamorphosed strata, and 
tremendous faults. 
    Diastrophism contributes enormously to the architecture of the Earth's 
crust. Although the term diastrophism has fallen out of widespread usage among 
earth scientists, the subject of deformation of the Earth's crust is of 
significant and growing interest and practical importance. Scientists study 
the products of diastrophism out of a fundamental historical interest and 
curiosity, but also as a means to carry out difficult operational tasks such 
as predicting earthquakes and volcanic eruptions, identifying places of 
concentration of metals and oil and gas, and analyzing the feasibility of safe 
disposal of radioactive waste in the Earth's interior. 
    Diastrophism is attributed to multiple origins, most of which are 
intimately linked to major crustal movements related to sea-floor spreading, 
transform faulting, and the collision, underthrusting (subduction), and 
general interference of shifting lithospheric plates which compose the Earth's 
outer shell. The fundamental plate configurations which establish the settings 
for diastrophism include the subduction of oceanic crust under oceanic crust 
at sites of deep ocean trenches; subduction of oceanic crust under continental 
margins; collision of two continents; and the movement of one plate past 
another along high-angle transform fault zones. See MOUNTAIN SYSTEMS; OROGENY; 
PLATE TECTONICS; STRUCTURAL GEOLOGY. 
 �34s[George H. Davis]
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�6v�2v�32sDiatom   �21m �33s 
    A markedly distinct group of algae belonging to the plant division 
Chrysophyta and the class Bacillariophyceae. They have long been a favorite 
with naturalists because of the beauty of the symmetry and sculpturing of the 
siliceous cell walls. They abound in all natural waters and are the basic food 
for many organisms. 
    The cells of diatoms are unicellular, although they may remain in loose 
chains or groups. The cell wall, formed in two halves which fit together like 
a shoe box and lid (illustration a and b), is largely composed of silicon 
dioxide and pectic compounds. The markings on the valves are explained by the 
presence of minute thin places or pores in the walls. In the different forms 
there is an almost endless diversity of patterns, but the constant arrangement 
of markings in any given species forms the basis for delimiting the numerous 
species (illustration c). Enormous beds of fossil diatoms, known as 
diatomaceous earth, are found in various parts of the world. See CHRYSOPHYTA; 
DIATOMACEOUS EARTH. 
 �34s[Paul A. Vestal]
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�6v�2v�32sDiatomaceous earth   �21m �33s 
    Earth consisting of a friable, porous silica deposit made up of the 
opaline silica tests (shells) of diatoms. It is the dry, relatively 
unconsolidated equivalent of diatom ooze found on some parts of the sea floor. 
It is usually white or cream-colored. Because this material is inert 
chemically and has unusual physical properties, it is admirably suited for 
many scientific and industrial purposes, such as filtering agent; insulator 
against heat, cold, and sound; catalyst carrier; absorbent; filler; building 
material; abrasive; ingredient in pharmaceutical preparations; and 
stratigraphic indicator. 
    Diatomite is the indurated equivalent of diatomaceous earth; the pores are 
partially or completely filled with silica. See CHERT; DIATOM. 
 �34s[Raymond Siever]
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�6v�2v�32sDiatrymiformes   �21m �33s 
    A group of extinct flightless birds known only from upper Paleocene and 
lower to middle Eocene deposits in North America and Europe. Diatryma was a 
huge bird (up to 2 m tall) with powerful legs, very small wings, and a large 
head and beak (see illustration). It lived during the period just after the 
extinction of dinosaurs and just before the major radiation of very large 
mammals. The apparently powerful nature of the  locomotory and feeding 
apparatus of Diatryma suggests that it was an active predator. In fact, 
Diatryma may have been one of the main enemies of the terrestrial mammals of 
its era. Among living birds, the Diatrymiformes are probably distantly related 
to the Gruiformes, which include the cranes, limpkins, trumpeters, and 
seriemas. See AVES; DINORNITHIFORMES; GRUIFORMES; NEORNITHES. 
 �34s[David W. Steadman]
�33s 
  
  
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�6v�2v�32sDiazotization   �21m �33s 
    The reaction between a primary aromatic amine and nitrous acid to give a 
diazo compound. Diazotization is important in organic chemical synthesis. Its 
most striking use is in the large-scale manufacture of the important azo class 
of dyes, but it has likewise been invaluable in general synthesis in both 
chemical manufacturing and in research. 
    The light sensitivity of aromatic diazo compounds has led to their use in 
photocopying processes. Azo dyes derived by coupling diazonium salts to 
aromatic amines and phenols remain an important class of dyes, although the 
use of some members of this class in food has been restricted in recent years 
because of carcinogenic properties observed in experiments with laboratory 
animals. See ANILINE; DYE. 
 �34s[Martin Stiles]
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�6v�2v�32sDibranchia �21m �33s
    A subclass of the Cephalopoda which contains the extinct Belemnoidea and
all living cephalopods (Decapoda, Octopoda, and Vampyromorpha) with the
exception of Nautilus. All members of this group possess two gills and, when
present, the shell is internal. The belemnites arose in the Upper
Mississippian and disappeared at the close of the Cretaceous. Belemnites is an
index fossil of Jurassic and Cretaceous rocks throughout the world. See
BELEMNOIDEA; DECAPODA (MOLLUSCA); INDEX FOSSIL; OCTOPODA; VAMPYROMORPHA.
    Most dibranchiates emit ink from a special gland. Contrary to popular
belief, in most species the ink discharged is not a smoke screen but coagulates
in the water as an object approximately the size of the animal. This is seized
by the enemy while the colorless cephalopod darts away. See CEPHALOPODA.
[Gilbert L. Voss]


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�6v�2v�32sDichroism   �21m �33s 
    In certain anisotropic materials, the property of having different 
absorption coefficients for light polarized in different directions. There are 
few natural materials which exhibit strong dichroism. One of the first to be 
discovered was tourmaline. Light transmitted by thin plates of dark forms of 
tourmaline is almost completely polarized. See POLARIZED LIGHT. 
    If the absorption in a dichroic material is different for different linear 
states of polarization, the material is termed linear dichroic. If it is 
different for right and left circularly polarized light, it is termed circular 
dichroic. Similarly, there can be elliptically dichroic crystals. 
 �34s[Bruce H. Billings]
�33s 
    The study of dichroism allows conclusions as to the submicroscopic fine 
structure of cells. In visible light only a few cellular components, such as 
chloroplasts, show absorption. An absorption can, however, be produced by 
staining. The dichroic staining of plant fibers is especially simple. The 
elongate stain particles of benzidine dyes, for example, congo red, are 
deposited in an oriented manner in the spaces between the microfibrils and 
produce an intrinsic dichroism of the fiber: colored for a vibration plane 
parallel, colorless for a plane perpendicular to the stain particles and 
fibrils. Therefore, the direction of strongest absorption indicates the course 
(parallel or helical) of the microfibrils in the fiber. 
    Ultraviolet dichroism gives direct information as to the orientation of 
the absorbing molecules or molecular groups in cell structures. The method has 
been especially helpful for studies of orientation of deoxyribonucleic acid in 
nuclei and chromosomes. Lignified plant cell walls show ultraviolet dichroism. 
It is pure form dichroism, a fact which eliminates the possibility that lignin 
is in an anisotropic state in the wall. 
    By irradiation with ultraviolet light, various compounds of the cell are 
caused to fluoresce. The fluorescent light is polarized if the object is 
anisotropic. This phenomenon, called difluorescence, is observable in 
lignified cell walls, and leads to the same conclusions as to lignin 
deposition as emerge from dichroism studies. 
 �34s[Fritz Ruch]
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�6v�2v�32sDichromate �21m �33s
    The Cr2O7 sup 2 sup + ion which is derived from dichromic acid, H2Cr2O7.
This acid and other polyacids are obtained when chromium trioxide, CrO3, is
dissolved in water. The dichromates are converted to chromates under basic
conditions.
    Most dichromates are water-soluble and give orange-colored solutions.
Dichromates are good oxidizing agents, and this property is utilized in
chemical processes, analytical chemistry, explosives, and safety matches.
Dichromates are also used in electroplating, tanning, photography, and
pigments. See CHROMATE.
 �34s[E. Eugene Weaver]
�33s

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�6v�2v�32sDicranales   �21m �33s 
    An order of mosses. The stems are erect, dichotomously branched, and 
densely foliated. The costate, narrow leaves vary from subulate to lanceolate, 
and in some species are falcate-secund. The seta is elongated. The single 
peristome consists of 16 teeth, usually two-parted into linear or awllike 
divisions from the apices to the middle or even beyond. See BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sDictyoceratida   �21m �33s 
    An order of sponges of the class Demospongiae which includes the bath 
sponges of commerce. Dictyoceratida are mostly of considerable size, and form 
massive, lobate, or branching colonies. Some are leaflike in shape or 
vase-shaped. A reticulate skeleton of spongin fibers is always present. In 
many species some or all the fibers enclose foreign bodies, such as sand 
grains, shell fragments, or spicules of other sponges. There are three 
families, Spongiidae, Thorectidae, and Dysideidae. 
    Dictyoceratid sponges are most abundant in tidal and shallow waters of 
tropical and subtropical regions. Relatively few species occur in Arctic and 
Antarctic seas, and most occur above the continental slope. A few species are 
found down to 3300 ft (1000 m). See DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sDicyemida   �21m �33s 
    An order of Mesozoa comprising minute, wormlike parasites of the renal 
organs of cephalopod mollusks. They are composed of a single layer of large, 
ciliated epithelial cells enclosing one or more elongate axial cells (see 
illustration). Each axial cell contains, in addition to its own nucleus, 
reproductive cells and developing larvae. These larvae, when fully developed, 
escape from the parent organism. 
    There are two reproductive phases termed the nematogens and rhombogens, 
respectively. During the nematogen phase vermiform larvae are formed asexually 
from the germ cells. They increase the infection in the same cephalopod. This 
is succeeded by the rhombogen phase in which infusoriform, or swarm, larvae 
are liberated. These are discharged into the sea with their host's urine and 
their fate is unknown. Zoologists are not in agreement regarding many points 
in the life cycle, particularly in respect to the interpretation of the 
infusorigen and its products. See MESOZOA. 
 �34s[Bayard H. McConnaughey]
�33s 
  
  
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�6v�2v�32sDielectric constant �21m �33s
    For a given dielectric material, the ratio of electrical capacitance of a
dielectric-filled capacitor to a vacuum capacitor of identical dimensions.
This is defined by the equation below, where C is the capacitance of the
dielectric-filled capacitor and C0 is the capacitance of the empty capacitor.

			      *** KAPPA = C/C sub 0 ***

    The dielectric constant KAPPA is also known as the specific inductive
capacity or as the relative permittivity. It is perhaps most familiar as the
proportionality constant in Coulomb's law of electrostatics. For a given
charge distribution, the dielectric constant expresses the ratio of electric
field strength in vacuum to that in a dielectric, the latter field being
reduced by the polarization of the dielectric medium. See CAPACITANCE;
CAPACITOR; COULOMB'S LAW; DIELECTRICS; ELECTRIC FIELD; PERMITTIVITY.
    The values of KAPPA for low frequency or static fields range from 1 to more
than 10,000 for typical dielectrics. The dielectric constants of gases are
only slightly greater than unity, while high values occur for many polar
liquids and certain ionic solids.
    The experimental methods of measuring dielectric constants depend on the
frequency range under investigation. For frequencies below about 10 sup 9 Hz,
the permittivity or impedance of a dielectric sample inserted in a
parallel-plate capacitor may be measured in suitable circuits; a Schering
bridge arrangement is commonly employed up to 10 sup 7 Hz, and resonant
circuits in the range 10 sup 4 - 10 sup 9 Hz. For frequencies above
10 sup 8 Hz, the dielectric constant may be determined by measuring the
interaction of electromagnetic waves with the medium. From about 10 sup 8 to
10 sup 11 Hz, the material is usually inserted in wave guides or coaxial lines,
and the standing-wave patterns measured. At still higher frequencies, optical
techniques involving reflection and transmission measurements are employed.
    Because the dielectric constant is related to chemical structure, it can
be used for both qualitative and quantitative analysis. From measurements of
the dielectric constant and refractive index taken together, it is possible to
compute the dipole moment of a species. See DIPOLE MOMENT.
 �34s[Robert D. Waldron]
�33s

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�6v�2v�32sDielectric heating   �21m �33s 
    The heating of a nominally electrical insulating material due to its own 
electrical (dielectric) losses, when the material is placed in a varying 
electrostatic field. 
    The material to be heated is placed between two electrodes (which act as 
capacitor plates) and forms the dielectric component of a capacitor (see 
illustration). The electrodes are connected to a high-voltage source of 2 - 
90-MHz power, produced by a high-frequency vacuum-tube oscillator. 
    The resultant heat is generated within the material, and in homogeneous 
materials is uniform throughout. Dielectric heating is a rapid method of 
heating and is not limited by the relatively slow rate of heat diffusion 
present in conventional heating by external surface contact or by radiant 
heating. For a discussion of dielectric losses see DIELECTRICS. 
    This technique is widely employed industrially for preheating in the 
molding of plastics, for quick heating of thermosetting glues in cabinet and 
furniture making, for accelerated jelling and drying of foam rubber, in 
foundry core baking, and for drying of paper and textile products. Its 
advantages over conventional methods are the speed and uniformity of heating, 
which offset the higher equipment costs. Because of the absence of high 
thermal gradients, an improved end-product quality is usually obtained. 
 �34s[George F. Bobart]
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�6v�2v�32sDielectrics   �21m �33s 
    Materials which are electrical insulators or in which an electric field 
can be sustained with a minimum dissipation of power. In a more general sense, 
dielectrics include all materials except condensed states of metals. See 
ELECTRIC INSULATOR. 
    Dielectrics are employed as insulation for wires, cables, and electrical 
equipment, as polarizable media for capacitors, in devices used for the 
propagation or reflection of electromagnetic waves, and for a variety of 
dielectric devices, such as rectifiers and semiconductor devices, 
piezoelectric transducers, dielectric amplifiers, and memory elements. 
    The electrical response of a normal dielectric can be described by its 
dielectric or breakdown strength, conductivity or dielectric loss, and 
dielectric constant. The behavior of nonlinear dielectrics depends also on the 
amplitude and time variation of the electric field. 
    Dielectric strength is defined as the maximum electric field which can be 
applied to a dielectric without causing breakdown, the abrupt irreversible 
drop in resistivity at high fields often accompanied by destruction of the 
material. Dielectric breakdown is caused by an enormous increase in the number 
of charge carriers because of collisions or thermal ionization and field 
emission. See FIELD EMISSION. 
    Dielectric loss is the power dissipated in a dielectric because of 
conduction processes. This power loss results from thermal dissipation of the 
electrical energy expended by the field. It is caused by molecular collisions. 
    The dielectric constant or permittivity relative to vacuum is important in 
many applications. Materials with high dielectric constants are desirable for 
capacitors, since they permit a reduction in size for a given capacitance, 
while low dielectric constants are usually preferred for cable and transformer 
insulation. See DIELECTRIC CONSTANT. 
    Vacuum or gaseous dielectrics other than air have had relatively little 
dielectric application except in electron tube devices, voltage regulators, 
and lightning arresters. Dielectric liquids are principally employed for 
impregnating porous insulation in high-voltage cables and capacitors and as 
insulating media for transformers and circuit breakers. In the latter 
applications, the heat transfer properties of the liquid are also important. 
Mineral oils, halogenated hydrocarbons, and silicone oils are the most 
important commercial dielectric liquids. Solid dielectrics are employed for 
the vast majority of commercial applications. Important solid dielectrics 
include many ceramics and glasses; plastics and rubber; minerals such as 
quartz, mica, magnesia, and asbestos; and paper and fibrous products. The 
mechanical and thermal properties as well as the electrical response are 
important in the choice of a dielectric for a particular product. 
 �34s[Robert D. Waldron]
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�6v�2v�32sDiels-Alder reaction �21m �33s
    The 1,4-addition of an alkene (the dienophile) to a conjugated diene. The
reaction, also known as the diene synthesis, is one of the most valuable and
versatile methods for the preparation of compounds containing a six-membered
ring, and proceeds most rapidly when the dienophile is substituted by
electron-attracting groups. An example is in the illustration. The Diels-Alder
reaction does not require a catalyst, nor is the reaction retarded by the
presence of oxidation inhibitors with which dienes are commonly treated to
prevent formation of peroxides.
    Industrially the diene synthesis is used in the production of the
insecticides aldrin and dieldrin. The adduct of butadiene and maleic anhydride
is used in the synthesis of the important fungicide captan.
 �34s[Paul E. Fanta]
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�6v�2v�32sDiesel cycle �21m �33s
    An internal combustion engine cycle in which the heat of compression
ignites the fuel. Compression-ignition engines, or diesel engines, are
thermodynamically similar to spark-ignition engines. The sequence of processes
for both types is intake, compression, addition of heat, expansion, and
exhaust. Ignition and power control in the compression-ignition engine are,
however, very different from those in the spark-ignition engine. See
THERMODYNAMIC CYCLE.
    Usually, a full unthrottled charge of air is drawn in during the intake
stroke of a diesel engine. A compression ratio between 12 and 20 is used, in
contrast to a ratio of 4 to 10 for the Otto spark-ignition engine. This high
compression ratio of the diesel raises the temperature of the air during the
compression stroke. Just before top center on the compression stroke, fuel is
sprayed into the combustion chamber. The high temperature of the air ignites
the fuel, which burns almost as soon as it is introduced, adding heat. The
combustion products expand to produce power, and exhaust to complete the cycle.
    In an actual engine with a given compression ratio, the Otto engine has the
higher efficiency. However, fuel requirements limit the Otto engine to a
compression ratio of about 10, whereas a diesel engine can operate at a
compression ratio of about 15 and consequently at a higher efficiency.
    In addition, heat can be added earlier in the cycle by injecting fuel
during the latter part of the compression process. This mode of operation is
the dual-combustion or semidiesel cycle. With most of the heat added near peak
compression, semidiesel efficiency approaches Otto cycle efficiency at a given
compression ratio. See DIESEL ENGINE.
[Theodore Baymeister]

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�6v�2v�32sDiesel engine   �21m �33s 
    An internal combustion engine operating on a thermodynamic cycle in which 
the ratio of compression of the air charge is sufficiently high to ignite the 
fuel subsequently injected into the combustion chamber. The engine differs 
essentially from the more prevalent mixture (Otto) engine in which an 
explosive mixture of air and gas or air and the vapor of a volatile liquid 
fuel is made externally to the engine cylinder, compressed to a point some 
200DGF (110DGC) below the ignition temperature, and ignited at will as by an 
electric spark. The diesel engine utilizes a wider variety of fuels with a 
higher thermal efficiency and consequent economic advantage under many service 
applications. See OTTO ENGINE. 
    The true diesel engine, as projected by R. Diesel and as represented in 
most low-speed engines, such as about 300 rpm, uses a fuel-injection system 
where the injection rate is delayed and controlled to maintain constant 
pressure during combustion. Adaptation of the injection principle to higher 
engine speeds, such as 1000 - 2000 rpm, has necessitated departure from the 
constant pressure specification because the time available for fuel injection 
is so short (milliseconds). Combustion proceeds with little regard to the 
constant-pressure specification. High peak pressures may be developed. Yet 
nonvolatile (distillate) fuels are burned to advantage in these engines which 
cannot be rigorously identified as true diesels but which properly should be 
called commercial diesels. In ordinary parlance all such engines are 
classified as diesels. See DIESEL CYCLE. 
    Distillate fuel prevails with locomotive, truck, bus, and automotive 
applications. Lower-speed engines (stationary and motorship service) burn 
heavier fuels. Alternative fuels are burned in dual-fuel and gas diesel 
engines for stationary service. The main fuel is typically natural gas (90 - 
95%) with oil (5 - 10%) used to control burning and to stabilize ignition. In 
the more prevalent liquid-fuel-injection system, the technical problems are 
numerous and embrace such elements as pumps, spray nozzles, and combustion 
chambers for the delivery, atomization, and burning of the fuel in the hot 
compressed air. There must be accurate timing (measured in milliseconds) for 
the entire process to give clean, complete combustion without undue excess 
air. See DIESEL FUEL.  
    Small size (<200 units) engines are conveniently started by an electric 
motor and storage battery. Larger engines use compressed air introduced 
through valves in the cylinder head. Starting, with engine-driven generator 
sets, may be accomplished by motoring the generator. 
    Cooling systems use water at 120 - 180DGF (49 - 82DGC) with radiators, 
cooling towers, and cooling ponds employed for conservation and reclamation. 
Lubrication costs can become prohibitive with inadequate engine maintenance. 
See INTERNAL COMBUSTION ENGINE. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sDiesel fuel �21m �33s
    A broad class of petroleum products which includes distillate or residual
materials (or blends of these two) from the refining of crude oil. Diesel fuel
generally has a distillation range between 374 and 716DGF (190 and 380DGC) and
a specific gravity (59DGF or 15DGC) range between 0.760 and 0.935. In addition
to these two primary criteria, other properties which are used to define
diesel fuel are: viscosity (1.4 to 26.5 mmsup2/s), sulfur content (usually
<1.0% by weight), cetane number (30 to 60), ash content (usually <0.10% by
weight), water and sediment content (usually <0.5% by volume), flash point
(usually >131DGF or 55DGC) and cloud point (that temperature at which wax
crystals begin to precipitate as the oil is cooled under prescribed
conditions) or pour point (the lowest temperature at which the oil will flow
when cooled under prescribed conditions). The properties of diesel fuel
greatly overlap those of kerosine, jet fuels, and burner fuel oils; thus all
these products are generally referred to as intermediate distillates. See FUEL
OIL; JET FUEL; KEROSINE.
    The diesel engine uses a method of ignition which requires that the fuel
ignite spontaneously and quickly (within 1 to 2 milliseconds in a high-speed
engine). The time lag between the initiation of fuel injection and the
initiation of combustion is called ignition delay characterized by two major
factors: a mechanical factor which is influenced by such things as compression
ratio, motion of the charge air during injection, and ability of the injector
to atomize the fuel; and a chemical factor which is influenced by such things
as the fuel's autoignition temperature, specific heat, density, thermal
conductivity, surface tension, and coefficient of friction. See DIESEL ENGINE.
    The fuel's effect on the chemical portion of ignition delay is expressed
by a quantity called the cetane number. Cetane, which has a high-ignition
quality (short chemical ignition delay), has arbitrarily been assigned a
cetane bumber of 100, whereas heptamethylnonane has been assigned a cetane
number of 0. The cetane number of a diesel fuel is determined by comparing it to
a blend of cetane and heptamethylnonane which has the same ignition quality. The
cetane number is the percentage by volume of cetane in the blend which has an
ignition quality equal to the test fuel. See CETANE NUMBER; COMBUSTION CHAMBER;
COMBUSTION KNOCK.
[Robert Teasley]
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�6v�2v�32sDifferential   �21m �33s 
    A mechanism which permits a rear axle to turn corners with one wheel 
rolling faster than the other. An automobile differential is located in the 
case carrying the rear-axle drive gear (see illustration). 
    The differential gears consist of the two side gears carrying the inner 
ends of the axle shafts, meshing with two pinions mounted on a common pin 
located in the differential case. The case carries a ring fear driven by a 
pinion at the end of the drive shaft. This arrangement permits the drive to be 
carried to both wheels, but at the same time as the outer wheel on a turn 
overruns the differential case, the inner wheel lags by a like amount. 
    Special differentials permit one wheel to drive the car by a predetermined 
amount even though the opposite wheel is on slippery pavement; they have been 
used on racing cars for years and are now used by a number of car 
manufacturers. See AUTOMOTIVE TRANSMISSION. 
 �34s[Harold Fischer]
�33s 
  
  
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�6v�2v�32sDifferential amplifier �21m �33s
    An amplifier whose output signal is proportional to the difference between
two input voltages. Amplifiers that use a single active device such as a
vacuum tube or transistor cannot usually achieve high dc gain, good bias
stability, and low noise simultaneously, because large emitter resistors must
be used for high bias stability. These resistors restrict the dc gain but may
be bypassed for high ac gain. The differential amplifier uses two active
devices in an arrangement that overcomes this problem, as illustrated in the
balanced transistor amplifier of Fig. 1. As the base of one is driven
positive, the other is driven negative. If the transistors are matched and
operate in their linear regions, the emitter-current increase of one
transistor is equal to the emitter-current decrease of the other. Therefore,
the current through the emitter resistor RsubE and the voltage drop across
RsubE remain constant. Thus, RsubE causes no degeneration and needs no
bypassing. On the other hand, when signals having the same magnitude and
polarity are applied to the two bases, the current change through both
transistors must flow through RsubE, and the resulting voltage change across
RsubE opposes the input voltage, causing degeneration. This is the same effect
observed with an unbypassed emitter resistor in a conventional amplifier. This
equal-polarity signal is known as a common-mode signal, and the voltage
amplification of a differential transistor amplifier with a common-mode input
signal is approximately RsubC/RsubE. Voltage or current changes due to
temperature changes are common-mode signals. Therefore, best thermal stability
is achieved when the common-mode voltage gain is very low. A figure of merit
for the differential amplifier is the ratio of differential voltage gain to
the common-mode voltage gain. This ratio is known as the common-mode rejection
ratio (CMRR).
    The input signal of the differential amplifier may be single-ended, or
referenced to ground, and the output signal may be either differential or
single-ended, as shown in Fig. 2. Besides being unbalanced, an additional
transistor T3 is included in the emitter circuit of this differential
amplifier. This transistor accurately controls the bias currents in the
differential amplifier and therefore acts as a current source, or very high
resistance, in the emitter circuit, thus providing a very high CMR. See
AMPLIFIER.
 �34s[Christos C. Halkias ;Charles L. Alley]
�33s


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�6v�2v�32sDifferential equation �21m �33s
    A relationship between a function and its derivatives.
    If there is one independent variable, the differential equation is called
an ordinary differential equation. The general form of such an equation is

		  F (t,u,u', . . . ,u (sup (n))) = 0                         (1)

shown in Eq. (1), where t is the independent variable; u is a function of t; u'
= du/dt, uDOUBLE PRIME = d sup 2 u/dt sup 2, . . . ,u(n) = d sup nu/dt sup n are
the derivatives of u; and F(t,u0,u1, . . . ,u sub n) is a given function of the
n + 2 variables t,u0, . . . ,u sup n. The positive integer n is called the order
of the differential  equation; that is, the order of the differential equation
is the order of the highest derivative that occurs in the equation. As an
example, consider Eq. (2), which is an ordinary differential equation of order
2. A function u is said to be a solution to Eq. (1) if, when u and its
derivatives up to order n re substituted into F, the identity expressed in Eq.
(1) is valid for t in some interval a < t < b. In Eq. (2), u = cos t and u =
sin t are solutions for -INFINITY < t < INFINITY, as can be immediately
verified. 

			    u" + u = 0                                       (2)

    If there are two or more independent variables, the equation is called a
partial differential equation. Again, the order of the equation is the order
of the highest partial derivative that occurs in the equation. Thus, the
general form of a partial differential equation of order 2, with two
independent variables, is given by Eq. (3), where x and y are the independent
variables;

	  F(x,y,u,u sub x,u sub y,u sub xx,u sub xy,u sub yy) = 0	     (3)

u = u(x,y) is a function: usubx = PARTIAL DERIVATIVEu/PARTIAL DERIVATIVEx,
usuby = PARTIAL DERIVATIVEu/PARTIAL DERIVATIVEy, usubxsubx = PARTIAL
DERIVATIVEsup2u/PARTIAL DERIVATIVExsup2, usubxsuby = PARTIAL
DERIVATIVEsup2u/PARTIAL DERIVATIVExPARTIAL DERIVATIVEy, and usubysuby =
PARTIAL DERIVATIVEsup2u/PARTIAL DERIVATIVEysup2 are the derivatives of u; and
F(x, y, u, p, q, r, s, t) is a given function of eight variables. For example,
Eq. (4) is a partial differential equation of order

				u sub xy = 0				     (4)

2. It is easily verified that u = h(x) + g(y) is a solution of Eq. (4) for any
choice of functions h and g.
    A differential equation is linear if the function u is nonlinear otherwise.
    Partial differential equations occur with more than two independent
variables. Another possibility is a system of differential equations. This
consists of two or more equations involving one or more unknown functions
which are to be solved simultaneously. The Cauchy-Riemann equations, Eqs. (5),
are a first-order linear system of partial differential equations. See COMPLEX

			    u sub x - v sub y = 0			     (5)

			    u sub y + v sub x = 0			     (6)

NUMBERS.


 �34s[John C. Polking]
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�6v�2v�32sDifferential geometry �21m �33s
    A branch of mathematics that deals with intrinsic properties of curves and
surfaces in three-dimensional euclidean space. The intrinsic properties are
those which are independent of the geometrical object's orientation or
location in space. The subject is also concerned with nets of curves and
families of surfaces, these having wide application in the arts.
    The space is referred to a rectangular cartesian coordinate system
(x,y,z). A space curve may be defined by a pair of independent equations
f(x,y,z) = 0 and g(x,y,z) = 0 or, more meaningfully, by parametric equations:
x = x(t), y = y(t), z = z(t). In this case the arc length between two points
t0 and t is given by Eq. (1).

s = INTEGRATION (t0 to t) SQUARE ROOT [(dx/dt)(sup 2) + (dv/dt)(sup 2) +
		       (dz/dt)(sup 2) + (dz/dt)(sup 2)] dt		   (1)


    Obviously, if s is chosen as parameter, Eq. (2) holds. In this case dx/ds,
dy/ds, dz/ds at a point P are the direction cosines of the tangent to the
curve at P.

       (dx/ds)(sup 2) + (dy/ds)(sup 2) + (dz/ds)(sup 2) = 1		     (2)

    Consider three nearby points P, P1, P2. Through them, in general, one
plane may be constructed. The limiting position of this plane as P1 and P2
approach P is the osculating plane of the curve at P. The limiting position of
the circle through P, P1, P2 as P1 and P2 approach P is the osculating circle.
Its center is the center of curvature and its radius is the radius of
curvature rho. The reciprocal of the radius of curvature kappa is the
curvature; its value is shown in expression (3). The perpendicular to the
tangent at P in the osculating plane is the principal normal and the
perpendicular at P to the osculating plane is the binormal.

SQUARE ROOT {[(d(sup 2) x)/ds(sup 2)](sup 2) + [(d(sup 2) y)/ds(sup 2)](sup 2)
		  + [(d(sup 2) z)/ds(sup 2)](sup 2)			    (3)

    A surface in three-dimensional euclidean space may be given by f(x,y,z) =
0 or, more conveniently, by Eqs. (4). For a fixed value of v, Eqs. (4)
describe a curve in the surface, a u line, and similarly for a fixed value of
u. Thus (u,v) are curvilinear coordinates of a point in the surface. The most
important quantity in the study of surfaces is the arc length of a curve. This
is given by Eq. (5), where E, F, G are functions of partial derivatives of
(x,y,z). In the real domain E, G, and EG - F(sup 2) are nonnegative and may be
zero only at singular points of the surface, or at points where the matrix of
the partial derivatives of (x,y,z) is of rank less than two. The right-hand
side of Eq. (5) is called the first fundamental form of the surface, where E,
F, G are the first fundamental quantities of the surface. All applicable
surfaces have the same first fundamental form. Thus any cylinder or cone which
is developable (applicable to a plane) has the fundamental form du(sup 2) +
dv(sup 2), where (u,v) are rectangular cartesian coordinates. Under an arbitrary
transformation of the surface coordinates (u,v) the fundamental quantities E,
F, G transform linearly so that many problems in the study of surfaces reduce
to the question whether a coordinate system exists on the surface in which E,
F, G satisfy desired conditions. See CALCULUS OF TENSORS; COORDINATE SYSTEMS;
RIEMANNIAN GEOMETRY.
 �34s[Morris S. Knebelman]
�33s

		 x = x(u,v)	y = y(u,v)     z = z(u,v)		    (4)


       ds(sup 2) = E du(sup 2) + 2F du dv + G dv(sup 2) 		     (5)

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�6v�2v�32sDifferential transformer   �21m �33s 
    An iron-core transformer with movable core. A differential transformer 
produces an electrical output voltage proportional to the displacement of the 
core. It is used to measure motion and to sense displacements. It is also used 
in measuring devices for force, pressure, and acceleration which are based on 
the conversion of the measured variable to a displacement. 
    Various available configurations, some translational and others 
rotational, all employ the basic circuit shown in the illustration: a primary 
winding, two secondary windings, and a movable core. The primary winding is 
energized with alternating voltage. The two secondary windings are connected 
in series opposition, so that the transformer output is the difference of the 
two secondary voltages. When the core is centered, the two secondary voltages 
are equal and the transformer output is zero. This is the balance or null 
position. When the core is displaced from the null point, the two secondary 
voltages are no longer alike and the transformer produces an output voltage. 
With proper design, the output voltage varies linearly with core position over 
a small range. Motion of the core in the opposite direction produces a similar 
effect with 180DG phase reversal of the alternating output voltage. See 
TRANSDUCER; TRANSFORMER. 
 �34s[Gerald Weiss]
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�6v�2v�32sDifferentiation �21m �33s
    A mathematical operation performed on a function to determine the effect
of a change in the value of the independent variable. Functions of one
variable are considered in this article. For differentiation of functions of
several variables see PARTIAL DIFFERENTIATION.
    If f is a function of x, defined on an interval containing x(sub 0), the
derivative at x(sub 0) is, by definition, that shown in Eq.  (1).  For
generalities about derivatives and calculus see CALCULUS.

f'(x[sub 0]) = lim (x -> x[sub 0]) [f(x) - f(x[sub 0])]/(x - x[sub 0])      (1)

    In the quotient on the right, in the definition of f'(x[sub 0]), x is
restricted to the interval on which f is defined.  If y = f(x), f'(x[sub 0]) is
also denoted by

			     (dy/dx)(for x - x[sub 0])

The limit defining f'(x[sub 0]) may not exist. If it does, f is called
differentiable at x[sub 0].
    The derivative of f'(x), called the second derivative, is denoted by

	       f"(x) or (d[sup 2] y)/dx[sup 2]

    A function f is called continuous at x(sub 0) if x(sub 0) is in the domain
of f and

			  lim (x -> x[sub 0]) f(x) = f(x[sub 0])

The precise formulation of the limit concept used here is the following: Let g
be a function and let A be a number. Then Eq. (2) means that to each positive

			    lim (x -> x[sub 0]) g(x) = A		   (2)
(2)

number epsilon corresponds some positive number delta such that |g(x) - A| <
epsilon whenever x is a number in the domain of g such that x NOT EQUAL
x(sub 0) and |x - x(sub 0)| < delta.  It is required of g that its domain shall
contain numbers x as close to x(sub 0) as desired, but different from x(sub 0).
The domain of g may also contain x(sub 0), but this is irrelevant.
      It is a theorem that, if f is differentiable at x(sub 0), then f is
continuous at x(sub 0).  However, a function can be continuous at x(sub 0), but
not differentiable there.  An example is f(x) = |x| at x(sub 0) = 0.  It is even
possible for a function to be continuous on an interval and yet not
differentiable at any point of this interval.

     The chief elementary applications of differentiation are (1) in expressing
rates of change (velocity, acceleration), and in solving problems where, through
functional relationship, the rate of change of one variable is calculated when
the rate of change of another variable is known; (2) in studying graphs of
functions and, more generally, in studying curves in the plane or in space of
three dimensions:  (3) in expressing scientific laws or principles in the form
of differential equations; and (4) in the expression of various extensions and
applications of the law of the mean, including such topics as l'Hospital's rule
and Taylor's formula or series.

			(d/dx)(u + v) = du/dx + dv/dx
		       (d/dx)(uv) = u(dv/dx) + v(du/dx)
		(d/dx)(u/v) = [v(du/dx) + u(dv/dx)]/v(sup 2)		   (3)

    The general technique of differentiation is built upon the rules for
differentiating combinations of differentiable functions. If u = f(x) and v =
g(x) are functions differentiable for the same values of x, then u + v and uv
are differentiable and so is u/v if v NOT EQUAL 0. The formulas are shown in
Eqs. (3). If u is constant in value, then du/dx = 0. A very powerful instrument
of technique is furnished in the chain rule for composite functions. If y is a
differentiable function of u and u is a differentiable function of x, then y
is a differentiable function of x, and

			   dy/dx = (dy/du)(du/dx)

In functional notation, if y = f(u) and u = h(x), then y = F(x), where F(x) =
f[h(x)], and then F'(x) = f'[h(x)]h'(x).
 �34s[Angus E. Taylor]
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�6v�2v�32sDiffraction �21m �33s
    The bending of light, or other waves, into the region of the geometrical
shadow of an obstacle. More exactly, diffraction refers to any redistribution
in space of the intensity of waves that results from the presence of an object
that causes variations of either the amplitude or phase of the waves. Most
diffraction gratings cause a periodic modulation of the phase across the
wavefront rather than a modulation of the amplitude. Although diffraction is
an effect exhibited by all types of wave motion, this article will deal only
with electromagnetic waves, especially those of visible light. For discussion
of the phenomenon as encountered in other types of waves see ELECTRON
DIFFRACTION; NEUTRON DIFFRACTION; SOUND.
    Diffraction is a phenomenon of all electromagnetic radiation, including
radio waves; microwaves; infrared, visible, and ultraviolet light; and x-rays.
The effects for light are important in connection with the resolving power of
optical instruments. See RADIO-WAVE PROPAGATION; X-RAY DIFFRACTION.
    There are two main classes of diffraction, which are known as Fraunhofer
diffraction and Fresnel diffraction. The former concerns beams of parallel
light, and is distinguished by the simplicity of the mathematical treatment
required and also by its practical importance. The latter class includes the
effects in divergent light, and is the simplest to observe experimentally.
    To illustrate the difference between the methods of observation of the two
types of diffraction, Fig. 1 shows the experimental arrangements required to
observe them for a circular hole in a screen S. The light originates at a very
small source O, which can conveniently be a pinhole illuminated by sunlight.
In Fraunhofer diffraction, the source lies at the principal focus of a lens
L1, which renders the light parallel as it falls on the aperture. A second
lens L2 focuses parallel diffracted beams on the observing screen F, situated
in the principal focal plane of L2. In Fresnel diffraction, no lenses
intervene. The diffraction effects occur chiefly near the borders of the
geometrical shadow, indicated by the broken lines. An alternative way of
distinguishing the two classes, therefore, is to say that Fraunhofer
diffraction concerns the effects near the focal point of a lens or mirror,
while Fresnel diffraction concerns those effects near the edges of shadows.
Photographs of diffraction patterns are shown in Fig. 2.

    Fraunhofer diffraction.  This class of diffraction is characterized by a
linear variation of the phases of the Huygens secondary waves with distance
across the wavefront, as they arrive at a given point on the observing screen.
At the instant that the incident plane wave occupies the plane of the
diffracting screen, it may be regarded as sending out, from each element of
its surface, a multitude of secondary waves, the joint effect of which is to
be evaluated in the focal plane of the lens L2. The analysis of these
secondary waves involves taking account of both their amplitudes and their
phases. The simplest way to do this is to use a graphical method, the method
of the so-called vibration curve, which can readily be extended to cases of
Fresnel diffraction. See HUYGENS' PRINCIPLE.
    The vibration curve results from the addition of a large (really infinite)
number of infinitesimal vectors, each representing the contribution of the
Huygens secondary waves from an element of surface of the wavefront. If these
elements are assumed to be of equal area, the magnitudes of the amplitudes to
be added will all be equal. They will, however, generally differ in phase, so
that if the elements were small but finite each would be drawn at a small
angle with the preceding one, as shown in Fig. 3a. The resultant of all
elements would be the vector A. When the individual vectors represent the
contributions from infinitesimal surface elements (as they must for the
Huygens wavelets), the diagram becomes a smooth curve, the vibration curve,
shown in Fig. 3b. The intensity on the screen is then proportional to the
square of this resultant amplitude. In this way, the distribution of the
intensity of light in any Fraunhofer diffraction pattern may be determined.
    The intensity distribution for Fraunhofer diffraction by a slit as a
function of the angle THETA from the center may be simply calculated by the
method of the vibration curve. The intensity at any angle is given by Eq. (1),
where I0 is the intensity at the center of the pattern, and BETA is given by
Eq. (2), where b is the width of the slit and LAMBDA is the wavelength. The
central maximum is twice as wide as the subsidiary ones, and is about 21 times
as intense as the strongest of these. A photograph of this pattern is shown
in Fig. 2a.

    *** I = [I sub 0 (sin (sup 2) BETA)] / BETA sup 2 ***	       (1)


	      *** BETA = (PI b sin THETA) LAMBDA ***		     (2)

    Fraunhofer diffraction by a circular aperture determines the resolving
power of instruments such as telescopes, cameras, and microscopes, in which
the width of the light beam is usually limited by the rim of one of the
lenses. The method of the vibration curve may be extended to find the angular
width of the central diffraction maximum for this case. An exact construction
of the curve or, better, a mathematical calculation shows that the extreme
phase differences required are PLUS/MINUS 1.220 PI, yielding Eq. (3) for the

  *** sin THETA APPROXIMATELY = THETA = PLUS/MINUS 1.220 LAMBDA / d ***  (3)

angle THETA at the first zero of intensity. Here d is the diameter of the
circular aperture. This pattern has circular symmetry and consists of a
diffuse central disk, called the Airy disk, surrounded by faint rings. The
angular radius of the disk, given by Eq. (3), may be extremely small for an
actual optical instrument, but it sets the ultimate limit to the sharpness
of the image, that is, to the resolving power. See RESOLVING POWER (OPTICS).

    Fresnel diffraction. The diffraction effects obtained when the source of
light or the observing screen are at a finite distance from the diffracting
aperture or obstacle come under the classification of Fresnel diffraction.
This type of diffraction requires for its observation only a point source, a
diffracting screen of some sort, and an observing screen. The latter is often
advantageously replaced by a magnifier or a low-power microscope. The observed
diffraction patterns generally differ according to the radius of curvature of
the wave and the distance of the point of observation behind the screen. If the
diffracting screen has circular symmetry, such as that of an opaque disk or a
round hole, a point source of light must be used. If it has straight, parallel
edges, it is desirable from the standpoint of brightness to use an illuminated
slit parallel to these edges. In the latter case, it is possible to regard the
wave emanating from the slit as a cylindrical one. For the purpose of deriving
the vibration curve, the appropriate way of dividing the wavefront into
infinitesimal elements is to use annular rings in the first case, and strips
parallel to the axis of the cylinder in the second case.
    The zone plate is a special screen designed to block off the light from
every other half-period zone, and represents an interesting application of
Fresnel diffraction. The Fresnel halfperiod zones are drawn, with radii
proportional to the square roots of whole numbers, and alternate ones are
blackened. The drawing is then photographed on a reduced scale. When light
from a point source is sent through the negative, an intense point image is
produced, much like that formed by a lens.
 �34s[Francis A. Jenkins/William W. Watson]
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�6v�2v�32sDiffraction grating   �21m �33s 
    An optical device consisting of an assembly of narrow slits or grooves, 
which by diffracting light produces a large number of beams which can 
interfere in such a way as to produce spectra. Since the angles at which 
constructive interference patterns are produced by a grating depend on the 
lengths of the waves being diffracted, the waves of various lengths in a beam 
of light striking the grating will be separated into a number of spectra, 
produced in various orders of interference on either side of an undeviated 
central image. By controlling the shape and size of the diffracting grooves 
when producing a grating and by illuminating the grating at suitable angles, a 
beam of light can be thrown into a single spectrum whose purity and brightness 
may exceed that produced by a prism. Gratings can now be made with much larger 
apertures than prisms, and in such form that they waste less light and give 
higher intrinsic dispersion and resolving power. See DIFFRACTION. 
    Transmission gratings consist of a large number of narrow transparent and 
opaque slits alternating side by side in regular order and with uniform 
separation, through which a beam of light will appear as a series of spectra 
in various orders of interference. Reflection gratings, either plane or 
concave, are used in most spectrographs. Such a grating may consist of an 
original ruling or of a metal-coated replica from an original. Large grating 
replicas can now be made which are practically indistinguishable in 
performance or permanence from an original. 
    Gratings are engraved by highly precise ruling engines, which use a 
diamond tool to press into a highly polished mirror surface a series of many 
thousands of fine shallow burnished grooves. If a grating is to give 
resolution approaching the theoretical limit, its grooves must be ruled 
straight, parallel, and equally spaced to within a few tenths of the shortest 
incident wavelength. Scattered light and false images may arise from local 
spacing error and groove shape variations of only a few hundredths of the 
diffracted wavelength. 
    A grating spectroscope usually consists of a slit, a lens or mirror to 
collimate the light sent through the slit into a parallel beam, a transmission 
or reflection grating to disperse the light, a lens or mirror to focus the 
light into spectrum lines (which are monochromatic images of the slit in the 
light of each wavelength passing through it), and an eyepiece for viewing the 
spectrum. If a camera is substituted for the telescope, the instrument becomes 
a grating spectrograph. If a photoelectric cell, a thermocouple, or other 
radiation-detecting device is used instead of a camera or telescope, the 
device becomes a grating spectrometer. See CONCAVE GRATING; ECHELETTE GRATING; 
ECHELLE GRATING; INFRARED SPECTROSCOPY. 
 �34s[George R. Harrison]
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�6v�2v�32sDiffuser �21m �33s
    A device to convert a high-velocity low-pressure stream of fluid (usually
air) into a low-velocity high-pressure flow. In doing so, a diffuser converts
the velocity or kinetic energy into pressure or potential energy. An efficient
diffuser operates by decelerating the high-velocity stream with as little loss
in total energy as possible.
    The shape of a diffuser depends on the Mach number of the entering fluid.
If a supersonic stream is to be decelerated, the diffuser cross-sectional area
must be progressively decreased (ideally to just sonic velocity) and then
increased until the flow reaches the desired velocity, as indicated in the
illustration. If the initial Mach number M is supersonic (greater than 1.0),
an area reduction is required to reduce the stream velocity; the converse is
true for an initial subsonic Mach number. See SUPERSONIC DIFFUSER.
    Probably the most dramatic use of diffusers is on jet aircraft. Diffusers
supplying air to the engine have had inlets in the nose, at the wing root,
along the airplane fuselage, and even in the tail section.
 �34s[James F. Connors ;Leonard J. Obey]
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�6v�2v�32sDiffusion in gases and liquids �21m �33s
    The spreading or scattering of matter under the influence of a
concentration gradient. For a related discussion see DIFFUSION IN SOLIDS.
    Consider a fluid confined in a space of dimensions which are large
compared to the mean free path of the fluid molecules. For gases at
atmospheric pressure and room temperature, the mean free path is on the order
of 10 sup -5 cm and varies inversely with pressure. Liquids in general have much
smaller free paths. Assume that one of the components of the fluid exists
initially at different concentrations, or more rigorously at different
activities, in two or more different locations in the confining space. At
constant temperature, and in the absence of external forces, there will be a
spontaneous movement, that is, diffusion, of the component in the direction of
establishing a uniform concentration of that component in all parts of the
enclosure.
    In a very qualitative way the cause of this spontaneous mixing may be
interpreted as follows. As a consequence of thermal agitation, molecules of a
fluid are in constant motion in all directions. The number of molecules of a
given kind moving in any given direction at a particular point in the fluid is
proportional to the number of these molecules present per unit volume. In the
absence of a concentration gradient, on the average as many molecules per unit
time leave any hypothetical plane in a given direction as return to the plane
from the opposite direction. However, if the number of molecules per unit
volume decreases in a given direction, more molecules on the average move into
the region of lower concentration than return from it. This results in a net
transfer of molecules of that kind toward the region of lower concentration.
    In general, the diffusion of one component of a mixture will be
accompanied by diffusion of one or more other components in the opposite
direction to maintain the volume of fluid constant in the region under
consideration. This may not be the case when special restrictions are placed
on the system, for example, in the selective absorption of one component from
a gas by a liquid acting as a boundary of the confining space, or for liquids
when there is a volume change on mixing.
    Fluid motion may be viscous or turbulent. In viscous flow, mass transfer
resulting from a concentration gradient occurs by molecular diffusion. In
turbulent flow, transfer of material occurs by the more rapid process of
mixing of the swirling eddies of fluid, a process termed turbulent diffusion
or eddy diffusion.
    S. Chapman demonstrated in 1916 as a consequence of the kinetic theory
that a temperature gradient in a mixed gas might give rise to a flow of one
constituent relative to the mixture as a whole. His finding was verified
experimentally by L. Chapman and F. W. Dootsen, who placed various mixtures in
a bulb which was heated at one end and cooled at the other. A similar effect
had been observed for liquids by Ludwig in 1856 and by J. L. Soret in 1879,
and is usually called the Soret effect. The phenomenon as it pertains to
fluids generally is called thermal diffusion.
    An external force acting upon each of a group of molecules may induce
molecular movement analogous to diffusion. Such movements, which may result
from gradients of pressure within a fluid, or from electric or magnetic
fields, may be termed forced diffusion. An important example is the process of
electrolytic migration of ions under an applied potential.
    Mass transfer through a turbulent fluid to an interface formed by some
other fluid with which it is immiscible or by a solid surface is commonly
encountered in processes of industrial importance. For such situations it is
necessary to resort to empirical equations to describe mass-transfer rates. As
an example, consider the dissolution of a slightly soluble solute, such as
benzoic acid, from the wall of a pipe into water in turbulent flow. Solute
concentrations in the fluid in the vicinity of the wall are shown
qualitatively in the illustration. At the interface the water is saturated
with solute at concentration C sub i corresponding to the solubility of the
solute. The concentration falls progressively from the wall until a value
corresponding essentially to the value in the bulk liquid, C sub 0, is reached.
In the steady state the mass-transfer flux at a given point is commonly
expressed by a mass-transfer coefficient, k sub C, as in Eq. (1). Similar
concepts can be 

	*** N (sub A) = k (sub C)(C (sub i) - C(sub o))  ***   (1) 

applied to both gases and liquids and for surfaces other than pipe walls.
Various mass-transfer coefficients may be defined corresponding to the units
used for mass-transfer rates, concentrations, and distance.
    An alternate method of describing the rate of mass transfer is to define
an equivalent film b of stagnant fluid which offers the same resistance to
transfer (that is, requires the same total drop in concentration between the
interface and bulk liquid) as the actual fluid region which is not completely
stagnant (see illustration). Integration of Eq. (1) for steady-state diffusion
gives the relation between the mass-transfer flux and the film thickness for
equivolume diffusion, Eq. (2).

	   *** N(sub A) = (D(sub A)[C(sub i) - C( sub o)]) / b ***		       (2)

See MASS-TRANSFER OPERATION.
 �34s[Charles R. Wilke]
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�6v�2v�32sDiffusion in solids   �21m �33s 
    The actual transport of mass, in the form of discrete atoms, through the 
lattice of a crystalline solid is called diffusion. Diffusion processes are 
fundamental to a large class of solid-state effects: solubility, phase 
transformations, nucleation, annealing, devitrification, recrystallization, 
oxidation, corrosion, creep, welding, sintering, tempering, the photographic 
process, and ionic conductivity. Almost any change in a solid - in its form, 
strength, or even its color - is usually associated with mass motion within 
the lattice. See IONIC CRYSTALS. 
    Scientific interest in diffusion has centered on elucidating the basic 
atomic mechanisms which permit diffusion to occur without violating the 
solidity of the lattice. The jump rates of individual atoms necessary to 
explain observed bulk effects is formidable: Well below the melting point, 
each atom in a solid must jump from one ``fixed'' lattice site to another over 
a million times per second. A large number of models have been proposed, and 
with precise radioactive tracer measurements it has finally been possible to 
define the basic atomic mechanisms. It is clear that diffusion occurs only by 
virtue of lattice imperfections. A perfect lattice would permit no mass 
motion. At temperatures below about half the absolute melting temperature, 
diffusional motion is largely confined to surfaces and regions of inherent 
disorder: dislocations, grain boundaries, and paths disarranged by the passage 
of high-enengy particles. True bulk diffusion, where atoms migrate through the 
whole volume, predominates at high temperatures. In most solids it has been 
shown that the principal defect responsible for bulk diffusion is the vacancy 
- a missing lattice atom; in a few cases the defect is the interstitial - an 
atom in an extra nonlattice site. These defects can be formed externally at 
free surfaces or internally along jogs on dislocations. In order to maximize 
the entropy, they will tend toward a random distribution by migration through 
the lattice. Results of high-pressure measurements show that vacancies and 
interstitials are ``many-body'' rather than pure ``point'' defects, since 
their sizes are clearly different from that of a single atom. This implies a 
considerable relaxation of the lattice around a defect, severely complicating 
theoretical attempts to calculate defect energies from first principles. See 
CRYSTAL DEFECTS. 
 �34s[David Lazarus]
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�6v�2v�32sDigenea   �21m �33s 
    A group of parasitic flatworms or flukes constituting a subclass or order 
of the class Trematoda, in the phylum Platyhelminthes. The name Digenea refers 
to the two types of generations in the life cycle: (1) the germinal sacs which 
parasitize the intermediate host (a mollusk or rarely an annelid) and 
reproduce asexually; and (2) the adult which is primarily endoparasitic in 
vertebrates and reproduces sexually. The adult usually is hermaphroditic, but 
many of the blood flukes and a few others are dioecious. Vertebrates of all 
classes, except the Cyclostomata, serve as definitive hosts. Those feeding on 
aquatic plants and animals harbor the greatest variety of digenetic 
trematodes, but several species occur in strictly terrestrial hosts. Effects 
on the vertebrate vary from no apparent harm to severe and even fatal injury. 
Control of flukes is by preventive measures, including interruption of their 
life cycles and treatment with drugs. See TREMATODA. 
    The adult differs from other trematodes in several respects, the most 
obvious being the absence of a posterior adhesive organ with sclerotized 
hooks, plates, or multiple suctorial structures. Usually the mouth is anterior 
and opens into an oral sucker. A second sucker, the ventral one, is at the 
posterior end of amphistomes, on the ventral surface of distomes, or absent in 
monostomes. Adult digenetic trematodes occur in any part of the vertebrate 
that provides egress for their eggs and also in the circulatory system, from 
which eggs work through the tissues and escape in the feces or urine. 
    There is no general agreement concerning the classification of the 
Digenea. The scheme of G. La Rue proposes two superorders, the 
Anepitheliocystidia and Epitheliocystidia. See PLATYHELMINTHES. 
 �34s[Raymond M. Cable]
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�6v�2v�32sDigestive system �21m �33s
    That system of structures in which food substances are digested; that is,
complex food materials are subjected to the action of digestive enzymes
throughout the system, in the presence of water, and broken down into simpler
chemical compounds. Absorption into the cells and tissues is then possible.
The digestive system of invertebrates varies among the various phyla. Many
protozoans have no permanent structures; such animals as sponges have
intracellular digestion while in coelenterates digestion is partly
extracellular in the body cavity; most animals have a complete digestive
system in that there is a mouth and anal opening. Among the vertebrate
classes, the structures of the digestive system are essentially similar,
although they may be highly modified according to the food habits of the
animal.

    Anatomy.  The major digestive organs form the posterior portions of the
digestive tract, while the mouth and pharynx are the reception point for food
and have numerous specialized structures and glands.
    The mouth is the expanded and structurally complex upper end of the
digestive tract. When lips and cheeks are present, as in humans, the space
anterior to the teeth and gums is termed the vestibule or buccal cavity.
Behind this is the much larger oral cavity, in the floor of which lies the
tongue; the roof of this cavity is termed the palate. The mouth is continuous
posteriorly with the pharynx. Although the mouth is primarily concerned with
feeding and varies accordingly, in terrestrial vertebrates it also
participates importantly in the production and modulation of sound, a function
which attains its highest development in the voice and articulate speech of
humans. The pharynx is a relatively short segment of the digestive tract lying
between the posterior part of the oral cavity and the upper end of the
esophagus. It is an important region because its passage is shared by both the
digestive and respiratory systems. Below the pharynx the digestive tract
proper (or gut) consists of an elongated muscular tube extending from the
mouth to the anus. Four successive segments of the gut are distinguished on
the basis of their structural and functional specializations (see
illustration) the esophagus, for rapid conveyance of food from the mouth to
the stomach; the stomach, where the food is held for a time and digestion
begins; the small intestine, where digestion and absorption are virtually
completed; and the large intestine, into which undigested and waste materials
are passed for elimination by way of the rectum and anus. See ESOPHAGUS;
INTESTINE; PHARYNX; STOMACH.
 �34s[Victor M. Emmel]
�33s

Phsiology.
The foodstuffs utilized by animals to satisfy their energy and growth
requirements consist of nutrients (complex carbohydrates, proteins, and fats),
vitamins, minerals, and water. The nutrients, to be utilized by body tissues,
must first be digested into smaller, absorbable units such as simple sugars,
amino acids, and fatty acids. In vertebrates digestion is an extracellular
process which takes place in the lumen of the alimentary tract. A number of
glands, including the pancreas, liver, and less commonly the salivary glands,
are connected to the lumen of the tube by ducts. The secretions from these
glands supply lubricating and food-softening fluids and digestive enzymes for
chemically splitting the foodstuffs into absorbable units. Other glands lodged
in the wall of the alimentary tube also supply fluids, acids, and other
important aids to the digestive process. Digestion is a highly complex, highly
integrated, and delicately controlled physiological process. Integration and
control of the functioning of the alimentary tract are achieved by means of an
interlocking system of nervous networks and hormones.
    In the tract food is masticated, triturated, mixed with hydrochloric acid,
digestive enzymes and fluid, and propelled toward the anus by peristaltic
action. The digestive enzymes have similar physiological actions throughout
the vertebrates. The amounts of each enzyme produced vary with the species and
appear to be functionally adapted to differences in dietary habits.

    Mouth.  In vertebrates the primary functions of the mouth in digestion are
the reception and swallowing of food. Other important functions subserved by
it in some, but not all, vertebrates are the mastication and moistening of
whole foods. Actual digestion in the mouth is of limited importance. The
functions of the mouth in the digestive process, then, are mainly mechanical;
that is, prehension, tearing, cutting and grinding of food, and mixing it with
a fluid medium.
    Salivary glands. The fluids which bathe the oral cavity are produced by
the salivary glands, which are found in many groups of animals, invertebrates
as well as vertebrates. Saliva performs a number of different functions.
During mastication it helps to form food into a convenient bolus for
mastication and swallowing. It also facilitates swallowing by keeping the
mouth and esophagus moist. Saliva aids the taste mechanism by dissolving the
substances in the food which stimulate the taste buds. Finally saliva helps
keep the oral tissues and teeth clean of food debris.
    In humans there are three pairs of large salivary glands: the parotid, the
submaxillary, and the sublingual. There are also numerous, smaller glands
lodged in the submucosal tissue of the lips, cheeks, tongue, and palate.
Secretion by the salivary glands is under the control of the autonomic nervous
system, especially the parasympathetic branch. The presence of food in the
mouth causes a reflex secretion of saliva. Salivary secretion is easily
conditioned and the sight, smell, and in humans even the thought of food cause
increased salivary secretion (``makes the mouth water''). Saliva is secreted
in much greater volume than is generally appreciated; the daily secretion for
humans is probably about 37 in.sup3 (600 milliliters) per day.
    Teeth. Along with their role as biologic weapons for offense and defense,
the teeth serve in the prehension, cutting, and mastication of food. It would
seem logical to suppose that mastication, because it increases the surface
area of the food exposed to digestive action, would be an aid to digestion.
This is definitely the case in ruminant forms. On the other hand, the food
swallowed whole by snakes or wolfed by carnivores is clearly subject to
complete digestion. See TOOTH.
 �34s[Herbert Wells]
�33s
    Tongue. The tongue is composed principally of skeletal muscle and is
enveloped by mucous membrane. The mucous membrane on the undersurface forms a
fold, termed the frenulum linguae or more commonly the frenulum, which binds
the tongue, with the exception of the tip, to the floor of the mouth. Normally
the upper surface of the tongue presents a moist, pink appearance. Under other
conditions it may appear yellow and dry because of the accumulation of shed
epithelial cells, remains of food, and organisms. The surface of the tongue is
characterized by the presence of papillae. The taste buds are scattered over
the upper surface of the tongue.
    The taste buds are chemoreceptors and as such are attuned to the chemical
composition of the oral cavity. There are specific substances which will
stimulate only one of the four types of taste buds. It is common experience
that more than four basic tastes can be easily differentiated. This ability
depends upon two factors: utilization of more than one type of taste bud in a
blend, so to speak, and derivation of clues from activation of extragustatory
receptors such as those for temperature, pressure, touch, and olfaction. For
example, if an individual were to chew a piece of uncooked potato it could be
distinguished from a raw carrot or an apple. Yet is it clear that none of
these foods can be described as truly acid or sour, bitter, or sweet. The
texture of the substance, its moisture content, the temperature, the relative
proportion of the various types of taste buds stimulated, and finally the
sense of smell all contribute. It is easy to demonstrate that the taste of a
substance depends on contrast. For example, coffee consumed after a very sweet
dessert does not taste as sweet as the same beverage does in the morning when
nothing else has preceded it. See CHEMORECEPTION; TASTE; TONGUE.

    Deglutition.  The act of swallowing is termed deglutition. It is a complex
process, involving both voluntary and reflex mechanisms, and occurring in
three stages.
    In the first stage of deglutition the food is ground and rolled into a
mass, called a bolus, and thoroughly soaked with saliva. The tongue then
directs the bolus to the back of the mouth and forces it to enter the pharynx.
All these acts are purely voluntary. Once the bolus is forced into the
pharynx, however, the voluntary phase of swallowing has ended, and all
subsequent mechanisms are purely reflex. During the second stage of
deglutition, the bolus passes through the pharynx to enter the esophagus.
    In the third phase of deglutition the food traverses the esophagus to
enter the stomach. The force of gravity assists in transporting the bolus
through the esophagus, but it is not indispensable to the swallowing act. The
esophagus is composed chiefly of muscles which contract in wavelike fashion
along the length of the tube. These are called peristaltic waves, or
movements. Thus the bolus is moved through the esophagus to enter the stomach.
 �34s[Leroy L. Langley]
�33s

    Esophagus.	The esophagus extends from the pharynx to the stomach in
mammals. It transfers swallowed material from the pharynx to the stomach, and
prevents the return of gastric contents to the esophagus and pharynx. The
esophagus contains four functionally distinct regions: the upper esophageal
sphincter, the part of the esophageal body composed of striated muscle, the
part of the esophageal body composed of smooth muscle, and the lower
esophageal sphincter.
    The upper esophageal sphincter is a segment, 1.2 - 1.6 in. (3 - 4 cm) long
in humans, between the pharynx and the esophageal body, in which the lumen is
completely closed at rest but opens during swallowing. The opening of the
esophageal lumen in swallowing occurs immediately after pharyngeal
contraction, so that the lumen offers low resistance to the movement of the
swallowed bolus. After the brief relaxation, resting tonic contraction is
resumed.
    The striated muscle of the proximal esophageal body is flaccid at rest.
After a swallow, an occlusive contraction begins at the most proximal level as
an extension of the closure of the upper esophageal sphincter, and moves as a
front throughout the length of the organ. The peristaltic contraction of the
esophageal body generated by swallowing passes smoothly from the striated
muscle to the smooth muscle, but the mechanism of the contraction in the
smooth muscle is different.
    The lower esophageal sphincter, 1.2 - 1.6 in. (3 - 4 cm) long in humans,
separates the esophageal body from the stomach. In this segment the lumen is
closed at rest but opens during swallowing. In humans, this segment lies in an
opening through the diaphragm, the esophageal hiatus of the diaphragm. The
stable contraction of the muscle at rest is due to a special property of the
muscle itself, although there may also be some contribution from constantly
active nerves to the region. After a swallow, luminal occlusion is transiently
obliterated through relaxation of the muscle. The relaxation persists until
esophageal peristalsis has been accomplished.
    The temporal interrelationships of these events are maintained by a
swallowing center in the brainstem. The swallowing center not only activates
the somatic nerves to the striated muscle but also triggers the autonomic
nerves, which initiate contractions in the esophageal body smooth muscle and
relaxation of the lower esophageal sphincter. Also, peristalsis in the smooth
muscle segment can be produced by esophageal distension. Thus, there are
neural sensory receptors in the esophageal wall which can, through nerve
reflex pathways, modify the process.
 �34s[James Christensen]
�33s

    Stomach.  Broadly speaking, there are three kinds of stomach found among
the vertebrates: the simple stomach, as found in humans, the dog, and the
great majority of higher and lower vertebrates; the complex stomach, as found
in the ruminants; and the stomachs found in animals with a gizzard, such as
birds and crocodiles. The basic functions are similar in all simple stomachs:
storage, secretion of digestive fluids, and mixing.
    Simple stomachs. The stimple stomach, as found in humans, is a bag-shaped
organ into which food passes from the esophagus. The partially digested food
is afterward released piecemeal to be dealt with by the digestive processes in
the small intestine. The main function of the stomach is to initiate the
digestion of proteins. This digestion is only partial, but sufficient to
disrupt the cells of foodstuffs and to release the intracellular contents,
rendering them susceptible to digestion in the small intestine. The gastric
secretions include mucus, which moistens the food and helps protect the
gastric mucosa from the action of the digestive juices and from the action of
hydrochloric acid and pepsinogen.
    Hydrocholoric acid is secreted in a surprisingly concentrated form. The
fresh secretion has a pH of about 1.0, but dilution of the acid with other
gastric contents gives an overall pH in the stomach of about 2.0 (which is
only one-tenth as acid). This extreme acidity is important in several
respects; it not only kills any live food, but also prevents bacterial
putrefaction if a large meal is stored in the stomach for a considerable time.
The low pH also denatures and precipitates soluble proteins, which then remain
longer in the stomach. The most essential role of the gastric acid, however,
is in providing suitably acid conditions for pepsin, a hydrolytic enzyme which
splits proteins into smaller units but does not carry the digestion all the
way to single amino acids.
    Digestion of carbohydrates in the stomach is negligible, but sometimes a
large starchy meal can form a dense mass in the stomach, in which the action
of salivary ptyalin can continue for some time before the acid gastric juices
can penetrate and deactivate the ptyalin. Digestion of fats proceeds to a
small extent. The stomach secretes a small amount of lipase which is adequate
to work on the type of fat found in egg yolk, for example. Additional active
lipase is frequently regurgitated from the duodenum.
    The mixing action of the stomach comes about through waves of powerful
contraction which push the food toward the pyloric sphincter. If the sphincter
remains closed, the inner part of the food mass in the antrum is pushed back
into the body and thereby mixed thoroughly. When the stomach contents are
digested enough to be largely fluid and if the first part of the small
intestine is empty (or nearly so), the pyloric sphincter relaxes and the
antral contraction empties that portion of the stomach. Contrary to what might
be expected, the antral muscles do not eject the stomach contents in vomiting.
This is actually accomplished by compression of the stomach against the liver
and diaphragm by the muscles of the abdominal wall.
    The stomach plays an extremely important role in the absorption of vitamin
B12, which is essential to the normal manufacture of red blood cells. The
stomach secretes Castle's intrinsic factor, which combines with dietary B12 to
form a complex that is resistant to digestion in the stomach and most of the
small intestine. This complex is absorbed into the blood much further down in
the intestine. See VITAMIN B12.
    Birds. The characteristic modification of the stomach in birds is a
gizzard derived from the posterior portion of the stomach. The anterior
portion, the proventriculus, is glandular and similar in function to the human
stomach. In most birds there is probably a minimum of digestion in the
proventriculus because the food reaches it in an undivided state and it is too
small for prolonged storage. The food is rapidly passed on to the gizzard,
where it is triturated for 1 - 12 min, depending on the consistency of the
food. This mixes the food thoroughly with the digestive enzymes previously
secreted in the proventriculus; digestion continues in the duodenum. In birds
the biliary and pancreatic ducts enter the duodenum at its distal end, so that
the full length of the duodenum is available for gastric digestion.
    The gizzard is most highly developed in seedeating birds, less so in birds
of prey. Birds do consume grit and small rocks, which are retained in the
gizzard. The gizzard is an advantage to birds because it enables them to
utilize the high nutrient value of plant seed. Mammals do not derive the full
benefit from this food source unless the seeds are first ground or cooked.
    Ruminants. The ruminants include cows (and wild relatives), sheep, goats,
deer, musk oxen, camels, and llamas. Of all the vertebrates, the ruminants
show the most spectacular modification of the stomach. The anterior portion is
enlarged to form a fermentation vat in which microorganisms break down the
otherwise indigestible cellulose of the animal's vegetable diet, and make the
products of the fermentation available for the nutrition of the animal.
    The rumen and reticulum together form a large fermentation vat in which
the ingesta are held in a very liquid suspension. Their total volume may be 80
gal (300 liters) in a large cow. The pH of the contents is about 6.0 - 7.5.
Fermented material from the rumen-reticulum passes through the omasum, where
it is subjected to squeezing and trituration, and then the omasum discharges
into the abomasum, which is generally similar to the simple stomach of other
mammals and is the only part of the ruminant's complex stomach that contains
digestive glands.
    When grazing, ruminants swallow the grass rapidly and it remains in the
rumen for some time. Later, boli of rumen contents are regurgitated and chewed
more thoroughly before being returned to the stomach. This permits
fermentation to begin disrupting the cellulose structure before it is chewed.
It is also likely that this practice of rumination has survival value in the
wild state, where the animal can rapidly eat its fill on the open plain and
then work it over in some safe retreat.
 �34s[Terence A. Rogers]
�33s

    Small intestine.  Digestion in the small intestine (that is, in the
duodenum, jejunum, and ileum) depends on the secretions of the liver,
pancreas, and intestinal glands, which provide liquid for dilution and
solution of the food, a reducer of surface tension for the emulsification of
fat, and enzymes for the breakdown of food into smaller molecules. Digestion
also depends on a great deal of metabolic activity within the columnar cells
lining the small intestine and on the movement of the intestine which mixes
the food with the secretions and propels it in a caudal direction toward
available absorptive surfaces.
    The mixture delivered to the duodenum is strongly acid. One of the first
functions of the duodenum is to bring the contents to, or close to, neutrality
in order to facilitate the activity of pancreatic hormones. Alkaline duodenal
secretions, bile, and pancreatic juice accomplish neutralization quickly. Once
neutrality and osmotic equilibrium have been established, digestion proceeds
rapidly.
    Pancreatic secretion. Pure pancreatic juice is a colorless, clear or
opalescent, rather viscous fluid, with a pH of 8.4 - 8.9. The inorganic
constituents include sodium, potassium, and bicarbonate ions; the bicarbonate
renders the juice alkaline. The organic constituents consist of various
proteins which contain the enzymes of the juice. The three main enzyme systems
in pancreatic secretions are proteolytic, amylolytic, and lipolytic. See
PANCREAS.
    Trypsin is the main proteolytic enzyme formed by the pancreas. It is
secreted in an inactive form, trypsinogen. Trypsinogen does not become active
until it enters the duodenum. This is a safety mechanism to prevent
autodigestion of the pancreas. Chymotrypsinogen is another proteolytic enzyme
that is secreted in an inactive form in the pancreatic juice. It is activated
by trypsin to form chymotrypsin. This enzyme clots milk and hydrolyzes casein
and gelatin.
    Carbohydrates are acted upon by pancreatic amylase, or amylopsin, which
acts like salivary amylase to hydrolyze starch first to dextrin and then to
maltose. A maltase capable of splitting maltose into glucose has been
identified in pure pancreatic juice.
    Fats are subjected to the action of pancreatic lipase, or steapsin, which
hydrolyzes neutral fat into glycerol and fatty acids. The digestion of fat is
enhanced considerably by the concomitant action of bile salts on the duodenal
contents. Bile salts lower the surface tension between water and neutral fat
so that the fat becomes emulsified into minute globules. Fat also aids its own
digestion. Liberated fatty acids unite with alkali in the bowel to form sodium
salts or soaps that have the same emulsifying action as the bile salts.
    Bile. Bile is an orange alkaline fluid. The principal components of bile
are bile salts, bile pigments (bilirubin and biliverdin), cholesterol, and
inorganic salts. The bile salts and cholesterol have a digestive function,
while the other components are waste products. The bile salts promote the
emulsification of fat in the intestine and also keep cholesterol and lecithin
in solution. Of the bile salts, 90% are reabsorbed in the small intestine and
remetabolized by the body. See BILE ACID; BILIRUBIN.
    Absorption. The average daily intake of protein in humans is about 1.5 oz
(45 grams). This is broken down and absorbed completely. Gastrointestinal
secretions and desquamated intestinal cells are important sources of
intestinal protein and can exceed the daily oral intake in amount. The small
intestine receives proteins after they have been broken down in the duodenum
to amino acids and polypeptides. Products of the digestion of protein are
absorbed in the first part of the jejunum under normal conditions. See AMINO
ACIDS; PROTEIN METABOLISM.
    The initial splitting of carbohydrates into disaccharides occurs in the
upper gastrointestinal tract. More amylase is added to the duodenal contents
by the pancreas. The disaccharides maltose, lactose, and sucrose are the
products of carbohydrate digestion which enter the small intestine. Further
digestion occurs in the columnar cells of the intestine; maltose is broken
down to glucose, sucrose becomes glucose and fructose, and lactose is split to
glucose and galactose. The sugars are absorbed rapidly and completely in the
first part of the jejunum. See CARBOHYDRATE METABOLISM.
    The absorption of dietary fat is almost complete, and very small amounts
appear in the feces. Fat is absorbed mainly in the terminal part of the
duodenum and upper jejunum, but the reserve capacity of the intestine to
absorb fat is considerable. Pancreatic lipase hydrolyzes fat in the intestine
first to diglycerides and then to glycerol and free fatty acids.
    The gastrointestinal membrance plays little part in the normal control of
the body's composition of fluid and electrolytes, even though the daily
transfer across the intestinal membrane is very great. Gastric secretion,
bile, pancreatic juice, and intestinal secretions provide 2 gal. (8 liters) of
fluid for the small intestine to deal with daily, yet only 6 in. 100
milliliters of fluid is actually excreted in the feces each day.
    Electrolytes pass both ways across the intestinal membrane in health, and
the rate of this transfer can be demonstrated easily by isotopes.
 �34s[Ivan D. A. Johnston]
�33s

    Colon.  The colon or large intestine may be described as a reservoir where
water and food material that have escaped digestion and absorption in the
small intestine may be stored and concentrated until the time for elimination.
Although there is no evidence of any secretion of digestive enzymes by the
colon, those enzymes that pass from the small intestine with the undigested
food materials into the colon continue digestion for a time.
    The colon and cecum of all mammals provide an excellent environment for
the growth and development of bacteria. These bacteria act effectively on
protein and carbohydrates that have escaped digestion and produce a number of
volatile compounds and gases that may be either absorbed or excreted. They can
also synthesize certain vitamins, especially the B vitamins.
    The principal secretion of the cells of the walls of the cecum, colon, and
rectum is mucin, which lubricates the accumulating feces and thus counteracts
the loss of water that is absorbed from the colon. The normal water content of
the passed feces varies considerably in different animals. Likewise, the
volume and weight of feces passed per day vary from animal to animal and are
related to the amount of undigestible cellulose consumed. The composition of
fecal material varies with the nature of the diet consumed and the
completeness of the digestion and absorption in the small intestine. Such
substances as mucin, live and dead bacteria, and sloughed-off epithelial cells
make up a considerable percentage of the feces.
    For effective absorption of water and certain end products of digestion,
it is necessary that the contents be well churned, squeezed, and moved to the
walls of the colon and cecum. Following this, the concentrated material must
be propelled down toward the rectum where it can be temporarily stored and
finally expelled.
 �34s[Joseph Callaway]
�33s

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�6v�2v�32sDigital computer   �21m �33s 
    Any device for performing mathematical calculations on numbers represented 
digitally; by extension, any device for manipulating symbols according to a 
detailed procedure or recipe. 
    This class of digital computers includes microcomputers, conventional 
adding machines and calculators, digital controllers used for industrial 
processing and manufacturing operations, store-and-forward data communication 
equipment, and electronic data-processing systems. See CALCULATORS; COMPUTER. 
    In this article emphasis is on electronic stored-program digital 
computers. These machines store internally many thousands of numbers or other 
items of information, and control and execute complicated sequences of 
numerical calculations and other manipulations on this information in 
accordance with instructions also stored in the machine. 
    A digital system can be considered from many points of view. At the lowest 
level it is a network of wires and mechanical parts whose voltages and 
positions convey coded information. At another level it is a collection of 
logical elements, each of which embodies certain rules, but which in 
combination can carry out very complex functions. At a still higher level, a 
digital system is an arrangement of functional units or building blocks which 
read (input), write (output), store, and manipulate information. 
  
    Codes.  Numbers are represented within a digital computer by means of 
circuits that distinguish various discrete electrical signals on wires inside 
the machine. In practice, the most reliable and economical circuit elements 
distinguish between only two signal levels. These two-valued signals make it 
necessary to represent numbers and symbols using a corresponding base-two or 
binary system. 
    Data are stored and manipulated within a digital computer in units called 
words. The binary digits (called bits), which make up a word, may represent 
either a binary number or a collection of binary-coded alphanumeric characters. 
  
    Logical circuit elements.  Two kinds of logical circuits are used in the 
design and construction of digital computers: decision elements and memory 
elements. A typical decision element provides a binary output as a function of 
two or more binary inputs. The AND circuit, for example, has two inputs and an 
output which is 1 only when both inputs are 1. A memory element stores a 
single bit of information and is set to the 1 state or reset to the 0 state, 
depending on the signals on its input lines. And because such a circuit can be 
caused alternately to store 0's and 1's from time to time, a memory element is 
commonly called a flip-flop. These two basic logical elements are all that are 
required to construct the most elaborate and complex digital arithmetic and 
control circuits. 
  
  
    Physical components.  The actual physical components which were used to 
realize conceptual gates and flip-flops in some specific piece of equipment 
are dependent on the status of electronic technology at the time the equipment 
was designed. In the 1950s the earliest commercial computers used vacuum 
tubes, resistors, and capacitors as components. A flip-flop typically required 
a dozen or more such components in these first-generation computers. Between 
the late 1950s and middle 1960s, solid-state transistors and diodes replaced 
the vacuum tubes, and the resulting second-generation systems were 
considerably more reliable than their first-generation predecessors; they were 
also smaller and consumed less power. But the number of electronic components 
per conceptual logical component remained about the same - a dozen or more for 
a flip-flop. 
    Since the mid-1960s the integrated circuit (IC) has been the principal 
logical building block for digital systems. Integrated circuit technology has 
consistently improved, and typical large-scale-integration (LSI) integrated 
circuits now contain thousands of flip-flops and gates. See INTEGRATED 
CIRCUITS.  
  
    System building blocks.  On a completely different conceptual level, a 
digital computer can be regarded as being composed of functional, system 
building blocks, containing (among other things) subassemblies of the 
fundamental logical components. A computer viewed at this level may be 
described in an oversimplified fashion by the diagram of Fig. 1. The 
computation and control block (often called the central processing unit, or 
CPU) is constructed entirely of logical elements of the kind described above. 
The main memory, which may store from a few thousand to several million binary 
digits, and the input/output and auxiliary memory devices (the socalled 
peripheral equipment) are specialized devices available over a range of speeds 
and operating characteristics. 
    Main memory is a building block capable of storing data or instructions in 
bulk for use by the computation and control portion of the computer. The 
important characteristics of a memory are capacity, access time, and cost. 
Starting in the early 1970s, the integrated circuit memory was introduced, and 
is now the most widely used technology. See COMPUTER STORAGE TECHNOLOGY. 
    Input/output and auxiliary memory peripherals represent the other major 
computer building blocks. Equipment is now available, and has been so from the 
beginning, for feeding information to the computer from paper tape and punched 
cards, and for receiving data from the computer and printing it, or punching 
it on tape or cards. But in the intervening years, designers have provided 
additional output devices which record computer data on microfilm, or plot 
data on graphs, or use data to control physical devices such as valves or 
rheostats. They have also designed input equipment which feeds the computer 
data from laboratory instruments, and from devices which scan documents and 
``read'' printed characters. Data can be transmitted to and from the computer 
over ordinary telephone lines, and a wide variety of devices, generally called 
terminals, make it possible for people to send data to, or receive requested 
data from, a computer system located hundreds or thousands of miles away. See 
CHARACTER RECOGNITION; COMPUTER GRAPHICS; DATA COMMUNICATIONS. 
  
    Stored program computer.  The concept of the stored-program computer is 
simple and can be described with reference to Fig. 1. Main memory contains, in 
addition to data and the results of intermediate computations, a set of 
instructions (or orders, or commands, as they are sometimes called); these 
specify how the computer is to operate in solving some particular problem. The 
computation and control section reads these instructions from the memory one 
by one and performs the indicated operations on the specified data. The 
instructions can control the reading of data from input or auxiliary memory 
peripherals, and (when the prescribed computations are completed) can send the 
result to auxiliary memory, or to output devices where it may be printed, 
punched, displayed, plotted, and so forth. The feature that gives this form of 
computer organization its great power is the ease with which instructions can 
be changed; the particular calculations carried out by the computer are 
determined entirely by a sequence of instructions stored in the computer's 
memory; that sequence can be altered completely by reading a new set of 
instructions into the memory through the computer input equipment. 
 �34s[Montgomery Phister, Jr.]
�33s 
    Each word in storage is assigned an address in order to refer to it, in a 
manner analogous to postal addresses. Each instruction contains two basic 
parts: (1) a coded number that dictates what operation is to be performed, for 
example, ADD, MULTIPLY, and STORE; and (2) the address of the word (data) on 
which to perform that operation. 
    The stored-program computer gains additional power from two additional 
concepts. The first concept is derived naturally in that the words of 
information referenced by an instruction can be other instruction words; thus, 
a computer can manipulate its own instructions and alter them dynamically, 
particularly in their address portions, during the execution of a program. The 
second concept concerns the ability to branch, based on the condition of the 
accumulator at any given moment. Basically, the machine can interrogate the 
accumulator and, on the basis of its contents being negative, zero, or 
positive, call for operating on the instruction counter so that the next 
executed instruction is not at the location that is one more than the last 
executed location, but is any other desired location. This branch of control 
may also be called for arbitrarily, that is, not based on the condition of the 
accumulator. For the available branch instructions the address portion is then 
the location of the next instruction rather than that of a data word. 
    The logic of any problem situation can be expressed graphically in a flow 
chart. For the problem of summing the contents of 10,000 words, the flow chart 
of Fig. 2 applies. This is an example of the programmed loop technique. 
    Programmers prefer to work in languages that are at a higher level than 
the machine language. The use of mnemonic operation codes and the replacement 
of all absolute machine addresses by symbols greatly speed up the work of the 
programmer at the modest cost of an extra computer run whose sole function is 
the translation back to the machine language. The translating program is 
called an assembler, and the language used is assembly language. Generally, 
assembly language follows the format of the machine language and is 
one-for-one with it; that is, for every instruction written in assembly 
language, one machine language instruction will be produced. 
    Most programming is done in the language of compilers, for example, 
FORTRAN or COBOL. Compiler language permits a format that fits the problems 
rather than the machine, and is many-to-one. As with assemblers, a separate 
machine run is needed to translate from the compiler language to the language 
of the machine; the translating program is called a compiler. 
    The looping technique shown in Fig. 2 is one of the two basic building 
blocks of the programmer. The other is the closed subroutine. Both techniques 
have the purpose of allowing a small number of instructions to be executed 
many times. The instructions of this program can be stored once, and simple 
arrangements can be made in the main program to cause their linkage and return 
at each of the points at which the operation is needed. 
 �34s[Fred J. Gruenberger]
�33s 
  
    Computer characteristics.  A computer installation is complex. 
Consequently it is difficult to describe a system or to compare the 
characteristics of two systems without listing their instruction types and 
describing their modes of operation at some length. Nevertheless, certain 
important descriptors are commonly used for comparison purposes and are shown 
in the table, where salient characteristics of two typical systems are shown. 
Memory cycle time is the time required to read a word from main memory. Add 
time is the time required to perform an addition, including the time necessary 
to extract the addition instruction itself and the operand from memory. 
    Evolution of capabilities.  The process by which new circuit and 
peripheral equipment technologies led to the development of a series of 
generations of computers was discussed above. But simultaneous with the 
changes in technology, there came changes in the structure or architecture of 
computers. These changes were introduced to improve the capability and 
efficiency of systems, as designers came to understand how computers were 
actually used. 
    In the first generation of computer equipment only one activity could be 
carried out at a time. Between jobs the computer was idle while an operator 
made ready for the next task. When the operator was ready, the program was 
read into the computer from some input device and the input data were then 
loaded. The program operated upon the data and performed necessary 
calculations. When the calculations were complete, the computer printed out 
answers, and the operator took steps to set up the next problem. 
  
    This series of operations was inefficient, and the designers of 
second-generation equipment removed some of the inefficiency by arranging 
input and output operations to be performed directly between the input/output 
peripherals and the computer memory without interfering with computations. As 
a result, second-generation computers were able to perform computations while 
reading in data and printing out replies, and efficiency was greatly enhanced. 
    First-generation equipment was most efficient while performing tedious and 
lengthy computations. The input/output capabilities of the second generation 
made them useful in applications where large volumes of data had to be handled 
with relatively little computation - applications such as billing, payroll, 
and inventory control. At the same time, the great capability and increased 
reliability of second-generation systems encouraged engineers to apply them to 
situations where the computer acts as a control element. 
    In the mid-1960s a new set of trends in computer applications was becoming 
apparent, and a third generation of computer systems became available to cope 
with those trends. The usefulness and flexibility of the stored-program 
computer, together with its improved cost-performance ratio, made it apparent 
that the computer had the basic power to perform a great variety of small and 
large tasks simultaneously. A computer system which serves a number of users 
in this way is called a time-sharing system. To perform in such complex 
applications, third-generation computers required elaborations of the features 
found in second-generation computers. 
    The operating system is the set of supervisory programs which manages the 
system, keeps it operating efficiently, and takes over many of the scheduling 
and monitoring functions which had previously been the function of the 
computer operator. 
    Computer memory has become increasingly important with the passing 
generations, as users have found it useful to store more and more business, 
government, engineering, and scientific data in machine-readable form. 
Starting with the third generation of computers, but increasingly with 
computers introduced in the early 1970s, computer-system architects have 
provided a hierarchy of memory devices to improve system performance and give 
the user access to very large memory capacities. 
    In all these hierarchical arrangements, the guiding principle is that the 
larger, slower, cheaper memory supplies data as needed to the smaller, faster, 
more expensive memory. The rious levels of memory are invisible to the user; 
the hardware and, where necessary, the operating system make the various 
hierarchical levels deliver data and programs to the user without the 
necessity for any special action on his or her part. 
    The versatility of the digital computer has led to its application in a 
wide variety of industries and activities. The evolution of the integrated 
circuit has made it possible for designers to provide even cheaper systems, 
and as a result the computer has become economical for use even in small 
organizations. The development of the microcomputer - the stored-program 
computer on an integrated-circuit chip - has made possible the introduction of 
the personal computer, which can supply entertainment, educational, 
record-keeping, and computing capabilities in the home at a relatively low 
cost. See MICROCOMPUTER; MICROPROCESSOR. 
 �34s[Michael Perch]
�33s 
  
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�6v�2v�32sDigital control   �21m �33s 
    The use of digital or discrete technology to maintain conditions in 
operating systems as close as possible to desired values despite changes in 
the operating environment. Traditionally, control systems have utilized analog 
components, that is, controllers which generate time-continuous outputs 
(volts, pressure, and so forth) to manipulate process inputs and which operate 
on continuous signals from instrumentation measuring process variables 
(position, temperature, and so forth). In the 1970s, the use of discrete or 
logical control elements, such as fluidic components, and the use of 
programmable logic controllers to automate machining, manufacturing, and 
production facilities became widespread. In parallel with these developments 
has been the accelerating use of digital computers in industrial and 
commercial applications areas, both for logic-level control and for replacing 
analog control systems. The development of inexpensive mini- and 
microcomputers with arithmetic and logical capability orders of magnitude 
beyond that obtainable with analog and discrete digital control elements has 
resulted in the rapid substitution of conventional control systems by digital 
computer - based ones. With the introduction of microcomputer-based control 
systems into major consumer products areas (such as automobiles and video and 
audio electronics), it is clear that the digital computer will be widely used 
to control objects ranging from small, personal appliances and games up to 
large, commercial manufacturing and production facilities. See MICROCOMPUTER; 
PROGRAMMABLE CONTROLLERS. 
    The object that is controlled is usually called a device or, more 
inclusively, process. A characteristic of any digital control system is the 
need for a process interface to mate the digital computer and process, to 
permit them to pass information back and forth. 
    Measurements of the state of the process often are obtained naturally as 
one of two switch states; for example, a part to be machined is in position 
(or not), or a temperature is above (or below) the desired temperature. 
Control signals sent to the process often are expressed as one of two states 
as well; for example, a motor is turned on (or off), or a valve is opened (or 
closed). Such binary information can be communicated naturally to and from the 
computer, where it is manipulated in binary form. For this reason the binary 
or digital computer/process interface usually is quite simple. 
    Process information also must be dealt with in analog form; for example, a 
variable such as temperature can take on any value within its measured range, 
or, looked at conceptually, it can be measured to any number of significant 
figures by a suitable instrument. Furthermore, analog variables generally 
change continuously in time. Digital computers are not suited to handle 
arbitrarily precise or continuously changing information; hence, analog 
process signals must be reduced to a digital representation (discretized), 
both in terms of magnitude and in time, to put them into a useful digital form. 
    The magnitude discretization problem most often is handled by transducing 
and scaling each measured variable to a common range, then using a single 
conversion device - the analog-to-digital converter (ADC) - to put the 
measured value into digital form. See ANALOG-TO-DIGITAL CONVERTER. 
    Discretization in time requires the computer to sample the signal 
periodically, storing the results in memory. This sequence of discrete values 
yields a ``staircase'' approximation to the original signal, on which control 
of the process must be based. Obviously, the accuracy of the representation 
can be improved by sampling more often, and many digital systems simply have 
incorporated traditional analog control algorithms along with rapid sampling. 
However, newer control techniques make fundamental use of the discrete nature 
of computer input and output signals. Analog outputs from a computer most 
often are obtained from a digital-to-analog converter (DAC), a device which 
accepts a digital output from the computer, converts it to a voltage in 
several microseconds, and latches (holds) the value until the next output is 
converted. Usually a single DAC is used for each output signal. See 
DIGITAL-TO-ANALOG CONVERTER. 
    In order to be used as the heart of a control system, a digital computer 
must be capable of operating in real time. Except for very simple 
microcomputer applications, this feature implies that the machine must be 
capable of handling interrupts, that is, inputs to the computer's internal 
control unit which, on change of state, cause the computer to stop executing 
some section of program code and begin executing some other section. The 
ability to initiate operations on schedule and to respond to process 
interrupts in a timely fashion is the very basis of real-time computing; this 
feature must be available in any digital control system. 
    Computer control systems for large or complex processes may involve 
complicated programs with many thousands of computer instructions. Several 
routes have been taken to mitigate the difficulty of programming control 
computers. One approach is to develop a single program which utilizes data 
supplied by the user to specify both the actions to be performed on the 
individual process elements and the schedule to be followed. Another approach 
is to develop a rather sophisticated operating system to supervise the 
execution of user programs, scheduling individual program elements for 
execution as specified by the user or needed by the process. See DIGITAL 
COMPUTER.  
    Many applications, particularly machining, manufacturing, and batch 
processing, involve large or complex operating schedules. Invariably, these 
can be broken down into simple logical sequences. Some applications - in the 
chemical process industries, in power generation, and in aerospace areas - 
require the use of traditional automatic control algorithms. 
    Attempts to expand the digital control medium through development of 
strictly digital control algorithms is an important and continuing trend. Such 
algorithms typically attempt to exploit the sampled nature of process inputs 
and outputs, significantly decreasing the sampling requirements of the 
algorithm. See CONTROL SYSTEMS. 
 �34s[Duncan A. Mellichamp]
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�6v�2v�32sDigital counter   �21m �33s 
    An instrument which, in its simplest form, provides an output that 
corresponds to the number of pulses applied to its input. Counters may be 
categorized into two types: the Moore machine or the Mealy machine. The 
simpler counter type, the Moore machine, has a single count input (also called 
the clock input or pulse input), while the Mealy machine has additional inputs 
that alter the count sequence. Digital counters take many forms, including 
geared mechanisms (tape counters and odometers are examples), relays (old 
pinball machines and old telephone switching systems), vacuum tubes (old test 
equipment), and solid-state semiconductor circuits (most modern electronic 
counters).  
    Most digital counters operate in the binary number system, since binary is 
easily implemented with electronic circuitry. Binary allows any integer (whole 
number) to be represented as a series of binary digits, or bits, where each 
bit is either a 0 or 1 (off or on, low or high, and so forth). See NUMBER 
SYSTEMS.  
    Digital counters are found in much modern electronic equipment, especially 
equipment that is digitally controlled or has digital numeric displays. A 
frequency counter, as a test instrument or a channel frequency display on a 
radio tuner, consists simply of a string of decade counters that count the 
pulses of an input signal for a known period of time, and display that count 
on a seven-segment display. A digital voltmeter operates by using nearly the 
same idea, except that the counter counts a known frequency for a period of 
time proportional to the input voltage. See ANALOG-TO-DIGITAL CONVERTER; 
DIGITAL VOLTMETER. 
    A digital watch contains numerous counter/dividers in its large-scale 
integration (LSI) chip, usually implemented with complementary metal oxide 
semiconductor (CMOS) technology. 
    Digital computers may contain counters in the form of programmable 
interval timers that count an integral number of clock pulses of known period, 
and then generate an output at the end of the count to signal that the time 
period has expired. Most of the counters in a microprocessor consist of 
arithmetic logic units that add one many-bit number to another, storing the 
results in a memory location. The program and data counters are examples of 
this kind of counter. See DIGITAL COMPUTER; MICROPROCESSOR. 
    Counters have progressed from relays to light-wavelength-geometry 
very-large-scale-integrated circuits. There are several technologies for 
building individual digital counters. Single counters are available as 
integrated circuit chips in emittercoupled logic, transistor-transistor logic, 
and CMOS. 
 �34s[Michael E. Wright]
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�6v�2v�32sDigitalis   �21m �33s 
    A genus of the figwort family (Scrophulariaceae) ranging from the Canary 
Islands to central Asia. Foxglove (Digitalis purpurea), a native of western 
Europe, is the source of the important drug digitalis, much used in the 
treatment of heart disorders. The active ingredient, digitalin, slows and 
regulates the heartbeat, improving the tone and rhythm, and making the 
contractions more effective. See SCROPHULARIALES. 
 �34s[Perry D. Strausbaugh /Earl L.]
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�6v�2v�32sDigital recording   �21m �33s 
    The application of digital techniques to the recording and reproduction of 
music on disk records. During recording, the output of the microphone 
amplifier is converted to a stream of digital bits. Typically the sampling 
rate lies between 44,000 and 55,000 samples per second with 16 bits per 
sample. The digitized program is recorded on magnetic tape. Special encoding 
schemes involving the insertion of additional bits permit correction in 
playback of errors due to loss or misinterpretation of bits. Thus the digital 
signal may be rerecorded through many generations without deterioration of 
quality. Editing and mixing are done with the signals in digital form. See 
ANALOG-TO-DIGITAL CONVERTER. 
    After editing and mixing, the digital signals are transferred from the 
tape to a disk rotating on the disk-recorder turntable. The recorded signal is 
carried in the form of microscopic indentations in a spiral track on the 
surface of the disk. Typically the indentations are less than 1 micrometer 
across and 0.15 micrometer deep. The disk format and the recording techniques 
are those developed for video-disk recording of television signals. The master 
disk is used to form stampers for pressing vinyl records following the same 
processes used in the production of conventional phonograph records. 
    Players for the digital records resemble, and in some cases may be 
identical to, video-disk players. Depending on the record format offered by 
the manufacturers, the pickup in the player may be a capacitive, mechanical, 
or optical type. The digital output of the pickup and player circuits is 
error-corrected and stored temporarily in a memory. It is removed from the 
memory at a bit rate controlled by a precise quartz crystal, and is converted 
to analog audio signals to drive loudspeakers. Because the output bit rate is 
precisely controlled, any effects due to small fluctuations in speed of the 
tape recorders or of the disk turntables are removed from the analog output. 
See DIGITAL-TO-ANALOG CONVERTER. 
 �34s[J. Guy Woodward]
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�6v�2v�32sDigital-to-analog converter   �21m �33s 
    A device for converting information in the form of combinations of 
discrete states of a signal, often representing binary number values, to 
information in the form of the value or magnitude of some characteristics of a 
signal, in relation to a standard or reference. Most often, it is a device 
which has electrical inputs representing a parallel binary number, and an 
output in the form of voltage or current. 
    Digital-to-analog (D/A) converters (sometimes called DACs) are used to 
present the results of digital computation, storage, or transmission, 
typically for graphical display or for the control of devices that operate 
with continuously varying quantities. D/A converter circuits are also used in 
the design of analog-to-digital converters that employ feedback techniques, 
such as successive-approximation and counter-comparator types. In such 
applications, the D/A converter may not necessarily appear as a separately 
identifiable entity. See COMPUTER GRAPHICS; DATA COMMUNICATIONS; DIGITAL 
COMPUTER.  
    The fundamental circuit of most D/A converters involves a voltage or 
current reference; a resistive ``ladder network'' that derives weighted 
currents or voltages, usually as discrete fractions of the reference; and a 
set of switches, operated by the digital input, that determine which currents 
or voltages will be summed to constitute the output. 
    The output of the D/A converter is proportional to the product of the 
digital input value and the reference. In many applications, the reference is 
fixed, and the output bears a fixed proportion to the digital input. In other 
applications, the reference, as well as the digital input, can vary; a D/A 
converter that is used in these applications is thus called a multiplying DAC. 
It is principally used for imparting a digitally controlled scale factor, or 
``gain,'' to an analog input signal applied at the reference terminal. See 
AMPLIFIER; ANALOG COMPUTER. 
 �34s[Daniel H. Sheingold]
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�6v�2v�32sDigital voltmeter   �21m �33s 
    An instrument for measuring potential difference and displaying the 
measurement directly in digital form. Digital voltmeters are used not only for 
their accuracy but also for their ease and speed of reading and their relative 
freedom from the operator error always present in reading moving-pointer 
voltmeters. Digital voltmeters are also widely employed in digital systems. 
See NUMERICAL INDICATOR TUBE; VOLTAGE MEASUREMENT. 
 �34s[Malcolm C. Holtje]
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�6v�2v�32sDihedron   �21m �33s 
    A geometric figure formed by two half planes that are bounded by the same 
straight line. This line is called the edge of the dihedron. Planes 
perpendicular to the edge cut the dihedron in equal plane angles AOB and 
A'O'B' whose common measure is called the dihedral angle of the dihedron (see 
illustration). See ANGLE. 
 �34s[James S. Frame]
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�6v�2v�32sDill �21m �33s
    A small annual or biennial herb, Anethum graveolens (Umbelliferae), of
high repute among ancient peoples and now cultivated in Europe, India, and the
United States. In the United States dill is used mainly as a flavoring for
pickles. In France, India, and other countries it is used in soups and sauces,
stews, and other dishes. Both the fruits and oil are used in medicine. See
UMBELLALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sDilleniales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), 
subclass Dilleniidae, class Magnoliopsida (dicotyledons). It consists of 3 
families: the Dilleniaceae, the Paeoniaceae, and Crossosomataceae. Within its 
subclass the order is marked by separate carpels; numerous stamens; and seeds 
that have a well developed endosperm and are provided with an aril (a partial 
or complete covering or appendage, usually derived from the funiculus of the 
ovule). The garden peony (Paeonia lactiflora) is a familiar member of the 
order. See DILLENIIDAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sDilleniidae   �21m �33s 
    A large subclass of the class Magnoliopsida (dicotyledons) of the division 
Magnoliophyta (Angiospermae), the flowering plants, consisting of 12 orders, 
69 families, and nearly 24,000 species. The subclass is morphologically 
ill-defined, but most of the orders and species have the carpels united to 
form a compound pistil. The petals are either separate or joined into a 
sympetalous corolla. The stamens, when numerous, are initiated in centrifugal 
sequence. Many of the species have numerous ovules on parietal placentas; that 
is, the placentas are borne along the walls of an ovary which is usually with 
a single chamber. 
    The largest orders included in the subclass are the Violales, Capparales, 
Malvales, Theales, and Ericales. Other orders include the Dilleniales, 
Lecythidales, Sarraceniales, Salicales, Diapensiales, Ebenales, and 
Primulales. See separate articles on each order. See MAGNOLIOPHYTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sDimensional analysis �21m �33s
    A technique that involves the study of dimensions of physical quantities.
Dimensional analysis is used primarily as a tool for obtaining information
about physical systems too complicated for full mathematical solutions to be
feasible. It enables one to predict the behavior of large systems from a study
of small-scale models. It affords a convenient means of checking mathematical
equations. Finally, dimensional formulas provide a useful cataloging system
for physical quantities.
    All the commonly used systems of units in physical science have the
property that the number representing the magnitude of any quantity (other
than purely numerical ratios) varies inversely with the size of the unit
chosen. Thus, if the length of a given piece of land is 300 ft, its length in
yards is 100. The ratio of the magnitude of 1 yd to the magnitude of 1 ft is
the same as that of any length in feet to the same length in yards, that is,
3. The ratio of two different lengths measured in yards is the same as the
ratio of the same two lengths measured in feet, inches, miles, or any other
length units. This universal property of unit systems, often known as the
absolute significance of relative magnitude, determines the structure of all
dimensional formulas. See UNITS OF MEASUREMENT.
    In defining a system of units for a branch of science such as mechanics or
electricity, certain quantities are chosen as fundamental and others as
secondary, or derived. The choice of the fundamental units is always arbitrary
and is usually made on the basis of convenience in maintaining standards. In
mechanics the fundamental units most often chosen are mass, length, and time.
    It can be proved that every secondary quantity which satisfies the
condition of the absolute significance of relative magnitude is expressible as
a product of powers of the primary quantities. Such an expression is known as
the dimensional formula of the secondary quantity. There is no requirement
that the exponents be integral.
    The technique of dimensional analysis has several important applications.
It is intuitively obvious that only terms whose dimensions are the same can be
equated. The equation 10 kg = 10 m/s, for example, makes no sense. A necessary
condition for the correctness of any equation is that the two sides have the
same dimensions. This is often a help in the verification of complicated
analytic expressions. Of course, an equation can be correct dimensionally and
still be wrong by a purely numerical factor.
    The application of dimensional analysis to the derivation of unknown
relations depends upon the concept of completeness of equations. An expression
which remains formally true no matter how the sizes of the fundamental units
are changed is said to be complete. Assume a group of n physical quantities
x1, x2, . . . , xn, for which there exists one and only one complete
mathematical expression connecting them, namely, PHI(x1, x2, . . . , xn) = 0.
Some of the quantities x1, x2, . . . , xn may be dimensional constants.
Assume further that the dimensional formulas of the n quantities are expressed
in terms of m fundamental quantities ALPHA, BETA, GAMMA, . . . . Then it will
always be found that this single relation PHI can be expressed in terms of
some arbitrary function F of n - m independent dimensionless products PI 1,
PI 2, . . . , PI (n - m), made up from among the n variables, as in the
equation below. This is known as the PI theorem. It was first rigorously proved

	*** F(PI sub 1, PI sub 2, . . . , PI sub [n - m]) = 0 ***

by E. Buckingham. The main usefulness of the PI theorem is in the deduction of
the form of unknown relations. The procedure is particularly useful in
hydraulics and aeronautical engineering, where detailed solutions are often
extremely complicated.
    A further application of dimensional analysis is in model design. Often
the behavior of large complex systems can be deduced from studies of small-
scale models at a great saving in cost. In the model each parameter is
reduced in the same proportion relative to is value in the original system.
See MODEL THEORY.
    Dimensional formulas also provide a convenient shorthand notation for
representing the definitions of secondary quantities and are helpful in
changing units from one system to another. See DYNAMICAL ANALOGIES.
 �34s[John W. Stewart]
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�6v�2v�32sDimensioning   �21m �33s 
    Assigning of dimensions on a mechanical drawing. A dimension on a drawing 
is a labeled measure in a straight line of the breadth, height, or thickness 
of a part, the angular position of a line, or the location of a detail such as 
a hole or boss. Dimensions specify the size and shape of the part as required 
by the designer and aid the worker in constructing the part. 
    Two plans of dimensioning are used on mechanical drawings, as illustrated. 
One is called point-to-point or chain dimensioning; each length is dimensioned 
from the end of the preceding one. These dimensions are usually taken directly 
from the designer's sketch; however tolerances are cumulative and, rather than 
averaging out, usually add or subtract so that intermediate clearances may be 
disturbed. In the other plan, called datum dimensioning or the reference-line 
method, all dimension lines in each direction extend from a datum or reference 
line or plane which is usually the first machined surface. Here, tolerances 
are not cumulative and a mating part will be within the specified limits.  See 
  DRAFTING; TOLERANCE. 
 �34s[Paul H. Black]
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�6v�2v�32sDimensionless groups �21m �33s
    A dimensionless group is any combination of dimensional or dimensionless
quantities possessing zero overall dimensions. Dimensionless groups are
frequently encountered in engineering studies of complicated processes or as
similarity criteria in model studies. A typical dimensionless group is the
Reynolds number (a dynamic similarity criterion), NsubRsube = VDRHO/MU. Since
the dimensions of the quantities involved are velocity V: [L//THETA];
characteristic dimension D: [L]; density RHO: [M/Lsup3]; and viscosity MU:
[M/LTHETA] (with M, L, and THETA as the fundamental units of mass, length, and
time), the Reynolds number reduces to a dimensionless group and can be
represented by a pure number in any coherent system of units. See DYNAMIC
SIMILARITY.
    Many important problems in applied science and engineering are too
complicated to permit completely theoretical solutions to be found. However,
the number of interrelated variables involved can be reduced by carrying out a
dimensional analysis to group the variables as dimensionless groups. See
DIMENSIONAL ANALYSIS.
    The advantages of using dimensionless groups in studying complicated
phenomena include:
    1. A significant reduction in the number of ``variables'' to be
investigated; that is, each dimensionless group, containing several physical
variables, may be treated as a single compound ``variable,'' thereby reducing
the number of experiments needed as well as the time required to correlate and
interpret the experimental data.
    2. Predicting the effect of changing one of the individual variables in a
process (which it may be impossible to vary much in available equipment) by
determining the effect of varying the dimensionless group containing this
parameter (this must be done with some caution, however).
    3. Making the results independent of the scale of the system and of the
system of units being used.
    4. Simplifying the scaling-up or scaling-down of results obtained with
models of systems by generalizing the conditions which must exist for
similarity between a system and its model.
    5. Deducing variation in importance of mechanisms in a process from the
numerical values of the dimensionless groups involved; for instance, an
increase in the Reynolds number in a flow process indicates that molecular
(viscous) transfer mechanisms will be less important relative to transfer by
bulk flow (``inertia'' effects), since the Reynolds number is known to
represent a measure of the ratio of inertia forces to viscous forces. See
FROUDE NUMBER; KNUDSEN NUMBER; MACH NUMBER; REYNOLDS NUMBER; UNITS OF
MEASUREMENT.
 �34s[John Catchpole ;George D. Fulford]
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�6v�2v�32sDimensions (mechanics) �21m �33s
    Length, mass, time, or combinations of these quantities serving as an
indication of the nature of a physical quantity. Quantities with the same
dimensions can be expressed in the same units. For example, although speed can
be expressed in various units such as miles/hour, feet/second, and
meters/second, all these speed units involve the ratio of a length unit to a
time unit; hence, the dimensions of speed are the ratio of length L to time T,
usually stated as LT- sup 1. The dimensions of all mechanical quantities can be
expressed in terms of L, T, and mass M. The validity of algebraic equations
involving physical quantities can be tested by a process called dimensional
analysis; the terms on the two sides of any valid equation must have the same
dimensions. See DIMENSIONAL ANALYSIS; UNITS OF MEASUREMENT.
 �34s[Dudley Williams]
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�6v�2v�32sDimethyl sulfoxide   �21m �33s 
    A versatile solvent (formula C2H6OS) abbreviated DMSO, used industrially 
and in chemical laboratories as a medium for carrying out chemical reactions. 
Its uses have been extended to that of a chemical reagent where DMSO itself is 
involved in a chemical change. It is the simplest member of a class of organic 
compounds which are typified by the polar sulfur-oxygen bond represented in 
the resonance hybrid shown below. The molecule is pyramidal in shape with the 
oxygen and the carbons at the corners. 
    DMSO is a colorless, odorless (when pure), and very hygroscopic stable 
liquid (bp 189DGC, mp 19.5DGC). It is manufactured commercially by reacting 
the black liquor, from digestion in the kraft pulp process, with molten sulfur 
to form dimethyl sulfide which is oxidized with nitrogen tetroxide to yield 
DMSO. This highly polar aprotic solvent is water- and alcohol-miscible and 
will dissolve most polar organic compounds and many inorganic salts. 
    As an aprotic solvent, DMSO strongly solvates cations, leaving a highly 
reactive anion. Thus in DMSO, basicity and nucleophilicity is enhanced, and it 
is a superior solvent for many elimination, nucleophilic substitution, and 
solvolysis reactions in which nucleophile and base strength are important. 
Nucleophilic substitution reactions in which halogens or sulfonate esters are 
displaced by anions such as cyanide, alkoxide, thiocyanate, azide, and others 
are accelerated 1000 to 10,000 times in DMSO over the reaction in aqueous 
alcohol. Dimethyl sulfoxide is also superior to protic solvents as a media for 
elimination reactions. Its high dielectric constant makes DMSO a useful 
solvent for dissolving resins, polymers, and carbohydrates. It is employed 
commercially as a spinning solvent in the manufacture of synthetic fibers. 
    The apparent low toxicity and high skin permeability have led to extensive 
studies in numerous biological systems, including humans. Inorganic salts or 
small-molecular-weight organic compounds dissolved in DMSO can be transported 
across skin membrane, indicating a potential hazard in commercial use. 
 �34s[W. W. Epstein /F. W. Sweat]
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�6v�2v�32sDinocerata   �21m �33s 
    An extinct order of large herbivorous mammals, often called uintatheres, 
from early Cenozoic deposits of North America and northern Asia. Members of 
this group have semigraviportal limbs, that is, adapted to bearing 
considerable weights, with hoofed, five-toed feet (see illustration). A 
saberlike canine tooth and protective lower jaw flange are present in all 
forms except the aberrant Gobiatherium. Horns are absent or very small on the 
most primitive forms. Middle and late Eocene uintatheres in North America 
developed an imposing array of six horns. One pair was on the tips of the 
nasal bones, another was above the root of the saberlike canine tooth, and a 
third pair above the ear region. 
    The order Dinocerata consists of one family, the Uintatheriidae, which is 
divided into three subfamilies: Prodinoceratinae, Uintatheriinae, and 
Gobiatheriinae. The Dinocerata left no descendants. They are believed to have 
arisen from the arctocyonid condylarths but from a different subfamily than 
that which gave rise to the order Pantodonta. See CONDYLARTHRA; MAMMALIA; 
PANTODONTA.  
 �34s[Malcolm C. McKenna]
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�6v�2v�32sDinoflagellida   �21m �33s 
    An order of the class Phytamastigophorea; also known as the 
Dinoflagellata. Although primarily marine, some dinoflagellates occur in fresh 
water. Some possess brown chromatophores, some are variously colored, and 
others are colorless. All types of nutrition exist, and some species are 
parasitic. Two flagella emerge laterally from a longitudinal depression or 
sulcus. An encircling flagellum in a groove or girdle divides the body into 
epicone and hypocone; the other, extending backward, propels the organism 
forward. The nucleus is very large. 
    Ceratium hirundinella blooms in fresh water, causing tastes and odors. 
Marine blooms of Gymnodinium breve produce the fish-killing red tides along 
the Gulf Coast of the southern United States. See PHYTAMASTIGOPHOREA. 
 �34s[James B. Lackey]
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�6v�2v�32sDinornithiformes   �21m �33s 
    An order of birds, the extinct moas, confined to New Zealand, containing 
two families, with about 13 species assigned to six genera. Moas are known 
only from late Quaternary deposits on both the North Island and the South 
Island.  
    Moas ranged in height from about 1 to 3 m (3 to 10ft), and there probably 
was sexual dimorphism in size within species. All moas had heavy bodies, 
powerful legs with four-toed feet, and long necks with relatively small heads. 
The wings were completely absent, and the scapula, coracoid, and unkeeled 
sternum were greatly reduced. The feathers had loose webs and an aftershaft. 
No intact specimens of moas are known.  
    It is believed that the moas represent an adaptive radiation of huge 
herbivorous birds that evolved from a flying ancestor which became large and 
flightless only after reaching New Zealand sometime in the Tertiary Period. 
See APTERYGIFORMES; AVES; NEORNITHES. 
 �34s[David W. Steadman]
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�6v�2v�32sDinosaur �21m �33s
    The term Dinosauria, meaning ``terrible lizard,'' was coined in 1841 by
Sir Richard Owen in reference to huge fossil bones then being discovered in
England. These bones were recognized as reptilian, but of extinct species much
larger than any living reptile. The term is no longer used in a formal sense,
the dinosaurs now being classified in two separate orders, Saurischia and
Ornithischia, under the subclass Archosauria. The diagnostic distinction
between representatives of these two orders lies in the arrangement of the
three pelvic bones, either in a triradiate pattern (saurischian), as in modern
crocodilians and lizards, or in a tetraradiate arrangement (ornithischian),
superficially ``birdlike.'' The name dinosaur is obviously appropriate, for
among these reptiles were the largest land animals that ever lived. Some were
so large, in fact, that it has been theorized that they could not have
supported their great weight without the buoyant effect of water in swamps or
lakes. And it still is a great mystery as to how such giants could have
obtained adequate food to grow to, and to sustain, such enormous bulk. See
ARCHOSAURIA.
    Not all dinosaurs were gigantic, however. One variety, Compsognathus, was
no larger than a pigeon, and several others were chicken- or turkey-sized.
Dinosaurian habits and life styles can only be inferred from their skeletal
anatomy, but that evidence clearly indicates a wide variety of living habits,
many of which were quite different from those of modern animals. As a whole,
dinosaurs were almost universal in distribution, having been found on all
continents except Antarctica, and were dominant among land animals throughout
the Mesozoic Era, a time span of approximately 150,000,000 years. There were
many kinds, some of which apparently existed in great numbers. But at the end
of the Cretaceous, dinosaurs became extinct as a consequence of unknown, but
probably multiple, causes.
    Both dinosaurian orders are believed to have evolved from advanced members
of the Triassic order Thecodontia known as pseudosuchians, probably from two
separate kinds, but perhaps from a common ancestor. Some advanced
pseudosuchians resemble some of the later saurischians, with marked tendencies
toward upright posture and bipedal locomotion. This upright posture previews
one of the distinctive conditions of all dinosaurs, where the limbs are
largely or completely beneath the body, rather than sprawled out to the side
as in lizards, turtles, and crocodilians. Erect posture permits longer strides
and more rapid and efficient locomotion. Development of erect carriage was
accompanied in all dinosaurs by enlargement of the sacrum, strengthening the
attachment of the pelvic bones to the backbone, and rearrangement of the limb
skeleton and musculature. Most dinosaurs were quadrupedal, but the trend
toward bipedal gait in some pseudosuchians was perfected in certain
saurischians and ornithischians (see Figs. 1 - 4). In the ornithischians
(Ornithopoda), bipedality seems to have been facultative only, with a
four-legged pose still possible for activities like grazing. Bipedal
saurischians (Theropoda), however, became obligatory bipeds, losing all
potential for assuming a quadrupedal stance. Such highly perfected bipedality
suggests a high level of agility and activity - and perhaps a higher
physiologic level than is typical of modern reptiles. See ORNITHISCHIA;
SAURISCHIA; THECODONTIA.
    Because all dinosaurs possessed certain skeletal features which also occur
in all living reptiles, they have been classified as reptiles. Consequently,
it is natural that they have been interpreted as reptilian in their habits,
physiology, and levels of activity. This view of dinosaurs as cold-blooded,
sluggish, and generally inactive animals has been challenged, and various
lines of evidence have been cited as indicative that they were probably
warm-blooded and avianlike or mammallike in their metabolism and activity
levels.
    Because dinosaurs are classified as reptiles, it has generally been
assumed that they deposited eggs like modern crocodilians, turtles, and
lizards rather than giving live birth. The 1922 discovery in Mongolia of
clutches of eggs associated with skeletons of Protoceratops confirmed that
some dinosaurs, at least, were egg layers rather than live bearers. Several
other kinds of dinosaur eggs have been found elsewhere in Mongolia, France,
Brazil, Argentina, and western North America, indicating that most, if not
all, types of dinosaurs laid eggs. In 1978 a nest of a dozen or more very
young hadrosaurs, associated with eggshell fragments, was discovered in
central Montana. The fact that all the young had worn teeth shows that they
were not fresh hatchlings, yet they were all crowded together in what appears
to have been a nest. This suggests that they may have had some parental care.
    At the end of the Mesozoic Era, the dinosaurs and many other kinds of
reptiles, including the flying reptiles, marine mosasaurs, ichthyosaurs, and
plesiosaurs, became extinct. Various other forms of life also died out at
about the same time. The causes of such widespread extinction of such a large
number of different kinds of organisms, all at about the same time, are among
the most perplexing problems in paleontology. Inability to adapt to changing
environments is the usual cause of extinction. Under conditions of change,
those animals with the narrowest specializations are most apt to be wiped out.
Contrarily, broad adaptations to a variety of foods and habitats may enable
species to survive such changing conditions. See EXTINCTION (BIOLOGY).
    Extinction of the dinosaurs has been attributed to many causes, including
diseases, evolutionary senility, exploding supernovae and cosmic radiation,
egg-eating mammals, falling temperatures, rising mountains, drifting
continents, receding sea level, and rising temperatures. None appears adequate
to account for the extinction of such a wide variety of animal kinds, and at
the same time to explain why other kinds were little, or not, affected. It is
known that the end of the Mesozoic was a time of many changes, including
sea-level and climatic changes, mountain building, and continental drifting,
but no one of these phenomena alone is judged capable of eliminating the
dinosaurs. The most logical explanation is that dinosaurian extermination
resulted from a combination of factors, including some of those listed here as
well as others that are still unknown to scientists. See REPTILIA.
 �34s[John H. Ostrom]
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�6v�2v�32sDioctophymoidea   �21m �33s 
    A group of nematodes characterized by the peculiar structure of the 
copulatory bursa of the male. Various species are parasites of fish, aquatic 
birds, and mammals. This group is considered to constitute either an order or 
superfamily, according to the viewpoint of the specialist. 
    Dioctophyme renale, the so-called giant kidney worm, is the best known 
species in the group. It is a fairly common parasite of dogs and a wide 
variety of mammals, especially the mink. The adult worms are among the longest 
nematodes, the females measuring up to 1 m (3 ft). The size, blood-red color, 
and characteristic location in the kidney led to the common name. They live in 
the kidney for 1 - 3 years, digesting its substance until it is only a shell. 
The infection may be fatal but is often symptomless. 
    Other genera include Eustrongylides and Hyrstrichis, which inhabit the 
proventriculus of aquatic birds and are apparently transmitted by fish. 
Species of Soboliphyme are parasites in the intestine of foxes, cats, and the 
wolverine. See NEMATODA. 
 �34s[J. Allen Scott]
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�6v�2v�32sDiode   �21m �33s 
    A two-terminal electron device exhibiting a nonlinear current-voltage 
characteristic. Although diodes are usually classified with respect to the 
physical phenomena that give rise to their useful properties, in this article 
they are more conveniently classified according to the functions of the 
circuits in which they are used. This classification includes rectifier 
diodes, negative-resistance diodes, constant-voltage diodes, light-sensitive 
diodes, light-emitting diodes, and capacitor diodes. 
    A circuit element is said to rectify if voltage increments of equal 
magnitude but opposite sign applied to the element produce unequal current 
increments. An ideal rectifier diode is one that conducts fully in one 
direction (forward) and not at all in the opposite direction (reverse). This 
property is approximated in junction and thermionic diodes. Processes that 
make use of rectifier diodes include power rectification, detection, 
modulation, and switching. See RECTIFIER. 
    Negative-resistance diodes, which include tunnel and Gunn diodes, are used 
as the basis of pulse generators, bistable counting and storage circuits, and 
oscillators. See NEGATIVE-RESISTANCE CIRCUITS; OSCILLATOR; TUNNEL DIODE. 
    Breakdown-diode current increases very rapidly with voltage above the 
breakdown voltage; that is, the voltage is nearly independent of the current. 
In series with resistance to limit the current to a nondestructive value, 
breakdown diodes can therefore be used as a means of obtaining a nearly 
constant reference voltage or of maintaining a constant potential difference 
betwween two circuit points, such as the emitter and the base of a transistor. 
Breakdown diodes (or reverse-biased ordinary junction diodes) can be used 
between two circuit points in order to limit alternating-voltage amplitude or 
to clip voltage peaks. See LIMITER CIRCUIT. 
    Light-sensitive diodes, which include phototubes, photovoltaic cells, 
photodiodes, and photoconductive cells, are used in the measurement of 
illumination, in the control of lights or other electrical devices by incident 
light, and in the conversion of radiant energy into electrical energy. 
Light-emitting diodes (LEDs) are used in the display of letters, numbers, and 
other symbols in calculators, watches, clocks, and other electronic units. See 
LIGHT-EMITTING DIODE; PHOTOCONDUCTIVE CELL; PHOTODIODE; PHOTOELECTRIC DEVICES; 
PHOTOTUBE; PHOTOVOLTAIC CELL. 
    Semiconductor diodes designed to have strongly voltage-dependent shunt 
capacitance between the terminals are called varactors. The applications of 
varactors include the tuning and the frequency stabilization of 
radio-frequency oscillators. See JUNCTION DIODE; MICROWAVE SOLID-STATE 
DEVICES; POINT-CONTACT DIODE; SEMICONDUCTOR DIODE; SILICON DIODE; VARACTOR. 
 �34s[Herbert J. Reich]
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�6v�2v�32sDiopside   �21m �33s 
    The monoclinic pyroxene mineral which in pure form has the formula 
CaMgSi2O6. Pure diopside melts congruently at 1391DGC (2536DGF) at atmospheric 
pressure. Diopside has no known polymorphs. Its structure consist of chains of 
SiO4 tetrahedrons in which each silicon ion shares an oxygen with each of its 
two nearest silicon neighbors. These chains are linked together by Ca and Mg 
ions in octahedral coordination. 
    Diopside forms gray to white, short, stubby, prismatic, often 
equidimensional, crystals. Small amounts of iron impart a greenish color to 
the mineral. Pure diopside is common and occurs as a metamorphic alteration of 
impure dolomites in medium and high grades of metamorphism. 
    Natural diopsidic pyroxenes which show extensive solid solution with 
jadeite and to a lesser extent with acmite are called omphacite. Omphacite is 
a principal constituent of eclogites, rocks of basaltic composition which have 
formed at high pressure. See DOLOMITE; ECLOGITE; PYROXENE. 
 �34s[F. R. Boyd]
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�6v�2v�32sDiopter   �21m �33s 
    A measure of the power of a lens or a prism. The diopter (also called 
dioptrie) is usually abbreviated D. Its dimension is a reciprocal length, and 
its unit is the reciprocal of 1 m (3.28 ft). See FOCAL LENGTH; LENS (OPTICS). 
    The dioptric power of a prism is defined as the measure of the deviation 
of a ray going through a prism measured at the distance of 1 m. A prism that 
deviates a ray by 1 cm in a distance of 1 m is said to have a power of one 
prism diopter. See OPTICAL PRISM. 
    Spectacle lenses in general consist of thin lenses, which are either 
spherical, to correct the focus of the eye for near and far distances, or 
cylindrical or toric, to correct the astigmatism of the eye. An added prism 
corrects a deviation of the visual axis. The diopter thus gives a simple 
method for prescribing the necessary spectacle for the human eye. See 
EYEGLASSES.  
 �34s[Max J. Herzberger]
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�6v�2v�32sDiorite   �21m �33s 
    A phaneritic (visibly crystallized) plutonic rock having intermediate SiO2 
content (53 - 66%), composed mainly of plagioclase (oligoclase or andesine) 
and one or more ferromagnesian minerals (hornblende, biotite, or pyroxene), 
and having a granular texture. Diorite is the plutonic equivalent of andesite 
(a volcanic rock). This dark gray rock is used occasionally as a building 
stone and is known commercially as black granite. See IGNEOUS ROCKS. 
    Gray or white plagioclase feldspar is the dominant mineral. Rocks with 
more calcic plagioclases and more abundant ferromagnesian minerals are 
gabbros. Rocks with greater proportions of alkali feldspar are called 
monzonite. Those with more quartz are called quartz-diorite or tonalite. 
    The texture of diorites is notably variable. Most often diorites are 
equigranular, with coarse, partly or mostly anhedral plagioclase and 
hornblende crystals, subordinate biotite, and interstitial quartz and 
orthoclase.  
    Diorite is found as isolated small bodies such as dikes, sills, and 
stocks, but it is also found in association with other plutonic rocks in 
batholithic bodies. It is closely associated with convergent plate boundaries 
where calc-alkalic magmatism and mountain building are taking place. See MAGMA. 
 �34s[William Ingersoll Rose, Jr.]
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�6v�2v�32sDiphosphopyridine nucleotide (DPN)   �21m �33s 
    An organic coenzyme and one of the most important components of the 
enzymatic systems concerned with biological  oxidation-reduction reactions. It 
is also known as DPN, diphosphopyridine dinucleotide, coenzyme I, 
codehydrogenase I, and nicotinamide adenine dinucleotide (NAD). DPN is found 
in the tissues of all living organisms. See COENZYME. 
    The nicotinamide, or pyridine, portion of DPN can be reduced chemically or 
enzymatically with the formation of reduced or hydrogenated DPN (DPNH). DPN 
functions as the immediate oxidizing agent for the oxidation, or 
dehydrogenation, of various organic compounds in the presence of appropriate 
dehydrogenases, which are specific apoenzymes, or protein portions of the 
enzyme. In the dehydrogenase reactions one hydrogen atom is transferred from 
the substrate to DPN, while another is liberated as hydrogen ion. 
    DPN and its reduced form, DPNH, serve to couple oxidative and reductive 
processes and are constantly regenerated during metabolism. Hence, they serve 
as catalysts and DPN is referred to as a coenzyme. In some enzymatic reactions 
a different coenzyme, triphosphopyridine nucleotide is required. 
Dehydrogenases are generally quite specific with respect to the coenzyme which 
they can utilize. See BIOLOGICAL OXIDATION; ENZYME; TRIPHOSPHOPYRIDINE 
NUCLEOTIDE (TPN). 
 �34s[Michael Doudoroff]
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�6v�2v�32sDiphtheria   �21m �33s 
    A communicable disease of humans caused by the growth of the bacterium 
Corynebacterium diphtheriae on the mucous membranes or, less commonly, in 
cutaneous tissues. Both toxinogenic and nontoxinogenic strains of C. 
diphtheriae are capable of inducing the formation of a spreading, 
pseudomembranous exudate which may cause serious obstruction in the larynx and 
trachea. The classic pattern of the disease, caused only by the toxinogenic 
strains, involves the spread of diphtheria toxin to target tissues far from 
the superficial site of the infection. 
    Although diphtheria has been of worldwide occurrence, its incidence has 
declined sharply in countries where there has been a general immunization of 
children with diphtheria toxoid. The immune status of an individual, with 
respect to diphtheria toxin, may be determined through the use of the modified 
Schick test. Approximately 0.0008 microgram of toxin protein is injected into 
the skin of the forearm, the test site. An equal amount of toxoid is injected 
at a control site. Necrosis at the test site indicates nonimmune state, while 
immunity, the presence of circulating antibody, is indicated by a lack of 
reaction at either site. Immunity complicated by allergy to diphtherial 
proteins results in a delayed inflammatory reaction at both sites. See 
IMMUNITY; SKIN TEST. 
 �34s[Lane Barksdale]
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�6v�2v�32sDiphyllidea   �21m �33s 
    An order of tapeworms of the subclass Cestoda. Species of this order 
belong to a single genus and live in the intestine of elasmobranch fishes. The 
scolex has a large muscular rostellum armed with hooks and two fused pairs of 
suckers; the neck is armed with T-shaped hooks. Larval stages have been found 
parasitizing marine mollusks and crustaceans, but the life history is not 
known. See CESTODA. 
 �34s[Clark P. Read]
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�6v�2v�32sDiplomonadida   �21m �33s 
    An order of the class Zoomastigophorea, phylum Protozoa. These are small, 
colorless flagellates, some of which are free-living, some parasitic. The body 
is bilaterally symmetrical, composed of two mirror halves, each with a nucleus 
and a full set of kinetic organelles. There are four flagella to a side, not 
all of the same length. 
    Trepomonas rotans is probably the most common of the free-living species 
(see illustration), and usually occurs in water of high organic, low (or no) 
oxygen content, such as sewage. It occurs in ocean water also. This genus also 
is found as a parasite or symbiont in other animals, such as amphibians. 
Hexamita is another genus containing both free-living and parasitic 
representatives; Spironucleus is parasitic; and Giardia has several species 
which parasitize vertebrates. In humans giardiasis results in severe 
dysentery; however, adequate medication is available. See ZOOMASTIGOPHOREA. 
 �34s[James B. Lackey]
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�6v�2v�32sDiplopoda   �21m �33s 
    A class of terrestrial, tracheate, oviparous arthropods. Diplopods 
(millipeds) are largely cryptic in habits, are saprophytic feeders, and are 
characterized by the development of a compact head with a pair of short, 
simple, eight-jointed antennae, and powerful mandibles. The body is not 
differentiated into thorax and abdomen, but is composed of a variable number 
of similar, cylindrical diplosomites, each of which (except the first two or 
three) bears two pairs of walking legs (see illustration). The body wall is 
chitinous, with a thick impregnation of calcium carbonate in the majority of 
species. Most millipeds are variously adapted for rolling into a closed spiral 
or nearly perfect sphere when threatened. 
    The sexes are separate and fertilization is internal, following prolonged 
clasping behavior. Eggs may be laid in a cluster and ``brooded'' by the 
mother, scattered singly in the humus environment, or enclosed in an 
igloo-shaped mud nest built by the mother. Postembryonic development is 
gradual, without major changes in appearance, and may require a year or more 
for completion. 
    More than 8000 species have been described, although so far only the fauna 
of Europe is well known. In general, about 11 orders and more than 111 
families are recognized. Probably as many as 25,000 species will eventually be 
recognized. Classification is based to a large extent upon shape of the male 
gonopods, which are quite constant and characteristic for each species. 
    Most species are local in distribution, because millipeds generally remain 
close to the parental habitat, and some may be restricted to a few square 
miles. Even genera are limited in distribution, and only a few occur on more 
than one continent. 
    The diplopods have a long geological history; they arose in the Early 
Devonian and were well developed by the Late Pennsylvanian. See ARTHROPODA. 
 �34s[Richard L. Hoffman]
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�6v�2v�32sDiploporita   �21m �33s 
    An extinct class of Echinodermata in which the thecal canals were 
associated in pairs; the canals ran perpendicularly to the surface and, when 
exposed in fossils by superficial abrasion, appear as paired pores termed 
diplopores. Exposure of solitary canals produces haplopores. These so-called 
pores are, of course, artifacts. The theca was ovoid or pear-shaped, built up 
of numerous small polygonal plates arranged without order. There was usually 
no stem. Diploporita probably gave rise to blastoids through transitional 
forms similar to Asteroblastus. None of them survived beyond the Devonian. See 
BLASTOIDEA; ECHINODERMATA. 
 �34s[Howard B. Fell]
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�6v�2v�32sDiplorhina   �21m �33s 
    The subclass of Agnatha that includes the jawless vertebrates with paired 
nostrils. Diplorhina, sometimes called Pteraspidomorphi, includes only the 
extinct ostracoderm order Heterostraci and some or all of the problematical 
fossil genera now referred to the order Thelodonti. See AGNATHA; HETEROSTRACI; 
MONORHINA; THELODONTI. 
 �34s[Robert H. Denison]
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�6v�2v�32sDiplura   �21m �33s 
    An order of primarily wingless insects, measuring normally 0.12 - 0.4 in. 
(3 - 10 mm) but occasionally larger, up to 2 in. (50 mm) in length. There are 
four families, given higher rank by some authors: Procampodeidae, Campodeidae, 
Projapygidae, and Japygidae. There are approximately 450 described species in 
this order, but many more are still to be found. 
    These insects generally lack pigment, and only a few campodeids have 
scales. The bodies are elongate, cylindrical, and highly flexible, in 
accordance with their subterranean habits (see illustration). The mandibles 
and maxillae are retracted into the head capsule when at rest; maxillary and 
labial palps are obsolescent. The antennal flagellum is provided with 
intrinsic musculature. Eyes and ocelli are always absent. Well-developed 
external genitalia are absent. All are subterranean and can be found in the 
soil, detritus, rotten wood, and under bark or rocks. Diplura are of no 
importance to the economy of humans. Campodeids and projapygids feed on 
detritus and fungal mycelia and spores; japygids attack and devour small soil 
arthropods. See APTERYGOTA; INSECTA. 
 �34s[Pedro W. Wygodzinsky]
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�6v�2v�32sDipnoi   �21m �33s 
    The lungfishes, sometimes called Dipneusti, which compose one of three 
subclasses of the class Osteichthyes, the bony fishes. Lungfishes have the 
palatoquadrate fused with the cranium (autostylic) and have three pairs of 
enlarged, hardened tooth plates developed internally. These function in 
crushing mollusks and other hard-shelled invertebrates and largely replace 
marginal teeth which, together with premaxillae and maxillae, were lost, and 
dentaries, which were reduced, early in lungfish history. Living, and 
presumably ancient, lungfishes have lungs that arise from a ventral connection 
with the gut. The paired fins are paddlelike or ribbonlike. 
    Lungfishes, which are probably derived from early crossopterygians, 
appeared by the Lower Devonian. They were common through the late Paleozoic 
and early Mesozoic, but have since become rare. The single order, 
Dipteriformes, includes a few living representatives. See CROSSOPTERYGII; 
DIPTERIFORMES; OSTEICHTHYES. 
    Dipteriformes are classified in some nine families, of which two very 
dissimilar types have living representatives. The Ceratodontidae, well known 
as Mesozoic fossils from all continents, differ but little from the Recent 
Australian species, Neoceratodus forsteri. This is a heavy-bodied lungfish 
with large scales and paddlelike paired fins. It has a single dorsal lung  
with a ventral connection to the gut and can survive in deoxygenated waters by 
breathing air, but does not estivate. The Lepidorsirenidae, with a fossil 
history going back to the Permian, survive today in South America (Lepidosiren 
paradoxa) and Africa (four species of Protopterus). These are slender, eellike 
fishes with small, thin scales and slender, ribbonlike paired fins (see 
illustration). They have paired ventral lungs like tetrapods and are obligate 
air breathers. They can survive desiccation by encasing themselves in a 
protective capsule of mud as swamps dry, and they breathe atmospheric air 
until freed by seasonal rains. The young of the Lepidosirenidae have external 
gills which disappear with age. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sDipole   �21m �33s 
    Any object or system that is oppositely charged at two points or poles, 
such as a magnet or a polar molecule. The properties of a dipole are 
determined by its dipole moment, that is, the product of one of the charges by 
their separation directed along an axis through the centers of charge. 
    An electric dipole consists of two electric charges of equal magnitude but 
opposite polarity (-Q and +Q in the illustration), separated by a short 
distance (d); or more generally, a localized distribution of positive and 
negative electricity without net charge whose mean positions of positive and 
negative charge do not coincide. For a discussion of electric dipole radiation 
see ELECTROMAGNETIC RADIATION. 
    Molecular dipoles which exist in the absence of an applied field are 
called permanent dipoles, while those produced by the action of a field are 
called induced dipoles. See POLAR MOLECULE. 
 �34s[Robert D. Waldron]
�33s 
    The term magnetic dipole originally referred to the fact that a magnet has 
two poles and, because of these two poles, experiences a torque in a magnetic 
field if its axis is not along a magnetic flux line of the field. It is now 
generalized to include electric circuits which, because of the current, also 
experience torques in magnetic fields. See MAGNET. 
 �34s[Ralph P. Winch]
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�6v�2v�32sDipsacales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of 5 families and about 1100 species. Within its subclass the group 
is marked by its inferior ovary; usually opposite, exstipulate (without 
stipules, the paired basal appendages found on many leaves) leaves; often 
anisomerous stamens (fewer in number than the petals); and frequently 
irregular corolla. The flowers are sometimes grouped in heads suggesting those 
of the Asterales, but then the ovules are either pendulous (instead of erect 
from the base) or more than one in each ovary. Elderberry (Sambucus) and 
honeysuckle (Lonicera), in the Caprifoliaceae, and teasel (Dipsacus) are 
familiar members of the Dipsacales. See ASTERIDAE; FLOWER. 
 �34s[Arthur Cronquist]
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�6v�2v�32sDiptera   �21m �33s 
    An order of the class Insecta known as the true flies and so named because 
they possess only two wings. This characteristic, together with a pair of 
balancers or halteres, distinguishes flies from all other orders of the 
Insecta. The three names for members of the order, known commonly as flies, 
gnats, or midges, form a part of the common names of most families, genera, 
and species of the order. The term fly also forms a part of the compound names 
of the insects in many other orders, such as butterfly, Mayfly, and chalcid 
fly, but when used alone it is correctly applied only to the members of the 
Diptera. The forms most commonly known as maggots are actually dipterous 
larvae, and keds (Hippoboscidae) are parasitic forms of flies that have lost 
their wings. The Diptera are the most important group of insects considered in 
medical entomology. Many, especially the mosquitoes, are the vectors for 
numerous parasites and diseases. 
    In number of species, it is the third largest order; only the Coleoptera 
and Lepidoptera are larger. There are approximately 100,000 kinds of flies in 
the world, of which over 18,500 inhabit America north of Mexico. 
    The order is ubiquitous and is more widespread than any other of the 
insects. Diptera are found on every continent and most islands of the world, 
except for the coldest parts of the Arctic and Antarctic, and at the tops of 
extremely high mountain ranges. They occur in almost all available ecological 
niches. Specimens have been taken at altitudes of many thousands of feet by 
specially constructed traps attached to airplanes. 
  
    Taxonomy.  For convenience, the families of Diptera may be divided into 
the Nematocera and the Brachycera, and the latter group further divided into 
the Orthorrhapha and the Cyclorrhapha. Adults of the Nematocera are 
characterized by their slow flight and long antennae. These are the more 
primitive Diptera, and the major groups are represented in fossil deposits as 
old as Jurassic and Cretaceous age. 
    The Brachycera are generally swift fliers, and their antennae are no 
longer than the head. In the Orthorrhapha, the adult fly emerges from the pupa 
through a T-shaped anterior opening, while in the Cyclorrhapha it emerges from 
a circular opening at the anterior end of the puparium. 
    Following is a list of scientific and vernacular names of the more common 
families of flies. 
Nematocera 
    Trichoceridae (winter crane flies) 
    Ptychopteridae (phantom crane flies) 
    Tipulidae (crane flies) 
    Psychodidae (moth flies and sand flies) 
    Blepharoceridae (net-winged midges) 
    Chironomidae (midges) 
    Ceratopogonidae (biting midges) 
    Simuliidae (black flies) 
    Chaoboridae (phantom midges) 
    Culicidae (mosquitoes) 
    Dixidae (dixid midges) 
    Anisopodidae (wood gnats) 
    Bibionidae (march flies) 
    Scatopidae (scatopsid flies) 
    Mycetophilidae (fungus gnats) 
    Sciaridae (black fungus gnats) 
    Cecidomyiidae (gall midges) 
Brachycera - Orthorrhapha 
    Stratiomyidae (soldier flies) 
    Rhagionidae (snipe flies) 
    Tabanidae (horse flies and deer flies) 
    Bombyliidae (bee flies) 
    Asilidae (robber flies) 
    Empididae (dance flies) 
    Dolichopodidae (long-legged flies) 
Brachycera - Cyclorrhapha 
    Lonchopteridae (spear-winged flies) 
    Phoridae (hump-backed flies) 
    Platypezidae (flat-footed flies) 
    Syrphidae (syrphid flies) 
    Pipunculidae (big-headed flies) 
    Lonchaeidae (lonchaeid flies) 
    Lauxaniidae (lauxaniid flies) 
    Drosophilidae (pomace or fruit flies) 
    Ephydridae (shore flies) 
    Anthomyiidae (anthomyiid flies) 
    Muscidae (house flies) 
    Calliphoridae (blow flies) 
    Sarcophagidae (flesh flies) 
    Oestridae (bot flies) 
    Tachinidae (tachinid flies) 
    Hippoboscidae (louse flies) 
    Micropezidae (stilt-legged flies) 
    Diopsidae (stalk-eyed flies) 
    Sciomyzidae (marsh or snail-killing flies) 
    Sepsidae (black scavenger flies) 
    Heleomyzidae (heleomyzid flies) 
    Sphaeoceridae (small dung flies) 
    Agromyzidae (leaf-miner flies) 
    Tethinidae (tethinid flies) 
    Chloropidae (frit flies) 
    Conopidae (thick-headed flies) 
    Tephritidae (fruit flies) 
    Piophilidae (skipper flies) 
751A 
  
    Morphology.  Adult flies vary from somewhat less than 1 mm (0.04 in.) to 
over 25 mm (1 in.) in body length. The head is vertical, usually with three 
ocelli at the dorsal vertex and the mouthparts ventral. In some cases, 
however, the head is distinctly longer than high when viewed laterally. The 
compound eyes are usually large and prominent. 
    The mouthparts are modified for either lapping or piercing, but never with 
the mandibles apposable and capable of chewing. The structure of the 
mouthparts varies greatly throughout the order so that their homologies are 
often rather difficult to determine. Throughout the Orthorrhapha there tends 
to be a retention of mandibles for piercing, especially in those flies that 
require blood meals. In the Cyclorrhapha, on the other hand, mandibles tend to 
be reduced or lost entirely, and with few exceptions the work of the 
mouthparts is largely sucking, with the maxillae and labium taking over the 
large share of work. Since all adult flies imbibe fluids only, the presence of 
a pharyngeal ``pump'' to transfer fluids from the external substrate to the 
gut is characteristic. 
    Each antenna typically has two basal segments and one or more additional 
segments called the flagellum. In the Nematocera the flagellum consists of a 
variable number of quite similar segments. In all remaining flies that are 
three distinct antennal segments. The third segment, representing the 
flagellum, is usually longer than the two basal ones and sometimes complex. 
    The prothorax and metathorax are small and closely united with the large 
mesothorax which contains the musculature for the single pair of wings. The 
legs are usually alike, except in species in which the prothoracic pair are 
raptorial and species in which the fore, mid, or hind legs of the males are 
modified by secondary sexual characters. 
    Only the first pair of wings, the mesothoracic pair, is developed. The 
second pair is reduced to a pair of club-shaped organs, or halteres, which 
serve as balancing organs in flight. They are present in most species, even 
when the mesothoracic  wings are absent. In many species, especially those of 
the Nematocera, abundant scalelike setae clothe the veins and sometimes the 
margins of the wings. The wings of some of the higher Diptera bear yellow, 
brown, or black markings that aid in species identification. 
    In certain families of Diptera, some of the setae covering the head and 
body are greatly enlarged and are used for identification. The location and 
relative size of these bristles are often characteristic of species, genera, 
or families. 
  
    Life cycle.  The adult stage in the life history may be regarded as a 
reproductive and dispersal phase, during which these insects increase both in 
number and often in geographic range. Females select suitable oviposition 
sites and lay a variable number of eggs, after which they usually die. The 
eggs, after a period of incubation of varying length, give rise to larvae, 
which represent a growth phase. During this period, the insect does almost 
nothing but nourish itself, thus providing the tissues necessary for a later 
transformation into an adult. This growth period is divided into stages, or 
instars, each of which terminates with the molting of the larval skin to allow 
increase in size for the stage that follows. In the Diptera there are four 
larval instars. In most of the Orthorrhapha, especially those with aquatic 
immature forms, all four instars are active; but in the Cyclorrhapha, at the 
third larval molt, a hard puparium is formed inside which a quiescent fourth 
larval stage occurs. At the conclusion of the final larval instar, there is a 
dramatic reorganization in tissue structure initiated by the action of 
appropriate hormones on the so-called ``imaginal buds,'' a process that 
eventually results in the formation of an adult insect. During this time, the 
insect is called a pupa because its external form is now altered and any 
external activity virtually ceases. 
    When the adult tissues are almost fully formed, the fly emerges from its 
pupal case and spends some time in drying, hardening, and expanding its wings, 
attaining color, and reaching sexual maturity. It is then ready to start its 
cycle once again. 
  
    Economic importance.  Diptera have probably more economic importance than 
any other insect order. This importance comes from the relatively few species 
that affect domestic animals, and plants. The vast majority of forms have no 
direct importance although a number are beneficial in one way or another. 
    Only a relatively few dipteran species cause severe economic loss to 
humans. Perhaps of most concern is the role played by them in disease 
transmission. About 70 species of Anopheles mosquitoes transmit an estimated 
500,000 cases of malaria each year. Yellow fever is transmitted principally by 
a single mosquito, Aedes aegypti. Dengue, or breakbone fever, a usually 
nonfatal disease of worldwide distribution that leaves its victims debilitated 
for several weeks, is transmitted by Aedes aegypti and A. albopictus. 
Filariasis, primarily a disease of peoples of Africa, the Orient, and the 
Pacific islands, is caused by a minute roundworm whose larvae are transmitted 
by a few species of Anopheles, Mansonia, Culex, and Aedes. With modern methods 
used by bacteriologists and virologists, a large number of virus diseases 
known to be transmitted by mosquitoes have been found in humans. Many of 
these, such as Sindbis virus of Egypt, may not produce clinical symptoms of 
disease, but others, such as the equine encephalitides, which include western, 
eastern, St. Louis, Japanese, and others, may be quickly fatal in humans. With 
the exception of the viruses, none of the diseases mentioned above are 
transmitted by any insects other than members of the order Diptera. 
    A few less well-known diseases are transmitted by other flies. Black flies 
transmit Onchocerca volvulus, the causative agent of onchocerciasis, a disease 
affecting the eyes of natives of Central and parts of South America. Sandflies 
of the genus Phlebotomus transmit organisms that cause kala azar, Oriental 
sore, pappataci fever, and Oroya fever. Even today large parts of Africa 
remain underdeveloped because of the presence of sleeping sickness, a fatal 
disease transmitted by tsetse flies. Species of the genus Chrysops (Tabanidae) 
are instrumental in carrying tularemia, primarily a disease of rodents. 
    Domestic animals, hides, meat, and dairy products are affected by disease 
transmission or by direct attack by flies. Anthrax, tularemia, botulism, many 
virus diseases, and nagana, a form of sleeping sickness, are some of the 
diseases transmitted by members of the Diptera that take an annual toll 
amounting to millions of dollars. Some flies wreak their damage by direct 
attack. The primary screwworm fly deposits eggs on hides of animals, and the 
larvae, upon hatching, burrow through the skin and into the flesh. The 
secondary screwworm gains entrance through holes, often infected, already 
present on the skin surface. Hypoderma lineata and H. bovis are botflies of 
special importance. Eggs of both species are laid on hairs of cattle, and the 
hatching larvae bore through the skin into the connective tissue. During their 
development they wander through the tissues of the animal, and when mature 
they escape through holes which they make along the spine of their host and 
drop to the ground to pupate. Horses are afflicted by a species of 
Gasterophilus. They lick the eggs from their bodies, and the larvae settle and 
dwell in their stomachs, often in large numbers. Sheep are victims of the 
sheep botfly, whose larvae live in their nasal passages and sinuses. All of 
these insects affect the meat, milk, and wool production of animals through 
debilitation and irritation, and hides may be rendered completely useless by 
their entrance and exit holes. An interesting dipteran is the bee louse, which 
is a wingless, ectoparasite of the honeybee. 
    Economic losses as a result of damage by flies to crops are perhaps not as 
great as those caused by other insects, yet their presence has necessitated 
the expenditure of large sums of money for control. Among these, perhaps the 
most important are members of the family Tephritidae, the larvae of which feed 
upon and ruin the succulent flesh and seeds of their hosts. The most important 
of these are the European and American cherry fruit flies; their relatives in 
the genus Rhagoletis that attack walnuts in North America; six or seven 
species of the genus Anastrepha which attack many kinds of fruit in the New 
World; the Mediterranean fruit fly which is a limiting factor in the 
production of many fruits, especially citrus; and several species of the genus 
Dacus, which attack olives, citrus fruits, many kinds of vegetables, and other 
edible plants and plant parts in Europe, the Orient, and Pacific islands. 
    Larvae of some flies mine the leaves of ornamentals thereby defacing them, 
reducing their growth potentials, and affecting their production and sale by 
nurseries. The larvae of some species of Muscidae are known as root maggots 
and burrow into the underground parts of plants with considerable loss to 
growers of truck crops over the world. See ARTHROPODA; INSECTA. 
 �34s[Raymond J. Gagne]
�33s 
  
A700000005002�254s�5v�1v560
\IMG\CI481.IMG
10669056
561
\IMG\CI481.IMG
10810880
�6v�2v�32sDirect-coupled amplifier   �21m �33s 
    A device for amplifying signals with direct-current components. There are 
many different situations where it is necessary to amplify signals having a 
frequency spectrum which extends to zero frequency. Some typical examples are 
amplifiers in electronic differential analyzers (analog computers), certain 
types of feedback control systems, and some medical instruments such as the 
electrocardiograph. Amplifiers which have capacitor coupling between stages 
are not usable in these cases, because the gain at zero frequency is zero. 
Therefore, a special form of amplifier, called a dc or direct-coupled 
amplifer, is necessary. These amplifers will also amplify alternative-current 
(ac) signals. See AMPLIFIER. 
    Some type of coupling circuit must be used between successive amplifer 
stages to prevent the relatively large supply voltage of one stage from 
appearing at the input of the following stage. These circuits must pass 
direct-current signals with the least possible amount of attenuation. 
    Transistor direct-coupled amplifiers have a number of advantages and 
disadvantages relative to vacuum-tube amplifiers. Interstage direct coupling 
is much easier with transistors because of the availability of both pnp and 
npn transistors and zener diodes. The main disadvantage of this amplifier is 
the temperature dependence of the transistor parameters. 
    It is generally recognized that the differential amplifier is the most 
stable direct-coupled amplifier circuit available. This is true because in 
this circuit the performance depends on the difference of the device 
parameters, and transistors can be manufactured using the planar epitaxial 
technique with very close matching of their parameters. 
    A method of amplifying direct-current (or slowly varying) signals by means 
of ac amplifiers is to modulate a carrier signal by the signal to be 
amplified, amplifying the modulated signal, and demodulating at the output. 
    A chopper-stabilized amplifier is composed of two amplifiers: One 
amplifier is a carrier direct-coupled amplifier employing a chopper to 
modulate and demodulate a portion of the signal to be amplified; the second 
amplifier is a straight directcoupled amplifier. The success of this circuit 
in reducing to a very low value the amount of drift appearing at the output 
depends upon the fact that the drift is a very-low-frequency phenomenon. 
 �34s[Christos C. Halkias]
�33s 
  
A700000002162�254s�5v�1v561
\IMG\CI481.IMG
10810880
�6v�2v�32sDirect current  �21m �33s 
    Electric current which flows in one direction only through a circuit or
equipment. The associated direct voltages, in contrast to alternating voltages,
are of unchanging polarity. Direct current corresponds to a drift or
displacement of electric charge in one unvarying direction around the closed
loop or loops of an electric circuit. Direct currents and voltages may be of
constant magnitude or may vary with time.
    Direct current is used extensively to power adjustable-speed motor drives in
industry and in transportation. Very large amounts of power are used in
electrochemical processes for the refining and plating of metals and for the
poreduction of numerous basic chemicals.
    Direct current ordinarily is not widely distributed for general use by
electric utility customers. Instead, direct current (dc) power is obtained at
the site where it is needed by the rectification of commercially available
alternating-current (ac) power to dc power. See DIRECT-CURRENT TRANSMISSION;
ELECTRIC CURRENT; ELECTRIC POWER SYSTEMS.
 �34s[D.D. Robb]
�33s

A700000004670�254s�5v�1v561
\IMG\CI481.IMG
10810880
�6v�2v�32sDirect-current generator �21m �33s
    A rotating electric machine which delivers a unidirectional voltage and
current. An armature winding mounted on the rotor supplies the electric power
output. One or more field windings mounted on the stator establish the
magnetic flux in the air gap. A voltage is induced in the armature coils as a
result of the relative motion between the coils and the air gap flux.
Faraday's law states that the voltage induced is determined by the time rate
of change of flux linkages with the winding. Since these induced voltages are
alternating, a means of rectification is necessary to deliver direct current
at the generator terminals. Rectification is accomplished by a commutator
mounted on the rotor shaft. Carbon brushes, insulated from the machine frame
and secured in brush holders, transfer the aramture current from the rotating
commutator to the external circuit. See COMMUNICATION; ELECTRIC ROTATING
MACHINERY; GENERATOR; WINDINGS IN ELECTRIC MACHINERY.
    The field windings of dc generators require a direct current to produce a
magnetomotive force (mmf) and establish a magnetic flux path across the air
gap and through the armature. Generators are classified as series, shunt,
compound, or separately excited, according to the manner of supplying the
field excitation current.
    In the separately excited generator, the field winding is connected to an
independent external source. Separately excited generators are among the most
common of dc generators, for they permit stable operation over a very wide
range of output voltages.
    Using the armature as a source of supply for the field current, dc
generators are also capable of self-excitation. Residual magnetism in the
field poles is necessary for self-excitation. Series, shunt, and
compound-wound generators are self-excited, and each produces different
voltage characteristics. The armature winding and field winding of a series
generator are connected in series. The field winding of a shunt generator is
connected in parallel with the armature winding. A compound generator has both
a series field winding and a shunt field winding. Both windings are on the
main poles with the series winding on the outside. The shunt winding furnishes
the major part of the mmf. The series winding produces a variable mmf,
dependent upon the load current, and offers a means of compensating for
voltage drop.
 �34s[Robert T. Weil, Jr.]
�33s

A700000010601�254s�5v�1v561
\IMG\CI481.IMG
10810880
562
\IMG\CI481.IMG
10946560
�6v�2v�32sDirect-current motor   �21m �33s 
    An electric rotating machine energized by direct current and used to 
convert electric energy to mechanical energy. It is characterized by its 
relative ease of speed control and, in the case of the series-connected motor, 
by an ability to produce large torque under load without taking excessive 
current. See ELECTRIC ROTATING MACHINERY. 
    The principal parts of a dc motor are the frame, the armature, the field 
poles and windings, and the commutator and brush assemblies. The frame 
consists of a steel yoke of open cylindrical shape mounted on a base. Salient 
field poles of sheet-steel laminations are fastened to the inside of the yoke. 
Field windings placed on the field poles are interconnected to form the 
complete field winding circuit. The armature consists of a cylindrical core of 
sheet-steel disks punched with peripheral slots, air ducts, and shaft hole. 
These punchings are aligned on a steel shaft on which is also mounted the 
commutator. The commutator, made of hard-drawn copper segments, is insulated 
from the shaft. Segments are insulated from each other by mica. Stationary 
carbon brushes in brush holders make contact with commutator segments. Copper 
conductors placed in the insulated armature slots are interconnected to form a 
reentrant lap or wave style of winding. See COMMUTATION; WINDINGS IN ELECTRIC 
MACHINERY.  
    Rotation of a dc motor is produced by an electromagnetic force exerted 
upon current-carrying conductors in a magnetic field. For basic principles of 
motor action see MOTOR. 
    Direct-current motors may be categorized as shunt, series, compound, or 
separately excited. 
    The field circuit and the armature circuit of a dc shunt motor are 
connected in parallel. The field windings consist of many turns of fine wire. 
The entire field resistance, including a series-connected field rheostat, is 
relatively large. The field current and pole flux are essentially constant and 
independent of the armature requirements. The torque is therefore essentially 
proportional to the armature current. Typical applications are for load 
conditions of fairly constant speed, such as machine tools, blowers, 
centrifugal pumps, fans, conveyors, wood- and metalworking machines, steel, 
paper, and cement mills, and coal or coke plant drives. 
    The field circuit and the armature circuit of a dc series motor are 
connected in series. The field winding has relatively few turns per pole. The 
wire must be large enough to carry the armature current. The flux of a series 
motor is nearly proportional to the armature current which produces it. 
Therefore, the torque of a series motor is proportional to the square of the 
armature current, neglecting the effects of core saturation and armature 
reaction. An increase in torque may be produced by a relatively small increase 
in armature current. Typical applications of this motor are to loads requiring 
high starting torques and variable speeds, for example, cranes, hoists, gates, 
bridges, car dumpers, traction drives, and automobile starters. 
    A compound motor has two separate field windings. One, generally the 
predominant field, is connected in parallel with the armature circuit; the 
other is connected in series with the armature circuit. The field windings may 
be connected in long or short shunt without radically changing the operation 
of the motor. They may also be cumulative or differential in compounding 
action. With both field windings, this motor combines the effects of the shunt 
and series types to an extent dependent upon the degree of compounding. 
Applications of this motor are to loads requiring high starting torques and 
somewhat variable speeds, such as pulsating loads, shears, bending rolls, 
plunger pumps, conveyors, elevators, and crushers. See DIRECT-CURRENT 
GENERATOR.  
    The field winding of a separately excited motor is energized from a source 
different from that of the armature winding. The field winding may be of 
either the shunt or series type, and adjustment of the applied voltage sources 
produces a wide range of speed and torque characteristics. Small dc motors may 
have permanent-magnet fields with armature excitation only. Such motors are 
used with fans, blowers, rapid-transfer switches, electromechanical 
activators, and programming devices. 
 �34s[L. F. Cleveland]
�33s 
  
A700000005102�254s�5v�1v562
\IMG\CI481.IMG
10946560
�6v�2v�32sDirect-current transmission   �21m �33s 
    The conduction of electric power by means of electric currents which flow 
in one direction only. After the almost complete replacement of direct current 
(dc) by alternating current (ac), interest in dc power transmission began 
increasing in 1954. One reason is that the cost of a dc line is about 
two-thirds that of the corresponding ac line. This is because the usual dc 
bipolar line has two conductors, compared with three conductors of an ac 
single-circuit three-phase line. See DIRECT CURRENT. 
    The dc line requires additional apparatus at its terminals, not required 
for an ac line, which increases the cost. These are the converters for 
changing ac to dc at the sending end and dc to ac at the receiving end. 
Considering the combined costs of the line and of the terminal equipment, the 
dc link is thus more expensive than ac for a short line but less expensive for 
a long line. However, the choice between ac and dc usually depends on other 
factors.  
    Direct-current lines are used to perform functions for which ac lines are 
unsuited, such as transmitting power across a wide body of water, using 
submarine cables. Cables have a much larger shunt capacitance per unit of 
length than overhead lines have. When an ac voltage is applied between the two 
conductors of a cable, an alternating ``charging'' current flows even if the 
distant end is not connected to a load. When the length of the cable exceeds 
some distance - about 30 mi (50 km) - the thermal limit is reached, even 
though no power is transmitted. This was a significant factor in the choice of 
dc for connecting the island of Gotland to the mainland of Sweden, Denmark to 
Sweden, England to France, and the South Island of New Zealand to the North 
Island.  
    Direct-current lines are also used to provide interconnections of two ac 
systems of different nominal frequencies, for example, 50 Hz and 60 Hz, as was 
done in Japan, or interconnections between two ac systems having the same 
nominal frequency but different frequency controls. An ac tie of the latter 
type might require a much higher power rating than the greatest power which 
was desired to be interchanged. In some cases, the two converters are 
installed in the same station, so that really there is no dc line. Such 
stations are called frequency-changer stations if there are two different 
nominal frequencies, and asynchronous ties if the nominal frequencies are 
equal, as in New Brunswick, Canada, or Steagle, Nebraska. 
 �34s[Edward W. Kimbark]
�33s 
  
A700000001150�254s�5v�1v562
\IMG\CI481.IMG
10946560
�6v�2v�32sDirectional coupler   �21m �33s 
    A waveguide network of four guide terminals A, B, C, and D (see 
illustration) such that there is a complete isolation between A and C and 
between B and D, but no isolation between the two terminals of any other 
combination.  
    A directional coupler is most useful in selectively measuring either the 
forward (incident) or the backward (reflected) wave. In the case of a small 
hole coupling (large decibel rating), this objective can be achieved with a 
negligible attrition of the main wave. 
 �34s[C. K. Jen]
�33s 
  
  
A700000004535�254s�5v�1v562
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10946560
�6v�2v�32sDirection-finding equipment   �21m �33s 
    Equipment used by aircraft and ships as an aid to navigation or used to 
obtain information for other purposes such as plotting the flight of 
meteorological balloons, locating storm fronts, discovering the location of 
illicit transmitters, locating the source of galactic noise, and determining 
source and characteristics of signals for military purposes. The equipment 
attempts to determine the direction of arrival of a radio wave by measuring 
the orientation of the wavefront or of the magnetic or electric vector of a 
radio wave. 
    Generally speaking, all radio direction finders consist of three 
components: (1) the directional antenna or collector system; (2) the radio 
receiver that detects the emission from which the directional information will 
be derived; and (3) the display and indicator system which transforms the 
output of the two previous components into usable information for presentation 
to the operator. 
    The directional antenna or collector systems are either signal-minimum 
(null) or signal-maximum types. 
    The common airborne direction finder (ADF) was derived from the right-left 
radio compass. The radio compass employs both a loop antenna and a 
nondirectional antenna. The loop antenna is mounted rigidly to the aircraft, 
with its plane at right angles to the longitudinal axis of the aircraft. By 
proper phasing within the receiver, the receiving antenna pattern is a 
cardioid which can be reversed so that its maximum moves from the right to the 
left side of the aircraft as the connection to the loop antenna is reversed. 
The output of the compass receiver is rectified and connected to a 
differential meter. The meter reads the relative output for the two 
loop-antenna connections. Therefore, it shows the relative strength of the 
signals received from one side of the aircraft as compared with those received 
from the other side. If the radio station is directly ahead, the reading of 
the meter will be zero. In actual practice the switching is accomplished by 
electronic methods as frequencies ranging from 30 to 90 Hz. 
    Automatic direction finders for marine services would employ solid-state 
components used in circuits consistent with modern technology. See ELECTRONIC 
NAVIGATION SYSTEMS. 
 �34s[Peter C. Sandretto]
�33s 
  
A700000002160�254s�5v�1v562
\IMG\CI481.IMG
10946560
563
\IMG\CI481.IMG
11065856
�6v�2v�32sDirectivity   �21m �33s 
    The general property of directional discrimination displayed by systems 
that receive or emit waves. Thus, loudspeakers, microphones, radio antennas, 
underwater sound projectors, hydrophones, and even telescopes all have the 
common property that their effectiveness depends upon the direction from which 
the wave is either emitted or received. The manner in which a sender or 
receiver is directional depends largely upon its geometrical shape and, in 
particular, upon its dimensions compared to those of the wavelength involved. 
    Directivity is a desirable property of a receiver because it permits the 
identification if the direction from which a signal comes and because noise 
from other directions is eliminated. It is desirable in a sender because the 
available energy can be concentrated in a given direction. A simple example of 
directivity is furnished by the megaphone, which effectively increases the 
size of the emitting area, thus increasing directivity. See MEGAPHONE. 
 �34s[William S. Cramer]
�33s 
  
A700000000722�254s�5v�1v563
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11065856
�6v�2v�32sDirigible   �21m �33s 
    A lighter-than-air craft equipped with means of propelling and steering 
for controlled flight in all three dimensions of the atmosphere. As a general 
designation, the term dirigible has gradually become associated with the term 
airship and has come to indicate primarily that type of airship with the hull 
of rigid structure. See AIRSHIP. 
 �34s[Karl Arnstein ;Robert S. Ross]
�33s 
  
A700000001073�254s�5v�1v563
\IMG\CI481.IMG
11065856
�6v�2v�32sDiscellaceae   �21m �33s 
    A family of fungi, also known as Excipulaceae, of the order 
Sphaeropsidales. Although most of the 225 species of the 70 genera reported 
are saprophytes, some are plant pathogens. Many of the species are conidial 
stages of Discomycetes (class Ascomycetes). 
    The fruit body bearing conidia (pycnidium) is closed early in development 
but later opens out into a black cup- or disk-shaped structure. See FUNGI 
IMPERFECTI; PLANT PATHOLOGY; SPHAEROPSIDALES. 
 �34s[N. F. Buchwald]
�33s 
  
A700000002073�254s�5v�1v563
\IMG\CI481.IMG
11065856
�6v�2v�32sDiscriminant �21m �33s
    For a polynomial f(x) = asubnxsupn + asubn xsupn + asubn-1 xsupn-1 + . . . +
a1x + a0, the discriminant is given by the expression D = a(subn)(sup2n-1) (x1 -
x2)sup2 (x1 - x3)sup2 . . . (x1 - xsubn)sup2 (x2 - x3)sup2 . . . (x2 -
xsubn)sup2 . . . (xsubn-1 - xsubn)sup2, where x1, x2, . . . , xsubn are the
roots of the equation f(x) = 0.
    The importance of the discriminant D lies in the fact that D vanishes if,
and only if, the equation f(x) = 0 has equal roots. Since the value of the
discriminant is unchanged if any two letters xsubi and xsubj are interchanged,
it is a symmetric function of the roots, and can be expressed in terms of the
coefficients of f(x). The discriminant of a quadratic polynomial f(x) =
a2xsup2 + a1x + a0 is D = a1sup2 - 4a2a0. The discriminant of a cubic
polynomial f(x) = a3xsup3 + a2xsup2 + a1x + a0 is

   D = 18a3a2a0 - 4a2sup3 a0 - a2sup2 a1sup2 - 4a3a1sup3 - 27a3sup2 a0sup2

See EQUATIONS, THEORY OF; POLYNOMIAL SYSTEMS OF EQUATIONS.
 �34s[Richard A. Bambach]
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�6v�2v�32sDiscriminator   �21m �33s 
    A circuit that transforms a frequency-modulated or phase-modulated carrier 
into a wave that is amplitude-modulated as well as frequency-modulated. The 
amplitude modulation is then detected by a linear diode detector. Commonly 
used discriminators are the Foster-Seely discriminator and the ratio detector, 
which are popularly used in fm and television receivers. For these and other 
circuits see FREQUENCY-MODULATION DETECTOR. 
 �34s[Charles L. Alley]
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�6v�2v�32sDisease   �21m �33s 
    A deleterious set of responses which occurs at the subcellular level, 
stimulated by some injury, and which is often manifested in altered structure 
or functioning of the affected organism. With advances in understanding and 
the development of sensitive probes, it has become clear that the fundamental 
causes of diseases are based on biochemical and biophysical responses within 
the cell. These responses are now being categorized and, slowly, the 
mechanisms are being understood. 
    The term homeostasis refers to functional equilibrium in an organism and 
to the processes that maintain it. There is a range of responses that is 
considered normal. If cells are pushed to respond beyond these limits, there 
may be an increase, a decrease, or a loss of normal structure or function. 
These changes may be reversible or irreversible. If irreversible, the cells 
may die. Thus, subcellular changes may be reflected in altered tissues, 
organs, and consequently organisms, and result in a condition described as 
diseased. See HOMEOSTASIS. 
    Lesions are the chemical and structural manifestations of disease. 
Subjective manifestations of a disease process such as weakness, pain, and 
fatigue are called symptoms. The objective measurable manifestations such as 
temperature, blood pressure, and respiratory rate changes are called signs or 
physical findings. Changes in the chemical or cellular makeup of an organ, 
tissue, or fluid of the body or its excretory products are called laboratory 
findings. To make a diagnosis is to determine the nature of the pathologic 
process by synthesizing information from these sources evaluated in the light 
of the patient's history and compared with known patterns of signs and 
symptoms. In common usage, the term disease indicates a constellation of 
specific signs and symptoms attributable to altered reactions in the 
individual which are produced by agents that affect the body or its parts. 
    Etiology is the study of the cause or causes of a disease process. 
Although a disease may have one principal etiologic agent, it is becoming 
increasingly apparent that there are several factors involved in the 
initiation of a disease process. Susceptibility of the individual is an ever 
present variable. The etiologic factors can conveniently be divided into two 
categories (see table). One group consists of endogenous (internal; within the 
body) factors, and may originate from errors in the genetic material. The 
other category of etiologic factors is exogenous (environmental). These 
account for the majority of disease reactions. Exogenous factors include 
physical, chemical, and biotic agents. 
    Pathogenesis refers to the mechanisms by which the cell, and consequently 
the body, responds to an etiologic agent. It involves biochemical and 
physiological responses which are reflected in ultrastructural, microscopic, 
or gross anatomic lesions. There are a limited number of ways in which cells 
respond to injury. The nature of the response is modified by the nature of the 
agent, dose, portal of entry, and duration of exposure, as well as many host 
factors such as age, sex, nutritional state, and species and individual 
susceptibility.  
    The diseases which are important in causing human death have changed in 
the last 80 years. In 1900 six of the ten leading causes of death in the 
United States were infectious (biotic) agents. At present, only one of the ten 
leading causes of death in the United States, influenza and pneumonia, is due 
to biotic agents. While most of the biotic causes of diseases were being 
brought under control, continued population growth (in large part, a 
consequence of the control of infectious disease) and the remarkable growth of 
industrialization have been associated with an increased prevalence of 
diseases caused by physical and chemical agents. These include cancer, 
cirrhosis, and cardiovascular disease. 
    General principles of the organism's response to toxic substances, some of 
which occur naturally in the environment, have evolved from a great number of 
investigations of agent-host interaction. They are: (1) All substances 
entering the organism are toxic; none is harmless. Dose rate of exposure and 
route of entry into the body determine whether a toxic response will occur or 
not. (2) All agents evoke multiple responses. (3) Most of the biological 
responses are undesirable, leading to the development of pathological changes. 
(4) A given dose of an agent does not produce the same degree of response in 
all individuals. Thus, when disease is viewed as interaction between the 
environment and the individual, the control of disease is largely the 
management of the environmental causes of disease. 
 �34s[N. Karle Mottet ;Carol Quaife]
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�6v�2v�32sDisease carrier   �21m �33s 
    A person who is infected with certain microorganisms and is therefore 
liable to transmit the disease produced by them, but who does not exhibit 
signs or symptoms of the infection. The individual may be in such an early 
phase of the disease that clinical manifestations are not apparent, and in 
this case is an incubatory carrier. If there has already been a clinical 
attack, the description is of a convalescent carrier. These carriers sometimes 
continue to harbor the infectious agent for such an extended period that they 
are called chronic carriers. It is also possible for a person to carry 
microorganisms, even without a manifestation of the disease; this association 
with the germ is an incidental one, and the person is described as a casual 
carrier.  
    The existence of healthy carriers is a complicating factor in the study of 
infectious diseases. Since they are without symptoms or signs of disease, they 
can be identified as carriers only by laboratory investigation. See DISEASE; 
EPIDEMIOLOGY.  
 �34s[Colin White]
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�6v�2v�32sDisk recording   �21m �33s 
    The process of inscribing suitably transformed acoustical or electrical 
signals on a flat circular plate that may be played back at a subsequent time. 
Virtually all modern disk recorders and reproducers are used to record or 
reproduce sound signals, mainly music and voice. 
  
  
    Monophonic system.  A monophonic disk recording system consists of a disk 
record rotated by a turntable mechanism  and a cutter for producing 
undulations in a groove in the disk corresponding to the sound signals. A 
monophonic disk reproducing system consists of a pickup and mechanism for 
rotating the disk record by means of which the recorded undulations in the 
disk record are converted into electrical signals of approximately like form. 
    The elements in a complete monophonic disk recording system are shown in 
Fig. 1. The first element is the acoustics of the studio. The output of each 
microphone is amplified and fed to a mixer, a device having two or more inputs 
and a common output. If more than one microphone is used (for example, when an 
orchestra accompanies a soloist, there is one microphone for the orchestra and 
one for the singer), the outputs of the two microphones may be adjusted for 
the proper balance by means of the mixers. An electronic compressor is used to 
reduce a large amplitude range to that suitable for reproduction in the home. 
A corrective electrical network called an equalizer provides the recording 
characteristics. The attenuator, or gain control, provides a given control on 
the overall level fed to the power amplifier. The cutter, actuated by the 
amplifier, cuts a wavy path in the groove of the revolving record 
corresponding to the undulations in the original sound wave striking the 
microphone. A monitoring system consisting of a volume indicator, 
complementary equalizer, attenuator or gain control, power amplifier, and 
loudspeaker is used to control the recording operation. 
    A phonograph recorder (Fig. 2) is an instrument for transforming 
acoustical or electrical signals into motion of approximately like form and 
inscribing such motion in an appropriate medium by cutting or embossing. The 
lacquer disk used in recording the master record is placed on the recording 
turntable. The turntable is heavy, to ensure against spurious rotational 
motions. A suitable mechanical filter is placed between the driving motor and 
the turntable so that uniform rotational motion of the turntable will be 
obtained. The lead screw drives the cutter in a radial direction so that a 
spiral groove is cut in the record. Lead screws of different pitches are used, 
ranging from 100 to 500 grooves per inch (4 to 20 per millimeter). In some 
recordings a variable pitch is used. In this procedure the spacing between the 
grooves is made to correspond to the amplitude - small spacing for small 
amplitudes and large spacing for large amplitudes. Under these conditions the 
maximum amount of information can be recorded on a record. 
    The electromechanical transducers used as cutters can be of either the 
lateral or the vertical type. In the lateral type of disk recording, the 
undulations are cut in a direction parallel to the surface of the record and 
perpendicular to the groove. The vibrating system is of the dynamic type with 
two coils, one the driving coil and the other the sensing (feedback) coil, 
wound on a common cylinder. The cutting stylus is attached to the coil 
cylinder.  
    The cutting stylus consists of a sapphire, synthetic ruby, or other hard 
material fashioned in the form of a pointed chisel. The stylus is heated in 
recording and thereby imparts a smooth sidewall to the groove. This expedient 
results in considerable reduction in noise in reproduction. 
    The original recording of disk records is made on a lacquer disk. The 
lacquer disk consists of a coating of an acetate plastic on two sides of an 
aluminum disk. The grooves are cut in the plastic. 
    In the vertical (hill-and-dale) type of disk recording, the undulations of 
the groove are cut in a direction perpendicular to the surface of the record 
and perpendicular to the groove. The vertical disk phonograph system is used 
to a limited extent in broadcasting stations, but it is not used in home disk 
phonograph systems. 
  
    In the mass production of disk phonograph records, the original lacquer 
disk, termed the original, is metalized and then electroplated. The plating is 
separated from the lacquer and reinforced by backing with a solid metal plate. 
The assembly, called the master, is electroplated. This plating is separated 
from the master and reinforced by backing with a solid plate. The resulting 
assembly, the mother, is electroplated and reinforced by a solid metal plate, 
forming an assembly termed the stamper. Several stampers are made from each 
mother. One stamper containing a sound selection to be placed on one side of 
the final record is mounted in the upper jaw, and another stamper containing a 
sound selection to be placed on the other side of the record is placed in the 
lower jaw of a hydraulic press equipped with means for heating and cooling the 
stampers. A preform, or biscuit, of thermoplastic material such as a shellac 
compound or vinyl compound is placed between the two stampers. 
    The stampers are heated, and the jaws of the press are closed to bring the 
two stampers against the thermoplastic material. When an impression of the 
stampers has been obtained in the thermoplastic material, the stampers are 
cooled, thus cooling and setting the plastic record. The jaws of the hydraulic 
press are opened, and the record is removed from the press. The modulated 
grooves in the record correspond to those in the original lacquer disk. The 
stamping procedure is repeated until the desired number of records is obtained. 
  
    Stereophonic system.  Two-channel disk phonograph sound reproduction was 
commercialized in 1958. 
    In a two-channel disk phonograph dynamic-type feedback cutter the two 
vibrating systems are arranged at right angles; therefore, the two channels in 
the groove are recorded at right angles. The same type of recorder described 
in the beginning of this article may be used in the recording of stereophonic 
records. See STEREOPHONIC SOUND. 
  
    Quadraphonic system.  There are two approaches to quadraphonic sound 
reproduction by means of the disk record: first, the matrixing and encoding 
from four channels, to recording on a two-channel disk, and reproducing from a 
two-channel disk and decoding and matrixing to four channels; and, second, the 
recording and reproducing of four discrete channels by means of the disk 
record. See QUADRAPHONIC SOUND SYSTEM. 
  
    Distortion and noise.  Distortion is any undesired change in the waveform 
of a signal in a sound-reproducing system. Noise is any foreign, erratic, 
intermittent, or statistically random signal produced within a 
sound-reproducing system. Distortion and noise are the two major problems that 
must be solved to achieve high-fidelity performance in disk sound-reproducing 
systems.  
    The recording and reproducing of a phonograph record constitute a 
complicated process, with many sources of nonlinear distortion. The record 
does not present an infinite mechanical impedance to the stylus. As a 
consequence, the vibrating system of the pickup is shunted by the effective 
mechanical impedance of the record at the stylus. Nonlinear distortion will be 
introduced if this impedance of the record is variable. Other sources of 
distortion are tracking error and tracing distortion. A nonlinear distortion 
due to a deviation in tracking is commonly termed tracking error. A form of 
distortion in lateral disk-record reproduction known as tracing distortion is 
a function of the diameter of the stylus, the lateral velocity, and the linear 
groove velocity. This distortion is due to the fact that there is not a 
one-to-one correspondence between the shapes of the cutting and reproducing 
styli.  
    The record surface noise, in the absence of any signal, is one of the 
factors which limits the volume range and the frequency range of shellac 
phonograph records. The noise of Vinylite records is extremely low and, in 
general, is not a problem. Adequate volume ranges can be obtained with 
Vinylite and similar plastics. See SOUND-REPRODUCING SYSTEM. 
 �34s[Harry F. Olson]
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�6v�2v�32sDispersion relations   �21m �33s 
    Relations between the real and imaginary parts of a response function. A 
response function relates a cause and its effect through an integral equation. 
The term dispersion refers to the fact that the index of refraction of a 
medium is a function of frequency. In 1926 H. A. Kramers and R. Kronig showed 
that the imaginary part of an index of refraction (that is, the absorptivity) 
determines the real part (that is, the refractivity); this is called the 
Kramers-Kronig relation. The term dispersion relation is now used for the 
analogus relation between the real and imaginary parts of the values of any 
response function. See COMPLEX NUMBERS. 
 �34s[Charles J. Goebel]
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�6v�2v�32sDispersion (radiation)   �21m �33s 
    The separation of a complex of electromagnetic or sound waves into its 
various frequency components. For example, a beam of white light can be 
separated into its monochromatic components by virtue of the different 
velocities of rays of different wavelength of the beam as it passes through a 
prism. The dispersion of a material, such as glass or water, at a given 
wavelength in the electromagnetic spectrum is defined as the rate of change of 
refractive index with wavelength at the wavelength in question. 
 �34s[William West]
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�6v�2v�32sDisplacement current   �21m �33s 
    The name given by J. C. Maxwell to the term PARTIAL DIFFERENTIAL D/PARTIAL 
DIFFERENTIAL t which must be added to the current density i to extend to 
time-varying fields A. M. Ampere's magnetostatic result that i equals the curl 
of the magnetic intensity H. In integral form this result is given by the 
equation below, where the unit vector n is perpendicular to the surface dS. 

           *** CONTOURED INTEGRAL H MULT ds =
                    INTEGRAL s (i = [PARTIAL DIFFERENTIAL D/
                              PARTIAL DIFFERENTIAL t]) MULT n dS ***

The concept of displacement current has important consequences for insulators 
and for free space where i vanishes. For conductors, however, the difference 
between the above equation and Ampere's result is negligible. See AMPERE'S LAW: 
MAXWELL'S EQUATIONS. 
    If one defines current as a transport of charge, the term displacement 
current is certainly a misnomer when applied to a vacuum where no charges 
exist. If, however, current is defined in terms of the magnetic fields it 
produces, the expression is legitimate. 
 �34s[William R. Smythe]
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�6v�2v�32sDisplacement pump   �21m �33s 
    A pump that develops its action through the alternate filling and emptying 
of an enclosed volume. There are five basic types: reciprocating, 
direct-acting steam, rotary, vacuum, and air-lift. 
    Positive-displacement reciprocating pumps have cylinders and plungers or 
pistons with an inlet valve, which opens the cylinder to the inlet pipe during 
the suction stroke, and an outlet valve, which opens to the discharge pipe 
during the discharge stroke. Reciprocating pumps may be power-driven through a 
crank and connecting rod or equivalent mechanism, or direct-acting, driven by 
steam or compressed air or gas. Power-driven reciprocating pumps are highly 
efficient over a wide range of discharge pressures. Except for some special 
designs with continuously variable stroke, reciprocating power pumps deliver 
essentially constant capacity over their entire pressure range when driven at 
constant speed. 
    A reciprocating pump is readily driven by a reciprocating engine; a steam 
or power piston at one end connects directly to a fluid piston or plunger at 
the other end. Steam pumps can be built for a wide range of pressure and 
capacity by varying the relative size of the steam piston and the liquid 
piston or plunger. The delivery of a steam pump may be varied at will from 
zero to maximum simply by throttling the motive steam, either manually or by 
automatic control. Reciprocating pumps are used for low to medium capacities 
and medium to highest pressures. They are useful for low- to medium-viscosity 
fluids, or high-viscosity fluids at materially reduced speeds. Specially 
fitted reciprocating pumps are used to pump fluids containing the more 
abrasive solids. 
    Another form of displacement pump consists of a fixed casing containing 
gears, cams, screws, vanes, plungers, or similar elements actuated by rotation 
of the drive shaft. Most forms of rotary pumps are valveless and develop an 
almost steady flow rather than the pulsating flow of a reciprocating pump. 
    Although vacuum pumps actually function as compressors, displacement pumps 
are used for certain vacuum pump applications. Simplex steam pumps with 
submerged piston pattern fluid ends are used as wet vacuum pumps in steam 
heating and condensing systems. Sufficient liquid remains in the cylinder to 
fill the clearance volume and drive the air or gas out ahead of the liquid. 
See VACUUM PUMP. 
    In handling abrasive or corrosive waters or sludges, where low efficiency 
is of secondary importance, air-lift pumps are used. The pump consists of a 
drop pipe in a well with its lower end submerged and a second pipe which 
introduces compressed air near the bottom of the drop pipe. The mixture of air 
and water in the drop pipe is lighter than the water surrounding the pipe. As 
a result, the mixture of air and water is forced to the surface by the 
pressure of submergence. See COMPRESSOR; PUMP. 
 �34s[Elliott F. Wright]
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�6v�2v�32sDisplacement (mechanics)   �21m �33s 
    When an object is moved from one position to another, it is said to be 
displaced, and the linear distance from the initial to the final position, 
regardless of the length of path followed, is called the displacement. The 
displacement is always in a particular direction, and consequently 
displacement is a vector quantity involving direction as well as magnitude. In 
rectilinear motion the magnitude of the displacement is also the length of 
path or the distance traversed, but since length of path does not involve 
direction, it is a scalar quantity. 
    When a body is rotated about any axis, it is said to undergo angular 
displacement. Angular displacement is commonly measured in radians or degrees. 
See ROTATIONAL MOTION. 
 �34s[Rogers D. Rusk]
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�6v�2v�32sDistance-measuring equipment   �21m �33s 
    An internationally standardized navigation system which allows an aircraft 
to measure its distance from a selected ground-based beacon. Such beacons are 
used throughout the world by all airliners, most of the military aircraft of 
the West, and a large number of general-aviation aircraft. The range of 
service is line-of-sight up to 300 mi (480 km) and system accuracy is usually 
0.1 mi (0.16 km) but precision equipment, intended for use during landing, has 
accuracy of 100 ft (30 m). 
    The airbone equipment, called an interrogator, transmits pulses of 1 kW 
peak power on 1 of 126 frequencies. These are in the 1025 - 1150-MHz band and 
are spaced 1 MHz apart. Each pulse is of 3.5 microseconds duration and is 
paired with another, spaced 12 or 36 microseconds later. The combination of 
frequencies and pulse spacings therefore provides 252 operating channels. 
    The beacon on the ground, called a transponder, receives these pulses, 
delays them by 50 MUs, and then retransmits them, usually with a power of 1 
kW, on 252 frequencies lying between 962 and 1213 MHz. The pulse-pair spacing 
is 12 MUs on those frequencies not used by the interrogator, and 30 MUs on 
those frequencies shared with the interrogator. The transponder transmission 
is called the reply. The frequency difference between interrogation and reply 
is always 63 MHz. This arrangement allows each transmitter frequency to act as 
the local oscillator for its associated superheterodyne receiver, the 
intermediate frequency of which is 63 MHz. For landing purposes, some 
transponders have powers as low as 100 W. 
    In the aircraft the replies to its own interrogations are recognized by 
their phase coherence with their own transmissions, and by the elapsed time 
measured between transmissions and reception (minus the 50-MUs transponder 
delay), usually by means of a crystal clock. This elapsed time is about 12 MUs 
for each nautical mile (7 MUs for each kilometer), and is displayed in the 
cockpit on a digital meter, which is usually calibrated in miles and tenths of 
miles. See ELECTRONIC NAVIGATION SYSTEMS; INSTRUMENT LANDING SYSTEM (ILS); 
POLAR-COORDINATE NAVIGATION SYSTEMS; RADIO RANGE; TACAN. 
 �34s[Sven H. Dodington]
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�6v�2v�32sDistillate fuel   �21m �33s 
    A broad term for any one of the wide variety of fuels obtained from 
fractions boiling above gasoline in the distillation of petroleum. The most 
important distillate fuels are kerosine, furnace oils, and diesel fuels. See 
DIESEL FUEL; KEROSINE; PETROLEUM PRODUCTS. 
 �34s[Mott Souders]
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�6v�2v�32sDistillation   �21m �33s 
    The process of producing a gas or vapor from a liquid or solid. However, 
the term sublimation is used ordinarily to describe the vaporization of a 
solid. Heat is generally supplied to the liquid during the distillation, 
although in special cases the latent heat required for the vaporization may be 
obtained from the internal energy of the liquid. 
    The main purpose of distillation is either the separation of volatile 
components from nonvolatile materials, or the separation of a mixture of 
volatile components. The separation of volatile components from nonvolatile 
materials is carried out by a simple distillation in which the material is 
placed in a still and heated, and the vapor removed and condensed. Simple 
distillation is similar to the process of evaporation, but the former term 
usually describes the operation in which the volatile material is a desired 
product, whereas evaporation generally is applied to aqueous solutions of 
nonvolatile materials in which the nonvolatile material is the desired 
product. Simple distillation is frequently used for high-boiling organic 
compounds; to prevent thermal degradation of the product, the operation is 
usually carried out either at reduced pressure, termed simple vacuum 
distillation, or with the addition of steam, termed steam distillation. See 
EVAPORATION; EVAPORATOR; HEAT TRANSFER ; SUBLIMATION. 
    Although simple distillation can be applied to mixtures of volatile 
components, the separation obtained is usually not complete, particularly if 
the components have boiling points that are close to each other. To obtain 
greater separations in such cases, fractional distillation is employed. In 
this process, the vapors from the still are permitted to come in contact with 
a portion of the condensate in a countercurrent or stepwise countercurrent 
operation. Because of its lower operating and capital costs, this 
countercurrent type of operation has completely replaced the multiple 
distillation-condensation method formerly employed. This process has been 
widely adopted for both laboratory and industrial operations because it is 
usually the most effective method of separating mixtures of miscible volatile 
liquids. It is so effective and efficient that it is also frequently employed 
to separate mixtures which are not normally liquids, such as fractional 
distillation of liquid air. 
    Separation of two liquids is possible only when the composition of the 
vapor is different from that of the liquid from which it was produced. The 
design of distillation equipment is usually based on vapor-liquid equilibrium 
compositions. The vapor-liquid equilibrium data needed for distillation work 
either obtained experimentally or estimated from physical chemistry 
relationships. See BOILING POINT; GAS; LIQUID; PHASE EQUILIBRIUM; VAPOR 
PRESSURE.  
    In steam distillation, steam is introduced directly into the liquid in the 
still. The method is usually limited to those cases in which the solubility of 
the steam in the liquid is low at the operating temperature and pressure. It 
is employed with relatively high-boiling organic materials which would 
decompose if they were distilled directly at atmospheric pressure or with 
liquids that have such poor heat-transfer characteristics that excessive local 
overheating would result with indirect heating. By steam distillation, a 
volatile material can be separated from nonvolatile impurities, or mixtures 
can be separated with results about equivalent to those obtained with simple 
distillation. See HEAT EXCHANGER; VAPOR CONDENSER. 
    In fractional distillation, the vapor produced in the still is brought 
into contact with a portion of the condensate in a countercurrent or stepwise 
countercurrent system. The operation can be batch or continuous. The unit 
consists of a still to which a vertical column is attached (see illustration). 
This column is filled with some type of packing or plate construction which 
will permit the descending liquid added at the top to come into contact with 
the vapor rising from the still. The vapor from the top of the column is 
condensed and a portion of it is removed as product; the remaining liquid is 
returned to the top of the column as liquid, called reflux. The flow rates of 
liquid and vapor are adjusted so that, at every place within the column, the 
liquid has a higher concentration of the more volatile components than 
corresponds to equilibrium with the vapor with which it is in contact. As a 
result, the more volatile components pass from the liquid to the vapor and the 
less volatile components pass in the reverse direction. The vapor becomes 
progressively more enriched in the volatile components as it flows up the 
column to the condenser, and the liquid becomes more concentrated in the less 
volatile components as it flows down the column to the still. The separation 
obtainable by such a system is much greater than that of a simple distillation. 
 �34s[Edwin R. Gilliland]
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�6v�2v�32sDistortion (electronic circuits)   �21m �33s 
    Any undesired change in the waveform of an electric signal passing through 
a circuit, including the transmission medium. In the design of any electronic 
circuit one important problem is to modify the input signal in the required 
way without producing distortion beyond an acceptable degree. Amplifier and 
loudspeaker systems are examples where maximum effort has been expended to 
produce a design for faithful amplification of speech and music input signals. 
There are four general types of distortion: amplitude, frequency, phase, and 
cross modulation. 
    Amplitude distortion is generally considered to mean distortion produced 
by a nonlinear relationship between the input and output amplitudes of a 
device. Amplitude distortion is  usually introduced by a transistor or vacuum 
tube. The change in collector current of a transistor is nearly proportional 
to the change in signal voltage only over a small range of signal voltage 
amplitude. For further increase, the collector current change begins to depart 
from proportionality. See TRANSISTOR. 
    Frequency distortion is an inherent feature of all amplifiers but can be 
minimized by proper design. It occurs because the reactive elements and 
inherent reactances in the amplifier circuit do not allow the same 
amplification for all frequencies, and therefore some components of a signal 
are amplified more than others. Furthermore, in the case of audio amplifiers, 
the loudspeaker and enclosure characteristics affect the load presented to the 
amplifier in a manner which depends upon frequency. 
    Like frequency distortion, phase distortion is caused by the reactive 
elements in the circuit producing a phase shift that is not the same for each 
frequency component of the input signal. It is possible for an input signal to 
have frequency components with approximately constant magnitude amplification 
but not with a phase shift for each component which is proportional to the 
frequency. As a result, the output is not an amplified replica of the input. 
Fortunately, the ear is more tolerant of phase distortion than frequency 
distortion.  
    Cross modulation, also called intermodulation, is caused by nonlinear 
device characteristics. If two signals of different frequencies are applied to 
the input of a nonlinear transistor or vacuum tube stage, the output will 
contain the fundamental and harmonic components of each signal, frequency 
components equal to the sum and difference of the input signal frequencies, 
and sums and differences of the harmonics of the two input signals. Therefore, 
if the input is a signal composed of several frequencies, the nonlinearity 
will produce new frequencies not integrally related to those of the input 
signal. The distortion, if bad enough, is generally more noticeable than 
harmonic distortion. See FEEDBACK CIRCUIT. 
 �34s[Christos C. Halkias]
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�6v�2v�32sDistortion (optics)   �21m �33s 
    The optical aberration arising from the variation in magnification over 
the field of an optical system. Distortion that is positive, the magnification 
increasing with field angle, is called pincushion distortion because the image 
of a square has concave sides and thus looks like a pincushion (see 
illustration). The opposite type, negative distortion, is called barrel 
distortion because the image of a square has bulging sides. It is possible to 
balance distortion in an optical system by balancing higher-order distortion 
through introducing some third-order distortion of opposite sign. 
 �34s[Max J. Herzberger]
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�6v�2v�32sDistributed control system   �21m �33s 
    A collection of modules, each with its own specific function, 
interconnected to carry out integrated data acquisition and control. Typical 
modules include data acquisition modules, direct digital controllers, operator 
consoles, and the supervisory computer. 
    Industrial control systems have evolved from totally analog systems 
through centralized digital computer-based systems to multilevel, distributed 
systems. In an analog system the individual process variables were each 
controlled by a single analog feedback controller. Analog control systems had 
many advantages: simplicity due to the separate nature of the realizations of 
the control loops; reliability from the independence of the analog control 
devices (separate transducers, transmitters, and controllers); and graceful 
degradation in the face of a device failure, since the failure of one element 
or loop has a limited effect on the rest of the control system. The major 
disadvantage of analog control is the lack of integrated process information 
display to the process operator and the difficulty in implementing complex 
control strategies. Supervisory control using digital computers alleviated 
these problems. This mode of control is called supervisory control because the 
computer itself is not directly involved in the dynamic feedback. Here the 
analog control system is maintained as before, but a digital computer is added 
which periodically scans, digitizes, and inputs process variables to the 
computer. See DIGITAL COMPUTER. 
    Direct digital control evolved from supervisory control and later merged 
with supervisory control to produce the best available system prior to the 
introduction of distributed control. Direct digital control replaces the 
analog control with a periodically executed equivalent digital control 
algorithm carried out in the central digital computer. The advantages of 
direct digital control are the ease with which complex dynamic control 
functions can be carried out and the elimination of the cost of the analog 
controllers themselves. The cost reduction which resulted from the 
introduction of direct digital control was offset by a number of 
disadvantages. The most notable of these were the decrease in reliability and 
the total loss of graceful degradation. Failure of a sensor or transmitter had 
the same effect as before, but failure of the computer itself threw the entire 
control system into manual operation. Hence it was necessary to provide analog 
controllers to back up certain critical loops which had to function even when 
the computer was down. 
    Increasing demand for higher and higher levels of supervisory control, 
coupled with increasing installation cost, highlighted two disadvantages of 
the centralized digital computer control of processes. First, process signals 
were still being transmitted from the process sensor to the central control 
room in analog form, meaning that separate wires had to be installed for every 
signal going to or from the computer. Cost of installing this cable is very 
high. Second, the digital computer system itself evolved into a very complex 
unit because of the number of devices attached to the computer and because of 
the variety of different programs needed to carry out the myriad control and 
management functions. The latter resulted in the need for an elaborate 
real-time operating system for the computer. Design, coding, installation, and 
checkout of centralized digital control systems was so costly and 
time-consuming that application of centralized digital control was limited. 
    Low-cost electronic hardware utilizing large-scale integrated (LSI) 
circuits provided the technology to solve both of these problems while 
retaining the advantages of centralized direct digital and supervisory 
control. The solution involved distributing control functions in such a way 
that software complexity was eliminated and process signals could share 
communication paths to the central computer. See INTEGRATED CIRCUITS. 
    Distributed systems have a number of valuable modern applications. 
Chemical processing plants, and electrical energy generating plants in 
particular, have stringent control requirements related to process quality and 
process safety which demand very reliable, very fast, and very complex control 
systems. These needs are being met by distributed systems. See CONTROL 
SYSTEMS; DIGITAL CONTROL. 
 �34s[James D. Schoeffler]
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�6v�2v�32sDistribution (probability) �21m �33s
    The results of a series of independent trials, random variables, or errors
often occur in fairly regular and predictable patterns that can be expressed
mathematically. The most important of them are called the binomial, normal,
and Poisson distributions.
    Consider n independent trials, each of which results in success S or
failure F, with corresponding probabilities p and q = 1 - p. Denote by S(sub n)
the number of successes. Because there are (n over k) possible ways to select k
places for S  and n - k places for F, the probability distribution of the
random variable S(sub n) is given by

		P{S(sub n) = k} = (n over k) p(sup k) q(sup n-k)

where k = 0, 1, . . . , n. This is the binomial distribution. Its expectation
is np, its variance npq. See PROBABILITY.
    The standard normal density is defined for -INFINITY < x < INFINITY by

		phi(x) = (2pi)(sup -1/2)    e(sup -x[sup 2])

    The standard normal distribution function or error function PHI(x) is its
integral from -INFINITY to x.  The normal distribution function with mean m and
variance s(sup 2) is PHI[(x - m)/s]; its density is (2pi)(sup -1/2) s(sup -1) e
[sup -(x-m)(sup 2)/(2s[sup 2])].  As x goes from -INFINITY to INFINITY, the
function increases from 0 to 1.  It plays an important role in many fields.  In
particular, u(t,x;XI) = (2 pi(sup -1/2) t(sup -1/2) e[sup -(x-XI)(sup 2)/4t] is
the fundamental solution of the heat (or diffusion) equation u(sub t) = u(sub
xx) and represents the heat distribution on the x axis at time t caused by a
unit heat source initially concentrated at x = XI.  Probabilistically, this
represents the transition probabilities in the Wiener process.	A random
variable whose distribution is normal is called normal or Gaussian.  Many
empirical quantities (for example, the amount of water in a reservoir, certain
inherited characteristics such as height, and the experimental error of physical
measurements) represent the cumulative effect of many small components, and the
statistical fluctuations of such quantities may be expected to follow the normal
distribution.
    The Poisson distribution with parameter lambda is the probability
distribution of a random variable assuming the values 0, 1, 2, .  .  .	with
probabilities p(sub k)(lambda) = (e[sup -lambda] lambda[sup k])/k!.  Both its
expectation and variance equal lambda.	This is one of the most important
distributions; it plays a basic role in the theory of stochastic processes and
in many applications.
    Consider a large number n of independent trials, each of which results in
success or failure with probabilities p and q = 1 - p. Ordinarily, interest is
restricted to the case where p is very small, but the expected number of
successes np = lambda is of moderate size. Typical examples may be obtained by
considering centenarians, color-blind people, or triplets in a large
population. The number of successes in the n trials is a random variable with
the binomial distribution, but under the present circumstances the binomial is
close to the Poisson distribution and may be replaced by it. In fact, the
probability of no success is qsupn = (1 - lambda/n)(sup n), which is close to
e(sup -lambda), the first term of the Poisson distribution.
    In other circumstances the Poisson distribution appears, not as an
approximation, but as the exact expression of a law of nature. This is true in
particular of processes where certain events, such as radioactive
disintegrations, mutations, power failures, and accidents, occur in time in
such a way that (1) the probability that an event occurs during any given time
interval of length dt is, asymptotically, lambda dt; and (2) there is no
interaction or aftereffect between nonoverlapping time intervals. A similar
argument applies to random distributions of points in space, with t interpreted
as volume; typical examples are stars, flaws of material, raisins in a cake,
and animal litters in a field. See ANALYSIS OF VARIANCE; STATISTICS.
 �34s[William Feller]
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�6v�2v�32sDistributor   �21m �33s 
    A rotary switch that directs the high-voltage ignition current in the 
proper firing sequence to the various cylinders of an internal combustion 
engine. In automotive practice the distributor housing usually contains, in 
addition, apparatus for timing the ignition to occur when each piston is at 
optimum position in the cycle. Timing is also varied with engine load, by the 
movement of a diaphragm exposed to the pressure in the engine intake manifold. 
See IGNITION SYSTEM. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sDistrict heating   �21m �33s 
    The supply of heat, either in the form of steam or hot water, from a 
central source to a group of buildings. Most district heating systems in the 
United States rely on separate steam generation facilities close to load 
centers. In some cities, notably New York, high-pressure district steam is 
used extensively to feed turbines that drive pumps and refrigerant 
compressors. Although some district heating plants serve detached residences, 
the cost of underground piping and the small quantities of heating service 
required makes this service generally unfeasible. District heating, apart from 
utility-supplied systems in downtown areas of cities, is accepted as efficient 
practice in many colleges, universities, and government complexes. 
    In Europe, a substantial part of the heat distribution is by hot water 
from plants combining power and heat generation. Hot water distribution 
systems have the advantage of high thermal mass (hence, storage for peak 
periods) and freedom from problems of condensate return. On the other hand, 
pumping costs are high and two mains are required, supply and return. See 
COMFORT HEATING. 
 �34s[Richard L. Koral]
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�6v�2v�32sDisulfide   �21m �33s 
    One of a group of organosulfur compounds, RSSR', that may be symmetrical 
(R = R') or unsymmetrical (R and R' different). They are of great biochemical 
interest, since the S-S link occurs in natural products. The cleavage of the 
S-S bond in proteins, for example, has important biological and industrial 
interests in the dehairing of hides, in the preparation of cysteine from wool, 
in wavesetting of hair, and in petroleum refining. Disulfides are also of 
interest in the manufacture of polysulfide rubbers and polymers. See 
ORGANOSULFUR COMPOUND. 
 �34s[Norman Kharasch]
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�6v�2v�32sDiverticulosis   �21m �33s 
    A degenerative process in the gastrointestinal tract disorder that results 
in saccular outpouchings of the colonic mucosa which extend to the serosa. 
These most frequently occur where the blood vessels penetrate the muscular 
wall to supply the mucosa. It has been now well documented that the presence 
of a diet low in bulk is one of the leading causes of this disease. As with 
other lesions of the gastrointestinal tract which become acutely inflamed 
(diverticulitis), the lesion may perforate, causing peritonitis, or may bleed, 
causing massive gastrointestinal hemorrhage. 
 �34s[H. Thomas Norris]
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�6v�2v�32sDiving �21m �33s
    Skin diving, scuba diving, saturation diving, and ``hard hat'' diving are
techniques used by scientists to investigate the underwater environment. Skin
diving is usually without breathing apparatus and is done with fins and
faceplate. The diver's under-water observation is limited to the time that
breath can be held (1 - 2 min). Diving with scuba (self-contained underwater
breathing apparatus) and ``hard hat'' provide the diver with a breathable gas,
thus expanding the submerged time and the depth range of underwater
observations. This type of diving is limited by human physiology and the
diver's reaction to the pressure and nature of the breathing gas. Saturation
diving permits almost unlimited time down to depths of 100 ft (30 m).

    Scuba diving.  Scuba is used by trained personnel as a tool for direct
observation in marine research and underwater engineering. This equipment is
designed to deliver through a demand-type regulator a breathable gas mixture
at the same pressure as that exerted on the diver by the overlying water
column. The gas which is breathed is carried in high-pressure cylinders (at
starting pressures of 2000 - 3000 psi) 14 - 20 megapascals) worn on the back.
    Scuba can be divided into three types: closed-circuit, semiclosed-circuit,
and open-circuit. In the first two, which use pure oxygen or various
combinations of oxygen, helium, and nitrogen, exhaled gas is retained and
passed through a canister containing a carbon dioxide absorbent for
purification and then recirculated to a bag worn by the diver. During
inhalation additional gas is supplied to the bag by various automatic devices
from the high-pressure cylinders. These two types of equipment are much more
efficient than the open-circuit system, in which the exhaled gas is discharged
directly into the water after breathing. Most open-circuit systems use
compressed air because it is relatively inexpensive and easy to obtain.
Although open-circuit scuba is not as efficient as the other types, it is
preferred because of its safety, the ease in learning its use, and its
relatively low cost.
    For physiological reasons scuba diving is limited to about 165 ft (50 m)
of water depth. Below this depth when using compressed air as a breathing gas,
the diver is limited, not by equipment, but by the complex temporary changes
which take place in the body chemistry while breathing gas (air) under high
pressure.

    Saturation diving.	This type of diving permits long periods of
submergence (1 - 2 weeks). It allows the diver to take advantage of the fact
that at a given depth the body will become fully saturated with the breathing
gas and then, no matter how long the submergence period, the decompression
time needed to return to the surface will not be increased. Using this method,
the diver can live on the bottom and make detailed measurements and
observations, and work with no ill effects. This type of diving requires
longer periods of decompression in specially designed chambers to free the
diver's body of the high concentration of breathing gas. Decompression times
of days or weeks (the time increases with depth) are common on deep dives of
over 200 ft (60 m).
 �34s[Robert F. Dill]
�33s

    Physiology.  When a skin diver descends into the sea, the pressure of the
surrounding water increases rapidly and compresses the gas-filled space of the
lungs. At some depth between 100 and 300 ft (30 and 90 m), depending on the
individual's lung capacity and shift of blood into the chest, a threshold is
reached where the thorax and lungs cannot withstand this pressure and
collapse. With the aid of diving equipment, helmet or scuba, air is supplied
to the diver under high pressure, thus preventing the lungs from collapsing.
The effects of increased pressure observed on divers in a water environment
are different from those found in caisson workers, who are exposed to a dry,
pressurized environment in which hydrostatic pressure differences are not
present.
    A diver breathing air at a pressure equivalent to that at 300 ft (90 m) of
water is subject to certain changes in personality and performance, ascribed
to nitrogen narcosis. The subjective symptoms and mental reactions consist of
euphoria, overconfidence accompanied by dulling of mental ability, and
difficulty in assimilating facts and making quick and accurate decisions.
These symptoms are similar to alcohol intoxication. J. Y. Cousteau called it
``rapture of the deep.''
    The use of nitrogen-oxygen gas mixtures limited the maximal depth of
diving to about 300 ft (90 m) because of nitrogen narcosis and resistance to
breathing. Helium was found superior to nitrogen as an inert gas diluent for
oxygen, and it is used for divers below 150 ft (45 m). It extended practical
diving operations to 500 ft (150 m). Helium is lighter than nitrogen, thus
reducing the resistance to breathing. Moreover, helium is less soluble in the
tissues than nitrogen. Therefore, smaller quantities of helium are absorbed in
the body during a dive, and less gas is eliminated from the tissues during
decompression to result in bubble formation. Nevertheless, in spite of these
advantages, helium-oxygen diving has not lessened incidents of bends during
decompression. Diving with helium-oxygen atmospheres requires special
equipment and special decompression tables.
    Inhalation of oxygen concentration above 65% at 1 atmospheric pressure has
deleterious effects in most warm-blooded animals; it leads eventually, even
after days or weeks, to death due to pulmonary oxygen toxicity. Inhalation of
oxygen at approximately twice the atmospheric pressure results in generalized
convulsions. With increasing depth the toxicity symptoms develop after shorter
exposure times. Under higher partial pressures oxygen toxicity manifests
itself primarily in the central nervous system. No permanent damages resulting
from oxygen convulsions in humans have been reported.
    With properly designed diving equipment and sufficient ventilation, one
would expect the danger of CO2 accumulation to be minimal. However, because of
increased resistance to breathing under higher pressures, CO2 retention does
occur in diving operations. Inhaling CO2 can cause symptoms of dyspnea,
headaches, dizziness, warmth, restlessness, or increased motor activity.
    The tissues of a diver who has been exposed to high pressure for a long
period will have accumulated a large amount of dissolved nitrogen. If the
diver were brought back to the surface immediately, the result would be bubble
formation, which can cause minor or severe damage, depending on the quantity
of bubbles and the organs in which they form. It is fortunate that the blood
and tissues can hold a certain amount of gas in supersaturation without
significant bubble formation. If bubbles are liberated from supersaturated
tissues, they can exert pressure on nerves and result in local pain to
muscles, joints, or bones. Bubbles formed in the spinal cord can produce
paralysis. If gas bubbles develop in the brain, they may cause dizziness,
blindness, paralysis, unconsciousness, or convulsions, depending on quantity
and location. Bubbles formed in the blood can interfere with the pulmonary
blood flow and result in asphyxia and the ``chokes.'' The first symptom is
often pain below the sternum during deep breathing, followed by shallow and
rapid breathing. The onset of symptoms varies greatly, occurring within 1 - 18
h following decompression. See DECOMPRESSION ILLNESS.
    Aside from narcosis, toxicity, and decompression sickness, the scuba,
helmet, and breath-hold divers are subject to the effects of unequal pressure
differences (barotrauma) across their air-containing structures, such as
middle ears, sinuses, lungs, and gastrointestinal tract. Swimmers and divers
often have difficulty in equalizing pressure in the middle ear during descent
because of blockage of the Eustachian tube. They will experience pain and upon
further descent will rupture the eardrum. If the sinuses do not equalize
during descent, transudation and hemorrhage can develop.
    During ascent the air contained in the lungs of the diver expands in
relation to the diminishing ambient pressure. If the diver uses a breathing
apparatus and breathes normally during ascent, the excess of expanding lung
air is discharged through the exhaust valve. But if the diver panics and holds
his breath, overexpansion of lung tissues may cause air embolism from gases
entering the pulmonary blood vessels from adjoining tissues. Air embolism
frequently results in death due to blockage of circulation.
 �34s[Karl E. Schaefer]
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�6v�2v�32sDiving animals   �21m �33s 
    Truly aquatic animals have no need for a specialized diving response, 
because they are able to obtain  oxygen directly from the water and thus 
sustain themselves indefinitely in the aquatic medium. Terrestrial or land 
animals, on the other hand, have in many cases gone back to the aquatic mode 
of living in varying degrees. In all of these has arisen the problem of 
obtaining the necessary gas exchange in an aquatic environment with only the 
physical attributes of a terrestrial animal. Many of these terrestrial animals 
have adapted so well as to achieve almost perfect freedom in the water. 
    All classes of terrestrial vertebrates (birds, mammals, and reptiles) have 
species which spend part of their lives under water or seek food or shelter by 
becoming submerged for a period of time. One example, a fresh-water mammalian 
species, is the North American beaver. One of the first changes noted in these 
and other animals that normally dive beneath the surface of the water is a 
reduction in heart rate, called bradycardia. However, those animals which 
traditionally utilize an aquatic habitat show a more pronounced reduction in 
heart rate in a much shorter time. In the other two classes of animals which 
contain divers, the reptiles and birds, some excellent examples reflect 
reductions in heart rate similar to the rates seen in mammals. 
    Warm-blooded animals show a more rapid response to submergence than do 
reptilian species. Those mammals, such as seals, which are traditionally found 
in open-ocean environments show the most dramatic responses of all. The 
reptiles in general utilize less oxygen than do warm-blooded animals and have 
slower resting heart rates, so that their response would be expected to be 
less spectacular. Also, reptilian reaction times are generally slower than 
those seen in mammals, and are usually modified by the temperature of the 
water in which they dive, so that a slower response to the diving stimulus is 
expected.  
    Specifically, the diving response enables the animal to remain underwater 
for longer periods of time. Not only does the heart rate slow down, but blood 
flow to specific muscles is decreased or stopped altogether. Lactic acid 
produced as a result of anaerobic glycolysis in muscles thus isolated is less 
likely to enter the systemic circulation and alter the functions of vital 
organs such as the brain and heart. This response also reduces the likelihood 
that hydrogen ions will build up in the blood and stimulate chemoreceptors 
(receptors which respond to changes in blood carbon dioxide levels and 
hydrogen ion concentration) which control breathing. Blood flow thus remains 
constant and unrestricted to the most essential organs, such as brain, heart, 
and lungs, during the dive, and arterial pressure to these organs remains at 
the normal level, so that perfusion of the capillaries with blood is not 
altered.  
    Other changes in the bodies of divers are also apparent, but are seen as 
changes in total body chemistry. For instance, glycogen and 
adenosinetriphosphate, which are the principal energy sources of muscles, are 
in more than ample supply in divers, and appear to have been stored for that 
time when they are most needed. Oxygen stores are essential to the length of 
time that a diver may stay submerged, and diving animals have concentrations 
of myoglobin in their muscles which gives them a remarkable ability to store 
oxygen. See DIVING. 
 �34s[J. Homer Ferguson]
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�6v�2v�32sDivision   �21m �33s 
    In arithmetic and algebra, the process of finding one of two factors of a 
number (or polynomial) when their product and one of the factors are given. 
The symbol / is used in elementary English and American arithmetics to denote 
division. Division is more often symbolized by the double do t:, the bar  - , 
or the solidus /; thus x : y, *EQN*|, or x/y indicates  division of a number x 
by a number y. Considered as an operation inverse to multiplication, x/y is a 
symbol denoting a number whose product with y is x. Another way to base 
division upon multiplication is provided by the concept of the reciprocal of a 
number. If y is any number (real or complex) other than 0, there is a number, 
denoted by 1/y and called the reciprocal of y, whose product with y is 1. Then 
x/y is the symbol for the product of x and 1/y. This view of division 
furnishes a means of extending the concept to objects other than real or 
complex numbers. See ALGEBRA; MULTIPLICATION. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sDocodonta �21m �33s
    One of the most primitive mammalian orders known, found in the Jurassic of
North America and England and possibly in Rhaetic deposits. In docodonts the
main jaw articulation was formed by the dentary and squamosal bones, but the
articular and quadrate bones formed a secondary jaw articulation. Early
members of other Mesozoic orders possessed the same double articulation, but
by Late Jurassic time only the docodonts still retained this transitional
reptile-mammal condition. See MAMMALIA.
 �34s[Malcolm C. McKennan]
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�6v�2v�32sDog �21m �33s
    Any breed of domestic or wild dogs, and related species, that belong to
the family Canidae. The origin of domestic dogs is obscure, but they seem to
be most closely related to the wolf. In many respects the dog is structurally
primitive and shows a genetic plasticity which accounts for the many varieties.

    Domestic breeds.  There are more than 100 breeds of domestic dogs, and
their classification is based principally on their uses. The breeds are the
sporting breeds, hounds, terriers, working breeds, and toy breeds.
    Sporting breeds are trained and employed for retrieving or finding game.
The largest group in this class is the spaniels, of which there are about 10
types. The spaniels have been trained to retrieve and to flush game, and they
hunt both birds and fur-bearing animals. The Brittany spaniel is the only
member of the group which points. Retrievers are specially trained to locate
and return game to hunters and are used most commonly in hunting waterfowl.
There are four varieties: the curly, the Chesapeake Bay, the golden retriever,
and the Labrador. With the exception of the golden retriever, the Labrador is
the origin of the others by crossbreeding with spaniels and setters. Pointers
and setters, used for hunting upland game birds, range ahead of the hunter,
point the game until the hunter arrives, and retrieve the fowl after it has
been flushed and shot. Among the varieties that are used in these pursuits are
the Weimaraner, the English and Irish setters, and the German short-haired
pointer.
    The hound group includes the basset, the bloodhound, the whippet (produced
by crossing a terrier with an Italian greyhound and then backbreeding to the
English greyhound), the dachshund, the wolfhound, and the beagle. The
greyhound is built for speed with its thin body and long legs. The bloodhounds
and foxhounds are used for hunting, mostly by scent. The beagle, now more a
pet than a hunter, can also follow a scent and is easier to follow on foot..
    The terriers originally were bred for hunting burrowing animals, such as
the badger and fox. The Boston terrier is the only breed to have originated in
the United States. The fox terrier was originally bred for fox hunting but is
an excellent ratter. Other terriers include the Airedale and the Cairn,
Yorkshire, Scottish, and Skye terriers.
    Most of the working breeds are large animals used as draught animals, for
police work, for herding, and as guide dogs for the blind. Draught breeds are
the Alaskan malamute, the Eskimo, the Samoyed, and the St. Bernard. Among the
animals used as guard dogs and for police work are the Doberman, the German
shepherd, and the Great Dane. The collies, Belgian sheep dogs, and English
sheep dogs are outstanding sheep herders. The bulldog is now more of a pet and
house dog than a guard dog. The poodle can be trained as a gun dog and was
originally used for duck shooting.
    Toy breeds are all quite small, some being miniatures of the larger
breeds. The chihuahua is the smallest. Some of the more popular varieties are
the Pomeranian, Pekingese, and pug.

    Wild species.  The wild species of the family, numbering about 36 and
having a wide distribution, include several wild dogs, wolves, coyotes, foxes,
and jackals. There are a number of wild dogs which have never been
domesticated, unlike the dingo of Australia (Canis dingo) that is believed to
have been a domestic dog introduced into Australia during prehistoric times
and then reverted to the wild state. The Asiatic wild dog occurs throughout
Asia, Java, and Sumatra and is considered by some authorities to be three
distinct species. The Cape hunting dog (Lycaon pictus) ranges throughout the
grasslands of eastern and southern Africa but has become reduced in numbers.
    The common European wolf (Canis lupus) is the species that once ranged
throughout the temperate forested regions of Europe, Asia, and North America.
While they occur in some European countries such as Italy, Spain, and the
Balkans, they are plentiful only in the Scandinavian countries. The gray wolf
or timber wolf, originally extremely common in North America, is now
restricted to Alaska and the subarctic regions of Canada.
    The coyote (Canis latrans), sometimes called the prairie wolf, is a close
relative of the true wolf, although it is smaller. Coyotes inhabit the
prairies, open plains, and desert areas of North America.
    Jackals are scavengers as well as menaces to domestic poultry. The
oriental jackal (Canis aureus) has spread from southeastern Europe and
northern Africa through Asia as far south as Burma. It prefers higher
elevations in contrast to the black-backed jackal (C. mesomelas), which is
found in the grasslands of eastern and southern Africa. The most common enemy
of the jackal is the leopard.
    Foxes have relatively short legs, long bodies, and long bushy tails. The
Old World red fox (Vulpes vulpes) is closely related to, but is smaller than,
the American red fox (V. fulva), which is found throughout North America. The
American species has undergone many color phases and mutations and includes
other varieties such as the silver fox and the cross fox. See CARNIVORA;
MAMMALIA.
 �34s[Charles B. Curtin]
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�6v�2v�32sDogwood �21m �33s
    A tree, Cornus florida, also known as flowering dogwood, found in the
eastern half of the United States and in southern Ontario, Canada. When this
tree is in full flower, the four large, white, notched bracts or petallike
growths surrounding the small head of flowers give an ornamental effect that
is unequaled by any native tree. Pink, rose, and cream-colored varieties are
commonly planted. The wood is very hard and is used for roller skates,
carpenters' planes, and other articles in which hardness is desired.
    The Pacific dogwood (C. nuttallii) grows in Idaho and from southwestern
British Columbia to southern California. The Japanese dogwood (C. kousa) is a
similar small tree that blooms in June. Other shrubby species of dogwood are
used as ornamentals. See CORNALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sDolomite   �21m �33s 
    The carbonate mineral CaMg(CO3)2. Often small amounts of iron, manganese, 
or excess calcium replace some of the magnesium: cobalt, zinc, lead, and 
barium are more rarely found. Dolomite is normally white or colorless with a 
specific gravity of 2.9 and a hardness of 3.5 - 4 on Mohs scale. It can be 
distinguished from calcite by its extremely slow reaction with cold dilute 
acid.  
    Dolomite is a very common mineral, occurring in a variety of geologic 
settings. It is often found in ultrabasic igneous rocks, notably in 
carbonatites and serpentinites, in metamorphosed carbonate sediments, where it 
may recrystallize to form dolomite marbles, and in hydrothermal veins. The 
primary occurrence of dolomite is in sedimentary deposits, where it 
constitutes the major component of dolomite rock and is often present in 
limestones. See DOLOMITE ROCK; LIMESTONE; SEDIMENTARY ROCKS. 
 �34s[Alan M. Gaines]
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�6v�2v�32sDolomite rock   �21m �33s 
    A limestone whose carbonate fraction contains more than 50% of the mineral 
dolomite. Most rocks called dolomite have more than 90% dolomite. Newer 
classifications of carbonate rocks based on textural elements introduce the 
amount of dolomite as a secondary criterion. See DOLOMITE; LIMESTONE. 
    Dolomites show gross textures similar to those of limestones, but the 
finer details differ. Most coarsely crystalline dolomite appears to be 
secondary. An original clastic texture may be revealed only by scattered 
quartz grains floating in a mosaic of dolomite crystals. The original outlines 
of fossils or oolites may show as faint dust outlines in the dolomite, but the 
original structure has been lost. A sharply contrasting type of dolomite is 
finely crystalline, shows little or no evidence of replacement origin, and 
preserves many sedimentary structures such as weak current bedding. 
    Dolomite and dolomitic limestone are known from rocks of all ages but are 
more common in older rocks, particularly the Paleozoic. Dolomite is most often 
found in association with limestone, with which it may be interbedded or 
laterally gradational. Modern dolomite has been found in supratidal flats in 
carbonate depositional areas such as the Bahamas and in a variety of 
hypersaline lagoons or arms of the sea in warm climates. The deposits are 
associated with precipitates of calcite, aragonite, and magnesian calcite as 
well as gypsum and anhydrite. See SEDIMENTARY ROCK. 
 �34s[Raymond Siever]
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�6v�2v�32sDolphin   �21m �33s 
    The name for about 33 species of cetacean mammals included in the family 
Delphinidae. These animals may be distinguished from the porpoises by the 
shape of the mouth, which is seen as a pronounced beak in the dolphin while 
the mouth of the porpoise is blunt. See PORPOISE. 
    The common dolphin (Delphinus delphis) is found in the North Sea and the 
Mediterranean and in all other warm and temperate waters. This dolphin is one 
of the swiftest cetaceans and swims in schools as large as 10,000. Dolphins 
surface frequently to breathe air which they take in through the dorsally 
located ``blowhole,'' which has a single aperture. 
    The bottle-nosed dolphin (Tursiops truncatus), restricted to the 
Mediterranean and North Atlantic, is known in the United States as a porpoise 
and is found performing tricks in most seaquariums. Other species of dolphins 
are T. nuuana and T. gilli, both closely related to the bottle-nosed dolphin 
and found along the North Pacific coast. The Amazonian white dolphin (Sotalia 
fluviatilis) is an estuarine species found in many of the rivers of South 
America, while other species of this genus occur in Africa and Asia. They are 
known collectively as the long-beaked river dolphins. See CETACEA; MAMMALIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sDomain (crystallography)   �21m �33s 
    A region in a solid within which elementary atomic or molecular magnetic 
or electric moments are uniformly arrayed. 
    Ferromagnetic domains are regions of parallel-aligned magnetic moments. 
Each domain may be thought of as a tiny magnet pointing in a certain 
direction. A ferromagnet is generally composed of many such domains, pointing 
in many different directions. See FERROMAGNETISM. 
    Antiferromagnetic domains are regions of anti-parallel-aligned magnetic 
moments. They are associated with the presence of grain boundaries, twinning, 
and other crystal inhomogeneities. 
    Ferroelectric domains are electrical analogs of ferromagnetic domains. See 
FERROELECTRICS.  
 �34s[Elihu Abrahams ;Frederic Keff]
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�6v�2v�32sDominance   �21m �33s 
    The development of the same phenotype in homozygotes with two doses of an 
allele and heterozygotes with one dose of the allele and one dose of another 
allele which in homozygotes produces a different phenotype. See MENDELISM. 
    Although complete dominance is common, there are exceptions in which the 
heterozygote is intermediate in character between the two parents. An example 
of this kind of exception is found in the plant Mirabilis: a red-flowered 
strain crossed with a white-flowered strain produces a pink-flowered hybrid. 
This is a case of semidominance. 
    When an inherited character is detectable at the protein or enzyme level, 
it is sometimes found that the heterozygote contains two slightly different 
polypeptides. Several aspects of dominance are well illustrated by sickle-cell 
anemia in humans; the homozygote sickle-cell HsupSHsupS is lethal before the 
individual reaches the age of maturity. The heterozygote HsupAHsupS is almost 
normal. Therefore, insofar as the criterion of viability is concerned, the 
normal is dominant to the sickle cell. The hemoglobins in the heterozygote, 
however, are of two types, normal and sickle-cell, which differ in one amino 
acid. At this level, both the HsupA and HsupS genes are expressed; this is an 
example of codominance. Similar codominance is found in the blood group 
antigens ABO, in which A and B are codominant to one another but A and B are 
both dominant to O, which lacks the antigen altogether. See BLOOD GROUPS. 
    Another expression of the heterozygote which has been classified as a type 
of dominance occurs when the heterozygote transcends both parents. This has 
been called overdominance. Examples of overdominance are rare and difficult to 
establish, but, again, sickle cell provides the best illustration. In an 
environment of falciparum malaria the heterozygote is more fit then either of 
the homozygotes: It is greatly resistant to this persistent malaria; the 
homozygote HsupAHsupA is sensitive, the homozygote HsupS HsupS is virtually 
lethal.  
    These examples have been chosen not only to show the different kinds of 
dominance but also to demonstrate that dominance is a phenomenon of the 
character (phenotype) and not of the gene (genotype). Since characters are the 
result of interactions between genotype and environment, in some cases, the 
type of dominance, as in the over-dominance of sickle cell, is expressed only 
in one environment. R. Goldschmidt called this type of dominance conditioned 
dominance. Despite the many factors affecting dominance, the type of dominance 
found in a gene system can be a powerful indication of the type of gene action 
in the system. See ALLELE; GENE ACTION; HETEROSIS. 
 �34s[D. Lewis]
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�6v�2v�32sDonkeys and allies   �21m �33s 
    Donkeys, asses, and mules are included in the family Equidae along with 
the horses and zebras. These are odd-toed ungulates and therefore belong to 
the mammalian order Perissodactyla. 
    Donkeys (Equus asinus) originated in Africa, and their only relatives in 
the wild state are found in Ethiopia and Somaliland. They have been 
domesticated and are used principally as pack animals throughout Asia and 
Europe. They are hardy animals but better suited to hot climates since they 
are sensitive to cold. 
    The wild ass of Asia (E. hemionus) is known by a variety of common names, 
and because of its extensive distribution it has been divided into a number of 
races or subspecies. In the central regions of Mongolia is the kulan (E. h. 
hemionus), the onager (E. h. onager) occurs in Persia and India, while the 
kiang (E. h. kiang), largest of all the races, is restricted to Tibet and the 
Sikkim. The Syrian wild ass (E. h. hemippus) is extremely rare and possibly 
extinct. They are the most abundant of the wild Equidae after the zebras. The 
African wild ass (E. asinus) is also becoming rare in its natural habitat, 
being found only between the Sudan and Somaliland. 
    The mule is a hybrid resulting from the cross of the male ass (E. asinus) 
and the mare or female horse (E. caballus). When the female ass is bred to a 
stallion, the cross is called a hinny. These crosses do not occur naturally 
and both mule and hinny are usually sterile. See PERISSODACTYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sDonnan equilibrium   �21m �33s 
    The particular equilibrium set up when two coexisting phases are subject 
to the restriction that one or more of the ionic components cannot pass from 
one phase into the other. This equilibrium was first recognized by F. G. 
Donnan in 1911. Commonly, the restriction is caused by a membrane which is 
permeable to the solvent and small ions but impermeable to colloidal ions or 
charged particles of colloidal size. The presence of a membrane is not 
essential, since the restriction of movement on the charged colloid can be 
provided by a centrifugal or gravitational field or by gel coherence. 
    An immediate consequence of such a restriction in a system is the uneven 
distribution of diffusible ions at equilibrium. This is apparent in the 
following example. 

        Na POSITIVE      R NEGATIVE   |   Na POSITIVE    Cl NEGATIVE
        c1               c1           |   c2             c2

Let the initial state of a system be a solution of the ions Na POSITIVE and 
R NEGATIVE of concentration c1, separated from a solution of sodium chloride
of concentration c2 by a membrane freely permeable to all but the R NEGATIVE    
ions. Then at equilibrium, a certain concentration of chloride ions, x, will
have diffused through the membrane, accompanied by the same number of sodium 
ions in order to preserve electrical neutrality on both sides of the membrane,
and the final equilibrium will be:
  
    Na POSITIVE  R NEGATIVE  Cl NEGATIVE |  Na POSITIVE  Cl NEGATIVE
    (c1 + x)                 x           |  (c2 - x)     (c2 - x)
                 (1)                     |             (2)

    It can be shown thermodynamically that at equilibrium the product of the 
concentrations, or more strictly, the activities of the sodium and chloride 
ions, will be the same on both sides of the membrane. Hence may be formed 

          [Na POSITIVE](sub 1) [Cl NEGATIVE] (sub 1) = 
                    [Na POSITIVE] (sub 2) [Cl NEGATIVE] (sub 2) 

or                         (c1 + x)x = (c2 -x) sup 2

    Obviously, the diffusion of the chloride ions (and an equal number of 
sodium ions) through the membrane has been hindered by the presence of the 
nondiffusible ion R NEGATIVE. Calculations based on this equation show that 
sodium chloride is almost completely prevented from diffusing through the 
membrane if it is present in small concentration relative to the concentration 
of the nondiffusible ion R NEGATIVE. As the relative concentration of sodium 
chloride is increased, more of it diffuses through the membrane. Finally, when 
the salt concentration is very high relative to that of R NEGATIVE, an even 
distribution of sodium and chloride ions on either side of the membrane is 
approached. A similar equilibrium is also attained when the diffusible salt 
has no ion common to the colloidal electrolyte. See COLLOID; DIALYSIS; 
ION-SELECTIVE MEMBRANES AND ELECTRODES. 
_FIELD034;-237[George S. Mill; W. O. Milligan]_FIELD034
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�6v�2v�32sDonor atom   �21m �33s 
    An impurity atom in a semiconductor which can contribute or donate one or 
more conduction electrons to the crystal by becoming ionized and positively 
charged. For example, an atom of column V of the periodic table substituting 
for a regular atom of a germanium or silicon crystal is a donor because it has 
one or more valence electrons which can be detached and added to the 
conduction band of the crystal. Donor atoms thus tend to increase the number 
of conduction electrons in the semiconductor. See ACCEPTOR ATOM: SEMICONDUCTOR. 
 �34s[Hsu Y. Fan]
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�6v�2v�32sDoppler effect �21m �33s
    A change in the observed frequency of sound, light, or other waves, caused
by motion of the source or of the observer. A familiar example for sound waves
is the increase (decrease) in pitch of a train whistle as the train approaches
(passes). The optical phenomenon is shown in the altered frequencies of
spectral lines in the light emitted from a moving star. If the star and the
Earth are moving closer to one another, more light pulses are received in a
given time interval, and the color emitted from the star appears to be shifted
toward the violent end of the spectrum. When the distance between the Earth
and the star is increasing, the observed light is shifted toward the red end
of the spectrum. The color shifts of remote galaxies are taken as evidence
that the universe is expanding. See REDSHIFT. For an important application of
the Doppler effect see DOPPLER RADAR.
    Three fundamental differences exist between the acoustical and the optical
Doppler effects.
    1. The optical frequency change does not depend upon whether it is the
source or the observer that is moving with respect to the other, whereas the
acoustical frequency change is different in the two cases.
    2. No effect is observable in the acoustical case when the source, or the
observer, moves at right angles to the line connecting the source and the
observer. An optical frequency change is observable under such conditions.
    3. The motion of the medium through which the waves are propagated does
not affect the observed optical frequency, whereas it does affect the observed
acoustical frequency.
 �34s[George W. Stroke]
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�6v�2v�32sDoppler radar �21m �33s
    A radar system used to measure the relative velocity of the system and the
radar target. The operation of these systems is based on the fact that the
Doppler frequency shift in the target echo is proportional to the radial
component of target velocity.  See  DOPPLER EFFECT.
    Airborne systems are used to determine the velocity of the vehicle
relative to the Earth for such purposes as navigation, bombing, and aerial
mapping, or relative to another vehicle for fire control or other purposes.
Ground or ship equipment is used to determine the velocity of vehicular
targets for fire control, remote guidance, intercept control, traffic control,
and other uses.
    Doppler navigation radar is a type of airborne Doppler radar system for
determining aircraft velocity relative to the Earth's surface. It is generally
used with a navigation computer. The signal from a single beam can provide only
the velocity component in the direction of that beam. Complete velocity
determination requires, therefore, the use of at least three beams. Most
systems use four beams for symmetry.
    A preferred technique for obtaining coherent detection is to employ
sinusoidal frequency modulation. A sideband of the detected beat between echo
and transmitter signal is used. Modulation index and rate and the sideband
order are chosen such that echoes from nearby objects are rejected, while those
from distant objects are accepted. Leakage noise is reduced at the expense of
lowered efficiency.						  [F.B.B.]
    Pulse Doppler radars are useful tools for the observation of the movements
of precipitation particles. The Doppler frequency shift associated with the
velocity of atmospheric targets, such as precipitation particles or artificial
chaff, is always a very small fraction (10 sup-6 to 10 sup-8 ) of the radar
operating frequency. The observation and measurement of such small frequency
shifts require excellent radar system frequency-stability characteristics that
are not usually found in conventional radars but can be added without a drastic
increase in equipment cost. See RADAR.
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�6v�2v�32sDoppler VOR   �21m �33s 
    A vhf omnidirectional radio range (VOR) employing the Doppler principle. 
If VOR beacons are installed in the vicinity of obstructions, or when aircraft 
using VOR signals fly over mountainous terrain, the bearing accuracy is 
deteriorated by reflections (site and enroute errors). The Doppler VOR solves 
this problem by two fundamental principles maintaining full compatibility of 
the radiated information with existing airborne receivers. It uses (1) a 
wide-base antenna array for suppressing the effects of multipath propagation 
and (2) the Doppler principle for determination of bearing. See DOPPLER 
EFFECT; RADIO RANGE. 
    Disturbances by multipath propagation can be reduced by wide-aperture 
systems because of integration or averaging between the information 
transferred over the direct and the reflected propagation path. Widebase 
systems, however, have the drawback that their directional information becomes 
ambiguous as soon as the aperture is larger than half a wavelength, 
approximately.  
    By applying the Doppler principle, one can overcome these limitations. A 
dipole rotating eccentrically on an orbit periodically changes the distance 
between transmitter and receiver, thus modulating the frequency of the 
radiated carrier by a deviation fD according to the equation below. Here D 
is the diameter of the array and f sub rot. is the number of rotations, or 

    *** f sub D = velocity/wavelength = [PI Df (sub rot.)] / LAMBDA ***

revolutions, per second. The resulting Doppler effect produces a sinusoidal 
frequency modulation of the carrier frequency; the phase of this FM contains 
the bearing information (variable phase). 
    The Doppler VOR presents two essential advantages over conventional VOR: 
the improvement of absolute accuracy in good and poor terrain, and the 
spectacular radial stability and smoothness even over mountains. 
 �34s[Ernst Kramar]
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�6v�2v�32sDormancy   �21m �33s 
    A periodic phase in the development of many plants characterized by their 
inability to grow, even in the presence of favorable environmental conditions. 
Dormancy is often considered a phase in the plant's life history during which 
the usual processes contributing to orderly growth and development are 
inhibited. Viewed in this restricted sense, the fact that a succession of 
important morphological and physiological stages precede dormancy and 
contribute to its development is overlooked. 
    This article is concerned primarily with dormancy and its regulation in 
buds and seeds of woody plants. While consideration is given only to these two 
dormancy systems, the dormancy process is widespread throughout the plant 
kingdom and many phenomena common to buds and seeds also may be active in 
regulating dormancy in other systems. 
  
    Buds.  A summary of the events occurring during the seasonal cycle of many 
woody plants illustrates the complexity of the dormancy process and emphasizes 
that both internal and external factors actively regulate specific phases of 
the developmental process. See BUD. 
    For most woody plants growing in the temperate zone, the termination of 
shoot extension and the appearance of buds in summer often provide the first 
visible evidence of dormancy development. During the initial weeks following 
bud formation, growth activity can be renewed by subjecting the plant to a 
regime of long days or favorable temperatures or both. This reversible phase 
of dormancy development has been termed summer dormancy. These observations 
suggest that the changes leading to the dormant state do not take place 
immediately but usually develop progressively during the autumn months. 
Ultimately, growth activity can no longer be stimulated by manipulating the 
environmental conditions and winter dormancy, or true dormancy, develops. 
    Many changes take place within the bud during the time of dormancy 
development. In some species of plants the shoot apex becomes active and 
produces all the leaves for the next season's growth. In other plants only a 
part of the total complement of leaves is produced before dormancy develops, 
the remainder being initiated during the growing season. For some time after 
the primordia have formed, they continue to develop within the bud. The bud 
remains in this condition throughout the dormant period of winter. 
    Release from dormancy may be brought about by a variety of factors, 
including an adequate period of low temperature, lengthening photoperiod, and 
the appropriate chemical stimulation. The bud is now exposed to the 
environmental conditions of spring, which promote growth. During spring and 
early summer, the bud scales open, the shoot elongates rapidly, leaf growth 
occurs, and in some species the shoot apex actively produces new leaves. Then, 
long before the low temperatures and short days of winter arrive, shoot 
extension ceases and dormant buds are produced, thus completing the seasonal 
cycle of bud development. See PHOTOPERIODISM; PLANT HORMONES. 
  
    Seeds.  Most seeds are shed in the mature condition and are 
morphologically complete and apparently physiologically capable of 
germinating. Those which germinate immediately or within a brief period after 
exposure to favorable environmental stimuli are termed quiescent and are 
distinguished from dormant seeds which will not germinate, even under optimal 
conditions, until they have undergone a rest period, that is, a period of 
afterripening. Afterripening, defined as the time during which the synthetic 
machinery of the seed is prepared for germination and growth, is considered to 
be necessary for releasing dormancy. The major causes of seed dormancy are 
those attributed to the seed coat, the embryo, and the presence of various 
physiological inhibitors. See SEED; SEED GERMINATION. 
    In a large number of seeds, dormancy is imposed by the presence of a hard 
seed coat which is impermeable to water and gases or is mechanically 
restrictive to embryo growth. Impermeable seed coats having chemical and 
structural properties preventing water absorption are characteristic of many 
families of plants, particularly the Leguminosae. Water imbibition is an 
important factor in that it is responsible for reactivation of metabolic 
processes leading to seed germination, and treatments which alter the chemical 
or structural barriers to water absorption can be effective agents for 
overcoming dormancy. 
    Probably the most common type of dormancy in seeds is associated either 
with immature embryos or with fully mature embryos which are dormant. In both 
cases a rest period is required by the seeds before germination takes place. 
For seeds with immature embryos, afterripening may be necessary before further 
morphological and physiological developments can occur. Low temperatures are 
usually necessary for embryo maturation and subsequent seed germination. In 
addition, there is a very close correlation between growth of the embryo and 
loss of stored materials from the endosperm. This suggests that low 
temperature stimulates the transfer of nutrients from endosperm to embryo and 
promotes growth. Once the embryo has matured, germination follows rapidly. 
    Inhibitors of seed germination are known to exist in a variety of fleshy 
and nonfleshy fruits and in seeds. They may occur in the pericarp, endosperm, 
seed coat, or embryo; when present in high concentrations, they prohibit 
embryo growth and seed germination. Removal of the tissue surrounding the 
embryo or extensive leaching of the seed or embryo often overcomes dormancy 
associated with inhibitors. See PLANT GROWTH. 
 �34s[Augustus E. de Maggio]
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�6v�2v�32sDorylaimoidea �21m �33s
    One of the commonest groups of soil- and freshwater-inhabiting nematodes
with the taxonomic status of an order or superfamily. The almost endless
series of morphological variations between families, genera, and species have
adapted them to live in every ecological complex in which other free-living
organisms are found. Predatory species feed on earthworms, nematodes, and
other microorganisms. Mycophagous forms browse on the mycelia of fungi,
whereas the intestines of other species are filled with algae and fragments of
mosses. Many are numbered among the most economically important plant
parasites, especially those of the subfamily Longidorinae. See NEMATA.
 �34s[Gerald G. Thorne]
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�6v�2v�32sDosimeter   �21m �33s 
    A meter used for measuring the dose of ionizing radiation received by the 
person wearing it. There are many types of dosimeters, the most common being 
the air-capacitor dosimeter and the thermo-luminescent dosimeter. See 
AIR-CAPACITOR DOSIMETER; FILM BADGE; MONITORING OF IONIZING RADIATION; 
THERMOLUMINESCENT DOSIMETER. 
 �34s[Karl Z. Morgan]
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�6v�2v�32sDoublet flow   �21m �33s 
    In hydrodynamics, a doublet is the combination of a source and a sink of 
equal strength which are allowed to approach each other in such a manner that 
the product of their strength and the distance between them remains constant 
in the limit. Doublets have directional properties, the line drawn from the 
sink toward the source being the axis of the doublet. The strength of a 
doublet is proportional to the product of strength of source and distance 
between source and sink before the limit is taken. See SINK FLOW; SOURCE FLOW. 
    The doublet is a flow element that is used in combination with other 
elements to build up special flow cases. For example, a uniform flow 
superposed on a two-dimensional doublet so that the axis of the doublet is 
directed upstream yields the flow case of uniform flow around a circular 
cylinder. In three-dimensional flow, uniform flow and a doublet directed 
upstream result in the case of uniform flow around a sphere. 
 �34s[Victor L. Streeter]
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�6v�2v�32sDouglas fir �21m �33s
    A large coniferous tree, Pseudotsuga menziesii, also known as red fir.
This tree grows in the Pacific Coast region and the Rocky Mountains of the
United States and differs from true fir in having short leaf stalks,
elliptical leaf scars, and pendant cones. The wood is hard and strong and is
used throughout the United States for construction timber, lumber, and plywood
veneer. It is also used for cooperage, mine timbers, mill work, railway car
construction, flooring, furniture, ships, and ladders. In addition, it is used
as a shade tree, an ornamental, and for shelter belts. Many cultivated
varieties are planted in the eastern United States and Europe. See PINALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sDown's syndrome   �21m �33s 
    A type of developmental disability historically known as mongolism because 
of the characteristic appearance of the eyes. This condition is due to an 
abnormality in either the number or structure of the chromosomes. The most 
common type of abnormality in Down's syndrome is due to an extra chromosome. 
Such persons have 47 chromosomes (instead of 46). 
    The characteristic physical features include almond-shaped eyes and a 
rounded, brachycephalic skull with a flattened occipital region; an enlarged, 
fissured tongue; broad hands with stubby fingers; hypotonic muscle 
development; a short nose and depressed nasal bridge; thick, everted, and 
cracked lips; dry, rough skin; subnormal height; and infantile genitalia. All 
these physical signs are not present in every case. 
    The degree of mental defect may be directly related to the number and 
gravity of the physical signs; however, few Down's children are classified as 
severely retarded. Most cases are moderate to mild, and are often educable; 
the rest are usually trainable by educational definition. 
    Down's syndrome does not appear to be a hereditary disease following a 
typical genetic pattern. The major factor which appears with consistency in 
studies of the etiology is that the probability of a Down's syndrome birth 
increases with maternal age after age 35, and in advanced paternal age. Other 
than the certainty that Down's syndrome is a manifestation of a defect in 
embryonic development and chromosomal abnormality, etiology is still in doubt. 
Prenatal determination of the presence of a Down's syndrome fetus can be 
ascertained through amniocentesis. See HUMAN GENETICS; MENTAL DEFICIENCY. 
 �34s[Henry Leland]
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�6v�2v�32sDrafting   �21m �33s 
    The making of drawings of objects, structures, or systems that have been 
visualized by engineers, scientists, or others. Such drawings may be executed 
in the following ways: manually with drawing instruments and other aids such 
as templates and appliques, freehand with pencil on paper, or with automated 
devices.  
    A drafting machine has two straight-edges which, although fixed at right 
angles to each other, maintain a preset angular relationship to the work when 
moved from place to place over the drafting surface. Engineering drawings 
often require making numerous parallel lines. Drafting machines simplify the 
making of these and other types of drawings and substantially reduce the 
worker-hours required. 
    On most drafting machines the straightedges can be pivoted to any desired 
angle, then locked. Two types of mechanisms are in use to allow the 
straightedges to move over the drafting surface. One consists of vertical and 
horizontal tracks or bars (see illustration). The other consists of mechanical 
linkages which may take the form of a double set of parallel four-bar linkages 
having a common member at the elbow, or two sets of equal-diameter pulleys 
tightly coupled by metal bands. See FOUR-BAR LINKAGE. 
    As the complexity of designs has increased and the use of standard parts 
has become widespread, drawings have become increasingly stylized. Templates 
carry frequently used symbols, from which the symbols in the required 
positions on the drawing can be quickly traced. 
    Where the design procedures from which drawings are developed are 
repetitive, computers can be programmed to perform the design and to produce 
their outputs as instructions to automatic drafting equipment. Essentially, 
automated drafting is a method for creating an engineering drawing or similar 
document consisting of line delineation either in combination with, or 
expressed entirely by, alphanumeric characters. See COMPUTER GRAPHICS; 
ENGINEERING DRAWING. 
 �34s[Tram C. Pritchard]
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�6v�2v�32sDragonfly   �21m �33s 
    A member of the suborder Anisoptera of the order Odonata of the class 
Insecta. The adults are large, attractive insects characterized by four 
similar-appearing elongate, membranous wings with numerous characteristic 
veins; very large compound eyes; tiny antennae; chewing mouthparts; and a 
long, slender abdomen terminating in a short pair of cerci. Adults are strong, 
agile fliers and are all predacious. In flight, their legs are held to form a 
``basket'' which entraps small insects. Dragonflies are considered very 
beneficial since they consume large numbers of mosquitoes, flies, gnats, and 
similar pests. Dragonflies are most commonly found near permanent water. Eggs 
are laid in the water or on the banks. The nymphs are all aquatic and 
predacious. See ODONATA. 
 �34s[Frank R. Voorhees]
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�6v�2v�32sDrawing of metal   �21m �33s 
    An operation wherein the workpiece is pulled through a die, resulting in a 
reduction in outside dimensions. This article deals only with bar and wire 
drawing and tube drawing. See SHEET-METAL FORMING. 
    Among the variables involved in the drawing of wires and bars are 
properties of the original material, percent reduction of cross-sectional 
area, die angle and geometry, speed of drawing, and lubrication. The operation 
usually consists of swaging the end of a round rod to reduce the 
cross-sectional area so that it can be fed into the die; the material is then 
pulled through the die at high speeds. Most wire drawing involves several dies 
in tandem to reduce the diameter to the desired dimension. Die materials are 
usually alloy steels, carbides, and diamond. Diamond dies are used for drawing 
fine wires. The purpose of the die land is to maintain dimensional accuracy 
(see illustration). 
    Tubes are also drawn through dies to reduce the outside diameter and to 
control the wall thickness. The thickness can be reduced and the inside 
surface finish can be controlled by using an internal mandrel (plug). Various 
arrangements and techniques have been developed in drawing tubes of many 
materials and a variety of cross sections. Dies for tube drawing are made of 
essentially the same materials as those used in rod drawing. 
 �34s[Serope Kalpakjian]
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�6v�2v�32sDredge   �21m �33s 
    A digging machine installed on a floating hull used to excavate underwater 
and discharge into a containment vessel or a transport pipeline. A dredge may 
be either mechanical or hydraulic. Mechanical dredges pick up material with a 
scoop, shovel, or bucket and deposit it in a barge for transport. They include 
the revolving crane with clamshell or dragline bucket, the backhoe, the 
front-opening dipper, and the endless-chain bucket dredge. Hydraulic dredges 
pick up material with a pump. The solids are carried up the pipe with the flow 
of water. They include the trailing suction hopper dredge, the cutterhead or 
pipeline dredge, and the dustpan dredge. See CONSTRUCTION EQUIPMENT. 
 �34s[Keith W. Lawrence]
�33s 
  
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�6v�2v�32sDrier (paint)   �21m �33s 
    A material added to paint formulations to facilitate the oxidation of 
oils. Driers are salts of metals. Acids used to render metals soluble in oil, 
and therefore useful as driers, may be fatty acids, rosin, or naphthenic or 
octoic acids derived from petroleum. Cobalt, the most reactive of drier 
metals, is generally regarded as a surface drier, and it is widely used as the 
only additive in thin-film paint formulations. Lead, while less reactive than 
cobalt, may have increasing restrictions in its applications because of 
ecological requirements. Numerous other metals, including cerium and vanadium, 
have been used occasionally, and are effective driers. Certain organic 
compounds also catalyze the drying of oils and have been used for this purpose 
when freedom from all metallic contamination is required. 
    In general, small amounts of drier are essential to the formation of a 
satisfactory paint film within a reasonable time. Addition of large amounts of 
drier, however, can lead to premature embrittlement and failure of the paint 
film. See DRYING OIL; PAINT. 
 �34s[C. R. Martinson ;C. W. Sisler]
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�6v�2v�32sDrill hole logging   �21m �33s 
    The technique of analyzing and recording the character of the formations 
penetrated by a drill hole in mineral exploration and exploitation work. 
    Drill-hole logs (records) are compilations of data obtained from (1) 
recordings of the action of drill and auxiliary equipment, (2) examination of 
samples produced by the drilling, in the form of cores, cuttings, and sludge, 
and (3) examination of the wall of the hole, by visual and geophysical methods. 
    Mineral drilling almost always produces samples in some form as a basis 
for logging. Core drilling is the most accurate because it recovers unaltered 
samples in the form of cores. In all other drilling methods, cuttings of some 
sort are produced as the hole progresses. But even where 100% core recovery is 
obtained, other methods are needed for complete logging. For example, the 
position of natural groundwater level cannot be determined from the core. 
Also, geophysical logging methods may disclose mineralization close to but not 
in the direct path of the drill bit.  See  BORING AND DRILLING (MINERAL); CORE 
DRILLING.  
    The driller's log is a record of all occurrences during drilling that 
might help in a complete logging of the hole or in determining the cost of the 
drilling. Observations of changes in rate of drilling, pump pressure, action 
of the tools, wear on the tools, and color of the circulating fluid may 
furnish the best clues as to depths at which formation changes occurred.  
    Core logging ranges from visual recognition of valueless rocks to the 
multitude of physical and chemical tests to which a mineral specimen may be 
subjected. Visual examination provides recognition of rock type, 
identification of mineral content, and determination of physical condition. 
    Cuttings-analysis logging involves interpretation of the driller's log and 
is commonly supplemented by other logging methods. Cuttings can be ascribed 
only to approximate depths, since they are either in a mixture from a section 
of the hole, as in cable tool drilling, or there is a time delay in their 
reaching the surface. 
    Visual wall logging may be done by actual examination of the wall of the 
hole by a geologist in drilled shafts and inspection holes 30 in. (75 cm) or 
larger in diameter. Logging of the wall has certain advantages over core 
logging in that sedimentation, faulting, and joint patterns may be studied 
with true orientation. 
    In rock mechanics studies and in other analyses such as mine subsidence, 
the condition of the rock strata is equally important as, or more important 
than, the types of formations. Fractured rock is difficult to core, and core 
losses occur frequently in the areas where it is most essential to know the 
condition of the rock. The condition must be seen to be interpreted, and this 
need has led to the development of borehole cameras, for use in drill holes as 
small as 3 in. (7.5 cm) in diameter in some cases. 
    Directional surveying logs the drift of a hole from the vertical and its 
deviation in azimuth. Directional logging has made possible the development of 
directional drilling, for such valuable uses, as drilling a number of 
deflected wells from an offshore drilling platform. 
    All logging methods utilizing electrical recording instruments run in a 
drill hole are frequently termed electric well logging. More specifically the 
term describes logging methods recording variations in electrical potential 
due to the natural electrical properties of subsurface formations or due to 
their reaction to applied electrical forces. Variations in resistivity, or the 
reciprocal conductivity, indicate changes in lithology, porosity, and fluid 
content.  
    Radioactivity logging measures natural or induced radioactivity in 
subsurface formations. Rock character is reflected by variations in the 
intensity of radioactive gamma rays emitted from all formations in varying 
degrees because of the presence of uranium, thorium, or potassium minerals, 
even though very minute in amount. Neutron logging measures the concentration 
of hydrogen atoms in the rock formations thereby indicating fluid content and 
porosity.  
    Other significant techniques include: caliper logging, in which variations 
in hole diameter due to erosion of the formations or to mud cake deposition 
are measured; dip logging, which determines the angle and azimuth of dip of 
nonhorizontal formations; sonic logging, a pulse-echo system utilized to 
measure the distance between the instrument and a sound-reflecting surface; 
acoustic logging, which is used to measure variations in sound wave 
transmission between a high energy source and a receiver spaced vertically in 
a drill hole; geophone (velocity) logging, a seismic method using a geophone 
to indicate formation changes by comparison of seismic readings at different 
depths; geothermal (temperature) logging, which measures variations of 
temperature with depth; photoelectric logging, a technique used to determine 
points of water entry into a drill hole; and geochemical well logging, in 
which characteristics of cores and samples are observed and analyzed for trace 
element patterns.  See  GEOPHYSICAL EXPLORATION; PROSPECTING. 
 �34s[Frank C. Sturges]
�33s 
  
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�6v�2v�32sDrilling machine   �21m �33s 
    A motor-driven device fitted with an end cutting tool that is rotated with 
sufficient power either to create a hole or to enlarge an existing hole in 
solid material. One or more flutes or grooves in the drill tool conduct 
coolant to the cutting lips and also provide chip relief. Spot-facing to 
finish the area around a hole, counterboring to enlarge the diameter over part 
of the depth, and countersinking to chamfer edges of a hole are operations 
frequently performed during drilling setups. See BORING; MACHINING OPERATIONS; 
REAMER.  
 �34s[Alan H. Tuttle]
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�6v�2v�32sDrone   �21m �33s 
    A pilotless aircraft capable of performing any nondestructive mission; 
when used destructively, the device is properly termed a missile. There are 
three basic types of drones: the preprogrammed drone, the smart drone, and the 
remotely piloted vehicle. See GUIDED MISSILE; MISSILE. 
    A preprogrammed drone responds to an on-board timer or scheduler on board 
the aircraft and has no sensor contact with the ground. The drone follows a 
set routine of maneuvers, altitude changes, speed changes, and course changes 
that are programmed through an autopilot to the drone's control surfaces and 
engine throttle. The drone is usually recovered by a parachute at the end of 
the mission. 
    A smart drone has any number of sensor contacts and is equipped with an 
on-board computer. The ability of a smart drone to make decisions governing 
course and altitude changes is limited only by its computer and sensor 
capacity. See GUIDANCE SYSTEMS; NAVIGATION. 
    The remotely piloted vehicle, probably the most common type of drone, is 
under the constant control of an operator or pilot through radio links. The 
pilot or pilots can be located on the ground, in other aircraft, or on boats. 
 �34s[R. Dale Reed]
�33s 
  
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�6v�2v�32sDrought   �21m �33s 
    A general term implying a deficiency of precipitation of sufficient 
magnitude to interfere with some phase of the economy. Agricultural drought, 
occurring when crops are threatened by lack of rain, is the most common. 
Hydrologic drought, when reservoirs are depleted, is another common form. The 
Palmer index is used by agriculturalists to express the intensity of drought 
as a function of rainfall and hydrologic variables. 
    The meteorological causes of drought are usually associated with slow, 
prevailing, subsiding motions of air masses from continental source regions. 
These descending air motions, of the order of 660 - 1000 ft (200 or 300 m) per 
day, result in compressional warming of the air and therefore reduction in the 
relative humidity. Since the air usually starts out dry, and the relative 
humidity declines as the air descends, cloud formation is inhibited - or if 
clouds are formed, they are soon dissipated. 
 �34s[Jerome Namias]
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�6v�2v�32sDrug addiction   �21m �33s 
    The state wherein a person is unable to live without the drug in question 
and is compelled to take it. The most dangerous addictive drugs are the 
amphetamine-like drugs, opiates, and cocaine. Various mechanisms of action for 
the addictive drugs have been proposed. It is considered that these drugs 
induce addiction by a direct action on the basic conditioning mechanisms of 
the limbic system so as to either block transmitters that reduce reward or 
increase punishment or to potentiate transmitters that have the opposite 
effect. In addition, addictive drugs induce withdrawal effects, but this 
appears to play a less important part in the establishment of the craving for 
the drug. The interference with the chemical mechanisms of conditioning causes 
the patterns of behavior that led to taking the drugs to become so powerfully 
reinforced, and the behavior-directing mechanism of the brain originating in 
the limbic circuits to become so altered, that these patterns become too 
powerful to be broken by an act of ``will,'' that is, by cortically orginated 
directives to the altered limbic system. It is well known that monkeys become 
more quickly addicted to morphine if they make the injection themselves; that 
is, there must be some behavior emitted that can be reinforced. See 
BARBITURATES; NARCOTIC. 
 �34s[John R. Smythies]
�33s 
  
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�6v�2v�32sDrug resistance   �21m �33s 
    A decreased reactivity of living organisms to the injurious actions of 
certain chemicals. Resistance is usually specific for one chemical or for a 
related group of chemicals, while the susceptibilities to others remain 
unchanged. Independent resistances to diverse chemicals may be accumulated. 
Many living organisms can acquire some degree of resistance through gradually 
increased exposures to a drug, as in human tolerance to arsenic. DDT-resistant 
insects and antibiotic-resistant microorganisms present important sanitary and 
medical problems. Resistance can be temporary, or it can be permanent and 
hereditable. It may be due to several factors, among which are decreased drug 
penetration, increased drug destruction, and alternative metabolic pathways, 
and several mechanisms may coexist. 
 �34s[Henry P. Treffers]
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�6v�2v�32sDrumlin   �21m �33s 
    A streamlined, oval-shaped hill which has been shaped by flowing glacial 
ice. The long axis is parallel to the direction of ice flow, the up-glacier 
slope is usually steeper than the lee slope, and composition includes a 
variety or combination of materials - till, outwash, or bedrock. Drumlins are 
highly localized, but where present, they occur in large numbers. Some 
drumlins are clearly erosional in origin, but in others till deposition 
appears to have been synchronous with drumlin formation. Thus, one or both 
processes must be operative at some time in the subglacial environment where 
drumlins form. 
 �34s[W. Hilton Johnson]
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�6v�2v�32sDry-air filter   �21m �33s 
    The broadest category of air filters in terms of the variety of designs, 
sizes, and shapes in which they are manufactured. The most common filter 
medium is glass fiber. Dry filters employ the principles of interception, in 
which particles too large to pass through the filter openings are literally 
strained from the airstream; impaction, in which particles strike and stick to 
the surfaces of the glass fibers because of natural adhesive forces, even 
though the fibers are not coated with a filter adhesive; and diffusion, in 
which molecules of air moving in a random pattern collide with very fine 
particles of airborne solids, causing the particles to have random movement as 
they enter the filter media. It is this random movement which enhances the 
likelihood of the particles coming in contact with the fibers of filter media 
as the air passes through the filter. Through the process of diffusion a 
filter is able to separate from the airstream particles much smaller than the 
openings in the medium itself. See AIR FILTER; MOLECULAR ADHESION. 
 �34s[Morton A. Bell]
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�6v�2v�32sDry cell   �21m �33s 
    A primary cell in which the electrolyte is absorbed in a porous medium, or 
is otherwise restrained from flowing. Common practice limits the term dry cell 
to the Leclanche cell, which is the major commercial type. Other dry cells, 
not discussed in this article, include the mercury cell, the 
alkaline-zinc-manganese dioxide cell, and the air-depolarized cell. These 
cells all use aqueous electrolytes immobilized in absorbent materials or gels. 
By order of the Federal Trade Commission, ``leakproof'' must not be printed on 
any dry cell. See PRIMARY BATTERY. 
    The Leclanche cell is made in a variety of sizes in either round or flat 
shapes. The negative electrode is usually solid  zinc. The carbon positive 
electrode is embedded in a black mixture of manganese dioxide and carbon 
black. The carbon is either a rod located in the center of a cylindrical cell 
or a coating on the back of the flat zinc electrode in the flat cell. The 
separator between the black mix and the zinc electrode consists of a paper 
barrier coated with cereal or methyl cellulose. The electrolyte is a solution 
of ammonium chloride and zinc chloride in water. The cell enclosure consists 
of a top seal in the cylindrical cell or thin plastic wrappings for the flat 
cell.  
    The service capacity of dry cells is not a fixed number of ampere-hours, 
but varies with current drain, operating schedule, cutoff voltage, operating 
temperature, and storage conditions prior to use. Most cells are tailor-made 
for their rated end use. The higher the temperature during discharge, the 
greater is the energy output. Conversely, the lower the temperature, the lower 
is the output. At -9.4DG (-23DGC) the battery is virtually inoperative. 
However, shelf life is influenced in the reverse direction by environmental 
temperatures. Shelf life is the period of time that a battery can be stored 
before it drops to 90% of its capacity when tested fresh at 70DGF (21DGC) and 
50% relative humidity. In general, shelf life decreases as the cell size 
becomes smaller. Tests show that, when batteries are frozen, they suffer no 
deterioration for about 10 years. They must, however, be allowed to reach room 
temperature before use. 
    It is possible to recharge dry-cell batteries for five or six cycles 
provided the following precautions are taken: The battery should be subject 
only to shallow discharges between cycles, used immediately after charging, 
charged over a 10- to 15-h period at constant current, and not overcharged. 
 �34s[Jack Davis ;Kenneth Franzese]
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�6v�2v�32sDrydocking   �21m �33s 
    A technique used to remove a ship from the water so that the underwater 
portion may be inspected, repaired, maintained, or altered. Occasionally 
underwater repairs may be undertaken while a ship is afloat; however, at 
regular intervals, or as dictated by emergency, it may be necessary to expose 
all of the underwater portion, regardless of whether the ship is a small 
harbor tug or a large transoceanic liner. 
    The four types of dry docks are known as marine railways, floating dry 
docks, graving docks, and mechanical lift docks. The size of the ship usually 
determines which type is used. 
    The marine railway consists of a cradle of wood or steel with rollers on 
which the ship may be hauled out of the water along a fixed inclined track 
leading up the bank of a waterway. The advantages of a marine railway lie in 
the economy of the original construction and the relative low cost of 
maintenance. A marine railway is ideal for ships up to 5000 tons. 
    The floating dry dock may be constructed of wood, steel, or concrete. The 
dock is submerged, to provide the required depth of water over the keel 
blocks, by partially filling its tanks with water. The ship to be drydocked is 
then positioned within, the tanks of the dock are rapidly pumped out by 
powerful pumps located within the dock walls, and the ship is lifted out of 
the water. 
    The graving dock consists of an excavation in the ground with a thick 
concrete base supported, if necessary, by piling and surrounded on three sides 
by earth held back by timbers, stone, cement, or steel supports, or a 
combination of these materials. The entrance, or seaward end of the dock, is 
usually closed by a caisson of the pontoon type which, when flooded, is 
trimmed down into position. The dock is flooded, the caisson is floated, and 
the ship enters the dry dock and is positioned over the keel blocks. The 
caisson is then replaced and submerged, the dock is pumped out, and the ship 
settles on the keel blocks. This process is reversed when the ship is ready to 
leave the dock. 
    The mechanical lift dock is somewhat similar in action to the floating dry 
dock. The vessel, after taking up on the keel and bilge blocks in the dock, is 
bodily lifted clear of the water. The mechanical platform dock has much more 
flexibility than other types and has increased greatly in size and use. 
 �34s[Lemuel C. Robertson]
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�6v�2v�32sDry ice   �21m �33s 
    A solid form of carbon dioxide, CO2, which finds its largest application 
as a cooling agent in the transportation of perishables. It is nontoxic and 
noncorrosive and sublimes directly from a solid to a gas, leaving no residue. 
At atmospheric pressure it sublimes at -109.6DGF (-78.7DGC). Slabs of dry ice 
can easily be cut and used in shipping containers for frozen foods, in 
refrigerated trucks, and as a supplemental cooling agent in refrigerator cars. 
See CARBON DIOXIDE; REFRIGERATION. 
 �34s[Carl F. Kayan]
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�6v�2v�32sDrying of gases   �21m �33s 
    The removal of 95 - 100% of the water vapor in air or other gases. Gases 
having a dew point of  - 40DGF (-40DGC) are considered commercially dry. The 
more important reasons for the removal of water vapor from air are: (1) 
comfort, as in air conditioning; (2) control of the humidity of manufacturing 
atmospheres; (3) protection of electrical equipment against corrosion, short 
circuits, and electrostatic discharges; (4) requirement of dry air for use in 
chemical processes where mositure present in air adversely affects the economy 
of the process; (5) prevention of water adsorption in pneumatic conveying; and 
(6) as a prerequisite to liquefaction. 
    Gases may be dried by the following processes: (1) absorption by use of 
spray chambers with such organic liquids as glycerin, or aqueous solutions of 
salts such as lithium chloride, and by use of packed columns with 
countercurrent flow of sulfuric acid, phosphoric acid, or organic liquids; (2) 
adsorption by use of solid adsorbents such as activated alumina, silica gel, 
or molecular sieves; (3) compression to a partial pressure of water vapor 
greater than the saturation pressure to effect condensation of liquid water; 
(4) cooling below dew point of the gas with surface condensers or coldwater 
sprays; and (5) compression and cooling, in which liquid desiccants are used 
in continuous processes in spray chambers and packed towers - solid desiccants 
are generally used in an intermittent operation that requires periodic 
interruption for regeneration of the spent desiccant. 
    The mechanical methods of drying gases, compression and cooling and 
refrigeration, are used in large-scale operations, and generally are more 
expensive methods than those using desiccants. Such mechanical methods are 
used when compression or cooling of the gas is required. See DESICCANT; GAS 
ABSORPTION OPERATIONS. 
 �34s[William R. Marshall, Jr.]
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�6v�2v�32sDrying of solids   �21m �33s 
    An operation in which a liquid, usually water, is removed from a wet solid 
in equipment termed dryers. The use of heat to remove liquids distinguishes 
drying from mechanical dewatering methods such as centrifugation, decantation 
or sedimentation, and filtration, in which no change in phase from liquid to 
vapor is experienced. Drying is preferred to the term dehydration, which is 
sometimes used in connection with the drying of foods. Dehydration usually 
implies removal of water accompanied by a chemical change. 
    Drying is a widespread operation in the chemical process industries. It is 
used for chemicals of all types, pharmaceuticals, biological materials, foods, 
detergents, wood, minerals, and industrial wastes. The materials dried may be 
in the form of thin solutions, suspensions, slurries, pastes, granular 
materials, bulk objects, fibers, or sheets. Drying may be accomplished by 
convective heat transfer, by conduction from heated surfaces, by radiation, 
and by dielectric heating. In general, the removal of moisture from liquids 
(that is, the drying of liquids) and the drying of gases are classified as 
distillation processes and adsorption processes, respectively. See ADSORPTION; 
CENTRIFUGATION; DISTILLATION; DRYING OF GASES; EVAPORATION; FILTRATION; 
SEDIMENTATION (INDUSTRY). 
    Equipment for drying solids may be conveniently grouped into three classes 
on the basis of the method of transferring heat for evaporation: direct 
dryers, indirect dryers, and radiant heat dryers. Batch dryers are restricted 
to low capacities and long drying times. Most industrial drying operations are 
performed in continuous dryers. The large numbers of different types of dryers 
reflect the efforts to handle the large numbers of wet materials in ways which 
result in the most efficient contacting with the drying medium. Thus, filter 
cakes, pastes, and similar materials, when preformed in small pieces, can be 
dried many times faster in continuous through-circulation dryers than in batch 
tray dryers. Similarly, materials which are sprayed to form small drops, as in 
spray drying, dry much faster than in through-circulation drying. 
 �34s[William R. Marshall, Jr.]
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�6v�2v�32sDrying oil   �21m �33s 
    An oil that readily undergoes autoxidation and polymerization to form a 
hard, dry film on exposure to air. Drying oils are relatively highly 
unsaturated; that is, they are composed of triglycerides constructed from 
unsaturated fatty acids. The best drying oils contain several nonconjugated 
double bonds per molecule. 
    Raw (untreated) drying oils are not suitable for paints and varnishes 
because they polymerize too slowly, and various methods have been introduced 
to improve the polymerization process. One method involves boiling the oil 
after addition of soluble resin-acid salts. Boiled oil, dries in approximately 
one-fifth the time in which raw oil dries. Blown oil, produced by blowing air 
through the oil (to which driers have been added) at about 120DGC (248DGF), is 
said to have superior wetting or surface-covering properties. Stand oil has 
been partially polymerized, with admixture of driers, by heating to 260 - 
280DGC (500 - 536DGF). This material is used extensively in antifouling 
paints, printing inks, and linoleum, as well as in varnishes and enamels. 
Linseed oil is the most widely used drying oil in paints and varnishes. See 
DRIER (PAINT). 
 �34s[Evans B. Reid]
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�6v�2v�32sDucted fan   �21m �33s 
    A propeller or multibladed fan inside a coaxial duct or cowling, also 
called a ducted propeller or a shrouded propeller, although in a shrouded 
propeller the ring is usually attached to the propeller tips and rotates. The 
duct serves to protect the fan blades from adjacent objects and to protect 
objects from the revolving blades, but more importantly, the duct prevents 
radial flow of the fluid at the blade tips. Fan efficiency remains high over a 
wider speed range with a properly shaped duct than without. However, fan 
efficiency is sensitive to duct shape at off-center design conditions. See 
DUCTED FLOW; FAN. 
    Ducted fans are used in axial-flow blowers or compressors of several 
stages for turbine engines. A ducted fan engine is a gas turbine arranged to 
move a larger mass of air than passes through the turbine, the additional air 
leaving at lower exit velocity and hence higher jet propulsion efficiency for 
moderate-speed aircraft than obtainable with a simple turbojet. See GAS 
TURBINE; TURBOFAN; TURBOJET. 
 �34s[Frank H. Rockett]
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�6v�2v�32sDucted flow   �21m �33s 
    Fluid flow with zero velocity at the boundary relative to the boundary. 
This condition is distinguished from jet flow, in which the boundary is a 
fluid, either liquid or gas, and does not remain stationary. 
    Under certain conditions a propeller has a housing or shrouding around it 
to control the fluid approaching it, as well as to control the induction of 
fluid into the jet downstream from the propeller. Shrouding a propeller may be 
used on a ship to decrease interference of propeller and hull, or it may be 
used to protect the propeller. See DUCTED FAN; JET FLOW. 
 �34s[Victor L. Streeter]
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�6v�2v�32sDumortierite   �21m �33s 
    A nesosilicate mineral with composition Al8BSi3O19(OH). Dumortierite 
crystallizes in the orthorhombic system but well-formed crystals are rare; the 
mineral usually occurs in parallel or radiating fibrous aggregates. The 
hardness is 7 on Mohs scale, and the specific gravity is 3.26 - 3.36. The 
mineral has a vitreous luster and a color that varies not only from one 
locality to another but in a single specimen. It may be pink, green, blue, or 
violet. In the United States it occurs at Dehesa, California, and at 
Rochester, Nevada, where it has been mined for the manufacture of high-grade 
porcelain. See SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sDune   �21m �33s 
    A mobile mound, hill, or ridge of windblown material, usually sand. Dunes 
develop where the velocity of sand-driving winds is checked or reduced, often 
as a result of barriers or obstructions. Dunes require not only a suitable 
site for development but an adequate sand source as well. Consequently, most 
dunes are found downwind from sandy lake or ocean beaches; dry stream beds, 
particularly in the desert; and barren, sandy, glacial outwash plains. See 
DESERT.  
    No commonly accepted classification of dunes exists, partly because of 
dune complexity but chiefly because numerous intermediate and special forms 
must be accommodated in any classification. Nevertheless, there are dune types 
that are widely recognized. 
    Draa are very large dune forms, found only in the North African desert, 
whose dimensions are measured in hundreds of meters and whose spacing is in 
kilometers.  
    Transverse dunes include three main types: barchan, akle, and transverse 
ridge. Crescent-shaped barchan dunes are those which develop on nearly flat, 
barren desert surfaces where wind direction is constant and sand supply 
moderate (Fig. 1). Akle dunes are sinuous transverse ridges of sand somewhat 
intermediate in form between the crescentic barchan and the simple transverse 
ridge. Transverse ridges are nearly straight in ground plan, and are oriented 
more or less at right angles to the sand-driving winds. Transverse ridges are 
most common in coastal localities where vegetation affects sand movement, but 
they may occur in desert regions as well. 
    Parabolic or blowout dunes are hairpin or spoon-shaped with horns pointing 
toward the wind, opposite to those of the barchan. 
    Longitudinal or seif dunes are sometimes hundreds of kilometers long, as 
in the Sahara or Australian deserts. Barchan chains pass gradually into seif 
ridges as winds change from unidirectional to multidirectional. As 
sand-driving winds come from increasingly divergent directions, the seif form 
is lost altogether (Fig. 2). 
 �34s[Robert M. Norris]
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�6v�2v�32sDunite   �21m �33s 
    An ultrabasic  rock consisting almost solely of olivine. The type locality 
is Dun Mountain, situated in the so-called Mineral Belt in New Zealand. 
Besides a magnesium-rich olivine, chromite and picotite are common 
constituents of dunites. Dunites are therefore important sources of chromium. 
Spinel, magnetite, ilmenite, pyrrhotite, and native platinum also occur in 
some dunites. Serpentinization of olivine in dunites is widespread. Some 
dunites have changed completely to serpentine bodies. See SERPENTINE; 
SERPENTINITE.  
    Dunites may form by fractional crystallization of basic magma or by 
replacement of rocks such as limestone and dolomites; metamorphic 
differentiation has also been suggested as a mechanism of formation. It is 
also possible that some dunites are directly derived from a deep-seated basic 
layer in the earth. See MAGMA; METAMORPHISM. 
 �34s[Hans Ramberg]
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�6v�2v�32sDust and mist collection   �21m �33s 
    The physical separation and removal of solid or liquid particles from a 
gas in which they are suspended. Such separation is required for one or more 
of the following purposes: (1) to collect a product which has been processed 
or handled in gas suspension, as in spray-drying or pneumatic conveying; (2) 
to recover a valuable product inadvertently mixed with processing gases, as in 
kiln or smelter exhausts; (3) to eliminate a nuisance, as a fly-ash removal; 
(4) to reduce equipment maintenance, as in engine intake air filters; (5) to 
eliminate a health, fire, explosion, or safety hazard, as in bagging 
operations or nuclear separations plant ventilation air; and (6) to improve 
product quality, as in cleaning of air used in processing pharmaceutical or 
photographic products. 
    All particle collection systems depend upon subjecting the suspended 
particles to some force which will drive them mechanically to a collecting 
surface. The known mechanisms by which such deposition can occur may be 
classed as gravitational, inertial, physical or barrier, electrostatic, 
molecular or diffusional, and thermal or radiant. There are also mechanisms 
which can be used to modify the properties of the particles or the gas to 
increase the effectiveness of the deposition mechanisms. For example, the 
effective size of particles may be increased by condensing water vapor upon 
them or by flocculating particles through the action of a sonic vibration. 
Usually, larger particles simplify the control problem. To function 
successfully, any collection device must have an adequate means for 
continuously or periodically removing collected material from the equipment. 
    Devices for control of particulate material may be considered, by 
structural or application similarities, in eight categories as follow: gravity 
settling chamber, inertial device, packed bed, cloth collector, scrubber, 
electrostatic precipitator, and air filter. See AIR FILTER; CHEMICAL 
SEPARATION TECHNIQUES; ELECTROSTATIC PRECIPITATOR; MECHANICAL SEPARATION 
TECHNIQUES; UNIT OPERATIONS. 
 �34s[Charles E. Lapple]
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�6v�2v�32sDust storm   �21m �33s 
    A strong, turbulent wind carrying large clouds of dust. In a large storm, 
clouds of fine dust may be raised to heights well over 10,000 ft (3000 m) and 
carried for hundreds or thousands of miles (1 mi = 1.6 km). 
    Sandstorms differ by the larger mass, more rapid settling speeds of the 
particles involved, and the stronger transporting winds required. The sand 
cloud seldom rises above 3.3 - 6.6 ft (1 - 2 m) and is not carried far from 
the place where it was raised. 
    Dust storms cause enormous erosion of the soil, as in the dust bowl 
disasters of 1933 - 1937 in the Great Plains of the United States. Besides 
causing acute physical discomfort, they present a severe hazard to 
transportation by reducing the visibility to very low ranges. Conditions 
required are an ample supply of fine dust or loose soil, surface winds strong 
enough to stir up the dust, and sufficient atmospheric instability for marked 
vertical turbulence to occur. 
    Small dust particles increase scattering of light, mainly in short (blue) 
wavelengths. The Sun often appears a deep orange or red when seen through a 
dust cloud; however, optical effects are variable. Large particles are 
effective reflectors, and an observer in an aircraft above a dust storm may 
see a solid sheet with an apparent dust horizon. 
 �34s[Chester W. Newton]
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�6v�2v�32sDwarf star �21m �33s
    The most common type of star in the Galaxy, also known as a main-sequence
star. The Sun is a typical dwarf, with surface temperature of 5750 K, radius
of 6.96 x 10 sup 5 km, mass of 2 x 10 sup 3 sup 0 kg, and luminosity of 4 x
10 sup 3 sup 3 ergs/s (that is, 4 x 10 sup 2 sup 3 kW). These numbers are fairly
typical as to radius and mass. However, in luminosity, main-sequence stars
occur over a range from O and B stars, up to more than 10 sup 4 times brighter
than the Sun, with surface temperatures of 35,000-15,000 K, down to the
faintest M dwarfs, 10- sup 4 as bright as the Sun, with a temperature of only
2500 K. Dwarfs form a single-parameter family in which the significant
variable is mass; members range in mass from 20 Suns down to less than 0.1
Sun. The M or red dwarfs are the most common stars in space, and because of
their low luminosity have the longest life. Almost all other types of stars
have evolved from main-sequence stars. See STAR.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sDyadic   �21m �33s 
    A mathematical abstraction corresponding to an expression of the type bd x 
ef x . . . , in which the elements (dyad symbols) consist of two vector 
symbols in juxtaposition without the intervention of either the dot (.) or 
cross (x). Essentially, a dyad is an ordered pair of vectors subject to 
certain rules of operation. The first symbolic factor in a dyad (b in bd, for 
example) is called the antecedent and the second the consequent. See CALCULUS 
OF TENSORS; CALCULUS OF VECTORS. 
 �34s[Homer V. Craig]
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�6v�2v�32sDye   �21m �33s 
    A colored substance, also called a dyestuff, which imparts more or less 
permanent color to other materials. See DYEING. 
    Customarily, colored water-insoluble substances are called pigments. Dyes 
are generally water-soluble, although some are soluble only during 
application, after which they become insoluble. See PIGMENT. 
    The mechanism by which soluble colored substances enter the internal 
structure of fibers and there become fixed has been variously explained in 
terms of the physical and chemical concepts of the times when the explanations 
were given. It is said to be an adsorption phenomenon, a salt formation, a 
quasi-chemical union caused by hydrogen bonding, or an ether linkage, and in 
some cases it is considered to be a true solution effect. The end result, 
however, is that the dye has imparted a color (not necessarily that of the 
solid dye itself) to the fiber which is more or less resistant to washing or 
removal by similar mechanical operations. The dye is said to be fixed on and 
to have affinity for the material it has colored. The material is designated 
as the substrate. If the color is quite resistant to washing and light, it is 
called a fast color; if the color is easily removed or fades quickly, it is a 
fugitive dye. 
    Dyestuffs may be classified in various ways: according to color (blue, 
red, and so on); origin (natural - from vegetable and animal matter - or 
synthetic); chemical structure (the most precise), kinds of material to which 
they are applied (cloth, paper, leather, plastic, food, biological specimens, 
etc.); and method of application (used most frequently by the practical dyer). 
 �34s[Wesley Minnis]
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�6v�2v�32sDyeing �21m �33s
    The application of color-producing agents to material, usually fibrous or
film, in order to impart a degree of color permanence demanded by the
projected end use. True dyeing covers mechanisms in which molecules of
material to be dyed become involved by various means with the molecules of the
coloring matter, or small aggregates thereof. There is some overlapping
between true dyeing and other methods of coloring, which are called dyeing in
the industry. Products which are commonly dyed include textile fibers, plastic
films, anodized aluminum, fur, wood, paper, leather, and some foodstuffs. See
DYE.
    The broad term affinity is used to describe the various types of
attraction between the material to be colored and the dye. Affinity may be
caused by attraction between charged dye particles and oppositely charged dye
sites on the material, by various types of chemical attraction, and by
formation of solid solutions of dye in the material.
    When affinity is involved, dyeing is an exothermic process. More dye will
be absorbed by a fiber or film at low temperatures than at higher
temperatures. However, the rate of dyeing increases geometrically, whereas
maximum dye absorption decreases only arithmetically with rise in temperature.
Since maximum dye absorption is rarely necessary or desirable, the trend in
modern practice is toward dyeing at high temperatures and short times.
    Dyeing is accomplished by dissolving or dispersing the colorant in a
suitable vehicle (usually water) and bringing this sytem into contact with the
material to be dyed. Although many dye molecules or aggregates may adhere to
the material surface when they meet, dyeing does not occur until the adhering
dye particles migrate within the fibers or films. All dyeing processes are
designed to accomplish ultimately penetration of the undyed substance by the
colorant.
    Assistants are materials which do not impart color to the product to be
dyed but, as the name suggests, promote or retard dyeing. Usually, but not
always, they affect the dye molecule.
    Swelling agents are assistants which open up the structure of the fiber
temporarily so that dye molecules or aggregates may enter more freely and
reach otherwise inaccessible dye sites.
    Carriers are agents (often solvents of low water solubility) which
accelerate dyeing by breaking up or dissolving dye aggregates and bringing
them to the fiber-water interface in a size small enough to be absorbed by the
material.
    Dye retarders are a class of dyeing assistants, usually inorganic or
organic salts, which slow up the dyeing process by forming evanescent
compounds with the dye, by buffering or depressing the ionization of an acid
assistant, or by temporarily occupying the more active or more accessible dye
sites on the fiber, later to be dislodged therefrom by the dye.
    Aftertreating agents are salts, resins, or other products (more frequently
applied to cellulosic fibers) to render the colored fabric more resistant to
the effects of washing, perspiration, or fading by ozone or combustion gases.
More often than not, their application causes a loss in light fastness of the
dyed material.

    Textiles.  Cellulose fibers, such as cotton and rayon, are most commonly
dyed by immersion of the fibers in a solution of direct dyes using an
electrolyte such as common salt as assistant and then boiling this dyebath.
Such dyeings usually exhibit only commercial (minimum) resistance to washing.
Treatment of the properly dyed fibers with resins and copper, for example,
increases the resistance to washing with minimum loss of light resistance.
    The dyeing of wool, silk, and fur (felt) involves the formation of salts
by reactive positively charged groups on the fiber and negatively charged
color groups on anionic dyes. Level-dyeing (acid or anionic) dyes, which
require a relatively large amount of strong acid to force the dye onto the
fiber, are used for carpet yarns, felt hats, and wherever even dyeing and
penetration instead of washing fastness is a paramount requirement. Milling
dyes, which demand less acid, are faster to washing and are used for blankets
and sweaters. Where a very high resistance to light and washing is required,
acid-dyeing or neutral-dyeing metallized dyes are employed.
    Synthetic fibers, such as cellulose acetates and triacetate (Arnel), are
dyed in a supension of solvent-soluble dyes by immersion. Polyamide synthetic
fibers are dyed like wool with acid, metallized acid, neutral metallized,
fiber-reactive mordant dyes, azoics, and selected direct dyes from an acid
bath. Special processes have also been developed for acrylic, polyester, and
propylene fibers. See TEXTILE CHEMISTRY.
    There are two fundamental types of textile-dyeing processes - the dye
liquor is pumped through stationary material, or the material to be dyed moves
through the dye liquor.
    Kiers are tanks which hold loose fiber or packaged yarn and the dye liquor
is pumped through the fiber at temperatures up to the boiling point or (if the
fiber can withstand it) under pressure at temperatures up to 270DGF (132DGC).
In continuous dyeing, cloth is impregnated with dye and then passed through a
series of developing, washing, and drying zones to a final take-up roll.

    Nontextile materials.  Dyeing anodized aluminum was formerly believed to
be accomplished by the formation of aluminum salts of dyes by the electrically
deposited aluminum oxide coating. However, many dyes having no salt-forming
groups were found to dye this film. Anodized aluminum is readily dyed by many
textile dyes. Light and weather resistance undreamed of in textile
applications of some of these same dyes is achieved.
    Paper pulp is usually dyed in the paper beater by dyes normally employed
for cotton; on occasion, it is tinted by wool dyes, and it is frequently
tinted by addition of pigments to the beater. Finished paper is also colored
by passing it over rollers which supply dye or colored coatings to its surface
(calender staining).
    Wood is normally stained with solutions of dyes or dispersions of pigments
in water, in solvents, or in lacquers. Wood, however, is frequently dyed by
the application of water solutions of dyes at high temperature under pressures
of 100 psi (6.9 x 10 sup 5 pascals) or more. Freshly felled trees have been dyed
by force-pumping dye solution through the length of the log. The dye solution
replaces the natural moisture and sap.
    Leather is dyed at low temperatures with the classes of dyes normally used
for wool and cotton. Formic acid is normally used to exhaust the dye. For
dress gloves, leather is usually colored by applying the dye on the grain
surface, leaving the flesh side undyed. Leather is also dyed with natural dyes
such as logwood, fustic, and quercitron. Leather fresh from tanning and
containing considerable moisture is dyed in Europe by tumbling with dry
water-soluble dye.
    Most food products which are artificially colored are not actually dyed.
Maraschino cherries, however, are dyed for several hours with food dyes, then
washed and placed in flavored syrup. Glazed citron, citrus, or watermelon
rinds are colored by long immersion followed by drainings in a series of
food-dye-colored, increasingly strong, sugar syrups. Pistachio nuts in the
shell are dyed by applying food-dye solutions with or without salt during
roasting operations.
    It is not generally realized that human hair is, in the final analysis, a
class of textile fiber. Cosmetic companies, appreciating this fact, have hired
textile experts to do research on dyeing, waving, and washing hair. The dyeing
of hair is of course limited to processes employing relatively low
temperatures. Food dyes (also carbon black) mixed with citric or malic acid
are used for so-called rinses. Other rinses employing basic dyes and salt or
textile dyes diluted with salts, detergents, and malic, citric, or tartaric
acids are on the market. Products selected, as a rule, from among the safer
fur-dye intermediates are used for more permanent and intense coloring
techniques; these employ hydrogen peroxide for developing. Natural dyes, such
as the age-old henna and some metallic salts, are still encountered in some
hairdyeing establishments. Rinses which are used to mask the yellowish tinge
in gray or white hair frequently use blue or violet food or textile dyes and
contain optical bleaches - substantially colorless compounds which receive
invisible ultraviolet light and reflect it to the eye as a blue or violet
fluorescence.
    Many plastic materials may be dyed by processes similar to those employed
for textiles. Nylon, cellulose acetate, polyethylene, polypropylene, and
polyester resins are dyeable with dyes which color these materials in yarn
form. See NATURAL FIBER; MANUFACTURED FIBER.
 �34s[James E. Loughlin]
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�6v�2v�32sDynamical analogies   �21m �33s 
    Analogies are useful when one is comparing a familiar system with an 
unfamiliar one. An electrical circuit can be considered to be a vibrating 
system, and this immediately suggests analogies between electrical circuits 
and vibrating systems. The work of an acoustical engineer involves the study 
of acoustical, electroacoustical, mechanoacoustical, or 
electromechanoacoustical systems, while a mechanical engineer studies 
vibrating systems involving masses, springs, and friction. The analogies 
between systems of these types and electrical circuits are known as dynamical 
analogies. See ALTERNATING-CURRENT CIRCUIT THEORY; ANALOG COMPUTER; 
KIRCHHOFF'S LAWS OF ELECTRIC CIRCUITS; VIBRATION DAMPING. 
 �34s[K. W. Johnson]
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�6v�2v�32sDynamic braking �21m �33s
    A technique of electric braking in which the retarding force is supplied
by the same machine that originally was the driving motor. Dynamic braking is
effective only in high inertial systems wherein the kinetic energy of the
motor rotor with its connected load is converted into electrical energy and
dissipated mainly as I sup 2 R losses or returned to the source.
    The commonest type of dynamic braking will be explained for a
direct-current (dc) motor. To accomplish braking action, the supply voltage is
removed from the armature of the motor but not from the field. The armature is
then connected across a resistor. The electromotive force generated by the
machine, now acting as a generator driven by the kinetic energy of the
rotating system, forces current in the reverse direction through the armature.
Thus a torque is produced to oppose rotation, and the load decelerates as its
kinetic energy is dissipated, mostly in the external resistor but to some
extent in core and copper losses of the machine.
    Electric braking can also be accomplished by causing the kinetic energy of
the rotating system to be converted in the armature to electrical energy and
then returned to the supply lines. This mode of operation, called regenerative
braking, occurs when the counter electromotive force exceeds the supply
voltage.
    Interchanging two of the lines supplying a threephase alternating-current
(ac) induction motor also produces braking. In this case, called plugging, the
direction of the electromagnetic torque on the rotor is reversed to cause
deceleration. Both the kinetic energy of the system and the energy drawn from
the supply lines are expended in copper and core losses in the machine. See
ALTERNATING-CURRENT GENERATOR; DIRECT-CURRENT MOTOR; INDUCTION MOTOR;
SYNCHRONOUS MOTOR.
 �34s[Arthur R. Eckels]
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�6v�2v�32sDynamic nuclear polarization   �21m �33s 
    The creation of assemblies of nuclei whose spin axes are not oriented at 
random, and which are in a steady state that is not a state of thermal 
equilibrium. Under commonly occurring conditions, the spin axes of nuclei 
(with nonzero spin) are oriented at random; where this is not so, the nuclei 
are said to be polarized. Assemblies of polarized nuclei are not in a state of 
thermal equilibrium except under rather extreme conditions (for example, 
temperatures below 10 millikelvins and magnetic fields greater than several 
teslas), and therefore schemes have been devised to produce polarized 
assemblies, in a steady state which is not a state of thermal equilibrium, 
under less extreme conditions of temperature and so forth. Such schemes 
constitute dynamic nuclear polarization. 
    Among the many applications of polarized nuclei are the following. Nuclear 
forces are spin-dependent, and although the spin-dependent part can be found 
by using unpolarized assemblies, the experiments are simpler and their 
interpretation is clearer if polarized nuclei are used. Assemblies of 
polarized nuclei have a lower geometrical symmetry than assemblies of randomly 
oriented nuclei, and so these have been used to investigate the fundamental 
symmetries of nature. Polarized nuclei have been used to enhance the signal in 
free precession magnetometers and similar instruments, and the use of an 
assembly of polarized nuclei as a gyroscope has also been suggested. See 
MAGNETOMETER; NUCLEAR ORIENTATION; PARITY (QUANTUM MECHANICS); SPIN (QUANTUM 
MECHANICS).  
 �34s[James M. Daniels]
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�6v�2v�32sDynamic programming   �21m �33s 
    A mathematical technique for the study of systems that can be represented 
by certain types of multistage processes. In such processes the output at each 
stage is a function of that at the preceding stage (the state), an action 
taken by a decision maker (the control), and possibly a random event. An 
example occurs in inventory control, in which the inventory of a given item 
depends on the previous day's inventory (the state), the amount produced (the 
control), and the external demand (random event). This gives rise to a 
constraint space of very high dimension. The technique of dynamic programming 
reduces this sequence to that of one-dimensional problems, which are generally 
much simpler to solve. 
 �34s[Guillermo Owen]
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�6v�2v�32sDynamics �21m �33s
    That branch of mechanics which deals with the motion of a system of
material particles under the influence of forces, especially those which
originate outside of the system under consideration. From Newton's third law
of motion, namely, to every action there is an equal and opposite reaction,
the internal forces cancel in pairs and do not contribute to the motion of the
system as a whole, although they determine the relative motion, if any, of the
several parts.
    Particle dynamics refers to the motion of a single particle under the
influence of external forces, particularly electromagnetic and gravitational
forces. The dynamics of a rigid body is the study of the motion, under given
forces, of a system of particles, the distances between which are postulated
to be constant throughout the motion.
    In classical dynamics the basic relation that enables the motion to be
determined once the force is known is Newton's second law of motion, which
states that the resultant force on a particle is equal to the product of the
mass of the particle times its acceleration. For a many-particle system it
becomes impracticable to write and solve this equation for each individual
particle and, in general, the motion may be computed only on a statistical
basis (that is, by the methods of statistical mechanics) unless, as for a few
particles or a rigid body, the number of degrees of freedom is sufficiently
small. See DEGREE OF FREEDOM (MECHANICS); KINEMATICS; KINETICS (CLASSICAL
MECHANICS); NEWTON'S LAWS OF MOTION; RIGID BODY DYNAMICS; STATISTICAL
MECHANICS.
 �34s[Herbert C. Corben /Bernard Goodman]
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�6v�2v�32sDynamic similarity   �21m �33s 
    A relationship existing between two homologous fluid-flow systems such 
that corresponding parts of the systems experience similar net forces. 
Dynamically similar flows about geometrically similar bodies will themselves 
be geometrically similar. Consequently, this concept is basic to the 
meaningful extrapolation of model results to full-scale performance. However, 
geometrically similar flows are not necessarily dynamically similar. 
    Dynamic similarity between two flow systems will occur if certain 
nondimensional parameters formed from the flow variables have the same values 
for both systems (see table). 
794A 
        In practice, it is often difficult to establish equality of all 
similarity parameters simultaneously for two flows. Equality of parameters 
corresponding to dominant flow properties is usually sufficient. See 
DIMENSIONAL ANALYSIS; DIMENSIONLESS GROUPS; FLUID MECHANICS; FROUDE NUMBER; 
MACH NUMBER; MODEL THEORY. 
 �34s[Arthur G. Hansen]
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�6v�2v�32sDynamitron accelerator �21m �33s
    A particle accelerator that utilizes a steady high-voltage direct-current
(dc) potential to accelerate charged particles. The process is similar to that
used in electrostatic accelerators; however, the high-voltage potential of the
Dynamitron accelerator is produced by electrical means rather than by the
mechanical charge transport system utilized in electrostatic accelerators. The
high voltage is produced by a large number of dc voltage sources, all of which
are connected in series. Each of these dc sources receives its power by means
of individual parallel capacitive coupling to a radio-frequency (rf) power
system operating at 100 kHz. This high voltage can then be used for the
acceleration of particles in a manner similar to that used in any other kind
of dc accelerator. Although other accelerators, for example, the
Cockcroft-Walton machines, use various types of cascade voltage-multiplying
circuits, the Dynamitron principle makes possible considerably higher
operating particle currents. The practical limit of size for a Dynamitron-type
accelerator is probably 8 - 10 MV; the largest machines constructed are rated
at 5 MV. The advantages of the Dynamitron arise from its use of high frequency
and low capacitance; other systems use low frequency and high capacitance, the
high-energy storage of which sometimes leads to component failure. Invented by
M. R. Cleland in 1956, Dynamitron accelerators are used in industrial,
medical, and basic research applications. See COCKCROFT-WALTON ACCELERATOR;
VAN DE GRAAF GENERATOR.
    A smaller, less powerful version of the Dynamitron tandem accelerator
called a Tandetron. These tandem accelerators are made in different sizes
varying from 1 to 3 MV and have a power supply capable of only 1 mA, in
contrast to the 100 mA or more of a Dynamitron. The Tandetron is used as an
analytical system by the semiconductor industry and in sup 1 sup 4C analysis as
an ultrasensitive mass spectrometer. See PARTICLE ACCELERATOR.
 �34s[Harvey E. Wegner]
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�6v�2v�32sDynamo   �21m �33s 
    An electric machine for the conversion of electrical energy into 
mechanical energy or, conversely, mechanical energy into electrical energy. It 
is called a generator if it converts mechanical into electrical energy, and it 
is called a motor if it converts electrical into mechanical energy. See 
ELECTRIC ROTATING MACHINERY; GENERATOR; MOTOR. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sDynamometer   �21m �33s 
    A special type of electric rotating machine used to measure the output 
torque or driving torque of rotating machinery. Most dynamometers consist of a 
direct-current (dc) machine with the stator cradle-mounted in antifriction 
bearings. The rotor is connected to the rotor of the machine under test. The 
field current is introduced through flexible leads. The stator is constrained 
from rotating by a radial arm of known length to which is attached a scale for 
measuring the force required to prevent rotation. The torque of the connected 
machine is found from the product of the lever arm length and the scale 
reading, after correcting the scale reading by the amount of the zero torque 
reading. See ELECTRIC ROTATING MACHINERY; TORQUE. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sDyschondroplasia   �21m �33s 
    A deforming nonhereditary disease of early youth also referred to as 
Ollier's disease or enchondromatosis. The fingers and the long bones of the 
extremities are most commonly affected, while skull, ribs, and pelvis are 
rarely involved. The disease is progressive and involves the growing portions 
of the affected bone. 
    In dyschondroplasia the growth rate of the epiphyseal cartilage is 
retarded and formation of new bone is defective, leaving irregular residual 
islands of abnormal cartilage scattered along the length of the shaft. These 
two factors result in an abnormally short bone and irregular nodular swellings 
along the shaft of the bone. The former results in arms or legs of unequal 
length; the latter may cause such irregular enlargement of fingers that 
function is severely impaired. The deformities produced increase steadily 
until skeletal maturity is reached, at which time the process slows down or 
stops entirely, always leaving some degree of residual disability. No cure is 
known.  
 �34s[W. Robert Adams]
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�6v�2v�32sDysprosium   �21m �33s 
    A metallic rare-earth element, Dy, atomic number 66 and atomic weight 
162.50. The naturally occurring element is composed of seven stable isotopes. 
Dysprosium forms a white oxide, Dy2O3, which dissolves in acid to give a 
yellowish-green solution. For properties of the metal See RARE-EARTH ELEMENTS. 
    The metal is attacked readily by air at high temperatures, but at room 
temperatures, in massive blocks, it is fairly stable in the atmosphere and 
remains shiny for long periods of time. Dysprosium is paramagnetic, but as the 
temperature is lowered, it becomes antiferromagnetic at the Neel point (178K 
or -139DGF) and ferromagnetic at the Curie point (85K or -306.4DGF). At very 
low temperatures, the metal shows strong anisotropic magnetic properties. 
 �34s[Frank H. Spedding]
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�6v�2v�32sEar �21m �33s
    The organ which is responsible for sending both auditory information and
space orientation information to the brain.

    Ear structure.  The basic ear structures common to all vertebrate ears are
those of the internal ear, embedded in the  ear capsule. The basic sensory
units in the inner ear are hair cells. These structures are all
morphologically similar but have either auditory or vestibular functions,
depending upon the superstructure that surrounds them. The superstructure
around the auditory receptor must facilitate the transmission of vibrations
from the environment to the hair cells. For the vestibular apparatus, the
superstructure must block external vibratory energy but must sensitize the
hair cells to the pull of gravity and to acceleratory and deceleratory
movements of the head.
    The other parts of the ear, namely, the middle ear and external ear, are
strictly auditory in function and vary considerably among vertebrate groups.
They are lacking among fishes; amphibians, reptiles, birds and mammals have an
external auditory meatus; and mammals have three middle-ear ossicles and
usually an external pinna.

    Fishes.  In bony fishes the ear is embedded in the otic bones of the
skull. An elaborate closed system of canals within these bones (or cartilages,
in the case of cartilaginous fish), termed the bony labyrinth, is filled with
perilymph. Within this fluid, its shape closely corresponding to that of the
bony labyrinth, is the membranous labyrinth; it too is hollow but is filled
with endolymph, a fluid of different chemical composition. Connective tissue
attaches the membranous labyrinth to the walls of the bony labyrinth at
several points, but most of its outer surface is in contact with the
perilymphatic fluid only. The fish's membranous and bony labyrinths correspond
to the structures of the same name in other vertebrates, and thus collectively
are termed the inner ear, even though there are no middle or external ears in
fishes. See OSTEICHTHYES.
    For many years fishes were assumed to be deaf. However, since early in the
20th century, there has been strong behavioral evidence to the contrary. Some
fishes can respond to frequencies as low as 16 Hz and as high as 7000 Hz.
    The members of one prominent group of bony fishes, the Ostariophysii
(including minnows, carp, and catfish), relay acoustical information to the
ear relatively directly, and there is behavioral and physiological evidence
that they hear better than do other fish. In all bony fishes a large
gas-filled chamber, the swim bladder, is located in the dorsal part of the
body cavity, where it helps control the fish's buoyancy. In the Ostariophysii
the anterior part of this gas bladder is connected by a set of tiny bones
(Weberian ossicles) to the membranous labyrinth. Vibratory energy causes
deformation of the swim bladder, and the Weberian ossicles carry energy from
this vibrating bladder to the inner ear. See SWIM BLADDER.

    Tetrapoda.	Many structural and functional features of the fish ear are
also found in the tetrapod ear (the tetrapoda include birds). The inner ear is
embedded in the otic bones of the skull, with the membranous labyrinth
attached to the bony labyrinth by connective tissue but suspended in
perilymphatic fluid. There are three semicircular canals, with cristae, and a
sacculus and utriculus, with maculae; there are usually also a lagena and a
lagenar macula (Fig. 1). In their morphology and physiology the vestibular
parts of fish and tetrapod ears are nearly the same; the tetrapod maculae,
however, function as vestibular organs only, rather than playing an auditory
role as they do in fish.

    Amphibians.  The tympanic membrane of frogs and toads is located on the
lateral surface of the head. Attached to its inner aspect is a small rodlike
bone, the stapes, or columella, which runs through the air space of the
middle-ear cavity and plugs a small hole, the oval window, beyond which are
the inner ear fluids. The frog's tympanic membrane collects sound energy and
transmits it through the columella to the inner ear fluids. In the lagenar
portion of the amphibian's membranous labyrinth are two areas of hair cells,
the lagenar macula and a sensory structure not found in fish ears, termed the
basilar papilla. In addition, amphibians have a second unique area of sensory
epithelium for hearing, the amphibian papilla. Vibratory energy enters the
inner ear at the oval window, passes through the basilar and amphibian
papillae causing them to vibrate, and then dissipates at the round window.

    Birds and reptiles.  In most reptiles and birds the tympanic membrane lies
not on the surface of the head but internally, at the end of the tube called
the external auditory meatus. A middle-ear cavity (with its Eustachian tube to
the mouth) lies medial to the tympanic membrane. A single ossicle, the
columella, crosses this cavity from the tympanic membrane to the oval window
at the inner ear.

    The lagena of birds and reptiles, sometimes termed a cochlea, is
elongated; there is a small maculae lagena at its tip. Along most of its
length there is an elongated basilar papilla, sometimes termed an organ of
Corti, which is usually somewhat shorter in reptiles than in birds but which
in both classes exhibits considerable species variation. Because of its
drawn-out structure, the bird and reptile lagenae contain greater numbers of
hair cells than does the amphibian structure.

    Mammals.  In mammals the ear consists of three parts: the external ear,
which receives the sound waves; the middle ear, which transmits the vibrations
by a series of three small bones; and the inner, or internal, ear, a complex
bony chamber placed deep in the skull (Fig. 2). The external auditory meatus
plus the newly evolved pinna, a cartilaginous structure projecting from the
ear, compose the external ear. The shape and  size of the pinna vary greatly;
in elephants it is a huge, flapping structure; in whales and moles it is
completely absent; and in other species it falls between the two extremes. Its
auditory function is similarly varied; it is an accurate and acute collector
of sound waves in animals such as the deer, whereas in man it plays little or
no auditory role.
    As in other tetrapods, the middle-ear cavity communicates with the pharynx
by way of the Eustachian tube. In other tetrapods this tube is permanently
open; in mammals it is usually closed, and in order to equalize the pressure
between the inside of the middle-ear cavity and the environment, the tube must
be opened by yawning or swallowing. This is the "popping" in the ears which is
experienced when one drives rapidly up a mountain or rides in an elevator.
    Instead of the single columella of other tetrapods, the mammalian middle
ear has three bones, closely articulated with one another. The innermost is the
stapes, fitting into the oval window of the inner ear and homologous with the
columella. Attached tot he tympanic membrane is the malleus, and lying between
the malleus and stapes is the incus.
    In the mammalian inner ear the vestibular apparatus is much like that of
other tetrapods; the auditory portion, however, is considerably different. The
lagena, greatly elongated and coiled into a snail shape, is now termed the
cochlea; in its center, like the center pole of a spiral staircase, is the bony
modiolus. The epithelium of the basilar papilla, more differentiated in
mammals than in other tetrapods, is termed the organ of Corti. The number of
turns in the cochlea varies, from less than one on the generalized montremes of
Australia, to two and a half in humans, to five in the guinea pig. At the base
of the cochlea is the oval window, which carries sound energy into the inner
ear, and the round window, where this energy is dissipated after traveling in
the cochlea.
    The mammalian auditory ear is structurally more complex than that of
reptiles, birds, amphibians, and fishes--for instance, in its larger size and
greater differentiation of cochlear cell types. It is probable that the overall
hearing ability of mammals is superior to that of other vertebrates and that
this is due to their large cochleae with more greatly differentiated cell
types. See HEARING (HUMAN); PHONORECEPTION.
[DOUGLAS B. WEBSTER]

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�6v�2v�32sEarphones   �21m �33s 
    A class of energy transducers capable of receiving alternating current and 
generating acoustic waves resembling very closely the characteristics of that 
current. The movement of an element (diaphragm) is accomplished by magnetic 
attraction, electrostatic attraction, or the piezoelectric effect (the 
expansion or contraction of certain crystalline substances in response to 
electric charges). Earphone systems include the driver element with its 
diaphragm and arrangements for magnetic flux, or electrostatic or direct 
electric charge, plus a casing, one or more acoustic cavities and ports, 
acoustic damping and insulation, and some arrangement for coupling the driver 
to the human ear. 
    In a magnetic earphone, a permanently magnetized diaphragm is moved in and 
out by an electromagnet energized by alternating current. Dynamic earphones 
are actually small dynamic loudspeakers. In some, a small coil fed by the 
sound source is bonded to the membranous diaphragm. In another configuration, 
the coil is relatively large and is attached to the diaphragm only at its 
edge. A miniature magnetic earphone is a small unit whose output port fits 
snugly into a plastic olive in the ear canal. These are widely used with small 
radios, with the more powerful hearing aids, by television commentators, in 
business transcription devices, and in many other communications situations.  
See  HEARING AID. 
    Efficiency is increased if the mass of the diaphragm is reduced to a 
minimum. The electrostatic earphone utilizes a thin metallized plastic film on 
which a large constant electrostatic charge is placed by an auxiliary unit. 
The motion of the diaphragm is controlled by the audio signal impressed on 
perforated wire mesh plates on either side. 
    The diaphragm of a dynamic-electrostatic (or orthodynamic) earphone is a 
permanently polarized electret of fluorocarbon. Consequently the need for an 
added source of polarization voltage (a drawback inherent to electrostatic 
earphones) is eliminated.  See  ELECTRET TRANSDUCER. 
    Certain crystalline substances expand and contract when alternating 
voltage is applied. In some piezoelectric earphones, a crystal element is 
coupled mechanically to the center of a small cone. Such earphones can be 
lightweight and cheap and may be acceptable for speech communication.  See  
PIEZO-ELECTRIC CRYSTAL; PIEZOELECTRICITY. 
 �34s[J. Donald Harris]
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�6v�2v�32sEar protectors   �21m �33s 
    Devices used to protect the human ear from noise that may be injurious to 
hearing. There are situations in industry, military services, and commercial 
aviation where persons must be exposed to sounds having levels in excess of 
those which may damage hearing. 
    There are three general classes of ear protectors: (1) earplugs that fit 
into the ear canal; (2) earmuffs or cushions that fit over the external ear, 
under which earplugs may be worn; and (3) rigid helmets that fit over the 
entire head, under which earplugs and earmuffs may be worn. 
    The amount of protection afforded by an ear protector, or combination of 
protectors, is measured in terms of the number of decibels it attenuates or 
reduces the intensity of a sound transmitted through it. In general, the 
higher-frequency components in a noise are attenuated more than are the 
low-frequency components. 
 �34s[Karl D. Kryter]
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�6v�2v�32sEarth �21m �33s
    The third planet, of nine, from the Sun. The Earth is unique, so far as is
known, in having life, although statistics strongly suggest that many similar
planets exist, and some probably also have life. The Earth is orbited by the
Moon and many artificial satellites. See MOON.
    The Earth's nearest neighbors in space, other than the Moon, are the
planets Venus, which is about 108 x 10 sup 6 km (67 x 10 sup 6 mi) from the Sun,
and Mars, 227 x 10 sup 6 km (141 x 10 sup 6 mi) from the Sun. Earth is 150 x
10 sup 6 km (93 x 10 sup 6 mi) from the Sun.

    Motions.  There are two types of Earth motions, orbital and rotational.
The Earth's orbit around the Sun is an ellipse, with the Sun at one focus.
Perihelion, or the closest approach to the Sun, occurs about January 3, and
aphelion, farthest from the Sun, about July 4. The Earth's velocity varies
systematically, being greatest at perihelion and least at aphelion. The
average velocity is 107,000 km/h (66,600 mi/h) or 29.6 km/s (18.5 mi/s). The
Earth's orbital period is the year, whose length is determined by the average
distance between Earth and Sun. The calendar year or tropical year is defined
as the time between two successive crossings of the celestial equator by the
Sun at the time of the vernal equinox. It is exactly 365 days 5 h 48 min 46 s.
See CALENDAR.
    As the Earth moves in its orbit around the Sun, its north spin axis, or
geographic pole, points in the direction of the star Polaris, making it the
North Star or polestar. One obvious result is that different parts of the
Earth receive differing amounts of sunlight; this is the primary cause of
seasons. Because of the gravitational attraction of the Moon and Sun on the
Earth's equatorial bulge, the direction in which the Earth's axis points moves
very slowly. This motion is called precession, and a complete cycle requires
about 25,800 years. See PRECESSION OF EQUINOXES.
    The Earth's period of rotation is the day, and the day has been used to
define the second, which is the basic unit of time. The length of the solar
day varies because the Earth's orbital velocity changes predictably with
distance from the Sun, and less regularly owing to tidal friction, changes in
the Earth's core, and seasonal atmospheric circulation. To avoid the obvious
problems in determining the length of the mean solar day, the second is now
defined atomically. See DAY; TIME.

    Structure.	The Earth is not quite a sphere, but has an equatorial bulge
caused by its rotation. Its shape therefore is an oblate spheroid. The
equatorial radius is 3963.205 mi (6378.160 km), and the polar radius is
3949.917 mi (6356.775 km).
    The Earth's mass is 2.108 x 10 sup 2 sup 6 oz (5.975 x 10 sup 2 sup 7 g),
giving it an average density of 3.20 oz/in sup 3 (5.53 g/cm sup 3). This figure
is about twice the density of the common rocks that form the Earth's surface and
strongly suggests that the interior is denser than the surface. Seismic
studies have confirmed that the Earth's interior has a layered structure  (see
illustration).
    The deepest layer is the core, which is divided into a solid inner core
and a liquid outer core. The core is believed to be composed of nickel-iron
and probably lighter elements such as sulfur. Electric currents moving in this
liquid conducting core are believed to be the origin of Earth's magnetic
field. The layer between the crust, or outermost rock layer, and the core is
called the mantle. The mantle is believed to be composed of silicate minerals.
The horizon that separates the mantle from the crust is known as the
Mohorovicic seismic discontinuity. See MOHO (MOHOROVICIC DISCONTINUITY).
    The crust is the thinnest of the rock layers, and is composed of 19 - 37
mi (30 - 60 km) of granitic rocks under the continents and 3 mi (5 km) of
basaltic rocks under the oceans. The continents are much older than the
oceans, and deformed rocks as old as 3,900,000,000 years indicate an eventful
history. See CONTINENT.
    Much of the surface of the continents is covered by a thin veneer of
sedimentary rocks. Where the underlying rocks of the plains and hills that
make up most of the continents are exposed, the rocks and their structures are
similar to those found in most mountain ranges. This leads to the theory that
continents are formed by the deeply eroded remnants of earlier mountain ranges.
    The topographic features underlying the oceans are similarly diverse and
reveal more evidence of a dynamic Earth. The continental shelf, an area
covered by shallow water, generally less than 150 m (500 ft) deep, surrounds
the continents at most places. Such areas are generally underlain by
continental, granitic rocks, and are submerged parts of the continents.
Continental slopes are the transition between the continental shelf and the
ocean floors. See CONTINENTAL MARGINS.
    The ocean floors are the most widespread surface feature of Earth.
Interrupting the ocean floor at many places are submarine mountains formed by
basalt volcanoes. Some of these volcanoes are very large and form oceanic
islands such as the Hawaiian Islands. The ocean floors rise gradually to the
mid-ocean ridges, a more or less continuous feature through all the oceans
with some branches and offsets. Lines of parallel volcanoes and steep scarps
mark the midocean ridges. See MARINE GEOLOGY; VOLCANO.


    Atmosphere.  The Earth's temperature and gravitation are such that an
atmosphere is present. The major constituents are nitrogen and oxygen. The
atmosphere, especially oxygen, and the presence of water, both at the surface
and in the atmosphere, make life possible. A thin ozone layer in the
atmosphere shields the Earth from lethal ultraviolet radiation from the Sun.
See ATMOSPHERE; ATMOSPHERIC OZONE; METEOROLOGY.

    Age.  The Earth with the rest of the solar system is believed to have
formed about 4,500,000,000 years ago. This age is determined by dating
radioactive isotopes in meteorites. Meteorites are believed to be fragments
produced by collisions among small bodies formed by the same process that
created the solar system. See COSMOLOGY; DATING METHODS; GEOCHRONOMETRY;
METEORITE; PLANET; SOLAR SYSTEM.
 �34s[Robert J. Foster]
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�6v�2v�32sEarth inductor   �21m �33s 
    An instrument for measuring the dip angle of the Earth's magnetic field 
(see illustration). Essentially, it is a coil of wire designed to be rotated 
about an axis parallel to the plane of the coil. As the coil turns in the 
Earth's field, a voltage is induced unless the axis of rotation is parallel to 
the field. With the help of a special compass needle temporarily mounted on 
the frame of the earth inductor, the instrument is oriented about a vertical 
axis so that the rotational axis is approximately (within a few minutes of 
arc) in the magnetic meridian. In that position the deviation of the 
rotational axis from the inclination of the magnetic field produces, upon 
rotation of the coil, a voltage readily detectable with a galvanometer. 
Electrical connection is made between the coil and the galvanometer through a 
two-segment commutator. The coil frame is mounted to turn upon conical 
bearings of agate and is equipped with a vertical circle, reading to about 10 
seconds of arc in the better instruments. Errors due to mechanical 
maladjustments of the coil axis are made negligible by taking a series of 
readings for plus and minus rotations of the coil, vertical circle east and 
west, and commutator up and down. See MAGNETOMETER. 
 �34s[James H. Nelson]
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�6v�2v�32sEarthmover   �21m �33s 
    Any of a variety of construction machines designed to move or transport 
earth. Earthmovers include heavy-duty trucks with high-sided dump bodies, 
self-propelled or towed scrapers, wagons, and bulldozers. The bulldozer 
mounted on a wheeled or crawler tractor is suitable for moving large 
quantities of earth for distances of several hundred feet. Scrapers and wagons 
are efficient for moving earth over relatively level terrain for distances up 
to 1 or 2 mi (2 or 3 km). For longer distances or for grades in excess of 5%, 
trucks are the most practical. See BULK-HANDLING MACHINES; CONSTRUCTION 
EQUIPMENT.  
 �34s[Edward M. Young]
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�6v�2v�32sEarthquake   �21m �33s 
    A phenomenon during the occurrence of which the Earth's crust is set 
shaking for a period of time. The shaking is caused by the passage through the 
Earth of seismic waves - low-frequency sound waves that are emanated from a 
point in the Earth's interior where a sudden, rapid motion has taken place. It 
is more proper today to use the term earthquake to refer to the source of 
seismic waves, rather than the shaking phenomenon, which is an effect of the 
earthquake.  
    Earthquakes vary immensely in size. Although thousands of earthquakes 
occur every day, and have for billions of years, a truly great earthquake 
occurs somewhere in the world only once every 2 or 3 years. When a great 
earthquake occurs near a highly populated region, tremendous destruction can 
occur within a few seconds. 
  
    Cause.  Earthquakes are not distributed randomly over the globe but tend 
to occur in narrow, continuous belts of activity. These earthquake belts link 
up so that they encircle large seismically quiet regions, which are known as 
plates. The plates are in continuous motion with respect to one another at 
rates on the order of centimeters per year; this plate motion is responsible 
for most geological activity. See PLATE TECTONICS. 
    As the plates move past each other, the motion at their boundaries does 
not occur by continuous slippage but in a series of rapid jerks. Each jerk is 
an earthquake. This happens because, under the pressure and temperature 
conditions of the shallow part of the Earth's lithosphere, the frictional 
sliding of rock exhibits a property known as stick-slip, in which frictional 
sliding occurs in a series of jerky movements, interspersed with periods of no 
motion - or sticking. In the geologic time frame, then, the lithospheric 
plates chatter at their boundaries, and at any one place the time between 
chatters may be hundreds of years. 
    The periods between major earthquakes is thus one during which strain 
slowly builds up near the plate boundary in response to the continuous 
movement of the plates. The strain is ultimately released by an earthquake 
when the frictional strength of the plate boundary is exceeded. See FAULT AND 
FAULT STRUCTURES. 
  
    Classification.  Most great earthquakes occur on the boundaries between 
lithospheric plates and arise directly from the motions between the plates. 
Although these may be called plate boundary earthquakes, there are many 
earthquakes, sometimes of substantial size, that can not be related so simply 
to the movements of the plates. 
    Near many plate boundaries, earthquakes are not restricted to the plate 
boundary itself, but occur over a broad zone - often several hundred miles 
wide - adjacent to the plate boundary. These earthquakes, which may be called 
plate boundary - related earthquakes, do not reflect the plate motions 
directly, but are secondarily caused by the stresses set up at the plate 
boundary.  
    While most earthquakes occur on or near plate boundaries, some also occur, 
although infrequently, within plates. These earthquakes, which are not related 
to plate boundaries, are called intraplate earthquakes, and can sometimes be 
quite destructive. Intraplate earthquakes are probably caused by the same 
convective forces which drive the plates; their immediate cause is not 
understood.  
    In addition to the tectonic types of earthquakes, some earthquakes are 
directly associated with volcanic activity. These volcanic earthquakes result 
from the motion of underground magma that leads to volcanic eruptions. 
  
    Sequences.  Earthquakes often occur in well-defined sequences in time. 
Tectonic earthquakes are often preceded, by a few days to weeks, by several 
smaller shocks (foreshocks), and are nearly always followed by large numbers 
of aftershocks. Foreshocks and aftershocks are usually much smaller than the 
main shock. Volcanic earthquakes often occur in flurries of activity, with no 
discernible main shock. This type of sequence is called a swarm. 
  
    Size.  Earthquakes range enormously in size, from tremors in which 
slippage of a few tenths of an inch occurs on a few feet of fault, to the 
greatest events, which may involve a rupture many hundreds of miles long, with 
tens of feet of slip. Accelerations as high as 1 g (acceleration due to 
gravity) can occur during an earthquake motion. The velocity at which the two 
sides of the fault move during an earthquake is only 1 - 10 mi/h (0.5 - 5 
m/s), but the rupture front spreads along the fault at a velocity of nearly 
5000 mi/h (2200 m/s). 
    The size of an earthquake is in terms of a scale of magnitude based on the 
amount of seismic waves generated. Magnitude 2.0 is about the smallest tremor 
that can be felt. Most destructive earthquakes are greater than magnitude 6; 
the largest shock known measured 8.9. The scale is logarithmic, so that a 
magnitude 7 shock is about 30 times more energetic than one of magnitude 6, 
and 30 x 30, or 900 times, more energetic than one of magnitude 5. 
    The intensity of an earthquake is a measure of the severity of shaking and 
its attendant damage at a point on the surface of the Earth. The same 
earthquake may therefore have different intensities at different places. The 
intensity usually decreases away from the epicenter (the point on the surface 
directly above the onset of the earthquake), but its value depends on the many 
factors in addition to earthquake magnitude. Intensity is considered more a 
measure of the earthquake's effect on humans than an innate property of the 
earthquake. See SEISMOLOGY. 
 �34s[Christopher H. Scholz]
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�6v�2v�32sEarth sciences   �21m �33s 
    A group of overlapping areas of study that consider all aspects of the 
Earth and its place in the universe. The principal Earth sciences are geology, 
oceanography, meteorology, astronomy, geophysics, and geochemistry. Each of 
these is subdivided into several disciplines. Geography, the study of all 
aspects of the Earth's surface, including political and economic as well as 
scientific aspects, and especially physical geography, are included in the 
Earth sciences by many. See ASTRONOMY; GEOCHEMISTRY; GEODESY; GEOGRAPHY; 
GEOMORPHOLOGY; GEOPHYSICS; METEOROLOGY; OCEANOGRAPHY. 
 �34s[Robert J. Foster]
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�6v�2v�32sEarth tides �21m �33s
    Cyclic motions of the Earth, sometimes over a foot or so in height,
depending on latitude, caused by the same lunar and solar forces which produce
tides in the sea. These forces also react on the Moon and Sun, and thus are
significant in astronomy in evaluations of the dynamics of the three bodies.
For example, the secular spin-down of the Earth due to lunar tidal torques is
best computed from the observed acceleration of the Moon's orbital velocity.
In oceanography, earth tides and ocean tides are very closely related. See
GEODESY; TIDE.
    By far the most widely used earth tide instruments are the tiltmeter and
the gravimeter. Both instruments have the merits of portability, high
potential precision, and low cost. Thus they are able to advance economically
an important mission - the global mapping of earth tides and ocean tides.
 �34s[Louis B. Slichter /John T. Kuo]
�33s

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�6v�2v�32sEast Indies   �21m �33s 
    The Indonesian islands and, for convenience of presentation, New Guinea 
and the Bismarck Archipelago, lying between Asia and Australia. They make up 
the territory of Indonesia, Brunei, part of Malaysia, and regions formerly 
under Australian trusteeship. Together with the intervening seas, the East 
Indies form a region of great relief and of geological diversity. The islands 
extend from western Sumatra to eastern New Guinea, a distance of about 4000 mi 
or 6400 km. 
    Geographically, three basic divisions are recognized: (1) Sundaland, or 
the islands west of Macassar and the Lombok Straits; (2) Papualand, or the 
islands east of the Aru Islands and New Guinea; and (3) Wallacea, or the 
islands between Sundaland and Papualand and south of the Philippines. Each 
division is an important biological and geomorphological region. 
    During the Quaternary, a lowering of the general level of the world's 
oceans is thought to have resulted in the emergence of dry land between the 
western edge of Wallacea and Asia, and on the east in exposed portions of the 
Sahul Shelf beneath the Arafura Sea. The islands of Wallacea, however, 
remained separated by deep water, which was an effective barrier to the 
migration of plants and animals. Thus, the flora and fauna of Sundaland are 
closely related to the flora and fauna of Asia, while the flora and fauna of 
Papualand show a relationship to those of Australia and a separation from Asia 
since early Cretaceous. Because of this isolation and the great variation of 
topography, soils, rainfall, humidity, and altitude within short distances, 
there have existed conditions favorable for the development of numerous 
species of plants. See ASIA; AUSTRALIA; ISLAND BIOGEOGRAPHY; NEW ZEALAND. 
    Since almost all the islands lie in a belt within 10DG of the Equator, an 
equatorial climate prevails, with high temperatures throughout the year, 
except at higher elevations. The diurnal variations are greater than the range 
in mean temperatures of the hottest and coldest months. Monsoons, representing 
an interplay of air masses between Asia and the Southern Hemisphere, control 
the seasonal change in winds and the variation in precipitation. See BORNEO; 
CELEBES; JAVA; NEW BRITAIN; NEW GUINEA; NEW IRELAND; SUMATRA; TIMOR; TROPICAL 
METEOROLOGY.  
 �34s[M. G. Audley-Charles]
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�6v�2v�32sEbenales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae) in the 
subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of 5 families and about 1700 species, the Sapotaceae (about 800 
species) and Ebenaceae (about 450 species) being the largest and most familiar 
families. The Ebenales are woody, chiefly tropical, sympetalous plants (those 
with flowers have the petals joined by their margins, at least toward the 
base, forming a basal tube, cup, or saucer) with usually twice as many stamens 
as corolla lobes. 
    Chicle (from Manilkara sapota, in the Sapotaceae) and ebony (Diospyros 
ebeneum) are obtained from members of the Ebenales. See CHICLE; DILLENIIDAE; 
EBONY.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sEbony �21m �33s
    A genus, Diospyros, of the ebony family, containing more than 250 species.
Some species are important for their succulent fruits, such as date plum, kaki
plum, and persimmon, and several for their timber, particularly the heartwood,
which is the true ebony of commerce. See EBENALES.
    Although it is popularly supposed to be a black wood, most species have a
heartwood that is only streaked and mottled with black. The heartwood is very
brittle, and is difficult to work, but it has long been in demand. The sapwood
is white, becoming bluish or reddish when cut.
    Black ebony is used for knife handles, piano keys, finger boards of
violins, hairbrush backs, inlays, and marquetry. Some of the woods called
ebony, however, belong to different families, especially the pulse family,
Leguminosae.
    Persimmon (D. virginiana), of the southeastern United States, is one of
numerous tropical or subtropical species. The species in tropical America are
too small or rare to be of economic value, although several of them have black
heartwood used locally for making walking sticks, inlays, and miscellaneous
articles of turnery and carving.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sEbriida   �21m �33s 
    An order of flagellate Protozoa, subphylum Sarcomastigophora, class 
Phytamastigophorea. Two genera, Ebria and Hermesinum, remain of the once 
numerous order, as determined by fossil remains. They possess an internal 
solid siliceous skeleton forming a shallow flattened or slightly arched 
structure that is enclosed by clear cytoplasm. Skeleton form and structure are 
distinctive. These phytoflagellates do not have chromatophores. 
    The organisms are common in the inshore waters of the Atlantic Ocean and 
the Gulf of Mexico. However, numbers sufficient to influence the general 
ecology have not been observed. Their only occurrence seems to be marine, 
although somewhat lowered salinities are certainly tolerated. See 
PHYTAMASTIGOPHOREA; PROTOZOA. 
 �34s[James B. Lackey]
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�6v�2v�32sEcdysone   �21m �33s 
    The molting hormone of insects. It is a derivative of cholesterol. The 
most striking physiological activity of ecdysone is the induction of puffs 
(zones of gene activity) in giant chromosomes of the salivary glands and other 
organs of the midge Chironomus. The induction of puffs has been visualized as 
primary action of the hormone, indicating that ecdysone controls the activity 
of specific genes. It has been shown that ecdysone stimulates the synthesis of 
messenger RNA, among which is the messenger for dopa decarboxylase. This 
enzyme is involved in the biosynthesis of the sclerotizing agent 
N-acetyl-dopamine. See INSECTA. 
 �34s[Peter Karlson]
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�6v�2v�32sEchelette grating   �21m �33s 
    A diffraction grating that has coarse groove spacing and is designed for 
the infrared region, the grooves being so shaped and of such size that most of 
the radiation is concentrated by reflection into a small angular coverage. 
Radiation of any given wavelength is thus concentrated largely into one order 
by shaping the point of the ruling diamond to give grooves with comparatively 
flat sides and by choosing a groove separation which minimizes diffraction 
effects. See DIFFRACTION GRATING. 
 �34s[George R. Harrison]
�33s 
  
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�6v�2v�32sEchelle grating   �21m �33s 
    A type of diffraction grating designed for use in high orders and at 
angles of illumination greater than 45DG to obtain high dispersion and 
resolving power by the use of high orders of interference. Echelles have 
properties lying midway between those of plane gratings of the ordinary type 
and interferometers of the reflection echelon type, orders of interference 
ranging from 100 to 1000 being used. Overlapping orders are separated by using 
crossed dispersion. See DIFFRACTION GRATING. 
 �34s[George R. Harrison]
�33s 
  
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�6v�2v�32sEchinacea   �21m �33s 
    A superorder of Euechinoidea, having a rigid test, the periproct within 
the apical system, keeled teeth, a complete perignathic girdle, and branchial 
slits. J. Durham and R. Melville (1957) include five orders in this group. 
These were formerly distributed among the Stirodonta and Camarodonta in the 
classification of R. Jackson (1912). See ARBACIOIDA; ECHINODERMATA; ECHINOIDA; 
EUECHINOIDEA; HEMICIDAROIDA; PHYMOSOMATOIDA; SALENIOIDA; TEMNOPLEUROIDA. 
 �34s[Howard B. Fell]
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�6v�2v�32sEchinococcosis   �21m �33s 
    The infestation by the hydatid (cyst) of Echinococcus granulosus, also 
known as hydatidosis. The adult worm occurs in the gut of dogs and other 
canines, and the hydatid occurs primarily in the liver and lungs of 
herbivores, and occasionally in humans. 
    Human infestations are found throughout the world in nontropical areas but 
are particularly common in certain areas of South America, Australia, New 
Zealand, and Africa, where dogs, sheep, and cattle are abundant and in 
intimate association. The normal cycle of the tapeworm involves the dog and 
the herbivore, but humans may become infested directly from the dog feces or 
from fecally contaminated food or water. Human infestations are rare in North 
America, except in the Far North. Here the parasite normally cycles from moose 
and other wild herbivores to wolves, but dogs become infested when eating 
offal from the herbivores. See CYCLOPHYLLIDEA. 
 �34s[Reino S. Freeman]
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�6v�2v�32sEchinocystitoida   �21m �33s 
    An extinct order of Perischoechinoidea which arose in the Ordovician and 
seems to have inhabited calm, shallow lagoons. There was no perignathic 
girdle, the number of columns of plates was variable, the ambulacral plates 
overlapped adorally, and the interambulacral plates overlapped one another 
toward the apex and also overlapped the adjacent ambulacrals (see 
illustration). The shape of the test varied from spherical (Echinocystis) to 
flattened pentagonal (Hyattechinus). The largest forms (Fournierechinus) 
reached a diameter of 30 cm (12 in.). All were extinct by the close of the 
Permian, but before extinction they had given rise to the Archaeocidaridae, 
from which all surviving echinoids probably stem. See ECHINOIDEA; 
PERISCHOECHINOIDEA.  
 �34s[Howard B. Fell]
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�6v�2v�32sEchinodermata �21m �33s
    A unique group of exclusively marine animals with peculiar body
architecture. They are headless with a fivefold radial symmetry. The body wall
contains the endoskeleton, made of numerous independent calcareous plates
which frequently support spines. The plates may be tightly interlocked or
loosely associated. The spines may protrude through the outer epithelium, and
are often used for defense. The skeletal plates of the body wall, together
with their associated muscles and connective tissue, form a tough and
sometimes rigid test which encloses the large coelom. There is a unique
multipurpose water-vascular system involved in locomotion, respiration, food
gathering, and sensory perception. This shows outside the body as rows of
fluid-filled tube-feet lying in conspicuous double lines or ambulacra. Within
the body wall lie the ducts and fluid reservoirs necessary to protract and
retract the tube-feet by hydrostatic pressure. The nervous system consists of
a ring around the mouth with connecting nerve cords associated with each
ambulacrum. There may also be diffuse nerve plexuses lying below the outer
epithelium. The coelom houses the alimentary canal and associated organs and
in most groups the reproductive organs.
    The larvae are usually planktonic with a bilateral symmetry, but the
adults are almost always sedentary and benthic. They inhabit all seas and
oceans of the world, ranging from the shores to the ocean depths.
    The phylum comprises about 6000 existing species and many fossils,
providing a good fossil record. Echinoderms first appeared in the Early
Cambrian and have been evolving over 600,000,000 years. During this vast time
several divergent patterns have arisen. The surviving groups show few
resemblances to the original stock. The existing representatives fall into
three subphyla: Crinozoa, Asterozoa, and Echinozoa. The fourth subphylum,
Homalozoa, has no living representatives. Following is the outline of
classification for the phylum.
Phylum Echinodermata
    Subphylum Homalozoa
    Subphylum Crinozoa
      Class: Cystoidea
	     Diploporita
	     Rhombifera
	     Blastoidea
	     Eocrinoidea
	     Paracrinoidea
	     Crinoidea
	Subclass: Inadunata
		  Camerata
		  Flexibilia
		  Articulata
    Subphylum Echinozoa
      Class: Edrioasteroidea
	     Helicoplacoidea
	     Ophiocistioidea
	     Cyclocystoidea
	     Holothuroidea
    Subclass Dendrochirotacea
      Order: Dactylochirotida
	     Dendrochirotida
	Subclass Aspidochirotacea
	   Order: Aspidochirotida
		  Elasipodida
	Subclass Apodacea
	   Order: Molpadida
		  Apodida
      Class Echinoidea
	Subclass Perischoechinoidea
	   Order: Bothriocidaroida
		  Echinocystitoida
		  Palaechinoida
		  Cidaroida
	Subclass Euechinoidea
	  Superorder Diadematacea
	    Order: Diadematoida
		  Pedinoida
		  Echinothurioida
		  Pygasteroida
	  Superorder Echinacea
	    Order: Salenioida
		   Hemicidaroida
		   Phymosomatoida
		   Arbacioida
		   Temnopleuroida
		   Echinoida
	  Superorder Gnathostomata
	    Order: Holectypoida
		   Clypeasteroida
		   Cassiduloida
	  Superorder Atelostomata
	    Order: Holasteroida
		   Spatangoida
    Subphylum Asterozoa
      Class Stelleroidea
	Subclass Somasteroidea
	Subclass Asteroidea
	    Order: Platyasterida
		   Hemizonida
		   Phanerozonida
		   Paxillosina
		   Valvatina
		     Suborder Valvatina
	    Order: Spinulosida
		   Euclasterida
		   Forcipulatida
	Subclass Ophiuroidea
	    Order: Stenurida
		   Oegophiurida
		   Phrynophiurida
		   Ophiurida

 �34s[Andrew C. Campbell]
�33s
    The origin of the echinoderm invertebrates surely belongs some time before
the beginning of the Paleozoic Era, for Lower Cambrian deposits contain such
divergent branches of the phylum as Homalozoa, Helicoplacoidea,
Edrioasteroidea, and Eocrinoidea. These are primitive sorts of echinoderms.
Cystoids, crinoids, and blastoids, as well as all recognized main groups of
asterozoans and echinozoans, appear in Ordovician strata. It is evident from
their already well-marked development of characteristic structures that a long
period of antecedent evolution must have operated to produce such widely
divergent forms. For example, each main division of the Paleozoic crinoids
(Inadunata, Flexibilia, Camerata), although seeming to appear very suddenly,
is fully differentiated in the Middle Ordovician. One can only guess as to
reasons for the absence of these echinoderms in older deposits. During the
Paleozoic numerous well-marked evolutionary trends are discernible in nearly
all echinoderm groups, including free-moving forms (especially echinoids) as
well as crinozoans. All groups of modern echinoderms have their origin in
early Paleozoic stocks, and the lines of their phylogeny are mostly indicated
by the fossil record. Echinoids predominate in Mesozoic and Cenozoic
echinoderms.
 �34s[Raymond C. Moore]
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�6v�2v�32sEchinoida   �21m �33s 
    An order of Echinacea with a camarodont lantern, smooth test, imperforate 
noncrenulate tubercles, ambulacral plates of echinoid type, and shallow 
branchial slits. There are numerous tropical and temperate species, some of 
them remarkably adapted to living on coral reefs. The four included families 
(Parasaleniidae, Echinidae, Strongylocentrotidae, and Echinometridae) are 
principally distinguished by characters of the pedicellariae. See ECHINACEA; 
ECHINODERMATA; ECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sEchinoidea   �21m �33s 
    A class of Echinozoa known as the sea urchins. These animals have a 
compact body enclosed in a hard shell, or test, formed from regularly arranged 
plates which bear movable spines (see illustration). There are no arms, but 
radii are represented by five double rows of tube feet arranged as meridians 
between the upper and lower poles of the body. 
    There are about 850 living species, and some 500 fossil species have been 
recorded, included  in 225 genera. They are classified in 18 orders, grouped 
in 2 subclasses. Sea urchins range in size from a few tenths of an inch (few 
millimeters) across the test to 8 in. (20 cm.) They differ from other 
echinoderms in possessing echinochromes in the pigmentation complex. Although 
many species are dull or dark in color, some are brilliant shades of purple, 
red, green, or orange. Others have particolored striped spines, and deep-sea 
forms may be white. 
    The test is globular, or nearly so, in those forms in which the anus lies 
within the apical system. These are often called Regularia. In exocyclic forms 
the test tends to assume a secondary bilateral symmetry. The radial symmetry 
is always evident, however, and is fivepart (pentamerous). See IRREGULARIA; 
REGULARIA.  
    The mouth lies on the lower (oral) side of the test, surrounded by soft 
skin, the peristome. In endocyclic forms the mouth is central, but in 
exocyclic forms it may suffer displacement into the radius and lie opposite 
the interamb which contains the anus. The radius is then termed anterior. In 
endocyclic forms and in some of the exocyclic forms the mouth is furnished 
with a ring of five powerful jaws, each with one large tooth. The jaws and 
teeth collectively comprise the so-called lantern, first described by 
Aristotle. The water-vascular system is highly developed and greatly 
influences the form of the ambulacral plates. 
  
    Small grasping organs, pedicellariae, are well developed in echinoids, in 
which they take the form of a beak carried on a stalk. The beak is made up of 
three (sometimes only two) movable jaws, operated by muscles and sometimes 
provided with venom glands. They respond to tactile stimuli and seize any 
small organisms or particles which may touch the skin. 
    The nervous system follows the same pattern as the water-vascular system. 
The tube feet evidently serve as tactile and taste organs. A few sea urchins 
have photosensitive eyespots on the upper surface of the test, scattered in 
the ectoderm. The ocular pores are not sensitive to light. Minute spherical 
stalked bodies attached to the skin in some echinoids are believed to be 
organs of balance. 
    The alimentary canal is tubular. It lies in the coelom, attached to the 
wall of the test by mesenteries. The stomach runs from the esophagus in a 
counterclockwise coil (as viewed from above), and the intestine retraces the 
route in reverse. The rectum passes upward to the anus. 
    Like starfishes, sea urchins seem to be sexually mature after 1 year, but 
continue to grow for several years. The life span is  unknown, but may average 
5 - 6 years in medium-sized species. The sexes are normally separate, although 
fertile hermaphrodites are known. Spines and some other organs are regenerated 
after injury. Autotomy and autoevisceration are unknown. 
    Sea urchins occur in all seas from low-tide level downward. Pourtalesia 
reaches a depth of more than 4 mi (7250 m) in the Banda Trench. The rounded 
(or regular) urchins feed  mainly on algae, often hiding by day under stones 
held over the test by tube feet, and emerging at night or at high tide. The 
heart urchins and other exocyclic forms live buried in mud or sand, feeding on 
organic matter in the mud or on selected detritus. 
    Sea urchins move slowly, using muscles at the bases of the spines to swing 
the spines like stilts. The suctorial tube feet are used to ascend steep 
surfaces and as anchors. See ECHINODERMATA; EUECHINOIDEA; PERISCHOECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sEchinothurioida   �21m �33s 
    An order of Diadematacea with solid or hollow primary radioles, diademoid 
ambulacral plates, noncrenulate tubercles, and the anus within the apical 
system. The extant members of the two included families, Pedinidae and 
Echinothuriidae, are all deep-water forms. The Pedinidae have solid radioles 
and simple diademoid plates. The Echinothuriidae have a large flexible test 
which collapses into a disk at atmospheric pressure, and the middle element of 
the diademoid plates is much larger than the other two elements. Some species 
carry venomous spines. See DIADEMATACEA; ECHINODERMATA; ECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sEchinozoa   �21m �33s 
    A subphylum of free-living echinoderms in which the body is essentially 
globoid with meridional symmetry. They lack arms, brachioles, or other 
appendages, and do not at any time exhibit pinnate structure. The Echinozoa 
range in time from the Lower Cambrian to the present day. There are six 
classes: Helicoplacoidea; Lower Cambrian; Edrioasteroidea (Lower to Middle 
Paleozoic); echinozoans, in which the mouth and anus were both directed upward 
and ambulacra (three to five in number) served as food-collecting areas; 
Echinoidea (sea urchins); Holothuroidea (sea cucumbers); Ophiocistoidea 
(Paleozoic); and Cyclosteroidea (Middle Ordovician to Middle Devonian). 
    The ancestry of the Echinozoa is unknown, but the oldest known members 
suggest forms similar to extant plated holothurians, the Dendrochirotacea, and 
may have arisen from cystoids or similar crinozoans. See CYCLOCYSTOIDEA; 
ECHINODERMATA; ECHINOIDEA; EDRIOASTEROIDEA; HELICOPLACOIDEA; HOLOTHUROIDEA; 
OPHIOCISTOIDEA.  
 �34s[Howard B. Fell]
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�6v�2v�32sEchiurida   �21m �33s 
    A small group of wormlike animals, once linked with the Sipuncula and 
Priapulida under the term Gephyrea, but now regarded as a separate phylum, 
with affinities to the annelid worms. They range from the topical to the polar 
seas, living buried in the sea floor from the intertidal area to depths of 
30,000 ft (9000 m). 
    Their classification has always presented difficulties. In the 
classification of W. Fisher three orders - Echiuroinea, Xenopneusta (4 
species), and Heteromyota (1 species) - are recognized, with one problematic 
species attached. By far the largest number of species are included in the 
Echiuroinea, which is divided into the families Bonellidae and Echiuridae. The 
classification of the Bonellidae is the most uncertain and now includes 21 
genera with about 35 species. The Echiuridae are more stable with only 8 
genera but more than 70 species. 
    The body is saclike or sausage-shaped and often highly colored. Anteriorly 
there is a prostomium which is readily detached. This may be very long and 
cleft at the tip (Bonellidae), or short and flaplike (Echiuridae; see 
illustration). The mouth, located at the base of the prostomium, leads into 
the gut, which is divided into several distinct regions. At the posterior end 
of the body are two very characteristic structures, the anal vesicles, for 
which an excretory function has been proposed. 
    The sexes are separate and similar in the Echiuridae. In the Bonellidae 
they are separate and very dissimilar, the male being minute and parasitic 
within or on the female. 
 �34s[Mary E. Rice]
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�6v�2v�32sEcho   �21m �33s 
    A sound wave which has been reflected or otherwise returned with 
sufficient magnitude and time delay to be perceived in some manner as a sound 
wave distinct from that directly transmitted. Multiple echo describes a 
succession of separately indistinguishable echos arising from a single source. 
When the reflected waves occur in rapid succession, the phenomenon is often 
termed a flutter echo. Echoes and flutter echoes are generally detrimental to 
the quality of the acoustics of rooms. They may be minimized through the 
proper selection of room dimensions, room shape, and distribution of 
sound-absorbing materials. 
    Echoes have been put to a variety of uses in measurement problems. For 
example, the distance between two points can be measured by timing the 
duration required for a direct sound originating at one location to strike an 
object at the other point and to return an echo to the location of the initial 
source. Ultrasonic echo techniques have achieved considerable success in 
nondestructive testing of materials. See ECHO SOUNDER; ECHOCARDIOGRAPHY; 
NONDESTRUCTIVE TESTING; REFLECTION OF SOUND; SONAR; SOUND; ULTRASONICS. 
 �34s[William J. Galloway]
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�6v�2v�32sEcho box   �21m �33s 
    A device used to check the output power and spectrum of a radar 
transmitter. It consists of a low-loss, tunable, resonant cavity connected to 
the antenna feed line through a fixed coupling circuit so that the fraction of 
output power supplied to the cavity is always constant. The signal level 
within the cavity depends on the strength of the portion of the transmitter 
output spectrum lying within the cavity's narrow passband, which can be tuned 
to traverse the entire frequency range of interest. A microammeter connected 
through a crystal rectifier to a loop within the cavity permits reading the 
signal level. The spectrum can be measured as the cavity is tuned to different 
frequencies. See RADAR. 
 �34s[Robert I. Bernstein]
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�6v�2v�32sEchocardiography   �21m �33s 
    A noninvasive diagnostic procedure in clinical and investigative 
cardiology that uses a transducer held against the chest to send 
high-frequency sound waves which pass into the heart; as they strike 
structures within the heart, they are reflected back to the transducer and 
recorded on an oscilloscope. See ULTRASONICS. 
    Mitral stenosis, idiopathic hypertrophic subaortic stenosis, atrial 
myxoma, and pericardial effusion are disorders that can be accurately 
diagnosed by echocardiography. If used in conjunction with the clinical, 
electrocardiographic, and roentgenologic findings, echocardiography is useful 
in the evaluation of numerous cardiac disorders. When combined with 
phonocardiography and cardiac catheterization, echocardiography has extended 
knowledge of the genesis of heart sounds, murmurs, and left ventricular 
function. In addition, the technique is useful in measuring the internal 
dimensions of the ventricles, atria, and outflow vessels. These measurements 
can be used with reasonable accuracy to estimate ventricular volumes and 
ejection fractions in normal subjects. However, in patients with grossly 
dilated ventricles or in those exhibiting localized contraction abnormalities, 
the correlation with ventriculography is unreliable. See CARDIAC 
ELECTROPHYSIOLOGY; PHONOCARDIOGRAPHY. 
    Other techniques, such as compound-B ultrasonography, which produces a 
two-dimensional cross-sectional image of intracardiac structures, and 
multiscan echocardiography, may provide a more accurate anatomic display of 
cardiac chambers and outflow vessels. Unique features of the multiscan 
technique are the capacity to display substantial segments of cardiac 
structures and to show them in continuous, real-time motion. See HEART 
DISORDERS.  
 �34s[Bernard L. Segal]
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�6v�2v�32sEcho sounder   �21m �33s 
    A marine instrument used primarily for determining the depth of water by 
means of an acoustic echo. A pulse of sound sent from the ship is reflected 
from the sea bottom back to the ship, the interval of time between 
transmission and reception being proportional to the depth of the water. An 
echo sounder is really a type of active sonar. It consists of a transducer 
located near the keel of the ship which serves (in most models) as both the 
transmitter and receiver of the acoustic signal; the necessary oscillator, 
receiver, and amplifier which generate and receive the electrical impulses to 
and from the transducer; and a recorder or other indicator which is calibrated 
in terms of the depth of water.  
    Echo sounders, sometimes called fathometers, are used by vessels for 
navigational purposes, not only to avoid shoal water, but as an aid in fixing 
position when a good bathymetric chart of the area is available. Some 
sensitive instruments are used by commercial fishermen or marine biologists to 
detect schools of fish or scattering layers of minute marine life. 
Oceanographic survey ships use echo sounders for charting the ocean bottom. 
See SCATTERING LAYER; SONAR; UNDERWATER SOUND. 
 �34s[Robert W. Morse]
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�6v�2v�32sEchovirus   �21m �33s 
    One of the divisions of the enterovirus subgroup, within the picornavirus 
group of viruses. The name is derived from the term enteric cytopathogenic 
human orphan virus. More than 34 antigenic types exist. Only certain types 
have been associated with human illnesses, particularly with aseptic 
meningitis and febrile disease. Their epidemiology is similar to that of other 
enteroviruses. Echoviruses resemble polioviruses and coxsackieviruses in size 
(about 28 nanometers) and in many other properties. Diagnosis is made by 
isolation and typing of the viruses in tissue culture. Antibodies form during 
convalescence. See ANIMAL VIRUS; ENTEROVIRUS; PICORNAVIRUS. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sEclipse �21m �33s
    The darkening of one celestial body by the shadow of another. A solar
eclipse occurs when the Moon passes directly between Earth and the Sun, hiding
at least a part of the Sun from view. The Moon is eclipsed when it goes into
Earth's shadow. This article deals only with these two kinds of eclipses, but
there are other similar phenomena. See ECLIPSING VARIABLE STARS; JUPITER;
OCCULTATION: TRANSIT (ASTRONOMY).
    The Moon's orbit around Earth is inclined 5DG to the plane of Earth's
orbit around the Sun. The latter is called the ecliptic, because eclipses can
occur only when the Moon is on or near it.
    The two points where the Moon's orbit crosses the ecliptic are called
nodes. The line connecting them and passing through Earth's center is the line
of nodes. When this line points to the Sun, the Moon's shadow strikes Earth,
producing a solar eclipse. About 2 weeks before or after that event, the Moon
can go through Earth's shadow, producing a lunar eclipse. This interval during
which the Earth-Sun line is approximately along the line of nodes is called an
eclipse season. Another one occurs about 6 months later.
    The Moon's orbit is not fixed in position, being disturbed by the
gravitational pull of the Sun. This causes the line of nodes to move in a
direction opposite to that of Earth's orbital motion. This effect is called
the regression of the nodes. It makes the eclipse seasons occur earlier each
year by about 20 days, resulting in an interval of 346.62 days which is called
the eclipse year. At least one solar eclipse must occur during an eclipse
season, making a minimum of two solar eclipses in a year.

    Solar eclipses.  A total eclipse of the Sun is one of the greatest
spectacles of nature and results from one of the most fortunate coincidences
of nature. In round numbers, the Sun's diameter is 400 times that of the Moon,
and the Sun's distance from Earth is also 400 times that of the Moon. Both
subtend nearly the same angle of about 0DG.5, so that the Moon's disk can
appear just large enough to cover the Sun's disk.
    Since the Moon and Earth are smaller than the Sun, the shadow of each is a
cone having its point directed away from the Sun. All sunlight is
geometrically excluded within this region, which is called the umbra. The
Moon's umbra is formed by tangents to the Sun and Moon, which intersect on or
near Earth's surface. Surrounding it is the larger inverted cone of the
penumbra, formed by tangents intersecting between the Sun and Moon. A total
eclipse can be observed within the umbra, and a partial eclipse within the
penumbra.
    The length of the Moon's umbra is very nearly 1/400 of its distance from the
Sun. At new moon it averages about 374,000 km (232,000 mi), and it varies
about 6400 km (4000 mi) either way. Since the Moon's distance ranges from
about 357,000 km (221,000 mi) to 407,000 km (253,000 mi), its umbra is
sometimes long enough to reach Earth, but more often falls short. In the first
case an observer located on the line of centers of the Sun and Moon can see a
total solar eclipse. In the second case the Moon appears smaller than the Sun
and the observer sees an annulus or ring of sunlight around the Moon. Hence
this is called an annular eclipse.
    In the most favorable case of a total eclipse, the diameter of the
circular umbra where Earth's surface cuts it squarely is 269 km (167 mi). This
occurs near the Equator. The path of totality traced out by the umbra is so
narrow compared with Earth's diameter that the average interval between total
eclipses at any one place on Earth is 360 years.
    Under the most favorable conditions a total solar eclipse can last at a
given point near the Equator for about 7 1/2 min. The maximum possible duration
of an annular eclipse is nearly 12 1/2 min.
    Since the Moon moves its own diameter in about 1 h, the complete
phenomenon of a total solar eclipse lasts about 2 h: 1 h each in covering and
uncovering the Sun and not more than a very few minutes for the total phase.

    With the arrival of totality, the pearly white corona appears (see
illustration). It is the Sun's outer atmosphere, extending in all directions
for millions of miles. It is bright at its inner edge and fades out in
irregular streamers. Flamelike prominences are often visible to the unaided
eye and usually with binoculars. They are protuberances from the red
chromosphere, which is the layer of gases just above the Sun's visible
surface. The brighter stars and planets are usually visible to the unaided eye.

    Lunar eclipses.  Although lunar eclipses do not occur as often as solar
eclipses, they are seen by many more people. They occur during full moon,
which is opposite the Sun and can be observed at any one instant by all of the
night half of Earth. Also, since a total lunar eclipse can last for more than
an hour, it can be seen from a little more than half of Earth's surface. On
the other hand, solar eclipses are visible even in the partial phases from
limited areas of Earth.
    When the Moon passes through the center of Earth's shadow, it takes about
5 h 40 min. The first hour is spent in the penumbra. No darkening is
noticeable until near the end of that period, because all of the Moon's side
facing Earth is still receiving some direct sunlight. Then as the Moon enters
the umbra, the edge of the Moon appears nearly black by contrast with the
bright parts of the lunar surface. The second hour is required for all of the
Moon to get into the umbra.
    The diameter of the umbra where the Moon crosses it is about 2 2/3 times the
Moon's diameter. The Moon can remain totally eclipsed for 1 h 40 min. The next
hour is spent in moving out of the umbra, and the final hour in passing
through the penumbra.
    If Earth had no atmosphere, the Moon would disappear from view while in
the umbra. However, the Earth's atmosphere acts like a lens and bends the
sunlight into the umbra. The longer waves of red light penetrate the
atmosphere better than the shorter waves of blue light, which are scattered to
form the blue of the sky. An observer on the Moon would see Earth's disk
surrounded by a thin ring of bright sunset colors. This accounts for the usual
reddish color of the totally eclipsed Moon.
 �34s[Clarence H. Cleminshaw]
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�6v�2v�32sEclipsing variable stars   �21m �33s 
    Double star systems in which the two components are too close to be seen 
separately but which reveal their duplicity by periodic changes in brightness 
as each star successively passes between the other and the Earth, that is, 
eclipses the other. Studies of the light changes and the radial velocity 
changes of each component permit the computation of the radii, masses, and 
densities of the components - important quantities that cannot be measured 
directly in single stars. In addition, these close double stars are useful in 
studies of mass loss and of stellar evolution. Since eclipsing stars are 
variable in light, they are included in general variable star catalogs under 
the same system of nomenclature. See BINARY STAR; VARIABLE STAR. 
    It is now believed that all explosive variables (novae, recurrent novae, 
and so forth), with the exception of supernovae, are members of close binary 
systems. At least some of the x-ray sources are close binaries in this state. 
In a few of the eruptive variables, particularly those known as dwarf novae, 
rapid scintillation is found, presumably in each system from a hot spot where 
the transferring mass collides violently with a circumstellar disk of 
relatively low-density material revolving around the collapsed star; the 
scintillation stops periodically when the spot is eclipsed by the other 
component. See  NOVA; STELLAR EVOLUTION. 
 �34s[Frank B. Wood]
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�6v�2v�32sEcliptic   �21m �33s 
    The path in the sky traced by the Sun in its apparent annual journey as 
Earth revolves around it. The ecliptic is a great circle on the celestial 
sphere, inclined about 23DG5 to the celestial equator, the angle of 
inclination being called the obliquity of the ecliptic. See ASTRONOMICAL 
COORDINATE SYSTEMS; SUN. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sEclogite   �21m �33s 
    A very dense rock composed of red-brown garnet and the grape-green 
pyroxene omphacite. Eclogites possess basaltic bulk chemistry, and their 
garnets are rich in the components pyrope, almandine, and grossular, while the 
pyroxenes are rich in jadeite and diopside. 
    Eclogite occurrences may be subdivided into three broad categories: Group 
a eclogites are found as layers, lenses, or boudins in schists and gneisses 
seemingly of the amphibolite facies. Quartz, together with zoisite or kyanite, 
commonly occurs in these rocks. Amphibole of barroisitic composition may also 
be present. Group b eclogites are found as inclusions in kimberlites and 
basalts. They are frequently accompanied by xenoliths of garnet peridotite. 
Group c eclogites are found as blocks and lenses in schists of the 
glaucophane-schist facies. Such eclogites do not contain kyanite, rarely 
contain quartz, but bear amphibole, epidote, rutile, or sphene. 
    Eclogite is the name given to the highest-pressure facies of metamorphism; 
the critical mineral assemblage defining this facies is garnet + omphacite, 
together with kyanite or quartz in rocks of basaltic composition. Where 
sedimentary and granitic rocks have been metamorphosed under eclogite facies 
conditions, they result in spectacular omphacite + garnet + quartz-bearing 
mica schists and metagranitic gneisses such as are found in the Sezia-Lanzo 
zone of the western Alps. See METAMORPHIC ROCKS. 
    The high density of eclogite, together with its elastic properties, makes 
it a candidate for upper mantle material. Large quantities of basaltic oceanic 
crust are returned to the mantle through the process of subduction, where 
prevailing high pressures convert it to eclogite. The quantity and 
distribution of eclogite within the mantle is not known; that it occurs is 
known from the nodules brought up in kimberlite pipes and in basalts. 
 �34s[Timothy J. B. Holland]
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�6v�2v�32sEcological interactions   �21m �33s 
    Relationships between members of different species belonging to a 
particular ecological community. Ecologists generally classify interactions 
according to the effect that members of one species have on the population 
growth rate of a second species. If an increase in one species' population 
increases the growth rate of a second species' population, the effect is 
positive or beneficial (denoted by +). If an increase in one species' 
population decreases the growth rate of a second species' population, the 
effect is negative or harmful (denoted by -). If neither occurs, the effect is 
neutral (denoted by 0). Effects may be indirect, in which case they involve 
one or more additional species in the community. For example, earthworms 
increase plant growth and thereby provide more food for caterpillars. 
Alternatively, effects may be direct; for example, injuries or death may 
result from territorial fighting between members of different species. 
    Interspecific competition occurs when two species negatively affect one 
another. Two principal kinds of competition are recognized. Exploitative 
competition between species occurs when individuals of one species, by 
consuming some resource, usually food, deprive individuals of another species 
of that resource and thereby lower the second species' population growth rate. 
Interference competition is more direct; it occurs when individuals of one 
species directly harm individuals of a second species. The degree to which 
such an effect is harmful can be quite varied. Organisms may kill other 
organisms by producing toxins or by fighting; fighting may also produce 
injuries. More subtle types of interference competition occur when individuals 
interact with one another in such a way as to use energy that might have been 
used for the production of offspring, or take up time that might have been 
used to gather additional energy for the production of offspring. 
    Predation occurs when individuals of one species, the predator, consume 
part or all of individuals of another species, the prey. In this interaction, 
the predator benefits and the prey is harmed. Broadly defined, the predator 
and prey may be animal or plant, and the predator may be a parasite. Examples 
of predation include (1) a hawk eating a sparrow, (2) a lizard eating an 
insect, (3) a sparrow eating a seed, (4) a sheep eating grass, (5) a Venus' 
flytrap eating an insect, (6) a tick drawing blood from a dog, and (7) a 
pathogenic bacterium afflicting a human. Some ecologists exclude plants from 
the definition, in which case examples 3-5 do not apply, or exclude 
parasitism, in which case examples 6 and 7 do not apply. 
    Mutualism occurs when two species positively affect one another. The 
degree to which the interaction is necessary for the survival of one or both 
species is sometimes emphasized; when the interaction is not necessary for 
either species, it is called protocooperation. Several well-known examples of 
mutualistic interactions that are necessary for the survival of both species 
exist. Various animals, such as termites and ruminants, that consume woody 
plant parts are dependent on the protozoa in their guts to digest cellulose. 
Various bacteria live in nodules on the roots of leguminous plants such as 
peas and clover. These bacteria are able to extract nitrogen directly from air 
spaces in the soil and make it available to the plant, which otherwise would 
have to acquire the nitrogen from nitrates. A third example is the lichen, 
which is composed of two kinds of plants, algae and fungi, living in a 
mutualistic association. The algae can photosynthesize and so provide various 
organic compounds. The fungi are believed to be able to dissolve nutrients 
from the rock on which the lichen lives; they also provide a structural 
matrix. This relationship is usually but not always obligatory. See LICHENES; 
NITROGEN CYCLE. 
    Commensalism occurs when one species positively affects another while 
being little affected itself. Several examples meet this criterion to various 
degrees. Plants such as bromeliads and orchids grow on other plants such as 
trees; the former are benefitted, while the latter are not usually seriously 
affected. Cattle egrets, which are insectivorous heronlike birds, have become 
adapted to following cattle and consuming the insects that the cattle disturb. 
The egrets are benefitted, and their actions seem to bother the cattle little. 
Certain fish (Amphiprion percula) are specialized to live among the poisonous 
tentacles of sea anemones. The anemones feed on other kinds of fish, and the  
Amphiprion feed on the leftovers from the anemones' meals. See ECOLOGY; 
ECOSYSTEM.  
 �34s[Thomas W. Schoener]
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�6v�2v�32sEcological succession �21m �33s
    A gradual process brought about by the change in the number of individuals
of each species of a community and by the establishment of new species
populations which may gradually replace the original inhabitants. Succession
depends upon several major factors: (1) the floristic and faunistic resources
of the general region, that is, what organisms are in a position to invade a
site; (2) the rate of change of the habitat and its receptivity to these
potential invaders; and (3) chance factors which may influence interactions.
    The term sere is used to describe all of the temporary communities which
occur during a successional sequence on a given site. Eventually succession
ends, changes in species composition of the community cease or fluctuate
within some bounds, and the result is a climax community.
    Successional sequences may be classified on the basis of starting habitat.
Thus, hydroseres are communities in which pioneer plants invade open water,
eventually forming some kind of soil such as peat or muck; xeroseres are
communities on dry, sterile ground, such as rock, sand, or clay.
    If an open habitat has never been occupied before, the way is opened for
primary for primary succession. If the habitat has been disturbed, leaving
vestiges of soil, seeds, and other organic debris from previous occupancy, a
secondary succession will begin.

    As an area is invaded and taken over by successive populations of plants
and animals, the physiognomy of the community changes. Isolated pioneer plants
eventually give way to a consolidation of the plant cover. With continued
exploitation of the habitat, new species take over which have life forms able
to utilize more fully the resources of the habitat than previous life forms
until finally the climax community is established. Such a community can
perpetuate itself indefinitely, at least in theory, because each species
reproduces and maintains a stable number of individuals.
    It was traditional to divide succession into pioneer, consolidation,
subclimax, and climax stages on the basis of physiognomy. However, such
discrete stepwise changes are not always shown by close study of the
populations, because there is a continuously changing array of species.
Therefore, the subdivision of a successional continuum must depend on local
conditions and purposes of study.
    An understanding of succession is important for effective human management
of communities. Human activities generally have the effect of stopping
succession at an early stage. Fire was used by most primitive people to keep
forests suitable for hunting by clearing away the underbrush or to clear land
for agriculture. Since almost all major crop plants are annuals of the pioneer
stage, regular plowing is needed to hold down succession. Other methods of
checking succession include mowing and grazing, which favor sodforming grasses
and kill most tree seedlings, and selective weed control with chemicals.
    Succession can be either retarded or accelerated by manipulation of
drainage. Such manipulation may be used to establish a cranberry bog or a
stand of productive timber. Many timber trees occur naturally as successional
species rather than in climaxes. Thus the forest must be managed so as to
permit these species to grow free from the competition of the climax species
which will eventually replace them. Fire, herbicides, and cutting practices
are all useful in regulating the growth and  composition of forest stands so
that early successional stages are maintained. See CONSERVATION OF RESOURCES;
ECOLOGY; VEGETATION MANAGEMENT.
 �34s[Arthur W. Cooper]
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�6v�2v�32sEcology �21m �33s
    A study of the relation of organisms to their environment, or in more
simple terms, environmental biology. Ecology is concerned especially with the
biology of groups of organisms and with functional processes on the lands, in
the oceans, and in fresh waters. Ecology is the study of the structure and
function of nature (humans being considered part of nature).
    Ecology is one of the basic divisions of biology which are concerned with
principles or fundamentals common to all life. Since biology may also be
divided into taxonomic divisions which deal with specific kinds of organisms,
ecology is not only a basic division of biology but also an integral part of
any and all of the taxonomic divisions. Both approaches are profitable. It is
often productive to restrict certain studies to taxonomic groups, insects, for
example, because different kinds of organisms require different methods of
study. (Some groups of organisms are of greater interest to humans than
others.) It is also important to seek and to test unifying principles which
may be applicable to nature as a whole.

    Approaches to ecology.  From the standpoint of general ecology, an
instructive subdivision is in terms of the concept of level of organization.
For convenience, a biological spectrum can be visualized as follows:
protoplasm, cells, tissues, organs, organ systems, organisms, populations,
communities, and ecosystems. Ecology is concerned largely with the levels
beyond that of the individual organism. In ecology the term of population,
originally used to denote a group of people, is broadened to include groups of
individuals of any one species of organism. Community in the ecological sense,
sometimes designated as biotic community, includes all of the species
populations of a given situation. The community or individual and the
nonliving environment function together as an ecological system or ecosystem.
The Earth as an ecosystem is conveniently designated as the biosphere. The
term autecology is often used to refer to the study of environmental relations
of individuals or species, whereas the term synecology refers to the study of
groups of organisms such as communities. Mathematical concepts have permeated
the basic foundations of ecology and have provided new means of exploring
qualitative and quantitative relations at all levels. This new brand of
ecology is known as systems ecology. See BIOSPHERE; COMMUNITY; SYSTEMS ECOLOGY.
    For convenience, subdivisions of ecology may also be based on the kind of
environment or habitat to be considered or studied. Marine ecology,
fresh-water ecology, and terrestrial ecology are the three broad divisions
from this point of view; estuarine ecology, stream ecology, or grassland
ecology represent more restricted interests. See FRESH-WATER ECOSYSTEM; MARINE
ECOSYSTEM; TERRESTRIAL ECOSYSTEM.
    Some attributes, obviously, become more complex and variable during the
procession from cells to ecosystems; however, it is often overlooked that
other attributes may become less complex and less variable from the small to
the large unit. The reason is that homeostatic mechanisms, which may be
defined as checks and balances or forces and califorces, operate all along the
line to produce a certain amount of integration, and smaller units function
within larger units. For example, the rate of photosynthesis of a whole forest
or a whole cornfield may be less variable than that of the individual trees or
corn plants within the communities, because when one individual or species
slows down, another may speed up in a compensatory manner. See BIOGEOCHEMICAL
CYCLES; ECOSYSTEM; HOMEOSTASIS; HUMAN ECOLOGY.
 �34s[Eugene P. Odum]
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    Applied ecology. Applied ecology involves the application of ecological
principles to the solution of human problems and the maintenance of a quality
life. It is assumed that humans are an integral part of ecological systems and
that they depend upon healthy, well-operating, and productive systems for their
continued well-being. For these reasons, applied ecology is based on a
knowledge of ecosystems, and the principles and techniques of ecosystem ecology
are used to interpret and solve specific environemental problems and to plan
new managment systems in the biosphere. Although a variety of management
fields, such as forestry, agriculture, wildlife management, environmental
engineering, and environmental design, are concerned with specific parts of
the environment, applied ecology is unique in taking a view of the whole system,
and attempting to account for all inputs to and outputs from the system--and
all impacts. In the past, applied ecology has been considered as being
synonymous with the above applied sciences. See ENVIRONMENTAL PROTECTION.
[FRANK B. GOLLEY]
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�6v�2v�32sEconomic geology �21m �33s
    Commercial application of the general knowledge of geology. A large number
of geologists receive special training for service in solving problems met in
the mining of metals and nonmetals, in discovering and producting petroleum
and natural gas, and in engineering projects of many kinds. See ENGINEERING
GEOLOGY; MINING; PETROLEUM GEOLOGY.
 �34s[Chester R. Longwell ;Peter J. Wyllie]
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�6v�2v�32sEcosystem   �21m �33s 
    A functional system which includes the organisms of a natural community 
together with their environment. The term, a contraction of ecological system. 
All natural communities, or assemblages of organisms which live together and 
interact with one another, are closely related to their environments. It is 
consequently appropriate to conceive of community and environment as forming a 
single, complex whole - the ecosystem. See COMMUNITY; ECOLOGY; ENVIRONMENT. 
    The term ecosystem can be applied to any size or level of environment 
chosen for study. Communities of small and distinctive environments, such as 
the organisms inhabiting an animal's intestine, a decaying log, or the foliage 
of a particular plant, are sometimes referred to as microcommunities, and 
their environments, as microcosms or microenvironments. These are generally 
parts of larger ecosystems, such as a forest and its environment or a lake and 
its organisms, for which the terms macrocommunity and macroenvironment are 
appropriate. At the opposite extreme from microcommunities, all the organisms 
of the world together may be regarded as a world community which, with its 
environment, forms a single world ecosystem. The living part of this world 
ecosystem is often termed the biosphere. The transfer and concentration of 
materials in the world ecosystem by both organisms and physical processes are 
the subject of biogeochemistry. Since humans are part of the world ecosystem, 
their place and effects on it are part of the study of human ecology. See 
BIOSPHERE; HUMAN ECOLOGY. 
    Any organism lives in a natural context which includes an environment and 
a community of other organisms. The natural context of the life of an organism 
is thus an ecosystem. Two aspects of an organism's relation to the ecosystem 
in which it lives are often distinguished, though they cannot be sharply 
separated. Its location, as described in terms of physical and chemical 
factors, and often, kind of community, constitute its functional habitat. Its 
place within the community, including relations to other organisms, is its 
niche.  
    Within an ecosystem certain major fractions, or divisions, may be 
recognized. A terrestrial ecosystem is often thought of as having four or five 
such subdivisions: (1) the physical environment, such as climate; (2) the 
soil, if distinguished from physical environment; (3) the vegetation or plant 
community; (4) the animal community; and (5) the saprobe community, which 
consists of bacteria and fungi. These same divisions may be recognized in a 
marine or fresh-water ecosystem, with the substitution of substrate or bottom 
material for soil. Aquatic ecosystems, however, are often divided in a 
different way. These divisions are the communities and environments of the  
open water, those of the shores, and those of the deeper bottom. See 
FRESH-WATER ECOSYSTEM; MARINE ECOSYSTEM; TERRESTRIAL ECOSYSTEM. 
 �34s[Robert H. Whittaker]
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�6v�2v�32sEczema �21m �33s
    A nonspecific term used to denote any skin disorder characterized by
redness, thickening, oozing from blisters or papules, and occasional formation
of fissures and crusts. The words eczema, eczematous, and chronic eczematous
are difficult to define and use. It is suggested that these terms be avoided
in favor of the specific type of dermatitis. Various authorities differ as to
the specific skin lesions which are considered under this catchall heading,
but it is most commonly used to mean acute dermatitis, while some also include
chronic dermatitis. Few eczemas represent pure forms if they have been present
for a while.
    Infantile eczema may be a form of contact dermatitis, atopic or
neurodermatitis, or combinations of these with others of known or unknown
cause.
    Contact dermatitis occurs in many individuals as the result of either
chronic irritation or sensitization to some external substance. Poison ivy,
industrial dermatoses, and cosmetic sensitivities are examples.
    Neurodermatitis may be either localized or general in its manifestations.
An initial itching, or pruritis, is followed by a thickening of the skin due
to repeated scratching. There is often a prior history of psychic disturbance
or a family history of allergy, such as hay fever or asthma.
    Stasis dermatitis is primarily the result of poor circulation and is found
principally on the legs and ankles. Older patients or those with an occupation
that requires prolonged periods of standing appear to be especially
susceptible.
    The senile eczema seen in elderly people is thought to be caused by
various factors, notably dryness of the skin, soap sensitivity, poor hygiene
or diet, or a preceding neurodermatitis. See SKIN DISORDERS.
 �34s[Edward G. Stuart /N. Karl Mottet]
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�6v�2v�32sEddy current   �21m �33s 
    An electric current induced within the body of a conductor when that 
conductor either moves through a nonuniform magnetic field or is in a region 
where there is a change in magnetic flux. It is sometimes called Foucault 
current. Although eddy currents can be induced in any electrical conductor, 
the effect is most pronounced in solid metallic conductors. Eddy currents are 
utilized in induction heating and to damp out oscillations in various devices. 
    It is possible to reduce the eddy currents by laminating the conductor, 
that is, by building the conductor of many thin sheets that are insulated from 
each other rather than making it of a single solid piece. The laminations do 
not reduce the induced emfs, but if they are properly oriented to cut across 
the paths of the eddy currents, they confine the currents largely to single 
laminae, where the paths are long, making higher resistance. See CORE LOSS. 
 �34s[Kenneth V. Manning]
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�6v�2v�32sEdema   �21m �33s 
    An abnormal accumulation of fluid in the cells, tissue spaces, or cavities 
of the body; also known as dropsy. An excess of fluid in the pleural spaces is 
referred to as hydrothorax; in the pericardial sac as hydropericardium; and in 
the peritoneal cavity as ascites. Anasarca is a generalized subcutaneous edema. 
    There are three main factors in the formation of generalized edema and a 
fourth which plays an important role in the formation of local edema. They are 
(1) permeability of the capillary wall, (2) colloid osmotic pressure of the 
plasma proteins, (3) hydrostatic pressure in the capillaries, and (4) 
lymphatic obstruction. 
 �34s[Romeo A. Vidone]
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�6v�2v�32sEdentata   �21m �33s 
    A mammalian order consisting of many strange fossil and living animals 
found exclusively in the Western Hemisphere. Included is the suborder 
Xenarthra. Xenarthrans include Gravigrada (sloths), Vermilingua (anteaters), 
and Cingulata (the armored animals, armadillos and glyptodonts). Edentate, 
meaning toothless, directly applies only to the infraorder Vermilingua (``worm 
tongue,'' the South American true anteaters). Other edentates have simple 
prismatic teeth with no enamel, except in a few extinct genera and in the 
embryos of living armadillos. There is little or no specialization of the 
dentition.  
    Some edentates were, or are, no larger than rats; others (the megatheriine 
ground sloths, which ranged from Patagonia to the southern United States) 
rivaled large elephants in size. All animals in this order are clawed (some 
were secondarily hoofed) and quadrupedal. Some live in trees; others are 
ambulatory-terrestrial; and others are burrowers. Many edentates can be termed 
omnivorous; some are more strictly insectivorous or herbivorous. See ANTEATER; 
ARMADILLO; SLOTH. 
    Armor seems to have been a very early development in xenarthrans, and 
there is some evidence to suggest that the families of sloths may have been 
derived from an armored armadillolike stock. South America was the center of 
early history and diversification of the Xenarthra. 
    Xenarthran edentates may have originated in North America in an early 
Paleocene or Late Cretaceous stock of small clawed quadrupedal eutherian 
mammals without dermal armor and with generalized skeleton. Such a group would 
most likely be included in the order Insectivora. See EUTHERIA; INSECTIVORA; 
MAMMALIA.  
 �34s[Donald E. Savage]
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�6v�2v�32sEdiacaran fauna   �21m �33s 
    The oldest known assemblage of fossil remains of soft-bodied marine 
animals. Its name is taken from the Ediacara Hills, an abandoned mining area 
some 380 mi (610 km) north of Adelaide, southern Australia. 
    The first fossils were found in these hills in 1947 by the Australian 
geologist Reginald Sprigg, who discovered abundant ``jellyfish'' remains in 
the uppermost strata of a thick sequence of quartzites. These quartzites were 
believed to be of Lower Cambrian age, but investigations proved them to be 
older than rocks containing early Lower Cambrian fossils. The age of the 
Ediacaran fauna is considered as late Precambrian for the following reasons: 
The beds with worm burrows and the younger calcareous beds contain fossils 
known elsewhere from Lower Cambrian strata, but no such fossils were found 
below. The Lower Cambrian fauna does not include any of the fossils of the 
Ediacaran fauna, even where abundant jellyfish remains occur with Cambrian 
shelly fossils, for example, in southern Sweden. However, future discoveries 
of Paleozoic survivors of some elements of this fauna are not unlikely. Some 
elements of the Ediacaran fauna have been found elsewhere below strata with 
Cambrian fossils and in rocks which have been dated as late Precambrian by 
isotopic dating of radioactive minerals. 
    Medusoid fossils in various stages of preservation comprise about 
two-thirds of the total number collected (Fig. 1). Annelid worms (Fig. 2) 
follow with less than one-quarter, the sea pens and representatives of several 
groups of totally extinct invertebrate organisms follow these. There are also 
traces of life activities of other animals which burrowed or fed on the 
surface of the sediment. In all, some 25 species of body fossils and traces of 
activities of at least six different kinds of other animals have been 
recognized.  
  
    The surprising occurrence of the remains of soft-bodied animals in 
sandstone is explained by a temporary reduction in the intensity of wave and 
current action during the time of deposition of the Pound sandstone. This 
allowed the bodies to settle together with fine clay or silt particles and, 
when covered by succeeding laminae of sand, either during or after decay, to 
be preserved as casts or molds. Bodies more resistant to decay remained long 
enough on sedimentary surfaces for imprints (external molds) of their upper 
surfaces to be formed in the overlying sand lamina. This preservation of 
soft-bodied animals is not unique in the geological record, but where remains 
of shelly fossils also occur, as in Paleozoic sandstones, attention was given 
to them in preference to the more obscure remains of jellyfish and worms. 
 �34s[Martin F. Glaessner]
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�6v�2v�32sEdrioasteroidea   �21m �33s 
    A class of extinct Echinozoa which arose early in Cambrian times and died 
out before the end of the  Carboniferous Period. They resemble the other 
Echinozoa in having the ambulacral radial areas bordered by tube feet  which 
emerge through pores in or between the ambulacral plates. They differ from 
extant Echinozoa in having the mouth and anus on the upper side of the theca. 
Hinged ambulacral plates bordered the groove on either side and could be 
erected over it to convert it into a tube (see illustration). The animals in 
some cases were cemented to the substrate by the lower surface; in other cases 
they were free and rested upon the sea bed. Brachioles are lacking. About 30 
genera have been described and grouped into 7 families. See ECHINODERMATA; 
ECHINOZOA.  
 �34s[Howard B. Fell]
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�6v�2v�32sEel   �21m �33s 
    The name for a number of unrelated fish included in the orders 
Anguilliformes and Cypriniformes. 
    The true eels are members of the Anguilliformes, which is also known as 
the Apodes (see table). There are several hundred species, most of which are 
marine. They are most common in the shallower waters of tropical and 
subtropical seas, although a few species do occur in colder waters or at 
considerable depths. See ANGUILLIFORMES. 
    The family Gymnotidae of the order Cypriniformes contains about 32 species 
which are found in fresh waters of Central and South America. The electric eel 
(Electrophorus electricus), the largest and best-known species of the family, 
may grow to a length of 8 ft (2.4 m) and weigh 60 lb (27 kg). It can produce 
an electric shock from its electric organs, which extend almost the length of 
the body. The bodies of the members of this family are eellike and may be 
naked or scaled. See CYPRINIFORMES; ELECTRIC ORGAN (BIOLOGY). 
 �34s[Charles B. Curtin]
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�6v�2v�32sEffective dose 50   �21m �33s 
    A term used chiefly to characterize the potency of a drug by the amount 
required to produce a response in 50% of the subjects to whom the drug is 
given; also known as ED50 or median effective dose. 
    Median effective dose is most commonly applied in connection with drugs, 
but it may be used when various other sources of stimuli, for example, x-rays, 
are under consideration. The response must be of the kind known as quantal, or 
all-or-nothing, where the investigator is simply able to report that the 
response either was or was not elicited. 
    The median effective dose is a well-defined quantity only when the test 
animal, the end response, and such factors as the route of injection of a drug 
and the state of nutrition of the animal are specified. 
 �34s[Colin White]
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�6v�2v�32sEffector systems �21m �33s
    Those organ systems of the animal body which mediate overt behavior.
Injury to an effector-system leads to loss or to subnormal execution of
behavior patterns mediated by the system, conditions termed paralysis and
paresis, respectively.
    Overt behavior consists of either movement or secretion. Movement results
from contraction of muscle. Secretion is a function of glands. Neither
muscular contraction nor glandular secretion is autonomous but is regulated by
an activating mechanism which may be either neural or humoral. In neurally
activated systems the effector organ, whether muscle or gland, is supplied by
nerve fibers originating from cell bodies situated in the central nervous
system or in peripherally located aggregates of nerve cell bodies known as
ganglia.
    In other effector systems (humeromuscular and humeroglandular) the
activating agent is normally a blood-borne chemical substance produced in an
organ distant from the effector organ. For example, uterine smooth muscle is
uninfluenced by the uterine nerve activity but contracts vigorously when the
blood contains pitocin, a chemical substance elaborated by the posterior lobe
of the hypophysis.
    Finally, some effector systems are hybrid in the sense that both nerves
and humors regulate their functions. The smooth muscle of arterioles contracts
in response to either nerve stimulation or epinephrine. Secretion of
hydrochloric acid by the gastric mucosa is increased by activation of the
vagus nerve or by the presence in the blood of histamine. Effector systems
with both neural and humoral regulation are never completely  paralyzed by
denervation but may be deficient in reaction patterns when the quick
integrated activation provided by neural regulation is essential. See NERVOUS
SYSTEM (VERTEBRATE).
 �34s[Theodore C. Ruch ;Harry D. Patton]
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�6v�2v�32sEfficiency   �21m �33s 
    The ratio, expressed as a percentage, of the output to the input of power 
(energy or work per unit time). As is common in engineering, this concept is 
defined precisely and made measurable. Thus, a gear transmission is 97% 
efficient when the useful energy output is 97% of the input, the other 3% 
being lost as heat due to friction. A boiler is 75% efficient when its product 
(steam) contains 75% of the heat theoretically contained in the fuel consumed. 
All automobile engines have low efficiency (below 30%) because of the total 
energy content of fuel converted to heat; only a portion provides motive 
power, while a substantial amount is lost in radiator and car exhaust. 
 �34s[F. R. E. Crossley]
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�6v�2v�32sEfflorescence   �21m �33s 
    The spontaneous loss of water (as vapor) from hydrated crystalline solids. 
The thermodynamic requirement for efflorescence is that the partial pressure 
of water vapor at the surface of the solid (its dissociation pressure) exceed 
the partial pressure of water vapor in the air. A typical efflorescent 
substance is Glauber's salt, Na2SO4.10H2O. The spontaneous loss of water 
normally requires that the crystal structure be rearranged, and consequently, 
efflorescent salts usually go to microcrystalline powders when they lose their 
water of hydration. See PHASE EQUILIBRIUM; VAPOR PRESSURE. 
 �34s[Robert L. Scott]
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�6v�2v�32sEggplant   �21m �33s 
    A warm-season vegetable (Solanum melongena) of Asiatic origin belonging to 
the plant order Polemoniales (formerly Tubiflorales). Eggplant is grown for 
its usually egg-shaped fleshy fruit and is eaten as a cooked vegetable. 
Popular purple-fruited varieties (cultivars) are Black Beauty and a number of 
hybrid varieties; fruits of other colors, including white, brown, yellow, and 
green, are used chiefly for ornamental purposes. Florida and New Jersey are 
important eggplant-producing states. See POLEMONIALES. 
 �34s[H. John Carew]
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�6v�2v�32sEgg (fowl)   �21m �33s 
    A single, large, living, female sex cell enclosed in a porous, calcareous 
shell through which gases may pass. Although they vary in size, shape, and 
color, the eggs of chickens, ducks, geese, and turkeys are essentially the 
same in structure and content (see illustration). Inward from the shell are 
the outer and inner shell membranes which are also permeable to gases. The 
membranes are constructed to prevent rapid evaporation of moisture from the 
egg but to allow free entry of oxygen, which is necessary for life. Air begins 
to penetrate the shell soon after the egg is laid, and it tends to accumulate 
in a space between the two membranes at the large end of the egg. See CELL 
(BIOLOGY); RESPIRATION. 
    The inner shell membrane surrounds a mass of fluid albumin which, in turn, 
encloses a body of dense albumin; these two types of protoplasm constitute the 
so-called egg white. The central part of the egg is occupied by the yolk, 
which contains the vital egg nucleus and its associated parts. The yolk 
consists of alternating layers of yellow and white yolk. The yolk, enclosed by 
the vitelline membrane, is held in place by the chalaza which is anchored at 
each end of the egg and prevents undue mechanical disturbance. See YOLK SAC. 
 �34s[James F. Ferry]
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�6v�2v�32sEigenvalue (quantum mechanics)   �21m �33s 
    If an equation containing a variable parameter possesses nontrivial 
solutions only for certain special values of the parameter, these solutions 
are called eigenfunctions and the special values are called eigenvalues. 
    The eigenfunction-eigenvalue relation is of particular importance in 
quantum mechanics because of its prominence in the equations which relate the 
mathematical formalism of the theory with physical results. See 
NONRELATIVISITC QUANTUM THEORY; QUANTUM MECHANICS. 
 �34s[David Park]
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�6v�2v�32sEinsteinium �21m �33s
    A chemical element, Es, atomic number 99, a member of the actinide series
in the periodic table. It is not found in nature but is produced by artificial
nuclear transmutation of lighter elements. All isotopes of einsteinium are
radioactive, decaying with half-lives ranging from a few seconds to about 1
year. See ACTINIDE ELEMENTS; RADIOACTIVITY.
    Einsteinium is the heaviest actinide element to be isolated in weighable
form. The metal is chemically reactive, is quite volatile, and melts at 860DGC
(1580DGF); one crystal structure is known. See TRANSURANIUM ELEMENTS.
 �34s[Stanley G. Thompson ;Glenn T. Seaborg]
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�6v�2v�32sElasipodida   �21m �33s 
    An order of aspidochirotacean holothurians in which there are no 
respiratory trees and the mesentery of the posterior loop of the intestine is 
attached in the right dorsal interradius. The order is composed almost 
exclusively of deep-sea forms, in which bilateral symmetry is often extremely 
conspicuous (see illustration). They are common in deep water and especially 
common in the faunas of deep trenches, ranging down to at least 5000 fathoms 
(9000 m). This distribution probably occurs because they are able to extract 
nutriment from mud in an environment almost devoid of other sources of food. 
See ASPIDOCHIROTACEA; ECHINODERMATA; HOLOTHUROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sElasmobranchii   �21m �33s 
    A subclass of the class Chondrichthyes, the cartilaginous fishes. The 
elasmobranchs are distinguished  by separate gill openings, amphistylic or 
hyostylic jaw suspension, and sensory ampullae (of Lorenzini) in the head 
region. Characters shared with members of the other subclass (Holocephali) 
include a variably calcified cartilaginous endoskeleton, placoid scales, 
urea-retention mechanism, clasper organs in the male for internal 
fertilization, and the absence of an air (swim) bladder. See BATOIDEA; 
CHONDRICHTHYES; CLADOSELACHII; SELACHII; SHARK. 
 �34s[Bobb Schaeffer]
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�6v�2v�32sElasticity �21m �33s
    The property whereby a solid material changes its shape and size under the
action of opposing forces, but recovers its original configuration when the
forces are removed. The theory of elasticity deals with the relations between
the forces acting on a body and the resulting changes in configuration, and is
important in many branches of science and technology, for instance, in the
design of structures, in the theory of vibration and sound, and in the study
of the forces between atoms in crystal lattices.
    The forces acting on a body are expressed as stresses and measured as
force per unit area. Thus if a bar ABCD of square cross section (see
illustration a) is fixed at one end and subjected to a force F uniformly
distributed over the other end DC, the stress is F/(DC) sup 2. This stress
causes the bar to become longer and thinner and to assume the shape A'B'C'D'.
The strain is measured by the ratio (change in length)/(original length), that
is, by (B'C' - BC)/(BC). According to Hooke's law, stress is proportional to
strain, and the ratio of stress to strain is therefore a constant, in this
case the Young's modulus, denoted by E, so that E = F(BC)/(DC)(sup 2)
9(B'C' - BC). See HOOKE'S LAW; STRESS AND STRAIN; YOUNG'S MODULUS.
    Poisson's ratio SIGMA is the ratio of lateral strain to longitudinal
strain so that SIGMA = BC(DC - D'C')/DC(B'C' - BC). The bar of illustration a
is in a state of tension, and the stress is tensile; if the force F were
reversed in direction, the stress would be compressive. Stresses of this type
are called direct or normal stresses; a second type of stress, known as
tangential or shear stress, is known in illustration b. In this case, the
configuration ABCD becomes ABC'D', with the shear forces F acting in the
directions AB and CD. The shear strain is measured by the angle THETA, and if
the body is originally a cube, the shear stress is F/(DC). The ratio of stress
to strain, F/(DC)(sup 2) THETA, is the shear or rigidity modulus G, which
measures the resistance of the material to change in shape without change in
volume.
    A further elastic constant, the bulk modulus k, measures the resistance to
change in volume without changes in shape, and is shown in illustration c. The
original configuration is represented by the circle AB, and under a
hydrostatic (uniform) pressure P, the circle AB becomes the circle A'B'. The
bulk modulus is then k = P NU/DELTA NU, where DELTA NU/NU is the volumetric
strain. The reciprocal of the bulk modulus is the compressibility.
    The elastic constants may be determined directly in the way suggested by
their definitions; for instance, Young's modulus can be determined by
measuring the relative extension of a rod or wire subjected to a known tensile
stress. Less direct methods are, however, usually more convenient and
accurate. Prominent among these are the dynamic methods involving frequency of
vibration and velocity of sound propagation. The elastic constants can be
expressed in terms of frequency of (or velocity in) regularly shaped
specimens, together with the dimensions and density, and by measuring these
quantities, the elastic constants can be found. The elastic constants can also
be determined from the flexure and torsion of bars. See ULTRASONICS.
    In practice, stress is only proportional to strain, and the strain is only
completely recoverable within certain limits called the elastic limits of the
material. Above the elastic limits, the material is subject to time-dependent
effects, and as the stress is further increased, the ultimate strength of the
material is approached. See ELASTIC LIMIT; PLASTICITY; STRENGTH OF MATERIALS.



 �34s[R. F. S. Hearmon]
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�6v�2v�32sElastic limit   �21m �33s 
    The largest stress that a material can sustain without permanent 
deformation or strain. The proportional limit, it may be noted, is the maximum 
stress at which the stress is directly proportional to the strain. 
    Actual materials are not perfectly elastic upon first application of load 
because they lack homogeneity and have residual stresses produced by the 
manufacturing process. Upon subsequent loadings, the material will exhibit 
elastic properties. See ELASTICITY; STRESS AND STRAIN. 
 �34s[John B. Scalzi]
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�6v�2v�32sElaterite   �21m �33s 
    A light-brown to black, naturally occurring carbonaceous substance having 
specific gravity 0.90 - 1.05. Elaterite is insoluble in carbon disulfide and 
infusible. It is moderately soft and elastic, and on heating leaves only 2 - 
5% fixed carbon. Elaterite is thought to be of algal origin and occurs in 
small quantities in Derbyshire County, England, in the Coorong District of 
South Australia, and in Turkestan, Soviet Union. 
 �34s[Irving A. Breger]
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�6v�2v�32sElective culture �21m �33s
    A type of microorganism grown selectively from a mixed culture by
culturing in a medium and under conditions selective for only one type of
organism. No single culture medium or environment will support growth of more
than a small proportion of the known types of microorganisms. Each organism
has characteristic nutritional and environmental requirements. Thus, in a
culture composed of more than one type of organism, the medium and conditions
are selective for only one type of organism present and the other types are
suppressed. These selective, or elective, culture principles and techniques
are invaluable for the detection and isolation of specified organisms from
natural sources. Elective culture techniques are more effective for obtaining
bacteria from natural sources than for obtaining fungi, actinomycetes, and
other groups; the faster growth rates, larger numbers, and greater variety of
bacteria usually confer a competitive advantage upon them. In general,
elective culture procedures are applicable to organisms capable of utilizing
particular sources of energy, carbon, nitrogen, and other nutrients in a given
physicochemical environment. See CULTURE MEDIA.
 �34s[Jackson W. Foster /R. E. Kallio]
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�6v�2v�32sElectret   �21m �33s 
    A solid dielectric possessing persistent dielectric polarization. An 
electret is the analog of a magnet. Electrets are made by cooling suitable 
dielectrics from elevated temperatures in strong electric fields. A special 
class called photoelectrets is produced by the removal of light from an 
illuminated photoconductor in an electric field. See DIELECTRICS; 
FERROELECTRICS; MAGNET; POLARIZATION OF DIELECTRICS. 
 �34s[Robert D. Waldron]
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�6v�2v�32sElectret transducer   �21m �33s 
    A device for the conversion of acoustical or mechanical energy into 
electrical energy, and vice versa, which utilizes a quasi-permanently charged 
dielectric material (electret). Examples are certain microphones, earphones, 
and phonograph cartridges. 
    In the simplest implementation, such a transducer consists of a metal 
backplate (first electrode) covered by a mechanically tensioned diaphragm. The 
diaphragm is a foil electret carrying a metal coating (second electrode) on 
the side facing away from the backplate. Provisions are made to maintain a 
shallow air gap between electret and backplate. The air gap is occupied by an 
electrostatic field originating from the electret charges. Upon acoustical or 
mechanical deflection of the diaphragm, such a device generates an electrical 
output signal between its two electrodes; similarly, application of an 
electrical signal results in diaphragm deflections. Electret devices are 
therefore self-biased electrostatic or condenser transducers. They thus 
exhibit all the advantages of this transducer class, such as wide dynamic 
range and flat response over a frequency range of several decades, without 
requiring the external bias necessary in conventional transducers of this 
kind. See ELECTRET; ELECTROSTATICS; TRANSDUCER. 
    The basic component of all modern electret transducers is a ``foil 
electret'' consisting of a thin film of insulating material that has been 
electrically charged to produce an external electric field. Strongly 
insulating materials capable of trapping charge carriers, such as the 
halocarbon polymers, in particular polyfluoroethylenepropylene (Teflon), are 
best suited for this purpose. Before charging, the material is either 
metallized on one side or backed up with a metal electrode. 
    A simple implementation of the most widely used electret transducer, the 
foil-electret microphone, is shown in the illustration. The diaphragm, 
typically 12- or 25-micrometers Teflon metallized on one surface, is charged 
to 10 - 20 nanocoulombs per square centimeter, corresponding to an external 
bias of about 200 volts. The nonmetallized surface of this foil electret is 
placed next to a backplate, leaving a shallow air layer, the thickness of 
which (about 20 micrometers) is controlled by ridges or raised points on the 
backplate surface. The stiffness of the air layer can be decreased (and thus 
the sensitivity of the microphone can be improved) by connecting the air layer 
to a larger cavity by means of small holes through the backplate. The 
backplate is either a metal disk or a metal-coated dielectric with a thermal 
expansion coefficient about equal to that of the diaphragm. The electrical 
output of the microphone is taken between the backplate, which is insulated 
from the outer case, and the metal side of the foil. The output is fed into a 
high-impedance preamplifier. See MICROPHONE. 
  
    Compared with conventional electrostatic transducers, electret microphones 
have the following advantages: they do not require a dc bias; they have three 
times higher capacitance per unit area, resulting in a better signal-to-noise 
ratio; and they are not subject to destructive arcing between foil and 
backplate in humid atmospheres and under conditions of water condensation. 
    Another group of electret transducers of interest are electret headphones. 
The above microphone design can also be utilized in this case. They produce a 
certain amount of nonlinear distortion due to the quadratic dependence of the 
force per unit area, exerted on the diaphragm, on the applied ac voltage. This 
drawback can, however, be minimized in a well-designed system, and 
high-voltage sensitivities have been achieved by making the air-gap thickness 
small. The distortion can be eliminated by the use of push-pull transducers 
and monocharge electrets. 
    Apart from its use in electroacoustic transducers, such as microphones and 
earphones, the electret principle has been applied to electromechanical 
transducers. Examples are phonograph cartridges, touch or key transducers, and 
impact transducers. 
    Because of their favorable properties, simplicity, and low cost, electret 
transducers have been used in many applications, both as research tools and in 
the commercial market. 
    Among the research applications are microphones for use in acousto-optic 
spectroscopy, applied to the detection of air pollution and to the study of 
reaction kinetics of gases and optical absorption of solids. Other 
applications of electret microphones have been reported in aeronautics and 
shock-tube studies, in which the low vibration sensitivity of these 
transducers is crucial. The wide frequency range of electret transducers, 
discussed above, made possible their application in infrasonic atmospheric 
studies and in ultrasonic investigations of liquids  and solids. In addition, 
ultrasonic arrays of electret microphones have been used in acoustic 
holography. See ACOUSTICAL HOLOGRAPHY; PHOTOACOUSTIC SPECTROSCOPY; ULTRASONICS. 
    Of all commercial applications of electret devices, the high fidelity 
electret microphone for amateur, professional, studio, and tape recorder use 
is most prominent. Although the electret microphone was introduced only about 
1970, tens of millions of the instruments are produced annually, accounting 
for about one-half of the entire output of high fidelity microphones. 
 �34s[Gerhard M. Sessler]
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�6v�2v�32sElectrical breakdown �21m �33s
    A large, usually abrupt rise in electric current in the presence of a
small increase in electric voltage. Breakdown may be intentional and
controlled or it may be accidental. Lightning is the most familiar example of
breakdown.
    In a gas, such as the atmosphere, the potential gradient may become high
enough to accelerate the naturally present ions to velocities that cause
further ionization upon collision with atoms. If the region of ionization does
not extend between oppositely charged electrodes, the process is corona
discharge. If the region of ionization bridges the gap between electrodes,
thereby breaking down the insulation provided by the gas, the process is
ionization discharge. When controlled by the ballast of a fluorescent lamp,
for example, the process converts electric power to light. In a gas tube the
process provides controlled rectification. See GAS TUBE.
    In a solid, such as an insulator, when the electric field gradient exceeds
10 sup 6 volts/cm, valence bonds between atoms are ruptured and current flows.
Such a disruptive current heats the solid abruptly. In a semiconductor if the
applied backward or reverse potential across a junction reaches a critical
level, current increases rapidly with further rise in voltage. This avalanche
characteristic is used for voltage regulation in the Zener diode. In a
transistor the breakdown sets limits to the maximum instantaneous voltage that
can safely be applied between collector and emitter. See BREAKDOWN POTENTIAL;
ELECTRICAL INSULATION; TRANSISTOR; ZENER DIODE.
 �34s[Frank H. Rockett]
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�6v�2v�32sElectrical codes   �21m �33s 
    Systematic bodies of rules governing the practical application, 
installation, and interconnection of electrically operated equipment, devices, 
and electrical wiring systems. 
    The basic code used throughout the United States is the National 
Electrical Code, prepared under the direction of the National Fire Protection 
Association (NFPA). It is approved by the American National Standards 
Institute. The National Electrical Code is purely advisory as far as the 
National Fire Protection Association is concerned, but it is very widely used 
for legal regulatory purposes. The code is administered by various local 
inspection agencies, whose decisions govern its application to individual 
installations. The National Electrical Code is incorporated bodily or by 
reference in many municipal building ordinances, often with additional 
provisions or restrictions applicable in the particular locality. Some large 
cities have independent electrical codes; however, the actual provisions in 
most such codes tend to be basically similar to the National Electrical Code. 
    Compliance with the provisions of the code can effectively minimize fire 
and accident hazards in any electrical design. It sets forth requirements that 
constitute a minimum standard for the framework of electrical design. As 
stated in its introduction, the code is concerned with the ``practical 
safeguarding of persons and of buildings and their contents from hazards 
arising from the use of electricity for light, heat, power, radio, signalling 
and for other purposes.'' The National Electrical Code is recognized as a 
legal criterion of safe electrical design and installation. It is used in 
court litigation and by insurance companies as a basis for insuring buildings. 
    In addition to the National Electrical Code itself, other standards and 
recommended practices are made available by the National Fire Protection 
Association. These cover such special subjects as hospital operating rooms, 
municipal fire alarm systems, garages, aircraft hangars, and other equipment 
with great potential hazards due to improper design. 
    The National Electrical Safety Code (to be distinguished from the National 
Electrical Code) is published by the Institute of Electrical and Electronic 
Engineers, Inc. This code applies to the outdoor circuits of electric utility 
companies and to similar systems or equipment on commercial and industrial 
premises.  
    Standards on the construction and assembly of many types of electrical 
equipment, materials, and appliances are set forth in literature issued by the 
Underwriters' Laboratories, Inc. The Underwriters' Laboratories examines, 
tests, and determines the suitability of materials and equipment to be used 
according to code regulations. See WIRING. 
 �34s[Joseph F. McPartland]
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�6v�2v�32sElectrical communications   �21m �33s 
    The science and technology by which information is collected from an 
originating source, transformed into electric currents or fields, transmitted 
over electrical networks or through space to another point, and reconverted 
into a form suitable for interpretation by a receiving entity. 
    Information to be transmitted comes from many sources. Some sources 
produce essentially continuously varying signals. Examples are speech, music, 
the output of TV cameras, and sensors of temperature, pressure, and the like. 
Others produce signals whose distinguishing characteristic is that they are on 
or off. Examples are the hand telegraph key, many operations of computers, 
above-or-below sensors of temperature, pressure, and so on. 
    For transmission over an electrical communication channel, a transducer is 
used to transform the original physical manifestation into an analogous 
electrical signal - a speech sound in air by a telephone transmitter, 
variations in light intensity in a scanned picture by a television camera, 
temperature observations by the opening and closing of the switch of a 
thermostat. See TRANSDUCER. 
  
    Transmission media.  Media commonly used for electrical communication can 
be divided into two classes: (1) Those carrying electrical currents, such as a 
pair of wires or coaxial cable. (2) Those in which a transducer must be used 
to change electrical currents into a form suited to the medium. Examples are 
electromagnetic fields in space, such as radio, for which the antennas are the 
transducers; electromagnetic fields in guiding structures, such as a 
waveguide; pressure waves in water (sonar); light waves in space or optical 
fibers. See ANTENNA (ELECTROMAGNETISM); COAXIAL CABLE; COMMUNICATION CABLES; 
ELECTROMAGNETIC WAVE TRANSMISSION; MICROWAVE TRANSMISSION LINES; OPTICAL 
COMMUNICATIONS; OPTICAL FIBERS; SONAR; TRANSMISSION LINES; WAVEGUIDE. 
  
    Network configurations.  Networks may be configured in several ways. 
Perhaps the most common is a channel between point A and point B. These may, 
as in the telephone network, be switched on a call or message basis among the 
very large number of points available. Another configuration is a treelike 
structure in which the channel from one source is divided and subdivided to 
connect to many receivers. A cable televison (CATV) system is an example of 
this type of configuration. The third configuration is typified by radio and 
TV broadcasting: a single source sends out radio waves, and any number of 
receivers in the area can pick them up on a one-way basis. See BUSINESS 
COMMUNICATIONS SYSTEM; CLOSED-CIRCUIT TELEVISION; SWITCHING SYSTEMS 
(COMMUNICATIONS).  
  
    Signal amplification.  Whatever the transmission medium, the signal 
becomes weaker with distance, dropping nearer to the inevitable noise in the 
medium, thus reducing the S/N ratio, and so limiting the information-carrying 
capacity of the channel. It becomes desirable to place one or more amplifiers 
at periodic intervals along the channel to raise the signal power and hence 
maintain the desired S/N at the receiving end. See AMPLIFIER. 
  
    Frequency division multiplex.  Radio broadcasting is the best-known 
example of many channels sharing a common medium, a technique known as 
frequency division multiplex. Each transmitter is assigned a position in the 
frequency spectrum and is permitted to occupy a specified bandwidth. The 
receivers include selective networks or filters, which, when tuned to a 
selected channel, accept that channel and reject the others. The process of 
shifting the input signal to the assigned frequency slot is called modulation, 
and the inverse process at the receiving end is called demodulation. The same 
processes are applied to the sharing of a common wire or coaxial cable medium. 
See ELECTRIC FILTER; MODULATION; RADIO BROADCASTING. 
  
    Time division multiplex.  There is another way for a multiplicity of 
channels to share a common medium, called time division multiplex. Each 
channel is assigned a short time slot during which it occupies the whole 
available bandwidth, with the channels occupying time slots in turn. When all 
channels have had their turn, a frame has been completed, and the roll call is 
repeated. Obviously, there has to be an identifying symbol at the beginning of 
each frame, so that the channel time slots can be identified. 
    The processing of continuously varying input sources is much more 
complicated than in frequency-division AM or FM systems. It involves sampling, 
quantizing, coding, and organizing into words and frames, with framing symbols 
and precise timing. When only vacuum tubes were available, the economics of 
the resulting tradeoffs were completely unfavorable. With the advent of the 
transistor, and the subsequent flowering of the solid-state art into 
integrated circuits, the whole economic picture changed dramatically. 
Complicated processing could be accomplished by tiny, inexpensive chips. The 
extra complexity in many cases was well justified by savings in the cable 
system and its amplifiers. 
    At the same time, there was a rapidly growing demand for communication of 
essentially on-off signals, as between computers and machines. The time 
division multiplex is very well suited to these signals, with little 
processing. For these reasons, there has been rapidly expanding use of time 
division systems. See ELECTRONICS; INTEGRATED CIRCUITS; PULSE MODULATION; 
TRANSISTOR.  
    For a discussion of major forms of electrical communication systems see 
RADAR; RADIO; SOUND-REPRODUCING SYSTEMS; TELEGRAPHY; TELEMETERING; TELEPHONY; 
TELEVISION.  
 �34s[A. C. Dickieson]
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�6v�2v�32sElectrical conduction in gases �21m �33s
    The process by means of which a net charge is transported through a
gaseous medium. It encompasses a variety of effects and modes of conduction,
ranging from the Townsend discharge at one extreme to the arc discharge at the
other. The current in these two cases ranges from a fraction of 1 microampere
in the first to thousands of amperes in the second. It covers a pressure range
from less than 10- sup 4 atm (10 pascals) to greater than 1 atm (100
kilopascals). See ARC DISCHARGE; TOWNSEND DISCHARGE.
    In general, the feature which distinguishes gaseous conduction from
conduction in a solid or liquid is the active part which the medium plays in
the process. Not only does the gas permit the drift of free charges from one
electrode to the other, but the gas itself may be ionized to produce other
charges which can interact with the electrodes to liberate additional charges.
Quite apparently, the current voltage characteristic may be nonlinear and
multivalued. See ELECTROLYTIC CONDUCTANCE; SEMICONDUCTOR.
    The applications of the effects encountered in this area are of
significant commercial and scientific value. A few commercial applications are
thyratrons, gaseous rectifiers, ignitrons, glow tubes, and gas-filled
phototubes. These tubes are used in power supplies, control circuits, pulse
production, voltage regulators, and heavy-duty applications such as welders.
In addition, there are gaseous conduction devices widely used in research
problems. Some of these are ion sources for mass spectrometers and nuclear
accelerators, ionization vacuum gages, radiation detection and measurement
instruments, and thermonuclear devices for the production of power.
 �34s[Glenn H. Miller]
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�6v�2v�32sElectrical connector   �21m �33s 
    A device that joins electric conductors mechanically and electrically to 
other conductors and to the terminals of apparatus and equipment. The term 
covers a wide range of devices designed, for example, to connect small 
conductors employed in communication circuits, or at the other extreme, large 
cables and bus-bars. 
    Electrical connectors are applied to conductors in a variety of ways. 
Soldered connectors have a tube or hole of approximately the same diameter as 
the conductor. The conductor and connector are heated, the conductor inserted, 
and solder flowed into the joint until it is filled. Solderless connectors are 
applied by clamping the conductor or conductors in a bolted assembly or by 
staking or crimping under great mechanical force. 
    Typical connector types are in-line splice couplers, T-tap connectors, 
terminal lugs, and stud connectors. Couplers join conductors end to end. T-tap 
connectors join a through conductor to another conductor at right angles to it 
(illustration a). Terminal lugs join the conductor to a drilled tongue for 
bolting to the terminals of equipment (illustration b). Stud connectors join 
the conductor to equipment studs; the stud clamp is threaded or smooth to 
match the stud. 
  
    Split-bolt connectors are a compact construction widely used for splices 
and taps in building wiring. The bolt-shape casting has a wide and deep slot 
lengthwise. The conductors are inserted in the slot and the nut is drawn up, 
clamping the conductors together inside the bolt (illustration c). 
    Expansion connectors or flexible connectors allow some limited motion 
between the connected conductors. The clamp portions of the connector are 
joined by short lengths of flexible copper braid and may also be held in 
alignment by a telescoping guide. 
    Separable types consist of matched plugs and receptacles, which may be 
readily separated to disconnect a conductor or group of conductors from the 
circuit or system. Separable connectors are commonly used for the connection 
of portable appliances and equipment to an electric wiring system. 
    Locking types are designed so that, when coupled, they may not be 
separated by a strain or pull on the cord or cable. In a typical construction 
the plug is inserted and twisted through a small arc, locking it securely in 
place.  
    Plug receptacles, sometimes called convenience outlets, are a type of 
wiring device distributed throughout buildings for the attachment of cord plug 
caps from portable lamps and appliances. In residences at least one such 
outlet must be provided for every 12 linear feet (3.66 m) or major fraction of 
wall perimeter. Grounding receptacles have an additional contact that accepts 
the third round or U-shaped prong of a grounding attachment plug. See WIRING. 
 �34s[Joseph F. McPartland]
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�6v�2v�32sElectrical degree   �21m �33s 
    A unit equal to 1/360 of a complete cycle of electric current or voltage. 
In an electric machine it is 1/360 of the angle subtended at the axis by two 
consecutive field poles of like polarity, since the voltage wave generated in 
a conductor completes one cycle when it traverses one pair of poles. The term 
mechanical degree is used to designate the space angle between two positions 
about the axis of the machine. The number of electrical degrees between two 
positions about the axis equals the number of mechanical degrees multiplied by 
the number of pairs of poles on the machine. See ALTERNATING CURRENT. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sElectrical engineering   �21m �33s 
    A branch of engineering dealing primarily with electricity and magnetism 
and devoted to utilization of the forces of nature and materials for the 
benefit of humans. Electrical engineering encompasses many phases of other 
engineering sciences and the physical sciences; it includes research, 
invention, development design, application, and education. Many phases of 
electrical engineering are based on applications of higher mathematics. See 
ENGINEERING.  
    The need for better communications between electrical engineers led to the 
establishment of the American Institute of Electrical Engineers (AIEE) in 
1884. In 1913 the Institute of Radio Engineers (IRE) was founded. In later 
years these two organizations were merged into a single organization which is 
now known as the Institute of Electrical and Electronics Engineers (IEEE). 
    Electrical engineers apply their abilities in other engineering fields 
that are not strictly electrical. There is hardly a field of technology to 
which electrical engineering has not made a contribution. For important 
related engineering fields see COMPUTER; CONTROL SYSTEMS; INSTRUMENTATION; 
SYSTEMS ENGINEERING. 
 �34s[Albert G. Conrad]
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�6v�2v�32sElectrical impedance   �21m �33s 
    The total opposition that a circuit presents to an alternating current. 
Impedance, measured in ohms, may include resistance, inductive reactance, and 
capacitive reactance. See REACTANCE. 
    Impedance may also be defined as the ratio of the rms voltage to the rms 
current, Z = E/I. This is a form of Ohm's law for ac circuits. 
 �34s[Burtis L. Robertson]
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�6v�2v�32sElectrical instability   �21m �33s 
    A persistent condition of unwanted self-oscillation in an amplifier or 
other electrical circuit. Instability is usually caused by excessive positive 
feedback from the output to the input of an active network. If, in an 
audio-frequency amplifier, instability is at a low audible frequency, the 
output will contain a putt-putt sound, from which such instability is termed 
motorboating. The instability may also be at a high audible frequency, or it 
may be at frequencies outside the audible range. Although such oscillations 
may not be heard directly, they produce distortion by driving the amplifier 
beyond its linear range of operation. See AMPLIFIER. 
 �34s[Frank H. Rockett]
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�6v�2v�32sElectrical insulation �21m �33s
    A nonconducting material that provides electric isolation of two parts at
different voltages. To accomplish this, an insulator must meet two primary
requirements: it must have an electrical resistivity and a dielectric strength
sufficiently high for the given application. The secondary requirements relate
to thermal and mechanical properties. Occasionally, tertiary requirements
relating to dielectric loss and dielectric constant must also be observed. A
complementary requirement is that the required properties not deteriorate in a
given environment and desired lifetime. See CONDUCTOR (ELECTRICITY).
    Electric insulation is generally a vital factor in both the technical and
economic feasibility of complex power and electronic systems. The generation
and transmission of electric power depend critically upon the performance of
electric insulation, and now plays an even more crucial role because of the
energy shortage.

    Requirements.  The important requirements for good insulation are as
follows.
    The basic difference between a conductor and a dielectric is that free
charge has high mobility on and in a conductor, whereas free charge has little
or no mobility on or in a dielectric. Dielectric strength is a measure of the
electric stress required to abruptly move substantial charge on or through a
dielectric. It deteriorates with the ingress of water and with elevated
temperature. For high-voltage (on the order of kilovolts) applications,
dielectric strength is the most important single property of the insulation.
See DIELECTRICS.
    Resistivity is a measure of how much current will be drained away from the
conductor through the bulk or along the surface of the dielectric. An
insulator with resistivity equal to or greater than 10 sup 1 sup3  ohm-cm may be
considered good. See ELECTRICAL RESISTIVITY.
    When a dielectric is subjected to an alternating field, a time-varying
polarization of the atoms and molecules in the dielectric is produced. The
alternation of both the permanent and induced polarization in the dielectric
results in power dissipation within the dielectric of which the power factor
is a measure. This dielectric power loss is proportional to the product of the
dielectric constant and the square of the electric field in the dielectric.
Although it may be a loss that is small relative to other losses in most
ambient-temperature applications, and even though it generally decreases at
low temperatures, it is a relatively important loss for dielectrics to be used
at cryogenic temperatures.
    The dielectric constant, also known as the relative permittivity or
specific inductive capacity, is a measure of the ability of the dielectric to
become polarized, taken as the ratio of the charge required to bring the
system to the same voltage level relative to the charge required if the
dielectric were vacuum. It is thus a pure number, but is in fact not a
constant, and may vary with temperature, frequency, and electric-field
intensity.
    In addition to the problem of intensification of the electric field in
regions of relatively lower dielectric constant, a low-dielectric-constant
insulation is desirable for two more compelling reasons. In ac transmission
cables, the lower the dielectric constant, the more the current and the
voltage will be in phase. This means that more usable power will be delivered,
without the need for reactive compensation. Furthermore, in reducing the
charging current (which is proportional to the dielectric constant),
concomitant power losses (related to the square of the charging current) are
also reduced. A high dielectric constant is desirable in capacitors, since the
capacitance is proportional to it. See CAPACITANCE; DIELECTRIC CONSTANT.

    Properties.  All insulators may be classified as either solid or fluid.
Solid insulation is further divided into flexible and rigid types.

    Solid insulation.Flexible hydrocarbon insulation is generally either
thermoplastic or thermosetting. Thermosets are initially soft, and can be
extruded by using only pressure. Following heat treatment, when they return to
ambient temperature, they are tougher and harder. After thermosetting,
nonrubber thermosets are harder, stronger, and have more dimensional stability
than the thermoplastics. Thermoplastics are softened by heating, and when cool
become hard again. They are heat-extruded.
    Cellulose paper insulation is neither thermoplastic nor thermosetting. It
is widely used in cables and rotating machinery in multilayers and impregnated
with oil. It has a relatively high dielectric loss that hardly decreases with
decreasing temperature, which rules it out for cryogenic applications. Because
of its high dielectric strength, the high loss has not been a deterrent to its
use in conventional ambient-temperature applications. However, the high
dielectric strength deteriorates quickly if moisture permeates the paper.
    Rigid insulation includes glass, mica, epoxies, ceramoplastics, porcelain,
alumina, and other ceramics. Rather than being used to insulate wires and
cables, except for mica, these materials are used in equipment terminations
(potheads) and as support insulators (in tension or compression) for overhead
lines whose primary dielectric is air. These rigid structures must be
shock-resistant, be relatively water-impervious, and be able to endure corona
discharges over their surfaces.

    Fluid insulation. Liquids, gases, and vacuum fall in the category of fluid
insulation. For all of these, the electrical structure must be such as to
contain the fluid in the regions of high electric stress.
    The main types of insulating liquids are the mineral oils, silicones,
chlorinated hydrocarbons, and the fluorocarbons with dielectric strengths on
the order of megavolts per centimeter. Many other liquids also have good
dielectric strength, such as carbon tetrachloride, toluene, hexane, benzene,
chlorobenzene, alcohol, and even deionized water.
    Most gases have a dielectric constant of about 1, and low dielectric loss.
Air is used as a dielectric in a wide variety of applications, ranging from
electronics to high-voltage (765-kV) and high-power (2000-MW) electric
transmission lines. Dry air is a reasonably good insulator. However, its
dielectric strength decreases with increasing gap.
    Vacuum (that is, pressures of less than 10- sup 5 torr or 10- sup 3 pascal)
has one of the highest dielectric strengths in the gap ranging 0.1 to 1 mm.
However, as the gap increases, its dielectric strength decreases rapidly. A
perfect vacuum might be expected to be a perfect insulator, since there would
be no charge carriers present to contribute to electrical conductance. That
this is not so in practice arises because of the effects of a high electric
field or high voltage at the surface of electrodes in vacuum, rather than
because a perfect vacuum is far from being realized in the laboratory. The
dielectric properties of vacuum can degenerate rapidly because vacuum offers
no resistance to the motion of charge carriers, once they are introduced into
the vacuum region.
 �34s[Mario Rabinowitz]
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�6v�2v�32sElectrical interference   �21m �33s 
    Any undesired electrical energy that tends to interfere with the reception 
of desired signals. It may be a signal generated by nearby electric motors, 
automotive ignition systems, radio transmitters, improperly operating 
television receivers, or a wide variety of other improperly operating electric 
devices. The interference signals either may be radiated through space as 
unguided electromagnetic waves or may travel along power lines as guided 
radiation. Interference may also be caused by atmospheric phenomena such as 
lightning. Such natural disturbances are sometimes termed sferics. Interfering 
signals are sometimes generated intentionally by powerful transmitters for the 
purpose of jamming communication and radar systems. See ELECTRICAL NOISE. 
    Interference from electrical devices is best suppressed at its source, by 
such means as minimizing sparking, connecting capacitors across sparking 
contacts, shielding the equipment with metal enclosures, and using choke coils 
or filters that prevent interfering signals from entering power lines. See 
ELECTRIC FILTER; ELECTRICAL SHIELDING; GROUNDING. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sElectrical loading   �21m �33s 
    The addition of inductance to a transmission line in order to improve its 
transmission characteristics over the required frequency band. Loading coils 
are often inserted in telephone lines, at spacings as close as one mile, to 
counteract the capacitance of the line and thus make the line impedance more 
closely equivalent to a pure resistance. Similar coils are used between 
sections of lines used for carrier transmission. See TRANSMISSION LINES. 
    With coaxial cables and waveguides, loading is placed at the end of the 
line to absorb all power reaching the end, thereby achieving a nonreflecting 
termination. See WAVEGUIDE. 
    When an antenna is too short to give resonance at the desired frequency, a 
coil can be inserted in series with the antenna to give the required amount of 
loading needed for resonance. As an example, practically all auto radios have 
a loading coil in the antenna circuit, because the usual whip antenna is much 
too short for broadcast-band frequencies. See ANTENNA (ELECTROMAGNETISM). 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sElectrical measurements �21m �33s
    The measurement of any one of the many quantities by which the behavior of
electricity is characterized. The knowledge of the quantitative behavior of
electricity is essential to scientific and technical progress. Electrical
measurements play a major role in industry, communications, and even in such
unrelated fields as medicine.
    Many electrical measurements can be made with direct-indicating
instruments merely by connecting the instrument properly in the circuit. Thus
a volt-meter provides a pointer which moves over a scale calibrated in volts,
and an ammeter in the same way presents a reading of current in amperes. Other
direct-reading instruments are wattmeters, frequency meters, power-factor or
phase-angle meters, and ohmmeters. Many electrical quantities are measured
both as instantaneous values and as values integrated over time. Some
electrical measurements must be made with various specialized devices or
systems requiring adjustment or balancing to obtain the measured value.
Typical of these are potentiometers and bridges in many standard and
specialized forms.
    Because of differences in instruments and techniques, it is convenient to
divide measurements into direct-current (dc) and alternating-current (ac)
classes.
    In dc circuits the measurement of voltage and current often suffices to
define the operation of the circuit. The product of the two represents power.
In the commercial sale of dc electricity the measurement of energy must be
made with a dc watt-hour meter. Occasional use is made of a dc ampere-hour
meter in battery-charging installations.
    To measure high values of current, shunts are used to bypass all but a
small fraction of the current around the measuring instrument. A newer
technique employs a form of saturable reactor energized by alternating current
to measure large direct currents. See CURRENT MEASUREMENT; ELECTRIC ENERGY
MEASUREMENT; ELECTRIC POWER MEASUREMENT; VOLTAGE MEASUREMENT.
    Alternating-current circuits involve more variables and hence more
measurements than dc circuits. The most common measurements are voltage,
current, and power; the last requires a wattmeter, as ac power cannot always
be calculated directly from voltage and current. Also measured are frequency
and power factor (or phase angle) and sometimes waveform or harmonic content.
Energy is measured by means of the ac induction watt-hour meter.
Direct-current instruments do not respond to ac quantities, but some may be
adapted by the addition of rectifiers to convert alternating current to
direct. The thermocouple is another form of convertor by which a dc instrument
may be made to read ac quantities.
    If alternating voltages and currents above the normal ranges of
self-contained instruments are to be measured, instrument transformers may be
used to extend the ranges of those instruments. In the study of ac waveform a
qualitative evaluation may be made with an oscillograph or a cathode-ray
oscilloscope. Quantitative measurement of harmonic content requires the use of
a harmonic analyzer. See HARMONIC ANALYZER; INSTRUMENT TRANSFORMER.
    In a laboratory, emphasis is normally placed on accuracy and on
completeness of facilities to deal with all types of measurements. There is
relatively little limitation on the size and complexity of equipment used.
    The term field measurements is used to designate all measurements made
outside a laboratory as in generating stations and substations, service shops,
factory testing areas, ships, and aircraft. For these uses equipment is chosen
to perform only specialized services. Accuracy well below that of laboratory
measurements is usually permissible. Convenience, compactness, and often
portability are prime considerations in choosing equipment.
    All electric instruments draw some power from the circuits to which they
are connected, and ac instruments generally take more power than dc
instruments. This circuit loading may alter appreciably the quantity being
measured. The magnitude of this error can usually be evaluated and minimized
by proper choice of instruments.
 �34s[Isaac F. Kinnard /Edward C. Starr]
�33s

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�6v�2v�32sElectrical noise �21m �33s
    Interfering and unwanted currents or voltages in an electrical device or
system. Electrical noise, usually simply called noise, has an important effect
on any electrical system which is used to gather, transmit, process, or
present information. In such systems as telephone, radio, television, radar,
radionavigation, telemetering, electronic control, or electronic computing,
the desired signals carrying intelligence may be masked or distorted by noise.
    Noise may originate either externally to the device in which it appears,
as atmospheric static, or internally, as thermal noise in a resistor. It may
result either from natural phenomena, as do both types of noise just
mentioned, or from interference from devices such as nearby electric motors or
generators.
    Interference from electric devices can usually be nearly eliminated by
good engineering design and proper location of equipment. Noise due to natural
phenomena often cannot be reduced below certain fixed levels, and good
engineering design can ensure only that the equipment will function as
effectively as possible in the face of this irreducible noise. For example, a
radio receiver cannot operate on received signals which are very weak
(compared to some level determined by the thermal noise in the receiver), no
matter how much amplification is used in the receiver, because the noise is
amplified along with the signal.
    Noise may conveniently be classified as either random noise or nonrandom
noise. Random noise is defined as noise which is not predictable, although it
may exhibit statistical regularities. Thermal noise is the random voltage
which appears at the terminals of a resistor or any component with internal
resistance because of the random motion of thermally excited electrons in the
resistor. Contact noise, the noise in carbon resistors and carbon microphones,
for example, is caused by randomly varying fluctuations in resistance.
    Random noise also originates in transistors and other semiconductor
devices. There are various mechanisms at work, and the terminology is not
completely standard. Thermal noise, as above, is the noise caused by the
random motion of thermally excited electrons. Shot noise, or
generation-recombination noise, is produced by fluctuations in free carrier
densities when an electric field is applied. Excess noise or modulation noise
is caused by slow fluctuations in conductivity. Noise also originates in
photoconductors. Most mechanisms of random noise generation share the
principle that observable gross currents and voltages are the result of many
random actions at microscopic level. See SEMICONDUCTOR.
    Any electric device which must receive electromagnetic radiation will pick
up radiated random noise as well as signals. Electrical disturbances in the
atmosphere cause noise which is very irregular in character, often appearing
in sharp bursts. Aside from atmospheric disturbances, an antenna receives a
steady background of noise which is of thermal origin, thermal radiation from
the gases of the atmosphere, and thermal radiation from heavenly bodies and
systems. This last is sometimes called interstellar noise. The Sun radiates
noise at all times, but during sunspot activity the intensity of its noise
radiation is greatly increased. See ATMOSPHERIC ELECTRICITY; RADIO ASTRONOMY;
STATIC.
    Nonrandom noise is usually the result of radiation from other electric
equipment, unwanted coupling with other systems, or spurious oscillations
within an electrical circuit. See CROSSTALK; ELECTRICAL INTERFERENCE.
 �34s[William L. Root]
�33s

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�6v�2v�32sElectrical noise generator   �21m �33s 
    A device which produces (usually random) electrical noise for use in 
electrical measurements. Electrical noise generators are commonly used in 
measuring the noise figure of a radio receiver or other amplifier. They are 
also used in other tests of the response of an electrical system to random 
noise, and in measurements of noise intensity. See ELECTRICAL NOISE. 
    Some standard types of noise generator are: hot-wire, diode, gas-discharge 
tube, and klystron. A hot-wire noise generator is commonly the filament of a 
lamp heated by a direct current. The filament is connected across the 
terminals where the noise is to be introduced, for example, the antenna 
terminals of a radio receiver. A reflex klystron, with reflector grid 
connected to the cavity to prevent oscillation, generates noise because of 
shot effect in the cathode current. See KLYSTRON; SCHOTTKY EFFECT. 
 �34s[William L. Root]
�33s 
  
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�6v�2v�32sElectrical resistance   �21m �33s 
    That property of an electrically conductive material that causes a portion 
of the energy of an electric current flowing in a circuit to be converted into 
heat. In 1774 A. Henley showed that current I flowing in a wire produced heat, 
but it was not until 1840 that J. P. Joule determined that the rate of 
conversion of electrical energy into heat in a conductor, that is, power 
dissipation H/t, could be expressed by the relation given in notation (1). 
  
                   *** H/t VARIES AS I (sup 2) R ***                     (1)
  
    The day-to-day determination of resistance R by measuring the rate of heat 
dissipation is not practical. However, this rate of energy conversion is also 
VI, where V is the voltage drop across the element in question and I the 
current through the element, as in Eq. (2), from which the more conventional 

                *** H/t VARIES AS I (sup 2) R = VI ***                   (2)

relationship implied by Ohm's law, Eq. (3), is apparent. See ELECTRICAL 

                              *** R = V/I ***                            (3)

RESISTIVITY; OHM'S LAW; RESISTANCE MEASUREMENT. 
 �34s[Charles E. Applegate]
�33s 
  
  

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�6v�2v�32sElectrical resistivity �21m �33s
    The electrical resistance offered by a homogeneous unit cube of material
to the flow of a direct current of uniform density between opposite faces of
the cube. Also called specific resistance, it is an intrinsic, bulk (not
thin-film) property of a material. Resistivity is usually determined by
calculation from the measurement of electrical resistance of samples having a
known length and uniform cross section according to the following equation,
where RHO is the resistivity, R the measured resistance, A the cross sectional
area, and l the length. In the mks system (SI), the unit of resistivity is the
ohm-meter. Therefore, in the equation, resistance is expressed in ohms, and
the sample dimensions in meters.

				 RHO = RA/l

    The room-temperature resistivity of pure metals extends from approximately
1.5 X 10 sup -8 ohm-meter for silver, the best conductor, to 135 X 10 sup -8
ohm-meter for manganese, the poorest pure metallic conductor. Most metallic
alloys also fall within the same range. Insulators have resistivities within
the approximate range of 10 sup 8 to 10 sup 16 ohm-meters. The resistivity of
semiconductor materials, such as silicon and germanium, depends not only on
the basic material but to a considerable extent on the type and amount of
impurities in the base material. Large variations result from small changes in
composition, particularly at very low concentrations of impurities. Values
typically range from 10 sup -4 to 10 sup 5 ohm-meters. See ELECTRICAL
INSULATION; ELECTRICAL RESISTANCE; SEMICONDUCTOR. 
    The temperature coefficients (changes with temperature) of resistivity of
pure metallic conductors are positive. Resistivity increases by about 0.4%/K
at room temperature and is nearly proportional to the absolute temperature
over wide temperature ranges. As the temperature is decreased toward absolute
zero, resistivity decreases to a very low residual value for some metals. The
resistivity of other metals abruptly changes to zero at some temperature above
absolute zero, and they become superconductors.
    Metals, and some semiconductors in particular, exhibit a change in
resistivity when placed in a magnetic field. Theoretical relations to explain
the observed phenomena have not been well developed.
 �34s[Charles E. Applegate]
�33s


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�6v�2v�32sElectrical shielding   �21m �33s 
    A means of avoiding pickup of undesired signals or noise, suppressing 
radiation of undesired signals, and confining wanted signals to desired paths 
or regions. These shielding objectives cannot be realized without some degree 
of modification of the electric and magnetic fields involved, although the 
effects of these modifications may not seriously interfere with wanted 
objectives.  
    A shield is a sheet, tube, screen, grid, or other object, usually of 
conducting material. Sometimes a shield is magnetic, or both magnetic and 
conducting, or even laminated. Fields may be largely confined to, or 
suppressed from, a specified region by shields. The ratio of the unwanted 
signal in a communication circuit when the source of shielding is present to 
the same signal when the shielding is absent is called the shield factor. 
    A change in an electric field gives rise to magnetic effects; similarly, a 
change in a magnetic field gives rise to electric effects. As a result, 
electromagnetic shielding is more commonly encountered than either 
electrostatic shielding or magnetostatic shielding. 
    Virtually complete shielding from external electrostatic fields is 
obtained by totally enclosing the space to be shielded with a highly 
conducting surface, usually grounded. 
    Substantially complete shielding from an external and virtually constant 
magnetic field is obtained by shunting the flux around the space to be 
shielded through a low-reluctance path. Ideally, the shield should present a 
complete magnetic path to the flux; that is, the space to be shielded should 
be surrounded by a ferromagnetic material, preferably of high permeability. 
    The magnetostatic shielding such as that described above is likewise 
effective when the magnetic field is changing with time. A different yet 
highly effective method of realizing electromagnetic shielding is by means of 
an electrostatic shield. 
    The cable sheath, which encloses many communication lines within a 
telephone cable, constitutes one of the most striking examples of 
electromagnetic shielding. See ELECTRICAL NOISE. 
 �34s[Harold S. Black]
�33s 
  
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�6v�2v�32sElectrical units and standards �21m �33s
    The standard in terms of which electrical quantities are evaluated, the
quantities so adopted being known as units. The electrical units in practical
use today, and also in extensive theoretical use, were designated by the
Eleventh General Conference of Weights and Measures in 1960 as members of the
International System of Units (Systeme International d'Unites, abbreviated SI
in all languages). This action by the General Conference was the culmination
of an effort initiated by A. Giorgi at the beginning of this century to bring
the practical electrical units into a coherent system with appropriate
mechanical units of the metric system.

    Units.  To accomplish the above objective, the base units for mechanical
quantities were arbitrarily selected: the meter for the unit of length, the
kilogram for the unit of mass, and the second for the unit of time. Units for
other mechanical quantities are derived from these units in accordance with
physical laws and concepts such as the unit of speed, the meter per second,
and the unit for acceleration, the meter per second per second.
    This system was originally called the mks system to distinguish it from
the cgs system (based on the centimeter, gram, and second).
    Two cgs systems of electric and magnetic units have been in use in
scientific circles for a long time but both are rapidly giving way to the
International System. They are the electrostatic system of units (esu) and the
electromagnetic system of units (emu).
    The electrostatic system defines a unit charge as that charge which exerts
1 cgs unit of force (1 dyne, which is equivalent to 10- sup 5 newton) on another
unit charge when separated from it by a distance of 1 centimeter in a vacuum.
All other units of the system are derived from this definition by assigning
unit coefficients in equations relating electric and magnetic quantities to
each other. The units so derived are often referred to in terms of the SI
units with the prefix ``stat,'' for example, statvolts, statohms, and
statamperes.
    The electromagnetic system defines a unit magnetic pole (a highly
fictitious concept) as that pole which exerts 1 cgs unit of force on another
unit pole when separated from it by a distance of 1 centimeter in a vacuum.
All other units of the system are derived from this definition in accord with
the principles set forth above for the electrostatic system. Units so derived
are often referred to in terms of the SI units with the prefix ``ab,'' for
example, abvolts, abohms, and abamperes. Special names are given to some
magnetic units of the emu system such as the maxwell and the gauss, which
correspond, respectively, to the weber and the tesla of SI, although differing
from them in magnitude.
    The magnitudes of corresponding units of the electrostatic and
electromagnetic systems differ from each other by a factor theoretically equal
to the speed of light, c (3 x 10 sup 1 sup 0 centimeters per second,
approximately), or its square.
    The esu system is found convenient for handling purely electrostatic
problems. A combination of the two systems in  which electrostatic quantities
are expressed in esu and magnetic and electromagnetic quantities in emu, with
appropriate use of the conversion constant c between the two systems, is
called the Gaussian system. All of these systems are rapidly giving way to use
of SI units in treatment of electric, magnetic, and electromagnetic phenomena.
    Ampere. The constant current which, if maintained in two straight parallel
conductors of infinite length, of negligible circular sections, and placed 1
meter apart in a vacuum, would produce between these conductors a force equal
to 2 x 10- sup 7 mks unit of force per meter of length. The ampere (A) is the
unit of electric current in the International System, and is one of the seven
SI base units. See ELECTRIC CURRENT.

    Volt. The difference of electric potential between two points of a
conducting wire carrying a constant current of 1 ampere, when the power
dissipated between these points is equal to 1 watt. The volt (V) is the unit
of electric potential in the International System.

    Ohm. The electric resistance between two points of a conductor when a
constant difference of potential of 1 volt, applied between these two points,
produces in the conductor a current of 1 ampere, the conductor not being the
seat of any electromotive force. The ohm (OMEGA) is the unit of electric
resistance in the International System. See ELECTRICAL RESISTANCE;
ELECTROMOTIVE FORCE (CELLS).

    Coulomb. The quantity of electricity transported in 1 second  by a current
of 1 ampere. The coulomb (C) is the unit of electric charge in the
International System. See ELECTRIC CHARGE.

    Farad. The capacitance of a capacitor between the plates of which there
appears a difference of potential of 1 volt when it is charged by a quantity
of electricity equal to 1 coulomb. The farad (F) is the unit of capacitance in
the International System. See CAPACITANCE.

    Henry. The inductance of a closed circuit in which an electromotive force
of 1 volt is produced when the electric current in the circuit varies
uniformly at a rate of 1 ampere per second. The henry (H) is the unit of
inductance in the International System.

    Weber. The magnetic flux which, linking a circuit of 1 turn, produces in
it an electromotive force of 1 volt as it is reduced to zero at a uniform rate
in 1 second. The weber (Wb) is the unit of magnetic flux in the International
System. See MAGNETIC FLUX; UNITS OF MEASUREMENT.

    Standards.	Electrical standards are the physical embodiments by means of
which the electrical units are realized and maintained. The only standards
generally maintained in national standards laboratories are those for the
volt, the ohm, the farad, and sometimes the henry, since very stable standards
for these quantities can be produced. The other electrical quantities are
determined from suitable combinations of these.
 �34s[Alvin G. McNish]
�33s

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�6v�2v�32sElectric charge �21m �33s
    A basic property of elementary particles of matter. One does not define
charge but takes it as a basic experimental quantity and defines other
quantities in terms of it.
    According to modern atomic theory, the nucleus of an atom has a positive
charge because of its protons, and in the normal atom there are enough
extranuclear electrons to balance the nuclear charge so that the normal atom
as a whole is neutral. Generally, when the word charge is used in electricity,
it means the unbalanced charge (excess or deficiency of electrons), so that
physically there are enough ``nonnormal'' atoms to account for the positive
charge on a ``positively charged body'' or enough unneutralized electrons to
account for the negative charge on a ``negatively charged body.''
    In line with this usage, the total charge q on a body is the total
unbalanced charge possessed by the body. For example, if a sphere has a
negative charge of 1 x 10-(sup 10) coulomb, it has 6.24 x 10(sup 8) electrons
more than are needed to neutralize  its atoms. The coulomb is the unit of
charge in the meter-kilogram-second (mks) system of units. See COULOMB'S LAW;
ELECTRICAL UNITS AND STANDARDS; ELECTROSTATICS.
 �34s[Ralph P. Winch]
�33s

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�6v�2v�32sElectric contact   �21m �33s 
    A part, in an electrical switching device, made of conducting material, 
for the purpose of closing, opening, or changing the conductive path of an 
electrical circuit. To open or close a circuit, an electric contact is made to 
come in contact with or separate from its mating part. Devices embodying 
contacts for these purposes are electric switches, relays, contactors, and 
circuit breakers. Contacts may be actuated directly or through a linkage that 
is driven either manually, mechanically, electromagnetically, hydraulically, 
or pneumatically. 
    An electric contact is also an essential part of a variable resistor. In 
such an application the contact or wiper provides a moving connection to the 
resistive element. See POTENTIOMETER (VARIABLE RESISTOR); RHEOSTAT. 
 �34s[Thomas H. Lee]
�33s 
  
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�6v�2v�32sElectric current �21m �33s
    The net transfer of electric charge per unit time. It is usually measured
in amperes. The passage of electric current involves a transfer of energy.
Except in the case of superconductivity, a current always heats the medium
through which it passes. See JOULE'S LAW.
    On the other hand, a stream of electrons or ions in a vacuum, which also
may be regarded as an electric current, produces no local heating. Measurable
currents range in magnitude from the nearly instantaneous 10 sup 5 or so amperes
in lightning strokes to values of the order of 10- sup 1 sup 6 ampere, which
occur in research applications.
    All matter may be classified as conducting, semiconducting, or insulating,
depending upon the ease with which electric current is transmitted through it.
Most metals, electrolytic solutions, and highly ionized gases are conductors.
Transition elements, such as silicon and germanium, are semiconductors, while
most other substances are insulators. See ALTERNATING CURRENT; CONDUCTION
(ELECTRICITY); DIRECT CURRENT; DISPLACEMENT CURRENT; ELECTRIC INSULATOR;
SEMICONDUCTOR; SUPERCONDUCTIVITY.
 �34s[John W. Stewart]
�33s

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�6v�2v�32sElectric-discharge machining   �21m �33s 
    A metal-removal process (also called electric-spark machining) in which 
materials that conduct electricity can be removed by an electric spark. The 
spark is a transient electric discharge through the space between the tool 
(cathode) and the workpiece (anode). It is used to form holes of varied shape 
in materials of poor machinability and to form cavities in steel dies. See 
MACHINING OPERATIONS. 
 �34s[Orlan W. Boston]
�33s 
  
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�6v�2v�32sElectric distribution system   �21m �33s 
    That part of an electric power system that supplies electric energy to the 
individual  user or consumer. The distribution system includes the primary 
circuits and the distribution substations that supply them; the distribution 
transformers; the secondary circuits, including the services to the consumer; 
and appropriate protective and control devices. The four general classes of 
individual users are residential, industrial, commercial, and rural. 
    The three-phase, alternating-current (ac) system is practically universal, 
although a small amount of two-phase and direct-current systems from early 
days are still in operation. Three-phase transmission and subtransmission 
lines require  three wires, termed phase conductors. Most of the three-phase 
distribution systems consist of three phase conductors and a common or neutral 
conductor, making a total of four wires. Single-phase branches (consisting of 
two wires) supplied from three-phase mains are used for single-phase 
utilization in residences, small stores, and farms. Loads are connected in 
parallel to common supply circuits. See ELECTRIC POWER SYSTEMS. 
    The distribution substation is an assemblage of equipment for the purpose 
of switching, changing, and regulating the voltage from subtransmission to 
primary distribution. More important substations are designed so that the 
failure of a piece of equipment in the substation or one of the 
subtransmission lines to the substation will not cause an interruption of 
power to the load. See ELECTRIC POWER SUBSTATION. 
    The primary system leaving the substation is most frequently in the 11,000 
- 15,000 volt range. A particular voltage used is 12,470-volt line-to-line and 
7200-volt line-to-neutral (conventionally written 12,470Y/7200 volts). Some 
utilities use a lower voltage, such as 4160Y/2400 volts. Secondary voltages 
are derived from distribution transformers connected to the primary system and 
they usually correspond to utilization voltages. Residential and most rural 
loads are supplied by 120/240-volt single-phase three-wire systems. Commercial 
and small industrial needs are supplied by either 208Y/120-volt or 
480Y/277-volt three-phase four-wire systems. The secondary voltage is usually 
used to supply multiple street lights, in addition to service to consumers. 
    Service continuity is the providing of uninterrupted electric power to the 
consumer; therefore, good continuity is doing this a high percentage of the 
time. This is accomplished for large industrial and commercial loads by use of 
some form of duplicate power supply. Downtown commercial areas are supplied 
from three-phase 208Y/120-volt grid networks. The system is arranged so that 
the failure of a primary feeder will not cause a loss of load on the 
secondary. Commercial buildings and shopping centers are often served by spot 
networks. All of the transformers and protectors are at the same location. 
Residential and rural loads are usually supplied by a radial system. Good 
continuity for them is obtained by sectionalizing the system with fuses, 
circuit breakers, and manual switches to reduce the extent of an outage due to 
a failure. 
    The majority of distribution systems already built in residential, 
industrial, and rural areas has been overhead construction. For aesthetic 
reasons, underground distribution systems are being installed in most new 
residential developments. See TRANSMISSION LINES. 
 �34s[Harold E. Campbell]
�33s 
  
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�6v�2v�32sElectric energy measurement �21m �33s
    The measurement of the integral, with respect to time, of the power in an
electric circuit. The absolute unit of measurement of electric energy is the
joule, or the charge in coulombs times the potential difference in volts. The
joule, however, is too small (1 watt-second) for use in commercial practice,
and the more commonly used unit is the watt-hour (3.6 x 10 sup 3 joules). The
most common measurement application is in the utility field.
    Electric energy is one of the most accurately measured commodities sold to
the general public. Many methods of measurement, with different degrees of
accuracy, are possible. Basically, measurements of electric energy may be
classified into two categories, direct-current power and alternating-current
power. The fundamental concepts of measurement are, however, the same for both.
    There are two types of methods of measuring electric energy: electric
instruments and timing means, and electricity meters.
    Electric instruments and timing means make use of conventional procedures
for measuring electric power and time. Typical methods are listed below. See
ELECTRIC POWER MEASUREMENT.
    1. Measurement of energy on a direct-current circuit by reading the line
voltage and load current at regular intervals over a measured period of time.
See CURRENT MEASUREMENT; VOLTAGE MEASUREMENT.
    2. Measurement of energy on a direct-current circuit by controlling the
voltage and current at constant predetermined values for a predetermined time
interval.
    3. Measurement of energy on an alternating-current circuit by reading the
watts input to the load at regular intervals over a measured period of time.
    4. Measurement of energy on an alternating-current circuit by controlling
the voltage, current, and watts input to the load at constant predetermined
values.
    5. Measurement of energy by recording the watts input to the load on a
linear chart progressed uniformly with time.
    Electricity meters are the most common devices for measuring the vast
quantities of electric energy used by industry and the general public. The
same fundamentals of measurement apply as for electric power measurement, but
in addition the electricity meter provides the time-integrating means
necessary for electric energy measurement. A single meter is sometimes used to
measure the energy consumed in two or more circuits. However, multistator
meters are generally required for this purpose.
    Watt-hour meters are generally connected to measure the losses of their
respective current circuits. These losses are extremely small compared to the
total energy being measured and are present only under load conditions.
Watt-hour meters  used for the billing of residential, commercial, and
industrial loads are highly developed devices. See ELECTRICAL MEASUREMENTS.
 �34s[William H. Hartwig]
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�6v�2v�32sElectric field   �21m �33s 
    A condition in space in the vicinity of an electrically charged body such 
that the forces due to the charge are detectable. An electric field (or 
electrostatic field) exists in a region if an electric charge at rest in the 
region experiences a force of electrical origin. Since an electric charge 
experiences a force if it is in the vicinity of a charged body, there is an 
electric field surrounding any charged body. 
    The electric field intensity (or field strength) E at a point in an 
electric field has a magnitude given by the quotient obtained when the force 
acting on a test charge q' placed at that point is divided by the magnitude of 
the test charge q'. Thus, it is force per unit charge. A test charge q' is one 
whose magnitude is small enough so it does not alter the field in which it is 
placed. The direction of E at the point is the direction of the force F on a 
positive test charge placed at the point. Thus, E is a vector point function, 
since it has a definite magnitude and direction at every point in the field, 
and its defining equation is Eq. (1). 
  
                            *** E = F/q' ***                             (1)
  
    Electric flux density or electric displacement D in a dielectric 
(insulating) material is related to E by either of the equivalent equations 
shown as Eqs. (2), where P is the polarization of the medium, and EPSILON is 
the permittivity of the dielectric which is related to EPSILON 0 by the 
equation EPSILON = k EPSILON 0, k being the relative dielectric constant of the 
dielectric. In empty space, D = EPSILON 0 E. 
  
               *** D = EPSILON 0 E + P       D = EPSILON E                (2)
  
    In addition to electrostatic fields produced by separations of electric 
charges, an electric field is also produced by a changing magnetic field. See 
ELECTRIC CHARGE; ELECTROMAGNETIC INDUCTION; POTENTIALS. 
 �34s[Ralph P. Winch]
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�6v�2v�32sElectric filter   �21m �33s 
    A transmission network used in electrical systems for the selective 
enhancement (or reduction) of specified components of an input signal. The 
filtering is accomplished by selectively attenuating those frequency 
components of the input signal which are undesired, relative to those which it 
is desired to enhance. 
    A typical filter network having two ports is shown in Fig. 1. Since the 
output is generally smaller than the input, it is customary to specify the 
amplitude information as the amount of reduction of the input signal; the loss 
or attenuation is measured in decibels (dB). See DECIBEL. 
    An optical filter transmits, or passes, certain colors in the light 
spectrum and rejects, or stops, others. Electrical filters perform in a 
similar way. A band of frequencies which is rejected or greatly attenuated is 
called a stopband; a band which is transmitted with little attenuation is 
called a passband. Ideally, there should be zero attenuation in the passband 
and infinite attenuation in the stopband. More practically, a small amount of 
attention can be tolerated in the passband, and a noninfinite attenuation in 
the stopband. 
  
  
    Applications.  Electrical filters are incorporated in almost all systems 
of electrical  communications and control. Radio and television receivers 
contain filters in order to pass the particular channel selected and to stop 
others. The telephone network incorporates millions of filters in order to 
pass each conversation, which is carried by a different band of frequencies, 
to the appropriate telephone receiver and to stop all others. Bandwidths vary 
from a fraction of a hertz in control systems, to 3 kilohertz (kHz) in 
telephone systems, to 20 kHz in AM radio systems, to 6 megahertz (MHz) in 
television systems. See AMPLITUDE-MODULATION DETECTOR; CONTROL SYSTEMS; 
FREQUENCY-MODULATION DETECTOR; RADIO RECEIVER; TELEVISION RECEIVER. 
    Narrow-band band-rejection filters are often used to reduce interference 
from extraneous electrical noise, which may come from power lines or from 
industrial equipment. One example of such a null network is the twin-tee 
filter shown in Fig. 2. See ELECTRICAL INTERFERENCE. 
    A common use of a filter is to smooth the output from a rectifier to make 
it more closely constant by attenuating the alternating-current component. The 
power supply in every radio and television receiver has such a low-pass 
filter. Another common use of a low-pass filter is as a decoupling network 
when several amplifier stages share the same power supply. A low-pass 
decoupling filter is placed between the power supply and each amplifier stage, 
thereby reducing interaction between the stages. 
  
    Special filters.  Special categories of filters can be classified in 
accordance with either their application or the components used in their 
construction. Piezoelectric crystalline materials such as quartz have the 
property of electrical resonance when mechanically stressed along certain 
axes. This property can be used to replace electrical components in a filter. 
The resulting crystal filters are almost always used in band-pass 
applications. The incoming (and outgoing) electrical signals must be coupled 
to the mechanical vibrations of the crystal through transducers, which are 
themselves electrical elements. Ceramic materials can also be used in place of 
crystals; the result is a ceramic filter. 
    Mechanical filters have some characteristics in common with crystal 
filters, in that the resonant elements are mechanical. But they differ from 
crystal filters in that the coupling mechanisms between mechanical vibrations 
and electrical signals (the transducers) are also mechanical. Ferrites and 
iron-nickel alloys whose elastic modulus is constant over a temperature range 
are common materials from which mechanical filters are constructed. See 
FERRITE DEVICES. 
    Microwave filters operate at microwave frequencies, generally considered 
to lie above 1 GHz. However, they can also be used in the much lower 
high-frequency (hf) range of 3 - 30 MHz. The components are no longer lumped 
elements, but consist of sections of transmission lines (called stubs), strip 
lines, coupled lines, and waveguide structures such as irises and posts. See 
MICROWAVE TRANSMISSION LINES; WAVEGUIDE. 
    Distributed RC filters are made of thin-film integrated circuits or 
various large-area semiconductor devices. 
    A filter that is matched to the characteristics of an input signal of 
known time dependence is called a matched filter. The term is also applied to 
describe an impedence match between a filter and its termination. A matched 
filter has the property that, when the input consists of noise in addition to 
the desired signal, the output signal-to-noise ratio is the maximum that can 
be obtained by any linear filter. Accordingly, such filters are used (in radar 
and similar systems) to optimize the detection of weak signals. 
  
    Switched filters are similar to other electrical filters except that a 
switch is incorporated into one of the branches. The switch is operated 
(turned on or off) in a periodic manner so that the switched filter is a 
time-varying linear network. Switched filters find particular application in 
modulators used in communications systems, both in frequency-division 
multiplexing (FDM) and in time-division multiplexing (TDM). 
  
    Digital filters.  An increasing amount of signal processing is being 
performed in digital form. A digital signal is one which takes on only a 
limited number of discrete values and only at discrete and equally spaced 
intervals of time. Signals may already be in digital form (for example, sent 
between digital computers and most business machines), or a continuous signal 
may be converted to digital form by sampling it at uniform intervals and 
digitally coding its values, for ease of transmission or some other reason. 
This digital signal is then processed by a digital filter in accordance with a 
set of instructions in order to regain the original information. 
 �34s[Norman Balabanian]
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�6v�2v�32sElectric furnace   �21m �33s 
    An enclosed space heated by electric power. The furnace may be in such 
forms as a refractory crucible, a large tiltable refractory basin with a 
capacity of 100 tons (91 metric tons) and a removable roof, or a long 
insulated chamber equipped with a continuous conveyor. Heat is provided by an 
arc to the charge or melt (direct-arc furnace), by an arc between electrodes 
(indirect-arc furnace), or by an arc confined for concentrated heating by an 
electromagnetic field (plasma-arc furnace). Heat may also be produced by 
current flowing in the melt. See ELECTROMETALLURGY; PYROMETALLURGY. 
    Because the source of heat is nonchemical, electric furnaces are 
especially desirable in melting alloys of controlled composition. Temperature 
is also readily controlled. The arc furnace may be used to smelt ores or to 
refine metals or alloys. Induction furnaces are widely used to melt alloys for 
castings. Because electric furnaces can be enclosed, they are used for 
operations that require controlled or inert atmospheres, such as growing 
crystals or annealing. When sealed and evacuated, they are used in degassing 
metals. Furnaces with hearth resistors are used for operations below melting 
temperatures, such as annealing, and with infrared heat lamps, for drying 
paints or setting glues. See ARC HEATING; ELECTRIC HEATING; HEAT TREATMENT 
(METALLURGY); KILN; REFRACTORY. 
 �34s[Frank H. Rockett]
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�6v�2v�32sElectric heating   �21m �33s 
    Methods of converting electric energy to heat energy by resisting the free 
flow of electric current. Electric heating has several advantages; it can be 
precisely controlled to allow a uniformity of temperature within very narrow 
limits; it is cleaner than other methods of heating because it does not 
involve any combustion; it is considered safe because it is protected from 
overloading by automatic breakers; it is quick to use and to adjust; and it is 
relatively quiet. For these reasons, electric heat is widely chosen for 
industrial, commercial, and residential use. There are four major types of 
electric heaters: resistance, dielectric, induction, and electric-arc. See ARC 
HEATING; DIELECTRIC HEATING; HEATING; INDUCTION HEATING; RESISTANCE HEATING. 
    In the past, electricity has not been a popular choice for heating houses. 
Power plants lose 60 - 65% efficiency in generating electricity, and another 
10% in transmission and distribution. With only a 35% efficiency rate, 
electricity has long been overlooked in favor of the direct use of gas and oil 
for heating homes. As these fossil fuels diminish, however, electric heating 
is a more attractive option. Also, if nuclear power becomes more widespread, 
so will electric heating because it will make electric heating far more 
economical than the direct use of gas and oil. 
    One type of electric heater for houses is the heat pump. The heat pump can 
be used alone or to supplement the output of direct resistance heating (heat 
strips). The heat pump works on a reversed refrigeration cycle; the pump pulls 
the heat from the cold outside air and forces it into the house. Since the 
heat pump performs both heating and cooling and is more efficient than other 
forms of electric heat, it can become the most economical way to heat houses 
in the future. See HEAT PUMP. 
    Common electric heating systems in houses are: central heating employing 
an electric furnace with forced air circulation; central heating employing an 
electric furnace with forced water circulation; central heating using radiant 
cables; electrical duct heaters; space (strip) heaters which use radiation and 
natural convection for heat transfer; and portable space heaters. 
 �34s[Mo-Shing Chen]
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�6v�2v�32sElectric insulator �21m �33s
    Ideal insulators are substances into which static electric fields
penetrate uniformly. Substances for which Ohm's law is valid are often classed
as insulators if their resistivities exceed about 10(sup14) ohm-cm.
Insulators used primarily as media for the storage of electrostatic energy,
for example, in capacitors, are called dielectrics. Important electrical
properties of insulators are resistivity, dielectric constant, and breakdown
voltage, the applied voltage above which a material ceases to act as an
insulator. See DIELECTRIC CONSTANT; DIELECTRICS.
    At high pressures gases are excellent insulators, with higher breakdown
voltages than most solids or liquids or high vacuum. See ELECTRICAL CONDUCTION
IN GASES.
    In solids electric current is carried by electrons or ions. Liquids show
many of the features of solids in their conducting mechanisms. See BAND THEORY
OF SOLIDS.
 �34s[B. Gale Dick]
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�6v�2v�32sElectricity   �21m �33s 
    Electricity comprises those physical phenomena involving electric charges 
and their effects when at rest and when in motion. Electricity is manifested 
as a force of attraction, independent of gravitational and short-range nuclear 
attraction, when two oppositely charged bodies are brought close to one 
another. It is now known that the elementary (nondivisible) electric charges 
are possessed by electrons and protons. The charge of the electron is equal in 
magnitude to that of the proton, but is electrically opposite. The electron's 
charge is arbitrarily termed negative, and that of the proton, positive. 
Magnetism, those physical phenomena involving magnetic fields and their 
effects upon materials, manifests itself in the presence of moving electric 
charge. For this reason, magnetism was originally considered to be a part of 
electricity.  See  ELECTRIC CHARGE; MAGNETISM. 
    The sources of electricity in modern technology depend strongly on the 
application for which they are intended. 
    The principal use of static electricity today is in the production of high 
electric fields. Such fields are used in industry for testing the ability of 
components such as insulators and condensers to withstand high voltages, and 
as accelerating fields for charged-particle accelerators. The principal source 
of such fields today is the Van de Graaff generator.  See  PARTICLE 
ACCELERATOR; VAN DE GRAAFF GENERATOR. 
    The major use of electricity today arises in devices using electric 
currents alternating at low or zero frequency. The use of alternating current 
allows power transmission over long distances at very high voltages with a 
resulting low percentage power loss followed by highly efficient conversion to 
lower voltages for the consumer through the use of transformers. Large amounts 
of zero-frequency current, that is, direct current, are used in the 
electrodeposition of metals, both in plating and in metal production, for 
example, in the reduction of aluminum ore. To avoid power transmission 
difficulties, such facilities are frequently located near sources of abundant 
power.  See  ALTERNATING CURRENT; DIRECT CURRENT; ELECTRIC CURRENT. 
    The principal sources of low-frequency electricity are rotary generators 
whose operation is based on the Faraday induction principle. The force to 
drive such generators derives from the flow of water or the expansion of 
gases, as in steam and internal combustion engines. Other sources, such as 
fusion, solar, and oceanic, still do not have significant application. 
    Many high-frequency devices, such as communications equipment, television, 
and radar, involve the consumption of only moderate amounts of power, 
generally derived from low-frequency sources. If the power requirements are 
moderate and portability is needed, the use of ordinary chemical batteries is 
possible.  See  BATTERY. 
 �34s[Walter Aron]
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�6v�2v�32sElectric organ (biology)   �21m �33s 
    An effector organ found in six different groups of fishes; output is an 
electric pulse. Voltages large enough to aid in prey capture or predator 
deterrence are produced by various strongly electric fishes. These include the 
electric eel (Electrophorus electricus) from South America; the electric 
catfish (Malapterurus electricus) from Africa; the family of electric rays, 
Torpedinidae, which are widely distributed in the world's oceans; and possibly 
the stargazer genus, Astroscopus, of the western Atlantic. Weakly electric 
fishes emit a lower voltage, the energy source for an active electrosensory 
system that monitors electrical impedance in the environment. These weak 
signals also serve in intra- and interspecific communication. There are three 
groups of weakly electric fishes. First, the South American knifefishes, the 
Gymnotiformes, are a large and diverse group of several families that also 
include Electrophorus. Second, the electrically active African Mormyriformes 
are composed of the numerous species of the family Mormyridae and the single 
species Gymnarchus niloticus in the family Gymnarchidae. Finally, many species 
of skates and rays of the family Rajidae occur in marine waters around the 
world.  
    The strongly electric fishes are remarkable for the high voltage and power 
of their discharges. For example the electric eel generates pulses in excess 
of 500 V. In contrast, a large torpedo generates a smaller voltage, about 50 V 
in air, but the maximum current is larger, and the peak pulse power can exceed 
1 kW (or about 1 hp). The electric organs may constitute a substantial 
fraction of the body mass. The weakly electric fishes have much smaller organs 
emitting pulses of a few tenths of a volt to several volts; such amplitudes 
that are still large compared to those recorded outside ordinary, nonelectric 
fishes.  
    Electric organ discharge is explicable in terms of the properties of 
ordinary excitable cells, that is, those of nerve and muscle. The single 
generating cells of electric organs are called electrocytes. They are modified 
striated muscle fibers that have lost the ability to contract, although they 
retain muscle filaments to varying degrees, and they produce electric signals 
as muscle fibers do. 
    A question often asked about the strongly electric fishes is how they keep 
from electrocuting themselves. When immersed in water, they do not appear to 
be at all affected by their own discharges, although their electroreceptors 
are undoubtedly activated. In air, when the organ is not loaded down by the 
conductivity of the water, they often do twitch when they discharge their 
organs, but as would be expected, they are much more resistant than other 
fishes. One factor is that their nerves and central nervous system are well 
insulated by many layers of connective and fatty tissue. Whether their brains 
are more resistant to stunning by a given current density has not been 
determined; certainly the excitability of individual cells is no different 
from that in animals in general. 
 �34s[Michael V. L. Bennett]
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�6v�2v�32sElectric power generation �21m �33s
    The production of bulk electric power for industrial, residential, and
rural use. Although limited amounts of electricity can be generated by many
means, including chemical reaction (as in batteries) and engine-driven
generators (as in automobiles and airplanes), electric power generation
generally implies large-scale production of electric power in stationary
plants designed for that purpose. The generating units in these plants convert
energy from falling water, coal, natural gas, oil, and nuclear fuels to
electric energy. Most electric generators are driven either by hydraulic
turbines, for conversion of falling water energy; or by steam or gas turbines,
for conversion of fuel energy. Limited use is being made of geothermal energy,
and work is progressing in the use of solar energy in its various forms.
Electric power generating plants are normally interconnected by a transmission
and distribution system to serve the electric loads in a given area or region.
See ELECTRIC DISTRIBUTION SYSTEM; GENERATOR.
    An electric load is the power requirement of any device or equipment that
converts electric energy into light, heat, or mechanical energy, or otherwise
consumes electric energy as in aluminum reduction, or the power requirements of
electronic and control devices. The total load on any power system is seldom
constant; rather, it varies widely with hourly, weekly, monthly, or annual
changes in the requirements of the area served. The minimum system load for a
given period is termed the base load or the unity load-factor component. Maximum
loads, resulting usually from temporary conditions, are called peak loads.
Electric energy cannot feasibly be stored in large quantities; therefore the
operation of the generating plants must be closely coordinated with fluctuations
in the load.
    Generating plants, often termed generating stations, contain apparatus
that converts some form of energy to electric energy in bulk. Three
significant types of generating plants are hydroelectric,
fossil-fuel-electric, and nuclear electric.
    The hydroelectric plant utilizes the potential energy released by the
weight of water falling through a vertical distance called head. The plant
consists basically of a dam to store the water in a forebay and create part or
all of the head, a penstock to deliver the falling water to the turbine, a
hydraulic turbine to convert the hydraulic energy released to mechanical
energy, an alternating-current generator (alternator) to convert the
mechanical energy to electric energy, and all accessory equipment necessary to
control the power flow, voltage, and frequency, and to afford the protection
required.
    Pumped storage hydroelectric plants are being used increasingly. Under
suitable geographical and geological conditions, electric energy can, in
effect, be stored by pumping water from a low to a higher elevation and
subsequently releasing this water to the lower elevation through hydraulic
turbines. These turbines and their associated generators are reversible. The
generators, operating in reverse direction as motors, drive their turbines as
pumps to elevate the water. When this water is released through the turbines,
electric power is produced by the generators. See HYDRAULIC TURBINE;
WATERPOWER.
    The fossil-fuel-electric plant utilizes the energy of combustion from
coal, oil, or natural gas. A typical large plant consists of fuel processing
and handling facilities, a combustion furnace and boiler to produce and
superheat the steam, a steam turbine, an alternator, and the accessory
equipment required for plant protection and for control of voltage, frequency,
and power flow. A steam plant can frequently be built near a convenient load
center, provided an adequate supply of cooling water and fuel is available,
and is usually readily adaptable to either base loading or intermediate or
peak loading. See STEAM TURBINE.
    In the nuclear electric plant one or more of the nuclear fuels are
utilized in a suitable type of nuclear reactor, which takes the place of the
combustion furnace in the typical steam electric plant. The heat exchangers
and boilers (if not combined in the reactor), the turbines, and
alternating-current generators, complete with controls, accessories, and
auxiliaries, make up the atomic electric plant. See NUCLEAR REACTOR.
    The size or capacity of electric utility generating units varies widely,
depending upon type of unit; duty required, that is base-, intermediate-, or
peak-load service; and system size and degree of interconnection with
neighboring systems. Base-load nuclear or coal-fired units may be as large as
1200 MW each, or more. Intermediate-duty generators, usually coal-, oil-, or
gas-fueled steam units, are typically of 200 to 600 MW capacity each. Peaking
units, combustion turbines or hydro, range from several tens of megawatts for
the former to hundreds of megawatts for the latter. Hydro units, in both
base-load and intermediate service, range in size up to 700 MW.
    The total installed generating capacity of a system is typically 20 to 30%
greater than the annual predicted peak load in order to provide reserves for
maintenance and contingencies.
    Voltage regulation is defined as change in voltage for specific change in
load (usually from full load to no load) expressed as percentage of normal
rated voltage. The voltage of an electric generator varies with the load and
power factor; consequently, some form of regulating equipment is required to
maintain a reasonably constant and predetermined potential at the distribution
stations or load centers. Since the inherent regulation of most
alternating-current generators is rather poor (that is, high percentagewise),
it is necessary to provide automatic voltage control. The rotating or magnetic
amplifiers and voltage-sensitive circuits of the automatic regulators,
together with the exciters, are all specially designed to respond quickly to
changes in the alternator voltage and to make the necessary changes in the
main exciter output, thus providing the required adjustments in voltage.
Electronic voltage control has been adapted to some generator and synchronous
condenser installations. See VOLTAGE REGULATOR.
    Computer-assisted (or online-controlled) load and frequency control and
economic dispatch systems of generation supervision are being widely adopted,
particularly for the larger new plants. Strong system interconnections greatly
improve bulk power supply reliability but require special automatic controls
to ensure adequate generation and transmission stability. Among the
refinements found necessary in large, long-distance interconnections are
special feedback controls applied to generator high-speed excitation and
voltage regulator systems. See ALTERNATING-CURRENT GENERATOR; ELECTRIC POWER
SYSTEMS.
 �34s[Eugene C. Starr]
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�6v�2v�32sElectric power measurement   �21m �33s 
    The measurement of the time rate at which work is done or energy is 
dissipated in an electric system. The work done in moving an electric charge 
is proportional to the charge and the voltage drop through which it moves. 
Charge per unit time defines electric current; electric power p is therefore 
defined as the product of the current i in a circuit and the voltage e across 
its terminals at a given instant. Expressed symbolically, p = ei. 
    A second important definition of power follows directly from Ohm's law. p 
= isup2R, where R is the resistance of the circuit. 
    The practical unit of electric power is the watt. The watt represents a 
rate of expending energy, and thus it is related to all other units of power; 
for example, in mechanics 1 watt = 1 joule/s and 746 watts = 1 horsepower. 
Commonly used small units are the milliwatt (0.001 watt) and the microwatt 
(0.000001 watt). Large units are the kilowatt (1000 watts) and the megawatt 
(1,000,000 watts). 
    Power measurements must cover the frequency spectrum from direct current 
through the conventional power frequencies, the audio and the lower radio 
frequencies, to the highest frequencies (up to 25,000 gigahertz). In general, 
different techniques are required in each frequency range. See ELECTRICAL 
MEASUREMENTS.  
  
    DC and ac power frequencies.  In the measurement of power in a dc circuit 
not subject to rapid fluctuations, there is usually no difficulty in making 
simultaneous observations of the true values of voltage and current using 
common types of dc voltmeters and ammeters. The product of these observations 
then gives a sufficiently accurate measure of power in the given circuit, 
except that, if great accuracy is required, allowance must be made for the 
power used by the instruments themselves. 
    In circuits with rapidly varying direct currents, pulsating rectified 
current, or, in general, alternating currents, the continuous averaging over 
short periods of time and the automatic multiplication of current and voltage 
values is accomplished by the wattmeter. See WATTMETER. 
    In ac circuits with steady effective values of voltage and current, the 
voltmeter-ammeter method may be used as in the dc case, except that, of 
course, ac meters are used, and a phase meter is also required to measure 
phase angle unless current and voltage are in phase. Because ac ammeters and 
voltmeters actually measure root-mean-square, or effective, values, these lead 
directly to values of average power. 
    Summation of power in the separate phases of a polyphase circuit is 
accomplished by combinations of single-phase wattmeters, or wattmeter 
elements, disposed according to the general rule, called Blondel's theorem, as 
follows: If energy is supplied to any system of conductors through N wires, 
the total power in the system is given by the algebraic sum of N wattmeters, 
so arranged that each of the N wires contains one current coil, the 
corresponding potential coil being connected between that wire and some point 
on the system which is common to all the potential circuits. If this common 
point is on one of the N wires and coincides with the point of attachment of 
the potential lead to that wire, the measurement may be effected by the use of 
N-1 wattmeters. 
  
    Audio and radio frequencies.  At frequencies above those used in the 
ordinary power-distribution systems, dynamometer-type wattmeters become 
inaccurate. For measurements of transmitted power at audio and the lower radio 
frequencies, no generally satisfactory substitute has been developed and 
measurements are therefore confined to determinations of power dissipated in a 
load, or available from a source, and are deduced from measurements of 
impedance and current or voltage. 
    Power-output meters combine resistive loads, which can be adjusted to 
various known values, and voltmeters calibrated to indicate the power 
dissipated therein. They are used at audio frequencies to determine the 
maximum power output that can be obtained from a source, or to measure output 
power in studies of harmonic distortion, intermodulation, overload, frequency 
characteristic, and so forth. 
    At radio frequencies the principle of impedance matching is frequently 
used to determine the power input to a receiver, with a standard-signal 
generator serving as source. 
    The power dissipated in a resistive load can be determined from the heat 
developed therein, and several ingenious schemes for measuring this heat have 
been derived. A widely used technique depends upon dissipating the power in a 
resistive element of relatively high temperature coefficient of resistivity. 
The power is deduced from the change in resistance by one of several methods. 
Measurements of this general nature are known as bolometric methods. See 
BOLOMETER.  
  
    Microwave frequencies.  Measurement of dissipated power at microwave 
frequencies can be carried out by methods similar to those used at lower 
frequencies. Because voltage and impedance become increasingly difficult to 
define precisely as the frequency is increased, however, calorimetric methods 
generally, and bolometric methods specifically, are almost universally used. 
 �34s[Donald B. Sinclair]
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�6v�2v�32sElectric power substation   �21m �33s 
    An assembly of equipment in an electric power system through which 
electrical energy is passed for transmission, distribution, interconnection, 
transformation, conversion, or switching. See ELECTRIC POWER SYSTEMS. 
    Specifically, substations are used for some or all of the following 
purposes: connection of generators, transmission or distribution lines, and 
loads to each other; transformation of power from one voltage level to 
another; interconnection of alternate sources of power; switching for 
alternate connections and isolation of failed or overloaded lines and 
equipment; controlling system voltage and power flow; reactive power 
compensation; suppression of overvoltage; and detection of faults, monitoring, 
recording of information, power measurements, and remote communications. Minor 
distribution or transmission equipment installation is not referred to as a 
substation.  
    Substations are referred to by the main duty they perform. Broadly 
speaking, they are classified as: transmission substations, which are 
associated with high voltage levels; and distribution substations, associated 
with low voltage levels. See ELECTRIC DISTRIBUTION SYSTEM. 
    Substations are also referred to in a variety of other ways, as follows: 
    1. Transformer substations are substations whose equipment includes 
transformers.  
    2. Switching substations are substations whose equipment is mainly for 
various connections and interconnections, and does not include transformers. 
    3. Customer substations are usually distribution substations located on 
the premises of a larger customer, such as a shopping center, large office or 
commercial building, or industrial plant. 
    4. Converter stations are complex substations required for high-voltage 
direct-current (HVDC) transmission or interconnection of two ac systems which, 
for a variety of reasons, cannot be connected by an ac connection. The main 
function of converter stations is the conversion of power from ac to dc and 
vice versa. The main equipment includes converter valves usually located 
inside a large hall, transformers, filters, reactors, and capacitors. 
    5. Most substations are installed as air-insulated substations, implying 
that the bus-bars and equipment terminations are generally open to the air, 
and utilize insulation properties of ambient air for insulation to ground. 
Modern substations in urban areas are esthetically designed with low profiles 
and often within walls, or even indoors. 
    6. Metal-clad substations are also air-insulated, but for low voltage 
levels; they are housed in metal cabinets and may be indoors or outdoors. 
    7. Acquiring a substation site in an urban area is very difficult because 
land is either unavailable or very expensive. Therefore, there has been a 
trend toward increasing use of gas-insulated substations, which occupy only 5 
- 20% of the space occupied by the air-insulated substations. In gas-insulated 
substations, all live equipment and bus-bars are housed in grounded metal 
enclosures, which are sealed and filled with sulfur hexafluoride (SF6) gas, 
which has excellent insulation properties. 
    8. For emergency replacement or maintenance of substation transformers, 
mobile substations are used by some utilities. 
    An appropriate switching arrangement for ``connections'' of generators, 
transformers, lines, and other major equipment is basic to any substation 
design. There are seven switching arrangements commonly used: single bus; 
double bus, single breaker; double bus, double breaker; main and transfer bus; 
ring bus; breaker-and-a-half; and breaker-and-a-third. Each breaker is usually 
accompanied by two disconnect switches, one on each side, for maintenance 
purposes. Selecting the switching arrangement involves considerations of cost, 
reliability, maintenance, and flexibility for expansion. 
    A substation includes a variety of equipment. The principal items are 
transformers, circuit breakers, disconnect switches, bus-bars, shunt reactors, 
shunt capacitors, current and potential transformers, and control and 
protection equipment. See BUS-BAR; CIRCUIT BREAKER; ELECTRIC PROTECTIVE 
DEVICES; ELECTRIC SWITCH; RELAY; TRANSFORMER; VOLTAGE REGULATOR. 
    Good substation grounding is very important for effective relaying and 
insulation of equipment; however, the safety of the personnel is the governing 
criterion in the design of substation grounding. It usually consists of a bare 
wire grid, laid in the ground; and all equipment grounding points, tanks, 
support structures, fences, shielding wires and poles, and so forth, are 
securely connected to it. The grounding resistance is reduced to be low enough 
that a fault from high voltage to ground does not create such high potential 
gradients on the ground, and from the structures to ground, to present a 
safety hazard. Good overhead shielding is also essential for outdoor 
substations, so as to virtually eliminate the possibility of lightning 
directly striking the equipment. Shielding is provided by overhead ground 
wires stretched across the substation or tall grounded poles. See GROUNDING; 
LIGHTNING AND SURGE PROTECTION. 
 �34s[Narain G. Hingorani]
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�6v�2v�32sElectric power systems   �21m �33s 
    A complex assemblage of equipment and circuits for generating, 
transmitting, transforming, and distributing electrical energy. 
    Electricity in the large quantities required to supply electric power 
systems is produced in generating stations, commonly called power plants. Such 
generating stations, however, should be considered as conversion facilities in 
which the heat energy of fuel (coal, oil, gas, or uranium) or the hydraulic 
energy of falling water is converted to electricity. See ELECTRIC POWER 
GENERATION; HYDRAULIC TURBINE; NUCLEAR REACTOR; POWER PLANT; STEAM TURBINE. 
    The transmission system carries electric power efficiently and in large 
amounts from generating stations to consumption areas. Such transmission is 
also used to interconnect adjacent power systems for mutual assistance in case 
of emergency and to gain for the interconnected power systems the economies 
possible in regional operation. 
    A relatively new approach to high-voltage long-distance transmission is 
high-voltage direct current (HVDC), which offers the advantages of less costly 
lines, lower transmission losses, and insensitivity to many system problems 
that restrict alternating-current systems. Its greatest disadvantage is the 
need for costly equipment for converting the sending-end power to direct 
current, and for converting the receiving-end direct-current power to 
alternating current for distribution to consumers. See TRANSMISSION LINES. 
    As systems grow and the number and size of generating units increase, and 
as transmission networks expand, higher levels of bulk-power-system 
reliability are attained through properly coordinated interconnections among 
separate systems. Most of the electric utilities in the contiguous United 
States and a large part of Canada now operate as members of power pools, and 
these pools (except one in Texas) in turn are interconnected into one gigantic 
power grid known as the North American Power Systems Interconnection. The 
operation of this interconnection, in turn, is coordinated by the North 
American Power Systems Interconnection Committee (NAPSIC). Each individual 
utility in such pools operates independently, but has contractual arrangements 
with other members in respect to generation additions and scheduling of 
operation. Their participation in a power pool affords a higher level of 
service reliability and important economic advantages. 
    The majority of large electric systems have computerized energy centers 
whose functions are to control operation of the system so as to optimize 
economy and security. In some large interconnected networks of individual 
utility companies, such as those in the Pennsylvania - New Jersey - Maryland 
power pool, regional control centers have been established to perform the same 
function for a group of companies. New generation centers now use digital 
computers, or hybrid analog-digital units, rather than the analog machines 
previously used, and redundant systems are required to ensure security. 
    Power delivered by transmission circuits must be stepped down in 
facilities called substations to voltages more suitable for use in industrial 
and residential areas. See ELECTRIC POWER SUBSTATION. 
    That part of the electric power system that takes power from a bulk-power 
substation to customers' switches, commonly about 35% of the total plant 
investment, is called distribution. See ELECTRIC DISTRIBUTION SYSTEM. 
 �34s[William C. Hayes]
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�6v�2v�32sElectric protective devices �21m �33s
    Equipment applied to electric power systems to detect abnormal and
intolerable conditions and to initiate appropriate corrective action.
Disturbances in the normal operation of electric power systems may be caused
by natural phenomena, such as lightning, wind, or snow; by accidental means
traceable to reckless drivers, inadvertent acts by plant maintenance
personnel, or other acts of human beings; or by conditions produced in the
system itself, such as switching surges, load swings, or equipment failure.
Protective devices must therefore be installed on a power system to ensure
continuity of electrical service, to limit injury to personnel, and to limit
damage to equipment when abnormal situations develop.
    Protective devices, like any type of insurance, are applied commensurately
with the degree of protection desired. For this reason, application of
protective devices varies widely.
    For the purpose of applying protection, the electric power system is
divided into five major protection zones: generators; transformers; buses;
transmission and distribution lines; and motors. Each block represents a set
of protective relays and associated equipment selected to initiate correction
or isolation of that area for all anticipated intolerable conditions or
trouble. The detection is done by protective relays with a circuit breaker
used to physically disconnect the equipment. For other areas of protection see
GROUNDING; LIGHTNING AND SURGE PROTECTION.

    Protective relays.	These are compact analog or digital networks connected
throughout the system to detect intolerable conditions within their assigned
area or zone. They operate on voltage, current, current direction, power
factor, power, impedance, temperature, and so forth, as well as combinations
of these. System faults for which the relays respond are generally short
circuits between the phase conductors, or between the phases and grounds. Some
relays operate on unbalances between the phases, such as an open or reversed
phase.
    The most fundamental and widely used protection technique is the
differential principle. The current flowing into the equipment to be protected
is compared with the current flowing out. For normal and permissible
operation, currents all sum to essentially zero. However, for internal
trouble, they add up to flow through the relay.
    While the application and protection principles are the same, the relay
units may be either electromechanical or solid-state (also known as static)
type. The plunger type (Fig. 1) is composed of a coil, plunger, and set of
contacts. When current I flows in the coil, a force is produced that causes
the plunger to move and close the relay contacts. The electromagnetic
induction-disk relay (Fig. 2) responds to alternating current only. The main
coil is connected to an external source. When current flows in the main coil,
transformer action induces current in the secondary circuit connected to the
upper poles. Fluxes produced by the currents flowing in the upper pole circuit
induce eddy currents in the rotor disk. Interaction between rotor eddy
currents and the flux from the lower pole produces torque on the rotor,
causing it to move and thus closing the contacts. See RELAY.

    Overcurrent protection. This must be provided on all systems to prevent
abnormally high currents from overheating and causing mechanical stress on
equipment. Overcurrent in a power system usually indicates that current is
being diverted from its normal path by a short circuit. In low-voltage,
distribution-type circuits, such as those found in homes, adequate overcurrent
protection can be provided by fuses that melt when current exceeds a
predetermined value. See FUSE (ELECTRICITY).
    Small thermal-type circuit breakers also provide overcurrent protection for
this class of circuit. As the size of circuits and systems increases, the
problems associated with interruption of large fault currents dictate the use
of power circuit breakers. See CIRCUIT BREAKER.

    Overvoltage protection. Lightning near power lines can cause
very-short-time overvoltages in the system and possible breakdown of the
insulation. Protection for these surges consists of lightning arresters
connected between the lines and ground. Normally the insulation through these
arresters prevents current flow, but they momentarily pass current during the
high-voltage transient to limit overvoltage. Overvoltage protection is seldom
applied elsewhere except at the generators, where it is part of the voltage
regulator and control system.

    Undervoltage protection.  This must be provided on circuits supplying
power to motor loads. Low-voltage conditions cause motors to draw excessive
currents, which can damage the motors. If a low-voltage condition develops
while the motor is running, the relay senses this condition and removes the
motor from service.

    Underfrequency protection.	A loss or deficiency in the generation supply,
the transmission lines, or other components of the system, resulting primarily
from faults, can leave the system with an excess of load. Solid-state and
digital-type under-frequency relays are connected at various points in the
system to detect the resulting decline in the normal system frequency. They
operate to disconnect loads or to separate the system into areas so that the
available generation equals the load until a balance is reestablished.

    Reverse-current protection.  This is provided when a change in the normal
direction of current indicates an abnormal condition in the system. In an ac
circuit, reverse current implies a phase shift of the current of nearly 180DG
from normal. This is actually a change in direction of power flow and can be
detected by ac directional relays.

    Phase unbalance protection.  This protection is used on feeders supplying
motors where there is a possibility of one phase opening as a result of a fuse
failure or a connector failure. One type of relay compares the current in one
phase against the currents in the other phases. Another type monitors the
three-phase bus voltages for unbalance.

    Reverse-phase-rotation protection.	Where direction of rotation is
important, electric motors must be protected against phase reversal. A
reverse-phase-rotation relay is applied to sense the phase rotation. This
relay is a miniature three-phase motor with the same desired direction of
rotation as the motor it is protecting. If the direction of rotation is
correct, the relay will let the motor start. If incorrect, the sensing relay
will prevent the motor starter from operating.

    Thermal protection.  Motors and generators are particularly subject to
overheating due to overloading and mechanical friction. Temperature-sensitive
elements, located inside the machine, form part of a bridge circuit used to
supply current to a relay. When a predetermined temperature is reached, the
relay operates, initiating opening of a circuit breaker or sounding of an
alarm.
 �34s[J. Lewis Blackburn]
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�6v�2v�32sElectric rotating machinery   �21m �33s 
    Any form of apparatus, having a rotating member, which generates, 
converts, transforms, or modifies electric power. The most common forms are 
motors, generators, synchronous condensers, synchronous converters, rotating 
amplifiers, phase modifiers, and combinations of these in one machine. 
    Most rotating machines consist of a stationary member, called the stator, 
and a rotating member, called the rotor. The rotor may be supported in 
bearings at both ends, or it may be supported at one or both ends by the shaft 
of another machine. See GENERATOR; MOTOR. 
    To function properly, rotating machines must have a magnetic circuit, 
usually involving both rotor and stator, and one or more insulated electrical 
circuits which interlink the magnetic circuit. To afford a low-reluctance 
magnetic path, the rotor and stator are separated only by a small clearance, 
called the air gap. 
    Rotating machinery must be ventilated to avoid overheating from internal 
losses. The principal cooling medium, usually air or hydrogen, is circulated 
by fans or blowers mounted on the rotor or separately driven. With 
conventional cooling, the cooling medium is blown over exposed surfaces of the 
insulated windings and core. In conductor cooling, the cooling medium flows in 
ducts within the major insulation wall. In large machines the superiority of 
conductor cooling is essential. 
    In all rotating machines, losses occur. Among them are Isup2R losses, 
called copper losses, in the windings, connections, and brushes; stray load 
losses in windings, solid metal structures, and frame; core loss in the 
magnetic material and structural parts; windage and friction loss; and exciter 
and rheostat losses. See CORE LOSS. 
 �34s[Leon T. Rosenberg]
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�6v�2v�32sElectric spark   �21m �33s 
    A transient form of gaseous conduction. This type of discharge is 
difficult to define, and no universally accepted definition exists. It can 
perhaps best be thought of as the transition between two more or less stable 
forms of gaseous conduction. For example, the transitional breakdown which 
occurs in the transition from a glow to an arc discharge may be thought of as 
a spark. See ELECTRIC CONDUCTION IN GASES. 
    Electric sparks play an important part in many physical effects. Usually 
these are harmful and undesirable effects, ranging from the gradual 
destruction of contacts in a conventional electrical switch to the large-scale 
havoc resulting from lightning discharges. Sometimes, however, the spark may 
be very useful. Examples are its function in the ignition system of an 
automobile, its use as an intense short-duration illumination source in 
highspeed photography, and its use as a source of excitation in spectroscopy. 
In the second case the spark may actually perform the function of the camera 
shutter, because its extinction renders the camera insensitive. See 
SPECTROSCOPY; STROBOSCOPIC PHOTOGRAPHY. 
 �34s[Glenn H. Miller]
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�6v�2v�32sElectric susceptibility �21m �33s
    A dimensionless parameter measuring the ease of polarization of a
dielectric. The susceptibility CHI is equal to the ratio of polarization P to
the product of electric field strength E and vacuum permittivity EPSILON 0, as
in Eq. (1) (EPSILON 0 = 1 in cgs electrostatic units, and 8.854 x 10 sup -12
farad/m in mks units).

		       *** CHI = P/EPSILON 0 E ***			 (1)

    The electric susceptibility is related to the dielectric constant KAPPA'
by Eq. (2), where GAMMA is a geometrical factor equal to 4 PI or 1 in the cgs
or mks systems, respectively. This ambiguity in the definition of CHI is

		   *** GAMMA = (KAPPA' - 1)/GAMMA ***                    (2)

unfortunate and must be considered in evaluating published data. See
DIELECTRIC CONSTANT; ELECTRICAL UNITS AND STANDARDS; POLARIZATION OF
DIELECTRICS.
 �34s[Robert D. Waldron]
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�6v�2v�32sElectric switch   �21m �33s 
    A device that makes, breaks, or changes the course of an electric circuit. 
Basically, an electric switch consists of two or more contacts mounted on an 
insulating structure and arranged so that they can be moved into and out of 
contact with each other by a suitable operating mechanism. 
    The term switch is usually used to denote only those devices intended to 
function when the circuit is energized or deenergized under normal manual 
operating conditions; as contrasted with circuit breakers, which have as one 
of their primary functions the interruption of short-circuit currents. 
Although there are hundreds of types of electric switches, their application 
can be broadly classified into two major categories; power and signal. 
    In power applications, switches function to energize or deenergize an 
electric load. On the low end of the power scale, wall switches are used in 
homes and offices for turning lights on and off; dial and push-button switches 
control power to electric ranges, washing machines, and dishwashers. On the 
high end of the scale are load-break switches and disconnecting switches in 
power systems at the highest voltages (several hundred thousand volts). 
    For power applications, when closed, switches are required to carry a 
certain amount of continuous current without overheating, and in the open 
position they must provide enough insulation to isolate the circuit 
electrically.  
    Load-break switches are required also to have the capability of 
interrupting the load current. Although this requirement is easily met in 
low-voltage and low-current applications, for high-voltage and high-current 
circuits, arc interrupters, similar to those used in circuit breakers are 
needed. In medium-voltage applications the most popular interrupter is the air 
magnetic type, in which the arc is driven into an arc chute by the magnetic 
field produced by the load current in a blowout coil. See BLOWOUT COIL; 
CIRCUIT BREAKER. 
    Some load-break switches may also be required to have the capability of 
holding the contacts in the closed position during short-circuit conditions so 
that the contacts will not be blown open by electromagnetic forces when the 
circuit breaker in the system interrupts the short-circuit current. 
    For signal applications, switches are used to detect a specified situation 
that calls for some predetermined action in the electrical circuit. For 
example, thermostats detect temperature; when a certain limit is reached, 
contacts in the thermostat energize or deenergize another electrical switching 
device to control power flow. 
    Switches for signaling purposes are often required to have long life, high 
speed, and high reliability. Contaminants and dust must be prevented from 
interfering with the operation of the switch. For this purpose, switches are 
usually enclosed and are sometimes hermetically sealed. 
    Switches frequently are composed of many single circuits elements, known 
as poles, all operated simultaneously or in a predetermined sequence by the 
same mechanism. Switches are often typed by the number of poles and referred 
to as single-pole or double-pole switches, and so on. It is also common to 
express the number of possible switch positions per pole, such as a 
single-throw or double-throw switch. 
 �34s[Thomas H. Lee]
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�6v�2v�32sElectric transient   �21m �33s 
    A temporary component of current and voltage in an electric circuit which 
has been disturbed. In ordinary circuit problems, a stabilized condition of 
the circuit is assumed and steady-state values of current and voltage are 
sufficient. However, it often becomes important to know what occurs during the 
transition period following a circuit disturbance until the steady-state 
condition is reached. Transients occur only in circuits containing inductance 
or capacitance. In general, transients accompany any change in the amount or 
form of energy stored in the circuit. 
    The study of transient phenomena is very broad. The mathematical 
requirements become severe and go far beyond the borders of all known 
mathematics. Transient analysis often requires the use of calculating 
machines, models, and tests. Fourier and Laplace transforms have proven 
indispensable in the modern treatment of transients and these disciplines need 
be mastered by anyone going far in the study of transients. The analysis of 
lumped-parameter circuits is comparatively easy. An electric circuit or system 
under steady-state conditions of constant, or cyclic, applied voltages or 
currents is in a state of equilibrium. However, the circuit conditions of 
voltage, current, or frequency may change or be disturbed. Also, circuit 
elements may be switched in or out of the circuit. Any change of circuit 
condition or circuit elements causes a transient readjustment of voltages and 
currents from the initial state of equilibrium to the final state of 
equilibrium. In a sense the transient may be regarded as superimposed on the 
final steady state, so that Eq. (1) applies. 
  
    (Instanteous condition) = (final condition) + (transient terms)      (1)
  
    Furthermore, since the instantaneous condition at the first instant of 
disturbance (time zero) must be the initial condition, it may be described by 
Eq. (2). 
  
(Initial condition) = (final condition) + (transient terms at time zero)  (2)
  
    In many practical cases transients are one of three types: 
    1. Single-energy transients, in which only one form of energy storage 
(either electromagnetic or electrostatic) is present; the transient exhibits 
simple exponential decay from the initial to the final conditions. 
    2. Double-energy transients, in which both forms of energy storage are 
present; the transient is either aperiodic or a damped sinusoid. 
    3. Combination of 1 and 2. 
 �34s[Loyal V. Bewley]
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�6v�2v�32sElectric uninterruptible power system   �21m �33s 
    A system that provides protection against commercial power failure and 
variations in voltage and frequency. Uninterruptible power systems (UPS) have 
a wide variety of applications where unpredictable changes in commercial power 
will adversely affect equipment. This equipment may include computer 
installations, telephone exchanges, communications networks, motor and 
sequencing controls, electronic cash registers, hospital intensive care units, 
and a host of others. The uninterruptible power system may be used on-line 
between the commercial power and the sensitive load to provide transient free 
well-regulated power, or off-line and switched in only when commercial power 
fails.  
    There are three basic types of uninterruptible power system. These are, in 
order of complexity, the rotary power source, the standby power source, and 
the solid-state uninterruptible power system. 
    The rotary power source consists of a battery-driven dc motor that is 
mechanically connected to an ac generator. The battery is kept in a charged 
state by a battery charger that is connected to the commercial power line. In 
the event of a commercial power failure, the battery powers the dc motor which 
mechanically drives the ac generator. The sensitive load draws its power from 
the ac generator and operates through the outage. 
    The standby power source consists of a battery connected to a dc-to-ac 
static inverter. The inverter provides ac power for the sensitive load through 
a switch. A battery charger, once again, keeps the battery on full charge. 
Normally, the load operates directly from the commercial power line. In the 
event of commercial power failure, the switch transfers the sensitive load to 
the output of the inverter. 
    The solid-state uninterruptible power system has a general configuration 
much like that of the standby power system with one important exception. The 
sensitive load operates continually from the output of the static inverter. 
This means that all variations on the commercial power lines are cleaned and 
regulated through the output of the uninterruptible power system. A commercial 
power line, known as a bypass, is provided around the uninterruptible power 
system through a switch. Should the uninterruptible power system fail at some 
point, the commercial power is automatically transferred to the sensitive load 
through the switch. This scheme is known as an on-line automatic 
reverse-transfer uninterruptible power system. 
    An uninterruptible power system consists of four major subsystems: a 
method to put energy into a storage system, a battery charger; an energy 
storage system, the battery; a system to convert the stored energy into a 
usable form, the static inverter; and a circuit that electrically connects the 
sensitive load to either the output of the uninterruptible power system or to 
the commercial power line, the transfer switch. The position of the transfer 
switch is controlled by a monitor circuit. Generally the switch in an 
uninterruptible power system is a high-speed solid-state device that can 
transfer the load from one ac source to another with little or no break in 
power. See ELECTRIC POWER SYSTEMS; ELECTRIC SWITCH. 
 �34s[John Sullivan]
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�6v�2v�32sElectroacoustics   �21m �33s 
    That part of electronics concerned with the conversion of acoustical 
energy into electrical energy or vice versa. The means for conversion from 
acoustical to lectrical systems and vice versa are termed electroacoustic 
transducers or electroacoustic systems. Electroacoustic systems are used in 
the telephone, phonograph, radio, sound motion picture and magnetic tape 
reproducers, and sound-reinforcing systems for converting acoustical waves at 
the input into corresponding electrical waves, and for converting electrical 
waves at the output into corresponding sound waves. 
 �34s[Harry F. Olson]
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�6v�2v�32sElectrocardiography   �21m �33s 
    The study of the electrical manifes-tations of the heart beat as obtained 
from the surface of the body. The electrocardiogram (ECG, or from German EKG) 
is the record obtained, and the electrocardiograph is the recording 
instrument. Clinical electrocardiography is that part of the science which 
deals with the diagnostic interpretation of records obtained in humans based 
in part on empirical correlations, in part on fundamental physiologic and 
physical principles. 
    Electrocardiographic ``leads'' are records of differences in potential 
occurring at two specific electrode positions. Standard bipolar limb leads 
refer to measurements of potential differences between two extremities when 
each is connected to one of the input terminals of the recoder. Unipolar leads 
are obtained when one electrode (exploring electrode) is coupled with an 
indifferent electrode, whose potential is close to the mean potential of the 
body during the entire cardiac cycle. When the exploring electrode of a 
unipolar lead is placed close to the heart over the anterior chest, certain 
portions of the recorded curve are preferentially influenced by electrical 
events subjacent to the electrode position. This is an observation of 
considerable clinical usefulness. Other leads have been designed in an attempt 
to approach an undistorted three-dimensional recording system. 
    The clinical analysis of electrocardiograms is concerned with alterations 
in the form of the complexes induced by abnormalities in conduction pathways, 
rate of cellular changes, myocardial electrolyte imbalance, cardiac chamber 
enlargement, and pathologic changes secondary to alterations in myocardial 
blood supply. Interpretations of changes in cycle sequence, by itself a cause 
of variations in shape of the complexes, have laid the basis for a precise 
study on the nature of the irregular heart. See HEART DISORDERS. 
 �34s[Hans H. Hecht]
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�6v�2v�32sElectrochemical equivalent   �21m �33s 
    The mass of a substance, according to Faraday's law, produced or consumed 
by electrolysis with 100% current efficiency during the flow of a quantity of 
electricity equal to 1 faraday or 96,487 coulombs (1 coulomb corresponds to a 
current of 1 ampere during 1 second). Electrochemical equivalents are 
essential in the calculation of the current efficiency of an electrode process. 
    The electrochemical equivalent of a substance  is equal to the gram-atomic 
or gram-molecular mass of this substance divided by the number of electrons 
involved in the electrode reaction. For example, the electrochemical 
equivalent of zinc, for which two electrons are required in order to deposit 
one atom, is Zn/2 or 65.37/2 g. Thus, the faraday is equal to the product of 
the charge of the electron times the number of electrons (the Avogadro number) 
required to react with 1 atom- or molecule-equivalent of substance.  See  
COULOMETER; ELECTROLYSIS. 
 �34s[Paul Delahay]
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�6v�2v�32sElectrochemical process   �21m �33s 
    The principles of electrochemistry may be adapted for use in the 
preparation of commercially important quantities of certain substances, both 
inorganic and organic in nature. See ELECTROCHEMISTRY. 
  
    Inorganic processes.  Inorganic chemical processes can be classified as 
electrolytic, electrothermic, and miscellaneous processes including electric 
discharge through gases and separation by electrical means. In electrolytic 
processes, chemical and electrical energy are interchanged. Current passed 
through an electrolytic cell causes chemical reactions at the electrodes. 
Voltaic cells convert chemicals into electricity. Electrothermic processes use 
electricity to attain the necessary temperature for reaction. For related 
information  see  ELECTROCHEMISTRY; ELECTROLYSIS; ELECTROLYTIC CONDUCTANCE; 
ELECTROMOTIVE FORCE (CELLS). 
  
    Electrolysis in aqueous solutions. The electrolysis of water to form 
hydrogen and oxygen, according to the reaction 2H2O ->2H2 + O2, may be 
considered as the simplest process for aqueous electrolytes. It does not 
compete with hydrogen from propane or from natural gas and with oxygen from 
liquid air, except in small installations. While simplicity, high hydrogen 
purity requirement, and lower capital cost (in small plants) have justified 
electrolytic plants, severely rising energy costs have limited such 
applications. Heavy water, or deuterium oxide, used in moderating nuclear 
reactors is also a by-product of the electrolysis of water.  See  DEUTERIUM; 
HEAVY WATER; HYDROGEN; OXYGEN. 
  
    Metallurgical applications. Protective or decorative coatings on a base 
metal such as steel are obtained by electroplating. Plating may also be used 
to replace worn metal or to provide a wear-resistant surface. 
Electrogalvanizing is preferred over hot dipping for applying zinc to steel. 
Tin plate for containers is electrolytic.  See  ELECTROPLATING OF METALS; 
METAL COATINGS. 
    Electroforming is a method of forming or reproducing articles by 
electrodeposition. In contrast to electroplating, the product is removed from 
the base surface or mold. Electrodeposition of metal powders is used to 
produce particles in the 1- to 1000-micrometer range for use in powder 
metallurgy and metallic pigments. Electrolytic polishing of metals is 
accomplished by making the article anodic in an electrolyte of mixed acids. 
Electrolytic machining of metals is accomplished by making the metal part 
anodic in a suitable electrolyte. Electrorefining is a process for purifying 
metals and recovering their impurities, which at times are more valuable than 
the original metal. Electrowinning, sometimes termed aqueous 
electrometallurgy, involves processing of metallic ores by leaching solutions 
to obtain metal-containing electrolytes which can be processed with insoluble 
anodes and metal cathodes.  See  ELECTROMETALLURGY; ELECTROPOLISHING. 
  
    Alkali-chlorine processes. Electrolysis of alkali halides is the basis of 
the alkali-chlorine and chlorate industries. Chlorine, Cl2, and caustic soda, 
NaOH (or caustic potash, KOH), are made by electrolysis of brine, a solution 
of sodium chloride, NaCl, in water. Hydrochloric acid electrolysis is of 
interest for recovery of chlorine from HCl resulting as a by-product from 
organic chlorinations.  See  CHLORINE. 
  
    Oxidations and reductions. These reactions occur in all cells, but in a 
narrower sense oxidation reactions are those in which oxygen or chlorine at 
the anode oxidizes some material to form a new compound; reduction reactions 
are those in which hydrogen, liberated at the cathode, reduces a material to a 
new product. There are no commercial applications of inorganic electrochemical 
reductions by this narrow definition. 
  
    Ion-permeable membrane cells. These utilize diaphragms made of 
ion-exchange resins. Cation-permeable membranes permit cations to pass through 
but not anions, whereas the reverse holds for anion-permeable membranes. 
Purification of sea water is the most important application. Salt has been 
recovered from sea water which has been concentrated in this way.  See  
ION-SELECTIVE MEMBRANES AND ELECTRODES. 
  
    Fused-salt electrolysis. Aluminum, barium, beryllium, cerium and misch 
metal, fluorine, lithium, magnesium, sodium, molybdenum, thorium, titanium, 
uranium, and zirconium are obtained by electrolysis of fused salts, because 
water interferes with the desired reaction. Raw materials must all be purified 
before addition to fused-salt cells, because purification of the electrolyte 
is not economical as in aqueous electrolytes. Metallizing is a process of 
depositing a metal as an alloy on a substrate from a fused complex metal salt. 
  
    Electrothermics. The manufacture of many products requires temperatures 
higher than can be obtained by combustion methods. Electric heat can usually 
be developed at, or close to, the point where it is required, so that it is 
relatively quick. It permits easy control of the atmosphere for oxidizing, 
reducing, or neutral conditions. 
    Products of the electric furnace include iron and steel; ferralloys; 
nonferrous metals and alloys; the exotic metals titanium, zirconium, hafnium, 
thorium, and uranium; and nonmetallic products such as calcium carbide, 
calcium cyanamide, sodium cyanide, silicon carbide, boron carbide, and 
graphite.  See  ELECTRIC FURNACE; PYROMETALLURGY. 
    Zone refining of metals for the electronics industry, such as silicon for 
diodes and transistors, is accomplished by induction melting of the metal in a 
narrow zone and slow movement of the molten zone in the metal ingot from one 
end to the other in an evacuated or inert gas - filled enclosure. Impurities 
move toward the end of the ingot. The operation is repeated until the desired 
purity is obtained.  See  ZONE REFINING. 
  
    Electrodialysis. This is the separation of low-molecular-weight 
electrolytes from aqueous solutions by migration of the electrolyte through 
semipermeable membranes in an electric field. It is used on an industrial 
scale for deashing starch hydrolyzates and whey, and in many municipalities 
for producing potable water from saline water. Its uses also include the 
concentration of liquid foods such as dairy products and citrus juices, the 
recovery of sulfite pulp waste and pickling acid, and the isolation of 
proteins.  See  COLLOID; DIALYSIS; SALINE WATER RECLAMATION. 
  
    Electrophoretic deposition. This is the deposition of a nonconductive 
material in a finely divided state from a suspension in an inert medium. 
Electrophoresis is the migration of colloidal particles, which acquire 
positive or negative charges in an electric field. The process is useful in 
electropainting; for instance, electropainting of automobile bodies and other 
objects has now been adopted on a large scale. Rubber latex is an example of a 
negatively charged colloid which can be plated on an anode. Electronic 
components can be coated with inorganic salts, oxides, and ceramics suspended 
in organic media.  See  ELECTROPHORESIS. 
  
    Electroendosmosis. This is the movement of a liquid with respect to an 
immobilized colloid in an electric field. The process is used in the 
dehydration of peat, dye pastes, and clay. It is also used commercially for 
dewatering soils in mining, road building construction, and other civil 
engineering works. 
  
    Electrostatic technique. The deposition of charged particles from 
suspension in gases has many useful applications. The Cottrell electrostatic 
precipitator removes dusts and mists from gases.  See  DUST AND MIST 
COLLECTION; ELECTROSTATIC PRECIPITATOR. 
    Spray painting with a high voltage between the spray gun and the work is 
particularly effective in providing an even coating with an economical use of 
paint on irregular and open surfaces, such as a screen. 
    In xerography a sheet of plain paper is electrically sensitized in those 
areas corresponding to an original so that  colored resin particles carrying 
an opposite charge are attracted and retained only on the sensitized areas, 
thus producing a visible image corresponding to the original.  
    Abrasive paper and cloth are coated with an adhesive and abrasive powders 
attracted to the base material in an electrostatic field. Pile fabrics can be 
produced in a similar manner, with the short fibers oriented by the electric 
field.  
  
    Organic processes.  Organic electrochemistry was once regarded as a 
tantalizing area with many important laboratory achievements but few successes 
in commercial practice. This situation is changing, however, in that 
electroorganic processes are likely to prove commercially advantageous if they 
can fulfill either of two conditions: (1) performance under conditions of 
voltage corresponding thermodynamically to the conversion of an organic group 
to a reduced or oxidized group, with the cell products relatively easy to 
isolate and purify; (2) performance of a highly selective, specific technique 
to make an addition at a double bond, or to split a particular bond (for 
example, between carbon atoms 17 and 18 of a complex molecule having 25 carbon 
atoms).  
    Selectivity and specificity are highly important in electroorganic 
processes for the manufacture of complicated molecules of vitamins and 
hormones - as well as for the medicinal products whose action on pathogenic 
organisms is  a function of their spatial arrangement, steric forms, and 
resonance.  
    The electrolytic approach can also be competitive for some low-cost, 
tonnage products. Here continuous processing is important, and only a single 
phase should be present, that is, a solution rather than an emulsion, 
dispersion, or mechanical mixture. Only for fairly valuable products is it 
practical to find a conducting solvent and then to engineer around it. 
    The electrolytic oxidation and reduction of organic compounds differ from 
the corresponding and more familiar inorganic reactions only in that organic 
reactions tend to be more complex and have low yields. The electrochemical 
principles are precisely those of inorganic reactions, while the procedures 
for handling the chemicals are precisely those of organic chemistry. 
 �34s[Charles L. Mantell]
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�6v�2v�32sElectrochemical series   �21m �33s 
    A series in which the metals are listed in the order of their chemical 
reactivity, the most active at the top and the less reactive or more ``noble'' 
metals at the bottom. In a broader sense such an activity series need not be 
limited to the metals but may be carried on through the electronegative 
(nonmetallic) elements as well. See the table for a list of common elements. 
    The electrochemical series as it applies to metals was first established 
by laboratory experiments in which the purpose was to determine which metals 
would displace others from solutions of their salts. By exhaustive experiments 
it becomes possible to draw up a complete list in the order of chemical 
activity, in which the metals at the top of the list are those which are found 
to give up their electrons most readily (that is, are the most electropositive 
elements). Such a list is shown in the table, where lithium exhibits the most 
reactivity as a metal. 
    To obtain an accurate and reproducible activity series, it is best to use 
the electrode potential, or oxidation-reduction potential, which is defined as 
the voltage developed by a sample of pure metal immersed in a solution of one 
of its salts (at unit activity and at 25DGC or 77DGF) versus a hydrogen 
electrode immersed in hydrochloric or sulfuric acid of equivalent 
concentration.  See  ELECTROCHEMISTRY;  ELECTRODE POTENTIAL; 
ELECTRONEGATIVITY; OXIDATION-REDUCTION. 
 �34s[Eugene C. Rochow]
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�6v�2v�32sElectrochemical techniques   �21m �33s 
    Experimental methods developed to study the physical and chemical 
phenomena associated with electron transfer at the interface of an electrode 
and solution. The objective is to obtain either analytical or fundamental 
information regarding electroactive species in solution. Fundamental electrode 
characteristics may be investigated also. 
    The physical and chemical phenomena important in electrode processes 
generally occur very close to the electrode surface. Mass transfer of species 
involved in an electrode process to and from the bulk of solution is one 
important aspect. Inclusion of a large excess of inert electrolyte in most 
electrochemical systems eliminates electrical migration as an important means 
of mass transfer for electroactive species, and only convection and diffusion 
are considered. 
    Important chemical aspects of electrode processes include the oxidation or 
reduction occurring as a result of electron transfer, and coupled chemical 
reactions. Coupled reactions are initiated by production or depletion of the 
primary products or reactants at the electrode surface. 
    The primary experimental variables involved in electrochemical techniques 
are the potential, the current, and the time. Either the potential or current 
at the working electrode is controlled and the other observed as a function of 
time. The many ways in which either may be controlled give rise to the wide 
variety of controlled-potential or controlled-current techniques.  See  
ELECTROCHEMICAL PROCESS; ELECTRODE; ELECTROLYSIS. 
 �34s[Sam P. Perone]
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�6v�2v�32sElectrochemistry   �21m �33s 
    The science dealing with the chemical changes accompanying the passage of 
an electric current, or the reverse process in which a chemical reaction is 
used as the source of energy to produce an electric current, as in a battery. 
Ionic conduction in electrolytes (liquid solutions, molten salts, and certain 
ionically conductive solids) is a phase of electrochemistry. Conduction in 
metals, semiconductors, and gases is generally considered a portion of 
physics. Other aspects of electrochemistry are described below.  See  
ELECTROLYTIC CONDUCTANCE. 
  
    Galvanic cells.  These are better known as electric batteries. Many 
chemical reactions can be arranged to produce electrical energy by physically 
separating the reaction into two half-reactions, one supplying electrons to an 
electrode forming the negative terminal of the cell, and the other removing 
the electrons from the positive terminal.  See  BATTERY; FUEL CELL; STORAGE 
BATTERY.  
  
    Electrodeposition.  The most important type of chemical reaction brought 
about by the passage of electric current is the deposition of a metal at a 
cathode from a solution of its ions. Electroforming is a variety of 
electrodeposition in which an article to be reproduced is rendered conductive 
by spraying a thin metallic coating, then electroplated with a metallic 
deposit that is stripped from its substrate and filled with backing to 
reproduce the original article. Electrowinning is used for the commercial 
production of active metals, such as aluminum, magnesium, and sodium, from 
molten salts and others, such as copper, manganese, and antimony, from aqueous 
solution. Electrorefining is commonly used to purify metals such as silver, 
lead, and copper. The impure metal is used as the anode, and purified metal is 
deposited at the cathode.  See  ELECTROPLATING OF METALS. 
  
    Electrolytic processes.  Many electrode reactions other than metal 
deposition are of commercial or scientific use. Electrolysis of brine to yield 
chlorine at the anode, hydrogen at the cathode, and sodium hydroxide in the 
electrolyte is an important industrial process. Many organic compounds can be 
prepared electrolytically.  See  ELECTROLYSIS. 
  
    Electroanalytical chemistry.  Many electrochemical measurements are useful 
for analytical purposes. Electrodes that are commonly used for analytical 
purposes through measurement of their potentials include the glass electrode 
for pH measurements, and ion-selective electrodes for certain ions, such as 
sodium or potassium ion (special glass compositions), calcium ion (liquid 
membrane), and fluoride ion (doped lanthanum fluoride single crystals). 
Polarography involves the use of a dropping mercury electrode as one electrode 
of an electrolytic cell. Qualitative analysis is carried out by measurement of 
characteristic potentials (half-wave potentials) for electrode processes, and 
quantitative analysis by measurement of diffusion-controlled currents. 
Coulometry involves the application of Faraday's law for analytical purposes. 
    Other analytical applications of electrochemistry include 
chronopotentiometry (measurement of potential-time transients under constant 
current conditions), and linear sweep and cyclic voltammetry (measurement of 
currents with linear voltage scan). Several titration methods involve 
electrochemical measurements, for example, conductometric, potentiometric, and 
amperometric titrations.  See  COULOMETRY; ELECTRODEPOSITION ANALYSIS; 
ION-SELECTIVE MEMBRANES AND ELECTRONS; POLAROGRAPHIC ANALYSIS; TITRATION. 
  
    Miscellaneous phenomena.  Electrochemical transport of ions through 
synthetic or natural membranes is important for processes, such as 
desalination of water and electrodialysis. In biological systems, the 
transmittal of nerve impulses and the generation of electrical signals, such 
as brain waves, are basically of electrochemical origin. A set of related 
phenomena can be grouped together under electrokinetic behavior, including the 
motion of colloidal particles in an electric field (electrophoresis), the 
motion of the liquid phase relative to a stationary solid under the influence 
of a potential gradient (electroosmosis), and the inverse generation of a 
potential gradient caused by a flowing liquid (streaming potential). 
Alternating-current phenomena, such as dielectric behavior, double-layer 
charging, and faradaic rectification, may also be included in a general 
definition of electrochemistry. Corrosion and passivation of metals are 
electrochemical in nature. 
 �34s[Herbert A. Laitinen]
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�6v�2v�32sElectrochromic displays   �21m �33s 
    Devices that employ a reversible electrochemical reaction to cause a 
change in color of segments patterned to form alphanumeric characters. 
    Electrochromic displays are passive devices that only modulate ambient 
light, in contrast to a light-emitting diode. Hence, they operate at low 
voltages, and have a low enough energy requirement that a watch-size display 
can be operated for about 1 year from a small commercial battery. In the off 
state, the segments are typically colorless; in the on state, they are 
brightly colored, for example, blue or purple.  See  LIGHT-EMITTING DIODE. 
    The structure of an electrochromic display package is shown in the 
illustration. It consists of a top glass piece with the transparent 
electrodes, typically vacuum-deposited indium and tin oxides, on the inside 
surface. The lead from each segment continues to the edge of the glass. An 
insulating layer of silicon dioxide is deposited onto the leads to prevent 
their coloration. A counter electrode is deposited on the bottom piece. The 
two pieces are held together with a solder-glass, or epoxy, seal. To operate 
the device, a dc potential of 1 - 1.5 V is applied between the segments on the 
top plate and the counter electrode. Coloration and bleaching times are in the 
200 - 500-millisecond range. 
    Electrochromic displays can be categorized by the type of reaction that 
occurs when the potential is applied. Viologen electrochromic displays operate 
in an electroplating mode; with tungsten trioxide displays, the electrode 
material itself undergoes a color change. Electrochromic displays would be 
most efficiently utilized in applications requiring the display of 
large-format and slowly changing information, such as in clocks or message 
boards.  See  ELECTROCHEMICAL PROCESS; ELECTRONIC DISPLAY; ELECTROOPTICS. 
 �34s[Gabriel G. Barna]
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�6v�2v�32sElectroconvulsive shock   �21m �33s 
    The technique is that of eliciting convulsions by applying an electric 
current through the brain of a human patient or experimental animal for a 
brief period. Low current produces clonic convulsions, while high current 
produces tonic convulsions followed by clonic convulsions. Tonic convulsions 
cause loss of consciousness for a brief period. In the early 1930s 
drug-induced convulsions were used as treatment for mental disorders. In 1938 
electroconvulsive shock treatments were introduced as a more convenient and 
reliable procedure. Electroconvulsive shock subsequently replaced convulsive 
drugs and is still used in some circumstances as a treatment for severe mental 
depression. The bases of the effectiveness of ECS as a clinical treatment are 
not known.  See  MENTAL ILLNESS. 
 �34s[James L. McGaugh]
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�6v�2v�32sElectrode   �21m �33s 
    An electrical conductor through which an electric current enters or leaves 
a conducting medium, whether it be an electrolytic solution, solid, molten 
mass, gas, or vacuum. For electrolytic solutions, many solids, and molten 
masses, an electrode is an electric conductor at the surface of which a change 
occurs from conduction by electrons to conduction by ions. For gases and 
vacuum, the electrodes merely serve to conduct electricity to and from the 
medium.  See  ELECTRODE POTENTIAL; ELECTRODEPOSITION ANALYSIS; ELECTROLYSIS; 
ELECTROMOTIVE FORCE (CELLS). 
 �34s[Walter J. Hamer]
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�6v�2v�32sElectrodeposition analysis   �21m �33s 
    An electroanalytical chemistry technique in which the product of the 
electrode reaction is insoluble and is either deposited upon or dissolved into 
the electrode. This method usually results in the complete depletion in 
solution of the deposited substance. Electrodeposition is applied most 
commonly to the separation of metals from solution and to their subsequent 
quantitative determination. However, it may be used simply as a separation 
technique.  See  ELECTROLYSIS. 
    The quantities of metals deposited may be determined by weighing a 
suitable electrode before and after deposition (electrogravimetry). Among the 
elements that may be determined by deposition are cadmium, nickel, zinc, 
copper, cobalt, antimony, tin, silver, and gold. At the anode the oxides of 
lead, PbO2, and manganese, MnO2, may be deposited. Halides may be deposited on 
a silver anode as the respective silver halides.  See  COULOMETRIC ANALYSIS. 
 �34s[George W. O'Dom]
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�6v�2v�32sElectrode potential   �21m �33s 
    The potential which a metal or gas electrode takes up relative to a 
solution of ions. 
    The various electrodes encountered in electrochemical work may be grouped 
into seven types: (1) metal - metal ion, (2) amalgam, (3) nonmetal nongas, (4) 
gas, (5) metal - insoluble salt, (6) metal - insoluble oxide, and (7) 
oxidation-reduction. Any of these electrodes may be combined with any other to 
give a cell, the electromotive force (emf) of which is equal to the algebraic 
sum of the potentials of the two electrodes. 
    In order to measure the potential of any electrode, it is necessary, in 
principle, to combine the electrode with a reference electrode such as the 
hydrogen electrode, which has an arbitrarily assigned potential, and to 
measure the total voltage across the two half-cells. The potential of the 
reference electrode is then subtracted to give the required electrode 
potential. For various reasons, such as the difficulty in setting up a 
hydrogen gas electrode, several subsidiary reference electrodes, whose 
potentials are known on the hydrogen scale, have been devised. The most common 
of these is the calomel electrode, which consists of mercury in contact with a 
solution of potassium chloride saturated with mercurous chloride.  See  
CALOMEL ELECTRODE. 
    A simple voltmeter cannot be used alone for measuring the emf of a small 
cell because the operation of the voltmeter draws some current, which causes 
chemical changes at the electrodes and produces a different voltage. To avoid 
these difficulties, the emf is measured with a potentiometer, that is, by 
balancing against a cell a known voltage under conditions such that 
practically no current flows.  See  POTENTIOMETER (VOLTAGE METER). 
    The standard cell that is widely employed for emf measurements is some 
form of the Weston cell. It is highly reproducible, its emf remains constant 
over long periods of time, and it has a small temperature coefficient.  See  
ELECTROCHEMISTRY; ELECTROMOTIVE FORCE (CELLS); ELECTRONEGATIVITY; 
OXIDATION-REDUCTION.  
 �34s[Richard Glicksman]
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�6v�2v�32sElectrodermal response   �21m �33s 
    A transient change in certain electrical properties of the skin, 
associated with the sweat gland activity and elicited by any stimulus that 
evokes an arousal or orienting response. Originally termed the psychogalvanic 
reflex, this phenomenon became known as the galvanic skin response. Phasic 
changes in skin conductance and potential (voltage) are designated as SCRs and 
SPRs, respectively, and electrodermal response has replaced galvanic skin 
response as the collective term. 
    The skin of a relaxed person has a low electrical conductance (high 
resistance), and the skin surface is some 40 mV negative with respect to 
interior tissues. Sweat gland activity changes these electrical properties by 
increasing skin conductance and by changing the balance of positive and 
negative ions in the secreted fluid. 
    Tonic skin conductance varies with psychological arousal, rising sharply 
when the subject awakens and rising further with activity, mental effort, or 
especially stress. Phasic skin conductance responses are wavelike increases in 
skin conductance that begin 1 - 2 seconds after stimulus onset and peak within 
about 5 seconds. The amplitude of the skin conductance response varies with 
the subjective impact of the eliciting stimulus, which in turn varies with the 
intensity of the stimulus, its novelty or unexpectedness for the subject, and 
its meaning or signal value. Aroused subjects display spontaneous skin 
conductance responses, generated apparently by mental events or other internal 
stimuli; their frequency, like the tonic skin conductance level, increases 
with the level of arousal. 
    Electrodermal responses are measured in studies of emotion and stress, 
conditioning, habituation, and cognitive processing: that is, when it is 
desired to assess the differential or changing impact of a series of stimuli. 
See LIE DETECTOR. 
 �34s[David T. Lykken]
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�6v�2v�32sElectrodiagnosis   �21m �33s 
    Diagnosis using electrical equipment. Three major kinds of 
electrodiagnostic procedures are used clinically. Electrical activity of the 
brain is observed and measured by electroencephalography, of the heart by 
electrocardiography, and of skeletal muscle by electromyography. All three are 
also used extensively in basic physiological, psychophysiological, and 
pharmacological research, and each is based on the principle that living cells 
have an electrical potential difference between their interior and external 
surfaces. Electrodiagnosis utilizes equipment that amplifies and records 
changes in potential, especially in those cells that are highly specialized 
electrical conductors (heart and skeletal muscle or nerve cells) and whose 
principal function is related thereto. See CARDIAC ELECTROPHYSIOLOGY; 
ELECTROCARDIOGRAPHY; ELECTROENCEPHALOGRAPHY; ELECTROMYOGRAM. 
 �34s[N. Karle Mottet]
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�6v�2v�32sElectrodialysis   �21m �33s 
    Dialysis rates of ionic low-molecular-weight solutes may be increased 
greatly by applying an electric field to achieve an electrophoretic movement 
of ions through the dialytic membrane. This combined process of dialysis and 
electrophoretic transport of solutes through a membrane is known as 
electrodialysis. See ELECTROPHORESIS. 
    Ions do not migrate readily through membranes that carry in their pores 
charges of the same sign as the ion; ions of charge opposite to that of the 
membrane are not prevented from passing through. Such a selective permeability 
to ions by charged membranes has been used to improve the efficiency of 
electrodialysis. Membranes of ion-exchange resins are commercially available 
in cationic and anionic forms. See IONSELECTIVE MEMBRANES AND ELECTRODES. 
    For desalting small quantities of solutions for chromatography and 
radioisotope tracer studies, micro- and semimicroelectrodialyzers have proved 
valuable. On a larger scale, electrodialysis combined with ion-exchange 
membranes has been used to remove salt from sea water. See COLLOID; SALINE 
WATER RECLAMATION. 
 �34s[Quentin Van Winkle]
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�6v�2v�32sElectrodynamic ammeter   �21m �33s 
    An ammeter in which a movable coil rotates in the magnetic field produced 
by a fixed coil. The instrument responds to alternating as well as direct 
current, and is thus a transfer instrument. Such instruments are precise and 
may be calibrated accurately and used for secondary standards, but the 
magnetic field of the fixed-coil system is relatively weak, 60 gauss (6 
milliteslas) being a common value. The controlling forces are thus much lower 
than in other types, and the electrodynamic ammeter is mainly used in the 
laboratory for calibrating the more rugged types. See AMMETER; ELECTRODYNAMIC 
INSTRUMENT.  
 �34s[John H. Miller /Jesse Beams]
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�6v�2v�32sElectrodynamic instrument   �21m �33s 
    A term used in preference to the older term electrodynamometer to refer to 
an instrument in which the electromagnetic reaction between two coils or sets 
of coils, each carrying electric current, can be measured. Because the 
reaction is proportional to the product of the current in the fixed and 
movable coils, the arrangement has found wide usage. It can be used to 
indicate ac or dc current, voltage, or power; with a crossed-coil movement it 
can be used for power-factor, phase-angle, frequency, and capacity 
measurements. For the crossed-coil instrument see PHASE METER. 
    A modern version of an electrodynamic instrument mechanism is shown in the 
illustration. Note the pair of fixed coils and the movable coil on a vertical 
staff with spiral countertorque springs for both control and current-carrying 
connections to the moving coil. 
    Electrodynamic instruments respond equally well to direct currents or to 
low-frequency alternating currents and therefore are used as transfer 
instruments to relate dc and ac currents. Transfer instruments are calibrated 
on direct current, using a standard cell and a standard ohm for reference, and 
are then used to measure correctly the rms value of an alternating current. 
    The pointer deflection of the electrodynamic instrument is a function of 
the product of two currents; thus the instrument can be connected as a 
wattmeter. The main current flows through the fixed coils, and a small current 
proportional to the voltage flows through the movable coil. On alternating 
current where the current may lag the voltage, the instrument responds only to 
the in-phase product of the two currents and hence indicates in terms of true 
watts, irrespective of power factor. See WATTMETER. 
 �34s[John H. Miller /Jesse Beams]
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�6v�2v�32sElectrodynamics   �21m �33s 
    The study of the relations between electrical, magnetic, and mechanical 
phenomena. This includes considerations of the magnetic fields produced by 
currents, the electromotive forces induced by changing magnetic fields, the 
forces on currents in magnetic fields, the propagation of electromagnetic 
waves, and the behavior of charged particles in electric and magnetic fields. 
Classical electrodynamics deals with fields and charged particles in the 
manner first systematically described by J. C. Maxwell, whereas quantum 
electrodynamics applies the principles of quantum mechanics to electrical and 
magnetic phenomena. Relativistic electrodynamics is concerned with the 
behavior of charged particles and fields when the velocities of the particles 
approach that of light. Cosmic electrodynamics is concerned with 
electromagnetic phenomena occurring on celestial bodies and in space. See 
COSMIC ELECTRODYNAMICS; ELECTROMAGNETISM; QUANTUM ELECTRODYNAMICS; 
RELATIVISTIC ELECTRODYNAMICS. 
 �34s[John W. Stewart]
�33s 
  
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�6v�2v�32sElectroencephalography   �21m �33s 
    The biomedical technology and science of recording the minute electric 
currents produced by the brains of humans and animals. Electroencephalography 
(EEG) has important clinical significance for the diagnosis of brain disease. 
The interpretation of EEG records has become a clinical specialty for 
neurological diagnosis. 
    The recording machine, the electroencephalograph, usually produces a 
16-channel ink-written record of brain waves, the electroencephalogram. It is 
interpreted by an electroencephalographer. The placement of about 20 equally 
spaced electrodes pasted to the surface of the scalp is in accordance with the 
standard positions adopted by the International Federation of EEG, and is 
called the 10/20 system. About 10 patterns or montages of combinations of 
electrode pairs are selected for transforming the spatial location from the 
scalp to the channels which are traced on the EEG pen writer. 
  
    Electrical voltage is transduced from the scalp by differential input 
amplifiers and amplified about a million times in order to drive the pens for 
the paper record. The recording usually takes 30 - 60 min during a relaxed 
waking state, and also during sleep when possible. Often, activating 
procedures are used, such as a flickering light stimulator and 
hyperventilation or overbreathing for about 3 min. 
    EEG waves are defined by form and frequency. Various frequencies are given 
Greek letter designations. Alpha rhythm is defined as 8 - 12-Hz sinusoidal 
rhythmical waves. Alpha waves are normally present during the waking and 
relaxed state and enhanced by closing the eyes. They are suppressed or 
desynchronized when the eyes are open, or when the individual is emotionally 
aroused or doing mental work. They are of highest amplitude in the posterior 
regions of the brain. The alpha rhythm develops with age, reaching maturity by 
about 12 years, stabilizes, and then declines in frequency and amplitude in 
old age (over 65). 
    Beta rhythms are faster, low-voltage sinuosidal waves, usually about 14 - 
30 Hz. They are more prominent in the frontal areas. They are prevalent during 
sedation with phenobarbital or with the use of tranquilizers and some sedative 
drugs.  
    Slower rhythms are theta and delta waves. Theta waves of 4 - 7 Hz usually 
replace the alpha rhythm during drowsiness and light sleep. Delta waves of 0.5 
- 4 Hz are present during deep sleep in normal people of all ages, and they 
are the primary waves present in the records of normal infants. Delta waves 
are almost always pathological in the waking records of adults. 
    Sleep stages are objectively defined by the use of EEG records. Theta 
waves are present in drowsiness and light sleep (stage I). Light sleep (stage 
II) is characterized by negative sharp waves at the vertex (top of head), 
called sleep humps, and spindle bursts of 13 - 15-Hz waves in the central 
regions of the head (over the Rolandic fissures). These are intermixed with 
theta waves, and seen only during light sleep, sometimes called the 
hump-and-spindle stage of sleep. Low-voltage, desynchronized delta waves are 
present during deep sleep (stage III), and higher-amplitude, slower, and more 
synchronized delta waves are seen in the deeper sleep stage (stage IV). Stage 
V sleep is called the dream stage or rapid-eye-movement (REM) state. That is 
when dreams take place, accompanied by rapid horizontal eye movements, as if 
the sleeper were scanning a real scene. The EEG pattern is more like waking, 
with alpha rhythm present. The REM state is the one in which it is hardest to 
wake up the subject, so that it is sometimes called paradoxical sleep. See 
SLEEP AND DREAMING. 
    The EEG reveals functional abnormalities of the brain, whether caused by 
localized structural lesions, essential paroxysmal states such as epilepsy, or 
toxic and abnormal metabolic conditions. The three major classes of 
abnormalities are asymmetries between the hemispheres, slow rhythms, and very 
sharp waves or spikes. Slow waves represent a depression of cerebral cortical 
activity or injury in the projection pathways beneath the recording 
electrodes. Sharp waves or spikes often indicate a hyperexcitable or irritable 
state of the cortex. During a full epileptic seizure attack, spikes become 
repetitive and synchronized over the whole surface of the brain. See EPILEPSY. 
    The EEG is frequently used for the evaluation of comatose states. The 
record is slowed in all areas in coma, with delta waves predominating. If the 
EEG becomes isoelectric or flat for several hours, brain function is not 
recoverable and the coma may be considered terminal. ``Brain death'' is 
determined by a flat EEG, recorded at the highest gain with widely spaced 
electrode positions and the absence of cerebral reflexes and spontaneous 
respiration. See DEATH. 
 �34s[Jerome Cohen]
�33s 
  
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�6v�2v�32sElectrokinetic phenomena   �21m �33s 
    Phenomena associated with the movement of charged particles through a 
continuous medium or with the movement of a continuous medium over a charged 
surface. The four principal electrokinetic phenomena are electrophoresis, 
electroosmosis, streaming potential, and sedimentation potential, or Dorn 
effect. These phenomena are related to one another through the zeta potential 
ZETA of the electrical double layer which exists in the neighborhood of the 
charged surface. See ELECTROPHORESIS; STREAMING POTENTIAL. 
 �34s[Quentin Van Winkle]
�33s 
  
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�6v�2v�32sElectroless plating   �21m �33s 
    A chemical reduction process which, once initiated, is autocatalytic. The 
process is similar to electroplating except that no outside current is needed. 
The metal ions are reduced by chemical agents in the plating solutions, and 
deposit on the substrate. An advantage of electroless plating with current is 
the more uniform thickness of the surface coating. 
    Electroless plating is used for coating nonmetallic parts. Decorative 
electroless plates are usually further coated with electrodeposited nickel and 
chromium. There are also applications for electroless deposits on metallic 
substrates, especially when irregularly shaped objects require a uniform 
coating. Electroless copper is used extensively for printed circuits, which 
are produced either by coating the nonmetallic substrate with a very thin 
layer of electroless copper and electroplating to the desired thickness or by 
using the electroless process only. Electroless iron and cobalt have limited 
uses. Electroless gold is used for microcircuits and connections to 
solid-state components. Deeply recessed areas which are difficult to plate can 
be coated by the electroless process. See ELECTROPLATING OF METALS. 
 �34s[Rolf Weil]
�33s 
  
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�6v�2v�32sElectroluminescence �21m �33s
    A luminescence resulting from the application of an electric field to a
material, usually solid. Several different types of electroluminescence can be
distinguished. For a discussion of the basic mechanism involved see
LUMINESCENCE.
    In 1936, G. Destriau in France observed that when suitably prepared zinc
sulfide phosphor powders, activated with small additions of copper, are
suspended in an insulator and an intense alternating electric field is applied
with capacitorlike electrodes, light emission results (the Destriau effect).
The effect is the basis for modern electroluminescent lamps. The insulator may
be either liquid (oil) or solid (glass or plastic). Thin films of tin oxide or
metal are used as transparent electrodes.
    Maximum efficiency is not obtained for the same operating conditions as
maximum brightness, and efficiency is normally low compared with other light
sources. See LIGHT PANEL.
    Injection electroluminescence results when a semiconductor pn junction or
a point contact is biased in the forward direction. It is the result of
radiative recombination of injected minority carriers with majority carriers
across the energy band gap of the material. Such emission has been observed in
a large number of semiconductors. In some cases the efficiency is high,
approaching one emitted photon for each carrier passing through the junction.
In suitable structures the excitation intensity can become so high that
stimulated rather than spontaneous emission predominates, and laser action
results, with spectral narrowing and coherent emission. See JUNCTION DIODE;
LASER; SEMICONDUCTOR; SEMICONDUCTOR DIODE.
 �34s[Clifford C. Klick ;James H. Schulman]
�33s

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�6v�2v�32sElectrolysis   �21m �33s 
    A method by which reactions are carried out in solutions of electrolytes 
or in molten salts by use of electricity. As shown in the illustration, the 
electrodes of an electrolytic cell are immersed in an electrolyte solution or 
in molten salts and are connected to a direct-current power supply. One or 
several reactions occur at each electrode when current flows through the cell. 
Reduction, a reaction in which electrons are consumed, occurs at the electrode 
called the cathode; oxidation occurs at the anode. For instance, sodium is 
produced at the cathode by reduction, and chlorine at the anode by oxidation 
in the electrolysis of molten sodium chloride. 
    Applications are important and varied: industrial production of chemicals, 
metallurgical extraction of metals, electroplating of metals, metal finishing, 
and production of electricity in batteries. Metallic corrosion often involves 
electrolytic processes. For application to analytical chemistry see 
ELECTRODEPOSITION ANALYSIS; POLAROGRAPHIC ANALYSIS. See also ELECTROCHEMISTRY; 
ELECTROMETALLURGY; ELECTROPLATING OF METALS. 
 �34s[Paul Delahay]
�33s 
  
  
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�6v�2v�32sElectrolyte   �21m �33s 
    A chemical compound which when fused or dissolved in certain solvents, 
usually water, will conduct an electric current. The passage of the current is 
always accompanied by decomposition of the electrolyte, called electrolysis, 
which takes place at the electrodes. All electrolytes in the fused state or in 
solution give rise to ions which conduct the electric current. All acids, 
bases, and salts are electrolytes. See ELECTRIC CURRENT; ELECTRODE; 
ELECTROLYSIS; ION; SOLVENT. 
    Electrolytes are divided into strong and weak electrolytes. Strong 
electrolytes usually contain a stable ionic bond and are wholly ionized in 
solution and usually in the crystalline state. Weak electrolytes are only 
partially ionized in solution. Metallic hydroxides and salts are usually 
strong electrolytes, for example, potassium hydroxide and sodium chloride. A 
weak electrolyte contains a covalent bond which on dissolving in a solvent 
such as water may be transformed into an ionic bond. A solution of a weak 
electrolyte contains both the ionic and the covalent forms in equilibrium; for 
example, acetic acid in water consists of a mixture of undissociated 
molecules, CH3COOH, and of the ions CH3COO- and Hsupp. See CHEMICAL BONDING; 
IONIC EQUILIBRIUM. 
 �34s[Thomas C. Waddington]
�33s 
  
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�6v�2v�32sElectrolytic conductance   �21m �33s 
    The transport of electric charges, under electric potential differences, 
by particles of atomic or larger size. This phenomenon is distinguished from 
metallic conductance, which is due to the movement of electrons. The charged 
particles that carry the electricity are called ions. 
    Positively charged ions are termed cations; the sodium ion, Nasupp, is an 
example. The negatively charged chloride ion, Cl-, is typical of anions. The 
negative charges are identical with those of electrons or integral multiples 
thereof. The unit positive charges have the same magnitude as those of 
electrons but are of opposite sign. Colloidal particles, which may have 
relatively large weights, may be ions, and may carry many positive or negative 
charges. Electrolytic conductors may be solids, liquids, or gases. 
Semiconductors have properties that are intermediate between the metallic and 
electrolytic types. See ELECTROCHEMISTRY; ELECTROMOTIVE FORCE (CELLS). 
    Conductances are usually reported as specific conductances KAPPA, which 
are the reciprocals of the resistances of cubes of the materials, 1 cm (0.39 
in) in each dimension, placed between electrodes 1 cm square, on opposite 
sides. These units are sometimes called mhos, that is, ohms spelled backward. 
Conductances of solutions are usually measured by a method in which a 
Wheatstone bridge is employed. See WHEATSTONE BRIDGE. 
 �34s[Herbert A. Laitinen]
�33s 
  
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�6v�2v�32sElectrolytic tank   �21m �33s 
    A special type of computing machine based on the fact that in ideal fluid 
flow the velocity potential PHI, and in planar flow the stream function 
PSI, satisfy Laplace's equations. Either the velocity potential or stream 
function or both can be related to the voltage through constants m and n 
called the scale factor as in the following equations: 
  
                         *** Analogy A: PHI = mV ***         
                         *** Analogy B: PSI = nV ***

The fluid velocity components are proportional to the respective voltage 
gradients. Every ideal fluid-flow problem, therefore, has an electrical 
counterpart; for this reason the electrolytic tank is also known as an 
electrical tank analogy or potential flow analyzer. Its solution includes 
construction of a scaled electrical-flow model, its installation in an 
electrical tank with proper simulation of the physical boundary conditions 
both on the model contour and on the field boundaries, measurement of the 
electrical variables as required, and finally the translation of the 
measurements by numerical computation into meaningful fluid-flow terms. See 
FLUID-FLOW; HYDRODYNAMICS; MAXWELL'S EQUATIONS. 
 �34s[William B. Brower, Jr.]
�33s 
  

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�6v�2v�32sElectromagnet   �21m �33s 
    A soft-iron core that is magnetized by passing a current through a coil of 
wire wound on the core. Electromagnets are used to lift heavy masses of 
magnetic material and to attract movable magnetic parts of electric devices, 
such as solenoids, relays, and clutches. 
    The difference between cores of an electromagnet and a permanent magnet is 
in the retentivity of the material used. Permanent magnets, initially 
magnetized by placing them in a coil through which current is passed, are made 
of retentive (magnetically ``hard'') materials which maintain the magnetic 
properties for a long period of time after being removed from the coil. 
Electromagnets are meant to be devices in which the magnetism in the cores can 
be turned on or off. Therefore, the core material is nonretentive 
(magnetically ``soft'') material which maintains the magnetic properties only 
while current flows in the coil. All magnetic materials have some retentivity, 
called residual magnetism; the difference is one of degree. See MAGNETIZATION. 
    In an engineering sense the word electromagnet does not refer to the 
electromagnetic forces incidentally set up in all devices in which an electric 
current exists, but only to those devices in which the current is primarily 
designed to produce this force, as in solenoids, relay coils, electromagnetic 
brakes and clutches, and in tractive and lifting or holding magnets and 
magnetic chucks. 
    Electromagnets may be divided into two classes: traction magnets, in which 
the pull is to be exerted over a distance and work is done by reducing the air 
gap; and lifting or holding magnets, in which the material is initially placed 
in contact with the magnet. Examples of the latter type are magnetic chucks 
and circular lifting magnets. For examples of the first type see BRAKE; 
CLUTCH; RELAY; SOLENOID (ELECTRICITY). 
 �34s[Jerome Meisel]
�33s 
  
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3319808
�6v�2v�32sElectromagnetic brake   �21m �33s 
    In commonly encountered electromagnetic brakes, the actuating force is 
applied by an electric current through a solenoid (see illustration). Direct 
current gives greater braking force than alternating current, and is therefore 
used almost exclusively; ac is rectified if used. Usually the electromagnetic 
force releases the brake against a compression spring, thus providing braking 
action if the power fails and overspeed protection when the brake is used with 
dc series motors. 
    The plunger and its stop are cone-shaped to provide a higher force than 
without a stop, but without an abrupt increase in force as the gap closes. 
Because the solenoid force decreases somewhat when the brake is applied and 
the gap is thereby increased, the retraction current is slightly higher than 
the current at which the spring overcomes the solenoid and applies the brake. 
This operation is stable. 
    The solenoid is wound with high impedance for shunt operation across a 
power line. Shunt operation is common where loads and motor speeds vary widely 
or where a compound motor is used. Where a series motor is used for high 
starting torque, the solenoid is wound with low impedance for series operation 
in the circuit of the motor it protects. In this way the brake will be applied 
if the motor starts to run away under light load. See BRAKE; SOLENOID 
(ELECTRICITY).  
 �34s[Harry P. Hale]
�33s 
  
  
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�6v�2v�32sElectromagnetic compatibility   �21m �33s 
    The capability of electronic equipment or systems to be operated in the 
intended electromagnetic environment at design levels of efficiency. The 
electromagnetic environment existing at the input or antenna terminal of a 
receiver is a mixture of signals from many sources. The mixture consists of 
both a desired signal and many undesired signals to which the receiver may be 
sensitive. These undesired or interference signals may come from equipment 
intended to radiate for the purpose of communications or from sources not 
intended to radiate electromagnetic energy but designed to transfer 
information. In addition to mutual interference between systems, the 
possibility of extraneous energy generated within a system interfering with 
its own operation exists. The subject of electromagnetic compatibility deals 
with reducing both the source of interference and its effect on the reception 
of signals. 
 �34s[Stanley H. Cohn]
�33s 
  
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3319808
�6v�2v�32sElectromagnetic field   �21m �33s 
    A changing magnetic field always produces an electric field, and 
conversely, a changing electric field always produces a magnetic field. This 
interaction of electric and magnetic forces gives rise to a condition in space 
known as an electromagnetic field. The characteristics of an electromagnetic 
field are expressed mathematically by Maxwell's equation. See ELECTRIC FIELD; 
ELECTROMAGNETIC RADIATION; ELECTROMAGNETIC WAVE; MAGNETIC FIELD; MAXWELL'S 
EQUATIONS.  
 �34s[Jesse Beams]
�33s 
  
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3319808
�6v�2v�32sElectromagnetic flowmeter �21m �33s
    A volume flow rate meter in which a magnetic field is applied across a
metering tube carrying a flowing liquid, and the voltage generated between two
perpendicularly located electrodes contacting the liquid is measured by a
secondary or transmitter instrument. Since this voltage is strictly
proportional to the average velocity, a linear volume flow signal results.
Practical flow measurements can be made on any liquid that is at all
conductive. The linear scale allows accurate flow measurements over a greater
range than is possible in differential-pressure meters. The lack of
obstruction to fluid flow makes it possible to measure thick slurries and
gummy liquids.	See  ELECTROMAGNETIC INDUCTION; FLOW MEASUREMENT; VOLUME FLOW
RATE METER.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

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�6v�2v�32sElectromagnetic induction   �21m �33s 
    The production of an electromotive force either by motion of a conductor 
through a magnetic field in such a manner as to cut across the magnetic flux 
or by a change in the magnetic flux that threads a conductor. See 
ELECTROMOTIVE FORCE (EMF); FARADAY'S LAW OF INDUCTION; MAGNETIC FLUX. 
    If the flux threading a coil is produced by a current in the coil, any 
change in that current will cause a change in flux, and thus there will be an 
induced emf while the current is changing. This process is called 
self-induction. The emf of self-induction is proportional to the rate of 
change of current. 
    The process by which an emf is induced in one circuit by a change of 
current in a neighboring circuit is called mutual induction. Flux produced by 
a current in a circuit A threads or links circuit B. When there is a change of 
current in circuit A, there is a change in the flux linking coil B, and an emf 
is induced in circuit B while the change is taking place. Transformers operate 
on the principle of mutual induction. See TRANSFORMER. 
    The phenomenon of electromagnetic induction has a great many important 
applications in modern technology. For example, see COUPLED CIRCUITS; 
GENERATOR; INDUCTION HEATING; MICROPHONE; MOTOR; SERVOMECHANISM. 
 �34s[Kenneth V. Manning]
�33s 
  
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�6v�2v�32sElectromagnetic propulsion   �21m �33s 
    Motive power for flight vehicles produced by high-speed discharge of a 
plasma fluid. Together with electrostatic (ion) propulsion, electromagnetic 
propulsion collectively designates several mechanisms capable of attaining 
specific impulses, exceeding by a considerable  margin those of thermal 
propulsion devices. To heat fluids beyond the level of chemical reactions, a 
separate power source must be provided. Much more power can be transferred to 
a body of matter by electrical means than by heating, and material limitations 
impose a principal barrier to the rate of power transfer (propellant heating) 
in thermal propulsion systems. By increasing the power transfer enough to 
ionize the propellant material, it is possible to apply electric and 
electromagnetic driving forces, thereby circumventing the thermal barrier. 
However, different problems are encountered in the resulting electromagnetic 
and electrostatic drives. See ION PROPULSION; SPACECRAFT PROPULSION. 
    Electromagnetic propulsion uses highly ionized gases (plasma), whose 
behavior is determined by interaction between electric currents in the plasma 
and magnetic fields from the vehicle. The discharged plasma is electrically 
neutral. Discharge density is, therefore, not limited by electrostatic forces 
(space charge) present in an ion beam. Consequently, electromagnetic 
propulsion devices offer promise of higher thrust per unit discharge area. See 
MAGNETOHYDRODYNAMICS.  
    Electromagnetic propulsion is adaptable to a wide range of specific 
impulses. The optimization of a propulsion system within the framework of an 
overall vehicle system and a space mission calls for a compromise between 
acceleration and payload capability that best serves the particular purpose. 
    The accelerator provides direction and speed to the plasma flow and thus 
represents the thrust-producing mechanism. Electromagnetic propulsion devices 
can be divided, on the basis of accelerating mechanism, into steady-flow 
systems and pulsed systems. Of the presently recognized electromagnetic 
drives, the magnetoplasmadynamic arcjet shows the greatest potential because 
it offers the widest range of high specific impulses, highest thrust density, 
and longest operating life. 
 �34s[Krafft A. Ehricke]
�33s 
  
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�6v�2v�32sElectromagnetic pumps   �21m �33s 
    Pumps that operate on the principle that a force is exerted upon a 
conductor (the fluid) carrying current in a magnetic field. The high 
electrical conductivity of liquid metals (used as heat-transfer media in some 
nuclear reactors) makes it possible to pump them by electromagnetic means. For 
use in nuclear reactors, where a minimal amount of maintenance is desirable, 
electromagnetic pumps are often preferable to conventional mechanical pumps 
because they have no moving parts, bearings, or seals. Various methods are 
employed to cause current to flow in the liquid metal. See NUCLEAR REACTOR; 
PUMP.  
 �34s[Leonard J. Koch]
�33s 
  
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�6v�2v�32sElectromagnetic radiation   �21m �33s 
    Energy transmitted through space or through a material medium in the form 
of electromagnetic waves. The term can also refer to the emission and 
propagation of such energy. Whenever an electric charge oscillates or is 
accelerated, a disturbance characterized by the existence of electric and 
magnetic fields propagates outward from it. This disturbance is called an 
electromagnetic wave. The frequency range of such waves is tremendous, as is 
shown by the electromagnetic spectrum in the table. The sources given are 
typical, but not mutually exclusive. 
    In theory, any electromagnetic radiation can be detected by its heating 
effect. This method has actually been used over the range from x-rays to 
radio. Ionization effects measured by cloud chambers, photographic emulsions, 
ionization chambers, and Geiger counters have been used in the GAMMA- and 
x-ray regions. Direct photography can be used from the GAMMA-ray to the 
infrared region. Fluorescence is effective in the x-ray and ultraviolet 
ranges. Bolometers, thermocouples, and other heat-measuring devices are used 
chiefly in the infrared and microwave regions. Crystal detectors, vacuum 
tubes, and transistors cover the microwave and radio frequency ranges. See 
ANTENNA (ELECTROMAGNETISM); DIFFRACTION; ELECTROMAGNETIC WAVE; GAMMA RAYS; 
HEAT RADIATION; INFRARED RADIATION; INTERFERENCE OF WAVES; LIGHT; MAXWELL'S 
EQUATIONS; MICROWAVE; POLARIZATION OF WAVES; RADIATION; RADIO-WAVE 
PROPAGATION; REFLECTION OF ELECTROMAGNETIC RADIATION; REFRACTION OF WAVES; 
SCATTERING OF ELECTROMAGNETIC RADIATION; TRANSMISSION LINES; ULTRAVIOLET 
RADIATION; WAVEGUIDE; WAVE MOTION; X-RAYS. 
 �34s[William R. Smythe]
�33s 
  
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�6v�2v�32sElectromagnetic wave   �21m �33s 
    A disturbance, produced by the acceleration or oscillation of an electric 
charge, which has the characteristic time and spatial relations associated 
with progressive wave motion. A system of electric and magnetic fields moves 
outward from a region where electric charges are accelerated, such as an 
oscillating circuit or the target of an x-ray tube. The wide wavelength range 
over which such waves are observed is shown by the electromagnetic spectrum. 
The term electric wave, or Hertzian wave, is often applied to electromagnetic 
waves in the radar and radio range. Electromagnetic waves may be confined in 
tubes, such as wave guides, or guided by transmission lines. They were 
predicted by J. C. Maxwell in 1864 and verified experimentally by H. Hertz in 
1884. See ELECTROMAGNETIC RADIATION; ELECTROMAGNETIC WAVE TRANSMISSION. 
 �34s[William R. Smythe]
�33s 
  
  
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�6v�2v�32sElectromagnetic wave transmission �21m �33s
    The transmission of electrical energy by wires, the broadcasting of radio
signals, and the phenomenon of visible light are all examples of the
propagation of electromagnetic energy. Electromagnetic energy travels in the
form of a wave. Its speed of travel is approximately 3 x 10 sup 8 m/s (186,000
mi/s) in a vacuum and is somewhat slower than this in liquid and solid
insulators. An electromagnetic wave does not penetrate far into an electrical
conductor, and a wave that is incident on the surface of a good conductor is
largely reflected. See ELECTROMAGNETIC WAVE.
    The waves emitted from a source are oscillatory in character and are
described in terms of their frequency of oscillation. The method of generating
an electromagnetic wave depends on the frequency used, as do the techniques of
transmitting the energy to another location and of utilizing it when it has
been received. The communication of information to a distant point is
generally accomplished through the use of electromagnetic energy as a carrier.
    The illustration shows the configuration of the electric and magnetic
fields about a short vertical antenna in which flows a sinusoidal current. The
picture applies either to an antenna in free space (in which case the
illustration shows only the upper half of the fields), or to an antenna
projecting above the surface of a highly conducting plane surface. In the
latter case the conducting plane represents to a first approximation the
surface of the Earth. The fields have symmetry about the axis through the
antenna. For pictorial simplicity only selected portions of the fields are
shown in this illustration. The magnetic field is circular about the antenna,
is perpendicular at every point to the direction of the electric field, and is
proportional in intensity to the magnitude of the electric field, as in a
plane wave. All parts of the wave travel radially outward from the antenna
with the velocity equal to that of a plane wave in the same medium.
    Often it is desired to concentrate the radiated energy into a narrow beam.
This can be done either by the addition of more antenna elements or by placing
a large reflector, generally parabolic in shape, behind the antenna. The
production of a narrow beam requires an antenna array, or alternatively a
reflector, that is large in width and height compared with a wavelength. The
very narrow and concentrated beam that can be achieved by a laser is made
possible by the extremely short wavelength of the radiation as compared with
the cross-sectional dimensions of the radiating system.
    The ground is a reasonably good, but not perfect, conductor; hence, the
actual propagation over the surface of the Earth will show a more rapid
decrease of field strength than that for a perfect conductor. Irregularities
and obstructions may interfere. In long-range transmission the spherical shape
of the Earth is important. Inhomogeneities in the atmosphere refract the wave
somewhat. For long-range transmission, the ionized region high in the
atmosphere known as the Kennelly-Heaviside layer, or ionosphere, can act as a
reflector. See RADIO-WAVE PROPAGATION.
    When an electromagnetic wave is introduced into the interior of a hollow
metallic pipe of suitably large cross-sectional dimensions, the energy is
guided along the interior of the pipe with comparatively little loss. The most
common cross-sectional shapes are the rectangle and the circle. The
cross-sectional dimensions of the tube must be greater than a certain fraction
of the wavelength; otherwise the wave will not propagate in the tube. For this
reason hollow waveguides are commonly used only at wavelengths of 10 cm or
less (frequencies of 3000 MHz or higher). A dielectric rod can also be used as
a waveguide. Such a rod, if of insufficient cross-sectional dimensions, can
contain the electromagnetic wave by the phenomenon of total reflection at the
surface. See WAVEGUIDE.

    Electromagnetic energy can be propagated in a simple mode along two
parallel conductors. Such a waveguiding system is termed a transmission line.
Three common forms are the coaxial cable, two-wire line, and parallel strip
line. As the wave propagates along the line, it is accompanied by currents
which flow longitudinally in the conductors. These currents can be regarded as
satisfying the boundary condition for the tangential field at the surface of
the conductor. The conductors have a finite conductivity, and so these
currents cause a transformation of electrical energy into heat. The energy
lost comes from the stored energy of the wave, and so the wave, as it
progresses, diminishes in amplitude. The conductors are necessarily supported
by insulators which are imperfect and cause additional attenuation of the
wave. See COAXIAL CABLE; TRANSMISSION LINES.
 �34s[Walter C. Johnson]
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�6v�2v�32sElectromagnetism   �21m �33s 
    The branch of science dealing with the observations and laws relating 
electricity to magnetism. Electromagnetism is based upon the fundamental 
observations that a moving electric charge produces a magnetic field and that 
a charge moving in a magnetic field will experience a force. 
    The magnetic field produced by a current is related to the current, the 
shape of the conductor, and the magnetic properties of the medium around it by 
Ampere's law. See AMPERE'S LAW. 
    The magnetic field at any point is described in terms of the force that it 
exerts upon a moving charge at that point. The electrical and magnetic units 
are defined in terms of the ampere, which in turn is defined from the force of 
one current upon another. See AMPERE. 
    The association of electricity and magnetism is also shown by 
electromagnetic induction, in which a changing magnetic field sets up an 
electric field within a conductor and causes the charges to move in the 
conductor. See EDDY CURRENT; ELECTRICITY; ELECTROMAGNET; ELECTROMAGNETIC 
INDUCTION; FARADAY'S LAW OF INDUCTION; HALL EFFECT; INDUCTANCE; LENZ'S LAW; 
MAGNETIC CIRCUITS; MAGNETIC FIELD; MAGNETIC FLUX; MAGNETIC INDUCTION; 
MAGNETISM; MAGNETOMOTIVE FORCE; MAXWELL'S EQUATIONS; RELUCTANCE. 
 �34s[Kenneth V. Manning]
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�6v�2v�32sElectrometallurgy   �21m �33s 
    The branch of process metallurgy dealing with the use of electricity for 
smelting or refining of metals. The electrochemical effect of an electric 
current brings about the reduction of metallic compounds, and thereby the 
extraction of metals from their ores (electrowinning) or the purification of 
the metals (electrorefining). See ELECTROREFINING; ELECTROWINNING. 
    In other metallurgical processes, electrically produced heat is utilized 
in smelting, refining, or alloy manufacturing. For a discussion of 
electrothermics, that is, the theory and applications of electric heating to 
metallurgy, see ELECTRIC FURNACE; ELECTRIC HEATING; ELECTROCHEMICAL PROCESS; 
STEEL MANUFACTURE. 
 �34s[Paul Duby]
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�6v�2v�32sElectrometer �21m �33s
    A highly sensitive instrument used to measure a voltage, without drawing
appreciable current, by measuring the electrostatic force exerted between two
bodies that are charged with the voltage. In one form two suspended parallel
strips of gold leaf are spread out at an angle proportional to the voltage to
which they are charged; the amount of movement is measured with a microscope
having a calibrated scale. In a string electrometer the gold strips are
replaced by lightly stretched metallized quartz fibers. Electrometers
involving mechanical movements have largely been replaced by electronic
devices, which are essentially voltage-measuring amplifiers having such a high
input resistance (usually above 10 sup 10 ohms) that they draw practically no
current. See ELECTROSCOPE; VOLTAGE MEASUREMENT; VOLTMETER.
 �34s[John Markus]
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�6v�2v�32sElectromotive force (cells) �21m �33s
    When two dissimilar electrodes are connected through an external
conducting circuit, a difference in electric potential exists between them.
Although this difference in potential is sometimes called the potential
difference of the electrode couple, it is customary to say that the galvanic
cell composed of the two dissimilar electrodes exhibits an electromotive
(driving) force. This electromotive force (emf) is the resultant of the
relative potential forces of the two dissimilar electrodes at which
electrochemical reactions occur during cell operation. The two dissimilar
electrodes need not be of unlike metals; for example, the metals may both be
copper, but with the two coppers immersed in solutions of different
concentration or composition. Likewise, both electrodes may be of the same
gas, but with the pressure of the gas different at the two electrodes. See
ELECTRODE POTENTIAL.
    Galvanic cells are of two general types, reversible and irreversible. If
the chemical reactions at the electrodes can be exactly reversed by reversals
in the direction of the current flow at the electrodes, the cell is said to be
reversible; if the chemical reactions cannot be reversed, or if entirely
different reactions occur on current reversal, the cell is then of the
irreversible type. An example of a reversible cell is the familiar lead-acid
storage cell (or battery), widely used for a variety of purposes, including
starting, ignition, and lighting for automobiles. A cell prepared by immersing
zinc and platinum electrodes in perchloric acid results in an irreversible
cell, since the electrode reactions at each electrode for the charge differ
from those obtained for the discharge. See STORAGE BATTERY.
    In measuring the emf of a cell, a reference cell of known emf must be
available to effect a comparison. This reference cell should have an emf that
is known in terms of physical laws and units, and not one chosen arbitrarily.
The standard cells for this purpose are the cadmium amalgam standard cells of
the saturated type proposed by Edward Weston in 1892. This type of cell is the
most reversible galvanic cell known and retains a constant emf to within a few
microvolts for many decades. This cell consists of a cadmium amalgam anode
(negative element), a mercury-mercurous sulfate cathode (positive element),
and a saturated solution of cadmium sulfate containing crystals of CdSO4
CTR DOT 8/3H2O. See BATTERY; CALOMEL ELECTRODE; ELECTROCHEMISTRY; POTENTIOMETER
(VOLTAGE METER); SALT BRIDGE.
 �34s[Walter J. Hamer]
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�6v�2v�32sElectromotive force (emf)   �21m �33s 
    The electromotive force, represented by the symbol f, around a closed path 
in an electric field is the work per unit charge required to carry a small 
positive charge around the path. It may also be defined as the line integral 
of the electric intensity around a closed path in the field. The abbreviation 
emf is preferred to the full expression since emf, also called electromotance, 
is not really a force. The term emf is applied to sources of electric energy 
such as batteries, generators, and inductors in which current is changing. 
    The magnitude of the emf of a source is defined as the electrical energy 
converted inside the source to some other form of energy (exclusive of 
electrical energy converted irreversibly into heat), or the amount of some 
other form of energy converted in the source into electrical energy, when a 
unit charge flows around the circuit containing the source. 
    For a discussion of motional emf see ELECTROMAGNETIC INDUCTION. For 
information on back, or counter, emf see DIRECT-CURRENT MOTOR. See also 
ELECTROMOTIVE FORCE (CELLS); MAGNETOMOTIVE FORCE. 
 �34s[Ralph P. Winch]
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�6v�2v�32sElectromyogram   �21m �33s 
    A record of the electrical response of a muscle to stimulation. Muscular 
contraction results from the rhythmical discharge of motor nerve cells, each 
of which controls a group of 10-500 muscle fibers, known as a motor unit. The 
intensity of contraction reflects the number of units activated among the many 
hundreds composing a muscle. Each nerve impulse in a series sets up a brief 
cycle of contraction in the corresponding muscle fibers. Some conditions 
synchronize the rhythms of many units (clonus) or group the corresponding 
impulses in regular bursts (tremor). The electromyogram is useful in the study 
of diseases that reduce the number of motor units (poliomyelitis or nerve 
injury), or diminish the number of muscle fibers in each unit (dystrophy or 
myasthenia), or abolish the nerve supply and leave isolated muscle fibers to 
twitch weakly, independently, and spontaneously (fibrillation). 
 �34s[Derek E. Denny-Brown]
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�6v�2v�32sElectron �21m �33s
    An elementary particle which is the negatively charged constiuent of
ordinary matter. The electron is the lightest known particle which possesses
an electric charge. Its rest mass is msube CONGRUENT 9.1 x 10- sup 2 sup 8 g,
about 1/1836 of the mass of the proton or neutron, which are, respectively, the
positively charged and neutral constituents of ordinary matter. Discovered in
1895 by J. J. Thomson in the form of cathode rays, the electron was the first
elementary particle to be identified. See ELECTRIC CHARGE; ELEMENTARY
PARTICLE; NUCLEAR STRUCTURE.
    The charge of the electron is -e CONGRUENT -4.8 x 10- sup 1 sup 0 esu= -1.6
x 10- sup 1 sup 9 coulomb. The sign of the electron's charge is negative by
convention, and that of the equally charged proton is positive. This is a
somewhat unfortunate convention, because the flow of electrons in a conductor
is thus opposite to the conventional direction of the current.
    Electrons are emitted in radioactivity (as beta rays) and in many other
decay processes; for instance, the ultimate decay products of all mesons are
electrons, neutrinos, and photons, the meson's charge being carried away by
the electrons. The electron itself is completely stable. Electrons contribute
the bulk to ordinary matter; the volume of an atom is nearly all occupied by
the cloud of electrons surrounding the nucleus, which occupies only about
10- sup 1 sup 3 of the atom's volume. The chemical properties of ordinary matter
are determined by the electron cloud. See BETA RAYS; MESON; RADIOACTIVITY.
    The electron obeys the Fermi-Dirac statistics, and for this reason is
often called a fermion. One of the primary attributes of matter,
impenetrability, results from the fact that the electron, being a fermion,
obeys the Pauli exclusion principle; the world would be completely different
if the lightest charged particle were a boson, that is, a particle that obeys
Bose-Einstein statistics. See BOSE-EINSTEIN STATISTICS; EXCLUSION PRINCIPLE;
FERMI-DIRAC STATISTICS; POSITRON.
 �34s[Charles J. Goebel]
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    Magnetic moment.  The electron has magnetic properties by virtue of (1)
its orbital motion about the nucleus of its parent atom and (2) its rotation
about its own axis. The magnetic properties are best described through the
magnetic dipole moment associated with 1 and 2. The classical analog of the
orbital magnetic dipole moment is the dipole moment of a small
current-carrying circuit. The electron spin magnetic dipole moment may be
thought of as arising from the circulation of charge, that is, a current,
about the electron axis; but a classical analog to this moment has much less
meaning than that to the orbital magnetic dipole moment. The magnetic moments
of the electrons in the atoms that make up a solid give rise to the bulk
magnetism of the solid.

    Spin.  That property of an electron which gives rise to its angular
momentum about an axis within the electron. Spin is one of the permanent and
basic properties of the electron. Both the spin and the associated magnetic
dipole moment of the electron were postulated by G. E. Uhlenbeck and S.
Goudsmit in 1925 as necessary to allow the interpretation of many observed
effects, among them the so-called anomalous Zeeman effect, the existence of
doublets (pairs of closely spaced lines) in the spectra of the alkali atoms,
and certain features of x-ray spectra. See SPIN (QUANTUM MECHANICS).
    The spin quantum number is s, where s is always **. This means that the
component of spin angular momentum along a preferred direction, such as the
direction of a magnetic field, is PLUS/MINUSJPLANCK'S CONSTANT where PLANCK'S
CONSTANT = h/2PI and h is Planck's constant. The spin angular momentum of the
electron is not to be confused with the orbital angular momentum of the
electron associated with its motion about the nucleus. In the latter case the
maximum component of angular momentum along a preferred direction is lPLANCK'S
CONSTANT, where l is the angular momentum quantum number and may be any
positive integer or zero. See QUANTUM NUMBERS.
    The electron has a magnetic dipole moment by virtue of its spin. The
approximate value of the dipole moment is the Bohr magneton MU0 which is equal
to eh/4PImc = 9.27 x 10- sup 2 sup 1 erg/ oersted, where e is the electron
charge measured in electrostatic units, m is the mass of the electron, and c is
the velocity of light. (In SI units, MU0 = 9.27 x 10- sup 2 sup 4 joule/tesla.)
The orbital motion of the electron also gives rise to a magnetic dipole moment
MUsubl that is equal to MU0 when l = 1. 
 �34s[Arthur Rich]
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�6v�2v�32sElectron affinity   �21m �33s 
    The amount of energy release when an electron at rest is captured by a 
species M, producing the negative ion M-. The electron affinity of a species M 
can also be thought of as the ionization potential of the negative ion M-. 
Stated in terms of a chemical equation, the electron affinity of a species M 
is equal to the exothermocity of the reaction e + M -> M-, where the negative 
ion M- is left in its lowest electronic, vibrational, and rotational state. 
See IONIZATION POTENTIAL. 
    If the electron affinity of M is negative, the M- ion is unstable with 
respect to decomposition into M + e. Most atoms have positive electron 
affinities, even though there is no net Coulomb attraction between the 
electron and the atom until the electron is close enough to be ``a part of the 
atom.'' The simple rules of chemical valency provide a qualitative guide to 
the magnitude of electron affinities. Thus the noble gases, which have a 
filled outer electronic shell and are chemically inert, are not capable of 
binding an additional electron to form a negative ion. The largest electron 
affinities are possessed by the halogens, atoms which require only one 
additional electron to fill the valence shell. See VALENCE. 
    The major exception to this concept is that multiply charged negative ions 
- for example, Osup2-, one of many multiply charged negative ions which are 
stable in solution - are not stable in the gas phase. The ability to place 
more than one additional electron in the valence shell of a neutral atom or 
molecule appears to come from the medium; the solvent shell surrounding the 
ion in liquid solutions and the amorphous or crystalline region surrounding 
the ion in solids. 
 �34s[W. C. Lineberger]
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�6v�2v�32sElectron beam welding   �21m �33s 
    A process wherein coalescence is produced by heat obtained from a 
concentrated beam composed primarily of high-velocity electrons impinging upon 
the surfaces to be joined. Electron beam welding equipment, which utilizes a 
heat source of electrons accelerated by an electric field to extremely high 
speeds and focused to a sharp beam by electrostatic or electromagnetic fields, 
is used for welding a wide range of metals in thicknesses ranging from foils 
to extremely thick sections. A weld of very high purity may be obtained by 
this technique. Of all welding techniques, electron beam welding seems most 
suited for use in outer space, where it has been successfully demonstrated in 
United States and Soviet tests. 
 �34s[Mel M. Schwartz]
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�6v�2v�32sElectron capture   �21m �33s 
    The process in which an atom or ion passing through a material medium 
either loses or gains one or more orbital electrons. In the passage of charged 
particles (defined here as nuclei having more or less than Z atomic electrons, 
where Z is the atomic number) through matter, the capture (and loss) of 
electrons is an important process in the slowing down of the particles and 
therefore has a strong influence on their range. Thus a neutral hydrogen atom 
loses only about half as much energy per centimeter as the positively charged 
proton in passing through matter consisting of light elements. 
    For the ordinary charged particles (alpha particles and protons) the 
capture process is important only at low energies, when the particle velocity 
is of the order of electron velocities in the stopping material, and thus is 
important at the end of the range. For fission fragments, however, which 
initially have a large excess of positive charges, electron capture occurs 
immediately and continues throughout the slowing-down process. This fact 
causes the energy-loss mechanisms at the latter part of the range to be 
different for fission fragments and protons or ALPHA-particles.  See  NUCLEAR 
FISSION.  
    The nuclear capture of electrons (K capture) occurs by a process quite 
different from atomic capture and is in fact a consequence of the general beta 
interaction. This general interaction includes BETA- decay (the oldest known 
beta transformation and hence the name), BETAsupp decay (or positron decay), 
and K capture, the latter so called because the electron captured by the 
nucleus is taken from the K shell (the shell nearest the nucleus) of atomic 
electrons. A second-order process, called L capture, can also occur, in which 
(to speak pictorially and thus somewhat imprecisely) an s electron (from the K 
shell) is captured with the simultaneous transition of a p electron (from the 
L shell) to the K shell with the emission of gamma radiation.  See  
RADIOACTIVITY.  
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sElectron configuration �21m �33s
    The orbital arrangement of an atom's electrons. Negatively charged
electrons are attracted to a positively charged nucleus to form an atom or
ion. Although such bound electrons exhibit a high degree of quantum-mechanical
wavelike behavior, there still remain particle aspects to their motion. Bound
electrons occupy orbitals that are somewhat concentrated in spatial shells
lying at different distances from the nucleus. As the set of electron energies
allowed by quantum mechanics is discrete, so is the set of mean shell radii.
Both these quantized physical quantities are primarily specified by integral
values of the principal, or total, quantum number n. The full electron
configuration of an atom is correlated with a set of values for all the
quantum numbers of each and every electron. In addition to n, another
important quantum number is l, an integer representing the orbital angular
momentum of an electron in units of h/2PI, where h is Planck's constant. The
values 1, 2, 3, 4, 5, 6, 7 for n and 0, 1, 2, 3 for l together suffice to
describe the electron configurations of all known normal atoms and ions, that
is, those that have their lowest possible values of total electronic energy.
The first seven shells are also given the letter designations K, L, M, N, O,
P, and Q respectively. Electrons with l equal to 0, 1, 2, and 3 are designated
s, p, d, and f, respectively. See QUANTUM MECHANICS; QUANTUM NUMBERS.
    In any configuration the number of equivalent electrons (same n and l) is
indicated by an integral exponent (not a quantum number) attached to the
letters s, p, d, and f. According to the Pauli exclusion principle, the
maximum is s sup 2, p sup 6, d sup 1 sup 0, and f sup 1 sup 4. See EXCLUSION
PRINCIPLE.
    An electron configuration is categorized as having even or odd parity,
according to whether the sum of p and f electrons is even or odd. Strong
spectral lines result only from transitions between configurations of unlike
parity. See PARITY (QUANTUM MECHANICS).
    Insofar as they are known from spectroscopic investigations, the electron
configurations characteristic of the normal or ground states of the first 103
chemical elements are shown in the table. In the next-to-last column of the
table, the spectral term of the energy level with lowest total electronic
energy is shown. The main part of the term symbol is a capital letter, S, P,
D, F, and so on, that represents the total electronic orbital angular
momentum. Attached to this is a superior prefix, 1, 2, 3, 4, and so on, that
indicates the multiplicity, and an anterior suffix, 0, 1/2, 1, 3/2, 2, 5/2, and
so on, that shows the total angular momentum, or J value, of the atom in the
given state. A sign DG above the J value signifies that the spectral term and
electron configuration have odd parity.
    The last column of the table presents the first ionization potential of
the atom, the energy required to remove from an atom its least firmly bound
electron and transform a neutral atom into a singly charged ion.  See  ATOMIC
STRUCTURE AND SPECTRA; IONIZATION POTENTIAL.
 �34s[James E. Bayfield]
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�6v�2v�32sElectron diffraction �21m �33s
    The phenomenon associated with interference processes that occur when
electrons are scattered by atoms to form diffraction patterns. The wave
character of electrons is shown most strikingly, and doubtless most
conclusively, by the phenomena of interference. For this reason, the
diffraction of electrons presents the most obvious confirmation of quantum
mechanics. Because of the dependence of the diffraction pattern on the
distances between the atoms, electron diffraction is also an important tool
for the study of the structure of crystals and of free molecules, analogous to
the use of x-rays for these purposes. See X-RAY CRYSTALLOGRAPHY; X-RAY
DIFFRACTION.

    According to energy E = eV, two major techniques of structure analysis are
distinguished: low-energy electron diffraction (LEED) [E =< 5-500 eV] and
high-energy electron diffraction (HEED) [E =< 5-500 keV]; medium-energy
electron diffraction (MEED) [E =< 500  e V-5keV] is of little importance.
Unlike neutrons and x-rays, electrons penetrate matter	only for a very short
distance before they lose energy (by inelastic scattering) or are scattered
elastically (diffracted). Due to the energy loss, the wavelength of an
electron changes so that it can no longer interfere with the other incident
electrons. This loss of coherence occurs in condensed matter typically after
mean free paths for inelastic scattering ranging from several tenths of
nanometers at low energies (E =< 50 eV) to several tens of nanometers at high
energies, which determines the application range of LEED and HEED. See
COHESION (PHYSICS); DE BROGLIE WAVELENGTH; DIFFRACTION; INTERFERENCE OF WAVES;
MEAN FREE PATH; NONRELATIVISTIC QUANTUM THEORY; QUANTUM MECHANICS.

    Low-energy electron diffraction.  LEED is used mainly for the study of the
structure of single crystal surfaces and of processes on such surfaces that
are associated with changes in the alteral periodicity of the surface. A
monochromatic, nearly parallel electron beam of 10-sup4 to 10-sup3 m in
diameter strikes the surface, usually at normal incidence. The elastically
backscattered electrons are separated from all other electrons by a retarding
field and detected with a suitable movable collector or, more frequently -
after acceleration to about 5 keV energy - on a hemispherical fluorescent
screen with the crystal in its center. Typical LEED patterns obtained in the
second detection mode are shown in the illustration. The numbers are indices
(h,k) labeling diffraction spots of the surfaces.
    An important contribution of LEED, is to the understanding of
chemisorption, which precedes corrosion and, in many cases, epitaxy. Here, not
only the structure of many adsorption systems - mainly of gases such as H2,
O2, N2 and CO on metals such as tungsten, nickel, copper, and platinum, or
metals on other metals and semiconductors - has been studied, but also the
kinetics of the adsorption and desorption process as well as changes in the
adsorption layer upon heating. The combination of LEED with Auger electron
spectroscopy and with work-function measurements has proven particularly
powerful in these studies, because such methods give the coverage and
information on the location of the adsorbed atoms normal to the surface.
Combining LEED with other complementary techniques such as ion scattering
spectroscopy, electron energy loss spectroscopy, or photoelectron spectroscopy
has become increasingly popular and can enable the elimination of ambiguities
in the interpretation of many LEED results. See ADSORPTION; CORROSION;
ELECTRON SPECTROSCOPY; SURFACE PHYSICS.

    High-energy electron diffraction.  HEED is used mainly for the study of
the structure of thin foils, films, and small particles (thickness or diameter
of 10-sup9 to 10-sup6 m), of molecules, and also of the surfaces of
crystalline materials. A monochromatic, usually nearly parallel, electron beam
with a diameter of 10-sup3 to 10-sup8 m is incident on the target. The
forward-scattered electrons (backscattering is negligible) are detected by
means of a fluorescent screen, a photoplate, or some other current-sensitive
detector, usually without the inelastically scattered electrons being
eliminated.

    The availability of ultra-high vacuum instruments has made it possible to
study the structure of surfaces under clean conditions by reflection HEED
(RHEED). Many experiments have confirmed the theoretical expectation that
RHEED has a sensitivity comparable to that of LEED. Therefore, similar to
LEED, RHEED can be used for the determination of the lateral arrangement of
the atoms in the topmost layers of the surface, including the structure of
adsorbed layers.
    Like LEED, RHEED gives little information on the chemical nature of the
atoms that produce the diffraction pattern. Therefore, RHEED has been combined
with analytical tools, such as Auger electron spectroscopy or x-ray emission
spectroscopy, in the same system. A second limitation inherent to both RHEED
and LEED is the difficulty of determining the atomic positions normal to the
surface. This is a consequence of multiple scattering effects and of
absorption.
    In scanning HEED (SHEED) the diffracted electrons are not recorded on
photographic film but are directly measured electronically with sensitive
detectors. By moving the detector across the diffraction pattern or by
deflecting the diffracted electrons across a stationary detector (scanning),
the intensity distribution in the diffraction pattern can be displayed
quantitatively on an XY recorder. The main application of SHEED is in the
study of processes which are accompanied by changes of the intensity
distribution, such as the growth of thin films and annealing and corrosion
processes.
    The technological importance of thin film and interface devices has led to
an upsurge of thin film growth studies by conventional transmission HEED
(THEED), usually combined with transmission microscopy. Information obtained
this way has been mainly on the orientation of the crystallites composing the
film.
 �34s[E. Bauer]
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    Gases and liquids.	Electron diffraction in gases and liquids is similar
in principle to that in solids; the differences arise from the lack in gases
and liquids of any highly regular arrangement of the component atoms. In gases
the low density makes it possible to study diffraction by individual atoms and
molecules. The results obtained from monatomic gases represent the density of
electronic charge in the atom as a function of the distance from the nucleus.
The results from gaseous polyatomic molecules represent the equilibrium
distances between the atomic nuclei and the average amplitudes of vibration
associated with these distances. Liquids have been studied much less
thoroughly, both in theory and in practice, than have gases. See SCATTERING
EXPERIMENTS (ATOMS AND MOLECULES); SCATTERING EXPERIMENTS (NUCLEI).
 �34s[Lawrence O. Brockway]
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�6v�2v�32sElectronegativity �21m �33s
    According to L. Pauling, ``the power of an atom in a molecule to attract
electrons to itself.'' Quantitative definitions and scales of
electronegativity have been based not on electron distribution itself but on
properties which were assumed to reflect electronegativity.
    The electronegativity of an element depends upon its valence state and
thus is not an invariant atomic property. As an example, the
electron-withdrawing ability of an spsupn hybrid orbital centered on carbon
and directed toward hydrogen increases as the percentage of s character in the
orbital increases in the series ethane < ethylene < acetylene. Thus, according
to this concept of orbital electronegativity, each element exhibits a range of
electronegativity values.
    The original scale, proposed by Pauling in 1932, is based upon the
difference between the energy of the A-B bond in the compound ABsubn and the
mean of the energies of the homopolar bonds A-A and B-B (see table). R. S.
Mulliken proposed that the electronegativity of an element is given by the
average of the valence-state ionization potential and electron affinity. The
Mulliken approach is consistent with Pauling's original definition and gives
orbital electronegativities, not invariant atomic electronegativities.
Electronegativity was defined by A. L. Allred and E. G. Rochow as the force of
attraction between a nucleus and an electron from a bonded atom. A
quantum-defect electronegativity scale has been developed from potentials
based on atomic spectral data, and a nonempirical scale has been calculated by
an ab initio method using floating gaussian orbitals.
    Other methods for calculating electronegativities utilize such observables
as bond-stretching force constants, electrostatic potentials, spectra, and
covalent radii. The measurement of electronegativities involves observations of
properties dependent upon electron distribution. Close agreement of
electronegativity values obtained from measurements of several diverse
properties lends confidence and utility to the concept.
 �34s[Albert Louis Allred]
�33s

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�6v�2v�32sElectron emission   �21m �33s 
    The liberation of electrons from a substance into vacuum. Since all 
substances are built up of atoms and since all atoms contain electrons, any 
substance may emit electrons; usually, however, the term refers to emission of 
electrons from the surface of a solid. 
    The process of electron emission is analogous to that of ionization of a 
free atom, in which the latter parts with one or more electrons. The energy of 
the electrons in an atom is lower than that of an electron at rest in vacuum; 
consequently, in order to ionize an atom, energy must be supplied to the 
electrons in some way or other. By the same token, a substance does not emit 
electrons spontaneously, but only if some of the electrons have energies equal 
to, or larger than, that of an electron at rest in vacuum. This may be 
achieved by various means, such as by heating, irradiation with light 
(photoemission), bombardment with charged particles (secondary emission), or 
use of a strong electric field (field, or cold, emission). See FIELD EMISSION; 
PHOTOEMISSION; SECONDARY EMISSION; THERMIONIC EMISSION. 
 �34s[Adrianus J. Dekker]
�33s 
  
  
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�6v�2v�32sElectronic air cleaner   �21m �33s 
    A type of air filter limited primarily to applications requiring high 
air-cleaning efficiency. These devices operate on the principle of passing the 
airstream through an ionization field where a 12,000-V potential imposes a 
positive charge on all airborne particles. The ionized particles are then 
passed between aluminum plates, alternately grounded and connected to a 6000-V 
source, and are precipitated onto the grounded plates. See AIR FILTER. 
 �34s[Morton A. Bell]
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�6v�2v�32sElectronic display �21m �33s
    An electronic component used to convert electric signals into visual
imagery in real time suitable for direct interpretation by a human operator.
It serves as the visual interface between human and machine. The visual
imagery is processed, composed, and optimized for easy interpretation and
minimum reading error. The electronic display is dynamic in that it presents
information within a fraction of a second from the time received and
continuously holds that information, using refresh or memory techniques, until
new information is received. The image is created by electronically making a
pattern from a visual contrast in brightness on the display surface without
the aid of mechanical or moving parts.
    Electronic display is widely used for presentation of graphs, symbols and
alphanumerics, and video pictures. Electronic displays are rapidly replacing
traditional mechanical and hardcopy (paper) means for presenting information.
This is due to the increased use of computers and microprocessors, both of
which employ low-cost large-scale integration (LSI) electronics, and digital
mass memories. The success of the hand-held calculator is directly
attributable to the availability of low-cost LSI electronics and low-cost
electronic numeric displays. See CALCULATORS; COMPUTER; COMPUTER GRAPHICS;
INTEGRATED CIRCUITS; MICROCOMPUTER; MICROPROCESSOR.
    Electronic transducers and four-digit (or more) flat-panel displays are
replacing the galvanometer movement, thermometer scale, barometer movement,
and other forms of scientific instrumentation. Large signs, arrival and
departure announcements, and scoreboards use electronic means to portray
changing messages and data. One of the major electronic displays applications
is in home color television. The computer terminal using a cathode-ray tube is
one of the most important industrial applications of electronic displays. The
standard computer terminal displays 25 lines of 80 characters, for a total of
2000 characters, which correspond to a typed page.
    Electronic displays can be categorized into four classifications, as shown
in the table. Each classification is defined by natural technical boundaries
and cost considerations. The categorization is useful in visualizing the
extent to which electronic displays are used.
    The primary applications of the cathode-ray tube are in home entertainment
television, scientific and electrical engineering oscilloscopes, radar
display, and alphanumeric and graphic electronic displays. See CATHODE-RAY
TUBE; OSCILLOSCOPE; RADAR; TELEVISION.
    Because of the depth dimension of the cathode-ray tube, there has been a
concentrated effort to invent a flat-panel display. A primary motivating
factor has been to achieve a flat television receiver which could be hung on a
wall. The electrical phenomena most extensively developed for flat-panel
displays are gas discharge (plasma), electroluminescence, light-emitting
diode, cathodoluminescence, and liquid crystallinity. The cost of flat-panel
displays is higher than cathode-ray tubes on a per-character basis for higher
information content displays such as the 2000-character computer terminal. The
flat-panel technologies are utilized extensively in portable displays and
numeric displays using several hundred characters and less. See
CATHODOLUMINESCENCE; ELECTROLUMINESCENCE; LIGHT-EMITTING DIODE; LIQUID
CRYSTALS.
    Color can be created on a cathode-ray tube equipped with a shadow mask
duplicating quite closely all colors that occur in nature. This is done in the
cathode-ray tube by using three different electron guns and three phosphors in
a triad of red, green, and blue on the screen at each pixel. The shadow mask
is a metal screen with a hole for each pixel. It is located in the path of the
beam between the deflection area and phosphor screen. Electron beams from each
of three guns are constrained by each shadow mask hole to hit each respective
phosphor dot.
    Penetration phosphors are also used to create color on cathode-ray tube
displays to eliminate the need for the shadow mask and extra guns. However,
the color is limited and the brightness is low. Normally, two phosphors are
placed on the screen in two layers or in microspheres of two layers. The gun
and cathode-ray-tube anode are operated in two energy states to produce either
a high-energy or a low-energy electron beam switchable in time.
    Monochromatic as well as full color is readily produced by flatpanel
display technologies. Full color is produced by using a three-color triad of
red, green, and blue for each pixel. Full color is very important for
entertainment television displays. Most industrial electronic displays do not
need or use full color. The most efficient monochrome from each display
technology is normally used; for example, red for light-emitting diode, orange
for gas discharge, and orange-yellow for electroluminescence. Limited color
displays are sometimes used in special industrial applications such as
aircraft weather radar and artificial horizons, computer-aided multilayer
circuit design, earth resources studies, air-route traffic control, and
medical electronic displays. In all of these applications, the display
instrument is usually a cathode-ray tube using the shadow-mask color technique.
    The essence of electronic displays is based upon the ability to turn on
and off individual picture elements (pixels; Fig. 1). The pixel is the
smallest controllable element of the display. A typical
high-information-content display will have a quarter million pixels in an
orthogonal array, each under individual control by the electronics. The pixel
resolution is normally just at or below the resolving power of the eye. Thus a
good-quality picture can be created from a pattern of activated pixels. The
pixel concept for electronic displays has evolved from the modern flat-panel
display technologies and digital electronics.
    With flat-panel displays and cathode-ray-tube digitial-raster displays,
alphanumeric character fonts are created by turning  on the appropriate pixels
in an array. One standard size is a 5 x 7 array with two pixels between
characters and two pixels between rows, as shown in Fig. 1. All the letters
and numbers can be created on this common array format. A very efficient and
elegant array has evolved for portraying numeric characters only, called the
seven-bar font (Fig. 2). Each bar is a pixel by definition. A similar 14-bar
font is used for alphanumeric characters.
 �34s[Lawrence E. Tannas, Jr.]
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�6v�2v�32sElectronic listening devices   �21m �33s 
    Devices which are used to capture the sound waves of conversation 
originating in an ostensibly private setting in a form, usually as a magnetic 
tape recording, which can be used against the target by adverse interests. 
    There are two kinds of electronic listening devices. One takes advantage 
of equipment already present on the target's premises, such as a telephone, 
radio, phonograph, television set, public-address loudspeaker, or tape 
recorder, to act as a microphone, transmitter, or power supply. The other does 
not. In the former case, the target's equipment is said to have been 
compromised.  
    Compromise of the target's own equipment takes advantage of the fact that 
any loudspeaker is capable of functioning just as well as a microphone, that 
convenient sources of dc power are available within the equipment, or that the 
equipment is connected to power or signal lines that can transmit the 
intercepted conversation to some place where recording can conveniently be 
accomplished. The equipment most frequently compromised is the telephone 
handset.  
    Eavesdropping devices that can stand alone are known commonly as ``bugs.'' 
They take advantage of many developments of modern technology, such as 
microcircuits, miniature ceramic microphones, and miniature batteries. 
Electronically a bug is often just a two-stage frequency-modulated 
transmitter: an audio amplifier and a variable-frequency radio-frequency (rf) 
oscillator. Bugs may operate on any frequency from 20 to 1000 MHz, but usually 
they snuggle up beside a powerful local FM or vhf television station. See 
AMPLIFIER; OSCILLATOR; RADIO TRANSMITTER. 
    A popular hybrid between a compromise device and a bug is the telephone 
drop-in. In this design, an FM transmitter is made in the form of a telephone 
microphone. The eavesdropper can casually unscrew the mouthpiece of his 
target's telephone handset and substitute the drop-in for the original 
microphone. The range of this device is about 240 ft (75 m). It has the added 
advantage of drawing its dc power from the telephone company central battery. 
 �34s[John M. Carroll]
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�6v�2v�32sElectronic navigation systems   �21m �33s 
    Systems deriving navigationally useful parameters through the application 
of the electronic sciences. The older term for these systems was ``radio aids 
to navigation.'' However, several of the modern devices, although employing 
electronics, do not employ radio waves. Therefore, the term electronic 
navigation systems is employed to cover all of the devices which use 
electronics. In electronic navigation systems there is a blending of the 
sciences of electronics and navigation. Therefore, the subject may be 
discussed from both points of view. 
  
    Electronic classifications.   From the electronic point of view, it is 
possible to classify all systems as classical or self-contained. Classical 
systems employ at least one radio transmitter and one radio receiver. The 
transmitter emits energy which travels over one or more paths to the receiver. 
The navigational parameter is derived by a measurement made on the delay 
incurred in the transmission. All classical systems are based on the 
assumption that wave propagation is rectilinear and that the velocity of 
propagation is constant. The table shows the total contribution of electronics 
to navigation systems. 
    The self-contained systems consist of devices which make observations on 
certain natural phenomena and computers which derive navigational parameters 
from the observations. 
    Classical systems have often been classified by the various types of 
position lines which they produce, but actually they are of only two 
fundamental types. These are the single-path and the multiple-path systems. 
Two main variations of each of these types exist. The single-path systems 
measure absolute transmission time and circular lines of position (Fig. 1). 
The multiple-path systems measure differences in, or otherwise compare, 
transmission times (Fig. 2). Since the locus of all points having a constant 
difference from two other points is a hyperbola, these systems should produce 
hyperbolic lines of positions. Actually, many of these multiple-path systems 
are so instrumented that they can only determine when the difference in 
transmission time is zero (that is, the times are equal). With these systems 
the line of position is a hyperbola of zero curvature, that is, a straight 
line. The simplified multiple-path systems are known as radial systems. These 
radial systems were the first type developed and included direction finders, 
radio ranges, and the directional portion of radar. The true hyperbolic 
systems must use transmissions which are diverse in either time or frequency. 
See HYPERBOLIC NAVIGATION SYSTEM. 
  
    There are four types of so-called self-contained navigational systems. The 
term is somewhat misleading, because these systems actually depend on 
observations made on external phenomena as the basis of their measurements. 
From these observations, course and speed are determined and used to compute 
an advanced position from a known position. The classification of these 
systems is based on the phenomena on which the observation is made. 
Self-contained systems include astro devices, which make observations on 
heavenly bodies; pressure devices, which observe the atmospheric pressure 
system; Doppler devices, which observe changes in radio frequency due to the 
Doppler effect; and inertial devices, which measure forces in inertial space. 
    Operational classifications.   Electronic navigation systems must also be 
discussed from the standpoint of navigation. Air operations are considered to 
take place in four zones: the en route long-distance zone, the en route 
short-distance zone, the approach and landing zone, and the airport zone. 
Electronic navigation devices serve three purposes in marine operations: 
anticollision, position fixing, and harbor control. See AIR NAVIGATION; 
AIR-TRAFFIC CONTROL; INSTRUMENT LANDING SYSTEM (ILS); MARINE NAVIGATION; 
UNDERWATER NAVIGATION. 
 �34s[Peter C. Sandretto]
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�6v�2v�32sElectronic power supply   �21m �33s 
    A source of electric energy employed to furnish the tubes and 
semiconductor devices of an electronic circuit with the proper electric 
voltages and currents for their operation. The more common sources of energy 
are chemical batteries and alternating-current mains or lines. Batteries are 
useful as portable sources but are expensive and have small capacities. 
Alternating-current mains are not portable but are relatively inexpensive and 
have a large capacity. See ALTERNATING CURRENT; BATTERY. 
    One common method of classifying power supplies is by their use in 
electronic circuits. Most vacuum and gas tubes require a source of energy to 
energize their filaments or heaters; this type is known as a filament or 
heater power supply. Tubes require plate or anode voltages, and transistors 
need voltages for their collectors and emitters. These voltages are from a 
direct-current power supply which may provide about 5 to 40 volts for circuits 
involving transistors and about 50 to 400 volts for circuits using tubes. See 
GAS TUBE; SEMICONDUCTOR; TRANSISTOR; VACUUM TUBE. 
    If the source is one of alternating voltage, a transformer is usually used 
to raise or lower the voltage to the required level. The alternating current 
(ac) usually must be changed to a direct current (dc); this is accomplished by 
a rectifier. A rectifier allows current to flow mostly in one direction, and 
pulsating current results. This pulsating direct current is not suitable for 
most purposes and must be smoothed by a power-supply filter or voltage 
regulator. See RECTIFIER; TRANSFORMER. 
    A power-supply filter stores energy when the current is high and gives it 
up when the current is lower. The net result is to smooth out the variations 
in the current. The voltage regulator performs a similar function, but its 
operation is quite different. The zener diode type of voltage regulator has a 
voltage which is nearly constant as the current is varied over a considerable 
range of values. The gas-tube type of voltage regulator has a characteristic 
constant voltage over a large range of current values. Vacuum-tube and 
transistor regulators usually operate as variable resistances; the resistance 
decreases when the load is heavy and increases when it is lighter. Voltage 
regulators deliver an almost constant output voltage in spite of variation in 
load or in input ac supply voltage. See VOLTAGE REGULATOR; ZENER DIODE. 
 �34s[Donald L. Waidelich]
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�6v�2v�32sElectronic publishing   �21m �33s 
    A technology encompassing a variety of activities which contain or convey 
information with a high editorial and value-added content in a form other than 
print. Included in the list of presently practiced electronic publishing 
activities are: on-line data bases, videotext, teletext, videotape cassettes, 
videodisks, cable TV programming (direct broadcast satellite delivery is 
optional), and electronic mail and messaging. 
    For convenience and ease of understanding, these may be classified 
according to the amount of interactivity the user is allowed. One-way systems 
deliver information in a continuous manner and allow the user only passive 
viewing and crude control over sequence or position. All television and 
sequential-access image-archiving media, like videotape, are one-way. 
One-way-plus systems are continuous but offer some option for sequencing and 
selective access, with teletext being the best example. Two-way systems access 
information randomly under direct user control. 
  
    Publishing environment.  The immense growth of interest in electronic 
publishing has resulted primarily from technological advances affecting the 
areas of information distribution, display, and storage, combined with a 
dramatic decrease in the cost, and increase in the speed and power, of 
telecommunications.  
    In the area of communications, satellites have already begun to 
revolutionize commercial long-distance data transmissions. In the late 1970s 
and early 1980s the United States witnessed the rise of a new industry, 
dedicated to creation of nationwide data communication networks using packet 
switching technology, such as GTE's Telenet and Tymeshare's Tymenet networks. 
These networks are easy and inexpensive to use when compared to long-distance 
telephone lines. For most users, access to the network is through a local 
telephone call on a regular telephone. The networks are designed to allow data 
communications and computing devices manufactured by many different companies 
to connect with one another intelligently, that is, without the users having 
to concern themselves with the compatibility of individual devices. Due to 
their low cost, ease of use, and effect on the device compatibility issue, 
packet switching networks have created a strong and stable data 
telecommunications industry.  See  DATA COMMUNICATIONS; SWITCHING CIRCUITS. 
    Since packet switching networks depend for most of their operation on 
satellites which are in a geostationary orbit 22,500 mi (36,000 km) above the 
Earth, surface distances are relatively immaterial to the cost of 
telecommunications. Other resources used are equivalent to those used in a 
local telephone call. 
    In this environment, electronic publishers can reach a national market of 
potential subscribers without the costs of logistical problems associated with 
the nationwide distribution of printed material. Since electronic publishing 
is based on a pay-per-use concept and the data base of text or images is 
continuously available, there are dramatically different economics from those 
associated with traditional publishing, where manufacturing and distribution 
of books, magazines, or newspapers represent by far the majority of costs. 
    Crucial to the economical production of new information packages is the 
ability to manipulate images and type in totally digital formats. This was 
available only experimentally and at great expense until mid-1981. Moreover, 
software and engineering expertise for type- and image-processing systems, 
most commonly referred to as pagination or image-assembly devices, are 
maturing at a rapid rate. 
  
    On-line data bases.  Most mature of the electronic publishing activities 
is that of on-line data base publishing. On-line data bases utilize standard 
magnetic media and are composed of text characters stored in ASCII. They are 
accessed remotely by using standard data terminals via existing public 
telecommunications facilities at low to moderate speed, with most hard-copy 
output being done on line printers at central facilities. Direct output of 
full-text data from electronic text-processing systems to on-line data bases 
is already a reality in highly automated newspaper or magazine publishing 
environments.  
    Inability to incorporate graphic material has been a major limitation and 
block to greater acceptance of on-line data bases. In 1982 the first attempt 
was made to use videodisk, attached locally to the access terminal, as a way 
of integrating image material into the on-line data base presentation. 
  
    Videotex.  Videotex is the generic term for electronic home information 
delivery systems. Within this broad term there are two specific approaches, 
called teletext and viewdata, or videotext. Teletext is a one-way 
communications medium. Images, each constituting a single frame of TV data in 
a special, compressed format, are transmitted in a continuous sequence. Users 
indicate which frame they would like to see by interaction with the decoding 
unit in their local TV set. Videotext, or viewdata, is a potentially more 
powerful service since it involves users' ability to directly access a central 
data base interactively from their local TV.  See  VIDEOTEXT AND TELETEXT. 
  
    Videodisk.  Laser videodisk is the most exciting of the new visual media. 
It has rich potential since it employs a random-access technique. Available 
videodisks store single-frame television images in the form of analog 
FM-encoded TV signals. These are decoded and played back by using a low-power 
laser. Future optical disks will be memory systems rather than frame storage 
devices. However, because of the enormous quantities of digital data they will 
be capable of storing in binary form, the creation of all-digital image 
archives will be their prime application.  See  CONSUMER ELECTRONICS; 
VIDEODISK RECORDING. 
 �34s[David H. Goodstein]
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�6v�2v�32sElectronics   �21m �33s 
    The branch of science and technology relating to the conduction and 
control of electricity flowing through semiconducting materials or through 
vacuum or gases. Electronics is concerned with the study and applications of 
the motions of charge carriers (electrons, holes, and ions) under the 
influence of externally applied voltage or current, or in relation to the 
incidence or production of radiant energy. While electronics is properly a 
part of electrical engineering, the latter term is often reserved for 
applications involving power generation, distribution, and use at low 
frequencies, for example, in utility systems and industry. Since the 1960s, 
the dominant segment of electronics has been that known as solid-state, which 
involves transistors and other semiconductor devices and assemblies. 
    The electronics industry is concerned with the design, manufacture, and 
application of: transistors, diodes, integrated circuits, and other 
semiconductor devices; and to a much lesser extent, electron tubes. The major 
uses of electron tubes, not taken over by solid-state devices, are the 
generation of large amounts of radio-frequency power and the perception and 
display of images, as in television camera and picture tubes and computer 
displays.  
    These devices are used in a rapidly expanding range of applications making 
use of so-called digital electronics, that basic to the electronic calculator 
and computer. Reductions in the size and cost of powerful computers have 
produced, in a single large-scale integrated circuit, devices used in the 
automotive field to control fuel use, in ``intelligent'' control of household 
appliances, satellite communications, financial transactions, industrial 
instrumentation, control and automation, medical devices and implants, and the 
aerospace industry and numerous aspects of national defense. To a degree not 
envisaged as late as 1970, electronics pervades nearly every avenue of human 
endeavor.  
    The manifold applications of electronic devices depend fundamentally on 
the physical fact that the force exerted on an electron or a hole by the 
voltage applied to such devices is very great in proportion to the mass of the 
charge carrier. Consequently the electric charge is very rapidly accelerated 
and can achieve its maximum velocity in a billionth of a second or less. This 
rapid reaction permits the device to amplify and generate currents alternating 
at frequencies of billions of cycles per second (gigahertz) or to respond to 
stimuli in less than a billionth of a second (nanosecond). 
    While these numbers apply primarily to low-power devices, lower-frequency 
semiconductor devices, such as the silicon-controlled rectifier (SCR), can 
control as much as 1000 MW of power at commercial frequencies. This requires 
the use of large arrays of silicon-controlled rectifiers. These devices are 
also finding use in the transformation and control of power in electric 
locomotives and in similar heavy-current uses. These have given rise to the 
term power electronics. See SEMICONDUCTOR RECTIFIER. 
 �34s[Donald G. Fink]
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�6v�2v�32sElectronic switch   �21m �33s 
    An electronic device to which two input waveforms can be applied and which 
delivers at a pair of output terminals a signal that is alternately a replica 
of each of the input signals (see illustration). The transmission gate 
performs the basic switching function of the electronic switch, and sometimes 
such a gate circuit is itself defined as an electronic switch. However, the 
electronic switch is usually considered to be a separate instrument to which 
periodically recurrent input signals, of arbitrary waveform but with 
synchronously related periods, are connected. The output is switched between 
the two waveforms at a rate that is synchronous with the period of the input 
waveforms. One of the input signals often provides synchronizing information 
to supply periodic trigger pulses, which are then applied to the gate signal 
generators within the switch. See GATE CIRCUIT. 
    A frequent use of the electronic switch is to provide means for displaying 
two time-related signals on the screen of a cathode-ray oscilloscope without 
requiring two independent deflection systems within the cathode-ray tube. See 
SWITCHING THEORY. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sElectronic warfare �21m �33s
    Use of techniques, devices, and equipment by an adversary to deny or
counteract the enemy's use of radar, communications, guidance, or other
radio-wave devices. Because of the use of optical and infrared techniques for
communications, guidance, detection, and control, electronic warfare
techniques are sometimes called electromagnetic, rather than electronic, to
convey more adequately the idea that countermeasures are not confined to the
portion of the spectrum where electronic techniques alone are applicable but
may be used throughout the electromagnetic spectrum.
    Traditionally, electronic warfare equipment and techniques are categorized
as active and passive, depending on whether or not they radiate their own
energy. The passive category includes reconnaissance or surveillance equipment
that detects and analyzes electromagnetic radiation from radar and
communications transmitters in a potential enemy's aircraft, missiles, ships,
satellites, and ground installations. The reconnaissance devices may be used
to identify and map the location of emitters without in any way altering the
nature of the signals they receive. Other types of passive electronic warfare
equipment do seek to enhance or change the nature of the energy reflected back
to enemy radars, but they do not generate their own energy.
    Active electronic warfare equipment generates energy, either in the form
of noise to confuse an enemy's electromagnetic sensors or by generating false
or time-delayed signals to deceive radio or radar equipment and their
operators. See ELECTRICAL NOISE.

    Passive systems.   Reconnaissance or surveillance electronic warfare
systems are carried by Earth-orbiting satellites, aircraft, ships, uncrewed
(drone) aircraft, and automotive vehicles. Some are located on the ground. A
few systems are small enough to be carried by a person. Reconnaissance systems
are interchangeably called ferret or electronic intelligence (elint) systems.
They consist of sensitive receivers electromechanically or electronically
tuned over desired portions of the spectrum in search of transmissions of
interest. Bearing to an intercepted signal can be determined by
direction-finding techniques. Once secured, the signals can be displayed for
analysis by an operator or stored on tape or other storage media for
subsequent analysis.
    Warning-receiver systems, which came into widespread use on United States
tactical and transport aircraft during the Vietnamese war, are a more limited
form of the elint system. Unlike the latter, which is intended to search for
signals over a broad range of the spectrum, the warning receiver is programmed
to alert a pilot when the aircraft is being illuminated by a specific radar
signal above predetermined power thresholds anticipated by elint systems. When
the pilot has been alerted, the aircraft can be maneuvered to evade the threat
or initiate counteraction with onboard electronic warfare capability.
    One of the oldest passive electronic warfare techniques, made famous by a
Royal Air Force raid on Hamburg in 1943, is the use of chaff. These are metallic
strips cut to lengths resonant at the defense radar frequency so that they
return spurious radar echoes to enemy radar. Chaff can confuse an enemy by
generating false targets, or noise, forcing him to take time to analyze the
returns and sort real from false targets. Chaff can screen or mask aircraft or
higher-speed ships so that the enemy is unable to determine their presence, or
chaff can help an aircraft break track once it is alerted by its warning
receiver that it is being tracked by radar.
    Other passive electronic warfare techniques include the use of special
radar-absorbing materials, such as ceramics or ferrites, which reduce
reflection coefficients so that the amount of radar energy returned to the
illuminating radar is reduced; the special shaping of bodies, specifically in
missile reentry systems, that reduces the vehicle's radar cross section; the
use of corner reflectors, or Luneberg lenses, which concentrate the energy
they reflect back to the radar.

    Active systems.   The many active electronic warfare techniques can be
classified broadly either as noise or deception jamming. The former is the
oldest, simplest, and most straightforward, but requires higher average power
levels and is more expensive. Deception jamming is the more artful and
sophisticated technique, operating on the characteristics of the pulse train
generated by threat radars.
    Deception-jamming techniques are predicated on the idea of operating on
pulses received from the enemy so that the signal reradiated from the target
deceives the enemy radar or its operators. For instance, the deception set may
receive an enemy radar pulse, circulate it through a delay line, amplify it,
and reradiate it back toward the enemy. Because the enemy determines the
position of the target by the round-trip transit time of the radar energy, his
radar decision circuitry will conclude that the target is at a greater
distance than it actually is because of the deceptive pulse delay inserted in
that round-trip period by the active set. Similarly, the deception set may
operate on the radar pulse train, returning many pulses instead of one, in an
effort to deceive the enemy into believing there are many targets spaced at
different positions.
 �34s[Barry Miller]
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�6v�2v�32sElectron lens   �21m �33s 
    An electric or magnetic field, or a combination thereof, which acts upon 
an electron beam in a manner analogous to that in which an optical lens acts 
upon a light beam. Electron lenses find application for the formation of 
sharply focused electron beams, as in cathode-ray tubes, and for the formation 
of electron images, as in infrared converter tubes, various types of 
television camera tubes, and electron microscopes. 
    Any electric or magnetic field which is symmetrical about an axis is 
capable of forming either a real or a virtual electron image of an object on 
the axis which either emits electrons or transmits electrons from another 
electron source. Hence, an axially symmetric electric or magnetic field is 
analogous to a spherical optical lens. 
    The lens action of an electric and magnetic field of appropriate symmetry 
can be derived from the fact that it is possible to define an index of 
refraction for electron paths in such fields. This index depends on the field 
distribution and the velocity and direction of the electrons. 
    Electron lenses differ from optical lenses both in the fact that the index 
of refraction is continuously variable within them and that it covers an 
enormous range. Furthermore, in the presence of a magnetic field, the index of 
refraction depends both on the position of the electron in space and on its 
direction of motion. It is not possible to shape electron lenses arbitrarily. 
See ELECTROSTATIC LENS; MAGNETIC LENS. 
 �34s[Edward G. Ramberg]
�33s 
  
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�6v�2v�32sElectron microscope �21m �33s
    A device for forming greatly magnified images of objects by means of
electrons. Electron microscopes serve primarily two purposes: (1) the visual
examination of structures too fine to be resolved with ordinary, or light,
microscopes, and (2) the study of surfaces that emit electrons. The first
function made transmission electron microscopes essential research tools in
biology, chemistry, and metallurgy. Beginning in the 1960s the scanning
electron microscope came to play an increasingly important role in the study
of the surfaces of solid objects at more moderate magnifications. Various
emission electron microscopes serve more specialized research purposes. See
EMISSION ELECTRON MICROSCOPE; SCANNING ELECTRON MICROSCOPE.
    A transmission electron microscope consists in its simplest form of a
source supplying a beam of electrons of uniform velocity, a condenser lens for
concentrating the electrons on the specimen, a specimen stage for displacing
the specimen which transmits the electron beam, an objective lens, a projector
lens, and a fluorescent screen on which the final image is observed. For
permanent record of the image, the fluorescent screen is replaced by a
photographic plate or film.
    Electrons are strongly scattered by all forms of matter including air.
Hence the entire instrument must be evacuated to about 10- sup 4 mmHg (10- sup
7 atm). Furthermore, the lenses cannot be material in nature. Instead, they are
electric or magnetic fields, symmetrical about the axis of the instrument,
that have the property of bending the electron paths toward the axis, just as
converging glass lenses bend light rays toward their axis. Lens strength is
varied by varying the current. Most electron microscopes employ magnetic
lenses of this type. These have yielded the highest resolution and
magnification attained. However, good results have also been obtained with
electron microscopes employing unipotential electrostatic lenses and magnetic
lenses excited by permanent magnets. See ELECTRON LENS; ELECTROSTATIC LENS;
MAGNETIC LENS.
    A microscope can, at best, permit the discrimination of two point objects
greater than 0.6LAMBDA/sin THETA apart. Here LAMBDA is the wavelength of the
illuminating radiation, and THETA is the aperture angle of the cone of
radiation that participates in forming the image. For green light LAMBDA = 500
nanometers. Even for ultraviolet radiation in an immersion medium of
refractive index 1.5, LAMBDA is no less than 170 nm. Since light and
ultraviolet microscope objectives can be designed to utilize practically all
the radiation passing through the specimen, sin THETA CONGRUENT 1 and the
least resolvable distance for the ultraviolet microscope is about 100 nm. For
50- to 100-kilovolt electrons, such as are commonly employed in electron
microscopes, the wavelength range is 0.0053-0.0037 nm. Hence, even though a
cone of radiation with an aperture angle less than 0.01 radian contributes to
an image of optimum sharpness, object separations smaller than 0.3 nm have
been resolved with the electron microscope. Thus the electron microscope has
several hundred times the resolving power of the light microscope. Similarly,
whereas the maximum useful magnification of the light microscope is about
2000, that of the electron microscope may approach 1,000,000. The maximum
useful magnification is the least magnification of the image that reveals to
the observer all the specimen detail that the microscope is capable of
conveying.
    Electrons are commonly emitted from the tip of a fine tungsten-wire
hairpin filament or, to further reduce the size of the effective electron
source, from a sharply pointed segment of wire welded to the filament tip.
Image contrasts are formed by the scattering of electrons out of the narrow
cone that contributes to the formation of the image; denser or thicker
portions of the specimen scatter more electrons and hence appear darker in the
image.
    In addition to the standard transmission microscopes operating at 50-100
kV, a number of very-high-voltage instruments have been constructed. A
microscope at the Laboratory for Electron Optics at Toulouse, France, is
designed for operation up to 1500 kV (see illustration). The advantage of
high-voltage electron microscopy does not lie in greater resolving power but
in increased penetration, which is particularly valuable in the direct study
of metal sections prepared with a microtome.
 �34s[Edward G. Ramberg]
�33s

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�6v�2v�32sElectron-nuclear double resonance   �21m �33s 
    A technique in which the magnetic resonance of a nucleus is detected by 
observing that of a nearby electron. The magnetic coupling between the nuclear 
and electronic magnetic moments gives rise to a back reaction on the electron 
resonance when the nuclei are brought into resonance. The sample under study 
is placed in a conventional electron resonance apparatus. In addition, an 
oscillator drives a coil to produce alternating magnetic fields at the sample 
under study, the frequency of alternation being in an appropriate range to 
produce nuclear transitions. Typically, the static magnetic field is adjusted 
to produce electron resonance. With the static magnetic field and the 
frequency of the electron resonance apparatus held fixed, the frequency of the 
nuclear resonance oscillator is then swept. As it passes through the resonant 
frequency of any nucleus that is coupled to the electron, a change in the 
electron absorption occurs.  See  MAGNETIC RESONANCE. 
 �34s[Charles P. Slichter]
�33s 
  
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�6v�2v�32sElectron optics �21m �33s
    The branch of physics concerned with the motion of free electrons under
the influence of electric and magnetic fields. The term electron optics is
derived from the fact that the laws governing electron paths in such fields
are formally identical with those governing light rays in media of varying
refractive index. Both may be derived from Fermat's law. This law states that
the actual light ray or electron path passing through two prescribed points A
and B is that which makes the integral

		     *** INTEGRATION (A TO B) n ds ***

carried out over it a minimum. The refractive index n is a function of
position and, in the presence of a magnetic field, of the direction of the
electron path. A similar dependence of the refractive index on the direction
of a light ray is encountered in crystal optics. See CRYSTAL OPTICS.
    The study of electron paths, analogous to the study of light rays, is more
properly called geometrical electron optics. The electron paths may be
regarded as normals to electron waves, whose amplitude determines the
statistical density of electrons, just as the amplitude of a light wave
determines the density of light quanta, or photons. The study of the wave
motion associated with electrons is called electron wave optics. It describes
diffraction and interference effects between electron beams which are in every
way similar to the diffraction and interference effects observed with light
and x-rays.
    Electron optics finds application in the formation of electron beams, as
in cathode-ray tubes and television camera tubes; in the deflection of such
beams by electric and magnetic fields; and in the formation of electron
images, as in electron microscopes and image tubes. See CATHODE-RAY TUBE;
ELECTRON DIFFRACTION; ELECTRON LENS; ELECTRON MICROSCOPE; ELECTROSTATIC LENS;
MAGNETIC LENS.
 �34s[Edward G. Ramberg]
�33s


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�6v�2v�32sElectron paramagnetic resonance (EPR) spectroscopy   �21m �33s 
    The study of magnetic resonance spectra of materials which show 
paramagnetism because of the magnetic moment of unpaired electrons. EPR 
spectra are usually presented as plots of the absorption or dispersion of the 
energy of an oscillating magnetic field of fixed radio frequency versus the 
intensity of an applied static magnetic field. See ELECTRON SPIN; MAGNETIC 
RESONANCE; PARAMAGNETISM. 
    EPR spectroscopy has been used for detection and identification of 
paramagnetic materials, for determinations of electronic structure, for 
studies of interactions between molecules, and for measurements of nuclear 
spins and moments. Among the wide variety of paramagnetic substances to which 
EPR spectroscopy has been applied are free radicals (including free atoms), 
impurity centers, and compounds of the transition elements, rare earths, and 
actinides. EPR spectra have been obtained from gases, liquids, and solids. 
    Spectra characteristic of individual paramagnetic molecules, uncomplicated 
by magnetic interactions with neighboring paramagnetic molecules, may be 
obtained only from dilute solutions in diamagnetic solvents. The required 
degree of dilution depends on the nature of the magnetic molecules. In some 
cases spin-lattice relaxation (exchange of magnetic energy with thermal 
motions of the environment) obscures the spectra. The effects are especially 
pronounced in inorganic magnetic ions and frequently require the use of low 
temperatures.  
    Greatly enhanced resolution may be obtained in many cases through 
simultaneous irradiation with several frequencies. The most useful of these 
methods is electron nuclear double resonance (ENDOR). The material is 
simultaneously irradiated at one of its EPR resonant frequencies and by a 
second oscillatory field whose frequency is swept over the range of nuclear 
frequencies. The intensity of the EPR response is measured as a function of 
the second frequency. Changes in EPR intensity accompany passage through 
nuclear resonance. Resonances as narrow as 10 kHz are observed. See NUCLEAR 
MAGNETIC RESONANCE (NMR). 
 �34s[Samuel Isaac Weissman]
�33s 
  
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�6v�2v�32sElectron-positron pair production   �21m �33s 
    A process in which a negative electron (negatron) and a positive electron 
(positron) are simultaneously created in the vicinity of a nucleus or an 
elementary particle. In external pair production, an electromagnetic wave 
(photon) is absorbed and creates an electron pair. The absorption of 
high-energy GAMMA-rays is due mainly to this effect. Internal pair production 
is not associated with observable electromagnetic radiation and may occur when 
an excited nucleus releases some of its internal energy. See  ELECTRON; 
POSITRON.  
    Pair production is of considerable theoretical interest, not only as an 
example of the materialization of energy, but also as a striking confirmation 
of the relativistic quantum theory proposed by P. A. M. Dirac. This theory has 
made possible quantitative predictions of production probability, differential 
electron distribution, and kinetic energy partition. The results are in 
satisfactory agreement with experimental findings. See RELATIVISTIC QUANTUM 
THEORY.  
 �34s[Gunnar Backstrom]
�33s 
  
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�6v�2v�32sElectron ring accelerator   �21m �33s 
    A device for imparting to protons or other positive ions kinetic energies 
of several to several hundred MeV per nucleon by the electron drag force of 
stable ringlike configuration of electrons that is accelerated to the 
requisite velocity. 
    Conventional particle accelerators are subject to limitations in the 
strengths of the fields that can feasibly be employed to guide and accelerate 
the desired particles, and accordingly may be massive and expensive. This 
situation has motivated the development of techniques for accelerating a small 
group of particles by means of their interaction with another group of charges 
or with some form of plasma wave. This ``collective acceleration'' employs 
fields developed by charged particles within the accelerator itself. Such 
fields can be quite large and are not subject to the technological limitations 
that restrict externally applied fields. 
    A direct and attractive method of collective acceleration is employed in 
the electron ring accelerator. The fields of significance in this accelerator 
are produced by a slender ring of electrons circulating in a magnetic field. 
The electron ring is formed by injecting an electron beam of several hundred 
amperes into a magnetic field that is rapidly deformed to trap one or more 
injected turns into closely spaced circular orbits and that then is further 
pulsed to shrink the ring dimensions to suitable size. The pulsed magnetic 
field serves both to increase the energy of the circulating electrons (through 
the transformer action of induced electric fields) and to reduce the major and 
minor dimensions of the ring (compression). In alternative experimental 
devices these rings are formed by injecting a hollow relativistic electron 
beam into a cusp-shaped magnetic field, wherein the forward motion of the 
electrons is converted into circulatory motion.  See  PARTICLE ACCELERATOR. 
 �34s[Uwe Schumacher]
�33s 
  
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�6v�2v�32sElectron spectroscopy �21m �33s
    A form of spectroscopy which deals with the emission and recording of the
electrons which constitute matter - solids, liquids, or gases. The usual form
of spectroscopy concerns the emission or absorption of photons (x-rays,
ultraviolet rays, visible or microwave wavelengths, and so on). Electron
spectra can be excited by x-rays, which is the basis for electron spectroscopy
for chemical analysis (ESCA), or by ultraviolet photons, or by ions
(electrons). By means of ESCA, complete sets of photoelectron lines can be
excited from the internal (core) levels as well as from the external (valence)
region. Also, complete sequences of the Auger electron lines are automatically
obtained in this mode. See AUGER EFFECT.
    The electron lines in an ESCA spectrum are extremely sharp and well suited
for precision measurements. With a high-resolving ESCA spectrometer which has
a magnetic or electrostatic focusing dispersive system, the electron lines
have widths which are set by the limit caused by the uncertainty principle
(the ``inherent'' widths of atomic levels). With a suitable choice of
radiation, electron spectroscopy reproduces directly the electronic level
structure from the innermost shells (core electrons) to the atomic surface
(valence or conduction band). Furthermore, all elements from hydrogen to the
heaviest ones can be studied even if the element occurs together with several
other elements and even if the element represents only a small part of the
chemical compound. See LINE SPECTRUM.
    When applied to solid materials, ESCA is a typical surface spectroscopy
with applications to problems such as chemical surface reactions, for example,
corrosion or heterogeneous catalysis. ESCA also reproduces bulk matter
properties such as valence electron band structures. Electron spectroscopy can
supply a detailed knowledge of the valence orbital structure for all molecules
which can be brought into gaseous form with pressures of 10- sup 5 torr (10-
sup 3 pascal) or more. Under certain conditions, liquids and solutions of
various compositions can be studied by ESCA techniques.
    A unique feature of ESCA is that, if the exact position of the electron
lines characteristic of the various elements in the molecule is measured, the
area of inspection can be moved from one atomic species to another in the
molecular structure. If the structure of the molecule is known, the charge
distribution can be estimated in a simple way by using, for example, the
electronegativity concept and assuming certain resonance structures. More
sophisticated quantum-chemical treatments can also be applied. Conversely, if,
by means of ESCA, the approximate charge distribution is known, conclusions
concerning the structure of the molecule can be drawn. See ATOMIC STRUCTURE
AND SPECTRA; ELECTRON CONFIGURATION; ELECTRONEGATIVITY; MOLECULAR ORBITAL
THEORY; SPECTROSCOPY.
 �34s[Kai Siegbahn]
�33s

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�6v�2v�32sElectron tube   �21m �33s 
    A device in which conduction takes place by the movement of electrons or 
ions between electrodes through a vacuum or ionized gas within a gas-tight 
envelope. Electron tubes include all partially evacuated tubes whose 
electrical characteristics are derived from the flow of electrons through the 
tube. Two subclasses of electron tubes are vacuum tubes and gas-filled tubes. 
Vacuum tubes are primarily used in applications where low noise and high 
frequency are involved. In contrast, gas tubes are used for high-current, 
low-frequency applications. See GAS TUBE; VACUUM TUBE. 
    The importance of electron tubes grew rapidly during the first half of the 
20th century. Although semiconductor devices have largely replaced vacuum 
tubes in many areas of application, tubes are still essential in a number of 
areas. Thus, cathode-ray tubes, which are a specialized form of vacuum tube, 
are at present indispensable to television receivers, oscilloscopes, and radar 
display units; camera tubes are used in television cameras; high-power tubes 
are used in broadcasting transmitters; and microwave tubes find many 
applications in radar, telephony, space communication and control, scientific 
research, and high-frequency ovens. See CATHODE-RAY TUBE; MICROWAVE TUBE; 
NUMERICAL INDICATOR TUBE; PHOTOTUBE; SEMICONDUCTOR; X-RAY TUBE. 
 �34s[Herbert J. Reich]
�33s 
  
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�6v�2v�32sElectronvolt   �21m �33s 
    A unit of energy used for convenience in atomic systems. Specifically, it 
is the change in energy of an electron, or of any particle having a charge 
numerically equal to that of an electron, when it is moved through a 
difference of potential of 1 mks volt. Its value (in mks units) is obtained 
from the equation W= qV, where W is energy in joules, q the charge in 
coulombs, and V the potential difference in volts. For a potential difference 
of 1 volt and the electronic charge of 1.601 x 10-sup1sup9 coulomb, the 
electronvolt is 1.601 x 10-sup1sup9 joule. See ELECTRON; IONIZATION POTENTIAL. 
 �34s[Glenn H. Miller]
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�6v�2v�32sElectron accelerators   �21m �33s 
    Electrical devices which accelerate electrons to high energies. Almost all 
electron linear accelerators are of the traveling-wave type. The wave is 
produced in a loaded waveguide, which is a waveguide excited in a TM mode 
(having a longitudinal electric field component) and loaded by disks or other 
means to produce the correct phase velocity to match the particle. An unloaded 
or uniform waveguide is not suitable since the phase velocity in such a guide 
exceeds the velocity of light.  See  PHASE VELOCITY; WAVEGUIDE. 
    The energy possibilities of the electron linear accelerator are not 
limited by fundamental considerations since electrons accelerated in a 
straight line do not lose energy by radiation by an appreciable amount. The 
performance of the accelerator is closely tied to the available power sources. 
Some of the earlier machines were powered by magnetron oscillators; newer 
machines are powered by klystron amplifiers driven from a common master 
oscillator. The largest electron linac is the 2-mi-long (3.2-km) 20-GeV 
machine located at Stanford University.  See  KLYSTRON; MAGNETRON; MICROWAVE 
TUBE.  
    The superconducting electron linac can operate continuously with 
dissipations of about a watt per meter. However, there have been severe 
metallurgical and electron loading problems with this device, and the initial 
hopes of very high accelerating gradients have not been realized. 
  
    Proton accelerators.  Existing proton accelerators are of the 
standing-wave type since protons, unlike electrons, do not reach relativistic 
velocities at injection voltages and traveling-wave structures are not 
feasible. The earliest and still popular design is the Alvarez structure. The 
drift tubes, supported by transverse stems, are metallic cylinders and no rf 
field is present inside. The distance between tube centers is called a cell 
and is one guide wavelength long. 
    The trend has been toward much higher beam currents, more precise energy, 
and much higher energies. These have been achieved largely by advances in 
structure design, in particular by the use of multiple resonances or cells of 
double periodicity which greatly relax the mechanical and beam-loading 
tolerances and allow efficient acceleration and precise control of the fields 
together with good power transfer along the structure. In modern designs the 
Alvarez structure with resonant posts or stems is preferred up to about 200 
MeV. Above 200 MeV, coupled-cavity structures of great precision and 
efficiency have been developed. The 200 - 800 MeV section of the proton 
accelerator at the Los Alamos Meson Physics Facility (LAMPF) uses these 
structures.  
  
    Heavy-ion accelerators.  Heavy-ion linacs are similar to low-energy proton 
accelerators, with differences dictated by the relatively low charge-to-mass 
ratios available from ion sources. For a given electric field and structure 
length, the ion velocities are much lower than for protons. After passing 
through one or more cavities and reaching velocities of about 4% of c 
(energies of 1 MeV per atomic mass unit, or amu), the ions are passed through 
a stripper - a very thin foil, gas-filled tube, or gas jet - and electrons are 
stripped from the ions; typically the ion charge is doubled, and about half 
the ions are in a single-charge state. Subsequent acceleration in later 
cavities is then twice as efficient. The final energy of the accelerators is 
in the range 7 to 15 MeV/amu (v/c of 0.12 to 0.18) and is chosen to allow 
nuclear reactions between any of the accelerated ions and the heaviest target 
nuclei.  
    Considerable development work has been undertaken to perfect a practical 
superconducting structure for heavy ions. Various problems remain to be solved 
with all of the proposed designs, but the principle is very attractive, 
particularly because of the very large power consumption of room-temperature 
heavy-ion linacs.  See  PARTICLE ACCELERATOR. 
 �34s[Robert Beringer]
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�6v�2v�32sElectrooptics   �21m �33s 
    The branch of physics which deals with the influence of an electric field 
on the optical properties of matter, especially in its crystalline form. These 
properties include transmission, emission, and absorption of light. 
    An electric field applied to a transparent crystal can change its 
refractive indexes and, therefore, alter the state of polarization of light 
propagating through it. When the refractive-index changes are directly 
proportional to the applied field, the phenomenon is termed the Pockels 
effect. When they are proportional to the square of the applied field, it is 
called the Kerr effect. See KERR EFFECT. 
    The Pockels effect is used in a light modulator called the Pockels cell. 
This device (see illustration) consists of a crystal C (usually potassium 
dihydrogen phosphate, or KDP) placed between two polarizers P1 and P2 whose 
axes are crossed. Ring electrodes bonded to two crystal faces allow an 
electronic driver V to apply an electric field parallel to the axis OZ along 
which a light beam (for example, a laser beam) is made to propagate. Pockels 
cells can be switched on and off in well under 1 nanosecond. See LASER; 
OPTICAL MODULATORS. 
 �34s[Michel A. Duguay]
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�6v�2v�32sElectrophilic and nucleophilic reagents   �21m �33s 
    Electrophilic reagents are chemical species which, in the course of 
chemical reactions, acquire electrons, or a share in electrons, from other 
molecules or ions. Although this definition embraces all oxidizing agents and 
all Lewis acids, electrophilic reagents are ordinarily thought of as cationic 
species, such as Hsupp, NO2supp, Brsupp, or SO3 (or carriers of these species 
such as HCl, CH3COONO2, or Br2), which can form stable covalent bonds with 
carbon atoms. Electrophilic reagents frequently are positively charged ions 
(cations). See ACID AND BASE. 
    Neucleophilic reagents are the opposite of electrophilic reagents. 
Nucleophilic reagents give up electrons, or a share in electrons, to other 
molecules or ions in the course of chemical reactions. Nucleophilic reagents 
frequently are negatively charged ions (anions). Typical nucleophilic reagents 
are hydroxide ion (OH-), halide ions (F-, Cl-, Br-, and I-), cyanide ion 
(CN-), ammonia (NH3), amines, alkoxide ions (such as CH3O-), and mercaptide 
ions (such as C6H5S-). See SUBSTITUTION REACTION. 
 �34s[Joseph F. Bunnett]
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�6v�2v�32sElectrophoresis �21m �33s
    The migration of electrically charged particles in solution or suspension
in the presence of an applied electric field. Each particle moves toward the
electrode of opposite electrical polarity. For a given set of solution
conditions, the velocity with which a particle moves divided by the magnitude
of the electric field is a characteristic number called the electrophoretic
mobility. The electrophoretic mobility is directly proportional to the
magnitude of the charge on the particle, and is inversely proportional to the
size of the particle. An electrophoresis experiment may be either analytical,
in which case the objective is to measure the magnitude of the electrophoretic
mobility, or preparative, in which case the objective is to separate various
species which differ in their electrophoretic mobilities under the
experimental solution conditions.
    The original apparatus known as the Tiselius cell detects electrophoretic
motion by the moving-boundary method, in which a boundary is created between
the solution of particles to be examined and a sample of pure solvent (see
illustration). As the particles migrate in an electric field, the boundary
between solution and solvent can be observed to move, and if there are a
number of species in the solution with different electrophoretic mobilities, a
series of boundaries of various shapes and magnitudes can be detected. The
moving-boundary method was used for three decades to separate complex mixtures
of charged macromolecules in solution and to study the physical
characteristics of solutions of proteins and other macromolecules of
biological and industrial importance.
    The resolving power of electrophoresis has been greatly improved by the
introduction of the use of gel supporting media. The gel matrix prevents
thermal convection caused by the heat which results from the passage of
electric current through the sample, which reduces greatly the mixing of the
various parts of the sample, and therefore allows for more stable separation.
This method is now the most common technique for the determination of
molecular weights of proteins in biochemical studies.
    The methodology of electrophoresis may be modified considerably when the
particles undergoing analysis are of sufficient size to be viewed either with
the naked eye or with the assistance of an optical microscope. This general
area of particle electrophoresis has its most important applications in the
analysis of the surface charge of living cells and in the study of various
types of particles used in industrial coating processes.
    Application of the optical laser to electrophoretic detection has resulted
in the development of a technique which can be used for analytical
electrophoresis experiments on particles of all sizes. The basic principle is
that the highly monochromatic laser light impinges upon the particles, and is
scattered from the particles in all directions. Observation of the laser light
which has been scattered form a moving particle permits detection of a slight
shift in the frequency of the light as a result of the motion of particle. This
is the Doppler effect. The application of the laser Doppler principle to
electrophoresis experiments, often called electrophoretic light scattering
(ELS), has become an important method for the rapid determination of
electrophoretic velocities. Electrophoretic light scattering has been used for
the study of many types of living cells, cell organelles, viruses, proteins,
nucleic acids, and synthetic polymers. See DOPPLER EFFECT; ELECTROLYTIC
CONDUCTANCE; ISOELECTRIC FOCUSING; ISOTACHOPHORESIS.  [Bennie R. Ware]

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�6v�2v�32sElectroplating of metals �21m �33s
    The process of electrodepositing metallic coatings to alter the existing
surface properties or the dimensions of an object. Electroplated coatings are
applied for decorative purposes, to improve resistance to corrosion or
abrasion, or to impart desirable electrical or magnetic properties. Plating is
also used to increase the dimensions of worn or undersized parts. An example
of a decorative coating is that of nickel and chromium on automobile bumpers.
However, in this application, corrosion and abrasion resistance are also
important. An example of electrodeposition used primarily for corrosion
protection is zinc plating on such steel articles as nuts, bolts, and
fasteners. Since zinc is more readily attacked by most atmospheric corrosive
agents, it provides galvanic or sacrificial protection for steel. An
electrolytic cell is formed in which zinc, the less noble metal, is the anode,
and steel, the more noble one, the cathode. The anode corrodes, and the
cathode is protected. See ELECTROCHEMISTRY.
    The electroplating process consists essentially of connecting the parts to
be plated to the negative terminal of a direct-current source and another
piece of metal to the positive pole, and immersing both in a solution
containing ions of the metal to be deposited. The part connected to the
negative terminal becomes the cathode, and the other piece the anode. In
general, the anode is a piece of the same metal that is to be plated. Metal
dissolves at the anode and is plated at the cathode. See ELECTROLYSIS.
    Most plating solutions are of the aqueous type. There is a limited use of
fused salts or organic liquids as solvents. Nonaqueous solutions are employed
for the deposition of metal with lower hydrogen overvoltages; that is,
hydrogen rather than the metal is reduced at the cathode in the presence of
water.
    In order for adherent coatings to be deposited, the surface to be plated
must be clean, that is, free from all foreign substances such as oils and
greases, as well as oxides or sulfides. The two essential steps are cleaning
and pickling.
    Three principal cleaning methods are employed to remove grease and
attached solids. (1) In solvent cleaning, the articles undergo vapor
degreasing, in which a solvent such as tri- or tetrachloroethylene is boiled
in a closed system, and its vapors are condensed on the metal surfaces. (2) In
emulsion cleaning, the metal parts are immersed in a warm mixture of kerosine,
a wetting agent, and an alkaline solution. (3) In electrolytic cleaning, the
articles are immersed in an alkaline solution, and a direct current is passed
between them and the other electrode, which is usually steel. Ultrasonic
cleaning is also used extensively, especially for blind holes or gears packed
with soils. Ultrasonic waves introduced into a cleaning solution facilitate
and accelerate the detachment of solid particles embedded in crevices and
small holes.
    In the pickling process, oxides are removed from the surface of the basis
metal. For steel, warm, dilute sulfuric acid is used in large-scale operations
because it is inexpensive; but room-temperature, dilute hydrochloric acid is
also used for pickling because it is fast-acting. Hydrogen embrittlement may
be caused by the diffusion of hydrogen in steel (especially high-carbon steel)
during pickling and also in certain plating operations. See EMBRITTLEMENT.
    Electropolishing is used when a thin, bright deposit is to be produced. In
this case, the substrate surface contours are essentially copied by the
deposit. The substrate surface therefore must have a bright finish which can
be attained by electropolishing. See ELECTROPOLISHING.
    Special processes, such as electroforming and anodizing, are required for
certain applications. Electroforming is a special type of plating in which
thick deposits are subsequently removed from the substrate, which acts as a
mold. The process is particularly suitable for forming parts which require
intricate designs on inside surfaces. Important applications of electroforming
are in the production of phonograph record masters, printing plates, and some
musical instruments and fountain pen caps as well in waveguides. In anodizing,
a process related to plating, an oxide is deposited on a metal which is the
anode in a suitable solution. The process is primarily used with aluminum, but
it can be applied to beryllium, magnesium, tantalum, and titanium. Colored
coatings can be produced by the incorporation of dyes. See METAL COATING.
 �34s[Rolf Weil]
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�6v�2v�32sElectropolishing   �21m �33s 
    A method of polishing metal surfaces by applying an electric current 
through an electrolytic bath in a process that is the reverse of plating. The 
metal to be polished is made the anode in an electric circuit. Anodic 
dissolution of protuberant burrs and sharp edges occurs at a faster rate than 
over the flat surfaces and crevices, possibly because of locally higher 
current densities. The result produces an exceedingly flat, smooth, brilliant 
surface.  
    Electropolishing is used for many purposes. The brilliance of the polished 
surface makes an attractive finish. Because the polished surface has the same 
structural properties as the base metal, it serves as an excellent surface for 
plating. Electropolishing avoids causing differential surface stresses, one of 
the requirements for the formation of galvanic cells which cause corrosion. 
Because no mechanical rubbing is involved, work hardening is avoided. 
Contaminants, which often are associated with the use of abrasives and 
polishing compounds, are also avoided. The surface is left clean and may 
require little or no preparation for subsequent treatment or use. 
Electropolishing also minimizes loss of high-temperature creep-rupture 
strength. See ELECTROPLATING OF METALS. 
 �34s[William W. Snow]
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�6v�2v�32sElectrorefining   �21m �33s 
    A purification process in which an impure metal anode is dissolved 
electrochemically in a solution of a salt of the metal being refined; the pure 
metal is recovered by electrode-position at the cathode. 
    Electrorefining is the most economical method for securing the high purity 
required for many uses of nonferrous metals. Since the same electrochemical 
reaction proceeds in opposite directions at the anode and the cathode, the 
overall chemical change is a small change in the activity of the metal at the 
two electrodes, and the reversible cell voltage is practically zero. The 
anodic and cathodic overvoltages, required for an economical reaction rate, 
are small, and the operating cell voltage consists mainly of the ohmic voltage 
drop through the electrolyte and at the electrode contacts. The power 
consumption is moderate, and electrorefining is less sensitive to the cost of 
electric power than are other electrometallurgical processes. 
    Electrochemical refining is a more efficient purification process than 
other chemical methods are because it is very selective. In particular, for 
metals such as copper, silver, gold, and lead, the operating electrode 
potential is close to the reversible potential, and a sharp separation is 
accomplished, both at the anode and at the cathode. Precious metals do not 
dissolve at the potential of the anode, and they remain as metals in the 
slimes, along with insoluble selenium and tellurium compounds and some 
metallic copper particles that fall from the anode. Metals less noble than 
copper do not deposit at the cathode. In particular, nickel and arsenic 
accumulate in the electrolyte, and their concentration is controlled by 
partial withdrawal and purification of the solution. See ELECTROCHEMICAL 
PROCESS.  
 �34s[Paul Duby]
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�6v�2v�32sElectroscope   �21m �33s 
    An instrument for detecting the presence and sign of an electric charge. 
It is the simplest type of ionization chamber. See IONIZATION CHAMBER. 
    The illustration shows a common type of simple gold-leaf electroscope. 
Gold leaf (L) is used because it is an extremely thin conducting foil which 
has low mass per unit area and is very flexible. Hence, it responds quickly 
and vigorously to small electrostatic forces. In the illustration H serves as 
a grounded electrostatic shield, as well as a shield against air currents. The 
hard-rubber rod R (illustration a) with its negative charge has set up the 
charge distribution by the process of electrostatic induction. The response 
shown is a test for the fact that R has a charge. See ELECTROSTATIC INDUCTION. 
    To leave the electroscope with a net charge, a grounded conductor is 
touched to K so that the surplus electrons on P and L go off to ground, 
leaving the bound positive charge on K. The ground connection is then broken 
and R is removed. At this stage (illustration b) the electroscope is said to 
have a positive charge because there is a positive charge on its leaf system. 
    If an electroscope has a charge of known sign, as in illustration b, it 
can be used to test the sign of an unknown charge, as in illustration c, where 
the metal test ball (T), with its insulating handle (J), has the unknown 
charge. In the situation pictured, L moves farther away from P as T is brought 
slowly up toward K, showing that T has a positive charge. If T had a negative 
charge, L would move toward P, as T slowly approaches K. The converse 
situation, if the leaf system in illustration c had a negative charge 
initially, can be readily visualized. 
    Although electroscopes have been built with a wide variety of geometries, 
the principle of operation is essentially the same for all. If an electroscope 
has a scale, permitting quantitative measurements, it is called an 
electrometer or electrostatic voltmeter. For information on electrometers see 
VOLTMETER.  
 �34s[Ralph P. Winch]
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�6v�2v�32sElectrostatic accelerator   �21m �33s 
    A device that accelerates atomic or subatomic particles to high energies 
by means of a high voltage potential that is maintained through some 
electrical or mechanical means of transporting charge from ground to the high 
voltage potential. Modern machines of larger size all use mechanical charge 
transport, but electrical transport systems are widely used in small 
Cockcroft-Walton or dynamitron-type electrostatic accelerator units. The most 
commonly used electrostatic generator was invented in the 1930s by R. J. Van 
de Graaff and utilizes a high-speed rubberized-fabric belt to transport the 
charge.  
    The Pelletron electrostatic accelerator utilizes a chain-charging system 
instead of the rubberized-fabric belt. A large Pelletron accelerator at the 
Oak Ridge National Laboratory is designed to operate at 25 MV.  See  
COCKCROFT-WALTON ACCELERATOR; DYNAMITRON ACCELERATOR; PELLETRON ACCELERATOR; 
RESONANCE TRANSFORMER; VAN DE GRAAFF GENERATOR. 
    All particle accelerations are carried out inside an evacuated tube so 
that the accelerated particles do not collide with air molecules or atoms and 
may follow trajectories characterized specifically by the electric fields 
utilized for the acceleration. Usually the evacuated tube is provided with a 
series of electrodes arranged to have gradually increasing potentials from 
ground to the maximum high-voltage potential. In this way, the high voltage is 
distributed uniformly along the acceleration tube, and the acceleration 
process is thereby simplified and better control permitted. 
    Most electrostatic accelerators are housed inside a large high-pressure 
vessel that is filled with very dry insulating gas. This high-pressure 
insulating gas allows the machine to be housed in a much smaller space than 
would be possible in air at atmospheric pressure. 
    Large electrostatic accelerators are constructed in the form of tandem 
electrostatic accelerators. This device gets its name from the fact that is it 
effectively two electrostatic accelerators back to back, arranged so that the 
high voltage potential is first used to accelerate negative ions from ground 
potential up to the high positive poential. The charge of the particle is then 
switched from negative to positive, and the positive ions are additionally 
accelerated from the high voltage potential back to ground, continuing in the 
same direction as the original acceleration. A third stage of acceleration can 
be accomplished by first accelerating negative particles from high voltage to 
ground and then injecting them into a conventional tandem. The charge-changing 
in the high-voltage terminal is accomplished by either a gas or foil stripper. 
    The folded tandem design has also been developed for large electrostatic 
accelerators. They are constructed like a large single-stage electrostatic 
accelerator; however, the column which supports the high-voltage terminal 
contains two acceleration tubes - one for the acceleration of negative ions 
from ground potential to the high-voltage terminal, and the other for 
acceleration of positive ions from the high-voltage terminal down to ground 
potential.  
    The larger electrostatic accelerators are used primarily for basic 
research in the study of nuclear structure and heavy-ion reactions, including 
fusion and fission processes. The smaller machines are used throughout 
industry for many applications, including neutron and x-ray production for 
diagnostic measurements, polymerization and other treatments of plastics and 
various materials, ion implantation, thin-film analysis, and many other 
related applications.  See  ION IMPLANTATION; PARTICLE ACCELERATOR. 
 �34s[Harvey E. Wegner]
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�6v�2v�32sElectrostatic induction   �21m �33s 
    A method whereby an electrical conductor becomes electrified when near a 
charged body. Its usefulness arises from the fact that for every electric 
charge there is somewhere an equal and opposite induced charge. For a detailed 
discussion see ELECTROSTATICS. 
 �34s[Ralph P. Winch]
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�6v�2v�32sElectrostatic lens   �21m �33s 
    An electrostatic field with axial or plane symmetry which acts upon beams 
of charged particles of uniform velocity as glass lenses act on light beams. 
The action of electrostatic fields with axial symmetry is analogous to that of 
spherical glass lenses, whereas the action of electrostatic fields with plane 
symmetry is analogous to that of cylindrical glass lenses. Plane symmetry as 
used here signifies that the electrostatic potential is constant along any 
normal to a family of parallel planes. 
    The action of an electrostatic lens on the paths of charged particles 
passing through it is most readily visualized with the aid of an equipotential 
plot of the fields in a plane of symmetry of the lens. The equipotential lines 
in the plot indicate the intersection with the plane of the drawing of 
surfaces on which the electrostatic potential is a constant. The paths of 
charged particles in the electrostatic field are bent toward the normals of 
the equipotentials as the particles are accelerated, and away from the normals 
as the particles are decelerated. 
    Axially symmetric lenses are generally formed at or between circular 
apertures and cylinders maintained at suitable potentials. For any of these it 
is possible to define focal points, principal planes, and focal lengths in the 
same manner as for light lenses and to determine with their aid image 
magnification for any object position. 
    Lenses of plane symmetry, analogous to cylindrical glass lenses, are 
formed between parallel planes and at slits, replacing the circular cylinders 
and apertures of lenses with axial symmetry. 
 �34s[Edward G. Ramberg]
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�6v�2v�32sElectrostatic precipitator �21m �33s
    A device used to remove liquid droplets or solid particles from a gas in
which they are suspended. The process depends on two steps. In the first step
the suspension passes through an electric discharge (corona discharge) area
where ionization of the gas occurs. The ions produced collide with the
suspended particles and confer on them an electric charge. The charged
particles drift toward an electrode of opposite sign and are deposited on the
electrode where their electric charge is neutralized. The phenomenon would be
more correctly designated as electrodeposition from the gas phase.
    The use of electrostatic precipitators has become common in numerous
industrial applications. Among the advantages of the electrostatic
precipitator are its ability to handle large volumes of gas, at elevated
temperatures if necessary, with a reasonably small pressure drop, and the
removal of particles in the micrometer range. Some of the usual applications
are: (1) removal of dirt from flue gases in steam plants; (2) cleaning of air
to remove fungi and bacteria in establishments producing antibiotics and other
drugs, and in operating rooms; (3) cleaning of air in ventilation and air
conditioning systems; (4) removal of oil mists in machine shops and acid mists
in chemical process plants; (5) cleaning of blast furnace gases; (6) recovery
of valuable materials such as oxides of copper, lead, and tin; and (7)
separation of rutile from zirconium sand.
 �34s[George S. Mill ;W. O. Milligan]
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�6v�2v�32sElectrostatics   �21m �33s 
    The study of electric charges at rest under conditions where the positive 
and negative charges are separated from each other. The term static 
electricity is often used to refer to electric charges at rest. 
    Electrification by friction or rubbing occurs when one substance is rubbed 
with another and a surplus of electrons, each with its negative charge, is 
rubbed onto one of the bodies, leaving a deficiency of electrons, that is, a 
positive charge, on the other body. The most commonly used materials for 
electrification by friction are hard rubber and fur. The hard rubber has a 
negative charge after being rubbed with cat fur. Only good insulators show a 
net charge after rubbing, because electrical conduction permits neutralization 
of the charge very rapidly in other materials. See ELECTRIC CHARGE; ELECTRICAL 
INSULATION.  
    Electrostatic induction affords another way of charging a body where a 
charged, hard-rubber rod is responsible for the charging process. Since like 
charges repel each other and unlike charges attract each other, the negative 
charge on the rod repels some electrons to the opposite side of a metal 
sphere. This leaves an equal positive charge on the part of the sphere near 
the rod. If a grounded lead (for example, the experimenter's finger) is 
touched to the metal sphere, the unbound electrons flow off to ground leaving 
the positive charge bound by the presence of the charge on the rod. Then if 
the ground lead is removed and next the rod is removed, the metal sphere is 
left with a net positive charge which, in the end, is uniformly distributed 
over the surface of the sphere. When the charge on a conductor is static, that 
is, after it has completed its redistribution, it is in any case all on the 
surface of the conductor. 
    Electrostatic generators, sometimes called static machines, depend on 
electrification by friction and by induction for their operation. The simplest 
electrostatic generator is the electrophorus shown in the illustration. The 
hard-rubber plate R has been given a negative charge by rubbing it with cat 
fur. The metal plate D has an insulating handle H. When D is set on R, it 
touches it at only a few high points because R is microscopically rough, even 
though it appears polished to the eye. A positive charge is induced on the 
lower surface of D, and an equal, negative charge appears on the upper surface 
of D in the induction process. Now if a grounded conductor is touched to D, 
the electrons go off to ground and the ground-connection is then removed. 
Next, by using the insulating handle H, D is lifted from the rubber plate, and 
there is a net positive charge on D which can be shared with a receiver and 
used for experimentation. Since no charge is removed from R while charging by 
induction, D can be recharged from R as many times as the above process is 
repeated, and a large charge can be built up on the receiver. A rotating 
machine which would carry out the repetition of the induction process might be 
built from the electrophorous. The Wimshurst machine is an electrostatic 
generator based on this method of operation. For information on the important 
electrostatic devices used in nuclear physics see COCKCROFT-WALTON 
ACCELERATOR; IMPULSE GENERATOR; PARTICLE ACCELERATOR; RESONANCE TRANSFORMER; 
VAN DE GRAAFF GENERATOR. 
 �34s[Ralph P. Winch]
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�6v�2v�32sElectrostriction   �21m �33s 
    A form of elastic deformation of a dielectric induced by an electric 
field; specifically, the term applies to those components of strain which are 
independent of reversal of the field direction. Electrostriction is a property 
of all dielectrics and is thus distinguished from the converse piezoelectric 
effect, a field-induced strain which changes sign upon field reversal and 
which occurs only in piezoelectric materials. See DIELECTRICS; 
PIEZOELECTRICITY.  
    The electrostrictive effect in certain ceramics is employed for commercial 
purposes in electromechanical transducers for sonic and ultrasonic 
applications. See MICROPHONE; UNDERWATER TRANSDUCER. 
 �34s[Robert D. Waldron]
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�6v�2v�32sElectrotherapy �21m �33s
    Medical treatment which uses electrical techniques to change structure or
function of the body or a body part. In some methods the electricity is
clearly not the major active agent; in others the mechanism of action is
unknown. Therefore the classification is of questionable significance except
as it indicates technique: (1) High-frequency currents are used within
physiological limits as in diathermy, or beyond such limits as in the
destruction or removal of tissue by so-called electrosurgery. Another
application is in closing small bleeding vessels in surgery,
electrocauterization. (2) Low-frequency currents are used for passive exercise
of paralyzed muscles. (3) Iontophoresis depends upon the movement of ions in a
voltage gradient. If two electrodes are in contact with tissue (such as skin
and mucous membranes), then positively charged ions can be driven into the
tissue by a positive electrode, and negatively charged ions by a negative
electrode. (4) Electroshock therapy is a method of producing violent
excitation of the central nervous system which seems beneficial in certain
types of psychopathology. See ELECTROCONVULSIVE SHOCK.
 �34s[Julian M. Tobias /N. Karle Mottet]
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�6v�2v�32sElectrowinning �21m �33s
    An electrometallurgical process involving the recovery of a metal, usually
from its ore, by dissolving a metallic compound in a suitable electrolyte and
reducing it electrochemically through passage of a direct electric current.
Metals recovered by this process include zinc, manganese, copper, aluminum,
alkali metals, alkaline-earth metals, rare-earth metals.
    Following beneficiation and, sometimes, chemical pretreatment, the
metal-bearing constituent of the ore is dissolved in an aqueous solution or in
a molten salt. The electrolysis of the purified electrolyte with direct
current yields the reduced metal at the cathode (negative electrode of an
electrolytic cell), and the nonmetal is the oxidation product at the anode
(positive electrode). Since the process is very selective, the purity of the
electrowon metal is high, and no further refining is needed. See
ELECTROMETALLRGY.
 �34s[Paul Duby]
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�6v�2v�32sElementary particle �21m �33s
    A particle which, in the present state of knowledge, cannot be described
as compound. The elementary particles are the fundamental constituents of all
matter. Ordinary matter and radiation is composed of only four kinds of
elementary particles, namely protons, neutrons (bound in stable nuclei),
electrons, and photons. (Plus, strictly speaking, gravitons and neutrinos, but
these interact so weakly as to be negligible.) This is because other known
elementary particles are unstable; they decay spontaneously into two or more
other particles in an average time, called their lifetime or mean life, which
ranges from 2 x 10- sup 6s (muon) to less than 10- sup 2 sup 3s (hadron
resonances). Free neutrons are also unstable. On the other hand, although free
positrons and antiprotons are stable, in the presence of ordinary matter they
are rapidly destroyed by annihilation on electrons or nucleons, respectively.
See ANTIPROTON; POSITRON; RADIOACTIVITY.
    The unstable elementary particles must be studied within a short time of
their creation, which occurs in the collision of a fast (high-energy) particle
with another particle. In this process some of the kinetic energy of the
colliding particles is transformed into the rest mass of the created
particles, in accordance with Einstein's mass-energy relation, E = mc sup 2; in
the decay of particles, the reverse energy transformation takes place. Such
fast particles exist in nature, namely the cosmic rays, but their flux is
small; thus most elementary particle research is based on high-energy particle
accelerators. See NUCLEAR REACTION; PARTICLE ACCELERATOR; PARTICLE DETECTOR;
REST MASS.
    A particle, by its quantum mechanical definition, has a definite mass and
spin. These quantities are eigenvalues of symmetry operators generating
translations and rotations of spacetime. A particle is also characterized by
other quantum numbers, such as parity and lepton number, which are eigenvalues
of further symmetry operators that commute with mass and spin. See EIGENVALUE
(QUANTUM MECHANICS); NONRELATIVISTIC QUANTUM THEORY; QUANTUM MECHANICS;
QUANTUM NUMBERS; SYMMETRY LAWS (PHYSICS).
    All the particles of one kind are identical, and can be described as
quanta of fields; for each kind of particle there is an associated field. Each
particle is either a boson or a fermion, according to whether its spin is an
integral or a half-odd-integral multiple of PLANCK'S CONSTANT, Planck's
constant divided by 2PI. See QUANTUM FIELD THEORY; QUANTUM STATISTICS; SPIN
(QUANTUM MECHANICS).
    To each kind of particle there corresponds an antiparticle, or
charge-conjugate particle; if self-conjugate, the particle and antiparticle
are identical. The elementary particles can be grouped conveniently into three
classes:
    1. Classons, massless bosons which are quanta of the two classical fields,
gravitational and electromagnetic.
    2. Leptons, fermions which have interactions with the classical fields and
also weak interactions (beta interactions). The leptons are the electron, the
muon, formerly called the mu meson, the tau, and their associated neutrinos.
    3. Hadrons, or strongly interacting particles, which in addition have
strong interactions. Boson hadrons are called mesons. Fermion hadrons are
called baryons and carry one unit of an exactly conserved quantity, the baryon
number. Baryons include nucleons (proton and neutron), quasistable hyperons,
and numerous resonances. For detailed information on the various particles see
BARYON; ELECTRON; GRAVITON; HADRON; HYPERON; J PARTICLE; LEPTON; MESON;
NEUTRINO; NEUTRON; NUCLEON; PROTON; STRANGE PARTICLES; UPSILON PARTICLES.
    The conventional restriction of the term elementary particle to the types
of particles named above excludes electromagnetically bound particles (such as
atoms, molecules, and macroscopic particles) and hadrons with baryon number
greater than unity (atomic nuclei). The former can be described very
accurately by means of quantum electrodynamics; the latter can be described,
though less accurately, as bound or resonant states of baryons. The success in
describing them as compound particles is due to the smallness of their binding
energy, which is at most a few percent of the mass of their constituent
particles. The weakness of the binding between baryons is attributed to a
``repulsive core'' in their interaction. This is lacking in other combinations
of hadrons (baryon-meson, meson-meson), so that their binding is much
stronger. Consequently, the latter particles, hadrons with baryon number B =
-1,0,1, cannot easily be described as compounds, and so are termed elementary.
    Hadrons can be divided into the quasistable or (semistable) and the
unstable. The quasistable hadrons are simply those that are too light to decay
into other hadrons by way of the strong interactions, such decays being
restricted by the requirement that isotopic spin, I, strangeness, s, and
charm, c, be conserved. The unstable hadrons are also called
elementary-particle resonances or excited states. Their lifetimes, of the
order of 10(sup 23)s, are much too short to be observed directly. Instead they
appear, through the uncertainty principle, as spreads in the masses of the
particles - that is, in their widths - just as in the case of nuclear
resonances. See UNCERTAINTY PRINCIPLE.
    A characteristic of the hadrons is that they are grouped into
isotopic-spin (i-spin) multiplets (for example, n, p; PI-, PI sup 0, PI sup +);
the masses of the particles in each multiplet differ by only a few million
electronvolts. The hadrons exhibit a further grouping into supermultiplets of
i-spin multiplets, the masses of the multiplets in a supermultiplet differing
by a few hundred million electronvolts. These supermultiplets appear to be the
consequence of a symmetry that is less exact than i-spin symmetry. This
symmetry, termed unitary symmetry, is the symmetry with respect to a group of
transformations SU3. See ISOTOPIC SPIN.
    Elementary particles are classified by the values of certain conserved
quantities, or quantum numbers, which they carry. The spin J and parity P
correspond to symmetries of ordinary space. Quantum numbers corresponding to
an internal symmetry space are charge Q, charge parity C, lepton number,
baryon number, and, for hadrons, the not exactly conserved quantities i-spin
I, strangeness s, charm c, G (if baryon number B = 0 and s = c = 0), and
unitary spin, that is, the SU3 supermultiplet to which the particle belongs,
such as octet or deket.
    There are more than 75 known hadron multiplets; the actual number may well
be infinite. Attempts have been made to construct a theory which will explain
the existence and properties of these particles in an economical way. Broadly
speaking, two kinds of such theories have been proposed, constituent and
self-consistent. In constituent theories, hadrons are supposed to be composed
of truly elementary particles, analogous to the way in which atomic nuclei are
composed of nucleons. On the other hand, in self-consistent theories, the
properties of hadrons are supposed to be determined by the requirement that
their scattering amplitudes contain resonances, that is, hadrons, with the
same properties.
    In the most popular constituent theories the constitutents are called
quarks. By assuming that there are three equivalent quarks, all states made
from them will have SU3 symmetry. If it is further assumed that mesons consist
of a quark and an antiquark, qq, and baryons consist of three quarks, qqq
(each q in these formulas stands for any one of the three quarks), then mesons
will comprise only the multiplets 1 and 8, and baryons will comprise only 1,
8, and 10, in agreement with the data. For the baryons to be fermions, the
quarks must be so too; it is simplest to assume that they have spin 1/2. A
curious consequence of the construction of baryons as qqq is that each quark
must carry one-third of a unit of baryon number and a charge which is a
multiple of one-third of the electronic charge e.
    Despite extensive searches, no observation of a free quark has been
independently confirmed. However, a theoretical structure, quantum
chromodynamics, has been developed to understand and predict the behavior of
hadrons, based on the interactions of their constituent quarks. It has been
conjectured that one consequence of this theory is quark confinement, whereby
it is impossible for two quarks to separate freely to large distances, so that
free quarks cannot be isolated. See QUANTUM CHROMODYNAMICS; QUARKS.
    In 1974 the first of a new class of mesons was discovered independently at
Brookhaven National Laboratory and the Stanford Linear Accelerator Laboratory,
and was named J and PSI, respectively, by the discoverers. The J/PSI is
distinct from the previously known mesons due to the combination of its large
mass and small decay rate, which together means that it is strongly forbidden
to decay hadronically. The quark model can describe the J/PSI if extended by
the supposition that there is a fourth type or flavor of quark, the so-called
charmed quark, c. The c quark must be supposed much more massive than the
other three in order to explain the large mass of the J/PSI, which is
described as the ground state of cc. The system cc is called charmonium (in
analogy to positronium, ee). Since it is a cc state the J/PSI has no net
charm. The lightest charmed mesons are the D sup + and D sup 0, respectively
c2]d and c2]u states, and their antiparticles, first observed in 1976. See
CHARM. 
    In 1977 a new meson resonance was discovered at an energy of about 9.4 GeV
in a study of the production of MU sup + MU- pairs in proton-nucleus collisions.
Subsequent studies of these reactions and of electron-positron annihilation
showed that the new particle, the UPSILON (upsilon), is evidence of a fifth
type or flavor of quark, called b (for bottom or beauty).
 �34s[Charles J. Goebel]
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�6v�2v�32sElement 104 �21m �33s
    The first element beyond the actinide series, and the twelfth transuranium
element. In 1964 G. N. Flerov and coworkers at the Dubna Laboratories in the
Soviet Union claimed the first identification of element 104 and a little
later suggested the name kurchatovium (symbol Ku). The Dubna group claimed the
preparation of element 104, mass number 260, by irradiating plutonium-242 with
neon-22 ions. See ACTINIDE ELEMENTS; TRANSURANIUM ELEMENTS.
    Attempts were soon made at the then Lawrence Radiation Laboratory,
University of California at Berkeley, by A. Ghiorso and coworkers to achieve
additional confirmation of the Dubna discovery. By 1969 the Berkeley group had
succeeded in unambiguously discovering two alpha-emitting isotopes of element
104 with mass numbers 257 and 259 by bombarding (sup 249)Cf with (sup 12)C
and (sup 13)C projectiles from the Berkeley heavy-ion linear accelerator
(HILAC). Since the Berkeley group felt they had demonstrated conclusively that
the Dubna discovery was not valid, they suggested that element 104 be named
rutherfordium with the symbol Rf in honor of Lord Rutherford.
    A number of years after the original discovery at Berkeley, a team at Oak
Ridge National Laboratory confirmed discovery of the isotope (sup 257)Rf by
detecting the characteristic nobelium x-rays following alpha decay. However,
the International Union of Pure and Applied Chemistry has not yet adopted an
official name for element 104 because of the conflicting American-Soviet
claims.
 �34s[Albert Ghiorso]
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�6v�2v�32sElement 105   �21m �33s 
    A chemical element. It was synthesized and identified unambiguously for 
the first time in March 1970 at the heavy-ion linear accelerator (HILAC) in 
the Lawrence Radiation Laboratory, Berkeley, California, by A. Ghiorso and 
colleagues. They suggested that the new element be called hahnium, symbol Ha, 
in honor of Otto Hahn, the discoverer of nuclear fission. This name has not 
been officially accepted in the international committee on nomenclature 
because of a conflicting claim by a Soviet group. The International Union of 
Pure and Applied Chemistry (IUPAC) will establish the matter of priority of 
discovery so as to decide whether element 105 will be called hahnium or 
nielsbohrium, the Soviet choice. 
    The hahnium isotope had a half-life of 1.6 s and decayed by emitting alpha 
particles with energies 9.06 (55%), 9.10 (25%), and 9.14 (20%) MeV. It was 
shown to be of mass 260 by identifying lawrencium-256 as its daughter by two 
different methods. See NUCLEAR CHEMISTRY; TRANSURANIUM ELEMENTS. 
 �34s[Albert Ghiorso]
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�6v�2v�32sElement 106   �21m �33s 
    The element with the atomic number 106 was synthesized and identified in 
1974. This is the fourteenth of the synthetic transuranium elements. This 
discovery of element 106 took place nearly simultaneously in two widely 
separated nuclear laboratories, the Lawrence Berkeley Laboratory at the 
University of California and the Joint Institute for Nuclear Research at Dubna 
(near Moscow). Two independent and different approaches, using bombardment 
with heavy ions, were used in this difficult accomplishment. In view of the 
near simultaneity of the experiments at the two laboratories, and their very 
different nature, neither group has suggested a name for the element. 
Consequently, for the time being, it will have the simple designation 
``element 106.'' 
    On the basis of its projected position in the periodic table, element 106 
is expected to have chemical properties similar to those of tungsten (atomic 
number 74). See NUCLEAR CHEMISTRY; TRANSURANIUM ELEMENTS. 
 �34s[Glenn T. Seaborg]
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�6v�2v�32sElements �21m �33s
    An element is a substance made up of atoms with the same atomic number.
Some common elements are oxygen, hydrogen, iron, copper, gold, silver,
nitrogen, chlorine, and uranium. Approximately 75% of the elements are metals
and the others are nonmetals. Most of the elements are solids at room
temperature, two of them (mercury and bromine) are liquids, and the rest are
gases. A few of the elements are found in nature in the free (uncombined)
state. Some of these are oxygen, nitrogen, the noble gases (helium, neon,
argon, krypton, xenon, and radon), sulfur, copper, silver, and gold. Most of
the elements in nature are combined with other elements in the form of
compounds.
    The elements are classified in families or groups in the periodic table.
Elements are also frequently classified as metals and nonmetals. A metallic
element is one whose atoms form positive ions in solution, and a nonmetallic
element is one whose atoms form negative ions in solution. See PERIODIC TABLE.
    Atoms of a given element have the same atomic number, but may not all have
the same atomic weight (see table). Atoms with identical atomic numbers but
different atomic weights are called isotopes. All the elements have isotopes,
although in certain cases only synthetic isotopes are known. Many of the
isotopes of the different elements are unstable, or radioactive, and hence
disintegrate to form stable atoms either of that element or of some other
element. See ATOMIC WEIGHT; ISOTOPE; RADIOACTIVITY.
    The origin of the chemical elements is believed to be the result of the
synthesis by fusion processes at very high temperatures (in the order of
180,000,000 DG F or 100,000,000 DG C and higher) of the simple nuclear particles
(protons and neutrons) first to heavier atomic nuclei such as those of helium
and then on to the heavier and more complex nuclei of the light elements
(lithium, boron, beryllium, and so on). The helium atoms bombard the atoms of
light elements and produce neutrons. The neutrons are captured by the nuclei
of elements and produce heavier elements. These two processes - fusion of
protons and neutron capture - are the main processes forming the chemical
elements. See NUCLEOSYNTHESIS.
     A number of elements, found in only very slight traces or not at all in
nature, have been synthesized.	Those elements are technetium, promethium,
astatine, francium, and all the elements with atomic numbers above 92.	See
CHEMISTRY; TRANSURANIUM ELEMENTS.
 �34s[Alfred B. Garrett]
�33s

    Cosmic abundance.  The abundance of the elements in surface rocks of the
Earth, in the Earth as a whole, in meteorites, in the solar system, in
galaxies, or in the total universe corresponds to the average relative amounts
of the chemical elements present, or, in other words, to the average chemical
composition of the respective object. Element abundances are given in numbers
of atoms of one element relative to a certain number of atoms of a reference
element. Silicon is commonly taken as the reference element in the study of
the composition of the Earth and the meteorites, and the data are given in
atoms per 10(sup 6) atoms of silicon. The results of astronomical determinations
of the composition of the Sun and of the stars are often expressed in atoms
per 10(sup 10) atoms of hydrogen. Ordinary chemical analyses, including
advanced techniques for trace element studies (such as neutron activation or
isotope dilution), are used for determination of the composition of rocks and
meteorites. The composition of the Sun and of stars can be derived by
quantitative spectral analysis. On the surface of the Earth, the most abundant
elements are oxygen, silicon, magnesium, calcium, aluminum, and iron. In the
universe as a whole, hydrogen and helium constitute more than 95% of the total
matter. See ACTIVATION ANALYSIS; ASTRONOMICAL SPECTROSCOPY; ISOTOPE DILUTION
TECHNIQUES.
     The isotopic composition of the elements is practically the same in all
terrestrial material and in meteorites. From the isotopic composition of an
element and its cosmic abundance, the nuclear abundances of its isotopes can
be calculated. See ISOTOPE; NUCLEAR STRUCTURE.
    Nuclear abundance values show a clear correlation with certain nuclear
properties, and can be assumed to represent in good approximation the original
yield distribution of the thermonuclear processes that led to the formation of
the elements. The empirical abundance values can therefore serve as the basis
for theoretical considerations about the origin of matter and of the universe,
and have led to the following conclusion: No simple, single mechanism exists
by which the elements in their observed isotopic composition can have formed.
The matter of the cosmos appears to be a mixture of material that formed under
different conditions by different types of nuclear processes. See
NUCLEOSYNTHESIS.
 �34s[Hans E. Suess]
�33s

    Geochemical distribution.  The distribution of chemical elements in major
zones (crust, mantle, and core) of the Earth is dependent upon the early
history and subsequent evolution of both the Earth and solar system. Since
these events occurred long ago and there is no direct evidence for what
actually happened, considerable speculation is necessarily involved in
explaining the postulated present-day distribution of elements among the major
zones of the Earth.
    The proto-solar system, before it evolved into the Sun and orbiting
planets, was probably a rotating, swirling, lens-shaped cloud of gas, dust,
and other matter. The interior of this cloud contracted and heated primarily
through gravitational attraction, until finally temperatures and pressures
were high enough to initiate nuclear reactions, giving off light and heat.
Matter in swirling vortices within peripheral regions of the cloud eventually
coalesced and formed individual planets. Portions of lighter, more volatile
elements (such as N, C, O, and H) escaped outward from the hotter interior of
the system and were preferentially enriched in the less dense, larger outer
planets (Jupiter, Saturn, Uranus, and Neptune). Heavier, less volatile
elements (such as Ca, Na, Mg, Al, Si, K, Fe, Ni, and S) tended to remain
closer to the center of the system, eventually becoming preferentially
enriched in the more dense, smaller inner planets (Mercury, Venus, Earth, and
Mars). See SOLAR SYSTEM.
    Accretion of the Earth is believed to have taken place from a cloud whose
composition was quite similar to that of the type of stony meteorites known as
chondrites. The protoearth was probably a homogeneous, unzoned spheroid of
approximately chondritic composition. See METEORITE.
    Assuming the hypothesis of an initially unzoned Earth, and the chrondritic
Earth model hypothesis, the Ni-Fe alloy eventually formed the core, and the
remaining phases formed the mantle. At a very early stage in its history [4 -
5 x 10(sup 9) years ago], the Earth probably was mostly in solid form. Most
earth scientists now believe that subsequent heating, due to adiabatic
contraction and radioactive decay, led to a widespread melting event.  The
lowest-melting component, the Ni-Fe alloy, is believed to have melted initially;
because of its much greater density, this alloy would have settled and formed
the core.  This event has been referred to as the iron catastrophe.  Continued
melting would have created three immiscible liquids:  silicate, sulfide, and
alloy.	The remaining silicates, sulfides, and other compounds then would have
formed the surrounding mantle.
    New oceanic crust, composed primarily of basaltic rocks, is thought to be
forming at the mid-oceanic ridges (spreading centers) by means of partial
melting of the underlying mantle.  Relative to the mantle, the basaltic crust is
comparatively enriched in Si, Al, Ca, Na, K, and large ionic lithophile
elements, but is depleted in Mg, Fe, and certain transition metals (group VIII
in particular).  The process of partial melting of upper mantle material and
ascent of magma, eventually forming new crust, may be the dominant mechanism for
concentrating the enriched-in-crust elements in the overlying crust at the
expense of the mantle.
    Partial melting also goes on within the continental crust, leading to the
formation and ascent of magmas comparatively rich in enriched-in-crust elements
and poor in enriched-in-mantle elements relative to the rocks from which the
magmas derive.	These magmas tend to move upward through time, eventually
solidifying and forming a zoned continental crust, with the upper zone (sial)
having an overall granitic composition and the lower zone (sima) an unknown
composition, probably similar to that of basalt.  The upper granitic crust is
even more abundant in enriched-in-crust elements.  Further modifications of the
upper continental crust can occur through processes such as weathering
sedimentation, metamorphism, and igneous differentiation.  See GEOCHEMISTRY;
PLANETOLOGY; PLATE TECTONICS.
 �34s[Paul G.  Ragland]
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�6v�2v�32sElephant   �21m �33s 
    The common name for two living species of mammals in the family 
Elephantidae, one of several families included in the order Proboscidea. The 
remaining families contain extinct animals, such as the mammoth. One of the 
living species (Loxodonta africana) is indigenous to Africa, and the other 
(Elephas maximus) ranges throughout Southeast Asia. 
    These animals are terrestrial and entirely herbivorous. The nostrils and 
upper lip are elongated into a proboscis, the trunk, which is a powerful and 
sensitive organ specialized to form a prehensile, food-gathering structure. 
The upper incisors protrude from the mouth on either side of the trunk as 
tusks, which continually grow throughout the life of the animal. The hard, 
thick skin of the elephant is sparsely covered with hair, and serves as an 
insulator. The eyes are tiny but vision is keen. The tail is short; large 
columnar legs support the massive body. 
    Elephants live and travel in herds which were originally composed of 
several hundred individuals, but the usual number is now around 20 animals, 
with a mature bull, a number of cows and calves, and some younger bulls. 
Elephants are at ease in water, and they bathe and roll in the mud as a 
protective measure since, despite the thickness of the skin, they are 
sensitive to intense sun and insects. 
    Bulls reach sexual maturity at the age of 15, while cows mature earlier. 
The gestation period averages 20 - 22 months with a single calf being born. 
Twins are rare. The newborn, which may be 3 ft (0.9 m), tall and weigh 200 lb 
(90 kg), grows rapidly. See MAMMALIA; PROBOSCIDEA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sEleutherozoa   �21m �33s 
    One of the two subphyla into which the phylum Echinodermata had been 
customarily divided. The  Eleutherozoa are now best considered as comprising 
at least two distinct subphyla: (1) the Echinozoa, spherical-bodied forms with 
meridional symmetry; and (2) the Asterozoa, starshaped forms with radially 
divergent axes of symmetry. See ASTEROZOA; ECHINODERMATA; ECHINOZOA. 
 �34s[Howard B. Fell]
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�6v�2v�32sElevating machines   �21m �33s 
    Materials-handling machines that lift and lower a load along a fixed 
vertical path of travel with intermittent motion. In contrast to hoisting 
machines, elevating machines support their loads instead of carrying them 
suspended, and the path they travel is both fixed and vertical. They differ 
from vertical conveyors in operating with intermittent rather than continuous 
motion. Industrial lifts, stackers, and freight elevators are the principal 
classes of elevating machines. 
    A wide range of mechanically, hydraulically, and electrically powered 
machines are classified as industrial lifts (Fig. 1). They are adapted to such 
diverse operations as die handling and feeding sheets, bar stock, or lumber. 
In some locations with differences in floor level between adjacent buildings, 
lifts take the form of broad platforms to serve as floor levelers to obviate 
the need for ramps. They are also used to raise and lower loads between the 
ground and the beds of carriers when no loading platform exists. Lifting tail 
gates attached to the rear of trucks are similarly used for loading or 
unloading merchandise on sidewalks or roads and at points where the lack of a 
raised dock would make loading or unloading difficult. 
    Stackers are tiering machines and portable elevators used for stacking 
merchandise with basically portable vertical frames that support and guide the 
carriage, to which is attached a platform, pair of forks, or other suitable 
lifting device (Fig. 2). Horizontal movement is effected by casters on the 
bottom of the vertical frame, and can be accomplished manually, or 
mechanically, by using the same power source as the lifting mechanism. These 
casters are usually provided with floor locks bolted in position during the 
elevating or lowering operation. Used in conjunction with cranes, stackers are 
widely applied to the handling of materials on storage racks and die racks. 
    Examples of industrial elevators range from those set up temporarily on 
construction jobs for moving materials and personnel between floors to 
permanent installations for mechanized handling in factories and warehouses. 
Dumbwaiters are a type of industrial elevator; they carry parts, small tools, 
samples, and similar small objects between buildings, but are not permitted to 
carry people. The most common and economical elevator employs electric motors, 
cables, pulleys, and counterweights. See MATERIALS-HANDLING MACHINES. 
 �34s[Arthur M. Perrin]
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�6v�2v�32sElevator (aircraft)   �21m �33s 
    The hinged rear portion of the longitudinal stabilizing surface or tail 
plane of an aircraft used to obtain longitudinal- or pitch-control moments. 
The angular setting of the elevator is controlled by the human or automatic 
pilot through the flight-control system. A typical elevator control surface is 
shown in the illustration. See FLIGHT CONTROLS. 
    The elevator is used to perform pitching maneuvers, or maneuvers in which 
the aircraft's plane of symmetry is not disturbed. These maneuvers include 
airspeed adjustments and acceleration normal to the flight path (pull-ups or 
push-downs). The elevator also serves to adjust the aircraft's attitude with 
respect to the ground for takeoff and landing. 
 �34s[Malcolm J. Abzug]
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�6v�2v�32sElevon   �21m �33s 
    The hinged rear portion of an aircraft wing, moved in the same direction 
on each side of the aircraft to obtain longitudinal control and differentially 
to obtain lateral control. The word elevon is a combination of the words 
elevator and aileron, to denote that elevons combine the functions of aircraft 
elevators and ailerons. See AILERON; ELEVATOR (AIRCRAFT). 
    Elevons are used exclusively on tailless aircraft, or specifically those 
without horizontal stabilizers. Aircraft in this category include the modern 
delta wing fighter aircraft. In addition, elevons are also required on 
lifting-body versions of crewed space vehicles. 
 �34s[Malcolm J. Abzug]
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�6v�2v�32sEllipse �21m �33s
    A member of the class of curves that are intersections of a plane with a
cone of revolution. The ellipse is obtained when the plane cuts all the
elements of one nappe, and does not go through the apex. In the figure, denote
the distance between two points F, F' of a plane by 2c, c  > 0, and let 2a be
a constant, with a > c. The ellipse with foci F and F' and major axis 2a is
the locus of points P of the plane such that PF + PF' = 2a, where PF denotes
the distance of P and F. This suggests the following construction of an
ellipse. Put pins at F and F', and slip over them a loop of thread of length
2a + 2c, pulling the thread taut with a pencil. If the pencil is moved,
keeping the thread taut, its point traces an ellipse. See CONIC SECTION.
    The midpoint of F, F' is the center O of the ellipse, and the chord
through O perpendicular to the major axis is the minor axis, whose length is
denoted by 2b. If B is a point in which the minor axis intersects the ellipse,
then BF = BF' = a, and so c sup 2 = a sup 2 - b sup 2. The ratio c/a =
EPSILON < 1 is the eccentricity of the ellipse. See ANALYTIC GEOMETRY.
    Figure caption: An ellipse, as described in the text.
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sEllipsoid and spheroid   �21m �33s 
    An ellipsoid is a quadric surface of finite extent representable in 
analytic geometry by an equation of the second degree in three variables (such 
as x, y, z). Ellipsoids include spheres, spheroids obtained by stretching or 
shrinking a sphere in one direction, and surfaces obtained by stretching a 
sphere in two mutually perpendicular directions. Every plane section of an 
ellipsoid is either an ellipse or a single point (provided that a circle is 
considered to be a special case of an ellipse). The centers of a set of 
parallel plane sections of an ellipsoid lie on a line segment called a 
diameter of the ellipsoid, whose midpoint is the center of the ellipsoid. 
Chords of the ellipsoid through the centers are diameters and are bisected by 
the center. See ANALYTIC GEOMETRY; SURFACE AND SOLID OF REVOLUTION. 
 �34s[James S. Frame]
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�6v�2v�32sElliptic function and integral   �21m �33s 
    In a certain sense, elliptic integrals are the simplest integrals not 
expressible in terms of elementary functions; elliptic functions arise as the 
inverse functions of certain elliptic integrals. 
    Let R be a rational function of x and y, and set I = INTEGRALR(x,y) dx. I 
can be expressed in terms of elementary functions if ysup2 is a polynomial of 
degree 2 or less in x. If ysup2 is a polynomial of degree 3 or 4 in x, I 
cannot in general be expressed in terms of elementary functions and is called 
an elliptic integral. The standard elliptic functions are analogous to 
trigonometric functions. Trigonometric functions may be defined as the inverse 
functions of certain integrals of the form I; they satisfy differential 
equations, are periodic functions, and may alternatively be obtained as the 
``simplest'' periodic functions. The standard elliptic functions are the 
inverse functions of certain elliptic integrals; they satisfy differential 
equations of order 1 and degree 2, are doubly periodic functions, and may 
alternatively be obtained as the ``simplest'' doubly periodic functions. 
    In geometry elliptic functions or integrals arise in determining the 
length of an arc of an ellipse, hyperbola, or lemniscate, the surface of an 
ellipsoid, geodesics on quadrics of revolution, parametric representations of 
plane cubic curves or, more generally, curves of genus 1, conformal 
representation problems, and other problems. In analysis there are 
applications to differential equations (Lame's equation, diffusion equation, 
and others); in number theory, in a great variety of problems. In the physical 
sciences elliptic functions or integrals appear in potential theory both 
through conformal representations and in the potential of an ellipsoid, in the 
theory of elastica, the pendulum, in rigid body motion, in Green's functions 
in heat conduction and diffusion theory, and many other problems. 
 �34s[A. Erdelyi]
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�6v�2v�32sElm �21m �33s
    Any species of Ulmus, a genus of hardwood trees in the Northern
Hemisphere, with simple, serrate, deciduous leaves. The American or white elm
(U. americana) is the most important species. It ranges from the eastern half
of the United States westward as far as the base of the northern Rockies and
southward through central Texas to the Gulf of Mexico. The tree is also found
in southern Canada.
    The tree was very popular as a shade tree, perhaps better adapted as a
street tree than any other species because the upper branches spread, joining
with elms across the street to form an arch. Once abundant, the elm has been
severely attacked by the lethal Dutch elm disease imported from Europe. The
future of the species is uncertain.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sElopiformes   �21m �33s 
    A primitive order of soft-rayed, teleost fishes, including the tarpons, 
bonefishes, and their relatives, that was formerly included in the 
Clupeiformes. The elopiforms have a single dorsal fin composed of soft rays 
only (see illustration). The most important characteristic of the order 
involves the occurrence in early development of a leptocephalous larval stage. 
This larva is translucent, ribbonlike, and strongly toothed; eventually it 
passes through a marked transformation to adult form. This developmental 
pattern characterizes only two other orders of teleosts, the Notacanthiformes 
(spiny eels and halosaurs) and the Anguilliformes (true eels), groups which 
are therefore believed to be descendants of elopiforms. See ANGUILLIFORMES; 
CLUPEIFORMES; NOTACANTHIFORMES. 
    The elopiforms have a well-represented paleontological history since the 
Jurassic, on all continents. They were especially abundant in the Cretaceous. 
Extant forms are classified in 2 families, 4 genera, and 12 species. Most 
inhabit tropical to temperate shore waters of all oceans, but tarpons invade 
rivers, sometimes for long distances. See ACTINOPTERYGII; TARPON; TELEOSTEI. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sEmbioptera   �21m �33s 
    A peculiar order of silk-spinning, orthopteroid insects related to 
termites, commonly called the embiids or web spinners. This order comprises 
about 1000 species which are chiefly tropical in distribution. The body is 
linear and supple (see illustration). The legs are short with three-segmented 
tarsi. The forelegs are adapted for spinning silk, and the hindlegs for 
reverse locomotion. Metamorphosis is incomplete. The females are neoteinic and 
wingless (apterous). Males are usually winged (alate), but in certain genera 
and species they are apterous. The wings are subequal and elongate. The wings 
are flexible when in repose and folded over the back, but are stiffened when 
extended for flight by the blood pressure in the saclike veins. Flight is a 
poorly directed, whirling flutter. 
    The silk galleries, which constitute a safe shelter for all embiid 
activities except adult dispersal, radiate on or in the food supply which also 
constitutes the habitat and consists of bark, lichens, moss, dead leaves, or 
grass. Many individuals, usually the brood of one female, may occupy one 
gallery system. See INSECTA; ORTHOPTERA. 
 �34s[Edward S. Ross]
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�6v�2v�32sEmbolism   �21m �33s 
    The sudden blocking of an artery or vein by a clot or other substance 
which has been brought to its place by the blood current. The material carried 
in the circulation in this process is an embolus. Emboli may be composed of 
thrombi, fat, air, tumor cells, masses of bacteria or parasites, bone marrow, 
amniotic fluid, or atheromatous material from the vessel wall. With complete 
obstruction of a vessel by an embolus an infarct may result. See EMBOLUS; 
INFARCTION.  
 �34s[Romeo A. Vidone]
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�6v�2v�32sEmbolus �21m �33s
    An abnormal mass or particles carried in the bloodstream to a site too
small to allow passage so that blood obstruction usually follows. A majority
of all emboli originate as fragments of thrombi which have broken off the
fixed clot. Clusters of tissue cells, bacteria, or parasites, and bubbles of
gas, air, or foreign materials may also become emboli. See EMBOLISM.
    Venous emboli, 95% of which originate in the leg veins, may pass through
the heart to lodge in the lungs. These frequently fatal pulmonary emboli
originate in patients with poor circulation produced by age, lack of activity,
pregnancy, or varicosities, or following operations or other conditions in
which the venous blood is slowed down or immobilized so that an embolus forms.
See CIRCULATION DISORDERS.
    Arterial emboli commonly arise from intracardiac or aortic thrombi. In
this type lodgement occurs in the brain, legs, spleen, and kidneys, and damage
may be severe or fatal.
    Air embolism may be a complication of a surgical procedure, particularly
those about the neck. It can also result from rapid decompression, as in
caisson disease, with the formation of bubbles of nitrogen in the blood.
Infected emboli can form new foci of infection at their sites of lodgement.
See DECOMPRESSION ILLNESS.
    Trauma to fatty tissues or the fracture of bones containing fatty marrow
may release small fat globules. Since most of the droplets can pass singly
through various circulatory beds, any effects are usually the result of
accumulation, which occurs most often in the lungs and in the brain with
variable results.
    Bacteria may, in certain cases, form septic emboli which are
characteristic of some infections. One common method of tumor spread is by
means of blood-borne, malignant emboli which have become dislodged from the
parent neoplasm. See ONCOLOGY.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sEmbrithopoda   �21m �33s 
    An order established for the unique mammal Arsinoitherium, which has been 
found only in early Oligocene deposits in northern Egypt. This animal was of 
rhinoceros size with a large body and short pillarlike legs (see 
illustration). There were two huge, scimitarlike horns over the nose and two 
much smaller, peglike horns over the eyes. The exact relationship of this 
exotic order to other orders is not clear, but similarities to both the 
uintatheres, or Dinocerata, and Hyracoidea have been interpreted by some 
authorities as evidence of kinship. See DINOCERATA; MAMMALIA. 
 �34s[Malcolm C. McKenna]
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�6v�2v�32sEmbrittlement   �21m �33s 
    A general set of phenomena whereby materials suffer a marked decrease in 
their ability to deform (loss of ductility) or in their ability to absorb 
energy during fracture (loss of toughness), with little change in other 
mechanical properties, such as strength and hardness. Embrittlement can be 
induced by a variety of external or internal factors, for example, (1) a 
decreasing or an increasing temperature; (2) changes in the internal structure 
of the material, namely, changes in crystallite (grain) size, or in the 
presence and distribution of alloying elements and second-phase particles; (3) 
the introduction of an environment which is often, but not necessarily, 
corrosive in nature; (4) an increasing rate of application of load or 
extension; and (5) the presence of surface notches. 
    Low-temperature embrittlement results from a competition between 
deformation and brittle fracture, with the latter becoming preferred at a 
critical temperature. For a material to be useful structurally, it is 
desirable that this critical temperature be below the minimum anticipated 
service temperature; in most cases, this is room temperature. At high 
temperatures, internal structural changes that lead to intergranular 
embrittlement can occur. Embrittlement usually occurs in the creep temperature 
range, a temperature at which deformation can occur under very low stresses; 
and the two processes are believed to be connected. 
    In many metals, particularly structural steels, annealing or heat treating 
in certain temperature ranges sensitizes the grain boundaries in such a way 
that intergranular embrittlement subsequently occurs during service. To reduce 
the brittleness, the steel undergoes an annealing treatment called tempering, 
which, while decreasing the strength, usually increases the toughness. The 
exception to this trade-off occurs when the steel is tempered at 1000DGF 
(538DGC). This can lead to a mode of intergranular fracture called temper 
embrittlement; such a process has led to catastrophic failures in turbines, 
rotors, and other high-strength steel parts. In other metals, there are less 
specific but similar types of embrittlement resulting from critical heat 
treatments. See HEAT TREATMENT (METALLURGY); TEMPERING. 
    Metals can fracture catastrophically when exposed to a variety of 
environments. These environments can range from liquid metals to aqueous and 
nonaqueous solutions to gases such as hydrogen. 
    If a thin film of a liquid metal is placed on the oxide-free surface of a 
solid metal, the tensile properties of the solid metal will not be affected, 
but the fracture behavior can be markedly different from that observed in air. 
Although many different liquid metals are capable of inducing embrittlement in 
a variety of solid metals, some of the more common couples, many of which have 
important engineering and design consequences, are mercury embrittlement of 
brass, lead embrittlement of steel, and gallium embrittlement of aluminum. 
    Stress corrosion cracking can occur when a metal is stressed and 
simultaneously exposed to an environment which may be, but is not necessarily, 
corrosive in nature. Both stress and environment are required; if only one of 
these elements is present, the metal usually displays no embrittlement. See 
CORROSION.  
    Hydrogen embrittlement is a form of embrittlement often considered to be a 
type of stress corrosion cracking. Hydrogen atoms can enter a metal, causing 
severe embrittlement, again with little effect on other mechanical properties. 
This phenomenon was originally observed, and is most critical in, steels, but 
it is not documented to occur in titanium and nickel alloys, and may lead to 
cracking in other alloy systems as well. 
    Factors such as notches and the rate of application of stress can modify 
the response of a material to a specific type of embrittlement. In general, 
notches or surface flaws always enhance embrittlement, both by acting as a 
stress raiser and by providing a preexisting crack. 
 �34s[I. M. Bernstein]
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�6v�2v�32sEmbryobionta   �21m �33s 
    One of the two plant subkingdoms, the other being the Thallobionta. The 
Embryobionta are here considered to include eight divisions, the Rhyniophyta, 
Bryophyta, Psilotophyta, Lycopodiophyta, Equisetophyta, Polypodiophyta, 
Pinophyta, and Magnoliophyta. The Rhyniophyta are represented only by 
Paleozoic fossils, but the other seven divisions have both modern and fossil 
representatives. See the separate articles on each division. 
    The Embryobionta differ from the green algae (Chlorophyta) and from most 
Thallobionta in that the normal life cycle of the Embryobionta shows a 
well-marked alternation of generations  in which the sporophyte 
(spore-producing, typically diploid) generation always begins its development 
as a parasite on the gametophyte (gamete-producing, typically haploid) 
generation. The young sporophyte is called an embryo. 
    The more primitive divisions of Embryobionta have the gametes produced in 
multicellular sex organs (archegonia and antheridia), in contrast to the 
unicellular oogonia and antheridia of the Thallobionta in general. In the more 
advanced divisions (Pinophyta and Magnoliophyta) of Embryobionta, the 
antheridia and archegonia are highly modified or entirely suppressed, in 
conformity with the general reduction of the gametophyte generation. 
    All divisions of Embryobionta except the Bryophyta have specialized 
conducting tissues (xylem and phloem) in the sporophyte. With the exception of 
most bryophytes, they also commonly have a characteristic stomatal apparatus 
which controls the opening and closing of numerous tiny pores (stomates) in 
the leaves and stems in response to environmental conditions. These 
specializations, together with the progressive reduction of the gametophyte in 
the more advanced divisions, reflect the progressive evolutionary adaptation 
of the Embryobionta to life on dry land instead of in the ancestral water. The 
Embryobionta are therefore often called the land plants, in spite of the fact 
that many of them, such as the water lilies, have returned to an aquatic 
habitat. The seven divisions of Embryobionta which characteristically have 
xylem and phloem have sometimes been treated as a single comprehensive 
division under the name Tracheophyta. See PLANT KINGDOM; THALLOBIONTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sEmbryogenesis   �21m �33s 
    The formation of an embryo from the egg, showing the basic pattern of 
organization of the animal, with the rudiments of most organ systems at least 
outlined. The axial organization of the embryo (anteroposterior and 
dorsoventral axes, giving the embryo its bilateral symmetry) can often be 
traced back to the egg before or after fertilization. See DEVELOPMENTAL 
BIOLOGY; FATE MAPS (EMBRYOLOGY); MEROGONY. 
 �34s[Gerhard Fankhauser]
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�6v�2v�32sEmbryology   �21m �33s 
    The study of the development of the organism from the zygote, or 
fertilized egg. It confines itself mainly to the study of the development of 
many-celled organisms, since reproduction in one-celled organisms is usually 
carried out by means other than through the formation of zygotes. Embryology 
classifies the processes of development into a number of overlapping phases. 
These are gametogenesis, fertilization, cleavage, and gastrulation. See 
BLASTULATION; CLEAVAGE (EMBRYOLOGY); GAMETOGENESIS; GASTRULATION; REPRODUCTION 
(ANIMAL).  
    Prior to gastrulation, the cells of any particular embryo may differ from 
each other in their size, inclusions, pigments, and so forth. However, they 
are still essentially similar in that they are undifferentiated; that is, they 
have not yet attained the structural and functional specialization that marks 
their adult state. The beginning manifestations of their differentiation 
become visibly apparent usually after gastrulation. The process of 
gastrulation therefore has received great attention from embryologists who are 
interested in investigating the factors that influence cellular 
differentiation.  
    Embryologists study not only the differentiation of individual cells but 
also changes in the size and shape of aggregates of cells constituting 
tissues, organs, and the whole organism. The sum total of these processes is 
called morphogenesis. Morphogenesis, or the development of form, occurs 
concomitantly with all the other phases of development and includes them all 
since it deals with the differentiation and growth of the whole and its parts. 
See ANIMAL MORPHOGENESIS; EMBRYONIC DIFFERENTIATION. 
    The constitution and organization of eggs, and their specific patterns and 
procedures of development, differ so markedly in different species that 
embryology has not succeeded in formulating any satisfactory general theory of 
development. The main generalizing concept to have emerged is the principle of 
progressive differentiation, which postulates that the events of each phase of 
development are intimately related to events of the phases immediately 
preceding and following. See DEVELOPMENTAL BIOLOGY. 
 �34s[Jane M. Oppenheimer]
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�6v�2v�32sEmbryonated egg culture   �21m �33s 
    Embryonated eggs are among the most useful and available forms of living 
animal tissue for the isolation and identification of animal viruses, for 
titrating viruses, and for quantity cultivation in the production of viral 
vaccines. The embryo proper, chorioallantoic membrane, yolk sac, allantoic 
sac, or amniotic sac may be inoculated in hen's eggs of various ages, so that 
a wide choice of types of tissue is available to fit the characteristics of 
the virus under study or for special studies. The chorioallantoic membrane is 
frequently used; in some infections, such as smallpox, vaccinia, and herpes 
simplex, characteristic lesions are produced which in some cases may resemble 
those in the natural host. See ANIMAL VIRUS; VACCINE. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sEmbryonic differentiation   �21m �33s 
    The process by which specialized and diversified structures arise during 
development of the embryo. The process involves (1) an increase in the number 
of cell types, and (2) an increase in morphological heterogeneity through the 
arrangement of cells into increasingly complex structural patterns in the form 
of tissues and organs. See HISTOGENESIS. 
    Differentiation begins in most organisms with fertilization of an egg with 
a sperm, after which the relatively large egg divides into many smaller cells 
called blastomeres. The blastomeres receive unequal portions of the 
cytoplasmic materials of the egg and are therefore initially somewhat 
different from each other. At the end of cleavage, the blastomeres are 
organized into a blastula, commonly either a hollow ball of cells or a 
flattened two-layered disk of cells. The cells of the blastula lie in 
different relative positions from those that will be occupied by their 
descendants in the adult organism. By a process known as gastrulation, they 
move to their approximate final positions and are arranged into three basic 
layers, called germ layers. However, only two layers form in the simpler 
multicellular organisms. The outer layer is the ectoderm, from which arise the 
nervous system and the epidermal layer of the skin. The innermost germ layer, 
the entoderm, forms the epithelial lining of the digestive tract and 
contributes the essential tissue of associated organs. In all but the most 
primitive animals a third germ layer, the mesoderm, is formed by cells which 
come to lie in the area between the other two layers. In higher animals the 
mesoderm gives rise to most of the cells of the organism, such as those found 
in the muscles, skeleton, blood, connective tissue, kidneys, gonads, and 
certain other organs. The molding of groups of embryonic cells into such 
diverse tissues and organs proceeds through a variety of morphogenetic 
processes, such as migration, aggregation, dispersion, delamination, folding, 
and differential local growth of cells. See BLASTULATION; GASTRULATION; GERM 
LAYERS.  
    The mechanisms by which the course of cellular differentiation is realized 
are not precisely known. The factors involved may, however, be divided into 
two classes: (1) intrinsic, those operating within the cell, and (2) 
extrinsic, those brought to bear upon the cell from outside. The relative 
importance of these factors varies considerably from one cell strain to 
another and also within the same cell at different stages in its development. 
    The fertilized egg begins development with a rich endowment, consisting of 
a nucleus with a set of paternal and maternal chromosomes together with a 
complexly organized cytoplasm. The emergence of new cell characteristics may 
be attributed to an oscillating interaction between the intrinsic gene makeup 
of the cell and the surrounding cytoplasm. The dynamic imbalance existing 
between these interacting components drives the cell along its path of 
differentiation. In certain kinds of invertebrate embryos, interactions within 
each separate cell seem sufficient for guiding differentiation to its terminal 
state. Such embryos exhibit mosaic development. By contrast, in the embryos of 
vertebrates and certain invertebrates such as echinoderms, influences from 
adjacent cells are an essential part of the differentiation process. These 
embryos show regulative development. See CELL LINEAGE; EMBRYONIC INDUCTION. 
 �34s[Clement L. Markert]
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�6v�2v�32sEmbryonic induction   �21m �33s 
    Each tissue of multicellular animals is formed during embryogenesis by a 
specific cell group located at a definite site in the early embryo. In many 
cases such a cell group is able to develop into the corresponding tissue only 
under a specific influence of a neighboring cell group. Such an influence is 
called embryonic induction. Thus in embryonic induction two cell groups 
interact: the reacting system which develops into the tissue in question, and 
is influenced by the other cell group in its developmental pathway; and the 
acting system which contributes to formation of the tissue in question by 
controlling the mode of development of the former group, but not by providing 
the induced tissue with cellular material. It should be emphasized, however, 
that in the absence of the acting system the reacting system can proceed in 
development and form a specific tissue which is different from that produced 
in the presence of the acting system. Thus embryonic induction consists in a 
shift in the developmental pathway of the reacting system, rather than in 
initiating development in the same system. The acting system is often called 
the inducer or inductor. These two expressions, however, are also used to 
denote chemical or physical factors which affect the developmental pathway of 
the reacting system. 
    The ability of the reacting system to respond to the inductive stimulus, 
often called competence, starts to appear at a developmental stage and 
disappears in many cases at a later stage. These critical stages are different 
in different types of embryonic induction. See DEVELOPMENTAL BIOLOGY. 
 �34s[Tuneo Yamada]
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�6v�2v�32sEmerald   �21m �33s 
    The medium- to dark-green gem variety of the mineral beryl, crystallizing 
in the hexagonal system. A flawless emerald with good color is one of the most 
sought after and highly prized of all precious gems. Emerald is restricted in 
its occurrence, and only infrequently are exceptional stones found; most 
emeralds are flawed and cloudy, and few stones command high prices. 
    In contradistinction to beryl and its other gem varieties, emeralds have 
only been found in mica schists or metasomatized limestones. The most 
outstanding occurrences include the Muzo and El Chivor mines in Colombia. 
Noteworthy occurrences in mica schists include Tokovoja in the Ural Mountains, 
where emerald occurs with the beryllium minerals chrysoberyl (and its gem 
variety alexandrite) and phenakite; Habachtal, Austria; Transvaal, South 
Africa; and Kaliguman, India. The ultimate source of an emerald can often be 
assessed by a study of its inclusions. See BERYL; GEM. 
 �34s[Paul B. Moore]
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�6v�2v�32sEmery   �21m �33s 
    A natural mixture of corundum with magnetite or with hematite and spinel. 
Because the mixture is very intimate and appears to be quite homogeneous, it 
was considered to be a single mineral species until the middle of the 19th 
century. The aggregate has a gray-to-black color and is extremely tough and 
difficult to break. The specific gravity varies from 3.7 to 4.3, depending 
upon the relative amounts of the constituent minerals. The hardness is about 8 
(Mohs scale), less than that of pure corundum which is 9, and is more 
dependent upon the physical state of aggregation than on the percentage of 
corundum. See CORUNDUM. 
    Although synthetic abrasives have replaced emery in many of its earlier 
uses, it is still used as an abrasive and polishing material by lapidaries and 
in the manufacture of lenses, prisms, and other optical equipment. Emery 
wheels, emery paper, and emery cloth are used not only by lapidaries but also 
by machinists in the grinding and polishing of steel. See ABRASIVE; GRINDING; 
MAGNETITE.  
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sEmission electron microscope   �21m �33s 
    An electron microscope in which electrons emitted from a metal surface are 
projected on a fluorescent screen, with or without focusing. These may take 
several forms (see illustration). In the immersion electron microscope the 
specimen is a flat conducting surface which may be heated, illuminated, or 
bombarded by high-velocity electrons or ions so as to emit low-velocity 
thermionic, photo-, or secondary electrons. These are accelerated to a high 
velocity in an immersion objective or cathode lens and imaged as in a 
transmission electron microscope. See ELECTRON MICROSCOPE. 
  
    Other emission microscopes employ simple projection without lenses. One 
example is the cylindrical microscope used for examining the thermionic 
emission from a thin wire stretched along the axis of a cylindrical tube whose 
walls at high positive potential are coated with a fluorescent substance. 
Another is the field-emission microscope, in which field electron currents are 
drawn from an extremely fine, rounded point on the end of a wire of a metal 
such as tungsten. The electrons drawn out of the point follow nearly straight 
lines toward a positive-potential screen. The display on the screen shows 
variations in the emission characteristics (as determined by the work function 
and surface nonuniformities) over the surface of the point. See FIELD-EMISSION 
MICROSCOPY; THERMIONIC EMISSION. 
 �34s[Edward G. Ramberg]
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�6v�2v�32sEmission spectrochemical analysis �21m �33s
    A technique conducted by monitoring and measuring the spectrum of light
caused to be emitted by the material to be analyzed. In general, there are
many ways in which to conduct an emission spectrometric measurement; the
differences among approaches result mainly from the choice of location within
the electromagnetic spectrum at which to observe emitted radiation. However,
emission spectrochemical analysis traditionally refers to those analytical
determinations based on radiation in the visible through vacuum ultraviolet
region of the electromagnetic spectrum (wavelengths about 800 to 100
nanometers). The technique is used principally to detect (qualitative
analysis) and determine (quantitative analysis) metals and some nonmetals.
Under optimum conditions, as little as 10-(sup 10) gram of an element per gram
of sample can be determined.
    The steps in emission spectrochemical analysis are: vaporization and
atomization of sample; excitation of atomic vapor; resolution of emitted
radiation; and observation and measurement of resolved radiation. See
SPECTROSCOPY.
 �34s[Andrew T. Zander]
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�6v�2v�32sEmissivity   �21m �33s 
    The ratio of the radiation intensity of a nonblackbody to the radiation 
intensity of a blackbody. This ratio, which is usually designated by the Greek 
letter EPSILON, is always less than or just equal to one. The emissivity 
characterizes the radiation or absorption quality of nonblack bodies. 
Published values are readily available for most substances. Emissivities vary 
with temperature and also vary throughout the spectrum. For an extended 
discussion of blackbody radiation and related information see HEAT RADIATION. 
 �34s[Heinz G. Sell ;Peter J. Walsh]
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�6v�2v�32sEmitter follower   �21m �33s 
    A circuit that utilizes a common-collector transistor which provides less 
than unity voltage gain but high input resistance and low output resistance. 
This circuit is used extensively to provide isolation or impedance matching 
between two electronic circuits. 
    The most common use for the emitter follower is as a device which performs 
the function of impedance transformation over a wide range of frequencies with 
voltage gain close to unity. In addition, the emitter follower increases the 
power level of the signal. 
 �34s[Christos C. Halkias]
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�6v�2v�32sEmotion   �21m �33s 
    An umbrella concept in the common language, typically defined by 
instantiation by reference to a variety of mental and behavioral states. These 
range from lust to a sense of liking, from joy to hostile aggression, and from 
esthetic appreciation to disgust. Emotions are usually considered to be 
accompanied by some degree of internal, frequently visceral, excitement, as 
well as strong evaluative components. Emotions are also often described as 
irrational, that is, not subject to deliberative cogitation, and as 
interfering with normal thought processes. 
    These latter qualities are often exacerbated in the emotional behavior and 
expression seen in clinical cases. The expression of strong emotions is 
typically considered to be symptomatic of some underlying conflict, and even 
the positive emotions are used as indices of unusually strong attachments and 
atypical earlier experiences. Sigmund Freud introduced the concept of 
repression to describe a defense mechanism against the occurrence of strong 
emotional experiences. From the psychoanalytic point of view, what is 
repressed is not the emotion itself, since the very concept of emotion implies 
conscious experience, but rather the memory of an event which, if it became 
conscious, would lead to strong conflicts and emotional consequences. Many 
other defense mechanisms, such as rationalization and compulsive or obsessive 
neurotic symptoms, are also seen as serving the purpose of avoiding conscious 
conflict and emotional sequelae. See NEUROSIS; PSYCHOANALYSIS. 
 �34s[George Mandler]
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�6v�2v�32sEmphysema   �21m �33s 
    A disorder of pulmonary inflation characterized by enlargement and 
destruction of the air spaces. The key element in this definition is the word 
destruction for it implies the irreversible loss of a given area of the 
pulmonary parenchyma. Certain variants of this condition do not necessarily 
imply irreparable destruction of pulmonary tissue but rather overdistention of 
air spaces, and consequently are not properly classified as emphysema. 
    Generalized emphysema probably has many causes; most share chronic 
bronchiolitis as a factor. Narrowing at this level would cause retention of 
air, leading to dilatation and rupture of alveolar septa. Increasing attention 
is being given to heavy cigarette smoking and air pollution as contributing 
factors. Given the dilatation of the air spaces, the total air space in the 
lungs is increased. However, the lungs cannot be properly emptied and are 
functionally impaired. 
    Emphysema, if widespread, will cause very serious limitation in physical 
activity. Many cases, however, are compatible with long survival. 
Complications of severe emphysema include right heart failure (cor pulmonale), 
respiratory acidosis, and rupture of bullae with development of pneumothorax. 
    The important variants of emphysema are as follows. Centrilobular 
emphysema affects predominantly respiratory bronchioles without involvement of 
the more peripheral elements. In diffuse vesicular emphysema, the most common 
form, all elements of the respiratory unit (respiratory bronchiole, alveolar 
ducts, alveolar sacs, and alveoli) are dilated. Senile emphysema was formerly 
applied to barrel-chested elderly people; however, functional impairment is, 
in most cases, inconspicuous. This condition is also known as aging lung. See 
BRONCHIAL DISORDERS. 
 �34s[Victor E. Goldenberg]
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�6v�2v�32sEmpirical method �21m �33s
    The empirical method is generally characterized by the collection of a
large amount of data before much speculation as to their significance, or
without much idea of what to expect, and is to be contrasted with more
theoretical methods in which the collection of empirical data is guided
largely by preliminary theoretical exploration of what to expect. The
empirical method is necessary in entering hitherto completely unexplored
fields and becomes increasingly less purely empirical as the greater is the
acquired mastery of the field. Successful use of an exclusively empirical
method demands a high degree of intuitive ability in the practitioner.
 �34s[Percy W. Bridgman /Gerald Holton]
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�6v�2v�32sEmulsion �21m �33s
    A dispersion of one liquid in a second immiscible liquid. Since the
majority of emulsions contain water as one of the phases, it is customary to
classify emulsions into two types: the oil-in-water (O/W) type consisting of
droplets of oil dispersed in water, and the water-in-oil (W/O) type in which
the phases are reversed. The continuous liquid is referred to as the
dispersion medium, and the liquid which is in the form of droplets is called
the disperse phase.
    A stable emulsion consisting of two pure liquids cannot be prepared; to
achieve stability, a third component, an emulsifying agent, must be present.
Generally, the introduction of an emulsifying agent will lower the interfacial
tension of the two phases. A large number of emulsifying agents are known;
they can be classified broadly into several groups. The largest group is that
of the soaps, detergents, and other compounds whose basic structure is a
paraffin chain terminating in a polar group. Some solid powders can act as
emulsifiers by being wetted more by one phase than by the other. Whichever
phase shows the greater wetting power will become the dispersion medium. Many
naturally occurring emulsions, such as milk or rubber latex, are stabilized by
proteins. Egg yolk proteins stabilize mayonnaise and salad dressing. Certain
hydrophilic colloids such as gum arabic or gelatin also stabilize water-in-oil
emulsions by a similar mode of action. See INTERPHASE OF PHASES.
    Emulsions may be prepared readily by shaking together the two liquids or
by adding one phase drop by drop to the other phase with some form of
agitation, such as irradiation by ultrasonic waves of high intensity. In
industry, emulsification is accomplished by means of emulsifying machines.
    The breaking of emulsions is necessary in many industrial operations, for
example, in the separation of water-in-oil emulsions in the petroleum industry
and in product recovery from emulsions produced by the steam distillation of
organic liquids. Emulsions may be broken by (1) addition of multivalent ions
of charge opposite to the emulsion droplet, (2) chemical action (addition of
acids to emulsions stabilized by soaps), (3) freezing, (4) heating, (5) aging,
(6) centrifuging, (7) application of high-potential alternating electric
fields, and (8) treatment with ultrasonic waves of low intensity. See COLLOID;
DETERGENT; SOAP.
 �34s[George S. Mill ;W. O. Milligan]
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�6v�2v�32sEnamel   �21m �33s 
    A type of paint usually formulated by the suspension of pigments in resin 
and having excellent flow and leveling properties. Early enamels cured to a 
hard, smooth film with high gloss that simulated porcelain enamel. Modern 
formulation techniques now make available semigloss enamel finishes comparable 
in sheen and texture to satin fabric and flat finishes, as well as the 
high-gloss materials, all of which level out free from brush marks and other 
surface irregularities. See PAINT. 
 �34s[C. R. Martinson ;C. W. Sisler]
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�6v�2v�32sEnantiomorph   �21m �33s 
    One of an isomeric pair of chemical compounds whose molecules are 
nonsuperimposable mirror images. One molecular configuration of such 
dissymmetric substances is capable of rotating plane-polarized light to the 
right, dextro or (+) form, while the mirror image rotates the light equally to 
the left, levo or (-) form. Each member of such an enantiomorphic pair 
(optical isomers) possesses identical chemical and physical properties except 
for interaction with other dissymmetric systems, that is, other optically 
active substances or plane-polarized or circularly polarized light. See 
OPTICAL ACTIVITY. 
 �34s[Wyman R. Vaughan]
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�6v�2v�32sEnargite   �21m �33s 
    A mineral having composition Cu3AsS4. In some places enargite is a 
valuable ore of copper. The mineral has perfect prismatic cleavage, metallic 
luster, and grayish-black color. The hardness is 3 on Mohs scale, and the 
specific gravity is 4.44. Enargite is one of the rarer copper ore minerals and 
it has been mined in Yugoslavia, Peru, the Philippines, and the United States 
at Butte, Montana, and Bingham Canyon, Utah. Probably the largest deposit is 
at Chuquicamata, Chile. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sEndocardial fibroelastosis   �21m �33s 
    A cardiomyopathy characterized by marked fibrous and elastic thickening of 
the endocardium (the lining of the heart wall) which extends into the 
immediately subjacent myocardium. It is an important cause of cardiac death in 
infants between the age of 1 and 2 years. Some are associated with congenital 
malformations of the heart or a major blood vessel. However, in most instances 
it occurs independently of other heart lesions. The etiology and pathogenesis 
of this lesion is completely unknown. As a result of the thickening of the 
endocardium, the efficiency of the contraction of the heart is markedly 
diminished, and congestive heart failure ensues. See HEART; HEART DISORDERS. 
 �34s[N. Karle Mottet]
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�6v�2v�32sEndocarditis �21m �33s
    An inflammation of the lining of the heart or endocardium, which also
includes the tissues which form the heart valves. The two main forms of
endocarditis are bacterial and nonbacterial. In the former there is active
infection on the surface of a valve. This is usually fatal within a period of
a few months unless successfully treated. The most common cause of
nonbacterial endocarditis is rheumatic fever, which is sometimes characterized
by a slowly progressive scarring of a heart valve. After some years this may
cause serious mechanical impairment of cardiac function, due either to
narrowing (stenosis) or incompetence (regurgitation) at the valve orifice. See
RHEUMATIC FEVER.
 �34s[Paul B. Beeson]
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�6v�2v�32sEndocrine system (invertebrate)   �21m �33s 
    The chemical integrating system in animals that lack a vertebral (spinal) 
column. An endocrine system consists of those glandular cells, tissues, and 
organs whose products (hormones) supplement the rapid, short-term coordinating 
functions of the nervous system. Almost all of the information about 
invertebrates pertains to the more highly evolved groups that will be 
discussed below, the annelids, echinoderms, mollusks, and most particularly 
two classes of arthropods, the insects and crustaceans. Several of the 
hormones in invertebrates are neurohormones, that is, they are produced by 
nerve cells. See NEUROSECRETION. 
  
    Insects.  Increase in linear dimensions of an insect can only occur at 
periodic intervals when the restricting exoskeleton is shed during a process 
known as molting. Once an insect becomes an adult, it ceases to molt. The 
orderly sequence of molts that leads from the newly hatched insect to the 
adult is controlled by three hormones. The brain produces a neurohormone which 
stimulates a pair of glands in the prothorax, the prothoracic glands, causing 
release of the molting hormone, ecdysone. A third hormone, the juvenile 
hormone, produced by a pair of glands near the brain, functions during the 
juvenile molts to suppress the differentiation of adult tissues. Juvenile 
hormone permits growth but prevents maturation. See ECDYSONE. 
    Two neurohormones with antagonistic actions are involved in regulating the 
water content of insects. One, the diuretic hormone, promotes water loss by 
increasing the volume of fluid secreted into the Malpighian tubules, the 
excretory organs. The second, the antidiuretic hormone, acts to conserve water 
by causing the wall of the rectum to increase the volume of water resorbed 
from its lumen while lowering the excretion rate from the Malpighian tubules. 
    Bursicon, a protein neurohormone, is responsible for the tanning and 
hardening of the newly formed cuticle. During the development of some insects, 
a period of arrested development occurs, termed the diapause. The mechanisms 
controlling the onset and the termination of diapause are largely unknown. 
However, in some insects there is evidence for a hormone, proctodone, that 
reinitiates development. A very few species of insects, most notably the stick 
insect (Carausius morosus), have the ability to change color. This insect 
becomes darker at night and lighter by day as a result of the rearrangement of 
pigment granules within the epidermal cells. Darkening is due to a hormone 
produced in the brain. 
  
    Crustaceans.  Higher crustaceans have a structure, the sinus gland, which 
in most stalk-eyed species lies in the eyestalk and is the storage and release 
site of a molt-inhibiting hormone. The Y-organs, a pair of structures found in 
the anterior portion of the body near the excretory organs, are the source of 
the crustacean molting hormone, crustecdysone. Chemically, crustecdysone is 
very similar to the ecdysone from insects; both substances can cause molting 
in crustaceans and insects. The sinus gland is a neuroendocrine structure, but 
the Y-organs are nonneural. 
    Crustacean pigment cells (chromatophores) are under hormonal control, as 
in insects. The active substances are released from the sinus glands and the 
postcommissural organs, which lie near the esophagus. Substances have been 
found that cause dispersion of the pigment within the chromatophore, as well 
as substances with the opposite action. See CHROMATOPHORE. 
    The compound eye of crustaceans has three retinal pigments. The movements 
of these pigments with illumination level control the amount of light 
impinging on the photosensitive cells. A substance has been found that causes 
the migration of these pigments toward the light-adapted positions and another 
substance that causes migration toward dark-adapted positions. 
    The pericardial organs, which are found near the heart, are neuroendocrine 
organs that cause an increase in the amplitude of the heart beat. The sinus 
gland contains the hyperglycemic hormone, which causes a rise in blood glucose. 
  
    Annelids.  Strong evidence for hormones in annelids has been obtained from 
studies of the reproductive system. One substance that has been found in some 
marine annelids inhibits maturation of the gametes. This substance was thought 
to have been produced by the brain, but studies show that another structure, 
the infracerebral gland, which lies ventral to the brain, may be involved. 
  
    Echinoderms.  The radial nerves of starfishes contain two substances that 
are required for the maintenance of a normal reproductive cycle. One, the 
shedding substance, induces spawning. The second, shedhibin, inhibits the 
former.  
  
    Mollusks.  The best-established endocrine organs in mollusks, the optic 
glands, occur in the octopus and squid. They are a pair of small structures, 
found near the brain, that produce a substance which causes gonadal 
maturation. The optic glands in turn are regulated by inhibitory nerves from 
the brain. There is also some evidence that a portion of the nervous system 
(the pleural ganglia) may secrete a hormone that affects water balance. 
Removal of the pleural ganglia from a fresh-water snail results in swelling of 
the animal due to the influx of water. 
 �34s[Milton Fingerman]
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�6v�2v�32sEndocrine system (vertebrate)   �21m �33s 
    A system that consists of localized groups of specialized cells, tissues, 
and discrete ductless endocrine organs that synthesize and secrete into the 
internal environment specific organic substances (hormones) that regulate 
other kinds of cells in the body. In performing this role, the endocrine 
system shares with the nervous system the responsibility for adjusting body 
structure, function, and behavior to the constantly changing external 
environment and for maintaining a constant and appropriate internal 
physiological state. See NERVOUS SYSTEM (VERTEBRATE). 
  
    Endocrine tissues.  Many endocrine glands form discrete organs. Among 
these are the pituitary, gonads, thyroid, parathyroids, adrenals, pineal body, 
ultimobranchial bodies, and thymus. Some endocrine cells are arranged more or 
less diffusely in restricted regions, often embedded in other organs. Most 
intensively studied of the diffuse tissues are islands of endocrine cells 
embedded in the pancreas, in the lining of the stomach, intestine, and uterus, 
and in the placenta. Additional endocrine tissue is embedded in the liver and 
kidneys, and groups of atypical neurons (neurosecretory neurons) in the brain 
and, in fish, in the spinal cord (urophysis) secrete impor-tant endocrine 
substances. Associated with all endocrine tissues are rich networks of 
capillaries from which endocrine cells receive raw materials and chemical 
instructions, and into which they secrete their products, hormonal and 
otherwise. See ADRENAL GLAND; OVARY; PANCREAS; PARATHYROID GLAND; PITUITARY 
GLAND; TESTES; THYMUS GLAND; THYROID GLAND. 
  
    Targets.  Vertebrate hormones are organic molecules of various sizes, 
ranging from amines and derivatives of fatty acids to small polypeptides, huge 
protein molecules, and steroids. Hormones are usually transported by the 
bloodstream, and thus bathe every living cell of the body. Nevertheless, not 
all cells are competent to respond. For example, thyroid cells respond to 
thyroid-stimulating hormone from the pituitary but not to antidiuretic hormone 
from the same source. Kidney cells, on the other hand, respond to antidiuretic 
hormone. Thus the thyroid, but not the kidney, is a ``target'' for thyrotropic 
hormone. A few hormones - general metabolic ones that influence basic cellular 
metabolism such as utilization of energy - may have as targets every cell of 
the body. See HORMONE. 
    In order for a cell to be a target for a hormone, the cell must contain 
``receptors'' for that specific hormone. Receptors are molecules of specific 
configuration that are capable of forming a bond with a specific hormone 
molecule. The resulting combination, hormone plus receptor, triggers a chain 
reaction that results in the characteristic response of the target cell. 
    Hormones are not always circulated by the bloodstream. When targets are 
close enough, the hormone may reach the target via microscopic intercellular 
spaces. For instance, male sex hormones produced by endocrine cells in a rat's 
testis diffuse to adjacent seminiferous tubules, where they stimulate 
formation of sperm. 
  
    Roles.  The endocrine and nervous systems of vertebrates share 
responsibility for integrating body activities. Rapid, but quickly reversible, 
changes in the external environment are compensated for partly through the 
mediation of the nervous system, which may evoke immediate but short-term 
contraction of muscles that drive an animal deeper into a friendly environment 
or cause shivering in cold, temporary alteration of heartbeat, or a spurtlike 
release of glucose into the bloodstream. More gradual environmental changes, 
such as seasonal fluctuations in day lengths, temperatures, food supply, 
moisture, or salinity, evoke more gradual and sustained responses; these are 
mediated ultimately by the endocrine system. Endocrine responses prepare the 
animal for the changing seasons, and since reproduction is usually seasonal, 
they regulate reproduction. Hormones maintain homeostasis, and regulate the 
long-term availability of energy-containing compounds and utilization of 
energy by the cells. Hormones in developing vertebrates regulate such 
processes as cell division, growth (by regulating protein synthesis), 
differentiation, and metamorphosis. It might be said that the nervous system 
copes with the environment on an immediate basis, whereas the endocrine system 
copes with it on a sustained basis. See HOMEOSTASIS. 
  
    Endocrine rhythms.  The concentration of most hormones in the bloodstream 
of vertebrates rises and falls rhythmically. Daily rhythms in serum levels of 
these hormones result in daily fluctuations in metabolism and behavior. As a 
result, human beings traveling by air through several time zones find their 
body rhythms affected, and animals placed in an artificial environment in 
which night and day are reversed continue to show daytime metabolism and 
behavior in the dark, and vice versa, until the hormone rhythms have been 
modified by the new lighting schedule. Annual rhythms also occur, and a 
hormone that peaks in the bloodstream just before dawn in summer may peak at 
the end of the daylight period in winter. Since different hormones peak at 
different times, ratios of one hormone to another vary daily and annually. 
Daily and annual fluctuations in adrenal steroid hormones, in particular, 
appear to play a prominent role in the temporal organization of metabolism and 
behavior in vertebrates. Birds that normally migrate northward in spring can 
be caused to migrate southward at that time from the same starting point if 
the ratio of corticosterone (an adrenal steroid) to prolactin is 
experimentally altered so as to mimic that of a nonexperimental bird about to 
migrate south. See BODY RHYTHM; PERIODICITY IN ORGANISMS. 
 �34s[George C. Kennedy]
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�6v�2v�32sEndocrinology   �21m �33s 
    The study of the glands of internal secretion, the endocrine glands, and 
the hormones which they synthesize and secrete. These glands are ductless; the 
hormones are secreted directly into the blood to be carried to the target 
tissue or organ. The hormones, or chemical messengers, are highly specific and 
their action may be selective or generalized. See ENDOCRINE SYSTEM 
(VERTEBRATE); HORMONE. 
 �34s[Sybil P. Parker]
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�6v�2v�32sEndocytosis   �21m �33s 
    The process of uptake or internalization of particles, macromolecules, and 
fluid droplets by living cells; the first observation of this process was of 
the uptake of fluid droplets into macrophages by W. H. Lewis in 1930, an event 
he termed pinocytosis. The term endocytosis includes very distinct mechanisms 
which cells use for very specific purposes, and can be divided into categories 
according to the morphologic processes that are involved, or into functional 
categories.  
    Cells have large protrusions on their surfaces, called pseudopods or 
ruffles, which are thin extensions of the cytoplasm into the surrounding 
medium. Within a few minutes a ruffle can be extended from the cell, fall back 
onto the adjacent cell surface, and the membranes of the ruffle and cell can 
fuse at the edges, trapping a bubble of fluid, which is then, at least 
functionally, inside the cell. The process is called macropinocytosis. 
    Another endocytosis mechanism also involves the internalization of fluid 
from the outside of the cell. However, this process is mediated by 
indentations in the surface membrane (caveolae) which they pinch off from the 
plasma membrane as very tiny internal vesicles called micropinosomes. 
    The third, and most unusual, morphologic structure is a very specialized 
area of the surface membrane that is coated on the inner cytoplasmic surface 
by a layer of spikes called a bristle coat. By some mechanism, the membrane of 
the coated pit enters the cell and becomes an intracellular vesicle. 
    The internalization of soluble materials and fluid from the outside of the 
cell is called fluid-phase or bulk-phase endocytosis. It is a process by which 
materials from the outside of the cell neither bind selectively to the cell 
surface nor are concentrated before entering the cell. Macromolecules, 
particularly proteins, which bind selectively to the cell surface, enter the 
cell by adsorptive endocytosis; that is, they bind to something on the cell 
surface which they recognize, and are taken into the cell. The third form of 
functional endocytosis is a very specialized type, concentrative adsorptive 
endocytosis.  
    Almost all animal cells have specific molecules on their surface which 
bind other molecules in the medium they wish to take in. These surface-binding 
molecules are called receptors, and for many physiologically important 
systems, the receptors are surface proteins that are part of the cell membrane. 
    While not all of the functions of endocytosis are clear, some guesses can 
be made about its significance. Fluid-phase pinocytosis brings about the 
release of nutrients in the cell by lysosomal degradation of materials in the 
medium. A more likely major role for this pinocytosis, however, is to recycle, 
or cleanse, the surface membrane of the cell to ensure a viable and 
functioning barrier between the inside of the cell and its environment. 
Receptor-mediated endocytosis may either deliver specifically valuable 
surface-bound materials into the interior or selectively remove receptors with 
their ligands from the surface. In some cases, such as some polypeptide 
hormones, the cell responds to a hormone only when it is bound to the surface 
on the outside of the cell. Endocytosis could therefore be a way of regulating 
the response to a hormone on the outside. See CELL PERMEABILITY; PHAGOCYTOSIS. 
 �34s[Mark C. Willingham]
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�6v�2v�32sEndodermis   �21m �33s 
    The single layer of plant cells that is located between the cortex and the 
vascular (xylem and phloem) tissues. It has its most obvious development in 
roots and subaerial stems. The endodermis has many apparent functions: 
absorption of water, selection of solutes and ions, and production of oils, 
antibiotic phenols, and acetylenic acids. 
    The endodermis has been found to have extra sets of chromosomes as 
compared with cortical and other cells in the plant. In some plants the 
chromosome numbers may be so high in the endodermis that four sets of 
chromosomes may occur in each endodermal cell. The larger amount of nuclear 
material and nucleic acid in the cells of the endodermis may in part account 
for the great capacity of endodermal cells to produce large amounts of 
chemical substances, such as acetylenic oils, high in caloric energy. See 
CORTEX (PLANT); HYPODERMIS; PLANT TISSUE SYSTEMS. 
 �34s[D. S. Van Fleet]
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�6v�2v�32sEndoplasmic reticulum   �21m �33s 
    A finely divided vacuolar system of the cytoplasm which, in differentiated 
cells, assumes a variety of forms and specialized functions. 
    The endoplasmic reticulum can be pictured as a complex of vesicles and 
connecting tubules that extends to almost all parts of the cytoplasm. In some 
cells, especially those most active in the manufacture of proteins, the 
predominant vesicles are thin (75 nanometers) and lamellar, like pancakes. 
These vesicles, or cisternae, are often arranged parallel and in stacks of 
several units. In other cells the system is constructed of tubules (75 - 100 
nm in diameter) which interconnect to form a complex three-dimensional lattice. 
    One of the most constant and remarkable features of the endoplasmic 
reticulum is its continuity with the nuclear envelope; the two are properly 
regarded as parts of a single system. This means that part of the reticulum, 
represented by the envelope, is in intimate contact with the genetic material 
of the cell. See CELL NUCLEUS; CHROMOSOME; NUCLEAR-CYTOPLASMIC INTERACTIONS. 
    Among several associations with other components of the cell, none has 
greater significance than that with ribosomes, the sites of protein synthesis 
(see illustration). As a result of this association the polypeptides assembled 
in the ribosomes are simultaneously pushed through the membranes and into the 
cavities of the endoplasmic reticulum where they undergo the secondary and 
tertiary foldings that give the protein molecule its mature functional form. 
See RIBOSOMES. 
  
    The ribosome-rich, or rough, form of the reticulum plays a role in protein 
synthesis and sequestration. Some fragments of the endoplasmic reticulum from 
muscle fibers have an unusual capacity to concentrate calcium ion, and this 
has helped define a role of the endoplasmic reticulum in muscle contraction 
and relaxation. The ribosome-free, or smooth, form of the endoplasmic 
reticulum in liver cells has the capacity to metabolize and detoxify a wide 
range of drugs and toxic substances. This points to the extraordinary fact 
that the membranes of special parts of the endoplasmic reticulum (usually free 
of ribosomes) contain enzymes capable of performing highly specialized 
functions.  
 �34s[Keith R. Porter]
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�6v�2v�32sEndopterygota   �21m �33s 
    A division of the insects which comprises part of the subclass Pterygota. 
The group includes those orders of insects which undergo a complete or 
holometabolous metamorphosis during their life cycle; that is, they undergo 
four stages of development during metamorphosis: the egg, larva, pupa, and 
imago or adult. 
 �34s[Bryant E. Rees]
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�6v�2v�32sEndorphins   �21m �33s 
    Peptides manufactured within the central nervous system which share with 
plant alkaloids derived from the opium poppy the ability to alleviate pain. 
Endorphins are relatively short chains of amino acids which function as 
neurohumoral transmitters or as neuromodulators in a system descending from 
the brain through the spinal cord which antagonizes painful impulses coming 
from the periphery toward the brain. The term endorphin (from elision of 
endogenously manufactured morphinelike substance) is now used generically to 
describe a family of endogenous opiates which include leucine enkephalin; 
methionine enkephalin; alpha, beta, gamma, and delta endorphin; and a growing 
number of synthetic compounds. Narcotic analgesic drugs are thought to mimic 
the actions of one or another of the endorphins by occupying stereospecific 
opiate receptors in the brain, spinal cord, and gut normally occupied by 
endorphins. See ALKALOID; ANALGESICS; NARCOTIC; OPIATES; PAIN. 
 �34s[Lawrence M. Halpern]
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�6v�2v�32sEndospores (bacteria) �21m �33s
    Resistant and metabolically dormant bodies produced by the gram-positive
rods of Bacillus (aerobic or facultatively aerobic) and Clostridia (strictly
anaerobic), by the coccus Sporosarcina, and by certain other bacteria.
Sporeforming bacteria are found mainly in the soil and water and also in the
intestines of human beings and animals. Some sporeformers are pathogens.
Endospores seem to be able to survive indefinitely. Spores kept for more than
50 years have shown little loss of their capacity to germinate and propagate
by cell division. The unique property of bacterial spores is their extreme
resistance to heat, radiation from ultraviolet light and x-rays, organic
solvents, chemicals, and desiccation.
    The endospore appears as a light-refractile body inside another cell
(sporangium). Each sporangium produces one endospore with a characteristic
size, shape, and position within the cell. The mature spore has a complex
structure which contains a number of layers. The outermost envelope,
surrounding the spore, is called the exosporium and is a thin, membranous
covering. Beneath the exosporium lies the spore coat, which is composed of
several layers, largely of a protein nature; each is about 2 - 2.5 nanometers
thick. Beneath these is a thin membrane which separates the spore coat from an
area of low electron density called the cortex, primarily a modified
peptidoglycan structure. It occupies approximately half the volume of the
spore. A wall and a thin membrane separate the cortex from the cytoplasm of
the dormant spore.
    The three processes involved in the coversion of the spore into a
vegetative cell are (1) activation (usually by heat or aging), which
conditions the spore to germinate in a suitable environment; (2) germination,
an irreversible process which results in the loss of the typical
characteristics of a dormant spore; and (3) outgrowth, in which new classes of
proteins and structures are synthesized so that the spore is converted into a
new vegetative cell.
 �34s[Harlyn O. Halvorson ;Keith Hutchison]
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�6v�2v�32sEnergy   �21m �33s 
    The ability of one system to do work on another system. There are many 
kinds of energy: chemical energy from fossil fuels, electrical energy 
distributed by a utility company, radiant energy from the Sun, and nuclear 
energy from a reactor. The units of energy include ergs, joules, foot-pounds, 
and foot-poundals. Work and heat have the same units as energy, but are 
entirely different physical concepts. See HEAT; WORK. 
    Any particle or system of particles subject to conservative forces has two 
kinds of energy, potential energy and kinetic energy. Potential energy is the 
energy due to position or configuration, and kinetic energy is the energy due 
to motion. See KINETIC ENERGY; POTENTIAL ENERGY. 
    Energy is conserved for all isolated mechanical systems. This is because 
if a system A is isolated, there is no other system B that it can give any 
energy to, and its total energy must remain constant. This system A can 
convert kinetic energy to potential energy, and it can convert one form of 
potential energy to another, but the total energy must remain the same. The 
meaning of conserved total energy is that the system has the same value of 
total energy at all times. See CONSERVATION OF ENERGY. 
    In 1905 A. Einstein showed that at high velocities near the speed of light 
important modifications must be made in physical concepts. One particularly 
radical idea which he advanced was that space and time are not independent, 
but rather are two aspects of the same object, a space-time manifold. This 
necessitated a reexamination of the concept of energy and led to the 
conclusion that the inertia, or mass m, depends upon its energy through the 
mass-energy relation shown below, where c is the speed of light in vacuum. 

                           *** E = mc sup 2 ***

Furthermore, energy and momentum conservation become joined in a single 
four-momentum conservation law in special relativity. See INERTIA OF ENERGY; 
INTERNAL ENERGY; RELATIVITY. 
 �34s[Brian DeFacio]
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�6v�2v�32sEnergy conversion   �21m �33s 
    The process of changing energy from one form to another. There are many 
conversion processes that appear as routine phenomena in nature, such as the 
evaporation of water by solar energy or the storage of solar energy in fossil 
fuels. In the world of technology the term is more generally applied to 
operations of human origin in which the energy is made more usable; for 
instance, the burning of coal in power plants to convert chemical energy into 
electricity, the burning of gasoline in automobile engines to convert chemical 
energy into propulsive energy of a moving vehicle, or the burning of a 
propellant for ion rockets and plasma jets to provide thrust. 
    There are well-established principles in science which define the 
conditions and limits under which energy conversions can be effected, for 
example, the law of the conservation of energy, the second law of 
thermodynamics, the Bernoulli principle, and the Gibbs free-energy relation. 
Recognizable forms of energy which allow varying degrees of conversion include 
chemical, atomic, electrical, mechanical, light, potential, pressure, kinetic, 
and heat energy. In some conversion operations the transformation of energy 
from one form to another, more desirable form may approach 100% efficiency, 
whereas with others even a ``perfect'' device or system may have a theoretical 
limiting efficiency far below 100%. See ENERGY SOURCES. 
 �34s[Theodore Baumeister]
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�6v�2v�32sEnergy level (quantum mechanics)   �21m �33s 
    Self-contained physical systems of molecular size or smaller are found to 
have states of internal motion or excitation in which their energies take on 
definite stationary values. The energies of these states are called energy 
levels.  
    The existence of stationary states was first clearly seen in the study of 
atomic spectra. In 1900 Johannes Rydberg remarked that the frequencies of many 
observed spectral lines could be expressed in terms of the differences between 
pairs of numbers in a certain set called spectral terms; in 1908 Walter Ritz 
proposed this as a fundamental physical law, which later became known as the 
combination principle. Five years later Niels Bohr explained the combination 
principle in dynamical terms: Atoms exist in stationary states; when an atom 
passes from one stationary state to another of lower energy, it delivers the 
extra energy to a single quantum of light. Since according to Max Planck the 
energy of a quantum is proportional to its frequency, it follows that the 
spectral terms, written in appropriate units, are the energy values of the 
stationary states and that Ritz's combination principle merely expresses the 
conservation of energy. See ATOMIC STRUCTURE AND SPECTRA. 
    For nuclear energy levels, the continuous spectrum begins rather low, that 
is, typical level spacings are a few MeV (a million times greater than those 
of atomic levels), and a few more MeV disrupt a nucleus. Further, there are 
several ways in which a nucleus in an excited energy level can lose energy in 
addition to the radiation of a photon: It can lose an electron, a positron, or 
more rarely one or two protons or neutrons, a deuteron, or an alpha particle. 
The emission of anything but a photon changes the identity of the nucleus. 
    In reactions involving the so-called elementary particles, the same 
considerations enter, but often in simpler form because the internal structure 
of these particles, whatever it may be, is presumably simpler than those of 
all but the simplest nuclei. The situation with particles is however 
complicated by the existence of transitions and selection rules that have no 
counterpart among nuclei. See ELEMENTARY PARTICLE. 
 �34s[David Park]
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�6v�2v�32sEnergy metabolism   �21m �33s 
    Living cells all carry on sequences of chemical reactions, the overall 
energetics of which are such as to label the processes as spontaneous, that 
is, exergonic, in terms of the change in free chemical energy. A few of the 
chemical intermediates in such sequences are sufficiently reactive as they are 
so formed that inorganic phosphate may in turn be ``activated'' chemically and 
thus be able to form substituted phosphoric acid anhydrides, such as 
adenosinetriphosphate (ATP) or its energetic equivalents. The ATP becomes a 
reactant in numerous energy-requiring reactions, both relatively simple (fatty 
acid activation) and relatively complex (muscle contraction). See 
ADENOSINETRIPHOSPHATE (ATP). 
    There is a remarkable unity and also a remarkable simplicity in the 
patterns of energy transformation in living cells. A surprisingly small number 
of chemical entities and appropriate catalysts (enzymes) and reaction 
sequences are found in the amazingly diverse forms of life. Living cells are 
chemical engines whose function is to synthesize ATP (or equivalent) 
accompanying the overall energy-yielding degradation of nutrients. The 
estimated thermodynamic efficiency is 60 - 70%, which is very high. See CELL 
(BIOLOGY); ENZYME; KREBS CYCLE; METABOLISM. 
 �34s[Carl S. Vestling]
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�6v�2v�32sEnergy sources   �21m �33s 
    Sources from which energy can be obtained to provide heat, light, and 
power. Sources of energy have evolved from human and animal power to fossil 
fuels, uranium, water power, wind, and the Sun. 
    The principal fossil fuels are coal, lignite, peat, petroleum, and natural 
gas; other potential sources of fossil fuels include oil shale and tar sands. 
As fossil fuels become depleted, nonfuel sources and fission and fusion 
sources will become of greater importance since they are renewable. Nuclear 
power is based on the fission of uranium, thorium, and plutonium, and the 
fusion power is based on the forcing together of the nuclei of two light atoms 
such as deuterium, tritium, or helium-3. See COAL; NATURAL GAS; NUCLEAR POWER; 
OIL SAND; OIL SHALE; PETROLEUM. 
    Nonfuel sources of energy include wastes, water, wind, geothermal 
deposits, biomass, and solar heat. See BIOMASS; GEOTHERMAL POWER; SOLAR 
ENERGY, WATER POWER; WIND POWER. 
    Fuels which do not exist in nature are known as synthetic fuels. They are 
synthesized or manufactured from varieties of fossil fuels which cannot be 
used conveniently in their original forms. Substitute natural gas (SNG) is 
manufactured from coal, peat, or oil shale. Synthetic liquid fuels can be 
produced from coal, oil shale, or tar sands. Both gaseous and liquid fuels can 
be synthesized from renewable resources, collectively called biomass. These 
carbon sources are trees, grasses, algae, plants, and organic waste. 
Production of synthetic fuels, particularly from renewable resources, 
increases the scope of available energy sources. 
    Energy management includes not only the procurement of fuels on the most 
economical basis, but the conservation of energy by every conceivable means. 
Whether this is done by squeezing out every Btu through heat exchangers, or by 
room-temperature processes instead of high-temperature processes, or by 
greater insulation to retain heat which has been generated, each has a role to 
play in requiring less energy to produce the same amount of goods and 
materials.  
 �34s[Gerard C. Gambs]
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�6v�2v�32sEnergy storage �21m �33s
    The general method and specific techniques for storing energy derived from
some primary source in a form convenient for use at a later time when a
specific energy demand is to be met, often in a different location.
    In the past, energy storage on a large scale had been limited to storage
of fuels. For example, large amounts of natural gas and petroleum are
routinely stored. On a smaller scale, electric energy is stored in batteries
that power automobile starters and a great variety of portable appliances. In
the future, energy storage in many forms is expected to play an increasingly
important role in shifting patterns of energy consumption away from scarce to
more abundant and renewable primary resources.
    An example of growing importance is the storage of electric energy
generated at night by coal or nuclear power plants to meet peak electric loads
during daytime periods. This is achieved by pumped hydroelectric storage, that
is, pumping water from a lower to a higher reservoir at night and reversing
this process during the day, with the pump then being used as a turbine and
the motor as a generator.
    Off-peak electric energy can also be converted into mechanical energy by
pumping air into a suitable cavern where it is stored at pressures up to 80
atm (8 megapascals). Turbines and generators can then be driven by the air
when it is heated and expanded.
    The development of advanced batteries (such as nickel-zinc, nickel-iron,
zinc-chloride, and sodium-sulfur) with characteristics superior to those of
the familiar lead-acid battery could result in use of battery energy storage
on a large scale. For example, batteries lasting 2000 or more cycles could be
used in installations of several-hundred-thousand - kilowatt-hour capacity in
various locations on the electric power grid, as an almost universally
applicable method of utility energy storage. Batteries combining these
characteristics with energy densities (storage capacity per unit weight and
volume) well above those of lead-acid batteries could provide electric
vehicles with greater range. See BATTERY; STORAGE BATTERY.
    Ceramic brick ``storage heaters'' that store off-peak electricity in the
form of heat have gained wide acceptance for heating buildings in Europe, and
the barriers to their increased use in the United States are more
institutional and economic than technological.
    Solar hot-water storage is technically simple and commercially available.
However, the use of solar energy for space heating requires relatively large
storage systems, with water or rock beds as storage media, and difficulties
can arise in integrating this storage with existing buildings while keeping
costs within acceptable limits. See SOLAR ENERGY; SOLAR HEATING AND COOLING.
    Heat or electricity may be stored by using these energy forms to force
certain chemical reactions to occur. Such reactions are chosen so that they
can be reversed readily with release of energy; in some cases the products can
be transported from the point of generation to that of consumption. For
example, reactions which produce hydrogen could become attractive since
hyrdogen could be stored for extensive periods of time and then conveniently
used in either combustion devices or in fuel cells. See FUEL CELL.
    Electrical energy can be stored directly in the form of large direct
currents used to create fields surrounding the superconducting windings of
electromagnets. In principle such devices appear attractive because their
storage efficiency is high. However, the need for maintaining the system at
temperatures approaching absolute zero and, particularly, the need to
physically restrain the coils of the magnet when energized require expensive
auxiliary equipment (insulation, vacuum vessels, and structural supports). See
SUPERCONDUCTING DEVICES.
    Storage of kinetic energy in rotating mechanical systems such as flywheels
is attractive where very rapid absorption and release of the stored energy is
critical. However, research indicates that even advanced designs and materials
are likely to be too expensive for utility energy storage on a significant
scale, and applications will probably remain limited to systems where high
power capacity and short charging cycles are the prime consideration.
 �34s[Fritz Kalhammer ;Thomas R. Schneider]
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�6v�2v�32sEngine   �21m �33s 
    A machine designed for the conversion of energy into useful mechanical 
motion. The principal characteristic of an engine is its capacity to deliver 
appreciable mechanical power. By usage an engine is usually a machine that 
burns or otherwise consumes a fuel, as differentiated from an electric machine 
that produces mechanical power without altering the composition of matter. 
Similarly, a spring-driven mechanism is said to be powered by a spring motor; 
a flywheel acts as an inertia motor. By this definition a hydraulic turbine is 
not an engine, although it competes with the engine as a prime source of 
mechanical power. See HYDRAULIC TURBINE; MOTOR; PRIME MOVER. 
    Traditionally, engines are classed as external or internal combustion. 
External combustion engines consume their fuel or other energy source in a 
separate furnace or reactor. A further basis of classification concerns the 
working fluid. If the working fluid is recirculated, the engine operates on a 
closed cycle. If the working fluid is discharged after one pass through boiler 
and engine, the engine operates on an open cycle. The commonest types of 
engine use atmospheric air in open cycles both as the principal constituent of 
their working fluids and as oxidizer for their fuels. DIESEL ENGINE; GAS 
TURBINE; INTERNAL COMBUSTION ENGINE; NUCLEAR REACTOR; ROTARY ENGINE; 
STEAM-GENERATING FURNACE; STIRLING ENGINE; TURBINE PROPULSION. 
 �34s[Frank H. Rockett]
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�6v�2v�32sEngine cooling   �21m �33s 
    Cylinder gas temperatures in internal combustion engines may reach 4500DGF 
(2500DGC). This is well above the melting point of the engine parts in contact 
with the gases, so that it is necessary to control the temperature of the 
parts or they will become too weak to carry the gas pressure stresses. 
    If the outer cylinder surface is placed in contact with a cool fluid such 
as air or water and there is sufficient contact area to cause a rapid heat 
flow, the resulting temperature drop produced by the heat flow in the inside 
boundary layer keeps the cylinder wall temperature much closer to the coolant 
temperature than to the combustion gas temperature. 
    If the coolant is water, it is usually circulated by a pump through 
jackets surrounding the cylinders and cylinder heads. The water is circulated 
fast enough to limit the water's temperature rise through the engine to about 
15DGF (8DGC). The warmed coolant is then piped to an air-cooled heat exchanger 
called a radiator. To prevent freezing, the water coolant is often mixed with 
alcohol or ethylene-glycol. See ANTIFREEZE MIXTURE. 
    Engines are often cooled directly by a stream of air without the 
interposition of a liquid medium. The heat-transfer coefficient between the 
cylinder and airstream is much less than with a liquid coolant, so that the 
cylinder temperatures must be much greater than the air temperature to 
transfer to the cooling air the heat flowing from the cylinder gases. To 
remedy this situation and to reduce the cylinder wall temperature, the outside 
area of the cylinder, which is in contact with the cooling air, is increased 
by finning. See HEAT EXCHANGER; HEAT TRANSFER; INTERNAL COMBUSTION ENGINE. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sEngineering   �21m �33s 
    Most simply, the art of directing the great sources of power in nature for 
the use and the convenience of people. In its modern form engineering involves 
people, money, materials, machines, and energy. It is differentiated from 
science because it is primarily concerned with how to direct to useful and 
economical ends the natural phenomena which scientists discover and formulate 
into acceptable theories. Engineering therefore requires above all the 
creative imagination to innovate useful applications of natural phenomena. It 
seeks newer, cheaper, better means of using natural sources of energy and 
materials.  
    The typical modern engineer goes through several phases of career 
activity. Formal education must be broad and deep in the sciences and 
humanities. Then comes an increasing degree of specialization in the 
intricacies of a particular discipline, also involving continued 
postscholastic education. Normal promotion thus brings interdisciplinary 
activity as the engineer supervises a variety of specialists. Finally, the 
engineer enters into the management function, weaving people, money, 
materials, machines, and energy sources into completed processes for the use 
of society. 
    For specific articles on various engineering disciplines see CHEMICAL 
ENGINEERING; CIVIL ENGINEERING; ELECTRICAL ENGINEERING; INDUSTRIAL 
ENGINEERING; MANUFACTURING ENGINEERING; MARINE ENGINEERING; MECHANICAL 
ENGINEERING; METHODS ENGINEERING; MINING; NUCLEAR ENGINEERING. 
 �34s[Joseph W. Barker]
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�6v�2v�32sEngineering design   �21m �33s 
    Engineering is concerned with the creation of systems, devices, and 
processes useful to, and sought by, society. The process by which these goals 
are achieved is engineering design. 
    The process can be characterized as a sequence of events as suggested in 
Fig. 1. The process may be said to commence upon the recognition of, or the 
expression of, the need to satisfy some human want or desire, the ``goal,'' 
which might range from the detection and destruction of incoming ballistic 
missiles to a minor kitchen appliance or fastener. 
  
    Concept formulation.  The first obligation of the engineer is to develop 
more detailed quantitative information which defines the task to be 
accomplished in order to satisfy the goal, labeled on Fig. 1 as task 
specification. At this juncture the scope of the problem is defined, and the 
need for pertinent information is established. Generation of ideas for 
possible solutions to the problem is the creative stage, called the concept 
formulation. When great strides in engineering are made, this represents 
ingenious, innovative, inventive activity; but even in more pedestrian 
situations where rational and orderly approaches are possible, the conceptual 
stage is always present. The concept does not represent a solution, but only 
an idea for a solution. It can only be described in broad, qualitative, 
frequently graphical terms. Concepts for possible solutions to engineering 
challenges arise initially as mental images which are recorded first as 
sketches or notes and then successively tested, refined, organized, and 
ultimately documented by using standardized formats. 
    Concepts are accompanied and followed by, sometimes preceded by, acts of 
evaluation, judgment, and decision. It is in fact this testing of ideas 
against physical, economic, and functional reality that epitomizes 
engineering's bridge between the art of innovation and science. The process of 
analysis is sometimes intuitive and qualitative, but it is often mathematical, 
quantitative, careful, and precise. 
    Production considerations can have a profound influence on the design 
process, especially when high-volume manufacture is anticipated. Evolutionary 
products manufactured in large numbers, such as the automobile, are tailored 
to conform with existing production equipment and techniques such as assembly 
procedures, interchangeability, scheduling, and quality control. New 
techniques such as those associated with space exploration, where volume 
production is not a central concern, factor into the engineering design 
process in a very different fashion. 
    Similarly, the design process must anticipate and integrate provisions for 
distribution, maintenance, and ultimate replacement of products. 
Well-conceived and executed engineering design will encompass the entire 
product cycle from definition and conception through realization and demise 
and will give due consideration to all aspects. 
  
    Hierarchy of design.  An adequate description of the engineering design 
process must have both general validity and applicability to a wide variety of 
engineering situations: tasks simple or complex, small- or large-scale, 
short-range or far-reaching. That is to say, there is a hierarchy of 
engineering design situations. 
    Systems engineering occupies one end of the spectrum. The typical goal is 
very broad, general, and ambitious, and the concepts are concerned with the 
interrelationships of a variety of subsystems or components which, taken 
together, make up the system to accomplish the desired goal. See SYSTEMS 
ENGINEERING.  
  
  
    Time=nworker-power resource dynamics.  Another dimension of the dynamics 
of the engineering design process is the elapse of time and expenditure of 
worker-hours in the evolution of an engineering design project. Figure 2 plots 
time as the abscissa and resources (worker-power or dollars) as the ordinate. 
The various stages of the engineering design process are set out in time 
sequence from left to right. 
    Goal refinement, task specification, and first-order concept and analyses 
iterations are conducted by one to a few engineers in the early stages to 
establish the feasibility of the idea and to block out possible approaches. 
This is usually called the advanced design stage. 
    As the design concept becomes more specific and substantive, more and more 
engineers, technicians, and draftsmen become involved in the project. In 
projects of significant size, the problem of coordinating and integrating the 
efforts of the many participants of different talents and skills becomes 
itself a major consideration. See PERT. 
  
    Use of the computer in design.  The use of the computer, both analog and 
digital, in the engineering design process has increased. Where economically 
justified, the overall engineering design process for a product is mechanized 
via computer programming. See COMPUTER; COMPUTER-AIDED DESIGN AND 
MANUFACTURING; COMPUTER STORAGE TECHNOLOGY; DIGITAL COMPUTER PROGRAMMING; 
MULTIACCESS COMPUTER. 
    The speed, memory, and accuracy of the computer to iteratively calculate, 
store, sort, collate, and tabulate have greatly enhanced its use in design and 
encouraged the study, on their own merits, of the processes and subprocesses 
exercised in the design process. These include optimization or sensitivity 
analysis, reliability analysis, and simulation as well as design theory. See 
DIGITAL COMPUTER. 
    Optimization analyses, given a model of the design and using linear and 
nonlinear programming, determines the best values of the parameters consistent 
with stated criteria and further studies the effects of variations in the 
values of the parameters. 
    Reliability is a special case of optimization where the emphasis is to 
choose or evaluate a system so as to maximize its probability of successful 
operation, for example, the reliability of electronics. See CIRCUIT 
(ELECTRONICS); RELIABILITY OF EQUIPMENT. 
    Simulation, as of dynamic systems, is mathematical modeling to study the 
response of a design to various inputs and disturbances. The analog computer 
has been widely used for simulation through its physical modeling of the 
mathematic analytical relationships of the proposed design. See ANALOG 
COMPUTER; SIMULATION. 
    Decision theory deals with the general question of how to choose between a 
great number of alternatives according to established criteria. It proposes 
models of the decision process as well as defining techniques, that is, 
programs or algorithms, of calculation by which to make choices. See DECISION 
THEORY.  
  
    Graphical input/output.  In many fields of design - notably architecture; 
design of airplanes, automobiles, and ships; and almost all mechanical design 
- the designer works largely in visual terms. A way has been found to set up 
the computer to interpret drawings. Moreover, the computer has become an 
active partner in the act of drawing, so that it can provide a certain 
superskill in preparing the drawing once the human operator has made 
intentions clear. See COMPUTER GRAPHICS; DATA-PROCESSING SYSTEMS. 
 �34s[Robert W. Mann]
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�6v�2v�32sEngineering drawing   �21m �33s 
    A graphical language used by engineers and other technical personnel 
associated with the engineering profession. The purpose of engineering drawing 
is to convey graphically the ideas and information necessary for the 
construction or analysis of machines, structures, or systems. See COMPUTER 
GRAPHICS; DRAFTING; GRAPHIC METHODS; NOMOGRAPHY; SCHEMATIC DRAWING. 
    The basis for much engineering drawing is orthographic representation 
(projection). Objects are depicted by front, top, side, auxiliary, or oblique 
views, or combinations of these. The complexity of an object determines the 
number of views shown. At times, pictorial views are also shown. See 
DESCRIPTIVE GEOMETRY; PICTORIAL DRAWING. 
    Engineering drawings often include such features as various types of 
lines, dimensions, lettered notes, sectional views, and symbols. They may be 
in the form of carefully planned and checked mechanical drawings, or they may 
be freehand sketches. Usually a sketch precedes the mechanical drawing. Final 
drawings are usually made on tracing paper, cloth, or Mylar film, so that many 
copies can be made quickly and cheaply by such processes as blueprinting, 
ammonia-developed (diazo) printing, or lithography. See PHOTOCOPYING PROCESSES. 
 �34s[Charles J. Baer]
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�6v�2v�32sEngineering geology   �21m �33s 
    The application of education and experience in geology and other 
geosciences to solve geological problems posed by civil engineering works. The 
branches of the geosciences most applicable are surficial geology, 
petrofabrics, rock and soil mechanics, geohydrology, and geophysics, 
particularly exploration geophysics and earthquake seismology. 
    The terms engineering geology and environmental geology often seem to be 
used interchangeably. Specifically, environmental geology is the application 
of engineering geology in the solution of urban problems; in the prediction 
and mitigation of natural hazards such as earthquakes, landslides, and 
subsidence; and in solving problems inherent in disposal of dangerous wastes 
and in reclaiming mined lands. 
    Another relevant term is geotechnics, the combination of pertinent 
geoscience elements with civil engineering elements to formulate the civil 
engineering system that has the optimal interaction with the natural 
environment.  
  
    Engineering properties of rock.  The civil engineer and the engineering 
geologist consider most hard and compact natural materials of the earth crust 
as rock, and their derivatives, formed mostly by weathering processes, as 
soil. A number of useful soil classification systems exist. Because of the 
lack of a rock classification system suitable for civil engineering purposes, 
most engineering geology reports use generic classification systems modified 
by appropriate rock-property adjectives. See ROCK; ROCK MECHANICS; SOIL 
MECHANICS.  
    The properties of a rock element can be determined by tests on cores 
obtained from boreholes. The rock properties most useful to the engineering 
geologist are compressive and triaxial shear strengths, permeability, Young's 
modulus of elasticity, erodability under water action, and density (in pounds 
per cubic foot, or pcf; or in kilograms per cubic meter). See YOUNG'S MODULUS. 
    The compressive (crushing) strength of rock generally is defined as the 
amount of stress required to fracture a sample unconfined on the sides and 
loaded on the ends. The wide variety of classification systems used for rock 
results in a broad range of compressive strengths for rocks having the same 
geologic name. The value of compressive strength to be used in an engineering 
design must be related to the direction of the structure's load and the 
orientation of the bedding, discontinuities, and structural weaknesses in the 
foundation rock. 
    Shearing stresses tend to separate portions of the rock (or soil) mass. 
Faults and folds are examples of shear failures in nature. In engineering 
structures, every compression is accompanied by shear stresses. The 
application of loads over long periods of time on most rocks will cause them 
to creep or even to flow like a dense fluid (plastic flow). See CREEP 
(MATERIALS); STRUCTURAL GEOLOGY. 
    Ambient stress in a rock system is actually potential energy, probably 
created by ancient natural forces, recent seismic activity, or nearby 
human-caused disturbances. Ambient (residual, stored, or primary) stress may 
remain in rock long after the disturbance is removed. An excavation, such as a 
tunnel or quarry, will relieve the ambient stress by providing room for 
displacement of the rock, and thus the potential energy is converted to 
kinetic energy. In tunnels and quarries, the release of this energy can cause 
spalling, the slow outward separation of rock slabs from the rock massif; when 
this movement is rapid or explosive, a rock burst occurs. See ROCK BURST; 
UNDERGROUND MINING. 
    One of two fundamental principles generally is used to predict the 
possible rock load on a tunnel roof, steel, or timber supports, or a concrete 
or steel lining: (1) The weight of the burden (the rock and soil mass between 
the roof and the ground surface) and its shear strength control the load, and 
therefore the resultant stresses are depth-dependent, or (2) the shear 
strength of the rock system and the ambient stresses control the stress 
distribution, so the resultant loading is only indirectly dependent upon 
depth. See TUNNEL. 
    Rock as a construction material is used in the form of dimension, crushed, 
or broken stone. Broken stone is placed as riprap on slopes of earth dams, 
canals, and river banks to protect them against water action. Also, it is used 
as the core and armor stone for breakwater structures. Dimension stone 
(granite, limestone, sandstone, and some basalts) is quarried and sawed into 
blocks of a shape and size suitable for facing buildings or for interior 
decorative panels. Crushed stone (primarily limestone but also some basalt, 
granite, sandstone, and quartzite) is used as aggregate in concrete and in 
bituminous surfaces for highways, as a base course or embankment material for 
highways, and for railroad ballast (to support the ties). See PETROGRAPHY; 
STONE AND STONE PRODUCTS. 
  
    Geotechnics.  Glacial and alluvial deposits contain heterogeneous mixtures 
of pervious (sand and gravel) and impervious (clay, silt, and rock flour) soil 
materials. The pervious materials can be used for highway subgrade, concrete 
aggregate, and filters and pervious zones in earth embankments. Dam reservoirs 
may be endangered by the presence of stratified or lenticular bodies of 
pervious materials or ancient buried river channels filled with pervious 
material. Deep alluvial deposits in or close to river deltas may contain very 
soft materials such as organic silt or mud. See DELTA; FLOODPLAIN. 
    Concrete or earth dams can be built safely on sand foundations if the 
latter receive special treatment. One requirement is to minimize seepage 
losses by the construction of cutoff walls (of concrete, compacted clay, or 
interlocking-steel-sheet piling) or by use of mixed-in-place piles 3 ft (0.9 
m) or more in diameter. 
    Loess, a relatively low-density (PLUS/MINUS 0.36 ton/ftsup3 or 1.4 metric 
tons/msup3) soil composed primarily of silt grains cemented by clay or calcium 
carbonate, has a vertical permeability considerably greater than the 
horizontal. When a loaded loess deposit is wetted, it rapidly consolidates, 
and the overlying structure settles. Aeolian sand deposits present the problem 
of stabilization for the continually moving sand. This can be done by planting 
such vegetation as heather or young pine or by treating it with crude oil. See 
LOESS.  
    Excessive settlement will occur in structures founded on muskeg terrain. 
Embankments can be stabilized by good drainage, the avoidance of cuts, and the 
removal of the organic soil and replacement by sand and gravel. Structures 
imposing concentrated loads are supported by piling driven through the soft 
layers into layers with sufficient bearing power. See MUSKEG; TUNDRA. 
    Residual soils are derived from the in-place deterioration of the 
underlying bedrock. There are regions (such as California, Australia, and 
Brazil) where disintegrated granite (locally termed ``DG'') may be hundreds of 
feet thick; although it may be competent to support moderate loads, it is 
unstable in open excavations and is pervious. Laterite (a red clayey soil) 
derived from the in-place disintegration of limestone in tropical to 
semitropical climates is another critical residual soil. It is unstable in 
open cuts on moderately steep slopes, is compressible under load, and when wet 
produces a slick surface that is unsatisfactory for vehicular traffic. 
    Clays supporting structures may consolidate slowly over a long period of 
time and cause structural damage. When clay containing montmorillonite is 
constantly dried and rewetted by climatic or drainage processes, it 
alternately contracts and expands. During the drying cycle, extensive networks 
of fissures are formed that facilitate the rapid introduction of water during 
a rainfall. This cyclic volume change of the clay can produce uplift forces on 
structures placed upon the clay or compressive and uplift forces on walls of 
structures placed within the clay. A thixotropic or ``quick'' clay has a 
unique lattice structure that causes the clay to become fluid when subjected 
to vibratory forces. Various techniques are used to improve the foundation 
characteristics of critical types of clay: (1) electroosmosis that uses 
electricity to force redistribution of water molecules and subsequent 
hardening of the clay around the anodes inserted in the foundation; (2) 
provision of adequate space beneath a foundation slab or beam so the clay can 
expand upward and not lift the structure; (3) belling, or increasing in size, 
of the diameter of the lower end of concrete piling so the pile will withstand 
uplift forces imposed by clay layers around the upper part of the pile; (4) 
treatment of the pile surface with a frictionless coating (such as Teflon or a 
loose wrapping of asphalt-impregnated paper) so the upward-moving clay cannot 
adhere to the pile; (5) sufficient drainage around the structure to prevent 
moisture from contacting the clay; and (6) replacement of the clay by a 
satisfactory foundation material. Where none of these solutions is feasible, 
the structure then must be relocated to a satisfactory site or designed so it 
can withstand uplift or compressive forces without expensive damage. See CLAY. 
    Geotechnical investigations for engineering projects may include 
preliminary studies, preconstruction or design investigations, consultation 
during construction, and the maintenance of the completed structure. 
    Preliminary studies are made to select the best location for a project and 
to aid in formulating the preliminary designs for the structures. The first 
step in the study is a search for pertinent published material in libraries, 
state and Federal agencies, private companies, and university theses. Sources 
of geologic information are listed in the Directory of Geological Material in 
North America by J. A. Howell and A. I. Levorsen (NAS-NRC Publ. 556, 1957). 
Air photos and other remote sensing techniques such as pulsed or side-looking 
radar or false color can be used to supplement map information (or may be the 
only surficial information readily available). Airbone geophysical techniques 
using magnetometers or gravimeters also may be useful to delineate surface and 
subsurface geologic conditions. See AERIAL PHOTOGRAPH; REMOTE SENSING; 
TOPOGRAPHIC SUR-VEYING AND MAPPING. 
    Field reconnaissance may include the collection of rock and soil 
specimens; inspection of road cuts and other excavations; inspection of the 
condition of nearby engineering structures such as bridges, pavements, and 
buildings; and location of sources of construction material. Aerial 
reconnaissance is essential at this stage and can be performed best in 
helicopters and second-best in slow-flying small planes. 
    Surface and subsurface investigations are required prior to design and 
construction. Surface studies include the preparation of a detailed map of 
surficial geology, hydrologic features, and well-defined landforms. For dam 
projects, a small-scale geologic map (for example, 1:5000) is made of the 
reservoir area and any adjacent areas that may be directly influenced by the 
project; in addition, a large-scale geologic map (for example, 1:500) is 
required of the various specific sites of the main structures (the dam, 
spillway, power plant, tunnels, and so on). 
    Subsurface investigations may include test pits, trenches, short tunnels 
(drifts or adits), and the drilling of vertical, horizontal, or oblique 
(angle) boreholes. The geology disclosed by subsurface investigations is 
``logged'' on appropriate forms. See WELL LOGGING (MINERAL). 
    Geotechnical supervision is desirable during construction in or on earth 
media. The engineering geologist must give advice and keep a record of all 
geotechnical difficulties encountered during the construction and of all 
geological features disclosed by excavations. The engineering geologist also 
is considered an important member of the team assigned to the task of 
assessing the safety of existing dams as now required by Federal legislation. 
An important consideration for the engineering geologist is the possibility of 
a contractor making legal claims for damages, purportedly because of 
unforeseen geologic conditions (generally referred to legally as charged 
conditions) encountered during construction. Legal support for such claims can 
be diminished if the engineering geologist supplies accurate and detailed 
geologic information in the specifications and drawings used for bidding 
purposes.  
  
    Special geotechnical problems.  In arctic zones, structures built on 
permafrosted soils may be heaved or may cause thawing and subsequent 
disastrous settlement. The growth of permafrost upward into earth dams 
seriously affects their stability and permeability characteristics. 
Sedimentation studies are required for the design of efficient harbors or 
reservoirs because soil carried by moving water will settle and block or fill 
these structures. In areas with known earthquake activity, aseismic 
(earthquakeproof) design requires knowledge of the intensity and magnitude of 
earthquake forces. The prevention and rehabilitation of slides (landslides) in 
steep natural slopes and in excavations are important considerations in many 
construction projects and are particularly important in planning reservoirs. 
    Geohydrologic problems can arise in the foundation material under a 
structure from pore water locked into the interstices or pores of the soil or 
rock, free water that is moving through openings in the earth media, or 
included water that is a constituent or chemically bound part of the soil or 
rock. When the structure load compresses the foundation material, the 
resulting compressive forces on the pore water can produce undesirable uplift 
pressures on the base of the structure. Included or pore water generally is 
determined by laboratory tests on cores; these are shipped from the borehole 
to the laboratory in relatively impervious containers that resist loss of 
moisture from the core. 
    Civilian and military structures may be designed to minimize the effects 
of nuclear explosions. The most effective protection is to place the facility 
in a hardened underground excavation. A hardened facility, including the 
excavation and its contents, is able to withstand the effects generated by a 
specified size of nuclear weapon such as displacement, acceleration, and 
particle velocity that occurs in the earth media and the adjacent structure. 
Desirable depths and configurations for hardened facilities are highly 
dependent upon the shock-wave characteristics of the surrounding earth media, 
for example, the type of rock, discontinuities in the rock system, free water, 
and geologic structure. See NUCLEAR EXPLOSION. 
    Projected uses of nuclear energy for the efficient construction of civil 
engineering include rapid excavation, increasing production of natural gas by 
opening fractures in the reservoir rock, expediting production of low-grade 
copper ore by causing extensive fracturing and possible concentration of the 
ore, and by the underground ``cracking'' of oil shale. The production of 
electrical energy by nuclear fission would require engineering geology inputs 
during the planning and design of the power plant; for example, a major 
question to be answered is the presence or absence of faults and an estimate 
of when the last movement on the fault occurred. 
    Another geotechnical problem occurs in the use of nuclear energy for 
generation of power or radioisotopes: safe disposal of the radioactive waste 
products. This is a problem that is under serious study. See NUCLEAR POWER; 
NUCLEAR REACTOR. 
 �34s[William R. Judd]
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�6v�2v�32sEngine lubrication   �21m �33s 
    Continuous supply of a viscous film between moving surfaces in an engine 
during its operation. Relative motion between unlubricated surfaces of engine 
parts would result in excessive wear, overheating, and seizure, with 
destruction of the contacting surfaces. The energy loss associated with the 
wear and overheating would be evident in reduced engine power and efficiency. 
The purpose of lubrication is to minimize these effects by interposing a 
viscous fluid called the lubricant between the parts. 
    Engines are generally lubricated with petroleum base oils containing 
chemical additives to improve their natural properties. The absolute viscosity 
is a measure of the force required to move one layer of the oil film over 
another. With low viscosity a protecting oil film between the parts is not 
formed. With high viscosity, the flow of oil through the engine is retarded. 
The viscosity of oil tends to decrease as its temperature is raised. It is 
therefore difficult to maintain the proper viscosity as the engine warms up, 
or as other operating conditions change. See LUBRICANT. 
    The function of the lubrication system is to supply clean oil cooled to 
the proper viscosity to critical points in the engine. In all but the most 
primitive lubrication systems, the oil, which has been heated by friction and 
by heat transfer from the hot engine parts, is collected, cooled, and pumped 
back into the engine. See INTERNAL COMBUSTION ENGINE. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sEngine manifold   �21m �33s 
    The branch pipe arrangement which connects the valve ports of 
multicylinder engines to a single carburetor or to a muffler. The manifold may 
have considerable effect on engine performance. 
    The intermittent nature of the gas flow through each cylinder may develop 
resonances (similar to the vibrations in organ pipes) in the airflow at 
certain speeds. These may increase the volumetric efficiency and thus the 
power at certain engine speeds at the expense of loss at other speeds, 
depending on manifold dimensions. 
    Another very important influence which intake manifolds may have on engine 
performance results from the fact that it is here that most of the heat is 
supplied to vaporize the fuel droplets metered to the airstream as it passes 
through the carburetor. The resulting temperature of the fuel mixture should 
be held within a comparatively narrow range for best engine performance with a 
fuel of a given volatility. If the temperature is not high enough, the 
unvaporized fuel in the manifold makes engine response to the throttle during 
acceleration sluggish and probably unsatisfactory. If the temperature is 
unnecessarily high, it causes loss of power, due to the reduced weight of the 
mixture supplied to the cylinders and the increased tendency of the engine to 
develop vapor lock from fuel boiling in the metering system; moreover, fuel 
knock can develop. See CARBURETOR; INTERNAL COMBUSTION ENGINE. 
 �34s[Neil MacCoull]
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�6v�2v�32sEnopla   �21m �33s 
    A class of the phylum Rhynchocoela which is divided into the orders 
Hoplonemertini (free-living) and Bdellonemertini or Bdellomorpha (symbiotic). 
In both orders the mouth is anterior to the brain, the nervous system is 
inside the body wall musculature, and the vascular system is well developed. 
See ANOPLA; BDELLONEMERTINI; HOPLONEMERTINI; RHYNCHOCOELA. 
 �34s[J. B. Jennings]
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�6v�2v�32sEnoploidea   �21m �33s 
    A superfamily of small to very large free-living marine nematodes 
comprising six well-defined families: Leptosomatidae, Oxystomatidae, 
Lauratonematidae, Phanodermatidae, Thoracostomopsidae, and Enoplidae. Amphids 
are generally pocketlike, opening to the exterior via slitlike aperatures; in 
exceptional instances the amphid aperature is wide and circular, or more or 
less elongated longitudinally (Oxystomatidae). The stoma is of variable form, 
with or without armature. The cuticle is nonstriated and usually without 
ornamentation. The esophagus is cylindrical to conoid. While the feeding 
habits are largely unproven, it has been postulated that the group is 
predacious or omnivorous. See NEMATA. 
 �34s[Bruce Hopper]
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�6v�2v�32sEnstatite   �21m �33s 
    The name given to the magnesian end member (MgSiO3) of the orthorhombic 
pyroxene solid-solution series. The mineral is usually yellowish-gray, 
becoming greenish with a little iron present, and transparent in thin sections. 
    Enstatite is an important mineral in the upper mantle of the Earth, and 
coexists with clinopyroxene, garnet, olivine, and plagioclase. Enstatite, 
clinopyroxene, and garnet rocks called eherzolites are common in certain 
alpine mountain terrains and as nodules in some diamond-bearing rocks called 
kimberlites. See PYROXENE. 
 �34s[George W. DeVore]
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�6v�2v�32sEnterobacter   �21m �33s 
    A genus of the bacteria, formerly called Aerobacter. Its members are 
motile, ferment lactose, form gas from glucose, decarboxylate ornithine, give 
a positive Voges-Proskauer reaction and a variable urease reaction, and 
utilize citrate, but do not produce indole or hydrogen sulfide. 
    The two recognized species, E. cloacae and E. aerogenes, are widely 
distributed in nature (soil, water sources, sewage) and are also found in 
human and animal feces. Human infections often follow primary infection with 
other pathogens (Escherichia coli, gram-positive cocci) or instrumentation, 
for example, of the urinary tract. Septicemia is rare. A majority of strains 
are hospital-acquired and resistant to the cephalosporins and to ampicillin. 
    A new species, E. agglomerans, has been proposed for strains formerly 
assigned to the genus Erwinia, species herbicola, lathyri, ananas, milletiae, 
uredovora, and cassavae. They have been observed as human pathogens in a 
variety of conditions as primary invaders, for example, after infusion of 
contaminated solutions. See ENTEROBACTERIACEAE. 
 �34s[Alexander W. C. von Graeveni]
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�6v�2v�32sEnterobacteriaceae   �21m �33s 
    A family of bacteria consisting of straight, rod-shaped gram-negative, 
non-spore-forming cells, motile with peritrichous flagella or nonmotile. All 
members are facultative chemosynthetic heterotrophs that obtain energy by the 
oxidation of a variety of simple organic compounds or by the fermentation of 
sugars and, in some cases, organic acids and polyalcohols. Most species grow 
in synthetic media with a single carbon source and an inorganic nitrogen 
source, although some species require specific amino acids or water soluble 
vitamins. The family contains many species of direct significance to human 
affairs, including important human and plant pathogens. 
    A stable classification of the group has yet to be achieved, but as now 
constituted in Bergey's Manual (eighth edition), the family is divided into 
five fairly distinct groups, which, for purposes of reference, were given 
tribe designations. The separations are based on general and specific 
metabolic characteristics and on DNA composition. Antigenic analysis, 
initially undertaken for epidemiological reasons, has shown the existence of 
about 2000 different Salmonella, with respect to the combinations of somatic 
and flagellar antigens present, and these differences have served as the basis 
of various schemes of classification. Although they have not been investigated 
as extensively, it appears that Escherichia are equally diverse antigenically, 
and Shigella somewhat less so. The presence of antigens common to species of 
different genera of the family has been noted frequently and has been 
interpreted as indicating a common phylogeny for the group. 
    The tribe Escherichieae is composed of species whose normal habitat is 
primarily the intestinal tract of warm-blooded animals including humans. 
Included are the Escherichia and Citrobacter, which are a characteristic part 
of the normal intestinal flora of humans. The Salmonella species are pathogens 
of mammals and birds, and the Shigella species are pathogens of primates. See 
ESCHERICHIA; SALMONELLA; SHIGELLA. 
    Members of the Klebsielleae differ from the Escherichieae in producing 
neutral solvents during sugar fermentation rather than the mixed acids 
characteristic of the Escherichieae. Although Enterobacter, Hafnia, and some 
strains of Klebsiella pneumoniae are normal to the intestinal tract, 
Klebsielleae are also respiratory and genitourinary tract parasites or 
pathogens. Other Klebsielleae, including the Serratia, are widely distributed, 
free-living forms common in soil, water, grain, and so on. See SERRATIA. 
    The tribe Proteeae, genus Proteus, includes forms common in feces and 
putrefying materials which may be involved in intestinal and urinary tract 
infections in humans. Characteristically, the representatives are motile, 
ferment glucose but not lactose with gas production, and produce urease. 
    The tribe Yersinieae, genus Yersinia, results from a subdivision of the 
previously accepted genus Pasteurella. Its most prominent species, Yersinia 
pestis, is the causative agent of plague in humans and rodents. See YERSINIA. 
    The tribe Erwinieae, with the single genus Erwinia, comprises a group of 
plant pathogens that causes a variety of galls, wilts, and soft rots, and 
includes the causative agent of fire blight of pear, the first bacterial plant 
pathogen to be described. The production of pectinases is characteristic of 
the soft rot group. 
 �34s[Sydney C. Rittenberg]
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�6v�2v�32sEnteropneusta   �21m �33s 
    A class of the Hemichordata, known as the acorn worms or tongue worms. 
They are free-living solitary animals without an exoskeleton. Numerous gill 
slits, a straight gut, and marked distinction between proboscis, collar, and 
trunk are characteristic of this class. (see illustration). Often, a 
free-swimming larva of the tornaria type occurs. Many of the dozen or so 
genera are widespread. Some species are locally abundant, but in general the 
group is uncommon. The distribution suggests that this is a group of great 
antiquity. Well-known genera include Balanoglossus and Saccoglossus. 
    These excessively fragile, long, wormlike animals are usually burrowers in 
sand or mud. They feed upon the organic matter in the substratum which is 
passed through the gut. Some species throw up conspicuous castings. Many are 
described as smelling like iodoform; one is recorded as swimming; another is 
luminous. See HEMICHORDATA. 
 �34s[Theodore H. Bullock]
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�6v�2v�32sEnterovirus   �21m �33s 
    One of the two subgroups of human picornaviruses. Enteroviruses include 
the polioviruses, the coxsackieviruses, and the echoviruses. They are small 
(17 - 28 nanometers in diameter), contain ribonucleic acid (RNA), and are 
resistant to ether. The enteroviruses multiply chiefly in the alimentary 
tract. The polioviruses, most echoviruses, and a number of the 
coxsackieviruses can be grown in cell cultures of monkey origin, as well as in 
human cells. 
    Enteroviruses are widespread during summer and fall in temperate climates, 
but may circulate throughout the year in tropical areas. The majority of 
enterovirus infections are benign and inapparent. However, when these viruses 
invade tissues other than the enteric tract, serious diseases may result, as 
when poliovirus invades the spinal cord or when some of the coxsackievirus 
types invade the heart muscle. See ANIMAL VIRUS; COXSACKIEVIRUS; ECHOVIRUS; 
PICORNAVIRUS; POLIOMYELITIS; RHINOVIRUS. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sEnterozoa   �21m �33s 
    Animals with a digestive cavity or tract. The term excludes only the 
one-celled Protozoa, the peculiar Mesozoa, and the Parazoa. The form and 
extent of the digestive mechanism vary widely, from a simple sac as in the 
Coelenterata to a diversified digestive tract in all higher groups. Tapeworms 
and a few other parasites lack a digestive tract. See MESOZOA; METAZOA; 
PARAZOA; PROTOZOA. 
 �34s[Tracy I. Storer]
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�6v�2v�32sEnthalpy �21m �33s
    For any system, that is, the volume of substance under discussion,
enthalpy is the sum of the internal energy of the system plus the system's
volume multiplied by the pressure exerted by the system on its surroundings.
The sum is given the special symbol H primarily as a matter of convenience
because this sum appears repeatedly in thermodynamic discussion. Previously,
enthalpy was referred to as total heat or heat content, but these terms are
misleading and should be avoided. Enthalpy is, from the viewpoint of
mathematics, a point function, as contrasted with heat and work, which are
path functions. Point functions depend only on the initial and final states of
the system undergoing a change; they are independent of the paths or character
of the change. For change in enthalpy with pressure or temperature see
THERMODYNAMIC PRINCIPLES. See also ENTROPY; THERMODYNAMIC PROCESSES.
 �34s[Harold C. Weber ;William A. Steele]
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�6v�2v�32sEntodiniomorphida   �21m �33s 
    An order of the Spirotrichia. These are strinkingly different-looking 
ciliates, covered with a smooth, firm pellicle. They are devoid of external 
ciliature except for the adoral zone of membranelles and, occasionally, one or 
two other tufts or zones of other specialized cilia (see illustration). 
Internal organization of the body is very specialized and complex. These 
organisms are considered to be highly evolved. Entodiniomorphids occur 
exclusively as endocommensals of herbivorous mammals, either in the rumen and 
reticulum of ruminants or in the colon of certain higher mammals. See 
CILIOPHORA; PROTOZOA; SPIROTRICHIA. 
 �34s[John O. Corliss]
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�6v�2v�32sEntomology   �21m �33s 
    The broad field of the biological sciences which treats all aspects of the 
study of insects. Because of the large number of insect species and their 
economic significance, many specialized areas of entomology have evolved. 
Numerous insects are of benefit to humans. They are necessary for flower 
pollination upon which much of the fruit crop depends. Many plants are 
attacked by fungal, viral, and bacterial diseases that are transmitted by 
insects. Insect parasites have been studied as a method of controlling 
destructive insects. Such studies form the basis of economic entomology. The 
specialized field of medical entomology is concerned with insects that are 
vectors for numerous diseases and parasitic infestations of humans and 
domestic animals. See ENTOMOLOGY, ECONOMIC; EPIDEMIOLOGY; INSECTA; PLANT 
PATHOLOGY.  
 �34s[Charles B. Curtin]
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�6v�2v�32sEntomology, economic   �21m �33s 
    The study of insects that have a direct influence on humanity. Though this 
includes beneficial as well as harmful species, most attention is devoted to 
the latter and how they become pests and are controlled. The emphasis on 
managing harmful insects reflects the immediacy and seriousness of pest 
problems, particularly the destruction of food and the transmission of 
disease. These are highly visible problems, whereas the benefits gained from 
useful insects are not so clearly understood, nor so well documented 
economically.  
    Central to the definition of a pest is determination of the economic 
threshold. Any insect population, when introduced into a favorable 
environment, increases numbers until reaching an environmental carrying 
capacity. In pest insects, there exists a density above which the insect 
population interferes with human health, comfort, convenience, or profits. 
When this economic threshold is reached, a decision must be made to utilize 
some control measure to prevent further increase in numbers. Often, the 
presence of even a single insect is sufficient to warrant control measures, 
for instance, when that insect is a flea harboring the plague bacillus, or a 
mosquito capable of transmitting malaria. Also, consumer expectations in most 
markets are for insect-free produce, so that the economic threshold can be 
very low on items that people eat. However, economic thresholds may be higher 
for insects that damage only the inedible portions of crop plants such as the 
leaves of beans, tomatoes, and apple trees. In any case, knowledge of the 
amount of injury which is due to different densities of insects is an 
important prerequisite for efficient management. 
  
  
    Pest management.  Management of insect pests begins with prevention. Many 
of the United States' most noxious insects have been imported from overseas: 
most domestic cockroaches, the gypsy moth, Japanese beetle, corn borer, 
housefly, cabbageworm, and codling moth are just a few. Some North American 
insects have spread elsewhere too - the Colorado potato beetle to Europe and 
the fall webworm to Japan. To stem the flow of insect invasions, the Federal 
government's Animal and Plant Health Inspection Service maintains inspection 
facilities for the examination of all incoming shipments of plant or other 
material that may harbor pests. 
    Once a pest is established, its spread can sometimes be slowed by an 
efficient system of local quarantines, early detection, and local eradication. 
A widely used eradication method is the application of synthetic chemical 
insecticides. Insecticides were once regarded as a panacea for pest problems, 
but the development of resistant strains of major insect pests, together with 
the rising cost of materials and application, has led to recognition that 
insecticides are more efficiently utilized in a program integrating them with 
other techniques in a framework of total crop management. See INSECTICIDE. 
    Another method is biological control, in which insects have had their 
numbers checked by natural enemies. Economic entomologists have effectively 
reduced densities of several pests by releasing parasites or predators. 
Natural enemies that are mobile and relatively restricted in diet do the best 
job of biological control. See INSECT CONTROL, BIOLOGICAL. 
    Crop rotation is a standard agronomic practice that often reduces damage 
due to insects. Rotation of alfalfa or soybeans with corn reduces populations 
of corn rootworms, wireworms, and white grubs. The physical disruption of 
autumnal plowing and disking destroys many insects that could overwinter in 
stubble or on the soil surface. 
    The cleanup of breeding and gathering sites is useful, especially in 
management of medically important insects, many of which have evolved 
resistance to the commoner insecticides. In general, the most successful 
programs of insect pest management rely on integrated control or the use of 
several methods in concert to control a complex of pests. 
  
    Beneficial insects.  It has been estimated that the dollar value gained 
from a single insect, the honeybee, equals the loss from damage plus cost of 
control for all pests combined. Honeybees are managed for their honey and 
beeswax, but their most valued service is pollination of crop plants. Bees of 
many species are the chief pollinators, though wasps, flies, moths, 
butterflies, and beetles pollinate as well. See POLLINATION. 
    Silk is produced by larvae of the silkworm, an insect so thoroughly 
domesticated that it cannot climb its food plant, mulberry, with its 
degenerated legs. The silkworm apparently no longer survives in the wild. Many 
uses of silk have been taken over by less expensive synthetic materials. 
    Other insects may be equally beneficial, but their value is not so easily 
calculated. Foremost among these are predatory insects of several orders. 
These predators may prevent other insects from ever reaching an economic 
threshold and thus becoming pests. Innumerable insect species are scavengers, 
quietly but efficiently breaking down the remains of dead plants and animals. 
A lack of scavenging insects would, however, result in a great increase of 
decomposing organic material lying about. Plant-eating insects have been set 
to beneficial use when their diets consist mainly of unwanted weeds. 
    Certain rare and showy butterflies and beetles are sought after so that 
they have considerable economic worth. Conservation of rare and endangered 
insects incurs some expense as well. Habitat management to conserve rare 
insects is a valid and growing concern of economic entomologists. 
    Finally, insects have rendered invaluable service to science, and thus to 
humanity, as easily reared experimental animals for investigation of basic 
principles of genetics, biochemistry, development, and behavior. See INSECTA. 
 �34s[David J. Horn]
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�6v�2v�32sEntoprocta   �21m �33s 
    A phylum of sessile, aquatic, often colonial invertebrates having a looped 
gut with both mouth and anus situated inside a circlet of tentacles, a 
pseudocoelomate body cavity, and no hardened skeleton. The phylum is divided 
into two orders, Loxosomatida (or Solitaria), with about 100 species, and 
Pedicellinida (or Coloniales), with about 30. No fossils are known. 
    Entoprocts are mainly marine, and may be encountered as small colonies on 
seaweeds and bryozoans near low tidemark. The colonies consist of threadlike, 
creeping stolons and erect zooids arising at intervals from them (see 
illustration). Noncolonial forms have similar zooids but lack the stolons. 
Each zooid is goblet-shaped, consisting of a basal stalk supporting a rounded 
or pyriform viscera-containing dilatation, the calyx. 
    The entoprocts feed on particles filtered from the surrounding water. Long 
lateral cilia drive water inward between the tentacles, while shorter cilia on 
the inner face convey particles trapped in mucus down the tentacles. These 
frontal tracts join an annular ciliated groove, which dips into the mouth. 
    Entoprocts reproduce asexually and sexually. Dispersal is by means of 
sexually produced larvae. Zooids may be unisexual or hermaphroditic. Gonads 
are paired, with the ducts uniting before opening into the coelom. 
Fertilization is internal. 
 �34s[John S. Ryland]
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�6v�2v�32sEntropy �21m �33s
    A function first introduced in classical thermodynamics to provide a
quantitative basis for the common observation that naturally occurring
processes have a particular direction. Subsequently, in statistical
thermodynamics, entropy was shown to be a measure of the number of microstates
a system could assume. Finally, in communication theory, entropy is a measure
of information. Each of these aspects will be considered in turn. Before the
entropy function is introduced, it is necessary to discuss reversible
processes.


    Reversible processes.  Any system under constant external conditions is
observed to change in such a way as to approach a particularly simple final
state called an equilibrium state. For example, two bodies initially at
different temperatures are connected by a metal wire. Heat flows from the hot
to the cold body until the temperatures of both bodies are the same. It is
common experience that the reverse processes never occur if the systems are
left to themselves; that is, heat is never observed to flow from the cold to
the hot body. Max Planck classified all elementary processes into three
categories: natural, unnatural, and reversible. Natural processes do occur,
and proceed in a direction toward equilibrium. Unnatural processes move away
from equilibrium and never occur. A reversible process is an idealized natural
process that passes through a continuous sequence of equilibrium states.

    Entropy function.  The state function entropy S puts the foregoing
discussion on a quantitative basis. Entropy is related to q, the heat flowing
into the system from its surroundings, and to T, the absolute temperature of
the system. The important properties for this discussion are:
    1. dS > q/T for a natural change.
       dS = q/T for a reversible change.
    2. The entropy of the system S is made up of the sum of all the parts of
the system so that S = S1 + S2 + S3 PRODUCT PRODUCT PRODUCT. See HEAT;
TEMPERATURE; THERMODYNAMIC PRINCIPLES.

    Nonconservation.  In his study of the first law of thermodynamics, J. P.
Joule caused work to be expended by rubbing metal blocks together in a large
mass of water. By this and similar experiments, he established numerical
relationships between heat and work. When the experiment was completed, the
apparatus remained unchanged except for a slight increase in the water
temperature. Work (W) had been converted into heat (Q) with 100% efficiency.
Provided the process was carried out slowly, the temperature difference
between the blocks and the water would be small, and heat transfer could be
considered a reversible process. The entropy increase of the water at its
temperature T is DELTAS = (Q/T) = (W/T). Since everything but the water is
unchanged, this equation also represents the total entropy increase. The
entropy has been created from the work input, and this process could be
continued indefinitely, creating more and more entropy. Unlike energy, entropy
is not conserved. See CONSERVATION OF ENERGY; THERMODYNAMIC CYCLE;
THERMODYNAMIC PROCESSES.

    Degradation of energy.  Energy is never destroyed. But in the Joule
friction experiment and in heat transfer between bodies, as in any natural
process, something is lost. In the Joule experiment, the energy expended in
work now resides in the water bath. But if this energy is reused, less useful
work is obtained than was originally put in. The original energy input has
been degraded to a less useful form. The energy transferred from a
high-temperature body to a lower-temperature body is also in a less useful
form. If another system is used to restore this degraded energy to its
original form, it is found that the restoring system has degraded the energy
even more than the original system had. Thus, every process occurring in the
world results in an overall increase in entropy and a corresponding
degradation in energy.
7
 �34s[William F. Jaep]
�33s

    Measure of information.  The probability characteristic of entropy leads
to its use in communication theory as a measure of information. The absence of
information about a situation is equivalent to an uncertainty associated with
the nature of the situation. This uncertainty is the entropy of the
information about the particular situation. See INFORMATION THEORY.
 �34s[Frank H. Rockett]
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�6v�2v�32sEnumeration of arrangements �21m �33s
    A branch of combinatorial theory which attempts to determine how many
arrangements of a given type exist, without regard to their properties. In
enumeration of arrangements it is always essential to know which arrangements
are considered the same and which different. When k objects are to be selected
from a set S and different orderings on the selected objects are to be counted
separately, the arrangements are called permutations or relabelings of the set
of k objects. But if order is to be disregarded, so that attention is focused
only on the subset consisting of the k selected elements, then the choice is
termed a combination. If elements of S are allowed to be selected more than
once, then the arrangements are called permutations with repetition or
combinations with repetition. If S has n different elements, Eqs. (1) - (4)
enumerate these four basic cases. The number of k-permutations is shown in Eq.
(1). In particular, P(n,n) = n! = n(n - 1) . . . (2)(1) is the number of ways to
order S. The number of k-combinations (or k-element subsets of S) is shown in
Eq. (2). When repetition is allowed, Eq. (3) gives the number of permutations
with repetition (or k-tuples) and Eq. (4) gives the number of combinations with
repetition. See CALCULUS.

	    P(n,k) = n(n-1)(n-2) . . . (n-k+1) = n!/(n-k)!		     (1)


	      C(n,k) = (n over k) = P(n,k)/k! = C(n,n-k)		     (2)


		    BAR P (n,k) = n(sup k)				     (3)


		   BAR C(n,k) = [(n+k-1) over k]			     (4)

    The symbol (n over k) in Eq. (2) is sometimes called a binomial coefficient,
because it is the coefficient of x(sup k) in the expansion of (1 + x)(sup n),
given in Eq.  (5).  When a finite or infinite sequence (a0, a1, .  .  .) is
exhibited in this way as the coefficients of a polynomial or series p(x) =
SUMMATION a(sub i) x(sup i), then p(x) is called a generating function for the
sequence.  The general method of generating functions was developed by P.  S.
Laplace in 1812 but had already been used extensively in the 18th century by L.
Euler.	For example, substituting x = -1 in Eq.  (5) shows that there are
exactly as many even subsets of a set as odd subsets (and thus the probability
of an even number of heads in n flips of a coin is 1/2).

C(n,x) = (1 + x)(sup n) = SUMMATION (FROM k = 0 TO n) (n OVER k) x(sup k)    (5)

See BINOMIAL THEOREM; SERIES.



	      c(sub n) = (1/n) [(2n-2) over (n-1)]

    Occasionally, only a rough estimate of the number of arrangements of a
certain kind is desired (for example, to estimate how long a computer will
take to list all such arrangements), and then an asymptotic enumeration
formula is sought. Thus, n!, which is the number of (complete) permutations
P(n,n), obeys for large n Stirling's formula (6), where

	    n! SIMILAR SQUARE ROOT 2pi n (n/e)(sup n)			 (6)

e = 2.71828 . . . [here f(n) SIMILAR  g(n) means that as n grows without bound
the ratio f(n)/g(n) approaches one]. See CATALAN NUMBERS; COMBINATORIAL THEORY;
E (MATHEMATICS); FIBONACCI NUMBERS; INTEGER PARTITION; MAGIC SQUARE; MOEBIUS
INVERSION.
 �34s[Thomas Brylawski]
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�6v�2v�32sEnvironment   �21m �33s 
    Ecologically, the environment is the sum of all external conditions and 
influences affecting the life and development of organisms. Various ecological 
principles and concepts have been developed in regard to the environment. Two 
main aspects of the environment are usually considered, the abiotic and the 
biotic. These divisions are artificial in the sense that neither can be 
separated when organisms are studied. All environmental aspects and their 
influences on living organisms must be considered together. 
  
    Abiotic environment.  The physical or abiotic environment includes all 
those physical and nonliving chemical aspects which exert an influence on 
living organisms. Among these factors are soils, water, and the atmosphere, as 
well as energy from various sources (gravity, pressure, sound, and so on). 
    Various forms of energy exert an influence on, or modify, the environment 
in which organisms are found. With minor exceptions, the ecologist is 
concerned with radiant energy received directly from the Sun, infrared 
radiation or heat, visible radiation, and ionizing radiation. These energies, 
coming from the same source, are subject to the same modifying factors, the 
primary one being daily and seasonal cycles which in turn are modified by 
latitude, altitude, slope, exposure, atmospheric conditions, and the color, 
texture, and cover of the ground surface. 
    The intensity aspect of heat energy is temperature, and, with moisture and 
light, it is one of the most familiar of environmental conditions. With the 
exception of birds and mammals, the body temperature of plant and animal life 
is determined primarily by the external environment. See HIBERNATION; 
HOMEOSTASIS; HYPOTHERMIA. 
    The source of light may be the Sun, Moon, stars, or luminescence, but 
usually only the first two are of sufficient intensity to influence life. In 
addition to the well-known uses of light for photosynthesis in plants and 
vision in more advanced animals, it also affects growth, development, and 
survival, and through many types of photoperiodicities, affects the behavior, 
life cycle, and distribution of many plants and animals. 
    Ultraviolet, nuclear, and cosmic radiations have sufficient energy to 
damage protoplasm on relatively short exposures. The atmosphere is opaque to 
all ultraviolet light shorter than 290 nanometers, so that little or no injury 
results at moderate altitudes. Nuclear radiation, composed of alpha and beta 
particles and gamma rays, comes from radioactive substances naturally 
occurring in the crust of the Earth, and from living and nonliving materials 
which have these incorporated in them. The intensity of this background 
radiation varies on an average from 0.1 to 0.5 roentgen unit (r) per year, 
although many areas give much higher readings. Cosmic rays come from space and 
vary appreciably with altitude. Some geneticists estimate that 10% of the 
mutations in geological time are attributable to these ionizing radiations. 
Very little is yet known about their direct effects at nautral levels on 
organisms. See COSMIC MICROWAVE RADIATION; ENVIRONMENTAL RADIOACTIVITY; 
RADIATION BIOLOGY. 
    Water and air are the fundamental media in which life exists. As such, 
they provide the basis for the division of the world into two major 
environments, aquatic and terrestrial. Water not only covers 70% of the 
Earth's surface, but also provides for the existence of life throughout its 
depths. As the chief components of protoplasm, water and air have many common 
physicochemical characteristics. See HYDROSPHERE. 
    The atmosphere is composed of 78% nitrogen, 21% oxygen, 0.03% carbon 
dioxide, several other gases in very small quantities, and varying amounts of 
water vapor. In addition to the physical characteristics of the atmosphere as 
a medium, its oxygen and carbon dioxide chemically affect all forms of life 
through photosynthesis and respiration, that is, through their reciprocal 
relationship of synthesis and decomposition. See ATMOSPHERE; PHOTOSYNTHESIS; 
RESPIRATION.  
    The substratum is any solid on whose surface an organism rests or moves, 
or within which it lives, thus providing anchorage, shelter, and food. Aquatic 
substrata are rock, sand, and mud, while terrestrial substrata are rock, sand, 
and soil, soil being the most complex. 
  
    Biotic environment.  The biotic environment consists of living organisms, 
which both interact with each other and are inseparably interrelated with 
their abiotic environment. 
    Within a population, interactions between organisms include such aspects 
as density, birth rate, death rate, age distribution, dispersion, and growth 
forms. At the interspecies population level interactions include competition, 
predation, parasitism, commensalism, cooperation, and mutualism. At the 
community levels, interactions include dominance, where one or several species 
control the habitat. In succession, there is an orderly process of community 
change. Rhythmic change in community activity is termed periodicity. See 
ECOLOGICAL INTERACTIONS; PERIODICITY IN ORGANISMS. 
    Interactions with abiotic environment include (1) the effects of organisms 
on the microenvironment through temperature, water, wind, and light, (2) their 
modification of the substrate, as through soil building, and (3) their 
modification of the medium, as in aquatic habitats. See BIOSPHERE; ECOSYSTEM. 
 �34s[Robert B. Platt]
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�6v�2v�32sEnvironmental engineering �21m �33s
    The discipline which evaluates the effects of humans on the environment
and develops controls to minimize environmental degradation.
    In the 1960s the United States became acutely aware of the deterioration
of its air, water, and land. The roots of the problem lie in the rapid growth
of the national population and the industrial development of natural resources
which has given Americans the highest standard of living in the world. Since
1964 there has been enactment of national, state, and local legislation
directed toward the preservation of these resources.
    It is the feeling of industry and governmental agencies that the ultimate
goals should be the design of processes and systems which need minimal
treatment for pollution control and the ultimate recycling of all wastes for
reuse. This philosophy is both logical and necessary in a society that is
rapidly depleting its nonrenewable natural resources.

    Governmental policy.  The Federal government enacts environmental air,
water, and land-use laws which in most cases require the individual states to
develop programs and enforcement policies to ensure that the goals are
fulfilled. Much environmental legislation was enacted in the 1970s by both the
state and Federal governments.
    The significant Federal legislation has included the Wilderness Act of
1964; the Air Quality Act of 1967, followed by the Clean Air Act of 1970 and
the very significant Clean Air Act Amendments of 1977; the Water Quality Act
of 1965, replaced by the Water Pollution Control Act of 1972 with amendments
to the act in 1977; the National Environmental Policy Act of 1969; Executive
Order No. 11574 of December 1970, restating the Refuse Act of the Rivers and
Harbors Act of 1899; the Noise Control Act and the Coastal Zone Management
Act, both of 1972; the Resource Conservation and Recovery Act of 1976; the
Surface Mining Control and Reclamation Act and the Toxic Substances Control
Act, both of 1977. The sweeping Federal legislation program for environmental
protection has also led to the establishment of complementary legislation,
regulations, and requirements at the state level. All states now have
extensive programs for environmental control and protection.
    The National Environmental Policy Act is an example of the
all-inclusiveness of government regulation. This act makes it mandatory that
an in-depth study of environmental impacts be made in connection with any new
industrial or government activity that may involve the Federal government,
directly or indirectly. Similarly, state environmental policy acts, patterned
after this Federal act, often require more detailed studies which must
withstand more vigorous scrutiny.
    In December 1970, by presidential order, the Environmental Protection
Agency (EPA) was formed. This agency embodies under one administrator the
responsibility for setting standards and a compliance timetable for air and
water qualities improvement. This agency also administers the Noise Control
Act, the Resource Conservation and Recovery Act, and the Toxic Substances
Control Act. In addition, the agency administers grants to state and local
governments for construction of wastewater treatment facilities and for air
and water pollution control.

    Industrial policy.	Through its interdisciplinary environmental teams,
industry is directing large amounts of capital and technological resources
both to define and resolve environmental challenges. Each air and water
problem has its own unique approach and solution. Restrictive standards
necessitate high retention efficiencies for all control equipment.
Off-the-shelf items, which were applicable in the past, no longer suffice.
Controls must now be specifically tailored to each installation. Liquid wastes
can generally be treated by chemical or physical means, or by a combination of
the two, for removal of contaminants with the expectation that the majority of
the liquid can be recycled. Air or gaseous contaminants can be removed by
scrubbing, filtration, absorption, or adsorption and the clean gas discharged
into the atmosphere. The removed contaminants, either dry or in solution, must
be handled wisely, or a new water- or air-pollution problem may result.
    Industries that extract natural resources from the earth, and in so doing
disturb the surface, are being called on to reclaim and restore the land to a
condition and contour that is equal to or better than the original state. The
Surface Mining Control and Reclamation Act, which now covers only the mining
of coal, requires the states to have an EPA-approved program for controlling
surface mining operations and the reclaiming of abandoned mined lands. Most
states already have reclamation laws which cover all types of mining
activities; most require an approved restoration plan and bonding to assure
that restoration is accomplished. See AIR POLLUTION; ENVIRONMENTAL PROTECTION;
STRIP MINING; WATER POLLUTION.
 �34s[Lewis N. Blair ;John C. Spindler]
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�6v�2v�32sEnvironmental protection   �21m �33s 
    The system of procedures which limit the impairment of the quality of 
water humans use, of the air they breathe, and of the land that sustains them. 
It includes the means to control the physical energies of ionizing radiations, 
nonionizing radiations, sound, air pressure changes, and heat and cold. Human 
activities produce wastes that are vapors or gases, solids, liquids, or energy 
states. Humans seek to disperse these to the open environment of water, air, 
or land. The receptors are all forms of life on Earth, with people the primary 
concern.  
  
    Objectives.  Environmental protection has three objectives. The first is 
to protect people from physiological damage from pathogenic organisms, from 
toxic chemicals, and from excesses of physical energies. The second is to 
spare humans annoyance, irritation, and discomfort from offensive conditions 
in water, in air, and on the land. The third objective is to safeguard the 
balances in the Earth's ecosystems and to conserve natural resources. Many 
people strongly advocate that this should be the primary goal of environmental 
protection. Fortunately, the three objectives are not incompatible, although 
conflicts arise. The drainage of a swamp which is a breeding place for 
anopheline mosquito vectors of malaria obviously changes the ecosystem that 
has existed there. Thus there are differences of opinions on which 
environmental actions should be given priority when the three objectives are 
not compatible. 
  
    Pollution.  When humans' waste-loads on the water, air, and land overwhelm 
the natural process of assimilation of such wastes, pollution occurs. The 
condition of pollution may jeopardize one or more of the three objectives of 
environmental protection. Large urban-industrial areas with the massive use of 
individual internal combustion engines for transportation have both chronic 
and acute conditions of air pollution. The airborne waste loads exceed the 
capacity of horizontal and vertical air movement to disperse the materials. 
Additionally, conditions of inversion, stagnation, and ultraviolet radiation 
produce reactants from the primary pollutants. On land and in water, not all 
wastes are usable as a food for the natural biota; these are labeled 
nonbiodegradable. The chlorinated hydrocarbon pesticides are high on the list 
of persistent contaminants which change slowly in the open environment. 
  
    Control of pollutants.  To keep the balance, the alternatives are: 
eliminate the source; eliminate the waste; treat the waste to reduce the 
deleterious load on the open environment; or augment the environmental 
capacity to assimilate the waste. All of these are applied in one way or 
another to manage liquid, solid, and airborne wastes. 
    Abatement action is taken to prevent injury to humans and animal and plant 
life, to protect property and resource values, and to limit conditions that 
are offensive to people apart from physiological damage. The mainstay of 
wastewater treatment is biooxidation. Trickling filters, activated sludge, and 
stabilization ponds depend on the same natural process as a well-aerated 
stream. That process is the biological feeding, primarily by bacteria, on the 
carbohydrates, proteins, and fats in the wastewater in the presence of ample 
oxygen. The settled solids, or sludge, are separated and made the food of 
anaerobic organisms in sludge digestors. Sewage sludge is an example of 
removing a waste from a mainstream and then having to deal with it further. 
The same holds for removing particulates from an airstream or from flue gas. 
The captured material requires further handling. There are always 
possibilities of recovery of the isolated material for economically useful 
purposes. See AIR-POLLUTION CONTROL; WATER TREATMENT. 
  
    Environmental Protection Agency.  The initiatives of environmental 
protection in the United States have passed to the Federal government. The 
National Environmental Protection Act (NEPA) consolidated all Federal 
activities on air and water pollution, solid wastes, pesticides, noise, and 
environmental radiation in a new organization, the Environmental Protection 
Agency (EPA). Many states followed suit by organizing all or most of the 
environmental protection work in an independent or autonomous unit. States 
follow the EPA leads as surrogates to keep out direct Federal intervention and 
to qualify for various forms of monetary subsidies. When the subsidies go to 
local projects, the state agency is the control gate. An extensive and 
sometimes intensive bureaucratic process has evolved to move the authorities 
and mandates of Congress to EPA, sometimes directly and sometimes through the 
states to the point where the problem can be defined and solved. 
    EPA regulations, standards, and requirements are nationwide. Much has been 
accomplished. Environmental protection in governmental agencies is firmly 
institutionalized. It has greater strength and support than ever. Pollutant 
sources are being controlled. Many streams and air sheds are showing 
improvement. Ocean dumping has been greatly reduced. Solid wastes are being 
managed much better than in the past. Air loading of particulates, 
hydrocarbons, and carbon monoxide on a national scale has been reduced. State 
agencies for environmental protection have been strengthened. 
  
    Low-level intakes through a lifetime.  The matter of low-level intakes of 
known or suspected toxicants through a life of 65 to 75 years cannot be 
resolved by present toxicological and epidemiological information or methods. 
Decisions on the use and control of such substances cannot be made solely on 
existing scientific evidence. These issues are politically and socially 
sensitive and highly emotional when laboratory animal tests indicate the 
possibility of the substances being carcinogenic, mutagenic, or teratogenic. 
See ENVIRONMENTAL ENGINEERING; HUMAN ECOLOGY. 
 �34s[Emil T. Chanlett]
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�6v�2v�32sEnvironmental radioactivity �21m �33s
    Radioactivity that originates from natural and anthropogenic sources,
including radioactive materials in food, housing, and air, radioactive
materials used in medicine, nuclear weapon tests in the open atmosphere, and
radioactive materials used in industry and power generation.
    Natural radioactivity, which is by far the largest component to which
humans are exposed, is of both terrestrial and extraterrestrial (cosmic)
origin. About 340 nuclides are known in nature, of which 70 are radioactive
and are found mainly among the heavy elements. Three nuclides which are
responsible for most of the terrestrial component are potassium-40,
uranium-238, and thorium-232.
    The average person in the United States receives 80 - 180 mrem/year (0.8 -
1.8 millisieverts/year) from natural sources of ionizing radiation. Most of
this dose originates from radioactive materials in the Earth's crust. The
external dose due to cosmic rays is an average of about 28 mrem/year (0.28
mSv/year), a value that increases with altitude due to reduced shielding of
cosmic radiation by the atmosphere.
    A number of radionuclides are produced in nature by the interaction of
cosmic rays with atmospheric gases, and these add slightly to the exposure
received from natural sources. Carbon-14 is the most important of these
nuclides which, like potassium-40, is present in the body to the extent of
about 0.1 microcurie (3.7 x 10 sup 3 Bq), but which delivers a dose of only
about 1 mrem/year (10 microsieverts/year) because of the unusual softness,
that is, low energy, of its beta emission.
    There are wide deviations from the average doses. Thus, at one extreme,
miners working underground in the presence of radioactive ore can be exposed
to such high levels of atmospheric radon that they develop lung cancer. There
are also geographical areas where the levels of natural radioactivity are
unusually high. Six types of anomalies that can be important from the point of
view of population exposure are: monazite sands and other placers, alkaline
intrusives and granites of the Conway type in New Hampshire, bauxites and
intensely weathered soils, uraniferous phosphate rock (and soils),
groundwaters enriched in radium and radon, and black shales and related
organic accumulations. The natural radioactive environment can also be altered
by human activities, such as building construction, combustion of fossil
fuels, aircraft travel, medical procedures, and nuclear weapons testing.
    Although various national and international regulatory organizations have
proposed guidelines that limit the per capita dose received by individuals in
the general population to 170 mrem/year (1.7 mSv/year), it has become evident
that nuclear power plants can be routinely operated so that the general
population will not be exposed to more than 1% of this limit. See NUCLEAR
REACTOR; RADIOACTIVE WASTE MANAGEMENT; RADIOACTIVITY.
 �34s[Merrill Eisenbud]
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�6v�2v�32sEnvironmental test �21m �33s
    A laboratory test conducted to determine the functional performance of a
component or system under conditions that simulate the real environment in
which the component or system is expected to operate. The performance of all
components and systems is influenced to varying degrees by the environment.
Moral, economic, and design considerations often preclude testing for
environmental effects by actual operation under real conditions.
    Temperature testing in the laboratory is conducted in a sealed chamber
which can be cooled or heated. The test chamber must be capable not only of
maintaining a desired temperature for extended periods of time, but also of
varying the temperature of the device under test. The test chamber should be
designed so that the component can actually operate during temperature testing.
    Many components and systems must perform satisfactorily during and after
exposure to wide ranges of humidity, when subject to fresh- and salt-water
spray, or complete immersion, and when blasted by windborne sand. Humidity
testing is usually done in conjunction with temperature testing. In the case
of spray, immersion, and sand testing, specific test conditions must be
determined.
    Components may be expected to operate at low pressures corresponding to
high altitudes or outer space, or at high pressures characteristic of great
depths beneath the surface of the sea or perhaps in high-pressure transients.
When such requirements are posed, a satisfactory environmental test procedure
must provide laboratory facilities for duplicating, insofar as is possible,
the actual pressure levels and pressure gradients to which the components will
be subjected.
    Many components and systems, as for example, the control components in a
missile rocket-engine orienting system, are subject in actual operation to
high accelerations, vibrations, and shocks. To provide some assurance of the
satisfactory performance of such a component in actual flight, it is essential
to duplicate in the laboratory as many as possible of these several
environmental factors and to explore the performance of the component while it
is subjected to one or more of these disturbing influences. Acceleration
testing is conducted on a centrifuge or a sled. Acceleration testing in pure
translation is accomplished by means of rocket-accelerated sleds on which the
components to be tested are mounted. Shock testing consists of exposing the
component to high rates of change of deceleration or acceleration, by either
dropping the component onto a resilient or absorbent surface such as rubber,
cork, or sand, or subjecting it to a pressure pulse from a compressed gas.
    Vibration tests measure the ability of a component to perform satisfactorily
while subjected to vibration by mounting it on a structure whose frequency and
amplitude of vibration can be controlled. See SYSTEMS ENGINEERING.
 �34s[Robert W. Mann]
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�6v�2v�32sEnzyme �21m �33s
    A catalytic protein produced by living cells. The chemical reactions
involved in the digestion of foods, the biosynthesis of macromolecules, the
controlled release and utilization of chemical energy, and other processes
characteristic of life are all catalyzed by enzymes. In the absence of
enzymes, these reactions would not take place at a significant rate. Several
hundred different reactions can proceed simultaneously within a living cell,
and the cell contains a comparable number of individual enzymes, each of which
controls the rate of one or more of these reactions. Some representative
enzymes, their sources, and reaction specificities are shown in the table.
    Chemical nature.  All enzymes are proteins. Their molecular weights range
from about 10,000 to more than 1,000,000. An enzyme molecule may contain one
or more polypetide chains. The sequence of amino acids within the polypeptide
chains is characteristic for each enzyme and is believed to determine the
unique three-dimensional conformation in which the chains are folded. This
conformation, which is necessary for the activity of the enzyme, is stabilized
by interactions of amino acids in different parts of the peptide chains with
each other and with the surrounding medium. These interactions are relatively
weak and may be disrupted readily by high temperatures, acid or alkaline
conditions, or changes in the polarity of the medium. Such changes lead to an
unfolding of the peptide chains (denaturation) and a concomitant loss of
enzymatic activity, solubility, and other properties characteristic of the
native enzyme. Enzyme denaturation is sometimes reversible. See PROTEIN.
    Many enzymes contain an additional, nonprotein component, termed a
coenzyme or prosthetic group. This may be an organic molecule, often a vitamin
derivative, or a metal ion. The coenzyme, in most instances, participates
directly in the catalytic reaction. Some enzymes have coenzymes that are
tightly bound to the protein and difficult to remove, while others have
coenzymes that dissociate readily. When the protein moiety (the apoenzyme) and
the coenzyme are separated from each other, neither possesses the catalytic
properties of the original conjugated protein (the holoenzyme). See COENZYME.

    Classification and nomenclature.  Enzymes are usually classified and named
according to the reaction they catalyze.
    Oxidoreductases are enzymes which catalyze reactions involving electron
transfer, and play an important role in cellular respiration and energy
production. Some of them participate in the process of oxidative
phosphorylation, whereby the energy released by the oxidation of carbohydrates
and fats is utilized for the synthesis of adenosinetriphosphate (ATP) and thus
made directly available for energy-requiring reactions. See BIOLOGICAL
OXIDATION; ENERGY METABOLISM.
    Transferases catalyze the transfer of a particular chemical group from one
substance to another. Thus, transaminases transfer amino groups,
transmethylases transfer methyl groups, and so on. An important subclass of
this group are the kinases, which catalyze the phosphorylation of their
substrates by transferring a phosphate group, usually from ATP, thereby
activating an otherwise metabolically inert compound for further
transformations.
    Hydrolases catalyze the hydrolysis of proteins (proteinases and
peptidases), of nucleic acids (nucleases), of starch (amylases), of fats
(lipases), of phosphate esters (phosphatases), and of other substances. Many
hydrolases are secreted by the stomach, pancreas, and intestines and are
responsible for the digestion of foods. Others participate in more specialized
cellular functions. For example, cholinesterase, which catalyzes the
hydrolysis of acetylcholine, plays an important role in the transmission of
nervous impulses.
    Lyases catalyze the nonhydrolytic cleavage of their substrate with the
formation of a double bond. Examples are decarboxylases, which remove carboxyl
groups as carbon dioxide, and dehydrases, which remove a molecule of water.
The reverse reactions are catalyzed by the same enzymes, but are difficult to
demonstrate in some cases because of an unfavorable equilibrium.
    Isomerases are enzymes which catalyze the interconversion of isomeric
compounds. For example, triose phosphate isomerase catalyzes the reaction
shown below.

	  CHO			     CH2OH
	   |			     |
	H--C--OH   O		     C==O    O
	   |	   ||		     |	     ||
	   CH2--O--P--OH EQUILIBRIUM CH2--O--P--OH
		   |			     |
		   OH			     OH

	D-Glyceraldehyde-	     Dihydroxyacetone
	   3-phosphate			phosphate

    Ligases or synthetases are enzymes that catalyze endergonic syntheses
coupled with the exergonic hydrolysis of ATP. They allow the chemical energy
stored in ATP to be utilized for driving reactions uphill.

    Active site.  Since enzymes are large molecules and their substrates are
often of low molecular weight, it appears probable that only a small portion
of the enzyme protein, the active site, comes into contact with the substrate
and is directly involved in catalyzing the reaction. The active site consists
of a few amino acid residues. The active site also includes any coenzyme which
may be required for activity. The function of the remainder of the protein
molecule may be primarily to hold the components of the active site in the
proper relative position and orientation. Part of the active site is involved
in binding the substrate, while another part is responsible for the making or
breaking of chemical bonds.

    Specificity.  The high degree of specificity exhibited by enzymes is one
of their most characteristic properties. The majority of enzymes catalyze only
one type of reaction and act on only one compound or on a group of closely
related compounds. There must exist between an enzyme and its substrate a
close fit, or complementarity. This relationship has been compared to that
between a lock and key. In many cases, a small structural change, even in a
part of the molecule remote from that altered by the enzymatic reaction,
abolishes the ability of a compound to serve as a substrate. On the other
hand, some enzymes exhibit a less restricted specificity and act on a number
of different compounds that possess a particular chemical group. This is
termed group specificity.
    A remarkable property of many enzymes is their high degree of
stereospecificity, that is, their ability to discriminate between asymmetric
molecules of the right-handed and lefthanded configurations. An example of a
stereospecific, as well as a group-specific, enzyme is L-amino acid oxidase.
See ALLOSTERIC ENZYME; ENZYME INHIBITION; ISOZYME; STEREOSPECIFICITY.
 �34s[Daniel Wellner]
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�6v�2v�32sEnzyme inhibition   �21m �33s 
    The prevention of an enzymic process as a result of the interaction of 
some substance with an enzyme so as to decrease the rate of the enzymic 
reaction. The substance causing such an effect is termed an inhibitor. Enzyme 
inhibitors are important as chemotherapeutic agents, as regulators in normal 
control of enzymic processes in living organisms, and as useful agents in the 
study of biochemistry. See ANTIBIOTIC; ANTIMETABOLITE. CHEMOTHERAPY; ENZYME. 
    Inhibitors have been classified as competitive, noncompetitive, and 
uncompetitive. The effect of a competitive inhibitor is to bind only free 
enzyme. This can be reversed by sufficiently increased substrate 
concentrations, so that essentially all of the enzyme is bound into an 
enzyme-substrate complex. Since both noncompetitive and uncompetitive 
inhibitors interact with the enzyme-substrate complex, their effects are not 
nullified by increased concentrations of substrate. An uncompetitive inhibitor 
exerts less effect (as percent of control) at low than at high substrate 
concentrations, since less of the enzyme is in the form of the 
enzyme-substrate complex, with which it interacts. A noncompetitive inhibitor, 
which reacts with both free enzyme and the enzyme-substrate complex, exerts 
comparable effects at all substrate concentrations. 
 �34s[William Shive]
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�6v�2v�32sEocanthocephala   �21m �33s 
    An order of the Acanthocephala characterized by the presence of a small 
number of giant subcuticular nuclei which are similar to the embryonic nuclei. 
Body spines may or may not be present and the chief lacunar vessels are dorsal 
and lateral. Proboscis hooks are few in number and arranged in circles. Eggs 
are ellipsoidal and thin shelled. These worms are parasitic in cold-blooded 
vertebrates (turtles, fish). The cystacanth occurs in crustaceans. See 
ACANTHOCEPHALA.  
 �34s[Donald V. Moore]
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�6v�2v�32sEocene   �21m �33s 
    The next to the oldest of the five major worldwide divisions (epochs) of 
the Tertiary Period (Cenozoic Era); the epoch of geologic time extending from 
the end of the Paleocene Epoch to the beginning of the Oligocene Epoch; the 
middle epoch of the older Tertiary (Paleogene or Nummulitic). See CENOZOIC; 
TERTIARY.  
    Eocene strata include all the common sedimentary types, varying from 
marine through estuarine to terrestrial in origin. Igneous activity was still 
not as widespread as in the later Tertiary, though it was notable in some 
areas (Oregon, Washington, British Columbia). 
    Eocene marine waters expanded far beyond those of the Paleocene. 
Generally, the early Eocene seas transgressed over land areas of low relief. 
Mid-Eocene diastrophic disturbances modified somewhat the previous low-relief 
surfaces; breaks in the depositional record do not indicate intensive folding, 
but in many areas diastrophism is reflected by a coarsening in texture of the 
sedimentary deposits. A widespread three-fold upper Eocene sequence produced 
strata of shallow-water origin below and above, and of deeper water between. 
    Eocene life was essentially an amplification of that of the Paleocene. 
Birds diversified and the modern families appeared. True bony fish and marine 
siphonate gastropods flourished. The pelecypod Venericardia planicosta 
characterized the world's last tropicopolitan faunas of bottom-living 
shellfish. Echinoids thrived. Nummulites multiplied and diversified, and the 
worldwide discocycline foraminifers made their last stand. Microscopic 
floating life (siliceous diatoms, calcareous coccoliths, and foraminifers) 
flowered in the widespread open oceans. 
    On land nipa palms grew where London is today, and the flora there was 
much as in Japan and Australia. On the North American Pacific Coast palms 
reached Puget Sound and southern Alaska. On the shrunken isolated Eocene land 
masses, large flightless birds developed locally. Bats, flying lemurs, 
creodont carnivores, perissodactyls, artiodactyls, notoungulates (mostly South 
American), and edentates reflected the diversification of primitive placental 
mammals; uintatheres, rhinoceroslike titanotheres, and other huge grotesque 
browsers eventually appeared alongside the last multituberculate mammals, and 
Eohippus (Hyracotherium), the multitoed, little ``dawn horse,'' was already 
present. Toward the close of the Eocene, the aquatic ancestors of the 
proboscidians developed in Egypt, and mammals began their trek into the 
free-swimming life of the ocean: Zeuglodonts (the first whales) and the first 
sea cows appeared. 
    Australia, already separated from Paleocene southeastern Asia, was further 
isolated by the expansion of the warm-water Eocene seas over much of the East 
Indies, laying the foundations for the most sharply drawn of all modern 
terrestrial life zones, Wallace's Line. Eocene marine expansions in Middle 
America cut off South America from North, isolating its fauna until toward the 
end of the Tertiary. See PALEOBOTANY; PALEONTOLOGY. 
 �34s[Robert M. Kleinpell]
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�6v�2v�32sEocrinoidea   �21m �33s 
    A class of extinct Pelmatozoa, in the subphylum Crinozoa, that had 
biserial brachioles like those of cystoids, combined with a theca like that of 
crinoids. Their affinities have been sought among both cystoids and crinoids. 
Eocrinoidea appeared in the Lower Cambrian until the Middle Ordovician. See 
CRINOIDEA; CRINOZOA; CYSTOIDEA; ECHINODERMATA; PELMATOZOA. 
 �34s[Howard B. Fell]
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 �34s[Howard B. Fell]
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�6v�2v�32sEosuchia   �21m �33s 
    The oldest, most primitive, and only extinct order of lepidosaurian 
reptiles; they are clearly ancestral of the other two orders (Rhynchocephalia 
and Squamata). They have a typical diapsid skull, with at most a partial 
reduction of the lower arcade. Tooth implantation is subthecodont. 
    Of the four suborders, the oldest is the Younginiformes, including forms 
such as Youngina (see illustration). These, ranging from the Middle Permian to 
the Lower Triassic and found mainly in South Africa, were small unspecialized 
lizardlike reptiles. The Prolacertiformes, also terrestrial, range throughout 
the Triassic. They include forms in which a reduction of the lower temporal 
arcade and other features indicate a transition toward the Squamata (lizards). 
The Thalattosauria include a few marine reptiles from the Middle Triassic. The 
Choristodera include only one genus, Champsosaurus, a much later reptile 
(Upper Cretaceous to Lower Eocene), which had amphibious habits and seems to 
have been an eosuchian offshoot. See LEPIDOSAURIA; REPTILIA; RHYNCHOCEPHALIA; 
SQUAMATA.  
 �34s[Alan J. Charig]
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�6v�2v�32sEphemeris   �21m �33s 
    A table of data, especially astronomical data, that depend on the time, 
usually arranged with values of the time in the left-hand column. A lunar 
ephemeris, for example, may give the right ascension and declination of the 
Moon for every hour of a particular year. The American Ephemeris and Nautical 
Almanac is an annual volume published by the Nautical Almanac Office, U.S. 
Naval Observatory, with ephemerides of the Sun, Moon, planets, and satellites, 
and other astronomical data. See NAUTICAL ALMANAC. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sEphemeris time   �21m �33s 
    A form of dynamical time, which appears in the equations of motion and in 
ephemerides of bodies moving in accordance with Newton's laws of dynamics and 
gravitation, as modified by relativity. The Earth's orbital motion about the 
Sun, for example, is a function of dynamical time, but not its rotational 
motion, which is affected by nongravitational forces, including tidal friction. 
    Astronomers had used dynamical time under names such as newtonian and 
gravitational. In 1958 the International Astronomical Union (IAU) adopted a 
specific definition based on the orbital motion of the Earth given by Simon 
Newcomb's Tables of the Sun of 1895 and designated it ephemeris time (ET). See 
EPHEMERIS.  
    From a joint experiment of 1955 - 1958, the frequency of the cesium-beam 
atomic clock constructed in 1955 by the National Physical Laboratory in 
Britain was found to be 9,192,631,770 cycles per second of ET. In 1967 the 
General Conference of Weights and Measures defined the International System 
(SI) unit of time, the second, as the duration of this number of periods of 
cesium radiation. The roles of dynamical and atomic times were thereby 
interchanged. The second of SI became the standard. See ATOMIC TIME. 
    In 1976 the IAU defined timelike arguments for use in dynamical astronomy, 
and in 1979 designated them as follows: Terrestrial Dynamical Time (TDT) is 
the scale for apparent geocentric ephemerides, and Barycentric Dynamical Time 
(abbreviated TDB in French) is the scale for equations of motion referred to 
the barycenter (center of mass) of the solar system. TDT and TDB superseded ET 
in national ephemerides as of January 1, 1984. See TIME. 
 �34s[William Markowitz]
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�6v�2v�32sEphemeroptera   �21m �33s 
    An order of insects commonly known as mayflies. They are aquatic and live 
in clean, fresh waters during their immature, or nymphal, lives. Nymphs are 
adapted to aquatic environments ranging from ponds to mountain streams. Most 
mayfly nymphs are vegetarians, and most species are present throughout the 
year as nymphs. As immature insects, they constitute a year-round basic food 
supply for carnivores of their communities, especially for fishes, and trout 
in particular. Nymphs and adults are models for artificial bait flies (see 
illustration).  
    Mayfly nymphs are distinguished from all other aquatic insects by the 
paired tracheal gills on the back of each of the first seven abdominal 
segments. The body ends in two or three finely segmented tailpieces. The 
mouthparts are highly interesting examples of adaptations for scraping, 
cutting, and crushing the various plant cells, which are the chief food. 
    As subadults, mayflies have the general adult form and useless mouthparts. 
They are clothed by a thin, furry, grayish skin which accounts for the common 
name, duns. The adult has transparent wings, shining body, and useless 
mouthparts; the males generally have extremely large eyes, which may be 
divided, and long front legs. This stage is brief, lasting from a few hours to 
several days. In a few mayflies, the legs of the female, and those of the 
male, except for the front ones, are nonfunctional and the brief adult life is 
spent entirely in flight. The quivering of the tailpieces during the mating 
flight gave them the name spinners. Thousands join in the twilight mating 
swarms, rhythmic dancing flights unequaled by other insects. See INSECTA. 
 �34s[Anne H. Morgan /Lewis Berner]
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�6v�2v�32sEpicaridea   �21m �33s 
    A suborder of the Isopoda which are parasitic on various crustaceans, 
mainly marine forms. The females are sometimes modified so strongly as to 
leave almost no indication of their isopod nature. The female pierces the 
host's skin with the aid of styliform mandibles to suck blood. The dwarf male 
attaches to the female, from which it takes nourishment, and retains its 
isopod structure. 
    The suborder is divided into two tribes, the Cryptoniscina and Bopyrina. 
The Cryptoniscina live on Entomostraca and are protandrous hermaphrodites. The 
cryptoniscium larva attaches to a suitable female, develops testes without any 
change in its external appearance, and acts as a male. After the death of the 
host, it metamorphoses into a female, ultimately undergoing anatomical 
degradation that leaves it a mere bulky sac distended with developing embryos. 
    The Bopyrina are dioecious, but it has been shown experimentally in some 
forms that the cryptoniscium is ambipotent, and a presumptive young male 
removed from the female develops into either a female or intersex. The tribe 
includes three families: Bopyridae, Entoniscidae, and Dajidae. See ISOPODA; 
PARASITIC CASTRATION. 
 �34s[Sueo M. Shiino]
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�6v�2v�32sEpicycloid   �21m �33s 
    A curve traced by a point P on a circle with radius r and center O' that 
rolls on the convex side of a fixed circle with radius R and center O (see 
illustration). The term is also occasionally applied to the curve generated by 
a point on the prolongation of the radius of a circle as the circle rolls on a 
straight line. It is a cardioid when the rolling circle has the same radius as 
the fixed circle. See CARDIOID. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sEpidemic   �21m �33s 
    Any sudden increase in the incidence rate of a disease. The rate must be 
in excess of the normal, but the actual value required to justify the name 
epidemic is not fixed. The usual practice is to compare the current incidence 
of the disease with the level in the same population, at the same time of the 
year, for several recent nonepidemic years. The judgment which is then made is 
influenced by various administrative considerations. A single case of a 
dreaded disease, such as smallpox, arising in an area from which it has been 
long absent, may be regarded as the potential beginning of an epidemic; 
whereas in the case of diseases calling for less community action, such as 
mumps or colds, there may be a considerable increase in the incidence rate 
without any reference being made to an epidemic. 
    A worldwide epidemic is given the name of pandemic. An example is the 
Black Death of the Middle Ages, a pandemic of plague, which in London alone 
killed 70,000 persons in a population of 500,000. A more recent example is the 
pandemic of influenza which occurred in 1918. In the United States there were 
20,000,000 cases and 500,000 deaths. See INFLUENZA; PLAGUE. 
    At the other end of the scale from the pandemic is the endemic state, 
which may be defined as the occurrence of cases of disease at a rate less than 
the epidemic level. The sporadic cases of the disease that occur in the 
endemic period provide a reservoir of infection. If conditions become suitable 
for more rapid transmission of the disease, an epidemic may break out. See 
EPIDEMIOLOGY; INFECTIOUS-DISEASE TRANSMISSION. 
 �34s[Colin White]
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�6v�2v�32sEpidemic neuromyasthenia   �21m �33s 
    A disease sometimes confused with mild poliomyelitis, as well as with 
other diseases in which there appears to be involvement of the nervous system. 
Various names (Iceland disease, Akureyri disease, benign myalgic 
encephalomyelitis, epidemic vegetative neuritis, acute infective 
encephalomyelitis) have been applied to the illness in different outbreaks 
reported first in Iceland, then in Europe and the United States, during 1950 - 
1960.  
    The main features of the illness are fatigue, headache, intense muscle 
pain, slight and transient paresis, emotional and mental disturbances, and 
objective evidence of diffuse involvement of the central nervous system. 
Sensory disturbance (paresthesia) may occur. The disease often takes a course 
which is unaccountably prolonged and debilitating, sometimes with 
recrudescences of the acute symptoms over a period of months following onset. 
No etiologic agent has been isolated, although viruses are believed to play a 
role.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sEpidemic typhus fever   �21m �33s 
    An acute, febrile human disease caused by Rickettsia prowazeki, and 
transmitted by the human louse, Pediculus humanus. It is the most universally 
known and longest recognized of the rickettsioses because of its past ravages 
during wars and famine. See MURINE TYPHUS FEVER; RICKETTSIALES; RICKETTSIOSES. 
    In humans, some 10 - 14 days after exposure, onset is usually abrupt with 
headache, malaise, and generalized aches and pains. Fluctuating but sustained 
fever between 100 - 105DGF (37.8 - 40.6DGC) lasts over 2 weeks. A generalized 
macular rash occurs about the fourth or fifth day, accompanying marked 
prostration. The second to third weeks of illness are critical, the patients 
frequently becoming stuporous and disoriented. Fatality rates vary but average 
about 20% and have reached as high as 70% in severe epidemics. Recovery is 
surprisingly rapid and sustained aftereffects are rare for so severe an 
illness accompanied by lesions in the brain. Immunity is generally lasting, 
but recrudescences many years afterward in the absence of lice are know to 
occur as Brill's disease. 
 �34s[Cornelius B. Philip]
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�6v�2v�32sEpidemic viral gasteroenteritis   �21m �33s 
    A clinical syndrome characterized by a watery diarrhea, anorexia, nausea, 
vomiting, and abdominal cramps, with little or no fever. Probably more than 
one virus is responsible. This disease is also known as acute infectious 
gastroenteritis, winter vomiting disease, and febrile nonbacterial 
gastroenteritis. It is a common syndrome of wide geographic distribution, 
highly communicable, occurring frequently in epidemic form, particularly in 
institutions, although sporadic cases also occur. Infants, as well as adults 
of all ages, are affected. The disease is of short duration, rarely lasting 
longer than 2 or 3 days, and the recovery, like the onset, is sudden. Immunity 
is not lasting. The disease appears to be distinct from epidemic diarrhea of 
the newborn, some outbreaks of which have been associated with the 
echoviruses. See INFANT DIARRHEA. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sEpidemiology �21m �33s
    The study of the mass aspects of disease. The word epidemic literally
means ``upon the people,'' and was coined to describe the way an infectious
disease, in spreading through a group of people, gives the impression of an
affliction that has been placed upon the community as a whole. See
INFECTIOUS-DISEASE TRANSMISSION.
    In recent years diseases other than infectious diseases have been included
in the subject matter of epidemiology. It is quite natural, therefore, that
these massive noninfectious diseases should be studied as group phenomena, and
that one should speak of the epidemiology of accidents, cancer, heart disease,
or mental disease. For both infectious and noninfectious diseases there are
similar general problems such as identifying cases in the population at risk,
and finding which circumstances lead to an increase and which to a decrease in
the disease. In spite of the differences in detail arising in a study of Asian
influenza and a study of suicide in a particular population, both are
fundamentally investigations of disease rates and a comparison of the way
these rates vary from one subgroup to another.
    The mortality rate for a given year is the number of deaths occurring in
the year per 1000 total midyear population. However, there are certain
diseases which cannot be studied by a means of mortality rates of any kind.
Mental diseases, arthritis, and the common cold are examples of conditions the
epidemiology of which is reflected inadequately by death rates. A more useful
approach is to consider morbidity rates which give the number of cases of
disease per 100,000 population, either as cases existing at a particular point
in time or as cases occurring in a particular period of time.
    The incidence rate is concerned with the cases of disease that develop or,
more strictly, that come under diagnosis in a given period of time. The
incidence rate per 100,000 population for a given year is equal to the number
of new cases occurring during the year times 100,000, divided by the midyear
population.
    The prevalence ratio is concerned with the cases of disease that exist at
a particular point in time. The concept is clearest when the point of time is
thought of as being instantaneous; for example, one might ask how many cases
of tuberculosis per 100,000 population existed at a specified moment, and this
would then be a prevalence ratio for the disease. Prevalence ratios are also
computed, especially for chronic diseases, over periods as long as a year.
    In addition to studying the presence or absence of a disease, the
epidemiologist often uses some measure of the severity of the disease. One
index of severity is the case fatality ratio, which is defined as the number
of deaths expressed as a percentage of the number of cases. See DISEASE;
EPIDEMIC.
 �34s[Colin White]
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�6v�2v�32sEpidermal ridges   �21m �33s 
    Minute corrugations of the skin. They compose a sculpturing, termed 
dermatoglyphics, which characterizes the palmar and plantar surfaces of all 
primates. These areas lack hair and sebaceous (oil) glands, but sweat glands 
are numerous. In certain kinds of monkeys, a portion of the undersurface of 
the tail bears similarly specialized skin. See INTEGUMENTARY PATTERNS. 
    The ridges over the human hand as a whole, in young adult males, average 
0.48 mm in breadth. They are slightly narrower in females, 0.43 mm. Ridges 
serve two functions: (1) They increase security of contact with objects, in 
the manner of the milling of a tool handle. Ducts of sweat glands open on the 
summits of ridges, and moistening of the skin augments the security of 
contact. (2) They enhance the sense of touch. In passing the fingers or palm 
over an object for judging its texture, the slight displacement of ridges 
heightens stimulation of the underlying nerve endings. 
    The characteristics of individual ridges and their collective 
configurations (see illustration) are not ordinarily studied by direct 
inspection of the skin. Instead, they are examined in prints, usually 
impressions of the inked surface, made for the purpose of record, or natural 
imprints made by chance contact. See FINGERPRINT. 
 �34s[Harold Cummins]
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�6v�2v�32sEpidermis (plant)   �21m �33s 
    The outermost layer (occasionally several layers) of cells on the primary 
plant body. Its structure is variable; this article singles out five 
structural components of the tissue: (1) cuticle; (2) stomatal apparatus 
(including guard cells and subsidiary cells); (3) bulliform (motor) cells; (4) 
trichomes; and (5) root hairs. 
    Leaves, herbaceous stems, and floral organs usually retain the epidermis 
through life. Most woody stems retain it for one to many years, after which it 
is replaced. In roots it is usually short-lived. See LEAF; PERIDERM. 
    Cutin is a mixture of fatty substances characteristically found in 
epidermal cells. It impregnates the outer cell walls and occurs as a 
continuous layer (cuticle) on the outer surface. The cuticle covers the 
surfaces of young stems, leaves, floral organs, and even apical meristems. 
Waxes appear as a deposit on the outside of the cuticle in many plants; the 
bloom on purple grapes and plums is an example. Most often the waxes are 
present in small quantity, but the leaves of some plants may be almost white 
with wax (Echeveria subrigida). The waxes of a few species are of great 
commercial value in the manufacture of polishes for floors, furniture, 
automobiles, and shoes. Other substances, such as gums, resins, and salts, 
usually in crystalline form, may be deposited on the outside of the cuticle. 
    The apertures in the epidermis which are surrounded by two specialized 
cells, the guard cells, are known as stomata. The singular form, stoma, is 
derived from the Greek word for mouth. However, some authorities prefer to 
include both aperture and guard cells within the concept of stoma. The 
apertures of stomata are contiguous with the intercellular space system of 
underlying tissues and thus permit gas exchange between internal cells and the 
external environment. The opening and closing of the stomatal aperture is 
caused by relative changes in turgor between the guard cells and surrounding 
epidermal cells. 
    Bulliform (motor) cells are large, highly vacuolated cells that occur on 
the leaves of many monocotyledons but are probably best known in grasses. They 
are thought to play a role in the unfolding of developing leaves and in the 
rolling and unrolling of mature leaves in response to alternating wet and dry 
periods.  
    Appendages derived from the protoderm are known as trichomes; the simplest 
are protrusions from single epidermal cells. Included in the concept, however, 
are such diverse structures as uniseriate hairs, multiseriate hairs (Begonia, 
Saxifraga), anchor hairs, stellate hairs, branched (candelabra) hairs, peltate 
scales, stinging hairs, and glandular hairs (see illustration). Cotton and 
kapok fibers are unicellular epidermal hairs. 
    Root hairs are thin-walled extensions of certain root epidermal cells. 
They develop only on growing root tips and may arise from any epidermal cell, 
or from specialized cells known as trichoblasts. The life of a given root hair 
is usually numbered in days. See PLANT ORGANS: PLANT TISSUE SYSTEMS; ROOT; 
SECRETORY STRUCTURES (PLANT). 
 �34s[Norman H. Boke]
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�6v�2v�32sEpidiorite   �21m �33s 
    A rock name applied to an altered diorite or gabbro. Gabbroic rocks 
exposed to mechanical pressure and hydrothermal action easily alter into green 
rocks. Many greenstones are in reality epidiorites. The term epidiorite has 
been generally used to cover rocks of this category. Minor recrystallized 
constituents may be epidote, garnet, sphene, and granular quartz. Epidiorites 
have a widespread distribution among low-metamorphic schists and phyllites of 
mountain ranges in Scotland, the Alps, the Appalachians, and the Rocky 
Mountains. See METAMORPHIC ROCKS. 
 �34s[T. F. W. Barth]
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�6v�2v�32sEpidosite   �21m �33s 
    A rare metamorphic rock composed of epidote and quartz. It may form by 
normal metamorphism of calcareous grits, but most epidosites are of 
metasomatic-hydrothermal origin. The truly metasomatic epidosites are found in 
connection with limestones that have reacted with adjacent silicate rocks 
under the conditions of the epidote-amphibolite facies. Or they form bands, 
veins, streaks, or nodules in amphibolites, hornblende schists, and masses of 
epidiorite. Epidosite from such occurrences is sometimes cut and polished and 
used for ornamental purposes. See EPIDOTE; METAMORPHIC ROCKS; METASOMATISM. 
 �34s[T. F. W. Barth]
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�6v�2v�32sEpidote   �21m �33s 
    A group of minerals having monoclinic symmetry and the ideal composition 
Ca2Fesup3suppAl2O(OH)[Si2O7][SiO4]. The common rock-forming epidote group 
minerals include zoisite (gemstone variety, tanzanite), clinozoisite, epidote, 
and piemontite (manganiferous epidote). 
    Epidote crystals are usually a distinctive yellowish-green (pistachio), 
but it may be gray, brown, or blackish-green. Hardness is 6 - 7 and specific 
gravity is 3.2 - 3.5. In petrographic thin section, epidote may have 
yellow-green pleochroism with spectacular color-zoning common. 
    Epidote is a common rock-forming mineral in regionally metamorphosed mafic 
rocks from low to medium-grade terranes. It is not characteristic of 
very-low-grade conditions, and in rocks of appropriate bulk compositions it 
replaces early-formed pumpellyite. Epidote usually occurs with quartz, 
sodium-rich plagioclase, chlorite, muscovite, and calcium-rich amphibole. It 
is a common constituent of igneous rocks, crystallizing as a late-stage 
mineral in cavities or as an alteration of primary calcium-aluminum silicates 
such as plagioclase feldspar. See SILICATE MINERALS. 
 �34s[George H. Myer]
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�6v�2v�32sEpilepsy   �21m �33s 
    A disorder of the central nervous system in which there is a chronic 
recurrence of seizures. A seizure (ictus) is an event in which there is a 
sudden alteration in function of nerve cells, most commonly involving an 
excessive electrical activity of the cells. This sudden change in nerve cell 
function is usually relatively brief, lasting seconds to minutes. Soon after a 
seizure, the brain may function quite normally. The manifestation of a seizure 
varies depending on which area of the brain is involved. 
    The following are examples of the four common seizure types. 
    1. Focal motor epilepsy is a type of seizure which is manifested by 
uncontrolled rhythmic jerking of the face, arm, or leg, caused by excessive 
abnormal discharges of nerve cells within the area of the brain which under 
usual circumstances controls movement in that part of the body. 
    2. Temporal lobe seizures may be manifested by a myriad of symptoms 
depending upon which part of the lobe is involved. Psychomotor seizures, the 
most common type, are characterized by an alteration in the state of 
consciousness, and performance of repetitive, patterned, non-goal-directed 
activity. Usually, the person is aware of having had the seizure, but may not 
remember what happened during it. 
    3. Grand mal seizures, also referred to as generalized tonic-clonic 
convulsive seizures, occur if the abnormal discharges involve the entire brain 
all at once. In this condition there are forceful, generalized, symmetrical 
musculature contractions accompanied by the loss of consciousness. A seizure 
comes to an end abruptly or gradually, and depending on the pattern and 
intensity of the attack, the person may return immediately to normal preictal 
conditions, or the person may continue to show various physical or mental 
deficits or both for some time. Amnesia is rather common, and after major 
seizures the subject often falls into a sleeplike state or goes through a 
period of confusion. 
    The relatively rare but serious life-threatening condition in which one 
seizure occurs after another without recovery between them and lasts for hours 
is called status epilepticus. 
    4. Generalized nonconvulsive seizures, also called absences or petit mal 
seizures, consists of brief periods of loss of consciousness and immediate 
recovery. The subjects are often unaware of having had the episode. 
    Epilepsy is not a disease in itself. It is a symptom of an underlying 
disease process. That disease process may be metabolic, such as uremia, 
decreased brain oxygen, or low calcium levels; or structural brain damage, 
such as from head trauma at birth, brain injuries, brain tumors, strokes, or 
congenital malformations, or previous encephalitis or meningitis. In some 
types of epilepsy there is apparently also a genetic component. 
    The ideal treatment of epilepsy is removal of the cause, such as excision 
of a tumor, or correction of a metabolic disorder, such as uremia or 
hypocalcemia. In many instances, however, the cause cannot be established or 
may not be amenable to direct treatment. When the cause cannot be removed, the 
symptoms (seizures) are treated. The following drugs are commonly used to 
control seizures. Phenytoin, carbamazepine, phenobarbital, or primidone are 
used for individuals with partial (focal) or generalized convulsive seizures. 
Ethosuximide, dipropylacetic acid, or trimethadione may be beneficial in 
generalized, nonconvulsive seizures (absences, petit mal). In individuals 
whose seizures are more difficult to control, combinations of these drugs 
might be used, or other agents may be added. 
    The occurrence of epilepsy can be decreased by measures to reduce head 
injury and birth defects. There are some indications that epilepsy can also be 
reduced by prophylactic administration of antiepileptic drugs in conditions 
which, if untreated, may lead to development of chronically recurring 
seizures. These include febrile convulsions of childhood, traumatic head 
injuries, and postoperative conditions of patients who have had brain surgery 
for other reasons. See CONVULSIVE DISORDERS. 
 �34s[Linda Moretti Ojemann]
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�6v�2v�32sEpinephrine   �21m �33s 
    A hormone which is the predominant secretion from the adrenal medulla; 
also known as adrenalin, it has the structure shown. Epinephrine is a 
sympathomimetic substance; that is, it acts on tissue supplied by sympathetic 
nerves, and generally the effects of its action are the same as those of other 
nerve stimuli. Conversely, the stimulation of the splanchnic or visceral 
nerves will cause the rapid release of the hormone from the medullary cells of 
the adrenal gland. Thus, epinephrine plays an important role in preparing the 
organism to meet conditions of physiologic emergency. 
    When injected intravenously, epinephrine causes an immediate and 
pronounced elevation in blood pressure, which is due to the coincident 
stimulation of the action of the heart and the constriction of peripheral 
blood vessels. The chief metabolic changes following the injection of 
epinephrine are a rise in the basal metabolic rate and an increase of blood 
sugar. These effects of epinephrine are transitory. See ADRENAL GLAND; 
CARBOHYDRATE METABOLISM. 
 �34s[Choh Hao Li]
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�6v�2v�32sEpithelium �21m �33s
    One of the four primary tissues of the body, which constitutes the
epidermis and the lining of respiratory, digestive, and genitourinary
passages. The major characteristic of epithelium is that the cells are close
together, separated by a very small amount of intercellular substance.
Epithelium may be derived from any of the three primary germ layers of the
very early embryo - ectoderm, entoderm, or mesoderm. With very few exceptions,
epithelium is free of blood vessels.
    The functions of epithelium are varied and include (1) protective
function, by completely covering the external surface (including the
gastrointestinal surface - and the surface of the whole pulmonary tree
including the alveoli); (2) secretory function, by secreting fluids and
chemical substances necessary for digestion, lubrication, protection,
excretion of waste products, reproduction, and the regulation of metabolic
processes of the body; (3) absorptive function, by absorbing nutritive
substances and preserving water and salts of the body; (4) sensory function,
by constituting important parts of sense organs, especially of smell and
taste; and (5) lubricating function, by lining all the internal cavities of
the body, including the peritoneum, pleura, pericardium, and the tunica
vaginalis of the testis.
    The forces which hold the epithelial cells together are not satisfactorily
understood. The intercellular substance between the cells, also called cement
substance, is undoubtedly important. The interdigitation of adjacent cell
surfaces and the occurrence of intercellular bridges in certain cells may also
be important in holding the cells together. Finally, in certain cells local
modifications of contiguous surfaces and the intervening intercellular
substances, which together form the terminal bars, may be effective in the
same way.
    The outstanding property of the arrangement of most of the epithelium of
the body is the economy of space achieved in the face of a broad exposure of
the cell surfaces. The efficiency is achieved by the presence of numerous
folds, which may be gross or microscopic and temporary or permanent. A part
must also be attributed to the surface specialization of the epithelial cells
themselves, such as their minute, fingerlike processes. Another specialization
of the surface or epithelial cell is the occurrence of motile cilia. See CILIA
AND FLAGELLA.
    Classification of epithelia is based on morphology, that is, on the shape of
the cells and their arrangement (see illustration):
I.   Single-layered.
     A. Squamous (mesothelium, descending loop of Henle in the kidney) - thin,
	flat.
     B. Cuboidal (duct, thyroid, choroid plexus) - cubelike.
     C. Columnar (intestine), sometimes ciliated (Fallopian tube, or oviduct) -
	tall.
II.  Multiple-layered or stratified.
     A. Squamous (skin, esophagus, vagina) - superficial cells thin and flat,
	deeper cells cuboidal and columnar.
     B. Columnar (pharynx, large ducts of salivary glands), sometimes ciliated
	(larynx) - two or more layers of tall cells.
III. Pseudostratified (male urethra), sometimes ciliated (respiratory passages)
     - all cells reach to basement membrane but some extend toward the surface
     only part of the way, while others reach the surface.
IV.  Transitional
     (urinary bladder) - like stratified squamous in the fully distended
     bladder; in the empty bladder, superficial cells rounded, almost 
     spherical. 

    An important property of epithelium is the ability of its cells to glide
over surfaces. This allows replacement of dead cells to take place in the
normal state, while presenting a closed surface to the external environment;
replacement is especially important in wound repair. Gliding ability is also
manifested normally in the movement of cells which slide over each other in
transitional epithelium, for example, when the urinary bladder is being
distended or contracted. See GLAND.
 �34s[Isidore Gersh]
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�6v�2v�32sEpoxidation   �21m �33s 
    The conversion of olefins (or other substances containing carbon-carbon 
double bonds) into epoxy (oxirane) compounds. Direct oxidation with oxygen to 
oxiranes has been used industrially only with ethylene, where oxidation over 
silver yields ethylene oxide [reaction (1)]. Epoxidation with peroxy acids is 
the most generally applied method [reaction (2)]. Other epoxidation procedures 
involve oxidation of the olefin with organic peroxides, permanganates, 
chromates, or dehydrochlorination of chlorohydrins with caustic alkalies. See 
ALKENE; SUBSTITUTION REACTION. 
 �34s[Frank Wagner]
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                             ******FORMULA****** 
  
  
  
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�6v�2v�32sEpoxy resin   �21m �33s 
    A polyether resin formed by the polymerization of bisphenol A and 
epichlorohydrin. Epoxy resins are used as coatings, adhesives, castings, and 
foams. Laminates of epoxy resin and glass cloth have been used to make pipes, 
to repair damaged automobile bodies, and to make small-boat hulls. See 
POLYETHER RESINS. 
 �34s[Marvin Yelles]
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�6v�2v�32sEpsomite   �21m �33s 
    A mineral with the chemical composition MgSO4PRODUCT7H2O. Epsomite, or 
epsom salt, occurs in clear, needlelike, orthorhombic crystals. More commonly 
it is massive or fibrous, although crystals from salt lakes on Kruger Mountain 
near Orville, Washington, are reported to be several feet long. Fracture is 
conchoidal. Luster varies from vitreous to silky. Hardness is 2 - 2.5 on Mohs 
scale and specific gravity is 1.68. The mineral has a salty bitter taste and 
is soluble in water. Epsomite is found as a capillary coating in limestone 
caves and in coal or metal mine galleries. It is also found associated with 
gypsum and in thin layers in salt deposits of oceanic origin or from salt 
lakes.  
 �34s[Edward C. T. Chao]
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�6v�2v�32sEpstein-Barr virus   �21m �33s 
    An antigenically distinct member of the herpesvirus group of viruses, 
whose genome is DNA. EB virus is the cause of one benign disease (infectious 
mononucleosis), and is associated with certain types of cancer; however, the 
great majority of EB virus infections are clinically inapparent. The virus was 
detected initially by electron microscopy in a small proportion of cells in 
continuous lymphoblastoid cell lines derived from Burkitt's lymphoma (but 
particles have not been seen in cells of the tumor itself). The virus also has 
been detected in cell lines derived from nasopharyngeal carcinomas, a type of 
cancer found with high frequency in persons from southern China. The virus is 
found in peripheral blood leukocytes from normal individuals and from patients 
with infectious mononucleosis. See INFECTIOUS MONONUCLEOSIS; LYMPHOMA. 
    If EB virus is indeed confirmed as having a role in the development of 
human malignancies, then one major question to be resolved is how a virus so 
ubiquitous can be involved in so wide a variety of responses. However, it 
should be recalled that many virus infections (for example, polio virus, 
hepatitis viruses, certain of the arboviral encephalitides) have a wide 
spectrum of outcomes, ranging from inapparent infection to severe syndromes. 
See ANIMAL VIRUS; ONCOLOGY. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sEquation of continuity �21m �33s
    An equation of continuity appears in many branches of physics. In dynamic
field theory, it is essentially a statement that charge is conserved or that the
rate of increase of charge in any region equals the current i flowing into that
region.  If v is the volume enclosed by the surface S, this statement may be
expressed in integral form, as given in Eq.  (1), where rho is the charge
density and n is a unit vector normal to S.  The second integral comes from the
first by Gauss' divergence theorem.  The volume in Eq.  (1) is arbitrary, so the
integrands in the last two integrals are equal.  This leads to the differential
form in Eq.  (2), where i(sub x), i(sub y), and i(sub z) are the rectangular
components of i.  Maxwell's equations satisfy Eq.  (1).  See GAUSS' THEOREM;
MAXWELL'S EQUATIONS.

 �34s[William R. Smythe]
�33s

INTEGRATION (in terms of S) BOLD i.BOLD n dS = INTEGRATION (in terms of v)
    DEL . BOLD i dv =- (CURLY D/CURLY Dt) INTEGRATION (in terms of v) rho dv
(1)


DEL . BOLD i = (CURLY Di[sub x])/CURLY Dx + (CURLY Di[sub y])/CURLYD y +
(CURLY Di[sub z])/CURLY Dz = -CURLY D rho/CURLY Dt
(2)

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�6v�2v�32sEquation of time   �21m �33s 
    The quantity added to mean solar time to obtain apparent solar time. 
Apparent solar time equals the hour angle of the true Sun (angle west of the 
meridian) plus 12 h. The Sun moves apparently with respect to the stars, in 
the ecliptic inclined 23**2DG to the Equator, and not at a uniform angular 
rate because of the Earth's elliptic orbit. In consequence, apparent solar 
time has annual variations. See TIME. 
    The introduction of fairly accurate clocks and watches about 1800 made it 
necessary to introduce the term mean solar time, defined as 12 h plus the hour 
angle of the mean sun. The mean sun is a fictitious body that moves in the 
celestial equator at the same average speed as the true Sun and has the same 
average position. 
 �34s[William Markowitz]
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�6v�2v�32sEquations, theory of �21m �33s
    The branch of mathematics concerned with finding facts concerning the
roots of algebraic equations and finding methods for obtaining them. The most
important type of algebraic equation is the polynomial equation in one unknown
which is an expression of the form f(x) = a(sup n)x(sup n) + a(sup n - 1) +
. . . + a(sub 1)x + a(sub 0) = 0, where x is called the unknown, or variable; n
is a positive whole number; and the a(sub i), with i = 0, 1, . . . , n, are
constants, or fixed numbers, called coefficients of the equation. The left
member of the equation is called a polynomial in one variable of degree n. A
root of such an equation is a number which, when substituted for the variable
x, makes the left member zero. For example, 3 is a root of the equation x(sup 3)
+ 2x(sup 2) - 13x - 6 = 0. In addition, systems of equations in one or more
variables are considered, and here the problem is to find values for the
variables which simultaneously satisfy each equation of the system. See LINEAR
SYSTEMS OF EQUATIONS; POLYNOMIAL SYSTEMS OF EQUATIONS.
    The topics covered in a systematic study of the theory of equations can be
placed in the following principal subdivisions: properties of a polynomial
which do not depend on the particular number system containing the
coefficients of the polynomial; factorization of polynomials; equations with
coefficients which are rational, real, or complex numbers; determination of
bounds for real roots, and systematic methods for approximating real roots of
equations; the solution of quadratic, cubic, and quartic equations by radicals.
    The factorization of a polynomial f(x) depends on the particular number
field F under consideration. For example, the polynomial x sup 5 - 1/2x sup 4 -
x sup 3 + 1/2x sup 2 - 2x + 1, having coefficients which are rational numbers,
factors as

		   (x sup 2 + 1) (x sup 2 - 2) (x - 1/2)

over the real numbers, as

     (x sup 2 + 1) (x - SQUARE ROOT [2]) (x + SQUARE ROOT [2]) (x - 1/2)

over the real numbers, and as

    (x + i) (x - i) (x - SQUARE ROOT [2]) (x + SQUARE ROOT [2]) (x - 1/2)

over the complex numbers. A polynomial f(x) with coefficients in F is
irreducible over F if it cannot be expressed as a product of polynomials of
lower degree. Every polynomial can be expressed in essentially one way as a
product of irreducible factors, although there is no general algorithm which
enables one to obtain this expression.
    The fundamental theorem of algebra states that a polynomial equation with
complex coefficients has a complex root. From this it follows immediately that
a polynomial of degree n with complex coefficients factors into n linear
factors over the complex numbers.
    Polynomial equations of degree 2, 3, and 4 are solvable by radicals. This
means that there are formulas which give the roots in terms of the
coefficients of the equation and that these formulas involve only the rational
operations and the operation of extraction of roots. By use of the Galois
theory of equations, it can be proved that for n > 4 there cannot exist a
formula involving only rational operations and root extractions for expressing
the roots of every polynomial equation of degree n in terms of the
coefficients. See DETERMINANT; MATRIX THEORY.
 �34s[Ross A. Beaumont]
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�6v�2v�32sEquator   �21m �33s 
    The great circle around the Earth, equally distant from the North and 
South poles, which divides the Earth into Northern and Southern hemispheres. 
It is the greatest circumference of the Earth because of centrifugal force 
from rotation, and resultant flattening of the polar areas. 
    The Earth's rotational axis is vertical to the plane of the Equator, and 
because the inclination of the axis is 66**DG from the plane of the ecliptic, 
the plane of the Equator is always inclined 23**DG from the ecliptic. 
    The celestial equator in astronomy is equally distant from the celestial 
poles and is the great circle in which the plane of the terrestrial Equator 
intersects the celestial sphere. See ASTRONOMY; MATHEMATICAL GEOGRAPHY. 
 �34s[Van H. English]
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�6v�2v�32sEquilibrium of forces   �21m �33s 
    In a mechanical system the condition under which no acceleration takes 
place. In newtonian mechanics the law of motion states that a particle of mass 
m acted on by resultant force F has acceleration a in accordance with the 
equation F = kma. Therein, k is a positive constant whose value depends upon 
the units in which F, m, and a are measured. 
    The action-reaction law states that when one particle exerts force on 
another, the other particle exerts on the one a collinear force equal in 
magnitude but oppositely directed. 
    A particle at rest or in uniform (unaccelerated) motion in a newtonian 
reference frame is in equilibrium. With few exceptions, a frame of reference 
fixed with respect to Earth is considered to be newtonian and the equation F = 
kma applied. Accordingly, when a particle is in equilibrium on Earth, a = 0 
and thus F = 0. Also, when F = 0, a = 0 because k and m are not zero. These 
considerations lead to a theorem and its corollary: The resultant of forces 
exerted on a particle in equilibrium is zero; and if the resultant of forces 
exerted on a particle is zero, the particle is in equilibrium. 
    A body acted upon by force is in equilibrium when its constituent 
particles are in equilibrium. The forces exerted on its particles (and 
therefore on the body) are either internal or external to the body. An 
internal force is one exerted by one particle on another in the same body. 
    An external force is one exerted on its particles (and therefore on the 
body) are either internal or external to the body. An internal force is one 
exerted by one particle on another in the same body. 
    An external force is one exerted on a particle or the body by a particle 
not of the body. 
    Being particle actions, internal forces obey the action-reaction law. They 
occur in pairs whose individual and combined resultants are zero. Hence a 
further theorem is that the resultant of all forces internal to a body is zero. 
    The resultant of forces on each particle and therefore on all particles of 
a body in equilibrium is zero. Because the forces are either internal or 
external and the resultant of internal forces is zero, it follows as a theorem 
that the resultant of all external forces acting upon a body in equilibrium is 
zero.  
    To determine the forces which act upon stated matter at rest on Earth is 
the usual problem of equilibrium. The problem is statically determinate if 
solution is possible by employing the equations of equilibrium only. These 
equations relate all external forces; hence accurate solution requires that 
external forces be recognized and described according to their agencies of 
application. To summarize this information the free-body diagram is 
conveniently employed (see illustration). A free-body diagram is the sketch of 
a body of interest with all external forces acting thereon symbolized by 
arrows drawn on the sketch. See DYNAMICS; FORCE; KINETICS (CLASSICAL 
MECHANICS); STATICS. 
 �34s[Nelson S. Fisk]
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�6v�2v�32sEquine encephalitis   �21m �33s 
    A mosquito-borne viral infection of horses and mules. Two types are known 
in the United States. Western equine encephalitis (WEE) occurs chiefly west of 
the Mississippi River, and Eastern equine encephalitis (EEE) in the eastern 
and southern parts of the country. Both viruses have been isolated in Canada, 
as well as in South America. The chief vector of WEE is a culicine mosquito, 
Culex tarsalis. The vector of EEE is not known. 
    The virus, a member of arbovirus group A, is believed to be maintained in 
nature by a cycle in birds and mosquitoes. If a human happens to be bitten by 
an infected mosquito, an encephalitic disease may be produced. See ARBOVIRAL 
ENCEPHALITIDES.  
    Venezuelan equine encephalitis (VEE) is distributed in northern South 
America, the Amazon valley, Trinidad, and Panama. It is a disease chiefly of 
equine animals, but it may be transmitted to humans, in whom it usually 
produces a mild, febrile disease. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sEquinox   �21m �33s 
    The date of the year and the point on the celestial sphere at which the 
Sun's rays at noon are 90DG above the horizon at the Equator, or at an angle 
of 90DG with the Earth's axis, and neither North nor South Pole is inclined 
toward the Sun. This phenomenon occurs on two days of the year, approximately 
March 21 and September 23. In the Northern Hemisphere the event in March is 
referred to as the vernal equinox, and that in September as the autumnal 
equinox. See ECLIPTIC. 
    With the Sun's rays at 90DG to the Earth's axis the rays illuminate half 
the globe. Therefore the Sun appears at all places on the Earth and gives 12 h 
of sunlight and 12 h of darkness, not considering twilight. In other words, at 
all places the Sun at the equinox appears in the east at 6 A.M., local Sun 
time, and sets at 6 P.M., in the west; thus the term equinox, meaning equal 
night. See ASTRONOMICAL COORDINATE SYSTEMS; PRECESSION OF EQUINOXES. 
 �34s[Van H. English]
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�6v�2v�32sEquisetales   �21m �33s 
    An order of the class Equisetopsida of the division Equisetophyta, 
subkingdom Embryobionta. The Equisetales, commonly known as horsetails, is 
represented by a single living genus, Equisetum, with about 25 species found 
both in moist and dry habitats. These plants grow throughout the world, except 
in Australia and New Zealand. The plants range from herbaceous to shrubby. 
    The plant body is commonly composed of perennial underground stems 
(rhizomes) with various types of aerial stems (see illustration) in different 
species. Some of these are perennial, others annual; some are unbranched and 
reproductive, others much branched and vegetative. The bushy structure of the 
latter is suggestive of the common name, horsetail. See EQUISETOPHYTA; 
EQUISETOPSIDA.  
 �34s[Paul A. Vestal]
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�6v�2v�32sEquisetophyta   �21m �33s 
    One of the eight divisions of the subkingdom Embryobionta. The division 
(formerly called Equisetineae) is divided into three classes: Hyeniopsida, 
known only from fossils, Sphenophyllopsida, and Equisetopsida. Because of 
deposits of silicon in the epidermis, the plants were formerly used for 
cleaning pots and pans and were called scouring rushes. This division had its 
beginnings in the Devonian, its peak in the Carboniferous, and exists today 
only as a remnant group. Its fossil members contributed greatly to the 
Carboniferous coal seams. See EQUISETOPSIDA; HYENIALES; SPHENOPHYLLALES. 
 �34s[Paul A. Vestal]
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�6v�2v�32sEquisetopsida   �21m �33s 
    One of the three classes of the division Equisetophyta, and the only one 
with any living species, the others (Hyeniopsida and Sphenophyllopsida) being 
represented by fossils only. The class consists of two orders, Equisetales and 
Calamitales, the latter order not being represented by any living species. The 
Equisetopsida made up a major part of the flora, especially in moist or swampy 
places, during the Carboniferous Period. See CALAMITALES; EQUISETALES; 
EQUISETOPHYTA; HYENIALES; SPHENOPHYLLALES. 
 �34s[Arthur Cronquist]
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�6v�2v�32sEquivalent circuit   �21m �33s 
    A representation of an actual electric circuit or device by a simplified 
circuit whose behavior is identical to that of the actual circuit or device 
over a stated range of operating conditions. These conditions depend on the 
elements contained in the actual circuit and may include such variables as 
frequency, temperature, and pressure, in addition to voltage and current. 
Equivalent circuits are often used by engineers to simplify circuit analysis 
since they show the relation between the variables more clearly than the 
actual circuit.  See  NETWORK THEORY. 
 �34s[Robert L. Ramey]
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�6v�2v�32sEquivalent weight   �21m �33s 
    The number of parts by weight of an element or compound which will combine 
with or replace, directly or indirectly, 1.008 parts by weight of hydrogen, 
8.00 parts of oxygen, or the equivalent weight of any other element or 
compound. For all elements, the atomic weight is equal to the equivalent 
weight times a small whole number, called the valence of the element. An 
element can have more than one valence and therefore more than one equivalent 
weight.  See  ATOMIC WEIGHT; ELEMENTS; VALENCE. 
    The concept of equivalent weight, together with that of gram-equivalent 
weight, tends to have been abandoned, and relations are expressed in terms of 
balanced stoichiometric chemical equations and relative numbers of moles 
reacting.  See  ELECTROCHEMICAL EQUIVALENT; MOLE; STOICHIOMETRY. 
 �34s[Thomas C. Waddington]
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�6v�2v�32sErbium   �21m �33s 
    A chemical element, Er, atomic number 68, atomic weight 167.26, belonging 
to the rare-earth group. The naturally occurring element is made up of the six 
stable isotopes. The rose-pink oxide, Er2O3, dissolves in mineral acids to 
give rose-colored solutions. The salts are paramagnetic and the ions are 
trivalent. At low temperatures the metal is antiferromagnetic and at still 
lower temperatures becomes strongly ferromagnetic. For properties of the metal 
  see  RARE-EARTH ELEMENTS. 
 �34s[Frank H. Spedding]
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�6v�2v�32sErgosterol   �21m �33s 
    A steroid that belongs to the class of unsaponifiable lipids. It was first 
isolated from ergot bodies. Ergosterol is a white crystalline compound, 
insoluble in water and soluble in organic solvents, with the structural 
formula shown. Ergosterol differs from cholesterol by having three positions 
of unsaturation, at carbon atoms 5-6, 7-8, and 22-23, and containing a methyl 
group (carbon atom 28) substituted for a hydrogen atom at carbon atom 24.  See 
  CHOLESTEROL. 
    Ultraviolet irradiation of ergosterol leads to the formation of calciferol 
(vitamin D2). Under controlled conditions, a more than 50% yield of vitamin D2 
can be obtained. The process is used commercially.  See  STEROID; VITAMIN D. 
 �34s[Willem J. Van Wagtendonk]
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�6v�2v�32sErgot   �21m �33s 
    The seedlike body of fungi (molds) of the genus Claviceps. Ingestion of 
these long, hard, purplish-black structures which contain poisonous alkaloids 
may lead to convulsions, abortion, hallucinations, or death. There are 32 
recognized species of  Claviceps,  most of which infect members of the grass 
family. Only three species are parasitic on the rushes and sedges. 
    The use of ergot in medicine has been reported in the past, but only in 
the 20th century did it become possible to produce stable ergot preparations 
(ergotoxine and ergotamine). However, because of side effects, researchers 
continued to look for other compounds and in 1935, a new water-soluble ergot 
alkaloid was reported. It is called ergonovine in the United States, 
ergobasine in Switzerland, and ergometrine in other countries. Ergonovine is a 
small lysergic acid derivative. Ergotamine is a large, tetracyclic peptide 
linked to lysergic acid. Ergotoxine was found to be a mixture of similar large 
alkaloids (ergocornine, ergokryptine, and ergocristine). Many other important 
lysergic acid derivatives were produced in the decade that followed. Of these 
derivatives, LSD-25 ( d -lysergic acid diethylamide) is undoubtedly the most 
famous.  
    Many uses for ergot alkaloids have been found. Ergotamine and 
dihydroergotamine are used to treat migraines, and methysergine is used in 
migraine prophylaxis. Dihydroergotoxine is prescribed for hypertension, high 
blood pressure, cerebral sclerosis, and peripheral vascular disorders. 
Ergocornine and the less toxic agroclavine have been reported as unusual 
experimental birth-control agents.  See  ERGOTISM. 
 �34s[Richard L. Mower]
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�6v�2v�32sErgotism   �21m �33s 
    A complex disease of humans and certain domestic animals caused by 
ingestion of grains and cereals infested with ergot. There are three types of 
ergotism (gangrenous, convulsive, and hallucinogenic). Their symptoms often 
overlap; the hallucinogenic form is usually observed in combination with one 
of the other two. High mortality rates have been associated with severe 
epidemics involving any of the three forms of ergotism. 
    The onset of gangrenous ergotism is generally characterized by lassitude, 
nausea, and pains in the limbs. Then, alternating sensations of intense heat 
and cold occur. As bodily extremities become numb, livid watery vesicles may 
appear on the affected parts (usually arms and legs). Finally, the diseased 
area turns black, dry, and becomes mummified. During large epidemics it was 
not uncommon for the whole rotted limb to spontaneously break off at the joint. 
    In convulsive ergotism, various parts of the body become grossly deformed 
as a result of clonic or tonic convulsions, or both. Generalized neurological 
stimulation causes epileptiform seizures, whereas specific stimulation might 
involve ravenous hunger, violent retching, tongue biting, or unusual breathing 
patterns. This form involves a longer recovery period and often results in 
permanent nerve damage and subsequent sensitization. 
    The hallucinogenic form often includes symptoms of one of the other types. 
In its more pure form, it is referred to as choreomania, St. Vitus's dance, or 
St. John's dance. Vivid hallucinations are accompanied by psychic intoxication 
reminiscent of the effects of many of the modern psychedelic drugs 
(nervousness, physical and mental excitement, insomnia, and disorientation). 
    The success with which the disease is controlled in humans has been 
brought about by (1) agricultural inspection, (2) use of wheat, potatoes, and 
maize instead of rye, (3) limited control of ergot, (4) reserves of sound 
grain, and (5) forecasting severe ergot years. The most recent and 
best-recorded epidemic was in southern France in 1951 when an unscrupulous 
miller used moldy grain to make flour.  See  ERGOT. 
 �34s[Richard L. Mower]
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�6v�2v�32sEricales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of seven families and a little more than 3000 species. The Ericaceae, 
with more than 2500 species, are by far the largest family, followed by the 
Epacridaceae, with about 400. The Ericales are sympetalous (having the petals 
joined by their margins, at least at the base) or less often polypetalous 
(having the petals separate from each other) plants. The stamens are typically 
twice as many as the petals and usually attached directly to the receptacle. 
Generally, there are many ovules in each locule of the ovary. The anthers very 
often have prominent appendages (like horns or tails) and open by terminal 
pores.  
    A large proportion of the species are strongly mycorrhizal, and some of 
them are so dependent on their fungal symbiont that they do not even produce 
chlorophyll. Heath  (Erica),  heather  (Calluna),  blueberries  (Vaccinium),  
Rhododendron, wintergreen  (Gaultheria procumbens),  and Indian pipe  
(Monotropa uniflora)  are familiar members of the Ericales.  See  BLUEBERRY; 
DILLENIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sEriocaulales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), 
subclass Commelinidae of the class Liliopsida (monocotyledons). The order 
consists of the single family Eriocaulaceae, with about 1200 species. The 
Eriocaulales are Commelinidae with a reduced (or no) perianth and with 
unisexual flowers aggregated into a dense, involucrate head that is elevated 
above the clustered, basal leaves on a long peduncle. Although the individual 
flowers are small and inconspicuous, the heads are more or less showy and 
pollination is usually by insects, in spite of the absence of nectar and 
nectaries. The order is of negligible economic importance.  See  COMMELINIDAE; 
LILIOPSIDA.  
 �34s[Arthur Cronquist]
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�6v�2v�32sErosion   �21m �33s 
    The loosening and transportation of rock debris at the Earth's surface. 
Erosion is one of the most important geologic processes at the surface and 
operates to move earth materials to lower levels. Agents of erosion include 
moving surface, ground, and ocean waters; ice; wind; organisms; temperature 
changes; and gravity. They work by moving earth materials in two ways: 
physically, that is, without change in chemical composition of the earth 
materials; and chemically, in which the rock materials have been dissolved or 
decomposed. Counteracting the effects of erosion are geologic processes which 
raise the Earth's surface, such as volcanism (volcanoes and intrusions) and 
diastrophism (folding, faulting, plate tectonics). See WEATHERING PROCESSES. 
    Within a given region, distinctive landforms, enjoyed as scenery, are 
produced by stream erosion, mountain glaciation, continental glaciation, wind 
(desert) erosion, landslides and mass wastage, and groundwater solution 
(karstic) features. See FLUVIAL EROSION CYCLE; GLACIOLOGY; MASS WASTING; 
STREAM TRANSPORT AND DEPOSITION. 
    Not only does erosion affect human beings, but the human race in turn is 
an almost violent agent of erosion, with its practices of agriculture, 
overgrazing, lumbering, and urban development on continental land slopes and 
coastal topographic features. See SOIL CONSERVATION. 
 �34s[Walter D. Keller]
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�6v�2v�32sErrantia   �21m �33s 
    A group of 34 families of Polychaeta in which the anterior, or cephalic, 
region is more or less fully exposed and the body is often long, linear to 
short, and depressed. The segments of these worms are similar or change 
gradually. The pharynx is often heavily muscularized and eversible, and its 
inner walls are fortified with calcified or chitinized plates or jaws. Some 
families are benthic throughout their life, others are entirely pelagic, and 
some have pelagic larval and reproductive stages and benthic trophic 
development. Errantia occur in all seas, at all depths, and in inland seas or 
lakes. They range in length from a few tenths of an inch (few millimeters) to 
6 ft (1.8 m). See POLYCHAETA. 
    There are 6 families of scale bearers and allies: Aphroditidae (9 genera 
and about 85 species) they include the sea mouse (Aphrodita), the giant 
cold-water Laetmonice. Polynoidae (75 genera, 600 species) are in all seas and 
at all depths. Most are benthic, some are pelagic (Drieschia, Nectochaeta), 
and others are commensal (Arctonoe, Hesperonoe, Polynoe); some are known only 
from abyssal depths (Macellicephala). Common genera are Harmothoe, 
Lepidonotus, and Halosydna. Polyodontidae (8 or 9 genera and about 35 species) 
are tubicolous and often large-bodied. Some secrete fibers to construct tubes. 
They are worldwide, occurring chiefly in tropical latitudes. Typical genera 
are Eupanthalis, Panthalis, and Polyodontes. Sigalionidae (10 genera and about 
130 species) are worldwide and occur in all latitudes, from littoral to 
abyssal depths. The best-known genera are Leanira, Psammolyce, Sigalion, 
Sthenelais, and Thalenessa. The other 5 families are small: Peisidicidae (1 
genus, 3 species); Pareulepidae (1 genus, 8 species); Pisionidae (5 genera, 12 
species); Chrysopetalidae (4 genera, 18 species); and Palmyridae (1 genus, 2 
species).  
    There are 3 Amphinomorpha families: Amphinomidae (18 genera and about 120 
species) comprise the stinging or fire worms; a burning sensation results from 
handling them. Common genera are the large, brilliantly colored Amphinome and 
Hermodice - the ``scorpions of the sea.'' Euphrosinidae (Fig. 1; 2 genera, 45 
species) and Spintheridae (1 genus, 8 species) are short, and depressed and 
are associated with sponges and other colonial animals. 
    The leaf-bearing and pelagic groups are composed of 8 families. The 
Phyllodocidae (30 genera, more than 240 species) are often brilliantly 
iridescent and are highly motile, with arresting colors and ornamentation. 
They occur in all seas and at all depths. Common genera are Anaitides, 
Eulalia, Eteone, and Phyllodoce. The pelagic families allied to the 
Phyllodocidae are Alciopidae (10 genera, 41 species); Lopadorrhynchidae (2 
genera, 5 species); Iospilidae (4 genera, 7 species); Pontodoridae (single 
genus and species); and Typhloscolecidae (3 genera, 14 species). The benthic 
families are Lacydonidae (3 genera, 6 species), and Tomopteridae, or glass 
worms (2 genera and about 40 species). 
    The Hesionidae (22 genera, more than 82 species), and the Pilargidae (6 
genera and about 25 species) differ from other families chiefly in cephalic 
parts. Most are small, short, and depressed; a few are long and linear. Some 
are free-living; others commensal. The best-known genera are Hesionie, 
Leocrates, Oxydromus, and Podarke in the Hesionidae and Ancistrosyllis and 
Pilargis in the Pilargidae. 
    Syllidae comprise the second largest of polychaete families; they have 55 
- 60 genera and more than 400 species. Most occur along intertidal rocky 
shores among epiphytic plants or animals or in encrusting sponges. They are 
chiefly foraging predators or scavengers, identified by their long, linear, 
translucent bodies with articulated cirri (Fig. 2). 
    Nereidae is known for about 27 genera and more than 360 species. Most are 
marine, occuring in worldwide seas and at all depths; a few are euryhaline, 
entering tidal streams or inland seas, or occur in thermal springs and high 
mountain lakes. Common genera are Nereis, Neanthes, Ceratocephale, 
Platynereis, and Perinereis. 
    Nephtyidae (4 genera and about 85 species) they are sometimes called 
sandworms for their occurrence in intertidal sands, where their highly 
opalescent colors and rapid movements render them almost invisible. They occur 
in worldwide seas at all depths. Common genera are Nephtys and Aglaophamus. 
    Sphaerodoridae (2 genera and about 18 species) are small, short- to 
long-bodied, and usually papillated; usually their occurrence is rare. 
    Glyceridae (3 genera and about 66 species) are characterized by the 
enormous eversible proboscis used in food capture and forward progression. 
Closely allied, the family Goniadidae (7 genera and more than 60 species) 
occurs in all seas and at all depths. Glycinde, Goniada, Goniadides, and 
Ophioglycera are worldwide in occurrence. 
    The superfamily Eunicea comprises 6 families. Onuphidae (10 genera and 
about 100 species) are tubicolous, grazing, herbivorous, scavenging, are 
worldwide in occurrence, and exist in intertidal to abyssal depths. Eunicidae 
have (7 genera and about 225 species) are mainly from tropical seas; included 
are the coral-eating Lysidice, and a bait worm, Marphysa. Lumbrineridae (3 
genera and about 130 species) are known in all seas. Arabellidae (9 genera and 
about 66 species) chiefly  include free-living and parasitic species. 
Lysaretidae (3 genera, 8 species) include large fish-bait worms (Lysarete and 
Halla) in tropical regions where they occur. Iphitime is an ectoparasite on 
large crabs. Dorvilleidae (3 genera and about 40 species) are small to minute 
and are chiefly intertidal in epiphytic growth. 
    Two parasitic families allied to the Eunicea are the Histriobdellidae, 
ectoparasites of crayfishes, (2 genera, 5 species) and the Ichthyotomidae, 
parasites of fishes, (single genus and species). See ANNELIDA. 
 �34s[Olga Hartman]
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�6v�2v�32sErysiphales   �21m �33s 
    An order of ascomycetous fungi which are obligate parasites of seed 
plants. In the Erysiphales the ascocarp consists of a tough, dark rind 
enclosing one ascus or several asci arising at the same level (see 
illustration). Asci and ascospores are liberated by rupture of the ascocarp 
wall. In most Erysiphales the filamentous mycelium is superficial except for 
absorptive branches (haustoria) which penetrate host cells. 
    About 70 genera and 1000 species are divided into two families: the 
Erysiphaceae (powdery mildews), with light-colored mycelium and conidia, and 
the Meliolaceae (sooty molds), with dark mycelium and conidia. 
    Erysiphaceae are common in temperate regions. Important powdery mildews 
include those of grape (Uncinula necator), apple (Podosphaera leucotricha), 
hops (Sphaerotheca humuli), roses (S. pannosa var. rosae), beans (Erysiphe 
polygoni), and grains (E. graminis). See ASCOMYCOTINA. 
 �34s[Robert M. Page]
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�6v�2v�32sErythroblastosis fetalis   �21m �33s 
    A form of hemolytic anemia affecting the fetus and newborn infant, caused 
by the presence in the pregnant mother of antibody directed against fetal 
erythrocyte antigens. 
    Approximately 85% of Caucasians, 95% of Negroes, and 99% of Orientals 
possess in their red blood cells an antigen, known as the rhesus, or Rh, 
factor, which was first discovered during studies with the rhesus monkey. If 
Rh-positive blood is introduced into the circulation of an Rh-negative person, 
it is recognized as foreign and the recipient may form antibodies capable of 
destroying Rh-positive red blood cells. An individual with such antibodies is 
thus sensitized to the Rh factor. Sensitization may result from leakage of 
Rh-positive red cells from a fetus into an Rh-negative mother, usually during 
delivery. The latter method is the usual manner in which a mother develops the 
antibodies responsible for erythroblastosis. Since sensitization usually 
occurs as the result of bleeding into the mother's circulation during 
delivery, erythroblastosis almost never occurs in the first Rh-positive 
pregnancy. However, subsequent Rh-positive infants are in danger if this first 
delivery results in sensitization of the mother. See ANTIBODY; ANTIGEN; BLOOD 
GROUPS.  
    It is important to recognize that not all incompatible pregnancies result 
in sensitization, and that only about 1 in 20 Rh-positive fetuses of an 
Rh-negative mother will develop erythroblastosis. Because of the rarity of 
Rh-negative Negroes and Orientals, Rh incompatibility is uncommon in these 
races. Erythroblastosis also results from incompatibility with other blood 
groups than the Rh system. The major blood groups belonging to the ABO system 
probably account for more cases than does the Rh system, but ABO 
incompatibility disease is in general less severe. 
    The symptoms of erythroblastosis are due to effects of red cell 
destruction. Anemia is one of the cardinal symptoms. Another major symptom 
produced by blood cell destruction is jaundice. Usually the destruction of red 
cells lasts only a few days, since maternal antibodies are no longer 
transferred to the baby after birth. See ANEMIA; JAUNDICE. 
    Erythroblastosis can usually be anticipated prior to birth by monitoring 
the development of Rh antibodies in the blood of a pregnant mother. It has 
become possible to detect severe cases by studying blood breakdown pigments in 
the amniotic fluid. The fluid is obtained by introducing a needle into the 
amniotic sac through the mother's abdominal wall, a comparatively safe 
procedure known as amniocentesis. If the disease is severe, the pregnancy is 
terminated as soon as is reasonably safe for the infant. Some medical centers 
have resorted to intrauterine transfusion of blood into the fetus to carry it 
over until pregnancy can safely be terminated. However, this procedure is not 
without its hazards and is available only in larger centers. 
  
    Treatment is determined by the severity of the disease. In mild cases no 
treatment at all may be necessary. In many cases a procedure known as exchange 
transfusion is used. The chief objective of exchange transfusion is to rid the 
body of excessive bilirubin. Exchange transfusion consists of replacing the 
infant's blood by compatible donor blood; in some cases this must be performed 
several times before the disease is controlled. If this measure is successful, 
no long-term effects of erythroblastosis need be feared. 
 �34s[J. Bruce Beckwith]
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�6v�2v�32sErythrocytosis   �21m �33s 
    An increase in the number of circulating red blood cells (erythrocytes). 
This condition is also known as polycythemia or erythremia. Since the 
erythrocyte is largely responsible for the viscosity of whole blood, serious 
circulatory complications are frequently associated with increases that are 
significantly above average normal values. 
    There are several forms of polycythemia. Absolute erythrocytosis is 
distinguished by an increase in the total mass of red corpuscles. It may be 
classified as primary (polycythemia rubra vera), of unknown cause, and 
secondary, which develops from a variety of factors. Some cases represent a 
physiological response to conditions of hypoxia; that is, defective oxygen 
saturation of arterial blood stimulates red blood cell production. Other cases 
are associated with renal disease (hydronephrosis and polycystic kidney) and 
tumors (renal and hepatic carcinomas, cerebellar aggregations of blood 
vessels, and adrenal cortical tumors related to aldosteronism and Cushing's 
disease.  
    Relative erythrocytosis occurs when the concentration of red blood cells 
in the circulating blood increases through loss of blood plasma. This is the 
consequence of dehydration, due to reduced fluid intake or loss of body fluid 
or stress. 
 �34s[M. Wurm]
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�6v�2v�32sErythromycin   �21m �33s 
    A relatively stable crystalline antibiotic produced by cultures of 
Streptomyces erythreus, and of primary importance in the treatment of 
gram-positive bacterial infections. It is especially useful in controlling 
diseases which have become resistant to other antibiotics, such as penicillin. 
    Erythromycin has been used successfully in the treatment of staphylococcal 
bacteremia, streptococcal bacteremia, and pneumococcal septicemia. The 
antibiotic has controlled infections of the nose, throat, tracheobronchial 
tree, the skin and soft tissue, and certain types of skeletal infections. It 
is useful for treating gonorrhea or syphilis in patients who are allergic to 
penicillin.  
    Erythromycin is given intravenously or, more usually, orally. The 
antibiotic is readily absorbed and diffuses throughout most of the body 
tissues. It tends to concentrate in the liver and is excreted in biologically 
active form in the bile, urine, and feces. See ANTIBIOTIC. 
 �34s[Frederick R. Hanson]
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�6v�2v�32sEscalator   �21m �33s 
    A continuously moving stairway and handrail. The escalator (see 
illustration) transports a continuous stream of passengers from floor to 
floor. Usually speed is 90 ft/min (0.46 m/s); slope is standardized at 30DG. 
Steps ride on resilient rollers running on tracks; endless roller chains 
propel the steps, the chains being driven by sprockets and a worm drive. An 
electric motor at the top of the escalator provides the motive force. The 
direction of travel can be reversed in accordance with the flow of traffic. 
 �34s[Frank H. Rockett]
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�6v�2v�32sEscapement   �21m �33s 
    A mechanism in which a toothed wheel engages alternate pallets attached to 
an oscillating member. The escapement is found principally in timepieces but 
may be employed wherever oscillating motion is required. 
    In a mechanical clock or watch, the escapement intervenes between the 
energy source (spring or elevated weight) and the regulating device (pendulum 
or balance wheel). It is acted upon by both. The escape wheel is mounted on 
the same shaft as the last wheel of the gear train, and impulses are delivered 
from the escape wheel to operate the regulating device. The regulating device, 
which has a natural period of oscillation, determines the rate at which it 
will receive these impulses, and thus regulates the rate of going of the 
timepiece.  
    The anchor recoil escapement (Fig. 1) is used with a pendulum and takes 
its name from the shape of its oscillating member and its action. In the 
position shown, pallet B has just received an impulse from the escape wheel, 
the impulse swinging the pendulum to the left. When pallet B has cleared the 
tooth, allowing the wheel to escape, pallet C will be in position to arrest 
the wheel. Recoil, or momentary reversal of the escape wheel, occurs just 
after it is arrested because the pendulum has not quite completed its swing. 
The wheel tooth in contact with pallet C will then give the oscillating parts 
an impulse in the opposite direction. 
    Modern watches generally employ a detached-lever escapement (Fig. 2), 
which has banking pins B to limit the oscillation of the anchor and its lever. 
An escapement is termed detached when the regulating device, in this case the 
balance wheel, is given an impulse during only a small part of its operating 
cycle. When the fork reaches the end of its swing, it is lightly locked by a 
wheel tooth and remains stationary until the returning impulse pin E causes 
sufficient recoil of the escape wheel to release the pallet. See CHRONOMETER; 
CLOCK.  
    Some modern watches employ a light ratchet powered by a vibrating reed at 
360 Hz. See WATCH. 
 �34s[Douglas P. Adams]
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�6v�2v�32sEscape velocity �21m �33s
    Minimum speed away from a parent body that a particle must acquire to
escape permanently from the gravitational attraction of the parent. Escape
velocity is also termed parabolic velocity. See ORBITAL MOTION.
    Earth retains an atmosphere because the escape velocity is considerably
higher than the mean velocity of the gas molecules in its atmosphere. For a
space ship to escape from Earth and travel to another planet or orbit about
the Sun, it must reach escape velocity. This velocity can be calculated by
equating the kinetic energy of the moving body to the work necessary to
overcome the gravity at the surface of the parent. For Earth,v(sub escape)
= 11.2 x 10 sup 3 m/s. See SATELLITE (SPACECRAFT).
 �34s[Raynor L. Duncombe]
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�6v�2v�32sEscarpment �21m �33s
    A long line of cliffs or steep slopes that break the general continuity of
the land by separating it into two level or sloping surfaces. Some very high
escarpments, or scarps, may form by vertical movement along faults. Often a
whole block of land may be forced upward while the adjacent block is
downfaulted. See FAULT AND FAULT STRUCTURES.
    Other types of escarpments form by differential weathering and erosion of
contrasted rock types. Less resistant rocks, such as clay or shale, are often
eroded from beneath resistant cap rocks, such as sandstone and limestone. With
support removed from below, the cap rock fails and the escarpment retreats.
Escarpments are often very prominent in arid regions, where hardened
weathering products may form extensive cap rocks known as duricrusts.
    Some of the largest known escarpments occur on the planet Mars, where
erosion has presumably been much slower than on the Earth in reducing primary
structural relief. See MARS.
[Victor R. Baker]



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�6v�2v�32sEscherichia �21m �33s
    A genus of the bacterial family Enterobacteriaceae, with only one species,
Escherichia coli. Most strains ferment lactose, form gas from glucose, and are
motile; they do not produce hydrogen sulfide. Indole is produced, urea is not
split, citrate is not utilized, and the Voges-Proskauer reaction is negative.
The term ``coliform bacteria'' generally refers to the group of genera made up
of Escherichia, Klebsiella, Enterobacter, Serratia, and Citrobacter.
    The bacterium is regularly present in the large intestine (and stool) of
humans and warm-blooded animals. Its presence in drinking water is considered
an indication of fecal contamination. E. coli is the most commonly isolated
gram-negative rod in the clinical laboratory, and the most common agent of
urinary tract infection in humans. It is one of the most studied
microorganisms and is extensively used in research. Genetically altered E.
coli are used for the production of insulin and interferon. See
ENTEROBACTERIACEAE.
 �34s[Alexander W. C. von Graevenitz]
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�6v�2v�32sEsker   �21m �33s 
    A sinuous ridge composed predominantly of sand and gravel deposited by 
glacial meltwater. Eskers vary in degree of continuity, and range in size from 
a few meters (1 m = 3.3 ft) to tens of meters high and from a few meters to a 
hundred or more kilometers long. They have steep ice-contact slopes and were 
deposited in channels confined by ice. Most eskers generally parallel the 
direction of ice flow, and while most follow valleys and have a normal 
down-drainage slope, some trend up a regional or local slope. 
 �34s[W. Hilton Johnson]
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�6v�2v�32sEsophagus �21m �33s
    A section of the alimentary canal that is interposed between the pharynx
and the stomach. In humans it is a tube running the full length of the neck
and the thorax, held in its position ventral to the vertebral centra by a
tunica adventitia of loose connective tissue. It has an inner lining of folded
mucous membrane with an exceptionally thick lamina propria, a submucosa of
elastic and collagenous connective tissue, and two layers of muscle. It is
supplied with autonomic nerve fibers.
    Although normally collapsed, the human esophagus is capable of
considerable distension during the rapid passage of swallowed material, under
which condition the folds of mucous membrane and lamina propria are
temporarily smoothed out. See DIGESTIVE SYSTEM.
 �34s[William W. Ballard]
�33s
    Pathology of the esophagus can be of congenital, inflammatory, vascular,
neoplastic, and mechanical origin. The two major symptoms of esophageal
disorders are dysphagia and hematemesis, or vomiting of blood.
    Congenital defects are infrequent, but stenosis (a narrowing) and atresia
(a failure of development) are seen with some regularity.
    Esophagitis, or inflammation, most commonly follows the accidental
swallowing of corrosive agents by children. Other causes are infections,
persistent vomiting, and reflux of stomach acid into the esophagus.
    Carcinoma of the esophagus is the most important of the neoplastic
processes, and it accounts for more than 5% of all malignancies other than
cutaneous neoplasms in men. It occurs most often in men over 50 years of age
and in any case of dysphagia, or difficulty in swallowing, carcinoma must be
excluded before another diagnosis is made.
    Mechanical lesions include diverticula which may be of two main types.
Traction diverticula are outpouchings of the esophageal wall caused by
adhesions following previous inflammatory disease of the adjacent lymph nodes
or mediastinal tissue, usually following tuberculosis. Pulsion diverticula are
small to large pouches which extrude through surrounding tissues, such as the
pharyngeal muscles or the diaphragm.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sEssential oils   �21m �33s 
    Odoriferous, oily products of plant origin which are distillable. The 
essential oils occur in the leaves, twigs, blossoms, fruit, trunk, and roots 
of plants. Relatively few of the great number of species of plants have been 
used to produce the oils of commerce. These plants are widespread and occur 
even in remote geographical regions. The tropic zone produces many of them. 
    The principal constituents of essential oils are the terpenes. Benzenoid 
and aliphatic compounds may also be present. Most of the constituents are 
hydrocarbons and oxygenated derivatives of hydrocarbons. A few contain 
nitrogen and sulfur. For example, oil of mustard contains organic 
isothiocyanates; garlic and onion oils contain organic sulfides. Indole, 
skatole, and anthranilates sometimes occur in small amounts. Most essential 
oils are exceedingly complex mixtures of terpene and nonterpene ingredients. 
For example, 75 identifiable compounds have been isolated from camphor oil. 
See TERPENE. 
    No essential oil has been successfully reconstructed. Synthetic methyl 
salicylate can be differentiated by odor and taste from oil of sweet birch 
even though the natural oil contains 98% methyl salicylate. However, synthetic 
methods have made possible the commercial production of important constituents 
of essential oils, such as linalool, linalyl acetate, nerolidol, and citral. 
 �34s[William Mosher]
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�6v�2v�32sEster �21m �33s
    The product of a condensation reaction (esterification) in which a
molecule of an acid unites with a molecule of alcohol with elimination of a
molecule of water as shown in the following reaction. See ESTERIFICATION.

	O		O
	||		||
	RCOH + HOR' -> RCOR' + H2O
	Acid  Alcohol  Ester

    At one time it was thought that esterification was analogous to
neutralization, and esters are still named as though they are ``alkyl salts''
of carboxylic acids.
    Esters are generally insoluble in water and have boiling points slightly
higher than hydrocarbons of similar molecular weight.
    Ethyl and butyl acetates are volatile industrial solvents, used
particularly in the formulation of lacquers. Higher-boiling esters such as
butyl phthalate are used as softening agents (plasticizers) in the compounding
of plastics. The natural waxes of biological origin are largely simple esters.
For example, a principal component of beeswax is myricyl palmitate. See
SOLVENT; WAX, ANIMAL AND VEGETABLE.
    Esters of cellulose (cellulose triacetate) are used in photographic film,
as a textile fiber (acetate rayon), and several have become important as
thermoplastic materials. Cellulose nitrate called celluloid pyroxylin, forms
celluloid, dynamite cotton, and gun cotton. Cordite and ballistite are made
from gun cotton. Dimethyl and diethyl sulfates (esters of sulfuric acid) are
excellent agents for alkylating organic molecules that contain labile hydrogen
atoms, for example, starch and cellulose.
    Esters of unsaturated acids, for example, acrylic or methacrylic acid, are
reactive and polymerize rapidly, yielding resins; thus, methyl methacrylate
yields a polymethyl methacrylate resin (Lucite). Analogously, esters of
unsaturated alcohols are reactive and readily react with themselves; thus,
vinyl acetate polymerizes to polyvinyl acetate.
    Many low-molecular-weight esters have characteristic, fruitlike odors:
banana (isoamyl acetate), rum (isobutyl propionate), and pineapple (butyl
butyrate). These esters are used to some extent in compounding synthetic
flavors and perfumes. See ALCOHOL: CARBOXYLIC ACID: DRYING OIL; FAT AND OIL;
POLYESTER RESINS.
 �34s[Paul E. Fanta]
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�6v�2v�32sEsterification   �21m �33s 
    In the broadest sense, any reaction in which at least one of the products 
is an ester. There are many routes to the formation of esters. Some of the 
more important reactions for preparing esters take place between the following 
pairs of compounds: (1) an acid and an alcohol, (2) an acid anhydride and an 
alcohol, (3) an acid chloride and an alcohol, (4) an acid and an unsaturated 
hydrocarbon such as an olefin or an acetylene, (5) an ester and an alcohol, 
(6) an ester and an acid, and (7) two different esters. See 
TRANSESTERIFICATION.  
    Esterification reactions are generally reversible and accompanied by 
relatively small heat effects of the order of a few kilocalories per mole of 
ester. Although the reactions generally take place in a single liquid phase in 
the presence of a catalyst, a limited number of esters have been prepared by 
passing the reactant vapors over a solid catalyst. 
    In a typical industrial procedure for the preparation of ethyl acetate, a 
mixture of acetic acid, excess ethanol, and sulfuric acid is passed into an 
esterifying column heated to reflux. A ternary azeotrope containing 70% 
ethanol, 20% ester, and 10% water separates into layers, one of which contains 
85% ethyl acetate. The ester may be purified by fractional distillation, and 
the recovered starting materials are recycled. Aspirin, the world's most used 
analgesic, is prepared by the reaction of salicylic acid with acetic anhydride 
below 190DGF (90DGC), and is purified by recrystallization. See ASPIRIN; 
AZEOTROPIC MIXTURE; ESTER. 
 �34s[Paul E. Fanta]
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�6v�2v�32sEstimation theory   �21m �33s 
    A branch of probability and statistics concerned with deriving information 
about properties of random variables, stochastic processes, and systems based 
on observed samples. Some of the important applications of estimation theory 
are found in control and communication systems, where it is used to estimate 
the unknown states and parameters of the system. 
    The estimation problem for dynamic systems may be divided into two parts: 
parameter estimation and state estimation. The basic difference between a 
parameter and the state is that the former either does not change at all or 
changes slowly in time, whereas the latter continuously evolves in time. For 
example, the state of a satellite is a six-dimensional vector consisting of 
three position variables and three velocity variables along the axes of an 
orthogonal coordinate system. The parameters of the satellite are its mass, 
inertia, and so on. In many control and communication problems, some of the 
system parameters are not known with desired accuracy. The problem of 
estimating these parameters from observed data is called parameter 
identification, though it is basically a problem of estimation. The more 
general problem of developing a mathematical model of the system from observed 
data is called system identification. On the other hand, the problem of state 
estimation is described by names such as signal processing, filtering, and 
smoothing. The problem belongs to the theory of stochastic processes and is 
also commonly known as time series analysis. See STOCHASTIC PROCESS. 
    Three basic approaches used for estimation are least-squares, 
maximum-likelihood, and Bayesian. An estimator is defined as a function of the 
observations possessing certain desirable properties such as unbiasedness, 
consistency, and minimum variance. A Kalman filter provides estimates that are 
optimal in the least-squares, maximum-likelihood and Bayesian sense for a 
Gauss-Markov model. (A stochastic process is Markov if, given its present 
state, its future is independent of its past.) 
    Since their introduction in 1960, Kalman filters and their extensions have 
found numerous applications. Initially, these filters were developed for space 
applications such as satellite orbit determination, inertial navigation, 
Apollo lunar landing module guidance, and so on. The applications to power 
systems and industrial processes were developed shortly thereafter. Kalman 
filters have been used for forecasting, water quality prediction, hurricane 
tracking, aircraft landing systems, and stochastic control. See CONTROL 
SYSTEMS; FLIGHT CONTROLS; GUIDANCE SYSTEMS; PROCESS CONTROL; STATISTICS; 
SYSTEMS ENGINEERING. 
 �34s[Raman K. Mehra]
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�6v�2v�32sEstrogen   �21m �33s 
    A substance that maintains the secondary sex characters and organs, such 
as mammary glands, uterus, vagina, and fallopian tubes, of mammalian females. 
Naturally occurring substances with this activity are steroid hormones. The 
principal estrogenic hormone substances are 17(BETA)-estradiol, estrone, and 
estriol. They are produced and secreted directly into the bloodstream by the 
ovary, testis, adrenal, and placenta of pregnancy. Two other naturally 
occurring estrogenic hormones, equilin and equilenin, have been obtained only 
from the urine of pregnant mares and are apparently peculiar to that species. 
Stilbestrol, a synthetic compound with considerable estrogenic activity, has 
been used extensively in medical practice. See HORMONE; STEROID. 
 �34s[Ralph I. Dorfman]
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�6v�2v�32sEstrus �21m �33s
    The period in mammals during which the female ovulates and is receptive to
mating. It is commonly referred to as rut or heat. From one estrus period to
the next there occurs a series of changes, particularly in the ovary, uterus,
and vagina, termed the estrous cycle. With reference to the ovary, the cycle
can be divided into a follicular phase, during which the Graafian follicles
are ripening, and a luteal phase, during which the corpora lutea develop in
the ovulated follicles. During these two phases mainly estrogen and
progesterone, respectively, are secreted, and these hormones control the
uterine and vaginal changes. The beginning of the follicular phase is termed
proestrus and the luteal phase metestrus. Following the latter, there is a
period of relatively little change, termed diestrus. In species in which the
latter is prolonged, it is termed anestrus. See ESTROGEN; MENSTRUATION; OVUM;
PROGESTERONE; REPRODUCTION (ANIMAL).
 �34s[Albert Tyler /Howard L. Hamilton]
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�6v�2v�32sEstuarine oceanography   �21m �33s 
    The study of the physical, chemical, biological, and geological 
characteristics of estuaries. An estuary is a semienclosed coastal body of 
water which has a free connection with the sea and within which the sea water 
is measurably diluted by fresh water derived from land drainage. Many 
characteristic features of estuaries extend into the coastal areas beyond 
their mouths, and because the techniques of measurement and analysis are 
similar, the field of estuarine oceanography is often considered to include 
the study of some coastal waters which are not strictly, by the above 
definition, estuaries. Also, semienclosed bays and lagoons exist in which 
evaporation is equal to or exceeds fresh-water inflow, so that the salt 
content is either equal to that of the sea or exceeds it. Hypersaline lagoons 
have been termed negative estuaries, whereas those with precipitation and 
river inflow equaling evaporation have been called neutral estuaries. Positive 
estuaries, in which river inflow and precipitation exceed evaporation, form 
the majority, however. 
    Within estuaries, the river discharge interacts with the sea water, and 
river water and sea water are mixed by the action of tidal motion, by wind 
stress on the surface, and by the river discharge forcing its way toward the 
sea. There is a small difference in salinity between river water and sea 
water, but it is sufficient to cause horizontal pressure gradients within the 
water which affect the way it flows. Salinity is consequently a good indicator 
of estuarine mixing and the patterns of water circulation. 
    Estuarine ecological environments are complex and highly variable when 
compared with other marine environments. They are richly productive, however. 
Because of the variability, fewer species can exist as permanent residents in 
this environment than in some other marine environments, and many of these 
species are shellfish that can easily tolerate short periods of extreme 
conditions. Motile species can escape the extremes. A number of commercially 
important marine forms are indigenous to the estuary, and the environment 
serves as a spawning or nursery ground for many other species. See MARINE 
ECOSYSTEM.  
    The patterns of sediment distribution and movement depend on the type of 
estuary and on the estuarine topography. The type of sediment brought into the 
estuary by the rivers, by erosion of the banks, and from the sea is also 
important; and the relative importance of each of these sources may change 
along the estuary. Fine-grained material will move in suspension and will 
follow the residual water flow, although there may be deposition and 
re-erosion during times of locally low velocities. The coarser-grained 
material will travel along the bed and will be affected most by high 
velocities and, consequently, in estuarine areas, will normally tend to move 
in the direction of the maximum current. 
 �34s[K. R. Dyer]
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�6v�2v�32sEthane   �21m �33s 
    A member of the alkane or paraffin series of hydrocarbons, formula CH3CH3. 
It occurs in small quantities in natural gas. It is a colorless, odorless, 
normally gaseous hydrocarbon having a freezing point of -183.3DGC (-297.9DGF) 
and a boiling point of -88.6DGC (-127.5DGF). Pyrolysis of the ethane and 
propane portion of natural gas is used as an industrial method for production 
of ethylene. See ALKANE. 
 �34s[Louis Schmerling]
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�6v�2v�32sEther �21m �33s
    One of a class of organic compounds characterized by the structural
feature of an oxygen atom linking two hydrocarbon groups, R--O--R'. Ethers are
used widely as solvents, both in chemical manufacture and in the research
laboratory.
    The hydrocarbon radicals R and R' may be identical (simple ether) or
different (mixed ether). They may be aromatic or aliphatic, and the names of
the ethers correspond to the hydrocarbon groups present. Thus, CH3--O--CH3 is
methyl ether, rarely dimethyl ether, and C6H5--O--CH3 is phenyl methyl ether.
    Simple ethers may be considered to be the anhydrides of alcohols and are
manufactured from alcohols by catalytic dehydration, as in reaction (1), or
from olefins by controlled catalytic hydration, as in reaction (2).

			       2ROH -> ROR + H2O			     (1)


2CH3CH==CH2 + HOH ->
		    (CH3)2 CH--O--CH(CH3)2				     (2)

    Ethers are less soluble in water than are the corresponding alcohols, but
are miscible with most organic solvents. Low-molecular-weight ethers have a
lower boiling point than the corresponding alcohols, but for those ethers
containing radicals larger than butyl, the reverse is true. Inertness at
moderate temperatures, an outstanding chemical characteristic of the saturated
alkyl ethers, leads to their wide use as reaction media.
    The best known of the ethers is ethyl ether, sometimes called diethyl
ether or simply ether, CH3CH2OCH2CH3. It is used in industry as a solvent and
in medicine as an anesthetic. When ethyl ether is used as a solvent, its high
volatility can cause loss. However this volatility is advantageous in that the
ether can be readily removed from the concentrated or crystallized product.
The toxicity to humans is low, and recovery from overexposure is rapid and
complete. It readily forms explosive mixtures with air, and on standing in
containers which have been opened, it forms dangerous peroxides.
    Several cyclic ethers (epoxides) are of special importance and interest.
The simplest of these is ethylene oxide or oxirane (I), made industrially by
the oxidation of ethylene with air over a silver catalyst. Dioxane or
1,4-dioxane (II) is prepared by the catalytic dimerization of ethylene oxide.
It is used extensively as an industrial solvent. Furan (III), made by the
decarbonylation of furfural, is the most important ether obtained from an
agricultural source. Most of it is hydrogenated to form the useful solvent
tetrahydrofuran (IV). See ETHYLENE OXIDE; FURAN.
    Certain large-ring polyethers, the crown ethers, are able to increase the
solubility of alkali metal salts in nonpolar organic solvents. Specific metal
complexes are formed by these crown ethers with alkali metal cations, the
specificity for a given cation depending upon the hole in the middle of the
crown ether structure. See CROWN ETHERS AND CRYPTANDS; HETEROCYCLIC COMPOUNDS.
 �34s[Paul E. Fanta]
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�6v�2v�32sEther hypothesis   �21m �33s 
    James Clerk Maxwell and his contemporaries in the 19th century found it 
inconceivable that a wave motion should propagate in empty space. They 
therefore postulated a medium, which they called the ether, that filled all 
space and transmitted electromagnetic vibrations. 
    Direct experimental attempts to establish the existence of an absolute 
ether frame of reference, in which Maxwell's equations hold and light has the 
velocity c, have failed. The best known of these is the Michelson-Morley 
experiment, in which an attempt was made to measure the velocity of the Earth 
relative to the ether. At present, there is no evidence whatever that the 
ether exists. See LIGHT; MAXWELL'S EQUATIONS; RELATIVITY. 
 �34s[William R. Smythe]
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�6v�2v�32sEthology   �21m �33s 
    The study of animal behavior. Ethology differs from psychology in that the 
interest is in behavior that occurs in a natural context. Although laboratory 
experiments are often done, the ethologist focuses on the adaptive value of 
behavior.  
    Some patterns of coordination show little variability while other 
movements are graded and more sensitive to changes in the environment. All 
degrees of intermediacy fall between these poles. Relatively form-constant 
behavior patterns that are easily recognized have been called modal action 
patterns. The best known are usually concerned with such social actions as 
courtship, fighting, or parental behavior. But they also serve nonsignal 
activities, such as the grooming movements of crickets or mice and the 
specialized feeding behavior of a variety of animals. 
    In animal behavior the same stimulus is responded to in differing ways at 
different times; there is no simple one-to-one relationship. The analysis of 
the factors leading to this variation in responsiveness is termed the study of 
motivation.  
    Stimulus filtering refers to apparent awareness of only a few of the great 
number of stimuli bombarding the animal. It may involve any sensory modality, 
such as the visual, chemical, auditory, or mechanical. 
    Among the classic works in ethology are Konrad Lorenz's experiences with 
young geese which adopted him as their mother. Lorenz characterized imprinting 
as follows. (1) There is a very early and brief sensitive period. (2) The 
object encountered in that time is the one that is followed, then and later. 
(3) This ``mother'' will become the object of sexual responses in the mature 
animal; thus the object of response is learned long before the performance of 
that response. (4) The learning is irreversible in that the animal will 
respond to the same object throughout life. (5) The response generalizes to 
all members of the species. While others before Lorenz had observed similar 
phenomena, none had dealt with it in a significant conceptual framework. 
    This concept has been hotly debated, in part because so many and widely 
different phenomena have been called imprinting; it is doubtful that these 
share the same causal mechanisms. Also a disputed issue is whether imprinting 
differs from more traditionally defined learning. Most research in this area 
is directed toward the sensitive period and what causes it to end. Lack of 
social contact is one way of prolonging it. While much argued, the development 
of fear responses, perhaps by becoming familiar with one's general 
environment, seems important in ending the sensitive period. 
 �34s[George W. Barlow]
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�6v�2v�32sEthyl alcohol   �21m �33s 
    Probably the best known of the alcohols, ethyl alcohol (C2H5OH) is also 
called alcohol, ethanol, grain alcohol, industrial alcohol, fermentation 
alcohol, cologne spirits, ethyl hydroxide, and methylcarbinol. Pure ethyl 
alcohol is a colorless, volatile liquid which is flammable and toxic and has a 
pungent taste. It boils at 78.4DGC (173DGF) and melts at -112.3DGC 
(-170.1DGF), has a specific gravity of 0.7851 at 20DGC (68DGF), and is soluble 
in water and most organic liquids. It is one of the most important industrial 
organic chemicals. Billions of pounds of it are produced annually. Ethyl 
alcohol is produced by chemical synthesis and by fermentation or biosynthetic 
processes.  
    Ethyl alcohol is used as a solvent, extractant, antifreeze, and 
intermediate in the synthesis of innumerable organic chemicals. It is also an 
essential ingredient of alcoholic beverages. 
    Various grades of ethyl alcohol are produced, depending on their intended 
use. U.S. Pharmaceutical grade is 95% ethyl alcohol by volume. National 
Formulary grade is 99+% ethyl alcohol by weight; it is also called absolute, 
or anhydrous, alcohol. Denatured alcohol contains a small amount of a 
malodorous or obnoxious material to prevent the use of this nontaxed grade of 
ethyl alcohol for beverage purposes. Ethyl alcohol which is employed in the 
manufacture of beverages, medicine, and flavoring is taxed and all ethyl 
alcohol production is closely supervised by the government. 
    The concentration of ethyl alcohol is often expressed as proof, which is 
simply twice the volume percent of ethyl alcohol. Thus, a 100-proof whiskey 
contains 50% by volume of ethyl alcohol. See ALCOHOL; FERMENTATION. 
 �34s[John W. Lynn]
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�6v�2v�32sEthylene   �21m �33s 
    A colorless gas, formula CH2DOUBLE BARCH2, with a boiling point of 
-103.8DGC (-272.9DGF) and a melting point of -169.4DGC (-154.8DGF). Ethylene 
is the most important synthetic organic chemical in terms of volume, sales 
value, and number of derivatives. Polyethylene, the most important derivative 
of ethylene, is produced by both high- and low-pressure processes to make 
high- and low-density, high-molecularweight thermoplastic polymers. Other 
important derivatives are ethylene chlorohydrin, ethylene dichloride, vinyl 
chloride, ethanol, vinyl acetate, and acetaldehyde. See ALKENE; POLYMER. 
 �34s[Robert K. Barnes]
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�6v�2v�32sEthylenediaminetetraacetic acid   �21m �33s 
    A chelating agent for metallic ions, abbreviated EDTA. Tetrasodium EDTA is 
the most common form in commerce, but other metallic chelates are marketed, 
for example, iron, zinc, and calcium. Tetrasodium EDTA is a white solid, very 
soluble in water and forming a basic solution. Prepared from ethylenediamine, 
formaldehyde, and sodium cyanide in basic solution, or from ethylenediamine 
and sodium chloroacetate, EDTA is a strong complexing and chelating agent. It 
reacts with many metallic ions to form soluble chelates, which are used in a 
variety of applications. See CHELATION; COMPLEX COMPOUNDS. 
 �34s[Frank Wagner]
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�6v�2v�32sEthylene glycol   �21m �33s 
    A colorless, nearly odorless, sweet-tasting, hygroscopic liquid, formula 
HOCH2CH2OH. It is relatively nonvolatile and viscous and is the simplest 
member of the glycol family. Ethylene glycol freezes at -13DGC (8.6DGF), boils 
at 197.6DGC (387.7DGF), and is completely soluble in water, common alcohols, 
and phenol. Low molecular weight, low volatility, water solubility, and low 
solvent action on automobile finishes make ethylene glycol ideal as a radiator 
antifreeze and coolant. Other uses for this commodity chemical are in 
polyester resins, explosives, brake and shock-absorber fluids, and alkyl-type 
resins. See ANTIFREEZE MIXTURE; GLYCOL; POLYESTER RESIN. 
    Dibasic acids or anhydrides react with ethylene glycol to form polyester 
condensation polymers. Reaction with terephthalic acid or its esters produces 
polyester resins which can be spun to fibers that find wide use in clothing 
and general fabrics applications. The polymer also has important film 
applications. See ETHYLENE OXIDE; POLYETHYLENE GLYCOL; POLYHYDROXY ALCOHOL. 
 �34s[Robert K. Barnes]
�33s 
  
A700000001776�254s�5v�1v690
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12897280
691
\IMG\CI601.IMG
13024256
�6v�2v�32sEthylene oxide �21m �33s
    The simplest cyclic ether or epoxide, with the formula C2H4O, and the
structure
			     ******FORMULA******

It is also called epoxyethane and oxirane. Commercial processes use either
air or oxygen to oxidize ethylene to ethylene oxide. See ETHER; ETHYLENE;
HETEROCYCLIC COMPOUNDS.
    Ethylene oxide is a colorless gas boiling at 10.4DGC (50.7DGF) and melting
at -112DGC (-169.6DGF). Its vapors are flammable and explosive, and it is
considered a relatively toxic liquid and gas. It is miscible in all
proportions with water, alcohols, ethers, and other organic solvents.
    About 50% of the ethylene oxide produced is converted to ethylene glycol;
about 30% is used in the manufacture of nonionic surfactants, glycol ethers,
and ethanolamines. Gaseous ethylene oxide in CO2 or difluoromethane is used as
a fumigant and a sterilizing agent for medical equipment. See ETHYLENE GLYCOL.
 �34s[Robert K. Barnes]
�33s

A700000004756�254s�5v�1v691
\IMG\CI601.IMG
13024256
�6v�2v�32sEubrya   �21m �33s 
    A subclass of the Bryopsida (Musci) which is composed of many orders of 
living mosses. There are approximately 80 families, 650 genera, and 14,000 
species. The leafy gametophytes arise from buds on the protonemata, which are 
nearly always filamentous or branched green threads and are attached to the 
substratum by rhizoids. 
    In a cross section of the stem an epidermis, a cortical area, and a 
central cylinder are commonly distinct (Fig. 1). The leaves are unistratose in 
the majority of species, bistratose in some, and may be of more than two 
layers in other species. A costa or midrib more than one cell in thickness is 
present in many species. 
    The sporophyte consists of the foot, seta, and capsule. The setae may be 
very short to much elongated, but in the Eubrya there is never a pseudopodium 
to elevate the capsule. 
    The columella (absent in Archidium) and spores develop from the 
endothecium. The columella penetrates the spore-producing layer, the 
archesporium. The sporogenous tissue, therefore, is not in the form of a dome 
over the upper portion of the columella, as in the Sphagnobrya and 
Andreaeobrya. An air cavity, not present in the Andreaeobrya and Sphagnobrya, 
separates the spore sac from the wall of the capsule. 
    In the Eubrya the mouth of the urn frequently bears an annulus and one or 
more rows or rings of processes, resembling a fringe, the peristome. When the 
spores are ripe and the operculum is shed or lost, the teeth of the peristome, 
which are hygroscopic, bend into the cavity of the urn (Fig. 2). As the teeth 
dry, they straighten, lifting the spores to disseminate them in the wind. If 
the peristome is single, there is one row of teeth. If the peristome is 
double, it consists of an outer ring of teeth, termed the outer peristome, and 
an inner ring of segments. Often one or more cilia occur between the segments. 
    For convenience in classification, the subclass Eubrya is often divided 
into plants acrocarpous (sporophyte at end of leafy gametophyte or terminal) 
and pleurocarpous (sporophyte in axils of leaves along the side of the stem or 
lateral).  
    The habitats of the plants in the Eubrya may be dry, moist, or wet; and 
the plants can exist in the tropics, temperate zones, or in the arctic 
regions. Some mosses grow in water but the majority live on rock, bark, wood, 
leaves, and soil, most frequently in shaded areas. See BRYOPSIDA. 
 �34s[Winona H. Welch]
�33s 
  
  
A700000001154�254s�5v�1v691
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�6v�2v�32sEubryales   �21m �33s 
    An order of mosses. The plants are acrocarpous, vary from small to robust 
(0.2-3 in. or 0.5 - 8 cm), and generally grow in tufts. The costate leaves, of 
various shapes, are arranged on the stem in several rows. The terminal 
sporophyte consists of an elongated seta and a cernuous, horizontal, or 
pendant capsule. The peristome is commonly double. The outer row of processes 
consists of 16 teeth, the inner of 16 segments which are carinate or 
carinately split, and cilia are often present. See BRYOPSIDA. 
 �34s[Winona H. Welch]
�33s 
  
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�6v�2v�32sEucalyptus �21m �33s
    A large genus of evergreen trees, with about 600 species, belonging to the
myrtle family (Myrtaceae), and occurring in Australia, New Guinea, and the
Sunda Islands. It has been widely planted as an ornamental and for wood
production. Some species are valuable for fuel and lumber. The leaves of blue
gum (E. globulus) contain an essential oil used in medicines and perfumes. See
MYRTALES.
 �34s[Perry D. Strausbaugh /Kenneth P. Davis]
�33s

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�6v�2v�32sEucarida   �21m �33s 
    A large and economically important superorder of higher Crustacea 
(Malacostraca), including shrimps, lobsters, hermit crabs, and crabs. They 
have the following characteristics: The shell is fused dorsally with all the 
thoracic segments; and the eyes are on movable stalks. Eggs are usually 
attached to the abdominal appendages. The liver is much ramified; the heart is 
short and thoracic. Development occurs as a rule with metamorphosis and, in 
primitive forms, a free-swimming nauplius stage. Eucarida comprises two 
orders, the Euphausiacea and Decapoda. See DECAPODA (CRUSTACEA); EUPHAUSIACEA; 
MALACOSTRACA.  
 �34s[Isabella Gordon]
�33s 
  
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�6v�2v�32sEuclasterida   �21m �33s 
    An order of Asteroidea (starfishes) in which there are many deciduous arms 
clearly differentiated from a small discoidal body. Euclasterids normally 
inhabit deep water and bear a superficial resemblance to ophiuroids, but they 
have typically open asteroid ambulacra (these are closed in ophiuroids). They 
range in time from the lower Oligocene to the present. See ASTEROIDEA; 
ECHINODERMATA.  
 �34s[Andrew C. Campbell]
�33s 
  
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�6v�2v�32sEuclidean geometry   �21m �33s 
    The chief subject matter of the monumental 13-volume work called The 
Elements, written about 300 B.C. by the Greek mathematician Euclid, who taught 
and founded a school of geometry at Alexandria. One of the milestones in the 
history of scientific thought, these books of Euclid still occupy an important 
position in mathematical instruction today. 
    Geometry, as developed in Euclid, was a systematic body of mathematical 
knowledge, built by deductive reasoning upon a foundation of three main 
pillars: (1) definitions of such things as points, lines, planes, angles, 
circles, and triangles; (2) the assumption of certain geometrical postulates 
regarded as true but perhaps not self-evident; and (3) the assumption of 
certain axioms or common notions which were taken to be self-evident truths. 
The body of Euclid's great work consists of a set of propositions or theorems, 
each derived systematically and logically from the definitions, axioms, and 
postulates of his foundation and from theorems already proved. 
    Five postulates may be paraphrased as: 
    1. A unique straight line can be drawn from any point to any other point. 
    2. A finite straight line can be extended continuously in either direction 
in a straight line. 
    3. A circle can be described with any given center and radius. 
    4. All right angles are equal. 
    5. If a straight line falling on two straight lines makes interior angles 
on the same side with a sum less than two right angles, the two straight 
lines, if produced indefinitely, meet on that side on which the angle sum is 
less than two right angles. 
    This fifth postulate is known as the parallel postulate and is essentially 
equivalent to the statement that ``a unique line parallel to a given line can 
be constructed through any point not on the line.'' For 2000 years many 
unsuccessful attempts were made to prove that this postulate was a consequence 
of Euclid's other postulates, definitions, and axioms. Only as recently as the 
19th century did N. I. Lobachevski (1793 - 1856), J. Bolyai (1802 - 1860), and 
G. F. B. Riemann (1826 - 1866) show that the parallel postulate was 
independent of the others, by constructing so-called noneuclidean geometries 
in which the fifth postulate is not valid. See NONEUCLIDEAN GEOMETRY. 
    Certain axioms were stated by Euclid and treated as self-evident truths. 
    1. Things which are equal to the same thing are equal to each other. 
    2. If equals be added to equals, the wholes are equal. 
    3. If equals be subtracted from equals, the remainders are equal. 
    4. Things which coincide with one another are equal to one another. 
    A fifth axiom, ascribed by Proclus to Euclid, is believed to have been 
added by later writers. 
    5. The whole is greater than the part. 
    This has also been replaced by the statement: The whole is equal to the 
sum of its parts. 
    The first six books of Euclid include most of the subject matter commonly 
taught in high school geometry. Book 1, after listing a large number of 
definitions and the postulates and axioms described above, contains 48 
propositions, including many properties relating to perpendicular and parallel 
lines, angles, and congruent triangles, and the book culminates with the 
important Pythagorean theorem. 
    Book 2 contains 14 propositions on geometrical algebra, establishing 
relationships between the areas of certain related squares and rectangles. 
Book 3 deals with circles, chords, inscribed angles, and other figures related 
to the circle; book 4 with inscribed and circumscribed circles and polygons; 
book 5 with ratios; and book 6 with areas and similar triangles. Books 7, 8, 
9, and 10 are concerned with number theory, square and cube roots, and 
incommensurable magnitudes; books 11 and 12 are concerned with solid geometry 
and mensuration; and book 13 is concerned with extreme and mean ratio and the 
five regular solids. See DIFFERENTIAL GEOMETRY; POLYHEDRON; PROJECTIVE 
GEOMETRY; SOLID (GEOMETRY). 
 �34s[James S. Frame]
�33s 
  
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�6v�2v�32sEucoccida   �21m �33s 
    The most important order of the protozoan subclass Coccidia. In this group 
there is alternation of asexual and sexual phases of the life cycle, and the 
parasitic stages occur within the cells of vertebrates or invertebrates. There 
are three suborders. 
    In the suborder Adeleina, the macrogamete and microgametocyte are joined 
during development (syzygy) and the microgametocytes produce only a few 
microgametes. Members of this suborder are parasites of invertebrates and 
lower vertebrates; a few even occur in higher vertebrates. 
    In the suborder Eimeriina, there is no syzygy and the microgametocytes 
produce a large number of microgametes. This group contains several hundred 
species of coccidia, most of which occur in the intestinal cells of 
vertebrates.  
    In the suborder Haemosporina, there is no syzygy and the microgametocytes 
produce a moderate number of microgametes. Asexual development takes place in 
the blood cells of vertebrates, and sexual development in blood-sucking 
insects or mites. This suborder contains Plasmodium, species of which cause 
malaria in humans, lower primates, birds, and other vertebrates. See COCCIDIA; 
MALARIA; PROTOZOA. 
 �34s[Norman D. Levine]
�33s 
  
A700000001027�254s�5v�1v692
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�6v�2v�32sEucommiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of a single family, genus, and species, Eucommia ulmoides, of China. 
It is a simpleleaved tree with reduced, solitary flowers that lack a perianth, 
and leaves without stipules, the paired basal appendages found on many leaves. 
See HAMAMELIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
A700000000766�254s�5v�1v692
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�6v�2v�32sEuechinoidea   �21m �33s 
    A subclass of Echinoidea (sea urchins, sand dollars, and heart urchins) in 
which the test plates are consistently arranged in double rows or columns. 
Five double rows bear the tube-feet (the ambulacral rows), and these alternate 
with five which do not (the interambulacral rows). See ATELOSTOMATA; 
DIADEMATACEA; ECHINACEA; ECHINODERMATA; ECHINOIDEA; GNATHOSTOMATA 
(ECHINODERMATA).  
 �34s[Andrew C. Campbell]
�33s 
  
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693
\IMG\CI601.IMG
13263872
�6v�2v�32sEugenics   �21m �33s 
    The study of factors that influence the hereditary qualities of future 
generations. It may be thought of as both a science and a social movement. 
Eugenics proposes to improve humanity's future by increasing the number of 
children produced by persons who are, by some definition, superior and by 
reducing the number produced by persons who are physically or mentally 
deficient. Attempts to encourage larger families from superior parents are 
called positive eugenics, attempts to reduce the number of children from 
defective parents negative eugenics. 
    There have been a number of proposals regarding negative eugenics in the 
United States. Several of the states have laws providing for sterilization of 
persons with severe mental or physical defects. Segregation of persons in 
public institutions also has the effect of reducing the number of births. 
Finally there is the advocacy of greater use of contraceptives by persons 
likely to transmit hereditary weaknesses. Positive eugenics has consisted 
largely of exhortations for the well endowed to have more children. 
    Such eugenic proposals have not had complete acceptance. There are 
objections on religious grounds. There are those who think that eugenic laws, 
particularly if they involve compulsory sterilization, are an unwarranted 
infringement on basic human rights. A different reservation comes from those 
who point out that knowledge of human heredity is not very complete and that 
prediction of the attributes of children from their parents is not very exact. 
    The spectacular success of cellular and molecular genetics and the 
possibility of directly changing, replacing, or otherwise influencing the 
deoxyribonucleic acid (DNA) in bacteria and viruses have led to considerable 
speculation about what humans can do to their heredity if and when the 
techniques known for microbes become available. However, such possibilities 
are still very much in the future, nobody knows how far away. See HUMAN 
GENETICS.  
 �34s[James F. Crow]
�33s 
  
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�6v�2v�32sEuglenida   �21m �33s 
    An order of the class Phytamastigophorea. This order of protozoans, also 
known as Euglenoidina, includes the largest green noncolonial flagellates, 
Euglena ehrenbergii, which are 400 micrometers long. Many of the colorless 
members are also large. They have one or two equal or subequal flagella. There 
are relatively few genera of Euglenida. See PHYTAMASTIGOPHOREA. 
    Euglenids generally show an anteroposterior elongation; however, 
Trachelomonas volvocina is practically spherical, and Euglena acus is 
extremely needlelike. Many are enclosed in a thick pellicle which is 
sculptured into keels. The pellicle is sometimes ornamented with striae or 
small wartlike protuberances. Colony formation is restricted to Colacium, in 
which a flagellated euglenoid cell usually attaches to a copepod by its 
anterior end and develops into an arboroid structure of several nonflagellated 
cells. Tests or shells are common. 
    Euglenids have two distinguishing features. They synthesize a starchlike 
polysaccharide, paramylum; and they have a gullet-reservoir system. 
    In habitat the Euglenida are widespread. Green members occur mostly in 
fresh water and frequently in such numbers as to form blooms. Unlike the 
Phytomonadida, the Euglenida appear in warmer waters but do not seem to occur 
in hot springs. They abound in citrus-waste lagoons and in later stages of 
sewage-treatment waters, an indication of their partially saprozoic nutrition. 
See MASTIGOPHORA; PROTOZOA. 
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sEuglenophyta   �21m �33s 
    A group of microscopic, one-celled, chiefly aquatic organisms. Because 
they possess the characteristic green plant pigment chlorophyll, they are 
classified as a phylum of the subkingdom Thallophyta. Zoologists, however, 
classify them as animals in the phylum Protozoa, chiefly because they have a 
gullet through which solid food particles may be taken into the body. With the 
aid of chlorophyll some euglenoids carry on photosynthesis; in this respect, 
they resemble all other green plants. Colorless members either ingest solid 
food or absorb soluble organic matter. See ALGAE; EUGLENIDA; PHOTOSYNTHESIS; 
PLANT KINGDOM; PROTOZOA. 
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sEugregarinida �21m �33s
    An order of the subclass Gregarinia. These protozoans are common parasites
of invertebrates such as the arthropods and annelids. Only sexual reproduction
occurs in the life history of these animals; schizogony is lacking. This order
is divided into the suborders Cephalina and Acephalina. In North America
grasshoppers of the genus Melanoplus are commonly the host for the cephaline
Gregarina rigida, inhabiting the lumen of the gut. See GREGARINIA;
INVERTEBRATE PATHOLOGY; PROTOZOA.
 �34s[Elery R. Becker /Norman D. Levine]
�33s

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�6v�2v�32sEuler angles   �21m �33s 
    Three angular parameters that specify the orientation of a body with 
respect to reference axes. They are used for describing rotating systems such 
as gyroscopes, tops, molecules, and nonspherical nuclei. They are not 
symmetrical in the three angles but are simpler to use than other rotational 
parameters.  
    Unfortunately, different definitions of Euler's angles are used, and 
therefore it is confusing to compare equations in different references. The 
definition given here is the majority convention according to H. Margenau and 
G. Murphy. 
    Let OXYZ be a right-handed cartesian (right-angled) set of fixed 
coordinate axes and Oxyz a set attached to the rotating body (see 
illustration).  
    The orientation of Oxyz can be produced by three successive rotations 
about the fixed axes starting with Oxyz parallel to OXYZ. Rotate through (1) 
the angle PSI counterclockwise about OZ, (2) the angle THETA counterclockwise 
about OX, and (3) the angle PHI counterclockwise about OZ again. The line of 
intersection OK of the xy and XY planes is called the line of nodes. 
    Denote a rotation about OZ, for example, by Z (angle). Then the complete 
rotation is, symbolically, given by the equation below where the rightmost 
operation is done first. 

          *** R(PSI, THETA, PHI) = Z(PHI) X(THETA) Z(PSI) ***

For another set of rotational parameters see CAYLEY-KLEIN PARAMETERS. 
 �34s[Bernard Goodman]
�33s 
  
  

A700000002541�254s�5v�1v693
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694
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�6v�2v�32sEuler's equations of motion   �21m �33s 
    A set of three differential equations expressing relations between the 
force moments, angular velocities, and angular accelerations of a rotating 
rigid body. They are equations of motion in the usual dynamical sense, having 
the form of Eqs. (1) - (3). 
  
** I (sub 1) [d OMEGA (sub 1) / dt] + 
        [I (sub 3) - I (sub 2)] OMEGA (sub 2) OMEGA (sub 3) = M (sub 1) ** (1)
  
** I (sub 2) [d OMEGA (sub 2) / dt] + 
        [I (sub 1) - I (sub 3)] OMEGA (sub 3) OMEGA (sub 1) = M (sub 2) ** (2)

** I (sub 3) [d OMEGA (sub 3) / dt] +
        [I (sub 2) - I (sub 1)] OMEGA (sub 1) OMEGA (sub 2) = M (sub 3) ** (3)

    The formulation employs as coordinate axes the three principal axes of 
rotational inertia of the body that can rotate about a body-fixed point, which 
is the center of mass if constraints are absent. These reference axes, which 
form a right-hand set, are indicated by subscripts 1, 2, and 3 in the 
equations, where I (sub 1), I (sub 2), and I (sub 3) represent the principal 
moments of inertia; OMEGA (sub 1), OMEGA (sub 2), and OMEGA (sub 3) the angular 
velocities about the axes; M (sub 1), M (sub 2), and M (sub 3) the 
corresponding force moments; and t the time. See RIGID-BODY DYNAMICS. 
 �34s[Russell A. Fisher]
�33s 
  

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�6v�2v�32sEuler's momentum theorem   �21m �33s 
    A principle of fluid mechanics which states that momentum of the particles 
in a moving frictionless, or inviscid, fluid is conserved. This theorem is 
expressed by Euler's hydrodynamical equations, a set of nonlinear partial 
differential equations. When viscous shear forces are included, the momentum 
equations are called the Navier-Stokes equations. The eulerian method of 
viewing fluid motion is to consider it as a velocity-pressure field; that is, 
the velocity of the fluid particles is considered as a vector function of 
position in space and time, and the pressure as a scalar function of position 
and time. Euler's equations are essentially a restatement of Newton's 
single-particle law, force equals mass times acceleration, adapted to the 
many-particle continuum concept of fluid motion. 
    The negative of pressure gradient may be regarded as a force per unit 
volume acting on a fluid particle. There may also be body forces such as 
gravity and electromagnetic forces (if the fluid is electrically conducting). 
See BERNOULLI'S THEOREM; GAS DYNAMICS; KELVIN'S CIRCULATION THEOREM; LAPLACE'S 
IRROTATIONAL MOTION; NAVIER-STOKES EQUATIONS. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
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�6v�2v�32sEumalacostraca   �21m �33s 
    A series of the class Crustacea which contains the superorders Syncarida, 
Peracarida, Eucarida, Hoplocarida, and Pancarida. These higher crustaceans are 
shrimplike in appearance and have several invariable characteristics. The 
thorax has eight segments and the abdomen has six segments in addition to the 
telson or tail fan. The last pair of pleopods has a specialized form and 
function. In many orders of Eumalacostraca the first or second pleopods in the 
male are modified for sexual purposes. The stomach is divided into two 
chambers, the anterior cardiac portion, containing the gastric mill which 
mechanically grinds the food, and the posterior pyloric portion, in which 
chemical digestion is initiated. Characteristic larval stages are the 
nauplius, contained within the egg, and the mysis and zoaea, which are 
free-swimming. These organisms are free-living or parasitic. The parasites 
show a great adaptability to their hosts which are fish or other crustaceans. 
The majority of the species are marine and many, except the Hoplocarida, show 
adaptive tendencies to brackish and fresh-water environments. The oviducts 
open on the sixth thoracomere while the sperm ducts open on the eighth.  See  
EUCARIDA; HOPLOCARIDA; PANCARIDA; PERACARIDA; SYNCARIDA. 
 �34s[Hans Jakobi]
�33s 
  
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�6v�2v�32sEumycetozoida   �21m �33s 
    An order of Mycetozoia. These are slime molds which form a plasmodium, a 
multinucleate stage that may in some species measure a foot or more across. 
The plasmodium, typically found on decaying plant material, is a migratory 
phagotroph preying on microorganisms. A mature plasmodium is a sheet of 
protoplasm containing a network of channels through which rapid endoplasmic 
streaming occurs. 
    Resistant cysts (spores), possibly haploid, are produced in sporangia. 
Liberated spores hatch into myxoflagellates or myxamebae (which may change 
into flagellates). Either type may represent gametes, and mating types have 
been reported in certain species.  See  MYCETOZOIA. 
 �34s[Richard P. Hall]
�33s 
  
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�6v�2v�32sEuphausiacea   �21m �33s 
    A well-defined marine order of the class Crustacea. All are shrimplike in 
appearance. The order contains two families, one with a single species, the 
other with 84 species divided among 10 genera. Euphausiids are predominantly 
pelagic organisms, but a few species are neritic. Euphausiids form a 
significant proportion of the total planktonic biomass. 
    Euphausiids, with the exception of  Bentheuphausia amblyops , possess 
photophores that emit a brilliant blue-green light. The eyes are compound and 
contain three pigments: a carotenoid (astaxanthin), a melanoid, and a 
photolabile substance that is probably a visual pigment. Respiration is by 
means of foliose, digitiform gills located at the bases of the second to 
eighth thoracic appendages. 
    The blood is a pale, leukocyte-bearing fluid with hemocyanin as the 
respiratory pigment. The heart is compact and has two pairs of ostia. 
    All species are strictly marine and do not occur in fresh-water or 
brackish environments. Species are found in all oceans and seas, with the 
exception of the brackish Baltic and Black seas. Known as krill to whalers, 
they constitute the diet of many whales, particularly the whalebone whales. 
The euphausiids can be processed to yield large quantities of protein. Krill 
pastes and meal can be manufactured to feed domestic animals or used in 
therapeutic diets. Krill sausages, krill-stuffed eggs, and shrimp butter are 
already marketable products in Japan, the Soviet Union, and West Germany.  See 
  CRUSTACEA; DECAPODA (CRUSTACEA); MALACOSTRACA. 
 �34s[John Mauchline]
�33s 
  
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�6v�2v�32sEuphorbiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of the large family Euphorbiaceae (about 7500 species) and four small 
satellite families which have fewer than 150 species among them. The 
Euphorbiales are a group in which the flowers have become unisexual and then 
undergone further reduction accompanied by aggregation. 
    The Christmas poinsettia ( Euphorbia pulcherrima ) and the para rubber 
tree ( Hevea brasiliensis ) are well-known members of the Euphorbiaceae. Many 
African euphorbiads are spiny stem-succulents which resemble cacti in habit. 
Aside from the pronounced floral differences, the euphorbiads have a milky 
juice, which the cacti do not.  See  MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sEurope   �21m �33s 
    Although long called a continent, in many physical ways Europe is but a 
great western peninsula of the Eurasian landmass. Its eastern limits are 
arbitrary and are conventionally drawn along the water divide of the Ural 
Mountains, the Ural River, the Caspian Sea, and the Caucasus watershed to the 
Black Sea. On all other sides Europe is surrounded by salt water. Of the 
oceanic islands of Franz Josef Land, Spitsbergen (Svalbard), Iceland, and the 
Azores, only Iceland is regarded as an integral part of Europe; thus the 
northwestern boundary is drawn along the Danish Strait. 
    Europe is not only peninsular but has a large ratio of shoreline to land 
area reflecting a notable interfingering of land and sea. Excluding Iceland, 
the maximum north-south distance is (3529 mi) (5680 km); and the greatest 
east-west extent is 2398 mi (3860 km). Of Europe's area of 3,881,000 misup2, 
(10,050,000 kmsup2) 73% is mainland, 19% peninsulas, and 8% islands. Also, 51% 
of the land is less than 155 mi (250 km) from shores and another 23% lies 
closer than 310 mi (500 km). This situation is caused by the inland seas that 
enter, like arms of the ocean, deep into the northern and southern regions of 
Europe, which thus becomes a peninsula of peninsulas. The most notable of 
these branching arms of salt water are the White Sea, the North Sea, the 
Baltic Sea with the Gulf of Bothnia, the English Channel (La Manche), the 
Mediterranean Sea with its secondary branches, and finally, the Black Sea. 
Even the Caspian Sea, presently the largest salt-water lake of the world, 
formed part of the southern seas before the folding of the Caucasus. The 
penetration of the landmass by these seas brings marine influences deep into 
the continent and provides Europe with a balanced climate favorable for human 
evolution and settlement. 
    Europe has a unique diversity of land forms and natural resources. The 
relief, as varied as that of other continents, has an average elevation of 980 
ft (300 m) as compared with North America's 1440 ft (440 m). The shape and the 
overall physiographic aspect of the great peninsula are controlled by geologic 
structure which delimits the major regional units. 
    Climate is determined by a number of factors. Probably the most important 
are a favorable location between 35DG and 71DGN latitudes on the western or 
more maritime side of the world's largest continental mass; the west-to-east 
trend (rather than north-south) of the lofty southern ranges and the Central 
Lowlands, as well as of the inland seas, which permit the prevailing westerly 
winds of these latitudes to carry marine influences deep into the continent; 
the beneficial influence of the North Atlantic Drift, which makes possible 
ice-free coasts far within the Arctic Circle; and the low elevation of the 
northwestern mountain ranges and the Urals, which allows the free shifting of 
air masses over their crests. 
    The intricate relief and the climates of Europe are well reflected in the 
drainage system. Extensive drainage basins with large slow-flowing rivers are 
developed only in the Central Lowlands, especially in the eastern part. 
Streams with the greatest discharge empty into the Black Sea and the North 
Sea, although Europe's longest river, the Volga, feeds the Caspian Sea. Second 
in dimension is the Danube, which crosses the Carpathian Basin and cuts its 
way twice through mountain ranges at the Gate of Bratislava and at the Iron 
Gate. The Rhine and Rhone are the two major Alpine rivers with headwater 
sources close to each other but feeding the North Sea and the Western 
Mediterranean Basin, respectively. Abundant precipitation throughout the year, 
as well as the permeable soils and the dense vegetation which temporarily 
store the water, provides the streams of Europe north of the Southern 
Highlands with ample water throughout the seasons. The combined effects of 
poor vegetation, rocky and desolate limestone karstlands, and slight annual 
precipitation result in intermittent flow of the rivers along the 
Mediterranean coast, especially on the eastern side of peninsulas. Only the 
Alpine rivers carry enough water, and if it were not for the Danube and Rhone, 
both originating in regions north of the Alps, the only major river of the 
Mediterranean basin would be the Po.  See  ATLANTIC OCEAN; BALTIC SEA; BLACK 
SEA; CONTINENT; MEDITERRANEAN SEA. 
 �34s[Geza Teleki]
�33s 
  
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�6v�2v�32sEuropium �21m �33s
    A chemical element, Eu, atomic number 63, atomic weight 151.96, a member
of the rare-earth group. The stable isotopes, sup 1 sup 5 sup 1 Eu and
sup 1 sup 5 sup 3Eu, make up the naturally occurring element. The metal is the
second most volatile of the rare earths and has a considerable vapor pressure
at its melting point. It is very soft, is rapidly attacked by air, and really
belongs more to the calcium-strontium-barium series than to the rare-earth
series.
    The element is attractive to the atomic industry, since the elements can
be used in control rods and as nuclear poisons. These poisons are materials
added to a nuclear reactor to balance the excess reactivity at start-up, and
are so chosen that the poisons burn out at the same rate as the excess
activity decreases. The television industry uses considerable quantities of
phosphors, such as europium-activated yttrium orthovanadates, and other
europium-activated yttrium phosphors have been patented. These phosphors give
a brilliant red color and are used in the manufacture of television screens.
See  RARE-EARTH ELEMENTS.
 �34s[Frank H. Spedding]
�33s

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�6v�2v�32sEurotiales   �21m �33s 
    An order of ascomycetous fungi bearing ascospores in globose or broadly 
oval, evanescent asci without a pore. The asci are arranged at various levels, 
typically within a closed fruiting body or cleistothecium. The order was 
previously known as Aspergillales, and Plectascales. The order is divided into 
five families: Gymnoascaceae, Eurotiaceae, Onygenaceae, Trichocomaceae, and 
Elaphomycetaceae.  See  ASCOMYCOTINA; FUNGI; MEDICAL MYCOLOGY. 
 �34s[Constantine J. Alexopoulos]
�33s 
  
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�6v�2v�32sEuryapsida   �21m �33s 
    A subclass of the reptiles, extinct since end of the Mesozoic Era. In the 
Euryapsida each side of the skull has only an upper temporal opening, bounded 
laterally by postorbital and squamosal bones (see illustration); the lower 
part of the cheek is never truly fenestrated. 
    In the classification adopted here, the Euryapsida comprise three orders: 
the Permian and Triassic Araeoscelidia, little-known forms, generally small 
and essentially terrestrial; the Placodontia of the Triassic which were 
mollusk-eating aquatic reptiles, usually with enormous flat crushing teeth; 
and the Sauropterygia, the small to medium-sized amphibious nothosaurs of the 
Triassic and the marine plesiosaurs which range throughout the Mesozoic.  See  
ARAEOSCELIDIA; PLACODONTIA; REPTILIA; SAUROPTERYGIA. 
 �34s[Alan J. Charig]
�33s 
  
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�6v�2v�32sEurypterida   �21m �33s 
    Primitive aquatic arthropods of the subphylum Chelicerata and class 
Merostomata. The Eurypterida were dominant from the Ordovician to the end of 
the Paleozoic. Like other Merostomata, their elongate-lanceolate bodies were 
encased in a chitinous exoskeleton comprising a cephalothorax (prosoma) and an 
abdomen (opisthosoma). The Eurypterida, presumably descended from aglaspid 
ancestors, apparently left no direct descendants.  See  MEROSTOMATA. 
 �34s[Gilbert O. Raasch]
�33s 
  
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�6v�2v�32sEutardigrada �21m �33s
    An order of tardigrades, lacking a cirrus lateralis, a sensory cephalic
appendage, and a clava, or club-shaped appendage. Pharyngeal pockets are
strengthened by separated rods or macroplacoids or are without thickenings.
Claws are of different size, arranged in two pairs in which a larger and
smaller claw are united. In  Milnesium	the claws are separated.
Haplomacrobiotus  has two simple claws.  See  HETEROTARDIGRADA; TARDIGRADA.
 �34s[Ernesto Marcus /Eveline Marcus]
�33s

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�6v�2v�32sEutectics   �21m �33s 
    The microstructures that result when a solution of metal of eutectic 
composition solidifies. The eutectic reaction must be distinguished from 
eutectic microstructures. The eutectic reaction is a reversible transformation 
of a liquid solution to two or more solids, under constant pressure 
conditions, at a constant temperature denoted as the eutectic temperature. 
Microstructures which are wholly eutectic in nature can occur only for a 
single, fixed composition in each alloy system demonstrating the reaction. 
  
    Although technologically important alloy systems, particularly the Fe-C 
system (all cast irons used commercially pass through a eutectic reaction 
during solidification), exhibit at least one eutectic reaction, there has been 
little exploitation of wholly eutectic microstructures for structural 
purposes. Some eutectic fusible alloys are used as solders, as heat-transfer 
media, for punch and die mold and pattern applications, and as safety plugs. A 
silver-copper eutectic alloy is also used for high-temperature soldering 
applications.  See  SOLDERING. 
    Directional solidification of eutectic alloys so as to create a 
microstructure well aligned parallel to the growth direction produces 
high-strength, multiphase composite materials with excellent mechanical 
properties. Among the major advantages of these alloys are extraordinary 
thermal stability of unstressed microstructures, retention of high strength to 
very close to the eutectic temperature of the respective alloys, and the 
ability to optimize strength by appropriate alloying additions to induce 
either solid-solution strengthening or intraphase precipitation of additional 
phases.  
    The most likely future applications for aligned eutectics are as gas 
turbine engine materials (turbine blades or stator vanes) or in nonstructural 
applications such as superconducting devices in which directionality of 
physical properties is important.  See  ALLOY; METAL; SOLID SOLUTION. 
 �34s[Norman S. Stoloff]
�33s 
  
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�6v�2v�32sEutheria �21m �33s
    An infraclass of therian mammals including all living mammals except the
monotremes and marsupials. Eutherians, often called placental mammals, are
distinguished from their nearest relatives, the marsupials, by numerous
characters. The braincase is large, and the angular process of the mandible is
not inflected. The young are born in a relatively advanced state of
development, and are never sheltered in a pouch after birth. Living eutherians
are divided into 16 orders; at least 10 additional orders are extinct and
known only as fossils. The largest orders are the Rodentia, Chiroptera,
Carnivora, and Primates.  See  MAMMALIA; METATHERIA; THERIA.
 �34s[D. Dwight Davis /Frederick Szalay]
�33s

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�6v�2v�32sEutrophication   �21m �33s 
    The deterioration of the esthetic and life-supporting qualities of lakes 
and estuaries, caused by excessive fertilization from effluents high in 
phosphorus, nitrogen, and organic growth substances. Algae and aquatic plants 
become excessive, and, when they decompose, a sequence of objectionable 
features arise. Water supplies drawn from such lakes must be filtered and 
treated. Diversion of sewage, better utilization of manure, erosion control, 
improved sewage treatment and harvesting of the surplus aquatic crops 
alleviate the symptoms. Prompt public action is essential.  See  WATER 
CONSERVATION; WATER POLLUTION. 
 �34s[Arthur D. Hasler]
�33s 
  
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�6v�2v�32sEvaporation   �21m �33s 
    The process by which a substance in the liquid state is converted into the 
vapor state. The molecules of substances in a condensed state are held to one 
another by strong forces of attraction, which are balanced by equally strong 
repulsive forces. Tending to overcome the potential energy of attraction is 
the escaping tendency of molecules, which arises from their kinetic energy. 
The kinetic energy, and therefore the escaping tendency of molecules, is a 
function of temperature. 
    The following factors affect the rate of evaporation of a liquid: the rate 
at which heat is supplied to the liquid to furnish the latent heat of 
vaporization; the rate at which the liquid is stirred to bring to the surface 
molecules having sufficient kinetic energy to escape; and the rate at which 
the vapor above the liquid is changed to provide the best conditions for 
escape of molecules from the surface of the liquid.  See  EVAPORATOR; 
EVAPOTRANSPIRATION; HUMIDIFICATION; LIQUID; VAPOR PRESSURE. 
 �34s[Norman H. Nachtrieb]
�33s 
  
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�6v�2v�32sEvaporator   �21m �33s 
    A device used to vaporize part or all of the solvent from a solution. The 
valuable product is usually either a solid or a concentrated solution of the 
solute. If a solid, the heat required for evaporation of the solvent must have 
been supplied to a suspension of the solid in the solution, otherwise the 
device would be classed as a drier. The vaporized solvent may be made up of 
several volatile components, but if any separation of these components is 
effected, the device is properly classed as a still or distillation column. 
When the valuable product is the vaporized solvent, an evaporator is sometimes 
mislabeled a still, such as water still, and sometimes is properly labeled, 
such as boiler-feedwater evaporator. In the great majority of evaporator 
installations, water is the solvent that is removed.  See  DISTILLATION; 
DRYING.  
    Evaporators are used primarily in the chemical industry. For example, 
common salt is made by boiling a saturated brine in an evaporator. The salt 
precipitates as a solid in suspension in the brine. This slurry is pumped 
continuously to a filter, from which the solids are recovered and the liquid 
portion returned for further evaporation. Evaporators are widely used in the 
food industry, usually as a means of reducing volume to permit easier storage 
and shipment. Evaporators are also the most commonly used means of producing 
potable water from sea water or other contaminated sources.  See  SALINE WATER 
RECLAMATION.  
    The vaporization of solvent requires large amounts of heat. Provisions for 
transferring this heat to the solution constitute the largest element of 
evaporator cost and the principal means of distinguishing between types of 
evaporators. Practically all evaporators fall into one of the following 
categories:  
    1. Submerged-combustion evaporators: those heated by a flame that burns 
below the liquid surface, and in which the hot combustion gases are bubbled 
through the liquid. 
    2. Direct-fired evaporators: those in which the flame and combustion gases 
are separated from the boiling liquid by a metal wall, or heating surface. 
    3. Stem-heated evaporators: those in which steam or other condensable 
vapor is the source of heat, and in which the steam condenses on one side of 
the heating surface and the heat is transmitted through the wall to the 
boiling liquid.  See  EVAPORATION. 
 �34s[Ferris C. Standiford]
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�6v�2v�32sEvapotranspiration   �21m �33s 
    The total process of water vapor transfer into the atmosphere from 
vegetated land surfaces. Evaporation is the change of liquid water into 
gaseous water vapor; it occurs from bodies of water such as lakes and streams 
or from other wet surfaces such as wet soil surfaces. Transpiration is the 
process whereby water is absorbed by plant roots, transported through the 
plant, and evaporated from plant surfaces, especially from leaves into the air 
above. The energy required to change liquid water into water vapor comes from 
sources traceable to the Sun. Energy for evapotranspiration may come from the 
transfer of heat from warm air to a cooler plant or soil surface, but the 
major source of energy is generally net radiation. Net radiation is the 
difference between the incoming and the outgoing shortwave and long-wave 
radiation streams. 
    In general, the amount of evapotranspiration will be less than or equal to 
the amount of precipitation that falls in a given region. Exceptions arise 
when precipitation is supplemented with irrigation, or in instances when plant 
roots may extract water from underground water tables located within a meter 
or two of the soil surface. 
 �34s[Blaine L. Blad]
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�6v�2v�32sEvergreen plants   �21m �33s 
    Plants that retain their green foliage throughout the year. Popularly, 
needle-leaved trees (pine, fir, juniper, spruce) and certain broad-leaved 
shrubs (rhododendron, laurel) are called evergreens. In warm regions many 
broad-leaved trees (magnolia, live oak) are evergreen, and in the tropics most 
trees are evergreen and nearly all have broad leaves. 
 �34s[Nelle Ammons]
�33s 
  
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�6v�2v�32sEvolution �21m �33s
    Stars spend most of their lifetime on the main sequence, generating energy
by conversion of hydrogen into helium in their cores. When the hydrogen in the
core is exhausted, the stars expand. Single stars encounter no problem in this
expansion, but the gravitational field of a nearby companion sets a definite
upper limit to the volume available for expansion to a binary star component;
it also considerably distorts the shape of the expanded star. Systems in which
evolution is substantially affected by the duplicity are called close binary
systems. When the expanding component reaches the limiting surface, the
evolution of both stars is completely changed. See STELLAR EVOLUTION.
    The sequence of shapes through which an expanding component passes is
shown in the illustration. The curves drawn are cross sections of
three-dimensional equipotential surfaces, for a system, with stars of masses M
and M', viewed edge-on. When a star is still small with respect to the
dimensions of the system, it is very nearly spherical. As it expands, its
shape becomes that of a distorted ellipsoid more and more extended toward the
companion, until the star reaches the critical surface marked with a heavy
line in the illustration. The next equipotential surface already surrounds
both stars. A star that has reached its critical surface will lose mass in the
vicinity of the point L1 (the first lagrangian point) where the effective
gravity is zero, and a gas stream will flow from there toward the other star.
The stream either impacts directly on the companion or leads to the formation
of an accretion disk surrounding it. In either case, the star gains (accretes)
mass either directly from the stream or through the disk, in which viscosity
makes some particles fall on the star.


    X-ray binaries.  Since 1970, x-ray satellites have discovered a number of
strong sources of x-rays clearly associated with binary systems: the x-ray
source undergoes eclipses or the optical counterpart is a binary, or both. The
picture which has been developed from study of these systems is as follows.
The optically visible star is unstable and loses mass, either by stellar wind
or by Roche lobe overflow. The stream flows to the compact star and falls into
a very deep ``potential well,'' since the compact star is extremely small.
Usually the accretion occurs through a viscous disk, but eventually always a
large fraction of the potential energy of the particles is released as
radiation. The hot gas reaches temperatures of order 10 sup 8 - 10 sup 9
DGF/(50 - 500 x 10 sup 6 DGC) and radiates mostly in x-rays. Some of the x-ray
binary sources are among the most powerful energy emitters in the Galaxy. The
regular rapid pulses of x-rays are believed to be generated by a rapid rotation
of the magmetized neutron star. See PULSAR; X-RAY STAR.
    The neutron stars found in binary x-ray sources are thought to be remnants
of supernova explosions that in the past terminated the normal stellar
lifetime of their parental stars. It is also believed that very massive stars
leave black holes as the remnants after their supernova explosion, rather than
neutron stars. See BLACK HOLE.

    Cataclysmic variables.  Probably all novae are close binary systems. Novae
are a special class (conspicuous because of a very high increase in light in
an outburst) of cataclysmic variables. Other classes include recurrent novae
and dwarf novae. All these objects appear to be binaries of a basically
similar nature: a cooler object, usually a red dwarf, is unstable and
transfers mass to a hotter and much more compact object, a degenerate white
dwarf or a similar body. A viscous disk surrounding the accreting star plays
an important role. Variable rates of mass transfer, variable viscosity, and
continual interaction between the star, the disk, and the stream lead to
instabilities and eruptive events. In the more violent cases, thermonuclear
reactions at the surface of the accreting star are believed to be the source
of the energy for a nova outburst. See NOVA; VARIABLE STAR.
 �34s[Mirek Plavec]
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�6v�2v�32sExanthem subitum   �21m �33s 
    A mild, sometimes epidemic viral disease with abrupt onset, high fever, 
and rash. It is also known as roseola infantum. It occurs almost exclusively 
in infants 6 months to 3 years old. Sometimes exanthem subitum may be confused 
with rubella, from which it may be distinguished by its abrupt onset; 
distinction from other exanthematous diseases is chiefly by abrupt termination 
of fever before the rash appears.  See  RUBELLA. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sExcavator   �21m �33s 
    Any of a variety of bucket-equipped construction machines used for digging 
earth and rock. The most common excavator is basically a modified crane. When 
modified for digging, the unit made up of crane and its digging attachment is 
commonly called by the name of the attachment. Thus a crane carrying a 
dragline bucket is called a dragline. 
    There are four principal modifications of cranes for use as an excavator: 
dragline, clamshell, power shovel, and backhoe. The dragline uses a long 
latticed crane boom, two working cables, and a bucket designed for horizontal 
digging. The clamshell uses a crane boom and a clamshell bucket. Digging with 
a clamshell is not a power operation; the clamshell is filled only by its own 
weight and the bite of its jaws as they are closed. The power shovel uses a 
heavy but relatively short boom and a dipper stick to which is attached a 
hinged-bottom digging bucket or dipper. The backhoe is a short-boomed machine 
with an inverted bucket at the end of its stick. 
    The hydraulic backhoe is a variation in which the operations are activated 
by hydraulic pistons rather than cables. The front-end loader consists of a 
continuous-track or wheeled tractor with a wide hydraulically operated bucket 
mounted on jointed arms at its front end. The trencher, designed especially 
for excavating trenches, is a crawler tractor mounting either a movable wheel 
or a continuous chain, on which there are numerous buckets.  See  
BULK-HANDLING MACHINES; CONSTRUCTION EQUIPMENT. 
 �34s[Edward M. Young]
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�6v�2v�32sExcitation   �21m �33s 
    Application of energy to one portion of a system or apparatus in a manner 
that enables another portion to carry out a specialized function. Excitation 
establishes a condition essential to that function. 
    Excitation of a system or apparatus serves either to permit a transfer of 
energy or to control a flow of energy elsewhere in the system. Excitation 
energy may differ from the output energy in source, form, level, or location. 
Excitation produces a primary effect that is linked through an intermediate 
physical phenomenon to a dependent secondary effect. 
 �34s[William W. Snow]
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�6v�2v�32sExcitation potential   �21m �33s 
    The difference in potential between an excited atomic or molecular state 
and the ground state. The term is most generally used in connection with 
electron excitation, but it can be applied to excited molecular vibrational 
and rotational states. 
    A closely related term is excitation energy. If the unit of potential is 
taken as the volt and the unit of energy as the electron volt, then the two 
are numerically equal. According to the Bohr theory, there is a relationship 
between the wavelength of the photon associated with the transition and the 
excitation energies of the two states. Thus the basic equation for the 
emission or absorption of energy is as shown below, where h is Planck's 

              *** hc / LAMBDA = [E (sub i) - E (sub f)]***

constant, c the velocity of light, LAMBDA the wavelength of the photon, and  
E (sub f) and  E (sub i) the energies of the final and initial states, 
respectively. See EXCITED STATE; GROUND STATE; IONIZATION POTENTIAL. 
 �34s[Glenn H. Miller]
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�6v�2v�32sExcited state   �21m �33s 
    In quantum mechanics, a stationary state of higher energy than the lowest 
stationary state or ground state of a particle or a system of particles. 
Customarily, only bound stationary states, which generally are at most 
denumerably infinite in number, are spoken of as excited, although the formal 
quantum theory often treats the noncountable unbound stationary states on an 
equal footing with the bound states.  See  GROUND STATE; METASTABLE STATE; 
STATIONARY STATE. 
 �34s[Edward Gerjuoy]
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�6v�2v�32sExciton   �21m �33s 
    In a nonmetallic crystal, a quantum of electronic excitation which 
transports energy but not charge. There are two customary ways of picturing an 
exciton, termed the Frenkel model and the Wannier model. 
    The Frenkel model applies to a solid consisting of weakly interacting 
atoms or molecules, a prime example being organic crystals such as anthracene 
and naphthalene. The excitons are very much like the excited electronic states 
of the isolated molecules which make up the crystal. Although largely 
localized on a single molecule, an exciton can hop readily from one molecule 
to another in the crystal. 
    The Wannier model begins at the opposite extreme, with an electron in the 
conduction band and a hole in the valence band which are bound weakly to each 
other by the Coulomb force. This exciton overlaps many atoms in the crystal 
and does not resemble the excitation of any constituent atom. The dielectric 
screening of the crystal serves to weaken the attraction between electron and 
hole, and thus to increase their average separation. The Wannier model is 
therefore most appropriate for materials with a high dielectric constant 
semiconductors such as germanium and silicon, in particular.  See  BAND THEORY 
OF SOLIDS; DIELECTRIC CONSTANT; SEMICONDUCTOR. 
    An exciton is not stable against electron-hole recombination and can thus 
exist for only a brief interval before it decays by emitting a photon 
(recombination luminescence) or by giving up its energy to the crystal as heat 
or as stored energy. In most materials, the lifetime is long enough that the 
exciton becomes trapped before it decays.  See  ELECTROLUMINESCENCE; 
LUMINESCENCE; TRAPS IN SOLIDS. 
    The production of a high concentration of excitons in Ge and Si crystals 
at very low temperatures leads to the phenomenon of exciton condensation, 
which is similar in many respects to the condensation of water molecules in 
air to form fog droplets. The condensed phase is referred to as an 
electron-hole droplet (see illustration) since the excitons lose their 
individual character, and the droplet becomes an electrically conducting 
plasma.  
 �34s[Milton N. Kabler]
�33s 
  
  
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�6v�2v�32sExclusion principle   �21m �33s 
    No two electrons may simultaneously occupy the same quantum state. This 
principle, often called the Pauli principle, was first formulated by Wolfgang 
Pauli in 1925 and, for time-independent quantum states, it means that no two 
electrons may be described by state functions which are characterized by 
exactly the same quantum numbers. In addition to electrons, all known 
particles having half-integer intrinsic angular momentum, or spin, obey the 
exclusion principle. It plays a central role in the understanding of many 
diverse phenomena, including the periodic table of the elements and their 
chemical activities, the electron contribution to the specific heat of metals, 
the shell structure in the atomic nucleus analogous to that of electrons in 
atoms, and certain symmetries in the scattering of identical particles.  See  
ANGULAR MOMENTUM; NONRELATIVISTIC QUANTUM THEORY; QUANTUM NUMBERS; SPIN 
(QUANTUM MECHANICS). 
    Using the fact that a system will try to occupy the state of lowest 
possible energy, the electron configuration of atoms may be understood by 
simply filling the single-particle energy levels according to the Pauli 
principle. This is the basis of Niels Bohr's explanation of the periodic 
table.  See  ATOMIC STRUCTURE AND SPECTRA; ELECTRON CONFIGURATION. 
 �34s[S. A. Williams]
�33s 
  
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�6v�2v�32sExobiology   �21m �33s 
    The study of the origin and distribution of life in the universe. With 
time, exobiology has grown to include the study of the origin, evolution, and 
distribution not only of life, but of the chemicals necessary for life, both 
on Earth and throughout the universe. It strives to determine the relationship 
of life to the evolution of planets. Planetary systems are now believed to be 
a companion event in the formation of stars, and interstellar gas clouds have 
been found to contain biologically important molecules made of carbon, 
hydrogen, and other elements. Thus there is reason to believe that life could 
be widespread in the universe. 
    In 1976 two Viking robot spacecraft landed on Mars. One instrument showed 
that the Mars soil contained no organic matter. It was concluded that the 
intense ultraviolet radiation from the Sun dissociates organic compounds 
exposed to it. A second instrument designed to detect biological reactions in 
the Martian soil indicated no life. Mars is still an enigma, since final 
conclusions cannot be drawn from such limited testing.  See  MARS. 
    It is quite possible that the outer planets (Jupiter, Saturn, Uranus, and 
Neptune) and some of their satellites may be veritable laboratories of 
chemical evolution, since they have atmospheres which appear to be very 
similar to that of the primitive Earth. Comets, meteorites, and asteroids, 
like the outer planets, may also contain evidence of chemical evolution. 
Organic geochemical analyses of carbonaceous chrondrites has revealed the 
presence of amino acids and other organic molecules of biological interest. 
Organic compounds have even been detected in the interstellar medium with the 
aid of powerful radio telescopes. The apparent ubiquity of abiotically 
produced organic molecules supports the universality of chemical evolution and 
suggests that the processes which led to the origin of life on the Earth may 
be quite commonplace throughout the universe. 
    It is now generally believed that the origin of life on Earth was closely 
related to the origin of the solar system. It is thought that soon after Earth 
formed, its primitive atmosphere, which contained no oxygen and was composed 
mainly of reduced gases like hydrogen, methane, ammonia, and water vapor, was 
activated by energy sources much as lightning and ultraviolet radiation from 
the Sun. The gas molecules broke apart and recombined repeatedly, forming 
newer and increasingly complex molecules, advancing through prebiotic 
synthesis of organic matter to a point where the assembled molecules exhibited 
properties of living things: respiration, metabolism, reproduction, and the 
transfer of genetic information.  See  LIFE, ORIGIN OF. 
    The accumulation of information supporting other life in the universe has 
stimulated a search for life beyond the solar system. Terrestrial radio 
telescopes listen for signals reaching Earth from different stars. 
Intelligently contrived signals would constitute proof of the existence of 
life elsewhere in the universe. 
 �34s[Thora W. Halstead]
�33s 
  
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�6v�2v�32sExopterygota   �21m �33s 
    One of the two major divisions of the subclass Pterygota, the winged 
Insecta, including the insects often referred to as the Hemimetabola. The 
developing wings appear as external pads during the early stages of the 
insect. The immature forms, or nymphs, usually live in the same environment as 
the adults and feed on the same type of food.  See  ENDOPTERYGOTA; INSECTA; 
PTERYGOTA.  
 �34s[Frank M. Carpenter]
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�6v�2v�32sExpert systems   �21m �33s 
    Methods and techniques for constructing human-machine systems with 
specialized problem-solving expertise. The pursuit of this area of artificial 
intelligence research has emphasized the knowledge that underlies human 
expertise and has simultaneously decreased the apparent significance of 
domain-independent problem-solving theory. In fact, new principles, tools, and 
techniques have emerged that form the basis of knowledge engineering. 
    Expertise consists of knowledge about a particular domain, understanding 
of domain problems, and skill at solving some of these problems. Knowledge in 
any specialty is of two types, public and private. Public knowledge includes 
the published definitions, facts, and theories which are contained in 
textbooks and references in the domain of study. But expertise usually 
requires more than just public knowledge. Human experts generally possess 
private knowledge which has not found its way into the published literature. 
This private knowledge consists largely of rules of thumb or heuristics. 
Heuristics enable the human expert to make educated guesses when necessary, to 
recognize promising approaches to problems, and to deal effectively with 
erroneous or incomplete data. The elucidation and reproduction of such 
knowledge are the central problems of expert systems. 
    Researchers in this field suggest several reasons for their emphasis on 
knowledge-based methods rather than formal representations and associated 
analytic methods. First, most of the difficult and interesting problems do not 
have tractable algorithmic solutions. This is reflected in the fact that many 
important tasks, such as planning, legal reasoning, medical diagnosis, 
geological exploration, and military situation analysis, originate in complex 
social or physical contexts, and generally resist precise description and 
rigorous analysis.  See  ALGORITHM. 
    The second reason for emphasizing knowledge is pragmatic: human experts 
achieve outstanding performance because they are knowledgeable. If computer 
programs embody and use this knowledge, they too attain high levels of 
performance. This has been proved repeatedly in the short history of expert 
systems. Systems have attained expert levels in several tasks: mineral 
prospecting, computer configuration, chemical structure elucidation, symbolic 
mathematics, chess, medical diagnosis and therapy, and electronics analysis. 
    The third motivation for focusing on knowledge is the recognition of its 
intrinsic value. Traditionally, the transmission of knowledge from human 
expert to trainee has required education and internship periods ranging from 3 
to 20 years. By extracting knowledge from humans and transferring it to 
computable forms, the costs of knowledge reproduction and exploitation can be 
greatly reduced. At the same time, the process of knowledge refinement can be 
accelerated by making the previously private knowledge available for public 
test and evaluation. 
    Most of the knowledge-engineering applications fall into a few distinct 
types, summarized in the table. 
    The illustration shows an idealized representation of an expert system. No 
existing expert system contains all components shown, but one or more 
components occur in every system. The ideal expert system contains: a language 
processor for problem-oriented communications between the user and the expert 
system; a blackboard for recording intermediate results; a knowledge base 
comprising facts plus heuristic planning and problem-solving rules; an 
interpreter that applies these rules; a scheduler to control the order of rule 
processing; a consistency enforcer that adjusts previous conclusions when new 
data or knowledge alter their bases of support; and a justifier that 
rationalizes and explains the system's behavior.  See  ARTIFICIAL 
INTELLIGENCE; DATA-PROCESSING SYSTEMS. 
 �34s[Frederick Hayes-Roth]
�33s 
  
  
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�6v�2v�32sExplosive forming   �21m �33s 
    The shaping or modifying of metals by means of explosions. The explosives 
may be of either the detonating or deflagrating type. Explosive gas mixtures 
or stored gas at high pressure may also provide the motive power. 
    Cold welds can be made between dissimilar metals by driving the two parts 
together under explosive impact. In other applications of explosive-forming 
methods, powders are pressed into solid billets. In a different application, 
high explosives are used to cut large blocks of metal and even to split thin 
sheets into two layers of exactly one-half the original thickness. Explosives 
can also be employed to extrude metal shapes and to punch hard metals with the 
aid of dies. Shapes produced explosively are very exact and free from the fine 
cracks that sometimes result when pressure is applied slowly.  See  METAL 
FORMING.  
 �34s[William E. Gordon]
�33s 
  
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�6v�2v�32sExponent �21m �33s
    In mathematics a symbol or number written to the right of and above
another symbol or number to denote how many times the latter is to be
multiplied by itself. For example, 7 sup 3 = 7 . 7 . 7, and a sup 3 =  a . a . a
. By use of this convenient mathematical device, the number
420,000,000,000,000 can be expressed unmistakably and more compactly as 4.2 x
10sup1sup4, which is read as, ``four and two-tenths times ten multiplied by
itself fourteen times'' or, more properly, ``four and two-tenths times ten to
the fourteenth power.'' This abbreviated notation is particularly valuable in
expressing the extremely large or extremely small numbers encountered in
modern scientific work. For example, 0.000000143 may be expressed as

	  (143/1,000,000,000) or (143/10 (sup 9)) or 1.43 x 10(sup -7)

    Operations with exponents are governed by the following rules:

1. x(sup 1)x(sup b) = x(sup a + b). Example, x(sup 2)x(sup 3) = xx CTR DOT
			DOT xxx = x(sup 5).

2. (x sup a)(sup b) = x(sup ab). Example, (x(sup 2))(sup 3) = (x(sup 2))
		     (x(sup 2)) (x(sup 2)) = x(sup 6).

3. (xy)(sup a) = x(sup a)y(sup a). Example, (xy)(sup 3) = xyxyxy = xxxyyy =
			   x(sup 3)y(sup 3).

4. If y NOT EQUAL 0, (x/y)(sup a) = (x(sup a)/y(sup a CTR DOT)). Example,
 (x/y)(sup 3) =
	(x/y) CTR DOT (x/y) CTR DOT (x/y) = (x(sup 3)/y(sup 3 CTR DOT))

5. If  x = 0, (x(sup a)/x(sup b)) = x(sup a - b). Example, (x(sup 5)/x(sup 2)) =
		       (xxxxx/xx) = xxx = x(sup 3).
See  ALGEBRA.
 �34s[Arnold N. Lowan /Salomon Bochner]
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�6v�2v�32sExposure meter   �21m �33s 
    An indicating instrument used in photography to determine lens aperture 
and shutter speed. An exposure meter may be used either in the darkroom to 
determine approximate printing time for a contact print or an enlargement or, 
more usually, with a camera to determine exposure of film. 
    Exposure meters are of two basic types, photovoltaic and photoconductive. 
In photovoltaic meters, a selenium cell converts photons to electrons, 
producing a current directly proportional to received light. A sensitive 
microammeter indicates this current to the user.  See  PHOTOVOLTAIC CELL. 
    In the photoconductive meter, a cadmium sulfide cell changes conductivity 
in proportion to the received light. A battery (usually a mercury cell) 
supplies power through the cadmium sulfide cell to the meter movement. Because 
the received light serves to control the power from the battery, this type of 
instrument is about three orders of magnitude more power-sensitive than the 
photovoltaic type. 
    Used at the camera, either type of meter serves as a brightness meter to 
measure reflected light from the subject. Used at the subject, with a diffuser 
head over the exposure meter lens, the instrument serves as an illumination 
meter to indicate incident light.  See  PHOTOGRAPHY; PHOTOMETRY. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sExterior ballistics   �21m �33s 
    The science dealing with the motions of projectiles not under propulsive 
power, such as a shell after it has left the cannon or a rocket after burning 
has ceased. The motion of a body projected at an angle to the Earth's surface 
is perturbed by several forces: drag, cross wind forces, and the Coriolis 
effect. Their magnitude depends upon the shape and mass of the body, the 
density of the air, the wind currents, and the length of the path along the 
trajectory. See TRAJECTORY. 
    The trajectory of a projectile under the influence of the Earth's 
gravitational field and in vacuum would be an arc of an ellipse with the 
center of the Earth as one of the foci. This arc can be closely approximated 
by a parabola if the motion does not carry the projectile so high that the 
gravitational force differs significantly from the force at the Earth's 
surface.  
    As a projectile moves through a fluid such as air, the fluid particles are 
accelerated as it collides with them, and this results in a force, called 
drag, which robs the projectile of some of its energy. The drag is a function 
of the density of the fluid, the area of the moving body, and the velocity of 
the body squared. 
    For projectiles, such as rockets, with high velocities and high quadrant 
angles of departure, the variations of the drag coefficient and of atmospheric 
density along the trajectory are more complex. Solutions for these 
trajectories involve the method of approximate numerical intergration, long 
used in astronomy. Computers with large data-handling and storage capacity 
have been used extensively to perform the necessary calculations. See 
NUMERICAL ANALYSIS. 
    When the range of the projectile becomes great, the ballistics analysis 
must be modified to accommodate the fact that the force of gravity varies with 
height, is directed toward the center of the Earth, and therefore changes 
direction as the projectile moves forward. Also, the impact point is depressed 
because of the Earth's curvature, and the range therefore is extended. These 
effects are customarily taken care of by the method of differential 
corrections.  
    Once the projectile is clear of, say, the gun muzzle or the lauching 
stand, it is a separate entity free of the Earth's rotation. One component of 
its initial velocity, however, is the velocity of the surface of the Earth at 
the point of firing. For this reason, when the projectile arrives at the 
impact point, it will miss by an amount dependent on the azimuth of the 
trajectory, the range, and the height of the trajectory. This is called the 
Coriolis effect. For intermediate trajectories, such as those with pronounced 
Earth curvature, the Coriolis effect is handled by means of differential 
corrections. for very-long-range missiles, the approach of celestial mechanics 
with nonrotating Earth-centered coordinates is used. See CORIOLIS ACCELERATION 
AND FORCE. 
    In order to stabilize a projectile in flight, it is frequently spun about 
its longitudinal axis. The spin of a shell is imparted by rifling th gun 
barrel, that of a rocket by canting the exhaust nozzles or the fins. While 
spin does tend to stabilize the orientation, in practice the spin axis will 
precess so that the nose of the projectile follows a spiral path. This causes 
troublesome complexities, but the equation of motion of a spinning projectile 
can be integrated by high-speed computing machines. 
 �34s[John P. Hagen]
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�6v�2v�32sExtinction (biology)   �21m �33s 
    The death and disappearance throughout the world of diverse groups of 
organisms under circumstances that suggest common or related causes. The time 
span involved in mass extinctions varies greatly, but commonly it is short 
compared with evolutionary history. It may, however, seem to be long as 
compared with human records, and there is no scientific evidence that such 
mass extinctions were cataclysmic or involved extraordinary processes. 
    One of the greatest of all mass extinctions started a few hundred years 
ago and is still in progress. Approximately 500 species of animals and many 
plants have disappeared in that time or may soon become extinct as the result 
of ecological disturbances caused by humans. This revolution in the organic 
world is expected to become climactic within a few decades because of the 
rapid spread of human population. 
    Mass extinctions are of two sorts. They may take place as a result of 
competition between native and immigrant faunas, or they may stem from 
external physical changes in environment in which competition is not a major 
factor. In the former case, the immediate cause of extinction stems from 
biological stresses between better- and poorer-adapted species that are using 
the same environmental facilities. 
    External physical changes may include meteorite storms, diseases, shifts 
of the Earth's axis or crust, variations in composition of the atmosphere, 
variations in supply of metallic trace elements, reduction of salinity in the 
ocean, racial old age, insufficient nutrient salts in the sea, excessive 
juvenile mortality, and climatic changes. All of these have been advanced as 
causes of mass extinctions, but mostly they are not supported by compelling 
evidence.  
 �34s[Norman D. Newell]
�33s 
    Some extinctions may be caused by factors operating on the level of genes. 
In evolutionary terms, the excess of deaths over births that leads to 
extinction reflects a reduction in adaptation, that is, a loss in 
characteristics that promote survival. The bulk of adaptation is brought about 
through natural selection, which favors the perpetuation of the genes that 
confer better-than-average survival or reproductive success (together termed 
fitness) on their possessors. Most species are more or less well adapted to 
their environments, since natural selection has been at work on them since 
their inception and was at work on their ancestors as well. Thus for most 
species the environment changes only for the worse. Selection under changed 
conditions leads to the establishment of new genes, new gene combinations, or 
new gene arrangements that promote adaptation to the new conditions. However, 
if these genetic changes cannot be made for some reason, then as new 
conditions arise, adaptation will deteriorate and extinction may ensue.  See  
POPULATION GENETICS. 
 �34s[J. W. Valentine]
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�6v�2v�32sExtraction �21m �33s
    A method of separating the constituents of a mixture utilizing
preferential solubility of one or more components in a second phase. Commonly,
this added second phase is a liquid, while the mixture to be separated may be
either solid or liquid. If the starting mixture is a liquid, then the added
solvent must be immiscible or only partially miscible with the original and of
such a nature that the components to be separated have different relative
solubilities in the two liquid phases.
    Solvent extraction processes can be divided into two broad categories
according to the origins of the differential solubility. On the one hand, it
arises from purely physical differences between the two solutes, such as
polarity, while in other cases it can be traced to definite chemical
interaction between solute and solvent. Categories of major importance for the
latter cases are ion-association systems and chelate compounds.
    Liquid/solid extraction may be considered as the dissolving of one or more
components in a solid matrix by simple solution, or by the formation of a
soluble form by chemical reaction. The largest use of liquid/solid extraction
is in the extractive metallurgical, vegetable oil, and sugar industries. The
field may be subdivided into the following categories: leaching, washing
extraction, and diffusional extraction. Leaching involves the contacting of a
liquid and a solid (usually an ore) and the imposing of a chemical reaction
upon one or more substances in the solid matrix so as to render them soluble.
In washing extraction the solid is crushed to break the cell walls, permitting
the valuable soluble product to be washed from the matrix. In diffusional
extraction the soluble product diffuses across the denatured cell walls (no
crushing involved) and is washed out of the solid.
    Liquid/liquid extraction separates the components of a homogeneous liquid
mixture on the basis of differing solubility in another liquid phase. Because
it depends on differences in chemical potential, liquid/liquid extraction is
more sensitive to chemical type than to molecular size. This makes it
complementary to distillation as a separation technique. One of the first
large-scale uses was in the petroleum industry for the separation of aromatic
from aliphatic compounds. Liquid/liquid extraction also has found application
for many years in the coal tar industry. On a smaller scale, extraction is a
key process in the pharmaceutical industry for recovery of antibiotics from
fermentation broths, in the recovery and separation of vitamins, and for the
production of alkaloids from natural products.	See  CHEMICAL SEPARATION
TECHNIQUES.
 �34s[Bruce M. Sankey]
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�6v�2v�32sExtrapolation   �21m �33s 
    A process in mathematics used to find the value of a function outside its 
tabulated values. This is done as in interpolation by assuming that over a 
small range of x the function may be closely approximated by a polynomial or 
some other readily computed function. Any of the interpolation formulas can be 
used, therefore, and the desired value of  x  substituted in them.  See  
INTERPOLATION.  
 �34s[Kaiser S. Kunz]
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�6v�2v�32sExtrasensory perception (ESP)   �21m �33s 
    The phenomenon of awareness or reaction to external events in the absence 
of any known physical energy conveying information about those events to the 
person perceiving or reacting to them. 
    Although a wide variety of unusual events occur which may represent forms 
of ESP, only telepathy, clairvoyance, and precognition have been extensively 
studied under laboratory conditions. Mental telepathy refers to the case in 
which one person is aware of or thinking about something and another, 
sensorially separated person, shows knowledge of the first person's thoughts. 
Clairvoyance refers to direct knowledge of physical-world events that are 
sensorially shielded from the knower when, at the time the knowledge is 
obtained, no one else has knowledge (no mental representations) of those 
events. Precognition refers to knowledge of future events which is not 
predictable by logical inference from known facts. 
    There seems to be a paranormal motor analog as well as sensory analog, 
namely psychokinesis (PK), or the direct effect of the mind on matter. The 
three kinds of ESP and psychokinesis are collectively referred to as psi. Two 
major problems hinder advances in understanding psi phenomena. First, since 
psi phenomena are paraconceptual, some scientists prefer to believe that all 
the evidence for them must be due to error and fraud, and psi is not a proper 
subject for scientific study. As a result, parapsychological research has been 
an extremely small-scale activity. The other major problem in understanding 
psi is the illusiveness of the phenomena. While experiments in general 
replicate in a statistical sense, no one knows how to produce psi phenomena on 
demand in any particular experiment. 
 �34s[Charles T. Tart]
�33s 
  
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�6v�2v�32sExtrusion   �21m �33s 
    The forcing of solid metal through a suitably shaped orifice under 
compressive forces. Extrusion is somewhat analogous to squeezing toothpaste 
through a tube, although some cold extrusion processes more nearly resemble 
forging, which also deforms metals by application of compressive forces. Most 
metals can be extruded, although the process may not be economically feasible 
for high-strength alloys. 
    The most widely used method for producing extruded shapes is the direct, 
hot extrusion process. In this process, a heated billet of metal is placed in 
a cylindrical chamber and then compressed by a hydraulically operated ram (see 
illustration). The opposite end of the cylinder contains a die having an 
orifice of the desired shape; as this die opening is the path of least 
resistance for the billet under pressure, the metal, in effect, squirts out of 
the opening as a continuous bar having the same cross-sectional shape as the 
die opening. By using two sets of dies, stepped extrusions can be made. 
    The extrusion of cold metal is variously termed cold pressing, cold 
forging, cold extrusion forging, extrusion pressing, and impact extrusion. The 
term cold extrusion has become popular in the steel fabrication industry, 
while impact extrusion is more widely used in the nonferrous field. 
    The original process (identified as impact extrusion) consists of a punch 
(generally moving at high velocity) striking a blank (or slug) of the metal to 
be extruded, which has been placed in the cavity of a die. Clearance is left 
between the punch and die walls; as the punch comes in contact with the blank, 
the metal has nowhere to go except through the annular opening between punch 
and die. The punch moves a distance that is controlled by a press setting. 
This distance determines the base thickness of the finished part. The process 
is particularly adaptable to the production of thin-walled, tubular-shaped 
parts having thick bottoms, such as toothpaste tubes. 
    Advantages of cold extrusion are higher strength because of severe 
strain-hardening, good finish and dimensional accuracy, and economy due to 
fewer operations and minimum of machining required.  See  METAL FORMING. 
 �34s[Ralph L. Freeman]
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�6v�2v�32sEyepiece   �21m �33s 
    A lens or optical system which offers to the eye the image originating 
from another system (the objective), at a suitable viewing distance. The image 
can be virtual.  See  OPTICAL IMAGE. 
    In modern instruments, most eyepieces (also called oculars) are not 
independently corrected for all errors. They are designed to balance out 
certain residual aberrations of the objective or (as in the microscope) of a 
group of objectives, for instance, chromatic difference of magnification.  See 
  ABERRATION (OPTICS). 
    The Ramsden eyepiece consists of two planoconvex lenses, the field lens 
and the eye lens, with their plane sides out. Both of these lenses have the 
same power and focal length; their separation is equal to their common focal 
length.  See  GEOMETRICAL OPTICS; LENS (OPTICS). 
    The Huygens eyepiece also consists of two planoconvex lenses, but the 
plane sides of both lenses face the eye. The focal length of the field lens is 
in general three times that of the eye lens, and the separation is twice the 
focal length of the eye lens. 
 �34s[Max J. Herzberger]
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�6v�2v�32sEye (invertebrate)   �21m �33s 
    An eye may be defined as an aggregation of photoreceptor cells. It is one 
of the most frequently encountered sense organs of animals, and is possessed 
by some species of virtually every major animal phylum. However, the 
complexity of eyes varies greatly and this sense organ undoubtedly evolved 
independently a number of times within the animal kingdom. 
  
    Structure and function.  The simplest invertebrate eyes are clusters of 
photoreceptor cells located on the surface of the body. Pigment cells are 
usually interspersed among the photoreceptors, giving the eye a red or black 
color. Simple eyes of this type, called pigment spot ocelli, are found in such 
invertebrates as jellyfish, flatworms, and sea stars. 
    The more common type of invertebrate eye probably arose from such patches 
of photoreceptor cells by an in-sinking of the sensory epithelium to form a 
cup. The cup, which represents the retina, may have remained as a pit open to 
the surface, may have sunk beneath the surface, or may have become closed in 
conjunction with the evolution of a cornea and lens. 
    Simple light detection is still the sole function of the great majority of 
invertebrate eyes, even those with lens and cornea. Cornea and lens merely 
provide the eye with a protective covering and concentrate the light on the 
retinal surface. Image formation has evolved as an additional function of the 
eyes of some invertebrates. The number of photoreceptor cells composing the 
retinal surface is of primary importance, since each photoreceptor cell or 
group of cells acts as the transmitter for one point of light. The great 
majority of invertebrate eyes cannot form an image because they do not possess 
a sufficient number of photoreceptor cells. The number of photoreceptor cells 
might be sufficient to detect movement of an object, but is inadequate to 
provide information about the object's form.  See  PHOTORECEPTION. 
    The focusing mechanisms of invertebrate eyes vary considerably. The focus 
of arthropod eyes tends to be fixed, that is, the distance between the 
light-bending structure - the cornealens - and the retina cannot be changed. 
Thus objects are in focus only a certain distance from the eye. The oceanic 
family of swimming polychaete worms, Alciopidae, have the most highly 
developed eyes of any of the segmented worms. The eyes of these worms are 
focused hydrostatically. A bulb to one side of the eye injects fluid into the 
space between the retina and the lens, forcing the lens outward. Another 
mechanism is employed in octopods whereby lens movement is brought about by a 
ciliary muscle attached to the lens. However, octopods tend to be rather 
nearsighted because of the somewhat flattened, or shortened, eye. 
    Visual object perception in invertebrates is correlated with a variety of 
behavioral adaptations. In fact, behavior frequently indicates that the eye is 
capable of some degree of image formation. Invertebrates which are predatory 
commonly possess complex eyes that are probably capable of detecting and 
recognizing objects, such as cursorial hunting spiders, squids and octopods, 
certain insects, some crabs and other crustaceans; those with highly 
directional flight, such as bees, wasps, flies, and some butterflies; those 
displaying courtship behavior, such as jumping spiders and fiddler crabs; or 
those whose orientation is by celestial clues, such as bees and some spiders. 
  
    Compound eyes.  The compound eye of crustaceans, insects, centipedes, and 
horseshoe crabs has a different construction from that of other invertebrates. 
The term compound is somewhat misleading, because the eye functions basically 
like that of other eyes and is not compound in the sense of representing an 
aggregate of eyes. Each light-transmitting photoreceptor unit has a refractive 
body instead of a single cornea-lens structure for the entire eye. 
    The structural unit of the compound eye is called ommatidium (see 
illustration). The outer end of the ommatidium is composed of a cornea, which 
appears on the surface of the eye as a facet. Beneath the cornea is an 
elongated, tapered crystalline cone; the cornea and cone together function as 
a lens. The receptor element at the inner end of the ommatidium is called the 
retinula and is composed of a central translucent cylinder (rhabdome), around 
which are located usually seven photoreceptor cells plus one eccentric cell. 
The photoreceptor element of each ommatidium functions as a unit and can 
respond only to one point of light. Thus image formation is dependent upon the 
number of photoreceptor units present. Pigment granules surround the 
ommatidium proximally and distally, forming a light screen that separates one 
ommatidium from another. The pigment granules migrate, depending upon the 
amount of light. 
    The advantage of the compound eye may well be its ability to detect 
movement. A slight shift in a point of light results in a corresponding shift 
in the ommatidia being stimulated; this feature is valuable in emphasizing 
movement since several ommatidia will respond to the same point of light. 
Another advantage of the compound eye of many arthropods is that the total 
corneal surface is greatly convex, resulting in a wide visual field. For 
example, the compound eye at the end of the eyestalk of a crab may cover an 
arc of 180DG. 
  
    Nocturnal vision.  The eyes of many nocturnal invertebrates are adapted 
for functioning in reduced light. Wolf spiders, hunters that run over the 
ground at night, illustrate eyes of this type. Of the eight eyes, six are 
indirect and are supplied with a layer of reflecting pigment which reflects 
light onto the photoreceptors. Like those of a cat, the eyes of these spiders 
appear to glow in the dark when facing a beam of light; the eyes of specimens 
the size of a quarter can be seen from a distance of 50 ft (15 m). 
 �34s[Robert D. Barnes]
�33s 
  
  
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�6v�2v�32sEye (vertebrate) �21m �33s
    A sense organ that acts as a photoreceptor capable of image formation. The
eye of vertebrates is constructed along a basic anatomical pattern which, in
the diversification of animals, has undergone a variety of structural and
functional modifications associated with different ecologies and modes of
living. Often compared with a camera, the vertebrate eye is conveniently
described in terms of its wall, cavities, and lens (see illustration).

    The wall.  The wall of the eye consists of three distinct layers or tunics
which, from outward to inward, are termed the fibrous, vascular, and sensory
tunics.

    Fibrous tunic. This continuous, outermost fibrous tunic comprises a
transparent anterior portion, the cornea, and a tough posterior portion, the
sclera. In the human, the cornea represents about one-sixth of the fibrous
tunic, the sclera five-sixths.
    The vertebrate cornea exhibits very few modifications in structure
regardless of environmental influences. Its major constituent is connective
tissue (both cells and fibers), regularly arranged and bordered on both
anterior and posterior surfaces by an epithelium. The anterior epithelium is
stratified, ectodermal in origin, and continuous with the (conjunctival)
epithelium lining the eyelids. The transparency of the cornea is attributed to
the geometric organization of its connective tissue elements, its constant
state of deturgescence, and its chemical composition. It is the first ocular
component traversed by the incoming light.
    The sclera, a tough connective tissue tunic, provides support for the eye
and serves for the attachment (insertions) of the muscles that move it.
    The limbus is located at the angle of the anterior chamber. This small,
circular transitional zone between the cornea and the sclera houses the major
route for the discharge of aqueous humor from the anterior chamber.
    Vascular tunic. The vascular tunic or uvea makes up the middle layer of
the wall of the eye. It does not form a continuous layer around the eye but is
deficient anteriorly, where the opening is termed the pupil. Beginning at the
pupil, three continuous components of the uvea can easily be recognized: the
iris, ciliary body, and choroid.
    The iris is a spongy, circular diaphragm of loose, pigmented connective
tissue separating the anterior and posterior chambers and housing a hole, the
pupil, in its center. When heavily pigmented, the human iris appears brown,
when lightly pigmented, blue.
    The ciliary body is continuous with the root of the iris. The posterior
epithelium of the iris continues along the internal surface of the ciliary body
as a double layer of cells (ciliary part of the retina) which assumes many
folds for the attachment of the suspensory ligament of the lens. This ligament
holds the lens in position and shape, and marks the posterior boundary of the
posterior chamber. The inner layer of the ciliary epithelium contains no
pigment. It produces aqueous humor which flows into the posterior chamber and
thence into the anterior chamber (via the pupil). The continual production and
removal of this fluid maintain the intraocular pressure of the eye (which is
increased in glaucoma).
    The choroid is the most posterior portion of the uvea. It is directly
continuous with the subepithelial portion of the ciliary body and consists
primarily of blood vessls embedded within deeply pigmented connective tissue.
    Sensory tunic. The retina is the sensory tunic of the eye. It has the form
of a cup closely applied to the inner portion of the choroid, and, internally,
it is slightly adherent to the semisolid vitreous body. The vertebrate retina
contains the light-sensitive receptors (visual cells) and a complex of
well-organized impulse-carrying nerve cells (neurons), all arranged into
discrete layers.
    Pigmented epithelium forms an important barrier between the
light-sensitive receptors (visual cells) and their blood supply, the choroid.
As in the choroid, the pigmentation serves to absorb light and prevent its
reflection.
    The rods and cones of vertebrates generally occur as single units, but
combinations of each type are frequently encountered in several vertebrate
classes. Cones appear to be adapted for photopic, or daylight, vision, based
on correlations with the visual habits of the animals involved. Rods, which
predominate in nocturnal vertebrates, are adapted for scotopic, or night,
vision. Except for their external process, the structure of these cells does
not reflect these differences.	See  PHOTORECEPTION; VISION.
    An important adaptation for improving visual detail in vertebrates is the
formation of circumscribed thickenings of the retina resulting from localized
increases in the number of visual cells and the other retinal neurons
associated synaptically with them. Such thickenings, termed areas of acute
vision, appear in some members of all vertebrate classes and reach their
greatest development in birds, in which one to three distinct areas may be
found in the same retina. Only a single area occurs in the human; it is
colored yellow and is called the macula. The macula is situated in the center
of the fundus and contains only cones.

    The cavities.  Three cavities or chambers are present within the
vertebrate eye: anterior, posterior, and vitreous (Fig. 1). The anterior and
posterior chambers are continuous with one another at the pupil and are filled
with the aqueous humor. The eye is normally maintained in a distended state by
the (intraocular) pressure created by this fluid. The vitreous cavity, on the
other hand, is filled with a semisolid material, the vitreous body, which is
fixed in amount and relatively permanent. Its consistency is not uniform in
all vertebrates, however.

    The lens.  The lens is a transparent body, supported by thin suspensory
fibers and by the vitreous body behind and by the iris in front. It is
completely cellular, the anterior cells forming a thin epithelium, and the
posterior cells, much elongated, forming the so-called lens fibers. The entire
lens is surrounded by an elastic capsule which serves for the attachment of
the ciliary zonule. In all vertebrates the lens functions in accommodiation,
either by moving backward and forward or by changing its shape. An opacity of
the lens is termed a cataract.
 �34s[David B. Meyer ;Ronan O'Rahilly]
�33s

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�6v�2v�32se (mathematics) �21m �33s


    The number e is usually defined as the limit approached by the expression

		     ( 1 + (1/n)(sup n))

as n approaches infinity. If the given expression is expanded  by the binomial
theorem, and if one uses the theorem that the limit of the quotient of two
polynomials of equal degree as the variable tends to infinity is equal to the
ratio of the coefficients of highest degree, one obtains the expansion in
relation (1). It can be shown by elementary methods that e is irrational; that
is, it cannot be represented as the quotient of two integers. Furthermore, e is
transcendental;

e = 1 + 1/1 + 1/(1x2) + 1/(1x2x3) + . . . + 1/(1x2x3x...xn) + . . .   (1)

it does not satisfy any algebraic equation with integral coefficients. By the
method outlined above, it may be shown that the limit of

				(1 + x/u)(sup n)

as n tends to infinity is e(sup x), and moreover that relation (2)
holds. The function esupx is of great importance in mathematical analysis and
is encountered in numerous problems in applied mathematics. BINOMIAL THEOREM;
CALCULUS; LOGARITHM.
 �34s[Arnold N. Lowan /Salomon Boch]
�33s

   e(sup x) = 1 + x/1 + x2/1x2 + . . . + x(sup n)/1x2x3x...xu + . . .	 (2)

 �34s[Arnold N. Lowan /Salomon Bochner]
�33s
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�6v�2v�32sFacies (geology)   �21m �33s 
    Any observable attribute of rocks, such as overall appearance, 
composition, or conditions of formation, and changes that may occur in these 
attributes over a geographic area. The term facies is widely used in 
connection with sedimentary rock bodies, but is not restricted to them. In 
general, facies are not defined for sedimentary rocks by features produced 
during weathering, metamorphism, or structural disturbance. In metamorphic 
rocks specifically, however, facies may be identified by the presence of 
minerals, that denote degrees of metamorphic change. 
  
    Sedimentary facies.  The term sedimentary facies is applied to bodies of 
sedimentary rock on the basis of descriptive or interpretive characteristics. 
Descriptive facies are based on lithologic features such as composition, grain 
size, bedding characteristics, and sedimentary structures (lithofacies) or on 
biological (fossil) components (biofacies), or on both. Individual lithofacies 
or biofacies may be single beds a few millimeters thick or a succession of 
beds tens to hundreds of meters thick. For example, a river deposit may 
consist of decimeter-thick beds of a conglomerate lithofacies interbedded with 
a cross-bedded sandstone lithofacies. The fill of certain major Paleozoic 
basins may be divided into units hundreds of meters thick comprising a shelly 
facies, containing such fossils as brachiopods and trilobites, and a 
graptolitic facies.  See  SEDIMENTARY ROCKS; SEDIMENTATION (GEOLOGY). 
    The term facies can be used also in an interpretive sense for groups of 
rocks that are thought to have been formed under similar conditions. This 
usage may emphasize specific depositional processes, such as a turbidite 
facies, or a particular depositional environment such as a shelf carbonate 
facies, encompassing a range of depositional processes. Finally, widespread 
use is made of two facies terms which are defined primarily on the basis of 
tectonic control. The flysch facies comprises marine sediments, typically 
turbidites and other mass-flow deposits, formed on continental margins under 
plate tectonic conditions of convergence, prior to the development of major 
orogenic uplifts. The molasse facies consists of nonmarine and shallow marine 
sediments formed within and flanking fold belts during and following their 
elevation into mountain ranges, under conditions of plate subduction.  See  
TURBIDITES.  
    Groups of facies (usually lithofacies) that are commonly found together in 
the sedimentary record are known as facies assemblages or facies associations. 
These groupings provide the basis for defining broader, interpretive facies 
for the purpose of paleogeographic reconstruction.  See  PALEOGEOGRAPHY. 
 �34s[Andrew D. Miall]
�33s 
  
    Metamorphic facies.  A metamorphic facies refers to a set of mineral 
assemblages produced from primary igneous or sedimentary parent rocks in 
response to changes in temperature and pressure during burial in the Earth's 
crust.  See  METAMORPHIC ROCKS. 
    The facies concept of metamorphism, with its reliance on stable mineral 
assemblages as indicators of metamorphic conditions, complements and refines 
the so-called index mineral approach to metamorphic petrogenesis, whereby 
individual minerals such as chlorite, kyanite, and lawsonite are used to 
signify relative metamorphic grade.  See  METAMORPHISM. 
    Metamorphic facies are often correlated with formation in a particular 
geological environment. For example, the blueschist facies is usually 
interpreted as having formed in the subduction zones of converging 
lithospheric plates and the eclogite facies rocks near the base of the 
continental crust.  See  BLUESCHIST METAMORPHISM; PETROLOGY; PLATE TECTONICS. 
 �34s[J. J. Fawcett]
�33s 
  
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�6v�2v�32sFacsimile   �21m �33s 
    Transmission of a fixed image by wire or radio. The image or subject copy 
is usually in the form of a photograph, handwriting, map, or drawing. 
Facsimile equipment in the United States is used for telegram pickup and 
delivery and for business document transmission. Almost all newspaper 
photographs of events outside the local city in which the newspaper is 
published are sent by facsimile. Complete newspapers are printed from 
facsimile transmissions originating in distant cities. Railroads employ 
high-speed facsimile for transmission of waybills. Cloud-cover photographs and 
other types of graphic data are recorded by facsimile from signals sent by 
orbiting and stationary satellites. X-ray photographs have been sent from 
hospitals to radiological interpretation centers. Another use that is becoming 
increasingly important is information storage and retrieval. Facsimile will 
provide rapid, remote access to documents stored on microfilm at central 
locations such as libraries. 
    In the black and white scanning and transmitting process the subject copy 
is divided into dots, or elemental areas. Each elemental area (1/100 x 1/100 
in. or 0.25 x 0.25 mm in a typical scanning raster) is transmitted as an 
electrical signal that represents its blackness. In an amplitude-modulated 
system, elemental areas are transmitted at a constant rate, usually 1800 - 
4000 per second, when transmission is over a voice-frequency circuit. 
Newspaper pages are transmitted at rates  of up to 1,500,000 elemental areas 
per second over microwave circuits. 
    Some circuits cannot retain the true amplitude relationship of the signal 
in transmission. For example, high-frequency radio circuits are subject to 
considerable fading. In such cases amplitude modulation is unsatisfactory. 
Frequency-shift modulation is used so that the received copy will not be 
degraded by fading or level changes. 
    The received signals are recorded on a record sheet by any one of many 
processes. In a direct-recording facsimile system, an image is visible while 
being recorded. In photofacsimile systems using photographic film or paper, 
processing is necessary before the image can be viewed. 
    For transmission of color photographs, three separate facsimile 
transmissions are made from the original color print. Color-separation filters 
in the optical system of the facsimile transmitter produce black-and-white 
color-separation negatives at the facsimile recorder. The technique is similar 
to that used for color-separation negatives made by photography. 
    One type of digital facsimile system sends only black or white picture 
information; no gray scale shades are reproduced. Because changes from black 
to white can occur only at discrete points along a recording line, vertical 
lines have a more ragged appearance than in comparable analog facsimile 
systems. However, noise on long circuits is less apt to degrade the received 
picture. Another advantage is secrecy. For example, digital signals may be 
connected to military encrypting equipment for sending secret information. 
 �34s[Ken R. McConnell]
�33s 
  
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�6v�2v�32sFactor analysis   �21m �33s 
    A method of quantitative multivariate analysis with the goal of 
representing the interrelationships among a set of continuously measured 
variables (usually represented by their intercorrelations) by a number of 
underlying, linearly independent reference variables called factors. Although 
the term factor analysis has come to represent a family of analysis methods, 
the two most commonly used approaches are the full component model, in which 
the entire variance of the variables (represented by unities inserted in the 
principal diagonal of the correlation matrix) is analyzed, and the common 
factor model, in which the proportion of the variance that is accounted for by 
the common factors (represented by communality estimates inserted in the 
principal diagonal) is analyzed. 
    The method was developed in England around the turn of the century and was 
first applied to the study of the structure of intellectual abilities. Since 
then it has been used in many disciplines, from agriculture to zoology, in 
which the underlying structure of multiple variables and their representation 
in that structure are of interest. Another application of factor analysis is 
to represent parsimoniously the variables in the set on which the observations 
are made by a smaller number of underlying reference variables or factors.  
See  STATISTICS. 
 �34s[Benjamin Fruchter]
�33s 
  
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�6v�2v�32sFagales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of three families, the Fagaceae, Betulaceae, and Balanopaceae. The 
Fagales are simple-leaved, woody plants with much reduced, mostly unisexual 
flowers, the pistillate ones with one or two anatropous ovules in each of the 
two or three locules of the ovary, the staminate ones usually grouped into 
catkins. Birch  (Betula) , beech  (Fagus) , and oak  (Quercus)  are well-known 
members of the Fagales.  See  BEECH; BIRCH; HAMAMELIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sFalconiformes   �21m �33s 
    The order containing the diurnal birds of prey. Five living families are 
usually recognized, as well as two extinct families, the Neocathartidae (Upper 
Eocene) and Teratornithidae (Pleistocene). The largest and most diverse family 
is the Accipitridae, including rapacious birds such as the hawks, eagles, and 
kites, adapted for capturing living prey ranging from ungulates to small 
invertebrates. It also includes a subfamily (Aegypiinae) of carrion-feeders, 
the Old World vultures. Usually placed in this subfamily is the only primarily 
vegetarian member of the order, the palm-nut vulture  (Gypohierax angolensis)  
of Africa. 
    Similar to the Aegypiinae in food habits and superficially similar in 
appearance are the New World vultures (Cathartidae), among which is the Andean 
condor, one of the largest of extant flying birds, attaining a wingspread of 
12 ft (3.6 m). The Cathartidae differ from other Falconiformes in many 
anatomical and behavioral characters. 
    The falcons (Falconidae) constitute another anatomically distinct family 
composed principally of long-winged rapacious birds. One New World subfamily, 
the caracaras (Caracarinae), has become secondarily adapted to carrion-feeding. 
    Two somewhat specialized species of this order are each given family rank. 
The widely distributed osprey or fish hawk (Pandionidae) has a reversible hind 
toe, especially well-developed claws, and spicules on the scales of its feet, 
all assisting in the capture of slippery prey. The long-legged, primarily 
terrestrial secretary bird (Sagittariidae) of Africa, named for a fancied 
resemblance of its nuchal plumes to quill pens stuck behind an ear, is famous 
for its ability to prey on poisonous snakes, although it also feeds on many 
other vertebrates.  See  AVES; BIRDS OF PREY. 
 �34s[Kenneth C. Parkes]
�33s 
  
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�6v�2v�32sFall line   �21m �33s 
    The zone or boundary between resistant rocks of older land and weaker 
strata of plains. Here the easily eroded strata of the plains lap onto the 
more resistant rocks of the old land. Streams that cross this boundary have 
steeper gradients along the width of the zone and make sudden descents over 
falls and rapids. In geologic and geographic literature, a line connecting 
such falls on several main rivers is referred to as a fall line. 
 �34s[Sheldon Judson]
�33s 
  
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�6v�2v�32sFallopian tube   �21m �33s 
    The upper part of the female oviduct present in the humans and other 
higher vertebrates. The Fallopian tube extends from the ovary to the uterus 
and transports ova from the ovary to the cavity of the uterus. Each tube is 
about 5 in (12.5 cm) long; one lies on either side of the uterus and is 
attached at the upper portion. Each curves outward to end in a hoodlike 
opening, the infundibulum, with many fingerlike projections; the cavity of the 
Fallopian tube is continuous with the cavity of the coelom. The ovaries lie 
below and inside the tubal curve. 
    The ovum remains viable in the oviduct for about 1 - 3 days only. If 
fertilization occurs, the ovum moves into the cavity of the uterus and then 
implants on its wall. If fertilization fails to occur, the ovum degenerates in 
the uterus. Occasionally, a fertilized ovum fails to enter the uterus, or may 
be freed into the abdominal cavity, so that an ectopic pregnancy results if 
the ovum finds a site for implantation.  See  PREGNANCY DISORDERS; 
REPRODUCTIVE SYSTEM. 
 �34s[William Bradford]
�33s 
  
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�6v�2v�32sFan   �21m �33s 
    A fan moves gases by producing a low compression ratio, as in ventilation 
and pneumatic conveying of materials. The increase in density of the gas in 
passing through a fan is generally negligible; the pressure increase or head 
is usually measured in inches of water. 
    Blowers are fans that operate where the resistance to gas flow is 
predominantly downstream of the fan. Exhausters are fans that operate where 
the flow resistance is mainly upstream of the fan. 
    Fans are further classified as centrifugal or axial (see illustration). 
The housing provides an inlet and an outlet and confines the flow to the 
region swept out by the rotating impeller. The impeller imparts velocity to 
the gas, and this velocity changes to a pressure differential under the 
influence of the housing and ducts connected to inlet and outlet.  See  DUCTED 
FLOW.  
 �34s[Theodore Baumeister]
�33s 
  
  
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�6v�2v�32sFaraday effect �21m �33s
    Rotation of the plane of polarization of a beam of linearly polarized
light when the light passes through matter in the direction of the lines of
force of an applied magnetic field. Discovered by M. Faraday in 1846, the
effect is often called magnetic rotation.  See	MAGNETOOPTICS.
    The Faraday effect is particularly simple in substances having sharp
absorption lines, that is, in gases and in certain crystals, particularly at
low temperatures. Here the effect can be fully explained from the fundamental
properties of the atoms and molecules involved. In other substances the
situation may be more complex, but the same principles furnish the explanation.
    Rotation of the plane of polarization occurs when there is a difference
between the indices of refraction  n supp for right-handed polarized light and
  n - for left-handed polarized light. Most substances do not show such a
difference without a magnetic field, except optically active substances such
as crystalline quartz or a sugar solution. It should be noted that the index
of refraction in the vicinity of an absorption line changes with the
frequency.  See  ABSORPTION; POLARIZED LIGHT.
 �34s[G. H. Dieke /William W. Watson]
�33s

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�6v�2v�32sFaraday's law of induction �21m �33s
    A statement relating an induced electromotive force (emf) to the change in
magnetic flux that produces it. For any flux change that takes place in a
circuit, Faraday's law states that the magnitude of the emf XI induced in the
circuit is proportional to the rate of change of flux as in the expression
below.

		     xi PROPORTIONAL to dPHI/dt

    The time rate of change of flux in this expression may refer to any kind
of flux change that takes place. If the change is motion of a conductor
through a field,  dPHI/dt  refers to the rate of cutting flux. If the change
is an increase or decrease in flux linking a coil,  dPHI/dt  refers to the
rate of such change. It may refer to a motion or to a change that involves no
motion.  See  ELECTROMAGNETIC INDUCTION; MAGNETIC FLUX.
 �34s[Kenneth V. Manning]
�33s

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�6v�2v�32sFascioliasis   �21m �33s 
    The accidental infection of humans with the cattle liver fluke  (Fasciola 
hepatica) . The leaflike fluke inhabits the bile ducts; infected animals pass 
the eggs in feces; the eggs embryonate in water. Humans are exposed by 
ingesting aquatic vegetables such as watercress. Incidence is generally low, 
but serious outbreaks have been recorded in humans from various parts of the 
world. Symptoms are usually chronic and directly referable to biliary 
malfunction, which may become confused with various liver diseases.  See  
TREMATODA.  
 �34s[Jose F. Maldonado]
�33s 
  
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�6v�2v�32sFasciolopsiasis   �21m �33s 
    The presence of the trematode  Fasciolopsis buski  in the small intestine. 
The parasite is native to eastern Asia and the southwestern Pacific; cases 
have been reported from Ecuador. Hogs act as natural reservoirs. Humans become 
parasitized by using their teeth to peel various aquatic raw vegetables on 
which the parasite encysts after its development in a molluscan host. The 
fluke lives attached to the intestinal mucosa by its two powerful suckers, 
producing inflammation and ulceration. A light infection is characterized by 
toxic diarrhea. A heavy one produces toxic symptoms leading to edema and heart 
failure.  See  TREMATODA. 
 �34s[Jose F. Maldonado]
�33s 
  
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�6v�2v�32sFat and oil �21m �33s
    Naturally occurring esters of glycerol and fatty acids that have
commercial uses. Since fats and oils are triesters, they are commonly called
triglycerides or simply glycerides. A glyceride may be designated a fat or
oil, depending on its melting point. A fat is solid and an oil is liquid at
room temperature. Some liquid waxes are incorrectly referred to as oils.  See
ESTER; TRIGLYCERIDE; WAX, ANIMAL AND VEGETABLE.
    The structure of triglycerides is shown below, where R1, R2, and R3
represent the alkyl chain of the fatty acid.

			H sub 2 COC(O)R sub 1
				|
			       HCOC(O)R sub 2
				|
			H sub 2 COC(O)R sub 3

    The physical and chemical properties of fats and oils are determined to a
large extent by the types of fatty acids in the glyceride. It is possible for
all the acids to be identical, but this is rare. Usually there are two or even
three different acids esterified to each glycerol molecule.
    In all commercially important glycerides, the fatty acids are
straight-chain, and nearly all contain an even number of carbon atoms. Most
fats and oils are based on C16 and C18 acids with zero to three ethylenic
bonds. There are exceptions, such as coconut oil, which is rich in
shorter-chain acids, and some marine oils, which contain acids with as many as
22 or more carbons and six or more ethylenic linkages.
    The majority of fats and oils come from only a few sources. Plant sources
are nuts or seeds, and nearly all terrestrial animal fats are from adipose
tissue. Marine oils come principally from the whole body, although a small
amount comes from trimmings. Plant fats and oils are obtained by crushing and
solvent extraction. Animal and marine oils are nearly all recovered by
rendering. This is a process of heating fatty tissue with steam or hot water
to melt and free the glyceride, followed by separating the oil or fat from the
aqueous layer.	See  SOLVENT EXTRACTION.
    There are some uses for fats and oils in their native state, but
ordinarily they are converted to more valuable products. The most important
changes are hydrolysis and hydrogenation. Hydrolysis is commonly called
splitting. The purpose is to hydrolyze the ester into its constituent glycerol
and fatty acids, which are valuable intermediates for many compounds.
Hydrogenation is the catalytic addition of hydrogen to ethylenic bonds, and is
applicable to both acids and glycerides. The purpose of hydrogenation is
usually to raise the melting point or to increase the resistance to oxidation.
  See  HYDROGENATION; HYDROLYTIC PROCESS.
 �34s[Howard M. Hickman]
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�6v�2v�32sFat and oil (food)   �21m �33s 
    One of the three major classes of basic food substances, the others being 
protein and carbohydrate. Fats and oils are a source of energy. They also aid 
in making both natural and prepared foods more palatable by improving the 
texture and providing a more desirable flavor.  See  FOOD. 
    Fats are grouped according to source. Animal fats are rendered from the 
fatty tissues of hogs, cattle, sheep, and poultry. Butter is obtained from 
milk. Vegetable oils are pressed or extracted from various plant seeds, 
primarily from soybean, cottonseed, corn (germ), peanut, sunflower, safflower, 
olive, rapeseed, sesame, coconut, oil palm (pulp and kernal separately), and 
cocoa beans. Marine oils, which are not consumed in the United States are 
obtained mostly from herring, sardine, and pilchard. 
    Fats and oils are important in the diet. They are the most concentrated 
form of food energy, contributing about 9 cal/g (38 joules/g), as compared to 
about 4 cal/g (17 joules/g) for carbohydrates and proteins. Fats make a meal 
more satisfying by creating a feeling of fullness, and also delay the onset of 
hunger. Contrary to popular belief, fats are highly digestible, with 94 - 98% 
of the ingested fat being absorbed from the intestinal tract. 
    The polyunsaturated fatty acids, primarily linoleic and arachidonic, are 
essential nutrients; that is, they are not synthesized by the body but are 
required for tissue development. Absence of these fatty acids from the diet 
results in an essential fatty acid syndrome and in a specific form of eczema 
in infants. Vegetable oils are an excellent source of linoleic acid, while 
meat fats provide arachidonic acid in small but significant amounts. Fats and 
oils are carriers of the oil-soluble vitamins A and D, and are the main source 
of vitamin E. They also have a sparing action on some of the B complex 
vitamins.  See  NUTRITION. 
    Several forms of deterioration may occur in fats and oils. Flavor and odor 
may develop after deodorization of a product to complete blandness. The flavor 
is generally characteristic of the oil source and is therefore usually 
acceptable. However, soybean oil can develop disagreeable flavors described as 
beany, grassy, painty, fishy, or like watermelon rind. Beef fat can become 
tallowy, which is also objectionable. Reversion is apparently caused by 
changes in substances which have been oxidized prior to, but not removed by, 
deodorization.  
    Oxidative rancidity is a serious flavor defect and highly objectionable. 
It starts with the formation of hydroperoxides which then decompose to form 
aldehydes which have a pungent, disagreeable flavor and odor. Retardation of 
oxidation is brought about by using opaque, airtight containers, or nitrogen 
blanketing if clear glass bottles are used. Antioxidants are required in meat 
fats, since lard, tallow, and so on contain no natural antioxidant material. 
Vegetable oils contain tocopherols. Additional antioxidant, with the exception 
of tertiary butyl hydroquinone (TBHQ), has little benefit for these oils. 
    Hydrolytic rancidity results from the liberation of free fatty acids by 
the reaction of fats and oils with water. While most fats show no detectable 
off flavors, coconut and other lauric acid oils develop a soapy flavor, and 
butter develops the strong characteristic odor of butyric acid. Packaged 
coconut-oil products and lauric-type hard butters sometimes contain added 
lecithin, which acts as a moisture scavenger, thereby retarding hydrolytic 
rancidity development. 
    Fats and oils used in deep fat frying can break down under adverse 
conditions, especially where frying is intermittent or the fryer capacity is 
not fully used. Deterioration ultimately results in the oil becoming very dark 
in color, viscous, foul-odored, and foaming badly during frying. The oil 
becomes oxidized and then polymerized, requiring that it be discarded, since 
it imparts strong off flavors to the fried food. 
    Crystal structure transformation of packaged shortening results in 
formation of a grainy, soft product, which may also lose incorporated gas and 
take on the appearance of petroleum jelly. Similar changes in crystal 
structure can cause bloom in chocolate coatings, a defect which gives a white 
haze or even open grain on an originally smooth, glossy surface. Chocolate 
bloom can be inhibited by the addition of lecithin or polysorbates or both.  
See  LIPID. 
 �34s[Theodore J. Weiss]
�33s 
  
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�6v�2v�32sFault and fault structures   �21m �33s 
    Products of fracturing and differential movements along fractures in 
continental and oceanic crustal rocks. Faults range in length and magnitude of 
displacement from small structures visible in hand specimens, displaying 
offsets of a centimeter or less, to long, continuous crustal breaks, extending 
hundreds of kilometers in length and accommodating displacements of tens or 
hundreds of kilometers. Faults exist in deformed rocks at the microscopic 
scale, but these are generally ignored or go unrecognized in most geological 
studies. Alternatively, where microfaults systematically pervade rock bodies 
as sets of very closely spaced subparallel, planar fractures, they are 
recognized and interpreted as a type of cleavage which permitted flow of the 
rock body. Fractures along which there is no visible displacement are known as 
joints. Large fractures which have accommodated major dilational opening (a 
meter or more) perpendicular to the fracture surfaces are known as fissures. 
Formation of fissures is restricted to near-surface conditions, for example, 
in areas of crustal stretching of subsidence.  See  JOINT (GEOLOGY). 
    In addition to describing the physical and geometric nature of faults and 
interpreting time of formation, it has been found to be especially important 
to determine the orientations of minor fault structures (such as striae and 
drag folds) which record the sense of relative movement. Evaluating the 
movement of faulting can be difficult, for the apparent relative movement 
(separation) of fault blocks as seen in map or outcrop may bear little or no 
relation to the actual relative movement (slip). The slip of the fault is the 
actual relative movement between two points or two markers in the rock that 
were coincident before faulting (see illustration). Strike-slip faults have 
resulted in horizontal movements between adjacent blocks; dip-slip faults are 
marked by translations directly up or down the dip of the fault surface; in 
oblique-slip faults the path of actual relative movement is inclined somewhere 
between horizontal and dip slip. 
    Recognizing even the simplest translational fault movements in nature is 
often enormously difficult because of complicated and deceptive patterns 
created by the interference of structure and topography, and by the absence of 
specific fault structures which define the slip path. While mapping, the 
geologist mainly documents apparent relative movement (separation) along a 
fault, based on what is observed in plan-view or cross-sectional exposures.  
See  TRANSFORM FAULT. 
 �34s[George H. Davis]
�33s 
  
  
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�6v�2v�32sFault-tolerant systems   �21m �33s 
    Systems, predominantly computing and computer-based systems, which 
tolerate undesired changes in their internal structure or external 
environment. Such changes, generally referred to as faults, may occur at 
various times during the evolution of a system, beginning with its 
specification and proceeding through its utilization. Faults that occur during 
specification, design, implementation, or modification are called design 
faults; those occurring during utilization are referred to as operational 
faults. The use of fault tolerance techniques is based on the premise that a 
complex system, no matter how carefully designed and validated, is likely to 
contain residual design faults and to encounter unpreventable operational 
faults.  
    Generally, fault tolerance techniques attempt to prevent lower-level 
errors (caused by faults) from propagating into system failures. By using 
various types of structural and informational redundancy, such techniques 
either mask a fault (no errors are propagated to the faulty subsystem's 
output) or detect a fault (via an error) and then effect a recovery process 
which, if successful, prevents a system failure. In the case of a permanent 
internal fault, the recovery process usually includes some form of structural 
reconfiguration (for example, replacement of a faulty subsystem with a spare 
or use of an alternate program) which prevents the fault from causing further 
errors. Typically, a fault-tolerant system design will incorporate a mix of 
fault tolerance techniques which complement the techniques used for fault 
prevention.  See  SOFTWARE ENGINEERING. 
 �34s[John F. Meyer]
�33s 
  
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�6v�2v�32sFeather   �21m �33s 
    A specialized keratinous outgrowth of the skin, which is a unique 
characteristic of birds. Feathers are highly complex structures that provide 
insulation, protection against mechanical damage, protective coloration, and 
also function significantly in behavior. One special functional role is in 
flight, where feathers provide propulsive surfaces and a body surface 
aerodynamically suitable for flight. Feathers are used in maintenance of 
balance and occasionally in the capture of prey and various specialized 
displays.  
    A representative definitive feather contains a single long central axis 
which supports a row of small branchlike structures along each side (barbs). 
Barbs form the vane, or web, of the feather. Individual barbs branch off at 
variable angles and point toward the outer tip of the feather. The barbules 
are small branches from the barbs. They lie in the same plane as the barbs and 
arise in rows from their anterior and posterior surfaces. The anterior 
barbules have a flattened base and a series of small hooklike projections 
which attach to the proximal ridge of the posterior barbules of the next barb, 
forming an interlocking structure characterized by its great strength and 
light weight. All feather types consist basically of these structural elements. 
    Most of the superficial feathers are contour feathers (pennae). These 
include the large flight feathers (remiges) of the wing and the long tail 
feathers (rectrices). Other common feather types include the down feathers 
(plumulae), intermediate types (semiplumes), and filoplumes (see illustration). 
  
    Feathers normally undergo attrition because of the physical abuse 
attendant to the normal activity of birds. In most species, feathers are 
replaced completely at least annually, and many of the feathers are replaced 
more frequently. The sequence of feather molt is surprisingly orderly. 
Penguins, which shed large patches of feathers in an irregular pattern, are an 
exception. In most species the power of flight is retained during molt. The 
molt, that is, the normal shedding of feathers and their replacement by a new 
generation of feathers, is a single growth process which is actively concerned 
only with the production of the new generation of feathers. The old feathers 
are pushed out of the follicles passively. 
    A major physiological role of feathers is to provide insulation. This is 
accomplished by regulating the configuration of feather and skin in such a way 
that differing amounts of air are trapped in the dead space so formed. A 
second mechanism for control of heat dissipation is the balance of the 
exposure of feathered and unfeathered body parts. 
    Feathers act as a protective boundary in their role of providing 
waterproofing. Water repellency is a structural feature of feathers and is the 
result of precise geometric relationships between the diameter and spacing of 
barbs and barbules. Preening appears to be more important in the maintenance 
of this structure than it is for the application of oils or any other natural 
product, as was once thought. 
    A third function of the surface configuration and overall pattern of 
feathers is in the area of behavioral adaptations. These may be of two types. 
First is concealment, when the bird is cryptically marked to match its 
background and escape detection. The second type consists of various types of 
advertisement.  See  PROTECTIVE COLORATION. 
 �34s[Alan H. Brush]
�33s 
  
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�6v�2v�32sFeces   �21m �33s 
    The waste material eliminated by the gastrointestinal tract. The 
components and form of feces vary with diet, metabolic activities, and the 
state of health; illness, changes in diet or water intake and excretion, 
emotional stimuli, and many drugs and chemical may alter the feces both 
qualitatively and quantitatively.  See  CONSTIPATION; DIARRHEA. 
    In general, the feces contain undigested food materials and excretions and 
secretions of the digestive system, including salts, pigments, bile products, 
and cellular debris. Food passes from the small intestine to the lower bowel 
(large intestine) in a semiliquid state; nutrients and water are absorbed in 
the bowel, and the feces become more solid. 
    Feces constitute important objective evidence in disease. Defecation 
patterns together with laboratory analyses of fecal components are helpful, 
though not definitive, in the differential diagnosis of digestive system 
disorders.  See  BILIRUBIN; GASTROINTESTINAL TRACT DISORDERS. 
 �34s[Sybil P. Parker]
�33s 
  
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�6v�2v�32sFederal Telecommunications System   �21m �33s 
    A private telephone network throughout the United States for the exclusive 
use of all Federal government offices. The Federal Telecommunications System 
(FTS) consists of leased private-line circuits arranged for automatic 
switching at centers located on telephone company premises. This unique system 
can interconnect agency telephones by using techniques similar to those 
employed in the Bell System Direct Distance Dialing (DDD) network. In 
addition, there are telephones that can have access to the network but are not 
directly connected. The network has been engineered to permit many 
simultaneous messages to be handled with ease. The network can be rapidly 
augmented when needed for emergencies such as floods and storms. The network 
stretches from coast to coast and covers approximately 11,000,000 route miles 
(18,000,000 route kilometers). In addition to serving the continental United 
States, FTS is also installed in Alaska, Puerto Rico, and Hawaii. The system 
is used to handle normal administrative telephone calls and data transmission. 
See TELEPHONE; TELEPHONE SERVICE; TELEPHONE SIGNALING; TELEPHONE SYSTEMS 
CONSTRUCTION; TELEPHONY. 
 �34s[Robert M. Sawyer]
�33s 
  
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�6v�2v�32sFeedback circuit   �21m �33s 
    A circuit that returns a portion of the output signal of an electronic 
circuit or control system to the input of the circuit or system. When the 
signal returned (the feedback signal) is at the same phase as the input 
signal, the feedback is called positive or regenerative. When the feedback 
signal is of opposite phase to that of the input signal, the feedback is 
negative or degenerative. 
    The use of negative feedback in electronic circuits and automatic control 
systems produces changes in the characteristics of the system which improve 
the performance of the system. In electronic circuits, feedback is employed 
either to alter the shape of the frequency-response characteristics of an 
amplifier circuit and thereby produce more uniform amplification over a range 
of frequencies, or to produce conditions for oscillation in an oscillator 
circuit. It is also used because it stabilizes the gain of the system against 
changes in temperature, component replacement, and so on. Negative feedback 
also reduces nonlinear distortion. In automatic control systems, feedback is 
used to compare the actual output of a system with a desired output, the 
difference being used as the input signal to a controller. See AMPLIFIER; 
FEEDBACK CONTROL SYSTEM; NEGATIVE-RESISTANCE CIRCUITS; SERVOMECHANISM. 
 �34s[Harold F. Klock]
�33s 
  
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�6v�2v�32sFeedback control system   �21m �33s 
    A system in which the value of some output quantity is controlled by 
feeding back the value of the controlled quantity and using it to manipulate 
an input quantity in such a manner as to bring the value of the controlled 
quantity closer to a desired value. Such a system is also called a closed-loop 
control system. See CONTROL SYSTEMS; FEEDBACK CIRCUIT. 
 �34s[John A. Hrones]
�33s 
  
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�6v�2v�32sFeedforward control system   �21m �33s 
    A system that converts measured changes in a disturbing variable into 
proportional and opposing changes to a manipulated variable, so that the 
effect of the disturbance on the controlled variable will be minimized. In 
order to accomplish this, the feedforward system must embody the same 
steady-state relationships and dynamic characteristics that appear in the 
process being controlled. 
    Consider the controlled variable c to be a linear combination of functions 
of the distrubing variable u and the manipulated variable m, as in Eq. (1), 
where t is time and a and g are steady-state and dynamic gain terms 
respectively. If it is desired that c not respond to u, the necessary 
  
*** dc(t) = (a sub 1) (g sub 1) dm(t) + (a sub 2) (g sub 2) du(t) ***    (1)
  
feedforward compensation can be found by setting Eq. (1) to zero and solving  
for dm(t), which yields Eq. (2). The illustration shows the position of the 

*** dm(t) = - [(a sub 2) (g sub 2)] / [(a sub 1) (g sub 1)] du(t) ***    (2)

feedforward compensation in the control loop. See CONTROL SYSTEMS.  
 �34s[F. G. Shinskey]
�33s 
  
  

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�6v�2v�32sFeedwater   �21m �33s 
    Water supplied to a boiler unit for the generation of steam. Feedwater 
should be virtually free of impurities that are harmful to the boiler and its 
associated system. Generally, natural waters are unsuitable for direct use as 
feedwater because of their contamination by contact with the earth, the 
atmosphere, or other sources of pollution. These contaminants can be removed 
or altered by chemical treatment and other means to provide satisfactory 
feedwater. See BOILER WATER; RAW WATER; WATER TREATMENT. 
 �34s[George W. Kessler]
�33s 
  
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�6v�2v�32sFeldspar   �21m �33s 
    A group of silicate minerals that constitute the most abundant group of 
minerals and are also important economic minerals, particularly in the ceramic 
and glass industries which utilize most of the output as raw materials. 
Feldspars are also used as gemstones if distinguished by an attractive luster 
or color. See SILICATE MINERALS. 
    The feldspars are silicates of aluminum, Al, with the metals potassium, K, 
sodium, Na, and calcium, Ca, and, rarely, barium, Ba. They occur as components 
of all kinds of rocks. Feldspars have no color of their own but are frequently 
colored (yellow, brown, reddish, or dirty green to black) by impurities. The 
cause of the beautiful bright green color of amazonite is unknown. The 
hardness is 6 on Mohs scale. Feldspars are brittle but usually show excellent 
cleavage.  
    Alkali feldspars is the name for feldspars composed of Or and Ab in any 
ratio. Usually some An is present as a minor constituent. There are many names 
for the different varieties of alkali feldspars, most of which include the 
following: sanidine, microcline, orthoclase, albite, analbite, monalbite, 
perthite, antiperthite, and anorthoclase. 
    Plagioclase feldspars form at high temperature a complete series of solid 
solutions, ranging from NaAlSi3O8(Ab) to CaAl2Si2O8(An). The plagioclase 
series is arbitrarily subdivided and named according to increasing An content 
as follows: Ab100An0 - Ab90An10 (albite); Ab90An10 - Ab70An30 (oligoclase); 
Ab70An30 - Ab50An50 (andesine); Ab50An50 - Ab30An70 (labradorite); Ab30An70 - 
Ab10An90 (bytownite); and Ab10An90 - Ab0An100 (anorthite). Because the content 
of silicic acid (in its anhydrous form SiO2) is relatively higher in NaAlSi3O8 
than in CaAl2Si2O8, the Abrich members albite-oligoclase are frequently called 
acid plagioclases and the An-rich members bytownite-anorthite basic 
plagioclases. See ALBITE; ANDESINE; ANORTHITE; BYTOWNITE; LABRADORITE; 
OLIGOCLASE.  
 �34s[Fritz H. Laves]
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�6v�2v�32sFeldspathoid �21m �33s
    A member of the feldspathoid group of minerals. Members of this group are
characterized by the following related features: (1) All are aluminosilicates
with one or more of the large alkali ions (for example, sodium, potassium) or
alkaline-earth ions (for example, calcium, barium). (2) The proportion of
aluminum relative to silicon, both of which are tetrahedrally coordinated by
oxygen, is high. (3) Although the crystal structures of many members are
different, they are all classed as tektosilicates. (4) They occur principally
in igneous rocks, but only in silica-poor rocks, and do not coexist with
quartz (SiO2). Feldspathoids react with silica to yield feldspars, which also
are alkali - alkaline-earth aluminosilicates. Feldspathoids commonly occur
with feldspars. See FELDSPAR; SILICATE MINERALS.
    The principal species of this group are the following:

	Nepheline		KNa3[AlSiO4]4
	Leucite 		K[AlSi2O6]
	Cancrinite		Na6Ca[CO3|(AlSiO4)6] CTR DOT 2H2O
	Sodalite		Na8[Cl2|(AlSiO4)6]
	Nosean			Na8[SO04|(AlSiO4)6]
	Haoeuyne		(Na,Ca)8-4[(SO4)2-1|(AlSiO4)6]
	Lazurite		(Na,Ca)8[(SO4,S,Cl)2|(AlSiO4)6]

    The last four species (sodalite group) are isostructural, and extensive
solid solution occurs between end members; but members of the sodalite group,
cancrinite, leucite, and nepheline have different crystal structures. See
CANCRINITE; LAZURITE; LEUCITE; LEUCITE ROCK; NEPHELINE SYENITE; NEPHELINITE;
SODALITE.
 �34s[Donald R. Peacor]
�33s

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�6v�2v�32sFelsite   �21m �33s 
    An igneous rock with a felsitic or aphanitic (not visibly crystalline) 
texture, composed largely of light-colored (felsic) minerals (quartz and 
feldspar). The constituents may consist of glass, very fine crystalline 
material, or both. See APHANITE; IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sFenestrata   �21m �33s 
    An extinct order of bryozoans in the class Stenolaemata. Fenestrates 
possess small delicate colonies, mostly latticelike or trellislike; the 
loosely grouped zooecia all open out onto one surface of the colony, which 
lacks distinct subdivision into exozone and endozone regions. The zooecia are 
very short, fat, and tubular, and moderately complex, with very thick, solid 
calcareous walls. See BRYOZOA; STENOLAEMATA. 
    Apparently marine, although some species may possibly have ranged into 
brackish water, the Fenestrata (sometimes called Fenestelloidea, and formerly 
included in the Cryptostomata) first appeared late in Ordovician time and 
became extinct in the earliest Triassic. See CRYPTOSTOMATA. 
 �34s[Roger J. Cuffey]
�33s 
  
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�6v�2v�32sFennel �21m �33s
    The culinary spice and the plant Foeniculum vulgare (Umbelliferae), a tall
perennial herb native to the Mediterranean region. It now occurs in all parts
of the world. The fruits are used in cookery, confectionary, and for flavoring
beverages. In modern French and Italian cooking, fennel is indispensable. Oil
of fennel is used in medicine, soaps, and perfumes. See UMBELLALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sFermentation   �21m �33s 
    Decomposition of foodstuffs generally accompanied by the evolution of gas. 
The best-known example is alcoholic fermentation, in which sugar is converted 
into alcohol and carbon dioxide. During fermentation organic matter is 
decomposed in the absence of air (oxygen); hence, there is always an 
accumulation of reduction products, or incomplete oxidation products. Some of 
these products (for example, alcohol and lactic acid) are of importance to 
humans, and fermentation has therefore been used for their manufacture on an 
industrial scale. There are also many microbiological processes that go on in 
the presence of air while yielding incomplete oxidation products. Good 
examples are the formation of acetic acid (vinegar) from alcohol by vinegar 
bacteria, and of citric acid from sugar by certain molds (for example, 
Aspergillis niger). These microbial processes, too, have gained industrial 
importance, and are often referred to as fermentations, even though they do 
not conform to Pasteur's concept of fermentation as a decomposition in the 
absence of air. See INDUSTRIAL MICROBIOLOGY. 
 �34s[Cornelis B. Van Niel]
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�6v�2v�32sFermi-Dirac statistics �21m �33s
    The statistical description of particles or systems of particles that
satisfy the Pauli exclusion principle. This description was first given by E.
Fermi, who applied the Pauli exclusion principle to the translational energy
levels of a system of electrons. It was later shown by P. A. M. Dirac that
this form of statistics is also obtained when the total wave function of the
system is antisymmetrical. See EXCLUSION PRINCIPLE; NONRELATIVISTIC QUANTUM
THEORY.
    Such a system is described by a set of occupation numbers {nsubi} which
specify the number of particles in energy levels epsilonsubi. It is important
to keep in mind that epsilon(sub i) represents a finite range of energies, which
in general contains a number, say gsubi, of nondegenerate quantum states. In
the Fermi statistics, at most one particle is allowed in a nondegenerate
state. (If spin is taken into account, two particles may be contained in such
a state.) This is simply a restatement of the Pauli exclusion principle, and
means that n(sub i) < or = g(sub i).  The probability of having a set {n(sub i)}
distributed over the levels epsilon(sub i), which contain g(sub i) nondegenerate
levels, is

W = PI PRODUCT (in terms of i)g(sub i)!/(g[sub i] - n[sub i])!(n[sub i])!   (1)

described by Eq. (1), which gives just the number of ways that n(sub i) can be
picked out of g(sub i), which is intuitively what one expects for such a
probability. The equilibrium state which actually exists is the set of ns that
makes W a maximum, under the auxiliary conditions given in Eqs. (2a) and (2b).

		      SUMMATION n(sub i) = N				  (2a)

	     SUMMATION n(sub i) epsilon(sub i) = E			  (2b)

These conditions express the fact that the total energy E and the total number
of particles N are given.  Equation (3) holds for this most probable
distribution.  Here A and beta are parameters, to be determined from Eq.  (3);
in fact, beta = 1/kT, where k is Boltzmann's constant and T is the absolute
temperature.  When the 1 in the denominator may be neglected, Eq.  (3) goes over
into the Boltzmann distribution.

    Classical conditions pertain when the volume per particle is much larger
than the volume associated with the de Broglie wavelength lambda of a particle.
For electrons in a metal at 300 K, the ratio of the volume per particle to
lambda(sup 3) has the value 10(sup -4), showing that classical statistics fail
altogether.  When the classical distribution fails, a degenerate Fermi
distribution results.  A somewhat lengthy calculation yields the result that in
this case the contribution of the electrons to the specific heat is negligible.
This resolves an old paradox, for, according to the classical equipartition law,
the electronic specific heat C should be (3/2)Nk, whereas in reality it is very
small.	See BOSE-EINSTEIN STATISTICS; KINETIC THEORY OF MATTER; QUANTUM
STATISTICS; STATISTICAL MECHANICS.

 �34s[Max Dresden]
�33s

   n(sub i) = g(sub i)/[(1/A)e(sup (beta epsilon[sub i]) + 1]			(3)


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�6v�2v�32sFermi surface   �21m �33s 
    A surface of constant energy in a space defined by the components of the 
wave vectors of a system of half-integral spin particles. This is not a real 
surface, but is a geometrical description of the dynamical behavior of 
conduction electrons in solids. Its name comes from the fact that 
half-integral spin particles obey Fermi-Dirac statistics and at the zero of 
temperature T fill energy levels up to a maximum energy, the Fermi energy, and 
no energy levels are occupied above this energy. The wave vectors of electrons 
having the Fermi energy at T = 0 define the Fermi surface. See FERMI-DIRAC 
STATISTICS.  
    Fermi surfaces are used in the discussion of the electrical conduction 
properties of solids. As a first approximation to consider, assume that the 
electrons responsible for electrical conduction in a solid constitute a 
free-electron gas. In this approximation the energy of the conduction 
electrons is entirely kinetic, with vanishingly small potential interactions 
with either the atomic electrons or other conduction electrons. The only 
interaction is the fact that they obey Fermi-Dirac statistics. Thus, the 
energy of electrons at the Fermi energy is given by the equation below, where 
  
   *** E (sub F) = [PLANCK'S CONSTANT (sup 2) k (sub F) (sup 2)] / 2m *** 
  
k sub F is the wave vector of electrons of this energy and m is the electronic 
mass. At T = 0 all states with k less than k sub F are filled, and those with 
k greater than k sub F are empty. This situation can be represented 
geometrically by a sphere of radius k sub F. All states inside the sphere are 
filled, and states outside are empty. See FREE-ELECTRON THEORY OF METALS. 
    Because of the Pauli exclusion principle, only a small number of electrons 
having energies near the Fermi energy can contribute to conduction processes, 
and this makes the details of the properties of the Fermi surface of prime 
importance to understanding electrical conduction in solids. Two phenomena not 
included in this free-electron approximation cause real metals to have Fermi 
surfaces which are geometrical shapes other than spheres: conduction electrons 
experience potential interactions with the atoms in the crystal, and they 
experience Bragg reflections at the boundaries of the Brillouin zone. See BAND 
THEORY OF SOLIDS; BRILLOUIN ZONE. 
    Other techniques for experimentally determining the dimensions and other 
detailed properties of Fermi surfaces of metals which involve the application 
of a magnetic field include the de Haas - van Alphen effect, the 
radio-frequency size effect, cyclotron resonance, acoustical geometrical 
resonance, and magnetoresistance measurements. See CYCLOTRON RESONANCE 
EXPERIMENTS; DE HAAS - VAN ALPHEN EFFECT; MAGNETORESISTANCE; ULTRASONICS. 
 �34s[R. G. Goodrich]
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�6v�2v�32sFermium �21m �33s
    A chemical element, Fm, atomic number 100, the eleventh element in the
actinide series. Fermium does not occur in nature; its discovery and
production have been accomplished by artificial nuclear transmutation of
lighter elements. Radioactive isotopes of mass number 244 - 259 have been
discovered. The total weight of fermium which has been synthesized is much
less than one-millionth of a gram. See ACTINIDE ELEMENTS; RADIOACTIVITY.
    Spontaneous fission is the major mode of decay for (sup 244)Fm, (sup 256)Fm,
and (sup 258)Fm. The longest-lived isotope is (sup 257)Fm, which has a half-life
of about 100 days. Fermium-258 decays by spontaneous fission with a half-life of
0.38 millisecond. This suggests the existence of an abnormality at this point
in the nuclear periodic table. See NUCLEAR CHEMISTRY; NUCLEAR REACTION;
TRANSURANIUM ELEMENTS.
 �34s[Glenn T. Seaborg]
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�6v�2v�32sFerret   �21m �33s 
    The name for the largest member of the weasel family, Mustelidae. This 
carnivorous animal, also known as the black-footed ferret (Mustela nigripes), 
is an inhabitant of the western states in the Rocky Mountain area, where it is 
referred to as the prairie dog ferret or prairie dog hunter. Though once 
abundant, ferrets are now very rare; they suffered from the poisoning 
campaigns against prairie dogs and larger carnivores. 
    Two litters are born each year after a gestation period of about 60 days, 
with 5 - 10 young in each litter. The maximum life is 13 years. See CARNIVORA; 
WEASEL.  
 �34s[Charles B. Curtin]
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�6v�2v�32sFerrimagnetism   �21m �33s 
    A specific type of ordering in a system of magnetic moments or the 
magnetic behavior resulting from such order. In some magnetic materials the 
magnetic ions in a crystal unit cell may differ in their magnetic properties. 
This is clearly so when some of the ions are of different species. It is also 
true for similar ions occupying crystallographically inequivalent sites. Such 
ions differ in their interactions with other ions, because the dominant 
exchange interaction is mediated by the neighboring nonmagnetic ions. They 
also experience different crystal electric fields, and these affect the 
magnetic anisotropy of the ion. A collection of all the magnetic sites in a 
crystal with identical behavior is referred to as a magnetic sublattice. A 
material is said to exhibit ferrimagnetic order when, first, all moments on a 
given sublattice point in a single direction and, second, the resultant 
moments of the sublattices lie parallel or antiparallel to one another. The 
notion of such an order is due to L. Neel, who showed in 1948 that its 
existence would explain many of the properties of the magnetic ferrites. See 
FERRITE; FERROMAGNETISM. 
    In general, there is a net moment, the algebraic sum of the sublattice 
moments, just as for a normal ferromagnet. However, its variation with 
temperature rarely exhibits the very simple behavior of the normal 
ferromagnet. For example, in some materials, as the temperature is raised over 
a certain range, the magnetization may first decrease to zero and then 
increase again. Ferrimagnets can be expected, in their bulk properties, 
measured statically or at low frequencies, to resemble ferromagnets with 
unusual temperature characteristics. See CURIE TEMPERATURE. 
 �34s[Laurence R. Walker]
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�6v�2v�32sFerrite   �21m �33s 
    Any of the class of magnetic oxides. Typically the ferrites have a crystal 
structure which has more than one type of site for the cations. Usually the 
magnetic moments of the metal ions on sites of one type are parallel to each 
other, and antiparallel to the moments on at least one site of another type. 
Thus ferrites exhibit ferrimagnetism. See FERRIMAGNETISM; MAGNETIC MATERIALS. 
    There are three important classes of commercial ferrites. One class has 
the spinel structure, with the general formula Msup2suppFe*EQN*suppO4, where 
Msup2supp is a divalent metal ion. So-called linear ferrites used in inductors 
and transformers are made of Mn and Zn (for frequencies up to 1 MHz) and Ni 
and Zn (for frequencies greater than 1 MHz). MgMn ferrites are used in 
microwave devices such as isolators and circulators. Until the late 1970s, 
ferrites with square loop shapes held a dominant position as computer 
memory-core elements, but these gave way to semiconductors. See COMPUTER 
STORAGE TECHNOLOGY. 
    The second class of commercially important ferrites have the garnet 
structure, with the formula M*EQN*suppFe*EQN*suppO12, where Msup3supp is  a 
rare-earth or yttrium ion. Yttrium-based garnets are used in microwave 
devices. Thin monocrystalline films of complex garnets have been developed for 
bubble domain memory devices. See MAGNETIC BUBBLE MEMORY. 
    The third class of ferrites has a hexagonal structure, of the 
Msup2suppFe*EQN*O19 magnetoplumbite type, where Msup2supp is usually Ba,  Sr, 
or Pb. Because of their large magnetocrystalline anisotropy, the hexagonal 
ferrites develop high coercivity and are an important member of the permanent 
magnet family. 
    Another magnetic oxide, GAMMA-Fe2O3, also has the spinel structure, but 
has no divalent cations. It is the most commonly used material in the 
preparation of magnetic recording tapes. 
    The largest usage of ferrite measured in terms of material weight is in 
the nonlinear B/H range, and is found in the form of deflecting yokes and 
flyback transformers for television receivers. The cores for these devices 
must have high saturation induction Bsubs along with high maximum permeability 
MUsubm at the knee of the B/H curve to frequencies as high as 100 kHz, the 
effective flyback frequency used in scanning a television tube. Again, MnZn 
and NiZn ferrites dominate the use in these devices. See TELEVISION RECEIVER. 
 �34s[Gilbert Y. Chin]
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�6v�2v�32sFerroalloy   �21m �33s 
    A member of an important group of metallic raw materials required for the 
steel industry. Ferroalloys are the principal source of such additions as 
silicon and manganese which are required for even the simplest plain-carbon 
steels; and chromium, vanadium, tungsten, titanium, and molybdenum, which are 
used in both low- and high-alloy steels. Ferroalloys are unique in that they 
are brittle and otherwise unsuited for any service application, but they are 
important as the most economical source of these elements for use in the 
manufacture of the engineering alloys. These same elements can also be 
obtained, at much greater cost in most cases, as essentially pure metals. The 
ferroalloys contain significant amounts of iron and usually have a lower 
melting range than the pure metals and are therefore dissolved by the molten 
steel more readily than the pure metal. In other cases, the other elements in 
the ferroalloy serve to protect the critical element against oxidation during 
solution and thereby give higher recoveries. Ferroalloys are used both as 
deoxidizers and as a specified addition to give particular properties to the 
steel. See STEEL. 
 �34s[Gerhard Derge]
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�6v�2v�32sFerrocene   �21m �33s 
    Dicyclopentadienyl iron, (C5H5)2Fe, an orange crystalline solid with a 
melting point of 174DGC (343DGF). The compound sublimes at 100DGC (212DGF), is 
diamagnetic, and has a dipole moment of zero. The structure of ferrocene, in 
which an atom of iron is sandwiched between two parallel cyclopentadiene 
rings, is shown below. 
    The most outstanding feature of the chemistry of ferrocene is its aromatic 
character. The aromaticity of ferrocene is evidenced by the manner in which it 
behaves toward electrophilic reagents. The molecule undergoes typical 
Friedel-Crafts acylation to yield either the mono- or diacetylated product. 
Ferrocene also undergoes other electrophilic substitution reactions typical of 
a reactive aromatic compound. See AROMATIC; METALLOCENE. 
 �34s[Frank D. Popp]
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�6v�2v�32sFerroelectrics   �21m �33s 
    Crystalline substances which have a permanent spontaneous electric 
polarization (electric dipole moment per cubic centimeter) that can be 
reversed by an electric field. In a sense, ferroelectrics are the electrical 
analog of the ferromagnets, hence the name. The spontaneous polarization is 
the so-called order parameter of the ferroelectric state. The names 
Seignette-electrics or Rochelle-electrics, which are also widely used, are 
derived from the name of the first substance found to have this property, 
Seignette salt or Rochelle salt. See FERROMAGNETISM. 
    From a practical standpoint ferroelectrics can be divided into two 
classes. In ferroelectrics of the first class, spontaneous polarization can 
occur only along one crystal axis; that is, the ferroelectric axis is already 
a unique axis when the material is in the paraelectric phase. Typical 
representatives of this class are Rochelle salt, monobasic potassium 
phosphate, ammonium sulfate, guanidinium aluminum sulfate hexahydrate, glycine 
sulfate, colemanite, and thiourea. 
    In ferroelectrics of the second class, spontaneous polarization can occur 
along several axes that are equivalent in the paraelectric phase. The 
following substances belong to this class: barium (IV) titanate-type (or 
perovskite-type) ferroelectrics; cadmium niobate; lead niobate; certain alums, 
such as methyl ammonium alum; and ammonium cadmium sulfate. 
    From a scientific standpoint, one can distinguish proper ferroelectrics 
and improper ferroelectrics. In proper ferroelectrics, the structure change at 
the Curie temperature can be considered a consequence of the spontaneous 
polarization. In improper ferroelectrics, the spontaneous polarization can be 
considered a by-product of another structural phase transition. Examples of 
such systems are gadolinium molybdate and boracites. 
    The spontaneous polarization can occur in at least two equivalent crystal 
directions; thus, a ferroelectric crystal consists in general of regions of 
homogeneous polarization that differ only in the direction of polarization. 
These regions are called ferroelectric domains. Ferroelectrics of the first 
class consist of domains with parallel and antiparallel polarization, whereas 
ferroelectrics of the second class can assume much more complicated domain 
configurations. The region between two adjacent domains is called a domain 
wall. Within this wall, the spontaneous polarization changes its direction. 
    As a rule, the dielectric constant EPSILON measured along a ferroelectric 
axis increases in the paraelectric phase when the Curie temperature is 
approached. In many ferroelectrics, this increase can be approximated by the 
Curie-Weiss law. See CURIE-WEISS LAW; DIELECTRIC CONSTANT; DIELECTRICS. 
    Ferroelectrics can be divided into two groups according to their 
piezoelectric behavior. The ferroelectrics in the first group are already 
piezoelectric in the unpolarized phase. Those piezoelectric moduli which 
relate stresses to polarization along the ferroelectric axis have essentially 
the same temperature dependence as the dielectric constant along this axis, 
and hence become very large near the Curie point. The spontaneous polarization 
gives rise to a large spontaneous piezoelectric strain which is proportional 
to the spontaneous polarization. 
    The ferroelectrics in the second group are not piezoelectric when they are 
in the paraelectric phase. However, the spontaneous polarization lowers the 
symmetry so that they become piezoelectric in the polarized phase. This 
piezoelectric activity is often hidden because the piezoelectric effects of 
the various domains can cancel. However, strong piezoelectric activity of a 
macroscopic crystal or even of a polycrystalline sample occurs when the 
domains have been aligned by an electric field. The spontaneous strain is 
proportional to the square of the spontaneous polarization. See PIEZOELECTRIC 
CRYSTAL; PIEZOELECTRICITY. 
    Antiferroelectric crystals are characterized by a phase transition from a 
state of lower symmetry (generally low-temperature phase) to a state of higher 
symmetry (generally high-temperature phase). The low-symmetry state can be 
regarded as a slightly distorted high-symmetry state. It has no permanent 
electric polarization, in contrast to ferroelectric crystals. The crystal 
lattice can be regarded as consisting of two interpenetrating sublattices with 
equal but opposite electric polarization. This state is referred to as the 
antipolarized state. In a certain sense, an antiferroelectric crystal is the 
electrical analog of an antiferromagnetic crystal. 
    The piezoelectric effect of ferroelectrics (and certain 
antiferroelectrics) finds numerous applications in electromechanical 
transducers. The large electrooptical effect (birefringence induced by an 
electric field) is used in light modulators. In certain ferroelectrics, light 
can induce changes of the refractive indices. These substances can be used for 
optical information storage and in real-time optical processors. The 
temperature dependence of the spontaneous polarization corresponds to a strong 
pyroelectric effect which can be exploited in thermal and infrared sensors. 
 �34s[Werner Kanzig]
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�6v�2v�32sFerromagnetism �21m �33s
    A property exhibited by certain metals, alloys, and compounds of the
transition (iron group), rare-earth, and actinide elements in which, below a
certain temperature called the Curie temperature, the atomic magnetic moments
tend to line up in a common direction. Ferromagnetism is characterized by the
strong attraction of one magnetized body for another.
    Atomic magnetic moments arise when the electrons of an atom possess a net
magnetic moment as a result of their angular momentum. The combined effect of
the atomic magnetic moments gives rise to a relatively large magnetization, or
magnetic moment per unit volume, for a given applied field. Above the Curie
temperature, a ferromagnetic substance behaves as if it were paramagnetic: Its
susceptibility approaches the Curie-Weiss law. The Curie temperature marks a
transition between order and disorder of the alignment of the atomic magnetic
moments. Some materials exhibit a special form of ferromagnetism below the
Curie temperature called ferrimagnetism. See CURIE TEMPERATURE; CURIE-WEISS
LAW; ELECTRON SPIN; FERRIMAGNETISM; MAGNETIC SUSCEPTIBILITY; PARAMAGNETISM.
    The characteristic property of a ferromagnet is that, below the Curie
temperature, it can possess a spontaneous magnetization in the absence of an
applied magnetic field. Upon application of a weak magnetic field, the
magnetization increases rapidly to a high value called the saturation
magnetization, which is in general a function of temperature. See
MAGNETIZATION.
    Small regions of spontaneous magnetization, formed at temperatures below
the Curie point, are known as domains. As shown in the illustration, domains
originate in order to lower the magnetic energy. In illustration b it is shown
that two domains will reduce the extent of the external magnetic field, since
the magnetic lines of force are shortened. On further subdivision, as in
illustration c, this field is still further reduced.
    An alternate way to describe the energy reduction is to note that the
interior demagnetizing fields, coming from surface poles, are much smaller in
the long, thin domains of illustration c than in the ``fat'' domain of
illustration a.
    The question arises as to how long this subdivision process continues.
With each subdivision there is a decrease in field energy, but there is also
an increase in Heisenberg exchange energy, since more and more magnetic
moments are aligning antiparallel. Finally a state is reached in which further
subdivision would cause a greater increase in exchange energy than it would
cause decrease in field energy, and the ferromagnet will assume this state of
minimum total energy.
    Materials easily magnetized and demagnetized are called ``soft''; these
are used in alternating-current machinery. The ideal cheap soft material would
be an iron alloy fabricated by some inexpensive technique which results in all
crystal grains being oriented in the same or nearly the same direction.
Various complicated rolling and annealing methods have been discovered in the
continued search for better grain-oriented or ``cube-textured'' steels. See
IRON-SILICON ALLOY; MAGNETIC MATERIALS.
    Materials which neither magnetize nor demagnetize easily are called
``hard''; these are used in permanent magnets. The fine-particle precipitates
have dominated the market. These alloys consist basically of small and
elongated magnetic particles embedded in a nonmagnetic matrix.
 �34s[Elihu Abrahams ;Frederic Keffer]
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�6v�2v�32sFerry   �21m �33s 
    A vessel having provision for passengers or vehicles, or both, operating 
on a short run with a frequent schedule between two points over the most 
direct water route and offering a public service of a type normally attributed 
to bridge or tunnel. With a few notable exceptions, passengers on ferry 
vessels are considered to be day passengers, meaning that seating is provided 
similar to a commercial airliner, bus, or passenger train without staterooms 
or overnight sleeping accommodations. Vehicles may include railroad rolling 
stock in addition to passenger automobiles, recreation vehicles, commercial 
trucks and trailers. 
    In the strict sense, ferry vessels are normally confined to crossing 
rivers, canals, lakes, bays, and sounds. However, with the expansion of ferry 
systems worldwide, the specific terms ocean ferry and coastwise ferry have 
been added to ferry vessel designations. Ocean ferries include all ferry 
vessels navigating any ocean or the Gulf of Mexico, the Caribbean Sea, or the 
Gulf of Alaska more than 20 nautical miles (37 km) offshore. Coastwise ferries 
include all ferry vessels normally navigating these same bodies of water 20 
n-mi or less offshore. Ocean and coastwise ferries are generally equipped with 
staterooms for overnight passenger accommodations. 
    In many cases, ferry vessels travel to foreign ports and therefore carry 
the additional classification of short international voyage. This means an 
international voyage in the course of which a vessel is not more than 200 n-mi 
(370 km) from a port or place in which the passengers and crew could be placed 
in safety, and which does not exceed 600 n-mi (1111 km) in length between the 
last port of call in the country in which the voyage begins and the final port 
of destination. 
    On short water routes where ferries shuttle back and forth across a river, 
canal, or other inland waterway, the vessels are generally designed to be 
double-ended. This term describes a vessel with a propeller and rudder at each 
end and a hull form which is identical at each end. 
    On longer water routes, where speed, weather conditions, or certain other 
operational features demand, a more normal hull form is adopted with the 
propellers and rudders at the stern. These vessels proceed in one direction 
and are designated as being single-ended. They must maneuver at the terminals 
in order to properly load and discharge their vehicles. 
 �34s[Philip F. Spaulding]
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�6v�2v�32sFertilizer �21m �33s
    Materials added to the soil, or applied directly to crop foliage, to
supply elements needed for plant nutrition. These materials may be in the form
of solids, semisolids, slurry suspensions, pure liquids, aqueous solutions, or
gases.
    The chemical elements nitrogen, phosphorus, and potassium are the
macronutrients, or primary fertilizer elements, which are required in greatest
quantity. Sulfur, calcium, and magnesium, called secondary elements, are also
necessary to the health and growth of vegetation, but they are required in
lesser amounts compared to the macronutrients. The other elements of agronomic
importance, called micronutrients and provided for plant ingestion in small
(or trace) amounts, include boron, cobalt, copper, iron, manganese,
molybdenum, and zinc. All these fertilizer elements, along with other chemical
elements, occur naturally in agricultural soils in varying concentrations and
mineral compositions which may or may not be in forms readily accessible to
root systems of plants. The addition of fertilizer to soils used for the
production of commercial crops is necessary to correct natural deficiencies
and to replace the components absorbed by the crops in their growth.
    Crop requirements of fertilizer components could be satisfied by the
spreading of individual materials for each element deficient in the soil.
However, economy favors the single application of a balanced mixture that
satisfies all nutritional needs of a crop. Many commercial fertilizers
therefore contain more than one of the primary fertilizer elements.
    The compositions of fertilizer mixtures, in terms of the primary
fertilizer elements, are identified by an N-P-K code: N denotes elemental
nitrogen; P denotes the anhydride of phosphoric acid (P2O5); K denotes the
oxide of potassium (K2O). All are expressed numerically in percentage
composition, or units of 20 lb each per short ton (10 kg per metric ton) of
finished fertilizer as packaged. Formula 8-32-16 thus contains a mixture
aggregating 8 wt % N in some form of nitrogen compounds, 32 wt % P2O5 in some
form of phosphates, and 16 wt % K2O in some form of potassium compounds, to
give a product with a total of 56 fertilizer units. The commercial N-P-K
formulas are generally in whole numbers. None of the N-P-K formulas totals
100% plant nutrients because the formulas indicate only the nutrient portions
of the primaryelement compounds and do not account for any other materials
present.
    Other unidentified materials present in the fertilizer products include
free water and water content of the hydrated primary components, uncredited
cation and anion portions of salts (sodium, silica, chloride, sulfate, and so
on), impurities remaining in the chemical ingredients as processed, and
deliberately added inert materials (called ballast), to make whole-number
formula compositions.
    Growing plants can assimilate only fertilizer elements in the combined
state of inorganic compounds that are amenable to osmotic absorption. Many
modern fertilizer materials consist of compounds that are immediately usable
by the crops to which they are applied. Others are quickly converted within
the soil to forms that can be assimilated. Some fertilizer chemicals are
specifically designed to dissolve slowly or to delay reaction within the soil
and therefore prolong the release of easily absorbed compounds to provide
sustained feeding over the growth cycle of the plants. The relatively few
plants for production of large quantities of these basic materials are usually
located near the mining operations. In the case of nitrogen fertilizer
products, the large production complexes are located near natural gas sources,
which may be wells or major natural gas pipelines.
    Aqueous solutions of urea, ammonia, and ammonium nitrate (UAN solutions)
are used directly by the farmers as well as in the preparation of granular
N-P-K products by mixing with other materials, such as normal superphosphate
and triple superphosphate. UAN solutions are also spread directly by field
application or used to prepare complete N-P-K fertilizer solutions or
suspensions. Suspension fertilizers consist of aqueous slurries of fine
crystals in saturated solutions that are stabilized by small amounts of
gelling materials, such as attapulgite clay. Suspensions can be maintained in
uniform composition during spreading on the fields, and give better dispersion
than granular material. See FERTILIZING.
 �34s[Alan Longacre]
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�6v�2v�32sFertilizing   �21m �33s 
    Addition of elements or other materials to the soil to increase or 
maintain plant yields. Fertilizers may be organic or inorganic. Organic 
fertilizers are usually manures and waste materials which in addition to 
providing small amounts of growth elements also serve as conditioners for the 
soil. Commercial fertilizers are most often inorganic. See FERTILIZER. 
    Methods of applying fertilizers vary widely and depend on such factors as 
kind of crop and stage of growth, application rates, physical and chemical 
properties of the fertilizer, and soil type. Two basic application methods are 
used, bulk spreading and precision placement. Time and labor are saved by the 
practice of bulk spreading, in which the fertilizer is broadcast over the 
entire area by using large machines which cover many acres in a short time. 
Precision placement, in which the fertilizer is applied in one or more bands 
in a definite relationship to the seed or plants, requires more equipment and 
time, but usually smaller amounts of fertilizer are needed to produce a given 
yield increase. 
    For some deep-rooted plants, subsoil fertilization to depths of 12 - 20 
in. (30 - 50 cm) is advantageous. This is usually a separate operation from 
planting, and uses a modified subsoil plow followed by equipment to bed soil 
over the plow furrow, thereby eliminating rough soil conditions unfavorable 
for good seed germination. Top-dressings are usually applied by broadcasting 
over the soil surface for closely spaced crops such as small grains. 
    Since solid fertilizers range from dense heavy materials to light powders 
and liquid fertilizers range from high pressure to zero pressure, a variety of 
equipment is required for accurate metering and placement. In addition, 
application rates may be as low as 50 lb/acre (56 kg/hectare) or as high as 6 
tons/acre (13.3 metric tons/hectare). Large bulk spreaders usually use drag 
chains or augers to force the material through a gate or opening whose size is 
varied to regulate the amount passing through and falling on the spreader. 
    Liquid fertilizer of the high-pressure type (for example, anhydrous 
ammonia) is usually regulated by valves or positive displacement pumps. The 
size of the orifice may be controlled manually or automatically by 
pressure-regulating valves. Low-pressure solutions may be metered by gravity 
flow through orifices, but greater accuracy is obtained by using compressed 
air or other gases to maintain a constant pressure in the tank. This method 
eliminates the effect of temperature and volume changes. Nonpressure solutions 
may be metered by gravity or by gear, roller, piston, centrifugal, or hose 
pumps. The accuracy of the gravity type can be improved by the use of a 
constant head device by which all air is introduced into the tank at the 
bottom.  
    Nonvolatile fertilizer solutions are often pumped into the supply lines of 
irrigation systems to allow simultaneous fertilization and irrigation. With 
the exception of bulk spreaders and other broadcasters, most fertilizer 
application devices are built as attachments which can be mounted in 
conjunction with planters, cultivators, and herbicide applicators. Often the 
tanks, pumps, and controls used for liquid fertilizers are also used for 
applying other chemicals such as insecticides. 
 �34s[J. G. Futral]
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�6v�2v�32sFescue   �21m �33s 
    A group of grasses, of which the most important species in agricultural 
use is tall fescue (Festuca arundinacea). Other important fescues are Meadow 
fescue (F. elatior), red fescue (F. rubra), Idaho fescue (F. idahoensis), 
sheep fescue (F. ovina), and hard fescue (F. diriuscula). 
 �34s[Howard B. Sprague]
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�6v�2v�32sFetal alcohol syndrome   �21m �33s 
    A spectrum of changes in the offspring of women who consume alcoholic 
beverages during pregnancy, ranging from severe growth deficiency, mental 
retardation, and abnormal facial features to mild mental changes. Physical 
abnormalities include limitation of joint motion, increased congenital heart 
malformation rate, short palpebral fissures, and maxillary hypoplasia with 
relative prognathism. 
    The frequency of adverse outcome of pregnancies of chronic alcoholic women 
is very high. There is an eightfold increase in perinatal mortality, and the 
frequency of the fetal alcohol syndrome in the surviving children is common. 
Almost half of the offspring have IQs below 80, and a smaller percentage have 
the dysmorphic features noted above. 
    Medical advice regarding the consumption of alcoholic beverages during 
pregnancy involves three principles: heavy drinking is clearly deleterious to 
the fetus; abstinence from alcoholic beverages immediately preceding and 
during pregnancy is the only assured safe course of action presently known; 
and therapeutic attention should also be directed to the problems leading to 
the alcoholism so that the psychological environment of the offspring may be 
improved. See ALCOHOLISM; BEHAVIORAL TOXICOLOGY. 
 �34s[N. Karle Mottet]
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�6v�2v�32sFetal membrane   �21m �33s 
    One of the membranous structures which surround the embryo during its 
developmental period. Since such membranes are external to the embryo proper, 
they are called extraembryonic membranes. They function in the embryo's 
protection, nutrition, respiration, and excretion. 
    There are four fetal membranes - the amnion, chorion, yolk sac, and 
allantois. In the course of development, the chorion becomes the outermost, 
and the amnion the innermost, membrane surrounding the developing embryo. As 
the allantois increases in size, it expands and becomes closely associated, if 
not fused, with the chorion. The two membranes together are known as the 
chorioallantoic membrane. 
    The amniotic cavity within which the embryo is enclosed becomes filled 
with an aqueous fluid which gives osmotic and physical protection to the 
embryo during the remainder of its fetal existence. Smooth muscle fibers in 
the amnion spontaneously contract and gently rock the embryo before it 
develops the capacity for spontaneous movement. 
    As the stored nutrients of the yolk are depleted during development, the 
yolk sac gradually decreases in size and is eventually incorporated into the 
mid-gut of the embryo. The yolk sac in the nonyolky eggs of placental mammals 
is vestigial. It has evolutionary but essentially no functional significance. 
    At the time of birth or hatching, the embryo becomes completely separated 
from the amnion and chorion and from the major portion of the allantois. The 
proximal portion of the latter remains within the embryo, however, as the 
urinary bladder. See ALLANTOIS; AMNION; CHORION; YOLK SAC. 
 �34s[Anna Ruth Brummett]
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�6v�2v�32sFever   �21m �33s 
    An elevation in the central body temperature of warm-blooded animals 
caused by abnormal functioning of the thermoregulatory mechanisms. Fever 
accompanies a wide variety of disease states, both infectious and 
noninfectious, and in the great majority of instances is due to an abnormality 
in the regulation of body temperature by the central nervous system. See 
THERMOREGULATION.  
    Experimental studies on the cause of fever suggest that leukocytes, as 
well as the fixed macrophages can be activated by various stimuli to produce a 
fever-inducing substance - endogenous pyrogen (EP). Endogenous pyrogen has 
been characterized as a protein of relatively low molecular weight (13,000) 
with an essential lipid moiety. 
    The mechanisms responsible for elevating body temperature include: 
reduction in heat loss by constriction of peripheral vessels whose tone is 
under control of the sympathetic nervous system; inhibition of panting and 
sweating, the latter by way of the cholinergic nerves; and increased heat 
production by means of shivering in voluntary muscles innervated by somatic 
motor nerves. 
    There is no clear evidence that elevated body temperature evoked by most 
infections is directly injurious to microbial invaders. As fever regularly 
accompanies inflammation, however, increased body temperature may well 
accelerate certain biochemical reactions of use to the host in combating 
infection. See HOMEOSTASIS; INFLAMMATION. 
 �34s[Elisha Atkins]
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�6v�2v�32sFeynman integral   �21m �33s 
    A technique, also called the sum over histories, which is basic to 
understanding and analyzing the dynamics of quantum systems. It is named after 
fundamental work of Richard Feynman. The crucial formula gives the quantum 
probability density for transition from a point q sub 0 to a point q sub 1 in 
time t as the expression below, where S(path) is the classical mechanical 

       *** INTEGRAL exp {[iS(path)] / PLANCK'S CONSTANT} d(path) ***

action of a trial path, and PLANCK'S CONSTANT is the rationalized Planck's 
constant. The integral is a formal one over the infinite-dimensional space 
of all paths which go from q sub 0 to q sub 1 in time t. Feynman defines it by 
a limiting procedure using approximation by piecewise linear paths. See 
NONRELATIVISTIC QUANTUM THEORY.  
    Feynman integral ideas are especially important in quantum field theory, 
where they not only are a useful device in analyzing perturbation series but 
are also one of the few nonperturbative tools available. See QUANTUM FIELD 
THEORY.  
    An especially attractive element of the Feynman integral formulation of 
quantum dynamics is the classical limit, PLANCK'S CONSTANT -> 0. Formal 
application of the method of stationary phase to the above expression says 
that the significant paths for small PLANCK'S CONSTANT will be the paths of 
stationary action. One thereby recovers classical mechanics in the hamiltonian 
stationary action formulation. See LEAST-ACTION PRINCIPLE. 
 �34s[Barry Simon]
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�6v�2v�32sFiber bundle �21m �33s
    A type of structure arising frequently in various branches of mathematics,
especially differential geometry, Lie groups, and algebraic geometry. A fiber
bundle is a principal tool for the applications of algebraic topology to these
subjects.
    A fiber bundle is a topological space B which is decomposed in a smooth
fashion into a family of closed disjoint subsets, called fibers, each of which
is homeomorphic to a fixed space Y. The base space X of the bundle is the
space whose points are the fibers. The projection p:B -> X is the function
assigning to each point the fiber which contains it, and X is topologized so
that p is continuous. The requirement of smoothness reads as follows: For each
x EPSILON X, there is a neighborhood V of x, and a homeomorphism SIGMA PHI:V x
Y -> p- sup 1(V) satisfying pSIGMA PHI(x',y) = x' for all x' EPSILON V and y
EPSILON Y. See TOPOLOGY.
    The product space X x Y and its projection into X satisfy these conditions
with each V = X. The Moebius band (see illustration) is one of the simplest
examples of a bundle which is not a product. The fibers are the line segments
perpendicular to its center line, and its base space is a circle. The
corresponding product space is a cylindrical surface. Because of such
examples, fiber bundles are sometimes called twisted products.
 �34s[Norman E. Steenrod]
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�6v�2v�32sFibonacci numbers �21m �33s
    A series of numbers defined by the elementary recursion f(sub n+2) = f(sub
n) + f(sub n+1); f(sub 0) = 0, f(sub 1) = 1.  Each f(sub n) counts, for
example, the number of sequences of n - 2 zeros and ones with no consecutive
zeros.	The generating function f(x) of the Fibonacci numbers satisfies the
identify (1 - x - x[sup 2])f(x) = x, from which the closed form f(sub n) =
[tau(sup n) - (1 - tau)(sup n)]/[SQUARE ROOT (5)], where TAU = [1 + SQUARE ROOT
(5)]/2, may readily be deduced.  See COMBINATORIAL THEORY; ENUMERATION OF
ARRANGEMENTS. 

 �34s[T.  Balderes]
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�6v�2v�32sFibrinogen   �21m �33s 
    A plasma protein synthesized by the parenchymal cells of the liver. 
Fibrinogen (factor I) is the precursor of fibrin, the insoluble fibrous 
protein that polymerizes to form the clot during blood coagulation. Congenital 
or acquired deficiencies of circulating fibrinogen may be associated with 
severe symptoms of a bleeding tendency. See HEMOPHILIA; HEMORRHAGE; PLASMIN. 
 �34s[Oscar D. Ratnoff]
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�6v�2v�32sFibrous protein   �21m �33s 
    One of the two main classes of protein molecules, the other being termed 
globular proteins. Examples of fibrous proteins are keratin in hair, wool, 
horn, and feathers; myosin in muscle; fibrinogen, a plasma protein that forms 
fibrin clots; collagen in tendon, cartilage, and fish scales; fibroin in silk; 
and elastin, the structural element of elastic fibers. These proteins are 
predominantly arranged as long fibers along a single axis. Alignment between 
fibers is accomplished by hydrogen bonding such that the polypeptide backbone 
of the protein assumes a ``pleated'' conformation. See GLOBULAR PROTEIN. 
    In contrast to globular proteins, which do not usually contain covalent 
bonds between molecules, the fibrous proteins can form a network through a 
small number of covalent bonds between each fiber. Thus, these tightly linked 
molecules are generally insoluble in aqueous solvents and are highly resistant 
to digestion by proteolytic enzymes. See COLLAGEN; GELATIN; PROTEIN. 
 �34s[James M. Manning]
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�6v�2v�32sFidelity   �21m �33s 
    The degree to which the output of a system accurately reproduces the 
essential characteristics of its input signal. Thus, high fidelity in a sound 
system means that the reproduced sound is virtually indistinguishable from 
that picked up by the microphones in the recording or broadcasting studio. 
Similarly, a televison system has a high fidelity when the picture seen on the 
screen of a receiver corresponds in essential respects to that picked up by 
the television camera. Fidelity is achieved by designing each part of a system 
to have minimum distortion, so that the waveform of the signal is unchanged as 
it travels through the system. See DISTORTION (ELECTRONIC CIRCUITS); 
SOUND-REPRODUCING SYSTEMS. 
 �34s[John Markus]
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�6v�2v�32sField emission   �21m �33s 
    The emission of electrons from a metal or semiconductor into vacuum (or a 
dielectric) under the influence of a strong electric field. In field emission, 
electrons tunnel through a potential barrier, rather than escaping over it as 
in thermionic or photoemission. The effect is purely quantum-mechanical, with 
no classical analog. It occurs because the wave function of an electron does 
not vanish at the classical turning point, but decays exponentially into the 
barrier (where the electron's total energy is less than the potential energy). 
Thus there is a finite probability that the electron will be found on the 
outside of the barrier. See NONRELATIVISTIC QUANTUM THEORY; PHOTOEMISSION; 
THERMIONIC EMISSION. 
    For a metal at low temperature, the process can be understood in terms of 
the illustration. The metal can be considered a potential box, filled with 
electrons to the Fermi level, which lies below the vacuum level by several 
electronvolts. The distance from Fermi to vacuum level is called the work 
function, PHI. The vacuum level represents the potential energy of an electron 
at rest outside the metal, in the absence of an external field. In the 
presence of a strong field, the potential outside the metal will be deformed 
along the line AB, so that a triangular barrier is formed, through which 
electrons can tunnel. Most of the emission will occur from the vicinity of the 
Fermi level where the barrier is thinnest. See ELECTRON EMISSION; 
FIELD-EMISSION MICROSCOPY. 
 �34s[Robert Gomer]
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�6v�2v�32sField-emission microscopy   �21m �33s 
    A technique that uses field emission of electrons or positive ions to 
produce a magnified image of the emitter surface on a fluorescent screen. The 
field-emission (or field-ion) microscope is the most powerful microscope 
known. The tip of the needle-shaped emitter shown in Fig. 1 is projected 
directly without lenses, with a magnification up to 1,000,000 diameters and a 
resolution, with electrons as the emitted particles, of 2 nanometers. With 
this microscope, adsorption of various gases has been studied in the 
temperature range from 4 to above 2000 K (-452 to above 3140DGF). See FIELD 
EMISSION.  
    The condensation and surface migration of evaporated materials and surface 
reactions important in catalysis have been investigated. Mediumsized 
individual molecules such as phthalocyanin have been made visible. 
  
  
    Field-ion microscope.  Rather than by electrons, the emitter tip surface 
can be imaged by positive ions. In the field-ion microscope (FIM), the tip is 
kept at a high positive potential, while the microscope vessel is backfilled 
with about 1 millitorr (0.1 pascal) of helium, neon, or hydrogen. Field 
ionization occurs when the electron tunnels out of an image gas molecule in 
the region of exceedingly high field strength above each protruding surface 
atom. When the tip is cooled by heat conduction through its leads, preferably 
by using liquid hydrogen or liquid helium, the reduced random velocity of the 
ions, combined with their short de Broglie wavelength, assures a resolution 
approaching 0.2 nm. Thus the atoms constituting the specimen surface can be 
seen individually, a feat not possible with any other type of microscope 
presently known. The FIM is used as a research tool for the study of crystal 
lattice defects in metals. Figure 2 shows the atomic structure of an iridium 
crystal surface. 
  
  
    Atom-probe FIM.  A modification is the atom-probe FIM. The tip of this FIM 
can be tilted to place an interesting image spot onto a small hole in the 
screen. When a high-voltage pulse has ripped off the selected atom, it travels 
through the probe hole into a time-of-flight mass spectrometer of single 
particle sensitivity, where it is chemically identified by its atomic mass. 
Thus the atom-probe FIM represents the ultimate in microanalysis, requiring 
only one atom selected at the discretion of the operator. See CRYSTAL. 
 �34s[Erwin W. Mueller]
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�6v�2v�32sField theory (mathematics) �21m �33s
    In algebra, the term field is used to designate an algebraic system or
structure containing at least two elements and having two binary rules of
composition:  addition and multiplication (that is, if a and b are any two
elements of the field, then a + b and ab are defined and are elements of the
field).  The structure rules are as follows:  The elements form an abelian
(commutative) group under addition with the additive identity denoted by 0; that
is, a + 0 = a for all elements a.  The set of nonzero elements (and there are
some since the field has at least two elements) form an abelian group under
multiplication with the multiplicative identity denoted by 1.  It follows that
all nonzero elements have a multiplicative inverse or reciprocal.  The two rules
of composition are related by the distributive law:  (a + b)c = ac + bc for all
elements a, b, c.  It follows from the distributive law that a x 0 = 0 for all
elements a, since 1 x a = (1 + 0)a = 1 x a + 0 x a, and consequently 0 = 0 x a.
See GROUP THEORY.

    The most familiar example of a field is the set BOLD Q of all rational
numbers (0, 1, 2, 1/2, -1, -2, -1/2, 3, 1/3, 2/3, .  .	.) with the usual rules
for equality, addition, and multiplication.  Other examples are the set BOLD R
of all real numbers and the set BOLD C of all complex numbers (both with the
usual rules of addition and multiplication).  The set of rational functions
(formal quotients of two polynomials with rational coefficients) also
constitutes a field under the usual rules for equality, addition, and
multiplication.  See ALGEBRA; COMPLEX NUMBERS; REAL VARIABLE.

    There are fields which contain only finitely many elements. The simplest
example is obtained by considering the residue class ring of the integers modulo
a prime p.  This field, which may be denoted by BOLD F(sub p), has as its
elements the p distinct arithmetic progressions or residue classes modulo the
prime p, namely I(sub 0), I(sub 1), .  .  ., I(sub p-1), where I(sub j) consists
of the integers j, j PLUS/MINUS p, j PLUS/MINUS 2p, .  .  .  .	The rules of
composition are given by Eqs.  (1) and (2).

	I(sub j) + I(sub k) = I(sub s) if j + k lies in I(sub s)	    (1)

	I(sub j) I(sub k) = I(sub t) if jk lies in I(sub t)		    (2)

    If F is a field, and a subset A of F is a field under the rules of
composition of F, then A is said to be a subfield of F, and F is said to be an
extension of A.  Thus the field of rational numbers BOLD Q is a subfield of the
field of real numbers BOLD R, and BOLD R is an extension of BOLD Q.  If a field
F is an extension of the field A, then F is a vector space over A.  If, as a
vector space, F is of finite dimension n over A, then F is said to be an
extension of A of degree n.  See LINEAR ALGEBRA.

    If a field F is such that every polynomial equation over F of positive
degree has a solution in F, then F is said to be algebraically closed.	The
field of all complex numbers is algebraically closed; but this is a theorem in
analysis.  The field of rational numbers, the fields of residue classes modulo a
prime p, and the field of real numbers are not algebraically closed.  Given any
field F, there exists a unique (to within isomorphism) minimal extension F of F
such that F is algebraically closed.

 �34s[D.  J.  Lewis]
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�6v�2v�32sFievre boutonneuse   �21m �33s 
    A generally mild febrile disease of humans due to Rickettsia conori. This 
condition is also called boutonneuse or Marseille fever. While the disease 
resembles Rocky Mountain spotted fever in some respects (for example, the 
rash), it differs in exhibiting a characteristic tache noire, or primary 
ulcer, at site of tick attachment, and swollen lymph glands which drain the 
area; furthermore, it does not exhibit localities of high virulence. The 
generalized rash has a similar maculopapular appearance and includes the palms 
and soles, but the case fatality rate is less than 3%. 
    The disease occurs in parts of the Old World, particularly the 
Mediterranean littoral, the Crimea, and parts of India. The primary cycle in 
most areas involves domestic dogs and the brown dog tick (Rhipicephalus 
sanguineus), which invades premises inhabited by their hosts. See 
RICKETTSIOSES; ROCKY MOUNTAIN SPOTTED FEVER. 
 �34s[Cornelius B. Philip]
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�6v�2v�32sFig   �21m �33s 
    A deciduous tree, Ficus carica, with a milky juice growing in subtropical 
regions of Asia, Africa, Europe, and the United States. The fruit consists of 
a fleshy hollow receptacle lined with pistillate, or female, flowers and with 
a small opening at the end. Most edible types and varieties develop fruit 
without pollination. In the United States figs are grown primarily in 
California with minor production in Arizona, Texas, and Florida. 
 �34s[Charles A. Schroeder]
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�6v�2v�32sFilariasis   �21m �33s 
    A disease caused by Filarioidea in humans or lower animals. More loosely, 
the term is used to indicate mere infection by such organisms. In human 
medicine, filariasis commonly refers to the disease caused by, or infection 
with, one of the mosquito-borne, elephantoid-producing Filarioidea - most 
frequently Wuchereria bancrofti, less frequently Brugia malayi, and more 
recently including B. timori. 
    Wuchereria bancrofti has worldwide distribution in the tropics and extends 
into both north and south temperate zones. However, the very high infection 
rates and the high prevalence of the resulting grotesque elephantiasis are 
localized at sea-level areas with year-round uncontrolled high mosquito 
densities in the tropics. Introduction into the temperate zones commonly 
results in a few secondary cases before the infection dies out. 
    In most areas of the world, the microfilariae of W. bancrofti have a 
conspicuous nocturnal periodicity. There are 50 to 150 times as many 
microfilariae in the midnight blood as in noon blood. Often none may be found 
in midday blood. This periodic variety of W. bancrofti is transmitted by 
domestic, anthropophilous, night-biting mosquitos, which readily enter houses 
to feed. Culex fatigans (C. quinquefasciatus) and some of the important 
malaria vectors such as Anopheles darlingi, A. funestus, and A. farauti are 
important vectors. See FILARIOIDEA. 
 �34s[Gilbert F. Otto]
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�6v�2v�32sFilarioidea   �21m �33s 
    A superfamily in the phylum Nematoda. These are highly specialized 
parasites of most groups of vertebrates. They are particularly common in 
amphibians, birds, and mammals. While they cannot be classified as completely 
harmless, most of the many hundreds of known species are not associated with 
any recognized disease. A limited number of species produce serious diseases 
in humans and a few others produce serious diseases in domestic or wild 
animals. The filarial parasites of humans are found almost exclusively in the 
tropics, with some extension into the subtropics. This is by no means the 
universal pattern. Heartworm disease in dogs has become established in the 
north temperate zone in United States and Canada and in the south temperate 
zone in Australia. See HEARTWORMS; NEMATA. 
    Among the Filarioidea there are no conspicuous divisions into distinct 
body regions. True lips are absent, and the mouth cavity is greatly reduced. 
The cuticule is devoid of prominent ornaments found on many nematodes. Sexual 
dimorphism is the rule; in common with other nematodes, the female filarioid 
is at least twice as long as the male. 
    Microfilariae, the precociously active embryos, are devoid of any 
alimentary canal. These are found in the blood of the host and may continue to 
circulate for months to years depending on the species and the level of host 
immune response. All the known filariae require a blood-sucking arthropod 
intermediate host, usually an insect and commonly a dipteron, in which to 
complete embryonation. The microfilariae are ingested as the arthropod feeds. 
After embryonation is completed, the resulting infective larvae gain entrance 
into the definitive vertebrate in association with the next feeding of the 
arthropod. See FILARIASIS; LOA LOA; ONCHOCERCIASIS. 
 �34s[Gilbert F. Otto]
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�6v�2v�32sFilbert   �21m �33s 
    A nut from any plant in the genus Corylus (Betulaceae); also called 
hazelnut. About 15 species are recognized, distributed widely over the north 
temperate zone and ranging in size from shrubs to tall trees. Filberts in 
commerce are derived mostly from the European species C. avellana and C. 
maxima. Hybrids of these with the American species, C. americana, are grown 
sparingly in the eastern United States. 
    Filberts are grown widely in Europe and the Mediterranean basin, where 
much of the crop is consumed locally. Imports to the United States come mostly 
from Turkey. In the United States filberts are grown in Oregon and Washington. 
Filberts are sold in shell, or the kernels are roasted and used in 
confectionery and baked goods. 
 �34s[Laurence H. MacDaniels]
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�6v�2v�32sFiller   �21m �33s 
    A coating material used to smooth and fill porous wood surfaces before 
finishing coats are applied. Fillers normally are colorless materials that 
harden rapidly with minimum shrinkage. A typical formulation includes silica 
or another hard pigment dispersed in fast-drying varnish. 
    Materials specially formulated to fill crevices before surfaces such as 
wallboard are painted are sometimes referred to as fillers, as are surfacers 
used to smooth dents or irregularities in metals before finishing. The term is 
sometimes used to describe transparent extender pigments in paint 
formulations. See PAINT; SURFACE COATING; WOOD FINISHING. 
 �34s[C. R. Martinson ;C. W. Sisler]
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�6v�2v�32sFilm badge   �21m �33s 
    A device worn for the purpose of indicating the absorbed dose of ionizing 
radiation received by the wearer. A film badge (or badge meter, as it is 
commonly called) usually is made of metal, plastic, or paper and is loaded 
with one or more film packets (ordinarily standard dental film packets). It is 
attached to the clothing by means of a clip or pin and is frequently combined 
with an identification badge. In some cases, it is made into a film ring or a 
film bracelet to indicate specifically the absorbed dose received by the hands 
or forearms. The simplest type of film badge consists of a small paper 
envelope containing a dental film, one-half of which is surrounded by a thin 
lead foil. 
    The badge must contain one or more filters (usually made of cadmium, lead, 
silver, aluminum, or copper) so that a comparison can be made of the relative 
blackening of the developed film from behind the various filters. This 
comparison reveals the extent of exposure to various types of radiation. See 
DOSIMETER; MONITORING OF IONIZING RADIATION. 
 �34s[Karl Z. Morgan]
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�6v�2v�32sFilosia   �21m �33s 
    A subclass of Rhizopodea characterized by slender filopodia which rarely 
anastomose. The two orders are Aconchulinida and Gromiida. See ACONCHULINIDA; 
GROMIIDA; PROTOZOA; RHIZOPODEA. 
 �34s[Richard P. Hall]
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�6v�2v�32sFiltration   �21m �33s 
    The separation of solid particles from a fluid-solids suspension of which 
they are a part by passage of most of the fluid through a septum or membrane 
that retains most of the solids on or within itself. The septum is called a 
filter medium, and the equipment assembly that holds the medium and provides 
space for the accumulated solids is called a filter. The fluid may be a gas or 
a liquid. The solid particles may be coarse or very fine, and their 
concentration in the suspension may be extremely low (a few parts per million) 
or quite high (>50%). 
    The object of filtration may be to purify the fluid by clarification or to 
recover clean, fluid-free particles, or both. In most filtrations the 
solids-fluid separation is not perfect. In general, the closer the approach to 
perfection, the more costly the filtration; thus the operator of the process 
cannot justify a more thorough separation than is required. 
    Gas filtration involves removal of solids (called dust) from a gas-solids 
mixture because: (1) the dust is a contaminant rendering the gas unsafe or 
unfit for its intended use; (2) the dust particles will ultimately separate 
themselves from the suspension and create a nuisance; or (3) the solids are 
themselves a valuable product that in the course of its manufacture has been 
mixed with the gas. 
    Three kinds of gas filters are in common use. Granular-bed separators 
consist of beds of sand, carbon, or other particles which will trap the solids 
in a gas suspension that is passed through the bed. Bag filters are bags of 
woven fabric, felt, or paper through which the gas is forced; the solids are 
deposited on the wall of the bag. Air filters are light webs of fibers, often 
coated with a viscous liquid, through which air containing a low concentration 
of dust can be passed to cause entrapment of the dust particles. See AIR 
FILTER; DUST AND MIST COLLECTION. 
    Liquid filtration is used for liquid-solids separations in the manufacture 
of chemicals, polymer products, medicinals, beverages, and foods; in mineral 
processing; in water purification; in sewage disposal; in the chemistry 
laboratory; and in the operation of machines such as internal combustion 
engines.  
    Liquid filters are of two major classes, cake filters and clarifying 
filters. The former are so called because they separate slurries carrying 
relatively large amounts of solids. They build up on the filter medium as a 
visible, removable cake which normally is discharged ``dry'' (that is, as a 
moist mass), frequently after being washed in the filter. It is on the surface 
of this cake that filtration takes place after the first layer is formed on 
the medium. The feed to cake filters normally contains at least 1% solids. 
Clarifying filters, on the other hand, normally receive suspensions containing 
less than 0.1% solids, which they remove by entrapment on or within the filter 
medium without any visible formation of cake. The solids are normally 
discharged by backwash or by being discarded with the medium when it is 
replaced. See CLARIFICATION. 
 �34s[Shelby A. Miller]
�33s 
  
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�6v�2v�32sFine structure (spectral lines) �21m �33s
    The closely spaced groups of lines observed in the spectra of the lightest
elements, notably hydrogen and helium. The components of any one such group
are characterized by identical values of the principal quantum number n, but
different values of the azimuthal quantum number l and the angular momentum
quantum number j.
    In atoms having several electrons, this fine structure becomes the
multiplet structure resulting from spin-orbit coupling. This gives splittings
of the terms and the spectral lines that are ``fine'' for the lightest
elements but that are very large, of the order of an electronvolt, for the
heavy elements.
 �34s[Francis A. Jenkins /William Watson]
�33s

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�6v�2v�32sFinger Lakes �21m �33s
    Long, narrow, comparatively straight lakes developed in the north border
of the Appalachian Plateau in western New York. These lakes resemble fiords;
the bottoms of some are below sea level. The lakes occupy preglacial or
interglacial valleys that were deepened by glacial erosion. Some tributary
valleys hang above the lakes, creating falls. See FIORD.
 �34s[Fredrik T. Thwaites /Richard F. Flint]
�33s

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�6v�2v�32sFingerprint   �21m �33s 
    Distinctive ridges that appear on the bulbs of the inside of the end 
joints of the fingers and thumbs. Fingerprints are an infallible means of 
identification. In addition to their value in criminal matters, fingerprints 
can ensure personal identification for humanitarian reasons, such as in cases 
of amnesia, missing persons, or unknown deceased. Fingerprints are invaluable 
in effecting identifications in tragedies such as fire, flood, and vehicle 
crashes. The vast majority of fingerprints maintained in the Identification 
Division of the Federal Bureau of Investigation of the United States, the 
largest repository of fingerprints in the world, are civil records. 
    Fingerprints fall into three general types of patterns (see illustration). 
Each group bears the same general characteristics or family resemblance. The 
three general pattern types may be further divided into subgroups by means of 
smaller differences existing between the patterns in the same general group. 
The arch group includes the plain arch and the tented arch. The loop group 
includes the radial and ulnar loops. The whorl group includes four types of 
whorls, the plain whorl, central pocket loop, double loop, and accidental 
whorl.  
    The pattern area is that part of a fingerprint in which appear the cores, 
deltas, and ridges that are used for classification. The pattern areas of 
loops and whorls are enclosed by type lines, which may be defined as the two 
innermost ridges which start parallel, diverge, and surround or tend to 
surround the pattern area. 
    Within the pattern areas of loops and whorls are the focal points which 
are used in the detailed classification. These points are called the delta and 
the core. The delta is that point on a ridge at or in front of and nearest the 
center of the divergence of the type lines. A core may be defined as that 
point on a ridge which is located in the approximate center of the finger 
impression.  
    The distinctive ridges of the palms of the hands and the soles of the feet 
closely resemble those of the fingerprint and can be identified in the same 
way. Footprints of persons having no fingers are taken for record purposes. 
Fingerprint, palm print, and sole print identifications have all been accepted 
as evidence by the courts. See EPIDERMAL RIDGES. 
 �34s[J. Edgar Hoover]
�33s 
  
  
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0
�6v�2v�32sFinite element method   �21m �33s 
    A numerical analysis technique for obtaining approximate solutions to many 
types of engineering problems. The need for numerical methods arises from the 
fact that for most practical engineering problems analytical solutions do not 
exist. While the governing equations and boundary conditions can usually be 
written for these problems, difficulties introduced by either irregular 
geometry or other discontinuities render the problems intractable 
analytically. To obtain a solution, the engineer must make simplifying 
assumptions, reducing the problem to one that can be solved, or a numerical 
procedure must be used. In an analytic solution, the unknown quantity is given 
by a mathematical function valid at an infinite number of locations in the 
body, while numerical methods provide approximate values of the unknown 
quantity only at discrete points in the body. In the finite element method, 
the region of interest is divided up into numerous connected subregions or 
elements within which approximate functions are used to represent the unknown 
quantity.  
    The physical concept on which the finite element method is based has its 
origins in the theory of structures. The idea of building up a structure by 
fitting together a number of structural elements (see illustration) was used 
in the early truss and framework analysis approaches employed in the design of 
bridges and buildings in the early 1900s. By knowing the characteristics of 
individual structural elements and combining them, the governing equations for 
the entire structure are obtained. The limitation on the number of 
simultaneous algebraic equations that could be solved posed a severe 
restriction to the analyses of that period. The introduction of the digital 
computer has made possible the solution of the large-order systems of 
equations.  
    The finite element method is one of the most powerful approaches for the 
approximate solution of a wide range of problems in mathematical physics. As 
such, its application has extended beyond structural analysis to soil 
mechanics, heat transfer, fluid flow, magnetic field calculations, and other 
areas.  
 �34s[T. Balderes]
�33s 
  
  
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�6v�2v�32sFinite mathematics   �21m �33s 
    These parts of mathematics which deal with finite sets. More generally, 
finite mathematics is taken to include those fields which make no use of the 
concept of limit; thus linear programming, which finds maxima and minima 
through the four arithmetic operations, is included, whereas classical 
optimization, which uses differential calculus, is excluded. Other subjects 
generally considered in the finite mathematics include finite set theory; 
logic; the study of finite groups, rings, and fields; combinatorial theory; 
linear algebra; probability; game theory; dynamic programming; and networks. 
See DYNAMIC PROGRAMMING; FIELD THEORY (MATHEMATICS); GAME THEORY; GROUP 
THEORY; LINEAR ALGEBRA; LINEAR PROGRAMMING; LOGIC; NETWORK (MATHEMATICS); 
OPTIMIZATION; PROBABILITY; RING THEORY; SET THEORY. 
 �34s[Guillermo Owen]
�33s 
  
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�6v�2v�32sFiord   �21m �33s 
    A segment of a troughlike glaciated valley partly filled by an arm of the 
sea. It differs from other glaciated valleys only in the fact of submergence. 
The floors of many fiords are elongate basins excavated in bedrock, and in 
consequence are shallower at the fiord mouths than in the inland direction. 
The seaward rims of such basins represent lessening of glacial erosion at the 
coastline, where the former glacier ceased to be confined by valley walls and 
could spread laterally. Some rims are heightened by glacial drift deposited 
upon them in the form of an end moraine. 
    Fiords occur conspicuously in British Columbia and southern Alaska, 
Greenland, Arctic islands, Norway, Chile, New Zealand, and Antarctica - all of 
which are areas of rock resistant to erosion, with deep valleys, and with 
strong former glaciation. 
 �34s[Richard F. Flint]
�33s 
  
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�6v�2v�32sFir �21m �33s
    Any tree of the genus Abies, of the pine family, characterized by erect
cones, by the absence of resin canals in the wood but with many in the bark,
and by flattened needlelike leaves which lack definite stalks. The leaves
usually have two white lines on the underside and leave a circular scar when
they fall.
    The native fir of the northeastern United States and adjacent Canada is A.
balsamea. Its principal uses are for paper pulp, lumber, boxes and crates, and
as a source of the liquid resin called Canada balsam. In the eastern United
States the fir is commonly used as a Christmas tree.
    The Fraser fir (A. fraseri) is a similar species found in the southern
Appalachians.
    Several species of Abies grow in the Rocky Mountains region and westward
to the Pacific Coast. The most important commercially is the white fir (A.
concolor), also known as silver fir. Other western species of commercial
importance are the subalpine fir (A. lasiocarpa), grand fir (A. grandis),
Pacific silver fir (A. amabilis), California red fir (A. magnifica), and noble
fir (A. procera). See PINALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sFire   �21m �33s 
    A rapid but persistent chemical reaction accompanied by the emission of 
light and heat. The reaction is self-sustaining, unless extinguished, to the 
extent that it continues until the fuel concentration falls below a minimum 
value. Most commonly, it results from a rapid exothermic combination with 
oxygen by some combustible material. Flame, the visible manifestation of fire, 
results from a heating to incandescence of minute particulate matter composed 
principally of incompletely burned fuel. See COMBUSTION; FLAME. 
 �34s[Francis J. Johnston]
�33s 
  
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�6v�2v�32sFire assaying   �21m �33s 
    Analysis of a metal-bearing material by heating a sample with a suitable 
flux and measuring the content of resultant metal beads by weighing or some 
instrumental technique. 
    Fire assaying is used almost exclusively for measuring the content of 
silver, gold, and platinum metals in ores, alloys, plating solutions, and 
reclaimed materials, such as sweepings, commonly called sweeps. Reasons for 
the continued use of the fire-assay method include complete decomposition of 
large refractory representative samples, high sensitivity, high selectivity, 
and ready adaptability to routine work. These reasons become apparent on 
considering the essentials of the method. 
 �34s[Silve Kallmann]
�33s 
  
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�6v�2v�32sFire detector   �21m �33s 
    A temperature-sensing device designed to sound an alarm, to turn on a 
sprinkler system, or to activate some other fire preventive measure at the 
first signs of fire. Sprinkler systems are usually equipped with fusible metal 
links, and these are perhaps the most common fire detector devices. They serve 
a double function, because they sense the rise in temperature due to the fire 
and put the defense measures into motion. More elaborate fire alarm systems 
use detectors similar to thermostats. They are often actuated electrically. 
    Smoke detectors are also common. These detectors are actuated when the 
smoke interrupts a light beam that shines on a photoelectric cell. 
    Photosensing devices respond to a stimulus in the infrared part of the 
spectrum and can thus detect the rise in temperature at the source of the fire 
itself. Because of the rapid response of such detectors, extinguishing 
apparatus can be actuated in a fraction of a second. Therefore, it is possible 
to stop the development of an explosion in the fuel tank of an aircraft, for 
example, by placing the detector and the extinguishing equipment inside the 
tank itself. See ALARM SYSTEMS; AUTOMATIC SPRINKLER SYSTEM; FIRE EXTINGUISHER. 
 �34s[William E. Gordon]
�33s 
  
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�6v�2v�32sFire extinguisher   �21m �33s 
    Fire may be extinguished by the following methods: (1) cooling the burning 
materials; (2) blanketing the fire with inert gas that chokes it for lack of 
oxygen; (3) introducing materials that inhibit combustion; and (4) covering 
the burning matter with a blanket or a layer of solid particles that prevent 
access of air. Fire extinguishers operate on one or a combination of these 
principles.  
    Water is the most effective cooling agent used in fire extinguishing. The 
generation of steam also drives away the air and forms a blanket, but being 
less dense than air, it is rapidly displaced. Wetting agents and foaming 
agents increase the effectiveness of water. In the small, first-aid, water 
fire extinguishers, a propellant must be provided. Usually this is carbon 
dioxide, which is either generated when needed or stored in a cartridge. Water 
should not be used on oil fires or on electrical fires. 
    Automatic water sprinkler systems are a common form of fire protection in 
industrial plants and large buildings. The installation of these systems is 
the greatest single factor to be credited for the sharply reduced incidence of 
disastrous fires in recent years. See AUTOMATIC SPRINKLER SYSTEM. 
    Carbon dioxide is a safe and effective extinguisher for all confined 
fires. It acts as an inert blanket, and because it is heavier than air, it 
will exclude oxygen very efficiently from a fire on the floor of a building or 
in a vat or similar vessel. It is not effective in an elevated location or 
outdoors where the wind can blow the gas away. 
    A dry powder, consisting principally of sodium bicarbonate, may also be 
used as a fire extinguisher. The powder must have the correct particle size 
and contain materials that prevent it from caking. The action of the powder is 
threefold: to generate carbon dioxide; to cool the burning material; and to 
provide a shielding to prevent access of air. Dry chemical is useful for small 
fires, and especially electrical fires. 
    Carbon tetrachloride, CCl4, has had a long history as a fire-extinguishing 
agent. As it is customarily used, in small quantities, the principal action is 
to supply a heavy blanket of vapor over the fire. In addition, carbon 
tetrachloride, in common with all the halogenated compounds, has a definite 
chemical inhibiting effect on combustion. Other halogenated hydrocarbons that 
have been used are chlorobromomethane, and several of the fluorinated 
hydrocarbons known as freons. The principal difficulty, however, with all the 
halogenated hydrocarbons, and with CCl4 in particular, is toxicity. 
    Other extinguishing methods should be mentioned that require no special 
equipment. For a household fire, especially involving clothing on a person, a 
blanket or a rug provides an effective means to smother the fire. Small fires 
around a kitchen stove may be snuffed out with salt or, better still, with 
bicarbonate of soda. A bucket of sand, strategically located, is also useful 
against domestic fire hazards. 
 �34s[William E. Gordon]
�33s 
  
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�6v�2v�32sFire-tube boiler   �21m �33s 
    A steam boiler in which hot gaseous products of combustion pass through 
tubes surrounded by boiler water. The water and steam in fire-tube boilers are 
contained within a large-diameter drum or shell, and such units often are 
referred to as shell-type boilers. Heat from the products of combustion is 
transferred to the boiler water by tubes or flues of relatively small diameter 
(approximately 3 - 4 in. or 7.5 - 10 cm) through which the hot gases flow. The 
tubes are connected to tube sheets at each end of the cylindrical shell and 
serve as structural reinforcements to support the flat tube sheets against the 
force of the internal water and steam pressure. Braces or tension rods also 
are used in those areas of the tube sheets not penetrated by the tubes. 
    Fire-tube boilers may be designed for vertical, inclined, or horizontal 
positions. One of the most generally used types is the horizontal-return-tube 
(HRT) boiler (see illustration). In the HRT boiler, part of the heat from the 
combustion gases is transferred directly to the lower portion of the shell. 
The gases then make a return pass through the horizontal tubes or flues before 
being passed into the stack. See STEAM-GENERATING UNIT.   
 �34s[George W. Kessler]
�33s 
  
  
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�6v�2v�32sFischer-Tropsch process   �21m �33s 
    The synthesis of hydrocarbons and, to a lesser extent, of aliphatic 
oxygenated compounds by the catalytic hydrogenation of carbon monoxide. The 
synthesis was discovered in 1923 by F. Fischer and H. Tropsch at the Kaiser 
Wilhelm Institute for Coal Research in Muelheim, Germany. The reaction is 
highly exothermic, and the reactor must be designed for adequate heat removal 
to control the temperature and avoid catalyst deterioration and carbon 
formation. The sulfur content of the synthesis gas must be extremely low to 
avoid poisoning the catalyst. See COAL GASIFICATION. 
 �34s[Joseph H. Field]
�33s 
  
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�6v�2v�32sFisher   �21m �33s 
    A carnivorous mammal that is a member of the family Mustelidae, which also 
includes the minks, ferrets, martens, weasels, badgers, otters, skunks, and 
wolverine. The American fisher (Martes pennanti) occurs in forested areas of 
North America. The fur, which is valuable, is variable in color, but always 
dark, and does not change color seasonally. The fisher has a long, lithe 
powerful body with short legs and a long muzzle. These animals breed once a 
year, and the two to four young are born in the nest high above the ground. 
This species is now nearly extinct in the eastern United States. See BADGER; 
CARNIVORA; MARTEN; OTTER; SKUNK; WEASEL; WOLVERINE. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sFishery   �21m �33s 
    The taking or propagation of fishes or other aquatic life in inland or 
oceanic waters. A fishery may be operated for subsistence, for pleasure (sport 
fishery), or for profit (commercial fishery). The ``fishes'' include: fin 
fishes; invertebrate shellfishes such as the mollusks (including clams, 
mussels, and oysters) and the shrimps and their relatives, the marine lobsters 
and the fresh-water crayfishes; aquatic plants; sponges; coral; sea cucumbers; 
amphibians, chiefly frogs; reptiles such as turtles, alligators, and 
crocodiles; and mammals, including whales, seals, and walruses. 
    Fisheries are important for the production of food and of raw materials 
for industry, and for recreation. They occur in all the many kinds of waters 
that together cover nearly three-fourths of the Earth's surface. In the 
international accounting system of world fish catch maintained by the UN Food 
and Agriculture Organization (FAO), there are three major fisheries - marine 
(salt-water), fresh-water, and diadromous (involving fishes that ascend 
fresh-water streams to spawn, like salmon, or descend from streams to 
reproduce in the oceans, like certain eels.). See AQUACULTURE; MARINE 
FISHERIES.  
 �34s[Karl F. Lagler]
�33s 
  
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�6v�2v�32sFissidentales   �21m �33s 
    An order of the Bryopsida. The plants appear flattened because the two 
rows of leaves are arranged on the stem in one plane. They are described as 
being distichous or two-ranked. Most of the species are terrestrial, but a few 
are aquatic. See BRYOPSIDA. 
 �34s[Winona H. Welch]
�33s 
  
  
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�6v�2v�32sFission track dating �21m �33s
    A method of dating geological and archeological specimens by counting the
radiation-damage tracks produced by spontaneous fission of uranium impurities
in minerals and glasses. During fission two fragments of the uranium nucleus
fly apart with high energy, traveling a total distance of SIMILAR 25
micrometers (SIMILAR 0.001 in.) and creating a single, narrow but continuous,
submicroscopic trail of altered material, where atoms have been ejected from
their normal positions. Such a trail, or track, can be revealed by using a
chemical reagent to dissolve the altered material, and the trail can then be
seen in an ordinary microscope. The holes produced in this way can be enlarged
by continued chemical attack until they are visible to the unaided eye.
    Track dating is possible because most natural materials contain some
uranium in trace amounts and because the most abundant isotope of uranium,
(sup 238)U, fissions spontaneously. Over the lifetime of a rock substantial
numbers of fissions occur; their tracks are stored and thus leave a record of
the time elapsed since track preservation began. The number of tracks produced
in a given volume of material depends on the uranium content as well as the
age, so that it is necessary to measure the uranium content before an age can
be determined.
    One feature unique to this dating technique is the time span to which it
is applicable. It ranges from less than 100 years for certain synthetic,
decorative glasses to approximately 4,500,000,000 years, the age of the solar
system. A second useful feature is that measurements can sometimes be made on
extremely minute specimens, such as chips of meteoritic minerals or fragments
of glass from the ocean bottom. A third useful feature is that each mineral
dates the last cooling through the temperature below which tracks are retained
permanently. Since this temperature is different for each mineral, it is
possible to measure the cooling rate of a rock by dating several minerals -
each with a different track-retention temperature. See CHEMICAL DATING;
GEOLOGICAL TIME SCALE; ROCK AGE DETERMINATION.
 �34s[R. L. Fleischer ;P. Buford Price]
�33s

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�6v�2v�32sFitzGerald-Lorentz contraction �21m �33s
    The contraction of a moving body in the direction of its motion. In 1892
G. F. FitzGerald and H. A. Lorentz proposed independently that the failure of
the Michelson-Morley experiment to detect an absolute motion of the Earth in
space arose from a physical contraction of the interferometer in the direction
of the Earth's motion. According to this hypothesis, as formulated more
exactly by Albert Einstein in the special theory of relativity, a body in
motion with speed v is contracted by a factor
SQUARE ROOT OF [1 - (v[sub 2]/c[sub 2])] in the direction of motion,
where c is the speed of light. See LIGHT;
RELATIVITY.
 �34s[Edward L. Hill]
�33s


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�6v�2v�32sFlabellifera   �21m �33s 
    The largest and morphologically most generalized suborder of isopod 
crustaceans. The biramous uropods are attached to the sides of the abdomen and 
may with the last abdominal segment form a caudal fan (see illustration). More 
than 1400 species and over 10 families are known. 
    Members of the family Cirolanidae are actively swimming predators and 
scavengers with biting mouthparts. They feed on dead and living animals and 
are notorious for their attacks on the bait of line fishermen and on fishes 
caught in nets. The legs of bathers are also sometimes bitten. 
    Members of the families Corallanidae and Excorallanidae are sometimes 
found parasitizing fishes, but are often free-living. The families Aegidae and 
Cymothoidae include fish parasites; the Aegidae feed by sucking blood. The 
Cymothoidae are more highly adapted for a parasitic life than other 
Flabellifera.  
    The gribbles, Limnoriidae, are well known for the damage they cause to 
marine structures of wood. In the family Sphaeromatidae, some species of 
Sphaeroma are economically important in tropical seas because of their habits 
of boring into wood, clay, and rock. Isopods of the family Serolidae are 
greatly flattened forms which live partially buried on sandy bottoms. See 
ISOPODA.  
 �34s[Thomas E. Bowman]
�33s 
  
  
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�6v�2v�32sFlame   �21m �33s 
    A reaction front (or wave) in a gaseous medium into which the reactants 
flow and out of which the products flow. Solid or liquid particles may also be 
carried in by the gas and provide fuel for the flame, and sometimes the 
product gases contain solid particles, such as soot. Flames produce heat and, 
usually, light. 
    A seeming paradox is worthy of note, however. A faint, luminous 
phenomenon, known as a cool flame, is observed when, for example, a mixture of 
ether vapor and oxygen is slowly heated. Such flames are not really cool; they 
do produce some heat. But unlike ordinary flames that advance from layer to 
layer in the gas mainly by heat conduction, cool flames proceed by diffusion 
of reactive molecules, or free radicals, which initiate chemical processes as 
they go. The light from cool flames is a chemiluminescence that originates 
from excited molecules created by the reaction. See CHEMILUMINESCENCE. 
    Combustion flames, in which oxygen or some other oxidizing substance 
combines with a fuel, are more common. But decomposition flames, in which a 
molecule such as ozone, O3, breaks down into simple molecules, in this case 
oxygen, O2, are also known. Not all combustion or decomposition reactions, 
however, produce a flame, even though they may evolve heat. For example, 
charcoal undergoes surface combustion and, although it glows, there is no 
flame. See COMBUSTION. 
 �34s[William E. Gordon]
�33s 
  
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�6v�2v�32sFlame arc lamp   �21m �33s 
    An arc lamp that radiates light from the arc instead of from the 
electrode. The carbon electrode is impregnated with chemicals, which are more 
volatile than the carbon and, when driven into the arc, become luminous. The 
chemicals commonly used are calcium, barium, titanium, and strontium, which 
produce their characteristic color of light as chemical flames and radiate 
efficiently in their specific wavelengths in the visible spectrum. Some flames 
use several chemicals, some for light and others for high arc temperature and 
steady arc operation. Other chemicals may be introduced to produce radiation 
outside the visible spectrum for specific commercial or therapeutic purposes. 
See ARC LAMP. 
 �34s[John O. Kraehenbuehl]
�33s 
  
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�6v�2v�32sFlame photometry   �21m �33s 
    A branch of spectrochemical analysis in which samples in solution are 
excited to luminescence by introduction into a flame. Flame photometry is 
particularly useful in determining small amounts of lithium, sodium, and 
potassium in solution. Only small samples are required, and an analysis 
usually can be carried out in relatively short time. 
    Most emission flame photometry in the United States is done with the 
Beckman burner, which aspirates solution directly into a hydrogen-oxygen or 
acetylene-oxygen flame. Since flame temperatures are comparatively low, only 
the more volatile compounds are vaporized from the residue. Also, only the 
less stable molecules are dissociated, and only the more easily excited 
spectral lines and bands are emitted. Flame line-emission spectra are thus 
relatively simple, although molecular band spectra caused by the fuel or 
metallic oxides sometimes cause spectral interference. The line or band of 
interest is isolated with a monochromator, and its intensity measured 
photoelectrically. The size and type of monochromator required depend on the 
spectral region studied and the complexity of the spectrum. 
    Emission flame photometry is more accurate, precise, and sensitive than 
wet chemical methods for the determination of small amounts of alkali metals, 
and it rivals wet methods in the determination of some other elements. See 
ATOMIC STRUCTURE AND SPECTRA; SPECTROCHEMICAL ANALYSIS. 
    Atomic absorption analysis is a method of determining the concentration of 
a metal, usually in solution. The solution is converted to a vapor in which at 
least some of the metal is in the form of individual neutral atoms. The 
absorbance of this vapor is measured at a resonant absorption wavelength of 
the metal in question. This absorbance is then compared with that given by 
standard solutions. For some elements (for example, zinc, cadmium, antimony, 
lead, bismuth, mercury, and tellurium) detection limits are much better than 
those achieved by emission flame photometry; for most elements the two methods 
are comparable in sensitivity. The atomic absorption method has fewer spectral 
line interference problems than does emission flame photometry. 
 �34s[Cyrus Feldman]
�33s 
  
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�6v�2v�32sFlameproofing   �21m �33s 
    The process of treating materials so that they will not support 
combustion. Although cellulosic materials such as paper, fiberboard, textiles, 
and wood products cannot be treated so that they will not be destroyed by long 
exposure to fire, they can be treated to retard the spreading of fire and to 
be self-extinguishing after the igniting condition has been removed. 
    Numerous methods have been proposed for flameproofing cellulosic products. 
One of the simplest and most commonly used for paper and wood products is 
impregnation with various soluble salts, such as ammonium sulfate, ammonium 
phosphate, ammonium sulfamate, borax, and boric acid. Special formulations are 
often used to minimize the effects of these treatments on the color, softness, 
strength, permanence, or other qualities of the paper. For some applications, 
these treatments are not suitable because the salts remain soluble and leach 
out easily on exposure to water. A limited degree of resistance to leaching 
can be achieved by the addition of latex, lacquers, or waterproofing agents. 
In some cases the flameproofing agent can be given some resistance to leaching 
by causing it to react with the cellulose fiber (for example, urea and 
ammonium phosphate). 
    Leach-resistant flameproofing may also be obtained by incorporating 
insoluble retardants in the paper during manufacture, by application of 
insoluble materials in the form of emulsions, dispersions, or organic 
solutions, or by precipitation on, or reaction with, the fibers in 
multiple-bath treatments. The materials involved are of the same general types 
as those used for flameproofing textiles and include metallic oxides or 
sulfides and halogenated organic compounds. See COMBUSTION; TEXTILE CHEMISTRY. 
 �34s[T. A. Howells]
�33s 
  
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�6v�2v�32sFlash-photolysis resonance fluorescence �21m �33s
    A technique of fluorometric analysis for measuring absolute rate constants
for gas-phase reactions involving free radicals (principally atoms and
diatomic radicals). The method combines pulsed (SIMILAR 10- sup 5 s duration)
photolytic production of the radical with real-time resonance fluorescence
monitoring of its subsequent temporal behavior. This technique revolutionized
gas-phase chemical kinetics research in the 1970s.
    The typical experimental apparatus includes a vacuum reaction chamber, a
pulsed photolysis source (flash lamp), an analytical light source (reasonance
lamp), and a detection system. In a typical experiment a gas mixture
consisting of a photolytic radical source, molecular reactant, and inert
diluent are flash-photolyzed in the cell under preselected conditions of light
intensity and wavelength distribution. In this way the desired uniform
concentration of transient reactive species is produced across the central
region of the cell. The analytical light source probes an intersection of this
central region continuously, thereby generating a fluorescence emission from
the radical species.
    The application of resonance fluorescence analysis for real-time kinetics
investigations is limited only by the fluorescing ability of the free radical
and the availability of a suitable resonance light source. This versatility
has led to the adaptation of this measurement technique to other kinetics
experiments utilizing entirely different production and analysis
methodologies. See CHEMICAL DYNAMICS; FLUORESCENCE; FLUOROMETRIC ANALYSIS;
OPTICAL METHODS OF CHEMICAL ANALYSIS.
 �34s[Michael J. Kurylo]
�33s

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�6v�2v�32sFlash point   �21m �33s 
    The temperature at which a flash of flame is first observed when a small 
lighted flame is passed over a sample of a flammable substance in a cup. The 
flash point is an important test for fire and safety regulations. So also is 
the fire point, or temperature at which sustained burning first occurs. These 
tests are indicative of the lowest-boiling components in the oil and are 
applied to solvents, distillate fuels, lubricating oils, and many other 
petroleum products. 
 �34s[Mott Souders]
�33s 
  
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�6v�2v�32sFlash welding   �21m �33s 
    A form of resistance welding that is used for mass production. The welding 
circuit consists of a low-voltage, high-current energy source (usually a 
welding transformer) and two clamping electrodes, one stationary and one 
movable.  
    The two pieces of metal to be welded are clamped tightly in the 
electrodes, and one is moved toward the other until they meet, making light 
contact. Energizing the transformer causes a high-density current to flow 
through small areas that are in contact with each other. Flashing starts, and 
the movable workpiece must accelerate at the proper rate to maintain an 
increasing flashing action. After a proper heat gradient has been established 
on the two edges to be welded, an upset force is suddenly applied to complete 
the weld. This upset force extrudes slag, oxides, molten metal, and some of 
the softer plastic metal, making a weld in the colder zone of the heated 
metal. See RESISTANCE WELDING. 
 �34s[Eugene J. Limpel]
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�6v�2v�32sFlat-bed chromatography �21m �33s
    A common form of liquid chromatography that takes place in flat
chromatographic beds. Two methods which belong in this category are paper
chromatography and thin-layer chromatography.
    In paper chromatography, a strip of filter paper that has been
equilibrated in a moist atmosphere is spotted near the bottom with a sample
(mixture) and placed inside the developing chamber. The water in the moist
atmosphere that has been adsorbed by paper is most commonly used as the
stationary phase (the paper is a support). Papers can also be impregnated by
other liquids. The mobile phase, usually an organic solvent or a mixture of
solvents, ascends the paper strip by means of capillary forces, and the
components of the sample mixture are thus separated by the partition effect.
    The stationary phase in thin-layer chromatography is a suspension which
forms a layer (approximately 250 MUm thick) on a metal or glass plate. It is
most frequently an adsorbent (with a particle size of several micrometers)
suspended in a suitable solvent, uniformly spread on a plate, and dried. The
mobile phase is a liquid that ascends the plate by capillary action. The
development of chromatograms is similar to that in paper chromatography in its
simplicity. But thin-layer chromatography is superior in terms of rapidity,
the variety of stationary phases that can be used, and detection methods.
See CHROMATOGRAPHY; LIQUID CHROMATOGRAPHY.
 �34s[Milos V. Novotny]
�33s

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�6v�2v�32sFlatfish   �21m �33s 
    A number of asymmetrical fish which make up the order Pleuronectiformes. 
The body of these fish is laterally compressed, and both eyes are on the same 
side of the head. The larva is symmetrical, but after further development and 
assumption to its bottom-dwelling habits, the young turns to one side or the 
other and there is a movement or migration of the lower eye to the upper 
surface of the head. In addition, the body surface of the fish that is in 
contact with the substratum is unpigmented; many species are able to modify 
the pattern and intensity of their pigmented skin to match their variable 
substrate. These are mainly marine species that live on the continental shelf. 
The more common flatfish are economically important species which include the 
turbot, halibut, plaice, flounder, and sole. See PLEURONECTIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sFlavonoid   �21m �33s 
    Any of a series of widely distributed plant constituents related in one 
way or another to the aromatic heterocyclic skeleton of 2-phenylbenzopyran, or 
flavan. The three most important classes are the anthocyanins, the flavones, 
and the flavon-3-ols, or flavonols. Other classes are the chalcones and 
aurones (yellow flower pigments); isoflavones (some are weak estrogens); 
flavanones (such as naringin, the bitter principle of the lemon); and 
leucoanthocyanidins (condensed tannins). The various flavonoid classes are 
biogenetically related, all being derived from a common precursor.  See  
ANTHOCYANIN.  
    Flavonoids have no single overriding function in plants. Many are highly 
colored and serve to attract insects and animals to the flowers and fruits for 
the purpose of pollination and seed dispersal. Others have been implicated in 
growth regulatory processes or as ``phytoalexins,'' (agents for protecting 
plants from fungal attack). Others present in leaves, because of their taste 
properties, protect plants from being eaten by insects or wild animals. 
    Flavonoids are found in all groups of plants except algae, bacteria, and 
fungi. Their occurrence in insects, in the wings of a number of butterflies, 
is due to their ingestion in the food and a subsequent failure of the 
caterpillar to metabolize them. All animals above insects break down the 
dietary flavonoids into small fragments, which are then excreted in the urine. 
    In contrast to the alkaloids, flavonoids are rather inactive 
pharmacologically; the quercetin glycoside rutin is the only one that is used 
much in medicine. It has a protective function in preventing capillary 
fragility. An economically significant property of flavonoids is their 
contribution to taste and flavor in foods. Some compounds are bitter, others 
very sweet, and yet others astringent. Bitterness is largely confined to 
certain flavanone glycosides of citrus fruits. 
    From the point of view of the food industry, the most important classes of 
flavonoids are the catechins and flavan-3,4-diols, or leucoanthocyanidins. 
These compounds are widely distributed in drinks such as cocoa, tea, beer, and 
cider and in many fruits and vegetables. They are responsible for astringency, 
a necessary counterbalance in certain foods to insipid sweetness. 
Leucoanthocyanidins also have the property of tanning protein, and most of the 
condensed tannins used in the leather industry are flavonoid polymers.  See  
TANNIN.  
 �34s[Jeffrey B. Harborne]
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�6v�2v�32sFlavor �21m �33s
    A generic term referring to quarks, the most elementary constituents of
matter. The evidence is conclusive that all matter is constituted of quarks of
at least five different types of flavors, with a sixth predicted. They are
labeled  u, d, s, c, b, and t. The last, the t quark, is predicted from symmetry
considerations. Corresponding mnemonics are up, down, strange, charmed, bottom,
and top. The u, c, and t flavors each carry a positive electric charge equal in
magnitude to two-thirds that of the electron, and the  d, s, and b flavors have
a negative charge one-third that of the electron.  See	ELEMENTARY PARTICLE;
QUARKS.
 �34s[Val Fitch]
�33s

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�6v�2v�32sFlax �21m �33s
    The flax plant ( Linum usitatissimum ) is the source of two products:
flaxseed for linseed oil and fiber for linen products. Plants with two
distinct types of growth are used for seed and fiber production.
    Flax for fiber requires fertile, well-drained, and well-prepared soil and
a cool, humid climate. The best-known use for flax fiber is in the manufacture
of fine linen fabrics; other uses are linen thread, linen twine, toweling, and
canvas. Seed from fiber flax is used for replanting and for oil. The major
producer of flax fiber is the Soviet Union, but the world's best fiber comes
from Belgium and adjoining countries. Most Irish linen is manufactured from
fiber produced in Belgium.  See  CANVAS; LINEN; NATURAL FIBER; TEXTILE.
    Seedflax is the source of linseed oil. The principal uses of linseed oil
are in the manufacture of protective coatings - paint, varnish, and lacquer.
It is also used in linoleum, printer's ink, and patent and imitation leathers,
and as core oil for making sand forms in metal casting. The linseed cake
remaining after the oil is extracted is used as a high-protein livestock feed.
Large quantities of clean (weed-free) straw from seedflax are used in the
production of a high-grade paper. This product is used to make cigarette and
other fine papers. The world's major producers of flaxseed are Argentina,
Canada, India, the Soviet Union, and the United States.
 �34s[Elton G. Nelson ;Joseph O. Culbertson]
�33s

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�6v�2v�32sFlea   �21m �33s 
    A member of the insect order Siphonaptera; a flattened, wingless 
ectoparasite. Most fleas are less than **5 in. (5 mm) long. They are 
intermittent parasites, readily changing hosts and able to live up to 4 months 
without feeding. Adults have piercing-sucking mouthparts with which they take 
blood from mammals and birds. Most fleas parasitize mammals, and while human, 
cat, dog, and rodent fleas each have their preferred host, they 
opportunistically may bite any species of mammal encountered. This can cause 
irritation to pet lovers, and a far more serious consequence is transmission 
of plague from rodents to humans.  See  PLAGUE; SIPHONAPTERA. 
 �34s[David J. Horn]
�33s 
  
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�6v�2v�32sFletcher-Munson contours   �21m �33s 
    A set of curved lines showing how the intensities of tones of different 
frequency should be adjusted so that all appear to be equally loud to the 
listener. This important characteristic of hearing is shown in the 
illustration in the form adopted as an international standard in 1961. 
    Equal loudness contours are obtained from tests in which a group of 
observers listen to electronically generated pure tones of different pitch 
coming from a loudspeaker. The tones are heard in pairs, one after the other, 
and volume controls are adjusted until the two tones of different frequency 
appear to be equally loud. Then the tones are measured with a sound level 
meter, and it is generally found that the sound pressures are not the same but 
differ systematically as the frequencies of the tones are changed. It follows 
that the ``sound level,'' measured with a meter, is not the same as the 
``loudness level,'' where the ear is used as the measuring device. The 
differences can be obtained from the equal loudness contours shown in the 
illustration. All tones represented by different points on the same contour 
line sound equally loud to the average listener.  See  HEARING (HUMAN); 
LOUDNESS.  
 �34s[W. A. Munson]
�33s 
  
  
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�6v�2v�32sFlexibilia   �21m �33s 
    An extinct subclass of stalked or creeping Crinoidea which includes some 
50 Paleozoic genera, ranging from Ordovician to Permian times. A flexible 
tegmen was present with open ambulacral grooves. One of the five oral plates 
served as a madreporite. The anus was at the tip of a short siphon. The 
uniserial arms branched freely, arching inward to form a globular crown (see 
illustration). The cylindrical stem lacked cirri and was sometimes discarded 
to produce a creeping adult.  See  CRINOIDEA; ECHINODERMATA. 
 �34s[Howard B. Fell]
�33s 
  
  
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�6v�2v�32sFlight   �21m �33s 
    Traditionally, flight has dealt with motion in or through the air, but 
flight now includes motion at distances so far from the Earth's surface that 
the air has negligible effect. Natural animal flight is necessarily restricted 
to the denser portion of the Earth's atmosphere. Artificial flight can either 
use the air or be independent of it.  See  AERONAUTICS; ASTRONAUTICS. 
    The ballistic flight of a bullet or missile is a consequence of energy 
imparted to it during its ejection from a gun barrel or during the burning 
period of its engine. At the opposite extreme is the soaring flight of a 
glider or a hawk that depends on the energy from local rising air currents. 
True natural flight, as for birds, bats, and insects, and true artificial 
flight, as for humans in aircraft, require the continuous expenditure of 
energy to achieve fully controlled progress through the air. Historically, 
insects flew fast, followed by reptile groups that are now extinct, and then 
birds, bats, and finally humans. 
    The greater the size of an animal, the greater is the problem of remaining 
airborne, because, although the weight increases as the cube of the linear 
dimensions, the surface area increases only as the square. The practical limit 
seems to have been reached in the largest birds. 
    In all flying animals both lift and thrust are provided by the 
predominantly up-and-down movement of a pair of laterally extended planes. The 
movements of animal wings in level flight usually show a forward component on 
the downstroke and a backward component on the upstroke, either relative to 
the body axis or to the vertical or both. These wing movements have the effect 
of giving a downward component to the main direction of the slipstream and 
hence contribute lift. In hovering flight the movements are usually 
tremendously exaggerated. 
 �34s[Brian Hocking]
�33s 
  
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�6v�2v�32sFlight controls   �21m �33s 
    The devices and systems which govern the attitude of an aircraft and, as a 
result, the flight path followed by the aircraft. Flight controls are 
classified as primary flight controls, auxiliary flight controls, and 
automatic controls. In the case of many conventional airplanes, the primary 
flight controls utilize hinged, trailing-edge surfaces called elevators for 
pitch, ailerons for roll, and the rudder for yaw. These hinged surfaces are 
operated by the human pilot in the cockpit or by an automatic pilot. In the 
case of vertically rising aircraft, a lift control is provided.  See  AILERON; 
AIRCRAFT RUDDER; ELEVATOR (AIRCRAFT); ELEVON. 
    Auxiliary flight controls may include trimming devices for the primary 
flight controls, as well as landing flaps, leading-edge flaps or slats, an 
adjustable stabilizer, a wing with adjustable sweep, and dive brakes or speed 
brakes.  
    Controls to govern the engine power and speed, while not usually 
classified as flight controls, are equally important in the overall control of 
the aircraft. 
    Automatic controls include systems which supplement or replace the human 
pilot as a means of controlling the attitude or path of the aircraft. Such 
systems include automatic pilots, stability augmentation systems, automatic 
landing systems, and active controls. Active controls encompass automatic 
systems which result in performance improvement of the aircraft by allowing 
reductions in structural weight or aerodynamic drag, while maintaining the 
desired integrity of the structure and stability of flight. 
    The control system incorporates a set of cockpit controls which enable the 
pilot to operate the control surfaces. Because of the approximately fixed size 
and strength of the human pilot and the need to standardize the control 
procedures for airplanes, the primary controls are similar in most types of 
airplanes. The cockpit controls incorporate a control stick (sometimes called 
the joy stick) which operates the elevators and ailerons, and pedals which 
operate the rudder. Sometimes a column/wheel arrangement is used to operate 
the elevators and ailerons, respectively. The cockpit controls for auxiliary 
control devices are not as completely standardized as those for the primary 
controls.  
    Control systems with varying degrees of complexity are required, depending 
on the size, speed, and mission of an airplane. In relatively small or 
low-speed airplanes, the cockpit controls may be connected directly to the 
control surfaces by cables or pushrods, so that the forces exerted by the 
pilot are transmitted directly to the control surfaces. In large or high-speed 
airplanes, the forces exerted by the pilot may be inadequate to move the 
controls. In these cases, either an aerodynamic activator called a servotab or 
spring tab may be employed, or a hydraulic activator may be used. In some 
airplanes, particularly those with swept wings and those which fly at high 
altitudes, the provision of adequate static stability and damping of 
oscillations by means of the inherent aerodynamic design of the airplane 
becomes difficult. In these cases, stability augmentations systems are used. 
These systems utilize sensors such as accelerometers and gyroscopes to sense 
the motion of the airplane. These sensors generate electrical signals which 
are amplified and used to operate the hydraulic actuators of the primary 
control surfaces to provide the desired stability or damping. 
    The weight and complication of mechanical control linkages and the 
extensive reliance on electrical signals in automatic controls have led to the 
development of control systems in which the control inputs from the pilot, as 
well as those from the stability augmentation sensors, are transmitted to the 
primary control actuators by electrical signals. Systems of this type are 
called electrical signaling systems or fly-by-wire systems. The electrical 
signals are readily compatible with computers, either analog or digital, which 
can perform the functions of combining the signals from the pilot and the 
sensors, as well as introducing additional stabilization, navigation, and 
guidance functions. Because of the need for high reliability, these systems 
employ the principle of redundancy; that is, the components are installed in 
multiple configurations, and provision is made for detecting a failed 
component and removing its influence from the system. 
    The availability of reliable, full-authority electronic control systems 
makes possible the use of closed-loop control systems to improve the 
performance of the airplane. The applications most frequently considered in 
order to take advantage of this capability are reduced static stability, 
maneuver load control, gust load alleviation, and structural mode damping. 
    The flight of an aircraft may be controlled automatically by providing the 
necessary signals for navigation as inputs to the control system. In practice, 
automatic pilots are used to relieve the human pilot of routine flying for 
long periods, and automatic control systems are used to make precision 
landings and takeoffs under conditions of reduced visibility. 
 �34s[William H. Phillips]
�33s 
  
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�6v�2v�32sFlight dynamics   �21m �33s 
    The study of the motion of an aircraft or missile. Flight dynamics is 
generally concerned with transient or short-term effects, which have to do 
with the stability and control of the vehicle, rather than with calculating 
such performance as ultimate range, altitude, or velocity of the vehicle. 
Sometimes, however, the two are treated together, particularly in the case of 
aircraft.  See  AEROELASTICITY; FLIGHT CONTROLS. 
 �34s[John R. Sellars]
�33s 
  
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�6v�2v�32sFlight science   �21m �33s 
    The sum total of all knowledge that enables humans to accomplish flight. 
Flight science is compounded of both science and engineering. It is concerned 
with airplanes, missiles, and crewed and uncrewed space vehicles. 
    The scope of flight science is illustrated by some of the diverse fields 
which are included within it, such as electronics, aerodynamics, propulsion 
engineering, structural engineering, nuclear engineering, metallurgy, 
chemistry, space medicine, certain parts of astronomy, mathematics, classical 
and modern physics, and other branches of engineering, such as civil 
engineering for the planning and construction of airports. These constitute 
the major branches of science and technology needed to solve modern flight 
problems.  
 �34s[John R. Sellars]
�33s 
  
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�6v�2v�32sFlocculation   �21m �33s 
    The formation of larger particles of a solid phase dispersed in a solution 
by the gathering together of smaller particles. The process whereby initial 
aggregates (having dimensions of a few unit cells) in a solution develop 
spontaneously into particles of a new stable phase is known as nucleation. 
When these particles grow to size sufficient to scatter visible light, they 
are known as colloids. By changing the ionic environment in which colloidal 
particles exist, the colloidal particles can be made to undergo further 
aggregation, or flocculation.  See  COLLOID; NUCLEATION; PRECIPITATION 
(CHEMISTRY).  
 �34s[Louis Gordon /Royce W. Murray]
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�6v�2v�32sFloodplain   �21m �33s 
    The relatively broad and smooth valley floor that is constructed by an 
active river and periodically covered with floodwater from that river during 
intervals of overbank flow. Engineers consider the floodplain to be any part 
of the valley floor subject to occasional floods that threaten life and 
property. Various channel improvements or impoundments may be used to restrict 
the natural process of overbank flow. Geomorphologist consider the floodplain 
to be a surface that develops by the active erosional and depositional 
processes of a river. Floodplains are underlain by a variety of sediments, 
reflecting the fluvial history of the valley. 
    Most floodplains consist of the following types of deposits: colluvium - 
slope wash and mass-wasting products from the valley sides, as is common in 
small, narrow floodplains; channel lag - coarse debris marking the bottoms of 
former channels; lateral accretion deposits - sand and gravel deposited as the 
meandering river migrates laterally; vertical accretion deposits - clay and 
silt deposited by overbank flooding of the river; crevasse-splay deposits - 
relatively coarse sediment carried through breaks in the natural river levees 
and deposited in areas that usually receive overbank deposition; and 
channel-fill deposits - fills of former river channels. Channel fills may be 
coarse for sandy rivers. The noncohesive character of coarse sediments allows 
these rivers to easily erode laterally.  See  STREAM TRANSPORT AND DEPOSITION. 
 �34s[Victor R. Baker]
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�6v�2v�32sFloor construction   �21m �33s 
    The selection of the type of construction for the floors of a building 
should be based on the building's architectural and structural requirements 
and on cost. 
    A medium-priced home might contain a wooden floor supported by wooden 
joists. A floor system frequently used in small commercial buildings, 
apartment buildings, and higher-priced homes consists of a solid concrete slab 
supported on steel or concrete beams or open-web steel joists. Open-web joists 
are available for clear spans up to 120 ft (36 m). A thin, lightly reinforced 
concrete slab cast in one piece with its supporting reinforced concrete joists 
is particularly economical for short spans. Light-gage steel cellular floor 
decking is often used for steel-framed buildings. 
 �34s[Charles N. Gaylord]
�33s 
  
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�6v�2v�32sFloriculture   �21m �33s 
    The segment of horticulture concerned with commercial production, 
marketing, and sale of bedding plants, cut flowers, potted flowering plants, 
foliage plants, flower arrangements, and noncommercial home gardening. 
    Commercial crops are grown either in the field or under protected 
cultivation, such as in glass or plastic structures. Field production is 
confined to warm climates or to summer months in colder areas. Typical field 
crops are gladiolus, peonies, stock, gypsophila, asters, and chrysanthemums. 
Greenhouse production is not as confined by climate or season, but most 
greenhouses are located in areas that have advantages such as high light 
intensity, cool night temperatures, or ready access to market. Jet air 
transportation resulted in major changes in international crop production. 
    Pronounced improvements in cultivars have been realized because of 
excellent breeding programs conducted by commercial propagators and by some 
horticulture departments. Modern cultivars have traits such as more attractive 
flower colors and forms, longer-lasting flowers, better growth habit, 
increased resistance to insects and disease organisms, or ability to grow and 
flower at cooler night temperatures.  See  BREEDING (PLANT). 
 �34s[Roy A. Larson]
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�6v�2v�32sFlotation   �21m �33s 
    A process used to separate particulate solids, which have been suspended 
in a fluid, by selectively attaching the particles to be removed to a light 
fluid and allowing this mineralized fluid aggregation to rise to where it can 
be removed. The principal use of the process is to separate valuable minerals 
from waste rock, or gangue, in which case the ground ore is suspended in water 
and, after chemical treatment, subjected to bubbles of air. The minerals which 
are to be floated attach to the air bubbles, rise through the suspension, and 
are removed with the froth which forms on top of the pulp. Although most 
materials subjected to flotation are minerals, applications to chemical and 
biological materials have been reported. 
 �34s[Ronald L. Atwood]
�33s 
  
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�6v�2v�32sFlower �21m �33s
    A higher plant's sexual apparatus in the aggregate, including the parts
that produce sex cells and closely associated attractive and protective parts
(Fig. 1). ``Flower'' as used in this article will be limited, as is usual, to
the angiosperms, plants with enclosed seeds and the unique reproductive
process called double fertilization. In its most familiar form a flower is
made up of four kinds of units arranged concentrically. The green sepals
(collectively termed the calyx) are outermost, showy petals (the corolla)
next, then the pollen-bearing units (stamens, androecium), and finally the
centrally placed seedbearing units (carpels, gynoecium). This is the
``complete'' flower of early botanists, but it is only one of an almost
overwhelming array of floral forms. One or more kinds of units may be lacking
or hard to recognize depending on the species, and evolutionary modification
has been so great in some groups of angiosperms that a flower cluster
(inflorescence) can look like a single flower.

    Flora diversity. Most botanical terms are descriptive, and a botanist
must have a large store of them to impart the multiformity of flowers. The
examples that follow are only a smattering. An extra series of appendages
alternating with the sepals, as in pruple loosestrife, is an epicalyx. A petal
with a broad distal region and a narrow proximal region is said to have a blade
and claw: the crape myrtle has such petals. The term perianth, which embraces
calyx and corolla and avoids the need to distinguish between them, is
especially useful for a flower like the tulip, where the perianth parts are in
two series but are alike in size, shape, and color. The members of such an
undifferentiated perianth are tepals. When the perianth has only one series of
parts, however, they are customarily called sepals even if they are petallike,
as in the windflower.
    A stamen commonly consists of a slender filament topped by a four-lobed
anther, each lobe housing a pollen sac. In some plants one or more of the
androecial parts are sterile rudiments called staminodes: a foxglove flower has
four fertile stamens and a staminode. Carpellode is the corresponding term for
an imperfectly formed gynoecial unit.
    A gynoecium is apocarpous if the carpels are separate (magnolia,
blackberry) and syncarpous if they are connate (tulip, poppy). Or the
gynoecium may regularly consist of only one carpel (bean, cherry). A solitary
carpel or a syncarpous gynoecium can often be divided into three regions: a
terminal, pollen-receptive stigma; a swollen basal ovary enclosing the
undeveloped seeds (ovules); and a constricted, elongate style between the two.
The gynoecium can be apocarpous above and syncarpous below; that is, there can
be separate styles and stigmas on one ovary (wood sorrel).
    Every flower cited so far has a superior ovary: perianth and androecium
diverge beneath it (hypogyny). If perianth and androecium diverge from the
ovary's summit, the ovary is inferior and the flower is epigynous (apple,
banana, pumpkin). A flower is perigynous if the ovary is superior within a cup
and the other floral parts diverge from the cup's rim (cherry). A syncarpous
ovary is unilocular if it has only one seed chamber, plurilocular if septa
divide it into more than one. The ovules of a plurilocular ovary are usually
attached to the angles where the septa meet; this is axile placentation, a
placenta being a region of ovular attachment. There are other ways in which
the ovules can be attached - apically, basally, parietally, or on a
free-standing central placenta - each characteristic of certain plant groups
(Fig. 2).
    The term bract can be applied to any leaflike part associated with one or
more flowers but not part of a flower. Floral bracts are frequently small,
even scalelike, but the flowering dogwood has four big petallike bracts below
each flower cluster. The broad end of a flower stalk where the floral parts
are attached is the receptacle. The same term is used, rather inconsistently,
for the broad base bearing the many individual flowers (florets) that make up
a composite flower like a dandelion or a sunflower.

    Sexuality. A plant species is diclinous if its stamens and carpels are in
separate flowers. A diclinous species is monoecious if each plant bears
staminate and carpellate (pistillate) flowers, dioecious if the staminate and
carpellate flowers are on different plants. The corn plant, with staminate
inflorescences (tassels) on top and carpellate inflorescences (ears) along the
stalk, is monoecious. Hemp is a well-known dioecious plant.

    Nectaries. Flowers pollinated by insects or other animals commonly have
one or more nectaries, regions that secrete a sugar solution. A nectary can be
nothing more than a layer of tissue lining part of a floral tube or cup
(cherry), or it can be as conspicuous as the secretory spur of a nasturtium or
a larkspur. It can be a cushionlike outgrowth at the base of a superior ovary
(orange blossom) or atop an inferior ovary (parsley family). Gladiolus and a
number of other monocotyledons have septal nectaries, deep secretory crevices
where the carpels come together. Substances that give off floral odors -
essential oils for the most part - ordinarily originate close to the
nectar-producing region but are not coincident with it. Production by the
epidermis of perianth parts is most common, but in some species the odor
emanates from a more restricted region and may even come from a special flap
or brush. Most insect-pollinated plants have visual cues, some of them outside
the human spectral range, as well as odor to bring the pollinators to the
flowers and guide them to the nectar. See SECRETORY STRUCTURES (PLANT).

    Inflorescence. Inflorescence structure, the way the flowers are clustered
or arranged on a flowering branch, is almost as diverse as floral structure.
To appreciate this, one need only contrast the drooping inflorescences
(catkins) of a birch tree with the coiled flowers of a forget-me-not or with
the solitary flower of a tulip. In some cases one kind of inflorescence
characterizes a whole plant family. Queen Anne's lace and other members of the
parsley family (Umbelliferae) have umbrellalike inflorescences with the flower
stalks radiating from almost the same point in a cluster. The stalkless
flowers (florets) of the grass family are grouped into clusters called
spikelets, and these in turn are variously arranged in different grasses.
Flowers of the arum family (calla lily, jack-in-the-pulpit), also stalkless,
are crowded on a thick, fleshy, elongate axis. In the composite family,
florets are joined in a tight head at the end of the axis; the heads of some
composites contain two kinds, centrally placed florets with small tubular
corollas and peripheral ray florets with showy, strap-shaped corollas (the
``petals'' one plucks from a daisy). See INFLORESCENCE.


    Anatomy.  Some of the general anatomical features of leaves can be found
in the floral appendages. A cuticle-covered epidermis overlies a core of
parenchyma cells in which there are branching vascular bundles (solitary
bundles in most stamens). Sepal parenchyma and petal parenchyma are often
spongy, but palisade parenchyma occurs only rarely in flowers and then only in
sepals. As in other parts of the plant, color comes mostly from plastids in
the cytoplasm and from flavonoids in the cell sap. Cells of the petal
epidermis may have folded side walls that interlock so as to strengthen the
tissue. In some species the outer walls of the epidermis are raised as
papillae; apparently, this is part of the means of attracting pollinators, for
the papillae are light reflectors.

    Stamen. As a stamen develops, periclinal divisions in the second cell
layer of each of its four lobes start a sequence that will end with the
shedding of pollen. The first division makes two cell layers. The outer
daughter cells give rise to the wall of the pollen sac, and the inner ones are
destined to become pollen after further divisions. When mature, a pollen sac
typically has a prominent cell layer just below a less distinctive epidermis.
The inner wall and the side walls of an endothecium cell carry marked
thickenings, but the outer wall does not. Splitting of the ripe anther is due
partly to the way in which these differentially thickened walls react to
drying and shrinking and partly to the smaller size of the cells along the
line of splitting. See POLLEN.

    Carpel. Like other floral parts, a carpel is made up of epidermis,
parenchyma, and vascular tissue. In addition, a carpel commonly has a special
tissue system on which pollen germinates and through which, or along which,
pollen tubes are transmitted to the ovules. Most angiosperms have solid
styles, and the transmitting tissue is a column of elongate cells whose
softened walls are the medium for tubal growth. The epidermis at the stigmatic
end of a carpel usually changes to a dense covering of papillae or hairs; the
hairs can be unicellular or pluricellular, branched or unbranched. In taxa
with hollow styles, the transmitting tissue is a modified epidermis running
down the stylar canal. There are two kinds of receptive surfaces, and they are
distributed among the monocotyledons and the dicotyledons with taxonomic
regularity. One kind has a fluid medium for germinating the pollen, and the
other has a dry proteinaceous layer over the cuticle. The proteins of the dry
stigmas have a role in the incompatibility reactions that encourage
outbreeding.

    Ovule. Ovule development usually takes place as the gynoecium forms, but
it may be retarded when there is a long interval between pollination and
fertilization (oaks, orchids). A typical ovule has a stalk (funiculus), a
central bulbous body (nucellus), and one or two integuments (precursors of
seed coats), which cover the nucellus except for a terminal pore (micropyle).
Orientation of the ovule varies from group to group. It can be erect on its
stalk or bent one way or another to differing degrees. There are also
taxonomic differences in the extent to which the ovule is vascularized by
branches from the gynoecial vascular system. See FRUIT; REPRODUCTION (PLANT).
 �34s[Richard H. Eyde]
�33s

A700000004156�254s�5v�1v728
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968192
�6v�2v�32sFlow measurement �21m �33s
    The determination of the quantity of a fluid, either a liquid, vapor, or
gas, that passes through a pipe, duct, or open channel. Flow may be expressed
as a rate of volumetric flow (such as liters per second, gallons per minute,
cubic meters per second, cubic feet per minute), mass rate of flow (such as
kilograms per second, pounds per hour), or in terms of a total volume or mass
flow (integrated rate of flow for a given period of time).
    Flow measurement, though centuries old, has become a science in the
industrial age. This is because of the need for controlled process flows,
stricter accounting methods, and more efficient operations, and because of the
realization that most heating, cooling, and materials transport in the process
industries is in the form of fluids, the flow rates of which are simple and
convenient to control with valve or variable speed pumps.  See	PROCESS
CONTROL.
    Measurement is accomplished by a variety of means, depending upon the
quantities, flow rates, and types of fluids involved. Many industrial process
flow measurements consist of a combination of two devices: a primary device
that is placed in intimate contact with the fluid and generates a signal, and
a secondary device that translates this signal into a motion or a secondary
signal for indicating, recording, controlling, or totalizing the flow. Other
devices indicate or totalize the flow directly through the interaction of the
flowing fluid and the measuring device that is placed directly or indirectly
in contact with the fluid stream.
    For discussion of various types of fluid meters  see  ELECTROMAGNETIC
FLOWMETER; FLUIDIC-FLOW MEASUREMENT; LASER DOPPLER VELOCIMETER; MASS FLOW RATE
METER; METERING ORIFICE; NUCLEAR MAGNETIC RESONANCE FLOWMETER; PITOT TUBE;
POSITIVE DISPLACEMENT FLOWMETER; QUANTITY FLOWMETER; ROTATING IMPELLER
FLOWMETER; TARGET FLOWMETER; THERMAL FLOW MEASUREMENT DEVICES; TRACER METHOD;
ULTRASONIC FLOW MEASUREMENT; VARIABLE-AREA FLOWMETER; VENTURI TUBE; VOLUME
FLOW RATE METER; VORTEX FLOWMETER.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

A700000001720�254s�5v�1v730
\IMG\CI721.IMG
1217024
�6v�2v�32sFluid coupling   �21m �33s 
    A device for transmitting rotation between shafts by means of the 
acceleration and deceleration of a hydraulic fluid. Structurally, a fluid 
coupling consists of an impeller on the input or driving shaft and a runner on 
the output or driven shaft. The two contain the fluid (see illustration). 
Impeller and runner are bladed rotors, the impeller acting as a pump and the 
runner reacting as a turbine. Basically, the impeller accelerates the fluid 
from near its axis, at which the tangential component of absolute velocity is 
low, to near its periphery, at which the tangential component of absolute 
velocity is high. This increase in velocity represents an increase in kinetic 
energy. The fluid mass emerges at high velocity from the impeller, impinges on 
the runner blades, gives up its energy, and leaves the runner at low velocity. 
See HYDRAULICS. 
 �34s[Henry J. Wirry]
�33s 
  
  
A700000000637�254s�5v�1v730
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1217024
�6v�2v�32sFluid dynamics   �21m �33s 
    The science of fluids in motion. Fluid dynamics attempts to describe the 
motion of a fluid as it is displaced and deformed by the action of moving or 
fixed boundaries. Fluid dynamics may be divided into two parts, hydrodynamics 
and aerodynamics. See FLUID MECHANICS; GAS DYNAMICS; HYDRODYNAMICS. 
 �34s[Victor L. Streeter]
�33s 
  
A700000006610�254s�5v�1v730
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1217024
�6v�2v�32sFluid flow   �21m �33s 
    Motion of a fluid as a continuum. Fluids flow whereas solids move as 
bodies. In flow, the individual particles of a substance move relative to each 
other as well as to their surroundings. A fluid is a substance that flows 
under the slightest stress. Thus, the pressure of a gas is sufficient to cause 
it to flow throughout a container to which it is admitted. The weight of a 
liquid is sufficient to cause it to flow in a container but without 
significant change in volume. 
    A fluid may be liquid, vapor, or gas. A liquid will fill the container 
which holds it, but it may have a free surface, that is, a surface from which 
all pressure is removed except that of its own vapor. All liquids are 
relatively incompressible. See FLUIDS. 
    A vapor is a gas whose temperature and pressure are such that it is very 
near the liquid phase. Thus, steam is considered to be a vapor because its 
state is not far from that of water. 
    A gas may be defined as a highly superheated vapor; that is, its state is 
far removed from the liquid phase. Thus, air is considered to be a gas because 
its state is normally very far from that of liquid air. A gas is very 
compressible, and when all external pressure is removed, it tends to expand 
indefinitely. A gas is therefore in equilibrium only when it is completely 
enclosed.  
    The volume of a liquid is altered only slightly by changes in either 
pressure or temperature unless the temperature change is considerable or 
unless the initial temperature is near the critical temperature, but the 
volume of a gas or a vapor is greatly affected by changes in either pressure 
or temperature. See GAS; LIQUID. 
    When there is no friction between adjacent moving particles, flow is 
termed ideal; that is, viscosity is zero. In ideal flow, internal forces at 
any section are always normal to the section. Forces are purely pressure 
forces. Such flow is approached but never achieved in reality. 
    In a real fluid, tangential or shearing forces always come into being 
whenever motion takes place, thus giving rise to fluid friction, because these 
forces oppose the sliding of one particle past another. These friction forces 
are due to a property called viscosity. See VISCOSITY. 
    All liquids are relatively incompressible, and for most purposes water in 
particular can be treated as incompressible; yet it is 10 times as 
compressible as steel. The passage through water of a sound wave, which is 
really a pressure wave, is a result of the water's compressibility. 
    In dealing with the flow of air or other gases when the change in pressure 
is small, so that the change in density is negligible, even gases can be 
treated as incompressible. For an airplane flying at speeds of less than 250 
mi/h (110 m/s) the air can be considered to be of constant density. However, 
as the speed approaches that of sound in air, which is of the order of 700 
mi/h (310 m/s) the pressure of the air adjacent to the body becomes materially 
different from that at some distance away, and air must then be considered to 
be compressible. See COMPRESSIBLE FLOW. For various types of fluid flow see 
also BOUNDARY-LAYER FLOW; IRROTATIONAL FLOW; ISENTROPIC FLOW; LAMINAR FLOW; 
NON-NEWTONIAN FLUID FLOW; ROTATIONAL FLOW; STEADY FLOW; TURBULENT FLOW; 
UNIFORM FLOW; UNSTEADY FLOW; WAKE FLOW. 
 �34s[Robert L. Daugherty]
�33s 
  
A700000002561�254s�5v�1v731
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1333760
�6v�2v�32sFluidic-flow measurement �21m �33s
    Use of fluidic phenomena to measure the rate of fluid flow. Two types of
flowmeters based on such phenomena have been developed: the fluidic oscillator
meter and the fluidic flow sensor.  See  FLUIDICS.
    The fluidic-oscillator meter works on the principle of the Coanda effect,
the tendency of fluid coming out of a jet to follow a wall contour. The fluid
entering the meter will attach to one of two opposing diverging side walls. A
small portion of the stream is split off and channeled back through the
feedback passage on that side to force the incoming stream to attach to the
other side wall. The frequency of the oscillation is directly proportional to
the volume flow through the meter. A sensor detects the oscillations and
transmits the signal. This principal is used in both liquid and gas flowmeters.
    The fluidic-flow sensor (also known as a deflected-jet fluidic flowmeter)
is used in gas flows, particularly in dirty environments. It consists of a jet
of air or other selected gas directed back from the outer nozzle onto two
adjacent small openings. The flow of the gas being measured will deflect the
jet to change the relative pressure on the two ports and thereby give a signal
corresponding to the gas velocity.  See  FLOW MEASUREMENT.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

A700000002121�254s�5v�1v731
\IMG\CI721.IMG
1333760
�6v�2v�32sFluidics   �21m �33s 
    A body of techniques which use flowing fluids as a signal-bearing medium 
and fluid dynamics phenomena as a basis for signal switching and 
amplification. Most work and interest in the field dates between 1960 and 
1968. The work has natural analogies to electronics, electronic devices such 
as transistors, and even electronic circuit board technology. See AMPLIFIER; 
ELECTRONICS; LOGIC CIRCUITS; TRANSISTOR. 
    Fluidic devices are capable of switching speeds in the kilohertz range. 
They function with a variety of fluids and can be included directly in 
hydraulic designs without transducers. In principle, fluidics is appropriate 
to applications where electronics break down, as in high electronic 
interference or nuclear radiation. The relative disadvantages of the devices 
are their potential to clog and the increasing availability of very-high-speed 
inexpensive electronic computation. For these latter reasons, the early 
interest has diminished. See FLUID DYNAMICS; HYDRAULICS. 
 �34s[Edgar H. Bristol]
�33s 
  
A700000004117�254s�5v�1v731
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1333760
�6v�2v�32sFluidization   �21m �33s 
    The processing technique employing a suspension or fluidization of small 
solid particles in a vertically rising stream of fluid - usually gas - so that 
fluid and solid come into intimate contact. This is a tool with many 
applications in the petroleum and chemical process industries. Suspensions of 
solid particles by vertically rising liquid streams are of lesser interest in 
modern processing, but have been shown to be of use, particularly in liquid 
contacting of ion-exchange resins. However, they come in this same 
classification and their use involves techniques of liquid settling, both free 
and hindered (sedimentation), classification, and density flotation. See ION 
EXCHANGE; MECHANICAL CLASSIFICATION. 
    The interrelations of hydromechanics, heat transfer, and mass transfer in 
the gas-fluidized bed involve a very large number of factors. Because of the 
excellent contacting under these conditions, numerous chemical reactions are 
also possible - either between solid and gas, two fluidized solids with each 
other or with the gas, or most important, one or more gases in a mixture with 
the solid as a catalyst. In the usual case, the practical applications in 
plants have far outrun the exact understanding of the physical, and often 
chemical, interplay of variables within the minute ranges of each of the small 
particles and the surrounding gas phase. 
    With such excellent opportunities for heat and mass transfer to or from 
solids and fluids, fluidization has become a major tool in such fields as 
drying, roasting, and other processes involving chemical decomposition of 
solid particles by heat. An important application has been in the catalysis of 
gas reactions, wherein the excellent opportunity of heat transfer and mass 
transfer between the catalytic surface and the gas stream gives performance 
unequaled by any other system. See CATALYSIS; CRACKING; FLUIDIZED-BED 
COMBUSTION, GAS ABSORPTION OPERATIONS; HEAT TRANSFER; MASS-TRANSFER OPERATION; 
UNIT OPERATIONS. 
 �34s[Donald F. Othmer]
�33s 
  
A700000003127�254s�5v�1v731
\IMG\CI721.IMG
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�6v�2v�32sFluidized-bed combustion   �21m �33s 
    A method of burning fuel in which the fuel is continually fed into a bed 
of reactive or inert material while a flow of air passes up through the bed, 
causing it to act like a turbulent fluid. Fluidized beds have long been used 
for the combustion of low-quality, difficult fuels and have become a rapidly 
developing technology for the clean burning of coal. See FLUIDIZATION. 
    A fluidized-bed combustor is a furnace chamber whose floor is slotted, 
perforated, or fitted with nozzles. Air is forced through the floor and upward 
through the chamber. The chamber is partially filled with particles of either 
reactive or inert material, which will fluidize at an appropriate air flow 
rate. When fluidization takes place, the bed of material expands (bulk density 
decreases) and exhibits the properties of a liquid. As air velocity increases, 
the particles mix more violently, and the surface of the bed takes on the 
appearance of a boiling liquid. If air velocity were increased further, the 
bed material would be blown away. 
    Once the bed is fluidized, its temperature can be increased with ignitors 
until a combustible material can be injected to burn within the bed. Proper 
selection of air velocity, operating temperature, and bed material will cause 
the bed to act as a chemical reactor. The three broad areas of application of 
fluidized-bed combustion are incineration, gasification, and steam generation. 
See COAL GASIFICATION; COMBUSTION; GAS TURBINE; STEAM GENERATING UNIT. 
 �34s[Michael Pope]
�33s 
  
A700000002141�254s�5v�1v730
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�6v�2v�32sFluid mechanics   �21m �33s 
    The science concerned with fluids, either at rest or in motion. It deals 
with pressures, velocities, and accelerations in the fluid, including fluid 
deformation and compression or expansion. Fluid mechanics may be divided into 
two branches, fluid statics and fluid dynamics; the first deals with pressure 
intensities and forces exerted by a fluid at rest, and the second with forces 
exerted on fluids and their resulting motions. See FLUID DYNAMICS; FLUID 
STATICS.  
    The laws of fluid mechanics control a great portion of natural phenomena. 
The flight of an insect or bird, the motion of a fish through water, the 
relative movement of air masses as in frontal weather systems, and the 
eruption of a volcano are examples of flow that follow the laws of fluid 
mechanics. The science of fluid mechanics is involved in many phases of 
aeronautical, chemical, civil, and mechanical engineering. It requires the 
combination of theoretical analysis and orderly experimentation. See FLUID 
FLOW.  
 �34s[Victor L. Streeter]
�33s 
  
A700000002346�254s�5v�1v732
\IMG\CI721.IMG
1459200
�6v�2v�32sFluids   �21m �33s 
    Substances with no reference configuration of permanent significance. 
Aggregates of matter in which the molecules are able to flow past each other 
without limit and without fracture planes forming are usually classified as 
fluids. The subdivisions of fluids known as gases, vapors, and liquids, each 
of which exhibits successively closer association of the molecules and is 
distinguished by different thermodynamic and mechanical properties, compose a 
group of easily distinguishable fluids, some having mainly newtonian and the 
rest mainly non-newtonian flow properties. 
    There are some substances, the semisolids, which appear to be able to flow 
without fracture and yet which also have some of the attributes of solids such 
as the ability to form free-standing figures. Butter in a temperate 
environment is an excellent example of a material with this dual nature. 
    Similarly, some fluids possess elastic properties, the so-called 
viscoelastic fluids. Such substances are not easy to classify in one or 
another category, and their properties are not well understood in depth. See 
GAS; LIQUID; NON-NEWTONIAN FLUID. 
 �34s[J. Harris ;W. L. Wilkinson]
�33s 
  
A700000003224�254s�5v�1v730
\IMG\CI721.IMG
1217024
731
\IMG\CI721.IMG
1333760
�6v�2v�32sFluid statics   �21m �33s 
    The determination of pressure intensities and forces exerted by liquids 
and bases at rest. Hydrostatics, although implying the statics of water alone, 
applies to liquids in general. By definition, a fluid at rest cannot sustain a 
shear stress; therefore, a fluid force exerted on an element of boundary area 
must act normal to the area. For example, consider the small free body of 
fluid shown in the illustration. Equilibrium requires that dp = - GAMMA dz, in 
which p is the absolute pressure, GAMMA is the specific weight (weight of a 
unit volume of fluid), and z is the elevation, measured vertically upward. For 
liquids, GAMMA is substantially constant, and the equation shows that pressure 
decreases linearly as the elevation increases. See HYDROSTATICS. 
    With gases, the variation of GAMMA with pressure or elevation must be 
known in order to integrate the equation. For an isothermal gas [p / GAMMA = 
(p sub 0) (GAMMA sub 0) = constant], the pressure variation with elevation is 
given by the equation below, in which (p sub 0) and (GAMMA sub 0) are the 
values of p and GAMMA at elevation (z sub 0). 

 *** p = (p sub 0) exp {- [z - (z sub 0)] / [(p sub 0) / (GAMMA sub 0)]} ***

    Fluid forces on plane surfaces may be determined by integration of p dA     
over the surface, with p the pressure intensity and dA an element of the 
surface area. For liquids, the magnitude of the force is the product of the
area and the pressure at the centroid of the area. See ARCHIMEDES' PRINCIPLE;
BUOYANCY; FLUID MECHANICS.
 �34s[Victor L. Streeter]
�33s 
  
  

A700000000766�254s�5v�1v732
\IMG\CI721.IMG
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�6v�2v�32sFluke   �21m �33s 
    Any of more than 40,000 species of parasitic flatworms that form the class 
Trematoda in the phylum Platyhelminthes. The class can be separated into two 
sharply demarcated subclasses: Monogenea, the majority of which are 
ectoparasites of cold-blooded vertebrates; and Digenea, the largest group of 
flatworms, endoparasites of vertebrates and invertebrates. See DIGENEA; 
MONOGENEA; TREMATODA. 
 �34s[Charles B. Curtin]
�33s 
  
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\IMG\CI721.IMG
1459200
�6v�2v�32sFluoborate   �21m �33s 
    In the broadest sense, the term fluoborate refers to a group of compounds 
related to the borates in which one or more oxygens have been replaced by 
fluorines. In a strict sense, fluoborate refers to the BF4- ion which is 
derived from tetrafluoroboric acid, HBF4. The fluoborates are used as fluxes 
and in electroplating. See BORATE; FLUORINE. 
 �34s[E. Eugene Weaver]
�33s 
  
A700000004011�254s�5v�1v732
\IMG\CI721.IMG
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�6v�2v�32sFluorescence �21m �33s
    Fluorescence is generally defined as a luminescence emission that is
caused by the flow of some form of energy into the emitting body, this
emission ceasing abruptly when the exciting energy is shut off. In attempts to
make this definition more meaningful it is often stated, somewhat arbitrarily,
that the decay time, or afterglow, of the emission must be of the order of the
natural lifetime for allowed radiative transitions in an atom or a molecule,
which is about 10- sup 8 s for transitions involving visible light. Perhaps a
better distinction between fluorescence and its counterpart, phosphorescence,
rests not on the magnitude of the decay time per se, but on the criterion that
the fluorescence decay is temperature-independent.
    In the literature of organic luminescence, the term fluorescence is used
exclusively to denote a luminescence which occurs when a molecule makes an
allowed optical transition. Luminescence with a longer exponential decay time,
corresponding to an optically forbidden transition, is called phosphorescence,
and it has a different special distribution from the fluorescence. See
PHOSPHORESCENCE.
    The decay time of fluorescent materials varies widely, from the order of 5
x 10- sup 9 s for many organic crystalline materials up to 2 s for the
europium-activated strontium silicate phosphor. Fluorescent materials with
decay times between 10- sup 9 and 10- sup 7 s are used to detect and measure
high-energy radiations, such as x-rays and gamma rays, and high-energy
particles such as alpha particles, beta particles, and neutrons. These agents
produce light flashes (scintillations) in certain crystalline solids, in
solutions of many polynuclear aromatic hydrocarbons, or in plastics
impregnated with these hydrocarbons. The so-called fluorescent lamps employ
the luminescence of gases and solids in combination to produce visible light.
See ABSORPTION; FLUORESCENT LAMP; LUMINESCENCE.
 �34s[James H. Schulman ;Clifford C. Klick]
�33s

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�6v�2v�32sFluorescence astronomy   �21m �33s 
    The study of astronomical objects in which certain elements absorb photons 
of energy greater than their ionization energy, thereby causing the elements 
to fluoresce, or to emit luminescence. The conversion of photons of high 
energy to lower energy occurs in a variety of astronomical objects. Examples 
are the astrophysically most abundant elements, such as hydrogen, helium, and 
ionized helium. The commonest physical environment is a low-density gas cloud 
near or surrounding a hot star. See ATOMIC STRUCTURE AND SPECTRA; 
FLUORESCENCE; STAR. 
 �34s[Jesse L. Greenstein]
�33s 
  
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�6v�2v�32sFluorescence microscope   �21m �33s 
    A variation of the compound laboratory light microscope which is arranged 
to transmit ultraviolet, violet, and sometimes blue radiations to a specimen. 
The specimen then fluoresces, that is, appears to be self-luminous and often 
colored. The phenomenon of fluorescence is thought to involve an electronic 
rearrangement in the irradiated substance. See FLUORESCENCE; OPTICAL 
MICROSCOPE.  
    Materials with absorption-spectra maxima below 320 nanometers require a 
quartz condenser in place of the usual glass condenser of the microscope. The 
microscope should have an aluminized front-surface mirror because silver is a 
less efficient reflector of ultraviolet. 
    The fluorescent image looks bright and has good contrast, although the 
amount of light is small. With weak fluorescence a monocular microscope must 
be used and the work is done in a darkened room. With more intense 
fluorescence a binocular microscope should be coated to decrease loss of light. 
    The absorbing filter is usually in the microscope between the objective 
and the observer's eye to remove any other exciting radiation not absorbed by 
the specimen. This may be a nearly colorless filter absorbing only ultraviolet 
when the specimen is to be observed in full color, or it may be of a color 
complementary to the radiation used. For example, with materials absorbing 
blue and ultraviolet radiation a yellow filter absorbs the blue radiation 
beyond the specimen and passes the yellow fluorescence of the specimen to the 
eye. The cross-filter combination is chosen to give the best visibility for 
the specimen. 
 �34s[Oscar W. Richards]
�33s 
  
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�6v�2v�32sFluorescence spectroscopy �21m �33s
    A technique of fluorometric analysis in which the emitted fluorescent
light is passed through a monochromator so that the fluorescence emission
spectrum can be recorded. It is possible to measure several fluorescing
compounds in the same solution, provided that they have sufficiently different
fluorescence emission spectra. In obtaining this selectivity, some sensitivity
is lost since some fluorescent light is lost in the monochromator, and only a
small portion of the total fluorescent energy emitted is measured at any one
wavelength. See FLUORESCENCE; FLUOROMETRIC ANALYSIS; OPTICAL METHODS OF
CHEMICAL ANALYSIS.
 �34s[Robert F. Goddu ;James N. Little]
�33s


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�6v�2v�32sFluorescent lamp   �21m �33s 
    A lamp which produces light largely by conversion of ultraviolet energy 
from a low-pressure mercury arc to visible light. Chemicals that absorb 
radiant energy of a given wavelength and reradiate at longer wavelengths are 
called phosphors. The phosphors produce most of the light provided by 
fluorescent lamps. 
  
    The fluorescent lamp consists of a glass tube containing two electrodes, a 
coating of powdered phosphor, and small amounts of mercury. The glass tube 
seals the inner parts of the lamps from the atmosphere. The electrodes provide 
a source of free electrons to initiate the arc and are connected to the 
external circuit through the ends of the lamp. The phosphor is a chemical or 
mixture of chemicals that converts short-wave ultraviolet energy into light. 
The mercury, when vaporized in the arc, produces the ultraviolet radiation 
that causes fluorescence. Argon gas, introduced in small quantities, provides 
the ions that facilitate starting of the lamp (see illustration). See 
FLUORESCENCE.  
    Fluorescent lamps are usually operated on ac circuits with a frequency of 
60 Hz. However, higher frequencies permit higher-efficiency lamp operation 
with simpler ballasts of lower power dissipation per watt of lamp. 
Consequently, systems have been developed for the operation of fluorescent 
lamps at frequencies from 360 to 3000 Hz. These systems employ various types 
of frequency converters to obtain the high-frequency power. 
    Fluorescent lamps provide light at several times the efficiency of 
incandescent lamps, the exact ratio depending on the fluorescent lamp color. 
Lamp color is determined by the selection of chemicals used in the phosphors. 
Several types of essentially white fluorescent lamps are available 
commercially, as well as a range of tinted and saturated colors. 
 �34s[Alfred Makulec]
�33s 
  
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�6v�2v�32sFluoride   �21m �33s 
    A compound derived from hydrofluoric acid, HF, in which the fluorine atom 
is in the 1 - oxidation state. The small size of the fluoride ion means that 
the extra electron is held tightly; it is difficult to convert the fluoride 
ion to free fluorine. The small size also means that there is less tendency to 
form covalent bonds in simple salts such as mercuric fluoride, HgF2, which has 
an ionic lattice, whereas its chlorine analog, mercuric chloride, HgCl2, has a 
molecular crystal. In direct contrast to this, many fluorides are known in 
which the central atom is in a higher oxidation state than that observed for 
other halides and these compounds are often covalent, for example, vanadium 
pentafluoride, VF5, chromium tetrafluoride, CrF4, bismuth pentafluoride, BiF5, 
and sulfur hexafluoride, SF6. See FLUOBORATE; FLUOROCARBON; FLUOSILICATE; 
HALOGENATED HYDROCARBON. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sFluorine �21m �33s
    A chemical element, symbol F, atomic number 9, the member of the halogen
family that has the lowest atomic number and atomic weight. Although only the
isotope with atomic weight 19 is stable, the other, radioactive isotopes
between atomic weight 17 and 22 have been artificially prepared. Fluorine is
the most electronegative element, and by a substantial margin the most
chemically energetic of the nonmetallic elements.

    Properties.  The element fluorine is a pale yellow gas at ordinary
temperatures. The odor of the element is somewhat in doubt. Some physical
properties are listed in the table. The reactivity of the element is so great
that it will react readily at ordinary temperatures with many other elementary
substances, such as sulfur, iodine, phosphorus, bromine, and most metals.
Since the products of the reactions with the nonmetals are in the liquid or
gaseous state, the reactions continue to the complete consumption of the
fluorine, frequently with the evolution of considerable heat and light.
Reactions with the metals usually form a protective metallic fluoride which
blocks further reaction, unless the temperature is raised. Aluminum, nickel,
magnesium, and copper form such protective fluoride coatings.
    Fluorine reacts with considerable violence with most hydrogen-containing
compounds, such as water, ammonia, and all organic chemical substances whether
liquids, solids, or gases. The reaction of fluorine with water is very
complex, yielding mainly hydrogen fluoride and oxygen with less amounts of
hydrogen peroxide, oxygen difluoride, and ozone. Fluorine displaces other
nonmetallic elements from their compounds, even those nearest fluorine in
chemical activity. It displaces chlorine from sodium chloride, and oxygen from
silica, glass, and some ceramic materials. In the absence of hydrofluoric
acid, however, fluorine does not significantly etch quartz or glass even after
several hours at temperatures as high as 390DGF (200DGC).
    Fluorine is a very toxic and reactive element. Many of its compounds,
especially inorganic, are also toxic and can cause severe and deep burns. Care
must be taken to prevent liquids or vapors from coming in contact with the
skin or eyes.

    Natural occurrence.  At an estimated 0.065% of the Earth's crust, fluorine
is roughly as plentiful as carbon, nitrogen, or chlorine, and much more
plentiful than copper or lead, though much less abundant than iron, aluminum,
or magnesium. Compounds whose molecules contain atoms of fluorine are widely
distributed in nature. Many minerals contain small amounts of the element, and
it is found in both sedimentary and igneous rocks.

    Uses.  Fluorine-containing compounds are used to increase the fluidity of
melts and slags in the glass and ceramic industries. Fluorspar (calcium
fluoride) is introduced into the blast furnace to reduce the viscosity of the
slag in the metallurgy of iron. Cryolite, Na2AlF6, is used to form the
electrolyte in the metallurgy of aluminum. Aluminum oxide is dissolved in this
electrolyte, and the metal is reduced electrically from the melt. The use of
halocarbons containing fluorine as refrigerants was patented in 1930, and
these volatile and stable compounds found a market in aerosol propellants as
well as in refrigeration and air-conditioning systems. However, use of
fluorocarbons as propellants has declined sharply because of concern over
their possible damage to the ozone layer of the atmosphere. A use for fluorine
that became prominent during World War II is in the enrichment of the
fissionable isotope (sup 235)U; the most important process employed uranium
hexafluoride. This stable, volatile compound was by far the most suitable
material for isotope separation by gaseous diffusion.
    While consumers are mostly unaware of the fluorine compounds used in
industry, some compounds have become familiar to the general public through
minor but important uses, such as additive to toothpaste and nonsticking
fluoropolymer surfaces on frying pans and razor blades (for example,  Teflon).

    Compounds.	In all fluorine compounds the high electronegativity of this
element suggests that the fluorine atom has an excess of negative charge. It
is convenient, however, to divide the inorganic binary fluorides into saltlike
(ionic lattice) nonvolatile metallic fluorides and volatile fluorides, mostly
of the nonmetals. Some metal hexafluorides and the noble-gas fluorides show
volatility that is frequently associated with a molecular compound. Volatility
is often associated with a high oxidation number for the positive element.
    The metals characteristically form nonvolatile ionic fluorides where
electron transfer is substantial and the crystal lattice is determined by
ionic size and the predictable electrostatic interactions. When the
coordination number and valence are the same, for example, BF3, SiF4, and WF6,
the binding between metal and fluoride is not unusual, but the resulting
compounds are very volatile, and the solids show molecular lattices rather
than ionic lattice structures. For higher oxidation numbers, simple ionic
lattices are less common and, while the bond between the central atom and
fluorine usually still involves transfer of some charge to the fluorine,
molecular structures are identifiable in the condensed phases.
    In addition to the binary fluorides, a very large number of complex
fluorides have been isolated, often with a fluoroanion containing a central
atom of high oxidation number. The binary saltlike fluorides show a great
tendency to combine with other binary fluorides to form a large number of
complex or double salts.
    The fluorine-containing compounds of carbon can be divided into
fluorine-containing hydrocarbons and hydrocarbon derivatives (organic fluorine
compounds) and the fluorocarbons and their derivatives. The fluorine atom
attached to the aromatic ring, as in fluorobenzene, is quite unreactive. In
addition, it reduces the reactivity of the molecule as a whole. Dyes, for
example, that contain fluorine attached to the aromatic ring are more
resistant to oxidation and are more light-fast than dyes that do not contain
fluorine. Most aliphatic compounds, such as the alkyl fluorides, are unstable
and lose hydrogen fluoride readily. These compounds are difficult to make and
to keep and are not likely to become very important. See FLUOROCARBON; HALOGEN
ELEMENTS.
 �34s[Irving Sheft]
�33s

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�6v�2v�32sFluorite   �21m �33s 
    A mineral of composition CaF2. It is the most abundant fluorine-bearing 
mineral, and occurs as cubes or compact masses and more rarely as octahedra 
with complex modifications. Fluorite has a perfect octahedral cleavage, 
hardness 4 (Mohs scale), and specific gravity 3.18. The color is extremely 
variable, the most common being green and purple; but fluorite may also be 
colorless, white, yellow, blue, or brown. Colors may result from the presence 
of impurity ions. Fluorite frequently emits a blue-to-green fluorescence under 
ultraviolet radiation, especially if rare-earth or hydrocarbon material is 
present. Some fluorites are thermoluminescent; that is, they emit light when 
heated.  
    Fluorite occurs as a typical hydrothermal vein mineral with quartz, 
barite, calcite, sphalerite, and galena. Crystals of great beauty from 
Cumberland, England, and Rosiclare, Illinois, are highly prized by mineral 
fanciers. It also occurs as a metasomatic replacement mineral in limestones 
and marbles. 
 �34s[Paul B. Moore]
�33s 
  
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�6v�2v�32sFluorocarbon   �21m �33s 
    An organic compound containing fluorine directly bonded to carbon. Simple 
fluorocarbons are made by the liquid-phase reaction of carbon tetrachloride 
with hydrogen fluoride catalyzed by antimony trifluoride. CCl3F is employed 
mainly as an aerosol propellant, and CCl2F2 is used as a refrigerant. They are 
low-boiling, odorless, nonflammable gases. Higher fluorocarbons are nontoxic. 
For example, tetrafluoroethylene, CF2CF2, undergoes polymerization in the 
presence of free-radical initiators to form the highly inert 
polytetrafluoroethylene.  
    Consumption of the fluorocarbon aerosol propellants has been declining 
because of concern over their possible effect on the stratospheric ozone 
layer, and there is a government prohibition for certain applications, such as 
deodorants, hairsprays, perfumes, polishes, some insecticides, and food 
products. The largest use for these products is as a refrigerant. Some 
fluorocarbons are used as blowing agents in the manufacture of both flexible 
and rigid polyurethane foams and in polystyrene and polyethylene foams. Almost 
all the remaining fluorocarbons are used as grease-removing solvents, 
especially in electronic and aerospace industries, and for the production of 
fluoropolymers. See FREON; HALOGENATED HYDROCARBON; POLYFLUOROOLEFIN RESINS. 
 �34s[Frank Wagner]
�33s 
  
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�6v�2v�32sFluorometric analysis �21m �33s
    A method of chemical analysis in which a sample, exposed to radiation of
one wavelength, absorbs this radiation and reemits radiation of the same or
longer wavelength. If this reemission occurs in about 10- sup 9 s, it is called
fluorescence. Reemission after about 10- sup 6 s or more is called
phosphorescence. Fluorescence analysis utilizes the reemitted light to
determine the material which is the source of the fluorescence. See
FLUORESCENCE; PHOSPHORESCENCE.
    The radiation source is usually a mercury arc, with one line of its
spectrum isolated by the primary filter. The light then passes through the
sample, and the fluorescence is measured at an angle, usually a right angle,
to the light beam. A diagram of the components involved in the technique is
shown in the illustration. Fluorometric analysis can also be employed in the
x-ray region by using a molybdenum or tungsten source, appropriate metal
filters, and, usually, a solid sample.
    For quantitative work at very low fluorescent intensities, the intensity
is almost directly proportional to the concentration of the fluorescing
material. At high fluorescent intensities there are usually appreciable
deviations from any linear function of fluorescent intensity and
concentration. For this reason, fluorescence analysis is usually carried out
in a very dilute solution, and a calibration curve of fluorescent intensity
versus concentration of emitter is carefully prepared.
    Fluorescence is widely used in analytical work because of its sensitivity
and selectivity in many systems. For example, vitamin B2 (riboflavin) is
usually determined by its fluorescence in dilute solutions. The fluorescence
technique has been extended to the determination of amino acids, proteins, and
nucleic acids. Solids are also often analyzed by fluorescence. In particular,
uranium compounds fluoresce with a yellow color, and uranium in ores is often
detected after fusion of the ore with sodium fluoride. Many sensitive
qualitative tests for the presence of various inorganic ions have been
developed, based on the selective formation of a fluorescing compound. See
FLASH-PHOTOLYSIS RESONANCE FLUORESCENCE; FLUORESCENCE SPECTROSCOPY.
 �34s[Michael J. Kurylo]
�33s

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�6v�2v�32sFluosilicate �21m �33s
    The SiF6 sup 2- ion which is derived from fluosilicic acid, H2SiF6. The
fluosilicates are prepared from solutions of fluosilicic acid. The salts of
barium, potassium, rubidium, cesium, and sodium are only sparingly soluble,
whereas many of the heavy-metal salts are quite soluble.
    Sodium fluosilicate, Na2SiF6, is used in the fluoridation of water
supplies and in laundry scours, enamel frits, insecticides, and wood
preservatives. See FLUORINE; SILICATE.
 �34s[E. Eugene Weaver]
�33s

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�6v�2v�32sFlutter and wow   �21m �33s 
    All sound recording and reproducing processes employ mechanical systems 
for the translation of electric waves or sound waves into and from the 
permanent record. If the speeds of either or both the recording mechanism and 
reproducing mechanism vary at some rate, then a constant frequency impressed 
upon the recorder will not be reproduced as a constant frequency by the 
reproducer. The frequency of the tone will vary above and below the normal 
pitch depending upon the extent of the speed variation, and in frequency at 
the rate of the speed variation. The manifestation of this phenomenon is 
termed flutter. The term wow is used to designate this phenomenon for lower 
rates of flutter, such as those encountered in phonograph recording. 
    Flutter and wow are produced in disk recording systems by variations in 
the rotational speed of the recording and reproducing turntables and by a 
nonsymmetrically located center hole in the disk record. Flutter and wow are 
produced in tape and film reproducers by a nonuniform rotational speed of the 
capstan in the recording and reproducing machines, by nonuniform tension in 
the take-up and pay-off reel systems, by the variety of idlers, and by a 
mechanical distortion of the tape in the recording or reproducing process. See 
DISK RECORDING; MAGNETIC RECORDING. 
 �34s[Harry F. Olson]
�33s 
  
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�6v�2v�32sFlutter (aeronautics)   �21m �33s 
    An aeroelastic self-excited vibration with a sustained or divergent 
amplitude, which occurs when a structure is placed in a flow of sufficiently 
high velocity. Flutter is an instability that can be extremely violent. At low 
speeds, in the presence of an airstream, the vibration modes of an aircraft 
are stable; that is, if the aircraft is disturbed, the ensuing motion will be 
damped. At higher speeds, the effect of the airstream is to couple two or more 
vibration modes such that the vibrating structure will extract energy from the 
airstream. The coupled vibration modes will remain stable as long as the 
extracted energy is dissipated by the internal damping or friction of the 
structure. However, a critical speed is reached when the extracted energy 
equals the amount of energy that the structure is capable of dissipating, and 
a neutrally stable vibration will persist. This is called the flutter speed. 
At a higher speed, the vibration amplitude will diverge, and a structural 
failure will result. See AEROELASTICITY. 
    Aircraft manufacturers now have engineering departments whose primary 
responsibility is flutter safety. Modern flutter analyses involve extensive 
computations, requiring the use of large-capacity, high-speed digital 
computers. Flutter engineers contribute to the design by recommending 
stiffness levels for the structural components and control surface actuation 
systems and weight distributions on the lifting surfaces, so that the aircraft 
vibration characteristics will not lead to flutter within the design speeds 
and altitudes. See AIRFRAME; WING. 
 �34s[William P. Rodden]
�33s 
  
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�6v�2v�32sFluvial sediments   �21m �33s 
    Deposits formed on land from the sediment load carried by rivers. Many 
such deposits are ephemeral and are reeroded as the river continues to degrade 
its bed. Geologically more significant deposits are those preserved in areas 
of net aggradation (vertical sediment accretion), where the sedimentary basin 
subsides at a rate faster than downward erosion. 
    Fluvial sediments range in grain size from boulder conglomerates to the 
finest mud and contain a variety of internal structures. The nature of any 
deposit depends on the morphology of the river which forms it, the river 
morphology itself being dependent on climate, discharge (amount and 
variability), sediment load, and slope. See DELTA; FLOODPLAIN; STREAM 
TRANSPORT AND DEPOSITION. 
    Many deposits of nonrenewable fuels and minerals occur in fluvial 
sediments. Placer gold, uranium, and diamond deposits (in the Yukon and South 
Africa, for example) consist of detrital fragments transported and 
concentrated on riverbeds. Porous gravel and sand units make excellent 
aquifers and may be conduits and traps for migrating fluids such as oil and 
gas (as in the Gulf Coast and Alberta Province) or metallic mineralizing 
fluids, including copper, tin, and uranium (as in Texas and the Rocky Mountain 
region). Coal is a floodplain deposit which typically occurs in coastal river 
and delta systems. See SEDIMENTATION (GEOLOGY). 
 �34s[Andrew D. Miall]
�33s 
  
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�6v�2v�32sFluxional compounds   �21m �33s 
    Molecules that undergo rapid intramolecular rearrangements among 
equivalent structures in which the component atoms are interchanged. The 
rearrangement process is usually detected by nuclear magnetic resonance (NMR) 
spectroscopy. With sufficiently rapid rates, a single resonance is observed in 
the NMR spectrum for a molecule that might be expected to have several 
nonequivalent nuclei on the basis of its instantaneous structure. 
    Within organic chemistry, degenerate Cope rearrangements represented some 
of the first examples of interconversions between equivalent structures, but 
these were relatively slow. The rate of this rearrangement is rapid in more 
complex molecules. The epitome of degeneracy is reached in bullvalene, which 
has more than 1,200,000 equivalent structures and rapidly interconverts among 
them.  
    Fluxional molecules are frequently encountered in organometallic 
chemistry, and rapid rearrangements which involve migrations about unsaturated 
organic rings are commonly observed. The best known (called ring-whizzers) are 
cyclopentadienyl and cyclooctatetraene complexes of iron. 
    Inorganic structures also exhibit fluxional phenomena, and five-coordinate 
complexes provide the greatest number of wellknown examples, one being 
phosphorus pentafluoride (PF5). 
    The rearrangement of PF5 involves interconversion of the trigonal 
bipyramidal molecule to a square pyramidal configuration and back. If two such 
nonequivalent structures are present in observable concentrations but 
interconvert rapidly to cause averaging in the NMR experiments, they are said 
to be stereochemically nonrigid. This term is generally taken to embrace all 
compounds that undergo rapid reversible intramolecular rearrangements. Thus, 
fluxional compounds are a subset of nonrigid compounds with equivalent 
structures. Nonequivalent structures, that is, tautomers, might be 
stereochemically nonrigid if they rearranged rapidly, but would not be 
considered fluxional. 
    Some workers prefer to reserve the term fluxional for molecules in which 
bonds are broken and reformed in the rearrangement process. Hence, of the 
examples above, only bullvalene and the iron complexes would be termed 
fluxional, whereas all would be considered stereochemically nonrigid. See 
NUCLEAR MAGNETIC RESONANCE (NMR); RESONANCE (MOLECULAR STRUCTURE); TAUTOMERISM. 
 �34s[J. W. Faller]
�33s 
  
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�6v�2v�32sFluxmeter   �21m �33s 
    An instrument designed to measure magnetic flux. The fluxmeter is 
essentially a moving-coil ballistic galvanometer with a long period and 
restoring torque reduced to minimum. The low restoring torque (ideally zero) 
may be obtained by substituting a silk fiber for the usual metallic strip and 
making connection to the moving coil by light metallic spirals designed to 
exert negligible control of the motion. See GALVANOMETER; MAGNETIC FLUX. 
    A search coil is connected to the fluxmeter, as shown in the illustration, 
and the change in the flux that produces the induced emf occurs within the 
search coil. This change may be caused by a change in the flux that links a 
stationary coil or by a motion of the search coil in the field so that there 
is a change in the flux linkage of the coil. See SEARCH COIL. 
 �34s[Kenneth V. Manning]
�33s 
  
  
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�6v�2v�32sFly   �21m �33s 
    A member of the insect order Diptera. Insects of other orders are 
popularly called flies; mayflies, stoneflies, dragonflies, dobsonflies, and 
caddis flies all have four wings and therefore are not true flies. Mosquitoes, 
gnats, and midges all have two wings and are therefore also true flies, order 
Diptera. Hindwings of all Diptera are greatly reduced balancing organs called 
halteres.  
    Flies are a numerous and diverse lot, with over 85,000 described species 
worldwide. North America has 16,000 species belonging to 107 families. Flies 
are old, their fossils dating from the Triassic, over 200,000,000 years ago. 
    The flies of greatest importance to humanity are those that suck blood 
from people or domestic animals. Females of many mosquitoes, deerflies, 
horseflies, blackflies, and gnats require a meal of blood before producing 
eggs. When biting, they may introduce pathogenic microorganisms. Mosquitoes 
may transmit malaria, yellow fever, viral encephalitis, or parasitic 
roundworms. Tropical blackflies transmit onchocerciasis (river blindness), and 
sandflies transmit leishmaniasis, a debilitating protozoan infection. Tsetse 
flies, in which both sexes bite, transmit African sleeping sickness. 
    Some flies that do not bite may become a nuisance because of their sheer 
numbers and association with human habitations. The ubiquitous housefly is 
bothersome and, in unsanitary situations, may contaminate food with the 
pathogens of hepatitis, polio, cholera, typhoid, or tuberculosis. 
    Many thousands of flies are predatory, and they doubtless help to suppress 
populations of insect pests. Especially important are larvae of the family 
Syrphidae that eat up to 50 aphids per day. Scavenging Diptera are quite 
important in aiding the quick breakdown of dead animals and plants. See 
DIPTERA; ENTOMOLOGY, ECONOMIC. 
 �34s[David J. Horn]
�33s 
  
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�6v�2v�32sFlyingfish   �21m �33s 
    About 65 species of marine fishes which form the family Exocoetidae in the 
order Atheriniformes. The enlarged pectoral fins are used for gliding; the 
fish are incapable of true flight. None of the species is economically 
important. _Flyingfish travel in large schools in the open seas. These fish 
attain a considerable speed under water, then suddenly rise to the surface and 
pass into the air, propelled by the large forked caudal fin. The pectoral 
fins, which are pressed to the body when in water, are spread out to form a 
gliding surface. Some species may glide as far as 400 yd (365 m), but always 
near the surface of the water. See ATHERINIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sFlywheel   �21m �33s 
    A rotating mass used to maintain the speed of a machine between given 
limits while the machine releases or receives energy at a varying rate. A 
flywheel is an energy storage device. It stores energy as its speed increases 
and gives up energy as the speed decreases. The specifications of the machine 
usually determine the allowable range of speed and the required energy 
interchange.  
    The difficulty of casting stress-free spoked flywheels leads the modern 
designer to use solid web castings or welded structural steel assemblies. For 
large, slow-turning flywheels on heavy-duty diesel engines or large mechanical 
presses, cast spoked flywheels of two-piece design are standard (see 
illustration). See ENERGY STORAGE. 
 �34s[L. Sigfred Linderoth, Jr.]
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�6v�2v�32sFoam   �21m �33s 
    A material made up of gas bubbles separated from one another by films of 
liquid. The bubbles are spherical when the liquid films separating them are 
thick (approximately 0.01 mm). Pure liquids do not foam; that is to say, they 
cannot produce liquid films of any permanence. Relatively permanent films are 
created only when a substance is present that is adsorbed at the surface of 
the liquid. Substances capable of being so adsorbed may be in true solution in 
the liquid or may be particles of a finely divided solid, which, because of 
poor wetting by the liquid, remain at the surface. In both cases, surface 
layers of the added substance are produced. The reluctance of the adsorbed 
substance to enter the bulk of the liquid preserves the surface and, hence, 
the thermodynamic stability of the foam. See SURFACE-ACTIVE AGENT. 
    Although thermodynamically stable, a foam is mechanically fragile. 
Offsetting this fragility to some extent are mechanisms that provide the 
liquid films with resiliency and plasticity. 
    Although foams of exceptional stability are desired in some commercial 
applications, foam is a nuisance in many situations. A common recourse is the 
addition of chemical antifoams, which are usually insoluble liquids of very 
low surface tension. When a droplet of such a liquid is sprayed onto the foam 
or is carried into it by mechanical agitation, it spreads spontaneously and 
rapidly at the surface of the film, virtually sweeping the film away as it 
does so. See ADSORPTION; INTERFACE OF PHASES; SURFACE TENSION. 
 �34s[Sidney Ross]
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�6v�2v�32sFocal length   �21m �33s 
    A measure of the collecting or diverging power of a lens or an optical 
system. Focal length, usually designated f' in formulas, is measured by the 
distance of the focal point (the point where the image of a parallel entering 
bundle of light rays is formed) from the lens, or more exactly by the distance 
from the principal point to the focal point. See GEOMETRICAL OPTICS. 
    The power of a lens system is equal to n'/f', where n' is the refractive 
index in the image space (n' is usually equal to unity). A lens of zero power 
is said to be afocal. Telescopes are afocal lens systems. See DIOPTER; LENS 
(OPTICS); TELESCOPE. 
 �34s[Max J. Herzberger]
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�6v�2v�32sFog   �21m �33s 
    Water droplets or, rarely, ice crystals suspended in the air so as to 
reduce visibility appreciably; a cloud resting on the ground. Fog is most 
common near seacoasts or large lakes, on mountains, in small valleys of 
nonarid regions, and over oceans where the water is relatively cold, as in 
summer off Newfoundland or California. On land, fog is most frequent in the 
early morning. 
    Fog results either when air is cooled to its dew point and below or from 
an increase of moisture through evaporation from water that is warmer than the 
air. Causes of cooling are the especially rapid nighttime radiation that 
develops with clear skies; advection (air moving over a colder surface); and 
expansional (adiabatic) cooling, as when air moves up a mountain slope. For 
radiation to cause fog, winds must be light or calm; otherwise, mixing 
prevents fog or produces low clouds. 
    Before fog can form, there must be sufficient condensation nuclei in the 
air. This condition is usually met, but ice nuclei are often lacking in 
sufficient quantity so that many fogs below freezing are water fogs. Fog with 
snow cover, however, is common only near freezing, and again, at temperatures 
below about -40DGC (-40DGF). See DEW; DEW POINT; HUMIDITY; PRECIPITATION 
(METEOROLOGY); SMOG. 
 �34s[J. R. Fulks]
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�6v�2v�32sFolic acid   �21m �33s 
    A yellow vitamin, slightly soluble in water, which is usually found in 
conjugates containing varying numbers of glutamic acid residues. It is also 
known as pteroylglutamic acid (PGA), and has the structural formula shown. 
                             ******FORMULA****** 
  
    Folic acid is so widespread in nature and intestinal synthesis is so great 
that a folic acid deficiency in humans because of low dietary intake is 
probably not very common. Deficiencies of other nutrients (particularly iron, 
ascorbic acid, or vitamin B12) may lead to a number of clinical conditions in 
which folic acid deficiency is involved. These include various nutritional 
macrocytic anemias, sprue, idiopathic steatorrhea, and pernicious anemia. See 
VITAMIN.  
 �34s[Stanley N. Gershoff]
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�6v�2v�32sFontechevade man   �21m �33s 
    A fossil man, known from a skullcap of an old adult and a brow fragment of 
an adolescent excavated in 1947 by Germaine Henri-Martin in the Fontechevade 
cave, Charente, France. The fossils, as reconstructed by Henri Vallois, 
appeared similar to modern humans who lack large browridges. Studies have 
shown the fossils to be similar to other Last Interglacial human fossils and 
possibly ancestral to the more recent Neandertals. See FOSSIL MAN; NEANDERTAL 
MAN.  
 �34s[Erik Trinkaus]
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�6v�2v�32sFood �21m �33s
    The material that enables humans to grow and to maintain and reproduce
themselves. Food is composed of chemicals (natural or synthetic) in various
combinations and is as essential to life as water and oxygen, which are also
important constituents of foods. See NUTRITION.
    Food scientists, technologists, and engineers are concerned with the
many-faceted problems of transfer of food from field to consumer in the most
wholesome and palatable condition possible. Their concern involves such
problems as freshness, appearance, stability, avoidance of contamination, and
prevention of spoilage, as well as development of new products from food
components. In particular, they are responsible for the maintenance or
enhancement of the palatability and nutritional value of the product through
various processing procedures. See FOOD ENGINEERING; FOOD MANUFACTURING.
    In any dietary regimen minimal amounts of three types of materials must be
provided: energy-producing compounds, structure-producing compounds, and
compounds necessary to	energy-exchanging reactions. These three types include
proteins, carbohydrates, fats, vitamins, and mineral elements, which
constitute the classes of chemical compounds used to describe the composition
of any particular food. Water may be considered essential for their
utilization. See CARBOHYDRATE; FAT AND OIL (FOOD); PROTEIN; VITAMIN.
ms. Pgs. 2665-26704:30
    The staple foods form the basis of the dietaries of civilized humans and
may be classified by origin (see table). The first step in the natural
synthesizing of chemical elements into food molecules takes place in plants.
The plant concentrates the foods so produced in seeds and nuts (less so in
fruits) and stores them in leaves and stems. These basic plant processes give
rise to the wide variety of cereals, vegetables, and other plant foods.
    Foods of animal origin are composed predominantly of protein, are low in
carbohydrate content, and contain varying amounts of fat. Of the many animal
foods, milk most nearly approaches the perfect food, containing
proportionately significant amounts of all nutrients except iron. Milk
by-products are cheese and butter.
 �34s[John H. Nair /Bernard E. Proctor]
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�6v�2v�32sFood additives   �21m �33s 
    Substances added to foods during processing to retain or improve desirable 
characteristics or quality. Use of food additives has extended beyond mere 
preservation to include all stages of production, processing, and distribution 
of food. Additives may enhance flavor, improve texture, heighten color, better 
the appearance, or retain or increase nutritive value; facilitate mass 
production of products on uniform quality and properties; prevent microbial 
spoilage or oxidative changes (especially during transportation and 
distribution); increase effectiveness of sanitary packaging and protection of 
foodstuffs for retail sale; or add to ease, rapidity, and uniformity of 
preparation in the home. Some additives serve more than one function when 
present in certain foods. However, they may be classified according to their 
intended purpose as described below. 
  
    Flavoring agents.  Flavoring agents make up the largest and most important 
group of food additives. Many of the most familiar of these are the natural 
spices and essential oils, and extracts, such as clove, ginger, citrus oils, 
and vanilla. Equally important, though less well known, are such synthetic 
aromatic chemicals as aldehydes, esters, and alcohols, which may be used to 
impart to foods such flavors as strawberry, cherry, pineapple, walnut, 
wintergreen, and many others. Other compounds, of which monosodium glutamate 
and sodium inosinate are examples, serve as flavor enhancers in meat products. 
  
    Nutrients.  Minerals and vitamins are added to foods to restore 
nutritional values lost in processing or to supplement natural content of 
nutrients. Iodine in the form of iodized table salt was the first nutritional 
supplement to be added to a food. Vitamin D added to fluid milk has made 
rickets rare in the United States. Flour and bread are enriched with iron, 
thiamine, riboflavin, and niacin. Calcium as carbonate or phosphate is added 
to special dietary foods. Sodium fluoride and silicon fluoride are introduced 
into water supplies deficient in fluorine in order to improve dentition.  
  
    Preservatives.  During storage and distribution, undesirable changes occur 
in flavor, color, texture, and appetite appeal. The food producer uses various 
preservatives to delay these changes. See FOOD PRESERVATION. 
  
    Antioxidants.  Oxygen in the air may cause off-colors or undesirable 
flavors; for example, browning of apple or peach slices or rancidity in lard. 
Antioxidants are used to retard such effects. Dipping of fresh fruit in 
ascorbic acid solutions (vitamin C) prevents enzymatic browning. Sodium 
ascorbate retards oxidation of cooked, cured, and comminuted meat products. 
Off-flavor development in lard, vegetable oils, hydrogenated shortening, 
crackers, biscuits, breakfast cereals, dry cake mixes, and soup mixes due to 
oxidation is safely retarded by addition of butylated hydroxyanisole, 
butylated hydroxytoluene, lecithin, and tocopherols. Sulfur dioxide and 
sulfites are useful antioxidants for wine and beer, sugar syrups, and cut, 
peeled, or dried fruits and vegetables. 
  
    Antimicrobials.  Molds, bacteria, and yeasts produce another kind of food 
spoilage, but good sanitary practices in the modern food-processing plant, 
together with additives effective in controlling such spoilage organisms, keep 
losses to a minimum. Common table salt (sodium chloride) has been used for 
centuries to keep meat and fish from spoiling. High concentration of sugars 
helps preserve fruit jams and jellies, as well as canned and frozen fruits, 
against microbial spoilage. Chlorine and hypochlorites control microorganisms 
in public water supplies. Sorbic acid and sorbates inhibit mold in cheese 
wraps, pie fillings, and fruit syrups. Benzoic acid and sodium benzoate have 
long been used as safe preservatives in pickles, catsup, relishes, soft 
drinks, jellies, jams, and preserves. Bread no longer becomes moldy or ropy 
quickly in hot weather because the baker adds calcium propionate or sodium 
diacetate to the dough. See ANTIOXIDANT. 
  
    Emulsifiers.  Desirable consistency in many foods is difficult to achieve 
or maintain unless an emulsifying agent is added. These surfactants facilitate 
and stabilize oil-in-water (or water-in-oil) dispersions. The most important 
emulsifiers are the mono- and diglycerides of various fatty acids and their 
derivatives.  
  
    Thickeners.  The appetizing mouth feel of many foods, as well as their 
flavor appeal, is traceable to smooth, uniform texture derived from added 
stabilizers or thickeners. Such agents impart body or viscosity to fluids or 
emulsions. They include glycerol, sorbitol, gelatin, pectin, vegetable gums 
(alginates, carrageenin, and karaya), and cellulose gums 
(carboxymethylcellulose). They are used in processed cheese and salad 
dressings; in ice cream and frozen desserts they help prevent formation of 
large, coarse ice crystals. 
  
    Coloring agents.  Coloring agents are often employed to improve food 
appearance. Coloring agents may be of natural origin, such as turmeric, 
annatto, caramel, carmine, and carotene; or they may be one of the synthetic 
certified food colors. Principal uses are in soft drinks, confectionery, 
icings, jellies, baked goods, fillings, cake mixes, cereals, popcorn, packed 
fruits, cheese, butter, margarine, and meat products. 
  
    Bleaching agents.  The yellow color of freshly milled flour gradually 
disappears on aging as a result of oxidation. This maturing makes flour 
satisfactory for baking. Bleaching with chemical agents accelerates this 
process and produces uniformity in color. For this purpose oxides of nitrogen, 
chlorine, chlorine dioxide, nitrosyl chloride, or ammonium persulfate are 
employed.  
  
    Anticaking agents.  Some powdered foods tend to lump or cake on storage. 
Anticaking agents or desiccants prevent this occurrence in table salt, 
confectioner's sugar, malted milk powder, onion or garlic salt, and 
meat-curing mixes. Among the chemicals used as anticaking agents are calcium 
phosphate, magnesium carbonate, calcium or magnesium silicate, silica gel, 
talc, and starch. 
  
    Clarifying agents.  Fruit juices, vinegar, wine, beer, and soft drinks 
frequently exhibit turbidity, damaging the appearance of the liquids. A 
variety of compounds are used as clarifying agents to improve quality. These 
include tannin, gelatin, albumin, methyl cellulose, pectinases, and 
proteinases (papain, bromelin, and fungal enzyme preparations). 
  
    Artificial sweeteners.  Nonnutritive sweeteners, such as saccharin, impart 
sweetness to foods without adding calories. Their sole use is in low-calorie 
or diabetic foods, such as beverages, canned fruits, and desserts. 
  
    Foam regulators.  Food processors use foam regulators either to stabilize 
foams or to depress them. Dextrins, peptones, and cellulose gums are added to 
pressure-packed toppings so that they will be propelled from the containers 
fully foamed. They ensure that beer and root beer will produce a head when 
dispensed.  
  
    Aerating agents.  Carbonated beverages depend on carbon dioxide, added 
under pressure, for their flavor appeal. Cream is dispensed from pressurized 
containers charged with nitrous oxide or food-grade Freon. 
  
    Buffering agents.  With many processed foods, quality depends on adjusting 
and stabilizing pH (hydrogen ion concentration) to an optimum value. This 
adjustment is accomplished by means of buffering agents. Sodium salts of 
acetic, citric, phosphoric, and pyrophosphoric acids are employed for this 
purpose. They are used in confectionery, jellies, jams, preserves, soft 
drinks, prepared cereals, meat cures, and evaporated milk. Lactic, citric, and 
acetic acids serve to adjust pH to the desired end point in processed cheese, 
pickles, and brines, canned vegetables, and some kinds of confectionery. 
  
    Enzymes.  In various food processing operations, enzymes also serve as 
valuable additives. In practice, enzymes are permitted to react only to an 
established optimum condition and are then inactivated by heat. Their function 
is to effect hydrolysis of such food components as proteins, carbohydrates, 
and hemicelluloses. See ENZYME. 
  
    Other additives.  Among additives of lesser importance are clouding agents 
used to impart turbidity to beverages; coating agents for glazing oranges and 
chocolate confectionery and for facilitating removal of baked goods from pans 
or confectionery from slabs; hydrolytic agents to effect cleavage of proteins, 
carbohydrates, and cellulosic materials; and growth stimulators, such as 
antibiotics or phenylarsonic acid to promote rapid growth of poultry and 
livestock. See FOOD ENGINEERING. 
 �34s[Virgil O. Wodicka]
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�6v�2v�32sFood engineering   �21m �33s 
    The technical discipline involved in food manufacturing and refined foods 
processing. It encompasses the practical application of food science in the 
efficient industrial production, packaging, storing, and physical distribution 
of nutritious and convenient foods that are uniform in quality, palatable, and 
safe. Controlled biological, chemical, and physical processes and the 
planning, design, construction, and operation of food factories and processes 
are usually involved. See FOOD MANUFACTURING. 
    Food engineering is the food industry equivalent of chemical engineering. 
Food science in industry converts agricultural materials into products that 
are marketable because they meet a consumer need and can be profitably sold at 
reasonable prices by virtue of being economically produced, packaged, and 
distributed.  
    Food engineering is a vital link, therefore, between farms and food 
stores. Without it, food would be available only at farms, in forms produced 
by nature, and only in season. 
    Many natural foods must be preserved in order to be stored and shipped 
long distances to population centers. To provide a varied, nutritious diet, 
many raw food materials must be refined, and others combined with various 
ingredients and processed to produce food in new forms, such as bread, cheese, 
ice cream, frankfurters, soft drinks, cake and dessert mixes, candy, many 
breakfast cereals, ketchup, and salad dressings. 
    Because food engineering is applied in food manufacturing and refined 
foods processing, it requires a knowledge of unit operations and processes 
such as cleaning, separating, mixing, forming, heat transfer, moisture 
removal, fermenting, curing, packaging, and materials handling. This 
requirement to a large extent differentiates food engineering from food 
science. Yet these operations involve applied food science, just as chemical 
engineering encompasses chemistry. That is why the food engineer must have a 
working knowledge of food chemistry, bacteriology, and industrial 
microbiology, as well as of physics, mathematics, and basic engineering 
disciplines. See FOOD MICROBIOLOGY. 
 �34s[Frank K. Lawler]
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�6v�2v�32sFood manufacturing   �21m �33s 
    The commercial production and packaging of food products that are 
fabricated by processing or combining various ingredients or both. 
Manufactured foods are basically different from any found in nature, but they 
are fabricated principally from natural ingredients, and often include small 
quantities of chemical food additives to improve nutritive value, taste, 
color, shelf life, and convenience in use. A limited quantity of simulated 
meatlike foods has been fabricated from soya protein and other ingredients. 
Development is being done on the production of intermediate-moisture foods for 
human consumption, using a process similar to the one for making 
intermediate-moisture pet foods. 
    Mechanical manipulation, chemical treatment, biologic processes, heat 
treatment, or freezing may be involved. For example, bread is manufactured by 
combining flour, shortening, yeast, and other minor ingredients to make dough. 
Kneading and fermentation of the dough change the characteristics and develop 
flavor. The dough is formed into loaf-sized pieces, put into bake pans, 
proofed, and baked to form bread, which is sliced and wrapped. 
    Some foods, however, are prepared for consumption by refining processes 
which do not essentially change the nature of the material. Milk, for example, 
is clarified by centrifuging and pasteurized by heat treatment to kill 
pathogenic organisms. This treatment makes it safe for human consumption, but 
does not change it basically. 
    Food manufacturing and processing involve many unit operations, such as 
separating, mixing, forming, application of heat or refrigeration, packaging, 
materials handling, and process control. These are carried out in plants 
ranging in size from small retail bakeries to a processing plant covering 35 
acres (141,600 msup2). 
    Originally an enlarged version of a kitchen, the typical food factory has 
been developed through engineering into a highly efficient, technical 
industrial plant. Many processes have been converted from batch to continuous 
methods and put under automatic control. Materials and finished products are 
handled with high efficiency and little labor, with the aid of bulk pneumatic 
conveyors, pumps, mechanical conveyors, and lift trucks. Modern food factories 
are built with sanitary floors and walls of tile and brick, and much of the 
equipment is made of stainless steel. Most of the newer plants are in suburban 
areas on attractively landscaped plots. 
    Food factories are inspected by Federal health authorities if they do 
interstate business, and by state and municipal authorities when they do 
business locally. Health agencies also require that additives and packaging 
materials used in food manufacturing be approved as safe before they may be 
employed.  
    Manufacturers usually sell their products to retail stores through 
wholesalers, though some sales are made direct to large chain-store 
organizations. Sale of processed and manufactured foods to restaurants and 
caterers is a large business; and so is sale of convenience products via 
vending machines. 
    Major branches of the food manufacturing and refining industry are those 
producing bakery products; flour and cereal products; milk and other dairy 
products; canned, frozen, and dehydrated fruits and vegetables; fruit and 
vegetable juices; preserves; meat; poultry; fish; soft drinks; beer and ale; 
wine; confections; coffee; tea; spices and condiments; sweeteners; syrups; 
baby foods; dessert mixes; roasted nuts and nut butters; oleomargarine; 
shortenings and edible oils; and mayonnaise and salad dressings. See FOOD; 
FOOD ENGINEERING; FOOD PRESERVATION; FOOD SCIENCE; FOOD SPOILAGE. 
 �34s[Frank K. Lawler]
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�6v�2v�32sFood microbiology   �21m �33s 
    The branch of biology that deals with the microorganisms involved in the 
spoilage, contamination, and preservation of food. Food microbiologists must 
know how to detect the disease-producing organisms, how to prevent their 
entrance into food, and how to destroy them if they are present. 
    Another aspect of food microbiology concerns food preservation. Some foods 
are preserved and desirably flavored by fermentation processes; therefore, the 
specialist needs to be familiar with the organisms involved as well as their 
physiology and behavior in fermentation. See BOTULISM; FOOD PRESERVATION; FOOD 
POISONING; FOOD SPOILAGE. 
 �34s[Harlyn O. Halvorson]
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�6v�2v�32sFood packaging   �21m �33s 
    A total system which assures separation of the contained product from the 
environment and protection against external chemical, physical, and biological 
influences. As a system it provides protection, containment, communication, 
and unitization while affording sanitation, retention of product quality, and 
consumer convenience. 
    Food packaging might be categorized by several schemes, such as by 
materials (metal, glass, paper, plastic, and composites); package form (can, 
bottle, pouch, bag, wrap, carton, and case); packaging process (canning, 
bottling, pouching, bagging, overwrapping, collating, filling, closing, 
labeling, coding, and casing.); food product; or packaging level (primary, 
secondary, tertiary, and quaternary). Each method of classification has its 
merits and disadvantages, and none is without considerable overlap into other 
categories.  
  
    Labeling.  The Truth in Labeling Act instituted in the mid-1960s 
stipulated that all food packages bear a uniform and accurate representation 
of the weight, volume, or count of the package contents. The law further 
provided for more accurate graphic depiction of the nature of the contents. 
Accompanying the mandate were provisions encouraging food packagers to 
voluntarily reduce the number and variety of sizes made available to consumers. 
  
    Indirect additives.  The 1958 Delaney Amendment to the 1938 Food, Drug and 
Cosmetic Act stated that any chemical additive capable of causing cancer was 
specifically prohibited from food products. Chemical additives are defined as 
any compounds which enter the product intentionally or unintentionally, or are 
entered directly or indirectly. Since many packaging materials are in intimate 
contact with the foods contained, components of the materials may be extracted 
into the food. These components or indirect additives may be compounds having 
a functional value in food packaging or may be compounds which are present 
incidentally.  
  
    Degradable packaging.  The environmental issue of litter has stimulated 
many studies in the development of biodegradable packaging, that is, packaging 
that would disintegrate to invisible components as a result of exposure to 
natural elements. Biodegradable packaging would be counter to the need for 
stable waste required for sanitary landfill. For effective use of solid-waste 
dumps, it is necessary that the waste be resistant to the elements so that the 
recovered land does not collapse due to biodegradation. Further, 
biodegradability contradicts the concept of the protective value of packaging. 
    Yet another direction or research has been to find better means of 
ultimate useful degradation of packaging, as in incineration. Much of the 
packaging which is functionally effective does not burn well when it is being 
destroyed or being used to provide thermal energy to, say, generate electrical 
power. Among the problems encountered are that glass and metal must be 
separated out as nonburnable, and that some plastics such as PVC might 
possibly emit quantities of corrosive HCl. 
  
    Universal Product Code (UPC).  In a long-sought answer to the problem of 
improving productivity in retail food store operations,  Universal Product 
Code was created to appear on all food packages. This code, designating the 
manufacturer, brand, product, and package size, was designed to be read by 
electronic reading devices, thus eliminating the need for package price 
marking and for manual checkout procedures in retail stores. These electronic 
devices scan the code and transmit their reading to a computer memory which 
records the item and price and totals the purchase automatically. 
  
    Packaging materials.  A broad spectrum of materials is employed for 
packaging, depending upon the ultimate package configuration and upon 
economics. Immediately containing the food product is the primary package, 
which is no longer a simple material. In a continuing effort to provide the 
most cost-effective functional package, developments have been directed to 
combining materials to provide an optimum synergy of the desired attributes of 
the basic materials. For example, in a glass package, the glass would 
contribute impermeability, rigidity, inertness in contact with the food,and 
vertical stacking strength in distribution. In the same glass package, drawn 
aluminum with a plastic core would provide the closure, paper and adhesive the 
label, and a thin plastic coating protection to the glass surface to minimize 
the potential hazard from breakage. 
    Primary packages might be generally classified as cans, bottles and jars, 
bags and pouches, wraps and cartons. Secondary packaging is usually composed 
of paperboard cartons or wraps, but considerable quantities of plastic 
unitizers are being increasingly employed. Tertiary packaging is usually the 
corrugated shipping container, but can also be paperboard or plastic wrap. 
 �34s[Aaron L. Brody]
�33s 
  
    Nutrition labeling.  This is a standardized way to tell the consumer 
through labeling the chief nutritional characteristics of foods. The 
nutritional properties stated are based on a serving of the food. The size of 
the serving in common household units is stated on the label and also, for 
packaged foods, the number of servings in the package. The label then states 
the number of calories and the number of grams each of protein, carbohydrate, 
and fat in the serving of food. If so desired, the purveyor may further 
qualify the declaration of fat by saying how much of it is saturated and how 
much is polyunsaturated (only for significant sources of fat). The amount of 
cholesterol present may also be stated if the food is a significant source. 
    The next section of the label sets forth the content of protein (restated 
in different units) and micronutrients per serving. Each nutritionally labeled 
product must declare the amount of vitamins A and C, thiamine, riboflavin, 
niacin, calcium, and iron. Optionally, it may also declare vitamins D, E, B6, 
and B12, folic acid, phosphorus, iodine, magnesium, zinc, copper, biotin, or 
pantothenic acid. In this section, the nutrient levels are declared as 
percentages of the United States Recommended Daily Allowances (RDA). These 
United States RDAs are based on but not quite identical to the Recommended 
Daily Dietary Allowances of the Food and Nutrition Board of the National 
Research Council (NRC). See NUTRITION. 
 �34s[Virgil O. Wodicka]
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�6v�2v�32sFood poisoning   �21m �33s 
    The two types of bacterial food poisoning are infection of the host by 
organisms carried by food; and intoxication by poisonous substances produced 
by bacteria in food before it is consumed. 
    Salmonella food poisoning is the most common food-borne infection. Large 
numbers of these bacteria are necessary to establish human infection; the 
infection is usually acute, but mortality is very low. Food becomes 
contaminated by contact with animal or human carriers. See SALMONELLA. 
    Food-borne intoxication is most frequently caused by ingestion of 
staphylococcus enterotoxin. Certain strains of Staphylococcus pyogenes are 
capable of producing this heat-stable toxin as an excretion, or exotoxin, 
during rapid growth in food products. The gastrointestinal symptoms are 
severe, but recovery is complete in 3 - 4 days. See STAPHYLOCOCCUS. 
    Botulism is a rare intoxication usually associated with foods of low acid 
content canned under insufficiently rigorous conditions. The heat-labile toxin 
formed during growth of Clostridium botulinum acts upon the central nervous 
system. Mortality averages about 60%. See BOTULISM. 
    Ptomaine poisoning is an archaic and erroneous term once used as a synonym 
for food poisoning. Ptomaines are diamines found in putrefying material. They 
are in part responsible for foul odors but their toxicity is low. They have 
long ceased to be regarded as important in food poisoning. 
 �34s[John H. Silliker]
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�6v�2v�32sFood preservation   �21m �33s 
    The science and art of employing processes which protect food from 
degradation by rodents, insects, microbes, enzymes, or undesirable chemical 
alteration. A variety of processes may be used, either alone or in 
combination, including refrigeration, freezing, canning, drying and 
dehydration, salting, fermentation, chemical additives, and radiation. 
    Commercial and household refrigeration units effectively maintain low 
nonfreezing temperatures which preserve the desirable characteristics of 
perishable foods for reasonable periods of time. The reduced temperature 
retards the growth of spoilage microorganisms such as bacteria, yeasts, and 
molds. See REFRIGERATION. 
    Frozen foods keep for long periods of time when properly prepared, 
packaged, and stored. Microbial action is arrested, but the microbes are not 
necessarily destroyed. Enzymatic action is markedly reduced but not eliminated. 
    In commercial canning (thermal processing) foods are placed in 
hermetically sealed containers and then heat-processed. The heat treatment 
must be severe enough to destroy the spores of toxigenic organisms that may be 
present. The thermal process depends on the particular food product and the 
container size. The equipment may be a pressure container (retort) or a more 
sophisticated apparatus which allows for a continuous operation. 
    Drying is one of the oldest methods of food preservation. The use of heat 
from an energy source to speed up the removal of water is a modern method and 
is referred to as dehydration. The water content of the food is reduced to a 
critical level below which microorganisms cannot grow. 
    Salt is used in the preservation of food, either alone or in combination 
with drying. Large quantities of salt are used in the brining of pickles and 
olives to ensure that proper fermentation develops. Salt and salt solutions 
serve a variety of functions in a great variety of food processing operations. 
    Fermentation is a biological process which depends on certain 
microorganisms to produce chemical substances which inhibit the growth of 
undesirable microorganisms. One such action is the inhibition of 
toxin-producing bacteria by the lactic acid produced from carbohydrates in the 
fermented food. Foods involved in lactic fermentation include soured milk, 
butter, cheeses, pickles, sauerkraut, olives, breads, and sausages. See 
FERMENTATION.  
    Food additives are generally nonnutritive substances added intentionally 
to food, generally in small quantities, to improve its appearance, flavor, 
texture, or storage properties. They must meet legal requirements. The uses 
and compounds are many and varied, but all serve to impart desirable qualities 
to the food, such as the prevention of rancidity or the inhibition of 
microbial growth. See FOOD ADDITIVES. 
 �34s[Z. John Ordal]
�33s 
    Irradiation processing serves mainly to kill microorganisms, parasites, 
and insects. Ultraviolet light of wavelengths shorter than 300 nanometers has 
been used to sterilize the surfaces of food products such as meat and bakery 
items. However, this application is limited by the low depth of penetration, 
which leaves microorganisms located under the surface unaffected. Ionizing 
radiation, in the form of gamma rays, and x-rays are much more effective, 
having the capability of penetrating packaged products, even cartons 11 in. 
(30 cm) thick. 
    Irradiation has other potential applications. The range of 0.03 to 0.15 
kilogray (1kGy = 1 joule/gram of absorbed radiation energy) is suitable for 
sprout inhibition of potatoes, onions, and garlic. The 0.15 - 1-kGy range is 
suitable for disinfestation of grains, fruits, and vegetables. The 0.5 - 
10-kGy range can be used for reducing or eliminating spoilage microflora such 
as mold and vegetative bacteria. See FOOD ENGINEERING; FOOD MICROBIOLOGY; FOOD 
SPOILAGE.  
 �34s[A. Brynjolfsson]
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�6v�2v�32sFood science �21m �33s
    The applied science which deals with the chemical, biochemical, physical,
physicochemical, and biological properties of foods. Chemical properties of
foods include: (1) composition, that is, constituents, such as proteins, fats,
and carbohydrates, which make up food; (2) chemical reactions which occur when
foods are processed (canned, frozen, dried, irradiated, and so on), packaged,
or stored; and (3) interactions of food constituents with chemical additives,
such as bleaching agents and antioxidants.
    Biochemical properties of foods are related to postharvest (plant) or
postmortem (animal) enzymatic activity, and to the presence of physiologically
active constituents, such as vitamins and other nutrients which are essential
to humans or animals. Physical properties include color, specific gravity,
refractive index, viscosity, texture, and various thermal constants.
Physicochemical properties relate to the behavior of colloids, solutions,
crystals, and so on, occurring in foods. Biological properties pertain to the
activity of macroorganisms, such as insects or parasites, and microorganisms,
including bacteria, molds, yeasts, and viruses, found in foods (which are
involved in spoilage and fermentation), as well as the effects of natural or
induced toxicants.
    Food science provides the basic knowledge for food technology, which is
the application of science and engineering to the research, production,
processing, packaging, distribution, preparation, and utilization of foods.
See FOOD TECHNOLOGY.
 �34s[Gideon E. Livingston ;Myron Solberg]
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�6v�2v�32sFood spoilage   �21m �33s 
    Foods undergo many different types of spoilage, resulting in abnormal 
colors, flavors, odors, or in other changes, such as consistency. Spoilage 
cannot be equated to danger to the consumer; indeed, in most instances, foods 
which contain poison of either chemical or biological origin show no external 
evidence of spoilage. 
    Chemical changes are responsible for spoilage. These changes are brought 
about through enzyme action. These enzymes may have their origin in the food 
material or may be elaborated by microorganisms such as yeasts, molds, and 
bacteria which contaminate the product. In general, enzymes derived from foods 
themselves are of secondary importance to those elaborated by microorganisms; 
however, when foods are held under conditions which preclude microbial action, 
the natural enzymes in the food materials may be the primary reason for 
spoilage. The fat-splitting enzymes in frozen foods cause this type of 
spoilage.  
    Abnormal or unusual spoilages of food products occur periodically. Such 
spoilage may be seasonal, regional, or entirely haphazard in appearance. In 
extreme cases a food plant may have to cease manufacture of a product due to 
contamination with undesirable microorganisms responsible for peculiar 
spoilage problems. Examples of such unusual spoilage include green 
discoloration of milk; gas pockets in, and in extreme cases the explosion of, 
cheese. In most cases, a particular organism or group is responsible for each 
of these conditions. See FOOD ENGINEERING; FOOD MICROBIOLOGY; FOOD 
PRESERVATION.  
 �34s[John H. Silliker]
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�6v�2v�32sFoot-and-mouth disease   �21m �33s 
    A highly contagious virus disease of economic importance, infecting 
cattle, pigs, sheep, and goats. It is characterized by fever, salivation, and 
formation of vesicles in the mouth and pharynx and on the feet. It may be 
transmitted to humans. 
    Attenuated-virus vaccines have been developed and used in the field with 
success. In serious epizootics all susceptible animals in the area are 
slaughtered, and strict quarantine is observed. To prevent human infection, 
boiling or pasteurizing of farm products under suspicion should be adequate. 
See ANIMAL VIRUS; EPIDEMIOLOGY. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sFoot (anatomy)   �21m �33s 
    The terminal portion of the lower, or inferior, extremity. Its bony 
framework consists of 7 tarsal or ankle bones, 5 metatarsal or foot bones, and 
14 phalanges or toe bones. The prominent heel is a projection of the 
calcaneus, a tarsal bone (see illustration). A strong but elastic system of 
ligaments holds the bones in position, producing the arched structure 
characteristic of the human foot, and strengthens the joints of the foot. 
Ankle, foot, and toe movements are produced by the action of three groups of 
leg muscles and six groups of foot muscles. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sForaminiferida   �21m �33s 
    An order of Granuloreticulosia in the class Rhizopodea. Foraminiferans are 
dominantly marine protozoans, with a secreted or agglutinated shell, or test, 
enclosing the continually changing ameboid body (see illustration) that 
characterizes this and other orders of the superclass Sarcodina. Their unique 
combination of long geologic history, ubiquitous geographic distribution, and 
exceptional diversity of test composition, form, and structure make the 
foraminiferans the most useful of all marine fossils for stratigraphic 
correlation, geologic age dating of sediments, and paleoecologic 
interpretation. Their tests accumulated in great numbers and are recoverable 
from small quantities of sediment, rock outcroppings, well cores or cuttings, 
or ocean dredging and submarine coring. 
    Pseudopodia from the ameboid body form a delicate anastomosing network of 
2 - 10 times the test diameter. The pseudopodial net may arise from the 
apertural region of the test alone in those with tectinous, porcelaneous, and 
agglutinated walls, or may radiate in all directions through many tiny 
perforations of the hyaline test wall. The pseudopodia variously serve in 
capture, ingestion, and digestion of food, in test and temporary cyst 
construction, for anchorage, and for locomotion. The characteristic granular 
streaming of the continuously moving cytoplasm differentiates the 
Foraminiferida from the other Sarcodina orders. 
    The foraminiferan constructs its own test. Growth of the individual may 
cease after test construction, or the test may enlarge by continued growth in 
one or more directions (tubular or branching tests) or by periodic formation 
of separate but always interconnected chambers. Growth may be either 
continuous or periodic and may be in a straight line, a planispiral or 
trochospiral coil, a cycle, or a zigzag. The latter (zigzag) is expressed by 
chambered forms in biseriality. Variations and combinations of these growth 
patterns occur repeatedly in different lineages (isomorphism). 
    As is true of most protists with skeletons or tests, systematic 
differentiation and classification of foraminiferans is based on test 
composition, microstructure, and gross morphology. Information currently 
available concerning cytoplasmic characters, life cycles, and so on has shown 
good agreement with this classification, although the function and origin of 
many shell characters believed to be of systematic importance (canal systems, 
pores, septal doubling, and apertural tooth plates) are yet undetermined. 
There are 5 suborders with 19 superfamilies that comprise 102 families and 
nearly 1400 genera. 
    Foraminiferans have shown a remarkable diversification and rapid 
evolutionary development over their 500,000,000-year known history. Of the 
34,000 living and fossil species, possibly 4000 are living. Some with 
uncomplicated tests have been reported to have a long geologic range, but most 
with diagnostic shell structure and morphology had relatively short histories, 
each successively replaced by others. The oldest-known and presumably 
primitive tests are the morphologically simple tectinous and agglutinated one 
of the Cambrian and Ordovician, of globular, tubular, branching, or irregular 
form.  
  
    Most foraminiferans are benthonic, living upon the sea floor, within the 
upper few centimeters of ooze, or upon benthic algae or other organisms. They 
occur from the intertidal zone to oceanic depths, in brackish, normal marine, 
or hypersaline waters, and from the tropics to the poles. Some modern 
Lagynacea live in fresh water, but none are known as fossils. Assemblages vary 
widely in response to local conditions, with the greatest diversity occurring 
in warm, shallow water. A smaller number, the Globigerinacea, are planktonic, 
living at various depths in the water column from the surface to the bottom, 
being most numerous between 20 and 100 ft (6 and 30 m). Vertical migration may 
be diurnal and may occur during ontogenetic development. The preferred depth 
range of a species may vary geographically in response to temperature 
differences or to changes in water density. See GRANULORETICULOSIA; 
RHIZOPODEA; SARCODINA. 
 �34s[Helen Tappan]
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�6v�2v�32sForce �21m �33s
    Force may be briefly described as that influence on a body which causes it
to accelerate. In this way, force is defined through Newton's second law of
motion.
    This law states in part that the acceleration of a body is proportional to
the resultant force exerted on the body and is inversely proportional to the
mass of the body. An alternative procedure is to try to formulate a definition
in terms of a standard force, for example, that necessary to stretch a
particular spring a certain amount, or the gravitational attraction which the
Earth exerts on a standard object. Even so, Newton's second law inextricably
links mass and force. See ACCELERATION; MASS.
    One may choose either the absolute or the gravitational approach in
selecting a standard particle or object. In the so-called absolute systems of
units, it is said that the standard object has a mass of one unit. Then the
second law of Newton defines unit force as that force which gives unit
acceleration to the unit mass. Any other mass may in principle be compared
with the standard mass (m) by subjecting it to unit force and measuring the
acceleration (a), with which it varies inversely. By suitable appeal to
experiment, it is possible to conclude that masses are scalar quantities and
that forces are vector quantities which may be superimposed or resolved by the
rules of vector addition and resolution.
    In the absolute scheme, then, the equation below is written

				  F = ma

for nonrelativistic mechanics; here boldface type denotes vector quantities.
This statement of the second law of Newton is in fact the definition of force.
In the absolute system, mass is taken as a fundamental quantity and force is a
derived unit of dimensions MLT- sup 2 (M = mass, L = length, T = time).
    The gravitational system of units uses the attraction of the Earth for the
standard object as the standard force. Newton's second law still couples force
and mass, but since force is here taken as the fundamental quantity, mass
becomes the derived factor of proportionality between force and the
acceleration it produces. In particular, the standard force (the Earth's
attraction for the standard object) produces in free fall what one measures as
the gravitational acceleration, a vector quantity proportional to the standard
force (weight) for any object. It follows from the use of Newton's second law
as a defining relation that the mass of that object is m = w/g, with g the
magnitude of the gravitational acceleration and w the magnitude of the weight.
The derived quantity mass has dimensions FT sup 2 L- sup 1. See FREE FALL.
 �34s[George E. Pake]
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�6v�2v�32sForced oscillation   �21m �33s 
    An oscillation produced in a simple oscillator or equivalent mechanical 
system by an external periodic driving force. Forced oscillations are to be 
contrasted with free oscillations which occur when the system is displaced 
from equilibrium and released. See HARMONIC MOTION; MECHANICAL VIBRATION; 
OSCILLATION; VIBRATION. 
    Application of the driving force results at first in two simultaneous 
oscillations, one at the frequency of the free oscillations, and one at the 
frequency of the impressed driving force. The former, the so-called transient, 
eventually decays, whereas the latter builds up to the steady-state value. 
Small damping is associated with slow decay of the transient and slow 
establishment of the steady state, whereas large damping is associated with 
fast decay. See DAMPING. 
 �34s[Martin Greenspan]
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�6v�2v�32sForce fit   �21m �33s 
    A means for holding mating mechanical parts in fixed position relative to 
each other. In a force fit of cylindrical parts, the inner member has a 
greater diameter than the hole of the outer member; that is, the metals of the 
two parts interfere. In a true force fit, the parts are highly stressed, the 
interference amounting to 0.002 or 0.003 in. (0.05 or 0.07 mm) for parts with 
a basic diameter of 1 in. See ALLOWANCE; SHRINK FIT. 
 �34s[Paul H. Black]
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�6v�2v�32sForcipulatida   �21m �33s 
    An order of Asteroidea characterized by straight or crossed pedicellariae 
or both. These may be carried on stalks, and in some cases are arranged in 
rosettes around the bases of the spines. The spines usually occur singly, and 
are ungrouped. The body is stellate in outline, but is not necessarily 
restricted to five arms or radial axes. 
    The order contains four families, whose genera are numerous and widely 
distributed. The Brisingidae and Zoroasteridae are characteristically 
deep-water families, and the former includes species with as many as 44 arms. 
The Asteridae is the largest family, and includes many well-known predatory 
starfishes, some of which are economically significant because of the damage 
they inflict upon oyster and clam beds. The fourth family is the 
Heliasteridae. See ASTEROIDEA; ECHINODERMATA. 
 �34s[Andrew C. Campbell]
�33s 
  
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�6v�2v�32sForensic chemistry   �21m �33s 
    The application of chemistry to the study of physical materials or 
theoretical problems, the results of which may be entered into court as 
technical evidence. Boundaries are not sharply defined for forensic chemistry, 
and it includes topics that are not entirely chemical in nature. 
    Some of the items most often encountered in crime laboratories, and the 
information sought in regard to them, are: (1) body fluids and viscera to be 
analyzed for poisons, drugs, or alcohol, quantitation of which may assist in 
determining the dosage taken or the person's behavior prior to death; (2) 
licit and illicit pills, vegetable matter, and pipe residues for the presence 
of controlled substances; (3) blood, saliva, and seminal stains, usually in 
dried form, to be checked for species, type, and genetic data; (4) hairs, to 
determine if animal or human; if human, the race, body area of origin, and 
general characteristics; (5) fibers, to determine type (animal, vegetable, 
mineral, or synthetic), composition, dyes used, and processing marks; (6) 
liquor, for alcoholic proof, trace alcohols, sugars, colorants, and other 
signs of adulteration; (7) paint, glass, plastics, and metals, usually in 
millimeter-sized chips, to classify and compare to known materials; (8) inks 
on documents, to determine type, dye content, or possible age; also chemical 
obliterations and restoration of charred papers; (9) swabs from the hands of 
suspects, to be checked for the presence of gunshot residue; (10) debris from 
a fire or explosion scene, for the remains of the accelerant or explosive 
used. See ANALYTICAL CHEMISTRY; FORENSIC MEDICINE; FORENSIC TOXICOLOGY. 
 �34s[Michael J. Camp]
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�6v�2v�32sForensic medicine   �21m �33s 
    The use of medical evidence or medical opinion for purposes of settling 
disputes in civil matters or providing medical evidence in issues of criminal 
action. All branches of medicine may provide such information. Medical 
specialists who frequently contribute evidence in legal disputes are from the 
disciplines of forensic pathology, forensic psychiatry, forensic dentistry, 
orthopedic surgery, general surgery, and general practice. The expert medical 
witness is called upon to state an opinion of the medical facts at issue. In 
industrial compensation cases, medical evidence is used to assess the nature 
and extent of injury or poor health resulting from employment. 
    All state and local governments in the United States establish by 
constitution or statute an office or agency to investigate deaths which are of 
public interest. Most states have a medical examiner, that is, a medical 
expert to investigate violent and sudden unexpected death. The physician has 
the responsibility of ruling the violent death an accident, a suicide, a 
homicide, or the result of violence of undetermined origin. About 25% of all 
deaths require some level of investigation. There may be only an investigation 
into the decedent's medical history and the circumstances surrounding death. 
Other deaths may require a complete investigation, including autopsy. 
    Special problems involved in forensic deaths but rarely encountered in 
other branches of medicine include identification of decomposed bodies, parts 
of bodies, and skeletal remains. Other forensic disciplines intersect with 
forensic pathology particularly in the area of identification, where forensic 
anthropology and forensic dentistry provide the means. Anthropological 
measurements are used to determine stature, age, race, and sex of skeletal 
remains. Dental records can provide the means to establish the identity of a 
badly burned body. In assessing the nature of violence, the forensic 
pathologist must be skilled in the examination of microscopic tissue to 
determine the age of injury. The pathologist also must approximate the time of 
death by assessing the general condition of the body. Other tests are used to 
establish evidence of sexual battery; blood group determination of stains 
present on clothing or the body can be most helpful in identifying a murder 
suspect. Recovery of bullets from tissue can provide ballistic evidence which 
conclusively establishes that a bullet was fired from a particular weapon. See 
ANTHROPOMETRY; BLOOD GROUPS; DEATH; FORENSIC CHEMISTRY; FORENSIC TOXICOLOGY; 
PUBLIC HEALTH. 
    The majority of deaths which are investigated by the local forensic 
authority are natural in character. Such deaths come under his or her purview 
when death occurs suddenly and unexpectedly in people who are in apparent good 
health.  
 �34s[Donald T. Reay]
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�6v�2v�32sForensic toxicology   �21m �33s 
    An interdisciplinary science dealing with the analysis and interpretation 
of drugs and chemicals in biological samples for medical-legal purposes. There 
are three major case load areas in most forensic toxicology laboratories: drug 
abuse - cases resulting from the illegal use of drugs; police cases - 
toxicological aspects of criminal investigations; and postmortem cases - 
analytical studies in support of the medical examiner to determine the cause 
of death. 
    In the United States poisons are responsible for approximately 10,000 
deaths annually (3% of all accidental deaths and 25% of all suicides). The 
most commonly encountered drugs and chemicals include ethyl alcohol, 
barbiturates, carbon monoxide, morphine (most often from heroin use), 
propoxyphene, and benzodiazepines (diazepam, chlordiazepoxide, and so on). The 
role of ethyl alcohol in death is rarely due to its direct toxic effect but to 
its indirect role in accidents. Similarly hallucinogens rarely cause death by 
their direct toxicity, but are indirectly responsible for accidents, suicides, 
and so forth. Carbon monoxide, barbiturates, and analgesics exert a direct 
toxic effect (asphyxia, central nervous system depression, and respiratory 
depression). See FORENSIC MEDICINE; TOXICOLOGY. 
 �34s[Jesse Bidanset]
�33s 
  
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�6v�2v�32sForest and forestry   �21m �33s 
    A forest is a community of trees, other plants, and animals which live in 
and thrive on the forest environment. Trees are the dominant form of 
vegetation. The forest community or ecosystem also includes shrubs, herbs, 
mosses, fungi, insects, reptiles, birds, and mammals. All these organisms live 
on or in the soil, water, and air of the forest. Each is a part of the 
community, and each reacts with all the other parts, but all require the 
warmth and energy provided by the Sun to survive. 
    The natural affiliation of some trees for specific environmental 
conditions in the forests of the United States has resulted in hundreds of 
groupings of trees known as forest types and subtypes. These can be classified 
into six major forests or natural forest regions. Three are east of the Great 
Plains: the Northern Forest, the Central Hardwood Forest, and the Southern 
Forest. The Rocky Mountain, Pacific Coast, and Alaskan Forests are west of the 
Prairie states. 
    To facilitate forest management, foresters have arbitrarily classified 
forest trees into several groups. One classification, based on the size of the 
individual trees, refers to them as seedlings, saplings, poles, standards, or 
veterans. A forest containing trees ranging from small seedlings to poles to 
large veterans is said to be all-aged. Forests with trees essentially the same 
size and age are called even-aged. Foresters talk of a pure forest if it is 
composed mainly of one species, and of a mixed forest if it contains several 
species. Botanists and foresters refer to tolerant trees as those which can 
grow in the shade of other trees. Species which cannot survive in shade are 
said to be intolerant. 
    The trees in a forest compete with each other for soil, water, and light 
on any given site. Some tree species may grow better on hills facing north, 
while others do better on slopes facing south. This forest competition may 
result in one combination of trees being more successful than another for a 
given type of environment. As new species invade a site, others may decline in 
importance and bring about a forest community change called forest succession. 
  
    Forestry.  In 1971 the Society of American Foresters defined forestry as 
``the science, the art and practice of managing and using for human benefit 
the natural resources that occur on and in association with forest lands.'' 
Other definitions might note specifically that the basic ``natural resources'' 
were wood, forage, water, wildlife, and recreation. The major divisions of 
forestry which deal specifically with biological fields are forest soils, 
dendrology, silvics, and silviculture. Forest pathology, forest zoology, and 
forest entomology also fall into this category. The part that forest soils 
play in providing growth and support of forest trees is essential. Soil 
composition, physical and chemical character, and the behavior of soils under 
different forest conditions must be studied and understood. Dendrology covers 
the description, classification, and identification of the numerous species of 
trees with which foresters must work. Silvics deals with the manner in which 
environmental factors such as climate, slope, soil, fire, and biological 
conditions affect forest sites. Silviculture deals with the technical problems 
of establishing new forests, naturally or artificially, making thinnings or 
cuttings to stimulate growth on the remaining trees, and making harvest cuts 
of forest stands in such a way as to assure their perpetuation. See 
DENDROLOGY; SILVICULTURE. 
  
    Forest management.  The fact that forest crops of timber take from 20 to 
100 years to mature makes it essential that planning and decision making 
should be based on the best facts and forecasts available. Planning for use of 
resources other than timber also requires careful long-term planning and 
integration with all the environmental factors of the forest. Forest 
management refers to the practical application of scientific, economic, and 
social principles affecting the administration and working of a forest estate 
for specified objectives. Forest management includes mensuration (forest 
measurement), regulation, and preparation of working plans. The working plans 
also involve forest finance, forest economics, and forest policy, and should 
meet the requirements of an environmental impact statement. This statement, 
which is now a prerequisite of all management plans of national forests, must 
show the probable impact that the management plan will have on the wildlife, 
watershed, and recreational resources. The statement may even include 
integration with range (grazing of livestock) management resources and, in 
some cases, with municipal and utility plans and resources. Management plans 
and policies for forests or public lands may have to submit their plans and 
environmental impact statements to a series of public hearings and amendments 
before they are finally adopted. 
  
  
    Utilization.  Forests cannot be used or timber harvested without a system 
of roads, bridges, culverts, and some administrative structures. Their 
planning and construction and the planning and conduct of logging operations 
make up a body of knowledge called forest engineering. In terms of volume and 
dollar value, the greatest direct product of most forests is wood. Study of 
its variations of structure, physical and mechanical properties, and behavior 
of wood under varying conditions is labeled wood technology. Utilization also 
includes the manufacture, transportation, and marketing of forest products, 
and it most certainly includes the utilization of all wood (and bark) 
materials left over after logging operations in the woods and manufacturing at 
the mills. The use of leftovers is resulting in a minimum of waste in the 
woods and provides a major source of increased yields of products and fuel for 
the forest. See LOGGING (FORESTRY); WOOD PRODUCTS. 
 �34s[Willard S. Bromley]
�33s 
  
    Ecosystem analysis.  The term ``ecosystem'' is the most concise 
formulation of the concept of an interacting system comprising living 
organisms together with their site. The goal of the study of whole ecosystems 
is to understand their processes and functions. Such an understanding is 
urgent because of the need to predict the long-term as well as the short-term 
consequences of humans' actions in ecosystems. Greatest emphasis has been 
placed on photosynthesis and respiration, on cycling of water and nutrients, 
and on the relationship of these to forest productivity. 
    Studies of nutrient cycling in hardwood forests of the northeastern United 
States show the enormous amount of nutrients available for uptake and 
regeneration of a new forest after cutting of the mature forest. Conventional 
forest cuttings pose little threat to site productivity, but short-rotation 
cropping and whole-tree harvesting can lead to nutrient deficiencies. This 
kind of integrated study will increasingly provide the basis for management of 
terrestrial and adjacent aquatic ecosystems. See ECOSYSTEM. 
  
    Forest succession.  Since the developing forest changes the forest site, 
it follows that the changed site may itself be more favorable to a new group 
of tree species other than those currently occupying it. As a result, 
successive forest communities occupy the site in the absence of fire, logging, 
windstorm, or other disturbance. Thus, in the northeastern United States early 
successional or pioneer communities characterized by aspens, cherries, 
birches, and pines occupy open sites. These in time are replaced on fertile 
sites by midsuccessional communities containing oaks, ash, and other species 
until, in several hundred years, the late successional or climax community is 
attained. This climax contains species, such as hemlock, sugar maple, and 
beech, that are capable of reproducing under their own cover in the absence of 
disturbance. On dry, sandy sites, especially, periodic fires enable pines to 
succeed themselves over long time periods, and a fire climax prevails. Thus, 
species composition of late successional communities is dependent on 
prevailing site factors; several climax types are typically recognized in each 
forest region. 
    Throughout the world, fire greatly influences forest succession, setting 
it back or maintaining fire species as climax types. In the southeastern 
United States, pine types predominate and constitute the most important 
commercial forests. Pines owe their existence to periodic fires; without fire 
they would be replaced successionally on many sites by mixed hardwood forests. 
Similarly, in western North America, the extensive pure stands of Douglas fir 
owe their existence to fire and periodic felling by wind. They would be 
replaced by less valuable, mixed coniferous types if these disturbances were 
eliminated. Also in western North America the esthetically pleasing and 
commercially important parklike stands of ponderosa pine are perpetuated and 
shaped by fire (see illustration). In present-day silviculture humans 
increasingly use controlled fire together with cutting, chemicals, and 
mechanical treatments to maintain the forest on a given site at a stage of 
forest succession most valuable to humans. See ECOLOGY. 
 �34s[Burton V. Barnes]
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�6v�2v�32sForest fire   �21m �33s 
    The term wildfire refers to all uncontrolled fires that burn surface 
vegetation (grass, weeds, grainfields, brush, chaparral, tundra, and forest 
and woodland); often these fires also involve structures. In addition to the 
wildfires, several million acres of forest land are intentionally burned each 
year under controlled conditions to accomplish some silvicultural or other 
land-use objective or for hazard reduction. 
    Most wildfires are caused by human beings, directly or indirectly. In the 
United States less than 10% of all such fires are caused by lightning, the 
only truly natural cause. In the West (the 17 Pacific and Rocky Mountain 
states) lightning is the primary cause, with smoking (cigarettes, matches, and 
such) the second most frequent. Combined they account for 50 to 75% of all 
wildfires. In the 13 southern states (Virginia to Texas) the primary cause is 
incendiary. This combined with smoking and debris burning make up 75% of the 
causes. The 20 eastern states have smoking and debris burning as causing close 
to 50% of all wildfires. Miscellaneous causes of wildfires are next in 
importance in most regions. The other causes of wildfires are machine use and 
campfires. Machine use includes railroads, logging, sawmills, and other 
operations using equipment. 
    The manner in which fuel ignites, flame develops, and fire spreads and 
exhibits other phenomena constitutes the field of fire behavior. Factors 
determining forest fire behavior may be considered under four headings: 
attributes of the fuel, the atmosphere, topography, and ignition. A forest 
fire may burn primarily in the crowns of trees and shrubs - a crown fire; 
primarily in the surface litter and loose debris of the forest floor and small 
vegetation - a surface fire; or in the organic material beneath the surface 
litter - a ground fire. The most common type is a surface fire. 
    The U.S. Forest Service has developed a National Fire Danger Rating System 
(NFDRS) to provide fire-control personnel with numerical ratings to help them 
with the tasks of fire-control planning and the suppression of specific fires. 
The system includes three basic indexes: an occurrence index, a burning index, 
and a fire load index. Each of these is related to a specific part of the 
fire-control job. These indexes are used by dispatchers in making decisions on 
setting up firefighting forces, lookout systems, and so forth. 
 �34s[Willard S. Bromley]
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�6v�2v�32sForging   �21m �33s 
    The plastic deformation of metals, usually at elevated temperatures, into 
desired shapes by compressive forces exerted through a die. Forging processes 
are usually classified either by the type of equipment used or by the geometry 
of the end product. The simplest forging operation is upsetting, which is 
carried out by compressing the metal between two flat parallel platens. From 
this simple operation, the process can be developed into more complicated 
geometries with the use of dies. A number of variables are involved in 
forging; among major ones are properties of the workpiece and die materials, 
temperature, friction, speed of deformation, die geometry, and dimensions of 
the workpiece. 
    In practice, forgeability is related to the material's strength, 
ductility, and friction. In terms of factors such as ductility, strength, 
temperature, friction, and quality of forging, various engineering materials 
can be listed as follows in order of decreasing forgeability: aluminum alloys, 
magnesium alloys, copper alloys, carbon and low-alloy steels, stainless 
steels, titanium alloys, iron-base superalloys, cobalt-base superalloys, 
columbium alloys, tantalum alloys, molybdenum alloys, nickel-base superalloys, 
tungsten alloys, and beryllium. See METAL. 
    Some of the terminology in forging is shown in the illustration. Draft 
angles facilitate the removal of the forging from the die cavity. The purpose 
of the saddle or land in the flash gap is to offer resistance to the lateral 
flow of the material so that die filling is encouraged. Die filling increases 
as the ratio of land width to thickness increases up to about 5; larger ratios 
do not increase filling substantilly and are undesirable due to increased 
forging loads and excessive die wear. The purpose of the gutter is to store 
excess metal. The flash is removed either by cold or hot trimming or by 
machining.  
  
    A number of methods produce the necessary force and die movement for 
forging. Two basic categories are open-die and closed-die forging. Drop 
hammers supply the energy through the impact of a falling weight to which the 
upper die is attached. Another type of forging equipment is the mechanical 
press. For large forgings the hydraulic press is the only equipment with 
sufficient force. However, the speed for such presses is about one-hundredth 
that of hammers. See METAL FORMING; SWAGING. 
 �34s[Serope Kalpakjian]
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�6v�2v�32sFormaldehyde   �21m �33s 
    The simplest aldehyde, formula HCHDOUBLE BARO. Because of its extreme 
reactivity, even with itself, it cannot be readily isolated or handled in the 
pure state. Therefore, it is produced and marketed as an aqueous solution 
(usually 37 - 50% formaldehyde by weight), sometimes known as Formalin. It is 
also sold as the solid hydrated polymer known as paraformaldehyde or paraform. 
    Formaldehyde is used principally to produce synthetic resins and adhesives 
by reaction with phenols, urea, and melamine. Other uses are in the 
manufacture of textiles, dyes, drugs, paper, leather, photographic materials, 
embalming agents, disinfectants, and insecticides. See ALDEHYDE; PHENOLIC 
RESIN.  
 �34s[L. Madestau]
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�6v�2v�32sFormic acid   �21m �33s 
    A colorless, pungent liquid having a melting point of 8.4DGC (47DGF) and 
the formula HCOOH. Formic acid is miscible with water, alcohol, ether, and 
glycerol. It is toxic, and causes painful skin wounds. It is a constituent of 
the stings of ants, stinging caterpillars, and stinging nettles. 
    Formic acid is stronger than acetic acid. With carbonates, oxides, and 
hydroxides of alkali or alkaline-earth metals, it gives formates (salts); with 
alcohols it forms esters (formates). See ESTER. 
    It is used as a reducing agent for metallic ions and dyes, in dehairing 
and tanning, as a latex coagulant, in the preparation of a large number of 
esters, as a source of allyl alcohol, and in the preparation of oxalic acid. 
 �34s[Evans B. Reid]
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�6v�2v�32sFossil   �21m �33s 
    The remains and traces, such as skeletons, animal tissues, footprints, and 
trails, of organisms preserved from the geologic past. The term fossil was 
originally used for a wide range of curios, both organic and mineral, found in 
rocks; but it came to be restricted to organic remains about the close of the 
17th century. The term fossil is still used as an adjective and, in a 
figurative sense, to indicate that nonorganic structures in the rocks, such as 
fossil mudcracks, rain imprints, ripple marks, and even fuels, were formed in 
the geologic past. 
    Several different modes of preservation are known. Generally only the hard 
parts, such as bones and shells, have been preserved, but in some instances 
even the soft tissues are preserved almost intact. Some organic objects are 
turned to stone. This petrifaction usually involves only the hard parts such 
as bones, shells, and wood. See EDIACARAN FAUNA; PETRIFACTION. 
    Mold, casts, and imprints form another category of fossils. They are 
formed when the sediment has solidified about an organic object and the latter 
is subsequently dissolved, leaving a hole in the rock. This is a mold. 
Deposition of mineral matter from underground solutions may fill the hole, 
producing a cast. Molds of thin objects such as leaves are commonly called 
imprints.  
    Tracks, trails, and burrows are traces that constitute another category of 
fossils. Although involving no remnant of the organism, they are structures 
made by a living animal and, as such, may give evidence of its size, shape, 
and habits. 
    Coprolites are petrified excrement. Undigested particles of food preserved 
in such fossils may indicate the feeding habits of animals long since extinct. 
See PALEOBOTANY; PALEONTOLOGY. 
 �34s[Carl O. Dunbar]
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�6v�2v�32sFossil fuel   �21m �33s 
    Carbon-containing materials that are burned with air, or with oxygen 
derived from the air to supply heat for any purpose. 
    Fuels are characterized (1) by their physical form (solid, liquid, or gas) 
at normal temperatures; (2) by their heating value, that is, by the amount of 
heat given off when a unit weight or volume of the material is burned under 
standard conditions; and (3) by their combustion characteristics, which cover 
such points as ease of ignition, rate of combustion, flame temperature, and 
flame luminosity. 
    Fuels may be used to supply heat directly, as in a furnace. They may also 
be used to supply heat to perform some other function, such as to produce 
steam in a boiler, the steam then being used to produce power in a turbine or 
engine. The heat may be used to raise the temperature and pressure of the 
products of the combustion to provide a driving force, as in the internal 
combustion engine or gas turbine. 
    Solid fuels comprise primarily the various ranks of coal (anthracite, 
bituminous, subbituminous, and lignite) and the coke or char derived from 
them. The important factors governing choice of coal for a given use are cost 
per unit of heating value, size range, moisture and ash content, and amount of 
smoke evolved. See COAL; COKE. 
    Liquid fuels are derived almost entirely from petroleum. In general, the 
liquid fuels suitable for internal combustion engines command a premium, and 
the fuels designed for external combustion are derived from those portions of 
the petroleum that have the least potential for gasoline and diesel fuel 
manufacture. Factors in choice of liquid fuels are heating value, fluidity, 
boiling range, impurities such as sulfur, and freedom from water and sediment. 
See DIESEL FUEL; FUEL OIL; GASOLINE; OIL SAND; OIL SHALE; PETROLEUM. 
    Of the gaseous fuels, the most important are those derived from natural 
gas or petroleum, chiefly methane. Other gaseous fuels are those that are 
by-products of some manufacturing process and therefore available at a cost 
that makes them competitive with natural gas. These include coke-oven gas, 
blast-furnace gas, and producer gas. See ENERGY SOURCES; FUEL GAS; NATURAL GAS. 
 �34s[Howard R. Batchelder]
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�6v�2v�32sFossil man �21m �33s
    All prehistoric skeletal remains of humans which are archeologically
earlier than Neolithic (necessarily an imprecise limit), regardless of degree
of mineralization or fossilization of bone, and regardless of whether the
remains may be classed as Homo sapiens sapiens, modern humans.
    Discoveries began in the early years of the 19th century, although their
meaning and antiquity were not recognized before the finding of Neandertal man
in 1856. Remains have come principally from Europe and adjacent portions of
Asia, from Africa, from China, and from Java. Because of the rather late entry
of humans into the New World, American Indian remains are all of relatively
recent origin and recognizable as H. s. sapiens.

    Prehuman ancestry.	The human is a catarrhine primate, part of a group
including Old World monkeys, apes, and various extinct forms. The oldest
certain representatives of the Catarrhini are fossils from the Fayum beds of
northern Egypt, dated around 30 million years (m.y.) old. The best-known of
these is Propliopithecus (= Aegyptopithecus) zeuxis, a possible ancestor for
apes and humans (see illustration). This animal was the size of a cat, with a
monkeylike body, apelike teeth, and a distinctive skull: a mosaic of
evolutionary features. Between about 23 and 15 m.y. ago, in the early Miocene,
several species of Dryopithecus (= Proconsul) in East Africa represent
relatives of the ancestors of humans. Evidence from both comparative
morphology and molecular studies of proteins shows that humans' closest living
relatives are the African apes: the chimpanzee and the gorilla. See
AEGYPTOPITHECUS; MONKEY; PRIMATES.
    The first evidence of a distinctive line of human evolution is provided by
a group of animals known colloquially as ground apes. These comprise the
genera Sivapithecus, Ramapithecus (= Kenyapithecus), and Gigantopithecus,
known by about seven species from East Africa, eastern Europe, Turkey,
Pakistan, India, and China in the middle and late Miocene (about 15 - 8 m.y.
ago), contemporary with the younger species of Dryopithecus from western
Europe. See KENYAPITHECUS; RAMAPITHECUS.

    Pliocene Homininae.  Australopithecus, the first truly humanlike beings,
appear in the fossil record in quantity some 4 m.y. ago, during the Pliocene.
One partial jaw with a single tooth has been found that may be as old as 6
m.y., and a few other fragments are a bit younger, but the majority of more
complete finds date between 1 and 4 m.y. It now appears that between 6 and 4
m.y. ago (when few fossils are known), a generalized type of Australopithecus
inhabited eastern (and	perhaps southern) Africa. Recently interpreted finds
from Hadar and Laetoli, between 4 and 3 m.y. in age, document such a creature,
which has been named A. afarensis. Most exciting was the find in 1974 of a
partial skeleton, identified as a female by its pelvic bones and named
``Lucy.''
    There is some disagreement as to the place of A. afarensis in human
evolution, whether a possible ancestor of all later forms or only related to
one of several lineages. By 2.5 to 3 m.y. ago, the two later varieties of
Australopithecus had diverged. Australopithecus africanus, the so-called
gracile form, probably was little changed from its putative ancestor; it is
known especially in South Africa, and less certainly in East Africa, between 3
and 2 m.y. ago. The second form, known as A. robustus (or also A. boisei, and
sometimes given generic or subgeneric rank as Paranthropus), lived in East and
South Africa between roughly 2.25 and 1.25 m.y. ago. At some sites, bones of
the two forms may occur together. Specimens of A. (P.) robustus may be
distinguished, however, by their larger size and craniodental specializations.
See AUSTRALOPITHECUS.

    Early Pleistocene. The only other genus of the subfamily Homininae (or
family Hominidae of many authors) is Homo, true humans, into which all later
forms (including ourselves) are placed. The identification of the earliest
specimens of Homo is a subject of debate among paleoanthropologists. In the
late 1970s the scientific pendulum had swung back to an idea proposed on less
secure grounds by L. S. B. Leakey and colleagues in 1964. They named a species
Homo habilis, based on several finds from Olduvai. Especially significant was
the discovery of the 1.8-m.y.-old remains of a juvenile's lower jaw, with teeth
much like those of A. africanus, and its partial skull, with an estimated
cranial capacity of about 650 to 700 cm sup 3. Although most experts did not
consider this find to be evidence of a new species, and showed that other
specimens involved here might really be a later type of human, newer
discoveries suggested that there was indeed an early form of Homo in East
Africa, and possible elsewhere, between 1.5 m.y. and more than 2 m.y. years
ago.

    Homo erectus. While H. habilis is apparently a short-lived and rare East
African species, its descendant, Homo erectus, is common, widespread, and
long-surviving. The first fossils were found in Java in 1893 and termed
Pithecanthropus erectus. Later finds in China and across Africa were each given
distinctive generic and specific names but are now all considered local
variants or subspecies of the single species H. erectus. See JAVA MAN; PEKING
MAN.
    The earliest known representatives are once again from East Africa, dating
from 1.5 to 1 m.y. ago. Homo erectus probably spread from Africa into Eurasia
more than 1 m.y. ago, perhaps the first type of human to do so in large
numbers. Fossils in Asia may date from over 1 m.y. through nearly 250,000
years ago. New finds in Africa document H. erectus as young as about 400,000
years ago. In Europe, however, there is definite evidence of human-made tools
between 1 m.y. and 500,000 years ago, but no human fossils are older than about
350,000 years. The earliest of these, in central and southeastern Europe,
already seem to be more advanced in some ways than Afro-Asian H. erectus.

    Pre-modern Homo sapiens. It is possible that the increased rigor of the
glacial climate in Europe at this time was the impetus leading to the evolution
of H. sapiens, modern-type humans. It is about equally likely that H. sapiens
may have evolved elsewhere, but between about 500,000 and 350,000 years ago,
the only known human fossils are those from Europe and those of typical of H.
erectus in China and North Africa. See HEIDELBERG MAN.
    For the next 200,000 years there is still a dearth of evidence, but it may
be suggested that local varieties of H. sapiens developed in Africa, Europe,
and perhaps northern Asia. The best-known of these varieties of Neanderthal
Man, mostly from Europe and the Near East, formally termed Homo sapiens
neanderthalensis. See FONTECHEVADE MAN; NEANDERTHAL MAN; STEINHEIM MAN;
SWANSCOMBE MAN.

    Spread of modern man. There is virtually no evidence of the area of origin
and early history of anatomically modern H. s. sapiens, characterized by a
small upright face, small teeth and brow ridges, chin, and high rounded skull.
There are some early (about 100,000 year old) fossils from East and especially
South Africa which suggest this region as a possible source. By about 35,000
years ago, modern Homo s. sapiens was the sole form of human to be found
anywhere. One reason for the success of H. s. sapiens may have been their
greater tool-making efficiency. In many parts of the world, they also engaged
in artistic pursuits, including carving small animal statues and perhaps
calendars as well as painting on the walls of deep caves.  [Eric Delson]

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�6v�2v�32sFoucault pendulum   �21m �33s 
    A pendulum or swinging weight, supported by a long wire, by which J. B. L. 
Foucault demonstrated in 1851 the rotation of Earth on its axis. Foucault used 
a 62-lb (28-kg) iron ball suspended on about a 200-ft (60-m) wire in the 
Pantheon in Paris. The upper support of the wire restrains the wire only in 
the vertical direction. The bob is set swinging along a meridian in pure 
translation (no lateral or circular motion). In the Northern Hemisphere the 
plane of swing appears to turn clockwise; in the Southern Hemisphere it 
appears to turn counterclockwise, the rate being 15 degrees times the sine of 
the local latitude per sidereal hour. Thus, at the Equator the plane of swing 
is carried around by Earth and the pendulum shows no apparent rotation; at 
either pole the plane of swing remains fixed in space while Earth completes 
one rotation each sidereal day. See DAY; INERTIAL GUIDANCE SYSTEM; PENDULUM; 
SCHULER PENDULUM. 
 �34s[Frank H. Rockett]
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�6v�2v�32sFoundations   �21m �33s 
    Structures or other constructed works are supported on the earth by 
foundations. The word ``foundation'' may mean the earth itself, something 
placed in or on the earth to provide support, or a combination of the earth 
and the elements placed on it. The foundation for a multistory office building 
could be a combination of concrete footings and the soil or rock on which the 
footings are supported. The foundation for an earth-fill dam would be the 
natural soil or rock on which the dam is placed. Concrete footings or piles 
and pile caps are often referred to as foundations without including the soil 
or rock on which or in which they are placed. The installed elements and the 
natural soil or rock of the earth form a foundation system; the soil and rock 
provide the ultimate support of the system. Foundations that are installed may 
be either soil-bearing or rock-bearing. The reactions of the soil or rock to 
the imposed loads generally determine how well the foundation system 
functions. In designing the installed portions, the designer must determine 
the safe pressure which can be used on the soil or rock and the amount of 
total settlement and differential settlement which the structure can withstand. 
    The installed parts of the foundation system may be footings, mat 
foundations, slab foundations, and caissons or piles, all of which are used to 
transfer load from a superstructure into the earth. These parts, which 
transmit load from the superstructure to the earth, are called the 
substructure (see illustration). 
    Footings or spread foundations are used to spread the loads from columns 
or walls to the underlying soil or rock. Normally, footings are constructed of 
reinforced concrete; however, under some circumstances they may be constructed 
of plain concrete or masonry. When each footing supports only one column, it 
is square. Footings supporting two columns are called combined footings and 
may be either rectangular or trapezoidal. Cantilever footings are used to 
carry loads from two columns, with one column and one end of the footing 
placed against a building line or exterior wall. Footings supporting walls are 
continuous footings. 
    Mat or raft foundations are large, thick, and usually heavily reinforced 
concrete mats which transfer loads from a number of columns or columns and 
walls to the underlying soil or rock. Mats are also combined footings, but are 
much larger than a footing supporting two columns. They are continuous 
footings and are designed to transfer a relatively uniform pressure to the 
underlying soil or rock. 
  
    Slab foundations are used for light structures wherein the columns and 
walls are supported directly on the floor slab. The floor slab is thickened 
and more heavily reinforced at the places where the column and wall loads are 
imposed. See CAISSON FOUNDATION; PILE FOUNDATION; RETAINING WALL. 
 �34s[Gardner M. Reynolds]
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�6v�2v�32sFour-bar linkage   �21m �33s 
    A basic linkage mechanism used in machinery and mechanical equipment. The 
term has been applied to three types of linkages: plane, spherical, and skew. 
    The plane four-bar linkage (Fig. 1) consists of four pin-connected links 
forming a closed loop, in which all pin axes are parallel. The spherical 
four-bar linkage consists of four pin-connected links forming a closed loop, 
in which all pin axes intersect at one point. The skew four-bar linkage (Fig. 
2) consists of four jointed links forming a closed loop, in which crank 2 and 
link 4 are pin-connected to ground 1 and the axes of the pins are generally 
nonparallel and nonintersecting; coupler 3 is connected to crank 2 and link 4 
by ball joints. 
    Four-bar linkages are most frequently used to convert a uniform, 
continuous rotation (the motion of crank 2) into a nonuniform rotation or 
oscillation (the motion of link 4). In instrument applications the primary 
function of the linkage is the conversion of motion, while in power 
applications both motion conversion and power transmission are fundamental. 
    Each of the above linkages can be proportioned for three types of motion, 
or linkage types: crank-and-rocker, drag, and double-rocker. 
    Crank-and-rocker linkages have a motion in which the crank (link 2) is 
capable of unlimited rotation, while the output link (link 4) oscillates or 
rocks through a fraction of one turn (usually less than 90DG). This is the 
most common form of the plane and the skew four-bar linkage, and is used in 
machinery and appliances of all types. 
    In drag linkages the motions of cranks 2 and 4 are both capable of 
unlimited rotations. The plane drag linkage has been used for quick-return 
motions. The most common drag linkage is the spherical drag linkage. One such 
linkage is the Hooke-type universal joint, or Hooke joint. See UNIVERSAL JOINT. 
    In double-rocker linkages, neither crank 2 nor 4 is capable of complete 
rotations. Such motions occur in hand tools and mechanical equipment in which 
only limited rotations are required. See LINKAGE (MECHANISM); SLIDER CRANK 
MECHANISM; STRAIGHT-LINE MECHANISM. 
 �34s[Ferdinand Freudenstein]
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�6v�2v�32sFourier series �21m �33s
    A mathematical tool for the study of periodic phenomena, indispensable in
the theory of wave motion (for example, light waves and sound waves),
oscillatory mechanical systems (such as vibrating strings), and celestial
orbits. Fourier series and related topics have important applications to other
branches of mathematics, of which the theories of probability and partial
differential equations are worthy of special mention. Finally, the disciplines
motivated by the subject itself enjoy a prominent position in pure
mathematical research.
    A function f of a real variable is said to be periodic with period T if
f(t + T) = f(t) for each t. The simplest examples of functions with a given
period T are pure harmonics, that is, functions of the form f(t) = asubn cos
nOMEGAt + bsubn sin nOMEGAt, where OMEGA = 2PIT- sup 1 is the fundamental
frequency and asubn, bsubn are constants. The basic idea for the applications
of Fourier series is that any function f of period T which satisfies rather
mild restrictions can be represented as a superposition of pure harmonics, as
in relation (1), or, in a more convenient form, using complex exponentials, as
relation (2).

f(t) SIMILAR SUMMATION (n = 0 to INFINITY) (a sub n cos nOMEGAt + b sub n    (1)
		sin nOMEGAt)

f(t) SIMILAR SUMMATION (- INFINITY to INFINITY) c sub n e sup inOMEGAt	     (2)

    In term-by-term integration of relation (2) is assumed to be legitimate,
an easy calculation shows that Eq. (3) can be formed. (The interval of

c sub n = T sup -1 INTEGRATION (-T/2 to T/2) f(t) e- sup inOMEGAt dt	     (3)

integration could be any interval of length T). This motivates the formal
definition of Fourier series. Suppose that f is a function of period T such
that expression (4) exists and is finite. The coefficients {csubn} defined by
Eq. (3) are the Fourier coefficients of f, and the series in relation (2), the

		    INTEGRATION (-T/2 to T/2) |f(t)|dt			     (4)

Fourier series expansion of f. The coefficients uniquely determine the
function; that is, if csubn = 0 for each n, then f is essentially the zero
function. In addition, many formal operations in which the series is
substituted term by term for the function can be justified. See SERIES.
 �34s[Carl S. Herz]
�33s

			     *****EQUATION***** 			     (3)


			     *****EQUATION***** 			     (4)


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�6v�2v�32sFourth dimension �21m �33s
    The designation frequently given to time in the theory of relativity. This
terminology arises because in relativity neither the time of occurrence of an
event nor the passage of time between events can be specified in an absolute
sense; in fact, the measurements of both time and distance depend on the state
of motion of the observer.
    It thus becomes necessary to conceive of an observer's time and space
measurements as only particular aspects of a four-dimensional world called
space-time, consisting of the set of all possible events. Observers in
relative motion characterize an event by different sets of four coordinates;
three coordinates give the event's relative position and the fourth coordinate
is x sup 0 = ct, where c is the speed of light and t is the time. If DELTAx is
the distance and DELTAx sup 0/c is the time between two events, then in special
relativity a space-time interval DELTAs between the events may be defined by
the relation -DELTAs sup 2 = (DELTAx) sup 2 - (DELTAx sup 0) sup 2. This
interval has a unique numerical value irrespective of the motion of the
observer, and plays a role in space-time analogous to that of length in ordinary
three-dimensional euclidean geometry. In ordinary space the x component of a
given line segment may be found only after specifying the coordinate system in
which the x component is to be determined. Similarly in space-time, the time
component of an interval may be uniquely determined only when the state of
motion of the observer performing the measurement is given. See RELATIVITY;
SPACE-TIME.
 �34s[Neil Ashby]
�33s

A700000001755�254s�5v�1v750
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�6v�2v�32sFractionating column   �21m �33s 
    An apparatus used widely for countercurrent contacting of vapor and liquid 
to effect separations by distillation or absorption. In general, the apparatus 
consists of a cylindrical vessel with internals designed to obtain multiple 
contacting of ascending vapor and descending liquid, together with means for 
introducing or generating liquid at the top and vapor at the bottom (see 
illustration).  
    Fractionating columns used in industrial plants range in diameter from a 
few inches to 40 ft (12 m) and in height from 10 to 200 ft (3 to 60 m). They 
operate at pressures as low as a few millimeters of mercury and as high as 
3000 psi (21 megapascals); at temperatures from -300 to 700DGF (-184 to 
1290DGF). They are made of steel and other metals, of ceramics and glass, and 
even of such materials as bonded carbon and plastics. See DISTILLATION; GAS 
ABSORPTION OPERATIONS. 
 �34s[Mott Souders]
�33s 
  
  
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�6v�2v�32sFrame of reference   �21m �33s 
    A base to which to refer physical events. A physical event occurs at a 
point in space and at an instant of time. Each reference frame must have an 
observer to record events, as well as a coordinate system for the purpose of 
assigning locations to each event. The latter is usually a three-dimensional 
space coordinate system and a set of standardized clocks to give the local 
time of each event. For a discussion of the geometrical properties of 
space-time coordinate systems see SPACE-TIME. See also RELATIVITY. 
    In the ordinary range of experience, where light signals, for all 
practical purposes, propagate instantaneously, the time of an event is quite 
distinct from its space coordinates, since a single clock suffices for all 
observers, regardless of their state of relative motion. The set of reference 
frames which have a common clock or time is called newtonian, since Isaac 
Newton regarded time as having invariable significance for all observers. 
    For discussion of other types of reference frames see ACCELERATED 
REFERENCE FRAME; ASTRONOMICAL REFERENCE FRAME; INERTIAL REFERENCE FRAME; 
ROTATING REFERENCE FRAME. 
 �34s[Bernard Goodman]
�33s 
  
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�6v�2v�32sFrancium   �21m �33s 
    A chemical element, Fr, atomic number 87, an alkali metal element falling 
below cesium in group Ia of the periodic table. Distinguished by nuclear 
instability, francium exists only in short-lived radioactive forms, the most 
durable of which has a half-life of 21 min. The chief isotope of francium is 
actinium-K, an isotope of mass 223, which arises from the radioactive decay of 
the element actinium. From the properties of the known isotopes, it is 
reasonably certain that no long-lived form of element 87 will ever be found in 
nature or synthesized artificially. 
    The chemical properties of francium can be studied only on the tracer 
scale. The element has all the properties expected of the heaviest alkali 
element. With few exceptions, all the salts of francium are water-soluble. See 
ACTINIUM; ALKALI METALS. 
 �34s[Earl K. Hyde]
�33s 
  
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�6v�2v�32sFranck-Condon principle �21m �33s
    In any molecular system the transition from one energy state to another
occurs so near to instantaneously that the nuclei of the atoms involved can be
considered to be stationary during the transition. This principle, proposed by
J. Franck in 1925 and developed on the basis of quantum mechanics by E. U.
Condon in 1928, is important in discussing systems where more than one atom is
involved. It is therefore valuable in problems of molecular spectroscopy and
in the interpretation of the optical properties of liquids and solids. See
LUMINESCENCE; MOLECULAR STRUCTURE AND SPECTRA.
 �34s[Clifford C. Klick ;James H. Schulman]
�33s

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�6v�2v�32sFranklinite �21m �33s
    A natural member of the spinel structure type, with composition
Zn sup 2 sup + Fe2 sup 3 sup + O4. The habit is octahedral, often modified by
the cube and dodecahedron, but the mineral usually occurs as bands of isolated
rounded grains, blebs, or compact masses. It is black with a metallic luster and
red internal reflections; its hardness is 6 (Mohs scale); specific gravity is
5.3; and it is weakly magnetic.
    Franklinite is confined in its occurrence to the unique ore bodies at
Franklin and Sterling Hill (Ogdensburg), Sussex County, New Jersey.
Franklinite is a major ore mineral and is still mined at Sterling Hill for
spiegeleisen and zinc. See WILLEMITE; ZINC; ZINCITE.
 �34s[Paul B. Moore]
�33s

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�6v�2v�32sFree-electron laser �21m �33s
    A device in which laser action involves passage of electrons through a
spatially varying magnetic field which causes the electron beam to ``wiggle''
and hence to radiate. Free-electron lasers are of interest because of their
potential for efficiently producing very high-power radiation, tunable from
the millimeter to the x-ray region. However, operation has been demonstrated
only at 3.4 nanometers and at very low-power levels. The large Doppler upshift
due to relativistic electron velocities can be adjusted, resulting in tunable
emission at optical frequencies.  See  DOPPLER EFFECT; LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
�33s

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�6v�2v�32sFree-electron theory of metals   �21m �33s 
    A model of a metal in which the free electrons, that is, those giving rise 
to the conductivity, are regarded as moving in a potential - the sum of the 
average potential due to the metal ions in the lattices and to all the 
remaining free electrons - which is approximated as constant everywhere inside 
the metal. The free electrons are thus assumed to move independently of one 
another throughout the space bounded by the surfaces of the metal. Interaction 
of the free electrons with the ion cores is assumed to be negligible. The free 
electrons are taken to be identical to the valence electrons of the free metal 
atoms; thus, alkali metals contain one free electron per atom, and aluminum 
contains three. The free electron theory should actually be termed the 
quasi-free-electron theory, to distinguish between electrons in a metal which 
behave nearly as do free electrons, and truly free electrons, such as those in 
a vacuum tube. 
    A number of important physical properties of some metals, in particular 
the simple monovalent metals, are explained satisfactorily in terms of the 
quasi-free-electron model. Among these are the electronic specific heat, the 
magnetic susceptibility, and the electrical and thermal conductivity. To 
understand the behavior of the more complex metals and of semiconductors, it 
became necessary to introduce the more complicated band theory of solids. See 
BAND THEORY OF SOLIDS. 
 �34s[Frank J. Blatt]
�33s 
  
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�6v�2v�32sFree energy   �21m �33s 
    A term in thermodynamics which in different treatments may designate 
either of two functions defined in terms of the internal energy E or enthalpy 
H, and the temperature-entropy product TS. 
    The function (E - TS) is the Helmholtz free energy and is the function 
ordinarily meant by free energy in European references. The Gibbs free energy 
is the function (H - TS). For the Lewis and Randall school of American 
chemical thermodynamics, this is the function meant by the free energy F. To 
avoid confusion with the symbol F as applied elsewhere to the Helmholtz free 
energy, the symbol G has also been used. Another development was the 
introduction of the name free enthalpy, with symbol G, for the Gibbs function. 
See WORK FUNCTION (THERMODYNAMICS). 
    For a closed system (no transfer of matter across its boundaries), the work 
which can be done in a reversible isothermal process is given by the series 
shown in Eq. (1). For these conditions, T DELTA S represents the heat given up 

** W (sub rev) = - DELTA A = - DELTA (E - TS) = - (DELTA E - T DELTA S) ** (1)

to the surroundings. Should the process be exothermal, T DELTA S < 0, then 
actual work done on the surroundings is less than the decrease in the internal 
energy of the system. The quantity (DELTA E - T DELTA S) can then be thought 
of as a change in free energy, that is, as that part of the internal energy 
change which can be converted into work under the specified conditions. This 
then is the origin of the name free energy. Such an interpretation of 
thermodynamic quantities can be misleading, however; for the case in which T 
DELTA S is positive, Eq. (1) shows that the decrease in ``free'' energy is 
greater than the decrease in internal energy. See CHEMICAL THERMODYNAMICS. 
    For constant temperature and pressure in a reversible process the decrease
in the Gibbs function G for the system again corresponds to a free-energy 
change in the above sense, since it is equal to the work which can be done by 
the closed system other than that associated with its change in volume DELTA V 
under the given constant pressure P. The relations shown in Eq. (2) can be 
formed since DELTA H = (DELTA E + P DELTA V). 

       *** DELTA G = - (DELTA H - T DELTA S) =
                W (sub net) = [W (sub rev) - P DELTA V] ***             (2)
  
    Each of these free-energy functions is an extensive property of the state 
of the thermodynamic system. For a specified change in state, both DELTA A and 
DELTA G are independent of the path by which the change is accomplished. Only 
changes in these functions can be measured, not values for a single state. 
    The thermodynamic criteria for reversibility, irreversibility, and 
equilibrium for processes in closed systems at constant temperature and 
pressure are expressed naturally in terms of the function G. For any 
infinitesimal process at constant temperature and pressure, -dG EQUAL TO OR 
GREATER THAN DELTA w (sub net). If DELTA w (sub net) is never negative, that 
is, if the surroundings do no net work on the system, then the change dG must 
be negative or zero. For a reversible differential process, -dG > DELTA w (sub 
net); for an irreversible process, -dG > DELTA w (sub net). The free energy G 
thus decreases to a minimum value characteristic of the equilibrium state at 
the given temperature and pressure. At equilibrium, dG = 0 for any differential 
process taking place, for example, an infinitesimal change in the degree of 
completion of a chemical reaction. A parallel role is played by the work 
function A for conditions of constant temperature and volume. Because 
temperature and pressure constitute more convenient working variables than 
temperature and volume, it is the Gibbs free energy which is the more commonly 
used in thermodynamics. See ENTROPY; THERMODYNAMIC PRINCIPLES. 
 �34s[Paul J. Bender]
�33s 
  

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�6v�2v�32sFree fall   �21m �33s 
    The accelerated motion toward the center of the Earth of a body acted on 
by the Earth's gravitational attraction and by no other force. If a body falls 
freely from rest near the surface of the Earth, it gains a velocity of 
approximately 9.8 m/s every second. Thus, the acceleration of gravity g equals
9.8 m/s (sup 2) or 32.16 ft/s (sup 2). This acceleration is independent of the
mass or nature of the falling body. For short distances of free fall, the value
of g may be considered constant. After t seconds the velocity v (sub t) of a
body falling from rest near the Earth is given by Eq. (1). 

                         *** v (sub t) = gt ***                          (1)
  
    If a falling body has an initial constant velocity in any direction, it 
retains that velocity if no other forces are present. If other forces are 
present, they may change the observed direction and rate of fall of the body, 
but they do not change the Earth's gravitational pull; therefore a body may 
still be thought of as freely ``falling'' even though the resultant observed 
motion is upward. 
    For a body falling a very large distance from the Earth, the acceleration 
of gravity can no longer be considered constant. According to Newton's law of 
gravitation, the force between any two bodies varies inversely with the square 
of the distance between them; therefore with increasing distance between any 
body and the Earth, the acceleration of the body toward the Earth decreases 
rapidly. The final velocity v sub f, attained when a body falls freely from an 
infinite distance to the surface of the Earth, is given by Eq. (2), where R is 
  
                   *** v (sub f) = SQUARE ROOT [2Gr] ***                 (2)
  
the radius of the Earth, which gives a numerical value of 11.3 km/s or 7 mi/s. 
This is consequently the ``escape velocity,'' the initial upward velocity for 
a rising body to completely overcome the Earth's attraction. 
    Because of the independent action of the forces involved, a ball thrown 
horizontally or a projectile fired horizontally with velocity v will be 
accelerated downward at the same rate as a body falling from rest, regardless 
of the horizontal motion. 
    At a sufficiently large horizontal velocity, a projectile would fall from 
the horizontal only at the same rate that the surface of the Earth curves away 
beneath it. The projectile would thus remain at the same elevation above the 
Earth and in effect become an earth satellite. See BALLISTICS; GRAVITATION. 
 �34s[Rogers D. Rusk]
�33s 
  

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�6v�2v�32sFree radical   �21m �33s 
    Any molecule or atom which possesses one unpaired electron. There are some 
molecules which contain more than one unpaired electron (for example, oxygen); 
they normally are not considered as free radicals. Free radicals can be 
chemically very reactive (for example, the methyl radical) or they can be very 
stable entities (for example, nitric oxide). 
    Free radicals can be grouped into three major classes: atoms (for example, 
H, F, and Cl), inorganic radicals (for example, OH, CN, NO2, and ClO3), and 
organic radicals (for example, CH3, CH3CH2, and C6H6-). Such radicals are of 
great importance since they often appear as intermediates in thermal and 
photochemical reactions. Radicals are also known to initiate and propagate 
polymerization and combustion reactions. 
    In general, free radicals are formed by the rupture of a bond in a stable 
molecule with the production of two fragments, each with an unpaired electron. 
The resulting free radicals may participate in further reactions or may 
combine to reform the original compound. 
    There are many ways in which radicals can be generated - among these are 
thermal decomposition, electric discharge photochemical reactions, 
electrolysis at an electrode such as mercury or platinum, rapid mixing of two 
reactants, and gamma- or x-ray irradiation. 
 �34s[James R. Bolton]
�33s 
  
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�6v�2v�32sFreon   �21m �33s 
    One of a group of polyhalogenated derivatives of methane and ethane 
containing fluorine and, in most cases, chlorine or bromine. These include 
such compounds as trichlorofluoromethane and dichlorodifluoromethane, also 
known as Freon-11 and Freon-12, respectively. The freons possess such 
noteworthy characteristics as nonflammability, excellent chemical and thermal 
stability, and low toxicity. Additional properties include high density, low 
boiling point, low viscosity, and low surface tension. 
    This unique combination of properties has made the freons particularly 
suitable for use as refrigerants. In addition to their use as refrigerants, 
the freons serve as propellants in aerosol products, as solvents, and as 
intermediates in the synthesis of other fluorine compounds. Further 
application has been in the field of polymers and plastics, where several of 
the freons are important intermediates. See FLUOROCARBON; HALOGENATED 
HYDROCARBON; REFRIGERATION. 
 �34s[Ellen Clarke ;Max G. Gergel]
�33s 
  
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�6v�2v�32sFrequency bridge �21m �33s
    A bridge network that is used to measure frequency. An ac bridge is
generally made up of a four-arm network, where an ac source is applied to a
pair of opposite terminals and a current-detecting device is connected to the
remaining two terminals. With the bridge network energized, the current in the
detecting device can be made zero by adjusting suitable values of resistance,
capacitance, or inductance in the four bridge arms, a process known as
balancing.
    Bridge networks in which the balancing action depends upon the supply
frequency may be used for frequency measurement. The choice of frequency
bridge is dependent on the frequency range, the available apparatus, and the
ease with which the bridge can be set up and balanced. See BRIDGE CIRCUIT;
FREQUENCY MEASUREMENT.
 �34s[Edward B. Curdts]
�33s

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�6v�2v�32sFrequency counter   �21m �33s 
    An electronic device capable of counting the number of cycles in an 
electrical signal during a preselected time interval. The modern high-speed 
electronic counter (see illustration) is a useful tool in the measurement of 
frequency when an accurate time base is available. It provides a digital 
counting or scaling device for registering the total number of events 
occurring during a given time interval. 
    The principal requirements are for signals of adequate level, free from 
interference and noise. With these conditions satisfied it is usually possible 
to obtain a precision of reading well beyond the stability of the overall 
system if a sufficiently long counting time is used. An extension of the 
frequency-measurement range is made possible by extending the range of the 
converter or heterodyning system. It is also feasible to add a transfer 
oscillator with harmonics available up into the microwave region. See 
ELECTRICAL MEASUREMENTS; FREQUENCY MEASUREMENT. 
 �34s[Frank D. Lewis]
�33s 
  
  
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�6v�2v�32sFrequency divider   �21m �33s 
    An electronic circuit that produces an output signal at a frequency that 
is an integral submultiple of the frequency of the input signal. 
    Several information-processing and transmitting techniques require 
frequency division. In television, for example, it is essential to maintain a 
precise relationship between the horizontal-scanning frequency and the 
vertical-scanning frequency. Frequency division can be conveniently 
accomplished in two ways, digital division and division by triggering a 
subharmonic.  
    Many circuits are available to count pulses. A bistable or flip-flop 
circuit produces one output pulse for every two input pulses. By cascading 
successive flip-flops, any desired degree of division can be obtained. This is 
the method of digital division. 
    Any circuit which has a characteristic resonance responds to certain types 
of input energy by ringing, that is, by going through one or more cycles of 
electrical activity caused by the nature of the circuit rather than by the 
nature of the input. This characteristic can be used to accomplish frequency 
division, provided the input frequency does not vary over any extensive 
frequency range. This is the method for triggering a subharmonic. 
 �34s[William W. Snow]
�33s 
  
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�6v�2v�32sFrequency measurement �21m �33s
    Measurement of the frequency of a periodic quantity, defined as the number
of times a cyclic phenomenon occurs per unit of time. The second is the
universally used unit of time. Conversely, time may be measured by observing
the number of cycles occurring at constant frequency. The ordinary pendulum
and household electric clocks are familiar examples of such time-measuring
devices.
    The only primary frequency standards acceptable for use in national
standards laboratories for frequency reference are atomic standards of the
cesium-beam type. Cesium-beam atomic clocks have superseded other types of
primary frequency standards as a consequence of the adoption of the atomic
second as the unit of time. See ATOMIC CLOCK; ATOMIC TIME.
    The frequency of a secondary standard is determined by comparison with a
primary standard or by comparison with another secondary standard originally
compared with a primary standard. Secondary standards include (1)
quartz-crystal frequency standards, with or without clock indicators, and (2)
gas-cell atomic standards such as rubidium-cell standards, and (3) ammonia
masers. Rubidium-cell standards are relatively rugged and can be made to
exhibit low drift rates, thus enabling relatively long intervals between
calibrations. Quartz-crystal frequency standards are the least expensive
initially and may provide the most satisfactory working standards if
calibration means are available. See QUARTZ CLOCK.
 �34s[Frank D. Lewis]
�33s

    Audio-frequency.  A fairly wide variety of audio-frequency meters has been
developed and is commercially available. Broadly speaking they can be grouped
into two classes, the resonant and ratiometer types. The resonant type may be
further subdivided into instruments employing resonant reeds and those having
electrically resonant circuits. The latter, as well as the ratiometer type,
are classified as deflection-type instruments embodying moving systems with
pointers and scales. See FREQUENCY BRIDGE; FREQUENCY COUNTER; MOVING-COIL
FREQUENCY METER; MOVING-IRON FREQUENCY METER; REED-TYPE FREQUENCY METER.
    The most widely used of the simpler methods is aural comparison. An
unknown frequency can be determined by direct comparison with a known
adjustable standard frequency by using the ear. Typically, headphones are
connected directly to the two audio sources. The unknown frequency is fed into
one earpiece while the standard frequency supplied by a resistance-tuned or a
beat-frequency oscillator is fed into another earpiece. A beat note between
the two frequencies can be detected by the ear when the frequencies are not
exactly equal. A meter or an electric-eye indicator can take the place of the
ear as a detector.
    Cathode-ray oscilloscopes provide a most convenient means of comparing two
frequencies. The usual method consists of applying a voltage of unknown
frequency to one pair of deflecting plates and a voltage of known frequency to
the other. The resulting patterns, known as Lissajous figures, are a function
of the difference in amplitude, frequency, phase, and the waveforms of the two
applied voltages and their frequency ratio. Knowing one of the frequencies and
determining the difference or the ratio of the frequencies, it is possible to
calculate the second frequency. See LISSAJOUS FIGURES.

    Radio-frequency. Precise mesurement of frequencies above the audible range
may be performed by various techniques. Basic measuring systems may consist
of (1) a calibrated oscillator with some means of comparison with the unknown
frequency, such as an oscilloscope or a heterodyne detector unit, (2) a
digital counting or scaling device which registers the total number of events
occurring during a given time interval, or (3) an electronic circuit for
producing a direct current proportional to the frequency of its input signal
which then may be indicated by a dc meter. In practice, each of these basic
systems has an upper frequency limit for practical operation.
    In order to extend the range of frequency measurement above the practical
limits mentioned above, it is customary to generate a fixed standard frequency
and to select a harmonic of it near the unknown frequency, after which the
unknown frequency may be reduced to a lower value by subtraction of the
standard-frequency harmonic in a mixer or heterodyne detector. This lower beat
frequency may then be measured by application of the basic methods outlined
above, which thus serve to interpolate between the known standard frequencies.
[Frank D. Lewis]

 �34s[Edward B. Curdts]
�33s

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�6v�2v�32sFrequency modulation �21m �33s
    A special kind of angle modulation in which the instantaneous frequency of
a sine-wave carrier is varied by an amount proportional to the magnitude of
the modulating wave. See ANGLE MODULATION; PHASE MODULATION.
    Either amplitude modulation (AM) or frequency modulation (FM) offers a
solution to the important problem of how to impress the message wave to be
communicated upon a high-frequency oscillation. However, FM offers important
advantages in exchange for extra bandwidth occupancy. Also, FM with negative
feedback minimizes noise problems and receiver distortion. See MODULATION.
    Frequency modulation is defined in terms of a generalized concept known as
instantaneous frequency which is directly proportional to the time rate of
change of the angle of a sine function, the argument of which is a function of
time. When the argument is expressed in radians and the time in seconds, the
instantaneous frequency in hertz is the time rate of change of the angle
divided by 2PI.
    In frequency modulation the instantaneous frequency is linearly
proportional to the magnitude of the modulating wave. Louder tones with AM
mean greater changes in amplitude. Louder tones with FM mean greater changes
in frequency.
    In radio broadcasting, provided the frequency deviation (peak difference
between instantaneous and carrier frequencies) is large and provided multipath
transmission effects are small, FM is capable of high-fidelity reception
combined with the advantages of reduced noise, less interference between
stations, and less transmitter power to cover a given area. Constant average
power and constant peak power that is only twice the average power are two
factors that permit a ready realization of a simple high-efficiency
transmitter, simplify problems of automatic volume control, and allow
amplifiers and other devices to operate closer to their maximum power
capability without the penalties normally associated with nonlinearities.
    Practically, when estimating approximate bandwidth occupancy, a rule of
thumb states that angle modulation requires the band traversed by the
instantaneous frequency plus the bandwidth of the modulating wave added at
both top and bottom. For some purposes an even wider band may be required.
    Moreover, unambiguous representation and recovery of the wanted message by
angle-modulation techniques also require that the unmodulated carrier
frequency comfortably exceed the sum of the frequency deviation in the down
direction plus the bandwidth of the modulating wave. In other words, in FM the
carrier frequency must be high compared to the maximum frequency deviation.
    For certain types of noise disturbance characterized by a noise spectrum
that is uncorrelated and independent of frequency, the ratio of average signal
power to average noise power in the output of the FM receiver will be
proportional to the square of the peak-frequency deviation. Therefore under
certain important conditions, the signal-to-noise ratio of an angle-modulation
system improves 6 decibels for each 2:1 increase in bandwidth occupancy.
    However, the noise advantage of FM cannot be increased indefinitely. As
the bandwidth occupancy is continually increased to accommodate an increased
frequency deviation, more noise reaches the FM detector. Presently, the
assumption that the noise is less than the so-called improvement threshold is
violated, whereupon the noise advantage of FM is quickly lost. FM with
negative feedback acts differently in that the improvement threshold is
minimized and held constant, independent of the bandwidth occupied by the
incoming FM signal. FM with negative feedback is accompanied by a decrease in
distortion originating within the FM receiver and by an increased tolerance to
noise falling within the frequency band occupied by the incoming FM signal.
These two important advantages are not possessed by nonfeedback receivers.
Substantial benefits are realized only when the amount of negative feedback is
large. In common with nonfeedback FM systems, any large reduction in noise
must be paid for by a corresponding increase in the bandwidth occupied by the
transmitted FM signal.
    Many schemes for production and detection are possible and nearly all use
spectrum translation. For the production of FM most schemes resort to spectrum
multiplication. Spectrum translation of an angle-modulated wave is
accomplished by single-sideband modulation. The translated spectrum, with or
without inversion, is centered about a new carrier. Otherwise its significant
properties are unchanged. Spectrum multiplication implies angle
multiplication. By generating the xth harmonic of an angle-modulated wave, the
angle is multiplied by x. If the required multiplication is too great, then
after a convenient number of multiplications the resulting spectrum may be
translated downward and multiplication resumed. See FREQUENCY-MODULATION
DETECTOR; FREQUENCY-MODULATION RADIO.
 �34s[Harold S. Black]
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�6v�2v�32sFrequency-modulation detector   �21m �33s 
    Detection or demodulation of a frequency-modulated (FM) wave. FM detectors 
operate in several ways. In one class of detector, known as a discriminator, 
the frequency modulation is first converted to amplitude modulation, which is 
then detected by an amplitude-modulation detector. Another type of FM detector 
employs a phased-locked oscillator to recover the modulation. A still 
different type converts the frequency modulation to pulse-rate modulation, 
which can be converted to the desired signal by use of an integrating circuit. 
See AMPLITUDE-MODULATION DETECTOR. 
 �34s[Charles L. Alley]
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�6v�2v�32sFrequency-modulation radio   �21m �33s 
    Radio transmission accomplished by symmetrical variation of the carrier 
frequency by an analog input signal. The amount of swing from center frequency 
is dependent upon the peak value of the modulating voltage, as well as upon 
its frequency. The frequency of the modulation signal governs the rate at 
which the changes in carrier frequency occur. Frequency-modulation (FM) 
sidebands are formed during the modulation process and are separated from one 
another by an amount equal to the audio frequency. The amplitude of the 
sidebands diminishes progressively as the sidebands occur farther and farther 
from the center frequency, and the number of significant sidebands depends on 
the amplitude of the modulating signal. The deviation ratio of the 
carrier-frequency variation to the highest signal frequency transmitted may be 
any selected value, from fractional to large values. 
    Because there is no amplitude change in the output of an FM transmitter, 
whatever the deviation ratio, this mode is an almost perfect cure for 
amplitude-related interference problems that plague radio-frequency reception. 
Another property of FM receivers is the relative freedom from interference 
between distant and local stations using the same channel; only the strongest 
signal is received, even if the wanted signal is only 3 - 6 dB stronger than 
the interfering one. This characteristic is known as the capture effect. In 
contrast, AM radio signals differing in strength by 35 dB result in noticeable 
interference.  
    Since most FM receivers are equipped with a muting circuit (squelch) that 
silences the audio channel when no signal is present, an irritating hiss noise 
is not emitted from the loudspeaker when the frequency (or channel) being 
monitored is not in use. 
    Frequency modulation is used mainly for transmissions above 25 megahertz 
(MHz). Typical uses are in broadcasting, television sound, mobile radio 
telephony, radio paging systems, space telemetry, intercity microwave relaying 
of all classes of public traffic including voice channels, teleprinting, 
facsimile, broadcast network programs, and television and computer data, and 
intercontinental telecommunications via satellite. Frequency modulation is 
used for both analog and digital communications, and phase modulation as well 
as frequency modulation is employed. 
  
    FM broadcasting.  The frequency band 88 - 108 MHz is allocated to FM 
broadcasting in a large part of the world by international agreement. For a 
channel spacing of 200 kHz, there are 100 allocable channels for transmission 
of an audio range of 50 - 15,000 Hz, with a frequency-deviation ratio of 5. 
This means that there are five significant sidebands above and below the 
carrier, the carrier is deviated a maximum of PLUS/MINUS75 kHz, and the 
emitted spectrum is twice this value. Because of the relatively small signal 
power in the modulating frequencies above 4 kHz, the received signal-to-noise 
ratio is improved substantially by preemphasis of the audio signal in 
transmission, necessitating complementary deemphasis in the receiver to 
restore natural program balance. In fact, preemphasis produces a sort of 
hybrid form of modulation, being pure frequency modulation at the lowest audio 
frequencies, and gradually changing to phase modulation at the highest. See 
FREQUENCY MODULATION. 
  
    FM mobile transmission.  Millions of land, maritime, and aeronautical 
mobile FM transceivers are employed by police, firemen, public safety 
agencies, industrial and commercial enterprises, private citizens, and radio 
amateurs who desire the benefits of enroute telephony. The intensity of such 
usage has grown exponentially, mainly because of the availability of reliable, 
small, low-power-consumption solid-state equipments that are economical, and 
also because of the public realization of the benefits of having such 
communications. See MOBILE RADIO. 
  
    Radio relaying.  Frequency modulation is used for microwave radio relaying 
over land, over water, and to great distances using satellites, sometimes 
carrying thousands of simultaneous telephone conversations or several 
television channels. 
    The advent of requirements for short-haul services, local distribution 
networks within cities, television relay, and a wide variety of optical 
communication services including high-speed computer communications, 
electronic mail, data transmission, and other services, where it may be 
cost-effective to avoid the local telephone loop, presents another application 
for FM radio relaying. 
  
    Telegraphy.  Telegraphy, including teleprinting and binary digital data 
transmission, is based on shifting the carrier frequency or its phase between 
two limiting values, one of which represents a mark signal and the other a 
space signal. This frequency shift (or phase shift) is a form of FM signaling 
used over a wire, cable, or radio. See TELEGRAPHY. 
  
    Facsimile.  Black-and-white images (line drawings and typed copy) can be 
transmitted by employing the principles used in FM telegraphy; one limit 
frequency corresponds to black, and the other to white, on the image to be 
transmitted. A continuous gray scale can be transmitted and recorded if, 
instead of just two frequencies, a continuous frequency shift is employed 
between some low frequency (say, 1500 Hz) and some higher frequency (say, 2700 
Hz), the exact frequency at any instant being proportional to the gray level 
of the image. See FACSIMILE. 
  
    Telemetry.  Frequency modulation is the preferred method for transmission 
of information or data from a remote or inaccessible location such as a rocket 
vehicle in flight. Each condition to be remotely observed actuates one 
subchannel, which, when multiplexed with other channels reporting other status 
conditions, modulates the radio carrier by frequency modulation. See 
TELEMETERING.  
 �34s[John S. Belrose]
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�6v�2v�32sFrequency modulator   �21m �33s 
    An electronic circuit or device producing frequency modulation. The 
frequency modulator changes the frequency of an oscillator in accordance with 
the amplitude of a modulating signal. If the modulation is linear, the 
frequency change is proportional to the modulating voltage. 
    High-frequency oscillators usually employ LC tuned circuits to establish 
the frequency of oscillation, and this frequency can be controlled by changing 
the effective capacitance or inductance in accordance with the modulating 
signal. Practical circuits usually employ either a reactance tube or a 
varactor diode to change the oscillator frequency in accordance with a 
modulating voltage. 
    The varactor diode is used commonly as a frequency modulator in modern 
equipment. A typical varactor modulator is shown in the illustration. In this 
circuit the collector voltage of the modulating amplifier controls the reverse 
bias across the diode. But since the junction capacitance of an abrupt 
junction diode is inversely proportional to the square root of the junction 
voltage, the oscillator tuning capacitance, which consists of capacitor C and 
the diode in series, is therefore controlled by the modulation signal. The 
resistance of R must be small in comparison with the reactance of either 
capacitor C or the diode at all modulating frequencies. Otherwise, the high 
modulating frequencies will be attenuated. Also, the resistance of R must be 
large in comparison with either the reactance of C or the reactance of the 
diode at the oscillator frequency. Otherwise the Q of the tuned circuit will 
be degraded. See FREQUENCY MODULATION; PHASE MODULATION. 
 �34s[Charles L. Alley]
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�6v�2v�32sFrequency multiplier   �21m �33s 
    An electronic circuit that produces an output frequency which is an 
integral multiple of the input frequency. There are two basic types of 
frequency multipliers. The first type is a nonlinear amplifier which generates 
harmonics in its output current and a tuned load that resonates at one of 
these harmonics. The second type uses the nonlinear capacitance of a junction 
(semiconductor) diode to couple energy from the input circuit, which is tuned 
to the fundamental, to the output circuit, which is tuned to the desired 
harmonic.  
    A highly efficient doubler can be devised by using two amplifiers, such as 
transistors, with their inputs driven with opposite polarity, obtained from 
opposite ends of a center-tapped coil, and their outputs connected in 
parallel. See AMPLIFIER; SEMICONDUCTOR. 
 �34s[Charles L. Alley]
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�6v�2v�32sFrequency-response equalization   �21m �33s 
    The process of obtaining a desired overall frequency-response 
characteristic in an audio-frequency circuit by introducing corrective 
electrical networks of various types, termed equalizers, into the circuit. 
    Equalizers are used both in communications networks and in systems for the 
recording and reproducing of photographic film, magnetic tape, and disk 
phonograph records. These equalizers are in the form of electrical networks of 
resistance, inductance, and capacitance. See AMPLIFIER; DISK RECORDING. 
 �34s[Harry F. Olson]
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�6v�2v�32sFrequency (wave motion)   �21m �33s 
    The number of times which sound pressure, electrical intensity, or other 
quantities specifying a wave vary from their equilibrium value through a 
complete cycle in unit time. The most common unit of frequency is the hertz 
(Hz); 1 Hz is equal to 1 cycle per second. In one cycle there is a positive 
variation from equilibrium, a return to equilibrium, then a negative 
variation, and return to equilibrium. This relationship is often described in 
terms of the sine wave, and the frequency referred to is that of an equivalent 
sine-wave variation in the parameter under discussion. See ANGULAR FREQUENCY; 
FREQUENCY MEASUREMENT; SINE WAVE; WAVE MOTION. 
 �34s[William J. Galloway]
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�6v�2v�32sFresh-water ecosystem   �21m �33s 
    An ecosystem is the functional unit of ecology, consisting of living 
organisms and the nonliving environment interacting upon each other to produce 
an exchange of energy and materials between the living and nonliving 
components. A lake or pond in its entirety is such an ecosystem. A stream is 
also an ecosystem, although less well differentiated. Limnology is the 
comprehensive study of all the components of inland aquatic ecosystems and 
their interrelationships. See ECOSYSTEM; LIMNOLOGY. 
    The energy of fresh-water ecosystems is derived mainly from photosynthesis 
accomplished by the algae suspended in the water and by the higher plants and 
algae growing on or in the bottom. A variable proportion of the total energy 
available is derived from allochthonous organic matter, such as leaves and 
pollen, produced by terrestrial communities. The materials cycled within an 
ecosystem are derived ultimately from the weathering of rocks and the leaching 
of soil in the watershed, and to a lesser extent from the air. 
    Fresh-water habitats are conveniently divided into a lenitic or basin 
series, such as lakes, reservoirs, ponds, and bogs, and a lotic or channel 
series, such as rivers, streams, brooks, springs, and groundwater. The lotic 
series is distinguished by a continual flow of water in one direction. 
 �34s[David G. Frey]
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�6v�2v�32sFriction   �21m �33s 
    Resistance to sliding, a property of the interface between two solid 
bodies in contact. Many everyday activities like walking or gripping objects 
are carried out through friction, and most people have experienced the 
problems that arise when there is too little friction and conditions are 
slippery. However, friction is a serious nuisance in devices that move 
continuously, like electric motors or railroad trains, since it constitutes a 
dissipation of energy, and a considerable proportion of all the energy 
generated by humans is wasted in this way. Most of this energy loss appears as 
heat, while a small proportion induces loss of material from the sliding 
surfaces, and this eventually leads to further waste, namely, to the wearing 
out of the whole mechanism. See WEAR; WORK. 
    In stationary systems, friction manifests itself as a force equal and 
opposite to the shear force applied to the interface. Thus, as in the 
illustration, if a small force S is applied, a friction force P will be 
generated, equal and opposite to S, so that the surfaces remain at rest. P can 
take on any magnitude up to a limiting value F, and can therefore prevent 
sliding whenever S is less than F. If the shear force S exceeds F, slipping 
occurs. During sliding, the friction force remains approximately equal to F 
and always acts in a direction opposing the relative motion. The friction 
force is proportional to the normal force L, and the constant of 
proportionality is defined as the friction coefficient f. This is expressed by 
the equation F = fL. 
  
    In prehistoric and early historic times, humans' main interest in friction 
was to reduce the friction coefficient, to reduce the labor involved in 
dragging heavy objects. This led to the invention of lubricants, the first of 
which were animal fats and vegetable oils. A great breakthrough was the use of 
rolling action, first in the form of rolling logs and then in the form of 
wheels, to take advantage of the lower friction coefficients of rolling 
systems. See LUBRICANT. 
    In modern engineering practice available materials and lubricants reduce 
friction to acceptable values. In special circumstances when energy is 
critical, determined efforts to minimize friction are undertaken. Friction 
problems of practical importance are those of getting constant friction in 
brakes and clutches, so that jerky motion is avoided, and avoiding low 
friction in special circumstances, such as when driving a car on ice or on a 
very wet road. Also, there is considerable interest in developing new bearing 
materials and new lubricants that will produce low friction even at high 
interfacial temperatures and maintain these properties for long periods of 
times, thus reducing maintenance expenses. Perhaps the most persistent problem 
is that of avoiding frictional oscillations, a constant cause of noise 
pollution of the environment. 
 �34s[Ernest Rabinowicz]
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�6v�2v�32sFriction brake �21m �33s
    A type of brake in which a fixed surface contacts a moving part that is to
be slowed or stopped. Friction brakes are classified according to the kind of
friction element employed and the means of applying the friction forces. See
FRICTION.
    The simplest form of brake, the single-block, consists of a short block
fitted to the contour of a wheel or drum and pressed up against the surface by
means of a lever on a fulcrum, as widely used on railroad cars. The block may
have the contour lined with friction-brake material, which gives long wear and
a high coefficient of friction. The fulcrum may be located with respect to the
lever in a manner to aid or retard the braking torque of the block. The lever
may be operated manually or by a remotely controlled force (illustration a).
    Two single-block brakes in symmetrical opposition, where the operating
force on the end of one lever is the reaction of the other, make up a
double-block brake (illustration b). External thrust loads are balanced on the
rim of the rotating wheel.
    An external-shoe brake operates in the same manner as the block brake, and
the designation indicates the application of externally contracting elements.
In this brake the shoes are appreciably longer, extending over a greater
portion of the drum (illustration c). It is used on elevator installations for
locking the hoisting sheave by means of a heavy spring when the electric
current is off and the elevator is at rest.
    An internal shoe has several advantages over an external shoe. Because it
works on the inner surface of the drum, it is protected from water and grit
(illustration d). It may be designed in a more compact package, is easily
activated, and is effective for drives with rotations in both directions. The
internal shoe meets the requirement of the automobile. Early mechanical brakes
had an elliptical cam; modern brakes have hydraulic piston-actuated shoes.
    Hoists, excavating machinery, and hydraulic clutch-controlled
transmissions have band brakes. They operate on the same principle as flat
belts on pulleys. In the simplest band brake, one end of the belt is fastened
near the drum surface and the other end is then pulled over the drum in the
direction of rotation so that a lever on a fulcrum may apply a tension to the
belt.
    Disk brakes have long been used on hoisting and similar apparatus. Because
more energy is absorbed in prolonged braking than in a clutch startup,
additional heat dissipation must be provided in equivalent disk brakes. A
significant application of disk brakes is on the wheels of aircraft. A
familiar application is the bicycle coaster brake.
    The caliper disk brake consists of a revolving disk which can be gripped
between two brake pads. A brake on a bicycle is an example. The automotive
caliper disk brake is hydraulically operated, and the pads cover between
one-sixth and one-ninth of the swept area of the disk. It is used on the front
axle of a car. See AUTOMOTIVE BRAKE.
[Harry P. Hale]

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�6v�2v�32sFriction factor   �21m �33s 
    The name given to the proportionality factor in the equation below 
relating the friction or head loss encountered when a fluid flows in a pipe to 

* E (sub f) = [4 PI (sub w) L]| RHO D = af (L | D) [(u sup 2) / (2g sub c)] *

the kinetic energy of the flowing fluid and the length and diameter of the 
pipe. Here E sub f is the energy converted into heat as the result of friction 
[(ft-lb force)/(lb fluid flowing) or J/kg of fluid flowing], TAU sub w is the 
shear stress at the wall (lb force/ft sup 2 or N/m sup 2), RHO is the fluid 
density (lb mass/ft sup 3 or kg/m sup 3), f is a dimensionless friction factor, 
L is pipe length (ft or m), D is pipe diameter (ft or m), u is average fluid 
velocity in the pipe (ft/s or m/s), g sub c is the conversion factor [32.17 
(lb mass/lb force)(ft/s sup 2) or 1 (kg/N)(m/s sup 2)], and a is an arbitrary 
dimensionless constant. See FLUID FLOW. 
 �34s[Charles E. Lapple]
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�6v�2v�32sFriedel-Crafts reaction   �21m �33s 
    A substitution reaction, catalyzed by aluminum chloride, in which an alkyl 
(R--) group or an acyl (RCO--) group replaces a hydrogen atom of an aromatic 
nucleus. This general reaction is the most important member of a larger group 
of aromatic substitution reactions known to be catalyzed by conventional or 
Lewis acids. 
    In the classical alkylation reaction, an alkyl halide (RX) serves as the 
alkylating agent. Alkenes may be substituted for alkyl halides. For acylation 
of aromatic hydrocarbons, acyl halides have proved most valuable although acid 
anhydrides have also been used. See AROMATIC HYDROCARBON; SUBSTITUTION 
REACTION.  
 �34s[Charles K. Bradsher]
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�6v�2v�32sFringe (optics) �21m �33s
    One of the light or dark bands produced by interference or diffraction of
light. Distances between fringes are usually very small, because of the short
wavelength of light. Fringes are clearer and more numerous when produced with
light of a single color.
    Diffraction fringes are formed when light from a point source, or from a
narrow slit, passes by an opaque object of any shape. Interference fringes are
obtained by bringing together two or more beams of light that have originated
from a common source. This is usually accomplished by means of an apparatus
especially designed for the purpose called an interferometer, although
interference fringes may also be seen in nature. Examples are the colors in a
soap film and in an oil film on water. See DIFFRACTION; INTERFERENCE OF WAVES;
INTERFEROMETRY; RESOLVING POWER (OPTICS).
 �34s[Francis A. Jenkins /William W. Watson]
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�6v�2v�32sFrit   �21m �33s 
    A ground glass used in making glazes and enamels. It is formed by melting 
the ingredients and rapidly cooling (quenching) them, for example, by pouring 
into water or onto water-cooled steel rolls. 
    The main reason for fritting is to convert water-soluble raw materials 
into an insoluble glass, making it easier to keep them uniformly distributed 
in the glaze or enamel during processing. Other advantages of fritting are the 
conversion of poisonous materials (for example, lead compounds) into less 
dangerous forms, the more even distribution of color, and the reduction in 
loss of volatile materials. See GLAZING; METAL COATINGS. 
 �34s[John F. McMahon]
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�6v�2v�32sFrog   �21m �33s 
    The name for a number of species of animals in the order Anura of the 
class Amphibia. These animals lack a tail in the adult stage, the hindlegs are 
adapted for jumping, and the skin is moist and smooth with no scales. The eyes 
are large and have a functional third eyelid, or nictitating membrane. 
External ears are absent, but there is a tympanum to receive vibratory 
stimuli. The mobile tongue, attached anteriorly to the floor of the mouth and 
free and notched at the posterior, can be projected at the prey. Insects 
adhere to the tongue, which is coated with sticky secretions from tongue 
glands.  
    The larval form, the tadpole has a tail and external gills at first, but 
these structures soon disappear as the gills are replaced by internal gills in 
the pharyngeal region and the tail is resorbed. Metamorphosis of the larva is 
complete soon after the appearance of the limbs. 
    Frogs are poikilothermous animals; that is, body temperature is dependent 
upon the environment. There are larger numbers of individuals and species in 
moist areas of temperate and tropical regions. See ANURA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sFront   �21m �33s 
    An elongated, sloping zone in the troposphere, within which changes of 
temperature and wind velocity are large compared to changes outside the zone. 
Thus the passage of a front at a fixed location is marked by rather sudden 
changes in temperature and wind and also by rapid variations in other weather 
elements such as moisture and sky condition. 
    In its idealized sense, a front can be regarded as a sloping surface of 
discontinuity separating air masses of different density or temperature. In 
practice, the temperature change from warm to cold air occurs mainly within a 
zone of finite width, called a transition or frontal zone. The 
three-dimensional structure of the frontal zone is shown in the illustration. 
In typical cases, the zone is about 1 km (0.6 mi) in depth and 100 - 200 km 
(60 - 120 mi) in width, with a slope of approximately 1/100. The cold air lies 
beneath the warm in the form of a shallow wedge. Temperature contrasts 
generally are strongest at or near the Earth's surface, the frontal zone 
usually being narrowest near the ground and becoming wider and more diffuse 
with height. 
    The surface separating the frontal zone from the adjacent warm air mass is 
referred to as the frontal surface, and it is the line of intersection of this 
surface with a second surface, usually horizontal or vertical, that strictly 
speaking constitutes the front. According to this more precise definition, the 
front represents a discontinuity in temperature gradient rather than in 
temperature itself. The boundary on the cold air side is often ill-defined, 
especially near the Earth's surface, and for this reason is not represented in 
routine analysis of weather maps. See WEATHER MAP. 
    The wind gradient, or shear, like the temperature gradient, is larger 
within the frontal zone than on either size of it. In well-developed fronts 
the shift in wind direction often is concentrated along the frontal surface, 
while a more gradual change in wind speed may occur throughout the frontal 
zone. An upper-level jet stream normally is situated above the frontal zone. 
See JET STREAM. 
 �34s[Stephen E. Mudrick]
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ms pg 2863-2867 
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�6v�2v�32sFrost   �21m �33s 
    A covering of ice in one of several forms produced by the freezing of 
supercooled water droplets on objects colder than 32DGF (0DGC). The partial or 
complete killing of vegetation, by freezing or by temperatures somewhat above 
freezing for certain sensitive plants, also is called frost. Air temperatures 
below 32DGF (0DGC) sometimes are reported as ``degrees of frost''; thus, 10DGF 
(-12DGC) is 22 degrees of frost (this usage is confined to the Fahrenheit 
scale and is not applied to Celsius temperatures). 
    Frost forms in exactly the same manner as dew except that the individual 
droplets that condense in the air a fraction of an inch from a subfreezing 
object are themselves supercooled, that is, colder than 32DGF (0DGC). When the 
droplets touch the cold object, they freeze immediately into individual 
crystals. When additional droplets freeze as soon as the previous ones are 
frozen, and hence are still close to the melting point because all the heat of 
fusion has not been dissipated, amorphous frost or rime results. 
    At more rapid rates of condensation, the drops form a thin layer of liquid 
before freezing, and glaze or glazed frost (``window ice'' on house windows, 
``clear ice'' on aircraft) generally follows. Glaze formation on plants, 
buildings and other structures, and especially on wires sometimes is called an 
ice storm, a silver frost storm, or thaw. 
    At slower deposition rates, such that each crystal cools well below the 
melting point before the next joins it, true crystalline or hoar frosts form. 
These include fernlike assemblages on snow surfaces, called surface hoar; 
similar feathery plumes in cold buildings, caves, and crevasses, called depth 
hoar; and the common window frost or ice flowers on house windows. 
    Killing frosts or freezes damage or kill vegetation depending on their 
duration and their intensity, that is, how far the plant temperatures go below 
32DGF (0DGC). Such conditions result from advection of much colder air, which 
then cools the plants, as in the infamous cold waves of the north-central 
United States; or from radiational cooling of the plants themselves, by 
long-wave radiation to clear skies at night. In either case, the extent to 
which plant fluids freeze determines the severity of the frost. See AIR 
TEMPERATURE; DEW; DEW POINT. 
 �34s[Arnold Court]
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�6v�2v�32sFrost action   �21m �33s 
    The mechanical weathering process caused by alternate or repeated cycles 
of freezing and thawing of water in pores, cracks, and other openings, usually 
at the surface, and the resulting effects of frost action on materials and 
structures. The term is loosely used to indicate any action in which frost is 
involved.  
    Frost action is initiated during growth of ice in the ground from which 
may result either hydrostatic pressure or directed pressure by individual ice 
crystals, usually growing in the direction of heat loss. The ice grows slowly 
as water migrates molecule by molecule through ``suction'' or capillary action 
to the freezing zone. Plant roots can be broken, soils stirred and sorted, and 
many small morphologic features may grow overnight or within a few years. In 
fine-grained unconsolidated materials, ice much in excess of the volume of 
pores is commonplace. Under these conditions thawing yields suspensions of 
soil or high pore-water pressures that promote loss of strength and subsequent 
slump and flow. Under such conditions the common potholes in roads are 
initiated. See EROSION; MASS WASTING; WEATHERING PROCESSES. 
 �34s[Robert F. Black]
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�6v�2v�32sFroude number �21m �33s
    In fluid mechanics, the ratio v sup 2/gd of the inertia force v sup 2 to the
gravitational force gd, where v is velocity, g is gravitational acceleration,
and d is a characteristic length. Froude number is significant in the design
of a model of any system in which the effect of gravity is important in
controlling the velocities or the flow pattern. For example, it is used in the
evaluation of the drag or the slipstream velocity of a ship producing surface
waves. See DYNAMIC SIMILARITY; MODEL THEORY.
 �34s[Glenn Murphy]
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�6v�2v�32sFructose   �21m �33s 
    A sugar that is the commonest of ketoses and the sweetest of the sugars. 
It is also known as D-fructose, D-fructopyranose, and levulose fruit sugar. It 
is found in free state, usually accompanied by D-glucose and sucrose in fruit 
juices, honey, and nectar of plant glands. D-Fructose is the principal sugar 
in seminal fluid. See CARBOHYDRATE. 
    Fructose is readily utilized by diabetic animals. In persons with diabetes 
mellitus or parenchymal hepatic disease, the impairment of fructose tolerance 
is relatively small and not at all comparable to the diminution in their 
tolerance to glucose. See MONOSACCHARIDE. 
 �34s[William Z. Hassid]
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�6v�2v�32sFruit   �21m �33s 
    A matured carpel or group of carpels (the basic units of the gynoecium or 
female part of the flower) with or without seeds, and with or without other 
floral or shoot parts (accessory structures) united to the carpel or carpels. 
Carpology is the study of the morphology and anatomy of fruits. The ovary 
develops into a fruit after fertilization and usually contains one or more 
seeds, which have developed from the fertilized ovules. Parthenocarpic fruits 
usually lack seeds. Fruitlets are the small fruits or subunits of aggregate or 
multiple fruits. Flowers, carpels, ovaries, and fruits are, by definition, 
restricted to the flowering plants (angiosperms), although fruitlike 
structures may enclose seeds in certain other groups of seed plants. The fruit 
is of ecological significance because of seed dispersal. See SEED. 
  
    Morphology.  A fruit develops from one or more carpels. Usually only part 
of the gynoecium, the ovary, develops into a fruit; the style and stigma 
wither. Accessory (extracarpellary or noncarpellary) structures may be closely 
associated with the carpel or carpels and display various degrees of adnation 
(fusion) to them, thus becoming part of the fruit. Such accessory parts 
include sepals (as in the mulberry), the bases of sepals, petals, and stamens 
united into a floral tube (apple, banana, pear, and other species with 
inferior ovaries), the receptacle (strawberry), the pedicel and receptacle 
(cashew), the peduncle (fleshy part of the fig), the involucre composed of 
bracts and bracteoles (walnut and pineapple), and the inflorescence axis 
(pineapple). See FLOWER. 
    A fruit derived from only carpellary structures is called a true fruit, 
or, because it develops from a superior ovary (one inserted above the other 
floral parts), a superior fruit (corn, date, grape, plum, and tomato). Fruits 
with accessory structures are called accessory (or inaptly, false or spurious) 
fruits (pseudocarps), or, because of their frequent derivation from inferior 
ovaries (inserted below the other floral parts), inferior fruits (banana, 
pear, squash, and walnut). 
    Fruits can be characterized by the number of ovaries and flowers forming 
the fruit. A simple fruit is derived form one ovary, an aggregate fruit from 
several ovaries of one flower (magnolia, rose, and strawberry). A multiple 
(collective fruit is derived from the ovaries and accessory structures of 
several flowers consolidated into one mass (fig, pandan, pineapple, and sweet 
gum).  
    The fruit wall at maturity may be fleshy or, more commonly, dry. Fleshy 
fruits range from soft and juicy to hard and tough. Dry fruits may be 
dehiscent, opening to release seeds, or indehiscent, remaining closed and 
containing usually one seed per fruit. Fleshy fruits are rarely dehiscent. 
    The pericarp is the fruit wall developed from the ovary. In true fruits, 
the fruit wall and pericarp are synonymous, but in accessory fruits the fruit 
wall includes the pericarp plus one or more accessory tissues of various 
derivation. Besides the fruit wall, a fruit contains one or more seed-bearing 
regions (placentae) and often partitions (septa). 
  
    Anatomy.  Anatomically or histologically, a fruit consists of dermal, 
ground (fundamental), and vascular systems and, if present, one or more seeds. 
After fertilization the ovary and sometimes accessory parts develop into the 
fruit; parthenocarpy is fruit production without fertilization. The fruit 
generally increases in size and undergoes various anatomical changes that 
usually relate to its manner of dehiscence, its mode of dispersal, or 
protection of its seeds. The economically important, mainly fleshy fruits have 
received the most histological and developmental study. 
    Size increase of fruits is hormonally controlled and results from cell 
division and especially from cell enlargement. Cell number, volume, and weight 
thus control fruit weight. Cell division generally is more pronounced before 
anthesis (full bloom); cell enlargement is more pronounced after. 
  
    Functional aspects.  Large fruits generally require additional anatomical 
modifications for nutrition or support or both. The extra phloem in fruit 
vascular bundles and the often increased amount of vascular tissue in the 
fruit wall and septa supply nutrients to the developing seeds and, especially 
in fleshy fruits, to the developing walls. Large, especially fleshy fruits 
(apple, gourd, and kiwi) usually contain proportionally more vascular tissue 
than small fruits. Vascular tissue also serves for support and in lightweight 
fruits may be the chief means of support. 
    Crystals, tannins, and oils commonly occur in fruits and may protect 
against pathogens and predators. The astringency of tannins, for example, may 
be repellent to organisms. With fruit maturation, tannin content ordinarily 
decreases, so the tannin  repellency operative in early stages is superseded 
in fleshy fruits by features (tenderness, succulence, sweetness through odor 
and increased sugar content, and so on) attractive to animal dispersal agents. 
Many fruits are dispersed by hairs, hooks, barbs, spines, and sticky mucilage 
adhering the fruit to the surface of the dispersal agent. Lightweight fruits 
with many air spaces or with wings or plumes may be dispersed by wind or 
water. Gravity is always a factor in dispersal of fruits and seeds. 
 �34s[Rudolf Schmid]
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�6v�2v�32sFruit, tree   �21m �33s 
    Tree fruits include temperate, subtropical and tropical zone species. Most 
temperate zone fruits are grown principally in regions protected from 
prolonged summer heat and severe winter cold (above -10 to -15DGF or -23 to 
-26DGC). The principal deciduous tree fruits grown in the United States are 
apple, peach, pear, plum, apricot, sweet cherry, tart cherry, and nectarine. 
Tree nuts, such as almond, pecan, walnut, and filbert, are sometimes 
classified as deciduous tree fruit crops. 
    Most subtropical fruit trees are evergreen. They will withstand 
temperatures somewhat below freezing during their dormant or semidormant 
season, but not the extreme temperatures tolerated by the temperate zone 
crops. Major subtropical fruits in the United States are the citrus group (for 
example, orange, grapefruit, lemon), olive, avocado, fig, and others are of 
lesser importance. The Japanese persimmon might be considered as a borderline 
case between the temperate and subtropical zone groups, and avocado is often 
classed as a tropical fruit. 
    The tree fruit crops are grown commercially in orchards or groves, usually 
in single rows which permit necessary cultural operations for each tree. 
Nearly all tree fruit crops have similar requirements of training, pruning, 
spraying to control diseases and insects, and cultivation or chemical control 
of weeds. See separate articles on the common named fruits. 
 �34s[R. Paul Larsen]
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�6v�2v�32sFuel cell   �21m �33s 
    An electric cell that converts the chemical energy of a fuel directly into 
electric energy in a continuous process. The efficiency of this conversion can 
be made much greater than that obtainable by thermal-power conversion. In the 
latter the chemical reaction is made to produce heat by combustion. The heat 
is then transformed partially into mechanical energy by a heat engine, which 
drives a generator to produce electric energy. Further loss is involved if the 
direct current generated is converted into alternating current. 
    Although, in principle, the nature of the reactants is not limited, the 
fuel-cell reaction almost always involves the combination of hydrogen with 
oxygen. If the reaction is harnessed in a galvanic cell working at 100% 
efficiency, a cell voltage of 1.23 volts results. Fuel cells are of 200 - 500 
watts capacity and 50 - 100 mA/cmsup2 current density. Larger prototypes have 
been produced. 
    In the present stage of development, it is difficult to make a 
classification of the fuel-cell types. The most popular and successful type 
remains the classical H2-O2 fuel cell of the direct or indirect type. In the 
direct type, hydrogen and oxygen are used as such, the fuel being produced in 
independent installations. The indirect type employs a hydrogen-generating 
unit which can use as raw material a wide variety of fuel. See DRY CELL; 
ELECTRODE POTENTIAL; ELECTROMOTIVE FORCE (EMF). 
 �34s[Jack Davis ;L. Rozeanu]
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�6v�2v�32sFuel gas   �21m �33s 
    A fuel in the gaseous state whose potential heat energy can be readily 
transmitted and distributed through pipes from the point of origin directly to 
the place of consumption. The types of fuel gases are natural gas, LP gas, 
refinery gas, coke oven gas, and blast-furnace gas. The last two are used in 
steel mill complexes. 
    Most fuel gases are composed in whole or in part of the combustibles 
hydrogen, carbon monoxide, methane, ethane, propane, butane, and oil vapors 
and, in some instances, of mixtures containing the inerts nitrogen, carbon 
dioxide, and water vapor. See COAL GASIFICATION; LIQUEFIED NATURAL GAS (LNG); 
LIQUEFIED PETROLEUM GAS (LPG); NATURAL GAS. 
 �34s[Jack Huebler]
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�6v�2v�32sFuel injection   �21m �33s 
    Implicitly, the delivery of fuel to an internal combustion engine by 
pressure from a mechanical pump. It must be used in diesel engines because of 
the high pressures necessary to deliver fuel against the highly compressed air 
in the engine cylinders at the end of the compression stroke, as required for 
combustion in the diesel cycle. In addition, considerable pressure is required 
to break up the fuel oil, which has low volatility and is often viscous, into 
the fine-spray pattern required for good combustion. See DIESEL ENGINE; 
INTERNAL COMBUSTION ENGINE. 
    Fuel injection has been used also for spark ignition engines with volatile 
fuels, notably racing automobile engines and reciprocating piston airplane 
engines. The advantages over the almost universally used aspirated carburetors 
on these engines are the additional power which may be obtained from an engine 
of a given size, elimination of the warm-up time, and freedom from stalling 
due to ice formation on the throttle valves. See CARBURETOR. 
 �34s[Neil MacCoull]
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�6v�2v�32sFuel oil   �21m �33s 
    Any of the petroleum products which are less volatile than gasoline and 
are burned in furnaces, boilers, or other types of heaters. The two primary 
classes of fuel oils are distillate and residual. Distillate fuel oils are 
composed entirely of material which has been vaporized in a refinery 
distillation tower. Consequently, they are clean, free of sediment, relatively 
low in viscosity, and free of inorganic ash. Residual fuel oils contain 
fractions which cannot be vaporized by heating. These fractions are black and 
viscous and include any inorganic ash components which are in the crude. In 
some cases, whole crude is used as a residual fuel. 
    Distillate fuel oils are used primarily in applications where ease of 
handling and cleanliness of combustion are more important than fuel price. The 
most important use is for home heating. They are also used in certain 
industrial applications where low sulfur or freedom from ash is important. 
Increasing amounts of distillate fuel oils have been burned in gas turbines 
used for electricity generation. See GAS TURBUNE. 
    Residual fuel oils are used where fuel cost is an important enough 
economic factor to justify additional investment to overcome the handling 
problems they pose. They are particularly attractive where large volumes of 
fuel are used, as in electric power generation, industrial steam generation, 
process heating, and steamship operation. See ELECTRIC POWER GENERATION; OIL 
FURNACE.  
 �34s[C. W. Siegmund]
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�6v�2v�32sFuel pump   �21m �33s 
    A pump for drawing fuel from a storage tank and forcing it to an engine or 
furnace. Choice of pump type for fuel depends to a great extent on the 
volatility of the liquid to be pumped. 
    In a gasoline engine the fuel is highly volatile at ambient temperature; 
therefore a sealed diaphragm positive-displacement pump is used (see 
illustration). With such a pump the fuel line is completely sealed from tank 
to carburetor to prevent escape of fuel and to enable the pump to purge the 
line of vapor in event of a ``vapor lock'' caused by fuel vaporizing due to 
abnormally high ambient temperature. See FUEL SYSTEM. 
    In a diesel engine, where fuel is injected at high pressure through a 
nozzle into the combustion cylinder, a piston serves as its own inlet valve 
and as the compression member of the fuel pump. See DIESEL ENGINE; FUEL 
INJECTION.  
    Fuel pumps must operate before the engine or furnace that they serve can 
function; consequently an auxiliary or starter motor is usually an essential 
adjunct to a fuel pump. 
 �34s[Frank H. Rockett]
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�6v�2v�32sFuel system   �21m �33s 
    A system which stores fuel for present use and delivers it as needed to an 
engine. The commonly used components for automobiles and stationary gasoline 
engines are fuel tank, remote reading fuel gage, filter, fuel pump, and 
carburetor. See CARBURETOR; FUEL PUMP. 
    The presence of multiple engines and multiple tanks complicates the 
aircraft fuel system. Components of a typical aircraft fuel system include one 
main and two auxiliary tanks with their gages; booster, transfer, and 
engine-driven pumps; various selector valves; and a fuel jettisoning or defuel 
valve and connection, which is typical also of what would be needed for either 
single-point ground or flight refueling. The arrangement is usually such that 
all the fuel supply will pass to the engines by way of the main tank, which is 
refilled as necessary from the auxiliary tanks. In case of emergency, the 
system selector valve may connect the auxiliary tanks to the engines directly. 
 �34s[Frank C. Mock /J. A. Bolt]
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�6v�2v�32sFugacity   �21m �33s 
    A function introduced by G. N. Lewis to facilitate the application of 
thermodynamics to real systems. Thus, when fugacities are substituted for 
partial pressures in the mass action equilibrium constant expression, which 
applies strictly only to the ideal case, a true equilibrium constant results 
for real systems as well. 
    The fugacity f sub i of a constituent i of a thermodynamic system is 
defined by the following equation (where MU sub i is the chemical potential and 

                *** MU sub i = MU sub i* + RT In (f sub i) *** 

MU sub i* is a function of temperature only), in combination with the require-
ment that the fugacity approach the partial pressure as the total pressure of 
the gas phase approaches zero. At a given temperature, this is possible only 
for a particular value for MU sub i*, which may be shown to correspond to the 
chemical potential the constituent would have as the pure gas in the ideal gas 
state at 1 atm pressure. This definition makes the fugacity identical to the 
partial pressure in the ideal gas case. For real gases, the ratio of fugacity 
to partial pressure, called the fugacity coefficient, will be close to unity 
for moderate temperatures and pressures. At low temperatures and appropriate 
pressures, it may be as small as 0.2 or less, whereas at high pressures at any 
temperature it can become very large. See ACTIVITY (THERMODYNAMICS); CHEMICAL 
EQUILIBRIUM; CHEMICAL THERMODYNAMICS; GAS. 
 �34s[Paul J. Bender]
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�6v�2v�32sFuller's earth   �21m �33s 
    Any natural earthy material (such as clay materials) which decolorizes 
mineral or vegetable oils to a sufficient extent to be of economic importance. 
It has no mineralogic significance. The clay minerals present in fuller's 
earth may include montmorillonite, attapulgite, and kaolinite. However, some 
materials containing these clay minerals often have very low decolorizing 
power. See CLAY MINERALS. 
    Fuller's earth deposits are distributed throughout the world, but the 
largest deposits have been found in the United States, England, and Japan. In 
the United States, principal deposits are found in Georgia, Florida, Illinois, 
Texas, Nevada, and California. 
 �34s[Floyd M. Wahl ;Ralph E. Grim]
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�6v�2v�32sFulminate   �21m �33s 
    A compound containing the --ONC group and derived from fulminic acid, 
HONC. Fulminates are isomeric with cyanates; that is, cyanates have the same 
atoms but in different arrangement, --OCN. 
    The fulminates have been commercially important because of the use of 
mercury fulminate, Hg(ONC)2, in priming compositions and as an initial 
detonating agent; lead azide is replacing mercury fulminate as a detonating 
agent. See CYANATE. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sFumigant   �21m �33s 
    A pesticidal chemical or chemical formulation that functions in a gaseous 
state. Chemical formulations are designed to increase toxicity, reduce 
flammability, give off warning odors, and provide for sorption at different 
rates.  
    Physical types of fumigants include gases, liquids, and solids. There are 
several chemical types of fumigants. These include: halogenated hydrocarbons, 
such as carbon tetrachloride and ethylene dibromide; sulfur-containing 
compounds, such as carbon disulfide and sulfur dioxide; cyanides, such as 
hydrogen cyanide and calcium cyanide; and others, such as phosphine and 
ethylene dioxide. 
    Fumigants are used in space fumigations to disinfest food-processing 
plants, warehouses, grain elevators, boxcars, shipholds, stores, and 
households, and in spot fumigations within those structures. They are used in 
atmospheric vaults and vacuum chambers and are applied extensively to stacked 
bags of grain or stored foods under polyethylene sheets, to trees under tents 
to control scale insects, and to areas of land to destroy weeds, 
soil-infesting insects, and nematodes. 
 �34s[Donald A. Wilbur, Sr.]
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�6v�2v�32sFunariales   �21m �33s 
    An order of mosses. The plants are usually annual, occasionally biennial, 
and terrestrial, growing in clusters or loose tufts. The stems are erect, 
short, simple, or sparingly branched. The leaves are soft, broad, and costate. 
The calyptra varies but is often inflated, usually cucullate and rostrate. The 
sporophyte is acrocarpous, with a rudimentary to elongated seta. The capsule 
is erect and symmetrical or cernuous and unsymmetrical, often with a distinct 
and narrower neck. See BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sFunction generator   �21m �33s 
    An electronic instrument which generates periodic voltage or current 
waveforms that duplicate various types of well-defined mathematical functions. 
The simplest function generator usually generates a combination of square 
waves, triangular waves, and sine waves. 
    One electronic circuit approach to the design of a simple function 
generator is to begin with a bistable multivibrator or ``flip-flop'' 
controlled in time by a succession of clock pulses which generates the square 
wave. The triangular waveform is obtained by integrating the square wave 
through the use of the operational amplifier integrator. The sine wave is 
obtained by applying the triangular wave to a shaping circuit consisting of a 
combination of resistors and diodes. See AMPLIFIER; MULTIVIBRATOR; 
WAVE-SHAPING CIRCUITS. 
    Alternatively the sine wave may be generated by a sinusoidal oscillator. 
From this output, the square wave may be obtained by amplication, limiting, 
and clipping of the sine wave. Then the triangular wave may be obtained using 
an integrator as before. See CLIPPING CIRCUIT; LIMITER CIRCUIT. 
    A combination of counters, programmed read-only memories (PROMS), and a 
digital-to-analog converter can be used as a function generator, generating 
almost any function desired to almost any degree of accuracy.  See COMPUTER 
STORAGE TECHNOLOGY; DIGITAL-TO-ANALOG CONVERTER. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sFundamental frequency   �21m �33s 
    The lowest frequency at which a system vibrates freely. The mode of 
vibration having this frequency is called the fundamental mode. For a complex 
wave consisting of a sum of sinusoidal components, that component having the 
lowest frequency is the fundamental and its frequency is the fundamental 
frequency. See HARMONIC (PERIODIC PHENOMENA); MODE OF VIBRATION; PARTIAL TONE. 
 �34s[Robert W. Young]
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�6v�2v�32sFundamental interactions �21m �33s
    Fundamental forces that act between the elementary particles of which all
matter is assumed to be composed. At present, four fundamental interactions
are distinguished. The gravitational interaction manifests itself as a
long-range force of attraction between all elementary particles. The
electromagnetic interaction is responsible for the long-range force of
repulsion of like and attraction of unlike electric charges. At comparable
distances, the ratio of gravitational to electromagnetic interactions (as
determined by the strength of respective forces between an electron and a
proton) is approximately 4 x 10- sup 3 sup 7. See  GRAVITATION; MAXWELL'S
EQUATIONS; QUANTUM ELECTRODYNAMICS.
    The third fundamental interaction is the weak nuclear interaction, whose
characteristic strength for low-energy phenomena is a thousand times weaker
than electromagnetic. Weak interactions are short-range, the range of the
force being much smaller than 10- sup 1 sup 5 cm. The strong nuclear interaction
between protons and neutrons is also short-range, though the range is
approximately 10- sup 1 sup 3 cm rather than /10- sup 1 sup 5 cm. As the name
implies, within this range of distances the strong force overshadows all other
forces between these particles.  See  QUANTUM CHROMODYNAMICS; STRONG NUCLEAR
INTERACTIONS; WEAK NUCLEAR INTERACTIONS. 
    Ever since the discovery and clear classification of these four
interactions, particle physicists have attempted to unify these interactions
as aspects of one basic interaction between all matter.
    Unification of weak and electromagnetic interactions was suggested by S.
Glashow and by Abdus Salam and J. C. Ward. This followed a parallel between
these two interactions, pointed out by J. S. Schwinger, providing one assumed
that the then known classes of weak interactions were mediated by exchanges of
Wsupp and W-, quanta of weak interactions, just as electromagnetism is
mediated by the exchange of quanta of light, the photons. The empirical
properties of weak interaction phenomena had revealed that if WPLUS/MINUS
existed, these quanta of the weak interaction must carry an intrinsic spin of
magnitude PLANCK'S CONSTANT (Planck's constant divided by 2PI), like the
photon.  See  QUANTUM FIELD THEORY.
    One important consequence of this unification is the prediction of the
existence of weak interactions exhibiting neutral-current phenomena, in which
the nature of the interacting particles is not changed during the interaction,
or, equivalently, the existence of a new heavy intermediate weak-quantum
(called Z sup 0) which, unlike the W sup + and W- (which carry the same electric
charges as the proton and the electron), would be electrically neutral. Since
1973, the verification of neutral current phenomena has given decisive support
to the notion of a basic unity between weak and electromagnetic interactions
into a single so-called electroweak interaction.
    The gauge unification of weak and electromagnetic interactions, which
started with the observation that the mediating quanta (WPLUS/MINUS, Z sup 0,
GAMMA) for these interactions possess intrinsic spin PLANCK'S CONSTANT, can be
carried further to include strong nuclear interactions as well, if strong
interactions are also mediated through quanta (gluons) carrying spin PLANCK'S
CONSTANT. Indirect experimental evidence exists for the existence of gluons,
and it should be possible to verify if their spin is indeed PLANCK'S CONSTANT.
If this is so, a complete gauge unification of all three forces,
electromagnetic, weak-nuclear, and strong-nuclear, into a single
electronuclear force seems plausible. Such a unification necessarily means
that the distinction between quarks on the one hand and neutrinos, electrons,
and muons on the other must disappear and that all interactions (weak,
electromagnetic, and strong) are facets of a universal force, with primitive
strength of order of ALPHA, this universality manifesting itself for
experiments carried out at sufficiently high energies. Another consequence of
this disappearance of distinction between quarks and neutrinos and electrons
appears to be the possibility of proton decays (with a lifetime of
approximately 10 sup 3 sup 1 years).
    It is possible that the gravitational interaction will also be unified
with the other three interactions by extensions of gauge ideas to the
so-called supersymmetry between particles and quanta.  See  ELEMENTARY
PARTICLE.
 �34s[Abdus Salam]
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�6v�2v�32sFundamental stars   �21m �33s 
    The term applied to a relatively small number of the brighter stars, 
distributed as uniformly as possible over the entire sky. The fundamental 
stars are the celestial analog of the so-called first-order triangulation 
points on the Earth. Thus the coordinates of the fundamental stars are first 
measured, using elaborate special techniques designed to measure large angular 
distances; the coordinates of the remaining stars are then inferred by other, 
less elaborate techniques. See  ASTROMETRY; ASTRONOMICAL COORDINATE SYSTEMS. 
    Since the coordinates of the stars are continuously changing as a result 
of the motion of the Earth and, to a lesser degree, the motions of the stars 
themselves, it is equally necessary to measure the changes as to measure the 
coordinates at a definite specified time. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sFungi   �21m �33s 
    Nucleated, usually filamentous, spore-bearing organisms devoid of 
chlorophyll; typically reproducing both sexually and asexually; living as 
parasites in plants, animals, or other fungi, or as saprobes on plant or 
animal remains, in aquatic, marine, terrestrial, or subaerial habitats. 
Yeasts, mildews, rusts, mushrooms, and truffles are examples of fungi. 
    In one system of classification, the fungi constitute the division of 
Eumycetes of the plant kingdom. The division is subdivided into eight classes: 
Chytridiomycetes, Hyphochrytridiomycetes, Plasmodiophoromycetes, Oomycetes, 
Zygomycetes, Trichomycetes, Ascomycetes, and Basidiomycetes, and two form 
classes, Deuteromycetes (Fungi Imperfecti) and Lichenes. The first six classes 
are grouped under the general term Phycomycetes. Some mycologists also include 
the class Myxomycetes (Mycetozoa, Myxomycophyta) in the fungi. 
    Another system recognizes the division of Eumycota with 16 classes in five 
subdivisions: Mastigomycotina (Chytridiomycetes, Hyphochytridiomycetes); 
Zygomycotina (Zygomycetes, Trichomycetes); Ascomycotina (Hemiascomycetes, 
Loculoascomycetes, Plectomycetes, Pyrenomycetes, Discomycetes); 
Basidiomycotina (Teliomycetes, Phragmobasiomycetes, Hymenomycetes, 
Gasteromycetes); and Deuteromycotina (Hyphomycetes, Coelomycetes). 
 �34s[Constantine J. Alexopoulos]
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    The characters of this group are described for the four classes of the 
older system of classification. 
    Phycomycetes, class I, are characterized by mycelia of nonseptate hyphae. 
Examples are water molds, downy mildews, and black bread molds. 
    Ascomycetes, class II, are characterized by ascospores, the final product 
of the sexual process. Examples are yeasts, powdery mildews, some blue and 
green molds, pink bread mold, cup fungi, morels, and truffles. 
    Basidiomycetes, class III, are characterized by basidiospores, the final 
product of the sexual process. Examples are rusts, smuts, bracket fungi, jelly 
fungi, mushrooms, puffballs, earth stars, and stink horns. 
    Fungi Imperfecti, class IV, are a group of vegetative molds, predominantly 
allied with class II, which lack sexual reproduction. Examples are most blue 
and green molds, some yeasts, and most human pathogenic fungi. 
    The mycelium, generally the vegetative body of fungi, is extremely 
variable. Most mycelia are white, but a wide variety of pigments can be 
synthesized by specific forms and may be secreted into the medium or deposited 
in cell walls and protoplasm. Mycelial consistency varies from loose, soft 
wefts of hyphae to compact, hardened masses that resemble leather. Each cell 
is usually able to regenerate the entire mycelium, and vegetative propagation 
commonly  results from mechanical fragmentation of the mycelium. 
    Asexual reproduction, propagation by specialized elements that originate 
without sexual fusion, occurs in  most species and is extremely diverse. The 
most common and important means of asexual reproduction are unicellular or 
multicellular spores of various types that swim, fall, blow, or are forcibly 
discharged from the parent mycelium. 
    Sexual reproduction occurs in a majority of species of all classes except 
the Fungi Imperfecti. Juxtaposition and fusion of compatible sexual cells are 
achieved by four distinct sexual mechanisms. 
 �34s[John R. Raper]
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�6v�2v�32sFungicide   �21m �33s 
    Synthetic or biosynthetic compounds used to control fungal diseases in 
plants. Chemical compounds are used to control plant diseases caused by fungi. 
Fungicides now used include both inorganic and organic compounds. Agricultural 
fungicides must have certain properties and conform to very strict 
regulations, and of thousands of compounds tested, few have reached the 
farmer's fields. 
    The manner in which these compounds are applied, that is, as wettable 
powders, dusts, or emulsions, is often essential to the success of  
agricultural fungicides. Raw fungicides must be pulverized to uniform 
particles of the most effective size, mixed with wetting agents, or dissolved 
in solvents. These carriers or diluents must not degrade the fungicides or 
injure the plants. 
    Foliage fungicides are applied to aboveground parts of plants, usually to 
prevent disease rather than cure it. Because they are intended to form a 
protective coating on the plant surface that kills fungus spores before 
infection occurs, foliage fungicides must adhere to foliage despite 
weathering. Protective fungicides must be insoluble in water in order to 
remain on foliage. Certain foliage fungicides, however, are water-soluble. 
These materials destroy the fungus in disease spots after infection. 
Fungicides of this type are called eradicant or contact fungicides. 
    Seeds and seedlings are protected against fungi in the soil by treating 
the seeds and the soil with fungicides. Seed-treating materials must be safe 
for seeds and must resist degradation by soil and soil microorganisms. Some 
soil fungicides are safe to use on living plants. 
    Systemic fungicides are compounds that permeate plants to protect new 
growth or to eliminate infections that have already occurred.  
    Formic acid, acetic acid, and propionic acid up through pelargonic acid 
and capric acid (the C1-C10 volatile fatty acids) possess significant 
fungicidal activity. Many of them are present in natural foodstuffs that are 
resistant to fungal attack.These lower volatile fatty acids are used 
extensively to prevent fungal growth and so preserve the nutritious quality of 
cattle fodders. Since these volatile fatty acids stop fungal growth, they 
prevent mycotoxin generation and lessen the risk of cancer from exposure to 
mycotoxins.  
 �34s[James G. Horsfall ;Saul Rich]
�33s 
  
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�6v�2v�32sFungi Imperfecti   �21m �33s 
    A class of the Fungi. Many members of the Fungi Imperfecti, or 
Deuteromycetes (Deuteromycotina), are pathogenic for plants, animals, and 
humans, causing diseases like leaf spot of maple, apple blotch and anthracnose 
in plants, moniliasis and chromoblastomycoses in humans, and microsporoses in 
animals.  See  FUNGI. 
    Fungi ordinarily reproduce by both sexual and asexual stages. L. Fuckel, a 
German mycologist, defined the Fungi Imperfecti as such fungi that do not have 
the sexual stage and thus are imperfect, that is, incomplete in their life 
cycle. The designations of perfect stage for the sexual stage and imperfect 
stage for the asexual stage are derived from this definition. The term Fungi 
Imperfecti is now used to designate the fungi whose sexual reproductive stages 
are unknown. There are 1825 genera and 15,000 species known in the Fungi 
Imperfecti; most of these are imperfect stages of the Ascomycetes, a small 
number have Basidiomycetes as their perfect counterparts, and some are Mycelia 
Sterilia, that is, they produce neither sexual nor asexual spores. Imperfect, 
or conidial, stages of the Zygomycetes, Erysiphales, Ustilaginales, and 
Uredinales are generally not put in the Fungi Imperfecti. 
    A classification of the Fungi Imperfecti follows. 
    Order Sphaeropsidales 
        Family:  Sphaerioidaceae 
                 Zythiaceae 
                 Leptostromataceae 
                 Discellaceae 
    Order Melanconiales 
        Family Melancoliaceae 
    Order Moniliales 
        Family: Moniliaceae 
                Dematiaceae 
                Stilbellaceae 
                Tuberculariaceae 
    Order Cryptococcales 
    Order Mycelia Sterilia 
    The genera, with the exception of those in the Cryptococcales and Mycelia 
Sterilia, are usually arranged in spore groups. The spore group classification 
depends on the number of cells in the spore and on its shape. Each spore group 
is further subdivided into groups with pale-colored, or hyaline, spores, and 
with dark spores.  See  CRYPTOCOCCALES; EUMYCETES; MELANCONIALES; MONILIALES; 
MYCELIA STERILIA; SPHAEROPSIDALES. 
 �34s[N. F. Buchwald]
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�6v�2v�32sFuran �21m �33s
    One of a group of organic heterocyclic compounds containing a
diunsaturated ring of four carbon atoms and one oxygen atom. Furan (I) is a
typical member of the group. Furfural (II) and some of its close relatives,
such as furfuryl alcohol, tetrahydrofurfuryl alcohol, and tetrahydrofuran, are
important chemicals of commerce.  See  FURFURAL; HETEROCYCLIC COMPOUNDS;
NITROFURAN; PYRAN.
    Furan (1) is a colorless, volatile liquid, bp 31.4 DEG C (88.5 DEG F),
which is stable to alkali but not to mineral acid. Its water solubility is
approximately 1% at room temperature. On exposure to air, furan decomposes
very slowly by autoxidation. Substituted furans, particularly negatively
substituted furans, are much less sensitive. The furan system is aromatic.
Nitration, halogenation, acylation, mercuration, and sulfonation reactions
occur with relative ease.					[W.J.Ge.]


 �34s[Walter J. Gensler]
�33s
			     ******FORMULA******


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�6v�2v�32sFurfural   �21m �33s 
    When pure, a colorless liquid aldehyde boiling at 161.7DGC (323.1DGF); 
also called furfuraldehyde, fural, 2-furaldehyde, or 2-furancarboxaldehyde. It 
is obtained by the digestion of corn cobs or oat hulls with dilute mineral 
acid. A wide variety of agricultural by-products also yield furfural if 
treated in a similar manner; thus, the potential supply of this highly 
reactive aldehyde is virtually limitless. 
    Furfural is used as a general synthetic intermediate in the preparation of 
chemicals, many of which compete with the same chemicals derived from coal and 
petroleum. Large amounts are also used in the preparation of molding resins 
and other polymers of value to the plastics industry. Its unusual solvent 
properties make it useful in the refining of vegetable and lubricating oils 
and in extracting certain components, such as butadiene, from cracked refinery 
gases. Other uses to which furfural has been put are as an insecticide, 
herbicide, fungicide, and embalming fluid.  See  ALDEHYDE; BENZALDEHYDE; FURAN. 
 �34s[Alex E. Brodhag, Jr.]
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�6v�2v�32sFurnace   �21m �33s 
    An apparatus in which heat is liberated and transferred directly or 
indirectly to a solid or fluid mass for the purpose of effecting a physical or 
chemical change. The source of heat is the energy released in the oxidation of 
fossil fuel (commonly known as combustion) or the flow of electric current 
through adjacent semiconductors or through the mass to be heated. According to 
the source of heat and method of its application, there are four categories of 
furnaces: combustion, electric, nuclear, and solar, in the order of their 
present commercial or industrial importance.  See  ELECTRIC FURNACE; 
STEAM-GENERATING FURNACE; TUBE-STILL HEATER. 
    Furnaces employing combustion vary widely in construction, depending upon 
the application of the heat released, whether direct or indirect. Direct heat 
transfer is used, in regenerative refractory-type heaters, in flow systems in 
which reactants are injected into the combustion gases. Indirect heat transfer 
is employed in heaters in which the mass to be heated is kept separate from 
the combustion gases and made to flow in tubes which absorb and transmit the 
heat to the fluid to be heated.  See  COMBUSTION. 
    Furnaces developed for indirect heat transfer can be divided into two 
classes. One class of heaters is used solely for general utility purposes, 
such as all types of boilers; and the second class is applied in the petroleum 
and chemical industries as an essential unit operations in refining or 
processing plants.  See  BOILER. 
 �34s[H. C. Schutt /H. L. Beggs]
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�6v�2v�32sFuselage   �21m �33s 
    The component of an aircraft that provides the payload containment and the 
structural connection for the wing and the empennage (tail assembly). The 
fuselage and the wing are major structural components of an aircraft. The 
fuselage is the mounting structure for the horizontal and tail surfaces that 
provides stability as well as the means of introducing pitch and yaw control 
to the aircraft. For some aircraft like fighter and private aircraft, the 
fuselage houses the engine or engines. The nose or tail gear and the main 
landing gear are often attached to the fuselage structure. 
    The history of the construction of aircraft fuselages has evolved through 
the early wood truss structural arrangements to the current metal 
semi-monocoque shell structures. A majority of aircraft fuselages are 
fabricated from aluminum alloys and are produced by a process of automatic 
machining of the skins and stringers (see illustration), with much of the 
assembly being done by automatic drilling, countersinking, and fastener 
installation. In some areas, adhesive bonding is used as a means of attaching 
doublers to reinforce skin panels. In many of the high-performance aircraft, 
such as fighters and bombers, extensive use is made of titanium and 
high-strength steel. See AIRFRAME. 
 �34s[John E. McCarty]
�33s 
  
  
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�6v�2v�32sFuse (electricity) �21m �33s
    An expendable device for opening an electric circuit when the current
therein becomes excessive. An electric fuse consists principally of a section
of conductor, known as a fusible element, of such properties and proportions
that excessive current melts it and thereby severs the circuit. Fuses are used
in nearly all types of electric circuits to protect circuit conductors and
apparatus from damage that could result from sustained excessive current.
    The most familiar type of fuse is the screw-plug fuse used in domestic
electric systems. It consists of a shell, similar to the base of an
incandescent lamp, a fusible element connected between the screw shell and the
center terminal, and a transparent protective cover.
    Another widely used type is the cartridge fuse, in which the fusible
element is connected between metal ferrules at either end of an insulating
tube. In an expendable cartridge fuse, this tube is usually filled with an
insulating material, such as chalk, sand, or a suitable liquid, which cools
and quenches the arc that forms when the fusible element melts. Some fuses,
known as renewable fuses, are so constructed that a new fusible element can be
inserted to replace one which has been melted. In some types the cartridge is
unfilled and open at one end to allow the arc gases to escape to the
atmosphere. These are known as expulsion fuses.
    In large power systems available short-circuit current can be enormous. In
such systems, it is often desirable to have a fuse that can melt and interrupt
rapidly. These fuses are known as current-limiting fuses. To accomplish the
current-limiting effect, the fuse must be able to melt rapidly at high
currents and to form an arc that the system voltage cannot sustain.  See  ARC
DISCHARGE; CIRCUIT BREAKER; ELECTRIC PROTECTIVE DEVICES.
 �34s[Thomas H. Lee]
�33s

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�6v�2v�32sFuse (explosive)   �21m �33s 
    A conduit that leads fire from one place to another. The firecracker fuse 
is a familiar example; it consists of a thin train of black powder wrapped in 
a tube of tissue paper. The miner's safety fuse is similar, the tissue paper 
being replaced by a woven and waxed fabric. 
    The burning rate of a black-powder fuse varies widely and depends on the 
grain size and physical distribution of the powder and especially on the cord 
construction.  
    In one type of fuse used with high explosives in mining and quarrying, a 
woven fabric tube, often reinforced with wrappings of fiber or metal, and 
impregnated with asphalt and wax, is filled with a core of high explosive, 
such as pentaerythritol tetranitrate (PETN). 
    The word fuse should not be confused with fuze (sometimes also spelled 
fuse), which applies to mechanisms used to fire a shell at the moment it 
strikes a target, for example, or a hand grenade at a certain interval after 
it is thrown.  See  DETONATOR; PYROTECHNICS. 
 �34s[William E. Gordon]
�33s 
  
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�6v�2v�32sFusulinacea   �21m �33s 
    An extinct superfamily of marine organisms in the phylum Protozoa, order 
Foraminiferida. Fusulinaceans first appeared late in the Mississippian Period, 
and in the succeeding 100,000,000 years evolved into more than 125 genera and 
6000 species before becoming extinct near the close of the Permian Period. 
Among protozoans fusulinaceans became giants, many reaching 0.4 in. (1 cm) in 
length and some even attaining 4 in. (10 cm) in length. The group is 
characterized by rapid evolution of distinctive morphological features, which 
have enabled paleontologists to establish a detailed fusulinacean phylogeny 
and biostratigraphic zonation. 
    In general, the calcareous shell of fusulinaceans is constructed on a 
simple plan. The initial chamber, the proloculus, is a small calcareous sphere 
with an opening on one side. As the individual grew and the volume of 
protoplasm increased, chambers were added successively in a planispiral coil 
around the proloculus. In most genera these chambers overlap the ends of the 
previous chambers and become elongate, parallel to the axis of coiling. The 
spiral wall (spirotheca) is composed of several layers. The initial wall is 
formed of a thin, dark organic-rich layer (tectum), beneath which is a thicker 
translucent layer (diaphanotheca) formed of calcium carbonate.  See  
FORAMINIFERIDA.  
 �34s[Charles A. Ross]
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�6v�2v�32sGabbro   �21m �33s 
    A rock made up of centimeter-sized crystals mostly of calcium-rich 
plagioclase (35 - 65 vol %) and pyroxene in subequal amounts. The terms 
gabbro, diabase (or dolerite), and basalt refer to mineralogically identical 
rocks which differ in grain size. The textures of most gabbros are indicative 
of solidification from a melt. Gabbro occurs widely, but is not as common as 
basalt (its extrusive equivalent). See BASALT. 
    Gabbro is primarily an intrusive igneous rock. It forms irregular bodies 
as large as about 39 misup2 (100 kmsup2) in area, as well as dikes, sills, and 
laccoliths. The contacts of bodies of gabbro with surrounding rocks commonly 
are sharp, and the gabbro generally cuts across trends in older adjacent rock. 
Many gabbros are massive, but many have either a foliation or a layering or 
both. See IGNEOUS ROCKS. 
    Gabbros occur in all major tectonic environments (oceanic ridge, 
continental margin, oceanic island, and continental interior). Many 
continental interior occurrences are in fossil environments of continental 
margins and, possibly, oceanic ridges. On the Moon, as on Earth, gabbros are 
less common than basalts are. The lunar gabbros have textures similar to those 
of terrestrial gabbros. 
 �34s[Alfred T. Anderson, Jr.]
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�6v�2v�32sGadiformes   �21m �33s 
    A well-defined order of actinopterygian fishes, also known as the 
Anacanthini, which includes the codfishes and grenadiers, or rattails. 
Structurally the group is more or less intermediate between typical soft-rayed 
and spiny-rayed fishes. As in the former, there are no fin spines, the scales 
are cycloid, and the pelvic fin often has many rays. As in typical perciforms, 
however, the swim bladder has no duct, the orbitosphenoid and mesocoracoid are 
absent, and the upper jaw is bordered only by the protractile premaxillae. The 
pelvic fins, if present, are jugular in position but are attached to the 
cleithra only by ligaments. See CODFISH; PERCIFORMES. 
    Gadiform fishes are known from the Paleocene. Recent forms are classified 
into 8 families, about 185 genera, and nearly 730 species. The best-known 
family, the Gadidae, comprises 60 species that live in northern seas, where 
cod, haddock, pollock, and hake form the basis for extensive commercial 
fisheries. See ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sGadolinium   �21m �33s 
    A metallic chemical element, Gd, atomic number 64 and atomic weight 
157.25, belonging to the rare-earth group. The naturally occurring element is 
composed of eight isotopes. It is named in honor of the Swedish scientist J. 
Gadolin. The oxide, Gd2O3, in powdered form is white, and solutions of the 
salt are colorless. Gadolinium metal is paramagnetic and becomes strongly 
ferromagnetic below room temperatures. The Curie point, where this transition 
occurs, is about 16 K. See RARE-EARTH ELEMENTS. 
 �34s[Frank H. Shelton]
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�6v�2v�32sGage   �21m �33s 
    A device for determining the relative size or shape of objects. The 
function of gages is to determine whether parts are within or outside of the 
specified tolerances, which are expressed in a linear unit of measurement. 
Gages are the most widely used production tools for controlling linear 
dimensions during manufacture and for assuring interchangeability of finished 
parts. A gage may be an indicating type that measures the amount of deviation 
from a mean or basic dimension, or it may be a fixed type that simply accepts 
parts within tolerance and rejects parts outside tolerance. See GAGE BLOCK; 
INDICATING GAGE; PLUG GAGE; RECEIVING GAGE; RING GAGE; SCREW-THREAD GAGE; SNAP 
GAGE; TAPER PIPE THREAD GAGE; TOLERANCE. 
 �34s[Rush A. Bowman]
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�6v�2v�32sGage block   �21m �33s 
    The standard of lineal measurement for most manufacturing processes is the 
gage block (see illustration). Gage blocks are used for setting gages, for 
setting machines, and for comparative measurements. See COMPARATOR. 
    Gage blocks are made of chrome steel and may be chrome-plated or have 
carbide wear faces. Individual blocks have two surfaces which are flat and 
parallel. The parallel distance between the surfaces is the size marked on the 
block to a guaranteed accuracy of 0.000002, 0.000005, or 0.000008 in. (0.05, 
0.13, or 0.20 micrometers) at an ambient temperature of 68DGF (20DGC). Blocks 
are sold in sets so that combinations of individual blocks, when wrung firmly 
together, will produce a new end standard equal to the sum of the sizes of the 
individual blocks. 
    Gage blocks are measured by interferometry. Their actual sizes are 
determined by comparison with the wavelength of red light in the color 
spectrum, the ultimate standard of lineal measurement being the wavelength of 
light. Gage blocks translate this basic lineal standard into a practical form 
for shop use. See OPTICAL FLAT. 
 �34s[Rush A. Bowman]
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�6v�2v�32sGain   �21m �33s 
    An increase in signal power or voltage produced by an amplifier in 
transmitting a signal from one point to another. The amount of gain is usually 
expressed in decibels above a reference level. See AMPLIFIER. 
    Antenna gain is a measure of the effectiveness of a directional antenna as 
compared to a nondirectional antenna. See ANTENNA. 
 �34s[John Markus]
�33s 
  
  
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�6v�2v�32sGalactose   �21m �33s 
    A monosaccharide and a constituent of oligosaccharides, notably lactose, 
melibiose, raffinose, and stachyose. It is also known as D-galactose and 
cerebrose (see illustration). Agar, gum arabic, mesquite gum, larch arabo 
galactan, and a variety of other gums and mucilages contain D-galactose. See 
AGAR; GUM; MONOSACCHARIDE. 
  
    L-Galactose (enantiomorph of D-galactose) occurs in several 
polysaccharides, including agar, flaxseed mucilage, snail galactogen, and 
chagual gum. Since D-galactose is usually also present, hydrolysis of these 
polysaccharides produces DL-galactose. See CARBOHYDRATE; POLYSACCHARIDE. 
 �34s[William Z. Hassid]
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�6v�2v�32sGalaxy �21m �33s
    The large aggregation of stars and interstellar gas and dust of which the
Sun is a member. Except for three nearby external galaxies, which appear as
hazy patches of light, all objects visible to the unaided eye are members of
the Galaxy. The Milky Way is the faint band of light which encircles the sky
and results from the combined light of many stars. The Milky Way provides a
view of the Galaxy seen from within and off-center. The astronomer must
uncover the structure of the entire Galaxy from this limited inside view.
    Studies of other galaxies reveal three main types: elliptical, spiral, and
irregular. Counts of stars of different brightnesses, or apparent magnitudes,
show that in the vicinity of the Sun the Galaxy is quite flat. This result and
the observation of interstellar material near the Sun indicate that the Galaxy
is probably a spiral. The central bulge, or nucleus, is in the direction of
the constellation Sagittarius. See GALAXY, EXTERNAL.
    Stars of different types occupy different regions of the Galaxy. The
youngest and brightest stars of population I, together with interstellar
material, are found in the spiral arms in an extremely flat distribution, the
galactic disk. Older population I stars form a slightly less flattened
subsystem, while the oldest stars, population II, occupy a spherical
distribution, or halo, centered about the galactic nucleus. Population II
stars are also found in the nucleus. See STAR.
    In addition to individual stars, the Galaxy contains many star clusters.
Open clusters contain population I stars and are concentrated in the disk of
the Galaxy near the spiral arms. Globular clusters are compact, symmetric
groups of population II stars and are scattered throughout the halo. See STAR
CLUSTERS.
    Until early in the 20th century, it was believed that the Galaxy was a
huge flattened stellar system with a diameter of about 10,000 parsecs (1
parsec = 3 x 10(sup 13) km = 3.26 light-years) with the Sun near its center.
However, studies of globular clusters revealed that the Sun lies far (about
8500 parsecs) from the galactic center. Interstellar material dims light
passing through it and makes a distance determination uncertain if the amount
of material is not well known.
    Several subdivisions of the Galaxy are recognized (see illustration). The
nuclear bulge of the Galaxy extends to about 5000 parsecs from the galactic
center in the galactic plane. The thickness of the nuclear bulge is about 3000
parsecs. The total mass of the central section of the Galaxy is of the order
of 1.4 x 10(sup 11) solar masses. The interstellar gas and cosmic dust and the
young stars of population I are found only within the inner curve of the
illustration, hence close to the central plane. The inner section is enveloped
by a thin halo probably of ellipsoidal shape. Its maximum radius is 20,000
parsecs. Its mass is probably close to another 1.4 x 10(sup 11) solar masses,
contained mostly in vast numbers of population II stars. The RR Lyrae variable
stars are the most conspicuous population II stars, especially those found in
the outlying globular clusters.
    The halo is enveloped in a vast corona of unknown composition but with a
very great mass. This corona stretches to at least 60,000 parsecs from the
center, more likely to 100,000 parsecs. Its estimated mass is in the range
between 1 and 2 x 10(sup 12) solar masses, far exceeding the mass of the
core region and the conventional halo.
    There are vast numbers of old and dying stars within 10 parsecs or so of
the very center of the Galaxy, and radio evidence suggests that there exists a
core object with a mass of 5 x 10(sup 6) solar masses, possibly a massive black
hole. See BLACK HOLE; SUPERMASSIVE STARS.

    The evolution of the Galaxy must be reconstructed from knowledge of
present observed motions and spatial distributions of different types of
stars. Initially the Galaxy was a large, essentially spherical mass of gas.
This sphere contracted and stars began forming from the gas. The gas continued
to contract, and star formation increased as the density of the gas increased.
It contracted rapidly to a disk with a central concentration. Star formation
is still going on in the spiral arms, where there is interstellar material,
but the rate may be slower than in the early days of the disk since the
interstellar material is being used up. However, the steady replenishment of
interstellar gas through supernova explosions and the like should be taken
into account. See INTERSTELLAR MATTER; STELLAR EVOLUTION; SUN.
 �34s[Bart J. Bok]
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�6v�2v�32sGalaxy, external �21m �33s
    One of the large, self-gravitating aggregates of stars, gas, and dust
which contain nearly all of the observed matter in the universe. Typical large
galaxies have symmetric and regular forms, are about 50,000 light-years (1 ly
= 9.46 x 10(sup15) m) in diameter, and are roughly 5 x 10(sup 10) times more
luminous than the Sun. The stars and other material within a galaxy move
through it, often in regular rotation, with periods of a few hundred million
years.
    The hundreds of billions of stars making up a galaxy are not generally
individually observable with current telescope technology because they are too
faint and distant. Only the brightest stars in the nearest galaxies can be
observed directly with large telescopes. Two general types of stellar
populations are distinguished: One type (population I) is characterized by the
presence of young stars and by ongoing star formation. It is usually
associated with the presence of gas. The second type (population II) shows an
absence of gas and young stars as well as other indications that star
formation ceased long ago. The Sun is a population I star. See STAR.
    Galaxies contain gas (mostly un-ionized hydrogen) in amounts varying from
essentially zero up to a considerable fraction of their total mass. Dust in
galaxies, although small in mass (typically 1% of the gas mass), is often
dramatic in appearance because it efficiently obscures the starlight. See
INTERSTELLAR MATTER.
    Galaxies generally display strikingly regular forms. The most common form
is a disk with a central bulge. Its appearance is characterized by radially
decreasing brightness with a superposed spiral or bar pattern, or both (see
illustration). The central bulge may vary in size from hundreds to many
thousands of light-years and may be spherical or barlike in shape. Such
galaxies are classified as spirals (S) or barred spirals (SB) and
subclassified a, b, or c (for example, Sa, SBc) to distinguish increasingly
open spiral structure and small bulge size. See GALAXY.
    Another common type of galaxy is a featureless ellipsoid with radially
decreasing brightness. These galaxies are classified as ellipticals. They may
vary in size from thousands to several hundred thousand light-years. They are
most commonly found in clusters of galaxies and rarely contain gas or dust.
The brightest galaxies are usually ellipticals.
    Although galaxies are scattered through space in all directions for as far
as they can be observed, their distribution is not uniform or random. Most
galaxies are found in associations containing from two to hundreds of
individual galaxies (with a median number of about five). The clustering of
galaxies may be a hierarchical process. This would mean that the clusters
themselves are clustered to form superclusters, the superclusters are in turn
clustered to form yet higher order clusters, and so on. Although this is the
most popular theory of galaxy clustering and is supported by the existence of
several observed superclusters, it is far from an established fact. See
SUPERCLUSTERS.
    In the very central regions (sizes at least as small as a light-year) of
galaxies, violent and apparently explosive behavior is often observed. This
activity is manifested in many ways, including the high-velocity outflow of
gas, strong nonthermal radio emission (implying relativistic particles and
magnetic fields), intense and often polarized and highly variable radiation at
infrared, optical, ultraviolet, and x-ray wavelengths, and ejection of jets of
relativistic material. In the most extreme cases (called Seyfert and N
galaxies) the energy in the nuclear activity surpasses that in the rest of the
galaxy combined. The ultimate source of this activity, whether in some extreme
stellar population or in some previously unobserved type of object (for
example, black holes) remains completely unknown. There are many astronomers
who suspect that active galactic nuclei are related to the even more extreme,
but in many ways similar, phenomenon of quasistellar objects. See BLACK HOLE;
QUASARS.
    The nearest galaxies to the Milky Way Galaxy are the Large and Small
Magellanic Clouds, two small irregulars lying about 75,000 light-years away.
The nearest bright galaxy is the Andromeda Nebula, M31, visible to the naked
eye in the constellation of the same name. It is at a distance of almost 2 x
10(sup 6) light-years. These galaxies and a handful of fainter ones (all within
a few million light-years) make up the Local Group, of which the Milky Way
Galaxy is a member. See ANDROMEDA GALAXY; MAGELLANIC CLOUDS.
 �34s[Edwin L. Turner ;Joyce B. Turner]
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�6v�2v�32sGalena   �21m �33s 
    A mineral with composition PbS (lead sulfide) and belonging to the rock 
salt (NaCl) structure type. Galena usually occurs as cubes, sometimes modified 
by the octahedral form, with perfect cubic cleavage, brilliant metallic 
luster, color lead gray, specific gravity 7.5, and hardness 2** on Mohs scale. 
    Galena is widely distributed and constitutes by far the most important ore 
for lead. Silver, antimony, arsenic, copper, and zinc minerals often occur in 
intimate association with galena; consequently, galena ores mined for lead 
also include many valuable by-products. Important localities include Broken 
Hill, Australia; the tristate district of Missouri, Kansas, and Oklahoma; and 
numerous occurrences in Colorado, Montana, and Idaho. 
 �34s[Paul B. Moore]
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�6v�2v�32sGalilean transformations   �21m �33s 
    The family of mathematical transformations used in newtonian mechanics to 
relate the space and time variables of uniformly moving (inertial) reference 
systems. In the simple case of two similarly oriented cartesian reference 
frames, moving along their common (x,x') axis, the transformation equations 
can be put in the form of the following equations, where x, y, z and x', y', 
z' are the space coordinates of a given particle, and v is the speed of one 

            *** x' = x - vt     y' = y     z' = z     t' = t ***

system relative to the other. See FRAME OF REFERENCE. 
 �34s[Edward L. Hill]
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�6v�2v�32sGallbladder   �21m �33s 
    A hollow muscular organ, present in humans and most vertebrates, which 
receives dilute bile from the liver, concentrates it, and discharges it into 
the duodenum. Although not a vital organ, it is of great importance in humans 
because it stores bile, regulates biliary tract pressures, and, when diseased, 
enhances precipitation of various constituents of the bile as gallstones. See 
CHOLECYSTITIS; GALLSTONES. 
    The system of bile ducts lying outside the liver is known as the 
extrahepatic biliary tract. In humans (see illustration) right and left 
hepatic ducts empty into the common hepatic duct, which continues to the 
duodenum as the common bile duct, or ductus choledochus. The gallbladder and 
cystic duct thus appear to be accessory organs and therefore are removable. 
However, they are converted into main-line structures by the presence of a 
sphincter at the choledochoduodenal junction. Tonic contraction of this 
sphincter between meals forces the bile to back up into the gallbladder. See 
LIVER.  
    In humans, evacuation of the gallbladder is accomplished by a trigger 
mechanism which is set off by the presence of fatty foods, meat, and 
hydragogue cathartics in the duodenum and upper jejunum. Absorption of these 
substances by the mucous membrane results in the release of cholecystokinin, a 
hormone which rapidly circulates in the bloodstream and simultaneously 
produces contraction of the gallbladder and relaxation of the sphincter of 
Oddi. The most effective food is egg yolk which contains certain l-amino acids. 
 �34s[Edward A. Boyden]
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�6v�2v�32sGalliformes   �21m �33s 
    The order of birds of great economic significance because it includes 
important domestic and game birds. Seven families of living Galliformes are 
usually recognized: Megapodiidae (mound birds and brush turkeys), Cracidae 
(chachalacas, guans, and curassows), Tetraonidae (grouse), Phasianidae 
(pheasants, partridge, and quail), Numididae (guinea fowl), Meleagrididae 
(turkeys), and Opisthocomidae (hoatzins). See AVES; BIRDS, UPLAND GAME. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sGallium �21m �33s
    A chemical element, Ga, atomic number 31 and atomic weight 69.72. Gallium
was discovered by Lecoq de Boisbaudran in France in 1875. Gallium has a great
temperature range in the liquid state, prompting its use to some extent in
high-temperature thermometers and manometers. In an alloy with silver and tin,
gallium helps to form a suitable replacement for amalgam in dental fillings.
Gallium is also used to solder nometallic materials, including gems, to
metals. Gallium arsenide can be used in a system for translating mechanical
motion into electrical impulses. Synthetic superconducting articles are
prepared by making a porous matrix of vanadium or tantalum and impregnating it
with gallium hydride. Gallium has given excellent results in a semiconductor
for use in rectifiers, transistors, photoconductors, light sources, laser or
maser diodes, and refrigerating devices.
    Solid gallium looks bluish gray when exposed to the atmosphere. Liquid
gallium is silver white with a bright mirror surface. The freezing point of
gallium is lower than that of any metal except mercury (-39DGC or -38DGF) and
cesium (28.5DGC or 83.3DGF). Some properties of gallium are given in the table.
    Gallium is chemically similar to aluminum. It is amphoteric but slightly
more acid than aluminum. The normal valence of gallium is 3+, and it forms
hydroxides, oxides, and salts. Gallium burns in air when heated to 500DGC
(930DGF). It reacts vigorously with boiling water, but only slightly in water
at room temperature. The salts of gallium are colorless; they are prepared
directly from the metal, since purification of the metal is simpler than
purification of the salts.
    Gallium forms low-melting eutectic alloys with several metals, and
intermetallic compounds with many others. All aluminum contains small amounts
of gallium as a harmless impurity, but intergranular penetration by larger
amounts of gallium at 30DGC causes catastrophic failure on loading.
 �34s[W. D. Wilkinson]
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�6v�2v�32sGallstones   �21m �33s 
    Round, oval, or faceted concretions formed within the gallbladder from the 
salts and pigment of bile. Such stones may also be formed in any of the bile 
ducts within or outside of the liver, but the incidence there is low compared 
to the number originating in the gallbladder. The occurrence of gallstones 
increases with age, and 20 - 30% of all elderly adults have one or more such 
stones. They are particularly common in women, blacks, and individuals with 
diabetes. Usually about the size of a pea or a marble, gallstones may be 
extremely tiny, or so large that a single stone completely fills the 
gallbladder. They may be composed solely of calcium, cholesterol, or 
bilirubin, but usually represent combinations or mixtures of these three 
ingredients precipitated from supersaturated bile. See CHOLESTEROL; 
GALLBLADDER.  
    Although the mechanism and reason for their formation is not clearly 
understood, the major predisposing factors are prolonged retention of bile in 
the gallbladder, abnormal composition of the bile (excessive amounts of 
cholesterol, bilirubin, or calcium), and infection. Passage of a gallstone 
through the ducts into the duodenum usually produces severe pain, called 
biliary colic. Lodged in the distal portion of the common bile duct, a stone 
may obstruct that structure and produce jaundice and finally cirrhosis of the 
liver. The most frequent complication of gallstones is cholecystitis. See 
CHOLECYSTITIS  
 �34s[Milton R. Hales]
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�6v�2v�32sGalvanizing   �21m �33s 
    The generic term for any of several techniques for applying thin coatings 
of zinc to iron or steel stock or finished products to protect the ferrous 
base metal from corrosion; more specifically, the hot dipping that is widely 
practiced with mild steel sheet and corrugated sheets. During dipping, molten 
zinc reacts with the steel to form a brittle zinc-iron alloy. For marine use, 
magnesium is added. 
    An electrolytic process (also called cold galvanizing or 
electrogalvanizing) is also used for wire, as well as for applications 
requiring deep drawing. An alloy layer does not form, hence the smooth 
electroplated coating does not flake in the drawing die. See METAL COATINGS. 
 �34s[Frank H. Rockett]
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�6v�2v�32sGalvanomagnetic effects   �21m �33s 
    Electrical and thermal phenomena occurring when a current-carrying 
conductor or semiconductor is placed in a magnetic field. The galvanomagnetic 
effects are closely related to the thermomagnetic effects. See THERMOMAGNETIC 
EFFECTS.  
    Let the electric current density j be transverse to the magnetic field 
Hsubz, for example, along x. Then the following transverse-transverse effects 
are observed: (1) Hall effect, an electric field along y. (2) Ettingshausen 
effect, a temperature gradient along y. Also the following 
transverse-longitudinal effects are observed: (3) Transverse 
magnetoresistance, an electrical potential change along x. (4) Nernst effect, 
a temperature gradient along x. See HALL EFFECT; MAGNETORESISTANCE. 
    Let the electric current density j be along H. Then, the most important 
effect is longitudinal magnetoresistance, or an electrical potential change 
along H. 
 �34s[Elihu Abrahams ;Frederic Keff]
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�6v�2v�32sGalvanometer   �21m �33s 
    A device for indicating very small electric currents. Although the 
deflection of a galvanometer results from current in the moving coil, the 
voltage in a closed circuit producing this current is frequently the quantity 
of interest to the user. Galvanometers may also be used ballistically to 
integrate a transient current, as from the discharge of a capacitor, or a 
transient voltage, as produced when a coil moves relative to a magnetic field. 
See CURRENT MEASUREMENT; VOLTAGE MEASUREMENT. 
    While great sensitivity is desirable, reasonably rapid and controlled 
response is also required so that readings may be taken promptly and 
accurately. The degree to which the movable system is damped is thus of great 
importance.  
    The moving systems of many galvanometers carry a small mirror which 
deflects a light beam to give an indication. This serves as a means for 
amplifying small motions, and numerous variations have been devised to obtain 
optimum results in limited space. 
    The d'Arsonval galvanometer is the most common type, and is widely used. 
Its indicating system consists of a light coil of wire suspended from a copper 
or gold ribbon a few thousandths of an inch wide and less than 0.001 in. 
(0.025 mm) thick. This coil, free to rotate between the poles of a permanent 
magnet, carries a small mirror which serves as an optical pointer and 
indicates the coil position by reflecting a light beam onto a fixed scale (see 
illustration). Current is conducted to and from the coil by the suspension 
ribbons. The torque which deflects the indicating element is produced by the 
reaction of the coil current with the magnetic field in which it is suspended. 
See AMMETER; ELECTRICAL MEASUREMENTS; VOLTMETER. 
 �34s[Forest K. Harris]
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�6v�2v�32sGame theory �21m �33s
    The theory of games of strategy can briefly be characterized as the
application of mathematical analysis to abstract models of conflict
situations. The first such models analyzed by the theory were parlor games
such as chess, poker, and bridge. Since then, models arising from the
behavioral sciences such as economics, sociology, and political science have
been analyzed. Game theory is used in or closely connected to other areas such
as linear programming, statistical decisions, management science, operations
research, and military planning. In certain areas, the language and concepts
of the theory are sometimes used even though the corresponding mathematics is
not. See LINEAR PROGRAMMING; OPERATIONS RESEARCH.

    Games in extensive form.  The players of a game are called persons, and
such a person may actually consist of one or more people (for instance, in
bridge the pairs of partners, east-west and north-south, each make up a player
in the game). Chance moves occur when hands are dealt from a shuffled pack,
dice are rolled, or pointers are spun. One says that all chance moves are
allotted to the chance player - a fiction that is useful in abstracting
properties of games.
    When specified by a list of rules, a game is said to be in extensive form.
For mathematical purposes, it is convenient to have games in normalized form,
and for that, the idea of a pure strategy is needed.
    A pure strategy for a player (not the chance player) is a complete list of
choices of legal moves that he or she will make for every possible situation
that can occur during the game. This is a much more complete list of decisions
than that commonly called a strategy. The number of pure strategies in a game
can be astronomical even for childish games such as tic-tac-toe. Because of
the enormous number of pure strategies, the actual applications of game theory
even to parlor games have been severely limited by computational difficulties.
Simplified versions of the games have been developed for which computations
have been completely carried out.

    Games in normalized form.  After players have chosen pure strategies in a
game, they need not physically play the game. Instead they could hand their
strategies to a neutral person, or umpire, who could then carry out their
instructions and make the moves they would have made. This intuitively obvious
idea leads naturally to the normalized form of the game.
    Assume for the moment there are no chance moves in the game, that is, that
there are n real players but no chance player. Denote by s1, s2, . . . ,
s(sub n) specific pure strategies for players 1, 2, . . . , n, respectively.
Given these, the game must be played in exactly one way and a unique outcome
will result. Let P(sub i)(s1,s2, . . . ,s[sub n]) be the monetary outcome to
player i for this play of the game.
    Before the effect of the chance player can be introduced, the important
concept of mathematical expectation must be explained. Suppose that O1, O2, .
. . , O(sub k) are mutually exclusive monetary outcomes of a chance event, and
suppose further that they happen with probabilities, p1, p2, . . . , p(sub k)
where p(sub i) > 0 and p1 + p2 + . . . + p(sub k) = 1. Then the mathematical
expectation E of the chance event is defined to be the sum E = p1 O1 + p2 O2 + .
. . + p(sub k) O(sub k). See PROBABILITY.
    If there are chance moves in the game, a set of pure strategies, one for
each player, will not determine a unique outcome of the game but merely a set
of possible outcomes. These outcomes will be mutually exclusive and have
probabilities depending on the chance moves associated with their occurrence.
Hence in this case one can let P(sub i)(s1,s2, . . . ,s[sub n]) be the expected
payoff to player i for each i = 1, 2,. . . , n.
    Now the normalized form of a game is defined as the list of all expected
payoffs to each player for every possible combination of pure strategies. In
the case of two-person games it is most convenient to list these in tables
called matrices. See MATRIX THEORY.

    Classification of games.  A game is called zero-sum if, for every possible
n-tuple of pure strategies s1, . . . , ssubn, the sum of the payoffs to all
players is zero, that is, Eq. (1) holds. If this

P1(s1, . . . , s[sub n]) + P2(s1, . . . , s[sub n]) + . . . + P[sub n]
(s1, . . . , s[sub n])	    =	0					(1)

sum is not zero for some n-tuple of pure strategies, the game is called
nonzero-sum.
    A game in extensive form is said to have perfect information if every
player knows and remembers each move of each of her or his opponents as they
are made. A game is said to have perfect recall if each player knows and
remembers everything he or she did (but not necessarily what the opponent
did). Bridge does not have perfect recall, because the personality of a player
(= team) is divided between two actual persons and, for instance, north does
not see what is in south's hand (except when south is dummy) even though they
are members of the same team.
    Games can then be classified according to the number of players they have,
whether or not they are zero-sum, and whether or not they have perfect
information or perfect recall.

    Nonzero-sum games.	By far the most satisfactory part of the theory of
games consists of the zero-sum two-person cases, that is, in matrix games.
Applications of the theory to such areas as economics, sociology, and
political science almost invariably lead to many-person nonzero-sum games.
Although no universally accepted theory has been developed to cover these
games, many interesting and useful attempts have been made to deal with them.
    When more than two persons are involved in a conflict situation, the
important feature of the game becomes the coalition structure of the game. A
coalition is a group of players who band together and, in effect, act as a new
player in the game. There are two extremes to be considered. One is the
noncooperative game in which such coalitions are banned by some means.
Equilibrium-point solutions, discussed below, provide reasonably satisfactory
solutions to such games. The other extreme is that in which all the players
join together in a coalition to maximize jointly their total payoff. A game in
which coalitions are permitted is called a cooperative game.
    In the noncooperative game, each player is solely interested in his own
payoff. By an equilibrium point in such a game is meant a set of mixed
strategies s1, . . . , s(sub n) such that Eq. (2)

 P(sub i) (s1, . . . , s[sub i], . . . , s[sub n]) > or =
    P(sub i) (s[sub i], . . . , s'[sub n], . . . , s[sub n])             (2)

holds for each i = 1, . . . , n for all strategies s'subi of player i. What
this means is that no player can, by changing strategy and assuming that the
other strategies stay fixed, improve the payoff. By a theorem of J. Nash,
every game has at least one equilibrium-point solution (commonly there are
several).

    Simple games.  An important class of n-person games for application to
political behavior are the so-called simple games. Each coalition in such a
game can be either winning, losing, or blocking. For instance, a winning
coalition may be a set of voters who can elect their candidate, or a group of
lawmakers who can pass their bill. The players not in a given winning
coalition form a losing coalition. Finally, a coalition is blocking if neither
it nor the players not in it can enforce their wishes.

    Continuous games.  If, in the normalized form of the game, each player is
permitted to have a continuous range of pure strategies and the payoff
function is permitted to be a function of the two real variables that range
over each player's strategies, the result is a continuous game.

    Game-playing machines.  One of the first applications of large electronic
computers to numerical problems was in solving large matrix games. Several
methods have been devised for finding such solutions. One such method is the
simplex method. So-called decomposition methods can extend these methods to
certain problems having thousands or even millions of variables and
constraints. The principal application of the method is to solve linear
programming problems, which can be shown to be equivalent to matrix games.
    Computers have also been programmed to play board games such as checkers
and chess. Strictly speaking they do not use the theory of games at all at
present, but instead use some of the game sense of the people who devised the
codes. See ARTIFICIAL INTELLIGENCE; DECISION THEORY; INFORMATION THEORY;
STOCHASTIC PROCESS.
 �34s[Gerald L. Thompson]
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�6v�2v�32sGametogenesis   �21m �33s 
    The meiotic process of formation of the gametes, either eggs in the female 
or sperm in the males, which is common to all animals. This process involves 
changes in the chromosome number from the diploid (2N) number in somatic cells 
to the haploid (N) number in the gametes. In the process, the chromosomes 
(deoxyribonucleic acid, or DNA) are duplicated to form four chromatids 
(tetrads) with DNA quantity equal to 4N. 
    Meiotic divisions in eggs generally occur in association with ovulation 
and are completed with fertilization. Meiotic divisions are fundamentally the 
same in plants, with the formation of asexual spores called tetraspores which 
produce the gametophyte with mitotic divisions. See MEIOSIS; OOGENESIS, 
SPERMATOGENESIS.  
 �34s[Gertrude W. Hinsch]
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�6v�2v�32sGamma function   �21m �33s 
    A particular mathematical function that can be used to express many 
definite integrals. There are, however, no significant applications where the 
gamma function by itself constitutes the essence of the solution. Instead it 
occurs usually in connection with other functions, such as Bessel functions 
and hypergeometric functions. 
    A special case of the gamma function is the factorial n! = 1 SMALL BULLET 
2 SMALL BULLET 3 SMALL BULLET . . . SMALL BULLET n (for example, 1! = 1, 2! = 
2, 3! = 6, 4! = 24). It is defined only for integral positive values of n. The 
factorial occurs, for instance, in the expansion 

*** exp z = 1 + z / [1! + (z sup 2)] / [2! + (z sup 3)] / [3! + . . . ] *** 

The binomial coefficient (N/n) can be expressed in terms of factorials as 
N!/[n!(N - n)!]. Many occurrences of the factorial are found in combinatorial
theory (for instance, n! is the number of permutations of n different 
elements), in probability theory, and in the applications of this theory to    
statistical mechanics. For large values of n, the factorial can be easily,
although only approximately, computed with Stirling's formula,

*** n! APPROXIMATELY EQUALS [(n/e) (sup n)] [(2 PI n) (sup 1/2)] ***

See BESSEL FUNCTIONS.
 �34s[Josef Meixner]
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�6v�2v�32sGamma-ray astronomy   �21m �33s 
    The study of gamma rays of cosmic origin. The field may be divided into 
the study of high-energy gamma rays and that of soft gamma-ray bursts. The 
gamma-ray bursts have a typical energy 100 times lower, and do not seem at the 
present time to be related to the high-energy gamma rays. See GAMMA-RAY 
BURSTS; GAMMA RAYS. 
    Because, in the high-energy gamma-ray range, the photons (simply called 
gamma rays in the following) are completely absorbed by the atmosphere, it is 
necessary to place the gamma-ray detectors aboard high-altitude balloons, or 
better still, artificial satellites. The heart of a gamma-ray telescope is a 
spark chamber, similar to chambers used with particle accelerators, but much 
smaller. The incoming gamma-rays are absorbed in the plates and converted into 
electron-positron pairs. Since the gamma rays are invisible, it is the 
analysis of the electron and positron tracks, visualized by sparks, that gives 
the directional and energetic information about the detected photons. The 
location of the sparks is obtained through a digitized wire array. See 
GAMMA-RAY DETECTORS; SPARK CHAMBER. 
    The measurement of the energy of the incoming gamma rays is not very 
precise. At present, it involves an uncertainty of about 50 - 100%. 
Astronomical limitations of gamma-ray telescopes result from the weak 
intrinsic fluxes of gamma rays and from background problems. 
    The gamma-ray image of the Milky Way appears as a bright lane of diffuse 
emission, much like the optical image, although thinner (2DG wide). It spans 
roughly the same angular extent, from -60DG to +60DG in galactic longitude. 
Two bright spots, in the constellations Cygnus and Vela, are also visible. 
There is a fainter emission in other parts of the galactic disk. 
    The gamma-ray sky is almost devoid of ``stars,'' except along the galactic 
plane, where there is a strong concentration of gamma-ray sources (that is, 
``stars'' of 1DG radius). This is remarkable, as it is very different from 
what is seen at other wavelengths, even in x-rays. 
 �34s[Thierry Montmerle]
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�6v�2v�32sGamma-ray bursts �21m �33s
    Brief, intense bursts of cosmic gamma rays which are not obviously
associated with any known class of astronomical objects. They were discovered
in 1967 by the Vela satellites, developed to monitor the Nuclear Test Ban
Treaty, and their discovery was announced in 1973.
    The soft gamma-ray bursts typically last 1 - 10 s; however, some are as
short as 100 - 300 milliseconds, and the longest burst observed so far had a
duration of about 80 s. Most of the bursts exhibit multiple (nonperiodic)
pulses with significant structure occurring on time scales of 100 ms. Some
even show variations on a time scale as short as the 16-ms resolution of the
Vela detectors. Such short time scales imply a source size less than 10(sup 9)
cm (10(sup 7)m) or a highly relativistic emission process.
    The duration and the fast-time-scale structure of the gamma-ray bursts lie
in the range that is characteristic of the dynamical time scales (10(sup -4) to
1 s) associated with degenerate dwarfs, neutron stars, and black holes. There
are believed to be a large number of such compact objects within the
100-parsec distance as characteristic if the gamma-ray-burst sources lie
within the Galaxy. These considerations have led theorists to propose models
involving such compact objects; several of these models invoke neutron star
starquakes. See BLACK HOLE; GRAVITATIONAL COLLAPSE; WHITE DWARF STAR.
 �34s[D. Q. Lamb]
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�6v�2v�32sGamma-ray detectors   �21m �33s 
    Devices for detecting gamma (GAMMA) rays. Most detectors are capable of 
detecting and registering the passage of individual GAMMA-rays, or photons. A 
detector which simultaneously determines the energy of a GAMMA-ray may be 
properly termed a spectrometer. The term ``detector'' is frequently used as a 
generic name for devices that are also spectrometers. See GAMMA RAYS. 
    The two most popular devices in this category are the (thallium-activated) 
sodium-iodide [NaI(Tl)] detector and the (lithium-drifted) germanium [Ge(Li)] 
detector (see illustration). Since GAMMA-rays have no intrinsic charge or 
mass, both detectors depend on the fact that the three processes whereby 
GAMMA-rays interact with matter (photoemission, Compton effect, and pair 
production) result in the production of energetic electrons whose total energy 
is exactly proportional to the original GAMMA-ray energy. The electrons, in 
turn, lose energy by the ionization of the material (Ge or NaI), producing 
thereby a number of ion pairs exactly proportional to the electron energy. In 
both the NaI(Tl) and Ge(Li) detectors, the ionization energy is ultimately 
converted to a voltage-current pulse proportional to the total ionization 
produced by the GAMMA-ray interaction. The output pulses from the detector 
(after amplification) are then analyzed by a pulse-height analyzer to provide 
a quantitative measure of the pulse-height spectrum. See ELECTRON-POSITRON 
PAIR PRODUCTION; PHOTOEMISSION. 
 �34s[J. W. Olness]
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�6v�2v�32sGamma rays   �21m �33s 
    Electromagnetic radiation emitted from excited atomic nuclei as an 
integral part of the process whereby the nucleus rearranges itself into a 
state of lower excitation (that is, energy content). See NUCLEAR STRUCTURE; 
RADIOACTIVITY.  
    The gamma (ray) is an electromagnetic radiation pulse - a photon - of very 
short wavelength. The electric (E) and magnetic (H) fields associated with the 
individual radiations oscillate in planes mutually perpendicular to each other 
and also to the direction of propagation with a frequency v which 
characterizes the energy of the radiation. The E and H fields exhibit various 
specified phase-and-amplitude relations, which define the character of the 
radiation as either electric or magnetic. Gamma rays range in energy from a 
few keV to 100 MeV, although most radiations are in the range 50 - 6000 keV.  
As such, they lie at the very upper high-frequency end of the family of 
electromagnetic radiations, which include also light rays and x-rays. See 
ELECTROMAGNETIC RADIATION; MULTIPOLE RADIATION; PHOTON. 
    The dual nature of GAMMA-rays is well understood in terms of the wavelike 
and particlelike behavior of the radiations. For a GAMMA-ray of intrinsic 
frequency v, the wavelength is LAMBDA = c/v, where c is the velocity of light; 
energy is E = hv, where h is Planck's constant. See LIGHT; QUANTUM MECHANICS; 
RELATIVITY.  
    Various nuclear species exhibit distinctly different nuclear 
configurations: the excited states, and thus the GAMMA-rays which they 
produce, are also different. Precise measurements of the GAMMA-ray energies 
resulting from nuclear decays may therefore be used to identify the 
GAMMA-emitting nucleus. This has ramifications for nuclear research and also 
for a wide variety of more practical applications. One of the most useful 
studies of the nucleus involves the bombardment of target nuclei by energetic 
nuclear projectiles, to form final nuclei in various excited states. 
Measurements of the decay GAMMA-rays are routinely used to identify the 
various final nuclei according to their characteristic GAMMA-rays. 
    In practical applications, the presence of GAMMA-rays is used to detect 
the location or presence of radioactive atoms which have been deliberately 
introduced into the sample. In irradiation studies, for example, the sample is 
activated by placing it in the neutron flux from a reactor. The resultant 
GAMMA-rays are identified according to isotope, and thus the composition of 
the original sample can be inferred. Such studies have been used to identify 
trace elements found as impurities in industrial production, or in ecological 
studies of the environment, such as minute quantities of tin or arsenic in 
plant and animal tissue. See ACTIVATION ANALYSIS. 
    In tracer studies, a small quantity of radioactive atoms is introduced 
into fluid systems (such as the human blood stream), and the flow rate and 
diffusion can be mapped out by following the radioactivity. Local 
concentrations, as in tumors, can also be determined. See RADIOACTIVE TRACER. 
    For the three types of interaction with matter which together are 
responsible for the observable absorption of GAMMA-rays, namely, Compton 
scattering, the photoelectric effect, and pair production, see COMPTON EFFECT; 
ELECTRON-POSITRON PAIR PRODUCTION; PHOTOEMISSION. 
 �34s[J. W. Olness]
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�6v�2v�32sGammaridea   �21m �33s 
    A suborder of Amphipoda. These crustaceans are commonly known as scuds in 
aquatic environments, and as sandhoppers on beaches. The suborder comprises 
nearly 4750 species in 880 genera and 91 families. The majority of the species 
are marine. The terrestrial species are confined mainly to beaches in high 
latitudes but in the Indo-Pacific insular region they penetrate far inland and 
to an altitude of 9900 ft (3000 m) in moist environments. 
    Gammaridea are usually compressed laterally and are poor walkers except 
for the families Corophiidae and Cheluridae, which have depressed bodies (see 
illustration). The head and segments are free, lacking a carapace. The head 
bears two pairs of antennae and six kinds of mouthparts, some of them paired. 
The mandibles usually bear cutting and trituration surfaces for chewing, but a 
few species lack these devices or have the mouthparts modified for sucking the 
tissues of animals such as compound ascidians. 
    Gammaridea are largely scavengers, feeding on organic debris or detritus 
which falls to the ocean bottom. Species in the families Ampeliscidae, 
Photidae, and Corophiidae build nesting tubes attached to solid intertidal 
objects or lying on the sea bottom. The tubes are spun either from secreting 
glands on the first two pairs of pereiopods or from cuticular glands on the 
body. These animals use their well-developed antennae to strain food 
particles. Semiparasitic and commensal species with sucking or lapping 
mouthparts are known in the families Stenothoidae, Leucothoidae, and 
Dexaminidae. See AMPHIPODA. 
 �34s[J. Laurens Barnard]
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�6v�2v�32sGanglion   �21m �33s 
    A group of nerve cell bodies, usually located outside the brain and spinal 
cord. A ganglion located inside the central nervous system is called a nucleus. 
    The dorsal root ganglia are rounded clusters of cell bodies and fibers, 
surrounded by a connective tissue covering, located on the dorsal, or sensory, 
root of each spinal nerve. Other ganglia are given specific names which 
indicate their function or location, such as acoustic, cardiac, carotid, 
jugular, celiac, and sympathetic ganglia. Sympathetic ganglia, lying on either 
side of the vertebral column, unite by fiber strands to form a sympathetic 
chain. See SYMPATHETIC NERVOUS SYSTEM. 
    The term ganglion may be applied to a tumorlike, often cystic growth found 
on tendons, joints, and other connective tissues, but this usage is rare. See 
BRAIN; SPINAL CORD. 
 �34s[William Bradford]
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�6v�2v�32sGangrene �21m �33s
    A form of tissue death, or necrosis, usually occurring in an extremity and
due to insufficient blood supply.
    If no bacterial contamination is present, the part becomes dry,
greenish-yellow, and finally turns brown or black. This is known as
mummification. A sharp inflammatory border marks the edge of the adjacent
viable tissue. This dry gangrene is seen most often in small portions of the
extremities, such as the fingers and toes. Senile gangrene is the form caused
by deterioration of blood supply in the elderly, usually as the result of
progressive arteriosclerosis. Similar types are often present in diabetes,
Reynaud's disease, and Buerger's disease (thromboangiitis obliterans). See
ARTERIOSCLEROSIS; BUERGER'S DISEASE; DIABETES.
    When bacterial infection intervenes, putrefaction ensues, thus producing
the moist or wet type of gangrene. Occasionally the invading microorganisms
are gas-producing species of the genus Clostridium, and rapidly progressive
gas gangrene is the result. Gas gangrene is characterized by extensive edema
with gas production and discoloration of the tissue, and often accompanied by
a putrefactive odor. Moist gangrene may occur anywhere in the body that the
blood supply is blocked and bacterial contamination occurs.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sGantt chart   �21m �33s 
    A visual control device developed by Henry L. Gantt and used in production 
planning and control. In concept, the Gantt chart is a bar chart of several 
interrelated activities with time on the horizontal axis. The chart specifies 
the work required by a project (comprising a series of tasks) and the order in 
which the tasks must be done. 
    There are two basic types of Gantt charts: the planning chart and the 
progress chart. Preparation for the chart begins by listing the tasks that 
must be done to complete the project. A technological sequence of the 
activities for each task is required. After this step is completed, time 
estimates for each activity must be obtained. Next the Gantt planning chart 
(Fig. 1) is prepared, making a definite plan for each task necessary. This is 
one of the advantages of Gantt charts. It forces a thinking-through of the 
things that will be encountered and for which provisions must be made. As work 
progresses, a bar is drawn on the chart with a length proportional to the 
amount of work done during each time interval. Work done is initially plotted 
on the time interval during which it is accomplished. Later a cumulative bar 
may be added to summarize the status of work for an extended period. This 
second type of Gantt chart is known as the progress chart (Fig. 2). See 
CRITICAL PATH METHOD (CPM); INDUSTRIAL COST CONTROL; PERT. 
 �34s[Nelson M. Fraiman]
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�6v�2v�32sGar   �21m �33s 
    The name for about seven species of bony ganoid fishes (sometimes called 
garpikes) of the genus Lepisosteus. All have a slim form and an elongate snout 
(see illustration) and are covered with thick, close-set scales. These fish 
are a small remnant of a formerly large group of otherwise fossil fishes, 
usually regarded by paleontologists as constituting the order Semionotiformes. 
See ACTINOPTERYG II; SEMIONOTIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sGarlic   �21m �33s 
    A hardy perennial, Allium sativum, of Asiatic origin and belonging to the 
plant order Liliales, garlic is grown for its pungent bulbs, segments of which 
are used primarily for seasoning. Popular varieties are Italian, Tahiti, and 
Creole or Mexican. Garlic salt is made from dehydrated cloves. California is 
the most important producing state; smaller acreages are planted to garlic in 
Louisiana and Texas. See LILIALES. 
 �34s[H. John Carew]
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�6v�2v�32sGarnet   �21m �33s 
    A hard, dense silicate mineral which occurs as crystals of cubic symmetry 
in a wide range of geologic environments. The general chemical formula of the 
silicate garnet group is A3B2(SiO4)3 where, in natural occurrences, the A 
cations are dominantly Fesup2, Mnsup2, Mg, and Ca, and the B cations are Al, 
Fesup3, and Crsup3. 
    The garnet mineral group is generally divided into a number of individual 
species on the basis of chemical composition. The more common of these species 
are pyrope, almandine, spessartine, grossular, andradite, and uvarovite. 
    Garnets are substantially denser than most chemically analogous silicates, 
with specific gravities ranging between 3.58 (pyrope) and 4.32 (almandine). 
They also have high refractive indices (1.71 - 1.89) and hardness, on Mohs 
scale, of 6** to 7**. The relative hardness, coupled with the absence of 
cleavage, has led to the use of garnet as an abrasive. The color of garnet is 
primarily controlled by its chemical composition. Uvarovite is emerald green; 
gem varieties of garnet are generally clear, deep red pyrope. See GEM; 
SILICATE MINERALS. 
    Garnets are widespread in their occurrence, particularly in rocks which 
formed at high temperatures and pressures. Because of the large, readily 
identifiable crystals which form, the first appearance of garnet is commonly 
used by geologists as an index of the intensity, or grade, of metamorphism. 
Garnets are strongly resistant to weathering and alteration and are hence 
widespread constituents of sands and sediments in areas of garnetiferous 
primary rocks. See METAMORPHIC ROCKS. 
 �34s[B. J. Wood]
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�6v�2v�32sGarnierite   �21m �33s 
    A mineral used as a minor ore of nickel. The hardness is 2 - 3 on Mohs 
scale, and the specific gravity is 2.2 - 2.8. The luster is earthy and the 
color apple green to white. The composition may be expressed by the formula 
(Ni,Mg)SiO3 . nH2O, but varies greatly in the relative amounts of nickel and 
magnesium and in the amount of water. Garnierite has been mined as an ore of 
nickel in New Caledonia, the Soviet Union, South Africa, and Madagascar. In 
the United States it is found at Riddle, Oregon, and Webster, North Carolina. 
See NICKEL. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sGas   �21m �33s 
    A phase of matter characterized by relatively low density, high fluidity, 
and lack of rigidity. A gas expands readily to fill any containing vessel. 
Usually a small change of pressure or temperature produces a large change in 
the volume of the gas. The equation of state describes the relation between 
the pressure, volume, and temperature of the gas. In contrast to a crystal, 
the molecules in a gas have no long-range order. 
    At sufficiently high temperatures and sufficiently low pressures, all 
substances obey the ideal-gas, or perfect-gas, equation of state, shown as the 
following equation, where p is the pressure, T is the absolute temperature, 

                              *** pv = RT ***

v is the molar volume, and R is the gas constant. Absolute temperature T is 
expressed on the Kelvin scale. The gas constant is 8.314 joules/(mole K). The 
molar volume is the molecular weight divided by the gas density. 
    At lower temperatures and higher pressures, the equation of state of a     
real gas deviates from that of a perfect gas. Various empirical relations 
have been proposed to explain the behavior of real gases. See GAS DYNAMICS.
 �34s[Charles F. Curtiss; J. O. Hirschfelder]
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�6v�2v�32sGas absorption operations   �21m �33s 
    The separation of solute gases from gaseous mixtures of noncondensables by 
transfer into a liquid solvent. This recovery is achieved by contacting the 
gas stream with a liquid that offers specific or selective solubility for the 
solute gas or gases to be recovered. The operation of absorption is applied in 
industry to purify process streams or recover valuable components of the 
stream. It is used extensively to remove toxic or noxious components 
(pollutants) from effluent gas streams. See ABSORPTION. 
    The absorption process requires the following steps: (1) diffusion of the 
solute gas molecules through the host gas to the liquid boundary layer based 
on a concentration gradient, (2) solvation of the solute gas in the host 
liquid based on gas-liquid solubility, and (3) diffusion of the solute gas 
based on concentration gradient, thus depleting the liquid boundary layer and 
permitting further solvation. The removal of the solute gas from the boundary 
layer is often accomplished by adding neutralizing agents to the host liquid 
to change the molecular form of the solute gas. This process is called 
absorption accompanied by chemical reaction. See DISTILLATION; MASS-TRANSFER 
OPERATIONS.  
 �34s[Aaron J. Teller]
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�6v�2v�32sGas chromatography �21m �33s
    A method for the separation and analysis of complex mixtures of volatile
organic and inorganic compounds. Most compounds with boiling points less than
about 250DGC (480DGF) can be readily analyzed by this technique. A complex
mixture is separated into its components by eluting the components from a
heated column packed with sorbent by means of a moving-gas phase. See
CHROMATOGRAPHY.
    Gas chromatography may be classified into two major divisions: gas-liquid
chromatography, where the sorbent is a nonvolatile liquid called the
stationary-liquid phase, coated as a thin layer on an inert, granular solid
support, and gas-solid chromatography, where the sorbent is a granular solid
of large surface area. The moving-gas phase, called the carrier gas, is an
inert gas such as nitrogen or helium which flows through the chromatographic
column packed with the sorbent. The solute partitions, or divides, itself
between the moving-gas phase and the sorbent and moves through the column at a
rate dependent upon its partition coefficient, or solubility, in the liquid
phase (gas-liquid chromatography) or upon its adsorption coefficient on the
packing (gas-solid chromatography) and the carrier-gas flow rate.

    The apparatus used in gas chromatography consists of four basic
components: a carrier-gas supply and flow controller, a sample inlet system
providing a means for introduction of the sample, the chromatographic column
and associated column oven, and the detector system. A schematic diagram of a
working instrument is shown in the illustration.
    Qualitative and quantitative information is obtained from analyzing the
peaks appearing on a chromatogram. Combination of gas chromatography with mass
spectrometry provides the ultimate in qualitative information and has been
used extensively in research. See MASS SPECTROMETRY; QUALITATIVE CHEMICAL
ANALYSIS.
 �34s[Richard Juvet]
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�6v�2v�32sGas constant   �21m �33s 
    Boyle's law and Charles' law may be combined into a single expression, 
pV/T = a constant, showing how the volume V of a given mass of gas depends 
upon its temperature T and pressure p. If the mass of gas chosen is 1 g-mole, 
then the constant, known as the gas constant, is written as R. Hence, pV = RT 
for 1 g-mole. See BOYLE'S LAW; CHARLES' LAW. 
    The numerical value of the gas constant R is obtained by dividing the 
molar or gram-molecular volume of a perfect gas at the melting point of ice 
and a pressure of 760 mm of mercury (101.325 kPa) at the same temperature (the 
standard atmosphere) by the absolute temperature at the ice point, 
R = (V sub 0)/(T sub 0). The value for R is given below. See GAS.

                    *** R = 8.314 joules / (mole K) ***

 �34s[Thomas C. Waddington]
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�6v�2v�32sGas-discharge laser �21m �33s
    A device which makes use of nonequilibrium processes in a gas discharge to
achieve the population inversion required for laser action. At moderately low
pressures (on the order of 1 torr or 10(sup 2) pascals) and fairly high
currents, the population of energy levels is far from an equilibrium
distribution. Some levels are populated especially rapidly by the fast electrons
in the discharge. Other levels empty particularly slowly and so accumulate large
numbers of excited atoms. Thus laser action can occur at many wavelengths in
any of a large number of gases under suitable discharge conditions. For some
gases, a continuous discharge, with the use of either direct or
radio-frequency current, gives continuous laser action. Wavelengths generated
span the ultraviolet and visible regions and extend out beyond 700 micrometers
in the infrared. They thus provide the first intense sources of radiation in
much of the far-infrared region of the spectrum.
    The earliest, and still most widely used, gas-discharge laser utilizes a
mixture of helium and neon. Various infrared and visible wavelengths can be
generated, but most commonly they produce red light at a 632.8-nanometer
wavelength. Argon and krypton ion gas-discharge lasers provide a number of
visible and near-ultraviolet wavelengths. Many molecular gases, such as
hydrogen cyanide, carbon monoxide, and carbon dioxide, can provide infrared
laser action.  See  LASER; PULSED GAS LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
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�6v�2v�32sGas-dynamic laser �21m �33s
    A device in which laser action is achieved by the sudden expansion of a
hot molecular gas through a nozzle. The gas cools quickly, but different
excited states lose energy at different rates. It can happen that, just after
cooling, some particular upper state has more molecules than some lower one,
so that amplification by stimulated emission can occur. Very high continuous
power outputs have been generated from carbon dioxide in large gas-dynamic
lasers.  See  LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
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�6v�2v�32sGas dynamics   �21m �33s 
    The study of the motion of gases which takes into account thermal effects 
generated by the motion. Gas dynamics combines fluid mechanics and 
thermodynamics and differs from gas statics in that there is motion and from 
gas kinematics in that the forces exerted on or by the gas are considered. See 
GAS KINEMATICS. 
    Several other names are used to define the subject. The most important 
ones are aerothermodynamics, aerothermochemistry, fluid dynamics, compressible 
fluid flow, and supersonic aerodynamics. This terminology reflects the fact 
that in each particular case different aspects of gas dynamics are emphasized. 
Magnetogasdynamics, which includes effects due to magnetism and electricity, 
has applications in rocket reentry, propulsion, and astrophysics. See 
AEROTHERMODYNAMICS; COMPRESSIBLE FLOW; FLUID DYNAMICS. 
    Gas dynamics deals with the motion of a continuous medium and is not 
concerned with the behavior of individual atoms or molecules which constitute 
the gas. At low pressures, however, such as may be encountered at very high 
altitudes, the particle mean free path is very large and continuum 
considerations may not be applicable. 
    The fundamental conservation principles of mechanics and thermodynamics 
constitute the theoretical basis of gas dynamics. The conservation laws can be 
derived in lagrangian form with respect to a specific mass of flowing gas or 
in eulerian form with respect to the rate at which gas enters and leaves a 
fixed control volume in space. The eulerian conservation laws are expressed by 
(1) the continuity equation (conservation of mass); (2) the Navier-Stokes 
equations (conservation of momentum); and (3) the energy equation 
(conservation of energy). See CONSERVATION OF ENERGY; EQUATION OF CONTINUITY; 
FLUID FLOW; NAVIER-STOKES EQUATIONS. 
 �34s[Joshua Menkes ;Ali B. Cambel]
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�6v�2v�32sGas field and gas well   �21m �33s 
    Petroleum gas, one form of naturally occurring hydrocarbons of petroleum, 
is produced from wells that penetrate subterranean petroleum reservoirs of 
several kinds. Oil and gas production are commonly intimately related, and 
about one-third of gross gas production is reported as derived from wells 
classed as oil wells. If gas is produced without oil, production is generally 
simplified, in part at least because the gas flows naturally without lifting, 
and also because of fewer complications in reservoir problems. As for all 
petroleum hydrocarbons, the term field designates an area underlain with 
little interruption by one or more reservoirs of commercially valuable gas. 
See NATURAL GAS; OIL AND GAS FIELD EXPLOITATION; OIL AND GAS WELL DRILLING; 
PETROLEUM; PETROLEUM GEOLOGY; PETROLEUM RESERVOIR ENGINEERING. 
 �34s[Charles V. Crittenden]
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�6v�2v�32sGas furnace   �21m �33s 
    An enclosure in which a gaseous fuel is burned. Domestic heating systems 
may have gas furnaces. Some industrial power plants are fired with gases that 
remain as a by-product of other plant processes. Utility power stations may 
use gas as an alternate fuel to oil or coal, depending on relative cost and 
availability. Some heating processes are carried out in gas-fired furnaces. 
Among the gaseous fuels are natural gas, producer gas from coal, blast furnace 
gas, and liquefied petroleum gases such as propane and butane. See FUEL GAS; 
FURNACE; STEAM-GENERATING FURNACE. 
 �34s[Ralph M. Hardgrove]
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�6v�2v�32sGasket   �21m �33s 
    Deformable material used to make a pressure-tight joint between stationary 
parts, such as cylinder head and cylinder, that may require occasional 
separation. Gaskets are known as static seals, as compared with packing or 
dynamic seals. In packings the parts are frequently in motion, as in piston 
rods and valve stems. 
    Gaskets are made of sheet materials such as natural or synthetic rubber, 
cork, vegetable fiber such as paper, asbestos and plastic pastes, or of soft 
metallic materials such as lead and copper. Rubber in the form of O-rings is 
used for light pressure. See PRESSURE SEAL. 
 �34s[Paul H. Black]
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�6v�2v�32sGas kinematics   �21m �33s 
    The motion of a gas considered by itself, without regard for the cause of 
the motion. Various flow phenomena arise in widely different applications, yet 
the phenomena dealt with independently of their causes can be classed into 
relatively few fundamental patterns. Gas kinematics then deals with these 
abstracted flows. See GAS DYNAMICS; SINK FLOW; SOURCE FLOW. 
 �34s[Frank H. Rockett]
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�6v�2v�32sGas kinetics   �21m �33s 
    The effects of gases due to motion. The method of science is to isolate 
one phenomenon at a time for study, holding all else constant. However, in 
actual gas flows, motion is accompanied by other phenomena, so that for 
practical purposes a more comprehensive view is necessary. See AERODYNAMIC 
FORCE; BERNOULLI'S THEOREM; BUOYANCY; COMPRESSIBLE FLOW; EULER'S MOMENTUM 
THEOREM; GAS DYNAMICS; VISCOSITY. 
 �34s[Frank H. Rockett]
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�6v�2v�32sGas mechanics   �21m �33s 
    The action of forces on gases. One of the simplest forces on a gas is 
illustrated by the gravitational attraction of the Earth for its atmosphere. 
From this and other observations, the weight of an ideal gas is found to be 
proportional to its molecular weight. Another mechanical property of a gas is 
its ability to transmit pressure in any direction. In this respect a gas and a 
liquid behave similarly. See AVOGADRO'S LAW; FLUID MECHANICS; PASCAL'S LAW. 
 �34s[Frank H. Rockett]
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�6v�2v�32sGasoline   �21m �33s 
    A mixture of hydrocarbons used to fuel the spark-ignited internal 
combustion engine. This mixture consists of more than 100 different 
hydrocarbons. The lightest, that is, lowest-boiling, is isobutane, while the 
heaviest hydrocarbons consist of a variety of substituted naphthalenes. 
    The volatility of gasoline is controlled to provide a balance between a 
lack of vehicle performance due to insufficient vaporization and poor 
performance due to an excess of vapor. In order for the spark-ignited internal 
combustion engine to function, there must be a gaseous mixture of fuel vapor 
and air that sustains combustion when ignited by the spark discharge. That is, 
in cold weather there must be enough low-boiling hydrocarbons to form a 
flammable mixture in the cold cylinder. Shortly after the engine is running, 
it is expected to produce power and propel the vehicle. As power is required, 
greater quantities of vapor (up through the middle of the boiling range) are 
required. However, once the engine is fully warmed up, this same gasoline 
which was volatile enough to start the cold engine must not generate so much 
vapor as to create another set of operating problems. See INTERNAL COMBUSTION 
ENGINE.  
    A gasoline's octane number is an indication of the fuel's ability to 
prevent the occurrence of spark knock in an engine. Spark knock takes place 
when chain-branching reactions lead to an increasingly rapid buildup of 
hydroperoxide radicals in the region ahead of the advancing flame front. 
Tetraethyllead is an effective antiknock agent, because its decomposition 
gives rise to a nonstoichiometric lead oxide which acts as a radical trap. See 
ANTIKNOCK AGENTS; OCTANE; OCTANE NUMBER; TETRAETHYLLEAD. 
    A host of properties exhibited by gasolines results from the use of 
additives. Gasolines are colored with oil-soluble dyes to differentiate 
between the various grades marketed. Oxidation inhibitors make gasolines less 
susceptible to oxidation; this allows the gasoline to be stored for many weeks 
without excessive ``gum'' formation. The ``gums,'' now only partially soluble 
in gasoline, can deposit on critical parts of the carburetor and cause various 
parts to stick and thus fail to operate properly. Numerous organic compounds 
with an affinity for polar surfaces are used as rust inhibitors. 
    Most of the desirable features of gasolines require the deliberate 
application of technology during the refining and marketing of gasoline. The 
particular balance of properties arrived at will not be the same in all 
instances. Technical capability and economic considerations both play a part 
in determining which properties are imparted and emphasized. 
 �34s[Hugh F. Shannon]
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�6v�2v�32sGasterosteiformes   �21m �33s 
    An order of actinopterygian fishes which includes the groups 
Solenichthyes, Thoracostei, Hemibranchii, Lophobranchii, and Syngnathiformes 
of other classifications. Members of this order, commonly known as 
sticklebacks, tube-snouts, snipefishes, pipefishes, and seahorses, may be 
classified in 3 suborders, 7 - 9 families, 50 - 55 genera, and 200 or more 
species. The group dates from the Eocene. 
    Common characters include a swim bladder without a duct;  pelvic fin, if 
present, with or without a spine and abdominal to subthoracic in position; 
pelvic girdle not in contact with the cleithrum; and articulation of the 
quadrate with the lower jaw in advance of the orbit; fin spines are present or 
absent. The snout is commonly produced, the mouth opening at the end of a long 
tube (see illustration). 
    Most species are inhabitants of warm seas, but some sticklebacks live in 
fresh waters of northern continents, and a few tropical pipefishes enter or 
reside permanently in rivers. See ACTINOPTERYGII; SEA HORSE; TELEOSTEI. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sGas thermometry   �21m �33s 
    A method of measuring temperatures with a gas as the thermometric fluid. 
It is the most accurate means available for determining thermodynamic 
temperatures over the range from 2.5 to 1337 K. The unit of thermodynamic 
temperature is one of the base units of the International System (SI), and is 
defined by assigning the value of 273.16 K to the temperature of the triple 
point of water. Any other temperature to be determined by gas thermometry must 
be measured relative to the temperature of that triple point. 
    Of the several types (constant-volume, constant-pressure, and 
constant-bulb-temperature), constant-volume gas thermometry is potentially the 
most accurate, both because the equipment is relatively simple and because 
highly accurate absolute volume measurements are unnecessary. Accurate results 
are painstakingly difficult to obtain by any type of gas thermometry, however, 
so that reliable determinations of thermodynamic temperatures require special 
projects of substantial duration. The results are disseminated by recording 
them on the International Practical Temperature Scale - the international 
laboratory scale capable of precise and relatively easy realization. It is 
revised periodically to bring it into as close conformity with thermodynamic 
values as is known at a given time. See LOW-TEMPERATURE THERMOMETRY; PHYSICAL 
MEASUREMENT; TEMPERATURE MEASUREMENT. 
 �34s[Leslie A. Guildner]
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�6v�2v�32sGastrolith   �21m �33s 
    Any of the pebbles swallowed by animals and retained for a time in the 
gizzard or stomach, where they serve to grind up the food and in so doing 
become rounded and highly polished. Birds generally use such pebbles, as do 
some living reptiles, notably the crocodile and certain lizards. Some of the 
Mesozoic reptiles also used gastroliths. 
 �34s[Carl O. Dunbar]
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�6v�2v�32sGastropoda   �21m �33s 
    The largest and most varied class in the phylum Mollusca, possibly 
numbering over 74,000 species and commonly known as snails. 
  
    General characteristics.  The shell is in one piece which, in the majority 
of forms, grows along a turbinate (equiangular) spiral (see illustration), but 
which is modified into an open cone in various limpets or is secondarily lost 
in various slugs. 
  
    All gastropods, at some time in their phylogeny and at some stage in their 
development, have undergone torsion. The process does not occur in any other 
mollusks. It implies that the visceral mass and the mantle shell covering it 
have become twisted through 180DG in relation to the head and foot. As a 
result of torsion, all internal organs are twisted into a loop. Similarly in 
gastropods, the mantle cavity (the semi-internal space enclosed by the pallium 
or mantle) containing the characteristic molluscan gills (ctenidia) has become 
anterior and placed immediately above and behind the head. The most primitive 
gastropods retain a pair of aspidobranch (bipectinate or featherlike) gills, 
each with alternating ctenidial leaflets on either side of a ctenidial axis in 
which run afferent and efferent blood vessels. Lateral cilia on the faces of 
the leaflets create a respiratory water current (toward the midline and 
anteriorly) in the direction opposite to the flow of blood through the gills, 
to create the physiological efficiency of a countercurrent exchange system. 
    As also occurs in other mollusks, the ctenidia form a gill curtain which 
functionally divides the mantle cavity into an inhalant part (lateral and 
below) with characteristic osphradia as water-testing sense organs, and an 
exhalant part into which the anus and renogenital ducts discharge. 
  
    Classification and diversity.  The usual systematic arrangement of the 
class Gastropoda involves three somewhat unequal subclasses. The first, the 
largest and most diverse, is the subclass Prosobranchia, which is made up 
largely of marine snails all retaining internal evidence of torsion. 
Prosobranchs are divided into four orders: Archaeogastropoda, Neritacea, 
Mesogastropoda and Neogastropoda (or Stenoglossa). The other two subclasses 
(Opisthobranchia and Pulmonata) are each considerably more uniform than the 
subclass Prosobranchia and, in both, the effects of torsion are reduced or 
obscured by secondary processes of development and growth. See 
OPISTHOBRANCHIA; PROSOBRANCHIA; PULMONATA. 
    More than half of all molluscan species are gastropods, and they encompass 
a range from marine zygobranchs, which can be numbered among the most 
primitive of all living mollusks, to the highly evolved terrestrial 
air-breathing slugs and snails. Pulmonates and certain mesogastropod families 
are the only successful molluscan colonizers of land and fresh waters. 
 �34s[W. D. Russell-Hunter]
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    Fossils.  Fossil gastropods have a long geologic history, being common 
throughout the Paleozoic and increasingly abundant in the Mesozoic and 
Cenozoic. All three subclasses are known in the fossil record; many 
superfamilies, particularly prosobranchs, are extinct. Average duration of a 
genus has been estimated to range from 30,000,000 to 90,000,000 years. 
    Marine gastropods are important stratigraphic indicators in Cenozoic 
strata and locally are abundant in Cretaceous rocks. They are less common and 
less useful in the Jurassic and Triassic. Although individual genera have 
stratigraphic utility within the Paleozoic, it is only in the Ordovician that 
they are significant for correlation. 
    In North America, late Cenozoic nonmarine gastropods are becoming 
increasingly important for correlation and ecological interpretation of 
terrestrial deposits. Marine Cenozoic and later Mesozoic gastropods are 
interpreted as having lived under essentially the same ecologic conditions as 
related Recent gastropods. The possible ecology of Paleozoic and earlier 
Mesozoic gastropods has not been thoroughly investigated. In the Ordovician 
and Silurian, gastropods are most common in limestones. From the Silurian 
onward, marine forms are more common in shales and some sandstones. Most 
occurrences of abundant fossil gastropods probably can be interpreted as 
indicating shallow water conditions. See MOLLUSCA. 
 �34s[Ellis L. Yochelson]
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�6v�2v�32sGastrotricha   �21m �33s 
    A phylum of microscopic animals formerly considered to be a class of the 
phylum Aschelminthes. These cuticulate animals are multicellular, 
pseudocoelomate, unsegmented, and somewhat wormlike. Two orders occur, the 
Macrodasyoidea, which are exclusively marine, and the Chaetonotoidea, which 
are mainly fresh-water forms. Characteristic features of the group are the 
cilia used mainly in locomotion, a cuticular pattern of plates, scales, and, 
in some species, spines. Externally, the body ranges from complete smoothness 
and transparency through a very intricate display of short to long spinous 
structures and scales which entirely clothes the cellular body (see 
illustration).  
    Little is known of the exact environmental factors under which 
gastrotrichs live. They are readily found in standing fresh waters where algae 
and Protozoa abound, most likely close to or within the delicate film of the 
substrate.  
    Gastrotrichs possibly evolved from the Turbellaria and are clearly related 
to the Nematoda. See CHAETONOTOIDEA; MACRODASYOIDEA; NEMATA; TURBELLARIA. 
 �34s[Charles E. Packard]
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�6v�2v�32sGastrulation   �21m �33s 
    The formation of the primordial gut, the archenteron, or digestive cavity 
of an early animal embryo. More generally, and originally, the term 
gastrulation referred to the process by which the gastrula stage of the embryo 
is formed. Thus to 19th-century embryologists, gastrulation was the process by 
which the single-layered blastula, a hollow ball of cells, is converted into 
the double-layered gastrula. The term has now come to have a still more 
general meaning, namely, the process by which the three germ layers, or 
primordial tissues of the embryo, are brought into the positions and relations 
characteristic of the late gastrula stage, with ectoderm (outer skin), 
mesoderm (middle skin), and endoderm (inner skin) from the outside to the 
inside. The terms epiblast, mesoblast, and hypoblast are also used to denote 
ectoderm, mesoderm, endoderm, respectively. See BLASTULATION. 
    Two general but not mutually exclusive methods of gastrulation have been 
recognized: epiboly and emboly. Epiboly is the growing or extending of one 
part, such as the upper hemisphere of a spherical blastula, over and around 
another part, such as the lower hemisphere. Emboly is the pushing or growing 
of one part into another. In many embryos, both types of cell movement may 
occur; in certain invertebrate embryos, one type may predominate almost to the 
exclusion of the other. Generally speaking, epiboly tends to be the major, but 
not the only, method of gastrulation in forms with large, yolky eggs. See GERM 
LAYERS.  
 �34s[Nelson T. Spratt, Jr.]
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�6v�2v�32sGas tube   �21m �33s 
    An electron tube that contains gas or vapor at low pressure in which an 
electrical discharge takes place. Gas tubes are of two general types: 
cold-cathode tubes, in which the phenomenon known as glow discharge serves to 
maintain a conducting path between the electrodes, and hot-cathode tubes, in 
which an arc discharge conducts the current. The cold-cathode glow tubes are 
characterized by a relatively high voltage drop and a low current, while the 
hot-cathode arc tubes are characterized by a low voltage drop and relatively 
high current. See ELECTRON TUBE. 
    Cold-cathode gas tubes, or glow tubes, require no cathode heater power and 
are therefore always ready for instant service. Glow tubes are commonly 
constructed as two-element (diode) or three-element (triode) tubes. 
    Three representative types of hot-cathode gas tubes may be distinguished: 
the Tungar (sometimes called a Rectigon), the phanotron, and the thyratron. 
The first two are simple rectifier tubes while the third is a control tube 
having one or two grids between cathode and anode. Both phanotrons and 
thyratrons may be built with glass, ceramic, or metal envelopes. 
 �34s[James D. Cobine]
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�6v�2v�32sGas turbine   �21m �33s 
    A heat engine that converts some of the energy of fuel into work by using 
gas as the working medium and that commonly delivers its mechanical output 
through a rotating shaft. In the usual gas turbine, the sequence of 
thermodynamic processes consists basically of compression, addition of heat in 
a combustor, and expansion through a turbine. The flow of gas during these 
thermodynamic changes is continuous in the basic, simple, open-cycle 
arrangement.  
    The various gas-turbine cycle arrangements can be operated as open, 
closed, or semiclosed types. In the open-cycle gas turbine, there is no 
recirculation of working medium within the structural confines of the power 
plant, the inlet and exhaust being open to the atmosphere. This cycle offers 
the advantage of a simple control and sealing system. It also can be designed 
for high power-to-weight ratios (aircraft units) and for operation without 
cooling water. Most gas turbine plants are of this type. 
    In the closed-cycle gas turbine, essentially all the working medium 
(except for seal leakage, bleed loss, and any addition or extraction of 
working medium for control purposes) is continuously recycled. Heat from a 
source such as fossil fuel (or, possibly, nuclear reaction) is transferred 
through the walls of a closed heater to the cycle. The closed cycle can be 
charged with gases other than air such as helium, carbon dioxide, or nitrogen. 
This is a particular advantage with a nuclear heat source. Other advantages of 
the closed cycle are (1) clean working fluid; (2) control of the pressure and 
composition of the working fluid; (3) high absolute pressure and density of 
the working fluid; and (4) constant efficiency over wide load range. 
    In the semiclosed cycle gas turbine, a portion of the working fluid is 
recirculated. This type requires a precooler for the recirculated gas, and a 
charging compressor to provide the necessary air for combustion. The 
semiclosed cycle can operate at high densities. The major disadvantages of 
this cycle are the corrosion and fouling which occur with the recirculation of 
the products of combustion, particularly when the fuels used have high sulfur 
or ash content. 
    To achieve overall performance, each process is carried out in the engine 
by a specialized component. Air for the combustion chamber is forced into the 
engine by a compressor. Fuel is mixed with the compressed air and burned in 
combustors. The heat energy thus released is converted by the turbine proper 
into rotary energy. To improve efficiency, heat exchangers can be added on the 
gas turbine exhaust to recover heat energy and to return it to the working 
medium after compression and prior to its combustion. 
    In the open-cycle plant, products of combustion come in direct contact 
with the turbine blading and heat-exchanger surfaces. This requires a fuel in 
which the products of combustion are relatively free of corrosive ash and of 
residual solids that could erode or deposit on the engine surfaces. Natural 
gas, refinery gas, blast-furnace gas, and distillate oil have proved to be 
ideal fuels for open-cycle gas turbines. In closed-cycle turbines, gaseous 
fuel, distillate oils, and coal are satisfactory. 
    Fuels for semiclosed-cycle turbines must be selected to minimize both 
erosion and corrosion. These requirements limit the use of the semiclosed 
plant to fuels of low ash and sulfur content. See TURBINE. 
 �34s[Thomas J. Putz /J. A. Bolt]
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�6v�2v�32sGas welding   �21m �33s 
    A fusion welding process which uses a flame provided by the combustion of 
a fuel gas. The gas flame is a less concentrated and lower-temperature source 
of heat than an arc. For this reason, a welding torch is often used for 
brazing, for welding thin material, and for applications requiring low 
gradients of temperature to avoid cracking, as in welding cast iron. The 
oxyacetylene flame, using a mixture of equal volumes of oxygen and acetylene, 
C2H2, has a higher temperature than any other commercially available fuel gas 
combination. High flame temperature is essential for the rapid localized 
heating required in welding.  See  TORCH. 
    Flux is not used in welding steel but is necessary for gas-welding 
nonferrous metals. The use of other fuel gases, such as hydrogen, is used in 
the welding of metals of low melting point, such as lead, aluminum, and 
magnesium.  
 �34s[Mel M. Schwartz]
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�6v�2v�32sGate circuit   �21m �33s 
    An electronic circuit having one output and one or more inputs, in which 
the output is a function of the inputs in a prescribed manner and in a 
controllable time sequence. Gate circuits may be classified as transmission 
gates or as switching gates, of which the logic gate is the most notable 
example.  
    The transmission gate, sometimes called a sampling gate, delivers an 
output waveform that is a replica of a selected input during a specific time 
interval. The control signal, which determines the time interval, is called 
the gating signal or waveform; the gating generator is often a multivibrator. 
An operational amplifier with a separate strobe input which internally 
disconnects the input from the output sometimes functions as a transmission 
gate. The term analog switch is sometimes used synonymously with transmission 
gate. See AMPLIFIER. 
    In the switching, or logical, gate an output having a constant amplitude 
is registered during a time interval if a particular combination of input 
signals exists. The output indicates only the existence of a particular 
combination of similar input signals. Examples are the OR, AND, NOR, and NAND 
circuits, which are basic building blocks of digital computers. See LOGIC 
CIRCUITS.  
 �34s[Glenn M. Glasford]
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�6v�2v�32sGauss' theorem �21m �33s
    The assertion, under certain light restrictions, that the volume integral
through a volume V of the divergence of vector function SCRIPT F(x,y,z) is equal
to the surface integral of the exterior normal component of f over the boundary
surface S of V, or in symbols TRIPLE INTEGRAL DEL DOT SCRIPT F dV = DOUBLE
INTEGRAL SCRIPT F DOT BOLD v dS, with BOLD v the unit exterior normal to S. This
theorem is also known as the divergence theorem and as Green's theorem.
    The divergence theorem plays an important role in a variety of subjects
such as electricity and magnetism, mechanics of continuous media (including
fluid dynamics), heat flow, partial differential equations, and potential
theory. See CALCULUS OF VECTORS; GREEN'S THEOREM.

 �34s[Homer V. Craig]
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�6v�2v�32sGavial   �21m �33s 
    The name of two species of reptiles which form the family Gavialidae in 
the order Crocodilia. The Indian gavial (Gavialis gangeticus) is confined to 
the Ganges River and its tributaries, while the Malayan gavial (Tomistoma 
schlegeli) is found in Borneo and Sumatra. 
    The gavial is distinguished from other members of the order by its 
extremely long, slender snout (see illustration). The tip of the snout is 
enlarged, and fleshy elevations surround the openings of the nostrils. The 
females lay about 30 - 40 eggs in nests which they have prepared on the 
riverbanks. The newly hatched young are about 1 ft (30cm) long and quite 
active. See ALLIGATOR; CROCODILE; CROCODILIA; REPTILIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sGaviiformes   �21m �33s 
    The order of birds containing the loons, which are known as divers in 
Europe. There is but a single family, Gaviidae, and one living genus, Gavia, 
with four species, all of which nest in cooler parts of the Northern 
Hemisphere. The best-known and most southerly species is the common loon, or 
great northern diver (G. immer), whose cry is legendary. 
    Loons are goose-sized, fish-eating, diving birds that are highly adapted 
for aquatic life. Their legs are placed so far back on the body that they can 
neither walk well nor gain the air from dry land. The nest is usually placed 
at the water's edge, where the incubating bird can slip off and swim away 
quietly if disturbed. The feet of loons are webbed, the tarsi compressed and 
bladelike, and the bill heavy and point. Although their wings are 
exceptionally small in relation to body size, loons regularly perform long 
migrations. See AVES; DIVING BIRDS. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sGear   �21m �33s 
    A machine element used to transmit motion between rotating shafts when the 
center distance of the shafts is not too large. Toothed gears provide a 
positive drive, maintaining exact velocity ratios between driving and driven 
shafts, a factor that may be lacking in the case of friction gearing which is 
subject to slippage. See BELT DRIVE; CHAIN DRIVE; ROLLING CONTACT. 
    The application of gears for power transmission between shafts falls into 
three general categories: those with parallel shafts, those for shafts with 
intersecting axes, and those whose shafts are neither parallel nor 
intersecting but skew. See BEVEL GEARS; GEAR TRAIN; SPUR GEARS; WORM GEARS. 
 �34s[John R. Zimmerman]
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�6v�2v�32sGear drive   �21m �33s 
    Transmission of motion or torque from one shaft to another by means of 
direct contact between toothed wheels. The active parts of the gear teeth are 
usually made in the form of involutes and transmit the force smoothly from a 
tooth on one gear to a tooth on another gear. See GEAR; GEAR TRAIN. 
 �34s[Richard M. Phelan]
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�6v�2v�32sGear train   �21m �33s 
    A combination of two or more gears used to transmit motion between two 
rotating shafts or between a shaft and a slide. In theory two gears can 
provide any speed ratio in connecting shafts at any center distance, but it is 
often not practical to use only two gears. If the ratio is large or if the 
center distance is relatively great, the larger of the two gears may be 
excessively large. Moreover, an additional gear may be necessary simply to 
give the proper direction to the output gear. Belt, rope, and chain drives are 
frequently used in conjunction with gear trains. See BELT DRIVE; CHAIN DRIVE; 
GEAR; PLANETARY GEAR TRAIN. 
    The most important distinction in classifying gear trains is that between 
ordinary and epicyclic gear trains. In ordinary trains (illustration a), all 
axes remain stationary relative to the frame. But in epicyclic trains 
(illustration b), at least one axis moves relative to the frame. In 
illustration b, gear B, whose axis is in motion, is called a planet. The gears 
A and C are sun gears. 
    A simple gear train is one in which each gear is fastened to a separate 
shaft, as in illustration a. If at least one shaft has two or more gears 
fastened to it, the train is said to be compound. The train is a reverted gear 
train when the input and output shafts are in line. If the input shaft does 
not line up with the output shaft, the train is said to be nonreverted. 
  
    If a machine must be operated at any one of several output speeds, a 
multiple-speed gearbox, or transmission, may be used as a component part. 
Machine tools and motor vehicles are familiar instances of the need for 
transmissions. The speed of the output shaft of a transmission can be varied 
by sliding gears in and out of contact or by connecting gears in continual 
mesh to shafts by means of clutches. See AUTOMOTIVE TRANSMISSION. 
 �34s[John R. Zimmerman]
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�6v�2v�32sGecko   �21m �33s 
    The name for about 300 species of reptiles that form the family Gekkonidae 
in the order Squamata. They are small lizards, primarily arboreal and 
nocturnal, which occur in the warm regions of the world. The body is 
flattened; most species have five digits, while some have only four; the toes 
often have adhesive pads to assist the animal when climbing on smooth 
surfaces. Geckos feed on small animals, especially insects, and all species 
have a long sensitive tongue to aid in capturing their prey. Most species of 
gecko are oviparous. 
    The largest, most aggressive species is the orange-spotted Tokay (Gekko 
gecko), which is indigenous to Southeast Asia. Another species is the flying 
gecko (Ptychozoon homalocephalum), which is essentially arboreal and is well 
adapted to leaping and gliding because it has folds of skin on either side of 
the body that can be opened out to form a planing surface. The banded gecko 
(Coelonyx variegatus) of the southwestern United States is one of the few 
species with movable eyelids, and with claws instead of pads on the toes. See 
SQUAMATA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sGeiger-Mueller counter   �21m �33s 
    A detector of ionizing radiation. When a fast-moving charged particle 
traverses a Geiger-Mueller counter, an electrical impulse is produced. These 
impulses can readily be counted by electronic circuits. Geiger-Mueller (GM) 
counters, usually referred to simply as Geiger counters, are widely used to 
indicate the presence and intensity of nuclear radiations. 
    A GM counter consists of a gas between two electrodes (see illustration). 
One electrode, usually cylindrical and hollow, is the cathode. The other 
electrode, a fine wire stretched along the axis of the cylinder, is the anode. 
A potential of about 1000 volts is placed on the wire. When an atom of the gas 
between the two electrodes is ionized by collision with a charged particle 
passing through the gas, the electron produced in the collision is drawn 
toward the central wire. The electron then collides with the atoms of the gas. 
Near the central wire the electric field is very intense, and the electron may 
acquire enough energy between two collisions to allow it to ionize another 
atom. A second electron is then set free, and by successive collisions, an 
avalanche of electrons is produced which is then collected as charge on the 
central wire. This charge produces an electrical impulse which in typical 
cases may be 50 volts. See IONIZATION CHAMBER; PARTICLE DETECTOR. 
 �34s[William B. Fretter]
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�6v�2v�32sGel   �21m �33s 
    A two-phase colloidal system consisting of a solid and a liquid. Gels 
behave as elastic solids and retain their characteristic shape, whereas sols 
(colloidal dispersions) possess the shape of the container. Commonly, gels 
have a low solid content. Gels include jellies or transparent elastic gels 
rich in liquid, and gelatinous precipitates which are believed to consist of 
minute particles of jelly. Gels or jellies which have dried until apparently 
solid may be called zerogels, and sometimes will swell or redisperse to form a 
sol when treated with a suitable solvent. See COLLOID; GELATIN; RHEOLOGY. 
 �34s[W. O. Milligan]
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�6v�2v�32sGelatin   �21m �33s 
    A protein extracted after partial hydrolysis of collagenous raw material 
from the skin, white connective tissue, and bone of animals. It is a linear 
polymer of amino acids, most often with repeating glycine-proline-proline and 
glycine-proline-hydroxyproline sequences in the polypeptide linkages. See 
COLLAGEN; PROTEIN. 
    The unique characteristics of gelatin are: reversible sol-to-gel 
formation, amphoteric properties, swelling in cold water, film-forming 
properties, viscosity-modifying properties, and protective colloid properties. 
Gelatin contains 26.4 - 30.5% glycine, 14.8 - 18% proline, 13.3 - 14.5% 
hydroxyproline, 11.1 - 11.7% glutamic acid, 8.6 - 11.3% alanine, and in 
decreasing order arginine, aspartic acid, lysine, serine, leucine, valine, 
phenylalanine, threonine, isoleucine, hydroxylysine, histidine, methionine, 
and tyrosine. Absence of only two essential amino acids - tryptophan and 
methionine - makes gelatin a good dietary food supplement. 
    The principal uses of gelatin are in foods, pharmaceuticals, and 
photographic industries. Other uses of industrial gelatin are in the field of 
microencapsulation, health and cosmetics, and plastics. 
 �34s[Felix Viro]
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�6v�2v�32sGel chromatography   �21m �33s 
    A form of liquid chromatography in which separation is based on a 
selective process of penetration of molecules of different sizes and shapes 
through a porous gel medium. In this process, different molecules are 
``sized'' and thus separated from each other. The largest molecules in the 
mixture are not allowed to penetrate the porous structure at all; the 
medium-size molecules can penetrate only some pores; and the small molecules 
can diffuse rather freely inside the medium. Consequently, if the porous 
material is contained in a column, mixtures of components with differing 
molecular weights can be effectively resolved. This molecular sieve effect is 
extremely useful in many important separations. Gel chromatography has 
resulted in tremendous progress in the chemistry of biomacromolecules. See 
CHROMATOGRAPHY; LIQUID CHROMATOGRAPHY; MOLECULAR SIEVE. 
 �34s[Milos V. Novotny]
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�6v�2v�32sGel permeation chromatography �21m �33s
    A separation technique involving passage of a liquid moving phase through
a column containing the stationary phase which consists of a porous material.
Gel permeation chromatography (GPC) affords a rapid method for separating
high-molecular-weight materials, with separation based on differences in
molecular size. It is of particular importance for research in biological
systems and synthetic polymers.
    The stationary phase is a porous inorganic solid or cross-linked gel
containing a range of pore sizes. The mobile phase is a liquid, usually water
or a buffered solution for biological systems. Organic solvents are used for
synthetic polymers. The mobile phase is circulated through the column either
by pumping or by gravity flow. The sample to be separated is introduced at the
head of the column. As it progresses through the column, small molecules can
enter all pores larger than the molecule, while larger molecules can fit into
a smaller number of pores, again only those larger than the molecule. Thus the
larger the molecules, the smaller the amount of pore volume available into
which the molecules can enter. The sample emerges from the end of the column
in inverse order of molecular size, with the larger molecules eluting first
followed by the smaller ones. In order to determine the amount of sample
emerging, a detector is used which produces a signal proportional to sample
concentration. The volume of the mobile phase is also recorded to provide a
means of characterizing the molecular size of the emerging molecule. See
CHROMATOGRAPHY.
 �34s[Anthony R. Cooper ;Julian F. Johnson]
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�6v�2v�32sGem   �21m �33s 
    A mineral or other material that has sufficient beauty for use as personal 
adornment and has the durability to make this feasible. With the exception of 
a few materials of organic origin, such as pearl, amber, coral, and jet, and 
inorganic substances of variable composition, such as natural glass, gems are 
lovely varieties of minerals. 
  
    Natural gems.  Each distinct mineral is called a species by the 
gemologist. Two stones that have the same essential composition and crystal 
structure but that differ in color are considered varieties of the same 
species. Thus ruby and sapphire are distinct varieties of the mineral species 
corundum, and emerald and aquamarine are varieties of beryl. See MINERAL; 
MINERALOGY.  
    Most gemstones are crystalline (that is, they have a definite atomic 
structure) and have characteristic properties, most of which are related 
directly to either beauty or durability. Each mineral has a characteristic 
hardness (resistance to being scratched) and toughness (resistance to cleavage 
and fracture). With few exceptions, the most important gemstones are those at 
the top of the Mohs hardness scale; for example, diamond is 10, ruby and 
sapphire are 9, chrysoberyl is 8**, and topaz, beryl (emerald and aquamarine), 
and spinel are 8. 
    Optical properties are particularly important to the beauty of the various 
gem materials. The important optical properties include color; dispersion (or 
``fire''); refractive index (relating the breaking up of white light into 
colors - a rough measure of brilliancy); and pleochroism (the property of some 
doubly refractive materials of absorbing light unequally in the different 
directions of transmission, resulting in color differences). Gemstones usually 
are cherished for their color, brilliancy, fire, or one of the several optical 
phenomena, such as asterism (the star effect caused by certain reflections of 
light); chatoyancy, or a cat's-eye effect; play of color, such as displayed by 
an opal; and adularescence (the billowy light effect seen in adularia or 
moonstone varieties of orthoclase feldspar). 
    Gemstones are commonly designated as precious or semiprecious. This is a 
somewhat meaningless practice, however, and often misleading, since many of 
the so-called precious gem varieties are inexpensive and many of the more 
attractivevarieties of the semiprecious stones are exceedingly expensive and 
valuable. For example, a piece of fine-quality jadeite may be valued at 
approximately 100 times the price per carat of a low-quality star ruby. Fine 
black opals, chrysoberyl cat's-eyes, and alexandrites are often much more 
expensive than many sapphires of certain colors. See PRECIOUS STONES. 
    More than 100 natural materials have been fashioned at one time or another 
for ornamental purposes. Of these, however, only a relatively small number are 
likely to be encountered in jewelry articles.See GEM CUTTING. 
    The table lists the important gem minerals and the properties most useful 
in identification. For further information on the individual species or groups 
see AMBER; AMETHYST; AZURITE; BERYL; CAMEO; CHRYSOBERYL; CORUNDUM; DIAMOND; 
EMERALD; FELDSPAR; GARNET; INTAGLIO (GEMOLOGY); JADE; JET (GEMOLOGY); 
LABRADORITE; LAZURITE; MALACHITE; MICROCLINE; OLIVINE; ONYX; OPAL; ORTHOCLASE; 
PEARL; PERIDOT; QUARTZ; RUBY; SAPPHIRE; SPINEL; SPODUMENE; TOPAZ; TOURMALINE; 
TURQUOIS; ZIRCON. 
    Manufactured gems.  A mineral or other material that has sufficient beauty 
and durability for use as a personal adornment can be manufactured. The term 
``manufactured,'' as used here, does not include such processes as shaping, 
faceting, and polishing, but only the processes that affect the material from 
which the finished gem is produced. These processes are (1) those that change 
the mineral in some fundamental characteristic, such as color, called a 
treated gem; (2) those by which a material is made that is identical with the 
naturally occurring mineral, called a synthetic gem; and (3) those that 
produce a simulated material with the appearance but not both the composition 
and structure of the natural gem, called an imitation gem. 
  
    Treated gems. There are four basic methods of treatment: (1) dyeing and 
staining, (2) plastic or other impregnation, (3) heat treatment, and (4) 
radiation. When the process to which a gem material is subjected changes its 
structure or adds something, such as a dye or a plastic binder, an effect on 
value takes place. When such changes are made, the nature of the alteration 
must be disclosed. An example of the second category of treatment, wherein 
there is no obvious effect on value, is gentle heating of amethyst to even its 
color, or stronger heating to change it to yellow or brown citrine. The change 
is permanent and nothing but termporary heat has been added. Many colored 
stones, including most green tourmaline, aquamarine, and colorless and 
flame-colored zircon and all pink topaz and blue zircon have been heated to 
improve their color. 
  
    Synthetic gems. The U.S. Federal Trade Commission has restricted the term 
synthetic gems to manufactured materials that have the same chemical, 
physical, and optical properties as their naturally occurring counterparts. 
Many gem materials, including diamond, have been synthesized, but in such 
small crystals or poor quality that they are unsatisfactory as gemstones. Some 
attempts to make gemstones have resulted in producing substances hitherto not 
known, many of which are of great importance industrially. Others have 
resulted in significant improvements in existing processes. 
  
    Imitation gems. Since prehistoric times glass has been the most widely 
used gem imitation. Since World War II colored plastics have replaced glass to 
a great extent in the least expensive costume jewelry. 
  
    Identification. Materials made by a flame-fusion process almost always 
contain gas bubbles, which are usually spherical or nearly so. Those with 
medium to dark tones of color often show color banding, or striae, with a 
curvature corresponding to that of the top of the boule. Natural gem materials 
are characterized by angular inclusions and straight color bands, if any are 
present.  
    Flux-fusion synthetic emeralds have distinctly lower refractive indices 
and specific gravities than natural emeralds and are characterized by wisplike 
or veillike flux inclusions. They show a red fluorescence under ultraviolet 
light, whereas most natural emeralds are inert. 
    Hydrothermally made synthetic emeralds have properties similar to many 
natural emeralds, but their inclusions differ and they are characterized by a 
very strong red fluorescence under ultraviolet. 
    The cheaper forms of glass are cast in molds and, under a hand lens, show 
rounded edges at the intersections of facets; the facets are often concave. 
The better grades, known as cut glass, have been cut and polished after first 
being molded approximately into the desired form. Cut glass has facets that 
intersect in sharp edges. Both types may contain gas bubbles or have a roiled 
appearance in the heart or have both, in contrast to most of the colored 
stones they imitate. 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sGem cutting   �21m �33s 
    The polishing of rough gem materials into faceted or rounded forms for use 
in jewelry is called lapidary or gem cutting. The term lapidary is limited in 
application to the cutting of colored stones. One who fashions colored stones 
is called a lapidary, lapidarist, or lapidist. 
    Commercial colored-stone and diamond cutting are separate and distinct 
fields. Mechanically, the problems of colored-stone cutting are made simpler 
by the fact that all colored stones are soft enough to be shaped readily using 
silicon carbide or alumina powder as an abrasive. In contrast to diamond, they 
may be sawed or polished in any direction. A commercial gem cutter determines 
by eye the angles to which facets should be cut in relation to the girdle 
plane or the top facet and creates by eye the perfect symmetry one sees when 
viewing the finished stone from above. 
    The first step in the usual cutting operation is to saw the crystal or 
piece of massive gem material to obtain the size and general shape desired. If 
the material is relatively inexpensive, it may be ground to shape rather than 
reduced by cutting. Next, if the stone is soft enough, it is usually ground 
into the approximate final shape against a silicon carbide wheel. Facet 
grinding is usually accomplished on copper, iron, or lead laps, using diamond 
or silicon carbide powder for ruby and sapphire or their synthetic 
counterparts, and silicon carbide powder for the softer materials. Polishing 
is accomplished on tin, wood, or plastic laps, with the wooden laps usually 
covered with leather or cloth. 
  
    Innumerable cutting styles are used to fashion gemstones. The two basic 
types are those that employ curved surfaces, which are called cabochons, and 
those with flat surfaces, which are said to be faceted. 
    The two basic arrangements of facets are known as the brilliant and the 
step cut, from which most cutting styles derive. Both are characterized by a 
large facet (table) topping the crown facets, which slope to the periphery of 
the stone (girdle), and by a base portion, which slopes from the periphery to 
a tiny facet (culet) at the lowest point (see illustration). The two styles 
differ in that the facets on step cuts are parallel to the table and the 
girdle, so that they are all trapeze-shaped, whereas brilliant-cut facets, 
other than the table and culet, are triangular or kite-shaped. See GEM; GEM 
MOUNTING.  
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sGemini   �21m �33s 
    The Twins, in astronomy, is a winter zodiacal constellation. Gemini is the 
third sign of the zodiac. It is conspicuous, containing first-, second-, and 
third-magnitude stars. These stars, ALPHA, BETA, GAMMA, and MU, form a rough 
quadrilateral figure (see illustration). The constellation is pictured as the 
figures of the twin heroes Castor and Pollux, with the two brightest stars of 
the same names representing the heroes' heads. Pollux, slightly brighter than 
Castor, is a navigational star. The Sun is in this constellation at the time 
of the summer solstice. See CONSTELLATION. 
 �34s[Ching-Sung Yu]
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�6v�2v�32sGem mounting   �21m �33s 
    Gemstones are usually set in rings or other jewelry pieces made of one of 
the precious metals. Those metals most widely used for jewelry purposes are 
yellow gold, white gold, platinum, and palladium. When a stone is set in a 
ring, the portion in which the stone is placed is called the setting (see 
illustration). The stone may be held by small beads of metal, which are pushed 
over the edges by the stonesetter; by prongs, which are also bent over the 
edges of the stone to hold it securely; or by a rim of metal, which the 
stonesetter turns over the edges all the way around the stone. Tiffany 
settings are those in which naked prongs hold the stone in place high above 
the finger. See GEM; GEM CUTTING. 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sGemology   �21m �33s 
    The science of those minerals and other materials which possess sufficient 
beauty and durability to make them desirable as gemstones. It is concerned 
with the identification, grading, evaluation, fashioning, and other aspects of 
gemstones. See GEM. 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sGene   �21m �33s 
    The basic unit in inheritance. There is no general agreement as to the 
exact usage of the term, since several criteria that have been used for its 
definition have been shown not to be equivalent. 
    The nature of this difficulty can be indicated most easily after a 
description of the earlier position. The facts of mendelian inheritance 
indicate the presence of discrete hereditary units that replicate at each cell 
division, producing remarkably exact copies of themselves, and that in some 
highly specific way determine the characteristics of the individuals that bear 
them. The evidence also shows that each of these units may at times mutate, to 
give a new equally stable unit, which has more or less similar but not 
identical effects on the characters of its bearers. Each unit can be shown to 
occupy a specific locus in a chromosome, and the new units (alleles) to which 
it gives rise occupy the same locus. See ALLELE; MENDELISM. 
    These hereditary units are the genes, and the criteria for the recognition 
that certain genes are alleles have been that they (1) arise from one another 
by a single mutative step, (2) have similar effects on the characters of the 
organism, and (3) occupy the same locus in the chromosome. It has long been 
known that there were a few cases where these criteria did not give consistent 
results, but these were explained by special hypotheses in the individual 
cases. However, such cases have been found to be so numerous that they appear 
to be the rule rather than the exception. See GENE ACTION; MUTATION; 
RECOMBINATION (GENETICS). 
    Some authorities use the term gene to indicate the smallest unit of 
recombination. This is a logical procedure, but a somewhat inconvenient one 
operationally, since there is no method of determining when the limit of 
divisibility has been reached. Other authorities use the term to designate an 
area in a chromosome made up of subunits that are closely related in their 
action and that must be present in an unbroken unit to give their 
characteristic effect. This definition has the disadvantage of being rather 
indefinite, since intermediate conditions are known. See CHROMOSOME; 
DEOXYRIBONUCLEIC ACID (DNA); NUCLEIC ACID; OPERON; TRANSPOSONS. 
 �34s[Alfred H. Sturtevant]
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    In molecular terms a gene usually corresponds to a region on the linear 
DNA molecule (a cistron) that codes for a polypeptide. This polypeptide, built 
up from at most 20 kinds of amino acids, contains in itself all the 
information needed to fold up either alone or in association with other 
polypeptides into a functional protein (such as an enzyme or a structural 
complex). The linear order of the amino acids is specified by the nucleotide 
sequence of the gene. This information, encoded in the DNA, is first copied 
into a messenger RNA. DNA as well as RNA contains a linear arrangement of four 
different monomers (deoxynucleotides in DNA, ribonucleotides in RNA). The 
messenger RNA is then translated sequentially by the protein-synthesizing 
machinery of each cell (ribosomes, transfer RNA, enzymes, and so on). A group 
of three nucleotides constitutes a code word that corresponds to a given amino 
acid. Because there are four different nucleotides in the genetic textbook, 
there are 64 different code words to code for 20 amino acids and for 
initiation and stop signals. Genes carry information not only to specify a 
protein but to control precisely their expression, often in various ways. See 
GENETIC CODE. 
 �34s[Roland Contreras ;W. Fiers]
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�6v�2v�32sGene action   �21m �33s 
    The functioning of genes (hereditary units) in determining the structural 
and functional characteristics of an individual, that is, its phenotype. Gene 
action is studied by two somewhat different, but complementary, approaches: 
(1) the analysis of changes which occur in the phenotype when a gene mutates, 
or is changed in dosage, or position relative to other genes; this is 
frequently called the study of phenogenetics; and (2) the more direct 
approach, which attempts to determine the actual means by which genes exert 
their control over metabolism. The more direct approach is best described as 
study of primary gene action, but includes study of the interaction of primary 
or secondary products of gene action. See GENE. 
    All genes, with the exception of those in ribonucleic acid (RNA) viruses, 
are constituted of deoxyribonucleic acid (DNA), and the primary action of the 
great majority of them is to initiate a series of events leading to the 
determination of the amino acid sequences of specific polypeptides. The base 
sequence of probably only one of the strands of the DNA double helix of the 
gene is transcribed in the presence of RNA polymerase or polymerases to an RNA 
molecule known as messenger RNA (mRNA), which leaves the site of synthesis on 
the gene and participates in protein synthesis through the participation of 
transfer RNA (tRNA), ribosomes, and the necessary enzymes, amino acids, and 
cofactors. Each gene determines a specific mRNA, which in turn determines by a 
translation of the code a specific polypeptide. This polypeptide becomes a 
functional protein by folding and in most cases aggregating into a multimer. 
Therefore, these must exist in each organism at least as many genes as there 
are different kinds of polypeptides. See DEOXYRIBONUCLEIC ACID (DNA); GENETIC 
CODE; PROTEIN; RIBONUCLEIC ACID (RNA); RIBOSOMES. 
    It should be recognized that transcription is in essence the primary 
action of the gene. It is this step which determines the specificity of a 
protein. Most changes in the DNA sequence will result in an altered protein. 
    In addition to coding for specific proteins, such as enzymes, which 
function in metabolism, genes also code for the RNA species such as tRNA, 
ribosomal RNA, and proteins involved as general tools in protein synthesis or 
in the control of gene action as discussed below. 
    The actual effects of gene action are recognized for the most part by 
noting the effects of gene mutation on the phenotype, but in complex 
multicellular organisms the final phenotypic effect observed superficially may 
be far removed from the initial action of the gene itself, for example, a 
change in shape of the ear or a change in eye color. However, through study of 
the lower organisms, such as bacteria and fungi, it has become evident that 
there is a close relationship between genes and proteins. This has also been 
borne out by studies of the higher organisms. It is apparent that some 
phenotypic changes produced by gene mutation are due to changes in the 
protein-synthesis machinery brought about by the genes controlling tRNA and 
ribosomal RNA synthesis. 
    Because of the ramifying effects of a change in a single protein, the 
phenotypic change may be quite drastic, leading to lethality or changes in a 
number of the characteristics of the organism (pleiotrophy) rather than just 
in one. See LETHAL GENE; MUTATION. 
    Under certain conditions, an increase in the number of times a particular 
gene is present has a direct quantitative effect on one or more aspects of the 
phenotype. This effect is considered to be a manifestation of quantitative 
gene activity or gene dosage. It means that the gene does something, and that 
the higher the dose with which it is present, the greater the physiological or 
chemical end result. See POLYPLOIDY. 
    Some heterozygous combinations of mutant alleles do not produce the 
phenotype expected from the phenotypes of the homozygotes. This is defined as 
a manifestation of allelic interaction. It is in contrast to those situations 
in which one allele is dominant over the other, so that the phenotype of the 
heterozygote is very similar or identical to that of the homozygous dominant. 
Also, it is different from those situations in which the two alleles show an 
additive effect, and the phenotype of the heterozygote is intermediate between 
those expected from the homozygotes. See ALLELE; DOMINANCE. 
    The final phenotype of an organism is the resultant of the action of all 
the active genes in its cell or cells. These genes may act independently in 
producing their respective primary products, but the primary products, and the 
products of their activity, that is, enzymes and other macromolecules, 
interact at the level of extragenic metabolism to give the final phenotype. 
Thus, there is really no one gene determining the shape of an organ, or even 
the production of a certain pigment. These end products are determined by many 
genes acting together through their respective immediate and then succeeding 
interrelated products. This is true even though the mutation of only a single 
one of the genes may cause a characteristic unit change in the phenotype. The 
manifestation of these interactions is called gene interaction. This term does 
not necessarily imply that the genes themselves interact. In general, the term 
is applied to apparent interactions between genes. 
    The phenotype which develops in any organism is the resultant of two 
guiding influences, the genotype and the environment. Hence, the environment 
must be considered in any analysis of gene action, if it is desired to arrive 
at an understanding of how genes act toward the production of the phenotype. 
    The environment of genes is obviously a complex one. For convenience, two 
areas can be defined: (1) that immediately around the genes, the intracellular 
environment of the rest of the cell; and (2) the extracellular and 
extraorganismal environment. The intracellular environment can be changed by 
the mutation of other genes, which may then modify the action of a gene under 
study. Gene interaction is thus seen to be in part an aspect of the study of 
the internal environment of the cell. Extracellular environmental factors, 
such as light and heat, may also influence the action of genes greatly. 
    Changes in phenotype which occur against a constant genetic background are 
in reality responses to the environment by the extragenic part of the living 
system. The genes themselves are not changed, as can be readily demonstrated 
by changing the environment back to the original condition, or by breeding the 
individual and showing that the offspring inherit the original parental 
genotype. See DEVELOPMENTAL GENETICS; GENETICS. 
 �34s[Robert P. Wagner]
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�6v�2v�32sGeneral aviation   �21m �33s 
    All aviation activity not associated with either certificated air carriers 
or the military. Consequently, general aviation consists of the largest number 
of aircraft and pilots and accounts for the largest number of flying hours 
among all classifications of aviation within the United States. 
    A general aviation aircraft can range in size from the smallest pleasure 
craft designed and built by an individual to the largest air transport 
manufactured by a large airframe manufacturer. Of the active general aviation 
aircraft registered by the Federal Aviation Administration (FAA), about 50% 
are four-to-six-place single-engine airplanes that are capable of speeds below 
230 mi/h (103 m/s). The fleet also includes multiengine aircraft, mostly 
powered by two wing-mounted intermittent-combustion power plants, jet 
aircraft, and turboprop vehicles. The remainder of the general aviation fleet, 
which numbers about 60,000, are gliders and other recreational-use aircraft 
and two-or-three-place training and sport aircraft. See GLIDER. 
    Business applications, usually involving the transportation of executive 
or technical personnel in aircraft that range from single-engine vehicles 
flown personally by a business person to corporate jets piloted by a 
professional crew, account for more flying hours than any other segment of 
general aviation. An area that has responded to the changing nature of 
certificated air carriers is the commercial sector of general aviation, 
specifically the commuter airline and air taxi operators. 
    Commuters are very similar to the certificated air carriers in intent, 
although many lack the sophistication of the major airlines in execution. 
Commuters are the only airlines serving many communities, and they provide 
many aspects of the service that once was offered by a certificated air 
carrier. Air taxis provide on-demand air transportation for hire in aircraft 
that cover the spectrum of general aviation vehicles. General aviation 
aircraft are used in a variety of other commercial applications, including law 
enforcement, aerial mapping, pipeline patrol, counting cattle, surveying farm 
lands, fighting fires, and providing air ambulance services. 
    About 25% of general aviation involves the use of small aircraft for 
personal transportation and pleasure flying. Some aviators use their own or a 
rental aircraft to conduct vacation travel, while others just enjoy the 
pleasure of being aloft on a sunny day. Not all activities in this segment are 
leisure-oriented, however, since some proficiency flying (which is required by 
the FAA for the maintenance of a pilot's license) falls into this category. 
See AVIATION. 
 �34s[John W. Olcott]
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�6v�2v�32sGenerator   �21m �33s 
    Any machine by which mechanical power is transformed into electric power. 
Generators fall into two main groups, alternating-current (ac) and 
direct-current (dc). They may be further classified by their source of 
mechanical power, called the prime mover. Generators are usually driven by 
steam turbines, hydraulic turbines, engines, gas turbines, or motors. Small 
generators are sometimes powered from windmills or through gears, belts, 
friction, or direct drive from parts of vehicles or other machines. See 
ALTERNATING-CURRENT GENERATOR; DIRECT-CURRENT GENERATOR; PRIME MOVER. 
    In practice, permanent-magnet fields are used only in small generators. 
Large generators, except induction generators, are equipped with dc field 
windings. The field coils are wound on the stators of most dc generators to 
permit mounting the armature coils and commutator on the rotor. On ac 
generators, the field coils are normally located on the rotors. 
    Generators driven by steam or gas turbines are sometimes called 
turbogenerators. Although in small sizes these may be gear-driven, and some 
may be dc generators, the term turbogenerator generally means an ac generator 
driven directly from the shaft of a steam turbine. See ELECTRIC POWER 
GENERATION; ELECTRIC ROTATING MACHINERY. 
 �34s[Leon T. Rosenberg]
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�6v�2v�32sGenetic code �21m �33s
    The key to translation from the four-letter (nucleotides) alphabet of
nucleic acid, in which genetic information is stored, to the 20-letter (amino
acids) alphabet of protein, where the genetic information finds its primary
expression. The nucleic acid language consists of a vocabulary of 64
three-nucleotide sequences, or codons; a sequence of such codons (on average
about 300) constructs the message for a polypeptide. Translation into the
protein language, which consists of a vocabulary of 20 amino acids, takes
place in the protein-synthesizing apparatus of the cell strictly following the
rules given by the genetic code. See RIBOSOMES.
    The genetic information of an organism is encoded and maintained in the
linear sequence of deoxyribonucleotides in its deoxyribonucleic acid (DNA),
although in some viruses it is in ribonucleotides in their ribonucleic acid
(RNA). A major problem of biology has been to elucidate how the information
content of a particular DNA molecule becomes expressed by the production of a
particular type of protein molecule. One only of the two antiparallel strands
of the DNA double helix contains genetic information, and this information is
first transcribed from the four-letter alphabet of DNA into the slightly
different four-letter alphabet of RNA. See DEOXYRIBONUCLEIC ACID (DNA);
PROTEIN; RIBONUCLEIC ACID (RNA).
    A codon in RNA is the basic unit which is translated into, and therefore
corresponds to, one amino acid in protein. The coding ratio is 3 and the
nucleic acid language is a vocabulary of 64 codons which construct different
messages depending on their numbers and their linear sequence. The genetic
code is completely deciphered for Escherichia coli. The full catalog of code
words is listed in the illustration. Apparently this code is universal, or
nearly so, for organisms ranging from viruses to mammals.
    The genetic code is a nonoverlapping triplet code; that is, a nucleotide
which is part of one codon is never also part of a preceding or a following
codon. The code is highly degenerate in a semisystematic way; that is, more
than one codon is usually assigned for a particular amino acid (the only
exceptions are the amino acids tryptophan: UGG; and methionine: AUG) and where
several codons are assigned for the same amino acid, almost invariably they
are closely related, differing in only one nucleotide, which is usually the
third (for example, valine: GUU, GUC, GUA, and GUG).
    As there are only four nucleotides in messenger RNA (A = adenylic acid; U
= uridylic acid; C = cytidylic acid; G = guanylic acid), there are 4(sup 3) = 64
possible codons, and of these, 61 are known to code for one amino acid or
another. The three remaining codons apparently do not code for an amino acid
but are signals for termination of the growing polypeptide chain. See GENE;
GENE ACTION; GENETICS.
 �34s[H. Subak-Sharpe]
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�6v�2v�32sGenetic engineering �21m �33s
    The joining together of two DNA molecules in a test tube and the
subsequent insertion of this recombinant molecule into a living system. It
does not necessarily imply that the two molecules never join together in
nature, since frequently genetic engineering is used to achieve quickly and in
high yield a result which occurs only very rarely in nature. When applied to
higher organisms, the term is usually taken to imply the introduction into an
organism of a function, usually a gene or genes which either have not
previously been detected in that organism or exist in it in a different form.

    Plants.  The transfer of genetic functions between plants by breeding
methods is possible only between closely related species. The barrier to
breeding distantly related species imposes limits on the extent to which
desirable genes can be introduced into many economically important plants. It
is therefore of importance to develop artificial methods which can bypass this
breeding barrier. See BREEDING (PLANT).
    The majority of genes that would be of interest to transfer between
distantly related plant species do not give a selectable phenotype. They
would, therefore, have to be coupled to a marker gene that would enable
transformed plants or cells to be selected from a population comprising both
transformed and untransformed individuals. In attempting to use auxotrophic
markers (for instance, a requirement for an exogenously applied amino acid),
considerable effort must be expended in producing mutant forms of the receptor
plant species. Little success has been achieved in the production of such
mutants.
    An entirely different but parallel approach attempts to make use of
natural systems which modify or interfere with the expression of plant
genomes. These systems are potential natural vectors in that they are based on
DNA molecules, which, as a result of natural processes, are transferred to and
replicate in plant cells. They are naturally marked in that they cause plant
diseases which in some cases do not directly kill the infected plant but only
cause a loss of vigor.
    The other natural plant modification system is the plant crown gall
system. Crown galls are plant tumors produced in response to infection through
a wound by the soil bacterium Agrobacterium tumefaciens. The ``agent'' or
tumor-inducing principle within the bacterium is a group of genes carried on a
tumor-inducing (Ti) plasmid. This relatively small (90-160 x 10(sup 6) daltons)
circular DNA molecule codes for several distinct types of function. In
Agrobacterium, there is a system which transfers a fairly clearly defined
piece of DNA to plant cells, where it is integrated into the plant genome. The
observation that bacterial transposons inserted into the T-DNA (T for
``transforming'') are carried by it into plant cells and integrated as part of
the T-DNA shows that the system is capable of acting as a vector for foreign
DNA. The fact that some tumors produce morphologically normal plants shows
that tumorigenic functions can be suppressed, while other, nononcogenic T-DNA
functions (such as nopaline synthesis) continue to be expressed, thus
suggesting that the deleterious side effects of the system can be removed. See
CROWN GALL.

    Animals.  In contrast, the genetic engineering of animal cells poses a
different set of problems but, at the same time, carries a number of
advantages. For mammalian species, genome modification has been shown to be
possible at many levels.
    Cell fusion techniques have been extensively used in human genetics for
locating individual genes on specific chromosomes. Where mouse and human cells
have been fused, it is usually the human chromosomes which are preferentially
lost. When the final stage has been reached of a mouse cell containing one
human chromosome, the cells can be screened for proteins characteristic of
human cells. The genes for these can then be assigned to the particular
chromosome contained in the mouse cell. See SOMATIC CELL GENETICS.
    Several tumor viruses are capable of integrating their DNA, or
complementary DNA, into the host cell genome. After infection, they transform
their hosts to a tumor phenotype essentially by synthesizing a protein or
proteins which interfere with the normal cell-division control processes. Such
viruses could be used as vectors for the introduction of foreign genes into
animal cells in rather the same way that the T-DNA of the Agrobacterium Ti
plasmid can be used for the modification of plant cells. In the case of the
animal tumor viruses, however, the extent of virus genome integration can be
variable. See TUMOR VIRUS.
    There is one final system of animal cell modification which has
considerable potential and involves the transformation of cells with pure DNA,
which results in the integration of that DNA into the genomes of a
subpopulation of integration-competent cells. However, the complete inability
to regenerate whole animals from cultured cells severely limits the use of the
system. This is in contrast to plant tissue cultures, where the ability to
regenerate whole plants from undifferentiated cultured cells is a
well-established phenomenon. See DEOXYRIBONUCLEIC ACID (DNA); GENE; GENETICS.
 �34s[Angus G. Hepburn]
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�6v�2v�32sGenetic homeostasis   �21m �33s 
    The tendency of mendelian populations to maintain a constant genetic 
composition in the face of external pressure. When subjected to such pressures 
as artificial selection (usually for some quantitative trait) or temporary 
environmental changes, genetic homeostatic mechanisms tend to restore to 
equilibrium gene frequencies that may have shifted from mean optimal values. 
See MENDELISM; POPULATION GENETICS. 
    The mechanisms responsible for genetic homeostasis are varied. The two 
most commonly invoked, which are also responsible for maintenance of genetic 
polymorphisms, are frequency-dependent selection and heterozygote advantage. 
Under the first, the selective value of a gene depends on its frequency in the 
gene pool with a loss of selective advantage by an allele which was rare and 
has become common. Under the second, when either single alleles or blocks of 
linked genes are involved, the relative fitness of homozygotes is lower than 
that of heterozygotes. See ALLELE; POLYMORPHISM (GENETICS). 
 �34s[I. Michael Lerner]
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�6v�2v�32sGenetic mapping �21m �33s
    The resolution of the genetic material of an organism into the linear
sequence of the material's elements. In decreasing order of size these
elements are: the chromosome, the genes, the codons, and, ultimately, the
nucleotides of deoxyribonucleic acid, DNA (ribonucleic acid, RNA, in certain
viruses). Complete mapping of the genetic material of an organism implies the
complete description of its DNA sequences. This has been achieved in a few
cases, with small viruses with sequences only a few thousand nucleotides long.
Higher plants or animals have vastly longer sequences; in mammals, for
example, some sequences are 10(sup 9) nucleotides long. Thus genetic mapping
must proceed by successive approximations. Once the chromosome set and a number
of genes have been identified, mapping proceeds by establishing (1) the
chromosome or linkage group to which each gene belongs; (2) the order of genes
in a specific chromosome or linkage group; and (3) the distances between the
genes. A further step is ``fine'' genetic analysis at the intragenic level,
that is, the resolution of the nucleotides within a single gene or a small
segment of DNA. See CHROMOSOME; DEOXYRIBONUCLEIC ACID (DNA); GENE; RIBONUCLEIC
ACID (RNA).
 �34s[Guido Pontecorvo]
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�6v�2v�32sGenetics   �21m �33s 
    The science that is concerned with biological inheritance, that is, with 
the causes of the resemblances and differences among related individuals. 
Genetics occupies a central position in biology, for essentially the same 
principles apply to all animals and plants, and an understanding of 
inheritance is basic for the study of evolution and for the improvement of 
cultivated plants and domestic animals. It has also been found that genetics 
has much to contribute to the study of embryology, biochemistry, pathology, 
anthropology, and other subjects. 
    Genetics may also be defined as the science that deals with the nature and 
behavior of the genes, the fundamental hereditary units. From this point of 
view, evolution is seen as the study of changes in the gene composition of 
populations, whereas embryology is the study of the effects of the genes on 
the development of the organism. See BREEDING (ANIMAL); BREEDING (PLANT); 
CHROMOSOME; CHROMOSOME ABERRATION; CYTOPLASMIC INHERITANCE; GENE ACTION; 
GENETIC CODE; HETEROSIS ; HUMAN GENETICS; MENDELISM; MOLECULAR BIOLOGY; 
MOLECULAR GENETICS. 
 �34s[Alfred H. Sturtevant]
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�6v�2v�32sGentamicin   �21m �33s 
    A broad-spectrum antibiotic produced by a species of microorganism which 
belongs to the genus Micromonospora. The producing organism has been named M. 
purpurea for the purple color produced in the mycelium when grown in submerged 
culture.  
    Gentamicin is active in animals against a wide variety of gram-positive 
and gram-negative bacteria, mycobacteria, mycoplasma, and rickettsial 
organisms. Gentamicin is used clinically in a topical and parenteral form 
against susceptible organisms. The topical preparation has been shown to be 
effective for use on primary and secondary skin infections and skin ulcers and 
in the treatment and prophylaxis of infections due to burns. The injectable 
form is used for the treatment of acute and chronic urinary tract infections, 
pneumonias, and septicemias due to susceptible organisms, particularly 
Pseudomonas and Proteus strains. See ANTIBIOTIC. 
 �34s[Marvin J. Weinstein]
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�6v�2v�32sGentianales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of four families and about 5600 species. The Apocynaceae and 
Asclepiadaceae have about 2000 species each, the Gentianaceae have about 1100, 
and the Loganiaceae have about 500. 
    The Gentianales have well-developed internal phloem and opposite, simple, 
mostly entire leaves. The flowers are hypogynous, with a regular corolla, the 
lobes usually contorted in bud. The stamens form a single cycle, alternate 
with the corolla lobes. Many of the genera contain alkaloids or (in the 
Gentianaceae) glucosides. See ASTERIDAE; RAUWOLFIA; STROPHANTHUS; STRYCHNOS. 
 �34s[Arthur Cronquist]
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�6v�2v�32sGeobotanical indicators   �21m �33s 
    Vegetation whose characteristics provide information useful in 
interpreting geologic phenomena. Species assemblages and plant density are 
used to map strata of different chemical composition and reservoir capacity. 
Plants are also used in prospecting by analyzing plant tissue for metal 
content and by observing the species association, toxicity effects, and 
morphological changes in plants whose roots are in contact with ore. 
    Variations in plant associations are often useful in mapping the areal 
distribution of specific rock formations. A significant relationship between 
the vegetation and underlying rocks is commonly found in arid regions where 
the soils bear a chemical similarity to the rocks from which they have been 
derived. Plants are especially useful in geologic mapping wherever the 
dominant plant species differ on the formations being mapped. 
    Some plant species exhibit dependence upon groundwater in contrast to the 
ability of others to survive on occasional rainfall. Therefore plants are 
useful as indicators of the quality of water and depth of groundwater; the 
depth can be estimated from the known depth of root penetration of the species 
present.  
    The use of plant distribution, plant appearance, and growth anomalies in 
prospecting for ore deposits has been called geobotanical prospecting, in 
contrast to biogeochemical prospecting, in which common plant species are used 
as a sampling medium for chemical analysis. Plants have a third-dimensional 
advantage in prospecting because the roots may extend through many feet of 
surficial material for contact with mineralized water or rock. See GEOCHEMICAL 
PROSPECTING.  
    A plant species that grows more extensively near an ore body because it 
requires one or more of the ore elements or because an element on which this 
particular species depends is more available, is called an indicator. Some 
useful indicator plants are given in the table. A plant's distribution may be 
useful even though it is controlled by an associated pathfinder element rather 
than the most economically valuable element of the suite. 
 �34s[Helen L. Cannon]
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�6v�2v�32sGeochemical prospecting �21m �33s
    The use of chemical properties of naturally occurring substances
(including rocks, glacial debris, soils, stream sediments, waters, vegetation,
and air) as aids in a search for economic deposits of metallic minerals or
hydrocarbons. In exploration programs, geochemical techniques are generally
integrated with geological and geophysical surveys. See GEOCHEMISTRY;
GEOPHYSICAL EXPLORATION.

    General principles.  Mineral deposits represent anomalous concentrations
of specific elements, usually within a relatively confined volume of the
Earth's crust. Most mineral deposits include a central zone, or core, in which
the valuable elements or minerals are concentrated, often in percentage
quantities, to a degree sufficient to permit economic exploitation. The
valuable elements surrounding this core generally decrease in concentration
until they reach levels, measured in parts per million (ppm) or parts per
billion (ppb), which appreciably exceed the normal background level of the
enclosing rocks. These zones or halos afford means by which mineral deposits
can be detected and traced; they are the geochemical anomalies being sought by
all geochemical prospectors.
    The zone surrounding the core deposit is known as a primary halo or
anomaly, and it represents the distribution patterns of elements which formed
as a result of primary dispersion. Primary dispersion halos vary greatly in
size and shape as a result of the numerous physical and chemical variables
that affect fluid movements in rocks. Some halos can be detected at distances
of hundreds of meters from their related ore bodies; others are no more than a
few centimeters in width.
    Abnormal chemical concentrations in weathering products are known as
secondary dispersion halos or anomalies and are more widespread. They are
sometimes referred to as dispersion trains. The shape and extent of secondary
dispersion trains depend on a host of factors, of which topography and
groundwater movement are perhaps most important. Groundwaters frequently
dissolve some of the constituents of mineralized bodies and may transport
these for considerable distances before eventually emerging in springs or
streams. Further dispersion may ensue in stream sediments when soil or
weathering debris that has anomalous metal content becomes incorporated
through erosion in stream sediment. Analysis of the fine sand and silt of
stream sediment can be a particularly effective method for detection of
mineralized bodies within the area drained by the stream.

    Survey design.  The degree of success of a geochemical survey in a mineral
exploration program is often a reflection of the amount of care taken with
initial planning and survey design. This phase of activity is often referred
to as an orientation survey; its practical importance cannot be overstressed.
    When a geochemical prospecting survey is contemplated, four basic
considerations must be addressed: the nature of the mineral deposits being
sought; the geochemical properties of the elements likely to be present in the
target mineral deposit; geological factors likely to cause variations in
geochemical background; and environmental, or landscape, factors likely to
influence the geochemical expression of the target mineral deposit.
Elucidation of these factors in an orientation survey will permit design of a
geochemical prospecting survey that is most likely to prove effective under
the prevailing conditions. See PROSPECTING.
    Geochemical prospecting surveys fall into two broad categories, strategic
or tactical, which may be further subdivided according to the material
sampled. Strategic surveys imply coverage of a large area (generally several
thousands of square kilometers) where the primary objective is to identify
districts of enhanced mineral potential; tactical surveys comprise the more
detailed follow-up to strategic reconnaissance. Typically the area covered by
a tactical survey is divided into discrete areas of high mineral potential
within the general anomalous district.
    Soil and glacial till surveys have been used extensively in geochemical
prospecting and have resulted in the discovery of a number of ore bodies.
Generally, such surveys are of a detailed nature and are run over a closely
spaced grid.
    Biogeochemical surveys are of two types. One type utilizes the
trace-element content of plants to outline dispersion halos, trains, and fans
related to mineralization; the other uses specific plants or the deleterious
effects of an excess of elements in soils on plants as indicators of
mineralization. The latter type of survey is often referred to as a
geobotanical survey. See GEOBOTANICAL INDICATORS.
    Rock geochemical surveys are reconnaissance surveys carried out on a grid
or on traverses of an area, with samples taken of all available rock outcrops
or at some specific interval. One or several rock types may be selected for
sampling and analyzed for various elements. Geochemical maps are compiled from
the analyses, and contours of equal elemental values are drawn. These are then
interpreted, often by using statistical methods. Under favorable conditions,
mineralized zones or belts may be outlined in which more detailed work can be
concentrated. If the survey is executed over a large expanse of territory,
geochemical provinces may be outlined.
    Isotopic surveys are applicable to elements which exist in two or more
isotopic forms. They employ the ratios between isotopes such as
(sup204)Pb, (sup206)Pb, (sup207)Pb, (sup208)Pb, or (sup32)S and (sup34)S to
``fingerprint'' or indicate certain types of mineral deposits which may share a
common origin. Isotopic ratios may also be used to determine the ages of
minerals or given rock types and may, thus, assist in elucidating questions of
ore formation.
    Geochemistry applied to hydrocarbon exploration differs from that utilized
in the search for metallic mineral deposits; the former chiefly involves
detection and study of organic substances found during drilling, the latter
chiefly involves detection and study of inorganic substances at the surface.
Once accumulations of hydrocarbons have been discovered, their classification
into geochemical families becomes important. The final stages of detailed
exploration may involve complex multivariate computer-aided modeling of all
available geological, geochemical, geophysical, and hydrological data; the
objective would be to determine the ultimate hydrocarbon potential of a given
basin.
 �34s[R. F. Horsnail]
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�6v�2v�32sGeochemistry   �21m �33s 
    The study of the abundance of elements and nuclides in the Earth, the 
distribution of elements among the geochemical phases of the Earth, and the 
laws governing these abundance and distribution relationships. With the 
emphasis on lunar materials by geochemists and other scientists, the 
definition should probably be expanded to include other parts of the Cosmos 
accessible to exact observations. 
    Geochemistry is subdivided into four divisions: bio-, atmo-, hydro-, and 
lithogeochemistry. The distribution of the elements depends on the atomic 
structure of the elements, that is, their chemistry, whereas the quantitative 
composition of the Earth (and other solar system bodies) depends on the 
nuclear structure of the elements. 
    Classical geochemistry was directed largely at defining the abundance of 
the elements in rocks and minerals. Such data were essential to the attempts 
to understand the underlying nuclear causes which produced the observed 
abundance of the elements. 
    Advances in instrumentation techniques, including mass spectrometry, 
neutron activation analysis, and ion microprobe analysis, have led to 
significant improvements in the sensitivity and accuracy of the analyses. 
Studies on phase equilibria of simple petrologic systems conducted under high 
temperature and pressure have increased the knowledge of chemical reactions in 
the Earth's crust and have provided new data on the origin and emplacement of 
ore bodies within the Earth. 
    Modern geochemical research places considerable stress on the quantitative 
determination of temperature, pressure, and time in geologic systems. Methods 
based on isotopic fractionation and radioactivity which utilize mass 
spectrometry have proved extremely useful in this regard. See ACTIVATION 
ANALYSIS; BIOSPHERE; COSMOCHEMISTRY; GEOCHEMICAL PROSPECTING; GEOCHRONOMETRY; 
GEOLOGIC THERMOMETRY; HYDROSPHERE; LEAD ISOTOPES (GEOCHEMISTRY); LITHOSPHERE; 
MASS SPECTROMETRY; PALEOBIOCHEMISTRY; PETROLOGY; SECONDARY ION MASS 
SPECTROMETRY (SIMS). 
 �34s[D. Aylmer ;Marvin W. Rowe]
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�6v�2v�32sGeochronometry �21m �33s
    The study of the absolute age of the rocks of the Earth using methods
based on the radioactive decay of such isotopes as sup2sup3sup8U,
(sup 235)U, (sup 232)Th, (sup 87)Rb, (sup 40)K, and (sup 14)C. Measurements
using these decay schemes permit definition of the age of the Earth and
meteorites (4.5 x 10(sup 9) years); the oldest available rocks (3 x 10(sup 9)
years); significant events in the geologic time scale; the rate of evolution of
organisms; the time and duration of major events in crustal evolution; the
advance and retreat of continental glaciers; and the evolution of human culture.
See FISSION TRACK DATING; GEOLOGICAL TIME SCALE; LEAD ISOTOPES (GEOCHEMISTRY);
RADIOCARBON DATING; ROCK AGE DETERMINATION.
 �34s[J. Laurence Kulp]
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�6v�2v�32sGeode   �21m �33s 
    A roughly spheroidal hollow body, lined on the inside with 
inward-projecting small crystals (see illustration). Geodes are found most 
frequently in limestone beds but may occur in some shales. Typically, a geode 
consists of a thin outer shell of dense chalcedonic silica and an inner shell 
of quartz crystals. Many geodes are filled with water; others, having been 
exposed for some time at the surface, are dry. Calcite or dolomite crystals 
line the interior of some geodes, and a host of other minerals are less 
commonly found. In some geodes there is an alternation of layers of silica and 
calcite, but almost all geodes show some banding suggestive of rhythmic 
precipitation. See CHALCEDONY. 
 �34s[Raymond Siever]
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�6v�2v�32sGeodesy   �21m �33s 
    The science of surveying and mapping the Earth's surface. Geodesy supplies 
locations, distances, directions, elevations, and gravity information for the 
needs of civil engineering, boundary demarcations, navigation, and 
prospecting, as well as for other geosciences. 
    The many-faceted subject matter of geodesy includes the size and shape of 
the Earth, its external gravity field, horizontal and vertical control 
networks, and time-dependent phenomena. Included also are the utilization and 
adaptation of advanced technology (such as satellite geodesy and high-speed 
electronic computers) and the refinement of theories. Other sciences depend 
heavily upon geodesy, so that the scope of geodetic concerns embraces certain 
aspects of astronomy (astronomical positioning, orbit theory), geography 
(cartography, map projections), geophysics (gravity), geodynamics (crustal 
movements, polar motion), and oceanography (marine geodesy) and it even 
extends to the Moon (selenodesy) and planets. See CARTOGRAPHY; TERRESTRIAL 
COORDINATE SYSTEMS; TOPOGRAPHIC SURVEYING AND MAPPING. 
 �34s[Irene K. Fischer]
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�6v�2v�32sGeoelectricity   �21m �33s 
    Electromagnetic phenomena and electric currents, mostly of natural origin, 
that are associated with the Earth. Geophysical methods utilize natural and 
artificial electric currents to explore the properties of the Earth's interior 
and to search for natural resources (for example, petroleum, water, and 
minerals). Geoelectricity is sometimes known as terrestrial electricity. All 
electric currents (natural or artificial, local or worldwide) in the solid 
Earth are characterized as Earth currents. The term telluric currents is 
reserved for the natural, worldwide electric currents whose origins are almost 
entirely outside the atmosphere. Geoelectromagnetism is a more comprehensive 
term than geoelectricity. Time variations of any magnetic field are associated 
with an electric field that induces electric currents in conducting media such 
as the Earth. 
    Magnetic fields, electric fields, and electric currents are the 
constituents of electromagnetism, and are related by Maxwell's equations. For 
instance, the illustration shows the time variations of the natural magnetic 
and electric fields simultaneously measured at one location at the surface of 
the Earth. These two traces are related to each other, not only by Maxwell's 
equations but also by the physical properties of the subsurface rocks in the 
vicinity of the measuring site. Either one of the two traces may be computed 
synthetically from the other if the properties of the subsurface rocks are 
known. Conversely, the two traces together can yield geologic information; 
this is a form of geophysical exploration or prospecting. Thus, the terms 
geoelectricity, geomagnetism, and geoelectromagnetism are essentially 
interchangeable, although each one may have a somewhat different emphasis. For 
example, the term geomagnetism is sometimes used for the study of the Earth's 
quasistationary main magnetic field. See GEOMAGNETISM; GEOPHYSICAL EXPLORATION. 
 �34s[Sulhi H. Yungul]
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�6v�2v�32sGeography   �21m �33s 
    The study of the Earth's landscapes and their change with time. Landscapes 
may include ever-changing natural physical or biological systems such as 
climate, landforms, or vegetation (physical geography), or they may include 
ever-changing human designs such as cities, road systems, or barn types (human 
geography). Geography is, therefore, a very broad science that deals with 
subject matter of a spatial nature that changes through time. As such, its 
boundaries often blur and overlap with other sciences. See PHYSICAL GEOGRAPHY. 
    Because the boundaries of geography blur and overlap with other sciences, 
geography is viewed by many as an interdisciplinary science. Thus geographers 
often find close allies in cognate sciences such as geology, economics, 
political science, and meteorology, to mention but a few. See BIOGEOGRAPHY; 
CLIMATOLOGY; GEOMORPHOLOGY; HYDROLOGY; MATHEMATICAL GEOGRAPHY; PEDOLOGY. 
    The word geography immediately seems to bring to mind maps and mapmaking. 
However, maps are simply useful tools of geographers, just as they are useful 
tools of, say, geologists and politicians. Other useful tools of geographers 
are air photos and other remotely sensed imagery, and statistics. In fact, one 
of the more recent important trends in geography is the increasing use of 
statistics and remotely sensed imagery (radar, photography, impulses) from 
high-flying aircraft and spacecraft. See AERIAL PHOTOGRAPH; CARTOGRAPHY; 
REMOTE SENSING. 
 �34s[Donald Lee Johnson]
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�6v�2v�32sGeological time scale   �21m �33s 
    The assumed absolute time intervals occupied by the various geological 
periods. The evolution of plants and animals and the sequence of depositional 
and orogenic events provide the data of Earth history. In order to relate 
these events to absolute time and therefore establish a geological time scale, 
an independent means of dating rocks is required. Before the discovery of 
radioactivity, crude estimates of the length of geologic history were made 
based on the total thicknesses of sedimentary rock and assumed rates of 
erosion and sedimentation. These estimates varied by as much as a factor of 10. 
    The modern geologic time scale is based on many measurements of various 
rock types by the three quantitative isotopic chronometers (uranium-lead, 
U-Pb; rubidium-strontium, Rb-Sr; and potassium-argon, K-Ar). Unfortunately it 
is not yet possible to measure directly the absolute age of a sedimentary 
rock. A modern time scale is shown in the table. See ROCK AGE DETERMINATION. 
 �34s[J. Laurence Kulp]
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�6v�2v�32sGeologic thermometry   �21m �33s 
    The measurement or estimation of temperatures at which geologic processes 
take place. Methods used can be divided into two groups, nonisotopic and 
isotopic. Nonisotopic methods involve measurements of earth temperatures 
either directly by surface and near-surface features or indirectly from 
various properties of minerals and fossils. The isotopic methods involve the 
determination of distribution of isotopes of the lighter elements between 
pairs of compounds in equilibrium at various temperatures, and application of 
these data to problems of the temperature at which these compounds (commonly 
minerals) form in nature. 
 �34s[Earl Ingerson]
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�6v�2v�32sGeology �21m �33s
    The study or science of the Earth. Geologists are concerned primarily with
rocks that make up the outer part of the Earth. Rocks are chemical systems
adjusting to the physical environment in attempts to maintain thermodynamic
equilibrium. Many rocks represent the consequences of processes occurring deep
within the Earth, and modern geology is firmly rooted in the geophysics and
geochemistry of the whole Earth. The study of geology involves the complex
interrelationships between the solid Earth and its fluid envelopes of water
and air, which are modified by the action of the living biosphere and
modulated by energy received from and lost to space. Geology itself is an
interdisciplinary subject, and it overlaps other disciplines. Study and
mapping of surface forms and rocks are shared by geology and geodesy. The
study of the Earth's waters as related to geologic processes is shared by
hydrology, oceanography, and meteorology. Paleontology, the study of records
left in rocks by animals and plants that lived in past ages, is an essential
part of geology, though it involves many fundamental aspects of biologic
science. See GEODESY; HYDROLOGY; METEOROLOGY; OCEANOGRAPHY.
    The geological sciences have classically been subdivided into physical
geology and historical geology, each with many branches. Physical geology is
concerned with understanding the composition of the Earth and the physical
changes occurring on it, which are derived from a study of the Earth as a
whole, and of minerals and rocks, their structures and formations, and their
processes of origin and alteration. Historical geology is based on two
complementary disciplines, stratigraphy and paleontology. Paleontology is the
study of fossilized plants and animals, with regard to their distribution in
time. The stratigraphic record shows a succession of events through eons of
time, with absolute dates supplied by geochemical studies of radiometric
isotopes. Geophysical studies of the magnetic properties of rocks have become
increasingly important in dating stratigraphic units. See PALEONTOLOGY; ROCK
MAGNETISM; STRATIGRAPHY.
    Mineralogy concerns the study of minerals, the basic units in the
composition of most rocks. Although many hundreds of mineral species are
known, comparatively few are important in kinds of rocks that are abundant in
the visible part of the Earth crust. See MINERAL; MINERALOGY.
    Petrology involves the study of rocks, their physical and chemical
properties, and their modes of origin. Known rocks are divisible into three
groups: igneous rocks, which have solidified from molten matter (magma);
sedimentary rocks, made of fragments derived from preexisting rocks, of
chemical precipitates, or of organic products; and metamorphic rocks derived
from igneous or sedimentary rocks under conditions that brought about changes
in mineral composition, texture, and internal structure. See IGNEOUS ROCKS;
METAMORPHIC ROCKS; PETROGRAPHY; PETROLOGY; ROCK; SEDIMENTARY ROCKS.
    An understanding of weathering, erosion, and sedimention is all important
in geology. Weathering prepares the way for removal of rock materials and
reshaping of land surfaces by several agents of erosion. The most obvious of
these agents is running water. Terrestrial sediments, widespread and highly
varied, are laid down chiefly through the agencies of mass wasting, running
water, glacier ice, and wind. The deposits are in large part temporary, as the
tendency is for them to shift seaward in continued erosion of the lands.
Marine sediments, deposited beneath oceans, comprise material derived from the
land, from shells and skeletons of marine animals and plants, from the ocean
by chemical precipitation, and from space (particles of meteorites). See
EROSION; MARINE SEDIMENTS; MASS WASTING; SEDIMENTATION (GEOLOGY); WEATHERING
PROCESSES.
    A geologic map represents the lithology and, so far as possible, the
geologic age of every important geologic unit in a given area. Each
distinctive unit that can be shown effectively to the scale of the map is a
geologic formation. A good topographic base map is essential for representing
relations of bedrock to land surface forms. See AERIAL PHOTOGRAPH; ECONOMIC
GEOLOGY; REMOTE SENSING; STRUCTURAL GEOLOGY; TOPOGRAPHIC SURVEYING AND MAPPING.
 �34s[Chester R. Longwell ;Peter J. Wyllie]
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�6v�2v�32sGeomagnetism   �21m �33s 
    A term signifying both the magnetism of the Earth and the branch of 
science that deals with the Earth's magnetism. The term geomagnetism is now 
given some preference over the older term terrestrial magnetism. 
    The main geomagnetic field is specified at any point O by its vector 
magnetic intensity F. Its direction is that of the line P'OP from the negative 
end P' to the positive end P of a magnetized needle P'P, perfectly balanced 
before it is magnetized, and freely pivoted about O, when in equilibrium. The 
positive pole P is the one that at most places on the Earth takes the more 
northerly position. Over most of the Northern Hemisphere, P is below O; the 
needle is said to dip below the horizontal by an angle I, called the magnetic 
inclination. Over about half the Earth, however, P will be above O; the 
inclination I (or magnetic dip) is then reckoned as negative. This is the case 
over most of the Southern Hemisphere. The value of I thus ranges from 90 to 
-90DG. A point where I = PLUS/MINUS90DG is called a magnetic pole of the Earth. 
    There are two main magnetic poles; their approximate positions are 
76.1DGN, 100DGW and 65.8DGS, 139DGE. In a few places, near strongly magnetized 
mineral deposits, there may be local magnetic poles. 
    The distribution of I over the Earth's surface can be indicated on a 
globe, or on a map (with any kind of projection), by lines called isoclinic 
lines or isoclines, along each of which I has the same value. The isocline for 
which I = 0 (where the balanced magnetized needle rests horizontal) is called 
the magnetic or dip equator. 
    A compass needle is magnetized and pivoted at a slightly noncentral point 
so as to rest and move in the horizontal plane. The deviation of its direction 
from geographic (gg) north is called the magnetic declination D. This is 
reckoned positive to the east and negative to the west; alternatively, 
declinations can be specified by a positive (eastward) value ranging up to 
360DG.  
    Over the greater part of the Earth D is numerically less than 90DG, but 
along any small circuit around the magnetic poles it takes all values. At the 
magnetic poles themselves the compass needle takes no definite direction. At 
the gg poles the needle takes a definite direction, but as the northward 
direction changes through a whole revolution along any small circuit around 
the pole, D likewise takes all values around the circuit. 
    The distribution of the declination over the Earth's surface can be 
indicated by lines along each of which D is constant. These are called 
isogonic lines, or isogones. The isogones for which D = 0 are called agonic 
lines. Along these lines, the compass points to true north. 
    The distribution of D over the Earth can also be indicated by magnetic 
meridians, which at each point P have the direction of the compass needle at 
P. These lines extend from the south magnetic pole to the north and have no 
complication (as do the isogones) at the gg poles. They indicate, more clearly 
than the isogones, the general distribution of the compass direction over the 
Earth. But the isogonic map is more convenient in enabling the compass 
direction at any point to be read or estimated without using an angle 
measurer. Hence it is the one used by navigators and travelers on sea and land 
and in the air. See MAGNETIC COMPASS. 
 �34s[Sydney Chapman]
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�6v�2v�32sGeometrical optics   �21m �33s 
    The geometry of light rays and their imagery through optical systems. The 
phenomena of diffraction due to the finite apertures of the lens systems are 
neglected in geometrical optics. See DIFFRACTION. 
    Light moves in straight lines through homogeneous media and changes its 
direction at the surface separating two such media, for instance, air and 
glass. An incident ray at the bounding surface is divided into two; one is 
reflected back into the first medium and the other penetrates the second 
medium after being bent or refracted. The incident, reflected, and refracted 
rays all lie in one plane containing the surface normal and form angles i, 
i sub r, and i' with the surface normal such that Eqs. (1) and (2) hold where n 

                            *** i sub r = PI - i ***                     (1)

                          *** n sin i = n' sin i' ***                    (2)

and n' are the refractive indices of the media separated by the refracting 
surface. In order to obtain a solution of Eq. (2), i and i' must be chosen so 
that they are in the same quadrant. See LIGHT; REFLECTION OF ELECTROMAGNETIC 
RADIATION; REFRACTION OF WAVES. 
    These formulas together with pure geometry make it possible to trace a ray 
through a system of lenses. The specific form of the ray-tracing formula 
should be adapted to the tools of the lens designer; that is, it will be 
different for a person using logarithm tables and for one using an electric 
desk machine or an electronic computer. 
    The aim of the optical designer is to see what happens to all the rays 
coming from every point of the object to be imaged. Moreover, the designer (1) 
wants to direct the rays so that all of them coming from a fixed object point 
are collected at a fixed image point (freedom from aberration); (2) wants all 
these image points to lie on a plane (freedom from field curvature); and (3) 
wants the image to be similar in shape to the object (freedom from 
distortion). Finally, correction should be achieved for light of different 
wavelengths (freedom from chromatic aberrations). See ABERRATION (OPTICS); 
CHROMATIC ABERRATION. 
 �34s[Max J. Herzberger]
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�6v�2v�32sGeometry   �21m �33s 
    A branch of mathematics concerned with the properties of space, including 
points, lines, curves, planes and surfaces in space, and figures bounded by 
them. For discussion of various branches of geometry see ALGEBRAIC GEOMETRY; 
DIFFERENTIAL GEOMETRY; EUCLIDEAN GEOMETRY; NONEUCLIDEAN GEOMETRY; PROJECTIVE 
GEOMETRY; RIEMANNIAN GEOMETRY. 
 �34s[James S. Frame]
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�6v�2v�32sGeomorphology   �21m �33s 
    The study of landforms, including the description, classification, origin, 
development, and history of planetary surface features. Emphasis is placed on 
the genetic interpretation of the erosional and depositional features of the 
Earth's surface. However, geomorphologists also study primary relief elements 
formed by movements of the Earth's crust, topography on the sea floor and on 
other planets, and applications of geomorphic information to problems in 
environmental engineering. 
    Geomorphologists analyze the landscape. Their purview includes the 
structural framework of landscape, weathering and soils, mass movement and 
hillslopes, fluvial features, eolian features, glacial and periglacial 
phenomena, coastlines, and karst landscapes. Processes and landforms are 
analyzed for their adjustment through time, especially the most recent 
portions of Earth history. Geomorphologists consider processes from the 
perspectives of pedology, soil mechanics, sedimentology, geochemistry, 
hydrology, fluid mechanics, remote sensing, and other sciences. The complexity 
of geomorphic processes has required this interdisciplinary approach, but it 
has also led to a theoretical vacuum in the science. At present many 
geomorphologists are organizing their studies through a form of systems 
analysis. The landscape is conceived of as a series of elements linked by 
flows of mass and energy. Process studies measure the inputs, outputs, and 
transfers for these systems. Systems analysis provides an organizational 
framework within which geomorphologists are developing models to predict 
selected phenomena. 
 �34s[Victor R. Baker]
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�6v�2v�32sGeophysical exploration �21m �33s
    Making, processing, and interpreting measurements of the physical
properties of the Earth with the objective of practical application of the
findings. Most exploration geophysics is conducted to find commercial
accumulations of oil, gas, or other minerals, but geophysical investigations
are also employed with engineering objectives, in studies aimed at predicting
the nature of the Earth for the foundations of roads, buildings, dams,
tunnels, nuclear power plants, and other structures, and in the search for
geothermal areas, water resources, archeological ruins, and so on.
    Geophysical exploration is also called applied geophysics or geophysical
prospecting. The physical properties and effects of subsurface rocks and
minerals that can be measured at a distance include density, electrical
conductivity, thermal conductivity, magnetism, radioactivity, elasticity, and
other properties. Exploration geophysics is often divided into subsidiary
fields according to the property being measured, such as magnetic, gravity,
seismic, electrical, thermal, or radioactive properties.

    Magnetic exploration.  Rocks and ores containing magnetic minerals become
magnetized by induction in the Earth's magnetic field so that their induced
field adds to the Earth's field. Magnetic exploration involves mapping
variations in the magnetic field to determine the location, size, and shape of
such bodies. The magnetic susceptibility of sedimentary rock is generally
orders of magnitude less than that of igneous or metamorphic rock.
Consequently, the major magnetic anomalies observed in surveys of sedimentary
basins usually result from the underlying basement rocks. Determining the
depths of the tops of magnetic bodies is thus a way of estimating the
thickness of the sediments. See GEOMAGNETISM; MAGNETOMETER; ROCK MAGNETISM.
    Except for magnetite and a very few other minerals, mineral ores are only
slightly magnetic. However, they are often associated with bodies such as
dikes that have magnetic expression so that magnetic anomalies may be
associated with minerals empirically. For example, placer gold is often
concentrated in stream channels where magnetite is also concentrated.

    Gravity exploration.  Gravity exploration is based on the law of universal
gravitation: the gravitational force between two bodies varies in direct
proportion to the product of their masses and in inverse proportion to the
square of the distance between them. Because the Earth's density varies from
one location to another, the force of gravity varies from place to place.
Gravity exploration is concerned with measuring these variations to deduce
something about rock masses in the immediate vicinity. Gravity surveys are
used more extensively for petroleum exploration than for metallic mineral
prospecting. The size of ore bodies is generally small; therefore, the gravity
effects are quite small and local despite the fact that there may be large
density differences between the ore and its surroundings. See GRAVITY METER;
PROSPECTING.

    Seismic exploration.  Seismic exploration is the predominant geophysical
activity. Seismic waves are generated by one of several types of energy
sources and detected by arrays of sensitive devices called geophones or
hydrophones. The most common measurement made is of the travel times of
seismic waves, although attention is being directed increasingly to the
amplitude of seismic waves or changes in their frequency content or wave
shape. See SEISMOLOGY.

    Electrical and electromagnetic exploration.  Variations in the
conductivity or capacitance of rocks form the basis of a variety of electrical
and electromagnetic exploration methods, which are used primarily in metallic
mineral prospecting. Both natural and induced electrical currents are
measured. Direct currents and low-frequency alternating currents are measured
in ground surveys, and ground and airborne electromagnetic surveys involving
the lower radio frequencies are made. See GEOELECTRICITY.

    Radioactivity exploration.	Natural radiation from the Earth, especially
of gamma rays, is measured both in land surveys and airborne surveys. Natural
types of radiation are usually absorbed by a few feet of soil cover, so that
the observation is often of diffuse equilibrium radiation. The principal
radioactive elements are uranium, thorium, and potassium; radioactive
exploration has been used primarily in the search for uranium and other ores,
such as columbium, which are often associated with them. The Geiger counter
and scintillation counter are instruments generally used to detect and measure
the radiation. See GEIGER-MUELLER COUNTER: SCINTILLATION COUNTER.

    Remote sensing.  Measurements of natural and induced electromagnetic
radiation made from high-flying aircraft and earth satellites are referred to
collectively as remote sensing. This comprises both the observation of natural
radiation in various spectral bands, including both visible and infrared
radiation, such as by photography and measurements of the reflectivity of
infrared and radar radiation. See REMOTE SENSING.

    Well logging.  A variety of types of geophysical measurements are made in
boreholes, including self-potential, electrical conductivity, velocity of
seismic waves, natural and induced radioactivity, and temperature variations.
Borehole logging is used extensively in petroleum exploration to determine the
characteristics of the rocks which the borehole has penetrated, and to a
lesser extent in mineral exploration. See DRILL HOLE LOGGING ; WELL LOGGING.
 �34s[Robert E. Sheriff]
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�6v�2v�32sGeophysics   �21m �33s 
    Those branches of earth sciences in which the principles and practices of 
physics are used to study the Earth. Geophysics is considered by some to be a 
branch of geology, by others to be of equal rank. It is distinguished from the 
other earth sciences largely by its use of instruments to make direct or 
indirect measurements of the parts of the Earth being studied, in contrast to 
the more direct observations which are typical of geology. See GEOLOGY. 
    Geophysics consists of several principal fields plus parallel and 
subsidiary divisions. These are commonly considered to include plutology, with 
geodesy, geothermometry, seismology, and tectonophysics as subdivisions; 
hydrospheric studies, hydrology (groundwater studies), oceanography, and 
glaciology; atmospheric studies, meteorology, and aeronomy; and several fields 
of geophysics which overlap one another, including geomagnetism and 
geoelectricity, geochronology, geocosmogony, and geophysical exploration and 
prospecting. Planetary sciences, the study of the planets and satellites aside 
from the Earth, are usually considered a branch of geophysics because the 
techniques used have been, until the first landings on each, entirely 
instrumental rather than directly observational. See GEOCHRONOMETRY; GEODESY; 
GEOELECTRICITY; GEOLOGIC THERMOMETRY; GEOMAGNETISM; GLACIOLOGY; HYDROLOGY; 
METEOROLOGY; OCEANOGRAPHY; SEISMOLOGY; TECTONOPHYSICS. 
 �34s[Benjamin F. Howell, Jr.]
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�6v�2v�32sGeostrophic wind   �21m �33s 
    A hypothetical wind based upon the assumption that a perfect balance 
exists between the horizontal components of the Coriolis force and the 
horizontal pressure gradient force per unit mass, with the implication that 
viscous forces and accelerations are negligible. The geostrophic wind blows 
parallel to the isobars (lines of equal pressure) with lower pressure to the 
left of the direction of the wind in the Northern Hemisphere and to the right 
in the Southern Hemisphere. It represents a good approximation to the actual 
wind at elevations greater than about 3000 ft (900 m), except in instances of 
strongly curved flow and in the vicinity of the Equator. 
    The term thermal wind denotes the net change in the geostrophic wind over 
some specific vertical distance. This change arises because the rate of change 
of pressure in the vertical is different in two air columns of different air 
density, so that the horizontal component of the pressure gradient force per 
unit mass varies in the vertical. The thermal wind is directed approximately 
parallel to the isotherms of air temperature with cold air to the left and 
warm air to the right in the Northern Hemisphere, and vice versa in the 
Southern Hemisphere. Thus, for example, the increasing predominance of 
westerly winds aloft may be viewed as a consequence of the warmth of tropical 
latitudes and the coldness of polar regions. See CORIOLIS ACCELERATION AND 
FORCE; GRADIENT WIND; WIND; WIND STRESS. 
 �34s[Frederick Sanders]
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�6v�2v�32sGeosyncline   �21m �33s 
    A linear part of the crust of the Earth that sagged deeply through time, 
that is, a great trough hundreds of kilometers long and tens of kilometers 
wide that subsided as it received thousands of meters of sedimentary and 
volcanic rocks through millions of years. Thicknesses are roughly 10 times 
greater than synchronous strata in adjacent stable regions. Linear 
geosynclinal belts were great subsiding tectonic divisions of the crust that 
lay between more stable parts of continents (cratons) or the deep abyssal 
ocean basins of their time, or both. Geosynclines generally contain volcanic 
(eugeosynclinal) and nonvolcanic (miogeosynclinal) zones (see illustration). 
Although defined on the basis of thickness and types of rocks, the geosyncline 
has always been so closely linked with the origin of mountains that it has 
little meaning in any other context. See CORDILLERAN BELT; OROGENY; SYNCLINE. 
 �34s[Robert H. Dott, Jr.]
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�6v�2v�32sGeothermal power   �21m �33s 
    Thermal or electrical power produced from the thermal energy contained in 
the Earth (geothermal energy). Use of geothermal energy is based 
thermodynamically on the temperature difference between a mass of subsurface 
rock and water and a mass of water or air at the Earth's surface. This 
temperature difference allows production of thermal energy that can be either 
used directly or converted to mechanical or electrical energy. 
    There are several types of natural geothermal reservoirs. All the 
reservoirs developed to date for electrical energy are termed hydrothermal 
convection systems and are characterized by circulation of meteoric (surface) 
water to depth. The driving force of the convection systems is gravity, 
effective because of the density difference between cold, downward-moving, 
recharge water and heated, upward-moving, thermal water. Depending on the 
physical state of the pore fluid, there are two kinds of hydrothermal 
convection systems: liquid-dominated, in which all the pores and fractures are 
filled with liquid water that exists at temperatures well above boiling at 
atmospheric pressure, owing to the pressure of overlying water; and 
vapor-dominated, in which the larger pores and fractures are filled with 
steam. Liquid-dominated reservoirs produce either water or a mixture of water 
and steam, whereas vapor-dominated reservoirs produce only steam, in most 
cases superheated. 
    Natural geothermal reservoirs also occur as regional aquifers. In some 
rapidly subsiding young sedimentary basins, porous reservoir sandstones are 
compartmentalized by growth faults into individual reservoirs that can have 
fluid pressures exceeding that of a column of water and approaching that of 
the overlying rock. The pore water is prevented from escaping by the 
impermeable shale that surrounds the compartmented sandstone. The energy in 
these geopressured reservoirs consists not only of thermal energy, but also of 
an equal amount of energy from methane dissolved in the waters plus a small 
amount of mechanical energy due to the high fluid pressures. See AQUIFER; 
GROUNDWATER HYDROLOGY. 
    The most conspicuous use of geothermal energy is the generation of 
electricity. Hot water from a liquid-dominated reservoir is flashed partly to 
steam at the Earth's surface, and this steam is used to drive a conventional 
turbine-generator set. Equally important worldwide is the direct use of 
geothermal energy, often at reservoir temperatures less than 212DGF (100DGC). 
Geothermal energy is used directly in a number of ways: to heat buildings 
(individual houses, apartment complexes, and even whole communities); to cool 
buildings (using lithium bromide absorption units); to heat greenhouses and 
soil; and to provide hot or warm water for domestic use, for product 
processing (for example, the production of paper), for the culture of 
shellfish and fish, for swimming pools, and for therapeutic (healing) purposes. 
 �34s[L. J. Patrick Muffler]
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�6v�2v�32sGeraniales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 5 families and about 2200 species; several other families have 
often been included in other systems of classification. The Geraniales are 
herbs or soft shrubs with a superior ovary and generally either with compound 
or deeply cleft leaves (see illustration) or with strongly irregular flowers 
in which one of the sepals has a conspicuous free spur. Wood sorrel (Oxalis, 
family Oxalidaceae), geranium (Geranium and Pelargonium, family Geraniaceae), 
garden nasturtium (Tropaeolum, family Tropaeolaceae), and touch-me-not 
(Impatiens, family Balsaminaceae) are familiar members of the Geraniales. See 
MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sGerbil   �21m �33s 
    The common name for 106 species of rodents, known as the sand rats, 
composing the subfamily Gerbillinae in the family Muridae. They inhabit the 
more arid regions of Africa and Asia. All species have light, soft fur and 
hindlegs that are longer than the front ones, enabling them to hop kangaroo 
fashion, although most frequently they run like typical rodents. These animals 
are active nocturnally and remain in their burrows during the heat of the day. 
In captivity they are gregarious and make good pets. See RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sGermanium   �21m �33s 
    A brittle, silvery-gray, metallic chemical element, Ge, atomic number 32, 
atomic weight 72.59, melting point 937.4DGC (1719DGF), and boiling point 
2830DGC (5130DGF), with properties between silicon and tin. Germanium is 
distributed widely in the Earth's crust in an abundance of 6.7 parts per 
million (ppm). Germanium is found as the sulfide or is associated with sulfide 
ores of other elements, particularly those of copper, zinc, lead, tin, and 
antimony.  
    Germanium has a metallic appearance but exhibits the physical and chemical 
properties of a metal only under special conditions since it is located in the 
periodic table where the transition from nonmetal to metal occurs. At room 
temperature there is little indication of plastic flow and consequently it 
behaves like a brittle material. 
    As it exists in compounds, germanium is either divalent or tetravalent. 
The divalent compounds (oxide, sulfide, and all four halides) are easily 
reduced or oxidized. The tetravalent compounds are more stable. 
Organogermanium compounds are many in number and, in this respect, germanium 
resembles silicon. Interest in organogermanium compounds has centered around 
their biological action. Germanium in its derivatives appears to have a lower 
mammalian toxicity than tin or lead compounds. 
    The properties of germanium are such that there are several important 
applications for this element, especially in the semiconductor industry. The 
first solid-state device, the transistor, was made of germanium. 
Single-crystal germanium is used as a substrate for vapor-phase growth of GaAs 
and GaAsP thin films in some light-emitting diodes. Germanium lenses and 
filters are used in instruments operating in the infrared region of the 
spectrum. Mercury-doped and copper-doped germanium are used as infrared 
detectors; synthetic garnets with magnetic properties may have applications 
for high-power microwave devices and magnetic bubble memories; and germanium 
additives increase usable ampere-hours in storage batteries. 
 �34s[Paul S. Gleim]
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�6v�2v�32sGermfree vertebrate   �21m �33s 
    Those gnotobiotic (known biota) animals which have no living 
microorganisms intimately associated with them. When a specific microorganism 
is introduced into a germfree vertebrate, interrelationships between macro- 
and microorganisms may be studied; inoculation of more than one species of 
microorganisms allows microecology studies. 
    Germfree animals may be obtained by transfer of the germfree embryo from 
the sterile environment of the womb or egg into a sterile isolator. A 
comparable transfer occurs by placing germicide-treated eggs from healthy 
stock into a germfree isolator; at hatch the young transfers itself from the 
sterile interior of the egg into the sterile isolator.  
    Germfree animals are used for the formation of disease-free stock. 
Germfree techniques are also used for handling highly contagious 
microorganisms. In this case, since the air pressure inside the cage is 
somewhat less than that outside, solid cages have an advantage over soft 
plastic isolators. Sterile surgery is done through plastic isolators to 
prevent outside contamination of the wounded area. Adaptation of germfree 
isolators is being considered for hospital use for highly contagious diseases 
and severe burn conditions. 
    Inoculation of germfree vertebrates with known species followed by 
examination to show that only those species inoculated are present provides 
important concepts in gnotobiology. Germfree vertebrates have been inoculated 
with viruses, bacteria, protozoans, or yeast to study the effect of specific 
microorganisms upon the host. See GNOTOBIOLOGY; PURE CULTURE. 
 �34s[Thomas D. Luckey]
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�6v�2v�32sGerm layers   �21m �33s 
    The primitive cell layers or first tissues, which appear early in the 
development of animals, and from which the embryo body and its auxiliary 
membranes, when present, are constructed. The germ layers are more or less 
distinct anatomically, but do not necessarily have sharp boundaries of 
demarcation. Germ layers are almost universal among animal embryos. 
    Three kinds of germ layers are recognizable: (1) the ectoderm or outer 
skin, (2) the endoderm or inner skin, and (3) the mesoderm or middle skin. The 
layers have thus been named in accordance with their definitive positions in 
the spherical type of gastrula such as that of the sea urchin or amphibian. 
The terms epiblast, mesoblast, and hypoblast are sometimes used as synonyms 
for ectoderm, mesoderm, and endoderm, respectively. The three primary germ 
layers are present as a basic structural plan in all Metazoa except the 
coelenterates and the Porifera, in which a distinct mesodermal layer is 
absent. See GASTRULATION. 
 �34s[Nelson T. Spratt, Jr.]
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�6v�2v�32sGerontology �21m �33s
    The study of the inevitable structural and functional changes that occur
with time in the cells and tissues of the body (aging), resulting in a
decreased ability to adapt to or overcome environmental changes (diseases of
the aging). The inevitable outcome is death. A distinction between the
intrinsic features of aging in contrast to the diseases or disorders
associated with aging is an essential factor. Improved medical and public
health care in the last century has led to a diminution of deaths at an early
age caused by infectious diseases. More people live on to grow old; however,
in spite of these medical advances, vital statistics and other biologic
evidence indicate that there has been no lengthening of the human life-span
(longevity) during recent decades. Medical and sociological advances have
allowed more people to reach the upper limits of the human life-span.
    There are several characteristics of the aging process that apply to all
multicellular living things: Aging is universal, occurring in all members of
each species. It is intrinsic, being an inherent property of living organisms.
It is progressive, that is, a continuous rather than a sudden event. The
changes associated with aging are deleterious to the survability of the
individual and are associated with a declining functional capacity. See DEATH.
 �34s[N. Karle Mottet]
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�6v�2v�32sGERT   �21m �33s 
    A procedure for the formulation and evaluation of systems using a network 
approach. Problem solving with the GERT (graphical evaluation and review 
technique) procedure utilizes the following steps: 
    1. Convert a qualitative description of a system or problem to a 
generalized network similar to the critical path method PERT type of network. 
    2. Collect the data necessary to describe the functions ascribed to the 
branches of a network. 
    3. Combine the branch functions (the network components) into an 
equivalent function or functions which describe the network. 
    4. Convert the equivalent function or functions into performance measures 
for studying the system or solving the problem for which the network was 
created. These might include either the average or variance of the time or 
cost to complete the network. 
    5. Make inferences based on the performance measures developed in step 4. 
    Both analytic and simulation approaches have been used to perform step 4 
of the procedure. GERTE was developed to analytically evaluate network models 
of linear systems through an adaptation of signal flow-graph theory. For 
nonlinear systems, involving complex logic and queuing situations, Q-GERT was 
developed. In Q-GERT, a simulation of the network is performed in order to 
obtain statistical estimates of the performance measures of interest. 
    GERT networks have been designed, developed, and used to analyze the 
following situations: claims processing in an insurance company, production 
lines, quality control in manufacturing systems, assessment of job performance 
aids, burglary resistance of buildings, capacity of air terminal cargo 
facilities, judicial court system operation, equipment allocation in 
construction planning, refueling of military airlift forces, planning and 
control of marketing research, planning for contract negotiations, risk 
analysis in pipeline construction, effects of funding and administrative 
strategies on nuclear fusion power plant development, research and development 
planning, and system reliability. See CRITICAL PATH METHOD (CPM); DECISION 
THEORY; PERT; SIMULATION. 
 �34s[A. Alan B. Pritsker]
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�6v�2v�32sGestation period   �21m �33s 
    That period in mammals from fertilization to birth. In mammals other than 
humans the gestation period may vary, since it depends on such factors as 
maternal care, metabolic rates, environmental conditions, and types of 
placentation (see table). In humans the gestation period lasts 270 - 295 days; 
for clinical purposes the period is divided into three trimesters. 
    For humans, loss of the embryo or young fetus prior to the stages of 
viability is termed abortion; later loss is termed premature delivery until 
the fetus weighs more than 88 oz (2500 g), the lower limit of normal weight of 
the new born. See FERTILIZATION; PREGNANCY; PREGNANCY DISORDERS. 
 �34s[Sybil P. Parker]
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�6v�2v�32sGeyser �21m �33s
    A natural spring or fountain which discharges a column of water or steam
into the air at more or less regular intervals. Perhaps the best-known area of
geysers is in Yellowstone Park, Wyoming, where there are more than 100 active
geysers and more than 3000 noneruptive hot springs.
    The eruptive action of geysers is believed to result from the existence of
very hot rock not far below the surface. The neck of the geyser is usually an
irregularly shaped tube partly filled with water which has seeped in from the
surrounding rock. Far down the pipe the water is at a temperature much above
the boiling point at the surface, because of the pressure of the column of
water above it. Its temperature is constantly increasing, because of the
volcanic heat source below. Eventually the superheated water changes into
steam, lifting the column of water out of the hole.
 �34s[Albert N. Sayre /Ray K. Linsley]
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�6v�2v�32sGiant nuclear resonances �21m �33s
    Systematic excitations of the atomic nucleus which occur with great
strength in a concentrated energy region. When high-energy electromagnetic
radiation (gamma radiation) impinges on a nucleus, it can be strongly absorbed
into a number of high-lying resonances: one in which the nucleus is excited in
a dipole mode of oscillation that is electric in nature (E1), a second dipole
mode that is magnetic in nature (M1), and a third excitation that can be
identified as an electric quadrupole oscillation (E2). When excited into these
resonances, the nucleus can deexcite by emitting gamma radiation or, if
energetically allowed, by emitting particles, especially protons and neutrons.
If proton and neutron emissions are allowed, it is possible to study these
resonances by the inverse process in which the proton or the neutron is
captured in the giant resonance and gamma radiation is emitted. These giant
resonances can also be excited by inelastic scattering of electrons and other
particles such as protons, deuterons, (sup 3)He, and ALPHA-particles. For
inelastic excitation with particles, greater momentum can be imparted to the
nucleus, and higher modes such as octupole oscillations can be excited. The
particles can also excite the electric monopole vibration (E0), which
corresponds to a ``breathing'' mode whose energy determines the
incompressibility of a nucleus, a property that is basic to understanding the
force between neutrons and protons. See GAMMA RAYS; MULTIPOLE RADIATION;
NUCLEAR MOMENTS; NUCLEAR REACTION; NUCLEAR SPECTRA; NUCLEAR STRUCTURE.
 �34s[Stanley S. Hanna]
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�6v�2v�32sGiant star   �21m �33s 
    A member of one of the families into which stars are divided in the 
Hertzsprung-Russell diagram; the term giant refers to true brightness rather 
than to size. Although giant stars are frequent among the brighter, naked-eye 
stars, such as Arcturus and Capella, they are comparatively rare in space. 
They have luminosities approximately 40 - 200 times that of the Sun. The 
common red giant stars have radii about 10 times that of the Sun, and masses 
from 1.5 to 4 times that of the Sun; they represent a comparatively late stage 
in stellar evolution. See HERTZSPRUNG-RUSSELL DIAGRAM; STAR; STELLAR EVOLUTION. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sGiardiasis   �21m �33s 
    A disease caused by the protozoan parasite Giardia lamblia, characterized 
by chronic diarrhea that usually lasts 1 or more weeks. The diarrhea may be 
accompanied by one or more of the following: abdominal cramps, bloating, 
flatulence, fatigue, or weight loss. The stools are malodorous and have a pale 
greasy appearance. Infection without symptoms is also common. 
    Giardiasis occurs worldwide. In community epidemics caused by contaminated 
drinking water, as many as 50 to 70% of the residents have become infected. 
Outbreaks also occur among backpackers and campers who drink untreated stream 
water. Both human and animal (beaver) fecal contamination of stream water has 
been implicated as the source of Giardia cysts in waterborne outbreaks. 
Giardia species in dogs and possibly other animals are also considered 
infectious for humans. Epidemics resulting from person-to-erson transmission 
occur in day-care centers for preschool-age children and institutions for the 
mentally retarded. Infants and toddlers in day-care centers are more commonly 
infected than older children who have been toilet-trained. See EPIDEMIOLOGY. 
 �34s[Dennis D. Juranek]
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�6v�2v�32sGibberellin   �21m �33s 
    Any member of a family of compounds found in fungi and plants, some being 
plant hormones that usually have a broad spectrum of growth-promoting activity. 
    The gibberellins (GAs) are cyclic diterpenes possessing 19 or 20 carbon 
atoms. Fifty-seven gibberellins have been identified, and the trivial system 
of nomenclature identifies them as GA1 - GA57 inclusively. 
    The gibberellins are of widespread occurrence, being found in bacteria, 
fungi, algae, mosses, ferns, gymnosperms, and angiosperms. Although they seem 
to be ubiquitous, the gibberellins are generally found in very small amounts. 
Exceptions to this rule include the fungus Gibberella fujikuroi and immature 
seeds of some plants, which are rich sources of gibberellins. 
    The gibberellins play an important role in the integration of growth and 
development of plants. It is only in the flowering plants, however, that their 
function as hormonal agents has been conclusively established. This function 
is inferred for the lower vascular plants and algae and bryophytes because 
application of gibberellin to these plants influences aspects of their growth 
and differentiation. See PLANT HORMONES. 
 �34s[Russell L. Jones]
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�6v�2v�32sGibbon   �21m �33s 
    A medium to very large animal which lacks a tail and is the least 
anthropoid of the order Primates and smallest of the apes. The seven species 
are included in the single genus Hylobates and have a general distribution 
throughout Southeast Asia. 
    Gibbons live in family groups which normally include an adult male and 
female with the young. Like many animals, each family has a territory which it 
defends. The gibbons live almost entirely in the trees. They are vegetarians 
and usually eat fruits and nuts but will on occasion eat bird eggs and small 
vertebrates. The young are mature at the age of 6 years. See PRIMATES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sGibbs function   �21m �33s 
    The Gibbs function G, also known as Gibbs free energy or free enthalpy, is 
defined in the equation shown, where E is the internal energy, p is the 

                          *** G = E + pv - TS ***

pressure, v is the volume, T is the absolute temperature and S is the 
entropy. The Gibbs function is most useful in analyzing systems held at 
constant temperature and pressure. Under these conditions, the change in the 
Gibbs function DELTA G of a system is a measure of the maximum attainable 
work, not including the work of displacing the environment. Since chemical 
processes frequently occur at constant temperature and pressure, the Gibbs 
function is extensively used in chemical engineering for calculating phase 
equilibrium and reaction equilibrium. See CHEMICAL THERMODYNAMICS; FREE ENERGY. 
 �34s[William F. Jaep]
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�6v�2v�32sGila monster   �21m �33s 
    The name for one of two species of reptiles in the family Helodermatidae 
that occurs in the southwestern United States and Mexico. Heloderma suspectum 
is the Gila monster; the other, larger species, H. horridum, is known as the 
beaded lizard. The body of both species has a rounded appearance and is 
covered with many-colored beadlike tubercles that form patterns of orange, 
red, or yellow on a dark-brown or black background. 
    These are the only known poisonous lizards. They are carnivorous and feed 
on small mammals, birds, and eggs but can survive for months without food as 
they store fat in their tails. They are oviparous and lay white, tough-shelled 
eggs which are large in proportion to the size of the parent. See REPTILIA; 
SQUAMATA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sGill �21m �33s
    A structure in many vertebrates (Acrania, elasmobranchs, Holocephali,
cyclostomes, Dipnoi, bony fishes, and amphibia) involved in external
respiration. The gills in fish are considered here because these animals
have been studied more extensively than others. The gills of fish serve for
the exchange of materials between the blood and water. There are two kinds
of gills, external and internal (see illustration). They are permeable to
certain substances of importance to the economy of the organism, notably
oxygen, carbon dioxide, and the nitrogenous wastes, ammonia and urea. These
are transferred through the gill by simple diffusion. In addition, the gills
are somewhat permeable to water, a circumstance for which most fish must
compensate by the active transport of certain ions to maintain their osmotic
balance. A significant part of this latter function is also carried out by the
gills. The gills cannot be considered by themselves alone, because their
functioning depends as much on the systems which bathe their external surface
with water and their internal surface with blood as it does on the gills
themselves.
    The fundamental unit fo the gill is the gill filament. The filaments are
long, flattened, slightly crescent-shaped structures extending laterally from
from openings in the side of the pharynx. Filaments on the opposite sides of a
given pharyngeal opening arch toward each other, at least at their lateral
extremities, their tips meeting and thus enclosing a space between the
filaments and the pharyngeal opening that may be termed the prebranchial
chamber. External to the arched free portions of the filaments is a
postbranchial chamber with an opening in the body wall.
    The gill filaments bear two important types of structure. The first of
these is the secondary lamella, which provides the actual surface for diffusion.
The lamellae are almost microscopic, somewhat semicircular, leaflike extensions
transverse on the float faces of the filament. They consist of a loose web of
supporting tissue covered by an epithelium of a single layer of cells so that
there are extensive blood lacunae within them, separated from the water by the
least barrier to diffusion. The other type of structure probably of great
importance to the special functions of the gill is the so-called
chloride-excreting cell. These are cells that are associated with the afferent
blood supply. They are clustered about the bases of the lamellae and occur on
the face of the filament itself.
    There are comparative differences in each component of the branchial system.
There are wide quantitative differences in the area of the gill surface and in
the dimensions and the spacing the lamellae. Such differences are largely
associated with adaptive radiation. Active pelagic species tend to have large,
finely divided surfaces, while more sluggish demersal species tend to have less
lamellar surface and somewhat coarser channels. Those fish which have organs
for breathing air, in addition to gills, have reduced gill surfaces. There are
also wide quantitative differences in the oxygen capacity of the blood and in
the nature of the loading curve of the hemoglobin, which also appear to be of
adaptive significance. Active fish tend to have blood of higher oxygen
capacity; the blood of those which live in well-aerated waters requires a
relatively high partial pressure of oxygen for saturation. There are also major
differences in the sensitivities of fish bloods with respect to the effect of
the presence of carbon dioxide on the oxygen capacity. These, too, probably
have their part in fitting the various species for the environments in which
they are found. See RESPIRATORY SYSTEM.

 �34s[Frederick E. J. Fry]
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�6v�2v�32sGilsonite   �21m �33s 
    An asphaltite that occurs in the Uinta Basin of northeastern Utah. 
Gilsonite is black and has high luster, coarsely conchoidal fracture, and a 
red-brown streak. The material is friable and in mining or crushing breaks to 
a fine, penetrating dust. 
    In the petroleum industry, the only major products from gilsonite are coke 
for use in the aluminum industry, gasoline for local consumption, and gas, 
valuable as a fuel. Gilsonite is used in the manufacture of paints, battery 
boxes, asphalt floor tiles, brake linings, printing inks, and electrical 
insulation. It is also used for waterproofing and insulating high-temperature 
piping. See ASPHALT AND ASPHALTITE. 
 �34s[Irving A. Breger]
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�6v�2v�32sGinger �21m �33s
    An important spice or condiment; also the plant from which it is obtained,
Zingiber officinale, of the ginger family (Zingiberaceae). The plant is a
native of southeastern Asia. It is an erect perennial herb having thick,
scaly, branched rhizomes which contain starch, gums, an oleoresin (gingerin)
responsible for the pungent taste, and an essential oil which imparts the
aroma. Ginger is used in medicine, in culinary preparations, and for flavoring
beverages such as ginger ale and ginger beer. The plant is grown in China,
Japan, Sierra Leone, Jamaica, Queensland, and Indonesia. See ZINGIBERALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sGinkgoales   �21m �33s 
    An order of nearly extinct gymnosperms (Pinophyta) having only one living 
species, the maidenhair tree (Ginkgo biloba). The fan-shaped leaves resemble 
those of the maidenhair fern. The leaves are dichotomously (forked) veined and 
often more or less lobed (biloba) [see illustration]. The species is 
dioecious; that is, the male and female reproductive structures occur on 
separate trees. Ginkgo is widely planted on street borders, on home grounds, 
and in city parks. See PINOPHYTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sGiraffe   �21m �33s 
    Member of the family Giraffidae represented by a single species, Giraffa 
camelopardalis. The giraffe occurs in the savanna regions of tropical Africa. 
Giraffes are ruminants and belong to the mammalian order Artiodactyla 
(even-toed ungulates). The giraffe is the tallest of all mammals and may reach 
a height of 18 ft or 5.5 m. There are two prominent horns on the forehead 
which are bony outgrowths covered by skin, and there is a short mane along the 
back of the neck. 
    The giraffe lives in small herds with many females and usually one mature 
and several immature males. Gestation for the giraffe lasts about 15 months, 
and a single young is born. The young is about 6 ft (1.8 m) tall at the time 
of birth. See ARTIODACTYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sGlacial epoch   �21m �33s 
    An informal reference to a time during the history of the Earth when there 
were larger ice sheets (continental size) and mountain glaciers than today. 
The most recent glacial epoch, better known as the Pleistocene glacial epoch, 
and also by the older period-term Quaternary, encompassed at least the last 
3,000,000 years. 
    Many side effects resulted from the existence of these ice sheets and 
glaciers, including climate changes, sea-level rise and fall, depressions of 
the Earth's crust, and large-scale migrations of plants, animals, and humans 
as well as mass extinctions. Mountain landscapes were sculptured by glaciers, 
and erosional and depositional landforms were formed. Ocean temperatures were 
cold during glaciations and warm at times of interglacials. Early human 
evolution, development, and migrations resulted from the ever-changing 
climates closely related to glacier advances and retreats. 
    Glacial epochs seem to recur at intervals of 200,000,000 to 250,000,000 
years. In overall occurrence, all the glacial epochs that have ever occurred 
occupy only 5 to 10% of all geologic time. During major glacial epochs, great 
ice sheets formed in the high latitudes and spread out to cover as much as 40% 
of the Earth's land surface. Accompanying drops in temperature during some 
glacial epochs may have been as much as 25DGF (14DGC) in the mid-latitudes. 
During a glacial epoch, major glaciations are short-lived, each lasting less 
than 10,000 years, with the interglacials persisting for only about 10,000 
years, so that for most of an epoch, the ice sheets either grow or diminish in 
size. The Pleistocene glacial epoch was distinguished by seven or eight 
glacial advances within the last 700,000 years. Its last glaciation ended 
about 9000 years ago in Fennoscandia, and less than 8000 years ago in 
north-central Canada. See GLACIAL GEOLOGY; GLACIOLOGY; PLEISTOCENE. 
 �34s[Sidney E. White]
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�6v�2v�32sGlacial geology   �21m �33s 
    The scientific study of the effects of glaciers on the broad land areas, 
on the oceans, and on climate, of their erosion and deposition, and of their 
modification of the Earth's surface in detail. Included in the realm of 
glacial geology is the history of glacial theory, consideration of the origin 
of glacial ages, extent and times of past glaciations, erosion and sculpturing 
of plains and mountains, deposition of ice-contact and meltwater sediments, 
and the consequences of glaciers on worldwide climate, and also on local 
climate around their edges. Quite distinct from glacial geology, however, is 
the separate, growing subscience of glaciology, the study of glaciers 
themselves. See GLACIOLOGY. 
    Features on the Earth's surface explained by former worldwide glaciation 
are numerous, embracing, for example, glacially eroded and molded valleys and 
mountains; ice-transported and deposited sediments and nonglacial sediments; 
abandoned stream channels with associated floodwater deposits; elevated silts 
and clays that collected around continental edges when sea level was higher; 
valleys eroded across and into continental shelves and slopes when sea level 
was much lower; communities of plants and animals similar to each other but 
separated by shallow seaways where land bridges once existed; fossil shells 
and microorganisms in deep-sea sediments reflecting colder or warmer water 
temperatures than today; vegetated sand dunes aligned to wind systems no 
longer operating; ancient shorelines and beach ridges ringing dry empty lake 
basins far inland; and orderly patterns of stones and fine sediments next to 
glacier margins in polar regions and high mountains. See CIRQUE; DRUMLIN; 
GLACIAL EPOCH; MORAINE. 
 �34s[Sidney E. White]
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�6v�2v�32sGlaciology   �21m �33s 
    The study of existing or modern glaciers in their entirety, involving all 
related scientific disciplines. Glaciology is largely concerned with present 
glacial characteristics and processes, as opposed to studies in glacial 
geology which relate to the nature and effects of former glaciation. See 
GLACIAL GEOLOGY. 
    A glacier is a naturally accumulating mass of ice that moves in the 
process of discharging from head or center to its margins or a terminal 
dissipation zone. Glaciers are nourished in areas of snow accumulation that 
lie above the mean climatological or orographical snow line, which on the 
glacier surface is referred to as the neve line or firn line. The most active 
glaciers are generally found in regions receiving the heaviest snowfall, such 
as the maritime flanks of high coastal mountain ranges. See SNOW LINE; 
SNOWFIELD AND NEVE. 
    Glaciers are composed of three substances: snow, firn, and ice. The main 
material of glaciers is bubbly glacier ice composed of myriads of interlocking 
crystals, hence it is a polycrystalline material containing air pockets and 
entrapped water bubbles. Below the late-summer glacier snow line (neve line) 
only bubbly glacier ice is exposed. Above the neve line, the other categories 
of snow and firn (and firn ice) exist to depths of a few to hundreds of feet. 
It is deeper in polar (colder) firn packs. (Firn is a consolidated granular 
transition of snow not yet changed to glacier ice.) 
    Glacier deformation is a composite of internal and external movement. The 
internal movement is dominated by a continuous plastic creep (flow 
deformation), and the external in some cases by a fracture type of 
discontinuous movement at the bed or near the glacier margins. As a general 
rule, flow deformation, hence the rate of movement, is greater in the upper 
portion of a glacier than in its basal section. 
    Research has suggested that glacier surges, expressed as sudden 
catastrophic advances or raising and lowering of the ice surface, are 
relatively common phenomena in some regions. Such abnormal surges are 
characterized by marked and seemingly anomalous increases in flow velocity and 
often by a rapid transfer of ice from the neve to the terminus. The upper 
glacier surface may sink or be substantially lowered with this volume loss, 
expressed as a comparable thickening in the lower valley sector. Such surges 
are believed to be kinematic in nature, in that the wave moves through the 
glacier at a substantially faster rate than the actual discharge of ice. 
 �34s[Maynard M. Miller]
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�6v�2v�32sGland �21m �33s
    A structure which produces a substance or substances essential and vital
to the existence of the organism and species. Glands are classified according
to (1) the nature of the product; (2) the structure; (3) the manner by which
the secretion is delivered to the area of use; and (4) the manner of cell
activity in forming secretion. A commonly used scheme for the classification
of glands follows.

I. Morphological criteria
   A. Unicellular (mucous goblet cells)
   B. Multicellular
      1. Sheets of gland cells (choroid plexus)
      2. Restricted nests of gland cells (urethral glands)
      3. Invaginations of varying degrees of complexity
	 a. Simple or branched tubular (intestinal and gastric glands) - no duct
	    interposed between surface and glandular portion
	 b. Simple coiled (sweat gland) - duct interposed between glandular
	    portion and surface
	 c. Simple, branched, acinous (sebaceous gland) - glandular portion
	    spherical or ovoid, connected to surface by duct
	 d. Compound, tubular glands (gastric cardia, renal tubules) - branched
	    ducts between surface and glandular portion
	 e. Compound tubular-acinous glands (pancreas, parotid gland) - branched
	    ducts, terminating in secretory portion which may be tubular or
	    acinar
II. Mode of secretion
    A. Exocrine - the secretion is passed directly or by ducts to the exterior
       surface (sweat glands) or to another surface which is continuous with the
       external surface (intestinal glands, liver, pancreas, submaxillary gland)
    B. Endocrine - the secretion is passed into adjacent tissue or area and then
       into the bloodstream directly or by way of the lymphatics; these organs
       are usually circumscribed, highly vascularized, and usually have no
       connection to an external surface (adrenal, thyroid, parathyroid, islets
       of Langerhans, parts of the ovary and testis, anterior lobe of the
       hypophysis, intermediate lobe of the hypophysis, groups of nerve cells of
       the hypothalamus, and the neural portion of the hypophysis)
    C. Mixed exocrine and endocrine glands (liver, testis, pancreas)
    D. Cytocrine - passage of a secretion from one cell directly to another
       (melanin granules from melanocytes in the connective tissue of the skin
       to epithelial cells of the skin)
III. Nature of secretion
     A. Cytogenous (testis, perhaps spleen, lymph node, and bone marrow) - gland
	``secretes'' cells B. Acellular (intestinal glands, pancreas, parotid
	gland) - gland secretes noncellular product
IV. Cytological changes of glandular portion during secretion
    A. Merocrine (sweat glands, choroid plexus) - no loss of cytoplasm
    B. Holocrine (sebaceous glands) - gland cells undergo dissolution and are
       entirely extruded, together with the secretory product
    C. Apocrine (mammary gland, axillary sweat gland) - only part of the
       cytoplasm is extruded with the secretory product
V. Chemical nature of the product
   A. Mucous goblet cells (submaxillary glands, urethral glands) - the secretion
      contains mucin
      B. Serous (parotid gland, pancreas) - the secretion does not contain mucin
 �34s[Olin E. Nelsen]
�33s

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�6v�2v�32sGlanders   �21m �33s 
    An uncommon disease of equines known since the days of Aristotle and 
Hippocrates, which is occasionally transmitted to humans. Glanders still 
exists in parts of Europe, Asia, and Africa in horses, mules, donkeys, and 
humans. The causative organism is Pseudomonas mallei. 
    Glanders in equines occurs in two forms: acute or chronic respiratory 
infection, and farcy, a chronic infection of skin and lymphatics. Transmission 
occurs by food and water contaminated with nasal or skin discharge. 
    Glanders in humans is acquired by contact with discharge from infected 
equines or through accidental laboratory exposure. The portal of entry may be 
respiratory, alimentary, or abraded skin. The usual acute form of the disease 
produces chills, fever, and marked prostration. Subacute and chronic forms 
produce semigranulomatous pyemic nodules and abscesses in many organs. 
 �34s[Winston R. Miller]
�33s 
  
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�6v�2v�32sGlass   �21m �33s 
    Materials made by cooling certain molten materials in such a manner that 
they do not crystallize but remain in an amorphous state, their viscosity 
increasing to such high values that, for all practical purposes, they are 
solid. Materials having this ability to cool without crystallizing are 
relatively rare, silica, SiO2, being the most common example. Although glasses 
can be made without silica, most commercially important glasses are based on 
it. The most important properties are viscosity; strength; index of 
refraction; dispersion; light transmission, (both total and as a function of 
wavelength); corrosion resistance; and electrical properties. 
    Chemically, most glasses are silicates. Silica by itself makes a good 
glass (fused silica), but its high melting point (1723DGC or 3133DGF) and its 
high viscosity in the liquid state make it difficult to melt and work. To 
lower the melting temperature of silica to a more convenient level, soda, 
Na2O, is added in the form of sodium carbonate or nitrate, for example. This 
has the desired effect, but unfortunately the resulting glass has no chemical 
durability and is soluble even in water (water glass). To overcome this 
problem, lime, CaO, is added to the glass to form the basic soda-lime-silica 
glass composition which is used for the bulk of common glass articles, such as 
bottles and sheet (window) glass. Although these are the main ingredients, 
commercial glass contains other oxides (aluminum and magnesium oxides) and 
ingredients to help in oxidizing, fining, or decolorizing the glass batch. 
    Special kinds of glass have other oxides as major ingredients. For 
example, boron oxide is added to silicate glass to make a 
low-thermal-expansion glass for chemical glassware which must stand rapid 
temperature changes, for example, Pyrex glass. Also, lead oxide is used in 
optical glass because it gives a high index of refraction. 
 �34s[John F. McMahon]
�33s 
  
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�6v�2v�32sGlass switch �21m �33s
    A glassy, solid-state device used to control the flow of electric current.
Useful solid-state devices can be made from glassy as well as crystalline
semiconductors. A glass is a special case of a noncrystalline class of
materials, namely, amorphous solids. These do not exhibit long-range order,
although they tend to have the same local structure (that is, short-range
order) as the corresponding crystal. A glass is an amorphous solid that is
formed by cooling rapidly from the liquid phase. See GLASS.
    The first applications of glassy semiconductors were switches made from
chalcogenide (that is, alloys containing tellurium, selenium, or sulfur)
glasses. The two basic structures are known as the Ovonic Threshold Switch
(OTS) and the Ovonic Memory Switch (OMS). They are active devices consisting
simply of a thin film (about 1 micrometer thick) of glass between two metallic
contacts. The device characteristics depend on the bulk properties of the
semiconductor material rather than on the contacts. Consequently, the switches
are symmetrical in that they respond identically to voltages and currents of
both polarities.
    Amorphous semiconductors, as opposed to crystalline semiconductors, can be
doped or made insensitive to the effects of impurities, depending on the
desired application. For switching devices, it is ordinarily preferable to use
an impurity-independent structure to take advantage of lower costs and stable
operation. Ovonic switches are also highly resistant to the effects of
radiation.
    Both the OTS and OMS show a rapid and reversible transition between a
highly resistive and a conductive state effected by applied electric fields.
The main difference between the two devices is that, after being brought from
the highly resistive state to the conducting state, the OTS returns to its
highly resistive state when the current falls below a holding current value.
On the other hand, the OMS remains in the conducting state until a current
pulse returns it to its highly resistive state. The OMS thereby remembers the
last applied switching command, and it is from this property that the device
receives its name.
    Integrated arrays of OMSs can be used as electronically alterable
read-only memories. They fill the gap in the computer memory spectrum between
permanent read-only memories and volatile random-access memories. Photographic
films have been developed in which the quality and amount of structural
changes can be controlled by the amount of energy incident upon the film.
Since these films do not contain silver, their cost does not vary with that of
any precious metal. A transistor, using an OTS as the emitter, has been
developed. This can be used as a threshold amplifier, as a threshold latching
amplifier, or as the basis for a computer using ternary logic. Other promising
application areas include ac control and microwave generation. See AMORPHOUS
SOLID; SEMICONDUCTOR.
 �34s[Stanford R. Ovshinsky ;David Adler]
�33s

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�6v�2v�32sGlaucoma   �21m �33s 
    A disease in which an elevated intraocular pressure damages the eye. It is 
one of the most common and most serious disorders of the eye, exceeded in the 
United States only by cataracts as a cause of blindness. See BLINDNESS. 
    Glaucoma is generally classified into four major groups: congenital or 
infantile glaucomas (occurring at birth or shortly after); primary open-angle 
glaucoma; primary angle-closure glaucoma (the acute, painful kind; can be 
subdivided into acute and subacute or chronic angle-closure glaucoma); and 
secondary glaucoma (associated with other ocular disease such as a swollen 
cataract, inflammations, trauma, or aftereffects of surgical procedures). 
    Infantile glaucoma results in many cases in a large eye because of the 
lack of the rigidity of the sclera, and this condition is known as 
buphthalmos. Angle-closure glaucoma occurs when there is a sudden increase in 
intraocular pressure due to a block of the anterior chamber angle in such a 
way that the root of the iris cuts off the aqueous outflow. A severe, sudden 
pressure rise causes pain and sudden visual loss. Such an eye will show a 
steamy or cloudy cornea, a semidilated and fixed pupil (that will not respond 
to a light shining onto it), and an elevated intraocular pressure. The chief 
threat of the most common kind of glaucoma, the so-called open-angle, 
symptomless glaucoma, is insidious visual impairment. The disease is 
bilateral, and there is progressive field loss, occurring mostly in the 
periphery, so that the person is unaware of losing vision until it is too late. 
    In most cases, blindness can be prevented if treatment is instituted 
early. The object of the treatment is to maintain the intraocular pressure at 
a normal level. In cases in which pressure cannot be controlled with 
medication, such as angle-closure glaucoma, surgery can be done to create an 
artificial outflow and lower the intraocular pressure. Congenital glaucoma, 
which occurs as a result of a defect in the development of the angle, most be 
treated surgically. 
 �34s[Jack Hartstein]
�33s 
  
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�6v�2v�32sGlauconite   �21m �33s 
    The term glauconite as currently used has a two-fold meaning. It is used 
as both a mineralogic and morphologic term. The mineral glauconite is defined 
as an illite type of clay mineral. A fundamental characteristic of glauconite 
is that the unit cell is composed of a single silicate layer rather than the 
double layer of most other dioctahedral micas. See CLAY MINERALS; ILLITE. 
    Glauconite is known to occur in flakes and as pigmentary materials. When 
used in the morphological sense, the term glauconite often refers to small, 
green, spherical, earthy pellets. Some of these pelletal varieties are 
composed solely of the mineral described above, others are a mixed-layer 
association of this mineral and other three-layer structures. 
    Glauconite forms during marine diagenesis, in relatively shallow water, 
and at times of slow or negative deposition. Glauconite has been identified in 
both recent and ancient sediments. It is a major component in some 
``greensand'' deposits and has been used commercially for the extraction of 
potassium from such sources. See AUTHIGENIC MINERALS; DIAGENESIS; MARINE 
SEDIMENTS.  
 �34s[Floyd M. Wahl ;Ralph E. Grim]
�33s 
  
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�6v�2v�32sGlaucophane   �21m �33s 
    The name given to the monoclinic sodium amphiboles of the general 
composition Na2Mg3Al2Si8O22(OH)2. Like most sodium amphiboles, glaucophane is 
blue to black with marked pleochroism (color change on rotation in 
plane-polarized light) in thin sections from yellowish green to deep blue. The 
crystals are prismatic and sometimes fibrous. Glaucophane occurs in a 
restricted group of metamorphic schists that commonly exhibit an unusual 
mineral association with dense minerals, such as lawsonite, jadeite, 
pumpellyite, and garnet, and less dense minerals, such as epidote, mica, 
chlorite, and albite. See AMPHIBOLE. 
 �34s[George W. DeVore]
�33s 
  
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�6v�2v�32sGlazing   �21m �33s 
    The application of finely ground glass, or glass-forming materials, or a 
mixture of both, to a ceramic body and heating (firing) to a temperature where 
the material or materials melt, forming a coating of glass on the surface of 
the ware. Glazes are used to decorate the ware, to protect against moisture 
absorption, to give an easily cleaned sanitary surface, and to hide a poor 
body color. 
    Glazes are classified and described by the following characteristics: 
surface - glossy or matte; optical properties - transparent or opaque; method 
of preparation - fritted or raw; composition - such as lead, tin, or boron; 
maturing temperature; and color. Opaque glazes contain small crystals embedded 
in the glass, but special glazes in which a few crystals grow to recognizable 
size are called crystalline glazes. See CERAMICS; FRIT; GLASS. 
 �34s[John F. McMahon]
�33s 
  
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�6v�2v�32sGlide-path indicator   �21m �33s 
    An aircraft landing instrument that provides the pilot with a set of 
vertical and horizontal cross pointers that indicate deviation from a 
radio-transmitted course to the threshold of the runway. A dual-frequency 
transmitter sends out one frequency to the right of the runway center line and 
a second frequency to the left of the runway center line. The reception of 
these signals in the aircraft biases the vertical needle to the left or right 
depending on the position of the aircraft relative to the transmitted 5DG 
localizer path. Simultaneously, another dual-frequency transmitter causes a 
horizontal needle to indicate high or low as the aircraft descent path is 
compared to the transmitted 3DG glide path. 
 �34s[James W. Angus]
�33s 
  
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�6v�2v�32sGlider   �21m �33s 
    An unpowered flying device that attempts to copy the flight of soaring 
birds as accurately as possible. In October 1911, Orville Wright made a 
gliding flight of nearly 10 min duration, and demonstrated that gliders could 
stay up for long periods in rising air. This condition of flight, called slope 
soaring, was the basic method of soaring flight until about 1930. Thermal 
soaring, the next step, was accomplished by flying in areas of rising 
convection currents. By the use of thermal flight, the modern glider can fly 
almost anywhere in the world for extended time and distances over 800 km (500 
mi) in one flight. Other methods of soaring make use of clouds and 
standing-wave phenomena in the atmosphere. High-performance gliders 
(sailplanes) may be launched by towing behind powered aircraft or by car 
towing, which is used to a lesser extent. Some gliders have been fitted with a 
small motor and propeller, which enables them to take off and climb to an 
altitude where rising air permits them to soar unpowered. See SAILPLANE. 
    Modern foot-launched hang gliders with aluminum tube frames have flown 
over 160 km (100 mi) in straight-line distance, have reached about 6000 m 
(20,000 ft) altitude, and have remained aloft more than 15 h. But it is not so 
much their performance that makes hang gliders popular, as their low cost, 
their convenience of folding into a small package for transport or storage, 
and the fact that no license is required for glider or pilot. 
    In 1977 a team of workers directed by Paul B. MacReady constructed a very 
large but very light hang glider capable of human-powered flight. This 
aircraft, the Gossamer Condor, was pedaled around a figure-eight course. 
MacReady and his team improved their design, and in 1979 made the first 
human-powered flight across the English Channel with the Gossamer Albatross 
(see illustration). 
 �34s[Francis M. Rogallo]
�33s 
  
  
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�6v�2v�32sGlobe (Earth)   �21m �33s 
    A sphere on the surface of which is a map of the world. The map may be 
drawn, engraved, or painted directly on the surface but is more commonly 
prepared as a series of gores, or segments in other designs, to be affixed to 
the globe ball (see illustration). 
    Globes are both artistically interesting and scientifically useful. Their 
principal value is in stimulating sound concepts of worldwide patterns and in 
rectifying errors induced by the limitations of flat maps. All flat maps 
distort the Earth's surface patterns, but carefully made globes constitute 
truer scale models of the Earth, with correct areas, shapes, and distances as 
well as continuity of surface. Globes have long been used as aids in 
navigation, in the teaching of earth sciences, and as library or parlor 
ornaments.  
    Many modern globes have special attachments to improve their utility. A 
meridian ring, extending from pole to pole, may be calibrated in degrees to 
measure latitude. The longitude of points directly beneath that ring will be 
indicated at the intersection of the ring with the equatorial scale. A horizon 
ring at right angles to the meridian ring may be calibrated in miles or in 
meters, degrees, and hours to expedite distance and time measurement. A hinged 
horizon ring may be lifted to serve as a meridian ring, or placed in an 
oblique position to show great circle routes and distances. 
 �34s[Arch C. Gerlach]
�33s 
  
  
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�6v�2v�32sGlobular protein   �21m �33s 
    One of the two main classes of protein molecules, the other class being 
termed fibrous proteins. Many important proteins are of the globular class, 
for example, hemoglobin, albumin, casein, and GAMMA-globulin; all enzymes and 
protein hormones are also globular proteins. These proteins generally have a 
more spherical shape than fibrous proteins do, and this property lends itself 
to their high degree of solubility in aqueous solvents. The shape of globular 
proteins is due to their high degree of three-dimensional structure; that is, 
the polypeptide backbone of these proteins is folded in a compact manner 
because of interactions between the side chains of amino acid residues. See 
ALBUMIN; ENZYME; FIBROUS PROTEIN; GLOBULIN; HEMOGLOBIN; PROTEIN. 
 �34s[James M. Manning]
�33s 
  
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�6v�2v�32sGlobule   �21m �33s 
    A roundish blob of cosmic dust (that is, a roundish dark nebula) seen 
projected against the starlit sky. The diameter of a typical nearby globule is 
generally about 3 - 5 minutes of arc. Globules look like ``holes in the 
heavens'' and are made detectable by the fact that the cosmic dust inside is 
so thick that the stars on the far side of the globule cannot be seen. 
    Theoretical calculations show that globules should collapse into 
protostars in times of the order of a half million years. There will probably 
be considerable mass loss in the process of collapse. The result of the 
collapse will probably be a star (or binary pair of stars) with masses of 1 to 
a few solar masses. It appears that about one in every five to eight stars in 
the Milky Way Galaxy is born from a globule.  See  INTERSTELLAR MATTER; 
STELLAR EVOLUTION. 
 �34s[Bart J. Bok]
�33s 
  
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�6v�2v�32sGlobulin   �21m �33s 
    A general name for any member of a heterogeneous group of serum proteins 
precipitated by 50% saturated ammonium sulfate. See PROTEIN; SERUM. 
    The introduction of electrophoresis during the 1930s permitted subdivision 
of the globulins into alpha, beta, and gamma globulins on the basis of 
relative mobility at alkaline pH (8.6). However, each of these subgroups, 
though electrophoretically homogeneous, consists of a great variety of 
proteins with different biological properties and markedly different sizes and 
chemical properties other than net charge. Thus the ALPHA2-globulins, for 
example, as defined by moving boundary or paper electrophoresis, contain 
proteins ranging in molecular weight from approximately 50,000 to 
approximately 1,000,000 (ALPHA2-macroglobulin), each with differing functions. 
See ELECTROPHORESIS; IMMUNOGLOBULIN. 
 �34s[H. Hugh Fudenberg]
�33s 
  
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�6v�2v�32sGlomerulonephritis   �21m �33s 
    A common kidney disorder affecting the whole nephron, including the 
surrounding interstitial tissue. There are many diseases with which the acute 
process may be associated, the most common being a streptococcal infection, 
usually of the pharynx. The signs of nephritis - hypertension, edema, 
decreased urine output, and red blood cells and protein in the urine - 
typically begin 10 - 14 days after the acute infection. This disease is common 
in children and complete healing without treatment is the general rule. In 
affected adults, nearly 40% develop some degree of chronic disease. The 
histologic lesion is characterized by an increased number of cells in swollen 
glomeruli.  
    The course of this disease averages 15.7 years from the time of diagnosis. 
Treatment is purely symptomatic entailing management of high blood pressure 
and restriction of salt intake to reduce the amount of solute that the kidneys 
must excrete. See KIDNEY; NEPHRITIS. 
 �34s[Gary E. Striker]
�33s 
  
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�6v�2v�32sGlow discharge �21m �33s
    A mode of electrical conduction in gases. Glow discharge commonly occurs
under conditions of relatively low pressure and generally in the pressure
range of 1 - 10 mm of mercury (10(sup 2) - 10(sup 3) pascals). The discharge
typically gives off light, so that the region of the discharge appears to glow
with considerable intensity. This glow is quite diffuse as contrasted to a
higher-pressure discharge, such as a high-pressure arc. Typical currents may
be of the order of tens or hundreds of milliamperes, whereas the potential
drop may be of the order of 100 volts.
    The most important application of the glow discharge is in the so-called
voltage regulator or voltage reference tube. This device maintains a
relatively constant difference of potential across itself as the current is
varied over an appreciable range, and consequently is very useful in cases
where a constant reference potential is required. See ELECTRIC SPARK;
ELECTRICAL CONDUCTION IN GASES.
 �34s[Glenn H. Miller]
�33s

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�6v�2v�32sGlucagon   �21m �33s 
    The protein hormone secreted by the pancreas which is known to influence a 
wide variety of metabolic reactions. Glucagon, along with insulin and other 
hormones, plays a role in the complex and dynamic process of maintaining 
adequate supplies of sugar in the blood. Glucagon has often been called the 
hyperglycemic-glycogenolytic factor because it causes the breakdown of liver 
glycogen to sugar (a process known as glycogenolysis) and thereby increases 
the concentration of sugar in the bloodstream (a condition known as 
hyperglycemia). Glucagon may also be involved in the regulation of protein and 
fat metabolism, gastric acid secretion and gut motility, excretion of 
electrolytes (such as sodium, potassium and chloride) by the kidney, 
contractility of heart muscle, and release of insulin from the pancreas. 
Glucagon is used in human medicine chiefly in certain diabetic conditions when 
a dangerously low blood sugar must be rapidly raised. See CARBOHYDRATE 
METABOLISM; DIABETES; GLYCOGEN; HORMONE; INSULIN; PANCREAS. 
 �34s[William W. Bromer]
�33s 
  
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�6v�2v�32sGlucose   �21m �33s 
    A monosaccharide also known as D-glucose, D-glucopyranose, grape sugar, 
corn sugar, dextrose, and cerelose. The structure of glucose is shown in the 
illustration.  
    Glucose in free or combined form is not only the most common of the sugars 
but is probably the most abundant organic compound in nature. It occurs in 
free state in practically all higher plants. It is found in considerable 
concentrations in grapes, figs, and other sweet fruits and in honey. In lesser 
concentrations, it occurs in the animal body fluids, for example, in blood and 
lymph. Urine of diabetic patients usually contains 3 - 5%. 
    Cellulose, starch, and glycogen are composed entirely of glucose units. 
Glucose is also a major constituent of many oligosaccharides, notably sucrose, 
and of many glycosides. It is produced commercially from cornstarch by 
hydrolysis with dilute mineral acid. The commercial glucose so obtained is 
used largely in the manufacture of confections and in the wine and canning 
industries. See CELLULOSE; GLYCOGEN; STARCH. 
  
    D-Glucose is the principal carbohydrate metabolite in animal nutrition; it 
is utilized by the tissues, and it is absorbed from the alimentary tract in 
greater amounts than any other monosaccharide. Glucose could serve 
satisfactorily in meeting at least 50% of the entire energy needs of humans 
and various animals. 
    Glucose enters the bloodstream by absorption from the small intestine. It 
is carried via the portal vein to the liver, where part is stored as glycogen, 
the remainder reentering the circulatory system. Another site of glycogen 
storage is muscle tissue. 
    Glucose is readily fermented by yeast, producing ethyl alcohol and carbon 
dioxide. It is also metabolized by many bacteria, resulting in the formation 
of various degradation products, such as hydrogen, acetic and butyric acids, 
butyl alcohol, acetone, and many others. See CARBOHYDRATE; MONOSACCHARIDE. 
 �34s[William Z. Hassid]
�33s 
  
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�6v�2v�32sGluons   �21m �33s 
    The hypothetical force particles which are believed to bind quarks into 
``elementary'' particles. Although theoretical models in which the strong 
interactions of quarks are mediated by gluons have been successful in 
predicting, interpreting, and understanding many phenomena in particle 
physics, free gluons remain undetected in experiments (as do free quarks). 
According to prevailing opinion, an individual gluon cannot be isolated. See 
ELEMENTARY PARTICLE; FUNDAMENTAL INTERACTIONS; QUARKS. 
 �34s[Chris Quigg]
�33s 
  
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�6v�2v�32sGlutamic acid   �21m �33s 
    An amino acid. Glutamic acid has many important functions, including the 
following: (1) It is the principal point by which ammonia enters organic 
compounds (reductive amination of ALPHA-ketoglutarate), and the central 
distribution point for amino nitrogen by transamination. (2) It is the 
precursor of glutamine, the added amide group serving as a storage form of 
nitrogen and as a precursor of certain nitrogen atoms in purines, histidine, 
and glucosamine. (3) It is the precursor of proline and of arginine. (4) It is 
incorporated into both glutathione and folic acid. See AMINO ACIDS; ARGININE; 
FOLIC ACID; HISTIDINE; PROLINE. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sGlutamine   �21m �33s 
    An amino acid. Glutamine with its amide group is an important storage form 
of nitrogen in plants and animals; it also serves as the precursor of certain 
ring nitrogen atoms in purines and histidine and of the amino group in 
glucosamine. See AMINO ACIDS. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sGlutelin   �21m �33s 
    A general name for a class of proteins which are insoluble in water, 
alcohol, or neutral salt solutions but readily soluble in dilute acids or 
alkali. Glutelins are mixtures of various similar proteins. The most 
extensively investigated glutelins are those obtained from the insoluble 
residue derived from wheat kernel, barley, and rice. Examples are glutenin 
from wheat and oryzenin from rice. See PROTEIN. 
 �34s[Gertrude E. Perlmann]
�33s 
  
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�6v�2v�32sGlycerol   �21m �33s 
    The simplest trihydric alcohol, with the formula CH2OHCHOHCH2OH. The name 
glycerol is preferred for the pure chemical, but the commercial product is 
usually called glycerin. It is widely distributed in nature in the form of its 
esters, called glycerides. The glycerides are the principal constituents of 
the class of natural products known as fats and oils. 
    When pure, glycerin is a colorless, odorless, viscous liquid with a sweet 
taste. It is completely soluble in water and alcohol but is only slightly 
soluble in many common solvents, such as ether, ethyl acetate, and dioxane. 
Glycerin is insoluble in hydrocarbons. It boils at 290DGC (554DGF) at 
atmospheric pressure and melts at 17.9DGC. Its specific gravity is 1.262 at 
25DGC (77DGF) referred to water at 25DGC, and its molecular weight is 92.09. 
It has a very low mammalian toxicity. 
    Glycerin is used in nearly every industry. With dibasic acids, such as 
phthalic acid, it reacts to make the important class of products known as 
alkyd resins, which are widely used as coating and in paints. It is used in 
innumerable pharmaceutical and cosmetic preparations; it is an ingredient of 
many tinctures, elixirs, cough medicines, and anesthetics; and it is a basic 
medium for toothpaste. In foods, it is an important moistening agent for baked 
goods and is added to candies and icings to prevent crystallization. It is 
used as a solvent and carrier for extracts and flavoring agents and as a 
solvent for food colors. Many specialized lubrication problems have been 
solved by using glycerin or glycerin mixtures. Many millions of pounds are 
used each year to plasticize various materials. 
    Several grades of glycerin are marketed, including high gravity, dynamite, 
yellow distilled, USP (U.S. Pharmacopoeia), and CP (chemically pure). USP 
grade is water-white and suitable for use in foods, pharmaceuticals, and 
cosmetics, or for any purpose where the product is designed for human 
consumption. See ALCOHOL; FAT AND OIL; POLYHYDRIC ALCOHOL. 
 �34s[Philip H. Cook]
�33s 
  
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�6v�2v�32sGlycine   �21m �33s 
    An amino acid. Glycine has many important functions, including the 
following: (1) It is incorporated intact into purines. (2) The ALPHA-carbon 
and nitrogen atoms are incorporated into the pyrrole rings of porphyrin. (3) 
It accepts an amidine group from arginine and a methyl group from methionine 
to form creatine. (4) It is a constituent of the tripeptide eoenzyme 
glutathionine (GAMMA-glutamylcysteinylglycine). (5) It can accept formaldehyde 
from hydroxymethyltetrahydrofolic acid to become serine. This reaction is 
reversible. See AMINO ACIDS. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sGlycogen �21m �33s
    The primary reserve polysaccharide of the animal kingdom. It is found in
the muscles and livers of all higher animals, as well as in the cells of lower
animals. Because of its close relationship to starch, it is often called
animal starch, although glycogen is found in some lower plants, fungi, yeast,
and bacteria. See STARCH.
    Glycogen is a nonreducing, white, amorphous polysaccharide which dissolves
readily in cold water, forming an opalescent, colloidal solution. The
molecular weight of glycogen is usually very high, and it varies with the
source and the method of preparation; molecular weights of the order of 1-20 x
10(sup 6) have been reported. Chemical studies show glycogen to possess a
branched structure similar to the amylopectin starch fraction.
    In its biochemical reactions, glycogen is similar to starch. It is
attacked by the same plant amylases that attack starch, and like starch, it is
degraded to maltose and dextrins. Both glycogen and starch are broken down by
animal or plant phosphorylase enzyme in the presence of inorganic phosphate
with the production of ALPHA-D-glucose-1-phosphate. See CARBOHYDRATE
METABOLISM.
    The metabolic formation of glycogen from glucose in the liver is
frequently termed glycogenesis. In fasted animals, glycogen formation can be
induced by the feeding, not only of materials that can be hydrolyzed to
glucose and other monosaccharides, such as fructose, but also of various other
materials. A number of L-amino acids, such as alanine, serine, and glutamic
acid, upon deamination in the liver give rise to substances, such as pyruvic
acid and ALPHA-ketoglutaric acid, that can be converted in the liver to
glucose units which are subsequently converted to glycogen. Furthermore,
substances such as glycerol derived from fats, dihydroxyacetone, or lactic
acid can all be utilized for glycogen synthesis in the liver. Such
noncarbohydrate precursors are termed glycogenic compounds. The process of
glycogen formation from these precursors is known as glyconeogenesis. The term
glycogenolysis is used to connote glycogen breakdown. See POLYSACCHARIDE.
 �34s[William Z. Hassid]
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�6v�2v�32sGlycol   �21m �33s 
    Any of a class of compounds characterized by having two hydroxyl (--OH) 
groups on separate carbons of an organic structure, usually linear and 
aliphatic. Products in this class are also called diols, dihydric alcohols, or 
in special cases, polyglycols. Glycols are generally water-soluble and 
hygroscopic, viscous liquids. Polyethylene glycols, that is, polyglycols 
derived by polymerization of ethylene oxide, are water-soluble at all 
molecular weights. Polypropylene glycols from propylene oxide become 
increasingly less water-soluble at molecular weights above 425. 
    Glycols undergo typical reactions of alcohols such as esterification, 
etherification, salt formation, and additions to activated double bonds. They 
also react with diacids or diesters to produce long-chain polyester polymers, 
an important family of products which find broad application in the 
manufacture of fibers, films, and molded plastic products. Reaction of 
glycols, especially polypropylene glycols, with diisocyanates produces another 
important family of products, polyurethanes, used in rigid and flexible foams 
and elastomers. See POLYESTER RESINS; POLYETHYLENE GLYCOL; POLYHYDRIC ALCOHOL; 
POLYMER; POLYVINYL RESINS. 
 �34s[Robert K. Barnes]
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�6v�2v�32sGlycolipid �21m �33s
    One of a class of complex lipids which contains carbohydrate residues.
Cerebrosides are glycosides of ceramide (fatty acid amide of sphingosine).
Galactose is the major constituent of animal cerebrosides. Glucocerebrosides
are also found normally and accumulate in large amounts in certain
pathological conditions. A glycoside of ceramide containing both galactose and
glucose has been isolated from a human carcinoma.
    Ganglioside, hematoside, and globoside are glycosides of ceramide
containing several sugar residues. Ganglioside, which is found in the brain,
contains glucose, galactose, galactosamine, and neuraminic acid as sugar
residue. See  GLYCOSIDE; LIPID; SPHINGOLIPID..
 �34s[Herbert E. Carter ;Roy H. Gigg]
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�6v�2v�32sGlycoside �21m �33s
    A member of a large class of carbohydrate derivatives characterized by the
replacement of the hydrogen atom of the hemiacetal hydroxyl group by an alkyl
or aryl moiety, the aglycon. In nature, most glycosides are those formed from
five-carbon (pentose) sugars or six-carbon (hexose) sugars occurring
predominantly as pyranoside (six-membered) rings (see illustration).
    Natural glycosides constitute a large and biologically important class of
sugar derivatives. Many natural pigments and dyestuffs, poisons, drugs, and
aromatic principles are glycosides. Heart stimulants, cerebrosides, and
gangliosides are glycosides. It is believed that the hydrophilic sugar moiety
in the glycosides serves to solubilize the hydrophobic aglycon. Frequently the
biochemical interest or biological activity of glycosides resides solely, or
largely, in the aglycon. See ANTHOCYANIN; ANTHRAQUINONE PIGMENTS; FLAVONOID;
GLUCOSIDE; STEROID.
 �34s[G. N. Richards ;Roy L. Whisteler]
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�6v�2v�32sGnathiidea   �21m �33s 
    A suborder of the Isopoda. These animals are characterized as having a 
much reduced second thoracomere which is incorporated with the cephalothorax. 
The antennules are short and each has a flagellum with four or five joints; 
the terminal three bear single sensory palps. The antennae are also short, 
with flagella having five to eight joints, but generally there are seven. 
Thoracomeres 3 - 7 have a normal appearance and bear pereiopods, while the 
eighth thoracomere is vestigial and lacks appendages (see illustration). 
    The group comprises marine forms exclusively. They are found in all 
latitudes and at all depths. The larvae are found as parasites on fish. See 
ISOPODA.  
 �34s[Theodore Monod]
�33s 
  
  
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�6v�2v�32sGnathostomata (Echinodermata)   �21m �33s 
    A superorder of Echinoidea, subclass Euechinoidea. These invertebrates are 
characterized by having a rigid, exocyclic test and a lantern or jaw apparatus 
developed sometime during the life history and usually persisting into the 
adult stage. The included orders are the Holectypoida, Clypeasteroida, and 
Cassiduloida. See CASSIDULOIDA; CLYPEASTEROIDA; ECHINODERMATA; ECHINOIDEA; 
EUECHINOIDEA; HOLECTYPOIDA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sGnathostomata (Vertebrata)   �21m �33s 
    A group of the subphylum Vertebrata which possesses jaws, teeth, paired 
appendages, and girdles (secondarily lost in some), as well as other advanced 
features in contrast to the more primitive Agnatha, which lack jaws and paired 
appendages. The Gnathostomata are divided into two major subdivisions, the 
superclasses Pisces and Tetrapoda, which respectively constitute roughly the 
aquatic and the terrestrial vertebrates. See AGNATHA; PISCES (ZOOLOGY); 
TETRAPODA; VERTEBRATA. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sGnathostomulida �21m �33s
    Microscopic marine worms of uncertain systematic relationship, mainly
characterized by cuticular structures in the pharynx and a monociliated skin
epithelium. It is the most recently described phylum of the animal kingdom.
The total number of species probably exceeds 1000.
    Gnathostomulids are worm-shaped, cylindrical or slightly depressed, and
semitransparent (or bright red), and sometimes have the external divisions of
head and tail (see illustration). The skin is a one-layered epithelium that is
completely monociliated; that is, each of the polygonal epidermal cells bears
only one cilium. The sensory system usually consists of 1 - 2 pairs of simple
and 3 - 4 pairs of compound bristles (frontally and laterally), and a bundle
of stiff cilia (dorsally on the head). The reproductive system consists of a
dorsal ovarium and in most cases two caudolateral groups of testes follicles
in the same specimen. Fertilization is internal.
    The distribution of gnathostomulids is worldwide, the majority of
localities being known from European coasts, some from the North American east
coast, and some scattered over the western Pacific.
 �34s[Rupert J. Riedel ;W. E. Sterrer]
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�6v�2v�32sGneiss   �21m �33s 
    An old term in mining and geology that was first applied by the miners of 
the Erzgebirge, Austria, to designate the country rock adjacent to the ore 
veins. In its widest sense, gneiss is used as a structural rather than 
compositional term. Thus it applies to a great variety of rocks with a banded 
or coarsely foliated structure. A modern version includes in the definition of 
gneiss (primary gneiss) that it is of plutonic igneous origin with banding 
caused by flow movements in a crystallizing heterogeneous magma. 
    Typical gneisses are highly metamorphic, coarse-grained, irregularly 
banded or foliated rocks composed mainly of quartz and feldspar with some 
biotite (= granitic composition). As the grain becomes finer and the amount of 
mica increases, gneisses pass into pelitic schists. With fine grain and 
decrease of mica, or garnet substituting for mica, they become less foliated 
and pass into granulite. See GRANULITE; SCHIST. 
    Primary gneiss is a term used for rocks of metasomatic, migmatitic origin 
which are related to regional processes of granitization. Paragneiss shows a 
sedimentary parentage. Orthogneisses are primary igneous rocks in which a 
gneiss structure has been induced by essentially kinetic metamorphism. 
Ordinarily the term gneiss refers to a granitic composition; anything else 
should be specially noted. See METASOMATISM; MIGMATITE. 
 �34s[T. F. W. Barth]
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�6v�2v�32sGneticae   �21m �33s 
    A subdivision of the division Pinophyta, represented by a single class, 
Gnetopsida, with three orders, Ephedrales, Gnetales, and Welwitschiales; each 
order is represented by a single genus: Ephedra, Gnetum, and Welwitschia, 
respectively.  
    The genus Ephedra has about 35 species, mostly of arid regions such as the 
southwestern United States. The plants are freely branched, low shrubs with 
reduced, scalelike leaves and green, photosynthetic twigs. See EPHEDRA. 
    The genus  Gnetum also has about 35 species, all of which are tropical, 
with more than half of them native to the Malaysian region. Most of the 
species are lianas (climbing, woody vines); only a few are shrubs or small 
trees.  
    A single species mirabilis, native to very dry deserts of southwestern 
Africa, is assigned to the genus Welwitschia. One of the most bizarre plants 
in the world, it has a very short, unbranched, woody main stem which is 
cushion-shaped or saucer-shaped and tapers quickly to a very long taproot. 
There are only two leaves and these persist throughout the life of the plant, 
perhaps 100 years. See PINOPHYTA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sGnotobiology   �21m �33s 
    That branch of biology dealing with known living forms; the study of 
higher organisms in the absence of all demonstrable, viable microorganisms 
except those known to be present. Germfree plants and animals are in the 
gnotobiotic state because they have been shown to harbor no living 
microorganisms. If a germfree plant or animal is inoculated with known species 
of bacteria, it is no longer gremfree; therefore, the term gnotobiotic was 
coined to include that state in which species live in intimate association 
with other known species. The host and the known microorganisms which it 
harbors may each be called a gnotobiote. See GERMFREE VERTEBRATE. 
 �34s[Thomas D. Luckey]
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�6v�2v�32sGoat   �21m �33s 
    A number of wild and domestic species of the mammalian order Artiodactyla. 
They are swift, climbing animals with a strong odor, particularly in the male. 
Goats are closely related to sheep, but differ in having a narrower head, a 
bearded chin in the male, horns that are close at the base and sweep up and 
back and are ringed or twisted, a short upturned tail, and no facial gland. 
Since these animals are herbivores, the upper incisors and canines are 
lacking. Some species of wild goats are listed in the table. 
    Domestic goats are hardy animals, resistant to disease, and produce two to 
three times the amount of milk that the cow does, relative to size. Goat's 
milk does not harbor the tubercular bacillus and provides excellent cheese and 
butter. The Alpine goat is the most important milk-producing breed. The Swiss 
or Alpine goat, usually divided into two breeds, the Saanen and Toggenburg, 
has been bred to local, native goats in many countries to improve milk 
production. The Nubian or Oriental goat has been imported into many countries 
also. It has long, drooping ears about the size of the head, and the horns are 
small or entirely absent. In South Africa, a Swiss-Nubian cross is referred to 
as the Boer goat. The Angora and Kashmir goats have been raised for wool. See 
ARTIODACTYLA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sGobiesociformes   �21m �33s 
    An order of bony fishes, also known as the Xenopterygii, or clingfishes, 
equipped with a thoracic sucking disk which serves for attachment to the 
substrate. There are single dorsal and anal fins that lack fin spines (see 
illustration). The body is scaleless, and there are no ribs and no swim 
bladder. The order consists of a single family that is classified into 8 
subfamilies, 33 genera, and nearly 100 Recent species. See ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sGoethite   �21m �33s 
    A mineral of composition FeOPRODUCTOH, crystallizing in the orthorhombic 
system. Crystals are rare, and the mineral is usually in reniform or 
stalactitic masses which have a radiating fibrous internal structure. The 
luster is adamantine to dull, and the color light to dark brown. The Mohs 
hardness is 5.0 - 5.5, and the density is 4.28 for crystals and 3.3 - 4.3 for 
massive material. Most of the common, yellow-brown, earthy ferric oxides known 
as limonite are mixtures composed largely of cryptocrystalline goethite. 
    Goethite is one of the most common minerals. It is the major constituent 
of the gossan at the surface of metalliferous deposits rich in iron-bearing 
sulfides, as at Bisbee, Arizona, and of laterites, as in Cuba. Well-formed 
crystals are found at Pribram, Bohemia, and Cornwall, England. It is an 
important iron ore in Alsace-Lorraine, in the Lake Superior hematite deposits, 
and in the southern Appalachians. See LIMONITE. 
 �34s[Lawrence Grossman]
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�6v�2v�32sGoiter �21m �33s
    An enlargement of all or part of the thyroid gland, due to various causes,
which may be accompanied by a hormonal dysfunction.
    Colloid, endemic, or simple goiter results from an iodine deficiency.
Endemic goiter is peculiar to many geographic areas in the world where iodine
is lacking in food, soil, or water. However, there is often a marked
difference in the incidence of the disease in endemic areas, thus indicating
other etiological factors.
    Diffuse primary hyperplasia, or exopthalmic goiter, results from an
overall glandular enlargement accompanied almost always by excess thyroxine
secretion. The exact cause of this disorder, also called Grave's disease, is
unknown.
    Nodular goiter, or multiple colloid goiter, is usually the result of
repeated cyclic changes in a part of the gland subjected to other abnormal
processes such as the two preceding conditions. Thyroid output may be
decreased but more often is normal or increased.
    Goiter does not necessarily mean thyroid dysfunction, since enlargement
may be seen in hypothyroid, euthyroid, and hyperthyroid states. Conversely,
dysfunction may occur in a gland that shows no visible change. See THYROID
GLAND.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sGold �21m �33s
    A chemical element, Au, atomic number 79 and atomic weight 196.967, a deep
yellow, soft, and very dense metal. Gold is classed as a heavy metal and as a
noble metal; commercially, it is the most familiar of the precious metals.
Copper, silver, and gold are in the same group of the periodic table of
elements. The Latin name for gold, aurum (glowing dawn), is the source of the
chemical symbol Au. There is only one stable isotope of gold, that of mass
number 197.

    Uses.  Consumption of gold in jewelry accounts for about three-fourths of
the world's production of gold. Industrial applications, especially
electronic, consume another 10 - 15%. The remainder is divided among medical
and dental uses, coinage, and bar stock for governmental and private holdings.
Gold coins and most decorative gold objects are actually gold alloys, because
the metal itself is too soft (2.5 - 3 on Mohs scale) to be useful with
frequent handling. See GOLD ALLOYS.
    Radioactive (sup 198)Au is used in medical irradiation, in diagnosis,
and in a number of industrial applications as a tracer. Another tracer use is
in the study of movement of sediment on the ocean floor in and around harbors.
The properties of gold toward radiant energy have led to development of
efficient energy reflectors for infrared heaters and cookers and for focusing
and retention of heat in industrial processes.

    Occurrence.  Gold occurs widely throughout the world, but usually very
sparsely, so that it is quite a rare element. Sea water contains low
concentrations of gold, on the order of 10 MUg per ton (10 parts of gold per
trillion parts of water). Somewhat higher concentrations accumulate on
plankton or on the ocean bottom. At present, no economically feasible process
is visualized for extracting gold from the sea. Native, or metallic, gold and
various telluride minerals are the only forms of gold found on land. Native
gold may occur in veins among rocks and ores of other metals, especially
quartz or pyrite, or it may be scattered in sands and gravel (alluvial gold).
See GOLD METALLURGY.

    Properties.  The density of gold is 19.3 times that of water at 20DGC
(68DGF), so that 1 ft(sup 3) of gold weighs about 1200 lb (1 m(sup 3), about
19,000 kg). Masses of gold, like those of other precious metals, are measured on
the troy scale, which counts 12 oz to the pound. Gold melts at 1064.43DGC
(1947.97DGF) and boils at 2860DGC (5180DGF). It is somewhat volatile well
below its boiling point. Gold is a good conductor of heat and electricity. It
is the most malleable and ductile metal. It can easily be made into
translucent sheets 0.0000039 in. (0.00001 mm) thick or drawn into wire
weighing only 0.00005 oz/ft (0.5 mg/m). The quality of gold is expressed on
the fineness scale as parts of pure gold per thousand parts of total metal, or
on the karat scale as parts of pure gold per 24 parts of total metal.Gold
readily dissolves in mercury to form amalgams. Gold is one of the least-active
metals chemically. It does not tarnish or burn in air. It is inert to strong
alkaline solutions and to all pure acids except selenic acid.

    Compounds.	Gold may be either unipositive or tripositive in its
compounds. So strong is the tendency for gold to form complexes that all the
compounds of the 3+ oxidation state are complex. The compounds of the 1+
oxidation state are not very stable and tend to be oxidized to the 3+ state or
reduced to metallic gold. All compounds of either oxidation state are easy to
reduce to the metal.
    In its complex compounds gold forms bonds most readily and stably with
halogens and sulfur, less stably with oxygen and phosphorus, and only weakly
with nitrogen. Bonds between gold and carbon are fairly stable, as in the
cyanide complexes and a variety of organogold compounds.
 �34s[William E. Cooley]
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�6v�2v�32sGold alloys �21m �33s
    Combinations of gold and other metals. Pure gold is soft. The addition of
copper hardens the gold, and ultimately gold-copper alloys became standard for
coinage. Gold coins in the United States contained 10% copper, the balance
gold.
    Pure gold is weak, having a tensile strength of less than 20,000 psi (138
megapascals) when annealed; however, by alloying with copper, sometimes in
conjunction with silver or nickel, and often a little zinc, gold alloys with
strengths of 60,000 - 100,000 psi (414 - 690 MPa) may be made. Addition of
these metals changes the color of gold so that red, yellow, greenish, and
white golds result. The proportion of gold in solid gold jewelry is designated
in karats (k); pure gold is 24 k, 18 k is 18/24 or 75% pure gold, and 14 k is
14/24 or 58.3% pure gold.
    Industrial uses of gold depend primarily upon the corrosion resistance and
secondarily upon the strength that can be secured by alloying alone or by
alloying and heat treating. Many alloys used in dentistry contain gold,
silver, and copper, often with small amounts of platinum and palladium; these
alloys can be heat-treated to develop strengths above 150,000 psi (1.0
gigapascals). The latter have good spring properties. Alloys of this type find
many electrical uses as contacts, particularly where rubbing is involved. Gold
electroplate, often thin, is employed on high-frequency conductors, such as
those in radar equipment, because of the high electrical conductivity and
tarnish resistance of gold. For the same reason gold is employed in the
construction of many transistors, microcircuits, printed circuits, and
integrated circuits. Most such devices are so small that the cost of gold is
relatively unimportant.
    Because gold does not oxidize when heated in air, appropriate gold
compounds can be decomposed by heat to liberate the metal. Compounds of this
type are used in the decoration of china and also for the production of
printed electrical circuits on ceramics. These materials, known as liquid
bright golds, are applied in the form of varnish, which is dried and then
heated to redness, leaving a thin film of gold firmly attached to the
underlying ceramic. See GOLD; GOLD METALLURGY.
 �34s[Charles R. Marsland ;George Sistare, Jr.]
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�6v�2v�32sGoldhaber triangle �21m �33s
    The phase space triangle, or Goldhaber triangle, corresponds to the
kinematically allowed boundary for a high-energy reaction leading to four or
more particles. In a high-energy reaction between two particles a and b
yielding four particles 1, 2, 3, and 4 in the final state (a + b -> 1 + 2 + 3
+ 4), it is convenient to consider the reaction in terms of the production of
two intermediate-state quasi-particle composites x and y, which then decay
into two particles each, as in expression (1).

		   a + b -> x			   + y
			    |			     |
			    -> 1 + 2		     -> 3 + 4		     (1)

    Most high-energy interactions indeed proceed through such intermediate
steps, in which, for specific values of the invariant masses m(sub x) = M(sub
x]* and m(sub y) = M(sub y)*, the quasi-particle composites may form resonances.
However, the description in terms of the composites x and y, with the invariant
masses m(sub x) and m(sub y) as variables, is valid irrespective of whether or
not these composites form resonances.
    The kinematical limits in this representation are particularly simple,
namely, they form a right-angle isosceles triangle.  A Goldhaber triangle plot
corresponds to plotting a point (m[sub x],m[sub y]) for each event occurring in
the above high-energy reaction.  Because of the kinematical constraints, these
points must all lie inside the triangle.
    If one considers the general reaction given in Eq. (1), then the length of
each of the two equal sides of the triangle is Q, defined in Eq.  (2).	Here W
is the total energy in the center of mass of particles a and b, and m(sub i),
for i = 1 to 4, is the mass of the particles 1 to 4.  Hence Q corresponds to the
total kinetic energy available in the reaction.

		     Q = W - SUMMATION (i=1 to 4) m(sub i)		     (2)

    In the triangle corresponding to the general reaction (see illustration),
the vertical and horizontal bands indicate resonances at masses M(sub x)* and
M(sub y)* with full width at half-maximum height or GAMMA(sub x) and GAMMA,
respectively.  The bands shown of width 2 GAMMA represent the regions usually
chosen if the events corresponding to a given resonance are selected.

 �34s[Gerson Goldhaber]
�33s


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�6v�2v�32sGold intoxication   �21m �33s 
    Most of the knowledge of human toxicity of gold is derived from its 
medicinal use (especially in rheumatoid arthritis) or from animal 
experimentation. Excessive medicinal intake of gold results in a skin rash or, 
in severe cases, exfoliation. Injury to the kidney tubules (nephritis with 
albumin in the urine) is sometimes produced. Rarely, disordered blood 
formation may occur, ranging from complete failure to form all blood elements 
to a deficiency of white blood cells or platelets. See GOLD; METALS, PATHOLOGY 
OF.  
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sGold metallurgy   �21m �33s 
    Extracting gold from ores, refining it, and preparing it for use. The 
principal gold-producing countries in order of productivity are South Africa, 
the Soviet Union, Canada, and the United States. 
    In placer deposits the gold particles are generally free, and may be 
concentrated by simple methods based on gravity (planning, rocking, and 
hydraulic mining). In ores mined from solid rock, now the chief source of 
gold, the metal is usually finely dispersed in quartz veins or lodes or 
disseminated in rock, and is usually associated with sulfide minerals. Such 
ores must be finely ground and subjected to chemical treatment for extraction 
of the gold. 
    Cyanidation is the predominant process for extracting gold from its ores. 
A dilute cyanide solution is added to the finely ground ore for dissolution of 
the gold and silver. After extraction of the gold, the solution is separated 
from the tailings by filtration or countercurrent decantation and then 
deaerated, and the gold is precipitated by the addition of zinc dust. The 
precipitate, containing gold as well as some silver and unreacted zinc, is 
smelted to eliminate the zinc. The resulting gold-silver alloy is refined by 
parting with nitric acid or hot concentrated sulfuric acid, by chlorination of 
the molten metal, or by electrolysis. See GOLD. 
 �34s[Douglas N. Halbe]
�33s 
  
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�6v�2v�32sGolgi apparatus �21m �33s
    A part of the endomembrane system of the cell, composed of regions of
stacked cisternae (saccules), or flattened saccules, lacking membrane-attached
ribosomes. A major role of the Golgi apparatus is to form vesicles whose
limiting membranes can fuse with the external or plasma membrane of the cell.
These vesicles function as vehicles for the transport of substances (membranes
and secretory products) to the external cell surface. See CELL MEMBRANES.
    The terms Golgi apparatus, Golgi component, Golgi zone, Golgi complex,
Golgi body, and dictyosome have been used for the apparatus or its components.
The term dictyosome denotes an individual stack of cisternae or saccules and
the term Golgi apparatus usually denotes a system of dictyosomes where the
dictyosomes are structurally and functionally associated (see illustration).
    In secretory cells a major function of the Golgi apparatus is to
compartmentalize products into membrane-bounded packets (the secretory
vesicles), which may function as discrete cell components (for example,
condensing vacuoles and primary lysosomes) before the vesicle contents are
discharged at the cell surface. During discharge the vesicle membranes become
part of the plasma membrane and, in certain rapidly growing or dividing cells,
may be an important source of plasma membrane. Depending on cell type,
secretory vesicles derived from the Golgi apparatus may contain enzymes,
structural proteins, glycoproteins, lipoproteins, complex carbohydrates,
polysaccharides, particles of high lipid content, or pigments. See LYSOSOME;
VACUOLE.
    Another important function of the Golgi apparatus is in the biosynthesis
and delivery of membranes and associated informational receptor molecules to
the surface of the cell.
 �34s[Hilton H. Mollenhauer ;D. J. Morre accent aigue]
�33s


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�6v�2v�32sGonad   �21m �33s 
    One of the paired primary sex glands - the testes in the male and the 
ovaries in the female. The hormones secreted by these glands are responsible 
for the development of differential secondary sex characteristics at puberty, 
for example, distribution of hair, skeletal differences, voice, and mammary 
development, as well as the more obvious influence on the primary sex organs. 
The growth, development, and functions of female and male gonads are under the 
control of gonadotropic hormones produced by the anterior lobe of the 
pituitary. See ADENOHYPOPHYSIS HORMONE; OVARY; TESTIS. 
 �34s[Choh Hao Li]
�33s 
  
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�6v�2v�32sGonorrhea   �21m �33s 
    A specific infection involving the mucous membrane of the genitourinary 
tract; the predominant disease caused by the gonococcus (Neisseria 
gonorrhoeae). Not only is gonorrhea the most prevalent of the venereal 
diseases but it is the most common specific bacterial infection of adults. The 
gonococcus may also cause disease in other organ systems by spread from the 
primary focus. Although the percentage of such complications is small, they 
are important because of their serious and crippling outcomes. Transmission of 
the gonococcus is almost wholly by sexual intercourse, and survival of the 
organism depends entirely on sexual promiscuity. The human is the only 
reservoir and natural host of this bacterium. 
    Primary infection usually involves the columnar epithelium of the male 
urethra, or the cervix in the female. In women, secondary surface spread to 
the paraurethral glands and rectal mucosa commonly occurs. Rectal and 
oral-pharyngeal infection are being noted with increasing frequency. 
    In males the usual incubation period is 2-5 days, but may be much longer. 
Typically, there is a sudden onset of frequent, painful urination accompanied 
by a profuse mucopurulent urethral discharge. After a variable period, 
infection spreads to the posterior urethra, prostrate gland, seminal vesicles, 
and epididymitis. Gonococcal epididymitis may result in eventual sterility. 
    Most females are asymptomatic; diagnosis is based on bacteriologic 
evidence or the history of sexual contact with an infected male. A minority of 
women may complain of vaginal discharge or urinary frequency and pain. Pelvic 
inflammatory disease (salpingitis) occurs if spread from the cervical glands 
to the Fallopian tubes takes place; it is accompanied by fever, nausea, 
vomiting, and lower abdominal pain. Chronic salpingitis leads to scarring and 
loss of patency, with subsequent sterility. 
    In either sex gonococci may enter the bloodstream and cause septicemia. 
These blood-borne organisms frequently localize in the joints, resulting in a 
severe form of arthritis which is the most common extragenital manifestation 
of gonorrhea. Meningitis and endocarditis are rare complications but important 
because of their severity. See ENDOCARDITIS; SEPTICEMIA. 
    Through the years gonococci have gradually developed some resistance to 
penicillin and many other commonly used antibiotics; thus the cure of 
gonorrhea requires large doses of these drugs. Penicillin G remains the 
treatment of choice. Persons allergic to penicillin may be treated with 
tetracycline or spectinomycin. See VENEREAL DISEASE. 
 �34s[James Lucas]
�33s 
  
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�6v�2v�32sGonorynchiformes   �21m �33s 
    A small order of soft-rayed teleost fishes which at one time were included 
in the large order Clupeiformes (or Isospondyli). Gonorynchiforms are fusiform 
or moderately compressed fishes, varying from less than 2 in. (5 cm) to 5 ft 
(1.5 m) in length. There are single short dorsal and anal fins, and no adipose 
fin; the caudal fin is usually forked; and the pelvic fins are placed well 
back. The jaws are weak and toothless. 
    Modern gonorynchiforms are classified in 2 suborders, 4 families, 6 
genera, and about 12 species. They have a wide fossil distribution that goes 
back to the Lower Cretaceous. Gonorynchus lives in marine shore waters of the 
Indo-Pacific area; the four genera of the Kneriidae and Phractolaemidae live 
in tropical African fresh waters; and the milkfish (Chanos chanos, family 
Chanidae) lives in marine and estuarine waters of the tropical Indo-Pacific. 
Milkfish are pond-cultured for food in Southeast Asia. See ACTINOPTERYGII; 
CYPRINIFORMES; TELEOSTEI. 
 �34s[Reeve M. Bailey]
�33s 
  
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�6v�2v�32sGooseberry   �21m �33s 
    A small fruit represented by about six species of the genus Ribes of the 
plant order Rosales. The gooseberry is a thorny, spreading bush which produces 
red, yellow, or green berries. The most desirable hardier types in the United 
States are of American parentage, or are hybrids between American and European 
species. Commercial culture is limited to a few states, notably Oregon, 
Michigan, and Washington. 
    The fruit is very acid and only a few European varieties, when fully ripe, 
are suitable for eating fresh. The fruit may be canned or frozen for use in 
pies or as preserves. See FRUIT; ROSALES. 
 �34s[J. Harold Clarke]
�33s 
  
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�6v�2v�32sGopher   �21m �33s 
    The name for the North American rodents of the family Geomyidae. There are 
39 species in 8 genera; 11 species are found in the United States. The 
distinctive name pocket gopher is applied to these animals, since they have 
large, fur-lined cheek pouches which open outward on the side of the face. Two 
common species are the northern pocket gopher (Thomomys talpoides), a small 
burrowing species, most numerous in the western United States and Mexico, and 
the prairie pocket gopher (Geomys bursarius). See RODENTIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sGorgonacea   �21m �33s 
    An order of the coelenterate subclass Alcyonaria. The Gorgonacea are the 
horny corals which often form fanlike or featherlike colonies with branches 
spread rapidly or oppositely in one plane. (see illustration). They attach to 
objects by somewhat enlarged bases or tufts of stolons. They are more widely 
distributed than the Alcyonacea and extend from the littoral zone to some 
great depth. See ALCYONARIA. 
 �34s[Kenji Atoda]
�33s 
  
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�6v�2v�32sGorilla   �21m �33s 
    The largest of the living primates of which a single species,  Gorilla 
gorilla,  is recognized. The gorilla is found in the forested areas of 
central-western Africa. There are two subspecies, the lowland gorilla, which 
is still found in fair numbers, and the mountain gorilla, which is 
comparatively rare. 
    The adult male may reach a height of 66 in. (1.68 m) and weigh more than 
400 lb (180 kg). In addition to its impressive size, the gorilla has ferocious 
features, with eyes sunk deeply under heavy brows. However, it never kills to 
eat and exists on a fruit and vegetable diet. 
    The male gorilla lives in a family unit with his females and young. He is 
extremely protective and dangerous when accompanied by a female and will not 
hesitate to attack an intruder. At times some familes band together to form a 
tribe of about 40 animals. 
    Gorillas become sexually mature at about the age of 12; since they seldom 
breed in captivity, little is known of their reproductive behavior.  See  
MAMMALIA; PRIMATES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sGout   �21m �33s 
    A hereditary disease due to abnormal purine metabolism. The disease is 
characterized by increased amounts of blood uric acid (hyperuricemia), acute 
and chronic inflammatory arthritis, tophaceous deposits of uric acid crystals, 
and renal insufficiency. The increase of uric acid is thought to be caused by 
increased production or decreased excretion of uric acid from the kidney or 
both.  See  ARTHRITIS; PURINE. 
    Primary gout occurs most frequently in middle-aged males and is passed as 
a familiar or hereditary trait which for some unknown reason does not appear 
as often in females. Secondary gout refers to the disease when associated with 
some underlying disorder causing an elevation in uric acid production (for 
example, myeloproliferative disorders). 
    The uric acid becomes deposited as urates in soft tissues, especially 
around the joints, in the cartilages of the ear, along the shafts of long 
bones, in the kidney, and occasionally on the heart valves. Such deposits, 
when superficial, can be seen grossly as reddish, inflamed masses called 
tophi. See PROTEIN METABOLISM; URIC ACID. 
 �34s[Robert Searles]
�33s 
  
  
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�6v�2v�32sGovernor   �21m �33s 
    A device used to control the speed of a prime mover. A governor protects 
the prime mover from overspeed and keeps the prime mover speed at or near the 
desired revolutions per minute. When a prime mover drives an alternator 
supplying electrical power at a given frequency, a governor must be used to 
hold the prime mover at a speed that will yield this frequency. An unloaded 
diesel engine will fly to pieces unless it is under governor control.  See  
PRIME MOVER. 
    A governor regulates the speed of a prime mover by properly varying the 
flow of energy to or from it. In the case of gas and steam turbines and 
internal combustion engines, the fuel furnishes the energy to the prime mover. 
For such applications, the governor usually controls the speed of the unit by 
regulating the rate at which fuel, and hence energy, is furnished to the prime 
mover. The governor controls the fuel flow so that the speed of the prime 
mover remains constant regardless of load and other disturbances, or changes 
in accordance with such operating conditions as changes in speed setting. 
    The speed of a prime mover is usually measured by a ballhead that contains 
flyweights driven at a speed proportional to the speed of the prime mover. The 
force from the flyweights is balanced, at least in part, by the force of 
compression of a speeder spring (see illustration). The upper end of this 
spring is positioned according to the speed setting of the governor. 
    To increase the power output of a governor, a hydraulic amplifier is often 
employed. A governor that keeps the speed of a prime mover constant is said to 
be isochronous. In a simple isochronous governor, the ballhead senses the 
speed and strokes a pilot valve plunger that regulates the flow of fluid to a 
servomotor. The performance of the simple isochronous governor is often 
greatly improved by the introduction of a dashpot in the feedback path from 
the output to the ballhead. If there is little damping in the prime mover, 
instability often occurs when the simple isochronous governor is used, whereas 
this instability is removed when the dashpot is incorporated. Acceleration 
governs are sometimes used in place of governors with dashpots. In such 
governors a flywheel is employed instead of a dashpot. The prime mover drives 
the flywheel through a spring. 
 �34s[Rufus Oldenburger]
�33s 
  
  
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�6v�2v�32sGrab bucket   �21m �33s 
    A digging and materials-handling tool with hinged sides or leaves. It is 
particularly suited for digging deep holes or for raising material to high 
places. A grab bucket may have two, three, or four movable sides. A two-sided 
bucket is somewhat rectangular in shape and is called a clamshell. A three-or 
four-leafed bucket is ball-shaped and is called an orange peel. For hard 
digging, clamshells are equipped with hardened prongs or teeth. Orange peels 
are used mainly for cleaning sewer catch basins, vertical pipes, caissons, and 
other small vertical openings. 
 �34s[Edward M. Young]
�33s 
  
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�6v�2v�32sGraben   �21m �33s 
    A block of the Earth's crust, generally with a length much greater than 
its width, that has been dropped relative to the blocks on either side (see 
illustration). The size of a graben may vary. The faults that separate a 
graben from the adjacent rocks are inclined from 50 - 70DG toward the 
downthrown block and have displacements ranging from inches to thousands of 
feet. The direction of slip on these indicates that they are gravity faults.  
See  FAULT AND FAULT STRUCTURES; HORST; RIFT VALLEY. 
 �34s[Philip H. Osberg]
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�6v�2v�32sGradient of a scalar   �21m �33s 
    The result of the application of the distributive vector differential 
operator INVERTED DELTA, INVERTED DELTA = d/dx+ d/dy+  d/dz, to a 
differentiable scalar function S(x,y,z); thus INVERTED DELTAS =  dS/dx + dS/dy 
+ dS/dz. The letters , ,  are symbols for the base vectors associated with x, 
y, z. The gradient of S is also denoted by grad  S,  while the symbol INVERTED 
DELTA is usually called del and less frequently nabla. See CALCULUS OF VECTORS. 
 �34s[Homer V. Craig]
�33s 
  
  
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�6v�2v�32sGradient wind   �21m �33s 
    A hypothetical wind based upon the assumption that the sum of the 
horizontal components of the Coriolis force and the atmospheric pressure 
gradient force per unit mass is equivalent to a wind acceleration which is 
normal to the direction of the wind itself (centripetal acceleration), with 
the implication that there are no viscous forces acting. The direction of the 
gradient wind is the same as that of the geostrophic wind. The gradient wind 
speed is less than the geostrophic speed when the air moves in a cyclonically 
curved path and greater when the air moves in an anticyclonically curved path. 
The gradient wind is a good approximation of the actual wind and is often 
superior to the geostrophic wind, particularly when the flow is strongly 
curved in the cyclonic sense.  See  CORIOLIS ACCELERATION AND FORCE; 
GEOSTROPHIC WIND. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sGrafting of plants   �21m �33s 
    Vegetative propagation by joining a scion (a short section of the stem) to 
an understock in such a manner that the two grow together and continue 
development as a single plant without change in stock or scion. 
    The various kinds of grafts may be included under either topworking or 
repair grafting. Topworking is done by three methods: root grafting, in which 
a scion is grafted on a root; crown grafting, in which a similar scion graft 
is made at the root crown just below ground level; and top grafting, including 
top budding, cleft grafting. Other methods of top grafting are whip or tongue, 
bark sawkerf, stub or notch, side, and veneer. 
    Repair grafting is of two kinds: bridge grafting, in which each of several 
scions is grafted in two positions on the stock, one above and the other below 
an injury; and inarching, in which two plants growing on their own roots are 
grafted together and one plant is served from its roots after the graft union 
is established. 
 �34s[James E. Gunckel]
�33s 
  
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�6v�2v�32sGrain boundaries   �21m �33s 
    The surfaces separating individual crystals, or grains, in a crystalline 
solid. Many solids, especially metallic and ceramic materials, are 
characterized by a periodic internal structure. This structure is defined by 
arrangement of atoms (or ions) in a precise three-dimensional array to form a 
crystal. The extent of the arrangement is large with respect to the size of 
the atom or ion. Although solids may be a single crystal, it is much more 
common for solids to be composed of many crystals, which retain their 
characteristic structure but are misoriented with respect to neighboring 
crystals by a relative rotation of their principal axes. The boundary 
separating these individual grains is known as a grain boundary and is 
characterized by a two-dimensional structure which may be related to the 
structure of neighboring grains. Therefore, grain boundaries represent planar 
defects in an otherwise perfect crystalline material.  See  CRYSTAL STRUCTURE. 
    The microscopic structure of grain boundaries is intimately related to 
many macroscopic properties, such as electrical resistivity, corrosion 
resistance, tensile strength, and fracture toughness. Perhaps the effect of 
grain boundaries on mechanical properties is more important than on any other 
class of properties. In general, strength and ductility are controlled by the 
motion of dislocations under the influence of applied forces, which is 
responsible for plastic deformation. Grain boundaries provide an effective 
barrier to this motion so that strength increases and ductility decreases with 
decreasing grain size. Curiously, the role of grain boundaries is reversed at 
elevated temperatures, at which small grain size, that is, a large amount of 
grain boundary area, facilitates plastic deformation by mass transport of 
constituent atoms along these boundaries. This phenomenon is known as creep. 
Fine-grained polycrystalline materials sometimes exhibit extraordinarily large 
degrees of plasticity at elevated temperatures. This large degree of 
plasticity, termed superplasticity, actually is an accelerated form of creep. 
See CREEP (MATERIALS); METAL, MECHANICAL PROPERTIES OF; PLASTIC DEFORMATION OF 
METAL; SUPERPLASTICITY. 
 �34s[Charles L. Bauer]
�33s 
  
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�6v�2v�32sGrain crops   �21m �33s 
    Crop plants that belong to the grass family (Gramineae), generally grown 
for their edible starchy seeds. They also are referred to as cereal crops and 
include wheat, rice, maize (corn), barley, rye, oats, sorghum (jowar), and 
millet. The grain of all these is used directly for human food and also for 
livestock, especially maize, barley, oats, and sorghum. 
    An important attribute of these grain crops is the easy manner in which 
they can be stored. The grain often dries naturally before harvest to a safe 
moisture content (10 - 12%), or can easily be dried with modern equipment. 
Grain placed in adequate storage facilities can then be protected against 
insect infestations and maintained in sound condition for years.  See  CORN; 
MILLET; OATS; RICE; SORGHUM; WHEAT. 
 �34s[E. G. Heyne]
�33s 
  
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�6v�2v�32sGram-equivalent weight �21m �33s
    The equivalent weight of an element or compound, expressed in grams (g),
on a scale in which (sup 12)C (the isotope of carbon of nuclear mass 12) has
an equivalent weight of 3 g in those compounds in which its formal valency is
4. This replaces the older definition in which the fixed point on the scale
was the equivalent weight of oxygen, 8.000 g. Because variable valency is much
more common in carbon than in oxygen, it is now necessary to specify the
formal valency in defining the equivalent weight concept. The gram-equivalent
weight of an electrolyte is usually the weight in grams which is associated
with 1 faraday of electricity.
    This concept, together with that of equivalent weight, tends to be
abandoned, and relations are expressed in terms of balanced stoichiometric
chemical equations and relative numbers of moles reacting.  See
ELECTROCHEMICAL EQUIVALENT; EQUIVALENT WEIGHT; GRAM-MOLECULAR WEIGHT; MOLE;
STOICHIOMETRY; VALENCE.
 �34s[Thomas C. Waddington]
�33s

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�6v�2v�32sGram-molecular weight �21m �33s
    The molecular weight of an element or compound expressed in grams (g),
that is, the molecular weight on a scale on which the atomic weight of the
(sup 12)C isotope of carbon is taken as 12 exactly. This replaces the earlier
scale on which the atomic weight of oxygen was taken as 16.00 g. In the
International System of Units, gram-molecular weight is replaced by the mole.
    The ratio of the gram-molecular weights of any two elements or compounds
must be identical with the ratio of the absolute weights of their individual
molecules. Therefore, the gram-molecular weights of all elements or compounds
contain the same number of molecules. This number, called the Avogadro number,
N, is 6.022 x 10(sup 23).  See	AVOGADRO NUMBER; MOLE; MOLECULAR WEIGHT;
RELATIVE MOLECULAR MASS.
 �34s[Thomas C. Waddington]
�33s

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�6v�2v�32sGram's stain   �21m �33s 
    A differential stain used in bacteriology as one of the criteria in the 
identification and classification of bacteria. The Gram's stain reaction is 
based on differences among stained bacteria in their resistance to 
decolorization by neutral solvents such as alcohol or acetone. The fixed smear 
is stained with a slightly alkaline solution of a suitable basic dye, treated 
with a mordant and then with a neutral decolorizing agent, and usually 
counterstained with a dye solution of a contrasting color. Cells or structures 
that hold the color of the primary stain are called gram-positive, and those 
that take the color of the counterstain, gram-negative. 
 �34s[Georges Knaysi]
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�6v�2v�32sGranite   �21m �33s 
    A crystalline igneous rock that consists largely of alkali feldspar 
(typically perthitic microcline or orthoclase), quartz, and plagioclase 
(commonly calcic albite or oligoclase). Its average grain size is 0.04 - 1.0 
in. (1 - 25 mm); finer-grained rocks of this composition include rhyolite and 
aplite, and coarser-grained ones are granite pegmatite.  See  APLITE; 
PEGMATITE; RHYOLITE. 
    The revised nomenclature of the International Union of Geological Sciences 
(IUGS) subcommission defines granite as containing 80 - 100% by volume quartz, 
alkali feldspar, and plagioclase in the proportions given in the illustration, 
and 20 - 0% accessory minerals. The three essential minerals must include 20 - 
60% quartz, and alkali feldspar must constitute 65 - 90% of the total 
feldspar. The variety alkali feldspar granite is similar except that alkali 
feldspar constitutes 90 - 100% of its total feldspar. The term granitic rocks 
includes granodiorite and tonalite as well as granite, and as used by some 
geologists may include quartz syenite to quartz diorite (see illustration).  
See  FELDSPAR; GRANODIORITE; HORNBLENDE; MUSCOVITE; TONALITE. 
    Granites may be divided into three major types: Calc-alkaline, 
peraluminous, and alkaline. Calc-alkaline granites typically are biotite or 
biotite-hornblende granites, some contain augite, and sphene is a common 
accessory.  
    Peraluminous granites, also known as S-type granites, contain aluminum in 
excess of that contained in feldspars and biotite; thus muscovite is an 
accessory mineral. Other aluminous minerals such as andalusite, sillimanite, 
cordierite, or garnet also may be accessory. Subalkaline to alkaline granites 
are characterized by iron-rich mafic minerals and relatively sodic alkali 
feldspar. The subalkaline type typically contains ferruginous biotite or 
hornblende, or both, but varieties containing ferrohedenbergite or fayalite 
are not uncommon. Allanite and zircon are common accessories. The alkaline 
type contains the Na-Fe minerals aegirine or riebeckite-arfvedsonite, or all 
three, and may also contain ferruginous biotite or even astrophyllite, 
eudialyte, or other rare minerals.  See  GRANITIZATION; IGNEOUS ROCKS. 
 �34s[Fred Barker]
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�6v�2v�32sGranitization   �21m �33s 
    The process whereby various types of rocks may be converted, essentially 
in the solid state, to granite or closely related material, such as 
granodiorite - a process which cannot be considered independently of 
metamorphism, metasomatism, migmatization, the partial fusion of metamorphic 
rocks, and the intrusion of granitic magma. The origin of granitic rocks has 
been the focus of controversy between petrologists (magmatists) who believed 
that the rocks crystallized for the most part from granitic magmas, and 
petrologists (transformists, granitizers) who believed that the rocks were 
formed by a metasomatic process of granitization, without the intervention of 
a silicate melt. The interpretation of migmatites has played a central role in 
the granite controversy. See MAGMA; METAMORPHIC ROCKS; METAMORPHISM; MIGMATITE. 
    Granitization is generally a large-scale change and is considered to have 
operated in orogenic zones essentially contemporaneously with metamorphism. 
Here stratified rocks, undergoing reconstitution and recrystallization, may 
suffer changes in bulk composition adequate to convert them to rocks of 
granitic character. Such stratified rocks have thus been granitized, and the 
granites formed thereby may be indistinguishable from those of magmatic 
origin. In fact, one type of granite may pass imperceptibly into the other. 
See GRANITE; GRANODIORITE. 
    Granitization on a small scale, more or less confined to contacts of 
magmatic bodies at high levels in the Earth's crust, might more appropriately 
be referred to as contact metasomatism or feldspathization. See CONTACT 
AUREOLE.  
    The change in bulk composition during granitization involves the 
introduction of certain constituents and removal of others. This substitution, 
in essentially solid rock and with no appreciable overall volume change, is 
known as metasomatism. Bodily movement of fluids (along fissures and openings) 
carrying requisite substances in solution may account for the compositional 
changes in the rocks. Where openings are too small to permit fluid flow, 
diffusion along intergrain surfaces may occur. For short distances diffusion 
through the crystal structure may be important. Extensive deformation 
accompanying metamorphism may maintain adequate channelways for the migrating 
material. See METASOMATISM. 
 �34s[Peter J. Wyllie]
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�6v�2v�32sGranodiorite   �21m �33s 
    A phaneritic (visibly crystalline) plutonic rock composed chiefly of sodic 
plagioclase (oligoclase or andesine), alkali feldspar (microcline or 
orthoclase, usually perthitic), quartz, and subordinate dark-colored (mafic) 
minerals (biotite, amphibole, or pyroxene). Granodiorite is intermediate 
between granite and quartz diorite (tonalite). For convenience granite and 
granodiorite are commonly grouped and referred to as granite. See GRANITE; 
IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sGranulite   �21m �33s 
    An important class of metamorphic rocks exposed at the surface of the 
Earth's crust, and inferred to make up a large portion of the deeper crust. 
Granulites are known to have formed at higher temperatures, and in many cases, 
higher pressures, than most other crustal rock assemblages. Thus, they are 
believed to have formed at considerable depths in the crust. See METAMORPHIC 
ROCKS.  
    Granulites may be of many different bulk compositions, inherited from 
precursor sedimentary, igneous, or lower-grade metamorphic rocks. The high 
temperatures of crystallization have resulted in very low water content, 
reflected in nearly anhydrous mineralogy. Characteristic minerals of granulite 
metabasalts are plagioclase, orthopyroxene, clinopyroxene, hornblende, and 
garnet. These minerals are also characteristic of granulites of intermediate 
to granitic composition, together with progressively greater amounts of quartz 
and potassium feldspar. The association of potassium feldspar with 
orthopyroxene is definitive for charnockite, a granulite of approximately 
granitic composition characteristic of ancient high-grade terrains. See 
BASALT; GRANITE. 
    Granulites characteristically contain CO2-rich fluid inclusions in the 
mineral grains, in contrast to the more aqueous fluid inclusions of other 
kinds of rock. This has suggested that action of volatiles low in H2O and rich 
in CO2, probably of subcrustal origin, were important in crustal metamorphism 
early in the Earth's history. See METAMORPHISM. 
 �34s[R. C. Newton]
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�6v�2v�32sGranuloma inguinale   �21m �33s 
    A mildly infectious, chronic, granulomatous disease principally affecting 
skin and subcutaneous tissues of the genital and rectal areas. The causative 
organism is Calymmatobacterium granulomatis (Donovania granulomatis). Although 
rare in the United States, the disease is very common in New Guinea, the 
Caribbean, and other tropical and subtropical areas. 
    Although the method of transmission is controversial, there is a definite 
correlation with sexual activity and a frequent association with homosexual 
behavior. Tetracyclines are the drugs of choice, with streptomycin an 
effective alternative. 
 �34s[Douglas S. Kellogg]
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�6v�2v�32sGranuloreticulosia   �21m �33s 
    A subclass of the Rhizopodea. These Protozoa produce reticulopodia which 
often fuse into networks greatly exceeding the area of the body, as in various 
Foraminiferida. Such nets are effective food traps and may also be important 
in construction of tests and cyst walls. A characteristic feature of 
reticulopodia is the bidirectional flow of cytoplasm. Granuloreticulosia 
include three orders: Athalamida, Foraminiferida, and Xenophyophorida. See 
ATHALAMIDA; FORAMINIFERIDA; RHIZOPODEA; XENOPHYOPHORIDA. 
 �34s[Richard P. Hall]
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�6v�2v�32sGrape   �21m �33s 
    The two genera of grapes are Vitis and Muscadinia. Vitis vinifera has 
intermittent forked tendrils, bark that sheds and elongated clusters with 
berries that adhere to the pedicels at maturity. This species also has thin, 
smooth, shiny leaves with three, five, or seven lobes. Berries may be round or 
oval and have edible skins that adhere to the flesh. In the American species 
skins slip from the pulp. Many American species have a characteristic musky or 
foxy odor and taste. Muscadinia can be easily distinguished from Vitis by bark 
that does not shed and simple tendrils that do not fork. 
    Viticulture is the science of grape production. In a broad sense, 
viticulture includes studies of grape varieties; methods of culture such as 
trellising, pruning, and training; insect and disease control; propagation; 
and raisin production. 
    In the United States, V. vinifera is grown on the west coast, and most of 
the grapes cultivated east of the Rocky Mountains have been derived from 
American native species such as V. labrusca and V. aestivalis, or from crosses 
between them and V. vinifera. There is also a native Caribbean species and 
several Asiatic species. There are three main species of Muscadinia that are 
found mostly in the southeast portion of the United States. 
    Table grapes are utilized for food and decorative purposes. Some of the 
leading table grapes in California are Emperor, Tokay, Thompson Seedless, 
Cardinal, and Perlette. Some of the principal commercial American varieties 
are Concord, Catawba, Delaware, and Niagara. Some of the important varieties 
of M. rotundifolia, the Muscadine grape, are Scuppernong, Thomas, and Hunt. 
    Important wine grapes in California include Cabernet Sauvignon, Carignane, 
Chardonnay, Grenache, French Colombard, and Zinfandel. Many of the North 
American and rotundifolia species that are used for eating purposes are also 
used for wine. 
 �34s[Robert J. Weaver]
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�6v�2v�32sGrapefruit   �21m �33s 
    A citrus fruit, Citrus paradisi, including both seedy and seedless 
varieties, with pink, white, or red flesh. The trees are vigorous and 
evergreen, with well-rounded tops. See EVERGREEN PLANTS. 
    Of the world supply of grapefruit, almost 80% is produced in the United 
States. Approximately one-half of this crop is marketed as fresh fruit; the 
rest is processed as canned juice and segments. Other countries producing 
significant amounts of grapefruit are Israel, Argentina, and the Republic of 
South Africa. 
 �34s[Frank E. Gardner]
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�6v�2v�32sGraphic methods   �21m �33s 
    Pencil and paper methods that employ the geometry of a plane to express 
mathematical relationships and to carry out mathematical operations in analog 
form. Some of the specific topics that arise under this broad definition are 
the plotting of experimental data, curve fitting and development of empirical 
formulas, graphical interpolation and extrapolation, nomographs and alignment 
charts, graphical differentiation and integration, and graphical statics. 
    Since graphical methods are often rapid and easily understood, they find 
widespread application in engineering. Graphical techniques are also useful in 
statistical studies to determine the correlation between two quantities, to 
obtain a frequency distribution from experimental data, to smooth data having 
an appreciable statistical fluctuation, and to represent the conclusions of a 
statistical study. See COORDINATE SYSTEMS; CURVE FITTING; EXTRAPOLATION; 
INTERPOLATION; NOMOGRAPH; NUMERICAL ANALYSIS; STATICS; STATISTICS. 
 �34s[Kaiser S. Kunz]
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�6v�2v�32sGraphic recording instruments   �21m �33s 
    Instruments that make a graphic record of one or more quantities as a 
function of another variable, usually time. Recording instruments may have any 
type of sensing device, such as pressure, temperature, voltage, or weight. The 
sole identifying feature is that they make a graphic record of the quantity 
being measured. An instrument with a recording device often uses the suffix 
graph, such as a barograph or oscillograph, in place of the suffix used with a 
similar nonrecording instrument, such as a barometer or oscilloscope. 
    Recording instruments are classified according to their principle of 
operation. They are either direct-acting, in which case the marking device is 
mechanically connected to and directly operated by the primary detector, or 
indirect-acting, in which case the measurement energy of the primary detector 
is increased through some intermediate means to actuate the marking device. 
These intermediate means are usually mechanical, electrical, electronic, or 
photoelectric.  
    In addition to the primary classification, recording instruments are 
classified by their exhibiting means, recording means, number of marking 
devices, and marking means. The exhibiting means are either circular charts or 
strip charts. The recording means may be continuous, intermittent (marking 
device retracted from chart between measurements), or sequential (more than 
one variable is intermittently recorded). The instrument may have one or more 
marking devices. The marking means may be an inking pen on a paper chart, an 
inked typed impression on a paper chart, a heated stylus on coated paper, a 
mechanical stylus on coated or chemically treated paper, an electric stylus on 
current-sensitive paper, or a light beam on film or light-sensitive paper 
which can be continuously developed during use. 
 �34s[A. Walter Jacobson]
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�6v�2v�32sGraphite   �21m �33s 
    A low-pressure polymorph of carbon (the common high-pressure polymorph 
being diamond). Graphite is metallic in appearance and very soft. Crystals of 
graphite are infrequently encountered since the mineral usually occurs as 
earthy, foliated, or columnar aggregates often mixed with iron oxide, quartz, 
and other minerals. See CARBON. 
    The sheetlike character of the graphite atomic arrangement results in 
distinctive physical properties. The mineral is very soft, with hardness 1**; 
it soils the fingers and leaves a black streak on paper, hence its use in 
pencils. The specific gravity is 2.23, often less because of the presence of 
pore spaces and impurities. The color is black in earthy material to 
steel-gray in plates, and thin flakes are deep blue in transmitted light. 
Graphite is a conductor of electricity. 
    The major sources of graphite are in gneisses and schists, where the 
mineral occurs in foliated masses mixed with quartz, mica, and so on. 
Noteworthy localities include the Adirondack region of New York, Korea, and 
Ceylon. In Sonora, Mexico, graphite occurs as a product of metamorphosed coal 
beds. Graphite is also observed in meteorites. 
 �34s[Paul B. Moore]
�33s 
  
    Synthetic graphite.  Commercially produced synthetic graphite is a mixture 
of crystalline graphite and cross-linking intercrystalline carbon. Its 
physical properties are the result of contributions from both sources. Thus, 
among engineering materials, synthetic graphite is unusual because a wide 
variation in measurable properties can occur without significant change in 
chemical composition. 
    At room temperature the thermal conductivity of synthetic graphite is 
comparable to that of aluminum or brass. An unusual property of graphite is 
its increased strength at high temperature. Graphite is resistant to thermal 
shock because of its high thermal conductivity and low elastic modulus. It is 
one of the most inert materials with respect to chemical reaction with other 
elements and compounds. It is subject to oxidation, reaction with and solution 
in some metals. 
    Graphite has many uses in the electrical, chemical, metallurgical, 
nuclear, and rocket fields: electrodes in electric furnaces producing carbon 
steel, alloy steel, and ferroalloys; anodes for the electrolytic production of 
chlorine, chlorates, magnesium, and sodium; motor and generator brushes; 
sleeve-type bearings and seal rings; rocket motor nozzles; missile nose cones; 
metallurgical molds and crucibles; linings for chemical reaction vessels; and, 
in a resin-impregnated impervious form, for heat exchangers, pumps, pipings, 
valves, and other process equipment. 
  
    Graphite (carbon) fibers.  Carbon fibers are filamentary forms of carbon, 
with a fiber diameter normally in the 6 - 10-micrometer range. The product is 
offered in the form of yarns or tows containing from 1000 to 500,000 filaments 
per strand. The fibers offer a unique combination of properties. They are 
flexible, lightweight, thermally and to a large extent chemically inert, and 
are good thermal and electrical conductors. In their high-performance 
varieties, carbon fibers are very strong and can be extremely stiff. 
    The principal use of high-performance carbon fibers is as the reinforcing 
component in structural epoxy matrix composites. Due to initially high cost, 
the original applications were almost exclusively for lightweight, 
high-stiffness, and high-strength composites for the aerospace industry. The 
second major usage of high-performance carbon fibers is in sporting goods, 
such as golf club shafts, tennis rackets, fishing rods, and sailboat 
structures. The major matrix material for both applications is epoxy. See 
COMPOSITE MATERIAL. 
 �34s[Herbert F. Volk]
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�6v�2v�32sGraph theory   �21m �33s 
    A branch of mathematics that belongs partly to combinatorial analysis and 
partly to topology. Its applications occur (sometimes under other names) in 
electrical network theory, operations research, organic chemistry, theoretical 
physics, and statistical mechanics, and in sociological and behavioral 
research. Both in pure mathematical inquiry and in applications, a graph is 
customarily depicted as a topological configuration of points and lines, but 
usually is studied with combinatorial methods. See COMBINATORIAL THEORY; 
TOPOLOGY.  
    A graph consists of a set of points, a set of lines, and an incidence 
relation that designates the end points of each line. In many applications no 
line starts and ends at the same point. (Such a line would be called a loop.) 
Also, no two lines have the same pair of end points. A graph whose lines 
satisfy these conditions is called simplicial. The valence of a point is the 
number of lines incident on it, calculated so that a loop is twice incident on 
its only end point. Two graphs are isomorphic if there is one-to-one 
correspondence from the point set and line set of one onto the point set and 
line set, respectively, of the other that preserves the incidence relation. An 
automorphism of a graph is an isomorphism of a graph with itself. Two graphs 
are homeomorphic if, after smoothing over all points of valence 2, the 
resulting graphs are isomorphic. See GROUP THEORY. 
    Drawing a graph on a surface decomposes the surface into regions. One 
colors the regions so that no two adjacent regions have the same color, rather 
like a political map of the world. It is a remarkable fact that for a given 
surface, there is a single number of colors that will always be enough no 
matter how many regions occur in a decomposition of the surface. The smallest 
such number is called the chromatic number of that surface. It is easy to draw 
a plane map that requires four colors. In 1976 K. Appel and W. Haken settled a 
problem dating back to about 1850, by showing that four colors are always 
enough for plane maps. 
    A graph is planar if it can be drawn in the plane so that none of its 
lines cross each other. Neither of the two graphs in the illustration can be 
drawn in the plane. K. Kuratowski proved in 1930 that a graph is planar if and 
only if it contains no subgraph homeomorphic to either of those two graphs. 
    In a directed graph, or digraph, each line ab is directed from one end 
point a to the other end point b. There is at most one line from a to b. 
    An oriented graph is obtained from an ordinary graph by assigning a unique 
direction to every line. If there is one line between every pair of points and 
no loops, an ordinary graph is called complete. An oriented complete graph is 
called a tournament. 
 �34s[Jonathan L. Gross]
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�6v�2v�32sGraptolithina   �21m �33s 
    A class of extinct marine colonial animals believed to be related to the 
Hemichordata. They have an organic skeleton which appears to be made of 
collagen fibrils. The skeleton forms a series of tubes varying in diameter 
from 0.0012 to 0.8 in. (30 micrometers to 2 millimeters) whose walls are built 
of a series of half rings usually strengthened by supplementary cortical 
material, mainly on the outside. This structure is similar to that seen in the 
extant Rhabdopleura and its allies. The colonies range in size from a few 
millimeters diameter to long, slender ones which may be more than a meter long 
but less than a centimeter wide. Graptolites are found in Paleozoic rocks of 
all continents, but are most abundant in dark shales of the Ordovician and 
Silurian which often contain few other fossils. 
    The class Graptolithina is divided into several orders, of which the 
Dendroidea and Graptoloidea are the most important. The other orders and some 
doubtfully associated groups are known from relatively few occurrences. 
 �34s[Isles Strachan]
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�6v�2v�32sGrass   �21m �33s 
    Grasses provide about 53% of the total feed units (forages and grains) 
consumed by all domestic livestock, in the form of pastures, native range 
grazing, and harvested forage (hay, green feed, and grass silage). Grasses 
fill other important roles in the national economy: in conservation of soils, 
in development and conservation of water resources, in upstream flood control, 
in wildlife and game management, and in a wide range of outdoor recreation. 
See GRASSLAND ECOSYSTEM. 
  
    Grass stems have solid joints (nodes) and leaves arranged in two rows, 
with one leaf at each joint (see illustration). The leaves consist of the 
sheath, which fits around the stem like a split tube, and the blade, which is 
commonly long and narrow. Seed heads are made up of minute flowers on tiny 
branchlets, often several crowded together, but always two-ranked like the 
leaves. The flowers are generally wind-pollinated. The seeds are enclosed 
between two bracts, or glumes, which remain on the seed when ripe. See FLOWER. 
    Grass species may be annual or perennial. Some perennial species are 
perpetuated by creeping stems as well as by seed. Stems that creep undergound 
are termed rhizomes, or rootstocks, whereas surface-creeping stems are called 
runners, or stolons. Species having creeping stems produce a well-knit sod. 
These generally tolerate continuous close grazing and are utilized primarily 
for pastures. Species that do not creep vegetatively tend to grow erect. They 
make an open or loose sod, and are best suited for hay or silage. In subhumid 
to semiarid regions, range species are predominantly perennial and are 
perpetuated mostly by seed. In other areas both annuals and perennials 
commonly occur. All grasses have a fibrous type of root system that permeates 
the soil extensively and is effective in preventing soil erosion and restoring 
soil humus content. Grasses are worldwide in distribution; adapted species are 
available for nearly all conditions of soil and climate. See separate articles 
for grasses under their common names. 
 �34s[Howard B. Sprague]
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�6v�2v�32sGrassland ecosystem �21m �33s
    Grassland vegetation comprises herbaceous (not woody) plant species;
graminoids (grasses and grasslike plants) usually dominate. Natural grasslands
once occupied at least one-third of the land surface of the Earth, occuring in
regions which were too arid for forest, but where the climate was not so
adverse as to prevent the development of a closed perennial herbaceous cover
that is lacking in desert. In addition, grassland with scattered trees
(savannas) occurs in transition zones between grassland and forest,
particularly in the tropics and subtropics, and in semidesert regions shrubs
are scattered throughout the grassland. Humans have converted some forest
areas into seminatural grasslands, and they have modified many natural
grasslands by tillage and by grazing domesticated livestock. The most
extensive areas of natural open grassland occur in the temperate zone
(south-central North America, north-central Asia, eastern Europe, southeastern
South America, and southern Africa). Savannas are widespread in the tropical
regions of Africa, South America, and Australia; shrubby grasslands lie
adjacent to deserts.
    While there are a large number of plant species in the grassland
ecosystems of the world, usually very few grasses dominate in any one
location: commonly 50 - 200 species of flowering plants occur in the same
grassland, but only a few of these are considered dominants. Grassland
complexity (in terms of numbers of plant species) tends to increase with
increasing favorableness of habitat and to decrease with increasing aridity.
    The treelessness of natural grassland is often the result of aridity.
Relatively low precipitation and high evaporation restrict the supply of soil
moisture. Most trees cannot compete effectively with grasses where upper soil
layers are intermittently moist, but deeper layers are continuously dry. Other
factors that have been cited to account for the absence of trees in various
grassland areas are impermeable soil, excessive salinity, and poor drainage.
    Natural grasslands have developed in areas where fires are characteristic,
since herbaceous species are much better adapted than trees and shrubs to
withstand the effects of fire. This is because their perennating buds are
located near the soil surface where they are less exposed, and because they
have no perennial stems.
    The two principal purposes for which natural grasslands are used are crop
production and domesticated livestock grazing. Only a small fraction of the
world's grasslands have been brought under tillage because of constraints
imposed by climate, aridity, soil conditions (including stoniness, salinity,
and erodibility), and topography. Most of the remainder are in some form of
ranching enterprise. See ECOSYSTEM; GRASS; RANGELAND CONSERVATION.
 �34s[Robert T. Coupland ;George Van Dyne]
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�6v�2v�32sGravel   �21m �33s 
    A loose, or unconsolidated, deposit of rounded pebbles, cobbles, or 
boulders, whose size is greater than 0.08 in. (2 mm) in diameter. Gravels may 
consist only of pebbles, cobbles, and boulders, with large voids between the 
particles; these are the openwork gravels. Commonly, however, gravels have 
sand filling the interstices between the pebbles. The sand filling the pores 
is referred to as the matrix and commonly makes up about 30% of the rock. 
 �34s[Raymond Siever]
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�6v�2v�32sGravimetric analysis   �21m �33s 
    That branch of quantitative chemical analysis in which a desired 
constituent is converted (usually by precipitation) to a pure compound or 
element of definite, known composition, and is weighed. In a few cases, a 
compound or element is formed which does not contain the constituent but bears 
a definite mathematical relationship to it. In either case, the amount of 
desired constituent can be determined from the weight and composition of the 
precipitate.  
    At least two weighings are required for each analysis - the original 
sample, and the dried or ignited residue. From these weights, the percentage 
or proportion of the desired constituent may be calculated from the equation 
below.  

       *** %A = (wt of residue X factor X 100) / (wt of sample) ***

    The factor is determined from a knowledge of the chemical relationships 
between the weight of substance A contained in, or equivalent to, a fixed 
weight of residue of known composition. See ANALYTICAL CHEMISTRY; 
ELECTRODEPOSITION ANALYSIS; QUANTITATIVE CHEMICAL ANALYSIS; STOICHIOMETRY. 
 �34s[Stephen G. Simpson]
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�6v�2v�32sGravitation �21m �33s
    The mutual attraction between all masses and particles of matter in the
universe. In a sense this is one of the best-known physical phenomena. During
the 18th and 19th centuries gravitational astronomy, based on Newton's laws,
attracted many of the leading mathematicians and was brought to such a pitch
that it seemed that only extra numerical refinements would be needed in order
to account in detail for the motions of all celestial bodies. In the present
century, however, Albert Einstein shattered this complacency, and the subject
is currently in a healthy state of flux.

    Newton's law of gravitation.  Newton's law of universal gravitation states
that every two particles of matter in the universe attract each other with a
force that acts in the line joining them, the intensity of which varies as the
product of their masses and inversely as the square of the distance between
them. Put into symbols, the gravitational force F exerted between two
particles with masses m1 and m2 separated by a distance d is given by the
equation below, where G is called the constant of gravitation.

			  F = Gm1m2/d(sup 2)=c

    Gravitational constant.  In 1774 N. Maskelyne determined G by measuring
the deflection of the vertical by the attraction of a mountain. This method is
much inferior to the laboratory method in which the gravitational force
between known masses is measured. In the torsion balance two small spheres,
each of mass m, are connected by a light rod, suspended in the middle by a
thin wire. The deflection caused by bringing two large spheres each of mass M
near the small ones on opposite sides of the rod is measured, and the force is
evaluated by observing the period of oscillation of the rod under the
influence of the torsion of the wire (see illustration). This is known as the
Cavendish experiment, in honor of H. Cavendish who achieved the first reliable
results by this method in 1797 - 1798. Modern determinations yield the
results: constant of gravitation G = 6.67 x 10-(sup 11) mks units; mass of
Earth = 5.98 x 10(sup 27)g. The uncertainty of these results is probably about
one-half unit of the last place given.
    In newtonian gravitation G is an absolute constant, independent of time,
place, and the chemical composition of the masses involved. Partial
confirmation of this was provided before Newton's time by the experiment
attributed to Galileo when different weights released simultaneously from the
top of the tower of Pisa reached the ground at the same time. Newton found
further confirmation, experimenting with pendulums made out of different
materials. Early in this century, R. Eoetvoes found that different materials
fall with the same acceleration to within 1 part in 10(sup 7). The accuracy of
this figure was extended by R. H. Dicke to 1 part in 10(sup 11), using
aluminum and gold, and by V. B. Braginskii and V. I. Panov to 0.9 x
10-(sup 12) with a confidence of 95%, using aluminum and platinum.

    Mass and weight.  A cosmology with changing physical ``constants'' was
first proposed in 1937 by P. A. M. Dirac. Field theories applying this
principle have since been proposed by P. Jordan and D. W. Sciama and by C.
Brans and R. H. Dicke. In these theories G is diminishing; for instance Brans
and Dicke suggest a change of about 2 x 10-(sup 11) per year. This would have
profound effects on phenomena ranging from the evolution of the universe to
the evolution of the Earth.
    In the equations of motion of newtonian mechanics, the mass of body
appears as inertial mass as a factor of the acceleration, and as gravitational
mass in the expression of the gravitational force. The equality of these
masses is confirmed by the Eoetvoes experiment. It justifies the assumption
that the motion of a particle in a gravitational field does not depend on its
physical composition. In Newton's theory the equality can be said to be a
coincidence, but not in Einstein's theory, where inertia and gravitation are
unified.
    While mass in newtonian mechanics is an intrinsic property of a body, its
weight depends on certain forces acting on it. For example, the weight of a
body on the Earth depends on the gravitational attraction of the Earth on the
body and also on the centrifugal forces due to the Earth's rotation. The body
would have lower weight on the Moon, even though its mass would remain the
same. See CENTRIFUGAL FORCE.

    Gravity.  This should not be confused with the term gravitation. Gravity
is the older term, meaning the quality of having weight, and so came to be
applied to the tendency of downward motion on the Earth. Gravity or the force
of gravity are today used to describe the intensity of gravitational forces,
usually on the surface of the Earth or another celestial body. So gravitation
refers to a universal phenomenon, while gravity refers to its local
manifestation. See GRAVITY.

    Accuracy of newtonian gravitation.	A discrepancy in newtonian gravitation
was discovered by Leverrier in the orbit of Mercury. Because of the action of
the other planets, the perihelion of Mercury's orbit advances. But allowance
for all known gravitational effects still left an observed motion of about 43
seconds of arc per century unaccounted for by Newton's theory. Attempts to
account for this by adding an unknown planet or by drag with an interplanetary
medium were unsatisfactory, and a very small change was suggested in the
exponent of the inverse square of force. This particular discordance was
accounted for by Einstein's general theory of relativity in 1916, but the
final word on the subject has yet to be said. See RELATIVITY.

    Gravitational lens.  Light is deflected when it passes through a
gravitational field, and an analogy can be made to the refraction of light
passing through a lens. It has been suggested that a galaxy situated between
an observer and a more distant source might have a focusing effect, and that
this might account for some of the observed properties of quasistellar
objects. The multiple images of one quasar (QSO, 0957 + 567 A,B) are almost
certainly caused by the light from a single body passing through a
gravitational lens. See GRAVITATIONAL LENS; QUASARS.

    Relativistic theories.  In spite of its success and the absence of a
reasonable alternative, Newton's theory was heavily criticized, not least with
regard to its requirement of ``action at a distance'' (that is, through a
vacuum). Newton himself considered this to be ``an absurdity,'' and he
recognized the weaknesses in postulating in his system of mechanics the
existence of preferred reference systems (that is, inertial reference systems)
and an absolute time.
    Einstein showed in his special theory of relativity that these postulates
were physically unacceptable. In his general theory of relativity he
incorporated gravitational phenomena in such a way that there was no longer
any preferred reference system. He treated the phenomenon of gravitation as a
consequence of the geometric properties of space-time, this geometry being
affected by the presence of gravitating matter. The acceleration of a body is
determined by the local geometry, and the newtonian concepts of gravitational
force and action at a distance are abandoned. Mathematically the theory is far
more complicated than Newton's. Instead of the single potential described
above, Einstein worked with 10 quantities that are members of a tensor. See
RELATIVITY; TENSOR.
    An important step in Einstein's reasoning is his ``principle of
equivalence'': that a uniformly accelerated reference system imitates
completely the behavior of a uniform gravitational field. This principle
requires that all bodies fall in a gravitational field with precisely the same
acceleration and this is confirmed by the Eoetvoes experiment mentioned
earlier. Also, if matter and antimatter were to repel one another, it would be
a violation of the principle. Einstein's theory requires that experiments
should have the same results irrespective of the location or time. This has
been said to amount to the ``strong'' principle of equivalence. See FREE FALL.

    Gravitational waves.  The existence of gravitational waves, or
gravitational ``radiation,'' was predicted by Einstein shortly after he
formulated his general theory of relativity. They are now a feature of any
relativity theory. Gravitational waves are ``ripples in the curvature of
space-time.'' In other words, they are propagating gravitational fields, or
propagating patterns of strain, traveling at the speed of light. They carry
energy and can exert forces on matter in their path, producing, for instance,
very small vibrations in elastic bodies. The gravitational wave is produced by
change in the distribution of some matter. It is not produced by a rotating
sphere, but would result from a rotating body not having symmetry about its
axis of rotation: a pulsar, perhaps. In spite of the relatively weak
interaction between gravitational radiation and matter, the measurement of
this radiation is now technically possible and may already have been achieved.
 �34s[J. M. A. Danby]
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�6v�2v�32sGravitational collapse   �21m �33s 
    The rapid implosion of a star or other astronomical body from its initial, 
normal size to a size hundreds or thousands of times smaller. Astrophysicists 
have devoted considerable effort to understanding the mechanisms behind such 
explosive astrophysical phenomena as supernovae, quasars, the origin of strong 
radio sources, and explosions in the nuclei of galaxies. Because the explosive 
energy in these objects may be a sizable fraction of the rest mass, and 
because thermonuclear reactions, such as occur in the Sun and in hydrogen 
bombs, can convert no more than 0.9% of the rest mass to energy, a search has 
been made for other, more powerful mechanisms of energy release. See NUCLEAR 
FUSION; PLASMA PHYSICS; QUASARS; RADIO SOURCES (ASTRONOMY); SUPERNOVA; 
THERMONUCLEAR REACTION. 
    The most likely alternative mechanism seems to be gravitational collapse. 
If a star or other body collapses to a small fraction of its original size and 
then releases the final kinetic energy of collapse as radiation and 
high-energy particles, a sizable fraction of its original mass is converted to 
energy. There is no fully compelling reason to believe that gravitational 
collapse is important in quasars, in the origin of strong radio sources, or in 
galactic explosions. However, theoretical and observational studies provided 
convincing evidence that collapse is important in supernovae. 
    The effectiveness of gravitational collapse as a mechanism for converting 
mass to explosive energy is severely limited by general relativistic 
phenomena. J. Robert Oppenheimer and his students showed in 1939 that any 
spherical star which collapses to a radius smaller than its Schwarzschild 
radius, given by the equation below, can no longer release its kinetic energy 

          *** R (sub Sch) =  3.0 [M/M (sub circled dot)] km ***

of collapse in any form to the outside universe; it must continue to collapse 
until gravitation forces have compressed it to infinite density (general 
relativistic singularity). The work of Oppenheimer and his students and of S. 
Chandrasekhar also revealed that any star exceeding about 1.5 solar masses 
(the critical value is uncertain by a factor 2) must eventually reduce its 
mass below this value - for example, by a supernova explosion - or face 
inevitable collapse past its Schwarzschild radius and into the singularity. A 
set of theorems due to R. Penrose and S. Hawking generalize to situations 
without spherical symmetry the concept of being inside the Schwarzschild 
radius, and the consequent inevitability of continued collapse to a 
singularity.  
    Evidence has suggested strongly that a black hole, produced millions of 
years ago by stellar collapse, is now in orbit around a massive, bright star
in the constellation Cygnus. The hole seems to be pulling gas off its 
companion. As the gas falls inward toward the hole's Schwarzschild radius, it
emits x-rays which have been detected by satellites above the Earth's 
atmosphere. The name of this possible black hole is Cygnus X-1. See BLACK
HOLE.
 �34s[Kip S. Thorne]
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�6v�2v�32sGravitational lens   �21m �33s 
    A massive body producing distorted, magnified, or multiple images of more 
distant objects when its gravitational fields bend the paths of light rays. 
Lenses have been observed when the light from very distant quasars is affected 
by intervening galaxies and clusters of galaxies, producing several different 
images of the same quasar. Albert Einstein predicted the occurrence of this 
phenomenon in 1936, but the discovery of real gravitational lenses did not 
occur until 1979. Gravitational lenses, in addition to being intrinsically 
interesting, can reveal the intrinsic properties of galaxies, active galaxies, 
and quasars, and provide information on the overall geometry and scale of the 
universe. See GRAVITATION. 
    Gravitational lenses can make distant quasars appear brighter than they 
would otherwise be, since their focusing action beams more light in the 
direction of the Earth. This effect could cloud the interpretation of the 
quasar statistics, which currently argues for a ``quasar era'' early in cosmic 
evolution.  
    When the brightness of the multiple images in gravitational lenses has 
been monitored for a long time, lenses may reveal the cosmos in some more 
fundamental ways. If the energy-emitting regions of quasars are small (as is 
believed), then stars in the halos of galaxies could also act as lenses, 
changing the intensity of the light from quasar images when the stars pass 
between the Earth and the quasar. The lens phenomenon could be used to detect 
the existence of these stars, even if they were too faint to be seen 
optically. Variations in the brightness of multiple images of the same quasar 
could also determine the distance to the lensing galaxy and thus the scale of 
the universe. See COSMOLOGY. 
 �34s[Harry L. Shipman]
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�6v�2v�32sGravitational redshift   �21m �33s 
    A shift toward longer wavelengths of spectral lines emitted by atoms in 
strong gravitational fields. One of three famous predictions of the general 
theory of relativity, this shift results from the slowing down of all periodic 
processes in a gravitational field. The amount of the shift is proportional to 
the difference in gravitational potential between the source and the receiver. 
For starlight received at the Earth the shift is proportional to the mass of 
the star divided by its radius. In the solar spectrum the shift amounts to 
about 0.001 nanometer at a wavelength of 500 nanometers. In the spectra of 
white dwarfs, whose ratio of mass to radius is about 30 times that of the Sun, 
the shift is about 0.03 nanometer, which can easily be measured if it can be 
separated from the Doppler effect. See WHITE DWARF STAR. 
 �34s[David Layzer]
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�6v�2v�32sGraviton   �21m �33s 
    A theoretically deduced particle postulated as the quantum of the 
gravitational field. According to Einstein's theory of general relativity, 
accelerated masses (or other distributions of energy) should emit 
gravitational waves, just as accelerated charges emit electromagnetic waves. 
And according to quantum field theory, such a radiation field should be 
quantized; that is, its energy should appear in discrete quanta, called 
gravitons, just as the energy of light appears in discrete quanta, namely 
photons. See ELEMENTARY PARTICLE; GRAVITATION; QUANTUM FIELD THEORY; 
RELATIVITY.  
 �34s[Charles J. Goebel]
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�6v�2v�32sGravity �21m �33s
    The gravitational attraction at the surface of a planet or other celestial
body. The quantity g is often referred to simply as ``gravity'' or ``the force
of gravity'' of Earth, both of which are incorrect. The force of gravity means
the force with which a celestial body attracts an object, that is, the weight
of the object. The letter g represents the acceleration caused by the
gravitational force and, of course, has the dimensions of acceleration. See
GRAVITATION.
 �34s[Dirk Brouwer /Gerald M. Clemence]
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�6v�2v�32sGravity meter �21m �33s
    An instrument that measures the acceleration due to gravity; it is also
known as a gravimeter. Different types of instruments are used, depending on
whether the information required is an absolute value, a gravity difference
between two or more locations, or long-term variations at a single point.
Gravity acceleration is usually measured in Gals [1 Gal = 1 cm/(s sup 2)],
which is the acceleration produced by a force of 1 dyne acting on a mass of
1 gram. For small differences, it is useful to use a smaller unit, the milligal
(mGal), which equals 10 (sup -3) Gal. The acceleration of gravity at the
Earth's surface (g) is approximately 980 Gals.
    Absolute gravity measurements are made by direct measurement of the
acceleration of bodies falling in a vacuum. Relative gravity measurements
between stations a few meters to a few thousand kilometers apart are made with
four types of apparatus: spring, vibrating string, force-balance, and
pendulum. Each type has certain advantages, depending on the mode of transport
on land, sea, or air or on the ability to fit within a narrow-diameter drill
hole. See ACCELEROMETER.
    Absolute gravity systems utilize falling reflectors in high vacuums. The
rate of fall is determined by an optical interferometer with a laser for a
light source. Multiple drops are made, usually at least 50, over a vertical
distance of about 1.5 ft (0.5 m). These systems provide absolute gravity
values to an accuracy of about 0.03 mGal (standard error).
    In principle, a spring-type gravity meter may be visualized as a simple
mass m on a spring. If the meter is taken to another location where gravity is
greater by an amount DELTA g, the spring will be stretched by some distance s,
and this distance is proportional to the difference in gravity, Eq. (1) or
(2), where k is the sensitivity of the spring.

			*** DELTA s = k DELTA g ***			 (1)

		       *** DELTA g = (DELTA s) / k ***			 (2)

    The natural frequency of a vibrating string depends on its tension and the
mass per unit length of the string. If the tension is provided by a suspended
mass, the frequency of the string changes with variations in gravity, and the
specific change in frequency can be monitored with a high precision.
    Changes in the gravitational pull on a ferromagnetic mass can be balanced
by electromagnetic forces, and these forces can be measured very accurately by
measuring electric current. This principle is employed in the Bell gravity
meter used on surface ships at sea, and in the cryogenic gravity meter. The
latter consists of an iron ball held in place by a magnetic field. This field
is produced by a superconducting electromagnet kept within 1 DG C of absolute
zero (--273 DG C). The stability of the magnetic field at this low temperature
makes it possible for the cryogenic meter to measure changes in the Earth's
gravity field as small as 1 microgal (10 sup -6 Gal). See GEOPHYSICAL
EXPLORATION; GRAVITY.
 �34s[H. W. Oliver]
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�6v�2v�32sGraybody   �21m �33s 
    An energy radiator which has a blackbody energy distribution, reduced by a 
constant factor, throughout the radiation spectrum or within a certain 
wavelength interval. The designation ``gray'' has no relation to the visual 
appearance of a body but only to its similarity in energy distribution to a 
blackbody. Most metals, for example, have a constant emissivity within the 
visible region of the spectrum and thus are graybodies in that region. See 
BLACKBODY; HEAT RADIATION. 
 �34s[Heinz G. Sell ;Peter J. Walsh]
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�6v�2v�32sGraywacke   �21m �33s 
    An argillaceous sandstone (``dirty'' sandstone) that is characterized by 
an abundance of unstable mineral and rock fragments and a fine-grained clay 
matrix binding the larger sand-size detrital fragments. One of the most 
important characteristics is the clay matrix, which constitutes a minimum of 
15% of the rock and may, in extreme cases, make up almost half of the bulk. 
    The unstable mineral fragments include feldspar, augite, hornblende, 
serpentine, biotite, chlorite, and magnetite. Rock fragments include many 
varieties of low- and high-grade metamorphic rocks (phyllite, slate, 
quartzite, and granulite), as well as aphanitic (microscopically crystalline) 
volcanic rocks, typically mafic in composition. The clay matrix is composed of 
a mixture of micaceous miners. 
    The sorting of graywackes is poor. Thus the size distribution curve is 
skewed toward the fine sizes. Even when the matrix is not considered, the 
larger detrital grains are poorly sorted. The grains tend to be sharp and 
angular and may show little or no evidence of abrasion.  See  SANDSTONE; 
SEDIMENTARY ROCKS; SUBGRAYWACKE. 
 �34s[Raymond Siever]
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�6v�2v�32sGreat circle, terrestrial   �21m �33s 
    A circle or near-circle described on the Earth's surface by a plane 
passing through the center of the Earth (see illustration). The most important 
such circle is the Equator, cut by the plane which bisects, and is 
perpendicular to, the Earth's axis. Planes through the poles cut the Earth 
along meridians. The described forms are not quite circular, being slightly 
flattened ellipsoids. All other great circles have shapes between those of the 
Equator and the meridians. Even the equatorial circle and the ellipsoids have 
minor irregularities.  See  GEODESY; MATHEMATICAL GEOGRAPHY. 
    Segments of great circles extending from beginning to terminus of proposed 
global travel make the shortest routes on the Earth's surface. Ships sailing 
for short distances, however, follow compass directions, also called rhumb 
lines, or loxodromes. Except where they coincide with meridians or the 
Equator, loxodromes are not segments of great circles. In transoceanic 
voyages, distance can be shortened by following great-circle routes called 
orthodromes. These are laid out on gnomonic charts which show every orthodrome 
as a straight line. Ships, however, navigate by compass; thus orthodromes are 
transferred to a Mercator chart on which compass directions appear as straight 
lines; steering is actually by a series of short segments of compass 
directions. The longer the distance, the greater the deviation between 
loxodromes and orthodromes.  See  CARTOGRAPHY; MAP PROJECTIONS; NAVIGATION. 
    Great circles are used in various technical fields. Radio waves propagate 
along great circles from transmitting to receiving stations. This is utilized 
in loran and similar navigational systems. Great-circle directions are 
important in seismology, ballistics, intercontinental missiles, satellites, 
and like technical aspects of travel and transmission. 
 �34s[Erwin Raisz /Wesley Calef]
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�6v�2v�32sGreenhouse effect   �21m �33s 
    The Earth's atmosphere acts as the glass walls and roof of a greenhouse in 
trapping heat from the Sun. Like the greenhouse, it is largely transparent to 
solar radiation, but it strongly absorbs the longer-wavelength radiation from 
the ground. Much of this long-wave radiation is reemitted downward to the 
ground, with the paradoxical result that the Earth's surface receives more 
radiation than it would if the atmosphere were not between it and the Sun. 
    The absorption of long-wave (infrared) radiation is effected by small 
amounts of water vapor, carbon dioxide, and ozone in the air and by clouds. 
Clouds are almost completely opaque to infrared radiation, unless they are 
extremely thin. The appearance even of cirrus clouds after a period of clear 
sky at night is enough to cause the surface air temperature to increase 
rapidly by several degrees because of radiation from the cloud. See OZONE; 
TERRESTRIAL RADIATION. 
 �34s[Lewis D. Kaplan]
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�6v�2v�32sGreenockite   �21m �33s 
    A mineral having composition CdS (cadmium sulfide). Greenockite usually 
occurs as earthy coatings with resinous luster and yellow-to-orange color. 
There is good prismatic cleavage; the hardness is 3 (Mohs scale) and specific 
gravity is 4.9. Greenockite and wurtzite, ZnS, are isostructural, and a 
complete solid-solution series exists between the two minerals. Although 
greenockite is the most common cadmium mineral, no deposits of it are 
sufficiently large to warrant mining it solely as a source of cadmium. See 
CADMIUM.  
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sGreen's theorem �21m �33s
    A term used variously in mathematical literature to denote either (1) the
Gauss divergence theorem TRIPLE INTEGRAL DEL DOT SCRIPT F dV = DOUBLE INTEGRAL
SCRIPT F DOT BOLD nu dS or some one of several forms or immediate consequences
of this theorem, or (2) the plane case of Stokes' theorem.
See CALCULUS OF VECTORS;  GAUSS' THEOREM; STOKES' THEOREM.
 �34s[Homer V. Craig]
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�6v�2v�32sGregarinia �21m �33s
    A subclass of the class Telosporea. These protozoans occur principally as
extracellular parasites in the digestive tracts and body cavities of
invertebrates. There are three orders: the Archigregarinida, whose life cycle
embraces both sexual and asexual phases; the Eugregarinida, which increases
only by sporogony; and the Neogregarinida, whose life cycle involves
schizogony and gamont formation. See ARCHIGREGARINIDA; EUGREGARINIDA;
NEOGREGARINIDA; TELOSPOREA.
 �34s[Elery R. Becker /Norman D. Levine]
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�6v�2v�32sGreisen   �21m �33s 
    A pneumatolytically altered granite consisting of mainly quartz and a 
light-green (white) mica; the feldspars and biotites of the original granite 
have been replaced more or less completely by muscovite, lithium mica, 
kaolinite, and tourmaline. Other substances usually introduced during 
alteration are topaz, apatite, fluorite, and ores of iron, and in some places 
ores of tungsten and tin. See GRANITE; PNEUMATOLYSIS. 
    Greisen occurs in bands and veins intersecting granite. The bands are 
rarely more than 2 ft (0.6 m) thick with indefinite margins that grade into 
unaltered granite. They were evidently formed by flow of vapors and gases 
through fissures. 
 �34s[T. F. W. Barth]
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�6v�2v�32sGrignard reaction   �21m �33s 
    A reaction between an alkyl or aryl halide and magnesium metal in a 
suitable solvent, usually absolute ether. The organomagnesium halides produced 
by this reaction are known as Grignard reagents and are useful in many 
chemical syntheses. The structure of a Gignard reagent is usually written 
RMgX, where X represents a halogen. 
    The scope of the Grignard reaction is extremely broad, and Grignard 
reagents have been prepared from many kinds of alkyl and aryl halides. In 
general, alkyl chlorides, bromides, and iodides and aryl bromides and iodides 
react readily. A few halides, such as aryl chlorides, react very sluggishly. 
See ORGANOMETALLIC COMPOUND. 
 �34s[Paul E. Fanta]
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�6v�2v�32sGrimmiales   �21m �33s 
    An order of mosses. In size, the plants vary from slender to somewhat 
robust. They commonly grow in dense tufts or cushions, generally upon rock. 
These mosses usually range in height from about ** in. (5 mm)to 3 in. (8 cm) 
in the erect species and in length from 1** to 12 in. (4 to 30 cm) in the 
procumbent species. The leaves are very hygroscopic and are arranged on the 
stem in many rows, crowded, costate, usually lanceolate to ovate-lanceolate, 
but occasionally linear-subulate. The single peristome consists of 16 teeth, 
which may be entire, cribrose, or irregularly 2-3-cleft at the apex. See 
BRYOPSIDA.  
 �34s[Winona H. Welch]
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�6v�2v�32sGrinding   �21m �33s 
    The process of removing material from workpieces by the cutting action of 
a solid, rotating, grinding wheel composed of abrasive grains or grit plus a 
bonding material. The abrasive grains of the wheel perform a multitude of 
minute machining cuts on the workpiece. Although grinding is sometimes used as 
the complete machining operation on surfaces, it is generally considered a 
finishing process used to obtain a fine surface finish and extremely accurate 
dimensional tolerances. See ABRASIVE. 
    The grinding process is used in machining a wide range of metals in 
addition to such materials as cemented carbide, marble, stone, and certain 
ceramics. Because commercial grinding abrasives are many times harder than the 
metals to be machined, the grinding process may be used on metals too hard to 
machine otherwise. See MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sGrinding mill   �21m �33s 
    A machine that reduces the size of particles of raw material fed into it. 
The size reduction may be to facilitate removal of valuable constituents from 
an ore or to prepare the material for industrial use, as in preparing clay for 
pottery making or coal for furnace firing. Coarse material is first crushed. 
    Grinding mills are of three principal types, as shown in the illustration. 
In ring-roller pulverizers, the material is fed past spring-loaded rollers. 
The rolling surfaces apply a slow large force to the material as the bowl or 
other container revolves. The fine particles may be swept by an air stream up 
out of the mill. In tumbling mills the material is fed into a shell or drum 
that rotates about its horizontal axis. The attrition or abrasion between 
particles grinds the material. The grinding bodies may be flint pebbles, steel 
balls, metal rods lying parallel to the axis of the drum, or simply larger 
pieces of the material itself. In hammer mills, driven swinging hammers reduce 
the material by sudden impacts. See BALL-AND-RACE-TYPE PULVERIZER; CRUSHING 
AND PULVERIZING; PEBBLE MILL; TUMBLING MILL. 
 �34s[Ralph M. Hardgrove]
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�6v�2v�32sGriseofulvin   �21m �33s 
    An antifungal antibiotic effective in the treatment of ringworm and fungus 
infections of the skin, hair, and nails. Griseofulvin is a colorless, neutral, 
thermostable substance which is only very slightly soluble in water. The 
antibiotic is a metabolic product of various species of Penicillium. 
    Griseofulvin is indicated in the treatment of infections caused by 
dermatophytic fungi of the hands, feet, fingernails, toenails, glabrous skin, 
beard, and scalp. Onychomycosis and the following tineas respond to oral 
therapy: capitis, barbae, cruris, corporis, pedis, tonsurans, and rubrum. See 
ANTIBIOTIC.  
 �34s[Marvin J. Weinstein]
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�6v�2v�32sGroin (engineering)   �21m �33s 
    A structure oriented perpendicular to a shoreline and extending into the 
sea for the purpose of retarding erosion or trapping littoral drift, thus 
establishing, stabilizing, or widening a beach. 
    A single structure may suffice, but a series of structures uniformly 
spaced are usually more efficient. Groins may be permeable or impermeable, 
high or low, fixed or adjustable, and may be constructed of timber, steel, 
stone, concrete, and other materials. See COASTAL ENGINEERING. 
 �34s[Edward J. Quirin]
�33s 
  
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�6v�2v�32sGromiida   �21m �33s 
    An order of the subclass Filosia. The test of these protozoa is mostly 
chitinous in some species, rather thin and a bit flexible in others. Siliceous 
particles of endogenous origin, or sometimes minute platelets, may be embedded 
in the chitinous layer (see illustration); some species typically reinforce 
the test with sand grains or other extraneous particles. A few species are 
marine, the rest fresh-water types. Little is known about most Gromiida. See 
FILOSIA.  
 �34s[Richard P. Hall]
�33s 
  
  
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�6v�2v�32sGround-controlled-approach (GCA) system   �21m �33s 
    A ground radar system providing information by which aircraft approaches 
may be directed via radio communication. A GCA system consists of a 
surveillance radar and an approach radar. See PRECISION APPROACH RADAR (PAR); 
SURVEILLANCE RADAR. 
 �34s[Peter C. Sandretto]
�33s 
  
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�6v�2v�32sGrounding   �21m �33s 
    Intentional electrical connections to a reference conducting plane, which 
may be earth (hence the term ground), but which more generally consists of a 
specific array of interconnected electrical conductors, referred to as the 
grounding conductor. The symbol which denotes a connection to the grounding 
conductor is three parallel horizontal lines, each of the lower two being 
shorter than the one above it (Fig. 1). The electric system of an airplane or 
ship observes specific grounding practices with prescribed points of 
grounding, but no connection to earth is involved. A connection to such a 
reference grounding conductor which is independent of earth is denoted by use 
of the symbol shown in Fig. 2. 
  
  
    The subject of grounding may be conveniently divided into two categories: 
system grounding and equipment grounding. System grounding relates to a 
grounding connection from the electric power system conductors for the purpose 
of securing superior performance qualities in the electrical system. Equipment 
grounding relates to a grounding connection from the various electric-machine 
frames, equipment housings, metal raceways containing energized electrical 
conductors, and closely adjacent conducting structures judged to be vulnerable 
to contact by an energized conductor. The purpose of such equipment grounding 
is to avoid environmental hazards such as electric shock to area occupants, 
fire ignition hazard to the building or contents, and sparking or arcing 
between building interior metallic members which may be in loose contact with 
one another. The design of outdoor open-type installations presents special 
problems.  
    Installations in which earth is used as a reference ground plane present 
special problems. To design an earth ``floor surface'' for an outdoor 
open-type substation which will be free of dangerous electric shock voltage 
exposure to persons around the station is a difficult task. 
 �34s[R. H. Kaufmann]
�33s 
  
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�6v�2v�32sGround state   �21m �33s 
    In quantum mechanics, the stationary state of lowest energy of a particle 
or a system of particles. The ground state may be bound or unbound; when 
bound, its energy generally is a finite amount less than the energy of the 
next higher or first excited state. In the typical circumstance that the 
potential energy is zero at infinite separation, the magnitude of the negative 
ground-state energy is the binding energy, that is, the energy required to 
separate all the particles infinitely. See ENERGY LEVEL (QUANTUM MECHANICS); 
EXCITED STATE; NONRELATIVISTIC QUANTUM THEORY; NUCLEAR BINDING ENERGY; 
STATIONARY STATE. 
 �34s[Edward Gerjuoy]
�33s 
  
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�6v�2v�32sGroundwater hydrology   �21m �33s 
    The occurrence, circulation, distribution, and properties of any liquid 
water residing beneath the surface of the earth. Generally groundwater is that 
fraction of precipitation which infiltrates the land surface and subsequently 
moves, in response to various hydrodynamic forces, to reappear once again as 
seeps or in a more obvious fashion as springs. Most of groundwater discharge 
is not evident because it occurs through the bottoms of surface water bodies. 
See SPRING (HYDROLOGY). 
    Groundwater can be found, at least in theory, in any geological horizon 
containing interconnected pore space. Thus a groundwater reservoir (an analogy 
to an oil reservoir) can be a classical porous medium, such as sand or 
sandstone; a fractured, relatively impermeable rock, such as granite; or a 
cavernous geologic horizon, such as certain limestone beds. Groundwater 
reservoirs which readily yield water to wells are known as aquifers; in 
contrast, aquitards are formations which do not normally provide adequate 
water supplies, and aquicludes are considered, for all practical purposes, to 
be impermeable. These terms are, of course, subjective descriptions; the flow 
of water which constitutes an economically viable supply depends upon the 
intended use and the availability of alternative sources. See AQUIFER. 
    To effectively utilize groundwater as a natural resource, it is necessary 
to be able to forecast the impact of exploitation on water availability. When 
groundwater is used for water supply, a concern is the potential energy in the 
aquifer as reflected in the water levels in the producing well or neighboring 
wells. When a groundwater reservoir which does not readily transmit water is 
tapped, the energy loss associated with flow to the well can be such that the 
well must be drilled to prohibitively great depths to provide adequate 
supplies. On the other hand, in a formation able to transmit fluid easily, 
water levels may drop because the reservoir is being depleted of water. This 
is generally encountered in reservoirs of limited areal extent or those in 
which natural infiltration has been reduced either naturally or through human 
activities.  
    Problems involving groundwater quantity were once the primary concern of 
hydrologists; interest is now focused on groundwater quality. Groundwater 
contamination is a serious problem, particularly in the highly urbanized areas 
of the United States. See HYDROLOGY; WATER POLLUTION. 
 �34s[George F. Pinder]
�33s 
  
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�6v�2v�32sGroup theory �21m �33s
    Any set of elements which is equipped with an operation (called
multiplication) that is associative, has an identity element, and has an
inverse element for each element of the set, is called a group. [These
requirements are stated explicitly in Eqs. (1), (2) and (3) below.] Group
theory is the branch of mathematics devoted to the properties of groups. Group
theory has applications in the theories of relativity, quantum mechanics, and
crystallography, and also in some branches of algebra and in analytic function
theory.
    The group operation is supposed to give for every pair of elements, for
example, g and h, in the group, another element gh (called their product).
Multiplication is supposed to satisfy the following requirements.
    1. If g1, g2, and g3 are any elements of the group, then Eq. (1) is
satisfied; that is, in forming a product of three elements one obtains the

** [(g sub 1) (g sub 2)] (g sub 3) = (g sub 1) [(g sub 2) (g sub 3)] **  (1)

same result by first multiplying g sub 1 and g sub 2 and then multiplying the
result and g sub 3, as first multiplying g sub 2 and g sub 3 and then
multiplying g sub 1 and the result. This is called associative law.
    2. There is in the group an element e (called the identity) with the
property that Eq. (2) is satisfied for every element g of the group.

			    ** eg = ge = g **				 (2)

    3. For each element g of the group there is an element g sup -1 (called
the inverse of g) which satifies Eq. (3).

		 ** (g sup -1) g = g (g sup -1) = e **			 (3)

    An example of a group is the set of positive real numbers equipped with
the operation of ordinary multiplication. The identity element is then the
number 1 and the inverse of an element is the reciprocal. Another example of
a group is the set of all real numbers equipped with the operation of
ordinary addition. The identity element is then the number 0, and the inverse
of an element is the negative.
    Both these examples are commutative groups (also called abelian groups
after the mathematician Niels Abel) because their multiplication law satisfies
Eq. (4) for every g and h in the group. An example of a noncommutative group

			     ** gh = hg **				 (4)

(also called nonabelian) is the group of rotations of a three-dimensional
rigid body around a point.
    These three examples are all of groups with an infinite number of elements,
that is, infinite groups. There are also finite groups. A simple example is the
group with two elements, the identity e and another e sub 1 satisfying
(e sub 1) (e sub 1) = e.
    For infinite groups, it often occurs that a group has a natural geometry.
For example, in the group of positive real numbers described above one has the
geometry of the real numbers. This geometry is compatible with the group
operations in the sense that the product gh is a continuous function of g and
h, and the inverse g sup -1 is a continuous function of g. Generalizing this
scheme to arbitrary groups, one arrives at the notion of a topological group
(or less frequently a continuous group) which is a set of elements equipped
not only with a group operation but also with a topology, the two notions
being required to be compatible in the above sense. See TOPOLOGY.
    There is a particular class of topological groups which is of great
importance. These are the so-called Lie groups (named after the mathematician
Sophus Lie). Most of the topological groups occurring in applications are Lie
groups. See LIE GROUP.
    The principal applications of group theory outside mathematics itself have
to do with the classification and exploitation of symmetries in physical
systems. The applications of group theory within mathematics itself are
numerous and important. The part of mathematics in which the notion of group
was first clearly isolated was the theory of algebraic equations.
 �34s[Arthur S. Wightman]
�33s



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�6v�2v�32sGroup velocity   �21m �33s 
    The velocity of propagation of a group of waves forming a wave packet; 
also, the velocity of energy flow in a traveling wave or wave packet. The pure 
sine waves used to define phase velocity vsubp do not ever really exist, for 
they would require infinite extent. What do exist are groups of waves, wave 
packets, which are combined disturbances of a group of sine waves having a 
range of frequencies and wavelengths. Good approximations to pure sine waves 
exist, provided the extent of the media is very large in comparison with the 
wavelength of the sine wave. In nondispersive media, pure sine waves of 
different frequencies all travel at the same speed vsubp, and any wave packet 
retains its shape as it propagates. In this case, the group velocity vsubg is 
the same as vsubp. But if there is dispersion, the wave packet changes shape 
as it moves, because each different frequency which makes up the packet moves 
with a different phase velocity. If vsubp is frequency-dependent, then vsubg 
is not equal to vsubp. See PHASE VELOCITY; SINE WAVE; WAVE MOTION; WAVE PACKET. 
 �34s[S. A. Williams]
�33s 
  
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�6v�2v�32sGrout   �21m �33s 
    A mixture of cement and water called neat cement grout, or a mixture of 
cement, water, and fine aggregate that hardens with hydration of the cement to 
form a solid. The latter mixture is also called mortar. Grout may be pumped 
through drilled holes under pressure to fill voids or cracks in foundations 
for structures in order to improve loadbearing properties or watertightness of 
the material. Grout serves as a filling for joints in dams and for cable ducts 
in post-tensioned concrete structures. It may also be used to solidify gravel 
or stone fill, as in Prepakt concrete. See CONCRETE. 
 �34s[Ivan L. Tyler]
�33s 
  
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�6v�2v�32sGrowing season   �21m �33s 
    Generally considered to be the period of the year when climatic conditions 
are favorable for plant growth, common to a place or an area. In middle 
latitudes, with sufficient moisture, the growing season generally falls 
between the date of the last freeze in spring and the time of first freeze of 
autumn. This would apply to most annual plants and crops and to many 
perennials and biennials. However, some more hardy plants extend their growing 
season beyond these limits and continue some growth until the soil freezes; 
but some less hardy plants require minimum temperatures of 40DGF (4DGC), as in 
the tropics or subtropics. See AGRICULTURAL METEOROLOGY. 
 �34s[Milton L. Blanc]
�33s 
  
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�6v�2v�32sGruiformes   �21m �33s 
    A rather heterogeneous order of birds, especially well represented in the 
fossil record; of some 22 families, 10 are known only from fossils, and many 
fossil genera and species are allocated to living families. The order as a 
whole is cosmopolitan, but among the living families are several of small size 
and limited distribution, notably the monotypic family Rhynochetidae, 
containing only the peculiar kagu of New Caledonia. The largest and most 
widely distributed family is the Rallidae, comprising rails, gallinules, and 
coots. Largest living members of the order are the cranes (Gruidae), which are 
virtually cosmopolitan except in the Americas south of Mexico. See AVES. 
 �34s[Kenneth C. Parkes]
�33s 
  
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�6v�2v�32sGuava �21m �33s
    A plant, Psidium guajava, of tropical America. It is a shrub or low tree
which belongs to the myrtle family (Myrtaceae). The fruit is a berry, yellow
when ripe, and quite variable in size depending on variety and growing
conditions. The guava is quite aromatic, sweet, and juicy. It is used mostly
for jellies and preserves, but also as a fresh fruit. See FRUIT; MYRTALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sGuayule   �21m �33s 
    A desert plant, Parthenium argentatum, of the composite family 
(Compositae), which produces rubber. It is a native perennial shrub growing in 
the Chihuahuan Desert of north-central Mexico and southwestern Texas. The 
plant is bushy with dense branches, thick clusters of silverlike leaves, a 
strong taproot, and a thick crown. 
    During the early decades of this century, guayule was cultivated as an 
alternative source for rubber. With the development of synthetic rubber and 
the end of World War II, economic and political incentives for guayule 
production disappeared, and all projects were terminated by 1959. Later, in 
1976, research was reinitiated as the rising price of oil and the accompanying 
political difficulties with oil-producing countries rekindled interest in 
guayule as a source of natural rubber. See CAMPANULALES; RUBBER. 
 �34s[David D. Rubis ;Linda J. Cass]
�33s 
  
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�6v�2v�32sGuidance systems   �21m �33s 
    Apparatus for guiding the path of a vehicle, often remotely and 
automatically. Guidance systems are necessary in missiles and spacecraft of 
all kinds, and in drone (pilotless) aircraft. They are also employed to assist 
the human navigator in piloted aircraft, ships, and submarines. 
    In common usage the term guidance implies supervision of the navigation of 
a vehicle on its path from present location to desired future location. The 
companion term, flight control, is used to connote control of the vehicle, 
such as in attitude, heading, and speed. The equipment that performs the 
flight-control function is called an automatic pilot, or autopilot, and is an 
essential component of every guidance system. See AUTOPILOT. 
    In addition to the autopilot, a guidance system consists of a command 
computer - for accepting information and commands and, from them, determining 
the desired flight path - and sensors for measuring the actual flight path of 
the vehicle. The command computer determines the path to be flown and 
instructs the autopilot accordingly; actual flight path is measured by the 
sensors to be sure the guidance commands are being carried out; and errors are 
corrected by the usual feedback technique, that is, by making a corrective 
change proportional to error. See CONTROL SYSTEMS. 
    A revolutionary trend in the development of guidance systems of all kinds 
has been the increasing proportion of the guidance computation burden 
performed on board the vehicle. This trend was made possible by the advent of 
small digital computers of truly large capacity and by the spectacular rate at 
which the technology of microcircuitry is shrinking their size and power 
requirements and increasing their reliability. See INTEGRATED CIRCUITS. 
    Drone aircraft are often guided by ground radar tracking and radio command 
to the autopilot. Piloted aircraft are sometimes similarly guided by ground 
equipment for special purposes, such as ground-controlled intercept (GCI) of 
another aircraft or ground-aided approach to an airport. Except for such 
special purposes, however, piloted aircraft are navigated from on board with 
numerous internal and external aids. See ELECTRONIC NAVIGATION SYSTEMS; 
INERTIAL GUIDANCE SYSTEM; RADAR; STAR TRACKER. 
    Guidance of a missile to intercept a moving target such as an aircraft can 
be accomplished by either remote (launcher-based) command, or by an on-board 
homing system. Alternatively, the desired missile course may simply be preset 
as a function of time in an internal computer, as is common with torpedoes. 
    A command guidance system has the advantage that the missile carries only 
a radar receiver and an autopilot; the computer, being on the ground, may be 
as sophisticated as desired. However, the range of the system is limited, as 
is also the accuracy with which it can resolve geometric angles involved in 
the missile-target tactics. 
    In homing guidance systems, radar, infrared, or optical sensing 
(target-seeking) equipment in the missile itself measures the direction from 
missile to target. Measurements are more accurate from the missile's vantage 
point, and computations based on relative motion are simpler. However, because 
the missile must carry the additional load of a seeker and computer, the 
success of the system depends on the degree to which these subsystems can be 
miniaturized and simplified. 
    In an active homing system the missile's own radar illuminates the target, 
thus making the missile entirely autonomous, once the missile's radar has 
captured the target. In a passive homing system the missile carries only a 
receiver, which may be radar, infrared, or optical. 
    A beam-rider system is even simpler than the passive seeker; a radar 
receiver in the missile simply seeks to keep the missile in the beam of a 
launcher-based radar. The launcher station in turn keeps the radar beam 
centered on the target, so that the missile is guided along the beam to the 
target. See GUIDED MISSILE. 
    Guidance of long-range cruise (nonballistic) missiles is a problem of 
geographic navigation. In special cases a series of ground stations may form a 
command guidance system. Usually, however, the navigation system is carried in 
the missile, and guides it along a programmed course to a geographic 
destination. See AIRPLANE; BALLISTIC MISSILE; DRONE; MISSILE; NAVIGATION; 
SPACE NAVIGATION AND GUIDANCE. 
 �34s[Robert H. Cannon, Jr.]
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�6v�2v�32sGuided missile   �21m �33s 
    An uncrewed, controlled-flight vehicle that is guided to a target by 
guidance and control equipment. This equipment may be carried in the missile 
vehicle itself, or guidance may be directed from the launch site. The term is 
generally reserved for aerodynamic maneuverable missiles that may be guided to 
predetermined targets for military purposes. 
    Guided missiles are classified by launch/target mode such as air to air 
(AAM), air to surface (ASM), surface to air (SAM), surface to surface (SSM), 
and other possible modes. Missiles may be classified by range (short, medium, 
long) or by techniques related to tracking and guidance (radar, infrared heat 
seeker, optical or television, laser, radio, wire control command, 
acoustical). Some missiles make use of terrain following, which permits the 
missile to look at the terrain, compare it with a predetermined mapped route, 
and in effect fly a course as if by following a road map. See GUIDANCE SYSTEMS. 
    Guided missiles are generally self-propelled, and may use rocket motors 
(liquid or solid), air-breathing turbojet engines, ramjets, or various types 
of combined-cycle engines. For some missions, particularly air-to-surface 
missions, unpowered, gliding, guided missiles may be used. See ELECTROMAGNETIC 
PROPULSION; ION PROPULSION; JET PROPULSION; ROCKET ENGINE. 
    The kill mechanism for a missile consists of some form of explosive 
warhead and a system for detonation (fusing and arming). Warheads are 
typically either a high explosive or nuclear. Warheads may be exploded upon 
contact with the target, by command from an external source, by a proximity 
fuse that senses the target, by preset timers, and so on. See MISSILE. 
 �34s[M. Leroy Spearman]
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�6v�2v�32sGuinea worm infection   �21m �33s 
    An infection in humans with the nematode Dracunculus medinensis; also 
called dracunculosis or dracontiasis. The females develop in 8 - 12 months in 
the body cavities and migrate finally to the subcutaneous tissues, preferably 
of the limbs. A blister with a central papule forms at a spot where the worm's 
mouth approaches the epidermis. It bursts open, particularly upon contact with 
water, and numerous larvae are liberated. A small crustacean, the water flea 
(Cyclops and related genera), ingests the larvae and acts as an intermediate 
host. Humans become parasitized upon drinking well water containing these 
crustaceans.  
    The disease is and has been associated for centuries with religious rites. 
Various animals act as reservoirs, even in America. The disease is treated 
with some success with phenothiazine injections. Another method of treatment 
requires rolling the worm out inch by inch on a small stick. Prevention is by 
avoiding drinking water that contains infected crustaceans. See DRACUNCULOIDEA. 
 �34s[Jose F. Maldonado]
�33s 
  
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�6v�2v�32sGulf of California   �21m �33s 
    A deep, elongated trough in northwestern Mexico, 900 mi (1500 km) long and 
60 - 120 mi (100 - 200 km) wide, separated from the Pacific Ocean by the 
mountainous peninsula of Baja California. A central constriction and a group 
of islands separate a shallow (<650 ft or 200 m) northern section from a deep 
region to the south which contains a series of basins 3000 - 12,000 ft (1000 - 
3600 m) deep (see illustration). 
  
    The Gulf is an arid region over most of its expanse. Baja California has 
an annual rainfall of 4.0 - 6.0 in. (10 - 15 cm); no permanent streams exist. 
On the eastern side rainfall increases from 4.0 in. (10 cm) in the northwest 
to 34 in. (85 cm) in the southeast; permanent streams occur only in the 
southern half. 
    The northern Gulf has sediments derived predominantly from the Colorado 
River. The southern Gulf receives sediments from the eastern side; a wide 
coastal plain and shelf indicate high rates of supply. Very little sediment is 
supplied from the peninsula, and the western shallow-water sediments consist 
of calcareous skeletal debris. 
 �34s[Stephen E. Calvert]
�33s 
  
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�6v�2v�32sGulf of Mexico   �21m �33s 
    A semienclosed marginal sea forming the southern marine boundary of the 
United States, the scene of extensive marine fisheries and important offshore 
gas, oil, and sulfur production. Covering an area of 650,000 misup2 (1,700,000 
kmsup2), with wide continental shelves on the northern and eastern boundaries, 
the basin has a large central area (Sigsbee Abyssal Plain) with depths in 
excess of 11,500 ft (3500 m). 
    The Florida shelf and the Campeche bank are carbonate areas with active 
reef formation currently taking place on some of the Florida Keys and the 
outer reefs of Campeche. The remainder of the Gulf has detrital sediments 
(mainly montmorillonite), deriving at present principally from the Mississippi 
and the Atchafalaya rivers. The Texas-Louisiana shelf and slope display a 
great number of salt domes similar to those occurring under the adjacent land 
areas. Associated petroleum pooling has made this a productive area for oil 
and gas recovery. 
    In the southeast the island of Cuba separates the two connections with the 
Atlantic - one via the Caribbean through the Yucatan Strait and the other 
through the Straits of Florida. The Yucatan current carries water northward in 
an intense flow with speeds typically 2 - 4 knots (103 - 206 cm/s) close to 
the western side of the Strait. This flow generally loops to the north and 
exists as the Florida Current, which is continuous with, and after 
augmentation becomes, the Gulf Stream. See CARIBBEAN SEA; GULF STREAM. 
    The major contributions of fresh water to the Gulf come from the 
Mississippi, Atchafalaya, and the lesser rivers of Texas. The low-salinity 
coastal waters hold to the shelf with a westward flow. Although the shallow 
Texas bays at times may be brackish, restricted circulation and local 
evaporation make them hypersaline over extended periods. 
    The most valuable of the food fisheries are for shrimp and oysters. 
Several high-quality fin fishes are also landed in quantity. There are large 
inshore and offshore sports fisheries. 
 �34s[Hugh J. McLellan]
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�6v�2v�32sGulf Stream   �21m �33s 
    A great ocean current transporting about 70,000,000 tons (63,000,000 
metric tons) of water per second (1000 times the discharge of the Mississippi 
River) northward from the latitude of Florida to the Grand Banks off 
Newfoundland. The Gulf Stream is thought of as a portion of a great horizontal 
circulation in the ocean, where each particle of water executes a closed 
circuit, sometimes moving slowly in midocean regions and other times rapidly 
in strong currents like the Gulf Stream. Thus the beginning and end of the 
Stream have arbitrary geographical limits (see illustration). See ATLANTIC 
OCEAN.  
  
    The Gulf Stream is a narrow 62 mi or (100 km) and swift (up to 5 knots or  
250 cm/s) eastward-flowing current jet which is embedded in a weaker and 
broader mean westward flow and which is surrounded by intense eddies. As it 
leaves the coast at Cape Hatteras, the Stream meanders from side to side like 
a river. 
    The near-surface Gulf Stream transports warm water from southern latitudes 
eastward to the Grand Banks, where the flow becomes broader and weaker, 
separating into several branches and eddies. About half the near-surface flow 
continues eastward across the Mid-Atlantic Ridge, and half recirculates 
southwestward, with part of the recirculation consisting of a countercurrent 
located south of the Stream. 
    The Gulf Stream is predominantly driven by the large-scale wind pattern, 
the westerlies in the north and the trades in the south. The winds exert a 
torque on the ocean that, due to the shape and rotation of the Earth, causes a 
large western-intensified gyre. Cold, deep water is formed in northern seas 
and flows southward as a western boundary current; warm water flows northward 
and replaces it. See OCEAN CURRENTS. 
 �34s[Philip Richardson]
�33s 
  
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�6v�2v�32sGum   �21m �33s 
    A class of high-molecular-weight molecules, usually with colloidal 
properties, which in an appropriate solvent or swelling agent are able to 
produce gels at low dry-substance content. The molecules are either 
hydrophilic or hydrophobic. The term gum is applied to a wide variety of 
substances of gummy characteristics, and therefore cannot be precisely 
defined. See GEL. 
    Various rubbers are considered to be gums, as are many synthetic polymers, 
high-molecular-weight hydrocarbons, or other petroleum products. Chicle for 
chewing gum is an example of a hydrophobic polymer which is termed a gum but 
is not frequently classified among the gums. Quite often listed among the gums 
are the hydrophobic resinous saps that often exude from plants and are 
commercially tapped in balsam (gum balsam) and other evergreen trees (gum 
resin). Incense gums such as myrrh and frankincense are likewise fragrant 
plant exudates. 
    Usually, however, the term gum, as technically employed in industry, 
refers to plant polysaccharides or their derivatives. Modern usage of the term 
includes water-soluble derivatives of cellulose and derivatives and 
modifications of other polysaccharides which in the natural form are 
insoluble. Usage, therefore, also includes with gums the ill-defined group of 
plant slimes called mucilages. See CELLULOSE; COLLOID; POLYSACCHARIDE. 
    Gums are used in foods as stabilizers and thickeners. They form viscous 
solutions which prevent aggregation of the small particles of the dispersed 
phase. In this way they aid in keeping solids dispersed in chocolate milk, air 
in whipping cream, and fats in salad dressings. Gum solutions also retard 
crystal growth in ice cream (ice crystals) and in confections (sugar 
crystals). Their thickening and stabilizing properties make them useful in 
water-base paints, printing inks, and drilling muds. Because of these 
properties they also are used in cosmetics and pharmaceuticals as emulsifiers 
or bases for ointments, greaseless creams, toothpastes, lotions, demulcents, 
and emollients. The adhesive properties of gums make them useful in the 
production of cardboard, postage stamps, gummed envelopes, and as pill 
binders. Other applications include the production of dental impression molds, 
fibers (alginate rayon), soluble surgery films and gauze, blood 
anticoagulants, plasma extenders, beverage-clarifying agents, bacteriological 
culture media, half-cell bridges, and tungsten-wire-drawing lubricants. 
 �34s[Roy L. Whistler]
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�6v�2v�32sGunsights   �21m �33s 
    Optical instruments which establish an optical line or axis for the 
purpose of aiming a weapon. The axis includes the observer's eye, a suitable 
mark in the instrument, and the target. Most gunsights employ as their basis 
either a telescope or a partially reflective mirror. See MIRROR OPTICS; 
TELESCOPE.  
    A typical rifle sight consists of a terrestrial telescopic system having 
an objective, an eyepiece, an erector lens, and a reticle. Sometimes a field 
lens is employed to ensure uniform illumination. Aircraft gunsights are 
usually of the reflector type (also known as reflex sights) and employ in 
their simplest form a lamp, a reticle, a collimating lens, and a glass plate 
or partially reflecting mirror (see illustration). The collimator images the 
reticle pattern at infinity, and the mirror superimposes this image over the 
target area. Artillery sights can assume various forms, the simplest of which 
is the collimator sight, consisting of an objective having a reticle at its 
focus. When the eye is so placed as to receive light simultaneously from the 
target and the reticle, the latter appears superimposed on the former and a 
line of sight is established. See LENS (OPTICS). 
 �34s[Edward K. Kaprelian]
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�6v�2v�32sGymnolaemata   �21m �33s 
    A class of bryozoans. Predominantly marine, gymnolaemates possess 
lophophores which are circular in basal outline and zooecia which are short, 
wide, vaselike or boxlike. Highly diverse in size and shape, most gymnolaemate 
colonies are small and delicate, but a few are large conspicuous growths. The 
individual zooids may be relatively isolated or loosely grouped side by side. 
    The gymnolaemates include several thousand species - mostly marine, some 
brackish, and a very few fresh-water - belonging to the two orders 
Ctenostomata and Cheilostomata. Appearing early in the Early Ordovician, when 
they included the possible ectoproct stem group, gymnolaemates remained quite 
inconspicuous until the Cretaceous, when they rose to the position of 
dominance among bryozoans which they still maintain. See BRYOZOA; 
CHEILOSTOMATA; CTENOSTOMATA. 
 �34s[Roger J. Cuffey]
�33s 
  
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�6v�2v�32sGymnostomatida   �21m �33s 
    An order of the Holotrichia which contains a large, widely distributed 
group of what are believed to be the most primitive ciliate protozoans. These 
organisms occur abundantly in sands of intertidal zones, as well as in the 
more usual fresh- and salt-water habitats. The body size is frequently large; 
ciliation is simple and plentiful; and the oral area lacks buccal ciliature, 
as the name of the order implies. See HOLOTRICHIA. 
 �34s[John O. Corliss]
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�6v�2v�32sGynogenesis   �21m �33s 
    The development of a fertilized egg through the action of the egg nucleus, 
without participation of the sperm nucleus. However, the spermatozoon 
penetrates the egg, as it does in normal fertilization, and usually 
contributes the division center (centrosome) for the establishment of a 
bipolar division apparatus. 
    Gynogenesis occurs as a regular phenomenon in nature in some nematodes, 
flatworms, and other invertebrates, and in some species of fishes. Gynogenesis 
also occurs spontaneously in species that usually have normal fertilization, 
and produces haploid embryos.  See  ANDROGENESIS; MEROGONY. 
 �34s[Gerhard Fankhauser]
�33s 
  
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�6v�2v�32sGypsum   �21m �33s 
    A mineral with the chemical composition CaSO4PRODUCT2H2O. Gypsum is the 
commonest of the sulfate minerals. It occurs in five varieties: (1) rock 
gypsum; (2) gypsite, an impure earthy form; (3) alabaster, a massive, 
fine-grained, translucent variety; (4) satin spar, a fibrous silky form; and 
(5) selenite, transparent crystals. Crystals of gypsum are monoclinic, clear, 
white to gray, yellowish, and brownish in color, with well-developed 
cleavages. Luster is subvitreous to pearly. Hardness is 2 on Mohs scale and 
specific gravity is 2.3.  See  CALCIUM. 
    Gypsum is calcined in kilns at temperatures of 190 - 200DGC (370 - 390DGF) 
to remove part of the water of crystallization. The calcined product is known 
as plaster of paris. Gypsum and gypsum plaster products are chiefly used in 
the building industry and as a retarder in portland cement. Ground gypsum and 
anhydrite are used in the southern United States as soil conditioners to 
improve permeability. Gypsum and anhydrite, particularly the latter, may be 
made into ammonium sulfate fertilizer. Calcined gypsum is also used for 
industrial plasters, such as those used in pottery, molding, dentistry, and 
statuary. Pure white uncalcined gypsum, known as terra alba, is used as a 
filler in paper and paints and as a nutrient in growing yeast. Other 
uncalcined ground gypsum is used as a filler in insecticide.  See  ANHYDRITE; 
PLASTER OF PARIS; PORTLAND CEMENT. 
    Gypsum is of world-wide distribution. Thick commercial deposits of calcium 
sulfate occur in Nova Scotia and in Stassfurt, Germany. In the western United 
States extensive commercial gypsum deposits occur in California, Nevada, Utah, 
Texas, Iowa, Oklahoma, Kansas, Montana, Colorado, Arizona, South Dakota, 
Wyoming, and New Mexico. In the East, large mines are located in Michigan, New 
York, Ohio, and Indiana. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sGypsum plank   �21m �33s 
    A structural precast unit designed for use as the roof deck of industrial 
or other steel-frame buildings, and in some cases for the floor system as 
well. The four edges of the plank are bound with a tongue-and-groove steel 
form which, when fitted into an adjacent unit, produces a rigid steel I beam 
at the joint. The gypsum core is reinforced with welded galvanized-steel mesh. 
The planks are usually 2 in. (5 cm) thick, 15 in. (38 cm) wide, and 8 - 10 ft 
(2.4 - 3.0 m) in length. Specially designed clips fasten the planks to the 
supporting purlins and to an asphalt-impregnated felt built-up roofing used to 
resist the weather. The planks are fire-resistant and light-weight, are easily 
erected, and may be readily cut or drilled.  See  ROOF CONSTRUCTION. 
 �34s[Charles M. Antoni]
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�6v�2v�32sGyrator   �21m �33s 
    A linear, passive, two-port electric circuit element whose transmission 
properties are such that it is effectively a half wavelength longer for one 
direction of transmission than for the other direction of transmission. Thus a 
gyrator is a device that causes a reversal of signal polarity for one 
direction of propagation but not for the other. (A two-port element has a pair 
of input terminals and a pair of output terminals.) This device is novel, 
since it violates the theorem of reciprocity.  See  RECIPROCITY PRINCIPLE. 
    Until the early 1950s, all known linear passive electrical networks obeyed 
the theorem of reciprocity. However, several different types of nonreciprocal 
networks are now widely applied, principally at microwave frequencies. These 
devices are used to control the direction of signal flow and to protect or 
isolate components from undesired signals. One common application of a 
three-port nonreciprocal network, called a circulator, is to permit connection 
of a transmitter and a receiver to the same antenna. This is accomplished with 
minimum interference and virtually no power loss of either transmitted or 
received signal.  See  CONTINUOUS-WAVE RADAR; NETWORK THEORY. 
    Perhaps the first passive nonreciprocal system was an optical one proposed 
by Lord Rayleigh, making use of the rotation of the plane of polarization of 
light when it passed through a transparent material in the presence of a 
magnetic field. This phenomenon is called Faraday rotation. 
    The microwave analogy of Lord Rayleigh's device was proposed by C. L. 
Hogan. The nonreciprocal medium used is ferrite. In such a material, 
infinitesimal magnetic dipole moments which arise from the electronic 
structure of the material act gyroscopically when a steady magnetic field is 
applied. They precess about the applied field direction in a counterclockwise 
sense, thus permitting strong coupling to the component of a 
microwave-frequency magnetic field which is circularly polarized in the same 
sense. The component with the opposite sense of polarization is weakly 
coupled. Thus energy exchange between the magnetic dipoles and the microwave 
field is polarization-sensitive.  See  FERRIMAGNETISM; FERRITE. 
 �34s[Fred J. Rosenbaum]
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�6v�2v�32sGyrocompass   �21m �33s 
    A north-seeking form of gyroscope used as a directional reference in 
navigation. Modern gyrocompasses are so reliable and so much more accurate 
than magnetic compasses that they are now used as the prime navigational 
instrument on nearly every ship and on major aircraft and missiles.  See  
MAGNETIC COMPASS. 
    A gyrocompass combines the action of two devices, a pendulum and a 
gyroscope, to produce alignment with the Earth's spin axis. The principle is 
demonstrated with the model shown in the illustration, which consists of (1) a 
rapidly spinning, heavy gyro rotor, (2) a pendulous case which permits the 
rotor axle to nod up and down (angle THETA), and (3) an outer gimbal which 
permits the axle to rotate in azimuth (angle PSI). For a gyroscope positioned 
at the Equator of the Earth, as the Earth rotates, the gimbal moves with it. 
So long as the rotor's spin axis is aligned with the Earth's axis, the gyro 
experiences no torque from Earth rotation. If there is misalignment, however, 
a sequence of restoring torques is initiated.  See  GYROSCOPE; PENDULUM. 
  
    In a shipboard installation the system must be mounted in a complete set 
of gimbals to isolate it from rolling, pitching, and yawing motions of the 
ship. Friction must be minimized. Moreover, Schuler tuning is employed to keep 
horizontal accelerations of the ship from producing false torques on the 
pendulum; the unique combination of gyro spin speed and pendulosity is chosen 
so that no acceleration of the instrument can disturb its vertical reference.  
See  SCHULER PENDULUM. 
 �34s[Robert H. Cannon, Jr.]
�33s 
    For many years the use of gyrocompasses in aircraft was impractical 
because of their high speed and large, rapid changes in attitude. The 
north-south component of vehicle velocity produces an error which depends on 
the velocity magnitude. In order to correct for this error, the true ground 
speed of the aircraft must be determined and used to precess the gyro in the 
opposite direction. In addition, the direction of the local vertical must be 
determined accurately. These capabilities are contained in modern military 
aircraft which are equipped with gyroscopes mounted in conjunction with 
accelerometers to provide a stable inertial platform. They also are equipped 
with Doppler navigation radar which measures true velocity over the Earth's 
surface.  See  DOPPLER RADAR; INERTIAL GUIDANCE SYSTEM. 
 �34s[Gus Stavis]
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�6v�2v�32sGyrocotylidea   �21m �33s 
    An order of tapeworms of the subclass Cestodaria. All species are 
intestinal parasites of chimaeroid fishes and have an anterior end with an 
eversible proboscis and a posterior end with a ruffled adhesive organ. 
Typically, only one sexually mature worm is found in an individual host. The 
life history is incompletely known.  See  CESTODARIA. 
 �34s[Clark P. Read]
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�6v�2v�32sGyromagnetic effect   �21m �33s 
    An effect arising from the relation between the angular momentum and the 
magnetization of a magnetic substance. It is the effect which is exploited in 
the measurement of the gyromagnetic ratio of magnetic materials. The 
gyromagnetic effect is demonstrated by a simple experiment in which a freely 
suspended magnetic substance is subjected to a magnetic field. Upon a change 
in direction of the magnetic field, the magnetization of the substance must 
change. In order for this to happen, the atoms must change their angular 
momentum. Since there are no external torques acting on the system, the total 
angular momentum must remain constant. Thus the sample must acquire a mass 
rotation which may be measured. In this way, the gyromagnetic ratio may be 
determined.  See  GYROMAGNETIC RATIO. 
 �34s[Elihu Abrahams ;Frederic Keff]
�33s 
  
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�6v�2v�32sGyromagnetic ratio   �21m �33s 
    The ratio of angular momentum to magnetic moment for atomic systems. This 
ratio is usually expressed in terms of the magnetomechanical factor g', as in 
Eq. (1). The ratio is written here in electromagnetic units; thus, e/c and m 

        *** Angular momentum / Magnetic moment = 2mc / g'e ***             (1)

are the charge and mass of the electron. The factor g' is sometimes loosely 
called the gyromagnetic ratio. 
    The magnetomechanical ratio is the inverse of the gyromagnetic ratio. It 
is usually denoted by GAMMA and is equal to g'e/2mc. The magnetomechanical 
ratio of a substance identifies the origin of the magnetic moment. For example,
for electron spin the angular momentum is 1/2 PLANCK'S CONSTANT, where PLANCK'S 
CONSTANT is Planck's constant divided by 2 PI. The magnetic moment is the Bohr 
magneton e PLANCK'S CONSTANT/2mc. Thus, the magnetomechanical ratio is given by 
Eq. (2). Since GAMMA = g'e/2mc, for electron spin g' = 2. For orbital angular
  
GAMMA = (e PLANCK'S CONSTANT / 2mc) / (PLANCK'S CONSTANT / 2) =  e / mc    (2)
  
momentum, GAMMA = e/mc and g' = 1. The experimental values of g' for most 
ferromagnetic materials are in the neighborhood of 2, showing that the major 
contribution to the magnetization comes from the electron spin. In supercon-
ductors, on the other hand, the fact that g' = 1 shows that the diamagnetic 
currents which cause the Meissner effect are caused by electrons. See ELECTRON 
SPIN; MEISSNER EFFECT; SUPERCONDUCTIVITY. 
 �34s[Elihu Abrahams; Frederic Keffer]
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�6v�2v�32sGyroscope   �21m �33s 
    An instrument, often called simply a gyro, that maintains an angular 
reference direction by virtue of a spinning mass. Gyroscopes are used in 
airplanes to ``remember'' the orientation of the horizon and the direction of 
north during maneuvers. Gyros measure the turning rate of gunsights to improve 
target tracking. The gyrocompass is a special north-seeking form of gyroscope 
developed to a high degree for shipboard use.  See  GYROCOMPASS. 
    Gyros are used in missiles, airplanes, ships, and torpedoes as the basic 
element in automatic steering systems. A set of gyroscopes may be used to 
stabilize instrument platforms in maneuvering vehicles. There are stringent 
accuracy requirements for gyroscopes used for the stable platforms in inertial 
navigation systems. All gyroscope applications depend upon a special form of 
Newton's law which states that a spinning body tends to react to a disturbing 
torque by precessing (rotating slowly) in a direction at right angles to the 
direction of the torque (see illustration). 
    The gyroscopic principle is used in instruments in several ways. In the 
free gyro, the spinning wheel is isolated from the airplane by gimbals. When 
the airplane banks, the gyro remains vertical and furnishes the pilot with an 
artificial horizon reference.  See  AUTOMATIC HORIZON. 
    The rate gyro is one of a class of instruments known as single-axis 
gyroscopes. It is mounted to the aircraft structure by only a single gimbal so 
that if the airplane rolls (about its fore-and-aft axis) the rate gyro is 
obliged to roll with it. This causes the gimbal to turn (precess) against the 
stiff spring by a small angle, and in this case the indication is the rate of 
roll. A set of three such gyros, mounted with input axes mutually orthogonal, 
will serve to determine completely the total angular velocity of the aircraft 
in space. 
    Families of gyroscopic instruments are based on the free or on the 
single-axis arrangement. 
 �34s[Robert H. Cannon, Jr.]
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�6v�2v�32sGyrotron   �21m �33s 
    One of a family of microwave generators, also called cyclotron resonance 
masers, in which cyclotron resonance coupling between microwave fields and an 
electron beam in vacuum is the basis of operation. This type of coupling has 
the advantage that both the electron beam and the associated microwave 
structures can have dimensions which are large compared with a wavelength. 
Thus, cyclotron resonance masers are potentially greatly superior to 
conventional microwave tubes with respect to power capability at short 
wavelengths.  
    The development of these power sources is particularly significant for 
magnetically confined plasma fusion experiments. Microwave heating is 
considered an attractive method of supplying the energy needed to bring a 
reactor to ignition temperature, and gyrotrons provide a potential means of 
producing sufficient microwave power at the very short wavelength required. 
Gyrotrons also have potential application in millimeter-wave radar and 
communications systems.  See  MICROWAVE TUBE; NUCLEAR FUSION; TRAVELING-WAVE 
TUBE.  
 �34s[Howard R. Jory]
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�6v�2v�32sHackberry �21m �33s
    A medium-sized to large tree, Celtis occidentalis. It occurs in the
eastern half of the United States, except in the extreme south, and is
characterized by corky or warty bark, by alternate, long-pointed serrate
leaves unequal at the base, and by a small drupaceous fruit, with thin, sweet,
edible flesh. It is used for furniture, boxes, and baskets, for shelterbelts,
and as a shade tree.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sHadromerida   �21m �33s 
    An order of sponges of the class Demospongiae with monactinal megascleres 
that usually have a terminal knob at one end. Microscleres in the form of 
streptasters, asters, sigmas, or small spined diactinals may occur. Spongin is 
usually sparse in occurrence. In shape, hadromeridan sponges include radially 
symmetrical forms (see illustration) as well as encrusting, massive, or 
branching types. They occur in tidal and shallow waters of all seas and extend 
down to depths of at least 18,000 ft (5500 m).  See  BORING SPONGES; 
DEMOSPONGIAE.  
 �34s[Willard D. Hartman]
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�6v�2v�32sHadron   �21m �33s 
    The generic name of a class of particles which interact strongly with one 
another. Examples of hadrons are protons, neutrons, the PI-, K-, and D-mesons, 
and their antiparticles. Protons and neutrons, which are the constituents of 
ordinary nuclei, are members of a hadronic subclass called baryons, as are 
strange and charmed baryons. Baryons have half-integral spin, obey Fermi-Dirac 
statistics, and are known as fermions. Mesons, the other subclass of hadrons, 
have zero or integral spin, obey Bose-Einstein statistics, and are known as 
bosons. The electric charges of baryons and mesons are either zero or 
PLUS/MINUS1 times the charge on the electron. Masses of the known mesons and 
baryons cover a wide range, extending from the pi-meson, with a mass 
approximately one-seventh that of the proton, to the upsilon-meson, with a 
mass about 10 times the proton mass. The spectrum of meson and baryon masses 
is not fully understood.  See  BARYON; BOSE-EINSTEIN STATISTICS; FERMI-DIRAC 
STATISTICS; MESON; NEUTRON; PROTON. 
    Much of the data relating to the properties of hadrons (both baryons and 
mesons) can be interpreted as if hadrons consist of more elementary 
constituents known as quarks and gluons. This conception of hadrons has 
widespread appeal in elementary particle physics.  See  ELEMENTARY PARTICLE; 
GLUONS; QUARKS. 
 �34s[Alfred K. Mann]
�33s 
  
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�6v�2v�32sHadronic atom �21m �33s
    A hydrogenlike system that consists of a strongly interacting particle
(hadron) bound in the Coulomb field and in orbit around any ordinary nucleus.
The kinds of hadronic atoms that have been made and the years in which they
were first identified include pionic (1952), kaonic (1966), SIGMA--hyperonic
(1968), and antiprotonic (1970). They were made by stopping beams of
negatively charged hadrons in suitable targets of various elements, for
example, potassium, zinc, or lead. The lifetime of these atoms is of the order
of 10- sup 1 sup 2 s, but this is long enough to identify them and study their
characteristics by means of their x-ray spectra. They are available for study
only in the beams of particle accelerators.  See  ELEMENTARY PARTICLE; HADRON;
PARTICLE ACCELERATOR.
    The hadronic atoms are smaller in size than their electronic counterparts
by the ratio of electron to hadron mass. For example, in pionic calcium,
atomic number Z = 20, the Bohr radius of the ground state is about 10 fermis
(1 fermi = 1 femtometer = 10- sup 1 sup 5 m) and in ordinary calcium it is about
2500 fermis. Thus the atomic electrons are practically not involved in the
hadronic atoms and the equations of the hydrogen atom are applicable. The
close approach of the hadrons to their host nuclei suggests that
hadron-nucleon and hadron-nucleus forces will be in evidence, and this is one
of the motivations for studying these new types of atoms.
 �34s[Clyde E. Wiegand]
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�6v�2v�32sHaemosporina   �21m �33s 
    A relatively small and generally rather compact group of protozoans in the 
subphylum Sporozoa. Authorities differ as to the group's taxonomic status; 
that assigned it by the Committee on Taxonomy and Taxonomic Problems of the 
Society of Protozoologists is followed here: a suborder of the order 
Eucoccida, subclass Coccidia, class Telosporea, subphylum Sporozoa. The 
Haemosporina are common protozoan parasites of vertebrates, and some of them 
are important as causes of illness and death. The best known of the group are 
the four species (genus Plasmodium) of malarial parasites of humans.  See  
MALARIA.  
    Transmission of these parasites is probably always effected in nature by 
the bite of some bloodsucking invertebrate. In the vertebrate host they 
reproduce asexually; sometimes this occurs in the tissues of certain internal 
organs, such as the lungs, liver, spleen, and brain; sometimes in the red 
blood cells (erythrocytes), or even in other types of blood and blood-forming 
cells; and often in both tissues and blood cells. The immature sex cells, 
gametocytes, always occur in erythrocytes or leukocytes (white blood cells). 
Gametocytes mature into gametes after ingestion by an intermediate host 
(arthropod). Fertilization ensues, with a subsequent period of development 
culminating in the production of numerous sporozoites. These tiny filamentous 
forms are infective for the vertebrate host. Since they can develop no further 
in the arthropod, infection in this host is self-limited, in the sense that no 
further buildup is possible; the insect is seldom harmed by the parasite. 
However, sporozoites may remain infective for a long time in the invertebrate 
host, perhaps as long as the insect lives.  See SPOROZOA. 
 �34s[Reginald D. Manwell]
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�6v�2v�32sHafnium   �21m �33s 
    A metallic element, symbol Hf, atomic number 72, and atomic weight 178.49. 
There are five naturally occurring isotopes. It is one of the less abundant 
elements in the Earth's crust. 
    Hafnium is a lustrous, silvery metal that melts at about 2222DGC 
(4032DGF). Reported values of the boiling point vary greatly, from about 2500 
to about 5100DGC (4530 to 9200DGF). There are virtually no uses of the metal 
other than in control rods for nuclear reactors. 
    The chemistry of hafnium is almost identical with that of zirconium. The 
similarity of hafnium to zirconium is a consequence of the lanthanide 
contraction, which brings the ionic radii to very nearly identical values. 
Before (and since) the discovery of hafnium, this element was extracted with 
zirconium from its ores and passed with zirconium into all derivatives. Since 
the chemical properties are so similar, there has been no incentive to 
separate the hafnium except for making nuclear studies and components of 
nuclear reactors.  See  ZIRCONIUM. 
 �34s[Warren B. Blumenthal]
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�6v�2v�32sHail   �21m �33s 
    Precipitation composed of chunks or lumps of ice formed in strong updrafts 
in cumulonimbus clouds. Individual lumps are called hailstones. Most 
hailstones are spherical or oblong, some are conical, and some are bumpy and 
irregular. Diameters range from **5 to 6 in. (5 to 150 mm) or more. The 
largest stones are grapefruit or softball size, and the smallest are pea size. 
    Very often hailstones are observed to be made of alternating rings of 
clear and white ice (see illustration). These rings indicate the growth 
processes of the hail. The milky or white portion of the growth occurs when 
small cloud droplets are collected by the hailstone and freeze almost 
instantaneously, trapping bubbles of air between the droplets and creating a 
milky appearance. The clear portion is formed when many droplets are collected 
so rapidly that a film of water spreads over the stone and freezes gradually, 
giving time for any trapped air bubbles to escape from the liquid. 
    The most favorable conditions for hail formation occur in the mountainous, 
high plains regions of the world. Hailstorms normally have relatively high, 
cool cloud bases and very strong updrafts within the clouds to carry the 
hailstones into the cooler regions of the cloud, where maximum growth occurs. 
Both small ice particles and supercooled liquid water (liquid water at 
temperatures below 32DGF or 0DGC) are needed for the ice particles to grow 
into hailstones.  See  CLOUD PHYSICS; PRECIPITATION (METEOROLOGY). 
 �34s[Harold D. Orville]
�33s 
  
  
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�6v�2v�32sHair   �21m �33s 
    Nonliving, specialized epidermal derivatives characteristic only of modern 
mammals. However, it is now thought that hair was present in at least some 
therapsid reptiles. It consists of keratinized cells, tightly cemented 
together, which arise from the matrix at the base of a follicle. A follicle is 
a tubular epidermal downgrowth that penetrates into the dermis and widens into 
a bulb (the hair root) at its deep end. The follicle, together with a lateral 
outgrowth called the sebaceous gland, forms the pilosebaceous system. Rapid 
cell production in the matrix, and differentiation in the regions immediately 
above, produces a hair shaft which protrudes from the follicle mouth at the 
skin surface.  See  GLAND. 
    Hairs are not permanent structures but are continually replaced throughout 
the life of a mammal. In some species, for example, the rat, hamster, mouse, 
chinchilla, and rabbit, the replacement pattern is undulant, and waves of 
follicular activity can be traced across the body. In other species, for 
example, humans, cats, and guinea pigs, each follicle appears to cycle 
independently of others in the immediate area. See INTEGUMENT. 
 �34s[Paul F. Maderson]
�33s 
  
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�6v�2v�32sHairworm   �21m �33s 
    An extremely elongate, nonsegmented worm (sometimes known as horsehair 
worm) belonging to the phylum Nematomorpha. The name is particularly applied 
to the approximately 200 species of the order Gordioidea, adults of which are 
found in smaller habitats of standing freshwater, such as wells, ponds, 
permanent swamp pools, and ditches. Adult hairworms, usually with black or 
dark brown pigmentation, are mostly about 0.04 in. (1 mm) in diameter and 
range from 4 to 40 in. (10 cm to 1 m) in length. 
    In their unusual life cycle, hairworm larvae are parasitic in arthropods, 
while the adults are free-living with a nonfunctional gut and two long, 
cylindrical gonads. The sexes are separate, and adult males are much more 
active.  
    Hairworms are found all over the world, and the majority of species belong 
to the gordioid genera Gordius, Paragordius, Chordodes, Parachordodes, and 
Gordionus. There are five other species of nematomorphan worms in the marine 
planktonic genus Nectonema, and they are separated as the order 
Nectonematoidea.  See  NEMATOMORPHA. 
 �34s[W. D. Russell-Hunter]
�33s 
  
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�6v�2v�32sHalf-life   �21m �33s 
    The time required for one-half of a given material to undergo chemical 
reactions; also, the average time interval required for one-half of any 
quantity of identical radioactive atoms to undergo radioactive decay. 
    The concept of the time required for all of the material to react is 
meaningless, because the reaction goes very slowly when only a small amount of 
the reacting material is left and theoretically an infinite time would be 
required. The time for half completion of the reaction is a definite and 
useful way of describing the rate of a reaction. 
    The specific rate constant k provides another way of describing the rate 
of a chemical reaction. This is shown in a first-order reaction, Eq. (1), 
  
                 *** k = (2.303 / t) log (c sub 0 / c) ***              (1)
  
where c sub 0 is the initial concentration and c is the concentration at time 
t. The relation between specific rate constant and period of half-life, 
t sub 1/2, in a first-order reaction is given by Eq. (2). See CHEMICAL 

         *** t sub 1/2 = (2.303 / k) log (1 / 1/2) = 0.693 / k ***      (2)

DYNAMICS. 
 �34s[Farrington Daniels]
�33s 
  
    The activity of a source of any single radioactive substance decreases to 
one-half in 1 half-period, because the activity is always proportional to the 
number of radioactive atoms present. For example, the half-period of 
sup 60 Co (cobalt-60) is t sub 1/2 = 5.3 years. Then a sup 60 Co source whose 
initial activity was 100 curies will decrease to 50 curies in 5.3 years. In 1 
additional half-period this activity will be further reduced by the factor 
1/2. Thus, the fraction of the initial activity which remains is 1/2 after one 
half-period, 1/4 after two half-periods, 1/8 after three half-periods, 1/16 
after four half-periods, and so on. The half-period is sometimes also called 
the half-value time or, with less justification, the half-life. See 
RADIOACTIVITY.  
 �34s[Robley D. Evans]
�33s 
  

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�6v�2v�32sHalichondrida   �21m �33s 
    A small order of sponges of the class Demospongiae, subclass 
Ceractinomorpha, with a skeleton of diactinal or monactinal siliceous 
megascleres or both. Spongin is present in small amounts; microscleres are 
absent. A skinlike dermis is present and is often reinforced with spicules. 
    Halichondrid sponges are encrusting, massive, lobate, or branching in 
shape (see illustration). Halichondrids inhabit all seas, occurring chiefly in 
tidal areas and shallow waters of the continental shelf. Some species occur 
down to depths of at least 5000 ft (1500 m). Fossil species are unknown.  See  
DEMOSPONGIAE.  
 �34s[Willard D. Hartman]
�33s 
  
  
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�6v�2v�32sHalide   �21m �33s 
    A compound of the type MX, where X may be fluorine, chlorine, bromine, 
iodine, or astatine, and M may be another element or organic radical. The 
compounds are considered derivatives of the hydrohalic acids, HX. All the 
halides form both covalent and ionic inorganic salts, coordination compounds, 
and many organic compounds. 
    Many properties show a gradual change as one compares the halides. The 
size of the ions, and the strength of the halide as a reducing agent, 
increases from F- to I-. It becomes more difficult to produce the element from 
the halide as one goes from I- to F-. The stability of complexes formed, and 
the solubilities of metal halides in water, increase going from I- to F-.  See 
BROMIDE; CHLORIDE; FLUORIDE; HALOGENATED HYDROCARBON; HALOGENATION; IODIDE. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sHalite �21m �33s
    The halite structure is one of the most fundamental in mineralogy. It is
based on the cubic close-packing of the chloride anions with the sodium
cations occupying all the available octahedral vacancies (see illustration).
The structure forms the basis for many compounds such as manganosite, MnO;
periclase, MgO; and galena, PbS. Fragments of the halite (rock-salt) structure
are found in a large number of basic manganese arsenates and in the lead-rich
sulfosalt minerals.
 �34s[Cornelius S. Hurlbut, Jr. ;Paul B. Moore]
�33s


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�6v�2v�32sHall effect �21m �33s
    The development of a transverse electric potential gradient in a
current-carrying conductor placed in a magnetic field when the conductor is
positioned so that the direction of the magnetic field is perpendicular to the
direction of current flow. Analysis of the Hall effect yields important
information on the band structure of metals and semiconductors and on the
nature of the conductivity process. In semiconductor research, the magnitude
of the Hall effect in simple cases provides a direct estimate of the
concentration of charge carriers. The Hall effect is one of the so-called
galvanomagnetic effects.  See  GALVANOMAGNETIC EFFECTS.
    The experimental arrangement for observing the Hall effect is shown
schematically in the illustration. A voltage VsubH appears between the sides
of the specimen whenever the current density Jsubx and the magnetic field
Bsubz are nonvanishing. The electric field EsubH = VsubH/d = Esuby, where d is
the width of the specimen using the coordinates of the illustration, is found
to be proportional to the product of Jsubx and Bsubz. The quantities VsubH and
EsubH are called the Hall voltage and Hall field, respectively. The Hall
coefficient R is defined by the equation below.

			R = E sub y/J sub x B sub z

    In semiconductors and metals, the sign and magnitude of R determine the
sign and number of charge carriers, provided only one type of carrier
(electrons or holes) is present. The Hall coefficient in semiconductors must
be calculated with the aid of a two-band model when more than one type of
carrier is responsible for charge transport. The Hall coefficient, especially
in a multicarrier situation, is field-dependent. This field dependence
provides valuable information on mobility ratios and relaxation times.	See
RELAXATION TIME OF ELECTRONS; SEMICONDUCTOR.
 �34s[Frank J. Blatt]
�33s

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�6v�2v�32sHalley's Comet   �21m �33s 
    The most famous of all comets; records of Halley's Comet appear at least 
as far back as 240 B.C. The comet's size, activity, and favorably placed orbit 
(with the perihelion roughly halfway between the Sun and the Earth's orbit) 
ensure its visibility to the naked eye at each apparition (see illustration). 
See COMET. 
    This comet was the first to have its return predicted, a feat accomplished 
by Edmond Halley in 1705. He computed the orbits of several comets with Isaac 
Newton's then new gravitational theory. The orbits of comets observed in 1531, 
1607, and 1682 were remarkably similar. Halley assumed that the sightings were 
of a single comet and predicted its return in 1758 - 1759. The prediction was 
verified, and the comet was named in his honor. The next perihelion passage of 
the comet is confidently expected on February 9, 1986. The best times for 
public viewing and for ground-based observation of the comet are during 
November 1985 and April 1986. 
 �34s[John C. Brandt]
�33s 
  
  
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�6v�2v�32sHalloysite   �21m �33s 
    A group of clay minerals. The major use of halloysite clay is in the 
manufacture of cracking petroleum catalysts. Pure halloysite clays are not 
abundant; hence their potential economic use has not been explored.  See  CLAY 
MINERALS; KAOLINITE. 
    There are two types of halloysite. Both are light-colored; one is porous 
and friable, while the other is dense, nonporous, and porcelainlike. The 
porous variety has the same chemical composition as kaolinite, whereas the 
nonporous hydrated form contains an added amount of water (2H2O). 
    The halloysite minerals are made up of the same structure units as 
kaolinite; that is, layers composed of single silica tetrahedral and alumina 
octahedral units. They differ in the presence of a single layer of water 
molecules between each silicate layer in the hydrated form. Electron 
micrographs show that halloysite is composed of elongate particles which 
appear to be tubes. The tubes of the hydrated 4H2O form appear to be made up 
of overlapping, curled-up sheets of the kaolinite type. 
    Halloysite is often found in hydrothermal deposits. It has also been 
reported in weathering products; however, it is a rare component of such 
materials. It is generally absent in sedimentary rocks, and it has been 
suggested that metamorphic processes would change halloysite to kaolinite. 
 �34s[Floyd M. Wahl ;Ralph E. Grim]
�33s 
  
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�6v�2v�32sHallucination   �21m �33s 
    An involuntary sensory experience of objectively nonexistent phenomena. 
The ability to distinguish hallucinations for perceptions requires contextual 
cues and the ability to use them. 
    Hallucinations can be elemental and meaningless (points of light, 
luminosities, noises, colors) or figural and identifiable (objects, sounds, 
voices). They may be static, repetitive, or erratic, or they may be 
developmental (sequential or unfolding). They may involve a clear sensorium 
(for example, the hallucinated event may be added to the perceived scene), or 
they may involve reduced consciousness (for example, the fusion of real and 
hallucinated events in the context of impaired attention and orientation, as 
in oneiric confusional states produced by neural degeneration or by acute 
alcohol withdrawal). They may be at least momentarily believed, as in the case 
of complex hallucinations superimposed on a context of otherwise normal 
awareness, consciousness, and feeling. They may be disbelieved, as in the case 
of psychomotor epileptic conditions that produce hallucinations in the context 
of strong feelings of recollection or dreamy unreality. On the other hand, 
reality testing may be completely suspended, as in oneiric confusional or 
normal (especially rapid-eye-movement or REM) dream states. 
    Hallucinations occur in abnormal conditions such as migraine headache, 
insulin hypoglycemia, high fever, schizophrenia (predominantly verbal 
hallucinations), psychomotor (temporal-lobe) epilepsy (visual, auditory, and 
gustatory-olfactory hallucinations), and narcolepsy (postural collapse, 
sleep-onset visual dream hallucinations, and postsleep paralysis, all 
suggesting an abnormal disinhibition of REM sleep). Hallucinations can be 
produced by acute or degenerative defects of subcortical and neocortical 
sensory projection mechanisms. In addition, hallucinations can be induced 
experimentally by sensory deprivation, by extended sleep deprivation, by 
direct electrical stimulation to sensory areas, or by psychoactive drugs (for 
example, LSD).  See  PERCEPTION. 
 �34s[David B. Cohen]
�33s 
  
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�6v�2v�32sHalo   �21m �33s 
    A system of luminous rings or arcs around the Sun or Moon, caused by 
refraction or reflection of light rays illuminating ice crystals floating in 
the air. These rays are seen as luminous circles around the Sun with a radius 
of either 22DG (inner halo) or 46DG (outer halo). Besides the inner and outer 
halos several similar arcs, luminous arcs, and bright spots are occasionally 
seen in the sky as a result of refraction or reflection of light rays 
illuminating ice crystals in the atmosphere. All these phenomena are usually 
considered parts of a more complex system, called the halo. The inner halo is, 
however, most frequently observed.  See  SUNDOG. 
 �34s[Zdenek Sekera]
�33s 
  
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�6v�2v�32sHalocline   �21m �33s 
    A layer of sea water within which a marked salinity gradient is present. A 
permanent halocline of decreasing salinity usually coincides with the top of 
the permanent thermocline. The oceanic halocline furnishes significant clues 
to the origin and movement of water masses. Haloclines are extremely important 
in estuarine oceanography, however, as their depth, strength, and shape 
greatly influence circulation, mixing, and flushing of these embayments. 
Estuarine haloclines have strong positive gradients which are formed as fresh 
water flows seaward over sea water entering along the bottom.  See  ESTUARINE 
OCEANOGRAPHY; SEA WATER; THERMOCLINE. 
 �34s[John J. Schule, Jr.]
�33s 
  
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�6v�2v�32sHaloform reaction   �21m �33s 
    The halogenation of acetaldehyde or a methyl ketone in aqueous basic 
solution. The reaction is characteristic of compounds that contain a CH3CO - 
group linked to a hydrogen or to another carbon atom. The halogen is dissolved 
in aqueous base (sodium hydroxide or carbonate) to form an alkali hypohalite, 
which is the halogenating agent. This is added to the ketone. The reaction 
occurs in two phases: the methyl group is fully substituted by halogen to give 
an intermediate trihaloketone; the trihaloketone breaks down in the presence 
of alkali to produce the haloform and the sodium salt of a monocarboxylic 
acid. The haloforms which can be produced by this method are chloroform, 
bromoform, and iodoform. 
    The haloform reaction was formerly applied industrially, specifically in 
the manufacture of chloroform, but it has been virtually displaced by more 
economical methods.  See  CHLOROFORM; IODOFORM. 
 �34s[Ellen Clarke ;Max G. Gergel]
�33s 
  
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�6v�2v�32sHalogenated hydrocarbon   �21m �33s 
    One of a group of halogen derivatives of organic hydrogen- and 
carbon-containing compounds. The group includes monohalogen compounds (alkyl 
or aryl halides) and polyhalogen compounds that contain the same or different 
halogen atoms. The halogenated hydrocarbons may be classified under the 
general headings of alkyl halides, unsaturated (vinyl and allyl) halides, 
polyhalogenated compounds, and halogenated aromatic hydrocarbons. 
    The alkyl halides, especially bromides and iodides, are used extensively 
as research intermediates. Several are essential pharmaceutical intermediates. 
Alkyl chlorides are important industrially in the manufacture of other organic 
compounds, especially alcohols and esters. 
    The polyhalogenated hydrocarbons find many uses. They are generally 
chemically inert, and are widely used in many industrial processes as 
solvents, degreasers, and dry-cleaning agents. The polybromides, more reactive 
than the polychlorides, are frequently used in the synthesis of other 
compounds. The polyfluorides, often with attached chorine and bromine, are 
called freons and are widely used as refrigerants. The unsaturated polyhalogen 
compounds are also valuable in a variety of organic syntheses. 
    The halogenated aryl hydrocarbons are used in organic syntheses as 
solvents and as reactants. Generally, the chlorine compounds are more inert 
than the bromine and iodine compounds. 
    The aralkyl- or benzyl-type halides are more reactive than the aryl 
halides. They are used chiefly as chemical intermediates.  See  HALOGENATION. 
 �34s[Ellen Clarke ;Max G. Gergel]
�33s 
  
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�6v�2v�32sHalogenation   �21m �33s 
    A chemical reaction or process which results in the formation of a 
chemical bond between a halogen atom and another atom. Reactions resulting in 
the formation of halogen-carbon bonds are especially important. The 
halogenated compounds produced are employed in many ways, for example, as 
solvents, intermediates for numerous chemicals, plastic and polymer 
intermediates, insecticides, fumigants, sterilants, refrigerants, additives 
for gasoline, and materials used in fire extinguishers.  See  HALOGEN ELEMENTS. 
    Halogenation reactions can be subdivided in several ways, for example, 
according to the type of halogen (fluorine, chlorine, bromine, or iodine), 
type of material to be halogenated (paraffin, olefin, aromatic, hydrogen, and 
so on), and operating conditions and methods of catalyzing or initiating the 
reaction.  
    Halogenation reactions with elemental chlorine, bromine, and iodine are of 
considerable importance. Because of high exothermocities, fluorinations with 
elemental fluorine tend to have high levels of side reactions. Consequently, 
elemental fluorine is generally not suitable for direct fluorination. Two 
types of reactions are possible with these halogen elements, substitution and 
addition.  
    Substitution halogenation is characterized by the substitution of a 
halogen atom for another atom (often a hydrogen atom) or group of atoms (or 
functional group) on paraffinic, olefinic, aromatic, and other hydrocarbons. A 
chlorination reaction of importance that involves substitution is that between 
methane and chlorine.  See  SUBSTITUTION REACTION. 
    Addition halogenation involves a halogen reacting with an unsaturated 
hydrocarbon. Chlorine, bromine, and iodine react readily with most olefins; 
the reaction between ethylene and chlorine, to form 1,2-dichloroethane is one 
of considerable commercial importance, since it is used in the manufacture of 
vinyl chloride. 
    Addition reactions with bromine or iodine are frequently used to measure 
quantitatively the number of --CHDOUBLE BARCH-- (or ethylenic-type) bonds in 
organic compounds. Bromine numbers or iodine values are measures of the degree 
of unsaturation of the hydrocarbons. 
    Substitution halogenation on the aromatic ring can be made to occur by 
means of ionic reactions. The chlorination reactions with elemental chlorine 
are similar to those used for addition chlorination of olefins.  See  
HALOGENATED HYDROCARBON. 
 �34s[Lyle F. Albright]
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�6v�2v�32sHalogen elements �21m �33s
    The halogen family consists of the elements fluorine, F; chlorine, Cl;
bromine, Br; iodine, I; and astatine, At. All the halogen elements except
astatine exist in the Earth's crust and atmosphere.  See  HALOGEN MINERALS.
    The halogens are the best-defined family of elements. They have an almost
perfect gradation of physical properties. The increase in atomic weight from
fluorine through iodine is paralleled by increases in density, melting and
boiling points, critical temperature and pressure, heats of fusion and
vaporization, and even in progressively deeper color (fluorine is pale yellow;
chlorine, yellow-green; bromine, dark red; and iodine, deep violet).
    Although all halogens generally undergo the same types of reactions, the
extent and ease with which these reactions occur vary markedly. Fluorine in
particular has the usual tendency of the lightest member of a family of
elements to exhibit reactions not comparable to the other members. Each
halogen must be considered individually, both in its preparation and in its
reaction.  See	ASTATINE; BROMINE; CHLORINE; FLUORINE; HALIDE; HALOGENATION;
IODINE.
 �34s[Richard J. Campana ;Albert A. Gunkler]
�33s

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�6v�2v�32sHalogen minerals   �21m �33s 
    Naturally occurring compounds containing a halogen as the sole or 
principal anionic constituent. There are over 70 such minerals, but only a few 
are common and can be grouped according to the following methods of formation. 
    1. Saline deposition by evaporation of sea water or salt lakes. Halite 
(rock salt), NaCl, is the most important of this type. Of the other minerals 
associated with halite, sylvite, KCl, and carnallite, KMgCl3.6H2O, are the 
most important.  See  ROCK SALT; SALINE EVAPORITE. 
    2. Hydrothermal deposition. Fluorite, CaF2, is the chief representative of 
this type. Cryolite, Na3A1F6, may be of primary deposition or may result from 
the action of fluorine-bearing solutions on preexisting silicates.  See  
CRYOLITE; FLUORITE. 
    3. Secondary alteration. Chlorides, iodides, or bromides of silver, 
copper, lead, or mercury may form as surface alterations of ore bodies 
carrying these metals. The most common are cerargyrite, AgCl, and atacamite, 
Cu2(OH)3Cl.  See  CERARGYRITE. 
    4. Deposition by sublimation. Halides formed as sublimation products about 
volcanic fumaroles include sal ammoniac, NH4Cl; malysite, FeCl3; and 
cotunnite, PbCl2. At Mount Vesuvius, Italy, is the most noted occurrence of 
such minerals.  See  HALIDE. 
    5. Meteorites. Lawrencite, FeCl2, has been found in iron meteorites. 
 �34s[C. S. Hammen]
�33s 
  
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�6v�2v�32sHalophilism (microbiology)   �21m �33s 
    The phenomenon of demand for salt (sodium chloride) for growth and 
maintenance displayed by certain microbes. These microbes comprise a variety 
of different types: bacteria, molds, algae, amebas, and protozoans. They may 
be classified according to the salt concentration supporting best growth: 
extreme halophiles, best growth at 20 - 30% salt; moderate halophiles, best 
growth at 5 - 20% salt; and slight halophiles, best growth at 2 - 5% salt. 
    The halophilic bacteria are widely distributed in nature in waters of very 
high salinity, for example, the Dead Sea. In evaporation ponds for the 
production of solar salt, they are often present in the strongly saline brine 
in such large numbers that they impart a red color to the brine. 
 �34s[Helge Larsen]
�33s 
  
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�6v�2v�32sHaloragales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 4 families, with less than 175 species in all. The Haloragales are 
herbs with perfect or often unisexual, more or less reduced flowers. Many of 
the species are aquatic. Pollen is commonly distributed by wind or water. The 
aquarium plant called parrot's feather (Myriophyllum, family Haloragaceae) and 
the very large-leaved specimen plant Gunnera (family Gunneraceae) are 
well-known members of the Haloragales.  See  MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sHamamelidae   �21m �33s 
    A small subclass of the class Magnoliopsida (dicotyledons) in the division 
Magnoliophyta (Angiospermae) consisting of 9 orders (Trochodendrales, 
Hamamelidales, Eucommiales, Urticales, Leitneriales, Juglandales, Myricales, 
Fagales, and Casuarinales), 23 families, and about 3400 species. They have 
strongly reduced, often unisexual flowers with poorly developed or no 
perianth. With the notable exception of some of the Urticales, they are all 
woody plants. Pollination is usually by wind. See articles on each order.  See 
also  FLOWER; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sHamamelidales   �21m �33s 
    A small order of flowering plants, division Magnoliophyta (Angiospermae), 
which gives its name to the subclass Hamamelidae in the class Magnoliopsida 
(dicotyledons). The family Hamamelidaceae contains about 100 species and the 
Platanaceae about 6 species; the other 4 families have only 2 species each. 
Within its subclass, the order is more advanced than the Trochodendrales in 
having vessels in the wood, but less advanced than the other orders in that 
the gynoecium consists either of separate carpels or of united carpels that 
open at maturity to release the seeds. Witch hazel (Hamamelis), sweet gum 
(Liquidambar), and the plane tree or sycamore (Platanus) are familiar members 
of the Hamamelidales.  See  HAMAMELIDAE; MAGNOLIOPSIDA; TROCHODENDRALES. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sHamilton's equations of motion �21m �33s
    The motion of a mechanical system may be described by a set of first-order
ordinary differential equations known as Hamilton's equations. Because of
their remarkably symmetrical form, they are often referred to as the canonical
equations of motion of a system. They are equivalent to Lagrange's equations,
but the fact that they are of first order and highly symmetrical makes them
advantageous for general discussions of the motion of systems.	See
LAGRANGE'S EQUATIONS.
    Hamilton's equations can be derived from Lagrange's equations. Let the
coordinates of the system be q(sub j) (j = 1, 2, . . . , f), and let the
dynamical description of the system be given by the lagrangian L(q,OVERDOTq,t),
where q denotes all the coordinates and a dot denotes total time derivative.
Lagrange's equations are then given by Eq.(1). The momentum p(sub j) canonically
conjugate to q(sub j) is defined by Eq. (2).

EQ.1:
(d/dt) (CURLY D L/CURLY D OVERDOT q[sub j]) - CURLY D L/CURLY D q(sub j) = 0

EQ.2:
	     p(sub j) = CURLY D L/CURLY D OVERDOT q(sub j)

    The hamiltonian H is defined by Eq. (3). Then Hamilton's canonical
equations are Eqs. (4).

Eq. 3:
H = SUMMATION (j-1 to f) p(sub j) OVERDOT q(sub j) - L(q,OVERDOT q,t)

Eq. 4:
	 OVERDOT q(sub j) = CURLY D H(q,p,t)/CURLY D p(sub j)
	 OVERDOT p(sub j) = -CURLY D H(q,p,t)/CURLY D q(sub j)			 (4)

    As they stand, Hamilton's equations are no easier to integrate directly
than Lagrange's. Hamilton's equations are of great advantage in more general
discussions, and they permit the making of canonical transformations which can
lead to simplifications. See CANONICAL TRANSFORMATIONS.
    The hamiltonian function H of classical mechanics is used to form the
quantum mechanical hamiltonian operator.  See  NONRELATIVISTIC QUANTUM THEORY.
 �34s[Philip M. Stehle]
�33s

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18262528
�6v�2v�32sHamilton's principle �21m �33s
    A variational statement known as Hamilton's principle forms a basis from
which the equations of motion of a classical dynamical system may be deduced.
Consider a mechanical system whose configuration is specified by f independent
generalized coordinates q1, .  .  .  , q(sub f).  Let the configuration at time
t1 be given by the values q(sub j)(t1) and that at time t2 by the values q(sub
j) (t2).  If the system is described dynamically by a Lagrangian function L(q,
OVERDOT q, t) where q stands for all the f coordinates, then, according to
Hamilton's principle, the trajectory or path of the system between the two
configurations at the two given times is that which makes the value of the
integral in the equation stationary relative to nearby paths PHI = INTEGRATION
(t1 to t2) L[q(t),OVERDOT q,t] dt between the same end points and taking the
same time.  Lagrange's equations may be derived from Hamilton's principle.

    Hamilton's principle is invariant under any transformation of coordinates.
It is also unaffected by the addition of a total time derivative to the
lagrangian, because the integral of such a total derivative is independent of
the path of integration.  This allows the coordinates qsubj and the momenta
p(sub j) = CURLY D L/CURLY OVERDOT q(sub j) to be subjected to arbitrary
canonical transformations without changing the form of Lagrange's equations.
See CANONICAL TRANSFORMATIONS; LAGRANGE'S EQUATIONS; LAGRANGIAN FUNCTION;
MINIMAL PRINCIPLES.

 �34s[Philip M.  Stehle]
�33s

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�6v�2v�32sHamster   �21m �33s 
    The common name for any of 14 species of rodents in the family Cricetidae. 
The natural range of most of these species is Asia but a few, such as the 
common hamster (Cricetus cricetus), are found in Europe. 
    The common hamster is a solitary, aggressive animal with interesting 
burrowing and hoarding habits. The burrows are not deep, rarely more than 2 ft 
(0.6 m), and consist of a large central chamber with radiating side chambers 
for special purposes, such as for hoarding food, for living quarters, and for 
excretion. A so-called summer chamber is used for breeding. The hamster goes 
into its burrow in the autumn, closes off the entrance, and goes to sleep. It 
does not hibernate deeply and wakes up from time to time to eat.  See  
HIBERNATION.  
    The golden hamster (Mesocricetus auratus) is extensively used as a 
laboratory animal for experimental purposes, especially in studies of the 
physiological aspects of hibernation.  See  RODENTIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sHand   �21m �33s 
    The terminal portion of the upper, or superior, extremity. Its bony 
framework consists of 8 carpal or wristbones arranged in 2 rows, 5 metacarpal 
or palm bones, and 14 phalanges or finger bones. These are held together, yet 
allowed considerable movement, by a complex system of ligaments. 
    Muscles of the forearm and hand act upon these bones, permitting a wide 
range of motion. The primate hand is characterized by the opposability of 
thumb and fingers, which permits grasping or prehension. In addition, fine 
voluntary control affords highly coordinated muscular action in man. The hand 
contains many nervous receptors, mediating sensations of touch, pressure, and 
temperature, and thus serves as a primary sensory apparatus. 
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sHaplosclerida   �21m �33s 
    An order of sponges of the class Demospongiae, including species with a 
skeleton made up of a single category of siliceous megascleres embedded in 
spongin fibers or joined together in a network by a spongin cement. 
Microscleres (never asters) are present or absent. A modified dermal skeleton 
is absent. Many species form large upright branching colonies, the branches 
often being hollow. 
    Haplosclerid sponges inhabit all seas and are especially abundant in tidal 
and shallow waters of the continental shelf. Some species occur down to depths 
of 6600 ft (2000 m). The family Spongillidae is restricted to fresh water 
except for a few species which have secondarily invaded brackish water.  See  
DEMOSPONGIAE.  
 �34s[Willard D. Hartman]
�33s 
  
  
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�6v�2v�32sHaplosporea   �21m �33s 
    A class of protozoa, often known as Acnidosporidia, in the subphylum 
Sporozoa. Haplosporea are distinguished from other similar groups by the 
production of spores lacking polar filaments. The spores are enclosed in a 
membrane, and each spore contains a single sporozoite. 
    Two orders make up the class according to the scheme proposed by the 
Committee on Taxonomy and Taxonomic Problems of the Society of 
Protozoologists. These are the Haplosporida and Sarcosporida. Haplosporida are 
parasites of invertebrates and primitive chordates (Ascidia). Sarcosporida are 
muscle parasites of vertebrates, particularly warm-blooded ones.  See  
HAPLOSPORIDA; SPOROZOA. 
 �34s[Reginald D. Manwell]
�33s 
  
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�6v�2v�32sHaplosporida   �21m �33s 
    A group of Protozoa usually regarded as an order within the class 
Haplosporea. The chief distinguishing characteristic of the Haplosporida, and 
the one from which their name is derived, is the production of uninucleate 
spores that lack polar capsules and polar filaments. 
    Haplosporida are mainly parasites of invertebrates, such as rotifers, 
annelids, crustacea, insects, and mollusks, but they also occur in the bodies 
of Ascidia (tunicates), which are primitive chordates. If the genus 
Icthyosporidium is properly included in the order, Haplosporida also 
parasitize fish. One species causes ``neon'' fish disease. 
    It is of considerable biological interest that some species of 
Haplosporida are hyperparasites, that is, parasitic on other parasites. They 
have been found in gregarines parasitizing annelids, and also in flukes.  See  
HAPLOSPOREA; SPOROZOA. 
 �34s[Reginald D. Manwell]
�33s 
  
A700000003217�254s�5v�1v833
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18262528
�6v�2v�32sHarbor   �21m �33s 
    Any body of water of sufficient depth for ships to enter and find shelter 
from storms or other natural phenomena. The modern harbor is a place where 
ships are built, launched, and repaired, as well as a terminal for incoming 
and outgoing ships. There are four principal classes of harbors: commercial, 
naval, fishery, and refuge for small craft. Most harbors are situated at the 
mouth of a river or at some point where it is easy to transfer cargoes inland 
by river barges, railroads, or trucks. 
    The term harbor is often confused with the term port. A port is a harbor 
with the necessary terminal facilities to expedite the moving of cargo and 
passengers at any stage of a journey. A good harbor must have a safe anchorage 
and a direct channel to open water, and must be deep enough for large ships. 
An efficient port must have enough room for docks, warehouses, and loading and 
unloading machinery. Geographically, a port or harbor is usually limited to a 
comparatively small area of usable berthing space rather than an extended 
coastline.  
    There are three types of harbors: land locked, natural harbors protected 
from the sea by a narrow inlet; unprotected harbors at which ships may dock 
even though unprotected from the hazards of changing tides, ocean waves, fogs, 
and ice; and artificial harbors carved out at sites where the natural features 
are unfavorable. The latter are fashioned by dredging and by constructing 
jetties, breakwaters, and sea basins to protect ships against unusually high 
or low tides.  See  COASTAL ENGINEERING. 
 �34s[Ben J. Wattenberg]
�33s 
  
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834
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�6v�2v�32sHardness scales �21m �33s
    Arbitrarily defined measures of the resistance of a material to
indentation under static or dynamic load, to scratch, abrasion, or wear, or to
cutting or drilling. Standardized tests compare similar materials according to
the particular aspect of hardness measured by the test. Widely used tests for
metals are Brinell, Rockwell, and Scleroscope tests, with modifications
depending upon the size or condition of the material. Indentation tests
compare species of wood or flooring materials, and abrasion tests serve as an
index of performance of stones and paving materials.
    Hardness tests are important in research and are widely used for grading,
acceptance, and quality control of manufactured articles. The hardness
designation or scale is associated with the test method or instrument used.
    Resistance to scratching is defined by comparison with 10 selected
minerals, which are numbered in the order of increasing hardness. This
mineralogical scale, called Mohs scale, is 1, talc; 2, gypsum; 3, calcite; 4,
fluorite; 5, apatite; 6, orthoclase; 7, quartz; 8, topaz; 9, corundum; and 10,
diamond. Minerals lower in the scale are scratched by those with higher
numbers.
    Materials are differentiated qualitatively according to resistance to
scratching or cutting by files especially selected for the purpose. Whether or
not a visible scratch is produced on the material indicates its hardness in
comparison with a sample of desired hardness.
    Resistance to indentation by a hardened steel or tungsten carbide ball
under specified load is the basis for Brinell hardness. Brinell hardness
number (Bhn), expressed in kilograms per square millimeter, is obtained by
dividing the load by the spherical surface area of the impression.
    Indentation of a square-based diamond pyramid penetrator with an angle
between opposite faces of 136DG measures Vickers hardness. Vickers hardness
number, also called diamond pyramid hardness, is equal to the load divided by
the lateral area of the pyramidal impression.
    Depth of indentation of either a steel ball or a 120DG conical diamond
with rounded point, called a brale, under prescribed load is the basis for
Rockwell hardness. The depth of impression is indicated on a dial whose
graduations represent the hardness number.
    The pressure in kilograms per square millimeter required to embed a
0.75-mm (0.0295-in.) hemispherical diamond penetrator to a depth of 0.0018 in.
(0.046 mm), producing an impression 0.36 mm (0.014 in.) in diameter, is the
measure of Monotron hardness.
    Height of rebound of a diamond-tipped weight or hammer falling within a
glass tube from a height of 10 in. (25.4 cm) and striking the specimen surface
measures Shore Scleroscope hardness.
    Resistance to indentation over very small areas (as on small parts, the
constituents of metal alloys, or for exploration of hardness variations) is
called microhardness.
 �34s[William J. Krefeld /Waldo G. Bowman]
�33s

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�6v�2v�32sHardy-Weinberg formula �21m �33s
    A basic mathematical relation used in population genetics. It gives the
proportion of the various genotypes in a randomly mating population in terms
of the frequencies of the genes. The formula was discovered independently in
1908 by G. H. Hardy, a British mathematician, and W. Weinberg, a German
physician. See HUMAN GENETICS; POPULATION GENETICS.
    In its simplest form the Hardy-Weinberg formula may be stated thus: If p
is the proportion of gene A in the population and q(= 1 - p) is the proportion
of gene a, then after one generation of random mating the three genotypes AA,
Aa, and aa will occur in the proportions p sup 2, 2pq, and q sup 2. In other
words the genotypes are given by the appropriate terms in the expansion of the
binomial (p + q) sup 2. The extension to multiple alleles is direct.
    The formula holds only for an infinite population and assumes random
mating in the absence of significant mutation pressure or gene transfer
between populations. However, it is an accurate approximation in many
populations. See GENETICS.
 �34s[James F. Crow]
�33s

A700000001170�254s�5v�1v834
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18392576
�6v�2v�32sHarelip �21m �33s
    A form of congenital defect, sometimes hereditary, marked by abnormal
clefts between the upper lip and the base of the nose. Many cases of harelip
occur in association with other defects of the facial area, most commonly
those of cleft palate. See CLEFT PALATE.
    Corrective measures (surgery) should be undertaken before the child begins
to talk, since phonation, or speech, is severely affected, particularly in
combination defects with penetration through the oral cavity into the nasal
chambers. See SPEECH.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sHarmonic analyzer   �21m �33s 
    A device for separating and measuring the frequencies and amplitudes of 
the Fourier-series components of a complex periodic wave. A complete harmonic 
analysis would include finding the phase of each component. Most electrical 
analyzers find  component amplitude only. See NONSINUSOIDAL WAVEFORM. 
    At low frequencies, harmonics may be measured with a wattmeter. The 
voltage to be analyzed is impressed upon the potential coil, and a current of 
adjustable frequency is passed through the current coil. By adjusting the 
frequency of the current to be close to that of the harmonic, a very slow beat 
can be produced and an accurate maximum reading can be obtained. See WATTMETER. 
    Two widely used types of instrument provide measurement of component 
amplitude at adjustable frequencies and nonmeasurement at other frequencies. 
The first of these, generally referred to as a distortion and noise meter, is 
designed to amplify uniformly over the frequency range of interest. The second 
type of instrument is a sound and vibration analyzer. Another widely used 
instrument employs a highly selective fixed-tuned filter in the heterodyne 
system.  
 �34s[Mark G. Foster]
�33s 
  
A700000003050�254s�5v�1v834
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�6v�2v�32sHarmonic motion   �21m �33s 
    A periodic motion that is a sinusoidal function of time. It is often 
called simple harmonic motion (SHM). It is the simplest possible type of 
vibratory motion. The motion is symmetric about its midpoint, at which the 
velocity is greatest and the acceleration is zero. At the extreme 
displacements or turning points, the velocity is zero, and the acceleration is 
a maximum. The motion is characterized by a unique frequency (without 
overtones).  
    Harmonic motion may be present in very simple mechanisms. For example, if 
a wheel is rotating at a constant speed about a fixed axis, the projection on 
any fixed line of the motion of a point on the wheel is simple harmonic. 
Harmonic motion may also result from the response of a vibrating system to a 
periodic - in particular a sinusoidal - force. Harmonic motion is the typical 
motion of most simple systems that have been displaced from a position of 
stable equilibrium and then released, provided that the damping is negligible. 
The motion of a pendulum is approximately simple harmonic for small 
amplitudes. See PENDULUM. 
    The realization that atoms are continually vibrating in motions that are 
nearly harmonic is essential for understanding many properties of matter, 
including molecular spectra, heat capacity, and heat conduction. See DAMPING; 
FORCED OSCILLATION; HARMONIC OSCILLATOR; LATTICE VIBRATIONS; MOLECULAR 
STRUCTURE AND SPECTRA; PERIODIC MOTION; VIBRATION; WAVE MOTION. 
 �34s[Joseph M. Keller]
�33s 
  
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�6v�2v�32sHarmonic oscillator   �21m �33s 
    Any physical system that is bound to a position of stable equilibrium by a 
restoring force or torque proportional to the linear or angular displacement 
from this position. If such a body is disturbed from its equilibrium position 
and released, and if damping can be neglected, the resulting vibration will be 
simple harmonic motion, with no overtones. The frequency of vibration is the 
natural frequency of the oscillator, determined by its inertia (mass) and the 
stiffness of its restoring force. 
    The harmonic oscillator is not restricted to a mechanical system, but 
might, for example, be electric. Typical electronic oscillators, however, are 
only approximately harmonic. 
    If a harmonic oscillator, instead of vibrating freely, is driven by a 
periodic force, it will vibrate harmonically with the period of the force; 
initially the natural frequency will also be present, but any damping will 
eventually remove the natural motion. See DAMPING; FORCED OSCILLATION; 
HARMONIC MOTION. 
    In both quantum mechanics and classical mechanics, the harmonic oscillator 
is an important problem. It is one of the few rigorously soluble problems of 
quantum mechanics. The quantum mechanical description of electromagnetic, 
electronic, mesonic, and other fields is usually carried out in terms of a 
(time) Fourier analysis. The individual Fourier components of noninteracting 
fields are independent harmonic oscillators. See ANHARMONIC OSCILLATOR; 
NONRELATIVISTIC QUANTUM THEORY. 
 �34s[Joseph M. Keller]
�33s 
  
A700000001672�254s�5v�1v835
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�6v�2v�32sHarmonic speed changer   �21m �33s 
    A mechanical-drive system used to transmit rotary, linear, or angular 
motion at high ratios and with positive motion. In the rotary version, shown 
schematically in the illustration, the drive consists of a rigid circular 
spline, an input wave generator, and a flexible spline. Any one of these can 
be fixed, used as the input, or used as the output. Any combination (fixed, 
driver, or driven) may be used. See SPLINES. 
    The advantages of this drive are (1) high-ratio gearing in small space, 
(2) speed reduction (or increase) in fixed ratio up to 1000:1 in one unit, (3) 
negligible wear of teeth, (4) balanced bearing loads, (5) negligible backlash, 
(6) efficiency of approximately 80% in a gear ratio of 400:1, and (7) 
adaptability to rotary-to-rotary, rotary-to-linear, and linear-to-linear 
drives. See GEAR DRIVE. 
 �34s[Paul H. Black]
�33s 
  
  
A700000001272�254s�5v�1v834
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�6v�2v�32sHarmonic (periodic phenomena)   �21m �33s 
    A sinusoidal quantity having a frequency that is an integral multiple of 
the frequency of a periodic quantity to which it is related. See MODE OF 
VIBRATION; PARTIAL TONE. 
    A harmonic series of sounds is one in which the basic frequency of each 
sound is an integral multiple of some fundamental frequency. An ideal string 
(or air column) can vibrate as a whole or in a number of equal parts, and the 
respective periods of vibration are proportional to the lengths. These 
increasingly shorter lengths or periods form a harmonic series. See HARMONIC 
ANALYZER.  
 �34s[Robert W. Young]
�33s 
  
A700000001406�254s�5v�1v835
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�6v�2v�32sHarpacticoida �21m �33s
    An order of the Copepoda, with about 1700 species known. Their form is
variable but is generally linear and more or less cylindrical (see
illustration). These animals are minute and vary in length from 1/64 to 1/8
in. (0.4 to 3 mm). As a rule, the first thoracic segment is incorporated with
the cephalothorax, and the last thoracic segment is included in the abdomen.
    Harpacticoids have a worldwide distribution. They occur in all kinds of
aquatic habitats, especially marine, but also among moss and leaves. In the
sea they range from the shore to abyssal depths. In general, they are
free-living and benthonic; some species are pelagic, parasitic, or commensal.
See COPEPODA.
[Karl Lang]


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�6v�2v�32sHarrowing   �21m �33s 
    Soil preparation for planting usually involves the pulling of an implement 
called a harrow over the plowed soil to break clods, level the surface, and 
destroy weeds. A wide variety of implements are classified as harrows; the 
most common kinds are the disk harrow, the spike-tooth harrow, the 
spring-tooth harrow, and the knife harrow. Previously the function of seedbed 
preparation was performed almost entirely by the implements classified as 
harrows. With the introduction of power, farming came to be performed in large 
part by field cultivators, rod weeders, rotary hoes, or treaders, subsurface 
packers, and various designs of rollers. Power-driven rotary tillers perform 
the function of both plowing and harrowing. 
    The spike-tooth is the oldest form of commercial harrow and consists of 
spikes or teeth (usually adjustable) extending downward from a frame. The 
teeth extend into the soil, and when the harrow is pulled forward, they cut 
through clods and break them. The teeth also stir and level the soil surface 
and kill weeds. This implement is most effective if used before clods dry; it 
is frequently attached behind the plow. 
    The spring-tooth harrow is similar to the spike-tooth type but has long 
curved teeth of spring steel. The spring action renders it suitable for rough 
or stony ground. It is particularly useful in bringing clods, roots of weeds, 
and obnoxious grasses to the surface for destruction, and to renovate and 
cultivate alfalfa fields. The knife harrow consists of a frame holding a 
number of knives which scrape and partly invert the surface to smooth it and 
destroy small weeds. The disk harrow is probably the most universally used 
type (see illustration). It cuts clods and trash effectively, destroys weeds, 
cuts in cover crops, and smoothes and prepares the surface for other farming 
operations. The penetration of the disk harrow depends largly upon weight. 
 �34s[Mark L. Nichols]
�33s 
  
A700000001135�254s�5v�1v836
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�6v�2v�32sHatchettite   �21m �33s 
    Mountain tallow, a yellow-white to yellow-green hydrocarbon occurring in 
Belgian coal seams; it is also called hatchettine. Hatchettite is translucent 
but darkens on exposure to air. It is soft, has no odor, is greasy to the 
touch, and consists of 85.5% carbon and 14.5% hydrogen. Its index of 
refraction is 1.47 - 1.50, it melts at 46 - 47DGC (115 - 117DGF), is sparingly 
soluble in alcohol or ether, decomposes in concentrated sulfuric acid, and has 
a specific gravity of 0.89 - 0.98. 
 �34s[Irving A. Breger]
�33s 
  
A700000001066�254s�5v�1v836
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�6v�2v�32sHausmannite �21m �33s
    A mineral with composition Mn sup 2 sup + Mn2 sup 3 sup + O4. Hausmannite is
most frequently massive-granular and possesses one perfect basal cleavage. The
color is black, and streak dark brown. Hardness is 5.5 on Mohs scale, and the
specific gravity is 4.81. It is an occasional ore of manganese, and it most
frequently occurs in metamorphosed sedimentary manganese ore deposits, such as
some small deposits in central Sweden and in the Central Provinces, India.
 �34s[Paul B. Moore]
�33s

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�6v�2v�32sHay fever   �21m �33s 
    An allergic disorder (also known as allergic rhinitis) of the nose and 
related structures due to sensitization by certain plant pollens. The term hay 
fever, although popularly used, is incorrect since hay is rarely implicated 
and fever is seldom present. 
    There is continuous or intermittent nasal obstruction accompanied by 
wheezing, sneezing, coughing, and the copious production of a watery 
discharge. The nasal mucosa becomes swollen and pale from vascular congestion 
and edema. Intense itching of the nose, mouth, and pharynx may occur, as well 
as excessive lacrimation, photophobia (light sensitivity), headache, 
irritability, insomnia, and gastric disturbances. 
    Skin testing and desensitization procedures are often helpful in 
identification of the inciting agent and in decreasing the severity of the 
reaction. Related forms of nasal allergy may be caused by exposure to other 
allergens such as foods, drugs, contactants, physical agents, and emotional 
crises in susceptible individuals. These may occur only in the nasal region 
but often are seen as part of a more generalized body reaction. See 
HYPERSENSITIVITY; SKIN TEST. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sHead   �21m �33s 
    The region of the body consisting of the skull, its contents, and related 
structures. Its two principal parts are the cranium, or braincase, and the 
face.  
    The skin, hair, and subcutaneous tissues over the top of the skull are 
collectively known as the scalp. The regions of the cranium take their names 
from the underlying bones, for example, the temporal, parietal, frontal, and 
occipital regions. 
    The intracranial contents include the brain and uppermost portion of the 
spinal cord with their coverings (meninges), blood vessels, and the important 
cranial nerves, as well as the cerebrospinal fluid system. Many openings, or 
foramina, afford means of passage from within the skull for nerves and blood 
vessels.  
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sHeadache   �21m �33s 
    A deep form of pain partaking of the aching quality of other deep pains 
and distinguished from them chiefly on the basis of locus of origin. See PAIN. 
    Most headaches may be shown to derive from distending forces applied to 
intracranial blood vessels. The presumption is, therefore, that the nervous 
structures responsible for initiating the massive patterns of sensation 
involved in headache are the free nerve endings embedded in the walls of 
vascular structures inside the head. Stretching of these fibers by agents 
producing traction, displacement, distention, or inflammation constitutes the 
stimulus, even as such forces, when exerted on visceral organs and muscle 
tendons, evoke deep aching pain and, when acting on superficial cutaneous 
tissues, arouse prickling or burning pain. 
    Among the most intense headaches are those associated with migraine, 
severe throbbing pains localized chiefly in the frontal, occipital, and 
parietal regions of the head, and often accompanied by visual patterns - 
streaked, striped, or scintillating ``sawtooth'' images of central origin. 
    Headaches may be induced by any set of conditions that will create and 
sustain strong contractile states of muscles in the head or neck region. This 
appears to be the basis of chronic headache from eyestrain. 
 �34s[Frank A. Geldard]
�33s 
  
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�6v�2v�32sHealth physics   �21m �33s 
    The science and profession that deals with problems of protection from the 
hazards of radiation or prevention of damage from exposure to this radiation 
while making it possible for humans to make full use of the various forms of 
energy. Initially health physics dealt only with ionizing radiations (ALPHA, 
BETA, GAMMA, neutrons, mesons, and so forth), but it has been extended to 
include nonionizing radiations (ultraviolet, visible, infrared, 
radio-frequency, microwave, long-wave, and sonic, ultrasonic, and infrasonic 
radiations). Health physics may be considered a border field of physics, 
biology, chemistry, mathematics, medicine, engineering, and industrial 
hygiene. See DOSIMETER; FILM BADGE; MONITORING OF IONIZING RADIATION; 
RADIATION BIOLOGY; RADIOMETRY. 
 �34s[Karl Z. Morgan]
�33s 
  
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�6v�2v�32sHearing aid   �21m �33s 
    An instrument used by a person who is hard of hearing to amplify sounds, 
particularly the sounds of speech. Modern electronic hearing aids are small 
enough to be worn comfortably and inconspicuously behind the ear, within the 
ear canal and concha, in the frames of spectacles, or in the clothing. A 
behind-the-ear aid is shown in the illustration. Essential components of a 
hearing aid are a microphone, an electronic amplifier, an earphone, and a 
plastic ear mold which serves to couple energy from the earphone to the 
eardrum either directly or through plastic tubes. Sometimes a vibrator is held 
by a spring headband behind the ear and delivers sound to the ear by bone 
conduction. See AMPLIFIER; DEAFNESS; EARPHONES; HEARING (HUMAN); MICROPHONE. 
 �34s[Arthur F. Niemoeller]
�33s 
  
  
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�6v�2v�32sHearing (human)   �21m �33s 
    The general perceptual behavior and the specific responses made in 
relation to sound stimuli. The perceptual behavior of hearing has many 
dimensions, including sound arousal, perceptual orientation, sound detection, 
sound discrimination, and perceptually motivated listening, such as music 
appreciation. Hearing behavior also involves the internal biochemical and 
physiological states associated with emotion, which add esthetic value and 
emotional tone to perception of speech, music, and noise. See PHONORECEPTION; 
PSYCHOACOUSTICS; SOUND. 
  
    Sound detection.  There are two aspects of sound detection which should be 
considered: detection of a sound stimulus in an otherwise soundfree field, and 
detection of a sound stimulus in the presence of a second masking sound. 
  
    Limits of hearing. The frequency and intensity patterns which are 
perceived as sound have definite limits, which vary to some extent from one 
individual to another and under different conditions. These limits of hearing 
also vary markedly from one animal species to another. See AUDIOMETRY. 
    The limits of hearing are represented graphically as the area of hearing 
(Fig. 1). To obtain such a chart, two curves are plotted: the audibility 
curve, which is the lower curve in Fig. 1, and the upper curve of ``pain'' or 
``feeling.'' The audibility curve is determined by measuring the absolute 
threshold for a series of frequencies within the hearing range. The absolute 
threshold for a given frequency is the lowest intensity at which that 
frequency can be heard. The upper limit of the area of hearing is determined 
by increasing the intensities of tones until they produce a perception of 
pain, tickle, or feeling. 
    Figure 1 shows that human individuals normally perceive as sound those 
frequencies between about 16 Hz, or lower, and 20,000 Hz, with the greatest 
sensitivity around 1000 - 4000 Hz. That is, the audibility curve, showing 
absolute thresholds for different frequencies, dips lowest for frequencies of 
around 1000 - 4000 Hz. The range of sound intensities is expressed in terms of 
units called decibels. See DECIBEL. 
    The area of hearing varies with age as well as with injuries and other 
defects. Sensitivity to higher frequencies typically decreases with age. Some 
animals, such as the bat, mouse, rat, cat, and dog, have a frequency range 
beyond that of humans. See DEAFNESS. 
  
  
    Auditory masking. It is sometimes impossible to detect one tone in the 
presence of another tone or noise. The first tone is said to be masked by the 
second tone or noise. The masking effect is defined as the difference between 
the absolute threshold of the masked tone and the threshold of detection of 
this tone in the presence of the masking sound. In general, the masking effect 
is greater for tones near the masking sound in frequency, although tones 
higher in frequency than the masking sound are affected more than lower tones. 
The range of frequencies affected is greater, the greater the intensity of the 
masking sound, and the lower the frequency of the masking sound. See LOUDNESS; 
MASKING OF SOUND. 
  
    Sound discrimination.  The differential threshold is a measure of ability 
to detect a difference between stimuli. The usual method for determining such 
thresholds for intensity and frequency of sounds is to introduce changes, 
either of intensity or frequency, into continuous sounds, and measure the 
smallest change that can be detected. 
    When white noise is used as a stimulus sound, the smallest change in 
intensity which can be detected remains fairly constant at slightly under 0.5 
dB, above an intensity level of about 20 dB. Below 20 dB, the just-detectable 
change in intensity increases sharply. When pure tones are used as stimuli, a 
similar threshold of 0.5 dB or less is found above levels of about 40 dB for a 
4000-Hz tone. The threshold is greater at lower intensities, and for tones of 
lower and higher frequencies. Since the decibel is a relative measure of 
intensity, one can say that the relative change in intensity which can be 
detected remains fairly constant above 20 dB for noise, and at somewhat higher 
intensity levels for pure tones. 
    The smallest change in frequency of pure tones which can be detected is 
fairly constant at about 2 - 3 Hz for frequencies up to 1000 Hz and at 
intensity levels over about 20 dB. Above 1000 Hz and 40 dB, the relative 
change in frequency which can be detected remains fairly constant, at about 
0.2 - 0.4%. 
    In general, loudness increases with increased intensity, but is also a 
function of frequency. The audibility threshold in Fig. 1 is an equal loudness 
contour of tones that can just barely be heard. At different frequencies such 
equal loudness tones require different intensities. 
  
    Binaural hearing.  Special characteristics of sound discrimination arise 
from binaural stimulation, or stimulation of the two ears. 
    The efficiency with which sound sources are localized in the environment 
is dependent on both learned cues and the ability of the auditory system to 
respond to slight differences in stimulation of the two ears. Success in 
judging distance depends on recognition of the sound, knowledge of how such a 
sound changes in loudness and timbre with distance, and corroboration from 
other sensory cues, especially visual. Even when other cues are eliminated, 
the direction of a sound which stimulates both ears can be judged fairly 
accurately except when it is exactly behind, above, or in front of the 
observer. Sounds to the right or left are localized in terms of differences at 
the two ears of intensity, time of arrival, or phase. Kinesthetic cues from 
head movements combine with binaural cues to enable observers to judge the 
elevation of sounds. 
    The ability to detect and discriminate sounds is increased with binaural 
stimulation. Tones heard with both ears sound louder than when heard with one. 
This summation effect accounts for the somewhat lower absolute thresholds 
found for binaural stimulation as opposed to monaural, and also for the lower 
intensity discrimination thresholds which are found for the two ears. 
Frequency discrimination is also better with binaural stimulation, especially 
for tones of very low and very high frequencies. 
  
    Response of auditory system.  The ear (Fig. 2) is a system for transducing 
the mechanical energy of sound into electrical energy which triggers nerve 
impulses in the auditory nerve. As sound strikes the eardrum, it causes the 
three small bones of the middle ear to vibrate. These movements set up 
corresponding vibrations in the oval window, and thence in the fluids of the 
cochlea. See EAR. 
  
    The auditory organ of the inner ear, the cochlea, is an enclosed structure 
that contains three fluid-filled canals (helicotrema).The gelatinous-fibrous 
basilar membrane makes up the base of the cochlear canal (the central canal 
and smallest of the three). Upon this membrane is the organ of Corti and its 
hair cells, which are thought to be the true receptors of hearing. 
Proliferations from the fibers of the auditory nerve extend up the center of 
the cochlea and connect with these hair cells. 
    The ability to hear many different frequencies as distinct pitches is 
related to the ability of the cochlea to resolve these frequencies. Between 
about 200 and 2000 Hz, this resolution may be accomplished by differential 
response of the basilar membrane. The cochlea has different resonance values 
at different points along its length, so that high tones cause the fluids and 
membrane to vibrate near the base, and low tones, near the apex. Vibrations of 
the oval window probably set up traveling waves in the fluids and membrane, 
which cause maximal vibrations at the place with a natural resonance value 
corresponding to the sound frequency. Below 200 Hz, the membrane vibrates as a 
whole, or nearly so. 
    Sensory nerve fibers, originating at the hair cells at all points along 
the organ of Corti, emerge from the cochlea to form the auditory nerve (VIIIth 
cranial nerve). This nerve enters the cochlear nucleus in the medulla 
oblongata, from which point connections are made with other centers in the 
midbrain, cerebellum, thalamus, and temporal lobe of the cortex. 
Discrimination of sound stimuli depends on differentiation of neural action at 
all levels. 
 �34s[Karl U. Smith]
�33s 
  
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�6v�2v�32sHeart disorders �21m �33s
    A group of structural and functional changes of the heart caused by
diverse etiologic agents such as toxic chemicals, bacteria, viruses, fungi,
parasites, and altered metabolism. These disorders are the leading cause of
illness and death in the industrialized nations.
    There are five general types of heart disorders based on the morphology of
the heart changes: coronary heart disease, cardiomyopathies, rheumatic fever,
infections, and congenital heart disease. Congestive heart failure is a common
result of these several lesions. Congestive heart failure is a clinical
syndrome resulting from diminished output of blood from the heart. The heart
is unable to keep pace with the venous blood returning to it, resulting in the
pooling of blood in the venous system with concomitant diminished filling of
the arterial vessels. The basic derangement may be an impaired contractility
of the heart muscle or an increased workload placed on the heart by extrinsic
factors.

    Coronary heart disease.  Heart disorders associated with coronary vessel
lesions are the product of insufficient blood supply to the heart muscle.
There are three forms of coronary vessel disease: arteriosclerotic heart
disease, myocardial infarction, and angina pectoris. With each there is
insufficient blood flow through the narrowed lumen of a coronary artery,
resulting in an insufficient oxygen supply to the heart muscle and subsequent
muscular cell injury or death (necrosis). See ANGINA PECTORIS;
ARTERIOSCLEROSIS; INFARCTION; THROMBOSIS.

    Cardiomyopathies.  Cardiomyopathies are a diverse group of heart disorders
which have in common the myocardium as the major target of injury. They are
divided into two subgroups: those which are inflammatory and those which are
degenerative. See COR PULMONALE; DEGENERATIVE CARDIOMYOPATHY; ENDOCARDIAL
FIBROELASTOSIS; HYPERTENSION; PERICARDITIS.

    Rheumatic fever. Rheumatic fever is a systemic, nonsuppurative, often
recurrent inflammatory process caused by group A beta hemolytic streptococci.
Joints are the most frequent site of lesions; however, the most severe and
important effect is injury to the heart. Most deaths from rheumatic fever occur
long after the acute disease has subsided, and are the result of endocardial
involvement at the heart valves. See RHEUMATIC FEVER.

    Infections. Bacterial and mycotic endocarditis are inflammatory lesions of
the lining of the chambers and valves of the heart caused by colonization of
the heart valves by bacteria or fungi, and are serious life-threatening
infections. Ordinarily the infection occurs in valves that have been previously
damaged by rheumatic heart disease or congenital malformation. However, in a
small proportion of cases there is no apparent preceding injury to the valve.
Occasionally the underlying disease process is atherosclerosis of the valves
or rarely syphilitic valvular disease. See ENDOCARDITIS.

    Congenital heart disease. Congenital malformations of the heart occur in
0.4-1.0% of live births in the United States. A high percentage of miscarriages
have evidence of heart malformations. The causes of congenital heart
malformations are not established in most instances. However, there is some
evidence that environmental chemicals may be important etiologic agents. The
clinical effect of heart malformation is the alteration of the flow of blood.
The large spectrum of anomalies of the heart are generally divided into two
kinds: those in which the blood is shunted from the pulmonary blood flow with
changes in the right atrium and ventricle, and those from the left atrium and
ventricle affecting systemic blood flow. When blood is shunted from the
pulmonary circuit without passing through the lungs, the blood is inadequately
oxygenated and the blood appears blue or cyanotic (the cause of a "blue baby").
See CONGENITAL ANOMALIES; FETAL ALCOHOL SYNDROME.
[N. Karl Mottet]

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�6v�2v�32sHeartwater disease   �21m �33s 
    A septicemic, infectious disease of cattle, sheep, and goats in equatorial 
and southern Africa caused by Cowdria ruminantium, a rickettsialike pathogen. 
It is carried chiefly by the bont tick (Amblyomma heberaeum). Humans are not 
susceptible.  See RICKETTSIALES. 
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sHeartworms   �21m �33s 
    Long filarioid nematodes (Dirofilaria immitis) of the superfamily 
Filarioidea which normally occur in the right heart chambers and adjacent 
vessels of the domestic dog. The progeny (microfilariae) begin to appear in 
the peripheral blood 7 to 9 months after infection. The microfilariae must 
complete their embryonation in mosquitoes. This has been demonstrated in over 
70 mosquito species, but probably fewer than 10 are important vectors. 
    The first discernible pathology is the enlarged pulmonary artery with a 
thickening of the lining. Continuing infection results in the occlusion of 
most of the pulmonary capillary bed, cirrhosis, and kidney pathology. The 
pathology is commonly well advanced in chronic heartworm disease before 
labored breathing, coughing, and unexplained exhaustion, particularly on 
exercise, alerts the owner. This standard chemotherapy is a course of 
intravenous injections of thiacetarsamide. This adulticide is later followed 
by a microfilaricide, such as orally administered dithiazanine. 
    Prevention is obviously very important. Currently the only 
chemoprophylactic agent is diethylcarbamazine (DEC). It must be administered 
daily (orally) from the beginning of the mosquito season, throughout the 
mosquito season, and for 2 months thereafter. It is very risky to start this 
preventive medication unless the microfilariae have already been eliminated.  
See  FILARIOIDEA. 
 �34s[Gilbert F. Otto]
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�6v�2v�32sHeart (vertebrate)   �21m �33s 
    The muscular pumping organ of the cardiovascular system. The heart 
typically lies ventrally, near the anterior end of the trunk; it is ventral 
and medial to the gills in fish and at the base of the neck or in the chest 
region of tetrapods. In humans it is located behind the breastbone and ribs 
between the third and fifth costal cartilages. Its anterior portion or base is 
directed to the right and dorsally and is the area where the great vessels 
enter and leave the heart. The lower muscular portion ends in a blunt apex 
which lies behind the fifth costal cartilage on the left. 
    The muscular wall of the heart, the myocardium, is lined by an inner 
endocardium and is covered externally by membranous visceral pericardium. 
There are coronary arteries and veins to and from the heart, which has a 
specialized neuromuscular conducting system and autonomic nerve supply. 
    In fishes the heart is basically a simple tube which becomes subdivided 
into four successive chambers, the sinus venosus, atrium, ventricle, and conus 
arteriosus. Blood from the body enters the sinus and leaves the conus to go to 
the gills to be oxygenated. The ventricle supplies the main pumping force. 
    When lungs are introduced into the system in lungfish and tetrapods, the 
mixing of oxygenated and nonoxygenated blood becomes a problem. In brief, the 
sinus venosus and conus arteriosus disappear, becoming incorporated into the 
other chambers or the bases of the great vessels. At the same time the atrium 
and later the ventricle become divided into right and left chambers by a 
median septum. 
    In birds and mammals including humans (see illustration) the medial 
fibromuscular septum divides the heart into two lateral halves, each 
consisting of a thin-walled receiving chamber or atrium and a thicker, 
muscular pumping chamber or ventricle. Blood enters the right atrium from the 
superior and inferior venae cavae which drain most of the body. It passes 
through the tricuspid valve to the right ventricle and is pumped to the lungs 
during systole, or contraction of the heart. Blood returns from the lungs by 
way of the pulmonary veins to the left atrium, passes into the left ventricle 
through the mitral valve, and during contraction is pumped out into the aorta. 
See CARDIOVASCULAR SYSTEM. 
 �34s[Thomas S. Parsons]
�33s 
  
  
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�6v�2v�32sHeat �21m �33s
    For the purposes of thermodynamics, it is convenient to define all energy
while in transit, but unassociated with matter, as either heat or work. Heat
is that form of energy in transit due to a temperature difference between the
source from which the energy is coming and the sink toward which the energy is
going. The energy is not called heat before it starts to flow or after it has
ceased to flow. A hot object does contain energy, but calling this energy heat
as it resides in the hot object can lead to widespread confusion.  See
ENERGY; INTERNAL ENERGY; TEMPERATURE; THERMODYNAMIC PRINCIPLES.
 �34s[Harold C. Weber ;William A. Steele]
�33s

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�6v�2v�32sHeat balance   �21m �33s 
    A particular form of an energy balance. The heat balance is generally 
useful in science but of special importance in process engineering. It is 
basic to the design and analysis of operating equipment since it provides a 
relationship between the energy terms in a process. As a simple example, the 
heat loss from a pipe carrying a hot fluid is difficult to measure directly 
but is easily calculated by a heat balance if the fluid properties at the ends 
of the pipe are known. 
    The conservation of energy requires that for any system the accumulation 
of energy within must equal the difference between energy entering and 
leaving. Energy terms are conveniently classified as: the Heat Q transferred 
into the system across the boundary, the work W put into the system, and the 
energy of the material streams flowing to and from the system. The flowing 
streams carry internal energy U, potential energy, kinetic energy, and a flow 
work term pV which arises from forcing a volume of material V into or out of 
the system under the restraint of the pressure p. 
    Frequently, the kinetic energy, potential energy, and work terms are 
negligible or cancel out so that the energy balance simplifies to the equation 
below, where subscripts 1 and 2 refer to input and output streams, respec-     

         *** U sub 1 = (p sub 1) (V sub 1) + Q = 
                  (U sub 2) + (p sub 2) (V sub 2) + DELTA E ***
  
tively, and DELTA E represents accumulation of energy in the system. This 
simplified energy balance is generally referred to as a heat balance. 
 �34s[William F. Jaep]
�33s 
  

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�6v�2v�32sHeat balance, terrestrial atmospheric   �21m �33s 
    The (heat) energy exchange between the planetary Earth-plus-atmosphere 
system and space, consisting of the reception, distribution, and 
transformation of heat received from the Sun as well as the emission, 
distribution, and transformation of thermal (infrared) radiation arising from 
the  Earth's surface and atmosphere. An area of 1 msup2 (10.76 ftsup2) at the 
Earth's mean distance from the Sun and perpendicular to the Sun's rays would 
receive about 1390 watts (or joules per second) if there were no intervening 
atmosphere. This quantity is the solar ``constant'' that depends on the energy 
output of the Sun and the Earth's distance from it.  
    Since the area of a sphere is four times that of the circle it presents to 
parallel radiation, the average meter-squared area outside the Earth's 
atmosphere receives about 350 W of solar energy (see illustration). Thirty 
percent of this incident solar energy is reflected and scattered back to 
space, mostly by clouds in the atmosphere; it is not available for use on 
Earth. This quantity, the percent reflected and scattered, is called the 
planetary albedo. The 70% of the solar energy that enters the planetary system 
is absorbed, mostly at the Earth's surface. The oceans absorb solar energy 
very well, and cover 70% of the planet's surface. This energy is partly 
transformed by plants, used to evaporate water, converted to thermal 
(infrared) radiation from the surface, and moved from the surface into the 
atmosphere by conduction and convection. The clouds and gases in the 
atmosphere also absorb some of the solar energy, thus evaporating clouds and 
heating the air.  See  GREENHOUSE EFFECT; OCEAN-ATMOSPHERE RELATIONS; SOLAR 
ENERGY; TERRESTRIAL RADIATION. 
 �34s[Thomas H. Vonder Haar]
�33s 
  
  
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�6v�2v�32sHeat capacity   �21m �33s 
    The quantity of heat required to raise a unit mass of homogeneous material 
one unit in temperature along a specified path, provided that during the 
process no phase or chemical changes occur is known as the heat capacity of 
the material in question. Moreover, the path is so restricted that the only 
work effects are those necessarily done on the surroundings to cause the 
change to conform to the specified path. The path is usually at either 
constant pressure or constant volume. 
    In accordance with the first law of thermodynamics, heat capacity at 
constant pressure C sub p is equal to the rate of change of enthalpy with 
temperature at constant pressure [(PARTIAL DIFFERENTIAL H/PARTIAL 
DIFFERENTIAL T) sub p]. Heat capacity at constant volume C sub v is the 
rate of change of internal energy with temperature at constant volume 
[(PARTIAL DIFFERENTIAL U/PARTIAL DIFFERENTIAL T) sub v]. Moreover, for 
any material, the first law yields the relation shown in the equation. 
  
   *** [(C sub p) - (C sub v)] = {[P + (PARTIAL DIFFERENTIAL U / 
          PARTIAL DIFFERENTIAL V)] sub T} [(PARTIAL DIFFERENTIAL U / 
                                       PARTIAL DIFFERENTIAL T) sub P]***
  
See ENTHALPY; INTERNAL ENERGY; THERMODYNAMIC PRINCIPLES. 
 �34s[Harold C. Weber]
�33s 
  

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�6v�2v�32sHeat exchanger   �21m �33s 
    A device used to transfer heat from a fluid flowing on one side of a 
barrier to another fluid (or fluids) flowing on the other side of the barrier. 
    When used to accomplish simultaneous heat transfer and mass transfer, heat 
exchangers become special equipment types, often known by other names. When 
fired directly by a combustion process, they become furnaces, boilers, 
heaters, tube-still heaters, and engines. If there is a change in phase in one 
of the flowing fluids - condensation of steam to water, for example - the 
equipment may be called a chiller, evaporator, sublimator, distillation-column 
reboiler, still, condenser, or cooler-condenser. 
    Heat exchangers may be so designed that chemical reactions or 
energy-generation processes can be carried out within them. The exchanger then 
becomes an integral part of the reaction system and may be known, for example, 
as a nuclear reactor, catalytic reactor, or polymerizer. 
    Heat exchangers are normally used only for the transfer and useful 
elimination or recovery of heat without an accompanying phase change. The 
fluids on either side of the barrier are usually liquids, but they may also be 
gases such as steam, air, or hydrocarbon vapors; or they may be liquid metals 
such as sodium or mercury. Fused salts are also used as heat-exchanger fluids 
in some applications. 
    Most often the barrier between the fluids is a metal wall such as that of 
a tube or pipe. However, it can be fabricated from flat metal plate or from 
graphite, plastic, or other corrosion-resistant materials of construction. 
    Heat exchangers find wide application in the chemical process industries, 
including petroleum refining and petrochemical processing; in the food 
industry, for example, for pasteurization of milk and canning of processed 
foods; in the generation of steam for production of power and electricity; in 
nuclear reaction systems; in aircraft and space vehicles; and in the field of 
cryogenics for the low-temperature separation of gases. Heat exchangers are 
the workhorses of the entire field of heating, ventilating, air-conditioning, 
and refrigeration.  See  CONDUCTION (HEAT); CONVECTION (HEAT); COOLING TOWER; 
DISTILLATION; EVAPORATOR; FURNACE; HEAT RADIATION; HEAT TRANSFER; TUBE-STILL 
HEATER; VAPOR CONDENSOR. 
 �34s[Raymond F. Fremed]
�33s 
  
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�6v�2v�32sHeat insulation   �21m �33s 
    Materials whose principal purpose is to retard the flow of heat. Thermal- 
or heat-insulation materials may be divided into two classes, bulk insulations 
and reflective insulations. The class and the material within a class to be 
used for a given application depend upon such factors as temperature of 
operation, ambient conditions, mechanical strength requirements, and economics. 
    Examples of bulk insulation include mineral wool, vegetable fibers and 
organic papers, foamed plastics, calcium silicates with asbestos, expanded 
vermiculite, expanded perlite, cellular glass, silica aerogel, and diatomite 
and insulating firebrick. They retard the flow of heat, breaking up the 
heat-flow path by the interposition of many air spaces and in most cases by 
their opacity to radiant heat. 
    Reflective insulations are usually aluminum foil or sheets, although 
occasionally a coated steel sheet, an aluminized paper, or even gold or silver 
surfaces are used. Refractory metals, such as tantalum, may be used at higher 
temperatures. Their effectiveness is due to their low emissivity (high 
reflectivity) of heat radiation. 
    The distinguishing property of bulk thermal insulation is low thermal 
conductivity; representative values are listed in the table. For a given 
thickness of material exposed to a given temperature difference, the rate of 
heat flow per unit area is directly proportional to the thermal conductivity 
of the material. See CONDUCTION (HEAT). 
 �34s[Harry F. Remde]
�33s 
  

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�6v�2v�32sHeat pump   �21m �33s 
    The thermodynamic counterpart of the heat engine. A heat pump raises the 
temperature level of heat by means of work input. In its usual form a 
compressor takes refrigerant vapor from a low-pressure, low-temperature 
evaporator and delivers it at high pressure and temperature to a condenser 
(see illustration). The pump cycle is identical with the customary 
vapor-compression refrigeration system.  See  REFRIGERATION CYCLE. 
    This dual purpose is accomplished, in effect, by placing the 
low-temperature evaporator in the conditioned space during the summer and the 
high-temperature condenser in the same space during the winter. Thus, if 70DGF 
(21DGC) is to be maintained in the conditioned space regardless of the season, 
this would be the theoretical temperature of the evaporating coil in summer 
and of the condensing coil in winter. The actual temperatures on the 
refrigerant side of these coils would need to be below 70DGF in summer and 
above 70DGF in winter to permit the necessary transfer of heat through the 
coil surfaces. If the average outside temperatures are 100DGF (38DGC) in 
summer and 40DGF (40DGC) in winter, the heat pump serves to raise or lower the 
temperature 30DG (17DGC) and to deliver the heat or cold as required.  
    The heat pump is also used for a wide assortment of industrial and process 
applications such as low-temperature heating, evaporation, concentration, and 
distillation.  
 �34s[Theodore Baumeister]
�33s 
  
  
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�6v�2v�32sHeat radiation �21m �33s
    The energy radiated by solids, liquids, and gases as a result of their
temperature. Such radiant energy is in the form of electromagnetic waves and
covers the entire electromagnetic spectrum, extending from the radio-wave
portion of the spectrum through the infrared, visible, ultraviolet, x-ray, and
GAMMA-ray portions. From most hot bodies on Earth this radiant energy lies
largely in the infrared region. See ELECTROMAGNETIC RADIATION; INFRARED
RADIATION.
    Radiation is one of the three basic methods of heat transfer, the other
two methods being conduction and convection. See CONDUCTION (HEAT); CONVECTION
(HEAT); HEAT TRANSFER.
    A hot plate at 260DGF (400 K) may show no visible glow; but a hand which
is held over it senses the warming rays emitted by the plate. A temperature of
more than 1340DGF (1000 K) is required to produce a perceptible amount of
visible light. At this temperature a hot plate glows red and the sensation of
warmth increases considerably, demonstrating that the higher the temperature
of the hot plate the greater the amount of radiated energy. Part of this
energy is visible radiation, and the amount of this visible radiation
increases with increasing temperature. A steel furnace at 2780DGF (1800 K)
shows a strong yellow glow. If a tungsten wire (used as the filament in
incandescent lamps) is raised by resistance heating to a temperature of
4580DGF (2800 K), it emits a bright white light. As the temperature of a
substance increases, additional colors of the visible portion of the spectrum
appear, the sequence being first red, then yellow, green, blue, and finally
violet. The violet radiation is of shorter wavelength than the red radiation,
and it is also of higher quantum energy. In order to produce strong violet
radiation, a temperature of almost 5000DGF (3000 K) is required. Ultraviolet
radiation necessitates even higher temperatures. The Sun emits considerable
ultraviolet radiation; its temperature is about 10,300DGF (6000 K). Such
temperatures have been produced on Earth in gases ionized by electrical
discharges. The mercury-vapor lamp and the fluorescent lamp emit large amounts
of ultraviolet radiation. Temperatures up to 36,000DGF (20,000 K), however,
are still much too low to produce x-rays or GAMMA-radiation. A gas maintained
at temperatures above 1.8 x 10 sup 6 K (10 sup 6 K), encountered in nuclear
fusion experiments, emits x-rays and GAMMA-rays. See NUCLEAR FUSION; SUN;
ULTRAVIOLET RADIATION.
    A blackbody is defined as a body which emits the maximum amount of heat
radiation. Although there exists no perfect blackbody radiator in nature, it
is possible to construct one on the principle of cavity radiation. See
BLACKBODY.
    A cavity radiator is usually understood to be a heated enclosure with a
small opening which allows some radiation to escape or enter. The escaping
radiation from such a cavity has the same characteristics as blackbody
radiation.
    Kirchhoff's law correlates mathematically the heat radiation properties of
materials at thermal equilibrium. It is often called the second law of
thermodynamics for radiating systems. Kirchhoff's law can be expressed as
follows: The ratio of the emissivity of a heat radiator to the absorptivity of
the same radiator is a function of frequency and temperature alone. This
function is the same for all bodies, and it is equal to the emissivity of a
blackbody. A consequence of Kirchhoff's law is the postulate that a blackbody
has an emissivity which is greater than that of any other body. See
KIRCHHOFF'S LAWS OF ELECTRIC CIRCUITS.
    Planck's radiation law represents mathematically the energy distribution
of the heat radiation from 1 cm sup 2 of surface area of a blackbody at any
temperature. Formulated by Max Planck early in the 20th century, it laid the
foundation for the advance of modern physics and the advent of quantum theory.
Equation (1) is the mathematical expression of Planck's radiation law, where
RLAMBDA is the total energy radiated from the body measured in watts per
square centimeter per unit wavelength, at the wavelength LAMBDA. The
wavelength in this formula is measured in micrometers. The quantity T is the
temperature in degrees Kelvin, and e is the base of the natural logarithms.
The illustration presents graphs of Planck's law for various temperatures and
shows substances which attain these temperatures. It should be noted that
these substances will not radiate as predicted by Planck's law since they are
not blackbodies themselves.

R sub LAMBDA = 37,418/LAMBDA (sup 5)(e[sup 14,388/LAMBDAT] - 1) 	(1)

    The Stefan-Boltzmann law states that the total energy radiated from a hot
body increases with the fourth power of the temperature of the body. This law
can be derived from Planck's law by the process of integration and is
expressed mathematically as Eq. (2), where RsubT is the total amount of energy
radiated from a blackbody in watts per square centimeter. When RsubT is
multiplied by the total emissivity, the total energy radiated from a real heat
radiator is obtained.
 �34s[Heinz G. Sell ;Peter J. Walsh]
�33s

	      R sub T = 5.670 x 10 - sup 1 sup 2 T sup 4		 (2)



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�6v�2v�32sHeat transfer   �21m �33s 
    Heat, a form of kinetic energy, is transferred in three ways: conduction, 
convection, and radiation. Heat can be transferred only if a temperature 
difference exists, and then only in the direction of decreasing temperature. 
Beyond this, the mechanisms and laws governing each of these ways are quite 
different. This article gives introductory information on the three types of 
heat transfer (also called thermal transfer) and on important industrial 
devices called heat exchanges. See CONDUCTION (HEAT); CONVECTION (HEAT); HEAT 
RADIATION.  
    By utilizing a knowledge of the principles governing the three methods of 
heat transfer and by a proper selection and fabrication of materials, the 
designer attempts to obtain the required heat flow. This may involve the flow 
of large amounts of heat to some point in a process or the reduction in flow 
in others. All three methods operate in processes that are common place. 
    In industry, for example, it is generally desired to extract heat from one 
fluid stream and add it to another. Devices used for this purpose have 
passages for each of the two streams separated by a heat-exchange surface in 
the form of plates or tubes and are known as heat exchangers. The automobile 
radiator, the hot-water heater, the steam or hot-water radiator in a house, 
the steam boiler, the condenser and evaporator on either the household 
refrigerator or air conditioner, and even the ordinary cooking utensils in 
everyday use are all heat exchangers. See HEAT; HEAT EXCHANGER. 
 �34s[Ralph H. Luebbers]
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�6v�2v�32sHeat treatment (metallurgy)   �21m �33s 
    A procedure of heating and cooling a material without melting. Plastic 
deformation may be included in the sequence of heating and cooling steps, thus 
defining a thermomechanical treatment. Typical objectives of heat treatments 
are hardening, strengthening, softening, improved formability, improved 
machinability, stress relief, and improved dimensional stability. Heat 
treatments are often categorized with special names, such as annealing, 
normalizing, stress relief anneals, process anneals, hardening, tempering, 
austempering, martempering, intercritical annealing, carburizing, nitriding, 
solution anneal, aging, precipitation hardening, and thermomechanical 
treatment.  
    All metals and alloys in common use are heat-treated at some stage during 
processing. Iron alloys, however, respond to heat treatments in a unique way 
because of the multitude of phase changes which can be induced, and it is thus 
convenient to discuss heat treatments for ferrous and nonferrous metals 
separately. See IRON; STEEL. 
  
    Ferrous metals.  Annealing heat treatments are used to soften the steel, 
to improve the machinability, to relieve internal stresses, to impart 
dimensional stability, and to refine the grain size. 
    Hardening treatments are used to harden steels by heating to a temperature 
at which austenite is formed and then cooling with sufficient rapidity to make 
the transformation to pearlite and/or ferrite unfavorable. 
  
    Some heat treatments are used to alter the chemistry at the surface of a 
steel, usually to achieve preferential hardening of a surface layer. 
Carburizing consists of subjecting the steel to an atmosphere of partially 
combusted natural gas which has been enriched with respect to carbon. In the 
nitriding treatment, nitrogen diffusing to the surface of the steel forms 
nitrides. Chromizing involves the addition of chromium to the surface by 
diffusion from a chromium-rich material packed around the steel or dissolved 
in molten lead. See SURFACE HARDENING OF STEEL. 
  
    Nonferrous metals.  Many nonferrous metals do not exhibit phase 
transformations, and it is not possible to harden them by means of simple 
heating and quenching treatments as in steel. Unlike steels, it is impossible 
to achieve grain refinement by heat treatment alone, but it is possible to 
reduce the grain size by a combination of cold-working and annealing 
treatments. Some nonferrous alloys can be hardened, but the mechanisms is one 
by which a fine precipitate is formed, and the reaction is fundamentally 
different from the martensitic hardening reaction in steel. There are also 
certain ferrous alloys that can be precipitation hardened. However this 
hardening technique is used much more widely in nonferrous than in ferrous 
alloys. In titanium alloys, the BETA phase can transform in a martensitic 
reaction on rapid cooling, and the hardening of these alloys is achieved by 
methods which are similar to those used for steels. 
 �34s[Benjamin L. Averbach]
�33s 
  
  
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�6v�2v�32sHeavy minerals   �21m �33s 
    Minerals with a density above 2.9, which is the density of bromoform, the 
liquid used to separate heavy from light minerals. Heavy minerals are 
sometimes used in igneous petrology, but their main importance is in the study 
of sedimentary rocks. See SEDIMENTARY ROCKS. 
    If not modified by weathering or sedimentation processes, the 
heavy-mineral suites of sedimentary rocks, primarily sandstones, offer clues 
to the composition of the source rock. For example, amphibole and 
amphibole-epidote suites suggest an igneous or high-grade metamorphic source 
and augite suites a volcanic source. 
    Sediments can be characterized by their heavy-mineral association. The 
study of the distribution of heavy-mineral associations in a depositional 
basin permits the tracing of the distribution in space and time of sediment 
supply from various sources and assists in the unraveling of the erosional 
history of the source areas. Since frequently several sources supply sediments 
with different heavy-mineral suites to a single basin, the pattern of 
heavy-mineral associations can be complex, in particular when mixing occurs. 
Thus, heavy minerals are an aid in the study of the depositional history and 
paleogeography of a basin. In some cases they have been used successfully in 
correlating sedimentary strata. See SEDIMENTATION (GEOLOGY); STRATIGRAPHY. 
 �34s[Tjeerd H. Van Andel]
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�6v�2v�32sHeavy water �21m �33s
    A form of water in which the hydrogen atoms of mass 1 (sup 1 H) ordinarily
present in water are replaced by deuterium (symbol D or sup 2 H), the heavy
stable isotope of hydrogen of mass 2. The molecular formula of heavy water is
D2O (or sup2H2O). See DEUTERIUM.
    Because the mass difference between sup 1 H and sup 2 H is the largest for
any pair of stable (nonradioactive) isotopes in the periodic table, many of the
physical and chemical properties of the pure isotopic species and their
respective compounds differ to a significant extent. Selected physical
properties of sup 1 H2O and sup 2 H2O are compared in the table.
    Heavy water, judging from its higher melting and boiling points, its
higher viscosity, and its surprisingly high temperature of maximum density, is
a distinctly more structured liquid than is ordinary water. Heavy water is
more extensively hydrogen-bonded, and the hydrogen bonds formed by sup 2 H are
somewhat stronger than are those of sup 1 H.
    The only large-scale use of heavy water in industry is as a moderator in
nuclear reactors. Small amounts of heavy water are used to grow fully
deuterated organisms, which serve as a source of fully deuterated compounds of
biological importance. These are finding increasing use in research techniques
such as small-angle neutron scattering, in high-resolution nuclear magnetic
resonance spectroscopy of immobilized samples, and in the study of isotope
effects. See ISOTOPE (STABLE) SEPARATION; NUCLEAR REACTOR; WATER.
 �34s[Joseph J. Katz]
�33s

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�6v�2v�32sHedgehog   �21m �33s 
    Members of the family Erinaceidae (order Insectivora), which includes 7 
genera and 19 species of medium- to large-size animals generally characterized 
by spines on their back and sides. This family, however, is subdivided into 
two subfamilies, the spiny hedgehog group which includes the European hedgehog 
(Erinaceus europaeus); and the hairy hedgehog or gymnuras, long-tailed species 
which lack spines. The typical spiny hedgehog (see illustration) has 
well-developed eyes and a rudimentary to moderately long tail. In most 
species, there are five toes on each foot, although in some species these are 
reduced to four on the hindfeet. The spines can be erected by strong muscles, 
and serve as a protection for the naked or hairy belly, head, and limbs when 
the animal rolls itself into a ball. See INSECTIVORA. 
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sHeidelberg man �21m �33s
    The European representative of the species Homo erectus which was
ancestral to the later Neandertals and anatomically modern H. sapiens.
Heidelberg man, or Homo heidelbergensis as it was originally called by O.
Schoetensack, was formerly represented only by a largely intact lower jaw from
a gravel pit near the village of Mauer, 16 km (10 mi) southeast of Heidelberg,
Germany. The Mauer jaw has been dated on faunal and geological grounds to the
early Middle Pleistocene, more than 400,000 years ago. Heidelberg man is now
also represented by several other fossil remains, including teeth and the
backs of craniums. See FOSSIL MAN.
 �34s[Erik Trinkaus]
�33s

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�6v�2v�32sHelicity (quantum mechanics) �21m �33s
    A fundamental quantized variable used in quantum mechanics to specify the
relative orientations of spin and linear momentum of massless particles. It is
a requirement of fundamental Dirac quantum mechanics that such particles have
their spins aligned either parallel or antiparallel to their linear momentum.
Particles having parallel alignment are arbitrarily assigned helicity +1;
those have antiparallel alignment, -1. See MOMENTUM; SPIN (QUANTUM MECHANICS).
    In a classic experiment on K electron capture by sup 1 sup 5 sup 2 Eu, M.
Goldhaber, L. Grodzins, and A. Sunyar first showed that the neutrino emitted
in the weak nuclear interaction had negative helicity - that its spin was
aligned antiparallel to its momentum. An equivalent description of this
situation is that these neutrinos are left-handed. Symmetry requires that
antineutrinos be right-handed and have positive helicity. See ELECTRON
CAPTURE; NEUTRINO; ELEMENTARY PARTICLE; NEUTRINO; QUANTUM MECHANICS; SYMMETRY
LAWS (PHYSICS).
 �34s[D. Allan Bromley]
�33s

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�6v�2v�32sHelicoplacoidea   �21m �33s 
    A class of free-living, spindle- or pear-shaped plated echinozoans, known 
only from the Lower Cambrian (Olenellus Zone) of California. The skeletal 
plates formed a flexible test or shell, evidently carried on a muscular 
integument, which was contractile and was pleated in helical spirals. The oral 
and anal apertures are believed to have lain at either end of the body (see 
illustration). Helicoplacoids were apparently related to early kinds of sea 
cucumbers. See ECHINODERMATA; ECHINOZOA. 
 �34s[Howard B. Fell]
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�6v�2v�32sHelicopter   �21m �33s 
    An aircraft characterized by its large-diameter, powered, rotating blades. 
The helicopter can lift itself vertically by the reactive force generated as 
the rotating blades accelerate air downward. It can both lift and propel 
itself by accelerating air downward at an angle to the vertical. The 
helicopter is the most successful vertical takeoff and landing (VTOL) aircraft 
developed, by virtue of its relatively high efficiency in performing hovering 
and low-speed flight missions. 
    The key to understanding the operation and control of a helicopter lies in 
a knowledge of the forces and resultant motion of each rotor blade as momentum 
is imparted to the air. Unlike a fixed-wing aircraft, which derives its lift 
from the translational motion of the fuselage and airfoil-shaped wing relative 
to the air, the helicopter rotates its wings (or rotor blades) about a 
vertical shaft and thus is able to generate lift when the fuselage remains 
stationary.  
    Many different rotor arrangements have been used, and most of the early 
attempts at vertical flight were made with machines having multiple or coaxial 
counterrotating rotors. Most modern helicopters employ the single rotor or the 
tandem rotor configurations. 
    In addition to the selection of the number and location of the lifting 
rotors, designers have developed varied methods for attaching the blades to 
the rotor hub. Very early experiments conducted with the blades rigidly 
attached to the hub were unsatisfactory because of the excessive moments 
applied to the rotor mast. Based on the success achieved by the introduction 
of hinged attachments for the rotor blades, several configurations have been 
successfully manufactured. The teetering rotor used on two-bladed 
configurations has one central hinge which allows the blades to move in unison 
(one up, one down) like a seesaw. The gimbaled rotor is essentially equivalent 
to the teetering rotor and has been used on rotors with three or more blades. 
The articulated rotor has each blade attached to the hub by its own flapping 
hinge.  
        The growth of the helicopter industry in the United States is founded 
in the uses made by the armed forces. The technology which evolved to meet the 
needs of the military provided the base for an impressive growth in commercial 
applications. With such diverse operations as crop spraying, logging, 
construction, police and ambulance service, and passenger and corporate 
transportation, the industry has responded with a variety of commercial 
helicopters. See VERTICAL TAKEOFF AND LANDING (VTOL). 
 �34s[Julian L. Jenkins]
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�6v�2v�32sHelicosporida   �21m �33s 
    An order of protozoa in the class Myxosporidea (subphylum Cnidospora). It 
is characterized by production of spores with a relatively thick, single 
intrasporal filament and three uninucleate sporoplasms. Helicosporidium 
parasiticum is the only species in the order. The parasite infests the body 
cavity, fat bodies, and ganglia of mites (Hericia hericia) and the body cavity 
of fly larvae (Dasyhelea obscura and Mycetobia pallipes) found in the sap of 
horse chestnut and elm trees. See MYXOSPORIDEA. 
 �34s[Ross F. Nigrelli]
�33s 
  
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�6v�2v�32sHelimagnetism   �21m �33s 
    A property possessed by some metals, alloys, and salts of transition 
elements in which the atomic magnetic moments, at sufficiently low 
temperatures, are arranged in a spiral or helix. Simple antiferromagnets and 
ferromagnets can be considered as nonconical helimagnets with helical angles 
of 180 and 0DG, respectively. In the same way, nonconical helimagnets may be 
considered as conical helimagnets with cone angle of 0DG. Some typical 
helimagnets are listed in the table. The magnetic structures have been 
detected by neutron diffraction. 
 �34s[Frederic Keffer]
�33s 
  
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�6v�2v�32sHeliozoia   �21m �33s 
    A subclass of the Actinopodea. Unlike Radiolaria, these protozoans have no 
central capsule. Most species live in fresh water. Pseudopodia may be either 
slender with an axial filament surrounded by cytoplasm (axopodia) or 
filamentous (filopodia). Certain floating species can roll along on the tips 
of their axopodia and also swim by moving their axopodia. Some species are 
naked; others have skeletal elements ranging from siliceous scales or spicules 
embedded in a gelatinous capsule to a reticulate chitinous  skeleton often 
impregnated with silica. A centroplast may or may not be present. The subclass 
has three orders: Actinophryida (see illustration), Centrohelida, and 
Desmothoracida. See ACTINOPODEA. 
 �34s[Richard P. Hall]
�33s 
  
  
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�6v�2v�32sHelium   �21m �33s 
    A gaseous chemical element, He, atomic number 2 and atomic weight 4.0026. 
Helium is one of the noble gases in group 0 of the periodic table. It is the 
second lightest element. The world's chief source of helium is a group of 
natural gas fields in the United States. See INERT GASES. 
    Helium is a colorless, odorless, and tasteless gas. It has the lowest 
solubility in water of any known gas. It is the least reactive element and 
forms essentially no chemical compounds. The density and the viscosity of 
helium vapor is very low. Thermal conductivity and heat content are 
exceptionally high. Helium can be liquefied, but its condensation temperature 
is the lowest of any known substance. The properties of helium are given in 
the table. See LIQUID HELIUM. 
    Helium was first used as a lifting gas in balloons and dirigibles. This 
use continues for high-altitude research and for weather balloons. The 
principal use of helium is in inert gas - shielded arc welding. The greatest 
potential for helium use continues to emerge from extreme-low-temperature 
applications. Helium is the only refrigerant capable of reaching temperatures 
below 14 K (-434DGF). The chief value of ultra-low temperature is the 
development of the state of superconductivity, in which there is virtually 
zero resistance to the flow of electricity. Other helium applications include 
use as a pressurizing gas in liquid-fueled rockets, in helium-oxygen breathing 
mixtures for divers, as a working fluid in gas-cooled nuclear reactors, and as 
a carrier gas for chemical analysis by gas chromatography. 
    Terrestrial helium is believed to be formed in natural radioactive decay 
of heavy elements. Most of this helium migrates to the surface and enters the 
atmosphere. The atmospheric concentration of helium (5.25 parts per million at 
sea level) could be expected to be higher. However, its low molecular weight 
permits helium to escape into space from the upper atmosphere at a rate 
roughly equal to its formation. Natural gases contain helium at concentrations 
higher than in the atmosphere. 
 �34s[Arthur W. Francis]
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�6v�2v�32sHelix   �21m �33s 
    Any nonplanar curve all of whose tangents make the same angle with a fixed 
line. Other characteristic properties are that all principal normals are 
parallel to a plane and that the ratio of torsion to curvature is constant. If 
a helix has constant curvature (and hence constant torsion), it is a circular 
helix; it lies on a circular cylinder whose elements it cuts at a constant 
angle. See ANALYTIC GEOMETRY; DIFFERENTIAL GEOMETRY. 
 �34s[Leonard M. Blumenthal]
�33s 
  
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�6v�2v�32sHelmholtz coils   �21m �33s 
    A pair of flat circular coils with equal numbers of turns and equal 
diameters, arranged with a common axis, and connected in series to have a 
common current (see illustration). The purpose of the arrangement is to obtain 
a magnetic field that is more nearly uniform than that of a single coil 
without the use of a long solenoid. The optimum arrangement is that in which 
the distance between the two coils is equal to the radius of one of the coils. 
See MAGNETIC FIELD; SOLENOID (ELECTRICITY). 
 �34s[Kenneth V. Manning]
�33s 
  
  
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�6v�2v�32sHematite   �21m �33s 
    The most important ore of iron, with composition ALPHA-Fe2O3. The crystals 
are thick tabular, usually flattened parallel to the base, and are frequently 
platy in habit. Hematite usually occurs as rouge-red earthy masses of finely 
divided particles. It is the major red-coloring agent in rocks and is a common 
interstitial cement in sediments. When mixed with quartzite or finely divided 
quartz, the mixture is called jasper, jaspilite, or taconite. Botryoidal 
masses are called ``kidney ore,'' and splinters of these masses are ``pencil 
ore.''  
    The color is steel gray, blood red in thin fragments, and streak and 
powder are rouge red; hardness is 6 on Mohs scale and specific gravity is 
5.25. The mineral is only weakly magnetic. 
    Hematite is the most widespread iron mineral. The most important ores are 
in low-to-medium-grade metamorphic rocks of sedimentary origin. Enormous beds 
occur in the Great Lakes region of the United States. Hematite also occurs in 
contact metamorphic and metasomatic deposits, often derived from the oxidation 
of magnetite and frequently associated with limestones. 
    Nearly every country in the world mines some hematite ore; the most 
important occurrences outside of the United States include India, Cuba, China, 
Chile, North Africa, and the Soviet Union. See IRON METALLURGY; REDBEDS. 
 �34s[Paul B. Moore]
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�6v�2v�32sHematologic disorders �21m �33s
    Those disorders marked by aberrations in structure or function of the
blood cells or the blood-clotting mechanism. Although many other diseases may
be reflected by the blood and its constituents, the abnormalities of red
cells, white cells, platelets, and clotting factors are considered to be
primary hematologic disorders.
    Red-cell abnormalities are principally represented by the anemias and
polycythemias. In a wide variety of conditions the many forms of white cell
present in the circulation, bone marrow, and lymphoid tissues of the body may
be altered in form or number. The suffixes ``-philia'' and ``-penia'' denote
increases and decreases, respectively, for the cells named. Leukopenia,
neutrophilia, eosinophilia, and pancytopenia are examples of the wide range of
possibilities. See ANEMIA; ERYTHROCYTOSIS; LEUKEMIA; LEUKOPENIA; POLYCYTHEMIA.
    The hemorrhagic disorders result from a large number of known and unknown
causes or contributing factors, often of a diverse nature. See HEMOPHILIA;
HEMORRHAGE.						[Cecil Hougie]

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�6v�2v�32sHematopoiesis   �21m �33s 
    The process by which the cellular elements of the blood are formed. 
    The formation of these cells is one of the most active and important 
processes in the body. Most of the circulating cells live only for a short 
time and must be replaced in order to maintain life. For instance, in the 
human adult a red blood cell has a life of 120 days; 250,000,000,000 new red 
cells have to be produced daily to replace those that are destroyed. 
    Blood cells originate in the reticuloendothelial tissue, which is a loose, 
fibrous, highly vascularized mesh of fibers, endothelial cells, and 
macrophages. Within the spaces of the tissue are found the precursor (blast) 
cells of the definitive adult types. For the sake of convenience, the 
reticuloendothelial tissue is divided into two general but imprecise types: 
lymphoid and myeloid tissue. 
    Lymphoid tissue is primarily localized in the lymph nodes of the lymphatic 
system and is also in the spleen, thymus, and bone marrow. Several classes of 
white cells are produced, including the lymphocytes, macrophages, and 
monocytes. Myeloid tissue is normally limited in humans to the red bone marrow 
of the ribs, sternum, vertebrae, and proximal ends of the long bones of the 
body. It is concerned with the production of the erythrocytes 
(erythropoiesis), granular leukocytes, and megakaryocytes. Fragments of 
megakaryocytes form the blood platelets (thrombocytes). See BLOOD. 
 �34s[Fred Wilt]
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�6v�2v�32sHemicellulose   �21m �33s 
    A term designating plant cell components which are made soluble by dilute 
alkali or which go into solution quite readily in hot dilute mineral acids 
with the formation of simple sugars. Hemicelluloses constitute about 
one-fourth of perennial plants and about one-third of annual plants. The term 
is usually applied to those polysaccharides in the cell wall of land plants 
which are extractable by dilute alkaline solutions. The term has also been 
used to include all the polysaccharide components of the cell wall other than 
cellulose. See CELL WALLS (PLANT); CELLULOSE. 
    Hemicelluloses extracted from different plant sources are rarely 
identical. In fact, many different hemicelluloses usually occur intermixed 
with each molecular type representing different degrees of polymerization. 
Because of this heterogeneity, few hemicelluloses have been isolated in a 
homogeneous state. Therefore, relatively little is known of the structure of 
these compounds that compose almost one-third of the carbohydrates in woody 
tissue.  
    D-Xylose is the dominant building unit of the hemicelluloses of most woods 
and annual plants. D-Mannose is also very abundant in hemicelluloses; the 
mannose content of softwoods is usually higher than that of hardwood. Often it 
occurs as a polymer, mannan, or in combination with D-glucose or Dgalactose as 
a glucomannan, galactomannan, or galactoglucomannan. See MANNANS. 
    Hemicelluloses are important to the paper industry. In chemical wood 
pulps, hemicellulose is needed for satisfactory pulp quality. Its presence 
aids the swelling of the pulp, the bonding of the fibers, the bursting 
strength, tensile strength, tear resistance, folding endurance, opacity, and 
specific surface of the pulp sheet. 
    Hemicelluloses also serve as nutrients for yeasts, and they can be used 
for raw material in the production of furfural and ethyl alcohol. 
 �34s[Roy L. Whistler]
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�6v�2v�32sHemichordata   �21m �33s 
    A group of animals thought by some authorities to be a phylum of 
deuterostomes and by others to be a subphylum of the Chordata. At present, 
this group is considered to include the Enteropneusta, Pterobranchia, and 
Graptolithina. All are marine species found mostly in shallow waters, 
distributed from the Arctic to the tropics. These animals are superficially 
dissimilar and are believed to be derived from ancestors which had a stomocord 
(a short extension of the gut forward into the proboscis), exemplified in 
living forms by the acorn worm. The closest affinities of this group are 
probably with the Tunicata (Urochordata). See CHORDATA; ENTEROPNEUSTA; 
GRAPTOLITHINA; PTEROBRANCHIA. 
 �34s[Theodore H. Bullock]
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�6v�2v�32sHemicidaroida   �21m �33s 
    An order of Echinacea with one very large tubercle on each interambulacral 
plate. In this respect they resemble cidarids, though the aboral tubercles may 
be reduced. There are two families: Orthopsidae, of the Jurassic and 
Cretaceous, and Hemicidaridae, of the Jurassic and Cretaceous. See 
DIADEMATOIDA; ECHINACEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sHemidiscosa   �21m �33s 
    An order of the subclass Amphidiscophora in the class Hexactinellida. 
These sponges are distinguished from the order Amphidiscosa in that the 
birotulates are hemidiscs with asymmetrical ends. See AMPHIDISCOPHORA; 
AMPHIDISCOSA.  
 �34s[Willard D. Hartman]
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�6v�2v�32sHemimorphite   �21m �33s 
    A mineral sorosilicate having the composition Zn4Si2O7(OH)2.H2O; an ore of 
zinc. Crystals are usually colorless and the aggregates white, but in some 
cases there are faint shades of green, yellow, and blue. The mineral has a 
vitreous luster, a hardness of 4J to 5 on Mohs scale, and a specific gravity 
of 3.45. 
    Hemimorphite has a wide distribution and has been mined in Belgium, 
Germany, Romania, England, Algeria, and Mexico. In the United States it is 
found at Sterling Hill, New Jersey: Friedensville, Pennsylvania; and Elkhorn 
Mountains, Montana.  See  SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sHemiptera   �21m �33s 
    An order of the class Insecta sometimes referred to as the Heteroptera. 
Both these names refer to the forewings, which are differentiated into a 
thickened basal area and a membranous apical region. These are the true bugs. 
Included in Hemiptera are such common insects as the bedbugs, stink bugs, 
plant bugs, lace bugs, and backswimmers. 
    The true bugs number about 25,000 species. They are known from all 
continents except Antarctica and occur on most islands. Hemiptera range in 
size from small aquatic and ground-inhabiting forms approximately 0.04-0.08 
in. (1 - 2 mm) in length to the giant water bugs that are 4 in. (100 mm) or 
more.  
    In habits, the true bugs range from strictly phytophagous types attached 
to a single host plant to general predators on other insects and even to 
specialized ectoparasites of bats. Many species are of economic importance as 
plant pests or vectors of disease. They occur in vegetation, on the ground, in 
and on the water, and in the nests of termites. Most water bugs depend on 
surface air held to the body by air spaces and hairs on the abdomen. As oxygen 
is depleted in the air bubble, it is replaced from the surrounding medium by 
diffusion.  
    Most Hemiptera are bisexual and oviparous, but parthenogenesis is known. 
Mating usually takes place on vegetation or on the ground, the pairing being 
end-to-end in stink bugs, squash bugs, chinch bugs, and similar species, and 
with the male above the female in most others. 
  
    Morphology.  In Hemiptera the mouthparts are elongate and slender, forming 
a sucking mechanism. The beak arises from the anterior part of the head and 
the head is commonly directed forward or downward. 
    Hemiptera are further characterized (see illustration) by antennae, 
usually of four or five segments, a pair of compound eyes, and often two 
ocelli. The thorax consists of a prominent pronotum, a triangular mesothoracic 
scutellum, and a broad metathorax which is partly fused with the first 
abdominal segment. The mesothoracic wings, or hemelytra, overlap at their 
membranous apices when at rest. The hindwings are hitched to the forewings in 
flight by grooves and pegs. Wings are sometimes reduced to short pads and may 
be lacking in certain groups or even in members of a single species. The front 
legs are frequently enlarged and sometimes chelate in predacious forms, and 
the middle and hindlegs are adapted for swimming in some groups. The abdomen 
is of 10 - 11 segments, but only seven are commonly seen. 
  
  
    Subdivisions.  Families of the Hemiptera and their distinguishing 
characteristics are listed in the table under the respective subdivisions. 
  
    Hydrocorisae. This subdivision contains nine families of water bugs with 
concealed antennae and without a bulbus ejaculatorius in the male. Many 
species are predaceous. 
    Corixoidea is a superfamily which contains the single family Corixidae, or 
water boatmen. Corixidae lack ocelli and have a unique type of mouthpart. 
Corixids swim with the dorsal side uppermost, using the oarlike middle and 
hind legs. Eggs are stalked and there are five nymphal instars. Adults 
sometimes fly to lights in great numbers. 
    In the superfamily Nepoidea, the Nepidae, or water scorpions, have a long 
breathing tube at the tip of the abdomen, through which they obtain air 
directly from the surface. The beak is short and stout to suck the juices of 
other insects on which they prey. Belostomatidae, or electric-light bugs, have 
short, straplike respiratory appendages at the tip of the abdomen. Giant water 
bugs of the genus Lethocerus are pests in fish ponds where they attack fry. 
They can inflict a painful bite when handled carelessly. 
    The superfamily Notonectoidea includes the Notonectidae or backswimmers: 
they swim ventral side uppermost, the hindlegs serving as oars. Breathing is 
facilitated by an air bubble, obtained by touching the tip of the abdomen to 
the surface. 
    The minute bugs of the families Pleidae and Helotrephidae (superfamily 
Pleoidea) are suboval, with legs not fitted for rowing. 
    The creeping water bugs (superfamily Naucoroidea) are sometimes separated 
into the Naucoridae and Aphelocheiridae. They are suboval in body form. 
Respiration is either by an air bubble or by means of a plastron; in the Old 
World genus Aphelocheirus, the plastron consists of an ultramicroscopic hair 
pile which acts as a physical gill; the bug is thus able to remain submerged 
permanently.  
    In the superfamilies Gelastocoroidea and Ochteroidea, the Gelastocoridae, 
or toad bugs, and the Ochteridae (or Pelogonidae) are shore-line or mud 
inhabitants. Both have ocelli. The former are cryptically colored, resembling 
the sand or mud background. Ochterids are black with a silky sheen. 
  
    Amphibicorisae. This subdivision contains surface water bugs with antennae 
exposed and without a bulbus ejactulatorius in the male. Only the single, 
diverse superfamily Gerroidea has been proposed for the surface water bugs. 
All have conspicuous antennae, and the body is clothed with hydrofuge hairs. 
All gerroidea are predacious. 
    Gerridae are the large water striders with long middle and hind legs and a 
median scent gland opening on the metasternum. The claws are inserted before 
the tips of the tarsi. The marine forms are always wingless but all others are 
polymorphic, with fully winged forms occurring together with short-winged or 
apterous types. 
    Veliidae are small water striders which have shorter legs and a 
longitudinal groove between the eyes. The claws are preapical. Like the 
Gerridae, these are pond inhabitants, stream-riffle bugs, and marine types, 
but the last are found only near shore in tropical reefs. 
    Hydrometridae are long, slender marsh treaders in which the head is longer 
than the thorax. The claws are apical. 
    Mesoveliidae and Hebridae are two small families which differ from others 
in having the well-developed ocelli and the single dorsal abdominal scent 
gland openings of the nymphs. 
  
    Geocorisae. This subdivision contains the land bugs with conspicuous 
antennae and an ejaculatory bulb in the male reproductive system. This 
subdivision can be divided into seven groups, each of which may be equivalent 
in rank to the hydrocorisae and Amphibicorisae. Superfamilies that do not fit 
the above groupings are the Reduvioidea, the Saldoidea, the Aradoidea, and the 
Enicocephaloidea and Dipsocoroidea, of isolated position in the system. 
    The superfamily Saldoidea comprise the shore bugs which have three pairs 
of trichobothria on the vertex and a single nymphal scent gland opening. 
Dipsocoroidea is a group of minute ground inhabitants, of which the 
Schizopteridae live in leaf mold and the Dipsocoridae are predators on small 
insects under bark or in rotten wood or amid stones at the edge of streams. 
    Enicocephaloidea is a unique group in which the head is bilobed, the 
pronotum trilobed, and the wings completely membranous. They live under 
stones, beneath bark, and in leaf mold and are predators. Some species are 
wingless; others can cast off their wings. 
    Reduvioidea includes only the assassin bugs or conenose bugs of the family 
Reduviidae. Nearly all have a stridulatory furrow on the prosternum, which is 
scraped by the rostrum to produce a squeaking sound. Ocelli are generally 
present and the beak is three-segmented. 
    Among the Aradoidea are flat bugs of the family Aradidae and their 
specialized relatives, Termitaphididae. They lack ocelli and have 
four-segmented antennae. Most, if not all, are fungus feeders. The 
Termitaphididae have no wings. They are known from termite nests in the Old 
and New World tropics. Aradidae are nearly cosmopolitan and fully winged, 
brachypterous, stenopterous, or apterous. 
    Cimicimorpha is the largest group of the Geocorisae and is divided into 
three superfamilies, the Cimicoidea, Tingoidea, and Thaumastocoroidea. 
    1. Cimicoidea. Anthocoridae, or the flower bugs, are small predators on 
thrips, mites, and similar species in vegetation, in stored products, and in 
the nests of birds. The ocelli are distinct, and the front wings have a 
marginal fracture. Cimicidae contains the bedbugs, which lack ocelli, and have 
the short wings reduced to pads. The food consists of blood of birds and 
mammals. The Polyctenidae are bat ectoparasites which lack eyes and have 
ctenidia and strong claws. The Miridae family contains a majority of the 
species of Hemiptera. Included are plant bugs of both herbivorous and 
predacious type. Ocelli are lacking, the antennae are four-segmented, and 
there is only one dorsal abdominal scent gland opening in nymphs. Two small 
families, the Plokiophilidae and Microphysidae, are predacious. The 
Plokiophilids live in the webs of spiders and embiids. 
    Nabidae are the long ``damsel'' bugs which are slender predators on other 
insects. The ocelli are well developed, and the rostrum is four-segmented. The 
tropical Velocipedidae are related but differ in the broader form and darker 
coloration.  
    2. Tingoidea. This superfamily contains the lace bugs of the family 
Tingidae which have wings with many lacelike areolae. They lack ocelli, have 
four segments in the antennae and beak, and commonly have one or more bulbous 
or hoodlike elevations on the thorax. Pests are known on ornamental plants. 
    3. Thaumastocoroidea. The only family, the Thaumastocoridae, occurs in 
Australia and the New World tropics; it includes the royal palm bug of Florida. 
    Pentatomorpha includes the superfamilies Lygaeoidea, Pyrrhocoroidea, 
Coreoidea, and Pentatomoidea. 
    1. Lygaeoidea. This is the first of the superfamilies with ventral 
trichobothria (bristles) on the abdomen. The antennae are four-segmented. 
Ocelli are present. The Lygaeidae include the chinch bug, the false chinch 
bugs, and the milkweed bugs. Small families related to the lygaeids are the 
thread-legged Neididae (Berytidae), the small Piesmidae, and the tropical 
Colobathristidae. Some of the last have a unique method of stridulation with a 
filelike arch on the sides of the head. 
    2. Pyrrhocoroidea. This group includes the cotton stainers, which attack 
cotton bolls in the southwestern United States and over most of the tropics, 
and stout, dark bugs with a reddish border. 
    3. Coreoidea. The squash bugs and their relatives include the Coreidae, 
Rhopalidae, Alydidae, and Hyocephalidae. They have four-segmented antennae, a 
beak, distinct ocelli, and many veins in the membrane. 
    4. Pentatomoidea. This large group has marginal trichobothria. The 
antennae are usually five-segmented and the beak is four-segmented. 
Scutelleridae, or shield bugs, are not injurious in the United States. 
Cydnidae include the ground-burrowing bugs, which attack strawberries in sandy 
soil, and the negro bugs. Smaller families of little or no economic importance 
in the United States are the large tropical Tessaratomidae, the 
Acanthosomatidae, the Dinidoridae, the Aphylidae, the Urostylidae, the 
barklike Phloeidae, the shining, oval Plataspidae, and the Lestoniidae. The 
true stink bugs, Pentatomidae, include the black-and-red harlequin cabbage 
bug, and several green stink bugs on cotton and other crops.  See  INSECTA. 
 �34s[Pedro W. Wygodzinsky]
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�6v�2v�32sHemizonida   �21m �33s 
    A Paleozoic order of Asteroidea embracing six families. The ambulacral 
groove is well defined by adambulacral ossicles, but the marginal plates 
either are not developed or are restricted to the interbrachial angles. The 
madreporite is either on the upper or the lower surface.  See  ASTEROIDEA; 
ECHINODERMATA.  
 �34s[Howard B. Fell]
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�6v�2v�32sHemlock �21m �33s
    The genus Tsuga of the pine family, characterized by flattened needles
with two white lines beneath the needlelike leaves, which have distinct short
stalks. The cones are small and pendent.
    Eastern hemlock (T. canadensis) occurs in eastern Canada, the Great Lakes
states, and the Appalachians. Minutely toothed leaves are characteristic of
this species. The wood is hard and strong, and is used for construction,
boxes, crates, and paper pulp. The bark is one of the principal domestic
sources of tannin. The eastern hemlock is a common ornamental tree.
    Carolina hemlock (T. caroliniana), a species found in the southern
Appalachians, has entire needles and is sometimes grown as an ornamental. The
western hemlock (T. heterophylla) grows in the extreme Northwest and in
Alaska. Its needles resemble those of the eastern hemlock, but the white lines
beneath are not so distinct. It is an important lumber tree, with uses similar
to these of the eastern species.  See  PINALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sHemoglobin   �21m �33s 
    The oxygen-carrying molecule of the red blood cells of vertebrates. This 
protein represents more than 95% of the solid constituents of the red cell. It 
is responsible for the transport of oxygen from the lungs to the other tissues 
of the body and participates in the transport of carbon dioxide in the reverse 
direction.  
    Each molecule of hemoglobin comprises four smaller subunits, called 
polypeptide chains. These are the protein or globin parts of hemoglobin. A 
heme group, which is an iron-protoporphyrin complex, is associated with each 
polypeptide subunit and is responsible for the reversible binding of one 
molecule of oxygen. The polypeptide chains and the heme are synthesized and 
combine together in nucleated red cells of the bone marrow. As these cells 
mature, the nuclei fragment and the cells, now called reticulocytes, begin to 
circulate in the blood. After sufficient hemoglobin has been formed in the 
reticulocyte, all nuclear material disappears and the cell is then called an 
erythrocyte, or red blood cell. Each hemoglobin molecule lasts as long as the 
red cell, which has an average life of 120 days. See PORPHYRIN. 
    Normal adult males and females have about 16 and 14 g, respectively, of 
hemoglobin per 100 ml of blood; each red cell contains about 29 X 10 sup -12 
g of hemoglobin. Red cells normally comprise 40 - 45% of the volume of whole 
blood.  
    The reversible combination of hemoglobin and oxygen can be represented by 
the reaction shown below. The equilibrium constants for each step are not the
                               
* Hb + 4O sub 2 EQUILIBRIUM (REACTION CONDITIONS = K sub 1)
  HbO sub 2 + 3O sub 2 EQUILIBRIUM (REACTION CONDITIONS = K sub 2) 
  Hb(O sub 2) sub 2 + 2O sub 2 EQUILIBRIUM (REACTION CONDITIONS = K sub 3) 
  Hb(O sub 2) sub 3 + O sub 2 EQUILIBRIUM (REACTION CONDITIONS = K sub 4) 
  Hb(O sub 2) sub 4 *

same because an oxygen molecule on one heme group changes the affinity of the 
other hemes for additional oxygen molecules. This alteration in binding 
affinity during oxygenation is called heme-heme interaction and is due to 
small changes in the three-dimensional structure of the molecule. 
    Hemoglobin combines reversibly with carbon monoxide about 210 times more 
strongly than with oxygen. This strong affinity for carbon monoxide accounts 
for the poisoning effects of this gas. 
    Hemoglobin binds carbon dioxide by means of free amino groups of the 
protein but not by the heme group. The reversible combination with carbon 
dioxide provides part of the normal blood transport of this gas. Hemoglobin 
serves also as a buffer by reversible reactions with hydrogen ions. The acidic 
property of oxyhemoglobin is greater than deoxygenated hemoglobin. The extra 
binding of hydrogen ion by deoxyhemoglobin promotes the conversion of tissue 
carbon dioxide into bicarbonate ion and thus increases the amount of total 
carbon dioxide which can be transported by blood. See BLOOD; RESPIRATION. 
 �34s[Richard T. Jones]
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�6v�2v�32sHemophilia   �21m �33s 
    A rare, hereditary blood disorder marked by a tendency toward excessive 
bleeding. It is almost entirely restricted to males, and is transmitted as a 
sex-linked mendelian recessive trait passing from an affected male through an 
unaffected or very mildly affected daughter to appear again in a grandson. 
Queen Victoria was a carrier, and several of her male descendants were 
affected.  See  HUMAN GENETICS; SEX-LINKED INHERITANCE. 
    Classical hemophilia (hemophilia A) is due to a deficiency of the 
antihemophilic factor or factor VIII, a clotting factor which is normally 
present in the blood in trace amounts and is essential for normal fibrin 
formation. Hemophilia B is a similar sex-linked bleeding disorder affecting 
males but characterized by a deficiency of another blood clotting factor, 
factor IX. It can only be distinguished from hemophilia A, which it closely 
resembles, by laboratory tests. Hereditary deficiencies of other clotting 
factors may give rise to bleeding disorders similar to hemophilia, but they 
are inherited as autosomal dominant or recessive characteristics. 
    Treatment consists in the intravenous administration of potent 
concentrates of factor VIII or IX prepared from human plasma and they are very 
effective in the management of hemophilia A and B, respectively.  See  BLOOD; 
HEMORRHAGE.  
 �34s[Cecil Hougie]
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�6v�2v�32sHemophilic bacteria   �21m �33s 
    Members of the genus Haemophilus, which consists of gram-negative 
pleomorphic coccobacilli, are facultative anaerobes and almost invariably 
ferment carbohydrates and reduce nitrates. Most significantly, they require 
for propagation one or both of two growth factors: X (an iron prophyrin), and 
V (nicotinamide - adenine dinucleotide [NAD] or one of its nucleoside 
precursors). The presence of both factors in blood accounts for the name 
Haemophilus (``blood-loving''); they are widely distributed in many plant, 
animal, and microbial cells.  See  BACILLACEAE. 
    All known members of the genus Haemophilus are obligate parasites of 
animals. Most of them are harmless commensal organisms living on the mucous 
membranes of the mouth, pharynx, and upper respiratory and lower genital 
tracts of humans and animals. The type species, H. influenzae, causes 
suppurative meningitis in infants, conjunctivitis (mainly in children), 
obstructive inflammation of the pharynx and larynx, inflammation of the middle 
ear, and various secondary infections, especially of the respiratory tract. 
Haemophilus ducreyi is the agent of a rather uncommon venereal disease, soft 
chancre.  
 �34s[Ernst L. Biberstein]
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�6v�2v�32sHemorrhage   �21m �33s 
    The escape of blood from within the vascular system. Hemorrhage may result 
from either trauma or disease of the vessel wall. In some cases there is no 
apparent break in the vessel wall, and the blood cells escape by a process 
called diapedesis. 
    The escape of blood following rupture of a vessel wall as a result of 
trauma is obvious and needs no further explanation. The causes other than 
trauma can be divided into three main groups. 
    The first group consists of those conditions in which there is a chronic 
disease process affecting the vessel wall, such as arteriosclerosis or 
aneurysm formation. Either of these conditions, in association with an 
elevated blood pressure, can result in a break in the wall and subsequent 
hemorrhage. An infarct, or tissue death from any cause, may also result in 
hemorrhage.  
    The second group consists of those causes in which there is an acute 
process affecting the vessel wall, such as septicemia, bacterial toxins or 
focal colonies of bacteria in the vessel wall, poisoning by metals such as 
mercury or arsenic, or anoxia. 
    The third group consists of those hemorrhagic conditions which result from 
some defect in the blood itself. Under this heading are included leukemia, 
pernicious anemia, thrombocytopenia, and the clotting disorders.  See  ANEMIA; 
APOPLEXY; CIRCULATION DISORDERS; HEMATOLOGIC DISORDERS. 
 �34s[Romeo A. Vidone]
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�6v�2v�32sHemorrhoids �21m �33s
    Soft, irregular venous abnormalities occurring in and above the anus.
External hemorrhoids lie outside the anal ring and are commonly seen in two
forms. The first is the acutely thrombosed hemorrhoid which gives great pain
or tenderness. The second form occurs as part of a more extensive network of
varicosities about the anal region. Symptoms vary greatly; often there is
merely a slight bleeding tendency while at stool.
    Internal hemorrhoids may be simple, hard-to-detect varicosities lying well
within the anal canal, or they may be huge, protruding, blood-filled vessels
that give intense pain. Severe hemorrhage may occur, as well as the passage of
lesser amounts of bright red blood.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sHemp   �21m �33s 
    The fiber and the plant Cannabis sativa. It should not be confused with 
Manila hemp, which is not related to true hemp. Hemp contains the drug 
marijuana.  See  ABACA; MARIJUANA. 
    Hemp fiber, which for many years was the major raw material used in the 
manufacture of rope, now is used mostly in the production of small twines, 
linenlike fabrics and canvases, and, to some extent, in making special types 
of paper.  See  CANVAS; NATURAL FIBER. 
    Hemp is an annual crop, most of which is produced in Eastern Europe and 
mainland China, with some production in South Korea, Turkey, and Italy. 
 �34s[Elton G. Nelson]
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�6v�2v�32sHenequen   �21m �33s 
    A fiber obtained from the leaves of Agave fourcroydes. It is produced only 
in Mexico, Cuba and El Salvador. Henequen is sometimes incorrectly called 
sisal, which is a closely related plant grown in Brazil and Africa.  See  
SISAL.  
    The greatest quantity of henequen fiber goes into farm twine, followed by 
industrial tying twine and then light-duty rope. Padding for innerspring 
mattresses is made from the lowest grades of fiber and from flume tow, the 
short, tangled fiber that can be recovered from the cleaning operation. 
Henequen is exported as manufactured twine, rope, or padding, not as raw 
fiber.  See  NATURAL FIBER. 
 �34s[Elton G. Nelson]
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�6v�2v�32sHeparin   �21m �33s 
    A highly sulfated mucopolysaccharide with blood anticoagulant activity, 
isolated from mammalian (chiefly beef) tissues. Heparin was first found in 
abundance in the liver, hence the name, but it is present in substantial 
amounts in the spleen, muscle, and lung as well. In the blood of most mammals, 
heparin is an antagonist to thrombin, prothrombin, and thromboplastin. It 
lessens the tendency of platelets to agglutinate. It is used in the treatment 
of venous thrombosis, embolism, myocardial infarction, and certain types of 
cerebral thrombosis.  See  POLYSACCHARIDE. 
 �34s[Frank Wagner]
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�6v�2v�32sHepatitis   �21m �33s 
    An inflammation of the liver caused by a number of etiologic agents, 
including viruses, bacteria, fungi, parasites, drugs, and chemicals. All types 
of hepatitis are characterized by distortion of the normal hepatic lobular 
architecture due to varying degrees of necrosis of liver cells, inflammation, 
and Kupffer cell enlargement and proliferation. There is usually some degree 
of disruption of normal bile flow, which causes jaundice. The severity of the 
disease is highly variable and often unpredictable.  See  LIVER. 
    The most common infectious hepatitis is of a viral etiology. The two 
well-recognized forms are infectious hepatitis (hepatitis A) and serum 
hepatitis (hepatitis B). Hepatitis A is orally acquired, has an incubation 
period of 15 to 45 days, occurs primarily in children and young adults, is 
usually a mild, often unrecognized, disease which as a duration of 5 to 8 
weeks, usually leaves no residual effects, and has a very low mortality rate. 
Hepatitis B is parenterally transmitted, has an incubation period of 50 to 160 
days, occurs at any age and is common in the elderly, is extremely common in 
hard-drug addicts, has an insidious onset and often produces a protracted 
disease, and has a mortality rate of up to 15%. It has been proven that many 
of these clinical concepts are inconstant. 
    Another frequently occurring form of hepatitis is caused by excessive 
ethyl alcohol intake and is referred to as alcoholic hepatitis. It usually 
occurs in chronic alcoholics and is characterized by fever, leukocytosis, and 
jaundice.  See  ALCOHOLISM. 
    Hepatitis can be caused by numerous commonly used drugs and can be 
pathogenetically related to a hypersensitivity allergic phenomenon or a 
dose-dependent phenomenon. 
    Chronic hepatitis is clinically defined in part as evidence of active 
liver disease for at least 3 consecutive months. The causes are multiple. It 
follows acute type B viral hepatitis in approximately 10% of the cases and is 
seen in association with the prolonged intake of certain drugs. Chronic 
hepatitis may also be found in young females, in whom it is possibly an 
autoallergic disease. 
    Clinical features of hepatitis include malaise, fever, jaundice, and serum 
chemical tests revealing evidence of abnormal liver function. In most mild 
cases of hepatitis, treatment consists of bedrest and analgesic drugs. In 
those individuals who develop a great deal of liver cell necrosis and 
subsequently progress into a condition known as hepatic encephalopathy, 
exchange blood transfusions are often used. This is done with the hope of 
removing or diluting the toxic chemicals thought to be the cause of this 
condition.  
 �34s[Samuel P. Hammar]
�33s 
  
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�6v�2v�32sHerbarium   �21m �33s 
    A collection of plant specimens, pressed and mounted on paper or placed in 
liquid preservatives, and systematically arranged for subsequent consultation. 
Most of the great botanical gardens have such collections.  See  BOTANICAL 
GARDENS.  
    The chief value of a herbarium lies in the bringing together of large 
numbers of specimens of plants for comparative studies. It is important for 
botanists to have a knowledge of the appearance of living plants in their 
customary habitats, but without collections of preserved plants, such works as 
monographic treatments of various groups or manuals of the flora of various 
regions could not be prepared. 
 �34s[Earl L. Core]
�33s 
  
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�6v�2v�32sHerbicide   �21m �33s 
    Any chemical used to destroy or inhibit plant growth, especially of weeds 
or other undesirable vegetation. There are well over a hundred chemicals in 
common usage as herbicides. Many of these are available in several 
formulations or under several trade names. The variety of materials are 
conveniently classified according to the properties of the active ingredient 
as either selective or nonselective. Selective herbicides are those that kill 
some members of a plant population with little or no injury to others. 
Nonselective herbicides are those that kill all vegetation to which they are 
applied. Further subclassification is by method of application, such as 
preemergence (soil-applied before plant emergence) or postemergence (applied 
to plant foliage). Additional terminology sometimes applied to describe the 
mobility of post-emergence herbicides in the treated plant is contact 
(nonmobile) or translocated (mobile - that is, killing plants by systemic 
action).  
    A rapidly expanding use for nonselective herbicides is the destruction of 
vegetation before seeding in the practice of reduced tillage or no tillage. 
Some are also used to kill annual grasses in preparation for seeding perennial 
grasses in pastures. Additional uses are in fire prevention, elimination of 
highway hazards, destruction of plants that are hosts for insects and plant 
diseases, and killing of poisonous or allergen-bearing plants. 
    Preemergence or postemergence application methods derive naturally from 
the properties of the herbicidal chemical. The distinction between pre- and 
postemergence is not always clear-cut. For example, atrazine can exert its 
herbicidal action either following root absorption from a preemergence 
application or after leaf absorption from a postemergence treatment. 
 �34s[Rodney O. Radke]
�33s 
  
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�6v�2v�32sHermaphroditism   �21m �33s 
    A condition in which components of both testes and ovaries are present in 
the same individual. Although true hermaphroditism is common among lower forms 
of animals such as annelids and mollusks, it is rare in humans. A more common 
condition in humans is pseudohermaphroditism, which simulates hermaphroditism. 
In female pseudohermaphroditism, or gynandry, the external sexual 
characteristics are in part or wholly of the male aspect, but internal female 
genitalia are present. In male pseudohermaphroditism, or androgyny, the 
individual has external sexual characteristics of female aspect, but has 
testes (usually undescended).  See  OVARY; REPRODUCTIVE SYSTEM DISORDERS; 
TESTIS.  
 �34s[Sybil P. Parker]
�33s 
  
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�6v�2v�32sHernia   �21m �33s 
    An abnormal protrusion of a portion of the body through its containing 
wall. It is sufficiently common to have acquired a popular name, rupture. 
    Most hernias occur in areas of innate weakness in the body wall. Although 
such a weakness, and even an area of faulty closure of the wall, may have been 
present from birth, there is often no obvious bulge until late in life. After 
a major physical effort or a series of minor strains, various organs may 
suddenly protrude through the weakened area as a noticeable and often painful 
lump. Hernias in other instances occur through areas weakened by injury. Most 
frequently this is surgical incision, but violent stress or infection may also 
be responsible. 
    The most common type is the inguinal hernia. The weakness lies in the 
inguinal canal through which the vas deferens passes from the abdominal cavity 
to the scrotum. Umbilical hernia, through the defect left by the passage of 
umbilical cord structures during fetal life, is particularly common in 
infancy, and often closes off spontaneously. Femoral hernia, originating in 
the weakness produced by passage of the major arteries and veins from the 
abdomen into the legs, presents below the inguinal ligament and is common in 
women. In diaphragmatic hernia, where a large defect exists in the muscular 
diaphragm separating chest from abdomen, the abdominal organs rise into the 
chest cavity and may seriously impair breathing. Esophageal hiatus hernia is a 
small bulging of part of the stomach through the diaphragm around the 
esophagus.  
 �34s[Roy N. Barnett]
�33s 
  
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�6v�2v�32sHerpes   �21m �33s 
    A mild vesicular eruption of the skin or mucous membranes caused by a 
virus. The most familiar form is seen as cold sores or fever blisters. 
    Primary infections are chiefly inapparent; the commonest clinical form of 
the first attack is acute herpetic gingivostomatitis, a severe febrile disease 
with mouth eruptions. Recurrent attacks in persons with antibodies produce 
local lesions, often at mucocutaneous junctions. In some instances herpes 
simplex virus may infect the eye, producing acute keratitis which may 
permanently affect the cornea. 
    There are two types of herpes simplex virus, differing both antigenically 
and biologically: an oral type (type 1) and a genital type (type 2). Herpes 
simplex virus type 1 is probably more constantly present in humans than is any 
other virus. Primary infection occurs early; 70 - 90% of adults have 
antibodies. However, the virus may be carried throughout life. Transmission is 
by contamination from saliva or stools. Evidence indicates that type 2 is 
venereally transmitted. There is a high degree of association between prior 
infection with type 2 herpesvirus and the development of cancer of the uterine 
cervix. It is not yet known whether the relationship with cervical cancer is a 
causal one.  See  ANIMAL VIRUS; CYTOMEGALOVIRUS INFECTION. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sHerrings   �21m �33s 
    The common name for a family (Clupeidae) of about 70 genera of fishes in 
the order Clupeiformes. They are used extensively as food all over the world, 
and occur in all seas except the Arctic and Antarctic. 
    These fishes are the most primitive of the higher bony fishes. The fins 
have no supporting spines and are soft-rayed. There are usually four gill 
clefts, with the pectoral fins behind the gill openings. Scales are present on 
the body but absent on the head, and the swim bladder and lateral line may be 
missing.  See  ANCHOVIES; SARDINE; SHAD. 
    The herring Clupea harengus has a circumpolar distribution. About eight 
other species of this genus are recognized, including the sprat or brisling 
(C. sprattus), which occurs in the Mediterranean and seas of western Europe, 
and the gizzard shad (Dorosoma cepedianum), which is a common species in the 
Potomac River. In Europe the herring is either salted, pickled, or smoked and 
cured as kippers. In Canada and the United States young herring are canned as 
``sardines.''  
    The sardine, Sardina pilchardus, is a herring known commonly as the 
pilchard and is found along the European coasts in the Atlantic. The entire 
fish may be processed and preserved in oil, since the bones are soft and all 
parts are edible. Shads, herrings of the genus Alosa, occur in northern waters 
on both sides of the Atlantic. Anchovies comprise a family of herringlike 
fish, the Engraudidae, which together with the Clupeidae belong to the 
suborder Clupoidea. These fishes are found in the Mediterranean and range 
along the European coast as far north as Norway.  See  CLUPEIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sHertzsprung-Russell diagram �21m �33s
    A graphical representation of the absolute magnitude and the surface
temperature of stars. The ordinate is commonly the absolute visual magnitude.
In the case of a cluster of stars, all of which may be assumed to be at the
same distance, the apparent magnitude may be used in place of the absolute
magnitude. For comparison with theory, the logarithm of the luminosity, in
units of the Sun's luminosity, is used. The abscissa is the spectral type or
the color index, both of which are related to the star's surface temperature.
For comparison with theory, the logarithm of the effective temperature is
used, the effective temperature being closely related to the surface
temperature. When the absolute or apparent magnitude is plotted against a
color index, the H - R diagram is also called a color-magnitude diagram.  See
MAGNITUDE (ASTRONOMY).
    An H - R diagram of all stars with known absolute magnitudes and surface
temperatures is shown in the illustration. By far the majority of the stars
fall within a band running from the lower right to the upper left. These stars
are called main-sequence stars. The region between the main sequence and the
giants and supergiants, where few stars are found, is called the Hertzsprung
gap.
    The H - R diagram is useful for exhibiting the effects of stellar
evolution. As a result of its evolution, a star's position in the H - R
diagram changes with time. The time scales for noticeable changes in the
position of a star in the H - R diagram generally are on the order of millions
to billions of years. Because of stellar evolution, the H - R diagram is
dynamic, rather than static, with stars moving into and out of each region.
Those regions in which there are few stars, such as the Hertzsprung gap, are
regions in which the stars are unstable and pass through relatively rapidly.
Those regions where there are many stars, such as the main sequence, are those
in which the stars stay for relatively long periods of time.  See  STAR;
STELLAR EVOLUTION.
 �34s[Donald S. Hayes]
�33s

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�6v�2v�32sHesperornithiformes   �21m �33s 
    An order of ancient extinct birds found from the Upper Cretaceous and 
frequently segregated in their own superorder, Odontognathae, on the basis of 
their possession of teeth. The hesperornithiforms were large, flightless, 
aquatic diving birds with the shoulder girdle and wings much reduced and the 
legs specialized for strong swimming. All aspects of the skeleton were typical 
of modern birds with the possible exception of teeth. The Hesperornithiformes 
almost certainly represent an evolutionary offshoot, not ancestral to any 
living birds; resemblances to loons and other modern diving birds are due to 
convergence.  See  AVES; DIVING BIRDS; ODONTOGNATHAE. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sHeterobasidiomycetidae   �21m �33s 
    A subclass of basidiomycetous fungi (Basidiomycetes). They are also known 
as Hemibasidiomycetes or Phragmobasidiomycetes. In the subclass, the basidium 
is either branched or is divided into cells by cross walls. In most 
Heterobasidiomycetidae the basidium consists of a first-formed portion, the 
hypobasidium, and a later-formed epibasidium which intervenes between the 
hypobasidium and the sterigmata (stalks) upon which basidiospores are borne.  
See  BASIDIOMYCOTINA; RUST (MICROBIOLOGY); SMUT (MICROBIOLOGY). 
 �34s[Robert M. Page]
�33s 
  
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�6v�2v�32sHeterocorallia   �21m �33s 
    A small, extinct, late Paleozoic order of fossil corals, known from 
Europe, North Africa, Asia, North America, and Australia, but limited to the 
Late Mississippian. They are found in calcareous shales and in limestones. 
Their very elongate skeletons, and the hooks on the outer septal edges of 
some, suggest a pseudoplanktonic existence attached to seaweeds. There are but 
rare indications of branching.  See  COELENTERATA. 
 �34s[Dorothy Hill]
�33s 
  
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�6v�2v�32sHeterocyclic compounds   �21m �33s 
    Cyclic compounds in which the rings include at least one atom of an 
element different from the rest. Most types of heterocyclic compounds studied 
to date are organic compounds. An example of an organic heterocyclic compound 
is oxazoline (I); an example of an inorganic heterocyclic compound is the 
phosphonitrilic chloride (II). The smallest possible ring is three-membered, 
for example, ethylene oxide (III), but very large rings are possible, as in 
the crown ethers, for example, 18-crown-6 (IV). The cycle may contain only 
single bonds and is thus saturated; it may include one or more double bonds; 
or it may possess aromatic unsaturation characteristics of benzene, that is, 
it is heteroaromatic. Heterocyclic compounds can contain more than one ring, 
either heterocyclic or homocyclic. 
    Naturally occurring heterocyclic compounds are extremely common as, for 
example, most alkaloids, sugars, vitamins, DNA and RNA, enzymic cofactors, 
plant pigments, many of the components of coal tar, many natural pigments 
(such as indigo, chlorophyll, hemoglobin, and the anthocyanins), antibiotics 
(such as penicillin and streptomycin), and some of the essential amino acids 
(for example, tryptophan), and many of the peptides (such as oxytocin). Some 
of the most important naturally occurring high polymers are heterocyclic, 
including starch and cellulose. The major groups of natural products that are 
not mainly heterocyclic are the fats and most of the terpenes, steroids, and 
essential ALPHA-amino acids, though exceptions do exist. 
    Heterocyclic compounds may be named systematically. Many heterocycles, 
however, have nonsystematic names that are usually preferred by practicing 
chemists over the systematic ones. In the systematic approach to nomenclature 
the ring size is denoted by the appropriate stem. For example, three-membered 
saturated rings without nitrogen would have a name ending in -irane. The 
nature of the heteroatom is denoted by such prefixes as oxa-, thia-, or aza-, 
for oxygen, sulfur, or nitrogen, respectively. Thus, ethylene oxide (III) 
becomes oxirane. A five-membered unsaturated ring would have a name ending in 
-ole. A six-membered unsaturated ring containing nitrogen would have a name 
ending in -ine according to this scheme. Actually, the trivial names for many 
systems are commonly accepted, and the systematic names are not often used. 
    For details about specific heterocyclic systems  see  ACRIDINE; AZOLE; 
CARBAZOLE; FURAN; HETEROCYCLIC POLYMER; IMIDAZOLE; INDOLE; ISOQUINOLINE; 
OXAZOLE; PYRAN; PYRAZOLE; PYRIDINE; PYRIMIDINE; PYRROLE; QUINOLINE; THIAZOLE; 
THIOPHENE.  
 �34s[Rudolph A. Abramovitch]
�33s 
  
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�6v�2v�32sHeterocyclic polymer   �21m �33s 
    Essentially, linear high polymers comprising heterocyclic rings, or groups 
of rings, linked together by one or more covalent bonds. As the search has 
continued for polymeric materials having useful properties at high 
temperatures (500DGC or 930DGF, or higher), much attention has been given to 
heterocyclic polymers. As a group such polymers are often both mechanically 
rigid and inherently resistant to thermal degradation. See POLYMER. 
    Some of these polymers form molecules in which the rings are fused 
together, as shown symbolically in the illustration (ladder polymers), and 
some form molecules in which fused rings are joined by single bonds 
(stepladder polymers). Similar considerations hold for simple aromatic 
systems, for example, linear polymers of benzene, but the heterocyclic systems 
have, in general, been more useful in application. 
    Three heterocyclic polymers have been developed to the point of commercial 
availability; polyimides, polybenzimidazoles, and polybenzothiazoles. At least 
the first two appear to have established specialized markets. 
    Major applications for these rather expensive polymers are as 
metal-to-metal adhesives and as laminating resins for fibrous composites for 
structural applications in the aerospace industry. Other applications 
requiring both strength and resistance to oxidation at elevated temperatures 
have developed, including valve seats, bearings, and turbine blades.  See  
POLYMERIC COMPOSITE. 
 �34s[John A. Manson]
�33s 
  
  
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�6v�2v�32sHeterodyne principle   �21m �33s 
    The basic principle underlying the operation of a superheterodyne radio, 
television, or other receiver, wherein two alternating currents that differ in 
frequency are mixed in a nonlinear device to produce two new frequencies, 
corresponding to the sum and the difference of the input frequencies. Only the 
difference frequency is commonly used in a superheterodyne receiver, where it 
serves as the input to the intermediate-frequency amplifier.  See RADIO 
RECEIVER; SUPERHETERODYNE RECEIVER. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sHeterogeneous catalysis �21m �33s
    A chemical process in which the catalyst is present in a separate phase.
In the usual case, the catalyst is a solid and the reactants and products are
in gaseous or liquid phases. See CATALYSIS.
    Heterogeneous catalysis proceeds by the formation and subsequent reaction
of chemisorbed complexes which can be considered to be surface chemical
compounds. A very simple case, where A -> B is slow in the absence of
catalyst, is shown in the following reaction.

      Chain propagating steps
	 * + A -> *A
	 A* -> B*
	 B* -> + *
	 A -> B

Reaction A -> B is fast if the three preceding steps are fast. Here, *
represents a catalytic site on the surface of the catalyst, A* -> B* is called
a surface reaction, * + A -> *A represents the chemisorption of A, and B* -> B
+ * represents desorption of B. See ADSORPTION.
    With most sets of reactants, more than one reaction will be
thermodynamically possible. The degree to which a given catalyst favors one
reaction compared with other possible reactions is called the selectivity of
the catalyst for the above reaction. Two aspects of a catalyst are of
particular importance: its selectivity, and its activity, which can be taken
as the rate of conversion of reactants by a given amount of catalyst under
specified conditions. Ideally, the rate will be proportional to the amount of
catalyst.
    The first important heterogeneous catalytic process to be used in the
chemical industry was the manufacture of sulfuric acid from sulfur trioxide by
the contact process in 1875. By the 1950s, heterogeneous catalytic processes
had come to dominate the petroleum, petrochemical, and chemical industries.
Today, about 70% of the crude oil refined in the United States is exposed to
at least one heterogeneous catalytic process. Heterogeneous catalysis is a
critical feature in energy conservation and interconversion, and is a key
feature in the production of synthetic fuels from coal and oil shale.  See
FISCHER-TROPSCH PROCESS; COAL GASIFICATION.
    Since catalytic activity will ordinarily be proportional to surface area,
most catalysts are used in forms with large specific areas. Higher-area metal
powders are often used for liquid-phase reactions in batch reactors. For
example, finely divided nickel is used for the hydrogenation of unsaturated
glycerides in the manufacture of margarine from vegetable oils. Supported
catalysts are widely used. In these, the catalytic ingredient is dispersed in
the internal porosity of such supports as silica gel, GAMMA-alumina, and
charcoals. These supports have large areas in the internal porosity, and their
average pore diameters are 2 - 20 nm. Supported catalysts have the advantage
that the area of the catalytic ingredient can be very large.
    One important type of catalyst exposes strongly acidic sites in its
internal porosity. Such catalysts are used to crack larger molecules of
hydrocarbon into smaller ones in petroleum refining. Other catalysts, called
dual-functional catalysts, have a hydrogenating catalytic ingredient on an
acidic support. These are also of major importance in processing petroleum.
See CATALYTIC REFORMING; CRACKING; HYDROCRACKING.
 �34s[Robert L. Burwell]
�33s


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�6v�2v�32sHeteronemertini   �21m �33s 
    An order of the class Anopla in the phylum Rhynchocoela, with an unarmed 
proboscis, a thick partly fibrous dermis, and a three-layered body musculature 
composed of outer longitudinal, median circular, and inner longitudinal 
strata. Cerebral organs, cephalic grooves and slits, and eyes are generally 
present. The alimentary system consists of a mouth, foregut, intestine with 
regular lateral diverticula but no cecum, and anus. Heteronemertini are mainly 
marine littoral in habit.  See  ANOPLA; RHYNCHOCOELA. 
 �34s[J. B. Jennings]
�33s 
  
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�6v�2v�32sHeterophile antigen   �21m �33s 
    The serologic reactions of the tissue and blood-cell antigens of most 
animals are normally characteristic of the species. Significant serologic 
cross reactions usually occur only with antisera to the corresponding antigens 
of closely related species. The numerous groups of heterophile antigens - of 
which the Forssman antigens are the best studied - constitute significant 
exceptions. Heterophile antigens link the species hog-ox-human (blood group 
A), cat-horse, and dog-hog-cat-human, while several heterophile groups link 
otherwise diverse microorganisms. Similarities between antigens in mammalian 
hearts and the cell walls of the group A hemolytic streptococcus are also 
known. The cross reactions between the Proteus bacillus and the Rickettsiae 
are important in the diagnosis of typhus fever.  See  ANTIBODY; ANTIGEN; 
SEROLOGY.  
 �34s[Margaret J. Polley]
�33s 
  
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�6v�2v�32sHeterophyiasis   �21m �33s 
    Presence, in the human small intestine, of the minute intestinal fluke 
Heterophyes heterophyes. The Nile Delta and Orient are endemic foci. Many 
reservoir hosts exist. Eggs in feces are ingested by snails, and cercariae 
finally emerge from these to encyst in the flesh of certain fish. Ingestion of 
the raw fish flesh by humans completes the cycle. 
    Large numbers of worms in humans damage the intestinal mucosa and cause 
diarrhea, colicky pains, and abdominal distress. Eggs may enter the lymph or 
blood to settle in the heart or brain, with serious complications.  See  
MEDICAL PARASITOLOGY. 
 �34s[Jose F. Maldonado]
�33s 
  
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�6v�2v�32sHeterosis   �21m �33s 
    Hybrid vigor or increase in size, yield, and performance found in hybrids, 
especially if the parents have previously been inbred. The application of 
heterosis has been one of the most important contributions of genetics to 
scientific agriculture in providing hybrid corn, and vigorous, high-yielding 
hybrids in other plants and in livestock.  See  BREEDING (ANIMAL); BREEDING 
(PLANT); GENETICS; MENDELISM. 
    There are two principal hypotheses to account for the association of size 
and vigor with heterozygosity, dominance and overdominance. The dominance 
hypothesis notes that any noninbred population carries a number of recessive 
genes that are harmful to a greater or lesser extent, but which are rendered 
ineffective by their dominant alleles. As they become homozygous through 
inbreeding, they exert their harmful effect. With hybridization, some of the 
detrimental recessives contributed to the hybrid by one parent are masked by 
dominant alleles from the other, and an increase in vigor is the result. The 
alternative hypothesis is that there are loci at which the heterozygote is 
superior in vigor to either homozygote. This, the overdominance hypothesis, 
also has the consequence that vigor is proportional to heterozygosity. The 
dominance hypothesis has been more widely accepted, but the two are very 
difficult to distinguish experimentally, and it is likely that overdominant 
loci are playing an appreciable role in heterosis, particularly in determining 
why one hybrid is better than another.  See  DOMINANCE. 
 �34s[James F. Crow]
�33s 
  
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�6v�2v�32sHeterostraci   �21m �33s 
    An extinct group of ostracoderms or armored, jawless vertebrates 
(Agnatha). The armor has a distinctive microscopic structure, consisting of 
bone lacking any cavities for bone cells, surmounted by tubercles or ridges of 
dentine. Fragments of such armor from the Middle Ordovician of Australia and 
North America are the earliest remains definitely attributable to vertebrates. 
Heterostraci became more common toward the end of the Silurian and persisted 
through the Devonian.  See  AGNATHA. 
    The anterior part of the body was covered with plates. The posterior part 
of the body and the tail were covered with thick scales. There were no jaws, 
but the mouth was bounded behind by a number of small plates that may have 
been used for nibbling.  See  OSTRACODERM. 
 �34s[Robert H. Denison]
�33s 
  
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�6v�2v�32sHeterotardigrada   �21m �33s 
    An order of the tardigrades; the majority of genera have widely varied 
structure. Cephalic appendages having a sensorial function are present, as 
well as cirrus lateralis and clava. This order of tardigrades is divided into 
two suborders, Arthrotardigrada and Echiniscoidea. 
    Members of the suborder Arthrotardigrada have toelike terminations of the 
legs. The tubular middle part of the leg telescopes into the broad proximal 
part. These animals are marine organisms found in sand or on algae. 
    In the suborder Echiniscoidea the legs terminate with claws. The middle 
part of the leg is partially retractable into the proximal part. At least the 
fourth pair of legs has a distinct fold. Frequently these animals are red 
because of the presence of carotenoid pigments.  See  CAROTENOID; TARDIGRADA. 
 �34s[Eveline Marcus]
�33s 
  
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�6v�2v�32sHeterotrichida   �21m �33s 
    A large order of the Spirotrichia. The buccal ciliature is well developed, 
although in a number of families the somatic, or body, ciliature is really 
holotrichous in nature. Heterotrichs have become adapted to all sorts of 
habitats, including the digestive tracts of a variety of invertebrate and a 
few vertebrate hosts. Because of their size and amazing regenerative powers, a 
number of heterotrichs have been widely used in experimental research. Common 
genera include Stentor, Blepharisma, Folliculina, Condylostoma, Climacostomum 
(see illustration), and Spirostomum.  See  SPIROTRICHIA. 
 �34s[John O. Corliss]
�33s 
  
  
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�6v�2v�32sHeulandite �21m �33s
    A mineral belonging to the zeolite family of silicates. It usually occurs
in crystals with prominent side pinacoid, often having a diamond shape. There
is perfect side pinacoid cleavage on which the luster is pearly; elsewhere the
luster is vitreous. The hardness is 3J to 4 Mohs scale; specific gravity is
2.18 - 2.20. The mineral is usually white or colorless but may be yellow or
red.  See  ZEOLITE.
    Heulandite is essentially a hydrous calcium aluminum silicate,
Ca(Al2Si7O18).6H2O. Heulandite is a secondary mineral found in cavities in
basalts associated with other zeolites and calcite. Notable localities are in
the Faeroe Islands, India, Nova Scotia, and West Paterson, New Jersey.
 �34s[Clifford Frondel ;Cornelius S. Hurlbut Jr.]
�33s

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�6v�2v�32sHexactinellida   �21m �33s 
    A class of the phylum Porifera which includes sponges with a skeleton made 
up basically of hexactinal siliceous spicules. Spongin is absent. 
Hexactinellida usually are radially symmetrical in form and have cylindrical, 
vase-, funnel-, or trumpet-shaped bodies. Some species are fan-shaped and 
others exhibit a dendritic or labyrinthine mode of branching. Some species 
attach to a hard substratum directly by a basal attachment region, but many 
have basal tufts or mats of spicules which serve to anchor them in bottom 
sediments. The Hexactinellida are exclusively marine and occur in the deeper 
waters of all seas. Their bathymetric range extends from depths of about 80 ft 
(25 m) down to 28,000 ft (8500 m), but very few descend into the hadal zone, 
the faunal zone of the deep trenches. 
    The body wall has a cavernous structure penetrated by intercommunicating 
cavities across which stretch the soft parts in the form of filmlike 
trabeculae. Thimble-shaped flagellated chambers, arranged side by side in a 
single layer, are supported in the midst of the trabeculae. Five regions may 
be recognized in the body wall. The outer dermal membrane is often filmlike 
and is perforated by more or less round pores which serve for the entrance of 
water into the intertrabecular lacunae. The subdermal trabecular network, 
which is continuous with the dermal membrane, is dense peripherally, but the 
lacunae become more spacious toward the interior and form subdermal cavities. 
These are in contact with the outermost flagellated chambers. Fairly distinct 
inhalant canals may lead to deeper-lying flagellated chambers in species with 
thick walls and a folded chamber layer. The flagellated chambers are composed 
of choanocytes whose bases are continuous and form a reticular membrane 
perforated with holes which allow passage of water from the subdermal 
trabecular lacunae into the flagellated chambers. Each chamber opens widely 
through an apopyle into the subspongocoelic trabecular system, which in turn 
opens into the central spongocoel by way of a perforated membrane. Fairly 
distinct exhalant channels are often present in the trabecular system, 
spreading between the chamber layer and the spongocoel. The spongocoel opens 
through a large terminal osculum, covered over in some genera by a sieve 
plate, the meshwork of which is reinforced by spicules. 
    The skeleton is made up of megascleres and microscleres. Although many 
forms of spicules occur, most of them can apparently be derived from the 
triaxon or hexactin, in which three axes meet at right angles. The megascleres 
are also classified, according to their position in the body of the sponge, as 
dermal, parenchymal, and gastral. 
    Other animals often live in association with Hexactinellida. Sea anemones 
and hydroids live on some sponges of this class, and isopods and brittle stars 
are known to inhabit the cavernous interior regions. 
    Hexactinellida are well known as fossils because their rigid skeletons, 
often composed of a lattice of fused spicules, increase the chances of their 
being preserved intact. Sponges of this class are reported from the Cambrian 
Period upward. There is a notable peak of abundance of fossil remains in the 
Cretaceous Period.  See  PORIFERA. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sHexactinosa   �21m �33s 
    An order of the subclass Hexasterophora in the class Hexactinellida. The 
parenchymal megascleres in this order of sponges are united to form a rigid 
framework and consist wholly of simple hexactins which are arranged in 
parallel linear series. The members of each series are united one to another 
by a secondary envelope of silica.  See  HEXACTINELLIDA; HEXASTEROPHORA. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sHexagon   �21m �33s 
    A closed figure formed by the six line segments, or sides, that join in 
order six ordered points, or vertices, of a plane. In elementary geometry it 
is usually assumed that the sides of a hexagon do not cross and even that the 
figure bounds a convex region of the plane. The ancient Greek geometers 
investigated regular hexagons, that is, hexagons with all sides equal and with 
all angles included by adjacent sides equal to 120DG. They found that a 
regular hexagon is obtained by applying the radius of a circle six times as a 
chord to the circumference, and that three regular hexagons completely fill up 
the plane about a point.  See  POLYGON; REGULAR POLYGON. 
 �34s[Leonard M. Blumenthal]
�33s 
  
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�6v�2v�32sHexasterophora   �21m �33s 
    A subclass of sponges of the class Hexactinellida, in which the 
parenchymal microscleres are typically hexasters (small, six-rayed spicules, 
often with branched ends). This is a diverse assemblage of sponges commonly 
firmly fixed to the substratum by the base, less commonly anchored by means of 
basal spicule tufts or mats. The spicules of the body are sometimes free and 
unconnected, but the parenchymal megascleres are often fused to form a rigidly 
connected skeleton. The following orders are recognized: Hexactinosa, 
Lychniscosa, Lyssacinosa, and Reticulosa.  See  HEXACTINELLIDA; HEXACTINOSA; 
LYCHNISCOSA; LYSSACINOSA; PORIFERA; RETICULOSA. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sHibernation   �21m �33s 
    A general term applied to a condition of dormancy and torpor found in 
cold-blooded (poikilotherm) vertebrates and invertebrates. This rather 
universal phenomenon can be readily seen when body temperatures of 
poikilotherm animals drop in a parallel relation to ambient environmental 
temperatures. In the strict sense of the word, hibernation is a term which 
physiologists apply to relatively few species of mammals and birds. 
  
    Cold-blooded animals.  Hibernation occurs with exposure to low 
temperatures and, under normal conditions, occurs principally during winter 
seasons when there are lengthy periods of low environmental temperatures. A 
related form of dormancy is estivation. Many animals estivate when exposed to 
prolonged periods of drought or during hot, dry summers. For all practical 
purposes, hibernation and estivation in animals are indistinguishable, except 
for the nature of the stimulus, which is either a cold or an arid environment. 
    There is no complete list of animals that hibernate; however, many 
examples can be found among the poikilotherms, both vertebrate and 
invertebrate. The poikilotherms are sometimes referred to as ectothermic, 
because their body temperatures are not internally regulated but follow the 
rise and fall of environmental temperatures. During hibernation and winter 
torpor, body temperatures reflect the environmental temperature, often to 
within a fraction of a degree. Among the classic examples of hibernators or 
estivators are reptiles, amphibians, and fishes among the vertebrates, and 
insects, mollusks, and many other invertebrates. 
 �34s[X. J. Musacchia]
�33s 
  
    Warm-blooded vertebrates.  Many mammals and some birds spend at least part 
of the winter in hiding, but remain no more drowsy than in normal sleep. On 
the other hand, some mammals undergo a profound decrease in metabolic rate and 
physiological function during the winter, with a body temperature near 0DGC. 
This condition, sometimes known as deep hibernation, is the only state in 
which the warm-blooded vertebrate, with its complex mechanisms for temperature 
control, abandons its warm-blooded state and chills to the temperature of the 
environment. Between the drowsy condition and deep hibernation are gradations 
about which little is known. The bear, skunk, raccoon, and badger are animals 
which become drowsy in winter. The deep hibernators are confined to five 
orders of mammals: the marsupials, the Chiroptera or bats, the insectivores, 
the rodents, and, probably, the primates. 
    During a preparation period for hibernation, the animals either become 
fat, like the woodchuck, or store food in their winter quarters, like the 
chipmunk and hamster. Prior to hibernation, there is a general involution of 
the endocrine glands, but at least part of this occurs soon after the breeding 
season and is not directly concerned with hibernation. Animals such as ground 
squirrels become more torpid during the fall, even when kept in a warm 
environment, indicating a profound metabolic change which may be controlled by 
the endocrine glands. 
    The hibernator is capable of waking at any time, using self-generated 
heat, and this characteristic clearly separates the hibernating state from any 
condition of induced hypothermia. During the total period of hibernation, the 
hibernator spontaneously wakes from time to time, usually at least once a 
week. In the period of wakefulness the stored food is evidently eaten, but 
animals which do not store food rely on their fat for the extra energy during 
the whole winter.  See  HYPOTHERMIA. 
 �34s[Charles P. Lyman]
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�6v�2v�32sHickory �21m �33s
    Any species of the genus Carya, formerly known botanically as Hicoria.
Hickories are mostly tall forest trees characterized by strong, terminal,
scaly winter buds, pinnately compound leaves (see illustration), solid pith
(not chambered), and fruit with an outer husk or exocarp which splits more or
less readily into four parts, revealing a nut with a hard shell or endocarp.
    The shagbark hickory (C. ovata) is found in the eastern half of the United
States and adjacent Canada. It is the most important species because of the
commercial value of its nuts, the hickory nuts of commerce, and of its wood.
The pecan (C. illinoensis) is also a valuable species because of its
commercially popular, thin-shelled, sweet nuts. Other species are the
mockernut, shellbark, and pignut hickories. The remarkably tough and strong
wood of all species makes it the world's best wood for tool handles. It is
also used for parts of furniture, flooring, boxes, and crates, and for smoking
meats.	See  FAGALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sHigh fidelity   �21m �33s 
    A term (often shortened to hi-fi) applied to sound reproduction, used to 
designate a state of performance in which realism is achieved. To achieve 
realism in sound reproduction, four fundamental conditions must be satisfied: 
(1) The frequency range must include without frequency discrimination all the 
audible components of the various sounds to be reproduced; (2) the volume 
range must permit noiseless and distortionless reproduction of the entire 
range of intensity associated with the sounds; (3) the reverberation 
characteristics of the original sound must be approximated in the reproduced 
sound; and (4) the spatial sound pattern of the original sound must be 
preserved in the reproduced sound.  See  DISK RECORDING; MAGNETIC RECORDING; 
QUADRAPHONIC SOUND SYSTEM; REVERBERATION; SOUND-REPRODUCING SYSTEMS; 
STEREOPHONIC SOUND. 
 �34s[Harry F. Olson]
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�6v�2v�32sHigh-frequency impedance measurements   �21m �33s 
    The electrical measurement of the complex ratio of voltage to current in a 
given circuit at frequencies from several hundred kilohertz to 100,000 
megahertz. This frequency range includes medium- and high-frequency bands.  
See  ELECTRICAL MEASUREMENTS. 
    At lower frequencies impedances may be accurately measured by standard 
techniques for measuring resistance, capacitance, and inductance. The most 
precise measurements are those in which the unknown impedance is compared with 
a resistance, capacitance, or inductance standard of nearly equal value; at 
lower frequencies such standards can be very accurate.  See  CAPACITANCE 
MEASUREMENT; INDUCTANCE MEASUREMENT; RESISTANCE MEASUREMENT. 
    At higher frequencies, details of measurement-standard shape, terminal 
geometry, and component-interconnection wiring provide series inductance and 
shunt capacitance as well as stray couplings that may dominate the situation 
unless great care is used. These undesired parameters cause particular 
difficulty if they have different values each time a connection is made. 
Highly repeatable connections may be made at high frequencies by the use of 
precision connectors. These connectors are coaxial to remove external effects 
and make a butt joint to ensure repeatable inductance and capacitance. Some 
resistance, inductance, and capacitance standards using modified low-frequency 
construction are usable up into vhf range. Distributed-parameter standards 
consisting of coaxial transmission lines are useful to over 10 gigahertz. At 
the highest frequencies, where mechanical dimensions become comparable with 
the wavelength, resonant cavities and waveguides are used because they 
incorporate simple boundary conditions for field calculations. 
    For measurements over the whole high-frequency range from below 1 MHz to 
over 10 GHz, the measurement methods used can be classified as 
voltmeter-ammeter methods, resonance methods, null methods, standing-wave 
methods, and reflection methods. 
 �34s[Mark G. Foster]
�33s 
  
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�6v�2v�32sHigh-power, short-pulse laser �21m �33s
    A pulsed laser whose output power has been greatly increased, with
correspondingly shorter pulse durations, by the Q-switch technique. In this
method, the optical path between one mirror and the amplification medium is
blocked by a shutter. The medium is then excited beyond the degree ordinarily
needed, but the shutter prevents laser action. At this time the shutter is
abruptly opened and the stored energy is released in a giant pulse (1 - 100 MW
peak power, lasting 1 - 30 nanoseconds for optically pumped solid-state
lasers). Still higher peak powers can be obtained by passing this output
through a traveling-wave laser amplifier (without mirrors). Peak powers in
excess of 100 MW have been obtained in this way.
    Still shorter, and higher-power, pulses can be generated by mode-locking
techniques. A typical laser without mode locking usually oscillates
simultaneously and independently at several closely spaced wavelengths. These
modes of oscillation can be synchronized so that the peaks of their waves
occur simultaneously at some instant. The result is a very short, intense
pulse which quickly ends as the waves of different frequency get out of step.
Mode-locked lasers have generated pulses shorter than 1 picosecond.  See
OPTICAL PULSES.
    For the highest peak power, the output may be further intensified by
additional stages of laser amplification. The beam diameter is increased by
some optical arrangement, such as a telescope, so as to expand (dilute) the
beam and thereby prevent damage to the laser material and optics.
    Development of very large multistage lasers has been undertaken for
research on thermonuclear fusion.  See	LASER; NUCLEAR FUSION; PULSED GAS
LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
�33s

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�6v�2v�32sHigh-pressure chemistry �21m �33s
    Chemistry at very high pressures, arbitrarily chosen to be above 10,000
bars (1 gigapascal), and mainly concerned with solid and liquid states. At
25DGC (77DGF) and 10 kilobars (1GPa), nearly all ordinary gases are liquid or
solid, and only a few liquids are not frozen; thus most high-pressure
chemistry involves either higher temperatures, at which chemical reactions can
occur at appreciable rates, or studies of internal arrangements in solids.
    From 1 bar (100 kilopascals) to about 10 sup 5 bars (10 GPa), normal
low-pressure chemical behavior prevails, and only minor departures from the
usual valence and coordination rules are found. However, many interesting
changes in materials can be effected in this pressure range as atoms are
forced into new bonding arrangements. From 10 sup 5 to 10 sup 9 bars (10 to 10
sup 5 GPa), the energy added by compression becomes comparable with chemical
bond energies, so that outer-shell electronic orbits are distorted and atoms and
molecules change in character. A general tendency toward more metallic
behavior is observed as the electrons become less strongly fixed to particular
atoms, and chemical bonds may be broken. Upward of about 10 sup 9 bars (10 sup 5
GPa), the delocalization of electrons is extensive, and the material consists
of a mixture of ions and electrons, so that chemical bonds are of little
importance. The boundaries on these three pressure ranges are, of course, only
approximate, and show some variation according to the temperature and the
atoms involved.
    The simplest effect of high pressure is the closer compression of atoms.
The noble gases and alkali metals are quite compressible, whereas most oxides
and the stronger metals are considerably stiffer. However, at a pressure
exceeding about 100 kilobars (10 GPa), most of the easily compressed
electronic clouds are tightened up, and the compressibilities of most
substances approach each other.
    Substances which consist of large molecules are easily stiffened or frozen
by high pressures. The mobility of the molecules is sharply decreased by a
sort of interlocking and tangling effect; thus for the substance to be
sheared, chemical bonds must be broken, a process which requires considerable
energy. This stiffening phenomenon limits the study of most reactions of
organic molecules to low pressures because they are rather large and
``freeze'' easily, but yet are usually not stable enough to withstand the
temperatures necessary for liquefaction or intermolecular reactions.
 �34s[Robert H. Wentorf, Jr.]
�33s

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�6v�2v�32sHigh-pressure physics   �21m �33s 
    High-pressure physics is concerned with the effects of high pressure on 
the properties of matter. Since most properties of matter are modified by 
pressure, the field of high-pressure physics encompasses virtually all 
branches of physics. 
    The ``high'' of high-pressure physics connotes experimental difficulty. At 
liquid-helium temperatures, pressures of several hundred bars are considered 
high. In general, however, the high-pressure range may be arbitrarily regarded 
as extending from about 1 kbar upward to the present experimental limit. 
Prolonged static high pressures of as much as 500 kilobars (50 gigapascals) at 
room temperature have been achieved in certain types of apparatus. Transient 
pressures as high as about 10,000,000 bars (1000 GPa) have been attained in 
shock waves produced by high explosives or by projectile impact. 
    The major effects of high pressure on matter include diminution of volume, 
phase transitions, changes in electrical, optical, magnetic, and chemical 
properties, increases in viscosity of liquids, and increases in the strength 
of most solids. In general solids are less compressible than liquids, and the 
compressibility of both solids and liquids decreases with increasing pressure. 
However, solids and liquids show wide individual variations in compressibility. 
    At high pressure many solids exhibit polymorphic phase changes, that is, a 
rearrangement of the atoms or molecules in the solid. There are no universally 
applicable rules governing the number of phase changes or the kind of phase 
change to be expected at high pressure, but there is a thermodynamic 
requirement that the phase that is stable at high pressure must have a smaller 
volume than the phase that is stable at low pressure.  See  THERMODYNAMIC 
PRINCIPLES.  
    Frequently, dramatic changes in physical properties result from phase 
changes. Ferromagnetic iron transforms to a paramagnetic form at pressures 
somewhat above 100 kilobars (10 GPa). In the same pressure range, the 
semiconducting element germanium transforms into a metallic phase that has an 
electrical conductivity greater than a million times that of the semiconductor. 
    Many phases that form at high pressure transform back to low-pressure 
phases as the pressure is released. However, some high-pressure phases may be 
retained in a metastable condition at low pressures, and some low-pressure 
phases can persist metastably at high pressure. Diamond, the high-pressure 
form of carbon, is thermodynamically unstable at room temperature and 
pressures below about 12 kilobars (1.2 GPa). Nonetheless diamond persists 
indefinitely as a metastable phase at low temperatures; it transforms to the 
stable form, graphite, only when heated to temperature in excess of 1000DGC 
(1800DGF) at low pressure. 
 �34s[Ronald K. Linde ;P. S. De Car]
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�6v�2v�32sHigh-temperature chemistry �21m �33s
    The study of chemical phenomena occurring above 500 K (227DGC or 440DGF).
High temperatures represent one of the important variables available to
scientists for increasing the variety of possible chemical reactions over that
expected for classical ground-state atoms and molecules. The relative
population of excited rotational, vibrational, and electronic states can be
enhanced by increasing the temperature and thus effectively create new species
and new mechanisms for reaction. The potentialities of this approach are well
illustrated by the three laws of high-temperature chemisty: (1) At high
temperatures everything reacts with everything. (2) The higher the
temperature, the faster the reaction. (3) The products may be anything.
    High temperatures also provide a common tie among the various options for
energy production, conversion, or storage. For maximum thermodynamic
efficiency, an energy production cycle should operate with a working fluid at
as high a temperature as possible, and exhaust the spent fluid at as low a
temperature as possible. Thus, in the combustion of coal to produce electric
power or in the combustion of gasoline or diesel fuel to propel a car or an
airplane, there is a need for materials of construction which allow operation
of such devices at higher temperatures.  See  HIGH-TEMPERATURE MATERIALS.
    It is convenient to discuss temperatures in terms of energy and to note
that 11,500 K (20,200DGF) corresponds to 1 electronvolt. In this sense, the
particles emitted by radioactive nuclei or accelerated in cyclotrons and
synchrotrons, which have energies in the keV, MeV, and BeV ranges, are
effectively at temperatures of SIMILAR 10 sup 7 K, SIMILAR 10 sup 1 sup 0 K, and
SIMILAR 10 sup 1 sup 3 K, respectively, and ``high-energy physics'' is
synonymous with ``ultrahigh temperature chemistry.''
    Traditional high-temperature chemistry of the past 25 years has been
mainly concerned with phenomena in the range of 500 - 3000 K, although exotic
flames can produce temperatures up to SIMILAR 6000 K, shock waves can generate
temperatures up to SIMILAR 25,000 K, electric arcs can be operated in
constricted modes to produce temperatures of SIMILAR 50,000 K, and nuclear
processes begin to occur at temperatures in the millions of degrees range.
Laser-excitation of selected energy states can produce species with effective
temperatures in the range of 10 sup 8 K.
 �34s[John L. Margrave]
�33s

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�6v�2v�32sHigh-temperature electronics   �21m �33s 
    The technology of electronic components capable of operating at high 
temperatures (above 572DGF or 300DGC). The need for electronic devices for 
geothermal well probes, planetary space probes, jet-engine controls, and 
nuclear power plant instruments is supplying the impetus for advancements in 
this area, since conventional silicon diodes, transistors, and integrated 
circuits will not function in this temperature range. The research has been 
carried out in three primary areas: new silicon devices, compound 
semiconductor devices, and integrated thermionic circuits. Some of these 
devices have the potential of extending the operational range of electronic 
circuits up to 1472DGF (800DGC). 
    Semiconductor devices can be divided into two categories: minority carrier 
devices, such as diodes and bipolar transistors, and majority carrier devices, 
such as the field-effect transistor (FET). Minority carrier devices made from 
silicon do not work well above 482DGF (250DGC). This is because the energy 
bandgap in silicon is only 1.1 electronvolt, and at temperatures above 250DGC 
a large number of minority carriers are generated by thermal excitation. 
    Majority carrier devices in silicon (such as FETs) do not depend on 
minority carriers for their operation and therefore can be made to operate at 
higher temperatures. Enhancement-mode metal-oxide semiconductor FET (MOSFET) 
devices have been made to operate at 662DGF (350DGC).  See  JUNCTION DIODE; 
TRANSISTOR.  
    The basic need for a simple high-temperature rectifier diode and moderate 
power devices with gain have led to high-temperature research on compound 
semiconductors. Many of the group III - V compound semiconductors have energy 
bandgaps larger than silicon. For example, the semiconductor bandgap in 
gallium phosphide (GaP) is 2.2 eV. The thermally generated minority carrier 
density in GaP at 752DGF (400DGC) is small enough that it is possible to build 
minority carrier devices at high temperatures by using this and other 
wide-bandgap semiconductor materials. 
    A dramatic departure from semiconductor technology is the integrated 
thermionic circuit (ITC). This technology combines photolithographically 
defined subminiature thin-film metal patterns with planar vacuum-tube 
technology. The result is a technology that allows fabrication of active 
circuits with a density approaching that of present room-temperature silicon 
integrated circuits. 
 �34s[Roger J. Chaffin]
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�6v�2v�32sHigh-temperature materials   �21m �33s 
    A metal or alloy which serves above about 1000DGF (540DGC). More 
specifically, the materials which operate at such temperatures consist 
principally of some stainless steels, superalloys, refractory metals, and 
certain ceramic materials. The giant class of alloys called steels usually see 
service below 1000DGF. The most demanding applications for high-temperature 
materials are found in aircraft jet engines, industrial gas turbines, and 
nuclear reactors. However, many furnaces, ductings, and electronic and 
lighting devices operate at such high temperatures. 
    In order to perform successfully and economically at high temperatures, a 
material must have at least two essential characteristics: it must be strong, 
since increasing temperature tends to reduce strength, and it must have 
resistance to its environment, since oxidation and corrosion attack also 
increase with temperature.  See  CORROSION; HIGH-TEMPERATURE CHEMISTRY. 
    High-temperature materials, always vital, have acquired an even greater 
importance because of developing crises in providing society with sufficient 
energy. The machinery which produces electricity or some other form of power 
from a heat source operates according to the basic Carnot cycle law, where the 
efficiency of the device depends on the difference between its highest 
operating temperature and its lowest temperature. Thus, the greater this 
difference, the more efficient is the device - a result giving great impetus 
to create materials that operate at very high temperatures.  See  CARNOT 
CYCLE; EFFICIENCY; METAL, MECHANICAL PROPERTIES OF. 
 �34s[Chester T. Sims]
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�6v�2v�32sHighway engineering   �21m �33s 
    Highway planning, location, design, and maintenance. Before the design and 
construction of a new highway or highway improvement can be undertaken, there 
must be general planning and consideration of financing. As part of general 
planning it is decided what the traffic needs of the area will be for a 
considerable period, generally 20 years, and what construction will meet those 
needs. To assess traffic needs, the highway engineer collects and analyzes 
information about the physical features of existing facilities, the volume, 
distribution, and character of present traffic, and the changes to be expected 
in these factors. The highway engineer must determine the most suitable 
location, layout, and capacity of the new routes and structures. The detailed 
design is normally begun only when the preferred location has been chosen. 
    In selecting the best route careful consideration is given to the traffic 
requirements, terrain to be traversed, value of land needed for the 
right-of-way, and estimated cost of construction for the various plans. The 
photogrammetric method, which makes use of aerial photographs, is used 
extensively to indicate the character of the terrain on large projects, where 
it is most economical. On small projects ground-mapping methods are preferred. 
  See  PHOTOGRAMMETRY; SURVEYING; TOPOGRAPHIC SURVEYING AND MAPPING. 
    Detailed design of a highway project includes preparation of drawings or 
blueprints to be used for construction. These plans show, for example, the 
location, the dimensions of such elements as roadway width, the final profile 
for the road, the location and type of drainage facilities, and the quantities 
of work involved, including earthwork and surfacing. Soil studies are made to 
analyze gradation and physical properties of the soil. Selection of the type 
and thickness of roadway surfacing depends upon the loads to be accommodated. 
Designs for drainage structures, small bridges, and sometimes tunnels must be 
included in the overall planning. Other work involves planning rest stops on 
major expressway routes and designing barriers to control and reduce noise 
along busy routes.  See  PAVEMENT; TUNNEL. 
    Highway maintenance and operation are important aspects of highway 
engineering. Involved are the repair and upkeep of surfacing and shoulders, 
bridges and drainage facilities, signs, traffic control devices, guard rails, 
traffic striping on the pavement, retaining walls, and side slopes. Additional 
operations include ice control and snow removal. 
 �34s[Archie N. Carter]
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�6v�2v�32sHill and mountain terrain   �21m �33s 
    Land surfaces characterized by roughness and strong relief. The 
distinction between hills and mountains is usually one of relative size or 
height, but the terms are loosely and inconsistently used. 
    Uplift of the Earth's crust is necessary to give mountain and hill lands 
their distinctive elevation and relief, but most of their characteristic 
features - peaks, ridges, valleys, and so on - have been carved out of the 
uplifted masses by streams and glaciers. Hill lands, with their lesser relief, 
indicate only lesser uplift, not a fundamentally different course of 
development. The features of hill and mountain lands are chiefly valleys and 
divides produced by sculpturing agents, especially running water and glacier 
ice. Local peculiarities in the form and pattern of these features reflect the 
arrangement and character of the rock materials within the upraised crustal 
mass that is being dissected. 
    Hill and mountain terrain occupies about 36% of the Earth's land area. The 
greater portion of that amount is concentrated in the great cordilleran belts 
that surround the Pacific Ocean, the Indian Ocean, and the Mediterranean Sea. 
Additional rough terrain, generally low mountains and hills, occurs outside 
the cordilleran systems in eastern North and South America, northwestern 
Europe, Africa, and western Australia. Eurasia is the roughest continent, more 
than half of its total area and most of its eastern portion being hilly or 
mountainous. Africa and Australia lack true cordilleran belts. The broad-scale 
pattern of crustal disturbance, and hence of rough lands, is now known to be 
related to the relative movements of a worldwide system of immense crustal 
plates.  See  MOUNTAIN; PLATE TECTONICS. 
 �34s[Edwin H. Hammond]
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�6v�2v�32sHip   �21m �33s 
    The region of the junction of the thigh and trunk. The bony framework 
consists of the large, flaring hipbone, or pelvis, which contains the hip 
socket, or acetabulum. This receives the head of the thighbone, or femur. 
Large muscle groups surround the joint and pass from trunk to thigh to impart 
motion to each. In addition, the region is rich in blood vessels, nerves, and 
lymphatics, which are important in the supply of the lower limb. See JOINT 
(ANATOMY).  
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sHippopotamus   �21m �33s 
    The name for two species of even-toed (artiodactylid) ungulates which form 
the family Hippopotamidae. Both species occur in Africa; the great African 
hippopotamus (Hippopotamus amphibius) inhabits the rivers of tropical Africa, 
and the pygmy hippopotamus (Choeropsis liberiensis) lives near the rivers of 
western Africa but is more terrestrial. 
    The great African hippopotamus is the largest living artiodactylid. The 
ears are small and flexible, and the nostrils and eyes protrude so that they 
are out of the water as the animal floats. The skin is almost devoid of hair. 
The feet end in four toes enclosed in round hoofs. The pygmy hippopotamus is 
about the size of a large pig and is a more solitary species and does not live 
in large herds, as does the common species. 
    These animals migrate regularly, following the river upstream during the 
rainy season and downstream during the dry season to new pastures. They come 
onto land, especially at night, to feed on vegetation. The males of the common 
species occupy and maintain territories within which are small herds of 
females and juveniles.  See  ARTIODACTYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sHirudinea   �21m �33s 
    A class of the annelid worms commonly known as leeches. These organisms 
are parasitic or predatory and have terminal suckers for attachment and 
locomotion. Most inhabit inland waters, but some are marine and a few live on 
land in damp places. The majority feed by sucking the blood of other animals, 
including humans. 
    Leeches differ from other annelids in having the number of segments in the 
body fixed at 34, chaetae or bristles lacking, and the coelomic space between 
the gut and the body wall filled with packing tissue (see illustration). In a 
typical leech the first six segments of the body are modified to form a head, 
bearing eyes, and a sucker, and the last seven segments are incorporated into 
a posterior sucker. 
    The mouth of a leech opens within the anterior sucker, and there are two 
main methods of piercing the skin of the host to obtain blood: an eversible 
proboscis or three jaws, each shaped like half a circular saw, placed just 
inside the mouth. The process of digestion is very slow, and a meal may last a 
leech for 9 months. The carnivorous forms have lost most or all of their gut 
diverticula and resemble earthworms in having a straight, tubular gut. Leeches 
are hermaphroditic, having a single pair of ovaries and several pairs of 
testes.  
    The importance of leeches as a means of making incisions for the letting 
of blood or the relief of inflammation is declining, and in developed 
countries the bloodsucking parasites of mammals are declining, because of lack 
of opportunity for contact with the hosts. In other countries they are still 
serious pests. See ARHYNCHOBDELLAE; RHYNCHOBDELLAE. 
 �34s[Kenneth H. Mann]
�33s 
  
  
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�6v�2v�32sHistamine   �21m �33s 
    A derivative of the amino acid histidine which is widely distributed in 
tissues. The effects produced by its release are manifold; almost every type 
of tissue or organ shows some response to it. 
    Histamine produces changes marked by loss of blood fluid and plasma 
protein from the vessels to adjacent tissue spaces. Histamine also causes 
either constriction or dilatation of the arteries and of portions of the 
circulatory system, depending on the animal species and other factors. This 
substance may also cause smooth muscle contraction, particularly of the 
uterus, the bronchioles, and the gallbladder. 
    Antihistamine compounds combat the general and specific effect of 
histamine release. These antihistamines are largely symptomatic in action and 
do not remove the inciting cause of the histamine release. See ANTIHISTAMINES. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sHistidine   �21m �33s 
    An amino acid widely distributed in proteins. Histidine is present in 
relatively large amounts in hemoglobin. Histidine derivatives include the 
dipeptides carnosine and anserine, which are abundant in muscle, and 
ergothioneine which is abundant in erythrocytes and seminal fluid; however, 
the function of these compounds is unknown. Decarboxylation of histidine 
results in histamine. See AMINO ACIDS; HISTAMINE. 
 �34s[David W. E. Smith]
�33s 
  
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�6v�2v�32sHistochemistry   �21m �33s 
    A science that provides information about the distribution and activities 
of chemical components in tissues. Its techniques are largely extensions of 
those used in histology but make direct use of the chemical properties of 
tissue constituents to localize them in relation to the structure of the 
tissue.  
    Most histochemical procedures involve microscopic examination of tissue 
sections or smears of cells that have been treated with reagents that convert 
specific cell constituents into visible derivatives or, in the case of 
enzymes, treatment with substrate solutions which yield a visible reaction 
product at their sites of activity. In this context visible means colored, and 
therefore seen by ordinary light microscopy, or colorless but detectable by 
electron, fluorescence, interference, phase, polarizing, ultraviolet, or x-ray 
microscopy.  
 �34s[Stanley J. Holt]
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�6v�2v�32sHistocompatibility   �21m �33s 
    The capacity to accept or reject a tissue graft. A set of genes, and the 
polymorphic cell surface antigens they control, provoke immunological 
rejection of grafts exchanged between genetically (and antigenically) 
dissimilar individuals. There are two categories of histocompatibility (H) 
genes (and antigens): major, which cause graft rejection in 2 - 3 weeks, and 
minor, where rejection usually takes more than a month. In mice the major 
histocompatibility gene complex (MHC) is designated the H-2 complex, and the 
antigens it controls, H-2 antigens. In humans the MHC is called the HLA 
complex.  
    Reactivity to foreign antigens is vested in a large population of 
lymphocytes, made up of many different clones, each with a different 
specificity of antigen-recognition capability. During the maturation of the 
immune system in fetal or neonatal animals, those clones with reactivity to 
self antigens are either deleted or suppressed, so that no destructive immune 
responses are permitted against self cells or tissues (self tolerance). See 
CELLULAR IMMUNOLOGY. 
    There are two broad classes of lymphocytes. T lymphocytes, which develop 
in the thymus, mediate graft rejection and ``cell-mediated immunity'' against 
viral and intracellular bacterial infections, but do not secrete free 
antibodies. B lymphocytes, which develop in bone marrow or the avian bursa of 
Fabricius, secrete free antibodies that neutralize bacterial toxins or viral 
infectivity, and promote phagocytosis and various other host defense 
reactions. The key point is that the need for self tolerance predicts that 
lymphocyte clones which recognize antigens very different from self antigens 
are likely to be more numerous than clones which recognize antigens closely 
related to self antigens. See TRANSPLANTATION BIOLOGY. 
 �34s[R. V. Blanden]
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�6v�2v�32sHistogenesis   �21m �33s 
    The developmental processes by which the definite cells and tissues which 
make up the body of an organism arise from embryonic cells. Among animals, the 
ectoderm, endoderm, and mesoderm, also known as the primary germ layers, 
provide the stem cells which gradually transform into distinctive kinds of 
cells and tissues. In the higher plants, meristematic cells, which occur 
wherever extensive growth takes place, provide the basis for tissue formation. 
See APICAL MERISTEM; EMBRYOLOGY; GERM LAYERS; HISTOLOGY; LATERAL MERISTEM. 
 �34s[Charles B. Curtin]
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�6v�2v�32sHistology   �21m �33s 
    The study of the structure and chemical composition of tissues of animals 
and plants as related to their function. The primary aim is to understand how 
tissues are organized at all structural levels, including the molecular and 
macromolecular, the entire cell and intercellular substances, and the tissues 
and organs. 
    The four tissues of the animal body include cells and intercellular 
substances. They are (1) epithelium, in which the cells are generally closely 
applied to each other and separated by very little intercellular substance; 
(2) connective tissue, in which the cells are usually separated by greater 
amounts of intercellular substance, which may indeed form the great bulk of 
the tissue; (3) muscular tissue, whose cells are primarily concerned with 
contractility; and (4) nervous tissue, whose components are concerned 
primarily with rapid conduction of impulses. See CONNECTIVE TISSUE; 
EPITHELIUM; MUSCULAR SYSTEM; NERVOUS SYSTEM. 
    The major fields of histological studies are: (1) morphological 
descriptions; (2) developmental studies; (3) histo- and cytophysiology; (4) 
histo- and cytochemistry; and (5) fine (or submicroscopic) structure. 
 �34s[Isidore Gersh]
�33s 
  
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�6v�2v�32sHistoplasmosis   �21m �33s 
    An infectious fungus disease primarily of humans, although it has been 
observed in animals. It is caused by a fungus, Histoplasma capsulatum, which 
has dimorphic properties. The infectious form is a mold whose spores are 
inhaled. As the organism invades tissues, the mold form is converted to a 
yeast form. This organism is endemic to the valleys of the Mississippi and 
Ohio rivers. See ASCOMYCOTINA; YEAST. 
    Histoplasmosis is primarily a disease of the lungs, usually in young 
children and in adults in their fifth or sixth decades. Approximately 50% of 
those individuals with the pulmonary form are asymptomatic. 
 �34s[Leanor D. Haley]
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�6v�2v�32sHistoradiography   �21m �33s 
    The technique for taking x-ray pictures of cells, tissues, or sometimes 
the whole animal or plant, if it is a small one. Soft x-rays, those with low 
penetrating power and relatively long wavelengths, are required for this type 
of picture. The best pictures are obtained when the tissues contain deposits 
of metallic elements which have a high absorption capacity for x-rays. See 
X-RAYS.  
    In applying the technique to tissues, a relatively thin section is placed 
against an x-ray film and irradiated with a beam of x-rays. When the film is 
developed, a picture of the object or section of tissue shows on the film. 
Another method attempts to focus the x-rays after they pass through the 
specimen. See X-RAY MICROSCOPE. 
 �34s[J. Herbert Taylor]
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�6v�2v�32sHodgkin's disease   �21m �33s 
    A malignant lymphoid neoplasm, usually arising in lymph nodes 
characterized by morphological heterogeneity and bizarre giant tumor cells 
referred to as Reed-Sternberg cells. The etiology of Hodgkin's disease is 
unknown, although current epidemiological data suggest an infectious (viral) 
etiology.  
    Persons with Hodgkin's disease usually seek medical advice because of 
enlarged painless lymph nodes. They may also have fever, weight loss, 
anorexia, pruritus, and anemia. The clinical extent of the disease is 
determined by a process of staging based on physical examination, various 
biopsies, and usually a laparotomy for examination of the spleen and liver. 
    Hodgkin's disease more commonly affects males than females, except for the 
nodular sclerosing variety, which occurs with equal frequency in both sexes. 
It generally is a disease of persons between the ages of 20 and 40, but it may 
affect the very young and the very old. Characteristically, persons with 
Hodgkin's disease exhibit a loss of cell-mediated immunity and become 
susceptible hosts for infection with a variety of microorganisms such as 
tubercle bacilli. 
    The treatment of Hodgkin's disease is dependent on its clinical stage and 
microscopic appearance. A combination of chemotherapy and radiation therapy is 
commonly used. See LYMPHATIC SYSTEM. 
 �34s[Samuel P. Hammar]
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�6v�2v�32sHoisting machines   �21m �33s 
    Mechanisms for raising and lowering material with intermittent motion 
while holding the material freely suspended. Hoisting machines are capable of 
picking up loads at one location and depositing them at another anywhere 
within a limited area. In contrast, elevating machines move their loads only 
in a fixed vertical path, and monorails operate on a fixed horizontal path 
rather than over a limited area. See ELEVATING MACHINES; MONORAIL. 
    The principal components of hoisting machines are: sheaves and pulleys, 
for the hoisting mechanisms; winches and hoists, for the power units; and 
derricks and cranes, for the structural elements. 
    Sheaves and pulleys or blocks are a means of applying power through a 
rope, wire, cable, or chain. Sheaves are wheels with a grooved periphery that 
change the direction or the point of application of a force transmitted by 
means of a rope or cable. Pulleys are made up of one or more sheaves mounted 
in a frame, usually with an attaching swivel hook, eye, or similar device at 
one or both ends. Pulley systems are a combination of blocks. See BLOCK AND 
TACKLE; PULLEY. 
    Normally, winches are designed for stationary service, while hoists are 
mounted so that they can be moved about, for example, on wheel trolleys in 
connection with overhead crane operations. A winch is basically a drum or 
cylinder around which cordage is coiled for hoisting or hauling. The drum may 
be operated either manually or by power, using a worm gear and worm wheel, or 
a spur gear arrangement. A ratchet and pawl prevent the load from slipping; 
large winches are equipped with brakes, usually of the external band type. 
    A derrick is distinguished by a mast in the form of a slanting boom 
pivoted at its lower end and carrying load-supporting tackle at its outer end. 
In contrast, jib cranes always have horizontal booms. Derricks are standard 
equipment on construction jobs; they are also used on freighters for loading 
and unloading cargo, and on barges for dredging operations. Hoisting machines 
with a bridgelike structure spanning the area over which they operate are 
overhead-traveling or gantry cranes. See BULK-HANDLING MACHINES; DERRICK. 
 �34s[Arthur M. Perrin]
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�6v�2v�32sHolasteroida   �21m �33s 
    An order of exocyclic Euechinoidea in which the apical system is elongated 
along the anteroposterior axis, the petals do not become sunken, and teeth 
occur only in the juvenile stages. There are seven families: Collyritidae, 
Disasteridae, Stenomasteridae, Holasteridae, Pourtalesiidae, Urechinidae, and 
Calymnidae.  See ECHINODERMATA; EUECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sHolectypoida   �21m �33s 
    An order of exocyclic Euechinoidea with keeled, flanged teeth, distinct 
genital plates, and with the ambulacra narrower than the interambulacra on the 
adoral side. Of the seven families, six are extinct: Holectypidae, Conulidae, 
Discoididae, Galeritidae, Conoclypidae, and Oligopygidae. The family 
Echinoneidae arose in the Jurassic, reached a worldwide climax in the 
Cretaceous, and survives today in two genera, Echinoneus and Micropetalon. See 
ECHINODERMATA; EUECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sHoles in solids   �21m �33s 
    Vacant electron energy states near the top of an energy band in a solid 
are called holes. A full band cannot carry electric current; a band nearly 
full with only a few unoccupied states near its maximum energy can carry 
current, but the current behaves as though the charge carriers are positively 
charged. See BAND THEORY OF SOLIDS. 
    The process of conduction in such a system may be visualized in the 
following way. An electron moves against an applied electric field by jumping 
into a vacant state. This transfers the position of the vacant state, or 
propagates the hole, in the direction of the field. 
    Hole conduction is important in many semiconductors, notably germanium and 
silicon. The occurrence of hole conduction in semiconductors can be favored by 
alloying with a material of lower valence than the ``host.'' Semiconductors in 
which the conduction is primarily due to holes are called p type. See 
SEMICONDUCTORS.  
 �34s[Joseph Callaway]
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�6v�2v�32sHolly �21m �33s
    The American species of holly (Ilex opaca) has evergreen leaves. It grows
naturally in the eastern and southeastern United States close to the Atlantic
and Gulf coasts, in the Mississippi Valley, and westward to Oklahoma and
Missouri. It is best known for its bright red berries. The heartwood takes a
high polish and is used for cabinet work and musical instruments; because it
resembles ivory, it is sometimes used for keys for pianos and organs.
    The English holly (I. aquifolium) is cultivated extensively in the extreme
northwestern United States, but is not hardy in the northeastern states. Its
spiny leaves are glossier than those of the American holly and have wavier
margins.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sHolmium �21m �33s
    A chemical element, Ho, atomic number 67, atomic weight 164.93, a metallic
element belonging to the rare-earth group. The stable isotope sup 1 sup 6 sup
5Ho makes up 100% of the naturally occurring element. The metal is paramagnetic,
but as the temperature is lowered, it changes to antiferromagnetic and then to
the ferromagnetic system. For properties of the metal see RARE-EARTH ELEMENTS.
See also ANTIFERROMAGNETISM.
 �34s[Frank H. Spedding]
�33s

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�6v�2v�32sHolocene   �21m �33s 
    An imprecise term defining geologic deposits and their times and processes 
of formation, postdating the Ice Age. Because the last deglaciation in middle 
and high latitudes was progressive, the boundary between the Holocene Epoch 
and the older Pleistocene Epoch has been set variously around a generally 
accepted value of 10,000 years. In its reference to this latest interval of 
geologic time, Holocene is essentially synonymous with Recent and Postglacial. 
See PLEISTOCENE. 
    The term is applied most commonly to sediments. Holocene strata represent 
virtually every environment of deposition, as they include all the sediments 
that are being deposited at present. Modern literature employs the term quite 
broadly as a time indicator for many other geological phenomena, such as 
uplift, ocean circulation, and volcanism. See QUATERNARY. 
 �34s[Roscoe G. Jackson, II]
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�6v�2v�32sHolocephali   �21m �33s 
    One of two Recent subclasses of the cartilaginous fishes, or 
Chondrichthyes. The Holocephali, or chimaeras, differ from the other subclass, 
the Elasmobranchii, in having only four pairs of gill arches and gills that 
open to the exterior from a single pair of apertures; in the erectile dorsal 
fin and spine (see illustration); in the naked skin in adults; and in the 
absence of a cloaca and of ribs. 
    Chimaeras date from the early Mesozoic. They are classified into a single 
order, the Chimaeriformes, 1 family, the Chimaeridae, 4 or 5 genera, and about 
24 species. All chimaeras are marine, most living in deep water. They are of 
little economic importance. See CHONDRICHTHYES. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sHolography   �21m �33s 
    A technique for recording, and later reconstructing, the amplitude and 
phase distributions of a coherent wave disturbance. Invented by Dennis Gabor 
in 1948, the process was originally envisioned as a possible method for 
improving the resolution of electron microscopes. While this original 
application has not proved feasible, the technique is widely used as a method 
of optical image formation, and in addition has been successfully used with 
acoustical and radio waves. See ACOUSTICAL HOLOGRAPHY. 
    The technique is accomplished by recording the pattern of interference 
between the unknown ``object'' wave of interest and a known ``reference'' wave 
(Fig. 1). In general, the object wave is generated by illuminating the 
(possibly three-dimensional) subject of concern with a highly coherent beam of 
light, such as supplied by a laser source. The waves reflected from the object 
strike a light-sensitive recording medium, such as photographic film or plate. 
Simultaneously a portion of the light is allowed to bypass the object, and is 
sent directly to the recording plane, typically by means of a mirror placed 
next to the object. Thus incident on the recording medium is the sum of the 
light from the object and a mutually coherent ``reference'' wave. See LASER. 
    The photographic recording obtained is known as a hologram (meaning a 
``total recording''); this record generally bears no resemblance to the 
original object, but rather is a collection of many fine fringes which appear 
in rather irregular patterns. Nonetheless, when this photographic transparency 
is illuminated by coherent light, one of the transmitted wave components is an 
exact duplication of the original object wave (Fig. 2). This wave component 
therefore appears to originate from the object (although the object has long 
since been removed) and accordingly generates a virtual  image of it, which 
appears to an observer to exist in three-dimensional space behind the 
transparency. The image is truly three-dimensional in the sense that the 
observer's eyes must refocus to examine foreground and background, and indeed 
can ``look behind'' objects in the foreground simply by moving his or her head 
laterally.  
    The holographic technique has a number of unique properties which make it 
of great value as a scientific tool. Although the field is young, and new 
applications are continually emerging, certain important areas can be 
identified.  
  
    Holography has been demonstrated to offer the capability of several unique 
kinds of interferometry. This capability is a consequence of the fact that 
holographic images are coherent; that is, they have well-defined amplitude and 
phase distributions. Any use of holography to achieve the superposition of two 
coherent images will result in a potential method of interferometry.  See  
INTERFEROMETRY.  
    Optical memories for storing large volumes of binary data in the form of 
holograms have been developed for commercial use. Such a memory consists of an 
array of small holograms, each capable of reconstructing a different ``page'' 
of binary data. When one of these holograms is illuminated by coherent light, 
it generates a real image consisting of an array of bright or dark spots, each 
spot representing a binary digit. See COMPUTER STORAGE TECHNOLOGY. 
    There has been interest in the use of holography for purposes of display 
of three-dimensional images. Applications have been found in the field of 
advertising, and there is increased use of holography as a medium for artistic 
expression.  
 �34s[Joseph W. Goodman]
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�6v�2v�32sHolostei   �21m �33s 
    One of three organizational levels (infraclasses) of the subclass 
Actinopterygii, or rayfin fishes. The holosteans are descended from the older 
Chondrostei and in turn are ancestral to the great mass of modern bony fishes, 
the Teleostei.  See  CHONDROSTEI; TELEOSTEI. 
    Holosteans made their first appearance in the Upper Permian as the order 
Semionotiformes; three additional orders arose in the Triassic Period, and the 
fifth and last order, the Aspidorhynchiformes, evolved in the Middle Jurassic. 
In the Jurassic and Lower Cretaceous, holosteans dominated actinopterygian 
fish life, but by the Late Cretaceous they had been largely replaced by 
teleosts.  
    Holosteans, although highly varied in body form, were structurally as well 
as temporally intermediate between chondrosteans, and teleosts, to which group 
they passed on substantial advances. In living holosteans the swim bladder is 
highly vascularized, and auxiliary aerial respiration is possible, a sometimes 
essential faculty in oxygen-poor waters of swamps.  See  ACTINOPTERYGII; 
AMIIFORMES; ASPIDORHYNCHIFORMES; PHOLIDOPHORIFORMES; PYCNODONTIFORMES; 
SEMIONOTIFORMES.  
 �34s[Reeve M. Bailey]
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�6v�2v�32sHolothuroidea   �21m �33s 
    A class of Echinozoa characterized by a cylindrical body and smooth 
leathery skin, and known as sea cucumbers. There are no arms, but a ring of 
five or more tentacles may surround the mouth, which is usually at one end of 
the body. There are no pedicellariae. Tube feet may be present or lacking. 
There are no ambulacral grooves.  See  ECHINOZOA. 
    Holothurians resemble worms because the pentamerous symmetry is largely 
concealed by a secondary bilateral symmetry, and the general absence of 
external spines distinguishes them from the other extant echinoderms (see 
illustration).  
  
    The 1100 living species have been grouped in 170 genera arranged in 6 
orders: the Dendrochirotida, Dactylochirotida, Aspidochirotida, Elasipodida, 
Molpadida, and Apodida. Colors vary widely; the most brilliant colors are 
found among the Synaptidae. Yellow, red, violet, and fawn tints occur, but 
many species are somber shades or black. See  APODACEA; ASPIDOCHIROTACEA; 
DENDROCHIROTACEA.  
    Holothurians occur in all seas, from low-tide level down to the greatest 
depths explored. At depths below 5J (8.85 km) holothurians comprise 90% of the 
total mass of living matter, the rest being mainly starfishes. Two pelagic 
genera are known. 
 �34s[Howard B. Fell]
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�6v�2v�32sHolotrichia �21m �33s
    A major subclass of the class Ciliatea. These protozoans have a fairly
uniform body ciliation, as the name implies. Separate articles appear on the
groups listed in the following classification:

		Subclass Holotrichia
		  Order: Gymnostomatida
			 Trichostomatida
			 Chonotrichida
			 Apostomatida
			 Astomatida
			 Hymenostomatida
			 Thigmotrichida

    The cilia are typically arranged in longitudinal rows over the body,
although scattered exceptions exist. A mouth is often, although not always,
present. The prototype of the Holotrichia is exemplified by the form portrayed
in the illustration.  See  CILIATEA.0]
 �34s[John O. Corliss]
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�6v�2v�32sHomalozoa   �21m �33s 
    One of the four subphyla of echinoderms, characterized by the complete 
absence of radial symmetry. All members are extinct; the group ranged from the 
Cambrian to the Devonian. Best-known members are the carpoids, some of which 
were bilaterally symmetrical. These animals may be related to the original 
stock from which all other echinoderms descend, but unfortunately sufficient 
information is so far lacking on the group.  See  CARPOIDS; ECHINODERMATA. 
 �34s[Howard B. Fell]
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�6v�2v�32sHomeostasis   �21m �33s 
    In living substance, the maintenance of internal constancy and 
independence of the environment. The essence of living substance is that it 
differs chemically from the surrounding medium and yet maintains a dynamic 
equilibrium with its environment. Homeostasis occurs in all living cells and 
in the fluids and organ systems of multicellular organisms. 
    The cells of multicellular organisms are bathed in fluids which are kept 
relatively constant in ionic composition, osmotic composition, pH, level of 
sugar, and organic composition. Two patterns of homeostasis of internal state 
in the face of environmental change are recognized: (1) An animal may alter a 
given property with reference to the environment; the property conforms to the 
medium, and homeostasis consists in cellular adjustment such that metabolism 
continues in the altered state. For example, in cold-blooded animals the body 
temperature conforms to that of the environment. In many marine animals and 
endoparasites the body fluids conform in osmotic concentration with the 
medium. (2) An animal may regulate its internal state and maintain internal 
constancy despite an altered environment. Such regulation is achieved by a 
series of automatic feedback controls as environmental stress is applied; at 
some environmental limits, regulation fails and the animal cannot long 
survive. For example, warm-blooded animals maintain a constant body 
temperature over a range limited by heat and cold. 
    Two types of homeokinetic response are also distinguished: (1) Capacity 
adaptations are changes in cellular functions such that an intact animal can 
maintain normal activity over a range of internal states. (2) Resistance 
adaptations are changes which permit survival at extremes of some physical 
parameter. Resistance limits are narrow for intact integrated organisms, less 
restricted for isolated cells and tissues, and much wider for critical 
molecules such as proteins (denaturation) and lipids (melting). For example, 
many poikilothermic animals show little or no regulation of body temperature, 
but they alter energy-yielding enzyme systems to provide for nearly constant 
activity over a wide temperature range. Also, these animals can, by 
acclimation, become more or less resistant to heat or cold. Similar changes in 
thermal resistance occur in some homeotherms (temperature regulators).  See  
HIBERNATION; HYPOTHERMIA; THERMOREGULATION. 
 �34s[C. Ladd Prosser]
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�6v�2v�32sHoming   �21m �33s 
    A process in navigation by which the destination is approached by keeping 
some navigational parameter constant. The parameter to be maintained constant 
is usually generated by the cooperative action of two sets of equipment, one 
on the vehicle and the other on the ground; that is, these equipments form a 
homing system.  
    Homing systems vary greatly in their complexity. One of the simpler 
systems is the radio compass, which operates with any suitably located radio 
beacon; the craft's heading is adjusted so that the bearing being read on the 
radio direction finder is kept constant. The loran system, on the other hand, 
is very complex. Specialized shipborne and ground-based installations are 
required to generate the parameter. In general, modern systems provide the 
homing feature while furnishing direct positional information.  See  LORAN. 
 �34s[Peter C. Sandretto]
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�6v�2v�32sHomobasidiomycetidae   �21m �33s 
    A subclass of basidiomycetous fungi, also known as Holobasidiomycetes or 
Eubasidiomycetes. In this subclass the basidium is not divided by cross walls. 
There are two main groups of Homobasidiomycetidae, the Hymenomycetes and the 
Gasteromycetes. In the Hymenomycetes, which include the mushrooms, pore fungi, 
and their relatives, the basidia are formed in an exposed layer (hymenium), 
and the basidiospores borne symmetrically on slender stalks (sterigmata) are 
forcibly discharged. In the Gasteromycetes, which include the puffballs, 
earthstars, stinkhorns, and their relatives, the basidia are enclosed during 
their formation, and the basidiospores borne symmetrically on long sterigmata 
are not forcibly discharged.  See  BASIDIOMYCOTINA; MUSHROOM. 
 �34s[Robert M. Page]
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�6v�2v�32sHomogeneous catalysis   �21m �33s 
    A process in which a catalyst is in the same phase as the reactant. A 
homogeneous catalyst is molecularly dispersed (dissolved) in the reactants, 
which are most commonly in the liquid state. Catalysis of the transformation 
of organic molecules by acids or bases represents one of the most widespread 
types of homogeneous catalysis. In addition, the catalysis of organic 
reactions by metal complexes in solution has grown rapidly in both scientific 
and industrial importance.  See  CATALYSIS. 
 �34s[Denis Forster]
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�6v�2v�32sHomoptera   �21m �33s 
    An order of the class Insecta related to the order Hemiptera. This is a 
major group of sucking insects, with more than 30,000 species, even though in 
Asia and Africa the number of undiscovered species probably still exceeds the 
discovered ones. Common examples are the cicadas, aphids, and leafhoppers. The 
group is difficult to characterize because of the large number and diverse 
forms of the species it contains. 
    The head of these insects is hypognathous or opisthognathous, the beak 
appearing to arise from the ventral posterior margin of the head or even from 
the prosternum. The gula is membranous or absent. As in the Heteroptera, the 
beak consists of two pairs of stylets, formed by the maxillae and the 
mandibles, ensheathed in the labium. The maxillary stylets fit together to 
form a double tube, one channel serving for the passage of food and the other 
for saliva. 
    Most winged species have four wings, but male scale insects have only two. 
In most forms both pairs of wings are membranous and transparent, but in some, 
the forewings are somewhat thickened and may then be either coriaceous and 
translucent, or opaque, and with or without an apical membranous area. When 
the insects are at rest, the forewings are usually held, rooflike, over the 
dorsum, with the apex of one of them slightly overlapping the apex of its 
complement (Fig. 1). 
    In most species metamorphosis is gradual, but in a few it is practically 
holometabolous. The adults and nymphs of most species are terrestrial, but a 
few species are subterranean in all stages and others are subterranean only in 
the immature stages. A number of species are vectors of virus diseases of 
plants.  
  
    Series Coleorrhyncha.  This group is characterized by the origin of the 
beak, formed at the anteroventral extremity of the face, and by the fact that 
the propleura form a sheath for the base of the beak. The hindwings are 
absent, and the forewings are held flat over the abdomen in repose. The flight 
function has been lost. They occur in Tasmania, New Zealand, and South America. 
  
    Series Auchenorrhyncha.  This series and the Sternorrhyncha are the major 
groups of the Homoptera. In the Auchenorrhyncha the beak arises at the 
anteroventral extremity of the face and is not sheathed by the propleura. The 
Auchenorrhyncha includes a large number of species. A number of 
classifications have been proposed; the classification adopted here is a 
common one. It divides the series into the superfamily Fulgoroidea and the 
families Cicadidae, Cercopidae, Membracidae, and Cicadellidae. These families 
are not subordinate to the superfamily Fulgoroidea. 
  
    Superfamily Fulgoroidea includes insects commonly known as lantern flies. 
This group is subdivided into 20 families and includes many species which are 
important because of the economic damage they do while feeding or because they 
carry virus diseases of plants. 
    Included in the family Cicadidae are the cicadas, harvest flies, and jar 
flies. The insects in this family are probably better known to the layman than 
any other homopterous family because of their large size and the strident 
songs of the males. Adults are usually found on trees or shrubs. At least in 
some species, the songs of the males assemble local populations. 
    Spittle bugs and froghoppers are common examples of the family Cercopidae. 
Insects in this group most often attract attention in the immature stages, 
during which they surround themselves with a mass of froth or spittle. One 
species, the meadow spittle bug, is very common in the temperate portion of 
the Northern Hemisphere, and its masses of spittle are familiar sights. Some 
species of Australia and the East Indies live within a calcium carbonate tube 
attached to stems or leaves. 
    The treehoppers (family Membracidae) are small to medium in size and 
seldom attract attention (Fig. 2a). Most of them feed on woody plants and are 
found on the stems in sunny locations. Frequently a number of specimens are 
arranged in a vertical row on the stem, all with their heads downward. 
    The greatest number of species occurs in the warmer regions of the world. 
In many species the enlarged pronotum has adornments, excrescences, and 
processes which are astonishing in appearance, some of them nearly as large as 
the remainder of the insect. The nymphs (Fig. 2b) leave the trees and feed on 
herbaceous plants, often occurring in great numbers in pastures. The adults 
return to woody plants before oviposition. 
  
    The leafhoppers are included in the large family Cicadellidae. These 
usually small insects are known to many people by sight but not by name, 
because of their common occurrence in great numbers at night near lights (Fig. 
1). Probably the greatest number of species occurs in tropical areas, but the 
majority of these have not been described. Leafhoppers occasionally bite 
humans, but apparently they have never been seen taking blood. Several species 
have been found to be vectors of virus diseases of plants. 
  
    Series Sternorrhyncha.  In this group of families, the beak appears to 
arise either between the fore coxae or behind them. The antennae are usually 
long, filamentous, and have no well-differentiated terminal setae. Wingless 
forms are common. 
    The Sternorrhyncha includes a large number of species, many of them of 
great economic importance. The winged forms are not strong flyers, but they 
are so light that they may be borne considerable distances by air currents. 
    The family Psyllidae are known as jumping plant lice. Its representatives 
resemble cicadas in appearance, but are much smaller. About 1000 species are 
known. Some psyllids produce severe damage to their food plants. This damage 
may result from the mere feeding by tremendous numbers of individuals, from 
the resulting yellowing or rolling of leaves, or from galls produced on the 
leaves. Indirect damage may result from the growth of fungi on leaves which 
have become coated with the sugary excrement, the honeydew, of the psyllids.  
    The whiteflies (family Aleyrodidae) are 0.28 in. (7 mm) or less in length 
and usually lightly covered with a white, powdery, waxy material which has led 
to their common name. Whiteflies directly damage plants by their feeding. 
Indirectly, damage results from spotting at the feeding site, growth of fungus 
on the excreted honeydew, or from increased susceptibility of leaves to winter 
damage.  
    Members of the large superfamily Aphidoidea have four wings, or none. The 
wings are usually membranous or whitish and opaque. The forewings are much 
larger than the hindwings. Honeydew is usually produced. This superfamily 
includes the families Aphididae and Chermidae. The true aphids (Aphididae) are 
very attractive to ants, and colonies of very small aphid species which 
otherwise might escape notice can often be located by observing the attending 
ants. In a few species, this relationship has progressed to the point where 
ants are necessary for the survival of the aphid species, as in the corn root 
aphid. Many aphid species are important because of damage done in feeding. 
    Chermidae is a small family of minute insects, the adelgids and 
phylloxeriids. Both winged and wingless forms occur. The grape phylloxera has 
been a severe pest of cultivated grapes and once threatened the entire wine 
industry of France. 
    The scale insects and mealy bugs (superfamily Coccoidea) are usually 
small. More than 4000 species have been described. In the males, the hindwings 
are reduced to clublike halteres. The wings are usually held flat over the 
back in repose, and the venation is greatly reduced. The females are wingless. 
    Scale insects injure the host plant by their feeding on leaves, stems, or 
roots, and a number of species are very important economically. Most species 
produce large quantities of honeydew. A few species have been shown to be 
vectors of virus diseases of cucumbers, tobacco, and cacao.  See  ENTOMOLOGY, 
ECONOMIC; INSECTA. 
 �34s[David A. Young]
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�6v�2v�32sHomosclerophorida   �21m �33s 
    An order of primitive sponges of the class Demospongiae, subclass 
Tetractinomorpha, with a skeleton consisting of equirayed, tetraxonid, 
siliceous spicules and their derivatives formed through reduction in number of 
rays.  
    Homosclerophorid sponges are mostly small in size and encrusting to 
massive in shape. They occur in tidal and shallow waters, down to depths of at 
least 1600 ft (500 m).  See  DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
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�6v�2v�32sHoney Dew melon   �21m �33s 
    A long-keeping cultivar of muskmelon,  Cucumis melo inodorus, of the gourd 
family, Cucurbitaceae. Vines are vigorous and prolific and have large leaves 
and stems. The fruits are large, slightly oval, (diameter 6 - 7 in. or 15 - 18 
cm), smooth, creamy yellow to ivory when ripe, and with little or no net. The 
flesh is thick, light green, tender, juicy, and very sweet with mild aroma and 
flavor; it contains 10% or more sugar when ripe and is rich in potassium and 
vitamin C, but not as rich in vitamin A as the orange-fleshed cantaloupe. 
    Honey Dew is of African origin, has been grown in France for many years, 
and was introduced into the United States in 1911. Most of the crop is 
harvested in the summer in California; almost all of the remaining crop is 
harvested in the spring in Texas, with less than 5% harvested in the fall in 
Arizona. See MUSKMELON; VIOLALES. 
 �34s[Oscar A. Lorenz]
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�6v�2v�32sHoning   �21m �33s 
    The process of removing a relatively small amount of material from a 
surface by means of abrasive stones to obtain a desired finish or extremely 
close dimensional tolerance. Seldom is more than a few thousandths of an inch 
(one thousandth of an inch equals 0.025 mm) of stock removed. The abrading 
action of the fine grit stones occurs on a wide surface area rather than on a 
line of contact as in grinding. As it applies to machining, honing refers 
primarily to work done on cylindrical surfaces. In addition to metals and 
carbides, materials such as plastics, ceramics, and glass may be honed. Honing 
is done with manually operated equipment as well as with vertical and 
horizontal honing machines.  See  GRINDING; MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sHookeriales   �21m �33s 
    An order of the mosses, many species of which occur in the tropics. The 
plants are small to robust, procumbent, and often flaccid, and frequently 
appear flattened. The stems are irregularly branched, and the cells of the 
peripheral layer are enlarged. The leaves vary in shape and size. They occur 
in many rows in some species, four rows in others, and appear to be in one 
plane (see illustration).  See  BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sHooke's law   �21m �33s 
    A generalization applicable to all solid materials, stating that stress is 
directly proportional to strain and expressed as 
  
             *** Stress / Strain = S / EPSILON = constant = E ***
  
where E is the modulus of elasticity, or Young's modulus, in pounds per square 
inch. The constant relationship between stress and strain applies only to 
stress below the proportional limit. See STRESS AND STRAIN; YOUNG'S MODULUS. 
 �34s[William J. Krefeld/Waldo G. Bowman]
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�6v�2v�32sHookworm   �21m �33s 
    A roundworm of the class Nematoda; an intestinal parasite of humans and 
other mammals. All species are included in the family Ancylostomidae and are 
phasmid nematodes; that is, there are phasmids, sensory (perhaps olfactory) 
structures, located posterior to the anal opening. Members of this family have 
a large mouth or buccal cavity armed either with chitinous cutting plates or 
teeth with which the worm attaches to the intestinal mucosa of the host.  
    Two species of hookworm are important to humans: the New World hookworm 
(Necator americanus) and the Old World hookworm (Ancylostoma duodenale). See 
HOOKWORM DISEASE; NEMATA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sHookworm disease   �21m �33s 
    Microcytic hypochromic anemia in humans produced by the nematodes  Necator 
americanus  or  Ancylostoma duodenale  in the intestine. These nematodes suck 
blood from the small intestine. Although clinical manifestations in both cases 
are similar,  A. duodenale  infections are usually more severe and are 
characterized by severe eosinophilia and diarrhea. The host, if malnourished, 
cannot replace the blood lost, and anemia ensues with all its symptoms.  See  
ANEMIA; HOOKWORM. 
    The life history of  Ancylostoma  is similar to that of  Necator . The 
eggs are eliminated in the host's feces. In areas of poor sanitation the soil 
becomes contaminated. With favorable temperature, humidity, shade, and 
aeration, eggs in soil embryonate in 24 hr. A first-stage larva hatches and 
molts twice within 1 week to become infective. The larva actively penetrates 
the skin of humans. It then enters the bloodstream, and moves to the pulmonary 
alveoli. Pneumonia and other respiratory diseases may result. Penetration of 
the skin by infective hookworm larvae produces a dermatitis known as dew itch 
or ground itch. Migrating via trachea and esophagus to the duodenum, the 
parasite reaches adulthood in 2 months. 
    The infective larva of  A. braziliense , found in dogs and cats, may 
infect the skin of humans and produce a creeping eruption that becomes 
contaminated with bacteria. The condition is known as larva migrans. The 
lesion may subsist for months. 
 �34s[Jose F. Maldonado]
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�6v�2v�32sHop   �21m �33s 
    A plant (Humulus lupulus) belonging to the family Urticaceae. The plant is 
perennial, with the clockwise twining vine dying back to the ground each year. 
The male and female flowers are borne on separate plants.  See  URTICALES. 
    The mature hop cone - the hop of commerce - consists of papery bracts and 
bracteoles. As the cone matures, numerous golden-yellow granules 
(lupulin)develop on the bracteoles. These granules contain the resins and 
essential oils to which the hop owes its brewing value.  See  MALT BEVERAGE. 
    Hops are found primarily in the temperate areas of the world. In the 
United States the cultivated hop is grown in Washington, Oregon, Idaho, and 
California.  
 �34s[C. B. Skotland]
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�6v�2v�32sHophornbeam �21m �33s
    The genus  Ostrya  of the birch family, represented in North America by
two species.  O. virginiana  is widely distributed in the eastern half of the
United States and in the highlands of southern Mexico and Guatemala. It can be
recognized by its fruit, which closely resembles that of the hop vine, and by
its very scaly bark. The scales usually occur in narrow, more or less
parallel, vertical strips. The leaves are sharply and doubly serrate. This is
one of several trees known as ironwood because of its hard, strong wood; it is
used for fence posts, tool handles, mallets, and other articles requiring
hardness and strength.
     O. knowltonii  is a small rare tree of the southwestern United States.
See  FAGALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sHoplocarida   �21m �33s 
    A superorder of the class Crustacea, with a single order, the  
Stomatopoda, commonly known as the mantis shrimps. The term Hoplocarida is 
used only in classifications; the animal group as such is usually indicated 
with the name Stomatopoda.  See  =-STOMATOPODA. 
 �34s[Lipke B. Holthuis]
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�6v�2v�32sHoplonemertini   �21m �33s 
    An order of the class Enopla of the phylum Rhynchocoela, characterized by 
possession of an elaborate armed proboscis consisting of an anterior 
thick-walled tube, a median portion armed with stylets, and a posterior blind 
tube. In some species the foregut (stomach) can be everted into the prey to 
achieve extracorporeal digestion prior to ingestion. There are two suborders, 
the Monostylifera and Polystylifera, separated on the number of stylets in the 
proboscis. Monostylifera include fresh-water, terrestrial, and symbiotic 
species, as well as the more common marine littoral forms.  See  ANOPLA; 
BDELLONEMERTINI; ENOPLA; RHYNCHOCOELA. 
 �34s[J. B. Jennings]
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�6v�2v�32sHorizon   �21m �33s 
    The visible horizon is the apparent boundary line between sky and earth or 
sea. The astronomical horizon is the great circle of the celestial sphere 90DG 
from the zenith and the nadir.  See  ASTRONOMICAL COORDINATE SYSTEMS. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sHormone   �21m �33s 
    One of the chemical messengers produced by endocrine glands, whose 
secretions are liberated directly into the bloodstream and transported to a 
distant part or parts of the body, where they exert a specific effect for the 
benefit of the body as a whole. The endocrine glands involved in the 
maintenance of normal body conditions are pituitary, thyroid, parathyroid, 
adrenal, pancreas, ovary, and testis. However, these organs are not the only 
tissues concerned in the hormonal regulation of body processes. For example, 
the duodenal mucosa, which is not organized as an endocrine gland, elaborates 
a substance called secretin which stimulates the pancreas to produce its 
digestive juices. The placenta is also a very important hormone-producing 
tissue. See separate articles on the individual glands. 
    The hormones obtained from extracts of the endocrine glands may be 
classified into four groups according to their chemical constitution: (1) 
phenol derivatives, such as epinephrine, norepinephrine, thyroxine, and 
triiodothyronine; (2) proteins, such as the anterior pituitary hormones, with 
the exception of adrenocorticotropic hormone (ACTH), human chorionic 
gonadotropin, pregnant-mare-serum gonadotropin, and thyroglobulin; (3) 
peptides, such as insulin, glucagon, ACTH, vasopressin, oxytocin, and 
secretin; and (4) steroids, such as estrogens, androgens, progesterone, and 
corticoids. Hormones, with a few exceptions like pituitary growth hormone and 
insulin, may also be classified as either tropic hormones or target-organ 
hormones. The former work indirectly through the organs or glands which they 
stimulate, whereas the latter exert a direct effect on peripheral tissues. 
#See  ENDOCRINE SYSTEM (VERTEBRATE). 
 �34s[Choh Hao Li]
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�6v�2v�32sHornbeam �21m �33s
    The genus  Carpinus  of the birch family, represented in the United States
by  C. caroliniana,  the American hornbeam or blue beech. Hornbeam is a small
tree, and it has a smooth, steel-gray, fluted bark. It grows throughout the
eastern half of the United States, especially in moist soil along banks of
streams; it is sometimes called water beech. When mature, it is easily
recognized by its peculiar bark, by the doubly serrate leaves resembling those
of sweet birch, and by the small, pointed, angular winter buds with scale in
four rows. The fruit is a small nutlet subtended by a three-lobed serrate
bract. The wood is very hard, giving rise to the name ironwood.
    The European hornbeam (C. betulus) is often cultivated in parks and
estates. It can be distinguished by its larger size, larger winter buds, and
larger three-lobed, almost entire fruiting bracts.  See  FAGALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sHornblende   �21m �33s 
    A general name given to the monoclinic calcium amphiboles that form 
extensive solid-solution series between the various metals in the generalized 
formula (Ca, Na)2(Mg, Fe, Al)5(Al, Si)8O22(OH,F)2. Hornblende has a widespread 
occurrence in metamorphis, igneous (intrusive), and volcanic rocks, ranging 
from dominantly ferromagnesian rocks to granites. The term common hornblende 
refers to the middle metamorphic grade hornblendes found in schists. Common 
hornblende usually forms as long, prismatic needles, dark green to black in 
color. The hornblendes of the high grades of metamorphism are usually short, 
stubby prisms, black to brown in color, and high in iron and aluminum, with 
some sodium.  See  AMPHIBOLE; METAMORPHISM; SCHIST. 
 �34s[George W. DeVore]
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�6v�2v�32sHornfels   �21m �33s 
    A common name given to a class of metamorphic rocks produced by contact 
metamorphism, also known as hornstone. Hornfelses were originally sedimentary 
rocks. As magma intruded into the sediments, the heat given off induced 
recrystallization and effected a complete alteration of the primary 
sedimentary strata into hard, often flinty rocks. Their fine-grained, 
constituent minerals usually can be discerned only with the microscope. The 
chemical composition of a hornfels depends only upon the composition of the 
original sediment, and consequently the mineral composition of hornfelses is 
predetermined by the nature of the original sediment.  See  METAMORPHIC ROCKS; 
METAMORPHISM.  
    According to the chemical composition of the sediments, there will be 10 
classes of silica-rich hornfelses: 
    1. Pure shale contains alumina and traces of ferromagnesia which form 
andalusite and cordierite (and an excess of silica which produces quartz). 
    2. A small addition of lime produces anorthite (in the actual hornfels 
represented by plagioclase, since some Na2O is present) in addition to 
andalusite and cordierite. 
    3. More lime produces more anorthite, whereas andalusite disappears. 
    4. Next, hypersthene will form in addition to cordierite and anorthite. 
    5. With still more lime, the cordierite disappears, with the formation of 
more anorthite and hypersthene. 
    6. Lime is now so dominating that it combines with magnesia to form 
diopside in addition to anorthite and hypersthene. 
    7. Hypersthene disappears. 
    8. More lime reacts with anorthite to form grossularite in addition to 
diopside.  
    9. Anorthite is completely used up in the grossularite reaction. 
    10. Pure lime silicate, that is, wollastonite, occurs with diopside and 
grossularite.  
 �34s[T. F. W. Barth]
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�6v�2v�32sHorn (geology)   �21m �33s 
    A high mountain peak steepened by glaciation on all sides, and a remnant 
of a former alpine landscape deeply eroded by surrounding mountain glaciers. 
Horns may have three or four near-vertical faces, determined by the positions 
of cirque glaciers carving into the mountain below the faces. Horns occur in 
glaciated mountains all over the world, or as nunataks standing above ice 
sheet surfaces. Excavation of a cirque headwall, which may become the face of 
a horn above, prevents rock debris from accumulating and keeps the cliff 
clean.  See  CIRQUE; GLACIAL GEOLOGY. 
    A majestic four-sided horn, the Matterhorn rises to 14,684 ft (4477 m) on 
the Swiss-Italian frontier above Zermatt, Switzerland (see illustration). From 
the Swiss side, it appears as an isolated obelisk, but actually is at the end 
of a long arete. 
 �34s[Sidney E. White]
�33s 
  
  
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�6v�2v�32sHorology   �21m �33s 
    Measurement of the time dimension. In practice, horology is: the search 
for a steady or repetitive action; and the design of an instrument to perform 
that action and to indicate (read out) a measure of the action. Until early in 
the 20th century, horology dealt with mechanical instruments, with effort 
distributed between improving accuracy and decreasing size of timepieces. 
Increasingly, however, electronic instruments have provided means for meeting 
these objectives.  See  ATOMIC CLOCK; CHRONOMETER; CLOCK; QUARTZ CLOCK; 
SUNDIAL; TIME; WATCH. 
 �34s[Frank H. Rockett]
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�6v�2v�32sHorse   �21m �33s 
    A mammal which is an odd-toed ungulate (Perissodactyla) of the family 
Equidae. These animals have a single functional digit, the middle one, with 
the second and fourth digits present as vestigial structures. The wild horse 
(Equus caballus) is now represented by a single race, the Mongolian or 
Przewalski's horse (E. c. przewalskii), which survives in small numbers in 
central Asia. It is not tall, standing only about 4J ft (1.4 m) high at the 
withers.  
    A mare may be bred at 1-2 years of age but usually not until 3 years old. 
One or two foals are born each year after a gestation period of about 11 
months. The maximum life-span recorded for the horse is 60 years, although 
average maximum life-span is 35. 
    During their evolution the horses have increased in size from small 
terrierlike animals to that of the modern varieties; the legs and feet 
increased in size and the original four digits gradually became reduced to 
one. There was a gradual change in the teeth as these animals developed into 
efficient browsers. 
    Throughout history many attempts were made to domesticate the various 
species of wild horses, but this accomplishment does not seem to have occurred 
before the Bronze Age. Modern horses may be arbitrarily classified into three 
groups bred for specific activities; saddle and race horses, draft breeds, and 
harness horses.  See  PERISSODACTYLA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sHorseradish   �21m �33s 
    A hardy perennial crucifer,  Armoracia rusticana , of eastern European 
origin belonging to the plant order Capparales. Horseradish is grown for its 
pungent roots, which are generally grated, mixed with vinegar and salt, and 
used as a condiment or relish. Production in the United States is limited to 
northern areas; Illinois, Wisconsin, and Missouri are important producing 
states.  See  CAPPARALES. 
 �34s[H. John Carew]
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�6v�2v�32sHorst   �21m �33s 
    A segment of the Earth's crust, generally long as compared to its width, 
that has been upthrown relative to the adjacent rocks (see illustration). 
Horsts range in size from those that have lengths and upward displacement of a 
few inches to those that are tens of miles long with upward displacements of 
thousands of feet. The faults bounding a horst on either side commonly have 
inclinations of 50-70DG toward the downthrown blocks, and the direction of 
movement on these displacements indicates that they are gravity faults. These 
relationships suggest that horsts develop in regions where the crust has 
undergone extension. They may form in the crests of anticlines or domes, or 
may be related to broad regional warpings.  See  FAULT AND FAULT STRUCTURES. 
 �34s[Philip H. Osberg]
�33s 
  
  
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�6v�2v�32sHospital infections   �21m �33s 
    Nosocomial infections; clinically apparent infections that develop while a 
person is hospitalized or that become apparent after a person is discharged 
from the hospital but were caused by microorganisms acquired during the stay 
there. Most nosocomial infections become clinically apparent while the 
individual is still hospitalized. However, as much as 25% of surgical wound 
infections, for example, appear after the person is discharged. Most 
nosocomial infections are probably not preventable, and many affect people 
whose resistance to infection is lowered by other means or by treatment for 
other diseases. 
    The four most frequently encountered types of nosocomial infection are, in 
descending order of frequency, urinary, wound and skin, respiratory, and 
bacteremic.  See  INFECTION. 
 �34s[Bruce Dan]
�33s 
  
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�6v�2v�32sHot-water heating system   �21m �33s 
    A heating system for a building in which the heat-conveying medium is hot 
water. A hot-water heating system consists essentially of water-heating or 
-cooling means and of heat-emitting means such as radiators, convectors, 
baseboard radiators, or panel coils. A piping system connects the heat source 
to the various heat-emitting units and includes a method of establishing 
circulation of the water or other medium and an expansion tank to hold the 
excess volume of water as it is heated and expands. 
    In a one-pipe system, radiation units are bypassed around a one-pipe loop. 
This type of system should only be used in small installations. In a two-pipe 
system (see illustration), radiation units are connected to separate flow and 
return mains, which may run in parallel or preferably on a reverse return 
loop, with no limit on the size of the system. In either type of system, 
circulation may be provided by gravity or pump. 
    One outstanding advantage of hot-water systems is the ability to vary the 
water temperature according to requirements imposed by outdoor weather 
conditions, with consequent savings in fuel. Radiation units may be above or 
below water heaters, and piping may run in any direction as long as air is 
eliminated. Hot water is admirably adapted to extensive central heating where 
high temperatures and high pressures are used and also to low-temperature 
panel-heating and -cooling systems.  See  COMFORT HEATING; DISTRICT HEATING; 
OIL BURNER. 
 �34s[Erwin L. Weber]
�33s 
  
  
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�6v�2v�32sHot-wire anemometer   �21m �33s 
    A device for measuring air velocity, used principally in research on air 
turbulence and boundary layers. There are two types, known as constant-voltage 
and constant-temperature. The cooling of an electrically heated fine wire 
placed in a gas stream which alters wire resistance depends on the fluid 
velocity. With the constant-voltage type, as soon as the gas starts flowing, 
the hot wire cools off, the voltage across a Wheatstone bridge connected to 
the wire is kept constant, and the calibrated galvanometer shows a reading 
related to the velocity. This is useful only for very low velocities but is 
extremely sensitive - velocities down to 0.2 in./s (5mm/s) are typical. 
    The constant-temperature, or constant-resistance, type is more useful. The 
resistance in the battery across the Wheatstone bridge is increased to such a 
value that the wire which was cooled by the stream is again brought to its 
original temperature. The current in the hot wire, read by a calibrated 
ammeter, gives a measure of the air velocity. Another important application is 
the measurement of turbulent fluctuations. These fluctuations show up as 
variations in wire resistance and are useful for boundary-layer studies. See 
BOUNDARY-LAYER FLOW; THERMOCOUPLE; WHEATSTONE BRIDGE. 
 �34s[Herman Feshbach]
�33s 
  
  
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�6v�2v�32sHubble constant �21m �33s
    The rate at which the velocity of recession of the galaxies increases with
distance. Edwin Hubble established the existence of this motion
observationally in 1929.  See  GALAXY EXTERNAL.
    The value of the expansion rate is still not agreed upon. The two values
which seem to have the widest acceptance are 15 km per second per million
light-years (1.6 x 10- sup 1 sup 8 s- sup 1) and 29 km per second per million
light-years (3.1 x 10- sup 1 sup 8 s- sup 1).
    If the expansion rate has been constant, then the age of universe is the
reciprocal of the Hubble constant. If, as seems more probable, the expansion
has decelerated due to the gravitational attraction of the galaxies, the
reciprocal of the Hubble constant gives an upper limit to the age,
approximately 19 and 10 x 10 sup 9 years respectively for the two values quoted
above. W. A. Fowler gave the age of the universe as determined from the oldest
globular clusters in the Galaxy as between 7 and 20 x 10 sup 9 years, and from
the abundances of the various radioactive element as between 10 and 15 x
10 sup 9 years. It therefore appears that either value of the Hubble constant
mentioned above is consistent with the other estimates of the age of the
universe.  See	COSMOLOGY; STAR CLUSTERS.
 �34s[Louis C. Green]
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�6v�2v�32sHuebnerite   �21m �33s 
    A mineral with the chemical composition MnWO4. Huebnerite is the manganese 
member of the wolframite solid-solution series. It commonly contains small 
amounts of iron. It occurs in monoclinic, short, prismatic crystals. Fracture 
is uneven. Luster varies from adamantine to resinous. Hardness is 4 on Mohs 
scale and specific gravity is 7.2. Huebnerite is transparent and yellowish to 
reddish-brown in color; streak is brown.  See  WOLFRAMITE. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sHuman-factors engineering   �21m �33s 
    The area of knowledge dealing with the capabilities and limitations of 
human performance in relation to design of machines, jobs, and other 
modifications of the physical environment. Human-factors engineering seeks to 
ensure that humans' tools and environment are best matched to their physical 
size, strength, and speed and to the capabilities of the senses, memory, 
cognitive skill, and psychomotor preferences. 
    Human-factors engineering has also been termed human engineering, 
engineering psychology, applied experimental psychology, ergonomics, and 
biotechnology. It is related to the field of human-machine systems engineering 
but is more general, comprehensive, and empirical and not so wedded to formal 
mathematical models and physical analysis.  
    Among the problems of human-factors engineering are design of visual 
displays for ease and speed of interpretation; design of tonal signaling 
systems and voice communication systems for accuracy of communication; design 
of knobs and control handles and pedals; design of seats, work places, 
cockpits, and consoles in terms of humans' physical size, comfort, strength, 
and visibility. Human-factors engineering addresses itself to problems of 
physiological stresses arising from such environmental factors as heat and 
cold, humidity, high and low atmospheric pressure, vibration and acceleration, 
radiation and toxicity, illumination or lack of it, and acoustic noise. 
Finally, the field includes psychological stresses of work speed and load and 
problems of memory, perception, decision-making, and fatigue.  See  
HUMAN-MACHINE SYSTEMS. 
 �34s[Thomas B. Sheridan]
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�6v�2v�32sHuman genetics   �21m �33s 
    The study of human heredity and variation includes the physical basis 
(chromosomes and genes), the mechanisms whereby hereditary factors are 
transmitted from one generation to the next, the processes whereby genes 
determine characteristics, and the distribution of alternative genes and 
characteristics in different populations and races and the action of 
evolutionary forces upon them. 
  
    Chromosomes.  The physical basis of human heredity is in all essential 
respects the same as in other organisms. Each cell contains in its nucleus 
threadlike chromosomes. Because of the nature of mitotic cell division, these 
chromosomes, barring some accident, are identical in number and kind with 
those present in the zygote formed by fusion of egg and sperm, and from which 
all other cells of the individual are derived. The chromosomes are 
nucleo-protein, and it is to be presumed that their deoxyribonucleic acid 
(DNA) is, as in other organisms, the actual chemical code of information 
transmitted via the germ cells from one generation to the next. The germ 
cells, formed by the meiotic divisions, carry one-half the chromosomes present 
in somatic cells and primordial germ cells; that is, the mature germ cells 
carry one chromosome of each kind instead of two. Fertilization restores the 
diploid chromosome number; each maternal chromosome is consequently matched by 
a similar paternal chromosome; and the corresponding genes at a particular 
site or locus in a given pair of chromosomes likewise form a pair. These 
paired genes, termed alleles, may be identical (homozygous) or different 
(heterozygous).  See  CHROMOSOME; MEIOSIS; MITOSIS. 
    The normal human chromosome number is 46, including 44 autosomes and 2 sex 
chromosomes. A diagram of the haploid chromosome set shows that the 12 largest 
chromosomes all have median, submedian, or subterminal centromeres (see 
illustration).  
  
    Among the smaller chromosomes there are 6, including the Y, with 
acrocentric, that is, nearly terminal, centromeres. The X is a large 
chromosome with a submedian centromere. Two smaller acrocentric chromosomes 
bear tiny satellites. 
    Sex is determined in the human species by a particular pair of 
chromosomes, represented in the female sex by two similar X chromosomes, and 
in the male sex by one X and one Y. Most of the Y chromosome is homologous 
with the proximal portion of the considerably larger X chromosome and crossing 
over might be expected to occur between the Y and this part of the X, because 
chiasmata are cytologically visible in this region. Eight or nine mutant loci 
have been assigned to this homologous portion of the X and Y chromosomes, on 
the basis of statistical evidence of their partial sex linkage. In order of 
closeness of linkage with sex, these are total color blindness, xeroderma 
pigmentosum, Oguchi's disease, spastic paraplegia, recessive epidermolysis 
bullosa, retinitis pigmentosa, hemorrhagic diathesis, and convulsive 
disorders. The validity of this partial sex linkage has also been questioned.  
See  SEX DETERMINATION; SEX-LINKED INHERITANCE. 
    The portion of the X chromosome present in two doses in females and in a 
single dose in males carries genes that show typical sex linkage, and of 
course must be linked with each other. The best-known case is the linkage 
between hemophilia and dichromatic (red-green) color blindness, which in some 
progenies of women heterozygous for both are closely linked. 
 �34s[H. Bentley Glass]
�33s 
  
    Determination of inheritance pattern.  The first step in all genetic 
analyses is to determine whether a character is inherited and if so, how it is 
inherited. Patterns of inheritance may be recognized only by the study of 
families. For genetic purposes, the term family means, as a minimum, parents 
and offspring; it may, however, include additional relatives. 
    By following the occurrence of certain characteristics over several 
generations it is possible to determine whether a characteristic is autosomal 
(dominant or recessive) or sex-linked (dominant or recessive). The inheritance 
is autosomal dominant when each affected individual has an affected parent. 
When affected males have affected sons, sex-linked dominance is excluded. 
    A simple type of autosomal inheritance concerns recessive characters such 
as albinism. An individual pedigree is not likely to contain sufficient 
information to permit the conclusion that inheritance of such a trait is 
recessive. Some features of pedigrees for rare recessive characters are that 
(1) the parents are usually normal (they are always normal if the character 
leads to early death, as in cystic fibrosis), (2) collateral and ancestral 
relatives are rarely affected, and (3) there may be an increased frequency of 
consanguineous matings. The last criterion is becoming of decreasing value in 
the United States. 
    Distinguishing features of pedigrees showing recessive sex linkage are 
that (1) affected males do not transmit the character (the gene) to their 
sons, (2) normal males have only normal descendants, (3) affected males other 
than sibs are related to each other via females, and (4) affected females do 
not occur. Exceptions may occur if the male in such a pedigree marries a 
carrier (heterozygous) female. In such a mating, affected males may have 
affected sons (the gene coming from the mother) and affected daughters, and 
normal males may have affected sons. The same features hold for sex-linked 
dominant characters. But, in addition, all daughters of affected males will be 
affected and normal females will have only normal descendants. 
    In the medicolegal uses of genetics only genetic factors which are 
widespread in the population and which are inherited by the way of a simple, 
well-understood, and adequately tested genetic mechanism are useful for 
medicolegal purposes. At present the only characters which fulfill these 
requirements are the ABO, MN, and Rh blood group systems. Accordingly, they 
are the only ones universally accepted for medicolegal use. 
    In cases of disputed paternity the question to be answered is whether this 
child of this mother could have this man as its father. If the child has an 
antigen not present in either its mother or alleged father, the man is 
excluded as the father; similarly, if the child does not have an antigen which 
the alleged father for genetic reasons had to transmit to his offspring, the 
man is excluded as the father.  See  BLOOD GROUPS. 
 �34s[Arthur G. Steinberg]
�33s 
  
    Population analysis.  A population, from the genetic viewpoint, 
constitutes a pool of genes with certain frequencies. The system by which the 
individuals mate determines how these genes are combined to form the 
genotypes. Two mating systems, panmixis or random mating, and assortative or 
nonrandom mating, are discussed in the following section. 
    Panmixis is a system of mating, also known as random mating, in which each 
member of one sex in a population is equally likely to mate with any member of 
the opposite sex. Such a population is said to be panmictic. A mating is then 
a random event, determined by chance. The frequency of certain types of 
matings is consequently determined by the frequency of the types of 
individuals in the population. The randomness of matings, however, refers only 
to certain characteristics of the mating individuals. The matings may be at 
random with respect to one characteristic (for example, blood groups) and yet 
not so with respect to some other characteristic (such as height). Random 
mating is not an absolute term and has meaning only with respect to specified 
characteristics of a given population. The result of panmixis in a large 
population with respect to one pair of autosomal genes (locus  A , a, for 
example) is known as the Hardy-Weinberg equilibrium. Another name is 
Castle-Hardy-Weinberg (CHW) law.  
    The Hardy-Weinberg law concerns the stability of the three genotypic  (AA, 
Aa, aa)  proportions in a large panmictic population in the absence of 
disturbing factors such as mutation. selection, migration, and sampling 
fluctuation. If a fraction  p  of all the genes at the  Aa  locus in the 
population is allele  A  and the rest ( q  = 1 -  p ) is allele  a  in both 
sexes, then a population consisting of  psup2(AA), 2pq(Aa),  and  qsup2(aa)  
will retain the same genotypic proportions in subsequent generations under the 
system of random mating. Such a population is said to be in an equilibrium 
state (of a neutral nature). The equilibrium law reflects the conservativeness 
of a mendelian population. 
    Assortative mating includes, in a broad sense, all systems of mating other 
than random mating. In the narrow but usual sense, it excludes the various 
systems of inbreeding (mating according to genetic relationship) and refers to 
nonrandom mating with respect to phenotypes. Positive assortative mating means 
that individuals of the same type tend to mate to a greater or lesser degree 
more often than they would by chance alone, and the matings between 
individuals of different types occur less often than dictated by chance alone. 
A preference based on phenotypic resemblance is involved. Familiar examples in 
man are the positive correlations between spouses in such characteristics as 
color, stature, physique, and intelligence level, all of which are controlled 
to a certain extent by heredity. Positive assortative mating leads to greater 
parent-offspring correlation. If the degree of assortativeness in mating 
remains the same for a large number of generations, the population will 
approach an equilibrium condition. Moreover, the genotypic composition of an 
equilibrium population reached through assortative mating may be quite 
different from that reached through an inbreeding system. Negative assortative 
mating means that the individuals of unlike phenotypes tend to mate more often 
than expected by chance alone. Continued negative assortative mating of the 
same degree also leads to an equilibrium condition. 
    The difference between racial groups is usually found not in one but in 
many biological characteristics. A race, from the genetic viewpoint, is an 
aggregate of individuals who form more or less a mating group among themselves 
so that the genic content of this group  differs from that of other mating 
groups. The fact that a marriage even between members of two different major 
races produces normal children who in turn also produce normal children shows 
that the chromosomes of all humans are homologous. Hence the racial 
differences must be attributed to the alleles of the various loci of the 
corresponding chromosomes. A particular allele may be present or absent in any 
race, but usually it occurs in all races. The racial differences arise from 
the differences in allelic frequencies. 
 �34s[Ching Chun Li]
�33s 
  
    Chromosome mapping.  Human chromosome mapping, the localization of human 
genes to specific chromosomes or even regions of chromosomes, has undergone 
explosive growth since the development of somatic cell genetics. Combined with 
the more traditional methods of mapping from family studies and some other 
techniques, this has resulted in the mapping of more than 260 genes to the 
human autosomes. Although it has been estimated that less than 1% of all human 
genes have as yet been assigned to their respective chromosomes, it is now 
possible to speak of a human gene map.  See  GENETIC MAPPING; SOMATIC CELL 
GENETICS.  
 �34s[Sue Povey]
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�6v�2v�32sHuman-machine systems   �21m �33s 
    The area of knowledge, also known as man-machine systems, dealing 
quantitatively with the communication and cooperation between people and 
machines in performing specific tasks. Human-machine systems engineering 
includes the experimentation techniques, the mathematical theory appropriate 
to handling data derived from experiments, and the engineering methods for 
applying this knowledge to the design of machines and systems. The pilot and 
the airplane, the driver and the automobile, the secretary and the typewriter, 
the talker and the telephone, the programmer and the computer are examples of 
human-machine systems.  See  SYSTEMS ENGINEERING. 
    Though it draws heavily on psychology and physiology, the field uses the 
language and concepts of engineering to analyze how people receive information 
through their vision, hearing, touch, and other senses, how they utilize this 
information to make decisions, and how they implement these decisions with 
their muscles. Often the response an individual makes in turn affects the new 
information which is displayed to the senses, thus completing a closed loop of 
information flow linking the person and machine. 
    As with pumps, gears, motors, and electronic circuits, engineers seek to 
describe the human operator in terms of definite input or stimulus quantities 
and output or response quantities and to connect these quantities by 
mathematical equations with a view to predicting the behavior of whole 
human-machine systems. The idea of predicting the behavior of a whole system 
from a knowledge of how the components are interconnected and what are the 
input-output equations of these components is at the root of what is commonly 
called systems theory. 
    Systems theory does not require understanding the inner workings of the 
components themselves; it is only necessary to know what that input-output 
relation is. This is why systems engineers often refer to their components as 
``black boxes.'' In the man-machine systems the engineer likewise need not 
understand the inner workings of the human nervous system - only what the 
stimulus-response relationship is in quantitative terms. The mathematical 
model of a person moreover need not correspond to the individual's anatomy or 
physioloboxes. Human-machine systems engineering is closely related to the 
field called human-factors engineering, which applies psychology and 
physiology to the design of machines but is not as concerned with compatible 
input-output equations for both humans and machine.  See  HUMAN-FACTORS 
ENGINEERING; SERVOMECHANISM. 
 �34s[Thomas B. Sheridan]
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�6v�2v�32sHumidification   �21m �33s 
    The process of increasing the water-vapor content (humidity) of a gas. 
This process and its reverse operation, dehumidification, are important steps 
in air conditioning for human comfort and in many industrial operations.  See  
AIR CONDITIONING; COMFORT HEATING; DEHUMIDIFIER; HUMIDITY. 
    Air (or other gas) can be humidified by direct injection of water vapor 
(steam) or, more commonly, by the evaporation of liquid water in contact with 
the airstream. When evaporation occurs, heat is required to provide the latent 
heat of vaporization. If no external source of heat is provided, either the 
water or the air, or both, will be cooled. The cooling of water by this 
process is the basis of operation for industrial cooling towers, whereas 
evaporative air coolers often used in hot, dry climates depend upon the 
air-cooling effect. In both these types of apparatus, humidification of the 
air occurs, although it is not the prime objective of the operation. In units 
designed primarily for humidification, the incoming air is usually heated to 
provide the latent heat of evaporation and to permit the air to leave the unit 
at controlled levels of both temperature and humidity.  See  COOLING TOWER. 
 �34s[Arthur L. Kohl]
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�6v�2v�32sHumidistat �21m �33s
    A controller that measures and controls relative humidity. A humidistat
may be used to control either humidifying or dehumidifying equipment by the
regulation of electric or pneumatic switches, valves, or dampers. Most methods
for measuring humidity rely upon the swelling and shrinking of materials, such
as human hair, silk, horn, goldbeater's skin, and wood, with increases and
decreases in relative humidity.
    An electronic humidistat includes a sensing element and a relay amplifier.
The sensing element consists of alternate metal conductors on a small, flat
plate with a plastic coating. An increase or decrease of the relative humidity
causes a decrease or increase in the electrical resistance between the two
sets of conductors; the change in resistance is measured by the relay
amplifier.  See  HUMIDITY; PSYCHROMETRICS.
 �34s[John E. Haines /Richard L. Koral]
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�6v�2v�32sHumidity   �21m �33s 
    Atmospheric water-vapor content, expressed in any of several measures, 
especially relative humidity, absolute humidity, humidity mixing ratio, and 
specific humidity.  
    Relative humidity is the ratio, in percent, of the moisture actually in 
the air to the moisture it would hold if it were saturated at the same 
temperature and pressure. It is a useful index of dryness or dampness for 
determining evaporation, or absorption of moisture.  See  PSYCHROMETRICS. 
    Absolute humidity is the weight of water vapor in a unit volume of air 
expressed, for example, as grams per cubic meter or grains per cubic foot. 
    Humidity mixing ratio is the weight of water vapor mixed with unit mass of 
dry air, usually expressed as grams per kilogram. Specific humidity is the 
weight per unit mass of moist air and has nearly the same values as mixing 
ratio.  
 �34s[J. R. Fulks]
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�6v�2v�32sHumite   �21m �33s 
    A homologous series of magnesium nesosilicate minerals with general 
composition Mg2subn+1(SiO4)subn (F,OH)2. The known species include norbergite, 
chondrodite, humite, and clinohumite.  
    The minerals of the humite group have similar physical properties. The 
luster is resinous, and the color usually light yellow, brown, orange, or red. 
The pure synthetic Mg end members are colorless. Hardness is 6 - 6** on Mohs 
scale, specific gravity is 3.1 - 3.2. They are typically found in regionally 
crystallized marbles. Typical sources are the Grenville-age marbles in New 
York and Ontario, marble ejecta from Mount Vesuvius, and marbles from central  
Sweden.  See  SILICATE MINERALS. 
 �34s[Paul B. Moore]
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�6v�2v�32sHumus   �21m �33s 
    The amorphous, ordinarily dark-colored, colloidal matter in soil, 
representing a complex of the fractions of organic matter of plant, animal, 
and microbial origin that are most resistant to decomposition. 
    Humus consists of the combined residues of organic materials which have 
lost their original structure following the rapid decomposition of the simpler 
ingredients and includes synthesized cell substance as well as by-products of 
microorganisms. It is not a definite substance and is in a continual state of 
flux, disappearing by slow decomposition, and being constantly renewed by 
incorporation of residual matter. With a balance between these processes, 
humus, though not static, remains relatively uniform in nature and amount in a 
given soil. It constitutes a reservoir of stabilizing material which imparts 
beneficial physical, chemical, and biological properties to soil. Fertile 
soils are rich in humus. 
    Humus improves the texture of soils. It exerts a binding effect on sandy 
soils, and loosens the harder, clayey soils, thus increasing their porosity 
and permeability. It increases the moisture-holding capacity and improves the 
granular structure by cementing mineral particles into stable crumbs. This 
helps soils resist the pulverizing and eroding action of wind, water, and 
cultivation. As a storehouse of elements important to plants, humus functions 
as a regulator of soil processes by liberating gradually nutrients that would 
otherwise drain away. A soil rich in humus provides optimum conditions for the 
development of beneficial microorganisms and constitutes the best medium for 
growth of plants. 
    Peat is a type of humus that results from the decomposition of plant 
material under conditions of excessive moisture or in areas submerged in 
water. It is an organic deposit formed in marshes and swamps by the partial 
decomposition of countless generations of a variety of plants. See BOG; PEAT. 
 �34s[Allan G. Lochhead]
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�6v�2v�32sHunger   �21m �33s 
    A term most commonly used to refer to the subjective feelings that 
accompany the need for food; however, the study of this topic has come to 
include consideration of the overall control of food intake. More 
specifically, experimental work on the problem of hunger has been concerned 
with: the sensory cues that give rise to feelings of hunger, the physiological 
mechanisms that determine when and how much food will be ingested, and the 
mechanism governing the selection of the food to be eaten. 
    Food consumption is basically controlled by the organism's nutritional 
status. Food  deprivation leads to eating, and the ingestion of food materials 
terminates hunger sensations. Blood-sugar level can be used as a case in 
point. The concentration of blood sugar varies in a general way with the 
periodicity of the food cycle. Moreover, hyperglycemia (extremely high blood 
sugar) and hypoglycemia (extremely  low blood sugar) have been observed to 
decrease and increase hunger respectively. Detailed analyses of normal life 
variations of blood sugar, however, reveal that the relation between the 
concentration of blood sugar and hunger is not sufficiently close for this 
single humoral factor to be able to control hunger in any simple and direct 
manner.  
    The evaluation of more local tissue utilization of food has proved a more 
promising approach to this problem. There is some evidence suggesting that the 
status of the liver is pivotal in the control of feeding. Depletion of liver 
glycogen stimulates feeding; its repletion terminates feeding in rats and 
rabbits.  
    Many stimuli that terminate feeding have been identified. Eating in 
food-deprived animals is inhibited by the reduction of either cellular water 
or of plasma fluid. It is also reduced by gastric distension and by infusing 
nutrients into the intestine and into the systemic, especially venous hepatic, 
circulation. Satiation produced by nutrient absorption from the intestine may 
be mediated, in part, by the gut hormone cholecystokinin. It is likely that 
the cholecystokinin is effective because it reduces the rate at which food 
passes through the stomach. 
 �34s[Elliott M. Blass]
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�6v�2v�32sHuntington's chorea   �21m �33s 
    A rare hereditary disease of the basal ganglia and cerebral cortex, 
resulting in choreiform (dancelike) movements, intellectual deterioration, and 
psychosis. The illness is strictly hereditary, as a dominant mendelian trait. 
All American patients are supposed to be descendants of two brothers with 
Huntington's chorea, who migrated to the United States from England. No 
effective treatment is known. See HUMAN GENETICS; PSYCHOSIS. 
 �34s[Fredrick C. Redlich]
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�6v�2v�32sHurricane �21m �33s
    A tropical cyclone whose maximum sustained winds reach or exceed a
threshold of 33 m/s (74 mi/h). In the western North Pacific ocean it is known
as a typhoon. Many tropical cyclones do not reach this wind strength.
    Maximum surface winds in hurricanes range up to about 200 mi (320 km) per
hour. However, much greater losses of life and property are attributable to
inundation from hurricane tidal surges and riverine or flash flooding than
from the direct impact of winds on structures.
    Tropical cyclones of hurricane strength occur in lower latitudes of all
oceans except the South Atlantic and the eastern South Pacific, where
combinations of cooler sea temperatures and prevailing winds whose velocities
vary sharply with height prevent the establishment of a central warm core
through a deep enough layer to sustain the hurricane wind system.
    In the United States, property losses resulting from hurricanes have
climbed steadily because of the increasing number of seashore structures.
However, the loss of life, which has been huge in many storms, has decreased
markedly. This is due mainly to the fact that warnings, aided by a more
complete surveillance from aircraft and satellite, and extensive programs of
public education, have become more accurate and more effective. Improvements
in methodology for hurricane prediction have reduced the error in pinpointing
hurricane landfall and have greatly reduced the probability of larger errors
in prediction. See STORM; TROPICAL METEOROLOGY.
 �34s[Robert Simpson ;Joanne Simpson]
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�6v�2v�32sHuygens' principle �21m �33s
    An assumption regarding the behavior of light waves, originally proposed
by C. Huygens in the 17th century to explain the fact that light travels in
straight lines and casts sharp shadows. Large-scale waves, such as sound waves
or water waves, bend appreciably into the shadow. The special behavior of
light may be explained by Huygens' principle, which states that ``each point
on a wavefront may be regarded as a source of secondary waves, and the
position of the wavefront at a later time is determined by the envelope of
these secondary waves at that time.'' Thus a wave WW originating at S is shown
in the illustration at the instant it passes through an aperture. If a large
number of circular secondary waves, originating at various points on WW, are
drawn with the radius r representing the distance the wave would travel in
time t, the envelope of these secondary waves is the heavily drawn circular
arc W'W'. This represents the wave after t. If, as Huygens' principle
requires, the disturbance is confined to the envelope, it will be 0 outside
the limits indicated by points W'.
    Careful observation shows that there is a small amount of light beyond
these points, decreasing rapidly with distance into the geometrical shadow.
This is called diffraction. See DIFFRACTION.
 �34s[Francis A. Jenkins /William W. Watson]
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�6v�2v�32sHyades   �21m �33s 
    A group of stars, scattered through the V in the constellation of Taurus. 
An estimated 350 stars form the Hyades cluster. Because of the proximity of 
this cluster, which permits intensive observations, its main sequence stars 
are used as luminosity standards for the zero-age main sequence, from which 
the ages and distances of other clusters may be determined. However, the 
precise distance is still controversial. Reliable distance determinations fall 
between 140 and 150 light-years, but even a small error in the distance of the 
Hyades will magnify errors in distance determinations for more distant 
clusters. See STAR; STAR CLUSTERS. 
 �34s[Helen S. Hogg]
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�6v�2v�32sHyaluronic acid   �21m �33s 
    A polysaccharide which is an integral part of the gel-like substance of 
animal connective tissue; it supposedly serves as a lubricant and shock 
absorbent in the joints. Hyaluronic acid has also been isolated from umbilical 
cord, synovial fluid, skin, certain fowl tumors, and other sources. Treatment 
of this polysaccharide with the enzyme hyaluronidase, followed by acid 
hydrolysis, yields a disaccharide consisting of N-acetyl-D-glucosamine and 
D-glucuronic acid. This disaccharide appears to be the basic repeating 
structural unit that constitutes the hyaluronic acid molecule. See 
HYALURONIDASE; POLYSACCHARIDE. 
 �34s[William Z. Hassid]
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�6v�2v�32sHyaluronidase   �21m �33s 
    Any one of a family of enzymes, also known as hyaluronate lyases or 
spreading factors, produced by mammals, reptiles, insects, and bacteria, which 
catalyze the breakdown of hyaluronic acid. Some hyaluronidases also attack 
other similar polysaccharides. Since all liquefy the polysaccharide gel which 
fills the tissue spaces, they effectively accelerate diffusion so that 
injected, dissolved, or particulate matter (bacteria, viruses, toxins, or 
pigments) can diffuse through a larger volume of tissue. See HYALURONIC ACID. 
    The biological importance of the enzyme depends upon its source. That 
found in the culture filtrates of many strains of virulent bacterial permits 
the microorganisms to gain access to a larger volume of the host's tissue and, 
hence, to additional nutriment. That found in the venom of certain snakes and 
bees permits the toxin to produce more extensive damage to the victim. See 
ENZYME.  
 �34s[R. H. Pearce]
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�6v�2v�32sHydrate   �21m �33s 
    A particular form of a solid compound which has water in the form of H2O 
molecules associated with it. An example is the hydrate of copper sulfate, 
CuSO4.5H2O. Four water molecules are attached to the copper ion in the manner 
of coordination complexes, and the fifth water molecule is related to the 
sulfate and presumably held by hydrogen bonding. 
    Water can also be present in definite proportions in the crystal without 
being associated directly with the anion or cation. The water occupies a 
definite place in the crystal lattice. 
    Gas hydrates are compounds with a definite composition obtained when water 
and certain gases are chilled sufficiently. Chlorine, carbon dioxide, and 
acetylene all form such icy hydrates. 
    Some crystalline hydrates do not have definite proportions of water. 
Zeolites and similar silicate minerals, certain clays, and metallic oxides 
have variable proportions of water in their hydrated forms. See COORDINATION 
CHEMISTRY; HYDRATION. 
 �34s[Frank Wagner]
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�6v�2v�32sHydration   �21m �33s 
    The incorporation of molecular water into a complex with the molecules or 
units of another species. The complex may be held together by relatively weak 
forces or may exist as a definite compound. Many salts form solid hydrates 
when exposed to water vapor under certain conditions of temperature and 
pressure. Water is lost from these compounds when they are heated or when the 
water vapor pressure falls below a minimum value. Solids forming hydrates at 
low pressures are used as drying agents. See DELIQUESCENCE; DESICCANT; 
EFFLORESCENCE; HYDRATE; SOLUTION; SOLVATION. 
 �34s[Francis J. Johnston]
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�6v�2v�32sHydraulic actuator   �21m �33s 
    A cylinder or fluid motor that converts hydraulic power into useful 
mechanical work. The mechanical motion produced may be linear, rotary, or 
oscillatory. Operation exhibits high force capability, high power per unit 
weight and volume, good mechanical stiffness, and high dynamic response. These 
features lead to wide use in precision control systems and in heavy-duty 
machine tool, mobile, marine, and aerospace applications. See CONTROL SYSTEMS; 
FLUIDICS.  
    Cylinder actuators provide a fixed length of straight-line motion. They 
usually consist of a tight-fitting piston moving in a closed cylinder. The 
piston is attached to a rod that extends from one end of the cylinder to 
provide the mechanical output. The double-acting cylinder (Fig. 1) has a port 
at each end of the cylinder to admit or return hydraulic fluid. A four-way 
directional valve functions to connect one cylinder port to the hydraulic 
supply and the other to the return, depending on the desired direction of the 
power stroke. 
    Limited-rotation actuators are used for lifting, lowering, opening, 
closing, indexing, and transferring movements by producing limited 
reciprocating rotary force and motion. Rotary actuators are compact and 
efficient, and produce high instantaneous torque in either direction. Figure 2 
shows a piston-rack type of rotary actuator. Hydraulic fluid is applied to 
either the two end chambers or the central chamber to cause the two pistons to 
retract or extend simultaneously so that the racks rotate the pinion gear. 
    Rotary motor actuators are coupled directly to a rotating load and provide 
excellent control for acceleration, operating speed, deceleration, smooth 
reversals, and positioning. They allow flexibility in design and eliminate 
much of the bulk and weight of mechanical and electrical power transmissions. 
    Motor actuators are generally reversible and are of the gear or vane type. 
 �34s[Charles Mangion]
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�6v�2v�32sHydraulic gradient   �21m �33s 
    The slope along a closed water conduit that measures the sum of elevation 
and pressure heads. Total head consists of these heads plus the velocity head. 
The elevation and pressure heads at a point can be measured directly by 
connecting a tube to a small hole in the conduit wall and observing the 
vertical height to which the water rises in the tube, called a piezometer. See 
BERNOULLI'S THEOREM. 
 �34s[Donald R. F. Harleman]
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�6v�2v�32sHydraulic jump �21m �33s
    An abrupt increase of depth in a free-surface liquid flow. A hydraulic
jump is characterized by rapid flow and small depths on the upstream side, and
by larger depths and smaller velocities on the downstream side. A jump can
form only when the upstream flow is supercritical, that is, when the fluid
velocity is greater than the propagation velocity c of a small, shallow-water
gravity wave (c = [SQUARE ROOT OF gh], where h is the depth).
See OPEN CHANNEL; WAVE MOTION IN LIQUIDS.
 �34s[Donald R. F. Harleman]
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�6v�2v�32sHydraulic press   �21m �33s 
    A combination of a large and a small cylinder connected by a pipe and 
filled with a fluid so that the pressure created in the fluid by a small force 
acting on the piston in the small cylinder will result in a large force on the 
large piston. The operation depends upon Pascal's principle, which states that 
when a liquid is at rest the addition of a pressure (force per unit area) at 
one point results in an identical increase in pressure at all points. 
    The principle of the hydraulic press is used in lift jacks, earth-moving 
machines, and metal-forming presses (see illustration). A comparatively small 
supply pump creates pressure in the hydraulic fluid. The fluid then acts on a 
substantially larger piston to produce the action force. Heavy objects are 
accurately weighed on hydraulic scales in which precision ground pistons 
introduce negligible friction. See MECHANICAL ADVANTAGE; SIMPLE MACHINE. 
 �34s[Richard M. Phelan]
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�6v�2v�32sHydraulics   �21m �33s 
    The behavior of water or other liquids, chiefly when in motion. As a part 
of fluid mechanics, hydraulics deals with the properties, behavior, and 
effects of all liquids at rest against, or in motion relative to, boundary 
surfaces or objects. Its laws also apply to bases when changes in density are 
small, that is, compressibility effects are negligible. See FLUID MECHANICS. 
    Common hydraulic applications encountered in civil engineering include the 
flow of water in pipes, canals, and rivers, in flood control, in land 
reclamation, and in hydroelectric power projects. Hydraulics also influences 
appurtenant structures and devices such as dams, dikes, locks, spillways, 
weirs, piers, ship hulls, gates, and valves. Mechanical and chemical 
engineering applications include the flow of gases, oils, or other liquids; 
lubrication; and fluid machinery such as pumps, turbines, propellers, fans, 
and fluid power transmission and control devices, including servomechanisms. 
    Characteristics of particular phenomena in hydraulics include fluid 
properties; the shapes, sizes, relative roughnesses and relative motions of 
the surfaces or objects involved; the relative times and distances (short or 
long); whether flow is closed as in a pipe or open as in a canal; and whether 
motion is in a relatively extensive fluid as a submarine deeply submerged or 
whether motion occurs where there is a free surface as a ship at sea. 
    Hydraulics deals with bulk fluids which are regarded as homogeneous; it is 
not concerned with molecular sizes. As distinct from solids, fluids, such as 
water, are unable to resist shearing forces while remaining in a state of 
equilibrium. Among fluids, a liquid mass will have a definite volume, varying 
only slightly with temperature and pressure, whereas a mass of gas will fill 
any space available to it, the pressure adjusting itself accordingly. See 
DENSITY; ELASTICITY; HYDROSTATICS; VISCOSITY. 
 �34s[William Allan]
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�6v�2v�32sHydraulic turbine   �21m �33s 
    A machine which converts the energy of an elevated water supply into 
mechanical energy of a rotating shaft. Most old-style waterwheels utilized the 
weight effect of the water directly, but all modern hydraulic turbines are a 
form of fluid dynamic machinery of the jet and vane type operating on the 
impulse or reaction principle and thus involving the conversion of pressure 
energy to kinetic energy. The shaft drives an electric generator, and speed 
must be of an acceptable synchronous value. See GENERATOR; IMPULSE TURBINE; 
PELTON WHEEL; TURBINE. 
    Efficiency of hydraulic turbine installations is always high, more than 
85% after all allowances for hydraulic, shock, bearing, friction, generator, 
and mechanical losses. Material selection is not only a problem of machine 
design and stress loading from running speeds and hydraulic surges, but is 
also a matter of fabrication, maintenance, and resistance to erosion, 
corrosion, and cavitation pitting. 
    Pumped-storage hydro plants have employed various types of equipment to 
pump water to an elevated storage reservoir during off-peak periods and to 
generate power during on-peak periods where the water flows from the elevated 
reservoir through hydraulic turbines. See WATERPOWER. 
 �34s[Theodore Baumeister]
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�6v�2v�32sHydraulic valve lifter   �21m �33s 
    A device that eliminates the need for mechanical clearance in the valve 
train of internal combustion engines. 
    Clearance is normally required to prevent the valve's being held open and 
destroyed as the valve train undergoes thermal expansion. However, clearance 
requires frequent adjustment and is responsible for much operating noise. The 
hydraulic lifter is a telescoping compression strut in the linkage between cam 
and valve, consisting of a piston and cylinder (see illustration). When no 
opening load exists, a weak spring moves the piston, extending the strut and 
eliminating any clearance. This action sucks oil into the cylinder past a 
check valve. The trapped oil transmits the valve-opening forces with little 
deflection. A slight leakage of oil during lift shortens the strut, assuring 
valve closure. The leakage oil is replaced as the spring again extends the 
strut at no load. See VALVE TRAIN. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sHydrazine   �21m �33s 
    A colorless liquid, H2NNH2 (boiling point 114DGC or 237DGF), with a musty, 
ammonialike odor. Physically it is similar to water, but chemically it is 
reducing, decomposable, basic, and bifunctional. Its derivatives range from 
simple salts to ring compounds, polymers, and coordination complexes. Major 
uses of hydrazine include such diverse applications as rocket fuels, corrosion 
inhibition in boilers, syntheses of biologically active materials and in 
rubber curing and foam-rubber production. 
 �34s[Theodore H. Dexter]
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�6v�2v�32sHydride   �21m �33s 
    A compound containing hydrogen and another element. Many of the elements 
combine with hydrogen to form compounds, and even though hydrogen is less 
negative than the second element in the compound, they are called hydrides. 
For example, H2S is called a hydride in the discussion here, although it is 
usually called hydrogen sulfide and properly so. Hydrides can be divided into 
three classes of compounds: covalent hydrides, saltlike hydrides, and metallic 
hydrides.  
    Covalent hydrides refer to compounds such as H2O, H2S, and NH3, which are 
volatile. Covalent hydrides are formed from the nonmetals. Carbon hydrides 
would refer to natural gas, CH4, and other hydrocarbons containing chains of 
carbons. Saltlike hydrides are ionic in nature, and nonvolatile. Lithium 
hydride, LiH, is a typical example. Metallic hydrides include compounds with 
simple formulas such as uranium hydride, UH3, or palladium hydride, PdHsubx. 
Some of these compounds are thought to contain the hydrogen in the holes in 
the metallic lattice. See HYDROCARBON; HYDROGEN; METAL HYDRIDES. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sHydrido complexes   �21m �33s 
    Complex hydrides containing a hydride ligand bonded to a central atom. The 
prefix hydro instead of hydrido is sometimes used. All are soluble in aromatic 
hydrocarbons. They are rather expensive, but their specific reducing powers 
make them attractive for synthesizing high-value products, such as 
pharmaceuticals, flavorings, fragrances, dyes, and insecticides. See COMPLEX 
COMPOUNDS; METAL HYDRIDES. 
 �34s[James C. Warf]
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�6v�2v�32sHydroboration   �21m �33s 
    The process of producing organoboranes by the addition of diborane to 
unsaturated organic compounds. In ether solvents the addition of diborane to 
such molecules is exceedingly rapid and essentially quantitative. This 
reaction therefore makes the organoboranes readily available. Such 
organoboranes are finding increasing application as intermediates for organic 
synthesis.  
    Diborane is highly soluble in tetrahydrofuran, where it exists as the 
addition compound tetrahydrofuran-borane. Such solutions are often used for 
hydroboration, and merely involve bringing the two reactants together as 
indicated by reaction (1). 

*** 3RCH DOUBLE BOND CH sub 2 + [(C sub 4) (H sub 8) O] : BH sub 3 ->
        [(RCH sub 2 CH sub 2) (sub 3) B] + [(C sub 4) (H sub 8) O]         (1)
  
    Alternatively, sodium borohydride may be utilized to achieve hydroboration 
by the addition of boron trifluoride etherate. This is shown by reaction (2). 

     *** 12RCH DOUBLE BOND CH sub 2 + (3NaBH sub 4) + 
              4[(C sub 2 H sub 5) (sub 2) O] : BF sub 3 -> 
              4[(RCH sub 2 CH sub 2) (sub 3) B] + 
              (3NaBF sub 4) + 4[(C sub 2 H sub 5) (sub 2) O] ***           (2)

Usually the organoborane is not isolated but is utilized in place, similar to 
applications of the Grignard reagent in synthesis. 
    Organoboranes are among the most versatile synthetic intermediates that 
are available to the organic chemist. The hydroboration reaction has made 
these intermediates readily available. See BORANE; ORGANIC CHEMICAL SYNTHESIS; 
ORGANOMETALLIC COMPOUND. 
 �34s[Herbert C. Brown]
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�6v�2v�32sHydrocarbon   �21m �33s 
    One of a group of chemical compounds composed only of hydrogen and carbon. 
The very large variety of hydrocarbons can best be divided into three classes 
(aliphatic, alicyclic, and aromatic), each of which may be further divided 
into a number of subclasses. See ALICYCLIC HYDROCARBON; ALIPHATIC HYDROCARBON; 
AROMATIC HYDROCARBON. 
    Of the natural sources of hydrocarbons, the largest are natural gas and 
petroleum. The composition of natural gas varies greatly depending on the area 
from which it is obtained. 
    Petroleum is a complex mixture of liquid and solid hydrocarbons, its 
composition also varying with the source. Its chief components are paraffinic 
hydrocarbons, saturated five- and six-carbon atom alicyclic hydrocarbons, and 
aromatic hydrocarbons. Olefins are usually absent. See NATURAL GAS; PETROLEUM. 
    Hydrocarbons are also obtained by the destructive distillation or 
carbonization of coal, producing coal gas and coal tar. Coal gas contains 
methane together with smaller amounts of ethane, ethylene, benzene, toluene, 
cyclopentadiene, and naphthalene. The normally liquid or solid hydrocarbons in 
the gas (namely the cyclopentadiene, benzene, toluene, and napthalene) can be 
recovered.  
    The volatile oils obtained from certain plants (caraway or dill), trees 
(pine), and citrus fruits (lemon or orange) contain members of a large group 
of hydrocarbons known as terpenes. See TERPENE. 
 �34s[Louis Schmerling]
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�6v�2v�32sHydrocarbon resin   �21m �33s 
    Brittle or gummy materials prepared by the polymerization of several 
unsaturated constituents of coal tar, rosin, or petroleum. The hydrocarbon 
resins are inexpensive and find many uses in rubber and asphalt formulations 
and in coating and calking compositions. Important classes are the 
coumarone-indene resins, the petroleum resins, and the polyterpene resins. 
    Coumarone and indene occur together in coal tar fractions. Polymerization 
may be readily effected in the presence of sulfuric acid to yield soft to 
hard, brittle polymers. The soft products are used as plasticizers and 
tackifiers in calking formulations and rubber compositions. The hard products 
are used to stiffen and strengthen synthetic rubber, and they also are used in 
floor coverings, roofing materials, varnishes, and metal paints, and in 
combination with other resins. 
    Petroleum resins are derived from mixtures of polymerizable dienes, for 
example, cyclopentadiene, and unsaturated compounds obtained from 
gasoline-refining operations. The properties and uses of the resins are 
generally similar to those of the coumarone-indene resins. 
    Polyterpenes are derived from the ALPHA- and BETA-pinene in turpentine. 
They polymerize in the presence of acid catalysts with the formation of soft 
to hard polymers of molecular weights up to about 2000. The properties and 
uses of the polyterpenes are generally similar to those of the other 
hydrocarbon resins described above. They are used in oil paints and adhesive 
formulations. See POLYMERIZATION. 
 �34s[John A. Manson]
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�6v�2v�32sHydrocharitales   �21m �33s 
    An order of aquatic flowering plants, division Magnoliophyta 
(Angiospermae), in the subclass Alismatidae of the class Liliopsida 
(monocotyledons). The order consists of the single family Hydrocharitaceae, 
with about 100 species. Within the subclass the order is marked by its 
inferior, compound ovary with basically laminar placentation. The ovules are 
scattered over the walls of the individual carpels. The familiar aquarium 
plant, Vallisneria spiralis, called tape grass or eelgrass, is not a grass but 
belongs to the Hydrocharitaceae.  See  ALISMATIDAE; LILIOPSIDA; FLOWER. 
 �34s[Arthur Cronquist]
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�6v�2v�32sHydrocracking   �21m �33s 
    A catalytic, high-pressure process flexible enough to produce either of 
the two major light fuels - high octane gasoline or aviation jet fuel. It 
proceeds by two main reactions: adding hydrogen to molecules too massive and 
complex for gasoline and then cracking them to the required fuels. The process 
is carried out by passing oil feed together with hydrogen at high pressure 
(1000 - 2500 pounds per square inch gage or 7 - 17 megapascals) and moderate 
temperatures (500 - 750DGF or 260 - 400DGC) into contact with a bifunctional 
catalyst, comprising an acidic solid and a hydrogenating metal component. 
Gasoline of high octane number is produced, both directly and through a 
subsequent step such as catalytic reforming; jet fuels may also be 
manufactured simply by changing conditions with the same catalysts. 
    Generally, the process is used as an adjunct to catalytic cracking. Oils, 
which are difficult to convert in the catalytic process because they are 
highly aromatic and cause rapid catalyst decline, can be easily handled in 
hydrocracking, because of the low cracking temperature and the high hydrogen 
pressure, which decreases catalyst fouling. However, the most important 
components in any feed are the nitrogen-containing compounds, because these 
are severe poisons for hydrocracking catalysts and must be almost completely 
removed.  
    The products from hydrocracking are composed of either saturated or 
aromatics compounds; no olefins are found. In making gasoline, the lower 
paraffins formed have high octane numbers. The remaining gasoline has 
excellent properties as a feed to catalytic reforming, producing a highly 
aromatic gasoline which, with added lead, easily attains 100 octane number. 
Another attractive feature of hydrocracking is the low yield of gaseous 
components, such as methane, ethane, and propane, which are less desirable 
than gasoline.  See  GASOLINE. 
    The hydrocracking process is being applied in other areas, notably, to 
produce lubricating oils and to convert very asphaltic and high-boiling 
residues to lower-boiling fuels.  See  AROMATIZATION; CRACKING; HYDROGENATION; 
REFORMING IN PETROLEUM REFINING. 
 �34s[Charles P. Brewer]
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�6v�2v�32sHydrodynamics �21m �33s
    That branch of continuum mechanics which treats of the laws of motion of
an incompressible fluid and of the interactions of the fluid with its
boundaries. For more detail on specific parts of the subject  see  FLUID FLOW;
FLUID MECHANICS.
    Fluids have many physical properties, including, besides thermodynamic and
electromagnetic properties, the dynamical ones of density, viscosity,
adhesion, and cohesion. Only the dynamical properties will be considered in
this article, and furthermore, because capillary effects will not be treated,
the last two properties will be of interest only insofar as they affect the
inception of cavitation.
    A flow field can be studied from either of two points of view, the
lagrangian, in which one is concerned with the history of each fluid particle,
and the eulerian, in which interest is focused, rather, on the velocity vectors
associated with each point. See EULER'S EQUATIONS OF MOTION; LAGRANGE'S
EQUATIONS.
    A fruitful concept in hydrodynamics is that of the ideal of inviscid fluid.
In such a fluid, across any element of area, the fluid on one side exerts a
normal stress or pressure on the fluid on the other side, the magnitude of which
is independent of the orientation of the element. Thus there is a scalar
pressure field associated with the vector velocity field. A principle problem
of hydrodynamics is to determine these fields corresponding to prescribed
boundary conditions.
    When viscosity is taken into account, the normal stress across an area
element are not, in general, independent of orientation, and in addition,
tangential stresses are present. Instead of a a scalar pressure field one must
consider now a tensor stress field. These stresses, or their integrals, must
be determined in order to solve the important problem of obtaining the force
and moment acting on a body moving through the fluid. See NEWTONIAN FLUID;
VISCOSITY.
    Since the viscosity of the most common fluids, air and water, is very
small, it is an excellent approximation to treat their flows as inviscid except
at very low Reynolds numbers (Vl/v<10) and in the neighborhood of the
boundaries. Because the velocity of a fluid at a wall is zero relative to that
of the boundary (nonslip condition), a so-called boundary layer in which
viscous effects are important is present, within which appreciable shear
stresses occur. Outside of this boundary layer the flow may be treated as
inviscid and to a good approximation, the pressure across the boundary layer
may be assumed to be that in the inviscid flow at the outer edge of the
boundary layer.
    A simplified form of the Navier-Stokes equations, called the boundary-layer
equations, yields laminar-flow solutions for the velocity profiles. It is
known, however, that boundary-layer flows are often turbulent. A theory of the
laminar boundary layer has been developed and current reseearch is shedding
considerable light on the processes whereby a disturbed laminar flow eventually
breaks down into the typical random motions of turbulence, but the mechanisms
are not yet clear. See BOUNDARY-LAYER FLOW; LAMINAR FLOW; NAVIER-STOKES
EQUATIONS; TURBULENT FLOW.					     [L.L.]

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�6v�2v�32sHydroelectric generator   �21m �33s 
    An electric rotating machine that transforms mechanical power from a 
hydraulic turbine or water wheel into electric power. Hydroelectric generators 
may have horizontal or vertical shafts, depending upon the turbine. The most 
common type in large ratings is the vertical-shaft, synchronous generator. The 
hydroelectric generator may be arranged to operate as a motor during periods 
of low power demand. In such a case the turbine serves as a pump, raising 
water to a high elevation for reuse to generate power at a period of peak 
load. Such a unit is termed a pump turbine.  See  ELECTRIC POWER GENERATION; 
GENERATOR; HYDRAULIC TURBINE; TURBINE; WATERPOWER. 
 �34s[Leon T. Rosenberg]
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�6v�2v�32sHydrofoil craft   �21m �33s 
    Marine craft of generally conventional hull form (as in planing or 
displacement craft), which are supported above the water by dynamic lift 
produced by winglike surfaces (hydrofoils) mounted below the hull. At low 
speed, these craft operate in the same manner as conventional hulls. As speed 
increases, they ``take off'' and ``fly'' supported by the hydrofoils. The lift 
principle employed by these foils in water is the same as that of an airfoil 
in air, although application of the principle entails consideration of the 
additional complexities caused by the air-water interface.  See  AIRFOIL. 
    The advantages which have encouraged the development of hydrofoil craft 
are (1) increased efficiency, that is, a hydrofoil craft offers about half the 
resistance of a planing boat of similar weight and speed; and (2) increased 
seakeeping ability. 
    There are two major types of hydrofoils: surface-piercing, which attain 
their stability by hydrodynamically balancing the amount of foil area above 
and below the water surface; and submerged-foil systems with autopilot which, 
by means of a height sensor and gyros, control the height and attitude of the 
craft above the water. The latter, more complicated system was devised to 
counter the effect of the orbital motion of the wave particles, which 
adversely affects surface-piercing systems, particularly in a following sea. 
 �34s[Thomas M. Buermann]
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�6v�2v�32sHydroformylation �21m �33s
    An aldehyde synthesis process that falls under the general classification
of a Fischer-Tropsch reaction but is distinguished by the addition of an
olefin feed along with the characteristic carbon monoxide and hydrogen. In the
oxo process for alcohol manufacture, hydroformylation of olefins to aldehydes
is the first step. The second step is the hydrogenation of the aldehydes to
alcohols. At times the term ``oxo process'' is used in reference to the
hydroformylation step alone. In the hydroformylation step, olefin, carbon
monoxide, and hydrogen are reacted over a cobalt catalyst to produce an
aldehyde which has one more carbon atom than the feed olefin. The olefin
conversion takes place by the addition of a formyl group (CHO) and a hydrogen
atom across the double bond. This is represented by the following reaction.
See  FISCHER-TROPSCH PROCESS.
			     ******FORMULA******

    The aldehyde is then treated with hydrogen to form the alcohol. In
commercial operations, the hydrogenation step is usually performed immediately
after the hydroformylation step in an integrated system.
    A wide range of carbon number olefins, C2 - C16, have been used as feeds.
Propylene, heptene, and nonene are frequently used as feedstocks to produce
normal and isobutyl alcohol, isooctyl alcohol, and primary decyl alcohol,
respectively. Feed streams to oxo units may be single-carbon-number or
mixed-carbon-number olefins.
    The lower-carbon-number alcohols such as butanols are used primarily as
solvents, while the higher-carbon-number alcohols go into the manufacture of
plasticizers, detergents (surfactants), and lubricants.
 �34s[Donald L. Holt]
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�6v�2v�32sHydrogen   �21m �33s 
    The first chemical element in the periodic system. Under ordinary 
conditions it is a colorless, odorless, tasteless gas composed of diatomic 
molecules, H2. The hydrogen atom, symbol H, consists of a nucleus of unit 
positive charge and a single electron. It has atomic number 1 and an atomic 
weight of 1.00797. The element is a major constituent of water and all organic 
matter, and is widely distributed not only on the Earth but throughout the 
universe. There are three isotopes of hydrogen: protium, mass 1, makes up 
99.98% of the natural element; deuterium, mass 2, makes up about 0.02%; and 
tritium, mass 3, occurs in extremely small amounts in nature but may be 
produced artificially by various nuclear reactions.  See  DEUTERIUM; ISOTOPE; 
TRITIUM.  
  
    Uses.  The largest single use of hydrogen is in the synthesis of ammonia. 
A rapidly expanding use for hydrogen is in petroleum-refining operations, such 
as hydrocracking and hydrogen treatment for removal of sulfur. Large 
quantities of hydrogen are consumed in the catalytic hydrogenation of 
unsaturated liquid vegetable oils to make solid fats. Hydrogenation is used in 
the manufacture of organic chemicals. Large quantities of hydrogen are used as 
a rocket fuel, in conjunction with oxygen or fluorine, and as a propellant for 
nuclear-powered rockets. 
  
    Properties.  Ordinary hydrogen has a molecular weight of 2.01594. The gas 
has a density at 0DGC and 1 atm of 0.08987 g/liter. Its specific gravity, 
compared to air, is 0.0695. Hydrogen is the lightest substance known. Some 
additional properties of hydrogen are listed in the table. 
    Hydrogen is somewhat more soluble in organic solvents than in water. Many 
metals adsorb hydrogen. The adsorption of hydrogen in steel may cause 
``hydrogen embrittlement,'' which sometimes leads to the failure of chemical 
processing equipment. 
    At ordinary temperatures hydrogen is a comparatively unreactive substance 
unless it has been activated in some manner, for example, by a suitable 
catalyst. At elevated temperatures it is highly reactive. 
    Although ordinarily diatomic, molecular hydrogen dissociates at high 
temperatures into free atoms. Atomic hydrogen is a powerful reducing agent, 
even at room temperature. It reacts with the oxides and chlorides of many 
metals, including silver, copper, lead, bismuth, and mercury, to produce the 
free metals. It reduces some salts, such as nitrates, nitrites, and cyanides 
of sodium and potassium, to the metallic state. It reacts with a number of 
elements, both metals and nonmetals, to yield hydrides such as NaH, KH, H2S, 
and PH3. With oxygen atomic hydrogen yields hydrogen peroxide, H2O2. With 
organic compounds atomic hydrogen reacts to produce a complex mixture of 
products. With ethylene, C2H4, for example, the products include ethane, C2H6, 
and butane, C4H10. The heat liberated when hydrogen atoms recombine to form 
hydrogen molecules is used to obtain very high temperatures in atomic hydrogen 
welding.  
    Hydrogen reacts with oxygen to form water. At room temperature this 
reaction is immeasurably slow, but is accelerated by catalysts, such as 
platinum, or by an electric spark, and then may take place with explosive 
violence.  
    With nitrogen, hydrogen undergoes an important reaction to give ammonia. 
Hydrogen reacts at elevated temperatures with a number of metals to give 
hydrides. The oxides of many metals are reduced by hydrogen at elevated 
temperatures either to the free metal or to lower oxides. Hydrogen reacts at 
room temperature with the salts of the less electropositive metals and reduces 
them to the metallic state. In the presence of a suitable catalyst hydrogen 
reacts with unsaturated organic compounds and adds to the double bond.  See  
HYDROGENATION.  
  
    Principal compounds.  Hydrogen is a constituent of a very large number of 
compounds containing one or more other elements. Such compounds include water, 
acids, bases, most organic compounds, and many minerals. Compounds in which 
hydrogen is combined with a single other element are commonly referred to as 
hydrides. For additional details on the compounds of hydrogen  see  ACID AND 
BASE; HYDRAZINE; HYDRIDE; HYDRIDO COMPLEXES; HYDROCARBON; HYDROGEN FLUORIDE; 
HYDROGEN PEROXIDE; WATER. 
  
    Preparation.  A large number of methods may be used to prepare hydrogen 
gas. The choice of method is determined by such factors as the quantity of 
hydrogen desired, the purity required, and the availability and cost of raw 
materials. Among the processes frequently used are the reactions of metals 
with water or acids, the electrolysis of water, the reaction of steam with 
hydrocarbons or other organic materials, and the thermal decomposition of 
hydrocarbons. The principal raw materials for hydrogen production are 
hydrocarbons, such as natural gas, oil refinery gas, gasoline, fuel oil, and 
crude oil. 
 �34s[Louis Kaplan]
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�6v�2v�32sHydrogenation   �21m �33s 
    The chemical reaction of hydrogen with another substance, generally an 
unsaturated organic compound, and usually under the influence of temperature, 
pressure, and catalysts. There are several types of hydrogenation reactions. 
They include: (1) the addition of hydrogen to reactive molecules; (2) the 
incorporation of hydrogen accompanied by cleavage of the starting molecules 
(hydrogenolysis); and (3) reactions in which isomerization, cyclization, and 
so on, result. 
    Hydrogenation is synonymous with reduction in which oxygen or some other 
element (most commonly nitrogen, sulfur, carbon, or halogen) is withdrawn 
from, or hydrogen is added to, a molecule. When hydrogenation is capable of 
producing the desired reduction product, it is generally the simplest and most 
efficient procedure. 
    Hydrogenation is used extensively in industrial processes. Important 
examples are the synthesis of methanol, liquid fuels, hydrogenated vegetable 
oils, fatty alcohols from the corresponding carboxylic acids, alcohols from 
aldehydes prepared by the aldol reaction, cyclohexanol and cyclohexane from 
phenol and benzene, respectively, and hexamethylenediamine for the synthesis 
of nylon from adiponitrile.  See  DEHYDROGENATION; FISCHER-TROPSCH PROCESS; 
HYDROFORMYLATION; HYDROGEN; OXIDATION-REDUCTION. 
 �34s[Roberto Lee]
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�6v�2v�32sHydrogen bomb �21m �33s
    A device in which an uncontrolled, self-sustaining thermonuclear fusion
reaction is carried out in heavy hydrogen (deuterium) gas to produce an
explosion. The hydrogen bomb is a fusion bomb. In a fusion reaction, the
collision of two energy-rich nuclei results in a mutual rearrangement of their
protons and neutrons to produce two or more reaction products, together with a
release of energy.  See  INERTIA OF ENERGY; NUCLEAR FUSION; THERMONUCLEAR
REACTION.
    For the hydrogen bomb reaction to become self-sustaining, a so-called
critical temperature of about 3.5 x 10 sup 7 K must be attained, possibly with
the aid of the enormous temperatures developed by the explosion of a fission
bomb. Once this temperature is achieved, the energy released in the initial
reactions maintains the temperature, and the chain proceeds either until the
supply of fusionable material is exhausted, or until sufficient expansion has
taken place that the gas is cooled below the critical temperature.  See
ATOMIC BOMB; CHAIN REACTION (PHYSICS); NUCLEAR EXPLOSION.
 �34s[Kenneth W. Perkins]
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�6v�2v�32sHydrogen bond   �21m �33s 
    The interaction which occurs when a hydrogen atom, covalently bonded to an 
electronegative atom (as in A - H), interacts with another atom to form the 
aggregate A - H . . . Y. The shortest and strongest bond is indicated as A - 
H, while the secondary and weaker interaction is written as H . . . Y. Thus A 
- H is a proton donor, while (Y) is a proton acceptor which often contains 
lone pair electrons and can act as a base. The strongest hydrogen bonds are 
formed between the most electronegative (A) atoms such as fluorine, nitrogen, 
and oxygen which interact with (Y) atoms having electronegativity greater than 
that of hydrogen (C, N, O, S, Se, F, Cl, Br, I). The weakest of hydrogen bonds 
are formed by acidic protons of C - H groups, as in chloroform and acetylene, 
and by olefinic and aromatic PI-electrons acting as (Y). 
    The weaker the hydrogen bond, the shorter the lifetime of the complex it 
forms. An important aspect of weak hydrogen bond formation is that the 
different molecular aggregates which do form can be easily and reversibly 
transformed. Thus the small energy changes resulting in the rapid making and 
breaking of hydrogen bonds in biological systems are of great importance; for 
example, hydrogen bonding determines the configuration of the famous 
ALPHA-helix of DNA, and the structures of most proteins, thereby serving an 
important function in determining the nature of all living things.  See  
CHEMICAL BONDING; DEOXYRIBONUCLEIC ACID (DNA); PROTEIN. 
 �34s[Jack M. Williams]
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�6v�2v�32sHydrogen electrode   �21m �33s 
    A noble metal of large surface area covered with hydrogen gas in a 
solution of hydrogen ion saturated with hydrogen gas. The hydrogen gas may 
also be bubbled continuously over the noble metal; in this case, the affluent 
gas must be saturated with water vapor. Platinum is generally used as the 
noble metal. A large surface area is achieved by plating the surface with 
platinum sponge in a solution of chloroplatinic acid. In general, the noble 
metal is used in foil form and is welded to a wire sealed in the bottom of a 
hollow glass tube which is partially filled with mercury. Electrical contact 
to an external circuit is made through the mercury usually by a copper wire 
(see illustration). 
    The hydrogen electrode is reversible to hydrogen ion and its electric 
potential is a logarithmic function of hydrogen-ion activity. Because of this 
relation, this electrode is used to measure hydrogen-ion activity. It cannot 
be used in reducing or oxidizing solutions or in solutions that poison the 
surface of the noble metal. Accordingly, it has limited application in the 
measurements of hydrogen-ion activity of solutions. 
    When the hydrogen-ion activity is unity and the hydrogen-gas pressure is 
at 1 atm (101 kilopascals), the potential of the hydrogen electrode is 
conventionally assigned a value of zero at all temperatures, and is used as a 
reference or standard electrode to which the potentials of all other 
electrodes are referred. All electrodes having a positive potential relative 
to the standard hydrogen electrode have greater reducing power than hydrogen; 
conversely, those of lower potential have a lower reducing power. Elements 
arranged in a series of their reducing power constitute what is called the 
electromotive-force series of the elements.  See  ELECTROCHEMICAL SERIES; 
ELECTRODE POTENTIAL; HYDROGEN ION. 
 �34s[Walter J. Hamer]
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�6v�2v�32sHydrogen fluoride   �21m �33s 
    The hydride of fluorine and the first member of the family of halogen 
acids. Anhydrous hydrogen fluoride is a mobile, colorless liquid that fumes 
strongly in air. It has the empirical formula HF, melts at -83DGC, and boils 
at 19.8DGC. The vapor is highly aggregated, and gaseous hydrogen fluoride 
deviates from perfect gas behavior to a greater extent than any other gaseous 
substance known. Aggregate formation in both the vapor and liquid phase arises 
from unusually strong hydrogen-bond interactions.  See  HYDROGEN BOND. 
    Anhydrous hydrogen fluoride is an extremely powerful acid, exceeded in 
this respect only by 100% sulfuric acid. Because anhydrous hydrogen fluoride 
is a superacid, many organic solutes dissolve in it to form stable carbonium 
ions. Alkali metal fluorides and silver fluoride dissolve readily in hydrogen 
fluoride to form conducting solutions. Anhydrous hydrogen fluoride dissolves a 
wide variety of organic compounds. Aqueous solutions of hydrogen fluoride 
(hydrofluoric acid) are relatively weakly acidic as compared to hydrochloric 
acid.  
    Hydrogen fluoride is a widely used industrial chemical. The largest use is 
in making fluorine-containing refrigerants (Freons, Genetrons). An 
increasingly important use of hydrogen fluoride is in the preparation of 
organic fluorocarbon compounds. 
    Both hydrogen fluoride and hydrofluoric acid cause unusually severe burns; 
appropriate precautions must be taken to prevent any contact of the skin or 
eyes with either the liquid or the vapor.  See  FLUORIDE; FLUORINE; 
HALOGENATED HYDROCARBON. 
 �34s[Joseph J. Katz]
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�6v�2v�32sHydrogen ion �21m �33s
    A proton combined with a number of water molecules. It is often written as
H3O sup + and called the hydronium ion. However, this species is best considered
as an excess proton on a tetrahedral group of four water molecules and so
would be designated as H9O4 sup +. For simplicity, it is most commonly written
as H sup + (aq).  See  PROTON.
    Since it is formed by the self-ionization of water, the hydrogen ion is
present in all aqueous solutions. This formation also means that H sup +(aq) is
always found in the company of the hydroxide ion. OH-(aq). The equilibrium
relationship between the concentrations of these two species is a very
important property of water. See IONIC EQUILIBRIUM.
    The H sup +(aq) and OH-(aq) concentrations in pure water are equal to each
other with a value of 10- sup 7 mole/liter. Any aqueous solution with this
concentration of H sup +(aq) is called a neutral solution. If the H sup + (aq)
concentration is greater than 10- sup 7 mole/liter, the solution is called
acidic. Basic solutions are those in which the H sup + (aq) concentration is
less than 10- sup 7 mole/liter. As the H sup + (aq) concentration increases the
OH-(aq) concentration must decrease, and vice versa. In the most straightforward
system, acids are substances that can donate an H sup +(aq), and bases are
substances that can accept one.  See  ACID AND BASE.
    Hydrogen ion concentration determines the course of many chemical
reactions that occur in living organisms and in the chemical industry. The
control of hydrogen ion concentration is achieved in living organisms and in
the laboratory by buffer systems. These are chemical mixtures designed to
resist change in hydrogen ion concentration.  See  BUFFERS (CHEMISTRY).
    Another property of the H sup + (aq) is important in both theoretical and
practical ways. H sup + (aq) is the best conductor of electricity of any ion in
aqueous solution. Its conductance at 77DGF (25DGC) is almost five times as
large as the next-most-conducting ion.	See  ELECTROLYTIC CONDUCTANCE;
HYDROGEN ELECTRODE.
    The hydrogen ion concentration can vary over fourteen powers of 10. To
avoid dealing with such exponentials, the concept of pH is used. Since all
aqueous solutions contain both hydrogen ion and hydroxide ion, it is possible
to define all degrees of acidity and basicity on the pH scale.	See  PH.
    Two general methods are used for the determination of hydrogen ion
concentrations. For relatively crude work, colorimetric methods are commonly
used. These methods depend on the fact that certain natural and synthetic dyes
have colors that depend on the hydrogen ion concentration. At times, paper is
impregnated with such an indicator. In most precise work, a potentiometric
method is used for the determination of hydrogen ion concentration. This
method depends on an electrode whose potential is sensitive to hydrogen ion
concentration. The only electrode commonly in use for practical pH
measurements is the glass electrode.  See  ACID-BASE INDICATOR; COLORIMETRY;
ION SELECTIVE MEMBRANES AND ELECTRODES; TITRATION; VOLUMETRIC ANALYSIS.
 �34s[Gordon Atkinson]
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�6v�2v�32sHydrogen maser clock �21m �33s
    A type of atomic clock which employs maser oscillation of hydrogen atoms
in a microwave cavity. In this active clock, hydrogen molecules are
dissociated into a beam of hydrogen atoms. Those atoms in the upper hyperfine
state are then focused into a Teflon-lined quartz bulb inside a cavity
resonant at the hyperfine frequency of 1420 megahertz. The Teflon coating
allows the atoms to bounce off the walls without much perturbation. As a
result of this and the small size of the entrance hole, the atoms can be
confined in the bulb for a long time, resulting in interaction times with the
exciting radiation of the order of a second. This produces a very narrow line
width. If the cavity losses are sufficiently low, maser oscillations take
place. The hydrogen maser oscillator is the most stable oscillator known.
Since the atoms do make collisions with the wall, there is a slight pulling
from the free atom frequency. This results in an uncertainty of only about 2
parts in 10 sup 1 sup 2, so that the hydrogen maser makes an excellent clock. By
reducing the hydrogen flux below the threshold for oscillation and providing
1420 MHz excitation, the apparatus can also be used as a passive clock. This
greatly reduces frequency pulling effects due to cavity tuning changes and
thus reduces the requirements for precise cavity temperature control at the
expense of losing the spectral purity of the maser oscillation. See ATOMIC
CLOCK; MASER.
 �34s[Leonard S. Cutler]
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�6v�2v�32sHydrogen peroxide   �21m �33s 
    A binary compound of hydrogen and oxygen, empirical formula H2O2, used 
mostly in dilute aqueous solutions as an oxidizing agent. Its most remarkable 
feature is its tendency to decompose readily into water and oxygen. 
    Anhydrous hydrogen peroxide is a clear, colorless liquid, of nearly the 
same viscosity and dielectric constant as water, but of greater density. Like 
water, it is strongly associated through hydrogen bonds. It boils at 150DGC 
(300DGF) with violent, sometimes explosive decomposition. Decomposition by 
light begins only in the near ultraviolet. As a solvent, hydrogen peroxide 
resembles water, except that acids and bases show much lower electrical 
conductivity. Although a fairly strong oxidant, it can act as a mild reducing 
agent, for example, with permanganates and perchromates. 
    Hydrogen peroxide is used mainly for bleaching cotton and other fibers, 
natural or synthetic. Increasing amounts are used in the pulp and paper 
industry. Its well-known cosmetic use as hair bleach consumes relatively 
little of the commercial 10% (30 volume) solution. In medicine it is useful 
for cleansing wounds and cuts, although its antiseptic action is rather slow. 
A limited but important use of the concentrated peroxide is for energy 
production in rockets, submarines (during submersion), airplanes (at takeoff), 
and the steering of space vessels.  See  BLEACHING; CHEMICAL FUEL. 
    Hydrogen peroxide, especially when concentrated, requires great care in 
handling and storing. When dropped on paper or wood, it can start a fire. 
Contact with the skin causes blotches that can be painful, but they disappear 
after a few hours without leaving traces.  See  HYDROGEN; OXYGEN; PEROXIDE. 
 �34s[Paul A. Giguere]
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�6v�2v�32sHydrography   �21m �33s 
    Now primarily the mapping of the ocean bottoms and the measuring of ocean 
currents; once broadly used to apply to studies of surface water of the Earth 
including measuring flow of streams and mapping of streams and lakes, as well 
as investigations of the ocean. The broader fields of hydrology and 
oceanography have absorbed much of the earlier scope of hydrography.  See  
HYDROLOGY.  
    Today the term is used mainly with respect to charting oceans and 
estuaries and coastal waters for navigational purpose. Every maritime nation 
of any importance has established a hydrographic office as one of its regular 
bureaus.  See  HYDROLOGY; OCEANOGRAPHY. 
 �34s[Ray K. Linsley]
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�6v�2v�32sHydroida   �21m �33s 
    An order of the coelenterates which includes the fresh-water hydras, the 
attached and usually colonial hydroids, and many of the smaller jellyfish. It 
is the largest order of the class Hydrozoa. 
    The order Hydroida includes two principal suborders, Gymnoblastea and 
Calyptoblastea. The Gymnoblastea are those hydroids which lack protective cups 
around the hydranths and gonozoids. Jellyfish produced by these athecate 
hydroids are Anthomedusae. The Calyptoblastea include the hydroids with 
protective cups around the hydranths (hydrothecae) and around the gonozoids 
(gonothecae). Jellyfish of these thecate hydroids are called Leptomedusae. Two 
minor suborders are Limnomedusae and Chondrophora. 
    Anthomedusae are typically ovoid jellyfish, often with eyespots. 
Leptomedusae are usually flattened or saucer-shaped, have statocysts (sense 
organs of balance), and lack eyespots. The gonads of Anthomedusae are 
generally on the wall of the stomach just above the mouth, and those of 
Leptomedusae below the radial canals. 
    Young hydranths of gymnoblastic hydroids are small with five or more 
tentacles, but subsequently grow much larger and add more tentacles. 
Calyptoblastic hydranths, in contrast, emerge from a bud with a full 
complement of parts; they do not grow, live for only about a week, undergo 
regression and absorption by the colony, and are then replaced by new 
hydranths.  
    The fresh-water hydras are simple, motile polyps which do not produce 
colonies (see illustration). Buds separate from the parent and become 
individual polyps. Simple gonads develop on the body and there is no medusa. 
Hydras are sometimes included in the Gymnoblastea and sometimes placed in a 
suborder by themselves, the Hydrida. 
    Hydroids are species in which the polyp stage is usually dominant. Most 
hydroids are found near the shore attached to various supports such as rocks, 
wharves, boats, mussels, barnacles, worm tubes, crab and snail shells, and 
seaweeds. Sometimes the medusa stage is well developed and the hydroid stage 
may be lacking. Medusae are abundant both in coastal waters and in the open 
sea.  
    Hydras and hydroids have been used extensively in research on problems of 
growth, development, and regeneration. They have a high capacity for 
reorganization. Missing parts are quickly replaced, a bit of stem can produce 
a new hydranth, and completely disorganized masses of cells can reconstitute a 
new polyp.  See  HYDROZOA; REGENERATION (BIOLOGY). 
 �34s[Sears Crowell]
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�6v�2v�32sHydrokinematics   �21m �33s 
    Motion of a liquid apart from the cause of the motion. A liquid is treated 
as a continuum, the elements of which move along continuous paths. At any 
instant the flow pattern may be delineated by a family of streamlines which 
are everywhere tangent to the paths, but do not in general coincide with them 
unless the flow is steady. Complete specification of a flow field requires 
that the distributions of sources and vorticity be known.  See  FLUID FLOW; 
HYDRODYNAMICS; STOKES STREAM FUNCTION. 
 �34s[Louis Landweber]
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�6v�2v�32sHydrokinetics   �21m �33s 
    Forces produced by a liquid as a consequence of its motion. The mass of a 
liquid gives rise to inertia forces which are manifested as normal stresses or 
pressures when the liquid flows. The viscosity and cohesion of a liquid give 
rise to tangential shear forces and also affect the normal stresses. The 
interaction of these properties on the motion of the fluid is formally 
expressed in the equations of Navier and Stokes. Because of the difficulty of 
solving these equations in general, approximate solutions are often obtained 
by neglecting the less important properties in a particular flow situation.  
See  FLUID FLOW; HYDRODYNAMICS. 
 �34s[Louis Landweber]
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�6v�2v�32sHydrology   �21m �33s 
    The science that treats of the waters of the Earth: their occurrence, 
circulation, and distribution; their chemical and physical properties; and 
their reaction with the environment, including their relation to living 
things. The domain of hydrology embraces the full life history of water on the 
Earth.  
    Water in liquid and solid form covers most of the crust of the Earth. By a 
complex process powered by gravity and the action of solar energy, an endless 
exchange of water, in vapor, liquid, and solid forms, takes place between the 
atmosphere, the oceans, and the crust. Water circulates in the air and in the 
oceans as well as over and below the surface of land masses. This global 
scheme is called the hydrologic cycle. Although, general patterns of 
circulation are present in the atmosphere, the oceans, and the land masses, 
regional features are very irregular and random in their detail. Therefore, 
while causal relations underlie the overall process, important elements of 
chance affect local hydrological events.  See  ATMOSPHERIC GENERAL 
CIRCULATION; EVAPOTRANSPIRATION; TERRESTRIAL WATER. 
    Water is essential for all living things. Its adequate supply is a key 
factor for urban, agricultural, and industrial development. Water can also be 
the recipient of pollutants which degrade its quality for all uses, and may be 
a destructive agent when it inundates valleys and causes death and great 
damage. The rising of groundwater levels in agricultural lands may cause 
deterioration of the soils and loss of fertility by waterlogging and increased 
salinity. Erosion of soil by flowing waters, and ultimate deposition of the 
sediment in lakes, reservoirs, stream channels, and harbors are also serious 
problems. Thus, the means whereby natural waters may be captured and 
controlled are of utmost importance for the development of human economy. The 
study of hydrology provides the information necessary for determining those 
means.  See  EROSION; WATER CONSERVATION; WATER POLLUTION. 
    A number of field measurements are performed for hydrologic studies. Among 
them are the amount and intensity of precipitation; the quantities of water 
stored as snow and ice, and their changes in time; discharge of streams; rates 
and quantities of infiltration into the soil, and movement of soil moisture; 
rates of production from wells and changes in their water levels as indicators 
of groundwater storage; dissolved chemical elements and compounds in surface 
and ground waters; amounts of water transferred by evaporation and 
evapotranspiration to the atmosphere from snow, lakes, streams, soils, and 
vegetation; and sediment lost from the land and transported by streams. In 
addition, hydrology is concerned with research on the phenomena and mechanisms 
involved in all physical and biological components of the hydrogeologic cycle, 
with the purpose of understanding them sufficiently to permit quantitative 
predictions and forecasting.  See  GROUNDWATER HYDROLOGY; HYDROMETEOROLOGY; 
HYDROSPHERE; SNOW HYDROLOGY; SURFACE WATER. 
 �34s[Jaime Amorocho]
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�6v�2v�32sHydrolysis   �21m �33s 
    Literally destruction, decomposition, or alteration of a chemical 
substance by water. In discussions of aqueous solutions of electrolytes the 
term hydrolysis is applied especially to reactions of cations (positive ions) 
with water to produce a weak base, or of anions (negative ions) to produce a 
weak acid. A salt of a weak acid or of a weak base, or of both a weak acid and 
weak base, is then said to be hydrolyzed. The degree of hydrolysis is the 
fraction of the ion which reacts with the water. The term solvolysis is 
employed for reactions of solutes with solvents in general. 
    A frequently quoted example is the hydrolysis of sodium acetate, which is 
highly dissociated in water, to yield sodium ion, Na sup POSITIVE, and acetate 
ion, CH sub 3 CO sub 2 NEGATIVE. The acetate ion is represented below by the 
symbol A NEGATIVE. Because acetic acid is a weak acid (it has a small 
dissociation constant), the acetate ion, A NEGATIVE, combines with some of the 
hydrogen ion present in aqueous solutions, as shown in reaction (1). 
  
  *** A NEGATIVE + (H sub 3) O POSITIVE EQUILIBRIUM HA + (H sub 2) O ***  (1)
  
    The concentration of the hydrogen ion is involved in the equilibrium, 
reaction (2), between water and the hydrogen ion and the hydroxide ion.        
  
   *** 2 (H sub 2) O EQUIVALENT (H sub 3) O POSITIVE + OH NEGATIVE ***    (2)
  
Because reaction (1) removes a product of reaction (2), each reaction proceeds 
to produce more of its products (the substances on the right side of each 
equation). The net result is reaction (3), which represents the reaction which
  
      *** A NEGATIVE + (H sub 2) O EQUILIBRIUM HA + OH NEGATIVE ***       (3)
  
is described as the hydrolysis of the acetate ion. When equilibrium is
attained, the concentrations are such as to conform to the equilibrium
constants of all three reactions (one of which is a mathematical consequence of
the other two). See ACID AND BASE; HYDROGEN ION; HYDROLYTIC PROCESSES; IONIC
EQUILIBRIUM. 
 �34s[Thomas F. Young]
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�6v�2v�32sHydrolytic processes   �21m �33s 
    Reactions of both organic and inorganic chemistry wherein water effects a 
double decomposition with another compound, hydrogen going to one component, 
hydroxyl to another, as in reactions (1) - (3). Although the word "hydrolysis"

                    *** XY + H2O -> HY + XOH ***                            (1)
                   *** KCN + H2O -> HCN + KOH ***                           (2)
*** [(C sub 5) (H sub 11) Cl] + H2O -> HCI + [(C sub 5) (H sub 11) OH] ***  (3)
  
means decomposition by water, cases in which water brings about effective 
hydrolysis unaided are rare, and high temperatures and pressures are 
usually necessary. See HYDROLYSIS. 
    Hydrolytic reactions may be classified as follows: (1) hydrolysis with 
water alone; (2) hydrolysis with dilute or concentrated acid; (3) hydrolysis 
with dilute or concentrated alkali; (4) hydrolysis with fused alkali with 
little or no water at high temperature. 
    In the field of organic chemistry, the term ``hydrolysis'' has been 
extended to cover the numerous reactions in which alkali or acid is added to 
water. An example of an alkaline-condition hydrolytic process is the 
hydrolysis of esters, reaction (4), to produce alcohol. An example of an 

          *** [(CH sub 3) (COOC sub 2) (H sub 5)] + NaOH -> 
              [(CH sub 3) (COONa)] + [(C sub 2) (H sub 5) OH] ***           (4)
  
acidic-condition process is the hydrolysis of olefin to alcohol in the presence 
of phosphoric acid, reaction (5). The addition of acids or alkalies hastens     
  
         *** [(C sub 2) (H sub 4)] + H2O -> 
                    [REACTION CONDITIONS = (H sub 3) (PO sub 4)]
                                     [(C sub 2) (H sub 5) OH] ***           (5)
 
such reactions even if it does not initiate the reaction. See ALCOHOLYSIS.  
    Perhaps some of the oldest and largest-volume hydrolysis technology is 
involved in soap manufacture. In the first step, glyceryl stearate acid, a 
fat, is hydrolyzed with water to yield stearic acid and glycerin. In the 
second step, the stearic acid is neutralized with caustic soda to give sodium 
stearate, the soap, and water. See FAT AND OIL; SAPONIFICATION. 
    Hydrolytic processes account for a huge product volume. Conversion of 
starch such as corn starch into maltose and glucose (sugar syrups) by 
treatment with hydrochloric acid is a major industry. Similarly, the 
production of furfural from pentosans of oat hulls or other cereal by-products 
such as corn cobs, rice hulls, or cottonseed bran is another commercial 
hydrolytic process. 
 �34s[Donald L. Holt]
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�6v�2v�32sHydromechanics   �21m �33s 
    That branch of physics which deals with liquids, traditionally water, as a 
medium for the transmission of forces. A body immersed in a liquid experiences 
a vertical force proportional to the volume of liquid that it displaces. A 
fluid whose velocity changes produces forces as a result of changing pressure 
and of changing momentum. Such phenomena are applied in various machines.  See 
  ARCHIMEDES' PRINCIPLE; BERNOULLI'S THEOREM; BUOYANCY; HYDRAULIC PRESS; 
HYDRAULIC TURBINE; HYDRAULICS; HYDROSTATICS. 
 �34s[William Allan]
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�6v�2v�32sHydrometallurgy   �21m �33s 
    The extraction and recovery of metals from their ores by processes in 
which aqueous solutions play a predominant role. Two distinct processes are 
involved in hydrometallurgy: putting the metal values in the ore into solution 
via the operation known as leaching; and recovering the metal values from 
solution, usually after a suitable solution purification or concentration 
step, or both. The scope of hydrometallurgy is quite broad and extends beyond 
the processing of ores to the treatment of metal concentrates, metal scrap and 
revert materials, and intermediate products in metallurgical processes. 
Hydrometallurgy enters into the production of practically all nonferrous 
metals and of metalloids, such as selenium and tellurium. Hydrometallurgical 
and pyrometallurgical processes complement each other.  See  LEACHING; 
PYROMETALLURGY.  
    Hydrometallurgy occupies an important role in the production of aluminum, 
copper, nickel, cobalt, zinc, gold, silver, platinum metals, selenium, 
tellurium, tungsten, molybdenum, uranium, zirconium, and other metals.  See  
METALLURGY.  
 �34s[W. Charles Cooper]
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�6v�2v�32sHydrometeorology   �21m �33s 
    The study of the occurrence, movement, and changes in the state of water 
in the atmosphere. The term is also used in a more restricted sense, 
especially by hydrologists, to mean the study of the exchange of water between 
the atmosphere and continental surfaces. This includes the processes of 
precipitation and direct condensation, and of evaporation and transpiration 
from natural surfaces. Considerable emphasis is placed on the statistics of 
precipitation as a function of area and time for given locations or geographic 
regions.  
    Water occurs in the atmosphere primarily in vapor or gaseous form. The 
average amount of vapor present tends to decrease with increasing elevation 
and latitude and also varies strongly with season and type of surface. 
Precipitable water, the mass of vapor per unit area contained in a column of 
air extending from the surface of the Earth to the outer extremity of the 
atmosphere, varies from almost zero in continental arctic air to about 6 
g/cmsup2 in very humid, tropical air. 
    Although a trivial proportion of the water of the globe is found in the 
atmosphere at any one instant, the rate of exchange of water between the 
atmosphere and the continents and oceans is high. Evaporation from the ocean 
surface and evaporation and transpiration from the land are the sources of 
water vapor for the atmosphere. Water vapor is removed from the atmosphere by 
condensation and subsequent precipitation in the form of rain, snow, sleet, 
and so on. The amount of water vapor removed by direct condensation at the 
Earth's surface (dew) is relatively small.  See  EVAPOTRANSPIRATION; 
HYDROLOGY; METEOROLOGY; PRECIPITATION (METEOROLOGY). 
 �34s[Eugene M. Rasmusson]
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�6v�2v�32sHydrometer   �21m �33s 
    A direct-reading instrument for indicating the density, specific gravity, 
or some similar characteristic of liquids. Almost all hydrometers are made of 
a high-grade glass tubing. The main body is the float section in the bottom of 
which ballast, such as small shot, is secured. A small-diameter tube, the 
stem, extends from the upper end of the float section. Inside the stem is the 
scale, printed on heavy-grade paper, and well-secured within the stem so its 
position will not change. When the hydrometer is placed in a liquid, the stem 
extends vertically above the surface for a portion of its length. 
    Hydrometers may be classified according to the indication provided by 
graduations of the scale as follows: (1) density hydrometers, to indicate 
densities at a particular temperature, and usually for a particular liquid; 
(2) specific gravity hydrometers to indicate specific gravity of a liquid, 
with reference to water, at a particular temperature; (3) percentage 
hydrometers to indicate, at a particular temperature, the percentage of a 
substance such as salt, sugar, or alcohol dissolved in water (alcoholometers 
are an example); and (4) arbitrary scale hydrometers, indicating the density, 
specific gravity, or concentration of a liquid in terms of an arbitrarily 
defined scale, at a defined temperature. The last group includes the 
saccharimeter (indicates percentage of pure sucrose solutions); the Baume 
hydrometer (measures specific gravity of liquids lighter than water); the 
lactometer (tests milk); and the barkometer (tests tanning extracts).  See  
DENSITY MEASUREMENT; SPECIFIC GRAVITY. 
 �34s[Howard S. Bean]
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�6v�2v�32sHydrophone   �21m �33s 
    A device which receives underwater sound waves and converts them to 
essentially equivalent electric waves. A hydrophone is the underwater analog 
of a microphone. Hydrophones are used in sonar apparatus, sonobuoys, and 
certain underwater weapons.  See  ACOUSTIC MINE; ACCOUSTIC TORPEDO; 
MICROPHONE; SONAR; SONOBUOY; UNDERWATER SOUND. 
 �34s[Harry F. Olson]
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�6v�2v�32sHydroponics   �21m �33s 
    One of several popular names given to the artificial culture of plants in 
inorganic salt solutions in open concrete tanks or other containers. The 
technique was devised in the mid-19th century as a means of studying mineral 
nutrients essential to the growth and development of plants, but for many 
years was only a laboratory procedure. 
    Mineral solution cultures are fully adequate for maximal growth if the 
essentials for plant development are provided, including light, air, favorable 
temperature, and a support for the plants. The size of such cultures has been 
increased and adapted to crop production. Means of root support have been 
provided by sand, gravel, and cinders; adequate aeration is maintained by 
pumps, by frequent drainage, or in other ways. Under these conditions, it is 
possible to obtain excellent crops in places where conventional methods of 
agriculture or gardening are unsuitable or impossible. Economic aspects of 
artificial culture, however, must be compared with ordinary soil cultivation 
in order to judge the feasibility of the soilless practice at a particular 
location. It is doubtful that nutriculture crop production will in a large 
measure supplant the usual agriculture method. 
 �34s[Theodore C. Broyer]
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�6v�2v�32sHydroquinone   �21m �33s 
    A dihydric phenol (C6H6O2; colorless crystals, melting point 169DGC or 
336DGF) in which the two hydroxyl groups are in the para or 1,4 positions. The 
important relationship between hydroquinone and p-benzoquinone (commonly 
called simply quinone) is illustrated in the reaction below. 
    The facile oxidation of hydroquinone is the basis for the most important 
uses of the compound and its derivatives. As a photographic developer, 
hydroquinone reacts with light-activated silver bromide crystals to form black 
metallic silver (reduction product) and quinone (oxidation product). Various 
alkylated derivatives of hydroquinone and the monomethyl ether 
(hydroxyanisole) find use as preservatives and antioxidants.  See  PHENOL. 
 �34s[Martin Stiles]
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�6v�2v�32sHydrosphere   �21m �33s 
    Approximately 74% of the Earth's surface is covered by water, in either 
the liquid or solid state. The following waters, combined with minor 
contributions from groundwaters, constitute the hydrosphere: world oceans, 
fresh-water lakes, saline lakes and inland seas, rivers, soil moisture and 
vadose water, groundwater to depth of 4000 m, and icecaps and glaciers. 
    The oceans account for about 97% of the weight of the hydrosphere, while 
the amount of ice reflects the Earth's climate, being higher during periods of 
glaciation. The circulation of the waters of the hydrosphere results in the 
weathering of the landmasses. The rainwater falling on the continents, partly 
taken up by the ground and partly by the streams, acts as an erosive agent 
before returning to the seas. 
    The unique chemical properties of water make it an effective solvent for 
many gases, salts, and organic compounds. Circulation of water and the 
dissolved material it contains is a highly dynamic process driven by energy 
from the Sun and the interior of the Earth. Each component has its own 
geochemical cycle or pathway through the hydrosphere, reflecting the 
component's relative abundance, chemical properties, and utilization by 
organisms. The introduction of materials by humans has significantly altered 
the composition and environmental properties of many natural waters.  See  
EROSION; GROUNDWATER HYDROLOGY; HYDROLOGY; LAKE; RIVER; SEA WATER; TERRESTRIAL 
WATER.  
 �34s[Edward D. Goldberg]
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�6v�2v�32sHydrostatics   �21m �33s 
    The study of liquids at rest. In the absence of motion, there are no shear 
stresses; the internal state of stress at any point is determined by pressure 
alone. Hence, the pressure at a point is the same in all directions. Pressure 
acts normally to all boundary surfaces. For equilibrium under gravity, 
regardless of the shape of the containing vessel, the pressure is uniform over 
any horizontal cross section. Pressure varies with height or depth. Two 
different reference levels are used in measuring pressure. For many 
engineering purposes, gage pressure is used with pressure measured relative to 
atmospheric pressure as zero. For most scientific purposes, pressure is 
referred to true zero. Normal atmospheric pressure at sea level caused by the 
weight of the air above is approximately 101 kilopascals or 14.7 pounds per 
square inch absolute. 
    The buoyant force is the force exerted vertically upward by a fluid on a 
body wholly or partly immersed in it. Its magnitude is equal to the weight of 
the fluid displaced by the body. This value is also the vertical component of 
the fluid pressure force acting upward against the bottom of the body minus 
the fluid pressure force component (if any) acting vertically downward against 
the top of the body. If this buoyant force equals the weight of the body, the 
body will remain at the given level. If it exceeds the weight of the body, the 
latter will rise, and vice versa. The buoyant force as a single magnitude acts 
vertically upward through the center of buoyancy which is the center of 
gravity of the displaced fluid.  See  ARCHIMEDES' PRINCIPLE. 
    Pressure applied to a confined liquid is transmitted with equal intensity 
throughout the liquid and by it to all surfaces of the confining vessel or 
piping. Hence, a small force applied to a small area of a confined liquid can 
create a large force against a large area. If the small and large areas are 
pistons the device may be a hydraulic press or jack. Because the transmitting 
liquid is practically incompressible and its volume virtually constant, the 
linear movement of the large piston will be to that of the small piston in 
inverse proportion to their areas. The principle of multiplying a force by 
means of liquid pressure applies also to hydraulic brakes, power steering, 
control systems, and the like; the actuating force may be a pump instead of a 
small piston.  See  HYDRAULICS; HYDROMECHANICS. 
 �34s[William Allan]
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�6v�2v�32sHydroxide   �21m �33s 
    One type of hydroxyl compound (OH-containing) whose solutions have a 
bitter taste, feel slippery to the touch, neutralize acids, and change the 
color of red litmus to blue. They are also called bases and have a high 
concentration of OH- ions in solution. When an oxide of an element, XO, X2O, 
dissolved in water, an hydroxyl compound of the type XOH is formed. Such 
species, XOH, can be ionized to give either OH- ions or Hsupp ions. The former 
are called bases and the latter acids. Ordinarily, when X is a metal, bases 
are obtained, for example, NaOH; and when X is a nonmetal, acids are obtained, 
for example, S(OH)6, which loses two molecules of water and exists as H2SO4. 
Some insoluble oxides, Al2O3, for example, have both acid and basic 
properties. This class of compounds is said to be amphoteric. 
    Hydroxides are very important compounds industrially, sodium hydroxide 
being the most important. It is used in making other chemicals, rayon, 
petroleum products, and soap.  See  ACID AND BASE; HYDRATE. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sHydroxylation reaction   �21m �33s 
    One of several types of reactions used to introduce one or more hydroxyl 
groups into organic compounds. Unsaturated compounds are the most common 
starting materials. Monohydroxylation, the introduction of one hydroxyl group, 
is especially useful when isomeric alcohols cannot form, as in the conversion 
of ethylene to ethyl alcohol. Reactions with longer-chain olefins produce 
primarily secondary and branched-chain alcohols. Dihydroxylation, the 
introduction of two hydroxyl groups, is used to produce glycol.  See  
EPOXIDATION; ETHYLENE GLYCOL; ETHYLENE OXIDE; GLYCOL; SUBSTITUTION REACTION. 
 �34s[Daniel Swern]
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�6v�2v�32sHydroxyproline   �21m �33s 
    An amino acid occurring in certain proteins. Hydroxyproline, the 4-hydroxy 
analog of proline, shares many of the chemical and biological features of 
proline. Unlike proline, hydroxyproline is not found generally in proteins but 
is essentially limited to structural proteins of the collagen type of animals, 
and to a similar protein in the cell walls of higher plants. It constitutes 
about 10% of most collagen in vertebrates, but is present in higher content in 
certain invertebrate collagens.  See  COLLAGEN; PROTEIN. 
    In humans hydroxyproline is excreted in the urine, predominantly in small 
peptides. Hydroxyproline excretion in urine is increased during increased 
breakdown of collagen, and occurs in certain disorders of bone growth or bone 
destruction and in some metabolic disorders, for example, hyperthyroidism or 
hyperparathyroidism.  See  AMINO ACIDS; PROLINE. 
 �34s[Elijah Adams]
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�6v�2v�32sHydrozoa   �21m �33s 
    A class of the phylum Coelenterata which includes the fresh-water hydras, 
the marine hydroids, many of the smaller jellyfish, a few special corals, and 
the Portuguese man-of-war. The Hydrozoa may be divided into five orders: the 
Hydroida, Milleporina, Stylasterina, Trachylina, and Siphonophora. See 
separate article on each order. 
    The form of the body varies greatly among the hydrozoans. This diversity 
is due in part to the existence of two body types, the polyp and the medusa. A 
specimen may be a polyp, a medusa, a colony of polyps, or even a composite of 
the first two. Polyps are somewhat cylindrical, attached at one end, and have 
a mouth surrounded by tentacles at the free end. Medusae are free-swimming 
jellyfish with tentacles around the margin of the discoidal body. 
    In a representative life cycle, the fertilized egg develops into a 
swimming larva which soon attaches itself and transforms into a polyp. The 
polyp develops stolons (which fasten to substrates), stems, and other polyps 
to make up a colony of interconnected polyps. Medusae are produced by budding 
and liberated to feed, grow, and produce eggs and sperm. 
    Most hydrozoans are carnivorous and capture animals which come in contact 
with their tentacles. The prey is immobilized by poison injected by stinging 
capsules, the nematocysts. Most animals of appropriate size can be captured, 
but small crustacea are probably the most common food.  See  COELENTERATA. 
 �34s[Sears Crowell]
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�6v�2v�32sHyena   �21m �33s 
    An African carnivore represented by three species of the family Hyaenidae. 
Hyenas resemble dogs superficially, but they are more closely related to the 
felids (cats). They are four-toed digitigrade (walking on the toes) animals 
with blunt, nonretractile claws that are used for digging and disinterring 
bodies. The gestation period is about 13 weeks, with two to four young 
composing the yearly litter. The maximum life-span of this animal is about 25 
years.  See  CARNIVORA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sHyeniales   �21m �33s 
    An order of Devonian plants considered to be related to the Sphenopsida. 
The small dichotomously forked leaves tend to be borne in whorls. Some leaves 
bear terminal sporangia, but these appendages are neither aggregated into a 
tight cone nor separated by bracts.  See  PALEOBOTANY; SPHENOPSIDA. 
 �34s[Harlan P. Banks]
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�6v�2v�32sHygrometer   �21m �33s 
    An instrument for giving a direct indication of the amount of moisture in 
the air or other gas, the indication usually being in terms of relative 
humidity as a percentage the moisture present bears to the maximum amount of 
moisture that could be present at the location temperature without 
condensation taking place. There are three major types of hygrometers: 
mechanical, electrical, and cold-spot or dew-point.  See  DEW POINT; HUMIDITY. 
    In a simple mechanical type of hygrometer the sensing element is usually 
an organic material which expands and contracts with changes in the moisture 
in the surrounding air or gas. The material used most is human hair. As shown 
in the illustration, the bundle of hair is held under a slight tension by a 
spring, and a magnifying linkage actuates a pointer. 
    In an electrical hygrometer the change in the electrical resistance of a 
hygroscopic substance is measured and converted to percent relative humidity. 
    In a third group of hygrometers, commonly called dew-point apparatus, the 
dew-point temperature is determined; this is the temperature at which the 
moisture in the gas is at the point of saturation, or 100% relative humidity. 
The usual procedure is to chill a polished surface until dew or a film of 
moisture just starts to appear and to measure the temperature of the surface. 
 �34s[Howard S. Bean]
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�6v�2v�32sHymenoptera �21m �33s
    The third largest order of insects, containing the sawflies, ants, wasps,
bees, and related forms. Conservative estimates suggest that the world fauna
may comprise well over 100,000 described species of this order, with many
thousands still to be described.  See  INSECTA.
    This order is of great importance to humans. Some members such as the
sawflies, certain chalcidoids, and most cynipoids, feed during the larval
stage on foliage or other plant tissues. Many species, such as the ichneumon
flies, most chalcid flies, and wasps, are parasites or predators of other
insects or spiders during their larval stage. Bees are indispensable in the
pollination of many fruits, vegetables, and forage crops.  See	BEE.
    Hymenoptera occur in all major faunal zones but are more abundant and have
greater diversity of species in the tropical and temperate zones.
    Adult Hymenoptera usually may be recognized by having two pairs of
membranous wings with reduced venation, the hind pair smaller than the front
pair, and by mouthparts formed for biting and often for lapping or sucking. In
the higher forms, the abdomen is constricted basally, its first segment fused
with the hind part of the thorax. Females always have an ovipositor modified
for sawing, piercing, or stinging. Metamorphosis is complete.
    The first four superfamilies of the Apocrita - the Ichneumonoidea,
Chalcidoidea, Cynipoidea, and Proctotrupoidea - are commonly called the
Parasitica, and the remaining superfamilies are known as the Aculeata. The
Aculeata are stinging forms and the Parasitica are parasites of other insects.
It is impossible to demarcate these two groups sharply because some Aculeata
are parasites and some Parasitica are phytophagous. However, except for the
phytophagous species of Parasitica, these insects lay their eggs in or on an
insect or spider host while the Aculeata place theirs in nests with a
provision of food. The table presents the major classification of the order as
recognized in North America.
    Morphology.  The adult hymenopteron has a clearly differentiated head,
thorax, and abdomen. Wings, when present, and legs are attached to the thorax.
    Typically the head is so oriented that mouthparts are directed downward;
however, all variations occur, and in some species the mouthparts are directed
forward. The large compound eyes occupy much of the sides of the head, though
they are reduced in size in many ants and some Parasitica. Three ocelli are
typically present on the top of the head, but may be reduced or absent in
wingless forms. The paired antennae arise from the face between the eyes, and
they may be close to the mouthparts or removed from them.
    The thorax consists of three segments, tightly fused together. Each
segment bears a pair of legs, and each of the last two segments bears a pair
of wings. In flight, the fore- and hindwings are joined by a row of tiny hooks
along the fore margin of the posterior wing, which fit into the downfolded
hind margin of the anterior wing. Flightless species with shortened,
nonfunctional wings, or no wings at all occur in most major groups, except the
sawflies and bees.
    The abdomen primitively consists of 10 segments, though the number appears
to be less because of modification or loss in the higher forms. The female
ovipositor, or sting, is formed from processes of the eighth and ninth sterna.
    In the Apocrita there is a pair of acid glands opening into a poison sac
connected with the ovipositor. The secretion of these glands produces either a
temporary paralysis when injected into their hosts by some Parasitica, or,
usually, permanent paralysis when injected into their prey by aculeate wasps.
Bees use their stings purely for defense. When a human is stung, the enzymes
react with the tissues to release histamine. Death may occasionally result
from anaphylactic shock, or from mechanical suffocation due to swelling of the
lymphatic system. Medical assistance should be sought if severe swelling
occurs following a sting, especially one on the face or throat.

    Biology.  Practically all hymenopterous adults are terrestrial forms,
living in, on, or near the Earth's surface. A few species are secondarily
aquatic, the adults swimming or walking under water to search out and
parasitize aquatic or subaquatic hosts.
    Most adults feed on plant nectar or honeydew secretions of various
insects. A few sawflies prey on other insects. Some species of Parasitica and
Aculeata imbibe body juices of the host or prey which they attack primarily
for oviposition.
    Mating takes place in a variety of situations, but it is always of rather
short duration. Most species are represented by both males and females. Males
are usually produced from unfertilized eggs and have half the normal number of
chromosomes, while females are produced from fertilized eggs and have the
normal number of chromosomes.
    Hymenoptera exhibit complete metamorphosis during development and pass
through an egg, larval, and pupal state. So far as is known, Hymenoptera
always lay eggs. These are deposited in a protected situation on or near the
supply of larval food. The hymenopterous egg is usually ovoid or
sausage-shaped, and many species in some groups have stalked eggs.
    Some species have only one generation a year in temperate zones, others
have two, and many breed continually during the warmer months. Hymenoptera
usually overwinter as prepupae but occasionally ants, social wasps, and some
bees overwinter as adults or as larvae, as in some Parasitica.
    Sexual dimorphism is often very marked. The two sexes of some species are
so dissimilar that earlier students placed them in different genera or
families. Even today there are many puzzles, and sexes have not been
associated for many of the species having wingless females and winged males.
Ordinarily the males are somewhat smaller than females, though the reverse is
true in most species having wingless females. See SEXUAL DIMORPHISM; SOCIAL
INSECTS.
 �34s[Karl V. Krombein]
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�6v�2v�32sHymenostomatida   �21m �33s 
    An order of the Holotrichia which contains many species that often are of 
small size and fairly uniform ciliation. Primarily, these protozoans are of 
importance as the first possessors of a definite, though inconspicuous, buccal 
ciliature. This ciliature consists of an undulating membrane on the right side 
of the buccal cavity and an adoral zone of membranelles that is primitively 
composed of three membranelles on the left side. This tetrahymenal, or 
four-part, buccal ciliary apparatus is considered the fundamental condition 
from which the oral ciliature of many subsequent higher groups evolved. See 
CILIOPHORA.  
    The majority of hymenostomes are free-living fresh-water forms. Paramecium 
(see illustration) is the best-known genus of ciliates. It is a good-sized, 
widely distributed ciliate, and is a much-studied form. Tetrahymena, beginning 
to rival Paramecium as a favorite ciliate in much experimental work, owes its 
scientific popularity primarily to its ability to grow axenically, that is, 
free from all other organisms, in a chemically defined medium. See PROTOZOA. 
 �34s[John O. Corliss]
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�6v�2v�32sHyperbola   �21m �33s 
    A curve cut from a cone or revolution by a plane that intersects both 
nappes of the cone and does not contain the apex. In analytic geometry it is 
shown (see illustration) that a hyperbola is the locus of points P in a plane, 
such that PF = NATURAL LOG . PD, where PF and PD denote the distances of P 
from a fixed point F (focus) and a fixed line (directrix) of the plane, 
respectively, and NATURAL LOG is a constant, greater than 1. It is also the 
locus of points P, the difference of whose distances from two fixed points F, 
F' (foci) PF - PF' is a constant 2a that is less than the distance 2c between 
the foci. The curve is symmetric to the line g(F,F') determined by F, F' and 
to O, their midpoint. It consists of two branches that are images of each 
other in the line g through O, perpendicular to g(F,F'). There are two lines 
through O, making equal angles with g(F,F'), and to each of which points on 
each branch get indefinitely close; that is, if point P traverses either 
branch of the hyperbola, its distance from these lines approaches zero. These 
lines are called asymptotes of the hyperbola. See ANALYTIC GEOMETRY; CONIC 
SECTION; HYPERBOLOID. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sHyperbolic function �21m �33s
    The hyperbolic sine and cosine of a real or complex variable z are defined

sinh z = (e[sup z] - e[sup -z])/z     cosh z = (e[sup z] + e[sup -z])/z      (1)

by Eqs. (1). Both sinh z and cosh z have a period 2pi i of e(sup z). From
De(sup z) = de(sup z)/dz = e(sup z), Eqs. (2) are obtained.

	D sinh z = cosh z		D cosh z = sinh z		     (2)

    Since e(sup z) = SUMMATION (n=0 to INFINITY) (zsupn)/n!  by definition, Eqs.
(3) hold and the series converge for all z and yield relations (4).  Thus
relations between the circular functions become hyperbolic (and vice versa) when
z is replaced by iz.

       sinh z = SUMMATION (n=0 to INFINITY) z(sup 2n + 1)/(2n + 1)!
       cosh z = SUMMATION (n=0 to INFINITY) (z[sup 2n])/2n!		    (3)

			    sinh iz = i sin z
			    sin iz = isinh z
			    cosh z = cos z
			    cos iz = cosh z				     (4)

    The hyperbolic tangent and cotangent are defined by Eqs. (5). They have
the period PIi and have simple poles at the zeros of cosh z and sinh z,
respectively. Moreover Eqs. (6) hold true.
See HYPERBOLA; TRIGONOMETRY.
 �34s[Lane Barksdale]
�33s

			tanh z = (sinh z)/(cosh z)
			coth z = (cosh z)/(sinh z)			     (5)


		      D tanh z = 1/cosh(sup 2) z
		      D coth z = -1/sinh(sup 2) z			     (6)

By definition Eqs. (7) hold.

			     sech z = 1/cosh z
			     csch z = 1/sinh z				     (7)

See HYPERBOLA; TRIGONOMETRY.
 �34s[Louis Brand]
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�6v�2v�32sHyperbolic navigation system   �21m �33s 
    A navigation system that produces hyperbolic lines of position through the 
measurement of the difference in times of transmission of radio signals from 
two or more synchronized transmitters at fixed points. When synchronized 
signals are received from two transmitting stations, the difference in the 
times of arrival is constant on a hyperbola having the two transmitting 
stations as foci. The measured time difference locates the receiver on the 
hyperbolic line of position for that time difference. Another pair of 
transmitters provides another hyperbolic line of position. The intersection of 
the lines of position provides a navigational fix, as shown in the 
illustration. See DECCA; LORAN; OMEGA. 
 �34s[Alexander A. McKenzie]
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�6v�2v�32sHyperboloid   �21m �33s 
    A quadric surface that has both elliptic and hyperbolic plane sections. It 
is symmetrical in each of a set of three mutually perpendicular planes that 
intersect pairwise in lines called axes of the hyperboloid. Two of these 
planes of symmetry intersect the hyperboloid in hyperbolas. If the third plane 
of symmetry does not intersect the hyperboloid, the latter consists of two 
separate but congruent parts and is called an hyperboloid of two sheets. If 
the third plane of symmetry intersects the hyperboloid, the curve of section 
in an ellipse (or circle) whose axes are also axes of the hyperboloid, and the 
latter is a connected surface called an hyperboloid of one sheet (see 
illustration). See ANALYTIC GEOMETRY; HYPERBOLA; QUADRIC SURFACE; SURFACE AND 
SOLID OF REVOLUTION. 
 �34s[James S. Frame]
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�6v�2v�32sHypercharge   �21m �33s 
    A quantized attribute, analogous to electric charge, introduced in the 
classification of a subset of elementary particles - the so-called baryons - 
including the proton and neutron as its lightest members. As far as is known, 
electric charge is absolutely conserved in all physical processes. Hypercharge 
was introduced to formalize the observation that certain decay modes of 
baryons expected to proceed by means of the strong nuclear force simply were 
not observed. See ELECTRIC CHARGE. 
    Unlike electric charge, however, the postulated hypercharge was found not 
to be conserved absolutely; the weak nuclear interactions do not conserve 
hypercharge - and indeed can change hypercharge by PLUS/MINUS1 or 0 units. 
    When the known baryons are classified according to their electric charge 
and their hypercharge, they naturally group into octets in the scheme first 
proposed by M. Gell-Mann and K. Nishijima. The quarks, hypothesized as the 
fundamental building blocks of matter, must have fractional hypercharge as 
well as electrical charge; the simplest quark model suggests values of ** and 
**, respectively. See BARYON; ELEMENTARY PARTICLE; QUARKS; SYMMETRY LAWS 
(PHYSICS); UNITARY SYMMETRY. 
 �34s[D. Allan Bromley]
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�6v�2v�32sHyperconjugation �21m �33s
    A phenomenon similar to conjugation in its formulation and manifestations,
but the ffects are weaker. It occurs when a CH2 or CH3 group (or in general an
AR2 or AR3 group, where A may be any polyvalent atom and R any atom or
radical) is adjacent to a multiple bond or to a group containing an atom with
a lone PI-electron, a PI-electron pair or quartet, or a PI-electron vacancy.
Hyperconjugation can be sacrificial (this is relatively weak) or isovalent
(stronger).
    Examples of molecules exhibiting first-order sacrificial hyperconjugation
are:
      Propylene
      Cylcopentadiene
      Methylamine
      Acetaldehyde


    The analogy to conjugation is brought out by rewriting the main structures
as

		  CHEMICAL STRUCTURE DRAWINGS

with minor resonance structures

		  CHEMICAL STRUCTURE DRAWINGS

    A still weaker effect is second-order sacrificial hyperconjugation, as
in ethane (H3 TRIPLE BOND C--C TRIPLE BOND H3: compare HC TRIPLE BOND C--C
TRIPLE BOND CH) or in ethylene (H2==XC==C==H2; compareH2C==C==C==CH2).
    Examples of isovalent hyperconjugation are H3 TRIPLE BOND C--C sup + H2
 (with hyperisovalent resonance structure H3+ ==C==CH2) and
H3 TRIPLE BOND C--CH2 (resonance structure H3==CCH2). The
number of bonds is the same in the two resonance structures, just as in
isovalent conjugation, but the second structure is energetically less
favorable than the first structure; for this reason the resulting stabilizing
resonance energy is less than in isovalent conjugation.
 �34s[Robert S. Mulliken]
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�6v�2v�32sHyperfine structure   �21m �33s 
    A closely spaced structure of the spectrum lines forming a multiplet 
component in the spectrum of an atom or molecule, or of a liquid or solid. In 
the emission spectrum for an atom, when a multiplet component is examined at 
the highest resolution, this component may be seen to be resolved, or split, 
into a group of spectrum lines which are extremely close together. This 
hyperfine structure may be due to a nuclear isotope effect, to effects related 
to nuclear spin, or to both. See ISOTOPE SHIFT; SPIN (QUANTUM MECHANICS). 
    The measurement of a hyperfine structure spectrum for a gaseous atomic or 
molecular system can lead to information about the nuclear magnetic and 
quadrupole moments, and about the atomic or molecular electron configuration. 
Important methods for the measurement of hyperfine structure for gaseous 
systems may employ an interferometer, or use atomic beams, electron spin 
resonance, or nuclear spin resonance. See ATOMIC STRUCTURE AND SPECTRA; 
ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY; INTERFEROMETRY; MAGNETIC 
RESONANCE; MOLECULAR BEAMS; NUCLEAR MAGNETIC RESONANCE (NMR); NUCLEAR MOMENTS. 
 �34s[Louis D. Roberts]
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�6v�2v�32sHypergeometric functions �21m �33s
    The analytic continuation of the function defined by the series in Eq.
(1), where the shifted factorial (a)subn is defined by Eq. (2). It satisfies
differential equation (3), and for |z| < 1, Re c > Re b > 0, is given by the
integral representation of Eq.	(4), where GAMMA represents the gamma function.
See COMPLEX NUMBERS; GAMMA FUNCTION; SERIES.

(sub 2)F1(a,b;c;z) = SUMMATION (n-0 to INFINITY) {[(a)(sub n) (b)(sub n)]/
			    [(c)(sub n) n!]} z(sup n)
			       |z| < 1					     (1)

		(a)(sub n) = a(a + 1) . . . (a + n - 1)
			n = 1,2, . . . , (a)0 = 1			     (2)

	      z(1 - z)y'' + [c - (a + b + 1)z]y' - aby = 0                   (3)

(sub 2)F1 (a,b;c;z) = [LAMBDA(c)/LAMBDA(b)LAMBDA(c - b)] INTEGRATION (0 to 1)
	   [(1 - zt)(sup -a) t(sup b-1) (1 - t)(sup c-b-1) dt]			(4)

    The interest in hypergeometric functions comes from the many important
functions which are special cases of the general hypergeometric function, the
rich theory which has been developed for the general hypergeometric function,
and the many times they occur in applications. Classically hypergeometric
functions have arisen in science as solutions to differential equations. As
discrete, rather than continuous, models of physical phenomena have become
increasingly useful, hypergeometric functions have continued to arise as
solutions to the equations governing these models. See DIFFERENTIAL EQUATION.
 �34s[Richard Askey]
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�6v�2v�32sHyperiidea   �21m �33s 
    A suborder of amphipod crustaceans. Most hyperiids can be recognized by 
the large eyes which cover nearly the entire surface of the head. The first 
maxillae and especially the maxillipeds are greatly reduced in comparison to 
the suborder Gammaridea. In the prehensile pereiopods, the claw is formed by 
the fifth and sixth segments rather than by the sixth and seventh segments, as 
in the Gammaridea. The second and third somites of the urosome are always 
fused, a condition rarely found in the Gammaridea. See AMPHIPODA; GAMMARIDEA. 
    The hyperiidea are exclusively pelagic and marine. They are found in all 
the oceans, from the surface to great depths. Most are characteristic of 
oceanic rather than neritic waters, although there are some species that 
frequent inshore waters in the tropics. After the Copepoda and Euphausiacea, 
the Hyperiidea are the most abundant planktonic crustaceans. Some species are 
frequently found in association with other animals. Other species are truly 
free-living. Some hyperiids are luminescent; the whole body glows with a 
greenish-yellow light. See BIOLUMINESCENCE. 
 �34s[Thomas E. Bowman]
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�6v�2v�32sHypermastigida   �21m �33s 
    An order of the Protozoa in the class Zoomastigophorea comprising the most 
complex flagellates, both structurally and in modes of division. All inhabit 
the alimentary canal of termites, cockroaches, and woodroaches. These 
organisms are multiflagellate. The nucleus is single and the organisms are 
plastic and slow-moving, generally ovoid to elongate. Flagella occur in spiral 
rows, in tufts, or over the entire body. These flagellates vary from 15 to 350 
micrometers in size. See PROTOZOA; ZOOMASTIGOPHOREA. 
 �34s[James B. Lackey]
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�6v�2v�32sHyperoartii   �21m �33s 
    One of the two superorders of primitive jawless vertebrates (Agnatha) 
belonging to the subclass Monorhina and containing the living lampreys, or 
Petromyzonida, as well as the extinct Osteostraci and Anaspida. They are 
distinguished from the other superorder, the Hyperotreti, by the fact that the 
single median nasal opening is placed dorsally on the head and leads into a 
blind hypophyseal sac that is separated from the mouth and pharynx. See 
AGNATHA; ANASPIDA; HYPEROTRETI; MONORHINA; OSTEOSTRACI; PETROMYZONIDA. 
 �34s[Robert H. Denison]
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�6v�2v�32sHyperon   �21m �33s 
    A collective name for any baryon with nonzero strangeness number s. The 
name hyperon has generally been limited to particles which are semistable, 
that is, which have long lifetimes relative to 10 sup -22 s and which decay 
by photon emission or through weaker decay interactions. Hyperonic particles 
which are unstable (that is, with lifetimes shorter than 10 sup -22 s) are 
commonly referred to as excited hyperons. The known hyperons with spin 
1/2 PLANCK'S CONSTANT (where PLANCK'S CONSTANT is Planck's constant divided by 
2 PI) are LAMBDA, SIGMA NEGATIVE, SIGMA sup 0, and SIGMA POSITIVE, with s = -1, 
and XI NEGATIVE and XI sup 0, with s = -2, together with the OMEGA NEGATIVE 
particle, which has spin 3/2 PLANCK'S CONSTANT and s = -3. The corresponding 
antihyperons have baryon number B = -1, opposite strangeness s, and charge Q; 
they are all known empirically, except for OVERBAR OMEGA POSITIVE.
    There is no deep distinction between hyperons and excited hyperons, beyond 
the phenomenological definition above. Indeed, the hyperon OMEGA (1672) 
NEGATIVE and the excited hyperons XI (1530) and SIGMA (1385), together with the 
unstable nucleonic states DELTA (1236), are known to form a unitary decuplet of 
states with spin 3/2 PLANCK'S CONSTANT. See BARYON; ELEMENTARY PARTICLE; 
SYMMETRY LAWS (PHYSICS); UNITARY SYMMETRY. 
 �34s[Richard H. Dalitz]
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�6v�2v�32sHyperotreti   �21m �33s 
    One of the two superorders of the subclass Monorhina containing the 
Myxinoidea, or hagfishes, a primitive group of jawless vertebrates (Agnatha). 
They are distinguished from the other superorder, the Hyperoartii, by, among 
other things, the fact that the nasal opening is at the tip of the snout and 
communicates by a long duct with the pharynx so that it can be used in 
respiration. See AGNATHA; HYPEROARTII; MONORHINA; MYXINOIDEA. 
 �34s[Robert H. Denison]
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�6v�2v�32sHyperplasia �21m �33s
    An increase in cell number. It should be distinguished from hypertrophy,
which is an increase in the size of cells, although both bring about an
increase in the size of an organ. See HYPERTROPHY.
    Certain types of hyperplasia may be extremely difficult to differentiate
from neoplasia, both grossly and microscopically. Not infrequently the mass of
hyperplastic cells causes a localized swelling which grossly resembles a
tumor. As in neoplasia, a rapid rate of proliferation is reflected in the
large number of mitoses which may be evident. Hyperplasia differs from
malignant neoplasia, however, in that the cells appear to be differentiating
in an orderly fashion and the features of anaplasia are lacking. See
ANAPLASIA; ATROPHY; NEOPLASIA.
 �34s[Wilson F. Powell /N. Karle Mottet]
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�6v�2v�32sHypersensitivity   �21m �33s 
    An abnormal reactivity in humans and animals to allergens (substances 
foreign to the body) induced by exposure through injection, inhalation, 
ingestion, or contact. It is usually referred to as allergy (altered 
reactivity), and its commonly known clinical forms include asthma, hay fever, 
hives, infantile eczema, and the contact skin lesions caused by poison oak and 
poison ivy. Since the allergen may itself be nontoxic, the first experience 
with it occasions no difficulties. Second and subsequent exposures, however, 
may give rise to a variety of reactions of different degrees of severity, 
ranging from transient hives to death. 
    The allergen, or antigen, induces the allergic state and subsequently 
becomes responsible for the occurrence of reactions. Allergens vary in 
chemical nature, ranging from proteins and polysaccharides through simpler 
drugs and chemical compounds, even the salts of metals See ANTIGEN. 
    Hypersensitivity may be classified into two categories, delayed and 
immediate. Delayed reactions begin after several hours, rather than in seconds 
or minutes as in immediate hypersensitivity. The two chief distinctions 
between these two kinds of hypersensitivity are: (1) In delayed 
hypersensitivity lymphoid cells become activated to react with antigen by 
means of an effector substance not yet characterized. Such reactions may occur 
in any tissue where these cells and antigen meet (such as skin, lung, or 
kidney), and damage may extend to include all the structures that make up such 
organs. In an immediate hypersensitive reaction, circulating antibodies react 
with antigen. As a result of this combination, intermediary substances, such 
as histamine, are released and the antigen-antibody complexes may have 
directly injurious effects by their affect on certain specific tissues, 
notably blood-vessel walls and smooth muscle. (2) Since sensitive lymphoid 
cells react without relation to circulating antibodies, the delayed 
hypersensitive state cannot be transferred from the allergic subject to a 
normal one by injections of serum, as can immediate hypersensitivity. See 
ANTIGEN-ANTIBODY REACTION; ASTHMA; ATOPIC ALLERGY; HISTAMINE. 
    The steroids are useful on the basis of their anti-inflammatory effects in 
alleviating reactions in already-sensitive subjects. They are used clinically 
in severe atopic hypersensitivities while attempted desensitization is 
underway or when the state is intractable to immunologic therapy. Steroids are 
applicable also in delayed hypersensitivity when the lesions are so 
troublesome as to require alleviation before they subside spontaneously. See 
IMMUNOLOGY; STEROID. 
 �34s[Sidney Raffel]
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�6v�2v�32sHypersonic flight   �21m �33s 
    Flight at speeds well above the local velocity of sound. By convention, 
hypersonic flight starts at about Mach 5 (five times the speed of sound) and 
extends upward in speed indefinitely. Supersonic vehicles also fly at speeds 
greater than the local speed of sound. However, when the Mach number is high, 
the flow field around an object exhibits a special behavior, which is worth 
studying separately from supersonic flight. This behavior is characteristic of 
hypersonic flight.  
    A body entering the Earth's atmosphere from space (for example, a 
meteorite, a ballistic reentry vehicle, or a spacecraft) has high velocity and 
hence large kinetic and potential energy. During reentry, drag forces act upon 
a reentry vehicle or spacecraft and cause it to decelerate, thus dissipating 
its kinetic and potential energy. The energy lost by the reentry vehicle is 
then transferred to the air within the flow field around the reentry vehicle. 
The flow field around the forward portion of a blunt-nosed vehicle (body of 
revolution or leading edge of a wing) generally exhibits (1) a distinct bow 
shock wave, (2) a shock layer of highly compressed hot gas, and (3) a highly 
sheared boundary layer over the surface. The flow field is defined as the 
region of disturbed air between the body surface and the shock wave. The 
temperature of the shock layer is so high that molecules begin to dissociate 
during collisions at about 4000DGF (2500 K), or at Mach 7. At about Mach 12, 
gas in the stagnation region reaches a temperature of 6700DGF (4000 K) and 
becomes ionized. 
    In general, at high hypersonic flight speed the characteristic temperature 
in the shock layer of a blunted body and in the boundary layer of a slender 
body is proportional to the square of the Mach number. Heat energy is then 
transferred from the hot gases to the vehicle surface by conduction and 
diffusion of chemical species in the boundary layer and by radiation from the 
shock layer near the nose. Heat energy is also radiated from the vehicle 
surface to space or to adjacent objects. One important problem confronting the 
designer of reentry vehicles or spacecraft is therefore to design a 
minimum-weight vehicle able to withstand large heat loads from adjacent 
hot-gas layers during reentry while retaining the ability to carry a given 
useful payload. See ATMOSPHERIC ENTRY. 
 �34s[Shih-Yuan Chen]
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�6v�2v�32sHypersonics   �21m �33s 
    A development of ultrasonics (also known as pretersonics) dealing with the 
production and utilization of sound waves of frequencies above 500 MHz. These 
frequencies are sufficiently high to approximate thermal waves, and they can 
interact directly with phonons. They also interact with electrons and holes 
and with spin waves in ferromagnetic materials. See HOLES IN SOLIDS; PHONON; 
ULTRASONICS; WAVE MOTION. 
    A considerable development occurred in the field of hypersonics due to the 
production of thin-film transducers of cadmium sulfide or zinc oxide, the use 
of lasers in Brillouin scattering or Bragg reflection from sound waves, and 
the use of thermal pulses. They have provided considerable evidence on the 
interaction of acoustic waves with other thermal and electrical waves in the 
media. Frequencies as high as 2.5 terahertz have been obtained. 
    One application of hypersonics is the acoustic microscope. This was 
suggested as early as 1936 by S. Sokolov, but it was not until sound waves 
with very high frequencies with wavelengths on the order of those for optical 
wavelengths were obtained that acoustic microscopes were able to compete with 
optical means. See ACOUSTIC MICROSCOPE. 
 �34s[Warren P. Mason]
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�6v�2v�32sHypertension   �21m �33s 
    The persistent elevation of blood pressure above normal level. Although 
there is no agreed-upon dividing line between ``normal'' and elevated blood 
pressure, generally the chronic elevation of diastolic pressure above 90 mm 
mercury (1 mmHg = 133.3 pascals), and of systolic pressure above 140 mm 
mercury, is considered abnormal by most physicians. Abnormal elevation of the 
blood pressure may occur transiently as a result of increased activity, 
emotion, or illness. The blood pressure also varies somewhat at different 
times of the day, state of tranquility or physical activity, level of 
nutrition, and so on. 
    Although hypertension may occur along with specific diseases, most cases 
are of the primary (essential or idiopathic) type. In these individuals, there 
is no known cause, although contributing and associated factors are now 
clearly recognized. Hypertension which occurs as part of many diseases is 
called secondary hypertension. Among these diseases are hyperthyroidism, 
increased intracranial pressure, and arteriosclerosis. Anxiety states may also 
raise blood pressure. See ARTERIOSCLEROSIS; CIRCULATORY SYSTEM; NEUROSIS; 
THYROID GLAND DISORDERS. 
 �34s[N. Karle Mottet]
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�6v�2v�32sHypertrophy �21m �33s
    The increase in size of a cell, tissue, or organ. Hypertrophy of an organ
or tissue is due either to an increase in the mass of the individual cells
(true hypertrophy) or to an increase in the number of cells making up the
tissue (hyperplasia) or to both. Certain organs such as the heart consist of
cells which lack the ability to divide, and in such cases hypertrophy results
in an increase in the size rather than in the number of the constituent cells.
In other tissues such as the liver, lymph nodes, and adrenal cortex,
hypertrophy of the organ is effected principally by an increase in the number
of cells.
    When the hypertrophy is the result of physiological rather than
pathological stimuli, it is termed physiological hypertrophy. Examples are
increased mass of skeletal muscles which may be produced by physical exercise,
enlargement of the uterus in pregnancy, and increased number of circulating
red blood cells found in people living at high altitudes. Compensatory
hypertrophy is the increase in size of an organ or organ system following
removal or destruction of a portion of it. See HYPERPLASIA.
 �34s[Wilson F. Powell /N. Karle Mottet]
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�6v�2v�32sHypnobryales   �21m �33s 
    An order of mosses composed of numerous families and genera. The plants 
are procumbent and pleurocarpous. The leaves are usually symmetrical but in 
some genera are often asymmetrical. They are arranged on the stem in more than 
two rows. The order has been divided into two suborders, the Leskeineae and 
the Hypnineae. See BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sHypnosis   �21m �33s 
    A presumed altered state of consciousness in which the hypnotized 
individual is usually more susceptible to suggestion than in his or her normal 
state. In this context, a suggestion is understood to be an idea or a 
communication carrying an idea that elicits a covert or overt response not 
mediated by the higher critical faculties (that is, the volitional apparatus). 
    Hypnosis cannot be physiologically distinguished from the normal awake 
state of an individual, and for this reason its existence has been questioned 
by some investigators. There are few phenomena observed in association with 
hypnosis, if any, that are specific to the hypnotic state. Most are directly 
or indirectly produced by suggestions. Through suggestions given to hypnotized 
individuals, it is possible to induce alterations in memory, perception, 
sensation, emotions, feelings, attitudes, beliefs, and muscular state. Such 
changes can be, and usually are, incorporated into the complex behavior of the 
individual, resulting in amnesias and paramnesias, fuguelike conditions, 
paralysis, loss of sensory functions, changes in attention, personality 
alterations, hallucinatory and delusional behavior, and even physiological 
changes. Enhanced recall is sometimes possible. Although sometimes remarkable, 
the effects produced through hypnosis with the majority of individuals are 
much less spectacular than popularly believed. See CONSCIOUSNESS. 
 �34s[Andre M. Weitzenhoffer]
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�6v�2v�32sHypochlorite �21m �33s
    The negative ion, ClO-, derived from hypochlorous acid, HClO. The
hypochlorite ion is best known as an oxidizing agent and as a constituent of
bleaching agents.
    Hypochlorites are prepared by passing chlorine gas into alkaline solution
and in this procedure the chlorine is simultaneously oxidized and reduced, as
in the following reaction.

		Cl sub 2 + 2NaOH -> NaCl + NaClO + H2O

    When chlorine is passed over slaked lime, a product called bleaching
powder or chlorinated lime is obtained which approximates the formula
CaCl(OCl). A related preparation, high-test hypochlorite (HTH), contains twice
as much available chlorine as bleaching powder. See BLEACHING; CHLORINE;
OXIDIZING AGENT.
 �34s[E. Eugene Weaver]
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�6v�2v�32sHypodermis   �21m �33s 
    The outermost cell layer of the cortex, also called the exodermis, of 
plants. It forms a prominent layer immediately under the epidermis in many but 
not all plants. Like the endodermis, it develops Casparian strips, suberin 
deposits, and cellulose deposits impregnated with phenolic or quinoidal 
substances. The hypodermis may produce substances that act as a barrier to the 
entry of pathogens, and in some plants it may function in the absorption of 
water and the selection of ions that enter the plant. See CORTEX (PLANT); 
ENDODERMIS.  
 �34s[D. S. Van Fleet]
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�6v�2v�32sHypofluorous acid   �21m �33s 
    An unstable acid with the chemical formula HOF. Hypofluorous acid is a 
colorless gas at room temperature. At very low temperature it is a white 
solid, melting at -117DGC (-179DGF) to a pale yellow liquid that boils 
somewhat below room temperature. The compound reacts rapidly with water to 
form H2O2 in acid solution and O2 in base. Most oxidizable substrates are 
attacked by HOF. Oxygen transfer from HOF provides a way of synthesizing a 
variety of compounds with isotopically labeled oxygen atoms. 
 �34s[Evan H. Appelman]
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�6v�2v�32sHypoplasia   �21m �33s 
    The failure of a tissue or organ to achieve complete development. Related 
but not synonymous terms are agenesis, the complete absence of a tissue, and 
aplasia, the virtual absence of a tissue or organ, only a remnant of the 
structure being present. Hypoplasia is usually apparent at birth. Examples of 
hypoplasia may be found in the kidney, the ovary, the testis, the heart, and 
in virtually every organ system. 
 �34s[N. Karle Mottet]
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�6v�2v�32sHypothermia   �21m �33s 
    The term which describes conditions of reduced body temperature in 
homeotherms, that is, those animals which routinely regulate and maintain body 
temperature (mammals and birds). Ordinarily the term hypothermia is not used 
in reference to conditions of reduced body temperature in poikilotherms 
(reptiles, amphibians, fishes, and invertebrates), cold-blooded animals which 
do not regulate and maintain body temperature. In those animals the effects of 
cold exposure are better termed cold torpor and loosely termed hibernation. 
See HIBERNATION. 
    An animal undergoing hypothermia reveals a number of dramatic biological 
events and conditions, for example, shivering, and reductions in body 
temperature and in respiration rate, and depression of cardiac activity. It is 
of note that many depressed physiological activities are comparable to 
processes of hibernation. If one assesses the true physiological meaning of 
these phenomena, it is evident that the animal has reduced its intake and 
transport of oxygen. Since the animal is still living, in its immobile state 
resembling dormancy, a remarkable slowing of metabolic activity is 
demonstrated. This state of depressed metabolism means that an animal's 
tissues and organs have reduced requirements for oxygen. Oxygen needed for 
oxidative cellular metabolism is now required in small amounts since the body 
chemistry has slowed drastically. This reduction in the tissue-oxygen 
requirement is one of the most useful aspects of hypothermia. 
    In the practice of medicine the depressed metabolism of hypothermia has 
become an important tool in various forms of surgery and in the treatment of 
several illnesses. For example, cardiovascular surgery, where there is need 
for repair of heart valves and corrections of congenital defects, it has been 
found adequate to reduce body temperature from normal levels (37 - 38DGC or 
98.6 - 100.4DGF) to about 30DGC (86DGF). This level of hypothermia can be 
achieved by surface cooling, in which the patient is placed in a modified ice 
bath or surrounded by ice blankets. The next level in surgical hypothermia is 
in bringing the body temperature down between 30 and 26DGC (86 and 79DGF). 
This is sometimes called moderate hypothermia, and at these levels it is 
necessary to cool the blood directly. The techniques usually involve cardiac 
bypass, where a portion of the circulation is shunted out of the body and 
blood-oxygen levels and flow rates are maintained by means of a highly 
sophisticated array of tubes, pumps, and oxygenators. Certain surgical 
procedures require deep levels of hypothermia, 25 - 15DGC (77 - 59DGF), with 
long periods of circulatory arrest. In these cases the heartbeat is markedly 
slowed, circulation is shunted and the body temperature is controlled outside 
of the body. See THERMOREGULATION. 
 �34s[X. J. Musacchia]
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�6v�2v�32sHypothesis �21m �33s
    A tentative supposition with regard to an unknown state of affairs, the
truth of which is thereupon subject to investigation by any available method,
either by logical deduction of consequences which may be checked against what
is known, or by direct experimental investigation or discovery of facts not
hitherto known and suggested by the hypothesis.
 �34s[Percy W. Bridgman /Henry Margenau]
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�6v�2v�32sHypotrichida   �21m �33s 
    An order of Spirotrichia. These protozoans are commonly considered to 
represent the pinnacle of specialized development in the evolution of 
ciliates. Somatic ciliature of the ventral surface has been replaced by cirri; 
on the dorsal surface it is absent or represented by inconspicuous sensory 
bristles (see illustration). The adoral zone of membranelles is very 
prominently developed, and the buccal area may occupy a large part of the 
ventral surface of the body. The whole body is generally rigid. Hypotrichs 
occur ubiquitously in fresh- and salt-water habitats. See SPIROTRICHIA. 
 �34s[John O. Corliss]
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�6v�2v�32sHyracoidea   �21m �33s 
    An order of ungulate mammals. Once abundant in the middle and late 
Tertiary faunas of Africa and reaching as far eastward as China during the 
Pliocene, they are represented today only by the small conies of Africa, 
Arabia, and Syria. 
    The short-tailed hyraxes keep their hindquarters in a crouch and are 
plantigrade, with the axis of symmetry of their feet going through the third 
digit. The strong hindlegs effectively propel the animals. The forefoot is 
four-fingered with only three functional fingers, and the hindfoot (pes) has 
three toes. The soles of the feet have soft, enlarged, elastic pads, which are 
kept moist by numerous glands allowing hyraxes to cling to rock surfaces and 
tree trunks. See DENTITION. 
    The living hyraxes (also called dassies or conies) are herbivorous. 
Ground-living forms live among rocks, and form colonies of up to 50 
individuals. They are mainly active during the day. Some genera are solitary, 
nocturnal, and almost completely arboreal, rarely leaving the trees. See 
EUTHERIA; MAMMALIA. 
 �34s[Frederick S. Szalay]
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�6v�2v�32sHysteresis �21m �33s
    A phenomenon wherein two (or more) physical quantities bear a relationship
which depends on prior history. More specifically, the response Y takes on
different values for an increasing input X than for a decreasing X.
    If one cycles X over an appropriate range, the plot of Y versus X gives a
closed curve which is referred to as the hysteresis loop. The response Y
appears to be lagging the input X.
    Hysteresis occurs in many fields of science. Perhaps the primary example
is of magnetic materials where the input variable H (magnetic field) and
response variable B (magnetic induction) are traditionally chosen. For such a
choice of conjugate variables, the area of the hysteresis loop takes on a
special significance, namely the conversion of energy per unit volume to heat
per cycle. For mechanical hysteresis, it is customary to take the variables
stress and strain, where the energy density loss per cycle is related to the
internal friction. Thermal hysteresis is characteristic of many systems,
particularly those involving phase changes, but here the hysteresis loops are
not usually related to energy loss. See FERROELECTRICS; MAGNETIC HYSTERESIS;
STRESS AND STRAIN; THERMAL HYSTERESIS.
 �34s[H. B. Huntington ;Robert K. MacCrone]
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�6v�2v�32sHysteresis motor   �21m �33s 
    A synchronous motor without salient poles and without dc excitation which 
makes use of the hysteresis and eddy-current losses induced in its 
hardened-steel rotor to produce rotor torque. The stator and stator windings 
are similar to those of an induction motor and may be polyphase, shaded-pole, 
or capacitor type. The rotor is usually made up of a number of hardened steel 
rings on a nonmagnetic arbor. The hysteresis motor develops constant torque up 
to synchronous speed. The motor can, therefore, synchronize any load it can 
accelerate. These motors are built in small sizes, for instance, for electric 
clocks.  See  INDUCTION MOTOR; SYNCHRONOUS MOTOR. 
 �34s[Loyal V. Bewley]
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�6v�2v�32sHysteriales (lichenized)   �21m �33s 
    An order of the Ascolichenes shared by the Ascomycetes, comprising those 
species with a so-called ascolocular structure. The hymenium consists of 
densely interwoven, branched pseudoparaphyses and irregularly scattered 
locules in which the asci are located. 
    The growth form of the Hysteriales is either crustose or fruticose. There 
are five or six families now included here. Arthoniaceae is the largest 
family, with two genera, Arthonia and Arthothelium, both widespread on bark, 
rarely on leaves, in temperate and tropical regions. The Roccellaceae is a 
family of conspicuous fruticose species that grow profusely on trees and rocks 
along the coastlines of Portugal, California, Baja California, and parts of 
western South America. These plants were collected by the ton in the Middle 
Ages for use as dyestuffs. These same lichens, on a smaller scale, now yield 
the dye used in litmus paper.  See  ASCOLICHENES; ASCOMYCOTINA. 
 �34s[Mason E. Hale, Jr.]
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�6v�2v�32sIceberg   �21m �33s 
    A large mass of glacial ice broken off and drifted from parent glaciers or 
ice shelves along polar seas. Icebergs should be distinguished from polar pack 
ice which is sea ice, or frozen sea water, though rafted or hummocked 
fragments of the latter may resemble small bergs.  See  GLACIOLOGY; SEA ICE. 
    Icebergs are classified by shape and size. The terms used are arched, 
blocky, dome, pinnacled, tabular, valley, and weathered for berg description, 
and bergy-bit and growler for berg fragments ranging smaller than cottage size 
above water. The lifespan of an iceberg may be indefinite while the berg 
remains in cold polar waters, eroding only slightly during summer months. But 
under the influence of ocean currents, an iceberg that drifts into warmer 
water will disintegrate rapidly. 
    In the Arctic, icebergs (see illustration) originate chiefly from glaciers 
along Greenland coasts. It is estimated that a total of about 16,000 bergs are 
calved annually in the Northern Hemisphere, of which over 90% are of Greenland 
origin; but only about half of these have a size or source location to enable 
them to achieve any significant drift. No icebergs are discharged or drift 
into the North Pacific Ocean or its adjacent seas, except a few small bergs 
each year that calve from the piedmont glaciers along the Gulf of Alaska. 
    In the Southern Ocean, bergs originate from the giant ice shelves all 
along the Antarctic continent. These result in huge, tabular bergs or ice 
islands several hundred feet high and often over a hundred miles in length, 
which frequent the entire waters of the Antarctic seas.  See  ICE ISLAND. 
 �34s[Robertson P. Dinsmore]
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�6v�2v�32sIce cream   �21m �33s 
    A commercial dairy food made by freezing while stirring a pasteurized mix 
of suitable ingredients. They may include but are not limited to milk fat, 
nonfat milk solids, or milk-derived ingredients; and sugars, corn syrup, 
water, flavoring, sometimes egg products, stabilizers, and emulsifiers. Air 
incorporated during the freezing process is also an important component. 
    Products classed as frozen dairy desserts include French ice cream, frozen 
custard, ice milk, sherbet, water ice, frozen dairy confection, dietary frozen 
dairy dessert, and mellorine or imitation ice cream. 
    In ice cream manufacture the basic ingredients are blended together; the 
liquid blend is called the mix. Then the mix is pasteurized, homogenized, 
cooled, and aged. The flavoring materials are usually added to the mix just 
before the freezing process, but fruits, nuts, and candies are not added until 
the ice cream is discharged from the freezer.  See  MILK. 
 �34s[Wendell S. Arbuckle]
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�6v�2v�32sIce field   �21m �33s 
    A network of interconnected glaciers or ice streams, with common source 
area or areas, in contrast to ice sheets and ice caps. (An ice sheet is a 
broad, cakelike glacial mass with a relatively flat surface and gentle relief. 
Ice caps are properly defined as domelike glacial masses, usually at high 
elevation.) Being generally associated with terrane of substantial relief, 
ice-field glaciers are mostly of the broad-basin, cirque, and mountain-valley 
type. Thus, different sections of an ice field are often separated by linear 
ranges, bedrock ridges, and nunataks. 
 �34s[Maynard M. Miller]
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�6v�2v�32sIce island �21m �33s
    One of the massive bodies of floating ice in the Arctic Ocean, from 20 to
200 ft (6 - 60 m) thick, irregular in shape, and from a few square miles to
300 mi sup 2 (680 km sup 2) in area. Ice islands originate in the landfast ice
along the high-latitude Northern shores of the Canadian archipelago and
Greenland. This distinguishes them from the smaller and more rugged icebergs
which originate in the glaciers of Greenland. The unbroken appearance of the
ice islands contrasts greatly with the surrounding pack ice, which normally
almost completely covers the Arctic Ocean with a maximum thickness of 20 ft (6
m).
    Ice islands are tabular features without the pressure ridges found on pack
ice. Long, shallow drainage channels are most evident in summer when they are
filled with meltwater. The presence of rock piles on the surface and of dust
and dirt layers within the ice, as well as plant and animal remains, testifies
that the ice islands were close to land at one time.
 �34s[Kenneth Hunkins]
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�6v�2v�32sIce point   �21m �33s 
    The temperature at which liquid and solid water are in equilibrium under 
atmospheric pressure. The ice point is by far the most important ``fixed 
point'' for defining temperatures scales and for calibrating thermometers. A 
closely related point is the triple point, which has gained favor as the 
primary standard since it can be attained with great accuracy in a simple 
closed vessel, isolated from the atmosphere.  See  TEMPERATURE; TRIPLE POINT 
(THERMODYNAMICS).  
 �34s[Ralph A. Burton]
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�6v�2v�32sIchneumon   �21m �33s 
    A member of the hymenopteran family Ichneumonidae. Included are more than 
10,000 species of minute to large parasitic wasps which have a worldwide 
distribution. Ichneumons parasitize their victims by depositing eggs on or 
within the host. Endoparasitic forms undergo a complete metamorphosis in the 
host body with a number of instars or developmental stages; the adults emerge 
from the pupal or chrysalis stage of the host.  See  HYMENOPTERA; INSECTA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sIchthyopterygia   �21m �33s 
    A subclass of extinct reptiles composed of predatory fish-finned and 
sea-swimming forms of the Mesozoic Era. All are much alike in body form: 
short-necked, streamlined, swordfishlike or porpoiselike in body, yet 
unrelated to either fishes or porpoises. A single order, Ichthyosauria, is 
included in this subclass.  See  REPTILIA. 
 �34s[Charles L. Camp]
�33s 
  
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�6v�2v�32sIchthyornithiformes   �21m �33s 
    An order of ancient fossil birds from the Upper Cretaceous marine chalk 
beds of the Niobrara formation. Two families are included: Ichthyornithidae, 
with seven species known from Kansas and Texas, and Apathornithidae, with one 
species each from Kansas and Wyoming. The Ichthyornithiformes possess all 
skeletal characteristics of modern birds (with some exceptions) and appear to 
be strong flying birds of inland seas, somewhat gull-like in their proportions 
and presumably in their habits.  See  AVES. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sIchthyostegalia   �21m �33s 
    An order comprising the oldest tetrapods yet found - labyrinthodont 
amphibians collected from Late Devonian fresh-water deposits of eastern 
Greenland. Two groups are distinguished, ichthyostegids and acanthostegids. 
The former includes the genus Ichthyostega (the ``four-legged fish''), the 
bony parts of which have been reconstructed. The fishlike tail with dermal fin 
rays and the vestigial intracranial joint are holdovers of their piscine 
ancestors, the osteolepiform crossopterygians. By these features, in 
combination with pentadactyl limbs and otic notches for the eardrums, the 
ichthyostegalians bridge the gap between fish and tetrapods. The humerus is 
complex; the scales exhibit a trend toward elimination; and the chest skeleton 
is composed of large, broad ribs forming a trunk armor.  See  AMPHIBIA; 
LABYRINTHODONTIA.  
 �34s[Erik Jarvik]
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�6v�2v�32sIctidosauria   �21m �33s 
    An order composed of a few extremely mammal-like reptiles (subclass 
Synapsida). Representatives occur in the uppermost Triassic (probably also 
Lower Jurassic) deposits. One group, the tritylodonts, is known from North 
America, Asia, Europe, and South Africa. Ictidosaurs proper are known from 
South Africa only. All ictidosaurs were small animals, no more than 3 ft (1 m) 
in length. The order included both carnivores and herbivores and seems to have 
been restricted to terrestrial life. Ictidosaurs represent one of the sources 
of mammals.  See  MAMMALIA; SYNAPSIDA. 
 �34s[Everett C. Olson]
�33s 
  
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�6v�2v�32sIdeal aerodynamics   �21m �33s 
    The oldest branch of analytical aerodynamics, also referred to as ideal 
fluid dynamics. Although limited in its application, it is useful for 
explaining some airflow problems and for providing approximate answers. It is 
also still studied because many other analytical methods in aerodynamics 
presuppose a knowledge of this earlier work. 
    The use of a set of five basic simplifying assumptions characterizes ideal 
aerodynamics: (1) The flow is two-dimensional; that is, there exists a 
direction in which the flow characteristics do not change. (2) The flow is 
steady; that is, the flow characteristics do not change with time. (3) The 
flow is incompressible; that is, density does not change. (4) The fluid is 
nonviscous; that is, the effects of viscosity are ignored except, in some 
cases, indirectly. (5) The flow is irrotational; that is, the curl of the 
velocity vector is zero. 
    The assumption of constant density limits application to flows with a Mach 
number below about 0.6, depending on the accuracy desired.  See  AERODYNAMICS; 
LAPLACE'S IRROTATIONAL MOTION; FLUID FLOW; SUBSONIC FLIGHT. 
 �34s[Frank McL. Mallett]
�33s 
  
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�6v�2v�32sIdocrase   �21m �33s 
    A sorosilicate mineral of complex composition, with formula 
Ca10Al4(Mg,Fe)2Si9O34(OH)4, crystallizing in the tetragonal system; also known 
by the name vesuvianite. Crystals, frequently well formed, are usually 
prismatic with pyramidal terminations. The luster is vitreous to resinous; the 
color is usually green or brown but may be yellow, blue, or red. Hardness is 
6** on Mohs scale; specific gravity is 3.35 - 3.45.  See  SILICATE MINERALS. 
    Idocrase is found characteristically in crystalline limestones resulting 
from contact metamorphism. Noted localities are Zermatt, Switzerland; 
Christiansand, Norway; River Vilui, Siberia; and Chiapas, Mexico. In the 
United States it is found at Sanford, Maine; Franklin, New Jersey; Amity, New 
York; and at many contact metamorphic deposits in western states. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sIgneous rocks �21m �33s
    Those rocks which have congealed from a molten mass. They may be composed
of crystals or glass or both, depending on the conditions of formation. The
molten matter from which they come is called magma; where erupted to the
surface, it is commonly known as lava. Solidification of the hot rock melt
occurs in response to loss of heat. Generated at depth the magma tends to
rise. It commonly breaks through the Earth's crust and spills out on the
Earth's surface or ocean floor to form volcanic or extrusive rocks. At the
surface where cooling is rapid, fine-grained or glassy rocks are formed.
    Where unable to reach the surface, magma cools more slowly, insulated by
the overlying rocks; and a coarser texture develops. The resulting igneous
rocks appear intrusive relative to adjacent rocks. In general, deeply formed
(plutonic) rocks display the coarsest texture. Igneous rocks formed at shallow
depths (hypabyssal) display features somewhat intermediate between those of
volcanic and plutonic types.  See  MAGMA; PLUTON; VOLCANO; VOLCANOLOGY.
    Texture refers to the mutual relation of the rock constituents within a
uniform aggregate. It is dependent upon the relative amounts of crystalline
and amorphous (glassy) matter as well as the size, shape, and arrangement of
the constituents. Rock textures are highly significant; they shed light on the
problem of rock genesis, and tell much about the conditions and environment
under which the rock formed.
    Schemes for classifying igneous rocks are numerous. Three principal
methods of classification are used. (1) Megascopic schemes are based on the
appearance of the rock-in-hand specimen or as seen with a magnifying glass
(hand lens). Such schemes are useful in the field study of rocks. (2)
Microscopic schemes (largely mineralogical) are employed in laboratory
investigations where more detailed information is needed. (3) Chemical schemes
are very useful but have more limited application. The mineral content and
texture of a rock generally tell much more about the rock's origin than does a
bulk chemical analysis. Igneous rocks show great variations chemically,
mineralogically, texturally, and structurally with few if any natural
boundaries.
    Plutonic rocks occur in large intrusive masses (batholiths, stocks, and
other large plutons). They form at great depth and are often referred to as
abyssal rocks. They are generated from large bodies of magma which cooled
slowly.
    Volcanic rocks are formed as lava flows or as pyroclastic rocks
(heterogeneous accumulations of volcanic ash and coarser fragmental matter).
They have solidified rapidly, and expanding gas bubbles formed by escaping
volatiles frequently create highly porous rocks.
    Hypabyssal rocks exhibit characteristics more or less intermediate between
those of volcanic and plutonic types. They differ from volcanic rocks in that
they are intrusive and generally free from glass and vesicular structures.
They differ from plutonic rocks in that they occur in small bodies (dikes and
sills) or in larger bodies formed at shallow depths (laccoliths) and they have
textures characteristically resulting from more rapid cooling.	See
ANORTHOSITE; BASALT; DIABASE; FELDSPAR; FELDSPATHOID; GABBRO; GRANITE;
GRANODIORITE; LABRADORITE; LEUCITE ROCK; MONZONITE; NEPHELINE SYENITE;
PYROXENITE; QUARTZ; RHYOLITE; SYENITE; THERALITE; TRACHYTE.
    Igneous rock-forming minerals may be classed as primary or secondary. The
primary minerals are those formed by direct crystallization from the magma.
Secondary minerals may form at any subsequent time. The principal primary
minerals are relatively few and may be classed as light-colored (felsic) or
dark-colored (mafic) varieties. Felsic is a mnemonic term for feldspathic
minerals (feldspar and feldspathoids) and silica (quartz, tridymite, and
cristobalite). Mafic is mnemonic for magnesium and iron-rich minerals
(biotite, amphibole, pyroxene, and olivine). Felsic minerals are composed
largely of silica, alumina, and alkalies. Mafics are rich in iron, magnesium,
and calcium.
    Secondary minerals include minerals formed by addition of material
subsequent to solidification of the rock or by alteration of minerals already
present in the rock. Alteration in which certain minerals become more or less
reconstituted is common and widespread. The common alteration products derived
from the essential primary minerals are as follows.

	Primary mineral 		Secondary mineral
	Quartz			Not altered
	Potash feldspar 	Kaolinite, sericite
	Plagioclase		Kaolinite, sericite (paragonite), epidote,
				  zoisite, calcite
	Nepheline		Cancrinite, analcite, natrolite
	Leucite 		Nepheline and potash feldspar
	Sodalite		Analcite, cancrinite
	Biotite 		Chlorite, sphene, epidote, rutile, iron oxide
	Hornblende		Actinolite, biotite, chlorite, epidote, calcite
	Orthopyroxene		Antigorite, actinolite, talc
	Clinopyroxene		Hornblende, actinolite, biotite, chlorite,
				epidote, antigorite
	Olivine 		Serpentine, magnetite, talc, magnesite

See  PETROLOGY.
 �34s[Carleton A. Chapman]
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�6v�2v�32sIgnition system   �21m �33s 
    The ignition system of an internal combustion engine initiates the 
chemical reaction between fuel and air in the cylinder charge. For maximum 
efficiency it is necessary to ignite the charge in each cylinder shortly 
before the piston reaches the top of its compression stroke. The best timing 
is determined experimentally for various engine speeds and loadings, and the 
ignition system is designed to provide this timing automatically. For smooth 
running, multicylinder engines are usually built so that the various pistons 
arrive at their firing top center positions in evenly spaced intervals. The 
sequence in which the cylinders reach their firing points depends upon the 
geometrical arrangement of the cylinders and crankshaft, and is called the 
firing order of the engine. The ignition system distributes the ignition 
impulse to the cylinders in this order. See INTERNAL COMBUSTION ENGINE. 
    A large class of engines, including most automobile engines, operate on a 
premixed charge of fuel vapor and air. In these engines the charge is ignited 
by passing a high-voltage electric current between two electrodes in the 
combustion chamber. The electrodes are incorporated in a removable unit. When 
a spark of sufficient energy jumps the gap between the electrodes, a 
self-propagating flame is produced in the fuel-air mixture which spreads 
rapidly throughout the charge.  See  COMBUSTION CHAMBER; DISTRIBUTOR; SPARK 
PLUG.  
    The electric energy for the spark is obtained from a storage battery in 
practically all spark-ignition engines. The battery, usually 6 or 12 volts, is 
part of a low-voltage primary circuit, which includes a switch, the primary 
winding of the ignition coil, a capacitor (or condenser), and breaker points.  
See  STORAGE BATTERY. 
    The magneto system is similar to the battery system except that the 
voltage required to cause a flow of current in the primary winding is 
generated by the rotation of a set of permanent magnets, instead of being 
supplied by a battery. The magneto is often used with small compact 
installations such as lawnmowers, chain saws, and small outboard engines, 
where a battery would represent undesirable weight and bulk. 
 �34s[Augustus R. Rogowski]
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�6v�2v�32sIguanid   �21m �33s 
    A member of the reptilian family Iguanidae, in the order Squamata. The 400 
species, found in the New World only, are adapted to various modes of life: 
terrestrial, arboreal, and fossorial (burrowing); some species are 
semiaquatic. They are mainly insectivorous, although there are herbivorous and 
omnivorous forms. The teeth are fixed to the inner edge of the jaws; the 
fleshy tongue is nonretractile; the body is compressed; the tail is long but 
rarely prehensile; the limbs have five clawed toes; and many species have 
dorsal or caudal spines, lappets, and crests. Many iguanids have the ability 
to change color and are commonly but erroneously referred to as chameleons.  
See  REPTILIA; SQUAMATA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sIleitis �21m �33s
    Regional ileitis is a sharply demarcated inflammation of the lower portion
of the small intestine. Actually, other portions of the small and large
intestine may be involved, so regional enteritis or regional enterocolitis may
be preferred terms.
    The disease is marked by intestinal obstruction, pain, cramps, diarrhea,
and constipation. There is often associated weight loss or anemia. The cause
is unknown, but faulty protein absorption, psychic factors, and abnormal lymph
drainage have been implicated. Although regional ileitis is found most often
in young adults, no age group is immune.
    In contrast to regional ileitis, ordinary ileitis is any inflammation of
the ileum from specific causes such as trauma or infections.  See  INTESTINE;
INTESTINE DISORDERS.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sIllite   �21m �33s 
    A general term for the mica-type clay minerals. Illite clays are used for 
the manufacture of structural clay products, such as brick and tile. Some 
degraded high plastic illites are used for bonding molding sands.  See  CLAY; 
CLAY MINERALS. 
    Illite is a common product of weathering if potash is present in the 
environment of alteration. It is a frequent constituent in many soil types, 
and may form in soils under certain conditions as a consequence of the 
addition of potash fertilizers. Illite is common in recent sediments and is 
particularly abundant in deep-sea clays. Because of its stability, illite is 
often found in ancient sediments. 
 �34s[Floyd M. Wahl ;Ralph E. Grim]
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�6v�2v�32sIllmenite   �21m �33s 
    A rhombohedral mineral with composition Fesup2suppTisup4suppO3. The 
hardness is 5** on Mohs scale, specific gravity 4.72. Color is black, and 
there is no cleavage. The mineral usually occurs massive or in thin plates. 
Two other minerals belonging to the ilmenite structure type are geikielite, 
MgTiO3, and pyrophanite, MnTiO3. 
    Ilmenite is the most abundant titanium mineral in igneous rocks and the 
most important ore of titanium. Important occurrences include the Ilmen 
Mountains, Soviet Union (whence the name); Kragero, Norway; and Allard Lake, 
Quebec. See  TITANIUM. 
 �34s[Paul B. Moore]
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�6v�2v�32sIlluminance   �21m �33s 
    The density of luminous flux incident on a surface. This word has been 
proposed by the Colorimetry Committee of the Optical Society of America to 
replace the term illumination. The definitions are the same. The symbol of 
illumination is E, and the equation is E = dF/dA, where A is the area of the 
illuminated surface and F is the luminous flux.  See  ILLUMINATION; PHOTOMETRY. 
 �34s[Russell C. Putnam]
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�6v�2v�32sIllumination �21m �33s
    In a general sense, the science of the application of lighting. Radiation
in the range of wavelengths of 0.38 - 0.76 micrometer produces the visual
effect, commonly called light, by the response of the average human eye for
normal (photopic) brilliance levels. Illumination engineering pertains to the
sources of lighting and the design of lighting systems which distribute light
to produce a comfortable and effective environment for seeing. In a specific
quantitative sense, illumination is the combination of the spatial density of
radiant power received at a surface and the effectiveness of that radiation in
producing a visual effect.  See  ILLUMINANCE.
 �34s[Warren B. Boast]
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�6v�2v�32sImage processing   �21m �33s 
    The enhancement of images by systems in which the input into the system is 
an image and the output is also an image that has been improved in some way. 
The image may consist of a photographic negative, a medical thermogram, or a 
nuclear medicine image comprising a collection of dots, with the density of 
dots related to the distribution of a radioactive pharmaceutical in the body. 
    An image-processing system may have one of two goals: it either makes the 
image more pleasing or more useful to a human observer, or it actually 
performs some of the interpretation and recognition tasks usually performed by 
humans. The latter goal is often referred to as image analysis or pattern 
recognition.  
    Of the various types of hardware that can be used for image enhancement, 
the digital computer is the most generally useful and perhaps also the easiest 
to understand conceptually. Suppose that the initial image is a photographic 
transparency. The first step is to convert the density levels in the image 
into numbers that can be entered into the computer memory. This can be 
accomplished with a microdensitometer in which a small, rectangular aperture 
is used to isolate a single picture element, or pixel. The light transmitted 
through this element is measured with a photodetector, digitized, and stored. 
Then the aperture is moved over by its width, and the density of the adjacent 
picture element is recorded. In this way the entire picture is converted into 
a matrix of numbers suitable for computer manipulation. The most common 
example of computer manipulation is contrast enhancement, which is very useful 
if the input image is underexposed or depicts a low-contrast scene. A second 
example is a pseudocolor display, in which each gray level in the input is 
encoded as a color in the output. Once the computer manipulations are 
complete, the processed image may be displayed on a cathode-ray tube, a 
mechanical printer, or any other suitable device.  See  DIGITAL COMPUTER. 
 �34s[Harrison H. Barrett]
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�6v�2v�32sImhoff tank   �21m �33s 
    A sewage treatment tank named after its developer, Karl Imhoff. Imhoff 
tanks differ from septic tanks in that digestion takes place in a separate 
compartment from that in which settlement occurs. The tank was introduced in 
the United States in 1907 and was widely used as a primary treatment process 
and also in preceding trickling filters. Developments in mechanized equipment 
have lessened its popularity, but it is still valued as a combination unit for 
settling sewage and digesting sludge.  See  SEPTIC TANK; SEWAGE. 
 �34s[William T. Ingram]
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�6v�2v�32sImidazole   �21m �33s 
    One of a group of organic heterocyclic compounds (also called iminazoles, 
glyoxalines, and 1,3-diazoles) containing a five-membered diunsaturated ring 
with two nonadjacent nitrogen atoms as part of the ring. Imidazole is a 
typical member of the group.  See  HETEROCYCLIC COMPOUNDS. 
    Imidazole itself is a water-soluble solid, mp 90DGC (194DGF), which is 
basic enough to form stable salts with both organic and inorganic acids. 
Imidazole is also weakly acidic, since the hydrogen at the 1 position may be 
replaced by metal. The low volatility of imidazole, bp 256DGC (493DGF), is 
indicative of considerable association by intermolecular hydrogen bonding. 
 �34s[Walter J. Gensler]
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�6v�2v�32sImmune complex disease �21m �33s
    Local or systemic tissue injury caused by the vascular deposition of
products of antigen-antibody interaction, termed immune complexes. Immune
complex formation with specific antibodies causes the inactivation or
elimination of potentially harmful consequences only when immune complexes
deposit in tissues, inciting various mediators of inflammation, leading to
injury and disease. When the reaction takes place in the extravascular fluids
near the site of origin of the antigen (by injection, secretion, and such),
focal injury can occur, as exemplified by the Arthus reaction or such
conditions as experimental immune thyroiditis. Systemic disease may occur when
soluble antigens combine with antibodies in the vascular compartment, forming
circulating immune complexes that are trapped nonspecifically in the vascular
beds of various organs, causing such clinical diseases as serum sickness or
systemic lupus erythematosus with vasculitis and glomerulonephritis. The term
immune complex disease usually signifies this systemic immune complex
formation and vascular deposition.  See  IMMUNOLOGY; IMMUNOPATHOLOGY.
 �34s[Edward H. Cole ;Curtis B. Wilson]
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�6v�2v�32sImmunity   �21m �33s 
    A state of resistance to an agent, the parasite, that normally produces an 
infection in one or more host species. Analogous immune responses also occur 
to tumors (cancers), in allergies and delayed hypersensitivity states, in 
autoimmune diseases, and in incompatible blood transfusions or tissue and 
organ transplantations. These responses are specifically lacking in immune 
deficiency diseases. Immunity is a complex resultant of many components, some 
native and hereditable, others acquired. 
    In acquired immunity both the invading parasite and the host may alter 
their offensive and resistive capacities with time. Many pathogenic organisms 
possess chemical components - enzymes, toxins, or surface antigens - which 
adversely affect the suceptible host. After contact with these substances 
(antigens) the host may, in turn, produce neutralizing or inhibiting 
substances (generally, specific antibodies), which check the infection. 
    Some organisms possess natural immunity. For example, animals, including 
humans, are generally insusceptible to even the most virulent plant pathogens, 
and plants are likewise insusceptible to animal pathogens. 
    Active immunity may be acquired upon recovery of an infection. Recovery 
from certain virus diseases, such as yellow fever, measles, and chickenpox, is 
usually attended by a lifelong immunity effective under the conditions of 
natural exposure. At the other extreme, immunity following recovery from the 
common cold is very short-lived or nonexistent.  See  CELLULAR IMMUNOLOGY; 
IMMUNOLOGY; VACCINATION. 
 �34s[Henry P. Treffers]
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�6v�2v�32sImmunoassay   �21m �33s 
    An assay that quantifies antigen or antibody by immunochemical means. The 
antigen can be a relatively simple substance such as a drug, or a complex one 
such as a protein or a virus.  See  ANTIBODY; ANTIGEN. 
    The reactants are first mixed so that a varying quantity of one (A) is 
added to a constant amount of the other (B). The formation of an immune 
(antigen-antibody) complex is measured as a function of the varied reactant 
(A). The result is represented by a ``standard curve'' for reactant A. An 
unknown sample is tested by adding it to reactant B. The extent of the 
measured change is referred to the standard curve, and thereby is obtained the 
amount of reactant A which produces a comparable change. The amount is 
represented as the content of reactant A in the unknown sample.  See  
IMMUNOFLUORESCENCE; IMMUNONEPHELOMETRY; IMMUNOLOGY; RADIOIMMUNOASSAY. 
 �34s[Alexander Baumgarten]
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�6v�2v�32sImmunochemistry �21m �33s
    A discipline concerned both with the structure of antibody
(immunoglobulin) molecules and with their ability to bind an apparently
limitless number of diverse chemical structures (antigens); with the
structure, organization, and rearrangement of the genes coding for the
immunoglobulin molecules; and with the structure and function of molecules on
the surface of animal cells, such as the transplantation (histocompatibility)
antigens, which recognize antibodies and the thymus-derived lymphocytes
mediating the cellular immune response.  See  ANTIGEN; IMMUNOASSAY;
IMMUNOGLOBULIN; RADIOIMMUNOASSAY; TRANSPLANTATION BIOLOGY.
 �34s[William H. Konigsberg ;Frank F. Richards]
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�6v�2v�32sImmunoelectrophoresis   �21m �33s 
    A combination of the techniques of electrophoresis and immunodiffusion 
used to separate the components of a mixture of antigens and make them visible 
by reaction with specific antibodies.  See  ELECTROPHORESIS. 
    A medium such as agar is deposited on a convenient base, for example, a 
microscopic slide. A small well is cut in the medium. A test solution is 
deposited in the well, and the contained substances are separated by 
electrophoresis along one axis of the plate. A trough is then cut in the 
medium parallel to, but at some distance from, the line of the separated 
substances. The trough is filled with antiserum which contains antibodies to 
one or more of the separated substances. The antiserum and substances diffuse 
toward one another and, where they meet, form curvilinear patterns of 
precipitation. These can be seen directly in clear media or can be visualized 
after washing out unreacted materials and staining in opaque media.  See  
IMMUNOASSAY.  
 �34s[Alexander Baumgarten]
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�6v�2v�32sImmunofluorescence   �21m �33s 
    A technique that uses a fluorochrome to indicate the occurrence of a 
specific antigen-antibody reaction. The fluorochrome labels either an antigen 
or an antibody. The labeled reactant is then used to detect the presence of 
the unlabeled reactant. The use of a labeled reactant (such as an antibody 
which both detects and indicates the antigen) to reveal the presence of an 
unlabeled one is termed direct immunofluorescence. The use of a labeled 
indicator antibody, which reacts with an unlabeled detector antibody that has 
previously reacted with an antigen, is termed indirect immunofluorescence. 
Substitution of a light meter for the human eye permits a quantitative 
measurement in immunofluorometry.  See  ANTIGEN-ANTIBODY REACTION; 
FLUORESCENCE; IMMUNOASSAY. 
 �34s[Alexander Baumgarten]
�33s 
  
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�6v�2v�32sImmunoglobulin   �21m �33s 
    Any of a set of serum glycoproteins which have the ability to bind other 
molecules with a high degree of specificity. Molecules which are foreign or 
nonself are called antigens; and when an antigen is introduced into a 
vertebrate, the immunoglobulin induced is called an antibody. The critical 
property of this antibody is that it will combine specifically with the 
inducing antigen.  See  ANTIBODY; ANTIGEN. 
    Human myeloma proteins were the first structurally discrete 
immunoglobulins known, and they have played a central role in the elucidation 
of the antigenic, structural, and functional features of immunoglobulins. 
Myeloma proteins appear to result from the malignant expansion of a normal 
clone of antibody-producing cells; but since they are spontaneous or 
nonspecifically induced, they have no predictable antibody activity. 
    Immunoglobulins are heterogeneous with respect to charge, size, 
antigenicity, and function.  Although charge heterogeneity was initially used 
to define the ``GAMMA-globulins,'' charge is the least useful of these 
criteria for precise characterization. The first separation of immunoglobulins 
by size was in 1937 when horse antipneumoccal antibody was separated into two 
different molecular weight groups: 900,000 daltons and 150,000 daltons. The 
larger antibody has been named immunoglobulin macro (IgM) and the smaller 
immunoglobulin gamma (IgG), reflecting its electrophoretic mobility. 
    Antigenic analysis is the most practical approach in differentiating the 
classes and subclasses of immunoglobulins. The three most abundant classes in 
normal human serum are IgG, IgA, and IgM. Two other classes of antigenically 
distinct immunoglobulins, IgD and IgE, were first recognized as myeloma 
proteins because their concentration in normal human serum was too low to 
detect by the then available techniques. 
    IgG is the most abundant immunoglobulin in serum. The IgG molecule has a 
molecular weight of 150,000 and consists of two pairs of polypeptide chains 
which are covalently linked by disulfide bonds. One is called the Fab, for 
``fragment, antigen binding.'' The other fragment is crystallizable, contains 
no light chain, and does not bind antigen. This is called Fc, for ``fragment, 
crystallizable.''  
    IgA is the second most abundant immunoglobulin in human serum but is the 
primary immunoglobulin of secretions. The nature of IgA differs widely among 
various mammals and between serum and secretions. 
    Immunoglobulin macro or IgM is the largest immunoglobulin, with a 
molecular weight of about 900 kilodaltons. Electron-microscopic studies show 
that the monomeric units are arranged like the spokes of a wheel, with the Fc 
portions at the hub and the antigen-binding Fab portions directed outward. IgM 
is the first immunoglobulin to appear during the primary immune response and 
is usually present only transiently. 
    IgD and IgE are present in minute amounts in normal human serum. No 
function has as yet been attributed to IgD. However, evidence suggests it may 
play a prominent role as a cell surface immunoglobulin and may serve as an 
antigen receptor.  See  IMMUNOLOGY. 
 �34s[J. Donald Capra]
�33s 
  
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�6v�2v�32sImmunology   �21m �33s 
    The division of biological science concerned with the native or acquired 
resistance of higher living forms, or hosts, to infection with microorganisms. 
Immunology is an eclectic science drawing on many other branches of knowledge. 
The comprehensive study of infection and resistance may be divided, 
arbitrarily, into data relating to the pathogen, the host, and their 
interactions during infection.  See  ANTIBODY; ANTIGEN; DISEASE; IMMUNITY; 
INFECTION; PATHOGEN; TOXIN. 
    Immunology is also heavily concerned with assaying the immune status of 
the host through a variety of serological procedures, and in devising methods 
of increasing host resistance through prophylactic vaccination. There has also 
been much important investigation of induced resistance and tolerance to 
transplants of skin and organs, including tumors.  See  ACQUIRED IMMUNOLOGICAL 
TOLERANCE; ANTIGEN-ANTIBODY REACTION; BLOOD GROUPS; CELLULAR IMMUNOLOGY; 
HYPERSENSITIVITY; IMMUNITY; IMMUNOASSAY; INTERFERON; ISOANTIGEN; PHAGOCYTOSIS; 
PRECIPITIN; SEROLOGY; TRANSPLANTATION BIOLOGY; VACCINATION. 
 �34s[Margaret J. Polley]
�33s 
  
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�6v�2v�32sImmunonephelometry   �21m �33s 
    An application of nephelometry to the quantification of antigen or 
antibody. The technique depends on the light-scattering properties of 
microparticles. The initial antigen-antibody complexes are macromolecular in 
size and do not scatter light. However, such complexes have the ability to 
aggregate. The size of the aggregates increases with time until they do 
scatter light. This process takes from several minutes to hours before it 
attains maximum, depending on various properties of the reactants. If other 
aspects are kept constant, including, for example, the concentration of one 
reactant, then both the rate at which the scatter of light increases and its 
maximal value increase with the concentration of the other reactant. Either 
the increase in the rate of scatter or its maximal value can be calibrated for 
various concentrations of a reactant to yield a concentration-response 
calibration curve. Unknown concentrations of that reactant can then be 
quantified by measuring the response and comparing it with the calibration 
curve.  See  ANTIBODY; ANTIGEN; IMMUNOASSAY; NEPHELOMETRIC ANALYSIS. 
 �34s[Alexander Baumgarten]
�33s 
  
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�6v�2v�32sImmunopathology   �21m �33s 
    A term used to describe various human and animal (expermental) diseases 
(autoimmune diseases) where humoral and cellular immune factors appear to be 
important in causing pathological damage to cells, tissues, and the host. The 
same immune mechanisms which cause immunopathology are also involved in 
protecting the host against exogenous toxic substances or microorganisms. 
    Certain human diseases have been thought to involve immunological 
mechanisms, especially where any of the following conditions exist: (1) 
Serological (antibodies, complement) abnormalities appear during the course of 
the disease; (2) gamma globulin and complement are found at the site of tissue 
injury (pathology); (3) ``delayed hypersensitivity'' (cellular immunity) 
appears; or (4) immunocompetent (lymphocytes and plasma cells) cells 
predominate at the site of tissue injury. 
    Most patients with rheumatoid arthritis have a circulating immune complex 
consisting of a macroglobulin antibody, the rheumatoid factor, and an antigen, 
the patient's own immunoglobulin G. This rheumatoid factor is therefore 
considered to be an autoantibody, that is, an antibody directed to a molecule 
of the host, to which the host, somehow deciding it is foreign, has made an 
antibody.  See  ARTHRITIS. 
    All patients with systemic lupus erythematosus have antibodies to nuclear 
antigens (deoxyribonucleic acid, nucleohistone, and glycoproteins), and many 
have antibodies to cytoplasmic organelles (mitochondria, lysosomes, and 
ribosomes). Many patients with lupus develop renal disease, especially those 
with antibodies to deoxyribonucleic acid.  See  LUPUS ERYTHEMATOSUS. 
    Patients with poststreptococcal glomerulonephritis also have depression of 
serum complement levels during acute disease. In addition, streptococcal 
antigen, gamma globulin, and complement have been found at the site of 
nephritis.  
    Pathological studies of acute rheumatic fever have demonstrated gamma 
globulin and complement on the myocardium. The disease is thought to involve 
the host making antibodies to some streptococcal antigens during an infection 
with streptococci which then crossreact with a closely related myocardial 
antigen.  See  ANTIBODY; ANTIGEN; IMMUNOLOGY. 
 �34s[Peter H. Schur]
�33s 
  
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�6v�2v�32sImpact   �21m �33s 
    A force, also known as impulsive force, which acts only during a short 
time interval but which is sufficiently large to cause an appreciable change 
in the momentum of the system on which it acts. The momentum change produced 
by the impulsive force is described by the momentum-impulse relation. For a 
discussion of this relation  see  IMPULSE (MECHANICS).  See also  COLLISION 
(PHYSICS).  
 �34s[Paul W. Schmidt]
�33s 
  
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�6v�2v�32sImpedance matching   �21m �33s 
    The use of electric circuits and devices to establish the condition in 
which the impedance of a load is equal to the internal impedance of the 
source. This condition of impedance match provides for the maximum transfer of 
power from the source to the load.  See  ELECTRICAL IMPEDANCE. 
    The maximum power transfer theorem of electric network theory states that 
at any given frequency the maximum power is transferred from the source to the 
load when the load impedance is equal to the conjugate of the generator 
impedance. When these conditions are satisfied, the power is delivered with 
50% efficiency; that is, as much power is dissipated in the internal impedance 
of the generator as is delivered to the load. In general, the load impedance 
will not be the proper value for maximum power transfer. A network composed of 
inductors and capacitors may be inserted between the load and the generator to 
present to the generator an impedance that is the conjugate of the generator 
impedance (see illustration). 
 �34s[Christos C. Halkias]
�33s 
  
  
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�6v�2v�32sImpetigo �21m �33s
    A contagious skin disease characterized by superficial, thin-walled
blisters which may develop into pustules. It is commonly seen in children but
also found in adults. It is caused by either streptococcal or staphylococcal
infections, or both. The disease is autoinoculable; that is, it can be quickly
spread from an original site to any part of the body. No immunity is conferred
by exposure.
 �34s[Edward G. Stuart ;N. Karle Mottet]
�33s

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�6v�2v�32sImpsonite   �21m �33s 
    A black, naturally occurring carbonaceous material having specific gravity 
1.10 - 1.25 and fixed carbon 50 - 85%. The origin of impsonite is not well 
understood, but it appears to be derived from a fluid bitumen that polymerized 
after it filled the vein in which it is found.  See  ASPHALT AND ASPHALTITE. 
 �34s[Irving A. Breger]
�33s 
  
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�6v�2v�32sImpulse generator   �21m �33s 
    An electrical apparatus which produces very short surges of high-voltage, 
or high-current, power. High impulse voltages are used to test the strength of 
insulators and of power equipment against lightning and switching surges. 
High-current impulses are produced by a discharge of capacitors connected in 
parallel. Such current surges may be used to magnetize permanent magnets or to 
produce the rising magnetic field in circular particle accelerators. 
 �34s[John G. Trump]
�33s 
  
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�6v�2v�32sImpulse turbine   �21m �33s 
    A prime mover in which fluid (water, steam, or hot gas) under pressure 
enters a stationary nozzle where its pressure (potential) energy is converted 
to velocity (kinetic) energy. The accelerated fluid then impinges on the 
blades of a rotor, imparting its energy to the blades to produce rotation and 
overcome the connected rotor resistance. The impulse principle is basic to 
many turbines (see illustration).  See  GAS TURBINE; HYDRAULIC TURBINE; PELTON 
WHEEL; REACTION TURBINE; STEAM TURBINE; TURBINE PROPULSION. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sImpulse (mechanics)   �21m �33s 
    The integral of a force over an interval of time. For a force F, the 
impulse J over the interval from t sub 0 to t sub 1 can be written as Eq. (1).
  
            *** J = INTEGRATION (t sub 0 TO t sub 1) F dt ***            (1)
  
The impulse thus represents the product of the time interval and the average 
force acting during the interval. Impulse is a vector quantity with the units 
of momentum.  
    The momentum-impulse relation states that the change in momentum of a mass 
m over a given time interval equals the impulse of the resultant force acting 
during that interval. The momentum change can be expressed in terms of the 
velocities v sub 1 and v sub 0 at times t sub 1 and t sub 0, respectively, 
giving Eq. (2). 
  
                     *** J = m[(v sub 1) - (v sub 0)] ***                (2)
  
See IMPACT; MOMENTUM. 
 �34s[Paul W. Schmidt]
�33s 
  

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�6v�2v�32sInadunata   �21m �33s 
    An extinct subclass of stalked Crinoidea comprising some 300 Paleozoic 
genera, ranging from the Upper Cambrian (Tremadocian) to the Permian. The arms 
(3 - 5 or more) were completely free and in no way incorporated into the calyx 
(see illustration). The calyx was either monocyclic or dicyclic, with 
conspicuous radials and basals, and all the thecal plates were sutured 
together into a rigid system.  See  CRINOIDEA; ECHINODERMATA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sInarticulata   �21m �33s 
    A class of phylum Brachiopoda. It is presently divided into four orders: 
Paterinida, Obolellida, Acrotretida, and Lingulida.  See  ACROTRETIDA; 
LINGULIDA; OBOLELLIDA; PATERINIDA. 
    The two valves of the shell are typically not articulated and are held 
together only by soft tissue of the living animal. A few genera developed 
hinge mechanisms posteriorly. The posterior sector of one or both valves is 
commonly flattened. Internally, muscle scars and mantle canal impressions are 
usually the only features present; a median ridge or elevated septum may be 
developed, particularly in the brachial valve. In all inarticulates, the 
fibrous secondary layer characteristic of the Articulata is undeveloped.  See  
ARTICULATA (BRACHIOPODA); BRACHIOPODA. 
 �34s[A. J. Rowell]
�33s 
  
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�6v�2v�32sIncandescence �21m �33s
    The emission of visible radiation by a hot body. A theoretically perfect
radiator, called a blackbody, will emit radiant energy according to Planck's
radiation law at any temperature. Prediction of the visual brightness requires
additional consideration of the sensitivity of the eye, and the radiation will
be visible only for temperatures of the blackbody which are above some
minimum. The relation between brightness and temperature is plotted in the
illustraton. As shown, the minimum temperature for incandescence for the
darkadapted eye is about 390DGC (730DGF). Under these ideal ob-serving
conditions, the incandescence appears as a colorless glow. The dull red light
commonly associated with incandescence of objects in a lighted room requires a
temperature of about 500DGC (930DGF).  See  BLACKBODY; HEAT RADIATION;
INCANDESCENT LAMP; VISION.
 �34s[Howard W. Russell /George R. Harrison]
�33s




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�6v�2v�32sIncandescent lamp   �21m �33s 
    An electric lamp that produces light by heating a metallic filament to 
intense heat by passing an electric current through it. It is designed to 
produce light in the visible portion (at wavelengths of 380 - 760 micrometers) 
of the electromagnetic spectrum. The filament is prepared of special materials 
(commonly tungsten) and is enclosed in either an evacuated enclosure or one 
filled with an inert gas. In addition to radiation in the visible spectrum, 
infrared and ultraviolet energy are emitted, thus lowering the luminous 
efficiency of the lamp. When either of these radiations is accentuated, 
however, the lamp may be used as a source of that radiant energy.  See 
INCANDESCENCE; LIGHT. 
    The important parts of an incandescent lamp are the lamp enclosure or 
bulb, the filament, and the base. Standard lamps have various bulb shapes, 
bases, and filament constructions. The bulb may be clear, colored, 
inside-frosted, or coated with diffusing or reflecting material. Most lamps 
have soft-glass bulbs; hard glass is used when the lamp will be subjected to 
sudden and severe temperature changes. 
    Lamps are built for various voltage conditions, the most common being 115, 
120, and 125 volts. High-voltage lamps are designed for ratings of 220 - 260 
volts and low-voltage lamps for 6 - 64 volts. Lamps for use in 525 - 625 volt 
systems are designed to operate in groups of five in series across the line. 
    Incandescent lamps have been developed for many services. Most common are 
those used in general service and the miniature lamp. Special types have been 
developed for rough service applications, bake-oven use, severe vibration 
applications, showcase lamps, multiple lights (three-way lamp), sign lamps, 
spotlights, floodlights, and insect-control lamps. For other types of 
incandescent lamps  see  ARC LAMP; INFRARED LAMP. 
 �34s[John O. Kraehenbuehl]
�33s 
  
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�6v�2v�32sIncendiary   �21m �33s 
    One of a number of flammable materials and devices that are used to set 
fire to tactical and strategic targets, such as buildings, industrial 
installations, and fuel and ammunition dumps. Flame warfare extends also to 
antipersonnel use in the case of flamethrowers and fire bombs. 
 �34s[Seymour D. Silver]
�33s 
  
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�6v�2v�32sInclined plane   �21m �33s 
    A plane surface inclined at an angle with the line of action of the force 
that is to be exerted or overcome. 
    In the free-body diagram shown here, three forces act on the object when 
no friction is present. The forces are its weight W, the force F sub p parallel 
to the surface, and a force F sub n normal to the surface. The summation of the 
forces acting in any direction on a body in static equilibrium equals zero; 
therefore, the summation of forces parallel to and forces normal to the surface 
are given by Eqs. (1) and (2). A force slightly greater than W sin THETA moves 

                       *** F sub p - W sin THETA = 0 ***                 (1)

                       *** F sub n - W cos THETA = 0 ***                 (2)

the object up the incline, but the inclined plane supports the greater part of 
the weight of the object. The principal use of the inclined plane is as ramps 
for moving goods from one level to another. See SIMPLE MACHINE. 
 �34s[Richard M. Phelan]
�33s 
  

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�6v�2v�32sInclinometer �21m �33s
    An instrument for measuring magnetic dip, or inclination, which is defined
as the angle between the magnetic field vector and the horizontal plane.
    In the form of a simple dip needle, the instrument is a thin, pointed,
mechanically balanced bar magnet mounted on the horizontal bearings. When
oriented so that its plane of rotation is in the magnetic meridian, the needle
aligns itself with the direction of the magnetic field and indicates on a
circular scale the angle of dip. It is used principally in mine surveying
where the minerals have a high content of magnetic materials.  See  EARTH
INDUCTOR; MAGNETOMETER.
 �34s[James H. Nelson]
�33s


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�6v�2v�32sInclusion blennorrhea   �21m �33s 
    Inclusion conjunctivitis of the newborn; an inflammation of the 
conjunctiva of the baby's eye caused by an agent of the PLT-Bedsonia group. 
The adult form of this disease is also called inclusion conjunctivitis. See 
PLT-BEDSONIA GROUP. 
    So-called swimming-pool conjunctivitis is of the same etiology as 
conjunctivitis of the newborn. Infection of the newborn's eyes comes from the 
cervix of the mother, and since the reservoir is the genital tract, water of 
swimming pools is probably infected by discharges from the genital tracts of 
infected bathers. Chlorination of the water eliminates this source of adult 
conjunctivitis.  
 �34s[Karl F. Meyer ;Julius Schacht]
�33s 
  
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�6v�2v�32sIncompressible flow   �21m �33s 
    Fluid motion without change in density. For practical purposes liquids are 
assumed to flow incompressibly. At low velocity this is nearly the case; 
however, even for liquids, abrupt changes in velocity produce compression or 
rarefaction. In general, whenever fluid velocities are low (less than a 
fourth) relative to the rate of propagation of a pressure wave in the fluid, 
density variations during flow will be negligible and the flow will be 
effectively incompressible. 
    Important applications of incompressible flow theory include the 
aerodynamics of aircraft moving at low airspeeds, surface vehicles, and 
airflow in various industrial processes such as steelmaking.  See  AERODYNAMIC 
FORCE; BERNOULLI`S THEOREM; COMPRESSIBLE FLOW; FLUID FLOW; GAS DYNAMICS. 
 �34s[James E. May]
�33s 
  
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�6v�2v�32sIndex fossil   �21m �33s 
    The ancient remains and traces of a plant or animal that lived during a 
particular span of geologic time and that geologically dates the containing 
rocks. Index fossils are almost exclusively confined to sedimentary rocks 
which originated in such diverse environments as open oceans, tropical 
lagoons, coral reefs, beaches, lakes, and rivers. The fossil groups most 
useful as index fossils are generally marine and either floaters or open ocean 
swimmers, such as cephalopods, or bottom dwellers that had a floating or 
swimming stage in their life cycles, such as the medusa stage in the 
brachiopods. Such characteristics are necessary for rapid dispersal of newly 
evolved forms. On land, such mobile forms as the horses or wind-borne pollen 
and spores were relatively unrestricted by environmental barriers and became 
widely dispersed. All of these groups have provided biochronological zones of 
worldwide extent. 
    During the Cambrian Period (600,000,000 years before present) the oldest 
highly developed animals appeared; among them the trilobites provide the first 
important group of index fossils. Small plantlike floating colonial animals 
called graptolites have proved useful in correlating Ordovician (500,000,000 
years B.P.) and Silurian (425,000,000 years B.P.) rocks. Ammonoids are a 
classic example of the internationally useful index fossil and are important 
beginning in the Devonian Period (405,000,000 years B.P.) and extending to the 
end of the Cretaceous Period (135,000,000 years B.P.). From the Pennsylvanian 
Period (310,000,000 years B.P.) fusulinids, a family of Foraminiferida, and 
pollen and spores from the coal forests are important indices.  See  
BRACHIOPODA; CEPHALOPODA; CONODONT; FORAMINIFERIDA; FOSSIL; FUSULINACEA; 
GEOLOGICAL TIME SCALE; GRAPTOLITHINA; STRATIGRAPHIC NOMENCLATURE; STRATIGRAPHY. 
 �34s[Charles Collinson]
�33s 
  
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�6v�2v�32sIndex mineral   �21m �33s 
    In metamorphic petrology, a characteristic mineral which by its presence 
in a rock indicates the mineral facies of the rock. An index mineral 
characterizes the mode of origin of a metamorphic rock in regard to 
temperature, pressure, and composition. A mineral such as quartz is not an 
index mineral because quartz is found in a great variety of rocks formed from 
low to high temperature and pressure. However, minerals with stability fields 
within restricted parts of the geologically important temperature-pressure 
range (by a given bulk composition of the containing rock) usually may be 
regarded as index minerals. In a crude way they are also geological 
thermometers and pressure gages.  See  GEOLOGIC THERMOMETRY; METAMORPHIC ROCKS. 
 �34s[T. F. W. Barth]
�33s 
  
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�6v�2v�32sIndian Ocean   �21m �33s 
    The smallest and geologically the most youthful of the three oceans. It 
differs from the Pacific and Atlantic oceans in two important aspects. First, 
it is landlocked in the north, does not extend into the cold climatic regions 
of the Northern Hemisphere, and consequently is asymmetrical with regard to 
its circulation. Second, the wind systems over its equatorial and northern 
portions change twice each year, causing an almost complete reversal of its 
circulation.  
    The eastern and western boundaries of the Indian Ocean are 147 and 20DGE, 
respectively. In the southeastern Asian waters the boundary is usually placed 
across Torres Strait, and then from New Guinea along the Lesser Sunda Islands, 
across Sunda Strait and Singapore Strait. 
    The ocean floor is divided into a number of basins by a system of ridges. 
The largest is the Mid-Ocean Ridge, the greater part of which has a rather 
deep rift valley along its center. It lies like an inverted Y in the central 
portions of the ocean and ends in the Gulf of Aden. The Sunda Trench, 
stretching along Java and Sumatra, is the only deep-sea trench in the Indian 
Ocean. East of the Mid-Ocean Ridge, deep-sea sediments are chiefly red clay; 
in the western half of the ocean, globigerina ooze prevails and, near the 
Antarctic continent, diatom ooze. 
    Atmospheric circulation over the northern and equatorial Indian Ocean is 
characterized by the changing monsoons. In the southern Indian Ocean 
atmospheric circulation undergoes only a slight meridional shift during the 
year. The surface circulation is caused largely by winds and changes in 
response to the wind systems. In addition, strong boundary currents are 
formed, especially along the western coastline, as an effect of the Earth's 
rotation and of the boundaries created by the landmasses. 
    North of 10DGS the changing monsoons cause a complete reversal of surface 
circulation twice a year. In February, during the Northeast Monsoon, flow 
north of the Equator is mostly to the west and the North Equatorial Current is 
well developed. Its water turns south along the coast of Somaliland and 
returns to the east as the Equatorial Countercurrent between about 2 and 
10DGS. In August, during the Southwest Monsoon, the South Equatorial Current 
extends to the north of 10DGS; most of its water turns north along the coast 
of Somaliland, forming the strong Somali Current. North of the Equator flow is 
from west to east and is called the Monsoon Current. Parts of this current 
turn south along the coast of Sumatra and return to the South Equatorial 
Current. During the two transition periods between the Northeast and the 
Southwest monsoons in April - May and in October, a strong jetlike surface 
current flows along the Equator from west to east in response to the westerly 
winds during these months.  See  OCEAN CURRENTS. 
    Both semidiurnal and diurnal tides occur in the Indian Ocean. The 
semidiurnal tides rotate around three amphidromic points situated in the 
Arabian Sea, southeast of Madagascar, and west of Perth. The diurnal tide also 
has three amphidromic points: south of India, in the Mozambique Channel, and 
between Africa and Antarctica. It has more the character of a standing wave, 
oscillating between the central portions of the Indian Ocean, the Arabian Sea, 
and the waters between Australia and Antarctica.  See  TIDE. 
 �34s[Klaus Wyrtki]
�33s 
  
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8289792
�6v�2v�32sIndicating gage   �21m �33s 
    A gage having contact points that move as they contact the part being 
inspected (see illustration). The movement is amplified on an indicator whose 
scale designates the limits of tolerance. Snap gages are commercially 
available with indicators instead of fixed contact points. Indicators are also 
used for inspecting out-of-roundness, centerline runout, or taper, by rotating 
the part on one axis or diameter and indicating the ``run out'' of another 
axis or diameter. 
 �34s[Rush A. Bowman]
�33s 
  
  
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8289792
�6v�2v�32sIndium   �21m �33s 
    A chemical element, In, atomic number 49, a member of group III and the 
fifth period of the periodic table. Indium has a relative atomic weight of 
114.82.  
    Indium occurs in the Earth's crust to the extent of about 0.000001% and is 
normally found in concentrations of 0.1% or less. It is widely distributed in 
many ores and minerals but is largely recovered from the flue dusts and 
residues of zinc-processing operations. 
    Indium is used in soldering lead wires to germanium transistors and as a 
component of the intermetallic semiconductor used for germanium transistors. 
Indium arsenide, antimonide, and phosphide are semiconductors with unique 
properties. Other uses of indium are sleeve-type bearings to reduce corrosion 
and wear, glass-sealing alloys, and dental alloys.  See  GERMANIUM. 
 �34s[Edwin M. Larsen]
�33s 
  
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�6v�2v�32sIndole �21m �33s
    The parent compound of a group of organic heterocyclic compounds
containing the indole nucleus, which is a benzene ring fused to a pyrrole ring
as in indole itself (I). The importance of the indole ring lies in its
presence in a large number of naturally occurring compounds.  See  CARBAZOLE;
PYRROLE.
    Indole can exist in two tautomeric forms, the more stable enamine form (I)
and the 3-H-indole or imine form (II). Unsubstituted 3-H-indoles (sometimes
called indolenines) and a structural isomer of indole, isoindole (III), are
not stable, but  have been shown to be reaction intermediates. They are
isolable when properly substituted. 
    Indole (I) is a steam-volatile, colorless solid, melting point 52.5DGC
(126.5DGF), boiling point 253DGC (487DGF). It is found in small amounts in
coal tar, feces, and flower oils. Despite the presence of nitrogen, indole is
not basic in the sense that it dissolves in aqueous acid or turns litmus blue.
In fact, the hydrogen on the nitrogen is about as acidic as an aliphatic
alcohol hydrogen. Indole is an aromatic compound and undergoes electrophilic
substitutions much like benzene, although it is much more reactive than
benzene. Its reactivity is comparable to that of phenol, and it undergoes a
number of reactions similar to those of phenol. Indole itself reacts slowly
with air and rapidly with most oxidizing agents to give intractable polymeric
tars.
    The indole alkaloids are a large group of substances containing the indole
nucleus and can be isolated from plants. They contain a number of
physiologically active materials, such as strychnine, reserpine, some forms of
curare poison, and the rye-fungus drug, ergot. Lysergic acid diethylamide
(LSD) is a synthetic, and not a naturally occurring, substance. There are many
hundreds of other indole alkaloids.  See  ALKALOID; HETEROCYCLIC COMPOUNDS.
 �34s[James M. Bobbitt]
�33s

A700000003414�254s�5v�1v902
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�6v�2v�32sInductance   �21m �33s 
    That property of an electric circuit or of two neighboring circuits 
whereby an electromotive force is induced (by the process of electromagnetic 
induction) in one of the circuits by a change of current in either of them. 
The term inductance coil is sometimes used as a synonym for inductor, a device 
possessing the property of inductance.  See  ELECTROMAGNETIC INDUCTION; 
ELECTROMOTIVE FORCE  ( EMF); INDUCTOR. 
    For a given coil, the ratio of the electromotive force of induction to the 
rate of change of current in the coil is called the self-inductance of the 
coil. An alternative definition of self-inductance is the number of flux 
linkages per unit current. Flux linkage is the product of the flux and the 
number of turns in the coil. Self-inductance does not affect a circuit in 
which the current is unchanging; however, it is of great importance when there 
is a changing current, since there is an induced emf during the time that the 
change takes place. For example, in an alternating-current circuit, the 
current is constantly changing and the inductance is an important factor. 
    The mutual inductance of two neighboring circuits is defined as the ratio 
of the emf induced in one circuit to the rate of change of current in the 
other circuit. 
    The mks unit of mutual inductance is the henry, the same as the unit of 
self-inductance. The same value is obtained for a pair of coils, regardless of 
which coil is the starting point. 
    The mutual inductance of two circuits may also be expressed as the ratio 
of the flux linkages produced in a circuit by the current in a second circuit 
to the current in the second circuit.  See  INDUCTANCE MEASUREMENT. 
 �34s[Kenneth V. Manning]
�33s 
  
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�6v�2v�32sInductance bridge �21m �33s
    A device for comparing inductances. The inductance bridge is a special
case of an alternating-current impedance bridge. Just as the Wheatstone bridge
is used to compare resistances, the impedance bridge is used to compare
impedances which may contain inductance, capacitance, and resistance.
    The inductance bridge shown in the illustration has resistors Rsuba and
Rsubb as ratio arms and compares an unknown impedance Zsubc to a standard
consisting of Rsubd and Lsubd. If the standard Lsubd is variable, it and Rsubd
are varied to reduce the detector (D) voltage to zero. This balances the
bridge, and the equations of balance become

	L sub c /L sub d = R sub c/R sub d = R sub a /R sub b

    Theoretically the condition for balance is independent of frequency. In
practice, the capacitance between turns and between layers of wire in the two
coils will be different, and at high frequencies the bridge can be balanced at
only one frequency at a time. See HIGH-FREQUENCY IMPEDANCE MEASUREMENTS;
INDUCTANCE MEASUREMENT.

 �34s[Harry Sohon /Edward C. Steven]
�33s

			     ******FORMULA******


A700000004713�254s�5v�1v902
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903
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�6v�2v�32sInductance measurement   �21m �33s 
    The determination of the electromagnetic parameter of inductance in an 
electric circuit.  See  INDUCTANCE. 
    Coils constructed so that their dimensions and consequently their 
inductance remain constant over long periods of time are used as inductance 
standards. If the dimensions are known, the inductance can be computed. Such 
an inductance is called a primary standard. Standards are usually maintained 
at constant temperature to keep the coil size and therefore the inductance 
from changing in value. When the inductance cannot be computed precisely, it 
may be measured by comparing it with a primary standard, and it would then be 
called a secondary standard.  See  INDUCTOMETER. 
    Impedance measurement is the determination of the total effect of a 
circuit element. The wire of which an inductor is wound has resistance, and 
there will be capacitance between turns. Thus every inductor exhibits 
resistance and capacitance as well as inductance. Also, every resistor 
exhibits some inductance and some capacitance, and every capacitor has some 
inductance and some resistance. Even if the capacitance of an inductor can be 
neglected, a measurement to determine the inductance is usually an impedance 
measurement giving the resistance as well as the inductance.  See  INDUCTANCE 
BRIDGE.  
    Measurement of distributed inductance of cables, lines, and waveguides is 
accomplished indirectly. For very low frequencies the cable currents are 
distributed uniformly throughout the conductors and the inductance per unit 
length is a maximum. At higher frequencies the skin effect causes the current 
to concentrate along the conductor surfaces. This concentration does not 
affect the magnetic field between the conductors, but it does tend to 
eliminate the field within each conductor itself. This reduction in magnetic 
field brings about a reduction in inductance. Consequently, as the frequency 
is increased the inductance decreases, approaching a fixed value. 
    The capacitance per unit length of a cable or open-wire transmission line 
is independent of frequency and can be measured at low frequencies. If direct 
measurements of attenuation and velocity of propagation are made, the 
inductance per unit length of transmission line may be computed at any 
frequency of operation.  See  ELECTRICAL MEASUREMENTS. 
 �34s[Harry Sohon /Edward C. Steven]
�33s 
  
A700000002271�254s�5v�1v903
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8720384
�6v�2v�32sInduction coil   �21m �33s 
    A device for producing a high-voltage alternating current or high-voltage 
pulses from a low-voltage direct current. The largest modern use of the 
induction coil is in the ignition system of internal combustion engines, such 
as automobile engines. Devices of similar construction, known as vibrators, 
are used as rectifiers and synchronous inverters.  See  IGNITION SYSTEM; 
VIBRATOR.  
    The illustration shows a typical circuit diagram for an induction coil. 
The primary coil, wound on the iron core, consists of only a few turns. The 
secondary coil, wound over the primary, consists of a large number of turns. 
    Induction coils of a different type are used in telephone circuits to step 
up the voltage from the transmitter and match the impedance of the line. The 
direct current in the circuit varies in magnitude at speech frequencies; 
therefore, no interrupter contacts are necessary. Still another type of 
induction coil, called a reactor, is really a one-winding transformer designed 
to produce a definite voltage drop for a given current.  See  REACTOR 
(ELECTRICITY).  
  
 �34s[Norman R. Bell]
�33s 
  
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8720384
�6v�2v�32sInduction generator   �21m �33s 
    A nonsynchronous ac generator that is driven above synchronous speed by 
external sources of mechanical power. The construction of these machines is 
identical to that of induction motors. They can operate either as motors or 
generators, depending on whether the speed is below or above synchronous 
speed. In some frequency-changer sets, induction generators may operate as a 
motor part of the time. This is accomplished by coupling them to a two-speed 
synchronous machine capable of operating at system frequency as a motor at the 
higher speed and as a generator at the lower speed. Induction generators are 
not common in large sizes because of their poor power factor. They can deliver 
only leading currents. Moreover, they require a power supply from which to 
obtain their magnetizing current. This normally requires that they be operated 
in parallel with a synchronous source. Capacitors may be used to minimize the 
current taken from the source. Under such conditions the frequency of the 
induction generator is determined by the frequency of the synchronous source, 
and the output of the generator is determined by the mechanical input to its 
shaft. Induction generators do not require any dc excitation, and their 
control can be very simple. For this reason they are well suited to small, 
unattended hydroelectric units, where they are easily operated by remote 
control. See ALTERNATING-CURRENT GENERATOR; INDUCTION MOTOR. 
 �34s[Leon T. Rosenberg]
�33s 
  
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�6v�2v�32sInduction heating   �21m �33s 
    The heating of a nominally electrical conducting material by eddy currents 
induced by a varying electromagnetic field. The principle of the induction 
heating process is similar to that of a transformer. In the illustration, the 
inductor coil can be considered the primary winding of a transformer, with the 
workpiece as a single-turn secondary. When an alternating current flows in the 
primary coil, secondary currents will be induced in the workpiece. These 
induced currents are called eddy currents. The current flowing in the 
workpiece can be considered as the summation of all of the eddy currents. 
    In the design of conventional electrical apparatus, the losses due to 
induced eddy currents are minimized because they reduce the overall 
efficiency. However, in induction heating, their maximum effect is desired. 
Therefore close spacing is used between the inductor coil and the workpiece, 
and high-coil currents are used to obtain the maximum induced eddy currents 
and therefore high heating rates.  See  CORE LOSS. 
    Induction heating is widely employed in the metalworking industry for a 
variety of industrial processes. While carbon steel is by far the most common 
material heated, induction heating is also used with many other conducting 
materials such as various grades of stainless steel, aluminum, brass, copper, 
nickel, and titanium products.  See  BRAZING; HEAT TREATMENT (METALLURGY); 
SOLDERING; WELDING AND CUTTING OF METALS. 
    The advantages of induction heating over the conventional processes (like 
fossil furnace or salt-bath heating) are the following: (1) Heating is induced 
directly into the material. It is therefore an extremely rapid method of 
heating. It is not limited by the relative slow rate of heat diffusion in 
conventional processes using surface-contact or radiant heating methods. (2) 
Because of skin effect, the heating is localized and the heated area is easily 
controlled by the shape and size of the inductor coil. (3) Induction heating 
is easily controllable, resulting in uniform high quality of the product. (4) 
It lends itself to automation, in-line processing, and automatic-process cycle 
control. (5) Startup time is short, and standby losses are low or nonexistent. 
(6) Working conditions are better because of the absence of noise, fumes, and 
radiated heat.  See  ELECTRIC HEATING. 
 �34s[George F. Bobart]
�33s 
  
  
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�6v�2v�32sInduction motor   �21m �33s 
    An alternating-current motor in which the currents in the secondary 
winding (usually the rotor) are created solely by induction. These currents 
result from voltages induced in the secondary by the magnetic field of the 
primary winding (usually the stator). An induction motor operates slightly 
below synchronous speed and is sometimes called an asynchronous (meaning not 
synchronous) motor.  See  ALTERNATING-CURRENT MOTOR. 
    Induction motors are the most commonly used electric motors because of 
their simple construction, efficiency, good speed regulation, and low cost. 
Polyphase induction motors come in all sizes and find wide use where polyphase 
power is available. Single-phase induction motors are found mainly in 
fractional-horsepower (1 horsepower = 746 W) sizes, and those up to 25 hp are 
used where only single-phase power is available. 
    There are two principal types of polyphase induction motors: squirrel-cage 
and wound-rotor machines. The differences in these machines is in the 
construction of the rotor. The stator construction is the same and is also 
identical to the stator of a synchronous motor. Both squirrel-cage and 
wound-rotor machines can be designed for two- or three-phase current. 
    Single-phase induction motors display poorer operating characteristics 
than polyphase machines, but are used where polyphase voltages are not 
available. They are most common in small sizes (** hp or less) in domestic and 
industrial applications. Their particular disadvantages are low power factor, 
low efficiency, and the need for special starting devices. 
 �34s[Albert G. Conrad]
�33s 
  
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�6v�2v�32sInductometer   �21m �33s 
    A coil of wire of known inductance. The inductance may be fixed, as in the 
case of primary standards; or the inductance may be adjustable by means of 
switches, or continuously variable by means of a movable-coil construction. 
    A standard self-inductance is constructed by winding a coil on a form 
having dimensions that are stable. Some materials dry out or absorb moisture 
over the years and consequently change continuously in size. The material on 
which a standard inductance is wound must not be subject to such change. 
Marble and some synthetics have been found suitable. A helical groove is cut 
in the material, and the coil is placed in this groove. See INDUCTANCE; 
INDUCTANCE BRIDGE; INDUCTANCE MEASUREMENT. 
 �34s[Harry Sohon /Edward C. Steven]
�33s 
  
A700000002741�254s�5v�1v904
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�6v�2v�32sInductor �21m �33s
    A device for introducing inductance into a circuit. The term covers
devices with a wide range of uses, sizes, and types, including components for
electric-wave filters, tuned circuits, electrical measuring circuits, and
energy storage devices.
    Inductors are classified as fixed, adjustable, and variable. All are made
either with or without magnetic cores. Inductors without magnetic cores are
called air-core coils, although the actual core material may be a ceramic, a
plastic, or some other nonmagnetic material. Inductors with magnetic cores are
called iron-core coils. A wide variety of magnetic materials are used, and
some of these contain very little iron.
    In fixed inductors coils are wound so that the turns remain fixed in
position with respect to each other. Adjustable inductors have either taps for
changing the number of turns desired, or consist of several fixed inductors
which may be switched into various series or parallel combinations. Variable
inductors are constructed so that the effective inductance can be changed.
Means for doing this include (1) changing the permeability of a magnetic core;
(2) moving the magnetic core, or part of it, with respect to the coil or the
remainder of the core; and (3) moving one or more coils of the inductor with
respect to one or more of the other coils, thereby changing mutual inductance.
  See  INDUCTANCE.
 �34s[Burtis L. Robertson ;Wilson S. Pritchett]
�33s

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�6v�2v�32sInductor alternator   �21m �33s 
    A synchronous generator in which the field winding is fixed in magnetic 
position relative to the armature conductors. There are two types: the 
homopolar, in which the dc field coil is concentric with the shaft, and the 
heteropolar, in which the dc windings are distributed. In both types the ac 
windings are distributed and generate their induced voltage from the pulsation 
in the flux caused by the change in position of the salient poles on the 
rotor. Inductor alternators are used for high-frequency power and, in 
conjunction with static rectifiers, as a maintenance-free power source for ac 
excitation systems. See SYNCHRONOUS GENERATOR.  
 �34s[Leon T. Rosenberg]
�33s 
  
A700000002426�254s�5v�1v904
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8828416
�6v�2v�32sIndustrial cost control   �21m �33s 
    A specific system or procedure for monitoring manufacturing costs to 
achieve profitability goals. The industrial cost control system should provide 
the framework for improving product profitability by identifying the 
variations between actual and target performance, discovering the causes of 
this variation, and eliminating the detrimental causes of cost variation. 
    In industrial cost control, performance of the control system is usually 
measured by the cost control performance of the factors of direct labor, 
material, and manufacturing burden or overhead. After creating the control 
accounts for these factors and after defining target costs for each account, 
those costs deviating from the desired standard can be quickly identified. 
Corrective action can then be taken to bring deviant costs back to the target 
level. Industrial cost control can be a simple and effective way to achieve 
goals of continued profitability.  See  INDUSTRIAL ENGINEERING; INSPECTION AND 
TESTING; INVENTORY CONTROL; MASS PRODUCTION; OPERATIONS RESEARCH; 
OPTIMIZATION; PERFORMANCE RATING; PERT; PROCESS ENGINEERING; PRODUCTION 
ENGINEERING; PRODUCTION METHODS; QUALITY CONTROL. 
 �34s[Raymond P. Lutz]
�33s 
  
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905
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�6v�2v�32sIndustrial engineering   �21m �33s 
    As defined by the American Institute of Industrial Engineers, a branch of 
engineering ``concerned with the design, improvement, and installation of 
integrated systems of people, material, equipment, and energy. It draws upon 
the specialized knowledge and skills in the mathematical, physical and social 
sciences together with the principles and methods of engineering analysis and 
design to specify, predict, and evaluate the results to be obtained from such 
systems.''  
    The major activities of industrial engineers include work methods analysis 
and improvement, work measurement and the establishment of standards, job and 
workplace design, plant layout, materials handling, wage rates and incentives, 
cost reduction, suggestion evaluation, production planning and scheduling, 
inventory control, maintenance scheduling, equipment evaluation, assembly line 
balancing, systems and procedures, overall productivity improvement, and 
special studies - all done, almost exclusively in the early days, in 
manufacturing industries. As the technology evolved, additional activities 
were added to this list. These include machine tool analysis; numerically 
controlled machine installation and programming; computer analysis, 
installation, and programming; linear programming; queueing and other 
operations research techniques; simulations; management information systems; 
value analysis; human factors engineering; human/machine systems design; 
ergonomics; biomechanics; and the use of robots and automation. The industrial 
engineer of today is using computers more in production and test equipment, 
controls, and systems for analyses and special studies. Minicomputers and 
microprocessors are in wide and growing use. 
 �34s[Albert G. Holzman]
�33s 
  
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�6v�2v�32sIndustrial meteorology   �21m �33s 
    The commercial application of weather information to the operational 
problems of business, industry, transportation, and agriculture in a manner 
intended to optimize the operation with respect to the weather factor. The 
weather information may consist of past weather records, contemporary weather 
data, predictions of anticipated weather conditions, or an understanding of 
physical processes which occur in the atmosphere. The operational problems are 
basically decisions in which weather exerts an influence. 
    Some of the applications of meteorological information are: professional 
meteorologists serving television or radio stations to explain to the public 
the changes in weather which either have occurred or are about to occur; 
forecasters working with airline dispatchers to utilize optimum flight paths 
and to avoid equipment tie-ups during lengthy adverse weather conditions; 
consultants who help utilities locate future power plant sites with minimal 
environmental impact; forecasters who advise shipping lines regarding ocean 
routing paths which avoid storms and decrease by a few hours the travel time 
for long-distance ocean freight movement; and consultant advisories which help 
industrial firms in the marketing of weather-sensitive products. In many cases 
the need for meteorological information is sufficiently important that a 
complete department within the operational segment of a company is established 
to serve the needs of that weather-affected firm.  See  WEATHER FORECASTING 
AND PREDICTION. 
 �34s[Loren W. Crow]
�33s 
  
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�6v�2v�32sIndustrial microbiology   �21m �33s 
    The study, utilization, and manipulation of those microorganisms (in the 
broad sense including bacteria, yeasts and other fungi, and some algae) 
capable of economically producing desirable substances, or changes in 
substances, and controlling unwanted microorganisms. 
    Since earliest times microorganisms have been used, intentionally or 
otherwise, as means of preserving foods or of making them more palatable, as 
in the making of such products as wine, cheese, butter, vinegar, and silage. 
They have also been used for such procedures as flax retting, removing 
mucilage from freshly harvested coffee beans, and copper leaching. 
    Microorganisms are used to produce chemical substances, which are then 
extracted from the culture mixture and purified. Included are (1) solvents 
such as ethanol, acetone, butanol, 2,3-butylene glycol, and other alcohols, 
such as glycerol, mannitol, erythritol, and polyhydric alcohols; (2) organic 
acids such as acetic, lactic, citric, gluconic, fumaric, propionic, itaconic, 
kojic, and gallic; (3) carbohydrates such as sorbose, fructose, dextrans, and 
other polysaccharides; (4) vitamins such as B12, thiamine, and riboflavin; (5) 
amino acids such as lysine, arginine, and glutamic acid; (6) antibiotics such 
as penicillin, streptomycin, tetracycline, and chloramphenicol; (7) enzymes 
such as amylases, proteases, and glucose oxidases; and (8) nucleosides.  
    Microorganisms are used to produce desired changes in certain types of 
compounds or materials, such as in the microbiological transformations of 
steroids. More ancient are the uses of microorganisms in the textile industry 
with cotton, wool, and flax. More modern are the applications of 
bacteriological processes to the generation of electric power in fuel cells 
and to studies of human wastes as nutrients in closed space ecologies. In 
addition, microorganisms that are genetically altered are being used to 
produce insulin and interferon. 
 �34s[Reginald H. Haskins]
�33s 
  
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�6v�2v�32sIndustrial trucks �21m �33s
    Manually propelled (handtrucks) or powered carriers for transporting
materials over level, slightly inclined, or slightly declined running
surfaces. Some industrial trucks can lift and lower their loads, and others
can also tier them. In any event, all such trucks maintain contact with the
running surface over which they operate and, except when towed by a chain
conveyor, follow variable paths of travel as distinct from conveying machines
or monorails.  See  MATERIALS-HANDLING MACHINES.
    In industry the principle of handling materials in unit loads has
developed in parallel with the increased use of powered industrial trucks,
particularly the forklift type. These mobile mechanical handling aids have
removed the limitations that existed when the weight and size of a load for
movement and stacking depended mainly on the ability of a person to lift it
manually. The unit-load principle of materials handling underlies the
skid-platform and the pallet-forklift methods of operation. Both methods,
especially the latter, have revolutionized handling techniques and equipment
and even production equipment.
 �34s[Arthur M. Perrin]
�33s

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�6v�2v�32sInert gases   �21m �33s 
    The inert gases, listed in the table, constitute group 0 of the periodic 
table of the elements. They are now better known as the noble gases, since 
stable compounds of xenon have been prepared. The noble gases are all 
monatomic.  
    All these gases occur to some extent in the Earth's atmosphere, but the 
concentrations of all but argon are exceedingly low. Argon is plentiful, 
constituting almost 1% of the air. 
    All the gases are colorless, odorless, and tasteless. They are all 
slightly soluble in water, the solubility increasing with increasing molecular 
weight. They can be liquefied at low temperatures, the boiling point being 
proportional to the atomic weight. All but helium can be solidified by 
reducing the temperature sufficiently, and helium can be solidified at 
temperatures of less than 2DGF above absolute zero (0 - 1 K) by the 
application of an external pressure of 25 atm (2.5 megapascals) or more.  See  
ARGON; HELIUM; KRYPTON; NEON; RADON; XENON. 
 �34s[Arthur W. Francis]
�33s 
  
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�6v�2v�32sInertia   �21m �33s 
    That property of matter which manifests itself as a resistance to any 
change in the motion of a body. Thus when no external force is acting, a body 
at rest remains at rest and a body in motion continues moving in a straight 
line with a uniform speed (Newton's first law of motion). The mass of a body 
is a measure of its inertia.  See  MASS. 
 �34s[Leo Nedelsky]
�33s 
  
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�6v�2v�32sInertial guidance system   �21m �33s 
    A self-contained system which can automatically determine the position, 
velocity, and attitude of a moving vehicle for the purpose of directing its 
future course. Based on prior knowledge of time, gravitational field, initial 
position, initial velocity, and initial orientation relative to a known 
reference frame, an inertial guidance system is capable of determining its 
present position, velocity, and orientation without the aid of external 
information. The generated navigational data are used to determine the future 
course for a vehicle to follow in order to bring it to its destination. Such 
systems have found application in the guidance and control of submarines, 
ships, aircraft, missiles, and spacecraft. See GUIDANCE SYSTEM. 
 �34s[William E. Howell]
�33s 
  
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�6v�2v�32sInertial reference frame   �21m �33s 
    A frame of reference in which a body moves with constant velocity when 
free of impressed forces. It follows that one inertial frame cannot be 
accelerating with respect to another. If x sub 1, y sub 1, z sub 1 and x sub 2,
y sub 2, z sub 2 are the coordinates of a particle P in parallel inertial 
frames 1 and 2, where 2 is moving with respect to 1 with velocity components 
v sub x, v sub y, v sub z, then Eq. (1) holds. (Newtonian frames are understood 

              *** x sub 2 = [(x sub 1) - (v sub x t)] ***
              *** y sub 2 = [(y sub 1) - (v sub y t)] ***                  (1)
              *** z sub 2 = [(z sub 1) - (v sub z t)] ***

here so that t sub 2 = t sub 1.) This is called a galilean transformation. 
Since the vs are independent of time, the component accelerations of P in 1 and 
2 are the same, that is, Eq. (2) is valid. See FRAME OF REFERENCE. 

*** A sub x sub 2 = [(d sup 2) (x sub 2)] / dt sup 2 = 
                    [(d sup 2) (x sub 1)] / dt sup 2 = A sub x sub 1 etc.  (2)

 �34s[Bernard Goodman]
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�6v�2v�32sInertial separator   �21m �33s 
    A type of air filter which utilizes the principle of momentum to separate 
larger particulates, micrometers in diameter, primarily those above 10 m 
altitude, from a moving airstream. The larger particles tend to keep going in 
the same direction in which they entered the device, while the light-density 
air changes direction. Commonly offered in manifolded multiples of V-shaped 
pockets with louvred sides or in small-diameter cyclones, the inertial 
separators are used most frequently in hot, arid, desertlike regions where 
winds generate significant airborne dust and sand particles. A secondary 
bleed-air fan is used to discharge the separated particulate to the outside of 
the building. See AIR FILTER. 
 �34s[Morton A. Bell]
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�6v�2v�32sInertia of energy �21m �33s
    The principle of inertia of energy states that the inertial properties of
matter determine, and are determined by, its total energy content. If E is the
total energy content, and m0 the rest mass of a piece of matter, c being the
speed of light, the mass-energy relation is E= m0c sup 2. This formula was
proposed on general grounds by H. Poincare in 1900 and was deduced from the
special theory of relativity by Albert Einstein in 1905.
    If the mass of a body changes by an amount DELTAm, the corresponding
energy change is DELTAE= c sup 2 . DELTAm. The existence of disintegration
processes in atomic nuclei has made it possible to test this formula with
great accuracy. Energy released from nuclear reactions is equivalent to the
difference in mass before and after the reaction.  See	MASS; MASS DEFECT;
RELATIVITY.
 �34s[Edward L. Hill]
�33s

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�6v�2v�32sInertia welding   �21m �33s 
    A welding process used to join similar and dissimilar materials at very 
rapid speed. It is, therefore, a very attractive welding process in mass 
production of good-quality welds. The ability to join dissimilar materials 
provides further flexibility in the design of mechanical components. The 
automotive and truck industry is the major user of this process. 
    Inertia welding is a type of friction welding which utilizes the 
frictional heat generated at the rubbing surfaces to raise the temperature to 
a degree that the two parts can be forged together to form a solid bond. The 
energy required for inertia welding comes from a rotating flywheel system 
built into the machine. Like an engine lathe, the inertia welding machine has 
a headstock and a tailstock. One workpiece held in the spindle chuck (usually 
with an attached flywheel) is accelerated rapidly, while the other is clamped 
in a stationary holding device of the tailstock. When a predetermined spindle 
speed is reached, the drive power is cut and the nonrotating part on the 
tailstock is pushed against the rotating part under high pressure. Friction 
between the rubbing surfaces quickly brings the spindle to a stop. At the same 
time the stored kinetic energy in the flywheel is converted into frictional 
heat which raises the temperature at the interface high enough to forge the 
two parts together without melting.  See  WELDING AND CUTTING OF METALS. 
 �34s[K. K. Wang]
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�6v�2v�32sInfant diarrhea   �21m �33s 
    Diarrhea and its complications are the most important causes of infant 
death in most developing regions. The causes of the illness vary from dietary 
incompatabilities to intestinal infection. The most important infectious 
causes are, in approximate order of importance: rotavirus, the bacteria 
Shigella (causing dysentery) and Salmonella, the parasite Giardia lamblia, and 
enteropathogenic Escherichia coli bacteria (a common cause of hospital nursery 
outbreaks). Breast-feeding is associated with a decreased occurrence of 
diarrhea and represents a major means of preventing infantile diarrhea in the 
developing world. See  DIARRHEA. 
 �34s[Herbert L. Dupont]
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�6v�2v�32sInfarction   �21m �33s 
    A process of vascular obstruction resulting in a region of dead tissue 
(necrosis) called an infarct. The usual cause is occlusion of an artery or 
vein by a thrombus or embolus and sometimes by severe arteriosclerosis; the 
development of the infarct depends to a great extent on the collateral 
circulation.  
    Infarcts commonly occur in the lungs, heart, brain, spleen, and kidneys. 
The common cause of infarcts of the heart is thrombosis of the coronary 
artery, usually secondary to arteriosclerosis. In addition, hemorrhage into an 
arteriosclerotic plaque can result in occlusion of the lumen.  See  EMBOLUS; 
HEART DISORDERS; THROMBOSIS. 
 �34s[Romeo A. Vidone]
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�6v�2v�32sInfection   �21m �33s 
    A term considered by some to mean the entrance, growth, and multiplication 
of a microorganism (pathogen) in the body of a host, resulting in the 
establishment of a disease process. Others define infection as the presence of 
microorganism in host tissues whether or not it evolves into detectable 
pathologic effects. The host may be a bacterium, plant, animal, or human 
being, and the infecting agent may be viral, rickettsial, bacterial, fungal, 
or protozoan.  See  EPIDEMIOLOGY; LYTIC INFECTION; MEDICAL BACTERIOLOGY; 
MEDICAL MYCOLOGY; MEDICAL PARASITOLOGY; PATHOGEN; VIRUS. 
 �34s[Daniel N. Lapedes]
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�6v�2v�32sInfectious-disease control   �21m �33s 
    Since infection depends on factors in the host, the parasite, and the 
environment, there are various means by which infectious-disease control may 
be brought about. Immunization strengthens the defenses of the host; 
chemoprophylaxis is directed at the parasite; and quarantine and sanitary 
measures remove the parasite from the environment of susceptible people.  See  
CHEMOTHERAPY; EPIDEMIOLOGY; SEWAGE DISPOSAL; VACCINATION. 
 �34s[Colin White]
�33s 
  
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�6v�2v�32sInfectious-disease transmission   �21m �33s 
    The transmission of infectious disease is accomplished in several ways. 
    1. There may be direct contact with an infected person. The venereal 
diseases are almost invariably, and measles is commonly, spread in this way.  
See  GONORRHEA; LYMPHOGRANULOMA VENEREUM; MEASLES; SYPHILIS; YAWS. 
    2. Certain diseases are spread by direct contact from animals to humans, 
an example being undulant fever.  See  ZOONOSES. 
    3. Milk-borne infectious diseases include typhoid fever, scarlet fever, 
summer diarrhea, and bovine tuberculosis. The milk responsible is almost 
always raw.  See  PASTEURIZATION; SCARLET FEVER; TUBERCULOSIS. 
    4. In the past there have been many serious epidemics of water-borne 
diseases, including typhoid fever, cholera, bacillary dysentery, and amebic 
dysentery.  See  WATER-BORNE DISEASE. 
    5. Gastroenteritis due to the Salmonella group of organisms is a 
food-borne disease in certain instances. Pork and poultry are particularly 
liable to be contaminated.  See  SALMONELLA. 
    6. A nosocomial infection is one that is spread in hospitals. An urgent 
problem is the spread of staphylococci, many of them antibiotic-resistant, in 
hospitals.  See  HOSPITAL INFECTIONS; STAPHYLOCOCCUS. 
    7. Articles which become contaminated by contact with a patient are known 
as fomites, and these in turn may cause fomite-borne infection. Among children 
there is considerable ``traffic in saliva'' by means of toys; other 
contaminated objects which serve as a means of indirect contact between a case 
and a susceptible are soiled clothing, bedding, and dressings.  See  
DIPHTHERIA; STREPTOCOCCUS. 
    8. Insect vectors are the most important means of spreading certain 
diseases such as yellow fever, sleeping sickness, plague, and typhus. See 
EPIDEMIC TYPHUS FEVER; MURINE TYPHUS FEVER; PLAGUE; SLEEPING SICKNESS. 
 �34s[Colin White]
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�6v�2v�32sInfectious mononucleosis   �21m �33s 
    A disease of children and young adults, characterized by fever and 
enlarged lymph nodes and spleen. EB (Epstein-Barr) herpesvirus is the 
causative agent.  
    Onset of the disease is slow and nonspecific with variable fever and 
malaise; later, cervical lymph nodes enlarge, and in about 50% of cases the 
spleen also becomes enlarged. The disease lasts 4 - 20 days or longer. 
Epidemics are common in institutions where young people live. EB virus 
infections occurring in early childhood are usually asymptomatic. In later 
childhood and adolescence, the disease more often accompanies infection - 
although even at these ages inapparent infections are common. See EPSTEIN-BARR 
VIRUS.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sInfectious myxomatosis   �21m �33s 
    A rapidly fatal viral disease of European rabbits, also known as rabbit 
pox, characterized by the formation of mucoid tumors in the skin and invasion 
of the blood and internal organs. It is spread by the bite of the mosquito. 
Rabbits in the area where the disease originated are immune. It has been 
successfully used as a method of extermination of unwanted rabbits in 
Australia where laboratory-infected rabbits have been released into the rabbit 
population.  
 �34s[Alice E. Moore]
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�6v�2v�32sInfectious papillomatosis   �21m �33s 
    A nonfatal disease of wild cottontail rabbits characterized by large, 
localized, fleshy tumors with horny surfaces, which occur on the neck and 
shoulders. It is caused by a virus, and natural infection occurs by 
inoculation of the skin surface as the rabbit enters its burrow. The disease 
is also known as Shope papilloma. 
 �34s[Alice E. Moore]
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�6v�2v�32sInfective dose   �21m �33s 
    The dose of microorganisms required to cause infection in 50% of the 
experimental animals; it is an important special case of the median effective 
dose and is also known as ID, or the median infective dose.  See  EFFECTIVE 
DOSE.  
 �34s[Colin White]
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�6v�2v�32sInfinity   �21m �33s 
    The terms infinity and infinite have a variety of related meanings in 
mathematics. The adjective finite means ``having an end,'' so infinity may be 
used to refer to something having no end. In order to give a precise 
definition, the mathematical domain of discourse must be specified. 
    Set theory provides a simple and basic example of an infinite collection - 
the class of natural numbers, or positive integers. A fundamental property of 
positive integers is that after each integer there follows a next one, so that 
there is no last integer. Now it is necessary in mathematics to treat the 
collection of all positive integers as an entity, and this entity is the 
simplest infinity, or infinite collection. 
    The term infinity appears in mathematics in a different sense in 
connection with limits of functions. For example, consider the function 
defined by y = 1/x. When x tends to 0, y approaches infinity, and the 
expression may be written as shown below. 
  
                    *** LIMIT (X TO 0) y = INFINITY *** 
  
    Precisely, this means that for an arbitrary number a > 0, there exists a 
number b > 0 such that when 0 < x < b, then y > a, and when -b < x < 0, then 
y < -a. This example indicates that it is sometimes useful to distinguish     
+INFINITY and -INFINITY. The points +INFINITY and -INFINITY are pictured at 
the two ends of the y axis, a line which has no ends in the proper sense of 
euclidean geometry. 
    In geometry of two or more dimensions, it is sometimes said that two 
parallel lines meet at infinity. This leads to the conception of just one 
point at infinity on each set of parallel lines and of a line at infinity on 
each set of parallel planes. With such agreements, parts of euclidean geometry 
can be discussed in the terms of projective geometry. For example, one may 
speak of the asymptotes of a hyperbola as being tangent to the hyperbola at 
infinity.  
 �34s[Lawrence M. Graves]
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�6v�2v�32sInflammation   �21m �33s 
    The local response to injury, involving small blood vessels, the cells 
circulating within these vessels, and nearby connective tissue. 
    The early phases of the inflammatory response are stereotyped: A similar 
sequence of events occurs in a variety of tissue sites in response to a 
diversity of injuries. The response characteristically begins with hyperemia, 
edema, and margination of the circulating white blood cells. The white cells 
then migrate between the endothelial cells of the blood vessel into the 
tissue. The subsequent development of the inflammatory process is determined 
by factors such as type and location of injury, condition of the host, and the 
use of therapeutic agents.  See  EDEMA. 
    A local inflammatory response is usually accompanied by systemic changes: 
fever, malaise, and an increase in circulating leukocytes (leukocytosis). Such 
signals and symptoms are often helpful to the physician, first as clues to the 
presence of inflammation and later as an indication of its course. 
    Inflammation is basically a protective mechanism. The leakage of water and 
protein into the injured area brings humoral factors, including antibodies, 
into the locale and may serve to dilute soluble toxic substances and wash them 
away. The margination and migration of leukocytes brings these cells to the 
local site to deal with the injurious agent, dead or alive. 
 �34s[David Lagunoff]
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�6v�2v�32sInflorescence   �21m �33s 
    A flower cluster segregated from any other flowers on the same plant, 
together with the stems and bracts (reduced leaves) associated with it. 
Certain plants produce inflorescences, whereas others produce only solitary 
flowers.  See  FLOWER. 
 �34s[George J. Wilder]
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�6v�2v�32sInfluenza   �21m �33s 
    An acute respiratory viral infection, usually epidemic in occurrence. This 
disease has been one of the world's greatest killers; over 21 million deaths 
are attributed to the 1918 - 1919 pandemic. Three immunologic types of 
influenza virus are known. Group A appears to be subject to continual 
antigenic changes, group B changes to a lesser degree, and group C seems 
antigenically stable. 
    In human infection, the virus enters the respiratory tract in airborne 
droplets; within 1 - 2 days inflammation of the respiratory tract occurs, with 
fever, chills, and muscular aches. The illness is usually mild, and 
asymptomatic infections may occur. However, a more serious course or even 
death may result in older persons and those with chronic debilitating 
diseases. Bacterial complications may produce more severe illnesses, 
especially pneumonia. The lethal impact of an influenza epidemic is reflected 
in an increase in deaths due to cardiovascular and renal disease, as well as 
those due to pneumonia or to influenza itself. 
    Influenza occurs in successive epidemic waves and also in pandemics. The 
antigenic shifts of influenza viruses usually occur gradually, so that 
infection with strains widespread in one year confers some immunity against 
those prevalent in the next several years. However, sudden wide antigenic 
shifts do occur at 10 - 15-year intervals.  See  EPIDEMIC. 
    Vaccines made up of current strains, grown in embryonated eggs and 
inactivated by formalin or ultraviolet light, confer immunity to the 
homologous strains in the vaccine, but only relatively transient resistance to 
other strains. Since the vaccine is prepared from infected chick embryos, 
persons hypersensitive to eggs or egg products may experience severe allergic 
reaction. Attenuated live virus vaccines have been developed and used widely 
in the Soviet Union, and to a much lesser extent in Great Britain. One of 
difficulties with the live vaccine is in obtaining a sufficiently high 
infection rate in those vaccinated. There is also the problem of antigenic 
shift that makes for obsolescence of the killed virus vaccines.  See  
VACCINATION.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sInformation theory   �21m �33s 
    A branch of communication theory devoted to problems in coding. A unique 
feature of information theory is its use of a numerical measure of the amount 
of information gained when the contents of a message are learned. Information 
theory relies heavily on the mathematical science of probability. For this 
reason the term information theory is often applied loosely to other 
probabilistic studies in communication theory, such as signal detection, 
random noise, and prediction.  See  ELECTRICAL COMMUNICATIONS; PROBABILITY. 
    In designing a one-way communication system from the standpoint of 
information theory, three parts are considered beyond the control of the 
system designer: (1) the source, which generates messages at the transmitting 
end of the system, (2) the destination, which ultimately receives the 
messages, and (3) the channel, consisting of a transmission medium or device 
for conveying signals from the source to the destination. The source does not 
usually produce messages in a form acceptable as input by the channel. The 
transmitting end of the system contains another device, called an encoder, 
which prepares the source's messages for input to the channel. Similarly the 
receiving end of the system will contain a decoder to convert the output of 
the channel into a form that is recognizable by the destination. The encoder 
and the decoder are the parts to be designed. In radio systems this design is 
essentially the choice of a modulator and a detector.  See  MODULATION. 
    A source is called discrete if its messages are sequences of elements 
(letters) taken from an enumerable set of possibilities (alphabet). Thus 
sources producing integer data or written English are discrete. Sources which 
are not discrete are called continuous, for example, speech and music sources. 
The treatment of continuous cases is sometimes simplified by noting that 
signal of finite bandwidth can be encoded into a discrete sequence of numbers. 
    The output of a channel need not agree with its input. For example, a 
channel might, for secrecy purposes, contain a cryptographic device to 
scramble the message. Still, if the output of the channel can be computed 
knowing just the input message, then the channel is called noiseless. If, 
however, random agents make the output unpredictable even when the input is 
known, then the channel is called noisy.  See  CRYPTOGRAPHY; PRIVACY SYSTEMS 
(SCRAMBLING).  
    Many encoders first break the message into a sequence of elementary 
blocks; next they substitute for each block a representative code, or signal, 
suitable for input to the channel. Such encoders are called block encoders. 
For example, telegraph and teletype systems both use block encoders in which 
the blocks are individual letters. Entire words form the blocks of some 
commercial cablegram systems. It is generally impossible for a decoder to 
reconstruct with certainty a message received via a noisy channel. Suitable 
encoding, however, may make the noise tolerable. 
    Even when the channel is noiseless, a variety of encoding schemes exists 
and there is a problem of picking a good one. Of all encodings of English 
letters into dots and dashes, the Continental Morse encoding is nearly the 
fastest possible one. It achieves its speed by associating short codes with 
the most common letters. A noiseless binary channel (capable of transmitting 
two kinds of pulse 0, 1, of the same duration) provides the following example. 
Suppose one had to encode English text for this channel. A simple encoding 
might just use 27 different five-digit codes to represent word space (denoted 
by #), A, B, . . . , Z; say # 00000, A 00001, B 00010, C 00011, . . . , Z 
11011. The word #CAB would then be encoded into 00000000110000100010. A 
similar encoding is used in teletype transmission; however, it places a third 
kind of pulse at the beginning of each code to help the decoder stay in 
synchronism with the encoder. 
 �34s[Edgar N. Gilbert]
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�6v�2v�32sInfrared astronomy   �21m �33s 
    Astronomy ranging from wavelength 1 micrometer to above 1 millimeter in 
the electromagnetic spectrum. Solid-state sensors, in conjunction with 
conventional and specialized telescopes, are used. Above 1 mm, radio 
telescopes are used. 
    The energy maximum of a blackbody is located at a wavelength of 2900 
MUm/T, where T is the body's thermodynamic temperature in kelvins. Because of 
their low temperature (700 - 100 K or +80DG to -280DGF) planets radiate 
largely in the infrared. 
    Surveys have revealed very red and, probably, very cool stars, of which 
some are nearly undetectable in visible light but extremely bright in the 
infrared. Colors suggest T = 800DGF (700 K) [red giants are seldom cooler than 
2200DGF (1500 K)]. One fundamental program of infrared astronomy is to 
establish the temperature scale for cool stars by the measurement of infrared 
magnitudes over wide spectral regions. Some infrared objects are also highly 
reddened by interstellar dust. Those observable spectroscopically in normal 
wavelengths are very cool red giants. However, still cooler objects exist; a 
large, cold nebula inside the very bright Orion Nebula has T = -330DGF (70 K), 
and a stellar object with T = 800DGF (700 K) was found near it. It seems 
probable that the cold nebula is associated with protostar formation, and 
infrared astronomy permits observation of such stars. See  STAR; STELLAR 
EVOLUTION.  
    Infrared radiation from dust has been found in hot, gaseous planetary 
nebulae. Seyfert galaxies, quasars, and the center of the Galaxy are strong 
emitters of infrared radiation.  See  GALAXY; GALAXY, EXTERNAL; NEBULA; 
QUASARS.  
 �34s[Alexander Hellemans]
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�6v�2v�32sInfrared imaging devices �21m �33s
    Devices that convert an invisible infrared image into a visible image. The
radiation available for imaging may be emitted from objects in the scene of
interest (usually at the longer wavelengths called thermal radiation) or
reflected. Reflected radiation may be dominated by sunlight or may be from
controlled sources such as lasers used specifically as illuminators for the
imaging device. The latter systems are called active, while those relying
largely on emitted radiation are called passive. Active optical imaging
systems were developed to achieve a nighttime aerial photographic capability,
and work during World War II pushed such systems into the near-infrared
spectral region. Development of passive infrared imaging systems came after
the war, but only the advent of lasers allowed creation of active infrared
imagers at wavelengths much longer than those of the photographic region.  See
  INFRARED RADIATION; LASER.
    Although developed largely for military purposes, infrared imaging devices
have been valuable in industrial, commercial, and scientific applications.
These range from nondestructive testing and quality control to earth resources
surveys, pollution monitoring, and energy conservation. Infrared images from
aerial platforms are used to accomplish ``heat surveys,'' locating points of
excessive heat loss. Calibration allows association of photographic tones with
values of apparent (that is, equivalent blackbody) temperatures. Dark areas
are ``colder'' than light ones.  See  NONDESTRUCTIVE TESTING; REMOTE SENSING.
 �34s[George J. Zissis]
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�6v�2v�32sInfrared lamp   �21m �33s 
    A special type of incandescent lamp that is designed to produce energy in 
the infrared portion of the electromagnetic spectrum. The lamps produce 
radiant thermal energy which can be used to heat objects that intercept the 
radiation. An infrared lamp with a filament operating at 4000DGF (2500 K) will 
release about 85% of its energy in the form of thermal radiant energy, about 
15% as visible light, and a tiny fraction of a percent as ultraviolet energy.  
See  HEAT RADIATION. 
  
    The lamps are supplied in three shapes. The most common shape, for general 
use, is that shown in illustration a, since the reflector unit is built in and 
needs only a suitable socket to form an infrared heating system. In industry, 
the lamp shown in illustration b is used with separate external reflectors 
designed to distribute the heat as desired. 
    The major advantage of infrared heating is that it is possible to heat a 
surface that intercepts the radiation without heating the air or other objects 
that surround the surface being heated. Infrared lamps have many uses, 
including paint drying, evaporative drying, farm heating of animals, heating 
of food, control heating, and therapeutic heating of portions of the body to 
relieve muscle strains.  See  INCANDESCENT LAMP; INFRARED RADIATION; 
THERMOTHERAPY.  
 �34s[G. R. Peirce]
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�6v�2v�32sInfrared radiation   �21m �33s 
    Electromagnetic radiation in which wavelengths lie in the range from about 
1 micrometer to 1 millimeter. This radiation therefore has wavelengths just a 
little longer than those of visible light and cannot be seen with the unaided 
eye. The radiation was discovered in 1800 by William Hershel. 
    An infrared source can be described by the spectral distribution of power 
emitted by an ideal body (a blackbody curve). This distribution is 
characteristic of the temperature of the body. A real body is related to it by 
a radiation efficiency factor or emissivity which is the ratio at every 
wavelength of the emission of a real body to that of a blackbody under 
identical conditions. The illustration shows curves for these ideal 
blackbodies radiating at a number of different temperatures. The higher the 
temperature, the greater the total amount of radiation.  See  EMISSIVITY. 
    Infrared detectors are based either on the generation of a change in 
voltage due to a change in the detector temperature resulting from the power 
focused on it, or on the generation of a change in voltage due to some 
photon-electron interaction in the detector material. This latter effect is 
sometimes called the internal photoelectric effect. 
    Infrared techniques have been applied in military, medical, industrial, 
meteorological, ecological, forestry, agricultural, chemical, and other 
disciplines. Weather satellites use infrared imaging devices to map cloud 
patterns and provide the imagery seen in many weather reports. Infrared 
imaging devices have also been used for breast cancer screening and other 
medical diagnostic applications. In most of these applications, the underlying 
principle is that pathology produces inflammation, and these locations of 
increased temperature can be found with an infrared imager. Airborne infrared 
imagers have been used to locate the edge of burning areas in forest fires.  
See  INFRARED IMAGING DEVICES; REMOTE SENSING. 
 �34s[William L. Wolfe]
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�6v�2v�32sInfrared spectroscopy   �21m �33s 
    The study of the interaction of material systems with electromagnetic 
radiation in the infrared region of the spectrum. The infrared region is 
valuable for the study of the structure of matter because the natural 
vibrational frequencies of atoms in molecules and crystals fall in the 
infrared range. Some gaseous molecules also have rotational frequencies in the 
far-infrared range, and certain frequencies corresponding to the energy levels 
of electrons in solids and in large molecules lie in the near infrared. 
    The infrared absorption spectrum of a molecule is highly characteristic, 
and often has been referred to as a molecular fingerprint. The spectrum can 
thus be used for molecular identification. Because the absorption of radiation 
at various infrared frequencies is quantitatively related to the number of 
absorbing molecules in a system, quantitative analysis is also possible. 
    The usual arrangement for measurement of an infrared spectrum consists of 
a source of continuous infrared radiation, an infrared spectrometer, and a 
detector. The infrared spectrometer disperses the radiation into a spectrum, 
and the detector converts the radiant energy into an electrical signal, which 
is amplified electronically and recorded by a chart recorder. To correct for 
variations caused by, among other factors, atmospheric absorption and the 
variation of source intensity with frequency, two spectra are recorded with a 
so-called double-beam spectrometer: one with a sample in the beam and one with 
the sample removed from the beam. The absorption by the sample as a function 
of frequency can then be computed from these two spectra. The comparison of 
the two spectra is obtained by rapid switching (10 times a second) of the beam 
from the infrared source back and forth between a path crossing the sample and 
a path passing by the sample. 
    The sharpness of the frequency and the high power per unit solid angle and 
unit spectral bandpass in a laser beam make it attractive for infrared 
spectroscopy at ultrahigh resolution. The main problem is the tuning of the 
laser frequency, that is, varying some parameter in the laser system so that 
its frequency may be varied continuously and accurately. Tunable infrared 
lasers have been used mainly to measure the energy levels due to the vibration 
and rotation of molecules in the gas phase at low pressures. By this means, 
very accurate values of such molecular parameters as internuclear distances, 
electric dipole moments, vibrational frequencies, internal force fields, and 
the like may be measured.  See  LASER SPECTROSCOPY. 
    Infrared spectra can be used to: identify pure chemical compounds by 
comparison of the spectrum of an unknown with previously recorded spectra of 
pure compounds; identify the constituents of mixtures; and show the presence 
of a group of atoms, a so-called functional group, in a molecule of unknown or 
doubtful structure. Also, since there is a linear quantitative relationship 
between the absorbance and the number of absorbing molecules, the quantitative 
analysis of mixtures by infrared means is feasible. Structures of molecules 
can be determined to varying degrees of refinement from infrared spectra. The 
infrared spectra of crystalline solids give information about modes of 
vibration of crystals, about hydrogen-bond vibrations when such bonds are 
present in them, and about electronic energy states in semiconductors and 
superconductors.  See  MOLECULAR STRUCTURE AND SPECTRA; SPECTROSCOPY. 
 �34s[Richard C. Lord]
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�6v�2v�32sInfrasound   �21m �33s 
    Sound waves, particularly in the atmosphere, whose frequencies of pressure 
variation and of vibration are below the audible range, that is, lower than 
about 20 Hz. Earthquake and seismic waves are elastic waves which occur at 
infrasonic frequencies in the Earth's crust and in the oceans and seas. The 
physical laws of propagation in the atmosphere are essentially the same as for 
audible sound. The local speed of infrasound in air at ambient temperatures 
near 20DGC (68DGF) is about 340 m/s (1115 ft/s), the same as for audible sound. 
    At frequencies less than about 1.0 Hz, infrasound propagates through the 
atmosphere for distances of thousands of kilometers without substantial loss 
of energy. Sounds at these frequencies are almost always present at measurable 
intensities. Those of natural origin have many causes, including tornadoes, 
volcanic explosions, earthquakes, the aurora borealis, waves on the seas, 
large meteorites, and lightning discharges. When the wind blows, turbulent 
pressure fluctuations in the atmosphere occur at amplitudes up to tens of 
pascals, at infrasonic frequencies. People are unaware of these pressures via 
the sensation of hearing. 
    But sufficiently strong infrasound is ``audible,'' contrary to simple 
acoustic tradition. However, there is no sensation of tone. Listeners 
variously describe audible infrasound as ``pumping,'' ``popping effect,'' or 
``chugging.'' The human body is particularly sensitive to vibrations and 
infrasound near 7 Hz, at which frequency there is an overall mechanical 
resonance of organs in the abdominal and chest cavities. Unpleasant 
physiological effects with pathological overtones occur when human subjects 
and animals are exposed to infrasound in the laboratory at sound pressure 
levels of 130 dB and greater. The equipment and machinery of modern technology 
can expose people to infrasound which is much stronger and occurs more often 
than in the natural environment.  See  SOUND. 
 �34s[Richard K. Cook]
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�6v�2v�32sInhibitor (chemistry)   �21m �33s 
    A substance which is capable of stopping or retarding a chemical reaction. 
To be technically useful, such compounds must be effective in low 
concentrations, usually under 1%. The type of reaction which is most easily 
inhibited is the free-radical chain reaction. Vinyl polymerization and 
autoxidation are two important examples of the class. Another reaction type 
for which inhibitors have been found is corrosion, particularly in aqueous 
systems.  See  ANTIOXIDANT; CORROSION; FREE RADICAL; POLYMERIZATION. 
 �34s[Lee R. Mahoney]
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�6v�2v�32sInk   �21m �33s 
    A dispersion of a pigment or a solution of a dye in a carrier vehicle, 
yielding a fluid, paste, or powder to be applied to and ``dried'' on a 
substrate. Writing, marking, drawing, and printing inks are applied by several 
methods to paper, metal, plastic, wood, glass, fabric, or other substrates. 
Often inks perform communicative, decorative, and even protective functions. 
    Printing ink can be classified according to its composition and texture, 
application, end use, and drying manner. Composition and texture may be any of 
the following types: oil or paste ink, solvent or liquid (aniline) ink, or 
specialty ink. The applications are letterpress, lithographic, flexographic, 
gravure, silk screen, stencil, duplicating, electrostatic, and jet. The end 
uses are news, publication, commercial, folding carton, book, corrugated box, 
paper bag, wrapper, label, metal container, plastic container, plastic film, 
foil, laminating, food insert, sanitary paper, and textile. The various drying 
manners are oxidizing, evaporating, penetrating, precipitating, polymerizing, 
reactive, gelling, and cold-setting or quick-setting. 
    Ultraviolet (radiation) cure inks are based on instant photopolymerization 
of monomers by a high-energy light source. They completely eliminate pollution 
from the web and spray powder on the sheet presses, and reduce energy 
consumption to 20% of that required by heat-set ovens previously employed in 
printing on paper or metal. 
    Inks developed for ball-point pens are Newtonian fluids of high tinctorial 
strength. These must be free of particles and premature drying so as to 
continue the feed to the paper without clogging. Rapid penetration into the 
paper accomplishes ``drying.'' Stamp-pad or marking inks are impregnated into 
a cloth or foam rubber pad and transferred by pressure to a rubber type which 
is then stamped or impressed against the substrate. The inks must remain 
nondrying on the pad, yet rapidly penetrate into the stamped substrate. 
 �34s[Bohdan V. Burachinsky]
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�6v�2v�32sInland waterways   �21m �33s 
    The internal commercial water routes of the United States except for the 
Great Lakes. Transportation on the inland waterways of the United States 
accounts for 10% of the nation's total movement of commerce. Barges move 7.2%; 
the remaining 2.8% is moved in deep-draft vessels. 
    The 25,380-mi (40,845-km) network of commercially navigable inland 
channels is used primarily by barges which are either pushed by towboats or 
pulled on a hawser by tugboats. Where improvements of the channels have been 
made for barge transportation, a standard operating depth of 9 ft (2.74 m) is 
provided. Certain sections of these inland waterways, as has been indicated, 
are deep enough to support oceangoing vessels. The primary routes where both 
barges and oceangoing vessels operate are the Hudson River to Albany, the 
Mississippi River to Baton Rouge, the Houston Ship Channel, and the Columbia 
River to Portland. 
    The system of commercially navigable inland waterways of the United States 
is made up of a series of Federal projects, with the exception of the New York 
State Barge Canal, which is a state project. Improvement, maintenance, and 
operation is the responsibility of the Army Corps of Engineers working with 
Federal funds appropriated by the Congress. 
 �34s[Braxton B. Carr]
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�6v�2v�32sInoculation �21m �33s
    The process of introducing a microorganism or suspension of microorganism
into a culture medium. The medium may be (1) a solution of nutrients required
by the organism or a solution of nutrients plus agar; (2) a cell suspension
(tissue culture); (3) embryonated egg culture; or (4) animals, for example,
rat, mouse, guinea pig, hamster, monkey, birds, or human being. When animals
are used, the purpose usually is the activation of the immunological defenses
against the organism. This is a form of vaccination, and quite often the two
terms are used interchangeably. Both constitute a means of producing an
artificial but active immunity against specific organisms, although the length
of time given by such protection may vary widely with different organisms. See
CULTURE MEDIA; IMMUNITY; VACCINATION.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sInorganic chemistry   �21m �33s 
    The chemical reactions and properties of all the elements in the periodic 
table and their compounds, with the exception of the element carbon. The 
chemistry of carbon and its compounds falls in the domain of organic 
chemistry. The boundaries of inorganic chemistry with the other major areas of 
chemistry are not precisely defined, and it is often a matter of taste as to 
whether a particular topic is to be included in the field of inorganic 
chemistry or is to be considered physical or even organic chemistry. 
Investigations into theoretical inorganic chemistry or the study of problems 
in inorganic chemistry by quantitative and sophisticated physical methods may 
be considered either inorganic or physical chemistry quite arbitrarily. In 
similar fashion, organometallic compounds may be considered to be in the 
sphere of either inorganic or organic chemistry. To an increasing extent, the 
inorganic chemist is concerned with problems that once were considered the 
prerogative of physical chemists, organic chemists, or even biochemists.  See  
BIOINORGANIC CHEMISTRY; CHEMICAL DYNAMICS; COORDINATION CHEMISTRY; 
GEOCHEMISTRY; NUCLEAR CHEMISTRY; ORGANOMETALLIC COMPOUND; PHYSICAL CHEMISTRY; 
SOLID-STATE CHEMISTRY. 
 �34s[Joseph J. Katz]
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�6v�2v�32sInorganic photochemistry   �21m �33s 
    The study of the light-induced behavior of various metal compounds. The 
physical and chemical properties of substances are generally altered by the 
absorption of light. Typical metal compounds have a characteristic number 
(coordination number) of molecules or ions (ligands) directly bonded to the 
metal center. For example a six-coordinate compound has the general formula 
ML6supnsupp. Many of these compounds are colored, and much interest has been 
aroused by speculation that some metal compounds could mediate the 
transformation of solar radiation into useful chemical or electrical energy.  
See  COORDINATION NUMBER. 
    The photochemistry of metal compounds has grown in concert with modern 
theories of the electronic structure of molecules and of chemical bonding in 
molecules. Photochemical studies are often designed to probe and test these 
theories. The range of pertinent studies spans most of the subdisciplines of 
chemistry and includes or bears on such topics as photophysics, the 
development of laser materials, catalysis, photosynthesis, oxidation-reduction 
chemistry, acid-base chemistry, organometallic chemistry, metalloenzyme 
chemistry, solid-state chemistry, and surface chemistry.  See  CHEMICAL 
BONDING; CHEMICAL DYNAMICS; COORDINATION CHEMISTRY; LASER PHOTOCHEMISTRY; 
PHOTOCHEMISTRY.  
 �34s[John F. Endicott]
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�6v�2v�32sInorganic polymer   �21m �33s 
    A giant molecule linked by covalent bonds but with an absence or 
near-absence of hydrocarbon units in the main molecular backbone; these may be 
included as pendant side chains. Carbon fibers, graphite, and so forth are 
considered inorganic polymers. 
    Some special characteristics of many inorganic polymers are a higher 
Young's modulus and a lower failure strain compared with organic polymers. 
Relatively few inorganic polymers dissolve in the true sense, or 
alternatively, if they swell, few can revert. Crystallinity and high glass 
transition temperatures are also much more common than in organic polymers. In 
highly cross-linked inorganic polymers, stress relaxation frequently involves 
bond interchange.  See  CHEMICAL BOND THEORY; YOUNG'S MODULUS. 
    Inorganic polymers can be classified in a number of ways. Some are based 
on the composition of the backbone, such as the silicones (Si--O), the 
phosphazenes (P--N), and polymeric sulfur (S--S). Others are based on their 
connectivity, that is, the number of network bonds linking the repeating unit 
into the network. Thus the silicones based on R2SiO, the phosphazenes based on 
NPX2, and polymeric sulfur each have a connectivity of two, while boric oxide 
based on B2O3 has a connectivity of three, and amorphous silica based on SiO2 
has one of four (see illustration). 
    Among the well-known inorganic polymers are silicones, chalcogenide 
glasses, graphite, boron polymers, and silicate polymers.  See  BORON 
POLYMERS; CHALCOGENIDE; GLASSES GRAPHITE; SILICATE POLYMERS; SILICONE RESINS. 
 �34s[Robert A. Shaw]
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�6v�2v�32sInositol �21m �33s
    A crystalline, water-soluble alcohol, often grouped with the vitamins.
Inositol has the structural formula shown below. Almost nothing is known of
its function in enzyme systems. It is metabolized as a carbohydrate, but its
availability is limited in the presence of dietary calcium. There is no
evidence of a human requirement for inositol in the diet, probably because a
considerable amount is synthesized by intestinal bacteria. Yeast cells and
mammalian tissues are capable of inositol synthesis.  See  CARBOHYDRATE.
 �34s[Frank Wagner]
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�6v�2v�32sInsecta �21m �33s
    A class of the phylum Arthropoda sometimes called the Hexapoda. This is
the largest class of animals, containing about 750,000 described species, but
there are possibly as many as 5,000,000 actual species of insects. Like other
arthropods, they have an external, chitinous covering. Fossil insects dating
as early as the Paleozoic have been found throughout the world.

    Classification.  The class Insecta is divided into orders on the basis of
the structure of the wings and the mouthparts, on the type of metamorphosis,
and on various other characteristics. There are differences of opinion among
entomologists as to the limits of some of the orders. The following are
considered herein as orders of insects.

  Subclass Pterygota
    Order: Ephemeroptera (Ephemerida) - mayflies
	   Odonata - dragonflies and damselflies
	   Blattaria (Dictyoptera) - cockroaches
	   Mantodea - mantises
	   Isoptera - termites
	   Orthoptera - grasshoppers, katydids, crickets, walking sticks,
	     cockroaches, and mantids
	   Phasmatoptera
	   Dermaptera - earwigs
	   Embiidina (Embioptera) - webspinners
	   Plecoptera - stoneflies
	   Zoraptera - zorapterans
	   Psocoptera - psocids
	   Mallophaga - chewing lice
	   Anoplura - sucking lice
	   Thysanoptera - thrips
	   Hemiptera - bugs
	   Homoptera - cicadas, hoppers, aphids, whiteflies, scale insects
	   Neuroptera - dobsonflies, fishflies, snakeflies, lacewings, antlions
	   Coleoptera - beetles
	   Strepsiptera
	   Mecoptera - scorpionflies
	   Siphonaptera - fleas
	   Diptera - true flies
	   Trichoptera - caddisflies
	   Lepidoptera - butterflies and moths
	   Hymenoptera - sawflies, ichneumons, chalcids, wasps, ants, bees

    Morphology.  Insects are usually elongate and cylindrical in form, and are
bilaterally symmetrical. The body is segmented, and the ringlike segments are
grouped into three distinct regions, the head, thorax, and abdomen. The head
bears the eyes, antennae, and mouthparts; the thorax bears the legs and wings,
when wings are present; the abdomen usually bears no locomotor appendages but
often bears some appendages at its apex. Most of the appendages of an insect
are segmented. The body wall serves not only as a covering, but also as a
supporting structure to which many important muscles are attached.
    A pair of compound eyes usually cover a large part of the head surface. In
addition to having compound eyes, most insects also possess two or three
simple eyes, the ocelli, usually located on the upper part of the head between
the compound eyes.  See  EYE (INVERTEBRATE).
    Insect mouthparts typically consist of labrum, or upper lip; a pair each
of mandibles and maxillae; a labium, or lower lip; and a tonguelike structure,
the hypopharynx. These structures are variously modified in different insect
groups and are often used in classification and identification. The type of
mouthparts an insect has determines how it feeds and what type of damage it is
capable of causing. A cricket is said to have chewing mouthparts because it
has heavily sclerotized mandibles that move sideways and can bite off and chew
particles of food. A bedbug has long, slender, needlelike mandibles which can
pierce the skin. Also, a pair of maxillae form a tube which is inserted
through the puncture and used to suck blood.
    Several forms of antennae are recognized, to which various names are
applied; they are used extensively in classification. The antennae are usually
located between or below the compound eyes and are often reduced to a very
small size. They are sensory in function and act as tactile organs, organs of
smell, and in some cases organs of hearing.
    Insects are the only winged invertebrates, and their dominance as a group
is probably due to their wings. Immature insects do not have wings, except in
the mayflies. The wings are solid structures and may be likened to the two
sides of a cellophane bag that have been pressed tightly together. The form
and rigidity of the wing are due to the stiff chitinous veins which support
and strengthen the membranous portion. At the base are small sclerites which
serve as muscle attachments and produce consequent wing movement. The wings
vary in number, size, shape, texture, and venation, and in the position at
which they are held at rest. There is a common basic pattern of wing venation
in insects which is variously modified and in general quite specific for
different large groups of insects. Much of the classification of insects
depends upon these variations. A knowledge of fossil insects depends largely
upon the wings, because they are among the more readily fossilized parts of
the insect body.

    _Internal anatomy.	The intake of oxygen, its distribution to the tissues,
and the removal of carbon dioxide are accomplished by means of an intricate
system of tubes called the tracheal system. The principal tubes of this
system, the trachea, open externally at the spiracles. Internally they branch
extensively, extend to all parts of the body, and terminate in simple cells,
the tracheoles. Many adaptations for carrying on respiration are known.
Aquatic insects may have tracheal gills located either on the external portion
of the body or in the rectal cavity. The larvae of certain insects, such as
mosquitoes, have respiratory tubes at the posterior end of the body. Some
aquatic insects with normal tracheal systems and active spiracles can go
beneath the water and remain for extended periods of time by carrying with
them a film of air on the surface of the body.
    Insects possess an alimentary tract consisting of a tube, usually coiled,
which extends from the mouth to the anus. It is differentiated into three main
regions, the foregut, midgut, and hindgut. Valves between the three main
divisions of the alimentary canal regulate the passage of food from one region
to another.
    The excretory system consists of a group of tubes with closed distal ends,
the Malpighian tubules. They vary in number from 1 to over 100, and extend
into the body cavity. Various waste products are taken up from the blood by
these tubules and passed out by way of the hindgut and anus.
    The circulatory system of an insect is an open one, as compared with a
closed system composed of arteries, veins, and capillaries, as in the
vertebrates. The only blood vessel is a tube located dorsal to the alimentary
tract and extending through the thorax and abdomen. The posterior portion of
this tube is called the heart; the anterior part of the tube is called the
dorsal aorta.
    The nervous system consists of a brain, often called the supraesophageal
ganglion, located in the head above the esophagus; a subesophageal ganglion,
connected to the brain by two commissures that extend around each side of the
esophagus; and a ventral nerve cord, typically double, extending posteriorly
through the thorax and abdomen from the subesophageal ganglion. In the nerve
cords there are enlargements, called ganglia. Typically, there is a pair to
each body segment. From each ganglion of the chain, nerves extend to each
adjacent segment of the body, and also extend from the brain to part of the
alimentary canal.
    Reproduction in insects is nearly always sexual, and the sexes are
separate. Variations from the usual reproductive pattern occur occasionally.
In many social insects, such as the ants and bees, certain females, the
workers, may be unable to reproduce because their sex organs are undeveloped;
in some insects, individuals occasionally occur that have characters of both
sexes, called gynandromorphs.  See  SOCIAL INSECTS.

    Metamorphosis.  After insects hatch from an egg, they begin to grow and
increase in size (growth) and will also usually change, to some degree at
least, in form (metamorphosis) and often in appearance. The growth of an
insect is accompanied by a series of molts, or ecdyses, in which the cuticle
is shed and renewed.


    The molt involves not only the external layers of the body wall, but also
the cuticular linings of the tracheae, foregut, and hindgut; the cast skins
often retain the shape of the insects from which they were shed. The shedding
process begins with a splitting of the old cuticle, usually along the midline
on the dorsal side of the thorax. This split grows and the insect eventually
wriggles out of the old cuticle. The new skin, at first wrinkled and folded,
remains soft and pliable long enough for the body to expand to its fullest
capacity before hardening.
    Insects have been grouped or classified upon the basis of the type of
metamorphosis which they undergo. All entomologists, however, do not agree
upon the same classification. These types of metamorphosis have been placed in
two groups, under the heading of simple metamorphosis and complete
metamorphosis. Some investigators include ametabolous, paurometabolous, and
hemimetabolous types in simple metamorphosis, and consider complete
metamorphosis to be synonymous with holometabolous. In order to cover the more
detailed pattern, the following outline is used.

1. Ametabolous or primitive: No distinct external changes are evident with an
   increase in size.
2. Metabolous: Distinct changes, both in size and form, are evident.
   a. Paurometabolous: Direct metamorphosis that is simple and gradual;
      immature forms resemble the adult except in size and are referred to as
      nymphs.
   b. Hemimetabolous: Metamorphosis is incomplete; gills are present in aquatic
      larvae, or naiads.
   c. Holometabolous: Complete or indirect metamorphosis; stages in this
      development are the egg, larva, pupa, and adult, or imago.

 �34s[Dwight M. DeLong]
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    Fossils.  Fossil insects have been found in about 100 different rock
formations throughout the world. One of the best known deposits is a shale bed
in the town of Florissant, Colo.
    The study of fossil insects has made significant contributions to the
understanding of the evolution of insects and of the relationships of existing
orders and families. It has become apparent that the groups now living are but
a small part of the total number that have existed on the Earth.
    The fossil record, combined with morphological studies of living types,
shows that insects have passed through three major evolutionary steps. The
first of these was the development of wings as simple but functional
outgrowths of the thoracic wall which could not be folded back over the
abdomen when the insect was at rest. The second step was marked by the
evolution of a complicated wing articulation, which enabled the wings to be
folded back over the abdomen at rest, and by the presence of immature stages
resembling the adults. The third step was the acquisition of a complicated
postembryonic development with an immature stage (larva) very different from
the adult. The fossils show that insects passed through these stages before
the beginning of the Permian Period, some 225,000,000 years ago, when
land-inhabiting vertebrates were only starting their evolutionary history.
 �34s[Frank M. Carpenter]
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�6v�2v�32sInsect control, biological   �21m �33s 
    The term biological control was proposed in 1919 to apply to the use or 
role of natural enemies in insect population regulation. The enemies involved 
are termed parasites (parasitoids), predators, or pathogens. This remains 
preferred usage, although other biological methods of insect control have been 
proposed or developed, such as the release of mass-produced sterile males to 
mate with wild females in the field, thereby greatly reducing or suppressing 
the pests' production of progeny. Classical biological control is an 
ecological phenomenon which occurs everywhere in nature without aid from, or 
sometimes even understanding by, humans. However, humans have utilized the 
ecological principles involved to develop the field of applied biological 
control of insects, and the great majority of practical applications have been 
achieved with insect pests. Additionally, such diverse types of pest organisms 
as weeds, mites, and certain mammals have been successfully controlled by use 
of natural enemies. 
    Biological control offers several advantages over chemical control. 
Application costs are minimal and nonrecurring and there are no environmental 
pollution problems connected with biological control, whereas insecticides 
cause severe problems related to toxicity to humans, wildlife, birds, and 
fish, as well as causing adverse effects in soil and water. Biological control 
causes no upsets in the natural balance of organisms, but these upsets are 
common with chemical control; biological control is permanent, chemical 
control is temporary, usually one to many annual applications being necessary. 
  See  ENTOMOLOGY, ECONOMIC; INSECTICIDE. 
 �34s[Paul De Bach]
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�6v�2v�32sInsecticide   �21m �33s 
    A material used to kill insects and related animals by disruption of vital 
processes through chemical action. Insecticides may be inorganic or organic 
molecules.  
    Insecticides are classified according to type of action as stomach 
poisons, contact poisons, residual poisons, systemic poisons, fumigants, 
repellents, attractants, insect growth regulators, or pheromones. Many act in 
more than one way. Stomach poisons are applied to plants so that they will be 
ingested as insects chew the leaves. Contact poisons are applied in a manner 
to contact insects directly, and are used principally to control species which 
obtain food by piercing leaf surfaces and withdrawing liquids. Residual 
insecticides are applied to surfaces so that insects touching them will pick 
up lethal dosages. Systemic insecticides are applied to plants or animals and 
are absorbed and translocated to all parts of the organisms, so that insects 
feeding upon them will obtain lethal doses. Fumigants are applied as gases, or 
in a form which will vaporize to a gas, so that they can enter the insects' 
respiratory systems. Repellents prevent insects from closely approaching their 
hosts. Attractants induce insects to come to specific locations in preference 
to normal food sources. Insect growth regulators are generally considered to 
act through disruption of biochemical systems or processes associated with 
growth or development, such as control of metamorphosis by the juvenile 
hormones, regulation of molting by the steroid molting hormones, or regulation 
of enzymes responsible for synthesis or deposition of chitin. Pheromones are 
chemicals which are emitted by one sex, usually the female, for perception by 
the other, and function to enhance mate location and identification; 
pheromones are generally highly species-specific.  See  FUMIGANT; PHEROMONE. 
    The use of inorganic insecticides has declined to almost nil. Probably the 
largest current uses are for inorganic arsenicals as constituents of 
formulations for wood impregnation and preservation. Organic insecticides 
began to supplant the arsenicals when DDT and lindane 
[1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] became available to the 
public during World War II. Historically, DDT and lindane were the two 
largest-volume organic insecticides. Since 1968, however, in the United States 
all nongovernmental and nonprescription uses of DDT have been canceled. 
Coincidentally, the use of lindane has declined both through development of 
resistant strains of insects and through official restriction. 
    Other chlorinated hydrocarbon insecticides which were available by or 
during the early 1970s included: TDE, methoxychlor, Dilan, chlordane, 
heptachlor, aldrin, dieldrin, endrin, toxaphene, endosulfan, Kepone, mirex, 
and Perthane. Registrations for all uses of TDE were canceled in 1971 because 
it is a metabolite of DDT. Chlordane, the first cyclopentadiene insecticide to 
achieve commercial status, was once the most effective chemical available for 
the control of cockroaches. As early as 1966, uses and registrations for 
aldrin and dieldrin were being curtailed. Most uses of aldrin and dieldrin 
were canceled by the U.S. Environmental Protection Agency (EPA) in 1975 
because of their risks to human health and the environment. The principal 
remaining registered use for these two chemicals is for the control of 
subterranean termites. The production of aldrin has been stopped. Most uses of 
chlordane and heptachlor were similarly canceled in 1978. Registrations of 
other insecticides are being reviewed. Registrations of other insecticides are 
being reviewed. 
    The development of organic phosphorus insecticides paralleled that of the 
chlorinated hydrocarbons. Since 1947, many thousands of organophosphorus 
compounds have been synthesized for evaluation as potential insecticides. A 
great diversity of activity against insects is found among organophosphorus 
insecticides. Unfortunately, many can be quite poisonous to humans and other 
warm-blooded animals, as well as to insects. Activity of organic phosphate 
insecticides results from the inhibition of the enzyme cholinesterase, which 
performs a vital function in the transmission of impulses in the nervous 
system.  
    The first commercial carbamate insecticide, carbaryl, was introduced 
during the mid-1950s and continues to enjoy broad use not only in the United 
States but worldwide. Carbamates, like organophosphates, are cholinergic. 
Several not only interfere with cholinesterase, but may also inhibit one or 
more enzymes known as aliesterases. 
    Diflubenzuron is another chemical with a new and different mode of action. 
It interferes with the action of chitin synthetase, an enzyme necessary for 
the synthesis of chitin, and as a consequence of that process, disrupts the 
orderly deposition of that vital element of the exoskeleton. Currently, it is 
registered for the control of the gypsy moth in hardwood forests of the 
eastern United States. 
    Pyrethrum is one of the oldest insecticides. Because it is highly active 
and quite safe for humans and animals, extensive effort was made during the 
period 1910 - 1945 to determine its chemical structure. These efforts 
culminated in the discovery of some of the most active insect control agents 
yet devised. The first product, allethrin, which was only partially synthetic, 
was introduced commercially during the early 1950s. Subsequent, entirely 
synthetic products have included: dimethrin, fenothrin, resmethrin, and 
tetramethrin.  
    Formulation of insecticides is extremely important in obtaining 
satisfactory control. Common formulations include dusts, water suspensions, 
emulsions, and solutions. Accessory agents, including dust carriers, solvents, 
emulsifiers, wetting and dispersing agents, stickers, deodorants or masking 
agents, synergists, and antioxidants, may be required to obtain a satisfactory 
product.  
    Proper timing of insecticide applications is important in obtaining 
satisfactory control. The application of insecticides should be correlated 
with the occurrence of the most susceptible or accessible stage in the life 
cycle of the pest involved. By and large, treatments should be made only when 
economic damage by a pest appears to be imminent.  See  INSECT CONTROL, 
BIOLOGICAL; MITICIDE; PESTICIDE. 
 �34s[George F. Ludvik]
�33s 
  
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�6v�2v�32sInsectivora �21m �33s
    An order of mammals including such familiar forms as the hedgehogs,
shrews, and moles. The Insectivora are of ancient origin and are difficult to
define; they are united by primitive characters that, for the most part, are
primitive for all placental mammals.
    Although their name implies a diet of insects, most insectivores feed, or
once fed, on a variety of invertebrate and even vertebrate prey; some were
even frugivorous or omnivorous. Many insectivores have spines, notably the
hedgehogs and tenrecs, and generally the senses of smell and hearing are far
more acute than eyesight. Echolocation is known in shrews and tenrecs, and
some shrews and Solenodon share the distinction of being the only mammals
whose bite is poisonous, at least to small prey. Almost all insectivores are
nocturnal or crepuscular, some are excellent tunnelers through the soil, and a
few are aquatic.
    Insectivores may be divided into two groups. The first group is composed
of insectivores in which the intestinal cecum is lacking and the stapedial
artery is a major source of blood supply to the brain. These are known as the
Lipotyphla, or Insectivora proper. All living insectivores (except tree
shrews, Tupaiidae) and most late Cenozoic, a few early Cenozoic, and possibly
Cretaceous insectivores belong to this first group. The second group,
sometimes called the Proteutheria, is generally more primitive and is
primarily early Cenozoic in known occurrence. Living members have an
intestinal cecum, and the blood supply to the brain is still primitively
complex. The stapedial artery is not as important as in the lipotyphlans.  See
  DELTATHERIDIA; EUTHERIA; HEDGEHOG; MAMMALIA; MOLE (ZOOLOGY); SHREW;
SOLENODON; TENREC.
 �34s[D. Dwight Davis /Malcolm C. McKenna]
�33s

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�6v�2v�32sInsectivorous plants �21m �33s
    Plants having variously modified, highly specialized leaves which capture
and digest insects. The proteins of the digested insect bodies supply
nitrogen, which otherwise may be unavailable to these plants in the places
where they grow. Sometimes they are also called carnivorous (flesh-eating)
plants.  See  PITCHER PLANT; SARRACENIALES; SECRETORY STRUCTURES (PLANT);
SUNDEW; VENUS' FLYTRAP.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sInsect pathology   �21m �33s 
    A biological discipline embracing the general principles of pathology 
(disease in the broadest sense) as applied to insects. It refers to human 
observations and actions concerning the cause, symptomatology, gross 
pathology, histopathology, pathogenesis, and epizootiology of the diseases of 
insects; it is concerned with whatever can go wrong or become abnormal in an 
insect. A diseased insect may be suffering from an infectious disease caused 
by a microorganism or a noninfectious disease, such as a metabolic 
disturbance, a genetic abnormality, a nutritional deficiency, a physical or 
chemical injury, or injury caused by parasites or predators. 
    Insect pathology draws upon and contributes to the general field of 
microbiology and provides understanding of certain of the biological 
relationships existing between insects and microorganisms not pathogenic to 
them. Insect pathology may be viewed as a part of invertebrate pathology and a 
part of the branch of entomology. Insect pathology finds applications in 
agriculture, medicine, and biology generally. Microbial control, the use of 
microorganisms in biological control, is one area of applied insect pathology. 
Microorganisms are introduced to control insect pests for the protection of 
humans, animals, and agricultural crops. However, the suppression of disease 
in beneficial insects, such as the silkworm, honeybee, and ladybird beetle, is 
also of significant practical importance.  See  INSECT CONTROL, BIOLOGICAL; 
INVERTEBRATE PATHOLOGY. 
 �34s[John D. Briggs]
�33s 
  
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�6v�2v�32sInsolation   �21m �33s 
    The amount of solar radiation which reaches a unit horizontal area of the 
Earth. It is the latitudinal variation of insolation which supplies energy for 
the general circulation of the atmosphere. Insolation outside the Earth's 
atmosphere depends on the angle of incidence of the solar beam and on the 
solar constant. The solar constant is the amount of energy which, in unit 
time, reaches a unit plane surface perpendicular to the Sun's rays outside the 
Earth's atmosphere when the Earth is at its mean distance from the Sun. 
    Solar energy is modified by the air, the clouds, and the land and water 
surface of the Earth. The energy absorbed at the surface depends also on the 
surface albedo. The reflectivity of the solid Earth varies with ground cover. 
Forests absorb nearly all the energy incident on them. Fresh snow, on the 
other hand, reflects up to 90% of the incident energy. In middle latitudes, 
snow albedos may decrease to less than 40% with time, especially after warm 
spells have modified the surface. Grass, fields, and other surfaces will have 
intermediate albedos. When all the factors are considered, the absorption by 
the atmosphere and Earth's surface can be estimated, as shown in the last line 
of the table. It is interesting to note that the gradient of absorbed 
radiation, which is the driving force of the circulation, is similar to the 
gradient of the extraterrestrial insolation.  See  ALBEDO. 
 �34s[Sigmund Fritz]
�33s 
  
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�6v�2v�32sInspection and testing   �21m �33s 
    Industrial activities which ensure that manufactured products, individual 
components, and multicomponent systems are adequate for their intended 
purpose. Complete inspection and testing programs cover evaluation of the 
product design prior to manufacturing, conformance to the design during 
manufacturing, and durability during subsequent storage, transport, and use. 
The terms inspection and testing are used interchangeably and jointly to 
describe closely related activities, but strictly speaking, inspection per se 
is the total activity dealing with: interpretation of product specifications; 
measurement and testing of the characteristics described in the 
specifications; methods of sampling and statistical analysis of results; and 
decisions on the disposition of the product evaluated.  See  GAGES; 
NONDESTRUCTIVE TESTING; QUALITY CONTROL. 
    The risks of occasionally making the wrong decisions in sampling 
inspection are an accepted part of industrial practice. The possibility of 
unsatisfactory product reaching the consuming public under such procedures is 
covered by guarantees which are now regulated by law. 
 �34s[John A. Clements]
�33s 
  
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�6v�2v�32sInstinctive behavior   �21m �33s 
    A relatively complex response pattern which is usually present in one or 
both sexes of a given species. These responses have a genetic basis, are 
essentially unlearned, and are generally adaptive. 
    Instinctive behavior occurs when an animal has a particular internal state 
while it is in the presence of a specific external stimulation called a 
releaser or a sign stimulus. Neither the internal state nor the external 
stimulus alone is adequate for the elicitation of the response. Many animals 
show particular instinctive behaviors only during the mating season, when 
hormonal changes associated with sexual behavior sensitize specific portions 
of the central nervous system, which will then be active in the presence of 
the releaser. The external stimulus may be relatively simple or incredibly 
complex.  
    Within limits, the instinctive behaviors can be modified by learning. 
There is evidence, for example, that some predators learn to attack their prey 
at the back of the neck because when held in that position the prey cannot 
counterattack.  See  MIGRATORY BEHAVIOR; REPRODUCTIVE BEHAVIOR. 
 �34s[Kenneth E. Moyer]
�33s 
  
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�6v�2v�32sInstrumentation �21m �33s
    Designing, manufacturing, and utilizing physical instruments or instrument
systems for detection, observation, measurement, automatic control, automatic
computation, communication, or data processing. The term instrumentation is
also used for the ensemble of instruments and auxiliary equipment used in an
experiment, test, or process, or in a plant, machine, or vehicle.
    Since all branches of experimental science and technology depend on
instrumentation, specialized instruments, with a corresponding body of
knowledge and practice, have been developed separately in many fields. Thus
chemical instrumentation, aeronautical instrumentation, medical
instrumentation, optical instrumentation, and many other similar terms
indicate areas of specialization in various industries or professions.
    Instruments are sometimes classified according to the field or purpose of
application, such as navigation instruments; according to their functions in
instrument systems, such as detection, measurement, recording, computing,
controlling, signal modification, or display; or according to the physical
quantity or property that is to be measured or controlled by the instrument,
such as flow, temperature, pressure, force, displacement, level, viscosity,
acceleration, electrical quantities (voltage, current, resistance, and
capacitance), or optical quantities (transmission, gloss, color, and
brightness).
    Other methods of describing or classifying instruments involve
distinguishing operating principle or type (for example, an electron
microscope of the transmission, emission, or scanning types), or specific
range of applicability (for example, spectrometers for the infrared, visible,
ultraviolet, or x-ray regions of electromagnetic radiation).
    All instruments have inherent deficiencies. No instrument responds
instantaneously to a change in the measurand; the lag is dependent on the
natural frequency of the instrument system and its degree of damping (internal
dissipation of energy). If the damping is too great or the natural frequency
too low, the response lag may be excessive and the indication or output
seriously in error during an interval of several times the natural period of
the system. Conversely, if the damping is too small, the dynamic response may
be excessive and the output may exhibit oscillations before attaining the
desired value. In either event a detailed analysis of the instrument
characteristics is essential to make appropriate corrections.  See  DAMPING.
    Many instrument elements exhibit drift, a slow change in properties with
time, resulting in a change in output. Many elements also exhibit the
phenomenon of hysteresis; that is, they exhibit a response to increasing
signals which is different from their response to decreasing signals.
Environmental factors, such as temperature or vibration, may also affect
performance of certain elements. The instrument maker must choose materials
and operating principles which minimize these effects; the user must develop
measurement techniques which either avoid these effects or correct for them.
See  HYSTERESIS.
    An important aspect of instrumentation is the estimation of the
uncertainties associated with measurements, and development of techniques to
minimize them. Instruments in industrial use may be linked to the National
Bureau of Standards through a calibration chain involving several other
similar instruments, usually of successively better performance.  See
PHYSICAL MEASUREMENT.
 �34s[William A. Wildhack]
�33s

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�6v�2v�32sInstrument landing system (ILS)   �21m �33s 
    A continuous-wave amplitude-modulated, horizontally-polarized fixed-beam 
system comprising the following aids to aircraft approaching a runway: (1) a 
vhf localizer to give azimuth guidance with reference to the runway center 
line, with an operating frequency in the band 108 - 112 MHz; (2) a uhf glide 
slope to give elevation guidance about an optimum approach angle, with an 
operating frequency in the band 328.6 - 335.4 MHz; and (3) two or three marker 
beacons operating at 75 MHz to give range with reference to the touchdown 
point. The system is shown in the illustration. 
    Since 1948, performance standards and recommended practices have been 
agreed on and formulated by the International Civil Aviation Organization 
(ICAO). In 1963, ICAO laid down three categories of ILS guidance criteria to 
standardize operations to lower approach visibility minima: 
  
    Category I. Operation down to a height of 60 m (200 ft) and 800 m (2600 
ft) visibility. 
    Category II. Operation down to 30 m (100 ft) and 400 m (1300 ft) 
visibility.  
    Category III. Operation down to and along the surface of the runway 
unrestricted by cloud base and visibility conditions.  See  ELECTRONIC 
NAVIGATION SYSTEMS; PRECISION APPROACH RADAR (PAR). 
 �34s[Frank H. Taylor]
�33s 
  
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�6v�2v�32sInstrument transformer �21m �33s
    Electric transformers specifically designed for use in measurement and
control circuits. The purpose of an instrument transformer is to convert
primary voltages or currents to secondary values suitable for use with relays,
meters, or other measuring equipment. A second purpose is to isolate the
high-voltage primary circuit from the measurement circuit. Thus the
construction of measuring devices and the protection of personnel using such
devices are greatly simplified by the use of instrument transformers. For
general discussion of transformers. See TRANSFORMER.
    There are two general types of instrument transformers: instrument
potential transformers, which are used in voltage measurements, and instrument
current transformers, which are used in current measurement.  See  ELECTRICAL
MEASUREMENTS.
 �34s[Isaac F. Kinnard]
�33s

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�6v�2v�32sInsulin   �21m �33s 
    The hormone, produced and secreted by the beta cells of the islets 
(insulae) of Langerhans of the pancreas, which regulates the use and storage 
of foodstuffs, especially the carbohydrates. When insufficient levels of 
insulin are produced, a state of diabetes occurs. Chemically insulin is a 
small, simple protein. The molecular weight is close to 6000, and the molecule 
consists of 51 amino acids, arranged in two linked polypeptide chains. 
Insulins from different species differ in their composition. These differences 
account for the fact that diabetics treated with animal insulins develop 
antibodies which may sometimes interfere with the action of the hormone.  See  
CARBOHYDRATE METABOLISM; DIABETES; PANCREAS. 
 �34s[Rachmiel Levine]
�33s 
  
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�6v�2v�32sIntaglio (gemology)   �21m �33s 
    The name given to the type of carved gemstone in which the figure is 
engraved into the surface of the stone, rather than left in relief by cutting 
away the background, as in a cameo. Intaglios are almost as old as recorded 
history, for this type of carving was popular in ancient Egypt in the form of 
cylinders. Intaglios have been carved in a variety of gem materials, including 
emerald, crystalline quartz, hematite, and the various forms of chalcedony.  
See  CAMEO. 
 �34s[Richard T. Liddicoat, Jr.]
�33s 
  
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�6v�2v�32sInteger partition �21m �33s
    An integer partition of n is a nonincreasing sequence of positive integers
whose sum is n; the number of these partitions is denoted by p(n).  Thus p(4) =
5, since 4, 3 + 1, 2 + 2, 2 + 1 + 1, and 1 + 1 + 1 + 1 all partition 4.  No
usable formula for p(n) is known, but Euler discovered an elegant expression for
the generating function:  p(x) = (1 - x)(sup -1) (1 - x[sup 2])(sup -1) .  .  .
(1 - x[sup m])(sup -1) .  .  .	.  Asymptotically, p(n) obeys the formula below,
where e(sup x) is the exponential function which uses the base e.  Thus p(n) is
eventually greater than any polynomial function of n but is eventually less than
asupn for any a > 1.

p(n) ASYMPTOTIC {1/[4n SQUARE ROOT (3)]} e(sup pi SQUARE ROOT (2n/3)]

    Associated with an integer partition n = a1 + .  .	.  + asubk is an array
of dots called its Ferrer's diagram, whose ith row contains asubi dots.  Thus 4
= 3 + 1 gives rise to the diagram three dots over a single dot, aligned at the
left.  A reflection of the diagram through its main diagonal interchanges rows
and columns, converting 4 = 3 + 1 into its dual partition 4 = 2 + 1 + 1.  This
correspondence proves a number of identities, such as that the number of
partitions of n into k parts equals the number of partitions of n which have
greatest part k.  See COMBINATORIAL THEORY; ENUMERATION OF ARRANGEMENTS.
 �34s[T.  Balderes]
�33s

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�6v�2v�32sIntegral equation �21m �33s
    An equation of the form typified by Eq. (1). The major problem is to

	INTEGRATION (a to b) K(x,y,phi(y))dy + f(x) = a(x)phi(x)	     (1)

decide when there is a function phi(x) which is a solution to the equation.
Equations such as (1) arise from the analysis of ordinary differential
equations. Integral equation (1) is an equation of the first kind if a(x)
IDENTITY 1, and an equation of the second kind if a(x) IDENTITY 0.
    If K(x,y,phi) = 0 for x < or = y < or = b, Eq. (1) is called a Volterra
equation.  Under mild assumptions the Volterra equation of the second kind can
be solved by the method of successive approximations (also known as the method
of Picard).
    A special case of some interest is that of linear integral equations. The
function K(x,y,phi) is a linear function of phi. The linear equations of the
first and second kind are shown in Eqs. (2) and (3) respectively. The function
K(x,y) is called the kernel, and the complex number lambda is called the
parameter. The equation of the second kind, Eq. (3), is homogeneous if f(x)
IDENTITY 0. In typical cases, the homogeneous equations will have only the
trivial solution phi(x) IDENTITY 0. For some values of the parameter lambda,
however, there will be nontrivial solutions. Such a value of the parameter
lambda is called a characteristic value for K, and the corresponding function
phi is called a characteristic function for K. These concepts are related to
corresponding concepts in linear algebra and operator theory.  See
EIGENFUNCTION; MATRIX CALCULUS; MATRIX THEORY; OPERATOR THEORY.

	       f(x) = INTEGRATION (a to b) K(x,y)phi(x) dy		     (2)

       phi(x) = f(x) + lambda INTEGRATION (a to b) K(x,y)phi(y) dy	     (3)

    The integral equation, Eq. (2) or (3), is called a Fredholm equation, and
K(x,y) is called a Fredholm kernel if ||K|| < INFINITY, where ||K||, the norm of
K, is

      ||K||sup2 = DOUBLE INTEGRATION (both a to b) |K(x,y)|sup2 dxdy	     (4)

defined by Eq. (4). This is certainly true if K is continuous or even bounded.
Even some infinite discontinuities are allowable. Fredholm equations of the
second kind for which the parameter satisfies the inequality |lambda| ||K|| <1
can be shown to have unique solutions by the method of Picard.	As a result
there are no characteristic values for which |lambda| < or = 1/||K||.  In
general, for any constant M > 0 there are only finitely many characteristic
values which satisfy |lambda| < or = M.  Further, only finitely many linearly
independent characteristic functions are associated with each characteristic
value.	See DIFFERENTIAL EQUATION; INTEGRAL TRANSFORM; INTEGRATION.
 �34s[John Polking]
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�6v�2v�32sIntegral transform �21m �33s
    An integral relation between two classes of functions. For example, a
relation such as Eq. (1) is said to define an integral transform. More
generally, the integral may be a multiple integral, and the functions f,G,phi

     f(x) = INTEGRATION (-INFINITY to INFINITY) G(x,y)phi(y) dy 	   (1)

may depend on a larger number of variables. Equation (1) is thought of as
transforming a whole class, or space, of functions phi(y) into another class
of functions f(x). The function G(x,y) is the kernel of the transform. One of
the important uses of such a transform is based on the fact that a problem
posed in one of the two spaces in question may be more easily solved in the
other. For example, a differential equation to be solved for the function
phi(y) may become an algebraic equation for the unknown function f(x).	See
LAPLACE TRANSFORM.
    The two basic problems for any integral transform are inversion and
representation. In inversion the aim is to recover phi(y) from f(x), the
kernel G(x,y) being known. That is, Eq. (1) is thought of as an integral
equation (of the first kind) to be solved for the unknown function phi(y). A
means of calculating phi(y) from f(x) is called an inversion formula, and in
its presence the transform achieves maximum utility, since explicit passage
from each space to the other is thus assured. In representation the question
is which functions f(x) may be written or represented in the form (1). That
is, one asks which functions f(x) will make Eq. (1) solvable for phi(y).
Usually this problem becomes more tractable when the solutions phi(y) are
restricted to some subspace such as the class of positive or bounded
functions.  See  INTEGRAL EQUATION.
    An important special case of Eq. (1) is the convolution transform, when
the kernel is a function of (x - y), Eq. (2). An equivalent form of Eq. (2) is

	f(x) = INTEGRATION (-INFINITY to INFINITY) G(x - y)phi(y) dy	     (2)


	     F(x) = INTEGRATION (0 to INFINITY) K(xy)PHI(y) dy		     (3)

Eq. (3), since the change of variable x = esupt, y = esup-u carries Eq. (3)
into Eq. (2) after a suitable change in notation.  See	INTEGRATION.
 �34s[David V. Widder]
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�6v�2v�32sIntegrated circuits �21m �33s
    Miniature electronic circuits produced within and upon a single
semiconductor crystal, usually silicon. Integrated circuits range in
complexity from simple logic circuits and amplifiers, about ** in. (1.3 mm)
square, to large-scale integrated circuits up to about ** in. (8 mm) square.
They contain tens of thousands of transistors and other components which
provide computer memory circuits and complex logic subsystems such as
microcomputer central processor units. See SEMICONDUCTOR.
    Integrated circuits consist of the combination of active electronic
devices such as transistors and diodes with passive components such as
resistors and capacitors within and upon a single semiconductor crystal. The
construction of these elements within the semiconductor is achieved through
the introduction of electrically active impurities into well-defined regions
of the semiconductor.
    Generally, integrated circuits are not straightforward replacements of
electronic circuits assembled from discrete components. They represent an
extension of the technology by which silicon planar transistors are made.
Because of this, transistors or modifications of transistor structures are the
primary devices of integrated circuits. Methods of fabricating good-quality
resistors and capacitors have been devised, but the third major type of
passive component, inductors, must be simulated with complex circuitry or
added to the integrated circuit as discrete components.  See  TRANSISTOR.
    Integrated circuits can be classified into two groups on the basis of the
type of transistors which they employ: bipolar integrated circuits, in which
the principal element is the bipolar junction transistor; and metal oxide
semiconductor (MOS) integrated circuits, in which the principal element is the
MOS transistor. Both depend upon the construction of a desired pattern of
electrically active impurities within the semiconductor body, and upon the
formation of an interconnection pattern of metal films on the surface of the
semiconductor. Bipolar circuits are generally used where highest logic speed
is desired, and MOS for largest-scale integration or lowest power dissipation.

    Bipolar integrated circuits.  Complete digital integrated circuits
generally contain tens to hundreds of inverters and gates interconnected as
counters, arithmetic units, and other building blocks. Hundreds of such
circuits may be fabricated on a single slice of silicon crystal. This feature
of planar technology - simultaneous production of many circuits - is
responsible for the economic advantages and wide use of integrated circuits.

    A bipolar integrated circuit, representative of the simplest logic
function, is illustrated schematically in Fig. 1. An inverter requires only a
transistor and register, shown in cross section. Both the transistor and the
resistor are formed within a separate pocket of n-type semiconductor
surrounded on all sides by p-type regions. When the inverter circuit is
operated, a reverse bias develops between the n-type pocket and its
surroundings. The depletion region separating the n- and p-type regions has a
very high resistance; consequently, the individual transistors and resistors
are electrically isolated from each other, even though both of them have been
formed within the same semiconductor crystal.  See  FUNCTION TRANSISTOR.
    Integrated circuits based on amplifiers are called linear because
amplifiers usually exhibit a linearly proportional response to input signal
variations. However, the category includes memory sense amplifiers,
combinations of analog and digital processing functions, and other circuits
with nonlinear characteristics. Some digital and analog combinations include
analog-to-digital converters, timing controls, and modems (data communications
modulator-demodulator units). Linear circuits cannot yet match the power and
frequency range of discrete transistors.
    In the continuing effort to increase the complexity and speed of digital
circuits, and the performance characteristics and versatility of linear
circuits, a significant role has been played by the discovery and development
of new types of active and passive semiconductor devices which are suitable
for use in integrated circuits. Among these devices is the pnp transistor
which, when used in conjunction with the standard npn transistors described
earlier, lends added flexibility to the design of integrated circuits.

    MOS integrated circuits.  The other major class of integrated circuits is
called MOS because its principal device is a metal oxide semiconductor
field-effect transistor (MOSFET). It is more suitable for large-scale
integration (LSI) than bipolar circuits because MOS transistors are
self-isolating and can have an average size of less than a millionth of a
square inch (6 x 10- sup 4 mm sup 2). This has made it practical to use over
100,000 transistors per circuit. Because of this high density capability, MOS
transistors are used extensively for high density RAMs, ROMs, and
microprocessors.
    Several major types of MOS device fabrication technologies have been
developed. They are: metal-gate p-channel MOS, which uses aluminum for
electrodes and interconnections (Fig. 2); silicon-gate p-channel MOS,
employing polycrystalline silicon for gate electrodes and the first
interconnection layer; n-channel MOS, which is usually silicon gate; and
complementary MOS, which employs both p-channel and n-channel devices. Silicon
gate n-channel and complementary MOS are the dominant technologies.
    The basic principles of MOS technology can be illustrated with the
metal-gate p-channel MOSFET in Fig. 2. It consists of two p-type diffused
regions (such as can be obtained by the process used to form the base region
of the npn transistors described earlier), separated by an n-type region which
is under control of an electrode, called the gate. When there is no voltage
applied to the gate, the two p-type regions, called source and drain, are
electrically insulated from each other by the n-type region which surrounds
them. When, however, a negative voltage is applied to the gate, the electric
field induces a very thin p-type region at the surface of the silicon. This
thin p-type region now connects the source and the drain, permitting the
passage of current between them. The path is called a channel.
    Both conceptually and structurally the MOS transistor is a much simpler
device than the bipolar transistor. What kept this simple device from
commercial utilization until 1964 is the fact that it depends on the
properties of the semiconductor surface for its operation, while the bipolar
transistor depends principally on the bulk properties of the semiconductor
crystal. Hence MOS transistors became practical only when understanding and
control of the properties of the oxidized silicon surface had been perfected
to a very great degree.
 �34s[Ron Burghard ;Neil Berglund ;Youssef El-Mansy]
�33s


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�6v�2v�32sIntegrated optics   �21m �33s 
    The study of optical devices, singly and in combinations, that are based 
on light transmission in waveguides, that is, structures that confine the 
propagating light to a region with one very small dimension (or sometimes 
two), of the order of the wavelength of the light. The principal motivation 
for these studies is to enable the combination of individual devices thus 
miniaturized, through waveguides or other means, into a functional optical 
system mounted on a small substrate. The resulting system is called an 
integrated optical circuit (IOC) by analogy with the semiconductor type of 
integrated circuit. An integrated optical circuit could, for example, include 
a laser, switches, polarizers, modulators, detectors, and so forth. An 
important use envisioned for integrated optical circuits is in connection with 
optical communications through the medium of glass fibers, which are 
themselves waveguides. Integrated optical circuits could be used in such a 
system as optical transmitters, repeaters, and receivers. See LASER; OPTICAL 
COMMUNICATIONS; OPTICAL FIBERS; OPTICAL MODULATORS; WAVEGUIDE. 
 �34s[Esther M. Conwell]
�33s 
  
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�6v�2v�32sIntegration �21m �33s
    An operation of the infinitesimal calculus which has two aspects. The
roots of one go back to antiquity, for Archimedes and other Greek
mathematicians used the ``method of exhaustion'' to compute areas and volumes.
A simple example of this is the approximation to the area of a circle obtained
by inscribing a regular polygon of known area, and then repeatedly doubling
the number of sides. The areas of the successive polygons are computable with
the help of elementary geometry. The limit of the sequence of these areas
gives the area of the circle. The area of each polygon can be regarded as
being made up of the sum of the areas of triangles with vertices at the center
of the circle, and so the process described is a constructive definition of an
integral which is the limit of a sum. Modern definitions of integrals as
limits of sums are discussed in this article.
    The other aspect of integration is the process of finding antiderivatives,
that is, for a given function f(x) to find another function g(x) whose
derivative is  f(x). This aspect is related to the first by the fundamental
theorem of integral calculus, so both processes are called integration.
    In the early 19th century A. L. Cauchy gave a clear-cut definition of the
definite integral for continuous functions and a proof of its existence.
Later, G. F. B. Riemann discussed the integral for discontinuous functions and
gave a necessary and sufficient condition for its existence. Thus the most
generally used definition of the integral as the limit of a sum has come to be
called the Riemann integral.
    The precise definition of the Riemann integral for a real function f of
one real variable x on a finite interval a < or = x < or = b may be formulated
as follows. Let P be a partition of the interval [a,b] into n subintervals by
points t(sub i), where t(sub i-1) < t(sub i), t0 = a, t(sub n) = b, and consider
a sum S of the

	   S = SUMMATION (n=1 to n) f(x[sub i]) (ti - ti - 1)		     (1)

form of Eq. (1), where t(sub i-1) < or = x(sub i) < or = t(sub i). The sum S
depends not only on the partition P but on the choice of the intermediate points
x(sub i).  It may happen that the sum S approaches a definite limit I when the
maximum of the numbers (t[sub i] - t[sub i-1]) tends to zero, and in this case I
is called the Riemann integral (or the definite integral) of f from a to b, and
is denoted by the Leibnitzian symbol (2).

		       INTEGRATION (a to b) f(x)dx			     (2)

    Also, f is said to be integrable on [a,b]. When f is a continuous function
with positive values on the interval [a,b], the integral has a simple
geometrical interpretation as the area bounded by the x axis, the ordinates x
= a and x = b, and the graph of y = f(x) (Fig. 1).

    It can be proved that a function f is integrable on [a,b] if and only if
the following two conditions are satisfied: f is bounded on [a,b]; and the set
of points where f is discontinuous can be enclosed in a series (possibly
infinite) of intervals, the sum of whose lengths is arbitrarily near to zero.
    To develop the fundamental theorem of integral calculus let u be a
variable in the interval [a,b], on which f is integrable; then formula (3)

		     INTEGRATION (a to u) f(x) dx			     (3)

defines a function of u which may be noted by I(u). If f is continuous on
[a,b], then f is integrable, and it is also true that I(u) has a derivative
I'(u) = f(u). Now let h be any antiderivative of f, that is, h'(u) = f(u) on
[a,b]. Then I'(u) - h'(u) = 0, so I(u) - h(u) = constant = -h(a), by the
theorem of the mean for derivatives and the fact that I(a) = 0, so relation

			I = I(b) = h(b) - h(a)				     (4)

(4) can be written. This is the fundamental theorem of integral calculus, and
it shows that definite integrals may be calculated by the process of finding
antiderivatives. For this reason antiderivatives are frequently called
indefinite integrals and denoted by INTEGRAL f(x) dx, and special methods of
finding indefinite integrals for frequently occurring functions occupy a large
part of elementary calculus. The standard notation for an indefinite integral
of f(x) is formula (5).

			 INTEGRATION f(x) dx				     (5)

    The term improper integral refers to an extension of the notion of
definite integral to cases in which the integrand is unbounded or the domain
of integration is unbounded.
    The concept called the Riemann integral can be extended to functions of
several variables. The case of a function of two variables illustrates
sufficiently the additional features which arise. To begin with, let f(x,y)
denote a real function defined on a rectangle of the form (6). Let P be a

	 R: a < or = x < or = b 	 c < or = y < or = d		     (6)

partition of R into n nonoverlapping rectangles Rsubi with areas A(sub i), and
let (x[sub i],y[sub i]) be a point of R(sub i). Define S by Eq. (7). In case the
sum S

S = SUMMATION (i = 1 to n) f(x[sub i], y[sub i])A[sub i]		    (7)

tends to a definite limit I when the maximum diagonal of a rectangle Rsubi
tends to zero, then f is said to be integrable over R, and the limit I is
called the Riemann integral of f over R. It will be denoted here by the
abbreviated symbol DOUBLE INTEGRAL (in terms of R) f.
    To define the integral of a function f(x,y) over a more general domain D
where it is defined, suppose that D is enclosed in a rectangle R, and define

			 F(x,y) = f(x,y) in D
			 F(x,y) = 0 outside D				     (8)

F(x,y) by Eqs. (8). Then f is said to be integrable over D in case F is
integrable over R, and Eq. (9) holds, by definition.

		DOUBLE INTEGRATION (in terms of D) f
		= DOUBLE INTEGRATION (in terms of R) F			     (9)

    When the function f(x,y) is continuous on D, and D is defined by

     a < or = x < or = b	     alpha(x) < or = y < or = beta(x)	    (10)

inequalities of the form (10), where the functions alpha(x) and BETA(x) are
continuous, the double integral of f over D always exists, and may be
represented in terms of two simple integrals by Eq. (11). In many cases, this
formula makes possible the evaluation of the double integral.

DOUBLE INTEGRATION (in terms of D) f
  = INTEGRATION (a to b) [INTEGRATION (alpha(x) to beta(x)) f(x,y) dy] dx   (11)

    In the study of the ``space'' of real functions defined, for example, on
the interval a < or = x < or = b, it is frequently useful to take formula (12)
as the distance between the functions f and g.	This distance already has a
meaning when f and g are Riemann-integrable, that is, bounded and not too
discontinuous, in the sense specified for the Riemann integral.  There is no
generally useful extension of the concept of integral to apply to all real
functions on [a,b], but it is desirable to extend it to apply to the functions
obtained from the continuous ones by certain limiting processes.  In particular,
it is desirable to have correspond to each sequence (f[sub n]) of functions
satisfying the Cauchy condition for convergence in terms of the distance (12),
namely Eq.  (13), a function g which is integrable (in the extended sense), and
for which Eq.  (14) holds.  An extended definition of integral having this
property was given by H.  L.  Lebesgue.

		    INTEGRATION (a to b) |f(x) - g(x)|dx		    (12)


lim (m=INFINITY, n=INFINITY) INTEGRATION (a to b)
|f(sub m) (x) - f(sub n) (x)|dx 					    (13)


lim (n=INFINITY) INTEGRATION (a to b) |f(sub n) (x) - g(x)|dx = 0	    (14)

    F. Riesz devised an equivalent definition which can be stated quite
simply, at least for the case of a bounded function g(x). As a first
definition, a point set S in the interval [a,b] has measure zero in case it
can be enclosed in a sequence (finite or infinite) of intervals, the sum of
whose lengths is arbitrarily small. Also, a step function f(x) is defined as
one which is constant on each interval of a partition of [a,b], as in Fig. 2.
The Riemann integral of a step function is expressible as a finite sum. Then a
bounded function g(x) is integrable in Lebesgue's sense in case it is the
limit of a uniformly bounded sequence of step functions f(sub n)(x), at each
point of [a,b] except those in a set S with measure zero, and by definition
Eq. (15) can be written. In case the step functions f(sub n) in Eq. (15) are
replaced by Lebesgue-integrable functions forming a uniformly bounded
sequence, no new functions g are obtained.
    The integral of Lebesgue is also defined for unbounded functions, but the
points of infinite discontinuity do not need to be considered one by one.  For
each function g(x) and each positive integer N let g(sub N) (x) denote the
lesser of g(x) and N.  If each gsubN (x) is Lebesgue-integrable in the sense
already defined, expression (16) forms a nondecreasing sequence, and so if
expression (16) is bounded, it tends to a finite limit, which is taken as the
value of integral (17).

     INTEGRATION (a to b) g(x) dx = lim (in terms of n) INTEGRATION (a to b)
     f(sub n) (x) dx							    (15)


     INTEGRATION (a to b) g(sub N) dx					    (16)


     INTEGRATION (a to b) g(x) dx					    (17)

    If alpha(x) is a fixed function defined on the interval [a,b], the sum S
defined by Eq. (1) may be replaced by that in Eq. (18). Then if the limit I of
S exists in this case, it is called the Stieltjes integral of f with respect
to alpha, and is denoted by integral (19). It has many of the properties of
the Riemann integral, especially in case the function alpha is nondecreasing.
See CALCULUS; FOURIER SERIES AND INTEGRALS; SERIES.
 �34s[Lawrence M. Graves]
�33s

S  = INTEGRATION (i=1 to n) f(x sub i) [alpha(t sub i) - alpha(t sub i-1)]
									   (18)


		   INTEGRATION (a to b) f(x) d alpha(x) 		   (19)



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�6v�2v�32sIntegument   �21m �33s 
    The skin, or outer covering, of the body. It is extremely diverse in its 
structure, varying both within species and from one species to another. It 
always consists of an outer epidermis and an innermost dermis (or corium), 
which rests on fatty connective tissue surrounding bone and muscle. The entire 
skin has a primary barrier function in protecting the soft internal tissues 
from physical trauma and bacteriological invasion, and in maintaining the 
internal salt and fluid levels. There are a variety of secondary functions 
which vary from species to species, namely, thermal insulation, sensory 
reception, scent secretion, respiration, attack and defense (claws, thick 
skins), and protective coloration. 
 �34s[Paul F. Maderson]
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�6v�2v�32sIntelligence   �21m �33s 
    General mental ability due to the integrative and adaptive functions of 
the brain that permit complex, unstereotyped, purposive responses to novel or 
changing situations, involving discrimination, generalization, learning, 
concept formation, inference, mental manipulation of memories, images, words 
and abstract symbols, eduction of relations and correlates, reasoning, and 
problem solving. 
    Intelligence tests are diverse collections of tasks (or items), graded in 
difficulty. The person's performance on each item can be objectively scored 
(for example, pass or fail); the total number of items passed is called the 
raw score. Raw scores are converted to some form of scaled scores which can be 
given a statistical interpretation. 
    The first practical intelligence test for children, devised in 1905 by the 
French psychologist Alfred Binet, converted raw scores to a scale of ``mental 
age,'' defined as the raw score obtained by the average of all children of a 
given age. Mental age (MA) divided by chronological age (CA) yields the 
well-known intelligence quotient or IQ. When multipled by 100 (to get rid of 
the decimal), the average IQ at every age is therefore 100, with a standard 
deviation of approximately 15 or 16. Because raw scores on mental tests 
increase linearly with age only up to about 16 years, the conversion of raw 
scores to a mental-age scale beyond age 16 must resort to statistical 
artifices. Because of this problem and the difficulty of constructing 
mental-age scales which preserve exactly the same standard deviation of IQs at 
every age, all modern tests have abandoned the mental-age concept and the 
calculation of IQ from the ratio of MA to CA. Nowadays the IQ is simply a 
standardized score with a population mean of 100 and a standard deviation 
(SIGMA) of 15 at every age from early childhood into adulthood. The middle 
50%, considered ``average,'' fall between IQs of 90 and 110. IQs below 70 
generally indicate ``mental retardation,'' and above 130, ``giftedness.'' 
 �34s[Arthur R. Jensen]
�33s 
  
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�6v�2v�32sIntercommunicating system   �21m �33s 
    A means of providing communication between two or more parties requiring 
either personal or business contact; also called an intercom. The calling 
parties are most often within the confines of the same building. The intercom 
may be a simple two-station system, such as the home ``baby-sitter,'' or a 
multistation system, which is usually wired during construction of a home or 
professional building and provides, among other features, music, station 
bridging, exclusion, and push-to-talk. 
 �34s[William Schiavoni]
�33s 
  
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�6v�2v�32sInterface of phases   �21m �33s 
    The boundary between any two phases. Among the three phases, gas, liquid, 
and solid, five types of interfaces are possible: gas-liquid, gas-solid, 
liquid-liquid, liquid-solid, and solid-solid. The abrupt transition from one 
phase to another at these boundaries, even though subject to the kinetic 
effects of molecular motion, is statistically a surface only one or two 
molecules thick. 
    A unique property of the surfaces of the phases that adjoin at an 
interface is the surface energy which is the result of unbalanced molecular 
fields existing at the surfaces of the two phases. Within the bulk of a given 
phase, the intermolecular forces are uniform because each molecule enjoys a 
statistically homogeneous field produced by neighboring molecules of the same 
substance. Molecules in the surface of a phase, however, are bounded on one 
side by an entirely different environment, with the result that there are 
intermolecular forces that then tend to pull these surface molecules toward 
the bulk of the phase. A drop of water, as a result, tends to assume a 
spherical shape in order to reduce the surface area of the droplet to a 
minimum. See ADSORPTION; FLOTATION; FOAM; PHASE EQUILIBRIUM; SURFACE TENSION. 
 �34s[Wendell H. Slabaugh]
�33s 
  
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�6v�2v�32sInterference filter   �21m �33s 
    An optical filter in which the wavelengths that are not transmitted are 
removed by interference phenomena rather than by absorption or scattering. In 
addition to being able to duplicate most of the spectral characteristics of 
absorption color filters, these devices can be made to transmit a very narrow 
band of wavelengths. They can thus be used as monochromators to examine a 
radiation source at the wavelength of a single spectrum line. For example, the 
solar disk can be observed in light of the hydrogen line HALPHA and thus the 
distribution of excited hydrogen over the disk can be determined. Most 
narrow-band interference filters are based on the Fabry-Perot interferometer.  
See  INTERFERENCE OF WAVES; INTERFEROMETRY. 
 �34s[Bruce H. Billings]
�33s 
  
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�6v�2v�32sInterference microscope   �21m �33s 
    A microscope used for visualizing and measuring differences in phase or 
optical path in transparent or reflecting specimens. It is closely allied to 
the phase-contrast microscope. See PHASE-CONTRAST MICROSCOPE. 
    In the phase-contrast microscope, the final image is produced by 
separating the incident wave and the wave diffracted by the object, 
introducing a phase difference between them, and then recombining the altered 
waves. The final resultant wave has a lower amplitude than the incident wave, 
and a transparent object thus becomes visible. The same result is achieved in 
interference microscopy by a different method. The incident and diffracted 
waves are not separated, but interference is produced between the transmitted 
wave and another wave which originates from the same source. In Fig. 1, A 
represents the incident wave, B the wave transmitted by a perfectly 
transparent object. Since no energy is absorbed, B has the same amplitude as 
A, but is slightly delayed or altered in phase relative to A. The interfering 
wave is represented by the broken line. According to the Abbe theory the final 
image is formed by the interference or summation of all waves that pass 
through the optical system. If the interfering wave and the transmitted wave B 
are added together algebraically, the resultant wave in Fig. 1 will have a 
lower amplitude than the incident wave A, so that the transparent object will 
now appear to absorb light. The resultant wave can be changed by altering the 
phase and amplitude of the interfering wave, thus enabling the appearance of 
the image to be varied. Since two waves can only interfere if they have the 
same wavelength and are derived from a common source, it is usual to divide 
the light from the source into two parts by means of a beam-splitting device, 
such as a semireflecting mirror (Fig. 2). One beam is made to traverse the 
object, the other travels along a similar path, but does not pass through the 
object. When the two beams are recombined under suitable conditions, there is 
interference, and the differences in optical path introduced by various parts 
of the object can be seen as variations in intensity or color. See 
INTERFERENCE OF WAVES. 
 �34s[Robert Barer]
�33s 
  
  
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�6v�2v�32sInterference of waves �21m �33s
    The process whereby two or more waves of the same frequency or wavelength
combine to form a wave whose amplitude is the sum of the amplitudes of the
interfering waves. The interfering waves can be electromagnetic, acoustic, or
water waves, or in fact any periodic disturbance.
    The most striking feature of interference is the effect of adding two
waves in which the trough of one wave coincides with the peak of another. If
the two waves are of equal amplitude, they can cancel each other out so that
the resulting amplitude is zero. This is perhaps most dramatic in sound waves;
it is possible to generate acoustic waves to arrive at a person's ear so as to
cancel out disturbing noise. In optics, this cancellation can occur for
particular wavelengths in a situation where white light is a source. The
resulting light will appear colored. This gives rise to the iridescent colors
of beetles' wings and mother-of-pearl, where the substances involved are
actually colorless or transparent.
    To observe interference with waves generated by atomic or molecular
transitions, it is necessary to use a single source and to split the light
from the source into parts which can then be recombined. In this case, the
amplitude and phase changes occur simultaneously in each of the parts at the
same time.
    The simplest technique for producting a splitting from a single source was
done by T. Young in 1801 and was one of the first demonstrations of the wave
nature of light. In this experiment, a narrow slit is illuminated by a source,
and the light from this slit is caused to illuminate two adjacent slits. The
light from these two parallel slits can interfere, and the interference can be
seen by letting the light from the two slits fall on a white screen. The
screen will be covered with a series of parallel fringes. The location of
these fringes can be derived approximately as follows: In the illustration, S1
and S2 are the two slits separated by a distance d. Their plane is a distance
l from the screen. Since the slit S0 is equidistant from S1 and S2, the
intensity and phase of the light at each slit will be the same. The light
falling on P from slit S1 can be represented by Eq. (1) and from S2 by Eq.
(2), where f is the frequency, t the time, c the velocity of light; x1 and x2
are the distances of P from S1 and S2, and A0 is the amplitude. This amplitude
is assumed to be the same for each wave since the slits are close together,
and x1 and x2 are thus nearly the same. The square of the amplitude or the
intensity at P can then be written as Eq. (3).

			  A = A0 sin 2pi f(t - x1/c)			     (1)

			  B = A0 sin 2pi f(t - x2/c)			     (2)

    In general, l is very much larger than y so that Eq. (3) can be simplified
to Eq. (4).

	 I = 4(A0)(sup 2) [cos(sup 2) (2pi f/c)] [x1 - x2]		    (3)


	  I = 4(A0)(sup 2) (cos(sup 2) pi) (yd/l lambda)		    (4)

    Equation (4) is a maximum when Eq. (5) holds and a minimum when Eq. (6)
holds, where n is an integer.

			      y = n lambda(l/d) 			     (5)


			   y = (n + 1/2)lambda (1/d)			     (6)

    Accordingly, the screen is covered with a series of light and dark bands
called interference fringes. If the source behind slit S0 is white light and
thus has wavelengths varying perhaps from 400 to 700 nanometers, the fringes
are visible only where x1 - x2 is a few wavelengths, that is, where n is
small. At large values of n, the position of the nth fringe for red light will
be very different from the position for blue light, and the fringes will blend
together and be washed out.
    The energy carried by a wave is measured by the intensity, which is equal
to the square of the amplitude. The total energy falling on the screen is not
changed by the presence of interference. The energy density at a particular
point is, however, drastically changed. This fact is most important for those
waves of the electromagnetic spectrum which can be generated by vacuum-tube
oscillators. The sources of radiation or antennas can be made to emit coherent
waves which will undergo interference. This makes possible a redistribution of
the radiated energy. Quite narrow beams of radiation can be produced by the
proper phasing of a linear antenna array. See ANTENNA; INTERFEROMETRY.
 �34s[Bruce H. Billings]
�33s


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�6v�2v�32sInterferometry �21m �33s
    The design and use of optical interferometers. Optical interferometers
based on both two-beam interference and multiple-beam interference of light
are extremely powerful tools for metrology and spectroscopy. A wide variety of
measurements can be performed, ranging from determining the shape of a surface
to an accuracy of less than a millionth of an inch (25 nanometers) to
determining the separation, by millions of kilometers, of binary stars. In
spectroscopy, interferometry can be used to determine the hyperfine structure
of spectrum lines. By using lasers in classical interferometers as well as
holographic interferometers and speckle interferometers, it is possible to
perform deformation, vibration, and contour measurements of diffuse objects
that could not previously be performed. There are two basic classes of
interferometers: division of wavefront and division of amplitude.

    Michelson interferometer.  The Michelson interferometer (Fig. 1) is based
on division of amplitude. Light from an extended source S is incident on a
partially reflecting plate (beam splitter) P1. The light transmitted through
P1 reflects off mirror M1 back to plate P1. The light which is reflected
proceeds to M2 which reflects it back to P1. At P1, the two waves are again
partially reflected and partially transmitted, and a portion of each wave
proceeds to the receiver R, which may be a screen, a photocell, or a human
eye. Depending on the difference between the distances from the beam splitter
to the mirrors M1 and M2, the two beams will interfere constructively or
destructively. Plate P2 compensates for the thickness of P1.
    The function of the beam splitter is to superimpose (image) one mirror
onto the other. When the mirrors' images are completely parallel, the
interference fringes appear circular. If the mirrors are slightly inclined
about a vertical axis, vertical fringes are formed across the field of view.
These fringes can be formed in white light if the path difference in part of
the field of view is made zero. Just as in other interference experiments,
only a few fringes will appear in white light.

    Twyman-Green interferometer. If the Michelson interferometer is used with a
point source instead of an extended source, it is called a Twyman-Green
interferometer. The use of the laser as the light source for the Twyman-Green
interferometer has made it an extremely useful instrument for testing optical
components. The great advantage of a laser source is that it makes it possible
to obtain bright, good-contrast, interference fringes even if the path lengths
for the two arms of the interferometer are quite different. See LASER.
    The Twyman-Green interferometer can be used to test a flat mirror. In this
case, M(sub 1) in Fig. 1 is a reference surface and M(sub 2) is the flat
surface being tested. If the test surface is perfectly flat, then straight,
equally spaced fringes are obtained. Departure from the straight, equally spaced
condition shows directly how the surface differs from being perfectly flat. A
height change of half a wavelength and a deviation from fringe straightness of
one fringe. Thus, the fringes give surface height information, just as a
topographical map gives height or contour information.
    The basic Twyman-Green interferometer can be modified to test
concave-spherical mirrors. In the interferometer, the center of curvature of
the surface under test is placed at the focus of a high-quality diverger lens
so that the wavefront is reflected back onto itself. Likewise, a
convex-spherical mirror can be tested. Also, if a high-quality spherical
mirror is used, the high-quality diverger lens can be replaced with the lens
to be tested.

    Fizeau interferometer.  One of the most commonly used interferometers in
optical metrology is the Fizeau interferometer, which can be thought of as a
folded Twyman-Green interferometer. In the Fizeau, the two surfaces being
compared, which can be flat, spherical, or aspherical, are placed in close
contact. The light reflected off these two surfaces produces interference
fringes. For each fringe, the separation between the two surfaces is a
constant. If the two surfaces match, straight, equally spaced fringes result.
Surface height variations between the two surfaces cause the fringes to
deviate from straightness or equal separation.

    Mach-Zehnder interferometer.  The Mach-Zehnder interferometer (Fig. 2) is
a variation of the Michelson interferometer and, like the Michelson
interferometer, depends on amplitude splitting of the wavefront. Light enters
the instrument and is reflected and transmitted by the semitransparent mirror
M1. The reflected portion proceeds to M3, where it is reflected through the
cell C2 to the semitransparent mirror M4. Here it combines with the light
transmitted by M1 to produce interference. The light transmitted by M1 passes
through a cell C1, which is similar to C2, and is used to compensate for the
windows of C2. The major application of this instrument is in studying airflow
around models of aircraft, missiles, or projectiles.

    Shearing interferometers.  In a lateral-shear interferometer a wavefront
is interfered with a shifted version of itself. A bright fringe is obtained at
the points where the slope of the wavefront times the shift between the two
wavefronts is equal to an integer number of wavelengths. That is, for a given
fringe the slope or derivative of the wavefront is a constant. For this reason
a lateral-shear interferometer is often called a differential interferometer.
    Another type of shearing interferometer is a radial-shear interferometer.
Here, a wavefront is interfered with an expanded version of itself. This
interferometer is sensitive to radial slopes.

    Michelson stellar interferometer.  A Michelson stellar interferometer can
be used to measure the diameter of stars which are as small as 0.01 second of
arc. This task is impossible with a ground-based optical telescope since the
atmosphere limits the resolution of the largest telescope to not much better
than 1 second of arc.

    Fabry-Perot interferometer.  All the interferometers discussed above are
two-beam interferometers. The Fabry-Perot interferometer is a multiple-beam
interferometer since the two glass plates are partially silvered on the inner
surfaces, and the incoming wave is multiply reflected between the two
surfaces. The position of the fringe maxima is the same for multiple-beam
interference as two-beam interference; however, as the reflectivity of the two
surfaces increases and the number of interfering beams increases, the fringes
become sharper.


    Holographic interferometry.  A wave recorded in a hologram is effectively
stored for future reconstruction and use. Holographic interferometry is
concerned with the formation and interpretation of the fringe pattern which
appears when a wave, generated at some earlier time and stored in a hologram,
is later reconstructed and caused to interfere with a comparison wave. It is
the storage or time-delay aspect which gives the holographic method a unique
advantage over conventional optical interferometry. See HOLOGRAPHY.

    Speckle interferometry.  A random intensity distribution, called a speckle
pattern, is generated when light from a highly coherent source, such as a
laser, is scattered by a rough surface. The use of speckle patterns in the
study of object displacements, vibration, and distortion is becoming of more
importance in the nondestructive testing of mechanical components. See
NONDESTRUCTIVE TESTING; SPECKLE.
 �34s[James C. Wyant]
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�6v�2v�32sInterferon   �21m �33s 
    A protein secreted by body cells when they are stimulated by viruses, 
bacteria, foreign cells, or antigenic macromolecules. It is the body's most 
rapidly produced defense against viruses. Interferon (IF) stimulates 
surrounding cells to produce other proteins which, in turn, may regulate viral 
multiplication, the immune response, and cell growth. Three different 
interferons have been identified: fibroepithelial, leukocyte, and immune. 
These are probably coded by three distinct cell genes. The three interferons 
have important differences in amino acid sequence, stimulus for induction, 
producer cell, and role in the body. 
    Interferon is under investigation as a treatment for viral infections, 
cancer, and autoimmune diseases. It functions in a number of animal phyla, 
ranging from fish to humans. Defense against viruses in plants is also related 
to interferon. 
    Interferon inhibits growth of many tumor cells more effectively than it 
inhibits growth of normal cells. Tumor inhibition by interferon may occur 
directly, through inhibition of tumor cell growth, or indirectly, through 
activation of cells that participate in the body's immune response 
(macrophages and lymphocytes). 
    Interferon is the most rapidly manufactured of the known body defenses 
against viruses. It appears within hours of viral infection and continues to 
be produced throughout the infection. A causal relationship between interferon 
and natural recovery from most viral infections is strongly supported by 
evidence developed since the discovery of interferon. 
 �34s[Samuel Baron]
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�6v�2v�32sInterhalogen compounds   �21m �33s 
    The elements of the halogen family (fluorine, chlorine, bromine, and 
iodine) possess an ability to react with each other to form a series of binary 
interhalogen compounds (halogen halides) of general composition given by 
XYsubn, where n can have the values 1, 3, 5, and 7, and where X is the heavier 
(less electronegative) of the two elements. All possible diatomic compounds of 
the first four halogens have been prepared. In other groups a varying number 
of possible combinations is absent. Although attempts have been made to 
prepare ternary interhalogens, they have been unsuccessful; there is 
considerable doubt that such compounds can exist. A list of known interhalogen 
compounds and some of their physical properties is given in the table.  See  
HALOGEN ELEMENTS. 
    The reactivity of the polyhalides reflects the reactivity of the halogens 
they contain. In general, they behave as strong oxidizing and halogenating 
agents. Most halogen halides (especially halogen fluorides) readily attack 
metals, yielding the corresponding halide of the more electronegative halogen. 
All halogen halides readily react with water. Such reactions can be quite 
violent and, with halogen fluorides, they may be explosive. They readily react 
with aliphatic and aromatic hydrocarbons and with oxygen- or 
nitrogen-containing compounds. 
 �34s[Terry Surles]
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�6v�2v�32sInterhemispheric integration   �21m �33s 
    The process that is responsible for the exchange of information between 
the two half brains. The human brain is a bilaterally symmetrical structure 
which is for the most part richly interconnected by two main bridges of 
neurons called the corpus callosum and anterior commissure. These structures 
can be surgically sectioned in humans in an effort to control the spread of 
epileptic seizures. Although there is no apparent change in everyday behavior 
of these patients, dramatic differences in cognitive function can be 
demonstrated under specialized testing conditions. Because of these studies it 
now can be said that in normal humans these cerebral commissures are largely 
responsible for behavioral unity; the neural mechanism keeps the left side of 
the body up to date with the activities of the right, and vice versa.  See  
BRAIN.  
 �34s[Michael S. Gazzaniga]
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�6v�2v�32sInterior ballistics   �21m �33s 
    The branch of applied physics which deals with the motions of projectiles 
while they are still inside the gun barrel or cannon, and are accelerated by 
the pressure of expanding gases resulting from the burning of the charge. The 
term is also used to refer to the similar events inside the casing of a 
solid-fuel rocket, in which case the exhaust blast is regarded as equivalent 
to the projectile. 
    If the propelling charge of the cartidge is changed into gas 
instantaneously, the gas pressure curve, from the instant of ignition to the 
instant the projectile leaves the muzzle, would be very nearly an adiabatic 
curve, quite similar to the curve showing the diminishing potential of a 
gravitational field. In reality, it takes some time for the powder charge to 
burn; furthermore, various resistance factors must be overcome. Therefore, the 
gas pressure in the gun barrel behind the projectile does not reach its 
maximum value until after the projectile has started moving inside the barrel. 
The pressure should reach a minimum value at the instant the projectile leaves 
the gun barrel, as shown in the illustration; hence, there is a definite 
relationship between powder charge, gas pressure, and barrel length. If the 
pressure is still rather high even when the projectile ``unseals'' the gun 
barrel, a portion of the propelling charge has been wasted. If the pressure 
drops too low before the projectile reaches the muzzle, the projectile is 
slowed down inside the barrel; in this case, shortening the barrel would 
actually improve the performance. 
    Even when the pressure drop has been properly calculated (and achieved), 
the exit velocity of the powder gases is still 2 to 21/2 times the muzzle 
velocity of the projectile. This fact can be utilized to prevent or to 
minimize the recoil of the weapon by means of the muzzle brake first developed 
by Schneider-Creusot in France. The muzzle brake deflects the direction of 
motion of the gases by 120 - 180DG, thereby creating what might be called a 
gas recoil opposite in direction to the projectile recoil. Recoilless guns 
eliminate recoil by discharging a quantity of the propelling gases through the 
rear.  
 �34s[Willy Ley /Daniel Wellner]
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�6v�2v�32sIntermediate-frequency amplifier   �21m �33s 
    A tuned amplifier employed in the amplification of the signals produced by 
the mixer in a radio receiver. Because the carrier frequency of the modulated 
signal from the mixer is essentially constant, the resonant frequency of the 
amplifier is fixed. 
    The proper design of the intermediate-frequency (i-f) amplifier is 
essential for good selectivity and reproduction of the original transmitted 
signal. If the amplifier is tuned too sharply, the high-frequency components 
of the modulating signal will be lost. To avoid this, stagger-tuning of the 
individual stages may be used. In a stagger-tuned amplifier the resonant 
frequency of each stage is slightly different from the carrier frequency, with 
the result that the gain is essentially constant over the bandwidth of the 
modulated signal. The gain decreases rapidly at frequencies outside this band. 
    The standard i-f frequency for broadcast radio receivers is 455 kHz; other 
frequencies used depend upon the application, such as television receivers or 
radar receivers.  See  AMPLIFIER. 
 �34s[Harold F. Klock]
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�6v�2v�32sIntermetallic compounds   �21m �33s 
    Materials composed of two or more types of metal atoms, which exist as 
homogeneous, composite substances and differ discontinuously in structure from 
that of the constituent metals. They are also called, preferably, 
intermetallic phases. Their properties cannot be transformed continuously into 
those of their constituents by changes of composition alone, and they form 
distinct crystalline species separated by phase boundaries from their metallic 
components and mixed crystals of these components; it is generally not 
possible to establish formulas for intermetallic compounds on the sole basis 
of analytical data, so formulas are determined in conjunction with 
crystallographic structural information. 
    The term ``alloy'' is generally applied to any homogeneous molten mixture 
of two or more metals, as well as to the solid material that crystallizes from 
such a homogeneous liquid phase. Alloys may also be formed from solid-state 
reactions. In the liquid phase, alloys are essentially solutions of metals in 
one another, although liquid compounds may also be present. Alloys containing 
mercury are usually referred to as amalgams. Solid alloys may vary greatly in 
range of composition, structure, properties, and behavior.  See  ALLOY; 
NONSTOICHIOMETRIC COMPOUNDS; SEMICONDUCTOR; SOLID-STATE CHEMISTRY. 
 �34s[John L. T. Waugh]
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�6v�2v�32sIntermolecular forces   �21m �33s 
    Attractive or repulsive interactions that occur between all atoms and 
molecules. These forces, which become significant at molecular separations of 
about 1 nm or less, are much weaker than forces associated with chemical bonds 
or electrostatic interactions of charged particles. They are important, 
however, since they are responsible for many of the physical properties of 
solids, liquids, and pressurized gases. 
    Intermolecular forces are responsible for many of the bulk properties of 
matter in all its phases. At increased pressures and sufficiently low 
temperatures the attractive forces between molecules in the gas will cause it 
to liquefy. The viscosity, surface tension, and diffusion of liquids are 
examples of physical properties which are a consequence of intermolecular 
forces. Repulsive forces prevent the molecules from approaching one another 
too closely and account for the high compressibility of liquids. 
Intermolecular forces between near and distant neighbors dictate the ordered 
molecular arrangements in crystalline solids. These forces also account for 
the elasticity of solids. Intermolecular forces also determine to an important 
extent the three-dimensional arrangement of biological molecules, polymers, 
and even smaller molecules. 
 �34s[George E. Ewing]
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�6v�2v�32sInternal combustion engine   �21m �33s 
    A prime mover, the fuel for which is burned within the engine, as 
contrasted to a steam engine, for example, in which fuel is burned in a 
separate furnace.  See  ENGINE. 
    The most numerous of internal combustion engines are the gasoline piston 
engines used in passenger automobiles, outboard engines for motor boats, small 
units for lawn mowers, and other such equipment, as well as diesel engines 
used in trucks, tractors, earth-moving, and similar equipment. For other types 
of internal combustion engines  see  GAS TURBINE; ROCKET ENGINE; ROTARY 
ENGINE; TURBINE PROPULSION. 
    The aircraft piston engine is fundamentally the same as that used in 
automobiles but is engineered for light weight and is usually air cooled.  See 
  RECIPROCATING AIRCRAFT ENGINE. 
    Characteristic features common to all commercially successful internal 
combustion engines include (1) the compression of air, (2) the raising of air 
temperature by the combustion of fuel in this air at its elevated pressure, 
(3) the extraction of work from the heated air by expansion to the initial 
pressure, and (4) exhaust. In 1862 Beau de Rochas proposed the four-stroke 
engine cycle as a means of accomplishing these conditions in a piston engine 
(see illustration). The engine requires two revolutions of the crankshaft to 
complete one combustion cycle. The first engine to use this cycle successfully 
was built in 1876 by N. A. Otto.  See  OTTO CYCLE. 
    Two years later Sir Dougald Clerk developed the two-stroke engine cycle by 
which a similar combustion cycle required only one revolution of the 
crankshaft. In 1891 Joseph Day simplified the two-stroke engine cycle by using 
the crankcase to pump the required air. Engines using this two-stroke cycle 
today have been further simplified by use of a third cylinder port which 
dispenses with the crankcase check valve used by Day. Such engines are in wide 
use for small units where fuel economy is not as important as mechanical 
simplicity and light weight. They do not need mechanically operated valves and 
develop one combustion cycle per crankshaft revolution. Nevertheless they do 
not develop twice the power of four-stroke cycle engines with the same size 
working cylinders at the same number of revolutions per minute (rpm). The 
principal reasons for this are (1) the reduction in effective cylinder volume 
due to the piston movement required to cover the exhaust ports, (2) the 
appreciable mixing of burned (exhaust) gases with the combustible mixture, and 
(3) the loss of some combustible mixture with the exhaust gases. 
  
    About 20 years after Otto first ran his engine, Rudolf Diesel successfully 
demonstrated an entirely different method of igniting fuel. Air is compressed 
to a pressure high enough for the adiabatic temperature to reach or exceed the 
ignition temperature of the fuel. Because this temperature is in the order of 
1000DGF (540DGC), compression ratios of 12:1 to 20:1 are used commercially 
with compression pressures generally over 600 psi (4.1 megapascals). This 
engine cycle requires the fuel to be injected after compression at a time and 
rate suitable to control the rate of combustion.  See  DIESEL ENGINE; FUEL 
INJECTION.  
    There are many characteristics of the diesel engine which are in direct 
contrast to those of the Otto engine. The higher the compression ratio of a 
diesel engine, the less the difficulties with ignition time lag. Too great an 
ignition lag results in a sudden and undesired pressure rise which causes an 
audible knock. In contrast to an Otto engine, knock in a diesel engine can be 
reduced by use of a fuel of higher cetane number, which is equivalent to a 
lower octane number.  See  CETANE NUMBER; OCTANE NUMBER. 
    The larger the cylinder diameter of a diesel engine, the simpler the 
development of good combustion. In contrast, the smaller the cylinder diameter 
of the Otto engine, the less the limitation from detonation of the fuel. 
    High intake-air temperature and density materially aid combustion in a 
diesel engine, especially of fuels having low volatility and high viscosity. 
Some engines have not performed properly on heavy fuel until provided with a 
supercharger. The added compression of the supercharger raised the temperature 
and, what is more important, the density of the combustion air. For an Otto 
engine, an increase in either the air temperature or density increases the 
tendency of the engine to knock and therefore reduces the allowable 
compression ratio. 
    Diesel engines develop increasingly higher indicated thermal efficiency at 
reduced loads because of leaner fuel-air ratios and earlier cutoff. Such 
mixture ratios may be leaner than will ignite in an Otto engine. Furthermore, 
the reduction of load in an Otto engine requires throttling, which develops 
increasing pumping losses in the intake system. 
 �34s[Neil MacCoull]
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�6v�2v�32sInternal energy   �21m �33s 
    A characteristic property of the state of a thermodynamic system, 
introduced in the first law of thermodynamics. For a static, closed system (no 
bulk motion, no transfer of matter across its boundaries), the change in 
internal energy for a process is equal to the heat absorbed by the system from 
its surroundings minus the work done by the system on its surroundings. Only a 
change in internal energy can be measured, not its value for any single state. 
For a given process, the change in internal energy is fixed by the initial and 
final states and is independent of the path by which the change in state is 
accomplished.  See  THERMODYNAMIC PRINCIPLES. 
 �34s[Paul J. Bender]
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�6v�2v�32sInternal wave   �21m �33s 
    Wave motions of stably stratified fluids in which the maximum vertical 
motion takes place below the surface of the fluid. The restoring force is 
mainly due to gravity; when light fluid from upper layers is depressed into 
the heavy lower layers, buoyancy forces tend to return the layers to their 
equilibrium positions. Internal waves have been found in the atmosphere as lee 
waves (waves in the wind stream downwind from a mountain) and as waves 
propagated along an inversion layer (a layer of very stable air). They are 
also associated with wind shears at the lower boundary of the jet stream. In 
the oceans internal oscillations have been observed wherever suitable 
measurements have been made, but it is not completely certain that all of 
these oscillations are manifestations of internal waves rather than turbulent 
eddies.  
 �34s[Charles S. Cox]
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�6v�2v�32sInternational Date Line   �21m �33s 
    The 180DG meridian, where each day officially begins and ends. A person 
traveling eastward, against the apparent movement of the Sun, gains 1 h for 
every 15DG of longitude; a person traveling westward loses time at the same 
rate. Two people starting from any meridian and traveling around the world in 
opposite directions at the same speed would have the same time when they meet, 
but would be 1 day apart in date. A traveler going west across the line loses 
a day; if it is Monday to the east, it will be Tuesday immediately across the 
International Date Line. In traveling eastward a day is gained; if it is 
Monday to the west of the line, it will be Sunday immediately across the line. 
    The 180DG meridian is ideal for serving as the International Date Line. It 
is exactly halfway around the world from the zero, or Greenwich, meridian, 
from which all longitude is reckoned. It also falls almost in the center of 
the largest ocean; consequently there is the least amount of inconvenience as 
regards population centers. A few deviations in the alignment have been made 
(see illustration). 
 �34s[Van H. English]
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�6v�2v�32sInterplanetary matter   �21m �33s 
    Gas and small solid particles in the space between the planets in the 
solar system. Most of the interplanetary gas originates in the Sun, flowing 
outward in the solar wind. The solid material originated in the asteroids and 
comets. See ASTEROID; COMET; INTERSTELLAR MATTER; SOLAR WIND. 
 �34s[Peter G. Martin]
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�6v�2v�32sInterplanetary propulsion   �21m �33s 
    Means of providing propulsive power for flight to the Moon or to a planet. 
A variety of different propulsion systems can be used. The space vehicles for 
these missions consist of a series of separate stages, each with its own set 
of propulsion systems. When the propellants of a given stage have been 
expended, the stage is jettisoned to prevent its mass from needlessly adding 
to the inertia of the vehicle. Although all propulsion systems actually used 
in interplanetary flight have been chemical rockets, several basically 
different propulsion systems have been studied.  See  ELECTROMAGNETIC 
PROPULSION; ION PROPULSION; ROCKET ENGINE; ROCKET STAGING; SPACECRAFT 
PROPULSION.  
 �34s[George P. Sutton]
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�6v�2v�32sInterpolation   �21m �33s 
    A process in mathematics used to estimate an intermediate value of one 
(dependent) variable which is a function of a second (independent) variable 
when values of the dependent variable corresponding to several discrete values 
of the independent variable are known. 
    Suppose, as is often the case, that it is desired to describe graphically 
the results of an experiment in which some quantity Q is measured, for 
example, the electrical resistance of a wire, for each of a set of N values of 
a second variable v representing, perhaps, the temperature of the wire. Let 
the numbers Qsubi, i = 1, 2, . . . , N, be the measurements made of Q and the 
numbers vsubi be those of the variable v. These numbers representing the raw 
data from the experiment are usually given in the form of a table with each 
Qsubi listed opposite the corresponding vsubi. The problem of interpolation is 
to use the above discrete data to predict the value of Q corresponding to any 
value of v lying between the above vsubi. If the value of v is permitted to 
lie outside these vsubi, the somewhat more risky process of extrapolation is 
used. See EXTRAPOLATION. 
 �34s[Kaiser S. Kunz]
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�6v�2v�32sInterstellar matter �21m �33s
    The material (gaseous, molecular, and particulate) between the stars,
constituting several percent of the total mass of the Galaxy. Being the
reservoir from which new stars are born in the Galaxy, interstellar matter is
of fundamental importance in understanding both the processes leading to the
formation of stars, including the solar system, and ultimately the origin of
life in the universe. Among the many ways in which interstellar matter is
detected, perhaps the most familiar are attractive photographs of bright
patches of emission-line or reflection nebulosity. However, these nebulae
furnish an incomplete view of the large-scale distribution of material,
because they depend on the proximity of one or more bright stars for their
illumination. Radio observations of hydrogen, the dominant form of
interstellar matter, reveal a widespread distribution throughout the thin disk
of the Galaxy, concentrating in the spiral arms. Mixed in with the gas are
small solid particles, called dust grains, of characteristic radius 0.1
micrometer. Although by mass the grains constitute less than 1% of the
material, they have a pronounced effect through the extinction of starlight.
Striking examples of this obscuration are the dark rifts seen in the Milky
Way. On average, the density of matter is only one hydrogen atom per cubic
centimeter (equivalently 10- sup 2 sup 4 g cm- sup 3), but because of the long
path lengths over which the material is sampled, this tenuous medium is
detectable. Radio and optical observations of other spiral galaxies show a
similar distribution of interstellar matter in spiral arms in the galactic
plane.
    A hierarchy of interstellar clouds, concentrations of gas and dust, exists
within the spiral arms. Many such clouds or cloud complexes are recorded
photographically. However, the most dense, which contain interstellar
molecules, are often totally obscured by the dust grains and so are detectable
only through their infrared and radio emission. These molecular clouds contain
the birthplaces of stars.  See	GALAXY, EXTERNAL; STELLAR EVOLUTION.
 �34s[Peter G. Martin]
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�6v�2v�32sIntestine �21m �33s
    The portion of the digestive tract beyond the stomach. It is the region of
the gut wherein digestion is completed through the combined action of bile,
pancreatic juice, and secretions from the intestinal wall itself, and wherein
the products of digestion are finally absorbed. Two segments are generally
distinguishable, a long slender small intestine and a wider terminal part, the
large intestine. To keep absorptive surface adequate to meet the nutritive
requirements of body mass, the ratio of the intestine length to body length is
greater in large animals than in small animals. In addition, plant food
appears to be difficult to digest, so that the intestine in herbivores is
longer than in carnivores. The small intestine of a cow is about 150 ft (45 m)
long and the larger intestine about 30 ft (9 m). The corresponding dimensions
for humans are about 22 and 5 ft (6.6 and 1.5 m).

    Small intestine.  In elasmobranchs and some primitive fishes the mucous
membrane of the rather short small intestine is thrown into a twisted fold,
the spiral valve, which effectively increases surface area for absorption and
lengthens the path which food must travel (Fig. 1). Among the higher
vertebrates, especially mammals, the mucous membrane of the small intestine is
thrown into numerous thin transverse folds, the plicae circulares, which
greatly increase the internal surface of the intestine.
    The villi, fingerlike outgrowths of the mucous membrane, are typical of
the entire small intestine in birds and mammals. They are present in great
numbers, and give the surface a soft velvety appearance. Humans have
approximately 5,000,000 villi, which provide an absorptive surface area of
about 10 msup2. Each villus is clothed with simple columnar epithelium and has
a core of delicate fibrous tissue. Within each villus is a network of blood
capillaries, a centrally located lymphatic capillary, or lacteal, and a few
smooth muscle fibers. The muscle fibers provide for movements of the villi,
which suggest a pumping action, and are believed to aid the forward movement
of absorbed materials which have entered the lacteals.

    The small intestine of mammals consists of three segments: duodenum,
jejunum, and ileum. Transitions are gradual and boundaries rather arbitrary.
The human duodenum is about 10 in. (25 cm) long, lies retroperitoneally, and
makes a C-shaped bend around the head of the pancreas. Piercing the wall of
the duodenum about 4 in. (10 cm) beyond the pylorus are the pancreatic and
bile ducts. The jejunum in the average human is about 9 ft (2.7 m) long and
the ileum about 13 ft (3.9 m). Both are suspended from the posterior abdominal
wall by a mesentery, and together their loops fill much of the abdominal
cavity. General structural features of the intestinal wall are similar
throughout these two segments.

    Large intestine.  The large intestine in nonmammalian species is
relatively short and often not clearly delimited from the small intestine. Its
expanded lower end forms the cloaca, a passage which is shared with the
reproductive and urinary systems. In reptiles and birds the cloaca absorbs
substantial amounts of water, and the digestive and urinary waste products are
discharged in a pastelike state.
    The large intestine in mammals comprises four principal regions: cecum,
appendix, colon, and rectum. At the opening of the small intestine into the
side of the cecum, regurgitation is prevented by the ileocecal sphincter and a
valvelike fold of the mucous membrane (Fig. 2). The human cecum lies in the
right lower quadrant of the abdomen and is a relatively short, saccular
structure terminating in the slender vermiform appendix. The colon in humans
is about 5 ft (1.5 m) long, and consists of ascending, transverse, descending
and sigmoid segments. The hepatic and splenic flexutres, which define the
extent of the transverse colon, lie beneath the liver and spleen, respectively.
    The two principal functions of the large intestine are concentration of
the feces by absorption of water, and production of mucus to facilitate the
forwarding of the feces.
    The rectum, terminal segment of the large intestine, serves to hold the
feces before discharge. In humans it is about 7 in. (17.5 cm) long and can be
distended to a diameter of about 3 in. (7.5 cm). At its lower end the circular
smooth muscle forms an internal (involuntary) sphincter. Below this is the
external (voluntary) sphincter of striated muscle. See DIGESTIVE SYSTEM.
 �34s[Victor M. Emmel]
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�6v�2v�32sIntestine disorders �21m �33s
    Disorders of the intestine include congenital defects and vascular,
mechanical, degenerative, inflammatory, and neoplastic disorders.
    The most common congenital defect is Meckel's diverticulum, a pouch or
tract which persists as the remnant of the fetal umbilical duct.
    Vascular lesions include mesenteric thrombosis, a blocking of the arterial
supply or venous drainage of the intestine by a thrombus or embolus. It is
frequently associated with trauma, heart failure, or arteriosclerosis.	See
EMBOLUS; THROMBOSIS.
    Common mechanical disorders are hernias, the protrusion of a loop of bowel
through a defect or weakness of the abdominal wall, and peritoneal adhesions,
resulting from recent or old peritonitis, which often lead to intestinal
obstruction.  See  HERNIA.
    The three degenerative lesions with the highest incidence are sprue,
mucoviscidosis, and Whipple's disease, but all are relatively uncommon. The
first is marked by fatty, foul-smelling, bulky stools (steatorrhea) and a
multiplicity of other clinical symptoms. The second is an inborn deficit of
pancreatic secretion that causes steatorrhea; other symptoms include salivary
and sweat gland abnormality and asthma. Whipple's disease is a rare disease of
middle age in which large fatty deposits accumulate in the intestinal walls
and in the mesenteries.
    Appendicitis is the most frequent provocation of surgery of the abdomen.
It probably arises from a combination of causes, including infection and
obstruction of the lumen, and also from parasitic infestations. Various
chemicals and poisons may also produce inflammations of the small intestine
(enteritis) or of the large bowel (colitis). Ulcerative colitis refers to a
disease of unknown cause producing intestinal ulcerations, particularly in the
colon.
    Malignant tumors of the small intestine are not common, but the large
intestine has a high incidence of carcinomas. Other malignancies are
lymphomas, Hodgkin's disease, malignant melanomas, sarcomas, and leukemic
infiltrations.	See  ONCOLOGY.
    Benign tumors are represented by polyps, lipomas, myomas, and fibromas and
usually occur in the colon, the small intestine rarely being involved.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sIntestine   �21m �33s 
    The tubular portion of the digestive tract, usually between the stomach 
and the cloaca or anus. The detailed functions vary with the region, but are 
primarily digestion and absorption of food. The structure of the intestine 
varies greatly in different vertebrates, but there are several common 
modifications, mainly associated with increasing the internal surface area. 
    In humans the intestine consists of the small and large intestines. The 
small intestine is further divided into three major parts: the duodenum, the 
jejunum, and the ileum. The duodenum, 10 - 12 in. (25 - 30 cm) long, begins at 
the pyloric sphincter of the stomach and curves around the head of the 
pancreas on the right side of the anterior part of the abdomen. It receives 
the ducts of the biliary system and the pancreas. The jejunum and ilium are 
about 19 ft (5.8 m) long and form a much-coiled tube that empties at right 
angles into the large intestine through the ileocolic valve. The large 
intestine, or colon, consists of five parts: the ascending, transverse, 
descending, and sigmoid regions, and the terminal rectum which empties into 
the anal canal.  See  DIGESTIVE SYSTEM. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sInventory control   �21m �33s 
    The process of managing the timing and the quantities of goods to be 
ordered and stocked, so that demands can be met satisfactorily and 
economically. Inventories are accumulated commodities waiting to be used to 
meet anticipated demands. Inventory control policies are decision rules that 
focus on the trade-off between the costs and benefits of alternative solutions 
to questions of when and how much to order for each different type of item. 
    The possible reasons for carrying inventories are: uncertainty about the 
size of future demands; uncertainty about the duration of lead time for 
deliveries; provision for greater assurance of continuing production, using 
work-in-process inventories as a hedge against the failure of some of the 
machines feeding other machines; and speculation on future prices of 
commodities. Some of the other important benefits of carrying inventories are: 
reduction of ordering costs and production setup costs (these costs are less 
frequently incurred as the size of the orders are made larger which in turn 
creates higher inventories); price discounts for ordering large quantities; 
shipping economies; and maintenance of stable production rates and work-force 
levels which otherwise could fluctuate excessively due to variations in 
seasonal demand. 
    The benefits of carrying inventories have to be compared with the costs of 
holding them. Holding costs include the following elements: cost of capital 
for money tied up in the inventories; cost of owning or renting the warehouse 
or other storage spaces; materials handling equipment and labor costs; costs 
of potential obsolescence, pilferage, and deterioration; property taxes levied 
on inventories; and cost of installing and operating an inventory control 
policy. Inventories, when listed with respect to their annual costs, tend to 
exhibit a similarity to Pareto's law and distribution. A small percentage of 
the product lines may account for a very large share of the total inventory 
budget (they are called class A items). 
    Continuous-review and fixed-interval are two different modes of operation 
of inventory control systems. The former means the records are updated every 
time items are withdrawn from stock. When the inventory level drops to a 
critical level called reorder point, a replenishment order is issued. Under 
fixed-interval policies, the status of the inventory at each point in time 
does not have to be known. The review is done periodically. 
    Uncertainties of future demand play a major role in the cost of 
inventories. That is why the ability to better-forecast future demand can 
substantially reduce the inventory expenditures of a firm. Conversely, using 
ineffective forecasting methods can lead to excessive shortages of needed 
items and to high levels of unnecessary ones. 
    Material requirements planning (MRP) systems (which are 
production-inventory scheduling softwares that make use of computerized files 
and data-processing equipment) are receiving widespread application. MRP 
systems have not yet made use of mathematical inventory theory. They recognize 
the implications of dependent demands in multiechelon manufacturing (which 
includes lumpy production requirements). Integrating the bills of materials, 
the given production requirements of end products, and the inventory records 
file, MRP systems generate a complete list of a production-inventory schedule 
for parts, subassemblies, and end products, taking into account the lead-time 
requirements. MRP has proved to be a useful tool for manufacturers, especially 
in assembly operations. 
 �34s[Ali Dogramaci]
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�6v�2v�32sInverse-square law   �21m �33s 
    Any law in which a physical quantity varies with distance from a source 
inversely as the square of that distance. When energy is being radiated by a 
point source (see illustration), such a law holds, provided the space between 
source and receiver is filled with a nondissipative, homogeneous, isotropic, 
unbounded medium.  See  POINT SOURCE. 
    Similar reasoning shows that the same law applies to mechanical shear 
waves in elastic media and to compressional sound waves. The term is also used 
for static field laws such as the law of gravitation and Coulomb's law in 
electrostatics.  
 �34s[William R. Smythe]
�33s 
  
  
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�6v�2v�32sInvertebrate pathology   �21m �33s 
    All studies having to do with the principles of pathology as applied to 
invertebrates. It includes studies of the disease process in its broadest 
sense, that is, investigations of any departures from the normal state of 
health. Disease in an invertebrate is initiated by the same factors causing 
disease in vertebrates; infection with microbes; parasites; and noninfectious 
factors. Various approaches are used in invertebrate pathology. These include 
(1) studies of the cause of disease, the symptomatology, the resulting 
pathology on a gross or microscopic level, and pathophysiology; (2) defense 
mechanisms used by the host to prevent infection, to overcome microbes and 
parasites, or to repair damaged body parts by wound healing or regeneration; 
and (3) the effects and progress of infectious disease in a population 
(epizootiology).  See  INSECT PATHOLOGY. 
 �34s[Phyllis T. Johnson]
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�6v�2v�32sInvolute   �21m �33s 
    A term applied to a curve C' that cuts at right angles all tangents of a 
curve C (see illustration). Each curve C has infinitely many involutes and the 
distance between corresponding points of any two involutes is constant. Let a 
length of string be coincident with a curve C, with one end fastened at a 
point P0 of C. If the string is unwound, remaining taut, the other end of the 
string traces an involute C' of C. By varying the length of the string, all 
involutes of C are obtained.   See  ANALYTIC GEOMETRY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sIodate   �21m �33s 
    A negative ion having the formula IO3-, and derived from iodic acid, HIO3. 
Some salts such as KH2I3O9 and KH(IO3)2 indicate that the acid may exist as 
polymers of the ion indicated by the empirical formula. Sodium and potassium 
iodates are the most important salts and are used in medicine. The iodates are 
more stable and are weaker oxidizing agents than bromates and chlorates.  See  
BROMATE; CHLORATE; IODINE. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sIodide   �21m �33s 
    A compound which contains the iodine atom in 1 - oxidation state and which 
is derived from hydriodic acid, HI. In comparing the iodide ion with the other 
halide ions, it should be pointed out that the iodides are more covalent, are 
the best reducing agents of the group, and form the least stable complexes. 
The aqueous solubilities of the metal iodides are much the same as the 
chlorides but in general a little lower. 
    Sodium or potassium iodide is added to table salt to prevent malfunction 
of the thyroid gland. Silver iodide is used in photographic films and papers.  
See  HALIDE; HALOGENATED HYDROCARBON; IODINE. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sIodine �21m �33s
    A nonmetallic element, symbol I, atomic number 53, relative atomic mass
126.9045, the heaviest of the naturally occurring halogens. Under normal
conditions iodine is a black, lustrous, volatile solid; it is named after its
violet vapor.  See  HALOGEN ELEMENTS.
    The chemistry of iodine, like that of the other halogens, is dominated by
the facility with which the atom acquires an electron to form either the
iodide ion I- or a single covalent bond  - I, and by the formation, with more
electronegative elements, of compounds in which the formal oxidation state of
iodine is +1, +3, +5, or +7. Iodine is more electropositive than the other
halogens, and its properties are modulated by: the relative weakness of
covalent bonds between iodine and more electropositive elements; the large
sizes of the iodine atom and iodide ion, which reduce lattice and solvation
enthalpies for iodides while increasing the importance of van der Waals forces
in iodine compounds; and the relative ease with which iodine is oxidized. Some
properties of iodine are listed in the table.  See  ASTATINE; BROMINE;
CHEMICAL BONDING; CHLORINE; FLUORINE.
    Iodine occurs widely, although rarely in high concentration and never in
elemental form. Despite the low concentration of iodine in sea water, certain
species of seaweed can extract and accumulate the element. In the form of
calcium iodate, iodine is found in the caliche beds in Chile. Iodine also
occurs as iodide ion in some oil well brines in California, Michigan, and
Japan.
    The sole stable isotope of iodine is sup 1 sup 2 sup 7I (53 protons, 74
neutrons). Of the 22 artificial isotopes (masses between 117 and 139), the
most important is sup 1 sup 3 sup 1 I, with a half-life of 8 days. It is widely
used in radioactive tracer work and certain radiotherapy procedures.  See
RADIOACTIVE TRACER.
    Iodine exists as diatomic I2 molecules in solid, liquid, and vapor phases,
although at elevated temperatures (> 200DGC or 390DGF) dissociation into atoms
is appreciable. Short intermolecular I . . . I distances in the crystalline
solid indicate strong intermolecular van der Waals forces. Iodine is
moderately soluble in nonpolar liquids, and the violet color of the solutions
suggests that I2 molecules are present, as in iodine vapor.
    Although it is usually less vigorous in its reactions than the other
halogens, iodine combines directly with most elements. Important exceptions
are the noble gases, carbon, nitrogen, and some noble metals. The inorganic
derivatives of iodine may be grouped into three classes of compounds: those
with more electropositive elements, that is, iodides; those with other
halogens; and those with oxygen. Organoiodine compounds fall into two
categories: the iodides; and the derivatives in which iodine is in a formal
positive oxidation state by virtue of bonding to another, more electronegative
element.  See  GRIGNARD REACTION; HALOGENATED HYDROCARBON; HALOGENATION;
IODIDE.
    Iodine appears to be a trace element essential to animal and vegetable
life. Iodide and iodate in sea water enter into the metabolic cycle of most
marine flora and fauna, while in the higher mammals iodine is concentrated in
the thyroid gland, being converted there to iodinated amino acids (chiefly
thyroxine and iodotyrosines). They are stored in the thyroid as thyroglobulin,
and thyroxine is apparently secreted by the gland. Iodine deficiency in
mammals leads to goiter, a condition in which the thyroid gland becomes
enlarged.  See	THYROID GLAND.
    The bactericidal properties of iodine and its compounds bolster their
major uses, whether for treatment of wounds or sterilization of drinking
water. Also, iodine compounds are used to treat certain thyroid and heart
conditions, as a dietary supplement (in the form of iodized salt), and for
x-ray contrast media.  See  ANTIMICROBIAL AGENTS; ANTISEPTIC; SALT (FOOD).
    Major industrial uses are in photography, where silver iodide is a
constituent of fast photographic film emulsions, and in the dye industry,
where iodine-containing dyes are produced for food processing and for color
photography.  See  DYE; PHOTOGRAPHIC MATERIALS.
 �34s[Chris Adams]
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�6v�2v�32sIodoform   �21m �33s 
    A yellow, hexagonal solid with a penetrating odor also called 
triiodomethane, CHI3. Its specific gravity is 4.08 and melting point 119DGC 
(246DGF). It is soluble in organic solvents and insoluble in water. It has 
weak bactericidal properties and exerts antiseptic action when applied to raw 
wounds, because of the liberation of free iodine. Iodoform also acts as an 
inhibitor for wound secretion, and this inhibits bacterial growth. Its chief 
use is in ointments for minor skin diseases. It is toxic when taken 
internally.  See  ANTIMICROBIAL AGENTS; HALOGENATED HYDROCARBON. 
 �34s[Elbert H. Hadley]
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�6v�2v�32sIon �21m �33s
    An atom, or group of atoms, which by loss or gain of one or more electrons
has acquired an electric charge. If the ion is formed from an atom of hydrogen
or an atom of a metal, it is usually positively charged; if the ion is formed
from an atom of a nonmetal or from a group of atoms, it is usually negatively
charged. The number of electronic charges carried by an ion is called its
electrovalence. The charges are denoted by superscripts which give their sign
and number; for example, a sodium ion, which carries one positive charge, is
denoted by Na sup + ; a sulfate ion, which carries two negative charges, by
SO4 sup 2-.  See  ATOMIC STRUCTURE AND SPECTRA; CHEMICAL BONDING.
    Salts are usually composed of orderly arrangements of ions which are not
free to move easily in the solid. However, when the salt is fused or dissolved
in water, the ions become free, and when an electric field is applied to the
salt in solution, the positively charged cations move to the cathode and the
negatively charged anions move to the anode. At the electrodes the ions lose
their electric charge. This process is called electrolysis.  See
ELECTROLYSIS; IONIC CRYSTALS; SALT (CHEMISTRY); VALENCE.
 �34s[Thomas C. Waddington]
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�6v�2v�32sIon exchange   �21m �33s 
    The reversible exchange of ions of the same charge sign between a 
solution, usually aqueous, and an insoluble solid in contact with it. The 
phenomenon was first recognized in soils, where it accounts for the absorption 
and retention of water-soluble fertilizers. When potash (potassium chloride) 
is applied to soil, potassium ions are taken into the soil, while chemically 
equivalent amounts of sodium and calcium ions are released. If the 
potassium-loaded soil is now washed with a solution of common salt (sodium 
chloride), some of the potassium is released, while sodium takes its place. 
Water alone will not remove potassium ions from the soil, because there are no 
negative ions in the water to balance the charges. 
    A solid ion exchanger has an extended, open molecular framework that 
includes electrically charged, ionic groups. A cation exchanger exchanges 
positive ions and therefore has negative ions built into its framework. An 
anion exchanger has positive ions in its framework. The ions of the lattice 
are called the fixed ions; the smaller ions of opposite charge that can change 
places with ions in the solution are called counterions; small ions having the 
same charge as the fixed ions, which may enter the exchanger under certain 
conditions, are called co-ions. 
    Sometimes exchange occurs at the surface, as in glass and some clays, but 
more commonly the exchanger has a porous structure that may be crystalline, 
with channels, cavities, or layered spaces that let ions move in and out, or 
may be amorphous, like a cross-linked organic polymer. Solvent molecules enter 
these structures when the exchanger is in contact with solutions. 
    Ion exchange is common in nature. Many synthetic ion exchangers have been 
made which have important uses in chemical processing and chemical analysis. 
These include ion-exchange resins, macroporous resins, and synthetic inorganic 
exchangers (such as molecular sieves). 
    Ion exchange has numerous applications for industry and for laboratory 
research. Water conditioning provides the largest market for ion exchange. 
Hard water is softened by contact with a cation exchanger that carries sodium 
ions. Calcium and magnesium ions enter the exchanger and displace sodium ions, 
which do not cause hardness. In sugar refining, ion exchange removes salts but 
not nonelectrolytes. Salts present in raw sugar juices interfere with the 
crystallization of sugar. 
    Ion exchange is used very widely on the laboratory scale for analytical 
purposes. In modern high-performance liquid chromatography, ion exchange has a 
limited role; it has been used extensively to analyze mixtures of amino acids. 
Ion-exchange membranes are used as separators in electrolytic and voltaic 
cells. See DIALYSIS; ION-SELECTIVE MEMBRANES AND ELECTRODES; SALINE WATER 
RECLAMATION.  
 �34s[Harold F. Walton]
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�6v�2v�32sIon-exchange chromatography   �21m �33s 
    A form of liquid chromatography based on selective ionic attractions 
between variously charged sample constituents and an ionized chromatographic 
matrix. There are two principal types of ion exchangers: cationic and anionic. 
The most commonly used ion exchangers consist of an organic polymeric backbone 
(usually a copolymer of styrene and divinylbenzene) with either acidic or 
basic exchange sites on its porous surface. The charged resins are capable of 
exchanging their cations or anions with those ions in the liquid phase which 
have a greater affinity for the matrix. Exchange interactions that take place 
during the passage of various ions through the column cause separation into 
discrete ionic zones. Ion-exchange chromatography is one of the widely 
employed forms of liquid chromatography. See CHROMATOGRAPHY; ION EXCHANGE; 
LIQUID CHROMATOGRAPHY. 
 �34s[Milos V. Novotny]
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�6v�2v�32sIonic crystals   �21m �33s 
    A class of crystals in which the lattice-site occupants are charged ions 
held together primarily by their electrostatic interaction. Such binding is 
called ionic binding. Empirically, ionic crystals are distinguished by strong 
absorption of infrared radiation, good ionic conductivity at high 
temperatures, and the existence of planes along which the crystals cleave 
easily. See CRYSTAL. 
    Compounds of strongly electropositve and strongly electronegative elements 
form solids which are ionic crystals, for example, the alkali halides, other 
monovalent metal halides, and the alkaline-earth halides, oxides, and 
sulfides. Crystals in which some of the ions are complex, such as metal 
carbonates, metal nitrates, and ammonium salts, may also be classed as ionic 
crystals.  
    As a crystal type, ionic crystals are to be distinguished from other types 
such as molecular crystals, valence crystals, or metals. The ideal ionic 
crystal as defined is approached most closely by the alkali halides (see 
illustration). Other crystals often classed as ionic have binding which is not 
exclusively ionic but includes a certain admixture of covalent binding. Thus 
the term ionic crystal refers to an idealization to which real crystals 
correspond to a greater or lesser degree, and crystals exist having 
characteristics of more than one crystal type.  See  CHEMICAL BONDING. 
    Ionic crystals, especially alkali halides, have played a very prominent 
role in the development of solid-state physics. They are relatively easy to 
produce as large, quite pure, single crystals suitable for accurate and 
reproducible experimental investigations. In addition, they are relatively 
easy to subject to theoretical treatment since they have simple structures and 
are bound by the well-understood Coulomb force between the ions. Being readily 
available and among the simplest known solids, they have thus been a frequent 
and profitable meeting place between theory and experiment. These same 
features of ionic crystals have made them attractive as host crystals for the 
study of crystal defects: deliberately introduced impurities, vacancies, 
interstitials, and color centers.  See  COLOR CENTERS.   
 �34s[B. Gale Dick]
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�6v�2v�32sIonic equilibrium   �21m �33s 
    An equilibrium in a chemical reaction in which at least one ionic species 
is produced, consumed, or changed from one medium to another. 
    The wide variety of types of ionic equilibrium possible include: (1) 
dissolution of an un-ionized substance, for example, the dissolution of 
hydrogen chloride (a gas) in water (an ionizing solvent), reactions (1) - (3); 
  
             *** HCl(g) EQUILIBRIUM H POSITIVE + Cl NEGATIVE ***           (1)
  
        *** HCl(g) + H2O EQUILIBRIUM H3O POSITIVE + Cl NEGATIVE ***        (2)
  
       *** HCl(g) + 4H2O EQUILIBRIUM H9O4 POSITIVE + Cl NEGATIVE ***       (3)
  
(2) dissolution of a crystal in  water, such as the dissociation of solid 
silver chloride, reaction (4); (3) dissociation of a strong acid, for example, 
nitric acid, HNO3, dissociates as it dissolves in water, as in reaction (5); 
  
        *** AgCl(crystal) EQUILIBRIUM Ag POSITIVE + Cl NEGATIVE ***        (4)
  
         *** HNO3 + H2O EQUILIBRIUM H3O POSITIVE + NO3 NEGATIVE ***        (5)
  
(4) dissociation of an ion in water, for example, the bisulfate ion, 
HSO4 NEGATIVE, dissociates in water, as in reaction (6); and dissociation of 
water itself, is represented by reaction (7).
  
*** HSO4 NEGATIVE + H2O EQUILIBRIUM H3O NEGATIVE + SO4 sup 2 POSITIVE ***  (6)
  
           *** 2H2O EQUILIBRIUM H3O POSITIVE + OH NEGATIVE ***             (7)
  
See ACID AND BASE; CHEMICAL EQUILIBRIUM; HYDROLYSIS. 
 �34s[Thomas F. Young]
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�6v�2v�32sIon implantation   �21m �33s 
    A process of introducing impurities into the near-surface region of solids 
by directing a beam of energetic ions at the solid. When ions of sufficient 
energy are directed toward a crystal surface, they will penetrate the surface 
and slow to rest within the solid. The advantages of the implantation process 
are: precise control of the type of impurity to be introduced, the amount of 
impurity introduced, and the impurity distribution in depth. In addition, 
since any atoms can be added to any solid, mixtures can be formed which would 
not normally be found in nature, that is, systems which are not in thermal 
equilibrium. A major area of application of ion implantation has been to 
semiconductor device fabrication, where the process is a standard technique. 
Additional applications have been developed in metals, where unexpectedly 
beneficial effects on surface-sensitive properties have been found, and in 
insulators, and research in ion implantation has been undertaken for the 
purpose of developing scientific understanding and technological applications. 
See CRYSTAL; DIFFUSION IN SOLIDS; ION; SEMICONDUCTOR. 
 �34s[Samuel T. Picraux]
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�6v�2v�32sIonization   �21m �33s 
    The process by which an electron is removed from an atom, molecule, or 
ion. This process is of basic importance to electrical conduction in gases and 
liquids. In the simplest case, ionization may be thought of as a transition 
between an initial state consisting of a neutral atom and a final state 
consisting of a positive ion and a free electron. In more complicated cases, a 
molecule may be converted to a heavy positive ion and a heavy negative ion 
which are separated. 
 �34s[Glenn H. Miller]
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�6v�2v�32sIonization chamber �21m �33s
    An instrument for detecting ionizing radiation by measuring the amount of
charge liberated by the interaction of ionizing radiation with suitable gases,
liquids, or solids.
    While the gold leaf electroscope is the oldest form of ionization chamber,
instruments of this type are still widely used as monitors of radiations by
workers in the nuclear or radiomedical professions. However, for many purposes
it is useful to measure the ionization pulse produced by a single ionizing
particle.  See	ELECTROSCOPE; MONITORING OF IONIZING RADIATION.
    The simplest form of a pulse ionization chamber consists of two conducting
electrodes in a container filled with gas (see illustration). A battery, or
other power supply, maintains an electric field between the positive anode and
the negative cathode. When ionizing radiation penetrates the gas in the
chamber - entering, for example, through a thin gastight window - this
radiation liberates electrons from the gas atoms leaving positively charged
ions. The electric field present in the gas sweeps these electrons and ions
out of the gas, the electrons going to the anode and the positive ions to the
cathode.
    In a chamber, such as that represented in the illustration, the current
begins to flow as soon as the electrons and ions begin to separate under the
influence of the applied electric field. The time it takes for the full
current pulse to be observed depends on the drift velocity of the electrons
and ions in the gas. Because the ions are thousands of times more massive than
the electrons, the electrons always travel several orders of magnitude faster
than the ions. As a result, virtually all pulse ionization chambers make use
of only the relatively fast electron signal.
    One of the most important uses of an ionization chamber is to measure the
total energy of a particle or, if the particle does not stop in the ionization
chamber, the energy lost by the particle in the chamber. In addition to energy
information, ionization chambers are now routinely built to give information
about the position within the gas volume where the initial ionization event
occurred. This information can be important not only in experiments in nuclear
and high-energy physics where these position-sensitive detectors were first
developed, but also in medical and industrial applications.
    Foremost among the other applications is the use of gas ionization
chambers for radiation monitoring. Portable instruments of this type usually
employ a detector containing approximately 60 in. sup 3 (1 liter) of gas, and
operate by integrating the current produced by the ambient radiation. Another
application of ionization chambers is the use of air-filled chambers as
domestic fire alarms. Yet another development in ion chamber usage is that of
two-dimensional imaging in x-ray medical applications to replace the use of
photographic plates.
    Gaseous ionization chambers have also found application as total-energy
monitors for high-energy accelerators. Such applications involve the use of a
very large number of interleaved thin parallel metal plates immersed in a gas
inside a large container.

    Ionization chambers can be made where the initial ionization occurs, not
in gases, but in suitable liquids or solids. In the solid-state ionization
chamber (or solid-state detector) the gas filling is replaced by a large
single crystal of suitably chosen solid material. In this case the incident
radiation creates electron-hole pairs in the crystal, and this constitutes the
signal charge. Silicon and germanium detectors have proved to be highly
successful and have led to detectors that have revolutionized low-energy
nuclear spectroscopy. The use of a liquid in an ionization chamber combines
many of the advantages of both solid and gas-filled ionization chambers; most
importantly, such devices have the flexibility in design of gas chambers with
the high density of solid chambers. During the 1970s a number of groups built
liquid argon ionization chambers and demonstrated their feasibility.
 �34s[William A. Lanford]
�33s

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�6v�2v�32sIonization gage   �21m �33s 
    An instrument for measuring vacuum by ionizing the gas present and 
measuring the ion current. There are two types of ionization gages. 
    In the hot-filament ionization gage (see illustration), electrons emitted 
by a filament are attracted toward a positively charged grid electrode. 
Collisions of electrons with gas molecules produce ions, which are then 
attracted to a negatively charged electrode. The current measured at this 
electrode is directly proportional to the pressure or gas density. 
    In the cold-cathode (Philips or Penning) ionization gage, a high voltage 
is applied between two electrodes. Fewer electrons are emitted, but a strong 
magnetic field deflects the electron stream, increasing the length of the 
electron path which increases the chance for ionizing collisions of electrons 
with gas molecules.  See  VACUUM MEASUREMENT. 
 �34s[Richard Comeau]
�33s 
  
  
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�6v�2v�32sIonization potential   �21m �33s 
    The potential difference through which a bound electron must be raised to 
free it from the atom or molecule to which it is attached. In particular, the 
ionization potential is the difference in potential between the initial state, 
in which the electron is bound, and the final state, in which it is at rest at 
infinity.  
    The ionization potential for the removal of an electron from a neutral 
atom other than hydrogen is more correctly designated as the first ionization 
potential. The potential associated with the removal of a second electron from 
a singly ionized atom or molecule is then the second ionization potential, and 
so on. 
 �34s[Glenn H. Miller]
�33s 
  
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�6v�2v�32sIonosphere   �21m �33s 
    That part of the upper atmosphere which is sufficiently ionized by solar 
ultraviolet radiation so that the concentration of free electrons affects the 
propagation of radio waves. See IONIZATION; RADIO BROADCASTING; RADIO-WAVE 
PROPAGATION.  
    The ionosphere is structured in the vertical direction. It was first 
thought that discrete layers were involved, referred to as the D, E, F1, and 
F2 layers; however, rocket measurements have shown that the ``layers'' merge 
with one another to such an extent that they are now normally referred to as 
regions rather than layers. Since a vertically incident radio wave is 
reflected from the ionosphere at that level where the natural frequency of the 
plasma equals the radio frequency, it is possible to identify the electron 
concentration at the point of reflection in terms of the radio frequency. See 
ATMOSPHERE.  
 �34s[Francis S. Johnson]
�33s 
  
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�6v�2v�32sIon propulsion   �21m �33s 
    Vehicular motion caused by reaction from the high-speed discharge of a 
beam of electrically equally charged minute particles. The charged particles, 
or ions, are accelerated in an electrostatic field produced within the vehicle 
that is propelled. Typical ion propellants include argon, sodium, xenon, 
cesium, and mercury. 
    Ion propulsion is characterized by high specific impulse but low thrust 
acceleration. Because high specific impulse means low propellant consumption, 
ion propulsion is attractive for a wide variety of applications. One 
functional category includes the use of ion thrusters on application 
satellites for orbit control, for station keeping (position maintaining of 
satellite in a given orbit), and attitude control. Another functional category 
involves the use of ion drives for cargo transports in Earth-Moon (geolunar) 
space. The third major functional application of ion propulsion is 
interplanetary transfer. 
    The power source for ion propulsion can be a nuclear reactor or a 
solar-radiation energy collector. In the first case, protection of the 
electrical equipment (and of personnel in the case of crewed vehicles) is 
required against neutron and GAMMA-radiation; protection of the main radiator 
against radioactivity induced in the working fluid by the reactor is also 
necessary. If solar radiation is the power source, no radiation shield is 
required. Solar radiation can be used to provide electric power directly, 
through photovoltaic solar cells, or indirectly, through a solar heater in 
which the nuclear reactor is replaced by a solar-radiation collector and a 
heat exchanger. 
    Ion or electrostatic thrust devices consist of three components: an 
ionizer, which furnishes the ions or other charged particles; an accelerator, 
which provides the properly shaped electric or electromagnetic field for 
accelerating and properly shaping the charged-particle beam; and a 
neutralizer, usually an electron emitter, which neutralizes the exhaust beam 
after ejection. See ELECTROMAGNETIC PROPULSION; INTERPLANETARY PROPULSION; 
SPACECRAFT PROPULSION. 
 �34s[Krafft A. Ehricke]
�33s 
  
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�6v�2v�32sIon-selective membranes and electrodes   �21m �33s 
    Membranebased devices, involving permselective, ion-conducting materials, 
used for measurement of activities of species in liquids or partial pressures 
in the gas phase. Permselective means that ions of one sign may enter and pass 
through a membrane. 
    Ion-selective electrodes are classified mainly according to the physical 
state of the ion-responsive membrane material, and not with respect to the 
ions sensed. 
    Glass membrane electrodes are mainly used for hydrogen ion activity 
measurements. They predate the wider variety of membrane electrodes developed 
after 1960. Electrodes based on water-insoluble inorganic salts include 
sensors for F-, Cl-, Br-, I-, CN-, SCN-, Ssup2-, Agsupp, Cusup2supp, 
Cdsup2supp, and Pbsup2supp. The compounds used are silver salts, mercury 
salts, sulfides of Cu, Pb, and Cd, and rare-earth salts. All of these are 
so-called white metals whose aqueous cations (except Lasup3supp) are labile. 
Electrodes using liquid-ion exchangers are supported in the voids of inert 
polymers such as cellulose acetate, or in transparent films of polyvinyl 
chloride, and provide extensive examples of devices for sensing. Electrodes 
with chemical reactions interposed between the sample and the sensor surface 
permit a new degree of freedom in design of sensors for species which do not 
directly respond at an electrode surface. Two primary examples are the 
categories of gas sensors and of electrodes which use enzyme-catalyzed 
reactions. See CALOMEL ELECTRODE; ELECTRODE; ION EXCHANGE. 
    Electrodes for many species are, for the most part, commercially 
available. Applications may be batch or continuous. Important batch examples 
are potentiometric titrations with ion-selective electrode end-point 
detection, determination of stability constants of complexes and speciation 
identity, solubility and activity coefficient determinations, and monitoring 
of reaction kinetics, especially for oscillating reactions. Ion-selective 
electrodes serve as liquid chromatography detectors and as quality-control 
monitors in drug manufacture. Applications occur in air and water quality 
(soil, clay, ore, natural-water, water-treatment, sea-water, and pesticide 
analyses); medical and clinical laboratories (serum, urine, sweat, 
gastric-juices, extracellular-fluid, dental-enamel, and milk analyses); and 
industrial laboratories (heavy-chemical, metallurgical, glass, beverage, and 
household-product analyses). See ANALYTICAL CHEMISTRY; CHROMATOGRAPHY; 
TITRATION.  
 �34s[Richard P. Buck]
�33s 
  
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�6v�2v�32sIon sources   �21m �33s 
    Devices which produce positive or negative electrically charged atoms or 
molecules. See ION. 
    In general, ion sources fall into three major categories: those designed 
for positive-ion generation, those for negative-ion generation, and a highly 
specialized type of source designed to produce a polarized ion beam. The 
positive-ion source category may further be subdivided into sources 
specifically designed to generate singly charged ions and those designed to 
produce very highly charged ions. 
    Ion sources have acquired a wide variety of applications. They are used in 
a variety of different types of accelerators for nuclear research; have 
application in the field of fusion research; and are used for ion 
implantation, in isotope separators, in ion microprobes, as a means of rocket 
propulsion, in mass spectrometers, and for ion milling. See ION IMPLANTATION; 
ION PROPULSION; ISOTOPE  (STABLE) SEPARATION; MASS SPECTROSCOPE; NUCLEAR 
FUSION; PARTICLE ACCELERATOR; SECONDARY ION MASS SPECTROMETRY (SIMS). 
 �34s[Roy Middleton]
�33s 
  
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�6v�2v�32sIon transport   �21m �33s 
    Movement of salts and other electrolytes in the form of ions from place to 
place within living systems. 
    In animals, ion transport may occur by any of several different 
mechanisms: electrochemical diffusion, active-transport requiring energy, or 
bulk flow as in the flow of blood in the circulatory system of animals or the 
transpiration stream in the xylem tissue of plants. The best-known system 
transporting ions actively is the sodium/potassium (Na/K) exchange pump which 
occurs in plasma membranes of virtually all cells. The physical basis of the 
Na/K pump has been identified as a form of the enzyme adenosinetriphosphatase 
(ATPase) which requires both Nasupp and Ksupp for its activity of catalyzing 
the hydrolysis of adenosinetriphosphate (ATP). This reaction provides the 
energy required for the extrusion of Nasupp from cells in exchange for Ksupp 
taken in, but the mechanism of coupling of the energy liberation with the 
transport process has not been established. 
 �34s[Bradley T. Scheer]
�33s 
    In plants, transport processes are involved in uptake and release of 
inorganic ions and in distribution of ions within plants, and thus determine 
ionic relations of plants. 
    In unicellular, filamentous, or simple thalloid algae, in mosses, in 
poorly differentiated aquatic higher plants, and experimentally in cell 
suspension cultures of higher plants, ion transport can be considered on the 
cellular level. In these systems all cells take up ions directly from the 
external medium. Intracellular distribution and compartmentation are 
determined by transport across other membranes within the cells. 
    Within tissues the continuous cell walls of adjacent cells form an 
apoplastic pathway for ion transport. A symplastic pathway is constituted by 
the cytoplasm extending from cell to cell via small channels crossing the cell 
walls. Transport over longer distances is important in organs (roots, shoots, 
leaves, fruits), which are composed of different kinds of tissues, and in the 
whole plant. Xylem and phloem serve as pathways for long-distance transport. 
Roots take up ions from the soil and must supply other plant organs. But there 
is also circulation within the plant; for example, potassium is readily 
transported both from root to shoot and in the opposite direction. See PHLOEM; 
XYLEM.  
 �34s[Ulrich Luttge]
�33s 
  
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�6v�2v�32sIR drop   �21m �33s 
    That component of the potential drop across a passive element (one which 
is not a seat of electromotive force) in an electric circuit caused by 
resistance of the element. This potential drop, by definition, is the product 
of the resistance R of the element and the current I flowing through it. The 
IR drop across a resistor is the difference of potential between the two ends 
of the resistor.  See  ELECTRICAL RESISTANCE; POTENTIALS. 
 �34s[John W. Stewart]
�33s 
  
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�6v�2v�32sIridium   �21m �33s 
    A chemical element, Ir, atomic number 77, and atomic weight 192.2. Iridium 
in the free state is a hard, white metallic substance.  See  TRANSITION 
ELEMENTS.  
    Iridium has considerably less oxidation resistance than platinum or 
rhodium, but more so than ruthenium or osmium. A thin, adherent oxide film, 
IrO2, forms above about 600DGC (1110DGF). It is the only metal which can be 
used unprotected in air up to 2300DGC (4170DGF) with any degree of life 
expectancy. Iridium is not attacked by any acid, including aqua regia. Iridium 
has a strong tendency to form coordination compounds. The table gives values 
for some important properties of iridium. 
    Principal compounds include iridium trichloride, IrCl3, a green, 
water-insoluble compound; sodium iridium(IV) chloride, Na2IrCl6.6H2O, a black, 
water-soluble crystalline solid; sodium iridium(III) chloride, Na3IrCl6.12H2O, 
an olive-green, water-soluble crystalline solid; and ammonium iridium(IV) 
chloride, (NH4)2IrCl6, a red-black, relatively insoluble crystalline solid. 
    Iridium's special properties have led to specialized applications, 
including crucibles for high-temperature growth of laser crystals, 
iridium-rhodium very-high-temperature thermocouples, and cladding applied over 
other materials. It is usually alloyed with platinum as the base, since a 
platinum  - 30% iridium alloy, for instance, is almost as corrosion-resistant 
as iridium and is a great deal easier to fabricate.  See  PLATINUM; RHODIUM. 
 �34s[Henry J. Albert]
�33s 
  
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�6v�2v�32sIron �21m �33s
    A chemical element, Fe, atomic number 26, and atomic weight 55.847. Iron
is the fourth most abundant element in the crust of the Earth (5%). It is a
malleable, tough, silver-gray, magnetic metal. It melts at 1540DGC, boils at
2800DGC, and has a density of 7.86 g/cm sup 3. The four stable, naturally
occurring isotopes have masses of 54, 56, 57, and 58. The two main ores are
hematite, Fe2O3, and limonite, Fe2O3 PRODUCT 3H2O. Pyrites, FeS2, and
chromite, Fe(CrO2)2, are mined as ores for sulfur and chromium, respectively.
Iron is found in many other minerals, and it occurs in groundwaters and in the
red hemoglobin of blood.
    The greatest use of iron is for structural steels; cast iron and wrought
iron are made in quantity, also. Magnets, dyes (inks, blueprint paper, rouge
pigments), and abrasives (rouge) are among the other uses of iron and iron
compounds. See CAST IRON; IRON ALLOYS; IRON METALLURGY; STAINLESS STEEL; STEEL
MANUFACTURE; WROUGHT IRON.
    There are several allotropic forms of iron. Ferrite or ALPHA-iron is
stable up to 760DGC (1400DGF). The change of BETA-iron involves primarily a
loss of magnetic permeability because the lattice structure (body-centered
cubic) is unchanged. The allotrope called GAMMA-iron has the cubic
close-packed arrangements of atoms and is stable from 910 to 1400DGC (1670 to
2600DGF). Little is known about DELTA-iron except that it is stable above
1400DGC (2600DGF) and has a lattice similar to that of ALPHA-iron.
    The metal is a good reducing agent and, depending on conditions, can be
oxidized to the 2+, 3+, or 6+ state. In most iron compounds, the ferrous ion,
iron(II), or ferric ion, iron(III), is present as a distinct unit. Ferrous
compounds are usually light yellow to dark green-brown in color; the hydrated
ion, Fe(H2O)6(sup 2)(sup+), which is found in many compounds and in solution, is
light green. This ion has little tendency to form coordination complexes
except with strong reagents such as cyanide ion, polyamines, and porphyrins.
The ferric ion, because of its high charge (3+) and its small size, has a
strong tendency to hold anions. The hydrated ion, Fe(H2O)6(sup 3)(sup +), which
is found in solution, combines with OH-, F-, Cl-, CN-, SCN-, N3-, C2O4(sup 2-),
and other anions to form coordination complexes. See COORDINATION CHEMISTRY.
    An interesting aspect of iron chemistry is the array of compounds with
bonds to carbon. Cementite, Fe3C, is a component of steel. The cyanide
complexes of both ferrous and ferric iron are very stable and are not strongly
magnetic in contradistinction to most iron coordination complexes. The cyanide
complexes form colored salts. See TRANSITION ELEMENTS.
 �34s[John O. Edwards]
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�6v�2v�32sIron alloys   �21m �33s 
    Solid solutions of metals, one metal being iron. A great number of 
commercial alloys have iron as an intentional constituent. Iron is the major 
constituent of wrought and cast iron and wrought and cast steel. Alloyed with 
large amounts of silicon, manganese, chromium, vanadium, molybdenum, niobium 
(columbium), selenium, titanium, phosphorus, or other elements, singly or 
sometimes in combination, iron forms the large group of materials known as 
ferroalloys that are important as addition agents in steelmaking. Iron is also 
a major constituent of many special-purpose alloys developed to have 
exceptional characteristics with respect to magnetic properties, electrical 
resistance, heat resistance, corrosion resistance, and thermal expansion. See 
ALLOY; FERROALLOY; STEEL. 
 �34s[Harold E. McGannon]
�33s 
  
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�6v�2v�32sIron metallurgy   �21m �33s 
    Extracting iron from ores and preparing it for use. Extraction involves 
the conversion of naturally occurring iron-bearing minerals into metallic 
iron. The term ironmaking is commonly used to include all of the industrial 
processes that convert raw materials into iron. The major process for the 
production of iron is the iron blast furnace. However, since 1970 the growth 
of alternative direct-reduction processes has been very significant. The 
principal difference between the blast furnace process and the 
direct-reduction processes is the temperature of operation. In the blast 
furnace, high operating temperatures enable the production of molten iron. At 
the lower operating temperatures of the direct-reduction processes, solid or 
sponge iron is produced. Most of the iron produced in the world is used in the 
production of steel. The remainder is converted to iron castings, ferroalloys, 
and iron powder. See FERROALLOY; IRON; PRESSURIZED BLAST FURNACE; 
PYROMETALLURGY; STEEL MANUFACTURE. 
 �34s[George R. St. Pierre]
�33s 
  
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�6v�2v�32sIronwood �21m �33s
    The name given to any of at least 10 kinds of tree in the United States,
including the American hornbeam (Carpinus caroliniana), eastern hophornbeam
(Ostrya virginiana), buckthorn bumelia (Bumelia lycioides), tough bumelia (B.
tenax,), buckwheat tree (Cliftonia monophylla), and swamp cyrilla or swamp
ironwood (Cyrilla racemiflora). Leadwood (Krugiodendron ferreum), a native of
southern Florida, has the highest specific gravity of all woods native to the
United States and is also known as black ironwood. See HOPHORNBEAM; HORNBEAM.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sIrregularia   �21m �33s 
    The name given by G. Cuvier in 1817 to an assemblage of echinoids in which 
the anus and periproct lie outside the apical system, the ambulacral plates 
remain simple, the primary radioles are hollow, and the rigid test shows more 
or less bilateral symmetry. The Irregularia are an artificial assemblage of 
similar but unrelated forms, and the name has no taxonomic validity. See 
DIADEMATACEA; ECHINOIDEA; PYGASTEROIDA; REGULARIA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sIrrigation (agriculture)   �21m �33s 
    The artificial application of water to the soil to produce plant growth. 
Irrigation also cools the soil and atmosphere, making the environment 
favorable for plant growth. Water is applied to crops by surface, subsurface, 
sprinkler, and drip irrigation. 
    Surface irrigation includes furrow and flood methods. The furrow method is 
used for row crops. Corrugations or rills are small furrows used on 
close-growing crops. The flow, carried in furrows, percolates into the soil. 
Flow to the furrow is usually supplied by siphon tubes, spiles, gated pipe, or 
valves from buried pipe. In the flood method, controlled flooding is done with 
border strips, contour or bench borders, and basins. 
    Subirrigation is a type of irrigation accomplished by raising the water 
table to the root zone of the crop or by carrying moisture to the root zone by 
perforated underground pipe. Either method requires special soil conditions 
for successful operation. 
    A sprinkler system consists of pipelines which carry water under pressure 
from a pump or elevated source to lateral lines along which sprinkler heads 
are spaced at appropriate intervals. Laterals are moved from one location to 
another by hand or tractor, or they are moved automatically. The side-roll 
wheel system, which utilizes the lateral as an axle (see illustration), is 
very popular as a labor-saving method. 
    Drip irrigation is a method of providing water to plants almost 
continuously through small-diameter tubes and emitters. It has the advantage 
of maintaining high moisture levels at relatively low capital costs. It can be 
used on very steep, sandy, and rocky areas and can utilize saline waters 
better than most other systems. The system has been most popular in orchards 
and vineyards, but is also used for vegetables, ornamentals, and for landscape 
plantings. See AGRICULTURAL SOIL AND CROP PRACTICES; LAND DRAINAGE 
(AGRICULTURE); TERRACING (AGRICULTURE). 
 �34s[Mel A. Hagood]
�33s 
  
  
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�6v�2v�32sIrrotational flow   �21m �33s 
    Fluid flow in which each infinitesimal particle or element of the fluid 
preserves its original orientation. Because an element of fluid can be caused 
to rotate about its axis only by the application of viscous forces, 
irrotational flow is possible only for an ideal or nonviscous fluid. For 
fluids of low viscosity, such as air or water, irrotational flow may be 
approached in a free vortex. In a free vortex, a body of fluid rotates without 
the application of external torque because of angular momentum previously 
imparted to it. Examples are the rotation of fluid after leaving the impeller 
of a centrifugal pump, a tornado, or the rotation of water entering the drain 
of a tub. See FLUID FLOW; VORTEX. 
 �34s[Robert L. Daugherty]
�33s 
  
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�6v�2v�32sIschemic colitis   �21m �33s 
    A broad spectrum of responses occurring in individuals who have 
compromised circulation to the colon. Three interrelated factors must be 
altered before the condition is clinically apparent: the vessels supplying the 
colon must be compromised; the state of the microcirculation must be 
compromised; and the ``virulence'' of the bacteria in the lumen of the bowel 
must be changed. If minimal alterations are present, transient mucosal 
necrosis occurs, manifested by episodes of bloody diarrhea, with complete 
recovery of the patient. If more severe alterations occur, the extent of 
mucosal necrosis is greater. The latter presentation heals by fibrosis, which 
is clinically manifested by the presence of a colonic stricture. In its 
severest presentation, transmural involvement occurs with the development of 
gangrene of the colon. The ischemic process can be present at any level of the 
gastrointestinal tract, and affect any age group. Examples of the spectrum of 
ischemic bowel disease include ischemic colitis, acute necrotizing 
enterocolitis of premature infants, hemorrhagic necrosis of the 
gastrointestinal tract, and pseudomembranous, staphylococcal, uremic, and 
radiation enterocolitis and stress ulceration. 
 �34s[H. Thomas Norris]
�33s 
  
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�6v�2v�32sIsentropic flow   �21m �33s 
    The flow of a fluid is isentropic when its entropy is identical at all 
points in the flow. Isentropic flow can be approached for fluids flowing 
either in a duct or over the outside of a body. Because the entropy of the 
fluid is a thermodynamic property, similar to the enthalpy or energy of a 
fluid, the value of the entropy is fixed by the state of the fluid. For a pure 
substance, in the absence of external forces, entropy is a function of two 
independent properties. For example, in the absence of gravity, capillarity, 
electricity, and magnetism, the entropy of a single-phase fluid is a function 
of the pressure and temperature. See ENTHALPY; ENTROPY; ISENTROPIC PROCESS; 
THERMODYNAMIC PRINCIPLES. 
 �34s[Philip E. Bloomfield]
�33s 
  
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�6v�2v�32sIsentropic process   �21m �33s 
    In thermodynamics, a process involving change without any increase or 
decrease of entropy. Since the entropy always increases in a spontaneous 
process, one must consider reversible or quasistatic processes. During a 
reversible process the quantity of heat transferred is directly proportional 
to the system's entropy change. Systems which are thermally insulated from 
their surroundings undergo processes without any heat transfer; such processes 
are called adiabatic. Thus during an isentropic process there are no 
dissipative effects and the system neither absorbs nor gives off heat. For 
this reason the isentropic process is sometimes called the reversible 
adiabatic process. See ADIABATIC PROCESS; ENTROPY; THERMODYNAMIC PROCESSES. 
 �34s[Philip E. Bloomfield]
�33s 
  
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�6v�2v�32sIsentropic surfaces   �21m �33s 
    Surfaces along which the entropy and potential temperature of air are 
constant. Potential temperature, in meteorological usage, is defined by the 
relationship  
  
   *** THETA = T (1000 / P) {sup [(c sub p) - (c sub v)] / c sub p} ***
  
in which T is the air temperature, P is atmospheric pressure expressed in 
millibars, c sub p is the specific heat of air at constant pressure and c sub v 
is the specific heat at constant volume. Since the potential temperature of an 
air parcel does not change if the processes acting on it are adiabatic (no 
exchange of heat between the parcel and its environment), a surface of constant 
potential temperature is also a surface of constant entropy. An advantage of 
representing meteorological conditions on isentropic surfaces is that there 
is usually little air motion through such surfaces, since thermodynamic 
processes in the atmosphere are approximately adiabatic. See ADIABATIC PROCESS; 
ATMOSPHERIC GENERAL CIRCULATION. 
 �34s[Frederick Sanders]
�33s 
  

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�6v�2v�32sIsing model �21m �33s
    A model which consists of a lattice of ``spin'' variables with two
characteristic properties: (1) each of the spin variables independently takes
on either the value +1 or the value -1; and (2) only pairs of nearest
neighboring spins can interact. The study of this model in two dimensions
forms the basis of the modern theory of phase transitions and, more generally,
of cooperative phenomena.
    A macroscopic piece of material consists of a large number of atoms, the
number being of the order of Avogadro's number (approximately 6 x 10 (sup 2)
(sup 3)). Thermodynamic phenomena all depend on the participation of such a
large number of atoms. Even though the fundamental interaction between atoms is
short-ranged, the presence of this large number of atoms can, under suitable
conditions, lead to an effective interaction between widely separated atoms.
Phenomena due to such effective long-range interactions are referred to as
cooperative phenomena. The simplest examples of cooperative phenomena are
phase transitions. The most familiar phase transition is either the
condensation of steam into water or the freezing of water into ice. Only
slightly less familiar is the ferromagnetic phase transition that takes place
at the Curie temperature, which, for example, is roughly 1043 K for iron. See
CURIE TEMPERATURE; FERROMAGNETISM; PHASE TRANSITION.
 �34s[Barry M. McCoy ;Tai Tsun Wu]
�33s

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�6v�2v�32sIsland biogeography   �21m �33s 
    Islands generally have fewer species of animals and plants than do 
comparable continental areas, at least if the islands lie some distance from 
continents. Often there are rather few genera, but some genera have many local 
species that sometimes exhibit strong adaptive radiation. The proportion of 
endemic species (found nowhere else) that are present generally increases with 
the degree of isolation and with the complexity of the habitat. Gigantism, 
flightlessness, and other unusual characteristics are relatively frequent. 
    Island floras and faunas are of far greater interest than either the 
number of species or their economic importance might seem to justify. How the 
plants and animals came to be on the islands, why so many of them are found 
nowhere else, why so many have special or strange forms, and where their 
ancestors came from, are fascinating questions which present problems of great 
scientific importance to the biologist. The theory of organic evolution 
emerged from studies of island faunas by Charles Darwin and Alfred Russel 
Wallace, and islands are still among the most advantageous places to study 
evolutionary processes. They also offer uniquely suitable sites for the 
investigation of the nature and functioning of ecosystems.  See  ECOLOGY; 
SPECIATION.  
 �34s[F. R. Fosberg]
�33s 
  
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�6v�2v�32sIsoantigen   �21m �33s 
    An immunologically active protein or polysaccharide present in some but 
not all individuals in a particular species. These substances initiate the 
formation of antibodies when introduced into other individuals of the species 
that genetically lack the isoantigen. Like all antigens, they are also active 
in stimulating antibody production in heterologous species. The ABO, MN, and 
Rh blood factors in humans constitute important examples. Consequently, 
elaborate precautions for typing are required in blood transfusion.  See  
ANTIBODY; ANTIGEN; BLOOD GROUPS. 
 �34s[Margaret J. Polley]
�33s 
  
A700000001152�254s�5v�1v937
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�6v�2v�32sIsobaric process �21m �33s
    A thermodynamic process in which the heat transfer to or from the gaseous
system causes a volume change at constant pressure. This process can be
illustrated by the expansion of a substance when it is heated. The system is
then capable of doing an amount of work on its surroundings. The maximum work
will be done when the external pressure of the surroundings on the system is
equal to the pressure of the system.  See  ISOMETRIC PROCESS; POLYTROPIC
PROCESS; THERMODYNAMIC PROCESSES.
 �34s[Philip E. Bloomfield /William A. Steele]
�33s

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�6v�2v�32sIsobar (atomic physics)   �21m �33s 
    One of two or more atoms which have a common mass number A but which 
differ in atomic number Z. Thus, although isobars possess approximately equal 
masses, they differ in chemical properties; they are atoms of different 
elements. Isobars whose atomic numbers differ by unity cannot both be stable; 
one will inevitably decay into the other.  See  ELECTRON CAPTURE; 
RADIOACTIVITY.  
 �34s[Henry E. Duckworth]
�33s 
  
A700000002347�254s�5v�1v937
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�6v�2v�32sIsobar (meteorology)   �21m �33s 
    A curve along which pressure is constant. Leading examples of its uses are 
in weather forecasting and meteorology. The most common weather maps are 
charts of weather conditions at the Earth's surface and mean sea level, and 
they contain isobars as principal information. Areas of bad or unsettled 
weather are readily defined by roughly circular isobars around low-pressure 
centers at mean sea level. Likewise, closed isobars around high-pressure 
centers define areas of generally fair weather.  See  AIR PRESSURE. 
    A principal use of isobars stems from the so-called geostrophic wind, 
which approximates the actual wind on a large scale. The direction of the 
geostrophic wind is parallel to the isobars, in the sense that if an observer 
stands facing away from the wind, higher pressures are to the person's right 
if in the Northern Hemisphere and to the left if in the Southern. Thus, in the 
Northern Hemisphere, flow is counterclockwise about low-pressure centers and 
clockwise about high-pressure centers, with the direction of the flow reversed 
in the Southern Hemisphere.  See  GEOSTROPHIC WIND; WEATHER MAP. 
 �34s[Frederick B. Shuman]
�33s 
  
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�6v�2v�32sIsobryales   �21m �33s 
    An order of mosses in which the plants are slender to robust. In some 
families, they are strongly flattened, mostly with creeping primary stem, and 
irregularly to pinnately divided into erect, ascending, dendroid, or pendulous 
branches. The leaves are in two, three, or many rows, generally symmetrical. 
The sporophytes are pleurocarpous or cladocarpous. The capsule varies from 
immersed to exserted and is commonly smooth.  See  BRYOPSIDA. 
 �34s[Winona H. Welch]
�33s 
  
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�6v�2v�32sIsocyanate �21m �33s
    One of a group of neutral derivatives of primary amines, formula
R--NDOUBLE BARCDOUBLE BARO, that are obtained by reaction with excess phosgene
and loss of hydrogen chloride as in the following.  See  AMINE.

RNH (sub 2) + COCl (sub 2) -> RNHCOCl -> R-N DOUBLEBOND C DOUBLEBOND O + HCl

    Isocyanates hydrolyze in air to the unstable carbamic acids (RNHCOOH)
which revert to the original amines by loss of carbon dioxide. Unhydrolyzed
isocyanate then reacts with the amine to give a symmetrical urea, RNHCONHR.
Normally an important reaction only because the nuisance must be avoided,
controlled hydrolysis of free isocyanate groups to liberate carbon dioxide is
the basis of one type of foamed plastic.  See  POLYURETHANE RESINS.
 �34s[Leallyn B. Clapp]
�33s

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�6v�2v�32sIsoelectronic sequence �21m �33s
    A term used in spectroscopy to designate the set of spectra produced by
different chemical elements ionized in such a way that their atoms or ions
contain the same number of electrons. The sequence in the table is an example.
Since the neutral atoms of these elements each contain Z electrons, removal of
one electron from scandium, two from titanium, and so forth, yields a series
of ions all of which have 20 electrons. Isoelectronic sequences are useful in
predicting unknown spectra of ions belonging to a sequence in which other
spectra are known. See ATOMIC STRUCTURE AND SPECTRA.
 �34s[Francis A. Jenkins /William W.Watson]
�33s

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�6v�2v�32sIsoelectric point   �21m �33s 
    The pH value of the dispersion medium of a colloidal suspension at which 
the colloidal particles do not move in an electric field; that is, the 
particles are electrophoretically inert. The isoelectric point is often 
employed to characterize colloidal material such as the proteins. However, a 
range of values is usually necessary, since the isoelectric point varies 
detectably with (1) the size of the particles, (2) the purity, and (3) the 
concentration of other than hydrogen ions.  See  COLLOID; ELECTROPHORESIS; PH; 
PROTEIN.  
 �34s[W. O. Milligan]
�33s 
  
A700000001451�254s�5v�1v938
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�6v�2v�32sIsoetopsida �21m �33s
    One of the two classes of the division Lycopodiophyta, the other being the
Lycopodiopsida. Like all Lycopodiophyta, the Isoetopsida are vascular
cryptogams with narrow, single-veined leaves and with the sporangia more or
less distinctly axillary to the sporophylls. They differ from the
Lycopodiopsida in being heterosporous (as opposed to homosporous), and in
having a small appendage, the ligule, on the upper side of the leaf near the
base. There are two living orders, the Isoetales and Selaginellales, and one
fossil order, the Lepidodendrales. There are probably not more than about 600
living species of Isoetopsida. See ISOETALES; LEPIDODENDRALES; LYCOPODIOPHYTA;
LYCOPODIOPSIDA; SELAGINELLALES.
 �34s[Arthur Cronquist]
�33s

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�6v�2v�32sIsolated singularity   �21m �33s 
    If a function f is analytic at all points of a disc OPEN TRIANGLE centered 
at a, except perhaps at the point a itself, then a is called an isolated 
singularity of the function f. There are three mutually exclusive possibilities 
for the nature of such a singularity: 

    1. It may happen that |f(z)| remains bounded near the point a. In that case 
the function can be extended to an analytic function even at the point a by 
suitably choosing the value of f(a); the point a is then called a removable 
singularity. 
    2. It may happen that LIMIT (z TO a)|f(z)| = INFINITY. In that case a is a 
removable singularity for the function g(z) = 1/f(z) = 1/g(z) where g(z) is an 
analytic function vanishing at the point a. The function f(z) is said to have 
a pole at the point a. 
    3. If neither of the two preceding cases arises, the function f(z) is said 
to have an essential singularity at the point a. Singularities of this type 
can occur and do so frequently. See ANALYTIC FUNCTION. 
 �34s[Robert C. Gunning]
�33s 
  

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�6v�2v�32sIsoleucine �21m �33s
    An amino acid with the structure shown, considered essential for normal
growth of animals. The biosynthesis of isoleucine occurs when pyruvate and
ALPHA-ketobutyrate react to form ALPHA-aceto-ALPHA-hydroxybutyrate, which
undergoes rearrangement and reduction to
ALPHA,BETA-dihydroxy-BETA-methylvalerate. Dehydration to the ALPHA-keto acid
and transamination complete the biosynthesis. See AMINO ACIDS.
 �34s[Edward A. Adelberg]
�33s

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�6v�2v�32sIsomerization �21m �33s
    Rearrangement of the atoms within hydrocarbon molecules. Isomerization
processes of practical significance in petroleum chemistry are (1) migration
of alkyl groups, (2) shift of a single-carbon bond in naphthenes, and (3)
double-bond shift in olefins. See ALKYLATION; AROMATIZATION; CRACKING;
MOLECULAR ISOMERISM.
 �34s[George E. Liedholm]
�33s

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�6v�2v�32sIsometric process �21m �33s
    A constant-volume thermodynamic process in which the system is confined by
mechanically rigid boundaries. No direct mechanical work can be done on the
surroundings by a system with rigid boundaries; therefore the heat transferred
into or out of the system equals the change of internal energy stored in the
system. This change in the internal energy, in turn, is a function of the
specific heat and the temperature change in the system. See POLYTROPIC PROCESS.
 �34s[Philip E. Bloomfield]
�33s

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�6v�2v�32sIsomorphism (crystallography)   �21m �33s 
    A similarity of crystalline form between substances of similar 
composition. Two substances which are isomorphous have a similar chemical 
formula, an equal or nearly equal ratio of cation to anion radius, and 
comparable polarizabilities of their ions. Isomorphism is morphotropism in a 
narrower, more precise sense. Similarity in the macroscopic characteristics of 
isomorphous crystals becomes so close that extreme precision is needed to 
distinguish between them.  See  CRYSTAL. 
 �34s[Willy C. Dekeyser]
�33s 
  
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�6v�2v�32sIsopoda   �21m �33s 
    An order of malacostracan crustaceans characterized by a cephalon, or 
head, bearing one pair of maxillipeds in addition to the antennae, mandibles, 
and maxillae (see illustration). A carapace is lacking. The peraeon, or 
thorax, consists of seven distinct somites each bearing a pair of peraeopods, 
the legs. The pleon, or abdomen, has six somites. The first five pairs of 
pleonal appendages are foliaceous. The last pair is modified into hardened 
appendages called uropods and the last somite is fused with the telson into a 
pleotelson.  
    The most familiar isopods are the terrestrial sow bugs or pill bugs. Many 
of the animals roll up into a compact ball when disturbed. Land isopods are 
usually found in moist environments, under decaying leaves and wood, and under 
rocks. The destructive marine wood-boring isopod Limnoria, the gribble, causes 
extensive damage to wharf piling in the United States, and one species is 
reported to attack treated timbers. 
    About 3000 species of isopods are known today, but it may be estimated 
that only one-half of the existing species have been described. See CRUSTACEA. 
 �34s[Robert J. Menzies]
�33s 
  
  
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�6v�2v�32sIsoprene �21m �33s
    A five-carbon, conjugated diolefin, or diene, having the structure shown.
It does not occur naturally but is obtained by the destructive distillation of
gas oil, naphthas, and rubber.
    The terpenes may be regarded as multiples of the isoprene unit, C5H8.
Indeed, isoprene may also be the foundation for other important plant products
such as phytol, the sterols, and the carotenoids.
    Isoprene is a mobile, colorless liquid boiling at 34.1DGC (93.4DGF),
having a specific gravity of 0.862. It exhibits all the characteristic
reactions of dienes of this type. Isoprene polymerizes readily to form dimers
and high-molecular-weight resins. See TERPENE.
 �34s[William Mosher]
�33s

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�6v�2v�32sIsopropylphenol   �21m �33s 
    One of thee isomeric compounds, having structures shown in the 
illustration. The nomenclature of the derivatives of these compounds is 
directly parallel to that of the cresol analogs. 
    In many of the applications of which cresols have historically been used, 
one or another of the isopropylphenols can be used. As a by-product of an 
extremely large industrial manufacturing operation, that is, phenol from 
cumene, isopropylphenols are potentially plentiful and are free of many 
troublesome impurities commonly found in the cresols or cresylic acids of 
commerce, which are largely derived from coal tars or waste products of 
petroleum-refining operation. See CRESOL; CUMENE; PHENOL. 
 �34s[Robert I. Stirton]
�33s 
  
                             ******FORMULA****** 
  
  
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�6v�2v�32sIsoptera   �21m �33s 
    An order of Insecta into which are grouped certain morphologically 
primitive forms commonly referred to as termites. They have a gradual 
metamorphosis, lacking true larval and pupal stages; the antennae are 
moniliform; the mouthparts are biting and prognathous; there are two pairs of 
subequal wings; and the abdomen is broadly joined to the thorax. All members 
of the Isoptera live in colonies. Termites perform a very important role in 
the breakdown of all types of cellulose debris. 
    Approximately 1900 species of Isoptera are recognized. Termites are often 
grouped into higher and lower forms. The higher termites include the family 
Termitidae, which contains about 80% of the species. 
    The mature termite (alate or imago) has membranous wings which extend 
beyond the end of the abdomen. In almost all termite species a second type of 
individual is produced in the colony. This is the soldier which lacks wings, 
is nonreproductive, and is variously modified for defense. 
    In the more advanced termites there is a third caste, the worker. Workers 
lack wings, are nonreproductive, and have mandibles which resemble those of 
the imagoes; they are usually blind. In addition to these definitive castes 
(alate, soldier, and worker), another type of individual may occur in the 
colony under certain circumstances. These individuals are the supplementary or 
replacement reproductives. Although the original pair (king and queen) may 
live for two or three decades, the life of the colony itself is not limited by 
their survival. If one or both are lost, other individuals in the colony 
become reproductive. See INSECTA; SOCIAL INSECTS; TERMITE. 
 �34s[Frances M. Weesner]
�33s 
  
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�6v�2v�32sIsopycnic   �21m �33s 
    The line of intersection of an atmospheric isopycnic surface with some 
other surface, for instance, a surface of constant elevation or of constant 
atmospheric pressure. An isopycnic surface is a surface in which the density 
of the air is constant. Such surfaces are also designated isosteric surfaces. 
On a surface of constant pressure, isopycnics coincide with isotherms, because 
on such a surface, density is a function solely of temperature. On a 
constant-pressure surface, isopycnics lie close together when the field is 
strongly baroclinic and are absent when the field is barotropic.  See  
BAROCLINIC FIELD; BAROTROPIC FIELD; SOLENOID (METEOROLOGY). 
 �34s[Frederick Sanders]
�33s 
  
A700000001766�254s�5v�1v939
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�6v�2v�32sIsoquinoline   �21m �33s 
    One of a group of organic compounds containing a benzene ring fused to the 
3,4 positions of pyridine, having the structure shown. Isoquinoline is a 
representative member of the group. Quinoline produced from coal tar contains 
approximately 1 - 4% of isoquinoline, and it is an important source of the 
latter material. Many plant alkaloids, especially those in the cactus, opium, 
and curare groups, are isoquinoline derivatives.  See  ALKALOID; HETEROCYCLIC 
COMPOUNDS; QUINOLINE. 
                             ******FORMULA****** 
  
    Isoquinoline is a colorless, odorous, liquid with bp 243.3DGC (469.9DGF) 
and mp 26.5DGC (79.7DGF). Its stability to acid, base, or heat is high. With 
some exception, the reactions of isoquinoline have analogous counterparts in 
the reactions of quinoline. Substitution reactions such as nitration, 
sulfonation, bromination, and mercuration are observed. 
 �34s[Walter J. Gensler]
�33s 
  
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�6v�2v�32sIsostasy   �21m �33s 
    The principle that variations in the height of the Earth's surface are 
compensated by mass distributions beneath, leading to a state of purely 
hydrostatic pressure, independent of geographic position, at some moderate 
depth beneath sea level. For ideal local compensation, any column of given 
cross-sectional area through the Earth's crust, from the surface down to the 
level at which the hydrostatic state is reached, contains the same total mass. 
 �34s[George D. Garland]
�33s 
  
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�6v�2v�32sIsotach   �21m �33s 
    A line along which the speed of the wind is constant. Isotachs are 
customarily represented on surfaces of constant elevation or atmospheric 
pressure, or in vertical cross sections. The closeness of spacing of the 
isotachs is indicative of the intensity of the wind shear on such surfaces. 
The term isovel is used synonymously with the term isotach.  See  WIND. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sIsotachophoresis �21m �33s
    A variant of electrophoresis in which ionic species move with equal
velocity in the presence of an electric field. The isotachophoretic condition
is maintained when particles of different mobilities form boundaries within
the solution so that the most mobile ions form the leading edge of the moving
sample, followed by the less mobile ions in the order of their mobility. The
reason that the ions with different mobility can move with the same velocity
is that the electric field in each of the regions is inversely proportional to
the mobility of the ionic species in order to maintain a constant current
throughout the sample. Isotachophoresis has a number of important analytical
and preparative applications which are featured by its advantages of high
resolving power, sensitivity, speed, and the ability to concentrate rather
than to dilute the components which are being analyzed. See ELECTROPHORESIS.
 �34s[Bennie R. Ware]
�33s

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�6v�2v�32sIsothermal chart   �21m �33s 
    A map showing the distribution of air temperature (or sometimes sea 
surface or soil temperature) over a portion of the Earth's surface, or at some 
level in the atmosphere. On it, isotherms are lines connecting places of equal 
temperature. The temperatures thus displayed may all refer to the same 
instant, may be averages for a day, month, season, or year, or may be the 
hottest or coldest temperatures reported during some interval. 
    Isothermal charts are drawn daily in major weather forecasting centers; 
5-day, 2-week, and monthly charts are used regularly in long-range 
forecasting; mean monthly and mean annual charts are compiled and published by 
most national weather services, and are presented in standard books, on, for 
example, climate, geography, and agriculture.  See  AIR TEMPERATURE; WEATHER 
MAP.  
 �34s[Arnold Court]
�33s 
  
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�6v�2v�32sIsothermal process �21m �33s
    A thermodynamic process which occurs with a heat addition or removal rate
just adequate to maintain constant temperature. The change in the internal
energy per mole U accompanying a change in volume in an isothermal process is

U2 - U1 = INTEGRATION (V1 to V2) [T(CURLY DP/CURLY DT)(in terms of V) - P]dV

given by the equation below, where T is the temperature, P is the pressure,
and V is the volume per mole. The integral in the equation is zero for an
ideal gas, and approximately zero for a condensed phase (solid or liquid) for
which the volume changes very little with pressure. Thus, in both these cases,
U2 = U1. For real gases, the integral is nonzero, and the internal energy
change is computed using the equation of state of the gas in the equation.
See  CHEMICAL THERMODYNAMICS; THERMODYNAMIC PROCESSES.
 �34s[Stanley I. Sandler]
�33s

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�6v�2v�32sIsotone   �21m �33s 
    One of two or more atoms which display a constant difference A - Z between 
their mass number A and their atomic number Z. Thus, despite differences in 
the total number of nuclear constituents, the numbers of neutrons in the 
nuclei of isotones are the same. The numbers of naturally occurring isotones 
provide useful evidence concerning the stability of particular neutron 
configurations. For example, the relatively large number (six and seven, 
respectively) of naturally occurring 50- and 82-neutron isotones suggests that 
these nuclear configurations are especially stable. On the other hand, from 
the fact that most atoms with odd numbers of neutrons are anisotonic, one may 
conclude that odd-neutron configurations are relatively unstable.  See  
NUCLEAR STRUCTURE. 
 �34s[Henry E. Duckworth]
�33s 
  
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�6v�2v�32sIsotope �21m �33s
    One of two or more nuclidic species of an element having identical number
of protons (Z) in the nucleus but different number of neutrons (N). Isotopes
differ in mass but chemically are the same element. All naturally occurring
elements have radioactive isotopes, and the majority have at least one stable
nuclide. Some elements which occur in nature, such as uranium, are radioactive
but have isotopes with long half-lives. Of the 83 elements present on Earth in
significant amounts, 20 possess only a single stable nuclide and are referred
to as mononuclidic or anisotopic. The others have 2 to 10 stable isotopes.
    Isotopic abundance usually refers to the isotopic composition of a
naturally occurring terrestrial element. Some elements are observed to vary in
isotopic composition. The variability ranges from a few per mil to a percent
or two, although variations greater than this are observed in some samples.
This variation occurs for several reasons. In the lighter elements - hydrogen,
lithium, and boron, for instance - the isotopes differ enough in mass and to
some extent in chemical reactivity that processes of distillation or chemical
exchange between different chemical compounds of the element can produce
significant differences in isotopic composition. Indeed, exchange reactions
are used in the case of hydrogen, lithium, boron, carbon, nitrogen, and oxygen
to separate isotopes of these elements on a relatively large scale.
    Isotopes of certain elements possess unique or peculiar properties, and
their separation or enrichment is desirable. Deuterium, (sup 2)H, which occurs
in an abundance of about 0.16% in terrestrial hydrogen, is useful for moderating
neutrons in heavy-water reactors and is expected to form the fuel of fusion
reactors in the future. Very large quantities of deuterated water have been
produced by a variety of distillative, exchange, and electrolysis processes.
The desirable fissionable isotope of uranium, (sup 2) (sup 3) (sup 5)U, is
enriched by gaseous diffusion in uranium hexafluoride gas in very large plants.
See DEUTERIUM; HEAVY WATER; ISOTOPE (STABLE) SEPARATION.
    Of the biologically important elements, only carbon and hydrogen have
radioisotopes of sufficiently long half-lives to be used in tracer studies in
living organisms. Studies of metabolism, drug utilization, and other reactions
in living organisms are best done with stable isotopes like (sup 1) (sup 3)C,
(sup 1) (sup 5)N, (sup 1) (sup 8)O, and (sup 2)H. Compounds are tagged by
introducing a high concentration of the isotope into the molecular structure,
and the metabolized products are studied using the mass spectrometer to measure
the altered isotopic ratios.  See  RADIOACTIVE TRACER.
 �34s[A. E. Cameron]
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�6v�2v�32sIsotope dilution techniques   �21m �33s 
    The introduction of radioisotopes into stable isotopes of an element in 
order to make volume, mass, and age measurements of the element. The technique 
may best be described by giving a simple example. If knowledge of the total 
amount of water present in a container is desired, a small amount of labeled 
water, D2O or T2O, is added to the H2O in the container. For the purposes of 
the measurement, this amount can be considered to be 100%. After the labeled 
deuterium or tritium is completely mixed with the water in the  container, a 
sample of the mixture of unit volume, perhaps 1 cmsup3, is removed, and the 
amount of labeled water in it determined. If this amount is found to be a%, 
then the total amount of water in the container will be 100/a. 
    In other applications the mass of an element may be determined. In this 
case the radioisotope is introduced and mixed, a sample is withdrawn, and both 
the amount of radioisotope and the amount of stable chemical present are 
measured. The ratio of these two amounts, called the specific activity, can be 
expressed as % dose/mg. For example, sodium-24 is administered to a patient 
and the specific activity of sodium in the blood (the amount of labeled sodium 
divided by the amount of stable sodium) determined. This permits measurement 
of the total amount of exchangeable sodium within the body. 
 �34s[Gordon L. Brownell]
�33s 
  
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�6v�2v�32sIsotope shift   �21m �33s 
    A small difference between the different isotopes of an element in the 
transition energies corresponding to a given spectral line transition. For a 
spectral line transition between two energy levels a and b in an atom or ion 
with atomic number Z, the small difference DELTAEsubasubb = Esubasubb(A') 
Esubasubb(A) in the transition energy between isotopes with mass numbers A' 
and A is the isotope shift. It consists largely of the sum of two 
contributions, the mass shift (MS) and the field shift (FS), also called the 
volume shift. The mass shift is customarily divided into a normal mass shift 
and a specific mass shift; each is proportional to the fractional mass 
difference (A' - A)/A'A. The normal mass shift is a reduced mass correction 
that is easily calculated for all transitions. The specific mass shift is 
produced by the correlated motion of different pairs of atomic electrons and 
is, therefore, absent in one-electron systems. The field shift is produced by 
the change in the finite size and shape of the nuclear charge distribution 
when neutrons are added to the nucleus.  See  ATOMIC STRUCTURE AND SPECTRA; 
NUCLEAR STRUCTURE. 
 �34s[Peter M. Koch]
�33s 
  
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�6v�2v�32sIsotope (stable) separation   �21m �33s 
    The physical separation of different stable isotopes of an element from 
one another. Many chemical elements always occur in nature as a mixture of 
several isotopes. The isotopes of any given element have identical chemical 
properties, but there are slight differences in their physical properties 
because of the differences in mass of the individual isotopes. Thus it is 
possible to separate physically the isotopes of an element to produce material 
of isotopic composition different from that which occurs in nature. Although 
these separation processes are all quite difficult and expensive to carry out, 
they are not inherently different from the usual operations employed in the 
chemical process industries.  See  ISOTOPE. 
    The separation of isotopes is particularly important in the nuclear energy 
field because individual isotopes may have completely different nuclear 
properties. For example, uranium-235 is used as a fuel for nuclear chain 
reactors, heavy water (deuterium oxide) is used as a neutron moderator in 
nuclear chain reactors, and deuterium gas is a possible fuel for thermonuclear 
reactors. Separated isotopes are also used widely for research concerning the 
structure and properties of the nucleus.  See  DEUTERTUM; HEAVY WATER; NUCLEAR 
FUELS; RADIOISOTOPE. 
    The process which is best suited for separating the isotopes of a given 
element depends upon the mass of the element and the desired quantity of 
separated material. Research quantities of separated isotopes are best 
prepared by electromagnetic separation in a mass spectrometer. When moderate 
quantities of a separated isotope are desired, thermal diffusion may be used. 
    In the large-scale separation of stable isotopes, the best processes are 
those which have the highest thermodynamic efficiencies. Reversible processes 
involving distillation and chemical exchange are best for separating the light 
isotopes such as deuterium. For heavy isotopes such as those of uranium, 
however, no appreciable separation is obtained by the reversible processes and 
some type of irreversible process such as gaseous diffusion must be used. 
 �34s[Richard M. Stephenson]
�33s 
  
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�6v�2v�32sIsotopic irradiation   �21m �33s 
    The subjection of a material to radiation from radioactive isotopes 
(radioisotopes). The radiation from radioisotopes produces essentially the 
same effect as the radiation from electron linear accelerators and other 
high-voltage particle accelerators, and the choice of which to use is based 
primarily on convenience and cost. Although the radioisotope radiation source 
does not require the extensive and complex circuitry necessary for a 
high-voltage radiation source, its radiation is always present and requires 
elaborate shielding for health protection and specialized mechanisms for 
bringing the irradiated objects into and out of the radiation beam. Further, 
the radiation output decreases with time according to the half-life of the 
radioisotope, which must therefore be replaced periodically.  See  PARTICLE 
ACCELERATOR.  
    The two main radioisotopes for industrial processing are cobalt-60 with a 
half-life of 5.261 years and an average gamma-ray energy of 1.25 MeV, and 
cesium-137 with a half-life of 30.3 years and a gamma-ray energy of 0.662 MeV. 
The application of industrial irradiation is increasing and includes 
cross-linking plastics, curing polymeric insulations on wires and cables, 
sterilizing medical devices, and food preservation. 
    Radioisotopes are used in the treatment of cancer by radiation. 
Encapsulated sources are used in two ways: the radioisotopes may be external 
to the body and the radiation allowed to impinge upon and pass through the 
patient (teletherapy), or the radiation sources may be placed within the body 
(brachytherapy). For teletherapy purposes cobalt-60 is the most commonly used 
isotope. Some cesium-137 irradiators have been built, but cesium-137 radiation 
is not as penetrating as that from cobalt-60. Radium-226 is now being replaced 
with cesium-137 sources as a brachytherapy encapsulated source. Iridium-192 is 
also being used for brachytherapy applications. Iodine-125 or gold-198 seeds 
are put directly in a tumor and permanently left in place. 
    For some medical applications the radioisotope is dispersed in the body; 
the most commonly used is iodine-131. When the radioisotope is administered 
either orally or intravenously in a highly purified form, it goes to the 
thyroid, where certain forms of thyroid disorders and cancers can be treated 
by the radiation.  See  RADIOACTIVITY AND RADIATION APPLICATIONS; 
RADIOISOTOPE; RADIOLOGY. 
 �34s[Peter R. Almond]
�33s 
  
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�6v�2v�32sIsotopic spin   �21m �33s 
    A quantum-mechanical variable or quantum number applied to hadrons, the 
strongly interacting fundamental particles, and compounds of hadrons (such as 
nuclear states) to facilitate consideration of the consequences of the charge 
independence of the strong (nuclear) forces. This variable is rarely labeled 
isobaric spin or isospin, and most commonly I-spin. 
    The many strongly interacting elementary particles (hadrons) and the 
compounds of these particles, such as nuclei, are observed to form sets or 
multiplets such that the members of the multiplet differ in their 
electromagnetic charge and electromagnetic properties but are otherwise almost 
identical. For example, the neutron and proton, with electric charges of zero 
and plus one fundamental unit (of the magnitude of the electronic charge), 
form a set of two such states. The pions, one with a unit of positive charge, 
one with zero charge, and one with a unit of negative charge, form a set of 
three. It appears that if the effects of electromagnetic forces and the 
closely related weak nuclear forces (responsible for BETA-decay) are 
neglected, leaving only the strong forces effective, the different members of 
such a multiplet are equivalent and cannot be distinguished in their strong 
interactions. The strong interactions are thus independent of the different 
electric charges held by different members of the set; they are 
charge-independent. See  ELEMENTARY PARTICLE; FUNDAMENTAL INTERACTIONS; HADRON. 
 �34s[Robert K. Adair]
�33s 
  
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�6v�2v�32sIsotropy (physics)   �21m �33s 
    A body is said to be isotropic if its physical properties are not 
dependent upon the direction in the body along which they are measured. A body 
displaying isotropy has only one refractive index, one dielectric constant, 
and so on. Most but not all liquids and aggregates made up of many small 
crystals randomly oriented in space are isotropic in all their properties. 
Depending on their symmetry, single crystals may or may not be isotropic with 
respect to a given property.  See  ANISOTROPY (PHYSICS); ELASTICITY. 
 �34s[David Turnbull]
�33s 
  
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�6v�2v�32sIsozyme   �21m �33s 
    Many enzyme preparations, even after crystallization, still consist of a 
number of distinct proteins, each capable of catalyzing the same reaction. 
These different enzymes, originating from the same organism or even from the 
same cells, have been named isozymes. They can usually be separated from each 
other by electrophoretic or chromatographic techniques. For example, in many 
tissues the oxidation of lactic acid to pyruvic acid is found to be catalyzed 
by five different isozymes. These isozymes differ not only in electrophoretic 
mobility but also in some catalytic properties. See ENZYME. 
 �34s[Daniel Wellner]
�33s 
  
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�6v�2v�32sItch   �21m �33s 
    Itch sensations are believed to be mediated by a set of unmyelinated nerve 
fibers different from those serving pain. Itch can be evoked by mechanical, 
electrical, and chemical stimuli. Certain chemical stimuli are the most 
effective in producing itch, and these may be externally applied to the skin 
or released within the skin under certain conditions. A naturally occurring 
substance is histamine, which is released from mast cells in the skin 
following local injury or in response to foreign substances that trigger an 
allergic reaction. Some chemicals produce itch by causing histamine to be 
released in the skin, while others act independently to evoke itch and can do 
so in regions of skin previously depleted of histamine. See CUTANEOUS 
SENSATION; HISTAMINE; SENSATION; TOUCH. 
 �34s[Rolf Landshoff]
�33s 
  
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�6v�2v�32sIxodides   �21m �33s 
    A suborder of the Acarina comprising the ticks. Ticks differ from mites, 
their nearest relatives, in their larger size and in having a pair of 
breathing pores, or spiracles, behind the third or fourth pair of legs. They 
have a gnathosoma, or head, which consists of a base, a pair of palps, and a 
rigid, elongated, ventrally toothed hypostome which anchors the parasite to 
its host. They also have a pair of protrusible cutting organs, or chelicerae, 
which permit the insertion of the hypostome. The 600 or so known species are 
all bloodsucking, external parasites of vertebrates including amphibians, 
reptiles, birds, and mammals. Ticks are divided into three families, 
Argasidae, Ixodidae (see illustration), and Nuttalliellidae.  
    In contrast to argasids, Ixodidae have a scutum covering most of the 
dorsal surface of the male but only the anterior portion of females, nymphs, 
and larvae. They are known as the hard ticks. The sexes are thus markedly 
dissimilar.  
    The numerous tick-borne diseases of animals cause vast economic losses, 
especially in tropical and subtropical regions. Aside from carrying disease, 
several species are extremely important pests of humans and animals. Heavy 
infestations of ticks produce severe anemia in livestock, and even death, from 
loss of blood alone.  See  ACARINA; FIEVRE BOUTONNEUSE; Q FEVER; QUEENSLAND 
TICK TYPHUS; ROCKY MOUNTAIN SPOTTED FEVER; SIBERIAN TICK TYPHUS; SOUTH AFRICAN 
TICK-BITE FEVER; TICK FEVER; TICK PARALYSIS. 
 �34s[Glen M. Kohls]
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�6v�2v�32sJade   �21m �33s 
    A name that may be applied correctly to two distinct minerals. The two 
true jades are jadeite and nephrite. In addition, a variety of other minerals 
are incorrectly called jade. Idocrase is called California jade, dyed calcite 
is called Mexican jade, and green grossularite garnet is called Transvaal or 
South African jade. The most commonly encountered jade substitute is the 
mineral serpentine. It is often called ``new jade'' or ``Korean jade.'' The 
most widely distributed and earliest known true type is nephrite, the less 
valuable of the two. Jadeite, the most precious of gemstones to the Chinese, 
is much rarer and more expensive.  See  JADEITE; NEPHRITE. 
 �34s[Richard T. Liddicoat, Jr.]
�33s 
  
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�6v�2v�32sJadeite   �21m �33s 
    The monoclinic sodium aluminum pyroxene, NaAlSi2O6. Free crystals are 
rare. Jadeite usually occurs as dense, felted masses of elongated blades or as 
fine-grained granular aggregates. It has a Mohs hardness of 6.5 and a density 
of 3.25 - 3.35. It has a vitreous or waxy luster, and is commonly green but 
may also be white, violet, or brown. 
    Jadeite is always found in metamorphic rocks. It is associated with 
serpentine at Tawmaw, Burma; Kotaki, Japan; and San Benito County, California. 
It occurs in metasedimentary rocks of the Franciscan group in California and 
in Celebes. It is found in Tibet; Yunan Province, China; and Guatemala. 
    Jadeite is the more cherished of the two jade minerals, because of the 
more intense colors it displays. It is best known in the lovely intense green 
color resembling that of emerald.  See  JADE; PYROXENE. 
 �34s[Lawrence Grossman]
�33s 
  
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�6v�2v�32sJahn-Teller effect   �21m �33s 
    A small distortion in the lattice structure of certain crystal defects 
and, in some cases, of entire crystals, and also in the structure of certain 
molecules, which reduces the symmetry and removes electronic degeneracy. 
    The Jahn-Teller effect is most commonly observed in the optical and 
microwave spectra of point defects and localized impurity centers in solids. 
Such a center has a certain point symmetry, that is, there is a group of 
coordinate transformations which leave the center and its surroundings 
unaltered. If this symmetry is high enough, some of the electronic states of 
the center will be orbitally degenerate. The Jahn-Teller theorem states that 
any such degenerate system is unstable against small distortions of the 
lattice framework which remove the degeneracy.  See  ATOMIC STRUCTURE AND 
SPECTRA; CRYSTAL; DEGENERACY (QUANTUM MECHANICS); MOLECULAR STRUCTURE AND 
SPECTRA.  
 �34s[M. D. Sturge]
�33s 
  
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�6v�2v�32sJamming   �21m �33s 
    Intentional generation of interfering signals by powerful transmitters as 
a countermeasure intended to block a communication or radar system or to 
impair its effectiveness appreciably. For example, radio broadcasts or radio 
messages can be jammed by beaming a more powerful signal on the same frequency 
at the area in which reception is to be impaired, using carefully selected 
noise modulation to give maximum impairment of intelligibility of reception.  
See  ELECTRONIC WARFARE. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sJapanning   �21m �33s 
    In the industrial sense, the finishing of metal objects with a black 
baking varnish that consists of a hard asphalt such as Gilsonite. Modern 
technology has made the practice obsolete. The term survives as an art form. 
Lacquers and high-gloss varnishes are hand-applied in layers of varying 
thickness for unusual effect to wood, ceramic, and metal creations in a 
variety of hues, often with filigree tracing of white or highlight color.  See 
  LACQUER; VARNISH. 
 �34s[C. Martinson ;C. W. Sisler]
�33s 
  
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�6v�2v�32sJasper   �21m �33s 
    An opaque, impure type of massive fine-grained quartz that typically has a 
tile-red, dark-brownish-red, brown, or brownish-yellow color. Jasper has been 
used since ancient times as an ornamental stone, chiefly of inlay work, and as 
a semiprecious gem material.  See  GEM; QUARTZ. 
    Jasper has a smooth conchoidal fracture with a dull luster. The specific 
gravity and hardness are variable; both values approach those of quartz. The 
color of jasper often is variegated in banded, spotted, or orbicular types. 
 �34s[Clifford Frondel]
�33s 
  
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�6v�2v�32sJaundice   �21m �33s 
    The yellow staining of the skin and mucous membranes associated with the 
accumulation of bile pigments, such as bilirubin, in the blood plasma. Bile 
pigments are the normal result of the metabolism of blood pigments, and are 
normally excreted from the blood into the bile by the liver. An increase in 
circulating bile pigments can, therefore, come about through increased 
breakdown of blood (hemolytic jaundice), through lack of patency of the bile 
ducts (obstructive jaundice), through inability or failure of the liver to 
clear the plasma (parenchymal jaundice), or through combinations of these.  
Jaundice occurs when the level of these circulating pigments becomes so high 
that they are visible in the skin and mucous membranes.  See  GALLBLADDER; 
LIVER.  
    Hemolytic jaundice results from certain morbid states that include various 
hemolytic anemias, hemolysis resulting from incompatible blood transfusion, 
severe thermal or electric injuries, or introduction of hemolytic agents into 
the bloodstream. Similar jaundice occurs in pulmonary infarction. 
    Chronic obstructive jaundice may be brought about through a variety of 
means. In the infant there may be a severe maldevelopment of the bile ducts, 
while in the adult obstructive jaundice is most commonly caused by impaction 
of a gallstone in the ducts, or benign and malignant tumors of the 
gallbladder, bile ducts, pancreas, or lymph nodes. 
    A wide variety of diseases exists in which part of the jaundice can be 
accounted for by actual damage to liver cells. This group comprises such 
conditions as inflammations of the liver, including viral hepatitis, Weil's 
disease, syphilis, parasitic infestations, and bacterial infections; toxic 
conditions, including poisoning and in a broader sense the toxemias associated 
with severe systemic diseases; tumorous conditions; and other miscellaneous 
conditions, the most common of which is congestive heart failure. 
 �34s[Rolla B. Hill]
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�6v�2v�32sJava   �21m �33s 
    An island in Sundaland. Java is one of the richest agricultural islands of 
the tropics; it is 670 mi (1080 km) long and varies from 24 to 120 mi (39 to 
190 km) in width, with an area of 48,800 misup2 (132,800 kmsup2). The 
mountains, which are most prominent in the southern portion of the island, 
were formed during the late Tertiary folding. The junction of the fold 
mountains with the northern lowlands, which are an extension of the Sunda 
Shelf, represents a fundamental structural line that is parallel to the length 
of the island and to the main structural trends. Intense volcanic activity at 
the end of the Pliocene and in the Quaternary resulted in the formation of 
more than 20 volcanoes, of which 13 are active. Petroleum is found in a belt 
extending from Semarang through the island of Madura. 
    Lying close to the Equator, between 6 and 9DGS latitude, the lowlands have 
high, even temperatures. Djakarta's mean monthly temperature ranges from 78 to 
79.7DGF (25.6 to 26.5DGC), and its average relative humidity is 78% during the 
driest month. Occasional frosts occur at elevations above 4500 ft (1400 m). 
The heaviest rainfall is in the mountains and the western lowlands. Eastern 
Java has a pronounced dry season from July through September. See EAST INDIES. 
 �34s[M. G. Audley-Charles]
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�6v�2v�32sJava man   �21m �33s 
    The Indonesian human fossils belonging to the species Homo erectus. The 
first remains of Java man were discovered in 1890 - 1891 by Eugene Dubois in 
central Java and consist of a skull cap, a fragment of a lower jaw, a few 
teeth, and a femur. Originally called ``Pithecanthropus erectus'' (meaning 
erect ape-man), these and related fossils were later recognized as 
sufficiently similar to modern humans to be called Homo.  See  FOSSIL MAN. 
 �34s[Erik Trinkaus]
�33s 
  
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�6v�2v�32sJerboa   �21m �33s 
    The common name for 25 species of rodents which make up the family 
Dipodidae. All species occur in desert or semiarid regions of Asia; three 
species extend into North Africa. All of these animals are adapted for 
jumping, and the hindlegs and feet are extremely long (see illustration). The 
most common species is Jaculus jaculus, the lesser Egyptian jerboa.  See  
RODENTIA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sJet flow   �21m �33s 
    A local high-velocity stream in a relatively stationary surrounding fluid. 
Fluid may be caused to flow in a jet for a variety of reasons. Propulsion of a 
body through a fluid may be by jet propulsion, by a propeller, or by a pump or 
compressor.  
    A jet may be formed by flow out of a closed conduit, downstream from a 
propeller, or through an orifice or nozzle. The formation of a jet requires 
that a force be exerted on the fluid in the direction of the jet. An equal and 
opposite force is exerted on the machine causing the jet, and this is the 
propulsive force in the case of an airplane or a ship.  See  JET PROPULSION. 
 �34s[Victor L. Streeter]
�33s 
  
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�6v�2v�32sJet fuel   �21m �33s 
    Fuel blended from the light distillates fractionated from crude petroleum. 
All jet fuels must meet the stringent performance requirements of aircraft 
turbine engines and fuel systems, which demand extreme cleanliness and freedom 
from oxidation deposits in high-temperature zones. Combustors require fuels 
that atomize and ignite at low temperatures, burn with adequate heat release 
and controlled radiation, and produce neither smoke nor attack of hot turbine 
parts. The operation of the aircraft in long duration flights at high altitude 
imposes a special requirement of good low-temperature flow behavior. 
 �34s[W. G. Dukek]
�33s 
  
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�6v�2v�32sJet propulsion   �21m �33s 
    Propulsion of a body by means of force resulting from discharge of a fluid 
jet. This fluid jet issues from a nozzle and produces a reaction (Newton's 
third law) to the force exerted against the working fluid in giving it 
momentum in the jet stream. Turbojets, ramjets, and rockets are the most 
widely used jet-propulsion engines.  See  RAMJET; ROCKET PROPULSION; TURBOJET. 
    In each of these propulsion engines a jet nozzle converts potential energy 
of the working fluid into kinetic energy. Hot high-pressure gas escapes 
through the nozzle, expanding in volume as it drops in pressure and 
temperature, thus gaining rearward velocity and momentum. This process is 
governed by the laws of conservation of mass, energy, and momentum and by the 
pressure-volume-temperature relationships of the gas-state equation.  See  
FLUID DYNAMICS; JET FLOW; NOZZLE. 
 �34s[J. W. Blanton]
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�6v�2v�32sJet stream   �21m �33s 
    A relatively narrow, fast-moving wind current flanked by more slowly 
moving currents. Jet streams are observed principally in the zone of 
prevailing westerlies above the lower troposphere and in most cases reach 
maximum intensity, with regard both to speed and to concentration, near the 
tropopause. At a given time, the position and intensity of the jet stream may 
significantly influence aircraft operations because of the great speed of the 
wind at the jet core and the rapid spatial variation of wind speed in its 
vicinity. Lying in the zone of maximum temperature contrast between cold air 
masses to the north and warm air masses to the south, the position of the jet 
stream on a given day usually coincides in part with the regions of greatest 
storminess in the lower troposphere, though portions of the jet stream occur 
over regions which are entirely devoid of cloud. 
 �34s[Frederick Sanders]
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�6v�2v�32sJetty   �21m �33s 
    A structure extending out from the shore at a river mouth or harbor 
entrance, located parallel to and at one or both sides of a navigation 
channel, at sites where littoral drift causes formation of bars across channel 
entrances. Jetties are used to keep a channel open by preventing littoral 
drift and by increasing velocity of flow in the channel. See COASTAL 
ENGINEERING; RIVER ENGINEERING. 
 �34s[Edward J. Quirin]
�33s 
  
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�6v�2v�32sJet velocity   �21m �33s 
    The velocity of the engine exhaust gases relative to the exhaust nozzle. 
In ideal air-breathing cycles, it is assumed that the exhaust gas mass rate 
equals the inlet air mass rate, the mass of fuel burned being neglected, and 
that the exhaust gases are expanded to ambient pressure in the nozzle. Under 
these conditions the thrust of an engine is directly proportional to the 
airflow rate and to the difference between the jet and vehicle velocities; 
thrust is greatest at zero flight speed and becomes zero when the flight speed 
is the same as the jet velocity. 
    In the non-air-breathing rocket cycle, thrust of a rocket is independent 
of flight speed. Rather, at constant mass flow, thrust is directly 
proportional to nozzle exhaust velocity plus a pressure times area term. See 
PROPULSION; ROCKET ENGINE. 
 �34s[Robert R. Hibbard]
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�6v�2v�32sJet (gemology)   �21m �33s 
    A black, opaque material that takes a high polish. Jet has been used for 
many centuries for ornamental purposes. It is a compact variety of lignite 
coal. It has a refractive index of 1.66, a hardness of 3 - 4 on Mohs scale, 
and a specific gravity of 1.30 - 1.35. Jet is compact and durable, and can be 
carved or even turned on a lathe. The principal source is Whitby, England, 
where it occurs in hard shales.  See  GEM; LIGNITE. 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sJewel bearing   �21m �33s 
    A bearing used in quality timekeeping devices, gyros, and instruments, 
usually made of synthetic corundum (crystallized Al2O3) which is more commonly 
known as ruby or sapphire. The extensive use of such bearings in the design of 
precision devices is mainly due to the outstanding qualities of the material. 
Sapphire's extreme hardness imparts to the bearing excellent wear resistance, 
as well as the ability to withstand heavy loads without deformation of shape 
or structure. The crystalline nature of sapphire lends itself to very fine 
polishing and this, combined with the excellent oil- and lubricant-retention 
ability of the surface, adds to the natural low-friction characteristics of 
the material. Ruby has the same properties as sapphire. See ANTIFRICTION 
BEARING; GEM; GYROSCOPE; WATCH. 
 �34s[Robert M. Schultz]
�33s 
  
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�6v�2v�32sJohne's disease   �21m �33s 
    A specific enteritis, or inflammation of the gastrointestinal tract, of 
cattle, sheep, and deer, caused by the bacterium Mycobacterium 
paratuberculosis. The disease is spread by animals having eaten the droppings 
of sick animals. Animals that have Johne's disease must be destroyed because 
they rarely recover and present a focus of infection for other animals in the 
herd. See TUBERCULOSIS. 
 �34s[Gardner Middlebrook]
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�6v�2v�32sJoint (anatomy)   �21m �33s 
    The contact surface between two individual bones, also known as an 
articulation, whose detailed structure depends largely upon the movement 
between the bones. Articulations can be grouped according to the intervening 
tissues: diarthrosis, when cartilaginous articular pads are present and an 
articular cavity with a capsule separates the two bones; and synarthrosis, 
when the connecting material is continuous. The synarthrosis can be divided 
into three types: synchondrosis (cartilage between the bones), syndesmosis 
(collagen fibers between the bones), and synostosis (bone is continuous). 
Articulations may be freely movable, of limited movement (amphiarthrosis), or 
rigid (some sutures). Bones that are completely fused to one another are 
ankylosed, in which the articulation is obliterated. 
    The type of motion permitted by various joints may be restricted to one 
kind of movement or may be a combination of movements depending largely upon 
the configuration of the joint surfaces, intervening tissues, and ligaments 
associated with the articulation; in many joints, the ligaments are the major 
or sole limitations to the movements of bones. Some individuals possess a 
wider range of motion, particularly if the joints are malformed or diseased or 
if the ligaments vary in length or position. Moreover the ligaments provide 
most of the strength against tensile forces acting on the joint. See LIGAMENT; 
SKELETAL SYSTEM. 
    About one person in 16 who is over 15 years old has some form of joint 
disturbance. The most common conditions are forms of arthritis, which cause 
the inflammation or degeneration of joint structures. See ARTHRITIS; GOUT; 
RHEUMATISM.  
 �34s[Walter J. Bock]
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�6v�2v�32sJoint (geology)   �21m �33s 
    A fracture that traverses a rock and does not show any discernible 
displacement of one side of the fracture relative to the other. The term joint 
refers primarily to the actual fracture as represented by a fine line or trace 
marking the intersection of the fracture and rock surfaces. Commonly, however, 
the joint is represented superficially by a cleft or fissure resulting from 
weathering, mechanical separation, or by one face of the fracture on an 
outcrop (see illustration). 
    Joints are the most abundant structures in the Earth's crust and are 
present everywhere. They constitute structural inhomogeneities in rock bodies 
and as such are influential in determining details of form in topographic 
relief. Locally, systematic joints determine the azimuth of topographic 
features such as hills, valleys, stream courses, and cliff faces and 
underground features such as cave passages. 
    At present no one theory for the origin of systematic joints is accepted 
universally. Most geologists believe systematic joints are related genetically 
to folding, faulting, and uplift and attribute them to tangential 
compressional or tensional forces acting regionally or locally in the Earth's 
crust.  
 �34s[Robert A. Hodgson]
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�6v�2v�32sJoint (structures)   �21m �33s 
    The surface at which two or more mechanical or structural components are 
united. Whenever parts of a machine or structure are brought together and 
fastened into position, a joint is formed. See STRUCTURAL CONNECTIONS. 
    Mechanical joints can be fabricated by a great variety of methods, but all 
can be classified into two general types, temporary (screw, snap, or clamp, 
for example), and permanent (brazed, welded, or riveted, for example). 
 �34s[William H. Crouse]
�33s 
  
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�6v�2v�32sJosephson effect   �21m �33s 
    The passage of paired electrons (Cooper pairs) through a weak connection 
(Josephson junction) between superconductors, as in the tunnel passage of 
paired electrons through a thin dielectric layer separating two 
superconductors.  
    Quantum-mechanical tunneling of Cooper pairs through a thin insulating 
barrier (on the order of a few nanometers thick) between two superconductors 
was theoretically predicted by Brian D. Josephson in 1962. Josephson found 
that a current of paired electrons (supercurrent) would flow in addition to 
the usual current that results from the tunneling of single electrons. 
Josephson predicted that if the current did not exceed a limiting value (the 
critical current), there would be no voltage drop across the tunnel barrier. 
This zero-voltage current flow is known as the dc Josephson effect. Josephson 
also predicted that if a constant nonzero voltage were maintained across the 
tunnel barrier, an alternating supercurrent would flow through the barrier in 
addition to the dc current produced by the tunneling of unpaired electrons. 
This phenomenon is known as the ac Josephson effect. See TUNNELING IN SOLIDS. 
    Josephson pointed out that the magnitude of the maximum zero-voltage 
supercurrent would be reduced by a magnetic field. In fact, the magnetic field 
dependence of the magnitude of the critical current is one of the more 
striking features of the Josephson effect. Circulating supercurrents flow 
through the tunnel barrier to screen an applied magnetic field from the 
interior of the Josephson junction just as if the tunnel barrier itself were 
weakly superconducting. The screening effect produces a spatial variation of 
the transport current, and the critical current goes through a series of 
maxima and minima as the field is increased. 
    Josephson junctions, and instruments incorporating Josephson junctions, 
are used in applications for metrology at dc and microwave frequencies, 
frequency metrology, magnetometry, measurement of absolute temperatures below 
about 1 K, detection and amplification of electromagnetic signals, and other 
superconducting electronics such as high-speed analog-to-digital converters 
and computers. A Josephson junction, like a vacuum tube or a transistor, is 
capable of switching signals from one circuit to another; a Josephson tunnel 
junction is the fastest switch known. Josephson junction circuits are capable 
of storing information. Finally, because a Josephson junction is a 
superconducting device, its power dissipation is extremely small, so that 
Josephson junction circuits can be packed together as tightly as fabrication 
techniques permit. All the basic circuit elements required for a Josephson 
junction computer have been developed. See LOW-TEMPERATURE THERMOMETRY; 
SUPERCONDUCTING DEVICES; SUPERCONDUCTIVITY. 
 �34s[Louis B. Holdeman]
�33s 
  
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�6v�2v�32sJoule's law �21m �33s
    A quantitative relationship between the quantity of heat produced in a
conductor and an electric current flowing through it. As experimentally
determined and announced by J. P. Joule, the law states that when a current of
voltaic electricity is propagated along a metallic conductor, the heat evolved
in a given time is proportional to the resistance of the conductor multiplied
by the square of the electric intensity. Today the law would be stated as H =
RIsup2, where H is rate of evolution of heat in watts, the unit of heat being
the joule; R is resistance in ohms; and I is current in amperes. This
statement is more general than the one sometimes given that specifies that R
be independent of I. Also, it is now known that the application of the law is
not limited to metallic conductors. See ELECTRIC HEATING; OHM'S LAW.
 �34s[Llewellyn G. Hoxton /John W. Stewart]
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�6v�2v�32sJ particle   �21m �33s 
    An elementary particle with an unusually long lifetime and large mass 
which does not fit into any of the schemes for classifying the large number of 
previously known particles. The discovery in 1974 of J particles in 
proton-proton (p,p) and electron-positron (e-,esupp) collisions was of great 
importance for elementary particle physics.  See  ELEMENTARY PARTICLE. 
    A family of particles has been identified which, by their emission and 
absorption of monochromatic gamma rays, transform into each other and into the 
J particle. The energy spectrum of the J-particle family is very similar to 
the positronium spectrum. The similarity between the J-particle spectrum and 
the positronium spectrum, together with the fact that the lifetime of the J 
particle is about 1000 to 10,000 times longer than that of other known 
elementary particles, indicates that the J particle is a bound state of a new 
type of quark and antiquark. This idea was first proposed by S. Glashow, who 
called it the charm quark. The J particle therefore is a charmonium state.  
See  CHARM; POSITRONIUM; QUARKS. 
 �34s[Samuel C. C. Ting]
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�6v�2v�32sJuglandales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of three families: the Juglandaceae with a little over 50 species, 
the Picrodendraceae with 3 species, and the Rhoipteleaceae with only 1 
species. Within its subclass the order is sharply set off by its compound 
leaves. Juglans (walnut and butternut) and Carya (hickory, including the 
pecan, C. illinoensis) are familiar genera of the Juglandaceae. See 
HAMAMELIDAE; HICKORY; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sJuncales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Commelinidae of the class Liliopsida (monocotyledons). The order 
consists of the family Juncaceae, with about 300 species, and the family 
Thurniaceae, with only 3. Within its subclass the order is marked by its 
reduced, mostly wind-pollinated flowers and capsular fruits with one to many 
anatropous ovules per carpel. The flowers have six sepals arranged in two more 
or less similar whorls, both sets chaffy and usually brown or green. See 
COMMELINIDAE; FLOWER; LILIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sJunction detector   �21m �33s 
    A device in which detection of radiation takes place in or near the 
depletion region of a reversebiased semiconductor junction. The electrical 
output pulse is linearly proportional to the energy deposited in the junction 
depletion layer by the incident ionizing radiation. See CRYSTAL COUNTER; 
IONIZATION CHAMBER. 
    Introduced into nuclear studies in 1958, the junction detector, or more 
generally, the nuclear semiconductor detector, revolutionized the field. In 
the detection of both charged particles and gamma radiation, these devices 
typically improved experimentally attainable energy resolutions by about two 
orders of magnitude over that previously attainable. To this they added 
unprecedented flexibility of utilization, speed of response, miniaturization, 
freedom from deleterious effects of extraneous electromagnetic (and often 
nuclear) radiation fields, low-voltage requirements, and effectively perfect 
linearity of output response. They are now used for a wide variety of diverse 
applications. They are used for general analytical applications, giving both 
qualitative and quantitative analysis in the microprobe and the scanning 
transmission electron microscopes. They are used in medicine, biology, 
environmental studies, and the space program. In the last category they 
continue to play a very fundamental role, ranging from studies of the 
radiation fields in the solar system to the composition of extraterrestrial 
surfaces. See PARTICLE DETECTOR; SEMICONDUCTOR. 
 �34s[James M. McKenzie]
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�6v�2v�32sJunction diode   �21m �33s 
    A semiconductor rectifying device in which the barrier between two regions 
of opposite conductivity type produces the rectification. Junction diodes are 
used in computers, radio and television, brushless generators, battery 
chargers, and electrochemical processes requiring high direct current and low 
voltage. Lower-power units are usually called semiconductor diodes, and the 
higher-power units are usually called semiconductor rectifiers. See 
SEMICONDUCTOR.  
    Junction diodes are classified by the method of preparation of the 
junction, the semiconductor material, and the general category of use of the 
finished device. By far the great majority of modern junction diodes use 
silicon as the basic semiconductor material. Germanium material was used in 
the first decade of semiconductor diode technology, but has given way to the 
all-pervasive silicon technology, which allows wider temperature limits of 
operation and produces stable characteristics more easily. Other materials are 
the group III - V compounds, the most common being gallium arsenide, which is 
used where its relatively large band-gap energy is needed. 
    In silicon units nearly all categories of diodes are made by self-masked 
diffusion, as shown in illustration a. Exceptions are diodes where special 
control of the doping profile is necessary. In such cases, a variety of doping 
techniques may be used, including ion implantation, alloying with variable 
recrystallization rate, silicon transmutation by neutron absorption, and 
variable-impurity epitaxial growth. The mesa structure shown in illustration b 
is used for some varactor and switching diodes if close control of capacitance 
and voltage breakdown is required. See DETECTOR; ELECTRONIC SWITCH; ION 
IMPLANTATION; SEMICONDUCTOR DIODE; RECTIFIER. 
 �34s[Lloyd P. Hunter]
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�6v�2v�32sJunction transistor   �21m �33s 
    A transistor in which emitter and collector barriers are formed by pn 
junctions between semiconductor regions of opposite conductivity type. These 
junctions are separated by a distance considerably less than a 
minority-carrier diffusion length, so that minority carriers injected at the 
emitter junction will not recombine before reaching the collector barrier and 
therefore be effective in modulating the collector-barrier impedance. Junction 
transistors are widely used both as discrete devices and in integrated 
circuits. The discrete devices are found in the high-power and high-frequency 
applications. Silicon is the most widely used semiconductor material, although 
germanium is still used for some applications. See TRANSISTOR. 
    Most modern transistors are fabricated by the silicon self-masked planar 
double-diffusion technique. The structure of a planar diffused epitaxial 
transistor is shown in section in the illustration. In this structure both 
collector and emitter junctions are formed by diffusion of impurities from the 
top surface. In modern technology the base and emitter diffusions are carried 
out in two steps: a predeposition step, in which a very thin layer of heavily 
doped oxide is chemically deposited over the open surface of the silicon in 
the hole opened in the masking oxide; and a drive-in diffusion step, in which 
the deposited dopant is diffused into the silicon at a higher temperature than 
that used for the predeposition. The chemical predeposition step is being 
replaced by ion implantation directly through the oxide. See ION IMPLANTATION. 
    Silicon planar technology is used in fabricating integrated circuit chips. 
The general form of the transistor structure displayed in the illustration is 
used in integrated circuits. Such a structure is used for diodes as well as 
transistors since, for example, it is necessary only to connect the base and 
collector contacts to use the collector junction as a diode. See INTEGRATED 
CIRCUITS.  
 �34s[Lloyd P. Hunter]
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�6v�2v�32sJupiter   �21m �33s 
    The largest planet in the solar system, and the fifth in the order of 
distance from the Sun. It is visible to the naked eye, except for short 
periods when in near conjunction with the Sun. Usually it is the second 
brightest planet in the sky; only Mars at its maximum luminosity and Venus 
appear brighter. Jupiter is brighter than Sirius, the brightest star. 
    Through an optical telescope Jupiter appears as an elliptical disk, 
strongly darkened near the limb, and crossed by a series of bands parallel to 
the equator (Fig. 1). Even fairly small telescopes show a great deal of 
complex structure in the bands and disclose the rapid rotation of the planet.  
The details observed, however, do not correspond to the solid body of a planet 
but to clouds in its atmosphere, and the rotation period varies markedly with 
latitude.  
    Apart from the constantly changing details of the belts, some permanent or 
semipermanent markings have been observed to last for decades or even 
centuries, with some fluctuations in visibility. The most conspicuous, and 
permanent marking is the great Red Spot (Fig. 2), intermittently recorded 
since the middle of the 17th century and observed continually since 1878, when 
its striking reddish color attracted general attention. However, the vivid red 
coloration remains unexplained. 
  
    Atmosphere.  The atmospheric pressure is probably a bit less than 3 atm 
(300 kilopascals). The atmosphere must be largely composed of hydrogen and 
helium. The radiometrically determined temperature of the visible disk of 
Jupiter, about -225 PLUS/MINUS 55DGF (130 PLUS/MINUS 30 K), is in fairly good 
agreement with the value theoretically estimated from the assumption that the 
visible cloud layer is mainly composed of ice crystals of solidified ammonia 
(about -170DGF or 160 K). Pioneer 10 and 11 flybys showed that Jupiter's zones 
and belts consist of gases at different altitudes and temperatures. The 
coloring agents of the bands are unknown. 
  
    The Voyager probes showed that the Jovian clouds are affected by the same 
varieties of weather patterns that produce terrestrial weather, though on a 
much larger scale. Measurements of the velocities of surface features show 
that the Jovian atmosphere consists of regular patterns of alternately east 
and west wind velocities. While the major atmospheric features no doubt 
persist due to inertia, it is unknown what agent powers the Jovian weather 
system, though undoubtedly it comes from within the planet. 
  
    Interior composition and structure.  Jupiter is primarily made up of 
liquid and metallic hydrogen. Its composition is similar to that of the Sun, 
namely 10 times as much hydrogen as helium (by numbers of atoms), and 
undoubtedly reflects the primordial composition of the solar system. Under the 
extreme pressures of the interior of the planet, the hydrogen would remain in 
a liquid metallic state, and this material is thought to make up the inner 60% 
of the radius of the planet. Above this layer it is speculated that there lies 
liquid hydrogen, with the pressure and temperature lowering sufficiently to 
become gaseous in nature at approximately 600 mi (1000 km) below the cloud 
tops. It seems likely that the core of Jupiter is made up of heavy 
iron-silicate molten material of several times the Earth's total mass and has 
a probable radius of a few thousand miles. 
  
    Jovian ring.  Voyager 2 discovered a faint ring about the planet on its 
closest passage. The brightest part of this ring is approximately 4000 mi 
(6000 km) in diameter, but the ring itself may possibly extend all the way to 
the planetary surface. 
  
    Satellites.  Jupiter has 15 known satellites of which the four largest, I 
Io, II Europa, III Ganymede, and IV Callisto, discovered by Galileo in 1610, 
are by far the most important. The four Galilean satellites are of fifth and 
sixth stellar magnitudes and would be visible to the naked eye if they were 
not so close to the much brighter parent planet. All the others are faint 
telescopic objects. 
 �34s[Elaine M. Hendry]
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�6v�2v�32sJurassic   �21m �33s 
    The system of rocks deposited during the middle part of the Mesozoic Era. 
The Jurassic System is normally underlain by the Triassic and overlain by the 
Cretaceous System. It was named after the Jura Mountains in Switzerland and 
consists mainly of sedimentary rocks. The duration of the Jurassic Period, 
which ended about 135,000,000 years ago, is estimated to be about 60,000,000 
years.  
    Different opinions have been expressed on the configuration and position 
of continents and oceans during the Jurassic Period. The existence of large 
continents or land bridges in the region of present-day oceans is now denied 
by many authorities. A tentative reconstruction of Gondwanaland for the 
Callovian, a stage of the Middle Jurassic, is shown in the illustration. 
    The Precambrian shields, which form the oldest and most stable parts of 
the Earth's crust, were land areas subject to erosion during the Jurassic 
Period. Thus in North America the Canadian Shield was land bordered by seas 
both to the north and west, and the Scandinavian Shield was surrounded by seas 
in Late Jurassic times. Smaller areas underlain by older rocks were lands, the 
coastlines of which are indicated in many places by conglomerates and 
sandstones. See PRECAMBRIAN. 
    The Jurassic flora is characterized by ferns, which occur abundantly in 
certain regions, and by numerous gymnosperms. The presence of angiosperms is 
indicated by pollen grains. Calcareous algae built reefs in some marine areas. 
    Reptiles were represented by a great number of different forms. On the 
land the dinosaurs were the dominating forms. Skeletons of the first bird, 
Archaeopteryx, were found in the Late Jurassic lithographic stones at 
Solenhofen, Bavaria. Practically all groups of modern fishes were present in 
the Jurassic. See DINOSAUR. 
  
    The most important marine invertebrates of the Jurassic are the ammonites. 
Their short vertical and wide horizontal range and their appearance in almost 
every facies have made them the best guide fossils of the Jurassic. Land 
invertebrates are represented by such insects as flies, butterflies, and 
moths. Among the crustaceans, ostracods are known from both marine and 
fresh-water beds and are used for age determinations. 
    Iron ores of Jurassic origin are present in western Europe. Upper Jurassic 
coal is known from Scotland, the Lofoten Islands, and northeast Greenland. 
Highly bituminous rocks and oil shales occur in western Europe. Lime and 
cement are made from calcareous beds in England and Germany and bricks from 
the clays. Many good building stones from various beds are quarried in the 
Jurassic limestones of western Europe. Petroleum is obtained from various 
Jurassic horizons in the United States, Canada, Mexico, Brazil, France, 
Germany, Morocco, and Saudi Arabia. 
 �34s[Hans Frebold]
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�6v�2v�32sJute   �21m �33s 
    A natural fiber obtained from two Asiatic species, Corchorus capsularis 
and C. olitorius, of the plant family Malvaceae. These are tall, slender, 
half-shrubby annuals. The fibers are not very strong and deteriorate quickly 
in the presence of moisture, especially salt water. Jute is made into gunny, 
burlap bags, sacks for wool, potato sacks, covers for cotton bales, twine, 
carpets, rug cushions, curtains, and a lineoleum base. Most of the commercial 
supply comes from plants grown in the Ganges and Brahmaputra valleys in 
Bangladesh and India. See FIBER CROPS; MALVALES; NATURAL FIBER. 
 �34s[Elton G. Nelson]
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�6v�2v�32sKale   �21m �33s 
    Either of two cool-season biennial crucifers, Brassica oleracea var. 
acephala and B. fimbriata, of Mediterranean origin and belonging to the plant 
order Capparales. Kale is grown for its nutritious green curled leaves which 
are cooked as a vegetable. Distinct varieties (cultivars) are produced in 
Europe for stock feed. Kale is a minor vegetable in the United States. 
Virginia is an important producing state.  See  CAPPARALES. 
 �34s[H. John Carew]
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�6v�2v�32sKaliophilite   �21m �33s 
    A rare mineral tectosilicate found in volcanic rocks high in potassium and 
low in silica. Kaliophilite is one of three polymorphic forms of KAlSiO4. It 
crystallizes in the hexagonal system in prismatic crystals. The hardness is 6 
on Mohs scale, and the specific gravity is 2.61. The principal occurrence of 
kaliophilite is at Monte Somma, Italy. See KALSILITE; SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sKalsilite   �21m �33s 
    A rare mineral found in volcanic rocks at Mafuru, in southwest Uganda. It 
is one of the three polymorphic forms of KAlSiO4. The mineral is hexagonal. 
The specific gravity is 2.59. In index of refraction and general appearance in 
thin section it resembles nepheline and is difficult to distinguish from it.  
See  KALIOPHILITE; SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sKame   �21m �33s 
    A round hill or small knoll of sand and gravel, a few meters to more than 
100 m high (330 ft). It is one of a family of stratified glacial sediments 
formed by meltwater in contact with a disintegrating ice sheet. Melting 
stagnant ice produces large volumes of water carrying boulders, sand, silt, 
and clay into holes melted in the wasting glacier. When the ice melts 
completely, the sediment is left standing as small hills. Mixtures of loose 
rock debris (till) carried above in the melting ice may be lowered onto the 
kame. Kames are common wherever ice sheets melted, as in New England, New 
York, the midwestern United States, the British Isles, and Sweden. 
 �34s[Sidney E. White]
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�6v�2v�32sKanamycin   �21m �33s 
    An antibiotic produced by an actinomycete which has found utility in 
medicine. The producing organism is Streptomyces kanamyceticus. The antibiotic 
has bactericidal  activity against a wide variety of gram-positive, 
gram-negative, and acid-fast bacteria. In general, its antimicrobial spectrum 
is similar to that of neomycin. Resistance to kanamycin develops slowly, and 
in stepwise fashion, except for the mycobacteria, which develop resistance 
rapidly.  See  DRUG RESISTANCE; NEOMYCIN. 
 �34s[Joseph Lein]
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�6v�2v�32sKangaroo   �21m �33s 
    The name for a number of Australian marsupials that are members of the 
family Macropodidae. This family also includes the wallabies. The kangaroos 
and their relatives occur principally in Australia, but are found in Tasmania 
and New Guinea as well. 
    Kangaroos have a long, thick tail that is used as a balancing organ, and 
enlarged hindlegs that are adapted for jumping in many species. The forelimbs 
are quite short, except in arboreal species such as the blacktree kangaroo 
(Dendrolagus ursinus) and its relatives, in which all four limbs are about the 
same length. The two largest species are the red kangaroo (Macropus rufus) and 
the great gray kangaroo (M. giganteus). 
    Kangaroos usually have one offspring each year. After the uterine 
gestation period of about 6 weeks, the very immature young is born and crawls 
into the marsupium. After an uninterrupted period of 2 months, it ventures out 
to find food and then returns to the safety of the marsupium. It may seek the 
protection of the pouch for up to 9 months.  See  MARSUPIALIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sKaolinite   �21m �33s 
    The principal mineral of the kaolinite group of clay minerals. Kaolinite 
is important in the ceramics industry because of its excellent firing 
properties and refractoriness. It is also used extensively as a filler in 
rubber products and for coating and filling paper products.  See  CERAMICS; 
CLAY MINERALS. 
    The kaolinite type of mineral forms under acid conditions at low 
temperatures and pressures. It can form under hydrothermal conditions or 
through the alteration of other clay minerals. Kaolinite is a principal 
component of lateritic-type soils. When calcium is present in the environment, 
the formation of kaolinite is retarded. The present-day marine environment 
does not favor the formation of kaolinite. The presence of a kaolinitic marine 
sediment is evidence of a kaolinitic source area, since this mineral does not 
form in the sea. It is also indicative of relatively rapid accumulation of 
material.  
 �34s[Floyd M. Wahl ;Ralph E. Grim]
�33s 
  
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�6v�2v�32sKapitza resistance   �21m �33s 
    A resistance to the flow of heat across the interface between liquid 
helium and a solid. The Kapitza resistance R sub K is given by the equation 
below, where dQ/dt is the heat flow, A is the area of the interface, and 

           *** R sub K = A DELTA T / dQ/dt     (cm sup 2 K/W) ***

DELTA T is the temperature discontinuity across the interface. The inverse of 
this equation is known as the Kapitza conductance. The effect was discovered in 
1941 by P. L. Kapitza.
    Kapitza resistance is an important problem in most experiments at less than 
2 DG F above absolute zero (1 K), since it hampers temperature equilibrium 
between refrigerator, sample, and thermometer. It is often the dominant thermal 
resistance at these temperatures. From a theoretical standpoint, Kapitza 
resistance provides information on the interaction of low-energy phonons and 
other excitations with each other and with surfaces. See CONDUCTION (HEAT); 
LIQUID HELIUM; LOW-TEMPERATURE PHYSICS; PHONON; QUANTUM SOLID.  
 �34s[R. Radebaugh]
�33s 
  

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�6v�2v�32sKapok tree �21m �33s
    Also called the silk-cotton tree (Ceiba pentandra), a member of the bombax
family (Bombacaceae). The tree has a bizarre growth habit and produces pods
containing seeds covered with silky hairs called silk cotton. It occurs in the
American tropics, and has been introduced into Java, Philippine Islands, and
Ceylon. The silk cotton is the commercial kapok used for stuffing cushions,
mattresses, and pillows.  See  MALVALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sKarman vortex street   �21m �33s 
    A double row of line vortices in a fluid. Under certain conditions a 
Karman vortex street is shed in the wake of bluff cylindrical bodies when the 
relative fluid velocity is perpendicular to the generators of the cylinder, as 
illustrated. This periodic shedding of eddies occurs first from one side of 
the body and then from the other, an unusual phenomenon because the oncoming 
flow may be perfectly steady. Vortex streets can often be seen, for example, 
in rivers downstream of the columns supporting a bridge. They can be created 
by steady winds blowing past smokestacks, transmission lines, bridges, 
missiles about to be launched vertically, and pipelines aboveground in the 
desert.  See  VORTEX. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
  
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�6v�2v�32sKarst topography   �21m �33s 
    Characteristic associations of minor, third-order, destructively developed 
land features resulting from subaerial and underground solution of limestone 
under conditions of humid climate. These pattern features are progressively 
carved into second-order structural forms, such as plains, plateaus, or even 
hilly and mountainous uplands, containing limestone layers at or near the 
surface. Virtually all limestone formations are products of biological or 
chemical precipitation of calcium carbonate, CaCO3, from aqueous solution. 
They are therefore susceptible in varying degrees to resolution in surface 
waters and groundwaters, particularly when such waters are charged with carbon 
dioxide, CO2, from vegetative sources or when the waters are under pressure.  
See  LIMESTONE. 
    The sequence of changes in landforms of karst regions, from initiation of 
sinkholes through stages of their increase in number, enlargement, and 
coalescence, with collapse of caverns, and eventual wasting away of 
intervening hills, leaves broad-bottomed lowlands (uvalas) and scattered 
surviving elevations. Although this karst cycle is still subject to 
theoretical debate, the concept seems valid because different stages of 
landform development are identifiable in various karsts of differing ages.  
See  CAVE; GEOMORPHOLOGY; WEATHERING PROCESSES. 
 �34s[J. Harlen Bretz]
�33s 
  
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�6v�2v�32sKata thermometer �21m �33s
    An instrument for measuring low air velocities in air-circulation
problems. An alcohol thermometer with a large bulb is heated above 100DGF
(37.8DGC), and its time to cool from 100 to 95 DEG F (37.8 to 35.0 DEG C),
or some other interval above the ambient temperature, is measured. This
time interval is a measure of the air current at the location. See
AIR-VELOCITY MEASUREMENT.		[Herbert Fox]

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�6v�2v�32sKelvin bridge �21m �33s
    A specialized version of the Wheatstone bridge network designed to
eliminate, or greatly reduce, the effect of lead and contact resistance and
thus permit accurate measurement of low resistance. The circuit shown in the
figure accomplishes this by effectively placing relatively
high-resistance-ratio arms in series with the potential leads and contacts of
the low-resistance standards and the unknown resistance. In this circuit R sub A
and R sub B are the main ratio resistors, R sub a and R sub b the auxiliary
ratio, R sub x the unknown, R sub s the standard, and R sub y a heavy copper
yoke of low resistance connected between the unknown and standard resistors.
    As with the Wheatstone bridge, the Kelvin bridge for routine engineering
measurements is constructed using both adjustable ratio arms and adjustable
standards. However, the ratio is usually continuously adjustable over a short
span, and the standard is adjustable in appropriate steps to cover the
required range.  See  BRIDGE CIRCUIT; RESISTANCE MEASUREMENT; WHEATSTONE
BRIDGE.
 �34s[Charles E. Applegate]
�33s


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�6v�2v�32sKelvin's circulation theorem   �21m �33s 
    A theorem in fluid dynamics that pertains to an incompressible, inviscid 
fluid. A direct consequence of this theorem is a great simplification in 
understanding and analyzing a large class of fluid flows called irrotational 
flows.  
    Kelvin's theorem states that in an incompressible, inviscid fluid the 
circulation along a closed curve, always consisting of the same fluid 
particles, does not change with time. An important consequence of this theorem 
relates to fluid motions starting from rest or uniform motion; in such flows 
the circulation is initially zero for every possible closed curve and hence 
remains equal to zero thereafter according to Kelvin's theorem.  See  
BERNOULLI'S THEOREM; FLUID FLOW; LAPLACE'S IRROTATIONAL MOTION. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
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�6v�2v�32sKelvin's minimum-energy theorem   �21m �33s 
    A principle of fluid mechanics which states that the irrotational motion 
of an incompressible, inviscid fluid occupying a simply connected region has 
less kinetic energy than any other fluid motion consistent with the boundary 
condition of zero relative velocity normal to the boundaries of the region.  
See  D'ALEMBERT'S PARADOX; FLUID FLOW; LAPLACE'S IRROTATIONAL MOTION. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
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�6v�2v�32sKenyapithecus   �21m �33s 
    The genus name given by L. S. B. Leakey to several fossil Hominoidea from 
the Miocene deposits of Kenya. The older specimens, referred to as 
``Kenyapithecus africanus,'' are now included in the genus Dryopithecus, which 
includes most of the Miocene and Pliocene Pongidae, or apes. Moderate-sized 
with short faces, these primates are the East African representatives of the 
genus Ramapithecus and are considered by many to be the earliest known 
hominids.  See  FOSSIL MAN; RAMAPITHECUS. 
 �34s[Erik Trinkaus]
�33s 
  
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�6v�2v�32sKeratitis   �21m �33s 
    Any inflammation involving the cornea of the eye. It may involve the 
cornea alone (keratitis), or more commonly, both cornea and conjunctiva 
(keratoconjunctivitis). The various forms of keratitis and 
keratoconjunctivitis together constitute a major portion of all eye disease; 
keratitis is second only to cataract and equal to glaucoma as a cause of 
blindness.  See  KERATOCONJUNCTIVITIS. 
    Keratitis may be caused by mechanical injury, infectious agents, extremes 
of heat or cold, drying, radiation, corrosive or irritating chemicals, or 
allergy, or it may occur in association with diseases affecting the entire 
body, such as a vitamin deficiency. Keratitis often accompanies other eye 
diseases, such as glaucoma. The degree of permanent impairment of vision is 
determined primarily by the extent of the injury rather than by the nature of 
the injurious agent.  See  GLAUCOMA. 
 �34s[W. Robert Adams]
�33s 
  
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�6v�2v�32sKeratoconjunctivitis   �21m �33s 
    A viral disease characterized by an acute conjunctivitis, followed by 
keratitis, which in some cases leaves round, superficial opacities in the 
cornea for up to 2 years. It is also known as shipyard eye. Sporadic cases and 
epidemics have occurred in many countries, particularly in shipyards and 
industrial plants. The etiological agent is adenovirus type 8.  See  KERATITIS. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sKernite �21m �33s
    A hydrated borate mineral with chemical composition Na2B4O6(OH)2 CTR DOT
3H2O. It occurs only very rarely in crystals but is found most commonly in
coarse, cleavable masses and aggregates. It is colorless to white; colorless and
transparent specimens tend to become chalky white on exposure to air.
    Boron compounds are used in the manufacture of glass, especially in glass
wool used for insulation purposes. They are also used in soap, in porcelain
enamels for coating metal surfaces, and in the preparation of fertilizers and
herbicides.  See  BORATE MINERALS.
 �34s[Cornelis Klein]
�33s

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�6v�2v�32sKerogen �21m �33s
    A name given to the complex organic matter present in carbonaceous shales
and oil shales. It is insoluble in all common solvents but on destructive
distillation yields oil, gas, and acidic and basic compounds. Kerogen is
formed by the conversion of plant and animal remains. It is the most common
form of organic carbon on Earth, and it has been estimated that there is 1000
times as much kerogen as coal.	See  OIL SHALE.
 �34s[Irving A. Breger]
�33s

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�6v�2v�32sKerosine   �21m �33s 
    A refined petroleum fraction used as a fuel for heating and cooking, jet 
engines, lamps, and as a base for insecticides. Kerosine, known also as lamp 
oil, is recovered from crude oil by distillation. Specifications are 
established for specific grades of kerosine by government agencies and by 
refiners. For use in lamps, for example, a highly paraffinic oil is desired 
because aromatics and naphthenes give a smoky flame. In order to avoid 
atmospheric pollution, sulfur content must be low; and a minimum flash point 
of 100DGF (38DGC) is desirable to reduce explosion hazards.  See  DISTILLATE 
FUEL; JET FUEL. 
 �34s[Harold C. Ries]
�33s 
  
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�6v�2v�32sKerr effect   �21m �33s 
    Electrically induced birefringence that is proportional to the square of 
the electric field. When a substance (especially a liquid or a gas) is placed 
in an electric field, its molecules may become partly oriented. This renders 
the substance anisotropic and gives it birefringence, that is, the ability to 
refract light differently in two directions. This effect, which was discovered 
in 1875 by John Kerr, is called the electrooptical Kerr effect, or simply the 
Kerr effect.  See  ANISOTROPY. 
    When a liquid is placed in an electric field, it behaves optically like a 
uniaxial crystal with the optical axis parallel to the electric lines of 
force. The Kerr effect is usually observed by passing light between two 
capacitor plates inserted in a glass cell containing the liquid. Such a device 
is known as a Kerr cell or optical Kerr shutter. Light passing through the 
medium normal to the electric lines of force (that is, parallel to the 
capacitor plates) is split into two linearly polarized waves. 
    In certain crystals there may be an electrically induced birefringence 
that is proportional to the first power of the electric field. This is called 
the Pockels effect. In these crystals the Pockels effect usually overshadows 
the Kerr effect, which is nonetheless present. In crystals of cubic symmetry 
and in isotropic solids (such as glass) only the Kerr effect is present.  See  
ELECTROOPTICS.  
 �34s[Michel A. Duguay]
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�6v�2v�32sKetone �21m �33s
    One of a class of chemical compounds of the general formula given below.
In the formula,
			     ******FORMULA******

R and R' are alkyl, aryl, or heterocyclic radicals. The groups R and R' may be
the same or different or incorporated into a ring as in (cyclopentanone):
			     ******FORMULA******

The ketones, acetone and methyl ethyl ketone, are used as solvents. Ketones
are important intermediates in the syntheses of organic compounds.
    By common nomenclature rules, the R and R' groups are named, followed by
the word ketone, for example, CH3CH2COCH2CH3 (diethyl ketone), CH3COCH (CH3)2
(methyl isopropyl ketone), and C6H5COC6H5 (diphenyl ketone). The nomenclature
of the International Union of Pure and Applied Chemistry uses the hydrocarbon
name corresponding to the maximum number of carbon atoms in a continuous chain
in the ketone molecule, followed by ``one,'' and preceded by a number
designating the position of the carbonyl group in the carbon chain. The first
two ketones above are named 3-pentanone and 3-methyl-3-butanone.
    The lower-molecular-weight ketones are colorless liquids. Acetone and
methyl ethyl ketone are miscible with water; the water solubility of the
higher homologs decreases with increasing number of carbon atoms. Because of
their characteristic odors, various ketones are of use in the flavoring and
perfumery industry.
    Addition to the carbonyl group is the most important type of ketone
reaction. Ketones are generally less reactive than aldehydes in addition
reactions. Methyl ketones are more reactive than the higher ketones because of
steric group effects.
 �34s[Paul E. Fanta]
�33s

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�6v�2v�32sKetosis �21m �33s
    An excessive accumulation of ketone bodies in the tissues, blood, and
urine. Ketone bodies are formed by the oxidation of fatty acids by the liver.
See  KETONE.
    When a disturbance in carbohydrate metabolism is present, ketone bodies
are produced in excess. The ketone bodies, by virtue of their acid properties,
hold an equivalent amount of base, thus producing a form of acidosis. This is
seen clinically in certain cases of diabetes, in starvation, occasionally in
hyperthyroidism, and in a variety of other disease states in which there is an
absolute or relative decrease in carbohydrate availability.  See  DIABETES;
THYROID GLAND DISORDERS.
    In ketosis, the breath has a fruity odor, and there is usually evidence of
dehydration; weakness, malaise, headache, nausea, and vomiting are common.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sKey telephone system   �21m �33s 
    A system which can be installed on a customer's premises to provide access 
to more than one central-office line as well as other special line features 
such as wide area telecommunication service (WATS) and foreign exchange (FX). 
Many small businesses make use of key telephone systems (KTS), which may also 
provide communication between members of the same business community, either 
through the serving central office (CO) or private branch exchange (PBX) or an 
intercom system not associated with regular telephone lines. 
    The telephone instrument used with a key telephone system is called a key 
telephone set and is distinguished by the appearance of key switches, which 
provide the means for implementing most of the key system features. The sets 
are typically provided with 6, 10, or 20 keys (buttons) which provide access 
to the key station lines and functions. In addition, an instrument with 
provision for the pickup of 30 lines may be used as an attendant console.  See 
  SWITCHING SYSTEMS (COMMUNICATIONS); TELEPHONE SERVICE. 
 �34s[William Schiavoni]
�33s 
  
  
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�6v�2v�32sKidney   �21m �33s 
    An organ involved with the elimination of water and waste products from 
the body. In vertebrates the kidneys are paired organs located close to the 
spine dorsal in the body cavity. They consist of a number of smaller 
functional units which are called urinary tubules or nephrons. The nephrons 
open to large ducts, the collecting ducts, which open into a ureter. The two 
ureters run backward to open into the cloaca or into a urinary bladder. In 
mammals, the kidneys are bean-shaped and found between the thorax and the 
pelvis. The number, structure, and function of the nephrons vary with 
evolution and, in certain significant ways, with the adaptation of the animals 
to their various habitats. 
    In its most primitive form, found only in invertebrates, the nephron has a 
funnel opening into the coelomic cavity followed by a urinary tubule leading 
to an excretory pore. In amphibians, some of the tubules have this funnel, but 
most of the tubules have a Bowman capsule (see illustration). In all higher 
vertebrates, the nephron has the Bowman capsule, which surrounds a tuft of 
capillary loops, called the glomerulus, constituting the closed end of the 
nephron. The inner epithelial wall of the Bowman capsule is in intimate 
contact with the endothelial wall of the capillaries. The wall of the 
capillaries, together with the inner wall of the Bowman capsule, forms a 
membrane ideally suited for filtration of the blood. 
    The blood pressure in the capillaries of the glomerulus causes filtering 
of blood by forcing fluid, small molecules, and ions through the membrane into 
the lumen of Bowman's capsule. This filtrate contains some of the proteins and 
all of the smaller molecules in the blood. As the filtrate passes down through 
the tubule, the walls of the tubule extract those substances not destined for 
excretion and return them to the blood in adjacent capillaries. Many 
substances which are toxic to the organism are moved in the opposite direction 
from the blood into the tubules. The urine thus produced by each nephron is 
conveyed by the collecting duct and ureter to the cloaca or bladder from which 
it can be eliminated. 
    In all classes of vertebrates the renal arteries deliver blood to the 
glomeruli and through a second capillary net to the tubules. The major blood 
supply to the kidney tubules comes, however, from the renal portal vein, which 
is found in all vertebrates except mammals and cyclostomes. Waste products 
from the venous blood can thus be secreted directly into the urinary tubules.  
See  URINARY SYSTEM. 
 �34s[Bodil Schmidt-Nielsen]
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�6v�2v�32sKidney disorders   �21m �33s 
    Kidney disorders can be divided into congenital, glomerular, tubular, 
infectious, and neoplastic for convenience of description; the clinical 
manifestations often overlap. 
    Congenital developmental anomalies are an important group of kidney 
disorders often clinically evident soon after birth. However, sometimes the 
anomalies may not produce clinically recognizable functional changes until 
later in life. Bilateral kidney agenesis, the complete absence of both 
kidneys, is rare, and incompatible with life. Unilateral kidney agenesis is 
more frequent than bilateral and is not necessarily life-threatening. 
Underdeveloped kidneys, often referred to as hypoplasia or dysplasia, is also 
a common congenital defect. Other groups of developmental anomalies are 
location and shape abnormality and polycystic kidney. See GLOMERULONEPHRITIS. 
    Diseases principally affecting the function of the filtration ability of 
the glomeruli of the kidneys are often referred to collectively as 
glomerulonephritis. There are two common mechanisms of glomerular injury, both 
of which involve immunological reactions. 
    Acute tubular necrosis is characterized by death of the epithelium in the 
portion of the renal tubules distal to the glomeruli. Shock, crush injury, 
thermal burns, and toxic chemicals are common causes for this type of injury. 
Chemical oliguria or anuria may result due to tubular blockage or reabsorption 
of the glomerular filtrate. If the patient survives the period of decreased 
urine production, complete repair of the tubules may ensue with normal renal 
function.  
    Diseases principally affecting the renal blood vessels may ultimately have 
effects comparable to those associated with primarily glomerular or tubular 
diseases. Prolonged restriction of renal blood flow may culminate in 
glomerular and tubular degenerative changes comparable to the preceding.  See  
KIDNEY.  
 �34s[N. Karle Mottet]
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�6v�2v�32sKiln   �21m �33s 
    A device or enclosure to provide thermal processing of an article or 
substance in a controlled temperature environment or atmosphere, often by 
direct firing, but occasionally by convection or radiation heat transfer. 
Kilns are used in many different industries, and the type of device called a 
kiln varies with the industry. 
    ``Kiln'' usually refers to an oven or furnace which operates at 
sufficiently high temperature to require that its walls be constructed of 
refractory materials. The distinction between a kiln and a furnace is often 
based more on the industry than on the design of the device. Generally the 
word ``kiln'' is used when referring to high-temperature treatment of 
nonmetallic materials such as in the ceramic, the cement, and the lime 
industries. When melting is involved as in steel manufacture, the term 
``furnace'' is used, as in blast furnace and basic oxygen furnace.  See  
FURNACE.  
 �34s[Burton B. Crocker]
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�6v�2v�32sKinematics �21m �33s
    That branch of mechanics which deals with the motion of a system of
material particles without reference to the forces which act on the system.
Kinematics differs from dynamics in that the latter takes these forces into
account.  See  DYNAMICS.
    For a single particle moving in a straight line (rectilinear motion), the
motion is prescribed when the position of the particle is known as a function
of the time. Plane kinematics of a particle is concerned with the
specification of the position of a particle moving in a plane by means of two
independent variables. The kinematics of a particle in space is concerned with
the ways in which three independent coordinates may be chosen to specify the
position of the particle at a given time, and with the relations between the
first and second time derivatives of these coordinates and the components of
velocity and acceleration of the particle.
    Among the coordinate systems studied in kinematics are those used by
observers who are in relative motion. In nonrelativistic kinematics the time
coordinate for each such observer is assumed to be the same, but in
relativistic kinematics proper account must be taken of the fact that lengths
and time intervals appear different to observers moving relative to each
other.	See  RELATIVITY.
 �34s[Herbert C. Corben /Bernard Goodman]
�33s

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�6v�2v�32sKinetic energy   �21m �33s 
    The energy of a body due to its motion. A point particle with mass m and 
velocity v has kinetic energy given by Eq. (1). Any body in motion has Eq. (1) 

                        *** E = 1/2 I mv sup 2 ***                       (1)

as its translational kinetic energy. The velocity must be specified with 
respect to some inertial reference frame. 
    A rigid body can move as in Eq. (1), but in addition it can rotate about 
its center. The rotational kinetic energy E sub R, for a body with moment of 
inertia I and angular frequency OMEGA, in units of radians per second, is 
given by Eq. (2). The most general kinetic energy for a solid body in motion 
is given by Eq. (3). See ROTATIONAL MOTION. 

                   *** E sub R = 1/2 I OMEGA sup 2 ***                   (2)

           *** E sub k = 1/2 mv sup 2 + 1/2 I OMEGA sup 2 ***            (3)

 �34s[Brian DeFacio]
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�6v�2v�32sKinetics (classical mechanics)   �21m �33s 
    That part of classical mechanics which deals with the relation between the 
motions of material bodies and the forces acting upon them. It is synonymous 
with dynamics of material bodies. See DYNAMICS. 
    Kinetics proceeds by adopting certain intuitively acceptable concepts 
which are associated with measurable quantities. These essential concepts and 
the measurable quantities used for their specification are as follows: 

    1. Space configuration refers to the positions and orientations of bodies 
in a reference frame adopted by the observer. It is expressed quantitatively 
by an arbitrarily chosen set of space coordinates, of which cartesian and 
polar coordinates are examples. All space coordinates rest on the motion of 
distance measurement. 
    2. Duration is expressed quantitatively by time measured by a clock or 
comparable mechanism. 
    3. Motion refers to change of configuration with time and is expressed by 
time rates of coordinate change called velocities and time rates of velocity 
change called accelerations. The classical assumption that coordinates behave 
as analytic functions of time permits representation of velocities and 
accelerations as first and second derivatives, respectively, of the space 
coordinates with respect to time. 
    4. Inertia is an attribute of bodies implying their capacity to resist 
changes of motion. A body's inertia with respect to linear motion is denoted 
by its mass. 
    5. Momentum is an attribute proportional to both the mass and velocity of 
a body. Momentum of linear motion is expressed as the product of mass and 
linear velocity. 
    6. Force serves to designate the influence exercised upon the motion of a 
particular body by other bodies, not necessarily specified. A quantitative 
connection between the motion of a body and the force applied to it is 
expressed by Newton's second law of motion, which is discussed later. 

    Distance, time, and mass are commonly regarded as fundamental, all other 
dynamical quantities being definable in terms of them. 
    A primary objective of classical kinetics is the prediction of the 
behavior of bodies which are subject to known forces when only initial values 
of the coordinates and momenta are available. This is accomplished by use of a 
principle first recognized by Isaac Newton. Newton's statement of the 
principle was restricted to the linear motion of an idealized body called a 
mass particle, having negligible extension in space. 
    The basic dynamical law set forth by Newton and known as his second law 
states that the time rate of change of a particle's linear momentum is 
proportional to and in the direction of the force applied to the particle. 
Stated analytically, Newton's second law becomes the differential equation, 
Eq. (1), in which m represents the particle's mass, v its velocity, F the 
  
                       ***  [d (mv)] / dt = F ***                        (1)
  
applied force, and t the time. Equation (1) provides a definition of force and 
of its units if units of mass, distance, and time have previously been adopted. 
The classical assumption of constancy of mass permits Eq. (1) to be expressed 
as Eq. (2), where a represents the linear acceleration. 
  
                              *** ma = F ***                             (2)
  
    The behavior of systems composed of two or more interacting particles is 
treated by Newtonian dynamics augmented by Newton's third law of motion which 
states that when two bodies interact, the forces they exert on one another are 
equal and oppositely directed. The important laws of momentum and energy 
conservation are derivable for such systems (the latter only for forces of 
special type) and useful in solution of problems. See ACCELERATION; FORCE; 
GRAVITATION; HARMONIC MOTION; MASS; MOMENTUM; RIGID-BODY DYNAMICS; VELOCITY. 
 �34s[Russell A. Fisher]
�33s 
  
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�6v�2v�32sKinetic theory of matter   �21m �33s 
    A theory which states that the particles of matter in all states of 
aggregation are in vigorous motion. In computations involving kinetic theory, 
the methods of statistical mechanics are applied to specific physical systems. 
The atomistic or molecular structure of the system involved is assumed, and 
the system is then described in terms of appropriate distribution functions. 
The main purpose of kinetic theory is to deduce, from the statistical 
description, results valid for the whole system. The distinction between 
kinetic theory and statistical mechanics is thus of necessity arbitrary and 
vague. Historically, kinetic theory is the oldest statistical discipline. 
Today a kinetic calculation refers to any calculation in which probability 
methods, models, or distribution functions are involved.  See  STATISTICAL 
MECHANICS.  
    Kinetic calculations are not restricted to gases, but occur in chemical 
problems, solid-state problems, and problems in radiation theory. Even though 
the general procedures in these different areas are similar, there are a 
sufficient number of important differences to make a general classification 
useful.  
    In classical ideal equilibrium problems there are no interactions between 
the constituents of the system. The system is in equilibrium, and the 
mechanical laws governing the system are classical. The basic information is 
contained in the Boltzmann distribution f (also called Maxwell or 
Maxwell-Boltzmann distribution) which gives the number of particles in a given 
momentum and positional range (d sup 3 x = dxdydz, d sup 3 v = [dv sub x] 
[dv sub y] [dv sub z], where x, y, and z are coordinates of position, and 
v sub x, v sub y, and v sub z are coordinates of velocity). In Eq. (1) EPSILON 

*** f[xyz, (v sub x) (v sub y) (v sub z)] = A e sup -BETA EPSILON ***    (1)

is the energy, BETA = 1/kT (where k is the Boltzmann constant and T is the 
absolute temperature), and A is a constant determined from Eq. (2). The 

     *** INTEGRATION INTEGRATION  (d sup 3) x (d sup 3) v f = N  
                        total number of particles ***                    (2)

calculations of gas pressure, specific heat, and the classical equipartition 
theorem are all based on these relations. See BOLTZMANN STATISTICS. 
    Many important physical properties refer not to equilibrium but to 
nonequilibrium states. Phenomena such as thermal conductivity, viscosity, and 
electrical conductivity all require a discussion starting from the Boltzmann 
transport equation. If one deals with states that are near equilibrium, the 
exact Boltzmann equation need not be solved; then it is sufficient to describe 
the nonstationary situation as a small perturbation superimposed on an 
equilibrium state. See BOLTZMANN TRANSPORT EQUATION. 
    The basic classical procedure for arbitrary systems (systems with 
interactions taken into account) that allows the calculation of macroscopic 
entities is that using the partition function. 
    Classical nonideal nonequilibrium theory is the most general situation 
that classical statistics can describe. In general, very little is known about 
such systems.  
    There are quantum counterparts to the classifications just described. In a 
quantum treatment a distribution function is also used for an ideal system in 
equilibrium to describe its general properties. For systems of particles which 
must be described by symmetrical wave functions, such as helium atoms and 
photons, one has the Bose distribution, Eq. (3), where BETA = 1/kT, and A is 
  
* f[(v sub x) (v sub y) (v sub z)] = 1 / [(1/A) e sup BETA EPSILON - 1] * (3)
  
determined by Eq. (2). See BOSE-EINSTEIN STATISTICS. 
    For systems of particles which must be described by antisymmetrical wave 
functions, such as electrons, protons, and neutrons, one has the Fermi 
distribution, Eq. (4). The application to electrons as an (ideal) Fermi-Dirac 
  
* f[(v sub x) (v sub y) (v sub z)] = 1 / [(1/A) e sup BETA EPSILON + 1] * (4)
  
gas in a metal is the basis of the Sommerfeld theory of metals.  See  
FERMI-DIRAC STATISTICS; FREE-ELECTRON THEORY OF METALS; QUANTUM STATISTICS. 
 �34s[Max Dresden]
�33s 
  
A700000002411�254s�5v�1v956
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�6v�2v�32sKinetoplastida   �21m �33s 
    An order of the class Zoomastigophorea in the phylum Protozoa, also known 
as Protomastigida, containing a heterogeneous group of colorless flagellates 
possessing one or two flagella in some stage of their life cycle. These small 
organisms (5 - 89 MU in length) typically have pliable bodies. Some species 
are holozoic and ingest solid particles, while others are saprozoic and obtain 
their nutrition by absorption. 
    There is disagreement on the division of the order into families. However, 
the five or more families can be divided into two general groups (see 
illustration). The first group contains simple organisms with no distinctive 
features save one or two flagella of equal or unequal length (includes the 
families Oikomonadidae, Amphimonadidae, Monadidae, and Bodonidae). The second 
group contains organisms which have an undulating membrane in addition to one 
or two flagella (includes Trypanosomatidae, and Cryptobiidae). The most 
important family is the Trypanosomatidae, since it includes several species 
that infect humans and domestic animals with serious diseases, such as African 
sleeping sickness. See CILIA AND FLAGELLA; TRYPANOSOMATIDAE. 
 �34s[M. M. Brooke]
�33s 
  
  
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�6v�2v�32sKinorhyncha   �21m �33s 
    A phylum of free-living marine invertebrates less than 0.04 in. (1 mm) 
long. They are segmented and lack external ciliation (see illustration). 
Kinorhynchs are benthonic, so-called because they generally dwell in mud or 
sand from intertidal to deep-sea habitats. Two orders are generally 
recognized, Cyclorhagida and Homalorhagida. 
    The body is covered by a transparent cuticle secreted by an underlying 
epidermis. The cuticle is molted only in the process of juvenile growth. Three 
body regions are recognized: a head segment, a neck segment and an 11-segment 
trunk. The head is completely retractable. When everted, it extends its five 
to seven circles of recurved spines called scalids. The neck consists of 
plates called placids which function to close the anterior opening of the 
trunk when the head is retracted. Most kinorhynchs have a pair of adhesive 
tubes on the ventral surface of the third or fourth segment. 
 �34s[Robert P. Higgins]
�33s 
  
  
A700000001741�254s�5v�1v956
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�6v�2v�32sKirchoff's laws of electric circuits �21m �33s
    Fundamental natural laws dealing with the relation of currents at a
junction and the voltages around a loop. These laws are commonly used in the
analysis and solution of networks. They may be used directly to solve circuit
problems, and they form the basis for network theorems used with more complex
networks. See NETWORK THEORY.
    One way of stating Kirchoff's voltage law is: ``At each instant of time,
the algebraic sum of the voltage rise is equal to the algebraic sum of the
voltage drops, both being taken in the same direction around the closed loop.''
    Kirchoff's current law may be expressed as follows: ``At any given
instant, the sum of the instantaneous values of all the currents flowing
toward a point is equal to the sum of the instantaneous values of all the
currents flowing away from the point.'' See CIRCUIT (ELECTRICITY).
 �34s[K. Y. Tang /Robert T. Weil, Jr.]
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957
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�6v�2v�32sKite   �21m �33s 
    A tethered flying device that supports itself and the cable that connects 
it to the ground by means of the aerodynamic forces created by the relative 
motion of the wind. This relative wind may arise merely from the natural 
motions of the air or may be caused by towing the kite through the agency of 
its connecting cable. 
    The lifting force of all kites is produced by deflecting the air downward, 
the resulting change in momentum producing an upward force. To be successful, 
a kite must have an extremely low wing loading (weight/area) so that it can 
fly even on days when the wind velocity is not high. It must be completely 
stable, since the only controls available to the operator are the length of 
cable and the rate at which it is taken in or let out. Efficient design 
requires that its lift-to-drag ratio be as high as possible. See AERODYNAMIC 
FORCE; AERODYNAMICS. 
 �34s[David C. Hazen]
�33s 
  
A700000001327�254s�5v�1v957
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�6v�2v�32sKlebsiella �21m �33s
    A genus of bacteria. Three species are recognized: K. pneumoniae, K.
ozaenae, and K. rhinoscleromatis.
    K. pneumoniae may be found in soil, in water, and on plants. Frequently,
it is found as a hospital-acquired bacterium which may colonize the throat or
intestine or cause urinary tract infection, pneumonia, septicemia (mostly in
debilitated patients), and meningitis (in newborns). K. ozaenae occurs in a
certain type of chronic rhinitis called ozaena. K. rhinoscleromatis causes
rhinoscleroma, a granulomatous disease of the upper respiratory tract
occurring in Eastern Europe. See ENTEROBACTERIACEAE.
 �34s[Alexander W. C. von Graevenitz]
�33s

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�6v�2v�32sKlystron   �21m �33s 
    An evacuated electron-beam tube in which an initial velocity modulation 
imparted to electrons in the beam results subsequently in density modulation 
of the beam. A klystron is used either as an amplifier in the microwave region 
or as an oscillator. 
    For use as an amplifier, a klystron receives microwave energy at an input 
cavity through which the electron beam passes. The microwave energy modulates 
the velocities of electrons in the beam, which then enters a drift space. Here 
the faster electrons overtake the slower to form bunches. In this manner, the 
uniform current density of the initial beam is converted to an alternating 
current. The bunched beam with its significant component of alternating 
current then passes through an output cavity to which the beam transfers its 
ac energy. See MICROWAVE. 
    Klystrons may be operated as oscillators by feeding some of the output 
back into the input circuit. More widely used is the reflex oscillator in 
which the electron beam itself provides the feedback. The beam is focused 
through a cavity and is velocity-modulated there, as in the amplifier. The 
cavity usually has grids to concentrate the electric field in a short space so 
that the field can interact with a slow, low-voltage electron beam. Leaving 
the cavity, the beam enters a region of dc electric field opposing its motion, 
produced by a reflector electrode operating at a potential negative with 
respect to the cathode. The electrons do not have enough energy to reach the 
electrode, but are reflected in space and return to pass through the cavity 
again. The points of reflection are determined by electron velocities, the 
faster electrons going farther against the field and hence taking longer to 
get back than the slower ones. Reflex oscillators are used as signal sources 
from 3 to 200 GHz. They are also used as the transmitter tubes in 
line-of-sight radio relay systems and in low-power radars. 
 �34s[Richard B. Nelson]
�33s 
  
A700000001231�254s�5v�1v957
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�6v�2v�32sKnudsen number �21m �33s
    In fluid mechanics, the ratio l/L of the mean free path length l of the
molecules of the fluid to a characteristic length L of the structure in the
fluid stream. When the mean free path of the fluid particles is short relative
to the size of the object being considered, the fluid can be treated as a
continuum. If the path length between molecular encounters is comparable to or
larger than a significant dimension of the flow region, the gas must be
treated as consisting of discrete particles. See GAS DYNAMICS; STATISTICAL
MECHANICS; SUPERAERODYNAMICS.
 �34s[Frank H. Rockett]
�33s

A700000001454�254s�5v�1v957
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�6v�2v�32sKoala   �21m �33s 
    A single species, Phascolarctos cinereus, which is a member of the family 
Phalangeridae in the mammalian order Marsupialia (pouch-bearing animals). It 
is a small animal that weighs from 11 to 17 lb (5 to 8 kg) when mature. They 
are restricted to eastern Australia. Not only do they have a specialized diet 
of eucalyptus leaves, but the leaves must be of a certain age from a specific 
species of tree, and the tree must grow upon a certain type of soil. 
    The koala breeds once each season, and the usual number of offspring is 
one. It remains in the pouch for 6 months; then it clings to the back of its 
mother and is carried around in this manner until 1 year old. See MAMMALIA; 
MARSUPIALIA.  
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sKohlrabi   �21m �33s 
    A cool-season biennial crucifer, Brassica caulorapa and B. oleracea var. 
caulo-rapa, of northern European origin belonging to the plant order 
Capparales. Kohlrabi is grown for its turniplike enlarged stem, which is 
usually eaten as a cooked vegetable (see illustration). A common cooked 
vegetable in Europe, especially Germany, kohlrabi is of minor importance in 
the United States. See CAPPARALES; TURNIP. 
 �34s[H. John Carew]
�33s 
  
  
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�6v�2v�32sKolbe hydrocarbon synthesis   �21m �33s 
    The production of an alkane by the electrolysis of a water-soluble salts 
of a carboxylic acid. The carboxylate ions are discharged at the anode, 
yielding carbon dioxide and alkyl radicals which couple to form the saturated 
hydrocarbon. Hydrogen is liberated at the cathode. Good yields of alkanes are 
obtained with the straight-chain acids containing 5 - 18 carbon atoms. See 
ALKANE.  
 �34s[Louis Schmerling]
�33s 
  
A700000003365�254s�5v�1v957
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�6v�2v�32sKondo effect   �21m �33s 
    The large anomalous increase in the resistance of certain dilute alloys of 
magnetic materials in nonmagnetic hosts as the temperature is lowered. This 
behavior is contrary to the decrease in electrical resistance with temperature 
observed in almost all other systems. It is believed that the interaction of 
the conduction electrons with the spin of the magnetic impurity creates rather 
long-range correlations in the electrons in the vicinity of the impurity and 
that this strong interaction tends to inhibit conductivity. This interaction 
is an exchange interaction similar to that in ferromagnets. See FERROMAGNETISM. 
    For temperatures above a few degrees absolute, however, thermal agitation 
tends to wash out these fairly delicate correlations, and on heating the 
sample further the contribution to the resistance diminishes. The temperature 
below which this effect predominates is referred to as the Kondo temperature, 
and it depends on the magnetic impurity and the host material. 
    It has been suggested that Kondo temperatures may range from millidegrees 
to hundreds of degrees, but for materials studied so far they are below about 
30 K (-405DGF). See CURIE TEMPERATURE; RESISTIVITY. 
    The importance of studying the Kondo effect lies in the fact that it is a 
property of what is, presumably, the simplest possible magnetic system - a 
single magnetic atom in a nonmagnetic environment. It is hoped that an 
understanding of this system will provide a key to the far more complex and 
important problem of the electronic structure of magnetic materials 
themselves, one of the greatest challenges in physics. 
 �34s[Michael Fowler]
�33s 
  
A700000002632�254s�5v�1v958
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�6v�2v�32sKrebs cycle   �21m �33s 
    A sequence of enzymatic reactions that is involved in the oxidative 
metabolism of many organisms, especially those that carry out respiration. The 
cycle is also known as the citric acid cycle and the tricarboxylic acid cycle. 
Through this sequence, or pathway, the two-carbon acetyl unit, an important 
metabolic intermediate derived from a variety of substrates, can be oxidized 
to two molecules of CO2. In aerobic metabolism, two molecules of oxygen are 
used for this oxidation. Organic acids containing four, five, and six carbons 
are formed in the course of the oxidation. These are consumed and regenerated 
continually through the operation of the entire sequence. Hence, this 
oxidative pathway is a catalytic mechanism which operates in a cyclic manner 
and is known as the citric acid cycle, because this tricarboxylic acid is one 
of the metabolic intermediates. The cycle is also named after H. A. Krebs, who 
recognized its functional role. The principal reactions of the Krebs cycle are 
outlined in the illustration.  See  METABOLISM. 
  
    The enzymatic reactions of the Krebs cycle are important, not only as a 
source of energy for living cells, but also as a source of essential metabolic 
intermediates which serve as starting materials for biosynthetic processes.  
See  BIOLOGICAL OXIDATION. 
 �34s[Michael Doudoroff]
�33s 
  
A700000001223�254s�5v�1v958
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�6v�2v�32sKronig-Penney model   �21m �33s 
    An idealized, one-dimensional model of a crystal which exhibits many of 
the basic features of the electronic structure of real crystals. Consider the 
potential energy V(x) of an electron shown in the illustration with an 
infinite sequence of potential wells of depth -Vsubo and width a, arranged 
with a spacing b. The width and the curvatures of the allowed bands increase 
with energy. The Kronig-Penney model has been extended to include the effects 
of impurity atoms.  See  BAND THEORY OF SOLIDS; NONRELATIVISTIC QUANTUM THEORY. 
 �34s[Joseph Callaway]
�33s 
  
  
A700000001676�254s�5v�1v958
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�6v�2v�32sKrypton   �21m �33s 
    A gaseous chemical element, Kr, atomic number 36, and atomic weight 83.80. 
Krypton is one of the noble gases in group 0 of the periodic table. Krypton is 
a colorless, odorless, and tasteless gas. The table gives some physical 
properties of krypton. The principal use for krypton is in filling electric 
lamps and electronic devices of various types. Krypton-argon mixtures are 
widely used to fill fluorescent lamps.  See  INERT GASES. 
    The only commercial source of stable krypton is the air, although traces 
of krypton are found in minerals and meteorites. A mixture of stable and 
radioactive isotopes of krypton is produced in nuclear reactors by the 
slow-neutron fission of uranium. It is estimated that about 2 x 10-sup8% of 
the weight of the Earth is krypton. Krypton also occurs outside the Earth. 
 �34s[Arthur W. Francis]
�33s 
u1] 
  
A700000001435�254s�5v�1v959
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�6v�2v�32sKumquat   �21m �33s 
    Shrubs or small trees that are members of the genus Fortunella, which is 
one of the six genera in the group of true citrus fruits. Kumquats are 
believed to have originated in China and the Malay Peninsula, but are now 
widely grown in all citrus areas of the world. Of the several species the most 
common are F. margarita, which has oval-shaped fruit, and F. japonica, which 
has round fruit. 
    Kumquats, with their brilliant orange-colored fruits and dense green 
foliage, are highly ornamental and are most frequently grown for this reason. 
Kumquat fruits can be eaten whole without peeling; they are also used in 
marmalades and preserves and as candied fruits.  See SAPINDALES. 
 �34s[Frank E. Gardner]
�33s 
  
A700000001660�254s�5v�1v959
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�6v�2v�32sKwashiorkor disease   �21m �33s 
    A nutritional deficiency disease in infants and young children, caused 
primarily by diet in which protein is of poor quality or inadequate quantity, 
or both, and carbohydrate intake is normal or high. The disease has frequently 
been referred to as protein-calorie malnutrition or deficiency. Kwashiorkor 
occurs throughout the world, but especially among the underdeveloped areas. 
According to the World Health Organization, it is the most widespread and 
important dietary disease in the world today. 
    Clinical features consist of poor growth and development, edema, 
dyspigmentation of skin and hair, and psychological changes characterized by 
apathy and irritability. Pathological changes consist of enlarged fatty liver; 
atrophy of pancreas, salivary glands, and intestinal glands; and wasting of 
muscles.  
 �34s[Herschel Sidransky]
�33s 
  
A700000001465�254s�5v�1v959
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�6v�2v�32sKyanite   �21m �33s 
    A nesosilicate mineral, composition Al2SiO5, crystallizing in the 
triclinic system. Crystals are usually long and tabular and commonly occur in 
bladed aggregates. The specific gravity is 3.56 - 3.66. The hardness of 
kyanite, one of its interesting features, is 5 (Mohs scale) parallel to the 
length of the crystals but 7 across the length. The luster is vitreous to 
pearly, and the color is usually a shade of blue but may be white, gray, or 
green.  See  SILICATE MINERALS. 
    Kyanite crystals, some of gem quality, are found at St. Gothard, 
Switzerland. Kyanite has been mined in India and in the United States in 
Georgia and North Carolina for the manufacture of highly refractory porcelain. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
A700000000631�254s�5v�1v961
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0
�6v�2v�32sLabradorite   �21m �33s 
    A plagioclase feldspar with composition range Ab50An50 to Ab30An70 (Ab = 
NaAlSi3O8; An = CaAl2Si2O8). In some labradorite samples brilliant colors, 
much like those seen in oil films on water, result from the interference of 
light reflected at successive lamellar interfaces. See FELDSPAR; IGNEOUS ROCKS. 
 �34s[Paul H. Ribbe]
�33s 
  
A700000001421�254s�5v�1v961
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0
�6v�2v�32sLabyrinthodontia �21m �33s
    An important subclass of fossil amphibians, first known in the Late
Devonian, common in the Carboniferous and Permian, and persisting to the Late
Triassic. They are named for infoldings of the enamel of their teeth, a
primitive feature which is not universally present in labyrinthodonts and
which is shared with rhipidistean crossopterygian fishes. It is generally
accepted that the labyrinthodonts are descended from rhipidisteans.
Labyrinthodonts are usually fairly large (up to 6.5 ft or 2 m long). Some were
totally aquatic (in fresh water). Most were probably semiaquatic, while some
(especially the Dissorophidae) appeared to be terrestrial, at least as adults.
See AMPHIBIA.
 �34s[Richard P. Hall]
�33s

A700000001014�254s�5v�1v961
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0
�6v�2v�32sLabyrinthulia   �21m �33s 
    Protozoa forming a subclass of Rhizopodea with obscure relationships to 
rest of the class. There is one order, Labyrinthulida. The mostly marine, 
ovoid to spindle-shaped, uninucleate organisms secrete a network of filaments 
(slime tubes) along which they glide, usually singly. This network inspired 
the name ``net slime molds'' sometimes applied to them. The mechanisms of 
locomotion is unknown. See RHIZOPODEA. 
 �34s[Richard P. Hall]
�33s 
  
A700000000773�254s�5v�1v961
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0
�6v�2v�32sLaccolith   �21m �33s 
    A geologic name for a body of igneous rock intruded into sedimentary rocks 
in such a way that the overlying strata have been notably lifted by the force 
of intrusion. In the most simple example the laccolith is circular or oval in 
plan, the subjacent sedimentary rocks and the floor of the laccolith are 
nearly horizontal, and the roof is dome-shaped with the sedimentary rocks 
arching over the dome. 
 �34s[James A. Noble]
�33s 
  
A700000001653�254s�5v�1v961
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0
�6v�2v�32sLacquer   �21m �33s 
    A fast-drying, hard, high-gloss surface coating. Lacquers are made by 
dissolving a cellulose derivative and other modifying materials in a solvent 
and adding pigment if desired. 
    The cellulose derivative most commonly used is nitrocellulose, but a 
cellulose ester such as cellulose acetate or cellulose butyrate, or a 
cellulose ether such as ethyl cellulose is often used in formulation. Lacquers 
dry by evaporation of the solvent. They usually are applied by spray because 
of their rapid drying properties. 
    Lacquers have been used extensively as fast-drying, weather-resistant 
finishes for automobiles and as coatings for furniture and other 
factory-finished items. Their use diminished, however, as less expensive 
coating materials which improved properties appeared on the market. See 
SURFACE COATINGS. 
 �34s[C. R. Martinson ;C. W. Sisler]
�33s 
  
A700000001631�254s�5v�1v961
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0
�6v�2v�32sLacrimal gland   �21m �33s 
    A tubuloalveolar or acinous skin gland, also known as the tear gland. Two 
types occur among the vertebrates, the lacrimal proper and the Harderian. The 
eye glands drain into the nasal cavity by means of the lacrimal duct. 
    In land mammals the lacrimal gland proper is highly developed as a complex 
tubuloalveolar structure with several ducts which pour their copious fluid 
into the outer, upper part of the conjunctival sac or cavity. The tear 
substance washes across the eyeball and eventually passes through two small 
openings, one on the margin of each lid, into the lacrimal ducts. The latter 
converge to form the lacrimal sac, from which the nasolacrimal duct leads into 
the nasal passageway. Tears contain a considerable quantity of the common 
salt, sodium chloride. See EYE (VERTEBRATE); GLAND. 
 �34s[Olin E. Nelsen]
�33s 
  
A700000000610�254s�5v�1v961
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0
�6v�2v�32sLactase   �21m �33s 
    An enzyme found in mammals, honeybee larvae, and some plants. It is a 
BETA-galactosidase which hydrolyzes lactose to galactose and glucose. In 
mammals, lactase appears in the intestinal secretion from the intestinal 
villi, and exerts its effect on lactose in chyme. See ENZYME; GLUCOSE. 
 �34s[Daniel N. Lapedes]
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�6v�2v�32sLactate    �21m �33s 
    A salt or ester of lactic acid in which the acidic hydrogen of the 
carboxyl group has been replaced by a metal or an organic radical (symbol X). 
The structural formulas of the two optical isomers are given in the 
illustration. Alkali metal salts have practical applications, for example, as 
blood coagulants in calcium therapy (calcium), as antiperspirants (aluminum), 
in treatment of anemia (iron), and as plasticizers (sodium). Esters are used 
as plasticizers and as solvents for lacquers, cellulose acetate, and cellulose 
nitrate. See ESTER; LACTIC ACID. 
 �34s[Elbert H. Hadley]
�33s 
  
                             ******FORMULA****** 
  
  
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�6v�2v�32sLactation   �21m �33s 
    The function of the mammary gland providing milk nourishment to the 
newborn mammal. This process is under the control of the endocrine and nervous 
systems. It involves transformation of an inactive duct system to a 
lobuloalveolar glandular structure during pregnancy, cellular production of 
the components of milk (galactopoiesis), secretion into the ducts, and 
ejection under the stimulus of milking or suckling. 
    Lactation makes demands on the maternal regulation of calcium metabolism. 
Resorption of bone increases in lactating rats and women, and there is a 
marked increase in the absorption of calcium from the intestine. The elevated 
need for calcium results in an increased role for parathyroid hormone, 
calcitonin, and vitamin D in the regulation of the absorption and utilization 
of calcium. In humans a concomitant phenomenon frequently associated with 
lactation is amenorrhea. Consequently in some societies prolonged nursing is 
used as a birth control technique. See MAMMARY GLAND; MILK. 
 �34s[Harry J. Lipner]
�33s 
  
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�6v�2v�32sLactic acid   �21m �33s 
    A slightly hygroscopic syrup with formula COOH|--|CHOH|--|CH3 and a 
specific gravity of 1.294 at 25DGC. The acid, also known as 
ALPHA-hydroxypropionic acid, contains one asymmetric carbon atom and exists as 
two optically active isomers that can be converted by an enzyme, racemase, 
into the racemic mixture. L(+)-Lactic acid, also called sarcolactic and 
paralactic acid, is metabolized by animals, whereas D(-)-lactic acid is not. 
The lactic acid of commerce is the racemic mixture, commonly known as i-lactic 
acid (optically inactive). Lactic acid is used in pharmaceutical preparations, 
in the textile and leather industry, in the production of inks, solvents, 
lacquers, and plastics, as the bread additive, calcium stearyl-2 lactylate, 
and as food preservative. 
 �34s[F. J. Simpson]
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�6v�2v�32sLactobacillaceae   �21m �33s 
    A family of gram-positive, asporogenous, rod-shaped bacteria with the 
single genus Lactobacillus. They occur singly, in pairs, or in chains. They 
are primarily saprophytic, rarely pathogenic. The bacteria are generally known 
as sugar fermenters, producing lactic acid as a major product. They are found 
in fermenting food products, in the mouth, in the intestinal tract, and in 
body lesions. The strains usually are nonmotile and show neither reduction of 
nitrates, gelatin liquefaction, catalase production, nor surface growth in 
liquid media. The species are microaerophilic or anaerobic, inactive toward 
protein sources but fastidious in their requirements. 
 �34s[Carl S. Pederson]
�33s 
  
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�6v�2v�32sLactose   �21m �33s 
    Milk sugar or 4-O-BETA-D-galactopyranosyl-D-glucose. This reducing 
disaccharide is obtained as the ALPHA-D anomer (see formula, where the 
asterisk indicates a reducing group); the melting point (mp) is 202DGC 
(396DGF). Lactose is found in the milk of mammals to the extent of 
approximately 2 - 8%. It is usually prepared from whey, which is obtained by a 
by-product in the manufacture of cheese. Upon concentration of the whey, 
crystalline lactose is deposited. 
 �34s[William Z. Hassid]
�33s 
                             ******FORMULA****** 
  
  
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�6v�2v�32sLagomorpha   �21m �33s 
    The order of mammals including rabbits, hares, and pikas. Lagomorphs have 
two pairs of upper incisors (the second pair minute), and enamel surrounds the 
tooth, which does not form a sharp chisel. Motion of the jaw is vertical or 
transverse. Lagomorphs have three upper and two lower premolars, the earliest 
fossil rodents have one less of each. The tibia and fibula are fused, the 
fibula articulating with the calcaneum as in artiodactyls. There is a spiral 
valve in the cecum, and the scrotum is prepenial. 
    The order includes three families: Leporidae (rabbits and hares); 
Ochotonidae (pikas, whistling hares, or American coneys); and Eurymylidae, an 
extinct family from the Paleocene of Mongolia. See MAMMALIA; PIKA; RABBIT. 
    Leporidae are the most familiar members of the order. There are, in 
general, two kinds: rabbits (such as the American cottontail), which are 
relatively small, with shorter hindlegs, shorter ears, and short tails; and 
hares, larger forms with longer legs, ears, and tails. Rapid locomotion is by 
leaps, using the hindlegs, combined (especially in rabbits) with abrupt 
changes of direction. Both types occur in the same region, with rabbits 
inhabiting brush, scrub, or woods and hares living in open grassland. In North 
America, hares are usually called jackrabbits. 
 �34s[Albert E. Wood]
�33s 
  
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�6v�2v�32sLagrange's equations   �21m �33s 
    Equations of motion of a mechanical system for which a classical 
(non-quantum-mechanical) description is suitable, and which relate the kinetic 
energy of the system to the generalized coordinates, the generalized forces, 
and the time. If the configuration of the system is specified by giving the 
values of f independent quantities q1, . . . , qf, there are f such equations 
of motion. 
    In their usual form, these equations are equivalent to Newton's second law 
of motion and are differential equations of the second order for the q's as 
functions of the time t. 
 �34s[Philip M. Stehle]
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�6v�2v�32sLagrangian function   �21m �33s 
    A function of the generalized coordinates and velocities of a dynamical 
system from which the equations of motion in Lagrange's form can be derived. 
The Lagrangian function is denoted by L(q1, . . . ,qf; dq/dt1, . . . ,qf; t). 
For systems in which the forces are derivable from a potential energy V, if the 
kinetic energy is T, the equation below holds. See LAGRANGE'S EQUATIONS. 

                            *** L = T - V ***

 �34s[Philip M. Stehle]
�33s 
  
  

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�6v�2v�32sLake   �21m �33s 
    An inland body of water, small to moderately large in size, with its 
surface exposed to the atmosphere. Most lakes fill depressions below the zone 
of saturation in the surrounding soil and rock materials. Generically 
speaking, all bodies of water of this type are lakes, although small lakes 
usually are called ponds, tarns (in mountains), and less frequently pools or 
meres. The great majority of lakes have a surface area of less than 100 misup2 
(259 kmsup2). More than 30 well-known lakes, however, exceed 1500 misup2 (3885 
kmsup2) in extent, and the largest freshwater body, Lake Superior, North 
America, covers 31,180 misup2 (80,756 kmsup2). Most lakes are relatively 
shallow features of the Earth's surface. Because of their shallowness, lakes 
in general may be considered evanescent features of the Earth's surface, with 
a relatively short life in geological time. 
    Lakes differ as to the salt content of the water and as to whether they 
are intermittent or permanent. Most lakes are composed of fresh water, but 
some are more salty than the oceans. Generally speaking, a number of water 
bodies which are called seas are actually salt lakes; examples are the Dead, 
Caspian, and Aral seas. All salt lakes are found under desert or semiarid 
climates, where the rate of evaporation is high enough to prevent an outflow 
and therefore a discharge of salts into the sea. 
    Lakes with fresh waters also differ greatly in the composition of their 
waters. Because of the balance between inflow and outflow, fresh lake water 
composition tends to assume the composite dissolved solids characteristics of 
the waters of the inflowing streams - with the lake's age having very little 
influence. Under a few special situations, as crater lakes in volcanic areas, 
sulfur or other gases may be present in lake water, influencing color, taste, 
and chemical reaction of the water.  See  FRESH-WATER ECOSYSTEM; HYDROSPHERE; 
MEROMICTIC LAKE; SURFACE WATER. 
    Both natural and artificial lakes are economically significant for their 
storage of water, regulation of stream flow, adaptability to navigation, and 
recreational attractiveness. A few salt lakes are significant sources of 
minerals.  See  EUTROPHICATION. 
 �34s[Edward A. Ackerman]
�33s 
  
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�6v�2v�32sLamellibranchia   �21m �33s 
    The largest subclass of the class Bivalvia of the phylum Mollusca. The 
name refers to the two enormously enlarged gills, each composed of great 
numbers of parallel ctenidial filaments held together to form an extensive 
folded lamella. Each gill, although homologous both functionally and 
morphologically with gills of more primitive gastropods, is far more extensive 
than is required for the respiratory needs of the animal. Here the gill has 
become an organ of food collection: the most elaborate and efficient 
ciliary-feeding mechanism in the animal kingdom. 
    Subdivision of the Lamellibranchia is done most suitably into 
superfamilies, each with characteristic shell structure, dentition, ligament, 
and degree of union of the mantle lobes, involving, in many cases, formation 
of siphons to conduct the inhalant and exhalant water currents. The more 
important superfamilies include the Mytilacea (mussels), Ostreacea (true 
oysters), Cardiacea (true cockles), Tellinacea, Myacea (including 
``soft-shell'' clams), Unionacea (fresh-water clams), Mactracea, and Adesmacea 
(or Pholadacea). See  BIVALVIA; MOLLUSCA; SHIPWORM. 
 �34s[C. Maurice Yonge]
�33s 
  
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�6v�2v�32sLamiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of 5 families and more than 7800 species: Labiatae (about 3200 
species; see illustration), Verbenaceae (about 2600 species), and Boraginaceae 
(about 2000 species) make up the bulk of the order. Within its subclass the 
order is marked by its characteristic gynoecium, consisting of usually two 
biovulate carpels, each carpel divided between the ovules by a ``false'' 
partition, or with the two halves of the carpel seemingly wholly separate. 
Except in some of the more primitive species, the fruit usually consists of 
four separate or separating nutlets. In the Labiatae and most of the 
Boraginaceae, the four segments of the ovary are nearly or quite distinct from 
each other and are united chiefly by the gynobasic style.  See  ASTERIDAE; 
MAGNOLIOPSIDA.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sLaminar flow   �21m �33s 
    Streamline flow of a viscous fluid which satisfies the Navier-Stokes 
equations of motion. In laminar flow, the fluid moves in layers without large 
irregular fluctuations. Laminar flow occurs at a low Reynolds number. This 
corresponds to the conditions of small velocities and dimensions of bodies, to 
very large viscosity, or to small density of the fluid. 
    Laminar flow plays an important role in several practical problems. The 
flow of oil in the bearings for lubrication is laminar. The theory of laminar 
flow shows that under great normal pressure, the oil in the bearing has only 
slight frictional resistance. Flow on the surface of modern aircraft and 
missiles flying at extremely high altitude may be laminar. Laminar 
boundary-layer flow determines the skin friction and the aerodynamic heating 
of these bodies.  See  NAVIER-STOKES EQUATIONS; REYNOLDS NUMBER; STREAMLINE 
FLOW.  
 �34s[Shih-I Pai]
�33s 
  
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�6v�2v�32sLamp   �21m �33s 
    An electric lamp is a device for converting electrical energy into 
illumination. The basic types include incandescent, vapor, fluorescent, and 
arc lamps.  See  ARC LAMP; FLUORESCENT LAMP; INCANDESCENT LAMP; MERCURY-VAPOR 
LAMP; VAPOR LAMP. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sLampridiformes   �21m �33s 
    An order of teleost fishes including the ribbonfishes, oarfishes, opah, 
and their allies, also known as the Allotriognathi. Although these fishes are 
diverse in form, they share characters that define this order. In most, the 
body is notably compressed, often ribbonlike (see illustration). The fins are 
composed of soft rays, or the dorsal has one or two anterior spines; the 
pelvic fin, if present, is thoracic, with 1 - 17 spineless rays; the pelvic 
girdle is inserted between the coracoids, or attached to them, or is lost; 
scales are small and cycloid or, commonly, absent; and the swim bladder is 
without a duct. Most species are large and colorful. They are all oceanic.  
See  ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sLamprophyre   �21m �33s 
    Any of a heterogeneous group of gray to black, mafic igneous rocks 
characterized by a distinctive panidiomorphic and porphyritic texture in which 
abundant euhedral, dark-colored ferromagnesian (femic) minerals (dark mica, 
amphibole, pyroxene, olivine) occur in two generations - both early as 
phenocrysts and later in the matrix or groundmass -  while felsic minerals 
(potassium feldspar, plagioclase, analcime, melilite) are restricted to the 
groundmass.  
    Many varieties of lamprophyre are known. Minettes, kersantites, vogesites, 
and spessartites are the most common and are sometimes collectively called 
calcalkaline lamprophyres. Camptonites and monchiquites are less common; 
alnoites are rare. These varieties, along with some others, are referred to as 
alkaline lamprophyres. 
    Lamprophyres are widespread but volumetrically minor rocks that apparently 
are restricted to the continents and are the last manifestation of igneous 
activity in a given area. They usually occur as subparallel or radial swarms 
of thin (SIMILAR 1.6 - 160 ft or SIMILAR 0.5 - 50 m) dikes or, less commonly, 
sills, volcanic neck fillings, or diatremes, or, rarely, lava flows.  See  
IGNEOUS ROCKS. 
 �34s[Sharon W. Bachinski]
�33s 
  
  
  
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�6v�2v�32sLancefield differentiation scheme   �21m �33s 
    A method developed by Rebecca C. Lancefield to classify streptococci into 
serological groups and types. The method is based on the detection of a 
carbohydrate antigen in hot acid extracts of the streptococcal cells with 
group-specific rabbit antisera and a laboratory capillary precipitin test. 
With this method, Lancefield described groups A to G, which (with the 
exception of group D) are composed primarily of BETA-hemolytic streptococci 
and which contain most of the groups pathogenic for humans. Later other 
investigators named groups H to V (two letters were not used: I and J), a 
mixture of BETA-, ALPHA-, and nonhemolytic streptococci. Serologic 
classification of groups other than A to G is usually unsatisfactory, and is 
therefore used infrequently.  See  STREPTOCOCCUS. 
 �34s[Hazel W. Wilkinson]
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�6v�2v�32sLand drainage (agriculture)   �21m �33s 
    The removal of water from the surface of the land and the control of the 
shallow groundwater table improves the soil as a medium for plant growth. The 
sources of excess water may be precipitation, snowmelt, irrigation water, 
overland flow or underground seepage from adjacent areas, artesian flow from 
deep aquifers, floodwater from channels, or water applied for such special 
purposes as leaching salts from the soil or for achieving temperature control. 
    The purpose of agricultural drainage can be summed up as the improvement 
of soil water conditions to enhance agricultural use of the land. Such 
enhancement may come about by direct effects on crop growth, by improving the 
efficiency of farming operations or, under irrigated conditions, by 
maintaining or establishing a favorable salt regime. Drainage systems are 
engineering structures that remove water according to the principles of soil 
physics and hydraulics. The consequences of drainage, however, may also 
include a change in the quality of the drainage water. Agricultural drainage 
is divided into two broad classes; surface and subsurface. Some installations 
serve both purposes. 
 �34s[James N. Luthin]
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�6v�2v�32sLanding gear   �21m �33s 
    Those parts of an aircraft structure which serve to support the aircraft 
when it is not in the air. Tires, wheels, brakes, shock struts, drag struts, 
and miscellaneous equipment, such as retracting mechanisms, steering 
mechanisms, shimmy dampers, and doors, are the components of an aircraft 
landing gear. For special purposes the wheels and tires may be replaced with 
skids or skis. A landing gear is required on all piloted aircraft. 
    The most widely accepted arrangement is the tricycle landing gear. This 
arrangement places a nose gear well forward of the center of gravity on the 
fuselage and two main gears slightly aft of the center of gravity, with a 
sufficient distance between them to provide stability against rolling over 
during a yawed landing in a cross wind, or during ground maneuvers. The nose 
gear prevents nosing over when brakes are applied. The nose gear, which is 
always castered, is often equipped with power steering to allow ground 
maneuvering.  
    Another arrangement still used extensively is the tail-wheel landing gear. 
Two main-gear struts are located slightly ahead of the center of gravity, 
well-spaced laterally to provide lateral stability; a third wheel is mounted 
on the fuselage near the aft end of the airplane. It suffers in comparison 
with a tricycle gear because of increased problems associated with vision, 
ground maneuvering, and braking. 
 �34s[John E. Stevens]
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�6v�2v�32sLanding ships and craft   �21m �33s 
    Combat vessels employed in amphibious operations to transport mobile 
equipment, amphibious vehicles, tanks, general cargo, and personnel, and to 
discharge them near or directly onto a beach. Landing ships are generally 
designated by the letters LS, landing craft by LC, and amphibious landing 
vehicles by LV. Landing craft, ranging from about 36 to 135 ft (11 to 41 m) in 
length, are not designed for long transoceanic voyages and are carried to the 
unloading area aboard landing ships. Landing ships, landing craft, and 
amphibious vehicles were first used during World War II, when a wide variety 
of types were developed. 
 �34s[Charles J. Papuchis]
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�6v�2v�32sLand reclamation   �21m �33s 
    The process by which seriously disturbed land surfaces are stabilized 
against the hazards of wind and water erosion. Surface mining for coal is 
responsible for almost one-half of the total land area disturbed in the United 
States. The drastic disturbance of the overburden severely changes the 
chemical and physical properties of the resulting spoils. These altered 
properties often create a hostile environment for seed germination and 
subsequent plant growth. Unless vegetative cover is established almost 
immediately, the denuded areas are subject to both wind and water erosion that 
pollute surrounding streams with sediment. 
    The removal of topsoil before mining and its replacement on the spoil 
surface after final grading have aided materially in the reclamation process. 
Surface grading techniques and seedbed preparation are very important in 
obtaining good vegetative cover for erosion and sedimentation control. Many 
plant species of economic importance can be used to produce hay, pasture, 
various horticultural crops, and major row crops. Commercial varieties of 
grasses that have shown promise on United States eastern stripmine spoils with 
moderate pH (5.0 - 6.0) include orchard grass, tall fescue, bromegrass, 
ryegrass, and timothy. Other varieties have shown promise on low-pH (4.5 or 
less) stripmine spoils. Most spoils in the western United States are returned 
to perennial grasses for eventual use by grazing livestock and wildlife. 
Almost any species of trees can be grown on stripmine areas as long as their 
nutrient and environmental needs are met.  See  STRIP MINING. 
 �34s[Orus L. Bennett]
�33s 
  
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�6v�2v�32sLandscape architecture   �21m �33s 
    Landscape architecture was defined by Charles Eliot, one of America's 
first landscape architects, as the art of arranging and fitting land for human 
use and enjoyment. It is an applied art founded on the premise that use and 
beauty are compatible and that neither is complete without the other. 
    The services of the landscape architect, similar to those of a building 
architect, involve consultation; preparation of reports, plans, 
specifications, and estimates; assistance in letting contracts; and 
supervision of the work done by the contractors. 
    Projects undertaken by landscape architects include large parks and 
recreation areas, real estate subdivisions, highways and parkways, and 
landscaping for industries, institutions, cemeteries, and large private 
estates.  
 �34s[Joseph P. Porter]
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�6v�2v�32sLandslide �21m �33s
    The perceptible downward sliding, falling, or flowing of masses of soil,
rock, and debris (mixtures of soil and weathered rock fragments). Landslides
range in size from a few cubic meters to over 10 (sup 9) m(sup 3) (3.5 x 10
(sup 1) (sup 0) ft (sup 3)), their velocities range from a few centimeters per
day to over 100 m/s (330 ft/s), and their displacements may be several
centimeters to several kilometers.  See  MASS WASTING.
    The U. S. Highway Research Board classification divides landsliding of
rock, soil, and debris, on the basis of the types of movement, into falls,
slides, and flows. Other classifications consider flows, along with creep and
other kinds of landslides, as general forms of mass wasting.
    Falls occur when soil or rock masses free-fall through air. Falls are
usually the result of collapse of cliff overhangs which result from
undercutting by rivers or simply from differential erosion. Slides invariably
involve shear displacement or failure along one or more narrow zones or
planes. Internal deformation of the sliding mass after initial failure depends
on the kinetic energy of the moving mass (size and velocity), the distance
traveled, and the internal strength of the mass. Flows have internal
displacement and a shape that resemble those of viscous fluids. Relatively
weak and wet masses of shale, weathered rock, and soil may move in the form of
debris flows and earthflows; water-soaked soils or weathered rock may displace
as mudflows.
    Mining and civil engineering works have induced myriads of landslides, a
few of them of a catastrophic nature. Open-pit mines and road cuts create very
high and steep slopes, often quite close to their stability limit. Local
factors (weak joints, fault planes) or temporary ones (surges of water
pressure inside the slopes, earthquake shocks) induce the failure of some of
these slopes. The filling of reservoirs submerges the lower portion of
natural, marginally stable slopes or old landslides. Water lowers slope
stability by softening clays and by buoying the lowermost portion, or toe, of
the slope.
    Advances in soil and rock engineering have improved the knowledge of slope
stability and the mechanics of landsliding. Small and medium-sized slopes in
soil and rock can be made more stable. Remedial measures include lowering the
slope angle, draining the slope, using retaining structures, compressing the
slope with rock bolts or steel tendons, and grouting.  See  ENGINEERING
GEOLOGY; EROSION; SOIL MECHANICS.
 �34s[Alberto S. Nieto]
�33s

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�6v�2v�32sLangevin function �21m �33s
    A mathematical function which is important in the theory of paramagnetism
and in the theory of the dielectric properties of insulators. The analytical
expression for the Langevin function is shown in the equation below. If x IS

		       *** L(x) = coth x - 1/x ***

MUCH LESS THAN 1, L(x) IS CONGRUENT TO x/3. The paramagnetic susceptibility of
a classical (non-quantum-mechanical) collection of magnetic dipoles is given by
the Langevin function, as is the polarizability of molecules having a permanent
electric dipole moment. See PARAMAGNETISM. 
 �34s[Elihu Abrahams; Frederic Keffer]
�33s



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�6v�2v�32sLangmuir-Child law �21m �33s
    A law governing space-charge-limited flow of electron current between two
plane parallel electrodes in vacuum when the emission velocities of the
electrons can be neglected. It is often called the three-halves power law, and
is expressed by the formula shown below.

j(amperes/cm[sup 2]) = (epsilon/9pi)(2e/m)(sup 1/2)   (V[sup 3/2])/(d[sup 2])
		  = (2.33 x 10[sup -6]){[V (volts)](sup 3/2)}/[d (cm)](sup 2)

    Here NATURAL LOG is the dielectric constant of vacuum, -e the charge of
the electron, m its mass, V is the potential difference between the two
electrodes, d their separation, and j the current density at the collector
electrode, or anode. The potential difference V is the applied voltage reduced
by the difference in work function of the collector and emitter. The
Langmuir-Child law applies, to a close approximation, to other electrode
geometries as well. See SPACE CHARGE.
 �34s[Edward G. Ramberg]
�33s

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�6v�2v�32sLanolin   �21m �33s 
    The hydrous sheep's-wool wax (primarily cholesterol esters of higher fatty 
acids) derived as a by-product from the preparation of raw wool for the 
spinner. Lanolin is widely used as a base for emollients in cosmetics and 
shampoos. Perhaps because of its slight antiseptic effect and high resistance 
to rancidity it has continued wide use as a base for skin ointments. Actually, 
lanolin hinders absorption of the medication through the skin, as compared to 
other substances such as olive oil or lard. See WAX, ANIMAL AND VEGETABLE. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sLanthanide contraction   �21m �33s 
    The name given to an unusual phenomenon encountered in the rare-earth 
series of elements. The radii of the atoms of the members of this series 
decrease slightly as the atomic number increases. As the charge on the nucleus 
increases across the rare-earth series, all electrons are pulled in closer to 
the nucleus so that the radii of the rare-earth ions decrease slightly as the 
compounds go across the rare-earth series. Any given compound of the rare 
earths is very likely to crystallize with the same structure as any other rare 
earth. However, the lattice parameters become smaller and the crystal denser 
as the compounds proceed across the series. This contraction of the lattice 
parameters is known as the lanthanide contraction. See RARE-EARTH ELEMENTS. 
 �34s[Frank H. Spedding]
�33s 
  
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�6v�2v�32sLanthanum �21m �33s
    A chemical element, La, atomic number 57, atomic weight 138.91. Lanthanum,
the second most abundant element in the rare-earth group, is a metal. The
naturally occurring element is made up of the isotopes	(sup 138)La and
(sup 139)La. The element occurs associated with other rare earths in
monazite, bastnasite, and other minerals. It is one of the radioactive
products of the fission of uranium, thorium, or plutonium. Lanthanum is the
most basic of the rare earths. It is an important ingredient in glass
manufacture. Lanthanum imparts a high refractive index to the glass and is
used in the manufacture of expensive lenses. For other properties of the metal
see RARE-EARTH ELEMENTS.
 �34s[Frank H. Spedding]
�33s

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�6v�2v�32sLaplace's differential equation �21m �33s
    Laplace's equation in two independent variables x and y is given as Eq. (1)

[CURLY D(sup 2)(x,y)]/[CURLY D x(sup 2)] =			     (1)
[CURLY D u(x,y)]/[CURLY D X(sup 2)] = 0

and is of central importance in both pure mathematics and mathematical
physics. A function u(x,y) having continuous first and second partial
derivatives and satisfying Laplace's equation in a neighborhood of a point is
called harmonic at that point. If a plane piece of tinfoil has its edges kept
at a temperature which varies from point to point but does not change with
time, and if the flow of heat in the tinfoil is steady (that is, independent
of the time), the temperature u(x,y) at interior points of the foil is
harmonic. Likewise Laplace's equation dominates the flow of electricity (the
potential is similarly harmonic) and the flow of any incompressible fluid.
    Many properties of Laplace's equation with two independent variables apply
also in three or more dimensions.  Thus, in three dimensions, a point
distribution of matter of masses m(sub k) at points (x[sub k],y[sub k],z[sub k])
has a potential defined by Eq.	(2), which

u(x,y,z) IDENTIDY SUMMATION M(sub k) [x - x(sub k)](sup 2) + (y - y(sub k))(sup
2) + (z - z(sub k))(sup 2)](sup -1/2)


is harmonic except in the points (x[sub k],y[sub k], z[sub k]).  Except at such
points, the force (Newtonian law of gravitation) exerted by the distribution on
a unit exploratory particle at (x,y,z) has the components (du/dx, du/dy, du/dz),
and the component of the force in any direction is the directional derivative of
u(x,y) in that direction.  See POTENTIALS; SPHERICAL HARMONICS.

 �34s[Joseph L. Walsh]
�33s

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�6v�2v�32sLaplace's irrotational motion   �21m �33s 
    Laplace's equation for irrotational motion of an inviscid, incompressible 
fluid is partial differential equation (1), where x1, x2, x3 are rectangular 
  
*** (PARTIAL DIFFERENTIAL sup 2 PHI / PARTIAL DIFFERENTIAL x sub 1 sup 2) +
    (PARTIAL DIFFERENTIAL sup 2 PHI / PARTIAL DIFFERENTIAL x sub 2 sup 2) +
    (2 sup 2 PHI / 2 x sub 3 sup 2) = 0 ***                                 (1)

cartesian coordinates in an inertial reference frame, and Eq. (2)  gives the 
  
                    *** PHI = PHI (x1, x2, x3, t) ***                       (2)
  
velocity potential. The fluid velocity components, u1, u2, u3 in the three 
respective rectangular coordinate directions are given by u sub i = PARTIAL
DIFFERENTIAL PHI / PARTIAL DIFFERENTIAL x sub i, i = 1,2,3. More generally, in 
any inertial coordinate system, the equation is div (grad PHI) = 0 and the 
velocity vector is v = grad PHI. Irrotational motion implies that the fluid 
particles translate without rotation (like the cars on a ferris wheel). See 
FLUID FLOW. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
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�6v�2v�32sLaplace transform �21m �33s
    An integral transform extensively used by P. S. Laplace in the theory of
probability. In simplest form it is expressed as the equation below. It is
thought of as transforming the determining function
phi(t) into the generating function f(s). The variable t is real, the variable
s may be real or complex, s = sigma + itau. As an example, if phi(t) = 1 the
integral converges for sigma > 0, and f(s) = 1/s.

  f(s) = INTEGRATION (0 to INFINITY) e(sup -st) phi(t) dt

    The Laplace transform is used for the solution of differential and
difference equations, for the evaluation of definite integrals, and in many
branches of abstract mathematics (functional analysis, operational calculus,
and analytic number theory). See INTEGRAL TRANSFORM.
 �34s[David V. Widder]
�33s

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�6v�2v�32sLaplacian   �21m �33s 
    The differential operator (PARTIAL DIFFERENTIAL sup 2 / PARTIAL 
DIFFERENTIAL x sup 2) + (PARTIAL DIFFERENTIAL sup 2 / PARTIAL DIFFERENTIAL 
y sup 2) + (PARTIAL DIFFERENTIAL sup 2 / PARTIAL DIFFERENTIAL z sup 2), in 
which the symbols x, y, z denote the variables of a rectangular cartesian 
coordinate system. The laplacian is frequently denoted by the symbol INVERTED 
DELTA sup 2 (read del square) in accordance with the fact that the laplacian of 
a scalar function S(x,y,z) is the divergence of the gradient of S, that is, 
the equation below applies. 
  
*** (PARTIAL DIFFERENTIAL sup 2 S / PARTIAL DIFFERENTIAL x sup 2) + 
    (PARTIAL DIFFERENTIAL sup 2 S / PARTIAL DIFFERENTIAL y sup 2) + 
    (PARTIAL DIFFERENTIAL sup 2 S / PARTIAL DIFFERENTIAL z sup 2) = 
    GRADIENT OF MULT (GRADIENT OF S) ***            

    The laplacian operator is involved in some of the most fundamental 
equations of mathematical physics, namely, Laplace's equation (INVERTED DELTA 
[sup 2] u = 0), Poisson's equation, various wave equations, and the heat flow 
and diffusivity equations. See CALCULUS OF VECTORS; GAUSS' THEOREM; GRADIENT 
OF A SCALAR; GREEN'S THEOREM; WAVE EQUATION. 
 �34s[Homer V. Craig]
�33s 
  
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�6v�2v�32sLapping   �21m �33s 
    A precision abrading process used to bring a surface to a desired state of 
refinement or dimensional tolerance by removal of an extremely small amount of 
material. Lapping is accomplished by abrading a surface with a fine abrasive 
grit rubbed about it in a random manner. Usually less than 0.0005 in. (0.012 
mm) of stock is removed. See GRINDING; MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sLarch �21m �33s
    A genus, Larix, of the pine family, with deciduous needles and short
spurlike branches, which annually bear a crown of needles. The cones are small
and persistent, varying by species in size, number, and form of the cone
scales. The tamarack (L. laricina), also called hackmatack, is a native
species. It has an erect, narrowly pyramidal habit, and grows in the
northeastern United States, west to the Lake states, and across Canada to
Alaska. The tough resinous wood is durable in contact with the soil and is
used for railroad ties, posts, sills, and boats. Other uses include the
manufacture of excelsior, cabinet work, interior finish, and utility poles.
See PINALES.
    The western larch (L. occidentalis), the most important and largest of all
the species, grows in the northwestern United States and southeastern British
Columbia.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sLarge systems control theory   �21m �33s 
    A branch of the theory of control systems concerned with the special 
problems that arise in the design of control algorithms (that is, control 
policies and strategies) for complex systems. Characteristically the systems 
have a great many inputs and outputs which interact in a complicated and 
usually imprecisely known manner. Some examples of such complex systems are 
the United States economy, the global telephone communication network, the 
electric power generation system for the western United States, large 
industrial plants, a modern military command, communication, control, and 
intelligence system, and a single advanced computer system composed of large 
arrays of interconnected microprocessors. See CONTROL SYSTEMS. 
 �34s[Michael G. Safonov]
�33s 
  
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�6v�2v�32sLarnite   �21m �33s 
    The alpha polymorph of calcium silicate (Ca2SiO4). Larnite is a mineral 
which crystallizes at high temperature. Its occurrences are practically 
confined to limestone or chalk zones in contact with semimolten basalts. At 
room temperature, larnite is metastable and inverts to its low-temperature 
polymorph calcio-olivine through shock. This leads to ``fall,'' or 
disintegration of slags with time, and presents problems in the cement 
industry. The mineral is very rare, known from its type locality at Scawt 
Hill, County Antrim, Ireland, and from Crestmore, near Riverside, California. 
See SILICATE MINERALS. 
 �34s[Paul B. Moore]
�33s 
  
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�6v�2v�32sLarvacea   �21m �33s 
    A class of the subphylum Tunicata consisting of minute planktonic animals 
in which the tail, with dorsal nerve cord and notochord, persists throughout 
life. Two gill slits are commonly present, but an atrium is lacking. The 
epidermis of the anterior region secretes a gelatinous tunic. In some species 
this tunic is highly complex and encases the whole body in a ``house'' 
equipped with chambers and strainers which enable the inhabitant to filter and 
concentrate very small plankton from the sea. About 60 species are known; 
Oikopleura (see illustration) and Fritillaria are the largest genera. See 
TUNICATA.  
 �34s[Donald P. Abbott]
�33s 
  
  
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�6v�2v�32sLarynx   �21m �33s 
    The complex of cartilages and related structures at the opening of the 
trachea, or windpipe, into the pharynx, or throat. In humans and most other 
mammals, the signet-shaped cricoid cartilage forms the base of the larynx and 
rests upon the trachea. The thyroid cartilage, which forms the prominent 
Adam's apple ventrally, lies anterior to the cricoid. Dorsally there are 
paired pivoting cartilages, the arytenoids. Each is pyramid-shaped and acts as 
the movable posterior attachment for the vocal cords and the laryngeal muscles 
that regulate the cords. Two other small paired cartilages, the cuneiform and 
the corniculate, also lie dorsal to the thyroid cartilage. The epiglottis, a 
leaf-shaped elastic cartilage with its stem inserted into the thyroid notch, 
forms a lid to the larynx. 
    Laryngitis is an inflammation of the mucous membrane of the larynx always 
associated with hoarseness. Hoarseness is also a manifestation of paralysis of 
the recurrent laryngeal nerve of the vagus, which is easily damaged upon 
surgical removal of a goiter. 
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sLaser �21m �33s
    A device that uses the principle of amplification of electromagnetic waves
by stimulated emission of radiation and operates in the infrared, visible, or
ultraviolet region. The term laser is an acronym for light amplification by
stimulated emission of radiation, or a light amplifier. However, just as an
electronic amplifier can be made into an oscillator by feeding appropriately
phased output back into the input, so the laser light amplifier can be made
into a laser oscillator, which is really a light source. Laser oscillators are
so much more common than laser amplifiers that the unmodified word ``laser''
has come to mean the oscillator, while the modifier ``amplifier'' is generally
used when the oscillator is not intended.  See	MASER.
    The process of stimulated emission can be described as follows. When
atoms, ions, or molecules absorb energy, they can emit light spontaneously (as
in an incandescent lamp) or they can be stimulated to emit by a light wave.
This stimulated emission is the opposite of (stimulated) absorption, where
unexcited matter is stimulated into an excited state by a light wave. If a
collection of atoms is prepared (pumped) so that more are initially excited
than unexcited, then an incident light wave will stimulate more emission than
absorption, and there is net amplification of the incident light beam. This is
the way the laser amplifier works.
    A laser amplifier can be made into a laser oscillator by arranging
suitable mirrors on either end of the amplifier. These are called the
resonator. Thus the essential parts of a laser oscillator are an amplifying
medium, a source of pump power, and a resonator. Radiation that is directed
straight along the axis bounces back and forth between the mirrors and can
remain in the resonator long enough to build up a strong oscillation. (Waves
oriented in other directions soon pass off the edge of the mirrors and are
lost before they are much amplified.) Radiation may be coupled out as shown in
the illustration by making one mirror partially transparent so that part of
the amplified light can emerge through it. The output wave, like most of the
waves being amplified between the mirrors, travels along the axis and is thus
very nearly a plane wave.  See	OPTICAL PUMPING.
    In contrast to lasers, all conventional light sources are basically hot
bodies which radiate by spontaneous emission. The overall wave produced by a
conventional light source is a jumble of waves from the numerous individual
atoms. The phase of the wave emitted by one atom has no relation to the phase
emitted by any other atom, so that the overall phase of the light fluctuates
randomly from moment to moment and place to place. The lack of correlation is
called incoherence.  See  HEAT RADIATION.
    In contrast to this, an ideal plane wave would have the same phase all
across any wavefront, and the time fluctuations would be highly predictable
(coherent). The output of a parallel-plate laser is very nearly such a plane
wave and is therefore highly directional. This arises because in the laser
oscillator atoms are stimulated to emit in phase with the stimulating wave,
rather than independently, and the wave that builds up between the mirrors
matches very closely the mirrors' surfaces. The output is powerful because
atoms can be stimulated to emit much faster than they would spontaneously. It
is highly monochromatic largely because stimulated emission is a resonance
process that occurs most rapidly at the center of the range of wavelengths
that would be emitted spontaneously. Since atoms are stimulated to emit in
phase with the existing wave, the phase is preserved over many cycles,
resulting in the high degree of time coherence of laser radiation.  See
COHERENCE; HEAT RADIATION.

    The various types of lasers are classified according to their pumping (or
excitation) scheme. The function of the pumping system is to maintain more
atoms in the upper than in the lower state, thereby assuring that stimulated
emission (gain transitions) will exceed (stimulated) absorption (loss
transitions). This so-called population inversion ensures net gain
(amplification greater than unity). The large classes of lasers include
optically pumped lasers, gas-discharge lasers, pulsed gas lasers, chemical
lasers, photodissociation lasers, nuclear lasers, gas-dynamic lasers,
semiconductor lasers, free-electron lasers, and high-power, short-pulse
lasers. See separate articles on each class.
    The variety of technological uses for lasers has increased steadily since
their appearance in 1960. Among the noticeable applications are those that
utilize high-speed controllability of the tiny focal spot of a laser beam. For
example, high-speed automatic scanners identify library cards, ski passes, and
supermarket purchases and perform a variety of functions known as optical
processing. Other uses for the laser beam's programmable control include
information storage and retrieval (including three-dimensional holography and
video disk reading), laser printing, micromachining, and automated cutting.
Further applications involving high power include weaponry, laser welding,
laser surgery (self-cauterizing), laser fusion, and materials processing.
Optical communications utilize the laser's high frequency, which makes
possible high information capacity. Bright laser beams are used by the
construction industry to align straight excavations and for surveying.	See
CHARACTER RECOGNITION; CONTINUOUS-WAVE RADAR; GYROSCOPE; HOLOGRAPHY;
INTEGRATED OPTICS; LASER PHOTOBIOLOGY; LASER PHOTOCHEMISTRY; LASER
SPECTROSCOPY; OPTICAL COMMUNICATIONS; OPTICAL FIBERS; PHOTOACOUSTIC
SPECTROSCOPY; VIDEO DISK RECORDING.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
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�6v�2v�32sLaser Doppler velocimeter �21m �33s
    A device that uses the Doppler effect to measure the velocity of fluid
flow. When light is scattered from a moving object, a stationary observer will
see a change in the frequency of the scattered light (Doppler shift)
proportional to the velocity of the object. In the laser Doppler velocimeter
this Doppler shift is used to measure the velocity of particles in a fluid.
From the particle velocity the velocity of the fluid is inferred. A laser is
used as light source because it is easily focused and is coherent and thereby
enables the measurement of a frequency shift due to a Doppler effect.  See
DOPPLER EFFECT; FLOW MEASUREMENT; LASER.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

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�6v�2v�32sLaser photobiology   �21m �33s 
    The interaction of laser light with biological molecules, and the 
applications to biology and medicine.  See  LASER. 
    Microirradiation is a useful technique for the study of cell function by 
alteration of a specific organelle or part of a cell. The laser beam is 
focused through the objective of a microscope onto the cell. Practically 
speaking, it is easy to obtain spots of about 1 micrometer in diameter. Ruby, 
neodymium, and argon lasers are used for this purpose. 
    Laser spectroscopy is used to probe biological processes in which very 
fast reactions are involved or to study structural changes of complex 
molecules. The two techniques used are flash photolysis (in the nanosecond and 
picosecond range) and Raman spectroscopy.  See  RAMAN EFFECT; LASER 
SPECTROSCOPY.  
    Continuous-wave lasers have been employed as a ``light knife,'' that is as 
a surgical cutting and coagulation tool. Generally, CO2 lasers, emitting in 
the infrared, are used for this purpose. When the laser energy is focused onto 
a tissue surface, a small volume of tissue is heated, and thus only this area 
is ``cut off.'' An advantage of this procedure is that small capillaries are 
coagulated, preventing hemorrhage resulting from cut blood vessels. Argon 
lasers are the most commonly used for treating retinopathies, but also for 
glaucoma and cataract. Laser irradiation is used for removal of foreign 
pigments in the skin (tattoos), for treatment of vascular disorders (``wine 
marks''), and for removal of various pigmented skin lesions. 
 �34s[Giuliana Moreno]
�33s 
  
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�6v�2v�32sLaser photochemistry �21m �33s
    The branch of physical chemistry in which chemical reactions are induced,
altered, or monitored by laser light.
    The main advantages of a laser over a conventional light source (such as
discharge or arc lamps) for study of photochemistry are threefold. First,
laser light has a very small divergence angle. Second, laser light is
exceptionally pure in color. Third, lasers are generally more powerful than
conventional light sources. All in all, the laser provides the photochemist
with a source of light between 10 and 20 orders of magnitude more spectrally
bright than previously available.
    One area of laser photochemistry is the study of single-collision chemical
reaction dynamics. A natural and practical application of this research is the
use of lasers to drive specific chemical reactions. One successful example of
laser-induced chemistry is carbon dioxide laser irradiation of diborane (B2H6)
to produce icosaborane (B20H16) in high yield; conventional pyrolysis
generates a mixture of products in which B20H16 is not found.
    Picosecond studies involve the use of state-of-the-art, ultrashort
(10- (sup 12) -s) laser pulses to probe the reaction dynamics of very fast
processes. A sample is illuminated by the initial pump pulse followed by a
staccato burst of probe pulses. By measurement of the absorption of the
delayed probe pulses, the fast chemical processes initiated by the pump pulse
can be monitored on an unprecedented time scale. See PICOSECOND MOLECULAR
PROCESSES.
    A long-standing goal of the laser photochemist has been to manipulate the
pathway of a photochemical process. One vigorously pursued approach to this
problem involves using extremely intense lasers to induce dissociation of a
specific bond in a molecule - in effect, molecular photosurgery.
    Another large-scale application of laser photochemistry is the separation
of isotopes. The laser preferentially dissociates molecules of a particular
isotope; the isotopically enriched radical fragments then react with added
scavengers, and can be removed. Application of this technique has achieved
spectacular isotopic separations for a number of elements, most notably
sulfur, boron, nitrogen, and hydrogen. See CHEMICAL DYNAMICS; ISOTOPE (STABLE)
SEPARATION; LASER; PHOTOCHEMISTRY.
 �34s[David J. Nesbitt]
�33s

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�6v�2v�32sLaser spectroscopy   �21m �33s 
    Spectroscopy with laser light or, re generally, studies of the interaction 
between laser radiation and matter. Lasers have led to a rejuvenescence of 
classical spectroscopy, because laser light can far surpass the light from 
other sources in brightness, spectral purity, and directionality, and if 
required, laser light can be produced in extremely intense and short pulses. 
The use of lasers can greatly increase the resolution and sensitivity of 
conventional spectroscopic techniques, such as absorption spectroscopy, 
fluorescence spectroscopy, or Raman spectroscopy. Moreover, interesting new 
phenomena have become observable in the resonant interaction of intense 
coherent laser light with matter. Laser spectroscopy has become a wide and 
diverse field, with applications in numerous areas of physics, chemistry, and 
biology. See LASER; SPECTROSCOPY. 
 �34s[Theo W. Hansch]
�33s 
  
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�6v�2v�32sLaser welding   �21m �33s 
    Welding with a laser beam. The primary apparatus is the continuous-wave, 
convectively cooled CO2 laser with either oscillator/amplifier (gaussian 
output beam) or unstable resonator (hollows output beam) optics. These lasers, 
available in output powers ranging from approximately 1000 to 15,000 W, have 
been used to demonstrate specific welding accomplishments in a variety of 
metals and alloys. Substantial advances in laser technology made possible the 
production of fully automated multikilowatt industrial laser systems which can 
be operated on a continuous production basis. These systems can be used for a 
variety of development programs and on-line production applications. See 
LASER; WELDING AND CUTTING OF METALS. 
 �34s[Edward M. Breinan]
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�6v�2v�32sLatent image   �21m �33s 
    An invisible image produced by a physical or chemical effect of light on 
the individual crystals (usually silver halide) of photographic emulsions. 
This image can be rendered visible by the process known as development. See 
PHOTOGRAPHY; PHOTOLYSIS. 
 �34s[R. H. Noble]
�33s 
  
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�6v�2v�32sLateral meristem   �21m �33s 
    Strips or cylinders of dividing cells located parallel to the long axis of 
the organ in which they occur. Radial enlargement of the cells derived from 
these meristems increases the diameter of the organ. The lateral meristem is 
concerned with secondary growth in the sense that its meristematic activity 
adds cells to the primary body which was derived from the apical meristems. 
See APICAL MERISTEM. 
 �34s[Vernon I. Cheadle]
�33s 
  
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�6v�2v�32sLaterite   �21m �33s 
    Originally the name for the iron-rich weathering product of basalt in 
southern India. The term is now used in a compositional sense for weathering 
products composed principally of the oxides and hydrous oxides of iron, 
aluminum, titanium, and manganese. Clay minerals of the kaolin group are 
typically associated with, and are genetically related to, laterite. Laterites 
range from soft, earthy, porous material to hard, dense rock. Concretionary 
forms of varying size and shape commonly are developed. The color depends on 
the content of iron oxides and ranges from white to dark red or brown, 
commonly variegated. See BAUXITE; CLAY MINERALS; KAOLINITE; WEATHERING 
PROCESSES.  
    Mature lateritic soils lack fertility for most systems of agriculture. 
Savannas or parklike grasslands are typical on laterite. Clay, not laterite, 
is found beneath rainforests and jungle vegetation. 
 �34s[Samuel S. Goldich]
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�6v�2v�32sLathe   �21m �33s 
    A machine for the removal of metal from a workpiece by gripping it 
securely in a holding device and rotating it under power against a suitable 
cutting tool. The tool may be moved radially or longitudinally in respect to 
the turning axis of the workpiece either manually or by attached power. Forms 
such as cones, spheres, and related concentric-shaped workpieces as well as 
true cylinders can be turned on a lathe. Machining operations such as facing, 
boring, and threading, which are variations of the turning process, can also 
be performed on a lathe. 
    Turning equipment may be classed as being either of the horizontal or 
vertical type referring to the turning axis of the workpiece in the machine. 
The basic engine lathe is primarily a manually operated machine. Filling the 
gap between the engine lathe and fully automatic turning equipment is the 
turret lathe. Fastest and best suited to high-quantity production is the 
automatic screw machine. See MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
  
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�6v�2v�32sLatin square   �21m �33s 
    A square n by n matrix with entries from the set N = {0,1, . . .,n - 1} so 
that each number occurs exactly once in each row and exactly once in each 
column. Two Latin squares are said to be orthogonal if when they are 
superposed the nsup2 cells contain each of the nsup2 pairs of numbers from N 
exactly once. For n = 3 one has the superposed orthogonal squares 
001122=c122001=c210210=c 
498a 
where the first digits form the first square, and the second digits the second 
square. This square may be used to design an agricultural experiment which 
tests the interaction of three varieties of grain with three types of 
fertilizers. A field is divided into nine plots in each of which the choice of 
grain is made according to the first digit and the choice of fertilizer 
according to the second digit. The Latin squares assure the even distribution 
of the varieties of grain and fertilizer in both directions, so that effects 
such as the variation of the soil are minimized, and the orthogonality allows 
the experimenter to try each fertilizer with each variety of grain. 
    For n EQUAL TO OR LESS THAN 10 each of these arrays can be viewed as a 
particular type of magic square whose entries are distinct two-digit numbers 
and such that each row and column sum is 11n(n - 1)/2 (33 for n = 3 and 495 
for n = 10). 
    If n is odd, it is easy to construct two orthogonal squares A and B. One 
takes A = [asubisubj] and B = [ bsubisubj] where asubisubj = i + j, bsubisubj 
= i + 2j, reducing these sums by n or 2n if necessary to put them in the range 
0, . . ., n - 1. If n is a multiple of 4, it is not much more difficult to 
make a construction. But for n of the form 4m + 2 it is considerably harder. 
It is clearly impossible when n = 2, and, by an exhaustive trial, G. Tarry 
showed it to be also impossible for n = 6. Euler had conjectured in 1782 that 
no pair of orthogonal Latin squares existed for any n of the form 4m + 2, but 
in 1959 orthogonal Latin squares were found for every 4m + 2 greater than or 
equal to 10. See COMBINATORIAL THEORY. 
 �34s[T. Balderes]
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�6v�2v�32sLatite   �21m �33s 
    An aphanitic (not visibly crystalline) rock of volcanic origin, composed 
chiefly of sodic plagioclase and alkali feldspar with subordinate quantities 
of dark-colored minerals (biotite, amphibole, or pyroxene). Latite is 
intermediate between trachyte and andesite. See ANDESITE; TRACHYTE. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sLatitude and longitude   �21m �33s 
    The latitude of a location specifies the angle between an imaginary line 
directed generally toward the center of the Earth and the Equator. The 
longitude measures the angle between the meridian (the plane defined by the 
Earth's axis and this local reference direction) and the plane of the 
Greenwich meridian. 
    Astronomical (or astronomic) latitude and longitude use the direction of 
gravity for the reference direction. This direction, known as the astronomical 
vertical, is perpendicular to the equipotential surface of the Earth's 
gravitational field at the location of the observer. 
    A particular geopotential surface approximating mean sea level in the open 
ocean is called the geoid. A mathematical surface in the form of an oblate 
ellipsoid may be constructed to approximate the geoid. The direction 
perpendicular to this reference ellipsoid at the observer's location is used 
as the reference direction in defining geodetic latitude and longitude. See 
GEODESY.  
    Geocentric latitude and longitude are defined by a reference direction 
which passes precisely through the center of mass of the Earth. These 
coordinates are determined mathematically from the geodetic latitude and 
longitude, assuming a fixed relationship between the center of the geodetic 
datum and the center of mass and knowing the mathematical shape of the 
ellipsoid.  
 �34s[Dennis D. McCarthy]
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�6v�2v�32sLattice constant   �21m �33s 
    A parameter defining the unit cell of a crystal lattice, that is, the 
length of the edges of the cell and the angle between edges. If the unit cell 
is a cube, the side of it is the lattice constant, and it is usually in this 
sense that the term is used. See CRYSTALLOGRAPHY. 
 �34s[Willy C. Dekeyser]
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�6v�2v�32sLattice vibrations   �21m �33s 
    Periodic oscillation of the atoms in a crystal lattice about their 
equilibrium positions. As the crystal is heated, the amplitude of the 
vibrations increases. If the heating is continued, the temperature of the 
crystal eventually reaches a value at which the vibrations are so violent that 
the atoms break away from their lattice sites, and the solid melts. On the 
other hand, if the crystal is cooled to absolute zero, the amplitude of the 
vibrations does not subside entirely. A residual vibration of the atoms, which 
is quantum-mechanical in origin, remains. It is called the zero-point 
vibration.  
    Lattice vibrations are involved in many of the temperature-dependent 
phenomena of a solid. For example, the electrical resistance of a metal at 
room temperature arises primarily from the scattering of the conduction 
electrons by the vibrating atoms. The higher the temperature, and hence the 
more violent the vibrations of the atoms, the more the electrons are 
scattered, and the higher the electrical resistance becomes. The 
Bardeen-Cooper-Schrieffer theory of superconductivity postulates that subtle 
interactions between lattice vibrations and conduction electrons are mainly 
responsible for this phenomenon. See SUPERCONDUCTIVITY. 
    Lattice vibrations conduct heat through a crystal; thus a knowledge of the 
vibration mechanism is essential to an understanding of the heat conductivity 
of crystalline solids. 
 �34s[Jules de Launay]
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�6v�2v�32sLattice (mathematics)   �21m �33s 
    Lattice theory deals with properties of order and inclusion, much as group 
theory treats symmetry. As a generalization of Boolean algebra, lattice theory 
was first applied to algebraic number theory; however, it was later recognized 
as a major branch of mathematics, unifying various aspects of algebra, 
geometry, and functional analysis, as well as of set theory, logic, and 
probability. See BOOLEAN ALGEBRA; GROUP THEORY; SET THEORY. 
    The most basic concept of lattice theory is that of a partial ordering of 
a set S of elements x, y, z, . . . . By this is meant a binary relation, 
usually denoted IS LESS THAN OR EQUAL TO (or IS GREATER THAN OR EQUAL TO), 
with the following properties: 

(P1)   x IS LESS THAN OR EQUAL TO x for all x EPSILON S 
(P2)   If x IS LESS THAN OR EQUAL TO y and y IS LESS THAN OR EQUAL TO x, 
           then x = y 
(P3)   If x IS LESS THAN OR EQUAL TO y and y IS LESS THAN OR EQUAL TO z, 
           then x IS LESS THAN OR EQUAL TO z

If LESS THAN OR EQUAL TO is any partial ordering of S, then its converse or 
dual GREATER THAN OR EQUAL TO, defined by statement (1), is also a partial 

      *** x IS GREATER THAN OR EQUAL TO y if and only if
                                  y IS LESS THAN OR EQUAL TO x ***       (1)
  
ordering of S. This easily verified fact provides a fundamental Duality 
Principle, which is useful in many connections. 
    A lattice is defined as a partially ordered set in which any two elements 
x and y have a meet x INTERSECTION y and a join x UNION y. These binary 
operations satisfy the four basic identities: 

(L1)   x INTERSECTION x = x UNION x = x 
(L2)   x INTERSECTION y = y INTERSECTION x and x UNION y = y UNION x 
(L3)   x INTERSECTION (y INTERSECTION z) = (x INTERSECTION y) 
                INTERSECTION z and x UNION (y UNION z) = (x UNION y) UNION z
(L4)   x INTERSECTION (x UNION y) = x UNION (x INTERSECTION y) = x 

The operations INTERSECTION and UNION are connected with the relation LESS THAN 
OR EQUAL TO by the condition that x IS LESS THAN OR EQUAL TO y, x INTERSECTION 
y = x, and x UNION y = y are three equivalent statements. Conversely, if L is 
an algebraic system with operations INTERSECTION and UNION satisfying (L1) to 
(L4) for all x, y, z, then the preceding condition defines LESS THAN OR EQUAL 
TO as a partial ordering of L, with respect to which INTERSECTION and UNION 
have the meanings defined above. 
 �34s[Garrett Birkhoff]
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�6v�2v�32sLaunch complex   �21m �33s 
    The composite of facilities and support equipment needed to assemble, 
check out, and launch a rocket-propelled vehicle. The term usually is applied 
to the facilities and equipment required to launch larger vehicles for which a 
substantial amount of prelaunch preparation is needed. Small operational 
rockets may require similar but highly simplified resources on a much smaller 
scale. For these, the term launcher is usually used. See SPACE FLIGHT. 
 �34s[Karl Kristofferson]
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�6v�2v�32sLava   �21m �33s 
    Molten rock material that reaches the Earth's surface through volcanic 
vents and fissures; also, the igneous rock formed by consolidation of such 
molten material. Relatively rapid cooling at the Earth's surface may transform 
fluid lava into a dense-textured volcanic rock composed of tiny crystals or 
glass or both. Molten rock material below the Earth's surface, however, 
usually known as magma and, upon cooling, gives rise to coarse-grained igneous 
rock, such as granite or gabbro. 
    During many volcanic eruptions the lava is so rapidly ejected that it is 
blown to bits by the explosive force of expanding gases. The small masses 
rapidly congeal and settle to the earth to form thick blankets of volcanic 
tuff and related pyroclastic rock. See IGNEOUS ROCKS; MAGMA; PYROCLASTIC 
ROCKS; TUFF; VOLCANIC GLASS; VOLCANO. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sLaw of the mean �21m �33s
    A theorem about derivatives, called the mean-value theorem of differential
calculus. It is stated in this form: Suppose f is continuous for x from x1 to
x2 inclusive, and suppose f is continuous for x from x1 to x2 inclusive, and
suppose f has a derivative for each x between x1 and x2. Then there is some x
of this kind for which the equation below holds true. This theorem enables one
to prove that, if f'(x) is always positive, then f(x) is always zero, then
f(x) remains constant as x changes. The first of these results is important
for applications to the investigations of graphs. The second is virtually
indispensable in the proof of the fundamental theorems of calculus and in
establishing the uniqueness of the solutions of certain problems which involve
antidifferentiation (the process of surmising what a function is from a
knowledge of its derivative). Such problems about in application of calculus
to problems of motion.
Then F has a derivative given by F'(x) = f(x).
 �34s[Angus E. Taylor]
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		f'(x) = f(x sub 2) - f(x sub 1)/x(sub 2) - x(sub 1)



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�6v�2v�32sLawrencium �21m �33s
    A chemical element, symbol Lr, atomic number 103. Lawrencium, named after
E. O. Lawrence, is the eleventh transuranium element; it completes the
actinide series of elements. See ACTINIDE ELEMENTS; TRANSURANIUM ELEMENTS.
    The nuclear properties of all the isotopes of lawrencium from mass 255 to
mass 260 have been established. (sup 260)Lr is an alpha emitter with a
half-life of 3 min and consequently is the longest-lived isotope known.
 �34s[Albert Ghiorso]
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�6v�2v�32sLawson criterion   �21m �33s 
    A necessary but not sufficient condition for the achievement of a net 
release of energy from nuclear fusion reactions in a fusion reactor. As 
originally formulated by J. D. Lawson, this condition simply stated that a 
minimum requirement for net energy release is that the fusion fuel charge must 
combust for at least enough time for the recovered fusion energy release to 
equal the sum of energy invested in heating that charge to fusion 
temperatures, plus other energy losses occurring during combustion. The Lawson 
criterion is to be thought of as only a rule of thumb for measuring fusion 
progress; detailed evaluation of all energy dissipative and energy recovery 
processes is required in order properly to evaluate any specific system. See 
NUCLEAR FUSION; PLASMA PHYSICS. 
 �34s[Richard F. Post]
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�6v�2v�32sLawsonite   �21m �33s 
    A metamorphic silicate mineral related chemically and structurally to the 
epidote group of minerals. Its composition is CaAl2(H2O) (OH)2[Si2O7]. It 
possesses two perfect cleavages; crystals are orthorhombic prismatic to 
tabular, and colorless to pale blue; specific gravity is 3.1, and hardness is 
6.5 on Mohs scale. See EPIDOTE; SILICATE MINERALS. 
 �34s[Paul B. Moore]
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�6v�2v�32sLayout drawing   �21m �33s 
    A design drawing or graphical statement of the overall form of a component 
or device, which is usually prepared during the innovative stages of a design. 
Since it lacks detail and completeness, a layout drawing provides a faithful 
explanation of the device and its construction only to inidividuals such as 
designers and drafters who have been intimately involved in the conceptual 
state. In a sense, the layout drawing is a running record of ideas and 
problems posed as the design evolves. In most cases the layout drawing 
ultimately becomes the primary source of information from which detail 
drawings and assembly drawings are prepared by other drafters under the 
guidance of the designer. See DRAFTING; ENGINEERING DRAWING. 
 �34s[Robert W. Mann]
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�6v�2v�32sLazurite   �21m �33s 
    The chief mineral constituent in the ornamental stone lapis lazuli. 
Lazurite is a feldspathoid. It crystallizes in the isometric system, but 
well-formed crystals, usually dodecahedral, are rare. Most commonly, it is 
granular or in compact masses. The hardness is 5 - 5.5 on Mohs scale, and the 
specific gravity is 2.4 - 2.5. There is vitreous luster and the color is a 
deep azure, more rarely a greenish-blue. Lazurite is a tectosilicate, the 
composition of which is expressed by the formula Na4Al3Si3O12S. 
    Lapis lazuli is a mixture of lazurite with other silicates and calcite and 
usually contains disseminated pyrite. It has long been valued as an ornamental 
material. Localities of occurrence are in Afghanistan; Lake Baikal, Siberia; 
Chile; and San Bernardino County, California. See FELDSPATHOID; SILICATE 
MINERALS.  
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sLeaching �21m �33s
    The removal of a soluble fraction, in the form of a solution, from an
insoluble, permeable solid with which it is associated. The separation usually
involves selective dissolving, with or without diffusion, but in the extreme
case of simple washing it consists merely of the displacement (with some
mixing) of one interstitial liquid by another with which it is miscible. The
soluble constituent may be solid (as the metal leached from ore) or liquid (as
the oil leached from soybeans).
    Leaching is closely related to solvent extraction, in which a soluble
substance is dissolved from one liquid by a second liquid immiscible with the
first. Both leaching and solvent extraction are often called extraction.
Because of its variety of applications and its importance to several ancient
industries, leaching is known by a number of other names: solid-liquid
extraction, lixiviation, percolation, infusion, washing, and
decantation-settling. The liquid used to leach away the soluble material (the
solute) is termed the solvent. The resulting solution is called the extract or
sometimes the miscella.
    Leaching processes fall into two principal classes: those in which the
leaching is accomplished by percolation (seeping of solvent through a bed of
solids), and those in which particulate solids are dispersed into the
extracting liquid and subsequently separated from it. In either case, the
operation may be a batch process or continuous. See EXTRACTION; FILTRATION;
MASS-TRANSFER OPERATION; SOLVENT EXTRACTION.
 �34s[Shelby A. Miller]
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�6v�2v�32sLead   �21m �33s 
    A chemical element, Pb, atomic number 82 and atomic weight 207.19. Lead is 
a heavy metal (specific gravity 11.34 at 16DGC or 61DGF), of bluish color, 
which tarnishes to dull gray. It is pliable, inelastic, easily fusible, melts 
at 327.4DGC (621.3DGF), and boils at 1740DGC (3164DGF). The normal chemical 
valences are 2 and 4. It is relatively resistant to attack by sulfuric and 
hydrochloric acids but dissolves slowly in nitric acid. Lead is amphoteric, 
forming lead salts of acids as well as metal salts of plumbic acid. Lead forms 
many salts, oxides, and organometallic compounds. 
    Industrially, the most important lead compounds are the lead oxides and 
tetraethyllead. Lead forms alloys with many metals and is generally employed 
in the form of alloys in most applications. Alloys formed with tin, copper, 
arsenic, antimony, bismuth, cadmium, and sodium are all of industrial 
importance. See LEAD ALLOYS; TETRAETHYLLEAD. 
    Lead compounds are toxic and have resulted in poisoning of workers from 
misuse and overexposure. However, lead poisoning is presently rare because of 
the industrial application of modern hygienic and engineering controls. The 
greatest hazard arises from the inhalation of vapor or dust. In the case of 
organolead compounds, absorption through the skin may become significant. Some 
of the symptoms of lead poisoning are headaches, dizziness, and insomnia. In 
acute cases there is usually stupor, which progresses to coma and terminates 
in death. The medical control of employees engaged in lead usage involves 
precise clinical tests of lead levels in blood and urine. With such control 
and the proper application of engineering control, industrial lead poisoning 
may be entirely prevented. 
    Lead rarely occurs in its elemental state. The most common ore is the 
sulfide, galena. The other minerals of commercial importance are the 
carbonate, cerussite, and the sulfate, anglesite, which are much more rare. 
Lead also occurs in various uranium and thorium minerals, arising directly 
from radioactive decay. Commercial lead ores may contain as little as 3% lead, 
but a lead content of about 10% is most common. The ores are concentrated to 
40% or greater lead content before smelting. See LEAD METALLURGY. 
    The largest single use of lead is for the manufacture of storage 
batteries. Other important applications are for the manufacture of 
tetraethyllead, cable covering, construction, pigments, solder, and ammunition. 
    Organolead compounds are being developed for applications such as 
catalysts for polyurethane foams, marine antifouling paint toxicants, biocidal 
agents against gram-positive bacteria, protection of wood against marine 
borers and fungal attack, preservatives for cotton against rot and mildew, 
molluskicidal agents, anthelmintic agents, wear-reducing agents in lubricants, 
and corrosion inhibitors for steel. 
    Because of its excellent resistance to corrosion, lead finds extensive use 
in construction, particularly in the chemical industry. It is resistant to 
attack by many acids because it forms its own protective oxide coating. 
Because of this advantageous characteristic, lead is used widely in the 
manufacture and handling of sulfuric acid. 
    Lead has long been used as protective shielding for x-ray machines. 
Because of the expanded applications of atomic energy, radiation-shielding 
applications of lead have become increasingly important. See RADIATION 
SHIELDING.  
    Lead sheathing for telephone and television cables continues to be a 
sizable outlet for lead. The unique ductility of lead makes it particularly 
suitable for this application because it can be extruded in a continuous 
sheath around the internal conductors. 
    The use of lead in pigments has been a major outlet for lead but is 
decreasing in volume. White lead, 2PbCO3PRODUCTPb(OH)2, is the most 
extensively used lead pigment. Other lead pigments of importance are basic 
lead sulfate and lead chromates. See PIGMENT. 
    A considerable variety of lead compounds, such as silicates, carbonates, 
and salts of organic acids, are used as heat and light stabilizers for 
polyvinyl chloride plastics. Lead silicates are used for the manufacture of 
glass and ceramic frits, which are useful in introducing lead into glass and 
ceramic finishes. Lead azide, Pb(N3)2, is the standard detonator for 
explosives. Lead arsenates are used in large quantities as insecticides for 
crop protection. Litharge (lead oxide) is widely employed to improve the 
magnetic properties of barium ferrite ceramic magnets. Also, a calcined 
mixture of lead zirconate and lead titanate, known as PZT, is finding 
increasing markets as a piezoelectric material. 
 �34s[Hymin Shapiro ;James D. Johns]
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�6v�2v�32sLead alloys   �21m �33s 
    Substances formed by the addition of one or more elements, usually metals, 
to lead. Lead alloys may exhibit greatly improved mechanical or chemical 
properties as compared to pure lead. The major alloying additions to lead are 
antimony and tin. The solubilities of most other elements in lead are small, 
but even fractional weight percent additions of some of these elements, 
notably copper and arsenic, can alter properties appreciably. 
    Lead is used as a sheath over the electrical components to protect power 
and telephone cable from moisture. Lead alloy grids are used in the lead-acid 
storage battery (the type used in automobiles) to support the active material 
composing the plates. Chemical-resistant alloys are used extensively in many 
applications requiring resistance to water, atmosphere, or chemical corrosion. 
Lead bearing metals (babbitt metals) find frequent application in cast sleeve 
bearings, and are used extensively in freight-car journal bearings. Lead-base 
solder contains large amounts of tin with selected minor additions to provide 
specific benefits. See ALLOY; LEAD; LEAD METALLURGY; SOLDERING; TIN ALLOYS. 
 �34s[Dudley Williams]
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�6v�2v�32sLead metallurgy   �21m �33s 
    The extraction of lead from ore, its subsequent purification and 
processing, and its alloying with other metals to achieve desired properties. 
    There are many types and several specifications for lead. Primary lead is 
lead which is extracted and refined from ore. Secondary lead, recycled mostly 
from old storage batteries, can be refined to meet the same specifications as 
the primary metal, but the remelting, drossing, and recasting involved may 
have a negative impact on composition. 
    In the United States most lead ore is smelted and refined to a minimum 
purity of 99.85%. At and above this level of purity, four different grades of 
lead are recognized by the American Society for Testing and Materials: 
corroding, chemical, acid-copper, and common desilverized lead. The major 
differences among these grades are the allowable concentrations of copper, 
silver, and bismuth. Even trace amounts of these elements can have a 
significant effect on the properties or cost of the lead and justify having 
the four grades. 
    The first step in the beneficiation of ores to raise the lead content and 
to separate the lead from the zinc and iron minerals is concentrating. The 
standard processing begins with crushing the raw ore, followed by wet 
grinding, and then by mixing with flotation chemicals that collect the lead 
minerals in a froth, which is thickened and filtrated. The lead concentrate 
produced in this first processing step has a lead metal content of around 70%. 
    Before smelting, lead concentrates are frequently blended with high-grade 
raw ores or returned intermediates (flue dusts, limerock, and so forth) drawn 
from proportioning bins. These materials are pelletized so that a homogeneous 
and carefully sized smelter feed is provided. The feed is then sintered. The 
sinter product is charged into the top of a blast furnace. As the charge 
descends in the furnace, the molten metal flows to the bottom, from where it 
is withdrawn for refining. See LEAD. 
 �34s[A. L. Ponikvar]
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�6v�2v�32sLeaf   �21m �33s 
    A lateral appendage which is borne on a plant stem at a node (joint) and 
which usually has a bud in its axil. In most plants, leaves are flattened in 
form, although they may be nearly cylindrical with a sheathing base as in 
onion. Leaves usually contain chlorophyll and are the principal organs in 
which the important processes of photosynthesis and transpiration occur. 
  
    Morphology.  A complete dicotyledon leaf consists of three parts: the 
expanded portion or blade; the petiole which supports the blades; and the leaf 
base. Stipules are small appendages that arise as outgrowths of the leaf base 
and are attached at the base of the petiole. The leaves of monocotyledons may 
have a petiole and a blade, or they may be linear in shape without 
differentiation into these parts; in either case the leaf base usually 
encircles the stem. The leaves of grasses consist of a linear blade attached 
to the stem by an encircling sheath. 
    Leaves are borne on a stem in a definite fixed order, or phyllotaxy, 
according to species (Fig. 1). For identification purposes, leaves are 
classified according to type (Fig. 2) and shape (Fig. 3), and types of margins 
(Fig. 4), tips, and bases (Fig. 5). The arrangement of the veins, or vascular 
bundles, of a leaf is called venation (Fig. 6). The main longitudinal veins 
are usually interconnected with small veins. Reticulate venation is most 
common in dicotyledons, parallel venation in monocotyledons. 
  
    Surfaces of leaves provide many characteristics that are used in 
identification. A surface is glabrous if it is smooth or free from hairs; 
glaucous if covered with a whitish, waxy material, or ``bloom''; scabrous if 
rough or harsh to the touch; pubescent, a general term for surfaces that are 
hairy; puberulent if covered with very fine, downlike hairs; villous if 
covered with long, soft, shaggy hairs; hirsute if the hairs are short, erect, 
and stiff; and hispid if they are dense, bristly, and harshly stiff. 
    The texture may be described as succulent when the leaf is fleshy and 
juicy; hyaline if it is thin and almost wholly transparent; chartaceous if 
papery and opaque but thin; scarious if thin and dry, appearing shriveled; and 
coriaceous if tough, thickish, and leathery. 
    Leaves may be fugacious, falling nearly as soon as formed; deciduous, 
falling at the end of the growing season; marcescent, withering at the end of 
the growing season but not falling until toward spring; or persistent, 
remaining on the stem for more than one season, the plant thus being 
evergreen. See DECIDUOUS PLANTS; EVERGREEN PLANTS. 
  
    Anatomy.  The foliage leaf is the chief photosynthetic organ of most 
vascular plants. Although leaves vary greatly in size and form, they share the 
same basic organization of internal tissues and have similar developmental 
pathways. Like the stem and root, leaves consist of three basic tissue 
systems: the dermal tissue system, the vascular tissue system, and the ground 
tissue system. However, unlike stems and roots which usually have radial 
symmetry, the leaf blade usually shows dorsiventral symmetry, with vascular 
and other tissues being arranged in a flat plane. 
    Stems and roots have apical meristems and are thus characterized by 
indeterminate growth; leaves lack apical meristems, and therefore have 
determinate growth. Because leaves are more or less ephemeral organs and do 
not function in the structural support of the plant, they usually lack 
secondary growth and are composed largely of primary tissue only. See APICAL 
MERISTEM; ROOT; STEM. 
    The internal organization of the leaf is well adapted for its major 
functions of photosynthesis, gas exchange, and transpiration. The 
photosynthetic cells, or chlorenchyma tissue, are noramlly arranged in 
horizontal layers, which facilitates maximum interception of the Sun's 
radiation. The vascular tissues form an extensive network throughout the leaf 
so that no photosynthetic cell is far from a source of water, and 
carbohydrates produced by the chlorenchyma cells need travel only a short 
distance to reach the phloem in order to be transported out of the leaf (Fig. 
7). The epidermal tissue forms a  continuous covering over the leaf so that 
undue water loss is reduced, while at the same time the exchange of carbon 
dioxide and oxygen is controlled. See EPIDERMIS (PLANT); PARENCHYMA; PHLOEM; 
XYLEM.  
 �34s[Nancy G. Dengler]
�33s 
  
A700000002341�254s�5v�1v973
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1808384
974
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1968128
�6v�2v�32sLeast-action principle �21m �33s
    Like Hamilton's principle, the principle of least action is a variational
statement that forms a basis from which the equations of motion of a classical
dynamical system may be deduced. Consider a mechanical system described by
coordinates q1, . . . , qsubf and their canonically conjugate momenta p1, . .
. , psubf. The action S associated with a segment of the trajectory of the
system is defined by the equation below, where the integral is evaluated along

	  S = INTEGRATION (c to  ) SUMMATION (over j) p(sub j) dq(sub j)

the given segment c of the trajectory. The action is of interest only when the
total energy E is conserved. The principle of least action states that the
trajectory of the system is that path which makes the value of S stationary
relative to nearby paths between the same configurations and for which the
energy has the same constant value. The principle is misnamed, as only the
stationary property is required. It is a minimum principle for sufficiently
short but finite segments of the trajectory. See HAMILTON'S EQUATIONS OF
MOTION; HAMILTON'S PRINCIPLE; MINIMAL PRINCIPLES.
 �34s[Philip M. Stehle]
�33s

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�6v�2v�32sLeast-squares method �21m �33s
    A method of obtaining the best values (the ones with least error) of
unknown quantities supposed to satisfy a system of linear equations of the
form shown as notation (1), where n > m. Since there are more equations

EQ.1:

M(sub 11)a(sub 1) + M(sub 12)a(sub 2) + . . . + M(sub 1m) a(sub m) = b1
M(sub 21)a(sub 1) + M(sub 22)a(sub 2) + . . . + M(sub 2m) a(sub m) = b2
. . . . . . . . . . . . . . . . . . . . .
M(sub n1) a(sub 1) + M(sub n2) a(sub 2) + . . . + M(sub nm) a(sub m) = b(sub n)

than unknowns, the system is said to be overdetermined. In the physical
situation, the bsubi are measured quantities, the Msubij are known (or assumed)
quantities, and the asubi are to be adjusted to their best values.
    Consider a simple example. A quantity y of interest is supposed (perhaps
for theoretical reasons) to be a linear function of an independent variable x.
For a series of selected values x1, x2, . . . of x the values y1, y2, . . . of
y are measured. The expected relation is shown as notation (2), and the problem
is to find the best values of alpha and beta, that is, respectively, the
slope and intercept of the line which graphically represents the function. The
best values of alpha and beta, in the least squares sense, are obtained by
writing Eq. (3) and asserting that term (4) shall be minimized with respect to

		      x(sub 1)alpha + beta = y1
		      x(sub 2)alpha + beta = y2
		      x(sub 3)alpha + beta = y3 			     (2)

	   ta(sub i) = y(sub i) - [x(sub i)alpha + beta]		     (3)

	      SUMMATION (i=1 to n)(eta[sub i])(sup 2)			     (4)

alpha and beta; that is, that Eqs. (5) hold. This leads to Eqs. (6) and (7),
which may be

(CURLY D/CURLY D alpha) SUMMATION (i=1 to n) (eta[sub i])(sup 2) = 0
CURLY D/CURLY D beta) SUMMATION (i=1 to n) (eta[sub i])(sup 2) = 0	     (5)

alpha SUMMATION (i=1 to n) (x[sub i] + eta beta) -
SUMMATION (i=1 to n) y1 = 0						     (6)

		alpha SUMMATION (i=1 to n) x(sub i)(sup 2)
		   + beta SUMMATION (i=1 to n) x(sub i)
		   - SUMMATION (i=1 to n) x(sub i) y(sub i) = 0 	     (7)

solved for alpha and beta. See CURVE FITTING.
 �34s[McAllister H. Hull, Jr.]
�33s

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�6v�2v�32sLecanicephaloidea   �21m �33s 
    An order of tapeworms of the subclass Cestoda. All species are intestinal 
parasites of elasmo-branch fishes. These tapeworms are distinguished by having 
a peculiar scolex divided into two portions. The lower portion is collarlike 
and bears four small suckers; the upper portion may be discoid or 
tentacle-bearing and is provided with glandular structures (see illustration). 
The scolex is usually buried in the intestinal wall of the host and may 
produce local pathology. The anatomy of the segments is very similar to that 
of the Proteocephaloidea.  See  CESTODA; TETRAPHYLLIDEA. 
 �34s[Clark P. Read]
�33s 
  
  
A700000001745�254s�5v�1v974
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�6v�2v�32sLecanorales   �21m �33s 
    An order of the Ascolichenes, also known as the Discolichenes. Lecanorales 
is the largest and most typical order of lichens and parallels closely the 
fungal order Helotiales. The apothecia are open and discoid, with a typical 
hymenium and hypothecium. There are four growth forms - crustose, squamulose, 
foliose, and fruticose - all showing greater variability than any other order 
of lichens. 
    The Lecanorales is divided into 25 families, about 160 genera, and 8000 - 
10,000 species. Family divisions are based on growth form of the thallus, 
structure of the apothecia, the species of symbiotic algae present, and spore 
characters. Species are separated by such characters as isidia, soredia, 
rhizines, and pores, and by chemistry. The larger families include: 
Cladoniaceae, Lecanoraceae, Lecideaceae, Parmeliaceae, Umbilicariaceae, and 
Usneaceae.  See  ASCOLICHENES. 
 �34s[Mason E. Hale, Jr.]
�33s 
  
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�6v�2v�32sLectins   �21m �33s 
    A group of proteins, also referred to as phytohemagglutinins and 
phytolectins, which possess the unique ability to agglutinate erythrocytes and 
other types of cells. Lectins are widely distributed in the plant kingdom, 
particularly among the legumes and to a lesser extent among the cereal grains. 
    All of the effects of lectins are believed to be a manifestation of the 
ability of lectins to bind to specific kinds of sugars present on the surface 
of cells. By virtue of this property, the lectins are an extremely useful 
tool, not only for the isolation and characterization of polysaccharides and 
glycoproteins, but also for probing the molecular structure of cell surfaces 
and the changes induced therein by chemical and biological agents. 
    There is some evidence to indicate that the lectins may serve to protect 
plants against certain pathogenic fungi and insect predators. Stimulation of 
pollen germination and seedling growth and regulation of cell wall extension 
are other physiological roles postulated for phytolectins. 
 �34s[Irvin E. Liener]
�33s 
  
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�6v�2v�32sLecythidales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of the single family Lecythidaceae, with about 450 species. They are 
tropical, woody plants with alternate, entire leaves, valvate sepals, separate 
petals, numerous centrifugal stamens, and a syncarpous, inferior ovary with 
axile placentation. Brazil nuts are the seeds of Bertholletia excelsa, a 
member of the Lecythidaceae.  See  BRAZIL NUT; DILLENIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sLegendre functions �21m �33s
    Solutions to the differential equation (1 - x[sup 2])y'' - 2xy' +
v(v + 1)y = 0.
    The most elementary of the Legendre functions, the Legendre polynomial
Psubn (x) can be defined by the generating function in Eq. (1). More explicit

(1 - 2xr + r[sup 2])(sup -1/2) =
SUMMATION (n=0 to INFINITY) P(sub n) (x)r(sup n)		       (1)

P(sub n) (x) = [(-1)(sup n)]/(2(sup n) n!)
(d[sup n])/(dx[sup n])/(1 - x[sup 2])(sup n)			       (2)

P(sub n) (x) = (sub 2)F1[-n, n + 1; 1; (1 - x)/2]		       (3)

representations are Eq. (2), and the hypergeometric function, Eq. (3).	See
HYPERGEOMETRIC FUNCTIONS.

(a[sup 2] - 2ar cos theta + r[sup 2])(sup -1/2)
= (1/a) SUMMATION (n=0 to INFINITY) Psubn (cos theta) (r/a)(sup n)	(4)

				 0 < r < a

    Generating function (1) implies Eq.  (4).  The function (a[sup 2] - 2ar
cos theta + r[sup 2])(sup -1/2) represents the potential in an inverse square
field at a point P of a source at A, where r and a are the distances from P and
A to a fixed point O, and theta is the angle between the segments PO and OA.
See DIFFERENTIAL EQUATION; ORTHOGONAL POLYNOMIALS; POTENTIALS.

 �34s[Richard Askey]
�33s

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�6v�2v�32sLegionnaire's disease   �21m �33s 
    A type of pneumonia caused by infection with the bacterium Legionella 
pneumophila. It is called Legionnaire's disease because it was first observed 
in an epidemic among those attending an American Legion convention in 
Philadelphia, Pennsylvania, in 1976. The initial symptoms are headache, muscle 
aches, and a generalized feeling of discomfort. The fever rises rapidly, 
reaching 102-105DGF (38.9-40.6DGC), and is usually accompanied by cough, 
shortness of breath, and chest pain. Abdominal pain and diarrhea are also 
often present. Ninety percent of Legionnaire's disease patients develop 
pneumonia, and approximately 15-20% of those die if they do not receive 
specific antibiotic treatment. Erythromycin is the antibiotic of choice. 
    The available epidemiologic evidence indicates that Legionnaire's disease 
is not contagious. Instead, it is thought that people become infected by 
inhalation of droplets of contaminated water, particularly the water that 
drifts down from cooling towers or evaporative condensers of air-conditioning 
systems.  
 �34s[Joseph E. McDade]
�33s 
  
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�6v�2v�32sLegume �21m �33s
    A dry, dehiscent fruit derived from a single, simple pistil. When mature,
it splits along both dorsal and ventral sutures into two valves. The term also
designates any plant of the order Rosales that bears this type of fruit.
    The family Leguminosae characteristically contains a single row of seeds
attached along the lower or ventral suture of the fruit. The seeds are highly
nutritious and several species of legumes furnish a large amount of food for
both humans and animals. Some more common and important legumes are alfalfa,
beans, clovers, kudzu, lespedeza, locust, peas, peanuts, soybeans, and vetch.
See  FRUIT; ROSALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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\IMG\CI961.IMG
2055680
�6v�2v�32sLeg (anatomy)   �21m �33s 
    The portion of the lower, or inferior, limb between the thigh and the 
foot. The leg bones are the tibia (a large, weight-supporting shinbone) and 
the slender, rodlike fibula. Compartmentalized muscle groups include 
functional sets which act on the foot, ankle, and thigh. Such muscles may 
either move the point to which they are attached or immobilize it to prevent 
movement, as when standing. 
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sLeishmaniasis   �21m �33s 
    An infection caused by any one of three pathogenic hemoflagellates of the 
genus Leishmania, transmitted to humans by blood-sucking sand flies 
(Phlebotomus). Three diseases are distinguished: visceral leishmaniasis, known 
also as kala azar, caused by L. donovani; cutaneous leishmaniasis (Oriental 
sore, Baghdad boil), caused by L. tropica; and mucocutaneous leishmaniasis 
(espundia), caused by L. braziliensis. 
    In cutaneous leishmaniasis the parasites invade only the skin and 
subcutaneous tissue, causing deep ulcers which upon healing confer a lasting 
immunity. The disease is widespread in the Near and Middle East, stretching to 
Asiatic Russia. Mucocutaneous leishmania affects the mucous membranes of the 
nose and mouth causing severe ulceration and disfigurement. It is prevalent in 
some areas of Central and South America. Visceral leishmaniasis is a severe, 
generalized, and often fatal infection affecting the organs rich in 
reticuloendothelial cells. There are endemic areas in India and China, in 
Central and North Africa, and along the Mediterranean littoral, where it 
affects mainly the young (infantile kala azar). 
 �34s[Meir Yoeli]
�33s 
  
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�6v�2v�32sLeitneriales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae) in the 
subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of a single family, genus, and species (Leitneria floridana) of the 
southeastern United States. The plants are simple-leaved, dioecious shrubs 
with the flowers in catkins.  See  HAMAMELIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sLemming   �21m �33s 
    The name applied to 11 species of rodents in the subfamily Microtinae, 
family Muridae. These animals have a northern circumpolar distribution. 
    The Norway lemming (Lemmus lemmus) is found in the mountainous wastelands 
of northern Norway and Lapland. It is usually nocturnal and timid in its 
habits, except when a population explosion occurs with its resultant 
migration, of which the causative factors are not known. Cyclic variations in 
fertility may be a factor. Usually there are two litters of five offspring 
each year, but often four litters of two to eight offspring occur.  See  
RODENTIA.  
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sLemniscate of Bernoulli   �21m �33s 
    A curve shaped like the figure eight (see illustration), referred to by 
Jacques Bernoulli in 1694. Let F sub 1, F sub 2 be points of a plane PI, with 
F sub 1 F sub 2 = 2a, a > 0. The locus of a point P of PI which moves so that 
(PF sub 1) PRODUCT (PF sub 2) = b sup 2, where b is a positive constant, is 
called an oval of Cassini. The lemniscate is obtained when b = a. Its equation 
in rectangular coordinates is [(x sup 2 + y sup 2) sup 2] = [a sup 2 
(x sup 2 - y sup 2)] and in polar coordinates RHO sup 2 = a sup 2 cos 2 THETA.  
See ANALYTIC GEOMETRY. 
 �34s[Leonard M. Blumenthal]
�33s 
  

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�6v�2v�32sLemon �21m �33s
    The fruit Citrus limon. The yellow fruits are medium-sized and elongate
with a prominent nipple. The lemon is more sensitive to cold than other major
citrus fruits, and thus its commercial culture is restricted to areas with
mild winter temperatures.
    The lemon is grown primarily for its acid flavor. Lemon juice, very high
in vitamin C, is used in beverages and has many culinary uses. It is also used
widely in proprietary soft drinks. The principal by-products are citric acid
from the juice and lemon oil from the peel.  See  ASCORBIC ACID.
    Commercial lemon production developed first in Italy, mainly in Sicily.
Italy is the largest producer, followed by California. Spain, Greece, and
Argentina are also significant producers.
 �34s[R. K. Soost]
�33s


l
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�6v�2v�32sLemur   �21m �33s 
    A primate of the family Lemuridae; all 16 species are indigenous to 
Madagascar. All lemurs have long tails, foxlike faces, and scent glands on the 
shoulder region and wrists. By constantly running its tail through its arms, 
the lemur creates a scent flag. Lemurs are largely dependent upon fruit as 
their diet; they also eat insects and eggs. A litter containing one or two 
young is born in the early spring after a gestation period of 2 months. The 
young cling to the underside of the mother for about the first 3 weeks and are 
then carried on her back for a period of time.  See  PRIMATES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sLength   �21m �33s 
    Extension in space. Length is one of the three fundamental physical 
quantities (the other two being mass and time), and therefore cannot be 
defined in terms of simpler quantities. It is measured by comparison with an 
arbitrary standard called the international meter, defined as 1,650,763.73 
times the wavelength of the orange light emitted when a gas of pure krypton-86 
is excited in an electrical discharge. 
 �34s[Dudley Williams]
�33s 
  
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�6v�2v�32sLens (optics)   �21m �33s 
    A curved piece of ground and polished or molded material, usually glass, 
used for the refraction of light. Its two surfaces have the same axis. Usually 
this is an axis of rotation symmetry for both surfaces; however, one or both 
of the surfaces can be toric, cylindrical, or a general surface with double 
symmetry (see illustration). The intersection points of the symmetry axis with 
the two surfaces are called the front and back vertices and their separation 
is called the thickness of the lens. There are three lens types, namely, 
compound, single, and cemented. A group of lenses used together is a lens 
system. Such systems may be divided into four classes: telescopes, oculars 
(eyepieces), photographic objectives, and enlarging lenses. 
  
    Lens types.  A compound lens is a combination of two or more lenses in 
which the second surface of one lens has the same radius as the first surface 
of the following lens and the two lenses are cemented together. Compound 
lenses are used instead of single lenses for color correction, or to introduce 
a surface which has no effect on the aperture rays but large effects on the 
principal rays, or vice versa. Sometimes the term compound lens is applied to 
any optical system consisting of more than one element, even when they are not 
in contact. 
    The diameter of a simple lens is called the linear aperture, and the ratio 
of this aperture to the focal length is called the relative aperture. This 
latter quantity is more often specified by its reciprocal, called the 
f-number. Thus, if the focal length is 50 mm and the linear aperture 25 mm, 
the relative aperture is 0.5 and the f-number is f/2.  See  FOCAL LENGTH.  
    A compound lens made of two or more simple thin lenses cemented together 
is called a cemented lens. 
  
    Lens systems.  A lens system consisting of two systems combined so that 
the back focal point of the first (the objective) coincides with the front 
focal point of the second (the ocular) is called a telescope. Parallel 
entering rays leave the system as parallel rays. The magnification is equal to 
the ratio of the focal length of the first system to that of the second.  See  
TELESCOPE.  
  
    A photographic objective images a distant object onto a photographic plate 
or film. The amount of light reaching the light-sensitive layer depends on the 
aperture of the optical system, which is equivalent to the ratio of the lens 
diameter to the focal length. The larger the aperture (the smaller the 
f-number), the less adequate may be the scene luminance required to expose the 
film. Therefore, if pictures of objects in dim light are desired, the f-number 
must be small. On the other hand, for a lens of given focal length, the depth 
of field is inversely proportional to the aperture. 
    In general, photographic objectives with large fields have small 
apertures; those with large apertures have small fields. 
    The basic type of enlarger lens is a holosymmetric system consisting of 
two systems of which one is symmetrical with the first system except that all 
the data are multiplied by the enlarging factor m. When the object is in the 
focus of the first system, the combination is free from all lateral errors 
even before correction. A magnifier in optics is a lens that enables an object 
to be viewed so that it appears larger than its natural size. The magnifying 
power is usually given as equal to one-quarter of the power of the lens 
expressed in diopters.  See  DIOPTER; MAGNIFICATION. 
 �34s[Max J. Herzberger]
�33s 
  
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�6v�2v�32sLentil   �21m �33s 
    A semiviny annual legume with slender tufted and branched stems. The 
lentil plant (Lens esculenta) was one of the first plants brought under 
cultivation. The world's lentil production is centered in Asia, with nearly 
two-thirds of the production from India, Pakistan, Turkey, and Syria. Whitman 
and Spokane counties in Washington, and Latah, Benewah, and Nez Perce counties 
in Idaho grow about 95% of the lentils produced in the United States. 
    The seeds grow in short broad pods, each pod producing two or three thin 
lens-shaped seeds. Seed color varies from yellow to brown and may be mottled, 
although mottled seeds are not desirable for marketing. 
    Lentil seed is used primarily for soups but also in salads and casseroles. 
Lentils are more digestible than meat and are used as a meat substitute in 
many countries.  See  LEGUME. 
 �34s[Kenneth J. Morrison]
�33s 
  
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�6v�2v�32sLenz's law   �21m �33s 
    A law of electromagnetism which states that, whenever there is an induced 
electromotive force (emf) in a conductor, it is always in such a direction 
that the current it would produce would oppose the change which causes the 
induced emf. If the change is the motion of a conductor through a magnetic 
field, the induced current must be in such a direction as to produce a force 
opposing the motion. If the change causing the emf is a change of flux 
threading a coil, the induced current must produce a flux in such a direction 
as to oppose the change. 
    Lenz's law is a form of the law of conservation of energy, since it states 
that a change cannot propagate itself.  See  CONSERVATION OF ENERGY; 
ELECTROMAGNETIC INDUCTION. 
 �34s[Kenneth V. Manning]
�33s 
  
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�6v�2v�32sLeo   �21m �33s 
    The Lion, in astronomy, is a magnificent zodiacal constellation appearing 
during spring and early summer. It is the fifth sign of the zodiac. Leo is 
well defined and bears a close resemblance to the creature it represents (see 
illustration). Associated with this constellation are the famous Leonids 
shower of meteors, which can be seen radiating from Leo in November of each 
year and appearing especially brilliant at intervals of about 33 years.  See  
CONSTELLATION.  
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sLepadomorpha   �21m �33s 
    A suborder of the Thoracica. These barnacles have a stalk or peduncle, 
which morphologically is the elongated, anterior, preoral region of the body, 
and a capitulum comprising the bivalved mantle enclosing the rest of the body 
(see illustration). The mantle folds are usually protected by a varying number 
of calcareous plates. The peduncle also may be protected by small calcareous 
scales or granules. Caudal furca and filamentary appendages are often present. 
These barnacles are hermaphroditic, or with separate sexes. 
    Several families are distinguished, the most primitive being Scalpellidae. 
  See  THORACICA. 
 �34s[H. G. Stubbings]
�33s 
  
  
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�6v�2v�32sLepidodendrales �21m �33s
    Giant club mosses, an order of extinct lycopods consisting primarily of
arborescent forms, which constituted an important element of the Carboniferous
flora. The group is characterized by branching, by spirally arranged
microphyllous leaves, by the production of small amounts of secondary vascular
tissues relative to the total bulk of the plant (several species are not known
to have produced secondary tissues), and by heterospory (bearing spores of two
types). In at least two genera heterospory attained a level of specialization
approaching the seed habit. The order existed from Late Devonian through
Permian times. Well-known genera are Lepidodendron, Lepidophloios, Sigillaria,
Bothrodendron, Stigmaria, Lepidostrobus, Lepidocarpon, Miadesmia, and
Mazocarpon.  See  LYCOPODIOPSIDA.
 �34s[Charles B. Beck ;Hermann W. Pfefferkorn]
�33s

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�6v�2v�32sLepidolite   �21m �33s 
    A mineral of variable composition that is also called lithium mica and 
lithionite. Lepidolite is uncommon, occurring almost exclusively in 
structurally complex granitic pegmatites, commonly in replacement units. 
Lepidolite is a commercial source of lithium, commonly used directly in 
lithium glasses and other ceramic products. 
    It usually forms small scales or fine-grained aggregates. Its colors, 
pink, lilac, and gray, are a function of the Mn/Fe ratio. It is fusible at 2, 
yielding the crimson (lithium) flame. It has a perfect basal cleavage. 
Hardness is 2.5 - 4.0 on Mohs scale; specific gravity is 2.8 - 3.0.  See  
MICA; SILICATE MINERALS. 
 �34s[E. William Heinrich]
�33s 
  
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978
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2563584
979
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2651648
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�6v�2v�32sLepidoptera �21m �33s
    The order of scaly-winged insects, including the butterflies, skippers,
and moths. This is one of the largest orders in the class Insecta. It has more
than 100,000 species, about 10,000 in North America, and between 125 and 175
families. The adults have a covering of hairs and scales on the wings, legs,
and body, and are often beautifully colored. With minor exceptions, the adults
are also characterized by two pairs of membranous wings and sucking
mouthparts, featuring a prominent, coiled proboscis. Butterflies and skippers
usually fly in the daytime, and most moths are nocturnal. The adults usually
take liquid food, such as nectar and juices of fruits. The caterpillars are
almost always herbivorous.

    Morphology.  The most unusual feature of the head of the adult animal is
the form of the mouthparts. The proboscis is extended by blood pressure
created by the retraction of the stipites of the maxillae. Diagonal muscles
within each proboscis unit cause the proboscis to coil. The liquid food is
sucked up by means of a muscular pump, formed from the pharynx, buccal cavity,
and cibarium, a food pocket of the mouth cavity. Ocelli are absent in many
groups. Antennae are quite variable in form.
    The prothorax is well developed in some lower groups but it is
considerably smaller than the pterothoracic, or wing-bearing, segments, and is
largely membranous in most Lepidoptera. The most prominent feature of the
dorsum of the prothorax, in most groups, is a pair of protuberant sclerotized
lobes.
    The scales are very variable in form. Generally, they are flat, thin,
sclerotized sacks, with striated outer surfaces. The vast spectrum of colors
seen in the Lepidoptera can be grouped into two categories, pigmentary and
structural colors. Pigmentary colors result from pigments which are present in
the scales. Structural colors are the result of either fine surface ridges on
the scales or layers within the cuticle, which interfere with or diffract the
light. The structural colors are generally metallic or iridescent.
    In most moths, the wings are coupled by a single spine formed by fused
setae which project forward from the base of the hindwing and are held by a
clasp on the forewing. The spine is known as the frenulum and the clasp is the
retinaculum. In the Homoneura, there is a lobe, the jugum, at the base of the
forewing, which engages the hindwing, or the frenulum, when it is present.
    The form of the external genitalia, especially that of the male, has been
widely used in the separation of species.

    Developmental stages.  Metamorphosis is complete. The larvae, commonly
called caterpillars, are mandibulate and cylindrical, with short thoracic legs
and a variable number of abdominal prolegs. They have one pair of thoracic and
eight pairs of abdominal spiracles. Pupae are variable in form and have
appendages that are usually firmly cemented down (obtect), though they are
sometimes partly or completely free (exarate). Pupae often are enclosed in a
silken cocoon.

    Classification.  Unfortunately, the classification of the Lepidoptera is
the subject of considerable controversy; much will doubtless be resolved when
more is known of the anatomy and life history of many groups.
    A rather conservative classification has been used here. In many
superfamilies only the more important families are mentioned. The table  lists
the important families.

    Homoneura (Jugatae). Fore- and hindwings are similar in shape and
venation. They are connected by a jugum and, sometimes, also by a frenulum.
Mouthparts are mandibulate, with mandibles vestigial or absent, and the galeae
forms a rudimentary proboscis. The female has a single genital opening. The
pupae are free or partially free.
    This small suborder, including less than 1% of the species in the order,
contains a diverse group of primitive forms showing certain features in common
with the Trichoptera, or caddis flies.
    The superfamily Micropterygoidea includes one small family, the
Micropterygidae, minute moths possessing toothed, functional mandibles and
lacking even the most rudimentary proboscis. The larvae, which feed on mosses,
are unusual in having eight pairs of abdominal prolegs.
    Superfamily Eriocranioidea is a group of tiny moths; the mandibles are
greatly reduced and untoothed. Three families, the Eriocraniidae,
Neopseustidae, and Mnesarchaeidea, have been recognized within the
superfamily. The leaf-mining larva lacks legs. The adults reportedly do not
feed. The females have a piercing ovipositor. Superfamily Hepialoidea contains
medium- to large-sized moths which possess rudimentary mouthparts. The larvae
are borers. The rapid flying adults are mostly crepuscular, thus the common
name swift, or ghost, moths. The only family of importance is the Hepialidae.
    Heteroneura (Frenatae). Fore- and hindwings are markedly different in
shape and venation. Usually they are connected by a frenulum and retinaculum.
Mouthparts are formed for sucking or, rarely, are vestigial. Adults with
functional mouthparts feed on nectar of flowers, juices of rotten fruits, and
other liquids. The female usually has two genital openings. Pupae are usually
obtect.
    One family, the Incurvariidae, comprises the superfamily Incurvarioidea.
The wings are covered with microscopic spines and the females have a single
genital opening. The venation is almost complete.
    Superfamily Nepticuloidea includes one family, the Nepticulidae. These
tiny moths have wing spines and the females have a single genital opening, but
they differ from the Incurvarioidea in having a reduced venation. The larvae
are generally miners in leaves, bark, and rarely, in fruits.
    Superfamily Cossoidea includes one family, the Cossidae, commonly called
the carpenter or goat moths. These are heavy-bodied moths, with the abdomen
extending well beyond the hindwings. Mouthparts are rudimentary except for
labial palpi.
    Superfamily Tineoidea contains 16-39 families; the number varies with the
author. This is a very large group, of uncertain composition. These moths are
of small size, usually with well-developed maxillary palpi. The labial palpi
have a slender, pointed third segment. Venation may be reduced, and the wings
may be divided into plumes.
    The small, wide-winged moths of the superfamily Tortricoidea belong to two
families, the Olethreutidae and the Tortricidae. The maxillary palpi are
vestigial or absent, and the third segment of the labial palpus is short and
usually obtuse. The hair fringes of the wings are always shorter than the width
of the wing.
    The family contains a number of agriculturally undesirable species.
Paramount among these is the codling moth (Carpocapsa pomonella L.), which is
a very serious pest of apples and other fruits. The large genus Laspeyresia
contains the interesting Mexican jumping bean moth (L. saltitans Westwood).
The violent movements of the larvae of this moth are responsible for the
action of the beans which they inhabit.
    Tortricidae is a family which generally lacks the fringe of long hairs
along the cubitus, characteristic of the Olethreutidae. The spruce budworm
(Choristoneura fumiferana Clemens) is probably the most important injurious
tortricid. In many places, especially in eastern Canada, it has defoliated
vast areas of coniferous forest.
    The superfamily Pyralidoidea is comprised of moths that are moderately
small to medium-sized, long-legged, and slender-bodied. The maxillary palpi
are usually well developed. Pyralididae is the second largest family of moths.
They are small and medium-sized, and a wing expanse of 20 - 35 mm is not
uncommon. The legs are usually long and slender. Pterophoridae is the family
known as the plume moths. The wings are divided into featherlike plumes, of
which there are usually two in the forewing and three in the hindwing. The
moths lack maxillary palpi and have slender bodies and long legs. The larvae
feed exposed or are borers.
    Superfamily Zygaenoidea includes moderately small- to medium-sized moths
that have complete venation, rudimentary palpi and, usually, a rudimentary
proboscis. The wings are broad with short fringes. The larvae are short and
more or less sluglike and are exposed feeders.
    Superfamily Castnioidea includes one family, the Castniidae. They are
large, diurnal, butterflylike moths with clubbed antennae, upright eggs, and
boring larvae. A proboscis may be either present or absent. These moths are
considered by some to be distantly related to the butterflies, but the
resemblances may very well be due to convergence.
    Members of the Superfamily Geometroidea are small to large moths with
reduced maxillary palpi and tympanal organs at the base of the abdomen. The
frenulum may be present or absent.
    Geometridae includes the measuring worms, loopers, and cankerworms, which
make up a very large family of small- and medium-sized moths with slender
bodies and relatively broad wings. The females are occasionally apterous. The
larvae have the anterior prolegs reduced or absent; usually only those on
segments 6 and 10 are well developed. They proceed with a characteristic
looping motion, which is the basis for the scientific name. The larvae
ordinarily are exposed feeders.
    Sphinx, hawk, or hummingbird moths constitute the one family Sphingidae of
the superfamily Sphingoidea. These medium-sized to very large, heavy-bodied
moths have extremely rapid flight. The adults are most crepuscular or
nocturnal, but a few genera are diurnal. The antennae are thickened and have a
pointed apex. The proboscis is well developed and often extremely long. The
wings are narrow, with the hindwing much shorter than the forewing. The larvae
are external feeders and usually have a characteristic caudal horn. The pupa
is in a cell in the ground or in a loose cocoon at the surface, and its long
proboscis is often in a projecting case resembling a pitcher's handle.
    Moths of the Superfamily Saturnioidea are medium-sized to very large moths
with the frenulum almost always reduced or absent. There is no tympanum and
the mouthparts are usually reduced. The antennae are ordinarily pectinate,
especially in the males.
    Noctuoidea is a large, rather uniform superfamily of more than 20,000
species. Most of them are moderately large moths with reduced maxillary palpi.
Tympanal organs are present in the metathorax.
    The superfamily Hesperioidea includes one rather large family, the
Hesperiidae. The skippers are small to moderately large, heavy-bodied, mostly
diurnal insects with a clubbed antenna, which is bent, curved, or reflexed at
the tip. The larvae have a prominent constriction, or neck, behind the head,
and often live in leaves drawn together by silk. Those of the giant skippers
are borers in yucca and agave. The pupa is usually enclosed in a slight cocoon.
    Butterflies of the superfamily Papilionoidea are small to large diurnal
insects with clubbed antennae, which are rounded at the tip and not bent or
reflexed. The forewings always have two or more veins, which are stalked.

    Biological aspects.  The Lepidoptera are a group of insects on which much
biological research remains to be done. A great deal is still unknown about
the genetics, physiology, and ecology of this group. Moreover, butterflies and
moths have proved useful as experimental animals in genetical research. The
larvae of many species are injurious to certain crops, causing severe economic
losses. Lepidoptera of all stages are subject to the attacks of a large number
of predators, including birds, mammals, lizards, frogs, spiders, and certain
other insects.
    The Lepidoptera penetrate almost every section of the globe, with the
major exception of Antarctica. Arctic and alpine tundra areas normally support
a lepidopteran fauna which, although relatively poor in species, is rich in
numbers. After rains, deserts are often alive with butterflies and moths.
Tropical areas are by far the richest in species. One of the strangest
habitats occupied by a lepidopteran is the hair of the neotropical three-toed
sloth, where the sloth moth (Bradypodicola hahneli), a pyralid, passes its
entire life cycle presumably feeding on algae which grow in the hair.
    Migration is the most spectacular behavior occurring in the order. It is
most frequent in the butterflies Phoebis, Danaus, Libytheana, and others, but
is also known in moths such as Chrysiridia. Huge migratory swarms are
frequently reported in many parts of the world, but this phenomenon, as well
as the related communal roosting of adults, daily use of flyways, hilltopping,
and so forth, is poorly understood.
    Aggregations of butterflies at a mud puddle are a frequent sight, and
moths have been observed ``pumping,'' an act which consists of sipping water
steadily from a puddle and ejecting it as a stream of drops or fine spray from
the anus.
    The phenomena of mimicry and protective resemblance are widespread in the
Lepidoptera. Eyespot patterns in certain species elicit escape responses in
passerine birds. Thus, they are an effective protective device. The appearance
of dark forms of various moths in heavily industrialized, and thus heavily
sooted, areas is a widespread phenomenon. This ``industrial melanism'' is
doubtless due to shifting selection pressures and is one of the best-known
examples of evolution in action. See  BUTTERFLY; CATERPILLAR; INSECTA; MOTH;
PROTECTIVE COLORATION.
 �34s[Paul R. Ehrlich]
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�6v�2v�32sLepidosauria   �21m �33s 
    A subclass of reptiles, both living and extinct, in which the structure of 
the skull is characterized by two temporal openings (diapsid condition) on 
each side (see illustration). Lepidosauria differ from Archosauria, which are 
also diapsid, in that the bony arcades bordering their temporal openings may 
suffer reduction or loss, causing the apparent disappearance of one or both 
openings. They differ also in that no lepidosaur skull has any antorbital 
opening in front of the orbit, and in that their teeth are typically fused to 
the jaw (acrodont or pleurodont) rather than implanted in sockets (thecodont). 
See  ARCHOSAURIA. 
    In the classification adopted here the Lepidosauria include three orders: 
Eosuchia, Rhynchocephalia, and Squamata. See  EOSUCHIA; REPTILIA; 
RHYNCHOCEPHALIA; SQUAMATA. 
 �34s[Alan J. Charig]
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�6v�2v�32sLepospondyli   �21m �33s 
    A term used to characterize Paleozoic amphibian groups in which the 
vertebral centra, basically spool-shaped, are formed by ossification directly 
around the notochord. In the Paleozoic, the larger amphibians were 
labyrinthodonts, but small lepospondyls appear to have been very abundant, 
particularly as water dwellers in the coal swamps. They are commonly 
considered to form three orders: Nectridia, Aistopoda, and Microsauria. The 
vertebral structure of the living amphibians is basically lepospondylous, and 
it is possible that part or all of the modern orders (unknown in the 
Paleozoic) have descended from lepospondyls, particularly the microsaurs.  See 
  AISTOPODA; AMPHIBIA; LABYRINTHODONTIA; MICROSAURIA; NECTRIDEA. 
 �34s[Alfred S. Romer]
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�6v�2v�32sLeprosy   �21m �33s 
    A chronic infectious disease of humans, also known as Hansen's disease, of 
worldwide occurrence but most commonly found in tropical areas where the 
incidence may be as high as 5 per 1000 population. Infection appears most 
often to result from intimate contact with a leprous individual. The 
incubation period is long, ranging from a few months to several years. The 
presence of bacilli in peripheral nerves is a hallmark of leprosy. The most 
debilitating form of the disease, lepromatous leprosy, is characterized by 
multiple nodular lesions (lepromata) of the skin. Microscopically these 
lesions contain masses of rodlike microorganisms, leprosy bacilli. 
 �34s[Lane Barksdale]
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�6v�2v�32sLeptolepiformes   �21m �33s 
    An order of small ray-finned fishes of the Mesozoic that are important to 
the understanding of the early evolution of the Teleostei. The principal 
family included in this order, the Leptolepidae, has been reported from the 
Middle Triassic of Europe and ranges into the Cretaceous. Leptolepids 
represent the first teleosts as defined on the structure of the caudal 
skeleton. They were either derived from the earliest Pholidophoridae (known 
from the European Middle Triassic sediments) or shared a common ancestry with 
them. In size, body shape, fin position, structure of head and jaws, and 
general habitat, leptolepids are similar to the pholidophorids. In many ways 
these two families appear to parallel one another.  See  TELEOSTEI. 
 �34s[Ted M. Cavender]
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�6v�2v�32sLepton �21m �33s
    An elementary particle having no internal constituents which interacts
through the electromagnetic, weak, and gravitational forces, but does not
interact through the strong (nuclear) force. Leptons are very small, less than
10-(sup 18)m in size. This is less than 1/1000 the size of a nucleus and less
than 10-(sup 8) the size of an atom. Leptons are much simpler than hadrons in
both their structure and their behavior; hence leptons are considered to be
more elementary than hadrons. At present the lepton family and the quark
family are thought to lie at the same level of elementariness.	See  HADRON;
QUARKS.
    There are three known charged leptons: the electron (e), muon (MU), and
tau (TAU). Associated with each charged lepton is a neutral lepton called a
neutrino.  See	ELECTRON; ELEMENTARY PARTICLE; NEUTRINO.
 �34s[Martin L. Perl]
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�6v�2v�32sLeptospira   �21m �33s 
    A genus of spirochetes of the family Spirochaetacea. Recognition of two 
species, Leptospira interrogans and L. Biflexa, has been proposed. Each 
comprises a large number of serological types (serovars). Leptospira biflexa 
serovars are apparently saprophytic organisms omnipresent in fresh surface 
waters and occasionally found in sea water. Leptospira interrogans comprises 
pathogenic leptospires which occur naturally in a large variety of wild and 
domesticated animals and cause acute, febrile, systemic disease in humans and 
other mammals.  See  LEPTOSPIROSIS; SPIROCHAETALES. 
 �34s[Aaron D. Alexander]
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�6v�2v�32sLeptospirosis   �21m �33s 
    An acute febrile disease of humans produced by spirochetes of many species 
of Leptospira. The incubation period is 6 - 15 days. Among the prominent 
features of the disease are fever, jaundice, muscle pains, headaches, 
hepatitis, albuminuria, and multiple small hemorrhages in the conjunctiva or 
skin. Meningeal involvement often occurs. The febrile illness subsides after 3 
- 10 days. Fatal cases show hemorrhagic lesions in the kidney, liver, skin, 
muscles, and central nervous sytem.  See  SPIROCHETE. 
    Wild rodents are the principal reservoirs, although natural infection 
occurs in swine, cattle, horses, and dogs and may be transmitted to humans 
through these animals. Humans are infected either through contact with the 
urine or flesh of diseased animals, or indirectly by way of contaminated water 
or soil, the organisms entering the body through small breaks in the skin or 
mucous membrane. 
 �34s[Thomas B. Turner]
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�6v�2v�32sLeptostraca   �21m �33s 
    A small and unimportant group of Crustacea, formerly ranked with the 
Branchiopoda but now regarded as the most primitive of the Malacostraca. Only 
a few genera, such as Nebalia, have survived to the present time, but numerous 
apparently related Paleozoic fossils are known. The fossil Leptostraca are the 
oldest known Malacostraca, ranging from the Cambrian to the Triassic epochs. 
Since they are already well differentiated in the earliest fossiliferous 
rocks, they shed no light on the possible origin of the Malacostraca.  See  
MALACOSTRACA; NEBALIACEA; PHYLLOCARIDA. 
 �34s[Isabella Gordon]
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�6v�2v�32sLeptostromataceae   �21m �33s 
    A family of fungi of the order Sphaeropsidales. Although most species are 
saprophytes, some are fruit-tree pathogens. Pycnidia, the fruit bodies 
containing conidia, are black, shield-shaped, circular or oblong, and slightly 
asymmetrical; the opening of the pycnidium is long and slitlike. There are 80 
genera and 375 species recognized. Many of these are conidial stages of 
Hemisphaeriales.  See  FUNGI IMPERFECTI; SPHAEROPSIDALES. 
 �34s[N. F. Buchwald]
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�6v�2v�32sLernaeopodoida   �21m �33s 
    A group of Crustacea known as the fish maggots, which are ectoparasites 
partially buried in the flesh of marine and fresh-water fish. They are 
characterized by (1) a modified postembryonic development, reduced to two or 
three recognizable stages, (2) a unique free-swimming larva, and (3) failure 
when sexually mature to show signs of external physical maturity typical of 
other copepods. The females, lacking distinct segmentation and functional 
thoracic appendages, appear in fish as comparatively large, wormlike, 
sometimes tufted outgrowths. The male (illustration a) is dwarfed and, though 
more distinctly segmented, does not exceed female morphological development. 
Using modified head appendages, the male clings to the female (illustration b) 
but retains some freedom of movement.  See  COPEPODA. 
 �34s[Abraham Fleminger]
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�6v�2v�32sLespedeza   �21m �33s 
    A warm-season legume with trifoliate leaves, small purple pea-shaped 
blossoms, and one seed per pod. There are 15 American and more than 100 
Asiatic species; two annual species and a perennial from Asia are grown as 
field crops in the United States. The American species are small shrubby 
perennials found in open woods and on idle land, rarely in dense stands; they 
are harmless weeds.  See  LEGUME. 
 �34s[Paul Tabor]
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�6v�2v�32sLethal dose 50 �21m �33s
    One special form of the effective dose 50, or median effective dose. The
lethal dose 50, or median lethal dose [LD(sup 50)], is used when the response
that is being investigated is the death of the experimental animal. The median
lethal dose is therefore the dose which is fatal to 50% of the test animals.
See  EFFECTIVE DOSE 50.
 �34s[Colin White]
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�6v�2v�32sLethal gene   �21m �33s 
    A gene which brings about the death of the organism carrying it. Lethal 
genes constitute the most common class of mutations and are reflections of the 
fact that the fundamental function of genes is the control of processes 
essential to the growth and development of organisms. In higher diploid forms 
lethals are usually recessive and expressed only in homozygotes. Dominant 
lethals, expressed in heterozygotes, are rapidly eliminated and thus rarely 
detected. Recessive zygotic lethals are retained with considerable frequency 
in natural populations of cross-fertilizing organisms, while gametic lethals 
(those affecting normal functioning of eggs and sperm among animals, or the 
pollen and ovules of plants) are subject to stringent selection and are 
accordingly rare.  See  GENE; MOLECULAR BIOLOGY; MUTATION. 
 �34s[Donald F. Poulson]
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�6v�2v�32sLettuce   �21m �33s 
    A cool-season annual, Lactuca sativa, of Asian origin and belonging to the 
tribe Cichorium of the Compositae family. Lettuce is grown for its succulent 
leaves, which are eaten raw as a salad. Four varieties of this leading salad 
crop are head lettuce (L. sativa var. capitata), leaf or curled lettuce (L. 
sativa var. crispa) cos or romaine lettuce (L. sativa var. longifolia), and 
stem or asparagus lettuce (L. sativa var. asparagina). There are two types of 
head lettuce: butterhead, and crisphead or iceberg. 
    California raises more lettuce than any other state; Arizona, Florida, and 
Texas are next in importance.  See  ASTERALES. 
 �34s[R. G. Grogan]
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�6v�2v�32sLeucettida   �21m �33s 
    An order of the subclass Calcinea in the class Calcarea. These sponges 
have either a radiate arrangement of the flagellated chambers or a leuconoid 
structure. A distinct dermal membrane or cortex is present. The spongocoel is 
not lined with choanocytes; these cells are restricted to the flagellated 
chambers. Two families are recognized, the Leucascidae and Leucaltidae.  See  
CALCAREA; CALCINEA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sLeucine   �21m �33s 
    An amino acid considered essential for normal growth of animals. In 
leucine biosynthesis, carboxyl and some carbon atoms of leucine are derived 
from acetate; the rest of the molecule is furnished by ALPHA-ketoisovalerate, 
which arises in the biosynthesis of valine.  See  AMINO ACIDS. 
 �34s[Edward A. Adelberg]
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�6v�2v�32sLeucite   �21m �33s 
    A framework structure silicate of the feldspathoidal mineral group with 
the chemical composition (K,Na)AlSi2O6. Leucite found in the natural 
environment is usually nearly pure KAlSi2O6. The crystals are typically white, 
and the luster varies from dull to vitreous. Mohs hardness lies between 5.5 
and 6. Density ranges from 2.45 to 2.5. 
    Crystals are common in the lavas of Nyiragongo volcano, Congo. Other 
well-known areas include: West Kimberley, Australia; Mount Vesuvius, Italy; 
Highwood Mountains and Bearpaw Mountains, Montana; and the Leucite Hills, 
Wyoming. See SILICATE MINERALS. 
 �34s[William Luth]
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�6v�2v�32sLeucite rock   �21m �33s 
    Igneous rocks rich in leucite but lacking or poor in alkali feldspar. 
Those types with essential alkali feldspar are classed as phonolites, 
feldspathoidal syenite, and feldspathoidal monzonite. The group includes an 
extremely wide assortment both chemically and mineralogically. 
    The rocks are generally dark-colored and aphanitic types of volcanic 
origin. They consist principally of pyroxene and leucite and may or may not 
contain calcic plagioclase or olivine. Leucite rocks are rare. They occur 
principally as lava flows and small intrusives (dikes and volcanic plugs).  
See  IGNEOUS ROCKS; LEUCITE. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sLeucosoleniida   �21m �33s 
    An order of the subclass Calcaronea in the class Calcarea. These sponges 
have an asconoid structure. A true dermal membrane or cortex does not develop 
in this order, and the spongocoel is lined with choanocytes. Leucosoleniidae 
is the one family recognized.  See  CALCAREA; CALCARONEA. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sLeukemia   �21m �33s 
    A disease of humans and animals characterized by generalized neoplastic 
proliferation of leukocytes, which are usually present in increased numbers in 
the bloodstream. As a result of uncontrolled proliferation, leukocytes or 
their precursors progressively replace the blood-forming tissues of the body, 
crowding out normal cellular elements so that anemia and thrombocytopenia 
ensue. Leukemic cells also tend to infiltrate certain other organs and tissues 
of the body, producing symptoms referable to the specific structures involved. 
    Leukemia includes a spectrum of clinical and cytological entities which 
may be subdivided according to three main criteria: the type of leukocyte 
affected; the degree of cytological differentiation; and the number of 
leukemic cells in the peripheral blood. 
    A great many factors have been suspected of causing leukemia, but few are 
supported by a substantial body of evidence. Probably the most firmly 
established causative factor for human leukemia is ionizing radiation. A 
number of chemical agents have been suspected of being leukemogenic, but proof 
is lacking, except for benzol, which seems capable of causing leukemia but 
only after years of heavy exposure. Studies on the etiology of leukemia have 
been particularly concentrated on possible viral agents. Also, two types of 
abnormality of the same pair of chromosomes (chromosome 21) have been found to 
be associated with leukemia. 
    The chief symptoms of leukemia are weakness and listlessness, severe 
anemia, hemorrhages into the skin or mucosal surfaces, and enlargement of 
spleen, liver, or peripheral lymph nodes. In a patient with symptoms 
suspiciously like those of leukemia, the usual first step in diagnosis is 
examination of the peripheral blood. Usually, however, additional diagnostic 
measures are utilized, the most important of which is examination of the bone 
marrow. In many cases biopsies of lymph nodes or liver are also obtained, and 
in some institutions puncture of the spleen in a manner similar to that used 
for the bone marrow has proven useful. 
    Leukemia is usually a fatal disease, but remarkable variations in duration 
of course and severity of symptoms are seen. These variations are to a large 
extent determined by the type of leukemia; however, within any given category 
of leukemia there are individual variations. 
    Several categories of drugs are in common use in the chemotherapy of 
leukemia, and new agents are being discovered at an impressive rate. Drugs in 
all the categories are capable of inducing remissions in individual cases. 
Each agent has a specific spectrum of effectiveness, and no one drug is useful 
in the treatment of all types of leukemia. These agents may be employed singly 
or in combination. 
    Bone marrow transplantation has become an accepted form of therapy for 
some cases of leukemia. Patients who receive bone marrow transplants are first 
treated with total-body irradiation and high-dose chemotherapy to destroy the 
leukemic cells (and also normal bone marrow cells). 
    Radiant energy in the form of conventional x-rays has a role in the 
therapy of leukemia; however, this therapy is limited by the fact that 
radiation of the entire body is highly toxic, and leukemia is a generalized 
disease. More useful, however, are certain radioactive isotopes which are 
selectively taken up by certain cell types, including leukemia cells. 
 �34s[J. Bruce Beckwith]
�33s 
  
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�6v�2v�32sLeukocidin   �21m �33s 
    A substance that destroys leukocytes, the white cells of blood. 
Leukocidins are extracellular substances which appear in the environment of 
certain species of growing bacteria. They are released into culture media, and 
their physical and chemical properties suggest that they are proteins. The 
most thoroughly studied leukocidins are those produced by staphylococci and 
streptococci.  See  STAPHYLOCOCCUS; STREPTOCOCCUS. 
 �34s[Alan W. Bernheimer]
�33s 
  
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�6v�2v�32sLeukopenia   �21m �33s 
    The condition of the blood characterized by a decrease in the number of 
circulating white cells, or leukocytes. It is not a disease in itself, but 
rather a symptom that may occur in the course of a great variety of diseases. 
Of the three kinds of white cells found in normal blood, it is the 
granulocytes that are largely affected in most forms of leukopenia, and these 
cells are the most important in resistance to infection.  See  BLOOD; 
INFECTION.  
    Leukopenia occurs as an incidental finding in a vast and heterogeneous 
group of pathologic conditions, especially infections. However, in most of the 
common bacterial infections the white count is elevated, reflecting the 
increased production of white cells. But even these infections, if 
overwhelming, may be associated with leukopenia if the demand for white cells 
is so great that the resources of the marrow are exhausted. 
    Leukopenia is regularly found in a group of diseases whose only common 
factor is enlargement of the spleen, for example, cirrhosis of the liver. 
Advanced and widespread cancer may produce leukopenia when it invades the bone 
marrow extensively and effectively crowds out the white-cell-producing tissues. 
    Leukopenia, sometimes severe and life-threatening, is regularly produced 
by certain physical and chemical agents when they are given in sufficient 
dosage. Among these are x-rays, benzene, arsenic, and a group of synthetic 
compounds used in the treatment of leukemia. 
    The only treatment that is undoubtedly useful is the administration of 
antibiotic drugs to control infection. These drugs have been life-saving in 
some cases, but their effectiveness is hampered by the almost complete lack of 
the patient's natural mechanisms of defense. 
 �34s[Peter B. Hukill]
�33s 
  
A700000012010�254s�5v�1v983
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�6v�2v�32sLevel measurement   �21m �33s 
    The determination of the linear vertical distance between a reference 
point or datum plane and the surface of a liquid or the top of a pile of 
divided solids. 
  
    Liquid level measurement.  Satisfactory measurements are possible only 
when the liquid is undisturbed by turbulence or wave action. When a liquid is 
too turbulent for the average level to be read, a baffle or stilling chamber 
is inserted in the tank or vessel to provide a satisfactory surface. 
    Stick, hook, and tape gages are used in open vessels where the surface of 
the liquid can readily be observed. The stick gage is a suitably divided 
vertical rod, or stick, anchored in the vessel so that the magnitude of the 
rise and fall of the liquid level may be observed directly. The hook gage 
provides a needle point, which is adjusted to produce a very tiny pimple in 
the liquid surface at the level reading, thereby minimizing the meniscus 
error. The tape gage reads the correct elevation when the point of a bob just 
touches the liquid surface. 
    Many forms of gage glass are available for the measurement of liquid 
level. Liquid in a tank or vessel is connected to the gage glass by a suitable 
fitting, and when the tank is under pressure the upper end of the glass must 
be connected to the tank vapor space. Thus the liquid rises to substantially 
the same height in the glass as in the tank, and this height is measured by 
suitable scale. 
    Various types of float mechanism are also used for liquid level 
measurement. The float, tape, and pulley gage provides an excellent method of 
measuring large changes in level with accuracy. It has the advantage that the 
scale can be placed for convenient reading at any point within a reasonable 
distance of the tank or vessel. 
    The change in buoyancy of a solid as its immersion in a liquid is varied 
is used to measure liquid level. This principle is used only when the 
densities of the liquid and vapor are substantially constant. Temperature 
changes will produce errors of greater magnitude than with the float 
mechanisms.  
    Hydrostatic head may also be used to measure liquid level. The pressure 
exerted by a column of liquid varies directly with its density as well as with 
its height, and thus this method of measurement requires that density be 
substantially constant. Densities of liquids vary with temperature; errors are 
therefore introduced with temperature changes, or the measuring element must 
be temperature compensated. 
    Electrode or probe systems are used in various forms for level indication 
and control. The number of electrodes and their design depend upon the 
characteristics of the liquid and the application. Fundamentally, a circuit 
through a relay coil is closed (or opened) when the liquid contacts a probe. 
    Capacitance-measuring devices can be used to measure levels of both 
dielectric (insulating) and conducting liquids. If the liquid being measured 
is a dielectric, one or two probes or rods, extending nearly to the bottom of 
the tank, are supported in an insulating mounting. The probes may be bare or 
covered with insulation. If the liquid is a conductor of electricity, only one 
probe is necessary but it must be covered with an insulating coating. 
    Nuclear level gages are used for difficult applications. Basically, all of 
the units involve a source of gamma (GAMMA) radiation and a detector separated 
by the vessel or a portion of the vessel in which a liquid level varies. As 
the level rises, the detector receives less GAMMA-radiation and thus the level 
is measured. 
    The sonic level detector is based on the time increment between the 
emission of a sound wave pulse and its reflection from liquid surface. The 
sound wave pulse is generated electronically, and its time in transit is 
measured very accurately by electronic means. If the speed of sound in the 
liquid or vapor is known accurately, the liquid level is known.  
  
    Solids level measurement.  Solids level detectors are used to locate the 
top of a pile of divided solids in large vessels or processing equipment. The 
instruments are designed for the different solids handled, and the 
installation must be carefully made to ensure proper measurements. Because 
solids funnel, cone, and vary in average density with the particle size, 
shape, distribution, moisture content, and other factors, these detectors 
provide only an approximate indication of the volume present or the top of the 
pile. Solids level detectors are classified as continuous or fixed point. 
Continuous detectors provide a continuous measurement of the level over the 
range for which they were designed. Their output is an analog representation 
of the level of the solids. Fixed-point detectors indicate when a specific 
level has been reached and are used mainly for actuating alarm signals. By 
installing a number of these, however, at different points, the combined 
response can be made to approach that of a continuous detector. 
 �34s[Howard S. Bean]
�33s 
  
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�6v�2v�32sLevel (communication and acoustics)   �21m �33s 
    Logarithm of the ratio of a given quantity to a reference quantity of the 
same kind. The base of the logarithm, the reference quantity, and the kind of 
level must be indicated. Frequently used levels, usually expressed in 
decibels, are power level, voltage level, and sound pressure level. Some other 
units of level are the neper (Np) and bel (B).  See  DECIBEL; NEPER. 
 �34s[Robert W. Young]
�33s 
  
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�6v�2v�32sLevel (surveying)   �21m �33s 
    An instrument for establishing a horizontal line of sight in order to find 
the difference of elevation between two points upon which a graduated rod is 
successively held to be sighted. The level usually consists of a telescope and 
attached spirit level, mounted on a tripod for rotation about a vertical axis 
(see illustration). The line of sight is fixed by cross hairs. Stadia hairs 
may be provided for convenience in keeping foresight and backsight distances 
balanced. The tubular spirit level vial is long, thus quite sensitive, with 
its upper interior surface being finely ground to a circular arc. A tangent to 
the center of the ground arc is parallel to the line of sight; thus the line 
of sight is horizontal (level) when the spirit bubble is centered. The 
instrument is leveled by manipulation of foot screws in the mounting. See 
SURVEYING.  
 �34s[B. Austin Barry]
�33s 
  
  
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�6v�2v�32sLever   �21m �33s 
    A pivoted rigid bar used to multiply force or motion, sometimes called the 
lever and fulcrum (see illustration). The lever uses one of the two conditions 
for static equilibrium, which is that the summation of moments about any point 
equals zero. The other condition is that the summation of forces acting in any 
direction through a point equals zero. See INCLINED PLANE. 
    If moments acting counterclockwise around the fulcrum of a lever are 
positive, then, for a frictionless lever, [(F sub B) b] - [(F sub A) a] = 0, 
which may be rearranged to give Eq. (1). 
  
                        *** F sub B = a/b (F sub A) ***                  (1)
  
If F sub B represents the output and F sub A represents the input, the 
mechanical advantage, MA, is then given by Eq. (2). 
  
                   *** MA = (F sub B) / (F sub A) = a/b ***              (2)
  
    Applications of the lever range from the simple nutcracker and paper punch 
to complex multiple-lever systems found in scales and in testing machines used 
in the study of properties of materials. See SIMPLE MACHINE. 
 �34s[Richard M. Phelan]
�33s 
  
  

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�6v�2v�32sL-Fucose   �21m �33s 
    A methyl pentose (also known as 6-deoxy-L-galactose, L-galactomethylose, 
L-rhodeose, and L-fucopyranose) present in some algae (seaweed), especially in 
Laminaria digitata, and in a number of gums; it has been identified in the 
polysaccharides of blood group - specific substances from animal and human 
sources. L-Fucose has also been found to be a constituent of certain bacterial 
polysaccharides. See BLOOD GROUPS; MONOSACCHARIDE. 
 �34s[William Z. Hassid]
�33s 
  
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�6v�2v�32sLibra   �21m �33s 
    The Balance, in astronomy, appearing during the spring. It is the seventh 
sign of the zodiac. The constellation consists of faint stars and is not 
conspicuous. It lies just west of the claws of Scorpius. The principal stars, 
ALPHA, BETA, GAMMA, and SIGMA, outline a foursided figure resembling a balance 
with beam and pans (see illustration).  See  CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sLichenes �21m �33s
    A group of organisms consisting of fungi and algae growing together
symbiotically. The fungal component is usually an ascomycete or, less
commonly, a basidiomycete. The algal component is green or blue-green. When
the two components grow together, they form a distinct, long-lived plant body
(see illustration) that may be foliose (leaflike), crustose (crustlike), or
fruticose (branched, shrubby, or bearded).
    Because their dual nature was not discovered until 1868, lichens were long
classified as a separate division of the plant kingdom between the fungi and
bryophytes. They are presently considered to be fungi, remarkable in their
ability to parasitize yet live symbiotically with algae. Eventually the
lichens will be assimilated among the orders and families of the true fungi,
but since their relationships are so poorly known, they are still maintained
out of necessity as a separate group. Three classes and five orders with
nearly 18,000 species are recognized:
Class Ascholichenes
    Order: Pseudosphaeriales
	   Hysteriales (Lichenized)
	   Pyrenulales
	   Lecanorales
	   Caliciales
Class Basidiolichenes
Class Lichenes Imperfecti
See separate articles on the classes and orders.
 �34s[Mason E. Hale, Jr.]
�33s

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�6v�2v�32sLichenes Imperfecti   �21m �33s 
    A class of the Lichenes containing species with no known method of sexual 
reproduction. This class parallels the Fungi Imperfecti, but differs 
significantly in absence of any asexual conidiospores. Reproduction is 
entirely by dispersal of fragments of the crusts by wind, water, or animals. 
These crusts are worldwide in distribution and often occur in shady, rather 
moist habitats on rocks and tree bases.  See  FUNGI IMPERFECTI; LICHENES. 
 �34s[Mason E. Hale, Jr.]
�33s 
  
  
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�6v�2v�32sLie detector   �21m �33s 
    A device intended to detect an involuntary physiological response that all 
persons exhibit when lying but never when telling the truth. Because there is 
no such specific lie response, actual ``lie detector'' tests used in the 
United States record breathing movements, blood pressure changes, and 
electrodermal responses on a polygraph while the respondent answers ``yes'' or 
``no'' to a series of questions. From the recordings, one can determine 
whether ``relevant'' questions had greater impact on the respondent than the 
interpolated ``control'' questions.  See  ELECTRODERMAL RESPONSE. 
 �34s[David T. Lykken]
�33s 
  
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�6v�2v�32sLie group   �21m �33s 
    A topological group with only countably many connected components whose 
identity component is open and is an analytic group. An analytic group or 
connected Lie group is a topological group with the additional structure of a 
smooth manifold such that multiplication and inversion are smooth. Many groups 
that arise naturally as groups of symmetries of physical or mathematical 
systems are Lie groups. The study of Lie groups has applications to analytic 
function theory, differential equations, differential geometry, Fourier 
analysis, algebraic number theory, algebraic geometry, quantum mechanics, 
relativity, and elementary particle theory.  See  GROUP THEORY; SMOOTH 
MANIFOLD; TOPOLOGY. 
 �34s[Anthony W. Knapp]
�33s 
  
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�6v�2v�32sLigament   �21m �33s 
    A strong, flexible connective tissue band usually found between two bony 
prominences. Most ligaments are composed of dense fibrous tissue formed by 
parallel bundles of collagen fibers. They have a shining white appearance and 
are pliable, strong, and noncompliant. A second kind of ligament, composed 
either partly or almost entirely of yellow elastic fibers, is extensible or 
compliant, thereby allowing the connected bones to move apart.  See  
CONNECTIVE TISSUE; JOINT (ANATOMY). 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sLight   �21m �33s 
    The term light, as commonly used, refers to the kind of radiant 
electromagnetic energy that is associated with vision. In a broader sense, 
light includes the entire range of radiation known as the electromagnetic 
spectrum. The branch of science dealing with light, its origin and 
propagation, its effects, and other phenomena associated with it is called 
optics. Spectroscopy is the branch of optics that pertains to the production 
and investigation of spectra.  See  OPTICS; SPECTROSCOPY. 
  
    Principal effects.  The electromagnetic spectrum is a broad band of 
radiant energy which extends over a range of wavelengths running from 
trillionths of inches to hundreds of miles; wavelengths of visible light are 
measured in hundreds of thousandths of an inch. Arranged in order of 
increasing wavelength, the radiation making up the electromagnetic spectrum is 
termed gamma rays, x-rays, ultraviolet rays, visible light, infrared waves, 
microwaves, radio and TV waves, and very long electromagnetic waves.  See  
ELECTROMAGNETIC RADIATION. 
    The fact that light travels at a finite speed or velocity is well 
established, both theoretically and experimentally. Numerous increasingly 
precise experiments have been conducted, leading to the currently accepted 
value of 299,792.458 km/s or 186,282.397 mi/s in a vacuum. In round numbers, 
the speed of light in vacuum or air may be said to be 186,000 mi/s or 300,000 
km/s.  
    One of the most easily observed facts about light is its tendency to 
travel in straight lines. Careful observation shows, however, that a light ray 
spreads slightly when passing the edges of an obstacle. This phenomenon is 
called diffraction. The reflection of light is also well known. Reflection of 
light from smooth optical surfaces occurs so that the angle of reflection 
equals the angle of incidence, a fact that is most readily observed with a 
plane mirror. When light is reflected irregularly and diffusely, the 
phenomenon is termed scattering. The scattering of light by gas particles in 
the atmosphere causes the blue color of the sky.  See  DIFFRACTION; REFLECTION 
OF ELECTROMAGNETIC RADIATION. 
    The type of bending of light rays called refraction is caused by the fact 
that light travels at different speeds in different media - faster, for 
example, in air than in either glass or water. Refraction occurs when light 
passes from one medium to another in which it moves at a different speed. 
Familiar examples include the change in direction of light rays in going 
through a prism, and the bent appearance of a stick partially immersed in 
water.  See  REFRACTION OF WAVES. 
    In the phenomenon called interference, rays of light emerging from two 
parallel slits combine on a screen to produce alternating light and dark 
bands. This effect can be obtained quite easily in the laboratory, and is 
observed in the colors produced by a thin film of oil on the surface of a pool 
of water. Polarization of light is usually shown with Polaroid disks. Such 
disks are quite transparent individually. When two of them are placed 
together, however, the degree of transparency of the combination depends upon 
the relative orientation of the disks. It can be varied from ready 
transmission of light to almost total opacity, simply by rotating one disk 
with respect to the other.  See  INTERFERENCE OF WAVES; POLARIZED LIGHT. 
    When light is absorbed by certain substances, chemical changes take place. 
This fact forms the basis for the science of photochemistry. 
  
    Theory.  Phenomena involving light may be classed into three groups: 
electromagnetic wave phenomena, corpuscular or quantum phenomena, and 
relativistic effects. The relativistic effects appear to influence similarly 
the observation of both corpuscular and wave phenomena.  See  RELATIVITY. 
  
    Wave phenomena. Interference and diffraction are the most striking 
manifestations of the wave character of light. Their fundamental similarity 
can be demonstrated in a number of experiments. The wave aspect of the entire 
spectrum of electromagnetic radiation is most convincingly shown by the 
similarity of diffraction pictures produced on a photographic plate, placed at 
some distance behind a diffraction grating, by radiations of different 
frequencies, such as x-rays and visible light. The interference phenomena of 
light are, moreover, very similar to interference of electronically produced 
microwaves and radio waves. 
    Polarization demonstrates the transverse character of light waves. Further 
proof of the electromagnetic character of light is found in the possibility of 
inducing, in a transparent body that is being traversed by a beam of 
plane-polarized light, the property of rotating the plane of polarization of 
the beam when the body is placed in a magnetic field.  See  FARADAY EFFECT. 
    The fact that the velocity of light had been calculated from electric and 
magnetic parameters (permittivity and permeability) was at the root of 
Maxwell's conclusion in 1865 that ``light, including heat and other radiations 
if any, is a disturbance in the form of waves propagated . . . according to 
electromagnetic laws.'' Finally, the observation that electrons and neutrons 
can give rise to diffraction patterns quite similar to those produced by 
visible light has made it necessary to ascribe a wave character to particles.  
See  ELECTRON DIFFRACTION; NEUTRON DIFFRACTION. 
  
    Corpuscular phenomena. In its interactions with matter, light exchanges 
energy only in discrete amounts, called quanta. This fact is difficult to 
reconcile with the idea that light energy is spread out in a wave, but is 
easily visualized in terms of corpuscles, or photons, of light. 
    The radiation from theoretically perfect heat radiators, called 
blackbodies, involves the exchange of energy between radiation and matter in 
an enclosed cavity. The observed frequency distribution of the radiation 
emitted by the enclosure at a given temperature of the cavity can be correctly 
described by theory only if one assumes that light of frequency NU is absorbed 
in integral multiples of a quantum of energy equal to hNU, where h is a 
fundamental physical constant called Planck's constant. 
    When a monochromatic beam of electromagnetic radiation illuminates the 
surface of a solid (or less commonly, a liquid), electrons are ejected from 
the surface in the phenomenon known as photoemission, or the external 
photoelectric effect. It is found that the emission of these photoelectrons, 
as they are called, is immediate, and independent of the intensity of the 
light beam, even at very low light intensities. This fact excludes the 
possibility of accumulation of energy from the light beam until an amount 
corresponding to the kinetic energy of the ejected electron has been reached. 
    The scattering of x-rays of frequency NU0 by the lighter elements is 
caused by the collision of x-ray photons with electrons. Under such 
circumstances, both a scattered x-ray photon and a scattered electron are 
observed, and the scattered x-ray has a lower frequency than the impinging 
x-ray. The kinetic energy of the impinging x-ray, the scattered x-ray, and the 
scattered electron, as well as their relative directions, are in agreement 
with calculations involving the conservation of energy and momentum.  See  
COMPTON EFFECT; HEAT RADIATION; PHOTOELECTRICITY; PHOTON. 
  
    Quantum theories. The need for reconciling Maxwell's theory of the 
electromagnetic field, which describes the electromagnetic wave character of 
light, with the particle nature of photons, which demonstrates the equally 
important corpuscular character of light, has resulted in the formulation of 
several theories which go a long way toward giving a satisfactory unified 
treatment of the wave and the corpuscular picture. These theories incorporate, 
on one hand, the theory of quantum electrodynamics, first set forth by P. A. 
M. Dirac, P. Jordan, W. Heisenberg, and W. Pauli, and on the other, the 
earlier quantum mechanics of L. de Broglie, Heisenberg, and E. Schroedinger. 
Unresolved theoretical difficulties persist, however, in the higher-than-first 
approximations of the interactions between light and elementary particles. 
    Dirac's synthesis of the wave and corpuscular theories of light is based 
on rewriting Maxwell's equations in a Hamiltonian form resembling the 
Hamiltonian equations of classical mechanics. Using the same formalism 
involved in the transformation of classical into wave-mechanical equations by 
the introduction of the quantum of action hNU, Dirac obtained a new equation 
of the electromagnetic field. The solutions of this equation require quantized 
waves, corresponding to photons. The superposition of these solutions 
represents the electromagnetic field. The quantized waves are subject to 
Heisenberg's uncertainty principle. The quantized description of radiation 
cannot be taken literally in terms of either photons or waves, but rather is a 
description of the probability of occurrence in a given region of a given 
interaction or observation.  See  HAMILTON'S EQUATIONS OF MOTION; 
NONRELATIVISTIC QUANTUM THEORY; QUANTUM ELECTRODYNAMICS; QUANTUM FIELD THEORY; 
QUANTUM MECHANICS; RELATIVISTIC QUANTUM THEORY; UNCERTAINTY PRINCIPLE. 
 �34s[George W. Stroke]
�33s 
  
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�6v�2v�32sLight amplifier   �21m �33s 
    In the broadest sense, a device which produces an enhanced light output 
when actuated by incident light. A simple photocell relay - light source 
combination would satisfy this definition. To make the term more meaningful, 
common usage has introduced two restrictions: (1) a light amplifier must be a 
device which, when actuated by a light image, reproduces a similar image of 
enhanced brightness; and (2) the device must be capable of operating at very 
low light levels without introducing spurious brightness variations (noise) 
into the reproduced image. The term is used synonymously with image 
intensifier. The light amplifier increases the brightness of an image which is 
below the visual threshold to a level where it can be readily seen with the 
unaided eye. It is, of course, impossible to see under conditions of complete 
darkness. Indeed, there is a fundamental lower limit of illumination under 
which an image of a given quality can be recognized. This limitation arises 
because of the corpuscular nature of light.  See  LIGHT; PHOTON. 
    In addition to their application for night vision, light amplifiers have 
been useful in many fields of science, such as astronomy, nuclear physics, and 
microbiology.  
 �34s[Dean R. Collins]
�33s 
  
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�6v�2v�32sLight curves   �21m �33s 
    Graphical descriptions of the light changes of variable stars. If the 
light changes occur in a regular manner, that is, if they are periodic or 
nearly so, a plot can be made of the brightness versus the phase. The phase is 
the time interval between an observation and the instant when the star was at 
some easily determined portion of the light curve. For eclipsing variables, 
this ``epoch'' will be at the time of greatest light loss when the star is 
faintest; for intrinsic variables the phase is usually reckoned from the time 
of maximum light when the star is brightest.  See  VARIABLE STARS. 
 �34s[Frank B. Wood]
�33s 
  
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�6v�2v�32sLight-emitting diode   �21m �33s 
    A rectifying semiconductor device which converts electric energy into 
electromagnetic radiation. The wavelength of the emitted radiation ranges from 
the green to the near infrared, that is, from about 550 to over 1300 
nanometers. Most commercial light-emitting diodes (LEDs) both visible and 
infrared, are fabricated from group III - V compounds. The most commonly used 
light-emitting diode is the red light-emitting diode, made of gallium 
arsenide-phosphide on gallium arsenide substrates. 
    Visible light-emitting diodes are used as solid-state indicator lights and 
as light sources for numeric and alphanumeric displays. Infrared 
light-emitting diodes are used in optoisolators and in optical fiber 
transmission in order to obtain the highest possible efficiency.  See  OPTICAL 
COMMUNICATIONS; OPTICAL FIBERS; OPTICAL ISOLATOR. 
    For discussion of the properties of pn junctions and light generation in 
solid-state devices see ELECTROLUMINESCENCE; JUNCTION DIODE; JUNCTION 
TRANSISTOR; LASER. 
 �34s[A. A. Bergh]
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�6v�2v�32sLighthouse   �21m �33s 
    A distinctive structure, built on or near a shore, which exhibits a light 
of distinctive characteristics to serve as an aid to navigation. Lesser lights 
may be displayed from fixed structures called beacons or from floating buoys 
or lightships. 
    The characteristics of the lights displayed by lighthouses are given in 
light lists available to mariners and, in abbreviated form, on charts. Some 
lights have one or more sectors in which the light appears red, usually to 
warn of some danger in this sector. In other sectors most lights are white.  
See  PILOTING. 
 �34s[Alton B Moody]
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�6v�2v�32sLightning   �21m �33s 
    The large spark produced by an abrupt discontinuous discharge of 
electricity through the air generally under turbulent conditions of the 
atmosphere.  
    Cumulonimbus clouds, which produce lightning, contain strongly 
upward-moving air and frequently extend to altitudes higher than 40,000 feet 
(12 km). Within these clouds, various factors act to cause the creation and 
separation of electric charge. The upper parts of thunderstorms are positively 
charged, whereas the central and lower parts are predominantly negative. 
    When cloud electrification has proceeded to the point where the potential 
gradient between adjacent charge centers, or between the cloud and ground, 
reaches the breakdown value, the large spark called lightning occurs. A 
breakdown potential of about 10,000 volts per centimeter has been suggested as 
an appropriate value for air containing water droplets.  See  CLOUD PHYSICS; 
GEOELECTRICITY; LIGHTNING AND SURGE PROTECTION; THUNDERSTORM. 
 �34s[Louis J. Battan]
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�6v�2v�32sLightning and surge protection   �21m �33s 
    Means of protecting electrical systems, buildings, and other property from 
lightning and other high-voltage surges. From studies of lightning, two 
conclusions emerge: (1) Lightning will not strike an object if it is placed in 
a grounded metal cage. (2) Lightning tends to strike, in general, the highest 
objects on the horizon.  See  ATMOSPHERIC ELECTRICITY; LIGHTNING. 
    One practical approximation of the grounded metal cage is the well-known 
lightning rod or mast. The effectiveness of this device is evaluated on the 
cone-of-protection principle. The protected area is the space enclosed by a 
cone having the mast top as the apex of the cone and tapering out to the base. 
If the radius of the base of the cone is equal to the height X of the mast, 
equipment inside this cone will rarely be struck. A radius equal to twice the 
height of the mast (2X) gives a cone of shielding within which an object will 
be struck occasionally. The cone-of-protection principle is shown in the 
illustration.  
    The probability that an object will be struck by lightning is considerably 
less if it is located in a valley. Therefore, electric transmission lines 
which must cross mountain ranges often will be routed through the gaps to 
avoid the direct exposure of the ridges. 
    There are a number of protective devices to limit or prevent lightning 
damage to electric power systems and equipment. The word protective is used to 
connote either one or two functions: the prevention of trouble, or its 
elimination after it occurs. Various protective means have been devised either 
to prevent lightning from entering the system or to dissipate it harmlessly if 
it does. Overhead ground wires and lightning rods are used to prevent 
lightning from striking the electrical system. Lightning arresters are 
protective devices for reducing the transient system overvoltages to levels 
compatible with the terminal-apparatus insulation.  See  SURGE ARRESTER.. 
    Immediate reclosure is a practice for restoring service after the trouble 
occurs by immediately reclosing automatically the line power circuit breakers 
after they have been tripped by a short circuit. The protective devices 
involved are the power circuit breaker and the fault-detecting and reclosing 
relays.  See  CIRCUIT BREAKER; ELECTRIC PROTECTIVE DEVICES. 
 �34s[Glenn D. Breuer]
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�6v�2v�32sLight panel   �21m �33s 
    A surface-area light source that employs the principle of 
electroluminescence to produce light. Light panels are composed of two sheets 
of electrically conductive material, one a thin conducting backing and the 
other a transparent conductive film, placed on opposite sides of a plastic or 
ceramic sheet impregnated with a phosphor, such as zinc sulfide, and small 
amounts of compounds of copper or manganese. When an alternating voltage is 
applied to the conductive sheets, an electric field is applied to the 
phosphor. Each time the electric field changes, it dislodges electrons from 
the edges of the phosphor crystals. As these electrons fall back to their 
normal atomic state, they affect the atoms of the slight ``impurities'' of 
copper or manganese, and radiation of the wavelength of light is emitted.  See 
  ELECTROLUMINESCENCE. 
    In contrast to incandescent, vapor-discharge, and fluorescent lamps, which 
are essentially point or line sources of light, the electroluminescent light 
panel is essentially a surface source of light. Complete freedom of size and 
shape is a fascinating aspect of luminescent cells (see illustration). See 
ILLUMINATION.  
 �34s[Warren B. Boast]
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�6v�2v�32sLight projector   �21m �33s 
    A device designed to produce controlled beams of light that can be 
projected over considerable distances. The function of a light projector may 
be to light a limited area, such as the face of a building or an actor on a 
stage, or to produce apparent brightnesses at the light source that make the 
source itself visible from considerable distances. 
    Equipment for the projection of light beams usually makes use of one or 
more of three principles of light control. (1) A specular paraboloidal 
reflector produces parallel rays of light from a source of brightness located 
at the focal point of the parabola. (2) A specular ellipsoidal reflector with 
a light source at the near focal point focuses rays at the opposite focal 
point of the ellipse. (3) A lens refracts the rays from a light source located 
at its focal point into parallel rays. 
    Searchlights employ the first of these principles, using carefully formed 
paraboloidal reflectors. The light source, either a carbon arc or a compact 
incandescent filament, is, for practical purposes, at the focal point. 
Spotlights, used in theatrical productions and in some forms of display 
lighting, usually use filament lamps and lenses, or ellipsoidal reflectors and 
lenses. Floodlights, used for outdoor lighting of buildings, parking lots, 
sports fields, and the like, usually use filament or mercury-vapor lamps in 
conjunction with parabolic reflectors.  See  ILLUMINATION. 
 �34s[Alfred Makulec]
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�6v�2v�32sLight-year �21m �33s
    A unit of measurement of astronomical distance. A light-year is the
distance light travels in 1 sidereal year. One light-year is equivalent to
9.461 x 10(sup 12) km, or 5.879 x 10(sup 12) mi. Distances to some of the
nearer celestial objects, measured in units of light time, are shown in the
table.
    This unit, while useful for its graphic presentation of the enormous scale
of stellar distances, is seldom used technically except in cosmology. See
ASTRONOMICAL UNIT; PARALLAX (ASTRONOMY); PARSEC.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sLignite   �21m �33s 
    Soft and porous carbonaceous material intermediate between peat and 
subbituminous coal, with a heat value less than 8300 Btu/lb (19,300 
kilojoules/kg) on a moist, mineral-matter-free basis. In North America, 
lignitic coals are classified as lignite A or lignite B, depending upon 
whether they have calorific value of more or less than 6300 Btu/lb (14,650 
kilojoules/kg). Elsewhere, lignites are commonly called brown coals, with two 
major classes being recognized, namely, hard and soft brown coals. 
    The main uses of lignite have been for generation of electrical energy and 
for domestic and industrial heating. If shipped any great distance, lignite 
commonly requires drying. See COAL; PEAT. 
 �34s[Jack A. Simon]
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�6v�2v�32sLignumvitae �21m �33s
    A tree, Guaiacum sanctum, also known as holywood lignumvitae, which is
cultivated to some extent in southern California and tropical Florida.
Lignumvitae is native in the Florida Keys, Bahamas, West Indies, and Central
and South America. It is an evergreen tree of medium size with abruptly
pinnate leaves. The tree yields a resin or gum known as gum guaiac or resin of
guaiac which is used in medicine. The very heavy black heartwood is used in
bowling balls, blocks and pulleys, and parts of instruments.  See  SAPINDALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sLiliales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), from 
whose name the names of the subclass Liliidae and class Liliopsida 
(monocotyledons) are derived. The order consists of some 13 families and 
nearly 7700 species, more than half of which belong to the family Liliaceae 
(4200 species). The Iridaceae, Dioscoreaceae, Agavaceae, and Smilacaceae are 
other good-sized families of the order. The ovary is either superior or 
inferior, the seeds are of ordinary number and structure, and most families 
and genera have septal nectaries in the flowers.  See  ASPARAGUS; GARLIC; 
LILIIDAE; LILIOPSIDA; ONION; ORNAMENTAL PLANTS; SISAL. 
 �34s[Arthur Cronquist]
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�6v�2v�32sLiliidae   �21m �33s 
    A subclass of the class Liliopsida (monocotyledons) of the division 
Magnoliophyta (Angiospermae), the flowering plants, consisting of 2 orders 
(Liliales and Orchidales), 17 families, and nearly 28,000 species. The 
Liliidae are syncarpous monocotyledons with both the sepals and the petals 
usually petaloid. The flowers generally have well-developed nectaries, and 
pollination is usually by insects or other animals.  See  FLOWER; LILIALES; 
LILIOPSIDA; ORCHIDALES. 
 �34s[Arthur Cronquist]
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�6v�2v�32sLiliopsida   �21m �33s 
    One of the two classes which collectively make up the division 
Magnoliophyta (Angiospermae), the flowering plants. The Liliopsida, often 
known as Monocotyledoneae, or monocotyledons, embrace 4 subclasses 
(Alismatidae, Commelinidae, Arecidae, and Liliidae), 18 orders, 61 families, 
and about 55,000 species.  See  ALISMATIDAE; ARECIDAE; COMMELINIDAE; LILIIDAE. 
    All of the characters which collectively distinguish the Liliopsida from 
the Magnoliopsida (dicotyledons) are subject to exception, but most of the 
Liliopsida have parallel-veined leaves, and when the embryo is differentiated 
into recognizable parts, there is only a single cotyledon. The vascular 
bundles are generally scattered or borne in two or more rings, so that the 
stems and roots do not have a well-defined pith and cortex. Monocotyledons 
never have an intrafascicular cambium, and most of them have no secondary 
growth at all. The mature root system of monocots is wholly adventitious. The 
floral parts of monocots, when of definite number, are most often borne in 
sets of 3, seldom 4, never 5. The pollen is uniaperturate or of 
uniaperturate-derived type.  See  CORTEX (PLANT); FLOWER; LEAF; MAGNOLIOPSIDA; 
PITH; STEM. 
 �34s[Arthur Cronquist]
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�6v�2v�32sLimburgite   �21m �33s 
    A dark, glass-rich igneous rock with abundant large crystals (phenocrysts) 
of olivine and pyroxene and with little or no feldspar. The glass is brown and 
usually alkali-rich. Phenocrysts are well formed. Limburgite is a rare rock 
and forms lava flows and small intrusive bodies (dikes, sills, and plugs). It 
is associated with basanite and alkali-basaltic rocks.  See  BASALT; IGNEOUS 
ROCKS.  
 �34s[Carleton A. Chapman]
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�6v�2v�32sLimestone   �21m �33s 
    A sedimentary rock composed dominantly of carbonate minerals, principally 
carbonates of calcium and magnesium. The chief minerals of limestones are 
calcite and aragonite, and in the dolomitic limestones, dolomite. Limestones 
are the most abundant of the nonclastic rocks. They are overwhelmingly the 
largest reservoir of the element carbon at or near the surface of the Earth. 
Much knowledge of invertebrate paleontology and consequently of the evolution 
of life and earth history comes from the fossils contained in them. Although 
the word limestone is used in the general sense above, specifically it refers 
to carbonate rocks dominated by the mineral calcite, CaCO3.  See  ARAGONITE; 
CALCITE; LIME (INDUSTRY). 
    The textures of limestones reveal much about their origin. The chief 
textural elements in a great many limestones are fossils, whole or fragmental. 
The species represented and their state of preservation are a guide to the 
ecology and environment of the site of deposition. In addition, the nature of 
the fossil debris, whether complete articulated shells or fragments in all 
stages of disarticulation, fragmentation, and comminution, is a guide to the 
presence of bottom currents. 
    Some mechanically deposited limestones show well sorted particles of 
calcite cemented by clear intergranular calcite. If the particles are 
sand-size, the rock is termed calcarenite. Secondary textural features that 
are found in limestones include stylolites, cross-cutting veinlets filled with 
calcite, and replacement effects (typically that of rhombohedrons of dolomite 
replacing calcite). 
    Limestones may be referred to two types of origin, autochthonous and 
allochthonous. Autochthonous limestones have been formed in place by biogenic 
precipitation from the water of the environment, usually sea water; 
allochthonous limestones have been transported from the site of original 
precipitation by current action.  See  CALCARENITE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sLime (botany) �21m �33s
    An acid citrus fruit, Citrus aurantifolia, usually grown in tropical or
subtropical regions because of its low resistance to cold. The two principal
groups of limes are the West Indian or Mexican and the Tahiti or Bearss. The
fruit of West Indian lime is very small (walnut size) and strongly acid, and
drops when fully colored. The Tahiti lime is seedless and its aroma is less
pronounced.
    Except in the United States, the commercial lime industry is restricted to
the West Indian group. The major producing areas are India, Mexico, Egypt, and
the West Indies. Commercial production of the Tahiti lime is largely confined
to the United States. It is grown mainly in Florida, with some plantings in
the warmer areas of southern California.  See  FRUIT; FRUIT, TREE.
 �34s[R. K. Soost]
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�6v�2v�32sLime (industry)   �21m �33s 
    A general term for burned (or calcined) limestone, also known as 
quicklime, hydrated lime, and unslaked or slaked lime. Its predominant usage 
(90%) is as a basic industrial chemical. It still enjoys its traditional 
building uses. In order of decreasing size uses are: steel fluxing, water 
treatment, nonferrous metals (alumina, magnesium, copper, and others), pulp 
and paper, refractories, soil stabilization, sewage and trade waste treatment, 
chemicals, and glass manufacture.  See  LIMESTONE. 
    Lime is not a mineral; it is manufactured from a mineral - limestone, 
coral, oystershell, all being sources of calcium carbonate. Dolomite, a 
calcium-magnesium carbonate, is used to produce dolomitic (magnesium) lime. 
Only the purest types of stone or shell are used for lime. 
 �34s[Robert S. Boynton]
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�6v�2v�32sLimiter circuit   �21m �33s 
    An electronic circuit used to prevent the amplitude of an electrical 
waveform from exceeding a specified level while preserving the shape of the 
waveform at amplitudes less than the specified level. The limiting action 
takes place by effectively shunting a normal load resistance with a much lower 
resistance at and above the specified limiting level. 
    Limiting action is usually accomplished by use of the highly nonlinear or 
``switching'' voltage-current characteristic of an electronic device. A 
semiconductor diode used as a clamp is often the essential element in a 
limiter circuit. The action of such a diode may be understood with reference 
to the voltage-current characteristic of a typical semiconductor diode shown 
in the illustration.  See  CLAMPING CIRCUIT; SEMICONDUCTOR DIODE. 
    Bipolar or field-effect transistors can also be used as limiters, in 
either the input or output circuits. Saturation characteristics of the output 
circuit of a field-effect transistor or bipolar transistor can also produce 
limiting action. 
    The term limiter is often defined in sufficiently broad terms to include 
the related operation of clipping, which also results in deletion of a portion 
of a waveform. Some authors even use the terms interchangeably. Limiters and 
clippers are often used together, rather than bidirectional limiters alone, 
where both negative and positive peaks of a waveform are to be removed.  See  
CLIPPING CIRCUIT. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sLimnology   �21m �33s 
    The study of lakes, ponds, rivers, streams, swamps, and reservoirs that 
make up inland water systems. Each of these inland aquatic environments is 
physically and chemically connected with its surroundings by meteorologic and 
hydrogeologic processes (see illustration). 
    Aquatic systems with excellent physical conditions for production of 
organisms and high nutrient levels may show signs of eutrophication. Eutrophic 
lakes are generally identified by large numbers of phytoplankton and aquatic 
macrophytes and by low oxygen concentrations in the profundal zone.  See  
ECOLOGY; EUTROPHICATION; FRESHWATER ECOSYSTEM; HYDROLOGY; LAKE; RIVER; STREAM. 
 �34s[James E. Schindler]
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�6v�2v�32sLimonite   �21m �33s 
    A field or generic term for natural hydrous iron oxides, the most common 
phase being the mineral goethite, ALPHA-FeO(OH). Limonite includes the 
so-called bog iron ores. It is the characteristic brown stain which coats 
rocks containing sulfide ores, such as pyrite and pyrrhotite, in the zone of 
weathering of these ores referred to as a gossan. It is formed by biogenic or 
inorganic precipitation in bog, spring, lacustrine, or marine deposits. 
 �34s[Paul B. Moore]
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�6v�2v�32sLimpet   �21m �33s 
    Name given to a variety of species of aquatic gastropod mollusks, all with 
a characteristic conical shell (see illustration) and a suckerlike foot. 
Limpets (like chitons) are well adapted for life on rocky surfaces exposed to 
wave action and, in the higher levels of the littoral zone, to alternating 
tidal submergence and aerial drying. All limpets move relatively slowly over 
rock or other hard surfaces, protecting themselves (against wave action or 
desiccation or predation) by clamping down the shell opening against the 
substrate by contraction of the enlarged shell muscles. There is never an 
operculum on the foot, and limpets are defenseless once detached. 
    Limpets usually graze slowly and continuously by radular scraping of 
attached algae and diatoms from rock surfaces. The structural and functional 
adaptations of the limpet form have arisen in many distinct groups of 
gastropods.  See  GASTROPODA; MOLLUSCA. 
 �34s[W. D. Russell-Hunter]
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�6v�2v�32sLinales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of three families: the Linaceae, with about 450 species; the 
Erythroxylaceae, with about 200 species; and the Humiriaceae, with about 50 
species. They are simple-leaved herbs or woody plants, with hypogynous, 
regular, syncarpous flowers that have five to many stamens. Linum 
usitatissimum (the source of flax fibers and linseed oil) and Erythroxylon 
coca (the source of cocaine) are well-known members of the Linales.  See  
COCA; FLAX; ROSIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sLine   �21m �33s 
    In axiomatic geometry, a line frequently is taken as a primitive 
(undefined) concept: an element in line geometry; a class of points, in a 
geometry with point as an element, whose essential properties are stated as 
axioms. In analytic geometry, a line is identified with (or is the graph of) a 
linear equation Ax + By + C = 0, with A, B = 0, 0, in cartesian coordinates 
(x,y). As a derived concept, line is defined in terms of primitive notions 
such as distance or betweenness.  See  ANALYTIC GEOMETRY; EUCLIDEAN GEOMETRY; 
PLANE; POINT. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sLinear algebra �21m �33s
    That branch of mathematics which deals with solutions of systems of linear
equations and the related geometric notions of vector spaces and linear
transformations. It is fundamental in the theory of the calculus of functions
of several variables and hence is of great importance in the application of
mathematics to physical and biological sciences, economics, and so on.
    The word linear is derived from the fact that the equation of a line in
two-dimensional analytic geometry has the form shown in Eq. (1) and a system

				ax + by = c				     (1)

of linear equations has the corresponding form shown in Eq. (2), where i = 1,

a(sub il)x(sub 1) + a(sub i2)x(sub 2) + . . . + a(sub in)x(sub n) = b(sub 1) (2)

2, . . . , m. The asubisubj and bsubi are fixed quantities belonging to a
specified field, for example, the field of real numbers, and solutions (x1,x2,
. . . ,xsubn) are sought in the same field.
 �34s[Nathan Jacobson]
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�6v�2v�32sLinearity   �21m �33s 
    The relationship that exists between two quantities when a change in one 
of them produces a directly proportional change in the other. Thus, an 
amplifier has good linearity when doubling of its input signal strength always 
doubles the output signal strength. A transistor displays good linearity when 
doubling of the instantaneous base voltage serves to double the instantaneous 
value of collector current. 
 �34s[John Markus]
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�6v�2v�32sLinear programming �21m �33s
    A mathematical subject whose central theme is finding the point where a
linear function defined on a convex polyhedron assumes its maximum or minimum
value. The largest class of applications of linear programming occurs in
business planning and industrial engineering. Its basic concepts are so
fundamental, however, that linear programming has been used in almost all
parts of science and social science where mathematics has made any
penetration. Furthermore, even in cases where the domain is not a polyhedron,
or the function to be maximized or minimized is not linear, the methods used
are frequently adaptions of linear programming.  See  NONLINEAR PROGRAMMING;
OPERATIONS RESEARCH.
    A typical linear programming problem is to maximize expression (1), where

		  c1x1 + . . . + c(sub n) x(sub n)			   (1)

x1, . . . , x(sub n) satisfy the conditions shown by notation (2). The a(sub
ij), c(sub j), and b(sub i) are constants; x1, . . . , x(sub n) are variables. 

		      x(sub j) > or = 0
 a(sub 11)x(sub 1) + . . . + a(sub 1n) x(sub n) < or = b1
 . . . . . . . . . . . . . . . . . . . .
 a(sub m1) x(sub 1) + . . . + alpha(sub mn) < or = b(sub m)		   (2)

    Sometimes the problem may be to minimize rather than to maximize;
sometimes some of the variables may not be required to be nonnegative;
sometimes some of the inequalities a(sub i1)x(sub 1) + .  .  .	+ a(sub in)
x(sub n) < or =  b(sub i) may be reversed, or be equalities.
    The linear inequalities, which the variables satisfy, correspond
algebraically to the fact that the variable point x = (x1, . . . ,x[sub n]) lies
in a convex polyhedron. Hence, the principal mathematical bases for linear
programming are the theory of linear equalities, a part of algebra, and the
theory of convex polyhedra, a part of geometry.
   If the function c1x1 + .  .	.  + c(sub n)x(sub n) does not get arbitrarily
large for points x = (x1, .  .	.  ,x[sub n]) on the convex polyhedron, then a
maximum is  attained at a vertex of the polyhedron.
    The most popular method for solving linear programs, that is, the finding
of the x = (x1, .  .  .  ,x[sub n]) which maximizes c1x1 + .  .  .  +
c[sub n]x[sub n],  is the simplex method, developed by Dantzig in 1947.  The
method is  geometrically a process of moving from vertex to neighboring vertex
on the convex polyhedron, each move attaining a higher value of c1x1 + .  .  .	+
c(sub n)x(sub n), until the vertex yielding the greatest value is reached.
Algebraically, the calculations are similar to elimination processes for solving
systems of algebraic equations.
    The popularity of the simplex method rests on the empirical fact that the
number of moves from vertex to vertex is a small multiple (about 2-4) of the
number of inequalities in most of the thousands of problems handled, although it
has never been proved mathematically that this behavior is characteristic of
``average'' problems.
 �34s[Alan J.  Hoffman]
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�6v�2v�32sLinear system analysis   �21m �33s 
    The study of a system by means of a model consisting of a linear mapping 
between the system inputs (causes or excitations), applied at the input 
terminals, and the system outputs (effects or responses), measured or observed 
at the output terminals. A system is a set or an arrangement of things or 
devices so related or connected as to form a unity or organic whole. For 
example, the inputs or controlling factors of the national economy (the 
system) are government spending, interest rates, tax levels, and monetary 
policy. The outputs are the gross national product, unemployment rate, 
inflation rate, and the consumer price index. Similar correspondences can be 
established for most systems, ranging from the more common variety of physical 
systems to the less-well-defined socioeconomic variety whose structure and 
control represent some of the major problem areas of society.  See  SYSTEMS 
ANALYSIS; SYSTEMS ENGINEERING. 
 �34s[C. T. Chen ;Patrick E. Barry]
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�6v�2v�32sLinear systems of equations �21m �33s
    Systems of mathematical equations of the form of system (1), where the

	    a11x11	    + a12x2	    + a(sub 1n) x(sub n) = b1
	    a21x1	    + a22x2	    + a(sub 2n) xsubn = b2
	    . . . . . . . . . . . . . . . . . . . . . . . . . .
	    a(sub m1) x1    + a(sub m) x2   + a(sub mn) x(sub n) = b(sub m)

a(sub ij), i = 1, 2, .	.  .  , m, j = 1, 2, .	.  .  , n, and the b1, i = 1, 2, .
.  .  , m, are constants, or fixed numbers, and the xsubi, i = 1, 2, .	.  .  ,
n are called unknowns.	System (1) is referred to as m linear equations in n
unknowns.  A solution of system (1) is a set of numbers c1, c2, .  .  .  , c(sub
n) such that when x1 is replaced by c1, x2 by c2, .  .	.  , x(sub n) by c(sub
n), every equation of system (1) becomes a true equality.  The problem posed by
such a system of equations is to find criteria for the existence of a solution
and, when solutions exist, to obtain systematic methods for finding the
solutions.

    Two linear systems of equations are equivalent if every solution of one of
the systems is a solution of the other and vice versa.

If m = n in system (1) and if the determinant |A| of

		     A = (a11	     a12	asub1n
			  a21	     a22	asub2n
			  . . . . . . . . . . . . . . .
			  a(sub m1)  a(sub m2)	a(sub mn))		 (2)


the matrix of the coefficients A, given by matrix (2), is not zero,
then system (1) has a unique solution which can be obtained by Cramer's rule,
which gives the values of the unknowns x1, x2, . . . , xsubn as the ratio of
two determinants.  See	DETERMINANT.
    Specifically, Cramer's rule gives

x1 = | b1	a12	 . . .	    a(sub 1n)
       b2	a22	 . . .	    a(sub 2n)
       . . . . . . . . . . . . . . . . . .
       b(sub n) a(sub n2). . .	    a(sub nn) |/|A|

x2 = | a11	b1	a13	  . . .       a(sub 1n)
       a21	b2	a23	  . . .       a(sub 2n)
	. . . . . . . . . . . . . . . . . . . . . . .
  a(sub n1)  b(sub n)	a(sub n3)    . . .    a(sub nn) |/|A|
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xsubn = |a11	. . .	a(sub n-1)     b1
	 a21	. . .	a(sub 2n-1)    b2
	 . . . . . . . . . . . . . . .
	 a(sub n1). .	a(sub nn-1)    b(sub n)|/|a|

In every case the determinant in the denominator is |A|, and for the value of
the unknown x(sub i), the determinant in the numerator is the determinant of the
matrix obtained from A by replacing the ith column of A by b1, b2, .  .  .  ,
b(sub n).  If |A| = 0 in this case of m equations in n unknowns, the system may
either be inconsistent or it may have infinitely many solutions.  See EQUATIONS,
THEORY OF; POLYNOMIAL SYSTEMS OF EQUATIONS.
 �34s[Ross A. Beaumont]
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�6v�2v�32sLine integral �21m �33s
    The line integral of a vector function BOLDF of position over a path C is
represented by Eq. (1), where Fsubx, Fsuby, Fsubz are the scalar components

		INTEGRATION BOLDF DOT dBOLDr =
		INTEGRATION (in terms of C) Fsubx (x,y,z) dx
		+ INTEGRATION (in terms of C) Fsuby (x,y,z) dy
		+ INTEGRATION (in terms of C) Fsubz (x,y,z) dz		     (1)

of BOLDF along the coordinate axes. The path C is supposed to be a curve, smooth

		  x = x(p)   y = y(p)	z = z(p)			     (2)

at least in part, defined parametrically by equations of form (2) for each
smooth portion. The functions Fsubx (x,y,z), etc., must be defined at all
points of C.
    When C is a closed curve, the line integral is called a circuit integral,
and is written as notation (3).

			LINE INTEGRAL BOLDF DOT dBOLDr			     (3)

See  INTEGRATION.
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sLines of force   �21m �33s 
    Imaginary lines in fields of force whose tangents at any point give the 
direction of the field at that point and whose number through unit area 
perpendicular to the field represents the intensity of the field. The concept 
of lines of force is perhaps most common when dealing with electric or 
magnetic fields. 
    Electric lines of force are drawn to represent, or map, an electric field 
graphically in the space around a charged body. They are of great help in 
visualizing an electric field and in quantitative thinking about such a field. 
A magnetic field may also be represented by lines of force. Magnetic lines of 
force due to magnets originate on north poles and terminate on south poles, 
both inside and outside the magnet.  See  ELECTRIC FIELD; MAGNETIC FIELD. 
 �34s[Ralph P. Winch]
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�6v�2v�32sLine spectrum   �21m �33s 
    A discontinuous spectrum characteristic of excited atoms, ions, and 
certain molecules in the gaseous phase at low pressures. If an electric arc or 
spark between metallic electrodes, or an electric discharge through a 
low-pressure gas, is viewed through a spectroscope, images of the spectroscope 
slit are seen in the characteristic colors emitted by the atoms or ions 
present.  See  ATOMIC STRUCTURE AND SPECTRA; SPECTROSCOPY. 
 �34s[George R. Harrison]
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�6v�2v�32sLingulida   �21m �33s 
    Inarticulate brachiopods forming an order that has persisted from the 
Early Cambrian to the present. One genus, Lingula, seems to be valid for both 
living animals and fossil species of the Ordovician. This implies an extremely 
low rate of evolution. 
    The pedicle of lingulids is very large and extremely contractile. All 
other brachiopods have smaller pedicles, and some groups have none at all. The 
pedicle is used as a prop to aid the valves in shoveling through the sediment 
to make a burrow. When the animal is in its usual living position, it is 
anchored in place by a mass of sand cemented together by a secretion from the 
base of the pedicle (see illustration).  See  BRACHIOPODA; INARTICULATA. 
 �34s[C. S. Hammen]
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�6v�2v�32sLink   �21m �33s 
    An element of a mechanical linkage. A link may be a straight bar or a 
disk, or it may have any other shape, simple or complex. It is assumed, for 
simple analysis, to be made of unyielding material; that is, its shape does 
not change. The frame, or fixed member, of a linkage is one of the links. 
    Two links of a kinematic chain meet in a joint, or pair, by which these 
links are held together. Just as each joint is a pair having two elements, one 
from each link, so a link in a kinematic chain is the rigid connector of two 
or more elements belonging to different pairs.  See  LINKAGE (MECHANISM); 
MECHANISM.  
 �34s[Douglas P. Adams]
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�6v�2v�32sLinkage (genetics)   �21m �33s 
    Failure of two or more genes to recombine at random as a result of their 
location on the same chromosome pair. Among the haploid products of a cell 
which has gone through meiosis, two genes located in the same chromosome pair 
remain in their two original combinations of alleles (``parental'') unless an 
odd number of exchanges of homologous segments occurred within the interval 
bounded by their loci. The incidence of exchanges of homologous segments at 
meiosis is roughly proportional to the length of the chromosome segment 
between two loci. The percentage of recombinants thus provides an estimate of 
this length and a basis for constructing gene maps on which linked loci are 
arranged in linear order and spaced out in proportion to the recombination 
percentages between them.  See  MEIOSIS. 
 �34s[Guido Pontecorvo]
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�6v�2v�32sLinkage (mechanism)   �21m �33s 
    A set of rigid bodies, called links, joined together at pivots by means of 
pins or equivalent devices. A body is considered to be rigid if, for practical 
purposes, the distances between points on the body do not change. Linkages are 
used to transmit power and information. They may be employed to make a point 
on the linkage follow a prescribed curve, regardless of the input motions to 
the linkage. They are also used to produce angular or linear displacement.  
See  LINK; MECHANISM. 
    If the links are bars the linkage is termed a bar linkage. A common form 
of bar linkage is one for which the bars are restricted to a given plane, such 
as a four-bar linkage. A commonly occurring variation of the four-bar linkage 
is the linkage used in reciprocating engines (see illustration). Slider C is 
the piston in a cylinder, link 3 is the connecting rod, and link 4 is the 
crank. (Link 1 is the fixed base, A and D are pivots, R is the length of the 
crank, L is the length of the connecting rod, and THETA denotes the angle of 
the crank.) This mechanism transforms a linear into a circular motion, or vice 
versa. The straight slider in line with the crank center is equivalent to a 
pivot at the end of an infinitely long link.  See  FOUR-BAR LINKAGE; 
PANTOGRAPH; SLIDER CRANK MECHANISMS. 
 �34s[Rufus Oldenburger]
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�6v�2v�32sLinoleum   �21m �33s 
    A floor covering made by applying a mixture of gelled linseed oil, 
pigments, fillers, and other materials to a burlap backing and curing to 
produce a hard, resilient sheet. Linoleum has been supplanted to a large 
extent by adhesive-backed vinyl or other resinous tiles, and by improved floor 
paints.  See  DRYING OIL; PAINT. 
 �34s[C. R. Martinson ;C. W. Sisler]
�33s 
  
  
  
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�6v�2v�32sLinseed oil   �21m �33s 
    Oil expressed or extracted from flaxseed (Linum usitatissimum). Freshly 
expressed oil is a dark amber color and has a powerful odor. The crude product 
is filtered or centrifuged and decolorized, and as an article of commerce 
usually has a light yellow color similar to other refined vegetable oils. The 
chief acid of linseed oil is linolenic acid, which makes up well over half the 
fatty acids. Oleic acid and linoleic acid are present in lesser amounts. 
    Much linseed oil has been displaced from paint and varnish formulations by 
synthetic polymers, but development of new types of linseed oil emulsions for 
paints and for the treatment of concrete to protect it from winter damage has 
created greater markets than ever. In addition, much linseed oil is consumed 
in the formulation of putties and calking compounds, in printing inks, and in 
linoleum. Some linseed oil is incorporated into various resins and plastics, 
and small amounts are used in specialty lubricants, greases, and core oils for 
foundries.  See  DRYING OIL; SURFACE COATING. 
 �34s[Frank Wagner]
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�6v�2v�32sLip   �21m �33s 
    A fleshy fold above and below the entrance to the mouth of mammals. Even 
the lower vertebrates with movable jaws, such as fishes, amphibians, and most 
reptiles, have folds of skin external to the jaws and teeth. Nevertheless, 
only the typical mammals (marsupials and placental mammals) possess a highly 
developed type of true lips. These are separated by deep clefts from the jaws 
and are made mobile by containing muscles belonging to the facial group. The 
separating cleft or space constitutes the vestibule of the mouth. 
 �34s[Leslie B. Arey]
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�6v�2v�32sLipid �21m �33s
    One of a class of compounds which contains long-chain aliphatic
hydrocarbons (cyclic or acyclic) and their derivatives, such as acids (fatty
acids), alcohols, amines, amino alcohols, and aldehydes. The presence of the
long aliphatic chain as the characteristic component of lipids confers
distinct solubility properties on the simpler members of this class of
naturally occurring compounds.
    The lipids are generally classified into the following groups:

A. Simple lipids
   1. Triglycerides or fats and oils are fatty acid esters of glycerol.
      Examples are lard, corn oil, cottonseed oil, and butter.
   2. Waxes are fatty acid esters of long-chain alcohols. Examples are beeswax,
      spermaceti, and carnauba wax.
   3. Steroids are lipids derived from partially or completely hydrogenated
      phenanthrene. Examples are cholesterol and ergosterol.
B. Complex lipids
   1. Phosphatides or phospholipids are lipids which contain phosphorus and, in
      many instances, nitrogen. Examples are lecithin, cephalin, and
      phosphatidyl inositol.
   2. Glycolipids are lipids which contain carbohydrate residues. Examples are
      sterol glycosides, cerebrosides, and plant phytoglycolipids.
   3. Sphingolipids are lipids containing the long-chain amino alcohol
      sphingosine and its derivatives. Examples are sphingomyelins, ceramides,
      and cerebrosides.

    Lipids are present in all living cells, but the proportion varies from
tissue to tissue. The triglycerides accumulate in certain areas, such as
adipose tissue in the human being and in the seeds of plants, where they
represent a form of energy storage. The more complex lipids occur closely
linked with protein in the membranes of cells and of subcellular particles.
More active tissues generally have a higher complex lipid content; for example,
the brain, liver, kidney, lung, and blood contain the highest concentration of
phosphatides in the mammal. See FAT AND OIL; FAT AND OIL (FOOD); GLYCOLIPID;
PHOSPHATIDE; SPHINGOLIPID; STEROID; TERPENE; TRIGYLCERIDE; VITAMIN; WAX; ANIMAL
AND VEGETABLE.�34s[Roy H. Gigg]
�33s

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�6v�2v�32sLipid metabolism   �21m �33s 
    The assimilation of dietary lipids and the synthesis and degradation of 
lipids; this article is restricted to mammals. 
    The principal dietary fat is triglyceride. This substance is not digested 
in the stomach and passes into the duodenum, where it causes the release of 
enterogastrone, a hormone which inhibits stomach motility. The amount of fat 
in the diet, therefore, regulates the rate at which enterogastrone is released 
into the intestinal tract. Fat, together with other partially digested 
foodstuffs, causes the release of hormones, secretin, pancreozymin, and 
cholecystokinin from the wall of the duodenum into the bloodstream. 
    Secretin causes the secretion of an alkaline pancreatic juice rich in 
bicarbonate ions, while pancreozymin causes secretion of pancreatic enzymes. 
One of these enzymes, important in the digestion of fat, is lipase. 
Cholecystokinin, which is a protein substance chemically inseparable from 
pancreozymin, stimulates the gallbladder to release bile into the duodenum. 
Bile is secreted by the liver and concentrated in the gallbladder and contains 
two bile salts, both derived from cholesterol: taurocholic and glycocholic 
acids. These act as detergents by emulsifying the triglycerides in the 
intestinal tract, thus making the fats more susceptible to attack by 
pancreatic lipase. In this reaction, which works best in the alkaline medium 
provided by the pancreatic juice, each triglyceride is split into three fatty 
acid chains, forming monoglycerides. The fatty acids pass across the membranes 
of the intestinal mucosal (lining) cells. Enzymes in the membranes split 
monoglyceride to glycerol and fatty acid, but triglycerides are reformed 
within the mucosal cells from glycerol and those fatty acids with a chain 
length greater than eight carbons: Short- and medium-chain fatty acids are 
absorbed directly into the bloodstream once they pass through the intestinal 
mucosa.  See  BILE ACID; CHOLESTEROL; DIGESTIVE SYSTEM; GALLBLADDER; LIVER; 
PANCREAS; TRIGLYCERIDE. 
    Obesity is a condition in which excessive fat accumulates in the adipose 
tissue. One factor responsible for this condition is excessive caloric intake. 
In starvation, uncontrolled diabetes, and many generalized illnesses the 
opposite occurs and the adipose tissue becomes markedly depleted of lipid.  
See  ADIPOSE TISSUE; DIABETES; LIPID; METABOLIC DISORDERS; OBESITY. 
 �34s[Martin A. Rizack]
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�6v�2v�32sLiquefaction of gases   �21m �33s 
    Several processes have been developed for liquefying gases. Although 
significant differences in efficiency exist among them, difficulties 
encountered in reducing the more efficient to practice have made them all 
roughly competitive. Consequently, the choice of the refrigerating cycle to be 
used for liquefaction of a given gas is made largely on the basis of 
convenience. In large industrial liquefaction plants, however, a significant 
reduction in production costs may be realized by use of the more efficient 
cycles.  
    Although any thermodynamic process which results in a lowering of the 
temperature could in principle be utilized in the liquefaction of some gas, 
only four have found practical employment. These are, in order of increasing 
complexity, vapor compression; use of refrigerators to cool the gas at 
constant pressure until liquefaction occurs; utilization of the adiabatic 
performance of work by the gas in a cyclic process with regenerative cooling; 
and use of the Joule-Thomson process in a cycle with regenerative cooling. 
Combinations of these four processes have also often been found effective. 
 �34s[Edward F. Hammel]
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�6v�2v�32sLiquefied natural gas (LNG)   �21m �33s 
    A product of natural gas which consists primarily of methane. Its 
properties are those of liquid methane, slightly modified by minor 
constituents. One property which differentiates liquefied natural gas (LNG) 
from liquefied petroleum gas (LPG) is the low critical temperature, about 
-100DGF(-73DGC). This means that natural gas cannot be liquefied at ordinary 
temperatures simply by increasing the pressure, as is the case with LPG; 
instead, natural gas must be cooled to cryogenic temperatures to be liquefied 
and must be well insulated to be held in the liquid state.  See  CRYOGENIC 
ENGINEERING; LIQUEFACTION OF GASES; LIQUEFIED PETROLEUM GAS (LPG). 
 �34s[Arthur W. Francis]
�33s 
  
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�6v�2v�32sLiquefied petroleum gas (LPG)   �21m �33s 
    A product of petroleum gases, principally propane and butane, which must 
be stored under pressure to keep it in a liquid state. At atmospheric pressure 
and above freezing temperature, these substances would be gases. Large 
quantities of propane and butane are now available from the gas and petroleum 
industries. These are often employed as fuel for tractors, trucks, and buses 
and mainly as a domestic fuel in remote areas. Because of the low boiling 
point (-47.2 to 32DGF or -44 to 0DGC) and high vapor pressure of these gases, 
their handling as liquids in pressure cylinders is necessary. Owing to demand 
from industry for butane derivations, LPG sold as fuel is made up largely of 
propane.  
    LPG has a high octane rating, making it useful in engines having 
compression ratios above 10:1. Another factor of importance in internal 
combustion engines is that LPG leaves little or no engine deposit in the 
cylinders when it burns.  See  INTERNAL COMBUSTION ENGINE; PETROLEUM PRODUCTS. 
 �34s[Mott Souders]
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�6v�2v�32sLiquid   �21m �33s 
    A state of matter intermediate between that of crystalline solids and 
gases. Macroscopically, liquids are distinguished from crystalline solids in 
their capacity to flow under the action of extremely small shear stresses and 
to conform to the shape of a confining vessel. Liquids differ from gases in 
possessing a free surface and in lacking the capacity to expand without limit. 
On the scale of molecular dimensions liquids lack the long-range order that 
characterizes the crystalline state, but nevertheless they possess a degree of 
structural regularity that extends over distances of a few molecular 
diameters. In this respect, liquids are wholly unlike gases, whose molecular 
organization is completely random. 
    Liquids possess important transport properties, notably their capacity to 
transmit heat (thermal conductivity), to transfer momentum under shear 
stresses (viscosity), and to attain a state of homogeneous composition when 
mixed with other miscible liquids (diffusion). These nonequilibrium properties 
of liquids are well understood in macroscopic terms and are exploited in 
large-scale engineering and chemical-process operations.  See  GAS; SOLID. 
 �34s[Norman H. Nachtrieb]
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�6v�2v�32sLiquid chromatography   �21m �33s 
    A form of chromatography involving passage of a liquid moving phase 
through a solid or a liquid stationary phase. The mobile phase is percolated 
through the column by means of either gravity, under pressure generated by a 
suitable pump, or centrifugal force. High-pressure pumps that generate up to 
several hundred atmospheres (1 atm = 0.1 megapascal) of inlet pressure are 
used in combination with chromatographic packings of only several micrometers 
of particle size. Since the viscosity of a liquid affects its flow at a given 
pressure, the pressure requirements vary for different liquids achieving 
identical flow. The two principal forms of liquid chromatography are 
ion-exchange and gel. See GEL CHROMATOGRAPHY; ION-EXCHANGE CHROMATOGRAPHY. 
    The mobile-phase composition is an important variable in chromatography. 
Sample solubility, competiton between the sample and the mobile-phase 
molecules for available adsorbent surface, and solvation effects on the 
structure and function of some chromatographic materials can all influence the 
separation process. In liquid-liquid chromatography, the mobile phase and the 
stationary phase must be immiscible. Since immiscibility implies that the two 
phases must have significantly different chemical properties, only 
combinations of either a nonpolar mobile phase and a polar stationary phase or 
a polar mobile phase and a nonpolar stationary phase are feasible. See 
CHROMATOGRAPHY.  
 �34s[Milos V. Novotny]
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�6v�2v�32sLiquid crystals   �21m �33s 
    A state of matter that mixes the properties of both the liquid and solid 
states. Liquid crystals may be described as condensed fluid states with 
spontaneous anisotropy. They are categorized in two ways: thermotropic liquid 
crystals, prepared by heating the substance, and lyotropic liquid crystals, 
prepared by mixing two or more components, one of which is rather polar in 
character (for example, water).  See  ANISOTROPY. 
    Liquid crystals are a state of matter that combines a kind of long-range 
order (in the sense of a solid) with the ability to form droplets and to pour 
(in the sense of waterlike liquids). They also exhibit properties of their own 
such as the ability to form monocrystals with the application of a normal 
magnetic or electric field; an optical activity of a magnitude without 
parallel in either solids or liquids; and a temperature sensitivity which 
results in a color change in certain liquid crystals. 
    Liquid crystals have many applications. They are used as displays in 
digital wristwatches, calculators, panel meters, thermometers, and industrial 
products. They can be used to record, store, and display images which can be 
projected onto a large screen. They also have potential use as television 
displays.  See  ELECTRONIC DISPLAY. 
 �34s[Glenn H. Brown]
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�6v�2v�32sLiquid helium �21m �33s
    Helium liquefies at a substantially lower temperature, 4.2 K (-452DGF or
-269DGC), than any other substance; and below 2.2 K (-455.8DGF) the liquid
exhibits the extraordinary properties of superfluidity, most notably the
ability to flow with complete absence of friction. Helium remains in a liquid
state at absolute zero. All of these characteristics are due to the weakness
of the attractive force between two helium atoms and to the small atomic mass,
which according to the laws of quantum mechanics makes the atoms difficult to
localize.  See	SUPERFLUIDITY.
    Liquid helium was first produced in 1908 by H. Kamerlingh Onnes. Although
extensive thermodynamic studies followed, the superfluid properties were not
discovered until 1938. In addition to the common isotope of atomic weight 4,
helium has a rare isotope of atomic weight 3 with quite different properties.
In 1972 a superfluid transition in liquid sup 3 He was discovered at a
temperature near 0.002 K (0.004DGF above absolute zero, -459.67DGF).
    The phase diagram of sup 4 He (see illustration) shows several remarkable
characteristics. Helium remains a liquid down to absolute zero unless a
pressure greater than 25.3 bars (1 bar = 0.987 atm = 10 sup 5 Pascals) is
applied. This is true despite the requirement of thermodynamics that the
system have the same degree of order as a perfect crystal. A more subtle
feature is a transition at the lambda line between two different liquid
phases. This LAMBDA-transition is so named because the specific heat has a
singularity resembling a backwards Greek lambda. There is no latent heat; such
a transition is called second-order. The high-temperature liquid phase, called
helium I, is a rather ordinary liquid. The LAMBDA-transition at 2.172 K
(-455.76DGF) (at vapor pressure) marks the onset of superfluidity, which is
the characteristic property of the low temperature phase, helium II.
 �34s[George B. Hess]
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�6v�2v�32sLiquid scintillation detector   �21m �33s 
    A particle detector in which the sensitive medium is a liquid 
scintillator.  See  SCINTILLATION COUNTER. 
    Liquid and plastic scintillators are the most extensively used forms of 
organic scintillator. They are noted for their fast response, for the ease 
with which they may be formed in arbitrary shapes, large or small, and for the 
economy they offer in achieving large, sensitive detecting volumes. Liquid 
scintillators also have the capacity to assimilate other liquids or substances 
and form homogeneous media, thereby providing an efficient and simple means 
for measuring the products of nuclear reaction or radioactive decay in those 
substances.  See  PARTICLE DETECTOR. 
 �34s[Frank D. Brooks]
�33s 
  
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�6v�2v�32sLissajous figures   �21m �33s 
    Plane curves traced by a point which executes two independent harmonic 
motions in perpendicular directions, the frequencies of the motion being in 
the ratio of two integers. Such figures are widely used in frequency and phase 
measurements (see illustration).  See  HARMONIC MOTION. 
    The cathode-ray oscilloscope furnishes the most important and practical 
means for the generation of the figures. The x-deflection plates of the tube 
are supplied with one alternating voltage, and the y-deflection plates with 
another. If the frequencies are incommensurable, the figure is not a closed 
curve and, except for very low frequencies, will appear as a patch of light 
because of the persistence of the screen. On the other hand, if the 
frequencies are commensurable, the figure is closed and strictly periodic; it 
is a true Lissajous figure, stationary on the screen and, if the persistence 
is sufficient, visible continuously as a complete pattern.  See  FREQUENCY 
MEASUREMENT; OSCILLOSCOPE; PHASE-ANGLE MEASUREMENT. 
 �34s[Martin Greenspan]
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�6v�2v�32sLissamphibia   �21m �33s 
    The subclass of Amphibia including all living amphibians (frogs, toads, 
salamanders, and apodans). The other two subclasses are the Labyrinthodontia 
and the Lepospondyli.  See  LABYRINTHODONTIA; LEPOSPONDYLI. 
    Living amphibians are grouped together by possession of a unique series of 
characters, the most important of which are (1) pedicellate teeth, consisting 
of two segments, a crown and a pedicel; (2) an operculum-plectrum complex of 
the middle ear; (3) the papilla amphibiorum, a special sensory area of the 
inner ear; (4) green rods in the retina of the eye; (5) similar skin glands; 
and (6) a highly vascular skin used in respiration (cutaneous respiration).  
See  AMPHIBIA; ANURA; APODA; URODELA. 
 �34s[Richard Estes]
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�6v�2v�32sListeriosis   �21m �33s 
    A disease of humans and animals caused by the bacterium Listeria 
monocytogenes. The most common clinical manifestation of listeriosis in humans 
are meningitis and septicemia. These are observed most often in the newborn 
within the first month of life, or in debilitated adults who usually are 40 or 
more years old. 
 �34s[Herbert J. Welshimer]
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�6v�2v�32sLithium   �21m �33s 
    A chemical element, Li, atomic number 3, and atomic weight 6.939. Lithium 
heads the alkali metal family in the periodic table. In nature it is a mixture 
of the isotopes Lisup6 and Lisup7. Lithium, the lightest solid element, is a 
soft, low-melting, reactive metal. In many physical and chemical properties it 
resembles the alkaline-earth metals as much as, or more than, it does the 
alkali metals.  See  ALKALI METALS; ALKALINE-EARTH METALS. 
    The major industrial use of lithium is in the form of lithium stearate as 
a thickener for lubricating greases. Other important uses of lithium compounds 
are in ceramics, specifically in porcelain enamel formulation; as an additive 
to give longer life and higher output in alkaline storage batteries, and in 
welding and brazing fluxes. 
    Lithium is a moderately abundant element and is present in the Earth's 
crust to the extent of 65 parts per million (ppm). This places lithium a 
little below nickel, copper, and tungsten, and a little above cerium and tin 
in abundance. 
    Noteworthy among lithium's physical properties are the high specific heat 
(heat capacity), large temperature range of the liquid phase, high thermal 
conductivity, low viscosity, and very low density. Lithium metal is soluble in 
liquid ammonia and is slightly soluble in the lower aliphatic amines, such as 
ethylamine. It is insoluble in hydrocarbons. 
    Lithium undergoes a large number of reactions with both organic, and 
inorganic, reagents. It reacts with oxygen to form the monoxide, Li2O, and the 
peroxide, Li2O2. Lithium is the only alkali metal that reacts with nitrogen at 
room temperature to form a nitride, Li3N, which is black. Lithium reacts 
readily with hydrogen at about 930DGF (500DGC) to form lithium hydride, LiH. 
The reaction of lithium metal with water is exceedingly vigorous. Lithium 
reacts directly with carbon to form the carbide, Li2C2. Lithium combines 
readily with the halogens, forming halides with the emission of light. While 
lithium does not react with paraffin hydrocarbons, it does undergo addition 
reactions with arylated alkenes and with dienes. Lithium also reacts with 
acetylenic compounds, forming lithium acetylides, which are important in the 
synthesis of vitamin A. 
    The most important lithium compound is lithium hydroxide. It is a white 
powder, and the material of commerce is actually lithium hydroxide 
monohydrate, LiOHPRODUCTH2O. Lithium carbonate, LiCO3, finds application in 
the ceramic industries and in medicine as an antidepressant. Both lithium 
halides, lithium chloride and lithium bromide, form concentrated brines having 
the ability to absorb moisture over a wide temperature range; these brines are 
used in commercial air conditioning systems. 
 �34s[Marshall Sittig]
�33s 
  
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�6v�2v�32sLithium primary cell   �21m �33s 
    A primary cell whose anode is composed of lithium. The lithium cell has a 
number of advantages over other primary cell systems, including high energy 
density, flat discharge characteristics, excellent service over a wide 
temperature range (as low as -40DGC or -40DGF), and good shelf life (up to 5 
years without refrigeration). Nonaqueous solvents are used as the electrolyte 
because of the solubility of lithium in aqueous solutions. This cell is 
designed primarily for cardiac pacemakers and works with a typical current 
drain of 30 microamperes.  See  BATTERY; PRIMARY BATTERY; SOLID-STATE BATTERY. 
 �34s[Jack Davis ;Kenneth Franzese]
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�6v�2v�32sLithosphere   �21m �33s 
    A term initially used in a general way to distinguish the rock phase of 
the Earth from the hydrosphere, biosphere, and atmosphere at the surface of 
the Earth. The term lithosphere has been used variously in recent geological 
literature to designate the crust (upper lithosphere), the crust plus the 
mantle (that is, the total silicate phase of the Earth), or the entire solid 
part of the Earth (crust + mantle + core). The lithosphere contains more than 
99% of the mass of the Earth; it is therefore by far the most important 
reservoir of almost all of the chemical elements, and the processes in the 
lithosphere exert a dominant influence on many surface phenomena. 
    Erosion and chemical weathering are evidence of the pronounced 
disequilibrium between the atmosphere and the lithosphere. Chemical changes 
are taking place at various levels in the lithosphere and at the boundary 
between the lithosphere and the atmosphere, biosphere, and hydrosphere. The 
processes taking place at the Earth's surface can be observed directly and 
therefore are more completely understood than the processes taking place 
within and below the Earth's crust. Yet it is just the processes at depth that 
are of primary interest as a key to the history of the crustal evolution of 
the Earth. 
    Many of these processes participate in what has come to be known as the 
major geochemical cycle, whose most important aspects are shown in the 
illustration. The weathering of igneous, sedimentary, and metamorphic rocks at 
the lithosphere-atmosphere boundary produces sediments. These are buried and 
compacted at depth to produce sedimentary rocks. At more elevated temperatures 
or pressures or both, sedimentary rocks recrystallize to metamorphic rocks. At 
still higher temperatures partial or complete melting may produce rock melts, 
which cool to form igneous rocks. Exposure of these rocks to weathering 
processes then renews the cycle.  See  ATMOSPHERE; BIOSPHERE; DIAGENESIS; 
EARTH; HYDROSPHERE; MAGMA; METAMORPHISM; SEDIMENTATION (GEOLOGY); WEATHERING 
PROCESSES.  
 �34s[Heinrich D. Holland]
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�6v�2v�32sLitopterna   �21m �33s 
    Hoofed herbivores confined to the Cenozoic of South America. The order was 
well represented from the Paleocene to the Pleistocene, and apparently arose 
on that continent from a condylarth ancestry. By later Paleocene time two main 
lines of descent were clearly demarcated. The Proterotheriidae displayed a 
remarkable evolutionary convergence with the horses in their dentition and in 
reduction of the lateral digits of their feet. In one group the foot was 
reduced to a single median toe by early Miocene time. The members of the 
Macraucheniidae were proportioned much as in the camels and by late Tertiary 
time had similarly lost the vertebral arterial canal of the cervical 
vertebrae.  See  CONDYLARTHRA; MAMMALIA. 
 �34s[Richard H. Tedford]
�33s 
  
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�6v�2v�32sLiver �21m �33s
    A large gland found in all vertebrates. It consists of a continuous
parenchymal mass arranged to form a system of walls through which venous blood
emanating from the gut must pass. This strategic localization between
nutrient-laden capillary beds and the general circulation is associated with
hepatic regulation of metabolite levels in the blood through storage and
mobilization mechanisms controlled by liver enzymes.

    Function.  The liver is the sole source of such necessary constituents of
the blood as fibrinogen, serum albumin, and cholinesterase (an enzyme which
may prevent harmful buildup of the neuronal transmitter agent acetylcholine).
In the embryonic stage of most vertebrates the liver serves as the major
manufacturing site of erythrocytes, a process known as erythropoiesis. The
liver also removes toxins from the systemic circulation and degrades them, as
well as excess hormones. Particulate material may be removed through a
phagocytic action of specialized cells (Kupffer cells) lining the lumen of the
hepatic ``capillary spaces,'' or sinusoids. In addition to the products which
the liver delivers directly to the general circulation (endocrine function),
it secretes bile through a duct system which, involving the gallbladder as a
storage chamber, eventually passes into the duodenum (exocrine function). Bile
functions as an emulsifier of fats to facilitate their digestion by
fat-splitting lipases, and may also activate the lipase directly.  See	GALL-
BLADDER.

    Anatomy.  Knowledge of the precise connections between liver and intestine
and of liver and systemic circulation provides the mechanical basis for
understanding hepatic function. The human liver is a massive wedge-shaped
organ divided into a large right lobe and a smaller left lobe. Its anterior
surface underlies the diaphragm. The upper portion of the liver is partially
covered ventrally by the lungs, whereas the lower portion overhangs the
stomach and intestine. The entire liver is covered by Glisson's capsule, an
adherent membranous sheet of collagenous and elastic fibers.
    Venous blood from the intestine, and to a lesser extent from spleen and
stomach, converges upon a short broad vessel, called the hepatic portal vein,
which enters the liver through a depression in the dorsocaudal surface termed
the porta hepatis. There the hepatic portal vein divides into a short right
branch and a longer left branch. These vessels then ramify into the small
branches which actually penetrate the functional parenchymal mass as the inner
tubes of the portal canals.
    The hepatic artery also enters at the porta hepatis and ramifies into
smaller branches, which flank the portal venules within the portal canals. The
branches of the portal vein and hepatic artery then empty into sinusoids,
which are major regions of hepatovascular exchange. They communicate with
small branches of the hepatic veins and, through the hepatic vein, the blood
is returned to the heart by way of the vena cava.
    The tiny bile canaliculi, which lie between grooves in adjacent
parenchymal cells, communicate with tiny intralobular bile ducts. These
intralobular bile ducts empty into increasingly larger interlobular bile ducts
which lie within the portal canals and make up the third element of the
so-called portal triad.
    Although the biliary system is intimately associated with the portal
system anatomically, even in terms of egress through the porta hepatis, it
should be stressed that bile flow within this biliary system is in the
opposite direction from that of blood flow. The hepatic duct arises in the
region of the porta from the confluence of the largest bile ducts.
 �34s[George H. Fried]
�33s

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�6v�2v�32sLiver disorders   �21m �33s 
    A heterogeneous group disease of particular importance because of the many 
essential physiological functions of the liver. A common condition 
characterized by accumulation of lipid (fat) within hepatocytes is fatty 
metamorphosis. The hepatocytes become swollen, and the liver can increase in 
size two to three times. The most common cause of a fatty liver in the United 
States is excessive alcohol intake; this is not due to nutritional deficiency 
but to changes produced in cellular enzymes by alcohol.  See  ALCOHOLISM. 
    Hepatitis is defined as inflammation of the liver. Viral hepatitis is the 
most common type of hepatitis, and is usually caused by two distinct viruses 
called A and B. Alcoholic hepatitis is frequently seen in individuals 
consuming excessive quantities of ethyl alcohol. Cirrhosis is the term applied 
to widespread fibrosis in the liver, with nodular regeneration of hepatic 
parenchymal cells.  See  CIRRHOSIS; HEPATITIS; LIVER. 
 �34s[Samuel P. Hammar]
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�6v�2v�32sLiving fossils   �21m �33s 
    Living species very closely resembling fossil relatives in most anatomical 
details. The term is a relative one and, applied loosely, could embrace nearly 
all extant animals and plants. In its more restricted usage, the term applies 
to living species with four additional characteristics: (1) truly close 
anatomical similarity to (2) an ancient fossil species - generally at least 
100,000,000 years old; (3) living members of the group are represented by only 
a single or at best a few species, which are (4) often found in a very limited 
geographic area. Examples are horseshoe crabs, ginkgo trees, and coelacanth 
fish.  
 �34s[Niles Eldredge]
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�6v�2v�32sLizard   �21m �33s 
    Any reptile in the suborder Sauria, order Squamata; there are about 3000 
living species in 20 families. Included in the group are a variety of diverse 
forms such as the iguanas, geckos, monitors, and chameleons. They have a 
cosmopolitan distribution but are most abundant in tropical regions and are 
absent in Antarctica. Lizards are found in all types of habitats, to which 
they are well adapted; although a few species are herbivorous, the majority 
are insectivorous or carnivorous. 
    Although most species have paired limbs, some lizards have elongated 
bodies with reduced limbs and thus resemble snakes. Typical of the lizards are 
movable eyelids and a fused lower jaw. The tongue may be extensile, even 
prehensile, or thick and attached to the floor of the mouth with only the 
anterior edge extensile. Dentition is homodont, and the teeth are inserted on 
the jaw ridge (acrodont condition) or on the inner surface of the jaws 
(pleurodont). Lizards have epidermal scales which may be modified to form 
tubercles or reinforced with bony plate.  See  CHAMELEON; DENTITION; GECKO; 
GILA MONSTER; IGUANID; MONITOR; REPTILIA; SKINK; SQUAMATA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sLlama �21m �33s
    A member of the camel family (Camelidae) found only in South America. The
llama is an artiodactyl with two toes on each foot. The upper lip is cleft and
prehensile; the neck is long (see illustration). A single young is born after
a gestation period of about 11 months. The maximum life-span is about 20
years. Like other members of the family, the llama is herbivorous.
    Lama glama has been domesticated since the Inca civilization by the
Peruvian Indians, who still keep large herds. Many interesting crosses have
occurred among the different breeds in South America:

Llama (male) + alpaca (female) = huarizo
Alpaca + llama = misti; machurga
Vicuna + llama = llamavicuna
Guanaco + llama = llamahuanaco
Vicuna + alpaca = vacovicuna

While the fur provides material for rugs, rope, and cloth, the main use of
this animal is as a beast of burden, especially in mining areas.  See  ALPACA;
ARTIODACTYLA.
 �34s[Charles B. Curtin]
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�6v�2v�32sLoad line   �21m �33s 
    A line drawn on the output characteristic curves of a vacuum tube or 
transistor, used to determine the operating range and the quiescent point (the 
operating point for zero signal input voltage or current). In the case of 
vacuum tubes, this graphical construction is necessary because the 
mathematical function relating plate current to plate voltage and grid voltage 
is not known analytically. Load lines may also be used with transistor 
circuits and other nonlinear devices to determine the quiescent point.  See  
TRANSISTOR; VACUUM TUBE. 
 �34s[Christos C. Halkias]
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�6v�2v�32sLoads, dynamic �21m �33s
    A force exerted by a moving body on a resisting member is called a dynamic
or energy load. Loads suddenly applied with appreciable striking velocity, as
when a falling weight strikes another body, are called impact loads. Such
loads are produced by moving machine parts, cars on a bridge, airplane landing
shock, falling weights, and other nonstationary loading conditions where
forces are applied in relatively short time intervals. Dynamic load is
expressed in terms of the amount of energy transferred or by an equivalent
static load which has the same stress-producing effect on the member.
    If a material or structure has sufficient elastic energy capacity as a
resisting member to absorb completely the energy load through elastic action
and recover completely from the deformation, it behaves elastically.  See
SHOCK ABSORBER.
    An energy load exceeding the elastic energy capacity of the member
produces inelastic deformation. Toughness is the energy absorbed per unit
volume when the material is stressed to fracture. Capacity to dissipate energy
overloads by plastic deformations is desirable in members subject to shock or
impact.  See  STRESS AND STRAIN.
 �34s[William J. Krefeld /Waldo G. Bowman]
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�6v�2v�32sLoads, repeated   �21m �33s 
    Forces reapplied many times and causing varying stresses as the load 
changes. Repeated loads exist in crankshafts, axles, springs, piston rods, 
rails, bridge members, and many other machine and structural elements. 
Repeated loads considerably smaller than similarly applied static loads cause 
failure by progressive fracture. Designs for repeated loads depend on the 
maximum value of repeated stress which the material can resist, called the 
fatigue strength, and the magnitude of localized stress concentrations. See 
STRESS CONCENTRATION. 
    The maximum stress is called a stress concentration. High local stresses 
exist at bolt threads, abrupt change of shaft diameter, notches, holes, 
keyways, or fillet wells. Internal discontinuities, such as blowholes, 
inclusions, seams or cracks, ducts and knots in wood, voids in concrete, and 
variable stiffness of component constituents, cause stress concentrations. At 
points of contact of ball or roller bearings, gear teeth, or other local load 
applications, the stress may be greatly increased.  See  STRESS AND STRAIN. 
 �34s[John B. Scalzi]
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�6v�2v�32sLoads, transverse   �21m �33s 
    Forces applied perpendicularly to the longitudinal axis of a member. 
Transverse loading causes the member to bend and deflect from its original 
position, with internal tensile and compressive strains accompanying change in 
curvature.  
    Concentrated loads are applied over areas or lengths which are relatively 
small compared with the dimensions of the supporting member. Examples are a 
heavy machine occupying limited floor area, wheel loads on a rail, or a tie 
rod attached locally. Loads may be stationary or they may be moving, as with 
the carriage of a crane hoist or with truck wheels. 
    Distributed loads are forces applied continuously over large areas with 
uniform or nonuniform intensity. Closely stacked contents on warehouse floors, 
snow, or wind pressures are considered to be uniform loads. Variably 
distributed load intensities include foundation soil pressures and hydrostatic 
pressures.  
    Members subjected to bending by transverse loads are classed as beams. The 
span is the unsupported length. Beams may have single or multiple spans and 
are classified according to type of support, which may permit freedom of 
rotation or furnish restraint (see illustration). 
 �34s[John B. Scalzi]
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�6v�2v�32sLoa loa   �21m �33s 
    The African eye worm, a filarioidean worm most commonly acquired by 
Caucasian immigrants, including missionaries, in Africa. Transmission is by 
daytime-feeding sylvan deer-flies, Chrysops. Fortunately, serious damage is 
rare even when the worm gets into the eye. The areas of pitting edema known as 
calabar swellings are painful and diagnostic. They commonly occur on the 
wrists, hands, arms, or orbital tissues. See FILARIOIDEA. 
 �34s[Gilbert F. Otto]
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�6v�2v�32sLobata   �21m �33s 
    An order of the Ctenophora in which the body is helmet-shaped. A pair of 
large lobe-shaped processes extend from the oral end in the pharyngeal plane. 
A pair of ``auricles'' are attached to the base of each process. The body is 
compressed, with the tentacular axis being shorter than the pharyngeal axis. 
The ribs are of unequal length, with the longest being the subpharyngeal 
pairs. The primary tentacles are degenerate and are replaced by secondary 
tentacles (see illustration). Examples of this order include Bolinopsis, 
Ocyropsis, Mnemiopsis, and Leucothea.  See  CTENOPHORA; TENTACULATA. 
 �34s[Taku Komai]
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�6v�2v�32sLobosia   �21m �33s 
    A subclass of Rhizopodea characterized by lobopodia predominantly, 
although certain of these protozoan species also may form slender pseudopodia, 
or even develop several different kinds. Lobosia are divided into two orders, 
Amoebida and Arcellinida, which differ in presence or absence of a test. 
Pseudopodia emerge through the aperture of the test.  See  AMOEBIDA; 
ARCELLINIDA; PROTOZOA; SARCODINA; SARCOMASTIGOPHORA. 
 �34s[Richard P. Hall]
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�6v�2v�32sLobster   �21m �33s 
    The name for a number of species of decapod crustaceans which are members 
of the family Homaridae and are commercially important because of their food 
value. Three species are commonly marketed: the European lobster (Homarus 
vulgaris), the Norwegian lobster (Nephrops norvegicus), and the American 
lobster (Homarus americanus). The American lobster is found in greatest 
abundance along the eastern coast of North America from Maine to New York, but 
ranges from Labrador to North Carolina. Migrations are chiefly to deeper 
waters in the fall, with a return to shallower waters in the spring.  See  
DECAPODA (CRUSTACEA). 
 �34s[Charles B. Curtin]
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�6v�2v�32sLocal Group   �21m �33s 
    The cluster of galaxies to which the Milky Way Galaxy belongs. It is now 
generally accepted that most or all galaxies are clustered. The great 
clusters, which contain thousands of member galaxies, are rare, but can be 
recognized to very large distances. Clusters of smaller population are far 
more numerous. Most common are systems of only a few dozen or less galaxies; 
these are usually called groups to distinguish them from the richer clusters. 
The Local Group is such an entity.  See  COSMOLOGY. 
    The total membership of the Local Group is about two dozen. Those galaxies 
generally agreed to belong to the Local Group include three spirals: the Milky 
Way Galaxy; the Andromeda Galaxy, also known as Messier 31 (M31) and New 
General Catalog Number 224 (NGC 224); and the spiral in Triangulum - M33 or 
NGC 598. There are four irregular galaxies, including the nearest galaxian 
neighbors, the Large and Small Magellanic Clouds. Four Local Group galaxies 
are relatively small ellipticals.  See  ANDROMEDA GALAXY; GALAXY, EXTERNAL; 
MAGELLANIC CLOUDS. 
 �34s[George O. Abell]
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�6v�2v�32sLocation fit   �21m �33s 
    Mechanical sizes of mating parts such that, when assembled, they are 
accurately positioned in relation to each other. Locational fits are intended 
only to determine the orientation of the parts. For normally stationary parts 
that require ease of assembly or disassembly, for parts that fit snugly, and 
for parts that move yet fit closely as in spigots, slight clearance is 
provided between parts. Where accuracy of location is important, transition 
fits are used. In these fits the holes and shafts are normally nearly the same 
diameter. For greater accuracy of location, the shafts are made slightly 
larger than the holes; such fits are termed location interference fits.  See  
ALLOWANCE.  
 �34s[Paul H. Black]
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�6v�2v�32sLocomotive   �21m �33s 
    A machine on flanged wheels, capable of propelling itself and other 
vehicles on railroad tracks by converting heat or electric energy into 
mechanical power. Most of the locomotive units in use on United States 
railroads are diesel-electric units. There are far fewer all-electric 
locomotives in service, and the steam locomotive has almost completely 
disappeared.  
    In addition to horsepower, locomotives are rated on the basis of tractive 
effort (measured at the rail) or drawbar pull (measured at the rear coupler of 
locomotive). A diesel or electric locomotive is designed to develop tractive 
effort equal to about 25% of the weight on its driving wheels when starting a 
train on clean dry rail. Greater tractive effort than this results in wheel 
slip. Three types of train resistance must be overcome by this tractive 
effort: resistance on grades, on curves, and during acceleration.  See  
RAILROAD ENGINEERING. 
 �34s[W. D. Edson]
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�6v�2v�32sLocust (forestry) �21m �33s
    A name commonly applied to two trees, the black locust (Robinia
pseudoacacia) and the honey locust (Gleditsia triacanthos). Both of these
commercially important trees have podlike fruits similar to those of the pea
or bean.
    The black locust is native in the Appalachian and the Ozark regions and is
now widely naturalized in the eastern United States, southern Canada, and
Europe. The wood is hard and durable in the soil and is used for fence posts,
mine timbers, poles, and railroad ties.
    The honey locust is native in the Appalachian and the Mississippi Valley
regions, but is also widely naturalized in the eastern United States and
southern Canada. The reddish wood is hard, strong, and coarse-grained and
takes a high polish. Because it is durable in contact with soil, it is used
for fence posts and railroad ties; it is also used for construction,
furniture, and interior finish.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sLoess   �21m �33s 
    An essentially unconsolidated, unstratified, calcareous silt. Most 
commonly it is homogeneous, permeable, buff to gray in color, and contains 
calcareous concretions and fossils. In natural and artificial excavations the 
loess maintains notably stable, vertical faces. 
    Although dominantly silt and often referred to as well sorted, loess is 
actually only moderately well sorted. The silt (and sand) grains are usually 
angular to subangular. Clay occurs as silt-size aggregates, as coatings on 
silt grains, and as interstitial filling. In texture and mineral composition, 
loess is comparable to the average mudstone or shale. 
    Loess is typically developed in areas peripheral to those covered by the 
last ice sheets in North America and Europe Most of the loess seems to have 
been deposited during the latest (Wisconsin) glacial stage.  See  
SEDIMENTATION (GEOLOGY). 
 �34s[Chalmer J. Roy]
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�6v�2v�32sLogarithm �21m �33s
    An exponent of a suitably chosen positive number (base) larger than unity.
Logarithms are of value in mathematical computation and in the equations and
formulas used in expressing natural phenomena.
    If b(sup l) = n, where b is a positive number larger than unity, then l is
called the logarithm of n to the base b and is written l = log(sub b) n. From
this definition it follows at once that all positive numbers larger than unity
have positive logarithms, all positive numbers smaller than unity have
negative logarithms, and the logarithm of unity is equal to 0. Since b(sup 0)
 = 1 irrespective of the value of b, it follows that the logarithm of unity is
equal to 0. From the known properties of exponentials expressed by relations
(1), it follows immediately that (1) the logarithm of a product of two or more
factors is equal to the sum of the logarithms of the factors; (2) the
logarithm of the ratio of two numbers is equal to the difference between the
logarithm of the numerator and the logarithm of the denominator; (3) the
logarithm of the mth power of a number is the product of m and the logarithm
of the number; and (4) the logarithm of the mth root of a number is equal to
the logarithm of the number divided by m.  See	EXPONENT.

b(sup l1) x b(sup l2) = b(sup l1+l2)

(b[sup l1])/b[sup l2] = b[sup l1-l2]

(b sup l)sup m = b sup lm (m ROOT [b sup l])   = b(sup l)/m		    (1)


    Although any positive number b larger than unity might have been chosen as
the base of a system of logarithms, actually two numbers have been chosen in
the construction of tables of logarithms, namely, the number b = 10 and the
number e = 2.718 . . . defined as the sum of infinite series (2). The system
of logarithms to the base 10 is usually referred to as common logarithms; the
system of logarithms to the base e is called natural logarithms. See
(mathematics).

     1 + 1/1 + 1/(1 x 2) + 1/(1 x 2 x 3) + . . .
				+ 1/(1 x 2 x 3 x . . . xn) + . . .	     (2)

    The integral part of the common logarithm of a number larger than unity is
one unit less than the number of digits before the decimal point.  This integral
part is called the characteristic; the decimal part is called the mantissa.
Similarly, because the common logarithms of 0.1, 0.01, and 0.001 are -1, -2, and
-3, it follows that the logarithm of a number smaller than unity having p zeros
after the decimal point may be expressed as the sum of a negative characteristic
equal to - (p + 1) and a positive mantissa.
 �34s[Arnold N.  Lowan/ Salomon Bochner]
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�6v�2v�32sLogging (forestry)   �21m �33s 
    The cutting and removal of the woody stem portions of forest trees. In 
effect, logging is the harvesting of timber from forests. Originally, logging 
produced only sawlog lengths for lumber sawmills, but with increased 
utilization of wood for other products such as pulp and paper, veneers, and 
poles and piles, the term logging includes practically all operations 
conducted in the forest in connection with felling trees, cutting the trunks 
to desired lengths, and transporting the merchantable material to processing 
mills or to shipping points. Most lumber log lengths are 12, 14 or 16 ft (3.6, 
4.2, or 4.8 m), corresponding to the standard lengths of lumber used in 
general construction; or in multiples of these lengths, in which event final 
log lengths are cut at the mill. Pulpwood generally is cut in lengths of 4, 5, 
or 8 ft (1.2, 1.5, or 2.4 m), although sawlog lengths may be produced for 
efficient mechanized handling of larger diameters. For other products, various 
lengths are produced. 
    The steps usually followed in logging are: (1) selection of trees; (2) 
felling the trees; (3) bucking the trunks (cutting into appropriate lengths) 
and removing limbs and unmerchantable tops; (4) skidding or forwarding the 
bucked lengths to roadside or waterside; (5) loading on hauling vehicles, 
usually trucks; and (6) hauling to destination.  See  LUMBER MANUFACTURE; WOOD 
PRODUCTS.  
 �34s[Albert E. Wackerman]
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�6v�2v�32sLogic �21m �33s
    The subject that investigates, formulates, and establishes principles of
valid reasoning.
    The first attempt to investigate systematically acceptable modes of
reasoning was made by Aristotle, in whose Organon reasoning was recognized as
the subject of a special science. He formulated three basic ``laws of
thought'': (1) the law of contradiction (no proposition is both true and
false), (2) the law of excluded middle (each proposition is either true or
false), and (3) the law of identity (each proposition implies itself).
Advanced as his views of logic were, it seems doubtful that the idea of
axiomatizing it ever occurred to him, despite the success of his contemporary
Euclid in organizing geometry. That aspect of logic which enunciates or
establishes valid reasoning (rather than merely investigating it) began with
the publication in 1854 of George Boole's An Investigation of the Laws of
Thought. The partly abstract treatment of logic presented in this work
initiated the completely abstract developments that were to follow. In it the
laws of thought are regarded as mere conventions which, like the postulates of
euclidean geometry, might be modified or even rejected to create new logics.
The only requirement that a ``new'' logic must satisfy is the one demanded of
every deductive system - consistency. Just as different geometries are useful
for different purposes, a logic that is appropriate in one environment might
not be so in another. There are a growing number of competent individuals, for
example, who consider that any logic containing Aristotle's law of excluded
middle is not suitable for mathematics.
    The unit of the propositional calculus is the proposition, which may be
defined as the meaning of an indicative or declarative sentence. Each
proposition has a truth-value; it is either true or false in the classical
logic, but may assume other truth-values (for example, uncertain) in some of
the new logics. It is convenient to use letters p, q, r, and so on to denote
propositions. Propositions are combined by logical connectives to form other
propositions. Chief among these are (1) negation (the negation of a
proposition p, symbolized by p', may be formed verbally by writing ``It is not
the case that'' before p); (2) conjunction (a binary operation symbolized by
PRODUCT or &, for example, p PRODUCT q, read ``p and q''); (3) disjunction (a
binary operation symbolized by + or OR; for example, p OR q, read ``p or q'');
(4) implication (a binary operation symbolized by CONTAINS or ->, for example,
p -> q, read ``p implies q''); and (5) equivalence (a binary operation denoted
by IDENTITY; for example, p IDENTITY q, read ``p is equivalent to q,''
means that each proposition implies the other). The proposition p PRODUCT q
asserts that both p and q are true. It is important to note that the
disjunctive ``or'' is always used in the inclusive sense; that is, p OR q
asserts that at least one, possibly both, of the propositions p and q is true.
In addition, the meaning of ``implication'' in logic differs markedly from the
ordinary sense of that term. The proposition p -> q asserts merely that it is
not the case that p is true and q is false; that is, in classical logic p -> q
means that either p is false or q is true. The logical connectives introduced
above are examples of logical constants. They are not independent notions.
Implication may be defined in terms of negation and conjunction, and this is
true for all of the connectives. In fact, all the connectives can be defined
in terms of one binary operation, the so-called Scheffer stroke function p|q,
which may be read ``p is false or q is false.''
    The logical connectives, the letters p, q, r, and so on, that stand for
propositions (propositional variables), and the parentheses, brackets, and
braces needed for punctuation are formal concepts. A propositional function is
a combination of concepts, formal or nonformal, involving at least one
variable, which is not a proposition but becomes one when all the variables
are given specified values. Examples are (1) p and q, (2) xsup2 + ysup2 = 1,
and (3) x is president of the United States. Unlike a proposition, a
propositional function has no truth-value. On the other hand, the statement
``For every integer x, xsup2 + 1 = 0'' is a proposition (not a propositional
function) even though it contains a (bound) variable. A formal function is a
propositional function that contains only formal concepts - for example [(p OR
q) PRODUCT r] CONTAINS s. A truth function is a propositional function in
which only propositional variables occur, and such that the truth value of
each proposition obtained from the function by substituting specific
propositions for the variables depends only on the truth values of those
propositions. Examples are (1) (p PRODUCT q) OR r', (2) Napoleon was a great
general and p (where p is a propositional variable). On the other hand,
``Jones stated that p'' is not a truth function.
    Tautologies are formal truth functions such that every proposition
obtained from them by substituting specific values (propositions) for the
variables they contain has the truth value T (true). A truth function is
contingent provided it assumes both truth values T and F (false) - here only
the classical two-valued (aristotelian logic) truth functions are considered -
and self-inconsistent provided it has only the value F. The character of a
truth function is determined when it is shown to be self-inconsistent,
contingent, or a tautology.
    A truth function may be analyzed by means of a truth table, or matrix.
Such tables are made first for the elementary truth functions p', p OR q, p
PRODUCT q, p CONTAINS q as follows:

(I)	p	p'
	T	F
	T	F

(II)	p	q	p OR q
	T	T	   T
	T	F	   T
	F	T	   T
	F	F	   F

(III)	p	q	p PRODUCT q
	T	T	     T
	T	F	     F
	F	T	     F
	F	F	     F

(IV)	p	q	p CONTAINS q
	T	T	     T
	T	F	     F
	F	T	     T
	F	F	     T

In every table each row of the last column gives that truth value of the
function which is determined by those truth values (assigned to the variables)
that are entered in the preceding columns of that row.
    All tautologies can be deduced from a set of just four of them:
P1 (p OR p) CONTAINS p
P2 q CONTAINS (p OR q)
P3 (p OR q) CONTAINS (q OR p)
P4 (q CONTAINS r) CONTAINS [(p OR q) CONTAINS (p OR r)]
    These simple tautologies were selected by Alfred North Whitehead and
Bertrand Russell in their monumental work Principia Mathematica as the basic
set from which all tautologies can be derived. This is analogous to what is
done in an axiomatic geometry in which all the theorems of the geometry are
deduced from a selected set of axioms or postulates that are assumed to be
true.
    It was proved by E. L. Post that postulates P1 - P4 are consistent; that
is, it is impossible to deduce from them truth functions X and X'. He also
proved that the four postulates form an independent set; that is, no one of
the postulates can be deduced from the others.
    In nonaristotelian logics the principle of the excluded middle (that is,
each proposition is either true or false) is not valid. There are, then, at
least three truth values possible for a proposition, and for this reason such
logics are known as many-valued. If ``true'' be interpreted in mathematics to
mean ``provable,'' and a proposition be called false provided its negation is
provable, then the law of excluded middle is demonstrably false, since it has
been shown that mathematics contains proposition p such that neither p nor its
negation p' is provable.  See  BOOLEAN ALGEBRA; EUCLIDEAN GEOMETRY;
MATHEMATICS; SCIENCE.
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sLogic circuits   �21m �33s 
    Electronic circuits which process information encoded as one of a limited 
set of voltage or current levels. Logic circuits are the basic building blocks 
used to realize consumer and industrial products that incorporate digital 
electronics. Such products include digital computers, video games, voice 
synthesizers, pocket calculators, and robot controls.  See  INTEGRATED 
CIRCUITS.  
    All logic circuits may be described in terms of three fundamental 
elements, shown graphically in the illustration. The NOT element has one input 
and one output; as the name suggests, the output generated is the opposite of 
the input in binary. In other words, a 0 input value causes a 1 to appear at 
the output; a 1 input results in a 0 output. (All signals are interpreted to 
be one of only two values, denoted as 0 and 1.) 
    The AND element has an arbitrary number of inputs and a single output. As 
the name suggests, the output becomes 1 if, and only if, all of the inputs are 
1; otherwise the output is 0. The AND together with the NOT circuit therefore 
enables searching for a particular combination of binary signals. 
    The third element is the OR function. As with the AND, an arbitrary number 
of inputs may exist and one output is generated. The OR output is 1 if one or 
more inputs are 1. 
    The operations of AND and OR have some analogies to the arithmetic 
operations of multiplication and addition, respectively. The collection of 
mathematical rules and properties of these operations is called boolean 
algebra.  See  BOOLEAN ALGEBRA. 
    While the NOT, AND, and OR functions have been designed as individual 
circuits in many circuit families, by far the most common functions realized 
as individual circuits are the NAND and NOR circuits of the illustration. A 
NAND may be described as equivalent to an AND element driving a NOT element. 
Similarly, a NOR is equivalent to an OR element driving a NOT element. 
    As the names of the logic elements described suggest, logic circuits 
respond to combinations of input signals. Logic networks which are 
interconnected so that the current set of output signals is responsive only to 
the current set of input signals are appropriately termed combinational logic. 
An important further capability for processing information is memory, or the 
ability to store information. The logic circuits themselves must provide a 
memory function if information is to be manipulated at the speeds the logic is 
capable of. Logic circuit networks that include feedback paths to retain 
information are termed sequential logic networks, since outputs are in part 
dependent on the prior input signals applied and in particular on the sequence 
in which the signals were applied. 
    Several alternatives exist for the digital designer to create a digital 
system. Two common realizations are ready-made catalog-order devices, which 
can be combined as building blocks, and custom-designed devices. Gate-array 
devices comprise a two-dimensional array of logic cells, each equivalent to 
one or a few logic gates. Programmable logic arrays have the potential for 
realizing any of a large number of different sets of logic functions. In table 
look-up, the collection of input signals are grouped arbitrarily as address 
digits to a memory device. Finally, the last form of logic network embodiment 
is the microcomputer.  See  MICROCOMPUTER. 
    There are basically two logic circuit families in widespread use: bipolar 
and metal-oxide-semiconductor (MOS).  See  TRANSISTOR. 
 �34s[R. R. Shively ;W. V. Robinson]
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�6v�2v�32sLophiiformes   �21m �33s 
    The anglerfishes and their relatives. The first dorsal fin is reduced to a 
few flexible rays, of which the first is placed on top of the head and bears a 
terminal bulb or tassel and functions as a fishing lure (see illustration). 
Luminescent organs may be present, and in some deep-sea species the males are 
dwarfed and attached as ectoparasites on the females. There are 3 suborders, 
15 families, nearly 60 genera, and about 195 known species.  See  
ACTINOPTERYGII.  
 �34s[Reeve M. Bailey]
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�6v�2v�32sLophophore   �21m �33s 
    The crown of tentacles which surrounds the mouth in the Bryozoa, 
Phoronida, and Brachiopoda. The numerous ciliated tentacles arise from a 
circular or horseshoe-shaped fold of the body wall. The tentacles are hollow 
outgrowths of the body wall, each containing fluid-filled extensions of the 
body cavity and extended hydraulically. The primary function of the lophophore 
is to gather food. On the tentacles are ciliary tracts which drive a current 
of food-particle-bearing water through the lophophore. While the lophophore is 
primarily a feeding organ, it may also play a role in reproduction, 
respiration, and larval locomotion.  See  BRACHIOPODA; BRYOZOA; PHORONIDA. 
 �34s[Judith E. Winston]
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�6v�2v�32sLoran   �21m �33s 
    A navigation system from which hyperbolic lines of position are determined 
by measuring the difference in times of arrival of pulses from widely spaced, 
synchronized transmitting stations. Since radio waves travel at the speed of 
light, a line of position represents a constant range difference from two 
transmitters.  
    In the illustration, it is assumed that the master station M transmits a 
pulse signal at time t = 0, and that the secondary station S transmits a 
similar pulse signal DELTA microseconds after receiving the master pulse 
signal. The secondary station receives the master signal at time t = BETA 
microseconds, and therefore transmits its pulse at time t = BETA + DELTA 
microseconds. A receiver at R measuring the difference in the times of arrival 
of the signals from the secondary and master stations measures the time 
difference TD = (BETA + DELTA) + tsubSsubR - tsubMsubR. The locus of all 
points with this common time difference is a hyperbola through the receiver 
position R. Similarly, every hyperbolic line of position is uniquely defined 
by a time difference. The intersection of two such lines of position gives a 
navigational fix, or location. Loran is used by commercial and military ships 
and aircraft. The name is derived from ``long range navigation.''  See  
HYPERBOLIC NAVIGATION SYSTEM. 
 �34s[James P. Van Etten]
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�6v�2v�32sLorentz transformations �21m �33s
    The family of mathematical transformations used in the special theory of
relativity to relate the space and time variables of uniformly moving (inertial)
reference systems.  They were discovered in part by H.	A.  Lorentz during his
studies of matter moving in an electromagnetic field and were formulated more
completely by H.  Poincare and by Albert Einstein.  If S and S' are two inertial
reference frames with space-time coordinates (x,y,z,t) and (x',y',z',t'),
respectively, the equations connecting these variables have the form (xsup0 =
ct, xsup1 = x, xsup2 = y, xsup3 = z) of Eq.  (1), where asup0,

x'(sup alpha) = SUMMATION (beta=0 to 3) (L[sub beta])(sup alpha) (x[sup beta] 
                - a[sup beta])    (alpha = 0, 1, 2, 3)			    (1)

a(sup 1), a(sup 2), a(sup 3) are arbitrary real constants.  Forming the two real
matrices (2) and (3),

	    L = [ L0sup0     L1sup0	 L2sup0       L3sup0
		  L0sup1     L1sup1	 L2sup1       L3sup1
		  L0sup2     L1sup2	 L2sup2       L3sup2
		  L0sup3     L1sup3	 L2sup3       L3sup3 ]		     (2)

	      eta = [ 1        0	   0		0
		      0       -1	   0		0
		      0        0	  -1		0
		      0        0	   0	       -1 ]		     (3)

the matrix L is subject to the condition shown as

	    L(sup -1) = eta L(sub t) eta				     (4)


Eq. (4), where Lsup-1 and Lsubt are the inverse and transposed ma
trices of L.  See  RELATIVITY.
 �34s[Edward L. Hill]
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�6v�2v�32sLoudness   �21m �33s 
    The perceptual intensity of sound. Loudness depends importantly on the 
physical intensity of sound, increasing when physical intensity increases and 
decreasing when physical intensity decreases. But loudness also depends on 
other physical properties of sound, such as frequency and duration. Sound 
waves with frequencies between 1000 and 5000 hertz are louder than sound waves 
that have the same intensity but lower or higher frequencies. The physical 
level of sound is expressed in decibels.  See  DECIBEL. 
    Because loudness depends on sound frequency as well as sound intensity or 
pressure, different stimuli with the same sound pressure level (SPL) may not 
be equally loud. One type of decibel scale, called the phon scale, overcomes 
this deficiency. The level of a sound in phons is the SPL in decibels of an 
equally loud 1000-Hz tone. Thus a 1000-Hz tone at 40dB SPL has a level of 40 
phons, as do all other sounds that equal its loudness, even though these other 
sounds may have SPLs much greater than 40 dB. 
    The phon scale is often designated as a scale of loudness level. It is a 
scale of equal loudness, in that all equally loud sounds take the same level 
in phons, regardless of the SPL. Nevertheless, the phon scale is not a true 
scale of loudness, because it is a physical (decibel) scale. That is, a sound 
of 80 phons is not necessarily twice as loud as a sound of 40 phons. In order 
to determine a scale of perceived loudness, observers were asked to set the 
level of one sound to make it appear twice or half as loud as standard sounds. 
Observers were also asked to make numerical judgments of the degree of 
loudness. From such judgments a scale of loudness in sones was established. 
One sone is defined as the loudness of a 1000-Hz tone at 40-dB SPL heard with 
both ears. (When heard with one ear, loudness is half as great.) Above 40 dB, 
loudness in sones doubles with every 10-dB increase in the level of sound. 
Below 40 dB, loudness falls by half with decrements smaller than 10 dB (see 
illustration).  See  HEARING (HUMAN); SOUND; SOUND PRESSURE. 
 �34s[Lawrence E. Marks]
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�6v�2v�32sLoudspeaker   �21m �33s 
    A device that converts electrical signal energy into acoustical energy, 
which it radiates into a bounded space, such as a room, or into outdoor space. 
The essential characteristic of the signal energy must be preserved in the 
energy-conversion process. The shorter term speaker is also used. The term 
unit, as in horn or driver unit, is applied to devices comprising a motor and 
diaphragm but lacking the acoustic element such as a horn to complete the 
speaker. The term speaker system is normally reserved for a plurality of 
speaker units and the associated accessories, horns, cabinets, and electrical 
networks required for an integrated system.  See  SOUND-REPRODUCING SYSTEMS. 
    Tests have shown that frequency ranges of 80 - 11,000 Hz for music and 150 
- 8000 Hz for speech, if uniform throughout the listening area, give 
high-quality reproduction in an otherwise distortionless system. The pressure 
range requirements are set primarily by listener preference.  See  
PSYCHOACOUSTICS.  
    Speakers are commonly classified by terms which describe their three 
important functional parts: the motor, diaphragm, and acoustic 
radiation-controlling element. The motor converts electrical energy into 
mechanical energy and couples the electrical signal source, commonly an 
amplifier, to the diaphragm. Speakers are also classified by their 
radiation-controlling elements into direct-radiator (hornless) and horn types. 
The direct-radiator types commonly employ baffles or enclosures. The diaphragm 
is the element which, vibrated by the motor, causes the air to vibrate; hence 
it couples the air load (radiation impedance) to the motor. 
    Direct-radiator speakers are the most widely used speakers. Of these, 
substantially all of the general-purpose and special-purpose low-frequency 
(woofer) speakers are of the moving-coil type. Electrostatic speakers are 
sometimes used as high-frequency speakers (tweeters) in broad-band systems and 
infrequently as general-purpose speakers. 
    Two or more identical speakers may be used to improve low-frequency 
efficiency and increase permissible input and output powers or, by proper 
relative orientation, improve the directional properties. Several 
substantially identical speakers may be used in combination to influence the 
directivity pattern. More commonly, the two or more speakers cover 
complementary frequency ranges. The more important advantages are (1) improved 
frequency response, (2) higher system efficiency, (3) improved directivity 
characteristic, (4) improved transient response, (5) reduced intermodulation, 
and (6) reduced frequency modulation. Coaxial units comprise two, or less 
commonly three, speaker units mounted on substantially the same axis in an 
integrated mechanical assembly. Appropriate wave filters or dividing networks 
control the amplitude and phase of the electrical signal reaching each unit. 
    In single-channel, or monophonic, systems speaker placement is largely 
controlled by the desirability of uniform response in the listening area. A 
corner location in a room is desirable, improving the low-frequency efficiency 
and reducing the angle the high-frequency radiation must cover. In 
stereophonic systems multiple speakers and signal channels are used to 
reproduce the spatial effects present in the original sound environment. The 
simpler conventional systems employ two channels and two speakers or speaker 
systems. These speakers are located symmetrically in the room, preferably 
along the shorter wall. In typical small listening rooms the speakers are 6 - 
10 ft (2 - 3m) apart. 
 �34s[Hugh S. Knowles]
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�6v�2v�32sLouse   �21m �33s 
    Any insect which is a member of the orders Anoplura (biting and sucking 
lice) or Mallophaga (biting or chewing lice). Lice are apterous (wingless) 
ectoparasites of birds and mammals. The leathery body is flattened 
dorsoventrally, with a head that is more or less distinct, and segmented legs 
(see illustration). The eggs, as they are laid, are attached to the hair or 
feathers of the host. See ANOPLURA; MALLOPHAGA. 
 �34s[Charles B. Curtin]
�33s 
  
A700000002656�254s�5v�1v1003
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�6v�2v�32sLow-level counting   �21m �33s 
    The measurement of very small amounts of radioactivity. The instruments 
used are special modifications of standard radioactivity measurement 
equipment, designed to yield greater detection efficiencies and lower 
background counting rates than those necessary for ordinary applications. 
These techniques are applied mainly to the measurement of long-lived natural 
radioactive isotopes and their daughter products, cosmic-ray-produced 
isotopes, and isotopes produced artificially during nuclear explosions.  See  
RADIOACTIVITY.  
    Because of the statistical error associated with the measurement of 
radioactivity, accurate assays of samples with small amounts of radioactivity 
cannot be made in the presence of large amounts of radiation from other 
sources. The error can be decreased by reducing the background or by 
increasing the sample count rate. Thus most low-level counters are designed 
(1) to hold as large a sample as it is practical to obtain and to prepare; (2) 
to record with as high an efficiency as possible the radiations from the 
sample; and (3) to have as low a background counting rate as possible. The 
instruments which have been most widely used are internal gas counters, solid 
source BETA-counters, and scintillation counters.  See  GEIGER-MUELLER 
COUNTER; PARTICLE DETECTOR; SCINTILLATION COUNTER. 
 �34s[Minze Stuiver]
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�6v�2v�32sLow-pressure fluid flow   �21m �33s 
    Flow of fluids below atmospheric pressures, following the perfect gas 
laws, in pipes, fittings, and other common configurations.  See  GAS. 
    The laws of fluid flow at subatmospheric pressures in the turbulent, 
transition, and laminar flow range are the same as the laws of fluid flow at 
any pressures. For liquids, the usual limitation applies that the downstream 
pressure should not drop to the vapor pressure of the liquid being handled. 
This imposes severe limitations on the velocity of liquid flow at reduced 
pressures.  
    The molecular flow range is characterized by a mean free path of molecules 
longer than the smallest linear dimension of the conduit. The laws of flow in 
this range (possible only for gases and vapors) are based on the assumption 
that the molecules have Maxwell-Boltzmann distribution of velocities.  See  
FLUID FLOW; KINETIC THEORY OF MATTER; LAMINAR FLOW; MEAN FREE PATH; 
SUPERAERODYNAMICS; TURBULENT FLOW. 
 �34s[Zbigniew C. Dobrowolski]
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�6v�2v�32sLow-temperature acoustics   �21m �33s 
    The application of acoustics to research on the properties of condensed 
matter at low temperatures. Acoustic techniques are readily adaptable to the 
cryogenic environment and make possible many measurements of the structural 
and thermodynamic properties of materials at temperatures approaching absolute 
zero (0 K, which is -273DGC). The study of sound propagation has also yielded 
major insights into the low-temperature phenomena of superconductivity in 
metals and superfluidity in liquid helium. 
  
    Solid materials.  Acoustic measurements have been used to characterize the 
properties of a wide variety of solid-state materials, such as metals, 
dielectric crystals, amorphous solids, and magnetic materials. A measurement 
of the velocity of sound in a substance gives information on its elastic 
properties, while the attenuation of the sound characterizes the interaction 
of the lattice vibrations with the electronic and structural properties of the 
material. Ultrasonic frequencies, in the range from 20 kHz to 100 MHz and 
above, are commonly employed in these measurements because of the ease of 
generating and detecting the sound with piezoelectric quartz crystals.  
    Because the sound velocity effectively measures elastic constants, such 
measurements are used to characterize phase transitions in crystals where the 
structure of the lattice changes. The attenuation of sound in many crystals is 
due to defects and impurities in the crystal lattice and provides information 
on such structures. In a metal at very low temperatures, the dominant source 
of attenuation is the interaction of the sound with the conduction electrons.  
See  CRYSTAL; LATTICE VIBRATIONS; PHASE TRANSITIONS; PHONON; SOUND ABSORPTION. 
  
    A large variety of magnetoacoustic effects are observed in metals and 
crystals. In these measurements, changes in the sound attenuation occur as the 
strength of a magnetic field applied to the sample is increased. One example 
is the phenomenon of nuclear acoustic resonance, resulting from the 
interaction of the nuclear spins in a crystal with vibrations of the lattice. 
There are also a number of other magnetoacoustic effects in metals which are 
useful in determining the orbits followed by the conduction electrons in the 
metal. See  DE HAAS - VAN ALPHEN EFFECT. 
    Sound propagation is becoming increasingly useful for studying amorphous 
materials, in which there is no regular crystal structure. In materials such 
as silica glass (amorphous silicon dioxide, SiO2), it has been found that at 
low temperatures there are only two quantum energy levels that are important. 
These levels correspond to two nearly equivalent arrangements of the atoms, 
with one arrangement having slightly higher energy. The stress from an imposed 
sound field can cause a transition from one arrangement to the other. If the 
relaxation rate back to the original configuration is comparable to the sound 
frequency, there will be a net absorption of energy from the sound wave.  See  
AMORPHOUS SOLID. 
    When a metal is cooled below its superconducting transition temperature, 
there are striking changes in the attenuation of sound. At the transition some 
of the electrons near the Fermi surface begin to pair together, due to the 
attractive electron-phonon coupling. Once this occurs, the electrons can no 
longer exchange momentum with the lattice, and hence have zero resistance. 
This also means that the paired electrons no longer absorb energy from the 
sound wave, and the attenuation is from the remaining unpaired normal 
electrons. As the temperature is lowered well below the transition, the 
density of the unpaired electrons drops rapidly, and the attenuation becomes 
very small.  See  SUPERCONDUCTIVITY. 
  
    Superfluid helium.  Sound propagation has been extensively used to probe 
many of the unusual properties of superfluid helium. The novel features of the 
superfluid (zero viscosity and entropy) give rise to a rich variety of 
different types of sound which can propagate in the superfluid helium. Five 
distinct sound modes have been identified and observed experimentally. The 
sound velocities of a number of these modes are shown in the illustration as a 
function of temperature. 
    First sound is a pressure wave which propagates in the bulk liquid. It is 
quite similar to sound in ordinary fluids. 
    Second sound is an unusual type of wave: it is a temperature wave in the 
bulk superfluid. In this mode the normal fluid and superfluid move in opposite 
directions. This keeps the density constant, and hence there are no pressure 
oscillations in the wave (as in first sound); but because only the normal 
fluid carries entropy, there are oscillations in the entropy and thus in the 
temperature of the liquid.  See  SECOND SOUND. 
    Third sound is a wave which propagates in very thin films of helium. The 
third sound is a wave in which the thickness of the film varies, somewhat like 
waves in a tank of water. Because the films are so thin, only the superfluid 
can move, the normal fluid being immobilized by its viscosity. 
    Fourth sound is a pressure wave which propagates in superfluid helium when 
it is confined in a porous material such as a tightly packed powder. In such a 
situation the normal fluid is immobilized, and only the superfluid can flow 
freely because of its zero viscosity (the porous materials are often called 
superleaks for this reason). The fourth sound is analogous to first sound 
because it involves density and pressure oscillations and can be generated and 
detected by pressure transducers. 
    Fifth sound is a temperature wave which can propagate in helium confined 
in a superleak. It is analogous to second sound, except that again only the 
superfluid component can flow.  See  LIQUID HELIUM; SUPERFLUIDITY. 
 �34s[Gary A. Williams]
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�6v�2v�32sLow-temperature physics   �21m �33s 
    A study of the properties of gross matter at such low temperature that the 
quantum character of the substance becomes observable. In a general way, this 
implies temperatures of -452DGF (4 K) and below. Quantum-mechanical coherence 
effects extend over readily measurable distances and give rise to 
superconductivity and superfluidity. Zero-point energy ``blows up'' light 
atoms and greatly influences the bulk properties of the hydrogens and the 
heliums in the solid state. The existence of quantized vortices plays a large 
role in the basic behavior of superconductors and the liquid heliums. The 
existence of discrete atomic energy levels shows up via heat capacity 
anomalies (the Schottky effect) and various magnetic and nuclear phenomena. 
The ``ortho-para conversion'' is a striking thermal phenomenon in hydrogen 
compounds. Minima in electrical resistance (the Kondo effect) and thermal 
boundary resistances (Kapitza resistance) are other peculiarities under active 
study. The topic of cryogenics, most strictly interpreted, is the method of 
producing low temperatures. In addition to the considerable technology 
involved in refrigeration, there is a very important section of 
low-temperature physics concerned with temperature measurement and related 
instrumentation.  See  CRYOGENICS; KAPITZA RESISTANCE; KONDO EFFECT; LIQUID 
HELIUM; LOW-TEMPERATURE THERMOMETRY; QUANTIZED VORTICES; QUANTUM SOLIDS; 
SCHOTTKY ANOMALY; SUPERCONDUCTIVITY; SUPERFLUIDITY. 
 �34s[Ralph P. Hudson]
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�6v�2v�32sLow-temperature thermometry �21m �33s
    The measurement of temperature below 32DGF (0DGC). Very few thermometers
are truly wide-range, and hence most of the conventional methods of
thermometry tend to fail the further one moves below room temperature (see
table). The defining instrument for the lower regions of the International
Practical Temperature Scale, the platinum resistance thermometer, rapidly
loses sensitivity below -405DGF (30 K), and its official limit is set at
-434.81DGF (13.81 K), the triple point of equilibrium hydrogen. This scale is
based upon measurements of thermodynamic temperature made with the gas
thermometer; the gas thermometer may be used down to about -456DGF (2 K).  See
  GAS THERMOMETRY.
    The low-temperature region is unique in having available several different
types of primary thermometers, all of which are quite practical. The least
practical, perhaps, is the acoustic thermometer, which uses the property that,
extrapolated to zero pressure, the speed of sound in a gas is proportional to
T1/2. This has been used in the range -456 to -424DGF (2 - 20 K) as an
alternative to, and check upon, the gas thermometer. The Johnson noise in a
resistor can be used with particular advantage at low temperatures when allied
with SQUID detector technology. See ELECTRICAL NOISE; SOUND.
    In suitable systems it is possible to spatially orient atomic nuclei at very
low temperatures, and if these nuclei are emitters of gamma rays, the emission
pattern is anisotropic to a degree which is a measure of the thermodynamic
temperature. Finally the magnetic susceptibility of suitable atomic nuclei may
also be employed via the Curie law. Nuclear magnetic resonance or static
SQUID-based techniques may be employed, but nuclear magnetic resonance is
preferable in being unaffected by magnetic impurities, to which the second
method falls hostage.  See  NUCLEAR MAGNETIC RESONANCE (NMR); SQUID;
TEMPERATURE; TEMPERATURE MEASUREMENT; THERMOCOUPLE; THERMOMETER.
 �34s[Ralph P. Hudson]
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�6v�2v�32sL-Rhamnose   �21m �33s 
    A methyl pentose, known also as 6-deoxy-L-mannose and L-mannomethylose. 
The free sugar, with the structure shown, occurs in leaves and flowers of 
poison ivy (Rhus toxicodendron). It is a constituent of many of the plant 
glycosides such as quercitrin and rutin. The type II Pneumococcus specific 
polysaccharides and a wide variety of gums and mucilages contain L-rhamnose, 
and there is evidence that this sugar is present in a number of bacterial 
polysaccharides.  See  MONOSACCHARIDE. 
 �34s[William Z. Hassid]
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�6v�2v�32sLubricant   �21m �33s 
    A gas, liquid, or solid used to prevent contact of parts in relative 
motion, and thereby reduce friction and wear. In many machines, cooling by the 
lubricant is equally important. The lubricant may also be called upon to 
prevent rusting and the deposition of solids on close-fitting parts. 
    Crude petroleum is an excellent source of lubricants because a very wide 
range of suitable liquids, varying in molecular weight from 150 to over 1000 
and in viscosity from light machine oils to heavy gear oils, can be produced 
by various refining processes (see table). In order to standardize on 
nomenclature for oils of differing viscosity, the Society of Automotive 
Engineers (SAE) has established viscosity ranges for the various SAE 
designations (see table). 
    It is often desirable to add various chemicals to lubricating oils to 
improve their physical properties or to obtain some needed improvement in 
performance. These include viscosity-index improvers, pour-point depressants, 
antioxidants, antiwear and friction-reducing additives, and dispersants. 
    Synthetic lubricants may be superior to mineral lubricants in some 
applications. The main advantage of synthetics is that they have a greater 
operating range than a mineral oil. Included in this class are esters, 
containing oxidation inhibitors and sometimes mild extreme pressure additives, 
silicones, and the polyglycols, such as polypropylene and ethylene oxides. 
    The most useful solid lubricants are those with a layer structure in which 
the molecular platelets will readily slide over each other. Graphite, 
molybdenum disulfide, talc, and boron nitride possess this property. A unique 
type of solid lubricant is provided by the plastic polytetrafluoroethylene 
(PTFE). The principal difficulty encountered with the use of solid lubricants 
is that of maintaining an adequate lubricant layer between the sliding metal 
surfaces.  
    A lubricating grease is a solid or semifluid lubricant comprising a 
thickening (or gelling) agent in a liquid lubricant. Other ingredients 
imparting special properties may be included. An important property of a 
grease is its solid nature; it has a yield value. This enables grease to 
retain itself in a bearing assembly without the aid of expensive seals, to 
provide its own seal against the ingress of moisture and dirt, and to remain 
on vertical surfaces and protect against moisture corrosion, especially during 
shut-down periods. 
 �34s[Robert G. Larsen]
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�6v�2v�32sLumber manufacture   �21m �33s 
    The production of lumber involves sawing logs lengthwise into boards, 
dimension lumber, and timbers of larger dimensions; seasoning or drying the 
wood; and planing it to exact size. 
    Softwood lumber is produced from coniferous trees and hardwood lumber from 
deciduous trees; actually, some ``softwood'' lumber is harder than some 
``hardwood'' lumber. Most softwood lumber is used in construction for framing, 
finish, and trim. Hardwood lumber is used mainly by converting industries, 
such as furniture, cabinet, and flooring manufacturers; however, some hardwood 
is used for interior finish when a particular color or grain is desired. Both 
hardwood and softwood lumber are used for paneling. 
 �34s[Albert E. Wackerman]
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�6v�2v�32sLuminaire   �21m �33s 
    An electric lighting fixture, wall bracket, portable lamp, or other 
complete lighting unit designed to contain one or more electric lighting 
sources and associated reflectors, refractors, housing, and such support for 
these items as is necessary.  See  ILLUMINATION; INCANDESCENT LAMP; LAMP. 
 �34s[Warren B. Boast]
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�6v�2v�32sLuminance �21m �33s
    The luminous intensity of any surface in a given direction per unit of
projected area of the surface viewed from that direction. The International
Commission on Illumination defines it as the quotient of the luminous
intensity in the given direction of an infinitesimal element of the surface
containing the point under consideration, by the orthogonally projected area
of the element on a plane perpendicular to the given direction. Simply, it is
the luminous intensity per unit area. Luminance is also called photometric
brightness.
    Since the candela is the unit of luminous intensity, the luminance, or
photometric brightness, of a surface may be expressed in candelas/cm sup 2,
candelas/in. sup 2, and so forth.
    The stilb is a unit of luminance (photometric brightness) equal to 1
candela/cm sup 2. It is often used in Europe, but the practice in America is to
use the term candela/cm sup 2 in its place.
    The apostilb is another unit of luminance sometimes used in Europe. It is
equal to the luminance of a perfectly diffusing surface emitting or diffusing
light at the rate of 1 lumen/m sup 2. See LUMINOUS INTENSITY; PHOTOMETRY.
 �34s[Russell C. Putnam]
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�6v�2v�32sLuminescence �21m �33s
    Light emission that cannot be attributed merely to the temperature of the
emitting body. Various types of luminescence are often distinguished according
to the source of the energy which excites the emission. When the light energy
emitted results from a chemical reaction, such as in the slow oxidation of
phosphorus at ordinary temperatures, the emission is called chemiluminescence.
When the luminescent chemical reaction occurs in a living system, such as in
the glow of the firefly, the emission is called bioluminescence. In the
foregoing two examples part of the energy of a chemical reaction is converted
into light. There are also types of luminescence that are initiated by the
flow of some form of energy into the body from the outside. According to the
source of the exciting energy, these luminescences are designated as
cathodoluminescence if the energy comes from electron bombardment;
radioluminescence or roentgenoluminescence if the energy comes from x-rays or
from GAMMA-rays; photoluminescence if the energy comes from ultraviolet,
visible, or infrared radiation; and electroluminescence if the energy comes
from the application of an electric field. By attaching a suitable prefix to
the word luminescence, similar designations may be coined to characterize
luminescence excited by other agents. Since a given substance can frequently
be made to luminesce by a number of different external exciting agents, and
since the atomic and electronic phenomena that cause luminescence are
basically the same regardless of the mode of excitation, the classification of
luminescence phenomena into the foregoing categories is only a matter of
convenience, not of fundamental distinction.
    When a luminescent system provided with a special configuration is
excited, or ``pumped,'' with sufficient intensity of excitation to cause an
excess of excited atoms over unexcited atoms (a so-called population
inversion), it can produce laser action. (Laser is an acronym for light
amplification by stimulated emission of radiation.) This laser emission is a
coherent stimulated luminescence, in contrast to the incoherent spontaneous
emission from most luminescent systems as they are ordinarily excited and
used.  See  LASER; OPTICAL PUMPING.
    A second basis frequently used for characterizing luminescence is its
persistence after the source of exciting energy is removed. Many substances
continue to luminesce for extended periods after the exciting energy is shut
off. The delayed light emission (afterglow) is generally called
phosphorescence; the light emitted during the period of excitation is
generally called fluorescence. In an exact sense, this classification, based
on persistence of the afterglow, is not meaningful because it depends on the
properties of the detector used to observe the luminescence. With appropriate
instruments one can detect afterglows lasting on the order of a few
thousandths of a microsecond, which would be imperceptible to the human eye.
The characterization of such a luminescence, based on its persistence, as
either fluorescence or phosphorescence would therefore depend upon whether the
observation was made by eye or by instrumental means. These terms are
nevertheless commonly used in the approximate sense defined here, and are
convenient for many practical purposes. However, they can be given a more
precise meaning. For example, fluorescence may be defined as a luminescence
emission having an afterglow duration which is temperature-independent, while
phosphorescence may be defined as a luminescence with an afterglow duration
which becomes shorter with increasing temperature. For details on various
types of luminescence see BIOLUMINESCENCE; CATHODOLUMINESCENCE;
CHEMILUMINESCENCE; ELECTROLUMINESCENCE; FLUORESCENCE; PHOSPHORESCENCE;
PHOTOLUMINESCENCE; THERMOLUMINESCENCE.
 �34s[Clifford C. Klick ;James H. Schulman]
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�6v�2v�32sLuminous efficacy   �21m �33s 
    There are three ways this term can be used: (1) The luminous efficacy of a 
source of light is the quotient of the total luminous flux emitted divided by 
the total lamp power input. Light is visually evaluated radiant energy. 
Luminous flux is the time rate of flow of light. Luminous efficacy is 
expressed in lumens per watt. (2) The luminous efficacy of radiant power is 
the quotient of the total luminous flux emitted divided by the total radiant 
power emitted. This is always somewhat larger for a particular lamp than the 
previous measure, since not all the input power is transformed into radiant 
power. (3) The spectral luminous efficacy of radiant power is the quotient of 
the luminous flux at a given wavelength of light divided by the radiant power 
at that wavelength. A plot of this quotient versus wavelength displays the 
spectral response of the human visual system. It is, of course, zero for all 
wavelengths outside the range from 380 to 760 nanometers. It rises to a 
maximum near the center of this range. Both the value and the wavelength of 
this maximum depend on the degree of dark adaptation present. However, an 
accepted value of 683 lumens per watt maximum at 555 nanometers represents a 
standard observer in a light-adapted condition.  See  ILLUMINATION; LUMINOUS 
EFFICIENCY; LUMINOUS FLUX; PHOTOMETRY. 
 �34s[G. A. Horton]
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�6v�2v�32sLuminous efficiency   �21m �33s 
    Visual efficacy of visible radiation, a function of the spectral 
distribution of the source radiation in accordance with the ``spectral 
luminous efficiency curve,'' usually for the light-adapted eye or photopic 
vision, or in some instances for the dark-adapted eye or scotopic vision. 
    The spectral luminous efficiency of radiant flux is the ratio of luminous 
efficacy for a given wavelength to the value of maximum luminous efficacy. It 
is a dimensionless ratio.  See  ILLUMINATION; LUMINOUS EFFICACY; PHOTOMETRY. 
 �34s[G. A. Horton]
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�6v�2v�32sLuminous energy   �21m �33s 
    The radiant energy in the visible region or quantity of light. It is in 
the form of electromagnetic waves, and since the visible region is commonly 
taken as extending 380 - 760 nanometers in wavelength, the luminous energy is 
contained within that region. It is equal to the time integral of the 
production of the luminous flux.  See  PHOTOMETRY. 
 �34s[Russell C. Putnam]
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�6v�2v�32sLuminous flux   �21m �33s 
    The time rate of flow of light. It is radiant flux in the form of 
electromagnetic waves which affects the eye or, more strictly, the time rate 
of flow of radiant energy evaluated according to its capacity to produce 
visual sensation. The visible spectrum is ordinarily considered to extend from 
380 to 760 nanometers in wavelength; therefore, luminous flux is radiant flux 
in that region of the electromagnetic spectrum. The unit of measure of 
luminous flux is the lumen.  See  PHOTOMETRY. 
 �34s[Russell C. Putnam]
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�6v�2v�32sLuminous intensity   �21m �33s 
    The solid angular luminous flux density in a given direction from a light 
source. It may be considered as the luminous flux on a small surface normal to 
the given direction, divided by the solid angle (in steradians) which the 
surface subtends at the source of light. Since the apex of a solid angle is a 
point, this concept applies exactly only to a point source. The size of the 
source, however, is often extremely small when compared with the distance from 
which it is observed, so in practice the luminous flux coming from such a 
source may be taken as coming from a point.  See  CANDLEPOWER; PHOTOMETRY. 
 �34s[Russell C. Putnam]
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�6v�2v�32sLuminous paint �21m �33s
    A type of paint that glows in the dark. Luminous paints may be either of
the self-luminous type (energized by a radioactive salt) or of the type that
requires preexcitation by an outside energy source such as light. Both types
are made by incorporating luminescent material into the paint formulation.
See  LUMINESCENCE; PAINT.
 �34s[Clifford C. Klick ;James H. Schulman]
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�6v�2v�32sLunar laser ranging   �21m �33s 
    An optical technique for monitoring the topocentric distance from Earth to 
specific points on the surface of the Moon with extremely high accuracy. While 
it is conceptually similar to terrestrial laser geodometry and artificial 
satellite applications of laser ranging (also called lidar, optical radar, or 
laser radar), there are significant differences in the techniques required in 
making the observations. The applications encompass the geometry and dynamics 
of both Earth and Moon. The features peculiar to lunar laser ranging, as 
contrasted with other laser applications, are related to the Moon's distance 
and motion. 
    The basic measurement consists of the precise time at which a light pulse 
is emitted from the laser, together with the elapsed time required for the 
pulse to travel from the transmitter to a known point on the Moon and return 
to an earthbound detector. A precise definition of the lunar target is 
provided by using a retroreflector (see illustration), an array of mirrors 
whose design ensures that the incoming light will be reflected through almost 
exactly 180DG, so that it will return to the emitting source.  See  LASER. 
 �34s[J. Derral Mulholland]
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�6v�2v�32sLung   �21m �33s 
    Paired, air-filled respiratory sacs, usually in the anterior or 
anteroventral part of the trunk of most tetrapods. They lie within the coelom 
and are covered by peritoneum. In mammals they are within special chambers of 
the coelom known as pleural cavities and the peritoneum is termed pleura. 
    Amphibian lungs are often simple sacs, with only small ridges on the 
internal walls. In higher forms the lungs become more and more subdivided 
internally, thus increasing greatly the surface areas across which the 
respiratory exchange takes place. However, even in many reptiles the lungs may 
be quite simple. Birds have especially complex lungs with a highly 
differentiated system of tubes leading into and through them to the air sacs 
which are contained in many parts of the bird's body. Mammalian lungs are 
simpler, but in them the internal subdivision into tiny sacs or alveoli is 
extreme; there may be over 350,000,000 of them in one human lung. 
    In humans the two lungs lie within the chest, separated by the heart and 
mediastinum. The right lung has three lobes and the left lung two. A bronchus, 
an artery, and a vein enter each lung medially at the hilum; each branches 
again and again as it enters the lobules and smaller divisions of the lungs 
(see illustration). The terminal airways or bronchioles expand into small 
clusters of grapelike air cells, the alveoli. The alveolar walls consist of a 
single layer of epithelium and collectively present a huge surface. A small 
network of blood capillaries in the walls of the alveoli affords surfaces for 
the actual exchange of gases.  See  RESPIRATION; RESPIRATORY SYSTEM. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sLung disorders   �21m �33s 
    Inflammatory processes resulting from infections of bronchi and lungs are 
daily events in medical practice; scarring and tissue destruction as well as 
impaired pulmonary circulation are frequently encountered. Since the lungs 
literally filter the venous blood drainage from the entire body, they often 
become involved secondarily by blood-borne processes; many malignant tumors 
eventually metastasize to the lungs. More important still is the steadily 
rising incidence of primary malignancy of the lung and its controversial 
causative relationship to cigarette smoking and environmental pollution. See 
ONCOLOGY.  
    Although congestion and edema may occur separately, they are so frequently 
associated in the lungs that they may be discussed together. Factors 
predisposing to their development are increased pulmonary venous pressure, 
increased capillary permeability, decreased lymphatic drainage, and sodium 
retention. Heart failure is the most frequent cause of acute pulmonary 
congestion and edema. This is often a life-threatening situation that requires 
prompt medical attention. Shock, lung inflammations, and central nervous 
system disorders may also lead to acute pulmonary congestion and edema. See 
EDEMA.  
    Pulmonary embolism is the occlusion of one or more pulmonary arteries by a 
clot. The clot usually originates in the deep veins of the legs, but femoral, 
iliac, and pelvic veins may also be the site of origin. Predisposing factors 
in the development of vein occlusion with subsequent embolism are heart 
failure, postoperative convalescence, and malignant tumors. Massive pulmonary 
embolism is a life-threatening situation. 
    The adult secondary type of atelectasis is the collapse of pulmonary 
alveoli. This process can be caused by complete bronchial occlusion with 
subsequent alveolar air resorption or by increased intrapleural pressure 
produced by air or fluid in the pleural cavity or by deviation of the 
diaphragm.  
    Pneumonia is a general term denoting solidification (consolidation) of the 
lung tissue caused by inflammation. The disease may be caused by bacteria, 
viruses, chemical or physical agents, or any combination of these and varies 
greatly in pattern and severity. See PNEUMONIA. 
    Tuberculosis is caused by Mycobacterium tuberculosis of various types. 
Modern therapy is highly effective against tuberculosis. However, the disease 
must be considered serious, and it is not infrequent. Autopsy examinations 
have revealed active tuberculosis in patients that were not even suspected of 
having the disease. See TUBERCULOSIS. 
    Pneumoconioses are a group of pulmonary diseases caused by inhalation of 
specific dusts. Most induce alveolar fibrosis, sometimes with granuloma 
formation. Anthracosis is coal dust accumulation in lungs; silicosis is caused 
by silica particles; and asbestosis is a pneumoconiosis caused by asbestos 
inhalation. Other forms of pneumoconiosis include byssinosis (cotton dust), 
berylliosis (beryllium dust or vapor), and bagassosis (sugarcane fiber). 
    Emphysema is a common pulmonary disease, almost exclusively occurring in 
heavy cigarette smokers, characterized by destruction of lung tissue distal to 
the terminal bronchial. It is commonly associated with chronic bronchitis. See 
EMPHYSEMA.  
    Lung tumors constitute a heterogeneous group of neoplasms, most of which 
are carcinomas. Lung cancer is the most common tumor in men, and it is 
estimated that by the mid-1980s it may surpass breast cancer as the most 
common female malignancy.  See  CANCER (MEDICINE); LUNG. 
 �34s[Victor E. Goldenberg]
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�6v�2v�32sLupus erythematosus   �21m �33s 
    One of the collagen diseases in which connective tissues show a similar 
form of biochemical and structural alteration. Each of the collagen diseases 
is a distinct clinical and pathologic entity and such a grouping exists more 
as a matter of convenience than because of a common etiologic factor. The 
causes are poorly understood, but hypersensitivity and nucleoprotein 
derangement appear to be involved. See HYPERSENSITIVITY; IMMUNOPATHOLOGY. 
    Constitutional symptoms and signs (fever, weakness, fatigability, or 
weight loss) may be the first manifestation of illness and are often 
insidious. The joints are affected in 90% of the patients. Severe joint pain 
with little or no objective evidence of arthritis is most common. The heart is 
involved in over one-half of the cases and there may be heart failure. Lupus 
nephritis is very frequent and is the most serious feature of lupus. It 
usually progresses to subacute glomerulonephritis with uremia and usually 
hypertension. Anemia is also a very common feature of lupus. Lupus often 
mimics other diseases because of the wide possibility of tissue damage. 
    The disease is treated with high doses of corticosteroids which often 
exert a remarkable effect, especially in the early stages of the illness. 
 �34s[N. Karle Mottet]
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�6v�2v�32sLutetium �21m �33s
    A chemical element, Lu, atomic number 71, atomic weight 174.97, a very
rare metal and the heaviest member of the rare-earth group. The naturally
occurring element is made up of the stable isotope sup 1 sup 7 sup 5Lu, 97.41%,
and the long-life BETA-emitter sup 1 sup 7 sup 6Lu with a half-life of 2.1 x 10
sup 1 sup 0 years.
    Lutetium, along with yttrium and lanthanum, is of interest to scientists
studying magnetism. All of these elements form trivalent ions with only
subshells which have been completed, so they have no unpaired electrons to
contribute to the magnetism. Their radii with regard to the other rare-earth
ions or metals are very similar so they form at almost all compositions either
solid solutions or mixed crystals with the strongly magnetic rare-earth
elements. Therefore, the scientist can dilute the magnetically active rare
earths in a continuous manner without changing appreciably the crystal
environment.  See  MAGNETOCHEMISTRY; RARE-EARTH ELEMENTS.
 �34s[Frank H. Spedding]
�33s

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�6v�2v�32sLychee �21m �33s
    The plant Litchi chinensis, also called litchi, a member of the soapberry
family (Sapindaceae). The fruit is a one-seeded berry. The thin, leathery,
rough shell or pericarp of the ripe fruit is bright red in most varieties.
Beneath the shell, completely surrounding the seed, is the edible aril or pulp.
    It is a native of southern China, where it has been cultivated for more
than 2000 years. It is grown in India, Union of South Africa, Hawaii, Burma,
Madagascar, West Indies, Brazil, Honduras, Japan, Australia, and the southern
United States.	See  SAPINDALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sLychniscosa   �21m �33s 
    An order of the subclass Hexasterophora in the class Hexactinellida. The 
parenchymal megascleres of these sponges are united to form a rigid framework. 
Examples of this order are Aulocystis and Dactylocalyx. See HEXACTINELLIDA; 
HEXASTEROPHORA.  
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sLycopodiales   �21m �33s 
    An order of the Lycopodiophyta with a fossil history extending back into 
the Paleozoic. These plants, commonly called club mosses, attained their 
greatest development during the Carboniferous Period and included several 
treelike forms comparable in size with the present-day pines. Today the order 
contains only two genera, Lycopodium and Phylloglossum, composed of small, 
herbaceous, evergreen plants with upright or trailing stems bearing numerous 
small, simple, spirally arranged leaves. The sporophylls (sporangium-bearing 
leaves) may be aggregated together into a cone or strobile (see illustration), 
or the sporangia may be borne singly on the upper surface of sporophylls 
scattered along the stem. The cones are often elevated on upright branches so 
that they occur above the leafy shoots. Because the cones resemble a club, 
these plants have been given the erroneous common name, club moss.  See  
LYCOPODIOPHYTA.  
 �34s[Paul A. Vestal]
�33s 
  
  
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�6v�2v�32sLycopodiophyta   �21m �33s 
    A division (formerly the subphylum Lycopsida) of the subkingdom 
Embryobionta (Embryophyta) having a long fossil history but now restricted to 
five living genera. The living members, commonly called club mosses, spike 
mosses, or quillworts, are significant in that the study of their structures 
and life cycles provides clues to the understanding of the many extinct 
species. Some of these plants, known only as fossils, were the dominant 
species of the extensive swampy forests of the Carboniferous.  See  
ISOETOPSIDA; LEPIDODENDRALES; LYCOPODIALES; LYCOPODIOPSIDA; PLEUROMEIALES. 
 �34s[Paul A. Vestal]
�33s 
  
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�6v�2v�32sLycopodiopsida   �21m �33s 
    One of the two classes of the division Lycopodiophyta, the other being the 
Isoetopsida; another name for this class is Lycopodineae. The plants in this 
class are commonly referred to as the lycopods. They have a very long history 
among vascular plants, dating back to the early Devonian and possibly 
Cambrian. The living members of this group are significant in that the study 
of their structures and life cycles provides clues to the understanding of the 
now extinct species of previous eras and the numerous changes which have 
occurred during the evolution of the present land flora. See PALEOBOTANY. 
    The two most common genera and the one having the widest distribution are 
Lycopodium and Selaginella. In Lycopodium the spores produced by the 
sporophyte are all alike (homosporous), whereas in Selaginella the spores are 
unlike (heterosporous). See ISOETALES; LEPIDODENDRALES; LYCOPODIALES; 
PLEUROMEIALES; SELAGINELLALES; TRACHEOPHYTA. 
 �34s[Paul A. Vestal]
�33s 
  
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�6v�2v�32sLymphadenitis   �21m �33s 
    An acute or chronic infection of the lymph nodes. Acute lymphadenitis may 
be caused by a variety of bacteria and toxic substances. Furthermore, lymph 
nodes draining an infected area show changes of acute inflammation. In 
comparatively mild reactions, white cells collect in the infected lymph nodes, 
and phagocytic activity is evident by ingestion by the inflammatory cells of 
bacteria, dead cells, tissue fragments, or any foreign material. With more 
severe involvement, pus collects in the nodes with abscess formation, and the 
infectious process often proceeds to neighboring lymph vessels, which are 
visible as red streaks leading to the nodes. The lymph nodes become enlarged 
and tender and the surrounding tissues are edematous. When pus formation and 
necrosis are marked, the lymph nodes open and drain on the body surface 
through sinus tracts. See EDEMA; PHAGOCYTOSIS. 
 �34s[Frank A. Carone]
�33s 
  
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�6v�2v�32sLymphangitis   �21m �33s 
    An infection of lymphatic channels which usually begins in an extremity 
and is frequently secondary to infection in the skin. The causative bacterium 
in many instances is a streptococcus. Several lymphatics draining the focus of 
infection in the skin become involved and these appear as red streaks that are 
firm, tender, and cordlike, especially if the lymph they contain coagulates. 
The infectious process usually extends only to the first chain of lymph nodes 
and then heals. In severe or untreated cases, the process reaches more distant 
lymph nodes, which may be destroyed by the inflammatory process, and finally 
enters the bloodstream, producing a bacteremia or infection of the blood. 
 �34s[Frank A. Carone]
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�6v�2v�32sLymphatic system   �21m �33s 
    A system of vessels in the vertebrate body, beginning in a network of 
exceedingly thin-walled capillaries in almost all the organs and tissues 
except the brain and bones. This network is drained by larger channels, mostly 
coursing along the veins and eventually joining to form a large vessel, the 
thoracic duct, which runs beside the spinal column to enter the left 
subclavian vein at the base of the neck. The lymph fluid originates in the 
tissue spaces by filtration from the blood capillaries. While in the lymphatic 
capillaries it is clear and watery. However, at intervals along the larger 
lymphatic vessels, the lymph passes through spongelike lymph nodes, where it 
receives great numbers of cells, the lymphocytes, and becomes turbid. 
    The lymph nodes of mammals vary in number, size, form, and structure in 
different species. The amount of connective tissue of the lymph nodes, that 
is, the degree of development of the capsule and trabeculae, also varies in 
different mammals. Other lymphoid organs include the tonsils, thymus gland, 
and spleen, and in certain classes and groups of animals, structures which are 
confined to such groups, for instance, the bursa of Fabricius in the birds, a 
diverticulum from the lower end of the alimentary canal.  See  LYMPH NODE; 
SPLEEN; THYMUS GLAND; TONSIL. 
    The functions of the lymphatics are to remove particulate materials such 
as molecular proteins and bacteria from the tissues; to transport fat from the 
intestine to the blood; to supply the blood with lymphocytes; to remove excess 
fluid; also to return to the bloodstream the protein which has escaped from 
the blood capillaries. Basically, the composition of lymph closely resembles 
that of the plasma; lymph contains all of the types of protein found in 
plasma, but in lower concentration. The composition of lymph varies to some 
extent from one part of the body to another. Thus,the lymph from the liver 
contains more protein than that from the skin. 
    The lymph nodes serve as filtering-out places for foreign particles, 
including microorganisms, because the lymph comes into intimate contact with 
the many phagocytic cells of the sinusoids. These macrophages are of both the 
fixed and free wandering types. In addition to the phagocytic function, 
lymphoid tissue produces antibodies, although the actual process of antibody 
formation is not well understood.  See  CELLULAR IMMUNOLOGY; PHAGOCYTOSIS. 
 �34s[Warren Andrew]
�33s 
  
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�6v�2v�32sLymph node   �21m �33s 
    Aggregations of lymphoid tissue found along the course of lymphatic 
vessels. Lymph nodes vary in length from 1 mm to 2 cm (0.04 to 0.8 in.) or 
more, and in shape from spherical to flattened on one or more sides. A fibrous 
capsule surrounds each gland and sends supporting sheets, or trabeculae, 
through the lymphoid tissue (see illustration). The lymhoid tissue is composed 
of germinal centers of lymphocytes together with other cells, particularly 
those of the reticuloendothelial system. Lymph glands are found in superficial 
and deep sites, including the neck, axilla, groin, mediastinum, abdomen, 
pelvis, and intestinal wall (Peyer's patches). At least two functions are 
apparent: to supply lymphocytes to the circulation, and to remove bacteria and 
particulate matter carried to the nodes by the lymphatic vessels.  See  
LYMPHATIC SYSTEM. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sLymphogranuloma venereum �21m �33s
    A venereal disease caused by a microorganism, a member of the PLT-Bedsonia
(Chlamydia) group. It is also known as venereal bubo. The disease manifests
itself as an enlargement of the inguinal lymph node and as such is seldom
overlooked in the male; however, mild atypical or asymptomatic infections may
be common. In the female the disease is more difficult to diagnose because the
lymphadenopathy may be retroperitoneal. Fever commonly accompanies the
development of the disease. Early diagnosis and treatment, supervision of
infected persons, and public education are important in prevention.  See
PLT-BEDSONIA GROUP; VIRUS.
 �34s[Karl F. Meyer ;Julius Schachter]
�33s


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�6v�2v�32sLymphoma   �21m �33s 
    A group of malignant tumors characterized by neoplastic proliferations of 
the various cell types endogenous to lymphoid tissue. Hodgkin's disease is 
considered to be a type of lymphoma. Lymphomas are classified on a 
morphological basis according to the cell of origin of the neoplasm, the 
degree of differentiation of the tumor cells, and the overall architecture of 
the involved lymphoid tissue. The etiology of lymphomas remains unknown. 
    The disease most commonly begins in the lymph nodes of the cervical (neck) 
and axillary (underarm) regions of the body; occasionally it originates in the 
lymphoid tissue of the intestine, and in Burkitt's lymphoma starts in the jaw. 
As the disease progresses, any organ in the body can become involved. Most 
commonly, the liver, spleen, and other lymph nodes become diseased as 
evidenced by their increase in size. 
    Most persons with lymphomas seek medical advice because of enlarged, 
nontender lymph nodes. These people usually have no other signs or symptoms, 
although occasionally they may be anemic and have a fever. Lymphomas are 
staged to determine the extent of involvement of the disease. Treatment is 
based on the stage of the disease and the tumor's microscopic appearance. The 
clinical course and prognosis of lymphomas are highly variable. When the 
disease is localized to a single group of lymph nodes in a well-defined region 
of the body, the therapy of choice is usually radiation. When the disease is 
disseminated, it is usually treated with a combination of drugs. Some long 
survivals and apparent cures have resulted from this chemotherapy.  See  
HODGKIN'S DISEASE; LYMPHATIC SYSTEM. 
 �34s[Samuel P. Hammar]
�33s 
  
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�6v�2v�32sLyophilization   �21m �33s 
    Solvent removal from the frozen state by sublimation; commonly referred to 
as freeze-drying. Lyophilization is accomplished by freezing the material to 
be dried below its eutectic point and then providing the latent heat of 
sublimation. Precise control of heat input permits drying from the frozen 
state without product melt-back. In practical application, the process is 
accelerated and more precisely controlled under reduced pressure conditions. 
 �34s[Samuel C. Tease]
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�6v�2v�32sLyra   �21m �33s 
    The Lyre, in astronomy, a summer constellation, small but important. Lyra 
has a first-magnitude star, Vega, a navigational star and the most brilliant 
star in this part of the sky. Vega forms, with two faint stars to the east, an 
almost perfect equilateral triangle. The southern one in turn forms, with 
three brighter stars to the south, an approximate parallelogram. The resulting 
overall figure resembles a tortoise more than a stringed musical instrument. 
However, according to legend, Mercury made the first lyre from a turtleshell 
by placing strings across it. Hence the two different representations are not 
incompatible.  See  CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
  
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�6v�2v�32sLysin   �21m �33s 
    A term used to describe substances that will disrupt a cell, with the 
release of some of its constituents. Unless the damage is minor, this action 
leads to the death of the cell. Lysins vary in the range of host species whose 
cells they will attack and in their requirements for accessory factors for 
lysis; the immune lysins are strictest in their requirements. Erythrocytes are 
lysed by a wide variety of chemicals, including water and hypertonic salt 
solutions, which displace the osmotic pressure from that of isotonicity. They 
are also susceptible to surface-active substances, such as saponin. Many 
bacteria, such as the staphylococcus and the streptococcus, elaborate one or 
more hemolysins that will lyse erythrocytes from certain, although not all, 
species of animals.  See  LYTIC REACTION. 
 �34s[Henry P. Treffers]
�33s 
  
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�6v�2v�32sLysine   �21m �33s 
    An amino acid constituent of proteins that contains a terminal amino group 
in addition to the usual ALPHA-amino group. Lysine must be supplied in the 
diet of animals and humans. It is widely distributed in proteins; however, it 
is absent in some such as zein (a grain protein). It is a precursor of 
5-hydroxylysine, an amino acid occurring in only a few proteins such as 
collagen, and it is found in biocytin, a complexed form of the vitamin biotin, 
which is found in yeast.  See  AMINO ACIDS. 
 �34s[David W. E. Smith]
�33s 
  
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�6v�2v�32sLysogeny   �21m �33s 
    Almost all strains of bacteria are lysogenic; that is, they have the 
capacity on rare occasions to lyse with the liberation of particles of 
bacteriophage (see illustration). Such particles can be detected by their 
ability to form plaques (colonies of bacteriophage) on lawns of sensitive 
(indicator) bacteria. The genetic determinant of the capacity of lysogenic 
bacteria to produce bacteriophage is a repressed phage genome (provirus) which 
exists in the bacterium in one of two states: (1) integrated into the 
bacterial chromosome (most cases), or (2) occupying some extra-chromosomal 
location (rare cases). 
    Bacteriophages which have the potential to exist as provirus are called 
temperate phages. When the provirus is integrated into the bacterial genome, 
it is called prophage. When the germinal substance (deoxyribonucleic acid or 
deoxyribonucleoprotein) of certain temperate phages enters a sensitive 
bacterium, the outcome may be death (lysis) for the bacterium as a result of 
phage multiplication, or it may result in the integration of the phage nucleic 
acid into the host genome (as a prophage), with the formation of a stable 
lysogenic bacterium. The lysogenic strain is designated by the name of the 
sensitive strain followed, in parentheses, by the strain of lysogenizing 
phage, for example, Escherichia coli (LAMBDA). Such a bacterium differs from 
its nonlysogenic ancestor in one very special way: It is immune to lysis by 
phage homologous to its carried prophage.  See  BACTERIOPHAGE. 
 �34s[Lane Barksdale]
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�6v�2v�32sLysosome   �21m �33s 
    A specialized cell part, of variable size and structure, surrounded by a 
single membrane and containing a mixture of hydrolytic (digestive) enzymes, 
most of which require an acid medium for optimal activity. Each enzyme splits 
a certain type of bond found in natural substances; together they have the 
ability to break down into small fragments the main constituents of living 
matter, that is, proteins, nucleic acids, polysaccharides, and lipids. Thus, 
the lysosomes make up the digestive system of the cell. They are found in most 
if not all animal cells, and in at least some plant cells. 
    There are two main types of lysosomes: primary lysosomes, packages of 
newly made enzymes that have not yet been involved in a digestive event; and 
secondary lysosomes, vacuoles derived from the primary lysosomes, within which 
digestion is taking place or has taken place. 
    Captured material is introduced into the cytoplasm of a cell as the 
content of a closed vesicle, called a phagosome or pinosome, surrounded by a 
membrane derived from the cell membrane. This vesicle later fuses with a 
lysosome in such a manner that their respective contents merge, while their 
membranes coalesce to form a single continuous boundary. The lysosomal enzymes 
and their substrates are brought together without any discontinuity affecting 
the membranes that surround them. Consequently, the enzymes cannot diffuse 
into the cytoplasm and attack the cell constituents indiscriminately. Even 
when lysosomes digest fragments of their own cells, they do so in a piecemeal 
and orderly fashion, without disruption of the cellular organization, at least 
under physiological conditions. Enzymes released from disrupted lysosomes can 
injure cells in pathological situations. 
 �34s[Christian de Duve]
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�6v�2v�32sLyssacinosa   �21m �33s 
    An order of the subclass Hexasterophora in the class Hexactinellida. In 
these sponges the parenchymal megascleres are typically free and unconnected 
but are sometimes secondarily united. Asconema, Euplectella, Rossella, and 
Rhabdocalyptus are examples of this order.  See  HEXACTINELLIDA; 
HEXASTEROPHORA.  
 �34s[Willard D. Hartman]
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�6v�2v�32sLytic infection   �21m �33s 
    Infection of a bacterium by a bacteriophage with subsequent production of 
more phage particles and lysis, or dissolution, of the cell. The viruses 
responsible are commonly called virulent phages. Lytic infection is one of the 
two major bacteriophage-bacterium relationships, the other being lysogenic 
infection. See BACTERIOPHAGE; LYSOGENY. 
 �34s[Philip B. Cowles]
�33s 
  
  
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�6v�2v�32sLytic reaction   �21m �33s 
    A term used in serology to describe a reaction that leads to the 
disruption or lysis of a cell. The best example is the lysis of sheep red 
blood cells by specific antibody and complement in the presence of Casup2supp 
(calcium ion) and Mgsup2supp (magnesium ion), a reaction that forms the 
indicator system of the standard Wassermann test for syphilis, as well as 
other complement-fixation reactions. In this example lysis results in the 
release of cellular hemoglobin into the medium; the reaction may be followed 
by visual or instrumental estimation of the decreased cell turbidity or the 
increased color of the medium due to the free hemoglobin. The initiation of 
lysis by complement can apparently proceed after the attachment of only one 
molecule of IgM or two molecules of IgG antibody to the red blood cell. IgM 
and IgG are both immunoglobulins.  See  ANTIBODY; COMPLEMENT (SERUM); 
COMPLEMENT-FIXATION TEST; SEROLOGY. 
 �34s[Henry P. Treffers]
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�6v�2v�32sMacadamia nut   �21m �33s 
    The fruit of a tropical evergreen tree, Macadamia ternifolia, native to 
Queensland and New South Wales and now grown commercially in Australia and 
Hawaii. The trees bear many small white or pinkish flowers in drooping 
racemes, each of which may mature from 1 to 20 fruits. These consist of a 
leathery outer husk (pericarp) which splits along one side at maturity, 
freeing the very hard-shelled, nearly round seed or nut. Two types of nuts are 
recognized: the most important commercially having a smooth shell and the 
other having a rough shell and sometimes referred to another species, M. 
integrifolia (see illustration). 
 �34s[Laurence H. MacDaniels]
�33s 
  
  
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�6v�2v�32sMachine   �21m �33s 
    A combination of rigid or resistant bodies having definite motions and 
capable of performing useful work. The term mechanism is closely related but 
applies only to the physical arrangement that provides for the definite 
motions of the parts of a machine. For example, a wristwatch is a mechanism, 
but it does no useful work and thus is not a machine. Machines vary widely in 
appearance, function, and complexity from the simple hand-operated paper punch 
to the ocean liner, which is itself composed of many simple and complex 
machines.  See  MACHINERY; SIMPLE MACHINE. 
 �34s[Richard M. Phelan]
�33s 
  
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�6v�2v�32sMachine key   �21m �33s 
    Generally, a device used to prevent relative rotation of a shaft and the 
member to which it is connected, such as the hub of a gear, pulley, or crank. 
Many types of keys  (see illustration) are available, and the choice in any 
installation depends on such factors as power requirements, tightness of fit, 
stability of connection, and cost. 
    Square keys are common in general industrial machinery. Flat keys are used 
where added stability of the connection is desired, as in machine tools. 
Square or flat keys may be of uniform cross section or they may be tapered. In 
tapered keys the width is uniform and the height of the key tapers. Tapered 
keys may have gib heads to facilitate removal. Other types of keys have been 
developed for special applications. 
 �34s[Paul H. Black]
�33s 
  
  
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�6v�2v�32sMachinery   �21m �33s 
    A group of parts arranged to perform a useful function. Normally some of 
the parts are capable of motion; others are stationary and provide a frame for 
the moving parts. The terms machine and machinery are so closely related as to 
be almost synonymous; however, machinery has a plural implication, suggesting 
more than one machine. Common examples of machinery include automobiles, 
clothes washers, and airplanes; machinery differs greatly in number of parts 
and complexity. 
    Some machinery simply provides a mechanical advantage for human effort. 
Other machinery performs functions that no human being can do for 
long-sustained periods.  See  MACHINE; MECHANICAL ENGINEERING; SIMPLE MACHINE. 
 �34s[Robert S. Sherwood]
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�6v�2v�32sMachining operations   �21m �33s 
    Methods for the removal of material from a workpiece by the use of a 
cutting medium. Machining operations are usually carried out in a power-driven 
arrangement which gives the piece a desired shape or finish. 
    Single- or multiple-point cutting tools, including broaching tools, 
milling cutters, honing stones, abrasives, and saw blades, are common forms of 
cutting devices. Although varied in configuration, the basic cutting action 
performed by each is much the same. Some tools remove material in large chips, 
shavings, or pieces; others such as the saw, honing stone, or abrasive grits 
remove stock by cutting away small particles. 
    The machines which hold the cutting tools and in some cases the 
workpieces, and furnish the power for cutting, vary greatly in size and 
configuration. Machines vary from small hand-held drilling or grinding devices 
to large, automatic, multioperation machine tools. Machining operations 
commonly performed are turning and facing, boring, milling, sawing, broaching, 
shaping, planing, drilling, threading, tapping, the various types of gear 
cutting, grinding, honing, lapping, superfinishing, buffing, polishing, and 
nibbling. Some of these overlap each other; operations such as polishing may 
not be commonly thought of as machining. 
    In power machining operations there must be a relative movement between 
cutting tool and workpiece. During some machining operations the workpiece 
moves; in others only the cutting tool moves; frequently both tool and 
workpiece are in motion. The various machining operations that employ cutting 
tools use either a rotating or a traversing motion. For example, if either a 
turning or boring operation is performed on a lathe, the workpiece rotates 
while the cutting tool traverses longitudinally along or through the piece. On 
a milling machine the opposite may be true; the cutting tool, or milling 
cutter, rotates while the workpiece is clamped to a table and made to traverse 
the path of the cutter teeth.  See  BORING; BROACHING; DRILLING MACHINE; 
GRINDING; LAPPING; LATHE; MILLING MACHINE; NIBBLING; PLANER; POLISHING; 
REAMER; SAWING; SHAPER; THREADING. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sMach number   �21m �33s 
    In fluid mechanics, the ratio of the free stream velocity to the velocity 
of sound in the fluid at the same condition, such as temperature and pressure. 
Mach number is also the ratio of the inertia force of the fluid to the force 
of compressibility or the elastic force. In most fluid systems compressibility 
effects become important for values of the Mach number greater than about 0.3. 
A body moving through a fluid at a velocity less than sonic is preceded by a 
region of gradually varying density and pressure that controls the flow around 
the body. At Mach numbers equal to or greater than unity the gradual 
transition of pressure cannot exist, and shock waves, or regions of abruptly 
altered pressure and density, form at critical sections on or near the surface 
of the body and extend outward.  See  SHOCK WAVE; STREAMLINING. 
 �34s[Glenn Murphy]
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�6v�2v�32sMackerel   �21m �33s 
    A fish which is a member of the order Perciformes, family Scombridae. 
There are about 50 carnivorous species found in the middle layer or near the 
surface of tropical and temperate seas. Mackerel are characterized by a long 
slender body, pointed head, and large mouth. 
    Scomber scombrus, the common mackerel, is an important fish commercially. 
It is a migratory species found on both sides of the North Atlantic. The 
Pacific mackerel (Pneumatophorous diego) is also an important commercial fish 
but differs from the common mackerel in having a swim bladder. The American 
Spanish mackerel (Scomberomorus maculatus) is a choice food fish.  See  
PERCIFORMES.  
 �34s[Charles B. Curtin]
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�6v�2v�32sMacrodasyoidea   �21m �33s 
    An order of wormlike invertebrates of the class Gastrotricha. It is 
characterized by distinctive adhesive tubes, which are cylindrical, cuticular 
projections moved by delicate muscle strands. Minute glands, clustered 
ventrally on the head surface or disposed in lateral or transverse 
longitudinal rows, produce a sticky secretion. Head differentiation is not 
usual. The sides of the body are parallel, and the shape of the animal is 
elongate. Hermaphroditism occurs in members of this group. Macrodasys, 
Turbanella, and Thaumastoderma are representative genera, comprising numerous 
Old World species.  See  GASTROTRICHA. 
 �34s[Charles E. Packard]
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�6v�2v�32sMacroevolution   �21m �33s 
    The larger course of evolution by which the categories of animal and plant 
classification above the species have been evolved from each other and have 
differentiated into the forms within each. Only that part of evolution for 
which concrete evidence exists will be considered. 
    The organisms observed, alive or as fossils, can be classified into large 
groups or phyla. Within each of these the body is organized in variations of a 
single fundamental type. Such groups are the annelids, mollusks, echinoderms, 
and other phyla. Macroevolution is the process by which evolution has taken 
place within these phyla. In some cases evidence can be given that two or more 
such phyla have earlier evolved from a single ancestral group. 
    The evidence for the reality of macroevolution is derived from many 
sources. Paleontology provides evidence from fossils of the course of 
evolution. Fossils preserved at intervals of geological time may be compared 
and deductions made of their relationships. Thus, phylogenetic trees may be 
constructed. Studies of comparative morphology and embryology give evidence of 
many kinds from the similarities and dissimilarities of living animals. 
Geographical distribution of forms, together with the known history of the 
lands, gives data which should be in agreement with beliefs concerning the 
evolution of the organisms.  See  PALEONTOLOGY. 
    Speciation or microevolution has continued throughout evolution and 
provides its background. But the question remains whether the course of change 
in the long progress of macroevolution has been controlled in other ways 
besides the mechanisms considered in speciation. Another question for 
discussion is whether macroevolution is merely long-continued speciation.  See 
  SPECIATION. 
  
    Throughout the course of evolution, change has occurred during the life of 
organisms. These organisms must always be competent to survive in competition 
with other organisms. The whole biology evolves; evolution is a phenomenon in 
natural history. Also, change of habit and habitat normally accompanies the 
evolution of structure. It is probably without meaning to ask which of these 
precedes the other. 
  
    Types of change.  In all evolution one may distinguish two types of 
change, cladogenesis and phyletic evolution. Cladogenesis is the type of 
evolution associated with altered habit and habitat, usually in populations of 
the species separated from the rest. Adaptation to the new conditions results 
in division of the originally homogeneous species into distinguishable parts. 
Phyletic evolution is the gradual evolution of a population in its environment 
without division into isolated parts. This consists partly in improved 
adaptation to the conditions, partly in readaptation when the conditions 
change, as they frequently do in natural environments. Usually change in 
environmental conditions is gradual, but sudden and irreversible change 
sometimes occurs and organisms often spread into new habitats where conditions 
differ from those to which they are adapted. The organisms must then adapt 
quickly to the new conditions if they are to survive. In such circumstances 
evolution will be rapid under the changed selection due to the new conditions. 
This process is known as quantum evolution and is considered as a special 
although extreme case of phyletic evolution. It may lead to new groups on any 
taxonomic level. The distinctions between these types of evolution are 
illustrated diagrammatically in the illustration. The evolutionary changes in 
the organisms are similar in all the types. 
  
    Adaptive radiation.  The course of evolution differs between successful 
groups, such as the vertebrates or the insects, which have enlarged their 
areas of dominance with time and evolved into many smaller groups, and other 
groups, such as many forms among the lower invertebrates, which show no 
progressive evolution and have often survived through long ages with little 
change.  
    The knowledge of vertebrate evolution is the most detailed. When the 
evolution of this successful group is examined it is found that a limited 
number of dominant types (several groups of fishes, amphibians, reptiles, 
birds, and mammals) have followed each other, each in a new mode of vertebrate 
life, and that each was evolved from the previous dominant group. Further, 
each dominant group divided soon after its appearance into a large number of 
subsidiary types adapted to more restricted modes of life within the range of 
the larger group. This diversification into different adaptive zones is known 
as adaptive radiation. In adaptive radiation the greater part of the changes 
of form are due to alterations of the relative sizes of the parts of the body 
by modifications of their growth rates during development and not to the 
evolution of new organs. In the mammals the radiating lines appear today as 
the orders of bats, primates, rodents, and so on. All of these can be traced 
back to near the beginning of the dominance of the mammals. Each line 
continues to radiate throughout its successful history. Formation of new 
organs, which are not modifications of organs previously present, occurs in 
the radiation but accounts for only a small part of the changes. The 
development of horns in the mammals is an example. 
    The evolution of a major dominant group from a preceding group, for 
example, the mammals or birds from the reptiles, or the amphibians from the 
fishes, demands much more fundamental changes than those of adaptive 
radiation. The whole form and function of the body is reorganized. It takes a 
considerable time, for example, about 40,000,000 years in the mammals and 
birds, and proceeds by successive alterations of one feature after another 
toward the new type. It occurs in one of the radiating lines of the preceding 
group, while its other lines, though they continue to radiate, clearly remain 
members of the preceding group.  See  ANIMAL EVOLUTION. 
 �34s[George S. Carter]
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�6v�2v�32sMacroscelidea �21m �33s
    A mammalian order, the elephant shrews and their allies, which consists of
a single African family, Macroscelididae. They are mostly hopping and
scampering mammals, divided among several subfamilies, some of which resemble
rodents and ungulates in certain features. Characteristic of all these animals
is an intestinal cecum and the arrangement of bones in the ear region. The
long proboscis of the first living macroscelidids to be discovered led to the
name elephant shrews.  See  MAMMALIA.
 �34s[Malcolm C. McKennan]
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�6v�2v�32sMadelung constant   �21m �33s 
    A numerical constant ALPHA sub M in terms of which the electrostatic energy 
U of a three-dimensional periodic crystal lattice of positive and negative 
point charges q sub +, -q sub -, N in number, is given by the equation below, 
where d is the nearest-neighbor distance between positive and negative charges 
and N is large. Knowledge of such electrostatic energies as given by the 
Madelung constant is of importance in the calculation of the cohesive energies 
of ionic crystals and in many other problems in the physics of solids. See 
IONIC CRYSTALS.  
  
           *** U = - 1/2 [N(q sub +)(q sub-)] / d (ALPHA sub m) ***
  
    The Madelung constants for a number of common ionic crystal structures are 
given in the table. For these cases d is chosen as the nearest-neighbor 
distance. See CRYSTAL. 
 �34s[B. Gale Dick]
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�6v�2v�32sMagellanic Clouds �21m �33s
    Of all easily observable external galaxies, the one nearest to the Milky
Way galactic system. The distance of the Large Magellanic Cloud (LMC) is
55,000 parsecs (180,000 light-years or 1.70 x 10(sup 18) km or 1.05 x
10(sup 18) mi); that of the Small Magellanic Cloud (SMC) is 63,000 parsecs
(205,000 light-years or 1.94 x 10(sup 18) km or 1.21 x 10(sup 18) mi); these
distances are uncertain by about 10%. Because of their proximity, many types
of galactic objects, such as bright main sequence stars, supergiants,
classical cepheid variables, novae, and planetary nebulae can be studied.  See
  CEPHEIDS; GALAXY, EXTERNAL; VARIABLE STAR.
    Both Clouds are visible to the naked eye. E. P. Hubble classified them as
irregular galaxies, although there is now evidence that supports the
hypothesis that the Large Magellanic Cloud (at least) may be a barred spiral.
The Large Magellanic Cloud is much more massive than the Small Magellanic
Cloud.
 �34s[Lawrence H. Allen]
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�6v�2v�32sMagic numbers   �21m �33s 
    Numbers of neutrons or protons in nuclei which correspond to particularly 
stable structures and closed shells. In nature the magic numbers for both 
neutrons and protons are 2, 8, 14, 20, 50, and 82. The next neutron magic 
numbers are 126 and 184; the next proton magic numbers are expected to be 114 
and perhaps 164. Nuclei having magic numbers of both neutrons and protons have 
been found to have spherical equilibrium shapes and special stability; they 
have anomalously low capture probabilities for additional neutrons and protons 
respectively.  See  NUCLEAR STRUCTURE. 
 �34s[D. Allan Bromley]
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�6v�2v�32sMagic square   �21m �33s 
    An n by n matrix of (not necessarily distinct) nonnegative integers all of 
whose row and column sums are equal to a prescribed number x. The number of 
such magic squares is 1 when n = 1, x + 1 when n = 2, and in general is a 
polynomial of the form [(a sub m) (x sup m)] + [(a sub m - 1) (x sup m-1)] + 
. . . + [(a sub 1) (x + 1)], where m = [(n - 1) (sup 2)]. Although the 
coefficients a sub i are not given explicitly in this formula, for each n they 
may be computed by the method of undetermined coefficients, so that a computer 
search of magic squares for small values of x will also determine the number of 
such squares for all values of x. See COMBINATORIAL THEORY; ENUMERATION OF 
ARRANGEMENTS; MATRIX THEORY. 
 �34s[T. Balderes]
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�6v�2v�32sMagma   �21m �33s 
    The hot material, partly or wholly liquid, from which igneous rocks form. 
Besides liquids, solids and gas may be present in magma. Most observed magmas 
are silicate melts with associated crystals and gas, but some inferred magmas 
are carbonate, phosphate, oxide, sulfide, and sulfur melts. 
    Strictly, any natural material which contains a finite proportion of melt 
(hot liquid) is a magma. However, magmas which contain more than about 60% by 
volume of solids generally have finite strength and fracture like solids. 
    Hypothetical, wholly liquid magmas which develop by partial melting of 
previously solid rock and segregation of the liquid into a volume free of 
suspended solids and gas are called primary magmas. Hypothetical, wholly 
liquid magmas which develop by crystallization of a primary magma and 
isolation of rest liquid free of suspended solids are called parental (or 
secondary) magmas. Although no unquestioned natural examples of either primary 
or parental magmas are known, the concepts implied by the definitions are 
useful in discussing the origins of magmas. 
    Bodies of flowing lava and natural volcanic glass prove the existence of 
magmas. Such proven magmas include the silicate magmas corresponding to such 
rocks as basalt, andesite, dacite, and rhyolite as well as rare carbonate-rich 
magmas and sulfur melts. Oxide-rich and sulfide-rich magmas are inferred from 
textural and structural evidence of fluidity as well as mineralogical evidence 
of high temperature, together with the results of experiments on the 
equilibrium relations of melts and crystals.  See  IGNEOUS ROCKS; LAVA. 
    Magma is presumed to underlie regions of active volcanism and to occupy 
volumes comparable in size and shape to plutons of eroded igneous rocks. 
However, it is not certain that individual plutons existed wholly as magma at 
one time; therefore, actual bodies of magma may be smaller than plutons.  See  
PLUTON.  
    Diverse origins are probable for various magmas. Basaltic magmas because 
of their high temperatures probably originate within the mantle several tens 
of kilometers beneath the surface of the Earth. Rhyolitic magmas may originate 
through crystallization of basaltic magmas or by melting of crustal rock. 
Intermediate magmas may originate within the mantle or by crystallization of 
basaltic magmas, by melting of appropriate crustal rock, and also by mixing of 
magmas or by assimilation of an appropriate rock by an appropriate magma.  See 
  PETROGRAPHIC PROVINCE; VOLCANO. 
 �34s[Alfred T. Anderson, Jr.]
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�6v�2v�32sMagnesite   �21m �33s 
    The mineral form of magnesium carbonate, MgCO3. Iron, manganese, and 
cobalt may replace some magnesium in magnesite. Magnesite has the same 
structure as calcite. It is usually massive and white, but iron impurities may 
give it a brownish tint. The specific gravity is 3 and the hardness is 4 on 
Mohs scale. 
    Magnesite may be associated with peridotites, soapstones, and dolomites, 
or it may occur as sedimentary deposits. Deposits are in Austria, Manchuria, 
California, Nevada, and Washington. Magnesite is a source of magnesia.  See  
CARBONATE MINERALS; MAGNESIUM. 
 �34s[Robert I. Harker]
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�6v�2v�32sMagnesium �21m �33s
    A metallic chemical element, Mg, in Group IIa of the periodic system,
atomic number 12, atomic weight 24.312. Magnesium is silvery white and
extremely light in weight. The specific gravity is 1.74, and the density is
1740 kg/m(sup 3) (0.063 lb/in.(sup 3) or 108.6 lb/ft(sup 3)). Because of this
lightness  combined with alloy strength suitable for many structural uses,
magnesium has long been known as industry's lightest structural metal.
    With a density only two-thirds that of aluminum, magnesium is used in
countless applications where weight saving is an important consideration. The
metal also has, however, many desirable chemical and metallurgical properties
which account for its extensive use in a variety of nonstructural applications.
    Magnesium is very abundant in nature, occurring in substantial amounts in
many rock-forming minerals such as dolomite, magnesite, olivine, and
serpentine. In addition, magnesium is also found in sea water, subterranean
brines, and salt beds. It is the third most abundant structural metal in the
Earth's crust, exceeded only by aluminum and iron.
    Some of the properties of magnesium in metallic form are listed in Table
1. Magnesium is very active chemically. It will actually displace hydrogen
from boiling water, and a large number of metals can be prepared by thermal
reduction of their salts and oxides with magnesium. The metal will combine
with most nonmetals and with practically all acids. Magnesium reacts only
slightly or not at all with most alkalies and many organic chemicals,
including hydrocarbons, aldehydes, alcohols, phenols, amines, esters, and most
oils. As a catalyst, magnesium is useful for promoting organic condensation,
reduction, addition, and dehalogenation reactions. It has long been used for
the synthesis of complex and special organic compounds by the well-known
Grignard reaction. Principal alloying ingredients include aluminum, manganese,
zirconium, zinc, rare-earth metals, and thorium.  See  MAGNESIUM ALLOYS.
    Magnesium compounds are used extensively in industry and agriculture.
Table 2 lists the major magnesium compounds and indicates some of their more
significant applications.
 �34s[William H. Gross ;Stephen C. Enclesor]
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�6v�2v�32sMagnesium alloys   �21m �33s 
    The most important alloying ingredients used in magnesium alloys are 
aluminum, zinc, manganese, silicon, zirconium, rare-earth metals, and thorium. 
The specific gravity of magnesium alloys ranges from 1.74 to 1.83. This low 
specific gravity has led to a great many structural applications in the 
aircraft, transportation, materials-handling, and portable-tool and -equipment 
industries. Magnesium alloys are most commonly used in the form of die 
castings, which account for 75% of their total usage in structural 
applications.  See  MAGNESIUM. 
 �34s[Thomas E. Leontis]
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�6v�2v�32sMagnet   �21m �33s 
    A piece of ferromagnetic material whose domains have been aligned 
sufficiently so that it produces a net magnetic field outside itself and can 
experience a net torque when placed in a magnetic field produced by some other 
source. For a discussion of ferromagnetic domains see FERROMAGNETISM. 
    If a bar of magnetically hard ferromagnetic material is magnetized to 
saturation in the magnetic field of a current flowing in a coil of wire, and 
the current is then turned off, the magnetic flux density in the bar drops 
down the hysteresis curve to a point below the retentivity point. The flux 
density goes below this point because the field becomes negative due to the 
demagnetizing field of the magnet's poles. The important point here is that 
the bar magnet does retain an appreciable magnetization and, if the material 
is well chosen, this magnetization will remain more or less permanently, 
provided the magnet is not heated, dropped, exposed to too strong a magnetic 
field, or in general, treated too roughly. Such a magnet is called a permanent 
magnet.  See  MAGNETIC HYSTERESIS. 
    A keeper is a piece of soft (easily magnetized) ferromagnetic material 
(such as soft iron) which (when the magnet is not in use) extends from one 
pole to the other, and through which the magnetic flux lines between the poles 
are concentrated. A keeper is used especially with U-shaped or horseshoe 
magnets. Its presence decreases the demagnetizing effect of the poles and thus 
helps to keep the magnet strong.  See  ELECTROMAGNET; MAGNETIC FIELD; MAGNETIC 
FLUX; MAGNETIC MATERIALS; MAGNETISM; MAGNETIZATION; MAGNETOSTATICS. 
 �34s[Ralph P. Winch]
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�6v�2v�32sMagnetic amplifier   �21m �33s 
    A device that employs saturable reactors to modulate the flow of 
alternating-current electric power to a load in response to a lower energy 
level direct-current input signal. Magnetic amplifiers are often referred to 
as power amplifiers, because they are well suited for use in driving electric 
motors and other output devices.  See  AMPLIFIER; SATURABLE REACTOR; 
SERVOMECHANISM.  
 �34s[J. Lowen Shearer]
�33s 
  
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�6v�2v�32sMagnetic bubble memory   �21m �33s 
    A memory storage technology which uses localized magnetized regions to 
store information. Magnetic bubble memory is the integrated-circuit analogy to 
rotating magnetic memories such as disks and tapes. 
    Magnetic bubble memories employ materials that are easily magnetized in 
one direction but are hard to magnetize in the orthogonal direction. The most 
commonly used materials are magnetic garnets which are deposited on a 
nonmagnetic substrate to form the basis of a bubble memory chip. A thin film 
of one of these materials, with the easy direction perpendicular to the 
surface, will allow only two natural directions of magnetization - up or down 
in the easy direction. When no external magnetic field is applied, the 
magnetic garnet film forms sepentine patterns of upward and downward 
magnetization. When a magnetic field is applied by sandwiching the film 
between two permanent magnets, cylindrical-shaped magnetic domains are formed 
in place of the serpentine structure. These cylinders are called magnetic 
bubbles and have a magnetization pointing in the opposite direction of the 
surrounding area. In effect, magnetic bubbles are magnetic islands in a 
magnetic sea of opposite magnetic polarity.  See  FERRIMAGNETIC GARNETS. 
    A bit of storage is represented as the presence or absence of a bubble at 
a given storage location. There are several methods of defining the storage 
locations, but the most common way is to use the chevron patterns. These 
chevrons are made of a soft ferromagnetic material such as permalloy and are 
deposited on the surface of the magnetic garnet. There can be one bubble for 
each chevron.  See  PERMALLOY. 
    The chevrons also serve as the path for bubble movement when the 
information is accessed. To organize the bubble memory bits, the chevron 
patterns are deposited to form loops along which the bubbles can travel. The 
simplest organization is one single, large loop. The drawback is that the 
bubbles must travel the entire length of the loop before the information can 
be retrieved, and this is time-consuming. Most bubble memory chips now use a 
more sophisticated architecture called the major/minor loop (see 
illustration). In these multiple storage loops, the length and the time taken 
to traverse a minor loop is much smaller than for one major, large loop. The 
information is retrieved by waiting until the desired bubbles are at the top 
of the minor loops. At that point one bubble from each minor loop is 
transferred to the major loop. This ``bubble train'' traverses the major loop, 
during which time the information can be read or new information can be 
substituted.  
    To move the bubbles along the loops, a rotating magnetic field must be 
generated. For each rotation of the magnetic field each bubble moves 
synchronously from one chevron to the next. In other words, the magnetic 
bubbles move within the material, but no physical motion takes place.  See  
COMPUTER STORAGE TECHNOLOGY; INTEGRATED CIRCUITS. 
 �34s[J. Egil Juliussen]
�33s 
  
  
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�6v�2v�32sMagnetic circuits   �21m �33s 
    Closed paths of magnetic flux; also a design method using such paths to 
compute the magnetic field of a core geometry that is often encountered, for 
instance, high-permeability flux-path segments (and their associated air 
gaps), each segment having reasonably definite length and area. Examples of 
magnetic circuits are transformer cores (see illustration), relay frames, and 
iron parts of electrical machinery. The magnetic-field equations for these 
devices look so similar to dc circuit equations that they are called magnetic 
circuits. See MAGNETIC FLUX. 
 �34s[Mark G. Foster]
�33s 
  
  
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�6v�2v�32sMagnetic compass   �21m �33s 
    A compass depending for its directive force upon the attraction of the 
Earth's magnetism for a magnet free to turn in any horizontal direction. A 
compass is an instrument used for determining horizontal direction. 
    The magnetic compass operates on the principle that like magnetic poles 
repel each other whereas unlike poles attract each other. The Earth has 
internal magnetism similar to that which would result from a short, powerful 
bar magnet at the center of the Earth. The lines of force connecting the two 
poles are vertical at two points on the surface of the Earth, called magnetic 
poles, and horizontal along the magnetic equator, a line approximating a great 
circle nearly midway between the magnetic poles. The horizontal component of 
the Earth's magnetic field varies from a maximum on or near the magnetic 
equator to zero at the magnetic poles. 
    A simple magnetic compass consists of a magnetized needle mounted so as to 
be free to align itself with the horizontal component of the Earth's field. A 
magnetic compass is unreliable or inoperative in the vicinities of the 
magnetic poles. 
 �34s[Alton B Moody]
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�6v�2v�32sMagnetic field   �21m �33s 
    A condition existing in the vicinity of a magnetic body (or a 
current-carrying medium) whereby the magnetic forces due to the body (or 
current) are detectable. In a magnetic field, there is a force on a moving 
charge in addition to the electrostatic (Coulomb) forces between charges, or 
there is a force on a magnetic pole. The description of the field may be in 
terms of magnetic induction (flux density) or in terms of magnetic field 
strength.  See  MAGNETIC INDUCTION. 
    A body can be magnetized by bringing it into a magnetic field due to 
currents or magnets. Except in the case of a ring magnetized along its 
circumference, the field associated with a magnetized body extends to the 
region surrounding the body. The external effect usually appears in limited 
regions of the body called poles. A magnetized bar of iron has two poles, one 
at either end; and from the fact that the bar will set itself in an 
approximate north-south direction in the Earth's field, it appears that there 
are two kinds of poles. The pole that is at the north end of the bar is called 
a north-seeking pole; that at the south end is called a south-seeking pole. 
The two poles at the ends are merely indications of the continuous 
magnetization within the body. An indication of the validity of this statement 
is the fact that when a bar magnet is broken into two parts, two new poles 
appear at the break, and the orientation of the poles in each fragment is the 
same as it was in the original magnet.  See  MAGNETIZATION. 
    It is observed that magnetic poles exert forces on each other and upon 
moving charges in the region near the poles. There is a field near the pole, 
and the pole may be considered as the cause of that field. 
    If the poles of a magnetized body are small enough that they may be 
considered point poles, the force that one pole exerts upon another is found 
to be proportional to the product of the pole strengths and inversely 
proportional to the square of the distance between them. This statement is 
called Coulomb's law of magnetostatics.  See  MAGNET; MAGNETOSTATICS. 
 �34s[Kenneth V. Manning]
�33s 
  
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�6v�2v�32sMagnetic flux   �21m �33s 
    Lines used to represent the magnetic induction B in a magnetic field.  See 
  MAGNETIC FIELD; MAGNETIC INDUCTION. 
    Lines of flux used to represent the field in magnitude and direction are 
selected so that they are parallel to B at each point, and the number of lines 
of flux per unit area of a surface perpendicular to the field is equal to the 
magnitude of the magnetic induction. The total number of lines of induction 
through a surface is the magnetic flux W. Magnetic induction is the flux per 
unit area or flux density. 
 �34s[Kenneth V. Manning]
�33s 
  
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�6v�2v�32sMagnetic hysteresis   �21m �33s 
    The lagging of magnetization behind magnetizing force as the magnetic 
condition of a ferromagnetic material is changed. 
    When a ferromagnetic sample that is initially demagnetized is subjected to 
a continuously increasing magnetizing force H, the relation between H and flux 
density B is shown by the normal magnetization curve Oab of the illustration. 
The point a indicates the magnetic condition as the increasing magnetic 
intensity has reached H1. If H is increased to a maximum value H2 and then 
decreased again to H1, the decreasing flux density does not follow the path of 
increase, but decreases at a rate less than that at which it rose. This lag in 
the change of B behind the change of H is called hysteresis. If the value of H 
is further reduced from H1 to zero, B is not reduced to zero but to a value 
Bsubr. The specimen has retained a permanent magnetism. This ordinate Bsubr is 
called the retentivity or remanence. The value of B may be reduced to zero at 
e by reversing the direction of H and increasing its value to Hsubc. This 
value Hsubc is called the coercive force or coercivity. 
    As H is increased in the negative direction, the magnetization proceeds 
along the curve of the illustration until at f the values of B and H are the 
same as those at b, but opposite in direction. When the reverse changes in H 
are made, the magnetization changes along the curve fghb. This entire loop 
bdefghb is called a hysteresis loop. If the hysteresis loop starts from 
another point on the normal magnetization curve, such as a, there will be a 
smaller hysteresis loop entirely within the larger, such as the dotted loop of 
the illustration.  See  MAGNETIZATION. 
 �34s[Kenneth V. Manning]
�33s 
  
  
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�6v�2v�32sMagnetic induction   �21m �33s 
    A vector quantity that is used as a quantitative measure of a magnetic 
field. It is defined in terms of the force on a charge moving in the field by 
Eq. (1), where B is the magnitude of the magnetic induction and F is the force 
  
                       *** B = F / (qv sin THETA) ***                    (1)
  
on the charge q, which is moving with speed v in a direction making an angle 
THETA with the direction of the field. The direction of the vector quantity B 
is the direction in which the force on the moving charge is zero. 
    The magnetic induction may also be expressed in terms of the force F on a 
current element of length l and current I, as in Eq. (2). The meter-kilogram-
  
                      *** B = F / (Il sin THETA) ***                     (2)
  
second (mks) unit of magnetic induction is derived from this equation by 
expressing the force in newtons, the current in amperes, and the length in 
meters. The unit of B is thus the newton/ampere-meter. 
    Magnetic flux density is the magnetic flux per unit area through a surface 
perpendicular to the magnetic induction. Magnetic flux density and magnetic 
induction are equivalent terms. See MAGNETIC FIELD; MAGNETIC FLUX. 
 �34s[Kenneth V. Manning]
�33s 
  
A700000002534�254s�5v�1v1019
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�6v�2v�32sMagnetic lens   �21m �33s 
    A magnetic field with axial symmetry capable of converging beams of 
charged particles of uniform velocity and of forming images of objects placed 
in the path of such beams. Magnetic lenses are employed as condensers, 
objectives, and projection lenses in magnetic electron microscopes, as final 
focusing lenses in the electron guns of cathode-ray tubes, and for the 
selection of groups of charged particles of specific velocity in velocity 
spectrographs.  
    Magnetic lenses may be formed by solenoids or helical coils of wire 
traversed by electric current, by axially symmetric pole pieces excited by a 
coil encased in a high-permeability material such as soft iron, or by similar 
pole pieces excited by permanent magnets. In the last two instances the 
armatures and pole pieces serve to concentrate the magnetic field in a narrow 
region about the axis. 
    Magnetic lenses are always converging lenses. Their action differs from 
that of electrostatic lenses and glass lenses in that they produce a rotation 
of the image in addition to the focusing action. For the simple uniform 
magnetic field within a long solenoid the image rotation is exactly 180DG. 
Thus a uniform magnetic field forms an erect real image of an object on its 
axis.  
 �34s[Edward G. Ramberg]
�33s 
  
A700000001733�254s�5v�1v1019
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�6v�2v�32sMagnetic materials   �21m �33s 
    A term generally applied to materials exhibiting ferromagnetism. The 
magnetic properties of all materials make them respond in some way to a 
magnetic field. Most materials are diamagnetic or paramagnetic and show almost 
no response. The materials which are most important to magnetic technology are 
ferromagnetic and ferrimagnetic materials.  See  FERRIMAGNETISM; 
FERROMAGNETISM; MAGNETISM. 
    Magnetic materials are further classified as soft or hard according to the 
ease of magnetization. Soft materials are used in devices in which change in 
the magnetization during operation is desirable, sometimes rapidly, as in ac 
generators and transformers. Hard materials are used to supply a fixed field 
either to act alone, as in a magnetic separator, or to interact with others, 
as in loudspeakers and instruments.  See  FERRITE; IRON ALLOYS; MAGNETIZATION. 
 �34s[A. Clarke Beiler]
�33s 
  
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�6v�2v�32sMagnetic moment   �21m �33s 
    The relationship between a magnetic field and the torque exerted on a 
magnet, a current loop, or a charge that is moving in the field. 
    When a magnet is placed in a magnetic field of strength H, there is a 
torque L exerted on the magnet by the field. The torque is a maximum when the 
axis of the magnet is perpendicular to the field. The ratio of the torque for 
this position to the strength of the field is called the magnetic moment M of 
the magnet. See MAGNET. 
    If a flat coil of wire of N turns and area A, in which there is a current 
I, is placed in a magnetic field of flux density B, the coil experiences a 
torque L given by Eq. (1), where THETA is the angle between the field and the 

                          *** L = NIAB sin THETA ***                     (1)

normal to the plane of the coil. The torque is maximum when THETA = 90 DG, that 
is, when the plane of the coil is parallel to the field. The ratio of the 
maximum torque to the flux density B is the magnetic moment of the coil, as 
shown in Eq. (2). If a charge is spinning, there is a charge in motion and 

                            *** M = L / B = NIA ***                      (2)

thus an electric current. The spin is equivalent to a tiny current loop which 
has a magnetic moment. Atomic nuclei also possess magnetic moments. See  
ELECTRON MAGNETIC MOMENT; ELECTRON SPIN; NUCLEAR MOMENTS. 
 �34s[Kenneth V. Manning]
�33s 
  

A700000003555�254s�5v�1v1020
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�6v�2v�32sMagnetic monopoles   �21m �33s 
    Magnetic charges; hypothetical entities designed to put magnetism on an 
equal footing with electricity. In this article, gaussian units are used 
throughout.  
    The name monopole or one-pole for a magnetic charge is meant as a contrast 
to dipole or two-pole. A magnetic dipole is a system producing a magnetic 
field which looks like that of two equal but opposite magnetic charges close 
together, when the field is measured at a large distance from the dipole. Such 
a field can be produced by a loop of wire carrying an electric current. In 
this case, it is obviously impossible to tear the dipole apart and get two 
magnetic monopoles. 
    Electric fields are produced by electric charges according to the formula 
E (r) = q r/r sup 3, where q is a point charge at the origin of coordinates, r 
is the position of observation at distance r from q, and E is the resulting 
field at r. They exert forces on point charges F = q E, where F is the force, 
q is the charge, and E is the electric field from all other charges. A 
magnetic field is produced by moving electric charges. The magnetic field 
about a wire carrying electric current is a well-known example. In turn, 
magnetic fields exert forces on charges according to F = q(v/c) x B, where v 
is the velocity, B is the magnetic field, and c is the speed of light. This 
force only acts on a moving charge and is perpendicular to the direction of 
motion and to the magnetic field. It is natural to speculate that there may be 
magnetic charges which act as sources of magnetic fields in the same way that 
electric charges produce electric fields. Two such magnetic charges would obey 
the famous rules that like charges repel and unlike charges attract each 
other. See ELECTRIC FIELD; ELECTROMAGNETIC FIELD. 
 �34s[Alfred S. Goldhaber]
�33s 
  

A700000004470�254s�5v�1v1020
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�6v�2v�32sMagnetic permeability �21m �33s
    A factor, characteristic of a material, that is proportional to the
magnetic flux density (magnetic induction) B produced in the material by a
magnetic field divided by the intensity of the field H. Permeability is
usually represented by the Greek letter MU.
    Consider a solenoid that has been bent into a circular form so that the
ends are joined together. This winding is called a toroid (see illustration).
For a closely wound toroid, almost all the flux is in the interior. The
intensity of the magnetic field inside the toroid is given by Eq. (1), where l

			      *** H = NI / I ***			 (1)

is the mean circumference of the toroid and I is the current in the toroid
coil. See MAGNETIC FIELD; SOLENOID (ELECTRICITY).
    The flux density B within the toroid is given, from Ampere's law, by Eq.
(2) if the toroid is in empty space. Then Eq. (3) holds. If a medium takes the
place of the empty space within the toroid, the value of B changes for the
same value of H. The ratio of B to H, given in Eq. (4), is called the absolute
permeability of the medium. See AMPERE'S LAW.

		       *** B sub 0 = MU sub 0 (NI / I) ***		 (2)

		      *** MU sub 0 = B sub 0 / H ***			 (3)

			    *** MU = B / H ***				 (4)

    It is convenient to define another quantity, called the relative
permeability, MU sub r, as the ratio of the permeability MU of the material to
the permeability MU sub 0 of empty space, as in Eq. (5). Relative permeability
is a 

		      *** MU sub r = MU / MU sub 0 ***			 (5)

pure number, and independent of the system of units used. The permeability of
free space has the numerical value of 4PI x 10(sup -7) henry per meter. See
ELECTRICAL UNITS AND STANDARDS.
    Materials may be classified in terms of their relative permeabilities.
Diamagnetic materials have values of MU sub r a little less than unity;
paramagnetic materials have MUsubr a little greater than unity. Ferromagnetic
materials are those that have relative permeabilities considerably greater
than unity, which are variable, depending upon the value of H and the previous
magnetic history. For some ferromagnetic materials, the maximum value of
MU sub r may be in the thousands. See DIAMAGNETISM; FERROMAGNETISM; MAGNETIC
MATERIALS; PARAMAGNETISM.
 �34s[Kenneth V. Manning]
�33s



A700000002253�254s�5v�1v1020
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�6v�2v�32sMagnetic reception (biology) �21m �33s
    Sensitivity to magnetic stimuli. A great variety of biological effects
resulting from exposure to fields many thousands of times more intense than
Earth's have been reported. Among these are changes in plant growth rates,
retardation of embryo development, changes in enzyme activity, alterations of
tumor growth, and other indications of stress.
    Evidence of magnetic detection has been given for honeybees and other
kinds of insects, including termites, beetles, and fruit flies (Drosophila);
for certain bacteria; and for birds. Most of the evidence for magnetic
detection by birds has come from studies of their migratory and homing
behavior. Results strongly suggest that birds possess a magnetic compass, that
is, they can determine compass bearings from the geomagnetic field. Birds'
sensitivity to magnetic stimuli is roughly similar to the honeybees'; they too
can probably detect fluctuations of less than 10(sup -4) gauss. The physical
mechanism for magnetic detection by living organisms is unknown, though a
variety of possibilities have been put forward.
 �34s[William T. Keeton]
�33s

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�6v�2v�32sMagnetic recording �21m �33s
    The technique of storing electric signals as a magnetic pattern on a
moving magnetic surface. In magnetic recording a time-varying electric current
produces a corresponding time-varying magnetic field in a recording head. This
field then lays down a spatially varying pattern of magnetization in a sheet
or strip of magnetic material moving past the head. When this recorded pattern
moves past a playback head, a time-varying electric output is generated in the
head corresponding to the original electric signal. A common form of magnetic
recorder is diagrammed in Fig. 1. Magnetic tape from the supply reel moves
past an erase head which removes any previously recorded pattern and leaves
the tape in a demagnetized condition. An electric signal fed to the recording
head lays down the desired new pattern on the tape. This recorded pattern
induces an electric output as it moves over the playback head. Since the
magnetization pattern remains in the tape until erased or otherwise altered,
the tape may be rewound onto the supply reel and replayed as often as desired,
with the erase and recording heads inactivated. The motordriven capstan and
pinch roller pull the tape over the heads at a uniform, controlled speed
during recording and playback.
    Magnetic recorders have become indispensable in innumerable areas of
modern society. Audio recorders are sold by the millions as consumer products
in reel-to-reel, cartridge, and cassette formats. Tape recorders are central
in professional sound-recording studios and in radio and television
broadcasting. They serve as mass-storage memories in computer systems and are
used as instrumentation recorders for collecting data in research, industry,
and military applications. They are employed in satellites and spacecraft for
delayed transmission of data to Earth stations.


    Principles of operation.  Regardless of the nature of the application and
of the mechanical configuration of the recorder, all magnetic recorders rely
on the same basic magnetic behavior for their operation.
    Thin plastic tapes are the medium now in universal use in tape recorders.
The magnetic coating consists of very tiny particles dispersed in a
thermosetting resin that is spread evenly on the surface of the plastic as a
viscous liquid. The most frequently used materials for the magnetically active
particles are iron oxide and chromium dioxide. Each of the particles in a tape
coating is a tiny permanent magnet embedded in the resin so it cannot move.

    Magnetic heads. While magnetic heads may differ widely in geometrical
details, they are basically of the form sketched in Fig. 2, whether they are
recording or playback heads. Frequently, the same head is used for both
functions. As indicated in the figure, a coil of wire is wound on a core of
material that very readily carries magnetic flux (that is, a high-permeability
material). A nonmagnetic gap is formed in the core at some point. When an
electric current passes through the coil, magnetic field lines associated with
the current follow paths around the high-permeability core. At the gap, some
of the lines spread outside the core to form a fringing field. The recording
medium passes through the fringing field above the gap and is magnetized there
during recordings.  See  MAGNETIZATION.

    Digital recording. Magnetic recording is naturally adapted to the use of
digital signals consisting of binary states represented by ones and zeros. The
two digital states are made to correspond to the two polarities of magnetic
saturation of the recording medium.  Since only saturation is involved, there
is no concern with linearity of recording and high-frequency bias is not
needed, nor is erasure of the recording medium. While the most widespread use
of digital recording is in computers, it is finding increasing use in
instrumentation recorders, satellite and telecommunication recorders,
machine-tool controllers, electronic games, and audio and video recorders.

    Audio recorders. The earliest significant use of magnetic recording was
in the audio-frequency domain, and today audio tape recorders greatly
outnumber other types. Machines for professional use in recording studios
operate at standard tape speeds of 15 in./s (38.1 cm/s) and 30 in./s
(76.2 cm/s), since short-wavelength losses are reduced at these speeds.
By contrast, the cassette recorders sold in the mass consumer market have a
tape speed of only 1 7/8 in./s (4.8 cm/s). Certain other recorders requiring
only low-grade voice quality, such as dictation machines and recorders for
long-term monitoring purposes, operate at even slower tape speeds.
   Of the numerous attempts to eliminate the inconvenience of threading tape
through its path from the supply reel to the take-up reel in a recorder, only
two have received widespread acceptance. They are the endless-loop cartridge
and the coplanar-hub cassette. In the cartridge, the tape is wound on a single
hub. Tape is drawn out of the wound stack at the hub, pulled past a playback
head by a capstan, and returned to the outside of the stack, with the two ends
of the length of tape being spliced together to form an endless loop (Fig. 3).
In an audio cassette the tape is wound in a stack on a hub, with one end of
the tape being permanently attached to the hub. The other end of the tape is
permanently attached to an identical hub in the same plane as the first and
symmetrically located in the flat plastic case. The tape path between the hubs
is determined by guide posts in the case, and the tape travels from one hub to
the other.

    Video recorders.  The purpose of video tape recorders is to record and
play back television signals. The television signal frequency-modulates a
carrier which is recorded on the magnetic tape, with frequencies in excess of
15 MHz being involved. The high head-to-tape speeds required for recording
these high frequencies are obtained by mounting the heads on a rapidly
rotating wheel that sweeps the heads more or less transversely across the tape
from edge to edge.

    Recorders for computers.  From the inception of electronic computers,
magnetic tape recorders have been an indispensable element in the hierarchy of
computer memories. During the 1960s and 1970s, magnetic disk files assumed an
equally important function in the hierarchy. All recording associated with
computers employs digital signals. Since the loss of even a single bit in a
digital number can have near-catastrophic results, much effort goes into
assuring precision and reliability in the manufacture and fabrication of
tapes, disks, heads, and mechanical structures involved in the recorders.  See
  COMPUTER STORAGE TECHNOLOGY.
 �34s[J. Guy Woodward]
�33s

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�6v�2v�32sMagnetic relaxation   �21m �33s 
    The relaxation or approach of a magnetic system to an equilibrium or 
steady-state condition as the magnetic field is changed. This relaxation is 
not instantaneous but requires time. The characteristic times involved in 
magnetic relaxation are known as relaxation times. Relaxation has been studied 
for nuclear magnetism, electron paramagnetism, and ferromagnetism. 
    Magnetism is associated with angular momentum called spin, because it 
usually arises from spin of nuclei or electrons. The spins may interact with 
applied magnetic fields, the so-called Zeeman energy; with electric fields, 
usually atomic in origin; and with one another through magnetic dipole or 
exchange coupling, the so-called spin-spin energy. Relaxation which changes 
the total energy of these interactions is called spin-lattice relaxation; that 
which does not is called spin-spin relaxation. (As used here, the term lattice 
does not refer to an ordered crystal but rather signifies degrees of freedom 
other than spin orientation, for example, translational motion of molecules in 
a liquid.) Spin-lattice relaxation is associated with the approach of the spin 
system to thermal equilibrium with the host material; spin-spin relaxation is 
associated with an internal equilibrium of the spins among themselves.  See  
MAGNETISM; SPIN (QUANTUM MECHANICS). 
 �34s[Charles P. Slichter]
�33s 
  
A700000002716�254s�5v�1v1022
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�6v�2v�32sMagnetic resonance   �21m �33s 
    A phenomenon exhibited by the magnetic spin systems of certain atoms 
whereby the spin systems absorb energy at specific (resonant) frequencies when 
subjected to alternating magnetic fields. The magnetic fields must alternate 
in synchronism with natural frequencies of the magnetic system. In most cases 
the natural frequency is that of precession of the bulk magnetic moment of 
constituent atoms or nuclei about some magnetic field. Because the natural 
frequencies are highly specific as to their origin (nuclear magnetism, 
electron spin magnetism, and so on), the resonant method makes possible the 
selective study of particular features of interest. For example, it is 
possible to study weak nuclear magnetism unmasked by the much larger 
electronic paramagnetism or diamagnetism which usually accompanies it. 
    Nuclear magnetic resonance (that is, resonance exhibited by nuclei) 
reveals not only the presence of a nucleus such as hydrogen, which possesses a 
magnetic moment, but also its interaction with nearby nuclei. It has therefore 
become a most powerful method of determining molecular structure. The 
detection of resonance displayed by unpaired electrons, called electron 
paramagnetic resonance, is also an important application.  See  
ELECTRON-NUCLEAR DOUBLE RESONANCE; MAGNETISM; NUCLEAR MAGNETIC RESONANCE;  
PARAMAGNETIC RESONANCE. 
 �34s[Charles P. Slichter]
�33s 
  
A700000002260�254s�5v�1v1022
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�6v�2v�32sMagnetic separation methods   �21m �33s 
    All materials possess magnetic properties. Substances that have a greater 
permeability than air are classified as paramagnetic; those with a lower 
permeability are called diamagnetic. Paramagnetic materials are attracted to a 
magnet; diamagnetic substances are repelled. Very strongly paramagnetic 
materials can be separated from weakly or nonmagnetic materials by the use of 
low-intensity magnetic separators. Minerals such as hematite, limonite, and 
garnet are weakly magnetic and can be separated from nonmagnetics by the use 
of high-intensity separators. 
    Magnetic separators are widely used to remove tramp iron from ores being 
crushed, to remove contaminating magnetics from food and industrial products, 
to recover magnetite and ferrosilicon in the float-sink methods of ore 
concentration, and to upgrade or concentrate ores. Magnetic separators are 
extensively used to concentrate ores, particularly iron ores, when one of the 
principal constituents is magnetic.  See  MAGNETIC MATERIALS; MAGNETISM; 
MECHANICAL SEPARATION TECHNIQUES; ORE DRESSING. 
 �34s[Fred D. DeVaney]
�33s 
  
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�6v�2v�32sMagnetic spherules   �21m �33s 
    Microscopic, black, magnetic dust particles found in the air or in 
sediments in remote and isolated parts of the world. The origin of these 
objects is a subject of controversy and extensive research. Particularly, most 
studies have been undertaken to discriminate between the spherules of 
terrestrial and those of extraterrestrial origin. Because magnetic spherules 
are found scattered near the places of fall of large meteorites, there is no 
doubt that at least some of the spherules found in the upper atmosphere and 
other isolated sites are meteoritic in origin. So far, a small number of these 
spherules have been identified as definitely of extraterrestrial origin, but 
the vast majority are still in doubt as to their place of formation. 
    Most evidence on the origin of magnetic spherules has centered about 
detailed and highly refined measurements of their chemical composition. Most 
of the spherules recovered from the environment of meteorite craters contain 
primarily iron and nickel. Compositions of the deep-sea spherules are rather 
similar to those of iron meteorites. For particles collected at the surface of 
the Earth, in the atmosphere, and in polar ice deposits, the composition shows 
a wide variation. 
 �34s[Paul W. Hodge]
�33s 
  
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9175552
�6v�2v�32sMagnetic susceptibility �21m �33s
    The magnetization of a material per unit applied field. It describes the
magnetic response of a substance to an applied magnetic field.	See
MAGNETISM; MAGNETIZATION.
    All ferromagnetic materials exhibit paramagnetic behavior above their
ferromagnetic Curie points. The general behavior of the susceptibility of
ferromagnetic materials at temperatures well above the ferromagnetic Curie
temperature follows the Curie-Weiss law. The paramagnetic Curie temperature is
usually slightly greater than the temperature of transition.  See  CURIE
TEMPERATURE; CURIE-WEISS LAW; FERROMAGNETISM.
    Most paramagnetic substances at room temperature have a static
susceptibility which follows a Langevin-Debye law. Saturation of the
paramagnetic susceptibility occurs when a further increase of the applied
magnetic field fails to increase the magnetization, because practically all
the magnetic dipoles are already oriented parallel to the field. See
PARAMAGNETISM.
    The susceptibility of diamagnetic materials is negative, since a
diamagnetic substance is magnetized in a direction opposite to that of the
applied magnetic field. The diamagnetic susceptibility is independent of
temperature. Diamagnetic susceptibility depends upon the distribution of
electronic charge in an atom and upon the energy levels.  See  DIAMAGNETISM.
    The susceptibility of antiferromagnetic materials above the Neel point,
which marks the transition from antiferromagnetic to paramagnetic behavior,
follows a Curie-Weiss law with a negative paramagnetic Curie temperature.
 �34s[Elihu Abrahams ;Frederic Keffer]
�33s

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�6v�2v�32sMagnetic thin films   �21m �33s 
    Sheets of magnetic material with thicknesses of a few micrometers or less, 
used in the electronics industry. Magnetic films can be single-crystalline, 
polycrystalline, or amorphous in the arrangement of their atoms. Applications 
include magnetic bubble technology, magnetoresist sensors, thin film heads, 
and recording media.  See  MAGNETIC BUBBLE MEMORY. 
    Both ferro- and ferrimagnetic films are used. The ferromagnetic films are 
usually transition-metal-based alloys. For example, permalloy is a nickel-iron 
alloy. The ferrimagnetic films, such as garnets or the amorphous films, 
contain transition metals such as iron or cobalt and rare earths. The 
ferrimagnetic properties are advantageous in bubble applications where a low 
overall magnetic moment can be achieved without a significant change in the 
Curie temperature. They are also useful for magnetooptic applications.  See  
FERRIMAGNETISM; FERROMAGNETISM; MAGNETIC MATERIALS; MAGNETISM; MAGNETIZATION. 
 �34s[Praveen Chaudhari]
�33s 
  
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�6v�2v�32sMagnetism �21m �33s
    Magnetism comprises those physical phenomena involving magnetic fields and
their effects upon materials. Magnetic fields may be set up on a macroscopic
scale by electric currents or by magnets. On an atomic scale, individual atoms
cause magnetic fields when their electrons have a net magnetic moment as a
result of their angular momentum. A magnetic moment arises whenever a charged
particle has an angular momentum. It is the cooperative effect of the atomic
magnetic moments which causes the macroscopic magnetic field of a permanent
magnet.  See  MAGNET; MAGNETIC FIELD; MAGNETIC MOMENT.
    Materials can be grouped according to their magnetic behavior, although
there is some overlap among groups.
    1. Diamagnetic substances have a negative magnetic susceptibility; they
are magnetized in a direction opposite to that of an applied magnetic field.
See  DIAMAGNETISM; MAGNETIC SUSCEPTIBILITY.
    2. Paramagnetic substances have a positive magnetic susceptibility such
that their magnetization is parallel and usually proportional to the applied
magnetic field.  See  PARAMAGNETISM.
    3. A ferromagnetic substance is one with net atomic moments which, within
a certain temperature range, tend to line up in such a way that there can
exist a net magnetization in the absence of an applied field. This category
includes all ferromagnets and ferrimagnets and some helimagnets. At
sufficiently high temperatures all these substances become paramagnetic.  See
CURIE TEMPERATURE; FERRIMAGNETISM; FERROMAGNETISM; HELIMAGNETISM.
    4. An antiferromagnetic substance is one with net atomic moments which,
within a certain temperature range, tend to line up in such a way that there
can exist no net magnetization in the absence of an applied field. This
category includes all antiferromagnets and some helimagnets.  See
ANTIFERROMAGNETISM; MAGNETIC MATERIALS; MAGNETIC PERMEABILITY; MAGNETIC
RESONANCE; MAGNON.
 �34s[Elihu Abrahams ;Frederic Keffer]
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�6v�2v�32sMagnetite   �21m �33s 
    A cubic mineral and member of the spinel structure type with composition 
[Fesup3supp]supIsupV [Fesup2suppFesup3supp]supVsupI O4. The color is opaque 
iron-black and streak black, the hardness is 6 (Mohs scale), and the specific 
gravity is 5.20. The habit is octahedral, but the mineral usually occurs in 
granular to massive form, sometimes of enormous dimensions. Magnetite is a 
natural ferrimagnet, but heated above 1072DGF (578DGC; the Curie temperature) 
it becomes paramagnetic. 
    The major magnetic ore of iron, magnetite may be economically important if 
it occurs in sufficient quantities. The most spectacular ore body occurs at 
Kiruna in northern Sweden. Other important occurrences are in Norway, the 
Soviet Union, and Canada.  See  IRON; SPINEL. 
 �34s[Paul B. Moore]
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�6v�2v�32sMagnetization �21m �33s
    The process of becoming magnetized; also the property and in particular
the extent of being magnetized. Magnetization has an effect on many of the
physical properties of a substance. Among these are electrical resistance,
specific heat, and elastic strain.  See  MAGNETOCALORIC EFFECT;
MAGNETORESISTANCE; MAGNETOSTRICTION.
    The magnetization M of a body is caused by circulating electric currents
or by elementary atomic magnetic moments, and is defined as the magnetic
moment per unit volume of such currents or moments. In the mks system of
units, M is measured in webers per square meter.
    The magnetic induction or magnetic flux density B is given by the
equation, where B and M are in webers/m(sup 2), H, the applied magnetic field,
is in ampere-turns/m, and MU(sub 0), the permeability of free space, is defined
as 4PI x 10(sup -7) henrys/m, that is, webers/(ampere-turn)(m).  See ELECTRICAL
UNITS AND STANDARDS; MAGNETIC INDUCTION.

		    B = MU(sub	0)(H + M) (mks)

    The topic of magnetization is generally restricted to materials exhibiting
spontaneous magnetization, that is, magnetization in the absence of H. All
such materials will be referred to as ferromagnets, including the special
category of ferrimagnets. A ferromagnet is composed of an assemblage of
spontaneously magnetized regions called domains. Within each domain, the
elementary atomic magnetic moments are essentially aligned, that is, each
domain may be envisioned as a small magnet. An unmagnetized ferromagnet is
composed of numerous domains, oriented in some fashion.
    The process of magnetization in an applied field H consists of growth of
those domains oriented most nearly in the direction of H at the expense of
others, followed by rotation of the direction of magnetization against
anisotropy forces.  See  FERROMAGNETISM.
    On removal of the field H, some magnetization will remain, called the
remanence M(sub r).
    Curves, sometimes called B-H curves, are used to describe magnetic
materials. They are plotted with H as abscissa and with either M or B as
ordinate. In the illustration, B(sub r) is the remanent induction (B(sub r) =
M(sub r)); H(sub )c is the coercive force, or reverse field required to bring
the induction B back to zero; and M(sub s) is the saturation magnetization, or
magnetization when all domains are aligned.  The saturation magnetization is
equal to the spontaneous magnetization of a single domain, except that it is
possible to increase this magnetization slightly by application of an extremely
large field.  Saturation magnetization is temperature dependent, and disappears
completely above the Curie temperature T(sub c) where a ferromagnet changes into
a paramagnet.  See CURIE TEMPERATURE.  The irreversible nature of magnetization
is shown most strikingly by the fact that the path of demagnetization does not
retrace the path of magnetization - the path 2 of the illustration does not
retrace path 1.  There is a tendency for the magnetization to show hysteresis,
that is, to lag behind the applied field, and the loop in the illustration is
called a hysteresis loop.  See MAGNETIC HYSTERESIS.�34s[Elihu Abrahams ;Frederic Keffer]
�33s


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�6v�2v�32sMagneto   �21m �33s 
    A type of permanent-magnet alternating-current generator frequently used 
as a source of ignition energy on tractor, marine, industrial, and aviation 
engines.  See  ALTERNATING-CURRENT GENERATOR. 
    Modern induction-type magnetos consist of a permanent-magnet rotor and 
stationary low- and high-tension windings, also called the primary and 
secondary windings. The energy output of a magneto is obtained as a result of 
a rapid rate of change of flux through the stationary windings. The primary 
winding has comparatively few turns and the secondary winding has many 
thousand turns of fine wire. One end of the secondary winding is connected to 
an end of the primary winding and grounded to the frame of the magneto. The 
primary winding is closed on itself through a breaker mechanism actuated by a 
cam on the magneto shaft. The breaker is mechanically set to interrupt the 
primary circuit each time the flux through the winding is changing at its 
greatest rate. The sudden collapse of the primary current induces a very high 
voltage in the secondary winding.  See  IGNITION SYSTEM; INTERNAL COMBUSTION 
ENGINE.  
 �34s[Robert T. Weil, Jr.]
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�6v�2v�32sMagnetocaloric effect �21m �33s
    The reversible change of temperature accompanying the change of
magnetization of a ferromagnetic or paramagnetic material. This change in
temperature may be of the order of 1DGC(2DGF), and is not to be confused with
the much smaller hysteresis heating effect, which is irreversible.  See
THERMAL HYSTERESIS.
 �34s[Elihu Abrahams ;Frederic Keffer]
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�6v�2v�32sMagnetochemistry �21m �33s
    The branch of chemistry which studies the interrelationship between a
magnetic field and atomic and molecular structures.
    A substance in a magnetic field acquires an intensity of magnetization
which may be either smaller or larger than that induced in a vacuum by the
same field. In the first case, the substance is said to be diamagnetic. In the
second case, the substance may be paramagnetic, ferromagnetic, or
antiferromagnetic.
    Diamagnetism, a universal property of matter, is usually of the order of
magnitude 10-sup6 to 10-sup5. Temperature-dependent paramagnetism, on the
other hand, arises only when an atom, ion, or molecule possesses a permanent
magnetic moment either in the ground state or in an excited state. A permanent
magnetic moment is the result of the presence of one or more unpaired
electrons. Paramagnetic susceptibilities are of the order of magnitude
10(sup -4) to 10(sup -3).
    A substance composed of atoms with permanent magnetic moments which are
very near to one another (for example, iron metal) may display ferromagnetism.
This phenomenon occurs when large numbers of the atoms with permanent magnetic
moments interact so that their individual moments align in a parallel fashion,
giving rise to a large resultant moment.
    On the other hand, a similar substance (for example, manganese metal) may
display antiferromagnetism. Here, the magnetic moments align in an
antiparallel fashion, thus largely canceling the individual magnetic moments
of the atoms. Parallel versus antiparallel alignment depends, among other
factors, upon interatomic distances.  See  ANTIFERROMAGNETISM; ATOMIC
STRUCTURE AND SPECTRA; ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY;
MAGNETIC RESONANCE; MAGNETISM; MOLECULAR STRUCTURE AND SPECTRA.
 �34s[Dieter M. Gruen]
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�6v�2v�32sMagnetohydrodynamic power generator   �21m �33s 
    A system for the generation of electrical power through the interaction of 
a flowing, electrically conducting fluid with a magnetic field. As in a 
conventional electrical generator, the Faraday principle of motional induction 
is employed, but solid conductors are replaced by an electrically conducting 
fluid. The interactions between this conducting fluid and the electromagnetic 
field system through which power is delivered to a circuit are determined by 
the electrical magnetohydrodynamic (MHD) equations, while the properties of 
electrically conducting gases or plasmas are established from the appropriate 
relationships of plasma physics. Major emphasis has been placed on MHD systems 
utilizing an ionized gas, but an electrically conducting liquid or a two-phase 
flow can also be employed.  See  ELECTROMAGNETIC INDUCTION; PLASMA PHYSICS. 
    When combined with a steam turbine system to serve as the high-temperature 
or topping stage of a binary cycle, an MHD generator has the potential for 
increasing the overall plant thermal efficiency to around 50%, and values 
higher than 60% have been predicted for advanced systems. This follows from 
the increased efficiency made available by the higher source temperature. 
Thus, the MHD generator is a heat engine or electromagnetic turbine which 
converts thermal energy to a direct electrical output via the intermediate 
step of the kinetic energy of the flowing working fluid.  See  ENGINE; 
MAGNETOHYDRODYNAMICS; THERMODYNAMIC PROCESSES. 
 �34s[William D. Jackson]
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�6v�2v�32sMagnetohydrodynamics   �21m �33s 
    The science that deals with the dynamics or motion of a fluid interacting 
with a magnetic field. The fluid must be a good conductor of electricity and 
hence can be a liquid metal or, more usually, an ionized gas or plasma. 
(Magnetohydrodynamics is alternately called hydromagnetics or magnetogas 
dynamics.) Magnetohydrodynamics is important in the development of controlled 
thermonuclear reactors. Other applications include simulation of hypersonic 
flight conditions, ionic thrust for outer-space propulsion, space-vehicle 
braking upon reentry to the atmosphere, high-energy particle accelerators, 
microwave generators, thermionic energy-conversion devices, application of 
thin metallic coatings, and the study of cosmic and upper atmospheric 
phenomena.  
    The conducting fluid and magnetic field interact through electric currents 
that flow in the fluid. The currents are induced as the conducting fluid and 
the magnetic field lines move across each other. In turn, the currents 
influence both the magnetic field and the motion of the fluid. Qualitatively, 
the magnetohydrodynamic interactions tend to link the fluid and the field 
lines so as to make them move together. 
    Although some early work in magnetohydrodynamics was concerned with liquid 
metals, a much wider interest has developed in phenomena which involve ionized 
gases or plasmas. A large electrical conductivity in a plasma requires a high 
density of high-energy electrons. This can occur for a plasma either in 
thermal equilibrium at relatively high temperatures, from a few electron volts 
upward (1 electronvolt is equivalent to a temperature of 11,600 K or 
20,900DGF), or in a nonequilibrium situation where the ions and molecules 
remain at a low temperature and the electrons are supplied with energy by an 
external source such as a microwave generator or ultraviolet radiation. 
    Plasmas are encountered in interstellar space, in the Sun and stars, and 
in the upper atmosphere, as well as in human-made devices into which energy is 
fed from electrical, chemical, or nuclear sources. Strong shock waves forming 
ahead of a blunt object traveling with hypersonic velocities through 
low-density air may heat the air sufficiently to ionize it. See MAGNETIC FIELD. 
 �34s[Rolf Landshoff]
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�6v�2v�32sMagnetometer   �21m �33s 
    Primarily, an instrument for measuring the intensity of a magnetic field. 
As related to the Earth's field, magnetometers are classed as absolute or 
relative, depending on whether they are capable of calibration without direct 
comparison with a standard magnetic instrument. Most types of magnetometers 
must be standardized by measuring the intensity of a field whose strength is 
accurately determined by other means, thus providing a relative measurement. 
 �34s[James H. Nelson]
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�6v�2v�32sMagnetomotive force �21m �33s
    The magnetomotive force (mmf) around a magnetic circuit is the work per
unit magnetic pole required to carry the pole once around the circuit. It is
the analog of electromotive force.  See  ELECTROMOTIVE FORCE (EMF); MAGNETIC
CIRCUITS.
    It is expressed mathematically in the following equation, where H cos THETA
is the component of magnetic field strength in the direction of a length of
path ds. The line integral is taken around any closed path in the field. 

	  mmf = CONTOURED INTERGRAL H cos THETA ds

See  MAGNETIC FIELD.
 �34s[Kenneth V. Manning]
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�6v�2v�32sMagneton �21m �33s
    A unit of magnetic moment used for atomic, molecular, or nuclear magnets.
See MAGNETIC MOMENT.
    The Bohr magneton MU (sub B) has the value of the classical magnetic moment
of the electron, which can theoretically be calculated as shown in Eq. (1),

       *** MU (sub B) = MU (sub O) = e PLANCK'S CONSTANT / 2mc
		    = 9.2741 MULT 10 (sup -2) erg/gauss
		    = 9.2741 MULT 10 (sup -24) joule/tesla ***		   (1)

where e and m are the electronic charge and mass, PLANCK'S CONSTANT is Planck's
constant divided by 2 PI, and c is the velocity of light. See ELECTRON SPIN.
    The magnetic moment of an atom or molecule results from contributions from
both the orbital angular momentum of the atomic electrons and the electronic
moments themselves (attributed to the electron spin). When certain groups of
atoms are compared, their moments show simple ratios. Observation of this fact
led to the definition of the Weiss magneton (before the Bohr magneton) on a
purely experimental basis as the unit for these moments. Its value is given by
Eq. (2).

	*** MU (sub W) = 1.853 MULT 10 (sup -21) erg/gauss
		     = 1.853 MULT 10 (sup -24) joule/tesla		   (2)

    The nuclear magneton is obtained from the Bohr magneton by replacing m by
the proton mass; it is thus 1836.15 times smaller than the Bohr magneton. The
neutron, which has no net charge, has a magnetic moment of the same order (but
opposite sign) as the proton. See NEUTRON; NUCLEAR MOMENTS.
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sMagnetooptic Kerr effect �21m �33s
    A magnetooptic effect that deals with the changes that are produced in the
optical properties of a reflecting surface of a ferromagnetic substance when
the substance is magnetized. In a typical case this will result in
elliptically polarized light appearing in reflection, when the ordinary rules
of metallic reflection would give only planepolarized light.  See
MAGNETOOPTICS.
 �34s[G. H. Dieke ;William W. Watson]
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�6v�2v�32sMagnetooptics �21m �33s
    That branch of physics which deals with the influence of a magnetic field
on optical phenomena. Considering the fact that light is electromagnetic
radiation, an interaction between light and a magnetic field would seem quite
plausible. It is, however, not the direct interaction of the magnetic field
and light that produces the known magnetooptic effects, but the influence of
the magnetic field upon matter which is in the process of emitting or
absorbing light.  See  COTTON-MOUTON EFFECT; FARADAY EFFECT; MAJORANA EFFECT;
MAGNETOOPTIC KERR EFFECT; VOIGT EFFECT; ZEEMAN EFFECT.
 �34s[G. H. Dieke ;William W. Watson]
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�6v�2v�32sMagnetoresistance �21m �33s
    The change of electrical resistance produced in a current carrying
conductor or semiconductor on application of a magnetic field H.
Magnetoresistance is one of the galvanomagnetic effects. It is observed both
with H parallel to and transverse to the current flow. The change of
resistance usually is proportional to Hsup2, except in very large fields,
where it becomes proportional to H. See  GALVANOMAGNETIC EFFECTS.
 �34s[Elihu Abrahams ;Frederic Keffer]
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�6v�2v�32sMagnetosphere   �21m �33s 
    A comet-shaped cavity or bubble around the Earth, carved in the solar 
wind. This cavity is formed because the Earth's magnetic field represents an 
obstacle to the solar wind, which is a supersonic flow of plasma blowing away 
from the Sun. As a result, the solar wind flows around the Earth, confining 
the Earth and its magnetic field into a long cylindrical cavity with a blunt 
nose. Since the solar wind is a supersonic flow, it also forms a bow shock a 
few earth radii away from the front of the cavity. The boundary of the cavity 
is called the magnetopause. The region between the bow shock and the 
magnetopause is called the magnetosheath. The Earth is located about 10 earth 
radii from the blunt-nosed front of the magnetopause. The long cylindrical 
section of the cavity is called the magnetotail, which is on the order of a 
few thousand earth radii in length, extending approximately radially away from 
the Sun.  See  MAGNETIC FIELD; SOLAR WIND; SUN. 
    The magnetosphere has been extensively explored by a number of satellites 
carrying sophisticated instruments. The satellite observations have indicated 
that the cavity is not an empty one, but is filled with plasmas of different 
characteristics. The Earth's dipolar magnetic field is considerably deformed 
by these plasmas and the electric currents generated by them.  See  VAN ALLEN 
RADIATION.  
    All other magnetic planets, such as Mercury, Jupiter, and Saturn, have 
magnetospheres which are similar in many respects to the magnetosphere of the 
Earth.  
 �34s[S. -I. Akasofu]
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�6v�2v�32sMagnetostatics   �21m �33s 
    The study of magnets and the fields produced by magnets. It should not be 
confused with the science of electromagnetism, which is concerned with the 
study of magnetic fields produced by currents. When considering 
magnetostatics, one looks upon magnet poles as seats of magnetism, and 
Coulomb's law of force between point poles in empty space is of fundamental 
importance. This law, expressed in rationalized meter-kilogram-second (mks) 
units, and using the Sommerfeld proposal, is written as the following 
equation, where MU sub 0 = 4 PI X 10 sup -7 weber/ampere-meter is the

       *** F = (MU sub 0 / 4 PI) [(m sub 1) (m sub 2)] / r sup 2 ***
  
permability of empty space; m sub 1 is the pole strength of one point pole and 
m sub 2 of the other, with pole strengths expressed in ampere-meters; F is the 
force in newtons; and r is the distance in meters between the point poles. See 
MAGNET; MAGNETIC FIELD.  
 �34s[Ralph P. Winch]
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�6v�2v�32sMagnetostriction �21m �33s
    The change of length of a ferromagnetic substance when it is magnetized.
More generally, magnetostriction is the phenomenon that the state of strain of
a ferromagnetic sample depends on the direction and extent of magnetization.
The phenomenon has an important application in devices known as
magnetostriction transducers.  See  FERROMAGNETISM.
    The magnetostrictive effect is exploited in transducers used for the
reception and transmission of high-frequency sound vibrations. Nickel is often
used for this application.  See  SONAR; ULTRASONICS.
 �34s[Elihu Abrahams ;Frederic Keffer]
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�6v�2v�32sMagnetron   �21m �33s 
    The oldest of a family of crossed-field microwave electron tubes wherein 
electrons, generated from a heated cathode, move under the combined force of a 
radial electric field and an axial magnetic field. By its structure a 
magnetron causes moving electrons to interact synchronously with 
traveling-wave components of a microwave standing-wave pattern in such a 
manner that electron potential energy is converted to microwave energy with 
high efficiency. Magnetrons have been used since the 1940s as pulsed microwave 
radiation sources for radar tracking. Because of their compactness and the 
high efficiency with which they can emit short bursts of megawatt peak output 
power, they have proved excellent for installation in aircraft as well as in 
ground radar stations. In continuous operation, a magnetron can produce a 
kilowatt of microwave power which is appropriate for rapid microwave cooking. 
See ELECTRON TUBE. 
    The magnetron is a device of essentially cylindrical symmetry (see 
illustration). On the central axis is a hollow cylindrical cathode. The outer 
surface of the cathode carries electron-emitting materials, primarily barium 
and strontium oxides in a nickel matrix. Such a matrix is capable of emitting 
electrons when current flows through the heater inside the cathode cylinder.  
See  VACUUM TUBE. 
    At a radius somewhat larger than the outer radius of the cathode is a 
concentric cylindrical anode. The anode serves two functions: (1) to collect 
electrons emitted by the cathode and (2) to store and guide microwave energy. 
The anode consists of a series of quarter-wavelength cavity resonators 
symmetrically arranged around the cathode.  See  CAVITY RESONATOR. 
    A radial dc electric field (perpendicular to the cathode) is applied 
between cathode and anode. This electric field and the axial magnetic field 
(parallel and coaxial with the cathode) introduced by pole pieces at either 
end of the cathode provide the required crossed-field configuration. 
 �34s[R. J. Collier]
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�6v�2v�32sMagnet wire �21m �33s
    The insulated copper or aluminum wire used in the coils of all types of
electromagnetic machines and devices. It is single-strand wire insulated with
enamel, varnish, cotton, glass, asbestos, or combinations of these. To meet
the immense variety of uses and to gain competitive advantage, a great number
of kinds of enamel and of fiber insulations have been developed and are widely
available.  See  ELECTRICAL INSULATION; MAGNET.
 �34s[Philip L. Alger ;C. J. Herman]
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�6v�2v�32sMagnification   �21m �33s 
    A measure of the effectiveness of an optical system in enlarging or 
reducing an image. For an optical system that forms a real image, such a 
measure is the lateral magnification m, which is the ratio of the size of the 
image to the size of the object. If the magnification is greater than unity, 
it is an enlargement; if less than unity, it is a reduction. 
    The angular magnification is the ratio of the angles formed by the image 
and the object at the eye. In telescopes the angular magnification (or, 
better, the ratio of the tangents of the angles under which the object is seen 
with and without the lens, respectively), can be taken as a measure of the 
effectiveness of the instrument. 
    Magnifying power is the measure of the effectiveness of an optical system 
used in connection with the eye. The magnifying power of a spectacle lens is 
the ratio of the tangents of the angles under which the object is seen with 
and without the lens, respectively. The magnifying power of a magnifier or an 
ocular is the ratio of the size under which an object would appear seen 
through the instrument at a distance of 10 in. or 250 mm (the distance of 
distinct vision) divided by the object size.  See  LENS (OPTICS); OPTICAL 
IMAGE.  
 �34s[Max J. Herzberger]
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�6v�2v�32sMagnitude (astronomy) �21m �33s
    The astronomical scale of brightness of stars and planets. A difference of
five magnitudes between two objects corresponds to a factor of 100 in their
ratio brightness. The basis of this system is that what the eye notes as equal
intervals of brightness are actually equal ratios. One magnitude corresponds
to a light ratio of (100)1/5 = 2.512. Magnitudes should be defined
monochromatically, or at least over a narrow and well-defined range of
wavelengths; the subscript LAMBDA indicates the band used. The zero point of
the magnitude system is defined so that the brightest stars are near zero.
    The energy received from a star depends on its distance r as well as on
its intrinsic luminosity. The absolute magnitude is defined as the apparent
magnitude a star would have if located at a standard distance of 10 parsecs
(3.0857 x 10[sup 17] m). See PARSEC.
    A wide variety of multicolor photoelectric systems are in use to measure
color; correlations between such color and brightness measures are defined by
the results for a group of standard stars. The color of a star is given, in
general, by a color index, on a particular base line.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sMagnolia �21m �33s
    A genus of trees with large, chiefly white flowers, and simple, entire,
usually large alternate leaves. In the winter the twigs may be recognized by
their aromatic odor when bruised.
    The most important species commercially is Magnolia acuminata, commonly
called cucumber tree, which grows in the Appalachian and Ozark mountains. The
fruit is red when ripe and resembles a small cucumber in shape.
    The wood of the magnolia is similar to that of the tulip tree and is
rather soft, but it is of such wide natural dimensions that it is valued for
furniture, cabinetwork, flooring, and interior finish.
    Magnolia species occur naturally in a broad belt in the eastern United
States and Central America, with a similar region in eastern Asia and the
Himalayas.  See  MAGNOLIALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sMagnoliales   �21m �33s 
    The nomenclaturally typical order of the subclass Magnoliidae in the class 
Magnoliopsida (dicotyledons) of the division Magnoliophyta (Angiospermae), the 
flowering plants. The Magnoliales are noteworthy as a loose aggregation of the 
most primitive existing families of angiosperms. The order consists of 19 
families and about 5600 species. The Lauraceae (about 2200 species) and 
Annonaceae (about 2100 species) are the largest families of the order, while 
the family Magnoliaceae is much smaller, with only a little over 200 species 
(see illustration). Within its subclass the order is characterized by the 
presence of spherical ``ethereal oil'' cells and by a well-developed perianth 
of separate tepals. All members of the order are woody.  See  AVOCADO; CAMPHOR 
TREE; MAGNOLIA; MAGNOLIOPSIDA; SASSAFRAS; TULIP TREE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sMagnoliidae   �21m �33s 
    A subclass of the class Magnoliopsida (dicotyledons) in the division 
Magnoliophyta (Angiospermae), the flowering plants. The subclass consists of 6 
orders, 36 families, and more than 11,000 species. The Magnoliidae are the 
most primitive subclass of flowering plants. In general, they have a 
well-developed perianth, which may or may not be differentiated into sepals 
and petals.  See  ARISTOLOCHIALES; FLOWER; MAGNOLIALES; MAGNOLIOPSIDA; 
NYMPHAEALES; PAPAVERALES; PIPERALES; RANUNCULALES. 
 �34s[Arthur Cronquist]
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�6v�2v�32sMagnoliophyta   �21m �33s 
    A division of seed plants consisting of about 220,000 widely distributed 
species, or about two-thirds of all known kinds of plants. Because the young 
seeds, called ovules, are enclosed in an ovary, the Magnoliophyta are often 
called angiosperms (Greek angeion, vessel, plus sperma, seed), in contrast to 
the gymnosperms (division Pinophyta), which lack an ovary and have ``naked'' 
ovules. They further differ in that most angiosperms have vessels (specialized 
water-conducting tubes) in the xylem, whereas most gymnosperms do not. 
Angiosperms range from tiny herbs to gigantic trees, and they dominate the 
land vegetation of most of the Earth. They also occur in lakes and ponds and 
streams, either rooted to the bottom or free-floating, but only a very few are 
truly marine, and these are rooted to the bottom near the shore of the ocean. 
    The angiosperms may be characterized as vascular plants with roots, stems, 
and leaves, usually with well-developed vessels in the xylem and with 
companion cells in the phloem. The central cylinder has leaf gaps or scattered 
vascular bundles; the ovules are enclosed in an ovary; and the female 
gametophyte is reduced to a few-nucleate embryo sac without an archegonium. 
The male gametophyte is reduced to a tiny pollen grain that gives rise to a 
pollen tube containing a tube nucleus and two sperms; one sperm fuses with the 
egg in the embryo sac to form a zygote, and the other fuses with two nuclei of 
the embryo sac to form a triple fusion nucleus that is typically the 
forerunner of the endosperm of the seed.  See  LEAF; PHLOEM; POLLEN; ROOT; 
SEED; STEM; XYLEM. 
    The angiosperms are often called flowering plants because the flower is a 
characteristic feature not found in other groups of plants.  See  FLOWER; 
REPRODUCTION (PLANT). 
    The angiosperms are usually considered to be the most highly evolved 
division of the subkingdom Embryobionta. Their highly specialized and 
relatively efficient conducting tissues, combined with the protection of their 
ovules in an ovary, give them a competitive advantage over most other groups 
of land plants in most regions. This general advantage is reflected in their 
abundance and in their great diversification, with different species adapted 
to a wide range of ecological niches.  See  EMBRYOBIONTA. 
    The Magnoliophyta consist of two classes, the Magnoliopsida and 
Liliopsida. The Magnoliopsida usually have two cotyledons in the embryo of the 
seed and are commonly called dicotyledons, or dicots; the Liliopsida usually 
have embryos with a single cotyledon and are commonly called monocotyledons, 
or monocots.  See  LILIOPSIDA; MAGNOLIOPSIDA. 
    All the important staples of the human diet are either angiosperms or are 
derived from animals (except for fish) which feed largely on angiosperms. In 
addition to food, they provide clothing and shelter and furnish raw materials 
for countless industrial processes.  See PLANT KINGDOM; TRACHEOPHYTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sMagnoliopsida   �21m �33s 
    One of the two classes of flowering plants which collectively make up the 
division Magnoliophyta (Angiospermae). The Magnoliopsida, often known as 
Dicotyledoneae or dicotyledons, embrace 6 subclasses, 56 orders, 293 families, 
and about 165,000 species.  See  ASTERIDAE; CARYOPHYLLIDAE; DILLENIIDAE; 
HAMAMELIDAE; MAGNOLIIDAE; ROSIDAE. 
    All of the characters which collectively distinguish the Magnoliopsida 
from the Liliopsida (monocotyledons) are subject to exception, but in general 
the Magnoliopsida have two cotyledons and net-veined leaves. The vascular 
bundles are typically borne in a ring (or cylinder) enclosing a pith. Increase 
in thickness of stems and roots, after the primary tissues have matured, 
results from meristematic activity of a cambial layer which passes through the 
vascular bundles. In about half of the species of the group, the cambium of 
the stem forms a continuous cylinder which produces a new layer of wood 
(secondary xylem) and bark (secondary phloem) each growing season for year 
after year. Such plants become trees or shrubs. 
    It is widely agreed that the most primitive existing angiosperms belong to 
the dicotyledons (especially the order Magnoliales) and that most of the 
characters which distinguish the monocotyledons as a group are derived rather 
than primitive.  See  LILIOPSIDA; MAGNOLIALES; MAGNOLIOPHYTA; PLANT KINGDOM. 
 �34s[Arthur Cronquist]
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�6v�2v�32sMagnon   �21m �33s 
    A quasi-particle which is introduced to describe small departures from 
complete magnetic ordering in ferro-, ferri-, antiferro-, and helimagnetic 
arrays.  See  ANTIFERROMAGNETISM; FERRIMAGNETISM; FERROMAGNETISM; 
HELIMAGNETISM.  
    A sinusoidal variation of that angular momentum which is associated with 
magnetism (mostly spin momentum of the electrons) is called a spin wave. A 
magnon is a quantized spin wave, much as a phonon is a quantized 
lattice-vibration wave or a photon is a quantized electromagnetic wave. 
    The low-temperature behavior of ordered magnetic materials is very 
accurately described in terms of the statistical excitation of magnons. In 
particular, the departure of ferromagnetic magnetization from complete 
saturation is found to fall off proportionally to T3/2 as the temperature T 
increases from absolute to zero to about half the Curie temperature. 
    The energy of a spin wave, or magnon, increases with increasing wave 
vector k, that is, with decreasing wavelength of  the ripple. The reason is 
that the so-called exchange forces which produce magnetic ordering are very 
short-range, causing only neighboring spins strongly to resist alignment. 
    The precessions of interacting spins are very complicated and, in general, 
the equations of motion contain nonlinear terms. These become important for 
the large-amplitude precessions induced by intense microwave fields. 
Large-amplitude precessions can also break up into acoustic vibrations, 
provided the acoustic eigenfrequencies equal the difference between the 
microwave (pump) frequency and that of some magnetostatic mode. This is called 
magnetoacoustic resonance. 
 �34s[Frederic Keffer]
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�6v�2v�32sMahogany �21m �33s
    A hard, red or yellow-brown wood which takes a high polish and is
extensively used for furniture and cabinetwork. The West Indies mahogany tree
(Swietenia mahagoni), a native of tropical regions in North and South America,
is a large evergreen tree with smooth pinnate leaves. Together with other
species it yields the world's most valuable cabinet wood. In the United States
it occurs naturally only in the extreme southern tip of Florida, but it is
planted elsewhere in the state as an ornamental and shade tree.  See
GERANIALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sMajorana effect �21m �33s
    A magnetooptic effect that deals with optical anisotropy of colloidal
solutions. The effect probably is caused by the orientation of the particles
in the magnetic field.	See  MAGNETOOPTICS.
 �34s[G. H. Dieke ;William W. Watson]
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�6v�2v�32sMalachite   �21m �33s 
    A basic carbonate of copper with the chemical formula Cu2(OH)2(CO3). Large 
quantities of malachite have been found in Siberia and used for ornamental 
stone. It has been mined as an ore of copper near Kolwezi, Republic of the 
Congo.  
    Malachite is monoclinic but usually occurs in massive forms or in bundles 
of radiating fibers. It is invariably green. It has a specific gravity of 4.05 
and a hardness of 3.5 - 4 on Mohs scale.  See  CARBONATE MINERALS; COPPER. 
 �34s[Robert I. Harker]
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�6v�2v�32sMalacobothridia   �21m �33s 
    A subclass of the class Trematoda. These worms typically have one or two 
soft flexible suckers and are endoparasitic in vertebrates and invertebrates. 
They begin their life cycles in mollusks and there may be one to four hosts in 
the cycle. There are two orders: Aspidobothrea, a monogenetic species, 
primarily parasites of mollusks; and Digenea, a species with alternation of 
sexual and asexual generations and multiple hosts.  See  TREMATODA. 
 �34s[Horace W. Stunkard]
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�6v�2v�32sMalacostraca   �21m �33s 
    The largest and most diversified subclass of the class Crustacea, which 
includes the shrimps, lobsters, crabs, sow bugs, beach hoppers, and their 
allies. The shell or carapace may be large, small, vestigial, or absent; the 
tail or abdomen is long or short; the eyes are generally set on movable stalks 
but may be sessile or even coalesced. Despite this diversity, the unity of the 
group is demonstrated by the following characteristics which all share (see 
illustration). The maximum number of appendages is 19 pairs, even where there 
is an additional body segment such as is found in the Leptostraca and 
embryonic Mysidacea. The trunk limbs are sharply differentiated into a 
thoracic series of eight pairs and an abdominal series of six pairs. 
Malacostraca are divided into two series, the Leptostraca and Eumalacostraca.  
See  EUMALACOSTRACA; LEPTOSTRACA. 
 �34s[Isabella Gordon]
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�6v�2v�32sMalaria   �21m �33s 
    A group of diseases caused by hemosporidian blood parasites of the genus 
Plasmodium, transmitted to humans by the bite of Anopheles mosquitoes. 
    Malarias have been known since ancient times. Their association with 
swamps and marshes has long been known, and the names given to these fevers - 
malaria (bad air), paludism (fever of the swamps), and Sumpfenfieber - bear 
witness to it. The worldwide prevalence of malaria, its great impact upon the 
health of the population of warm climates, and its toll in mortality and 
morbidity have made it the outstanding single global health problem. 
    With the bite of an infected mosquito, a few hundred parasites are 
introduced into the body. They find their way to the liver, where they develop 
in hepatic cells and grow to schizonts, each containing several thousand 
minute parasites. From the liver, the released parasites find their way to the 
bloodstream to attack red blood cells. Their synchronic development in 
erythrocytes and their division and simultaneous bursting produce the typical 
malaria attack. Together with this parasitic growth other, more resistant, 
forms appear in the blood, destined to undergo sexual development 
(gametocytes). When taken up by a female Anopheles during her blood meal, the 
fertilized gametes undergo a process of sporogony in the midgut of their 
insect vector and subsequently penetrate the salivary glands as invasive 
sporozoites; from there they reach their human host in the act of the mosquito 
bite.  
    Until 1930 quinine was the only specific remedy available. Since 1930 a 
number of synthetic chemotherapeutic compounds have been used in the therapy 
and prophylaxis of malaria: atabrine, chloroquine, primaquine, paludrine, 
pyrimethamine, and others. 
    A global program to eradicate malaria under the auspices of the World 
Health Organization has been underway since the end of World War II. By 
applying DDT and other contact insecticides, it has been possible to eliminate 
malaria from Greece, Italy, and a number of endemic countries.  See  
HAEMOSPORINA.  
 �34s[Meir Yoeli]
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�6v�2v�32sMallophaga   �21m �33s 
    A comparatively small order of insects numbering perhaps 3000 known 
species which are commonly called the bird lice or, more correctly, the biting 
lice. Most of them occur among the feathers of birds. Only a comparatively 
small number perhaps 300 species, occur on mammals. The group can be 
distinguished from the sucking lice by the fact that they always possess 
mandibles. Unlike sucking lice, they never have claws enlarged or modified to 
close about a hair or a feather. The significant difference between biting and 
sucking lice is the fact that the Mallophaga rarely transmit any disease of 
their host, while the sucking lice are well-known vectors of organisms that 
cause certain diseases. 
    The group may be technically defined as follows (see illustration): 
relatively small insects, much flattened and permanently without wings. The 
antennae are five-segmented; mandibles are distinctly developed and their 
apices cross. The prothorax is developed as a distinct segment, while the 
mesothorax and metathorax at times are closely fused. Legs possess either one 
or two terminal claws. The ovipositor is greatly reduced in size. 
    Like the sucking lice, the Mallophaga are closely adapted to life upon a 
single host species or, at most, upon a group of closely related hosts. The 
physical adaptation is not as close as in the sucking lice. There is enough 
adaptation, however, to preclude the ready passage of a mallophagan from one 
host species to another, particularly at times other than in the nest or when 
the hosts are in close bodily contact. The result is that they are usually 
passed from an animal to its offspring as a sort of racial inheritance, and 
the insects live as upon an ``island'' which is the host. Because of this, the 
classification of the Mallophaga reflects in a way the classification of their 
hosts.  See  ANOPLURA; INSECTA. 
 �34s[Dwight M. DeLong]
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�6v�2v�32sMalnutrition   �21m �33s 
    A compromised state of nutritional health resulting from inadequate intake 
of food or inability to assimilate and metabolize nutrients properly. 
Malnutrition may arise as a primary malady, due to insufficient intake of food 
or of any of many nutrients essential for the body to grow and maintain good 
health; this is by far the most common type of malnutrition. Malnutrition may 
also develop as a secondary consequence of many underlying disease states and 
psychic disorders. See NUTRITION. 
    Primary malnutrition may be further complicated when the available food is 
of compromised quality. Individuals and large populations suffer from 
inadequate food supplies throughout the nonindustrialized areas of the world: 
as many as one-half to two-thirds of the Earth's population have too little 
food.  
    An individual's stage of growth is a major determinant of vulnerability to 
malnutrition. The groups of individuals that are experiencing most rapid 
growth (infants, preschool children, pregnant women, and adolescents) are at 
highest risk. Lack of a specific essential nutrient is manifested most 
prominently in the body's most rapidly growing and dividing cells. These 
include cells of the skin and hair, the mucous membranes lining the 
gastrointestinal tract, and the blood cells. Thus lesions of the skin, 
gastrointestinal symptoms, and anemia are commonly encountered in the 
malnourished person.  See  KWASHIORKOR DISEASE. 
    Disease states are commonly associated with malnutrition in individuals 
who are ill for significant periods. A variety of diseases may be responsible 
for markedly restricted intakes of food, called secondary malnutrition. The 
ill person may lose interest in ingesting an adequate diet, even though 
nourishing and attractive food is available. Repeated episodes of vomiting may 
make food intake impossible. Mental illness may be expressed as an abnormal 
aversion to food as in anorexia nervosa. The alcoholic patient may misuse his 
or her economic resources and not have food available or ignore the necessity 
of eating.  See  ALCOHOLISM; METABOLIC DISORDERS. 
    Disease of the gastrointestinal tract may cause symptoms of secondary 
malnutrition, even though there is a normal dietary intake. Inability to 
digest food, or to absorb the digested nutrients, will eventually lead to 
malnutrition. Infectious diseases, as well as cancers and other malignant 
neoplasms, increase the metabolic activity of the body, raising the demand for 
certain essential nutrients and food energy. 
 �34s[Nathan J. Smith]
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�6v�2v�32sMaltase   �21m �33s 
    An enzyme which breaks down (hydrolyzes) the disaccharide maltose into 
glucose. Maltase has been found in the pancreas, intestine, liver, kidney, and 
blood serum of animals. It has also been found in the bacteria, fungi, and 
monocotyledonous and dicotyledonous plants.  See  CARBOHYDRATE METABOLISM; 
ENZYME.  
 �34s[Daniel N. Lapedes]
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�6v�2v�32sMalt beverage   �21m �33s 
    A fermented beverage produced from grain. Beer is a generic term used to 
describe alcoholic beverages made from cereal grains, especially barley, in 
the form of malt. Ale, lager, porter, and stout are different kinds of beer 
made by recognizably similar processes. The United States is the largest 
producer of beer in the world. 
    The manufacture of beer is a complex natural process of three general 
parts: the preparation of barley by germination, or the malting process; the 
actual digestion of barley (now malt) starch to produce a solution of sugars 
(called wort) and the adjustment of flavor with hops, which are the brewhouse 
processes; and the fermentation of these sugars by yeast to yield alcohol, 
carbon dioxide gas, and flavor compounds to produce beer. 
 �34s[M. J. Lewis]
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�6v�2v�32sMaltose   �21m �33s 
    An oligosaccharide, known as malt sugar, a reducing disaccharide (see 
illustration). It is fermentable by yeast in the presence of D-glucose. 
    The action of animal (salivary and pancreatic) as well as plant 
(germinating cereals, sweet potato) amylases on starch, dextrin, and glycogen 
produces maltose as the main end product. Maltose is hydrolyzed by acids and 
the enzyme maltase to two molecules of D-glucose.  See  GLUCOSE; MALTASE; 
OLIGOSACCHARIDE.  
 �34s[William Z. Hassid]
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�6v�2v�32sMalvales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of 6 families and more than 3500 species. The Malvaceae, with more 
than 1500 species (see illustration), and the Sterculiaceae, with about 1000 
species, are the largest families of the order. The Malvales have hypogynous 
flowers with valvate calyx, mostly separate petals that are often convolute in 
bud, usually numerous and centrifugal stamens that are often connate by their 
filaments, and a basically syncarpous pistil with axile (rarely parietal) 
placentation and often numerous carpels. 
    Cotton (Gossypium), the linden tree (Tilia), and such garden ornamentals 
as hollyhock (Althaea rosea) and rose of Sharon (Hibiscus syriacus) are 
familiar members of the Malvales.  See  BALSA; BASSWOOD; BURLAP; COLA; COTTON; 
DILLENIIDAE; FLOWER; JUTE; MAGNOLIOPSIDA; OKRA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sMammalia   �21m �33s 
    The vertebrate class Mammalia contains thousands of living and fossil 
animals that can be characterized by a complex of skeletal, soft anatomical, 
and physiological features. 
    In living mammals the allantois and amnion are present during embryonic 
development. As a rule hair covers the body, but it can be secondarily reduced 
in many orders such as cetaceans, proboscideans, perissodactyls, and 
sirenians. The fur cover, in addition to serving as protection, also insulates 
to retain body heat. The four-chambered heart (two ventricles and two 
auricles) keeps the circulation of the lungs separate from the body 
circulation, thus resulting in more efficient transport of oxygen to body 
tissues. The left aortic arch is dominant. The live-born young (monotremes are 
born from reptilelike eggs) feed on milk produced by the specialized skin 
glands of the female. The relatively large brain of mammals is the result of 
selection for an improved memory and for increased intelligence.  See  FETAL 
MEMBRANE; HAIR; LACTATION; MAMMARY GLAND; THERMOREGULATION. 
    The teeth of mammals are highly diagnostic even on the species level, and 
an understanding of dental evolution is the chief key to comprehending 
mammalian phylogeny. The longest and most crucial span of mammalian evolution, 
that during the Jurassic and Cretaceous (about 180,000,000 - 63,000,000 years 
ago) is deciphered mainly on the basis of cheek teeth (molars). Tracing the 
various homologous cusps and crests of different mammalian groups enables 
scientists to establish relatively firm evolutionary ties between groups.  See 
  DENTITION; TOOTH. 
    As a rule mammals have seven cervical vertebrae. During growth of the 
skeleton most mammalian long bones and some of the vertebrae, unlike those in 
reptiles, have ossified terminal ends (epiphyses) separated by the growing 
cartilage layer from the middle part of the bone. The cartilage layer in which 
growth occurs ossifies when growth is completed. The epiphyses enable mammals 
to have strongly articulating joints while the individual is still growing. 
Mammals tend to have ribs only in the thoracic region. These ribs are firmly 
attached to the sternum, forming the lung enclosing rib cage, covered 
posteriorly by the muscular diaphragm. See SKELETAL SYSTEM. 
    Oral digestion, control of internal body temperatures, a more efficient 
circulatory system, superior kidneys to excrete wastes of an increased 
metabolism, a skeletal and muscular system capable of more rapid locomotion, 
and in general, more efficient functioning by all the bodily organs rendered 
the mammals competitively superior to the reptiles. Instead of laying eggs 
like their reptile ancestors, mammals evolved development of the young inside 
the mother, thus protecting the offspring during their most vulnerable stage 
of development. Postnatal care of the young probably evolved concurrently with 
internal development. As a result of sustaining high metabolic activity, a 
greater awareness of the environment became necessary. Mammals developed 
keener, more sensitive noses and ears, and to coordinate this increased 
sensory information the brain became larger and more complex. See DIGESTIVE 
SYSTEM; KIDNEY; MUSCULAR SYSTEM; REPRODUCTIVE SYSTEM. 
    The mammals are placed in three subclasses as given in the following 
classification scheme. 
Class Mammalia 
    Subclass Prototheria 
        Order Monotremata 
    Subclass Allotheria 
        Order Multituberculata 
    Subclass Theria 
        Infraclass Triconodonta 
        Order: Triconodonta 
               Docodonta 
        Infraclass Pantotheria 
        Order: Pantotheria 
               Symmetrodonta 
        Infraclass Metatheria 
        Order Marsupialia 
        Infraclass Eutheria 
        Order: Insectivora 
               Deltatheridia 
               Dermoptera 
               Tillodontia 
               Taeniodonta 
               Chiroptera 
               Macroscelidea 
               Primates 
               Carnivora 
               Condylarthra 
               Pantodonta 
               Dinocerata 
               Pyrotheria 
               Proboscidea 
               Sirenia 
               Desmostylia 
               Hyracoidea 
        Order: Embrithopoda 
               Notoungulata 
               Astrapotheria 
               Xenungulata 
               Litopterna 
               Perissodactyla 
               Artiodactyla 
               Edentata 
               Pholidota 
               Tubulidentata 
        Order: Cetacea 
               Rodentia 
               Lagomorpha 
See separate articles on each group. 
 �34s[Frederick S. Szalay]
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�6v�2v�32sMammary gland   �21m �33s 
    One of the distinguishing and unique anatomical structures of Mammalia, 
designed to secrete a nutrient for the newborn suitable to bridge the 
transition from intra- to extrauterine life. The most primitive mammal, the 
duckbill or platypus, resembles reptiles in laying eggs. Its mammary glands 
are unique in lacking nipples, which are present in all other mammals. The 
milk oozes out of two mammary gland areas and is lapped up by the young. See 
MILK.  
    The mammary glands are considered skin glands because their embryonic 
origin stems from the proliferation of the ectoderm which gives rise to the 
skin. At birth, the mammary gland consists of the nipple and one or more ducts 
arising from primary and secondary sprouts. Until the approach of puberty, 
little further extension and sprouting of the duct system occurs. Then, 
through the stimulus of estrogen and recurring estrous cycles, considerable 
duct growth occurs to form fatty pads or breast tissue. 
    When animals become pregnant, the growth of the duct system continues with 
the development of many side branches. At the sides and ends of these branched 
ducts, a new type of spherical cellular growth occurs, termed alveoli, 
transforming the tubular gland into a typical compound tubuloalveolar gland. 
The alveoli may be compared to individual grapes and the bunch of grapes to a 
lobule, each of which is supported and surrounded by connective tissue. The 
alveoli are lined internally by epithelial cells capable of secreting milk 
into the central expandable cavity called the lumen. During mammary gland 
lobule-alveolar growth, the lumen of the alveoli do not show because milk is 
not secreted. 
    Lobule-alveolar growth is stimulated during the first two-thirds of 
pregnancy by the simultaneous secretion of estrogen and progesterone, first by 
the ovary and later by the placental membranes as well. It has been shown that 
additional lobulealveolar growth occurs during early lactation, stimulated by 
anterior pituitary hormones or their target glands. See OVARY; PITUITARY GLAND. 
    During the final third of pregnancy, the cells of the mammary gland begin 
to secrete a fluid called colostrum. The colostrum accumulates in the glands 
and causes a gradual enlargement of the breast or udder. At the time of 
parturition the mammary glands are fully prepared to secrete. The functional 
activity of the glands is controlled by the lactogenic hormone secreted by the 
anterior pituitary.  See  LACTATION. 
 �34s[Charles W. Turner]
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�6v�2v�32sMammography   �21m �33s 
    A radiological technique used to image breast tissues. Conventional 
mammographs (plain film) and xeromammographs are used most commonly, with 
ultrasound, computerized tomography, and enhanced mammography offering 
supplementary information. Both plain-film mammography and xeromammography use 
X-rays of low energy which are effective in imaging the breast skin, 
parenchyma, fat, and calcifications. An X-ray beam exiting the breast contains 
structural information that can be recorded by either photochemical or 
conductive processes. The photochemical system of conventional mammography 
uses film containing silver halide crystals, whereas the conductive system of 
xeromammography relies on a charged selenium plate to record the radiographic 
image. Both methods can also be used to image the lactiferic ducts after they 
have been injected with a radiopaque dye (contrast material). This examination 
is called galactography or ductography.  See  RADIOGRAPHY. 
 �34s[Nicole-Fr. Bolender]
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�6v�2v�32sMandarin   �21m �33s 
    A name used to designate a large group of citrus fruits in the species 
Citrus reticulata and some of its hybrids. This group is variable in the 
character of trees and fruits since the term is used in a general sense to 
include many different forms, such as tangerines, King oranges, Temple 
oranges, tangelos (hybrids between grapefruit and tangerine), Satsuma oranges, 
and Calamondin, presumably a hybrid between a mandarin and a kumquat.  See  
KUMQUAT; ORANGE; TANGERINE. 
    Although tangerines are the most extensively planted of the mandarin 
group, others, particularly the Temple orange, the Murcott orange, and the 
tangelos, of which there are several varieties, are important commercial 
fruits in the United States.  See  FRUIT; FRUIT, TREE; SAPINDALES. 
 �34s[Frank E. Gardner]
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�6v�2v�32sMandibulata   �21m �33s 
    A subphylum of the phylum Arthropoda. The Mandibulata, also known as the 
Antennata, includes the classes Crustacea, Insecta, Chilopoda, Diplopoda, 
Symphyla, and Pauropoda. The mandibulate arthropods possess a pair of 
mandibles, or jaws, which characterize the group.  See  CHILOPODA; CRUSTACEA; 
DIPLOPODA; INSECTA; PAUROPODA; SYMPHYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sManganese   �21m �33s 
    A chemical element, Mn, of atomic number 25 and atomic weight 54.9380. 
Manganese is one of the transition elements of the first long period of the 
periodic table, falling between chromium and iron. It has certain properties 
in common with both of these metals. Although relatively little known or used 
in its pure form, it has great practical importance in the manufacture of 
steel.  
    Manganese melts at 1244 PLUS/MINUS 3DGC (2271 PLUS/MINUS 5.4DGF) and boils 
at 2095DGC (3803DGF). In the solid state it exists in four allotropic 
modifications designated ALPHA, BETA, GAMMA, and DELTA. Manganese in either 
the ALPHA or BETA modifications at room temperature is both hard and brittle 
and cannot be plastically deformed to any significant extent. Ductility is, 
however, associated with the simpler structure of GAMMA-manganese. 
    Manganese readily oxidizes in air to form a brown oxide coating. It also 
readily oxidizes at elevated temperatures. In this respect its behavior is 
more akin to its neighbor in the periodic table of higher atomic number, iron, 
than its neighbor with lower atomic number, chromium.  See  TRANSITION 
ELEMENTS.  
 �34s[Arthur H. Sully]
�33s 
    Manganese is a fairly reactive metal. Although the massive metal is 
somewhat slow to react, the powdered metal reacts with ease and, in some 
cases, quite vigorously. When it is heated in air or oxygen, powdered 
manganese forms a red oxide, Mn3O4. With water, at room temperature, hydrogen 
and manganese(II) hydroxide, Mn(OH)2, are formed. In the case of acids, 
because manganese is such a reactive metal, hydrogen is liberated along with 
the formation of a manganese(II) salt. Manganese reacts at elevated 
temperatures with the halogens, sulfur, nitrogen, carbon, silicon, phosphorus, 
and boron. 
    In its many different compounds, manganese has oxidation states of 1+ 
through 7+. The most common oxidation states are 2+, 4+, and 7+. All the 
compounds, except those containing MnsupIsupI, are deeply colored. For 
example, potassium permanganate, KMnO4, forms aqueous solutions that are 
reddish-purple; potassium manganate, K2MnO4, forms deep-green solutions. 
    The compounds of manganese have many uses in industry. Manganese dioxide 
is used as a drying agent or catalyst in paints and varnishes, as a 
decolorizer in glass manufacturing, and in dry cells. Potassium permanganate 
is used for bleaching purposes, for decolorization of oils, and as an 
oxidizing agent in preparative and analytical chemistry.  See  DRY CELL; 
OXIDIZING AGENT. 
 �34s[Wesley W. Wendlandt]
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�6v�2v�32sManganese toxicosis   �21m �33s 
    Chronic manganese poisoning is an environmental disease of manganese 
miners and of workers in manganese mills. It occurs most frequently in the 
manganese mining villages of the Soviet Union, India, northern Africa, 
Yugoslavia, Cuba, and Chile. Its estimated incidence is as high as 25% of the 
exposed population in some areas and as low as 2 - 4% in Chile. It occurs 
after variable periods of inhalation of ore dust. 
    Chronic manganese poisoning has both psychiatric and neurological 
manifestations, the most crippling of which have been related to the 
extrapyramidal system of the brain. The psychiatric symptoms characterized by 
hallucinations, delusions, and compulsions, are transient, but the 
neurological damage may be irreversible, although some patients experience 
partial regression of their symptomatology after early removal from exposure. 
Toward the end of the psychotic period, or immediately after it, neurological 
symptoms characteristic of extrapyramidal involvement emerge. These included 
loss of facial expression, rigidity, slowness of movements, diminution of 
postural reflexes, and impairment of speech. Some therapeutic effects have 
been obtained with the administration of L-dopa.  See  MANGANESE. 
 �34s[N. Karle Mottet]
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�6v�2v�32sManganite   �21m �33s 
    A mineral having composition MnO(OH) and crystallizing in the orthorhombic 
system in prismatic crystals with deep vertical striations. The hardness is 4 
on Mohs scale, and the specific gravity is 4.3. The luster is metallic and the 
color iron black. Fine crystals have been found in the Harz Mountains; in 
Cornwall, England; and in the United States at Negaunee, Michigan. Manganite 
is a minor ore of manganese.  See  MANGANESE. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sMango   �21m �33s 
    A tree (Mangifera indica) of the family Anacardiaceae that originated in 
the Indo-Burma region and is now grown throughout the world. The mango is a 
medium to large evergreen tree; it produces a dense, round canopy, with leaves 
which are reddish brown when young and dark green when mature. 
    In the United States, mangoes are grown only in Florida and on a small 
scale in Hawaii, usually as backyard trees. Their greatest importance is in 
India, where almost 2,000,000 acres (800,000 hectares) are grown constituting 
75% of the world area devoted to mango production. The ripe fruit is eaten raw 
as a dessert or used in the manufacture of juice, jams, jellies, and 
preserves. Unripe fruit can be made into pickles or chutneys. Mangoes are a 
good source of vitamins A and C.  See  FRUIT, TREE; SAPINDALES. 
 �34s[Robert M. Warner]
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�6v�2v�32sMangrove swamp   �21m �33s 
    A swamp forest of low to tall trees and some shrubs, commonly associated 
with some salt marsh herbs. Swamp forests occur along the borders of many 
tropical shores where wave action is not intense and mud and peat are 
deposited. Most plants of this community are halophytes that are well adapted 
to salt water and fluctuations of tide level. Some have stilt or prop roots to 
help hold them on the shifting sediments and other have erect root structures 
(pneumatophores) that crop out above the surface (see illustration). 
    The mangrove community often develops as a distinct halosere. In such 
areas it is zoned from open water landward in a series of different species. 
The landward zone species develop on sediments and peats which were initially 
deposited in the seaward zone. These changes due to deposition in the swamp 
often extend the coast outward and form incipient islands in shallow, quiet 
waters. The swamps when dense also afford some protection against erosion 
resulting from violent storms. Thus, mangrove swamps have a significant 
geologic role. See ECOLOGICAL SUCCESSION; ECOLOGY. 
    The composition of mangrove swamps is strikingly different in two regions. 
Only four (Western) species predominate in the Americas and along the west 
coast of Africa, but over 10 (Eastern) species are frequent in swamps of 
eastern Africa, Asia, and the western Pacific region. The genera belong to a 
number of families. They are examples of convergent adaptations to their 
saline habitat. 
 �34s[John H. Davis]
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�6v�2v�32sManic-depressive psychosis   �21m �33s 
    A severe disturbance of affect (feeling or emotion); hence it is referred 
to as an affective psychosis. It is characterized by extreme and pathological 
elation alternating with severe dejection, both of which may last for months 
or years. There may be normal intervals between these states, and cases are 
known in which only the manic or the depressive attack occurs once or 
repeatedly.  See  PSYCHOSIS. 
    The etiology of the disorder is obscure. Endocrine and constitutional 
factors probably play a role, although specific data to prove this are 
lacking. Hereditary factors, as shown in research on twins, are of importance. 
 �34s[Fredrick C. Redlich]
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�6v�2v�32sManifold (mathematics)   �21m �33s 
    An n-dimensional manifold is a connected, locally compact space with a 
countable basis, each point of which has a neighborhood homeomorphic to 
euclidean n-space. Examples are simple closed curves (one-dimensional 
manifold) and surface of a sphere or a torus (two-dimensional manifolds). A 
fundamental problem of topology (solved only for n = 1,2) is to classify 
n-dimensional manifolds into types such that two manifolds are homeomorphic if 
and only if they belong to the same type. If a long rectangular ribbon is 
given half a turn and the two ends glued together, the resulting surface is 
called a Moebius band (see illustration). A manifold is orientable provided no 
portion of it is homeomorphic to a Moebius band; otherwise, it is 
nonorientable.  See  TOPOLOGY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sMannans   �21m �33s 
    A group of polysaccharides composed chiefly or entirely of D-mannose 
units. Mannans occur in the thickened cell walls of palm seeds as reserve 
polysaccharides, which disappear during germination. The structural formula 
below shows that the molecule is linear and is somewhat similar to cellulose. 
Glucomannans and galactoglucomannans are generally prominent components of 
coniferous woods, but they occur in only very small quantities in deciduous 
woods.  See  CELL WALLS (PLANTS); POLYSACCHARIDE. 
 �34s[Roy L. Whistler]
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�6v�2v�32sManometer   �21m �33s 
    A double-leg liquid-column gage used to measure the difference between two 
fluid pressures. Micromanometers are precision instruments which typically 
measure from very low pressures to 50 mm of mercury (6.7 kilopascals). The 
barometer is a special case of manometer with one pressure at zero absolute. 
See BAROMETER. 
    The various types of manometers have much in common with the U-tube 
manometer, which consists of a hollow tube, usually glass, a liquid partially 
filling the tube, and a scale to measure the height of one liquid surface with 
respect to the other (see illustration). If the legs of this manometer are 
connected to separate sources of pressure, the liquid will rise in the leg 
with the lower pressure and drop in the other leg. The difference between the 
levels is a function of the applied pressure and the specific gravity of the 
pressurizing and fill fluids. 
    A well-type manometer has one leg with a relatively small diameter, and 
the second leg is a reservoir. The cross-sectional area of the reservoir may 
be as much as 1500 times that of the vertical leg, so that the level of the 
reservoir does not change appreciably with a change of pressure. Mercurial 
barometers are commonly made as well-type manometers. 
    The inclined-tube manometer is used for gage pressures below 10 in. (250 
mm) of water differential. The leg of the well-type manometer is inclined from 
the vertical to elongate the scale. Inclined double-leg U-tube manometers are 
also used to measure very low differential pressures.  See  PRESSURE 
MEASUREMENT.  
 �34s[John H. Zifcak]
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�6v�2v�32sManufactured fiber   �21m �33s 
    One of a number of textile fibers and filaments manufactured from chemical 
substances of natural, inorganic, or synthetic origin. Although the 
nomenclature has varied in the past, the correct technical usage makes a 
distinction based on the origin of the raw material. Accordingly, rayon is a 
manufactured fiber but not a synthetic fiber. Nylon is a synthetic fiber that 
also falls into the broader classification of manufactured fibers. In addition 
to names based on their chemical composition, manufactured fibers are 
customarily known by the manufacturer's name or trademark, or by terms 
referring to characteristic properties, for example, thermoplastic fibers, or 
manufacturing process, for example, wet spun fibers and viscose rayon. 
    Manufactured fibers, with the exception of the inorganic ones, are 
composed of long, chainlike molecules called linear polymers. The arrangement 
(orientation) of these polymer chains with respect to the fiber axis, as well 
as the manner in which they fit or pack together (crystallinity), is 
controlled both by their chemical nature and by the manufacturing steps. In 
turn, these three variables - chemical nature, orientation, and crystallinity 
- establish the fiber properties, such as strength, flexibility, 
resilience,and durability. Fiber properties are important factors in the 
appearance, comfort, and wear of garments, home furnishings, and other textile 
applications.  
    To spin filaments, the starting material is made fluid by melting or 
dissolving a solvent, extruded through small holes, and then solidified by 
cooling (melt spinning), by chemical treatment (wet spinning), or by 
evaporation of the solvent (dry spinning). The filaments are produced in 
various sizes, and they range in diameter from finer than a human hair to 
large bristles used for toothbrushes. 
    Filaments are combined to form yarns of two different types: continuous 
filament yards and spun yarns. Spun yarns are made from short fibers called 
staple. These are cut in lengths from 1 to 4J in. (2.5 to 11.4 cm), and are 
held together in the yarn by crimp and twist in a manner similar to that used 
for natural fibers. The choice of yarn depends on the intended purpose; 
fabrics of spun-staple yarns are usually more bulky and less smooth and 
lustrous than fabrics of continuous filament yarns. 
    The fiber types of major importance in the United States are classified by 
composition as follows: (1) cellulosic, composed of regenerated cellulose, 
cellulose diacetate, and cellulose triacetate; (2) synthetic, composed of 
polyamide, polyester, polyacrylic, polyvinyl, and polyolefin resins; and (3) 
inorganic, composed of glass and metal. Numerous natural sources other than 
cellulose have been tried. Modern sources, including petroleum, natural gas, 
coke, acetylene, and related chemicals, yield the monomers, or starting units, 
for addition polymerization (polyvinyls, acrylics, dinitriles, and olefins 
such as polyethylene and polytetrafluoroethylene), and for condensation 
polymerization (polyamides, polyesters, and polyurethanes).  See  POLYMER; 
TEXTILE.  
 �34s[R. E. Seaman]
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�6v�2v�32sManufacturing engineering   �21m �33s 
    A function of industry that deals with all aspects of the production 
process. The function of manufacturing engineering is to develop and optimize 
the production process. This can best be understood by considering the total 
process through which a designer's concept becomes a marketable product: (1) 
The product designer (or product design department) conceptualizes a product. 
Drawings and one or more prototypes of this product are produced, usually by 
manual methods, but increasingly by computer-aided design and manufacturing. 
(2) The finalized prototype and its part drawings are released to the 
manufacturing engineering department, which starts designing and building an 
economically justifiable process by which the product will be produced. (3) 
When the manufacturing process developed by manufacturing engineering has been 
thoroughly tried and proved workable, it is turned over to the production 
department, which assumes responsibility for product manufacture. 
    In brief, the manufacturing engineering department bridges the gap between 
product design and full production. It discharges responsibilities in a number 
of subordinate fields of engineering: process engineering, tool engineering, 
material handling, plant engineering, and standards and methods. It must be 
noted that this list of component disciplines, while typical, is not 
universal. In virtually all manufacturing facilities the composition of 
subordinate engineering functions is adjusted to meet the needs of the product 
to be made. In all cases, however, the objective of manufacturing engineering 
is the same: to design and develop the manufacturing process in the most 
economical manner possible.  See  COMPUTER-AIDED DESIGN AND MANUFACTURING; 
ENGINEERING DESIGN; INDUSTRIAL ENGINEERING; METHODS ENGINEERING; PROCESS 
ENGINEERING; PRODUCTION ENGINEERING; TOOLING. 
 �34s[Daniel B. Dallas]
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�6v�2v�32sManure   �21m �33s 
    Any plant and animal residue that may contain excreta of animals. Manures, 
classified into animal manure, plant manure, and compost, are added to the 
soil in various stages of decomposition. In the soil they undergo further 
degradation through the action of soil microorganisms, raising the soil 
fertility level and improving soil texture by increasing humus content.  See  
HUMUS.  
    Animal manure includes plant residues like straw, used as litter, as well 
as solid and liquid excreta. Compost consists of plant and animal residues 
allowed to rot before being applied to soil. Green manure is plant material in 
the form of a growing crop plowed into the soil. Because the object of 
manuring is to increase the supply of nitrogen, leguminous crops are grown and 
then plowed into the soil.  See  SOIL MICROBIOLOGY. 
 �34s[Allan G. Lochhead]
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�6v�2v�32sMaple �21m �33s
    A genus, Acer, of broad-leaved, deciduous trees including about 115
species in North America, Asia, Europe, and North Africa. This genus is
characterized by simple, opposite, usually palmately lobed (rarely pinnate)
leaves, generally inconspicuous flowers, and a fruit consisting of two
long-winged samaras or keys (see illustration).
    The most important commercial species is the sugar or rock maple (A.
saccharum), called hard maple in the lumber market. This tree grows in the
eastern half of the United States and adjacent Canada. It can be recognized by
its gray furrowed bark, sharp-pointed scaly winter buds, and symmetrical oval
outline of the crown.
    Maples rank third in the production of hardwood lumber. Hard maple is used
for flooring, furniture, boxes, crates, woodenware, spools, bobbins, motor
vehicle parts, veneer, railroad ties, and pulpwood. It is the source of maple
sugar and syrup and is planted as a shade tree.  See  SAPINDALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sMap projections �21m �33s
    Methods of transforming a spherical representation of the Earth's surface
to a nonspherical surface, usually a plane. These methods are useful because
globes and curved-surface reproductions are cumbersome, expensive, and
difficult to measure upon. An infinite number of methods is possible. Each
method deforms the geometric relationships among points on the sphere in some
way, since a sphere and a plane are not applicable surfaces. Consequently,
directions, distances, areas, and angular relationships on the Earth are never
completely re-created on a map.

    Cylindrical projections.  These result from symmetrical transfer of the
spherical surface to a tangent or intersecting cylinder. True or correct scale
will obtain along the great circle of tangency or the two homothetic small
circles of intersection. If the axis of the cylinder is made parallel to the
axis of the Earth, the parallels and meridians will appear as right,
perpendicular lines. Points on the Earth equally distant from the tangent
great circle (Equator) or small circles of intersection (parallels equally
spaced on either side of the Equator) will have equal scale departure. The
pattern of deformation will therefore parallel the parallels, as change in
scale occurs in a direction perpendicular to the parallels. If the cylinder is
turned 90DG with respect to the Earth's axis, the projection is said to be
transverse, and the pattern of deformation will be symmetric with respect to a
great circle through the poles. If the turn of the cylinder is less than 90DG,
an oblique projection results (Fig. 1).

    Conic projections.	Transfer to a tangent or intersecting cone is the
basis of these projections. True scale will obtain along one or two small
circles in the same hemisphere. Conic projections are usually arranged with
the axis of the cone parallel to the Earth's axis. Consequently, meridians
appear as radiating straight lines and parallels as concentric arcs which
intersect with the meridians at right angles. Conical patterns of deformation
parallel to parallels, that is, scale departure will be uniform along any
parallel (Fig. 2).

    Azimuthal projections. These result from the transfer to a tangent or
intersecting plane established perpendicular to a right line passing through
the center of the Earth. All geometrically developed azimuthal projections
are transferred from some point on this line. Points on the Earth
equidistant from the point of tangency or the center of the circle of
intersection have equal scale departure. Hence the pattern of deformation
is circular and concentric to the aforementioned center. All azimuthal
projections, whether geometrically or mathematically derived, have two aspects
in common: (1) All great circles that pass through the center of the projection
appear as straight lines. (2) All azimuths from the center are truly displayed
(Fig. 3).

[Arthur H. Robinson]

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�6v�2v�32sMap scale   �21m �33s 
    The relation between the length of a line on a map to the corresponding 
line in nature. Map scale is expressed in three ways: 
    1. Representative fraction, as 
  
             ***  1/R = length on map / length in nature ***
  
Thus, if on a map 1 in. represents 10 mi, it is expressed by the representative 
fraction 1/633,600 or 1:633,600, as there are 63,360 in. in 1 mi.  
    2. Miles-per-inch scale indicates the number of miles represented and is 
shown on maps as 

                     *** Scale: 1 in. to 10 mi ***

    3. Graphic scale is a graduated line that portrays the actual length of     
miles or kilometers as they appear on the map. Graphic scales have the 
advantage of remaining true after the map has been enlarged or reduced in
reproduction (see illustration).
    In countries using the metric system, the common scales are 1:25,000, 
1:50,000, 1:100,000, and so on. British maps generally use scales based on 
multiples of 1:63,360; for example, the so-called quarter-inch maps which mean 
  
               *** 1 / (4 MULT 63,360) = 1 / 253,440 ***
  
 �34s[Erwin Raisz/Wesley Calef]
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�6v�2v�32sMaquis   �21m �33s 
    A type of vegetation composed of shrubs, or ``scrub,'' usually not 
exceeding 10 ft (3 m) in height, the majority having relatively small, hard, 
leathery, often spiny or needlelike drought-resistant leaves and occurring in 
areas with a Mediterranean climate. In this climate precipitation occurs 
principally in winter and relative humidity is low and evaporation high in 
summer. The life form of the plants together with the climatic regime, not the 
species composition, characterizes maquis. Species composition of maquis may 
be very uniform over wide areas, or may vary considerably locally, and between 
widely separated areas. 
    Californian chaparral is closely related ecologically to maquis and is 
included in the concept by some authorities.  See  CHAPARRAL. 
 �34s[Robert K. Godfrey]
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�6v�2v�32sMarattiales   �21m �33s 
    An ancient order of ferns which in some respects is intermediate between 
the orders Ophioglossales and Polypodiales. Whereas their fossil remains 
indicate they were formerly worldwide and abundant, they now comprise only 7 
genera and about 150 species limited to the humid tropical forests. The 
highest concentration of genera and species is in southeastern Asia. They are 
mostly large ferns with persistent leaf bases.  See  OPHIOGLOSSIDAE; 
POLYPODIALES.  
 �34s[Paul A. Vestal]
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�6v�2v�32sMarble   �21m �33s 
    A term applied commercially to any limestone or dolomite taking polish. 
Marble is extensively used for building and ornamental purposes.  See  
DOLOMITE; LIMESTONE. 
    In petrography the term marble is applied to metamorphic rocks composed of 
recrystallized calcite or dolomite. Schistosity, often controlled by the 
original bedding, is usually weak except in impure micaceous or 
tremolite-bearing types. Calcite (marble) deforms readily by plastic flow even 
at low temperatures. Therefore, granulation is rare, and instead of 
schistosity there develops a flow structure characterized by elongation and 
bending of the grains concomitant with a strong development of twin lamellae.  
See  METAMORPHIC ROCKS; MINERALOGY; SCHIST. 
    Pure marbles attaining 99% calcium carbonate, CaCO3, are often formed by 
simple recrystallization of sedimentary limestone. Dolomite marbles are 
usually formed by metasomatism.  See  CALCITE; DOLOMITE; METASOMATISM. 
 �34s[T. F. W. Barth]
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�6v�2v�32sMarcasite   �21m �33s 
    A mineral having composition FeS2 and crystallizing in the orthorhombic 
system. Marcasite frequently has a radiating structure and may be globular or 
stalactitic. There is poor prismatic cleavage. The hardness is 6 - 6.5 on Mohs 
scale and the specific gravity is 4.89. The luster is metallic and the color 
pale bronze-yellow to nearly white on a fresh fracture. Marcasite and pyrite 
are dimorphous; both have the composition FeS2. Because marcasite is whiter, 
it is called white iron pyrite. 
    Marcasite is found in metalliferous deposits associated with lead and zinc 
ores, as replacement deposits in limestone, and in concretions in clays and 
shales. The nodular and lenticular masses in coal known as brasses are in part 
marcasite and in part pyrite.  See  PYRITE. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sMarchantiopsida   �21m �33s 
    A class of lower green plants (formerly Hepaticae) called liverworts that 
belong to the division Bryophyta. The class is divided into approximately 225 
genera and 8500 species. Although there is a great diversity of external form, 
most of the gametophytes (gamete-producing plants) are dorsoventrally 
differentiated. These plants are considered the most primitive of the existing 
land plants. Fossil remains of liverworts have been found in both the Devonian 
and Carboniferous. Liverworts are widely distributed over the world, but have 
their greatest diversity in the tropics of the Americas and East Indies.  See  
BRYOPSIDA.  
    Except when the plants occur in masses, they are quite inconspicuous and 
are usually confused with mosses, which they resemble somewhat in their 
external appearance. In the presence of adequate moisture, they grow on soil, 
rocks, and tree trunks. Usually the plant body is a thin, prostrate thallus, 
sometimes having a short central axis with leaflike appendages. On the lower 
surface are rhizoids (rootlike structures) which function in anchorage and 
absorption.  
    The Marchantiopsida are divided into a number of orders of which the best 
known are the Jungermanniales and the Marchantiales. The Jungermanniales, with 
190 genera and 8000 species, are called the leafy liverworts because of their 
chlorophyll-containing ribbonlike or leaflike bodies. The Marchantiales, with 
33 genera and 450 species, are called thallose liverworts. The flat plant body 
is composed of several distinct layers of tissue, having the 
chlorophyll-containing cells of the uppermost layer. 
    In the life cycle of liverworts, the gametophyte, or gamete-producing 
generation, is the dominant phase. The plants are dioecious; that is, they 
have male and female sex organs on different plants. As a result of 
fertilization a zygote is formed which develops into a sporophyte. Spores are 
produced which, when falling upon a suitable substrate, germinate and produce 
new gametophytes. 
    In Marchantia there are also two methods of vegetative reproduction. The 
first results from the growth habit of the plant. As it grows and branches, 
the older portions of the plant decay, thus separating the branches into 
individual plants. The second method is in the formation of gemmae cups on the 
dorsal surface. Within each cup several small, notched, lens-shaped bodies are 
produced which, when detached, grow from both ends simultaneously and develop 
into new plants. This provides for rapid dispersal during the growing season.  
See  TRACHEOPHYTA. 
 �34s[Paul A. Vestal]
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�6v�2v�32sMargarine   �21m �33s 
    An emulsified fatty food product used as a spread and a baking and cooking 
fat, consisting of an aqueous phase dispersed in the fat as a continuous 
phase. Developed originally as a butter substitute, margarine is now 
considered a food in its own right and is manufactured in forms unknown to 
butter, such as plastic, soft, or fluid. However, margarine is colored, 
flavored, fortified with vitamins, and otherwise formulated to have the same 
or similar taste, appearance, and nutritional value as butter.  See  BUTTER. 
    Currently, margarine produced in the United States must contain not less 
than 80% fat. The fats and oils must be edible but may be from any vegetable 
or animal carcass source, natural or hydrogenated. The required aqueous phase 
may be water, milk, or solutions of dairy or vegetable protein, and must be 
pasteurized. Vitamin A must be added to yield a finished margarine with not 
less than 15,000 international units per pound (0.45 kg). Optional ingredients 
include salt or potassium chloride for low-sodium diets, nutritive sweeteners, 
fatty emulsifiers, antioxidants, preservatives, edible colors, flavors, 
vitamin D, acids, and alkalies.  See  FAT AND OIL (FOOD). 
 �34s[Theodore J. Weiss]
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�6v�2v�32sMarijuana �21m �33s
    The Spanish name for the dried leaves and flowering tops of the hemp
plant, Cannabis sativa (Cannabinaceae). The narcotic ingredients allegedly
have stimulating effects, and after smoking two or three cigarettes, the
smoker often has a feeling of well-being and increased power and ability.
After excessive amounts of the drug, illusions are often common, as well as
pleasing, fanciful hallucinations. Sometimes the excessive user experiences
disorientation and even delirium.  See	HEMP; URTICALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sMarine boiler   �21m �33s 
    A steam boiler designed to suit the marine environment and generally 
arranged to supply steam to the main propulsion machinery, ship's service 
electric generators, feed-pump drivers, and other auxiliary services. 
    Marine boilers are usually of the two-drum water-tube type with 
water-cooled furnaces, superheaters, desuperheaters, and heat recovery 
equipment of the economizer or air-heater type. The majority of ships are 
fitted with two boilers, although some large passenger ships may have three or 
more. Some cargo ships are fitted with only one boiler, and in some of these 
cases a smaller auxiliary boiler may be fitted for emergency or in-port 
steaming use. 
    Marine boilers are generally arranged for oil firing. Oil is used 
extensively because of its simplicity of handling and storing and the fact 
that it can be stored in spaces that often cannot be used for carrying cargo. 
However, the shortage of fuel oil, coupled with its ever-increasing cost, is 
bringing back coal as a fuel for marine boilers.  See  BOILER; MARINE 
MACHINERY; STEAM-GENERATING UNIT. 
 �34s[Robert P. Giblon]
�33s 
  
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�6v�2v�32sMarine containers   �21m �33s 
    Standardized rectangular boxes for the transport of marine cargo. Since 
1960, the ocean transportation of general cargo or freight has undergone a 
revolutionary technological change. The innovation was containerization - the 
development of standard marine cargo containers for consolidating packages 
into units of interchange between ships, docks, trucks, and railcars, and the 
development of special ships and handling systems to transport these 
containers at sea. This innovation brought economies of scale to marine 
cargo-handling operations, introduced capital-intensive processes to the 
labor-oriented stevedoring tasks, reduced cargo theft and damage, reduced the 
time a ship spent in port, and provided the means for efficient intermodal 
transport of cargoes. 
    Basic to the change to containerization was a new approach to loading and 
unloading cargoes. Instead of using nets or slings to lift individual bales, 
boxes, sacks, or pallets of cargo in and out of the ship's holds, the new 
systems employ standard cargo containers and special handling equipment to 
place the containers aboard (see illustration). The containers are loaded or 
stuffed at an inland factory or terminal and usually are never opened until 
they reach their ultimate destination. The ship carrying these containers 
becomes an extension of a truckline or a railroad. 
    Intermodability, moving containerized cargo by more than one mode of 
transport without the need for intermediate reloading, is the essence of this 
technology. The containers can be moved to the ship via regular highway 
trailers or trailer chassis, or by flatbed railcar or rail piggyback. Once at 
the marine terminal, they can be lifted or rolled on the ships. Upon arrival 
at the discharge port, all land and water modes are again available to move 
them, with cargo undisturbed, to the consignee. 
 �34s[John C. Couch]
�33s 
  
  
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�6v�2v�32sMarine ecosystem   �21m �33s 
    Marine ecology comprises the ecology of the world's oceans with their 
shores and estuaries. Individual organisms, groups of organisms, and the ocean 
environments in which they exist constitute ecological systems, termed 
ecosystems. Examples vary in size from a particular seaweed ecosystem, to a 
tidepool ecosystem, to an estuarine bay ecosystem. The size limits, or 
boundaries, of a particular ecosystem that distinguish it from others may 
depend upon physical barriers to community dispersal, such as a submarine 
mountain; environmental factors that restrict the area of the biotic 
community, such as the salinity factor; the productivity cycles within the 
community; or the reproductive and dispersal potentials of the community.  See 
  COMMUNITY; ECOSYSTEM. 
    The nonliving, or abiotic, materials in a marine ecosystem cycle comprise 
not only a variety of water-soluble inorganic nutrient salts, such as the 
phosphates, nitrates, and sulfates of calcium, potassium, and sodium and the 
dissolved gases oxygen and carbon dioxide, but also organic compounds, such as 
the various amino acids, vitamins, and growth substances. Most of the solid 
material dissolved in the sea originated from the weathering of the crust of 
the Earth. 
    The marine ecosystem can be divided into two large areas, the pelagic and 
benthic divisions. Each of these consists of various zones. The pelagic 
division embodies all the waters of the oceans and their adjacent salt-water 
bodies. It is divisible into the neritic zone, extending offshore to the edge 
of the continental shelf to a depth of over 650 ft (200 m), and the oceanic 
zone, embracing the remaining offshore waters. 
    The benthic division of the marine environment embraces the entire ocean 
floor, both coastal bottom and deep-sea bottom, properly termed the littoral 
and deep-sea systems, respectively. The littoral system consists of the 
eulittoral zone and the sublittoral zone. The eulittoral zone extends from 
high-tide level to about 200 ft (60 m), the approximate depth below which 
attached plants do not grow abundantly. This is probably the richest zone of 
the marine ecosystem in respect to numbers and kinds of organisms, as well as 
in variety of ecological types and habitat modifications. The sublittoral zone 
terminates at depths varying in different latitudes between about 650 to 1300 
ft (200 to 400 m), depending upon light and temperature factors that modify 
the distribution of benthic animals and plants. 
    The deep-sea system of the benthic division is subdivided into an upper 
part, the archibenthic zone or, more meaningfully, the continental deep-sea 
zone, extending from the edge of the continental shelf (650 - 1300 ft or 200 - 
400 m) to depths of about 2600 - 3600 ft (800 - 1100 m), and the 
abyssal-benthic zone that embraces the remainder of the benthic deep-sea 
system. The zones have little or no light, relatively constant conditions of 
salinity and temperature, and steadily decreasing numbers and kinds of 
organisms.  
 �34s[Curtis L. Newcombe]
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�6v�2v�32sMarine engine   �21m �33s 
    An engine that propels a waterborne vessel. Even in small craft the marine 
engine must have the following characteristics: reliability, light weight, 
compactness, fuel economy, low maintenance, long life, relative simplicity for 
operating personnel, ability to reverse, and ability to operate steadily at 
low or cruising speed.  See  BOAT PROPULSION; MARINE MACHINERY; SHIP POWERING 
AND STEERING. 
    Steam is a common type of propulsion for large ships. The diesel engine 
has gained wide acceptance in foreign merchant ships, but in the United States 
the majority of seagoing vessels use steam propulsion. 
    The gas turbine generally consists of an axial compressor discharging 
compressed air to a combustion chamber where fuel is burned, adding heat. The 
products of combustion at high temperature and pressure then pass through a 
gas turbine that drives the compressor and load. Generally, the term ``gas 
turbine'' is applied to the entire plant.  See  GAS TURBINE. 
    Both diesel and gasoline internal combustion engines are used in marine 
applications. Many moderate- and low-power marine installations use automotive 
or locomotive engines designed for variable load and intermittent service. 
High-power marine propulsion units normally are called on to operate 
continuously under load. 
    The turboelectric type of drive, comprising one or more steam turbine 
generators and ac propulsion motors, is also used for ship propulsion. The 
diesel-electric type of drive, composed of one or more dc diesel generator 
sets and often a double-armature propulsion motor, is used in tugs, dredges, 
Coast Guard cutters, and icebreakers, where maneuvering and a wide range in 
propeller speed are necessary. 
 �34s[Jens T. Holm]
�33s 
  
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�6v�2v�32sMarine fisheries   �21m �33s 
    The harvest of animals and plants from the ocean to provide food and 
recreation for people, food for animals, and a variety of organic materials 
for industry. It is now generally agreed that the world catch is approaching a 
maximum, which may be less than 100,000,000 metric tons per year. If methods 
can be devised to harvest smaller organisms not heretofore used because they 
have been too costly to catch and process, it has been estimated that the 
yield could perhaps be increased severalfold. The Soviet Union is said to have 
succeeded in developing an acceptable human food product from Antarctic krill. 
  See  FISHERY CONSERVATION. 
 �34s[J. L. McHugh]
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�6v�2v�32sMarine geology �21m �33s
    The study of the portion of the Earth beneath the oceans. More than 70% of
the Earth's surface is covered by marine waters.  Of the oceanic area (3.61 x
10(sup 8) km(sup 2) or 1.39 x 10(sup 8) mi(sup 2)) approximately 3 x 10(sup 8)
km(sup 2) (1.2 x 10(sup 8) mi(sup 2)) is contributed by the deep-sea floor; the
remaining 6 x 10(sup 7) km(sup 2) (2.3 x 10[sup 7] mi[sup 2]) represents the
submerged margins of the continents.  The distribution of elevations on the
Earth is shown in Fig.	1.

    Physiographic provinces.  The Earth relief lies at two dominant levels
(Fig. 1); one, within a few hundred meters of sea level, represents the normal
surface of the continental blocks; the other, between 4000 and 5000 m (2.5 and
3.0 mi) below sea level, and comprising more than 50% of the Earth's surface,
represents the ocean-basin floor. The topographic provinces beneath the sea
can be included under three major morphologic divisions: continental margin,
ocean-basin floor, and mid-oceanic ridge.

    Continental margins. The continental margin includes those provinces
associated with the transition from continent to ocean floor. The continental
margin in the Atlantic and Indian oceans is generally composed of continental
shelf, continental slope, and continental rise. The continental slopes are cut
by many submarine canyons. Submarine alluvial fans extend out from the seaward
ends of the larger canyons.  See  CONTINENT; CONTINENTAL MARGINS; SUBMARINE
CANYON.

    The continental margin can be divided into three categories of provinces.
Category I includes the continental shelf, marginal plateaus, and shallow
epicontinental seas, all slightly submerged portions of the continental block.
Category II includes the continental slope, marginal escarpments, and the
landward slopes of marginal trenches, all expressions of the outer edge of the
continental block. Category III includes the continental rise, the ridge-basin
complex, and the ridge-trench complex.

    Ocean-basin floor. Excluding the marginal trenches and mid-oceanic ridges,
the ocean-basin floor includes the deepest portions of the ocean.
Approximately one-third of the Atlantic and three-fourths of the Pacific fall
under this heading. The ocean-basin floor can be divided into three categories
of provinces; the abyssal floor, oceanic rises, and seamounts and seamount
groups.

    The abyssal floor includes the broad, deep areas of the central portion of
the ocean. An abyssal plain is a smooth portion of the deep-sea floor where
the gradient of the bottom does not exceed 1:1000. Abyssal plains adjoin all
continental rises and can be distinguished from the continental rise by a
distinct change in bottom gradient. At their seaward edge, most of the abyssal
plains gradually give way to abyssal hills, which are thought to represent
tectonic or volcanic relief of a type identical with that buried beneath the
abyssal plains. Abyssal plains are also found in the marginal trenches,
marginal basins, and in epicontinental marginal seas. Features of exactly the
same morphology and origin are found in some lakes. Of similar origin are
archipelagic aprons, which spread out from the base of oceanic islands.  See
MARINE SEDIMENTS; OCEANIC ISLANDS.

    Oceanic rises are areas slightly elevated above the abyssal floor which do
not belong to the continental margin or the mid-oceanic ridges. In contrast to
the mid-oceanic ridges, oceanic rises are nonseismic; their relief is more
subdued, and they are asymmetrical in cross section.

    A seamount is any submerged peak more than 900 m (500 fathoms) high.
Seamounts are distributed through all the physiographic provinces of the
oceans. Seamounts sometimes occur randomly scattered but more often lie in
linear rows. Virtually all conical seamounts are believed to be extinct or
active volcanoes.  See	SEAMOUNT AND GUYOT.

    Mid-Oceanic Ridge. The middle third of the Atlantic, Indian, and South
Pacific oceans is occupied by a broad, fractured swell known as the
Mid-Oceanic Ridge. In the Atlantic, it is known as the Mid-Atlantic Ridge; in
the southern Indian Ocean, the Mid-Indian Ridge; in the Arabian Sea, the
Carlsberg Ridge and Murray Ridge; and in the South Pacific, the Easter Island
Ridge.

 �34s[Bruce C. Heezen]
�33s


    Underlying structure.  Because approximately 70% of its surface is covered
by the oceans, the typical structure of the Earth is found in the oceanic and
not in the land areas. Statistical examination shows that most of the Earth's
solid surface is either at the elevation of the ocean floors or at the
elevation of the continents. The anomalous areas, those of extreme or of
intermediate elevation, are long, narrow features - the mountain ranges,
island arcs, deep-sea trenches, and continental margins. With the exception of
a few intermediate areas, such as the Red Sea, the crustal structure of the
ocean basins is distinctly different from the crustal structure of the
continents, and it appears that this has been the case for most of the Earth's
history.

    Ocean basins and continental margins. Figure 2 is a structure section
across an Atlantic type (rifted) margin based on an interpretation of geologic
and geophysical data within the framework of plate tectonics and sea-floor
spreading. On the continent and continental shelf, the rocks beneath the
sedimentary layers are mainly of the acidic type, such as granite, gneiss, or
schist. Beneath the rocks is an intermediate layer (layer 2) believed to be
gabbroic or basaltic. The mantle is probably composed of ultramafic rocks,
such as peridotite, enstatite, or eclogite. This is the most prominent layer
in the Earth, extending from near the surface approximately halfway to the
center. There are some variations in the upper parts of the mantle between
continental and oceanic areas, but the most apparent difference is in the
thickness and composition of the crust. The continental crust is six or seven
times thicker than the oceanic crust (layer 3) and contains almost all of the
acidic rocks, such as granites, whereas the oceanic crust is almost entirely
composed of basic rock.  See  EARTH.

    The thickness of the crust of the continental shelves is intermediate
between that of the continents and oceans, and its composition is continental.
In some places, such as parts of the east coast of North America and South
America, the continental shelf and continental slope are broad areas where
erosion of the continental masses has resulted in the deposition of many
thousands of meters of sediment during the past several million years. In
other areas which do not receive much sediment, notably the west coast of the
Americas, there is only a narrow continental shelf.

    Submarine ridges.There are two types of oceanic ridges, seismic and
aseismic. The mid-ocean ridge system is of the former type and is the largest
single feature on the Earth. Over its entire length the ridge is characterized
by a narrow zone of shallow-focus earthquake epicenters. Usually associated
with this zone are a rift valley and a large positive magnetic anomaly. In
addition, parallel linear bands of alternating positive and negative magnetic
anomalies are found on the flanks in many areas. The sediment cover is
typically thin and in some places in the crest region is entirely absent in a
zone 100 - 500 km (60 - 300 mi) wide. On the flanks an appreciably thicker
cover is found; in some places it covers the basement rock more or less
uniformly, while in others it is collected in pockets separated by exposed
basement peaks or ridges.

    Deep-sea trenches. Deep-sea trenches are important structural features
associated with some continental margins, island arcs, earthquake belts, and
areas of volcanic activity. Most of them are confined to the margins of the
Pacific Ocean, although there are trenches in the Atlantic and Indian oceans.
In the bottom are layers of sediments which are generally thickest landward of
the trench axes. The great depth of the underlying dense layers causes a
pronounced deficiency of gravity, a characteristic feature of all deep-sea
trenches. Earthquake foci tend to lie on inclined planes dipping landward from
the trenches.  See  MARINE SEDIMENTS.

 �34s[Maurice Ewing ;John Ewing]
�33s


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�6v�2v�32sMarine influence on weather and climate   �21m �33s 
    The moderating, energizing, and redistributing effects that bodies of 
water have on air temperature, weather, and climate by releasing energy and 
heat to the atmosphere in a number of characteristic ways. 
    To a greater extent than is commonly realized, the climates of the oceans 
determine the climates of the Earth. The oceans constitute the prime source of 
energy for all water-vaporinduced atmospheric processes. The latent heat 
liberated by the condensation of water vapor evaporated from the oceans 
supplies the energy for tropical cyclone formation; and such water vapor from 
the oceans is otherwise a primary stage in the energy cycle of the general 
atmospheric circulation.  See  ATMOSPHERIC GENERAL CIRCULATION; STORM. 
    The oceans affect weather and climate, then, because they are a tremendous 
reservoir for the storage of solar energy. This energy is absorbed during 
periods of the year (or in regions) of surplus solar energy. It is then stored 
or transported by ocean currents, to be released to the atmosphere at times 
(or in regions) of deficiency in solar energy. In this sense the oceans become 
equalizers or moderators of climates.  See  CONTINENTALITY (METEOROLOGY). 
 �34s[Woodrow C. Jacobs]
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�6v�2v�32sMarine machinery   �21m �33s 
    The engines, blowers, and other moving devices used on ships, boats, and 
related vessels. Practically all machinery used on ships has shore-based 
counterparts. Machinery is often developed for marine use prior to shore 
development and use. The marine environment places many demands on marine 
machinery. These include an extra measure of reliability, performance on a 
rolling and pitching platform, and limited space and weight requirements, as 
well as anticorrosion properties. These factors make necessary the frequent 
use of special designs for marine service. In addition, the purposes served by 
different types of ships and boats call for machinery of a number of types and 
characteristics.  See  BOAT PROPULSION; MARINE BOILER; MARINE ENGINE; 
PROPELLER (MARINE CRAFT); SHIP POWERING AND STEERING. 
 �34s[Jens T. Holm]
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�6v�2v�32sMarine microbiology   �21m �33s 
    The study of the microscopic organisms living in the sea. Marine 
microorganisms were probably among the first (cellular) living entities and 
have played a tremendous part in geology and in the biology of the oceans. 
They are still of paramount importance as transformers of organic and 
inorganic substances and as food for other, often larger, organisms. 
    Marine microbes may be considered as (1) autotrophs and (2) heterotrophs 
or phagotrophs. Autotrophs live on inorganic materials and use carbon dioxide 
as the sole carbon source, obtaining their energy from sunlight (phototrophs) 
or from chemical reactions (chemoautotrophs). The heterotrophs and phagotrophs 
(bacteria, heterotrophic algae and fungi, and protozoa) require a source of 
carbon more complex than carbon dioxide; this they obtain, along with energy, 
by breaking down or ingesting other microorganisms, plants, and animals. 
    Marine microoganisms are the food of a large number of marine animals 
including many crustaceans of the zooplankton, molluscans, and plant-eating 
fish.  
 �34s[Edward J. F. Wood]
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�6v�2v�32sMarine mining   �21m �33s 
    The process of recovering mineral wealth from sea water and from deposits 
on and under the sea floor. While mineral resources to the value of trillions 
of dollars do exist in and under the oceans, their exploitation is not simple. 
Many environmental problems must be overcome and many technical advances must 
be made before the majority of these deposits can be mined in competition with 
existing land resources. 
    The mineral resources of the marine environment are of three basic types: 
the dissolved minerals of the ocean waters; the unconsolidated mineral 
deposits of marine beaches, continental shelf, and deep-sea floor; and the 
consolidated deposits contained within the bedrock underlying the seas. As 
with land deposits, the initial stages preceding the production of a 
marketable commodity include discovery, characterization of the deposit to 
assess its value and exploitability, and mining, including beneficiation of 
the material to a salable product. 
 �34s[Michael J. Cruickshank]
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�6v�2v�32sMarine navigation   �21m �33s 
    The process of directing a watercraft to a destination in a safe and 
expeditious manner. From a known present position, a course is determined 
which avoids dangers, and on this course estimates are made of time schedules. 
The task is to periodically effect en route checks and to make required 
adjustments.  
    The method used will depend on the type of vessel and on its role or 
mission. The devices available range from a simple compass to a host of 
sophisticated electronic systems. In all cases, the navigator must plan and 
prepare by setting instruments in order and by checking for predictable 
current and tidal effects and hazards to navigation en route. This preparation 
includes having the latest correct charts and reviewing pertinent sections of 
sailing directions, light lists, and tide and current tables. 
    The methods used to fulfill the requirements of navigation come under one 
of the following broad categories: dead reckoning, piloting, celestial 
navigation, and electronic navigation. The first three categories have become 
somewhat standardized; the fourth category, on the other hand, has been under 
constant and innovative development.  See  CELESTIAL NAVIGATION; DEAD 
RECKONING; ELECTRONIC NAVIGATION; PILOTING. 
    With the unprecedented expansion of the world's merchant fleets and the 
ever-increasing number of very large deepdraft tankers with poor maneuvering 
characteristics and high-speed containerships, come additional hazards to 
shipping through collisions at sea. Hence, the international maritime 
community is constantly seeking ways to put to use available technology to 
provide the navigator with the tools necessary to cope with the problem of 
collision avoidance. 
    Radar can only present certain information and this information must be 
used properly and prudently by the navigator in traffic situations. To avoid 
this condition of task overload on the navigator and to reduce or eliminate 
the human error in calculations or interpretation, collision avoidance aids or 
systems (CAS), also known as automatic radar plotting aids (ARPA), of varying 
degrees of sophistication have been developed. 
    Integrated navigation-conning systems have the basic function of helping 
reduce the number of accidents between ships by having the computer correlate 
most of the data available during crowded conditions and thus help unburden 
the navigator by presenting this data in more usable form for better and 
quicker decision making. 
    In a program established for the purpose of reducing the number of 
collisions and strandings and at the same time safeguarding the environment, 
the U.S. Coast Guard, under the authority of the Ports and Waterways Safety 
Act of 1972, has instituted vessel traffic services (VTS) in a number of 
heavily trafficked United States ports. Generally the service provides marine 
traffic management of an advisory nature, but in an especially hazardous 
situation it may be necessary for the VTS to exercise emergency control of 
vessel movements.  See  NAVIGATION. 
 �34s[A. E. Fiore]
�33s 
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�6v�2v�32sMarine refrigeration   �21m �33s 
    The application of refrigeration on shipboard for the preservation of 
perishables in transit and for the preservation of foodstuffs to be used by 
the passengers and crew. Marine refrigeration is also used for maintaining 
certain cargo products in liquid form that would otherwise evaporate when 
stored at or near ambient conditions. Refrigeration is also required during 
warm weather for air-conditioning the quarters for passengers and crew. For 
air conditioning, similar but larger compressors than those used for ship's 
stores are generally employed, and for larger systems chilled water is 
circulated through the coils situated in various fan rooms throughout the 
quarters area.  See  AIR CONDITIONING; REFRIGERATION. 
 �34s[Robert P. Giblon]
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�6v�2v�32sMarine sediments   �21m �33s 
    The accumulation of minerals and organic remains on the sea floor. Marine 
sediments vary widely in composition and physical characteristics as a 
function of water depth, distance from land, variations in sediment source, 
and the physical, chemical, and biological characteristics of their 
environments. The study of marine sediments is an important phase of 
oceanographic research and, together with the study of sediments and 
sedimentation processes on land, constitutes the subdivision of geology known 
as sedimentology.  See  OCEANOGRAPHY; SEDIMENTATION (GEOLOGY). 
    Traditionally, marine sediments are subdivided on the basis of their depth 
of deposition into littoral 0 - 66 ft (0 - 20 m), neritic 66 - 660 ft (20 - 
200 m), and bathyal 660 - 6600 ft (200 - 2000 m) deposits. This division 
overemphasizes depth. More meaningful, although less rigorous, is a 
distinction between sediments mainly composed of materials derived from land, 
and sediments composed of biological and mineral material originating in the 
sea. Moreover, there are significant and general differences between deposits 
formed along the margins of the continents and large islands, which are 
influenced strongly by the nearness of land and occur mostly in fairly shallow 
water, and the pelagic sediments of the deep ocean far from land. 
  
    Sediments of continental margins.  These include the deposits of the 
coastal zone, the sediments of the continental shelf, conventionally limited 
by a maximum depth of 330 - 660 ft (100 - 200 m), and those of the continental 
slope. Because of large differences in sedimentation processes, a useful 
distinction can be made between the coastal deposits on one hand (littoral), 
and the open shelf and slope sediments on the other (neritic and bathyal). 
Furthermore, significant differences in sediment characteristics and 
sedimentation patterns exist between areas receiving substantial detrital 
material from land, and areas where most of the sediment is organic or 
chemical in origin. 
    Coastal sediments include the deposits of deltas, lagoons, and bays, 
barrier islands and beaches, and the surf zone. The zone of coastal sediments 
is limited on the seaward side by the depth to which normal wave action can 
stir and transport sand, which depends on the exposure of the coast to waves 
and does not usually exceed 66 - 100 ft (20 - 30 m); the width of this zone is 
normally a few miles. The sediments in the coastal zone are usually 
land-derived. The material supplied by streams is sorted in the surf zone; the 
sand fraction is transported along the shore in the surf zone, often over long 
distances, while the silt and clay fractions are carried offshore into deeper 
water by currents. Consequently, the beaches and barrier islands are 
constructed by wave action mainly from material from fairly far away, although 
local erosion may make a contribution, while the lagoons and bays behind them 
receive their sediment from local rivers. 
    The types and patterns of distribution of the sediments are controlled by 
three factors and their interaction: (1) the rate of continental runoff and 
sediment supply; (2) the intensity and direction of marine transporting 
agents, such as waves, tidal currents, and wind; and (3) the rate and 
direction of sea level changes. The balance between these three determines the 
types of sediment to be found.  See  DELTA; ESTUARINE OCEANOGRAPHY. 
    On most continental shelves, equilibrium has not yet been fully 
established and the sediments reflect to a large extent the recent rise of sea 
level. Only on narrow shelves with active sedimentation are present 
environmental conditions alone responsible for the sediment distribution. 
Sediments of the continental shelf and slope belong to one or more of the 
following types: (1) biogenic (derived from organisms and consisting mostly of 
calcareous material); (2) authigenic (precipitated from sea water or formed by 
chemical replacement of other particles, for example, glauconite, salt, and 
phosphorite); (3) residual (locally weathered from underlying rocks); (4) 
relict (remnants of earlier environments of deposition, for example, deposits 
formed during the transgression leading to the present high sea level stand);  
(5) detrital (products of weathering and erosion of land, supplied by streams 
and coastal erosion, such as gravels, sand, silt, and clay). 
    Much of the fine-grained sediment transported into the sea by rivers is 
not permanently deposited on the self but kept in suspension by waves. This 
material is slowly carried across the shelf by currents and by gravity flow 
down its gentle slope, and is finally deposited either on the continental 
slope or in the deep sea. If submarine canyons occur in the area, they may 
intercept these clouds, or suspended material, channel them, and transport 
them far into the deep ocean as turbidity currents. If the canyons intersect 
the nearshore zone where sand is transported, they can carry this material 
also out into deep water over great distances.  See  CONTINENTAL MARGINS; REEF. 
 �34s[Tjeerd H. Van Andel]
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    Deep-sea sediments.  In general, classifications are difficult to apply 
because so many deep-sea sediments are widely ranging mixtures of two or more 
end-member sediment types. However, they can be divided into biogenic and 
nonbiogenic sediments. 
    Biogenic sediments, those formed from the skeletal remains of various 
kinds of marine organisms, may be distinguished according to the composition 
of the skeletal material principally either calcium carbonate or opaline 
silica. The most abundant contributors of calcium carbonate to the deep-sea 
sediments are the planktonic foraminiferids, coccolithoforids and pteropods. 
Organisms which extract silica from the sea water and whose hard parts 
eventually are added to the sediment are radiolaria, diatoms, and to a lesser 
degree, dilicoflagellates and sponges. The degree to which deep-sea sediments 
in any area are composed of one or more of these biogenic types depends on the 
organic productivity of the various organisms in the surface water, the degree 
to which the skeletal remains are redissolved by sea water while settling to 
the bottom, and the rate of sedimentation of other types of sediment material. 
Where sediments are composed largely of a single type of biogenic material, it 
is often referred to as an ooze, after its consistency in place on the ocean 
floor.  
    The nonbiogenic sediment constituents are principally silicate materials 
and, locally, certain oxides. These may be broadly divided into materials 
which originate on the continents and are transported to the deep sea 
(detrital constituents) and those which originate in place in the deep sea, 
either precipitating from solution (authigenic minerals) or forming from the 
alteration of volcanic or other materials. The coarser constituents of 
detrital sediments include quartz, feldspars, amphiboles, and a wide spectrum 
of other common rock-forming minerals. The finer-grained components also 
include some quartz and feldspars, but belong principally to a group of 
sheet-silicate minerals known as the clay minerals, the most common of which 
are illite, montmorillonite, kaolinite, and chlorite. The distributions of 
several of these clay minerals have yielded information about their origins on 
the continents and, in several cases, clues to their modes of transport to the 
oceans.  
 �34s[Pierre E. Biscaye]
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�6v�2v�32sMarl   �21m �33s 
    An argillaceous, nonindurated calcium carbonate deposit that is commonly 
gray or blue-gray. It is somewhat friable, and in some respects resembles 
chalk, with which it is interbedded in some localities. It is formed in some 
fresh-water lakes, partially by the action of some aquatic plants. The clay 
content of marls varies, and all gradations between small amounts of clay 
(marly limestones) and large amounts (marly clay) are found. Marlstone, or 
marlite, is an indurated rock of the same composition as marl. The marlstones 
are not fissile but blocky and massive with subconchoidal fracture.  See  
LIMESTONE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sMarmoset   �21m �33s 
    Any of 10 species of primate which are members of the family 
Callithricidae and occur in South America. They are the most primitive of all 
primates, having claws rather than nails, a nonprehensile tail, and litters of 
more than two. 
    The pygmy marmoset (Cebuella pygmaea), found in the forests of the Amazon, 
is the smallest of all living monkeys. In some species, such as the common 
marmoset (Callithrix jacchus), the male carries the young about and returns 
them to the female only at feeding time. The diet is varied and consists of 
insects, fruits, seeds, and small vertebrates.  See  PRIMATES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMarmot   �21m �33s 
    Several species of rodents of the genus Marmota which are members of the 
squirrel family, Sciuridae. These mammals are found in Europe, Asia, and North 
America.  
    One of the best-known marmots is the alpine marmot (Marmota marmota). It 
has a broad round head, a short furry tail, prominent eyes, and medium-sized 
ears; it is covered with thick, coarse, dull-colored fur. Other marmots are 
the red marmot (M. caudata) of Central Asia, the red-bellied marmot (M. 
flaviventris), ranging from New Mexico to British Columbia, and the woodchuck, 
or groundhog (M. monax), of North America. All species seem to prefer high 
altitudes except the woodchuck, which lives in woodlands and on farms. In many 
ways the marmots resemble the prairie dog.  See  PRAIRIE DOG; RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMars   �21m �33s 
    The planet fourth in distance from the Sun. It is visible to the naked eye 
as a bright red star, except for short periods near its conjunctions with the 
Sun. Having a mean (synodic) period between oppositions of 780 days, Mars is 
in opposition with the Sun every other year.  See  PLANET.  
    In a telescope, Mars usually appears as a bright reddish disk marked by 
complex, semipermanent dark regions and variable white polar caps. Scientists 
have long known of atmospheric phenomena, variable polar caps of ice and 
clouds, and large bright and dark regions on the surface with dimensions of 
several hundred kilometers or more. But a multitude of other small surface 
formations with characteristic scales less than 125 mi (200 km) manifest their 
existence by the optical illusion of ``canals'' and ``oases.'' 
  
    Surface.  The Martian surface, which initially resembled the heavily 
cratered lunar uplands, has been modified extensively by the processes of 
volcanism, faulting, and the effects of wind and water. The northern 
hemisphere tends to be characterized by large volcanic lava plains, whereas 
the southern hemisphere appears to be older and more heavily cratered. 
    Among the more impressive features on the Martian surface are several 
enormous shield volcanoes, larger than any mountains on Earth. The largest and 
perhaps the youngest is Olympus Mons, which is nearly 1375 mi (600 km) across 
at its base and stands approximately 16 mi (26 km) above the surrounding 
terrain. Three other large shield volcanoes, Ascraeus Mons, Pavonis Mons, and 
Arsia Mons, lie along the nearby Tharsis Ridge. In shape and structure the 
Martian shield volcanoes bear a strong resemblance to their Hawaiian 
counterparts. At the summit of each shield is a complex of calderas, collapsed 
craterlike features that were once vents for lava. 
  
    Perhaps the most spectacular features on the Martian surface are the huge 
canyons located primarily in the equatorial regions. Valles Marineris, 
actually a system of canyons, extends for over 3000 mi (5000 km) along the 
equatorial belt. (Fig. 1). In places, the canyon complex is as much as 300 mi 
(500 km) wide and drops to more than 4 mi (6 km) below the surrounding 
surface. Dwarfing the Grand Canyon, Valles Marineris is comparable in size to 
the great East African Rift Valley. 
    An astonishing class of features on the Martian surface is a widespread 
network of channels that bear a very strong resemblance to dry river beds 
(Fig. 2). Ranging from broad, sinuous features nearly 40 mi (60 km) wide, to 
small, narrow networks less than 300 ft (100 m) wide, the channels present the 
strong impression that they were created by water erosion. 
  
    The seasonal cycle of growth and decay of the bright polar caps has long 
been taken as evidence of the presence of water on Mars. However, evidence 
from the Mariner spacecraft identified carbon dioxide as the principal 
constituent of the polar snow, with lesser amounts of water ice also present. 
Formed during autumn and winter by condensation and deposition of the icy mist 
covering the polar regions, the polar caps at the end of winter cover a vast 
area extending down to latitude 60DG in the southern hemisphere and 70DG in 
the northern hemisphere. The residual caps, which never completely disappear, 
are believed to be composed almost entirely of water ice. 
  
    Atmosphere.  The Martian atmosphere is very thin. It is composed 
principally of carbon dioxide, but contains nitrogen, argon, oxygen, and a 
trace of water vapor totaling about 5%. Both carbon dioxide and ice form 
clouds in the Martian atmosphere. 
    Localized dust storms appear quite frequently on Mars. Because of the 
extreme thinness of the Martian atmosphere, wind velocities greater than 90 - 
110 mi/h (40 - 50 m/s) are needed to set surface dust grains in motion, 
although they can then be carried to high altitudes by winds of lesser 
velocity. Some dust storms develop with such intensity that their total extent 
may be hemispheric or even global. 
  
    Viking spacecraft.  In 1976 two American spacecraft - the Viking landers - 
successfully dropped onto the surface of Mars. The spacecraft collected more 
than 1400 photographs and large quantities of other data concerning the 
character, composition, and organic content of the surface, and the 
composition and climatic patterns of the lower atmosphere. Three biological 
investigations and a gas chromatograph/mass spectrometer attempted to settle 
the question of whether life existed on the planet. The latter instrument 
detected no organic compounds at all (within the limits of its sensitivity). 
One of the trio of biological experiments gave a positive result - the release 
of oxygen from a soil sample when humidified - but there is no consensus among 
the researchers as to the source of this reaction. 
  
    Satellites.  Mars has two small satellites, Phobos and Deimos. Neither 
satellite is massive enough to be gravitationally contracted to a spherical 
shape. Both satellites are saturated with impact craters. Both satellites have 
the same low albedo, approximately 0.05. This low reflectivity suggests that 
the Martian satellites may have had their origin in the asteroid belt.  See  
ASTEROID.  
 �34s[J. Kelly Beatty]
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�6v�2v�32sMarsileales   �21m �33s 
    A small order of heterosporous, leptosporangiate ferns (division 
Polypodiophyta) which grow in water or wet places and are rooted to the 
substrate. The leaves arise on long stalks from the rhizome and typically have 
floating blades with four leaflets, suggesting a four-leaved clover. The order 
contains a single family and only 3 genera, about 50 species in all, most of 
them belonging to the widespread genus Marsilea (water clover).  See  
POLYPODIOPHYTA.  
 �34s[Arthur Cronquist]
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�6v�2v�32sMarsupialia   �21m �33s 
    An order of animals, long considered the only order of the mammalian 
infraclass Metatheria. 
    The marsupials are characterized by the presence of a pouch (marsupium) in 
the female, a skin pocket whose teat-bearing abdominal wall is supported by 
epipubic bones. The young are born in the embryonic state and crawl unaided to 
the marsupium, where they attach themselves to the teats and continue their 
development. In a few species the pouch is vestigial or has disappeared 
completely.  
    A wide variety of terrestrial adaptations are found among the 82 genera of 
living marsupials. Some spectacular examples of evolution convergent upon 
placental modes of life are exemplified by the Australian marsupial ``moles'' 
(Notoryctes), ``wolves'' (Thylacinus), and ``flying squirrels'' (Petaurus). 
For the most part, the adaptive radiation of the marsupials has paralleled 
that of the placentals, yielding many ecological analogs, such as kangaroos 
and wallabies, which are the counterparts of the placental deer and antelopes, 
although these groups differ widely in structure.  See  ANTEATER; EUTHERIA; 
KANGAROO; KOALA; MAMMALIA; METATHERIA; OPOSSUM. 
 �34s[Richard H. Tedford]
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�6v�2v�32sMarten   �21m �33s 
    The name applied to seven species of carnivores which are members of the 
family Mustelidae. This family also includes the skunk, weasel, otter, badger, 
and wolverine. 
    The American marten (Martes americana) inhabits the cooler forests of 
North America. Its brown pelt, known as the American sable, is highly prized. 
It is small and has scent glands, as do most members of the family. Other 
species are the pine marten (M. martes), yellow-throated marten (M. 
flavigula), stone marten (M. foina), Japanese marten (M. melampus), and South 
Indian yellow-throated marten (M. gwatkinsi). These are all found in Asia; the 
pine and stone martens occur in Europe as well. 
    The adult weighs a maximum of 3 lb (1.4 kg), and for the size of the 
animal, the gestation period of about 38 weeks is unusually long. This is 
apparently because these species, as many other members of the family do, have 
delayed implantation of the fertilized egg; that is, after fertilization there 
is a lapse of several months before embryonic activation is initiated. Mating 
occurs in the summer and the litter of three or four is born the following 
spring.  See  BADGER; CARNIVORA; FISHER; MAMMALIA; OTTER; SABLE; SKUNK; 
WEASEL; WOLVERINE. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMaser �21m �33s
    A device for coherent amplification or generation of electromagnetic waves
by use of excitation energy in resonant atomic or molecular systems. The word
is an acronym for microwave amplification by stimulated emission of radiation.
The device uses an unstable ensemble of atomic or molecular particles which
may be stimulated by an electromagnetic wave to radiate excess energy at the
same frequency and phase as a stimulating wave, thus providing coherent
amplification. Masers, however, are not limited to the microwave region; this
type of amplification has been extended to include a frequency range from
audio to infrared or optical frequencies. Maser-type amplifiers and
oscillators are also sometimes referred to as molecular, or
quantum-mechanical, since they involve processes on a molecular scale, and
since some types cannot be adequately described by classical mechanics, but
show characteristic quantum-mechanical phenomena.
    Maser amplifiers can have exceptionally low noise, and come close to
effectively amplifying a single quantum of radiation in the microwave region.
Their inherently low noise makes maser oscillators that use very narrow atomic
or molecular resonances extremely monochromatic, providing a basis for
frequency standards.  See  ATOMIC CLOCK.
    Because of their low noise and high sensitivity, maser amplifiers are
particularly useful for reception and detection of very weak signals in radio
astronomy, microwave radiometry, long-distance radar, and long-distance
microwave communications. They also provide research tools for very sensitive
amplification or detection of electromagnetic radiation.
    Thermodynamic equilibrium of an ensemble of particles - such as atoms,
molecules, electrons, or nuclei - which have discrete energy levels and which
may radiate electromagnetic energy, requires that the number n1 of particles in
a lower level 1 be related to the number n2 in an upper level 2 by the Boltzmann
distribution condition n(sub 1)/n(sub 2) = e[sup (E1-E2)kT], where E(sub 1) and
E(sub 2) are the respective energies of the two levels; k is Boltzmann's
constant; and T is the absolute temperature, a positive number.  Thermodynamic
equilibrium also requires that phases of oscillation of the particles, or
relative phases of quantum-mechanical wave functions for the various states, be
random.  A violation of either condition can result in instabilities which may
release electromagnetic radiation.  The frequency v of radiation released is
characteristically given by hv = E(sub 2) - E(sub 1), where h is Planck's
constant.  See BOLTZMANN STATISTICS.
    The particles may be stimulated by an electromagnetic wave to make
transitions from the lower to the upper level, absorbing energy from the wave,
or from the upper to the lower level, imparting energy to the wave, and
thereby increasing the wave amplitude coherently. Stimulated transitions from
the upper to the lower state and those from the lower to the upper state are
equally probable. For equilibrium at any positive temperature, the Boltzmann
distribution requires that n1 be greater than n2. Therefore, there is a net
absorption of energy from the wave, because particles which absorb are more
numerous than those which emit. If the condition n(sub 2) > n(sub 1) occurs,
the system may be said to have a negative absolute temperature, because the
Boltzmann condition is fitted only by a negative value of T. If there are not
too many counterbalancing losses from other sources, this condition allows a
net amplification, because particles which emit energy are more numerous than
those which absorb. 

    Gas masers.  The beam-type maser (see illustration) was the first type of
maser to be suggested. Ammonia gas issues from a small orifice into a vacuum
system to form a molecular beam. Molecules in the lower of the two states are
deflected away from the axis of the state sorter or focuser by inhomogeneous
electric fields which act on their dipole moments. Those molecules in the
upper state are deflected toward the axis and sent into the microwave-resonant
cavity. If losses in the cavity walls and coupling holes are sufficiently
small, or if the number of molecules is sufficiently large, amplification or
oscillation will occur. This maser is particularly useful as a frequency or
time standard because of the relative sharpness and invariance of resonances
of the ammonia beam.


    Solid-state masers. Solid-state masers usually involve the electrons of
paramagnetic atoms or molecules in a static or slowly varying magnetic field.
In the simplest case, the two-level solid-state maser, only one electron on
each molecule is affected. The energy of the electron is quantized into two
levels, according to whether the magnetic moment, associated with the electron
spin, is parallel or antiparallel to the magnetic field. At thermal
equilibrium there are more magnetic moments parallel than antiparallel to the
field. This situation may be reversed by interchanging the two populations of
particles. See ELECTRON SPIN.

    Optical and infrared masers. Optical and infrared masers utilize a variety
of principles for excitation of the nonequilibrium distribution. All involve
multimode cavities, usually consisting of two optically flat plates between
which the radiation is reflected many times through the excited medium. Such
masers are particularly valuable as oscillators, producing coherent light that
is extremely monochromatic and can be focused either to a very intense small
spot or radiated in a remarkably parallel beam. See LASER

[Charles H. Townes / J. P. Gordon]
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�6v�2v�32sMasking of sound   �21m �33s 
    Interference with the audibility of a sound caused by the presence of 
another sound. More specifically, the number of decibels (dB) by which the 
intensity level of a sound (signal) must be raised above its threshold of 
audibility, to be heard in the presence of a second sound (masker), is called 
the masking produced by the masker on the signal. The masker and the signal 
may be identical or may differ in frequency, complexity, or time. 
    Four major efforts are to be noted when both the masker and signal are 
pure tones and are presented simultaneously. First, the higher the level of 
the masker, the greater the masking. Second, masking is greatest when the 
frequency of the masker is in the vicinity of that of the signal. Third, the 
masking caused by a tone is much greater on frequencies higher than that of 
the masker than on frequencies below it. Fourth, in addition to the two tones 
themselves, other tones are heard which do not exist except in the listener's 
hearing. These tones are caused by nonlinear effects in the hearing mechanism. 
    The most widely studied complex masking sound is random noise which has 
energy at all frequencies and is said to be flat if the level for each 1-Hz 
bandwidth of the noise is the same. When random-flat noise is used to mask a 
pure tone, only a narrow frequency band (critical band) of the noise centered 
at the tonal frequency causes masking. When noise masks speech, either the 
detectability of speech or speech intelligibility can be measured. The level 
for speech intelligibility is about 10 - 14 dB higher than for speech 
detectability.  See  ACOUSTIC NOISE; HEARING (HUMAN); SOUND. 
 �34s[W. A. Yost]
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�6v�2v�32sMasonry   �21m �33s 
    A built-up construction or combination of building units of such materials 
as clay, shale, concrete, glass, gypsum, or stone, set in mortar or plain 
concrete. Common usage of the term masonry implies the use of cut stone set in 
mortar and patterned conventionally. In modern engineering practice, masonry 
includes unit construction not only of natural stone but also of brick and 
tile - burned-clay manufactured products. Plant-made bricks and blocks of 
gypsum, glass, concrete and sand-lime mixes are also regularly used in unit 
masonry construction. 
    Unit masonry may be plain (unreinforced) or reinforced by the use of steel 
mesh or small bars placed in the mortar beds as soon as the mortar is in 
place, or through vertical cavities in bricks or tile, in accordance with 
recognized design procedures. Such reinforcement gives to unit masonry 
increased resistance to tensile and shear forces, thus enhancing its value in 
resisting lateral forces such as those from seismic action. Multistory 
buildings of bearing-wall masonry construction are found throughout the world. 
 �34s[Robert F. Legget]
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�6v�2v�32sMass �21m �33s
    The quantitative or numerical measure of a body's inertia, that is, of its
resistance to being accelerated.
    Because it is often necessary to compare masses of such dissimilar bodies
as a sample of sugar, a sample of air, an electron, and the Moon, it is
necessary to define mass in terms of a property that not only is inherent and
permanent but is also universal in that it is possessed by all known forms of
matter. All matter possesses two properties, gravitation and inertia. The
property of gravitation is that every material body attracts every other
material body. The property of inertia is that every material body resists any
attempt to change its motion. A body's motion is said to change if the body is
accelerated, that is, if it increases or decreases its speed or changes the
direction of its motion. Because of its inertia a body cannot be accelerated
unless a force is exerted on it. The greater the inertia of a body, the less
will be the acceleration produced by a given force.  See  GRAVITATION; INERTIA.
    The present definition of mass is in terms of inertia. The masses of two
bodies are compared by applying equal forces to the bodies and measuring their
accelerations. For example, the two bodies may be allowed to collide.
According to Newton's third law, each body will then experience an equally
strong force. If there are no external forces, and if a1 and a2 are the
measured accelerations of the two bodies, the ratio of the masses of the two
bodies is by definition given by the following equation.

		   m (sub 1) / m (sub 2) = a (sub 2) / a (sub 1)


    This equation gives only ratios of masses; it is therefore necessary to
designate the mass of some one body as the standard mass to which the masses
of all other bodies can be compared. The body that has been chosen for this
purpose is a cylinder of platinum-iridium alloy. It is known as the
international standard of mass; its mass is called 1 kilogram (kg), and it is
kept at the International Bureau of Weights and Measures near Paris, France.
Replicas of the standard mass, kept at various national laboratories, are
periodically compared with this standard.
    Einstein's special theory of relativity predicts that the inertia of a
body should increase if the energy of the body increases. This prediction has
been conclusively verified experimentally. It follows that the mass of a body
will increase if, for example, the body gains speed (addition of kinetic
energy), or its temperature rises (addition of heat energy), or the body is
compressed (addition of elastic energy).  See  CONSERVATION OF MASS; INERTIA
OF ENERGY.
 �34s[Leo Nedelsky]
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�6v�2v�32sMass defect   �21m �33s 
    The difference between the mass of an atom and the sum of the masses of 
its individual components in the free (unbound) state. The mass of an atom is 
always less than the total mass of its constituent particles; this means, 
according to Albert Einstein's well-known formula, that an energy of E = 
mcsup2 has been released in the process of combination, where m is the 
difference between the total mass of the constituent particles and the mass of 
the atom, and c is the velocity of light.  See  INERTIA OF ENERGY. 
    The mass defect, when expressed in energy units, is called the binding 
energy, a term which is perhaps more commonly used.  See  NUCLEAR BINDING 
ENERGY.  
 �34s[William W. Watson]
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�6v�2v�32sMass flow rate meter �21m �33s
    An instrument that measures the mass of fluid flowing through a pipe,
duct, or conduit, per unit time. There have been many attempts to develop
practical industrial mass flowmeters. The most consistently satisfactory
method of determining mass flow has been through calculation using volume flow
and density determined either by measurement or inference from pressure and
temperatures. Only a few of the mechanical types will be mentioned here.
    In one type, the fluid in the pipe is made to rotate at a constant speed
by a motor-driven impeller. The torque required by a second, stationary
impeller to straighten the flow again is a direct measurement of the mass flow.
    A second type, described as gyroscopic/Coriolis mass flowmeter, employs a
C-shaped pipe and a T-shaped leaf spring as opposite legs of a tuning fork. An
electromagnetic forcer excites the tuning fork, thereby subjecting each moving
particle within the pipe to a Coriolis-type acceleration. The resulting forces
angularly deflect the C-shaped pipe an amount that is inversely proportional
to the stiffness of the pipe and proportional to the mass flow rate within the
pipe. The angular deflection (twisting) of the C-shaped pipe is optically
measured twice during each cycle of the tuning-fork oscillation (oscillating
at natural frequency). A digital logic circuit converts the timing of the
signals into a mass-flow-rate signal.  See  CORIOLIS ACCELERATION AND FORCE.
    Another type is based on the principle of the gyroscope (see
illustration). The flowing stream is sent through a pipe of suitable shape so
that the mass of material flowing corresponds to a gyro wheel rotating about
axis C. The entire gyrolike assembly is rotating about axis A, and a torque is
produced about axis  B	proportional to the angular momentum of the simulated
gyro wheel. This torque is therefore directly proportional to the mass rate of
flow. The pretzellike configuration of pipe is introduced to eliminate
centrifugal and other extraneous effects.  See	FLOW MEASUREMENT; GYROSCOPE.
 �34s[Mead Bradner ;Lewis P. Emerson]
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�6v�2v�32sMassif   �21m �33s 
    A block of the Earth's crust commonly consisting of crystalline gneisses 
and schists, the textural appearance of which is generally markedly different 
from that of the surrounding rocks. Common usage indicates that a massif has 
limited areal extent and considerable topographic relief. Structurally, a 
massif may form the core of an anticline or may be a block bounded by faults 
or even unconformities. In any case, during the final stages of its 
development a massif acts as a relatively homogeneous tectonic unit which to 
some extent controls the structures that surround it. Numerous complex 
internal structures may be present; many of these are not related to its 
development as a massif but are the mark of previous deformations. 
 �34s[Philip H. Osberg]
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�6v�2v�32sMass-luminosity relation   �21m �33s 
    The relation, observed or predicted by theory, between the quantity of 
matter a star contains (its mass) and the amount of energy generated in its 
interior (its luminosity). Because of the great sensitivity of the rate of 
energy production in a stellar interior to the mass of the star, the 
mass-luminosity relation provides an important test of theories of stellar 
interiors.  
    For a family of stars with different masses but with the same mixture of 
chemical elements uniformly distributed throughout the stellar volumes, there 
will be a unique mass-luminosity relation. Since most of the stars in the 
solar neighborhood have about the same chemical composition, the observed 
relation, obtained from binary stars for which masses and luminosities can be 
observationally evaluated, conforms reasonably well with theory.  See  BINARY 
STAR; STAR. 
 �34s[Daniel M. Popper]
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�6v�2v�32sMass number �21m �33s
    The mass number A of an atom is the total number of its nuclear
constituents, or nucleons, as the protons and neutrons are collectively
called. The mass number is placed before and above the elemental symbol, thus
(sup 238)U. The mass number gives a useful rough figure for the atomic
mass; for example, (sup 1)H = 1.00814 atomic mass units (amu), (sup 238)U =
238.124 amu, and so on.  See  ATOMIC NUMBER.
 �34s[Henry E. Duckworth]
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�6v�2v�32sMass production   �21m �33s 
    The manufacture of products in large quantities. The products may be 
machine-made individual parts, assembled items, or process items such as 
chemicals, coal, oil, cement, or cereals. 
    Eli Whitney, one of the foremost inventors of the early 1800s, is credited 
with being the father of mass production through his development of the 
concept of interchangeable manufacture. Whitney invented machines, tools, 
gages, jigs, and fixtures so that all parts could be made to close dimensional 
tolerances. Today it is difficult to realize that until about 1920 if anyone 
broke an automobile axle there was usually a wait of at least a week for the 
new one to be sent out from the factory. 
    In order for an item to warrant mass production, there must be a large, 
stable market to justify the type of specialized equipment required for 
economical production. Even this concept has evolved to what is called 
flexible manufacturing, which involves control of many operations, whether 
machining or assembly, by computers, programmable controllers, or numerical 
control systems. Robots can be quite easily ``trained'' to change from the 
requirements of one design to another.  See  INDUSTRIAL ROBOTS. 
    The effects of mass production are not all technical. Automation, by 
lowering the unit cost of a product enables more people to buy the product, 
creates a need for new maintenance services, calls for more raw materials, and 
increases shipping and handling jobs both to and from the factory, with 
consequent overall increases in employment.  See  OPERATIONS RESEARCH; 
PRODUCTION ENGINEERING; ROBOTICS. 
 �34s[Roy A. Lindberg]
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�6v�2v�32sMass spectrometry �21m �33s
    An analytical technique for identification of chemical structures,
determination of mixtures, and quantitative elemental analysis, based on
application of the mass spectrometer. Organic and inorganic molecular
structure determination is based on the fragmentation pattern of the ion
formed when the molecule is ionized; further, because such patterns are
distinctive, reproducible, and additive, mixtures of known compounds may be
quantitatively analyzed. Quantitative elemental analysis of organic compounds
requires exact mass values from a high-resolution mass spectrometer; trace
analysis of inorganic solids requires a measure of ion intensity as well.  See
  MASS SPECTROSCOPE.

    Organic compounds.	For analysis of organic compounds the principal
methods of ion production are electron impact, chemical ionization, field
ionization, and field desorption.
    In electron impact, a gaseous sample of a molecular compound is ionized
with a beam of energetic electrons and part of the energy is transferred to
the ion formed by the collision: For most molecules the production of cations
is favored over the production of anions by a factor of about 10 (sup 4). The
cation corresponding to the simple removal of the electron is commonly called
the molecular ion and normally will be the ion of greatest m/e ratio in the
spectrum. In the ratio m is the mass of the ion in atomic mass units and e is
the charge of the ion measured in terms of the number of electrons removed (or
added) during ionization. Occasionally the ion is of vanishing intensity, and
sometimes it collides with another molecule to abstract a hydrogen or another
group. In these cases an incorrect assignment of the molecular ion may be made
unless further tests are applied. Proper identification gives the molecular
weight of the sample. The remaining techniques were devised generally to
circumvent the problem of the weak or vanishingly small intensity of a
molecular ion. In chemical ionization, the ions to be analyzed are
produced by transfer of a heavy particle (Hsupp, H-, or heavier) to the sample
from ions produced from a reactant gas. Field ionization and field desorption
techniques are used for less volatile material. The sample is ionized when it
is in a very high field gradient (several volts per angstrom) near an
electrode surface. In field ionization, the sample is admitted as a gas and
ionized within a few angstroms of the surface. In field desorption, the sample
is coated on the metal surface.  See  IONIZATION POTENTIAL; LASER
SPECTROSCOPY; PLASMA CHROMATOGRAPHY.

	       A-B-C + e -->  A-B-C (sup +) + e + e

    Mixtures.  For a given instrument, the mass spectrum of a compound serves
as a fingerprint of a compound under standard conditions; it varies with
temperature, ionizing voltage, and parameters of the construction of the
instrument. A mixture of gases may be analyzed because spectra are additive.
    A further powerful technique for analysis of mixtures containing unknowns
is mass spectral analysis of gas chromatographic effluents either by direct
hookup of the gases exiting from the chromatograph, including carrier gas, to
the introduction system of the spectrometer or by analysis of trapped effluent
fractions.
    Similarly, mass spectrometers may be coupled to liquid chromatographs; the
solvent may be removed by evaporation of the solution on a moving belt which
next passes into the ion source and is then heated to evaporate the sample, or
by freezing solvent on liquid-nitrogen-cooled fingers surrounding the source;
or the solvent may be used itself as a reagent gas for chemical ionization.
Ion cyclotron resonance spectroscopy has become of special importance because
of the development of Fourier-transform ion cyclotron resonance techniques.
See  GAS CHROMATOGRAPHY; LIQUID CHROMATOGRAPHY.

    Inorganic solids.  The analysis of solid inorganic samples can be made
either by vaporization at very high temperatures or by volatilization of the
sample surface so that particles are atomized and ionized with a high-energy
spark. The wide range of energies given to the particles requires a
double-focusing mass spectrometer for analysis.
    Secondary ion mass spectrometry is most commonly used for surface
analysis. A primary beam of ions accelerated through a few kilovolts is
focused on a surface; ions are among the products sputtered from this surface,
and they may be directly analyzed in a quadrupole filter.  See	SECONDARY
ION-MASS SPECTROMETRY (SIMS); SPECTROSCOPY; TRACE ANALYSIS.
 �34s[Maurice M. Bursey]
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�6v�2v�32sMass spectroscope   �21m �33s 
    An instrument used for determining the masses of atoms or molecules found 
in a sample of gas, liquid, or solid. It is analogous to the optical 
spectroscope, in which a beam of light containing various colors (white light) 
is sent through a prism to separate it into the spectrum of colors present. In 
a mass spectroscope, a beam of ions (electrically charged atoms or molecules) 
is sent through a combination of electric and magnetic fields so arranged that 
a mass spectrum is produced (see illustration). If the ions fall on a 
photographic plate which after development shows the mass spectrum, the 
instrument is called a mass spectrograph; if the spectrum is allowed to sweep 
across a slit in front of an electrical detector which records the current, it 
is called a mass spectrometer. 
    Mass spectroscopes are used in both pure and applied science. Atomic 
masses can be measured very precisely. Because of the equivalence of mass and 
energy, knowledge of nuclear structure and binding energy of nuclei is thus 
gained. The relative abundances of the isotopes in naturally occurring or 
artificially produced elements can be determined. 
    Empirical and theoretical studies have led to an understanding of the 
relation between molecular structure and the relative abundances of the 
fragments observed when a complex molecule, such as a heavy organic compound, 
is ionized. When a high-resolution instrument is employed, the masses of the 
molecular or fragment ions can be determined so accurately that identification 
of the ion can frequently be made from the mass alone.  See  MASS SPECTROMETRY. 
    Because chemical compounds may have mass spectra as unique as 
fingerprints, mass spectroscopes are widely used in industries such as oil 
refineries, where analyses of complex hydrocarbon mixtures are required.  See  
ATOMIC WEIGHT; BETA RAYS. 
 �34s[Alfred O. Nier]
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�6v�2v�32sMass-transfer operation   �21m �33s 
    An operation in chemical engineering that involves transfer of material 
from one phase to another, or from one place to another within a single phase. 
Mass transfer may occur for various purposes, for example, to effect a 
chemical reaction, to obtain a separation of components, or to obtain uniform 
distribution of material within a phase. The most common types are (1) 
fluid-fluid transfer, occurring typically in absorption, distillation, and 
liquid-liquid extraction, and (2) fluid-solid transfer, occurring typically in 
crystallization and gas adsorption.  See  ADSORPTION; COUNTERCURRENT TRANSFER 
OPERATIONS; CRYSTALLIZATION; DIALYSIS; DIFFUSION IN GASES AND LIQUIDS; 
DISTILLATION; GAS ABSORPTION OPERATIONS; ION EXCHANGE; LEACHING; SOLVENT 
EXTRACTION.  
 �34s[Charles R. Wilke]
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�6v�2v�32sMass wasting   �21m �33s 
    A generic term for downslope movement of soil and rock, primarily in 
response to gravitational body forces. Mass wasting is distinct from other 
erosive processes in which particles or fragments are carried down by the 
internal energy of wind, running water, or moving ice and snow. 
    The stability of slope-making materials is lost when their shear strength 
(or sometimes their tensile strength) is overcome by shear (or tensile) 
stresses, or when individual particles, fragments, and blocks are induced to 
topple or tumble. The shear and tensile strength of earth materials depends on 
their mineralogy and structure. Processes that generally decrease the strength 
of earth materials include one or more of the following: structural changes, 
weathering, groundwater, and meteorological changes. Stresses in slopes are 
increased by steepening, heightening, and external loading due to static and 
dynamic forces. Processes that increase stresses can be natural or result from 
human activities. Although other classifications exist, these movements can be 
conveniently classified according to their velocity into two types: creep and 
landsliding.  See  SOIL MECHANICS. 
    Geologically, creep is the imperceptible downslope movement at rates as 
slow as a fraction of millimeter per year; its cumulative effects are 
ubiquitously expressed in slopes as the downhill bending of bedded and 
foliated rock, bent tree trunks, broken retaining walls, and tilted 
structures. There are two varieties of geologic creep. Seasonal creep is the 
slow, episodic movement of the uppermost several centimeters of soil, or 
fractured and weathered rock. It is especially important in regions of 
permanently frozen ground. Rheologic creep, sometimes called continuous creep, 
is a time-dependent deformation at relatively constant shear stresses of 
masses of rock, soil, ice, and snow. This type of creep affects rock slopes 
down to depths of a few hundred meters, as well as the surficial layer 
disturbed by seasonal creep. Continuous creep is most conspicuous in weak 
rocks and in regions where high horizontal stresses (several tens of bars or 
several megapascals) are known to exist in rock masses at depths of 330 to 660 
ft (100 to 200 m). 
    Landsliding includes all perceptible mass movements. Three types are 
generally recognized on the basis of the type of movement: falls, slides, and 
flows. Falls involve free-falling material; in slides the moving mass 
displaces along one or more narrow shear zones; and in flows the distribution 
of velocities within the moving mass resembles that of a viscous flow.  See  
LANDSLIDE.  
    Mass wasting is an important consideration in the interaction between 
humans and the environment. Deforestation accelerates soil creep. Engineering 
activities such as damming and open-pit mining are known to increase 
landsliding. On the other hand, enormous natural rock avalanches have buried 
entire villages and claimed tens of thousands of lives. 
 �34s[Alberto S. Nieto]
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�6v�2v�32sMastigomycotina   �21m �33s 
    Zoosporic fungi; an artificial grouping of fungi with all members, except 
a few advanced ones, producing free-living zoospores in some part of their 
life cycle. The thallus may be unicellular, multicellular, or mycelial with 
many reproductive bodies. The thallus has a chitinaceous or cellulosic wall. 
In a few endoparasitic species the thallus is naked (lacks a wall). Forms of 
sexual reproduction are many and varied. 
    The three classes are distinguished by the type of flagellation: single 
whiplash (Chytridiomycetes), single tinsel (Hyphochytridiomycetes), or 
biflagellate (Oomycetes). 
    Distribution is cosmopolitan; many are aquatic, occurring in fresh water 
or brackish water, and a few are marine. They are also abundant in soils, and 
some have part of their life cycles on vascular plants. Many are saprobes, but 
some are parasitic on insects or fish, and some are destructive plant 
pathogens.  
 �34s[Donald J. S. Barr]
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�6v�2v�32sMastigophora   �21m �33s 
    A superclass of the Protozoa also known as the Flagellata. The common 
morphological flagellate type is spherical to cylindrical on an 
anteroposterior axis. Despite their common morphological plan, based on the 
flagellum as a means of locomotion, Flagellata are very diverse in shape, 
colony formation, internal structure, external shell or test, color, 
physiology, reproduction, and choice of environment. 
    The Mastigophora exhibit marked plantlike characteristics, so that texts 
and references in botany always treat at least some of them. Some workers 
include all colorless flagellates in the algae; however, flagellates display 
many distinctly animal features and sometimes are treated as Protozoa. Thus, 
flagellates are regarded as a link between the plant and animal kingdoms.  See 
  PHYTAMASTIGOPHOREA; ZOOMASTIGOPHOREA. 
    Mastigophora possess one common feature, locomotion by means of one or 
more whiplike protoplasmic extrusions termed flagella. The flagellate body is 
typically monaxial and elongate to some degree. Cells are practically 
spherical in Monas, but ovoid, cordiform, pyriform, fusiform, acicular, 
tubular, or flattened cells are more common. Flattened species typically glide 
along a surface rather than swim. A particular shape is normally maintained by 
Ochromonas, whereas Mastigamoeba and some others form pseudopodia, and certain 
species (Euglena agilis) undergo frequent distortions termed euglenoid or 
metabolic changes of shape. 
    Structurally the flagellate cell is not simple. The cytoplasm is sometimes 
quite vacuolated (Collodictyon or Trepomonas) and occasionally colored, 
although color is generally confined to chromatophores when these are present. 
One or more contractile vacuoles may be located near the flagellar base. 
    Most flagellates are single-celled but colony formation is frequent. Cells 
may be naked (Oicomonas), enclosed in a thick cellulose cell wall or pellicle 
(Euglena spirogyra), or in a chitinous, calcareous, or silicious test, the 
lorica or shell (Trachelomonas, Distephanus speculum). 
    Flagellates adjust to wide ranges of pH, osmotic pressure, light, and 
temperature but optima are found upon investigation. Some flagellate species 
occur in both fresh and sea water, but cannot be transferred directly from 
fresh to sea water. Almost any aqueous ecological niche contains Flagellata. 
    Flagellata are near the base of the food pyramid, probably next to 
bacteria. Their varied synthetic abilities, fast reproductive rate, and huge 
numbers in both fresh and salt water compensate for their usual small size. 
The oceanic blooms which kill fish and other animals are principally 
flagellates, but green flagellates reaerate polluted water. They often cause 
tastes and odors in potable water, but they readily attack organic matter in 
natural water as do bacteria. They include many parasites dangerous to humans 
and animals and are themselves frequently parasitized.  See  PROTOZOA. 
 �34s[James B. Lackey]
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�6v�2v�32sMatch   �21m �33s 
    A short piece of wood or other material at the end of which has been 
placed a substance that can be ignited by friction. Matches are of two types, 
strike-anywhere and safety. The strike-anywhere match, also called the kitchen 
match, contains potassium chlorate (KClO3) and phosphorus sesquisulfide (P4S3) 
as the main ingredients, with added quantities of iron oxide, zinc oxide, and 
glue. In addition, it includes ground glass to create friction. Safety matches 
involve two separate compositions, the first coated on the matchstick and the 
second on the side of the container. The first contains potassium chlorate, 
potassium dichromate, manganese dioxide, sulfur, iron oxide, ground glass, and 
glue. The second contains red phosphorus, antimony sulfide, and an abrasive. 
The heat produced by the ignition of a minute amount of the red phosphorus 
when the match is struck is sufficient to ignite the match head. The wood or 
cardboard of the matchstick is coated with paraffin to give a longer and more 
ready flame and is impregnated with borax to prevent afterglow.  See  
COMBUSTION; FLAME; PHOSPHORUS; PYROTECHNICS. 
 �34s[William E. Gordon]
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�6v�2v�32sMaterials handling   �21m �33s 
    The loading, moving, and unloading of materials. The hundreds of different 
ways of handling materials are generally classified according to the type of 
equipment used. For example, the International Materials Management Society 
has classified equipment as (1) conveyor, (2) cranes, elevators, and hoists, 
(3) positioning, weighing, and control equipment, (4) industrial vehicles, (5) 
motor vehicles, (6) railroad cars, (7) marine carriers, (8) aircraft, and (9) 
containers and supports.  See  MATERIALS-HANDLING MACHINES. 
 �34s[Richard Muther]
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�6v�2v�32sMaterials-handling machines   �21m �33s 
    Devices used for handling materials in an industrial distribution 
activity. The equipment moves products as discrete articles, in suitable 
containers, or as solid bulk materials which are relatively free-flowing. 
    Many different types of machines result from combinations and permutations 
of the following factors: (1) The route over which the product is moved may be 
fixed or variable; (2) the path of travel may be horizontal, inclined, 
declined, or vertical; (3) motion may be imparted to the product manually, by 
the force of gravity, by air pressure, by vacuum, by vibration, or by 
power-actuated components of the machine; (4) the motion may be continuous or 
intermittent (reciprocating); and (5) the product may be supported or carried 
suspended during the handling operation. Based upon their most common 
characteristics, materials-handling machines can be grouped into six broad 
categories, listed in the following cross references.  See  BULK-HANDLING 
MACHINES; CONVEYING MACHINES; ELEVATING MACHINES; HOISTING MACHINES; 
INDUSTRIAL TRUCKS; MONORAIL. 
 �34s[Arthur M. Perrin]
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�6v�2v�32sMathematical biology   �21m �33s 
    In its broadest sense, mathematical biology encompasses all applications 
of mathematics, computer technology, and quantitative theorizing to biological 
systems and the underlying processes within them. 
    Mathematical biology arose in the application of mathematical equations to 
biological situations involving chemical or physical processes already 
understood quantitatively by the methods of classical physics and chemistry. 
An example is the process of diffusion, by which oxygen enters the bloodstream 
from the lungs and subsequently passes into the tissues to be utilized in 
energy production, a fundamental phenomenon found in all living systems. An 
understanding of all biological processes in which diffusion plays a role is 
substantially improved by the fact that mathematical equations describing 
diffusion have been available to physicists and chemists for some time and can 
be adapted to the particular circumstances involved in biology. 
    As a result of the extensive amount of research, mathematical models of 
biological systems have become prominent and sometimes even decisive tools in 
the development of understanding of living systems. These models range in 
scope from the submicroscopic domain of intracellular biochemical reactions 
and metabolism to the treatment of the problems of population growth; they 
range in interest from highly abstract problems arising from control and 
communication processes in the brain to more immediate practical problems such 
as computer diagnosis of human diseases. In a disciplinary sense they range 
through anatomy, physiology, pharmacology, psychology, clinical medicine, 
genetics, taxonomy, botany, and a variety of other areas. A large number of 
mathematical models of the heart, brain, kidney, lungs, bloodstream, bone, and 
endocrine and muscular systems have been developed.  See  BIOELECTRIC MODEL. 
    Thus, with the computer the ability of the mathematician to treat 
component atomic processes quantitatively can be extended to the construction 
of very complex systems of these component processes. With computers, in other 
words, the process of reductionism, whereby biological systems are reduced to 
simpler physical and chemical processes, has been reversed, and biological 
phenomena can be treated at an appropriate level of complexity. 
 �34s[George P. Moore]
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�6v�2v�32sMathematical geography �21m �33s
    The branch of physical geography that deals with attributes of the Earth,
such as its size, shape, and movements, and with those relations to the
heavenly bodies, especially the Sun, that affect the features of the face of
the Earth.
    The Earth is a nearly spherical body, 24,901.57 mi (40,075.19 km) in
circumference, rotating on an axis, and revolving in an orbit around the Sun.
The equatorial diameter is 7,926.41 mi (12,756.33 km) and the polar diameter
is 7,899.84 mi (12,713.56 km). Its shape may be better classed as an oblate
spheroid.  See	EARTH; GEODESY.
    Rotation and revolution are the two major motions of the Earth. The Earth
rotates on an axis, the ends of which are the poles, making one complete
rotation in 24 h, with a speed at the Equator of about 1000 mi/h (464 m/s).
The rotation is from west to east (in Earth-bound terms), making night and
day, and the impression that heavenly bodies rise in the east and set in the
west.
    The rotating Earth revolves in an orbit around the Sun, making one
complete revolution in 365 days, 5 h, 48 min, 45.15 s, or about 365 1/4 days.
See  INTERNATIONAL DATE LINE; TIME.
    The positions and changes of the Earth's axis are most important factors
in Earth-Sun relationships. As the Earth moves through its orbit, the axis of
rotation is constantly inclined at about 66 1/2 DG (66DG 55') from the plane of
the ecliptic, 23 1/2 DG (23 DG 45') from a line perpendicular to the ecliptic.
In addition to	maintaining its inclination with the ecliptic, the polar axis
of rotation always points to the same spot in the heavens throughout its
yearly revolution around the Sun.  See	PRECESSION OF EQUINOXES.
    From June 21, when the Sun's rays are vertical at the Tropic of Cancer
(summer solstice), summer continues in the Northern Hemisphere until September
23, when the Sun's rays strike directly over the Equator (autumnal equinox).
Fall begins with the Sun overhead at the Equator and continues until December
22 (winter solstice), when the Sun has reached its southern zenithal position
at the Tropic of Capricorn, 23JDG south. Winter finds the overhead Sun's rays
moving back toward the Equator, which is attained on March 21 (vernal
equinox). Spring is that period when the Sun is again overhead north of the
Equator, from March 21 to June 21.  See  EQUINOX; NORTH POLE; SOLSTICE; SOUTH
POLE.
    Although less important to humans than Earth-Sun relationships, the Moon
plays a significant role as the major force in the production and timing of
tides. The tide is due essentially to the gravitational pull of the Moon; the
greatest force will be on the Earth right under the Moon and at a directly
opposite point on the other side of the Earth. The Sun also exerts
gravitational pull on the Earth's waters and produces tidal effects, but these
are much less marked than the lunar tides.  See  TIDE.
 �34s[Van H. English]
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�6v�2v�32sMathematical physics   �21m �33s 
    The area of physics aimed at deducing the consequences of the more 
established physical theories by relying mainly on the method of mathematical 
solution, presuming that the basic laws of physics are known. A fruitful 
approach is possible largely because there is close analogy between the 
mathematical problems arising in different fields of theoretical physics. The 
same partial differential equations are encountered in many different contexts. 
    Through World War II, the main technique of mathematical physics was the 
analytical mathematical solution of problems. Since World War II, high-speed 
computing machines have become increasingly important and have made numerical 
solutions possible for many problems where the analytical technique did not 
work.  
    The term mathematical physics is sometimes used synonymously with 
theoretical physics.  See  THEORETICAL PHYSICS. 
 �34s[Hans A. Bethe]
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�6v�2v�32sMathematics   �21m �33s 
    Mathematics is frequently encountered in association and interaction with 
astronomy, physics, and other branches of natural science, and it also has 
deep-rooted affinities to the humanities. It is a realm of knowledge entirely 
unto itself, and one of considerable scope. 
  
    Relation to science.  Mathematics is not a branch of natural science 
itself. It does not deal with phenomena and objects of the external world and 
their relations to each other but, strictly speaking, only with objects and 
relations of its own imagery. One can practice meaningful mathematics without 
being concerned with science at all, and philosophical attempts to reduce all 
origin of mathematics to utilitarian motives are wholly unconvincing. However, 
mathematics is the language of science in a deep sense. Mathematics is an 
indispensable medium by which and within which science expresses, formulates, 
continues, and communicates itself. And just as the language of true literacy 
not only specifies and expresses thoughts and processes of thinking but also 
creates them in turn, so does mathematics not only specify, clarify, and make 
rigorously workable concepts and laws of science, but also at certain crucial 
instances becomes an indispensable constituent of their creation and emergence 
as well. 
  
    Creative formulas.  A formula is a string of mathematical symbols subject 
only to certain general rules of composition. To a working mathematician a 
string of symbols is a formula if it is something worth remembering. Much 
mathematics is concentrated in and propelled by certain formulas of unusual 
import.  
  
    Foundations--mathematical logic.  A prime demand on mathematics is that it 
be deductively rigorous, and a traditional model for intended rigor is 
Euclid's presentation of mathematical assertions in theorems. A theorem is a 
proposition which has been proved, excepting certain first theorems called 
axioms, which are admitted without proof; and to prove a theorem means to 
obtain it from other theorems by certain procedures of deduction or inference. 
It had long been commonplace that each branch of mathematics was based on its 
own axioms, but during the 19th century, mathematicians arrived at the insight 
that even the same branch might have alternate axioms. Specifically, there 
were envisaged alternate versions of two- and three-dimensional geometry, the 
axiom varied being the axiom on parallels. It was also recognized that a set 
of axioms becomes mathematically possible if it is logically consistent, that 
is, if one cannot deduce from the axioms to theorems one of which, as a 
proposition, is the negation of the other. See EUCLIDEAN GEOMETRY; NONEUCLIDEAN 
GEOMETRY. 
    At the same time certain developments led to the realization that not only 
the axioms but the rules of inference themselves might be, and even ought to 
be, subjected to variations. Now if axioms and rules of inferences are both 
viewed as subject to change, it is customary to speak of a mathematical system 
or also a formal system, and, of course, an irreducible first requirement is 
that the system be consistent after the manner just stated. Consistency alone 
is a somewhat negative property. There is a further property, called 
completeness, which is more positive, and which, if present, is very welcome. 
A system is complete if for any proposition which can be formulated it either 
can be proved that it holds or that its negation holds. 
    Some of the developments that led to doubt as to whether the traditional 
rules of inference are inviolate were the following. 
    1. G. Boole had found in 1854 that the classical Aristotelian connectives 
``and,'' ``or,'' ``negation of'' for propositions follow rules similar to 
those which the operations addition, multiplication, ``the negative of'' obey 
in ordinary algebra (Boole's algebra of propositions); his conclusions took 
from rules of inference the status of untouchability. 
    2. G. Cantor, the founder of the theory of sets and operations between 
them, defined a set (intuitively or naievely) as the collection of all objects 
having a certain property which is verbally expressible. Especially, ``the set 
of all sets'' is again a set and it has the peculiarity of being a set which 
contains itself as one of its elements. But this leads to the following 
contradictory situation (Russell's paradox): Divide the totality of all 
possible sets into two categories. A set shall belong to category I if it does 
not contain itself as an element, and to category II if it does contain itself 
as an element. Now form the set M whose elements are the sets of category I. 
It can now be reasoned by deductive steps admissible in Cantor's own theory 
that the set M cannot belong to either of the two categories, although the 
original division into categories did assign each set to one of them. 
    3. In 1904 E. Zermelo formulated the following axiom of choice: Given any 
family of nonvacuous sets {S}, no matter how (infinitely) large the family may 
be, it is possible to choose simultaneously an element x = x sub S from each 
given set S and thus to consider the set M consisting of precisely these 
elements. By the use of this axiom some striking theorems in classical 
mathematics could be proved which, without the use of the axiom, seemed to be 
logically out of reach entirely. Mathematicians began to wonder whether a 
theorem based on the axiom of choice is indeed valid or, at any rate, whether 
it has the same level of validity as one without it, and as a consequence, 
theorems employing the axiom of choice were frequently labeled as such. See  
SET THEORY. 
    Some of the doubts were resolved eventually; the most striking results are 
the following ones of K. Goedel. (1) Any consistent mathematical system which 
is sufficient for classical arithmetic must be incomplete. (2) Any such system 
remains consistent if one adds to it the axiom of choice, so that working 
mathematics cannot disprove the axiom of choice. In 1963 P. Cohen showed that 
the axiom cannot be proved, either. See LOGIC. 
  
    Constructiveness.  Some mathematicians object to mere existence proofs, 
and they demand that any proof also be constructive. The interpretations of 
this demand differ widely. Some proofs closely approach what a practical 
mathematician welcomes; if, for instance, a theorem asserts the existence of a 
number or a function, then the proof must also embody a procedure for actual 
computation of the solution, approximately, at least. Other versions are 
little more than the negative requirement that certain combinations of 
inference be avoided. There are also views which combine both; the best known 
among the last is the intuitionist view. It firmly demands a certain kind of 
constructiveness, which, however, does not necessarily guarantee the 
calculation by present-day computing machines. However, the actual stricture 
by which intuitionism became widely known is that proof by contradiction is 
not admissible. Proof by contradiction is also called proof by double 
negation, and it is equivalent to the Aristotelian law of the excluded middle. 
It assumes tentatively that the proposition to be proved is false and from 
this assumption deduces a contradiction to a previously established theorem. 
  
    Space in mathematics.  If geometry is the mathematics of space, then, in a 
superficial sense, all mathematics began with geometry, because apparently it 
began with measurements of figures: length, area, volume, and size of angles. 
It did not concern itself with questions of shape but with clarifying and 
deciding when figures are equal or substantially equal with regard to form. 
The first true theory of geometry was that of the Greeks, whose primary 
concern was study of the basic concept of equality of figures - their 
congruence and similarity - and the Greeks were so determined to dissociate 
their theory from the preceding phase of merely making measurements that 
Euclid's extensive work, for instance, avoids to a fault any kind of actual 
measurements. But, for all its lofty purposes, Greek geometry was too rigid 
and circumscribed to be able really to cope with the mathematical problem of 
space. Geometry did not progress further until, with the advent of coordinate 
systems, introduced by Descartes and his predecessors, a better mathematics of 
space could be initiated. 
    If a cartesian coordinate system is etched into two- or three-dimensional 
euclidean space, then the space becomes a point set, each point being a pair 
(x sup 1, x sup 2) or a triple (x sup 1, x sup 2, x sup 3) of real numbers, and 
any figure a suitable subset of it. This is a deliberate process of 
arithmetization of space which unifies space and number at the base. It does 
not hamper geometry in its task of pursuing problems of shape but instead aids 
it. In the cartesian plane, two figures are similar if the points of one can 
be obtained from the points of the other by means of a transformation, Eqs. 
(1), where Eq. (2) applies and where, for some RHO > 0, Eq. (3) can be 
written. The similarity is a congruence if, and only if, RHO = 1 (orthogonal 
transformation). Now this analytic representation of congruence and similarity 
suggests a geometric examination of the most general linear transformations, 
Eqs. (1), which are nonsingular, that is, for which the determinant 
|ALPHA sub q sup p| IS NOT EQUAL TO 0. They were virtually unknown to the 
Greeks, although they highlight the axiom of parallels of Euclid's geometry. A 
one-to-one transformation of the cartesian plane is such a linear 
transformation if, and only if, it carries a straight line onto a straight 
line and parallel straight lines into parallel straight lines. 
  
    y sup 1 = a sup 1 + [(ALPHA sub 1 sup 1) (x sup 1)]
                      + [(ALPHA sub 2 sup 1) (x sup 2)]    
  
    y sup 2 = a sup 2 + [(ALPHA sub 1 sup 2) (x sup 1)]
                      + [(ALPHA sub 2 sup 2) (x sup 2)]                    (1)
  
    (ALPHA sub 1 sup 1 MULT ALPHA sub 1 sup 2) 
  + (ALPHA sub 2 sup 1 MULT ALPHA sub 2 sup 2) = 0                         (2)
  
  
    [(ALPHA sub 1 sup 1) sup 2] + [(ALPHA sub 2 sup 1) sup 2] 
  = [(ALPHA sub 1 sup 2) sup 2] + [(ALPHA sub 2 sup 2) sup 2] = RHO sup 2  (3)
  
    The family of all linear transformations constitutes a transitive group, 
and the subfamily of orthogonal transformations is already a transitive group. 
F. Klein made the pronouncement, which is generally accepted, that there 
arises a geometry on a space if on the space there is given a transitive group 
of transformations; two figures are considered equal whenever one figure can 
be carried into the other figure by one of the transformations. 
    The arithmetization of space led to a purely mathematical creation of 
n-dimensional space, euclidean and other, for any integer dimension n, by 
defining its points generally as n-tuples of real numbers (x sup 1, . . . , 
x sup n) with suitable definitions for various geometrical relations between 
such points. The best-known application of this was the four-dimensional space 
of the theory of relativity, but actually multidimensional geometry had been 
playing a part in physics before that. If a mechanical system involves M mass 
points, it was customary in effect to introduce the space of dimension n = 3M, 
whose points are the states of the system, that is, the n-tuples of coordinates 
{x sub m sup 1, x sub m sup 2, x sub m sup 3}, m = 1, . . . , M, at any one 
time point. Also, if there are restraints operative in the system, then the 
Lagrange-Hamilton theory suitably reduced the dimension of the space by the 
use of the free parameters of the system instead of the original n coordinates 
themselves. The use of free parameters spread from mechanical systems to other 
systems in physics and chemistry, and all so-called equations of state are 
geared to this. Finally, in quantum theory a state of a system has infinitely 
many coordinates and the infinitely dimensional space representing it is a 
Hilbert space. Also, partly under the influence of Hilbert space, 
mathematicians have become fascinated with infinitely dimensional spaces in 
general. They are being studied intensively, and large parts of mathematics 
are being pressed into these new frames of reference. 
    The arithmetization of space is also reflected in the ever-widening use of 
graphs and charts. See ALGEBRA; ANALYTIC GEOMETRY; CALCULUS OF VECTORS; 
COORDINATE SYSTEMS; GEOMETRY; PROBABILITY; STATISTICS; TOPOLOGY. 
 �34s[Salomon Bochner]
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�6v�2v�32sMatrix isolation   �21m �33s 
    A technique for providing a means of maintaining molecules at low 
temperature for spectroscopic study. This method is particularly well suited 
for preserving reactive species in a solid, inert environment. Elusive 
molecular fragments, such as free radicals that may be postulated as important 
controlling intermediates for chemical transformations used in industrial 
reactions, high-temperature molecules that are in equilibrium with solids at 
very high temperatures, and molecular ions that are produced in plasma 
discharges or by high-energy radiation all can be examined by using absorption 
(infrared, visible, and ultraviolet), electron-spin resonance, and 
laser-excitation spectroscopes. 
    The experimental apparatus for matrix isolation experiments is designed 
with the method of generating the molecular transient and performing the 
spectroscopy in mind. The illustration shows the cross section of a vacuum 
vessel used for absorption spectroscopic measurements. The matrix sample is 
introduced through the spray-on line; argon is the most widely used matrix 
gas, although neon, krypton, xenon, and nitrogen are also used. The reactive 
species can be generated in a number of ways: mercury-arc photolysis of a 
trapped precursor molecule through the quartz window, evaporation from a 
Knudsen cell in the heater, chemical reaction of atoms evaporated from the 
Knudsen cell with molecules deposited through the spray-on line, and 
vacuum-ultraviolet photolysis of molecules deposited from the spray-on line by 
radiation from discharge-excited atoms flowing through the tube. For laser 
excitation studies, the sample is deposited on a tilted copper wedge which is 
grazed by the laser beam, and light emitted or scattered at approximately 90DG 
is examined by a spectrograph. In electron-spin resonance studies, the sample 
is condensed on a sapphire rod that can be lowered into the necessary 
waveguide and magnet. 
    The matrix isolation technique enables spectroscopic data to be obtained 
for reactive molecular fragments, many of which cannot be studied in the gas 
phase.  
 �34s[Lester Andrews]
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�6v�2v�32sMatrix mechanics   �21m �33s 
    A formulation of quantum theory in which the operators are represented by 
time-dependent matrices.  See  MATRIX THEORY; NONRELATIVISTIC QUANTUM THEORY. 
    Matrix mechanics is not useful for obtaining quantitative solutions to 
actual problems; on the other hand, because it is concisely expressed in a 
form independent of special coordinate systems, matrix mechanics is useful for 
proving general theorems. 
 �34s[Edward Gerjuoy]
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�6v�2v�32sMatrix theory �21m �33s
    A matrix is a rectangular array of numbers, or other elements, of the form

	       A =
		   ( a11	a12	  . . .       asub1n
		     a21	a22	  . . .       asub2n
		     . . . . . . . . . . . . . . . . . . . .
		     asubm1	asubm2	  . . .       asubmn )

below. The array A is an m by n matrix with m rows and n columns, and the size
of A is said to be m by n.  The rows of a matrix are always numbered from the
top down and the columns from left to right.  The position of each element in
the array is given by its subscripts; that is, asubij is the element in the ith
row and jth column.  Since every element of A is represented by asubij as i
takes on the values 1, 2, .  .	.  , m and j the values 1, 2, .  .  .  , n,
asubij is called the typical element of A, and the compact notation A = (asubij)
is used when the size of A is given.
    Matrices have application as computational devices in such widely
diversified fields as economics, psychology, statistics, engineering, physics,
and mathematics. In mathematics, matrices are useful tools in the study of
linear systems of algebraic equations, linear differential equations, linear
mappings and transformations, and bilinear and quadratic forms.
    If m = n in (1), A is called a square matrix of order n. If m = 1, A is a
row matrix, and if n = 1, A is a column matrix. The elements asubij of A,
for which i = j, are the principal diagonal elements. A diagonal matrix is a
matrix such that asubij = 0 if i = j, and a scalar matrix is a square
diagonal matrix with equal diagonal elements. An identity matrix is a scalar
matrix in which the common diagonal element is the number 1. An n by n
identity matrix is denoted by Isubn.
    Matrices can be regarded as generalized numbers, and their utility in
applications depends on the possibility of combining them in certain definite
ways. The matrix operations of addition, subtraction, and multiplication are
defined in terms of these same operations for the elements, and they satisfy
some, but not all, of the rules of ordinary algebra. In discussing these
matrix operations it is assumed that the elements of the matrices are numbers.
    Two matrices A = (asubij) and B = (bsubij) are equal if they have
the same size m by n and asubij = bsubij for all i, j.	Matrices A = (asubij)
and B = (bsubij) of the same size m by n are added by adding correspondingly
placed elements; that is, A + B = C = (csubij) is an m by n matrix where csubij
= asubij + bsubij for all i, j.  The m by n matrix with 0 in every position is
denoted by 0, and is called a null matrix.  The matrix -A = (-asubij) is the
negative of the matrix A = (asubij), and A + (-A) = 0.	Subtraction of m by n
matrices is defined by B - A = B + (-A) = (bsubij - asubij).
    A matrix B = (bsubij) is conformable with respect to a matrix A = (asubij)
if B has size n by q and A has size m by n; that is, B has the same number of
rows as A has columns.	The matrix product AB is defined only when B is
conformable with respect to A.	The product C = AB is an m by q matrix and the
element in the i, j position of C is obtained by multiplying the n elements in
the ith row of A into the n elements in the jth column of B, term by term, and
adding these products.	Thus if

	A = (2	       0	  -1
	     4	       1	  1/2)

    and B = (5	       1	  -2)

then   AB = (12       20)

If A and B are square matrices of the same size, then both AB and BA are
defined.  Unlike the case for numbers, it may happen for matrices that AB NOT
EQUAL BA and AB = 0 with A NOT EQUAL 0 and B NOT EQUAL 0.
    The product of a matrix A and a number a is called a scalar product and is
obtained by multiplying every element asubij of A by a.
    The transpose of an m by n matrix A is the n by m matrix B which has as
its ith row the ith column of A and as its jth column the jth row of A for all
i, j. If the transpose of a matrix A is denoted by A', and B is conformable
with respect to A, then (AB)' = B'A'. A matrix A is symmetric if A = A'. A
symmetric matrix is necessarily square.
    A square n by n matrix is nonsingular if the determinant of A is not zero.
Otherwise A is singular. A nonsingular matrix A has a unique inverse, that is,
a matrix Asup-1 such that AAsup-1 = Asup-1 A= Isubn. The inverse of a
nonsingular matrix is easily described but is difficult to compute for
matrices of large size. Many important applications require the calculation of
the inverse, and numerical methods are used to approximate the elements of the
inverse.  See  DETERMINANT.
 �34s[Ross A. Beaumont]
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�6v�2v�32sMatter (physics)   �21m �33s 
    The substance composing bodies perceptible to the senses. The 
distinguishing properties of matter are gravitation and inertia. Any entity 
exhibiting these properties when at rest is matter. Although electromagnetic 
radiation also possesses these properties to some extent, this radiation 
always moves with the speed of light. All material bodies have mass, which is 
a measure of inertia; every material body near the Earth's surface has weight, 
which is a measure of the Earth's gravitational attraction for the body.  See  
GRAVITATION; INERTIA; MASS; WEIGHT. 
 �34s[Dudley Williams]
�33s 
  
  
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�6v�2v�32sMatthiessen's rule   �21m �33s 
    An empirical rule which states that the total resistivity of a crystalline 
metallic specimen is the sum of the resistivity due to thermal agitation of 
the metal ions of the lattice and the resistivity due to the presence of 
imperfections in the crystal. This rule is a basis for understanding the 
resistivity behavior of metals and alloys at low temperatures. 
    The resistivity of a metal results from the scattering of conduction 
electrons. Lattice vibrations scatter electrons because the vibrations distort 
the crystal. Imperfections such as impurity atoms, interstitials, 
dislocations, and grain boundaries scatter conduction electrons because in 
their immediate vicinity the electrostatic potential differs from that of the 
perfect crystal. 
 �34s[Frank J. Blatt]
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�6v�2v�32sMaxwell's equations �21m �33s
    Four differential equations proposed by James Clerk Maxwell in 1864 as the
basis of the theory of electromagnetic waves.  They may be written, in vector
notation, as Eqs.  (1) - (4), where BOLD D is the electric displacement, BOLD B
the magnetic flux density, BOLD E the electric field strength or intensity, BOLD
H the magnetic field strength or intensity, rho the charge density, and BOLD i
the current density.

			    DEL DOT BOLD D = rho			     (1)


			     DEL DOT BOLD B = 0 			     (2)


	 DEL CROSS PRODUCT BOLD E = - CURLY D BOLD B/CURLY Dt		     (3)


       DEL CROSS PRODUCT BOLD H = BOLD i + CURLY D D/CURLY Dt		     (4)

    The first equation states that electric flux lines, if they end at all,
will do so on electric charges.  The second states that magnetic flux lines
never terminate.  The third is a form of Faraday's law of induction, which
states that the rate of change of the magnetic flux threading a circuit equals
the electromotive force or line integral of BOLD E around the circuit.	The
fourth integral is based partially on A.  M.  Ampere's experiments on steady
currents which show that the line integral of the magnetic intensity BOLD H (or
BOLD B/mu, where mu is the permeability) around a closed curve equals the
current encircled.  See DISPLACEMENT CURRENT; EQUATION OF CONTINUITY; STOKES'
THEOREM.

 �34s[William R.  Smythe]
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�6v�2v�32sMcLeod gage   �21m �33s 
    A type of instrument used to measure vacuum by application of the 
principle of Boyle's law. 
    A known volume of a gas whose pressure is to be measured is trapped by 
raising the level of a fluid (mercury or oil) by means of a plunger, by 
lifting a reservoir, by using pressure, or by tipping the apparatus. As the 
fluid level is further raised, the gas is compressed into the capillary tube 
(see illustration). Obeying Boyle's law, the compressed gas now exerts enough 
pressure to support a column of fluid high enough to read. Readings are 
somewhat independent of the composition of the gas under pressure.  See  
VACUUM MEASUREMENT. 
 �34s[Richard Comeau]
�33s 
  
  
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�6v�2v�32sMean effective pressure   �21m �33s 
    A term commonly used in the evaluation for positive displacement machinery 
performance which expresses the average net pressure difference in pounds per 
square inch (psi) on the two sides of the piston in engines, pumps, and 
compressors. It is also known as mean pressure and is abbreviated as mep or mp. 
    In an engine (prime mover) it is the average pressure which urges the 
piston forward on its stroke. In a pump or compressor it is the average 
pressure which must be overcome, through the driver, to move the piston 
against the fluid resistance. 
    The criterion of mep is a vitally convenient device for the evaluation of 
a reciprocating engine, pump, or compressor design as judged by initial cost, 
space occupied, and deadweight.  See  COMPRESSOR; DIESEL CYCLE; THERMODYNAMIC 
CYCLE; VAPOR CYCLE. 
 �34s[Theodore Baumeister]
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�6v�2v�32sMean free path   �21m �33s 
    The average distance traveled between two similar events. The concept of 
mean free path is met in all fields of science and is classified by the events 
which take place. The concept is most useful in systems which can be treated 
statistically, and is most frequently used in the theoretical interpretation 
of transport phenomena in gases and solids, such as diffusion, viscosity, heat 
conduction, and electrical conduction. The types of mean free paths which are 
used most frequently are for elastic collisions of molecules in a gas, of 
electrons in a crystal, of phonons in a crystal, and of neutrons in a 
moderator.  See  KINETIC THEORY OF MATTER. 
 �34s[W. Dexter Whitehead]
�33s 
  
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�6v�2v�32sMeasles   �21m �33s 
    An acute, highly infectious viral disease, with cough, fever, and 
maculopapular rash. It is of worldwide endemicity. 
    The virus enters the body via the respiratory system, multiplies there, 
and circulates in the blood. Prodromal cough, sneezing, conjunctivitis, 
photophobia, and fever occur, with Koplik's spots in the mouth. A rash appears 
after 14 days' incubation and persists 5 - 10 days. Serious complications may 
occur in 1 of every 15 persons; these are mostly respiratory (bronchitis or 
pneumonia), but neurological complications are also found. Encephalomyelitis 
occurs, but it is rare. Permanent disabilities may ensue for a significant 
number of persons. 
    Live attenuated virus vaccine can effectively prevent measles; 
vaccine-induced antibodies persist for years. In unvaccinated populations, 
immunizing infections occur in early childhood during epidemics which recur 
after 2 - 3 years' accumulation of susceptible children. Transmission is by 
coughing or sneezing. Measles is spread chiefly by children during the 
catarrhal prodromal period; it is infectious from the onset of symptoms until 
a few days after the rash has appeared. By the age of 20 over 80% of persons 
have had measles. Second attacks occur but are very rare. Treatment is 
symptomatic.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sMeasure   �21m �33s 
    A reference sample used in comparing lengths, areas, volumes, masses, and 
the like. The measures employed in scientific work are based on the 
international units of length, mass, and time - the meter, the kilogram, and 
the second - but decimal multiples and submultiples are commonly employed. 
Prior to the development of the international metric system, many 
special-purpose systems of measures had evolved and many still survive, 
especially in Great Britain and the United States.  See  METRIC SYSTEM; 
PHYSICAL MEASUREMENT; TIME; UNITS OF MEASUREMENT; WEIGHT. 
 �34s[Dudley Williams]
�33s 
  
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�6v�2v�32sMeasured daywork   �21m �33s 
    A tool used primarily in manufacturing facilities as a control device to 
measure productive output in relation to labor input within a specific time 
period. The measurement of the work content is accomplished through the use of 
time standards which are usually the result of a stopwatch time study, 
predetermined time standards (methods-time measurement, the work-factor 
system), or some other form of work-measurement technique designed to measure 
tasks of labor under normal and average conditions. 
    A measured daywork plan is similar to incentive pay plans inasmuch as in 
both plans, time standards are used as a device to measure operator 
performance and also for various forms of management planning. However, in a 
measured daywork plan, worker income is based on a fixed hourly rate 
established by management, and is usually affected only by job classification, 
shift premiums, and overtime adjustments. Because of the fixed hourly rate in 
a measured daywork plan, there is little incentive for a worker to exceed a 
normal or standard level of performance or productivity. On the other hand, 
time standards are more readily acceptable, and become less an item of 
contention to the employee and bargaining unit (union). 
    The term daywork as used in industry denotes a fixed hourly rate that is 
not raised or lowered by varying worker performance levels. The hourly rate 
for a particular job should be a fair one relative to other jobs in the shop, 
and should also be comparable to rates of pay for similar jobs in the 
industrial community. 
    Once measured labor time standards have been established for shop 
operations, in addition to evaluating operator performance and identifying 
labor costs, new-product costs can also be determined prior to release to 
production, worker-power planning and scheduling can be done, equipment 
capacity requirements can be identified, and planning and make/buy decisions 
can be facilitated.  See  PERFORMANCE RATING; PRODUCTIVITY; WAGE INCENTIVES; 
WORK MEASUREMENT. 
 �34s[Dave Scott]
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�6v�2v�32sMeasure theory �21m �33s
    A branch of mathematical analysis connected with the theory of
integration. In order to discuss this subject, a formal definition of the term
measure must be given.
    Let X be an arbitrary set. Let m be a fixed collection of subsets of X
satisfying the following conditions:
    1.	PHI epsilon m.	(phi is the empty set.	The symbol epsilon indicates
that phi is an element of m.)
    2. If A epsilon m, then Asupc epsilon m. (Asupc is the complement
of A. It consists of those elements of X which do not belong to A.)
    3. If A1, A2, . . . epsilon m, then

	     UNION (k=1 to INFINITY) A(sub k) epsilon m 		     (1)

(This set is the union of A1, A2, . . . . It consists of those elements of X
which belong to at least one of the sets A1, A2, . . . .)
    In this situation the collection m is called a sigma-algebra. Here the
term algebra refers to the various set operations (complementation, union,
intersection), and the prefix sigma to the fact that countably many such
operations can be performed with sets in m and still result in sets in m. For
example, the three properties mentioned above imply another property:
    4. If A1, A2, . . . epsilon m, then

	  INTERSECTION (k=1 to INFINITY) A(sub k) epsilon m		    (2)

(This set is the intersection of A1, A2, . . . . It consists of those elements
of X which belong to all of the sets A1, A2, . . . .)  See  SET THEORY.
    Now suppose that X is a set and m is a particular sigma-algebra of subsets
of X. A measure mu is a function which assigns to each set in m a certain
nonnegative real number (or + INFINITY) and which satisfies the following
conditions:
    1. mu(phi) = 0.
    2. If A and B epsilon m and are disjoint (A INTERSECTION B = phi), then
Eq. (3) holds.

		  mu(A INTERSECTION B) = mu(A) + mu(B)			     (3)

    3. If A1, A2, . . . epsilon m, then Eq. (4) holds.

mu[INTERSECTION (k=1 to INFINITY) Asubk] < or = SUMMATION (k=1 to INFINITY)
			     mu(A[sub k])				    (4)

    Various other properties can be derived from these conditions, such as:
    4.	If A and B epsilon m and A CONTAINED IN B (that is, A is a subset of B),
then mu(A) < or = mu(B).
    5. If A1, A2, . . . epsilon m and are mutually disjoint (Asubi INTERSECTION
A(sub j) = phi if i NOT EQUAL j), then Eq. (5) holds.

mu[UNION (k=1 to INFINITY) A(sub k)] =
SUMMATION (k=1 to INFINITY) mu(A[sub k])				     (5)

The last property is of crucial importance and is called the countable
additivity property of the measure MU.
    Measure theory has a great number of important applications. Undoubtedly,
the most important is the application to integration.  See  INTEGRATION.
 �34s[B. Frank Jones, Jr.]
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�6v�2v�32sMechanical advantage   �21m �33s 
    Ratio of the force exerted by a machine (the output) to the force exerted 
on the machine, usually by an operator (the input). The term is useful in 
discussing a simple machine, where it becomes a figure of merit. It is not 
particularly useful, however, when applied to more complicated machines, where 
other considerations become more important than a simple ratio of forces.  See 
  EFFICIENCY; SIMPLE MACHINE. 
 �34s[Richard M. Phelan]
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�6v�2v�32sMechanical classification   �21m �33s 
    A sorting operation in which mixtures of particles of mixed sizes, and 
often of different specific gravities, are separated into fractions by the 
action of a stream of fluid.  Water is ordinarily used as the sorting fluid, 
but other liquids or air or other gases may be used (see illustration). 
    The main objective of classification is to separate the particles 
according to size. This function is identical to that of screening, but 
classification is applicable to smaller particles, especially those that are 
undersize. For small particles it is more economical than screening. In 
classification the oversize and undersize are called sands and slimes, 
respectively.  
    Material also may be mechanically classified by specific gravity, a method 
that separates substances differing in chemical composition. This is called 
hydraulic separation. Such classification is based on the fact that, in a 
fluid, particles of the same specific gravity but of different size or shape 
settle at different constant speeds. Large, heavy, round particles settle 
faster than small, light, needlelike ones. If the particles also differ in 
specific gravity, the speed of settling is further affected. This is the basis 
for the separation of particles by kind rather than by size alone.  See  
FLOTATION; MECHANICAL SEPARATION TECHNIQUES; UNIT OPERATIONS. 
 �34s[Warren L. McCabe]
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�6v�2v�32sMechanical engineering   �21m �33s 
    One of several recognized fields of engineering. To grasp the meaning of 
mechanical engineering, it is desirable to take a close look at what 
engineering really is. The Engineers' Council for Professional Development has 
defined engineering as the profession in which a knowledge of the mathematical 
and physical sciences gained by study, experience, and practice is applied 
with judgment to develop ways to utilize economically the materials and forces 
of nature for the progressive well-being of mankind. It is a profession in 
which study in mathematics and science is blended with experience and judgment 
for the production of useful things. 
    Formal training of a mechanical engineer includes mastery of mathematics 
through the level of differential equations. Training in physical science 
embraces chemistry, physics, mechanics of materials, fluid mechanics, 
thermodynamics, statics, and dynamics.  See  ENGINEERING; MACHINE DESIGN; 
MACHINERY; MASS PRODUCTION; TECHNOLOGY. 
 �34s[Robert S. Sherwood]
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�6v�2v�32sMechanical impedance   �21m �33s 
    For a system executing simple harmonic motion, the mechanical impedance is 
the ratio of force to particle velocity. If the force is that which drives the 
system and the velocity is that of the point of application of the force, the 
ratio is the input or driving-point impedance. If the velocity is that at some 
other point, the ratio is the transfer impedance corresponding to the two 
points.  
    Mechanical impedance is a complex quantity. The real part, the mechanical 
resistance, is independent of frequency if the dissipative forces are 
proportional to velocity; the imaginary part, the mechanical reactance, varies 
with frequency, becoming zero at the resonant and infinite at the antiresonant 
frequencies of the system.  See  FORCED OSCILLATION; HARMONIC MOTION. 
 �34s[Martin Greenspan]
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�6v�2v�32sMechanical rectifier   �21m �33s 
    A device which uses a synchronously operated mechanical switch to convert 
a single-phase or polyphase alternating voltage to a direct voltage. 
Single-phase mechanical rectifiers are made for small current output and are 
normally called vibrators. For large values of power where low voltages (less 
than 600 volts) are desired, the polyphase mechanical rectifier is used. This 
low-voltage device has higher efficiency than electronic rectifiers, which 
have appreciable voltage drop across the arc. These devices are not commonly 
used in power system applications.  See  RECTIFIER; VIBRATOR. 
 �34s[Albert G. Conrad]
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�6v�2v�32sMechanical separation techniques   �21m �33s 
    A group of laboratory and production operations whereby the components of 
a polyphase mixture are separated by mechanical methods into two or more 
fractions of different mechanical characteristics. The separated fractions may 
be homogeneous or heterogeneous, particulate or nonparticulate. 
    The techniques of mechanical separation are based on differences in phase 
density, in phase fluidity, and in such mechanical properties of particles as 
size, shape, and density; and on such particle characteristics as wettability, 
surface charge, and magnetic susceptibility. Obviously, such techniques are 
applicable only to the separation of phases in a heterogeneous mixture. They 
may be applied, however, to all kinds of mixtures containing two or more 
phases, whether they are liquid-liquid, liquid-gas, liquid-solid, gas-solid, 
solid-solid, or gasliquid-solid. 
    Methods of mechanical separations fall into four general classes: (1) 
those employing a selective barrier such as a screen or filter cloth; (2) 
those depending on difference in phase density alone (hydrostatic separators); 
(3) those depending on fluid and particle mechanics; and (4) those depending 
on surface or electrical characteristics of particles. A wide variety of 
separation devices have been devised and are in use. The more important kinds 
of equipment are listed in the table, grouped according to the phases 
involved.  See  CENTRIFUGATION; CLARIFICATION; DUST AND MIST COLLECTION; 
ELECTROSTATIC PRECIPITATOR; FILTRATION; FLOTATION; MAGNETIC SEPARATION 
METHODS; MECHANICAL CLASSIFICATION; SCREENING; SEDIMENTATION (INDUSTRY); 
THICKENING.  
 �34s[Shelby A. Miller]
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�6v�2v�32sMechanical vibration �21m �33s
    The continuing motion, repetitive and often periodic, of a solid or liquid
body within certain spatial limits. Vibration occurs frequently in a variety
of natural phenomena such as the tidal motion of the oceans, in rotating and
stationary machinery, in structures as varied in nature as buildings and
ships, in vehicles, and in combinations of these various elements in larger
systems. The sources of vibration and the types of vibratory motion and their
propagation are subjects that are complicated and depend a great deal on the
particular characteristics of the systems being examined. Further, there is
strong coupling between the notions of mechanical vibration and the
propagation of vibration and acoustic signals through both the ground and the
air so as to create possible sources of discomfort, annoyance, and even
physical damage to people and structures adjacent to a source of vibration.

    Mass-spring-damper system.	Although vibrational phenomena are complex,
some basic principles can be recognized in a very simple linear model of a
mass-spring-damper system (see illustration). Such a system contains a mass M,
a spring with spring constant k that serves to restore the mass to a neutral
position, and a damping element which opposes the motion of the vibratory
response with a force proportional to the velocity of the system, the constant
of proportionality being the damping constant c. This damping force is
dissipative in nature, and without its presence a response of this mass-spring
system would be completely periodic.  See  DAMPING.

Ms. Pages 282-2847:50

    Complex systems.  The foregoing model of the linear spring-mass-damper
system contains within it a number of simplifications that do not reflect
conditions of the real world in any obvious way. These simplifications include
the periodicity of both the input and, to some extent, the response; the
discrete nature of the input, that is, the assumption that it is temporal in
nature with no reference to spatial distribution; and the assumption that only
a single resonant frequency and a single set of parameters are required to
describe the mass, the stiffness, and the damping. The real world is far more
complex. Many sources of vibration are not periodic. These include impulsive
forces and shock loading, wherein a force is suddenly applied for a very short
time to a system; random excitations, wherein the signal fluctuates in time in
such a way that its amplitude at any given instant can be expressed only in
terms of a probabilistic expectation; and aperiodic motions, wherein the
fluctuation in time may be some prescribed nonperiodic function or some other
function that is not readily seen to be periodic.

    Sources of vibration.  There are many sources of mechanical and structural
vibration that the engineer must contend with in both the analysis and the
design of engineering systems. The most common form of mechanical vibration
problem is motion induced by machinery of varying types, often but not always
of the rotating variety. Other sources of vibration include: groundborne
propagation due to construction; vibration from heavy vehicles on conventional
pavement as well as vibratory signals from the rail systems common in many
metropolitan areas; and vibrations induced by natural phenomena, such as
earthquakes and wind forces. Wave motion is a source of vibration in
mechanical and structural systems associated with offshore structures.

    Effect of vibrations.  The most serious effect of vibration, especially in
the case of machinery, is that sufficiently high alternating stresses can
produce fatigue failure in machine and structural parts. Less serious effects
include increased wear of parts, general malfunctioning of apparatus, and the
propagation of vibration through foundations and buildings to locations where
the vibration of its acoustic realization is intolerable either for human
comfort or for the successful operation of sensitive measuring equipment.  See
  ACOUSTIC NOISE; SOUND; VIBRATION; WEAR.
 �34s[Clive L. Dym ;J. P. Den Hartog]
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�6v�2v�32sMechanics   �21m �33s 
    In its original sense, mechanics refers to the study of the behavior of 
systems under the action of forces. Mechanics is subdivided according to the 
types of systems and phenomena involved. 
    An important distinction is based on the size of the system. Those systems 
that are large enough can be adequately described by the newtonian laws of 
classical mechanics; in this category, for example, are celestial mechanics 
and fluid mechanics. On the other hand, the behavior of microscopic systems 
such as molecules, atoms, and nuclei can be interpreted only by the concepts 
and mathematical methods of quantum mechanics. 
    Mechanics may also be classified as nonrelativistic or relativistic 
mechanics, the latter applying to systems with material velocities comparable 
to the velocity of light. This distinction pertains to both classical and 
quantum mechanics. 
    Finally, statistical mechanics uses the methods of statistics for both 
classical and quantum systems containing very large numbers of similar 
subsystems to obtain their large-scale properties.  See  CLASSICAL FIELD 
THEORY; CLASSICAL MECHANICS; DYNAMICS; FLUID MECHANICS; QUANTUM MECHANICS; 
RELATIVISTIC MECHANICS; STATICS; STATISTICAL MECHANICS. 
 �34s[Bernard Goodman]
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�6v�2v�32sMechanism   �21m �33s 
    Classically, a mechanical means for the conversion of motion, the 
transmission of power, or the control of these. Mechanisms are at the core of 
the workings of many machines and mechanical devices. In modern usage, 
mechanisms are not always limited to mechanical means. In addition to 
mechanical elements, they may include pneumatic, hydraulic, electrical, and 
electronic elements. In this article, the discussion of mechanism is limited 
to its classical meaning. See MACHINE. 
    Most mechanisms consist of combinations of a relatively small number of 
basic components. Of these, the most important are cams, gears, links, belts, 
chains, and logical mechanical elements. The latter includes such devices as 
ratchets, trips, detents, and interlocks. In order to understand how any 
mechanism works, their degree of freedom, structure, and kinematics must be 
considered. See BELT DRIVE; CAM MECHANISM; CHAIN DRIVE; ESCAPEMENT; GEAR; 
LINKAGE (MECHANISM); RATCHET. 
    Degree of freedom is conveniently illustrated for mechanisms with rigid 
links. The discussion is limited to mechanisms which obey the general 
degree-of-freedom equation, 

            *** F = LAMBDA (l - j - 1) + SUMMATION f sub i ***
  
where F = degree of freedom of mechanism, l = number of links of mechanism,
j = number of joints of mechanism, f sub i = degree of freedom of relative
motion at ith joint, SUMMATION = summation symbol (summation over all joints),
and LAMBDA = mobility number (the most common cases are LAMBDA = 3 for plane 
mechanisms and LAMBDA = 6 for spatial mechanisms). See DEGREE OF FREEDOM 
(MECHANICS).
    The kinematic structure of a mechanism refers to the identification of the 
joint connection between its links. Just as chemical compounds can be 
represented by an abstract formula and electric circuits by schematic diagrams, 
the kinematic structure of mechanisms can be usefully represented by abstract 
diagrams. The structure of mechanisms for which each joint connects two links 
can be represented by a structural diagram, or graph, in which links are
denoted by vertices, joints by edges, and in which the edge connection of 
vertices corresponds to the joint connection of links; edges are labeled 
according to joint type, and the fixed link is identified as well. Thus the 
graph of the slider-crank mechanism of illustration a is as shown in 
illustration b. In this figure the circle around vertex 1 signifies that link 
1 is fixed. 
    Kinematics is divided into kinematic analysis (analysis of a mechanism of 
given dimensions) and synthesis (determination of the proportions of a 
mechanism for given motion requirements). It includes the investigation of 
finite as well as infinitesimal displacements, velocities, accelerations and 
higher accelerations, and curvatures and higher curvatures in plane and 
three-dimensional motions. See KINEMATICS. 
    The design of mechanisms involves many factors. These include their 
structure, kinematics, dynamics, stress analysis, materials, lubrication, 
wear, tolerances, production considerations, control and actuation, vibrations, 
critical speeds, reliability, costs, and environmental considerations. Modern 
trends in the design of mechanisms emphasize economical design analysis by 
means of computer-aided design techniques. See COMPUTER-AIDED DESIGN AND 
MANUFACTURING; DESIGN ENGINEERING. 
 �34s[Ferdinand Freudenstein]
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�6v�2v�32sMechanoreceptors �21m �33s
    Receptors that provide the organism with information about such mechanical
changes in the environment as movement, tension, and pressure. In higher
animals receptors are actually the only means by which information of the
surroundings is gained and by which reactions to environmental changes are
stated.  See  SENSATION.
    In primitive forms of life, such as unicellular organisms and sponges, the
receptor and effector functions are built into the same cell. The cell is
directly excited by and reacts to the environmental stimulus. At higher levels
of organization, the receptor separates from the effector and appears
specialized in function and structure in the receptor cell of the primitive
nerve system of actinians (sea anemones). Stimuli of the environment excite
the receptor cell, and this excitation is conveyed to another specialized
cell, the effector. In another step of phylogenetic development, a third
element appears, the motor nerve cell, which serves as a link between the
receptor and effector. This arrangement appears clearly in coelenterates and
is carried through in its essential features up to the vertebrates.
    Mechanoreceptors are excited by mechanical disturbances of their
surroundings through deformation of their structure, through pressure or
tension, or through a combination of these. In general, little energy is
required for mechanical stimuli to cause a detectable excitation in
mechanoreceptors.
    From a physical point of view, mechanoreceptors are energy transducers;
they convert mechanical into electrical energy, which in turn triggers the
nerve impulse. Deformation leads to a sequence of events which may be
summarized by the following scheme:


Mechanical stimulus --> Generator Current --> Nerve Impulse (action potential)


The generator current is the earliest detectable sign of excitation. The most
salient characteristic of the generator current is its graded nature; its
amplitude increases continuously, without visible steps, if the stimulus
strength is progressively increased. When the generator current reaches a
certain critical amplitude, an all-or-nothing potential is discharged in the
sense organ which may then propagate as an all-or-nothing nerve impulse along
the afferent axon of the receptor.  See  NERVOUS SYSTEM (INVERTEBRATE);
NERVOUS SYSTEM (VERTEBRATE).
 �34s[Werner R. Loewenstein]
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�6v�2v�32sMecoptera   �21m �33s 
    A small order of insects called the scorpion flies. Characteristic of the 
adult insect is the peculiar prolongation of the head into a beak, which bears 
chewing mouthparts. They are small to medium in size. The insects either have 
two pairs of large, net-veined wings of equal size, often with dark areas, or 
have short and aborted wings. The legs are long and slender. In some species, 
the male abdomen has a terminal enlargement which is held recurved over the 
back so that he resembles a scorpion, thus the common name. The Mecoptera are 
found in moist habitats within densely wooded areas. The adults are omnivorous 
but feed chiefly on small insects.  See  INSECTA. 
 �34s[Bryant E. Rees]
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�6v�2v�32sMedical bacteriology   �21m �33s 
    The study of bacteria which affect human health, especially those which 
produce diseases known as infections, such as typhoid fever, plague, and 
tuberculosis. Sometimes called pathogens, certain bacteria are able to 
establish themselves in or on the body, where they grow and multiply, 
producing local or general functional upsets of the person thus infected. 
Medical bacteriologists study the whole process of infection but are primarily 
concerned with the role played by bacteria. They isolate suspected bacteria, 
relate them to the disease at hand, analyze their pertinent characteristics, 
study genetic changes which involve pathogenicity, and weigh other phases of 
prevention or therapy. 
    The identification of pathogenic bacteria is frequently used to establish 
the nature of a disease in a patient or for testing a response to antibiotics. 
It consists of searching for characteristics which will quickly and 
efficiently distinguish pathogenic bacteria from those with which they might 
be confused. Characteristics fall into four natural groups: morphology, 
metabolism, pathogenicity, and antigenicity. One or more of these groups may 
be necessary to establish identification.  See  PATHOGEN. 
    Bacteria may be members of a pathogenic species and still vary sharply in 
virulence. A strain or culture of bacteria which infects readily and produces 
severe effects is called virulent. One which, though similar in all other 
known ways, does not infect is called avirulent. A virulent organism which has 
been reduced in some degree from its expected virulence is termed attenuated.  
See  VIRULENCE. 
    Whereas virulence focuses on the bacteria, immunity refers to variations 
in receptiveness (susceptibility) on the part of the infected person. Virulent 
bacteria may affect two persons quite differently. Immunity is a complex 
constitutional state which varies in degree both as to individual mechanisms 
and as to each specific infection.  See  IMMUNITY. 
 �34s[Max S. Marshall]
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�6v�2v�32sMedical control systems   �21m �33s 
    Systems that adapt and utilize for medical application the methodologies, 
techniques, and devices developed for the control of physical systems. The 
concept of control usually suggests an autonomous or semiautonomous process 
that measures the difference between actual and desired performance of a 
system and, according to an analytically derived algorithm, manipulates energy 
to affect the system behavior to minimize the difference. Two types of 
application to medicine are apparent. The first incorporates the clinician 
(human operator) in the control loop. The second involves the development of 
an autonomous process for the control of one or more physiological variables. 
In the sequence leading to control action, namely, measurement - analysis - 
control, it is possible to make measurements of many medical indicators, 
expressed in physical terms, with almost arbitrary accuracy, speed, and 
precision.  
    Considerable advances have been made in the development of mechanized and 
semiautomated methods for measurement and analysis, the necessary precursors 
to effective control. Physiological monitoring during surgery, intensive 
therapy, or coronary care are representative examples. Another important area 
of measurement relates to the mechanization of routine medical diagnostic 
procedures.  
    Computer-based mathematical models are becoming available as decision aids 
to the clinician in diagnosing and treating disease. From the many 
physiological and clinical measures available, the clinician selects a set of 
observations for the particular patient. These are fed into the diagnostic 
subsystem. The resultant diagnostic statements and raw clinical observations 
are then fed to a decision maker which selects the next action by the 
clinician, either to carry out further tests or to start treatment. Again 
medical data are used as in diagnosis, but here local treatment objectives are 
employed. The treatment selected is fed to the relevant medical effector which 
applies the relevant responses to patient and environment. 
    The field of artificial organs, prostheses, and ortheses (orthopedic 
braces or devices), is an area where examples of direct-feedback control are 
being implemented. Developments in cardiac pacemakers imply that the 
autonomous regulation of heart rate may soon replace the current fixed-rate 
pacer and the variable-rate device that must be adjusted by the wearer. Other 
examples are the myoelectric-feedback control of artificial limbs and active 
devices that route neural signals around damaged pathways to operate distal 
members.  See  CONTROL SYSTEMS; PROSTHESIS. 
 �34s[E. R. Carson]
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�6v�2v�32sMedical mycology   �21m �33s 
    The study of fungi (molds, yeasts) having pathogenic properties for 
humans. Some of these fungi are called opportunistic pathogens because they 
can cause disease only in those individuals whose immunological systems have 
been compromised either through administration of certain drugs or because of 
some other disease state such as a malignancy. There are fungi that can cause 
disease in a noncompromised host and are considered to be primary pathogens, 
even though they may also cause an opportunistic mycotic disease.  See  
OPPORTUNISTIC INFECTIONS. 
    Most fungi pathogenic for humans reside in soil as saprophytes. Humans are 
not a natural environment for them and when infected by these organisms are 
considered to be accidental hosts.  See  ACTINOMYCOSIS; ASPERGILLOSIS; 
BLASTOMYCOSIS; CANDIDIASIS; CHROMOMYCOSIS; COCCIDIOIDOMYCOSIS; 
DERMATOPHYTOSIS; HISTOPLASMOSIS; NOCARDIOSIS. 
 �34s[Leanor D. Haley]
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�6v�2v�32sMedical parasitology   �21m �33s 
    The branch of medical microbiology which deals with the relationships 
between humans and those animals which live in or on them. 
    The most important human parasites fall into four phyla: the Protozoa, 
Nematoda, Platyhelminthes, and Arthropoda.  See  ACARINA; ANOPLURA; CESTODA; 
DIPTERA; MALLOPHAGA; NEMATA; SIPHONAPTERA. 
    Different organisms show extreme diversity in their localization within 
the host body. Some are exclusively intracellular, most are extracellular 
only, and some are both. Nearly every organ, tissue, and fluid of the body can 
be invaded: Trypanosomes live in the blood, lymph nodes, and cerebrospinal 
fluid; filaria live in the lymph channels and eye fluids; other worms inhabit 
the intestine, bronchi, and bile ducts; and a variety of organisms occur in 
the heart, brain, liver, spleen, and glands of internal secretion. In these 
sites the effect on the tissue varies from complete destruction to almost 
complete harmlessness, and the same parasite at different times may have 
either of these effects. 
    Also, the relationships between microorganisms and humans may be 
permanent, or temporary, the microorganism being known, accordingly, as either 
an obligatory or a facultative parasite. For diseases due to protozoa see 
AMEBIASIS; BALANTIDIASIS; CHAGAS' DISEASE; LEISHMANIASIS; MALARIA; 
PNEUMOCYTOSIS; TRICHOMONIASIS. For diseases due to helminths see ASCARIASIS; 
CLONORCHIASIS; FASCIOLIASIS; FASCIOLOPSIASIS; GUINEA WORM INFECTION; 
HETEROPHYIASIS; HOOKWORM DISEASE; PARAGONIMIASIS; PINWORM INFECTION; 
SCHISTOSOMIASIS; STRONGYLOIDIASIS; TAPEWORM DISEASE; TRICHINOSIS. 
 �34s[Meir Yoeli]
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�6v�2v�32sMedicine �21m �33s
    The field of science devoted to healing. Many subdivisions exist and more
ramifications appear almost daily. Included in the area of medicine are the
clinical specialties of surgery, pediatrics, psychiatry, obstetrics, and
others. Internal medicine is the specialization which deals with internal
diseases of a nonsurgical nature.
    Related to the clinical specialties, particularly in regard to medical
education and research, are the basic medical sciences. These include, among
others, anatomy, physiology, psychology pharmacology, biochemistry, and
microbiology. Midway between the basic and the clinical sciences lies
pathology, the study of the structural and functional alterations caused by
diseases or abnormal states.
    An important area in all specialties is preventive medicine and public
health. This form of medicine supplies a necessary link with the community,
state, or large geographic region in matters of prevention, mass treatment,
and statistical appraisals of health matters. It is also concerned with
socioeconomic factors related to physical and mental well-being.  See  PUBLIC
HEALTH.
    Socialized medicine is that form which exists under the direct control and
financing of the state. The National Health Service of Great Britain is the
best-known example, and other systems exist.
    Other subdivisions of medicine, with names that are largely
self-explanatory, include veterinary, legal, tropical, and military medicine.
See  FORENSIC MEDICINE.
    Although medicine is based primarily upon scientific information and
method, an important feature is the relationship between the physician and the
patient. It is at this point that the necessary scientific background of
medicine gives way to the art of healing.  See	SURGERY.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sMediterranean Sea   �21m �33s 
    The Mediterranean Sea lies between Europe, Asia Minor, and Africa. It is 
completely landlocked except for the Strait of Gibraltar, the Bosporus, and 
the Suez Canal. The Mediterranean is conveniently divided into an eastern 
basin and a western basin, which are joined by the Strait of Sicily and the 
Strait of Messina. 
    The total water area of the Mediterranean is 965,900 misup2 (2,501,000 
kmsup2), and its average depth is 5040 ft (1536 m). The greatest depth in the 
western basin is 12,200 ft (3719 m), in the Tyrrhenian Sea. The eastern basin 
is deeper, with a greatest depth of 18,140 ft (5530) m in the Ionian Sea about 
55 km off the Greek mainland. The Atlantic tide disappears in the Strait of 
Gibraltar. The tides of the Mediterranean are predominantly semidiurnal. 
 �34s[John Lyman]
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�6v�2v�32sMegaphone   �21m �33s 
    A device for increasing the efficiency of sound radiation and confining 
the sound of a speaker along a preferred direction. In the configuration most 
often used, the megaphone consists of a conical horn. The user speaks into the 
smaller end of the horn, which is of approximately the same cross-sectional 
area as a human's mouth. 
    Functionally, the megaphone acts as an acoustical transformer. The factors 
which influence its design are mouth diameter, throat diameter, length, and 
rate of flare. 
 �34s[William J. Galloway]
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�6v�2v�32sMeiosis �21m �33s
    A special type of cell division that occurs in diploid or polyploid
tissues and brings about a reduction in chromosome number. It is usual for the
chromosome number to be halved. It is therefore the opposite of fertilization,
for meiosis produces the nuclei which themselves, or their distant mitotic
descendants, fuse to form a zygote.  See  MITOSIS.
    Meiosis compensates for the doubling of the chromosome number brought
about at fertilization by reducing it again before the next fertilization.
Meiosis occurs immediately prior to gamete formation in most animals. There
are usually a further two or three mitotic divisions before gametes are formed
in higher plants.  See	GAMETOGENESIS; REPRODUCTION (ANIMAL); REPRODUCTION
(PLANT).

    First division.  Meiosis is a continuous sequence of events, but for ease
of description and reference certain readily recognizable stages have been
named (see illustration). The first division involves the separation of
homologs that have paired with each other.
    Prophase 1. There are five stages delineated for the sequence of events
occurring during prophase 1.
    1. Leptotene. This is the earliest stage; the chromosomes appear as long
thin threads and are differentiated along their length into beadlike
structures named chromomeres. In homologous chromosomes these chromomeres
correspond in size, number, and position, and together with the position of
centromeres, nucleolar organizers, and heterochromatic regions; by pachytene
they enable the cytologist to recognize particular chromosomes.
    2. Zygotene. Here homologous chromosomes begin to pair in a highly
specific way. The pairing, or synapsis, may begin at any place along the
length of the chromosome, but more usually it starts near the ends. The
precision of pairing is such that homologous chromomere pairs with homologous
chromomere. While the pairing is taking place, continued shortening,
thickening, and coiling result in the homologs being twisted about each other
so that there is close alignment along their entire length. When pairing is
complete, the diploid complement is reduced to the haploid number of bivalent
chromosomes or bivalents.
    3. Pachytene. The term refers to the more obvious thickening of the
chromosomes which reflects their paired nature and the fact that chromosome
duplication has occurred.
    Another event of great importance occurs during pachytene, although the
cytological consequences cannot be seen until the next stage. By a process
which is far from clear, chromatids can break, and it is of consequence when
two nonsister chromatids break at equivalent positions. These positions may be
within or between genes and usually, as far as can be ascertained, concern
breakage between the equivalent chemical bondings in the two chromatids.
Rejoining occurs between the original partners to the break, or an exchange
may take place.
    4. Diplotene. The attraction between the homologous chromosomes observed
during zygotene gives way to repulsion. The bivalents are however held
together by the points of exchange which, when opened out by the repulsion,
appear as crosses or chiasmata. It is also clear that each bivalent consists
of four strands at this stage, each chromosome having duplicated to form two
chromatids.
    5. Diakinesis. Intense internal coiling of the sister chromatids now
occurs and this results in greater shortening and thickening than has been
seen hitherto. One consequence is that the chromatid pairs on either side of a
chiasma twist through approximately 90DG at the point of exchange, revealing
the characteristic loops and half loops. The nucleoli and the nuclear membrane
break down.
    Prometaphase 1. Following the breakdown of the nuclear membrane, a spindle
is formed. Sister kinetochores of each chromosome face the same pole (syntelic
orientation). Unpaired chromosomes (univalents) which sometimes occur when
pairing or chiasma formation fails may show either amphitelic or syntelic
behavior. Also, the orientation of one bivalent on the spindle is independent
of the others. Following this autoorientation, congression occurs and this
leads to full metaphase.
    Metaphase 1. In the first metaphase of meiosis the centromeric regions of
the homologs come to lie equidistant on either side of the equator; the actual
distance is dependent upon the position of the proximal chiasma. There is
considerable tension on the bivalents at this stage, so that the position of
the chiasmata may have slipped toward the chromosome ends.
    Anaphase 1. In meiosis this duplication does not occur until the anaphase
of the second nuclear division. Instead, the homologs (each consisting of a
chromatid pair) are separated by the contraction of the spindle fibers pulling
them to opposite poles of the spindle. In meiosis there is often an elongation
of the spindle at late anaphase 1, which ensures that the products of the
first division are well separated before the onset of the second.
    Telophase 1 and interkinesis. These stages, which may not occur in some
organisms, involve the regrouping of the haploid sets of chromatid pairs into
organized nuclei. There may be a relaxation of the intense coiling begun at
diakinesis.

    Second division.  The second division is mechanically the same as a normal
mitotic division, but at metaphase sister chromatids are held together only at
the centromere, so that chromosomes appear X- or V-shaped. Chromosomes are
present in a haploid instead of a diploid number. At anaphase 2, centromeres
of sister chromatids move to opposite poles where the end result is four
haploid nuclei, and the reduction in chromosome number has been accomplished.

    Significance.  Apart from the reduction of chromosome number, there are
two other major consequences of meiosis. The independent orientation of
bivalents at metaphase 1 ensures random assortment of whole chromosomes, while
chiasma formation results in nonsister chromatids exchanging lengths of
chromosome material, thereby recombining genes which they carry, leading to
the breakup of linked allelic combinations. As a result, it is highly unlikely
that any of the meiotic products of an individual will be exactly the same as
any other, unless it contains devices for restricting crossing-over
autoorientation.  See  CELL DIVISION; GENE.
 �34s[Graham G. Nicholson]
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�6v�2v�32sMeissner effect   �21m �33s 
    The expulsion of magnetic flux from the interior of a superconducting 
metal when it is cooled in a magnetic field to below the critical temperature, 
near absolute zero, at which the transition to superconductivity takes place. 
It was discovered by Walther Meissner in 1933, when he measured the magnetic 
field surrounding two adjacent long cylindrical single crystals of tin and 
observed that at - 452.97DGF (3.72 K) the Earth's magnetic field was expelled 
from their interior. This indicated that at the onset of superconductivity 
they became perfect diamagnets. This discovery showed that the transition to 
superconductivity is reversible, and that the laws of thermodynamics apply to 
it. The Meissner effect forms one of the cornerstones in the understanding of 
superconductivity, and its discovery led F. London and H. London to develop 
their phenomenological electrodynamics of superconductivity.  See  
DIAMAGNETISM; THERMODYNAMIC PRINCIPLES. 
    The magnetic field is actually not completely expelled, but penetrates a 
very thin surface layer where currents flow, screening the interior from the 
magnetic field. 
    The Meissner effect is subject to limitations. Full diamagnetism is not 
observed in polycrystalline samples, and the effect is not observed in impure 
samples or samples with certain geometrics, such as a round flat disk, with 
the magnetic field parallel to the axis of rotation.  See  SUPERCONDUCTIVITY. 
 �34s[Hans W. Meissner]
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�6v�2v�32sMelanconiales   �21m �33s 
    An order of fungi of the class Fungi Imperfecti. A great number of species 
are plant pathogens commonly causing anthracnose. About 90 genera and 1000 
species are recognized. Conidiophores, or hyphae bearing conidia, are densely 
aggregated on a cushion- or disk-shaped mass of hyphae, an acervulus (see 
illustration). Acervuli, at first covered by the epiderm or the bark, form a 
pustule and later burst through the substratum, that is, are erumpent. The 
conidia are always slimy spores. This order contains only one family, 
Melanconiaceae.  
    The genera of the Fungi Imperfecti are usually arranged into spore groups, 
depending on characteristics of the spore, such as the number of cells in the 
spore, shape of the spore, and whether the spore is bright or dark.  See  
FUNGI IMPERFECTI; PLANT PATHOLOGY. 
 �34s[N. F. Buchwald]
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�6v�2v�32sMelanterite   �21m �33s 
    A mineral having composition FeSO4 PRODUCT 7H2O. Melanterite occurs mainly 
in green, fibrous or concretionary masses, or in short, monoclinic, prismatic 
crystals. Luster is vitreous, hardness is 2 on Mohs scale, and specific 
gravity is 1.90. 
    Melanterite is a common secondary mineral derived from oxidation and 
hydration of iron sulfide minerals such as pyrite and marcasite. Its occurence 
is widespread. It is not an ore mineral.  See  MARCASITE; PYRITE. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sMelilite   �21m �33s 
    A complete solid solution series ranging from gehlenite, Ca2Al2SiO7, to 
akermanite, Ca2MgSi2O7, often containing appreciable Na and Fe. The Mohs 
hardness is 5 - 6, and the density increases progressively from 2.94 for 
akermanite to 3.05 for gehlenite. The luster is vitreous to resinous, and the 
color is white, yellow, greenish, reddish, or brown. Akermanite-rich varieties 
occur in thermally metamorphosed siliceous limestones and dolomites, but more 
gehlenite-rich ones result if A1 is present. Melilites are found instead of 
plagioclase in silica-deficient, feldspathoid-bearing basalts.  See  SILICATE 
MINERALS.  
 �34s[Lawrence Grossman]
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�6v�2v�32sMelioidosis   �21m �33s 
    An endemic disease of animals and humans. The causative organism, 
Pseudomonas pseudomallei, exists primarily as a saprophyte in the soil and 
water of rice-growing areas of southeastern Asia. 
    The multiple forms of melioidosis in humans present confusing symptoms. A 
2 - 5 day fulminating, pulmonary, septicemic disease is seldom diagnosed 
before death. An indolent pulmonary form with thin-walled abscesses resembles 
tuberculosis except for chills and fever. A chronic systemic form produces 
widespread semigranulomatous abscesses in many organs. 
 �34s[Winston R. Miller]
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�6v�2v�32sMelting point �21m �33s
    The temperature at which a solid changes to a liquid. For pure substances,
the melting or fusion process occurs at a single temperature, the temperature
rise with addition of heat being arrested until melting is complete.
    Melting points reported in the literature, unless specifically stated
otherwise, have been measured under an applied pressure of 1 atm (10 sup 5
pascals), usually 1 atm of air. (The solubility of air in the liquid is a
complicating factor in precision measurements.) Upon melting, all substances
absorb heat, and most substances expand; consequently an increase in pressure
normally raises the melting point. A few substances, of which water is the
most notable example, contract upon melting; thus, the application of pressure
to ice at 32DGF (0DGC) causes it to melt. Large changes in pressure are
required to produce significant shifts in the melting point.
    For solutions of two or more components, the melting process normally
occurs over a range of temperatures, and a distinction is made between the
melting point, the temperature at which the first trace of liquid appears, and
the freezing point, the higher temperature at which the last trace of solid
disappears, or equivalently, if one is cooling rather than heating, the
temperature at which the first trace of solid appears.	See  PHASE
EQUILIBRIUM; SOLUTION; SUBLIMATION; TRIPLE POINT (THERMODYNAMICS).
 �34s[Robert L. Scott]
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�6v�2v�32sMemory   �21m �33s 
    The recollection of past events or the performance of previously learned 
skills without practice after the passage of time are evidence of memory 
function. In order to say that an organism remembers, the present behavior of 
that organism must be influenced by its past experience. However, this is not 
a sufficient condition since it should include changes brought about by 
physical growth or fatigue. For one to say that memory has occurred, the 
organism must compare a given stimulus with a stimulus which has occurred 
previously, and its behavior must reflect this comparison. The concept of 
memory thus indicates the formation of some kind of associative link between 
two stimuli separated in time. For this to occur the first stimulus must 
produce a persistent change in the organism. This change is often referred to 
as the memory trace or engram. To demonstrate that this change has occurred, 
the performance of the organism must be altered in some measurable way. In 
experimental studies of memory, the organism is presented with controlled 
stimuli, and in some quantitative way the behavioral output is measured after 
the passage of time. Such studies are designed both to define the 
psychological laws of memory and to investigate the neurological substrate in 
which memory occurs. 
 �34s[Anne Geller ;Murray E. Jarvik]
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�6v�2v�32sMendelevium   �21m �33s 
    A chemical element, Md, atomic number 101, the twelfth member of the 
actinide series of elements. Mendelevium does not occur in nature; it was 
discovered and is prepared by artificial nuclear transmutation of a lighter 
element.  
    Known isotopes of mendelevium have mass numbers from 248 to 258 and 
half-lives from a few seconds to about 55 days. They are all produced by 
charged-particle bombardments of more abundant isotopes. The amounts of 
mendelevium which are produced and used for studies of chemical and nuclear 
properties are usually less than about a million atoms; this is of the order 
of a million times less than a weighable amount. 
    Studies of the chemical properties of mendelevium have been limited to a 
tracer scale. The behavior of mendelevium in ion-exchange chromatography shows 
that it exists in aqueous solution primarily in the 3+ oxidation state 
characteristic of the actinide elements. However, it also has a dipositive 
(2+) and a monopositive (1+) oxidation state. See ACTINIDE ELEMENTS; 
TRANSURANIUM ELEMENTS. 
 �34s[Glenn T. Seaborg]
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�6v�2v�32sMendelism �21m �33s
    The basic laws of inheritance  discovered in 1865 by Gregor Mendel, a monk
in the Augustinian monastery at Bruenn, Moravia. Mendel's laws were formulated
to explain the basis for results of his experiments on the inheritance of
seven pairs of alternative characteristics in the garden pea. He studied each
character separately by making hybrids between plants which were constant for
one of a pair of alternative traits, such as round versus wrinkled seeds,
green versus yellow pods, and tall versus short stems. In the first hybrid
generation Mendel noted that each character of the hybrid resembles that of
one of the parental forms. These characters he called dominant, and the
contrasting hidden ones, recessive.

    Law of segregation.  In a second generation, bred by self-pollinating the
hybrid, Mendel observed that both the dominant and the recessive character
appeared unaltered and in the proportion of three dominant to one recessive.
He extended his experiments to subsequent generations and found that plants
showing a recessive characteristic bred true, whereas among the dominants one
bred true to two that bred like the original hybrid. Mendel formulated the
following hypothesis to account for these results. The characters are
controlled in their inheritance by units - now called genes - which occur
paired in each parent and are separated so that they occur singly in the
mature reproductive cells, the gametes. The units become paired once again
following the union of gametes in the formation of the next generation. When
the paired units are dissimilar a hybrid results. In the hybrid the paired
units remain unaltered and subsequently are segregated into approximately
equal numbers of gametes. These recombine randomly with reference to the
particular units they contain in forming the next generation.  See  GENE;
RECOMBINATION (GENETICS).
    Diagrammatically this may be represented as follows:
Parent generation:
AA (dominant) aa (recessive)=cGametes of parents:
		 A	       a
First hybrid generation:
		 Aa (resembles dominant parent)
Gametes of hybrid:
		 (1/2 A - 1/2 a) X (1/2 A + 1/2 a)
		      Eggs	    Pollen
Second hybrid generation:
		 1/4 AA + 1/4 Aa + 1/4 Aa + 1/4 aa
		     3/4 dominant: 1/4 recessive

    Law of independent assortment.  Mendel also investigated the offspring of
hybrids involving several alternative characters. To cite one experiment, he
pollinated a seed parent of round yellow peas (AA and BB, both dominant
characters) with pollen from a parent plant of wrinkled green peas (aa and
bb). The first hybrid generation formed round yellow seeds and the second
hybrid generation yielded seeds of four sorts in the following numbers: 315
round and yellow, 101 wrinkled and yellow, 108 round and green, and 32
wrinkled and green. Mendel adduced that this ratio resulted from independent
assortment of the hereditary units controlling these two characters. That is,
in the formation of the gametes of the hybrid one-half contain A and one-half
a; similarly one-half contain B and one-half b.  Each pair is sorted out
independently of the other so that the hybrid produces four kinds of gametes
in approximately equal numbers: The four kinds of gametes unite as random to
form nine different genetic types in the second generation. With dominance,
only four differently appearing types are observed.
    Thus Mendel's hypothesis predicts four differently appearing types in the
proportions 9 round, yellow seeds: 3 wrinkled, yellow seeds: 3 round, green
seeds: 1 wrinkled, green seed. See GENETICS

 �34s[Harold H. Smith]
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�6v�2v�32sMeninges   �21m �33s 
    In mammals, the three membranes that cover the brain and spinal cord: the 
dura mater, the arachnoid membrane, and the pia mater. 
    The outermost, the dura mater, is a tough, fibrous, double-layered 
structure that is adherent to the skull. The inner layer of the dura mater 
sends separating sheets between the cerebral hemispheres and between the 
cerebrum and cerebellum. It also contains large venous sinuses and forms 
sheaths for nerves leaving the skull. The middle layer, the arachnoid, is a 
delicate serous layer loosely investing the brain. Below this is the spongy 
subarachnoid cavity which contains the circulating cerebrospinal fluid. The 
innermost layer, the pia mater, is a vascular layer which closely follows each 
convolution of the brain. Together the meninges furnish protection, blood 
supply, drainage, and cerebrospinal channels for the brain.  See  NERVOUS 
SYSTEM (VERTEBRATE). 
 �34s[Thomas S. Parsons]
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�6v�2v�32sMeningitis   �21m �33s 
    An inflammation of the meninges and brain, caused by a variety of agents, 
including viral, bacterial, and protozoan. It is a common disease which may 
result in serious complications.  See  MENINGOCOCCUS. 
    Meningitis usually results from a previous infection of the bloodstream, 
but sometimes it results from direct extension of an infection of a contiguous 
site, such as the mastoid or the sinuses. A fracture of the skull or a 
congenital defect of the meninges may contribute; a respiratory infection 
frequently precedes meningitis. 
    Diagnosis is based upon clinical signs, such as headache, vomiting, 
convulsions, fever, and in the case of the meningococcus, a rash. Diagnosis is 
confirmed by examination of a sample of spinal fluid withdrawn by lumbar 
puncture.  
 �34s[William Bradford]
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�6v�2v�32sMeningococcus   �21m �33s 
    A major human pathogen belonging to the bacterial genus Neisseria, and the 
cause of meningococcal meningitis and meningococcemia. The official 
designation is N. meningitidis. The meningococcus is a gram-negative, aerobic, 
nonmotile diplococcus. It is fastidious in its growth requirements and is very 
susceptible to adverse physical and chemical conditions. 
    Humans are the only known natural host of the meningococcus. Transmission 
occurs by droplets directly from person to person. Fomites and aerosols are 
probably unimportant in the spread of the organism. The most frequent form of 
host-parasite relationship is asymptomatic carriage in the nasopharynx. 
    The most common clinical syndrome caused by the meningococcus is 
meningitis, which is characterized by fever, headache, nausea, vomiting and 
neck stiffness and has a fatality rate of 15% (higher in infants and adults 
over 60). Disturbance of the state of consciousness quickly occurs, leading to 
stupor and coma. Many cases also have a typical skin rash consisting of 
petechiae or purpura.  See  MENINGITIS. 
 �34s[Ronald Gold]
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�6v�2v�32sMenopause   �21m �33s 
    The irreversible cessation of regular monthly uterine bleeding in the 
adult human female, marking the end of her ability to become pregnant. The 
mean age of menopause (occurrence of the last menses) in the United States is 
51 plus or minus 2.5 years. Menopause probably occurs because the ovary runs 
out of eggs and the cyclic rise and fall of brain and ovarian hormones 
designed to prepare the uterus to receive and nourish pregnancy no longer 
occurs.  
    Menopause is one event in the climacteric, the period of time during which 
the reproductive capability slows down and finally stops. The biochemical 
hallmark of this period is a reduction in estrogen production by the ovary. 
Some estrogen continues to be produced by the adrenal gland and the fatty 
tissues throughout the body, but this amount is very small compared with 
premenopausal levels. A whole host of psychological problems have been 
attributed to estrogen deprivation, but well-documented proof of these 
relationships is lacking.  See  HORMONE; MENSTRUATION. 
 �34s[G. Guzinski]
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�6v�2v�32sMenstruation   �21m �33s 
    Periodic sloughing of the uterine lining in women of reproductive age. 
Menstrual bleeding indicates the onset of the menstrual cycle, which lasts an 
average of 27 30 days, although ranges of 21 - 60 days have been recorded. 
    The principal control mechanism is a complex hormonal regulation. The 
follicle-stimulating hormone (FSH) of the anterior pituitary gland stimulates 
the developing ovarian follicle, which secretes increasing amounts of 
estrogens during the first half of the menstrual cycle. The estrogenic 
hormones cause a proliferation and thickening of the uterine mucosa together 
with blood vessel and gland enlargement. During the middle of the cycle the 
ovum is extruded (ovulation) from the ovarian follicle. The remaining empty 
follicle becomes the corpus luteum, formed by the ingrowth of connective 
tissue, which begins to secrete progesterone to prepare the uterus for 
pregnancy. If the ovum is not fertilized, the corpus luteum degenerates, 
progesterone secretion is thus greatly reduced, and the vascularized mucosa of 
the uterus is sloughed off.  See  ADENOHYPOPHYSIS HORMONE; ESTROGEN; 
FERTILIZATION; PROGESTERONE; UTERUS. 
    The onset of menstruation (menarche) occurs between the ages of 9 and 16, 
with the majority of females beginning at 12 - 14 years. Duration of the flow 
and cycle length may be quite variable during these early years. Gradually, a 
degree of regularity is established but the flow may last 3 - 7 days; this, as 
well as the copiousness of the flow, may vary from one period to another. 
Cessation of menses (menopause) occurs predominantly in women 49 - 53 years 
old.  See  MENOPAUSE. 
    There is no menstrual bleeding during pregnancy. This amenorrhea sometimes 
continues during nursing until weaning; however, ovulation and conception are 
still possible at this time.  See  ESTRUS; LACTATION; PREGNANCY. 
 �34s[Sybil P. Parker]
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�6v�2v�32sMental deficiency   �21m �33s 
    A condition, also known as mental retardation, which is characterized by 
intellectual retardation, social inadequacy, and persistent dependency. This 
condition is evidence of an arrest in development, and is apparent from birth 
or an early age. The causes range from genetic factors to metabolic 
dysfunction, specific injury to the central nervous system, and environmental 
factors.  
    Mental deficiency is divided into a number of subgroups on the basis of 
cause, degree of manifest intellectual retardation, and ability to be trained. 
From the aspect of cause, the mentally deficient are classified into nine 
groups: infections and intoxications; trauma or physical agent; metabolism or 
nutrition; gross brain disease (postnatal); unknown prenatal influences; 
chromosomal abnormality; gestational disorders; following psychiatric 
disorder; and environmental influences. 
    Mentally retarded individuals are often classified in terms of their 
intelligence quotient (IQ). The highest groups, designated as ``mild,'' in 
general has IQ limits of 55 - 70. The next lowest group in degree of 
retardation, with IQ limits of 40 - 54, is known as ``moderate.'' The lowest 
groups, with demonstrable IQs generally below 39, are known as ``severe'' and 
``profound,'' and include approximately 3% of the known cases.  See  
INTELLIGENCE.  
    Classification terms more commonly used in public schools, and which 
roughly correspond to those referred to above, are educable, trainable, and 
multihandicapped. The educable are those who, by special training, can be 
expected to attain relative independence. The trainable are those who, with 
special training, are capable of a minor degree of self-help and social 
participation, but in a sheltered environment. 
 �34s[Henry Leland]
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�6v�2v�32sMental illness   �21m �33s 
    An imprecise term commonly applied to a variety of disturbances of 
thinking, emotional reaction, and conduct. Types of disturbances are: social 
maladaptations, psychoses, psychoneuroses, mental retardation (amentia), and 
intellectual deterioration (dementia). Some are inherited, others acquired. 
Some are clearly related to disease of the brain, and there is a demonstrable 
structural change (lesion); others are probable diseases, but a structural 
change has never been seen.  See  DEMENTIA; MENTAL DEFICIENCY; PSYCHONEUROSIS; 
PSYCHOPATHY; PSYCHOSIS. 
 �34s[Raymond D. Adams]
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�6v�2v�32sMenthol   �21m �33s 
    A monocyclic, saturated, secondary terpene alcohol with the formula below. 
Menthol contains three asymmetric carbon atoms (starred in the formula). It 
can exist in four externally compensated and eight optically active forms. The 
only forms encountered in nature are the l-menthol and d-neomenthol. 
Commercial l-menthol is isolated principally from the oil of Mentha arvensis. 
It possesses a distinct peppermint flavor and gives the impression of cooling 
the mouth and skin.  See  TERPENE. 
 �34s[William Mosher]
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�6v�2v�32sMercaptan   �21m �33s 
    One of a group of organosulfur compounds which are also called thiols or 
thio alcohols and which have the general structure RSH. Aromatic thiols are 
called thiophenols, and biochemists often refer to thiols as sulfhydryl 
compounds. The unpleasant odor of volatile thiols causes them to be classed as 
stenches, but the odors of many solid thiols are not unpleasant. 
    Mercaptans (1) form salts with bases, (2) are easily oxidized to 
disulfides and higher oxidation products such as sulfonic acids, (3) react 
with chlorine (or bromine) to form sulfenyl chlorides (or bromides), and (4) 
undergo additions to unsaturated compounds, such as olefins, acetylenes, 
aldehydes, and ketones. The insoluble mercury salts (mercaptides) are used to 
isolate and identify mercaptans.  See  ORGANOSULFUR COMPOUND. 
 �34s[Norman Kharasch]
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�6v�2v�32sMerchant ship   �21m �33s 
    A power-driven ship employed in commercial transport on the oceans and 
large inland bodies of water such as the Great Lakes. The relatively small 
craft used for inland waterway transportation are not commonly referred to as 
ships.  
    Commodities transported by water are classified as break-bulk, unitized, 
or bulk (dry or liquid) cargoes.  Generally, water cargo transportation is 
cheaper per ton-mile than land or air transportation; approximately 90% of the 
United States overseas trade revenue is waterborne, while the rest is 
airborne.  See  MARINE CONTAINERS. 
    A passenger ship, as defined by International Safety of Life at Sea 
(SOLAS) rules, carries more than 12 passengers on international voyages. 
Passenger vessels that also transport cargo are called passenger-cargo ships. 
 �34s[Amelio M. D'Arcangelo]
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�6v�2v�32sMercury battery   �21m �33s 
    A primary dry-cell battery consisting of a zinc anode, a cathode of 
mercuric oxide (HgO) mixed with graphite, and an electrolyte of potassium 
hydroxide (KOH) saturated with zinc oxide (ZnO). With carefully purified 
materials and balanced amounts of ZnO and HgO, the cell has very low 
self-discharge and makes efficient use of the active materials. In some cells 
which require long-term continuous drains, for example, in hearing aid use, 
MnO2 is added to the HgO. 
    Contained in a steel can, the active materials are separated by a porous 
material which prevents migration of conducting particles from the mercuric 
oxide pellet. The electrolyte is completely absorbed in the active materials, 
separator, and absorbent materials. The steel can serves as the contact to the 
HgO. A metal top with a concentric neoprene grommet closes off the top of the 
can and serves as the contact to the zinc. 
    The ampere-hour capacity of mercury cells is relatively unchanged with 
variation of discharge schedule and to some extent with variation of discharge 
current. Outstanding features of mercury cells include flat discharge curve, 
small variation in capacity with intermittent or continuous discharge, shelf 
life of several years, and good high-temperature characteristics.  See  DRY 
CELL; PRIMARY BATTERY; RESERVE BATTERY. 
 �34s[Jack Davis]
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�6v�2v�32sMercury poisoning   �21m �33s 
    Chronic mercury poisoning (mercurialism) presents five main symptoms: 
numbness, staggering gait, constriction of visual field (``tunnel vision''), 
garbled speech (dysarthria), and tremor. In addition, emotional disturbances 
have been noted in childhood cases, and infants with congenital intoxication 
also suffered from impaired chewing or swallowing. Convulsions and mental 
retardation often occurred in the childhood cases.  
    These symptoms of chronic poisoning are almost totally traceable to 
lesions in the central nervous system, principally in the brain. Autopsies of 
those who died soon after exposure revealed degenerative lesions in the liver, 
heart muscle fibers, and tubules of the kidneys.  See  MERCURY. 
 �34s[N. Karle Mottet]
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�6v�2v�32sMercury-vapor lamp   �21m �33s 
    A vapor or gaseous discharge lamp in which the arc discharge takes place 
in mercury vapor. This lamp is widely used for roadway and all other forms of 
illumination, and as a source of ultraviolet radiation for industrial 
applications.  
    The arc discharge takes place in a transparent tube of fused silica, or 
quartz, and this quartz tube is usually mounted inside a larger bulb of glass 
(see illustration). The outer bulb reduces the ultraviolet radiation of the 
inner arc tube, encloses and protects the mount structure, and can be coated 
with phosphors that greatly improve the color of the light emitted. 
    The arc tube has electrode assemblies mechanically sealed into each end. 
At one end of the arc tube, a smaller starting electrode is located close to 
the main electrode. The starting electrode is connected through a high 
resistance to the opposite electric polarity of the adjacent main electrode. 
The arc tube itself is filled with argon gas and a small amount of pure 
mercury before being sealed. 
    Radiation from the mercury arc is confined to four specific wavelengths in 
the visible portion of the spectrum and several strong lines in the 
ultraviolet. The visible radiation from clear mercury lamps provides light 
with a distinct blue-green appearance and poor color rendition. Red objects, 
for example, look brown or black, and human skin looks unattractive. For this 
reason, most mercury lamps have a phosphor color-correcting coating, and clear 
bulbs are used only where appearance of colors is secondary to some gain in 
efficiency.  See  ILLUMINATION; LAMP; ULTRAVIOLET LAMP; VAPOR LAMP. 
 �34s[T. F. Neubecker]
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�6v�2v�32sMercury (element)   �21m �33s 
    A chemical element, Hg, atomic number 80 and atomic weight 200.59. Mercury 
is a silver-white liquid at room temperature (melting point -38.89DGC or 
-37.46DGF); it boils at 357.25DGC (675.05DGF) under atmospheric pressure. It 
is a noble metal that is soluble only in oxidizing solutions. Solid mercury is 
as soft as lead. The metal and its compounds are very toxic. With some metals 
(gold, silver, platinum, uranium, copper, lead, sodium, and potassium, for 
example) mercury forms solutions called amalgams.  See  AMALGAM; MERCURY 
POISONING; TRANSITION ELEMENTS. 
    In its compounds, mercury is found in the 2+, 1+, and lower oxidation 
states, for example, HgCl2, Hg2Cl2, or Hg3 (AsF6)2. Often the mercury atoms 
are doubly covalently bonded, for example, Cl--Hg--Cl or Cl--Hg--Hg--Cl. Some 
mercury (II) salts, for example, Hg(NO3)2 or Hg(ClO4)2, are quite soluble in 
water and dissociate normally. The aqueous solutions of these salts react as 
strong acids because of hydrolysis. Other mercury (II) salts, for example, 
HgCl2 or Hg(CN)2, also dissolve in water, but exist in solution as only 
slightly dissociated molecules. There are compounds in which mercury atoms are 
bound directly to carbon or nitrogen atoms, for example, H3C--Hg--CH3 or 
H3C--CO--NH--Hg--NH--CO--CH3. In complex compounds, for example, K2(HgI4), 
mercury often has three or four bonds. 
    Metallic mercury is used as a liquid contact material for electrical 
switches, in vacuum technology as the working fluid of diffusion pumps, for 
the manufacture of mercury-vapor rectifiers, thermometers, barometers, 
tachometers, and thermostats, and for the manufacture of mercury-vapor lamps. 
It finds application for the manufacture of silver amalgams for tooth fillings 
in dentistry. Of importance in electrochemistry are the standard calomel 
electrode, used as the reference electrode for the measurement of potentials 
and for potentiometric titrations, and the Weston standard cell.  See  CALOMEL 
ELECTRODE.  
    Mercury is commonly found as the sulfide, HgS, frequently as the red 
cinnabar and less often as the black metacinnabar. A less common ore is the 
mercury (I) chloride. Occasionally the mercury ore contains small drops of 
metallic mercury.  See  CINNABAR. 
    The surface tension of liquid mercury is 484 dynes/cm, six times greater 
than that of water in contact with air. Hence, mercury does not wet surfaces 
with which it is in contact. In dry air metallic mercury is not oxidized. 
After long standing in moist air, however, the metal becomes coated with a 
thin layer of oxide. In air-free hydrochloric acid or in dilute sulfuric acid, 
the metal does not dissolve. Conversely, it is dissolved by oxidizing acids 
(nitric acid, concentrated sulfuric acid, and aqua regia). 
 �34s[Klaus Brodersen]
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�6v�2v�32sMercury (planet)   �21m �33s 
    The planet nearest to the Sun. It is visible to the naked eye shortly 
after sunset or shortly before sunrise when it is near its greatest angular 
distance from the Sun. The diameter is 3031PLUS/MINUS1 mi (4878PLUS/MINUS2 
km). The mass is 0.055 times the mass of the Earth. Mercury has the highest 
density of any planet except Earth, indicating that iron and nickel are its 
main chemical constituents. The surface of the planet is composed of silicate 
rock, heavily cratered from exposure to impacting meteoritic debris. There is 
no atmosphere, so that the surface retains the record of nearly the entire 
past geological history. 
    Understanding of Mercury was greatly advanced in 1974 and 1975 when the 
United States spacecraft Mariner 10 made three passes close to the planet. 
    On Mercury, the maximum surface temperature varies with distance from the 
Sun, at perihelion reaching 700 K (800DGF) at the equator. At night, however, 
the unshielded surface cools rapidly to below 110 K (-340DGF), producing the 
greatest diurnal variations in temperature in the solar system. A meter below 
the surface, the temperature hardly varies from an average value of about 350 
K (170DGF). Mariner 10 obtained photographic coverage of nearly half of 
Mercury, with 20% of the surface imaged at a resolution of 1 km or better. The 
face of Mercury shows evidence of internal (endogenic) geologic processes as 
well as the scars of impacts (see illustration). Much of the surface appears 
to be flooded with lava, in the same manner as the lunar maria, probably as 
the result of extensive past volcanism. There are also many great 
compressional scarps that cut across other topographic features and may have 
been caused by shrinkage of the planet as its interior cooled. 
 �34s[David Morrison]
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�6v�2v�32sMeridian   �21m �33s 
    That half of a great circle on Earth that passes through points having the 
same longitude and terminates at the North and South poles. The meridian from 
which longitudes are measured is the one passing through Greenwich, near 
London, England; it is called the prime meridian. 
    The celestial meridian is a great circle on the celestial sphere passing 
through the two celestial poles and the observer's zenith. Two branches of it 
are distinguished, each extending from pole to pole, the upper branch 
containing the zenith and the lower the nadir.  See  ASTRONOMICAL COORDINATE 
SYSTEMS.  
 �34s[Gerald M. Clemence]
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�6v�2v�32sMermithoidea   �21m �33s 
    A superfamily of nematodes containing two families: the Tetradonematidae, 
which are parasites all their life; and the more common Mermithidae, which are 
parasites only as juveniles. The Mermithidae have the esophagus embedded in a 
stichosome and lack an anus as adults, the intestine serving as a storage 
organ. Hosts, which include insects, spiders, and snails, are infected by 
eating eggs or by the attack of infective juveniles. The juveniles grow in the 
host's hemocoele and emerge from the insects usually prior to host pupation, 
at which time the host usually dies. Maturation, mating, and oviposition occur 
in soil and fresh water.  See  NEMATA. 
 �34s[Bruce Hopper]
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�6v�2v�32sMerogony   �21m �33s 
    The normal or abnormal development of a part of an egg following cutting, 
shaking, or centrifugation of the egg before or after fertilization. Depending 
on the character of the nucleus present in the egg fragment, the following 
types of merogony are recognized: (1) diploid merogony, the nucleus being the 
normal fusion product of the egg and sperm nuclei and thus diploid; (2) 
andromerogony, development with the haploid sperm nucleus; (3) gynomerogony, 
development of a fragment of a fertilized egg with the haploid egg nucleus; 
(4) parthenomerogony, development of a nucleated fragment of an unfertilized 
egg following parthenogenetic stimulation.  See  ANDROGENESIS; GYNOGENESIS; 
PARTHENOGENESIS.  
 �34s[Gerhard Fankhauser]
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�6v�2v�32sMeromictic lake   �21m �33s 
    A lake whose water is permanently stratified and therefore does not 
circulate completely throughout the basin at any time during the year. 
Normally lakes in the temperate zone mix completely during the spring and 
autumn when water temperatures are approximately the same from top to bottom. 
In meromictic lakes there are no periods of overturn or complete mixing 
because seasonal changes in the thermal gradient are either small or 
overridden by the stability of a chemical gradient, or the deeper waters are 
physically inaccessible to the mixing energy of the wind. Most commonly, the 
vertical stratification is stabilized by a chemical gradient in meromictic 
lakes.  
    The upper stratum of water in a meromictic lake is mixed by the wind and 
is called the mixolimnion. The bottom, denser stratum, which does not mix with 
the water above, is referred to as the monimolimnion. The transition layer 
between these strata is called the chemocline. 
    Of the hundreds of thousands of lakes on the Earth, only about 120 are 
known to be meromictic. In general, meromictic lakes in North America are 
restricted to: sheltered basins that are proportionally very small in relation 
to depth and that often contain colored water, basins in arid regions, and 
isolated basins in fiords.  See  FRESH-WATER ECOSYSTEM; LAKE; LIMNOLOGY. 
 �34s[Gene E. Likens]
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�6v�2v�32sMerostomata   �21m �33s 
    A class of the phylum Arthropoda, subphylum Chelicerata. Merostomes are 
aquatic chelicerates, characterized by abdominal appendages bearing 
respiratory organs. Most merostomes are extinct; only the horseshoe crabs, 
comprising four species and three genera (Carcinoscorpio and Tachypleus of 
eastern Asia and Limulus polyphemus of eastern North America) survive. 
    The body of a merostome consists of a prosoma, or head, which lacks 
antennae, has a pair of compound eyes and a pair of simple median eyes, and 
bears the chelicerae (pincers) and five pairs of uniramous walking legs with 
gnathobases for mastication. The opisthosoma, or trunk, consists of 12 or 
fewer segments which may be freely articulating or partly or entirely fused 
into a solid shield; the opisthosoma bears the respiratory appendages. The 
telson (tail) is a solid, usually spikelike, structure.  See  CHELICERATA; 
LIVING FOSSILS. 
 �34s[Niles Eldredge]
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�6v�2v�32sMeson �21m �33s
    The generic name for any hadronic particle with baryon number zero. Such
particles were first envisaged in 1935 by H. Yukawa. In 1937 C. Anderson
established the existence of positively and negatively charged MU-mesons (now
known as muons) of mass 105.6 MeV in cosmic radiations, but these were soon
shown to have very weak coupling with nucleons, that is, to be nonhadronic and
therefore not ``mesons,'' as this name is used today. The nuclear force mesons
turned out to be the PI-mesons (pions), of mean mass about 138 MeV, first
identified in cosmic radiation by C. F. Powell in 1947.  See  BARYON; COSMIC
RAYS; HADRON; LEPTON.
    In 1948, studies of the cosmic radiation indicated the existence of
heavier mesons (masses about 495 MeV), now known as K mesons (kaons).  There are
four K mesons, K+, K-, and two neutral particles, K(0 OVER S) and K(0 OVER L),
and strong K+, K-, and K(0 OVER L), beams are obtainable from higher-energy
(MORE THAN OR EQUAL TO 30 GeV) proton accelerators.  Since the PI PLUS/MINUS, K
PLUS/MINUS, and K PLUS/MINUS mesons have relatively long lifetimes (about 10[sup
-8] to 10[sup -10]s), they are referred to as semistable.
    In 1961 the OMEGA-meson of mass 783 MeV was discovered through its rapid
decay to three pions in a bubble chamber investigation.  It is highly unstable,
with lifetime 6.6 x 10(sup -23)s, as corresponds to the natural width 10.1 MeV
observed in its mass value.  Such a short lifetime implies that the OMEGA-meson
decays through hadronic interactions.  Many such highly unstable heavy mesons
are not established, with lifetimes shorter than 10(sup -22)s, decaying
hadronically to lighter mesons, and more continue to be discovered.
    In 1974 the J/PSI meson of mass 3097 MeV was discovered.  Although it decays
to mesons, its natural width is only 0.063 MeV (lifetime 10[sup -20] s).  Also,
it decays electromagnetically, yielding an e+ e- or MU+ MU- pairs, at a rate
comparable with those for mesonic final states.  This new phenomenon gave the
first indication of the existence of heavy new quarks.	In consequence, its
study has often been termed the ``new physics.'' See J PARTICLE; QUARKS.
    Hadrons are now considered to be composite, consisting of spin-1/2 quarks
(q), corresponding antiquarks (q), and some number of gluons (g), the last being
the quanta of the intermediate field which binds the quarks and antiquarks to
form hadrons.  Baryon number B = +1/3 holds for a quark q, B = -1/3 for
antiquark q, while B = 0 holds for a gluon.  In this view, the simplest
possibility is that each meson is a quark-antiquark (q- BAR q) pair bound
together by the gluon field, and this model accounts quite well for most of the
known mesons and their properties.  See ELEMENTARY PARTICLE; GLUONS.
 �34s[Richard H.  Dalitz]
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�6v�2v�32sMesosauria   �21m �33s 
    An order of extinct aquatic reptiles, also known as Proganosauria, of 
latest Carboniferous or earliest Permian time, about 280,000,000 years ago. 
The best-known genus is Mesosaurus. Like many aquatic reptiles, mesosaurs have 
a very long snout. The teeth are numerous and long, and appear very delicate. 
They may have served for filter feeding on soft-bodied invertebrates. 
    Mesosaurs, an early offshoot of the Carboniferous ``stem reptiles'' or 
captorhinomorphs, were the first reptiles to invade marine waters, but 
apparently soon became extinct, leaving no descendants.  See  COTYLOSAURIA; 
REPTILIA.  
 �34s[Robert L. Carroll]
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�6v�2v�32sMesosphere   �21m �33s 
    An atmospheric layer stretching from the stratopause, at an altitude of 
about 30 mi (50 km), to the mesopause at about 53 mi (85 km). Like the 
troposphere, from which it is separated by the stratosphere, it is a layer in 
which the temperature generally decreases with height. Many of the atmospheric 
variations encountered in the mesosphere are linked to complicated processes 
in the underlying layers.  
    Special features of the mesosphere include a region of extremely strong 
winds centered near 40 mi (65 km), tidal oscillations and gravity waves of 
appreciable amplitude, meteors, and noctilucent clouds. The clouds are 
observed in middle and high latitudes near an altitude of 50 mi (80 km), in 
summer, when the Sun is at a small angle below the horizon. A light-scattering 
cloud prevails near the mesopause, over the summer pole. Meteors, especially 
numerous near 59 mi (95 km), commonly burn up before reaching the stratopause. 
Also in the domain of the mesosphere is the greater part of the D region, the 
lowest ionized layer of the atmosphere.  See  ATMOSPHERE; IONOSPHERE; METEOR; 
NOCTILUCENT CLOUDS; STRATOSPHERE. 
 �34s[Roderick S. Quiroz]
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�6v�2v�32sMesostigmata �21m �33s
    A suborder of the Acarina (also called Parasitiformes), commonly referred
to as the mites. Mesostigmata are characterized by a single pair of breathing
pores, or stigmata, located laterally in the middle of the idiosoma between
the second and third, or third and fourth, legs. A two- or three-tined palpal
claw and a tritosternum, which is located ventrally on the idiosoma just
behind the gnathosoma, are also characteristic for most mesostigmatid mites.
    Many of the Mesostigmata are predacious on other small arthropods or are
scavengers and fungus-feeders that make up an important segment of the soil
fauna. Some members of the family Phytoseiidae are beneficial to humans as
predators of the destructive spider mites, Tetranychidae. Other forms are
parasitic on insects and other invertebrates.
    Terrestrial arthropods are the only invertebrate hosts affected by
mesostigmatid mites. As a rule, phoresy is the predominant reason for the
association. Vertebrate hosts are restricted to mammals, birds, and reptiles.
The Mesostigmata that parasitize them are of two main sorts: They are either
internal parasites of the respiratory passages, or they are ectoparasitic
nidicoles; that is, they reproduce in the nests of their hosts.  See  ACARINA;
PHORESY.
 �34s[Joseph H. Camin ;Russell W. Strandtmann]
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�6v�2v�32sMesozoa   �21m �33s 
    A division of the animal kingdom sometimes ranked as intermediate between 
the Protozoa and the Metazoa. The Mesozoa comprise two orders of small, 
wormlike organisms, the Dicyemida and the Orthonectida. Both are parasitic in 
marine invertebrates. The body consists of a single layer of ciliated cells 
enclosing one or more reproductive cells. These body cells are rather constant 
in number and arrangement for any given species. The internal cells do not 
correspond to the entoderm of other animals, as they have no digestive 
function. The life cycles are complex, involving both sexual and asexual 
generations (metagenesis).  See  ANIMAL KINGDOM. 
 �34s[Bayard H. McConnaughey]
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�6v�2v�32sMesozoic   �21m �33s 
    In geology, the system of rocks younger than the Paleozoic and older than 
the Cenozoic; also, the geologic era during which those rocks were formed. The 
era is now believed to have extended from about 225,000,000 to 64,000,000 
years ago. The Mesozoic is commonly referred to as the Age of Reptiles 
inasmuch as these animals dominated the land, sea, and air. The era is divided 
into three systems or periods which are, from oldest to youngest, Triassic, 
Jurassic, and Cretaceous.  See  CRETACEOUS; JURASSIC; TRIASSIC. 
 �34s[William A. Cobban]
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�6v�2v�32sMessier catalog   �21m �33s 
    An early listing of nebulae and star clusters. Charles Messier (1730 - 
1817) was primarily interested in discovering comets, so he compiled a list of 
objects that might be confused with comets in small telescopes. His first 
catalog was published in the Royal Academy of Sciences of France in 1771. The 
compilation contains both open and globular star clusters, diffuse galactic 
nebulae, so-called planetary nebulae, and external galaxies. 
    The New General Catalog (NGC) and the supplementary Index Catalogs (IC) 
published by J. L. E. Dreyer at the end of the 19th century were far more 
extensive than that of Messier. Consequently the NGC numbers are generally 
used for all except a few objects whose Messier numbers had become firmly 
established.  See  GALAXY, EXTERNAL; NEBULA; STAR CLUSTERS. 
 �34s[Lawrence H. Allen]
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�6v�2v�32sMetabolic disorders   �21m �33s 
    Disorders which involve an alteration in the normal metabolism of 
carbohydrates, lipids, proteins, water, and nucleic acids. Deviations in 
normal metabolic processes are evidenced by various syndromes and diseases.  
See  MALNUTRITION; METABOLISM. 
  
    Abnormal protein metabolism.  The diseases associated with abnormal 
protein metabolism have been classified as follows: (1) diseases associated 
with increased production of proteins, (2) those associated with decreased 
production of proteins, (3) diseases associated with the production of 
abnormal proteins, and (4) diseases associated with the excretion of unusual 
amounts of amino acids. 
    Hyperproteinemia is an increase of protein, usually one type of plasma 
protein. When it occurs, more BETA- or GAMMA-globulins are produced while 
fewer total proteins, including ALPHA-globulins and albumins, are synthesized. 
The diseases usually associated with hyperglobulinemia are multiple myeloma, 
kala-azar, Hodgkin's disease, lymphogranuloma inguinale, sarcoidosis, liver 
cirrhosis, and amyloid. The mechanisms which stimulate the production of 
proteins, particularly of one type, are not known, but the plasma cell has 
been implicated.  See  CIRRHOSIS; HODGKIN`S DISEASE. 
    A decreased amount of protein, hypoproteinemia, may be the result of a 
lack of essential amino acids for protein synthesis, a metabolic block, or 
other interference with the normal synthesis mechanism. Increased excretion of 
protein, particularly in chronic renal disease with a loss of albumin in the 
urine (albuminuria), is another common cause of hypoproteinemia. Kwashiorkor 
is the best example of hypoproteinemia resulting from dietary deficiency.  See 
  HEPATITIS; KWASHIORKOR DISEASE. 
    Another group of abnormal proteins have been associated with the 
hemoglobins. Hemoglobin functions in the transport of oxygen and carbon 
dioxide in the blood. Several different hemoglobins in red blood cells in 
mammals have been described, one of which produces the disease called 
sickle-cell anemia.  See  HEMOGLOBIN. 
    Amino aciduria is a group of disorders in which there appears to be an 
increase in the amount of amino acids excreted in the urine. They are due to 
abnormal protein metabolism and result either from an overflow mechanism where 
the concentration of amino acids in the serum surpasses the renal threshold of 
the glomerular membrane, or from defective absorption of amino acids in the 
renal tubules. The overflow amino acidurias include phenylketonuria, 
alkaptonuria, and liver disease. See PHENYLPYRUVIC OLIGOPHRENIA;  PROTEIN 
METABOLISM.  
  
    Abnormal nucleic acid metabolism.  Since nucleic acids are closely 
associated with chromosomes and the process of cell division as well as with 
protein synthesis, they are one of the most important of cell constituents. 
They have been shown to be very susceptible to injury by various agents, and 
interruption of division is one of the earliest signs of cell damage. A 
disturbance in nucleic acid metabolism is of extreme importance to many other 
aspects of cell metabolism. The whole field of virus infection of mammalian 
cells is undoubtedly largely concerned with a disturbance in nucleic acid or 
nucleoprotein metabolism.  See  GOUT; LUPUS ERYTHEMATOSUS. 
  
    Abnormal lipid metabolism.  Obesity in general may be attributed to an 
excess intake of calories and only rarely may glandular deficiencies such as 
hypopituitarism and hypothyroidism be implicated as its cause. Increased 
caloric intake results in an increase in the size and number of fat cells 
distributed throughout normal lipid deposits and many organs of the body.  See 
  OBESTY; PITUITARY GLAND DISORDERS; THYROID GLAND DISORDERS. 
    Hyperlipemia, an excess of lipid in the blood, is usually secondary to 
uncontrollable diabetes, hypothyroidism, biliary cirrhosis, or lipid 
nephrosis. A primary type of lipemia is caused by a genic defect. There is 
marked delay in the clearing of ingested fat so that a milky serum with 
elevation of all the lipid fractions, especially of the neutral fat, occurs. 
Severe sclerosis, and occasionally xanthomatosis, is common in this disease in 
early adulthood. Another primary hyperlipemia, called idiopathic 
hypercholesteremia, has also been described. Sclerosis of all the vessels is 
marked, with a high incidence of myocardial infarct. 
    There are small tumors composed of fat cells called lipomas which are 
commonly found in the subcutaneous tissue and which occasionally become 
malignant liposarcomas. 
    There are four common major diseases which are associated with 
accumulations of lipid in and around damaged cells. These are, in order of 
importance, arteriosclerosis, myocardial infarct, cirrhosis of the liver, and 
nephrosis.  See  ARTERIOSCLEROSIS; CIRRHOSIS; INFARCTION. 
    There is another group of diseases in which lipid accumulates because of a 
disturbance in lipid metabolism, not dependent as far as is known on external 
stimuli. These are the so-called lipid storage diseases which are in some ways 
very similar to the glycogen storage disease. In all of these diseases a large 
accumulation of lipids appears in many cells, but particularly in the 
reticuloendothelial cells of the lymph nodes, liver, spleen, and bone marrow. 
The lipid seems to be distributed throughout the cell and is not well 
localized. See LIPID METABOLISM. 
  
    Abnormal carbohydrate metabolism.  There are five pathological states 
which are reasonably well elucidated in terms of deficient mechanisms or in 
terms of the specific carbohydrate involved: diabetes mellitus, glycogen 
storage disease (von Gierke's disease), Hurler-Pfaundler's disease, 
galactosemia, and malignant neoplasm. The end effects of these diseases have 
been known for many years; the causes still remain obscure. See DIABETES. 
    In glycogen storage disease extraordinarily large amounts of glycogen are 
deposited in essential organs such as the heart, liver, kidney, and skeletal 
muscle. The symptoms of the disease arise when the accumulation of glycogen 
interferes with the normal cells' metabolism. When this occurs in the heart, 
the muscle fibers appear split by large amounts of glycogen, muscle 
contractility is impaired, and heart failure and death may eventually result. 
    In Hurler-Pfaundler's disease there is excess storage of a 
mucopolysaccharide in the spleen, liver, and other tissues. The symptoms, as 
in glycogen storage disease, result from interference with the cells' 
function. The enzymatic deficiency in this disease is still unknown. 
    Galactosemia is a disease of the newborn resulting from an inherited 
deficiency of the enzyme concerned with the conversion of galactose to 
glucose, phosphogalactose transferase. The principal food of newborns is milk 
which contains large quantities of lactose. Lactose is usually broken to 
galactose and glucose, and the glucose is quickly utilized. The galactose, 
however, must first be converted to glucose. If the galactose is not 
converted, it accumulates in cells and interferes with their normal functions. 
See CARBOHYDRATE METABOLISM. 
 �34s[Donald W. King]
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�6v�2v�32sMetabolism   �21m �33s 
    All the physical and chemical processes by which living, organized 
substance is produced and maintained and the transformations by which energy 
is made available for use by an organism. 
    In defining metabolism, it is customary to distinguish between energy 
metabolism and intermediary metabolism, although the two are, in fact, 
inseparable. Energy metabolism is primarily concerned with overall heat 
production in an organism, while intermediary metabolism deals with chemical 
reactions within cells and tissues. In general, the term metabolism is 
interpreted to mean intermediary metabolism.  See  ENERGY METABOLISM. 
    Metabolism thus includes all biochemical processes within cells and 
tissues which are concerned with their building up, breaking down, and 
functioning. The synthesis and maintenance of tissue structure generally 
involves the union of smaller into larger molecules. This part of metabolism, 
the building of tissues, is termed anabolism. The process of breaking down 
tissue, of splitting larger protoplasmic molecules into smaller ones, is 
termed catabolism. Growth or weight gain occurs when anabolism exceeds 
catabolism. On the other hand, weight loss results if catabolism proceeds more 
rapidly than anabolism, as in periods of starvation, serious injury, or 
disease. When the two processes are balanced, tissue mass remains the same. 
    The metabolism of the three major foodstuffs, carbohydrates, fats, and 
proteins, is intimately interrelated, so any clearcut division of the three is 
arbitrary and inaccurate. Thus the metabolism of protoplasm is concerned with 
all three of these foodstuffs. The metabolic pathways of carbohydrates, fats, 
and proteins cross at many points; thus certain pathways of metabolism are 
shared in common by fragments of these different classes of foodstuffs. 
    Some of the metabolic processes of the protoplasm of both plant and animal 
cells occur along common pathways; carbohydrate metabolism in plants is 
similar in many details to carbohydrate metabolism in animals. Therefore the 
study of metabolism in any organism is, in a sense, the study of metabolism in 
all protoplasm.  See  CARBOHYDRATE METABOLISM; KREBS CYCLE; LIPID METABOLISM; 
PROTEIN METABOLISM. 
 �34s[Mary B. McCann]
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�6v�2v�32sMetachlamydeae   �21m �33s 
    An artificial group of flowering plants, division Magnoliophyta 
(Angiospermae), recognized in the Englerian system of classification, 
consisting of those families of dicotyledons (Magnoliopsida) in which the 
petals are characteristically fused, forming a sympetalous corolla. This is in 
contrast to the Archichlamydeae, which have separate petals or lack petals. 
The group has also been called Sympetalae.  See  ARCHICHLAMYDEAE; 
MAGNOLIOPHYTA.  
 �34s[Arthur Cronquist]
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�6v�2v�32sMetal   �21m �33s 
    An electropositive chemical element. Physically, a metal atom in the 
ground state contains a partially filled band with an empty state close to an 
occupied state. Chemically, upon going into solution a metal atom releases an 
electron to become a positive ion. Consequently in biotic systems metal atoms 
function prominently in ionic transport and electron exchange. In bulk a metal 
has a high melting point and a correspondingly high boiling temperature; 
except for mercury, metals are solid at standard conditions. Direct 
observation shows a metal to be relatively dense, malleable, ductile, 
cohesive, highly conductive both electrically and thermally, and lustrous. 
When crystals of the elements are classified along a scale from plastic to 
brittle, metals fall toward the plastic end. Furthermore, molten metals mixed 
with each other over wide ranges of proportions form, upon slowly cooling, 
homogeneous close-packed crystals. In contrast, a metal mixed with a nonmetal 
completely combines into a homogeneous crystal only in one or a few discrete 
stoichiometric proportions. 
    For detailed discussions of metals, in particular, exceptions to generic 
behavior, see separate articles on each metal.  See also  ELEMENTS; PERIODIC 
TABLE.  
 �34s[Frank H. Rockett]
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�6v�2v�32sMetal-base fuel   �21m �33s 
    A fuel containing a metal of high heat of combustion as a principal 
constituent. High propellant performance in either a rocket or an 
air-breathing engine is obtained when the heat of combustion of the fuel is 
high. Chemically, high heats of combustion are attained by the oxidation of 
the low-atomic-weight metals in the upper left-hand corner of the periodic 
table. The generally preferred candidates are lithium, beryllium, boron, 
carbon, magnesium, and aluminum. 
    The metallized additive can be used in either a liquid or solid 
propellant. When the pure metal is added to liquid fuels, an emulsifying or 
gelling agent is employed which maintains the particles in uniform suspension. 
When used in composite solid rocket propellants, the metal powder is usually 
mixed with the oxidizer and unpolymerized fuel, and the propellant is then 
processed in the usual way. 
    Early compounding of metallized propellants employed the free metal 
itself. As a result of extensive research in metalloorganic compounds, 
however, several classes of metallic compounds have been employed. Two major 
reasons for the use of such compounds are that the solubility of the metal in 
the fuel can be realized, resulting in a homogeneous propellant, and 
performance higher than that for the pure metal can be obtained in some cases. 
    The major classes of metallic compounds of interest as high-performance 
propellants include the hydrides, amides, and hydrocarbons. Additional classes 
include mixtures either of two metals or of two chemical groups, such as an 
amine hydrocarbon. 
    Most metallic fuels are costly and many, because of particle-size 
requirements or synthesis in a specific compound, are in limited supply. The 
combustion gases all produce smoky exhausts which may be objectionable in use. 
Engine development problems are also increased because of the appearance of 
smoke and deposits in the engine. 
 �34s[David Altman]
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�6v�2v�32sMetal carbonyl   �21m �33s 
    A compound of a metal combined with carbon monoxide (CO). The structure 
involves a coordinate link between the metal atom and the CO group, with each 
CO donating a pair of electrons to the metal. Metal carbonyls are usually 
low-melting crystalline solids, and several are liquids at room temperature. 
They are generally soluble in organic solvents but insoluble in water. The 
primary decomposition products are the metal and carbon monoxide. Metal 
carbonyls are quite reactive. 
 �34s[Hymin Shapiro]
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�6v�2v�32sMetal casting   �21m �33s 
    The introduction of molten metal into a cavity or mold where, upon 
solidification, it becomes an object whose shape is determined by mold 
configuration. Casting offers several advantages over other methods of metal 
forming: it is adaptable to intricate shapes, to extremely large pieces, and 
to mass production; it can provide parts with uniform physical and mechanical 
properties throughout; and, depending on the particular material being cast, 
the design of the part, and the quantity being produced, its economic 
advantages can surpass other processes. 
    Two broad categories of metal-casting processes exist: ingot casting 
(which includes continuous casting) and casting to shape. Ingot castings are 
produced by pouring molten metal into a permanent or reusable mold. Following 
solidification these ingots (or bars, slabs, or billets, as the case may be) 
are then further processed mechanically into many new shapes. Casting to shape 
involves pouring molten metal into molds in which the cavity provides the 
final useful shape, followed only by machining or welding for the specific 
application. There are four basic types of these casting processes: sand, 
permanent-mold, die, and centrifugal. 
 �34s[Karl Rundman]
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�6v�2v�32sMetal cluster compounds   �21m �33s 
    A compound in which two or more metals aggregate so as to be within 
bonding distance of one another. Heterogeneous metal clusters of the 
transition metals deposited on supports such as alumina, silica, and zeolites 
have been widely used in catalysis. However, research involving soluble metal 
cluster compounds in homogeneous solution indicates that they can mimic the 
heterogeneous counterpart. Since solution chemistry is more amenable to 
characterization, the soluble clusters should lead to an intimate 
understanding of catalytic mechanisms and to the design of very selective 
catalysts.  
    Metal clusters in solution fall into two general categories: those 
encumbered with a coordination sphere of chemically bound ligands (ligated 
clusters; see illustration), and those surrounded only by a sphere of solvent 
molecules (the so-called naked clusters). Clusters of the first class are 
found primarily in the case of transition metals and are critical to catalysis 
since the substrate undergoing catalytic transformation must be chemically 
bound to the cluster some time during the catalytic cycle. See HETEROGENEOUS 
CATALYSIS; HOMOGENEOUS CATALYSIS. 
 �34s[Ralph W. Rudolph]
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�6v�2v�32sMetal coatings   �21m �33s 
    Thin films of material bonded to metals in order to add specific surface 
properties, such as corrosion or oxidation resistance, color, attractive 
appearance, wear resistance, optical properties, electrical resistance, or 
thermal protection. This article discusses various methods of applying either 
metallic coatings, or nonmetallic coatings, such as vitreous enamel and 
ceramics, and the conversion of surfaces to suitable reaction-product 
coatings. For other methods for the protection of metal surfaces  see  
CLADDING; ELECTROLESS PLATING; ELECTROPLATING OF METALS; LACQUER; PAINT. 
    Hot-dipped coatings of low-melting metals provide inexpensive protection 
to the surfaces of a variety of steel articles. Thoroughly cleaned work is 
immersed in a molten bath of the coating metal. The coating consists of a thin 
alloy layer together with relatively pure coating metal that adheres to the 
work as it is withdrawn from the bath. 
    Sprayed coating permits the coating of assembled steel structures to 
obtain corrosion resistance, the building up of worn machine parts for 
rejuvenation, and the application of highly refractory coatings with melting 
points in excess of 3000DGF (1650DGC). 
    Cementation coatings are surface alloys formed by diffusion of the coating 
metal into the base metal, producing little dimensional change. Parts are 
heated in contact with powdered coating material that diffuses into the 
surface to form an alloy coating, whose thickness depends on the time and the 
temperature of treatment. 
    In vapor deposition a thin specular coating is formed on metals, plastics, 
paper, glass, and even fabrics. Coatings form by condensation of metal vapor 
originating from molten metal, from high-voltage discharge between electrodes 
(cathode sputtering), or from chemical means such as hydrogen reduction or 
thermal decomposition (gas plating) of metal halides. 
    Immersion coatings are produced either by direct chemical displacement or 
for thicker coatings by chemical reduction (electroless coating). Metal ions 
plate out of solution onto the workpiece. 
    Vitreous enamel coatings are glassy but noncrystalline coatings for 
attractive durable service in chemical, atmospheric, or moderately 
high-temperature environments. In wet enameling a slip is prepared of a water 
suspension of crushed glass, flux, suspending agent, refractory compound, and 
coloring agents or opacifiers. The slip is applied by dipping or flow coating; 
it is then fired at a temperature at which it fuses into a continuous vitreous 
coating. Dry enameling is used for castings, such as bathtubs. The casting is 
heated to a high temperature, and then dry enamel powder is sprinkled over the 
surface, where it fuses.  See  FRIT. 
    Essentially crystalline, ceramic coatings are used for high-temperature 
protection above 1100DGC (2000DGF). The coatings may be formed by spraying 
refractory materials such as aluminum oxide or zirconium oxide, or by the 
cementation processes for coatings of intermetallic compounds such as 
molybdenum disilicide.  See  CERMET. 
    Surface-conversion coatings provide an insulating barrier of low 
solubility formed on steel, zinc, aluminum, or magnesium without electric 
current. The article to be coated is either immersed in or sprayed with an 
aqueous solution, which converts the surface into a phosphate, an oxide, or a 
chromate.  
    Anodic coatings of protective oxide may be formed on aluminum or magnesium 
by making them the anode in an electrolytic cell. If permanent color is 
required, the coating is impregnated with a dye before sealing.  See  
CORROSION.  
 �34s[William W. Bradley]
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�6v�2v�32sMetal forming   �21m �33s 
    Manufacturing processes by which parts or components are fabricated from 
metal stock. In the specific technical sense, metal forming involves changing 
the shape of a piece of metal. In general terms, however, it may be classified 
roughly into five categories: mechanical working, such as forging, extrusion, 
rolling, drawing, and various sheet-forming processes; casting; powder and 
fiber metal forming; electroforming; and joining processes.  See  DRAWING OF 
METAL; EXTRUSION; FORGING; MACHINING OPERATIONS; METAL CASTING; METAL ROLLING; 
POWDER METALLURGY; SHEET-METAL FORMING. 
 �34s[Serope Kalpakjian]
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�6v�2v�32sMetal hydrides   �21m �33s 
    A compound in which hydrogen is bonded chemically to a metal or metalloid 
element. The compounds are classified generally as ionic, transition metal, 
and covalent hydrides. Covalent hydrides are of two subtypes, binary and 
complex. Certain hydrides have achieved a position of modest industrial 
importance, but most are of theoretical interest only. 
    Under extreme conditions such as in electric discharges, many metals form 
volatile, short-lived transient hydrides of the general formula type MH. 
Although some of these can be prepared experimentally, most are observed only 
by their spectra. They are important in studying molecular bonding. The action 
of atomic hydrogen at low temperatures forms surface films of unstable 
hydrides with many metals.  See  HYDRIDO COMPLEXES; HYDROGEN. 
 �34s[James C. Warf]
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�6v�2v�32sMetallic-disk rectifier   �21m �33s 
    A rectifier that consists of one or more disks of metal in contact with 
coatings or layers of a semiconductor material. Alternating current is changed 
into pulsating direct current by the rectifying action that occurs at the 
junction interface between a metal disk and its mating semiconductor layer. 
The most common examples are the selenium and copper-oxide rectifiers.  See  
SEMICONDUCTOR RECTIFIER. 
 �34s[John Markus]
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�6v�2v�32sMetallic electrode arc lamp   �21m �33s 
    An arc lamp whose positive electrode is solid copper, and whose negative 
electrode is formed of magnetic iron oxide with titanium as the 
light-producing element and other chemicals to control steadiness and 
vaporization. In this type of lamp, light is produced by luminescent vapor 
introduced into the arc by conduction from the cathode. These lamps are 
limited to direct-current use. A regulating mechanism adjusts the arc. See ARC 
LAMP.  
 �34s[John O. Kraehenbuehl]
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�6v�2v�32sMetallic glasses   �21m �33s 
    Alloys having amorphous or glassy structures. A glass is a solid material 
obtained from a liquid which does not crystallize during cooling. It is 
therefore an amorphous solid, which means that the atoms are packed in a more 
or less random fashion similar to that in the liquid state. The word glass is 
generally associated with the familiar transparent silicate glasses containing 
mostly silica and other oxides of aluminum, magnesium, sodium, and so on. 
These glasses are not metallic; they are electrical insulators and do not 
exhibit ferromagnetism. In 1960 the first true metallic glass, Au80Si20, an 
alloy containing 80 at. % Au and 20 at. % Si, was obtained.  See  ALLOY; 
AMORPHOUS SOLID. 
 �34s[Pol E. Duwez]
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�6v�2v�32sMetalloacid elements   �21m �33s 
    The chemical elements with the following atomic numbers and names: 23, 
vanadium, V; 41, niobium, Nb; 73, tantalum, Ta; 24, chromium, Cr; 42, 
molybdenum, Mo; 74, tungsten, W; 25, manganese, Mn; 43, technetium, Tc; and 
75, rhenium, Re. These elements are the subgroup members of groups V, VI, and 
VII, respectively, of the periodic table. In the elemental state all are 
metals of relatively high density, high melting point, and low volatility. The 
classification metalloacid elements refers to the fact that their oxides react 
with water to give somewhat acidic solutions, in contrast to the more typical 
behavior of the oxides of other metals which yield basic solutions.  See  
PERIODIC TABLE; TRANSITION ELEMENTS. 
 �34s[Burris B. Cunningham]
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�6v�2v�32sMetallocenes   �21m �33s 
    A group of organometallic compounds, bis-cyclopentadienyl metals, which 
generally possess a sandwich structure (see illustration). 
    Almost all transition metals have been found to form metallocenes. 
According to the center metal atom, metallocenes are called ferrocenes (Fe), 
ruthenocenes (Ru), and so on. In a brief definition, both cyclopentadienyl 
groups bind with the metal atom in PI fashion (termed pentahapto, abbreviated 
hsup5).  
    Metallocenes have been used for coating or plating metal by their 
vapor-phase decomposition. Applications of metallocenes are versatile and have 
potential for further developments.  See  FERROCENE; ORGANOMETALLIC COMPOUNDS. 
 �34s[Minoru Tsutsui]
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�6v�2v�32sMetallography �21m �33s
    The study of the structure of metals and alloys by various methods,
especially optical and electron microscopy. The term metallography once
designated the science relating the properties of metals to their structure,
but this science is now called physical metallurgy.  See  METALLURGY.
    The optical microscopy of metals is conducted with reflected light on
surfaces suitably prepared to reveal structural features (see illustration); a
resolution of about 200 nanometers and a linear magnification of at most 2000x
can be obtained. Electron microscopy is generally carried out either with
replicas of specimen surfaces or with thin foils prepared from bulk specimens.
The replica technique provides a resolution of detail as small as 5 nm and
magnifications of up to approximately 50,000x. The thin-foil technique
provides a resolution of better than 1 nm and a useful magnification of
500,000x.  See	ELECTRON MICROSCOPE; OPTICAL MICROSCOPE.
    Metallography serves both research and industrial practice. Optical
microscopy has long been a standard method for the investigation of the phase
constitution of metallic systems and the microstructure of metals. Phase
transformations and the mechanisms by which metals deform plastically are
becoming better understood through the application of electron microscopy.
Metallographic methods are used in industry for the control of production
processes, especially heat treatments. These methods are frequently
indispensable in the analysis of the causes of service failures of metallic
objects. Metallographic methods have been adapted to the examination of
nonmetallic materials, especially ceramics, semiconductors, and minerals.
 �34s[Michael B. Bever ;John B. VanderSande]
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�6v�2v�32sMetalloid   �21m �33s 
    An element which exhibits the external characteristics of a metal but 
behaves chemically both as a metal and as a nonmetal. Arsenic and antimony, 
for example, are hard crystalline solids that are definitely metallic in 
appearance. They may, however, undergo reactions that are characteristic of 
both metals and nonmetals. However, only when this dualistic chemical behavior 
is very marked and the external appearance metallic is the element commonly 
called a metalloid.  See  METAL; NONMETAL. 
 �34s[Francis J. Johnston]
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�6v�2v�32sMetallurgy   �21m �33s 
    The technology and science of metallic materials. Metallurgy as a branch 
of engineering is concerned with the production of metals and alloys, their 
adaptation to use, and their performance in service. As a science, metallurgy 
is concerned with the chemical reactions involved in the processes by which 
metals are produced and the chemical, physical, and mechanical behavior of 
metallic materials. 
    The field of metallurgy may be divided into process metallurgy (production 
metallurgy, extractive metallurgy) and physical metallurgy. In this system 
metal processing is considered to be a part of process metallurgy and the 
mechanical behavior of metals a part of physical metallurgy. 
    Process metallurgy, the science and technology used in the production of 
metals, employs some of the same unit operations and unit processes as 
chemical engineering. These operations and processes are carried out with 
ores, concentrates, scrap metals, fuels, fluxes, slags, solvents, and 
electrolytes. Different metals require different combinations of operations 
and processes, but typically the production of a metal involves two major 
steps. The first is the production of an impure metal from ore minerals, 
commonly oxides or sulfides, and the second is the refining of the reduced 
impure metal, for example, by selective oxidation of impurities or by 
electrolysis.  See  ELECTROMETALLURGY; HYDROMETALLURGY; IRON METALLURGY; ORE 
DRESSING; PYROMETALLURGY; PYROMETALLURGY, NONFERROUS; STEEL MANUFACTURE. 
    Physical metallurgy investigates the effects of composition and treatment 
on the structure of metals and the relations of the structure to the 
properties of metals. Physical metallurgy is also concerned with the 
engineering applications of scientific principles to the fabrication, 
mechanical treatment, heat treatment, and service behavior of metals.  See  
ALLOY; HEAT TREATMENT (METALLURGY). 
    The structure of metals consists of their crystal structure, which is 
investigated by x-ray, electron, and neutron diffraction, their 
microstructure, which is the subject of metallography, and their 
macrostructure. Crystal imperfections, which provide mechanisms for processes 
occurring in solid metals, are investigated by X-ray diffraction and 
metallographic methods, especially electron microscopy. The microstructure is 
determined by the constituent phases and the geometrical arrangement of the 
microcrystals (grains) formed by those phases. Macrostructure is important in 
industrial metals. It involves chemical and physical inhomogeneities on a 
scale larger than microscopic. Examples are flow lines in steel forgings and 
blowholes in castings.  See  METALLOGRAPHY; X-RAY DIFFRACTION. 
    Phase transformations occurring in the solid state underlie many 
heat-treatment operations. The thermodynamics and kinetics of these 
transformations are a major concern of physical metallurgy. Physical 
metallurgy also investigates changes in the structure and properties resulting 
from mechanical working of metals.  See  PLASTIC DEFORMATION OF METAL. 
    For more information on metallurgy and some associated techniques see 
articles on individual metals and their metallurgy.  See also ELECTROPLATING 
OF METALS; METAL COATINGS; METAL FORMING. 
 �34s[Michael B. Bever]
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�6v�2v�32sMetal matrix composite   �21m �33s 
    A material consisting of two or more substances, one of which is a metal 
or metallic alloy, and which exhibits its own distinctive behavior. The latter 
criterion sets a composite apart from conventional metallic materials such as 
steels and nonferrous alloys, both of which contain two or more constituents. 
For example, the stiffness or strength of a composite may be higher than that 
of the individual components, or radically different from either. 
    Typically, metal matrix composites consist of wires, filaments, fibers 
(fine wires or filaments), whiskers, or thin sheets of a strong, relatively 
brittle material embedded in the weaker but more ductile metallic constituent. 
This combination and particular distribution of the constituents result in 
enhanced stiffness and strength coupled with ductility and toughness (the 
ability to withstand impact or shock loading). The need for a fine-scale 
distribution of the reinforcement, and the role of the matrix in stress 
transfer, crack propagation, and protection of the reinforcement are 
considerations which apply to metal matrix composites. The reinforcement may 
be either continuous, that is, extending the full length of the composite, or 
discontinuous, consisting of chopped lengths having a particular ratio of 
length to diameter (aspect ratio). 
    The overall property combinations of metal matrix composites are 
particularly attractive in aerospace and marine engineering, propulsion, and 
transportation. Unlike polymer matrix composites, metal matrix composites can 
be utilized at both ambient temperature and high temperatures.  See  ALLOY; 
COMPOSITE MATERIAL; POLYMERIC COMPOSITE. 
 �34s[Alan Lawley]
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�6v�2v�32sMetal rolling   �21m �33s 
    Reducing or changing the cross-sectional area of a workpiece by the 
compressive forces exerted by rotating rolls. The original material fed into 
the rolls is usually an ingot from a foundry. The largest product in hot 
rolling is called a bloom; by successive hot- and then cold-rolling operations 
the bloom is reduced to a billet, slab, plate, sheet, strip, and foil, in 
decreasing order of thickness and size. The initial breakdown of the ingot by 
rolling changes the coarse-grained, brittle, and porous structure into a 
wrought structure with greater ductility and finer grain size. 
    A schematic presentation of the rolling process, in which the thickness of 
the metal is reduced as it passes through the rolls, is shown in illustration 
a. The speed at which the metal moves during rolling changes, as shown in 
illustration b, to keep the volume rate of flow constant throughout the roll 
gap. Hence, as the thickness decreases, the velocity increases; however, the 
surface speed of a point on the roll is constant, and there is therefore 
relative sliding between the roll and the strip. The normal pressure 
distribution on the roll and hence on the strip is of the form shown in 
illustration c. Because of its particular shape this pressure distribution is 
known as the friction hill. 
    A great variety of roll arrangements and equipment are used in rolling. 
The proper reduction per pass in rolling depends on the type of material and 
other factors; for soft, nonferrous metals, reductions are usually high, while 
for high-strength alloys they are small. Requirements for roll materials are 
mainly strength and resistance to wear. Common roll materials are cast iron, 
cast steel, and forged steel. 
 �34s[Serope Kalpakjian]
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�6v�2v�32sMetals, pathology of   �21m �33s 
    Metals of particular toxic significance in humans are arsenic, zinc, gold, 
silver, copper, mercury, lead, cadmium, manganese, chromium, cobalt, and 
molybdenum. Heavy metals have a density at least five times greater than that 
of water. They are of particular toxologic importance. Some metals such as 
iron, zinc, copper, molybdenum, chromium, cobalt, and manganese are essential 
for life but can be toxic in high doses. A dietary deficiency of these metals 
can lead to pathologic changes. Others, such as platinum and tantalum, appear 
to be inert within the human body. Still others such as mercury, nickel, tin, 
lead, and cadmium have not yet been shown to be essential to life but are 
present in human tissues and are toxic in high doses. Although heavy metals 
have many physical properties in common, their chemical reactivity is quite 
diverse and their toxic effects on biologic systems even more diverse. 
    A metal can be regarded as toxic if it injures the growth or metabolism of 
cells when it is present above a given concentration. Almost all metals are 
toxic at high concentrations, and some are severe poisons even at very low 
concentrations. Copper, for example, is an essential constituent of cells, but 
if the copper intake is increased it becomes highly toxic. Like copper, each 
metal has an optimum range of concentration, in excess of which the element is 
toxic. When the optimum range for a particular metal is narrow, the risk of 
toxicity increases; thus even a minor environmental increase can have serious 
effects.  
    Metals exert their toxicity on cells by interfering with cell metabolism 
in any of several ways. The most important of these are their effects on 
enzyme systems. The more strongly electronegative metals (such as copper, 
mercury, and silver) bind with amino, imino, carboxylate, and sulfhydryl 
groups of enzymes, thus blocking enzyme activity. Another mechanism of action 
of some heavy metals (gold, cadmium, copper, mercury, lead) is to combine with 
the cell membranes, altering membrane permeability. Others displace elements 
that are important structurally or electrochemically to cells which then can 
no longer perform their biologic functions.  See  ARSENIC INTOXICATION; 
CADMIUM DISEASE; CHROMIUM INTOXICATION; CHROMOSOME ABERRATION; COBALT 
TOXICITY; COPPER INTOXICATION; GOLD INTOXICATION; LEAD TOXICOLOGY; MANGANESE 
TOXICOSIS; MERCURY POISONING; MOLYBDENUM INTOXICATION; NICKEL POISONING; 
SILVER INTOXICATION; THALLIUM DISEASE; ZINC INTOXICATION. 
 �34s[N. Karle Mottet]
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    Wide variations in the physical, chemical, and biologic properties of 
heavy metals exist. They have a common property of forming complexes with a 
wide variety of anions. However, alkali and alkaline-earth metals undergo 
similar reactions, and so this property is not unique to heavy metals. 
Generally, heavy metals tend to form stable complexes with sulfur and nitrogen 
rather than with oxygen. Many heavy metals are known to exert their biologic 
effects through the combination with sulfhydryl groups. Others have not been 
shown to have such predilections. 
  
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�6v�2v�32sMetamerism   �21m �33s 
    The serial repetition of parts along the length of the body axis in 
bilaterally symmetrical animals. The successive subdivisions are called 
metameres, somites, or segments, hence the synonym, segmentation. Common 
examples are the muscles and spinal nerves in the human body and in the body 
and tail of many mammals, snakes and lizards, salamanders, and fishes. It also 
occurs in other chordates, and in arthropods and annelid worms.  See  ANIMAL 
SYMMETRY.  
 �34s[Howard L. Hamilton]
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�6v�2v�32sMetamict state   �21m �33s 
    The amorphous state of substances that have lost their original cystalline 
structure because of the radioactivity of uranium or thorium. The process of 
rendering crystalline substances partly or wholly amorphous is known as 
metamictization. Minerals whose crystal structure has been disrupted by this 
process are known as metamict minerals. Some minerals which show crystal form 
are nevertheless structurally amorphous but have attained this state in a 
different manner than glasses or rigid gels. This third class of amorphous 
substances is known as metamict (mixed otherwise). 
    The more significant features of metamict minerals are: 
    1. Anomalous optical behavior. Many metamict substances are heterogeneous, 
being partly isotropic and partly anisotropic (doubly refractive). 
    2. Pyrognomic behavior. The minerals readily become incandescent on 
heating, although this varies greatly. 
    3. Glasslike properties. The minerals lack cleavage but exhibit conchoidal 
fracture, some being particularly brittle. 
    4. Density. The density is increased by heating. 
    5. Reconstitution of crystalline structure by heating. 
    6. Resistance to attack by acids. This is increased by 
heating. 
    7. Presence of uranium (U) or thorium (Th). In addition the presence of 
rare earths and related elements has been emphasized by some observers.  See  
RARE-EARTH ELEMENTS. 
    8. Existence of some minerals in both the crystalline and metamict states. 
In these cases little, if any, chemical difference can be found. 
    9. Amorphous behavior when exposed to x-rays. This is regarded as the 
crucial test. Some traces of x-ray diffraction may be observed, though the 
minerals have become optically quite isotropic.  See X-RAY DIFFRACTION. 
 �34s[Patrick M. Hurley]
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�6v�2v�32sMetamorphic rocks   �21m �33s 
    Preexisting rock masses in which new minerals, or textures, or structures 
are formed at higher temperatures and greater pressures than those normally 
present at the Earth's surface.  See  IGNEOUS ROCKS; SEDIMENTARY ROCKS. 
    Two groups of metamorphic rocks may be distinguished; cataclastic rocks, 
formed by the operation of purely mechanical forces; and recrystallized rocks, 
or the metamorphic rocks properly so called, formed under the influence of 
metamorphic pressures and temperatures. Cataclastic rocks are mechanically 
sheared and crushed. They represent products of dynamometamorphism, or kinetic 
metamorphism. Chemical and mineralogical changes generally are negligible. The 
rocks are characterized by their minute mineral grain size. Each mineral grain 
is broken up along the edges and is surrounded by a corona of debris or strewn 
fragments.  See  METAMORPHISM. 
    Metamorphic rocks, properly so called, are recrystallized rocks. The laws 
of recrystallization are not the same as those of simple crystallization from 
a liquid, because the crystals can develop freely in a liquid, but during 
recrystallization the new crystals are encumbered in their growth by the old 
minerals. Consequently, the structures which develop in metamorphic rocks are 
distinctive and of great importance, because in many ways they reflect the 
physiochemical environment of recrystallization and thereby the genesis and 
history of the metamorphic rock. 
    The metamorphic minerals may be arranged in an idioblastic series 
(crystalloblastic series) in their order of decreasing force of 
crystallization as follows: (1) sphene, rutile, garnet, tourmaline, 
staurolite, kyanite; (2) epidote, zoisite; (3) pyroxene, hornblende; (4) 
ferrogmagnesite, dolomite, albite; (5) muscovite, biotite, chlorite; (6) 
calcite; (7) quartz, plagioclase; and (8) orthoclase, microcline. Crystals of 
any of the listed minerals tend to assume idioblastic outlines at surfaces of 
contact with simultaneously developed crystals of all minerals of lower 
position in the series. 
    Igneous magma at high temperature may penetrate into sedimentary rocks, it 
may reach the surface, or it may solidify in the form of intrusive bodies 
(plutons). Heat from such bodies spreads into the surrounding sediments, and 
because the mineral assemblages of the sediments are adjusted to low 
temperatures, the heating-up will result in a mineralogical and textural 
reconstruction known as contact metamorphism.  See  CONTACT AUREOLE; PLUTON. 
    The effects produced do not depend only upon the size of the intrusive. 
Other factors are amount of cover and the closure of the system, composition 
and texture of the country rock, and abundance of gaseous and hydrothermal 
magmatic emanations. The heat conductivity of rocks is so low that gases and 
vaporous emanations become chiefly responsible for transportation and transfer 
of heat into the country rock. 
    Crystalline schists, gneisses, and migmatites are typical products of 
regional metamorphism and mountain building. If sediments accumulate in a 
slowly subsiding geosynclinal basin, they are subject to down-warping and deep 
burial, and thus to gradually increasing temperature and pressure. They become 
sheared and deformed, and a general recrystallization results. However, 
subsidence into deeper parts of the crust is not the only reason for 
increasing temperature. It is not known what happens at the deeper levels of a 
live geosyncline, but obviously heat from the interior of the Earth is 
introduced regionally and locally, partly associated with magmas, partly in 
the form of ``emanations'' following certain main avenues, determined by a 
variety of factors. From this milieu rose the lofty mountain ranges of the 
world, with their altered beds of thick sediments intercalated with tuffs, 
lava, and intrusives, all thrown into enormous series of folds and elevated to 
thousands of feet. Thus were born the crystalline schists with their variants 
of gneisses and migmatites.  See  GEOSYNCLINE; OROGENY. 
    Well-defined series of mineral facies have been singled out. Sedimentary 
rocks of the lowest metamorphic grade have recrystallized to give rocks of the 
zeolite facies. At slightly higher temperatures the greenschist facies 
develops - chlorite, albite, and epidote being characteristic minerals. A 
higher degree of metamorphism produces the epidote-amphibolite facies, and a 
still higher degree the true amphibolite facies in which hornblende and 
plagioclase mainly take the place of chlorite and epidote. Representative of 
the highest regional metamorphic grade is the granulite facies, in which most 
of the stable minerals are water-free, such as pyroxenes and garnets. Any 
sedimentary unit will recrystallize according to the rules of the several 
mineral facies, the complete sequence of events being a progressive change of 
the sediment by deformation, recrystallization, and alteration in the 
successive stages: greenschist facies->epidote-amphibolite facies->amphibolite 
facies->granulite facies.  See  FACIES (GEOLOGY); GRANULITE. 
    The normal continental crust is entirely made up of metamorphic rocks; 
where thermal, mechanical, and geochemical equilibrium prevails, there are 
only metamorphic rocks. Border cases of this normal situation occur in the 
depths where ultrametamorphism brings about differential melting and local 
formation of magmas. When equilibrium is restored, these magmas congeal and 
recrystallize to (metamorphic) rocks. At the surface, weathering processes 
oxidize and disintegrate the rocks superficially and produce sediments as 
transient products. Thus the cycle is closed; petrology is without a break. 
All rocks that are found in the continental crust were once metamorphites.  
See  AMPHIBOLITE; EPIDIORITE; EPIDOSITE; GREISEN; MARBLE; MICA SCHIST; 
MIGMATITE; PHYLLITE; PHYLLONITE; QUARTZITE; SANIDINITE; SCAPOLITE; 
SERPENTINITE; SOAPSTONE. 
 �34s[T. F. W. Barth ;R. C. Newton]
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�6v�2v�32sMetamorphism   �21m �33s 
    The alterations and transformations in preexisting rock masses effected by 
temperature and pressure, but excluding changes produced by weathering and 
sedimentation. The changes may include the production of new minerals, 
structures, or textures, or all three. They give a distinctive new character 
to the rock as a whole, but they do not involve the loss of individuality of a 
rock mass, such as changes brought about by fusion. Quantitatively, the 
metamorphic rocks, including gneisses and migmatites, are the most important 
group of rocks in the crust of the continents.  See  METAMORPHIC ROCKS; 
GNEISS; MIGMATITE. 
    Different kinds of metamorphism may be defined according to genetic 
criteria, such as the geologic processes that were assumed to have caused the 
metamorphism, or the physical and chemical conditions that appear to have been 
predominant in determining the course of metamorphism. Using these criteria 
three general kinds of metamorphism are noted below. 
    1. Dislocation, mechanical, or dynamic metamorphism is the result of 
pressure (or stress) along dislocations in the Earth's crust. The deformed 
rocks commonly show marked zones of extremely fine-grained rocks, such as 
mylonites, whose structures are determined by crushing and movement of the 
grains without important recrystallization of old, or growth of new, minerals. 
This type of metamorphism is local and restricted in occurrence. 
    2. Contact or thermal metamorphism occurs in response to increased 
temperature induced by adjacent intrusions of magma. Chemical reconstitution 
of the rocks is due to magmatic exhalation; other conditions, such as 
confining pressure, exert subordinate influence. 
    3. Regional metamorphism, the most widespread type, is brought about by an 
increase in both temperature and pressure in orogenic regions, which are vast 
segments of the crust represented by the folded mountain ranges. Heat and 
pressure are mainly consequences of downwarping and deep burial. Pressure is 
also generated by shearing stresses accompanying the orogenic movements.  See  
OROGENY.  
 �34s[T. F. W. Barth /R. C. Newton]
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�6v�2v�32sMetaplasia   �21m �33s 
    A type of epithelial or connective-tissue cellular reaction in which a 
pathway of cell specialization (differentiation) is selected other than the 
usual, resulting in the replacement of the original specialized type of tissue 
by another type. For example, the trachea is normally lined by mucus-secreting 
columnar epithelium. Following prolonged irritation, the primordial cells of 
the respiratory epithelium produce daughter cells that differentiate into 
stratified squamous epithelium. 
    Physiologic metaplasia occurs in normal developmental processes, such as 
in the ciliated mucous columnar epithelium in the esophagus during embryonic 
development. It is replaced by a stratified squamous epithelium by the time of 
birth. Pathologic metaplasia represents those types of changes of epithelial 
or connective tissue cells associated with alteration of the cell environment 
by toxic agents, bacteria, chronic inflammation, parasites, and avitaminosis 
A. Frequent sites of pathologic epithelial metaplasia are the gallbladder, 
urinary bladder, oral mucosa, uterine cervix, and stomach. Pathologic 
connective-tissue metaplasia, such as bone formation in fibrous scars, 
blood-vessel walls, or cartilage, is also frequently seen.  See  ANAPLASIA; 
HYPERPLASIA; HYPOPLASIA; NEOPLASIA. 
 �34s[N. Karle Mottet]
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�6v�2v�32sMetasomatism   �21m �33s 
    A process by which the chemical composition of rocks is altered. Every 
transformation of minerals by chemical replacement is designated as 
metasomatism, and in this sense most of the newly formed constituents of the 
metamorphic rocks would be metasomatic minerals.  See  METAMORPHIC ROCKS; 
METAMORPHISM; MINERAL. 
    Metasomatism is fundamentally a change, atom for atom, in a rock mineral. 
It postulates a reaction between a solid mineral and a disperse or fluid 
phase. Two types of metasomatism are recognized: mineral metasomatism and rock 
metasomatism. The mechanism of the mineral metasomatism in a metamorphic rock 
is the same as that of the rock metasomatism of large bodies, but in mineral 
metasomatism the fluid phase, which is generated of and by the metamorphic 
rock, would cause only a relocation of the chemical elements within the rock 
without changing the chemical composition of the whole rock. In the 
metasomatic rock, however, the fluid is introduced into the rock and acts 
metasomatically, thereby changing the initial chemical composition of the 
rock. Thus the mineral metasomatism taking place in a metamorphic rock may be 
considered as a metabolism of rock, that is, a relocation of the chemical 
elements which have to migrate over only short distances; whereas true 
metasomatism of rocks should be restricted to only those replacements by which 
the chemical composition of the whole rock, as well as that of the individual 
minerals, is essentially changed through an immigration of foreign chemical 
elements and an expulsion of other elements over long distances, up to several 
kilometers. Metasomatism in this sense would include, for example, the 
alteration of limestone into dolomite. Distinction may also be made between 
mutative, additive, and expulsive metasomatism.  See  DOLOMITE ROCK. 
    Metasomatism is widespread in silicate rocks. In contact zones of igneous 
intrusions the replacements may affect not  only the intruded rocks but also 
the crystallized magma from which the fluids are derived. Such magmatic fluids 
are often gaseous, giving rise to the so-called pneumatolytic metasomatism 
with formation of skarn rocks.  See  PNEUMATOLYSIS; SKARN. 
 �34s[T. F. W. Barth]
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�6v�2v�32sMetastable state   �21m �33s 
    In quantum mechanics, an excited stationary energy state whose lifetime is 
unusually long.  See  EXCITED STATE; STATIONARY STATE. 
    The stationary states of atoms and nuclei, or indeed of any material 
system, ordinarily are computed ignoring the interactions between matter and 
the electromagnetic radiation field. When these interactions are included, a 
system in an excited state always has some probability of emitting radiation 
in the form of one or more photons, and thereby decaying to a lower or less 
excited state. For a metastable state, the probability of making such a 
transition is unusually small.  See  NONRELATIVISTIC QUANTUM THEORY; NUCLEAR 
ISOMERISM.  
 �34s[Edward Gerjuoy]
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�6v�2v�32sMetatheria �21m �33s
    An infraclass of therian mammals including a single order, the
Marsupialia. The Metatheria are distinguished from the Eutheria (the placental
mammals) by numerous characters. The braincase is small, the angular process
of the mandible is inflected, and a pair of marsupial bones articulates with
the pelvis. Almost all living marsupials have a pouch on the belly of the
female in which the young are carried after birth. The early marsupials were
unable to compete with the more progressive later placental forms and died out
except in South America and Australia, where they were isolated by water
barriers.  See	EUTHERIA; MAMMALIA; THERIA.
 �34s[D. Dwight Davis /Frederick S. Szalay]
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�6v�2v�32sMetazoa   �21m �33s 
    The multicellular or tissue animals that make up all of the animal kingdom 
except the one-celled or acellular Protozoa. The cells of the metazoan body 
are organized in layers or groups as tissues or organ systems variously 
specialized for performing different bodily functions. The cellular 
organization into tissues varies widely in different groups of animals and in 
the parts of any one animal. In some instances the cells lose their boundaries 
after growth.  See  ENTEROZOA. 
 �34s[Tracy I. Storer]
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�6v�2v�32sMeteor   �21m �33s 
    Observable phenomenon resulting from entry of a particle of matter, called 
a meteoroid, into the Earth's atmosphere at a speed between 7 and 45 mi/s (11 
and 72 km/s). Meteoric material originates in the solar system and is 
attributed to asteroidal and cometary debris. 
    The mass of individual particles extends from micrometer-size dust specks 
to complete asteroids with weights in the thousands of tons. These extreme 
cases represent special fields of study.  See  METEORITE. 
    Meteoroids become visible at altitudes of from 50 to 75 mi (80 to 120 km), 
depending on their velocities. At these altitudes the energy transferred to 
meteoroids by collisions with atmospheric atoms and molecules becomes 
sufficient to vaporize the solids. Further collisional interactions of the 
vaporization products with the air produce luminosity and ionization. The 
detectable portions of trajectories, during which the meteoroids are luminous, 
are called trails. These phenomena may be observed by visual, photographic, 
telescopic, or radar techniques. 
    The meteor orbit for the few tens of kilometers of trail may almost always 
be described as a straight line. This straight-line trajectory is a great 
circle as seen projected on the celestial sphere. If the same meteor is 
observed from several different locations, the extension, backward along the 
trail, of the great circle observed at each station intersects the same point 
on the celestial sphere. This is the meteor's radiant point and is determined 
solely by the direction of motion of the meteoroid. In the same way, if a 
number of meteors traveling parallel to one another are observed from a single 
location, the backward extensions of their trails intersect at the common 
radiant point. Meteors with parallel motion and identical velocities must have 
identical orbits, and, very likely, identical origins. Groups of meteors with 
these characteristics are called meteor showers. 
    Apart from the elements they contain, visual meteors may bear no 
similarity to meteorites. There is good evidence that most meteoroids fragment 
as they pass through the atmosphere, and some evidence that nearly complete 
disruption of certain meteoroids occurs whenever the dynamic pressures on the 
meteoroids exceed 0.5 pound per square inch (3.4 kilopascals). The destruction 
of an entire meteoroid by so small a pressure, and not by thermal effects, is 
indicative of extreme fragility. Other observations indicate that the density 
of meteoroids may possibly be less than that of water. It is presumed that the 
low density and extreme fragility are due to porosity of the meteoric material. 
 �34s[Richard E. McCrosky]
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�6v�2v�32sMeteorite   �21m �33s 
    A natural, nonterrestrial object which survives passage through the 
Earth's atmosphere and arrives as a solid at the Earth's surface. As the 
meteorite passes through the atmosphere, it may be viewed as a falling star, 
with all of the optical brilliance and auditory effects characteristic of 
meteors. However, only an extremely small fraction of the meteors survive 
their atmospheric entry. Hence, meteorites represent rare events, but meteors 
can be seen on almost any clear night if moonlight or the lights of a city do 
not overilluminate the nighttime sky.  See  METEOR. 
    Meteorites consist mostly of metal and silicate material. They can be 
classified as irons, stony irons, or stones, depending on the relative 
proportions of the iron and the silicates. There are only about 2500 
meteorites known. These do not include the tiny micrometeorites that have been 
found in oceanic sediments and in high-altitude dust. Micrometeorites include 
both interplanetary dust and some of the numerous bits of debris that are 
generated when larger objects pass through the atmosphere.  See  
MICROMETEORITE.  
    Meteorites represent samples of the primitive interplanetary rubble that 
orbits the Sun but did not aggregate into a planetary-sized object. This 
rubble consists of bodies that range from hundreds of kilometers in diameter 
to tiny particles of interplanetary dust. Collisions between these bodies are 
frequent, particularly in the thickly populated region between the orbits of 
Mars and Jupiter. The larger bodies that are located between the orbits of 
Mars and Jupiter are called asteroids. More than 1600 asteroids are known, and 
they appear to have the same range of densities and chemical compositions that 
are observed in meteorites.  See  ASTEROID. 
    Comets are another possible source of meteorites. Cometary debris is 
probably the source of many meteors, including the more spectacular meteor 
showers, but there is no compelling evidence that comets are the source of any 
known meteorites.  See  COMET; SOLAR SYSTEM. 
    The impact of a large meteorite, one weighing more than a few hundred 
tons, releases so much energy that the meteorite explodes. This produces a 
depression called a meteorite crater. There was little interest in terrestrial 
meteorite craters prior to the observation that impact craters are responsible 
for many of the surface features of the Moon and Mars. 
 �34s[Oliver K. Manuel]
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�6v�2v�32sMeteorological optics   �21m �33s 
    A branch of atmospheric physics or physical meteorology in which optical 
phenomena occurring in the atmosphere are described and explained. 
    Meteorological optics can be divided into two main parts. In the first 
part, the atmosphere is considered as a continuous medium in which the speed 
of propagation of electromagnetic waves (light) varies with the density. In 
the second part, attention is given to the presence of particulate matter in 
the atmosphere, such as air molecules, dust and haze particles, cloud and 
raindrops, and ice crystals, and to the phenomena resulting from the 
interaction of light with these particles. In a third, minor part, 
applications to measurement of atmospheric properties are considered. 
    Several technical problems can be solved by direct application of the 
results of meteorological optics, such as the amount of illumination of 
objects on the ground or high in the atmosphere and the conditions of their 
visibility under different atmospheric conditions. 
 �34s[Zdenek Sekera]
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�6v�2v�32sMeteorological rockets   �21m �33s 
    Synoptic exploration of the stratospheric circulation through use of small 
rocket systems. The data obtained from rocket investigations has produced 
dramatic changes in the scientific view of this frontier region of the 
atmosphere, with a resulting alteration of the structural concepts into an 
atmospheric model which is primarily characterized by intense dynamics.  See  
STRATOSPHERE.  
    The Meteorological Rocket Network (MRN) has roughly doubled the observed 
atmospheric volume and its conception and operation has epitomized the 
international cooperation. The MRN began at North American missile ranges in 
1959, was expanded into a global network in 1968, and now includes 56 sites. 
 �34s[Willis L. Webb]
�33s 
  
 �34s[Walter R. Nagel]
�33s 
  
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�6v�2v�32sMeteorological satellites   �21m �33s 
    Earth-orbiting spacecraft carrying a variety of instruments for measuring 
visible and invisible radiation from the Earth and its atmosphere. The 
instruments of a single polar-orbiting satellite can view the entire Earth 
twice each 24 h - a striking contrast to the small fraction of the global 
atmosphere that is sampled by the combination of all other methods by all 
nations.  See  WEATHER FORECASTING AND PREDICTION. 
    The space age of meteorology began when Tiros 1 (television and infrared 
observation satellite) was launched on April 1, 1960. From that point on the 
United States launched a large number of satellites which produced a 
prodigious flow of weather data. Most satellites provide information for 
routine analysis; others are intended for research and development. 
    Pictures from orbiting spacecraft are assembled with a sophisticated 
communication-computer system. Pictures are tape-recorded while the vehicle 
transits remote regions. Once within (FM) radio range of a receiving station 
the tape is read out and the signals sent by land lines to the National 
Environmental Satellite Service near Washington, DC. There they are fed into a 
high-speed computer for assembling and mapping. Pictures displayed for 
photographing on a cathode-ray tube are overlaid with latitude-longitude lines 
and coastlines. These are fitted to the picture by a computer that calculates 
their shape and placement from the satellite message, and from the known 
height and position of the camera at a picture time (see illustration).  See  
WEATHER MAP. 
    While visible pictures are irreplaceable for some purposes, the 1970s 
witnessed an accelerated use of infrared images and data. Infrared radiation, 
heat energy, is highly dependent on the temperature of the radiating material, 
a characteristic that makes it possible to deduce a myriad of information. 
Visible and infrared images exhibit an important difference. Thick clouds, 
regardless of their altitude, reflect most of the sunlight that falls on their 
upper surface so that they are very bright when photographed from space. On 
the other hand, low clouds, lying in the warmer atmosphere, and high clouds, 
in the cold upper atmosphere, radiate at quite different intensities and 
therefore exhibit different brightnesses on film. 
    Ocean-surface temperatures, critical to both meteorology and oceanography, 
can be deduced from infrared measurements in cloudless regions. Sophisticated 
computer programs analyze infrared data from all ocean areas each day, 
deducing which measurements were made in cloud-free areas, using only these 
for computing sea-surface temperatures. Other sensors respond to water vapor 
in the atmosphere, while those measuring a broad spectrum of wavelengths 
observe the total heat lost to space. This debit side of the Earth's heat 
budget is a critical variable for long-range forecasting. 
 �34s[Lester F. Hubert]
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�6v�2v�32sMeteorology   �21m �33s 
    The science concerned with the atmosphere and its phenomena. Meteorology 
is primarily observational; its data are generally ``given.'' The 
meteorologist observes the atmosphere - its temperature, density, winds, 
clouds, precipitation, and other characteristics - and aims to account for its 
observed structure and evolution (weather, in part) in terms of external 
influence and the basic laws of physics.  See  ATMOSPHERE. 
    Synoptic meteorology is the branch of meteorology comprising the knowledge 
of atmospheric phenomena connected with the weather, applied mainly in weather 
forecasting, and based on data acquired by the synoptic method. This method 
involves the study of weather processes through representations of atmospheric 
states determined by synchronous observations at a network of stations, most 
of which are at least 6 mi (10 km) apart. By international agreement, the data 
taken from the Earth's surface and aloft at certain international hours of 
observation are inserted on weather maps, upper-air maps, vertical cross 
sections, time sections, and sounding diagrams with international symbols 
according to fixed rules. These crude representations are then analyzed and 
critically evaluated in accordance with the knowledge of existing structure 
models in the (lower) atmosphere, in order to ascertain the best approximation 
to a three-dimensional image of the true atmospheric state at the hour of 
observation. From one such representation, or a series of them, future 
atmospheric states are then derived with the aid of empirical knowledge of 
their behavior and by application of the theoretical results of dynamic 
meteorology.  See  WEATHER MAP. 
    The branch of meteorology known as dynamical meteorology is the science of 
naturally produced motions in the atmosphere and of the related distributions 
in space of pressure, density, temperature, and humidity. Based on 
thermodynamic and hydrodynamic theories, it forms the main scientific basis of 
weather forecasting and climatology.  See  CLIMATOLOGY; WEATHER FORECASTING 
AND PREDICTION. 
 �34s[Tor Bergeron]
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�6v�2v�32sMetering orifice   �21m �33s 
    A wall opening with sharp inlet edges, through which fluid flows, the wall 
thickness being less than an eighth of the area of the opening. An orifice 
serves basically to control or to meter the rate of fluid flow. It may be used 
as an instrument for the measurement of fluid flow, including oils, air, 
gases, steam, and other vapors, or as an element in a machine to limit the 
flow of fluid. Common shapes are sharp-edge, with bevel facing out, and 
square. Often, fuel enters a chamber through an orifice, as in a carburetor.  
See  FLOW MEASUREMENT. 
 �34s[Raymond E. Sprenkle]
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�6v�2v�32sMeterological balloon   �21m �33s 
    A balloon used to carry radiosondes and other lightweight instruments 
aloft, also known as a sounding balloon. It is an extensible balloon made from 
natural or synthetic rubber and filled with either hydrogen or helium. 
Hundreds of these balloons are launched daily by meterological stations 
throughout the world in support of the upper-air program of the World 
Meteorological Organization, as well as local weather forecasting. These 
flights are vertical sounding probes of the atmosphere for measuring pressure, 
temperature, humidity, and wind velocity. They can carry up to 18 kg (40 lb) 
of useful payload. High-altitude types carrying much lighter payloads may 
exceed 40 km (25 mi) before expanding to the bursting point. See 
METEOROLOGICAL INSTRUMENTATION. 
 �34s[Walter R. Nagel]
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�6v�2v�32sMethane �21m �33s
    A member of the alkane or paraffin series of hydrocarbons with the formula
shown below. Methane is called marsh gas because it forms by anaerobic
bacterial decomposition of vegetable matter in swampy land. Coal miners know
it as firedamp because mixtures with air are combustible. It is a major
constituent of natural gas (50 - 90%) and of coal gas. It forms in large
amounts in sewage disposal processes, especially in anaerobic digestion. As a
liquid it freezes at -182.6DGC (-296.7DGF) and boils at -161.6DGC (-258.9DGF).

				H
				|
			     H--C--H
				|
				H

    In addition to its use as a fuel, methane is important as a source of
organic chemicals and of hydrogen. Its reaction with steam at high
temperatures in the presence of catalysts yields carbon monoxide and hydrogen
(synthesis gas), which can be catalytically converted to liquid alkanes
(Fischer-Tropsch process) or to methanol and other alcohols.  See  ALKANE;
FISCHER-TROPSCH PROCESS; HYDROFORMYLATION; NATURAL GAS.
 �34s[Louis Schmerling]
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�6v�2v�32sMethane-producing bacteria   �21m �33s 
    Bacteria characterized by the unique ability to produce the gaseous 
hydrocarbon methane (CH4) as the major product of metabolism. These bacteria 
are found in nature wherever plant material is undergoing anaerobic 
decomposition. Although methane-producing bacteria are among the most 
oxygen-sensitive bacteria known, they are widespread in habitats such as the 
digestive tracts of animals (intestine, rumen, cecum), sediments, swamps, 
marshes, sanitary landfills, sewage sludge digesters, and decaying heartwood 
of certain trees. In these habitats other bacteria consume the available 
oxygen, thus making it possible for anaerobic zones of low reducing potential 
to be established. Methane-producing bacteria are terminal organisms in the 
microbial metabolic food chain in which organic matter is decomposed. However, 
the amount of methane formed in covered sanitary landfills is large enough 
that fills should be safely vented.  See  METHANE. 
 �34s[Ralph S. Wolfe]
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�6v�2v�32sMethanol �21m �33s
    The simplest alcohol (alkanol). Methanol has the formula shown below.
Formerly obtained from wood as a coproduct of charcoal production, it was
called wood spirit or wood alcohol.

			      H
			      |
			   H--C--OH
			      |
			      H

    Methanol is a colorless, poisonous liquid with essentially no odor and
very little taste. Its lack of odor is an important hazard in handling, and
often strong-smelling wood alcohol or kerosine is added so that workers will
become aware of leaks or spills. Methanol boils at 64.7DGC (149DGF). It is
miscible with water and most organic liquids, including gasoline. It is
extremely flammable, burning with a nearly invisible blue flame.
    Methanol has become a major feedstock for the synthesis of acetic acid,
and large amounts of methanol are consumed in the production of methyl esters.
Methanol itself is employed as an extractant and solvent for many substances,
and is sometimes blended with gasoline in cold weather to reduce condensation
problems.  See	ALCOHOL.
 �34s[Frank Wagner]
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�6v�2v�32sMethionine   �21m �33s 
    An amino acid considered essential for normal growth of animals. 
Methionine is an important methyl group donor in transmethylation reactions. 
For this purpose it must first be activated by adenosinetriphosphate (ATP), 
forming S-adenosylmethionine. This compound readily transfers its methyl group 
to suitable acceptors, leaving S-adenosylhomocysteine. Active methyl groups 
originate during methionine biosynthesis. A hydroxymethyl group is reduced to 
a methyl group through the mediation either of coenzyme B12 or of 
tetrahydrofolic acid.  See  AMINO ACIDS. 
 �34s[Edward A. Adelberg]
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�6v�2v�32sMethods engineering   �21m �33s 
    A technique used by progressive management to improve productivity and 
reduce costs in both direct and indirect operations of manufacturing and 
nonmanufacturing business organizations. Methods engineering is applicable in 
any enterprise wherever human effort is required. It can be defined as the 
systematic procedure for subjecting all direct and indirect operations to 
close scrutiny in order to introduce improvements that will make work easier 
to perform and will allow work to be done smoother in less time, and with less 
energy, effort, and fatigue, with less investment per unit. The ultimate 
objective of methods engineering is profit improvement. See OPERATIONS 
RESEARCH; PRODUCTIVITY. 
    Methods engineering includes five activities: planning, methods study, 
standardization, work measurement, and controls. Methods engineering, through 
planning, first identifies the amount of time that should be spent on a 
project so as to get as much of the potential savings as is practical. 
Invariably the most profitable jobs to study are those with the most 
repetition, the highest labor content (human work as distinguished from 
mechanical or process work), the highest labor cost, or the longest life-span. 
Next, through methods study, methods are improved by observing what is 
currently being done and then by developing better ways of doing it. The 
standardization phase includes the training of the operator to follow the 
standard method. Then the number of standard hours in which operators working 
with standard performances can do their job is determined by measurement. 
Finally, the established method is periodically audited, and various 
management controls are adjusted with the new time data. The system may 
include a plan for compensating labor that encourages attaining or surpassing 
a standard performance. 
 �34s[Benjamin W. Niebel]
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�6v�2v�32sMetric system   �21m �33s 
    A system of units used in scientific work throughout the world and 
employed in general commercial transactions and engineering applications in 
most of the developed nations of the world except for Great Britain and the 
United States. The basic units of the metric system define length (meter), 
mass (kilogram), and time (second). 
    The chief advantage of the metric system is that it is based on standards 
that have been accepted by international agreement, and it therefore provides 
a common basis for all scientific measurements. A second advantage of the 
metric system lies in the fact that only decimal multiples and submultiples of 
the fundamental length and mass units and of other derived units are employed. 
  See  PHYSICAL MEASUREMENT; TIME; UNITS OF MEASUREMENT. 
 �34s[Dudley Williams]
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�6v�2v�32sMica   �21m �33s 
    Any one of a group of hydrous aluminum silicate minerals with platy 
morphology and perfect basal (micaceous) cleavage. The most common micas are 
muscovite, paragonite, phlogopite, biotite, and lepidolite. 
K(Li,Al)2.5-3.0(Al1.0-0.5Si3.0-3.5O10)(OH)2. Ca, Ba, Rb, and Cs can sometimes 
substitute for Na and K; Mn, Cr, and Ti for Mg, Fe, and Li; and F for OH. 
    Mica is commonly found as small flakes or lamellar plates without a 
crystal outline. Muscovite and biotite sometimes occur in thick books, tabular 
prisms with a hexagonal outline that can be up to several feet across. The 
prominent basal cleavage is a consequence of the layered crystal structure. 
Thin cleavage sheets of micas, particularly muscovite and phlogopite, are 
flexible, elastic, tough, and translucent to transparent (isinglass). They 
have low electrical and thermal conductivity and high dielectric strength. 
    Micas have Mohs hardnesses of 2 - 3 and specific gravities of 2.8 - 3.2. 
They have a vitreous to pearly luster. Muscovite is colorless to pale shades 
of brown, green, or gray. Paragonite is colorless to pale yellow. Phlogopite 
is pale yellow to brown. Biotite is dark green, brown, or black. Lepidolite is 
most often pale lilac, but it can also be colorless, pale yellow, or pale gray. 
    Commercial mica is of two main types: sheet, and scrap or flake. Sheet 
muscovite, mostly from pegmatites, is used as a dielectric in capacitors and 
vacuum tubes in electronic equipment. Lower-quality muscovite is used as an 
insulator in home electrical products such as hot plates, toasters, and irons. 
Scrap and flake mica is ground for use in coatings on roofing materials and 
waterproof fabrics, and in paint, wallpaper, joint cement, plastics, 
cosmetics, well drilling products, and a variety of agricultural products.  
See  CAPACITOR; ELECTRIC INSULATOR; SILICATE MINERALS. 
 �34s[Lawrence Grossman]
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�6v�2v�32sMica schist   �21m �33s 
    A widely distributed group of rocks of medium to high metamorphic grade, 
composed essentially of mica and quartz and exhibiting a foliated or schistose 
structure which is easily revealed by the parallel orientation of the mica 
flakes. Varieties are biotite schists, biotite-sericite schists, 
biotite-chlorite schists, and albite-biotite schists. Of lower metamorphic 
grade and of fine grain are phyllites or chlorite-sericite schists, without 
biotite but otherwise similar to mica schists, into which they pass by gradual 
transitions.  See  MICA; PHYLLITE. 
    Mica schists are most widely distributed in Precambrian areas and in the 
younger eroded mountain ranges where they usually represent metamorphic 
equivalents of aluminum-rich sediments (clays and argillites). However, tuffs 
and certain acid igneous rocks (rhyolites, granites) may form part of the 
primary constituents.  See  METAMORPHIC ROCKS. 
 �34s[T. F. W. Barth]
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�6v�2v�32sMicelle   �21m �33s 
    A colloidal aggregate of a unique number (50 -> 100) of amphipathic 
molecules, which occurs at a well-defined concentration called the critical 
micelle concentration. In polar media such as water, the hydrophobic part of 
the amphiphiles forming the micelle tends to locate away from the polar phase 
while the polar parts of the molecule (head groups) tend to locate at the 
polar micelle solvent interface. A micelle may take several forms, depending 
on the conditions and composition of the system, such as distorted spheres, 
disks, or rods (see illustration a, b, and c). Micelles are formed in nonpolar 
media such as benzene, where the amphiphiles cluster around small water 
droplets in the system, forming an assembly known as a reversed micelle 
(illustration d). 
    Micellar systems have the unique property of being able to solubilize both 
hydrophobic and hydrophilic compounds. They are used extensively in industry 
for detergency and as solubilizing agents.  See  DETERGENT; SOAP. 
 �34s[J. Kerry Thomas]
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�6v�2v�32sMicrobiological methods   �21m �33s 
    Methods customarily employed in the laboratory for the study of 
microorganisms include techniques to describe, enumerate, identify, and study 
the physiological processes of microscopic forms of life. 
    Microscopes that provide approximately a thousandfold magnification are 
used to observe bacteria. Lesser magnification is required for yeasts, molds, 
algae, and protozoa, but much higher magnification (up to 
several-hundred-thousand-fold) as provided by electron microscopes is utilized 
for visualization of viruses. Electron microscopy is also used to view thinly 
sliced sections of bacteria and other organisms in order to study details of 
their internal structure.  See  ELECTRON MICROSCOPE. 
    Special staining methods are used to provide information concerning the 
chemical nature of the surface and some of the subcellular structures of 
microorganisms. Bacteria are separated into two large groups (gram-negative or 
gram-positive) by virtue of their staining by sequential application of basic 
dyes and a decolorizing agent that make up Gram's stain.  See  GRAM'S STAIN; 
STAIN (MICROBIOLOGY). 
    Counting of microorganisms generally involves inoculation of a series of 
tenfold dilutions into sterile culture agar media that are presumed to be 
adequate for the initiation of growth and subsequent multiplication of each 
living organism in the sample being counted. Each living microbial cell 
produces a mass of cells (called a colony) that is visible to the unaided eye. 
A count of these colonies multiplied by the reciprocal of the dilution 
inoculated into the dish provides a quantitative measure of the microbial 
population of the original specimen.  See  CULTURE MEDIA; CULTURE TECHNIQUE. 
    Identification and other studies of a microorganism normally must be 
preceded by its isolation and cultivation of a pure culture. This is usually 
accomplished by transferring some of the cells from a colony growing on an 
agar medium to a sterile liquid medium. (A fully grown culture, after one day 
of incubation, may exceed a billion cells per milliliter.) Characterization of 
a pure culture of a microorganism consists primarily of determinations of 
various physiological capabilities of this large population of cells.  See  
PURE CULTURE. 
    Research studies on microorganisms may involve all of the methods of 
modern cellular and molecular biology. Basic studies of the genetic processes 
of microorganisms have yielded much knowledge of fundamental genetics. 
Biochemical studies of cellular processes frequently involve the use of both 
physical and chemical methods to separate and purify specific intracellular 
components from homogeneous populations of microbial cells. The physical 
structure of a preparation may be examined by electron microscopy and by a 
variety of other sophisticated physicochemical methods. Biochemical activity 
may be studied by the methods of enzyme chemistry. 
 �34s[James B. Evans]
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�6v�2v�32sMicrobiology   �21m �33s 
    The science and study of microorganisms. These include the protozoa, 
algae, fungi, bacteria, viruses, and rickettsiae.  See  ALGAE; BACTERIA; 
FUNGI; PROTOZOA; RICKETTSIAE; VIRUS. 
    Microbiology at first developed largely as an applied science with 
emphasis on the role of microorganisms as pathogens and their association with 
the spoilage of food. Little attention was paid to the essential biological 
properties of the microorganisms themselves. However, the studies on their 
physiology and biochemistry, carried out in the 1920s, pointed to the 
fundamental unity underlying the behavior of all living organisms. The 
enormous diversity encountered among microorganisms and the specificity of 
their physiological patterns helped make them favorite objects for biochemical 
investigations. This, in turn, stimulated an interest in all sorts of 
microorganisms, with the result that microbiology became acknowledged as an 
important branch of biological science. 
    For medical and sanitary aspects of microbiology  see  EPIDEMIOLOGY; 
IMMUNOLOGY; MEDICAL BACTERIOLOGY; MEDICAL MYCOLOGY; MEDICAL PARASITOLOGY; 
RICKETTSIOSES; VIRUS. For food products spoilage see FOOD MICROBIOLOGY; MILK. 
For the transformation of microorganisms in water and soil see SOIL 
MICROBIOLOGY; WATER MICROBIOLOGY. For industrial aspects see INDUSTRIAL 
MICROBIOLOGY. For methodology see MICROBIOLOGICAL METHODS. 
 �34s[Cornelis B. Van Niel]
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�6v�2v�32sMicrocline   �21m �33s 
    The name for a triclinic potassium-rich feldspar. The microcline structure 
is built up of rather pure KAlSi3O8. Microcline is usually twinned with 
typical crosshatching, indicating that it usually grew (stably or metastably) 
as sanidine (monoclinic) and subsequently changed during geological time into 
the triclinic modification by passing through a state called normal 
orthoclase. Microcline is typically found in granites and pegmatites. Green 
microcline is known as amazon stone or amazonite.  See  FELDSPAR. 
 �34s[Fritz H. Laves]
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�6v�2v�32sMicrococcaceae   �21m �33s 
    A family of gram-positive cocci, all producing the enzyme catalase 
(splitting hydrogen peroxide to water and oxygen) and with a respiratory 
metabolism. Endospores are not formed. Bright yellow, red, green, and purple 
pigments are formed by many of the organisms in this group. The family 
contains three genetically nonrelated genera: Micrococcus, Staphylococcus, and 
Planococcus. See STAPHYLOCOCCUS. 
 �34s[Manley Mandel]
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�6v�2v�32sMicrocomputer   �21m �33s 
    A digital computer whose central processor unit (CPU) is a microprocessor 
(see illustration). A microcomputer can be a 4-bit-, 8-bit, or 16-bit-word 
machine depending upon the word size of its microprocessor. A microcomputer 
used to be a 4-bit- or 8-bit-word machine, and a minicomputer a 16-bit-word 
machine. Because of the advance of integrated circuit technology, even though 
minicomputers are still basically 16-bit-word computers, their physical size, 
weight, and cost have been reduced considerably. On the other hand, the 
microcomputer has expanded from 8-bit- to 16-bit-word machines with 
sophisticated supporting hardware and software. They can interface with mass 
storage media such as cassette tapes and floppy disks. Furthermore, in 
addition to machine and assembly programming languages, microcomputers can be 
programmed using high-level programming languages, such as BASIC, FORTRAN, or 
PASCAL, which once were available only on minicomputers. Hence, the difference 
between microcomputers and minicomputers has become very small.  See  COMPUTER 
STORAGE TECHNOLOGY; DIGITAL COMPUTER. 
    Based on their sizes and applications, microcomputers are classified into 
three categories: one-card (or minimum) microcomputers; personal (home or 
business) microcomputers; and microcomputer development systems. Because of 
their low cost, the use of personal microcomputers in education, home 
entertainment, personal record keeping, and business has become very popular. 
And, they can have peripherals such as keyboard and cathode-ray-tube display, 
audio cassette tape recorders, disk drives, and printers. A complete 
microcomputer system that is used to test both the software and hardware of 
other microcomputer-based systems is called a microcomputer development 
system. Such a system provides hardware and software support from initial 
program development to debugging the prototype system. 
  
ms. Pgs. 4436-44372:52 
    A microcomputer is generally used for a dedicated task as part of a 
system. Because of its low cost, the microcomputer has begun to replace the 
minicomputer in many dedicated systems, such as data acquisition systems, 
processor control systems, data-processing systems, and remote-terminal 
control systems.  See  MICROPROCESSOR. 
 �34s[Samuel C. Lee]
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�6v�2v�32sMicrocomputer development system   �21m �33s 
    A complete microcomputer system used to test and develop both the hardware 
and software of other microcomputer-based systems, from initial development 
through debugging of final prototypes. 
    A typical microcomputer development system includes assembler facilities, 
a text editor, debugging facilities, and hardware emulation capabilities. The 
assembler permits instructions for the microcomputer to be written as 
mnemonics, which are then translated by the assembler into hexadecimal or 
binary machine language. The machine language program, known as an object 
file, is loaded into the microcomputer for execution. The program written in 
mnemonics is known as the source code, and is generated by the text editor.  
See  DIGITAL COMPUTER PROGRAMMING. 
    A key component is the emulator, a combination of hardware and software 
which permits the microcomputer development system to emulate programs written 
for other microcomputer systems. Many emulators can execute source code 
programs for different microprocessors. A few emulators can also execute 
high-level languages such as BASIC, FORTRAN, and PASCAL.  See  MICROCOMPUTER; 
MICROPROCESSOR; PROGRAMMING LANGUAGES. 
 �34s[Samuel C. Lee]
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�6v�2v�32sMicrofilming   �21m �33s 
    The technique of making, on a reduced scale, a photographic film record of 
documents, printed matter, and the like, which can be enlarged for reading. 
Reductions may range from 8 to 40 times, although in extreme cases a reduction 
of 60 times has been used.  See  PHOTOCOPYING PROCESSES; PHOTOGRAPHY. 
 �34s[Walter Clark]
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�6v�2v�32sMicromanipulation   �21m �33s 
    The techniques and science of microdissection, microvivisection, 
microisolation, and microinjection. This field of knowledge is also known as 
micrurgy. Applications of micrurgy are many, especially in biology, cancer 
research, microchemistry, biophysics, colloid technology, and metallurgy. 
    The basic instruments required for micrurgy are (1) micromanipulators or 
micropositioners which convert the relatively crude hand movements to the 
exquisitely fine movements of the microtools; (2) microtools such as needles, 
pipes, hooks, loops, electrodes, scalpels, or forceps; and (3) microscopes and 
accessories.  
 �34s[M. J. Kopac]
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�6v�2v�32sMicrometeorite   �21m �33s 
    A meteorite less than about 0.1 mm in diameter. These bodies are usually 
called micrometeoroids while still in space. Micrometeorites can be expected 
to survive the atmospheric encounter since their size permits them to radiate 
the tremendous heat energy generated by friction with the atmosphere before it 
can cause vaporization. The Earth collects micrometeorites at a rate of about 
10 tons (9 metric tons) per day. Those larger than micrometeorite size are 
destroyed during atmospheric entry and produce the visible phenomena known as 
meteors. Studies have shown that micrometeoroids yield important clues to the 
nature, origin, and evolution of the solar system and contribute significantly 
to the physical and chemical nature of the Earth's upper atmosphere.  See  
METEOR; METEORITE. 
 �34s[Robert K. Soberman]
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�6v�2v�32sMicrometeorology   �21m �33s 
    A branch of atmospheric dynamics and thermodynamics which deals primarily 
with the interaction between atmosphere and ground; the interchange of masses, 
momentum, and energy at the earth-air interface; in short, with the lower 
boundary conditions of atmospheric processes. Micrometeorology, along with 
other branches of meteorology, is also concerned with investigating and 
predicting the transport and dispersion of pollution from such sources as 
smokestacks and automotive exhausts in the lower atmosphere. Micrometeorology 
typically deals with the transport (or flux) of air properties in the vertical 
direction (conduction and convection) and the vertical variation of these 
fluxes.  See  AIR POLLUTION; OCEAN-ATMOSPHERE RELATIONS; SEA WATER; SOIL. 
    Micrometeorological research is important in that it supplies detailed 
information about the physical processes in the region of the atmosphere where 
life is most abundant. It closes the gaps of information from synoptic 
networks. It produces results useful for applications in various fields such 
as climatology, oceanography, soil physics, agriculture, biology, chemical 
warfare, and air pollution. Among the branches of atmospheric physics, 
micrometeorology is the one whose subjects are most amenable to fairly 
complete experimental description, and to testing of the theoretical models.  
See  AGRICULTURAL METEOROLOGY; CLIMATOLOGY; CROP MICROMETEOROLOGY; INDUSTRIAL 
METEOROLOGY.  
 �34s[Heinz H. Lettau]
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�6v�2v�32sMicrometer   �21m �33s 
    A precision instrument used to measure small distances and angles. A 
common use is on a machinist's caliper, as in the illustration.  See  CALIPER. 
    The spindle of the caliper is an accurately machined screw, which is 
rotated by the thimble or the ratchet knob until the object to be measured is 
in contact with both spindle and anvil. The ratchet slips after correct 
pressure is applied, ensuring consistent, accurate gaging. The number 1 on the 
sleeve represents 0.1 in.; the smallest divisions are 0.025 in. A vernier 
scale allows accurate reading to 0.0001 in.  See  VERNIER. 
 �34s[Frank H. Rockett]
�33s 
  
 �34s[Harry L. Helms]
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�6v�2v�32sMicroorganisms   �21m �33s 
    A heterogeneous assemblage of simple organisms, consisting of the 
protozoa, algae, fungi, rickettsiae, viruses, and bacteria. All these forms 
have in common a relatively simple organization which sets them apart from 
true plants and true animals: They are either unicellular or, if 
multicellular, their tissues are relatively undifferentiated. They are 
presumed to be evolutionary offshoots of the ancestral groups from which the 
plant and animal kingdoms have evolved. 
    The belated discovery of microorganisms and the failure at first to 
recognize their unique properties led to their arbitrary classification within 
the plant and animal kingdoms. This practice is still in use, although many 
biologists are now aware of the misconceptions which it engenders.  See  
BACTERIA; FUNGI; MICROBIOLOGY; PROTOZOA; RICKETTSIAE; VIRUS. 
 �34s[Edward A. Adelberg]
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�6v�2v�32sMicropaleontology   �21m �33s 
    A branch of paleontology dealing with the fossilized microscopic organic 
remains (microfossils) of the geologic past, their structure, biology, 
phylogenetic relations, and distribution in space and time. The study of these 
microfossils has become an independent scientific field largely because: (1) 
The size of these fossils requires special methods for collection and 
examination. (2) Their abundance in geologic formations makes it possible to 
analyze their spatial distribution and the rates of morphological changes 
during the course of evolution by means of statistical methods which can be 
used only under exceptional circumstances in the study of larger fossils. (3) 
Microfossils have become indispensable tools in certain branches of applied 
geology, especially in the exploration for oil-bearing strata, because 
countless numbers of these minute fossils may be obtained from small pieces of 
subsurface rock recovered from drill holes. (4) The diversity of microfossils, 
their wide spatial distribution in varied environments, and their distinctive 
steps in evolution and the ease of studying them have contributed to make 
micropaleontology one of the most actively studied branches of the earth 
sciences.  
    The material subjected to micropaleontological studies forms a spectrum 
from primitive plants to advanced vertebrates (see illustration). The only 
prerequisite for organisms to become the subject of micropaleontological 
studies is their possession of resistant skeletal components ensuring their 
preservation in sedimentary strata as fossilized remains even after 
biological, chemical, or mechanical processes have destroyed the organisms' 
soft parts. 
    Most major groups of organisms incorporate, besides organic compounds, 
hard resistant materials that serve for structural support or protection. The 
more common substances found among the microfossils are calcium carbonate, 
silicon dioxide (or silica), calcium phosphate in the form of the mineral 
apatite (typical of bones and teeth), sporonine (principal constituent of 
pollen and spore walls), and various complex organic compounds. 
 �34s[Tsunemasa Saito]
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�6v�2v�32sMicrophone   �21m �33s 
    An electroacoustic device containing a transducer which is actuated by 
sound waves and delivers essentially equivalent electric waves. Modern 
conventional microphones may be classified as pressure, gradient, combination 
pressure-gradient, and wave types.  See  TRANSDUCER. 
    In the pressure microphone the electrical response is caused by variations 
in sound pressure. Pressure microphones are inherently nondirectional 
(omnidirectional), because pressure is a scalar and not a vector quantity. 
    A pressure-gradient microphone is one in which the electrical response 
corresponds to some function of the difference in pressure between two points 
in space. In general, when the distance between the two points is small 
compared to the wavelength, the pressure gradient corresponds to the particle 
velocity in the sound wave. A velocity microphone is one in which this 
condition holds. 
    The wave microphone consists of a system in which the directivity depends 
upon some type of wave interference. The most common wave microphones are 
line, reflector, and lens types. Line and reflector microphones are highly 
directional in the speech-frequency range and may therefore be used for 
long-distance pickup of speech under conditions of high ambient noise and 
excessive reverberation. High-sensitivity transducers are employed in line and 
reflector microphones to provide an adequate signal-to-noise ratio. 
    The hot-wire microphone consists of a fine wire heated by the passage of 
an electric current. The cooling due to the motion of air past the wire causes 
a change in electrical resistance of the wire. In a sound wave the particle 
velocity cools the wire. There are also minor cooling effects produced by the 
sound pressure. The change in resistance due to the passage of a sound wave 
may be used to detect its presence. However, the frequency of the electrical 
output is twice the frequency of the sound wave because the wire is cooled 
equally by both positive and negative particle velocities. Therefore, this 
microphone cannot be used for the reproduction of sound. 
 �34s[Harry F. Olson]
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�6v�2v�32sMicrophonics   �21m �33s 
    The transformation of the energy of mechanical vibration into an 
electrical signal. The term microphonics is most commonly applied to the 
production of an undesirable electrical output by an electronic device 
undergoing mechanical vibration. 
    Under certain conditions, electrical wires and cables are microphonic in 
that they produce undesirable electrical outputs in response to mechanical 
excitation. This is most commonly encountered with coaxial cables. In 
addition, microphonic effects have been noted in ceramic capacitors, variable 
resistors, point-contact transistors and diodes, and switches. 
    In order to avoid microphonics, special consideration is given to the 
electromechanical design of many electronic components. In addition, 
electronic equipment designed for use in high-vibration environments, such as 
aircraft radios, are often mounted with vibration isolators to reduce the 
mechanical excitation of the equipment.  See  SHOCK ISOLATION. 
 �34s[David N. Keast]
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�6v�2v�32sMicroprocessor   �21m �33s 
    A central processor unit (CPU) on a single integrated circuit (IC) chip. 
The physical size of a typical microprocessor wafer is around 0.2 x 0.2 in. (5 
x 5 mm), but may vary from one microprocessor to another. The illustration is 
a magnified view of a typical microprocessor. Based on its word size (bits), 
the commercially available microprocessors can be classified as 4-bit, 8-bit, 
16-bit, or bit-sliced. The physical size and the number of pins of a 
microprocessor chip are proportional to its word size. 
    Each microprocessor has its own architecture, which encompasses the 
general layout of its major components, the principal features of these 
components, and the manner in which they are interconnected. However, the 
major components of all microprocessors are the same: (1) the clock; (2) 
control unit, comprising the program counter (PC), instruction register (IR), 
processor status word (PSW), and stack pointer (SP); (3) control memory; (4) 
bus control; (5) working register; (6) arithmetic/logic unit (ALU); (7) 
internal memory or stack. They are also called the hardware of a 
microprocessor. Besides the hardware, a microprocessor has its instruction set 
or software. According to the type of operations that the instructions 
perform, an instruction set can be subdivided into group operations for 
arithmetic, data transfer, branching, logic, and input/output (I/O). A set of 
logically related instructions stored in memory is referred to as a program. 
The microprocessor ``reads'' each instruction from memory in a logically 
determinate sequence, and uses it to initiate processing actions. 
    The main difference between the microprocessor digital system design and 
the hard-wired logic digital system design is that the former uses the 
microprocessor to replace hard-wired logic by storing program sequences in the 
read-only memory (ROM) rather than implementing these sequences with gates, 
flip-flops, counters, and so on. After the design is completed, any 
modifications or changes may be made by simply changing the program in the 
ROM. The microprocessor digital system design is now widely used because of 
the following advantages: 
    1. Manufacturing costs of industrial products can be significantly reduced. 
    2. Products can get to the market faster, providing a company with the 
opportunity to increase product sales and market share. 
    3. Product capability is enhanced, allowing manufacturers to provide 
customers with better products, which can frequently command a higher price in 
the marketplace. 
    4. Development costs and time are reduced. 
    5. Product reliability is increased, while both service and warranty costs 
are reduced.  See  SEMICONDUCTOR MEMORIES. 
    Application areas include: industrial sequence controllers; machine tool 
controllers; point-of-sale terminals; intelligent terminals; instrument 
processors; traffic light controllers; weather data collection systems; and 
process controllers.  See  CONTROL SYSTEMS; DIGITAL COMPUTER; INTEGRATED 
CIRCUITS; MICROCOMPUTER. 
 �34s[Samuel C. Lee]
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�6v�2v�32sMicroradiography   �21m �33s 
    The process of producing enlarged radiographic images on photographic 
film. Microradiography is applicable to small metallurgical, biological, and 
other specimens with gross structures not easily visible on conventional 1:1 
radiographs. Historadiography is a term used for the microradiography of 
biological specimens, and hence is a branch of histology. Microradiography is 
a type of x-ray microscopy.  See  HISTORADIOGRAPHY; OPTICAL MICROSCOPE; 
RADIOGRAPHY; X-RAY MICROSCOPE. 
    An enlarged radiographic image can be obtained in two ways: (1) by 
projection microradiography, also termed shadow microscopy, in which the 
specimen is placed at various distances from the film to secure inherent 
enlargement of divergent beams, and (2) by contact microradiography, in which 
the specimen is in contact with a fine-grained photographic emulsion so that 
the absorption image is photographically enlarged or viewed with a microscope. 
    In microradiography it is convenient to be able to generate a wide range 
of essentially monochromatic beams, instead of relying upon a series of x-ray 
tubes with different targets. It is possible to generate such beams at will by 
exciting secondary fluorescent radiation from a variety of elements with a 
single high-intensity source of primary rays. The experimental arrangement is 
shown in the illustration. Obviously the fluorescent beams are much lower in 
intensity than primary rays directly used, but exposure times are by no means 
prohibitive, and there is a large gain in homogeneity of the beam and hence in 
sharpness and resolution of the microradiographic image.  See  X-RAY TUBE. 
    Since microradiography is a combination of radiography and micrography, 
analyses derived from it are at three levels: (1) for structure, which is most 
common and simplest; (2) for mass distribution; and (3) for mass-chemical 
analysis. The last two are accomplished best with thin biological specimens 
and with monochromatic beams of long wavelength. 
 �34s[George L. Clark]
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�6v�2v�32sMicrosauria   �21m �33s 
    A diverse order of small, extinct amphibians, known only from the 
Pennsylvanian and lower Permian of North America and Europe. They range from 
obligatorily aquatic, perennibranchiate genera with lateral-line canal 
grooves, to fully terrestrial lizardlike forms. Several families include 
long-bodied, possibly burrowing, species. Limbs are always retained, and the 
tail is never specialized as a swimming organ. Microsaurs are recognized by 
the possession of a broad, strap-shaped occipital condyle, and no more than a 
single bone in the temporal series. The trunk vertebrae are spool-shaped. 
    The specific origin of the group remains unknown. Although intermediate 
forms are not known, microsaurs appear to be the most probable group of 
Paleozoic amphibians from which two modern amphibian orders, the apodans and 
the salamanders, have evolved.  See  AMPHIBIA; LEPOSPONDYLI. 
 �34s[Robert L. Carroll]
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�6v�2v�32sMicroscope   �21m �33s 
    An instrument used to obtain an enlarged image of a small object. The 
image may be seen, photographed, or sensed by photocells or other receivers, 
depending upon the nature of the image and the use to be made of the 
information of the image. 
    A simple microscope, hand lens, or magnifier usually is a round piece of 
transparent material, ground thinner at the edge than at the center, which can 
form an enlarged image of a small object. Commonly, simple microscopes are 
double convex or planoconvex lenses, or systems of lenses acting together to 
form the image. 
    The compound microscope utilizes two lenses or lens systems. One lens 
system forms an enlarged image of the object and the second magnifies the 
image formed by the first. The total magnification is then the product of the 
magnifications of both lens systems (see illustration). 
    The typical compound microscope consists of a stand, a stage to hold the 
specimen, a movable body-tube containing the two lens systems, and mechanical 
controls for easy movement of the body and the specimen. The lens system 
nearest the specimen is called the objective; the one nearest the eye is 
called the eyepiece or ocular. A mirror is placed under the stage to reflect 
light into the instrument when the illumination is not built into the stand. 
For objectives of higher numerical aperture than 0.4, a condenser is provided 
under the stage to increase the illumination of the specimen. Various optical 
and mechanical attachments may be added to facilitate the analysis of the 
information in the doubly enlarged image.  See  CENTRIFUGE MICROSCOPE; 
ELECTRON MICROSCOPE; FLUORESCENCE MICROSCOPE; INTERFERENCE MICROSCOPE; LENS 
(OPTICS); OPTICAL MICROSCOPE; PHASE-CONTRAST MICROSCOPE; REFLECTING 
MICROSCOPE; X-RAY MICROSCOPE. 
 �34s[Oscar W. Richards]
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�6v�2v�32sMicrosporidea   �21m �33s 
    A class of Cnidospora characterized by the production of minute spores 
with a single intrasporal or one or two intracapsular filaments and a single 
sporoplasm. The spore membrane of these protozoans is usually a single piece. 
Microsporidians are mainly intracellular parasites of arthropods and fishes. 
Microsporida is the only order of this class.  See  CNIDOSPORA. 
 �34s[Ross F. Nigrelli]
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�6v�2v�32sMicrotechnique   �21m �33s 
    The art of preparing objects for examination under the microscope and of 
preserving objects so prepared. Few objects yield useful information if 
examined without such preparation, which may involve, in addition to 
preliminary preservation, hardening, rendering transparent, selective 
coloration of parts, and cutting into thin slices. 
    The four types of microscope slide commonly made are wholemounts, smears, 
squashes, and sections. The last three methods are merely devices to make 
thinner, or smaller, objects unsuitable for the first method. In all four 
methods, the objects are permanently preserved in a mounting medium between a 
glass slide, about 0.04 in. (1 mm) thick, and a glass cover slip 0.04 to 0.008 
in. (1 to 2/10 mm) thick. Preliminary fixation and preservation, staining, and 
the final mounting media are common to all four types. 
 �34s[Peter Gray]
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�6v�2v�32sMicrowave   �21m �33s 
    An electromagnetic wave which has a wavelength in the centimeter range. 
Microwaves occupy a region in the electromagnetic spectrum which is bounded by 
radio waves on the side of longer wavelengths and by infrared waves on the 
side of shorter wavelengths (see illustration). There are no sharp boundaries 
between these regions except by arbitrary definition. In the microwave region, 
a further delineation is sometimes made with such names as decimeter, 
centimeter, or millimeter waves. 
  
    All modern microwave generators are electronic devices which produce 
continuous-wave oscillations of a single tunable frequency. Some important 
microwave generators are known as klystrons, magnetrons, and traveling-wave 
oscillators. Their power outputs range from microwatts to thousands of 
kilowatts, depending upon the type and design of the generator and the 
operating frequency.  See  KLYSTRON; MAGNETRON; MICROWAVE SOLID-STATE DEVICES; 
MICROWAVE TUBE; TRAVELING-WAVE TUBE. 
  
    Circuit elements.  Any particular grouping of physical elements which are 
arranged or connected together to produce certain desired effects on the 
behavior of microwaves is known as a microwave circuit. 
    A waveguide is a circuit element which constrains or guides the 
propagation of microwaves along a path defined by the physical construction of 
the guiding element. It can be, for instance, a coaxial cable having an outer 
conductor of annular cross section coaxially placed with respect to an inner 
conductor. By prevailing usage, however, a microwave waveguide usually means a 
hollow metallic tube which can confine and guide the propagation of microwaves. 
    A microwave attenuator is a device that causes the field intensity of a 
wave to decrease by absorbing a part of the incident power. A phase changer is 
a device that causes the field intensity to shift its phase without 
attenuating its amplitude. 
    A microwave detector is a device that can demonstrate the presence of a 
microwave by a specific effect that the wave produces. One of the most 
effective means of detection makes use of a nonlinear device which converts 
the microwave field intensity into either a direct current or a low-frequency 
alternating current. A silicon crystal making a pinpoint contact with a 
tungsten wire is perhaps the most commonly used detector for microwaves. 
    Another type of microwave detector in common use is called a bolometer. It 
is a device whose resistance changes sensitively with temperature, the latter 
being a function of the absorbed microwave power.  See  BOLOMETER. 
    A microwave wavemeter is usually made of a tunable resonant cavity which 
measures the free-space wavelength (or frequency) of a wave by the position of 
tuning at resonance.  See  CAVITY RESONATOR. 
  
    Transmission.  A microwave transmitter is similar in all its principal 
aspects to an ordinary radio transmitter. It consists of a microwave 
generator, a power amplifier (if necessary), a circuit containing all 
necessary elements, means of modulation to impart some form of information or 
program to the waves, and an antenna network to send the waves out into space. 
Since World War II, klystron amplifiers have come into practical use at high 
powers, and traveling-wave amplifiers at low powers.  See  ANTENNA 
(ELECTROMAGNETISM).  
    Methods involving amplitude modulation (AM), frequency modulation (FM), or 
phase modulation (PM) can be used to transmit information. Transmission by 
pulsed waves is an efficient method of sending an extraordinarily large peak 
power at the expense of a small average power. Even apart from the power 
considerations, pulse transmission is the most effective way of securing echo 
signals by reflection, as in radar.  See  AMPLITUDE MODULATION; FREQUENCY 
MODULATION; PHASE MODULATION; PULSE MODULATION. 
  
    Propagation in space.  In space, a wave tends to travel along a straight 
line in the absence of obstructions. However, if the transmitted beam either 
wholly or partially hits the Earth's surface or any physical obstacle, it will 
be scattered, with the particular phenomenon variously described as 
reflection, refraction, or diffraction. A microwave has too high a frequency 
to be reflected by the ionosphere at angles near vertical incidence. However, 
for microwaves slanted toward the horizon, reflection by the ionosphere 
becomes possible.  See  MICROWAVE OPTICS. 
  
    Advantages.  From the point of view of radio communication, a great 
advantage of microwaves lies in the immense spaciousness of useful 
frequencies. For instance, the frequency difference between the S-band 
(wavelength around 10 cm) and K-band (wavelength around 1 cm) is roughly 
20,000 MHz, about 100 times the combined frequency range of present-day radio 
broadcasting, communication, and television. Another advantage of microwaves 
is related to the high directivity and resolving power of microwave 
wavelengths. Narrow microwave beams can be readily formed by antennas of 
physically convenient sizes. 
  
    Applications.  The most important practical application of microwaves is 
radar. In fact, microwave techniques have been developed mainly under the 
incentive of radar work. Next to radar the use of microwaves as carrier waves 
in relay links for multichannel transmission of telephone, telegraph, and 
television has already become practically important. 
    Microwave spectroscopy has become an established science chiefly because 
of the availability of microwave instruments and techniques. Another important 
advance came with the development of the so-called atomic clocks, which use 
microwave resonance interactions with either cesium atoms or ammonia 
molecules. The introduction of the ammonia maser led to the development of the 
solid-state maser, which is virtually a noiseless amplifier. One of the most 
important applications of the maser is in the field of radio astronomy, which 
is in its own right a fertile ground for microwave application. In the field 
of nuclear physics instrumentation, microwave electronics has been applied in 
a most significant manner to high-energy linear accelerators and similar 
devices.  See  ATOMIC CLOCK; MASER; MICROWAVE SPECTROSCOPY; PARTICLE 
ACCELERATOR; RADIO ASTRONOMY. 
 �34s[C. K. Jen]
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�6v�2v�32sMicrowave optics   �21m �33s 
    The study of those properties of microwaves which are analogous to the 
properties of light waves in optics. The fact that microwaves and light waves 
are both electromagnetic waves, the major difference being that of frequency, 
already suggests that their properties should be alike in many respects. But 
the reason microwaves behave more like light waves than, for instance, very 
low-frequency waves for electrical power (50 or 60 Hz) is primarily that the 
microwave wavelengths are usually comparable to or smaller than the ordinary 
physical dimensions of objects interacting with the waves. 
    As is the case with light, a beam of microwaves propagates along a 
straight line in a perfectly homogeneous infinite medium. This phenomenon 
follows directly from a general solution of the wave equation in which the 
direction of a wave normal does not change in a homogeneous medium.  See  WAVE 
EQUATION.  
    With some modification the laws of reflection and refraction can be 
applied to the propagation of microwaves inside a dielectric-filled metallic 
waveguide. Another interesting application is associated with the microwave 
analog of total internal reflection in optics. A properly designed dielectric 
rod (without metal walls) can serve as a waveguide by totally reflecting the 
elementary plane waves. Still another case of interest is that of a microwave 
lens.  See  ANTENNA (ELECTROMAGNETISM); REFLECTION OF ELECTROMAGNETIC 
RADIATION; REFRACTION OF WAVES; WAVEGUIDE. 
    In an analogous manner to light, a microwave undergoes diffraction when it 
encounters an obstacle or an opening which is comparable to or somewhat 
smaller than its wavelength.  See  MICROWAVE. 
 �34s[C. K. Jen]
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�6v�2v�32sMicrowave reflectometer   �21m �33s 
    A form of directional coupler that is used for measuring the power flowing 
in both directions in a waveguide. A pair of single-detector couplers 
appropriately positioned on opposite sides of the waveguide can be used for 
this purpose, with one detector positioned to monitor transmitted power and 
the other detector positioned to measure power reflected back from a 
discontinuity in the line. Each coupler receives a constant small fraction of 
the energy flowing in one direction in the waveguide.  See  HIGH-FREQUENCY 
IMPEDANCE MEASUREMENTS. 
 �34s[John Markus]
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�6v�2v�32sMicrowave solid-state devices   �21m �33s 
    Semiconductor devices for the generation, amplification, detection, and 
control of electromagnetic energy in the frequency range of about 1 - 100 GHz. 
Devices that generate or amplify microwave energy are referred to as active 
microwave devices. The most important active solid-state microwave devices are 
microwave transistors, transferred electron (Gunn) devices, avalanche diodes, 
and tunnel diodes. Devices that detect or control microwave energy are 
referred to as passive microwave devices. The most important passive 
solidstate microwave devices include PIN diodes, varactor diodes, and 
point-contact and Schottky-barrier detector diodes.  See  MICROWAVE; MICROWAVE 
TUBE; SEMICONDUCTOR DIODE; TRANSISTOR. 
  
    Active devices.  At the lower microwave frequencies, silicon bipolar 
transistors and gallium arsenide (GaAs) field-effect transistors (FETs) are 
the most widely used active solid-state devices. At the higher microwave 
frequencies, the usefulness of transistors is severely limited by 
high-frequency effects, and two-terminal negative resistance devices such as 
transferred-electron devices, avalanche diodes, and tunnel diodes predominate. 
  See  NEGATIVE-RESISTANCE DEVICES. 
    Most modern silicon bipolar microwave transistors are epitaxial diffused 
npn structures (Fig. 1). The basic principles of operation of bipolar 
microwave transistors are the same as those of low-frequency bipolar 
transistors. However, because of the higher frequencies involved, the 
requirements on dimensions, process control, packaging, heat sinking, and 
radio-frequency (rf) circuitry are much more severe for microwave transistors 
than for low-frequency transistors. 
    Microwave GaAs FETs are a type of microwave transistor that provides 
improved performance over silicon microwave transistors. Low-noise, low-power, 
and medium-power GaAs FETs are commercially available. 
    In transferred-electron devices (TEDs), negative resistance is achieved by 
taking advantage of the negative differential mobility of electrons in certain 
n-type III - V compounds, particularly GaAs. Structurally, TEDs are 
particularly simple semiconductor devices. They consist merely of a bar of 
transferred electron material with ohmic cathode and anode contacts. 
    Figure 2 shows graphs of electron drift velocity versus electric field for 
silicon and GaAs. The drift velocity for silicon exhibits a ``normal'' 
behavior, increasing monotonically with increasing electric field. In GaAs, on 
the other hand, the drift velocity decreases with increasing field above about 
3 kV/cm. The negative differential mobility of GaAs is caused by the transfer 
of electrons from high- to low-mobility energy bands, hence the name 
``transferred electron devices.'' See SEMICONDUCTOR. 
    Avalanche diodes are junction devices that produce a negative resistance 
by appropriately combining impact avalanche breakdown and charge-carrier 
transit time effects. Avalanche breakdown in semiconductors occurs if the 
electric field is high enough for the charge carriers to acquire sufficient 
energy from the field to create electron-hole pairs by impact ionization. 
Transit time effects occur if the time the charge carriers spend traversing 
the diode becomes an appreciable fraction of an rf period. The two most 
important classes of avalanche diodes are IMPATT diodes (impact avalanche and 
transit time diodes) and TRAPATT diodes (trapped-plasma avalanche-triggered 
transit diodes). 
    Tunnel diodes were first described by L. Esaki in 1958. They are heavily 
doped pn junctions that exhibit a negative differential resistance over part 
of their current-voltage characteristics. The negative resistance is caused by 
processes (quantum-mechanical tunneling) that are so fast that there are no 
transit time effects even at the highest microwave frequencies. See TUNNEL 
DIODE.  
  
  
    Passive devices.  The oldest passive microwave device is the point-contact 
diode, a diode that was widely used in World War II radars. More modern 
passive microwave solid-state devices include Schottky-barrier diodes, PIN 
diodes, and varactor diodes. 
    Point-contact and Schottky-barrier diodes are majority-carrier rectifiers 
that are used as the nonlinear element in passive microwave-frequency 
down-converters and in rectifiers. The rectification characteristics of these 
diodes are similar to those of pn junction diodes, but unlike pn junction 
diodes they do not exhibit any minority-carrier charge storage capacitance. 
This lack of charge-storage capacitance makes point-contact and 
Schottky-barrier diodes particularly attractive for down-converter 
applications, since variations in capacitance are in general undesirable in 
down-conversion. In point-contact diodes, a pointed tungsten wire is forced 
down on a semiconductor wafer (usually silicon). In Schottky-barrier diodes, 
the rectifying junction is formed at the interface of a deposited metal layer 
and a semiconductor crystal, usually silicon or gallium arsenide.  See  
POINT-CONTACT DIODE. 
    PIN diodes are junction diodes in which the p and n regions are separated 
by a relatively thick high-resistivity intrinsic layer. These diodes are 
widely used in microwave switches, electronically variable microwave 
attenuators, and microwave limiters.  See  JUNCTION DIODE. 
    A varactor (variable-capacitance) diode is a pn junction diode that is 
specifically designed for applications that make use of the voltage dependence 
of the diode depletion-layer capacitance.  See  VARACTOR. 
 �34s[Fred Sterzer]
�33s 
  
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�6v�2v�32sMicrowave spectroscopy   �21m �33s 
    The study of the interaction of matter and electromagnetic radiation in 
the microwave region of the spectrum.  See  MICROWAVE; SPECTROSCOPY. 
    The interaction of microwaves with matter can be detected by observing the 
attenuation or phase shift of a microwave field as it passes through matter. 
These are determined by the imaginary or real parts of the microwave 
susceptibility (the index of refraction). The absorption of microwaves may 
also trigger a much more easily observed event like the emission of an optical 
photon in an optical double-resonance experiment or the deflection of a 
radioactive atom in an atomic beam.  See  MOLECULAR BEAMS. 
    At room temperature, the relative population difference between the states 
involved in a microwave transition is a few percent or less. The population 
difference can be close to 100% at liquid helium temperatures, and microwave 
spectroscopic experiments are often performed at low temperatures to enhance 
population differences and to eliminate certain line-broadening mechanisms. 
The population differences between the states involved in a microwave 
transition can also be enhanced by artificial means. When the molecules or 
atoms with inverted populations are placed in an appropriate microwave cavity, 
the cavity will oscillate spontaneously as a maser (microwave amplification by 
stimulated emission of radiation).  See  MASER. 
    The magnetic dipole and electric quadrupole interactions between the 
nuclei and electrons in atoms and molecules can lead to energy splittings in 
the microwave region of the spectrum. Thus, microwave spectroscopy has been 
used extensively for precision determinations of spins and moments of nuclei.  
See  HYPERFINE STRUCTURE; MOLECULAR BEAMS; NUCLEAR MOMENTS. 
1645a     The rotational frequencies of molecules often fall within the 
microwave range, and microwave spectroscopy has contributed a great deal of 
information about the moments of inertia, the spin-rotation coupling 
mechanisms, and other physical properties of rotating molecules.  See  
MOLECULAR STRUCTURE AND SPECTRA. 
    The magnetic resonance frequencies of electrons in fields of a few 
thousand gauss (a few tenths of a tesla) lie in the microwave region. Thus, 
microwave spectroscopy is used in the study of electron-spin resonance or 
paramagnetic resonance.  See  ELECTRON PARAMAGNETIC RESONANCE (EPR) 
SPECTROSCOPY; MAGNETIC RESONANCE. 
    The cyclotron resonance frequencies of electrons in solids at magnetic 
fields of a few thousand gauss (a few tenths of a tesla) lie within the 
microwave region of the spectrum. Microwave spectroscopy has been used to map 
out the dependence of the effective mass on the electron momentum.  See  
CYCLOTRON RESONANCE EXPERIMENTS. 
    For other applications  See  ATOMIC CLOCK; COSMIC MICROWAVE RADIATION; 
RADIO ASTRONOMY. 
 �34s[William Happer]
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�6v�2v�32sMicrowave transmission lines   �21m �33s 
    Structures used for transmission of electromagnetic energy at microwave 
frequencies from one point to another. A transmission line may be defined more 
precisely as a system of material boundaries forming a continuous path from 
one place to another and capable of directing the transmission of 
electromagnetic energy along this path. See TRANSMISSION LINES. 
    The structures in most widespread use as microwave transmission lines are 
coaxial lines and hollow-pipe waveguides. Other structures, including 
striplines, open-wire lines, and dielectric rods, are used in special 
applications.  
    Coaxial lines are widely used at lower frequencies, and are satisfactory 
for many applications at microwave frequencies. In a coaxial line (Fig. 1) the 
electromagnetic waves are transmitted through the dielectric medium bounded by 
two conducting, coaxial cylinders. Because of skin effect, the currents in the 
conductors are concentrated in the surfaces of the conductors bounding the 
dielectric medium. 
    The hollow-pipe waveguide and other types of transmission lines are 
increasingly preferred at the higher frequencies. Electromagnetic waves are 
transmitted through the interior of hollow metal pipes, and electric currents 
flow on the inner surfaces. In contrast to coaxial lines, hollow-pipe 
waveguides are characterized by cutoff frequencies; that is, for a waveguide 
of given dimensions, the operating frequency must be higher than a critical 
cutoff frequency for energy to propagate. Waveguides therefore become 
impractically large at lower frequencies, and the dimensions are reasonably 
small only in the microwave range. 
    Dielectric rods and tubes can also be used as microwave transmission 
lines, as can single-conductor lines, which are sometimes coated with 
dielectric. These structures can have very low attenuation. Striplines (Fig. 
2), which consist of a metal strip supported above a metal plane or between 
two metal planes, have somewhat higher attenuation than hollow-pipe 
waveguides. They are, however, frequently used where structures of unusual 
compactness and physical simplicity are desired. 
  
  
    An important characteristic of a transmission line is its characteristic 
impedance, defined as the input impedance to a uniform line of infinite 
length. If the terminal or load impedance of a transmission line is equal to 
the characteristic impedance of the line, all of the energy in the wave 
traveling toward the line is absorbed in the load impedance, and the line is 
said to be matched. If the load impedance is not equal to the characteristic 
impedance, part of the traveling wave is reflected. 
    For most applications a matched transmission line is an optimum condition, 
as it minimizes the undesired losses of energy in the line itself and 
maximizes the energy delivered to the load impedance. Frequently, however, the 
load impedance does not match the characteristic impedance of the transmission 
line, and various matching networks and devices must then be employed.  See  
IMPEDANCE MATCHING; MICROWAVE. 
 �34s[Theodore Moreno]
�33s 
  
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�6v�2v�32sMicrowave tube   �21m �33s 
    A high-vacuum tube designed for operation in the frequency region from 
approximately 3000 to 300,000 MHz. Two considerations distinguish a microwave 
tube from vacuum tubes used at lower frequencies: the dimensions of the tube 
structure in relation to the wavelength of the signal that it generates or 
amplifies, and the time during which the electrons interact with the microwave 
field.  See  VACUUM TUBE. 
    In the microwave region wavelengths are in the order of centimeters; 
resonant circuits are in the forms of transmission lines that extend a quarter 
of a wavelength from the active region of the microwave tube. With such short 
circuit dimensions the internal tube structure constitutes an appreciable 
portion of the circuit. For these reasons a microwave tube is made to form 
part of the resonant circuit. Leads from electrodes to external connections 
are short, and electrodes are parts of surfaces extending through the envelope 
directly to the external circuit that is often a coaxial transmission line or 
cavity.  See  CAVITY RESONATOR; TRANSMISSION LINES. 
    At microwaves the period of signal is in the range of 0.001 - 1 
nanosecond. Only if transit time is less than a quarter of the signal period 
do significant numbers of electrons exchange appreciable energy with the 
signal field. Transit time is reduced in several ways. Electrodes are closely 
spaced and made planar in configuration, and high interelectrode voltages are 
used.  
    Tubes designed by the foregoing principles are effective for wavelengths 
from a few meters to a few centimeters. At shorter wavelengths different 
principles are necessary. To obtain greater exchange of energy between the 
electron beam and the electromagnetic field several alternative designs have 
proved practical. 
1646     Instead of collecting the electron beam at a plate formed by the 
opposite side of the resonant circuit, the beam is allowed to pass into a 
field-free region before reacting further with an external circuit. The 
electron cloud can be deflected by a strong static magnetic field so as to 
revolve and thereby react several times with the signal field before reaching 
the plate.  See  KLYSTRON; MAGNETRON. 
    Instead of producing the field in one or several resonant circuits, the 
field can be supported by a distributed structure along which it moves at a 
velocity comparable to the velocity of electrons in the beam. The electron 
beam is then directed close to this structure so that beam and field interact 
over an extended interval of time.  See  BACKWARD-WAVE TUBE; MICROWAVE; 
TRAVELING-WAVE TUBE. 
 �34s[Frank H. Rockett]
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�6v�2v�32sMidnight sun   �21m �33s 
    A phenomenon observed in the polar zones of the Earth near the time of the 
summer solestice, when the Sun remains visible above the horizon at midnight 
and reaches its minimum altitude without setting. The midnight sun is a 
consequence of the inclination of the rotation axis of the Earth, by which the 
Earth presents in turn each pole to the Sun for 6 months. 
 �34s[Gerard De Vaucouleurs]
�33s 
  
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�6v�2v�32sMigmatite   �21m �33s 
    Rocks originally defined as of hybrid character due to intimate mixing of 
older rocks (schist and gneiss) with granitic magma. Now most plutonic rocks 
of mixed appearance, regardless of how the granitic phase formed, are called 
migmatites. Commonly they appear as veined gneisses. 
    Several modes of origin have been proposed. (1) Granitic magma may be 
intercalated between thin layers of schist (lit-par-lit injection) to form a 
banded rock called injection gneiss. (2) The granitic magma may form in place 
by selective melting of the rock components. (3) The granitic layers may 
develop by metamorphic differentiation (redistribution of minerals in solid 
rock by recrystallization). (4) The granitic layers may represent selectively 
replaced or metasomatized portions of the rock.  See  GRANITIZATION; 
METAMORPHISM; METASOMATISM. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sMigratory behavior   �21m �33s 
    Regularly occurring, oriented seasonal movements of individuals of many 
animal species. The term migration is used to refer to a diversity of animal 
movements, ranging from short-distance dispersal and one-way migration to 
round-trip migrations occurring on time scales from hours (the vertical 
movements of aquatic plankton) to years (the return of salmon to their natal 
streams following several years and thousands of kilometers of travel in the 
open sea). 
    Many temperate zone species, including many migrants, are known to respond 
physiologically to changes in the day length with season (photoperiodism). For 
example, many north temperate organisms are triggered to come into breeding 
condition by the interaction between the lengthening days in spring and their 
biological clocks (circadian rhythms). Similar processes, acting through the 
endocrine system, bring animals into migratory condition. 
    To perform regular oriented migrations, animals need some mechanism for 
determining and maintaining compass bearings. Animals use many environmental 
cues as sources of directional information. Work with birds has shown that 
species use several compasses. 
    Many species of vertebrates and invertebrates possess a time-compensated 
Sun compass. With such a system, the animal can determine absolute compass 
directions at any time of day; that is, its internal biological clock 
automatically compensates for the changing position of the Sun as the Earth 
rotates during the day. Many arthropods, fish, salamanders, and pigeons can 
perceive the plane of polarization of sunlight, and may use that information 
to help localize the Sun even on partly cloudy days. 
    Only birds that migrate at night have been shown to have a star compass. 
Unlike the Sun compass, it appears not to be linked to the internal clock. 
Rather, directions are determined by reference to star patterns which seem to 
be learned early in life. 
    Evidence indicates that several insects, fish, a salamander, certain 
bacteria, and birds may derive directional information from the weak magnetic 
field of the Earth.  See  MAGNETIC RECEPTION (BIOLOGY). 
    Wind provides directional information to birds, but because it changes 
with time, it cannot yield compass information directly. Nocturnal songbird 
migrants nearly always fly with a following wind; in part this is because they 
select a night with favorable winds to initiate migration. 
    Three beach-dwelling amphipod crustaceans seem to possess a 
time-compensated lunar compass in addition to a Sun compass. 
    Many kinds of animals show the ability to return to specific sites 
following a displacement. The phenomenon can usually be explained by 
familiarity with landmarks near ``home'' or sensory contact with the goal. For 
example, salmon are well known for their ability to return to their natal 
streams after spending several years at sea. Little is known about their 
orientation at sea, but they recognize the home stream by chemical cues 
(olfactory) in the water. The young salmon apparently imprint on the odor of 
the stream in which they were hatched. Current evidence indicates that birds 
imprint on or learn some feature of their birthplace, a prerequisite for them 
to be able to return to that area following migration. On its first migration, 
a young bird appears to fly in a given direction for a programmed distance. 
Upon settling in a wintering area, it will also imprint on that locale and 
will thereafter show a strong tendency to return to specific sites at both 
ends of the migratory route. 
    Only in birds can an unequivocal case be made for the existence of true 
navigation, that is, the ability to return to a goal from an unfamiliar 
locality in the absence of direct sensory contact with the goal. This process 
requires both a compass and the analog of a map. Present evidence suggests 
that the map is not based on information from the Sun, stars, landmarks, or 
magnetic field. Other possibilities such as olfactory, acoustic, or 
gravitational cues are being investigated, but the nature of the navigational 
component of bird homing remains the most intriguing mystery in this field. 
 �34s[Kenneth P. Able]
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�6v�2v�32sMilitary aircraft   �21m �33s 
    Aircraft that are designed or modified for highly specialized military 
applications. Balloons, gliders, airplanes, and helicopters have been used for 
military purposes. 
    Basic types of military aircraft include bombers, fighters, transports, 
patrol, trainers, and reconnaissance and observation aircraft (Figs. 1 and 2). 
Bombers are usually characterized by relatively long range, low 
maneuverability, and large weapon-carrying capability. Bombers may be equipped 
to deliver conventional or nuclear weapons in day, night, or adverse weather. 
    Fighters are relatively short-range, highly maneuverable, fast airplanes 
designed to destroy enemy aircraft and to attack ground targets. They can 
carry machine guns, cannons, rockets, guided missiles, and bombs, depending 
upon the missions. Some fighters called fighter bombers, can carry 
conventional or nuclear weapons several hundreds of kilometers behind enemy 
lines to strike priority ground targets. 
    Transport aircraft carry troops and war supplies. Many are adaptations of 
airplanes used by commercial airlines. Cargo-carrying aircraft have been 
developed especially for the military services. These include the world's 
largest aircraft, capable of moving large quantities of people and material 
rapidly to distant points. A special-purpose type of transport airplane is the 
aerial tanker. By means of special refueling fittings together with other 
equipment, fighters, bombers, and helicopters can take on fuel to fill their 
tanks while in flight. 
    Reconnaissance aircraft are used to observe the enemy visually or by radar 
or photography. These aircraft may be specially designed or may be modified 
from a basic fighter or bomber type. 
    Helicopters deserve special mention in military use. They are unexcelled 
for rescue work and for delivery of people and material to otherwise 
inaccessible areas.  See  HELICOPTER; VERTICAL TAKEOFF AND LANDING (VTOL). 
    Cruise missiles are small, long-range, uncrewed aircraft (Fig. 3). 
Although termed missiles, they have more of the features of aircraft than of 
missiles. Cruise missiles have wings and a turbine engine, giving them ranges 
of a few hundred miles to over 1200 mi (2000 km). They are autonomous, using 
inertial navigation systems for en route navigation with periodic updates from 
space or terrain match correlation schemes. Cruise missiles permit the crewed 
aircraft to stand off from the target and strike it with the small, low-flying 
cruise missiles.  See  MISSILE. 
 �34s[Leland M. Nicolai]
�33s 
  
  
  
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�6v�2v�32sMilitary satellites   �21m �33s 
    Artificial Earth satellites used for a variety of military purposes. Many, 
if not most, crewed and uncrewed satellites successfully orbited by the 
various countries of the world have been either specifically military in 
nature or usable for military purposes. 
    The Soviet Union has launched almost twice as many satellites as the 
United States. Virtually all of the uncrewed satellites that the Soviet Union 
has sent into Earth orbit have been launched by the military. A simple 
comparison of numbers is not sufficient to draw conclusions about the two 
nations' space programs because of differences in the effectiveness of 
particular kinds of flight hardware. 
    The major functions of military satellite missions are: communications, 
navigation, meteorology, surveillance, geodesy, nuclear test and radiation 
detection, and research and technology. Future space shuttle programs will 
include all or some of the above.  See  APPLICATIONS SATELLITES; 
COMMUNICATIONS SATELLITE; METEOROLOGICAL SATELLITES; SATELLITE NAVIGATION 
SYSTEMS; SPACE SHUTTLE. 
 �34s[Henry W. Brandli]
�33s 
  
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�6v�2v�32sMilk   �21m �33s 
    The lacteal secretion, practically free from colostrum, obtained by the 
complete milking of one or more healthy cows and containing not less than 
8.25% milk solids (not fat) and not less than 3.25% milk fat. Among mammals, 
humans utilize milk as a source of food. The dairy cow supplies the vast 
majority of milk for human consumption, particularly in the United States; 
however, milk from goats, water buffalo, and reindeer is also consumed in 
other countries. Without qualification, the general term milk refers to cow's 
milk.  
    Average composition of milk is 87.2% water, 3.7% fat, 3.5% protein, 4.9% 
lactose, and 0.7% ash. Whole milk and skim milk are classified as excellent 
sources of calcium, phosphorus, and riboflavin because 10% of the daily 
nutritional requirement is supplied by not over 100 kcal (420 kilojoules). 
These two beverages are also classified as good sources of protein and 
thiamine; and whole milk is a good source of vitamin A. To be classified as 
good, the source must contribute 10% of a nutrient in not over 200 kcal (840 
kilojoules). Milk is a good source of protein rich in all the essential amino 
acids.  
  
    Processing.  Most raw milk collected at farms is pumped from calibrated 
and refrigerated stainless steel tanks into tank trucks for delivery to 
processing plants. The actual processing of raw milk begins with either 
separation or clarification. These machines are essentially similar except 
that in the clarifier the cream and skim milk fractions are not separated. 
Many processers have units called standardizer-clarifiers which separate only 
a small fraction of the fat from the raw whole milk; the amount of fat removed 
can be regulated. This facilitates the production of milk of standard fat 
content even though that in the raw product may vary. 
    Milk is rendered free of pathogenic bacteria by pasteurization. This is 
accomplished in a manner so that every particle of milk is heated to a 
specified temperature and held at that temperature for a specified time.  See  
PASTEURIZATION.  
    Fat globules in fluid milk products are broken by homogenization into 
sizes that are 2 micrometers or less and thus are relatively unaffected by 
gravitational forces. The U.S. Public Health Service specifies that the fat 
content of the upper 6 in. (100 ml) of a quart of homogenized milk that has 
been undisturbed for 48 h cannot differ by more than 10% from that of the 
remainder.  
    Most milk is fortified with 400 international units (IU) of vitamin D per 
quart, and some skim milk is fortified also with 2000 IU of vitamin A per 
quart. These vitamins as concentrates are added either by automatic dispensing 
into a continuous flow of milk prior to pasteurization or as a single quantity 
in a batch operation. 
  
    Products.  Many fermented or cultured products are produced from milk. 
These fermentations require the use of bacteria that ferment lactose or milk 
sugar.  
    Cultured buttermilk consists of skim milk or low-fat milk which is 
pasteurized at 180DGF (82DGC) for 30 min, cooled to 72DGF (22DGC), and 
inoculated with an active starter culture containing S. lactis and Leuconostoc 
citrovorum. The mixture is incubated at 70DGF (21DGC) and cooled when acidity 
is developed to approximately 0.8%. This viscous product is then agitated, 
packaged, and cooled. The desired flavor is created by volatile acids and 
diacetyl, the latter is produced by L. citrovorum. 
    One of the oldest fermented milks known is yogurt. Yogurt is prepared 
using whole or low-fat milk with added nonfat milk solids. The milk is heated 
to approximately 180DGF (82DGC) for 30 min, homogenized, cooled to 115DGF 
(46DGC), inoculated with an active culture, and packaged. Yogurt cultures are 
mixtures of S. thermophilus and Lactobacillus bulgaricus in a 1:1 ratio. 
Balance of these organisms in the culture is important for production of a 
quality product. 
  
    Concentrated and dried milk products.  To reduce costs of transportation 
and handling, either part or all of the water is removed from milk. Moreover, 
the partly dehydrated milk can either be sterilized or dried to permit 
unrefrigerated storage for prolonged periods. Many different milk products 
(such as dry whole milk, evaporated milk, and condensed milk) are produced for 
these specific reasons. The composition of some of these is controlled by 
standards of identity and some by request of the commercial buyer. 
 �34s[Robert L. Bradley, Jr.]
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�6v�2v�32sMilleporina   �21m �33s 
    An order of the class Hydrozoa of the phylum Coelenterata. These are the 
``string corals'' of shallow tropical seas. Their structure is similar to that 
of hydroids except for the addition of a calcareous exoskeleton (illustration 
a). Because of this skeleton, they resemble true corals (Anthozoa). The 
skeleton is covered by a thin layer of tissue, is penetrated by 
interconnecting tubes, and is perforated by tiny holes through which the 
bodies of the ``coral animals,'' or polyps, are extended (illustration b). 
    The polyps are of two types (illustration c): nutritive gastrozooids with 
tentacles and mouth; and protective polyps, which are long and armed with 
stinging cells but have no mouth. Millepores produce medusae, or jellyfish, in 
which sex cells develop.  See  HYDROZOA. 
 �34s[Sears Crowell]
�33s 
  
  
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�6v�2v�32sMillerite   �21m �33s 
    A mineral having composition NiS and crystallizing in the hexagonal 
system. Millerite usually occurs in hairlike tufts and radiating groups of 
slender to capillary crystals. The hardness is 3 - 3.5 (Mohs scale) and the 
specific gravity is 5.5. The luster is metallic and the color pale brass 
yellow. Millerite is found in many localities in Europe, notably in Germany 
and Czechoslovakia. In the United States it is found with pyrrhotite at the 
Gap Mine, Lancaster County, Pennsylvania; with hematite at Antwerp, New York; 
and in geodes in limestone at Keokuk, Iowa. In Canada large cleavable masses 
are mined as a nickel ore in Lamotte Township, Quebec. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sMillet   �21m �33s 
    A common name applied to at least five related members of the grass family 
grown for their edible seeds: foxtail millet (Setaria italica), proso millet 
(Panicum miliaceum), pearl or cat-tail millet (Pennisetum typhoideum), 
Japanese barnyard millet (Echinochloa frumentacea), raggee or finger millet 
(Eleusine coracana), and koda millet (Paspalum scrobiculatum). 
    As a crop for human food, pearl millet is grown widely in the tropics and 
subtropics in regions of limited rainfall where there is a growing season of 
90 to 120 days. The naked seeds are yellowish to whitish in color and about 
the size of wheat grain. The dried grain is usually pulverized to make a meal 
or flour and then cooked in soups, in porridge, or as cakes. 
 �34s[Howard B. Sprague]
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�6v�2v�32sMilling machine   �21m �33s 
    A machine for the removal of metal by feeding a workpiece through the 
periphery of a rotating circular cutter. The multitoothed cutter of a milling 
machine produces a milled surface as each revolving tooth removes a portion of 
metal from the passing workpiece. 
    Milling machines may generally be classed as horizontal or vertical, 
depending on the power spindle's axis of rotation. A movable, horizontal table 
holds the workpiece and carries it through the path of the cutter teeth (see 
illustration). On the universal mill, the table may also be rotated 
horizontally; on machines with a universal head, the cutter may be tilted to 
the desired angle.  See  MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sMineral �21m �33s
    A naturally occurring substance with a characteristic chemical composition
expressed by a chemical formula. Although organic substances such as coal and
oil are usually listed under mineral resources, they are not minerals, being
complex mixtures without definite chemical formulas. Minerals constitute an
extremely important natural resource. Most metals and inorganic chemicals, and
many other products essential to civilization, are derived from minerals. Both
forests and farms are dependent upon soils, which are composed chiefly of
minerals.


    Occurrence. Minerals may occur as individual crystals or they may be
disseminated in some other mineral or rock. Most minerals at the face of the
earth occur in rocks. Minerals may be attached to the surface of some opening.
Geodes are cavities in a rock which are lined with a mineral, or in some cases,
completely filled. The term vug applies to irregular openings containing ore
minerals. Veins are fissures of varying size which have been filled with one
or more minerals. The term lode is used for a vein or group of veins in a
definite area. Gangue minerals are the worthless minerals associated with a
valuable mineral or ore. The term ore is usually restricted to minerals from
which a metal is obtained. A placer is a concentration of relatively heavy
and durable minerals which have been transported and redeposited in a stream
bed where the water velocity is lowered. See GEODE.


    Names.  Minerals usually have both a chemical name and a mineral name.
Thus lead sulfide occurring in nature is called galena, and sodium chloride is
called halite. Most modern mineral names end in ``ite.'' Some have a chemical
connotation, as molybdenite, MoS2, and zincite, ZnO. Physical properties are
the basis for the names magnetite, graphite (to write), rhodonite (rose
color), and cryolite (ice stone). Some minerals have variety names, as
amethyst, agate, and jasper for quartz. There are also colloquial names, such
as fool's gold for pyrite.

    Classification.  Minerals are classified first with respect to chemical
composition, and then so far as possible by isomorphism, or similarity of
crystalline form. In general the cation is of less significance than the
anion. The major groups in mineral classification are:

	    Native elements		    Sulfate type
	    Sulfide and sulfo minerals	      Chromates
	    Oxides and hydrated oxides	      Molybdates
	    Halogen minerals		      Tungstates
	    Nitrates			    Phosphate type
	    Carbonates			      Arsenates
	    Borates			      Vanadates
					      Silicates

See BORATE MINERALS; CARBONATE MINERALS; HALOGEN MINERALS; NATIVE ELEMENTS;
NITRATE MINERALS; SILICATE MINERALS.

    Groups.  Any complete classification of minerals must be based on chemical
composition. However, limited groups of minerals may be of interest for
various special purposes. Such groups may be based on origin, type of
occurrence, certain physical properties, or use.
    Primary minerals are those formed from the magma, including the pegmatite
and hydrothermal phases. Others are secondary. Rock-forming minerals are those
which make up the great bulk of the igneous, sedimentary, and metamorphic
rocks. Thus quartz, feldspar, and mica are found in granite and are
rock-forming minerals. They are also called essential minerals, in contrast to
the occasional tiny crystals of pyrite, zircon, or apatite which might occur
in granite, and which are called accessory minerals.
    Classes of minerals are those which make up a certain chemical group, as
carbonates, sulfates, or oxides.
    Isomorphous groups are those whose members are strictly isomorphous, as
the garnet group.  See	ISOMORPHISM (CRYSTALLOGRAPHY).
    Families of minerals include certain closely related minerals which have
close chemical and physical similarities, but are not necessarily isomorphous,
as the feldspars or pyroxenes.
    Economic minerals are those which are of economic importance, and include
both metallic (ore minerals) and nonmetallic minerals, as cryolite and sulfur,
and gem minerals.
    Clay minerals have certain common physical and chemical properties. They
are fine-grained, plastic when wet, and become hard when dried or fired. They
are chiefly hydrous silicates of aluminum.  See  CLAY MINERALS.
    Stable minerals are those resistant to both chemical and mechanical
weathering, being both insoluble and hard. Heavy minerals are those which
collect in placers and beach sands because of their higher specific gravities,
or those which can be separated in the laboratory by gravity methods. Detrital
minerals are rock fragments which are essentially unaltered. Authigenic
minerals are those generated in the place of formation, and allogenic minerals
are those which have been transported.	See  AUTHIGENIC MINERALS.
    Mineral associations refer to minerals formed by the same process and
hence closely associated. Mineral sequence refers to a series of associated
minerals formed at successive stages.
    A mineral suite is a general term which may apply to a group of associated
minerals in one deposit; a representative group from a certain locality; or a
group of specimens showing variations, as in color or form, in a single
mineral species.  See  MINERALOGY.
 �34s[Lewis S. Ramsdell]
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�6v�2v�32sMineralogy   �21m �33s 
    The science which concerns the study of natural inorganic substances, 
whether of terrestrial or extraterrestrial origin, called minerals. Mineralogy 
is a science that cannot be easily defined. It is most properly a branch of 
inorganic chemistry, but the discipline concentrates on the origin, 
description, and classification of minerals.  See  MINERAL. 
    Thus, four main categories may be considered: crystal chemistry 
(composition and atomic arrangement of minerals); paragenetic mineralogy (the 
study of mineral association and occurrence both in natural and synthetic 
systems); descriptive mineralogy (the study of the physical properties of 
minerals and the means for their identification); and taxonomic mineralogy 
(mineral classification, systematization, and nomenclature). 
  
    Crystal chemistry.  This is the most vital aspect of mineralogy because it 
is the basis for the other studies. The fields of mineralogy, crystallography, 
inorganic chemistry, geochemistry, petrology, and geology are connected in the 
domain of crystal chemistry (Fig. 1). 
    A particular mineral, called a mineral species, is defined on the basis of 
a specified chemical composition and a specified crystal structure (atomic 
arrangement). These two criteria provide almost sufficient knowledge for 
characterization of a mineral since in principle all other properties can be 
derived from them. 
    Crystalline substances, that is, periodic arrangements of matter in three 
dimensions, can be divided into six crystal systems: triclinic, monoclinic, 
orthorhombic, tetragonal, hexagonal (including the trigonal and rhombohedral 
subdivisions), and cubic. These crystal systems can be considered as structure 
cells, each of which contains a certain integral number of atoms of a 
substance. Figure 2 depicts these systems and offers the criteria for 
distinguishing them.  See  CRYSTALLOGRAPHY. 
  
  
    Any crystalline substance has a certain integral number of its essential 
chemical formula units (loosely called molecules) in its structure cell. Thus, 
each unique atomic position in the structural unit can be occupied by a 
particular kind (or kinds) of atom(s). Consider the ideal cell formula (Aa   
)(Bb   ) . . . (Pp   ). Each of the parentheses specifies a unique atomic 
position. The capital letters specify the element present and the small 
subscripts the number of times it occurs in the cell. If the small subscripts 
have a factor in common, it is factored out and what remains is the formula 
unit. The ideal formula is further defined in terms of the atomic element 
which occurs in excess of 50 mole % within each of the parentheses. The 
structure type, along with the ideal formula unit, defines a mineral species. 
    This strict definition of a species is required since a particular mineral 
may have a range of compositions. The range of compositions is called a 
series. The ideal limiting compositions are called end members and each of the 
end members has a specific name. 
  
    Paragenetic mineralogy.  Paragenetic mineralogy is the study of mineral 
paragenesis, or the association and order of crystallization of minerals. The 
problem may concern mineral association within a single hand specimen or may 
embrace a much larger region, such as an entire ore body, in which case many 
representative specimens are judiciously collected. This study usually 
accompanies the analysis of the general geological structures within and 
around the ore body, such as the bedding, folding, and faulting. Included 
among the important aspects of paragenetic mineralogy are ore mineralogy, the 
mineralogy of a sequence of phases crystallized from a parent magma, the 
sequence of minerals crystallized in a vein, and so forth.  See  GEOLOGY; 
PETROLOGY.  
    Mineral paragenesis is usually considered in relative time and the 
absolute difference in time between the oldest and youngest minerals is often 
not known. Absolute age differences can be obtained in some instances, for 
example, by lead isotope age dating of a sequence of crystallized lead-bearing 
minerals.  See  ROCK AGE DETERMINATION. 
  
    Descriptive mineralogy.  Mineral recognition directly by the senses is 
very subjective and requires considerable experience. Gross features of a 
mineral such as color, form, hardness, and specific gravity are important 
criteria for identification in the field where a well-equipped laboratory is 
usually not available. More objective criteria such as the optical properties 
and x-ray powder diffraction spectra of a mineral require specialized 
equipment, but the results are usually certain since these data are known for 
most mineral species and are extensively tabulated. Older methods such as 
fusibility, flame tests, and blowpipe analysis have been largely abandoned. 
  
    Taxonomic mineralogy.  There are approximately 3000 distinct mineral 
species known to science. About 60 new mineral species are discovered each 
year. For a new species to be properly defined, the chemical analysis, 
structure cell and space group, crystal morphology, powder pattern, optical 
data, all physical properties, and paragenesis must be given as completely as 
possible.  
 �34s[Paul B. Moore]
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�6v�2v�32sMinimal principles   �21m �33s 
    In the treatment of physical phenomena, it can sometimes be shown that, of 
all the processes or conditions which might occur, the ones actually occurring 
are those for which some characteristic physical quantity assumes a minimum 
value. These processes or conditions are known as minimal principles. The 
application of minimal principles provides a powerful method of attacking 
certain problems that would otherwise prove formidable if approached directly 
from first principles. 
    One simple minimal principle asserts that the state of stable equilibrium 
of any mechanical system is the state for which the potential energy is a 
minimum. Other general theorems of classical dynamics that are related to 
minimal principles are Hamilton's principle and the principle of least action. 
  See  HAMILTON'S PRINCIPLE; LEAST-ACTION PRINCIPLE. 
 �34s[Dudley Williams]
�33s 
  
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�6v�2v�32sMinimal surfaces   �21m �33s 
    A branch of mathematics belonging to the calculus of variations, 
differential geometry, and geometric measure theory. A surface, interface, or 
membrane is called minimal when it has assumed a geometric configuration of 
least area among those configurations into which it can readily deform. Soap 
films spanning wire frames or compound soap bubbles enclosing volumes of 
trapped air are common examples.  See  CALCULUS OF VARIATIONS; DIFFERENTIAL 
GEOMETRY; MEASURE THEORY. 
    Geometrically, the mean curvature of a surface S at a point is the 
difference between the maximum upward curvature there and the maximum downward 
curvature; in particular, a surface of zero mean curvature has such principal 
curvatures equal and opposite and hence typically appears ``saddle-shaped.'' 
It turns out that S is a minimal surface; that is, it cannot be perturbed to 
less area leaving its boundary fixed, provided the mean curvature is zero at 
each of its points; such a surface could occur, for example, as a soap film 
spanning a wire frame. The corresponding minimal surface equation is partial 
differential equation. If, alternatively, S were part of a soap bubble 
enclosing trapped air, the mean curvature of S would be proportional to the 
difference in air pressure between the two sides of S. In the calculus of 
variations, typically area-minimizing properties of minimal surfaces are 
emphasized. In differential geometry, minimal surfaces are defined as surfaces 
of zero mean curvature; surfaces of constant mean curvature are also 
extensively studied. 
    The two-dimensional surface is dominant in determining shape whenever the 
energy of a system is changed significantly by a displacement or a change in 
area of the surface. Such surfaces include the interfaces between crystals in 
a typical rock or metal, the film of soapy water between the air cells in a 
soap froth, the membrane separating the cells in living tissue, and the cracks 
separating basalt columns. Minimization of surface area plays a role in 
determining the shape of many living organisms.  See  FOAM; GRAIN BOUNDARIES. 
 �34s[Frederick J. Almgren, Jr.]
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�6v�2v�32sMining   �21m �33s 
    The taking of minerals from the earth, including production from surface 
waters and from wells. Usually the oil and gas industries are regarded as 
separate from the mining industry. The term mining industry commonly includes 
such functions as exploration, mineral separation, hydrometallurgy, 
electrolytic reduction, and smelting and refining, even though these are not 
actually mining operations.  See  IRON METALLURGY; METALLURGY; ORE DRESSING. 
    Mining is broadly divided into three basic methods: opencast, underground, 
and fluid mining. Opencast, or surface mining, is done either from pits or 
gouged-out slopes or by strip mining, which involves extraction from a series 
of successive parallel trenches. Dredging is a type of strip mining, with 
digging done from barges. Hydraulic mining uses jets of water to excavate 
material.  
    Underground mining involves extraction from beneath the surface, from 
depths as great as 10,000 ft (3 km), by any of several methods. 
    Fluid mining is extraction from natural brines, lakes, oceans, or 
underground waters; from solutions made by dissolving underground materials 
and pumping to the surface; from underground oil or gas pools; by melting 
underground material with hot water and pumping to the surface; or by driving 
material from well to well by gas drive, water drive, or combustion. Most 
fluid mining is done by wells. In another type of well mining, insoluble 
material is washed loose by underground jets and the slurry is pumped to the 
surface.  See  OPEN-PIT MINING; PLACER MINING; PETROLEUM ENGINEERING; STRIP 
MINING; UNDERGROUND MINING. 
 �34s[Evan Just]
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�6v�2v�32sMink   �21m �33s 
    Any of three species of aquatic carnivorous mammals that are members of 
the family Mustelidae in the genus Mustela. They are found in the forested 
areas of North America, Europe, and Siberia. The mink is an excellent swimmer, 
aided by its slightly webbed hindfeet, and feeds on crayfish, frogs, snakes, 
and fish. The body is elongate and the legs are short (see illustration). 
    There is one litter each year, with three or four young being born in 
April or May after a gestation period of about 8 weeks. The young are mature 
the following season. The pelts of these animals are valuable; many mink farms 
have been established throughout the world. This animal is a natural predator 
on the muskrat, over whose numbers it exerts a control.  See  BADGER; 
CARNIVORA; MARTEN; OTTER; SKUNK; WEASEL. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMinnow   �21m �33s 
    A fresh-water fish in the family Cyprinidae, order Cypriniformes. There 
are over 200 species in North America, but as a group they are more abundant 
in tropical Africa and southeastern Asia. 
    This family is highly variable in morphological characteristics. 
Pharyngeal teeth are usually present, the body may be naked or covered with 
cycloid scales, there is a large swim bladder, and most species have spineless 
fins. Sexual dimorphism is common, and breeding males may be brightly colored 
or have nuptial tubercles on the head. The majority of species are small in 
size; many other small or immature species of fishes are erroneously called 
minnows.  See  CYPRINIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMiocene   �21m �33s 
    The next to the youngest of the five major worldwide divisions (epochs) of 
the Tertiary Period (Cenozoic Era); the epoch of geologic time extending from 
the end of the Oligocene to the beginning of the Pliocene; and the rocks 
(series) formed during this epoch and the fossils therein.  See  CENOZOIC; 
OLIGOCENE; PLIOCENE; TERTIARY. 
    Miocene time embraces two widespread reexpansions of the sea, separated by 
an interval of localized mountain making and shiftings of the seaways. Miocene 
strata include all the normal marine and terrestrial deposits. Extrusive 
volcanic rock is common; Miocene plutonic rocks are locally (Philippines) 
exposed.  
    In Miocene seas certain gastropod families suddenly diversified and 
multiplied, and other well-established gastropods persisted; large scallops of 
Oligocene inspiration, and clams expanded; oysters attained great size; cake 
urchins and sand dollars diversified and multiplied; diatoms, corals, 
cephalopods, crustacea, insects, and large-toothed sharks were locally 
important; and offshore small foraminifers proliferated. Mammalian marine 
adaptations (seals, sea lions, walruses, and sea-cowlike extinct desmostylids) 
appeared, or became modernized (sea cows, dugongs, and whales). 
    On land, except for rodents, all but two or three recognized mammal 
families already existed. Eurasian faunas included deer, hyenas, the earliest 
giraffes, and the bovines. A huge bear-dog, Amphicyon, was conspicuous, and 
mastodonts spread into North America for the first time. As grasslands spread, 
grazing life became dominant over browsing life; horses and camels, 
especially, evolved and spread rapidly. Australian, South American, and 
Madagascar mammal faunas remained isolated. 
    Ancestral redwoods flourished in China and California. Responding to a 
progressive cooling of climates through the Cenozoic, floras shifted 
equatorward along the north-south - trending mountain ranges bordering the 
Pacific, though species piled up where strandlines interfered (Malaysia and 
the Caribbean). Where mountains run east-west through the Old World, however, 
a Miocene Mediterranean flora was trapped and is ancestral to the Recent flora 
upon which human agricultural, high civilizations first developed. 
 �34s[Robert M. Kleinpell]
�33s 
  
  
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�6v�2v�32sMira   �21m �33s 
    The first star recognized to be a periodic variable. Called ``The 
Wonderful'' by the ancients, Mira was discovered by Fabricius in 1596 and has 
a period of 332 PLUS/MINUS9 days. It is the prototype of long-period 
variables, and like most of its class, changes considerably from epoch to 
epoch. At times the maximum is as bright as second magnitude, and sometimes it 
is barely fifth magnitude. Mira (o Ceti) has a faint companion discovered 
after its presence was predicted from a study of the spectrum.  See  VARIABLE 
STAR.  
 �34s[Margaret W. Mayall]
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�6v�2v�32sMirage   �21m �33s 
    A name for a variety of unusual images of distant objects seen as a result 
of the bending of light rays in the atmosphere during abnormal vertical 
distribution of air density. If the air closer to the ground is much warmer 
than the air above, the rays are bent in such a way that they enter the 
observer's eyes along a line lower than the direct line of sight. The object 
is then seen below the horizon, the inferior mirage. If the air closer to the 
ground is much colder than the air above, the rays are bent in the opposite 
direction, arriving at the observer's eyes above the line of sight; the object 
then seems to be elevated or floating in the air, the superior mirage. Mirages 
can be seen most frequently along an overheated highway surface; the inferior 
mirage of the sky gives the impression of water reflection over a wet 
pavement, which disappears upon a closer viewing. 
 �34s[Zdenek Sekera]
�33s 
  
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�6v�2v�32sMirror optics   �21m �33s 
    The use of plane or curved reflecting surfaces for the purpose of 
reverting, directing, or forming images. An optical surface which specularly 
reflects the largest fraction of the incident light is called a reflecting 
surface. Such surfaces are commonly fabricated by polishing of glass, metal, 
or plastic substrates, and then coating the surface of the substrate with a 
thin layer of metal, which may be covered in addition by a single or multiple 
layers of thin dielectric films. The law of reflection states that the 
incident and reflected rays will lie in the plane containing the local normal 
to the reflecting surface and that the angle of the reflected ray from the 
normal will be equal to the angle of the incident ray from the normal.  See  
GEOMETRICAL OPTICS. 
    The formation of images in the plane mirrors is easily understood by 
applying the law of reflection. The illustration shows the formation of the 
image of a point formed by a plane mirror. Each of the reflected rays appears 
to come from a point image located a distance behind the mirror equal to the 
distance of the object point in front of the mirror. In the illustration, the 
face of the observer can be considered as a set of points, each of which is 
imaged by the plane mirror. Since the observer is viewing the facial image 
from the object side of the mirror, the face will appear to be reversed left 
for right in the virtual image formed by the mirror. The illustration also 
indicates the redirection of light by a plane mirror, in that a viewer who 
cannot observe the object point directly can observe the virtual image of the 
point formed by the mirror. A simple optical device which is based on this 
principle is the simple mirror periscope, which uses two mirrors to permit 
viewing of scenes around an obstacle.  See  PERISCOPE. 
    A curved mirror, either spherical or conic in form, will produce a real or 
virtual image in much the same manner as a lens, but generally with reduced 
aberrations. There will be no chromatic aberrations since the law of 
reflection is independent of the color or wavelength of the incident light.  
See  ABERRATION (OPTICS); OPTICAL IMAGE. 
    Both concave and convex spherical mirrors are commonly encountered. Convex 
mirrors are commonly used as wide-angle rearview mirrors in automobiles or on 
trucks.  A common application of concave mirrors is the magnifying shaving 
mirror frequently found in bathrooms. 
    A spherical mirror will form an image which is not perfect, except for 
particular conjugate distances. The use of a mirror which has the shape of a 
rotated conic section, such as a parabola, ellipsoid, or hyperboloid, will 
form a perfect image for a particular set of object-image conjugate distances 
and will have reduced aberrations for some range of conjugate relations. The 
most familiar applications for conic mirrors are in reflecting telescopes.  
See  OPTICAL PRISM; OPTICAL SURFACES; OPTICAL TELESCOPE; REFLECTION OF 
ELECTROMAGNETIC RADIATION; TELESCOPE. 
 �34s[Robert R. Shannon]
�33s 
  
  
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�6v�2v�32sMissile   �21m �33s 
    An object capable of being projected or hurled, usually with the intent of 
striking some distant object. More particularly, a missile is usually a weapon 
that is self-propelled after leaving the launching device. The term thus 
excludes projectiles fired from guns as well as free-falling bombs. The 
propulsion criterion is not an absolute test, however, since gliding bombs, 
especially those guided after launch, are also frequently classed as missiles. 
    There are two primary categories of modern missiles; unguided and guided. 
All missiles, to be effective, must be directed in some sense, but those 
subject to no further control after leaving the launching device are usually 
classed as unguided. 
    Typical of unguided missiles are the ground-launched and air-launched 
free-flying rockets used in enormous number in World War II. 
    The growth of the sciences of instrumentation, electronics, and automatic 
control has led to the development of devices for the guidance of missiles in 
flight. Certain targets contrast with their surroundings by emitting or 
reflecting radiation in a distinctive manner. (Airplanes and ships are typical 
of the targets falling into this class.) The direction, and sometimes the 
distance, of targets of this kind can frequently be sensed by radiation 
receiving instruments. These instruments, located either in the missile or on 
the ground, can be used to guide the missile continuously toward the target. 
    Other targets cannot be readily distinguished from their surroundings by 
nonhuman means. Guided missiles used against these are frequently directed to 
the predetermined geographical location of such targets. Guidance errors are 
likely to be substantial, particularly as the distance from the guidance 
station to the target becomes great. In these cases, use of nuclear warheads 
becomes mandatory on economic grounds.  
    Missiles are classified in many ways. A common classification is according 
to the medium from which the missile is launched and to which it is directed. 
Thus, there are surface-to-air missiles, surface-to-surface missiles, 
air-to-surface missiles, and so on. They may also be classified according to 
range.  
    Missiles are also classified according to flight profile. The two 
categories are aerodynamic missiles (sometimes called cruise missiles) and 
ballistic missiles. Cruise missiles usually have wings or enlarged fins to 
give lift and maneuverability. A ballistic missile has no wings. It must be 
aimed sufficiently high to permit it to fall freely under the influence of 
gravity until it reaches the target.  See  BALLISTIC MISSILE. 
    High-speed missiles present problems of protecting sensitive components 
from the high temperatures produced by aerodynamic drag and sometimes from the 
high inertial forces produced by high accelerations. Only during the reentry 
phase is heat protection necessary. 
    The shape of the reentering body is sometimes quite blunt. This shape 
causes a greater fraction of the kinetic energy of the missile to be 
transferred to the surrounding atmosphere. However, blunt bodies slow down 
greatly before striking the ground, and they also have large radar 
reflectivity. These qualities are undesirable if antiballistic missiles are 
defending the target. As a consequence, the trend is toward sharper cones, 
accepting the greater heat transfer by providing greater protection. The 
ablative heat shield has almost entirely superseded the heat-sink type.  See  
NOSE CONE. 
 �34s[Robert C. Truax]
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�6v�2v�32sMississippian   �21m �33s 
    A large division of late Paleozoic geologic time, varyingly considered to 
rank as an independent period or as a main subdivision (termed epoch) of the 
Carboniferous Period. It is named Mississippian for a succession of highly 
fossiliferous marine strata consisting largely of limestones found along the 
Mississippi River between southeastern Iowa and southern Illinois. The rocks 
formed during the term defined as Mississippian are classed as a geologic 
system by most American geologists, and this practice is increasingly accepted 
outside of North America even though foreign usage favors Lower Carboniferous. 
Widespread marine transgression of the continents, accompanied especially by 
accumulation of limestones, characterized Mississippian time. Locally, this 
was preceded or succeeded by mountain building.  See  CARBONIFEROUS. 
 �34s[Raymond C. Moore]
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�6v�2v�32sMistletoe �21m �33s
    The name given to several species of the mistletoe family (Loranthaceae).
The true mistletoe of Europe is Viscum album, and among the early nations this
was an important ceremonial plant, which probably accounts for the origin of
the custom of kissing under the mistletoe. In the United States, the common
representative of the group is Phoradendron flavescens. All of the mistletoes
are green hemiparasites; that is, they obtain water and minerals from the host
plant but manufacture their own food.  See  SANTALALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sMiticide   �21m �33s 
    A chemical useful for killing phytophagous and parasitic members of the 
order Acarina of the class Arachnida. Although these animals are not insects, 
their control has become a part of economic entomology. Several species of 
mites are parasitic, but a considerable number are economically important as 
plant pests. Miticides are chemicals used specifically for the control of 
plant-feeding mites.  See  ACARINA; INSECTICIDE; PESTICIDE. 
 �34s[George F. Ludvik]
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�6v�2v�32sMitochondria   �21m �33s 
    Cell structures, sometimes called chondriosomes, which are minute 
cytoplasmic organelles found in all cells. They are clearly visible in the 
living cell when examined with the dark-field or phase-contrast microscope, 
and can be identified with the bright-field microscope when specifically 
stained by Janus green, applied supravitally. They may appear as small 
spherical granules, short rods, or long filaments. Spherical forms range from 
0.2 to more than 2.0 micrometers in diameter and filamentous forms may vary 
from 1 to 7 micrometers in length. 
    Mitochondria may change form in different physiological conditions and are 
sensitive indicators of cell injury. In living cells they are in constant 
motion, executing slow flexuous movements as they move from one part of the 
cell to another. As a rule they maintain their characteristic shape during 
these movements, but occasionally filamentous forms break up into short rods 
or spheres. In rare instances they elongate and thrust out lateral branches 
which coalesce temporarily with other parts of the same mitochondrion to form 
annular or plexiform (network) structures. New mitochondria arise only from 
preexisting mitochondria by a process of budding or fission. 
    In electron micrographs of thin sections of cells the mitochondria are 
seen to be limited by two dense lines SIMILAR 5 nanometers thick and SIMILAR 5 
nanometers apart (Fig. 1). The outermost of these is smooth and continuous, 
whereas the inner turns inward at more or less regular intervals along the 
length of each mitochondrion to form a series of thin folds, the cristae 
mitochondriales, that project into the interior of the organelle. According to 
the interpretation of the electron microscopic images, the mitochondrion is 
limited by two separate membranes, the innermost of which is unfolded to form 
the cristae. Although internal membranes usually take the form of folds or 
cristae, they may instead form slender villous or tubular projections called 
tubuli mitochondriales. 
    In negatively stained, fragmented fresh mitochondria, the inner surface of 
the organelle is found to be studded with small particles 9 nm in diameter 
which are connected to the membrane by a slender stalk 3.5 - 4.0 nm long (Fig. 
2). These particles, called inner membrane subunits, are believed to be sites 
of important enzymes of the mitochondrion, but are not preserved by routine 
methods of preparation of tissues for electron microscopy. 
    The interior of the mitochondrion is occupied by an amorphous matrix of 
appreciable density. A variable number of dense particles SIMILAR 50 nm in 
diameter, called matrix granules, and smaller granules SIMILAR 12 nm in 
diameter that contain ribonucleic acid (RNA). Deoxyribonucleic acid (DNA) has 
also been found, and this appears to reside in slender filaments that can be 
visualized in the matrix in electron micrographs of the mitochondria in some 
tissues.  
    Mitochondria contain highly integrated enzyme systems providing energy for 
cell metabolism. Among the most important of these is the tricarboxylic acid 
cycle and its associated respiratory enzymes. These produce the energy-rich 
compound adenosinetriphosphate which is essential for many vital cellular 
activities, including cell motility and muscular contraction.  See  
ADENOSINETRIPHOSPHATE (ATP); CELL (BIOLOGY); CELL MEMBRANES; KREBS CYCLE; 
PROTEIN.  
 �34s[Don W. Fawcett]
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�6v�2v�32sMitosis   �21m �33s 
    A division of the cell nucleus in which a spindle and chromosomes are 
involved. The process results in two daughter nuclei which are genetically 
identical to each other and to the cell from which they were derived. It 
involves the exact duplication of the chromosome threads, followed by a 
precise separation of the products of the duplication to each of the daughter 
cells. This is brought about by a series of mechanical movements during which 
the entire nucleus and cytoplasm are temporarily reorganized. Mitosis is the 
characteristic mode of division of eukaryotic cells, although other less 
precise forms of division are known, for example, amitosis.  See  CELL 
DIVISION.  
  
    Mitotic cycle.  The first indication that a cell is about to divide is the 
appearance within the nucleus of long chromosome threads (see illustration). 
At the outset these are divided longitudinally into pairs of sister 
chromatids, the consequence of reduplication of the chromosomal DNA during the 
preceding interphase. During prophase the chromosomes condense and spiralize, 
becoming shorter and thicker; sister chromatids often show relational coiling. 
The spindle apparatus forms, and at metaphase centromeres congress at the 
equator of the spindle. In anaphase, sister chromatids, led by their 
centromeres, pass to opposite poles. Following reorganization of nuclei in 
telophase, the cell may undergo cytokinesis, which in animal cells normally 
involves constriction and ultimate division of the cytoplasm, while in plant 
cells a new wall is laid down by the phragmoplast in the cytoplasm between the 
daughter nuclei. In the absence of cytokinesis, multinucleate cells or 
coenocytes may be formed. 
    The spindle apparatus is shaped somewhat like a football and in many 
species, especially in animals, is associated with centriolar asters at each 
of its poles. When fully formed at metaphase, the spindle is a discrete, 
distinct structure, and when suitably illuminated, can be seen without 
difficulty in the normal living cell. 
  
  
    Significance.  The chromosomes are permanent structures capable of 
accurate self-duplication which, in turn, depends upon the exact replication 
of the individual DNA molecules composing the chromosomes. Before replication, 
the chromosomes consist of two subunits of DNA, corresponding to the two 
chains of the DNA double helix. During interphase, the replication of DNA 
involves the separation of the complementary strands over a short distance, 
followed by the synthesis of complementary strands on each of the parental 
strands. During mitosis the products of DNA and chromosome duplication are 
distributed equally between daughter nuclei. After the nucleus has divided, 
the cytoplasm divides, producing two cells from the original one. Each 
daughter nucleus should then carry exactly the same heritable information as 
the other and the parent nucleus.  See  CYTOKINESIS. 
    Mitotic crossing over may well be an important method of redistributing 
genetic material in organisms which do not have a sexual stage, for example, 
imperfect fungi. Linkage groups could be broken up by a cycle of 
diploidization, recombination, and nonmeiotic haploidization, a sequence which 
has been named the parasexual cycle by Pontecorvo. In this way recombination 
of whole chromosomes and parts of chromosomes can occur.  See  CHROMOSOME; 
DEOXYRIBONUCLEIC ACID (DNA). 
 �34s[Graham G. Nicholson]
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�6v�2v�32sMixer   �21m �33s 
    A device having two or more signal inputs, usually adjustable, and one 
common output. It is used in audio amplifiers to combine the outputs of 
individual microphones and other audio-signal sources linearly and in desired 
proportions to produce one audio output signal. In television, a mixer serves 
similarly to combine the outputs of two or more television cameras or other 
video signal sources. The mixer stage in a superheterodyne receiver combines 
the incoming modulated radio-frequency (rf) signal with the signal of a local 
rf oscillator to produce a modulated intermediate-frequency (i-f) signal. 
Crystal diodes are widely used as mixers in radar and other microwave 
equipment.  
 �34s[John Markus]
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�6v�2v�32sMixing   �21m �33s 
    A common operation to effect distribution, intermingling, and homogeneity 
of matter. Actually the operation is called agitation, with the term mixing 
being applicable when the goal is blending, that is, homogeneity. Other 
processes, such as reaction, mass transfer (includes solubility and 
crystallization), heat transfer, and dispersion, are also promoted by 
agitation. The type, extent, and intensity of agitation determine both the 
rates and adequacy of a particular process result. The agitation is 
accomplished by a variety of equipment. 
    Most liquid mixing is done by rotating impellers in vertical cylindrical 
vessels. A typical impeller-type liquid mixer with a variety of features is 
shown in the illustration. The internal features, including the vessel itself, 
are considered as a whole, that is, as the agitated system. The forces applied 
by the impeller develop overall circulation or bulk flow. Superimposed on this 
flow pattern, there is molecular diffusion, and if turbulence is present, also 
turbulent eddies. These provide micromixing. Solids, granular to powder, are 
mixed in a variety of contrivances. 
    Solids of different density and size are mixed in tumblers (a double cone 
turning end on end) or with agitators (a helical ribbon rotating in a 
horizontal trough). The duration of mixing is an important additional variable 
because classification and separation often occur after attainment of the 
desired distribution if the operation is carried on too long. 
 �34s[Vincent W. Uhl]
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�6v�2v�32sMixture   �21m �33s 
    An aggregate composed of two or more distinct chemical components which 
retain their identities regardless of the degree to which they have become 
mingled. The substituents may be present in any proportion and may form a 
homogeneous or heterogeneous system. There is no chemical interaction between 
the components of a mixture, and it is possible, at least theoretically, to 
separate them by physical means. Attractive or repulsive forces between the 
different substituents, however, may affect the gross characteristics of the 
aggregate. For example, the total volume of a mixture of several liquids may 
not be the sum of the separate volumes. 
 �34s[Francis J. Johnston]
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�6v�2v�32sMizar   �21m �33s 
    A multiple star in the constellation Ursa Majoris. Mizar (ZETA Ursa 
Majoris) is located at the bend in the handle of the big Dipper. A very wide 
visual double, Mizar (the horse) and Alcor (the rider), is visible to the 
naked eye. Mizar is also a telescopic double star; both components are also 
spectroscopic binary stars. Mizar was the first spectroscopic binary to be 
discovered (1889).  See  BINARY STARS; URSA MAJOR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sModel theory   �21m �33s 
    The theory underlying the design and use of models to predict the 
characteristics of any system. It is particularly valuable when the desired 
system, or prototype, is large and complex. A model can be built, tested, and 
modified at comparatively low cost. If the model is properly designed, the 
results obtained from it may be used with a high degree of confidence in 
predicting the performance of the prototype. Models are thus widely used in 
the design of bridges, dams, unusual buildings, aircraft, river and harbor 
works, agricultural machinery, and a variety of network systems. 
    If the results obtained from a model are to be applicable to the 
prototype, a rigorous set of conditions on the model must be satisfied or else 
the deviations from these conditions must be taken into account in the 
predictions of the prototype behavior. The similitude relationships that must 
exist between model and prototype may be developed conveniently by using 
dimensional analysis. See DIMENSIONAL ANALYSIS. 
    The first step is to identify all of the independent variables in the 
system which will influence the magnitude of the quantity that is to be 
predicted. A set of dimensionless variables is then formed from these 
variables, using dimensional analysis. These dimensionless variables, or pi 
terms, are grouped in an equation, called the characteristic equation, in the 
form of Eq. (1), in which PI sub 1 is the dimensionless term containing the 
  
*** PI sub 1 = f (PI sub 2, PI sub 3, PI sub 4, . . . , PI sub s) ***      (1)
  
variable to be predicted, and the other PI terms are dimensionless quantities 
based on the independent variables. A similar equation may be written for the 
model, which involves the same variables as the prototype; this is shown in 
Eq. (2). If the two systems involve the same phenomenon, the forms of the two
  
*** PI sub 1m = f (PI sub 2m, PI sub 3m, PI sub 4m, . . . , PI sub sm) *** (2)
  
functions will be identical. If the model is so designed that Eqs. (3) hold, it 
follows the Eq. (4) is true. Therefore, if the design equations, Eqs. (3), are
  
    *** PI sub 2m = PI sub 2, PI sub 3m 
                  = PI sub 3, PI sub 4m
                  = PI sub 4, . . . . , PI sub sm = PI s ***               (3)
  
                          *** PI sub 1 = PI sub 1m ***                     (4)
  
satisfied in the construction and operation of the model, the prediction 
equation, Eq. (4), may be used to predict the magnitude of PI sub 1, and hence 
the desired variable in the prototype, from the measured magnitude of PI sub 1m 
in the model. 
    In some situations two design conditions may lead to conflicting 
requirements that cannot be satisfied simultaneously. In situations of this 
kind the design of the model is based on the predominant number, and the 
influence of the other number or numbers must be distorted. 
 �34s[Glenn Murphy]
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�6v�2v�32sMode of vibration   �21m �33s 
    A characteristic manner in which vibration occurs. In a freely vibrating 
system, oscillation is restricted to certain characteristic frequencies; these 
motions are called normal modes of vibration. 
    An ideal string, for example, can vibrate as a whole with a characteristic 
frequency defined in the equation below, where L is the length of string 

                  *** f = (1/2L) SQUARE ROOT [T/m] ***

between rigid supports, T the tension, and m the mass per unit length of the 
string. The displacements of different parts of the string are governed by a 
characteristic shape function. The frequency of the second mode of vibration 
is twice that of the first mode. Similarly, modes of higher order have 
frequencies that are integral multiples of the fundamental frequency. 
    Because the frequencies are in the ratios 1:2:3 . . . , the modes of 
vibration of an ideal string are properly called harmonics. Not all vibrating 
bodies have harmonic modes of vibration, however. See HARMONIC (PERIOD 
PHENOMENA); VIBRATION. 
 �34s[Robert W. Young]
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�6v�2v�32sModulation �21m �33s
    The process or result of the process whereby a message is changed into
information-bearing signals that not only unambiguously represent the message
but also are suitable for propagation over the transmitting medium to the
receiver. Demodulation or detection is the process whereby, in response to the
received wave, either the original message or information pertaining to the
original message is made available in the form desired.
    Modulation implies bandwidth occupancy. For any signal to change in a way
that cannot be predicted implies necessarily that the signal occupy a nonzero
band of frequencies. For example, the spoken word occupies a band from a few
hundred to several thousand hertz.
    Defined broadly, modulation is the process or result of the process
whereby some parameter of one wave is varied in accordance with another. As is
customary in the treatment of modulation, the word ``wave'' is used as a
generic term intended to include such concepts as signal, voltage, current,
pressure, displacement, and the like, whether these are constant or changing.
    This broad definition of modulation implies three fundamental concepts:
modulating wave, carrier, and modulated wave. A modulating wave changes some
parameter of the wave to be modulated; a carrier is a wave suitable for
modulation by the modulating wave; a modulated wave has some parameter changed
in accordance with the modulating wave.
    Common forms of modulation are amplitude modulation, angle modulation
(which includes frequency modulation and phase modulation), and pulse
modulation (see illustration).	See  AMPLITUDE MODULATION; ANGLE MODULATION;
FREQUENCY MODULATION; PHASE MODULATION; PULSE MODULATION.
 �34s[Harold S. Black]
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�6v�2v�32sModulator   �21m �33s 
    Any device or circuit by means of which a desired signal is impressed upon 
a higher-frequency periodic wave known as a carrier. The process is called 
modulation. The modulator may vary the amplitude, frequency, or phase of the 
carrier.  See  MODULATION. 
    There are many ways to accomplish amplitude modulation, but in all cases a 
nonlinear element or device must be employed. The modulating signal controls 
the characteristics of the nonlinear device and thereby controls the amplitude 
of the carrier.  See  AMPLITUDE MODULATION; AMPLITUDE-MODULATION DETECTOR; 
AMPLITUDE MODULATOR. 
    The frequency modulator usually changes the effective capacitance or 
inductance in the frequency-determining LC circuit of the oscillator. However, 
other techniques can be used. For example, a multivibrator can be used to 
generate carrier frequencies up to a few megahertz, and the multivibrator 
frequency can be modulated by controlling the base, gate, or grid bias supply 
voltage.  See  FREQUENCY MODULATION; FREQUENCY-MODULATION RADIO; FREQUENCY 
MODULATOR.  
 �34s[Charles L. Alley]
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�6v�2v�32sMoebius inversion   �21m �33s 
    A useful counting technique exploits the relationship between a pair of 
functions f and g defined on a lattice L. Suppose that g satisfies Eq. (1), 

       *** g(s) = SUMMATION (s EQUAL TO OR LESS THAN t) f(t) ***           (1)

and that the values of g are known; then the values of f can be found if the 
calculation in Eq. (1) can be ``inverted'' to define f in terms of g. The 
principle of Moebius inversion states that there is a function MU (s,t), called 
the Moebius function of the lattice, defined for pairs of elements (s,t) in L 
and which depends only on the lattice, such that Eq. (2) holds. 

   *** f(s) = SUMMATION (s EQUAL TO OR LESS THAN t) MU (s,t) g (t) ***     (2)

See COMBINATORIAL THEORY; ENUMERATION OF ARRANGEMENTS. 
 �34s[T. Balderes]
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�6v�2v�32sMoessbauer effect �21m �33s
    Recoil-free gamma-ray resonance absorption. The Moessbauer effect, also
called nuclear gamma resonance fluorescence, has become the basis for a type
of spectroscopy which has found wide application in nuclear physics,
structural and inorganic chemistry, biological sciences, the study of the
solid state, and many related areas of science.
    The fundamental physics of this effect involves the transition (decay) of
a nucleus from an excited state of energy E(sub e) to a ground state of energy
E(sub g) with the emission of a gamma ray of energy E(sub GAMMA).  If the
emitting nucleus is free to recoil, so as to conserve momentum, the emitted
gamma ray energy is E(sub GAMMA) = ( E[sub e] - E[sub g]) - E(sub r), where
E(sub r) is the recoil energy of the nucleus.  The magnitude of E(sub r) is
given classically by the relationship E(sub r) = E(sub GAMMA)(sup )2/2 mc(sup )2
, where m is the mass of the recoiling atom.  Since E(sub r) is a positive
number, the E(sub GAMMA) will always be less than the difference E(sub e) -
E(sub g) , and if the gamma ray is now absorbed by another nucleus, its energy
is insufficient to promote the transition from E(sub g) to E(sub e).
    In 1957 R. L. Moessbauer discovered that if the emitting nucleus is held
by strong bonding forces in the lattice of a solid, the whole lattice takes up
the recoil energy, and the mass in the recoil energy equation given above
becomes the mass of the whole lattice. Since this mass typically corresponds
to that of 10sup1sup0 to 10(sup 20) atoms, the recoil energy is reduced by a
factor of 10(sup -10) to 10(sup -20), with the important result that E(sub r)
APPROX EQUAL 0 so that E(sub GAMMA) = E(sub e) - E(sub g) ; that is, the emitted
gamma-ray energy is exactly equal to the difference between the nuclear
ground-state energy and the excited-state energy.  Consequently, absorption of
this gamma ray by a nucleus which is also firmly bound to a solid lattice can
result in the ``pumping'' of the absorber nucleus from the ground state to the
excited state.	See ENERGY LEVEL (QUANTUM MECHANICS); EXCITED STATE; GAMMA RAYS;
GROUND STATE.
     In a typical Moessbauer experiment the radioactive source is mounted on a
velocity transducer which imparts a smoothly varying motion (relative to the
absorber, which is held stationary), up to a maximum of several centimeters per
second, to the source of the gamma rays.  These gamma rays are incident on the
material to be examined (the absorber).  Some of the gamma rays are absorbed and
reemitted in all directions, while the remainder of the gamma rays traverse the
absorber and are registered in an appropriate detector.  A typical display of a
Moessbauer spectrum, which is the result of many repetitive scans through the
velocity range of the tranducer, is shown in the illustration.	In certain
nuclides the Moessbauer resonance line displays splitting that arises from the
coupling of the nuclear electric quadrupole moment with the electric field
gradient or of the nuclear magnetic dipole moment with the magnetic field at the
nucleus, providing information on the magnitude of these interactions.

    Moessbauer effect experiments have been used to elucidate problems in a
very wide range of scientific disciplines. Applications include the
measurement of nuclear magnetic and gradrupole moments and of excited-state
lifetimes involved in the nuclear decay process; study of the chemical
consequences of nuclear decay; study of the nature of magnetic interactions in
iron-containing alloys and of the dependence of the magnetic field in these
alloys on various parameters; study of the effects of high pressure on
chemical properties of materials; investigation of the relationship between
chemical composition and structure on the one hand and the superconductive
transition on the other; investigation of the structure of compounds; and
study of the structure and bonding properties of metal atoms in complex
biological molecules.
 �34s[Rolfe H. Herber]
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�6v�2v�32sMohair   �21m �33s 
    The long, lustrous hair of the Angora goat, which originated in the area 
around Ankara (Angora), Turkey. Mohair is a smooth, strong, durable, and 
resilient fiber. It enhances softness and luster in fabrics. Mohair absorbs 
dye evenly and brilliantly, retains color well, and permits unusual decorative 
effects. It is mainly used as an apparel fiber but may be used in upholstery, 
draperies, wigs, hairpieces, and rugs. Leather produced from the skin is 
useful for gloves, purses, and novelties.  See  WOOL. 
 �34s[Clair E. Terrill]
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�6v�2v�32sMoho (Mohorovicic discontinuity)   �21m �33s 
    A seismic discontinuity at the base of the Earth's crust inferred from 
travel time curves indicating that seismic waves undergo a sudden increase in 
velocity. Immediately below the discontinuity the velocity of seismic 
compressional waves increases to a little over  5 mi/s (8 km/s). The 
discontinuity is named after its discoverer, the Yugoslavian geophysicist A. 
Mohorovicic. Such a discontinuity seems to exist every place that 
geophysicists seismologically investigate the Earth's structure. Under the 
ocean basins its depth is generally 6 - 7 mi (10 - 12 km); under the 
continents it is usually at a depth of 20 - 22 mi (33 - 35 km). It usually 
lies deeper under mountain masses and shallower under lowlands and plains, 
sloping upward near coastal margins. Islands and island arcs are also marked 
by a downward bend of the Moho under the ocean basins.  See  CONTINENT; 
SEISMOLOGY.  
 �34s[Gordon G. Lill]
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�6v�2v�32sMoire pattern   �21m �33s 
    When one family of curves is superposed on another family of curves, a new 
family called the moire pattern appears. 
    To produce moire patterns, the lines of the overlapping figures must cross 
at an angle of less than about 45DG. The moire lines are then the locus of 
points of intersection. The illustration shows the case of two identical 
figures of simple gratings of alternate black and white bars of equal spacing. 
When the figures are crossed at 90DG, a checkerboard pattern with no moire 
effect is seen. At crossing angles of less than 45DG, however, one sees a 
moire pattern of equispaced lines, the moire fringes. The spacing of the 
fringes increases with decreasing crossing angle. This provides one with a 
simple method for measuring extremely small angles (down to 1 second of arc). 
As the angle of crossing approaches zero, the moire fringes approach 90DG with 
respect to the original figures. 
    Even when the spacing of the original figures are far below the resolution 
of the eye, the moire fringes will still be readily seen. This phenomenon 
provides a means of checking the fidelity of a replica of a diffraction 
grating.  See  DIFFRACTION GRATING. 
    Moire techniques are widely used in the stress analysis of metals, in the 
examination of large optical surfaces, in investigating aberrations of lenses, 
and in determining a refractive index gradient (for example, that of sugar 
molecules diffusing into water). 
 �34s[Gerald Oster]
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�6v�2v�32sMoisture-content measurement   �21m �33s 
    Measurement of the ratio or percentage of water present in a gas, a 
liquid, or a solid (granular or powdered) material. Nearly all materials 
contain free water, the relative amount being dependent upon the physical and 
chemical properties of the material. The primary purpose of determining and 
maintaining moisture contents within specified limits can usually be traced to 
economic factors, trade practices, or legal requirements. 
    Moisture content has a number of synonymous terms, many of which are 
specific to certain industries, types of product, or material. The water 
content in solid, granular, or liquid materials is usually referred to as 
moisture content on either the wet or dry basis; the wet basis is common to 
most industries. Specifically, moisture content on the wet basis refers to the 
quantity of water per unit weight or volume of the wet material. A weight 
basis is preferred. The textile industry uses the dry basis for moisture 
content of textile fibers. Often referred to as regain moisture content, the 
dry basis or regain refers to the quantity of water in a material expressed as 
a percentage of the weight of the bone-dry (thoroughly dried) material. 
    The moisture content in air is referred to as humidity, either absolute or 
relative. Absolute humidity is the number of pounds of water vapor associated 
with 1 lb (0.5 kg) of dry air, also called just humidity. Relative humidity is 
the ratio, usually expressed as a percentage, of the partial pressure of water 
vapor in the actual atmosphere to the vapor pressure of water at the 
prevailing temperature. Relative humidity is customarily reported by the U.S. 
Weather Bureau because it essentially describes the degree of saturation of 
the air. However, air which is saturated (100% RH) at 50DGF (10DGC) is quite 
dry (19% RH) when heated to 100DGF (38DGC). A changing basis of this type is 
not convenient for many purposes such as computations used in airconditioning, 
combustion, or chemical processing; therefore absolute units, such as dew 
point or grains of water per pound of dry air, are more acceptable. Dew point 
is the temperature at which a given mixture of air and water vapor is 
saturated with water vapor.  See  DEW POINT; HUMIDITY. 
  
    Gases.  The measurement of water content in gases and mixtures of air and 
gases is important in industry. A number of commercially manufactured 
instruments are available for these measurements; their principles of 
operation include condensation, used in dew- or fog-point indicators; 
dimensional change, used by hygrometers; thermodynamic equilibrium, used by 
wet-bulb psychrometers; and absorption methods, which serve as the basic 
principle for gravimetric and electric conductivity or dielectric types.  See  
HYGROMETER; PSYCHROMETER; PSYCHROMETRICS. 
    The importance of humidity in relation to personal comfort is well known. 
The air conditioning industry produces equipment to maintain comfortable 
conditions of temperature and humidity. Considerable industrial air 
conditioning is also done for process reasons. Control of humidity is also 
important in the preservation of materials, especially those which are 
hygroscopic, and in the storage of food products.  See  HUMIDITY CONTROL. 
  
    Liquids and solids.  The development of instrumentation for the 
measurement and control of moisture in liquids and solids has been due in a 
large part to the great need that exists in many processes where the control 
of a precise moisture is critical. The desirability of a specific moisture 
content in a product during its preliminary manufacturing process is often 
required. In general, however, the rigid control of moisture content occurs 
most frequently in the final product to assure its quality and the fulfillment 
of legal or trade practices for the individual product. 
    Instruments suitable for the measurement of moisture content may be 
classified as periodic and continuous. In general, only those instruments 
offering continuous measurement are practical for the automatic control of 
moisture content in a product. The periodic instrument types are generally 
automated versions of conventional laboratory moisture-analysis procedures. 
    Moisture measuring instruments may also be classified by operating 
principle. Those instruments employing electrical conductivity (either dc or 
ac), absorption of electromagnetic energy (radio-frequency regions), 
electrical capacitance (dielectric constant change), and infrared energy 
radiations are more readily adapted to continuous measurements inasmuch as the 
response of these instruments to moisture changes is very fast. Those 
instruments employing automatic oven drying, chemical titrations, equilibrium 
hygrometric methods, distillation methods, and so forth are usually of the 
intermittent, or periodic, type. 
 �34s[Lee E. Cuckler]
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�6v�2v�32sMolecular adhesion   �21m �33s 
    A particular manifestation of intermolecular forces which causes solids or 
liquids to adhere to each other. The term is usually used with reference to 
adhesion between two different materials, in contrast to the term cohesion, 
which refers to the action of intermolecular forces whereby solid and liquid 
substances are held to substantially constant volume. Thus adhesive forces are 
not different in kind from cohesive forces. The behavior of different 
substances in contact is greatly affected by the relative strength of adhesive 
forces between different materials or cohesive forces between the same 
materials.  See  COHESION (PHYSICS); INTERMOLECULAR FORCES. 
 �34s[E. U. Condon]
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�6v�2v�32sMolecular beams   �21m �33s 
    Well-directed streams of atoms or molecules in vacuum. Utilization of 
molecular beams is a cornerstone technique in the investigation of molecular 
structure and interactions. Molecular beams are usually formed at sufficiently 
low particle density for the interaction of one beam molecule with another to 
be negligible. This ensemble of truly isolated molecules is available for the 
spectroscopic study of molecular energy levels using photon probes from the 
radio-frequency to optical portions of the electromagnetic spectrum. Some of 
the best-determined fundamental knowledge of physics comes from spectroscopic 
molecular-beam experiments. Beyond this, beams can be applied as probes of the 
multifaceted nature of gases, plasmas, surfaces, and even the structure of 
solids. An application intermediate in complexity is the study of molecular 
interactions by means of two colliding beams, where one might be a beam of 
charged particles such as ions or electrons.  See  SCATTERING EXPERIMENTS 
(ATOMS AND MOLECULES). 
    One simple means of forming a beam is to permit gas from an enclosed 
chamber to escape through a small orifice into a second chamber maintained at 
high vacuum by means of large pumps (illustration a). A useful number of 
molecules passes forward along the horizontal axis of the apparatus. A 
well-collimated beam is then formed by requiring that those molecules entering 
the test chamber where an experiment is to be performed pass not only through 
the orifice but also through a second small hole separating the collimating 
and test chambers. 
    If higher velocities are desired, then a charge-exchange beam system can 
be used. In this scheme (illustration b), ions are produced by some ionizing 
process such as electron impact on atoms within a gas discharge. Since the 
ions are electrically charged, they can be accelerated to the desired velocity 
and focused into a beam using electric or magnetic fields. The last step in 
neutrally charged beam formation is to pass the ions through a neutralizing 
gas where electrons from the gas molecules are transferred to the beam ions in 
charge-exchange molecular collisions.  See  ION SOURCES. 
  
    Much of molecular spectroscopy involves the absorption or emission of 
light by molecules in a gas sample. The frequency of the light photon is 
proportional to the separation of molecular energy levels involved in the 
spectroscopic transition. However, the molecule density in typical gas samples 
is so high that the energy levels are slightly altered by collisions between 
molecules, with the transition frequency no longer characteristic of the free 
molecule. The use of low-density molecular beams with their sensitive 
detection techniques can reduce this collision alteration problem, with the 
result that atomic properties can be measured to accuracies of parts per 
million or even better. If the very simplest atoms or molecules are employed, 
then the basic electromagnetic interactions holding the component electrons 
and nuclei together can be precisely studied. This is of great importance to 
fundamental physics, since theoretical understanding of electromagnetic 
interactions through quantum electrodynamics represents the most successful 
application of quantum field theory to elementary particle physics problems.  
See  QUANTUM ELECTRODYNAMICS; QUANTUM FIELD THEORY. 
    The development of tunable, strong laser sources of single-frequency light 
beams has added another dimension to molecular-beam experiments. With laser 
radiation resonantly tuned to excite a molecule from its normal ground state 
to one of its infinite number of vibrationally, rotationally, and 
electronically excited states, the number of possible studies and applications 
of excited molecular beams becomes enormous.  See  LASER SPECTROSCOPY; 
MOLECULAR STRUCTURE AND SPECTRA; NUCLEAR STRUCTURE. 
 �34s[James E. Bayfield]
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�6v�2v�32sMolecular biology   �21m �33s 
    In its broadest context, molecular biology is that part of biology which 
attempts to interpret biological events in terms of the molecules in the cell. 
The subject matter merges indistinguishably with older and more traditional 
areas of biology, such as biochemistry and genetics. In a general way 
molecular biology may be described as that subject which grew out of the 
merging of biochemistry and genetics, since in this discipline special 
emphasis is placed on the molecular basis of genetics and of macromolecular 
replication.  
    Molecular biophysics is a related discipline that analyzes various 
biological phenomena on their most fundamental level, that is, on the level of 
the behavior of molecules. Because the primary emphasis is on a molecular 
explanation or description, one of the major tools in this discipline is 
chemistry, which is the science of molecules. On the other hand, many of the 
techniques used in molecular biophysics are taken directly from physics, such 
as the use of x-ray diffraction or nuclear magnetic resonance to study 
molecular structure. Thus molecular biophysics can be characterized as a 
discipline which utilizes the selective aspect of both chemistry and physics 
to study phenomena of biology on the molecular level.  See  BIOCHEMISTRY; 
BIOPHYSICS; GENETICS. 
    It is possible to describe a biological system in terms of various types 
of flux or flow in the living cell. The most obvious is a flow of matter, in 
which substances are taken up by the cell and a series of transformations are 
carried out within it; for example, glucose is converted into carbon dioxide 
and water. The determination of the detailed pathways of this conversion has 
been the traditional subject matter of biochemistry. In intermediary 
metabolism, the interconversions of a large variety of organic and inorganic 
molecules are described in great detail. A general conclusion reached in this 
type of investigation is the fact that a special role is played by the 
macromolecular proteins, which are polymers of amino acids. Proteins form the 
enzymes which have catalytic activity, and in a general way their presence or 
absence governs the metabolic pathways in living cells.  See  PROTEIN. 
    Another type of flux or flow in living tissue is that of energy. 
Associated with the flow of matter into cells and its conversion into other 
types of molecules is a conversion of chemical energy. For example, energy is 
contained in the chemical bonds of the glucose molecule, and the controlled 
breakdown of glucose into carbon dioxide and water leads to a controlled 
release of this chemical energy and its conversion into chemical energy 
associated with other types of molecules. Chemical energy is necessary to 
carry out many types of work in living cells, such as the work of chemical 
synthesis in which large molecules are built up through polymerization. Other 
types of work include mechanical work or osmotic work in which molecules are 
made to move against concentration gradients within the cell. The flow of 
chemical energy through the cell has been an integral part of intermediary 
metabolism, and a large part of biochemistry consists of describing metabolic 
pathways in terms of both molecular and energy conversions which are carried 
out within the cell.  See  ENERGY METABOLISM. 
    There is a third type of flux which is of essential importance in 
understanding the total molecular activity of the cell. This is the flow of 
information. By information is meant the directions or blueprints by means of 
which the cell knows which types of molecules to make. Since the control of 
intermediary metabolism is dependent upon the presence or absence of the 
protein enzymes, the information flow may be regarded as the system in the 
cell whereby the directions for assembling particular protein molecules are 
stored and transmitted through the cell. There is thus a system for 
controlling the metabolic activities of cells. This feature is characteristic 
of living matter, and it sharply divides such systems from nonliving material. 
In some ways, it may be said that the core of molecular biology is the study 
of the molecular representation of information in living cells and its manner 
of distribution and reproduction.  See  DEOXYRIBONUCLEIC ACID (DNA); 
RIBONUCLEIC ACID (RNA). 
 �34s[Alexander Rich]
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�6v�2v�32sMolecular distillation   �21m �33s 
    A process by which substances are distilled in high vacuum at the lowest 
possible temperature and with the least damage to their composition. In a 
conventional distillation carried out at atmospheric pressure, molecules that 
leave a liquid surface as a vapor in their movement toward a condensing 
surface travel in straight paths until they collide with a molecule of 
residual gas (usually air), with another vapor molecule, or with the surface 
of the distilling vessel. These collisions either return the vapor to the 
liquid or greatly slow its rate of travel from the liquid to the condenser. 
    If the residual gas is virtually removed from a distillation vessel (for 
example, to pressures approximating 0.001 mmHg), and if the condensing surface 
is located close to the evaporating liquid, then conditions are favorable for 
molecular distillation (see illustration). 
    Materials composed of compounds with molecular weights in the approximate 
range of 400 to 1200 (there are exceptions) are effectively distilled by the 
molecular distillation process. Since this method can distill these compounds 
at temperatures 90 - 270DGF (50 - 150DGC) lower than by any other means, their 
thermal decomposition is at a minimum, which is very important when dealing 
with heat-sensitive materials. Also, any tendency toward oxidation during 
distillation is eliminated, since there is no atmospheric oxygen present. 
    At molecular distillation pressure, evaporation takes place at all 
temperatures, for there is no boiling point as there is in atmospheric 
pressure or in conventional vacuum distillation. Since evaporation is slow at 
lower temperatures, the liquid temperature is usually raised until the rate of 
evaporation meets practical requirements. 
    Molecular distillation is widely used in the laboratory for analytical 
work, as well as in the chemical industry for the improvement of plasticizers, 
silicone fluids, flavors and fragrances, pharmaceuticals, and edible products. 
In the latter category, high-purity monoglycerides used in baked goods and 
confectionery, as well as cosmetics, are purified by molecular distillation. 
 �34s[Edward S. Barnitz]
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�6v�2v�32sMolecular isomerism �21m �33s
    The property of compounds (isomers) which have the same molecular formula
but different physical and chemical properties. The difference in properties
is caused by a difference in molecular structure (that is, molecular
architecture). A typical example is dimethyl ether, CH3OCH3, a chemically
quite inert gas which condenses at -24DGC, and ethyl alcohol, CH3CH2OH, a
liquid of substantial chemical reactivity which boils at 78DGC; both compounds
have the molecular formula C2H6O.
    Isomers may be classified as constitutional isomers or stereoisomers.
Constitutional isomers differ in constitution or connectedness, relating to
the question as to which atoms are linked to which others and how. Dimethyl
ether and ethanol (Fig. 1) are constitutional isomers. In dimethyl ether each
carbon is connected to three hydrogen atoms and the one oxygen atom; the two
carbon atoms are thus equivalent. In ethyl alcohol (ethanol) one carbon is
linked to three hydrogen atoms and the other carbon; the second carbon is
linked to the first carbon, two hydrogens, and the oxygen atom which, in turn,
is linked to the sixth hydrogen atom; the two carbon atoms are not equivalent.
Stereoisomers, in contrast, have the same constitution but differ in the
three-dimensional array of the atoms in space, called configuration.


    Constitutional isomers.  Constitutional isomers have been subdivided into
functional isomers, positional isomers, and chain isomers.
    Functional isomers (Fig. 1) differ in functional group, that is, the group
(or groups) most material in determining chemical behavior. In the third
example shown in Fig. 1 (propionaldehyde) the three compounds all correspond
to the molecular formula C3H6O, but the first one has an aldehyde function,
the second combines a double bond with an alcohol function, and the third one
has an epoxide function.
    Positional isomers (Fig. 2) have the same functional group but differ in
its position along a chain or in a ring. Closely related are chain isomers
which also have the same functional group or groups but differ in the shape of
the carbon chain (Fig. 3a); quite similar are ring isomers (Fig. 3b) which
differ in the size of one or more rings. Ring and chain isomers together are
sometimes called skeletal isomers.


    Stereoisomers. Compounds which have not only the same molecular formula
but also the same constitution (connectivity of atoms) but which differ in the
disposition of the atoms in space are called stereoisomers. Stereoisomers, in
turn, are subdivided into two types: those that are mirror images of each
other, called enantiomers, and those which are not mirror images, called
diastereomers or diastereoisomers.
    Enantiomers are unique in that they always come in pairs. Either a
molecule is superposable with its mirror image, in which case it does not have
an enantiomer, or it is not superposable with its mirror image, in which case
it has one and only one enantiomer (since an object can have only one mirror
image). Molecules which are not superposable with their mirror images are
called chiral; those which are so superposable are called achiral.
Enantiomers are much more alike than are other sets of isomers (constitutional
isomers or diastereomers); thus they have the same melting point, boiling
point, free energy, spectral properties, x-ray diffraction pattern, and so on.
    Diastereomers have the same constitution but different spatial arrangement
and are not mirror images. They resemble constitutional isomers in that there
may be more than two isomers in a set and that their physical, energetic, and
spectral properties are generally quite distinct. See CONFORMATIONAL ANALYSIS;
OPTICAL ACTIVITY; STEREOCHEMISTRY; TAUTOMERISM.

[Ernest L. Eliel]

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�6v�2v�32sMolecular orbital theory �21m �33s
    A quantum-mechanical (mathematical) model for the electronic structure and
chemical bonding of a molecule. Each electron is associated with its
particular wave function psi, which spans the entire molecule. Each molecular
orbital, as these wave functions are called, is definable by certain quantum
numbers which govern its shape and energy. In addition to the spatial quantum
numbers, each electron possesses a spin quantum number which has a value of
either PLUS/MINUS 1/2.	See  QUANTUM NUMBERS.
    It is reasonable that the form of the molecular orbital in the vicinity of
a particular atom should be similar to that of an atomic orbital located on
that atom, since the forces on the electron will be chiefly those due to the
nucleus of the atom and other electrons near that nucleus. Hence, a frequently
used approximation is to express the molecular orbital as a linear combination
of atomic orbitals which include all the atomic centers. Thus, Eq. (1) can be

     psi = SUMMATION (in terms of i) C(sub i) phi(sub i)		   (1)

written, where the phi(sub i) are normalized atomic orbitals located on the
various atoms and the C(sub i) are coefficients which determine the contribution
of the atomic orbitals to the molecular orbital. The set of atomic orbitals
phisubi used to construct the molecular orbital is called the basis set for
the molecular orbital.	See  NONRELATIVISTIC QUANTUM THEORY.

E = [INTEGRATION psi*SCRIPT H psi d tau]/[INTEGRATION psi*psi d tau]	    (2)

    The energy of the system is given by Eq.  (2), where SCRIPT H is the
hamiltonian operator which takes into account all the kinetic and potential
energy contributions, and dtau is the volume element for the electron.	Since
the energy will depend upon the coefficients C(sub i), application of the
variation principle will specify those coefficients for which the energy is a
minimum.  This approach requires the solution of the secular determinant, Eq.
(3), where S(sub ij) is the overlap integral,

		   |H(sub ij) - ES(sub ij)| = 0 			   (3)

INTEGRATION (phi(sub i)*phi(sub j) d TAU, and H(sub ij) = INTEGRATION
(phi[sub i])*SCRIPT H phi[sub j] d TAU.
Evaluation of the secular determinant results in
n values of the energy E, where n is the number of phi functions used to
construct the molecular orbital.  Each energy value will generate its own set of
coefficients C(sub i), so there will be a set of n molecular orbitals,
PSI(sub n), one  associated with each energy Esubn.  Therefore, the number of
molecular orbitals  will be identical with the number of atomic orbitals
used.	See MATRIX MECHANICS; QUANTUM CHEMISTRY. 
    Applied to the simple system H2+ in which only 1 s	orbitals on each of
the hydrogen atoms A and B are used to construct the molecular orbitals, the
theory results in two molecular orbitals, Eqs. (4) and (5).

(b) = (1/SQUARE ROOT[2]) phi(sub A)
(1s) + (1/SQUARE ROOT[2]) phi(sub B) (1S)				     (4)

psi(a) = (1/SQUARE ROOT[2]) phi(sub A) (1s) -
(1/SQUARE ROOT[2]) phi(sub B) (1S)					     (5)

(5)Contour diagrams of the electron densities psi*psi for the two functions
are given in Fig. 1. The numerical values on the diagrams are for psi*psi =
constant evaluated in atomic units. In the case of the bonding orbital
psisubb, the electron density is concentrated between the two nuclei while the
antibonding orbital psisuba possesses a nodal plane in this region and the
charge is pushed away from the nuclei. Note that both orbitals have axial
symmetry along the line connecting the two nuclei. Such orbitals are called
sigma (sigma) orbitals and can result from combinations of atomic orbitals
whose charge densities prior to bonding lie along the line. Some typical
examples of such orbitals are shown in Fig. 2.
    Molecular orbitals, bonding and antibonding, can also be constructed by
linear combinations of atomic orbitals such that charge densities occur above
and below the line connecting the two nuclei, but a nodal plane exists along
the line. For example, combinations of psubx orbitals on adjacent atoms can
result in bonding and antibonding orbitals as illustrated in Fig. 3. The
resultant orbitals are called pi orbitals. The psuby orbitals on each
of the atoms in a diatomic molecule are capable of analogous combinations. In
addition, pi-bonding can occur between p and d or between d and d
orbitals. Generally, pi-interactions are not as strong as sigma-interactions,
so that the bonding orbital is not lowered nor the antibonding orbital raised
in energy as much in pi-bonding compared to sigma-bonding.  See  CHEMICAL
BONDING; MOLECULAR STRUCTURE AND SPECTRA.
 �34s[Richard F. Fenske]
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�6v�2v�32sMolecular pathology   �21m �33s 
    Pathology is a branch of biology that deals with the nature of disease. 
Molecular pathology is a segment of the more general discipline and deals 
specifically with the study of the bases and mechanisms of disease on a 
molecular or chemical level. The field includes the study of protein 
alterations, metabolic disorders, and chromosome aberrations.  See  CHROMOSOME 
ABERRATION; METABOLIC DISORDERS; PROTEIN. 
    The major problems that face molecular pathology involve the elucidation 
of the mechanism of cellular injury, including cell death, cellular 
regeneration, malignant transformation of cells to cancerous tissue, 
multicellular interactions (including the immune response), autoimmune disease 
and inflammation, renal disease, and birth defects. Molecular pathology also 
must unwind the mysteries of aging, degenerative disease, and central nervous 
system dysfunction, including mental illness.  See  CONGENITAL ANOMALIES; 
DEATH; GERONTOLOGY; IMMUNOPATHOLOGY; MENTAL ILLNESS; ONCOLOGY; PATHOLOGY. 
 �34s[Edward A. Smuckler]
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�6v�2v�32sMolecular physics   �21m �33s 
    The study of the physical properties of molecules. Molecules possess a far 
richer variety of physical and chemical properties than do isolated atoms. 
This is attributable primarily to the greater complexity of molecular 
structure, as compared to that of the constituent atoms. Molecules also 
possess additional energy modes because they can vibrate; that is, the 
constituent nuclei oscillate about their equilibrium positions and rotate when 
unhindered. These modes given rise to additional spectroscopic properties, as 
compared to those of an atom; molecular spectroscopy in the optical, infrared, 
and microwave regions is one of the physical chemist's most powerful means of 
identifying and understanding molecular structure. Molecular spectroscopy has 
also given rise to the rapidly growing field of molecular astronomy. 
    Molecular physics is primarily concerned with the study of properties of 
isolated molecules, as contrasted to the more general study of molecular 
reactions, which is the domain of physical chemistry. Such properties, in 
addition to the broad field of spectroscopy, include electron affinities (for 
the formation of molecular negative ions); polarizabilities (the 
``distortability'' of the molecule along its various symmetry axes by external 
electric fields); magnetic and electric multipole moments, attributable to the 
distributions of electric charge; currents and spins of the molecule; and the 
(nonreactive) interactions of molecules with other molecules, atoms, and ions. 
  See  COSMOCHEMISTRY; INFRARED SPECTROSCOPY; INTERMOLECULAR FORCES; MICROWAVE 
SPECTROSCOPY; MOLECULAR BEAMS; MOLECULAR STRUCTURE AND SPECTRA; SPECTROSCOPY. 
 �34s[Benjamin Bederson]
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�6v�2v�32sMolecular sieve �21m �33s
    Any one of the crystalline metal aluminosilicates belonging to a class of
minerals known as zeolites. An important characteristic of the zeolites is
their ability to undergo dehydration with little or no change in crystal
structure. The dehydrated crystals are honeycombed with regularly spaced
cavities interlaced by channels of molecular dimensions which offer a very
high surface area for the absorption of foreign molecules.
    The basic formula for all crystalline zeolites can be represented as


      M (sub 2/n)O:Al(sub 2)O(sub 3):xSiO(sub 2):yH(sub 2)O

where M represents a metal ion and  n  its valence. The crystal structure
consists basically of a three-dimensional framework of SiO4 and AlO4
tetrahedrons (see illustration). The tetrahedrons are cross-linked by the
sharing of oxygen atoms, so that the ratio of oxygen atoms to the total of
silicon and aluminum atoms is equal to 2. The electrovalence of the
tetrahedrons containing aluminum is balanced by the inclusion of cations in
the crystal. One cation may be exchanged for another by the usual ion-exchange
techniques. The size of the cation and its position in the lattice determine
the effective diameter of the pore in a given crystal species.

    The properties of molecular sieves as adsorbents which distinguish them
from nonzeolitic adsorbents are (1) the relatively strong coulomb fields
generated by the adsorption surface and (2) the uniform pore size; the pore
size is controlled, in a given crystal species, by the associated cation.
    The basic characteristics of molecular sieves are utilized commercially in
several production and research applications. Their absorption properties make
them useful for drying, purification, and separations of gases and liquids.
Conversely, molecular sieves can be preloaded with chemical agents, which are
thereby isolated from the reactive system in which they are dispersed until
released from the adsorbent either thermally or by displacement by a more
strongly adsorbed compound. They are also used as cation exchange media and as
novel catalysts and catalyst supports. See ADSORPTION; GAS CHROMATOGRAPHY; ION
EXCHANGE; ZEOLITE.
 �34s[Rolland L. Mays]
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�6v�2v�32sMolecular structure and spectra �21m �33s
    Until the advent of quantum theory, ideas about the structure of molecules
evolved gradually from analysis and interpretation of the facts of chemistry.
Chemists developed the concept of molecules as built from atoms in definite
proportions, and identified and constructed (synthesized) a great variety of
molecules. Later, when the structure of atoms as built from nuclei and
electrons began to be understood with the help of quantum theory, a beginning
was made in seeing why atoms can combine in definite ways to form molecules;
also, infrared spectra began to be used to obtain information about the
dimensions and the nuclear motions (vibrations) in molecules. However, a
fundamental understanding of chemical bonding and molecular structure became
possible only by application of the present form of quantum theory, called
quantum mechanics. This theory makes it possible to obtain from the spectra of
molecules a great deal of information about the nature of molecules in their
normal as well as excited states, and about dissociation energies and other
characteristics of molecules.  See  CHEMICAL BONDING.

    Molecular sizes.  The size of a molecule varies approximately in
proportion to the numbers and sizes of the atoms in the molecule. Simplest are
diatomic molecules. These may be thought of as built of two spherical atoms of
radii  r  and  r ', flattened where they are joined. The equilibrium value  R
sube of the distance  R  between their nuclei is then smaller than the sum of
the atomic radii. However, the nuclei of atoms in two different molecules
cannot normally approach more closely than a distance  r + r ';  r  and  r '
are called the van der Waals radii of the atoms.
    To describe a polyatomic molecule, one must specify not merely its size
but also its shape or configuration. For example, carbon dioxide (CO2) is a
linear symmetrical molecule, the O - C - O angle being 180DG. The H - O - H
angle in the nonlinear water (H2O) molecule is 105DG. Many molecules which are
essential for life contain thousands or even millions of atoms. Proteins are
often coiled or twisted and cross-linked in ways which are important for their
biological functioning.

    Dipole moments.  Most molecules have an electric dipole moment. In atoms,
the electron cloud surrounds the nucleus so symmetrically that its electrical
center coincides with the nucleus, giving zero dipole moment; in a molecule,
however, these coincidences are disturbed, and a dipole moment usually results.
    Thus, when the atoms of HCl come together, there is some shifting of the
H-atom electron toward the Cl. A complete shift would give HsuppCl-, which
would constitute an electric dipole of magnitude  eR sube, where  e  is the
electronic charge. But in fact the dipole moment is only 0.17  eR sube. This
is because the actual electronic shift is only fractional. For further
discussion  see  ELECTRONEGATIVITY.

    Molecular polarizability.  In the preceding consideration of dipole
moments, the discussion has been in terms of atoms and molecules free from
external forces. An electric field pulls the electrons of an atom or molecule
toward it and pushes the nuclei away, or vice versa. This action creates a
small induced dipole moment, whose magnitude per unit strength of the field is
called the polarizability.

    Molecular energy levels.  The states of motion of nuclei and electrons in
a molecule, or of electrons in an atom, are restricted by quantum mechanics to
special forms with definite energies. The state of lowest energy is called the
ground state; all others are excited states. In analogy to water levels, one
speaks of energy levels. Excited states exist only momentarily, following an
electrical or other stimulus.  See  QUANTUM CHEMISTRY; QUANTUM MECHANICS.

ms pgs. 4534-453511:50
    Excitation of an atom consists of a change in the state of motion of its
electrons. Electronic excitation of molecules can also occur, but
alternatively or additionally, molecules can be excited to discrete states of
vibration and rotation.
    In a diatomic vibration, R varies periodically above and below Rsube. The
possible vibration energies are related to the potential curve, which shows
how the energy of attraction U(R) of the atoms varies with R. The U(R) curve
and vibrational levels for the ground electronic state of H2 are shown in the
illustration. For stable (attractive) U(R) curves, the vibrational levels
decrease in spacing as v increases, until finally, as the spacing approaches
zero, a maximum v is reached; in the illustration, this is 14. After a small
gap, a dissociation continuum of energy levels then sets in. Here the atoms
have enough mutual kinetic energy to fly apart.


	     E = E (sub el) + E (sub v) + (E (sub r) + E (sub fs) +	   (1)
		E (sub hfs) + E (sub ext))

    The total energy of any molecule can be written as Eq. (1).
Both the electronic energy Esubesubl and vibration energy Esubv can be
discrete or continuous. The quantities Esubr, Esubfsubs, and Esubhsubfsubs
denote rotational, fine-structure, and hyperfine-structure energies. The last
two appear as small or minute splittings of the rotation levels. The spacings
DELTAE of adjacent discrete levels of each type are usually in the order given

	    DELTA E (sub el) > DELTA E (sub v) > DELTA E (sub r) >	    (2)
	      DELTA E (sub fs) > DELTA E (sub hfs)

in notation (2). The Esubesubxsubt term in
Eq. (1) refers to additional fine structure which appears on subjecting
molecules to external magnetic fields (Zeeman effect) or electric fields
(Stark effect).  See  FINE STRUCTURE (SPECTRAL LINES); HYPERFINE STRUCTURE;
STARK EFFECT; ZEEMAN EFFECT.
    Polyatomic molecules have much more complicated patterns of vibrational
and (usually) rotational energy levels than diatomic molecules.

    Molecular spectra.	The frequencies cv of electromagnetic spectra obey the
Einstein-Bohr equation, Eq. (3), where h is


			 hcv = E' - E"


Planck's constant. Molecular emission spectra accompany jumps in energy from
higher to lower levels; absorption spectra accompany jumps from lower to
higher levels.
    Molecular spectra can be classified as fine-structure or low-frequency
spectra, rotation spectra, vibration-rotation spectra, and electronic spectra.
Low-frequency spectra are discussed elsewhere.	See  ELECTRON PARAMAGNETIC
RESONANCE (EPR) SPECTROSCOPY; MAGNETIC RESONANCE; MICROWAVE SPECTROSCOPY;
MOLECULAR BEAMS; SPECTROSCOPY.
    Transitions between energy levels differing only in rotational state give
rise to pure rotation spectra. These typically consist of a sequence of lines
spaced almost equidistantly, and lying in the far infrared or the microwave
region.
    Spectra involving only vibrational and rotational state changes consist of
bands which lie mainly in the infrared. Each band consists of two sets of
closely spaced rotational lines, one on each side of a central frequency.
Vibration-rotation absorption bands of liquids and solutions are widely used
in chemical analysis. Here the rotational structure is blurred out, and only
an ``envelope'' is seen.  See  INFRARED SPECTROSCOPY.
    Electronic band spectra are the most general type of molecular spectra.
For any one electronic transition, the spectrum consists typically of many
bands.	See  ATOMIC STRUCTURE AND SPECTRA; ELECTRON SPIN; INTERMOLECULAR
FORCES; MOLECULAR WEIGHT; RAMAN EFFECT; RESONANCE (MOLECULAR STRUCTURE);
SCATTERING EXPERIMENTS (ATOMS AND MOLECULES); VALENCE.
 �34s[Robert S. Mulliken]
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�6v�2v�32sMolecular weight �21m �33s
    The sum of the atomic weights of all the atoms in a molecule. Atomic
weights (and therefore molecular weights) are relative weights arbitrarily
referred to an assigned atomic weight of 12.0000 exactly for the most abundant
isotope of carbon, sup1sup2C.  See  ATOMIC WEIGHT.
    The amount of a substance which has a weight in grams equal to the
molecular weight of the substance is called a gram-mole or a gram-molecular
weight. The number of molecules in a gram-mole is exceedingly large (6.02 x
10[sup 23]) and is called the Avogadro number.	See  AVOGADRO NUMBER.
    The ultimate analysis of a substance provides precise knowledge of formula
weight, namely, the sum of the atomic weights of the atoms in the simplest
formula for the substance. The molecular weight is an exact multiple of the
formula weight. For example, analysis shows that the simplest formula of
benzene is CH and that the formula weight, the sum of the atomic weights of
carbon and hydrogen, is 13.02. Each benzene molecule is a hexagonal structure
of true formula C6H6 and molecular weight 78.11. Some substances, such as
sodium chloride, contain no discrete molecules and are best assigned formula
weights but no molecular weights. Known molecules range in molecular weight
from two (hydrogen) to several billion.
    Molecular weights are determined by a variety of methods. Historically,
molecular-weight determinations were essential in establishing the
stoichiometry which forms the basis for the fundamental laws of chemistry;
today, they are made to interpret chemical reactions, to aid in determining
molecular structure and molecular shapes, to give a better understanding of
solvent-solute interaction, and to provide data for the design and control of
a great variety of useful industrial processes.
 �34s[John R. Anderson]
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�6v�2v�32sMolecular genetics   �21m �33s 
    In classical genetics the gene was regarded as a unit of information which 
directed the expression of certain features, such as the color of flowers or 
eyes or the shape of a leaf. This view of the gene has changed considerably in 
molecular genetics, since the gene is now recognized as the information 
necessary to assemble a protein molecule, which, in turn, regulates the 
production of a pigment in the eye or the growth of a leaf cell. The protein 
itself is a complicated molecule; accordingly, the gene today is seen to have 
a great deal of structure as compared to that visualized in classical 
genetics. The basic unit of information is that piece of deoxyribonucleic acid 
which makes the messenger ribonucleic acid which codes for a particular 
polypeptide chain.  See  GENE. 
    One of the characteristic features of the molecules that contain genetic 
information is the fact that they are capable  of self-replication but that 
this replication process is not entirely perfect. Occasionally during DNA 
replication an incorrect base is inserted into the newly synthesized DNA 
chain. When this DNA replicates further, it produces altered DNA and organisms 
which differ slightly from those produced by the original strain. This is the 
phenomenon of mutation, and it is the mechanism by which genetic information 
changes. Mutants make messenger RNA molecules with a modified sequence, and 
they are expressed in the formation of a protein molecule which differs 
slightly from that of the original molecule. This modification may be trivial: 
The substitution of one amino acid for another may leave the protein largely 
unchanged. In other circumstances the modified protein molecule may have some 
chemical advantage in the cell over the original molecule; the mutation may 
thus produce a cell with a selective advantage over the original cell, better 
able to survive in its environment.  See  DEOXYRIBONUCLEIC ACID (DNA); 
MUTATION; PROTEIN. 
    Genetic information is encoded linearly in the DNA molecule. An important 
activity in molecular genetics is that of determining the sequence of genes 
found in the DNA. This activity has been very successful in small organisms 
such as bacteria or viruses in which genetic maps have been made relating 
positions in the bacterial genome to the production of specific proteins. Such 
genetic maps are useful in the study of the overall phenomena of cell division 
and gene expression.  See  GENETIC MAPPING. 
    Genes are often expressed in functionally related groups. For example, if 
a bacterial cell is exposed to a new kind of sugar molecule which it must 
utilize in order to grow, the cell often has the capacity to adapt to the new 
type of nutrient by forming proteins capable of metabolizing this particular 
type of sugar. A great deal of study has been carried out on the mechanisms 
which control gene expression. This work has shown the existence of regulator 
genes that act in such a way as to modify the expression of other genes. Thus 
not all genes in the bacterial cell are structural in the sense that they 
define proteins used in enzymatic activities. Other types of genes exist which 
control the expression of structural genes.  See  GENETICS. 
 �34s[Arthur Roberts]
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�6v�2v�32sMolecule   �21m �33s 
    A molecule may be thought of either as a structure built of atoms bound 
together by chemical forces or as a structure in which two or more nuclei are 
maintained in some definite geometrical configuration by attractive forces 
from a surrounding swarm of negative electrons. Besides chemically stable 
molecules, short-lived molecular fragments called free radicals can be 
observed under special circumstances.  See  CHEMICAL BONDING; FREE RADICAL; 
MOLECULAR STRUCTURE AND SPECTRA. 
 �34s[Robert S. Mulliken]
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�6v�2v�32sMole (chemistry) �21m �33s
    One mole is the mass numerically equal (in grams) to the relative
molecular mass of a substance. It is the amount of a substance that contains
the same number of molecules as there are atoms in 0.012 kilogram of
carbon-12. The mole is an individual unit of mass, that is, it relates only to
a given substance. If the relative molecular mass is MU, 1 mole = MU grams,
and the M of one mole is MU g/mol.
    Moles of different elements or compounds contain the same number of
molecules. This number, called the Avogadro number N0, is 6.023 x 10(sup 23).
    Since they contain the same number of molecules, moles of ideal gases
occupy the same volume at the same temperature and pressure. At 0DGC and 1
atmosphere (101,325 pascals) pressure, this volume, called the molar volume,
is 22.4 liters.  See  AVOGADRO NUMBER; GAS; RELATIVE MOLECULAR MASS.
 �34s[Thomas C. Waddington]
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�6v�2v�32sMole (medicine)   �21m �33s 
    A local growth of the skin which is usually rounded, raised, and 
pigmented. Related to birthmarks, it is correctly called a nevus. Nevi may be 
present at birth or may develop later. They are almost universally present but 
vary greatly in size, color, and distribution, as well as in components. Most 
nevi are soft, but some are corrugated and warty. Those present at birth 
represent a form of birthmark, cellular in nature, in contrast to vascular 
birthmarks, such as port-wine stains. 
    All nevi may become malignant, but controversy exists over which nevi are 
precursors of malignant melanomas, malignant tumors, of melanin-producing 
cells. Mechanical irritation, trauma, and inadequate removal may predispose 
nevi to malignant changes. Any nevus which shows a change in color, size, or 
hardness should be referred to a physician. 
 �34s[N. Karle Mottet]
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�6v�2v�32sMole (zoology)   �21m �33s 
    A mammal belonging to the family Talpidae. There are 19 species 
distributed on all continents except Australia. Moles are insectivores, 
feeding mainly on earthworms and insect larvae, and are highly specialized for 
their burrowing habits. 
    The body is stout and cylindrical and with a short neck (see 
illustration). The eyes and external ears are small or vestigial, and the 
conical head terminates a long naked muzzle. The forelimbs are especially 
adapted for digging, having very powerful muscles and a spadelike bony 
structure. Mating occurs in the spring; after a gestation period of 30 - 40 
days a litter of 3 - 7 young are born. 
    Moles are solitary animals and rarely come aboveground. Molehills are seen 
where temporary burrows are made, but permanent runs are made by compressing 
the earth so there are no molehills to betray their presence. Each mole lives 
in its own fortress constructed as a central chamber around which are two 
circular passages, one at a higher and the other at a lower level, connected 
by short passages. The upper passage connects with the central chamber and the 
lower one leads to the main exit. There is also an emergency exit leading to 
the main exit from below the central chamber.  See  INSECTIVORA; MAMMALIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMollicutes   �21m �33s 
    The class of bacteria comprising the mycoplasmas, with the single order 
Mycoplasmatales, and two families, Mycoplasmataceae and Acholeplasmataceae. 
    The organisms are gram-negative; though motility has been described for a 
few species, they are usually nonmotile; they are nonsporing; serum is 
required for growth. 
    Several Mycoplasma species are important pathogens of humans and animals, 
while others are common saprophytes of the mucous membranes of the lower 
genital tract and the oropharynx. M. pneumoniae, previously regarded as a 
virus, and known under the name of the Eaton agent, causes cold agglutin - 
associated primary atypical pneumonia. M. hominis, which is a common 
inhabitant of the lower genital tract, appears to be a potential pathogen. M. 
salivarium and M. orale are common members of the normal oral microflora. The 
role of M. fermentans, found rarely in the human genital tract, is obscure. 
The same holds true for the T-mycoplasmas (T for tiny) that are very common 
inhabitants of the genital tracts of males and females, and of which at least 
seven different serotypes are known. Two varieties of the type species M. 
mycoides are the etiological agents of contagious pleuro-pneumonia of cattle 
and goats. 
    Considerable interest has been shown in mycoplasmas as possible pathogens 
of plants. In plant diseases such as yellows-type disease, corn stunt, and 
alfalfa witches, mycoplasma-like structures have been demonstrated in great 
numbers by electron microscopy of thin sections of both diseased plant tissues 
and insect vectors. 
 �34s[E. A. Freundt]
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�6v�2v�32sMollusca �21m �33s
    A major phylum of the animal kingdom comprising an extreme diversity of
external body forms (oysters, clams, chitons, snails, slugs, squid, and
octopuses among others), all based on a remarkably uniform basic plan of
structure and function. The phylum name is derived from mollis, meaning soft,
referring to the soft body within a hard calcareous shell, which is usually
diagnostic. Soft-bodied mollusks make extensive use of ciliary and mucous
mechanisms in feeding, locomotion, and reproduction.
    The Mollusca constitute a successful phylum; there are probably over
110,000 living species of mollusks, a number second only to that of the phylum
Arthropoda, and more than double the number of vertebrate species. More than
99% of living molluscan species belong to two classes: Gastropoda (snails) and
Bivalvia. Ecologically, these two classes can make up a dominant fraction of
the animal biomass in many natural communities, both marine and fresh-water.
Certain bivalve species are the most abundant marine benthic animals, and
computations have suggested that one of these may encompass a larger
``standing crop'' biomass of animal tissue than any other single animal
species on the planet.
    The phylum Mollusca is divided into seven distinct extant classes, three
of which (Gastropoda, Bivalvia, and Cephalopoda) are of major significance in
terms both of species numbers and of ecological bioenergetics, and one extinct
class. An outline of their classification follows.

Class Monoplacophora (mainly fossil;
      but one living genus, Neopilina)
Class Aplacophora
Class Polyplacophora
Class Scaphopoda
Class Rostroconchia (fossil only)
Class Gastropoda
    Subclass: Prosobranchia
	      Opisthobranchia
	      Pulmonata
Class Bivalvia (or Pelecypoda)
    Subclass: Protobranchia
	      Lamellibranchia
	      Septibranchia
Class Cephalopoda (or Siphonopoda)
    Subclass: Nautiloidea (Tetrabranchia)
	      Ammonoidea (fossil only)
	      Coleoidea (Dibranchia)


    Functional morphology.  The unique basic plan of the Mollusca involves the
different modes of growth and of functioning of the three distinct regions of
the molluscan body (see illustration). These are: the head-foot with some
nerve concentrations, most of the sense organs, and all the locomotory organs;
the visceral mass (or hump) containing organs of digestion, reproduction, and
excretion; and the mantle (or pallium) hanging from the visceral mass and
enfolding it and secreting the shell. In its development and growth, the
head-foot shows a bilateral symmetry with an anterioposterior axis of growth.
Over and around the visceral mass, however, the mantle-shell shows a biradial
symmetry, and always grows by marginal increment around a dorsoventral axis.
It is of considerable functional importance that a space is left between the
mantle-shell and the visceral mass forming a semi-internal cavity; this is the
mantle cavity or pallial chamber within which the typical gills of the
mollusk, the ctenidia, develop. This mantle cavity is almost diagnostic of the
phylum; it is primarily a respiratory chamber housing the ctenidia, but with
alimentary, excretory, and genital systems all discharging into it.
    The shell is always underlain by the mantle, a fleshy fold of tissues
which has secreted it. The detailed structure of the shell and of the mantle
edge (with three functionally distinct lobes) is also consistent throughout
the Mollusca. The shell is made up of calcium carbonate crystals enclosed in a
meshwork of tanned proteins. It is always in three layers.
    Each of the eight classes of the Mollusca has a characteristic body form
and shell shape. Two classes are enormous (Gastropoda and Bivalvia), one of
moderate extent (Cephalopoda), the others being minor by comparison. The
Gastropoda constitute a diverse group with the shell usually in one piece.
This shell may be coiled as in typical snails - that is, helicoid or turbinate
- or it may form a flattened spiral, or a short cone as in the limpets, or it
may be secondarily absent as in the slugs. Most gastropods are marine, but
many are found in fresh waters and on land; in fact, they are the only
successful nonmarine mollusks.
    The cardiac structures of mollusks are also closely linked to the pallial
complex. If there is a symmetrical pair of ctenidia,  there will be a
symmetrical pair of auricles on either side of the muscular ventricle of the
heart; if one ctenidium, one auricle; if four ctenidia, four auricles. Body
fluids in mollusks are almost all blood, and body cavities are almost all
hemocoel. The respiratory pigment is usually hemocyanin in solution, so that
neither circulatory efficiency nor blood oxygen-carrying capacity is high.
    Uniquely molluscan is the use of cilia in ``sorting surfaces,'' which can
segregate particles into different size categories and send them to be
disposed of in different ways in several parts of the organism. In a simpler
type of sorting surface, the epithelium is thrown into a series of ridges and
grooves, the cilia in the grooves beating along them and the cilia on the
crests of the ridges beating across them. Thus, fine particles impinging on
the surface can be carried in the direction of the grooves, while larger
particles are carried at right angles. Such sorting surfaces occur both
externally on the feeding organs and internally in the gut of many mollusks.
For example, on the labial palps of bivalves, they are used to separate the
larger sand grains (which are rejected) from the smaller microorganisms which
then pass to the mouth.
    The range in levels of complexity of molluscan nervous systems is
comparable to that found in the phylum Chordata. The four-strand nervous
system with one pair of tiny ganglia found in chitons is not dissimilar to the
neural plan in turbellarian flatworms. In contrast, the nervous system and
sense organs of a cephalopod like an octopus are equaled and exceeded only by
those of some birds and mammals.
    In all primitive molluks, the sexes are separate, and external
fertilization follows the spawning of eggs and sperm into the sea. In more
advanced mollusks, eggs are larger (and fewer), fertilization may become
internal (with complex courtship and copulatory procedures), and larval stages
may be sequentially suppressed. A remarkably large number of mollusks
(including many higher snails) are hermaphroditic. Although some are truly
simultaneous hermaphrodites, many more show various kinds of consecutive
sexuality. Most often the male phase occurs first, and these species are said
to show protandric hermaphroditism.

    Distributional ecology.  Mollusks are largely marine. The extensive use of
ciliary and mucous mechanisms in feeding, locomotion, reproduction, and other
functions demands a marine environment for the majority of molluscan stocks.
Apart from a small number of bivalve genera living in brackish and fresh
waters, all nonmarine mollusks are gastropods.
    Despite the soft, hydraulically moved bodies and relatively permeable
skins typical of all mollusks, some snails are relatively successful as land
animals, although they are largely limited to more humid habitats. The primary
physiological requirements for life on land concern water control, conversion
to air breathing, and temperature regulation.
    In the sea, all classes of mollusks are found, and all habitats have
mollusks. Protobranchiate bivalves are found at depths of over 30,000 ft (9000
m). Although ecologically cephalopod mollusks are limited to the sea, there
are sound reasons for claiming modern cephalopods as the most highly organized
invertebrate animals. The functional efficiencies of jet propulsion and of
massive brains in squid, cuttlefish, and octopuses have not been paralleled in
their other physiological systems.
    In addition to the extreme diversity of external body form exhibited by
different mollusks, they show a remarkable diversity in their ecological
distribution and life styles. However, the basic molluscan plan of structure
and function always remains recognizable.  See	APLACOPHORA; BIVALVIA;
CEPHALOPODA; GASTROPODA; LAMELLIBRANCHIA; MONOPLACOPHORA; POLYPLACOPHORA;
SCAPHOPODA; SNAIL.
 �34s[W. D. Russell-Hunter]
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�6v�2v�32sMolluscum contagiosum   �21m �33s 
    A skin disease caused by a virus and characterized by the appearance of 
small discrete lesions in groups on the face, arms, or genitalia. The lesions 
are firm and pearly white with a sharply indented central core, and yield an 
infectious filtrate which produces the disease when inoculated into human 
volunteers. The disease, which may be epidemic in children, occurs in all ages 
and is worldwide in distribution. 
 �34s[Alice E. Moore]
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�6v�2v�32sMolpadida   �21m �33s 
    An order of sea cucumbers belonging to the Apodacea and characterized by a 
short, plump body bearing a taillike prolongation. Simple finger-shaped or 
feathery tentacles surround the mouth, and respiratory trees are present. The 
tube feet are restricted to a zone about the cloaca. They feed on bottom 
material and live buried in sand or mud, with the body somewhat bent into a 
U-shape so that the anterior and posterior ends are directed upward and 
project through the surface of the sediment. Molpadida are sedentary and 
seldom emerge from the burrow, though they may be disturbed by storms and then 
cast ashore in great numbers. The one family, the Molpadidae, comprises 12 
genera with species in most seas and usually found at moderate depths.  See  
APODACEA; ECHINODERMATA; HOLOTHUROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sMolybdenite   �21m �33s 
    A mineral having composition MoS2. Molybdenite is the chief ore of 
molybdenum. It crystallizes in the hexagonal system, but crystals are rare and 
when found are hexagonal plates. It is commonly in scales or foliated masses. 
The mineral has a greasy feel. The hardness is 1.5 (Mohs scale) and the 
specific gravity is 4.7. The luster is metallic and the color lead gray. 
Molybdenite and graphite have long been confused because of their nearly 
identical physical properties. They can be distinguished by the streak left on 
glazed paper, black for graphite and green for molybdenite. Molybdenite has 
been used as a lubricant. 
    Molybdenite occurs in various places in Norway, Sweden, Australia, 
England, China, and Mexico. In the United States, molybdenite is found in 
small amounts at many localities but the most important occurrence is at 
Climax, Colorado.  See  MOLYBDENUM. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sMolybdenum �21m �33s
    A chemical element, Mo, atomic number 42, and atomic weight 95.95; one of
the transition elements. A silver-gray metal with a density of 10.22 g/cm(sup 3)
(5.907 oz/in.[sup 3]), molybdenum melts at 2610DGC (4730DGF). The table gives
selected physical properties for metallic molybdenum.  See  TRANSITION
ELEMENTS.
    Molybdenum is found in many parts of the world, but relatively few
deposits are rich enough to warrant recovery costs. Most molybdenum comes from
mines where its recovery is the primary objective of the operation. The
remainder is recovered as a by-product of certain copper-mining operations.
See  MOLYBDENITE.
    Molybdenum forms compounds in which it displays valence states of 0, 2+,
3+, 4+, 5+, and 6+. Molybdenum as an ionizable cation has not been observed,
but cationic species, such as molybdenyl, MoO2(sup 2+), are known to exist.
The chemistry of molybdenum is extremely complex and, with the exception of
the halides and chalcogenides, very few simple compounds are known.
    Molybdenum dioxide and trioxide are the most common and most stable
oxides; other reported oxides are metastable and are essentially laboratory
species.
    Molybdic acid, H2MoO4 (or MoO3.H2O) forms a series
of stable, normal salts of the types M2(sup 2+)MoO4, M(sup 2+)MoO4, and
M2(sup 3+)(MoO4)3. Polymeric or isopoly molybdates may be formed by
acidification of a molybdate solution or, in some cases, by heating the normal
molybdates. Hydrogen peroxide reacts with a number of molybdates to form a
series of anionic peroxy compounds. Another group of molybdenum compounds are
the heteropoly electrolytes, a large, fundamental family of salts and free
acids, each member containing a complex and high-molecular-weight anion.
Molybdenum also forms halides and oxyhalides, representing a wide range in
stability, and a series of homologous compounds with S, Se, and Te that are
somewhat similar to the oxides.
 �34s[Hugh Morrow, III]
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�6v�2v�32sMolybdenum intoxication   �21m �33s 
    Traces of molybdenum are found in virtually all plant and animal tissues, 
and many authorities regard molybdenum as essential in nutrition. Its presence 
in the diet increases the activity of an essential enzyme called xanthine 
oxidase, and it is an essential element in various flavin-dependent enzymes 
such as aldehyde oxidase, hydrogenase, and nitrate reductase. 
    Acute or chronic excessive intake of molybdenum does not produce a defined 
pathologic lesion in humans; however, in lower animals, particularly the 
ruminants, pastures containing excessive amounts of molybdenum may produce a 
disease called teart. This is characterized by anemia, poor growth, and 
diarrhea.  
    Exposure to molybdenum disulfide, such as occurs in lubricant sprays, 
molybdenum hydroxide mist, or during electroplating, produces a wide variety 
of symptoms. In general the toxicity of these compounds is very low, but 
soluble products are considerably more toxic. Exposure to these substances may 
produce irritation of the eyes and mucous membranes of the nose and throat. An 
anemia may be a feature of molybdenum toxicity.  See  METALS, PATHOLOGY OF; 
MOLYBDENUM.  
 �34s[N. Karle Mottet]
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�6v�2v�32sMoment of inertia   �21m �33s 
    A relation between the area of a surface or the mass of a body to the 
position of a line. The analogous positive number quantities, moment of 
inertia of area and moment of inertia of mass, are involved in the analysis of 
problems of statics and dynamics respectively. 
    The moment of inertia of a figure (area or mass) about a line is the sum 
of the products formed by multiplying the magnitude of each element (of area 
or of mass) by the square of its distance from the line. The moment of inertia 
of a figure is the sum of moments of inertia of its parts. 
    For a body of mass distributed continuously within volume V, the movement 
of inertia of the mass about the X axis is given by either IsubX = 
INTEGRALrsubXsup2 dm or IsubX = INTEGRALrsubxsup2RHO dV, where dm is the mass 
included in volume element dV at whose position the mass per unit volume is 
RHO (see illustration). Similarly IsubY = INTEGRALrsubysup2RHO dV and IsubZ = 
INTEGRALrsubZsup2RHO dV. 
    The moments of inertia of a figure about lines which intersect at a common 
point are generally unequal. The moment is greatest about one line and least 
about another line perpendicular to the first one. A set of three orthogonal 
lines consisting of these two and a line perpendicular to both are the 
principal axes of inertia of the figure relative to that point. If the point 
is the figure's centroid, the axes are the central principal axes of inertia. 
The moments of inertia about principal axes are principal moments of inertia.  
See  CENTROIDS (MATHEMATICS); PRODUCT OF INERTIA; RADIUS OF GYRATION. 
 �34s[Nelson S. Fisk]
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�6v�2v�32sMomentum �21m �33s
    Linear momentum is the product of the mass and the linear velocity of a

			     P = mv					    (1)

body. It is defined by Eq. (1),
where m is the mass and v is the linear velocity. Since linear momentum is the
product of a scalar and a vector quantity, it is a vector and hence has both
magnitude and direction.
    According to the general statement of Newton's second law, for a force F,

			     F = dP/dt					   (2)

a momentum P, and a time t, Eq. (2) holds.
Thus Newton's second law involves the time rate of change of momentum. Changes
of momentum are important in collision processes.  See	COLLISION (PHYSICS).
    When a group of bodies is subject only to forces that members of the group
exert on one another, the total momentum of the group remains constant.  See
ANGULAR MOMENTUM; CONSERVATION OF MOMENTUM; IMPULSE (MECHANICS).
 �34s[Paul W. Schmidt]
�33s

A700000002107�254s�5v�1v1106
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�6v�2v�32sMonazite   �21m �33s 
    A phosphate (mineral) of the cerium metals, (Ce,La,Y,Th) (PO4). Ordinarily 
lanthanum, La, is present in about 1:1 ratio with cerium. Small amounts of the 
yttrium, Y, earths substitute for Ce and La. Thorium substitutes for Ce and La 
and generally ranges up to 10% ThO2. A series of monazite minerals ranging up 
to 30% ThO2 probably exists. Thorium-free monazite is rare. Uranium, U, in 
small amounts has been reported. 
    Monazite crystallizes in the monoclinic system. Crystals are prismatic and 
generally minute but occasionally large. Colors range from white through 
shades of yellow, green, and brown. 
    Monazite occurs in many regions of the world, but the major production 
comes from placer deposits in Idaho, South Carolina, and Florida in the United 
States, and in India, Brazil, and the Union of South Africa. Recovered 
monazite concentrates are sources of thorium, thorium compounds, and cerium 
metals.  See  CERIUM; RADIOACTIVE MINERALS; RARE-EARTH ELEMENTS. 
 �34s[Wayne R. Lowell]
�33s 
  
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�6v�2v�32sMongoose   �21m �33s 
    The name for about 39 species of carnivorous mammals which are members of 
the family Viverridae. This family also includes the civets and genets. 
Mongooses are restricted in their distribution to the warmer regions of the 
Old World, ranging from the Mediterranean into Africa and Southeast Asia. 
These are plantigrade animals about the size of a cat and have a long slender 
body, short legs, nonretractile claws, and scent glands. Some species are fair 
climbers even though the claws are nonretractile. Mongooses are predators, 
feeding on snakes, frogs, fishes, and crabs, and they are especially fond of 
bird and crocodile eggs. The mongoose cannot legally be brought into the 
United States because of its destructive habits.  See  CARNIVORA; CIVET. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sMonhysteroidea   �21m �33s 
    A superfamily of free-living nematodes with single or paired outstretched 
ovaries, amphids that are circular to cryptospiral or variants thereof, and a 
stoma which is usually shallow and unarmed. The stoma of the Sphaerolaimidae 
is spacious and deep, and that of the Siphonolaimidae is modified into a 
narrow, elongate, hollow spearlike apparatus. The majority of the species are 
around 0.04 in. (1 mm) in length. Most species probably feed on bacteria, 
yeasts, diatoms, and so on. Species inhabit marine and freshwater 
environments.  See  NEMATA. 
 �34s[Bruce Hopper]
�33s 
  
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�6v�2v�32sMoniliaceae   �21m �33s 
    A family of fungi of the order Moniliales. Mucedinaceae is the synonym for 
Moniliaceae. The sporophores, or fruit bodies, are usually lacking, but when 
present they are not aggregated into fascicles (small bundles). The hyphae and 
spores are hyaline or brightly colored, differentiating this family from the 
Dematiaceae, also in the Moniliales. The family comprises 200 genera and 2000 
species, some of which are plant or human pathogens. Important genera include 
Monilia, Penicillium, and Aspergillus.  See  DEMATIACEAE; MONILIALES. 
 �34s[N. F. Buchwald]
�33s 
  
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�6v�2v�32sMoniliales   �21m �33s 
    An order of fungi of the Fungi Imperfecti containing many plant pathogens. 
The Moniliales are divided into four families, the Moniliaceae (Mucidinaceae), 
Dematiaceae, Stilbellaceae (Stilbaceae), and Tuberculariaceae. There are 
approximately 650 genera and 5000 species known.  See  DEMATIACEAE; 
MONILIACEAE; STILBELLACEAE; TUBERCULARIACEAE. 
    Asexual (imperfect) spores are always formed free on the surface of the 
material on which the organism is living and never occur in either pycnidia or 
acervuli. There are two main types of asexual spores: thallospores and true 
conidia. Thallospores are formed by the transformation of preexisting elements 
of the thallus or vegetative hyphae. There are three kinds of thallospores: 
arthrospores, blastospores, and aleuriospores. 
    The true conidia arise on the thallus as newly formed elements and are 
always deciduous; that is, they do not remain on the hyphae. There are two 
kinds of true conidia: radula spores and phialospores. See FUNGI IMPERFECTI. 
 �34s[N. F. Buchwald]
�33s 
  
A700000002262�254s�5v�1v1107
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�6v�2v�32sMonitor   �21m �33s 
    Any of 27 species of the reptilian family Varanidae in the order Squamata, 
distributed in Africa, India, Malaysia, and Australia. They are carnivorous 
and voracious and are the largest and heaviest known lizards. They have long, 
slender forked tongues which can be protruded well beyond the mouth or 
withdrawn into a sheath at its base. The head is mobile and can be turned in 
all directions, The tail, which is round in terrestrial species and flattened 
in aquatic species, serves as a weapon. Depending on their habitat, these 
reptiles eat fish, snakes, amphibians, lizards, mammals, and birds. 
    The largest of all known lizards, the Komodo dragon (see illustration), 
was not discovered until 1912, when it was found inhabiting the small island 
of Komodo in the Dutch East Indies. It feeds on small deer and wild pigs, as 
well as members of its own species and dead animals. It has been protected, 
since it is considered to be a relict of extinct giant lizards. It is an 
extremely active and savage species, much feared by the natives.  See  LIZARD; 
REPTILIA; SQUAMATA. 
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sMonitoring of ionizing radiation   �21m �33s 
    The use of meters and special techniques to determine the absorbed dose of 
ionizing radiation received by individuals; also, the use of meters and other 
devices to determine the type of radiation, its energy spectrum and direction, 
and the absorbed dose in the various areas and inside the human body. 
Monitoring frequently is divided into personnel monitoring, building surveys, 
and area monitoring. 
    Personnel monitoring involves those operations directly associated with 
the measurement and recording of the absorbed dose received by the individual. 
Personnel monitoring includes the issuing of dosimeters; the reading, 
maintenance, and calibration of these devices; the keeping of exposure 
records; and personal contacts with individuals to determine the causes of 
exposure and to recommend procedures to limit the recurrence of exposures.  
See  DOSIMETER; FILM BADGE; HEALTH PHYSICS. 
    Building surveys are made with many types of survey or monitoring 
instruments which, for the most part, can be grouped into three classes: 
Geiger-Mueller counters, scintillation counters, and ionization chambers. This 
equipment is used to measure the dose rate and accumulated dose in various 
work areas and to estimate the surface contamination on floors, walls, 
furniture, and equipment. One of the hazards of greatest concern in many types 
of work with radioactive materials is the inhalation of airborne dusts, fumes, 
and gases. As a consequence, various types of equipment, such as air filters, 
precipitators, impingers, and charcoal collectors, have been developed to make 
collections of airborne contamination for radioisotope analysis.  See  GEIGER 
COUNTER; IONIZATION CHAMBER; SCINTILLATION COUNTER. 
    Area monitoring is concerned with the measurement of the buildup and 
spread of radioactive contamination in the air, water, and soil outside work 
areas. Many instruments used for building surveys are used in area monitoring. 
 �34s[Karl Z. Morgan]
�33s 
  
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�6v�2v�32sMonkey   �21m �33s 
    An adaptive or evolutionary grade among the primates, represented by 
members of two of the three modern anthropoid superfamilies. The New World, 
platyrrhine monkeys (Ceboidea) and Old World, catarrhine forms 
(Cercopithecoidea) probably reached a monkey level of adaptation independently 
some time after their separation from a common ancestor, perhaps 50 million 
years ago. The term ``monkey'' is not indicative of taxonomic or phylogenetic 
relationship: the closest relatives of the cercopithecoids are not the ceboid 
monkeys but the Old World apes and the humans. 
    The Ceboidea comprise two families, while the Cercopithecoidea are today 
considered to comprise only one family, with two subfamilies. A modern 
classification of the suborder Anthropoidea follows. 
1m]Suborder Anthropoidea 
    Infraorder Platyrrhini 
        Superfamily Ceboidea 
        Family Cebidae 
Subfamily Cebinae (capuchin and squirrel monkeys) 
Subfamily Callitrichinae (marmosets and tamarins) 
Family Atelidae 
Subfamily Atelinae (howler and spider monkeys) 
Subfamily Pitheciinae (saki, owl, and titi monkeys 
    Infraorder Catarrhini 
        Superfamily Parapithecoidea 
        (extinct Egyptian monkeys) 
        Superfamily Cercopithecoidea 
        Family Cercopithecidae 
Subfamily Cercopithecinae (cheek-pouched monkeys, macaques, baboons, guenons, 
and mangabeys) 
Subfamily Colobinae (leaf eaters, langurs, and colobus monkeys) 
    Superfamily Hominoidea (gibbons, great apes, and humans) 
0]    Monkeys are hard to characterize as a group because of their great 
diversity, and because much of the discussion reflects a comparison with the 
apes. Both monkeys and apes contrast with the prosimian grade in that they are 
typically large, diurnal animals that live in social groups. Monkeys differ 
from apes in their possession of a tail, a smaller brain, quadrupedal 
pronograde posture, and a usually longer face. They are generally smaller than 
apes, but large monkeys outweigh gibbons. Like almost all primates, monkeys 
are pentadactyl, with nails rather than claws on the digits in most cases. 
They have pectoral mammary glands and well-developed vision. Monkeys are 
primarily vegetarian and inhabit forested tropical or subtropical regions of 
Africa, Asia, and South America. The differences between the New and Old World 
monkeys are summarized in the table. 
    Old World species are found throughout all the warmer regions of the 
eastern Hemisphere, except Australia and Madagascar. Many of the familiar 
monkeys are included in this family, such as the rhesus macaque, Barbary 
``ape,'' mangabey, baboon, and mandrill. 
    New World species or ceboids occupy forested areas from southern Mexico to 
Argentina. They are divided into two main groups, or families (their major 
characteristics are given in the table). All are arboreal, including a few 
with prehensile tails; there is no living form, nor any evidence of a fossil 
form, that has come to the ground habitually. Familiar monkeys included in 
this group are marmosets, capuchins, sakis, howler monkeys, and spider and 
wooly monkeys.  See  PRIMATES. 
 �34s[Eric Delson]
�33s 
  
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�6v�2v�32sMonogenea   �21m �33s 
    A subclass of the Trematoda which are ectoparasites of the gills, skin, 
and orifices of fishes and, less frequently, of the esophageal tracts and 
bladders of amphibians and turtles. They have conspicuous anterior and 
posterior holdfasts, the latter usually armed. The terminal genitalia are 
frequently sclerotized. The group is characterized by sexual reproduction, 
direct development, and a single host in the life cycle. 
    The most widely used classification employs two orders, the 
Monophisthocotylea, in which the posthaptor is without discrete multiple 
suckers or clamps, and the Polyopisthocotylea, with suckers or clamps on the 
posthaptor.  
    Body shapes of the various genera are distinctive, sometimes bizarre, as 
in Vallisia, which is sickle-shaped. Paired external suckers or buccal cavity 
suckers and adhesive glands occur anteriorly. The posterior holdfast is either 
solid and armed with central anchors and marginal hooks (Gyrodactyloidea), 
sucker-shaped with anchors and hooks (Capsaloidea; see illustration), or solid 
and bearing suckers or clamps (Polyopisthocotylea). 
    Monogenea usually have direct development involving simple metamorphosis 
from the ciliated larval stage to the nonciliated juvenile. Juvenile anchors 
and hooks may be retained or replaced by adult suckers or clamps. 
Cross-fertilization or, perhaps less frequently, self-fertilization of 
hermaphroditic individuals resulting in egg capsules which hatch on the host 
or in its environment is most common.  See  TREMATODA. 
 �34s[William J. Hargis, Jr.]
�33s 
  
  
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�6v�2v�32sMonogononta   �21m �33s 
    A class of the phylum Rotifera which contains the majority of species in 
this invertebrate class. The organisms of this order are characterized by the 
presence of a single gonad in both males and females. There is a striking 
degree of sexual dimorphism, with the males being small and degenerate. The 
order is made up of three suborders: Ploima, Flosculariacea, and Collothecacea. 
    In suborder Ploima there is an exceptional diversity of form, varying from 
soft-bodied wormlike rotifers to species with variously ornamented, loricate 
shells (see illustration). Most of the free-swimming benthonic and pelagic 
rotifers belong to this suborder. Locomotion is by the ciliated corona. 
    The suborder Flosculariacea contains the spectacular sessile rotifers 
formerly known as melicertaceans of the family Flosculariidae, as well as a 
number of equally notable free-swimming forms included in the family 
Testudinellidae.  
  
    The suborder Collothecacea contains but a single family, the 
Collothecidae, made up of five genera. Most species of Collothecidae are 
sessile, and many are encased in gelatinous tubes. See ROTIFERA. 
 �34s[Elbert H. Ahlstrom]
�33s 
  
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�6v�2v�32sMonomolecular film   �21m �33s 
    A film one molecule thick; often referred to as a monolayer. Films that 
form at surfaces or interfaces are of special importance. Such films may 
reduce friction, wear, and rust, or may stabilize emulsions, foams, and solid 
dispersions. Thin films on water surfaces reduce evaporation losses, which are 
important in arid regions throughout the world. Nevertheless, the removal of 
thin films of contaminants is one of many problems in the control of 
pollution. The broad field of catalysis, which is basic to petroleum refining 
and many chemical industries, involves chemical reactions that are accelerated 
in the thin films of reactants at interfaces. Moreover, thin films containing 
proteins, cholesterol, and related compounds constitute biological membranes, 
the internal interfaces that control the complex processes of life.  See  
CATALYSIS; CELL MEMBRANES. 
    Monolayers on solids, or at liquid interfaces, may be formed by adsorption 
from the adjacent bulk phases; the process may show high specificity for 
particular chemical species. Measurements of the extent of adsorption have 
historically provided information on the composition and structure of 
monolayers formed in this way. A variety of surface-sensitive instrumental 
techniques, such as diffraction and scattering of low-energy electrons, 
neutrons, and ions, and spectroscopy of adsorbed species, have been brought to 
bear to obtain information about the structure of the surface layer and 
chemical perturbations in it.  See  ADSORPTION; SPECTROSCOPY. 
    In order to form spread monolayers which are sufficiently stable to study, 
a substance must combine low solubility and volatility with some moiety which 
attracts it to the liquid surface; for films on water, this generally means 
one or more polar functional groups. Totally nonpolar substances, such as the 
higher-molecular-weight paraffin hydrocarbons, will not spread on water 
(although they can spread on liquids of very high surface tension, such as 
mercury). Typical among the large group of substances which do form insoluble 
monolayers on water are the long-chain fatty acids and their derivatives such 
as glycerides, sterols, and many lipid substances of biological origin, 
including the fat-soluble vitamins and natural pigments such as chlorophyll. 
Many polar synthetic polymers, including polyvinyl acetate and polymethyl 
methacrylate, can be made to spread as monolayers on water; so can many 
proteins, because their tertiary structure unfolds at the air-water interface. 
  See  COLLOID; EMULSION; FOAM; INTERFACE OF PHASES; LUBRICANT; SURFACE 
TENSION; WATER CONSERVATION. 
 �34s[George L. Gaines]
�33s 
  
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�6v�2v�32sMonoplacophora   �21m �33s 
    A class of the phylum Mollusca. These univalved organisms are rare, but 
they have been found in the Atlantic and Indian oceans; the original discovery 
was in the deep Pacific trench off Costa Rica. Probably, they are 
morphologically the most primitive of living mollusks, but this does not mean 
they are ancestral to other classes.  See  MOLLUSCA. 
 �34s[Ellis L. Yochelson]
�33s 
  
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�6v�2v�32sMonopulse radar   �21m �33s 
    A radar that obtains a complete measurement of the target's angular 
position from a single echo pulse. Together with the range measurement 
performed with the same pulse, the target position in three dimensions is 
determined completely. Usually a train of echo pulses is then employed to make 
a large number of repeated measurements and produce a refined estimate, but 
this is not intrinsically necessary. An important advantage of a monopulse 
tracking radar over one employing conical scan is that the instantaneous 
angular measurements are not subject to errors caused by target scintillation. 
An additional advantage of monopulse tracking as compared to conical scan is 
that no mechanical action is required in monopulse.  See  RADAR. 
 �34s[Robert I. Bernstein]
�33s 
  
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�6v�2v�32sMonorail   �21m �33s 
    A distinctive type of materials-handling machine that provides an 
overhead, normally horizontal, fixed path of travel in the form of a trackage 
system and individually propelled hand or powered trolleys which carry their 
loads suspended freely with an intermittent motion. Because monorails operate 
over fixed paths rather than over limited areas, they differ from 
overhead-traveling cranes, and they should not be confused with such overhead 
conveyors as cableways.  See  BULK-HANDLING MACHINES; MATERIALS-HANDLING 
MACHINES.  
 �34s[Arthur M. Perrin]
�33s 
  
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�6v�2v�32sMonorhina   �21m �33s 
    The subclass of Agnatha that includes the jawless vertebrates with a 
single median nostril. The Monorhina, sometimes called Cephalaspidomorphi, 
includes the superorder Hyperotreti, containing the modern Myxinoidea or 
hagfishes, and the superorder Hyperoartii, containing the living Petromyzonida 
or lampreys, as well as the extinct Osteostraci and Anaspida.  See  AGNATHA; 
DIPLORHINA; HYPEROARTII; HYPEROTRETI. 
 �34s[Robert H. Denison]
�33s 
  
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�6v�2v�32sMonosaccharide �21m �33s
    A class of simple sugars containing a chain of 3 - 10 carbon atoms in the
molecule, known as polyhydroxy aldehydes (aldoses) or ketones (ketoses). They
are very soluble in water, sparingly soluble in ethanol, and insoluble in
ether. The number of monosaccharides known is approximately 70, of which about
20 occur in nature. The remainder are synthetic. The existence of such a large
number of compounds is due to the presence of asymmetric carbon atoms in the
molecules. Aldohexoses, for example, which include the important sugar
glucose, contain no less than four asymmetric atoms, each of which may be
present in either D or L configuration. The number of stereoisomers rapidly
increases with each additional asymmetric carbon atom.
    A list of the best-known monosaccharides is given below:

Trioses:   CH(sub 2)OH.CHOH.CHO, glycerose (glyceri aldehyde)
	   CH(sub 2)OH.CO.CH(sub 2)OH, dihydroxy acetone
Tetroses:  CH(sub 2)OH.(CHOH)(sub 2).CHO< erythrose
	   CH(sub 2)OH.CHOH.CO.CHO, erythrulose
Pentoses:  Ch(sub 2)OH.(CHOH)(sub 3).CHO, xylose, arabinose, ribose
	   CH(sub 2)OH.(CHOH)(sub 2).CO.CH(sub 2)OH, xylulose, ribulose
Methyl pentoses (6-deoxyhexoses):
	   CH(sub 3)(CHOH)(sub 4)CHO, rhamnose, fucose
Hexoses:   CH(sub 2)OH.(CHOH)(sub 4).CHO, glucose, mannose, galactose
	   CH(sub 2)OH.(CHOH)(sub 3).CO.CHOH, fructose, sorbose
Heptoses:  CH(sub 2)OH.(CHOH)(sub 5).CHO, glucoheptose,
	    galamannoheptose
	   CH(sub 2)OH.(CHOH)(sub 4).CO.CH(sub 2)OH, sedoheptulose,
	    mannoheptulose

0]Aldose monosaccharides having 8, 9, and 10 carbon atoms in their chains have
been synthesized.  See	CARBOHYDRATE; KETONE; OPTICAL ACTIVITY;
STEREOCHEMISTRY.
 �34s[William Z. Hassid]
�33s

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�6v�2v�32sMonosodium glutamate �21m �33s
    The single sodium salt of glutamic acid used in foods to accentuate
flavors. It is also known as MSG. Molecular structure is represented below.

			  H  H	H
			  |  |	|
		   HO--C--C--C--C--C--ONa
		      ||  |  |	|  ||
		       O  H  H	N  O
			       / \
			      H   H

    The crystal form available in commerce is the monohydrate, with structure
as represented plus one molecule of water of hydration.  See  GLUTAMIC ACID.
    Originally produced from seaweed in the Orient, it is now made principally
from cereal glutens, such as those of wheat, corn, and soybeans, from
solutions evolved in the manufacture of beet sugar, and by microbiological
fermentation of carbohydrates. The two raw materials used for the greater
proportion of commercial production are wheat gluten and desugared beet-sugar
molasses.
    Monosodium glutamate is recognized as a standard of identity ingredient in
several commercial food preparations. Its principal use is in the preparation
of canned and dried soups, but it also enters into the production of some
meat, vegetable, fowl, and fish products. It is the so-called secret
ingredient used by many of the famous restaurant and hotel chefs.
 �34s[Paul D. V. Manning]
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�6v�2v�32sMonotremata   �21m �33s 
    The single order of the mammalian subclass Prototheria. Two living 
families, the Tachyglossidae and the Ornithorhynchidae, make up this unusual 
order of quasi-mammals, or mammallike reptiles. 
    The Tachyglossidae comprise the echidnas (spiny anteaters), which have 
relatively large brains with convoluted cerebral hemispheres. The known 
genera, Tachyglossus and Zaglossus, are terrestrial, feeding on termites, 
ants, and other insects. They are capable diggers, both to obtain food and to 
escape enemies. Like hedgehogs, they can erect their spines and withdraw their 
limbs when predators threaten. Commonly one egg, but occasionally two or even 
three, is laid directly into the marsupium (pouch) of the mother where it is 
incubated for up to 10 days. Species of Tachyglossus live in rocky areas, 
semideserts, open forests, and scrublands. They are found in Australia, 
Tasmania, New Guinea, and Salawati Island. Species of Zaglossus are found in 
mountainous, forested areas. 
    The duck-billed platypus, comprising the Ornithorhynchidae, has a 
relatively small brain with smooth cerebral hemispheres. The young have 
calcified teeth, but in the adult these are replaced by horny plates which 
form around the teeth in the gums. The snout, as the name indicates, is 
duck-billed. The semiaquatic platypus is a capable swimmer, diver, and digger. 
Two eggs are usually laid by the female into a nest of damp vegetation. After 
incubating the eggs for about 10 days the female leaves, returning only when 
the eggs are hatched. The platypus is found in Australia and Tasmania in 
almost all aquatic habitats.  See  MAMMALIA; PROTOTHERIA. 
 �34s[Frederick S. Szalay]
�33s 
  
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�6v�2v�32sMonsoon   �21m �33s 
    A large-scale wind system which predominates or strongly influences the 
climate of large regions, and in which the direction of the wind flow reverses 
from winter to summer. The outstanding example of a monsoon circulation is the 
vast wind system which dominates the weather over most of the Asian continent, 
especially the southern and eastern parts, the north Indian Ocean, and the 
extreme western portion of the northern Pacific Ocean. The monsoon influence 
is felt more or less distinctly on all the continental landmasses outside 
equatorial latitudes, except Antarctica. The monsoonal circulation in the 
lower troposphere is cyclonic around the margins of a continental area in 
summer, and anticyclonic in winter. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sMonstrilloida   �21m �33s 
    A suborder of microscopic crustaceans, the least frequently encountered 
among the subclass Copepoda. The life history is unusual. After a 
free-swimming nauplius larval stage, the immature forms are parasitic upon 
marine invertebrates. Adults emerge from the hosts, and again become 
free-swimming in the plankton. Adults lack a second antenna and mouthparts, 
and the digestive tract is vestigial. See COPEPODA; CRUSTACEA. 
 �34s[Charles C. Davis]
�33s 
  
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�6v�2v�32sMonte Carlo method �21m �33s
    A technique for estimating the solution x of a numerical mathematical
problem by means of an artificial sampling experiment. The estimate is usually
given as the average value, in a sample, of some statistic whose mathematical
expectation is equal to x. In most of the useful applications, the
mathematical problem itself arises in a problem of probability, either in
physics or in operational research.
    The importance of the method arises primarily from two sources: the
practical need to solve equations that are too complicated to solve by
analytic methods alone, and the increased importance of all numerical methods
because of the advent of the electronic computer.
    The main advantage of Monte Carlo is that many numerical problems are in
practice too complicated to solve in any other method. A familiar example is
the estimation of the probability of winning a game of pure chance: Sometimes
the only reasonably simple method of estimation is to play the game several
times. There are also numerical problems that can be solved by deterministic
methods but are logically simpler to solve approximately by the Monte Carlo
method. Sometimes poor approximations are satisfactory because the aim is
merely to determine the strategic variables of a problem. This is likely to be
a fruitful technique in mathematical economics. Another situation where a poor
approximation is satisfactory occurs when there is available an iterative
method of calculation, that is, a method of successive approximation, which
converges to the right answer in a reasonable time provided that the first
trial solution is not too far from the truth. The Monte Carlo method may then
be used for obtaining a first trial solution.
    The main disadvantage of the Monte Carlo method is that for each extra
decimal place required it is necessary to multiply the sample size by 100. To
calculate PI to five decimal places by throwing a needle would require about
10(sup 10) throws. See OPERATIONS RESEARCH; PROBABILITY; PROBABILITY
(PHYSICS); QUEUEING THEORY; STATISTICS; STOCHASTIC PROCESS.
 �34s[Irving J. Good]
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�6v�2v�32sMonth   �21m �33s 
    Any of the several units of time based on the revolution of the Moon 
around Earth. 
    The calendar month is one of the 12 arbitrary periods into which the 
calendar year is divided. 
    The synodic month is the average period of revolution of the Moon with 
respect to the Sun, the same as the average interval between successive full 
moons. Its duration is 29.531 days. 
    The tropical month is the period required for the mean longitude of the 
Moon to increase 360DG, or 27.322 days. 
    The sidereal month, 7 seconds longer than the tropical month, is the 
average period of revolution of the Moon with respect to a fixed direction in 
space.  
    The anomalistic month, 27.555 days in duration, is the average interval 
between closest approaches of the Moon to Earth. 
    The nodical month, 27.212 days in duration, is the average interval 
between successive northward passages of the Moon across the ecliptic. See 
CALENDAR; TIME. 
 �34s[Gerald M. Clemence]
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�6v�2v�32sMontmorillonite   �21m �33s 
    A group name for all clay minerals with an expanding structure, except 
vermiculite, and also a specific mineral name for the high alumina end member 
of the group.  See  CLAY MINERALS; VERMICULITE. 
    Montmorillonite clays have wide commercial use. The high colloidal, 
plastic, and binding properties make them especially in demand for bonding 
molding sands and for oil-well drilling muds. They are also widely used to 
decolorize oils and as a source of petroleum cracking catalysts.  See  CLAY. 
    Members of the montmorillonite group of clay minerals vary greatly in 
their modes of formation. Alkaline conditions and  the presence of magnesium 
particularly favor the formation of these minerals. Several important modes of 
occurrence are in soils, in bentonites, in mineral veins, in marine shales, 
and as alteration products of other minerals. Recent sediments have a fairly 
high montmorillonite content.  See  BENTONITE; MARINE SEDIMENTS. 
 �34s[Floyd M. Wahl ;Ralph E. Grim]
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�6v�2v�32sMonzonite   �21m �33s 
    A phaneritic (visibly crystalline) plutonic rock composed chiefly of sodic 
plagioclase (oligoclase or andesine) and alkali feldspar (microcline, 
orthoclase, usually perthitic), with subordinate amounts of dark-colored 
(mafic) minerals (biotite, amphibole, or pyroxene). Monzonite is more or less 
intermediate between syenite and diorite.  See  SYENITE. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sMoon   �21m �33s 
    The Earth's natural satellite. United States and Soviet spacecraft have 
obtained lunar data and samples, and American astronauts have orbited, landed 
upon, and roved upon the Moon. 
    The Earth and Moon now make one revolution about their barycenter, or 
common center of mass (a point about 4670 km from the Earth's center), in 
27supd 7suph 43supm 11.6sups. This sidereal period is slowly lengthening, and 
the distance (now about 60.27 earth radii) between centers of mass is 
increasing, because of tidal friction in the oceans of the Earth. 
    The Moon's present orbit is inclined about 5DG to the plane of the 
ecliptic. As a result of differential attraction by the Sun on the Earth-Moon 
system, the Moon's orbital plane rotates slowly relative to the ecliptic (the 
line of nodes regresses in an average period of 18.60 years) and the Moon's 
apogee and perigee rotate slowly in the plane of the orbit (the line of 
apsides advances in a period of 8.850 years). Looking down on the system from 
the north, the Moon moves counterclockwise. It travels along its orbit at an 
average speed of nearly 0.6 mi/s (1 km/s) or about 1 lunar diameter per hour. 
    As a result of the Earth's annual motion around the Sun, the direction of 
solar illumination changes about 1DG per day, so that the lunar phases do not 
repeat in the sidereal period given above but in the synodic period, which 
averages 29supd 12suph 44supm. 
    When the lunar line of nodes coincides with the direction to the Sun and 
the Moon happens to be near a node, eclipses can occur.  See  ECLIPSE. 
    The relation between the Moon's shape and its mass distribution is very 
important to theories of lunar origin and the history of the Earth-Moon 
system. By radio altimetry, Apollo confirmed that the Moon's surface on the 
far side is higher on the average than the near side; that is, the center of 
mass is offset from the center of figure. The offset is about 2 km (1.2 mi) 
toward the Earth. These observations suggest that the Moon's crust is thicker 
on the far side than on the near side. 
  
    The Moon's small size and low mean density result in surface gravity too 
low to hold a permanent atmosphere, and therefore it was to be expected that 
lunar surface characteristics would be very different from those of Earth. 
However, the bulk properties of the Moon are also quite different - the 
density alone is evidence of that. The Moon is too small to have compressed 
its silicates into a metallic phase by gravity; therefore, if it has a dense 
core at all, the core should be of nickel-iron. Available data suggest that 
the Moon's iron core may have a diameter of at most a few hundred kilometers. 
    As can be seen from the Earth with the unaided eye, the Moon has two major 
types of surface: the dark, smooth maria and the lighter, rougher highlands. 
Photography by spacecraft shows that, for some unknown reason, the Moon's far 
side consists mainly of highlands (Fig. 1). Both maria and highlands are 
covered with craters of all sizes. Numerous different types of craters can be 
recognized. Most prominent at full moon are the bright ray craters whose 
grayish ejecta appear to have traveled for hundreds of miles across the lunar 
surface. Observers have long recognized that some erosive process has been and 
may still be active on the Moon. Bombardment of the airless Moon by meteoritic 
matter and solar particles, and extreme temperature cycling, are now 
considered the most likely erosive agents, but local internal activity is also 
a possibility. 
    The lunar mountains, though very high (26,000 ft or 8000 m), are not 
extremely steep, and lunar explorers see rolling rather than jagged scenery. 
Though a widespread network of fault traces is visible, there is no evidence 
on the Moon of the great mountain-building processes seen on the Earth. 
    Basins on the Moon's nearside, namely, Imbrium, Serenitatis, and Crisium, 
appear fully flooded. These were maria created by giant impacts, followed by 
subsidence of the ejecta and (probably much later) upwelling of lava from 
inside the Moon. Examination of small variations in Lunar Orbiter motions has 
revealed that each of the great circular maria is the site of a positive 
gravity anomaly (excess mass). The old argument about impact versus vulcanism 
as the primary agent in forming the lunar relief appears to be entering a new, 
more complicated phase with the confirmation of extensive flooding of impact 
craters by lava on the Moon's near side, while on the far side, where the 
crust is thicker, the great basins remain mostly empty. 
    In some of the Moon's mountainous regions bordering on the maria are found 
sinuous rilles (Fig. 2). These winding valleys were shown in Lunar Orbiter 
pictures to have an exquisite fineness of detail. No explanation for them yet 
offered has proved entirely convincing. 
    The Moon seems to be totally covered, to a depth of at least tens of 
meters, by a layer of rubble and soil with very peculiar optical and thermal 
properties. This layer is called the regolith. The observed optical and radio 
properties all point to a highly porous or underdense structure for at least 
the top few millimeters of the lunar surface material. A dark-gray, fine soil 
that appears to mantle the entire Moon, softening most surface contours and 
covering everything except occasional fields of rocks (Fig. 3). This soil, 
with a slightly cohesive character like that of damp sand and a chemical 
composition similar to that of some basic silicates on the Earth, is a product 
of the radiation, meteoroid, and thermal environment at the lunar surface. 
 �34s[James D. Burke]
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�6v�2v�32sMoose   �21m �33s 
    An even-toed ungulate (Artiodactyla) which is a member of the deer family, 
Cervidae. Alces alces is the largest member of the family and ranges in the 
boreal forested areas throughout North America and in northern Eurasia. The 
moose is known as the elk in Europe and is believed by some authorities to be 
a race of the American moose (A. americana). The legs are long, making the 
animal well-adapted for its feeding habits of wading for aquatic plants and 
browsing on trees and bushes. During the rutting season in the early fall, the 
male gathers a number of cows together, and mating takes place. After a 
gestation period of about 37 weeks, one or two calves are born.  See  
ARTIODACTYLA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sMoraine   �21m �33s 
    An accumulation of glacial debris, usually till, with distinct surface 
expression related to some former ice front position. End moraine, the most 
common form, is an uneven ridge of till built in front of or around the 
terminus of a glacier margin, and reflects some degree of equilibrium between 
rate of ice motion, supply of rock debris at the ice front, temperature of the 
glacier base, and shape and resistance of underlying bedrock (see 
illustration).  
    If an end moraine represents the farthest forward position a glacier ever 
moved, it is a terminal moraine. It demonstrates a steady-state condition for 
a period of time within the ice body where constant forward motion is balanced 
by frontal melting; and a continual supply of debris, as on an endless 
conveyor belt, is brought forward to the glacier terminus. If the ice front 
then melts farther back than it moves forward, till is spread unevenly over 
the land as ground moraine. If a retreatal position of steady-state 
equilibrium is maintained again, a recessional moraine may be constructed. 
    Drumlins, produced by glacier streamlining of ground moraine, are probably 
the best-known moraine forms.  See  DRUMLIN. 
 �34s[Sidney E. White]
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�6v�2v�32sMordant   �21m �33s 
    A substance or combination of substances that facilitates the fixing of a 
dye to a fiber. A mordant enables the production of a more permanent and often 
deeper color. Metallic salts or hydroxides are most frequently used as 
mordants.  
    Certain mordants act directly on the fiber, making it more susceptible to 
the dye. Fabrics are then pretreated with the mordant before exposure to the 
dye. Other mordants function through the formation of a complex with the dye. 
The complex acts as the dyeing agent. Mordant and dye in this case are exposed 
simultaneously to the fabric.  See  DYE; DYEING. 
 �34s[Francis J. Johnston]
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�6v�2v�32sMorphactin   �21m �33s 
    One of a group of fluorene-carboxylic acid derivatives that are 
morphogenetically active substances in higher plants. The most active 
compounds, used as herbicides, are 9-hydroxyfluorene-(9)-carboxylic acid 
(fluorenol) and especially the 2-chloro derivative. 
    Morphactins affect the adult organs of plants very little, although leaf 
abscission or shedding of the stem apex may be caused. Much more frequently it 
is the new growth that is affected: Stem internodes are shortened, leaf 
laminae are reduced, and apical dominance (suppression of lateral shoots by 
the apical bud) is deranged, so that branching is increased. Plants that have 
developed under the influence of morphactin show, therefore, a dwarf bushy 
habit with numerous morphological anomalies; but, protected from competition 
with other plants, they normally survive and may bloom (though sometimes with 
deformed flowers) and even form fruit. 
    Other noticeable effects are retardation of germination, strong inhibition 
of lateral root formation, inhibition of the nodule formation on the roots of 
legumes, stopping of the spreading of rosette plants, change in the number and 
placing of flower buds, alternation in flower sex expression, induction of 
parthenocarpy, and abnormal development of embryos within the seeds. 
 �34s[Hubert Ziegler]
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�6v�2v�32sMorphine �21m �33s
    The most important opium alkaloid. High-quality opium contains 10 - 15%
morphine (sometimes as high as 25%) along with smaller amounts of codeine,
thebaine, papaverine, narcotine, and other bases. Its formula is C17N19NO3,
and it has the structure depicted below.

			 ***CHEMICAL STRUCTURE***

    Clinically, the great usefulness of the drug lies in its depressant
action, which causes a raising of the threshold of pain, sleepiness, and
euphoria. While repeated use sometimes leads to addiction, morphine is an
excellent analgesic and has certain well-defined and indispensable uses in
medicine and surgery.  See  ALKALOID; ANALGESIC.
 �34s[Frank Wagner]
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�6v�2v�32sMorphotropism   �21m �33s 
    Similarity of structure, axial ratios, and angles between faces of one or 
more zones in crystalline substances whose formulas can be derived one from 
another by substitution. Examples of morphotropic substances are compounds of 
the type, M2SO4, where M stands for Li, Na, K, Rb, Cs, Ag, Tl, or NH4. 
Structurally, this is explained as follows. The crystal structure is 
essentially determined by the large SO4 groups. The oxygen ions form a 
tetrahedron, with the sulfur in its center. The other ions are in cavities in 
between four or six of the oxgyen ions according to their dimensions. Organic 
compounds also show morphotropism. No definite general rules, however, can be 
given for its occurrence.  See  COORDINATION NUMBER; CRYSTAL; CRYSTALLOGRAPHY; 
ISOMORPHISM (CRYSTALLOGRAPHY). 
 �34s[Willy C. Dekeyser]
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�6v�2v�32sMortar   �21m �33s 
    A plastic mixture of solids and water used to bond masonry units together. 
Mortar exhibits hardening characteristics (or sets) as reactions of the 
cementitious materials occur with water. 
    The principal solids in mortar are sand and cementitious materials, such 
as hydrated lime (or slaked quicklime) and portland cement. The latter is 
absent in a pure lime mortar. The portland cement reacts rapidly with water, 
giving the mortar its initial set. Lime reacts more slowly, both with carbon 
dioxide (CO2) from the atmosphere to form CaCO3, and with sand (SiO2) to form 
calcium silicates. Mortar thus hardens with age. 
    Lime is used in mortars mainly because of the good workability it imparts, 
leading to well-filled joints.  See  LIME (INDUSTRY); PORTLAND CEMENT. 
 �34s[Joseph A. Pask]
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�6v�2v�32sMosaicism   �21m �33s 
    The coexistence of two or more genetically distinct cell populations 
derived originally from a single zygote. Mosaics may arise at any stage of 
development, from the two-cell stage onward, or in any tissue which actively 
proliferates thereafter. The phenomenon is commonly observed in many species 
of animals and plants and may be caused by somatic mutation or chromosomal 
nondisjunction. An individual animal or plant may exhibit mosaicism, or it may 
occur in a culture of a single cell-or tissue-type obtained from an individual. 
  
    Chromosome nondisjunction is probably the principle cause of chromosomal 
aberration, which in turn may lead to the development of mosaicism. During 
cell division, the two sister chromatids usually separate completely, each 
chromatid going to opposite poles of the cell guided by the spindle apparatus. 
In some cases, one chromatid will fail to completely separate, or it may lag 
behind. This nondisjunction will lead to the presence of both sister 
chromatids in the same daughter cell instead of one in each of the daughter 
cells (Fig. 1). Somatic crossing-over leads to the production of a recombinant 
mosaic where chromosome segments, with their corresponding blocks of genes, 
are exchanged between homologous chromosomes during mitosis. The occurrence of 
this process leads to mosaicism, mostly manifested as spots (clones of variant 
cells) on the cuticle of insects or on leaves, petals, or stamen hairs.  See  
CHROMOSOME ABERRATION. 
    Sex chromosome mosaicism, the presence of a mixture of cell populations 
with different X and Y chromosome constitutions, is not uncommon and is often 
seen in individuals with ovarian dysgenesis. The presence of a significant 
proportion of chromosomally abnormal cells in any such mosaic will tend to 
lead to a clinically expressed syndrome. The proportion of each constituent 
clone may vary from tissue to tissue, but is relatively stable in each 
individual site throughout adult life. Sex mosaics (gynandromorphs) are 
particularly striking where a difference in the secondary sexual 
characteristics exists between the normal sexes. For example, in a butterfly 
with bilateral gynandromorphism, the left side may show the characteristic 
wing color and pattern of the male, and the right wing, typical female 
patterning (Fig. 2).  See  CHIMERA; GENETICS; SEX-LINKED INHERITANCE. 
 �34s[Andrew W. Wayne]
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�6v�2v�32sMosquito   �21m �33s 
    Any member of the dipterous subfamily Culicinae in the family Culicidae. 
These insects are slender and fragile, with long legs, a long slender abdomen, 
and narrow wings. As with other members of the Diptera, distinct halteres 
(balancing organs) are present. Mouthparts, adapted for piercing and sucking, 
are well developed in the females, since they alone are blood feeders, and 
weakly developed in the males, which feed on nectar, ripe or decaying fruit, 
and other sweet substances. The head is small with large compound eyes, but no 
ocelli (simple eyes) are present. Antennae are long and beadlike in the 
female, and plumose or feathery in the male. 
    The larvae of all mosquitoes are aquatic and, although they vary in 
structural details, all have a breathing tube or similar structure which 
allows them to obtain air while floating just below the water surface. Some 
larvae are predacious, though generally food consists of organic matter 
present in the water. The pupae, which are also aquatic, differ radically from 
the larvae in morphology, and eventually metamorphose into the adult, or 
imago, stage. All three stages are important in identification of the species. 
    In addition to being biting pests, these insects form an important group 
in medical entomology, since they are vectors of many diseases of humans. Of 
the 90 genera and subgenera of mosquitoes, three are commonly known because of 
their medical importance:  Culex , the common house mosquito;  Aedes , the 
yellow-fever mosquito; and  Anopheles , the malaria mosquito. These common 
names are misleading, since various species of each genus may transmit a 
variety of diseases.  See  DIPTERA; MALARIA; YELLOW FEVER. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMoth   �21m �33s 
    An insect of the order Lepidoptera, which also includes the butterflies. 
Moths are principally nocturnal, heavy-bodied insects. The wings are folded in 
a rooflike position, wrapped around the body, or held in a horizontal 
position. The antennae have varied forms and shapes, and are rarely enlarged 
at the tip. The majority of adults have mouthparts adapted for sucking nectar 
from flowers. In a few species the mouthparts are vestigial; these species do 
not feed as adults. 
    Moths have a complete metamorphosis during development. The eggs are 
attached to plants where the wormlike larvae, known as caterpillars, develop. 
They are of considerable economic importance, since they are phytophagous, and 
many are serious pests of cultivated plants; also, some feed on fabrics, 
others on stored foods such as grains. Most caterpillars spin a cocoon in 
which they pupate and from which, after a period of rest or diapause, the 
imago or adult emerges.  See  LEPIDOPTERA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMotion �21m �33s
    If the position of a material system as measured by a particular observer
changes with respect to time, that system is said to be in motion with respect
to the observer. Absolute motion, then, has no significance, and only relative
motion may be defined; what one observer measures to be at rest, another
observer in a different frame of reference may regard as being in motion.  See
  FRAME OF REFERENCE; RELATIVE MOTION.
    The time derivatives of the various coordinates used to specify the system
may be used to prescribe the motion at any instant of time. How the motion
develops in subsequent instants is then determined by the laws of motion. In
classical dynamics it is supposed that in principle the motion and
configuration of the system may be specified to an arbitrary precision,
although in quantum mechanics it is recognized that the measurement of the one
disturbs the other.
    The most general theory of motion that has yet been developed is quantum
field theory, which combines both quantum mechanics and relativity theory, as
well as the experimentally observed fact that elementary particles can be
created and annihilated.  See  DEGREE OF FREEDOM (MECHANICS); DYNAMICS;
EULER'S EQUATION OF MOTION; HAMILTON'S EQUATIONS OF MOTION; HARMONIC MOTION;
KINEMATICS; KINETICS (CLASSICAL MECHANICS); LAGRANGE'S EQUATIONS; NEWTON'S
LAWS OF MOTION; OSCILLATION; PERIODIC MOTION; QUANTUM FIELD THEORY; QUANTUM
MECHANICS; RECTILINEAR MOTION; RELATIVITY; ROTATIONAL MOTION.
 �34s[Herbert C. Corben /Bernard Goodman]
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�6v�2v�32sMotor �21m �33s
    An electric rotating machine that converts electric energy into mechanical
energy. Because of its many advantages, the electric motor has largely
replaced other motive power in industry, transportation, mines, business,
farms, and homes. Electric motors can meet a wide range of service
requirements - starting, accelerating, running, braking, holding, and stopping
a load. They are available in sizes from a small fraction of a horsepower (1
hp = 0.75 kilowatts) to many thousands of horsepower, and in a wide range of
speeds. The speed may be fixed (or synchronous), constant for given load
conditions, adjustable, or variable. Many motors are self-starting and
reversible.  See  ELECTRIC ROTATING MACHINERY.
    Electric motors may be alternating-current (ac) or direct-current (dc).
There are many types of each. Although ac motors are more common, dc motors
are unexcelled for applications requiring simple, inexpensive speed control or
sustained high torque under low-voltage conditions.  See  ALTERNATING-CURRENT
MOTOR; DIRECT-CURRENT MOTOR; INDUCTION MOTOR; REPULSION MOTOR; SYNCHRONOUS
MOTOR; UNIVERSAL MOTOR.
    When a conductor located in a magnetic field carries current, a mechanical
force is exerted upon it (see illustration). This force has the value shown in

			     F = Bil	    newtons			   (1)

Eq. (1), where	i  is the current in amperes,  B  is the magnetic density in
webers per square meter, and  l  is the conductor length in meters. The
illustration shows the relative directions of current, field, and force. The
force reverses with either current or field reversal, but not when both are
reversed. The torque  T  is the product of this force and the rotor radius.
    If the conductor moves in the direction of	F , an emf  e  is generated
which opposes the current (motor action). If the conductor is moved against
F, this emf will assist the current (generator action). Its value is shown in

			  e = vBl	 volts				   (2)

Eq. (2), where v = velocity of conductor across the flux in meters per second.
    The product  ei  represents the power converted, in watts, shown in Eqs.
(3) and (4), which are the bases for the emf

			  Motor output = ei - rotative loss

			  Generator shaft input = ei + rotative loss

and output formulas of dynamo machinery. Many machines will operate as either
a motor or a generator, but they should be designed for the particular
service.  See  GENERATOR.
 �34s[Albert F. Puchstein]
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�6v�2v�32sMotorboating   �21m �33s 
    A form of oscillation that occurs at a very low audio frequency in a 
system, circuit, or component. It is caused by excessive amount of audio 
feedback at low frequencies. This oscillation is a succession of pulses; when 
these occur in a circuit that is feeding a loudspeaker, the pulses result in 
putt-putt sounds resembling those made by a motorboat. 
 �34s[John Markus]
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�6v�2v�32sMotor-generator set   �21m �33s 
    A motor and one or more generators, with their shafts mechanically 
coupled, used to convert an available power source to another desired 
frequency or voltage. The motor of the set is selected to operate from the 
available power supply; the generators are designed to provide the desired 
output.  
    The principal advantage of a motor-generator set over other conversion 
systems is the flexibility offered by the use of separate machines for each 
function. Since a double energy conversion is involved, electrical to 
mechanical and back to electrical, the efficiency is lower than in most other 
conversion methods.  See  GENERATOR; MOTOR. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sMotor systems   �21m �33s 
    Those portions of nervous systems that regulate and control the 
contractile activity of muscle and the secretory activity of glands. Muscles 
and glands are the two types of organ by which an organism reacts to its 
environment; together they constitute the machinery of behavior. Cardiac 
muscle and some smooth muscle and glandular structures can function 
independently of the nervous system but in a poorly coordinated fashion. 
Skeletal muscle activity, however, is entirely dependent on neural control. 
Destruction of the nerves supplying skeletal muscles results in paralysis, an 
inability to move. The somatic motor system includes those regions of the 
central nervous system involved in controlling the contraction of skeletal 
muscles in a manner appropriate to environmental conditions and internal 
states.  See  GLAND; MUSCLE. 
  
    Skeletal muscle.  The nerve supply to skeletal muscles of the limbs and 
trunk is derived from large nerve cells called motoneurons, whose cell bodies 
are located in the ventral horn of the spinal cord. Muscles of the face and 
head are innervated by motoneurons in the brainstem. The axons of the 
motoneurons traverse the ventral spinal roots (or the  appropriate cranial 
nerve roots) and reach the muscles via peripheral nerve trunks. In the muscle, 
the axon of every motoneuron divides repeatedly into many terminal branches, 
each of which innervates a single muscle fiber. The region of innervation, 
called the neuromuscular junction, or motor end plate, is a secure synaptic 
contact between the motoneuron terminal and the muscle fiber membrane.  See  
SYNAPTIC TRANSMISSION. 
    Since synaptic transmission at the neuromuscular junction is very secure, 
an action potential in the motoneuron will produce contraction of every muscle 
fiber that it contacts. For this reason, the motoneuron and all the fibers it 
innervates form a functional unit, are called the motor unit. The number of 
muscle fibers in a single motor unit may be as small as six (for intrinsic eye 
muscles) or over 700 (for motor units of large limb muscles). For a given 
muscle the average number of muscle fibers per motor unit can be determined by 
dividing the total number of fibers in the muscle by the number of motoneurons 
in its pool. The typical innervation ratio for limb muscles is on the order of 
120 - 150. In general, muscles involved in delicate rapid movements have fewer 
muscle fibers per motor unit than large muscles concerned with gross movements. 
  
    Components of skeletal motor system.  Motoneurons are activated by nerve 
impulses arriving through many different neural pathways. Some of their neural 
input originates in peripheral receptor organs located in the muscles 
themselves, or in receptors in skin or joints. Many muscle receptors discharge 
in proportion to muscle length or tension; such receptors have relatively 
potent connections to motoneurons, either direct monosynaptic connections or 
relays via one or more interneurons. Similarly, stimulation of skin and 
joints, particularly painful stimulation, can strongly affect motoneurons. 
Such simple segmental pathways constitute the basis for spinal reflexes. The 
other major source of input to motoneurons arises from supraspinal centers. 
The illustration shows the main nervous system centers involved in controlling 
the input to motoneurons. 
  
    Segmental circuits. At the spinal level, the motoneurons and muscles have 
a close reciprocal connection. Afferent connections from receptors in the 
muscles return sensory feedback to the same motoneurons which contract the 
muscle. Connections to motoneurons of synergist and antagonist muscles are 
sufficiently potent and appropriately arranged to subserve a variety of 
reflexes. In animals with all higher centers removed, these segmental circuits 
may function by themselves to produce simple reflex responses. Under normal 
conditions, however, the activity of segmental circuits is largely controlled 
by supraspinal centers. Descending tracts arise from two major supraspinal 
centers: the cerebral cortex and the brainstem. 
    Brainstem. The brainstem, which includes medulla and pons, is a major and 
complex integrating center which combines signals descending from other higher 
centers, as well as afferent input arising from peripheral receptors. The 
descending output from brainstem neurons affects motor and sensory cells in 
the spinal cord. Brainstem centers considerably extend the motor capacity of 
an animal beyond the stereotyped reflex reactions mediated by the spinal cord. 
In contrast to segmental reflexes, these motor responses involve coordination 
of muscles over the whole body. Another major motor function of the brainstem 
is postural control, exerted via the vestibular nuclei of the ear. 
    Besides neurons controlling limb muscles, the brainstem also contains a 
number of important neural centers involved in regulating eye movements. These 
include motor neurons of the eye muscles and various types of interneurons 
that mediate the effects of vestibular and visual input on eye movement. 
    Cerebellum. Another important coordinating center in the motor system is 
the cerebellum, an intricately organized network of cells closely 
interconnected with the brainstem. The cerebellum receives a massive inflow of 
sensory signals from peripheral receptors in muscles, tendons, joints, and 
skin, as well as from visual, auditory, and vestibular receptors. Higher 
centers, particularly the cerebral cortex, also provide extensive input to the 
cerebellum via pontine brainstem relays. The integration of this massive 
amount of neural input in the cerebellum somehow serves to smooth out the 
intended movements and coordinate the activity of muscles. Without the 
cerebellum, voluntary movements become erratic, and the animal has difficulty 
accurately terminating and initiating responses. The output of the cerebellum 
affects primarily brainstem nuclei, but it also provides important signals to 
the cerebral cortex. 
    Basal ganglia. At another level of motor system are the basal ganglia. 
These massive subcortical nuclei receive descending input connections from all 
parts of the cerebral cortex. Their output projections send recurrent 
information to cerebral cortex via thalamus, and their other major output is 
to brainstem cells. 
  
    Cerebral cortex.  At the highest level of the nervous system is the 
cerebral cortex, which exerts control over the entire motor system. The 
cerebral cortex performs two kinds of motor function: certain motor areas 
exert relatively direct control over segmental motoneurons, via a direct 
corticospinal pathway, the pyramidal tract, and also through extrapyramidal 
connections via supraspinal motor centers. The second function, performed in 
various cortical association areas, involves the programming of movements 
appropriate in the context of sensory information, and the initiation of 
voluntary movements on the basis of central states. Cortical language areas, 
for example, contain the circuitry essential to generate the intricate motor 
patterns of speech. Limb movements to targets in extrapersonal space appear to 
be programmed in parietal association cortex. Such cortical areas involved in 
motor programming exert their effects via corticocortical connections to the 
motor cortex, and by descending connections to subcortical centers, 
principally basal ganglia and brainstem. 
    As indicated in the illustration, the motor centers are all heavily 
interconnected, so none really functions in isolation. In fact, some of these 
connections are so massive that they may form functional loops, acting as 
subsystems within the motor system. For example, most regions of the cerebral 
cortex have close reciprocal interconnections with underlying thalamic nuclei, 
and the corticothalamic system may be considered to form a functional unit.  
Another example is extensive connection from cerebral cortex to pontine 
regions of the brainstem, controlling cells that project to the cerebellum, 
which in turn projects back via the thalamus to cerebral cortex. Such 
functional loops are at least as important in understanding motor coordination 
as the individual centers themselves. See BRAIN; NERVOUS SYSTEM (VERTEBRATE). 
 �34s[Eberhard E. Fetz]
�33s 
  
  
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�6v�2v�32sMountain   �21m �33s 
    A feature of the Earth's surface that rises high above its base and has 
generally steep slopes and a relatively small summit area. Commonly the 
features designated as mountains have local heights measurable in thousands of 
feet, lesser features of the same type being called hills, but there are many 
exceptions.  See  HILL AND MOUNTAIN TERRAIN. 
    Mountains rarely occur as isolated individuals. Instead they are usually 
found in roughly circular groups or massifs, such as the Olympic Mountains of 
northwestern Washington, or in elongated ranges, like the Sierra Nevada of 
California. An array of linked ranges and groups, such as the Rocky Mountains, 
the Alps, or the Himalayas, is a mountain system. North America, South 
America, and Eurasia possess extensive cordilleran belts, within which the 
bulk of their higher mountains occur.  See  CORDILLERAN BELT; MASSIF; MOUNTAIN 
SYSTEMS.  
    As a rule, mountains represent portions of the Earth's crust that have 
been raised above their surroundings by upwarping, folding, or buckling, and 
have been deeply carved  by streams or glaciers into their present surface 
form. Some individual peaks and massifs have been constructed upon the surface 
  by outpourings of lava or eruptions of volcanic ash.  See  OROGENY. 
 �34s[Edwin H. Hammond]
�33s 
  
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�6v�2v�32sMountain systems   �21m �33s 
    Long, broad, linear to arcuate belts in the Earth's crust where extreme 
mechanical deformation and thermal activity have been (or are being) 
concentrated.  
    Mountain systems in the general sense occur both on continents and in 
ocean basins, but the geological properties of the systems in continental as 
opposed to oceanic settings are distinctly different. The mechanical strain in 
classical, continental mountain systems is expressed in the presence of major 
folds, faults, and intensive fracturing and cleavage. Thermal effects are in 
the form of vast volcanic outpourings, intruded bodies of igneous magma, and 
metamorphism. Uplift and deformation in young mountain systems are 
conspicuously displayed in the physiographic forms of topographic relief. 
Where mountain building is presently taking place, the dynamics are partly 
expressed in warping of the land surface and significant shallow or deep 
earthquake activity. Locations of ancient mountain systems in continental 
regions now beveled flat by erosion are clearly disclosed by the presence of 
highly deformed, intruded, and metamorphosed rocks. 
    Two basic classes of oceanic mountain systems exist. A world-encircling 
oceanic rift mountain system has been built along the extensional tectonic 
boundary between plates diverging at rates of 0.8 - 2.4 in. or 2 - 6 cm per 
year from the midoceanic ridges. This rift mountain system is exposed to 
partial view in Iceland. The second type, island arc mountain systems, occur 
in oceanic basins where the crust dives downward at trench sites, thus 
underthrusting adjacent oceanic crust.  See  MARINE GEOLOGY; OCEANIC ISLANDS. 
    The classical, conspicuous mountain systems of the Earth occur at the 
continent/ocean interface, for this is the site where plate convergence has 
led to major sedimentation, subduction of oceanic crust under continents, 
collision of island arc mountain systems with continents, and head-on 
collision of continents.  See  OROGENY; PLATE TECTONICS. 
 �34s[George H. Davis]
�33s 
  
A700000002552�254s�5v�1v1117
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�6v�2v�32sMouse   �21m �33s 
    The name associated with any species of animals which are members of the 
families Muridae, Heteromyidae, Cricetidae, and Zapodidae in the order 
Rodentia. Some of the more common species are listed in the table. Many 
species are used for research in both biology and medicine. In addition to 
their use in studying the mechanisms of genetics, they are important in the 
study of carcinogenesis, effects of drugs, and virology. They are also 
important experimental animals in studying cell physiology, such as for cell 
and tissue culture research. The familiar white mouse is an albino form of the 
house mouse and is used extensively in research. 
    The common house mouse is one of the oldest known species of domestic 
rodent pests. It usually has a maximum life-span of 4 years with four to six 
litters of four to eight young each per year. The gestation period is about 3 
weeks. These rodents begin to breed at 3 months of age. Adults have a pointed 
snout, compact body, and an equally long tail. The ears are fairly large, as 
are the legs. While omnivorous, they have a preference for grains and other 
vegetable foods. Of the 44 species known, only one species occurs in the 
United States, and it has become wild in some parts of the country.  See  
RODENTIA.  
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sMoving-coil frequency meter   �21m �33s 
    An instrument that employs one or more pivoted moving coils and resonant 
circuits to measure frequency. 
    The illustration shows one design of a moving-coil resonant-type 
instrument. Two coils, A and B, are placed at right angles to form a moving 
element and are supplied through a resonant circuit. The coils are tuned to 
different frequencies, coil A to a frequency slightly below the lowest scale 
point and coil B an equal amount above the upper scale point. A fixed coil C 
divided into two parts carries the sum of the currents of the two circuits of 
coils A and B. When the frequency of the applied voltage equals the midscale 
frequency the currents in the two resonant circuits are practically equal and 
the phase lead of one coil equals the phase lag of the other with respect to 
the fixed-coil current. At frequencies near the low end of the scale, the 
moving element comes to rest with coil A approaching a position parallel to 
the fixed coil. As the circuit of coil A approaches resonance, the current in 
coil A is relatively high and in phase with the fixed-coil current and the 
current in coil B leads with respect to the fixed-coil current. At frequencies 
in the higher range, coil B approaches resonance and takes an equilibrium 
position parallel to the plane of the fixed coil. If the supply frequency is 
very low, the impedance of the circuit A and B being equal, the pointer will 
indicate a fictitious midscale reading. To rectify this misleading 
measurement, a third coil D is introduced in parallel with coil A and 
resonates at frequencies much less than the lowest scale point. See FREQUENCY 
MEASUREMENT.  
 �34s[Edward B. Curdts]
�33s 
  
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�6v�2v�32sMoving-iron frequency meter   �21m �33s 
    An instrument in which the movement of a pivoted iron vane or needle under 
the influence of current-carrying coils is used to measure frequency. 
    A resonant-type frequency meter operating on the moving-iron principle is 
shown in Fig. 1. Two field coils are mounted opposite to each other and are 
connected so that their fluxes oppose. If i1 and i2 are moving-coil currents, 
the resultant field current will be i1 - i2. The moving system which lies 
between the field coils is composed of an iron vane of magnetic material 
centered within the armature coils and rigidly attached to the shaft. Both the 
field coils are connected to series-resonance circuits in which resonance is 
produced below the operating range for one circuit and above the operating 
range for the other. 
    Deflecting torque is produced by the reaction of the armature, or moving 
coil, field with the in-phase component of the field coils. In addition to 
this deflecting torque there is a countertorque produced by the action of the 
field flux in the magnetic vane of the moving element. Equilibrium occurs when 
the magnitude of the countertorque is equal to the deflecting torque, which 
causes the pointer to take a position indicating the frequency. 
  
    Another design of a moving-iron frequency meter, but of the ratiometer 
type, is shown in Fig. 2. Two coils A and B are arranged at right angles to 
each other. Coil A is in series with a resistance RsubA, and this combination 
is in parallel with an inductance LsubA. Similarly, coil B is in series with 
LsubB, and the combination is in parallel with RsubB. The complete circuit 
thus acts as a bridge network. When the bridge is balanced by suitable values 
of parameters, the currents in the two coils are equal and the moving element, 
a pivoted soft-iron needle, will indicate the mean-frequency position as shown 
in the diagram. See FREQUENCY MEASUREMENT. 
 �34s[Edward B. Curdts]
�33s 
  
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�6v�2v�32sMoving-target indication   �21m �33s 
    A method of presenting pulse-radar echoes in a manner that discriminates 
in favor of moving targets and suppresses stationary objects. Moving-target 
indication (MTI) is almost a necessity when moving targets are being sought 
over a region from which the ground clutter echoes are very strong. The most 
common presentation of the output of a radar with MTI is a plan-position 
indicator (PPI) display. The moving targets appear as bright echoes, while 
ground clutter is suppressed.  See  RADAR. 
 �34s[John M. Carroll]
�33s 
  
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�6v�2v�32sMucilage   �21m �33s 
    A naturally occurring, high-molecular-weight (200,000 and up), organic 
plant product of unknown detailed structure. The term is loosely used, often 
interchangeably with the term gum. Chemically, mucilage is closely allied to 
gums and pectins but differs in certain physical properties. Although gums 
swell in water to form sticky, colloidal dispersions and pectins gelatinize in 
water, mucilages form slippery, aqueous colloidal dispersions. Mucilages are 
formed in normal plant growth within the plant by mucilage-secreting hairs, 
sacs, and canals, but they are not found on the surface as exudates as a 
result of bacterial or fungal action after mechanical injury, as are gums. 
Mucilages occur in nearly all classes of plants in various parts of the plant, 
usually in relatively small percentages, and are not infrequently associated 
with other substances, such as tannins. The chief industrial sources of 
mucilages are Icelandic and Irish moss, linseed, locust bean, slippery elm 
bark, and quince seed.  See  ADHESIVE; GLUE; GUM; PECTIN. 
 �34s[Elbert H. Hadley]
�33s 
  
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�6v�2v�32sMucormycosis   �21m �33s 
    A severe infection caused by ubiquitous molds of the order Mucorales which 
occurs only in people who have deficient immune responses. The mold, is found 
in soil and on fruits and may grow on bread. While most people have contact 
with these fungi without becoming infected, certain individuals with immune 
defects develop invasive infections which result in disease. 
    The types of disease that result from Mucorales infection are limited. The 
mold first invades the nasal passages and then spreads directly through 
sinuses and the orbit to the brain or via a major blood vessel. One common 
finding is arterial clotting with tissue necrosis or venous clotting with 
obstruction.  
 �34s[Donald Armstrong]
�33s 
  
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�6v�2v�32sMud puppy   �21m �33s 
    A urodele amphibian which has both lungs and gills as an adult. The two 
genera, Proteus and Necturus, make up the family Proteidae. These aquatic 
animals have small lidless eyes. They have vomerine teeth as well as palatine 
and pterygoid teeth, which in this animal are not separate. The vertebrae are 
amphicoelous, that is, biconcave. Fertilization is internal, and there is a 
sperm receptacle in the female.  See  AMPHIBIA; URODELA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sMud volcano   �21m �33s 
    A conical landform composed  of clay and silt with variable admixtures of 
sand and rock fragments, the whole resulting from eruption of wet mud and 
impelled upward by fluid or gas pressure. The form of such features depends 
upon the ratio of mineral matter to the fluids and gases in the mobile 
material.  
    Associated features include mud pits, developed where the mobile material 
contains excessive amounts of fluid and gas, and pitch lakes, formed where the 
bitumens of petroleum residues are at least as voluminous as the clay 
particles in the viscous mud. Minor amounts of petroleum residues are often 
found in mud volcanoes and associated features, but pitch lakes are 
comparatively rare phenomena. 
 �34s[Hans H. Suter]
�33s 
  
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�6v�2v�32sMuffler   �21m �33s 
    A devide used to attenuate sound being propagated in conjunction with a 
moving stream of fluid (usually gas). Mufflers may be placed in two general 
classifications, depending upon the mode by which sound energy is removed from 
the gas stream. Those devices which attenuate by reflecting sound back to the 
source are called nondissipative, or reactive. Devices which absorb sound 
energy as the gas passes through them are called dissipative devices. 
    Reactive types are predominantly used in the intake and exhaust of 
automotive and other reciprocating engines and compressors. Reflection is 
provided through the use of acoustic filters, resonators, and change in 
direction by bends in the pipe confining the gas stream.  These mufflers are 
particularly useful for low-frequency applications and for those installations 
where high temperatures or flammable gases restrict the use of dissipative 
materials. See ACOUSTIC FILTER; ACOUSTIC RESONATOR. 
    Dissipative types are used in a wide range of applications where low 
pressure drop and high attenuation at predominantly middle and high 
frequencies are required. Many different configurations and types of materials 
can be employed, depending upon the individual application. In a simple 
ventilating system for a home or office the structure might consist of lining 
the interior walls of the duct with a blanket of glass fiber material of the 
order of 1 in. (25 mm) in thickness. On the other hand, a large wind tunnel or 
jet-engine test cell exhaust might require a complicated design of ducts, 
absorbent baffles in a parallel stack, cylinders of absorbing material mounted 
as a spatial array, or one of a number of proprietary designs combining these 
elements.  See  SOUND ABSORPTION. 
 �34s[William J. Galloway]
�33s 
  
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�6v�2v�32sMulberry �21m �33s
    A genus (Morus) of trees characterized by milky sap and simple, often
lobed, alternate leaves. White mulberry (M. alba) was introduced into the
United States from China during the 19th century as a source of food for
silkworms. The silkworm project was unsuccessful, but the trees remained and
are common in cities and on the borders of forests. Red mulberry (M. rubra)
grows in the eastern half of the United States and in southern Ontario. The
wood is used for fence posts, furniture, interior finish, agricultural
implements, and barrels.  See  URTICALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sMule   �21m �33s 
    The mule is a hybrid sired by a male ass (Equus asinus) out a female horse 
(E. caballus). The opposite cross, very seldom made, produces the hinny (a 
hybrid between a stallion and a female ass). The mule and the hinny are 
usually sterile, but two authenticated cases of mules producing living progeny 
are known. Male mules, often called horse mules, are almost always castrated 
to make them more tractable as work animals. 
    Mules are noted for their endurance, surefootedness, and ability to stand 
hard work in hot weather. They can safely be self-fed in lots or corrals, 
whereas horses cannot. Usually steady and free from nervous excitability, 
mules can be handled by inexperienced or careless farm labor. 
 �34s[John M. Kays]
�33s 
  
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�6v�2v�32sMultiaccess computer   �21m �33s 
    A computer system in which computational and data resources are made 
available simultaneously to a number of users. Users access the system through 
terminal devices, normally on an interactive or conversational basis. A 
multiaccess computer system may consist of only a single central processor 
connected directly to a number of terminals (that is, a star configuration), 
or it may consist of a number of processing systems which are distributed and 
interconnected with each other as well as with the user terminals. 
    The primary purpose of multiaccess computer systems is to share resources. 
The resources being shared may be simply the data-processing capabilities of 
the central processor, or they may be the programs and the data bases they 
utilize. The earliest examples of the first mode of sharing are the 
general-purpose, time-sharing, computational services. Examples of the latter  
mode are airlines reservation systems in which it is essential that all ticket 
agents have immediate access to current information.  See  DATA-BASE 
MANAGEMENT SYSTEMS. 
  
    System components.  The major hardware components of a multiaccess 
computer system are terminals or data entry/display devices, communication 
lines to interconnect the terminals to the central processors, a central 
processor, and on-line mass storage. Terminals may be quite simple, providing 
only the capabilities for entering or displaying data, or they may have an 
appreciable amount of ``local intelligence'' to support simple operations like 
editing of the displayed text without requiring the involvement of the central 
processor. The interconnecting communication lines can be provided by 
utilizing the common-user telephone system or by obtaining leased, private 
lines from the telephone company or a specialized carrier. 
  
    System operating requirements.  A multiaccess system must include the 
following functional capabilities: (1) multiline communications capabilities 
that will support simultaneous conversations with a reasonably large number of 
remote terminals; (2) concurrent execution of a number of programs with the 
ability to quickly switch from executing the program of one user to executing 
that of another; (3) ability to quickly locate and make available data stored 
on the mass storage devices while at the same time protecting such data from 
unauthorized access. 
    The ability of a system to support a number of simultaneous sessions with 
remote users is an extension of the capability commonly known as 
multiprogramming. In order to provide such service, certain hardware and 
software features should be available in the central processor. Primary among 
these is the ability to quickly switch from executing one program to another 
while protecting all programs from interference with one another. 
    Memory sharing is essential to the efficient operation of a multiaccess 
system. A popular memory management technique is the utilization of paging. 
The program is broken into a number of fixed-size increments called pages. 
Similarly, central memory is divided into segments of the same size called 
page frames. (Typical sizes for pages and page frames are 512 to 4096 bytes.) 
Under the concept known as demand paging, only those pages that are currently 
required by the program are loaded into central memory. 
  
    Software capabilities.  The control software component of most interest to 
an interactive user is the command interpreter. This routine interacts 
directly with users, accepting requests for service and translating them into 
the internal form required by the remainder of the operating system, as well 
as controlling all interaction with the system. 
    The capability to page the memory as outlined above can be utilized to 
provide users with the impression that each has available a memory space much 
larger than is actually assigned. Such a system is said to provide a virtual 
memory environment. Similarly, the ability of the operating system to quickly 
change context from one executing program to another will result in users' 
receiving the impression that each has an individual processor.  See  
DATA-PROCESSING SYSTEMS; DIGITAL COMPUTER. 
 �34s[Philip H. Enslow, Jr.]
�33s 
  
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�6v�2v�32sMultilevel control theory   �21m �33s 
    An approach to the control of large-scale systems based on (1) 
decomposition of the complex overall control problem into simpler and more 
easily managed subproblems, and (2) coordination of the subproblems so that 
overall system objectives and constraints are satisfied. 
    The controllers are organized in a multilevel hierarchical structure 
according to three basic criteria: functional decomposition, plant 
decomposition, and temporal decomposition. 
    In the functional decomposition approach the overall control problem is 
partitioned into a nested set of generic control functions. 
    In the plant decomposition approach the controlled system (plant) is 
partitioned into subsystems along lines of weak interaction. Each subsystem 
has its own (first-level) controller which acts to satisfy local objectives 
and constraints. A second-level controller (coordinator) influences the 
actions of the local controllers to compensate for subsystem interactions so 
that overall objectives and constraints are satisfied. 
    The multilevel structure induces an ordering with respect to time scale; 
specifically, the mean period of control action tends to increase as one 
proceeds from a lower to a higher level of the hierarchy. This motivates the 
concept of a temporal control hierarchy wherein a control or decision-making 
problem is partitioned into subproblems based on the different time scales 
relevant to the associated action functions. These time scales reflect such 
factors as the response time of the plant, the bandwidth characteristics of 
the disturbance inputs, and trade-off considerations relating the benefit of 
control action to its cost. The temporal hierarchy may embrace a broad 
spectrum of control/decision-making activities, including process control, 
production control, scheduling, and planning functions. These may range in 
time scale from seconds to weeks to years.  See  CONTROL SYSTEMS; DISTRIBUTED 
CONTROL SYSTEM. 
 �34s[Irving Lefkowitz]
�33s 
  
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�6v�2v�32sMultimeter �21m �33s
    A common term for a volt-ohm-milliammeter, also called an analyzer or
circuit analyzer. A much less common usage applies to a self-contained test
instrument containing two or more single- or multi-scale indicating
instruments for measuring simultaneously two or more electrical quantities.
See  VOLT-OHM-MILLIAMMETER.
 �34s[Isaac F. Kinnard /Edward C. Stevenson]
�33s

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�6v�2v�32sMultiple cropping   �21m �33s 
    A farming technique for growing several food crops on the same field in 
one year. Multiple cropping is most widely used in the tropical regions of 
South and Central America, Africa, and Asia. The advantages of multiple 
cropping in the densely populated sections of these regions enables farmers to 
produce on their own small farms the food needed to feed their families. Often 
four or five different food crops can be grown in succession on the same land 
in one calendar year. Such practices make it possible to support very dense 
populations.  
 �34s[Richard Bradfield]
�33s 
  
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�6v�2v�32sMultiple proportions, law of   �21m �33s 
    This law states that, when two elements combine together to form more than 
one compound, the weights of one element that unite with a given weight of the 
other  are in the ratio of small whole numbers. The law can be illustrated by 
the composition of the five oxides of nitrogen. One gram of nitrogen is 
combined with 2.85 g of oxygen in nitrogen pentoxide, N2O5; with 2.28 g in 
nitrogen dioxide, NO2; with 1.71 g in nitrogen trioxide, N2O3; with 1.14 g in 
nitric oxide, NO; and with 0.57 g in nitrous oxide, N2O. These numbers are in 
the simple ratio of 5:4:3:2:1.  See  DEFINITE COMPOSITION, LAW OF. 
 �34s[Thomas C. Waddington]
�33s 
  
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�6v�2v�32sMultiple sclerosis �21m �33s
    A disease which affects otherwise healthy young adults with multiple
lesions (demyelinated plaques) of the central nervous system; blindness,
urinary incontinence, incoordination, dysarthria (slurred speech), numbness,
and paralysis are common. These signs may develop rapidly or slowly and may
disappear just as mysteriously. Complete recovery from the acute episode
occurs in about two-thirds of all cases, so that evaluation of any treatment
is difficult. Although most cases are characterized by repeated remissions and
exacerbations, about 25% are steadily and slowly progressive without the
typical ups and downs. The average duration of illness is about 27 years, so
that it frequently puts a severe psychological and economic stress upon the
patient and the family.
 �34s[Ellsworth C. Alvord, Jr. ;Chen-MeiShaw]
�33s

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�6v�2v�32sMultiplexing �21m �33s
    The means or result of the means whereby a number of independent messages,
each unambiguously represented by information-bearing signals, may be sent and
received over a common transmitting medium. Viewing broadly, a multiplex
system is any part of a communication system in which two or more distinct
signal channels are combined. Multiplexing is generic to most present-day
systems of telecommunications. Many types of multiplexing are possible.
Typical examples are time division, frequency division, and phase
discrimination.  See  ELECTRICAL COMMUNICATIONS.
 �34s[Howard S. Black]
�33s

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�6v�2v�32sMultiplication   �21m �33s 
    One of the fundamental operations of arithmetic and algebra. The symbol x 
is commonly employed in arithmetic to denote multiplication. Because of its 
resemblance to the letter x, it is rarely used in algebra, where 
multiplication is frequently denoted by a dot (as in a PRODUCT b) or, most 
often, merely by juxtaposition of letters (for example, ab). Multiplication of 
numbers (real or complex) is associative, a(bc) = (ab)c; commutative, ab = ba; 
and distributive with respect to addition, a(b + c) = ab + ac; but the term 
has been extended to denote binary operations on many other kinds of objects, 
and these operations need not possess all  the properties of ordinary 
multiplication listed above (for example, multiplication of matrices is not 
commutative).  See  ADDITION; ALGEBRA; DIVISION; NUMBER SYSTEMS; SUBTRACTION. 
 �34s[Leonard M. Blumenthal]
�33s 
  
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�6v�2v�32sMultipole radiation   �21m �33s 
    Gamma rays, internal conversion electrons, or positron-electron pairs of 
defined characteristics emitted from an atom when the nucleus makes a 
transition between two energy states. The multipole order is the number of 
units of angular momentum removed by the radiation. This number is not 
necessarily equal to the difference between the spins of the nucleus in its 
initial and final states because the nuclear spin direction may change. Thus a 
quadrupole radiation will result when a state of spin 2 makes a transition to 
a state of spin 0, but a transition from a state of spin 2 to one of spin 1 
may also result in quadrupole radiation if there is an appropriate change in 
nuclear spin direction. 
    From multipole radiation measurements, the static and dynamic properties 
of nuclear energy states may be determined, and this information may be used 
in theories of nuclear structure. 
    In addition to the energy and angular momentum values, a third 
characteristic of a nuclear state is the parity. There are two classes of 
multipole radiation, the electric and the magnetic, and the designation of a 
given radiation, the electric and the magnetic, and the designation of a given 
radiation depends upon both the angular momentum change and whether the 
parities of the initial and final states are the same or different.  See  
NUCLEAR RADIATION; PARITY (QUANTUM MECHANICS); SPIN (QUANTUM MECHANICS). 
 �34s[David E. Alburger]
�33s 
  
  
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�6v�2v�32sMultituberculata   �21m �33s 
    The single order of the nominally mammalian subclass Allotheria. This 
extremely successful order of mammals is known to have flourished on northern 
continents from Late Jurassic time until the end of the Eocene, finally dying 
out about 40,000,000 years ago. The only mammalian herbivores of the Mesozoic, 
these animals differentiated by Late Cretaceous time into a number of distinct 
phyla which persisted generally through the Paleocene but which, during the 
Eocene, succumbed to competition from condylarths, primates, and finally 
rodents. Three suborders of the order Multituberculata are known, 
Plagiaulacida, Ptilodontoidea, and Taeniolabidoidea.  See  ALLOTHERIA. 
 �34s[Malcolm C. McKenna]
�33s 
  
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�6v�2v�32sMultivariable control   �21m �33s 
    The control of systems characterized by multiple inputs, which are usually 
referred to as the controls; or by multiple outputs, which are often the 
measured variables to be controlled (see illustration); or by both multiple 
inputs and outputs. Automobiles, chemical processing plants, aerospace 
vehicles, biological systems, and the national economy are all examples of 
multivariable systems which require and receive some form of regulation or 
control, be it mathematically contrived or not. In many cases, control of such 
systems is implemented without a previously given design or explicit 
mathematical model, such as in the manual control of relatively simple 
motorized vehicles, or in a person's control of arm and leg movements. In more 
complex tasks, however, such as the manual manipulation of an inherently 
unstable helicopter, automatic control devices (autopilots) are employed to 
assist the operator. 
    The control objectives to be achieved in the multivariable case include 
those control objectives which are generally sought in the scalar, that is, 
single input/output case, and also depend on the particular application 
involved. In particular, stability is usually a primary concern in the design 
of any control system, along with various measures of the degree of stability. 
While controller failure in the scalar case can lead to catastrophic failure 
of the overall system, such need not be true in the multivariable case due to 
the interactions between various input/output pairs. More specifically, in 
certain applications it may be possible to design a multivariable controller 
``driven'' by all p outputs which performs satisfactorily even when feedback 
information from one or more outputs is lost. The integrity of such a 
multivariable controller would be much better than that of its scalar 
counterpart.  See  CONTROL SYSTEM STABILITY. 
    In many control applications, a primary objective is that of tracking, or 
ensuring that the system output or outputs track a desired input or inputs 
with little or no steady-state errors. In addition, simultaneous regulation of 
external disturbances is also often desired; that is, ensuring that any 
external disturbances affect the plant outputs as little as possible. It is 
also obvious that simplicity of controller design is a highly desirable design 
objective both in the scalar and multivariable cases. One final design 
objective unique to the multivariable case is that of minimizing or 
eliminating the interaction between loops, which is often referred to as 
decoupling; that is, perfect decoupling would imply a controlled system where 
each input affects only one output or, otherwise stated, a system whose 
compensated transfer matrix is diagonal and nonsingular.  See  CONTROL 
SYSTEMS; PROCESS CONTROL. 
 �34s[William Wolovich]
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�6v�2v�32sMultivibrator   �21m �33s 
    A form of relaxation oscillator consisting normally of two or more active 
devices, such as transistors, interconnected by electric networks. In a 
multivibrator a portion of the output voltage or current of each active device 
is applied to the input of the other with such magnitude and polarity as to 
maintain the devices alternately conducting over controllable periods of time. 
The transition time of each device from one state to the other is extremely 
short. As a consequence the voltage waveform from the output of each of the 
devices is essentially rectangular in form. 
    Multivibrators are classified by the manner in which the reversal-of-state 
action of each device is initiated and by the method of control of the time 
interval in each state, whether from external sources or from the decay of 
voltage across a capacitor in circuits containing RC time constants within the 
multivibrator itself. 
  
    Symmetrical bistable multivibrator.  In bistable multivibrators either of 
the two devices may remain conducting, with the other nonconducting, until the 
application of an external pulse. Such a multivibrator is said to have two 
stable states. 
    The original form of bistable multivibrator made use of vacuum tubes and 
was known as the Eccles-Jordan circuit. It was sometimes called a flip-flop or 
binary circuit because of the two alternating output voltage levels. The 
junction field-effect transistor  (JFET) circuit (Fig. 1) is a modern version 
of the Eccles-Jordan circuit. Its resistance networks between positive and 
negative supply voltages are such that, with no current flowing to the drain 
of the first JFET, the voltage at the gate of the second is slightly negative, 
zero, or limited to, at most, a slightly positive value. The resultant current 
in the drain circuit of the second JFET causes a voltage drop across the drain 
load resistor; this drop in turn lowers the voltage at the gate of the first 
JFET to a sufficiently negative value to continue to reduce the drain current 
to zero. This condition of the first device OFF and second ON will be 
maintained as long as the circuit remains undisturbed. 
    If a fast negative pulse is then applied to the gate of the ON transistor, 
its drain current decreases and its drain voltage rises. A fraction of this 
rise is applied to the gate of the OFF transistor, causing some drain current 
to flow. The resultant drop in drain voltage, transferred to the gate of the 
ON transistor, causes a further rise at its drain. The action is thus one of 
positive feedback, with nearly instantaneous transfer of conduction from one 
device to the other. There is one such reversal each time a pulse is applied 
to the gate of the ON transistor. Normally pulses are applied to both 
transistors simultaneously so that whichever device is ON will be turned off 
by the action. 
    A bipolar transistor counterpart of the JFET bistable multivibrator with 
npn bipolar transistors may be used. The base of the transistor corresponds to 
the gate, the emitter to the source, and the collector to the drain. 
Insulated-gate field-effect transistors (IGFETs) may also be used in 
multivibrators.  
  
    Unsymmetrical bistable circuits.  Bistable action can be obtained in the 
emitter- or source-coupled circuit with one of the set of cross-coupling 
elements removed (Fig. 2). In this case, regenerative feedback necessary for 
bistable action is obtained by the one remaining common coupling element, 
leaving one emitter or gate free for triggering action. Biases can be adjusted 
such that device 1 is ON, forcing device 2 to be OFF. In this case, a pulse 
can be applied to the free input in such a direction as to reverse the states. 
Alternatively, device 1 may initially be OFF with device 2 ON. Then an 
opposite polarity pulse is required to reverse states. Such an unsymmetrical 
bistable circuit, historically referred to as the Schmitt trigger circuit, 
finds widespread use in many applications. 
  
    Monostable multivibrator.  A monostable multivibrator has only one stable 
state. If one of the normal active devices is in the conducting state, it will 
remain so until an external pulse is applied to make it nonconducting. The 
second device is thus made conducting and remains so for a duration dependent 
upon RC time constants within the circuit itself. 
  
    Astable multivibrator.  The astable multivibrator has capacitance coupling 
between both of the active devices and therefore has no permanently stable 
state. It will generate a periodic rectangular waveform at the output with a 
period equal to the sum of the OFF periods of the two devices. 
    Astable multivibrators, although normally free-running, can be 
synchronized with input pulses recurrent at a rate slightly faster than the 
natural recurrence rate of the device itself. If the synchronizing pulses are 
of sufficient amplitude, they will bring the internal waveform to the 
conduction level at an earlier than normal time and will thereby determine the 
recurrence rate. 
  
  
    Logic gate multivibrators.  Multivibrators may be formed by using two 
cross-coupled logic gates, with the unused input terminals used for triggering 
purposes. Such circuits are called flip-flops.  See  LOGIC CIRCUITS; 
TRANSISTOR.  
 �34s[Glenn M. Glasford]
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�6v�2v�32sMumps   �21m �33s 
    An acute contagious viral disease, characterized chiefly by enlargement of 
the parotid glands (parotitis). 
    Besides fever, the chief signs and symptoms are the direct mechanical 
effect of swelling on glands or organs where the virus localizes. One or both 
parotids may swell rapidly, producing severe pain when the mouth is opened. In 
orchitis, the testicle is inflamed but is enclosed by an inelastic membrane 
and cannot swell; pressure necrosis produces atrophy, and if both testicles 
are affected, sterility may result. The ovary may enlarge, without sequelae. 
    An attenuated live virus vaccine can induce immunity without parotitis. It 
is recommended particularly for adults exposed to infected children, for 
students in boarding schools and colleges, and for military troops. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sMuonium �21m �33s
    An exotic atom, Mu or (MU+, e-), formed when a positively charged muon
(MU+) and an electron are bound by their mutual electrical attraction.	It is a
light, unstable isotope of hydrogen, with a muon replacing the proton.	Muonium
has a mass 0.11 times that of a hydrogen atom due to the lighter mass of the
muon, and a mean lifetime of 2.2 microseconds, determined by the spontaneous
decay of the muon (MU+ --> e+ v[sub e][BAR v sub MU]).	Muonium is formed when
beams of MU+ produced in particle accelerators are stopped in certain
nonmetallic targets.
    Since muonium is a system consisting only of leptons, it serves as a
testing ground for the theory of quantum electrodynamics (QED), which
describes the electromagnetic interaction between particles. Muonium chemistry
and muonium spin rotation (MSR) are two developing subfields which seek to
understand the chemical and physical behavior of a light hydrogen isotope in
matter and to probe the structure of materials.  See  POSITRONIUM; QUANTUM
ELECTRODYNAMICS.
 �34s[Patrick O. Egan]
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�6v�2v�32sMurine typhus fever   �21m �33s 
    An infectious disease closely related to epidemic typhus fever. This 
generally milder, febrile disease with a more gradual onset, shorter duration, 
and lower mortality than the epidemic form is caused by Rickettsia typhi 
(synonyms manchuriae, mooseri).  See  EPIDEMIC TYPHUS FEVER; RICKETTSIALES; 
RICKETTSIOSES.  
    The primary disease cycle occurs between rats and their fleas with 
sporadic involvement of humans. The vector is chiefly the tropical rat flea 
Xenopsylla cheopis, though other fleas can be infected. As in epidemic typhus, 
contamination by the feces of the vector, rather than its bites, is 
responsible for transmission.  See  SIPHONAPTERA. 
    Because of the primary cycle in commensal rats and their fleas, prevention 
and control are directed to them. The sporadic human incidence does not 
indicate need for a vaccination program, and protection of personnel in 
food-handling establishments in urban and suburban areas, where most cases 
occur, is best accomplished by rat control. 
 �34s[Cornelius B. Philip]
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�6v�2v�32sMuscle   �21m �33s 
    The tissue in the body in which cellular contractility has become most 
apparent. Almost all forms of protoplasm exhibit some degree of contractility, 
but in muscle fibers specialization has led to the preeminence of this 
property. In vertebrates three major types of muscle are recognized: smooth, 
cardiac, and skeletal. 
  
    Smooth muscle.  Smooth muscle, also designated visceral and sometimes 
involuntary, is the simplest type. These muscles consist of elongated fusiform 
cells which contain a central oval nucleus. The size of such fibers varies 
greatly, from a few microns up to 0.02 in. (0.5 mm) in length. These fibers 
contract relatively slowly and have the ability to maintain contraction for a 
long time. Smooth muscle forms the major contractile elements of the viscera, 
especially those of the respiratory and digestive tracts, and the blood 
vessels. Smooth muscle fibers in the skin regulate heat loss from the body. 
Those in the walls of various ducts and tubes in the body act to move the 
contents to their destinations, as in the biliary system, ureters, and 
reproductive tubes. 
    Smooth muscle is usually arranged in sheets or layers, commonly oriented 
in different directions. The major physiological properties of these muscles 
are their intrinsic ability to contract spontaneously and their dual 
regulation by the autonomic nerves of the sympathetic and parasympathetic 
systems.  See  AUTONOMIC NERVOUS SYSTEM. 
  
    Cardiac muscle.  Cardiac muscle has many properties in common with smooth 
muscle; for example, it is innervated by the autonomic system and retains the 
ability to contract spontaneously. Presumably, cardiac muscle evolved as a 
specialized type from the general smooth muscle of the circulatory vessels. 
Its rhythmic contraction begins early in embryonic development and continues 
until death. Variations in the rate of contraction are induced by autonomic 
regulation and by many other local and systemic factors. 
    The cardiac fiber, like smooth muscle, has a central nucleus, but the cell 
is elongated and not symmetrical. It is a syncytium, a multinuclear cell or a 
multicellular structure without cell walls. Histologically, cardiac muscle has 
cross-striations very similar to those of skeletal muscle, and dense 
transverse bands, the intercalated disks, which occur at short intervals.  See 
  HEART (VERTEBRATE). 
  
    Skeletal muscle.  Skeletal muscle is also called striated, somatic, and 
voluntary muscle, depending on whether the description is based on the 
appearance, the location, or the innervation. The individual cells or fibers 
are distinct from one another and vary greatly in size from over 6 in. (15 cm) 
in length to less than 0.04 in. (1 mm). These fibers do not ordinarily branch, 
and they are surrounded by a complex membrane, the sarcolemma. Within each 
fiber are many nuclei; thus it is actually a syncytium formed by the fusion of 
many precursor cells. 
    The transverse striations of skeletal muscle form a characteristic pattern 
of light and dark bands within which are narrower bands. These bands are 
dependent upon the arrangement of the two sets of sliding filaments and the 
connections between them.  See  MUSCLE PROTEINS; MUSCULAR SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sMuscle proteins   �21m �33s 
    Specialized proteins in muscle cells are the building blocks of the 
structures constituting the moving machinery of muscle. They are disposed in 
myofilaments which are discernible by electron microscopy. These myofilaments 
are of two kinds, and their regular arrangement within the cell gives the 
striated pattern to skeletal muscle fibers (see illustration). It is 
recognized that the sliding of the two sets of filaments relative to each 
other is the molecular basis of muscle contraction. To understand the ultimate 
mechanism that causes the movement of these filaments (relative to each 
other), it is necessary to consider the features of the individual molecules 
making up these filaments. Evidence has been accumulating that practically all 
nonmuscle cells, although lacking the filaments of muscle, contain proteins 
similar to those found in muscle; these proteins are likely to be involved in 
cell motility and in determining properties of cell membranes. 
  
    Molecules of myosin, amounting to about 60% of the total muscle protein, 
are arranged in filaments occupying the central zone of each segment 
(sarcomere) or the fibril, the A band. Myosin is an elongated molecule made up 
of two intertwined heavy peptide chains (molecular weight of about 200,000) 
whose  ends form two separate globular structures. The intertwined portion 
forms a rigid rod; each of the two globular portions (heads) contains a center 
capable of combining with, and splitting off, the terminal phosphate of 
adenosinetriphosphate (ATP); ATP is the ultimate source of energy for muscle 
contraction. Each globular head can combine with actin. 
    The chief constituent of the filaments originating in the Z band of each 
sarcomere is actin. Actin filaments are made up of globular units in a double 
helix; there are 13 to 15 of these units in each strand for every complete 
turn of this helix. 
    These two proteins, or rather protein complexes, are associated with the 
actin filaments. Tropomyosin is an ALPHA-helical protein containing two 
intertwined polypeptide chains that extend over about seven globular actin 
units. Each tropomyosin molecule is associated with one troponin molecule, 
which in turn consists of three different types of subunits. In contrast to 
the tropomyosin molecule, the troponin complex and its subunits are thought to 
be essentially spherical in shape. Both tropomyosin and troponin are required 
to make the interaction of actin and myosin sensitive to calcium ions. 
    A great deal of information has come to light concerning the process by 
which the interaction between actin and myosin is regulated in the living 
cell. In higher organisms troponin and tropomyosin participate in this 
regulation. In the presence of these proteins the combination of actin and 
myosin cannot take place. However, if a small amount of ionized calcium is 
present, the inhibitory effect of the tropomyosin-troponin system is reversed, 
and the interaction with actin and myosin becomes fully effective. 
    This regulation involves a movement of tropomyosin molecules within the 
thin filaments from positions in which they block the combination of actin 
with the myosin moiety. Within the muscle cell the amount of calcium available 
is regulated by certain membranous structures, collectively the sarcoplasmic 
reticulum, that appear to have the ability to store calcium in the resting 
state. Upon excitation, the outer membrane of the muscle cell undergoes a 
change in permeability. This change spreads inside the muscle through the 
so-called transverse tubular system and causes a release of calcium within the 
cell. The calcium ions thus released permit the interaction of actin and 
myosin, leading to the liberation of chemical energy from ATP and its 
transformation into mechanical work.  See  MOTOR SYSTEMS; MUSCLE. 
 �34s[John Gergely]
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�6v�2v�32sMuscovite   �21m �33s 
    One of the mica group of minerals with the composition 
K2Al4(Si6Al2)O20(OH)4. It is also called white, or potash, mica. Muscovite 
occurs in some granites and is especially abundant in pegmatites, the chief 
commercial deposits. It is a widespread constituent of slates, phyllites, 
schists, and gneisses. Physical properties include easy perfect basal cleavage 
to sheets that are flexible and elastic; specific gravity of 2.7 - 3.1; and 
hardness of 2 - 2.5 on Mohs scale. Colors in sheets are in shades of brown, 
green, and yellow; viewed microscopically, muscovite is colorless.  See  MICA; 
SILICATE MINERALS. 
 �34s[E. William Heinrich]
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�6v�2v�32sMuscular dystrophy   �21m �33s 
    A group of muscle diseases which are hereditary and characterized by 
progressive muscle weakness and wasting. The etiology of the various muscular 
dystrophies is unknown. Muscular dystrophy is a primary disease of the muscle 
cells themselves, and not the result of abnormality in their innervation. In 
some types of muscular dystrophy the disease appears to be restricted to 
skeletal muscles alone, but in some others cardiac muscle may also be 
affected, and in still others the skeletal muscle involvement appears to be 
part of a more generalized disorder, with abnormalities in other organ systems 
as well. 
    The most common forms of muscular dystrophy are the sex-linked recessive 
Duchenne's pseudohypertrophic muscular dystrophy and the dominantly inherited 
myotonic dystrophy. Other, less common forms include a milder form of 
sex-linked pseudohypertrophic muscular dystrophy (Becker's type), dominantly 
inherited facioscapulohumeral muscular dystrophy, and recessively inherited 
limb-girdle muscular dystrophy. The ocular myopathies appear to include 
several different disease entities, all of which have as a common feature the 
progressive paralysis of extraocular muscles. 
    As the basic defect in any of the muscular dystrophies is unknown, 
treatment is purely symptomatic. Unfortunately, no treatment has been shown to 
alter the course of the muscular dystrophies. 
 �34s[S. Mark Sumi]
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�6v�2v�32sMuscular system �21m �33s
    The muscular system consists of muscular cells, the contractile elements
with the specialized property of exerting tension during contraction, and
associated connective tissues. The three morphologic types of muscles are
voluntary muscle, involuntary muscle, and cardiac muscle. The voluntary,
striated, or skeletal muscles are involved with general posture and movements
of the head, body, and limbs. The involuntary, nonstriated, or smooth muscles
are the muscles of the walls of hollow organs of the digestive, circulatory,
respiratory, and reproductive systems, and other visceral structures. Cardiac
muscle is the intrinsic muscle tissue of the heart.  See  MUSCLE.

    Anatomy.  Muscle groups are particularly distinct in elasmobranchs and
other primitive fishes, and they are generally defined on the basis of their
embryonic origin in these animals. Two major groups of skeletal muscles are
recognized, somatic (parietal) muscles, which develop from the myotomes, and
branchiomeric muscles, which develop in the pharyngeal wall from lateral plate
mesoderm. The somatic musculature is subdivided into axial muscles, which
develop directly from the myotomes and lie along the longitudinal axis of the
body, and appendicular muscles, which develop within the limb bud from
mesoderm derived phylogenetically as buds from the myotomes.
    The vertebrate muscular system is the largest of the organ systems, making
up 35 - 40% of the body weight in humans. The movement of vertebrates is
accomplished exclusively by muscular action, and muscles play the major role
in transporting materials within the body. Muscles also help to tie the bones
of the skeleton together and supplement the skeleton in supporting the body
against gravity.  See  SKELETAL SYSTEM.
    Most of the axial musculature is located along the back and flanks of the
body, and this part is referred to as trunk musculature. But anteriorly the
axial musculature is modified and assigned to other subgroups. Certain of the
occipital and neck myotomes form the hypobranchial muscles, and the most
anterior myotomes form the extrinsic ocular muscles.
    The hypaxial musculature of tetrapods can be subdivided into three groups:
(1) a subvertebral (hyposkeletal) group located ventral to the transverse
processes and lateral to the centra of the vertebrae, (2) the flank muscles
forming the lateral part of the body wall, and (3) the ventral abdominal
muscles located on each side of the midventral line. The subvertebral
musculature assists the epaxial muscles in the support and movement of the
vertebral column. Most of the flank musculature takes the form of broad, thin
sheets of muscle that form much of the body wall and support the viscera. The
midventral hypaxial musculature in all tetrapods consists of the rectus
abdominis, a longitudinal muscle on each side of the midline that extends from
the pelvic region to the anterior part of the trunk.
    The hypobranchial musculature extends from the pectoral girdle forward
along the ventral surface of the neck and pharynx to the hyoid arch, chin, and
into the tongue. It is regarded as a continuation of part of the hypaxial
trunk musculature.
    Limb muscles are often classified as intrinsic if they lie entirely within
the confines of the appendage and girdle, and extrinsic if they extend from
the girdle or appendage to other parts of the body. In fishes, movements of
the paired fins are not complex or powerful and the appendicular muscles in
the strictest sense are morphologically simple. In terrestrial vertebrates,
the limbs become the main organs for support and locomotion, and the
appendicular muscles become correspondingly powerful and complex. The muscles
are too numerous to describe individually, but they can be sorted into dorsal
and ventral groups, because tetrapod muscles originate embryonically in
piscine fashion from a dorsal and a ventral premuscular mass within the limb
bud. In general, the ventral muscles, which also spread onto the anterior
surface of the girdle and appendage, act to protract and adduct the limb and
to flex its distal segments; the dorsal muscles, which also extend onto the
posterior surface of the girdle and appendage, have the opposite effects
(retraction, abduction, and extension). The limb muscles also serve as
flexible ties or braces that can fix the bones at a joint and support the body.
    Flight in birds has entailed a considerable modification of the
musculature of the pectoral region. As one example, the ventral adductor
muscles are exceedingly large and powerful, and the area from which they arise
is increased by the enlargement of the sternum and the evolution of a large
sternal keel. Not only does a ventral muscle, the pectoralis, play a major
role in the downstroke of the humerus, but a ventral muscle, the
supracoracoideus, is active in the upstroke as well.
    In a number of terrestrial vertebrates, particularly amniotes, certain of
the more superficial skeletal muscles of the body have spread out beneath the
skin and inserted into it. These may be described as integumentary muscles.
Integumentary muscles are particularly well developed in mammals and include
the facial muscles and platysma, derived from the hyoid musculature, and often
a large cutaneous trunci (Fig. 1). The last is derived from the pectoralis and
latissimus dorsi and fans out beneath the skin of the trunk. The twitching of
the skin of an ungulate is caused by this muscle.


    Muscle mechanics. Many of the bones serve as lever arms, and the
contractions of muscles are forces acting on these arms (Fig. 2). The joint,
of course, is the fulcrum and it is at one end of the lever. The length of the
force arm is the perpendicular distance from the fulcrum to the line of action
of the muscle; the length of the work arm is the perpendicular distance from
the fulcrum to the point of application of the power generated in the lever.
Compactness of the body and physiological properties of the muscle
necessitates that a muscle attach close to the fulcrum; therefore, the force
arm is considerably shorter than the work arm. Most muscles are at a mechanical
disadvantage, for they must generate forces greater than the work to be done,
but an advantage of this is a small muscular excursion can induce a much
greater movement at the end of the lever. See SKELETAL SYSTEM.
    Slight shifts in the attachments of a muscle that bring ti toward or away
from the fulcrum, and changes in the length of the work arm, can alter the
relationship between force and amount or speed of movement.
    In general, the force of a muscle is inversely related to the amount and
speed of movement that it can cause. Certain patterns of the skeleton and
muscles are adapted for extensive, fast movement at the expense of force,
whereas others are adapted for force at the expense of speed. In the limb of
a horse, which is adapted for long strides and speed, the muscles that move
the limb insert close to the fulcrum and the appendage is long. This provides
a short force arm but a very long work arm to the lever system (Fig. 2). In
the front leg of a mole, which is adapted for powerful digging, the distance
from the fulcrum to the insertion of the muscles is relatively greater and the
length of the appendage is less, with the result that the length of the force
arm is increased relative to the length of the work arm.
[Warren F. Walker, Jr.]

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�6v�2v�32sMuscular system disorders   �21m �33s 
    Disorders affecting skeletal (voluntary) muscle. The normal functioning of 
the skeletal muscle is dependent not only on the integrity of the muscle 
fibers themselves, but also on that of the motor cortex, the pyramidal tract, 
and the extrapyramidal system (including the cerebellum). It also depends on 
innervation by the motoneurons of the brainstem and the spinal cord. In 
addition, the proper functioning of the other organ systems, such as the 
endocrine system, and variations in the concentration of various electrolytes, 
may also affect muscle function. 
    Damage to the motor cortex or the pyramidal tract produces the type of 
weakness seen in humans after a stroke or spinal cord injury. Although the 
paralyzed limb may initially be flaccid (hypotonic), spasticity (hypertonia) 
eventually develops. Despite the weakness, muscle atrophy is usually not 
striking. When the extrapyramidal system or the cerebellum is the site of 
damage, instead of weakness there are uncontrolled movements, difficulty with 
coordination, or both. In either of these situations there is no 
characteristic change in the muscle, either grossly or microscopically. At 
most, atrophy of type 2 fibers is seen.  See  ATROPHY. 
  
    Motoneuron damage.  With damage to the spinal motoneuron or its axon, 
there is flaccid weakness of the muscle with proportionate wasting. Direct 
involvement of the spinal motoneuron was typically seen in poliomyelitis, but 
is now seen more commonly in the progressive spinomuscular atrophies of 
infancy and childhood, and in amyotrophic lateral sclerosis in adults. 
Spontaneous twitching of groups of muscle fibers (fasciculation) innervated by 
the same motoneuron (motor unit) is frequently seen in these disorders.  See  
POLIOMYELITIS.  
    Progressive diseases of unknown cause and may occur in infancy 
(Werdnig-Hoffmann disease) or later in childhood (Kugelberg-Wielander 
disease). Both diseases result from degeneration of the motoneurons and appear 
to be inherited in an autosomal recessive pattern. In the infantile form, the 
baby is often floppy from birth with generalized weakness, a poor cry, and 
difficulty in sucking and breathing. Many children succumb in early childhood. 
In the later childhood forms, the rate of progression is slower and the 
outlook better. In these children weakness is more marked in the proximal 
muscles of the limbs. 
    Amyotrophic lateral sclerosis is a progressive disease of unknown cause, 
in which both the brainstem and spinal motoneurons, as well as the 
corticospinal tracts, undergo degeneration. This is a relatively rapid 
progressive disease, with death usually occurring within 3 years of diagnosis 
due to swallowing difficulty and respiratory failure. Although some cases of 
ALS appear to be inherited, most cases occur sporadically. 
  
    Neuromuscular junction.  The most common example of disease at the 
neuromuscular junction is myasthenia gravis. Other, less common diseases are 
the myasthenic Eaton-Lambert syndrome and botulism.  See  MYASTHENIA GRAVIS. 
  
    Myopathy.  Abnormalities of the muscle itself (myopathy) obviously result 
in muscle weakness, and muscle diseases fall into two large groups: those with 
a genetic basis and those which are nongenetic. 
    Congenital myopathies and the muscular dystrophies constitute the genetic 
muscle diseases. Congenital myopathies are characterized by a generalized 
weakness which is present at birth. The weakness is usually not progressive 
and often improves with time. Muscle fiber necrosis, a characteristic of the 
muscular dystrophies, is not seen in the congenital myopathies.  See  MUSCULAR 
DYSTROPHY.  
    Nongenetic or acquired diseases are all characterized by rapidly 
progressive weakness of the proximal muscles of the limbs, and so resemble the 
limb-girdle form of muscular dystrophy in the distribution of weakness. They 
are often included as inflammatory myopathies. Individuals with these 
disorders have difficulty arising from a recumbent or sitting position, in 
climbing stairs, and in lifting heavy objects onto a shelf. They also often 
have tender and painful muscles, may be febrile, and may have other 
manifestations of a systemic illness. 
  
    Metabolic diseases.  A number of metabolic diseases have been associated 
with muscle symptoms. These include thyroid diseases and certain endocrine 
diseases, particularly Cushing's syndrome, which are either spontaneous or 
secondary to therapeutically administered adrenocorticosteroid hormones. No 
specific histologic changes have been described in the muscle in these 
disorders. Muscular symptoms are also seen with some of the glycogen storage 
diseases. Muscle weakness and paralysis may also be associated with 
alterations in the level of serum potassium. 
  
    Myotonia.  A delayed relaxation of the muscle after forceful contraction 
(myotonic) is another symptom of muscle disease. This phenomenon is a 
characteristic feature of myotonic dystrophy and of congenital myotonia. 
Myotonia is usually present from early life and is often associated with 
muscle hypertrophy, but muscle weakness is not a feature of this disease. The 
muscle shows no definite histologic changes.  See  MUSCULAR SYSTEM. 
 �34s[S. Mark Sumi]
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�6v�2v�32sMushroom   �21m �33s 
    A fungus belonging to the basidiomycetous order Agaricales, or the 
fruiting body (basidiocarp) of such a fungus. The generalized life history of 
a mushroom begins with the germination of a basidiospore which produces a 
primary mycelium composed of filaments (hyphae) with one nucleus per cell. If 
two compatible primary mycelia fuse, a secondary mycelium with two nuclei in 
each cell develops. This underground secondary mycelium, which may be 
extensive and perennial, forms at its periphery small masses of compacted 
hyphae which enlarge and differentiate into immature basidiocarps, or mushroom 
buttons (see illustration). These buttons enlarge rapidly, burst through the 
soil and become mature basidiocarps (see illustration). Basidia form on plates 
of tissue (gills) underneath the cap of the mushroom. Each basidium produces 
basidiospores (usually four) which are shot off and dispersed by wind. Since 
mushrooms develop near the margin of the circular mycelium, they sometimes 
form ``fairy rings.''  See  BASIDIOMYCOTINA; FUNGI. 
    Basidiocarps of some mushrooms are prized as food; others are poisonous. 
The term toadstool is commonly applied to basidiocarps of poisonous species. 
The only way to distinguish edible from poisonous species is by identification. 
 �34s[Robert M. Page]
�33s 
  
  
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�6v�2v�32sMusical instruments   �21m �33s 
    Instruments for producing musical sounds have long been classified as 
woodwinds, brass, percussion, or strings; to these must be added electrical 
and electronic instruments. In a sense, all these instruments implement and 
extend the capability of the original musical instrument, the singing voice. 
The classes mentioned are useful in grouping instruments in a general way for 
the kinds of sounds they produce, even though woodwind instruments are not 
necessarily made of wood, nor are brass instruments always made of metal. 
  
    Woodwind instruments.  Woodwind instruments are distinguished primarily by 
the fact that the effective length of the vibrating air column is shortened by 
opening lateral side holes in succession. Two distinctly different means of 
generating the sound are employed. For the flute, and its half-size version 
the piccolo, the player blows across the embouchure hole near one end in such 
a way as to cause periodic puffs of air to enter the tube; after a turbulent 
turning these puffs excite the air column longitudinally. This method of 
excitation leaves the tube acoustically open in the sense that the contained 
air vibrates much as it does in a simple tube with both ends open to the 
atmosphere.  
    For the double-reed oboe or bassoon, the player holds between the lips a 
pair of thin reeds (pieces of cane appropriately thinned, shaped, and bound 
together) that beat against each other to change the player's breath to puffs 
of air. For clarinets and saxophones, a single reed attached to a mouthpiece 
by a ligature functions in a similar way. The portion of the mouthpiece (the 
lay) against which the reed beats must be appropriately curved; the character 
of the sound is modified somewhat by the volume of the mouthpiece as well as 
by the shape and material of the reed. 
    For both the single- and double-reed instruments, the reeds vibrate under 
the influence of sound waves reflected back from the distant end of the air 
column and allow the puffs of air to enter when the sound pressure within the 
instrument is large. Thus (in contrast to the flute) the air vibration at the 
reed end is that associated with an acoustically closed end. 
  
    Brass instruments.  The typical brass instrument consists of a cup-shaped 
mouthpiece, a slightly tapered mouthpipe, cylindrical tubing including valves, 
and a roughly hyperbolic bell. Puffs of air are introduced by the player via 
vibrating lips stretched over the mouthpiece. The action is comparable to that 
of the clarinet, in that the mouthpiece end is nearly closed acoustically. The 
length of the air column is increased by tubing switched in by use of valves, 
either piston or rotary: a common arrangement is such that the first valve 
lowers the intonation by two semitones, the second by one semitone, and the 
third by three semitones. For a given length of tubing, different tones are 
produced by tensioning the lips to excite different modes of vibration whose 
frequencies are approximately in the ratios 2;rc3;rc4;rc5;rc6;rc8. 
  
    Percussion instruments.  Instruments such as the timpani (kettledrums) and 
xylophone are called percussion instruments because the sound is initiated by 
a blow. Two kinds of sound producers are involved: a membrane under tension, 
associated with a cavity that can influence the frequency of vibration, as in 
the case of the timpani; and a rigid bar or plate vibrating transversely, 
whose frequency is little affected by any resonator that may be attached. Some 
percussion instruments give a well-defined sound that excites a sensation of 
definite pitch, such as does a church bell; others, such as drums, cymbals, 
and triangles, are useful primarily for rhythm effects. 
  
    Stringed instruments.  For the guitar and harp, strings are set into 
vibration by plucking; for the other stringed instruments the vibration is 
usually initiated and maintained by bowing. The frequency of vibration is 
primarily established by the length, tension, and mass per unit length of the 
string. A string vibrates not only at the lowest (fundamental) frequency, but 
also at the same time at higher frequencies which tend toward integer 
multiples of the fundamental frequency. The sound radiated from the instrument 
is thus complex. 
    The radiation of sound from a stringed instrument is enhanced by a 
resonator consisting of an almost closed air cavity. Some of the energy of the 
vibrating string is transmitted via the bridge to the walls of the cavity. In 
a carefully constructed violin the resonances of the air cavity and its 
vibrating walls are distributed in frequency in order to afford a relatively 
uniform response throughout the playing range of the instrument. 
  
    Keyboard instruments.  Instruments such as the celesta, pipe organ, 
accordion, and piano are usually put in a group called keyboard instruments, 
because the respective vibrating bars, pipes, reeds, and strings in these 
instruments are selected by use of keys in a keyboard. The celesta and piano 
could also be described as percussion instruments, because hammers strike the 
bars and strings; the pipe organ and the accordion, with its wind-driven free 
reeds, are wind instruments. By its multiple keyboards (and pedal board) the 
pipe organ puts under the control of a single player thousands of sources 
whose distinctive sounds can be reproduced on command. 
  
    Electrical and electronic types.  Musical instruments of the kinds already 
described become quasi-electrical instruments by the addition of a microphone 
to pick up the airborne sound, an amplifier, and a loudspeaker. Alternately, a 
vibration pickup can be used to generate an electrical signal directly from 
the vibration of a string; this is the case in the electric guitar and 
electric piano. The signal can be modified electrically before being radiated 
as sound from a loudspeaker. Since the resonance box or sounding board is not 
needed for these electrical stringed instruments, the damping is very slight 
and the vibration can last for a long time so that relatively sustained sounds 
can be obtained if desired. Rigid vibrators, such as reeds, rods, bars, or 
tubes, with associated pickups are also exploited as electrical musical 
instruments.  See  ACOUSTIC RESONATOR; RESONANCE (ACOUSTICS AND MECHANICS); 
SOUND-REPRODUCING SYSTEMS; SYMPATHETIC VIBRATION; VIBRATION. 
 �34s[Robert W. Young]
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�6v�2v�32sMuskeg   �21m �33s 
    A term derived from Chippewan Indian for ``grassy bog.'' In North America 
ecologists apply diverse usage, but most include peat bogs or tussock meadows, 
with variable woody vegetation such as spruce or tamarack. Plant remains 
accumulate when trapped in the water or in media such as sphagnum moss which 
inhibit decay. The peat might accrue indefinitely or might approach a steady 
state of raised bogs, blanket bogs, or forest if input became balanced by 
erosion or by loss as methane and CO2. 
    Organic terrain occurs on every continent. The largest expanses of it are 
in the Soviet Union (especially western Siberia) and Canada. Typical bog or 
spruce-larch muskeg develops in cool temperature and subarctic to arctic 
lands. In subtropics and the tropics there is considerable organic terrain, as 
in Paraguay, Uruguay, and Guyana in South America. Thus, despite differences 
in climate or floristics, the phenomenon of peat formation persits if local 
aeration or biochemical conditions hinder decay. Despite climatic and biotic 
differences, gross peat structure categories seem comparable the world over.  
See  BOG; PEAT. 
 �34s[Norman W. Radforth]
�33s 
  
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�6v�2v�32sMuskmelon   �21m �33s 
    The edible fruit of Cucumis melo, belonging to the gourd family, 
Cucurbitaceae, as do other vine crops such as cucumber, watermelons, pumpkin, 
and squash. The muskmelon appears to be indigenous to Africa.  See  
CANTALOUPE; HONEY DEW MELON; PERSIAN MELON; VIOLALES. 
    The plants are annual, trailing vines, with three to five runners. The 
runners produce short fruiting branches, which bear the perfect flowers and 
later the fruits. Muskmelons maturing on the vine without becoming overripe 
are superior in quality to those harvested immature. The sugar content, 
flavor, and texture of the fresh flesh improves very rapidly as the fruit 
approaches maturity. When mature, the melon is sweet, averages 6 to 8% sugar, 
and has a slight to distinctly musky odor and flavor, depending upon cultivar 
and environment. The flesh is rich in potassium, in vitamin C and, when deep 
orange, also in vitamin A. 
 �34s[Frank W. Zink]
�33s 
  
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�6v�2v�32sMusk-ox   �21m �33s 
    An even-toed ungulate, Ovibos moschatus, which is a member of the family 
Bovidae in the mammalian order Artiodactyla. This single species is the 
northernmost representative of the family, ranging through the tundra areas 
and snowfields of Canada and Alaska, as well as Greenland. 
    The musk-ox derives its name from the musky odor it emits. It is a stoutly 
built animal (see illustration) and has a coat of long dense hair that is 
resistant to the extreme cold of the windswept treeless tundra. They do not 
hibernate and are usually found in herds of 20 - 100 animals huddled together 
for warmth. As protection against its natural enemy, the wolf, the musk-ox 
will form a circle with the young inside. Since a cow produces a single calf 
every 2 years, the numbers have been so reduced that they are now protected by 
the Canadian government.  See  ARTIODACTYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sMuskrat   �21m �33s 
    The largest member of the rodent subfamily Microtinae in the family 
Muridae, it is also known as the musquash. Also included in this subfamily are 
the lemmings and voles. Ondatra zibethica, the single species, is about the 
size of a rabbit. It is economically important for its fur, which is shiny, 
soft, and thick. It received its name because of its inguinal glands which 
produce a characteristic musky odor. The muskrat is omnivorous, eating various 
types of vegetation, such as reeds and tuberous roots of aquatic plants, and 
animals, such as mussels, insects, and fish. These animals become active in 
March, when mating occurs. The female has two or three litters of 5 - 9 young 
each year, and the last litter of the season remains in the parental abode 
through winter.  See  LEMMING; RODENTIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sMustard   �21m �33s 
    Any one of a number of annual crucifer species of Asiatic origin belonging 
to the plant order Capparales. Mustards eaten as greens are Brassica juncea, 
B. juncea var. crispifolia, and B. hirta. Table mustard and oils are obtained 
from B. nigra. Important production centers for mustard greens are in the 
South, where the crop is popular. Montana and the West Coast states are 
important sources of mustard seed.  See  CAPPARALES. 
 �34s[H. John Carew]
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�6v�2v�32sMutagens and carcinogens   �21m �33s 
    Mutagens are agents capable of altering the genetic material of an 
organism, leading to inherited differences; carcinogens are agents that cause 
cancer in animals, including humans. 
    Occupational skin cancer among workers exposed to mineral oils and tars 
was well known over a century ago; the first pure chemical compounds 
(carcinogens) that could be shown to induce tumors in experimental animals 
were isolated from coal tar and synthesized in the laboratory in 1930. Since 
then an enormous effort has been expended on the study of chemical 
carcinogenesis (the induction of cancer with chemicals). Well over 3000 
compounds are carcinogenic in animals, and of these a few are known to be 
active also in humans. For example, the high incidence of bladder cancer among 
former workers in dyestuff factories is directly attributable to their daily 
contact with aromatic amines such as 2-napthylamine and benzidine, both 
carcinogens. The great majority of chemical carcinogens are organic compounds 
belonging to a number of diverse structural types (polycyclic hydrocarbons, 
aromatic amines, nitrosamines, and so on). Some natural products, such as the 
aflatoxins which can occur in peanuts and certain grains as a result of 
contamination with the common yellow mold Aspergillus flavus, and a few 
inorganic materials, notably asbestos, are also carcinogenic.  See  AFLATOXIN; 
ASBESTOS.  
    Under the correct conditions almost all carcinogens are mutagens. This 
positive correlation is probably the best indication that the hypothesis that 
cancers are caused by somatic mutations is essentially correct. 
    It has slowly become apparent that a high proportion of cancers are caused 
by environmental factors, and therefore are preventable, at least in theory. 
This conclusion has been reached largely as a result of cancer epidemiology. 
For example, the incidence of stomach cancer is much higher in Japan than in 
the United States, but that among Japanese who have emigrated to California 
tends toward that of the indigenous population. This becomes noticeable in the 
second and third generations, suggesting that although the disease has an 
environmental cause, this is connected with cultural differences rather than 
some general factor to which the whole Japanese population in California is 
exposed.  
    There is no mystery about one major disease, lung cancer; all the 
epidemiological evidence agrees that this is largely caused by the cultural 
habit of cigarette smoking.  Cigarette tar is readily shown to be mutagenic 
and carcinogenic in appropriate tests.  See  CANCER (MEDICINE); MUTATION; 
ONCOLOGY.  
 �34s[M. M. Coombs]
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�6v�2v�32sMutation   �21m �33s 
    Any alteration capable of being replicated in the genetic material of an 
organism. When the alteration is in the nucleotide sequence of a single gene, 
it is referred to as gene mutation; when it involves the structures or number 
of the chromosomes it is referred to as chromosome mutation, or rearrangement. 
Mutations may be recognizable by their effects on the phenotype of the 
organism (mutant). 
  
    Gene mutations.  Two classes of gene mutations are recognized: point 
mutations and intragenic deletions. Two different types of point mutation have 
been described. In the first of these, one nucleic acid base is substituted 
for another. The second type of change results from the insertion of a base 
into, or its deletion from, the polynucleotide sequence. These mutations are 
all called sign mutations or frame-shift mutations because of their effect on 
the translation of the information of the gene.  See  NUCLEIC ACID. 
    More extensive deletions can occur within the gene which are sometimes 
difficult to distinguish from mutants which involve only one or two bases. In 
the most extreme case, all the informational material of the gene is lost. 
    A single-base alteration, whether a transition or a transversion, affects 
only the codon or triplet in which it occurs. Because of code redundancy, the 
altered triplet may still insert the same amino acid as before into the 
polypeptide chain, which in many cases is the product specified by the gene. 
Such DNA changes pass undetected. However, many base substitutions do lead to 
the insertion of a different amino acid, and the effect of this on the 
function of the gene product depends upon the amino acid and its importance in 
controlling the folding and shape of the enzyme molecule. Some substitutions 
have little or no effect, while others destroy the function of the molecule 
completely.  
    Single-base substitutions may sometimes lead not to a triplet which codes 
for a different amino acid but to the creation of a chain termination signal. 
Premature termination of translation at this point will lead to an incomplete 
and generally inactive polypeptide. 
    Sign mutations (adding or subtracting one or two bases to the nucleic acid 
base sequence of the gene) have a uniformly drastic effect on gene function. 
Because the bases of each triplet encode the information for each amino acid 
in the polypeptide product, and because they are read in sequence from one end 
of the gene to the other without any punctuation between triplets, insertion 
of an extra base or two bases will lead to translation out of register of the 
whole sequence distal to the insertion or deletion point. The polypeptide 
formed is at best drastically modified and usually fails to function at all. 
This sometimes is hard to distinguish from the effects of intragenic 
deletions. However, whereas extensive intragenic deletions cannot revert, the 
deletion of a single base can be compensated for by the insertion of another 
base at, or near, the site of the original change.  See  GENE; GENETIC CODE. 
  
    Chromosomal changes.  Some chromosomal changes involve alterations in the 
quantity of genetic material in the cell nuclei, while others simply lead to 
the rearrangement of chromosomal material without altering its total amount.  
See  CHROMOSOME ABERRATION. 
  
    Origins of mutations.  Mutations can be induced by various physical and 
chemical agents or can occur spontaneously without any artificial treatment 
with known mutagenic agents. 
    Until the discovery of x-rays as mutagens, all the mutants studied were 
spontaneous in origin; that is, they were obtained without the deliberate 
application of any mutagen. Spontaneous mutations occur unpredictably, and 
among the possible factors responsible for them are tautomeric changes 
occurring in the DNA bases which alter their pairing characteristics, ionizing 
radiation from various natural sources, naturally occurring chemical mutagens, 
and errors in the action of the DNA-polymerizing and correcting enzymes. 
    Spontaneous chromosomal aberrations are also found infrequently. One way 
in which deficiencies and duplications may be generated is by way of the 
breakage-fusion-bridge cycle: During a cell division one divided chromosome 
suffers a break near its tip, and the sticky ends of the daughter chromatids 
fuse. When the centromere divides and the halves begin to move to opposite 
poles, a chromosome bridge is formed, and breakage may occur again along this 
strand. Since new broken ends are produced, this sequence of events can be 
repeated. Unequal crossing over is sometimes cited as a source of duplications 
and deficiencies, but it is probably less important than often suggested. 
    In the absence of mutagenic treatment, mutations are very rare. In 1927 H. 
J. Muller discovered the x-rays significantly increased the frequency of 
mutation in Drosophila. Subsequently, other forms of ionizing radiation, for 
example, gamma rays, beta particles, fast and thermal neutrons, and alpha 
particles, were also found to be effective. Ultraviolet light is also an 
effective mutagen. The wavelength most employed experimentally is 253.7 nm, 
which corresponds to the peak of absorption of nucleic acids. 
    Some of the chemicals which have been found to be effective as mutagens 
are the alkylating agents which attack guanine principally although not 
exclusively. The N7 portion appears to be a major target in the guanine 
molecule, although the Osup6 alkylation product is probably more important 
mutagenically. Base analogs are incorported into DNA in place of normal bases 
and produce mutations probably because there is a higher chance that they will 
mispair at replication. Nitrous acid, on the other hand, alters DNA bases in 
place. Adenine becomes hypoxanthine and cytosine becomes uracil. In both cases 
the deaminated base pairs differently from the parent base. A third 
deamination product, xanthine, produced by the deamination of guanine, appears 
to be lethal in its effect and not mutagenic. Chemicals which react with DNA 
to generate mutations produce a range of chemical reaction products not all of 
which have significance for mutagenesis. 
  
    Significance of mutations.  Mutations are the source of genetic 
variability, upon which natural selection has worked to produce organisms 
adapted to their present environments. It is likely, therefore, that most new 
mutations will now be disadvantageous, reducing the degree of adaptation. 
Harmful mutations will be eliminated after being made homozygous or because 
the heterozygous effects reduce the fitness of carriers. This may take some 
generations, depending on the severity of their effects. Chromosome 
alterations may also have great significance in evolutionary advance. 
Duplications are, for example, believed to permit the accumulation of new 
mutational changes, some of which may prove useful at a later stage in an 
altered environment. 
    Rarely, mutations may occur which are beneficial: Drug yields may be 
enhanced in microorganisms, the characteristics of cereals can be improved. 
However, for the few mutations which are beneficial, many deleterious 
mutations must be discarded. Evidence suggests that the metabolic conditions 
in the treated cell and the specific activities of repair enzymes may 
sometimes promote the expression of some types of mutation rather than others. 
  See  DEOXYRIBONUCLEUIC ACID (DNA). 
 �34s[Brian J. Kilbey]
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�6v�2v�32sMyasthenia gravis   �21m �33s 
    A disease resulting from an abnormality in neuromuscular transmission, 
characterized by a fluctuating degree of muscle weakness. The weakness is 
usually aggravated by activity, and there is partial or complete restoration 
of strength after a period of rest or the administration of anticholinesterase 
medications.  See  ACETYLCHOLINE. 
    It has been shown that the basic defect in myasthenia gravis is a 
reduction in the number of acetylcholine receptor sites in the postsynaptic 
membrane of the neuromuscular junction. It has also been shown that many 
myasthenic patients have immunoglobulins in the serum that partially block 
acetylcholine receptors.  See  IMMUNOGLOBULIN; IMMUNOPATHOLOGY. 
    Abnormalities have been demonstrated in the thymus gland and skeletal 
muscle in myasthenia gravis. There is an increased incidence of thymoma in 
myasthenia gravis, and in those without a thymoma, hyperplasia of the germinal 
centers is a common finding in the thymus gland. 
    Although the disease affects young women more commonly, usually in the 
third decade, it can occur in either sex at any age. In the majority of 
persons, weakness affects muscles of head, neck, and limbs (generalized 
myasthenia), but in some the weakness is restricted to the muscles of the eyes 
(ocular myasthenia), in which case the disease is usually benign. 
    The standard treatment for myasthenia gravis has been the use of 
longer-acting anticholinesterase agents; thymectomy and immunosuppressive 
drugs are reserved for those patients with generalized myasthenia that does 
not respond sufficiently to these agents.  See  SYNAPTIC TRANSMISSION. 
 �34s[S. Mark Sumi]
�33s 
  
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�6v�2v�32sMycelia Sterilia   �21m �33s 
    An order of fungi of the class Fungi Imperfecti. Certain members of the 
Mycelia Sterilia are pathogens of potatoes, onions, garlic, and other wild and 
cultivated plants. There are approximately 20 genera with 200 species. 
    The Mycelia Sterilia are a heterogeneous collection of Fungi Imperfecti 
which have no spores. Early mycologists gave names to collections of mycelial 
structures, such as sclerotia and rhizomorphs, for classification purposes, 
but these names were discontinued when it was seen that the structures did not 
produce spores.  See  FUNGI IMPERFECTI. 
 �34s[N. F. Buchwald]
�33s 
  
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�6v�2v�32sMycetozoia   �21m �33s 
    A subclass of protozoan class Rhizopodea. There are three orders: 
Eumycetozoida, producing freeliving plasmodia; Acrasida, forming 
pseudoplasmodia (aggregates of amebas); and Plasmodiophorida, endoparasitic in 
plants. Life cycles vary.  See  ACRASIDA; EUMYCETOZOIDA; PLASMODIOPHORIDA; 
RHIZOPODEA.  
 �34s[Richard P. Hall]
�33s 
  
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�6v�2v�32sMycobacterial diseases   �21m �33s 
    Humans are subject to infection by species of Mycobacterium, a genus of 
acid-fast, nonmotile, rod-shaped bacteria, some of which may produce disease. 
Studies completed in many countries demonstrate that mycobacterial infections 
with no recognizable disease are common; in many areas reactions to tuberculin 
very often are due to infections with mycobacteria other than tubercle 
bacilli; and a number of mycobacterial diseases, some of which mimic 
tuberculosis, are prevalent in some countries and very rare in others. Their 
epidemiological relationships are strikingly different from those of 
tuberculosis. Some mycobacterial diseases are characteristically localized in 
the superficial, cooler parts of the body. Each species of Mycobacterium is 
distinct in structure and in biochemical and other reactions (see table). 
Although these species are demonstrably distinct by immunological tests, 
prominent cross reactions occur; diagnostic serologic tests with patients' 
sera have not been available for identification of the diseases.  See  
IMMUNOLOGY; LEPROSY; SEROLOGY; SKIN TEST; TUBERCULOSIS. 
 �34s[Ernest H. Runyon]
�33s 
  
A700000001606�254s�5v�1v1130
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�6v�2v�32sMycology �21m �33s
    A field of microbiology concerned with the study of fungi, those
microorganisms included in the division Thallobionta (Thallophyta) of the
plant kingdom. Fungi lack photosynthetic pigments and include such familiar
plants as the molds, yeasts, and bacteria.  See  FUNGI; THALLOBIONTA.
    Medical mycology is a specialized field of study concerned with fungi that
cause disease. Practically all of the pathogenic fungi affecting humans belong
to either the Schizomycetes or the Fungi Imperfecti; a few species are
Phycomycetes. From a clinical viewpoint two major categories of fungus disease
are recognized: those which cause superficial skin infections, the
dermatophytes, and those which may produce deep invasive infections.  See
FUNGI IMPERFECTI; MEDICAL MYCOLOGY; SCHIZOMYCETES.
 �34s[Edward G. Stuart /R. E. Kallio]
�33s

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�6v�2v�32sMycorrhizae   �21m �33s 
    Joint or dual organs of absorption formed when healthy absorbing organs 
(roots, rhizomes, or thalli) of many, perhaps most, higher plants are 
inhabited by symbiotic fungi. 
    Mycorrhizae are of several types of structure and physiology as expected 
from the fact that the host species may belong to the Bryophyta, 
Polypodiophyta, Pinophyta, or Magnoliophyta, and the fungi to Basidiomycetes, 
Ascomycetes, Phycomycetes, and to the imperfect fungi. Two main structural 
types are usually recognized: (1) Ectomycorrhizae (ectotrophic mycorrhizae), 
in which the fungus forms a mantle or sheath about the host organ and its 
hyphae only penetrate between the cells, and (2) Endomycorrhizae (endotrophic 
mycorrhizae), in which the fungal hyphae penetrate into the cells of the host, 
inhabit them for a time, and are usually eventually eliminated by digestion. 
Some mycorrhizae seem to be intermediate between these two structural groups. 
    The work on the physiology of mycorrhizae has sufficed to show that 
although most kinds of these organs are more efficient in nutrient absorption 
than uninfected roots, they may be divided into two groups of carbon nutrition 
- two groups which do not conform to the structural groups. In all 
ectomycorrhizae, and in the endomycorrhizae of green Ericaceae and those 
endomycorrhizae with Endogonaceae (phycomycetes) as fungi 
(vesicular-arbuscular mycorrhizae), the symbiotic system depends for carbon on 
photosynthesis by the host. In endomycorrhizae caused by fungi with septate 
hyphae (Basidiomycetes and imperfect fungi), such as occur in those members of 
the Orchidaceae, Ericales, and other taxa which have a saprophytic phase or 
are saprophytes in the adult stage, the symbiotic system is supplied with 
carbon compounds by the fungus. 
 �34s[J. L. Harley]
�33s 
  

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�6v�2v�32sMycotoxin   �21m �33s 
    Any of the mold-produced substances that may be injurious to vertebrates 
upon ingestion, inhalation, or skin contact. The diseases they cause, known as 
mycotoxicoses, need not involve the toxin-producing fungus. Diagnostic 
features characterizing mycotoxicoses are: the disease is not transmissible; 
drug and antibiotic treatments have little or no effect; in field outbreaks 
the disease is often seasonal; the outbreak is usually associated with a 
specific foodstuff; and examination of the suspected food or foodstuff reveals 
signs of fungal activity.  See  AFLATOXIN; ERGOTISM; MEDICAL MYCOLOGY. 
 �34s[Alex Ciegler]
�33s 
  
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�6v�2v�32sMyelin   �21m �33s 
    A highly refractile fatty sheath surrounding many nerve fibers. Myelin is 
composed of alternating dense and light strata repeating radially at a period 
of 16 - 18 nanometers. Every other dense stratum is less dense and often 
appears as a double line. The very dense strata are called major dense lines, 
and the less dense ones intraperiod lines. 
    The main function of the myelin sheath is to facilitate conduction of the 
nerve action potential. The action potential is a propagated depolarization of 
the axon membrane that extends throughout the length of the axon. This spreads 
as a flow current carried by ions from point to point along the axon surface. 
Ions are exchanged with the gap between the axon and the Schwann cell 
membrane. In nonmyelinated nerve fibers these ions rapidly diffuse into the 
extracellular space, and propagation of the action potential is relatively 
slow, less than 3.3 ft/s (1 m/s). In myelinated nerve fibers the ions have no 
free pathway to the extracellular space since the membranes of the mesaxons 
are closely apposed and the gap between the axon and the Schwann cell membrane 
at the node is also sealed off. The only place where there is free access to 
the extracellular space is at the nodes of Ranvier. The consequence of this is 
that current flows primarily through the axon membrane at the node and jumps 
via the extracellular space to the next node to complete the circuit. 
 �34s[J. David Robertson]
�33s 
  
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�6v�2v�32sMyiasis   �21m �33s 
    The infestation of vertebrates by the larvae, or maggots, of numerous 
species of flies. These larvae may invade different parts of the bodies of 
these animals or may appear externally. The Diptera of medical and veterinary 
importance are largely confined to the families Oestridae, Calliphoridae, and 
Sarcophagidae.  
    In cutaneous myiasis, the larvae are found in or under the skin. There may 
be a migration of some species of these larvae through host tissues, resulting 
in a swelling with intense itching. Such a condition is known as larva 
migrans, or creeping eruption, and may require surgical treatment. 
    Intestinal myiasis in humans is usually the result of accidentally 
swallowing the eggs or larvae of these flies. It occurs commonly in many 
herbivores who ingest the eggs when feeding on contaminated herbage. The 
larvae settle in the stomach or intestinal tract of the animal host. 
    Cavity, or wound, myiasis occurs when the larvae invade natural orifices, 
such as the nasopharynx, vulva, and sinuses, or artificial openings such as 
wounds. External myiasis includes infestation by those maggots which are blood 
feeders.  See  DIPTERA; MEDICAL PARASITOLOGY. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sMylonite   �21m �33s 
    A rock that has suffered extreme mechanical deformation and granulation 
but has remained chemically unaltered. In the early stages of this process, 
cataclasites or flaser rocks are produced. These rocks are characterized by 
the fact that the nature of the parent rock is easily recognized. The 
mylonites, however, are altered beyond recognition. In spite of their 
pulverized condition they are hard, coherent, and often black and 
glassy-looking rocks, because the crushing took place under confining 
pressures so high that the comminuted particles have been welded tightly 
together.  See  METAMORPHIC ROCKS. 
 �34s[T. F. W. Barth]
�33s 
  
A700000001757�254s�5v�1v1130
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�6v�2v�32sMyocarditis   �21m �33s 
    Cardiomyopathies characterized by a prominent infiltration of inflammatory 
cells interstitially between the myocardial fibers. Most of these inflammatory 
cardiomyopathies are secondary to systemic disease and thus are termed 
secondary inflammatory cardiomyopathies. Primary myocarditis is rare and the 
causes are not well understood. The secondary myocarditis is associated with 
systemic bacterial, viral, fungal, and parasitic diseases, such as typhoid, 
diphtheria, scarlet fever, coxsackie virus, influenza, polio, measles 
toxoplasmosis, and trichinosis. Irrespective of the cause, the morphological 
appearance of inflammatory myocarditis is similar. Grossly the heart is 
normal; however, microscopically there is an interstitial inflammatory 
infiltrate with edema. The cellular infiltrate consists of patches of 
eosinophils, giant cells, and some lymphocytes. See HEART DISORDERS. 
 �34s[N. Karle Mottet]
�33s 
  
A700000002161�254s�5v�1v1131
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�6v�2v�32sMyodocopa   �21m �33s 
    A suborder of the order Myodocopida (subclass Ostracoda) which includes 
widespread marine ostracods provided with a heart. It is also called 
Myodocopina. In contrast to other living ostracods, nearly all myodocopines 
have a carapace with a permanent anterior aperture when the valves are 
otherwise drawn together. Lateral eyes are well developed, each with several 
lenses, and a frontal organ (a kind of median eye) projects from the center of 
the forehead (see illustration). Through the permanent aperture or gape, the 
antenna project for locomotion. Some forms are benthonic but most are pelagic; 
in the latter, the carapace may be very thin, containing little or no calcite. 
    Fossil myodocopines are known from Devonian to Recent. They are relatively 
rare but very diverse. Many of the extinct Entomoconchacea had two permanent 
openings, the larger anterior and the smaller posteroventral, evidently an 
adaptation to maintain efficient circulation when the valves were drawn 
together.  See  OSTRACODA. 
 �34s[Robert V. Kesling]
�33s 
  
  
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�6v�2v�32sMyriangiales   �21m �33s 
    An order of parasitic fungi of the class Ascomycetes which produce their 
asci at various levels in uniascal locules within stromata, which are masses 
of tightly interwoven somatic hyphae. The stroma weathers away and exposes the 
asci which contain the spores. Examples of Myriangiales are Elsinoee veneta 
and E. fawcetii, which cause raspberry and citrus anthracnose, respectively, 
and Myriangium duriaei, which parasitizes scale insects. Piedraia hortai, 
which attacks human hair causing black piedra, has also been placed here.  See 
  ASCOMYCOTINA; FUNGI; MEDICAL MYCOLOGY; PLANT PATHOLOGY. 
 �34s[Constantine J. Alexopoulos]
�33s 
  
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�6v�2v�32sMyricales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of the single family Myricaceae, with about 50 species. Within its 
subclass the order is marked by its simple, resinous-dotted, aromatic leaves 
and unilocular ovary with two styles and a single ovule. The plants are trees 
or shrubs, and the flowers are much reduced and borne in catkins. The fruit is 
a small, waxy-coated drupe or nut. Several species of Myrica are occasionally 
cultivated as ornamentals.  See  FLOWER; HAMAMELIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001765�254s�5v�1v1131
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�6v�2v�32sMyrtales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
contains 13 families and about 9000 species. The two largest families are the 
Melastomataceae, with about 4000 species, and the Myrtaceae, with about 3000. 
The Onagraceae, with about 650 species, are common in temperate regions, 
especially of western North America, in contrast to the larger families of the 
order, which are chiefly tropical. 
    The Myrtales usually have opposite, simple, entire leaves and perigynous 
to epigynous flowers with a compound pistil and most commonly axile 
placentation. Allspice and cloves are obtained from Pimenta dioica and 
Syzygium aromaticum, respectively, which are trees belonging to the Myrtaceae. 
Eucalyptus is another genus of the same family.  See  ALLSPICE; CLOVE; 
EUCALYPTUS; MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
�33s 
  
A700000004176�254s�5v�1v1131
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�6v�2v�32sMysidacea   �21m �33s 
    An order of free-swimming Crustacea. Mysidacea contains two suborders, the 
Lophogastrida and Mysida. Most species are marine, many are euryhaline, and 
some are fresh-water. They are known from inshore and surface waters down to 
depths of over 23,000 ft (7000 m) [deepest record 23,430-23,960 ft or 7100 - 
7260 m in a closing net], attaining a much greater size in the greater depths. 
    The adult body consists of 19 somites; 5 head, 8 thoracic, and 6 abdominal 
with a tail fan composed of uropods and a telson (see illustration). Each 
somite bears one pair of functionally modified, biramous appendages. The 
carapace envelops most of the thorax and is fused dorsally with not more than 
4 of the anterior thoracic somites. The eyes are pedunculate and movable. 
    Locomotion is effected by a paddling action of the pleopods in lower 
forms. In higher groups thoracic exopods set up backwardly directed currents 
which propel the animal forward. The tail fan and abdomen assist in swimming 
and can produce springing movements, the ``escape mechanism.'' Walking may be 
accomplished by the thoracic endopods. 
    Mysids are not combatant, relying on agility, camouflage, sheltering under 
rocks or weeds, or burrowing in the sea floor to escape enemies. Six species, 
all blind and representing the three most primitive genera of Mysida, are 
cavernicolous in fresh or brackish subterranean waters. Nine species of 
Heteromysis, almost the most highly developed genus of mysids, are known to 
live as commensales. No parasitic forms are known. 
    Pelagic species in upper waters are usually transparent and, except for 
the cornea, are colorless. Most littoral species have a chromatophore system 
whereby color changes are made to harmonize with the background. 
    During daylight most mysids remain feeding at, or near, the bottom, 
migrating at dusk toward upper levels where the young are liberated. Toward 
dawn, adults descend again, while the juveniles usually remain nearer the 
surface until mature.  See  MALACOSTRACA; PERACARIDA. 
 �34s[Olive S. Tattersall]
�33s 
  
  
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�6v�2v�32sMystacocarida   �21m �33s 
    An order of primitive Crustacea. The three species are in the genus 
Derocheilocaris. The body is wormlike and about 0.2 in. (5mm) long. The 
cephalothorax bears first antennae, second antennae, mandibles, first 
maxillae, and second maxillae. Maxillipeds are on a separate segment, and four 
additional free thoracic segments bear platelike appendages. The six abdominal 
segments are without appendages. The labrum is enormous, mouthparts and 
nervous system are primitive, and the genital pore is on the thorax.  See  
CRUSTACEA.  
 �34s[Robert W. Pennak]
�33s 
  
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�6v�2v�32sMyxinoidea   �21m �33s 
    The hagfishes, sometimes called Myxiniformes, an order of eellike, jawless
vertebrates (class Agnatha), often grouped with the Petromyzonida as
Cyclostomata. They resemble the Petromyzonida in a number of respects. However,
they differ from Petromyzonida in having the nasal opening at the tip of the
snout and leading to the pharynx so that it can be used in respiration; in
having barbels around the mouth; in having 6 - 15 pairs of gill pouches which
open to the exterior either separately or by a common aperture; in having only
one pair of semicircular canals; in having degenerate eyes lacking lens and
iris and not visible externally; in having a continuous dorsal-caudal-ventral
fin with fin rays only in the caudal portion; in having both male and female
organs in one individual, though only one matures; and in being exclusively
marine. 
    Hagfishes live on or near muddy bottoms in fairly deep waters, where they 
feed largely on dead or disabled fishes and polychaete worms, into which they 
bore with their rasplike tongues. 
    Hagfishes are divided into three genera: Myxine (see illustration), known 
in both the Atlantic and Pacific oceans; Paramyxine, known from Japan; and 
Bdellostoma, known in the Pacific.  See  AGNATHA; CYCLOSTOMATA (CHORDATA); 
PETROMYZONIDA.  
 �34s[Robert H. Denison]
�33s 
  
  
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�6v�2v�32sMyxobacterales   �21m �33s 
    An order of gliding bacteria. The Myxobacterales or myxobacters are also 
known as the slime bacteria. They are found mainly in the surface layers of 
soil, rotting wood, compost, and manure. 
    The vegetative cells are flexible, slender rods 1 - 2 micrometers wide and 
3 - 5 micrometers long. The cells are not enclosed by a rigid cell wall; thus, 
they are not as refractile as the cells of true bacteria and are somewhat 
difficult to see microscopically. On a surface the cells display a 
characteristic gliding motility, although neither flagella nor other motility 
organelles are present. Because of this, motility myxobacterial colonies are 
not limited like bacterial colonies but are, rather, rapidly spreading, thin 
``swarms,'' often with irregular, veinlike thickenings. The cells tend to 
arrange themselves in parallel rows, forming small tongues and islets at the 
edge of the swarm. 
    The fruiting bodies formed by the myxobacters are unique among the 
bacteria. After a period of vegetative growth on a suitable medium, the cells 
of the swarm begin to aggregate toward several centers. The aggregating 
myxobacters form tongues of parallel rows of cells which converge on the 
center. Arriving at the center, the cells heap up on themselves and become 
transformed into resting cells called microcysts. The fruiting body may be a 
more or less formless mass of microcysts embedded in a mass of slime, or it 
may become variously branched and twisted, bearing clusters of cysts at the 
tips of the branches. Mature fruiting bodies are usually pigmented.   
 �34s[William R. Sistrom]
�33s 
  
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�6v�2v�32sMyxomycetes   �21m �33s 
    A group of microorganisms belonging to the Myxomycota classified with the 
fungi and given the class name Myxomycetes because of the funguslike 
reproductive phase, or sometimes classified with the protozoans (one-celled 
animals) and given the class name Mycetozoa because of the animallike 
assimilative phase. Evolutionary origins are obscure and controversial, but 
many believe the Myxomycetes are descendants of protozoan ancestors. 
    The reproductive phase is characterized by sedentary funguslike fruiting 
structures containing one to many spores. The assimilative phase is initiated 
by spore germination that gives rise to one or more naked myxamoebae or swarm 
cells which may serve as gametes. Fusion of gametes forms the zygote, which 
develops into a free-living, multinucleate, acellular mass of protoplasm, the 
plasmodium, exhibiting protoplasmic streaming and amoeboid movement. The mode 
of nutrition is animallike, with ingestion of particulate matter (usually 
bacteria) by engulfment. The plasmodium undergoes a series of developmental 
stages, eventually becoming transformed into various types of fructifications. 
    Most slime molds are universally distributed and live in moist and dark 
places on decaying organic matter such as standing dead trees, and old 
decaying logs and dead twigs and leaves on the forest floor. 
    The Myxomycetes have little direct economic importance, but their 
beautiful design and brilliant colors have attracted and fascinated many 
collectors.  See  MYXOMYCOTA. 
 �34s[Harold W. Keller]
�33s 
  
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�6v�2v�32sMyxomycota   �21m �33s 
    A division of microorganisms containing four classes - Acrasiomycetes 
(cellular slime molds), Myxomycetes (plasmodial, acellular, or true slime 
molds), Protosteliomycetes, Plasmodiophoromycetes (endoparasitic slime molds) 
- and classified with the fungi, following the rules of botanical 
nomenclature. These same groups may be classified in the kingdom Protista at 
various taxonomic ranks, following zoological nomenclature. These slime mold 
groups are not necessarily closely related, but they all do have primitive 
amoebalike colonial stages somewhere in their life cycles.  See  
ACRASIOMYCETES; MYXOMYCETES; PLASMODIOPHOROMYCETES; PROTISTA. 
 �34s[Harold W. Keller]
�33s 
  
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�6v�2v�32sMyxosporida   �21m �33s 
    An order of the protozoan class Myxosporidea (subphylum Cnidospora). It is 
characterized by the production of spores with one or more valves and polar 
capsules, and by possession of a single sporoplasm with or without an 
iodinophilous vacuole. Myxosporidians are mainly parasites of fishes. They 
infect all parts of the body, including the heart and brain, and often induce 
considerable pathological changes in the host tissue. 
    Infection begins with the ingestion of the spore by a host fish. The 
digestive fluids cause the polar filaments to be extruded, and at the same 
time the sporoplasm is released from the spore (see illustration). The 
sporoplasm, or amebula, reaches the specific site of infection directly 
through the gut wall or by way of the bloodstream. The amebula becomes a 
trophozoite when it starts feeding on the host tissues. The trophozoite then 
goes through a series of nuclear divisions and, by a process of budding, gives 
rise to a number of cells, each of which eventually develops into a sporont. A 
sporont is a monosporoblast if one spore is produced and a pansporoblast if 
two or more spores are formed. The sporont undergoes a series of nuclear 
divisions, in which the number of nuclei produced will determine the number of 
spores and polar capsules to be formed. That is, in every spore one nucleus is 
involved in the formation of each valve and each polar capsule. Two nuclei 
become the gametic nuclei, which then fuse to form the zygotic nucleus of the 
sporoplasm.  See  CNIDOSPORA; MYXOSPORIDEA. 
 �34s[Ross F. Nigrelli]
�33s 
  
  
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�6v�2v�32sMyxosporidea   �21m �33s 
    A class of the protozoan subphylum Cnidospora. Members of this class, 
which includes the orders Myxosporida, Actinomyxida, and Helicosporida, are 
parasites in fish, a few amphibians, and invertebrates. The Myxosporida, are 
divided into two suborders, the Unipolarina and Bipolarina. 
    Unipolarina is characterized by spores with one to six (never five) polar 
capsules located at the anterior end, except in some genera in which the 
capsules are widely separated or located in the central part of the spore but 
in which the polar filament is attached near the anterior end. The Unipolarina 
contains nine or more families. 
    Bipolarina, containing a single family with three genera, is characterized 
by the presence of one capsule at or near each end of a fusiform or ellipsoid 
spore. See ACTINOMYXIDA; CNIDOSPORA; HELICOSPORIDA; MYXOSPORIDA. 
 �34s[Ross F. Nigrelli]
�33s 
  
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�6v�2v�32sMyxovirus   �21m �33s 
    A group of animal viruses that originally included the human influenza 
viruses, mumps virus, and Newcastle disease virus. Its name was based on a 
special affinity of these members for certain mucins, in a reaction concerned 
with the adsorption and penetration of virus into the cell. A number of other 
viruses have been found also to share characteristics of the group, 
particularly in their morphology, although not all have a demonstrated 
affinity for mucins. The group now includes influenza and fowl plague viruses, 
and the paramyxovirus subgroup, in which mumps, measles, parainfluenza, 
respiratory synctial (RS) disease, and Newcastle disease viruses have been 
placed. Rinderpest and canine distemper viruses are related to measles virus. 
    The myxoviruses range from 80 to 300 nanometers in size; they possess a 
coiled inner helix of ribonucleoprotein surrounded by an ether-sensitive 
lipoprotein envelope studded with projections. 
    All myxoviruses except canine distemper, rinderpest, and RS viruses 
agglutinate red blood cells. Such hemagglutination by the virus, and the 
development in the patient of specific antibodies which inhibit 
hemagglutination, are properties useful in diagnosis.  See  ANIMAL VIRUS; 
INFLUENZA; MEASLES; MUMPS; NEWCASTLE DISEASE; PARAINFLUENZA VIRUS; 
PARAMYXOVIRUS.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sMyzostomaria   �21m �33s 
    An aberrant group of Polychaeta recognized for four families, each with 
one genus. Myzostomidae with Myzostomum comprise about 130 species from 
worldwide areas. Mesomyzostomdae with Mesomyzostomum has two species, from 
Japan and the Aru Islands. Protomyzostomidae with Protomyzostomum is known for 
three species, from Japan and the Murman Sea, and Stelechopidae with 
Stelechopus has a single species, from Crozet Island, Antarctic Ocean. All are 
either external or internal parasites of echinoderms, chiefly crinoids, and 
hence from deep water. Most are greatly depressed, broad and very small, 
measuring at most a few millimeters. The separation of this group of 
polychaetes into families and species is based on external and internal 
characters.  See  ANNELIDA; POLYCHAETA. 
 �34s[Olga Hartman]
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�6v�2v�32sNailing   �21m �33s 
    The driving of nails in a manner that will position and hold two or more 
members, usually of wood, in a desired relationship to each other. The contact 
pressures between the surfaces of the nails and the surrounding wood fibers 
hold the nails in position. For some types of nails see illustration. 
    Factors that determine the strength and efficiency of a nailed joint are 
(1) the type of wood, (2) the nail used, (3) the conditions under which the 
nailed joint is used, and (4) the number of nails. In general, hard, dense 
woods hold nails better than soft woods. The better the resistance of a nail 
to direct withdrawal from a piece of wood, the tighter the joint will remain. 
To increase resistance to withdrawal or loosening, nails may be coated, 
etched, spirally grooved, annularly grooved, or barbed, as illustrated. 
    Blunt-pointed nails are often used to prevent the wood from splitting. 
Using nails of a smaller diameter also tends to prevent splitting but requires 
a greater number of nails per joint. Beeswax is sometimes applied to nail 
points to make them drive more easily, but it also reduces the holding power 
of the nail. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sNajadales   �21m �33s 
    An order of aquatic and semiaquatic flowering plants, division 
Magnoliophyta (Angiospermae), in the subclass Alismatidae of the class 
Liliopsida (monocotyledons). The order consists of 8 families and a little 
more than 200 species. The Potamogetonaceae, with about 90 species, are the 
largest family of the order, and the name Potamogetonales is sometimes used 
instead of Najadales for the group. The Najadales are Alismatidae in which the 
perianth, when present, is not differentiated into evident sepals and petals. 
Usually the flowers are not individually subtended by bracts. The Zosteraceae 
of this order are unique among flowering plants, in that they grow submersed 
in the ocean, albeit in shallow water near the shore. Zostera marina, or 
eelgrass, is a common member of the family.  See  ALISMATIDAE; LILIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sNaphtha   �21m �33s 
    Any one of a wide variety of volatile hydrocarbon mixtures. They are 
sometimes obtained from coal tar but are more often derived from petroleum. 
Physical properties vary widely. The initial boiling point may be as low as 
27DGC (80DGF), and end points may reach 260DGC (500DGF). Boiling ranges are 
sometimes as narrow as 11DGC (20DGF) or as wide as 110DGC  (200DGF). Products 
sold as naphthas find their greatest use as solvents, thinners, or carriers. 
    There is a fairly sharp differentiation between aliphatic and aromatic 
naphthas. Aliphatic naphthas are relatively low in odor and toxicity and tend, 
also, to be low in solvent power. The aromatic naphthas are highly solvent. 
Their main components are toluene and xylenes; benzene is less desirable 
because of the extreme toxicity of its vapors.  See  PETROLEUM PRODUCTS. 
 �34s[J. K. Roberts]
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�6v�2v�32sNaphthalene   �21m �33s 
    A colorless crystalline aromatic hydrocarbon, C10H8, with the familiar 
odor of mothballs, melting point 80.1DGC (176DGF), boiling point 218DGC 
(424DGF). It is almost insoluble in water but soluble in nearly all organic 
solvents. Structurally it is best represented as two benzenoid rings fused 
together.  
    One gallon of coal tar yields approximately one pound of naphthalene 
(approximately 120 grams per liter). It is present also in certain fractions 
obtained by the reforming or steam cracking of certain petroleum distillates 
and is recovered commercially on a large scale. 
    Of the naphthalene consumed, the majority is converted to phthalic 
anhydride. Aside from a few percent used in the manufacture of mothballs, the 
remainder is converted to naphthalene compounds, which are used as dye 
intermediates, tanning agents, and surface-active agents.  See  AROMATIC 
HYDROCARBON; POLYNUCLEAR HYDROCARBON. 
 �34s[Charles K. Bradsher]
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�6v�2v�32sNarcotic   �21m �33s 
    A drug which diminishes the awareness of sensory impulses, especially 
pain, by the brain. This action makes narcotics useful therapeutically as 
analgesics. While they are the most powerful pain-relieving agents available, 
their use is complicated by a number of undesirable side actions.  See  
ANALGESIC.  
    All of the generally used narcotics are in some way related to opium, and 
the term opiate is sometimes used interchangeably with the term narcotic. 
Opium is a gummy exudate obtained from the unripe seed capsules of the opium 
poppy. Crude opium contains over a dozen alkaloids, all of which have been 
isolated and identified as to their structural chemistry. From this knowledge 
chemists have developed a number of synthetic chemical compounds, some of 
which have important advantages over the naturally occurring alkaloids. 
Therapeutically important natural alkaloids are morphine, codeine, and 
papaverine. Among the important synthetic narcotics are meperidine (Demerol), 
dihydromorphine (Dilaudid), oxymorphone (Numorphan), alphaprodine (Nisentil), 
anileridine (Leritine), piminodine (Alvodine), levorphanol (Levo-Dromoran), 
methadone (Dolophine), and phenazocine (Prinadol).  See  ALKALOID; OPIATES; 
POPPY.  
    Nalorphine (Nalline) is a narcotic antagonist and is used in the treatment 
of acute overdosage from narcotics; it is dangerous to drug addicts. Heroin is 
a highly addicting narcotic, and is so dangerous in this regard that the drug 
has been completely banned by both Federal and state laws under all 
circumstances.  
  
    Pharmacology.  The pharmacology of narcotics is generally similar to that 
of morphine, the principal narcotic used for its analgesic effects. 
Differences among them lie in the potency of their action and in the degree 
and variety of the side actions which they produce. Effects are those of 
analgesia, accompanied by a state of euphoria characterized by drowsiness and 
a change of mood from anxiety and tension to calmness and equanimity. It 
should be remembered that whatever narcotic is used, the effects are 
dose-related, and in higher doses all narcotics produce deep sleep and 
eventually general depression of all brain functions. Death from overdosage is 
due to depression of the respiratory centers with resultant failure of 
respiration.  
    The predominant pharmacological effect of morphine (and the other 
narcotics) is on the central nervous system. From the standpoint of its 
medicinal use, its most important action is relief of pain. Along with its 
valuable medicinal use morphine produces a great many undesirable side 
actions; the most frequent are depressed respiratory activity, the production 
of nausea and vomiting, and the inhibition of defecation and urination. 
  
    Drug dependence.  All narcotics have the potential for producing 
dependence and addiction when used repeatedly over a period of time. Drug 
dependence results from compulsive, continued use of the drug, and is 
characterized by one or more of the following conditions: habituation, 
tolerance, or addiction. 
    Like any other habit pattern, habitual use of a drug can develop. Common 
examples are the use of nicotine in the form of cigarettes, or caffeine in the 
form of coffee or tea. Such habituation is generally regarded as innocuous. 
    Repeated ingestion of a drug in which the effect produced by the original 
dose no longer occurs results in tolerance. To produce the original effect, it 
is necessary to increase the dose. 
    When the body develops a dependence for the drug, addiction occurs. If the 
drug is suddenly stopped after a period of frequent use, a withdrawal syndrome 
develops, which is characterized by physical pain and widespread body 
reactions. The addict comes to dread the development of such painful and 
distressing reactions, and is trapped into continuing the drug. 
    All narcotics can produce habituation, tolerance, and addiction to a 
greater or less degree. Addiction to codeine is relatively rare but possible. 
Addiction to heroin develops rapidly, and this narcotic is therefore 
exceedingly dangerous.  See  DRUG ADDICTION. 
 �34s[James M. Dille]
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�6v�2v�32sNative elements   �21m �33s 
    Those elements which occur in nature uncombined with other elements. Aside 
from the free gases of the atmosphere there are about 20 elements that are 
found as minerals in the native state. These are divided into metals, 
semimetals, and nonmetals. Gold, silver, copper, and platinum are the most 
important metals and each of these has been found abundantly enough at certain 
localities to be mined as an ore. Rarer native metals are others of the 
platinum group, lead, mercury, tantalum, tin, and zinc. Native iron is found 
sparingly both as terrestrial iron and meteoric iron. 
    The native semimetals can be divided into (1) the arsenic group, including 
arsenic, antimony, and bismuth; and (2) the tellurium group, including 
tellurium and selenium. 
    The native nonmetals are sulfur, and carbon in the forms of graphite and 
diamond. Native sulfur is the chief industrial source of that element. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sNatrolite �21m �33s
    A fibrous or needlelike mineral belonging to the zeolite family of
silicates. Most commonly it is found in radiating fibrous aggregates. The
hardness is 5 - 5J on Mohs scale, and the specific gravity is 2.25. The
mineral is white or colorless with a vitreous luster that inclines to pearly
in fibrous varieties. The chemical composition is Na2(Al2Si3O10)PRODUCT2H2O
but some potassium is usually present substituting for sodium.
    Natrolite is a secondary mineral found lining cavities in basaltic rocks.
Its outstanding locality in the United States is at Bergen Hill, New Jersey.
See  ZEOLITE.
 �34s[Clifford Frondel ;Cornelius S. Hulber, Jr.]
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�6v�2v�32sNatural fiber �21m �33s
    A fiber obtained from a plant or animal. For many centuries, wool and flax
were the chief fibers for apparel in temperate climates, and cotton in the
tropics.

    Vegetable fibers.  The basic biological unit of vegetable fibers is the
plant cell, with a length frequently more than a thousand times its diameter.
The cotton fiber is a single cell. The bast and hard fiber strands are
composed of many overlapping, parallel fiber cells held together in a fiber
bundle that may be as long as the leaf or stem from which it comes.

    Cotton. The cotton plant is a member of the mallow family (Malvaceae). The
grading of cotton fibers is based on various considerations, but the length of
fiber is the most important. The most important universal standard grades are
determined mainly by staple length. For example, while the Sea Island cotton
extends up to 2 in. (5 cm) in staple length, Indian varieties range from 7/8 to
5/8 in. (2.2 to 1.7 cm). Yarn quality depends on inherent fiber properties of
strength, length, length uniformity, and fineness. In order to ensure a
desired uniformity and product quality, batches of raw fiber, different in
staple length and other characteristics, are processed by blending together.
    After weaving, the resulting cotton fabric may receive further processing,
which can also be complex, especially because modern textile technology can
change cotton fabric more than any other material. Bleaching, dyeing, and
printing can be considered among these, and with the introduction of chemical
finishes, cotton can be given many properties, such as durable press, crease
resistance, water repellence, and flame retardance.  See  BLEACHING; COTTON;
DYEING.

    Bast fibers. These fibers which come from the phloem or bast of
dicotyledonous plants are also called soft fibers. The greatest quantity of
these fibers goes into fabrics, but they have other uses such as thread,
twine, or paper. The more widely used bast fibers are jute, flax, hemp, and
ramie.
    Flax is grown mostly in northern and eastern Europe. The fiber is
separated from the stem of the plant by a system of retting and scutching. It
is used primarily for linen fabrics, but many other products are made from it.
The line (long, parallel) fiber is used principally for fine damask table
linen, apparel fabrics, sheeting, lace, thread, and special twine. The tow
(short, tangled fiber produced in the scutching and in the hackling
operations) and some of the coarser grades of line fiber are used for
toweling, canvas, mail bags, meat wraps, twine, and for high-grade papers such
as cigarette, bond, and currency. The fiber from flax raised for seed is also
used for high-grade papers (but not for textiles).  See  FLAX.
    Hemp comes from the stems of the plant Cannabis sativa and should not be
confused with Manila hemp, Mauretius hemp, sunn hemp, and others that are not
related to true hemp. The fiber is produced mainly in eastern Europe and
central Asia. It is removed from the stem by retting and scutching. For
centuries, hemp was the principal fiber used for marine cordage until replaced
by abaca and sisal. It is still used extensively for twine and for many of the
same products as flax.	See  HEMP.
    Ramie fiber is removed from the stem of the plant (Bohmeria nivea) by a
beating and scraping operation that leaves it in ribbons. These ribbons are
dried and later chemically degummed. Thus, ramie is not retted as other bast
fibers are. The world's largest producer is believed to be mainland China. The
greatest use for ramie fiber is in fabric and twine. Also made from ramie are
thread, gas mantles, belting, canvas, packing for propeller-shaft bearings,
and knitting yarn for blends with wool and synthetic fibers.  See  RAMIE.
    Jute fiber is obtained from the stems of white jute (Corchorus capsularis)
and from tossa and daisee (C. olitorius). The fiber is removed from the stem
by retting and then stripping the retted fiber from the stems by hand. Jute is
grown mainly in Bangladesh and India, but Burma, Nepal, and Brazil produce
sizable amounts. Its suitability for uses requiring strong, bulky fabric or
twine and its low cost compared to competitive fibers have made jute second
only to cotton among the natural fibers in world consumption.  See  JUTE.

    Hard fibers. Hard fibers, also known as leaf or cordage fibers, come from
the leaves of monocotyledonous plants. The fiber of the leaves of many species
is used but the three consumed in greatest quantity are abaca, sisal, and
henequen.
    Frequently known as Manila hemp, abaca comes from the pseudostem (modified
leaf petioles that look like a tree trunk) of Musa textilis. Most abaca is
grown in the Philippines, but other countries have produced it from time to
time. Abaca is traditionally used in marine cordage and in other applications
that require a strong, water-resistant rope. However, abaca is now being used
more for pulp which goes into tea bags, mimeograph mats, air filters, sausage
casings, and other products.  See  ABACA.
    The sisal plant (Agave sisalana) is grown mainly in Africa and Brazil,
while henequen (A. fourcroydes) is grown mainly in Mexico and Cuba. These are
cordage fibers removed from the leaves by defibering. The greatest use for
Agave fibers is for twine and rope. Upholstery padding, especially that used
in innerspring mattresses, is made from the lower grades of fiber. It is also
used for pulping, plastic reinforcement, and patio rugs.  See  HENEQUEN; SISAL.
 �34s[Elton G. Nelson]
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    Animal fibers.  Many animal species produce fibrous structures in a
variety of forms and with different functionalities. These include the open
webs of spiders, the more closed cocoons of moths, the fibrous part of bird
feathers, and body hairs found on species from insects to mammals, including
humans. The common characteristic of these fibrous structures is that they are
all denatured proteins. Silk and mammalian hairs have achieved commercial and
technical importance, although the utility of feathers is due largely to their
fibrous qualities.

    Mammalian fibers. Mammalian fibers are used by humans in the form of fur,
all types of textiles, fiber fills, ropes, brushes, wigs, and so on. All
mammalian fibers are closely related. Many species produce a double coat,
containing both long, coarse, and somewhat sparse guard hairs, and a shorter,
fine, and dense underfur.  See	HAIR.
    Mammalian hairs can reversibly sorb more water than any other commercially
used fibers, with the exception of some regenerated cellulose. Part of the
comfort factor of wool-based textiles is due to this fact. Mammalian fibers
are more elastically extensible than any other natural fibers. In water they
can be repeatedly stretched to 30%, with full recovery of their length and
strength.
    The most important commercial animal fiber is wool. Some of the more
important fiber qualities are fineness, length, crimp, color, handle,
soundness, and cleanness. The finest and most valuable wool is produced by the
Australian Merino sheep, developed from a Spanish breed. Medium- and
coarse-variety fibers are produced by other breeds.  See  SHEEP; WOOL.
    Mohair is produced by the Angora goat mostly in Turkey, South Africa, and
the United States. Luster is its most important quality. The Cashmere goat in
Asia grows fur fibers which are the source of cashmere; softness is the most
important characteristic. The fine underfur is the valuable component of all
goat hairs.  See  CASHMERE; MOHAIR.
    The fiber known as camel originates from the Bactrian two-humped species
from Asia. For textile purposes only the fine underfur is collected by
plucking or shearing. The alpaca and vicuna are South American members of the
camel family. The wild vicuna grows a shorter fleece but the finest fibers;
the softest wool type materials are made from it.  See	ALPACA; VICUNA.
    The Angora rabbit hair is very fine, and relatively long. It is used
mostly for blending with wool for soft fabric effects both in woven and
knitted textiles. The fur hair of common rabbits is still very fine but
shorter. Their most important use is in felted hats. Other animal fibers from
rodents, pigs, and horses have only limited uses in felts, brushes, and fiber
fills.

    Silk. Although it has lost ground to new synthetics, silk is still an
important luxury fiber. It is the secretory product of the larval stage of
many moth varieties. The cultivated breed is the Bombyx mori. The worm, before
changing into the chrysalis stage, spins a cocoon around itself from the silk
fiber. A whole cocoon is made up from a single fiber. Silk has a few unique
properties: it is the finest animal fiber; it has no cellular structure since
it was never a part of body tissue; and it is a continuous filament. The most
important qualities of silk fabrics are softness, smoothness, luster, and
drape. These are based mostly on the small fiber diameter and lack of surface
texture.  See  SILK; TEXTILE; TEXTILE CHEMISTRY; TEXTILE PRINTING.
 �34s[William S. Tolgyesi]
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�6v�2v�32sNatural gas   �21m �33s 
    A combustible gas that occurs in porous rock of the Earth's crust and is 
found with or near accumulations of crude oil. Being in gaseous form, it may 
occur alone in separate reservoirs. More commonly it forms a gas cap, or mass 
of gas, entrapped between liquid petroleum and impervious capping rock layer 
in a petroleum reservoir. Under conditions of greater pressure it is 
intimately mixed with, or dissolved in, crude oil. 
    Typical natural gas consists of hydrocarbons having a very low boiling 
point. Methane makes up approximately 85% of the typical gas. Ethane may be 
present in amounts up to 10%; and propane up to 3%. Butane, pentane, hexane, 
heptane, and octane may also be present. While normal hydrocarbons having 5 - 
10 carbon atoms are liquids at ordinary temperatures, they may be present in 
the vapor form in natural gas. Carbon dioxide, nitrogen, helium, and hydrogen 
sulfide may also be present. 
    Natural gas has no distinct odor. Its main use is for fuel, but it is also 
used to make carbon black, natural gasoline, certain chemicals, and liquefied 
petroleum gas. Propane and butane are obtained in processing natural gas.  See 
  LIQUEFIED PETROLEUM GAS (LPG); PETROLEUM PRODUCTS. 
 �34s[Michel T. Halbouty]
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�6v�2v�32sNautical Almanac   �21m �33s 
    A book published annually by the governments of the principal maritime 
nations which contains the astronomical data required for navigation by 
observations of celestial objects. An almanac (from the Arabic word for 
``calendar'') generally contains calendrical tables of astronomical phenomena, 
or ephemerides, and may also include diverse information of nonastronomical 
character. The United States Nautical Almanac has been published since 1855. 
    With the Nautical Almanac, and a chronometer or wireless time signals to 
determine Greenwich Mean Time (GMT), the navigator needs only the means of 
measuring positions of celestial objects with respect to the local vertical as 
defined by the physical horizon or by an artificial horizon. Such an 
instrument is the mariner's sextant. The angular distance between the local 
vertical and the celestial object is equivalent to the angular distance on the 
surface of the Earth between the observer and that point on the Earth where 
the celestial object is directly overhead, that is, the ground point of the 
celestial object.  See  SEXTANT. 
    The purpose of the tables in the Nautical Almanac is to allow the 
navigator to derive the ground point of the celestial object for any moment of 
time. By measuring the distance from two or more ground points and solving the 
navigational triangle for each observation, the navigator establishes the 
ship's position uniquely.  See  CELESTIAL NAVIGATION. 
    A similar publication but with slightly reduced tabular accuracy is 
especially designed to facilitate celestial air navigation.  See  AIR ALMANAC; 
EPHEMERIS.  
 �34s[Raynor L. Duncombe]
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�6v�2v�32sNautiloidea   �21m �33s 
    An order of the tetrabranchiate cephalopods considered to be the most 
primitive of the class. The shells are external and smooth, being straight or 
coiled and chambered with curved transverse septa; they have simple sutures 
and simple exterior sculpture. There are about 300 genera and more than 2500 
species. The first records are from the Upper Cambrian. The group reached its 
zenith in the Ordovician and had largely disappeared by the Triassic. The sole 
living genus is Nautilus (see illustration).  See  CEPHALOPODA; TETRABRANCHIA. 
 �34s[Gilbert L. Voss]
�33s 
  
    The perfectly symmetrical planospiral shell of Nautilus is divided into 
about 25 chambers by transverse partitions. Only the outermost and largest 
chamber is occupied by the animal, the rest being gas-filled and serving as a 
buoyancy organ. Each partition, or septum, has a perforation reinforced with a 
shelly ``neck'' through which passes the siphuncle, a cord of living tissue 
containing blood vessels and surrounded by cornified layers, which can adjust 
the buoyancy by absorption of water from the penultimate chambers balanced by 
gas secretion. Healthy specimens of Nautilus can move to and from the surface 
water from depths of 1150 ft (350 m) by such adjustment alone. 
    Apart from its buoyancy control, which allows it to live near the surface 
at night and on the bottom or at greater depths during the day, Nautilus is 
capable of horizontal movements by jet propulsion. As in squid and cuttlefish, 
this is achieved by water propelled by muscular contractions of the mantle 
cavity.  
    The head of Nautilus bears rather primitive, lenseless, ``pinhole'' eyes 
and 38 retractile, prehensile tentacles, which lack the characteristic suckers 
found on the arms of other modern cephalopods. Compared to octopus, squid, and 
the rest, Nautilus obviously represents the persistent line of a more 
primitive cephalopod stock. 
 �34s[W. D. Russell-Hunter]
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�6v�2v�32sNaval architecture   �21m �33s 
    An engineering discipline which involves the design of ships, boats, 
drilling rigs, submarines, and other floating or submerged craft. The naval 
architect is responsible for the overall concept of a new ship and 
specifically responsible for the design of the hydrodynamic shape of the hull, 
the structure, and the general arrangement, including the cargo-handling 
system. On the other hand, the propulsion plant and all its auxiliaries are 
the responsibility of the marine engineer. 
    The education of a naval architect is often combined with that of a marine 
engineer, so that both students receive some of the same training. In small 
ship-design offices, the naval architect - marine engineer may handle both 
hull and machinery design. The education of a naval architect also covers much 
of the same fundamental engineering and mathematics as that of the mechanical 
engineer, and the same types of degree programs are offered.  See  MARINE 
ENGINEERING; SHIP DESIGN. 
 �34s[Richard B. Couch]
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�6v�2v�32sNaval armament   �21m �33s 
    A general term which covers the ordnance and control systems used in naval 
ships and by naval aircraft. It includes a wide spectrum of weapons designed 
for use against targets in the air, on land or sea, or under the surface of 
the ocean. The weaponry used by naval forces thus runs from nuclear warheads 
to small arms and includes weapons that are intended for use against a 
particular type of target as well as the more general-purpose weapons. 
    Naval armament may be air-launched, surface-launched, or 
submarine-launched. It includes guns, guided missiles, rockets, bombs, depth 
charges, torpedoes, and mines. 
  
    Guided missiles.  Naval missiles may be launched from ships, submarines, 
or aircraft. Research has been aimed at producing missiles which can be used, 
say, by planes as well as by surface ships, or at modifying missiles designed 
for one type of work for another use. Advanced radars and computerized control 
systems can increase missile capabilities. 
    ASROC (Fig. 1) and SUBROC (submarine rocket) are antisubmarine weapons, 
launched respectively by surface ships and submarines. The weapon is aimed by 
a shipboard computer using target information obtained by sonar. The unguided 
rocket is fired from a launcher (ASROC) or a torpedo tube (SUBROC) and follows 
a ballistic trajectory through the air to the target's predicted position. 
    Polaris (Fig. 2), Poseidon, and Trident are solid-fueled ballistic 
missiles launched from submerged submarines. They are ejected from their 
vertical launch tubes by compressed air or by a gas-steam system. The missile 
is guided by a self-contained inertial-navigation system to the correct point 
in space for an unguided (ballistic) trajectory to its target. Poseidon, with 
greater range and a multiple warhead, is replacing Polaris in most of the 
Navy's nuclear-powered ballistic-missile submarines (SSBN). Trident also has a 
multiple warhead, with a considerably greater range than Poseidon.  See  
GUIDED MISSILE; MISSILE. 
  
    Guns.  Though missiles are widely used by ships and aircraft, guns 
continue to be an integral part of the naval arsenal. While missiles are 
superior for long-range attack and for defense against supersonic planes and 
missiles at high altitudes and long ranges, the opposite is often true for 
such missions as shore bombardment, gunfire support of land forces, and 
defense against small craft. Renewed attention has been given to lighter guns, 
with high rates of fire, and to quick-reaction control systems for close-in 
defense against aircraft and cruise missiles. 
  
    Bombs.  Bombs of many types and sizes, from small antipersonnel weapons to 
2000-pound bombs designed for blast effect, as well as nuclear weapons, can be 
carried by naval aircraft. ``Smart bombs'' can be guided to their target from 
a standoff distance. Homing bomb system (HOBOS) types are television-guided 
adaptations of standard ``dumb'' bombs with added guidance systems and 
controllable tail surfaces. Laser-guided bombs (LGB) use laser homing. The 
antipersonnel-antimateriel (APAM) bomb contains a cluster of small bomblets. 
If these hit a hard surface, they have a shaped-charge effect. If they strike 
a soft surface, such as earth or sand, they rebound for a fragmentation 
airburst.  
  
    Torpedoes.  Torpedoes travel underwater under their own power to intercept 
and destroy surface ships and submarines. Modern naval torpedoes are fast, 
far-ranging, and armed with a heavy explosive warhead. Torpedoes may be homing 
(guided acoustically to their target) or nonhoming (following a preset 
course). Torpedoes may be launched by aircraft, submarines, or surface 
warships.  See  ACOUSTIC TORPEDO. 
  
    Mines.  A mine is a thin-cased, non - self-propelled weapon filled with 
high explosive and planted underwater, where it is designed to explode when 
struck or closely approached by a ship. Mines can be contact type (fired by 
actually striking the hull of a passing ship) or influence type (detonated by 
the close approach of a ship). Influence mines thus are much harder to sweep 
than contact mines. Mines can be planted by surface ships, submarines, or 
aircraft. Specially designed minehunters and minesweepers use a variety of 
detection and clearance devices to locate and dispose of mines.  See  ACOUSTIC 
MINE; NAVAL SHIP. 
 �34s[John C. Reilly, Jr.]
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�6v�2v�32sNaval meteorology   �21m �33s 
    Those aspects of meteorology that are relevant to naval operations. 
However, in more general terms, naval meteorology refers to marine or maritime 
meteorology, which is the study of weather phenomena over the oceans and other 
large bodies of water, including the interactions between the atmosphere and 
the oceans, particularly the exchange of energy. 
  
    Weather over the sea.  Some weather phenomena occur mainly over the sea, 
including fog, tradewinds, tropical storms, and waterspouts. 
    Although fog is prevalent over the land and sea, certain types of fog are 
more extensive over the sea. Steam fog results from evaporation of relatively 
warm water into overlying cooler air followed by almost immediate condensation 
into minute liquid droplets, much like steam from a pan of hot water. A form 
known as advection fog may occur when relatively warm, moist air moves over 
colder water, cooling the air, saturating it, and eventually forming fog. 
Extensive areas of advection fog reduce visibility drastically and make 
navigation hazardous. This danger has been considerably reduced through the 
use of radar.  See  FOG. 
    Tradewinds, or simply trades, refer to an extensive tropical wind system 
found primarily over oceanic areas between the subtropical high-pressure belts 
centered near 30DGN and 30DGS latitude. On the average, the trades blow from 
the northeast or east-northeast in the Northern Hemisphere and from the 
southeast or east-southeast in the Southern Hemisphere, converging more or 
less toward the Equator into a rather narrow belt of sometimes intense cumulus 
convection and showers known as the intertropical convergence zone (ITCZ). 
    Tropical storms are warm-center storms that develop in low latitudes and 
derive their energy from warm tropical waters. The winds are nearly circular, 
spiraling inward to form a vortex rotating in counterclockwise direction in 
the Southern Hemisphere. In intense storms, known as hurricanes in the 
Atlantic, Gulf of Mexico, and the eastern North Pacific, and as typhoons in 
the western North Pacific, the winds may reach 150 knots (75 m/s) or more, 
with a central calm area or eye roughly 10 - 40 mi (15 - 70 km) in diameter.  
See  HURRICANE. 
    In addition to the tropical cyclones, much smaller vortices known as 
waterspouts occur over the sea, mainly in tropical and subtropical waters. 
These whirlwinds have a tornadolike appearance, but are less intense. 
  
    Air-sea interaction.  There is a two-way interaction between the 
atmosphere and the sea (or land) which results in the exchange of energy in 
the form of heat, water vapor, and momentum. A consequence of the interactions 
is the formation of boundary layers from which energy is eventually 
transmitted to the deepest parts of the ocean, the outer limits of the 
atmosphere, and space.  See  OCEAN - ATMOSPHERE RELATIONS. 
 �34s[George J. Haltiner]
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�6v�2v�32sNaval ship �21m �33s
    A ship designed primarily for use in warfare, either directly in combat
operations or to provide services and support such operations. United States
naval ships can be categorized as either combatants or noncombatants. They
also can be classified by their performance capabilities and by their intended
mission. Specific military features in warfare areas such as antiair warfare
(AAW), antisubmarine warfare (ASW), and surface warfare (SUW) are performance
attributes; whereas mission groupings include battleships, cruisers,
destroyers, frigates, aircraft carriers, mine craft, patrol craft, amphibious
ships, auxiliary ships, service craft, and high-speed or high-performance
ships.

    Combatant ships.  This category includes battleships, cruisers,
destroyers, frigates (formerly destroyer escorts), patrol craft, and aircraft
carriers. Cruisers and aircraft carriers now are the capital ships of the
United States fleet.
    The primary purpose of a cruiser is to provide antiaircraft and
antisubmarine protection to an aircraft carrier task force or group. These
ships are multimission warships suited for independent operations. Nuclear
cruisers (Fig. 1) generally are deployed with nuclear-powered aircraft
carriers. This mix provides the high speed and unlimited endurance necessary
to exploit the full capabilities of the task group.
    The DD 963 Spruance Class destroyer is primarily an antisubmarine warfare
ship, designed to detect and destroy submarines. Internal changes to the hull
and equipment have made this destroyer the quietest ship in the world. The USS
Charles F. Adams Class (DDG 2) of guided-missile destroyers went through a
modernization which brought it up to the fighting capability required to
counter the latest known offensive threats.

    Another group of combatants is called frigates. The frigate is generally a
single-purpose ship which provides the fleet with adequate escort vessels.
    The largest warships are aircraft carriers (Fig. 2) which, through the
aircraft they support, can project power many miles from their position. An
aircraft carrier is a floating mobile air station. Its flight deck provides
the runways, its island is the control tower, the large hangar area below the
flight deck contains maintenance and repair shops, and deep in the hull are
tanks for aircraft fuel storage and magazines for stowing aircraft weapons and
ammunition.

    Patrol ships constitute a naval combatant ship subcategory which augments
surface forces, particularly in coastal areas and narrow seas where
surveillance is impractical for conventional ships. Technical interest is
highest in high-performance patrol craft which strive for maximum capability
in the smallest package.
    The ships used for the transport, landing, support, and control of assault
troops collectively constitute the amphibious warfare force. With the
exception of the ships, boats, and craft that actually ground on the beach,
the amphibious ships used in World War II and Korea were conversions of
commercial ships. With the advent of the helicopter and the successful
adaptation of it for landing assault troops, the nature of amphibious warfare
changed and with the change came new ships.  See  LANDING SHIPS AND CRAFT.
    Mine warfare ships constitute a subcategory of combatant naval ships for
laying or for locating, removing, and neutralizing naval mines. Emphasis in
postwar years has not been on mine laying but on the latter functions, which
are loosely called minesweeping. Mine countermeasures support ships carry, in
addition to the mine warfare command functions, a number of minesweeping
launches and minesweeping helicopters.

    Auxiliary ships.  Auxiliary ships are those ships which provide services
and support naval operations. Other floating configurations which provide
general services are not auxiliary ships but are service craft. Included in
the auxiliaries are the tenders for destroyers and submarines and the ships
that replenish the fleet with supplies of ammunition, oil, naval stores, and
combat support items. Also included are ships for cargo, troops, and various
fields of research involving submarines, hydrofoils, oceanography, and
surveying.
 �34s[William H. Hunley ;Daniel J. Weiler]
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�6v�2v�32sNaval stores   �21m �33s 
    Rosin and turpentine obtained from the industrial processing of resinous 
(pine) woods. Southern slash pine is the source of much of the domestic naval 
stores. Rosin and turpentine are obtained either from the exudate from 
incisions in the trees or from the destructive or steam distillation of wood. 
Products from the former process are called gum rosin and gum turpentine, and 
from the latter, wood rosin and wood turpentine.  See  ROSIN; TURPENTINE; WOOD 
CHEMICALS.  
 �34s[William Mosher]
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�6v�2v�32sNavier-Stokes equations �21m �33s
    Three scalar partial differential equations that describe conservation of
momentum for the motion of a viscous, incompressible fluid. They may be
expressed vectorially as one equation,

	   rho (CURLY D BOLD v/CURLY Dt) + rho(BOLD v DOT DEL) BOLD v
	       = -DEL p + rho f + mu DEL(sup 2) BOLD v

where rho is fluid density, BOLD v is fluid velocity vector, p is fluid
pressure, f is body force (such as gravity) per unit mass, mu is fluid
viscosity coefficient, and t is time.  These equations, together with the
continuity relation, 'DEL DOT BOLD v=0, and suitable boundary conditions
determine the flow field; for example, BOLDv and p are determined as functions
of position in space and of time.  One of these boundary conditions is that of
no slip at the surface of a body; that is, the fluid immediately at the body
surface ``sticks'' to it and thus has the same velocity as the surface itself. 
See NEWTONIAN FLUID. 

    Few mathematical solutions to this complicated set of nonlinear partial
differential equations are known, except for simple geometrics. The importance
of viscosity in determining the flow depends on the relative size of the body.
Approximations to the Navier-Stokes equations for small Reynolds number give
good results. For Re >> 1, the effects of viscosity are confined to a thin
layer near the surface of bodies in the fluid.	See  BOUNDARY-LAYER FLOW;
D'ALEMBERT'S PARADOX; FLUID FLOW; REYNOLDS NUMBER.
 �34s[Arthur E. Bryson, Jr.]
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�6v�2v�32sNavigation   �21m �33s 
    The process of directing the movement of a craft from one place to 
another. Usually modern navigation is considered to encompass four procedures 
- piloting, position fixing, homing, and dead reckoning. Some consider 
celestial navigation to be a separate activity from other position-fixing 
methods. If this classification is accepted, the number of procedures 
increases to five.  See  AIR NAVIGATION; CELESTIAL NAVIGATION; DEAD RECKONING; 
MARINE NAVIGATION; NAVIGATION INSTRUMENTS; PILOTING; TERRESTRIAL COORDINATE 
SYSTEMS.  
 �34s[Alton B Moody]
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�6v�2v�32sNavigation instruments   �21m �33s 
    Measuring devices used in ships and aircraft to determine geographic 
position. The accuracy of such devices has improved steadily since the 
beginnings of travel, with occasional spectacular new inventions suddenly 
enlarging the navigator's domain. Notable milestones are the magnetic compass, 
the sextant, the gyrocompass, radio systems, inertial guidance, and satellite 
navigation.  See  ELECTRONIC NAVIGATION SYSTEMS; GYROCOMPASS; INERTIAL 
GUIDANCE SYSTEM; MAGNETIC COMPASS; SATELLITE NAVIGATION SYSTEMS; SEXTANT. 
 �34s[Robert H. Cannon, Jr.]
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�6v�2v�32sNeandertal man   �21m �33s 
    A group of archaic Homo sapiens from Europe, the Near East, and North 
Africa. The first Neandertal remains were found at Engis, Belgium, in 1829 and 
at Forbes' Quarry, Gibraltar, in 1848, but it was not until workers unearthed 
a skeleton in the Feldhofer cave in the Neander Valley near Duesseldorf, 
Germany, in 1856 that the Neandertals were recognized as a distinct group of 
archaic humans. Since then the remains of several hundred Neandertals have 
been found. The Neandertals were originally placed in the taxon ``Homo 
neanderthalensis,'' but they are now considered members of the species H. 
sapiens and referred to as H. sapiens neanderthalensis. See RHODESIAN MAN; 
SOLO MAN. 
    Most of the Neandertals can be dated to the period between the Riss/Wuerm 
Interglacial and the middle of the most recent glacial, the Wuerm 
(approximately 150,000 to 35,000 years ago). Their living sites exhibit 
advanced cultural development, with semipermanent dwellings, the systematic 
exploitation of resources, and the ritual burial of the dead. 
    Physically, the Neandertals (see illustration) possessed robust bodies 
with brains of modern human proportions, projecting faces with large brows and 
nasal regions and receding cheekbones, teeth slightly larger than those of 
modern Europeans, and trunks and limbs massively built but functionally 
identical to those of modern humans.  See  FOSSIL MAN. 
 �34s[Erik Trinkaus]
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�6v�2v�32sNearshore sedimentary processes   �21m �33s 
    The processes that shape the shore features of coastlines and begin the 
mixing, sorting, and transportation of sediments and runoff from land; 
particularly those interactions among waves, winds, tides, currents, and land 
that relate to the waters, sediments, and organisms of the continental shelf 
and nearshore areas.  See  MARINE SEDIMENTS. 
    The energy for nearshore processes comes from the sea and is produced by 
the force of winds blowing over the ocean, by the gravitational attraction of 
Moon and Sun acting on the mass of the ocean, and by various impulsive 
disturbances at the atmospheric and terrestrial boundaries of the ocean. These 
forces produce waves and currents that transport energy toward the coast. The 
configuration of the land mass and adjacent shelves modifies and focuses the 
flow of energy and determines the intensity of wave and current action in 
coastal waters. Rivers and winds transport erosion products from the land to 
the coast, where they are sorted and dispersed by waves and currents.  See  
OCEAN CURRENTS; OCEAN WAVES. 
    The dispersive mechanisms operative in the nearshore waters of oceans, 
bays, and lakes are all quite similar, differing only in intensity and scale, 
variables that are determined primarily by the nature of the wave action and 
the dimensions of the surf zone. The most important mechanisms are the orbital 
motion of the waves, which is the basic mechanism by which wave energy is 
expended on the shallow sea bottom, and the currents of the nearshore 
circulation system that produce a continuous interchange of water between the 
surf zone and offshore. The dispersion of water and sediments near the coast 
and the formation and erosion of sandy beaches are some of the more common 
manifestations of nearshore processes. 
    Erosional and depositional nearshore processes play an important role in 
determining the configuration of coastlines. Erosion is usually dominant off 
headlands and along coastal sections backed by alluvium and other 
unconsolidated material, whereas deposition is most common along indentations 
between headlands. The overall effect of such processes is usually a 
straightening and smoothing of the coastline. However, this is not always the 
case; differential wave erosion may cause a rapid erosion of material between 
headlands and thus cause irregularities in the coastline.  See  COASTAL 
LANDFORMS.  
    Whether deposition or erosion will be predominant in any particular place 
depends upon a number of interrelated factors: the amount of available beach 
sand and the location of its source; the configuration of the coastline and of 
the adjoining ocean floor; and the effects of wave, current, wind, and tidal 
action. The establishment and persistence of natural sand beaches are often 
the result of a delicate balance among a number of these factors, and any 
changes, natural or human-made, tend to upset this equilibrium. 
 �34s[Douglas L. Inman]
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�6v�2v�32sNebaliacea   �21m �33s 
    A small, exclusively marine order of the higher Crustacea (Malacostraca), 
belonging to the subclass Leptostraca. Only 5 Recent genera, including less 
than 20 species and varieties, are known; they are the last surviving members 
of a group that flourished in Paleozoic times. They are mostly small, 0.16 - 
0.50 in. (4 - 12 mm) long, bottom-living species that occur in shallow water 
or at moderate depths. 
    The large bivalve shell or carapace, without any definite hinge line, 
loosely envelops the thorax and part of the abdomen and in most cases quite 
conceals the thoracic limbs. Anteriorly the shell is produced into a movably 
articulated beak or rostrum. All eight thoracic somites are free. The abdomen 
has seven somites, the last without appendages. The telson bears a pair of 
movably articulated furcal rami. Stalked eyes, antennules, and antennae are 
well developed. Some of the fossils are of large size, reaching a length of 2 
ft (60 cm). Nothing is known of the structure of their limbs, and it is not 
possible to define their relationship to the living forms.  See  LEPTOSTRACA; 
MALACOSTRACA; PHYLLOCARIDA. 
 �34s[Isabella Gordon]
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�6v�2v�32sNebula   �21m �33s 
    Originally, any fixed, extended, and usually fuzzy luminous object seen in 
a telescope. Nebulae are now distinguished from star clouds that can be 
resolved into individual stars, but earlier workers were unable to 
differentiate between white nebulae, which are stellar systems so remote as to 
show no individual stars, and gaseous or diffuse nebulae in the Milky Way 
Galaxy.  See  STAR CLOUDS. 
    Extragalactic nebulae are stellar systems comparable with the Milky Way 
Galaxy or the Magellanic Clouds in size and number of stars, and are more 
properly termed external galaxies.  See  GALAXY, EXTERNAL. 
    This article deals with gaseous nebulae. This class of objects includes 
diffuse nebulae which contain dust and gas of the interstellar medium, excited 
and caused to fluoresce by embedded stars. Gaseous nebulae are members of the 
Milky Way galactic system, and small compared with its overall dimensions. 
    Diffuse nebulae range from huge masses of relatively high surface 
brightness, such as the Orion Nebula, down to faint, milky structures a 
hundred times less dense that are detectable only with long exposures and 
special filters. Diffuse nebulae may contain both dust and gas or may be 
purely gaseous, such as the California Nebula. The nebulosity found in the 
Pleiades and elsewhere consists of dust with no luminous gas, although it is 
probable that there is also a great quantity of neutral hydrogen.  See  ORION 
NEBULA.  
    Variable-brightness nebulae are associated with abnormal variable stars 
and are frequently fan-shaped in appearance. Nebulae of this type are 
associated with T Tauri variables, which are believed to be stars in the 
process of formation. 
    Planetary nebulae are so denoted because they often show small greenish 
disks in the telescope, not unlike the images of the planets Uranus and 
Neptune. The energy emitted by planetary nebulae is derived from the rich 
ultraviolet radiation of stars embedded within them. 
    The detonation of a star in a supernova event causes the ejection of the 
outer layers into the surrounding interstellar medium. In early stages as in 
the Crab Nebula, the radiating material consists of ejecta from the star. In 
the later stages this rapidly moving material is slowed down as it mixes with 
the surrounding dust and gas of the interstellar medium. Supernovae remnants 
characteristically emit nonthermal radio-frequency emission, whereby they are 
often detected in nearby galaxies as well as in the Milky Way system.  See  
CRAB NEBULA; SUPERNOVA. 
 �34s[Lawrence H. Allen]
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�6v�2v�32sNectarine �21m �33s
    A smooth-skinned, fuzzless form of peach, Prunus persica. The nectarine's
lack of pubescence is a simple recessive genetic characteristic. Classically,
the fruits were thought of as being somewhat smaller, softer, and richer in
flavor than those of the peach. More recently developed cultivars, however,
approximate fresh-market peaches in size and firmness but are not usually
superior in flavor.
    California is practically the sole commercial producer of nectarines.
There is a considerable number of plantings in irrigated areas in
south-central Washington.  See	FRUIT; FRUIT, TREE; PEACH; ROSALES.
 �34s[L. F. Hough ;Catherine H. Bailey]
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�6v�2v�32sNectridea   �21m �33s 
    An order of extinct (Carboniferous and early Permian) lepospondylous 
amphibians characterized by vertebrae in which large fan-shaped hemal arches 
grow directly downward from the middle of each caudal centrum; the neural 
arches of the tail, and sometimes of the trunk as well, have a similar shape. 
Most nectridians can be clearly separated into two types. In one, represented 
by Urocordylus, the skull is long and slender, the body and tail elongated, 
and the limbs reduced. In the other type, the head and trunk are broad and 
flattened and the black corners of the skull table taper into long slender 
``horns'' (see illustration).  See  AMPHIBIA; LEPOSPONDYLI. 
 �34s[Alfred S. Romer]
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�6v�2v�32sNegative-resistance circuits �21m �33s
    Circuits or devices whose static (dc) current-voltage characteristic at
one or more ports has a range in which the slope is negative. Figure 1 shows
two typical negative-resistance characteristics in which a positive increment
of voltage is accompanied by a negative increment of current. Strictly, the
term also includes circuits whose static current-voltage characteristics do
not have negative-slope ranges, but in which the input impedance or admittance
at one or more ports has a negative real component. The negative resistance
may arise from inherent properties of a device or from the proper choice of
electrical circuit configuration and operating conditions.
    Negative-resistance circuits and the circuits based upon them find many
applications in counting or information storage, in the generation of pulses
or alternating voltages, in instrumentation, in the control of current and
power, and in physical realization of idealized (lossless) or hypothetical
circuits.

    Basic circuits.  A port having a characteristic of the form of Fig. 1a is
said to be voltage-stable (voltage-controllable) because at any value of
voltage there is only one value of current, but over a range of current there
are three values of voltage, between which transition may take place
spontaneously or under excitation. Similarly, a port having a characteristic
of the form of Fig. 1b is said to be current-stable (current-controllable)
because for each value of current there is only one value of voltage, but over
a range of voltage there are three values of current, between which transition
may take place. In general, negative-resistance circuits may have both
voltage-stable and current-stable ports.

    Figure 2 shows one type of negative-resistance circuit. The indicated
direction of input current DELTAi resulting from a positive increment of input
voltage DELTA v is that which would obtain if the input resistance of the port
were positive (dissipative). If the voltage amplification A of the amplifier
is positive and greater than unity, the output voltage A DELTA V is of the same
sign as the input voltage and greater in magnitude. The current DELTA i(sub r)
through the resistance R is then in the direction shown. If this current
exceeds the current DELTA i(sub i) into the amplifier input, DELTA i must be
opposite in direction to that shown. In other words, the positive increment of
impressed voltage produces a negative increment of current, and the slope of
the characteristic relating current i with voltage v is negative. Because the
range of input voltage over which the amplifier has an amplification greater
than unity is limited, the negative-slope portion of the characteristic is
also limited with respect to voltage, and the characteristic is the
voltage-stable form shown in Fig. 1a. A similar analysis shows that a
voltage-stable port can be formed across the output terminals of the amplifier
and that current-stable ports can be formed in place of resistor R or by
opening the connection between the lower input and output terminals.
Voltage-stable and current-stable ports can in general also be formed within
the amplifier itself.  See  AMPLIFIER.
    The two essentials of a negative-resistance circuit are amplification and
positive feedback; any circuit modification that tends to increase the
open-loop amplification of the amplifier and feedback network is favorable to
the production of negative resistance. In order that negative resistance can
be obtained under static conditions, it is necessary that the amplifier be
capable of amplifying increments of direct voltage.  See  FEEDBACK CIRCUIT.
    Bistable, astable and monostable circuits, and sine-wave oscillators, can
be formed from negative resistance circuits.  See  MULTIVIBRATORS; OSCILLATOR.

    Negative-resistance devices.  Two examples of current-stable
negative-resistance devices are the pnpn transistor and the unijunction
transistor; two examples of voltage-stable devices are the tunnel (Esaki)
diode and the Gunn diode. An advantage of negative-resistance devices over
circuits that incorporate two transistors to achieve negative resistance is
the simplicity of bistable, monostable, and oscillator circuits in which they
are used.  See	MICROWAVE SOLID-STATE DEVICES; TRANSISTOR; TUNNEL DIODE.
 �34s[Herbert J. Reich]
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�6v�2v�32sNegative temperature   �21m �33s 
    The property of a thermodynamical system which satisfies certain 
conditions and whose thermodynamically defined absolute temperature is 
negative. The essential requirements for a thermodynamical system to be 
capable of negative temperature are: (1) the elements of the thermodynamical 
system must be in thermodynamical equilibrium among themselves in order for 
the system to be described by a temperature at all; (2) there must be an upper 
limit to the possble energy of the allowed states of the system; and (3) the 
system must be thermally isolated from all systems which do not satisfy both 
of the first two requirements, that is, the internal thermal equilibrium time 
among the elements of the system must be short compared to the time during 
which appreciable energy is lost to or gained from other systems. 
    The second condition must be satisfied if negative temperatures are to be 
achieved with a finite energy. Most systems do not satisfy this condition; for 
example, there is no upper limit to the possible kinetic energy of a gas 
molecule. Systems of interacting nuclear spins, however, have the 
characteristic that under suitable circumstances they can satisfy all three of 
the above conditions, in which case the nuclear spin system can be at negative 
absolute temperature.  See  KINETIC THEORY OF MATTER; STATISTICAL MECHANICS. 
    The transition between positive and negative temperatures is through 
infinite temperature, not absolute zero; negative absolute temperatures should 
therefore not be thought of as colder than absolute zero, but as hotter than 
infinite temperature.  See  ABSOLUTE ZERO; TEMPERATURE. 
 �34s[Norman F. Ramsey]
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�6v�2v�32sNeisseriaceae   �21m �33s 
    A family of aerobic, gram-negative cocci and coccobacilli. The most 
predominant cellular arrangement is in pairs; when the cells are spheroid, the 
adjacent sides are flattened. Many of the known species are parasitic, and 
several cause disease in humans. Neisseriaceae contains four genera: 
Neisseria, Branhamella, Moraxella, and Acinetobacter. 
 �34s[Manley Mandel]
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�6v�2v�32sNemata �21m �33s
    A phylum of unsegmented worms. A classification of nematodes follows:

Phylum Nemata
    Class Adenophorea (Aphasmidia, Aphasmidea)
	Subclass Enoplia
	Order: Enoplida
	       Isolaimida
	       Mononchida
	       Dorylaimida
	       Trichocephalida
	       Mermithida
	       Muspiceida
	Subclass Chromadoria
	Order: Araeolaimida
	       Chromadorida
	       Desmoscolecida
	       Desmodorida
	       Monhysterida
    Class Secernentea (Phasmidia, Phasmidea)
	Subclass Rhabditia
	Order: Rhabditida
	       Strongylida
	       Ascaridida
	Subclass Spiruria
	Order: Spirurida
	       Camallanida
	Subclass Diplogasteria
	Order: Diplogasterida
	       Aphelenchida
	       Tylenchida

    Diagnosis.	The Nemata are unsegmented or pseudosegmented (any superficial
annulation limited to the cuticle) bilaterally symmetrical worms with a
basically circular cross section. The body is covered by a noncellular
cuticle. The cylindrical body is usually bluntly rounded anteriorly and
tapering posteriorly. The body cannot be easily divided into head, neck, and
trunk or tail, although a region posterior to the anus is generally referred
to as the tail. The oral opening is terminal (rarely subterminal) and followed
by the stoma, esophagus, intestine, and rectum which opens through a
subterminal anus. Females have separate genital and digestive tract openings.
In males the tubular reproductive system joins posteriorly with the digestive
tract to form a cloaca. The sexes are separate and the gonads may be paired or
unpaired. Females may be oviparous or ovoviviporous.
    Adult nematodes are extremely variable in size, ranging from less than
0.012 in. (0.3 mm) to over 26 ft. (8 m). Nematodes are generally colorless
except for food in the intestinal tract or for those few species which have
eyespots.

    Life cycle.  Reproduction among nematodes is either amphimictic or
parthenogenetic (rarely hermaphroditic). After the completion of oogenesis the
chitinous egg shell is formed and a waxy vitelline membrane forms within the
egg shell; in some nematodes the uterine cells deposit an additional outermost
albuminoid coating. Upon deposition or within the female body, the egg
proceeds through embryonation to the eellike first- or second-stage larva, but
following eclosion the larva proceeds through four molts to adulthood. This
represents a direct life cycle, but among parasites more diversity occurs.

    Distribution.   Nemata comprise the third largest phylum of invertebrates,
being exceeded only by Mollusca and Arthropoda. In sheer numbers of
individuals they exceed all other metazoa. As parasites of animals they exceed
all other helminths combined. Nematodes have been recovered from the deepest
ocean floors to the highest mountains, from the Arctic to the Antarctic, and
in soils as deep as roots can penetrate.
 �34s[Armand R. Maggenti]
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�6v�2v�32sNematicide   �21m �33s 
    A type of chemical used to kill plant-parasitic nematodes. Nematicides may 
be classed as soil fumigants or soil amendments, space fumigants, surface 
sprays, or dips. Soil treatments are commonly used because most 
plant-pathogenic species spend part or all of their life cycle in the soil, in 
or about the roots of plants. Nematicides may be liquids, gases, or solids, 
but on a field scale, liquids are most practical.  See  NEMATA; PESTICIDE. 
 �34s[Dewey J. Raski]
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�6v�2v�32sNematomorpha   �21m �33s 
    A phylum of worms formerly considered to be a class of the phylum 
Aschelminthes; commonly called the hairworms, and closely allied to the 
nematodes. The adults are free-living in aquatic habitats, while the juveniles 
are parasitic in arthropods. The nematomorphs are found all over the world. 
They are divided into two classes, the Nectonematoidea and Gordioidea, with a 
total of 225 species.  See  NEMATA. 
    The body is long and slender with a maximum length of 5 ft (1.5 m) and a 
diameter of 0.02 - 0.12 in. (0.5 - 3 mm). The females are longer than the 
males. The posterior end may be rounded with a terminal cloaca, or it may form 
two or three lobes in a forklike structure. The body color is yellowish, 
brown, or almost black. The body wall consists of three layers: an outer, 
rather thick fibrous cuticle; an epidermis consisting of a single layer of 
cells; and innermost, a muscle layer with longitudinal fibers only. 
    The body cavity extends the length of the body. It may be filled with 
tissue so that only minor spaces are left around the digestive system and the 
gonads.  
    The sexes are always separate, and the gonads are paired and stringlike 
extending the length of the body. During copulation the male coils itself 
around the female and places a drop of sperm near the cloacal opening of the 
female. The sperm cells actively enter the seminal receptable. The eggs are 
laid in water in strings, and the adults die after egg laying. When hatched, 
the larvae swim to an aquatic arthropod. They penetrate the body wall of the 
host by means of their characteristic proboscis, which is armed with hooks and 
three long stylets. The gradual development in the host lasts some months 
without any metamorphosis. When mature, the worms leave the host. 
 �34s[Bent J. Muus]
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�6v�2v�32sNematophytales   �21m �33s 
    An enigmatic group of fossil plants, in mid-Silurian to lower Upper 
Devonian rocks, composed of intertwined, branching tubes of two sizes: the 
larger 10 - 50 micrometers in diameter, the smaller 1 - 10 micrometers. 
Although referred to the algae by some authors, the occurrence of this group 
in inland swamps, coastal plain deposits, and marine deposits close to shore 
indicates that they were terrestrial organisms, unrelated to any known groups, 
perhaps at an intermediate level between algae and bryophytes.  See  
PALEOBOTANY.  
 �34s[Harlan P. Banks]
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�6v�2v�32sNeodymium   �21m �33s 
    A metallic chemical element, Nd, atomic number 60, atomic weight 144.24. 
Neodymium belongs to the rare-earth group of elements. The naturally occurring 
element includes the six isotopes. The oxide, Nd2O3, is a light-blue powder. 
It dissolves in mineral acids to give reddish-violet solutions. For properties 
of the metal  see  RARE-EARTH ELEMENTS. 
    The salts have found application in the ceramic industry for coloring 
glass and for glazes. The glass is particularly useful in goggles used by 
glass blowers, since it absorbs the intense yellow D line of sodium present in 
the flame. The element has found commercial application in the manufacture of 
lasers.  
 �34s[Frank H. Spedding]
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�6v�2v�32sNeogastropoda   �21m �33s 
    An order of gastropods, also known as the Stenoglossa, which contains the 
most highly developed snails (see illustration). Respiration is by means of 
ctenidia. The nervous system is concentrated, an operculum is usually present, 
and the sexes are separate. All families in this order are marine. The family 
Muricidae contains the rock snails, which are all predatory; Buccinidae, most 
abundant in northern seas, contains the whelks in the genus Buccinum, which 
are frequently used for fish bait; other families, such as the Volutidae, 
Olividae, and Harpidae, are prized by shell collectors for their beautiful 
coloration. The family Conidae contains the poisonous cone shells, which have 
caused several deaths by their poisonous bite.  See  GASTROPODA. 
 �34s[William J. Clench]
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�6v�2v�32sNeognathae   �21m �33s 
    One of the two recognized superorders making up the subclass Neornithes of 
the class Aves. They are characterized as flying birds with fully developed 
wings and sternum with a keel, caudal vertebrae fused into a pygostyle, and 
absence of teeth in both jaws, or modifications of these conditions in 
secondary flightless birds. 
    This superorder includes all living birds and all known fossil birds since 
the Late Cretaceous; only the ancestral Jurrasic Archaeopteryx and the 
specialized Cretaceous Hesperonis and its allies do not belong to the 
Neognathae.  See  ARCHAEORNITHES; AVES; ODONTOGNATHAE; RATITES. 
 �34s[Walter J. Bock]
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�6v�2v�32sNeogregarinida   �21m �33s 
    An order of the protozoan subclass Gregarinia, class Telosporea, subphylum 
Sporozoa. All gregarines are parasites of the digestive tract and body cavity 
of invertebrates or lower chordates; their large, mature trophozoites 
(vegetative stages) live outside the the host's cells. The Neogregarinida are 
thought to be relatively advanced gregarines which live in insects. There are 
only about 29 species of about 12 genera, and 4 families. See GREGARINIA. 
 �34s[Norman D. Levine]
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�6v�2v�32sNeomycin �21m �33s
    A colorless antibiotic produced by certain strains of Streptomyces fradiae
in submerged culture. Chemically, the structure of neomycin, C23N46N6O13, is
analogous to that of streptomycin. Neomycin is active against a great variety
of gram-positive cocci and rods, gram-negative rods, and acid-fast bacteria,
notably the tuberculosis organism. It is not active against anaerobic
bacteria, fungi, most protozoa, rickettsiae, and viruses.
    Clinically, neomycin is used topically in the treatment of bacterial
infections of the skin, and orally in the treatment of bacterial infections
of the digestive tract. It can also be sued systemically in the treatment of
deep-seated bacterial infections that are resistant to other, less toxic
antibiotics. See ANTIBIOTIC; STREPTOMYCIN. [Selman A. Waksman]





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�6v�2v�32sNeon �21m �33s
    A gaseous chemical element, Ne, with atomic number 10 and atomic weight
20.183. Neon is a member of the family of noble gases. The only commercial
source of neon is the Earth's atmosphere, although traces of neon are found in
natural gas, minerals, and meteorites.	See  INERT GASES.
    Considerable quantities of neon are used in high-energy physics research.
Neon fills spark chambers used to detect the passage of nuclear particles.
Liquid neon can be utilized as a refrigerant in the temperature range about 25
- 40 K (-416 to -387DGF). Neon is also used in some kinds of electron tubes,
in Geiger-Mueller counters, in spark-plug test lamps, and in warning
indicators on high-voltage electric lines. A very small wattage produces
visible light in neon-filled glow lamps; such lamps are used as economical
night and safety lights.  See  NEON GLOW LAMP.
    Neon is colorless, odorless, and tasteless; it is a gas under ordinary
conditions. Some of the other properties of neon are given in the table.
Neon does not form any chemical compounds in the ordinary sense of the word;
there is only one atom in each molecule of gaseous neon.

[Arthur W. Francis]

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�6v�2v�32sNeon glow lamp   �21m �33s 
    A low-wattage lamp often used as an indicator light or as an electronic 
circuit component. The neon lamp usually consists of a pair of electrodes 
sealed within a bulb containing neon gas at a low pressure. Some of the 
smaller bulbs are equipped with wire leads that are connected directly into 
the electrical supply circuit; others are equipped with conventional bases 
that vary with the size of the lamps (see illustration). 
    Electrodes sealed in a neon atmosphere will emit electrons if a sufficient 
voltage difference is impressed across them. In glow lamps the electrodes are 
usually treated to emit electrons freely. With a sufficiently high voltage 
between electrodes, the velocity of electron flow is high enough to ionize the 
neon nearest the negative electrode (cathode). The neon then emits a 
reddish-orange glow similar to the color of neon sign tubing. With direct 
current the glow is restricted to the immediate vicinity of the negative 
electrode. With alternating current, both electrodes act alternately as 
cathodes, and the glow appears alternately at both surfaces. At usual 
frequencies, the alternations occur so rapidly that both electrodes appear to 
glow constantly. In dc circuits, the voltage across the electrodes may be 
reduced significantly, once the lamp has started, without causing the lamp to 
go out. 
 �34s[Alfred Makulec]
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�6v�2v�32sNeoplasia   �21m �33s 
    An abnormal proliferation of somatic cells resulting in a mass of tissue 
called a neoplasm or tumor. Neoplasia literally means new growth. The involved 
cells are characterized by an abnormality which allows them to escape the 
normal regulatory events. This anomaly is passed on to daughter cells. 
    The mass of tissue which forms the neoplasm can be divided into two 
components. One is the parenchyma, which consists of the abnormal, 
proliferating cells. The other is the stroma, which is connective tissue 
providing support. It includes the vascular tissues. The relative amount of 
parenchyma and stroma varies in different types of tumors. 
    Neoplasia can result in a tumor whose parenchyma have a high degree of 
differentiation. This means that in appearance and function the cells are very 
like normal tissue. These neoplasms typically remain localized and generally 
can be successfully removed. They are referred to as benign. Conversely, there 
are neoplasms whose parenchyma have varying degrees of abnormalities in 
structure and behavior. They tend to invade adjacent tissue. Small pieces may, 
by various means, locate in other places in the body where they form secondary 
tumors called metastases. These kinds of neoplasms are referred to as 
malignant or cancerous. 
    Thus, there is a whole range of neoplastic responses. The type of tumor 
produced is partially dependent upon the cell type involved in the change and 
the position in the body. Most cells are capable of undergoing neoplastic 
changes.  See  ONCOLOGY; TUMOR. 
 �34s[N. Karle Mottet ;Carol Quaife]
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�6v�2v�32sNeornithes   �21m �33s 
    The subclass of the class Aves containing all known birds except the 
Jurassic fossil Archaeopteryx, which alone constitutes the subclass 
Archaeornithes. Some authorities divide the Neornithes into two superorders: 
Odontognathae, containing only the fossil order Hesperornithiformes; and 
Neognathae, containing the remaining orders. Additional superorders that have 
been segregated by other authors are: Ichthyornithes, for the fossil order 
Ichthyornithiformes; Palaeognathae, for the Ratites or large running birds 
(ostrich, emu, cassowary, kiwi, moa, elephant bird, and rhea) and the tinamous 
(Tinamiformes); and Impennes, for the Sphenisciformes or penguins. All of 
these birds are now believed to represent portions of the evolutionary 
radiation of the Neognathae, and not separate phyletic lines as would be 
indicated by recognition of these additional superorders. See AVES; 
NEOGNATHAE; ODONTOGNATHAE. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sNeoteny   �21m �33s 
    A phenomenon among some salamanders, in which larvae of large size, while 
still retaining the gills and other larval features, become sexually mature, 
mate, and produce fertile eggs. In certain lakes of Mexico, only the neotenous 
larvae are present and are called axolotls. Neoteny occurs in certain species 
of the family Ambystomidae.  See  PEDOGENESIS. 
 �34s[Tracy I. Storer]
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�6v�2v�32sNeper �21m �33s
    A unit of attenuation used in transmission-line theory. On a uniform
transmission line having waves traveling in only one direction, the magnitudes
of voltage E and of current I decrease with distance x traveled, as given by

		    E/E0 = I/I0 =epsilonsup-alphax			     (1)

Eq.  (1), where E0, I0, and ALPHA are constants.  The attenuation in nepers
between the points where E0 and I0 are measured and where E and I are measured
is given by Eq.

		    alphax = ln (E0/E) = ln (I0/I)			     (2)

(2), in which ln denotes the natural (or napierian) logarithm.	One neper
equals 8.686 dB, the decibel being the practical unit of attenuation.  See
DECIBEL; TRANSMISSION LINES.
 �34s[Edward W. Kimbark]
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�6v�2v�32sNepheline syenite �21m �33s
    A phaneritic (visibly crystalline) plutonic rock with granular texture,
composed largely of alkali feldspar (orthoclase, or microcline, usually
perthitic), nepheline, and dark-colored (mafic) minerals (biotite,
soda-amphibole, and soda-pyroxene). Nepheline syenites have many features in
common with syenites into which they grade, but chemically, mineralogically,
and texturally they are much more variable.  See  SYENITE.
    The alkali feldspar is soda-rich and usually exhibits perthitic texture
(intergrown potash and soda feldspars). Nepheline, a gray mineral with a
greasy appearance, is also a major constituent. It may be highly altered and
is commonly converted to bright-yellow cancrinite. The most common accessory
minerals are sphene, zircon, apatite, ilmenite, and magnetite.
    Nepheline syenite and related rocks are rare and generally occur in small
bodies (dikes, sills, laccoliths, stocks, and small irregular plutons). Only a
few large bodies are known. Three of the largest bodies are in southern
Greenland, Pilaansberg in South Africa, and Kola Peninsula, Soviet Union.  See
IGNEOUS ROCKS.
 �34s[Carleton A. Chapman]
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�6v�2v�32sNephelinite   �21m �33s 
    A dark-colored, aphanitic (very finely crystalline) rock of volcanic 
origin, composed essentially of nepheline (a feldspathoid) and pyroxene. 
    The texture is usually porphyritic with large crystals (phenocrysts) of 
augite and nepheline in a very-fine-grained matrix. Augite phenocrysts may be 
diopsidic or titanium-rich and may be rimmed with soda-rich pyroxene 
(aegirine-augite). Microscopically the matrix is seen to be composed of tiny 
crystals or grains of nepheline, augite, aegirite, and sodalite with 
occasional soda-rich amphibole, biotite, and brown glass. Accessories usually 
include magnetite, ilmenite, apatite, sphene, and perovskite. 
    Nephelinite and related rocks are very rare. They occur as lava flows and 
small, shallow intrusives. A great variety of these feldspathoidal rocks is 
displayed in Kenya.  See  FELDSPATHOID; IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sNephelometric analysis �21m �33s
    A method of chemical analysis based on measurement of the cloudiness of
solutions. In nephelometry the intensity of the light scattered at right
angles to the light beam by a suspension is measured, rather than the decrease
in the intensity of directly transmitted light as in turbidimetry.
    Simple nephelometers consist of a light source focused on the sample cell
and a detecting element positioned at right angles to the light beam (see
illustration). Since a clear solution will scatter no light at right angles to
the beam, nephelometry measures the difference between no light and various
amounts of scattered light, making it sensitive to the detection of small
amounts of turbidity.  See  OPTICAL METHODS OF CHEMICAL ANALYSIS;
TURBIDIMETRIC ANALYSIS.
 �34s[Robert F. Goddu ;James N. Little]
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�6v�2v�32sNephrite   �21m �33s 
    One of the amphibole group of rock-forming minerals. The minutely fibrous 
structure of nephrite makes it exceedingly durable. It occurs in a variety of 
colors, mostly of low intensity, including medium and dark green, yellow, 
white, black, and blue-gray. Nephrite has a hardness of 6 to 6**2 on Mohs 
scale, a specific gravity near 2.95, and refractive indices of 1.61 to 1.64. 
Nephrite occurs throughout the world; important sources include the Soviet 
Union, New Zealand, Alaska, several provinces of China, and a number of states 
in the western United States. See AMPHIBOLE; GEM; JADE. 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sNephritis   �21m �33s 
    A general term referring to an inflammatory process affecting the kidney. 
Included in this broad category are diseases caused by specific infectious 
agents such as viruses, spirochetes, bacteria, fungi, or parasites. A large 
group of common diseases affecting the kidney have no known cause, but may be 
associated with diseases elsewhere in the body such as streptococcal 
pharyngitis, lupus erythematosus, or arteriosclerosis. Finally, there are 
those conditions which are seen following exposure to toxins such as heavy 
metals, carbon tetrachloride, and certain drugs. See GLOMERULONEPHRITIS; 
PYELONEPHRITIS.  
 �34s[Gary E. Striker]
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�6v�2v�32sNeptune   �21m �33s 
    The outermost of the four giant planets, a near twin of Uranus in size, 
mass, and composition. Its discovery in 1846 within a degree from the 
theoretically predicted position was one of the great achievements of 
celestial mechanics. 
    Through a small telescope, Neptune appears as a tiny greenish disk. Its 
linear diameter of 30,900 mi (49,400 km) is very similar to that of Uranus. 
The mass of Neptune is 17.21 times the mass of Earth, corresponding to a mean 
density of 1.66, somewhat above that of its sister planet. This suggests that 
the enrichment of heavy elements (when compared with Jupiter and Saturn, which 
exhibit a solar mixture) is somewhat greater in Neptune than in Uranus, but 
both must still be composed primarily of hydrogen and helium. 
    The temperature that this distant object would assume as a result of 
heating by solar radiation can be calculated as -380DGF (45 K). But 
temperatures measured near a wavelength of 13 micrometers are very much 
higher, over -280DGF (100 K). At radio frequencies, which permit observations 
of radiation from the lower atmosphere, one finds temperatures increasing with 
depth.  
    Neptune has two known satellites. The larger and brighter one, Triton, was 
found visually by W. Lassell in 1846; Nereid was discovered in a photographic 
search by G. P. Kuiper in 1949. 
 �34s[Tobias C. Owen]
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�6v�2v�32sNeptunium �21m �33s
    A chemical element, symbol Np, atomic number 93. Neptunium is a member of
the actinide or 5f series of elements. It was synthesized as the first
transuranium element in 1940 by bombardment of uranium with neutrons to
produce neptunium-239. The lighter isotope (sup 237)Np, a long-lived alpha
emitter with half-life 2.14 x 10(sup 6) years, is particularly important
chemically.
    Neptunium metal is ductile, low-melting (637DGC or 1179DGF), and in its
alpha form is of high density, 20.45 g/cm(sup 3) (11.82 oz/in.[sup 3]). The
chemistry of neptunium may be said to be intermediate between that of uranium
and plutonium. Neptunium metal is reactive and forms many binary compounds,
for example, with hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, and
the halogens.  See  ACTINIDE ELEMENTS; NUCLEAR CHEMISTRY; TRANSURANIUM
ELEMENTS.
 �34s[Robert A. Penneman]
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�6v�2v�32sNerve   �21m �33s 
    A group of nerve fibers coursing together as a bundle in the peripheral 
nervous system. The individual fibers are covered by Schwann cells, many of 
which contain large amounts of myelin, which makes the nerve appear shiny 
white. The nerve fibers with their Schwann cell sheaths are held together by 
connective tissue. In most nerves, some of the fibers are sensory (carrying 
information to the central nervous system) and some are motor (carrying 
information from the central nervous system to peripheral glands and muscles). 
When both sensory and motor fibers are in a nerve, it is called a mixed nerve. 
  See  MYELIN. 
    In the central nervous system (brain and spinal cord) a group of nerve 
fibers running together is called a tract. Glial cells, not Schwann cells, 
form the sheaths of tract fibers, and there is no connective tissue holding 
the bundle together. Whereas most nerves are mixed, there is functional 
segregation in the central nervous system so that most tracts have only one 
functional type of fiber.  See  MOTOR SYSTEMS; NERVOUS SYSTEM (VERTEBRATE). 
 �34s[Douglas B. Webster]
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�6v�2v�32sNervous system disorders �21m �33s
    A satisfactory classification of diseases of the nervous system should
include not only the type of reaction (congenital malformation, infection,
trauma, neoplasm, vascular diseases, and degenerative, metabolic, toxic, or
deficiency states) but also the site of involvement (meninges, peripheral
nerves or gray or white matter of the spinal cord, brainstem, cerebellum, and
cerebrum). This article will consider mostly the various types of reactions
and only incidentally the sites affected. Clinical signs and symptoms of
dysfunction of the nervous system depend not only on the type and location of
lesions but also on the severity and rapidity of development of the lesion.

    Malformations.  The central nervous system develops as a hollow neural
tube by the fusion of the crests of the neural groove, beginning in the
cervical area and progressing rostrally and caudally, the last points to close
being termed the anterior and posterior neuropores. If the anterior neuropore
fails to close (about 24 days of fetal age), anencephaly develops. The poorly
organized brain is exposed to amniotic fluid and becomes necrotic and
hemorrhagic, leaving a froglike head on an infant who usually dies within
hours after birth. The eyes, brainstem, cerebellum, and spinal cord are
usually normal.  See  CONGENITAL ANOMALIES.
    If the posterior neuropore fails to close (about 26 days of fetal age),
the lumbosacral neural groove is exposed to amniotic fluid. Without the
protective coverings of meninges, spines, muscles, and skin, the nervous
tissue becomes partially necrotic and incorporated in a scar. Such a
meningomyelocele is readily infected unless buried surgically within a few
hours after birth. In addition, in about 95% of such infants hydrocephalus (a
dilatation of the ventricles) occurs. The hydrocephalus can usually be
adequately treated by shunting the ventricular fluid into the venous system or
peritoneal cavity.

    Infections.   Infections of the nervous system may occur through a defect
in the normal protective coverings caused by certain congenital malformations,
as mentioned above, but also through other defects as the result of trauma,
especially penetrating wounds or fractures opening into the paranasal sinuses
or mastoid air cells. Subsequent infection of the nervous system may be the
major complication of such ``open head'' injuries.
    Viral infections vary widely geographically, generally related to the
necessity for intermediate hosts and vectors (animal reservoirs) by which the
virus is spread from animals to humans, or from one human to another.
Poliomyelitis is primarily an intestinal infection which occasionally spreads
to the nervous system. Herpes zoster has a similarly restricted preference for
infecting sensory nerve cells and producing an acute skin eruption in the
distribution of the affected sensory cells. Herpes simplex is closely related
to herpes zoster, resides in the trigeminal or sacral sensory nerve cells, and
intermittently produces eruptions in the distribution of these cells; ``fever
blisters'' in and around the mouth in type I herpes, or similar blisters in
the genital area in type II herpes.  See  HERPES; POLIOMYELITIS.
    Complications of infections of the meninges include: hydrocephalus,
cranial nerve paralyses, and focal destructions of neural tissue.

    Inflammations.   Certain viruses, such as lymphocytic choriomeningitis and
mumps, frequently produce a meningitis in humans from whose cerebrospinal
fluid the virus is relatively easily grown. Certain other viruses, such as
measles and varicella, occasionally produce meningitis or encephalomyelitis,
but the cerebrospinal fluid does not contain the virus, and the hypothesis is
that an allergic reaction is responsible. See MENINGITIS.
    Allergy to one's own tissue elements is an interesting possibility that
has evoked many experimental approaches. One of the best-studied examples is
experimental allergic encephalomyelitis. Two diseases, multiple sclerosis
affecting the central nervous system and the Landry-Guillain-Barre syndrome
affecting the peripheral nervous system, are considered likely candidates
eventually to be related to experimental allergic encephalomyelitis and
experimental allergic neuritis, respectively. See MULTIPLE SCLEROSIS.

    Vascular diseases.	 Vascular diseases of the nervous system are commonly
called strokes, a term which emphasizes the suddenness of onset of
neurological disability, as though the individual were struck down by a blow
to the head. Such a cataclysmic onset is characteristic of vascular diseases,
since the nerve cell can function without nutrients for only a matter of
seconds and will die if not renourished within several minutes.  See
CARDIOVASCULAR SYSTEM DISORDERS; HEMORRHAGE.
    Small and large strokes may be suffered clinically as the result of small
or large hemorrhages or ischemic episodes occurring in or around the brain.
One of the common ways the brain reacts to these injuries is by swelling. If
the person survives the first critical week after a massive stroke with
swelling, the neurologic deficit itself is rarely fatal, but the accompanying
prolonged convalescence may contribute to the development of pneumonia or
pyelonephritis, which may be fatal.

    Degenerative diseases.   Degenerative, metabolic, toxic, and deficiency
states include the largest numbers of both common and rare diseases of the
nervous system. Since neurons in the brain may be destroyed after birth and
cannot be replaced, mental deterioration, deafness and blindness,
incoordination and adventitious movements, and other neurologic signs so
typical of these disorders are generally not reversible even if the basic
metabolic defect is corrected.	See  HUNTINGTON'S CHOREA; METABOLIC DISORDERS;
PARKINSON'S DISEASE; PHENYLPYRUVIC OLIGOPHRENIA.

    Neoplasms.	 Neoplasms of the nervous system can be conveniently divided
into primary and metastatic, the primary into gliomas and others, and the
metastatic into bronchogenic and others. These four groups each account for
about 25% of all intracranial neoplasms.
 �34s[Ellsworth C. Alvord, Jr. ;Chen-Meg Shaw]
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�6v�2v�32sNervous system (invertebrate)   �21m �33s 
    All multicellular organisms have a nervous system, which may be defined as 
assemblages of cells specialized by their shape and function to act as the 
major coordinating organ of the body. Nervous tissue underlies the ability to 
sense the environment, to move and react to stimuli, and to generate and 
control all behavior of the organism. Compared to vertebrate nervous systems, 
invertebrate systems are somewhat simpler and can be more easily analyzed. 
Invertebrate nerve cells tend to be much larger and fewer in number than those 
of vertebrates. They are also easily accessible and less complexly organized; 
and they are hardy and amenable to revealing experimental manipulations. 
However, the rules governing the structure, chemistry, organization, and 
function of nervous tissue have been strongly conserved phylogenetically. 
Therefore, although humans and the higher vertebrates have unique behavioral 
and intellectural capabilities, the underlying physical-chemical principles of 
nerve cell activity and the strategies for organizing higher nervous systems 
are already present in the lower forms. Thus neuroscientists have taken 
advantage of the simpler nervous systems of invertebrates to acquire further 
understanding of those processes by which all brains function.  See  NERVOUS 
SYSTEM (VERTEBRATE). 
    Invertebrate and vertebrate nerve cells differ more in quantity, or 
degree, than in qualitative features. Aside from differences in size and 
numbers, the most striking difference is that invertebrate neurons have a 
unipolar shape, whereas most vertebrate neurons are multipolar. An additional 
general contrast between invertebrate and vertebrate nervous systems is that 
invertebrates tend to have more neurons displaced to the periphery (outside 
the central nervous system) and to perform more integrative and processing 
functions in the periphery. Vertebrates perform almost all their integration 
within the central nervous system, using interneurons. Invertebrate nervous 
systems also seem to have a greater potential for regrowth, regeneration, or 
repair after damage than do vertebrate nerve cells. Many invertebrates 
continue to add new nerve cells to their ganglia with age; vertebrates, in 
general, do not. Only vertebrate neurons have myelin sheaths, a specialized 
wrapping of glial membrane around axons, increasing their conduction speed. 
Invertebrates tend to enhance conduction velocity by using giant axons, 
particularly for certain escape responses.  See  MYELIN. 
 �34s[James E. Blankenship]
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�6v�2v�32sNervous system (vertebrate)   �21m �33s 
    A coordinating and integrating system which functions in the adaptation of 
an organism to its environment. An environmental stimulus causes a response in 
an organism when specialized structures, receptors, are excited. Excitations 
are conducted by nerves to effectors which act to adapt the organism to the 
changed conditions of the environment. In animals, humoral correlation is 
controlled by the activities of the endocrine system. 
    The central nervous system consists of the brain and spinal cord; most 
peripheral nerves are connected to the spinal cord. Peripheral nerves that are 
connected directly to the brain are called cranial nerves.  See  BRAIN; 
CENTRAL NERVOUS SYSTEM; CRANIAL NERVE; SPINAL CORD. 
    The nervous system is divided into the cerebrospinal (also called 
voluntary or somatic) system and the autonomic (also called visceral or 
involuntary) system. The cerebrospinal system controls all voluntary muscle 
motions, while the operation of organs (heart muscle and so on) is controlled 
by the autonomic system. The autonomic system is divided into the 
parasympathetic system, which plays an inhibitory role, and the sympathetic 
system, which is responsible for visceral reflexes, especially in emergency 
situations.  See  AUTONOMIC NERVOUS SYSTEM; PARASYMPATHETIC NERVOUS SYSTEM; 
SYMPATHETIC NERVOUS SYSTEM. 
 �34s[Alexander Hellemans]
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�6v�2v�32sNetwork (mathematics)   �21m �33s 
    A collection of points (nodes), together with arcs joining certain pairs 
of nodes which may represent communications links between various locations, 
roads joining certain cities, and so on. Numbers representing, for example, 
the capacity or length of the arcs are frequently assigned. Problems 
considered may be of a combinatorial nature (finding the number of possible 
connections between two nodes) or of an optimization nature (finding the 
shortest path, or maximizing the amounts shipped between two nodes). 
 �34s[Guillermo Owen]
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�6v�2v�32sNeural crest   �21m �33s 
    A strip of ectodermal material in the early vertebrate embryo inserted 
between the prospective neural plate and epidermis. After closure of the 
neural tube the crest cells migrate into the body and give rise to parts of 
the neural system: the main part of the visceral cranium, the mesenchyme, the 
chromaffin cells, and pigment cells. The fact that mesenchyme arises from this 
ectodermal organ was directly contrary to the doctrine of the specificity of 
the germ layers.  See  GERM LAYERS. 
    Neural crest no doubt exists in all vertebrate groups, including the 
cyclostomes. It has been most thoroughly studied in amphibians and the chick. 
 �34s[Sven Horstadius]
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�6v�2v�32sNeurobiology   �21m �33s 
    The development and function of the nervous system, with emphasis on how 
nerve cells generate and control behavior. The major goal of neurobiology is 
to explain at the molecular level how nerve cells differentiate and develop 
their specific connections and how nerve networks store and recall 
information. Ancillary studies on disease processes and drug effects in the 
nervous system also provide useful approaches for understanding the normal 
state by comparison with perturbed or abnormal systems. The functions of the 
nervous system may be studied at several levels: molecular, subcellular 
(organelle), cellular, simple multicellular interacting systems, complex 
systems, and higher functions (whole animal behavior).  See  ENDORPHINS; 
NERVOUS SYSTEM (INVERTEBRATE); NERVOUS SYSTEM (VERTEBRATE); NEURON. 
 �34s[James R. Bamburg]
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�6v�2v�32sNeurohypophysis hormone   �21m �33s 
    Extracts of the neurohypophysis, or posterior lobe of the pituitary, 
exhibit the following biological properties when administered to mammals: (1) 
The extracts affect blood pressure by constricting the blood vessels leading 
from the arteries (pressor action); (2) they stimulate the contraction of 
smooth muscle and specifically of the musculature of the uterus (oxytocic 
action); (3) they cause ejection of milk from the mammary gland; and (4) they 
cause inhibition of water excretion through the kidneys (antidiuretic action). 
These effects are due to the presence of two peptide hormones, oxytocin and 
vasopressin.  
    From a quantitative standpoint, vasopressin is the more versatile of the 
two, since it exercises appreciable uterine-stimulating, avian vasodepressor, 
and milk-ejecting actions in addition to its pressor and antidiuretic effects. 
Oxytocin has a slight but definite pressor and antidiuretic action in addition 
to its own characteristic effects. These overlapping biological activities of 
the two hormones reflect their close structural relationship.  See  HORMONE; 
PITUITARY GLAND. 
 �34s[Choh Hao Li]
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�6v�2v�32sNeuron   �21m �33s 
    A nerve cell: the functional unit of the nervous system. Structurally, the 
neuron is made up of a cell body or soma and one or more long processes: a 
single axon and dendrites. The cell body contains the nucleus and usual 
cytoplasmic organelles with an exceptionally large amount of rough endoplasmic 
reticulum, called Nissl substance in the neuron. The longest cell process is 
the axon, which is capable of transmitting propagated nerve impulses. There 
may be none, one, or many dendrites composing part of a neuron. If there is no 
dendrite, it is a unipolar neuron; with one dendrite, it is a bipolar neuron; 
if there is more than one dendrite, it is a multipolar neuron. In most neurons 
only the axon propagates nerve impulses; the dendrites and somas are also 
irritable but do not propagate nerve impulses.  See  NERVOUS SYSTEM 
(VERTEBRATE).  
 �34s[Douglas B. Webster]
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�6v�2v�32sNeuroptera   �21m �33s 
    An order of delicate insects having endopterygote development, chewing 
mouthparts, and soft bodies. Included are the insects commonly termed 
lacewings, ant lions, dobsonflies, and snake flies. The order consists of 
about 25 families and is widely distributed.  See  ANT LION. 
    The adults have long, slender antennae and usually four similar wings, 
although the front pair is generally slightly larger than the hind pair. The 
adults of most species are strongly attracted to lights. The larvae are 
aggressive predators. The larvae of lacewings are especially destructive to 
aphids, scale insects, and mites. See ENDOPTERYGOTA; INSECTA. 
 �34s[Frank M. Carpenter]
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�6v�2v�32sNeurosecretion   �21m �33s 
    The synthesis and release of hormones by neurons (nerve cells). Such 
neurons are specifically called neurosecretory cells, and their products are 
called neurohormones. Like conventional (nonglandular) neurons, neurosecretory 
cells are able to receive signals from other neurons and to generate and 
conduct impulses. There is some evidence that these impulses trigger the 
release of the neurohormones. Because of the glandular function of 
neurosecretory cells, they are able to send messages to structures that are 
distant from their axonal terminals, whereas conventional neurons can only 
send messages locally. Neurosecretory cells, like conventional neurons, have 
an axon, Nissl granules, and neurofibrillae. Furthermore, like conventional 
neurons, neurosecretory cells of vertebrates have dendrites, but the 
neurosecretory cells of invertebrates may lack dendrites.  See  NEURON. 
    In organisms with a well-developed circulatory system, the neurohormones 
are typically sent by the vascular route, whereas in lower invertebrates that 
lack an organized circulatory system, the neurohormones simply diffuse to the 
target. At one time there was thought to be a sharp line of distinction 
between the nervous and endocrine systems. However, experimentation revealed 
not only that the nervous and endocrine systems interact in many ways, but 
also that some neurons are by themselves actually capable of producing 
hormones. The fact that some neurons are hormone producing shows how closely 
related the nervous and endocrine systems really are. In fact, there is a 
definite trend toward treating both of these systems merely as the two 
components of a single coordinating system called the neuroendocrine system.  
See  ENDOCRINE SYSTEM (INVERTEBRATE); ENDOCRINE SYSTEM (VERTEBRATE); NERVOUS 
SYSTEM (VERTEBRATE). 
 �34s[Milton Fingerman]
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�6v�2v�32sNeurosis �21m �33s
    A term generally used to refer to disabling psychological states that
impair normal mental functioning but at the same time do not totally disrupt
and disorganize the grasp of the realities with which the person must cope.
The processes that lead to neurosis are regarded as aggravated reactions to
life problems, the aggravated nature of the reaction being attributable to
internal conflicts and inappropriate defenses that make it extemely difficult
for the individual to cope with either external demand or the demands of his
or her own needs.
    The symptoms expressed in the neurosis may often be desperate attempts to
deal with these problems - attempts that do not or cannot resolve the
difficulties. In the sense that neurotic symptomatology is consistent with the
pattern of the person's general mode of functioning, neurosis may be conceived
of as reflecting a continuation of the psychodynamic pattern responsible for
character formation in the individual.
    Four general patterns of neurotic reaction, corresponding to different,
though overlapping, modes of defense, may be distinguished: the hysteric, the
phobic and the counterphobic, the neuroses involving somatization, and the
obsessional-compulsive. These neurotic patterns may be either precipitated by
chronic conditions or seemingly traumatic in origin.
    The major forms of therapy used in dealing with neurotic problems are
psychological rather than physiological in nature, although mild tranquilizers
and sedatives are occasionally used to alleviate disruptive anxiety states.
See  ABNORMAL BEHAVIOR; HYSTERIA; OBSESSIVE-COMPULSIVE REACTION; PHOBIC
REACTION; SOMATIZATION.
 �34s[Jerome S. Bruner ;Walter Mischel]
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�6v�2v�32sNeutralization reaction (immunology)   �21m �33s 
    A procedure in which the chemical or biological activity of a reagent or a 
living organism is inhibited, usually by a specific neutralizing antibody. As 
an example, the lethal or the dermonecrotic actions of diphtheria toxin on 
animals may be completely neutralized by an equivalent amount of diphtheria 
antitoxin.  
    Antibodies to bacterial, snake-venom, and other enzyme preparations 
regularly precipitate them from solution so that the supernates are devoid of 
enzyme activity; however, the neutralization of activity in the precipitate 
may range from complete to negligible.  See  IMMUNOLOGY; NEUTRALIZING 
ANTIBODY; SEROLOGY. 
 �34s[Henry P. Treffers]
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�6v�2v�32sNeutralizing antibody   �21m �33s 
    An antibody that reduces or abolishes some biological activity of a 
soluble antigen or of a living microorganism. Thus, diphtheria antitoxin is a 
neutralizing antibody that, in adequate amounts, abolishes the pathological 
effects of diphtheria toxin in animals. This is only one characteristic; the 
other general properties of the antibody are those of the immunoglobulin 
family (IgG, IgA, or IgM) to which it belongs.  See  ANTIBODY; IMMUNOGLOBULIN. 
 �34s[Henry P. Treffers]
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�6v�2v�32sNeutrino �21m �33s
    An elementary particle designated by the Greek symbol NU, with zero rest
mass and zero electric charge. It is classified as a lepton. The neutrino is
the only known elementary particle that has only weak interactions. Thus, the
neutrino is a unique tool in the study of weak forces, for this reason, NU
interactions have been the subject of active study at large particle
accelerators.  See  ELEMENTARY PARTICLE; LEPTON; WEAK NUCLEAR INTERACTIONS.
    The existence of the neutrino was postulated in 1930 to explain the
apparent nonconservation of energy in beta-decay process n -> p + e- + BAR NU.
It was not until 1953 that the existence of the neutrino was experimentally
verified, by observation of the interactions caused by free neutrinos.
Quantitative studies of NU interactions resulted in the discovery that there are
two distinct neutrinos - the electron neutrino NU(sub e), associated with beta
decay, and the muon neutrino, associated with pion decay, PI+ -> MU+ + NU(sub
MU).  From the absence of neutrinoless double beta decay, it can be inferred
that NU(sub e) is not identical to its antiparticle BAR NU(sub e).  It is thus
believed that there are four distinct neutrinos, namely, NU(sub e), NU(sub MU),
BAR NU(sub e), and BAR NU(sub MU).  See RADIOACTIVITY.

    In the late 1970s a new heavy charged lepton, the tau (TAUPLUS/MINUS), was
discovered to exist in addition to the previously known charged leptons, the
electron (e PLUS/MINUS) and the muon (MU PLUS/MINUS). There is some theoretical
speculation that two new additional types of neutrinos, the tau-neutrino
(NUTAU) and the anti-tau-neutrino (NU[sub TAU]), are associated with this new
heavy lepton.

    Considerations of angular momentum conservation in pion decay establish
the spin of the neutrino to be 1/2 in units of h/2PI, where h is Planck's
constant. There has been some experimental indication and theoretical
speculation that the mass of at least some of the neutrinos is not identically
zero but has some finite, although very small, value. This possibility has
fundamental cosmological implications since astrophysicists believe that there
are a very large number of neutrinos in the universe, and if the neutrinos had
a nonzero mass they would constitute a significant fraction of the total mass
of the universe.  See  SPIN (QUANTUM MECHANICS).

    According to the rules of quantum mechanics, a massless spin-1/2 particle
such as the neutrino can have its spin lined up along its direction of motion
or opposite to its direction of motion; these are called the helicity states,
right-handed and left-handed respectively. Thus, in general, such a particle
has four components - the particle and its antiparticle, right-handed and
left-handed. It was, however, predicted by the two-component theory of the
neutrino, and shown by experiment, that only two of the four components exist
for the neutrinos - left-handed neutrinos and right-handed antineutrinos.  See
  HELICITY (QUANTUM MECHANICS); RELATIVISTIC QUANTUM THEORY.

 �34s[Charles Baltay]
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�6v�2v�32sNeutron   �21m �33s 
    An elementary particle having approximately the same mass as the proton, 
but lacking a net electric charge. It is indispensable in the structure of the 
elements, and in the free state it is an important reactant in nuclear 
research and the propagating agent of fission chain reactions. 
    Neutrons and protons are the constituents of atomic nuclei. The number of 
protons in the nucleus determines the chemical nature of an atom, but without 
neutrons it would be impossible for two or more protons to exist stably 
together within nuclear dimensions. The protons, being positively charged, 
repel one another by virtue of their electrostatic interactions. The presence 
of neutrons weakens the electrostatic repulsion, without weakening the nuclear 
forces of cohesion. In light nuclei the resulting balanced, stable 
configurations contain protons and neutrons in almost equal numbers, but in 
heavier elements the neutrons outnumber the protons; in sup2sup3sup8U, for 
example, 146 neutrons are joined with 92 protons. Only one nucleus, sup1H, 
contains no neutrons. For a given number of protons, neutrons in several 
different numbers within a restricted range often yield nuclear stability - 
and hence the isotopes of an element.  See  ISOTOPE; NUCLEAR STRUCTURE; PROTON. 
    Free neutrons have to be generated from nuclei, and since they are bound 
therein by cohesive forces, an amount of energy equal to the binding energy 
must be expended to get them out. Nuclear machines, such as cyclotrons and 
electrostatic generators, induce many nuclear reactions when their ion beams 
strike target material. Some of these reactions release neutrons, and these 
machines are sources of high neutron flux. Neutrons are released in the act of 
fission, and nuclear reactors are unexcelled as intense neutron sources.  See  
NUCLEAR BINDING ENERGY; NUCLEAR REACTION; NUCLEAR REACTOR. 
    Having no electric charge, neutrons interact so slightly with atomic 
electrons in matter that energy loss by ionization and atomic excitation is 
essentially absent. Consequently they are vastly more penetrating than charged 
particles of the same energy. The main energy-loss mechanism occurs when they 
strike nuclei. As with billiard balls, the most efficient slowing-down occurs 
when the bodies that are struck in an elastic collision have the same mass as 
the moving bodies; hence the most efficient neutron moderator is hydrogen, 
followed by other light elements: deuterium, beryllium, and carbon. The great 
penetrating power of neutrons imposes severe shielding problems for reactors 
and other nuclear machines, and it is necessary to provide walls, usually of 
concrete, several feet in thickness to protect personnel.  See  RADIATION 
DAMAGE TO MATERIALS; RADIATION SHIELDING; RADIOLOGY. 
    Free neutrons are themselves radioactive, each transforming spontaneously 
into a proton, an electron (BETA-particle), and an antineutrino. This 
instability is a reflection of the fact that neutrons are slightly heavier 
than hydrogen atoms. Neutrons are, individually, small magnets. This property 
permits the production of beams of polarized neutrons, that is, beams of 
neutrons whose magnetic dipoles are aligned predominantly parallel to one 
direction in space. Neutrons spin with an angular momentum of ** in units of 
h/2PI, where h is Planck's constant. The negative sign which is attached to 
the magnetic moment indicates that the magnetic moment vector and the angular 
momentum vector are oppositely directed. See MAGNETON; NUCLEAR MOMENTS; 
NUCLEAR ORIENTATION; SPIN (QUANTUM MECHANICS). 
    When neutrons are completely slowed down in matter, they have a maxwellian 
distribution in energy that corresponds to the temperature of the moderator 
with which they are in equilibrium. The maxwellian distribution has a tail 
extending to very low energies, and a few neutrons (about 10-sup1sup1 of the 
main neutron flux) at this extreme have energies less than 5 x 10-sup8 
attojoule (3 x 10-sup7 eV), and hence velocities of less than about 7 m/s. The 
de Broglie wavelength of these ultracold neutrons is greater than 50 
nanometers, which is so much larger than interatomic distances in solids that 
they interact with regions of a surface rather than with individual atoms, and 
as a result they are reflected from polished surfaces at all angles of 
incidence. Ultracold neutrons can be expected to be important in basic physics 
and to have applications in studies of surfaces and of the structure of 
inhomogeneities and magnetic domains in solids.  See  ANTINEUTRON; ELEMENTARY 
PARTICLE; THERMAL NEUTRON. 
 �34s[Arthur H. Snell]
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�6v�2v�32sNeutron diffraction   �21m �33s 
    The phenomenon associated with the interference processes which occur when 
neutrons are scattered by the atoms within solids, liquids, and gases. The use 
of neutron diffraction as an experimental technique is relatively new compared 
to electron and x-ray diffraction, since successful application requires high 
thermal-neutron fluxes, which can be obtained only from nuclear reactors. 
These diffraction investigations are possible because thermal neutrons have 
energies with equivalent wavelengths near 0.1 nanometer and are therefore 
ideally suited for interatomic interference studies. 
    In the scattering of neutrons by atoms, there are two important 
interactions. One is the short-range, nuclear interaction of the neutron with 
the atomic nucleus. This interaction produces isotropic scattering because the 
atomic nucleus is essentially a point scatterer relative to the wavelengths of 
thermal neutrons. Strong resonances associated with the scattering process 
prevent any regular variation of the nuclear scattering amplitudes with atomic 
number. The other important process for the scattering of neutrons by atoms is 
the interaction of the magnetic moment of the neutron with the spin and 
orbital magnetic moments of the atom.  See  SCATTERING EXPERIMENTS (ATOMS AND 
MOLECULES); SCATTERING EXPERIMENTS (NUCLEI). 
    Since the nuclear scattering amplitudes for neutrons do not vary uniformly 
with atomic number, there are certain types of chemical structures which can 
be investigated more readily by neutron diffraction than by x-ray diffraction. 
Moreover, since neutron scattering is a nuclear process, when the scattering 
amplitude of an element is not favorable for a particular investigation, it is 
frequently possible to substitute an enriched isotope which has scattering 
characteristics that are markedly different. The most significant application 
of neutron diffraction in chemical crystallography is the structure 
determination of composite crystals which contain both heavy and light atoms, 
and the most important compounds in this general classification are the 
hydrogen-containing substances. 
    The interaction of the magnetic moment of the neutron with the orbital and 
spin moments in magnetic atoms makes neutron scattering a unique tool for the 
study of a wide variety of magnetic phenomena, because information is obtained 
on the magnetic properties of the individual atoms in a material. This 
interaction depends on the size of the atomic magnetic moment and also on the 
relative orientation of the neutron spin and of the atomic magnetic moment 
with respect to the scattering vector and with respect to each other. 
Consequently, detailed information can be obtained on both the magnitude and 
orientation of magnetic moments in any substance which displays magnetic 
properties.  
    The investigation of antiferromagnetic and ferrimagnetic substances is one 
of the most important applications of the neutron diffraction technique, 
because detailed information on the magnetic configuration in these systems 
cannot be obtained by other methods. 
    One of the most important uses of inelastic neutron scattering is the 
study of thermal vibrations of atoms about their equilibrium positions, 
because lattice vibration quanta, or phonons, can be excited or annihilated in 
their interactions with low-energy neutrons. The measurements provide a direct 
determination of the dispersion relations for the normal vibrational modes of 
the crystal and do not require the large corrections necessary in similar 
x-ray investigations. These measured dispersion relations furnish the best 
experimental information available on interatomic forces that exist in 
crystals.  See  ELECTRON DIFFRACTION; MAGNON; NEUTRON SPECTROMETRY. 
 �34s[Michael K. Wilkinson]
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�6v�2v�32sNeutron optics   �21m �33s 
    A title by analogy of certain phases of neutron physics in which the wave 
character of neutrons dominates and leads to behavior similar to that of 
light. Neutrons can be reflected at small glancing angles from plane surfaces; 
they show various scattering phenomena with similarity to light, and they can 
be diffracted by crystals. Although they can also be polarized, the analogy 
with light is in this case invalid because the polarization of neutrons 
depends upon their possession of a constant magnetic moment, which light waves 
lack.  See  NEUTRON; NEUTRON DIFFRACTION. 
 �34s[Arthur H. Snell]
�33s 
  
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�6v�2v�32sNeutron spectrometry   �21m �33s 
    A generic term applied to experiments in which neutrons are used as the 
probe for measuring excited states of nuclides and for determining the 
properties of these states. The term neutron spectroscopy is also used. The 
strength of the interaction between a neutron and a target nuclide can vary 
rapidly as a function of the energy of the incident neutron, and it is 
different for every nuclide. At particular neutron energies the interaction 
strength for a specific nuclide can be very strong; these narrow energy 
regions of strong interactions are called resonances (see illustration). The 
strength of the interaction, expressing the probability that an interaction of 
a given kind will take place, can be considered as the effective 
cross-sectional area SIGMA presented by a nucleus to an incident neutron. 
  
    Neutron spectroscopy can be carried out by two different techniques (or a 
combination): (1) by the use of a time-pulsed neutron source which emits 
neutrons of many energies simultaneously, combined with the time-of-flight 
technique to measure the velocities of the neutrons; this time-of-flight 
technique can be used for neutron measurements from 10-sup3 eV to about 200 
MeV; (2) by the use of a beam of nearly monoenergetic neutrons whose energy 
can be varied in small steps approximately equal to the energy spread of the 
neutron beam; however, useful ``monoenergetic'' neutron sources are not 
available from about 10 eV to about 10 keV. 
    Neutron spectroscopy has yielded a mass of valuable information on nuclear 
systematics for almost all nuclides. The distribution of the spacings between 
nuclear levels and the average of these spacings have provided valuable tests 
for various nuclear theories. The properties of these levels, that is, the 
probabilities that they decay by neutron or gamma-ray emission, or by fission, 
and the averages and distribution of these probabilities have stimulated much 
theoretical effort. 
    In addition, knowledge of neutron cross sections is fundamental for the 
optimum design of thermal fission power reactors and fast neutron breeder 
reactors, as well as fusion power reactors now in the conceptual stage. Cross 
sections are needed for nuclear fuel materials such as 235U or 239Pu, for 
fertile materials such as 238U, for structural materials such as iron and 
chromium, for coolants such as sodium, for moderators such as beryllium, for 
shielding materials such as concrete.  See  NUCLEAR STRUCTURE; REACTOR PHYSICS. 
 �34s[John A. Harvey]
�33s 
  
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�6v�2v�32sNeutron star �21m �33s
    A star containing about 1** solar masses of material compressed into a
volume approximately 10 km (6 mi) in radius (1 solar mass = 2 x 10[sup 30] kg,
or 4.4 x 10[sup 30] lb). Neutron stars are one of the end points of stellar
evolution and are probably the final states of medium-mass and possibly of
high-mass stars. The density of neutron star material is 10(sup 14) to
10(sup 15) times the density of water and exceeds the density of matter in the
nuclei of atoms. Neutron stars are pulsars (pulsating radio sources) if they
rotate sufficiently rapidly and have strong enough magnetic fields.  See
PULSAR; STELLAR EVOLUTION.
    Neutron stars play a role in astrophysics which extends beyond their
status as strange, unusual types of stellar bodies. The interior of a neutron
star is a cosmic laboratory in which matter is compressed to densities which
are found nowhere else in the universe. Neutron stars in double-star systems
can cause the emission of x-rays when matter flows toward the neutron star,
swirls around it, and heats up. Neutron stars are probably formed in supernova
explosions. A few pulsars are found in double-star systems, and careful timing
of the pulses they emit can test Einstein's general theory of relativity.  See
GRAVITATION; RELATIVITY; SUPERNOVA.
    If Einstein's theory of gravitation is the correct one, a neutron star
with a mass larger than some limiting value will collapse catastrophically,
because its internal pressure will be insufficient, and become a black hole.
The exact value of this limiting mass is not known precisely, but lies between
3 and 5 solar masses.  See  BLACK HOLE.
    Most of the interior of a neutron star consists of matter which is almost
entirely composed of neutrons. In the bulk of the star, this matter is in a
superfluid state, where circulation currents can flow without resistance. This
material is under pressure, since it must be able to support the tremendous
weight of the overlying layers at each point in the neutron star. This
pressure, called degeneracy pressure, is caused by the close packing of the
neutrons rather than by the motion of the particles. As a result, neutron
stars can be stable no matter what the internal temperature is, because the
pressure that supports the star is independent of temperature.	See  NEUTRON;
SUPERFLUIDITY.
 �34s[Harry L. Shipman]
�33s

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�6v�2v�32sNew Britain   �21m �33s 
    The largest island of the Bismarck Archipelago. Also known as Neu Pommern, 
New Britain is 370 mi (595 km) long and 90 mi (145 km) at its maximum width, 
and it has an area of about 13,000 misup2 (34,000 kmsup2). Some of the 
mountains of the rugged range are over 7000 ft (2100 m) and are active 
volcanoes. Rabaul, the capital of the Bismarcks, is located in the caldera of 
an active volcano, one of whose walls has collapsed and is open to the sea. 
See EAST INDIES. 
 �34s[M. G. Audley-Charles]
�33s 
  
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�6v�2v�32sNewcastle disease   �21m �33s 
    An epizootic viral disease of fowls, with respiratory, gastrointestinal, 
and central nervous system involvement. It may be transmitted to humans who 
work with fowls, usually appearing as a conjunctivitis; recovery is complete 
in 10 - 14 days. The virus is related to other paramyxoviruses in 
hemagglutination characteristics, as well as in size and internal structure.  
See  MYXOVIRUS; PARAMYXOVIRUS. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sNew Guinea �21m �33s
    The second largest island in the world. West Irian, together with eastern
New Guinea and Papua, is nearly 1500 mi (2400 km) long and 390 mi (630 km)
wide at its maximum. with an area of approximately 340,000 mi(sup 2) (880,000
km[sup 2]).
    The southern lowland area is part of the Sahul Shelf. The high mountains
of the central part of the island are a Tertiary fold belt with a long
geosynclinal history. Peaks of the central mountains rise to more than 16,000
ft (4900 m) in the Nassau and Orange ranges. Since the snow line is at about
14,500 ft (4400 m), many of these are snow-capped, although within 5DG of the
Equator. North of the central mountains there is a broad valley with a coastal
range to its north. Important deposits of minerals have been found: gold,
petroleum, coal, iron ore, tin, copper, platinum, manganese, osmiridium, and
others.
    Heavy rains are received from the two annual monsoons. These rains, with
the high temperatures (the mean monthly high in December is 78DGF or 25.6DGC
and the low in August is 74DGF or 23.3DGC), result in a tropical rainforest
over most of the island, except where savannas have been culturally induced.
See EAST INDIES.
 �34s[M. G. Audley-Charles]
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�6v�2v�32sNew Ireland   �21m �33s 
    New Ireland, or Neu Mecklinberg, has an area of 3340 misup2 (8650 kmsup2); 
it is 200 mi (320 km) long, with an average width of 20 mi (32 km). It is very 
mountainous but lacks active volcanoes. See EAST INDIES. 
 �34s[M. G. Audley-Charles]
�33s 
  
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�6v�2v�32sNewtonian fluid   �21m �33s 
    A fluid in which the state of stress at any point is a linear function of 
the time rate of strain at that point. The fluid thus bears a direct analogy 
to a hookean solid, for which the state of stress is a linear function of the 
strain. Many gases and liquids are closely newtonian over a wide range of 
pressures and temperatures. 
  
    The simplest example of newtonian fluid flow is Couette flow, the 
low-speed steady motion of a viscous fluid between two infinite plates moving 
parallel to each other with relative velocity U, as illustrated. The shear 
stress in the fluid is constant and equals MU(du1/dx2) = MU(U/DELTA), where 
DELTA is the distance between the plates, MU is the viscosity, usubi are 
velocity components of fluid, and xsubi are rectangular cartesian coordinates 
with i = 1, 2, 3.  See  FLUID FLOW; FLUIDS. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
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�6v�2v�32sNewton's laws of motion   �21m �33s 
    Three fundamental principles which form the basis of classical, or 
newtonian, mechanics. They are stated as follows: 
    First law: A particle not subjected to external forces remains at rest or 
moves with constant speed in a straight line. 
    Second law: The acceleration of a particle is directly proportional to the 
resultant external force acting on the particle and is inversely proportional 
to the mass of the particle. 
    Third law: If two particles interact, the force exerted by the first 
particle on the second particle (called the action force) is equal in 
magnitude and opposite in direction to the force exerted by the second 
particle on the first particle (called the reaction force). 
    The newtonian laws have proved valid for all mechanical problems not 
involving speeds comparable with the speed of light and not involving atomic 
or subatomic particles.  See  DYNAMICS; FORCE; KINETICS (CLASSICAL MECHANICS). 
 �34s[Dudley Williams]
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�6v�2v�32sNew Zealand   �21m �33s 
    A group of islands in the Southern Hemisphere off the southeast coast of 
Australia in the South Pacific Ocean with a total area of 103,736 misup2 
(268,675 kmsup2). The two largest islands are South Island, with an area of 
58,093 misup2 (150,460 kmsup2), and North Island, with an area of 44,281 
misup2 (114,687 kmsup2). This is a land of high mountains and deeply dissected 
high country. The two main islands, narrow and elongated, have lengthy 
coastlines in proportion to their areas. Sheltered bays, harbors, and sounds, 
with wide sandy beaches, fringe the east coast. The west coast is rock strewn 
and bar bound at harbor and river mouths. In the south the coast is indented 
with deep-cut fiords. See AUSTRALIA. 
    Two-thirds of New Zealand lies at an elevation between 650 and 3500 feet 
(200 and 1070 m) and nearly as much of the surface is over 3500 feet as is 
below 650 feet. Mountains occupy more than one-half of South Island. The 
southernmost portion, along the west coast, is the ancient crystalline mass of 
Fiordland with its deep, steeply walled, canyonlike valleys, occupied on the 
coast by saltwater fiords and inland by fresh water lakes. 
    East of Fiordland is the mountainous interior of Otago. By contrast with 
Fiordland, which is the wettest part of New Zealand, this is the driest, and 
the basin floors are especially desertlike. To the north extends the 
succession of lofty alpine ranges which comprise the Southern Alps. Many 
summits, mantled with permanent snow, are above 10,000 feet (3000 m), and the 
highest peak, Mt. Cook, is 12,349 feet (3764 m). 
    In the North Island a belt of mountains of a general elevation of 3500 
feet (1070 m) lies to the east, but snow cover is seasonal and sparse on these 
bush-clad ranges. Three active volcanoes stand in the center of the island, 
and much of the interior is occupied by an ash-showered plateau, which 
contains active thermal areas (being developed for power generation) and 
numerous deep, but small, crater lakes. Beneath the ash-mantled plateau are 
vast sources of geothermally heated steam and water, which are tapped through 
deep bores and harnessed for the generation of electricity. 
    In both islands lowland areas are small and infrequent and are found along 
the coasts. Numerous short and swift-flowing rivers drain to all coasts. With 
its heavy rainfall and mild temperatures, primeval New Zealand was densely 
covered by a mixed rainforest. Since the beginning of European settlement in 
1840, the greater part of the forest disappeared. 
 �34s[Kenneth B. Cumberland]
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�6v�2v�32sNiacin �21m �33s
    A vitamin also known as nicotinic acid, a component of the vitamin B
complex. It is a white water-soluble powder stable to heat, acid, and alkali,
with the structure shown below.

			 ***CHEMICAL STRUCTURE***

    Many animals, including humans, are capable of synthesizing niacin in
varying degrees from the amino acid tryptophan. Niacin is widely distributed
in foods. Yeasts, wheat germ, and meats, particularly organ meats, are rich
sources of the vitamin. Some foods such as milk are relatively poor sources of
niacin but contain generous quantities of tryptophan.  See  TRYPTOPHAN.
    Niacin-deficiency disease is known as pellagra and is particularly
prevalent among the poor people whose diet is largely corn. The recommended
dietary allowance for niacin is 6.6 niacin equivalents per 1000 kcal.  See
PELLAGRA; VITAMIN.
 �34s[Stanley N. Gershoff]
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�6v�2v�32sNibbling   �21m �33s 
    The cutting of material by the action of a reciprocating punch. The 
nibbler takes repeated small bites as the work is passed beneath it. The 
workpiece must be backed up by a support or die. 
    Nibbling machines are constructed with considerable distance, or throat, 
between the punch and its supporting upright. This distance, plus the use of a 
round punch which allows the workpiece to be moved about, permits the cutting 
of irregular shapes. Duplicate pieces may be made by using templates as guides 
for the punch.  See  MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sNiccolite   �21m �33s 
    A minor ore of nickel. Niccolite is a mineral having composition NiAs. 
Crystals are rare, and niccolite usually occurs in massive aggregates with 
metallic luster and pale copper-red color. Because of the color, not the 
composition, it is called copper nickel. The hardness is 5.5 on Mohs scale and 
the specific gravity is 7.78. It is found in vein deposits with cobalt and 
silver minerals as in the silver mines of Saxony, Germany, and Cobalt, 
Ontario, Canada.  See  NICKEL; PYRRHOTITE. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sNickel �21m �33s
    A chemical element, Ni, atomic number 28, a silver-white, ductile,
malleable, tough metal. The atomic mass of naturally occurring nickel is 58.71.
    Nickel consists of five natural isotopes having atomic masses of 58, 60,
61, 62, 64. Seven radioactive isotopes have also been identified, having mass
numbers of 56, 57, 59, 63, 65, 66, and 67.
    Most commercial nickel goes into stainless steel and other
corrosion-resistant alloys. Nickel is also important in coins as a replacement
for silver. Finely divided nickel is used as a hydrogenation catalyst.	See
NICKEL ALLOYS.
    Nickel is a fairly plentiful element, making up about 0.008% of the
Earth's crust and 0.01% of the igneous rocks. Appreciable quantities of nickel
are present in some kinds of meteorite, and large quantities are thought to
exist in the Earth's core. Two important ores are the iron-nickel sulfides,
pentlandite and pyrrhotite (Ni,Fe)(sub x)S(sub y); the ore garnierite,
(Ni,Mg)SiO3 .nH2O, is also commercially important. Nickel occurs in small
quantities in plants and animals. It is present in trace amounts in sea water,
petroleum, and most coal.
    Nickel metal is of moderate strength and hardness (3.8 on Mohs scale).
When viewed as very small particles, nickel appears black. The density of
nickel is 8.90 times that of water at 20DGC (68DGF). Nickel melts at 1455DGC
(2651DGF) and boils at 2840DGC (5144DGF). Nickel is only moderately reactive.
It resists alkaline corrosion and does not burn in the massive state, although
fine nickel wires can be ignited. Nickel is above hydrogen in the
electrochemical series, and it dissolves slowly in dilute acids, releasing
hydrogen. In metallic form nickel is a moderately strong reducing agent.
    Nickel is usually dipositive in its compounds, but it can also exist in
the oxidation states 0, 1+, 3+, and 4+. Besides the simple nickel compounds,
or salts, nickel forms a variety of coordination compounds or complexes. Most
compounds of nickel are green or blue because of hydration or other ligand
bonding to the metal. The nickel ion present in water solutions of simple
nickel compounds is itself a complex, [Ni(H2O)6](sup 2+).
 �34s[William E. Cooley]
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�6v�2v�32sNickel alloys   �21m �33s 
    Combinations of nickel with other metals. Nickel-base alloys may be melted 
in open-hearth, electric-arc, or induction furnaces in air, under inert gas, 
or in vacuum. Casting may also be done under these same ambient conditions. 
    Nickel 211 and Duranickel alloy 301 are essentially binary alloys with 
4.75% manganese and 4.5% aluminum, respectively. A characteristic use of 
nickel 211 is as wire for spark-plug electrodes. Duranickel alloy 301 is well 
suited to the manufacture of springs and diaphragms. 
    Monel alloy 400 contains about two-thirds nickel and one-third copper and 
is the oldest of the commercial nickel-base alloys, dating from about 1905. 
Nickel-chromium binary alloys are used primarily in specialty high-temperature 
service. Other nickel alloys contain combinations of other metals in varying 
quantities.  See  NICKEL. 
 �34s[E. N. Skinner ;Gaylord Smith]
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�6v�2v�32sNickel metallurgy   �21m �33s 
    The extraction and refining of nickel from its ores. Nickel's properties 
of strength, toughness, and resistance to corrosion have been used to 
advantage in alloys since ancient times. Although nickel ranks twenty-fourth 
in order of abundance of the elements, there are relatively few nickel 
deposits of commercial importance. Nickel ores are of two generic types, 
sulfides and laterites. Explorations of the ocean bottoms have revealed vast 
deposits of manganese oxide nodules which contain significant values of 
nickel, copper, and cobalt. 
    Selection of processes for nickel extraction is largely determined by the 
type of ore to be treated. Sulfide ores are amenable to concentration by such 
methods as flotation or magnetic separation. The state of combination of 
nickel in the lateritic ores usually precludes such enrichment, thus requiring 
treatment of the total ore. 
    Sulfide ores are first crushed and ground to liberate the mineral values 
and then subjected to froth flotation or magnetic separation to concentrate 
the valuable constituents and reject the gangue or rock fraction. The nickel 
concentrate is treated by pyrometallurgical processes. The major portion 
undergoes partial roasting in multihearth or fluidized-bed furnaces to 
eliminate about half of the sulfur and to oxidize the associated iron. The hot 
calcine, plus flux, is smelted in natural gas - coal - fired reverberatory 
furnances operating at about 1200DGC to produce a furnace matte, enriched in 
nickel, and a slag for discard. The furnace matte is transferred to converters 
and blown with air in the presence of more flux to oxidize the remaining iron 
and associated sulfur, yielding Bessemer matte containing nickel, copper, 
cobalt, small amounts of precious metals, and about 22% sulfur. The molten 
Bessemer matte is cast into 25-ton (22.5-metric ton) molds in which it 
undergoes controlled slow cooling. After crushing and grinding, the metallics 
are removed magnetically and treated in a refining complex for recovery of 
metal values. 
    The bulk of the nickel originating from lateritic ores is marketed as 
ferronickel. The process employed is basically simple and involves drying and 
preheating the ore usually under reducing conditions. The hot charge is then 
further reduced and melted in an electric-arc furnace, and the crude metal is 
refined and cast into ferronickel pigs. A substantial amount of nickel is 
produced from lateritic ores by the nickel sulfide matte technique. In this 
process the ore is mixed with gypsum or other sulfur-containing material such 
as high-sulfur fuel oil, followed by a reduction and smelting operation to 
form matte. The molten furnace matte is upgraded in either conventional or 
top-blown rotary converters to a high-grade matte, which can be further 
refined by roasting and reduction to a metallized product.  See  NICKEL; 
PYROMETALLURGY, NONFERROUS. 
 �34s[Alexander Illis]
�33s 
  
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�6v�2v�32sNickel poisoning   �21m �33s 
    Depending on the dose, the organism involved, and the type of compound 
involved, nickel may be beneficial or toxic. It appears to be essential for 
the survival of some organisms. 
    The use of nickel in heavy industry has increased markedly over the last 
few decades, principally in the production of stainless steel and other alloys 
and in plating. Because of the excellent corrosion resistance of nickel and 
high-nickel alloys, these metals are used widely in the food processing 
industry. In fact, foods can be contaminated with nickel during handling, 
processing, and cooking by utensils containing large quantities of nickel. 
Nickel is also used frequently as a catalyst; one of its most significant 
catalytic applications is in the hydrogenation of fat. Nickel carbonyl, 
Ni(CO)4, is one of the most toxic nickel compounds and is a major industrial 
hazard.  
    Nickel usually is not readily absorbed from the gastrointestinal tract 
except as nickel carbonyl. This compound has caused most of the acute toxicity 
of nickel. It has been shown that nickel has a carcinogenic property and may 
be involved in hypersensitivity reactions. Nickel dermatitis is reported with 
increasing frequency in industrial workers, especially nickel platers. It 
produces an allergic dermatitis on almost any skin area. Nickel workers have 
approximately 150 times the cancer of the nasal passages and sinuses as the 
general population, and approximately five times the lung cancer. 
    Within the body the main storage depots of nickel are the spinal cord, 
brain, lungs, and heart, with lesser amounts widely distributed throughout the 
organ systems. Acute poisoning causes headache, dizziness, nausea and 
vomiting, chest pain, tightness of the chest, dry cough with shortness of 
breath, rapid respiration, cyanosis, and extreme weakness. The lesions 
resulting from acute exposure are mainly in the lung and brain.  See  NICKEL. 
 �34s[N. Karle Mottet]
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�6v�2v�32sNicotine   �21m �33s 
    A liquid alkaloid from the dried leaves of tobacco. Present in Nicotiana 
tabacum and N. rustica, nicotine is recovered by extraction of cigarette 
wastes. Chemically it is C10H14N2, a colorless to amber oil which takes up 
moisture from the air very avidly. It has a strong tobacco odor and intensely 
bitter, burning taste. Quite soluble in water, nicotine forms salts with 
mineral acids, such as hydrochloride, hydroiodide, and sulfate. The sulfate is 
used as an agricultural insecticide. 
    Nicotine is highly toxic. Symptoms include extreme nausea, vomiting, 
evacuation of intestines and urinary bladder, mental confusion, and 
convulsions.  See  ALKALOID. 
 �34s[Frank Wagner]
�33s 
  
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�6v�2v�32sNiobium �21m �33s
    A chemical element, Nb, atomic number 41 and atomic weight 92.906. In the
United States this element was originally called columbium. The metallurgists
and metals industry still use this older name.
    Most niobium is used in special stainless steels, high-temperature alloys,
and superconducting alloys such as Nb3Sn. Niobium is also used in nuclear
piles.
    Niobium metal has a density of 8.6 g/cm(sup 3) (5.0 oz/in[sup ]3.) at 20DGC
(68DGF), a melting point of 2468DGC (4474DGF), and a boiling point of 4927DGC
(8900DGF). Metallic niobium is quite inert to all acids except hydrofluoric,
presumably owing to an oxide film  on the surface. Niobium metal is slowly
oxidized in alkaline solution. It reacts with oxygen and the halogens upon
heating to form the oxidation state V oxide and halides, with nitrogen to form
NbN, and with carbon to form NbC, as well as other elements such as arsenic,
antimony, tellurium, and selenium.
    The oxide Nb2O5, melting point 1520DGC (2768DGF), dissolves in fused
alkali to yield a soluble complex niobate, Nb6O19(sup -8).  Normal niobates such
as NbO4(sup -3) are insoluble.	The oxide dissolves in hydrofluoric acid to give
ionic species such as NbOF5(sup -2) and NbOF6(sup -3), depending on the fluoride
and hydrogen-ion concentration.  The highest fluoro complex which can exist in
solution is NbF6-.�34s[Edwin M.  Larsen]
�33s

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�6v�2v�32sNippotaeniidea   �21m �33s 
    An order of tapeworms of the subclass Cestoda. The few known species are 
intestinal parasites of Eurasian fresh-water fishes. The head bears a single 
terminal sucker. The segmental anatomy shows relationships to the 
Pseudophyllidea and Cyclophyllidea. The life history is unknown. It is 
probable that this order is related to the proteocephalids.  See  CESTODA; 
CYCLOPHYLLIDEA; PSEUDOPHYLLIDEA. 
 �34s[Clark P. Read]
�33s 
  
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�6v�2v�32sNiter   �21m �33s 
    A potassium nitrate mineral with chemical composition KNO3. Niter 
crystallizes in the orthorhombic system, generally in thin crusts and delicate 
acicular crystals; it occurs in massive, granular, or earthy forms. It is 
brittle; hardness is 2 on Mohs scale; specific gravity is 2.109. The luster is 
vitreous, and the color and streak are colorless to white.  See  NITRATE 
MINERALS.  
    Niter is commonly found, usually in small amounts, as a surface 
efflorescence in arid regions and in caves and other sheltered places. Niter 
occurs associated with soda niter in the desert regions of northern Chile, and 
in similar occurrences in Italy, Egypt, the Soviet Union, the western United 
States, and elsewhere. 
 �34s[George Switzer]
�33s 
  
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�6v�2v�32sNitrate   �21m �33s 
    The negative ion NO3-, derived from nitric acid, HNO3. Because almost all 
metallic nitrates are water-soluble, they are not found in nature but are 
produced from nitric acid. One notable exception to this is the impure sodium 
nitrate, called Chile saltpeter, which occurs in large amounts because of the 
aridity and very small rainfall of the Chilean coastal plain. 
    Because nitrates contain nitrogen in its highest oxidation state (5+), the 
ion is a useful oxidizing agent. Because of this property, nitrates are often 
constituents of matches and explosives. Nitrates are an important source of 
nitrogen in fertilizers.  See  FERTILIZER; NITRIC ACID; NITROGEN. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sNitrate minerals   �21m �33s 
    These minerals are few in number and with the exception of soda niter are 
of rare occurrence. Normal anhydrous and hydrated nitrates occurring as 
minerals are soda niter, NaNO3; niter, KNO3; ammonia niter, NH4NO3; 
nitrobarite, Ba(NO3)2; nitrocalcite, Ca(NO3)2|PRODUCT|4H2O; and 
nitromagnesite, Mg(NO3)2PRODUCT6H2O. In addition there are three known 
naturally occurring nitrates containing hydroxyl or halogen, or compound 
nitrates. They are gerhardtite, Cu2(NO3)(OH)3; buttgenbachite, 
Cu19(NO3)2Cl4(OH)32PRODUCT3H2O; and darapskite, Na3(NO3)(SO4)PRODUCTH2O.  See  
NITER; SODA NITER. 
    The natural nitrates are for the most part readily soluble in water. For 
this reason they occur most abundantly in arid regions, particularly in South 
America along the Chilean coast.  See  FERTILIZER; NITROGEN. 
 �34s[George Switzer]
�33s 
  
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�6v�2v�32sNitration �21m �33s
    An important chemical conversion (unit process) in which a nitro group
(--NO2) is introduced into a hydrocarbon compound. Three types of nitration
are common, and these are classified according to their chemical structure:
    1. C-nitration in which the nitro group attaches itself to a carbon atom,
as indicated in reaction (1).

	     \			      \
	    --C--H + HNO (sub 3) --> --C--NO (sub 2) + H (sub 2) O	  (1)
	     /			      /

    2. O-nitration (really esterification) in which an O-N bond is formed to
produce a nitrate, as seen in reaction (2).

	     \			       \
	    --C--OH + HNO (sub 3) --> --C--O--NO (sub 2) + H (sub 2) O	  (2)
	     /			       /

    3. N-nitration, in which there is formation of a N-N bond, such as shown
by reaction (3).

	     \			     \
	    --N--H + HNO (sub 3) -->  N--NO (sub 2) + H (sub 2) O	  (3)
	     /			     /

    Nitration of aromatics is employed commercially for the production of
numerous nitroaromatics. Explosives such as TNT, tetryl, ammonium picrate, and
cyclonite (RDX) are produced by nitration. Olefins can be nitrated with nitric
acid to produce nitroolefins, nitro alcohols, and the subsequent nitronitrate
esters.  See  NITRIC ACID; NITRO AND NITROSO COMPOUNDS; NITRO PARAFFIN;
OXIDIZING AGENT.
 �34s[Lyle F. Albright ;R. Norris Shreve]
�33s

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�6v�2v�32sNitric acid   �21m �33s 
    A strong mineral acid having the formula HNO3. Pure nitric acid is a 
colorless liquid with a specific gravity of 1.52 at 25DGC (77DGC); it freezes 
at -47DGC (-53DGF). Nitric acid is used in the manufacture of ammonium nitrate 
and phosphate fertilizers, nitro explosives, plastics, dyes, and lacquers. The 
principal commercial process for the manufacture of nitric acid is the Ostwald 
process, in which ammonia, NH3, is catalytically oxidized with air to form 
nitrogen dioxide, NO2. When the dioxide is dissolved in water, 60% nitric acid 
is formed. Production of 90 - 100% nitric acid is based on processes such as 
the reaction of sulfuric acid with sodium nitrate (an older method of nitric 
acid manufacture), dehydration of 60% acid, and oxidation of nitrogen dioxide 
in a solution of dilute nitric acid.  See  AMMONIA; NITROGEN. 
 �34s[Francis J. Johnston]
�33s 
  
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�6v�2v�32sNitride   �21m �33s 
    A binary compound of nitrogen with elements less electronegative than 
nitrogen. Common practice, however, is to exclude azides (such as NaN3), in 
which specialized bonding exists among the nitrogen atoms, and the binary 
compounds with hydrogen, the halogens, and the oxygen group elements. With 
this limitation, essentially solid nitrides will be encountered. 
    Direct reaction of alkali metals with nitrogen yields azides, which can be 
decomposed by heating to form the nitrides Li3N, Na3N, K3N, Rb3N, and Cs3N. 
These nitrides decompose to nitrogen gas and the elements in the vicinity of 
400DGC (752DGF); they react with water vapor to liberate ammonia (NH3) and 
form the metal hydroxides. 
    The nitrides of the alkaline earth elements (Be3N2, Mg3N2, Ca3N2, Sr3N2, 
and Ba3N2) may be prepared by direct reaction of the element with nitrogen or 
with ammonia at elevated temperatures. The other metals also form nitrides 
except for the elements Ag, Au, Hg, Tl, Sn, Pb, Sb, and Bi.  See  AZIDE; 
NITROGEN; OXIDE. 
 �34s[Russell K. Edwards]
�33s 
  
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�6v�2v�32sNitrile   �21m �33s 
    One of a group of organic chemical compounds of general formula 
RCIDENTICALN. A nitrile is named from the acid to which it can be hydrolyzed 
by adding the suffix -onitrile to the acid stem, for example, acetonitrile 
from acetic acid. An alternative system names the group attached to CN, thus 
CH3CN is also named methyl cyanide. In more complex structures the CN group is 
named as a substituent, cyano. 
    Industrially, nitriles are formed by heating carboxylic acids with ammonia 
and a dehydration catalyst under pressure. For the preparation of 
acrylonitrile, which is used on a large scale in the plastics industry, a 
vapor-phase catalytic ammoxidation of propylene has been developed.  See  
ACRYLONITRILE; AMINE; CARBOXYLIC ACID. 
 �34s[Paul E. Fanta]
�33s 
  
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�6v�2v�32sNitrite   �21m �33s 
    The negative ion NO2- derived from the unstable nitrous acid HNO2. Because 
of the intermediate oxidation state of nitrogen in nitrites (3+), the ion can 
act as either an oxidizing or reducing agent. Most nitrites are water-soluble. 
Sodium and potassium nitrites are quite stable and find extensive use in 
dyestuff manufacture and organic synthesis.  See  NITRATE; NITROGEN. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sNitro and nitroso compounds �21m �33s
    Nitro compounds are derivatives of organic hydrocarbons having one or more
--NO2 groups with nitrogen-to-carbon bonding. They differ from the
oxygen-linked nitrites, which are esters. The group lacks enough electrons to
form double bonds with both oxygens. However, both oxygens react alike; hence
the bond is regarded as a resonance hybrid of single and double bonds.
    Aromatic nitro compounds have been used chiefly as dye intermediates,
explosives, and pharmaceuticals. They are formed readily by the reaction of
aromatic compounds with nitric acid; H is replaced by the --NO2 group, for
example,

		     ***CHEMICAL STRUCTURE***

Aliphatic nitro compounds are prepared with difficulty and have grown in
importance only since the development of vapor-phase nitration of hydrocarbons
with nitric acid vapors at 420DGC.
    Nitroso compounds contain the --NO group attached to carbon or nitrogen.
Many are unstable intermediates, for example, nitrosobenzene formed during the
reduction of nitrobenzene.  See  NITRATION.
 �34s[Allen L. Hanson]
�33s

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�6v�2v�32sNitrobacteriaceae   �21m �33s 
    A family of gram-negative chemolithotrophic bacteria which live 
autotrophically, derive energy from the oxidation of either ammonia to 
nitrite, or nitrite to nitrate, and obtain carbon for growth from carbon 
dioxide. They are the agents of nitrate formation in nature. Seven genera are 
recognized. Four genera oxidize ammonia to nitrite: Nitrosomonas, 
Nitrosococcus, Nitrosospira, and Nitrosolobus. Three genera oxidize nitrite to 
nitrate: Nitrobacter, Nitrospina, and Nitrococcus.  See  CHEMOLITHOTROPHIC 
BACTERIA; NITROGEN CYCLE; SOIL MICROBIOLOGY. 
 �34s[Jane Meiklejohn]
�33s 
  
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�6v�2v�32sNitrobenzene �21m �33s
    A very-pale-yellow liquid with a sweet but sickening odor. The structural
formula is shown below. Nitrobenzene boils at 210.9DGC (411.6DGF) and freezes
at 5.6 to 5.7DGC (42.1 to 42.2DGF). It is produced by the nitration of benzene
with a mixture of nitric and sulfuric acids.

		     ***CHEMICAL STRUCTURE***

    Most of the nitrobenzene produced is reduced to aniline, but other dye
intermediates including benzidine and metanilic acid are also prepared from
it. The great toxicity of nitrobenzene impairs its usefulness as a solvent for
organic compounds. It is absorbed through the skin and if the vapor is
inhaled, it may produce cyanosis.  See	AROMATIC HYDROCARBON; BENZENE.
 �34s[Charles K. Bradsher]
�33s

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�6v�2v�32sNitrofuran �21m �33s
    One of a family of yellow crystalline compounds possessing antimicrobial
activity, characterized by a nitro group in the 5-position of a 2-substituted
furan ring. Most nitrofurans have a broad antibacterial spectrum, while
relatively few exhibit clinically useful antifungal and antiprotozoan activity.
    Nitrofurans of importance in medicine include nitrofurazone, mainly used
as a topical antibacterial agent; furazolidone, used primarily for its local
antitrichomonal action in the vagina; nifuroxime, a topical antifungal agent;
and nitrofurantoin, used particularly for genitourinary tract infections.  See
CHEMOTHERAPY.
 �34s[Julian J. Jaffe]
�33s

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�6v�2v�32sNitrogen �21m �33s
    A chemical element, N, atomic number 7, atomic weight 14.0067. Nitrogen is
a gas under normal conditions.
    Molecular nitrogen is the principal constituent of the atmosphere (78% by
volume of dry air), in which its concentration is a result of the balance
between the fixation of atmospheric nitrogen by bacterial, electrical
(lightning), and chemical (industrial) action, and its liberation through the
decomposition of organic materials by bacteria or combustion. In the combined
state, nitrogen occurs in a variety of forms. It is a constituent of all
proteins (both plant and animal) as well as of many other organic materials.
Its chief mineral source is sodium nitrate.
    Because of the importance of nitrogen compounds in agriculture and the
chemical industry, much of the industrial interest in elementary nitrogen has
been in processes for converting elemental nitrogen into nitrogen compounds.
Nitrogen is also used for filling bulbs of incandescent lamps and, in general,
wherever a relatively inert atmosphere is required.
    Nitrogen, as it occurs in nature, consists of two isotopes, 14N and 15N,
in the abundance ratio of 99.635 to 0.365. In addition the radioactive
isotopes 12N, 13N, 16N, and 17N have been made by a variety of nuclear
reactions. At standard temperature and pressure, elemental nitrogen exists as
a gas with a density of 1.25046 g per liter. The molecular formula is N2. Some
physical properties of elemental nitrogen are listed in Table 1.
    Elemental nitrogen has a low reactivity toward most common substances at
ordinary temperatures. At high temperatures, molecular nitrogen reacts with
chromium, silicon, titanium, aluminum, boron, beryllium, magnesium, barium,
strontium, calcium, and lithium to form nitrides; with O2 to form NO; and at
moderately high temperatures and pressures in the presence of a catalyst, with
hydrogen to form ammonia. Above 1800DGC (3270DGF), nitrogen, carbon, and
hydrogen combine to form hydrogen cyanide.
    When molecular nitrogen is subjected to the actions of a condensed
electrode discharge or to a high-frequency electrodeless discharge, it is
partially changed to an activated, unstable condition, from which on standing
it returns to its normal state with the emission of a golden-yellow afterglow.
    The elements of the nitrogen family exhibit in their compounds three
principal oxidation states, -3, +3, and +5, though other oxidation states are
also exhibited. All the elements of the nitrogen family form hydrides of the

				 H
				 ..
			       H:M:H
				 ..

type shown in the formula below and +3 oxides, +5 oxides, +3 halides (MX3),
and, except for nitrogen and bismuth, +5 halides (MX5). Nitrogen is the most
electronegative element of the nitrogen family. Table 2 lists the principal
classes of inorganic nitrogen compounds. Thus, in addition to the typical
oxidation states of the family (-3, +3, and +5), nitrogen forms compounds
with a variety of additional oxidation states.	See  AMINE; AMMINE; AMMONIA;
HYDRAZINE; NITRIC ACID; NITRIDE; NITROGEN OXIDES.
 �34s[Harry H. Sisler]
�33s

    Compounds containing the dinitrogen molecule, N2, bound to a metal are
called nitrogen complexes or dinitrogen complexes. Outstanding in their
ability to form coordination compounds with nitrogen are a number of metals
which belong to the group VIII transition metal family. For each metal of this
group, several nitrogen complexes have been identified. Nitrogen complexes of
these metals occur in low oxidation states, such as Co(I) or Ni(O). The other
ligands present in these complexes besides N2 are usually of a type known to
stabilize low oxidation states; phosphines appear to be particularly prominent
bonding partners in this respect. The illustration shows the structure of a
typical N2 complex, elucidated by a crystal structure determination. The N-N
bond axis in this complex is aimed, within the limits of experimental error,
directly toward the position of the metal atom.  See  COMPLEX COMPOUNDS;
COORDINATION CHEMISTRY.
 �34s[Hans Brintzinger]
�33s

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�6v�2v�32sNitrogen cycle   �21m �33s 
    The collective term given to the natural biological and chemical processes 
through which inorganic and organic nitrogen are interconverted. It includes 
the process of ammonification, ammonia assimilation, nitrification, nitrate 
assimilation, nitrogen fixation, and denitrification. 
    Nitrogen exists in nature in several inorganic compounds, namely N2, N2O, 
NH3, NO2-, and NO3-, and in several organic compounds such as amino acids, 
nucleotides, amino sugars, and vitamins. In the biosphere, biological and 
chemical reactions continually occur in which these nitrogenous compounds are 
converted from one form to another. These interconversions are of great 
importance in maintaining soil fertility and in preventing pollution of soil 
and water. 
    An outline showing the general interconversions of nitrogenous compounds 
in the soil-water pool is presented in the illustration. There are three 
primary reasons why organisms metabolize nitrogen compounds: (1) to use them 
as a nitrogen source, which means first converting them to NH3, (2) to use 
certain nitrogen compounds as an energy source such as in the oxidation of NH3 
to NO2- and of NO2- to NO3-, and (3) to use certain nitrogen compounds (NO3-) 
as terminal electron acceptors under conditions where oxygen is either absent 
or in limited supply. The reactions and products involved in these three 
metabolically different pathways collectively make up the nitrogen cycle. 
    There are two ways in which organisms obtain ammonia. One is to use 
nitrogen already in a form easily metabolized to ammonia. Thus, nonviable 
plant, animal, and microbial residues in soil are enzymatically decomposed by 
a series of hydrolytic and other reactions to yield biosynthetic monomers such 
as amino acids and other small-molecular-weight nitrogenous compounds. These 
amino acids, purines, and pyrimidines are decomposed further to produce NH3 
which is then used by plants and bacteria for biosynthesis, or these 
biosynthetic monomers can be used directly by some microorganisms. The 
decomposition process is called ammonification. 
    The second way in which inorganic nitrogen is made available to biological 
agents is by nitrogen fixation (this term is maintained even though N2 is now 
called dinitrogen), a process in which N2 is reduced to NH3. Since the vast 
majority of nitrogen is in the form of N2, nitrogen fixation obviously is 
essential to life. The N2-fixing process is confined to prokaryotes (certain 
photosynthetic and nonphotosynthetic bacteria). The major nitrogen fixers 
(called diazotrophs) are members of the genus Rhizobium, bacteria that are 
found in root nodules of leguminous plants, and of the cyanobacteria 
(originally called blue-green algae).  See  NITROGEN FIXATION. 
 �34s[Leonard E. Mortenson]
�33s 
  
  
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�6v�2v�32sNitrogen fixation �21m �33s
    The conversion of dinitrogen (N2) from the atmosphere into a form
available to plants and hence to animals and humans. This is an important step
in the terrestrial nitrogen cycle. Both chemical and biological processes
contribute, to turn over some 2 x 10sup8 metric tons of nitrogen per year over
the planet as a whole: industrial fertilizer production accounts for about
one-third; roughly two-thirds of the global input of fixed nitrogen arises
from the biological process.

    Chemical fixation.	 Three basic processes are used that give different
products containing fixed nitrogen. All require a considerable input of
thermal or electrical energy.
    In arc processes air is passed through an electric arc and about 1% nitric
oxide is formed, which can be converted to nitrates by established chemical
procedures. In the widely used Haber-Bosch process, hydrogen is generated from
natural gas or petroleum, mixed with air, and burned to yield a
nitrogen-hydrogen mixture. This reacts (under 100 - 800 atm pressure [1 atm =
10(sup 5) pascals] at 200 - 700DGC or 390 - 1290DGF with a metal oxide catalyst)
to give ammonia. In the cyanamide process, calcium cyanamide is generated by
heating calcium carbide in nitrogen; when moistened, it hydrolyzes to give
urea and ammonia. These are the major industrial processes for obtaining
nitrogenous fertilizer.  See  AMMONIA; CYANAMIDE; ELECTROCHEMICAL PROCESSES;
FERTILIZER.

    Biological fixation.  Only prokaryotes (bacteria and blue-green algae) fix
nitrogen. Nitrogen-fixing microbes fall into two main groups, free-living and
symbiotic.
    The free-living microbes can be subclassified. The aerobes require air for
growth and fix nitrogen where air is present. Nitrogen-fixing, free-living
facultative bacteria are widespread: the bacterial genera Klebsiella and
Bacillus include many strains that can fix nitrogen, but they do so only in
the absence of air. They grow perfectly well in air if prefixed nitrogen such
as ammonium salt is present, but they do not then fix nitrogen. Some
Clostridium strains, some methanogenic bacteria, and some sulfate-reducing
bacteria are free-living nitrogen-fixing anaerobes; none grows in the presence
of air, whether they fix nitrogen or not. Finally there are the free-living
phototrophs: many blue-green algae fix nitrogen in air; many photosynthetic
bacteria can fix nitrogen, but only when growing anaerobically. The blue-green
algae are ecologically the most important of the free-living, nitrogen-fixing
organisms, particularly in regard to the productivity of arid, tundra, and
water-logged regions (such as rice paddies).
    The best-known symbiotic bacteria belong to the genus Rhizobium. Species
of Rhizobium colonize the roots of leguminous plants and stimulate the
formation of nodules, within which they fix nitrogen aerobically. Rhizobial
symbioses are of great agricultural importance in soybean, pulse, and similar
legume crops. Of comparable ecological importance are the symbiotic endophytes
of nonlegumes, such as Alnus (alder), Casuarina (Australian pine), or
Ceanothus, which are found in root nodules, where they fix nitrogen.  See
SOIL MICROBIOLOGY.
 �34s[John Postgate]
�33s

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�6v�2v�32sNitroparaffin   �21m �33s 
    One of a series of aliphatic compounds with relatively high boiling points 
and high molecular polarity. Such compounds are used as solvents for cellulose 
derivatives, waxes, and many substances of high molecular weight. Their chief 
use is in the synthesis of other organic compounds. The commercially available 
nitroparaffins include nitromethane, nitroethane, 1-nitropropane, and 
2-nitropropane.  
    Polynitroparaffins such as 2,2-dinitropropane increase the octane number 
in diesel fuels and also reduce the smoke nuisance in diesel motors.  See  
NITRATION.  
 �34s[Leallyn B. Clapp]
�33s 
  
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�6v�2v�32sNobelium   �21m �33s 
    A chemical element, No, atomic number 102. Nobelium is a synthetic element 
produced in the laboratory. It decays by emitting an alpha particle, that is, 
a doubly charged helium ion. Only atomic quantities of the element have been 
produced to date. Nobelium is the tenth element heavier than uranium to be 
produced synthetically. It is the thirteenth member of the actinide series, a 
rare-earth-like series of elements.  See  ACTINIDE ELEMENTS: RADIOACTIVITY; 
RARE-EARTH ELEMENTS; TRANSURANIUM ELEMENTS. 
 �34s[Paul R. Fields]
�33s 
  
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�6v�2v�32sNocardiosis   �21m �33s 
    An actinomycetous disease of humans caused by several species of Nocardia. 
The lesions are purulent and granulomatous, usually involving the subcutaneous 
tissues, although a pulmonary form is also recognized. The latter form of 
nocardiosis is the most serious, often metastasizing to the viscera and 
central nervous system. Nocardia asterioides, the primary etiological agent, 
is easily found in the soil. 
 �34s[Leanor D. Haley]
�33s 
  
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�6v�2v�32sNociceptors   �21m �33s 
    Cutaneous receptors that have bare nerve endings in or beneath the skin 
and are selectively responsive to noxious stimuli described as painful when 
applied to human skin. There are different sets of thinly myelinated nerve 
fibers and unmyelinated fibers whose endings are differentially responsive to 
only noxious thermal stimuli (hot or cold, or both) or only to noxious 
mechanical stimuli (such as pricking or pinching the skin). Other types of 
nociceptors are less specific, and the most common of these is one which 
responds to any noxious stimulus, whether it be chemical, thermal, or 
mechanical, and thus is called the polymodal nociceptor. Following injury to 
the skin, such as that produced by a superficial burn, the threshold responses 
to mechanical or thermal stimuli of many nociceptors may be greatly lowered. 
In such a state of heightened sensitivity to pain, termed hyperalgesia, 
normally innocuous stimulation such as gentle rubbing or warming can evoke a 
sensation of pain. See CUTANEOUS SENSATION. 
 �34s[Rolf Landshoff]
�33s 
  
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�6v�2v�32sNoctilucent clouds   �21m �33s 
    High-altitude clouds which are seen at the mesopause and are still 
illuminated by the Sun when it is already well below the observer's horizon 
during astronomical twilight. Noctilucent clouds occur only during the summer 
months at latitudes from about 45 to 70DG. Generally, they resemble high 
cirrus clouds with pronounced band or wave structures and are usually so 
tenuous that stars can be seen through them; they are always pure white to 
blue-white.  
    It is now agreed that the cold temperatures are a necessary, but not a 
sufficient, condition for noctilucent-cloud formation. It is also generally 
accepted that the clouds are composed of ice crystals or water droplets which 
condense from water vapor brought up to the mesopause by the turbulence 
present during the arctic summer.  See  CLOUD PHYSICS. 
 �34s[Robert K. Soberman]
�33s 
  
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�6v�2v�32sNoeggerathiales �21m �33s
    An incompletely known and poorly defined group of vascular plants whose
geologic range extends from Upper Carboniferous to Triassic. Their taxonomic
status and position in the plant kingdom are uncertain since morphological
evidence (because of the paucity of the fossil record) does not make it
possible to place the group confidently in any recognized major subdivision of
the vascular plants. The Noeggerathiales have been proposed in the
evolutionary scheme in two remotely related groups of vascular plants, the
Pteropsida and the Sphenopsida.  See  PALEOBOTANY; PLANT KINGDOM.
 �34s[Elso S. Barghoorn]
�33s

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�6v�2v�32sNomograph   �21m �33s 
    A graphical relationship between a set of variables that are related by a 
mathematical equation or law. The fundamental principle involved in the 
construction of a nomographic or alignment chart consists of representing an 
equation containing three variables, f(u,v,w) = 0, by means of three scales in 
such a manner that a straight line cuts the three scales in values of u, v, 
and w, satisfying the equation. The cutting line is called the isopleth or 
index line. Numbers may be quickly and easily read from the scales of such a 
chart even by one unfamiliar with the construction of the chart and the 
equation involved. The illustration shows such an example. Assume that it is 
desired to find the value of E when D= 2 and Q = 50. Lay a straightedge 
through 50 on the Q scale and through 2 on the D scale and read 11.8 at its 
intersection with the E scale. As another example, it might be desired to know 
what value or values of D should be used if E and Q are required to be 10 and 
60, respectively. A straightedge through E = 10 and Q = 60 cuts the D scale in 
two points, D = 2.8 and D = 9.4. This is equivalent to finding two positive 
roots of the cubic equation Dsup3 - 10Dsup2 + 56.25 = 0. It is assumed that g 
= 32 ft/ssup2 in this equation. 
 �34s[Raymond D. Douglass]
�33s 
  
  
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�6v�2v�32sNondestructive testing   �21m �33s 
    The use of tests to examine an object or material to detect imperfections, 
determine properties, or assess quality without changing its usefulness. 
Nondestructive testing is a term generally applied to certain tests that are 
utilized in industrial operations, although some of the same technologies are 
used in the medical field, such as radiography (x-ray) and ultrasonics. 
    Some nondestructive tests involve complex high-technology systems, while 
others, which employ the same basic test principle, are easily applied by 
unskilled operators. Nearly every form of energy is used in nondestructive 
tests, including all wavelengths of the electromagnetic spectrum, as well as a 
broad range of frequencies of vibrational mechanical energy. Although there 
are many methods used for nondestructive testing, six are applied widely in 
industry: visual-optical, liquid-penetrant, magnetic-particle, eddy-current, 
ultrasonic, and radiographic. 
  
    Visual-optical method.   This method can be applied to the examination of 
surfaces and transparent materials. A visual test might be as simple as an 
inspector looking at an object to check color or surface quality. At the other 
extreme, laser beams and complex image-recognition equipment can be used in 
automated scanning systems to perform an inspection. Endoscopes are rigid or 
flexible (fiber-optic) devices used to inspect internal surfaces of objects. 
They can be very small in diameter (G in. or 3 mm) and usually include their 
own illumination source. 
  
    Liquid-penetrant method.   Testing with liquid penetrants is useful for 
locating imperfections that are open to the surface of a wide variety of 
nonporous materials. To perform the test, a thin oillike liquid containing a 
dye is applied to the material and it moves into small openings or cracks. 
After the excess liquid on the surface is carefully removed and the object is 
dried, a developer is applied to the surface. This material is typically a 
fine powder, such as talc, usually in suspension in a liquid. It acts like a 
blotter and pulls the liquid penetrant up out of surface imperfections. The 
liquid tends to spread in the developer, thereby enlarging the indication. 
  
    Magnetic-particle method.   Magnetic-particle testing is used for 
detecting discontinuities at or near the surface in magnetic materials. Steel 
and cast iron parts such as those used in automobiles, aircraft, and railroad 
equipment are subjected to magnetic-particle testing to detect processing or 
fatigue cracks. To perform the test, the test object is properly magnetized, 
and then finely divided magnetic particles are applied to its surface. When 
the object is properly oriented to the induced magnetic field, a discontinuity 
creates a leakage field which attracts and holds the particles, forming a 
visible indication. 
  
    Eddy-current method.   Eddy current tests are based upon correlation 
between electromagnetic properties and physical or structural properties of a 
test object. Eddy currents are induced in metals whenever they are brought 
into an ac magnetic field. These eddy currents create a secondary magnetic 
field, which opposes the inducing magnetic field. The presence of 
discontinuities or material variations alters eddy currents, thus changing the 
apparent impedance of the inducing coil or of a detection coil. Coil impedance 
indicates the magnitude and phase relationship of the eddy currents to their 
inducing magnetic field current. Conditions such as heat treatment, 
composition, hardness, phase transformation, case depth, cold working, 
strength, size, thickness, cracks, seams, and inhomogeneities are indicated by 
eddy-current tests. 
  
    Ultrasonic method.   Ultrasonic waves can detect and locate structural 
discontinuities or differences and measure the thickness of a variety of 
materials. To perform a contact test, a transducer is placed on an object to 
which a coupling material has been applied, and the results are interpreted 
from cathode-ray-tube indications. To accomplish this, an electric pulse is 
generated in a test instrument and transmitted through a cable to a 
transducer, which converts the electric pulse into mechanical vibrations. 
These low-energy-level vibrations are transmitted through a cable to a 
transducer, which converts the electric pulse into mechanical vibrations. 
These low-energy-level vibrations are transmitted through a coupling material 
into the test object, where the ultrasonic energy is attenuated, scattered, or 
reflected to indicate conditions within the object. Reflected or transmitted 
sound energy is usually reconverted to electrical energy by the same 
transducer and returned to the test instrument. The received energy is then 
usually displayed on a cathode-ray tube. The presence, position, and amplitude 
of echoes indicate conditions of the test-object material.  See  ULTRASONICS. 
  
    Radiographic method.   Radiography can be used to reveal internal 
structure in industrial materials and assemblies, just as it does for a doctor 
or dentist who inspects parts of the body. The most widely used inspection 
approach involves the transmission of x-rays through an object to produce an 
image on x-ray film. This inspection method can be applied to many materials 
and assemblies. Proper placement of components in assemblies, detection of 
porosity, inclusions, or cracks, and differentiation of different materials, 
thicknesses, or densities are among the useful results obtained by 
radiography.  See  RADIOGRAPHY. 
 �34s[Donald D. Dodge]
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�6v�2v�32sNonelectrolyte   �21m �33s 
    A chemical compound which does not conduct electricity either when fused 
or when dissolved in a solvent such as water. The bonding in nonelectrolytes 
is covalent. Most organic compounds, with the exception of the acids and 
amines, are nonelectrolytes, as are many inorganic compounds such as the 
halides of the nonmetals.  See  CHEMICAL BONDING; ELECTROLYTE; SOLUTION. 
 �34s[Thomas C. Waddington]
�33s 
  
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�6v�2v�32sNongonococcal urethritis (NGU)   �21m �33s 
    Human urethral inflammation not associated with common bacterial 
pathogens. A significant cause of nongonococcal urethritis may be bacteria of 
the Bedsonia (Chlamydia) group, particularly trachoma inclusion conjunctivitis 
(TRIC) or lymphogranuloma venereum agents. The infective agent is in most 
cases the agent of inclusion conjunctivitis, which depends upon venereal 
transmission and a genital-tract reservoir for its perpetuation. The disease 
may be a severe purulent urethritis, but it is more often mild with a 
remitting course. In the female the same agent causes cervicitis.  See  
LYMPHOGRANULOMA VENEREUM; PLT-BEDSONIA GROUP. 
 �34s[Karl F. Meyer]
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�6v�2v�32sNonlinear control theory �21m �33s
    The theory of control systems that do not have the property of
superposition, that is, of systems in which some or all of the outputs are not
linear functions of the inputs. All physical systems, although in many cases
modeled on linear systems, exhibit some degree of nonlinearity. An electronic
amplifier will suffer output saturation if the input exceeds its allowable
limits, producing the nonlinear characteristic shown in Fig. 1. As a result of
backlash, the input-output characteristic of a gear train exhibits hysteresis,
which can significantly degrade the performance of a closed-loop control
system. A relay, with ideal characteristic as shown in Fig. 2, is widely used
in practice because of its high reliability and low cost.  See	HYSTERESIS.
    The behavior of nonlinear control systems is quite different from that of
linear control systems. A stable nonlinear system may sustain a
fixed-amplitude oscillation without the application of an input. The unforced
response of a nonlinear system may be entirely different for different initial
conditions. The state trajectory (the path followed by the state vector in the
course of time) of a nonlinear system may exhibit jump discontinuities and
possess two or more equivalent states (states that are physically identical
even though the values of the state variables describing them are different).
Frequencies can exist in the output that are not present in the input. These
phenomena do not occur in linear control systems.

    Most nonlinear control systems can be described by a set of first-order

			     .						     (1)
			     x = f(x,u)

differential equations of the form in Eq. (1), where x is the n-dimensional
state vector and u the m-dimensional input vector. Except in special cases,
the quantitative analysis of Eq. (1) is carried out on a digital computer.
See  DIFFERENTIAL EQUATION; LINEAR ALGEBRA.
    The state xsube is called an equilibrium state of the system described by

			     f(x (sub e),0) = 0 			     (2)

Eq. (1) if Eq. (2) is satisfied. At an equilibrium state, x(sub e) = 0, and if
no input is applied the state trajectory will remain indefinitely at the
equilibrium state. One aspect of the stability of nonlinear control systems is
defined in terms of its equilibrium states. An equilibrium state is said to be
stable if, for any initial state in the neighborhood of xsube, the resulting
trajectory remains in the vicinity of xsube. If, for all such initial states,
the resulting trajectories all approach xsube as time tends to infinity, the
equilibrium state is said to be asymptotically stable. These definitions
characterize the local behavior of the state trajectory in the vicinity of
equilibrium points. If the properties hold for any initial state, the
equilibrium state is said to be stable or asymptotically stable in the large.
See  CONTROL SYSTEMS; RELAY CONTROL SYSTEMS.
 �34s[Patrick E. Barry ;C. T. Chen]
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�6v�2v�32sNonlinear optics   �21m �33s 
    A field of study concerned with the interaction of electromagnetic 
radiation and matter in which the matter responds in a nonlinear manner to the 
incident radiation fields. The nonlinear response can result in 
intensity-dependent variation of the propagation characteristics of the 
radiation fields or in the creation of radiation fields at new frequencies. 
Nonlinear effects can take place in solids, liquids, gases, and plasmas, and 
may involve one or more electromagnetic fields as well as internal excitations 
of the medium. The wavelength range of interest generally coincides with the 
spectrum covered by lasers, extending from the far infrared to the vacuum 
ultraviolet, but some nonlinear interactions have been observed at wavelengths 
extending into the x-ray range. Historically, nonlinear optics precedes the 
laser, but most of the work done in the field has made use of the high powers 
available from lasers.  See  LASER. 
  
    Nonlinear materials.   Nonlinear effects of various types are observed in 
all materials. It is convenient to characterize the response of the medium 
mathematically by expanding it in a power series in the electric and magnetic 
fields of the incident optical waves. The linear terms in such an expansion 
give rise to the linear index of refraction, linear absorption, and the 
magnetic permeability of the medium, while the higher-order terms give rise to 
nonlinear effects. The coefficients in the expansion that describe the 
strength of the nonlinear interactions are now linear susceptibilities. 
    In general, nonlinear effects associated with the electric field of the 
incident radiation dominate over magnetic interactions. Effects involving the 
electric dipole response of the medium, termed the polarization, are generally 
the most important of the electric field interactions. Generally the 
magnitudes of the nonlinear susceptibilities decrease rapidly as the order of 
the interaction increases.  See  ELECTRIC SUSCEPTIBILITY; POLARIZATION OF 
DIELECTRICS.  
  
    Intensity-dependent effects.  Nonlinear polarization components at the 
same frequencies as those in the incident waves can result in effects that 
change the index of refraction or the absorption coefficient, quantities that 
are constants in linear optical theory. Materials that are transparent at low 
optical intensities can have their absorption increase at high intensities. 
This effect involves simultaneous absorption of multiple numbers of photons 
from one or more incident waves. 
    In materials which have a strong linear absorption at the incident 
frequency, the absorption can decrease at high intensities. This effect, 
termed saturable absorption, was observed long before lasers were available. 
Saturable absorbers are useful in operating Q-switched and mode-locked lasers. 
    Intensity-dependent changes in the refractive index can affect the 
propagation characteristics of a laser beam. For many materials the index of 
refraction increases with increasing optical intensity. If the laser beam has 
a profile that is more intense in the center than at the edges, the profile of 
the refractive index corresponds to that of a positive lens, causing the beam 
to focus. This effect, termed self-focusing, can cause an initially uniform 
laser beam to break up into many smaller spots with diameters of the order of 
a few micrometers and intensity levels that are high enough to damage many 
solids. In other materials the refractive index decreases as the optical 
intensity increases. The resulting nonlinear lens causes the beam to defocus, 
an effect termed self-defocusing. 
  
    Phase conjugation.   The same interactions that give rise to the 
intensity-dependent effects can also cause nonlinear interactions between 
waves that are at the same frequency but are otherwise distinguishable, for 
example, by their direction of polarization or propagation. Termed degenerate 
four-wave mixing, this interaction gives rise to a number of effects such as 
amplification of a weak probe wave and generation of a fourth wave that 
propagates in a direction opposite to that of a probe wave. In one of the most 
interesting aspects of this interaction, the backward wave has the same 
distribution of phase variations as the probe wave but with their sense 
reversed. This effect, termed phase conjugation or time reversal, can allow 
the correction of phase distortions of light beams caused by propagation 
through inhomogeneous media, by aberrated optics, or by mode dispersion in 
optical waveguides. It has important applications in the area of image 
transmission through optical fibers and in various forms of optical processing 
and information transfer. 
  
    Nonlinear spectroscopy.   The variation of the nonlinear susceptibility 
near the resonances that correspond to sum- and difference-frequency 
combinations of the input frequencies forms the basis for various types of 
nonlinear spectroscopy which allow study of energy levels that are not 
normally accessible with linear optical spectroscopy. Nonlinear spectroscopy 
can be performed with many of the interactions discussed earlier. Many types 
of four-wave mixing interactions can also be used in nonlinear spectroscopy.  
See  LASER SPECTROSCOPY; RAMAN EFFECT. 
  
    Inelastic scattering.   Light can scatter inelastically from fundamental 
excitations in the medium, resulting in the production of radiation at new 
frequencies that differ from the incident frequencies by an amount that is 
equal to the frequency of the fundamental excitation. The difference in photon 
energy between the incident and scattered fields is taken up or given up by 
the medium. 
  
    Coherent effects.   Another class of effects involves excitations of the 
medium in which the phase of the atomic wave function is preserved. These 
effects are generally observed only for short light pulses, of the order of 
several nanoseconds or less. In one interaction, termed self-induced 
transparency, a pulse of light of the proper shape, magnitude, and duration 
can propagate unattenuated in a medium which is otherwise absorbing. 
 �34s[John F. Reintjes]
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�6v�2v�32sNonlinear programming �21m �33s
    An area of applied mathematics concerned with finding the values of the
variables which give the smallest or largest value of a specified function in
the class of all variables satisfying prescribed conditions. The function
which is to be optimized is called the objective function, and the functions
defining the prescribed conditions are referred to as the constraint functions
or constraints. This general problem is called the nonlinear programming
problem. The study of the theoretical and computational aspects of the
nonlinear programming problem is called nonlinear programming, mathematical
programming, or optimization theory. When there are no constraints, the
nonlinear programming problem is said to be unconstrained; otherwise the
problem is said to be constrained. If the objective function and the
constraint functions are linear, the nonlinear programming problem is said to
be a linear programming problem. When the objective function is quadratic and
the constraints are linear, the nonlinear programming program is said to be a
quadratic programming problem.	See  LINEAR PROGRAMMING.
    The general nonlinear programming problem can be stated as below, where x

		minimize f(x)
		subject to g(sub i)(x) > or = 0, i = 1, . . . , m
			 h(sub j)(x) = 0, j = 1, . . . , p

= (x1, . . ., x[sub n]) are the variables of the problem, f is the objective
function, g(sub i) are the inequality constraints, and h(sub j) are the equality
constraints. By changing f to -f, a maximization problem is transformed to a
minimization problem. Moreover, an inequality constraint in the form g(sub i)(x)
> or = 0 is equivalent to -g(sub i)(x) < or = 0. Consequently the format of the
problem above can handle both types of inequality constraints and both
minimization and maximization problems.
    Many of the quantitative problems in business, economics, and engineering
design can be expressed as nonlinear programming problems. General
computational methods have been designed and implemented on large digital
computers. While considerable progress has been made in both the design and
algorithms and computer technology, there are still many practical large
nonlinear programs which cannot be solved today. Research is aimed at taking
advantage of the structure of the particular problem in question. One
important example of structure is sparseness. Sparseness would occur if the
nonlinear programming problem had many variables, but the objective function
and each constraint function involved only relatively few variables.  See
OPERATIONS RESEARCH; OPTIMIZATION; PROCESS CONTROL.
 �34s[Richard A. Tapia]
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�6v�2v�32sNonmetal   �21m �33s 
    The elements are conveniently, but arbitrarily, divided into metals and 
nonmetals. The nonmetals do not conduct electricity readily, are not ductile, 
do not have a complex refractive index, and in general have high ionization 
potentials.  
    If the periodic table is divided diagonally from upper left to lower 
right, all the nonmetals are on the right-hand side of the diagonal. Examples 
of elements which do not fit neatly into this useful but arbitrary 
classification are tin, which exists in two allotropic modifications, one 
definitely metallic and the other with many properties of a nonmetal, and 
tellurium and antimony. Such elements are called metalloids.  See  METAL; 
METALLOID; PERIODIC TABLE. 
 �34s[Thomas C. Waddington]
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�6v�2v�32sNon-newtonian fluid   �21m �33s 
    A fluid whose flow behavior departs from that of an ideal newtonian fluid. 
In a newtonian fluid the rate of shear in the fluid under isothermal 
conditions is proportional to the corresponding stress at the point under 
consideration. A non-newtonian fluid is one for which this rather strict 
requirement is not met. There are several ways in which real fluids can depart 
from the relatively simple newtonian condition, but it is better to think in 
terms of non-newtonian regions rather than non-newtonian fluids because many 
fluids exhbit both newtonian and non-newtonian behavior, depending on the 
conditions of flow, and cannot be strictly classified as being exclusively one 
type or the other.  See  NEWTONIAN FLUID. 
    Non-newtonian fluids can be broadly classified into three types as follows: 
    1. Time-independent fluids for which the rate of shear at any point in the 
fluid is some function of the shear stress at that point and depends on 
nothing else. (A newtonian fluid is a special case of a fluid in this 
category.)  
    2. More complex time-dependent fluids for which the relationship between 
shear stress and shear rate depends on the time the fluid has been sheared, 
that is, on its previous history. 
    3. Fluids which have the characteristics of both viscous liquids and 
elastic solids and exhibit partial elastic recovery after deformation, the 
socalled viscoelastic fluids. 
  
    Time-independent fluids.  These fluids may be conveniently divided into 
three distinct types. These types, the flow curves for which are shown in Fig. 
1, are as follows. 
    Bingham plastics exhibit a yield stress which is the stress that must be 
exceeded before flow starts. Thereafter the rate-of-shear curve is linear. 
This behavior is found in such materials as toothpaste, oil paints, and 
drilling muds. There are other materials which also exhibit a yield stress, 
but the flow curve is thereafter not linear. These are usually called 
generalized Bingham plastics. 
    Pseudoplastic fluids show no yield value, but the ratio of shear stress to 
the rate of shear, which may be termed the ``apparent viscosity,'' falls 
progressively with shear rate. This phenomenon of shear thinning is 
characteristic of suspensions of asymmetric particles or solutions of polymers 
such as cellulose derivatives. 
    Dilatant fluids are similar to pseudoplastics in that they show no yield 
stress, but the apparent viscosity for dilatant materials increases with 
increasing rates of shear. Such ``shear thickening'' is observed with 
suspensions of solids at high solids content (approaching point of tightest 
packing). Corn-flour pastes can be dilatant. 
  
    Time-dependent fluids.   These fluids may be subdivided into two classes - 
thixotropic fluids and rheopectic fluids - according to whether the shear 
stress decreases or increases with time when the fluid is sheared at a 
constant rate. 
    The structure breakdown of thixotropic materials is a function of the 
duration of shear as well as of the rate of shear. Consider a thixotropic 
material which is sheared at a constant rate after a period of rest. The 
structure will be progressively broken down, and the apparent viscosity will 
decrease with time. The rate of breakdown of structure during shearing at a 
given rate will depend on the number of linkages available for breaking and 
therefore must decrease with time. Also, the simultaneous rate of reformation 
of structure will increase with time as the number of possible new structural 
linkages increases. Eventually a state of dynamic equilibrium is reached when 
the rate of buildup of structure equals the rate of breakdown. This 
equilibrium position depends on the rate of shear and moves toward greater 
breakdown at increasing rates of shear. 
    Rheopectic fluids are fluids for which the structure builds up on 
shearing, and this phenomenon can be regarded as the reverse of thixotropy - 
in fact, rheopectic behavior is often referred to as antithixotropy. Such 
materials are comparatively rare, and few if any are of industrial 
significance.  
  
    Viscoelastic fluids.   A viscoelastic material is one which possesses both 
viscous and elastic properties; that is, although the material might be 
viscous, it exhibits a certain elasticity of shape and is capable of storing 
energy of deformation. 
    This type of behavior, which is typical of solutions of macromolecules and 
molten polymers, is analogous to mechanical springs and dashpots in series. 
Real materials behave as though they comprised a large number of such 
spring-dashpot combinations, each with its own relaxation time defined as the 
ratio of viscosity to the corresponding rigidity modulus in the combination. 
  
  
    Flow.  The flow behavior of non-newtonian fluids cannot be predicted 
quantitatively except in relatively simple situations. In the case of 
time-independent non-newtonian fluids the behavior under laminar flow in tubes 
can be treated rigorously, but turbulent flow is not well understood. Although 
time-dependent fluids, particularly of the thixotropic type, are of 
considerable commercial importance, there is little progress of engineering 
significance on problems where the changes of fluid properties with time are 
important. The steady flow of viscoelastic fluids can be reasonably well 
predicted, but when unsteady conditions exist, such as the flow through valves 
and fittings and in the whole field of turbulent flow, the elastic properties 
of the fluid are of major importance and quantitative treatments are not yet 
well developed. At a free surface, viscoelastic effects can manifest 
themselves in the form of the Weissenberg effect, in which an elastic liquid 
tends to climb up a rotating rod, as shown in Fig. 2a, or as the swelling of a 
jet on extrusion from a die, as in Fig. 2b. These phenomena are the result of 
the normal stresses set up in viscoelastic fluids under shear.  See  FLUID 
FLOW; FLUIDS. 
 �34s[J. Harris ;W. L. Wilkinson]
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�6v�2v�32sNonsinusoidal waveform �21m �33s
    The representation of a wave that does not vary in a sinusoidal manner.
Electric circuits containing nonlinear elements, such as electron tubes,
iron-core magnetic devices, and transistors, commonly produce nonsinusoidal
currents and voltages. When these are repetitive functions of time, they are
called nonsinusoidal electric waves. Oscillograms, tabulated data, and
sometimes mathematical functions for segments of such waves are often used to
describe the excursions throughout one cycle. A cycle corresponds to 2PI
electrical radians and covers the time interval T seconds in which the wave
repeats itself.
    These electric waves can be represented by a constant term, the average or
dc component, plus a series of harmonic terms in which the frequencies of the
harmonics are integral multiples of the fundamental frequency. The fundamental
frequency f1, if it does exist, has the time span T = 1/f1 seconds for its
cycle. The second-harmonic frequency f2 then will have two of its cycles
within T seconds, and so on.
    The series of terms stated above is known as a Fourier series and can be
expressed in the form of the equation below,


	  y(t) = B (sub 0) + C (sub 1) sin (OMEGA t + PHI (sub 1)) + . . .
		+ C (sub n) sin (n OMEGA t + PHI (sub n)) + . . .	   (1)
	    = SIGMA C (sub n) sin (n OMEGA t + PHI (sub n))
	      n = 0

where y(t), plotted over a cycle of the fundamental, gives the shape of the
nonsinusoidal wave. The radian frequency of the fundamental is OMEGA = 2PIf1,
and n is an integer. C1 is the amplitude of the fundamental (n = 1), and
succeeding C sub n's are the amplitudes of the respective harmonics having
frequencies corresponding to n = 2, 3, 4, and so on, with respect to the
fundamental. The phase angle of the fundamental with respect to a chosen time
reference axis is PHI1, and the succeeding PHI sub n's are the phase angles of
the respective harmonics.  See	FOURIER SERIES AND INTEGRALS.
    The equation for y(t) shows all its separate components, which, in
general, include an infinite number of terms. In practical problems the first
several terms usually yield an approximate result sufficiently accurate for
portrayal of the actual wave. The degree of accuracy desired in representing
faithfully the actual wave determines the number of terms that must be used in
any computation.
 �34s[Burtis L. Robertson ;Wilson S. Pritchett]
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�6v�2v�32sNonstoichiometric compounds �21m �33s
    Chemical compounds in which the relative number of atoms is not
expressible as the ratio of small whole numbers, hence compounds for which the
subscripts in the chemical formula are not rational (for example, Cu1.987S).
Sometimes they are called berthollide compounds to distinguish them from
daltonides, in which the ratio of atoms is generally simple. Nonstoichiometry
is a property of the solid state and arises because a fraction of the atoms of
a given kind may be (1) missing from the regular structure (for example,
Fe1-DELTAO), (2) present in excess over the requirements of the structure (for
example, Zn1+DELTAO), or (3) substituted by atoms of another kind (for
example, Bi2Te3PLUS/MINUSDELTA). The resulting materials are generally of
variable composition, intensely colored, metallic or semiconducting, and
different in chemical reactivity from the parent stoichiometric compounds from
which they are derived.
    Nonstoichiometry is best known in the binary compounds of the transition
elements, particularly the hydrides, oxides, chalcogenides, pnictides,
carbides, and borides. It is also well represented in the so-called insertion
or intercalation compounds, in which a metallic element or neutral molecule
has been inserted in a stoichiometric host. Nonstoichiometric compounds are
important in some solid-state devices (such as rectifiers, thermoelectric
generators, and photodetectors) and are probably formed as chemical
intermediates in many reactions involving solids (for example, heterogeneous
catalysis and metal corrosion).
    The simplest way to classify nonstoichiometric compounds is to consider
which element is in excess and how this excess is brought about. A
classification scheme largely based on this distinction but which also
includes some examples of ternary systems is as follows.
Binary compounds:
I. Metal: nonmetal ratio greater than stoichiometric
(a) Metal in excess, for example, Zn1+DELTAO
(b) Missing nonmetal, for example UH3-DELTA, WO3-DELTA
II. Metal: nonmetal ratio less than stoichiometric
(a) Metal-deficient, for example, Co1-DELTAO
(b) Nonmetal in excess, for example, UO2+DELTA
III. Deviations on both sides of stoichiometry, for example,
TiO1PLUS/MINUSDELTA
Ternary compounds (insertion compounds):
IV. Oxide ``bronzes,'' for example, MDELTAWO3, MDELTAV2O5
V. Intercalation
compounds, for example, K1.5+DELTAMoO3, LiDELTATiS2
Excluded from consideration are the recognized impurity materials, such as
Na1-2subxCasubxCl, which are best considered as conventional solid solutions
wherein ions of one kind and perhaps vacancies have replaced an equivalent
number of ions of another kind.
 �34s[Michell J. Sienko]
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�6v�2v�32sNormal (mathematics)   �21m �33s 
    A term generically synonymous with perpendicular, which often refers 
specifically to a line that goes through a point P of a curve C and is 
perpendicular to the tangent to C at P. If curve C is not a plane curve, all 
normal lines of C at point P on C lie in a plane, the normal plane of C at P.  
See  ANALYTIC GEOMETRY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sNorth America �21m �33s
    The northern of the two continents of the New World or Western Hemisphere,
extending from narrow parts in the tropics to progressively broader portions
in middle latitudes and Arctic polar margins. This protuberance of the Earth's
crustal shell stands with some 9.363 x 10(sup 6) mi(sup 2) (2.426 x 10(sup 7)
km(sup 2)) of land above the sea and ranks third in size among the continents.
The continental mass upstands with deep submarine continental scarps toward the
ocean basins at a marginal break or change in slope which appears at varying
shallow depths of continental submarine shelf. These shelf areas of shallow
sea bottom extend offshore for distances up to several hundred miles. Most of
this outline of North American physical geography emphasizes physiography,
especially land surface character, but it also includes some aspects of
continental shelves considered as coastal-zone character. This focus is
intended to present a basis for understanding other environmental and human
features with which the land character is commonly interrelated.  See
CONTINENT.

    Location.  North America, the continent with adjacent islands,
traditionally includes Greenland and the West Indies. The North American
continent extends from southeastern Panama (6DG6'N) at its isthmian frontier
with South America to the northern tip of the Boothia Peninsula (72DG10'N),
northeast of the magnetic pole. This makes an extreme north-south distance of
approximately 4554 mi (7327 km). If the Arctic islands are included, this
distance increases to 5272 mi (8483 km); the northern tip of Grant Land on
Ellesmere Island is at 82DG30'N. The maximum longitudinal spread is from
easternmost Greenland (18DGW) to the westernmost of the Aleutian Islands
(172DGE), a total of 170DG. Canada and the United States, exclusive of Alaska,
lie between longitude 52DG and 142DGW. Since the narrower portion of the
continent lies equatorward of 30DGN latitude, relatively little of its area is
in the tropics, whereas the greatest area occurs in mid-latitudes with
considerable portions extending into the subpolar and polar regions.  See
ARCTIC AND SUBARCTIC ISLANDS; WEST INDIES.

    Physiographic provinces.   The land surface is the base upon which all the
other geographical elements are placed. North America can be reasonably
divided into some six broad physiographic divisions, based on their general
geologic structure and subsequent landform development. These are, in turn,
further subdivided on the basis of more localized variations in topography.

    Canadian Shield.   The Canadian Shield, also known on the mainland as the
Laurentian Upland province, is the core around which the rest of North America
has been built. In general, this province occupies the northeast quarter of
North America, being defined by the St. Lawrence Gulf and River, the upper
Great Lakes, and the series of large lakes in central Canada extending north
from the United States border, including Winnipeg, Athabaska, Great Slave, and
Great Bear lakes. The two extensions of this region into the United States are
the Adirondack Mountains in northern New York State and the Superior Highland
circling the western end of Lake Superior.
    The Canadian Shield is composed of ancient crystalline rocks which have
been subjected to geological processes such as volcanism, folding, faulting,
warpage, and erosion. Extensive erosion over long periods has resulted in a
surface that has been somewhat smoothed and lowered in relief so it resembles
an undulating plain, except along the southern and eastern borders. Along the
Atlantic Coast there is an escarpment that has been serrated by the tongues of
mountain glaciers into fiords, making this area the most rugged section of the
province.  See	FIORD; GEOLOGICAL TIME SCALE.
    The Canadian Shield is one of the great mineralized areas of the world,
containing a variety of both ferrous and nonferrous ores. Detailed prospecting
and utilization of the minerals are generally limited to the outer border of
this province, especially along the south fringe where economical
transportation is provided.

    Appalachian Highlands.   A great variety of highlands and associated
lowlands in Eastern United States and southeastern Canada are considered as
composing the Appalachian Highlands, making it a very complex region. It is
generally divided into the New England - Maritime Canada province and the
Appalachian province, which in turn is further subdivided into older and newer
sections.
    Rocks ranging from ancient crystalline to Recent sediments are found in
the same geographic area in various sections of this highland region. The
crystalline rocks predominate in the northern and eastern portions, whereas
the northwest and west are characterized by sediments of Paleozoic time.
    Geologic structures of almost every type, which have been subjected to a
variety of erosional and depositional processes, are found somewhere in this
region. Doming, folding, faulting, and tilting, as well as volcanic
activities, contribute to the variety of surface features. Major uplifts
followed by long periods of erosion have left what might otherwise have been a
very high and rugged mountain area as a generally subdued highland with
maximum elevation only slightly exceeding 6000 ft (1800 m) in a few summits.
Even the peaks are smoothed and rounded by erosional forces. The
northeast-southwest trend of the province from Newfoundland to northern
Alabama appears in most of the individual structures.
    This highland area is rich in mineral resources which tend to be
localized. The newer Appalachians, as a whole, have the greatest bituminous
coal reserve in the world and are the site of the first developed petroleum
field in the United States. Elsewhere, minor coal deposits are found in Nova
Scotia; iron ores in Newfoundland, New England, New Jersey, and Alabama; lead,
zinc, and wolframite in New Jersey; asbestos in Quebec; limestone, granite,
marble, and slate in various localities; and gold in North Carolina.

    Southeastern coastal plain.   A distinctive coastal plain margins North
America along the coastal zone of the Atlantic Ocean and the Gulf of Mexico.
This plain varies in width but is nearly unbroken from Cape Cod and Long
Island to Yucatan in southeastern Mexico. In most parts, very flat seaward
margins of low-lying and most recent sedimentary deposits are followed inland
by a tendency for a coastwise banding of features. A landward increase in
elevation and erosional dissection also tends to contribute toward rolling and
irregular plains with local relief up to a very few hundreds of feet. A
gradual gentle uplift or upwarp of the geologically recent sedimentary rock
layers means that they are little disturbed from their original horizontal
attitude and dip slightly seaward and that such regions have most uplift
inland toward bordering backland areas of differing character. Erosion,
therefore, bares lower and somewhat older sedimentary layers progressively in
a landward direction. Landward-facing cuestas or bands of low hills may
develop on more-resistant rock exposures, whereas lower relief and occasional
flat intervales develop by differential erosion in exposures of less-resistant
rock layers. The farthest inland of these, commonly developed in Cretaceous
clays, are termed inner lowland. The landward coastal plain boundary is
commonly the inner margin of the Cretaceous clays, and the border between the
coastal plain layers and the oldland or backland rocks is known as the fall
line or fall zone.  See  ESCARPMENT; FALL LINE.
    A predominance of land subsidence marks the region from North Carolina to
Cape Cod with many embayments and produces shorelines irregular in contrast to
most other parts of recent upraisal or stillstand of the land relative to the
sea. A strongly marked variance from north to south in climatic
characteristics with an especially steep temperature gradient is reflected in
such aspects as natural vegetation and length of growing season and
agricultural production, as well as the pattern contrasts in winter resorts of
Florida and the Gulf Coast versus the definite winter influences of the New
York and New Jersey coastal zones.
    Within the essential unity of this great coastal plain form and structure,
six or seven subdivisions are useful in outlining the regional variations
characterizing its long extent: Northern embayed, Sea Island, Peninsular
Florida, East Gulf section, Mississippi alluvial plain, West Gulf, and Yucatan
section.

    Arctic margin plains.   Two plains units that are commonly considered
parts of the interior North American lowlands are the Arctic slope of Alaska
and the Mackenzie plains of Canada. They open out to Arctic Ocean shores from
which the polar ice pack recedes for an unpredictable duration during the late
part of the warm season. These plains are subject to arctic cold and long
hours of darkness during the low-sun period, and conversely to long hours of
daylight with short but surprisingly warm periods during the high sun. Both
are strongly marked in their lower parts by small lakes and ponds, polygonal
ground, and other features of permanently frozen ground. They are also subject
to the widespread flooding of lower and flatter parts which characterizes
subarctic regions, especially when the headwaters thaw earlier than the ice
breakup in the main streams to the northward.  See  PERMAFROST.

    Interior provinces.   Interior North America is so predominantly plains
land, although it contains a few highland parts, that other physical
attributes commonly contribute as much to regional differences as do structure
and surface form of the plains. The aspects of great area and huge distances
often cause the extent and character of climate and its closely associated
problems of natural vegetation and soil to make the greatest regional physical
contrasts in many parts of the plains. Recent (Pleistocene) glaciation north
of the Ohio and Missouri river valleys leaves an impress on patterns of
drainage, surface forms, and even soil materials quite different from areas
not recently affected by glaciers.
    The interior provinces include the interior low plateaus, the Ozark
province, the Ouachita province, the Central Lowland province, and the Great
Plains province.

    Cordilleran North America.	 A large proportion of western North America
is a great north-south highland of mountain masses, ranges, and systems and of
elevated basins and plateaus. With the Central American ranges and mountains
of somewhat different structure and axial orientation, the North American
Cordillera continues from those of Andean South America, the great mountain
rim of the Pacific. These Pacific-encircling mountains turn westward in Alaska
and swing toward the Asian shores of the Pacific in the great arcuate festoons
of the Aleutian, Kuril, and Japanese islands.
    The consequences in the physical and human geography of the continent are
pronounced but somewhat in contrast with those of South America. The great
orographic barrier made difficult an articulation of settlement and
development between the eastern regions and the western highlands and coastal
zones.
    Although bold in plan and extent, the Cordilleran lands are diverse in
component highland parts and commonly intricate in local details of highland
relief. One classification uses three or four levels of category to outline
the highland characteristics. The largest parts may be designated as three
systems: Rocky Mountain system, ranges extending in various parts on the
eastern margin of the Cordilleran region from New Mexico to Arctic shores of
Alaska; intermontane plateau system, westward of the Rockies and extending
even further (from southern Mexico to Alaska); and Pacific mountain system,
from the Aleutians to peninsular Lower California.

    Central America.   This narrowest part of the North American mainland has
predominantly mountainous landforms, but it is geologically and structurally
different from the great ranges, basins, and plateaus of western Cordilleran
North America. It is perhaps best considered as a composite of at least five
rather separate and different parts, more closely related to the Antillean
mountain system.
 �34s[Charles V. Crittenden]
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�6v�2v�32sNorth Pole   �21m �33s 
    That end of the Earth's axis which points toward the North Star, Polaris 
(Alpha Ursae Minoris). It is the geographical pole where all meridians 
converge, and should not be confused with the north magnetic pole, which is in 
the Canadian Archipelago. The North Pole's location falls near the center of 
the Arctic Sea. 
    The North Pole has phenomena unlike any other place except the South Pole. 
For 6 months the Sun does not appear above the horizon, and for 6 months it 
does not go below the horizon. As there is a long period (about 7 weeks) of 
continuous twilight before March 21 and after September 23, the period of 
light is considerably longer than the period of darkness. 
 �34s[Van H. English]
�33s 
  
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�6v�2v�32sNorth Sea   �21m �33s 
    A flooded portion of the northwest continental margin of Europe occupying 
an area of over 190,000 misup2 (500,000 kmsup2). The North Sea is the scene of 
extensive marine fisheries and important offshore oil and gas. In the south, 
water is shallower than 160 ft (50 m), but north of 58DGN it deepens gradually 
to 650 ft (200 m) at the top of the continental slope. A band of deep water of 
up to 1300 ft (400 m) extends around the south and west coast of Norway and is 
known as the Norwegian Trench. 
    The nontidal residual current circulation of the southern North Sea is 
mainly determined by wind velocity, but in the north, well-defined 
non-wind-driven currents have been identified, especially in summer. Two of 
these currents bring in water from outside the North Sea; one flows through 
the channel between Orkney and Shetland (the Fair Isle current), and the other 
follows the continental slope north of Shetland and merges with the Fair Isle 
current southwest of Norway before entering the Skagerrak. 
    In some areas of the North Sea, especially in the shallow parts of the 
south, and also within parts of the permanent inflowing currents, 
phytoplankton production persists throughout the summer, due to the continuous 
mixing of nutrients into the surface waters. There is a rich diversity of 
zooplankton within the North Sea. Copepods are of particular importance in the 
food web. There are many commercially important fish stocks, notably herring, 
mackerel, haddock, cod, whiting, saithe, and plaice. 
 �34s[H. D. Dooley]
�33s 
  
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�6v�2v�32sNorthwest Passage   �21m �33s 
    The northern sea route between the Atlantic and Pacific oceans through the 
Canadian Archipelago. The entire route is frozen over in winter. As the ice 
cap retreats northward in summer, two routes are opened. The eastern approach 
through Davis and Hudson straits is navigable in June, at which time the 
partially melted ice floes drift into the Atlantic Ocean on the Labrador 
Current. This approach remains open until November. 
    In the Canadian Archipelago the ice is landlocked. These passages remain 
closed until the ice melts. The tortuous southern route, close along the 
Canadian mainland coast, is usually passable from mid-August to mid-October. 
The more direct northern route, through Lancaster Sound and the Barrow and 
McClure straits, has never been found entirely ice-free. This passage has only 
been made by Wind-class icebreakers.  See  ARCTIC AND SUBARCTIC ISLANDS; 
BERING SEA; SEA ICE. 
 �34s[John P. Tully]
�33s 
  
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�6v�2v�32sNose   �21m �33s 
    The nasal cavities and the structures surrounding and associated with 
them. The nose functions primarily as the organ of smell and in most tetrapods 
also assumes a respiratory function, forming the anterior end of the air 
passage through which air is drawn in and in which it is warmed and moistened. 
    In humans the nasal cavities are triangular openings that pass from the 
external nares back to the dorsal part of the pharynx (see illustration). The 
lateral walls are composed principally of portions of the ethmoid and sphenoid 
bones and projections of three turbinate bones, or conchae, on each side. The 
floor of the nose is formed by the palate, which is also the roof of the 
mouth. The nasal cavities are lined with respiratory epithelium, which also 
lines the paranasal sinuses. The latter are cavities in the frontal, ethmoid, 
sphenoid, and maxillary bones which communicate with the nasal passages. The 
external nose consists of the two nasal bones that form the bony bridge and 
two pairs of lower nasal cartilages. These together with the tightly adherent 
skin determine the individual shape and size of the human nose.  See  
OLFACTION.  
 �34s[Thomas S. Parsons]
�33s 
  
  
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�6v�2v�32sNose cone   �21m �33s 
    The forward portion of a spacecraft that is designed for atmospheric 
entry. Nose cones are utilized for intercontinental ballistic missles and 
spacecraft such as Apollo and space shuttles. The nose cone is required to 
withstand heating encountered during atmospheric entry, maintain the 
structural integrity of the spacecraft, prevent overheating of the payload, 
and usually maintain the aerodynamic characteristics of the spacecraft. 
    Even for a properly designed shape, it is inevitable that some fraction of 
the spacecraft's initial kinetic energy will finally reach the nose cone in 
the form of heat. The design of the heat shield for the nose cone is a complex 
procedure, which is highly dependent on the heating level. There are a variety 
of surface-protection or cooling systems which have been used. Generally these 
systems consist of heat sinks of various types: the absorption of heat by 
virtue of a material's sensible heat capacity, latent heat capacity, or 
chemical heat capacity. That is, heat absorption is accomplished by a 
temperature rise, a phase change, or a chemical reaction. Aerodynamic lift is 
employed by such vehicles as reusable space shuttles to lower heating rates so 
that the nose cone material can radiate away much of the incident heating. 
    Ablation is used to provide surface protection. The designer can divert 
heat from the spacecraft by allowing the nose cone's outer layer of material 
to melt, vaporize, or sublime. While large ablation rates provide excellent 
thermal protection, the resulting change in profile due to surface recession 
can adversely change the aerodynamic characteristics of the spacecraft. The 
designer must account for this change.  See  SPACE SHUTTLE; SPACECRAFT 
STRUCTURE.  
 �34s[Philip R. Nachtsheim]
�33s 
  
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�6v�2v�32sNotacanthiformes   �21m �33s 
    An order of actinopterygian fishes known also as the Heteromi (spiny eels 
or notacanths) and the Lyopomi (halosaurs). The body is elongated, tapers 
posteriorly, and has no caudal fin (see illustration). 
    This small order, which has a history extending back to the Upper 
Cretaceous, includes 3 families, about 8 Recent genera, and about 25 species. 
Spiny eels and halosaurs inhabit deep seas of all oceans; some have 
photophores. They are like true eels (Anguilliformes) in that they lack a firm 
suspension of the pectoral girdle from the skull, but some have fin spines 
like the perciform fishes. See ACTINOPTERYGII; ANGUILLIFORMES; PHOTOPHORE 
GLAND.  
 �34s[Reeve M. Bailey]
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�6v�2v�32sNotomyotina   �21m �33s 
    A suborder of Phanerozonida in which the upper marginals alternate in 
position with the lower marginals to impart a degree of flexibility to the 
arm, and each of the tube feet has a terminal sucking disk. Paxillae are 
present on the upper surface. Each arm usually contains a pair of dorsal 
muscles, whose contraction enables the arm to be turned upward over the disk. 
These are mainly deep-water forms.  See  PHANEROZONIDA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sNotostraca   �21m �33s 
    An order of crustaceans of moderate size (20 - 90 mm) generally referred 
to the Branchiopoda. They are called the tadpole shrimps. The cylindrical 
trunk consists of 25 - 44 body segments, and the number varies slightly within 
a species. The first 11 somites each bear one pair of legs. Behind these are a 
varying number of segments with an increasing number of legs; a varying number 
of legless segments; and finally a telson with two narrow, cylindrical, caudal 
filaments (cercopods). The dorsal shield is rounded and flattened 
dorsoventrally, and usually covers only part of the animal. The animals feed 
mainly on detritus, and the food is moved to the mouth in a ventral groove. 
The young hatch in the nauplius larval stage. 
    The group contains only two genera, Triops, or Apus, and Lepidurus (see 
illustration), and about a dozen species. They live in nonpermanent waters, 
sometimes occurring in great numbers, and are distributed all over the world.  
See  BRANCHIOPODA. 
 �34s[Folke Linder]
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�6v�2v�32sNotoungulata   �21m �33s 
    An order of dominant, hoofed herbivores of the Cenozoic of South America 
that are abundantly represented in Paleocene through Pleistocene nonmarine 
sedimentary rocks of that continent. Diverging from a primitive condylarth 
ancestry at an early date, they radiated into a wide diversity of forms, some 
of which were convergent with Northern Hemisphere ungulates. 
    Notoungulates were characterized by a skull with an expanded temporal 
region due to the presence of a large sinus in the squamosal and no 
postorbital bar. The feet were primitive, with five toes (three or two in some 
advanced forms), and the weight was borne mainly by the third digit.  See  
EUTHERIA.  
 �34s[Richard H. Tedford]
�33s 
  
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�6v�2v�32sNova �21m �33s
    The brightening of a seemingly undistinguished star within a few days to
thousands or even hundreds of thousands of times its previous luminosity.
These events, which occur about 10 times a year in a large spiral galaxy like
the Milky Way, are the classical novae (short for novae stellae, or new stars,
so called because the pre-flare-up stars are too faint to have been seen in
the days before telescopes). A much rarer and still more violent class of
stellar flare-up is given the name supernova.  See  SUPERNOVA.
    The classical novae are defined and distinguished from other kinds of
explosive stellar events by the following observed properties: (1) brightening
by more than a factor of 4000 in a few days; (2) noticeable fading in less
than a few years (more often weeks to months); (3) a complex, changing
spectrum, showing emission lines of common gases at high temperature; (4)
ejection of gas at speeds of 100 to 5000 km/s (60 to 3000 mi/s); and (5) no
historical record of a similar previous event involving that particular star.
    The single most important step in understanding nova explosions occurred
in 1964, when Robert Kraft showed that most previously observed novae had
occurred in close (that is, stars not much farther apart than their own sizes)
binary systems, containing one hot, compact star (like a white dwarf) and one
cooler, more extended star (like a main sequence or subgiant star). The
systems typically have large amounts of stray gas and masses near 1 solar mass
(2 x 10(sup 30) kg).  See  BINARY STAR; STAR; WHITE DWARF STAR.
 �34s[Virginia Trimble]
�33s

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�6v�2v�32sNovobiocin �21m �33s
    A moderately broad-spectrum acid antibiotic produced by strains of
Streptomyces niveus and S. spheroides. It is a dibasic acid with the structure
below.
    The antibiotic is relatively nontoxic unless it is given for more than 1
week. The most frequent side effect is a skin rash which may subside with
continued therapy. The rash may or may not recur upon subsequent dosage.
Novobiocin is highly active against staphylococci and certain strains of
Proteus. It is also active against pneumococci, streptococci, Brucella, and
certain strains of Escherichia, Aerobacter, and Pseudomonas.  See  ANTIBIOTIC.

		      ***CHEMICAL STRUCTURE***



 �34s[George M. Savage]
�33s



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�6v�2v�32sNozzle   �21m �33s 
    A projecting opening that directs the flow of fluid into an open space. 
Some nozzles maintain the fluid in a jet; an example is the needle nozzle that 
directs water against the buckets of an impulse turbine. Other nozzles 
disperse the fluid in an atomized mist; an example is the cone nozzle that 
sprays liquid fuel into a combustion chamber. The nozzle may be an integral 
part of a machine, as the nozzle in a steam turbine, or it may be a separate 
interchangeable piece as on a fire truck. 
    As used in a wind tunnel, a nozzle increases fluid velocity but with the 
added requirement that the higher-velocity stream be uniform and parallel. 
Physically the nozzle consists of a contracting section. If the final fluid 
velocity is to be supersonic, a divergent portion downstream of the 
contraction is also required (see illustration). The region at the minimum 
section is called the throat. 
 �34s[Fred D. Kochendorfer]
�33s 
  
  
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�6v�2v�32sNuclear battery �21m �33s
    A battery that converts the energy of particles emitted from atomic nuclei
into electric energy. Two basic types have been developed: (1) A high-voltage
type, in which a beta-emitting isotope is separated from a collecting
electrode by a vacuum or a solid dielectric, provides thousands of volts but
the current is measured in picoamperes (pA); (2) a low-voltage type gives
about 1 volt with current in microamperes (MUA).
    In the high-voltage type, a radioactive source is attached to one
electrode, emitting charged particles. The source might be strontium-90,
krypton-85, or hydrogen-3 (tritium), all of which are pure beta emitters. An
adjacent electrode collects the emitted particles. A vacuum or solid
dielectric separates the source and the collector electrodes. The principal
use of the high-voltage battery is to maintain the voltage of a charged
capacitor. The current output of the radioactive source is sufficient for this
purpose.
    Three different concepts have been employed in the low-voltage type of
nuclear batteries: (1) a thermopile, (2) the use of an ionized gas between two
dissimilar metals, and (3) the two-step conversion of beta energy into light
by a phosphor and the conversion of light into electric energy by a photocell.
See  BATTERY.
 �34s[Jack Davis ;L. Rozeanu ;Kenneth Franzese]
�33s

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�6v�2v�32sNuclear binding energy �21m �33s
    The amount by which the mass of an atom is less than the sum of the masses
of its constituent protons, neutrons, and electrons expressed in units of
energy. This energy difference accounts for the stability of the atom. In
principle, the binding energy is the amount of energy which was released when
the several atomic constituents came together to form the atom. Most of the
binding energy is associated with the nuclear constituents (protons and
neutrons), or nucleons, and it is customary to regard this quantity as a
measure of the stability of the nucleus alone.	See  NUCLEAR STRUCTURE.
    A widely used term, the binding energy (BE) per nucleon, is defined by the
equation below, where (sub Z)M(sup A) represents the mass of an atom of mass
number A and atomic number Z, H and n are the masses of the hydrogen atom and

	  BE/nucleon = [ZH + (A - Z)n - (sub Z)M(sup A)]c (sup 2) / A

neutron, respectively, and c is the velocity of light. The binding energies of
the orbital electrons, here practically neglected, are not only small, but
increase with Z in a gradual manner; thus the BE/nucleon gives an accurate
picture of the variations and trends in nuclear stability.
    The binding energy, when expressed in mass units, is known as the mass
defect, a term sometimes incorrectly applied to quantity M - A, where M is the
mass of the atom.
 �34s[Henry E. Duckworth]
�33s
    The term binding energy is sometimes also used to describe the energy
which must be supplied to a nucleus in order to remove a specified particle to
infinity, for example, a neutron, proton, or ALPHA-particle. A more
appropriate term for this energy is the separation energy. This quantity
varies greatly from nucleus to nucleus and from particle to particle.
 �34s[Denys H. Wilkinson]
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�6v�2v�32sNuclear chemistry   �21m �33s 
    An interdisciplinary field that, in general, encompasses the application 
of chemical techniques to the solution of problems in nuclear physics. The 
discovery of the naturally occurring radioactive elements and of nuclear 
fission are classical examples of the work of nuclear chemists. 
    Although chemical techniques that are employed in nuclear chemistry are 
essentially the same as those in radiochemistry, these fields may be 
distinguished on the basis of the aims of the investigation. Thus, a nuclear 
chemist utilizes chemical techniques as a tool for the study of nuclear 
reactions and properties, whereas a radiochemist utilizes the radioactive 
properties of certain substances as a tool for the study of chemical reactions 
and properties. For the application of radioactive tracers to chemical 
problems see RADIOCHEMISTRY. For the chemical effects of radiation on various 
systems see RADIATION CHEMISTRY. 
    The chemical identification of radioactive nuclides and the determination 
of their nuclear properties has been one of the major activities of nuclear 
chemists. Such studies have produced an extensive array of radioisotopes, and 
present studies are concerned mainly with the more difficult identification of 
nuclides of very short half-life. Nuclear chemical investigations led to the 
discovery of the synthetic radioactive elements which do not have any stable 
isotopes and are not formed in the natural radioactive series (technetium, 
promethium, astatine, and the transuranium elements). Other major areas of 
nuclear chemistry include studies of nuclear structure and spectroscopy and of 
the probability and mechanisms of various nuclear reactions.  See  NUCLEAR 
FISSION; NUCLEAR REACTION; NUCLEAR STRUCTURE. 
 �34s[Ellis P. Steinberg]
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�6v�2v�32sNuclear engineering   �21m �33s 
    That branch of engineering that deals with the production and use of 
nuclear energy. It is concerned with the development, design, construction, 
and operation of power plants which convert energy produced by fission or 
fusion to other useful forms such as heat or electrical energy. Development of 
these unique sources of energy requires novel solutions to difficult 
mechanical, electrical, and materials problems. Because many of the components 
and systems operate in the presence of intense high-energy radiation, special 
problems that are generated by the interaction of radiation with various 
materials are encountered. Such problems are unique to nuclear engineering. 
Training of nuclear engineers places special emphasis on this area.  See  
DECONTAMINATION OF RADIOACTIVE MATERIALS; HEALTH PHYSICS; MONITORING OF 
IONIZING RADIATION; NONDESTRUCTIVE TESTING; NUCLEAR FISSION; NUCLEAR FUEL 
CYCLE; NUCLEAR FUELS; NUCLEAR FUELS  REPROCESSING; NUCLEAR FUSION; NUCLEAR 
POWER; NUCLEAR  REACTOR; RADIATION DAMAGE TO MATERIALS; RADIATION SHIELDING; 
RADIOACTIVE WASTE MANAGEMENT; REACTOR PHYSICS. 
 �34s[William Kerr]
�33s 
  
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�6v�2v�32sNuclear explosion   �21m �33s 
    An explosion for which the energy is produced by a nuclear transformation 
(either fission or fusion). 
    Although the terms atomic explosion and atomic weapon are occasionally 
used, the energy is derived in processes that involve the nuclear forces of 
matter rather than the atomic forces; hence, nuclear explosion and nuclear 
weapon are more appropriate designations. The energy released in a nuclear 
explosion is usually stated in terms of the equivalent energy release of a 
given mass of TNT using convenient units such as kiloton (KT) or megaton (MT). 
    The early nuclear explosions - such as the Alamogordo event (Trinity) or 
the Hiroshima, Japan, bomb - were about 20 KT. Tactical fission weapons in the 
sub-KT range have been announced by the United States. Fusion weapons are 
sometimes called hydrogen bombs or thermonuclear weapons, terms that describe 
their constituents and explosive mechanisms. 
    The prompt energy release of a nuclear explosion vaporizes the 
constituents and products, as well as the bomb container. These extremely hot 
gases are also at extremely high pressure and, following the explosion, exert 
enormous forces on their surrounding medium, such as air, earth, or water, 
thus initiating a complex series of effects which depend upon the surrounding 
medium. Four types of bursts are distinguished: (1) air, (2) underground, (3) 
underwater, and (4) very high altitude (exoatmospheric).  See  ATOMIC BOMB; 
HYDROGEN BOMB; NUCLEAR FISSION; NUCLEAR FUSION; RADIOACTIVE FALLOUT. 
 �34s[Harold Brown]
�33s 
  
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�6v�2v�32sNuclear fission �21m �33s
    An extremely complex nuclear reaction representing a cataclysmic division
of an atomic nucleus into two nuclei of comparable mass. This rearrangement or
division of a heavy nucleus may take place naturally (spontaneous fission) or
under bombardment with neutrons, charged particles, gamma rays, or other
carriers of energy (induced fission). Although nuclei with mass number A of
approximately 100 or greater are energetically unstable against division into
two lighter nuclei, the fission process has a small probability of occurring,
except with the very heavy elements. Even for these elements, in which the
energy release is of the order of 200,000,000 electronvolts (eV), the
lifetimes against spontaneous fission are reasonably long.  See  NUCLEAR
REACTION.

    Liquid-drop model.	 The stability of a nucleus against fission is most
readily interpreted when the nucleus is viewed as being analogous to an
incompressible and charged liquid drop with a surface tension. Long-range
Coulomb forces between protons act to disrupt the nucleus, whereas short-range
nuclear forces, idealized as a surface tension, act to stabilize it. The
degree of stability is then the result of a delicate balance between the
relatively weak electromagnetic forces and the strong nuclear forces. Although
each of these forces results in potentials of several hundred million
electronvolts, the height of a typical barrier against fission for a heavy
nucleus, because they are of opposite sign but do not quite cancel, is only
5,000,000 or 6,000,000 eV. Investigators have used this charged liquid-drop
model with great success in describing the general features of nuclear fission
and also in reproducing the total nuclear binding energies.  See  NUCLEAR
BINDING ENERGY; NUCLEAR STRUCTURE; SURFACE TENSION.


    Shell corrections.	The general dependence of the potential energy on the
fission coordinate representing nuclear elongation or deformation for a heavy
nucleus such as sup2sup4sup0Pu is shown in Fig. 1. The expanded scale used in
this figure shows the large decrease in energy of about 200 MeV as the
fragments separate to infinity. It is known that (sup 240)Pu is deformed in
its ground state, which is represented by the lowest minimum of -1813 MeV near
zero deformation. This energy represents the total nuclear binding energy when
the zero of potential energy is the energy of the individual nucleons at a
separation of infinity. The second minimum to the right of zero deformation
illustrates structure introduced in the fission barrier by shell corrections,
that is, corrections dependent upon microscopic behavior of the individual
nucleons, to the liquid-drop mass. Although shell corrections introduce small
wiggles in the potential-energy surface as a function of deformation, the
gross features of the surface are reproduced by the liquid-drop model. Since
the typical fission barrier is only a few million electronvolts, the magnitude
of the shell correction need only be small for irregularities to be introduced
into the barrier. This structure is schematically illustrated for a heavy
nucleus by the double-humped fission barrier in Fig. 2, which represents the
region to the right of zero deformation in Fig. 1 on an expanded scale. The
fission barrier has two maxima and a rather deep minimum in between. For
comparison, the single-humped liquid-drop barrier is also schematically
illustrated. The transition in the shape of the nucleus as a function of
deformation is schematically represented in the upper part of the figure.

    Experimental consequences.	 The observable consequences of the
double-humped barrier have been reported in numerous experimental studies. In
the actinide region more than 30 spontaneously fissionable isomers have been
discovered between uranium and berkelium, with half-lives ranging from
10(sup -11) to 10(sup -2)s. These decay rates are faster by 20 to 30 orders of
magnitude than the fission half-lives of the ground states, because of the
increased barrier tunneling probability (see Fig. 2). Several cases in which
excited states in the second minimum decay by fission are also known. Normally
these states decay within the well by gamma decay; however, if there is a
hindrance in gamma decay due to spin, the state (known as a spin isomer) may
undergo fission instead.


    Fission probability. The cross section for particle-induced fission
Sigma(y,f) represents the cross section for a projectile y to react with a
nucleus and produce fission, as shown by the equation below. The quantities
Sigma (sub R) (y), GAMMA (sub f), and GAMMA (sub t) are the total

	Sigma(y,f) = Sigma (sub R) (y) (GAMMA (sub f)/GAMMA (sub t)


reaction cross sections for the incident particle y, the fission width, and
the total level width, respectively where GAMMA sub t = GAMMA sub f +
GAMMA sub n +  GAMMA sub y + . . . is the sum of all partial-level widths.
All the quantities in the above equation are energy-dependent.
    When the incoming neutron has low energy, the likelihood of reaction is
substantial only when the energy of the neutron is such as to form a compound
nucleus in one or another of its resonance levels. The requisite sharpness of
the ``tuning'' of the energy is specified by the total level width GAMMA. The
nuclei (sup 233)U, (sup 235)U, and (sup 239)Pu have a very large
cross section to take up a slow neutron and undergo fission because both their
absorption cross section and their probability for decay by fission are large.
The probability for fission decay is high because the binding energy of the
incident neutron is sufficient to raise the energy of the compound nucleus
above the fission barrier. The very large, slow neutron fission cross sections
of these isotopes make them important fissile materials in a chain reactor.
See  CHAIN REACTION (PHYSICS); REACTOR PHYSICS.

    Postscission phenomena.   After the nuclear fragments are separated, they
are further accelerated as the result of the large Coulomb repulsion. The
initially deformed fragments collapse to their equilibrium shapes, and the
excited primary fragments lose energy by evaporating neutrons. After neutron
emission, the fragments lose the remainder of their energy by gamma radiation,
with a lifetime of about 10(sup -11)s. The variation of neutron yield with
fragment mass is directly related to the fragment excitation energy. Minimum
neutron yields are observed for nuclei near closed shells because of the
resistance to deformation of nuclei with closed shells. Maximum neutron yields
occur for fragments that are ``soft'' toward nuclear deformation.
    After the emission of the prompt neutrons and gamma rays, the resulting
fission products are unstable against BETA-decay. For example, in the case of
thermal neutron fission of (sup 235)U, each fragment undergoes on the
average about three BETA-decays before it settles down to a stable nucleus.
For selected fission products (for example, (sup 87)Br and (sup 137)I)
BETA-decay leaves the daughter nucleus with excitation energy exceeding its
neutron binding energy. The resulting delayed neutrons amount, for thermal
neutron fission of (sup 235)U, to about 0.7% of all the neutrons given off
in fission. Though small in number, they are quite important in stabilizing
nuclear chain reactions against sudden minor fluctuations in reactivity.  See
DELAYED NEUTRON; NEUTRON; THERMAL NEUTRONS.
 �34s[John R. Huizenga]
�33s

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�6v�2v�32sNuclear fuel cycle �21m �33s
    The steps by which fissionable (for example (sup 233)U, (sup 235)U,
(sup 239)Pu) and fertile (for example, (sup 138)U, (sup 232)Th)
materials are prepared for use in, and recycled or discarded after discharge
from, the nuclear reactor. These steps include mining of uranium- or
thorium-bearing ore and milling of the ore to form concentrates. The uranium
concentrate is converted to the volatile uranium hexafluoride (UF6) that is
used in the separation of isotopes to produce uranium enriched in the fissile
(sup 235)U. Another part of the fuel cycle is the fabrication of the
enriched uranium into fuel assemblies. After the fuel has liberated the
desired amount of heat in the reactor, the spent assemblies are reprocessed to
separate the remaining fissionable and fertile material (uranium and
plutonium) from the nuclear wastes. Other steps in the fuel cycle include the
various transportation operations that move materials from one step to
another, often connecting plants many hundreds of miles apart. Finally, waste
management includes the treatment, storage, and disposal of radioactive wastes
from the many other parts of the fuel cycle. The fuel cycle for a
thorium-based reactor requires, in addition to the uranium fuel cycle steps
outlined above, the mining, milling, and purification of thorium and the
reprocessing of thorium-containing fuel into its components, which include
unused thorium, uranium in the form of fissionable sup2sup3sup3U, and nuclear
waste.	See  NUCLEAR FUELS REPROCESSING; NUCLEAR REACTOR; RADIOACTIVE WASTE
MANAGEMENT; THORIUM; URANIUM.
 �34s[Martin J. Steindler]
�33s

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�6v�2v�32sNuclear fuels �21m �33s
    The fissionable and fertile elements and isotopes used as the sources of
energy in nuclear reactors. Although many heavy elements can be made to
fission by bombardment with high-energy alpha particles, protons, deuterons,
or neutrons, only neutrons can provide a self-sustaining reaction.
    The number of neutrons released in the fission process varies from one per
many fissions for elements just beyond the fission point (silver) to two or
more per fission for the heavier elements, such as thorium and uranium. Even
in such elements, neutron capture by the nucleus accompanied by the release of
excess energy in the form of a gamma ray occurs in many cases, rather than
nuclear fission. This reduces the number of neutrons available for further
fission. The ratio of neutron capture to neutron fission varies from nucleus
to nucleus and changes with the energy of the bombarding neutrons. Only a few
isotopes of the heavy elements have a higher probability of fission than
capture. These fissionable isotopes, (sup 233)U, (sup 235)U, and
(sup 239)Pu, are the only materials that can sustain the fission reaction
and are therefore called nuclear fuels.  See  NUCLEAR REACTOR; THORIUM;
URANIUM.
 �34s[Ronald L. Beatty]
�33s

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�6v�2v�32sNuclear fuels reprocessing   �21m �33s 
    The treatment of spent reactor fuel elements to recover fissionable and 
fertile material. Spent fuel is usually discharged from reactors because of 
chemical, physical, and nuclear changes that make the fuel no longer efficient 
for the production of heat, rather than because of the complete depletion of 
fissionable material. Therefore, discharged fuel usually contains fissionable 
material in sufficient amounts to make its recovery attractive. In the case of 
breeder reactors, in order to take advantage of the characteristic of the 
breeder reactor to produce more fissionable material than is used, 
reprocessing of fuel must be done to recover the fissile material bred into 
part of the fuel. If fertile material is also contained in the fuel, it is 
ordinarily recovered and purified during fuel reprocessing. Purification of 
the valuable constituents consists of the removal of fission products and 
extraneous structural material present in the fuel.  See  NUCLEAR FUELS; 
NUCLEAR REACTOR. 
    There are several basic steps involved in fuel reprocessing. After fuel 
has been discharged from a nuclear reactor, it is common practice to store the 
fuel submerged in 15 - 20 ft (4.6 - 6.1 m) of water (for cooling and 
radiation-shielding purposes) for a period of 50 - 200 days; this allows the 
short-lived fission products to decay radioactively. Following the cooling 
period, the fuel is mechanically cut or disassembled into convenient sizes. 
    The specific steps next undertaken depend upon the particular reprocessing 
method employed to separate the desired products from each other, from fission 
products, and from extraneous structural materials. Although many separation 
methods exist, the one based upon solvent extraction principles is most 
frequently used for fuel reprocessing. 
    The operating experience to date with fuel-reprocessing plants has shown 
them to be relatively safe in spite of hazards from radiation and nuclear 
criticality, as well as other hazards of a more conventional nature.  See  
HEALTH PHYSICS; RADIOACTIVE WASTE MANAGEMENT. 
 �34s[Martin J. Steindler]
�33s 
  
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�6v�2v�32sNuclear fusion �21m �33s
    One of the primary nuclear reactions, the name usually designating an
energy-releasing rearrangement collision which can occur between various
isotopes of low atomic number.	See  NUCLEAR REACTION.
    Interest in the nuclear fusion reaction arises from the expectation that
it may someday be used to produce useful power, from its role in energy
generation in stars, and from its use in the fusion bomb. Since a primary
fusion fuel, deuterium, occurs naturally and is therefore obtainable in
virtually inexhaustible supply, solution of the fusion power problem would
permanently solve the problem of the present rapid depletion of chemically
valuable fossil fuels. As a power source, the lack of radioactive waste
products from the fusion reaction is another argument in its favor as opposed
to the fission of uranium.  See  HYDROGEN BOMB; NUCLEAR FISSION.
    In a nuclear fusion reaction the close collision of two energy-rich nuclei
results in a mutual rearrangement of their nucleons (protons and neutrons) to
produce two or more reaction products, together with a release of energy. The
energy usually appears in the form of kinetic energy of the reaction products,
although when energetically allowed, part may be taken up as energy of an
excited state of a product nucleus. In contrast to neutron-produced nuclear
reactions, colliding nuclei, because they are positively charged, require a
substantial initial relative kinetic energy to overcome their mutual
electrostatic repulsion so that reaction can occur. This required relative
energy increases with the nuclear charge Z, so that reactions between low-Z
nuclei are the easiest to produce. The best known of these are the reactions
between the heavy isotopes of hydrogen, deuterium, and tritium.  See DEUTERIUM;
TRITIUM. 
    Nuclear fusion reactions can be self-sustaining if they are carried out at
a very high temperature. That is to say, if the fusion fuel exists in the form
of a very hot ionized gas of stripped nuclei and free electrons termed a
plasma, the agitation energy of the nuclei can overcome their mutual
repulsion, causing reactions to occur. This is the mechanism of energy
generation in the stars and in the fusion bomb. It is also the method
envisaged for the controlled generation of fusion energy.  See	PLASMA PHYSICS.
    The cross sections (effective collisional areas) for many of the simple
nuclear fusion reactions have been measured with high precision. It is found
that the cross sections generally show broad maxima as a function of energy
and have peak values in the general range of 0.01 barn (1 barn = 10[sup -24]
cm[sup 2]) to a maximum value of 5 barns, for the deuterium-tritium (D-T)
reaction. The energy releases of these reactions can be readily calculated
from the mass differences between the initial and final nuclei or determined
by direct measurement.
    Some of the important simple fusion reactions, their reaction products,
and their energy releases in millions of electronvolts (MeV) are given by the
reactions below.

	D	   + D --> He(sup 3) + n +  3.25 MeV
	D	   + D --> T	     + p +  4.0  MeV
	T	   + D --> He(sup 4) + n + 17.6  MeV
	He(sup 3)  + D --> He(sup 4) + p + 18.3  MeV
	Li(sup 6)  + D --> 2He(sup 4)	 + 22.4  MeV
	Li(sup 7)  + p --> 2He(sup 4)	 + 17.3  MEV

    If it is remembered that the energy release in the chemical reaction in
which hydrogen and oxygen combine to produce a water molecule is about 1 eV
per reaction, it will be seen that, gram for gram, fusion fuel releases more
than 1,000,000 times as much energy as typical chemical fuels.
 �34s[Richard F. Post]
�33s

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�6v�2v�32sNuclear isomerism   �21m �33s 
    The existence of excited states of atomic nuclei with unusually long 
lifetimes. If the lifetime of a specific excited state is unusually long, 
compared with the lifetimes of other excited states in the same nucleus, the 
state is said to be isomeric. The definition of the boundary between isomeric 
and normal decays is arbitrary, and the term is therefore used loosely.  See  
EXCITED STATE; PARITY (QUANTUM MECHANICS); SPIN (QUANTUM MECHANICS). 
    The predominant decay mode of excited nuclear states is by GAMMA-ray 
emission. The rate at which this process occurs is determined largely by the 
spins, parities, and excitation energies of the decaying state and of those to 
which it is decaying. In particular, the rate is extremely sensitive to the 
difference in the spins of initial and final states and to the difference in 
excitation energies. Both extremely large spin differences and extremely small 
energy differences can result in a slowing of the GAMMA-ray emission by many 
orders of magnitude, resulting in some excited states having unusually long 
lifetimes and therefore being termed isomeric. 
    In addition to spin isomers, two other types of isomers have been 
identified. The first of these arises from the fact that some excited nuclear 
states represent a drastic change in shape of the nucleus from the shape of 
the ground state. In many cases this extremely deformed shape displays unusual 
stability, and states with this shape are therefore isomeric. A particularly 
important class of these shape isomers is observed in the decay of heavy 
nuclei by fission, and the study of such fission isomers has been the subject 
of intensive effort.  See  NUCLEAR FISSION. 
    A more esoteric form of isomer has also been observed, the so-called 
pairing isomer which results from differences in the microscopic motions of 
the constituent nucleons in the nucleus. A state of this type has a quite 
different character from the ground state of the nucleus, and is therefore 
also termed isomeric. See NUCLEAR STRUCTURE. 
 �34s[Russell Betts]
�33s 
  
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�6v�2v�32sNuclear laser �21m �33s
    A device in which laser action in a gas is excited by the fast-moving ions
produced in nuclear fusion. These fusion products excite and ionize the gas
atoms, and make it possible to convert directly from nuclear to optical
energy.  See  LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
�33s

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�6v�2v�32sNuclear magnetic resonance flowmeter �21m �33s
    An instrument that uses the phenomenon of nuclear magnetic resonance to
measure the velocity of fluid flow. Nuclei of the flowing fluid are resonated
by a radio-frequency field superimposed on an intense permanent magnetic
field. A detector downstream measures the amount of decay of the resonance and
thereby senses the velocity of the fluid. The most effective fluids are
hydrocarbons, fluorocarbons, and water-bearing liquids because this resonance
is most pronounced in fluorine and hydrogen nuclei.  See  FLOW MEASUREMENT;
NUCLEAR MAGNETIC RESONANCE (NMR).
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

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�6v�2v�32sNuclear magnetic resonance (NMR)   �21m �33s 
    A phenomenon exhibited by a large number of atomic nuclei which is based 
upon the existence of nuclear magnetic moments which are associated with 
quantized nuclear spins. These nuclear moments, when placed in a magnetic 
field, give rise to distinct nuclear Zeeman energy levels between which 
spectroscopic transitions can be induced by radio-frequency radiation. Plots 
of these transition frequencies, called spectra, furnish important information 
about molecular structure and sample composition. 
    Spinning nuclei with nonzero angular momentum behave like tiny magnets. 
When placed in an external magnetic field, these tiny nuclear magnets assume 
certain allowed orientations (Zeeman energy levels) with respect to it. The 
nuclei can be reoriented by adding energy of the exact transition frequency. 
    As the separation between the nuclear Zeeman levels is directly 
proportional to the strength of the perturbing magnetic field, the transition 
frequency can be varied for a given nucleus by merely changing the applied 
magnetic field. In this regard, nuclear magnetic resonance (NMR) spectroscopy 
is unlike other spectroscopic methods, in which the investigator is unable to 
control the frequency of the spectral transition. Thus an NMR spectrum may be 
secured by varying the magnetic field to bring the separation of the Zeeman 
levels into correspondence with a constant irradiating frequency; or the 
alternative experimental method may be used, in which a constant magnetic 
field is employed and the irradiating frequency is varied over the range of 
spectroscopic frequencies.  See  ZEEMAN EFFECT. 
    It is this unique field-frequency relationship that makes possible the 
application of NMR spectroscopy in molecular studies. Although identical 
nuclei have the same frequency dependence upon the magnetic field, a 
difference in the chemical environment can modify the applied magnetic field, 
so that all such nuclei in the same sample do not experience the same net 
magnetic field, and thus they do not resonate at exactly the same frequency. 
The corresponding spectral shift in the transition frequencies between two 
such chemically nonequivalent nuclei is referred to as the chemical shift. 
Being directly proportional to the total applied field, this parameter is 
recorded in the relative units of parts per million (ppm). See  SPECTROSCOPY. 
 �34s[George Slomp]
�33s 
  
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�6v�2v�32sNuclear medicine   �21m �33s 
    A subspecialty of medicine based primarily on the use of radioactive 
substances in medical diagnosis, treatment, and research. In a typical 
examination a radioactive tracer is given, usually by injection into an arm 
vein, and the distribution of the radioactive substance within the body or a 
part of the body is portrayed in a series of nuclear images. The imaging is 
based on the emission of gamma rays by the tracer substance that pass out of 
the body and are recorded by a scintillation camera. The scintillation camera 
is a radiation detection device consisting of a large sodium iodide crystal 
(nearly 1 m in diameter in some cases) which detects the gamma rays that 
interact with the crystal and locates where on the detector face the 
interaction has occurred. These interactions are used to produce a picture or 
image of where the gamma rays originated within the body. The methods are 
related to those in routine x-ray examinations, except that gamma rays are 
emitted from the body to provide the diagnostic information, rather than being 
transmitted through the body. Nuclear images depend on specific radioactive 
elements or compounds labeled with radioactive elements being selectively 
concentrated in an organ, making it possible to obtain pictures that provide 
information about regional function within the organ.  See  GAMMA-RAY 
DETECTORS; SCINTILLATION COUNTER. 
    Nuclear imaging techniques make it possible to examine functions of the 
human body noninvasively, in ways that at times surpass the perception of 
pathologists at the autopsy table and surgeons at the operating table. The 
body can be examined as a series of physiologic processes, not simply as 
static structures.  See  RADIOACTIVE TRACER; RADIOISOTOPE (BIOLOGY); RADIOLOGY. 
 �34s[Henry N. Wagner, Jr.]
�33s 
  
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�6v�2v�32sNuclear molecule   �21m �33s 
    A quasistable entity of nuclear dimensions formed in nuclear collisions 
and comprising two or more discrete nuclei that retain their identities and 
are bound together by strong nuclear forces. Whereas the stable molecules of 
chemistry and biology consist of atoms bound through various electronic 
mechanisms, nuclear molecules do not form in nature except possibly in the 
hearts of giant stars; this simply reflects the fact that all nuclei carry 
positive electrical charges, and that under all natural conditions the 
long-range electrostatic repulsion prevents nuclear components from coming 
within the grasp of the short-range attractive nuclear force which could 
provide molecular binding. But in energetic collisions this electrostatic 
repulsion can be overcome.  See  NUCLEAR STRUCTURE. 
 �34s[D. Allan Bromley]
�33s 
  
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�6v�2v�32sNuclear moments   �21m �33s 
    Intrinsic properties of atomic nuclei: electric moments result from 
deviations of the nuclear charge distribution from spherical symmetry; 
magnetic moments are a consequence of the intrinsic spin and the rotational 
motion of nucleons within the nucleus. The classical definitions of the 
magnetic and electric multipole moments are written in general in terms of 
multipole expansions.  See  NUCLEAR STRUCTURE; SPIN (QUANTUM MECHANICS). 
    In special cases nuclear moments can be measured by direct methods 
involving the interaction of the nucleus with an external magnetic field or 
with an electric field gradient produced by the scattering of high-energy 
charged particles. In general, however, nuclear moments manifest themselves 
through the hyperfine interaction between the nuclear moments and the fields 
or field gradients produced by either the atomic electrons' currents and 
spins, or the molecular or crystalline electronic and lattice structures.  See 
  HYPERFINE STRUCTURE. 
 �34s[Noemie Koller]
�33s 
  
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�6v�2v�32sNuclear orientation   �21m �33s 
    The directional ordering of an assembly of nuclear spins I with respect to 
some axis in space. Under normal conditions nuclei are not oriented; that is, 
all directions in space are equally probable. For a system of nuclear spins 
with rotational symmetry about an axis, the degree of orientation is 
completely characterized by the relative populations asubm of the 2I + 1 
magnetic sublevels m(= I, I - 1, . . . , -I). 
    Nuclear orientation can be achieved in various ways. The most obvious way 
is to modify the energies of the 2I + 1 magnetic sublevels so as to remove 
their degeneracy and thereby change the populations of these sublevels. The 
spin degeneracy can be removed by a magnetic field interacting with the 
nuclear magnetic dipole moment, or by an inhomogeneous electric field 
interacting with the nuclear electric quadrupole moment. Significant 
differences in the populations of the sublevels can be established by cooling 
the nuclear sample to low temperatures. This means of producing nuclear 
orientation is called the static method. In contrast, there is the dynamic 
method, which is related to optical pumping in gases. There are other ways to 
produce oriented nuclei; for example, in a nuclear reaction such as the 
capture of polarized neutrons (produced by magnetic scattering) by unoriented 
nuclei.  See  DYNAMIC NUCLEAR POLARIZATION; OPTICAL PUMPING. 
    Oriented nuclei have been used to measure nuclear properties, for example, 
magnetic dipole and electric quadrupole moments, spins, parities, and mixing 
ratios of nuclear states. Oriented nuclei have been used to examine some of 
the fundamental properties of nuclear forces, for example, nonconservation of 
parity in the weak interaction. Measurement of hyperfine fields, 
electric-field gradients, and other properties relating to the environment of 
the nucleus have been made by using oriented nuclei. Nuclear orientation 
thermometry is one of the few sources of a primary temperature scale at low 
temperatures. Oriented nuclear targets used in conjunction with beams of 
polarized and unpolarized particles have proved very useful in examining 
certain aspects of the nuclear force.  See  LOW-TEMPERATURE THERMOMETRY; 
NUCLEAR MOMENTS; NUCLEAR STRUCTURE; PARITY (QUANTUM MECHANICS). 
 �34s[Harvey Marshak]
�33s 
  
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�6v�2v�32sNuclear physics   �21m �33s 
    The discipline involving the structure of atomic nuclei and their 
interactions with each other, with their constituent particles, and with the 
whole spectrum of elementary particles that is provided by very large 
accelerators. The nuclear domain occupies a central position between the 
atomic range of forces and sizes and those of elementary-particle physics, 
characteristically within the nucleons themselves. As the only system in which 
all the known natural forces can be studied simultaneously, it provides a 
natural laboratory for the testing and extending of many fundamental 
symmetries and laws of nature. Containing a reasonably large, yet manageable 
number of strongly interacting components, the nucleus also occupies a central 
position in the universal many-body problem of physics.  See  ATOMIC NUCLEUS; 
ATOMIC STRUCTURE AND SPECTRA; ELEMENTARY PARTICLE; SYMMETRY LAWS (PHYSICS). 
    Nuclear physics is unique in the extent to which it merges the most 
fundamental and the most applied topics. Its instrumentation has found broad 
applicability throughout science, technology, and medicine; nuclear 
engineering and nuclear medicine are two very important areas of applied 
specialization.  See  NUCLEAR ENGINEERING; NUCLEAR RADIATION (BIOLOGY); 
RADIOLOGY.  
    Nuclear chemistry, certain aspects of condensed matter and materials 
science, and nuclear physics together constitute the broad field of nuclear 
science; outside the United States and Canada elementary particle physics is 
frequently included in this more general classification.  See  ANALOG STATES; 
COSMIC RAYS; FUNDAMENTAL INTERACTIONS; ISOTOPE; NUCLEAR CHEMISTRY; NUCLEAR 
FISSION; NUCLEAR FUSION; NUCLEAR ISOMERISM; NUCLEAR MOMENTS; NUCLEAR REACTION; 
NUCLEAR REACTOR; NUCLEAR SPECTRA; NUCLEAR STRUCTURE; PARTICLE ACCELERATOR; 
PARTICLE DETECTOR; RADIOACTIVITY; SCATTERING EXPERIMENTS (NUCLEI); WEAK 
NUCLEAR INTERACTIONS. 
 �34s[D. Allan Bromley]
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�6v�2v�32sNuclear power   �21m �33s 
    Power derived from fission or fusion nuclear reactions. More 
conventionally, nuclear power is interpreted as the utilization of the fission 
reactions in a nuclear power reactor to produce steam for electrical power 
production, for ship propulsion, or for process heat. Fission reactions 
involve the breakup of the nucleus of heavy-weight atoms and yield-energy 
release which is more than a millionfold greater than that obtained from 
chemical reactions involving the burning of a fuel. Successful control of the 
nuclear fission reactions provides for the utilization of this intensive 
source of energy, and with the availability of ample resources of uranium 
deposits, significantly cheaper fuel costs for electrical power generation are 
attainable. Safe, clean, economic nuclear power has been the objective both of 
the Federal government and of industry's programs for research, development, 
and demonstration. Critics of nuclear power seek a complete ban or at least a 
moratorium on new commercial plants.  See  NUCLEAR FISSION. 
    The inherent dangers associated with nuclear power which involves 
unprecedented quantities of radioactive materials, including possible 
wide-scale use of plutonium, have been recognized, and extensive programs for 
safety, ecological, and biomedical studies, research, and testing have been 
integral with the advancement of the engineering of nuclear power. Criteria 
for siting a nuclear power station involve thorough investigation of the 
region's geology, seismology, hydrology, meteorology, demography, and nearby 
industrial, transportation, and military facilities. Also included are 
emergency plans to cope with fires or explosions, and radiation accidents 
arising from operational malfunctions, natural disasters, and civil 
disturbances.  See  NUCLEAR REACTOR; RADIOACTIVE WASTE MANAGEMENT. 
 �34s[Herbert S. Isbin]
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�6v�2v�32sNuclear quadrupole resonance �21m �33s
    A selective absorption phenomenon observable in a wide variety of
polycrystalline compounds containing nonspherical atomic nuclei when placed in
a magnetic radio-frequency field. Nuclear quadrupole resonance (NQR) is very
similar to nuclear magnetic resonance (NMR), and was originated as an
inexpensive (no stable homogeneous large magnetic field is required)
alternative way to study nuclear moments. It later gained a modest popularity.
See  MAGNETIC RESONANCE; NUCLEAR MAGNETIC RESONANCE (NMR).
    In the simplest case, for example, (sup 35)Cl in solid Cl2, NQR is
associated with the precession of the angular momentum I of the nucleus,
depicted in the illustration as a flat ellipsoid of rotation, around the
symmetry axis (taken as the z axis) of the Cl2 molecule fixed in the
crystalline solid. The precession, with constant angle THETA between the
nuclear axis and symmetry axis of the molecule, is due to the torque which the
inhomogeneous molecular electric field exerts on the nucleus of electric
quadrupole moment eQ. The absorption occurs classically when the frequency of
the rf field and that of the precessing motion of the angular momentum
coincide.
    NQR spectra have been observed in the approximate range 1 - 1000 MHz. Most
of the NQR work has been on molecular crystals. For such crystals the coupling
constants found do not differ very much from those measured for the isolated
molecules in microwave spectroscopy. The most precise nuclear information
which may be extracted from NQR data are quadrupole moment ratios of isotopes
of the same element. If values for the axial gradient of the molecular
electric field can be estimated from atomic fine structure data, then fair
values of the quadrupole moment may be obtained. However, it has also proved
very productive to use the quadrupole nucleus as a probe of bond character and
orientation and crystalline electric fields and lattice sites, and a large
body of data has been accumulated in this area.  See  MICROWAVE SPECTROSCOPY.
 �34s[Hans Dehmelt]
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�6v�2v�32sNuclear radiation   �21m �33s 
    All particles and radiations emanating from an atomic nucleus due to 
radioactive decay and nuclear reactions. Thus the criterion for nuclear 
radiations is that a nuclear process is involved in their production. The term 
was originally used to denote the ionizing radiations observed from naturally 
occurring radioactive materials. These radiations were alpha rays (energetic 
helium nuclei), beta rays (negative electrons), and gamma rays 
(electromagnetic radiation with wavelength much shorter than visible light).  
See  ALPHA RAYS; BETA RAYS; GAMMA RAYS. 
    Nuclear radiations have traditionally been considered to be of three types 
based on the manner in which they interact with matter as they pass through 
it. These are the charged heavy particles with masses comparable to that of 
the nuclear mass (for example, protons, alpha particles, and heavier nuclei), 
electrons (both negatively and positively charged), and electromagnetic 
radiation. For all of these, the interactions with matter are considered to be 
primarily electromagnetic. The behavior of mesons and other particles is 
intermediate between that of the electron and heavy charged particles.  See  
CHARGED PARTICLE BEAMS. 
    A striking difference in the absorption of the three types of radiations 
is that only heavy charged particles have a range. That is, a monoenergetic 
beam of heavy charged particles, in passing through a certain amount of 
matter, will lose energy without changing the number of particles in the beam. 
Ultimately, they will be stopped after crossing practically the same thickness 
of absorber. For electromagnetic radiation (gamma rays) and neutrons, on the 
other hand, the absorption is exponential. The difference in behavior reflects 
the fact that charged particles are not removed from the beam by individual 
interactions, whereas gamma radiation photons (and neutrons) are removed. 
Electrons exhibit a more complex behavior. See ELECTRON; NUCLEAR REACTION. 
 �34s[Dennis G. Kovar]
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�6v�2v�32sNuclear radiation (biology) �21m �33s
    Nuclear radiations are used in biology because of their common property of
ionizing matter. This makes their detection relatively simple, or makes
possible the production of biological effects in any living cell.  See
NUCLEAR RADIATION.
    All ionizing radiations produce biological changes, directly by ionization
or excitation of the atoms in the molecules of biological entities, such as in
chromosomes, or indirectly by the formation of active radicals or deleterious
agents, through ionization and excitation, in the medium surrounding the
biological entities. Ionizing radiation, having high penetrating power, can
reach the most vulnerable part of a cell, an organ, or a whole organism, and
is thus very effective. The radiations emitted by radioactive isotopes of the
various elements are used in biological research.  See	AUTORADIOGRAPHY;
HISTORADIOGRAPHY.
    The medical uses of nuclear radiations may be divided into three distinct
classes:
    1. The radiations, which are principally x-rays, are used to study the
anatomical configuration of body organs, usually for the purpose of detecting
abnormalities as an aid in diagnosis.
    2. The radiations are used for therapeutic purposes to produce biological
changes in such tissues as tumors.
    3. The radiations are used as a simple means of tracing a suitable
radioactive substance through different steps in its course through the body,
in the study of some particular physiological process.	See  RADIOLOGY.
 �34s[Gioacchino Failla /Edith H. Quimby]
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�6v�2v�32sNuclear reaction �21m �33s
    A process that occurs as a result of interactions between atomic nuclei
when the interacting particles approach each other to within distances of the
order of nuclear dimensions (APPX 10(sup -12)cm). While nuclear reactions occur
in nature, understanding of them and use of them as tools have taken place
primarily in the controlled laboratory environment. In the usual experimental
situation, nuclear reactions are initiated by bombarding one of the
interacting particles, the stationary target nucleus, with nuclear projectiles
of some type, and the reaction products and their behaviors are studied.

    Types of nuclear interaction.   As a generalized nuclear process, consider
a collision in which an incident particle strikes a previously stationary
particle, to produce an unspecified number of final products. If the final
products are the same as the two initial particles, the process is called
scattering. The scattering is said to be elastic or inelastic, depending on
whether some of the kinetic energy of the incident particle is used to raise
either of the particles to an excited state. If the product particles are
different from the initial pair, the process is referred to as a reaction.
    The most common type of nuclear reaction, and the one which has been most
extensively studied, involves the production of two final products. Such
reactions can be observed, for example, when deuterons with a kinetic energy
of a few million electronvolts (MeV) are allowed to strike a carbon nucleus of
mass 12. Protons, neutrons, deuterons, and alpha particles are observed to be
emitted, and reactions (1) - (4) are responsible. In these equations the
nuclei are indicated by the usual chemical symbols; the subscripts indicate
the atomic number (nuclear charge) of the nucleus, and the superscripts the
mass number of the particular isotope. These reactions are conventionally
written in the compact notation (sup 12)C(d,d)(sup12)C,
(sup 12)C(d,p)(sup 13)C, (sup 12)C(d,n)(sup 13)N, and
(sup 12)C(d,ALPHA)(sup 10)B, where d represents deuteron, p proton, n neutron,
and ALPHA alpha particle. In each of these cases the reaction results in the
production of an emitted light particle and a heavy residual nucleus. If the
residual nucleus is formed in an excited state, it will subsequently emit this
excitation energy in the form of gamma rays or, in special cases, electrons.
The residual nucleus may also be a radioactive species, in which case it will
undergo further transformation in accordance with its characteristic
radioactive decay scheme.  See	RADIOACTIVITY.

     (2 over 1) H + (12 over 6) C --> (2 over 1) H  + (12 over 6) C	  (1)

     (2 over 1) H + (12 over 6) C --> (1 over 1) H  + (13 over 6) C	  (2)

     (2 over 1) H + (12 over 6) C --> (1 over 0) n  + (13 over 7) N	  (3)

     (2 over 1) H + (12 over 6) C --> (4 over 2) He + (10 over 5) B	  (4)


    Nuclear cross section.   In general one is interested in the probability
of occurrence of the various reactions as a function of the bombarding energy
of the incident particle. The measure of probability for a nuclear reaction is
its cross section. Consider a reaction initiated by a beam of particles
incident on a region which contains N atoms per unit area (uniformly
distributed), and where I particles per second striking the area result in R
reactions of a particular type per second. The fraction of the area bombarded
which is effective in producing the reaction products is R/I. If this is
divided by the number of nuclei per unit area, the effective area or cross
section SIGMA = R/IN. This is referred to as the total cross section for the
specific reaction, since it involves all the occurrences of the reaction. The
dimensions are those of an area, and total cross sections are expressed in
either square centimeters or barns (1 barn = 10[sup -24] cm[sup 2]). The
differential cross section refers to the probability that a particular
reaction product will be observed at a given angle with respect to the beam
direction. Its dimensions are those of an area per unit solid angle (for
example, barns per steradian).

    Reaction mechanism.   Various reaction models have been extremely
successful in describing certain classes or types of nuclear reaction
processes. In general, all reactions can be classified according to the time
scale on which they occur, and the degree to which the kinetic energy of the
incident particle is converted into internal excitation of the final products.
A large fraction of the reactions observed has properties consistent with
those predicted by two reaction mechanisms which represent the extremes in
this general classification. These are the mechanisms of compound nucleus
formation and direct interaction.
    Compound nucleus formation is envisioned to take place in two distinct
steps. In the first step the incident particle is captured by (or fuses with)
the target nucleus, forming an intermediate or compound nucleus which lives a
long time (=<10-sup1sup6 s) compared to the approximately 10-sup2sup2 s it
takes the incident particle to travel past the target. During this time the
kinetic energy of the incident particle is shared among all the nucleons, and
all memory of the incident particle and target is lost. The compound nucleus
is always formed in a highly excited unstable state, is assumed to approach
themodynamic equilibrium involving all or most of the available degrees of
freedom, and will decay, as the second step, into different reaction products,
or through so-called exit channels. The essential feature of the compound
nucleus formation or fusion reaction is that the probability for a specific
reaction depends on two independent probabilities: the probability for forming
the compound nucleus, and the probability for decaying into that specific exit
channel.
    Some reactions have properties which are in striking conflict with the
predictions of the compound nucleus hypothesis. Many of these are consistent
with the picture of a mechanism where no long-lived intermediate system is
formed, but rather a fast mechanism where the incident particle, or some
portion of it, interacts with the surface, or some nucleons on the surface, of
the target nucleus. These direct reactions are assumed to involve only a very
small number of the available degrees of freedom. Most direct reactions are of
the transfer type where one or more nucleons are transferred to or from the
incident particle as it passes the target, leaving the two final partners
either in their ground states or in one of their many excited states. Such
transfer reactions are generally referred to as stripping or pick-up
reactions, depending on whether the incident particle has lost or acquired
nucleons in the reaction.
    Inelastic scattering is also a direct reaction. Whereas the states
preferentially populated in transfer reactions are those of specific
single-particle or shell-model structure, the states preferentially excited in
inelastic scattering are collective in nature.	See  NUCLEAR STRUCTURE;
SCATTERING EXPERIMENTS (NUCLEI).
 �34s[Dennis G. Kovar]
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�6v�2v�32sNuclear reactor   �21m �33s 
    A system utilizing nuclear fission in a controlled and self-sustaining 
manner. Neutrons are used to fission the nuclear fuel, and the fission 
reaction produces not only energy and radiation, but also additional neutrons. 
Thus a neutron chain reaction ensues. A nuclear reactor provides the assembly 
of materials to sustain and control the neutron chain reaction, to 
appropriately transport the heat produced from the fission reactions, and to 
provide the necessary safety features to cope with the radiation and 
radioactive materials produced by the operation of the nuclear reactor.  See  
CHAIN REACTION (PHYSICS); NUCLEAR FISSION. 
    Nuclear reactors are used in a variety of ways as sources for energy, for 
nuclear radiations, and for special tests and feasibility demonstrations. The 
generation of electric energy by a nuclear power plant requires the use of 
heat to produce steam or to heat gases to drive turbogenerators. Direct 
conversion of the fission energy into useful work is possible, but an 
efficient process has not yet been realized to accomplish this. Thus, in its 
operation the nuclear power plant is similar to the conventional coal-fired 
plant, except that the nuclear reactor is substituted for the conventional 
boiler.  See  NUCLEAR POWER. 
  
    Fuel and moderator.   The fission neutrons are released at very high 
energies and are called fast neutrons. The average kinetic energy is 2 MeV, 
with a corresponding neutron speed of 1/15 the speed of light. Neutrons slow 
down through collisions with nuclei of the surrounding material. This 
slowing-down process is made more effective by the introduction of 
light-weight materials, called moderators, such as heavy water (deuterium 
oxide), ordinary (light) water, graphite, beryllium, beryllium oxide, 
hydrides, and organic materials (hydrocarbons). Neutrons that have slowed down 
to an energy state in equilibrium with the surrounding material are called 
thermal neutrons. The probability that a neutron will cause the fuel material 
to fission is greatly enhanced at thermal energies, and thus most reactors 
utilize a moderator for the conversion of fast neutrons to thermal neutrons. 
    With suitable concentrations of the fuel material, neutron chain reactions 
also can be sustained at higher neutron energy levels. The energy range 
between fast and thermal is designated as intermediate. Fast reactors do not 
have moderators and are relatively small. Reactors have been built in all 
three categories. 
    Only three isotopes - uranium-235, uranium-233, and plutonium-239 - are 
feasible as fission fuels, but a wide selection of materials incorporating 
these isotopes is available. 
  
    Heat removal.   The major portion of the energy released by the fissioning 
of the fuel is in the form of kinetic energy of the fission fragments, which 
in turn is converted into heat through the slowing down and stopping of the 
fragments. For the heterogeneous reactors this heating occurs within the fuel 
elements. Heating also arises through the release and absorption of the 
radiations from the fission process and from the radioactive materials formed. 
The heat generated in a reactor is removed by a primary coolant flowing 
through the reactor. 
  
    Reactor coolants.   Coolants are selected for specific applications on the 
basis of their heat-transfer capability, physical properties, and nuclear 
properties.  
    Water has many desirable characteristics. It was employed as the coolant 
in the first production reactors and most power reactors still utilize water 
as the coolant. In a boiling-water reactor (see illustration) the water is 
allowed to boil and form steam that is piped to the turbine. In a 
pressurized-water reactor the coolant water is kept under increased pressure 
to prevent boiling, and transfers heat to a separate stream of water in a 
steam generator, changing that water to steam.  See  COOLING TOWER; 
RADIOACTIVE WASTE MANAGEMENT; VAPOR CONDENSER. 
    For both boiling-water and pressurized-water reactors, the water serves as 
the moderator as well as the coolant. Both light and heavy water are excellent 
neutron moderators, although heavy water (deuterium oxide) has a 
neutron-absorption cross section approximately 1/500 that for light water. The 
most serious limitation of water as a coolant for power reactors is its high 
vapor pressure. The high pressure necessary for water-cooled power reactors 
imposes severe design problems.  See  NUCLEAR REACTION. 
    Gases are inherently poor heat-transfer fluids as compared with liquids 
because of their low density. This situation can be improved by increasing the 
gas pressure; however, this introduces other problems. Helium is the most 
attractive gas (it is chemically inert and has good thermodynamic and nuclear 
properties) and has been selected as the coolant for the development of 
high-temperature gas-cooled reactor (HTGR) systems, in which the gas transfers 
heat from the reactor core to a steam generator. Gases are capable of 
operation at extremely high temperature, and they are being considered for 
special process applications and direct-cycle gas-turbine applications. 
    Diphenyl and terphenyl possess good neutron-moderating properties and have 
lower vapor pressures than water. Organic coolants are noncorrosive and 
relatively inexpensive. Their major disadvantage is dissociation or 
decomposition under irradiation. 
    The alkali metals have excellent heat-transfer properties and extremely 
low vapor pressures at temperatures of interest for power generation. Sodium 
is the most attractive because of its relatively low melting point and high 
heat-transfer coefficient. It is also abundant, commercially available in 
acceptable purity, and relatively inexpensive. It is not particularly 
corrosive, provided low oxygen concentration is maintained. Its nuclear 
properties are excellent for fast reactors. In the liquid metal fast breeder 
reactor sodium in the primary loop collects the heat generated in the core and 
transfers it to a secondary sodium loop in the heat exchanger, from which it 
is carried to the steam generator. 
    Molten salts have been used as reactor coolants because they have 
favorable high-temperature properties and because mixtures of salts containing 
fuel permit fluid fuel-coolant systems. 
  
    Fluid flow and hydrodynamics.   Because heat removal must be accomplished 
as efficiently as possible, considerable attention must be given to the 
fluid-flow and hydrodynamic characteristics of the system. 
    The heat capacity and thermal conductivity of the fluid at the temperature 
of operation have a fundamental effect upon the design of the reactor system. 
The heat capacity determines the mass flow of the coolant required. The fluid 
properties (thermal conductivity, viscosity, density, and specific heat) are 
important in determining the surface area required for the fuel to permit 
transfer of the heat generated at reasonable temperature differences. This, in 
turn, affects the design of the fuel - in particular, the amount and 
arrangement of the fuel elements. These factors combine to establish the 
pumping characteristics of the system because pressure drop and 
coolant-temperature rise are directly related. 
  
  
    Thermal stress.   The temperature of the reactor coolant increases as it 
circulates through the reactor. Fluctuations in power level or in coolant flow 
rate result in variations in the temperature rise. A reactor is capable of 
very rapid changes in power level, particularly, reduction in power level. 
Therefore, reactor-coolant systems must be designed to accommodate the 
temperature transients that may occur because of rapid power changes. In 
addition, they must be designed to accommodate temperature transients that 
might occur as a result of a coolant-system malfunction, such as pump 
stoppage. The consequent temperature stresses induced in the various parts of 
the system are superimposed upon the thermal stresses that exist under normal 
steady-state operations. 
  
    Coolant-system components.   The development of reactor systems has 
necessitated concurrent development of special components for reactor coolant 
systems. These have been required even for systems employing conventional 
coolants, such as water or air. 
    Because of the hazard of radioactivity, leak-tight systems and components 
are a prerequisite to safe, reliable operation and maintenance. Special 
problems are introduced by many of the fluids employed as reactor coolants. 
    More extensive component developments have been required for the alkali 
metals which are chemically very active and are extremely poor lubricants. 
Centrifugal pumps employing unique bearings and seals must be specially 
designed. Liquid metals are excellent electrical conductors, and, in some 
special cases, electromagnetic-type pumps have been developed. These pumps are 
completely sealed, contain no moving parts, and derive their pumping action 
from electromagnetic forces imposed directly on the fluid. 
  
    Core design.   A typical reactor core for a power reactor consists of the 
fuel element rods supported by a grid-type structure inside a vessel. 
    Structural materials employed in reactor systems must possess suitable 
nuclear and physical properties and must be compatible with the reactor 
coolant under the conditions of operation. The most common structural 
materials employed in reactor systems are aluminum, stainless steel, and 
zirconium alloys. Aluminum and zirconium alloys have favorable nuclear 
properties, whereas stainless steel has favorable physical properties. 
Aluminum is widely used in low-temperature reactors; zirconium and stainless 
steel are used in high-temperature reactors. Zirconium is relatively 
expensive, and its use is therefore confined to applications where neutron 
absorption is critical. 
    Heterogeneous reactors maintain a separation of fuel and coolant by 
cladding the fuel. The cladding material must be compatible with both the fuel 
and the coolant. 
    The cladding materials must also have favorable nuclear properties. The 
neutron-capture cross section is most significant because the parasitic 
absorption of neutrons by these materials reduces the efficiency of the 
nuclear fission process. Aluminum is a very desirable material in this 
respect; however, its physical strength and corrosion resistance in water 
decrease very rapidly above about 300DGF (149DGC). Zirconium has very 
favorable neutron properties, and in addition can be made reasonably 
corrosion-resistant in high-temperature water. It is extensively used for 
water-cooled power reactors. Stainless steel is used for the fuel cladding in 
fast reactors. 
  
    Control.   The control of reactors requires the measurement and adjustment 
of the critical condition. A reactor is critical when the rate of production 
of neutrons equals the rate of consumption in the system. The neutrons are 
produced by the fission process and are consumed in a variety of ways, 
including absorption to cause fission, nonfission capture in fissionable 
materials, capture in fertile materials, capture in structure or coolant, and 
leakage from the reactor. A reactor is subcritical (power level decreasing) if 
the number of neutrons produced is less than the number consumed. The reactor 
is supercritical (power level increasing) if the number of neutrons produced 
exceeds the number consumed. 
    Reactors are controlled by adjusting the balance between neutron 
production and neutron consumption. Normally, neutron consumption is 
controlled by varying the absorption or leakage of neutrons; however, the 
neutron-generation rate can be controlled by varying the amount of fissionable 
material in the system. 
    The reactor control system requires the movement of neutron-absorbing rods 
(control rods) in the reactor under very exacting conditions. They must be 
arranged to increase reactivity (increase neutron population) slowly and under 
absolute control. They must be capable of reducing reactivity, both rapidly 
and slowly. 
    Control drives are normally electromechanical devices that impart linear 
or swinging motion to the control rods. Normal operation of the control drives 
can be accomplished manually by the reactor operator or by automatic control 
systems. Reactor scram (very rapid reactor shutdown) can be initiated 
automatically by one or more system scram-safety signals, or it can be started 
manually by depressing a scram button convenient to the operator in the 
control room. 
  
    Applications.   Reactor applications include production of fissionable 
fuels (plutonium and uranium-233); mobile, stationary, and packaged power 
plants; research, testing, teaching-demonstration, and experimental 
facilities; space and process heat; dual-purpose designs; and special 
applications. The potential use of reactor radiation for sterilization of food 
and other products, for chemical processes, and for high-temperature 
applications has been recognized. 
 �34s[Herbert S. Isbin]
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    ms. Pgs. 4795-479710:22 
  
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�6v�2v�32sNuclear spectra   �21m �33s 
    The distribution in energy or momentum of radiations emitted by 
radioactive nuclei or in a nuclear reaction; also, the graphical display of 
data from instruments used to measure such radiations (for example, magnetic 
spectrometers and scintillation detectors).  See  NUCLEAR RADIATION. 
    Nuclear spectra are widely used in both basic and applied research. For 
the former, it is usually the information which can be inferred from studies 
of such spectra rather than the radiations themselves which are of primary 
interest. From experimental determinations of the type of radiation, its 
energy, and the changes that it causes in angular momentum and parity, one is 
able to deduce information pertaining to the static and dynamic properties of 
the initial and final nuclear configurations. This, in turn, enables one to 
gain insight into the structure of nuclei as well as the forces between 
nucleons. In applied research, it is the radiations themselves or the effects 
which they produce that are of most importance.  See  NUCLEAR STRUCTURE. 
    The distribution in energy (momentum) of radiations emitted during 
transitions between nuclear configurations can be discrete or continuous. 
Transitions which give rise to discrete spectra are those in which a single 
type of radiation is emitted. The emitted particle then has a specific energy 
determined by the two nuclear configurations and type of radiation involved. 
On the other hand, if two or more radiations are emitted during a transition, 
the energy will be shared among them. In this case, the energy of each 
particle emitted can take on a continuum of values between well-defined 
limits. This then gives rise to a continuous spectrum.  See  NUCLEAR REACTION; 
RADIOACTIVITY.  
 �34s[D. J. Horen]
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�6v�2v�32sNuclear structure �21m �33s
    The atomic nucleus is at the center of the atom and contains 99.975% of
the total mass of the atom. Its average density is about 3 x 10(sup 11)
kg/cm(sup 3); its diameter is about 10-sup1sup2 cm, and thus much smaller than
the diameter of the atom, which is about 10(sup -8) cm.
    The nucleus is composed of protons and neutrons. The number of protons is
usually denoted by Z, while that of neutrons is denoted by N. The number of
protons is equal to the number of electrons in the atom. Since the proton is
positively charged, the electron is negatively charged, and the neutron is
neutral, the atom as a whole is neutral under normal conditions. The total
number of protons and neutrons in a nucleus is called the mass number and
denoted by A = N + Z. Nuclei having the same proton number but different
neutron number are called isotopes. Nuclei having the same neutron number but
different proton number are called isotones. Finally, nuclei with the same
mass number are called isobars.  See  ATOMIC STRUCTURE AND SPECTRA; ELECTRON;
NEUTRON; PROTON.


    Properties. The nucleus is a quantum-mechanical system. Its properties
are best described by a diagram, called the energy-level diagram, in which
its quantum states are listed, together with the expectation values of all
measurable quantities. Three of these are particularly important: the energy,
the angular momentum, and the parity. In addition, other properties, such as
magnetic and electric moments, are often measured. See ANGULAR MOMENTUM;
ENERGY LEVEL (QUANTUM MECHANICS); PARITY (QUANTUM MECHANICS).


    Nuclear models.   A number of models have been developed to account for
the properties of nuclei.
    The basic model for the description of nuclear properties is the shell
model. The strong nuclear force binds together protons and neutrons, called by
the single name nucleons. Each individual nucleon moves in the average
potential generated by all the others. This potential has the same shape as
the nuclear matter distribution of the illustration, but upside down. The
states of a single nucleon in the average potential cluster together into
layers or shells, much like the single-particle states in atoms. The closed
shells appear at nucleon numbers 2, 8, 20, 28, 50, 82, 126. The numbers at
which closed shells occur are called magic numbers.  See  MAGIC NUMBERS; SPIN
(QUANTUM MECHANICS).
    Closed-shell nuclei behave, in many respects, as inert. Most properties of
nuclei can be described by considering as active only nucleons outside the
closed shells. These are called valence nucleons. In addition to the average
potential in which they move, it is assumed that there is a residual
interaction between valence nucleons. When the number of valence nucleons is
small (up to about four), the effects of the residual interaction can be
calculated easily.
    When the number of valence nucleons is large (greater than 5 - 6), some
striking regularities develop in the level scheme. These regularities indicate
the presence of collective features in nuclei. The collective features arise
from two special properties of the residual interaction between valence
nucleons: first, its pairing property which tends to pair off nucleons, one
with angular momentum up and one with angular momentum down, to give zero
resultant; and second, a quadrupole property which favors a distortion of the
nuclear surface in the shape of an ellipsoid.
    The number of energy levels of a nucleus below an excitation energy of 2
MeV is usually small. These levels can be described by using the shell model
and allowing only for excitations of the valence nucleons from some
single-particle levels to others nearby in energy. At higher excitation
energies, the number of observed states increases considerably. These states
arise from the excitation of the valence nucleons to higher single-particle
levels and from excitations of nucleons from the closed shells to the valence
shell. Because of their large number, it is no longer possible to describe
properties of individual states. Thus, statistical methods are employed to
describe average properties of states of this sort.
    Simple collective modes of excitation occur even at higher excitation
energy. Unlike the low-lying collective modes, these are no longer necessarily
associated with pairing and quadrupole correlations. They arise from some
coherent excitation of nucleons to shells other than the valence shell. These
modes are called giant resonances, and are observed in inelastic scattering of
strongly interacting particles off nuclei (ALPHA-scattering, for example), in
inelastic scattering of electrons off nuclei, and in photoabsorption.  See
GIANT NUCLEAR RESONANCES.

    Nuclear forces.   The nucleons which form the nucleus are held together by
the strong nuclear forces. The forces which act between two nucleons when they
are free (not inside the nucleus) have been studied by performing scattering
experiments of one nucleon on the other, and by analyzing the properties of
the deuteron, the only bound state of two nucleons. Because the proton and the
neutron share many properties, it is convenient to consider them as two
different states of the same particle, the nucleon. In order to distinguish
protons from neutrons, the nucleon is given a sort of intrinsic angular
momentum, called isospin.  See	ISOTOPIC SPIN.
    Nuclear structure calculations require the knowledge of the residual
interaction between nucleons. This interaction is different from the free
nucleon-nucleon interaction because of the modifications introduced by the
presence of the other nucleons in the nucleus. The interaction inside nuclei
is called the effective interaction, and its relationship to the free
interaction is not fully understood. One property of the effective
interaction, its charge independence, arises from the charge independence of
the free nucleon-nucleon interaction.  See  ANALOG STATES; NUCLEAR REACTION;
NUCLEAR SPECTRA; STRONG NUCLEAR INTERACTIONS.
 �34s[Franco Iachello]
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�6v�2v�32sNucleation   �21m �33s 
    The formation within an unstable, supersaturated solution of the first 
particles of precipitate capable of spontaneous growth into large crystals of 
a more stable solid phase. These first viable particles, called nuclei, may 
either be formed from solid particles already present in the system 
(heterogeneous nucleation) or be generated spontaneously by the supersaturated 
solution itself (homogeneous nucleation).  See  SUPERSATURATION. 
    Nucleation is significant in analytical chemistry because of its influence 
on the physical characteristics of precipitates. Processes occurring during 
the nucleation period establish the rate of precipitation, and the number and 
size of the final crystalline particles.  See  COLLOID; FLOCCULATION; 
PRECIPITATION (CHEMISTRY). 
 �34s[David H. Klein ;Louis Gordon]
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�6v�2v�32sNucleic acid   �21m �33s 
    An acidic, chainlike biological macromolecule consisting of multiply 
repeated units of phosphoric acid, sugar, and purine and pyrimidine  bases. 
Nucleic acids as a class are involved in the preservation, replication, and 
expression of hereditary information in every living cell. There are two types 
of nucleic acid: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).  See  
DEOXYRIBONUCLEIC ACID (DNA); RIBONUCLEIC ACID (RNA). 
 �34s[Edward Johnson]
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�6v�2v�32sNucleon   �21m �33s 
    A collective name for a proton or a neutron. Protons and neutrons are the 
main consitutents of the nuclei of atoms and have considerable similarity 
between themselves. They have the same spin, the same statistics, 
approximately the same mass, and, through the process of beta decay, can 
transform into each other. Thus it is convenient to have a common term to 
designate them both. 
    Occasionally neutrons and protons are considered as two states of a single 
particle called the nucleon, the two states being distinguished by the special 
value of an internal variable which can then assume only two values and is 
called the third component of the isotopic spin. See ISOTOPIC SPIN; NEUTRON; 
PROTON.  
 �34s[Emilio Segre]
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�6v�2v�32sNucleoprotein   �21m �33s 
    A generic term for any member of a large class of proteins associated with 
nucleic acid molecules. Nucleoprotein complexes occur in all living cells and 
in viruses, where they play vital roles in reproduction and protein synthesis. 
    Classification of the nucleoproteins depends primarily upon the type of 
nucleic acid involved - deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) 
- and on the biological function of the complex. Deoxyribonucleoproteins 
(complexes of DNA and proteins) constitute the genetic material of all 
organisms and of many viruses. They function as the chemical basis of heredity 
and are the primary means of its expression and control. Most of the mass of 
chromosomes is made up of DNA and proteins whose structural and enzymatic 
activities are required for the proper assembly and expression of the genetic 
information encoded in the molecular structure of the nucleic acid.  See  
DEOXYRIBONUCLEIC ACID (DNA). 
    Ribonucleoproteins (complexes of RNA and proteins) occur in all cells as 
part of the machinery for protein synthesis. This complex operation requires 
the participation of messenger RNAs (mRNAs), amino acid transfer RNAs (tRNAs), 
and ribosomal RNAs (rRNAs), each of which interacts with specific proteins to 
form functional complexes called polysomes, on which the synthesis of new 
proteins occurs.  See  RIBONUCLEIC ACID (RNA). 
    In simpler life forms, such as viruses which infect animal and plant cells 
and bacteriophages which infect bacteria, most of the mass of the viral 
particle is due to its nucleoprotein content. The material responsible for the 
hereditary continuity of the virus may be DNA or RNA, depending on the type of 
virus, and it is usually enveloped by one or more proteins which protect the 
nucleic acid and facilitate infection.   See  BACTERIOPHAGE; VIRUS. 
 �34s[Vincent G. Allfrey]
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�6v�2v�32sNucleosome   �21m �33s 
    The fundamental nucleohistone structural subunit of eukaryotic 
chromosomes. Nucleosomes (or NU-bodies) are particles about 10 nanometers in 
diameter, composed of a complex of DNA and histones, periodically positioned 
on a long DNA molecule like beads on a string. They represent the first level 
of DNA packaging in a hierarchy of organization that progresses up to the 
chromosomes visible in a light microscope. Nucleosomes appear to be universal 
among eukaryotic organisms. More than 90% of the DNA in any particular cell 
nucleus is contained in this form of packaging. 
    Neither the histones nor the nucleosomes have any apparent enzymatic or 
catalytic activity. At the minimum, a major function of the nucleosome is to 
package the DNA into a more compact form (leading to a sevenfold contraction 
of DNA length), at the same time permitting most of the DNA considerable 
exposure to the nuclear milieu.  See   CHROMOSOME; DEOXYRIBONUCLEIC ACID (DNA). 
 �34s[Donald E. Olins ;Ada L. Olins]
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�6v�2v�32sNucleosynthesis �21m �33s
    Theories of the origin of the elements involve synthesis with nucleons
(neutrons and protons), the elementary building blocks of the nucleus. The
nuclear theory, comprising nine distinct processes, is commonly called
nucleosynthesis. Any acceptable theory of nucleosynthesis must lead to an
understanding of the cosmic abundance of the elements observed in the solar
system, stars, and the interstellar medium. The curve of these abundances is
shown in the illustration.
    Observations of the expanding universe and of the 3K (-454DGF) background
radiation lend credence to the belief that the universe originated in a
primordial event known as the ``big bang'' some 1 - 2 x 10(sup 10) years ago.
Absence of stable mass 5 and mass 8 nuclei preclude the possibility of
synthesizing the major portion of the nuclei of masses greater than 4 in those
first few minutes of the universe, when density and temperature were
sufficiently high to support the necessary nuclear reactions to synthesize
elements. See BIG BANG THEORY; COSMOLOGY.
    The principal source of energy in stars is certainly nuclear reactions
which release energy by fusion of lighter nuclei to form more massive nuclei.
There is considerable evidence that nucleosynthesis is going on in stars, and
has been doing so for billions of years.
    Hydrogen burning, the first of the nine processes of nucleosynthesis,
converts hydrogen nuclei into helium. In stars of 1.2 or less solar masses,
hydrogen burning proceeds by the proton-proton chain. Such reactions occurring
during the big bang are believed to be responsible for the helium and
deuterium, and possibly some of the (sup 7)Li observed today. In more massive
stars where temperatures are equal to or greater than 2 x 10(sup 7) K, hydrogen
burning is accomplished through proton captures by carbon, nitrogen, and
oxygen nuclei, in the carbon-nitrogen-oxygen (CNO) cycles, to form (sup 4)He.
See CARBON-NITROGEN-OXYGEN CYCLES; NUCLEAR FUSION; PROTON-PROTON CHAIN.
    When the hydrogen fuel is exhausted in the central region of the star, the
core contracts and its temperature and density increase. Helium then becomes
the fuel for further energy generation and nucleosynthesis. The basic reaction
in this thermonuclear phase is the three-alpha process in which three (sup 4)He
nuclei (three alpha particles) fuse to form a carbon nucleus of mass 12
([sup 12]C).
    Upon exhaustion of the helium supply, if the star has an initial mass of
at least 7.5 solar masses, gravitational contraction of the core can lead to a
temperature of about 9 x 10(sup 8) DG F (5 x 10[sup 8] K), where it becomes
possible for two (sup 12)C nuclei to overcome their high mutual
Coulomb-potential barrier and fuse to form (sup 20)Ne, (sup 23)Na, and (sup
24)Mg. 
    Carbon burning is immediately followed by a short-lived stage, sometimes
referred to as neon burning, in which sup2sup0Ne is photodisintegrated. The
eventual result is that most of the carbon from helium burning becomes oxygen,
which supplements the original oxygen formed in helium burning. This is all
followed by the fusion of oxygen nuclei at significantly higher temperatures.
    Silicon burning commences when the temperature reaches 5 x 10(sup 9)DGF
(3 x 10[sup 9] K). In this phase, photodisintegration of (sup 28)Si and other
intermediate-mass nuclei in the neighborhood of A = 28 produces copious
supplies of protons, alpha particles, and neutrons. Capture of these by other
intermediate-mass nuclei synthesizes nuclei up to a mass A of about 60, and
results in the buildup of the iron-group peak near A = 56.
    Because neutrons are neutral particles, their capture is not affected by
the Coulomb barrier that inhibits charged-particle capture by the massive
nuclei. If the number of neutrons per seed nucleus is small, so that time
intervals between neutron captures are long compared to the BETA-decay
lifetimes of unstable nuclei that are formed, the s-process (slow process)
takes place. The seed nuclei are probably primarily those in the iron peak,
but neutron processes probably alter some of the lighter-mass nuclei as well.
In this slow process, after neutron capture a nucleus that is unstable because
of excess neutrons decays by electron and antineutrino emission to a stable
isobar which may capture another neutron, leading to a chain of captures and
decays to synthesize nuclei of masses up to about A = 209, when alpha decay
becomes a deterrent to further synthesis by the s-process. The s-process is
believed to occur in red giant stars.
    The r-process occurs when a large neutron flux allows rapid captures of
neutrons. Only in the massive explosions of supernovae are such large neutron
fluxes conceivable. With such rapid successive captures, it is possible to
proceed through unstable nuclei before they can either b- or ALPHA-decay. In
this manner it is possible to synthesize nuclei all the way into the
transuranic elements.
    The p-process produces proton-rich heavier elements probably by modifying
a small fraction of the s-process and r-process nuclei through (p,GAMMA),
(p,n), and (GAMMA,n) reactions.
    Spallation of more abundant nuclei such as carbon, nitrogen, and oxygen by
protons can account for the low-abundance nuclides (sup 6)Li, (sup 9)Be,
(sup 10)B, and (sup 11)B and for some (sup 7)Li. This breaking up of the more
massive nuclei to form the light nuclei is called the l-process. The most
probable site for the l-process is in the interstellar medium, where
high-energy galactic cosmic rays bombard the interstellar gas. See NUCLEAR
REACTION; STELLAR EVOLUTION. 
 �34s[Georgeanne R. Caughlan]
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�6v�2v�32sNuclide   �21m �33s 
    A species of atom that is characterized by the constitution of its 
nucleus, in particular by its atomic number Z and its neutron number A - Z, 
where A is the mass number. The total number of stable nuclides is 
approximately 275. About a dozen radioactive nuclides are found in nature, and 
in addition, hundreds of other have been created artificially. 
 �34s[Henry E. Duckworth]
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�6v�2v�32sNudibranchia   �21m �33s 
    An order of the gastropod subclass Opisthobranchia containing about 2500 
living species of carnivorous sea slugs. They occur in all the oceans, at all 
depths, but reach their greatest size and diversity in warm shallow seas. 
    In this, the largest order of the Opisthobranchia, there is much evidence 
indicating polyphyletic descent from a number of long-extinct opisthobranch 
stocks. In all those lines which persist to the present day, the shell and 
operculum have been discarded in the adult form. In many of them the body has 
quite independently become dorsally papillate. In at least two suborders these 
dorsal papillae have acquired the power to nurture nematocysts derived from 
their coelenterate prey so as to use them for the nudibranch's own defense. In 
other cases such papillae (usually called cerata) have independently become 
penetrated by lobules of the adult digestive gland, or they may contain 
virulent defensive glands or prickly bundles of daggerlike calcareous 
spicules.  See  OPISTHOBRANCHIA. 
 �34s[T. E. Thompson]
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�6v�2v�32sNumber systems �21m �33s
    Integral numbers may be represented as linear combinations of powers of
any convenient and arbitrarily chosen base. The choice of the base is not
always made on a rational basis; number systems have been based on 5, 6, 10,
and 60. More recently, systems based on 2 and 8 have proved quite useful in
computer applications. The duodecimal number system, in which numbers are
represented as linear combinations of powers of 12, has certain advantages
because 12 has the factors 1, 2, 3, 4, 6, and 12.

    Decimal system.   Every positive integer is uniquely a polynomial in 10
with coefficients, called digits, taken from 0, 1, .  .  .  , 9.  The fact that
205714 = 4 + 1 x 10 + 7 x 10(sup 2) + 5 x 10(sup 3) + 0 x 10(sup 4) + 2 x 10(sup
5) is nearly always lost in present-day teaching, and in the hurried application
of ordinary arithmetic.  In fact, numbers are likely to be thought of as merely
an orderly arrangement of decimal digits.

    The operations of addition and multiplication consist of the corresponding
operations with polynomials, together with rules that serve to keep the
results inside the system so that they can be used in future operations.
Subtraction introduces negative numbers that may be handled by introducing a
special digit called a sign digit with its own rules of combination, or by
introducing complementation in which the digits of a number are subtracted
from 9, except for the last nonzero digit which is subtracted from 10.
    Division is a process that can be carried out only rarely with absolute
exactness in the decimal system, the process usually being nonterminating.
This introduces the notion of infinite decimal expansions and the more or less
theoretical operations with such numbers. In practice, truncation and rounding
are used as in 2/3 = .66667, with consequent errors and departure from the
axioms of arithmetic.

    Binary system.  In the binary system every positive integer is the sum of
distinct powers of 2 in just one way.  Thus 434 = 2(sup 8) + 2(sup 7) + 2(sup 5)
+ 2(sup 4) + 2(sup 1), and this is expressed by writing 110110010.  The digits
corresponding to 2(sup 0), 2(sup 2), 2(sup 3), and 2(sup 6) are zero, since
these powers do not occur in 434.  The first dozen integers are written as
follows:

	 1	  1	 4	 100	 7	  111	 10	 1010
	 2	 10	 5	 101	 8	 1000	 11	 1011
	 3	 11	 6	 110	 9	 1001	 12	 1100

The great advantage of the binary system lies in the fact that there are only
two kinds of binary digits, or ``bits,'' namely 0 and 1. This not only gives a
simplified arithmetic but provides a language in which to treat two-valued
functions or bistable systems. Among its disadvantages is the fact that the
binary system requires nearly three times as many digits to represent a given
number as does the familiar decimal system.
    Digital computers invariably use the binary system. The so-called decimal
computers code the decimal digits into binary form, while the purely binary
machines use full binary arithmetic.
    Arithmetic in the binary system is remarkably simple. For addition, only 1
+ 1 = 10 is needed, while the multiplication table reduces to 1 x 1 = 1.  Such
simple operations are readily performed electronically with extreme rapidity and
reliability.
    The binary system is useful not only to represent numbers but also to
record and process information. In fact the unit of information is a binary
digit. For example, given a set S of objects and a property P, it is possible
to record which objects have the property P, and which do not, by assigning a
binary position to each object of S and recording there a 1 or 0 according as
the property P is, or is not, possessed by the corresponding object. A binary
computer, with its ability to extract and examine a given binary digit, can
use this compact method of storing information.

    Octal system.   To write a number in the octal system, once it has been
expressed in the binary system, one merely groups the binary digits by threes,
beginning at the binary point and working to the left and right. The octal
system with its eight digits 0, 1, . . . , 7 affords a convenient way of
condensing the lengthier display of the binary system. Arithmetic in the octal
system resembles the familiar decimal arithmetic. The octal system requires
only 10% more digits than the decimal system to represent the same amount of
information. Some computing systems use base 16, in which case binary
information is handled in sets of four bits.
 �34s[Derrick H. Lehmer]
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�6v�2v�32sNumber theory �21m �33s
    The primary objects of number theory are the properties and mutual
relations of the natural numbers 1, 2, 3, . . . and more generally of the
integers, which include also the zero and the negative integers. The integers
form a ring, that is, a domain within which addition, subtraction,
multiplication (but not necessarily division) can always be carried out. The
numbers can be classified in many ways: for example, odd and even numbers,
square numbers, prime numbers, perfect numbers.

    Elementary number theory.	The main concern of this branch of number
theory is divisibility. A number d is called a divisor of n (in symbols: d|n)
if there exists an integer t such that n = dt.	A prime number is a number that
has only 1 and itself as divisors.  Euclid proved that there is no last prime
number in the sequence of integers.  Indeed, if 2, 3, 5, .  .  .

		       N = 2 x 3 x 5 x . . . x p + 1			     (1)

, p are known prime numbers, then the number shown in Eq. (1) is
divisible by none of them and is therefore either a new prime number itself or
is divisible by prime numbers not used in the construction of N. The
fundamental theorem of number theory  states that a number n can be factored
into prime numbers in only one way when the order of the prime factors is
disregarded.
    A perfect number is defined as a number equal to the sum of its proper
divisors, or divisors smaller than the number. Euclid showed that (2[sup n] -
1)2[sup n-1] is a perfect number if 2(sup n - 1) is a prime number, a so-called
Mersenne prime. If 2(sup n - 1) is a prime, then n must itself be a prime. At
present 24 Mersenne primes and thus 24 perfect numbers are known, the first of
which are 6, 28, and 496.
    Eratosthenes (in the 3d century B.C.) showed how all prime numbers up to
some large integer x can be found if only the primes p up to SQUARE ROOT [x] are
already known.	It is indeed sufficient to strike out from the list of all
integers, starting with 2, the multiples 2p, 3p, .  .  .  , etc., of all the
primes up to SQUARE ROOT [x].  The remaining integers are precisely all the
primes up to x.
    If a - b is divisible by m, then a is called congruent b modulo m, as

			   a IDENTITY b (mod m) 			     (2)

shown in symbols in formula (2). This relation is an
equivalence; that is, it is reflexive, symmetric, and transitive, and defines,
therefore, equivalence classes of numbers congruent to each other, called
residue classes. There are evidently m residue classes modulo m. The number of
those residue classes which contain only numbers coprime to m is called
Euler's function phi(m).

    Diophantine equations.   There are infinitely many integral solutions for

		  x(sup 2) + y(sup 2) = z(sup 2)			    (3)

the pythagorean equation, Eq. (3). The solutions

x = u(sup 2) - v(sup 2)       y = 2uv	     z = u(sup 2) + v(sup 2)	    (4)

are all contained in Eqs.  (4), where u and v are positive integers. Pierre de
Fermat made the famous statement that Eq.  (5) is not solvable in integers for
any integral exponent n > 2.  Besides n = 4, only prime exponents n need to be
considered.

		 x(sup n) + y(sup n) = z(sup n) 			    (5)

    Single equations, or systems of equations in more unknowns than the number
of equations, but with added conditions, such as allowing only solutions in
integers, are called diophantine. Equations (3) and (5) are examples of
diophantine equations.

    Algebraic number theory.   The results concerning Fermat's last theorem
are obtained through the methods of algebraic number theory. Karl F. Gauss
carried over the concepts of number theory to the ring R[i] of all complex
integers a + bi, where a and b are ordinary integers. The law of unique prime
factorization is preserved in this ring. Ordinary prime numbers p IDENTITY 3
(mod 4) are also prime numbers in R[i], whereas 2 = -i(1 + i)(sup 2) and the
primes p IDENTITY 1 (mod 4) are split p = (a + bi) (a - bi). An algebraic
number field R(theta) of degree n is generated by the root theta of an
algebraic equation F(x) = 0 of degree n, having rational coefficients. A
number alpha in this field is called an (algebraic) integer if it satisfies an
algebraic equation with rational integer coefficients, the highest of which is
1. The algebraic integers in an algebraic number field form again an integral
domain. However, the prime factorization is not necessarily unique, as the
example 21 = 3 x 7 = {1 + 2(SQUARE ROOT [-5])}{1 - 2(SQUARE ROOT [-5])} = (4 +
(SQUARE ROOT [-5]))(4 - (SQUARE ROOT [-5])) shows, where each product is
irreducible in the ring R(SQUARE ROOT [-5]).  Uniqueness is restored through the
introduction of ideals.

    Analytic number theory.   For certain problems of number theory, the
methods of analysis, that is, of calculus and function theory, have to be
used. The most famous problem of this sort is the number pi(x) of prime
numbers p < or = x. About 1793, Gauss conjectured the relation in Eq. (6). This

		  lim (x -> INFINITY) [pi (x) log x]/x = 1		     (6)

equation
was proved a century later. The methods were created by G. F. B. Riemann who
investigates the function zeta(s) defined by the series shown in Eq. (7). Here

	       zeta(s) = SUMMATION (n=1 to INFINITY) 1/n(sup s) 	     (7)

s = sigma + it is a complex
variable; the series in Eq. (7) is convergent for sigma > 1. By a method of
analytic continuation, the function can be defined in the whole complex plane
of s. It is a meromorphic function with only a simple pole of residue 1 at s =
1. It can be shown that zeta(s) has no zeros for sigma = 1. This fact and the
pole at s = 1 suffice to establish Eq. (6). Riemann also proposed the still
unproved ``Riemann hypothesis'': All zeros of zeta(s) have a real part not
exceeding 1/2.	See  COMPLEX NUMBERS; NUMBER SYSTEMS; ZERO.
 �34s[Hans Brintzinger /Emil Grosswald]
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�6v�2v�32sNumerical analysis �21m �33s
    The development and analysis of computational methods (and ultimately of
program packages) for the minimization and the approximation of functions, and
for the approximate solution of equations, such as linear or nonlinear
(systems of) equations and differential or integral equations. Originally part
of every mathematician's work, the subject is now often taught in computer
science departments because of the tremendous impact which computers have had
on its development. Research focuses mainly on the numerical solution of
(nonlinear) partial differential equations and the minimization of functions.
    Numerical analysis is needed because answers provided by mathematical
analysis are usually symbolic and not numeric; they are often given implicitly
only, as the solution of some equation, or they are given by some limit
process. A further complication is provided by the rounding error which
usually contaminates every step in a calculation (because of the fixed finite
number of digits carried).
    Even in the absence of rounding error, few numerical answers can be
obtained exactly.  Among these are (1) the value of a piecewise rational
function at a point and (2) the solution of a (solvable) linear system of
equations, both of which can be produced in a finite number of arithmetic steps.
Approximate answers to all other problems are obtained by solving the first few
in a sequence of such finitely solvable problems.  A typical example is provided
by Newton's method:  A solution c to a nonlinear equation f(c) = 0 is found as
the limit c = lim (n-> INFINITY x(sub n), with x(sub n+1) a solution to the
linear equation f(x sub n)f'(x sub m)(x sub n+1) - (x sub n) = 0, that is,
xsubn+1 = (x sub n) - f(x sub n)/f'(x sub n), n = 0, 1, 2, .  .  .  .  Of
course, only the first few terms in this sequence x0, x1, x2, .  .  .  can ever
be calculated, and thus one must consider when to break off such a solution
process and how to gauge the accuracy of the current approximation.
    An otherwise satisfactory computational process may become useless,
because of the amplification of rounding errors. A computational process is
called stable to the extent that its results are not spoiled by rounding
errors. The extended calculations involving millions of arithmetic steps now
possible on computers have made the stability of a computational process a
prime consideration.

    Interpolation and approximation.   Polynomial interpolation provides a
polynomial p of degree n or less which uniquely matches given function values
f(x0), . . . , f(x sub n) at corresponding distinct points x0, . . . , x sub n.
The interpolating polynomial p is used in place of f, for example in
evaluation, integration, differentiation, and zero finding. Accuracy of the
interpolating polynomial depends strongly on the placement of the
interpolation points, and usually degrades drastically as one moves away from
the interval containing these points (that is, in case of extrapolation).  See
EXTRAPOLATION.
    When many interpolation points (more than 5 or 10) are to be used, it is
often much more efficient to use instead a piecewise polynomial interpolant or
spline. Suppose the interpolation points above are ordered, x0 < x1 < . . . <
x sub n. Then the cubic spline interpolant to the above data, for example,
consists of cubic polynomial pieces, with the ith piece defining the
interpolant on the interval [x(sub i-1),x(sub i)] and so matched with its
neighboring piece or pieces that the resulting function not only matches the
given function values (hence is continuous) but also has a continuous first and
second derivative.
    Interpolation is but one way to determine an approximant. In full
generality, approximation involves several choices: (1) a set P of possible
approximants, (2) a criterion for selecting from P a particular approximant,
and (3) a way to measure the approximation error, that is, the difference
between the function f to be approximated and the approximant p, in order to
judge the quality of approximation.

    Solution of linear systems.   Solving a linear system of equations is
probably the most frequently confronted computational task. It is handled
either by a direct method, that is, a method which obtains the exact answer in
a finite number of steps, or by an iterative method, or by a judicious
combination of both. Analysis of the effectiveness of possible methods has led
to a workable basis for selecting the one which best fits a particular
situation.
    Direct methods require a number of operations which increases with the
cube of the number of unknowns. Some types of problems arise wherein the
matrix of coefficients is sparse, but the unknowns may number several
thousand; for these, direct methods are prohibitive in computer time required.
One frequent source of such problems is the finite difference treatment of
partial differential equations. A significant literature of iterative methods
exploiting the special properties of such equations is available. For certain
restricted classes of difference equations, the error in an initial iterate
can be guaranteed to be reduced by a fixed factor, using a number of
computations that is proportional to n log n, where n is the number of
unknowns. Since direct methods require work proportional to n(sup 3), it is not
surprising that as n becomes large, iterative methods are studied rather
closely as practical alternatives.

    Differential equations.   Classical methods yield practical results only
for a moderately restricted class of ordinary differential equations, a
somewhat more restricted class of systems of ordinary differential equations,
and a very small number of partial differential equations. The power of
numerical methods is enormous here, for in quite broad classes of practical
problems relatively straightforward procedures are guaranteed to yield
numerical results, whose quality is predictable.  See  DIFFERENTIAL EQUATION.
 �34s[Carl de Boor]
�33s

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�6v�2v�32sNumerical indicator tube   �21m �33s 
    Any electron tube capable of visually displaying numerical figures. Some 
varieties also display alphabetical characters and commonly used symbols. In 
many electronic circuits and equipments it is desirable to indicate numbers or 
characters. Such a tube can be one of a set which displays, for instance, the 
magnitude and polarity of voltage in digital form. This is similar to 
electrical clocks which display the time in numerical form. 
    A simple and extensively used form is a cold-cathode gas tube in which 
there is a series of cathodes which light up because of the cathode glow that 
surrounds any cathode in a gas discharge. A common anode is used. The desired 
cathode is selected by any suitable switching scheme. The cathodes are made by 
photoetching the desired shape from a suitable metal. They are insulated and 
stacked one above the other, so that any particular number can be read when 
the corresponding cathode is energized.  See  ELECTRICAL CONDUCTION IN GASES; 
GAS TUBE. 
 �34s[Roger W. Wolfe]
�33s 
  
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�6v�2v�32sNumerical taxonomy   �21m �33s 
    The grouping by numerical methods of taxonomic units based on their 
character states. The application of numerical methods to taxonomy, dating 
back to the rise of biometrics in the late 19th century, has received a great 
deal of attention with the development of the computer and computer 
technology. Numerical taxonomy provides methods that are objective, explicit, 
and repeatable, and is based on the ideas first put forward by M. Adanson in 
1763. These ideas, or principles, are that the ideal taxonomy is composed of 
information-rich taxa based on as many features as possible, that a priori 
every character is of equal weight, that overall similarity between any two 
entities is a function of the similarity of the many characters on which the 
comparison is based, and that taxa are constructed on the basis of diverse 
character correlations in the groups studied.  See  TAXON. 
    In the early stages of development of numerical taxonomy, phylogenetic 
relationships were not considered. However, numerical methods have made 
possible exact measurement of evolutionary rates and phylogenetic analysis. 
Furthermore, rapid developments in the techniques of direct measurement of the 
homologies of deoxyribonucleic acid (DNA), and ribonucleic acid (RNA), between 
different organisms now provide an estimation of ``hybridization'' between the 
DNAs of different taxa and, therefore, possible evolutionary relationships. 
Thus, research in numerical taxonomy often includes analyses of the chemical 
and physical properties of the nucleic acids of the organisms, the data from 
which are correlated with phenetic groupings established by numerical 
techniques.  See  PHYLOGENY; TAXONOMIC CATEGORIES. 
    Several formulas have been suggested for calculating similarity between 
organisms. Resemblance is expressed by coefficients of similarity usually 
ranging between unity and zero, the former for perfect agreement and the 
latter for no agreement. Coefficients of dissimilarity (distance) can also be 
employed, in which case the range is between zero and an undefined positive 
value, the former for identity and the latter for maximal distance or 
disparity. Resemblance coefficients are usually tabulated in the form of a 
matrix, with one coefficient for every pair of taxonomic entities. The 
similarity index is the simplest of the coefficients employed to calculate 
similarity: S = nsubs/(nsubs + nsubd), where nsubs represents the number of 
positive features shared by two strains being compared and nsubd  represents 
the number of features positive for one strain and negative for the other (and 
vice versa). An example of an S-value matrix is shown in the illustration. 
    A variety of clustering methods has been employed in numerical taxonomy, 
including sequential, agglomerative, hierarchic, and nonoverlapping 
clustering. Dendrograms are most commonly employed to represent taxonomic 
structure resulting from cluster analysis. These have the advantage of being 
readily interpretable as conventional taxonomic hierarchies. Dendrograms are, 
in general, distinguished into phenograms representing phenetic relationships 
and cladograms representing evolutionary branching sequences. 
 �34s[Rita R. Colwell]
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�6v�2v�32sNummulites   �21m �33s 
    An extinct genus (properly known as Camerina) of the relatively large (up 
to 1.4 in. or 35 mm in diameter) unicellular protozoans of the order 
Foraminiferida. Individuals of this genus developed a lenticular, discoidal, 
planispirally coiled, involute, multichambered test (shell) composed of 
calcium carbonate. They inhabited shallow, warm marine waters of the tropical 
zone during the early Tertiary Period.  See  FORAMINIFERIDA. 
    The pyramids of Egypt are made of blocks of an Eocene nummulitic 
limestone. The numerous species are used in geologic correlation. 
 �34s[W. Storrs Cole]
�33s 
  
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�6v�2v�32sNursing   �21m �33s 
    The application of the principles of physical, biological, and social 
sciences in the physical and mental care of people, sick and well. Nursing 
includes therapy as directed by the doctor; physical and emotional care; and 
patient and family education in rehabilitation, health maintenance, and 
disease prevention. When several persons are simultaneously involved in the 
nursing care of a person or persons, nursing may also include the application 
of the principles and skills involved in management.  See  MEDICINE. 
 �34s[Ruth Sleeper]
�33s 
  
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�6v�2v�32sNutation (astronomy and mechanics)   �21m �33s 
    In mechanics, the term nutation refers to a bobbing or nodding up-and-down 
motion of a spinning rigid body, such as a top or a gyroscope, as it precesses 
about its vertical axis. Astronomical nutation refers to irregularities in the 
precessional motion of the equinoxes caused by the varying torque applied to 
the Earth by the Sun and Moon. Astronomical nutation, which is sometimes 
called nutational wandering of the terrestrial poles, should not be confused 
with nutation as defined in mechanics; the latter is present even if the 
source of the torques is unvarying.  See  PRECESSION. 
 �34s[Ray E. Bolz]
�33s 
  
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�6v�2v�32sNutmeg �21m �33s
    A delicately flavored spice obtained from the nutmeg tree (Myristica
fragrans), a native of the Moluccas, or Spice Islands. The tree is a
dark-leafed evergreen, and is a member of the nutmeg family (Myristicaceae).
The golden-yellow, mature fruits resemble apricots (see illustration). They
gradually lose moisture and when completely ripe, the husk (pericarp) splits
open, exposing the shiny brown seed which is the nutmeg of commerce. Nutmeg
oil is used in medicine, perfumery, and dentifrices, and in the tobacco
industry.  See	MAGNOLIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s


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�6v�2v�32sNutrition �21m �33s
    The science of nourishment, including the study of the nutrients that each
organism must obtain from its environment in order to maintain life and
reproduce. Mammals need for their nutrition (aside from water and oxygen) a
highly complex mixture of chemical substances, including amino acids;
carbohydrates; certain lipids; a great variety of minerals; and vitamins.  See
AMINO ACIDS; CARBOHYDRATE METABOLISM; LIPID METABOLISM; PROTEIN METABOLISM;
VITAMIN.
    One of the bases for current and continued interest in nutrition is the
fact that individuals who have differing genetic backgrounds have differing
nutritional needs, considered quantitatively; for this reason various human
ills may arise because the individuals concerned do not get all of the
nutrients in amounts compatible with their own distinctive requirements.  See
DISEASE; MALNUTRITION; METABOLIC DISORDERS.
 �34s[Rodger J. Williams]
�33s

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�6v�2v�32sNut (engineering)   �21m �33s 
    In mechanical structures, an internally threaded fastener. Plain square 
and hexagon nuts for bolts and screws are available in three degrees of 
finish: unfinished, semifinished, and finished. There are two standard 
weights: regular and heavy. For specific applications, there are other 
standard forms such as jam nut, castellated nut, slotted nut, cap nut, wing 
nut, and knurled nut. 
    Wing and knurled nuts are designed for applications where a nut is to be 
tightened or loosened by using finger pressure only.  See  SCREW FASTENER. 
 �34s[Warren J. Luzadder]
�33s 
  
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�6v�2v�32sNylon   �21m �33s 
    A family of synthetic, nonaromatic polyamides. Nylon was the first major 
synthetic fiber made entirely of a condensation polymer. 
    Textile fiber is made from the polymer by feeding polymer chips into a 
chip melter, in which a screw pushes them over a heated cylinder and forces 
the melt into a gear pump. The melt is then filtered and extruded through a 
spinneret. The polymer solidifies into fibers and is drawn out of the 
spinneret duct on a drum. Staple is made by taking the extruded and drawn 
fibers and pulling them together as tow, which is further treated before 
shipping to textile mills. Staple must be prepared, because few textile mills 
are equipped to handle filament yarn. 
    Blending nylon with other fibers gives improved tensile strength and 
abrasion resistance. By appropriate physical treatments, nylons can be formed 
into fibers resembling the best-quality wool or mohair, silk, and even cotton 
flannels and linens. Unlike the natural fibers, nylon is resistant to 
biological action by bacteria, fungus, insects, or rodents.  See  POLYAMIDE 
RESIN.  
 �34s[Frank Wagner]
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�6v�2v�32sNymphaeales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Magnoliidae of the class Magnoliopsida (dicotyledons). The order 
consists of three families, the Nymphaeaceae, Nelumbonaceae, and 
Ceratophyllaceae, with scarcely a hundred species in all. They are all aquatic 
herbs without cambium or vessels. Water lilies (Nymphaea; see illustration) 
and the lotus lily (Nelumbo) are familiar members of the order.  See  
MAGNOLIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sNystatin   �21m �33s 
    An antifungal antibiotic useful in the therapy of a wide variety of 
nonsystemic fungal infections and also as an ingredient in animal feeds for 
enhanced growth rates with poultry and swine. It is active against a wide 
range of yeasts and other fungi, including Candida, Aspergillus, Penicillium, 
Botrytis, and others, but it is without activity against bacteria. Chemically 
it is a polyene. It is produced biosynthetically by fermentation with a strain 
of Streptomyces noursei. 
 �34s[Ralph E. Bennett]
�33s 
  
  
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�6v�2v�32sOak �21m �33s
    A genus (Quercus) of trees, some of which are shrubby, with about 200
species, mainly in the Northern Hemisphere. About 50 species are native in the
United States. All oaks have scaly winter buds, usually clustered at the ends
of the twigs, and single at the nodes. The fruit is a nut (acorn). The leaves
are simple and usually lobed.
    Oaks furnish the most important hardwood lumber in the United States.
Principal uses are for charcoal, barrels, building construction, flooring,
railroad ties, mine timbers, boxes, crates, vehicle parts, ships, agricultural
implements, caskets, woodenware, fence posts, piling, and veneer. Oak is also
used for pulp and paper products.  See	FAGALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sOasis �21m �33s
    An isolated fertile area, usually limited in extent and surrounded by
desert. The term was initially applied to small areas in Africa and Asia
typically supporting trees and cultivated crops with a water supply from
springs and from seepage of water originating at some distance. However, the
term has been expanded to include areas receiving moisture from intermittent
streams or artificial irrigation systems. Thus the floodplains of the Nile and
Colorado rivers can be considered vast oases, as can arid irrigated areas.
    Among the native plants restricted to waste areas and abandoned plots are
a wide variety of phreatophytes, including tamarisk (Tamarix sp.), cattail
(Typha latifolia), sedges, and rushes; farther away many halophytes are found
where drainage is poor; and beyond this fringe, desert vegetation takes over.
 �34s[William G. McGinnies]
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�6v�2v�32sOats �21m �33s
    An agricultural crop (genus Avena) grown for its grain and straw in most
countries of the temperate zones of the world. In general, oats are a
cool-season crop which requires a moist climate. They grow well on both light
and heavy soils if sufficient moisture and fertility nutrients are available.
    Oat grain usually contains 10 - 16% protein, which makes it especially
good for rations for young livestock and for human food. Dehulled oat seeds
are used extensively for making breakfast cereals, such as porridge made from
rolled oats. In general, oat grain contains adequate quantities of minerals
and B vitamins for normal diet. The fat content is low. Oat hulls are used to
make furfural, an important chemical in nylon manufacturing.
 �34s[Kenneth J. Frey /John A. Shellenberger]
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�6v�2v�32sObesity �21m �33s
    The presence of an excessive amount or abnormal distribution of fat in an
individual. This tendency toward obesity is enhanced by rich diets, lack of
regular exercise, and the rejection of certain fundamental health rules. In
addition, emphasis has been placed upon the relationship between the
overindulgence in food and tension-producing situations. People are said to
eat too much because they are anxious, shy, insecure, or hostile, or have some
other need for which eating can at least partly compensate.  See  NEUROSIS.
    Studies in nutrition and metabolism indicate that, although such factors
certainly may play a role in many cases, more subtle differences occur among
individuals. As a person ages, the metabolic rate tends to slow down somewhat;
more important, however, there are changes in hormonal balance which affect
fat utilization by the body.  See  GERONTOLOGY; HORMONE; NUTRITION.
    Obesity occasionally appears in relation to specific disease processes.
These include certain disorders of the hypothalamus or injury to it resulting
from changes in adjacent structures, such as in the formation of some
pituitary tumors. Froehlich's syndrome, characterized by a feminine pattern of
obesity and sexual dysfunction, is thought to result from associated damage to
the neighboring hypothalamus, although it was formerly believed to be the
result of hypopituitarism. The influence of hereditary factors cannot be
overlooked because it has been established that obesity and other body
characteristics tend to be transmitted from one generation to the next.  See
HUMAN GENETICS.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sObolellida   �21m �33s 
    A small order of inarticulate brachiopods which are predominantly Early 
Cambrian but which are known in Middle Cambrian rocks; they are among the 
oldest known representatives of the phylum.  See  INARTICULATA. 
    Superficially, they show the closest similarity to obolids, which belong 
to the order Lingulida. Like the obolids, they are biconvex and subcircular to 
elongate oval in outline and have marginal beaks. They differ, however, in a 
number of features, most conspicuously in shell composition; the shell of the 
Obolellida is of calcium carbonate in contrast to the calcium phosphate shell 
of the obolids.  See  BRACHIOPODA; LINGULIDA. 
 �34s[A. J. Rowell]
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�6v�2v�32sObsessive-compulsive reaction �21m �33s
    A type of neurosis. The characteristic symptom pattern of the obsessive
compulsive is the irrational persistence of ideation and enactment which is
recognized as irrational but which the person cannot alter in spite of his
efforts. Seemingly irrational and fragmentary phrases, admonitions, and
melodies may haunt the person in an unshakable way in the case of obsessive
symptoms and, in compulsive behavior, the person is driven to perform acts
whose relevance for his problems remains utterly obscure to him, for example,
repeated handwashing, sometimes to a self-injurious point.  See  ABNORMAL
BEHAVIOR; NEUROSIS.
 �34s[Jerome S. Bruner ;Walter Mischel]
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�6v�2v�32sObsidian   �21m �33s 
    A volcanic glass, usually of rhyolitic composition, formed by rapid 
cooling of viscous lava. The color is jet-black because of abundant 
microscopic, embryonic crystal growths (crystallites) which make the glass 
opaque except on thin edges. Iron oxide dust may produce red or brown obsidian. 
    Obsidian usually forms the upper parts of lava flows. Well-known 
occurrences are Obsidian Cliffs in Yellowstone Park, Wyoming; Mount Hekla, 
Iceland; and the Lipari Islands off the coast of Italy.  See  IGNEOUS ROCKS; 
VOLCANIC GLASS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sOccultation   �21m �33s 
    The apparent disappearance of a star or planet behind the surface of the 
Moon, or of a satellite behind the disk of the parent planet. Occultations by 
the Moon are observed to determine the position of the Moon at the time of the 
phenomenon. They are also used for precise determinations of longitude. 
Because of the eastward motion of the Moon against the background of stars, 
immersion (or disappearance) of the star or planet occurs at the eastern limb, 
and emersion (or reappearance) at the western limb. A single occultation is 
visible only from a certain region of Earth. Outside that region, the effect 
of parallax causes the star or planet to remain clear of the lunar disk. 
 �34s[Simone Daro Gossner]
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�6v�2v�32sOcean-atmosphere relations   �21m �33s 
    A field of investigation concerned with the boundary zone between sea and 
air and the dynamic relationships between oceanographic and meteorologic 
studies. The largest fraction of the heat energy the atmosphere receives for 
maintaining its circulation is derived from the condensation of water vapor 
originating primarily from oceanic evaporation. Therefore, an understanding of 
processes occurring at the air-sea boundary is fundamental to an understanding 
of atmospheric behavior. The oceanic energy supply to the atmosphere is highly 
regionalized because of the character of ocean currents. The atmospheric 
circulation itself (and resulting weather) is greatly influenced by the 
oceanic circulation. Conversely, the oceanic circulation represents a state of 
equilibrium in which the effects of the frictional stresses of the wind on the 
sea surface are balanced by changes in the distribution of density of oceanic 
waters. These compensating density changes are in turn related to time and 
space variations in radiation, heat conduction, evaporation, and 
precipitation. An understanding of the oceanic circulation (and the resulting 
distribution of properties within the ocean) requires a thorough knowledge of 
atmospheric processes at the air-sea boundary. 
 �34s[Woodrow C. Jacobs]
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�6v�2v�32sOcean currents   �21m �33s 
    The general circulation of the ocean. The term is usually understood to 
include large-scale, nearly steady features, such as the Gulf Stream, as well 
as current systems which change seasonally but are persistent from one year to 
the next, such as the Davidson Current, off the northwestern United States 
coast, and the equatorial currents in the Indian Ocean. A great number of 
energetic motions have periods of a month or two and horizontal scales of a 
few hundred kilometers - a very-low-frequency turbulence, collectively called 
eddies. These are transient features, but locally have all the characteristics 
of currents. They are ubiquitous features in the ocean, although their energy 
varies widely, being highest near Western Boundary Currents. Energetic motions 
are also concentrated near the local inertial period (24 h, at 30DG latitude) 
and at the periods associated with tides (primarily diurnal and semidiurnal).  
See  TIDE. 
    The greatest single driving force for currents, as for waves, is the wind. 
Furthermore, the ocean absorbs heat at low latitudes and loses it at high 
latitudes. The resultant effect on the density distribution is coupled into 
the large-scale wind-driven circulation. Some subsurface flows are caused by 
the sinking of surface waters made dense by cooling or high evaporation.  See  
OCEAN WAVES. 
  
    Surface currents.   Except in western boundary currents, and in the 
Antarctic Circumpolar Current, the system of strong surface currents is 
restricted mainly to the upper 330 - 660 ft (100 - 200 m) of the sea. The 
mid-latitude anticyclonic gyres, however, are coherent in the mean well below 
3300 ft (1000 m). The average speeds of the open-ocean surface currents remain 
mostly below 20 cm/s (0.4 knot). Exceptions to this are found in the western 
boundary currents, such as the Gulf Stream, and in the Equatorial Currents of 
the three oceans, all of which have velocities of 1 - 2 m/s (2 - 4 knots). 
  
    Deep circulation.   The deep circulation results in part from the wind 
stress and in part from the internal pressure forces which are maintained by 
the budgets of heat, salt, and water. Both groups of forces are dependent upon 
atmospheric influences. Apart from Coriolis and frictional forces, the 
topography of the sea bottom exercises a decisive influence on the course of 
deep circulation. 
    The deep circulation in the oceans is more difficult to perceive than the 
circulation in the marginal seas. In addition to the internal pressure forces, 
determined by the distribution of density and the piling up of water by the 
wind, there are also the influences of Coriolis forces and large-scale 
turbulence. There are areas in tropical latitudes in which the surface water, 
as a result of strong evaporation, has a relatively high density. In 
thermohaline convection, the water sinks while flowing horizontally until it 
reaches a density corresponding to its own, and then spreads out horizontally. 
In this way the colder and deeper levels of the oceans take on a layered 
structure consisting of the so-called bottom water, deep water, and 
intermediate water. 
 �34s[Wilton Sturges]
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�6v�2v�32sOceanic birds   �21m �33s 
    Any species of birds which are capable of living for long periods of time 
away from land and obtaining food from the open sea, that is, pelagic birds. 
They come to the land only during the breeding season. The avian order 
Procellariformes contains the largest number and variety of pelagic forms, all 
of which live beyond the continental edge, the home range of the true deep-sea 
species. These birds depend on a diet of various aquatic forms, such as 
mollusks and planktonic crustaceans, rather than upon fish. They are adapted 
to their salt-water environment because of the large nasal glands known as 
salt glands, which control salt secretion and maintain a physiological 
balance.  See  PROCELLARIFORMES. 
    Most species of oceanic birds are long-lived and have a long period of 
maturation and a low reproductive rate; many species produce a single egg each 
year. The young are helpless when hatched and are fed by regurgitation. 
Included in this group are the albatrosses, fulmars, petrels, and shearwaters. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sOceanic islands �21m �33s
    Those islands which rise from the deep-sea floor rather than from shallow
continental shelves. Most islands in gulfs and seas that fringe the great
ocean basins are geologically similar to the nearby continents. On the other
hand, almost all islands that rise from the ocean basins are volcanoes with or
without coral reef and, geologically, bear little relation to the continents.
Volcanic islands are only the tops of much larger undersea volcanoes, most of
which are associated with great submarine structures, such as submarine ridges
and fractures in the Earth's crust (see illustration).  See  MARINE GEOLOGY.
    On the deep-sea floor, volcanoes begin as lava flows from fissures in the
Earth's crust under 2 to 3 mi (3 to 5 km) of water. Gradually they build
upward through the water, but about nine-tenths of the submarine volcanoes
become inactive and stop their growth before they reach the sea surface. The
others burst from the deep water into a new realm, where wave and subaerial
erosion combat their upward growth. At first the volcanoes tend to produce ash
and cinders, which are easily eroded. Gradually as the pile becomes broader,
volcanoes rise above the waves and more resistant fluid lava flows build a
solid island. Where several nearby volcanoes merge together, as in Hawaii, a
great island may form.	See  VOLCANO; VOLCANOLOGY.
    Volcanoes are active for no more than a few million years, however, and
inactive volcanoes are inevitably worn down to shallow submarine banks by
erosion which never stops. In addition to these worn-down volcanoes, drowned
former islands called guyots or tablemounts have been discovered in all the
ocean basins, mostly at depths of 1000 - 7000 ft (300 - 2000 m).  See
SEAMOUNT AND GUYOT.
 �34s[Edwin L. Hamilton ;Henry W. Menard, Jr.]
�33s



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�6v�2v�32sOceanographic platforms �21m �33s
    Supporting structures (stations) for instruments used in sensing and
recording the various parameters of the oceanic environment. The platforms
come in assorted shapes and sizes, ranging from moored buoys to fixed offshore
towers and from surface survey ships to deep-diving submersibles. The former
group collects a history of data over long periods of time in a specific
location, whereas the latter group collects data over a large area, usually
for a specific instant of time.  See  OCEANOGRAPHIC SUBMERSIBLES;
OCEANOGRAPHIC VESSELS.
 �34s[J. J. Hromadik]
�33s

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�6v�2v�32sOceanographic submersibles �21m �33s
    Small research vessels for underwater transport of people and equipment
and for use as underwater platforms for observation, sampling, measurement,
and performing various work tasks. They are also referred to as undersea
vehicles.
    Undersea vehicle systems are designed in accordance with user
requirements. A major trend involves design of a completely integrated system,
which, in addition to the submersible, includes support ship, handling gear
for launch and retrieval, and logistic and maintenance support. The objective
is to obtain an effective, high utilization rate under varying weather
conditions. The major vehicle operating problem is handling during launch and
retrieval in heavy seas.  See  OCEANOGRAPHIC VESSELS.
    Most small submersibles need buoyancy in excess of that produced by their
pressure hulls to attain neutral buoyancy while submerged. Low-density solids
that may be considered for this purpose include polyethylene, polypropylene,
expanded plastics, inorganic foams, and syntactic foam.
    As a source of power, the lead-acid storage battery is the most widely
employed. It places severe limitations on small submersibles because of its
relatively high weight-to-energy ratio. However, its characteristics are well
known.	See  STORAGE BATTERY.
    Sensors are instruments required on small submersibles to determine
position, communicate with other units, and make observations of the
environment both for a record and while performing work. Life support systems
on the small submersibles take into consideration the following services for
atmospheric control and monitoring: breathing mixture supply system; carbon
dioxide removal system; hydrogen, carbon monoxide, and toxin removal systems;
air purification and filtering system; atmospheric monitoring system; and
emergency breathing supply system.
    Since 1965, the Alvin has been probably the best-known submersible. It is
equipped with an extensive instrumentation suite, including navigation and
obstacle avoidance sonar, underwater telephone, radios, underwater lights,
35-mm cameras and strobes, underwater television system, and a mechanical
manipulator capable of lifting up to 50 lb (22.7 kg) of bottom samples.
Maximum operating depth is 12,000 ft (3600 m). Normal operations carry a crew
of one or two scientific observers. Alvin is normally supported by its tender,
Lulu, and is carried on board when not diving.
 �34s[D. C. Beumariage ;R. Frank Busby]
�33s

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�6v�2v�32sOceanographic vessels   �21m �33s 
    The largest and most basic tools used in the scientific study and 
exploration of the oceans. The term oceanographic vessels includes both 
conventional and special-purpose research ships and specialized research 
vehicles, such as deep submersibles and moored buoys. Uncrewed platforms and 
equipment, such as buoys, are commonly considered instruments rather than 
vehicles.  See #OCEANOGRAPHIC PLATFORMS; OCEANOGRAPHIC SUBMERSIBLES.  
    The primary purpose of a research ship is to carry scientists and 
technicians and their equipment to locations at sea where their investigations 
are to take place. Conventional oceanographic ships are used for 
multidiscipline research. Special-purpose oceanographic vessels, such as 
research submarines, crewed buoys, fisheries research ships, special ships to 
tend and carry deep-submergence vehicles (see illustration), research drilling 
ships, and routine survey ships, are often limited by the special requirements 
of their primary tasks from being economically suitable for multidiscipline 
operation in oceanographic investigations. 
 �34s[Jonathan Leiby]
�33s 
  
  
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�6v�2v�32sOceanography �21m �33s
    The scientific study and exploration of the oceans and seas in all their
aspects, including the sediments and rocks beneath the seas; the interaction
of sea and atmosphere; the body of sea water in motion and subject to internal
and external forces; the living content of the seas and sea floors and the
behavior of these organisms; the chemical composition of the water; the
physics of the sea and sea floor; the origin of ocean basins and ancient seas;
and the formation and interaction of beaches, shores, and estuaries. Hence
oceanography, sometimes called the science of the seas, consists of the marine
aspects of several disciplines and branches of science: geology, meteorology,
biology, chemistry, physics, geophysics, geochemistry, fluid mechanics, and in
its more theoretical aspects, applied mathematics. Oceanography is also an
environmental science which describes and attempts to explain all processes in
the ocean, and the interrelation of the ocean with the solid and gaseous
phases of the Earth and with the universe.  See  MARINE ECOSYSTEM; MARINE
GEOLOGY; OCEAN ATMOSPHERE RELATIONS; OCEAN CURRENTS; OCEAN WAVES; SEA WATER;
TIDE.
    Indirect benefits of oceanography arise from the application of marine
meteorology to weather prediction, not only over the sea areas but also over
the land. The study of marine geology and marine ecology aid in the
understanding of the character of oil-bearing sedimentary rocks found on land.
  See  MARINE INFLUENCE ON WEATHER AND CLIMATE.
 �34s[William A. Nierenberg]
�33s

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�6v�2v�32sOcean waves �21m �33s
    The irregular moving bumps and hollows on the ocean surface. Winds blowing
over the ocean, in addition to producing currents, create surface water
undulations called waves or a sea. The characteristics of these waves (or the
state of the sea) depend on the speed of the wind, the length of time that it
has blown, the distance over which it has blown, and the depth of the water.
If the wind dies down, the waves that remain are called a dead sea. Waves can
travel hundreds and thousands of miles from where they were generated into
areas where the wind is light. These waves are called swell.  See  SEA STATE.
For a discussion of wave characteristics  see  CAPILLARY WAVE; INTERNAL WAVE;
NEARSHORE SEDIMENTARY PROCESSES; SEICHE; STORM SURGE; TSUNAMI; WAVE MOTION IN
LIQUIDS.
    Wave spectral components first grow linearly in resonance with turbulent
pressure components advected by the winds of the atmosphere. The components
then grow exponentially by extracting energy from the wind profile. As
saturation at a particular frequency is reached, the formation of whitecaps
limits further growth. Although not accepted by all scientists, there is
considerable evidence that, if the wind blows long enough at constant velocity
over a large enough area, the spectrum of the waves will depend only on wind
speed.
    The rate at which waves are generated depends very much on whether or not
there is a background spectrum present when the wind speed increases. With no
background, a fairly long time is required to generate a fully developed sea.
With a background, the time required is some variable fraction, often one-half
or less, of the time required from a truly zero initial condition. Variable
background conditions imply that the spectrum at all points over the ocean
must be tracked accurately for accurate determination of changes.  See	WIND
STRESS OVER SEA.
 �34s[Willard J. Pierson, Jr. ;Gerhard Neumann]
�33s
    Instruments designed to measure ocean waves can be grouped into two
classifications: those that sense the elevation of the surface water, and
those that sense the subsurface pressure fluctuations generated by the waves.
Surface elevation sensing gages include (1) surface float devices mechanically
connected to a recording mechanism, (2) devices that record the buoyant force
on a vertical cylinder, (3) recorders actuated by vertical accelerometers
mounted in surface buoys, (4) inverted echo sounders (fathometers) fixed below
the surface to echo off the water surface, (5) accurate absolute altimeter
recorders operated in aircraft flying at a fixed elevation, (6)
stereophotographs, and (7) electrical elements whose resistance or capacitance
is a function of the elevation of the water surface.
 �34s[Frank E. Snodgrass]
�33s

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�6v�2v�32sOctagon �21m �33s
    A figure formed by the eight line segments (sides) that join in order
eight ordered points (vertices) of a plane. In elementary geometry it is
assumed that the sides do not cross and, usually, even that the finite region
of the plane bounded by the sides is convex (convex octagon). A regular
octagon has each two of its sides congruent, and each angle made by adjacent
sides equal to 135 DG (see illustration). The area of a regular octagon of side
a is 2([SQUARE ROOT OF 2] + 1)a(sup 2).  See  POLYGON; REGULAR POLYTOPES.
 �34s[Leonard M. Blumenthal]
�33s

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�6v�2v�32sOctahedron �21m �33s
    In geometry, a solid with 8 faces. The faces of a regular octahedron are
all congruent equilateral triangles. These 8 triangles (see illustration),
have a total of 6 vertices and 12 edges - the vertices and edges of the
octahedron. The solid is dual to the cube, which has 8 vertices, 6 faces, and
12 edges. The centers of the faces of a regular octahedron are the vertices of
a cube; conversely, the centers of the faces of a cube are the vertices of a
regular octahedron. The volume of an octahedron with edge a is
a(sup 3)([SQUARE ROOT OF 2]/3).
See  POLYHEDRON; REGULAR POLYTOPES.
 �34s[Leonard M. Blumenthal]
�33s

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�6v�2v�32sOctane   �21m �33s 
    An alkane having the formula C8H18. The 18 possible isomers range in 
boiling point from 99.3DGC or 2170DGF (2,2,4-trimethylpentane) to 125.6DGC or 
258.1DGF (n-octane). 
    The branched-chain octanes are obtained commercially by the catalytic 
alkylation of isobutane with the n-butylenes or isobutylene. They have high 
antiknock rating and are therefore valuable constitutents of gasoline.  See  
ALKANE; GASOLINE; OCTANE NUMBER. 
 �34s[Louis Schmerling]
�33s 
  
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�6v�2v�32sOctane number   �21m �33s 
    A standard laboratory measure of a fuel's ability to resist knock during 
combustion in a spark-ignition engine. A single-cylinder four-stroke engine of 
standardized design is used to determine the knock resistance of a given fuel 
by comparing it with that of primary reference fuels composed of varying 
proportions of two pure hydrocarbons, one very high in knock resistance and 
the other very low. A highly knock-resistant isooctane 
(2,2,4-trimethylpentane, C8H18) is assigned a rating of 100 on the octane 
scale, and normal heptane (C7H16), with very poor knock resistance, represents 
zero on the scale. Octane number is defined as the percentage of isooctane 
required in a blend with normal heptane to match the knocking behavior of the 
gasoline being tested.  See  COMBUSTION KNOCK; OCTANE. 
    For fuels with a rating higher than 100 octane, the rating is usually 
obtained by determining the amount of tetraethyllead compound that needs to be 
added to pure isooctane to match the knock resistance of the test fuel.  See  
ANTIKNOCK AGENTS. 
 �34s[John C. Lane]
�33s 
  
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�6v�2v�32sOctave   �21m �33s 
    The interval between two sounds having a basic frequency ratio of 2:1; 
also, the interval in pitch between two tones such that one tone may be 
regarded as duplicating at the next higher pitch the basic musical import of 
the other tone. The first definition given is concerned with physical 
oscillations, and in this sense, the interval in octaves between any two 
frequencies is the logarithm to the base 2 of the frequency ratio. The second 
definition deals with sensations. Historically, a ratio of 2:1 in frequency 
was considered to correspond to the pitch interval of an octave, but 
psychological research of the 20th century has demonstrated that this is not 
necessarily the case.  See  PITCH; SCALE (MUSIC). 
 �34s[Robert W. Young]
�33s 
  
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�6v�2v�32sOctopoda   �21m �33s 
    An order of the dibranchiate cephalopods containing the octopus, argonaut, 
blanket octopus, and others. They possess eight arms equipped with one to 
three rows of suckers. The common octopus (Octopus vulgaris) and its allies 
are secretive bottom dwellers in shallow to moderate depths. They feed upon 
bivalve mollusks and crustaceans and are often serious predators on lobsters 
and crabs. Other octopods, such as Argonauta argo with its fragile egg case, 
live in the open ocean. Many are small gelatinous forms occurring down to 9900 
ft (3000 m), while larger deep-sea forms may have feeble paddle-shaped fins, 
and one, Cirrothauma, is blind. The largest octopus recorded had an arm spread 
of 32 ft (9.6 m) but most are much smaller. There are about 150 species.  See  
CEPHALOPODA; DIBRANCHIA. 
 �34s[Gilbert L. Voss]
�33s 
  
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�6v�2v�32sOculosida   �21m �33s 
    An order of Radiolaria. Pores of these protozoans are restricted to 
certain areas in the central capsule, which may be a single or double membrane 
in different groups. The order is subdivided on the basis of arrangement of 
the pores. In the Monopylina (Nassellarina) pores lie at one pole of the 
single-layered capsule. In the Tripylina (Phaeodorina) the major opening 
(astropyle) usually contains a perforated plate. A common feature is the 
presence, near the astropyle, of olive-colored material (possibly it is 
partially digested food) sometimes known as phaeodium.  See  RADIOLARIA. 
 �34s[Richard P. Hall]
�33s 
  
  
  
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�6v�2v�32sOdonata   �21m �33s 
    An order of the class Insecta known as dragonflies. There are probably 
less than 3000 species known throughout the world. The order is divided into 
the Anisoptera, or true dragonflies, and the Zygoptera, or damselflies. 
    The young inhabit ponds, streams, and marshes; the adults fly over these 
localities or adjacent land. The adult structure is unique, characterized by a 
head with large compound eyes and wings with clear or transparent membranes 
traversed by networks of veins (see illustration); the male has accessory 
genital organs possessed by no other insects. 
    There are three general stages in the life history: the egg, the nymph 
(see illustration), sometimes called naiad, and the adult. Development is 
usually slow, often requiring 3 - 5 years. Rarely is there more than one 
generation a year in the northern range. Adults may live for an extended 
period in summer. Eggs are laid by insertion into plant stems, either beneath 
the water or just above the surface. Others are dropped directly into the 
water and sink to the bottom, where they hatch and the nymphs develop. 
    Dragonflies constitute one of the oldest insect orders; they can be traced 
back through fossil records to the Carboniferous and Permian. Surprisingly few 
changes have occurred since then, although the order has diversified and 
specialized to counter competition and to avoid enemies; for example, wing 
venation is more complex and body structure more varied. 
 �34s[Philip Garman]
�33s 
  
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�6v�2v�32sOdontognathae   �21m �33s 
    One of the two superorders making up the subclass Neornithes, or true 
birds. It was originally erected to include fossil birds that could fly or 
that were secondarily flightless but still possessed teeth; it was regarded 
therefore as an evolutionary step between Archaeopteryx and fully evolved 
modern birds. It is now considered to contain only a simple order, 
Hesperornithiformes, of which the best-known family is the Hesperornithidae 
containing several species from the Upper Cretaceous of North America.  See  
ARCHAEORNITHES; AVES; HESPERORNITHIFORMES; NEORNITHES. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sOdontostomatida   �21m �33s 
    An order of the Spirotrichia which represents a minor group of small, 
bizarre-looking protozoan species. The odontostomes are compressed laterally 
and possess very little ciliature. Even the adoral zone of membranelles is 
reduced in prominence. These ciliates are found in sewage disposal environs 
and other fresh- or salt-water habitats which have a very low oxygen content.  
See  CILIOPHORA; SPIROTRICHIA. 
 �34s[John O. Corliss]
�33s 
  
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�6v�2v�32sOedogoniales   �21m �33s 
    An order of fresh-water algae in the division Chlorophyta. These plants 
are branched or unbranched microscopic filaments with a basal holdfast cell. 
These plants have characteristics which sharply set them apart from all other 
green algae. The cells are slightly larger at the anterior end and undergo a 
peculiar type of cell division involving the building in of a cylindrical 
section between the older parts of the original cell. In Bulbochaete, however, 
cell division and growth of the filament occur at the basal cell. The 
chloroplast is a parietal network with many pyrenoids.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
�33s 
  
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�6v�2v�32sOegophiurida   �21m �33s 
    An order of Ophiuroidea comprising eight families. Seven of these are 
represented by Paleozoic fossils, but one has a living representative, 
Ophiocanops. Oegophiurids appeared in the Ordovician and were apparently the 
first ophiuroids in which the ambulacral ossicles fused to form the typical 
ophiuroid arm vertebrae. Dorsal and ventral arm plates are lacking, and the 
ambulacral groove remains as in their somasteroid ancestors. The gastric ceca 
extend into the arms, and the gonads are located there too, arranged in 
series.  See  OPHIUROIDEA; SOMASTEROIDEA. 
 �34s[Andrew C. Campbell]
�33s 
  
  
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�6v�2v�32sOhmmeter   �21m �33s 
    A small, portable instrument using a microammeter and associated circuitry 
to measure resistance by the voltmeter-ammeter method. Additional circuits are 
usually included to measure alternating and direct-current volts and amperes, 
and the instrument is called a volt-ohm-milliammeter, or multimeter.  See  
AMMETER; RESISTANCE MEASUREMENT; VOLTMETER. 
 �34s[Charles E. Applegate]
�33s 
  
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�6v�2v�32sOhm's law �21m �33s
    The direct current flowing in an electrical circuit is directly
proportional to the voltage applied to the circuit. The constant of
proportionality R, called the electrical resistance, is given by the equation
below, in which V is the applied voltage and I is the current.

			     V = RI

Numerous deviations from this simple, linear relationship have been discovered. 
CONDUCTIVITY; ELECTRICAL RESISTANCE.
 �34s[Charles E. Applegate]
�33s



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�6v�2v�32sOil and gas field exploitation �21m �33s
    In the petroleum industry, a field is an area underlain without
substantial interruption by one or more reservoirs of commercially valuable
oil or gas, or both. A single reservoir (or group of reservoirs which cannot
be separately produced) is a pool. Several pools separated from one another by
barren, impermeable rock may be superimposed one above another within the same
field. Pools have variable areal extent. Any sufficiently deep well located
within the field should produce from one or more pools. However, each well
cannot produce from every pool, because different pools have different areal
limits.
    Development of a field includes the location, drilling, completion, and
equipment of wells necessary to produce the commercially recoverable oil and
gas in the field.

    General considerations.   Oil and gas production necessarily are
intimately related, since approximately one-third of the gross gas production
in the United States is produced from wells that are classified as oil wells.
However, the naturally occurring hydrocarbons of petroleum are not only liquid
and gaseous but may even be found in a solid state, such as asphaltite and
some asphalts.
    Where gas is produced without oil, the production problems are simplified
because the product flows naturally throughout the life of the well and does
not have to be lifted to the surface. However, there are sometimes problems of
water accumulations in gas wells, and it is necessary to pump the water from
the wells to maintain maximum, or economical, gas production. The line of
demarcation between oil wells and gas wells is not definitely established.
Most gas wells produce quantities of condensable vapors, such as propane and
butane, that may be liquefied and marketed for fuel, and the more stable
liquids produced with gas can be utilized as natural gasoline.

    Production methods in producing wells.   The common methods of producing
oil wells are (1) natural flow; (2) pumping with sucker rods; (3) gas lift;
(4) hydraulic subsurface pumps; (5) electrically driven centrifugal well
pumps; and (6) swabbing.
    Natural flow is the most economical method of production and generally is
utilized as long as the desired production rate can be maintained by this
method. It utilizes the formation energy, which may consist of gas in solution
in the oil in the formation; free gas under pressure acting against the liquid
and gas-liquid phase to force it toward the well bore; water pressure acting
against the oil; or a combination of these three energy sources.
    However, most wells are not self-flowing and various lifting methods must
be employed. Approximately 90% of the wells made to produce by some artificial
lift method in the United States are equipped with sucker-rod - type pumps. In
these the pump is installed at the lower end of the tubing string and is
actuated by a string of sucker rods extending from the surface to the
subsurface pump. The two common variations are mechanical and hydraulic
long-stroke pumping. Other lifting mechanisms are the gas lift, hydraulic
subsurface pumps, swabs, bailers, jet pumps, and sonic pumps.  See  OIL AND GAS
WELL COMPLETION. 

    Production instruments.   The commoner and more important instruments
required in petroleum production operations are the following:
    1. Gas meters, which are generally of the orifice type, are designed to
record the differential pressure across the orifice, and the static pressure.
    2. Recording subsurface pressure gages small enough to run down 2-in.
(3-cm) ID (inside diameter) tubing are used extensively for measuring pressure
gradients down the tubing of flowing wells, recording pressure buildup when
the well is closed in, and measuring equilibrium bottom-hole pressures.
    3. Subsurface samplers designed to sample well fluids at various levels in
the tubing are used to determine physical properties.
    4. Oil meters of various types are utilized to meter crude oil flowing to
or from storage.
    5. Dynamometers are used to measure polished-rod loads.
    6. Liquid-level gages and controllers are used. They are similar to those
used in other industries, but with special designs for closed lease tanks.
 �34s[Roy L. Chenault]
�33s

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�6v�2v�32sOil and gas, offshore �21m �33s
    Oil and natural gas prospecting and exploitation on the continental
shelves and slopes. Exploration on the more than 8,000,000 misup2 (2.07 x
10(sup 7) km(sup 2)) of the world's continental shelves and slopes, lying
between the shore and 1000-ft (300-m) water depth includes exploration or
drilling or both off the coasts of more than 75 nations.
    For many years petroleum companies stopped at the water's edge or sought
and developed oil and gas accumulations only in the shallow seas bordering
onshore producing areas. Exploration deeper under the sea did not begin in
earnest until the world's burgeoning appetite for energy sources, coupled with
a lessening return from land drilling, provided the incentives for the huge
investments required for drilling in the open sea. There has been a steady
annual increase in the number of wells drilled in deep water beyond the
continental shelves, although the costs of developing the actual producing
systems are enormous.
    The underwater search has been made possible only by vast improvements in
offshore technology. A wide variety of rig platforms has evolved, some
designed to cope with specific hazards of the sea and others for more general
work. The world's mobile platform fleet can be divided into four main
groupings: self-elevating platforms, submersibles, semisubmersibles, and
floating drill ships.
    The most widely used mobile platform is the self-elevating, or jack-up,
unit (see illustration). It is towed to location, where the legs are lowered
to the sea floor, and the platform is jacked up above wave height. These
self-contained platforms are especially suited to wildcat and delineation
drilling.
    The submersible platforms are towed to location and then submerged to the
sea bottom. They are very stable and can operate in areas with soft sea
floors. Difficulty in towing is a disadvantage, but this is partially offset
by the rapidity with which they can be raised or lowered, once on location.
Semisubmersibles are a version of submersibles. They can work as
bottom-supported units or in deep water as floaters.
    Floating drill ships are capable of drilling in 60-ft (18-m) to abyssal
depths. They are built as self-propelled ships or with a ship configuration
that requires towing.

    The move of exploration into the open hostile sea has required not only
the development of drilling vessels but a host of auxiliary equipment and
techniques. A whole new industrial complex has developed to serve the offshore
industry. Of particular interest is the development of diving techniques and
submersible equipment to aid in exploration and the completion of wells.
Economics will eventually require sea-bottom completions in which robots will
make the necessary pipe and well connections.
 �34s[G. R. Schoonmaker]
�33s

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�6v�2v�32sOil and gas well completion �21m �33s
    The operations that prepare for production of a well drilled to an oil or
natural gas reservoir. Various problems of well casing during and at the end
of drilling are related to modes of completing connection between the proper
reservoirs and the surface. Tubing inside the casing and valves and a pumping
unit at the surface must deliver reservoir products to the surface at a
controlled rate. Variations in reservoir and overlying formations may require
special techniques to keep out water or sand or to increase the production
rate.  See  OIL AND GAS FIELD EXPLOITATION; OIL AND GAS WELL DRILLING.

    Casing.   Oil and gas wells are walled with steel tubing (casing) which is
cemented in place. Tubing lengths, called joints, are threaded and coupled so
that they may be joined together in a continuous string in the well bore.
Properly placed and cemented in the hole, casing protects fresh-water
reservoirs from contamination, supports unconsolidated rock formations,
maintains natural separation of formations, aids in the prevention of blowouts
and waste of reservoir energy, and acts as a conduit for receiving pipe of
smaller diameters through which the well effluent may be brought to the
surface under controlled conditions. It may be necessary to set many strings
of casing in one hole before reaching the objective (Fig. 1).


    Production-flow control.   A steel tube, smaller in diameter than the
casing, serves as a production flow line within the well. The tubing is run
inside the casing and is either suspended in the hole or set on a production
packer at or near the producing interval. The top of the tubing string
terminates at the surface in a sealing element in the wellhead assembly to
which the so-called Christmas tree is attached. A manifold constructed of
steel valves and fittings, placed on top of the casings protruding above the
surface, is called a Christmas tree (Fig. 2). Its purpose is to maintain the
well under proper control, to receive the formation products under pressure,
and to control the rate of daily production from the reservoir and direct it
into a pipeline, generally at reduced pressures, to the oil-gathering station.
    Most oil wells require secondary means of removing the reservoir product.
The most common of several methods is the pumping unit.  See
PETROLEUM-ENHANCED RECOVERY.

    Production-stimulation techniques. Any method designed to increase the
production rate from a reservoir is defined as production stimulation. Three
of the methods used are acidizing, fracturing, and employing explosives.
    In acidizing, varied volumes of hydrochloric acid are used in limestone
and dolomite or other acid-soluble formations to dissolve the existing
flow-channel walls and enlarge them. Formation fracturing is a hydraulic
process aimed at the parting of a desired section of formation. Selected
grades of sand or particles of other materials are added to the fracturing
fluid in varied quantities. These particles pack and fill the fracture, acting
as a propping agent to hold it open when the applied pressure is released.
Explosives are used to remove reservoir-blocking material from the reservoir
face and to create fractures in the rock to increase production. See
PETROLEUM RESERVOIR ENGINEERING. [Harry S. Brigham]


A700000002417�254s�5v�1v1192
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�6v�2v�32sOil and gas well drilling   �21m �33s 
    The drilling of holes for exploration and extraction of crude oil and 
natural gas. Deep holes and high pressures are characteristics of petroleum 
drilling not commonly associated with other types of drilling. In general, it 
becomes more difficult to control the direction of the drilled hole as the 
depth increases, and additionally, the cost per foot of hole drilled increases 
rapidly with the depth of the hole. Drilling-fluid pressure must be 
sufficiently high to prevent blowouts but not high enough to cause fracturing 
of the bore hole. Formation-fluid pressures are commonly controlled by the use 
of a high-density clay-water slurry, called drilling mud. The primary 
disadvantage in the use of drilling muds is the relatively low drilling rate 
which normally accompanies high bottom-hole pressure. Drilling rates can often 
be increased by using water to circulate the cuttings from the hole; when 
feasible, the use of gas as a drilling fluid can lead to drilling rates as 
much as 10 times those attained with mud.  See  BORING AND DRILLING (MINERAL); 
OIL AND GAS WELL COMPLETION; PETROLEUM GEOLOGY; ROTARY TOOL DRILL; TURBODRILL; 
WELL LOGGING. 
 �34s[J.B. Cheatham, Jr.]
�33s 
  
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�6v�2v�32sOil burner   �21m �33s 
    A device for converting fuel oil from a liquid state into a combustible 
mixture. A number of different types of oil burners are in use for domestic 
heating. These include sleeve burners, natural-draft pot burners, forced-draft 
pot burners, rotary wall flame burners, and air-atomizing and 
pressure-atomizing gun burners. The most common and modern type that handles 
80% of the burners used to heat United States homes is the 
pressure-atomizing-gun - type burner shown in the illustration. 
    The sleeve burner, commonly known as a range burner because of its use in 
kitchen ranges, is the simplest form of vaporizing burner. The natural-draft 
pot burner relies on the draft developed by the chimney to support combustion. 
A modification of this burner is the forced-draft pot burner which supplies 
its own air for combustion and does not rely totally on the chimney. The 
rotary wall flame burners have mechanically assisted vaporization. The 
gun-type burner uses a nozzle to atomize the fuel so that it becomes a vapor, 
and burns easily when mixed with air. 
    The oil burner is used for a wide assortment of heating, air conditioning, 
and processing applications. Oil burners heat commercial buildings such as 
hospitals, schools, and factories. Air conditioners using the absorbtion 
refrigeration system have been developed and fired with oil burners. Oil 
burners are used to produce CO2 in greenhouses to accelerate plant growth. 
They also produce hot water for many commercial and industrial applications.   
See  AIR COOLING; COMFORT HEATING; HOT-WATER HEATING SYSTEM; OIL FURNACE. 
 �34s[Robert A. Kaplan]
�33s 
  
  
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�6v�2v�32sOil field model   �21m �33s 
    A small-scale and commonly simplified replica of subsurface conditions of 
interest and value in petroleum prospecting and oil-field development. The 
term model has been applied by geologists to a simplified diagram and by 
mathematical physicists to a formal analysis with special boundary conditions 
and related attributes. These and somewhat more complex physical models have 
value in transmitting concepts and relationships to the nonspecialist, but 
they are generally designed by geologists and petroleum engineers to aid 
investigation of a particular problem. To do so, the model is scaled to size, 
shape, and like attributes, or according to forces upon it, relative to its 
archetype.  
    Physical models present the earth features in a readily visible fashion 
easier to grasp than mathematical analysis. For a discussion of mathematical 
models  see  PETROLEUM RESERVOIR MODELS. 
 �34s[Paul Weaver]
�33s 
  
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�6v�2v�32sOil field waters   �21m �33s 
    Waters of varying mineral content which are found associated with 
petroleum and natural gas or have been encountered in the search for oil and 
gas. They are also called oil field brines, or brines. They include a variety 
of underground waters, usually deeply buried, and have a relatively high 
content of dissolved mineral matter. These waters may be (1) present in the 
pore space of the reservoir rock with the oil or gas, (2) separated by gravity 
from the oil or gas and thus lying below it, (3) at the edge of the oil or gas 
accumulation, or (4) in rock formations which are barren of oil and gas. 
Brines are commonly defined as water containing high concentrations of 
dissolved salts. Potable or fresh waters usually are not considered oil field 
waters but may be encountered, generally at shallow depths, in areas where oil 
and gas are produced. 
    Probably the most important geological use of oil field water analyses is 
their application to the quantitative interpretation of electrical and neutron 
well logs, particularly micrologs.  See  PETROLEUM GEOLOGY; WELL LOGGING. 
 �34s[Preston McGrain]
�33s 
  
A700000001320�254s�5v�1v1193
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�6v�2v�32sOil furnace   �21m �33s 
    A combustion chamber in which oil is the heat-producing fuel. Fuel oils, 
having from 18,000 to 20,000 Btu/lb (42 - 47 megajoules/kg), which is 
equivalent to 140,000 to 155,000 Btu/gal (39 - 43 megajoules/liter), are 
supplied  commercially. The lower flash-point grades are used primarily in 
domestic and other furnaces without preheating. Grades having higher flash 
points are fired in burners equipped with preheaters.  See  FUEL OIL. 
    Domestic oil furnaces with automatic thermostat control usually operate 
intermittently, being either off or operating at maximum capacity.  See  OIL 
BURNER.  
 �34s[Frank H. Rockett]
�33s 
  
A700000001431�254s�5v�1v1193
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�6v�2v�32sOil mining   �21m �33s 
    The surface or subsurface excavation of petroleum-bearing sediments for 
subsequent removal of the oil by washing, flotation, or retorting treatments. 
Oil mining also includes recovery of oil by drainage from reservoir beds to 
mine shafts or other openings driven into the oil rock, or by drainage from 
the reservoir rock into mine openings driven outside the oil sand but 
connected with it by bore holes or mine wells. 
    Surface mining has been used primarily for the removal of oil shale or 
bituminous sands lying at or near the surface. Subsurface mining is used for 
the removal of oil sediments, oil shale, and Gilsonite.  See  OIL SAND; OIL 
SHALE; PETROLEUM GEOLOGY. 
 �34s[A. L. Ponikvar]
�33s 
  
A700000001274�254s�5v�1v1193
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�6v�2v�32sOil sand   �21m �33s 
    A loose to consolidated sandstone or a porous carbonate rock, impregnated 
with a heavy asphaltic crude oil, too viscous to be produced by conventional 
methods; also known as tar sand or bituminous sand.  See  ASPHALT AND 
ASPHALTITE.  
    Oil sands are distributed throughout the world but the largest proven 
accumulation occurs in Alberta, Canada (the Athabasca deposit). A large 
accumulation appears to be present in the Orinoco Basin in Venezuela, and far 
smaller deposits occur in the Soviet Union, the United States, Madagascar, 
Albania, Trinidad, and Romania.  See  OIL MINING. 
 �34s[G. Ronald Gray]
�33s 
  
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�6v�2v�32sOil shale �21m �33s
    A sedimentary rock containing solid, combustible organic matter in a
mineral matrix. The organic matter, often called kerogen, is largely insoluble
in petroleum solvents, but decomposes to yield oil when heated. Although ``oil
shale'' is used as a lithologic term, it is actually an economic term
referring to the rock's ability to yield oil. No real minimum oil yield or
content of organic matter can be established to distinguish oil shale from
other sedimentary rocks. Additional names given to oil shales include black
shale, bituminous shale, carbonaceous shale, coaly shale, cannel shale, cannel
coal, lignitic shale, torbanite, tasmanite, gas shale, organic shale, kerosine
shale, coorongite, maharahu, kukersite, kerogen shale, and algal shale.  See
OIL MINING.
 �34s[John Ward Smith ;Howard B. Jensen]
�33s

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�6v�2v�32sOkra   �21m �33s 
    A warm-season annual, Hibiscus esculentus, of Ethiopian origin. Okra, also 
called gumbo, is grown for its immature pods (see illustration), which are 
generally used for preparing soups but are also eaten as a freshly cooked 
vegetable. It is a member of the order Malvales and is related to cotton. 
Georgia, Florida, and Louisiana are important producing states.  See  MALVALES. 
 �34s[H. John Carew]
�33s 
  
  
A700000001324�254s�5v�1v1193
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�6v�2v�32sOleate �21m �33s
    A salt or ester of oleic acid, whose formula is given below, in which the
acid hydrogen of the carboxyl group is replaced by a metal or an organic
radical. Oleates occur in nature chiefly as the glyceryl ester, found in
substantial amounts in animal and vegetable fats. A few of the simpler esters
have commercial applications in textile, leather, cosmetics, and
pharamaceuticals. Alkalimetal oleates are water-soluble and are the chief
components of toilet and laundry soaps.  See  CARBOXYLIC ACID; DETERGENT;
ESTER; SOAP.
								O
							       /
     CH (sub 3)(CH (sub 2)) (sub 7)CH--CH(CH (sub 2))(sub 7) C --OH


 �34s[Elbert H. Hadley]
�33s



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�6v�2v�32sOlefin sulfide �21m �33s
    One of a number of organosulfur compounds, also called episulfides or
thiiranes, that are the sulfur analogs of olefin oxides. The homologous series
is represented by the structural formula shown below. The episulfides are far
less known than the epoxides. They undergo some reactions which are analogous
to those of epoxides, for example, ring opening with HCl, H2S, RSH, and
alcohols, ROH. The tendency of olefin sulfides to polymerize (to polysulfides)
is quite marked, however, and frequently complicates their use in other
reactions.  See  ORGANOSULFUR COMPOUNDS.

		   R (sub 2) C -- CR (sub 2)
			     \	  /
			       S

 �34s[Norman Kharasch]
�33s



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�6v�2v�32sOlfaction �21m �33s
    One of the chemical senses. In mammals the olfactory receptors are located
in the upper parts of the nasal cavity. In humans they are located in a small
patch in each nasal cavity, consisting of about 0.4 in.sup 2 (2.5 cm sup 2) of
mucous membrane, colored with a yellow pigment. The olfactory cells are long
and ovoid in shape, terminating at the distal end in several delicate hairs
projecting into and possibly through the mucous covering the nasal epithelium.
The proximal end consists of fine unmyelinated nerve fibers which pierce the
bony cribriform plate to enter directly the olfactory bulb of the brain.  See
BRAIN.
    Nerve endings of the trigeminal nerve fibers, utilized in common chemical
sensitivity, are widely distributed throughout the nasal and olfactory area.
Many odorants, for example, ammonia, stimulate both the olfactory and free
nerve endings.
    Most odorous compounds are organic. Both the arrangement and structure of
the molecule as well as the presence of certain groups within the molecule
appear to influence odor. In spite of the relative inaccessibility of the
olfactory end organs, odor materials can be detected at extremely low
concentrations. It has been estimated that olfaction is 10,000 times more
sensitive than taste. Threshold concentrations for well-known odorants, such
as ethylmercaptan, have been cited in the range of 4 x 10(sup -8)mg/liter of
air. 
    There is extensive literature on theories of olfaction, ranging from the
purely chemical to the purely physical. The factors considered are the
chemical or physical nature of an adequate stimulus and the nature of the
receptor; for example, the receptor may be composed of only a few basic kinds
of endings or many specialized different endings. Though many theories are
provocative, there is, as yet, insufficient ground for accepting one theory
over the other.  See  CHEMICAL SENSES.
 �34s[Carl Pfaffman]
�33s

A700000003220�254s�5v�1v1194
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�6v�2v�32sOligocene   �21m �33s 
    The third of the five major worldwide divisions (epochs) of the Tertiary 
Period (Cenozoic Era); the epoch of geologic time extending from the end of 
the Eocene to the beginning of the Miocene. Oligocene also denotes the series 
of rocks formed during the epoch and the fossils therein.  See  CENOZOIC; 
EOCENE; MIOCENE; TERTIARY. 
    Marine faunal provinces were fragmented in the Oligocene. Conversely, some 
isolated Northern Hemisphere landmasses became reconnected, permitting 
interchange of some mammals (opossums, anthracotheres, chalicotheres, tapirs, 
rhinocerids). Cool-water marine shellfish moved equatorward, certain pelecypod 
and gastropod stocks arriving for the first time in habitats they still occupy 
today. The marine planktonic foraminiferal faunas of the preceding Eocene have 
been shown to have suffered an entire eclipse, having become restricted to a 
single lineage. 
    With the rise of grasses the earlier huge browsers, such as the 
brontotheres and other archaic mammals, made their last stand. Hyracoids 
appeared and have left a lone relic descendant coney or ``rock rabbit.'' 
Primitive primates (lemurs) and insectivores carried on in isolated 
Madagascar. Ten living genera of birds already existed. The first New and Old 
World monkeys and Great Apes appeared. Carnivora modernized (dogs, cats, 
saber-toothed cats) alongside their disappearing creodont ancestors. Beavers, 
pangolins, three-toed horses, and pig, peccary, and tapir families appeared 
and other hoofed mammals diversified.  See  PALEOBOTANY; PALEONTOLOGY. 
 �34s[Robert M. Kleinpell]
�33s 
  
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�6v�2v�32sOligochaeta   �21m �33s 
    A class of the phylum Annelida including worms such as the earthworms. 
There are 21 families with over 3000 species. These animals exhibit both 
external and internal segmentation. They usually possess setae which are not 
borne on parapodia. Oligochaetes are hermaphroditic. The gonads are few in 
number and situated in the anterior part of the body, the male gonads being 
anterior to the female gonads. The gametes are discharged through special 
ducts, the oviducts and sperm ducts. A clitellum is present at maturity. There 
is no larval stage during development. 
    The oligochaetes are primarily fresh-water and burrowing terrestrial 
animals. A few are marine and several species occur in the intertidal zone. 
    Oligochaetes are cylindrical, elongated animals with the anterior mouth 
usually overhung by a fleshy lobe, the prostomium, and the anus terminal. The 
body plan is that of a tube within a tube. Externally, the segments are marked 
by furrows. The setae or bristles are borne on most segments. Other external 
features are the pores of the reproductive systems opening on certain 
segments, the openings of the nephridia, and in many earthworms dorsal pores 
which open externally from the coelom. Some aquatic species have extensions of 
the posterior part of the body which function as gills. 
    The oligochaetes have been used in studies of physiology, regeneration, 
and metabolic gradients. Some aquatic forms are important in studies of stream 
pollution as indicators of organic contamination. Earthworms are important in 
turning over the soil and reducing vegetable material into humus. It is likely 
that fertile soil furnishes a suitable habitat for earthworms, rather than 
being a result of their activity.  See  ANNELIDA. 
 �34s[Perry C. Holt]
�33s 
  
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�6v�2v�32sOligoclase   �21m �33s 
    A plagioclase feldspar with a composition ranging from Ab90An10 to 
Ab70An30 where Ab = NaAlSi3O8 and An = CaAl2Si2O8. If Fe2O3 is present as thin 
flakes oriented parallel to certain structurally defined planes, such 
oligoclase is called aventurine or sunstone.  See  FELDSPAR; IGNEOUS ROCKS. 
 �34s[Fritz H. Laves]
�33s 
  
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18431488
�6v�2v�32sOligosaccharide �21m �33s
    A sugar composed of 2 or more monosaccharide units. Those sugars
containing up to 6 units, many of which occur in nature, have been isolated as
crystalline compounds. Fragments, obtained by controlled hydrolysis of various
polysaccharides with acid and consisting of monosaccharides up to 10 units,
are also termed oligosaccharides.  See	MONOSACCHARIDE.
    The oligosaccharides may be considered as glycosides in which a hydroxyl
(OH) group of one monosaccharide is condensed with the reducing group of
another, with the loss of n - 1 molecules of water (n = number of
monosaccharide residues). This condensation process is shown below.

	C6H12O6 + C6H12O6 - H2O = C12H22O11	      Disaccharide
		3C6H12O6 - 2H2O = C18H32O16	      Trisaccharide



    If two sugar units are joined in this manner, a disaccharide results; a
linear array of three monosaccharides thus joined by glycosidic bonds is a
trisaccharide, and so forth. On the basis of the number of constituent
monosaccharide units, the oligosaccharides are classified as disaccharides,
trisaccharides, tetrasaccharides, and so on. No sharp distinction can be drawn
between the oligosaccharides and polysaccharides; it is chiefly a matter of
the latter's possessing higher molecular weights.  See  POLYSACCHARIDE.
    The monosaccharide units of an oligosaccharide may be alike, as in
maltose, which on hydrolysis gives two molecules of D-glucose, or different,
as in sucrose or raffinose. Sucrose consists of D-glucose and D-fructose, and
raffinose consists of D-glucose, D-fructose, and D-galactose residues.	See
MALTOSE; SUCROSE.
 �34s[William Z. Hassid]
�33s

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�6v�2v�32sOligotrichida   �21m �33s 
    A minor order of the Spirotrichia. If somatic ciliature is present, it is 
sparse. The bodies are round in cross section, and the adoral zone of 
membranelles is often highly developed at the anterior, or oral, end of the 
organism. Species are found in fresh- and salt-water habitats, and none occurs 
as a parasite. Halteria (see illustration) has long bristles which are used in 
a kind of jumping movement.  See  CILIOPHORA; SPIROTRICHIA. 
 �34s[John O. Corliss]
�33s 
  
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�6v�2v�32sOlive   �21m �33s 
    Olive fruits, which are produced by a small to medium-sized evergreen tree 
(Olea europaea), can be eaten, after processing, as table olives, or can be 
extracted for oil that is used on salads, for cooking, for body lotions, or 
for medicinal purposes. The olive tree is a historically ancient cultivated 
plant, having been domesticated by early civilizations in the eastern 
Mediterranean regions. Olive culture later spread to all the Mediterranean 
countries and subsequently to South America, California, South Africa, and 
Australia.  
  
    Olive fruits are harvested for table olives in the autumn, when the fruits 
change from green to straw or to a slightly red color. Raw fruits contain a 
bitter glucoside which makes them inedible, but treatment with an alkali such 
as sodium hydroxide (lye) neutralizes the bitterness. The lye must 
subsequently be leached out of the fruits with water. In another method most 
of the bitterness can be removed by leaching with salt water. A lactic acid 
fermentation process is widely used in the Mediterranean countries to preserve 
the olives. 
    For oil production, the fruits are harvested in midwinter when they have 
become black and have reached their maximum oil content - 15 to 25% of the 
fresh weight, depending on the variety. In producing olive oil, the freshly 
harvested fruits including pits are ground, after which this material is 
placed in burlap or cloth bags and placed under high pressure. Oil and water 
are extracted and transferred to tanks, where the oil rises to the top and is 
removed.  See  FAT AND OIL (FOOD). 
 �34s[Hudson T. Hartman]
�33s 
  
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�6v�2v�32sOlivine �21m �33s
    A name given to a group of magnesium-iron silicate minerals crystallizing
in the orthorhombic system. The luster is vitreous and the color olive-green,
giving rise to the name olivine. Hardness is 6 1/2 - 7 on Mohs scale; specific
gravity is 3.27 - 3.37, increasing with increase in iron content.  See
SILICATE MINERALS.
    Olivine is a nesosilicate with composition (Mg, Fe)2SiO4. It comprises a
complete solid solution series from the pure iron member fayalite, Fe2SiO4, to
the pure magnesium member forsterite, Mg2SiO4. Minerals of intermediate
composition have been given their own names but are usually designated simply
as olivine.
    Olivine occurs chiefly as a rock-forming mineral in igneous rocks.
Although it may be present in granites and other light-colored rocks, it is
found chiefly in the dark rocks such as gabbro, basalt, and peridotite. The
rock dunite is composed almost completely of olivine. Olivine is one of the
first minerals to form upon crystallization of a magma. It is believed that
this early-formed olivine accumulated through the process of magmatic
differentiation to form the large dunite masses.
    At a few localities, notably on St. John's Island in the Red Sea and in
Burma, olivine is found in transparent crystals. These are cut into gemstones
which go under the name of peridot. Olivine is a major constituent of many
stony meteorites.
 �34s[Cornelius S. Hurlbut, Jr.]
�33s

A700000005002�254s�5v�1v1195
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�6v�2v�32sOmega �21m �33s
    A global radio navigation system developed by the U.S. Navy. The
development of the system was the result of a Department of Defense
requirement for a worldwide, all-weather, continuous radio aid to navigation.
It provides position information by measuring the phase difference between
signals radiated by a network of transmitting stations. Omega utilizes eight
strategically located terrestrial very-low-frequency transmitting stations.
The locus of constant phase difference for any pair of stations is a
hyperbolic line of position. Geographical position is obtained by plotting or
computing the intersection of two or more such hyperbolic lines of position.
Omega provides moderate position-fixing accuracy (2 - 4 nautical miles or 3.7
- 7.4 km) anywhere on the globe.
    Omega is a hyperbolic navigation system using phase comparison of
very-low-frequency (10 to 14 kHz) continuous-wave radio signals. The system
can be used by aircraft, ships, and land vehicles, and also by submarines
operating at moderate antenna depths of about 40 to 50 ft (12 to 15 m). All
Omega transmitters are synchronized in phase and transmit on each of four
common navigation frequencies: 10.2, 11.05, 11 1/3, and 13.6 kHz. Each station
also has its own unique frequency on which it transmits, with all
transmissions on a time-shared basis according to the Omega signal format.
    Omega receivers vary in complexity according to the degree of performance
to be achieved and the number of automatic features included in each design.
The simplest semiautomatic equipment may include only provisions for measuring
phase difference at 10.2 kHz. However, most receivers are able to make phase
difference measurements on multiple frequencies (for example, 10.2 and 13.6
kHz, or 10.2, 13.6, and 11 1/3 kHz). The most advanced receivers utilize all
four navigation frequencies and, in addition, receive and utilize the unique
frequencies for acquisition and tracking purposes.
    Differential Omega - in which a receiver located in a known and fixed
geographical position monitors the Omega signals and broadcasts corrections on
a real-time basis to mobile users in the vicinity - can improve
position-fixing accuracy to better than 1/2 nautical mile (0.9 km) within a
range of 50 mi (80 km) of the monitor receiver. Such an implementation has
been established along the coast of France by the French government.  See
ELECTRONIC NAVIGATION SYSTEMS; HYPERBOLIC NAVIGATION SYSTEM.
 �34s[James P. Van Etten]
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�6v�2v�32sOmphacite   �21m �33s 
    The pale to bright green monoclinic pyroxene found in eclogites and 
related rocks. Omphacites are essentially members of the solid solution series 
between jadeite (NaAlSi2O6) and diopside (CaMgSi2O6). The density ranges from 
3.16 to 3.43 g/cmsup3; hardness is 5 - 6. 
    Omphacite is stable only at the relatively high pressures of the 
blueschist and eclogite facies of metamorphism, where it is associated with 
minerals such as glaucophane and lawsonite or pyropic garnet, respectively. In 
such environments it also occurs in veins, either on its own or with quartz.  
See  GLAUCOPHANE; PYROXENE. 
 �34s[Timothy J. B. Holland]
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�6v�2v�32sOnchocerciasis   �21m �33s 
    A disease caused by the microfilariae worm Onchocerca volvulus in the 
subcutaneous lymphatics. It is characterized by subcutaneous nodules which are 
most conspicuous where the skin lies closely over bony structures, via 
cranium, pelvic girdle, joints, and shoulder blades. When they are on the 
head, the microfilariae reach the eyes. Ocular disturbances vary from mild 
transient bleary vision to total and permanent blindness. This is a major 
problem in the provinces of Oaxaca and Chiapas in Mexico and the nearby 
Pacific slopes of Guatemala, where infection is an occupational hazard 
associated with coffee growing. The blackfly is the vector. 
    Termed ``river blindness,'' onchocerciasis also affects field workers in 
the Ivory Coast, Ghana, Mali, Togo, and Upper Volta. See FILARIOIDEA. 
 �34s[Gilbert F. Otto]
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�6v�2v�32sOncofetal antigens   �21m �33s 
    Antigens that are commonly present both in fetal tissue during early 
development of life and in adult tissue when cancer occurs. Immunization of 
animals such as rabbits or goats with these substances induces their immune 
system to produce antibodies; therefore, these substances are antigenic. 
    These antigens, primarily protein in nature and produced by cancer cells, 
are the products of one or more genes that normally are expressed only during 
fetal development and then are repressed in adult life. Production of these 
proteins as a result of activation of the controlled genes by a yet unknown 
mechanism in association with cancer has been called retrogenetic expression. 
Very minute but significant changes of these fetal antigens in body fluids can 
serve in detecting the early oncogenic process and in monitoring the efficacy 
of, and in developing new modalities of, cancer treatment. 
    Although several oncofetal antigens have been investigated, two of these, 
alpha fetoprotein (AFP) and carcinoembryonic antigen (CEA), have received the 
most attention. 
    Alpha fetoprotein is a normal embryonic product during fetal development. 
Alpha fetoprotein in the fetus is primarily synthesized in the liver, yolk 
sac, and gastrointestinal tract. In the adult, alpha fetoprotein in the serum 
is elevated in individuals with primary hepatocellular carcinoma and with 
teratocarcinomas of the ovary or testes. As a tumor marker, alpha fetoprotein 
has been used for large-scale screening in Africa, China, and Japan. 
    Since its discovery in 1965, the carcinoembryonic antigen of human 
digestive cancer has been the most studied oncofetal antigen. This 
glycoprotein was initially detected by immunoprecipitation techniques in the 
embryonic and fetal gut, pancreas, and liver in the first two trimesters of 
gestation, and also in entodermally derived digestive-system cancers. As an 
oncofetal antigen, carcinoembryonic antigen is less valuable as a cancer test 
than initially reported. Although it is generally accepted that the 
carcinoembryonic antigen assay should not be used as a screening test for 
cancer in the general population, it does have an adjunctive role in 
diagnostic procedures. 
    A few other less defined oncofetal antigens have been investigated. The 
fetal antigens associated with pancreatic cancer, named pancreatic oncofetal 
antigen and pancreas cancer - associated antigen, have been described. These 
pancreatic oncofetal antigens are found in extracts of the fetal pancreas, 
adult human pancreatic tumors and sera from individuals with carcinoma of the 
pancreas, and to a lesser degree in persons with other malignancies.  See  
ANTIGEN; ONCOLOGY. 
 �34s[T. Ming Chu]
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�6v�2v�32sOncology   �21m �33s 
    The study of the causes, development, characteristics, and treatment of 
tumors. The word tumor as commonly used is synonymous with neoplasm. Tumors 
are new growths of masses of tissue cells in excess of the needs of the tissue 
from which the cell masses are derived.  See  NEOPLASIA; TUMOR. 
    Benign tumors are usually slow-growing, localized, and circumscribed. 
Microscopically they have a relatively orderly architecture, and the cells 
generally resemble adult body cells. In contrast, malignant tumors, regardless 
of their appearance or tissue of origin, tend to grow rapidly, to invade 
surrounding tissues, and to spread to distant sites in the body. 
    Though cancers may start at any age, they occur with progressively 
increasing frequency in age groups beyond 40. In men the five most common 
sites for cancer are skin, lungs, prostate, stomach and colon. In women they 
are breast, uterus, skin, colon, and stomach. Three types of cancer (leukemia, 
neuroblastoma, and embryonal carcinoma of the kidneys) are significant 
contributors to infant mortality.  See  CANCER (MEDICINE). 
  
    Etiology.   Etiology is the study of the causes of the transformation of 
normal cells to neoplastic ones. The transformation involves a change in 
cellular heredity. The daughter cells of a neoplastic cell inherit their 
altered form and behavior from their parent cell and pass the change on to 
subsequent generations of cells. Normally, cells apportion their energy 
between proliferation and specialized function to meet the needs of the body. 
Neoplastic cells subordinate specialized functions to proliferation. The 
change from normal to neoplastic usually involves a stepwise series of 
chemical, morphological, and functional changes. Carcinogenesis is the term 
applied to the process. 
    The causes of the transformation of normal cells to neoplastic can be 
divided into two general groups, genetic and environmental. For the vast 
majority of cancers the relative importance of inherited and environmental 
factors in the genesis of cancers remains unknown. The general genetic makeup 
of individuals may enhance or diminish their susceptibility to exogenous 
agents. A whole population may be exposed to a similar dose of an agent, yet 
only some individuals develop cancer. Different races and strains of 
individuals have a varying susceptibility to the development of cancer. 
    General estimates suggest that approximately 80 - 90% of the neoplasms in 
human beings have an exogenous environmental agent as a prime causative 
factor. A major opportunity to control cancer in the human rests with 
improving the environment. More than 400 agents are known, and many others are 
suspected. The agents include viruses, chemicals, radiation, parasites, fungi, 
and bacteria. 
    Viruses that cause neoplasms (oncogenic viruses) occur in many species of 
animals. About two-thirds of oncogenic viruses are of the ribonucleic acid 
(RNA) type, and one-third are DNA viruses. The usual viral infections 
encountered, such as influenza, kill the cells attacked, and consequently 
destroy the virus too, and therefore no lasting effects are produced.  See  
TUMOR VIRUS. 
    Several radiations have been proved to be potent carcinogens, although the 
mechanism of action on the cell is not established. Whether the radiation 
directly alters the DNA of the genome or whether it alters some other aspect 
of cell metabolism remains moot. 
    The existence of cancer-producing chemicals in the environment was first 
conclusively shown by Sir Percival Potts in 1775, when he astutely observed 
that cancers of the scrotum occurred frequently in chimney sweeps. The 
chemical substance involved was isolated 140 years later when coal tars, 
painted on the ears of rabbits, produced carcinomas. Subsequently, numerous 
carcinogenic hydrocarbons have been extracted from coal tar. Among the known 
carcinogenic compounds are the polycyclic aromatic hydrocarbons, such as 
7,12-dimethyl benz(a)anthracene or benzo(a)pyrene; aromatic amines such as 
dimethyl amino azobenzene (``butter yellow''); urethane; and alkylating agents 
such as epoxides and beta-propriolactone. Some food contaminants and 
additives, estrogens, several metals, and asbestos fiber have carcinogenic 
effects. Air pollution and tobacco are known causes of lung cancer.  See  
MUTAGENS AND CARCINOGENS. 
  
    Tumor biology.   Benign and malignant tumors have some attributes in 
common. Both may begin at almost any site within the body. One cell or a small 
group of cells may begin to proliferate in an autonomous manner. Consequently, 
tumors are primarily composed of living cells, and they are deranged 
descendants of body cells. Through more or less resembling the cell of origin, 
tumor cells usually vary from the normal in size, shape, and staining 
properties and have defective physical and spatial relationships with the 
surrounding tissue. Most significant is their accentuated proliferative vigor. 
This is a basic feature of all tumors, for without it the prime 
characteristic, growth, would be lacking. 
    Benign tumors are composed of cells that usually closely resemble the 
tissue of origin and differ from normal tissue primarily in general 
architecture and excess of cells. Malignant tumors (cancers), on the other 
hand, have in addition to some similarities with benign tumors many unique 
features. The three cardinal properties of malignancy are anaplasia, invasion, 
and metastasis. These properties refer to the appearance and behavior of 
cancer cells, in which they usually differ greatly from benign tumor cells. 
They reflect the independence of cancer cells, that is, the ability to grow at 
sites other than the locus of origin. 
    Anaplasia is the structural aberration of cancer cells which results in a 
more primitive (embryonic) appearance and diminished or absent adult function. 
Their size, shape, staining properties, and spatial relationships to one 
another are usually markedly deranged. Two consequences of the development of 
autonomy by neoplastic cells are invasion and metastasis. These newly acquired 
properties of malignant cells are responsible for destructive growth and 
spread to the detriment or death of the host. Some cancers appear to be 
independent of regulation from their inception and are able to invade and 
metastasize immediately. Others undergo a stepwise series of precancerous 
changes before becoming autonomous (malignant). Sometimes this may take many 
years. Invasion precedes metastasis in tumor progression to autonomous growth. 
Individual or small groups of cancer cells, no longer dependent on their site 
of origin for survival, are capable of infiltrating the surrounding host 
tissues and remaining viable. 
  
    Classification.  Because of their widely differing growth patterns, each 
type of benign and malignant tumor has a different implication of illness and 
life expectancy. 
  
    Carcinoma. The table is a list of common tumors based on the tissue of 
origin, cell type, and the presence or absence of malignant features. Those 
tumors derived from the internal and external body surface coverings and their 
derivatives (such as the skin; sweat glands; breast; and lining of the 
respiratory, gastrointestinal, urinary, and genital systems) are epithelial 
tumors. Malignant tumors derived therefrom are called carcinomas. When a 
carcinoma takes origin from the skin or comparable epithelium, it is called 
epidermoid carcinoma. Benign skin tumors are called papillomas. When 
malignancies are derived from glandular epithelium, they are called 
adenocarcinomas. Adenomas are benign glandular tumors. For example, a benign 
tumor of bronchial glands is called a bronchial adenoma. A malignancy derived 
from these glands is called bronchial adenocarcinoma. Though remotely 
descended from embryonal ectoderm, tumors of the nervous system have many 
features peculiar to themselves and represent a separate subgroup. For the 
same reason tumors derived from the placental epithelium also form a separate 
subgroup.  
  
    Sarcoma. The term sarcoma is applied to malignancies arising from the 
derivatives of embryonal mesoderm, such as connective tissues, bone, muscle, 
cartilage, blood vessels, and blood cells. A benign fibrous connective tissue 
tumor is a fibroma, whereas a malignant one is a fibrosarcoma. A major 
subgroup is made up of the tumors composed of blood cells, the lymphomas and 
leukemias. Mixed tumors and teratomas comprise a third major category of 
tumors. They are composed of mixtures of cells of the above categories. In all 
three categories the Greek word root ``oma,'' meaning tumor, is applied to the 
end of the name of the involved tissue cell type. The table lists only a few 
representative tumors in each category. 
  
    Basal cell carcinoma. One of the most common tumors, the so-called basal 
cell carcinoma, is not included in the table because of the difficulty of 
classification. It begins as a small nodule in the skin which ulcerates and it 
has a ragged, dirty, irregular appearance and gradually increases in size. The 
tumor remains local, rarely if ever metastasizing, but will invade, compress, 
and destroy surrounding tissues. It erodes through cartilage and bone; hence 
the term rodent ulcer is sometimes applied. Therefore the invasive properties 
of the tumor are consistent with malignancy. However, its failure to 
metastasize suggests a benign nature. 
    In contrast, epidermoid carcinoma is a truly malignant skin tumor. It may 
arise wherever the stratified squamous type of epithelial cells may be found, 
such as skin, mouth, esophagus, bronchus, and uterine cervix. Beginning as an 
elevated, papillary, hard nodule in the epithelium, it more or less rapidly 
infiltrates the deeper connective and adipose tissues and spreads to the 
regional lymph nodes. If the primary tumor is not treated, metastases may 
occur to internal organs, principally lungs or liver. 
  
    Adenocarcinoma. Adenocarcinomas may arise wherever glandular tissue is 
found. They frequently arise in the breast, rectum, colon, stomach, pancreas, 
prostate, and uterus. Breast adenocarcinoma, derived from the lining of breast 
ducts, begins as a small, single, firm nodule which has an irregular border. 
It can be moved about only with difficulty and to a limited degree. 
    Unlike breast carcinomas, which are often noticeable when they are small, 
adenocarcinomas of the colon or rectum may achieve considerable size before 
being discovered. They tend to infiltrate and metastasize with vigor. 
    At the other extreme, adenocarcinomas of the prostate are exceedingly 
common in elderly men, but they uncommonly cause symptoms or spread outside 
the prostate gland. 
    Neuroblastoma, a malignant tumor derived from nervous system cells, is the 
most common cancer of infants. Occasionally the tumor is present at birth. 
  
    Lymphoma-leukemia tumors. The lymphoma-leukemia group of tumors has many 
unique features. Delineation into benign and malignant forms is difficult.  
See  LEUKEMIA; LYMPHOMA. 
  
    Diagnosis and treatment.   The most important tumor diagnostic tool is the 
oldest and simplest, a careful physical examination. Tumors being 
space-occupying masses, they are frequently detected by direct vision or 
palpation. Cancers of the skin and female breast are among the most frequently 
occurring types. 
    Diagnostic x-rays are another means of visualizing tumors. Blood chemistry 
studies and blood counts may at times reveal clues of the existence of a 
cancerous process. Because noncancerous lesions such as inflammations and 
degenerations may visibly resemble cancer, the diagnosis of cancer is not 
proved until a biopsy is done. 
    Exfoliative cytology is used extensively as an aid to the early diagnosis 
of cancer. Cells that flake off from various body surfaces are stained and 
observed under the microscope. When the Papanicolaou smear from the uterus is 
observed microscopically, the exfoliated normal cells surround several 
epidermoid carcinoma cells. This technique has been most effective in the 
early diagnosis of carcinoma of the cervix of the uterus. 
    The three major types of weapons for the treatment of cancer available to 
physicians are surgery, radiation, and chemotherapy. Complete surgical 
excision of a benign tumor is usually sufficient to effect a cure. Malignant 
tumors can also be cured if they have not infiltrated adjacent tissues or 
metastasized by the time of excision. Radiation therapy, including x-ray and 
radioactive metals (cobalt, radium, phosphorus, iodine, and others), has 
application in the treatment of many types of cancers. Strong beams of 
radiation are focused on the tumor, causing the death of susceptible cancer 
cells and scar formation. Thus radiation which may produce cancer at one 
dosage level can destroy it at another. Whether radiation or surgery or both 
are used in the treatment of a particular neoplasm depends on many variables, 
including the exact type of neoplasm and its size, location, and rate of 
spread and the general conditions of the patient. 
 �34s[N. Karle Mottet]
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�6v�2v�32sOnion   �21m �33s 
    A cool-season biennial, Allium cepa, of Asiatic origin and belonging to 
the plant order Liliales. The onion is grown for its edible bulbs. 
    Related species are leek (A. porrum), garlic (A. sativum), Welsh onion (A. 
fistulosum), shallot (A ascalonicum), and chive (A. schoenoprasum). 
    Onion varieties (cultivars) are classified mainly according to pungency 
(mild or pungent) and use (dry bulbs or green bunching). Bulbs may be white, 
red, or yellow. Varieties differ markedly in their keeping quality and in 
their response to length of day. Hybrid varieties, with increased disease 
resistance, longer storage life, and improved quality, are rapidly displacing 
older varieties. Texas, New York, and California are important producing 
states.  See  LILIALES. 
 �34s[H. John Carew]
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�6v�2v�32sOniscoidea   �21m �33s 
    A suborder of the Isopoda which contains the terrestrial members of these 
crustaceans. They are popularly known as sow bugs, slaters, wood lice, or, in 
the case of those that roll themselves into a ball, pill bugs. Land isopods 
commonly occur under rocks, loose bark, leaf mold, and similar moist places. 
They abound in humid tropical and warm-temperate regions, particularly in the 
Old World. A few are practically cosmopolitan and have probably been 
accidentally transported by humans with plants, soil, or building materials. 
When abundant in gardens or greenhouses, they sometimes do considerable damage 
by gnawing on plants. 
    The body is either flattened dorsoventrally or in pill bugs is highly 
vaulted like an armadillo. Its three subdivisions, the head, thorax, and 
abdomen, are broadly joined (see illustration). The lateral margins generally 
form a continuous oval outline, except in types with the abdomen abruptly 
narrower than the thorax. The surface may be smooth or variously sculptured 
with tubercles, ridges, or spines. Sexual dimorphism is rare. 
    Six fused segments, including the first thoracic somite, make up the head. 
It bears two pairs of antennae (the first being vestigial), two sessile 
compound eyes, and four pairs of mouthparts. The thorax has seven free 
segments, each bearing a pair of similar seven-jointed walking legs. The 
abdomen has five segments plus a terminal telson. Its appendages are biramous 
and include five pairs of platelike pleopods and one pair of uropods.  See  
ISOPODA.  
 �34s[Milton A. Miller]
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�6v�2v�32sOnychophora   �21m �33s 
    A phylum of terrestrial animals, frequently referred to as Peripatus. The 
80 or more species, divided into 13 genera, live in the tropics of both 
hemispheres and in the subtropical regions of the Southern Hemisphere. They 
undoubtedly represent the only survivors of a much more diversified phylum. 
    The adults of Onychophora species vary considerable in size of the 
individuals of a species. The body is annulated and covered with minute 
papillae, elongate, and wormlike, with 25 - 45 pairs of short, ventral legs 
that are no longer than the thickness of the body. Most species are gray, tan, 
or brown, but some are green, others purple, and a few are red or 
reddish-orange. They may be striped or speckled. 
    The body is distinctly segmented internally, which corresponds with the 
pairs of legs externally. The main body cavity is a hemocoel, quite different 
from the simple coelom of the Annelida. The heart is a tube, open at both 
ends, with paired openings or ostia along the sides of the tube. The excretory 
system consists of a series of metanephridia in each segment except the 
antennal and mandibular segments. A small bladder is present in each leg. This 
empties through a nephridial duct to a small nephropore between the ventral 
pads on the foot. 
    A pair of oral papillae, located one on each side of the mouth opening, 
squirt out a fluid toward prospective prey. This hardens into a sticky net 
which tangles the legs of the prey, mostly small, soft-bodied insects, but 
sometimes even large grasshoppers. The onychophoran then places its mouth on 
the body of the prey and exerts suction. The mandibles are extended from 
inside the mouth cavity. These paired structures are directed backward and are 
used to penetrate the integument of the prey and shred the tissue beneath. 
Digestive fluids are secreted into the hole. Somewhat liquified tissue is then 
sucked up by the host and swallowed. 
 �34s[Ross H. Arnett, Jr.]
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�6v�2v�32sOnyx   �21m �33s 
    Banded chalcedonic quartz, in which the bands are straight and parallel, 
rather than curved, as in agate. Unfortunately, in the colored-stone trade, 
gray chalcedony dyed in various solid colors such as black, blue, and green is 
called onyx, with the color used as a prefix. Because the color is permanent, 
the fact that it is the result of dyeing is seldom mentioned. 
    The natural colors of true onyx are usually red or brown with white, 
although black is occasionally encountered as one of the colors. When the 
colors are red-brown with white or black, the material is known as sardonyx; 
this is the only kind commonly used as a gemstone. Its most familiar gem use 
is in cameos and intaglios.  See  CAMEO; CHALCEDONY; INTAGLIO (GEMOLOGY). 
 �34s[Richard T. Liddicoat, Jr.]
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�6v�2v�32sOogenesis   �21m �33s 
    The sequence of events that leads to the formation of the female gamete. 
Oogenesis commences with proliferation of primoridal germ cells which give 
rise to oogonia, and it terminates with the mature oocyte (egg). The oocytes 
develop from oogonia and enter a growth phase that is initiated by the 
follicle-stimulating hormone (FSH), produced by the anterior portion of the 
pituitary gland, or adenohypophysis. Since the hormone stimulates the 
production of gametes, it is referred to as a gonadotropin. In insects the 
production of oocytes is stimulated by the juvenile hormone fabricated by the 
gland known as the corpus allatum. In other forms the agent responsible for 
the growth of oogonia is unknown. Growth of the oocyte is manifested by an 
increase in its volume and often by a change in its shape. The growth phase 
also encompasses a period during which the oocyte must differentiate into a 
mature egg, that is, an egg that possesses the capability, upon fertilization, 
of producing an embryo.  See  ADENOHYPOPHYSIS HORMONE; VITELLOGENESIS. 
 �34s[Everett Anderson]
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�6v�2v�32sOolite and pisolite   �21m �33s 
    Oolites are small, more or less spherical particles commonly found in 
limestones and dolomites. Most oolites are 0.5 - 1.0 mm (0.02 - 0.04 in.) in 
diameter, but their size range is much greater. Pisolites are similar 
particles that are greater than 2.0 mm (0.08 in.) in diameter. Oolites show 
varying degrees of departure from sphericity; some may be ellipsoidal, others 
may be appreciably flattened or distorted. The term oolite has been used to 
denote both the small, spherical bodies and the rock composed of an aggregate 
of these bodies. Some geologists prefer to call the particles oolite and the 
rock by its common lithologic name, prefixed by the word oolitic, for example, 
oolitic limestone.  See  DOLOMITE; LIMESTONE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sOpal �21m �33s
    A natural hydrated form of silica. There are many different varieties of
opal, but the best known are those which are highly prized as gemstones.
Precious opal displays the property of opalescence, a fine play of spectral
colors resulting from the interference of light rays within the stone. Fire
opal shows intense orange-to-red reflections against a yellow-to orange body
color. Black opal has a black background against which the colors are
displayed. Common opal is milk white, yellow, green, or red, but without
opalescence. The variety hyalite is clear and colorless with a globular
surface.  See  OPALESCENCE.
    Opal has a hardness of 5 - 6 on Mohs scale. The specific gravity varies
from 1.9 to 2.2, depending upon the water content. Opal is found in cavities
in igneous and sedimentary rocks and in fossil wood in which it is the
petrifying substance. Its largest deposits are in sedimentary beds as
diatomite which result from the accumulation of tiny opalean tests of diatoms.
Such a deposit is at Lompoc, California. See SILICATE MINERALS.
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sOpalescence   �21m �33s 
    The milky iridescent appearance of a dense transparent medium when the 
system (or medium) is illuminated by polychromatic radiation in the visible 
range, such as sunlight. Slight changes in the rainbowlike color of the system 
can occur, depending on the scattering angle, that is, the angle between the 
directions of incident radiation and of observation. 
    Opalescence is a general term which applies to the optical phenomenon of 
intense scattering in the visible range of the electromagnetic radiation by a 
system with strong local optical inhomogeneities. The iridescence, or 
rainbowlike display of interference of colors, arises because the intensity of 
scattered light is approximately proportional to the reciprocal fourth power 
of the wavelength of incident light (Rayleigh's law).  See  SCATTERING OF 
ELECTROMAGNETIC RADIATION. 
 �34s[Benjamin Chu]
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�6v�2v�32sOpalinata   �21m �33s 
    A superclass of Sarcomastigophora, including the single order Opalinida, 
with the single family Opalinidae. This group was formerly classified as 
ciliates because of the uniform covering of ``cilia''; these structures are 
actually short flagella. There is no cytostome, no contractile vacuole, and no 
cytoproct. The body size is large, some forms having a length greater than 
2000 micrometers. All species are found in the large intestine or rectum of 
cold-blooded vertebrate hosts.  See  SARCOMASTIGOPHORA. 
 �34s[John O. Corliss]
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�6v�2v�32sOpaque medium   �21m �33s 
    A medium which is impervious to rays of light, that is, not transparent to 
the human eye. By extension, a medium may be described as opaque if it does 
not transmit infrared waves or other regions of the electromagnetic spectrum, 
such as the x-ray, ultraviolet, and microwave regions. The property of zero 
transmittance does not necessarily imply total reflectance; that is, opacity 
can result both from reflection and from absorption of incident rays.  See  
ABSORPTION OF ELECTROMAGNETIC RADIATION. 
 �34s[M. G. Mellon]
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�6v�2v�32sOpen channel   �21m �33s 
    A covered or uncovered conduit in which liquid (usually water) flows with 
its top surface bounded by the atmosphere. Typical open channels are rivers, 
streams, canals, flumes, reclamation or drainage ditches, sewers, and 
water-supply or hydropower aqueducts. 
    Open-channel flow is classified according to steadiness, a condition in 
relation to time, and to uniformity, a condition in relation to distance. Flow 
is steady when the velocity at any point of observation does not change with 
time; if it changes from instant to instant, flow is unsteady. At every 
instant, if the velocity is the same at all points along the channel, flow is 
uniform; if it is not the same, flow is nonuniform. Nonuniform flow which is 
steady is called varied; nonuniform flow which is unsteady is called variable. 
    Flow occurs from a higher to a lower elevation by action of gravity. If 
the phenomenon is short, wall friction is small or negligible, and gravity 
shapes the flow behavior. Gravity phenomena are local; they include the 
hydraulic jump, flow over weirs, spillways, or sills, flow under sluices, and 
flow into culvert entrances. 
    If the phenomenon is long, friction shapes the flow behavior. Friction 
phenomena include flows in rivers, streams, canals, flumes, and sewers. 
 �34s[William Allan]
�33s 
  
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�6v�2v�32sOpen circuit   �21m �33s 
    A condition in an electric circuit in which there is no path for current 
between two points; examples are a broken wire and a switch in the OPEN, or 
OFF, position.  See  CIRCUIT (ELECTRICITY). 
    Open-circuit voltage is the potential difference between two points in a 
circuit when a branch (current path) between the points is open-circuited. 
Open-circuit voltage is measured by a voltmeter which has a very high 
resistance (theoretically infinite), such as a vacuum-tube voltmeter. 
 �34s[Clarence F. Goodheart]
�33s 
  
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�6v�2v�32sOpen-loop control system   �21m �33s 
    A control system in which the system outputs are controlled by the system 
inputs only. In such systems no account is taken of the actual system output. 
This is in contrast to closed-loop control systems, in which the system output 
is controlled by some combination of the system input and output. A simple 
example of this system is the electric-light control system in the home, in 
which the presence or absence of light from a lamp is controlled by the 
position of an electrical switch.  See  CLOSED-LOOP CONTROL SYSTEM.  
    There are two classes of open-loop control system in use: those without 
power amplification and those with power amplification. An open-loop control 
system without power amplification is one in which all the output power is 
supplied by the controlling input, for example, the ordinary household 
thermometer. An example of a control system that employs power amplification 
is a grinding wheel driven by an electric motor. In this system the 
controlling input is the position of an on-off electrical switch that 
energizes the motor, the controlled system is the grinding wheel, and the 
controlled output is the velocity of the surface of the grinding wheel. The 
power required to move the grinding wheel does not come directly from the 
controlling input but from some intermediate device (the electric motor), 
which is directly controlled by the switch. For examples of open-loop control  
see  ALARM SYSTEMS; CLOCK CONTROL SYSTEMS; REMOTE-CONTROL SYSTEM; SYNCHRO 
REPEATER; TELEMETERING; TRAFFIC CONTROL SYSTEMS. 
    Open-loop control system are in general considerably simpler than 
closed-loop systems. This simplicity usually results in a more economical 
control system. A more important advantage of open-loop control is the 
elimination of stability problems, which exist in many closed-loop systems.  
See  CONTROL SYSTEMS. 
 �34s[John G. Truxal]
�33s 
  
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�6v�2v�32sOpen-pit mining   �21m �33s 
    The extraction of ores of metals and minerals by surface excavations. This 
method of mining is applicable for near-surface deposits which do not have a 
ratio of overburden (waste material that must be removed) to ore that would 
make the operation uneconomic. Where these criteria are met, large-scale 
earth-moving equipment can be used to give low unit mining costs. 
    Most open-pit mines are developed in the form of an inverted cone, with 
the base of the cone on the surface. Exceptions are open-pit mines developed 
in hills or mountains (see illustration). The walls of most open pits are 
terraced with benches to permit shovels, front-end loaders, or bucket-wheel 
excavators to excavate the rock and to provide access for trucks, trains, or 
belts to transport the rock out of the pit. Numerous benches or terraces in 
the pit permit a number of areas to be worked at one time and are necessary 
for giving slope, typically less than a 2-to-1 slope, to the sides of the pit 
and for ore blending. Using multiple benches also permits a balanced operation 
in which much of the overburden is removed from upper benches at the same time 
that ore is being mined from lower benches. 
    The principal operations in mining are drilling, blasting, loading, and 
hauling. These operations are usually required for both ore and overburden. 
Occasionally the ore or the overburden is soft enough that drilling and 
blasting are not required. 
 �34s[Carl D. Broadbent]
�33s 
  
  
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�6v�2v�32sOperating system   �21m �33s 
    A set of programs to control and coordinate the operation of hardware and 
software in a computer system. Operating systems typically consist of two sets 
of programs. Control programs direct the execution of user application 
programs and support programs. They also supervise the location, storage, and 
retrieval of data (including input and output to various peripherals) and 
allocate the resources of the computer system to the different tasks entered 
into the system. Processing programs include applications programs, utility 
programs, system diagnostics, and language translators, such as compilers and 
interpreters. The growth of microcomputers has resulted in the development of 
operating systems specifically designed for them.  See  MICROCOMPUTER; 
MICROPROCESSOR.  
    There are five general types of operating systems: 
    1. Real-time. Processing functions are executed within narrow time limits, 
as in updating a data bank or reading and evaluating input from a sensor or 
transducer.  
    2. Serial or batch. Tasks or jobs are run one at a time according to a 
schedule determined by the operating system. 
    3. Multiprogramming. Two or more jobs may be run concurrently in the same 
system.  
    4. Time sharing. Multiple users have simultaneous access to the system. A 
user is not aware that others have access to the system; the impression is 
that of exclusive use of the system.  See  MULTIACCESS COMPUTER. 
    5. Multiprocessing. Two or more processing units are coupled together to 
execute a single job.  See  DIGITAL COMPUTER; MULTIPROCESSING. 
 �34s[Harry L. Helms]
�33s 
  
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�6v�2v�32sOperational amplifier �21m �33s
    An amplifier with very high voltage gain, high input impedance, and low
output impedance. The term operational amplifier was coined by people in the
analog computer field. This type of amplifier is used as the basic element in
the performance of the basic mathematical operations such as addition,
subtraction, multiplication, integration, differentiation, and logarithms.
These operations can be performed with high accuracy, providing the amplifier
has a voltage gain of the order of 100,000, an input impedance of the order of
1 megohm or higher, and an output impedance no greater than a few hundred
ohms. Thus, the operational amplifier is a special type of cascade amplifier
having these characteristics. See ANALOG COMPUTER.
    Operational amplifers, called op amps, are used extensively in the areas
of electronic instrumentation and communication in addition to analog
computing devices. These amplifiers are available in integrated-circuit, or
solid-state-chip, form and come in a variety of bandwidths. The op amp is
normally direct-coupled, so it may be used for either dc or ac applications
and its gain may be accurately controlled by negative feedback. Thus, the op
amp has become a universal amplifying device. See INTEGRATED CIRCUITS.
    The Norton amplifier is a type of operational amplifier that yields some
cost benefits because of its simplicity. See AMPLIFIER.
 �34s[Christos C. Halkias ;Charles L. Alley]
�33s

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�6v�2v�32sOperations research   �21m �33s 
    The application of scientific methods and techniques to decision-making 
problems. A decision-making problem occurs where there are two or more 
alternative courses of action, each of which leads to a different and 
sometimes unknown end result. Operations research is also used to maximize the 
utility of limited resources. The objective is to select the best alternative, 
that is, the one leading to the best result. 
    To put these definitions into perspective, the following analogy might be 
used. In mathematics, when solving a set of simultaneous linear equations, one 
states that if there are seven unknowns, there must be seven equations. If 
they are independent and consistent and if it exists, a unique solution to the 
problem is found. In operations research there are figuratively ``seven 
unknowns and four equations.'' There may exist a solution space with many 
feasible solutions which satisfy the equations. Operations research is 
concerned with establishing the best solution. To do so, some measure of 
merit, some objective function, must be prescribed. 
    In the current lexicon there are several terms associated with the subject 
matter of this program: operations research, management science, systems 
analysis, operations analysis, and so forth. While there are subtle 
differences and distinctions, the terms can be considered nearly synonymous.  
See  SYSTEMS ENGINEERING. 
  
    Methodology.   The success of operations research, where there has been 
success, has been the result of the following six simply stated rules: (1) 
formulate the problem; (2) construct a model of the system; (3) select a 
solution technique; (4) obtain a solution to the problem; (5) establish 
controls over the system; and (6) implement the solution. 
    The first statement of the problem is usually vague and inaccurate. It may 
be a cataloging of observable effects. It is necessary to identify the 
decision maker, the alternatives, goals, and constraints, and the parameters 
of the system. A statement of the problem properly contains four basic 
elements that, if correctly identified and articulated, greatly eases the 
model formulation. These elements can be combined in the following general 
form: ``Given (the system description), the problem is to optimize (the 
objective function), by choice of the (decision variable), subject to a set of 
(constraints and restrictions).'' 
    In modeling the system, one usually relies on mathematics, although 
graphical and analog models are also useful. It is important, however, that 
the model suggest the solution technique, and not the other way around. 
    With the first solution obtained, it is often evident that the model and 
the problem statement must be modified, and the sequence of 
problem-model-technique-solution-problem may have to be repeated several 
times. The controls are established by performing sensitivity analysis on the 
parameters. This also indicates the areas in which the data-collecting effort 
should be made. 
    Implementation is perhaps of least interest to the theorists, but in 
reality it is the most important step. If direct action is not taken to 
implement the solution, the whole effort may end as a dust-collecting report 
on a shelf. 
  
    Mathematical programming.   Probably the one technique most associated 
with operations research is linear programming. The basic problem that can be 
modeled by linear programming is the use of limited resources to meet demands 
for the output of these resources. This type of problem is found mainly in 
production systems, but is not limited to this area.  See  LINEAR PROGRAMMING. 
  
    Stochastic processes.   A large class of operations research methods and 
applications deals with stochastic processes. These can be defined as 
processes in which one or more of the variables take on values according to 
some, perhaps unknown, probability distribution. These are referred to as 
random variables, and it takes only one to make the process stochastic. 
    In contrast to the mathematical programming methods and applications, 
there are not many optimization techniques. The techniques used tend to be 
more diagnostic than prognostic; that is, they can be used to describe the 
``health'' of a system, but not necessarily how to ``cure'' it.  See  
PROBABILITY; QUEUEING THEORY; SAMPLING TECHNIQUES; STOCHASTIC PROCESS. 
  
    Scope of application.   There are numerous areas where operations research 
has been applied. The following list is not intended to be all-inclusive, but 
is mainly to illustrate the scope of applications: optimal depreciation 
strategies; communication network design; computer network design; simulation 
of computer time-sharing systems; water resource project selection; demand 
forecasting; bidding models for offshore oil leases; production planning; 
classroom size mix to meet student demand; optimizing waste treatment plants; 
risk analysis in capital budgeting; electric utility fuel management; optimal 
staffing of medical facilities; feedlot optimization; minimizing waste in the 
steel industry; optimal design of natural-gas pipelines; economic inventory 
levels; optimal marketing-price strategies; project management with 
CPM/PERT/GERT; air-traffic-control simulations; optimal strategies in sports; 
optimal testing plans for reliability; optimal space trajectories.  See  
DECISION THEORY; GERT; INVENTORY MANAGEMENT; PERT. 
 �34s[William G. Lesso]
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�6v�2v�32sOperator theory �21m �33s
    At one level of abstraction an operator is simply a function whose
arguments and values are real- (or complex-) valued functions of one or more
real variables; in more naive terms an operator is a rule for converting such
real- (or complex-) valued functions into others. The following are simple
examples: (i) the operator which takes each differentiable real-valued
function of one variable into its derivative; (ii) the operator which takes
each twice-differentiable function f of one variable into expression (1);
(iii) the operator which takes each twice-differentiable function f of three
variables into expression (2); and (iv) the operator which takes the
continuous function f of one real variable into the function g where relation
(3) holds.

 (df/dx)(sup 2) + (x[sup 2])[(d[sup 2]f)/dx(sup 2)]			(1)

(CURLY D[sup 2]f)/CURLY Dx[sup 2] + (CURLY D[sup 2]f)/CURLY Dy[sup 2]
	 + (CURLY D[sup 2]f)/CURLY Dz[sup 2]				(2)


       g(x) IDENTITY INTEGRATION (0 to 1) SQUARE ROOT [x + y] f(y) dy	     (3)

    Since an operator is a function, the usual functional notation is
applicable. L(f) may be used to denote the result of operating on f with the
operator L. The set of all functions f for which L(f) is defined is called the
domain of L, and the set of all functions g such that L(f) = g for some f in
the domain of L is called the range of L. It is obvious that solving a
differential or integral equation is equivalent (in many ways) to solving an
operator equation L(f) = g, where g and L are given and it is required to find
f. Moreover the operator concept can be very useful both in theory and
practice, producing a great variety of illuminating insights.  See
DIFFERENTIAL EQUATION; INTEGRAL EQUATION.
    In large part the fruitfulness of the operator concept can be traced to
two sources. One of these is the possibility of adding and multiplying
operators in such a way that many, though not all, of the laws of ordinary
algebra hold. The other is the fact that the ranges and domains of operators
behave in many respects like ordinary space and, indeed, may be regarded as
contained in infinite dimensional generalizations of the familiar
three-dimensional space of solid geometry. This makes it possible to think of
an operator as a geometrical transformation and to exploit one's spatial
intuition.
 �34s[George W. Mackey]
�33s

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�6v�2v�32sOperator training   �21m �33s 
    The specialized education of an organization's employees in the general 
knowledge, specific skills, and overall mental attitude required to do their 
jobs effectively. It is regarded as an essential function of an enterprise. As 
science advances and technology becomes more complex, competent and continuous 
training increases in importance. 
    Operator training can be of new employees and can include introduction to 
the company, orientation to the work situation, indoctrination in the rules 
and regulations of the firm, instruction in specific job skills, knowledge of 
particular subjects, and attitude improvement. Training can also be directed 
to existing employees to prepare them for job transfers or promotions, to 
maintain or upgrade their existing skills levels, to keep them current on new 
tools, techniques, or technologies, to aid in their personal or professional 
growth, or to improve their morale and attitudes. Training can also be of 
employees who have become injured, aged, or infirm in some way so that they 
can no longer do the job for which they were originally trained. 
    Productive industrial work requires the presence of at least six elements: 
(1) proper tools, machines, and equipment; (2) proper raw materials and 
supplies; (3) an operator skilled in the work to be done; (4) an environment 
suitable for the above three elements to operate; (5) correct instructions 
clearly communicated to the operator; and (6) motivation on the operator's 
part to perform. 
    A distinction may be made between training and development. Training may 
be said to be the imparting to the operator the correct skills, knowledge, and 
attitude to do the present or immediately prospective job. Development may be 
said to be the more general education and upgrading of the employee in 
subjects not as much required in the present job but anticipated to be needed 
in future work, usually an advancement or a promotion. Human resources 
development (HRD) is a term used by some firms to encompass these functions. 
 �34s[Vincent M. Altamuro]
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�6v�2v�32sOperon �21m �33s
    A contiguous linear array of genes whose expression is regulated as a
unit. Genes in an operon are preceded by two regulatory sites in the
chromosome: the promoter and the operator.
    An operon is characterized by several features as follows.
    1. The genes of an operon encode proteins that have related functions,
such as the successive enzymatic steps in the degradation of the sugar lactose
or in the biosynthesis of the amino acid histidine. Presumably, such genes are
organized into an operon so that their expression can be coordinated.
    2. The genes of an operon are contiguous to one another on the chromosome,
adjacent to the promoter and operator sites.
    3. The protein products of the genes of an operon are always produced in a
fixed ratio. For example, if a given operon encodes three proteins, A, B, and
C, the ratios of the rates of synthesis of each of these proteins to the other
is constant. A doubling in the rate of production of A is accompanied by a
doubling in the rate of production of B and C. Thus, all of the genes of an
operon are turned on and off together.
    4. Genes are expressed by transcription of the information stored in the
DNA into a short-lived nucleic acid molecule called messenger RNA. The mRNA is
translated in a complex series of reactions to yield a protein product that
carries out a specific cellular function. All of the genes of an operon are
transcribed into one large mRNA molecule, which is translated into the
individual protein products.
    5. A certain class of mutations in the genes of an operon affects the
expression not only of the gene in which the mutation resides, but also of
genes located to one side of the mutated gene. These mutations, termed polar
mutations, reduce the amount of mRNA corresponding to genes distal to the
mutated site.
    6. The genes of an operon are all regulated by the same mechanism.
Therefore, mutations that alter this common mechanism affect all genes of the
operon equally.  See  CHROMOSOME; DEOXYRIBONUCLEIC ACID (DNA); GENE; GENE
ACTION; RIBONUCLEIC ACID (RNA).
 �34s[Forrest G. Chumley ;H. Mark Johnston]
�33s

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�6v�2v�32sOphiocistioidea   �21m �33s 
    A small class of extinct Echinozoa which ranged from Ordovician to 
Devonian times. The domed aboral surface of the test was roofed by polygonal 
plates and carried an anal pyramid (see illustration). Below, at the center of 
the flat adoral side lay the mouth, with five interradial jaws surrounded by a 
flexible peristome. From it radiated the five ambulacra, each one with three 
rows of ambulacral plates. About eight pairs of large tube feet emerged from 
each ambulacrum, by way of pores lying between the median and the outer rows 
of plates.  See  ECHINOZOA. 
 �34s[Howard B. Fell]
�33s 
  
  
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�6v�2v�32sOphioglossidae   �21m �33s 
    A subclass of the class Polypodiopsida known as the adder's-tongue ferns. 
It is a small group with only 3 genera and about 80 species. Two genera, 
Ophioglossum and Botrychium, are widely distributed in tropical and temperate 
regions and have about the same number of species; the third genus, 
Helminthostachys, is represented by a  single species confined to southeastern 
Asia and Polynesia. These are considered the most primitive of the present-day 
ferns. No fossils have been reported for this group. 
    The chromosome number is high in the species of Ophioglossum. Ophioglossum 
petiolatum, a tropical species, has a chromosome count of over 1000, the 
largest number observed in a naturally occurring species of vascular plants.  
See  POLYPODIOPSIDA. 
 �34s[Paul A. Vestal]
�33s 
  
  
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�6v�2v�32sOphiurida   �21m �33s 
    An order of Ophiuroidea in which the vertebrae articulate by means of 
ball-and-socket joints, and the arms, which do not branch, move mainly from 
side to side and do not coil in the vertical plane. The disk and arms are 
usually sheathed in regularly arranged plates. These are disposed in four 
series on the arms, namely, one dorsal, one ventral, and two lateral. There is 
a single madreporite. The order embraces most of the known genera of brittle 
stars and includes 13 families.  See  OPHIUROIDEA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sOphiuroidea   �21m �33s 
    A subclass of the Asterozoa, known as the brittle stars, in which the arms 
are usually clearly demarcated from a central disk and perform whiplike 
locomotor movements, and the tube feet are nonsuctorial sensory tentacles. In 
all existing ophiuroids the ambulacral plates fuse together in pairs to form 
articulating joints termed vertebrae, and the ambulacral groove is converted 
into an internal epineural canal.  See  ASTEROZOA. 
    There are about 1900 extant species referred to 230 genera, arranged to 
form 3 orders: Oegophiurida, Phrynophiurida, and Ophiurida. There is also one 
Paleozoic order, the Stenurida.  See  OEGOPHIURIDA; OPHIURIDA; PHRYNOPHIURIDA; 
STENURIDA.  
    Ophiuroids are usually five-armed, with a few species being regularly six- 
or seven-armed. In some Euryalae the arms may branch repeatedly; these are the 
so-called basket fishes. Tropical species are often patterned in contrasting 
colors, but most ophiuroids tend to match their environment. Some species are 
luminescent, although not constantly so. 
    Ophiuroids occur in all the oceans from low-tide level downward, often in 
dense populations which number millions to the hectare. Six families range 
below a depth of 2 mi (3.2 km); the genera Ophiura, Amphiophiura, and 
Ophiacantha range below 4 mi (6.4 km). The shallow-water forms hide among 
algae, under stones, or within sponges or bury the disk in sand or mud, 
leaving only the arms protruding. Deep-water forms lie in or on the bottom 
material or adhere to corals or cidarids. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sOpiates   �21m �33s 
    Drugs derived from opium, the dried juice of the oriental poppy seed. The 
pharmacologically active substances, which constitute approximately 25% of the 
extract, are the alkaloids morphine, codeine, and papaverine. The newer 
synthetic compounds which resemble morphine in their action are called opioids. 
    The principal effect of opium and opioids is to relieve pain. Even today 
morphine remains the best analgesic. It also assuages anxiety and causes 
slight drowsiness, relaxation, and a euphoric state of mind. These psychic 
effects are so agreeable that many troubled individuals seek solace by 
ingesting, smoking, or injecting opiates.   See  MORPHINE. 
    Codeine has an action similar to morphine, but its analgesic effects are 
less. Papaverine has almost no analgesic action, and is used as an 
antispasmodic to relieve vascular spasm and undesirable contraction of smooth 
muscle.  See  ANALGESIC; DRUG ADDICTION; NARCOTIC. 
 �34s[Raymond D. Adams]
�33s 
  
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�6v�2v�32sOpilioacariformes   �21m �33s 
    The smallest and most primitive of the three orders of Acari, comprising 
only one suborder, the Notostigmata, and one family, the Opilioacaridae. These 
mites are characterized by the possession of a pretarsus, or apotele, on the 
pedipalp, with prominent claws, which are usually relatively unmodified and 
paired like those on the legs. There are two pairs of eyes on the propodosoma, 
above the second pair of legs.  See  ACARINA. 
 �34s[Joseph H. Camin]
�33s 
  
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�6v�2v�32sOpisthobranchia   �21m �33s 
    A subclass in the class Gastropoda containing about 4000 living species, 
arranged in nine orders, including the herbivorous Aplysiomorpha (sea hares) 
and Sacoglossa and the carnivorous Thecosomata (sea butterflies) and 
Nudibranchia (sea slugs). Primitive members of many of the orders show 
adaptations for burrowing beneath sand or mud; more advanced members are 
always active surface-living or pelagic forms. This trend is accompanied by a 
decrease in the importance of the shell and operculum for passive defense. 
These are replaced by more dynamic chemical (some species secrete decinormal 
sulfuric acid through the skin if annoyed), physical (daggerlike calcareous 
epidermal spicules), or biological (redirected nematocysts derived from 
coelenterate prey) defensive mechanisms. 
    The adult shells of the primitive opisthobranchs living today are often 
strongly developed and sometimes colorful; a more typical opisthobranch shell 
is fragile, inflated, and egg-shaped. In rather more advanced forms, the 
external shell has a very wide gape, and in animals like Berthella the widely 
gaping shell is wholly internal, covered by the mantle. The most varied shells 
are found in the Sacoglossa. In the highest sacoglossans, some bullomorphs and 
aplysiomorphs, and in all the nudibranchs, the true shell is completely lost 
after larval metamorphosis.  See  GASTROPODA; MOLLUSCA; NUDIBRANCHIA; 
SACOGLOSSA.  
 �34s[T. E. Thompson]
�33s 
  
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�6v�2v�32sOpossum   �21m �33s 
    Any member of the family Didelphidae in the order Marsupialia which 
includes about 65 species found in the New World. These mammals are arboreal 
and are mainly omnivorous. 
    The common opossum (Didelphis marsupialis) is an extremely adaptable 
mammal that ranges from Argentina through Central America into the United 
States. The uterine gestation period is quite short, about 12 days, and the 
young are born no larger than a bee. They then make their way through the 
mother's fur to the pouch (marsupium), where they remain and continue to 
develop for as long as 100 days. There may be two litters of 9 - 12 each year. 
The opossum is a sedentary animal and the adults, living in solitary pairs, 
rarely leave their territory.  See  MARSUPIALIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sOpportunistic infections �21m �33s
    Infections that owe their emergence to generalized or local defects in the
host's defense mechanisms. In general, chances for acquiring an infection in
humans are proportionate to the number of microbes at the site of infection,
proportionate to their virulence, and inversely proportionate to the strength
of the host's defenses.
    One possible classification of defects in the host's defense mechanisms
would list: defects in the production of antibodies; defects in cell-mediated
immunity; defects in phagocytosis; and defects in certain local defense
factors. Few human diseases are associated with only one such defect. In most
diseases underlying opportunistic infection, multiple mechanisms are at work
to affect host defenses. One form of immunosuppression, for instance, is
treatment with steroid hormones; it affects cell-mediated immunity,
phagocytosis, and (to a lesser degree) antibody production. Another form of
immunosuppression, cytotoxic chemotherapy, causes the number of phagocytes to
drop, diminishes antibody production, and causes damage to mucous membranes.
    Among other conditions favoring opportunistic infections are burns,
alcoholism, chronic lung diseases, chronic heart failure, cirrhosis of the
liver, renal failure, cystic fibrosis, diabetes mellitus, hypoparathyroidism,
sickle-cell disease, leukemias, lymphomas, and various defects in phagocytic
mechanisms, as well as the status after splenectomy. Certain manipulative
procedures such as catheterization, intravenous cannulation, and surgery
predispose as well, often to local infections.	See  CHEMOTHERAPY; INFECTION.
 �34s[Alexander W. C. von Graevenitz]
�33s

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�6v�2v�32sOpsonin   �21m �33s 
    A term used in serology and immunology to refer to a substance that 
enhances the phagocytosis of bacteria by leukocytes. Opsonin is generally 
synonymous with the bacteriotropin of F. Neufeld and coworkers (1904 - 1905), 
a relatively thermostable antibody, increased in amount during specific 
immunization, that renders the corresponding bacterium more susceptible to 
phagocytosis. There is evidence that this action can be promoted to some 
extent by antibody alone, but that it is substantially increased by the 
further addition of the thermolabile complement system.  See  AGGLUTINATION 
REACTION; ANTIBODY; LYTIC REACTION; NEUTRALIZATION REACTION (IMMUNOLOGY); 
PHAGOCYTOSIS; PRECIPITIN TEST; SERUM. 
 �34s[Henry P. Treffers]
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�6v�2v�32sOptical activity   �21m �33s 
    The effect of asymmetric compounds on polarized light. To exhibit this 
effect, a molecule must be nonsuperimposable on its mirror image, that is, 
must be related to its mirror image as the right hand is to the left hand. An 
optically active compound and its mirror image are called enantiomers or 
optical isomers (see illustration). Enantiomers differ only in their geometric 
arrangements; they have identical chemical and physical properties. The 
right-handed and left-handed forms of a molecule can be distinguished only by 
their optical activity or by their interactions with other asymmetric 
molecules. Optical activity can be used to probe other aspects of molecular 
geometry, as well as to identify which enantiomer is present and its purity. 
    The physical basis of optical activity is the differential interaction of 
asymmetric substances with left versus right circularly polarized light. If 
solids and substances in strong magnetic fields are excluded, optical activity 
is an intrinsic property of the molecular structure and is one of the best 
methods of obtaining structural information from a sample in which the 
molecules are randomly oriented. The relationship between optical activity and 
molecular structure results from the interaction of polarized light with 
electrons in the molecule. Thus the molecular groups that contribute most 
directly to optical activity are those that have mobile electrons which can 
interact with light. Such groups are called chromophores, since their 
absorption of light is responsible for the color of objects. For example, the 
chlorophyll chromophore makes plants green.  See  FARADAY EFFECT; 
MAGNETOOPTICS; POLARIZED LIGHT; STEREOCHEMISTRY. 
    Optical activity is measured by two methods, optical rotation and circular 
dichroism. The optical rotation method depends on the different velocities of 
left and right circularly polarized light beams in the sample. The velocities 
are not measured directly, but both beams are passed through the sample 
simultaneously. This is equivalent to using plane-polarized light. The 
differing velocities of the left and right circularly polarized components 
yield a rotation of the plane of polarization. Circular dichroism is the 
difference in absorption of left and right circularly polarized light. Since 
this difference is about a millionth of the absorption of either polarization, 
special techniques are needed to determine it accurately. Circular dichroism 
is reported as a difference in absorption, or as an ellipticity (a measure of 
the elliptical polarization of the emergent beam). 
 �34s[Vincent Madison]
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�6v�2v�32sOptical bistability   �21m �33s 
    A phenomenon in which an optical device can exist in either of two stable 
states. Optical bistability is an expanding field of research because of its 
potential application to all-optical logic and because of the interesting 
physical phenomena it encompasses. A bistable optical device can function as a 
variety of logic devices. It can have two stable output states labeled 0 and 1 
for the same input, as shown in illustration a. Thus it can serve as an 
optical memory element. Under slightly modified operating conditions, the same 
device can exhibit the optical transistor characteristic of illustration b. 
For input intensities close to Igain, small variations in the input light are 
amplified, in much the same way that a vacuum tube triode or transistor 
amplifies electrical signals. The characteristic of illustration b can also be 
used as a discriminator; inputs above Igain are transmitted with far less 
attenuation than those  below. Finally, there is limiting action above Igain: 
large changes in the input hardly change the output. There is hope that 
bistable devices will revolutionize optical processing, switching, and 
computing. Since the first observation of optical bistability  in a passive, 
unexcited medium in 1974, bistability has been observed in many different 
materials. Current research is focused on optimizing these devices and 
developing smaller devices of better materials which operate faster, at higher 
temperatures, and with less power. 
    Optical bistability has also attracted the attention of physicists 
interested in fundamental phenomena. In fact, a bistable device often consists 
of a nonlinear medium with an optical resonator; it is thus similar to a laser 
except that the medium is unexcited (except by the coherent light incident on 
the resonator). Such a device constitutes a simple example of a strongly 
coupled system of matter and radiation. Counterparts of many of the phenomena 
studied in lasers such as fluctuations, regenerative pulsations, and optical 
turbulence can be observed under better-controlled conditions in passive 
bistable systems.  See  LASER. 
    The transmission of information as signals impressed on light beams 
traveling through optical fibers is replacing electrical transmission over 
wires. With optical pulses and optical transmission, the missing component of 
an all-optical signal processing system is an optical logic element in which 
one light beam or pulse controls another. Because of the high frequencies of 
optical electromagnetic radiation, such all-optical systems have the potential 
for subpicosecond switching and room-temperature operation.  See  INTEGRATED 
OPTICS; OPTICAL COMMUNICATIONS. 
 �34s[Hyatt M. Gibbs]
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�6v�2v�32sOptical communications   �21m �33s 
    The transmission of speech, data, pictures, or other information by light. 
An information-carrying light wave signal originates in a transmitter, passes 
through an optical channel, and enters a receiver which reconstructs the 
original information. The ensemble of these three items constitutes an optical 
communication system. 
    Optical communication is one of the most advanced forms of communication 
by electromagnetic waves. Light waves are generally understood to occupy the 
part of the electromagnetic spectrum with wavelengths in the range 0.2 - 100 
micrometers. The advantages sought in using light waves instead of 
electromagnetic waves with longer wavelengths are threefold: 
    1. Because of its short wavelength, light can be focused into narrower 
beams or confined in smaller waveguides than radiation of longer wavelengths. 
    2. The information-carrying capacity is potentially greater than that of 
longer-wavelength radiation. 
    3. The highest transparency for electromagnetic radiation achieved in any 
solid material is that of silica glass in the wavelength range 1 - 2 MUm. This 
transparency is orders of magnitude higher than that of any other solid 
material in any other part of the spectrum.  See  ELECTROMAGNETIC RADIATION; 
LIGHT.  
    Until the 1960s, the only available sources generated light from a 
multitude of independent atomic radiators. Such light cannot be focused 
effectively to form a narrow beam or to propagate along the axis of a 
waveguide. The demonstration of the first laser removed this shortcoming and 
made feasible in the optical wavelength region all the communication 
techniques formerly limited to microwaves and other electromagnetic waves of 
longer wavelengths.  See  LASER. 
  
    Systems.   Although optical fiber systems presently appear as the most 
significant outgrowth of intense exploration in the field of optical 
communications, they were not the only or first optical systems considered or 
utilized. A free-space optical communications channel exists, for example, 
between orbiting satellites. For satellite-to-satellite communications, the 
region between the information source and information use is ideal, 
essentially a vacuum, which does not distort or attenuate the light beam. For 
this application, the laser is the best optical source because of its ability 
to radiate spatially coherent light. For satellite-to-Earth communications and 
other communication through air, the Earth's atmosphere strongly influences 
the light transmission between information source and destination. In neither 
case would the transmission reliability be considered satisfactory for most 
communication purposes. On the other hand, the probability of outage of an 
atmospheric propagation system can be arbitrarily low if the transmission path 
is short. For example, optical communication links confined to rooms or 
buildings for the remote control of television sets or video recorders have 
become quite popular.  See  OPTICAL FIBERS. 
  
    Optical transmitters.   The preferred sources for use in optical 
communication systems are semiconductor diodes which generate light in 
forward-biased operation. The simplest device is the light-emitting diode 
which radiates in all directions from a fluorescent area located in the diode 
junction. Since fibers accept only light entering the core at a small angle 
with respect to the axis, only a small part of the light radiated from a 
light-emitting diode can be captured and transmitted by the fiber.  See  
LIGHT-EMITTING DIODE. 
    In the case of the semiconductor laser, two ends of the junction plane are 
furnished with mirror surfaces which form an optical resonator and enhance the 
light bouncing back and forth between the mirrors by stimulated emission. As a 
result, the light emitted through the partially transparent end mirrors is 
well collimated within a narrow cone of angles. A large fraction of the light 
can be captured and transmitted by optical fibers. In practical semiconductor 
lasers, the semitransparent mirrors are simply formed by careful cleaving of 
two sides of the junction. 
  
    Optical receivers.   The simplest photodetectors are designed to capture 
light in a reverse-biased junction where it generates electron-hole pairs. The 
electrical carriers are swept out by the internal electrical field and induce 
a photocurrent in the external circuit. Greater receiver sensitivity may be 
achieved by multiplying the photo-generated carriers in the diode junction. 
This can be done by virtue of an internal electrical field that is 
sufficiently high to cause avalanche multiplication of free carriers.  See  
OPTICAL DETECTORS. 
 �34s[Detlef C. Gloge]
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�6v�2v�32sOptical detectors �21m �33s
    Devices that generate a signal when light is incident upon them. The
signal may be one observed visually in reflected or transmitted light, or it
may be electrical.
    One of the most widely used optical detectors is photographic film. This
method of detection is based on a photochemical process in particles embedded
in the film. This fairly sensitive technique is more effective than others in
detecting or recording, or both, the large amount of information usually
contained in an image or a complicated optical spectrum.  See  PHOTOGRAPHY.
    In many cases, fast detector response is needed to detect rapid changes in
incident light; furthermore, quite often an electrical output signal is
desired. In such cases, the optimum detector is one based on either external
or internal photoemission.
    In an externally photoemitting device, the light incident upon a surface
causes an emission of electrons from that surface into a vacuum. These
electrons can be amplified in number by using electric and magnetic fields to
cause them to impinge on other surfaces, thereby producing secondary
electrons. These detectors are limited to wavelengths in the visible region or
shorter, but are capable of a sensitivity and speed of response higher than
any other type.  See  IMAGE ORTHICON; PHOTOMULTIPLIER; PHOTOTUBE.
    In an internally photoemitting device, incident light produces free charge
carriers within a body and is detected through the effect of these charge
carriers on the electrical impedance of the body. In its simplest and most
widely used form, the body is a photoconductor whose impedance is high in the
absence of incident light. Such detectors are less sensitive and slower than
external photoemitters, but they require no vacuum envelope and operate at low
voltages. They also have the advantage that some of them are sensitive to all
wavelengths shorter than about 40 micrometers, far into the infrared.  See
OPTICAL COMMUNICATIONS; OPTICAL MODULATORS; PHOTOCONDUCTIVE CELL;
PHOTOCONDUCTIVITY; PHOTODIODE; SEMICONDUCTOR DIODE.
 �34s[John K. Galt]
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�6v�2v�32sOptical fibers   �21m �33s 
    Flexible transparent fiber devices, used for either image or data 
transmission, in which light propagates by total internal reflection. The 
optical fiber, or light guide, has a core of material with a refractive index 
higher than that of the surrounding cladding material. Fiber properties and 
requirements for image transfer, in which information is continuously 
transmitted over relatively short distances, are quite different than for data 
transfer where on-off pulses of light are used to transmit information over 
much longer distances. 
    There are three basic types of light guides. Multimode, stepped refractive 
index profile fibers (illustration a) are typically used for conventional 
image transfer as well as for short-distance data transmission. The number of 
rays or ``modes'' of light which are guided is determined by the core size and 
core/clad refractive index difference. In such a fiber, an initially sharp 
pulse made up of many modes broadens as it travels long distances through the 
fiber, since high-angle modes have a longer distance to travel than low-order 
modes. This limits the data transmission rate and distance because it 
determines how closely input pulses can be spaced without overlap at the 
output end. 
    A graded index multimode fiber (illustration b), where the core refractive 
index decreases with increased radial distance, can be used to minimize pulse 
broadening due to mode dispersion. Light rays travel more slowly near the core 
center than near the edge, resulting in the speed of the high-order modes 
approximating the speed of the low-order modes. This type of fiber is suitable 
for intermediate-distance, intermediate-data-rate transmission systems. 
    In the single-mode fiber type (illustration c), with low refractive index 
difference and small core size, the effect of mode dispersion on pulse 
dispersion is eliminated since only one mode is guided. This fiber is useful 
for long-distance, high-data-rate applications.  See  OPTICAL COMMUNICATIONS; 
REFLECTION OF ELECTROMAGNETIC RADIATION; REFRACTION OF WAVES; WAVEGUIDE. 
 �34s[Suzanne R. Nagel]
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�6v�2v�32sOptical flat   �21m �33s 
    A disk of high-grade quartz glass approximately 3/4 in. (2 cm) thick, 
having at least one side ground and polished with a deviation in flatness 
usually not exceeding 50 nanometers all over, and a surface quality of 5 
microfinish or less. When two surfaces of this quality are placed lightly 
together so that the air is not wrung out from between them, they are 
separated by a film of air and actually touch at only one point. This point is 
the vertex of a wedge of air separating the two pieces. 
    If parallel beams of light pass through the flat, part will be reflected 
against the surface being inspected, while part will be reflected directly 
back through the flat. Because the distance between the surfaces is constantly 
increasing along the angle, the beams reflected from the flat and the beams 
reflected from the workpiece will alternately reinforce and interfere with 
each other, producing a pattern of alternate light and dark bands (see 
illustration). Each succeeding full band from a point of contact means the 
distance between surfaces is one wave-length thicker. If the light is 
relatively monochromatic, the wavelength is known. Red with a wavelength of 
295 nm is commonly used. Thus a definite relationship is established between 
lineal measurement and light waves. Optical flats are used for two general 
purposes, determination of surface contour and comparison of lineal 
measurement.  
 �34s[Richard A. Bambach]
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�6v�2v�32sOptical image   �21m �33s 
    The image formed by the light rays from a self-luminous or an illuminated 
object that traverse an optical system. The image is said to be real if the 
light rays converge to a focus on the image side and virtual if the rays seem 
to come from a point within the instrument (see illustration). 
    The optical image of an object is given by the light distribution coming 
from each point of the object at the image plane of an optical system. The 
ideal image of a point according to geometrical optics is obtained when all 
rays from an object point unite in a single image point. However, diffraction 
theory teaches that even in this case the image is not a point but a minute 
disk.  See  DIFFRACTION. 
    From the standpoint of geometrical optics, if this most desirable type of 
image formation cannot be achieved, the next best objective is to have the 
image free from all but aperture errors (spherical aberration). In this case 
the light distribution in the image plane is still circular, resembling the 
point image; there is a true coordination of object point and image, although 
the image may be slightly unsharp. If the aperture errors are small, or if the 
image is viewed from a distance, such an image formation may be very 
satisfactory.  See  ABERRATION (OPTICS). 
    Asymmetry and deformation errors may be very disturbing if not held in 
check, because the light distribution of the image of a point in this case has 
a decidedly undesirable shape. 
 �34s[Max J. Herzberger]
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�6v�2v�32sOptical isolator   �21m �33s 
    A very small four-terminal electronic circuit element that includes in an 
integral package a light emitter, a light detector, and, in some devices, 
solid-state electronic circuits. The emitting and detecting devices are so 
positioned that the majority of the emission from the emitter is optically 
coupled to the light-sensitive area of the detector. The device is also known 
as an optoisolator, optical-coupled isolator, and optocoupler. The device is 
housed in an integral opaque package so that the only optical emission 
impinging on the detector is that produced by the emitter. This configuration 
of components can perform as a solid-state electronic transformer or relay, 
since an electronic input signal causes an electronic output signal without 
any electrical connection between the input and the output terminals. 
    Optical isolators are particularly useful to electronic circuit and 
systems designers because two circuits often have a large voltage difference 
between them and yet it is necessary to transfer a small signal between them 
without changing the basic voltage level of either. 
    The optical emitter most commonly selected for use in optical isolators is 
the gallium arsenide light-emitting diode (LED), which emits in the infrared 
or near-infrared regions of the spectrum. The other input devices commonly 
used are gas-discharge and incandescent lamps. These emitters tend to be 
somewhat larger and slower than LEDs, but are used when their particular 
electronic characteristics are desired, and in conjunction with detectors that 
have a spectral peak in the visible portion of the spectrum.  See  ELECTRIC 
LAMP; LIGHT-EMITTING DIODE. 
    The light detectors that are used in the construction of optical isolators 
include light-dependent resistors (such as photocells), light-sensitive 
devices that generate a voltage without any electrical input (such as 
photovoltaic devices), light-sensitive devices that switch from one state to 
another (such as photothyristors), and light-sensitive devices that modify a 
voltage or current (such as phototransistors, photodiodes, and 
photodetector-amplifier combinations).  See  CIRCUIT (ELECTRONICS); OPTICAL 
DETECTORS.  
 �34s[Robert D. Compton]
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�6v�2v�32sOptically pumped laser �21m �33s
    A device in which the population inversion necessary for laser action is
achieved by concentrating light (for example, from a flash lamp, the Sun or
another laser) onto the amplifying medium.
    Many such lasers are three-level lasers; that is, ground-state atoms are
excited by absorption of light to a broad upper-energy stage, from which they
quickly relax to the emitting state. Laser action occurs as they are
stimulated to emit radiation and so return to the original ground state. Solid
three-level lasers usually make use of ions of a rare-earth element, such as
neodymium, or of a transition metal, such as chromium, dispersed in a
transparent crystal or glass. The ions have broad absorption bands in which
pumping radiations can be absorbed. Thus broad-band white light can be used to
excite the atoms.
    Many rare-earth ion lasers use a fourth level above the ground state. This
level serves as a terminal level for the laser transition, and is kept empty
by rapid nonradiative relaxation to the ground state. This means that,
relative to three-level systems, a population inversion is easier to maintain,
and therefore such materials require relatively low pumping light intensity
for laser action.
    Liquid lasers have structures generally like those of optically pumped
solid-state lasers, except that the liquid is generally contained in a
transparent cell. Some liquid lasers make use of rare-earth ions in suitable
dissolved molecules, while others make use of organic dye solutions. The dyes
can lase over a wide range of wavelengths, depending upon the composition and
concentration of the dye or solvent. Thus tunability is obtained throughout
the visible, and out to a wavelength of about 0.9 micrometer. Fine adjustment
of the output wavelength can be provided by using a diffraction grating or
other dispersive element in place of one of the laser mirrors.
    In several infrared regions, tunable laser action can be obtained by using
an infrared gas laser to pump a semiconductor crystal in a magnetic field,
giving amplification by stimulating spin-flip Raman scattering from the
electrons in the semiconductor. Tuning is achieved by varying the magnetic
field. See LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
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�6v�2v�32sOptical materials   �21m �33s 
    Generally, all substances used to reflect, refract, filter, polarize, 
modulate, detect, or disperse infrared, visible, or ultraviolet radiation or 
light; more specifically, the transparent materials usually used for windows 
and lenses. These materials are often some formulation of glass, a plastic in 
either bulk or thin-film form, or crystals either single or in aggregates. The 
most important optical properties are the degree and spectral region of 
transparency, the value of the refractive index n over the same spectral 
region, and the uniformity of the sample. Desirable materials are usually 
hard, strong, and relatively insensitive to temperature variations.  See  
ABSORPTION OF ELECTROMAGNETIC RADIATION; REFRACTION OF WAVES; SCATTERING OF 
ELECTROMAGNETIC RADIATION. 
    The optical material most widely used is optical glass, which is available 
in a wide range of refractive indices and dispersions and differs from 
ordinary glass in its freedom from imperfections. It must be free from 
unmelted particles or ``stones,'' from bubbles, and from chemical 
inhomogeneity, which gives rise to regions of variable refractivity known as 
cords or striae.  See  GLASS. 
    Because of certain limitations of glasses, single-crystal or 
polycrystalline aggregates are used for special applications. The most useful 
of these are of cubic symmetry, so that the refractive index and other 
physical properties are isotropic. 
    The most significant properties of plastics (compared to glass) are their 
softness and molecular complexity. In the visible spectrum they can be used 
for relatively cheap lenses but must be protected from abrasion. They can be 
used in the infrared at relatively short and very long wavelengths. 
    For the ultraviolet region of the spectrum, the main refractors are LiF, 
CaF2, and SiO2. The region short of this is known as the vacuum ultraviolet, 
where the atmosphere and all known materials are opaque. In the very far 
infrared most of the alkali halides ``open up'' and transmit reasonably well. 
Many plastics are useful if not thicker than several thousandths of an inch. 
These are used primarily as windows in the form of membranes to maintain a 
prescribed interior atmosphere. 
 �34s[William L. Wolfe]
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�6v�2v�32sOptical methods of chemical analysis �21m �33s
    Methods that deal with the measurement of the extent of the interaction of
light (electromagnetic radiation) with matter. Both the manner of interaction
and the type of electromagnetic radiation utilized vary widely. The common
wavelength units are the angstrom (A), the micrometer (MUm), and the nanometer
(nm), which have values as shown below.

		       1A = 10 (sup -10) m
		    1 MUm = 10 (sup -6) m
		     1 nm = 10 (sup -9) m

    Frequency units are usually given in cm-sup1, called wave numbers of
kaysers, so that the frequency which corresponds to 1 MUm is equal to a value
of 10,000 cm(sup -1.)
    A broader interpretation of optical methods also includes the
corresponding techniques using higher-energy x-rays and lower-energy
microwaves. For discussions of the principal methods  see  FLAME PHOTOMETRY;
FLUOROMETRIC ANALYSIS; INFRARED SPECTROSCOPY; MICROWAVE SPECTROSCOPY;
NEPHELOMETRIC ANALYSIS; POLARIMETRIC ANALYSIS; REFRACTOMETRIC ANALYSIS;
SPECTROCHEMICAL ANALYSIS; SPECTROPHOTOMETRIC ANALYSIS; TURBIDIMETRIC ANALYSIS;
X-RAY FLUORESCENCE ANALYSIS.
 �34s[James N. Little]
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�6v�2v�32sOptical microscope �21m �33s
    An instrument used to obtain an enlarged image of a small object. In
general, a compound microscope consists of a light source, a condenser, an
objective, and an ocular or eyepiece, which can be replaced by a recording
device such as a photoelectric tube or a photographic plate. The optical
microscope is limited by the wavelengths of the light used and by the
materials available for manufacturing the lenses. Some optical errors, for
instance, curvature of field and lateral color, are not generally corrected in
the objective but neutralized in the ocular.  See  ABERRATION (OPTICS); LENS
(OPTICS).

    Magnifying power.	The magnifying power of a compound microscope is the
product of the magnification of the objective and the magnifying power of the
eyepiece. The latter is computed like that of any magnifier. The magnification
of the objective is equal to the distance from the second focal point to the
image formed by the objective, divided by the focal length. An objective of
18-mm focal length thus has a power of 10x. It is customary to specify
objectives in terms of magnifying power instead of focal length. The distance
mentioned is called the optical tube length (generally 180 mm), and is to be
distinguished from the mechanical tube length, which is the length of the
mechanical tube itself.  See  MAGNIFICATION.

    Catadioptric systems.   Catadioptric systems have been developed for
microscopes. Their great advantage is their comparatively small chromatic
aberration. A type consisting of a single element is shown in Fig. 1a. Pure
mirror systems have no color aberrations. All microscopic work in the
ultraviolet region is done with catadioptric systems. A type designed for this
purpose is sketched in Fig. 1b.


    Condensers.   An external auxiliary lens is used to condense the light
from a light source so that the object is brightly and uniformly illuminated.
The usual purpose of a condenser system is to make sure that as much light as
possible coming from the object goes through an optical system. Condensers are
used in macroscopic projection, in which an illuminated film or slide is
imaged with the help of a projection objective or magnifier. In microscope
systems, they are used to direct the light from a light source so that the
rays from any object point fill most of the entrance pupil.
    In microscope practice, it is sometimes advantageous to increase the
contrast of small objects by making them appear as bright objects on a dark
background. This is achieved by arranging the condenser so that direct light
is cut off and only light diffracted or dispersed from the object enters the
microscope. The cardioid condenser sketched in Fig. 2 is an example of such a
dark-field condenser.
 �34s[Max J. Herzberger]
�33s


    Light microscope. The mirror, condenser, oculars, and body tube of the
light microscope are frequently known as the optical train. The stand, stage,
and adjustments comprise the mechanical part of the microscope.
    A mirror is usually attached to the substage of the microscope to reflect
light along the optic axis of the microscope. When no condenser is used, the
concave mirror is used because it concentrates more light on the specimen; a
plane mirror is used with a condenser.
    The objective is the basic part of the microscope; it forms the image
that is again enlarged by the eyepiece. Photomicrographic objectives are
designed to produce a flat image with little distortion. For convenience, two
to five objectives can be mounted on a revolving nosepiece to be parfocal and
parcentric, so that the specimen remains almost in focus at the center of the
field as the objectives are changed.
    The commonly used Huygenian ocular has a fairly flat field with marked
pincushion distortion. Compensating oculars complete the color correction for
apochromatic objectives and have less distortion, but they do have curvature
of field.  See	EYEPIECE.
    The monocular body tube may be of adjustable length. American microscopes
are designed for a mechanical tube length of 160 mm (6.3 in.) and a cover
glass thickness of 0.18 mm (0.007 in.). The draw tube is lengthened for
thinner and shortened for thicker cover glasses to correct for the spherical
aberrations from cover glasses of incorrect thickness.
    Binocular bodies are designed for the use of both eyes. Most binocular
bodies use prisms to reflect one-half of the light to each eye. Because each
eye sees the same field, these binocular bodies do not give stereoscopic
vision. The binocular is often longer than the monocular body and the proper
tube length is maintained with a compensating lens.

    Inverted microscope.   The inverted microscope has the body of the
microscope, including the objective and the ocular, below the stage and the
illumination above the stage for transmitted light. The inverted microscope is
especially useful for the examination of surfaces. Large and awkward specimens
can be moved over the stage more readily than with the usual microscope. The
inverted microscope is also useful for microdissection and the observation of
hanging-drop preparations.

    Comparison microscope.   The comparison microscope is an arrangement of
two microscopes connected by a special viewing ocular so that the field of one
microscope is seen at one side of a vertical dividing line and the field of
the other microscope on the opposite side of the dividing line; or it may be a
projection type of microscope in which the image is compared with a template
or known pattern.

    Dissecting microscope.   Dissecting microscopes are of two types. The
simplest is a magnifying glass mounted on a support above a glass plate, used
for the dissection of materials.
    The more usual dissecting microscope, often called a Greenough microscope,
is a stereoscopic microscope composed of two separate microscopes fastened
together and used as a single unit on one stand (Fig. 3). This is a truly
stereoscopic instrument because the right eye sees the specimen from the right
side and the left eye from the left side. Prisms are usually included in the
body tube to erect the image; thus movements of the specimen are direct and
are not reversed as with the monobjective microscope.

    Metallurgical microscope. The metallurgical microscope is a laboratory
microscope with a focusing stage and a vertical illuminator, used primarily
for the examination of metal surfaces. See MICROSCOPE. [Oscar W. Richards]
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�6v�2v�32sOptical modulators �21m �33s
    Devices that serve to vary some property of a light beam. The direction of
the beam may be scanned as in an optical deflector, or the phase or frequency
of an optical wave may be modulated. Most often, however, the intensity of the
light is modulated.
    Rotating or oscillating mirrors and mechanical shutters can be used at
relatively low frequencies (less than 10[sup 5]Hz). However, these devices have
too much inertia to operate at much higher frequencies. At higher frequencies
it is necessary to take advantage of the motions of the low-mass electrons and
atoms in liquids or solids. These motions are controlled by modulating the
applied electric fields, magnetic fields, or acoustic waves in phenomena known
as the electrooptic, magnetooptic, or acoustooptic effect, respectively.  See
ACOUSTOOPTICS; ELECTROOPTICS; KERR EFFECT; MAGNETOOPTICS.
 �34s[Ivan P. Kaminow]
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�6v�2v�32sOptical phase conjugation �21m �33s
    A process that involves the use of nonlinear optical effects to precisely
reverse the direction of propagation of each plane wave in an arbitrary beam
of light, thereby causing the return beam to exactly retrace the path of the
incident beam. The process is also known as wavefront reversal or
time-reversal reflection. The unique features of this phenomenon suggest
widespread application to the problems of optical beam transport through
distorting or inhomogeneous media.
    Optical phase conjugation is a process by which a light beam interacting
in a nonlinear material is reflected in such a manner as to retrace its
optical path. As the illustration shows, the image-transformation properties
of this reflection are radically different from those of a conventional
mirror. The incoming rays and those reflected by a conventional mirror
(illustration a) are related by inversion of the component of the k-vector or
wave vector k normal to the mirror surface. Thus a light beam can be
arbitrarily redirected by adjusting the orientation of a conventional mirror.
In contrast, a phase-conjugate reflector (illustration b) inverts the vector
quantity k so that, regardless of the orientation of the device, the reflected
conjugate light beam exactly retraces the path of the incident beam. This
retracing occurs even though an aberrator (such as a piece of broken glass)
may be in the path of the incident beam. Looking into a conventional mirror,
one would see one's own face, whereas looking into a phase-conjugate mirror,
one would see only the pupil of the eye.
    These remarkable image-transformation properties (even in the presence of
a distorting optical element) open the door to many potential applications in
areas such as laser fusion, atmospheric propagation, fiber-optic propagation,
image restoration, real-time holography, optical data processing, nonlinear
microscopy, laser resonator design, and high-resolution nonlinear
spectroscopy.  See  HOLOGRAPHY; LASER; NONLINEAR OPTICS; OPTICAL
COMMUNICATIONS.
 �34s[Robert A. Fisher ;Barry J. Feldman]
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�6v�2v�32sOptical prism   �21m �33s 
    An optical system consisting of two or more usually plane surfaces of a 
transparent solid or embedded liquid at an angle with each other. Prisms are 
used for deviating light. Since the amount of deviation depends on the 
refractive index of the prism, which varies with wavelength, prisms can also 
be used for dispersing light, and they can therefore be used to separate white 
light into its monochromatic parts. Prisms can also be used instead of mirrors 
for deviating light, with the added advantage that the reflecting surfaces are 
protected against corrosion.  See  DISPERSION (RADIATION); MIRROR OPTICS; 
REFRACTION OF WAVES. 
    Dispersing prisms deviate light of different wavelengths by different 
amounts. To increase the dispersion, several prisms with their refracting 
edges parallel can be used. The Rayleigh prism, shown in illustration a, is an 
example of such a system. By using a prism made of a material, such as flint 
glass, that has a high dispersion, and adding one or more prisms made of a 
material having a low dispersion, such as crown glass (illustration b), the 
deviation can be neutralized without neutralizing the dispersion to give a 
direct-vision prism system. The arrangement shown in illustration b is known 
as the Amici prism system. By using a similar arrangement but adjusting the 
angle so that the dispersion, but not the deviation, is neutralized, it is 
possible to make a prism system that is achromatic over a small part of the 
spectrum, like an achromatic lens.  See  BINOCULARS; DIOPTER; GEOMETRICAL 
OPTICS; LENS (OPTICS); OPTICAL MATERIALS; PERISCOPE; RESOLVING POWER (OPTICS). 
 �34s[Max J. Herzberger]
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�6v�2v�32sOptical projection systems   �21m �33s 
    Optical projection is the process whereby a real image of a suitably 
illuminated object is formed by an optical system in such a manner that it can 
be viewed, photographed, or otherwise observed. Essential equipment in an 
optical projection system consists of a light source, a condenser, an object 
holder, a projection lens, and (usually) a screen on which the image is formed 
(Fig. 1). For some important applications of optical projection  see  
CINEMATOGRAPHY; LANTERN SLIDES. 
    The luminance of the image in the direction of observation will depend 
upon (1) the average luminance of the image of the light source as seen 
through the projection lens from the image point under consideration, (2) the 
solid angle subtended by the exit pupil of the projection lens at this image 
point, and (3) the reflective or transmissive characteristics of the screen. 
Usually it is desirable to have this luminance as high as possible. Therefore, 
with a given screen, lens, and projection distance, the best arrangement is to 
have the light source imaged in the projection lens, with its image filling 
the exit pupil as completely and as uniformly as possible. 
    The object is placed between the condenser and the projection lens. If 
transparent, it can be inserted directly in the light beam; however, it should 
be positioned, and the optical system should be so designed that it does not 
vignette (cut off) any of the image of the light source in the projection 
lens. If the object is opaque, an arrangement known as an epidiascope (Fig. 2) 
is used. 
 �34s[Armin J. Hill]
�33s 
  
  
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�6v�2v�32sOptical pulses �21m �33s
    Short flashes of light, used to isolate moments of time. Spark or flash
photography can freeze the most rapid movement of macroscopic objects with
flashes of 10(sup -7)s in duration. High-speed flash lamps and electronics play
an important role in the study of fast photophysical and photochemical
processes with a resolution of 10(sup -10)s. In 1966 a laser technique was
used to generate the first optical pulses of less than 10 picoseconds in
duration. Pulses as short as a tenth of a picosecond have been produced with
the continuous dye laser system.  See  LASER; STROBOSCOPIC PHOTOGRAPHY.
    The more frequencies or modes oscillating in a laser, the shorter the
optical pulse that can be generated. Organic dyes are ideal for generating
pulses, because dyes are capable of oscillating over a broad range of
frequencies or colors. To make a pulse, each of these oscillating frequencies
must maintain a fixed phase relationship with one another to be coherently
added to produce a short optical pulse. This process is called mode locking.
 �34s[C. V. Shank]
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�6v�2v�32sOptical pumping   �21m �33s 
    The process of causing strong deviations from thermal equilibrium 
populations of selected quantized states of different energy in atomic or 
molecular systems by the use of optical radiation (that is, light of 
wavelengths in or near the visible spectrum), called the pumping radiation. 
    Optical pumping is vital for light amplification by stimulated emission in 
an important class of lasers. For example, the action of the ruby laser 
involves the fluorescent emission of red light by a transition from an excited 
level E2 to the ground level E1. In this case E2 is relatively high above E1 
and the equilibrium population of E2 is practically zero. Amplification of the 
red light by laser action requires that number of atoms N2 exceed N1 
(population inversion). The inversion is accomplished by intense green and 
violet light from an external source which excites the chromium ion in the 
ruby to a band of levels, E3, above E2. From E3 the ion rapidly drops without 
radiation to E2, in which its lifetime is relatively long for an excited 
state. Sufficiently intense pumping forces more luminescent ions into E2 by 
way of the E3 levels than remain in the ground state E1, and amplification of 
the red emission of the ruby by stimulated emission can then occur.  See  
LASER.  
 �34s[William West]
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�6v�2v�32sOptical recording   �21m �33s 
    The process of recording sound signals on a photosensitive medium so that 
they may be reproduced at a subsequent time. Photographic film has been widely 
used as the photosensitive medium, but in the late 1970s development of 
another kind of medium, the so-called optical disk, was undertaken. The 
introduction of the laser as a light source greatly improves the quality of 
reproduced sound signals. The pulse code modulation (PCM) techniques make it 
possible to obtain extremely high-fidelity reproduction of sound signals in 
optical disk recording systems. 
  
    Optical film recording.  Optical film recording is also termed motion 
picture recording or photographic recording. A sound motion picture recording 
system consists basically of a modulator for producing a modulated light beam 
and a mechanism for moving a light-sensitive photographic film relative to the 
light beam and thereby recording signals on the film corresponding to the 
electrical signals (Fig. 1). A sound motion picture reproducing system is 
basically a combination of a light source, optical system, photoelectric cell, 
and mechanism for moving a film carrying an optical record by means of which 
the recorded photographic variations are converted into electrical signals of 
approximately like form.  See  CINEMATOGRAPHY; SOUND RECORDING. 
  
    Laser-beam film recording.  An optical film system which utilizes a laser 
as a light source and a combination of an acoustooptical modulator (AOM) and 
an acoustooptical deflector (AOD) instead of a galvanometer was developed in 
the late 1970s. A  100-kHz pulse width modulation (PWM) circuit converts the 
audio input signal into a PWM signal. The laser beam is made to continuously 
scan the sound track area at right angles to the direction of the film 
transport. This is done by means of the AOD, which in turn is driven by a 
100-kHz sawtooth signal. Simultaneously, the laser beam is 
pulse-width-modulated by means of the AOM, which is driven by a 100-kHz PWM 
signal. The AOD scanning signal and the AOM pulse-width-modulated signal 
combine and generate the variable-area sound track exposure on the film (Fig. 
2). Because the actual distance between successive scans is very small, the 
traces of successive scans are fused into a pattern of variable-area 
recording.  See  PULSE MODULATION. 
  
    PCM optical sound disk player.  An optical disk allows a color television 
program of about 30 minutes' duration to be reproduced from a recording on a 
phonographlike disk 12 in. (30 cm) in diameter. Owing to its wide frequency 
range characteristics, up to several MHz, it can also serve to record or 
reproduce digitized (pulse code modulation or PCM) audio signals. The PCM 
optical sound disk player has the following superior characteristics which are 
not available from the conventional disk: (1) wide-frequency-range (up to 20 
kHz) audio signals; (2) extremely large dynamic range; and (3) the complete 
absence of various distortions inherent in conventional records such as 
tracing distortion, pinch effect, and flutter and wow. These are direct 
results of PCM techniques applied to audio signal recording. 
    In the recording system, an originating disk is obtained by coating a thin 
layer of evaporated metal on one side of a glass disk. The audio signal is 
passed through a PCM coding circuit for conversion into a digital signal which 
is then frequency-modulated. This frequency-modulated signal is then applied 
to the optical modulator which chops the continuous beam of light from the 
argon laser. The laser-beam intensity corresponding to the frequency 
modulation is then focused into a spot of about 0.8 micrometer diameter by the 
optical condenser. The information is thus recorded as a series of pits in 
thin metal film on the disk. 
  
    The PCM laser sound disk player rotates the disk at the correct speed. Its 
optical system directs and focuses a low power (1 mW) helium-neon laser beam 
to a small read spot on the surface of the disk and then collects the 
reflected optical energy and directs it to a single photodetector. The 
received energy at the photodetector changes according to the presence of 
recorded pits. Its electronic circuits process the received signal into 
digitized form, and the subsequent digital-to-analog converter converts the 
digital signals into the audio output signals with  high fidelity. See 
DIGITAL-TO-ANALOG CONVERTER. 
 �34s[H. Date]
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�6v�2v�32sOptical surfaces   �21m �33s 
    Many common optical instruments contain optical materials bounded by 
spherical surfaces with a wide range of curvatures. A surface that is not a 
sphere is called an aspherical surface. Examples of such aspherical surfaces 
with symmetry of rotation that are occasionally used in optical instruments 
are conic sections (ellipsoids, hyperboloids, and paraboloids). Cylindrical 
and toroidal lenses are used in anamorphotic systems. 
    Since it is much easier to grind and polish spherical surfaces than 
aspherical ones, most optical systems consist of lenses that are bounded by 
two spherical surfaces. These surfaces are arranged so that their centers lie 
on a line known as the axis of the system. The point of intersection  of the 
surface with the axis is called the vertex. A plane surface is generally 
considered as a special case of a spherical surface which has its center at 
infinity and which can therefore be  said to have an infinite radius. 
    Aspherical surfaces are difficult to manufacture. When many identical 
aspherical elements are to be made in series, special methods of grinding and 
polishing must be used. For instance, templates are used for spectacle lenses 
and for paraboloidal and ellipsoidal mirrors. Where extreme correction is 
desired, for instance, in astronomical telescopes, retouching by hand is the 
proper procedure. For condensers and for other lenses where extreme accuracy 
is not needed, a molding process is used. Large paraboloidal mirrors for 
searchlights and ellipsoidal mirrors for arc lamps used to project motion 
pictures are made by a ``dropping'' process. A sheet of plate glass is laid on 
a suitable concave mold and heated until it softens and can be sucked into the 
mold.  See  GEOMETRICAL OPTICS; LENS (OPTICS); OPTICAL TELESCOPE. 
 �34s[Max J. Herzberger]
�33s 
  
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�6v�2v�32sOptical telescope   �21m �33s 
    An instrument that collects light energy from a distant source and focuses 
it into an image that can then be studied by a number of different techniques. 
This definition of an optical telescope must be narrowed somewhat. 
Satellite-borne telescopes operated to study celestial x-rays or ultraviolet 
radiation and ground-based telescopes used to study radio radiation can be 
called optical telescopes in the sense that they operate according to the 
principles of geometrical optics. Ground-based telescopes are used to study 
radiation from celestial objects in the wavelength range from the Earth's 
atmospheric cutoff in the near ultraviolet to the infrared, that is, from 300 
to 1000 nanometers. Such instruments may be classified as (1) large 
astronomical telescopes, used to study the nature of astronomical objects 
themselves, and (2) astronomical transit instruments, used to study positions 
and motions of astronomical objects and in the accurate determination of time. 
  See  ASTRONOMICAL TRANSIT INSTRUMENTS; GEOMETRICAL OPTICS; RADIO TELESCOPE; 
TELESCOPE.  
    There are basically three types of optical systems in use in astronomical 
telescopes; refracting systems whose main optical elements are lenses which 
focus light by refraction; reflecting systems, whose main imaging elements are 
mirrors which focus light by reflection; and catadioptric systems, whose main 
elements are a combination of a lens and a mirror. The most notable example of 
the last type is the Schmidt camera.  See  SCHMIDT CAMERA. 
    Astronomers seldom use large telescopes for visual observations. Instead, 
they record their data for future study. Modern developments in photoelectric 
imaging devices are supplanting photographic techniques for many applications. 
The great advantages of detectors such as charge-coupled devices is the fact 
that they are very sensitive,and the images can be read out onto a 
computer-compatible magnetic tape or disk for immediate analysis.  See  
ASTRONOMICAL PHOTOGRAPHY; CHARGE-COUPLED DEVICES. 
    Light received from most astronomical objects is made up of radiation of 
all wavelengths. The spectral characteristics of the radiation emitted from or 
reflected by a body may be extracted by special instruments called 
spectrographs.  See  ASTRONOMICAL SPECTROSCOPY. 
    For many applications the Earth's atmosphere limits the effectiveness of 
larger telescopes. The most obvious deleterious effect of the Earth's 
atmosphere is image scintillation and motion, collectively known as ``poor 
seeing.'' On the very best nights at ideal observing sites, the image of a 
star will be spread out over a 0.25-arc-second seeing disk; on an average 
night, the seeing disk may be between 0.5 and 2.0 seconds of arc. Telescopes 
larger than about 20 in. (50 cm) are built, not for resolution, but for 
light-gathering power, which depends on the area of the primary mirror or 
objective lens. Once again, however, the atmosphere limits what can be 
observed.  
    The 200-in. (5.08-m) Hale telescope at Palomar Mountain, California, was 
completed in 1950. The primary mirror of this reflector is 200 in. in diameter 
with a 40-in. (1.02-m) hole in the center. The Hale telescope was the first 
that was large enough to have a prime-focus cage where the observer could sit 
inside the main tube. 
    The 158-in. (4.01-m) Mayall reflector at the Kitt Peak National 
Observatory was dedicated in 1973 (see illustration). The prime focus has a 
field of view six times greater than that of the Hale reflector. An identical 
telescope was subsequently installed at Cerro Tololo Inter-American 
Observatory, in Chile. 
    The Smithsonian Astrophysical Observatory/University of Arizona 
Multi-Mirror Telescope, or MMT, is one of the most innovative of the newer 
telescopes. It uses six 16-ft (1.8-m) mirrors working together to provide 
light-gathering power equivalent to a 15-ft (4.5-m) mirror. 
 �34s[Robert D. Chapman]
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�6v�2v�32sOptical tracking instruments �21m �33s
    A family of optical instruments used for precise time-correlated
observation of distant airplanes, missiles, and artificial satellites, all of
which travel at apparent velocities much greater than those of most
astronomical objects. The instruments supply permanent engineering records for
the determination of spatial position, missile attitude, structural behavior,
and performance of specific mechanisms during test flights. These observations
enable engineers to correct design, improve performance, and collect
scientific data from missiles at extreme distances and altitudes.
    The instruments used fall by function into two classes, those which
determine spatial position and those which record engineering events. Tracking
telescopes are the basic engineering-event recording systems, while
cinetheodolites and ballistic cameras are used for the precise determination
of spatial position.
    Spatial position determination can be considered the determination of the
position of a moving target using dynamic adaptations of the methods of civil
surveying. The classical techniques utilize a minimum of four instruments on a
precisely measured baseline to locate a moving target. Each instrument records
the data for computing the direction of the line of sight to the target for
each instant of time. This information, in the form of analog or digital
elevation and azimuth angles, the times of observation, and the known location
of each instrument, is used to triangulate for the location of the missile as
a function of time.

    Cinetheodolite.  Most of the spatial position work is performed by
cinetheodolites,(see illustration), which are surveying theodolites  having
35-mm motion-picture cameras with 60- to 240-in. (1.5 - to 6.1-cm)
focal-length lenses substituted for the surveyor's eye and telescope. The
system of cameras is synchronized up to a maximum of 30 frames/s from a master
control station for simultaneous exposure as the cinetheodolites follow the
moving missile. Each photograph records the elevation angle, the azimuth
angle, the missile image, and the reticle lines which define the instrumental
axis.
    The rapid evolution of laser systems led to the development of a laser
automatic-tracking range-finding digital cinetheodolite, which is an optical
radar capable of single station solution of the target location. The
requirements for automatic tracking and range-finding have led to the
development of a cinetheodolite with 1-second accuracy.

    Ballistic cameras.	These are fixed-axis, wide-angle, photographic-plate
cameras, capable of more precise spatial position determination by recording
on one plate multiple exposures of the missile against a stellar background.
Use of a static system and precisely cataloged star positions decreases
necessity for long-term mechanical stability and accuracy, allowing ballistic
cameras to achieve 2 - 5 seconds angular accuracy. Pyrotechnic flares or
electronic stroboscopic lamps at the missile are used to indicate the missile
positions against the night sky.


    Tracking telescopes.  These are long-focal-length telescopes mounted to
track missiles in flight precisely while collecting missile performance data.
In the normal aided-tracking mode of operation, the operator observes the
missile through a tracking sight and controls the motion of the telescope with
a control knob. The system must track at rates up to 10DG/s with a precision
of 1 - 2 minutes of arc if the system resolution of 1/2 second is not to be
lost because of relative motion of the image during the exposure.
    The precise tracking of modern tracking telescopes has permitted the use
of slit spectrographs for infrared and visible-light spectroscopy.  See
ASTRONOMICAL PHOTOGRAPHY; CAMERA; LENS (OPTICS).
 �34s[George A. Economou]
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�6v�2v�32sOptics   �21m �33s 
    Narrowly, the science of light and vision; broadly, the study of the 
phenomena associated with the generation, transmission, and detection of 
electromagnetic radiation in the spectral range extending from the long-wave 
edge of the x-ray region to the short-wave edge of the radio region. This 
range, often called the optical region or the optical spectrum, extends in 
wavelength from about 1 nanometer to about 1 millimeter.  See  GEOMETRICAL 
OPTICS; METEOROLOGICAL OPTICS; PHYSICAL OPTICS; VISION. 
    The discoveries of the experimentalists of the early 17th century formed 
the basis of the science of optics. The statement of the law of refraction, 
the development of the astronomical telescope, observations of diffraction, 
and the principles of the propagation of light all came in this relatively 
short period. The publication of Isaac Newton's Opticks in 1704, with its 
comprehensive and original studies of refraction, dispersion, interference, 
diffraction, and polarization, established the science. 
    In the early 19th century many productive investigators established the 
transverse-wave nature of light. The relationship between optical and magnetic 
phenomena led to the crowning achievement of classical optics - the 
electromagnetic theory of J. C. Maxwell. Maxwell's theory, which holds that 
light consists of electric and magnetic fields propagated together through 
space as transverse waves, provided a general basis for the treatment of 
optical phenomena. In particular, it served as the basis for understanding the 
interaction of light with matter and, hence, as the basis for treatment of the 
phenomena of physical optics.  See  ELECTROMAGNETIC RADIATION; LIGHT; 
MAXWELL'S EQUATIONS. 
    In the 20th century optics has been in the forefront of the revolution in 
physical thinking caused by the theory of relativity and especially by the 
quantum theory.  
    The science of optics finds itself in a position that is satisfactory for 
practical purposes but less so from a theoretical standpoint. The theory of 
Maxwell is sufficiently valid for treating the interaction of high-intensity 
radiation with systems considerably larger than those of atomic dimensions. 
The modern quantum theory is adequate for an understanding of the spectra of 
atoms and molecules and for the interpretation of phenomena involving 
low-intensity radiation, provided one does not insist on a very detailed 
description of the process of emission or absorption of radiation. However, a 
general theory of relativistic quantum electrodynamics valid for all 
conditions and systems has not been worked out. 
    The development of the laser has been an outstanding event in the history 
of optics. The theory of electromagnetic radiation from its beginnings was 
able to comprehend and treat the properties of coherent radiation, but the 
controlled generation of coherent monochromatic radiation of high power was 
not achieved in the optical region until the work of C. H. Townes and A. L. 
Schawlow in 1958 pointed the way. Many achievements in optics, such as 
holography and interferometry over long paths, have resulted from the laser.  
See  HOLOGRAPHY; INTERFEROMETRY; LASER. 
 �34s[Richard C. Lord]
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�6v�2v�32sOptimal control theory �21m �33s
    An extension of the calculus of variations for dynamic systems with one
independent variable, usually time, in which control (input) variables are
determined to maximize (or minimize) some measure of the performance (output)
of a system while satisfying specified constraints. Theory is conveniently
divided into two parts: optimal programming, where the control variables are
determined as functions of time for a specified initial state of the system,
and optimal feedback control, where the control variables are determined as
functions of the current state of the system.  See  CALCULUS OF VARIATIONS.
    Examples of optimal control problems are: (1) determining paths of
vehicles between two points to minimize fuel or time, and (2) determining
feedback control logic for vehicles or industrial processes to keep them near
a desired operating point in the presence of disturbances with acceptable
control magnitudes.
    Dynamic systems are conveniently divided into two categories: continuous
dynamic systems, where the control and state variables are functions of a
continuous independent variable, such as time or distance, and discrete
dynamic systems, where the independent variable changes in discrete
increments. Many discrete systems are discretized versions of continuous
systems; the discretization is often made so that (1) the system can be
analyzed or controlled by digital computers (or both), or (2) measurements of
continuous outputs are made at discrete intervals of time (sampled-data
systems) in order to share data transmission channels.
    A large class of interesting optimal control problems can be described as
follows: the dynamic system is described by a set of coupled first-order
ordinary differential equations of the form of Eq. (1), where x (the state

			      OVERDOTx = f(u,x,t)			   (1)

vector) represents the state variables of the system (such as position,
velocity, temperature, voltage, and so on); u (the control vector) represents
the control variables of the system (such as motor torque, control-surface
deflection angle, valve opening, and so on); t is time; and f represents a set
of functions of x, u, and t. The performance index J, which one desires to
minimize, is a scalar function of the final time tsubf and the final state
x(tsubf) plus an integral from the initial time t0 to the final time tsubf of
a scalar function of the state and control vectors and time, as given in Eq.

J = phi [x(t sub f), t sub f] + INTEGRATION (t0 to tf) L[x(t),u(t),t]dt  (2) 


(2). Possible constraints include: (a) specified vector functions constraining
the initial time t0 and the initial state x(t0), as in Eq. (3); (b) specified

			   alpha [x(t0),t0] = 0 			     (3)

vector functions constraining the final time and final state, as in Eq. (4);
(c) specified vector functions constraining.

			 psi [x(t sub f),t sub f] =0   (4) 


			   C[u(t),t] < or = 0				     (5)

the control variables [inequality (5)]; (d) specified vector
functions constraining both control and state variables [inequality (6)]; and

			 CS[x(t),u(t),t] < or = 0			     (6)

(e) specified vector functions constraining only the state variables

			   S[x(t),t] < or = 0				     (7)

[inequality (7)]. Constraints a and b are equality constraints, whereas
constraints c, d, and e are inequality constraints.  See  LINEAR SYSTEM
ANALYSIS.

    Classes of problems.  There are several important classes of optimal
control problems.
    Bang-bang control is a type of control in which the control variables are
either at their maximum or at their minimum values, never in between. This
type of control is optimal, for example, in certain minimum-fuel problems
where the control variables enter linearly into system equations (1), the
performance index in Eq. (2), and the constraints in Eq. (5).  See  NONLINEAR
CONTROL THEORY.
    Bang-zero-bang control is a type of control in which the control values
are at their maximum values, zero, or their minimum values. This type of
control is optimal, for example, in certain minimum-fuel problems where the
control variables enter linearly into system equations (1) and the constraints
in Eq. (5), and the performance index depends on the magnitude (but not the
sign) of the control variables.
    Linear-quadratic problems are problems in which system equations (1) are
linear and the performance index in Eq. (2) is quadratic in the states x and
the control u.	See  OPTIMAL CONTROL (LINEAR SYSTEM).
    Neighboring optimum feedback control (NOFC) about a nominal optimal path
is a viable alternative to a ``complete'' optimal feedback control solution.
NOFC constitutes an approximation to the optimal feedback control solution in
a region neighboring the nominal optimal path.

    Stochastic optimal control theory.	This takes into consideration, in an
ensemble average sense, random disturbances on the dynamic system, random
initial conditions, and random errors in measurements. A control design is
sought that maximizes (or minimizes) the performance criterion on the average
and satisfies the constraints within certain tolerances, also on the average.
See  ESTIMATION THEORY; STOCHASTIC CONTROL THEORY; STOCHASTIC PROCESS.

    Differential games.  These are two-sided optimal control problems.
Typically, control system A tries to minimize the performance criterion, while
control system B tries to maximize it. This may lead to a mini-max feedback
strategy in which both systems do their best, taking into account the other
controller's intelligence in seeking the opposite goal.  See  CONTROL SYSTEMS;
DECISION THEORY; GAME THEORY.
 �34s[Arthur E. Bryson, Jr.]
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�6v�2v�32sOptimal control (linear systems) �21m �33s
    A branch of modern control theory which deals specifically with the
problem of determining optimal control for linear systems when the performance
index to be minimized depends quadratically on the system's variables.
    Although the class of problems encompassed by linear optimal control
theory may at first seem restrictive, its results have found wide application
in many different control problems for both linear and nonlinear systems and,
because of the linear nature of the resulting optimal control laws, are
particularly simple to implement.

    Regulator problem.	The simplest type of linear optimal control problem
with a quadratic cost is the so-called regulator problem. Its solution
contains all of the essential characteristics of linear optimal control
problems in general and establishes a framework from which more advanced
topics in the field can be examined. Consider the system described by state
equation (1), where x(t) is the n-dimensional state vector and u(t) is the

	    OVERDOTx = A(t)x + B(t)u	    x(t0) specified		     (1)

p-dimensional input vector. The performance index to be minimized, J, is

J = (1/2)x(sup T) (t1)S1x(t1) + (1/2) INTEGRATION x(sup T) Q(t)x +
u(sup T) R(t)udt							     (2)

defined by Eq. (2). The symmetric matrices S1 and Q(t), which penalize the
system for large excursions in both the final and intermediate values of the
system state, are assumed to be non-negative definite; that is, x(sup T) S1x and
x(sup T) Qx are non-negative for any vector x with real components. The
symmetric matric R(t) which reflects the penalty associated with excessive
control effort is assumed to be positive definite; that is, u(sup T) Ru>0 for
any vector u  with real components not all equal to zero. The problem is to
determine a control vector u(t) which minimizes the performance index J subject
to constraint equation (1). 

    Tracking systems.  The optimal regulator problem is a special case of the
more general problem of determining a control which causes the system output
or state to track a desired trajectory as closely as possible without undue
control effort. In the regulator problem the desired trajectory is x(t)
IDENTITY 0. The general tracking problem can be stated as follows: Given a
system described by Eq. (1), determine a control u(t) which causes the system
output y(t) = C(t)x(t) to track the desired output z(t) by minimizing the
performance index defined by Eq. (3) where e(t) IDENTITY z(t) - y(t).

J = (1/2)(e sup T)(t1) S1e(t1) + (1/2) INTEGRATION (t0 to t1)(e sup T) Q(t)e +
(u sup T)R(t)udt							   (3)

    Problem variations.  The regulator and tracking problems can be extended
to include a number of special variations of practical importance. Penalties
on u can be included in the performance index and will result in dynamic
feedback structures capable of operating satisfactorily in the presence of
slowly varying plant disturbances. Controller constraints such as piecewise
constant gains or a requirement that the system employ output feedback lead to
optimization problems which are solvable by numerical search techniques.
Requiring the control input to assume only those values which are constrained
in magnitude (for example, |u(t)| < or = 1 leads to a control law which is
nonlinear over most of the state space (u assumes values on the extremity of the
constraint set) and linear in the vicinity of the state space origin.  This type
of system is said to use dual-mode control.
    The extension of the linear optimal control problem to include the effects
of random noise in the plant and measurement equations yields a feedback
structure which contains an optimal state estimator followed by the linear
feedback structure associated with the deterministic regulator problem.  See
CONTROL SYSTEMS; LINEAR SYSTEM ANALYSIS; OPTIMAL CONTROL THEORY; STOCHASTIC
CONTROL THEORY.
 �34s[Patrick E. Barry]
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�6v�2v�32sOptimization   �21m �33s 
    In its most general meaning, the efforts and processes of making a 
decision, a design, or a system as perfect, effective, or functional as 
possible. Formal optimization theory encompasses the specific methodology, 
techniques, and procedures used to decide on the one specific solution in a 
defined set of possible alternatives that will best satisfy a selected 
criterion. Because of this decision-making function the term optimization is 
often used in conjunction with procedures which more appropriately belong in 
the more general domain of decision theory. Strictly speaking, formal 
optimization techniques can be applied only to a certain class of decision 
problems known as decision making under certainty.  See  DECISION THEORY. 
  
    Concepts and terminology.  Conceptually, the formulation and solution of 
an optimization problem involves the establishment of an evaluation criterion 
based on the objectives of the optimization problem, followed by determination 
of the optimum values of the controllable or independent parameters that will 
best satisfy the evaluation criterion. This determination is accomplished 
either objectively or by analytical manipulation of the so-called criterion 
function, which relates the effects of the independent parameters on the 
dependent evaluation criterion parameter. In most optimization problems there 
are a number of conflicting criteria and a compromise must be reached by a 
trade-off process which makes relative value judgments among the conflicting 
criteria. Additional practical considerations encountered in most optimization 
problems include so-called functional and regional constraints on the 
parameters. The former represents physical or functional interrelationships 
which exist among the independent parameters (that is, if one is changed it 
causes some changes in the others); the latter limits the range over which the 
independent parameters can be varied. 
    Optimization techniques work only for a specific system or configuration 
which has been described to a point where all criteria and parameters are 
defined within the system and are isolated and independent from other 
parameters outside the boundaries of the defined system. Formal optimization 
is not a substitute for creativity in that  it depends on a clear definition 
of the system to be optimized, and even though an optimum solution for a given 
system configuration is obtained, this does not guarantee that a better 
solution is not available. 
  
    Problem formulation.  Definition is a critical part of problem formulation 
and consists of: 
    (1) A description of the system configuration to be optimized, including 
definition of system boundaries to an extent that system parameters become 
isolated and independent of external parameters. 
    (2) Definition of a single, preferably quantitative, parameter which will 
serve as the overall evaluation criterion for the specific optimization 
problem. This is the dependent parameter (it depends upon the choice of the 
optimum solution) which measures how well the solution satisfies the desired 
objectives of the problem and this is the parameter which will be maximized or 
minimized to satisfy the objectives.  
    (3) Definition of controlled or independent parameters that will have an 
effect on the criterion. These are the parameters whose values determine the 
value of the criterion parameter, and these should include all controllable 
parameters which influence the criterion and are within the boundaries of the 
defined problem. 
    The most critical aspect of formulating a formal optimization problem is 
the establishment of a satisfactory criterion function that describes the 
behavior of the evaluation criterion as a function of the independent 
parameters. The criterion function, often also referred to as the pay-off or 
objective function, usually takes the form of a penalty or cost function 
(which attempts are made to minimize) or a merit, benefit or profit function 
(which attempts are made to maximize by choosing the optimum values of the 
independent variables). In order to apply formal solution methods, the 
criterion function should be expressed graphically or analytically. 
    Physical principles of operation which govern the relationship among the 
various independent parameters of the problem represent functional 
constraints. Most optimization problems involve practical limits on the range 
over which each parameter or function of the parameters can be varied. They 
represent the regional constraints. 
  
    Solution of optimization problems.   The reliability and the 
sophistication of the solutions of optimization problems increase as the 
problem becomes better defined. In the early stages of optimization of a 
complex problem, the considerations may be primarily objective judgments based 
on the optimizer's judgments of the relevant parameters. As alternative 
subsystem and system configurations evolve through a process of 
conceptualization, analysis and evaluation, and elimination of undesirable 
system configurations, a clearer configuration of parts of the problem can be 
defined, together with the relevant criteria, parameters, and constraints. 
Procedures for defining competing systems or alternate strategies and 
formulating the problem in order to apply formal optimization techniques are 
also common to operations research, systems analysis, and systems design.  See 
  GAME THEORY: METHODS ENGINEERING; MONTE CARLO METHOD; PRODUCTION 
ENGINEERING; PRODUCTION PLANNING. 
 �34s[Igor Paul]
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�6v�2v�32sOral glands   �21m �33s 
    Glands located in the mouth that secrete fluids to moisten and lubricate 
the mouth and food and may initiate digestive activity, and that may perform 
other specialized functions. Fishes and aquatic amphibians have only solitary 
mucus-secreting cells in the epithelium of the mouth cavity. Multicellular 
glands first appeared in land animals to keep the mouth moist and make food 
easier to swallow. Some glands of terrestrial amphibians have a lubricative 
secretion; others serve to make the tongue sticky for use in catching insects. 
Some frogs secrete a serous fluid that contains ptyalin, a digestive enzyme. 
The oral glands of reptiles are much the same, but are more distinctly 
grouped. In poisonous snakes and the single poisonous lizard, the Gila 
monster, certain oral glands of the serous type are modified to form venom. 
Also many of the lizards have glands that are mixed in character, containing 
both mucous and serous cells. Oral glands are poorly developed in crocodilians 
and sea turtles. Birds bolt their food, yet grain-eaters have numerous glands, 
some of which secrete ptyalin. 
    All mammals except aquatic forms are well supplied with oral glands. There 
are numerous small glands, such as the labial glands of the lips, buccal 
glands of the cheeks, lingual glands of the tongue, and palatine glands of the 
palate. Besides these, there are larger paired sets in mammals that are quite 
constant from species to species and are commonly designated as salivary 
glands (see illustration). The parotid gland, near each ear, discharges into 
the vestibule. The submaxillary or submandibular gland lies along the 
posterior part of the lower jaw; its duct opens well forward under the tongue. 
The sublingual gland lies in the floor of the mouth. It is really a group of 
glands, each with its duct.  See GLAND. 
 �34s[Leslie B. Arey]
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�6v�2v�32sOrange �21m �33s
    The sweet orange (Citrus sinensis) is the most widely used species of
citrus fruit and commercially is the most important. The sour or bitter
oranges, of lesser importance, are distinct from sweet oranges and are
classified as a separate species, C. aurantium. The United States is the
largest producer of oranges, followed by Spain, Italy, and Brazil. The orange
is also a major crop in several other countries.
    Sweet orange fruit is consumed fresh or as frozen or canned juice. A large
portion of the crop, particularly in the United States, is used as frozen
concentrate. After the juice is extracted, the peel and pulp are used for
cattle feed. Peel oil is used in perfumes and flavoring, and citrus molasses
is used as a livestock feed.
    The sweet orange tree is a moderately vigorous evergreen with a rounded,
densely foliated top. The fruits are round or somewhat elongate and usually
orange-colored when ripe. They can be placed in four groups: the common
oranges, acidless oranges, pigmented oranges, and navel oranges. They may also
be distinguished on the basis of early midseason, and late maturity.  See
FRUIT; FRUIT, TREE.
 �34s[R.K. Soost; Jack D. Whiteside]
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�6v�2v�32sOrangutan   �21m �33s 
    The largest species of great ape, Pongo pygmaeus, occurring outside 
Africa, in the low-lying forest areas of Borneo and Sumatra. This primate may 
grow to more than 4 ft (1.2 m) in height and weigh about 150 lb (68 kg), the 
male being larger than the female. The arms are longer than the legs. The 
canines are more fanglike than those of humans, and the orangutan is 
distinguished from humans by a large laryngeal cavity which appears as a large 
pouch below the chin. 
    These animals are slow and not very agile on the ground or in trees. Their 
diet consists mainly of fruit. Adult males live apart from the females except 
during the mating season. At this time a temporary tree nest is built to be 
used by a solitary adult or a female and young. Both sexes are sexually mature 
at 8 - 10 years of age. A single young is born each year after a gestation 
period of not quite 8 months. Males assume no responsibility in caring for the 
young.  
    Of all the anthropoids, this animal has a head which has developed in such 
a manner that there is a high, arched forehead like that of humans. There is 
some fear that this animal faces extinction; estimates place the present 
population at less than 5000.  See  PRIMATES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sOrbital motion �21m �33s
    In astronomy the motion of a material body through space under the
influence of its own inertia, a central force, and other forces. Johann Kepler
found empirically that the orbital motions of the planets about the Sun are
ellipses. Sir Isaac Newton, starting from his laws of motion, proved that an
inverse-square gravitational field of force requires a body to move in an
orbit that is a circle, ellipse, parabola, or hyperbola.
    Two bodies revolving under their mutual gravitational attraction, but
otherwise undisturbed, describe orbits of the same shape about a common center
of mass. The less massive body has the larger orbit. In the solar system, the
Sun and Jupiter have a center of mass just outside the visible disk of the
Sun. For each of the other planets, the center of mass of Sun and planet lies
within the Sun.
    For this reason, it is convenient to consider only the relative motion of
a planet of mass m about the Sun of mass M as though the planet had no mass
and moved about a center of mass M + m. The orbit so determined is exactly the
same shape as the true orbits of planet and Sun about their common center of
mass, but it is enlarged in the ratio (M + m)/M.  See  CENTER OF MASS; PLANET.
    Orbital velocity v of a planet moving in a relative orbit about the Sun

			 v (sup 2) = G(M + m) x (2/r - 1/a)		    (1)

may be expressed by Eq. (1) where a is the semimajor axis, and r is the
distance from the planet to the Sun. In the special case of a circular orbit,
r = a, and the expression becomes Eq. (2). When the eccentricity of an orbit

			 v (sup 2) = G(M + m) / a			     (2)

is exactly unity, the length of the major axis becomes infinite and the
ellipse degenerates into a parabola. The expression for the velocity then

			 v (sup 2) = G(M + m) x (2/r)			     (3)

becomes Eq. (3). This parabolic velocity is referred to as the velocity of
escape, since it is the minimum velocity required for a particle to escape
from the gravitational attraction of its parent body.  See  ESCAPE VELOCITY.
    Eccentricities greater than unity occur with hyperbolic orbits. Because in
a hyperbola the semimajor axis a is negative, hyperbolic velocities are
greater than the escape velocity.
    Parabolic and hyperbolic velocities seem to be observed in the motions of
some comets and meteors. Aside from the periodic ones, most comets appear to
be visitors from cosmic distances, as do about two-thirds of the fainter
meteors. It is possible that many of the ``parabolic'' comets are actually
moving in elliptical orbits of extremely long period. The close approach of
one of these visitors to a massive planet, such as Jupiter, could change the
velocity from parabolic to elliptical if retarded, or from parabolic to
hyperbolic if accelerated. It is possible that many of the periodic comets,
especially those with periods under 9 years, have been captured in this way.
See  COMET; GRAVITATION; PERTURBATION (ASTRONOMY); STELLAR ROTATION.
 �34s[Raynor L. Duncombe]
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�6v�2v�32sOrchid �21m �33s
    Any member of the orchid family (Orchidaceae), one of the largest families
of plants, with 450 genera and perhaps 20,000 species. Orchids have a wide
distribution, being most abundant in tropical forests where the majority are
epiphytes. In the temperate and arctic regions, the genera are terrestrial.
Unusual features of this group are the complex and highly specialized flowers,
the minute microscopic seeds having no endosperm, and the great number of
seeds in an orchid capsule. The extraordinary beauty of the flowers makes
orchids the basis of a multimillion-dollar floral industry. Vanilla is
obtained from the pods of Vanilla planifolia, and the tubers of some Asiatic
species are collected, dried, and marketed as salep, which is used both as a
medicine and a food.  See  ORCHIDALES; VANILLA.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sOrchidales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Liliidae of the class Liliopsida (monocotyledons). The order 
consists of 4 families: the Orchidaceae, with about 20,000 species; the 
Burmanniaceae, with about 130 species; the Cordiaceae, with 10 species; and 
the Geosiridaceae, with 1 species. The Orchidales are mycotrophic, sometimes 
nongreen Liliidae with very numerous tiny seeds that have an undifferentiated 
embryo and little or no endosperm. The ovary is always inferior and apparently 
lacks the septal nectaries found in many Liliatae, although other kinds of 
nectaries are present.  See  FLOWER; LILIIDAE; LILIOPSIDA; ORCHID. 
 �34s[Alice Chenault]
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�6v�2v�32sOrdovician �21m �33s
    The second-oldest period in the Paleozoic Era. The rocks deposited during
this time interval (these are termed the Ordovician System) overlie those of
the Cambrian and underlie those of the Silurian. The Ordovician Period was
about 60,000,000 to 65,000,000 years long, and lasted from about 495,000,000 -
500,000,000 to about 435,000,000 years ago.
    The type area for the Ordovician System is those parts of Wales and
England that include rocks bearing the fossils that composed the second of the
three major Lower Paleozoic faunas cited by C. Lapworth. The Ordovician System
in Britain was divided to five major units called series, each distinguished
by a unique fossil fauna. The Ordovician System may be recognized in nearly
all parts of the world, because the fossils that characterize it are so
broadly defined and so widely distributed that some may be found wherever
fossiliferous Ordovician rocks occur.
    The present-day continental areas of Asia, Australia, North America, and
part of Argentina in South America appear to have been mostly under shallow
continental shelf seas that were in tropical climates during the Ordovician.
Ordovician rocks over much of these areas are typified by considerable
thicknesses of carbonate (lime) rocks that accumulated in shallow subtidal and
intertidal environments similar to those seen today in the Bahamas, Bermuda,
and the Florida Keys.
    Life abounded in Ordovician shelf seas and in the surface waters over
shelf margins and ocean basins. Those environments in which carbonates
accumulated included many different types of organisms that lived on the sea
floor. Most of the animals were invertebrates that took foods from suspension
in the surrounding sea water. Bottom-dwelling marine animals were closely
spaced in many shallow tropical environments, suggesting that nutrients were
plentiful in them.
    Late Ordovician glacial deposits are found across nearly all of northern
Africa. Glacial beds of apparent Late Ordovician age are present in several
localities in South America as well. Remanent magnetism and sedimentary rock
evidence suggests that a pole, probably the South Pole, was located in
northern Africa and that continental-type glaciation occurred about it in the
Late Ordovician.
    Significant tectonic events took place during the mid-Ordovician. Uplifts
took place across most of the areas that had been under shallow shelf seas.
Mid-Ordovician uplift in northern Africa appears to have been a precursor to
glaciation. Much of mid-Ordovician North America became land, and a karst
topography with a relief of as much as 430 ft (130 m) developed on exposed
carbonate rocks. A tectonic highland (Taconica) formed along the margin of
what is today eastern North America, probably as a result of plate collision.
See  PALEOGEOGRAPHY; PALEOZOIC; PLATE TECTONICS.
 �34s[William B.N. Berry]
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�6v�2v�32sOre and mineral deposits �21m �33s
    Ore deposits are naturally occurring geologic bodies that may be worked
for one or more metals. The metals may be present as native elements, or, more
commonly, as oxides, sulfides, sulfates, silicates, or other compounds. The
term ore is often used loosely to include such nonmetallic minerals as
fluorite and gypsum. The broader term, mineral deposits, includes, in addition
to metalliferous minerals, any other useful minerals or rocks. Minerals of
little or no value which occur with ore minerals are called gangue. Some
gangue minerals may not be worthless in that they are used as by-products; for
instance, limestone for fertilizer or flux, pyrite for making sulfuric acid,
and rock for road material.  See  MINERAL.
    Mineral deposits that are essentially as originally formed are called
primary or hypogene. The term hypogene also indicates formation by upward
movement of material. Deposits that have been altered by weathering or other
superficial processes are secondary or supergene deposits. Mineral deposits
that formed at the same time as the enclosing rock are called syngenetic, and
those that were introduced into preexisting rocks are called epigenetic.
    The distinction between metallic and nonmetallic deposits is at times an
arbitrary one since some substances classified as nonmetals, such as
lepidolite, spodumene, beryl, and rhodochrosite, are the source of metals. The
principal reasons for distinguishing nonmetallic from metallic deposits are
practical ones, and include such economic factors as recovery methods and uses.
    The Earth's crust consists of igneous, sedimentary, and metamorphic rocks.
The table gives the essential composition of the crust and shows that 10
elements make up more than 99% of the total. Of these, aluminum, iron, and
magnesium are industrial metals. The other metals are present in small
quantities, mostly in igneous rocks.
    Most mineral deposits are natural enrichments and concentrations of
original material produced by different geologic processes. Economic
considerations, such as the amount and concentration of metal, the cost of
mining and refining, and the market value of the metal, determine whether an
ore is of commercial grade.
 �34s[A. F. Hagner]
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�6v�2v�32sOre dressing �21m �33s
    Treatment of ores to concentrate their valuable constituents (minerals)
into products (concentrate) of smaller bulk, and simultaneously to collect the
worthless material (gangue) into discardable waste (tailing). The fundamental
operations of ore-dressing processes are the breaking apart of the associated
constituents of the ore by mechanical means (severance) and the separation of
the severed components (beneficiation) into concentrate and tailing, using
mechanical or physical methods which do not effect substantial chemical
changes.
    Comminution is a single- or multistage process whereby ore is reduced from
run-of-mine size to that size needed by the beneficiation process. The process
is intended to produce individual particles which are either wholly mineral or
wholly gangue, that is, to produce liberation. Since the mechanical forces
producing fracture are not susceptible to detailed control, a class of
particles containing both mineral and gangue (middling particles) are also
produced. Comminution is divided into crushing (down to 6- to 14-mesh) and
grinding (down to micrometer sizes).  See  SIZE REDUCTION.
    Screening is a method of sizing whereby graded products are produced, the
individual particles in each grade being of nearly the same size. In
beneficiation, screening is practiced for two reasons: as an integral part of
the separation process, for example, in jigging; and to produce a feed of such
size and size range as is compatible with the applicability of the separation
process.  See  SCREENING.
    Beneficiation consists of two fundamental operations: the determination
that an individual particle is either a mineral or a gangue particle
(selection); and the movement of selected particles via different paths
(separation) into the concentrate and tailing products. When middling
particles occur, they will either be selected according to their mineral
content and then caused to report as concentrate or tailing, or be separated
as a third product (middling), which is reground to achieve further
liberation.  See  FLOTATION; LEACHING; MECHANICAL SEPARATION TECHNIQUES.
    Separation is achieved by subjecting each particle of the mixture to a set
of forces which is usually the same irrespective of the nature of the
particles excepting for the force based upon the discriminating property. This
force may be present for both mineral and gangue particles but differing in
magnitude, or it may be present for one type of particle and absent for the
other. As a result of this difference, separation is possible, and the
particles are collected in the form of concentrate or tailing.
    Magnetic separation utilizes the force exerted by a magnetic field upon
magnetic materials to counteract partially or wholly the effect of gravity.
Thus under the action of these two forces, different paths are produced for
the magnetic and nonmagnetic particles.  See  MAGNETIC SEPARATION METHODS.
 �34s[M. D. Hassialis]
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�6v�2v�32sOrganic chemical synthesis   �21m �33s 
    The preparation of a desired organic compound by altering the molecular 
structure of another compound in a rational and predetermined manner. This 
process may require one or more individual chemical reactions wherein each 
reaction results in a discrete modification of molecular structure. The 
reactions in organic synthesis may require the use of catalysts or inorganic 
reagents and frequently involve the combination of two or more molecules of 
different substances. 
    Adaptation of chemical reactions to the stepwise construction of complex 
compounds is one of the most important facets of organic chemistry from both a 
theoretical and a practical point of view. An almost unlimited number of 
different organic compounds are theoretically capable of existence. Although 
many organic compounds occur in nature, only a small fraction of the 
theoretically possible compounds have been isolated from natural sources. 
Consequently, desired organic compounds that do not occur naturally or cannot 
be obtained economically from natural sources must be synthesized. The 
synthesis of organic compounds forms the basis of the plastics and synthetic 
fiber industries. Many valuable dyes, flavors, fragrances, vitamins, and drugs 
are also prepared by synthesis. Organic synthesis is also used to confirm the 
structures of organic compounds that have been postulated on the basis of 
evidence derived from chemical behavior and physical measurements.  See  
CONDENSATION REACTION; ORGANIC CHEMISTRY; ORGANIC REACTION MECHANISM; 
POLYMERIZATION; STEREOCHEMISTRY; STERIC EFFECT (CHEMISTRY); SUBSTITUTION 
REACTION.  
 �34s[Richard A. Kretchmer]
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�6v�2v�32sOrganic chemistry �21m �33s
    The chemistry of the compounds of carbon. Organic chemistry owes its
peculiar and important position to the fact that carbon, almost alone among
the elements, is capable of uniting with itself indefinitely to form
compounds. Other elements, notably boron, display similar tendencies, but
carbon forms a far greater number of compounds. Secondly, carbon, almost
without exception, displays a constant valence of 4. On these two principles
the science of organic chemistry is built. The number of carbon compounds that
are theoretically capable of existence is staggering, and this very fact poses
problems of major magnitude in connection with nomenclature, molecular
structure, and arrangement in space of the atoms comprising organic molecules.
  See  CARBON.
    Organic compounds in general differ from inorganic compounds by the nature
of the bonds by which the component atoms of a molecule are united. The
valence bonds of most inorganic compounds are of the ionic or electrovalent
type. In contrast, covalent or electron pair bonds occur in some inorganic
compounds, such as ammonia, and in almost all carbon compounds. Covalent bonds
can be altered to give them some ionic character. In general, bonds range from
those that are essentially covalent to those that are entirely ionic. There is
no sharp boundary between ionic and covalent bonds. However, covalent bonds do
share certain characteristics which cause striking differences between the
physical and chemical properties of inorganic compounds, such as salts, and
those of typical organic compounds. In general, organic compounds possess
relatively low melting points, can frequently be distilled, are sparingly
soluble or insoluble in water but soluble in nonaqueous solvents, and with a
few exceptions do not conduct electricity.
    Over the years certain conventions for expressing molecular structures of
organic compounds have been accepted. Standard practice represents a shared
electron bond involving a single pair of electrons (a single bond) by a single
line. Double bonds, involving two pairs of shared electrons, are represented
by a double line, and triple bonds, involving three pairs of shared electrons,
are represented by a triple line.  See	CHEMICAL BOND THEORY; CHEMICAL
BONDING; CONJUGATION AND HYPERCONJUGATION; RESONANCE (MOLECULAR STRUCTURE).
    Further simplification is often achieved by omission of bond lines when
the meaning is obvious, as illustrated below with the hydrocarbon propene.

			    H  H  H
			    |  |  |
			 H--C--C==C--H	 H3C--CH==CH2
			    |
			    H
				  H3CCH==CH2

    An organic compound containing only single bonds between carbon atoms is
said to be saturated; a compound containing one or more multiple bonds between
carbon atoms is said to be unsaturated.
    The ability of carbon to combine with itself leads to the existence of
series of compounds, the formulas of which differ by a constant increment.
Such a series, known as a homologous series, is illustrated as follows by the
alkanes, each member of which differs from the next lower member by the
increment CH2.	See  CHEMICAL STRUCTURES.


			CH4	  CH3CH3  or  C2H6
		       Methane	     Ethane

			  CH3CH2CH3  or C3H8
			      Propane

    Classification of organic compounds.   Because of the great number and
variety of organic compounds (well over 1,000,000 are known), some systematic
classification scheme and systematic method of nomenclature for dealing with
them becomes mandatory. The problem is complex, and no completely satisfactory
system has yet been devised.
    Carbon atoms may combine in the form of long chains, either straight or
containing branches, or they may combine to form rings. Further, since carbon
is also capable of covalent bonding with other atoms, incorporation of such
so-called hetero atoms into carbon rings is possible. This situation furnishes
the basis for a broad classification of organic compounds into three main
groups depending upon the arrangement of the carbon atoms and the presence or
absence of atoms other than carbon in the cyclic compounds. Under these terms
of reference, organic compounds may be classified as acyclic compounds, which
contain no ring structural arrangements of the constituent atoms; carbocyclic
compounds, which contain one or more rings consisting solely of carbon atoms;
and heterocyclic compounds, the rings of which contain one or more atoms other
than carbon. These principles are illustrated by the following:

		      CH3--CH2--CH2--CH2--CH3

		      CH3
			 \
			  CH--CH2--CH3
		      CH3/

			      Acyclic

		CH2			 CH
	      /    \		      //    \
	     CH2   CH2		     CH      CH
	     |	   |		     |	     ||
	     CH2   CH2		     CH      CH
	      \     /		      \\     /
		CH2			 CH

			    Carbocyclic

    Compounds which contain only carbon and hydrogen are known as hydrocarbons.
In the hydrocarbons, one or more of the hydrogen atoms, at least in principle,
may be replaced by any other atom or group of atoms capable of entering into
a covalent bond. If a group of atoms replaces a hydrogen atom of a given
hydrocarbon it is considered a functional group since its presence changes
the chemical properties of the substituted compound from those of the parent
substance (usually the saturated hydrocarbon). The common functional groups
are:
					 CH
				      //    \
		     CH2--CH2	      N     CH
		     |	  |	      |     ||
		     CH2  CH2	      CH    CH
		     \	  /	      \\    /
		       S		 N
			  Heterocyclic

    Nomenclature of organic compounds.	 When the molecular structure of a
compound becomes known, assignment of a logical systematic name becomes
possible, at least in principle. After many attempts to develop a rational
system of nomenclature, the matter was finally placed in the hands of a
committee of the International Union of Pure and Applied Chemistry. From the
efforts of this committee, an international system, the IUPAC system, is
emerging, by which organic compounds can be named in a logical manner.
However, a certain amount of nationalism persists and practice does not
uniformly conform to the IUPAC rules.
    The system of nomenclature for acyclic hydrocarbon derivatives,
carbocyclic compounds, and heterocyclic compounds is outlined below.

    Acyclic hydrocarbons. The acyclic hydrocarbons are subdivided on the basis
of the presence or absence of double or triple bonds. Acyclic compounds
containing no multiple bonds are known as alkanes or paraffins; those
containing one or more double bonds are known as alkenes or olefins; and those
containing one or more triple bonds are known as alkynes or acetylenes.  See
ALIPHATIC HYDROCARBON; ALKANE; ALKENE; ALKYNE.

    Acyclic hydrocarbon derivatives. In general, the same principles which
govern the naming of branched-chain alkanes, alkenes, and alkynes hold. The
parent compound is that which contains the longest continuous straight carbon
chain.
    For halogen compounds, the prefixes fluoro, chloro, bromo, and iodo are
used with the name of the parent alkane:

		   CH3CHCICH2CH3 is 2-chlorobutane

		CH3CHBrCH2CH2CH2Br is 1,4-dibromopentane

    Hydroxyl derivatives are commonly known as alcohols, and many of them are
named as such. CH3OH is methyl alcohol, and CH3CH2OH is ethyl alcohol.
    Aldehydes, particularly the lower members of the series, are frequently
named by replacing the terminal -ic of the related acid by -aldehyde. Ketones
are named by three methods. The lower symmetrical ketones are named by
replacing the terminal -ic of the acid which would yield them on pyrolysis by
one. For naming mixed ketones, the word ketone is prefixed by the names of the
radicals joined to the carbonyl carbon atom. In the Geneva system the terminal
-e of the parent alkane is replaced by -one. The carbonyl group position must
be shown by a number.  See  ALDEHYDE; KETONE.
    The carboxyl function is characteristic of organic acids. Since many acids
were originally isolated from natural products, trivial names associated with
their sources are common. For example, HCOOH, formic acid, is named from the
Latin formica (ant). Branched-chain acids are named frequently as derivatives
of acetic acid, CH3COOH, or as derivatives of the parent straight-chain acid.
    Ethers are generally named by reference to the alkyl radicals present and
addition of the word ether; CH3CH2--O--CH3 is ethyl methyl ether. By the
Geneva system they are named as alkoxy derivatives of the parent hydrocarbon.
See  ETHER.
    Amines are alkyl derivatives of ammonia. They are classified into primary,
secondary, or tertiary amines according to whether 1, 2, or 3 hydrogen atoms
of ammonia have been replaced by organic radicals. They are conveniently named
by using the names of the radicals attached to nitrogen as prefixes to the
word amine.  See  AMINE.

    Carbocyclic compounds. The carbocyclic compounds are subdivided into the
aromatic compounds and the alicyclic compounds by certain structural features
and chemical properties.  See  ALICYCLIC HYDROCARBON; AROMATIC HYDROCARBON.
    The aromatic carbocyclic compounds are characterized structurally by an
alternating system of single and double bonds. The parent aromatic
hydrocarbons are almost always known by trivial names (such as benzene or
phenol) many of which indicate the source from which they were originally
obtained. For convenience, it is not customary to write the individual carbon
and hydrogen atoms in the structural formulas of aromatic hydrocarbons.
Rather, each angular position is assumed to represent a carbon atom, and the
presence of sufficient hydrogen atoms to bring the carbon atoms to the
quadrivalent state is also assumed. Typical examples are as follows:

		      ******CHEMICAL STRUCTURE******

    The problem of nomenclature of the alicyclic compounds is formidable and
is complicated by the occurrence of structural, positional, and geometrical
isomerism, as well as stereoisomerism. Definitive rules for naming and
numbering many of the polycyclic compounds have been formulated by the
International Union of Pure and Applied Chemistry.  See  ALICYCLIC
HYDROCARBON; MOLECULAR ISOMERISM.

    Heterocyclic compounds. One or more carbon atoms (with or without
accompanying hydrogens) in a carbocyclic compound may be replaced by a hetero
atom (sometimes in combination with hydrogen). A systematic method which can
be applied to the nomenclature of any heterocyclic compound has been
developed. In this the related aromatic hydrocarbons are taken as reference
standards, the nature of the hetero atom(s) is indicated by a prefix (oxa- for
O, thia- for S, and aza- for N), and the position of the hetero atom or atoms
is indicated by a number corresponding to the accepted numbering of the
aromatic hydrocarbons. Thus

		       ***CHEMICAL STRUCTURE***

    In compounds carrying more than one hetero atom, numbers are selected so
that oxygen bears the lowest number, followed by sulfur and, finally, by
nitrogen. The most highly unsaturated compound is taken as the parent.
Derivatives in which one or more double bonds are saturated are referred to as
dihydro or tetrahydro derivatives of the parent.  See  HETEROCYCLIC COMPOUND.
 �34s[Robert C. Elderfield]
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�6v�2v�32sOrganic geochemistry   �21m �33s 
    The study of the composition and processes of origin, movement, and 
alteration of biologically derived carbonaceous substances in rocks, waters, 
and the atmosphere. These may be highly dispersed organic materials in 
fine-grained rocks (including petroleum source rocks and oil shales); 
continuous phases (petroleum) occupying pore networks in reservoir rocks; or 
solid layers of mainly carbonaceous material, for example, coals.  See  COAL; 
OIL SHALE; PETROLEUM. 
    Another branch of organic geochemistry deals with the prebiologic or 
abiogenic formation of organic substances. For example, inorganic gases can be 
converted to essentially organic molecules when subjected to various forms of 
energy, particularly radiation.  See  GEOCHEMISTRY. 
 �34s[Richard D. McIver]
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�6v�2v�32sOrganic quantitative analysis   �21m �33s 
    The determination of elements, functional groups, or molecules in organic 
materials. The type of analysis used is determined by the information 
required. For example, if the total nitrogen content is desired, elemental 
analysis is used, if only amino nitrogen is desired, then only the amino group 
is determined. Organic analyses are made on a wide range of materials from 
pure compounds to mixtures such as blood and fertilizer. The identification of 
structures in conjunction with purity criteria, analyses of isotopic mixtures 
of organic molecules, and other examples of structure-related problems are 
increasingly important analytically. Proton nuclear magnetic resonance and 
other classes of spectrometric measurement are prominent for such purposes.  
See  ANALYTICAL CHEMISTRY; QUANTITATIVE CHEMICAL ANALYSIS. 
 �34s[Charles L. Rulfs]
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�6v�2v�32sOrganic reaction mechanism �21m �33s
    A detailed description of an overall organic process, R -> P, in which the
participants and their interconversion rates are characterized. All of the
steps from reactants R via possible intermediates I to products P are
included, as in reactions (1). To characterize the reaction path, it is
possible to use a variety of experimental techniques and invoke theoretical
and quasitheoretical exclusions.

R REVERSIBLE REACTION (REACTION CONDITIONS -1 and 1) I(sub 1)
I(sub 1) REVERSIBLE REACTION (REACTION CONDITIONS -2 and 2) I(sub 2)
I(sub 2) REVERSIBLE REACTION (REACTIONS CONDITIONS -3 and 3) P		(1)

    From an energy viewpoint, a mechanism may be represented schematically in
a free energy (G) and reaction coordinate (Q DOUBLE DAGGER) diagram (Fig.  1).
Energy minima are associated with observable species (R, P, I), while energy
maxima are associated with fleeting transients or activated complexes (AC).
Here all of the participants as well as barrier heights, for example, DELTA
G(sub 1)(DOUBLE DAGGER) = G(sub 1)(DOUBLE DAGGER) - G(R) for step 1 or DELTA
G(sub -2)(DOUBLE DAGGER) = G(sub 2)(DOUBLE DAGGER) - G(I[sub 2]) of Eqs.  (1),
are indicated and Q(DOUBLE DAGGER) relates in a general way to the progress of a
multistep reaction.  Unfortunately, this view of a mechanism is somewhat akin to
reconstructing a race from snapshots of the horses at the start and finish
lines.
     In a more comprehensive picture, chemical changes among species may be
regarded as taking place on a hypersurface representing the changes in potential
energy V as a function of atom configurations or a related set of orthogonal
coordinates (Q[sub i]).  Figure 2 is a portion of this surface which focuses on
the change I(sub 1) EQUILIBRIUM I(sub 2).  Q(sub x) stands for all of the
coordinates except Q(DOUBLE DAGGER); the activated complex is identified with a
saddle point or one dimensional maximum along Q(DOUBLE DAGGER).  While there is
an energy barrier to overcome, the energy is minimized along Q(DOUBLE DAGGER). 
Individual steps by which changes occur are termed elementary.  In such a
reaction, one or two - rarely more - molecules become the energized aggregate
labeled activated complex.  To illustrate, reactions (2) and (3) show the
activated complexes with partial bonds. 

Unimolecular
HCCI3 --> (HCI2C . . . CI) --> HCI2C . + CI.				     (2)

Bimolecular
HO (sup -) + H3CI --> (HO . . . CH3 . . . .I)(sup -) -->
							HOCH3 + 1 (sup -)    (3)

    Special techniques are employed to determine the elementary step or steps
involved in a reaction. At the outset it is necessary to decide what the
process is, and determine that it does in fact take place.  See  CHEMICAL
DYNAMICS.
 �34s[Sidney I. Miller]
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�6v�2v�32sOrganoactinides   �21m �33s 
    Organometallic compounds of the actinides - elements 90 and beyond in the 
periodic table. Both the large sizes of actinide ions and the presence of 5f 
valence orbitals are unique features which differ distinctly from most, if not 
all, other metal ions. 
    Organometallic compounds have been prepared for all actinides through 
curium (element 96), although most investigations have been conducted with 
readily available and more easily handled natural isotopes of thorium (Th) and 
uranium (U). Organic groups (ligands) which bind to actinide ions include both 
PI- and SIGMA-bonding functionalities. The importance of this type of compound 
reflects the ubiquitous character of metal-carbon two-electron sigma bonds in 
both synthesis and catalysis.  See  CATALYSIS; CHEMICAL STRUCTURES. 
    The molecular structures of a number of organoactinides have been 
determined by single-crystal x-ray and neutrondiffraction techniques. In 
almost all cases the large size of the metal ion gives rise to unusually high 
(as compared to a transition-metal compound) coordination numbers. That is, a 
greater number of ligands or ligands with greater spatial requirements can be 
accommodated within the actinide coordination sphere. The sandwich complex bis 
(cyclooctatetraenyl)uranium (uranocene), an example of this latter type, is 
shown in the illustration.  See   ACTINIDE ELEMENTS; COORDINATION CHEMISTRY; 
COORDINATION NUMBER; METALLOCENES; NEUTRON DIFFRACTION; ORGANOMETALLIC 
COMPOUNDS; X-RAY DIFFRACTION. 
 �34s[Tobin J. Marks]
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�6v�2v�32sOrganoarsenic compound �21m �33s
    A derivative of arsenic containing at least one organic radical attached
through carbon to the arsenic atom by means of a covalent bond. All
organoarsenic derivatives can be derived schematically by formal substitution
or oxidation from the primary, secondary, and tertiary arsines. This is shown
in the following notation. Depending on the state of oxidation, organoarsenic

		RAsH2		       R2AsH		     R3As
	      Alkylarsine	     Dialkylarsine	   Trialkylarsine
		  |			 |		       |
		RAsO		       (R2As)2O 	     R3AsO
		Alkyl		       Dialkyl- 	    Trialkyl-
	      arsenoxide	      arsinoxide	    arsinoxide
		  |			 |
	       RAs(OH)2 	       R2AsOH
	      Alkylarsonous	     Dialkylarsinous
		 acid			acid
		  |			 |
	       RAs(O)(OH)2	      R2As(O)OH
		Alkylarsonic	      Dialkylarsinic
		 acid			 acid

compounds are derivatives of trivalent or pentavalent arsenic. A special type
of organoarsenic compounds which do not have an equivalent in the
organophosphorus series are the arseno compounds, RAsDOUBLE BARAsR.
    All organoarsenic compounds exhibit physiological activity. The high
mammalian toxicity of the trivalent derivatives makes a number of them
potential chemical warfare agents. Several of the less toxic derivatives were
used as chemotherapeutics for combating certain infectious diseases before the
discovery of the antibiotics.  See  INSECTICIDE; ORGANOPHOSPHORUS COMPOUND.
 �34s[Friedrich W. Hoffmann /Thomas C. Simmons]
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�6v�2v�32sOrganometallic compound   �21m �33s 
    One of a group of substances containing carbon-metal bonds. This 
definition can be broadened to include all compounds of elements that possess 
metallike properties and that are combined chemically with one or more carbon 
atoms. In this respect, metal alkoxides, chelates, salts of organic acids, and 
related compounds are not discussed in this article.  See  CHELATION. 
    Organometallic compounds have found many commercial applications. The use 
of tetraethyllead as a gasoline antiknock additive is a good example. 
Organometallic compounds have played an important role in the development of 
silicone polymers, polyvinylchloride, and polyethylene.  See  POLYOLEFIN 
RESINS; TETRAETHYLLEAD. 
    A systematic classification of all known organometallic compounds is very 
difficult. For the sake of convenience, however, they can be divided into 
three main classes. The more electropositive metals of groups I and II of the 
periodic table form organometallic compounds that are nonvolatile, usually 
poorly soluble in organic solvents, and essentially ionic in nature. Lithium, 
beryllium, and magnesium compounds are much less ionic than those of the 
remaining metals, however. The metals (excepting transition metals) and 
metalloids of groups III, IV, V, and VI form organometallic compounds which 
are mainly volatile, soluble in organic solvents, and principally covalently 
bonded. The transition metals constitute a third main group, possessing 
bonding to aromatic or aromatic-type groups which is mainly of a special 
d-orbital, or ``sandwich,'' type.  See  CHEMICAL BONDING; FERROCENE; 
METALLOCENE; ORGANOARSENIC COMPOUND; ORGANOPHOSPHORUS COMPOUND; REFORMATSKY 
REACTION.  
    The reaction of metals or metalloids with organic halides is widely used 
in the synthesis of organometallic compounds. Another means of preparing 
organometallic compounds involves the reaction of highly reactive metals with 
hydrocarbons that contain active hydrogen atoms. The displacement of a metal 
in an organometallic compound by a more reactive metal to produce a new 
organometallic compound has been widely used, although reactions of this type 
are usually reversible. 
    The reaction of organometallic compounds with metal salts has found 
extensive application in the synthesis of many types of organometallic 
compounds. Examples include the reactions of alkylsodium compounds, 
alkyllithium compounds, and Grignard reagents with halides of elements of 
groups III, IV, V, and VI. Other reactions include methods by which an 
organometallic compound of a given metal is used to prepare other 
organometallic derivatives of the same metal.  See  GRIGNARD REACTION. 
 �34s[Marvin D. Rausch]
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�6v�2v�32sOrganophosphorus compound   �21m �33s 
    One of a series of derivatives of phosphorus that have at least one 
organic (alkyl or aryl) group attached to the phosphorus atom linked either 
directly to a carbon atom or indirectly by means of another element (for 
example, oxygen). The mono-, di-, and trialkylphosphines (and their aryl 
counterparts) can be regarded formally as the parent compounds of all 
organophosphorus compounds. 
    Considering the large number of organic groups that may be joined to 
phosphorus as well as the incorporation of other elements in these materials, 
the number of combinations is practically unlimited. A vast family in itself 
is composed of the heterocyclic phosphorus molecules, in which phosphorus is 
one of a group of atoms in a ring system. 
    Some organophosphorus compounds have been used as polymerization 
catalysts, lubricant additives, flameproofing agents, plant growth regulators, 
and insecticides. Organophosphorus compounds were made during World War II for 
use as chemical warfare agents in the form of nerve gases (Sarin, Trilon 46, 
Soman, and Tabun).  See  PHOSPHORUS. 
 �34s[Sheldon E. Cremer]
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�6v�2v�32sOrganosulfur compound   �21m �33s 
    One of a group of substances which contain both carbon and sulfur. Oxygen, 
nitrogen, the halogens, and phosphorus are also often present. Thousands of 
such compounds are well known. See SULFUR. 
    Many medicinal and natural products are organosulfur compounds, for 
example, the penicillins, the sulfa drugs, insulin, and amino acids such as 
cysteine and methionine. The utilizations of organosulfur compounds involve 
enormous commercial developments in medicinals, detergents, sulfide rubbers 
and polymers, sulfur dyes, solvents, and agricultural chemicals (herbicides, 
fungicides, and insecticides). Besides their practical uses, studies of 
organosulfur compounds helped to establish fundamental theory on the nature of 
valence bonding, of molecular geometry, and of reaction mechanisms of organic 
chemistry.  
    The most common classes of organosulfur compounds are: mercaptans, RSH; 
thiophenols, ArSH; disulfides, RSSR', where R may be an aliphatic, aromatic, 
or heterocyclic radical; sulfides, including cyclic sulfides, RSR; sulfenyl 
chlorides, RSCl; sulfoxides; sulfones; sulfonyl chlorides, RSO2Cl; 
sulfonamides, R--SO2NH2; thioaldehydes and thioketones, RC(H)DOUBLE BARS and 
R2CDOUBLE BARS; sulfinic acids, RSO2H; sulfonic acids, RSO3H; esters of 
sulfonic acids, RSO2OR'; thiol esters and thio acids. See  DIMETHYL SULFOXIDE; 
HETEROCYCLIC COMPOUNDS; MERCAPTAN; PETROLEUM PROCESSING; SULFAMIC ACIDS; 
SULFANILIC ACID; SULFENYL CHLORIDES; SULFONAMIDE; SULFONE; SULFONIC ACID; 
SULFONYL CHLORIDE; SULFOXIDE; THIO COMPOUNDS; THIOETHER. 
 �34s[Norman Kharasch]
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�6v�2v�32sOriental vegetables   �21m �33s 
    Oriental vegetables are very important in Asian countries, but are 
considered as minor crops in the United States and Europe. However, in recent 
years there has been an increased interest in these crops because of their 
unusual flavors and textures and in some cases their high nutritional values. 
Some of the more common ones are described below. 
    Chinese cabbage, celery cabbage, napa, or pe-tsai (Brassica campestris, 
pekinensis group; B. rapa, pekinensis group; B. pekinensis) belongs to the 
mustard (Cruciferae) family, and is a biennial leafy plant but is grown as an 
annual. The harvested part is a head which is composed of broad crinkled 
leaves with a very wide, indistinct, white midrib. The outer leaves are pale 
green, and the inner leaves of the head are blanched. It is a good salad 
vegetable because of the mild flavor and crisp texture. 
    There are many varieties of pak choy, bok choi, Chinese mustard, or celery 
mustard (Brassica campestris, chinensis group; B. rapa, chinensis group; B. 
chinensis). The crop is grown as an annual. Pak choy does not form a head like 
most varieties of Chinese cabbage, but forms a celerylike stalk of tall, dark 
green leaves, with prominent white veins and long white petioles (see 
illustration). The base of the petiole may be expanded and spoon-shaped; the 
blade of the leaf is smooth, and not crinkled like that of Chinese cabbage. 
    The large, long, white radish (Raphanus sativus, longipinnatus group) is 
often called oriental winter radish, daikon, Chinese winter radish, lobok, and 
lob paak. Radish is a dicotyledonous herbaceous plant grown for its long, 
enlarged roots. 
  
    Edible podded peas, China peas, sugar peas, or snow peas (Pisum sativum, 
macrocarpon group) belong to the Leguminosae family. The primitive forms of 
pea are slightly bitter and have a tough seed coat, which allows for long 
dormant periods. In the garden pea the pod swells first as the seeds enlarge, 
but in the edible podded pea the immature seeds bulge the pod, which demarks 
the developing seeds. Unlike the regular garden peas which have tough fibery 
seed pods, the edible podded peas were selected for the tender pods and not 
for the seeds. 
    Yard-long bean or asparagus bean (Vigna sinensis, sesquipedelis group) 
belongs to the Leguminosae family. It is an annual climbing plant. It is a 
relative of the cow pea (V. unguiculata). 
    Mung bean or green gram (Phaseolus aureus) sprouts have been used by the 
Chinese for their remarkable healing qualities for thousands of years. Only 
recently has the Western world recognized the value of sprouted leguminous 
seeds (mung, soy, and alfalfa sprouts). The unsprouted mung bean seeds contain 
negligible amounts of vitamin C (ascorbic acid), whereas the sprouted seeds 
contain 20 mg per 100 g of sprouts, which is as high as tomato juice. 
    Jicama (Pachyrrizus erosus), also called the yam bean, is indigenous to 
Mexico and Central America. It belongs to the pea (Leguminosae) family. The 
crop is grown for its enlarged turnip-shaped root, which is eaten raw or 
cooked, has a crisp texture, and is sweetish in taste. 
    Chinese winter melon, wax gourd, winter gourd, white gourd, ash gourd, 
Chinese preserving melon, ash pumpkin, or tung kwa (Benincasa hispida; B. 
cerifera) is a viny annual cucurbit. The flesh of mature fruits is used in 
making Chinese soups. It can be eaten raw or made into sweet preserves similar 
to citron or watermelon rind preserves. 
    Balsam pear, alligator pear, bitter gourd, bitter melon, bitter cucumber, 
or fu kwa (Momordica charantia) is an annual herbaceous vine. The heart-shaped 
to cylindrical fruits are extremely bitter; some of the bitterness is removed 
before cooking by peeling and steeping in salt water. Immature fruits are less 
bitter than mature ones. 
    Chinese okra or angled loofa (Luffa acutangula) is called zit kwa by the 
Chinese; it is a close relative of L. cylindrica, known as loofa, sponge 
gourd, or dishcloth gourd. Chinese okra is an annual climbing vine grown for 
the immature fruits. The thoroughly mature fruits can be made into vegetable 
sponge.  
    Water spinach, water convolvulus, swamp cabbage, kang kong, or shui ung 
tsoi (Ipomoea aquatica) is a perennial semiaquatic plant grown for its long, 
tender shoots. It is a relative of the sweet potato (I. batatas), but does not 
produce an enlarged root. 
 �34s[M. Yamaguchi]
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�6v�2v�32sOrion   �21m �33s 
    The Warrior, in astronomy, and undoubtedly the finest of all 
constellations in the sky. Orion is a winter group near the celestial equator. 
Four of the most prominent stars, ALPHA, GAMMA, BETA, and KAPPA, form a huge 
crude rectangle (see illustration). The group is pictured as the figure of a 
warrior, holding a shield with his left hand and swinging a club with his 
raised right arm ready to strike the charging Bull. Betelgeuse (meaning 
armpit), one of the largest stars known, is the red star at the right 
shoulder, Bellatrix is at the left shoulder, and Rigel, the bluewhite star, is 
at the left leg. Three bright stars in a straight line in the middle of the 
rectangle represent the warrior's belt. The center star is Alnilam.  See  
CONSTELLATION; ORION NEBULA; TAURUS. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sOrion Nebula   �21m �33s 
    A portion of a vast complex of ionized hydrogen, cool molecular clouds, 
and solid grains which is found in the club (or sword) of the constellation of 
Orion, and which shines because of radiation received from hot stars embedded 
within it (see illustration). Designated Messier 42 or NCG 1976, it is the 
brightest and perhaps one of the densest ionized hydrogen (HII) regions known. 
  See  ORION. 
    Radio-frequency and infrared observations have shown the Orion Nebula to 
be extremely intricate, with dense cool clouds in which numerous, often 
complex molecules abound. The hot Trapezium stars, which excite the nebula, 
are very young and only recently formed from the interstellar medium. This and 
various other lines of evidence show the luminous Orion Nebula to be very 
young, less than a million years old, and possibly only a few tens of 
thousands of years of age. Radio observations indicate that the Orion Nebula 
and the entire complex of hot stars, gas, and grains are surrounded by an 
expanding shell of neutral hydrogen.  See  INTERSTELLAR MATTER; NEBULA. 
 �34s[Lawrence H. Allen]
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�6v�2v�32sOrnithischia   �21m �33s 
    One of two orders of extinct reptiles popularly known as dinosaurs, the 
other being the Saurischia. The two orders are distinguished by various 
anatomical differences, but the shape of the pelvis is the primary 
distinction, tetraradiate or superficially ``birdlike'' in the Ornithischia 
and triradiate in the Saurischia. The ordinal names refer to these distinctive 
conditions: ``bird-hipped'' and ``reptile-hipped.'' Like the saurischians, 
ornithischians probably originated from the Thecodontia in Early or Middle 
Triassic times. Ornithischian remains are known from rock strata ranging from 
Middle Triassic to latest Cretaceous in age and have been found on all 
continents except Antarctica. On the basis of tooth shape, all ornithischians 
are judged to have been herbivores. Most kinds had peculiar beaks or bills, 
featuring a unique predentary bone at the front of the lower jaw, for plucking 
vegetation. Many possessed highly specialized batteries of teeth for crushing, 
grinding, or slicing plant tissues.  See  SAURISCHIA; THECODONTIA.  
    Traditional classifications recognize four suborders: Ornithopoda, 
Stegosauria, Ankylosauria, and Ceratopsia. A fifth sub-order, 
Pachycephalosauria, was suggested as a result of new discoveries in Mongolia.  
See  DINOSAUR. 
 �34s[John H. Ostrom]
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�6v�2v�32sOrnithopter   �21m �33s 
    An aircraft whose wings beat like those of a bird, bat, or insect. Models 
are recorded as early as 400 B.C. (Archytas). Since the mid-19th century some 
have been flown successfully; a number of machines of person-carrying size 
have been flown for short distances.  See  FLIGHT. 
    The beating surfaces may be nonlifting (that is, propelling only), in 
which case they are properly called fins, or they may be wings which lift as 
well as propel. 
 �34s[James L. G. FitzPatrick]
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�6v�2v�32sOrogeny   �21m �33s 
    The process by which mountainous tracts such as the Alpine-Himalayan, 
Appalachian, and Cordilleran orogenic belts are formed, also known as 
orogenesis. Characteristically, orogenic belts are long and linear or arcuate, 
with distinctive zones of sedimentary, deformational, and thermal patterns 
that are, in general, parallel but asymmetric to the belts. Orogenic belts 
have complex internal geometry, involving extensive mass transport of very 
dissimilar rock sequences of dominantly marine sediments and continental crust 
(sial) and oceanic crust and mantle (ophiolite suite). Intensive deformation 
and metamorphism of a particular orogenic belt is relatively short-lived when 
compared to the time during which much of the sedimentary rock of the belt was 
deposited. Typically, orogenic belts are sites of abnormally thick 
accumulations of sedimentary and volcanic rock and severe deformation and 
thermal alterations.  See  MOUNTAIN SYSTEMS. 
    Seismological and oceanographic studies have revealed a globe-encircling, 
seismically active belt of mountains, called oceanic ridges, and trenches with 
associated volcanic island arcs. Models propose that orogeny is a consequence 
of the evolution of oceanic ridges and trenches and continental drift. 
Although oceanic ridges and trench - island arc systems are mountainous, they 
are not orogenic belts in the classical sense which restricts orogenic belts 
to continental masses. Orogeny results from interactions of this global, 
continuously evolving system of oceanic ridges and trenches, according to 
concepts of sea-floor spreading or, currently, lithosphere plate tectonics. 
    Fundamental concepts of orogeny based on plate tectonics are that 
continental margins of the Atlantic type, which develop during ocean opening, 
are converted to island arc - cordilleran-type (Andean) orogenic belts as 
lithosphere is consumed in subduction zones, below trenches, along the 
continental margin during closing of an ocean. A collision-type (Himalayan) 
orogenic belt is superimposed on a cordilleran belt upon final ocean closing, 
as opposing continental margins suture.  See  CORDILLERAN BELT; OCEANIC 
ISLANDS; PLATE TECTONICS. 
    The plate tectonics model indicates that mountain building results from 
two fundamental mechanisms. Island arc - cordilleran orogeny, for the most 
part thermally driven, occurs along leading lithosphere plate edges, above 
subduction zones. Continent - island arc or continent-continent collision 
orogeny is for the most part mechanically driven and occurs subsequently to 
island arc development or during final closing of an ocean or both. Actual 
orogenic belts are usually the result of complex combinations of these basic 
mechanisms.  
 �34s[John M. Bird ;John F. Dewey]
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�6v�2v�32sOrpiment   �21m �33s 
    A mineral having composition As2S3. Crystals are small, tabular, and 
rarely distinct; the mineral occurs more commonly in foliated or columnar 
masses. The hardness is 1.5 - 2 (Mohs scale) and the specific gravity is 3.49. 
The luster is resinous and pearly on the cleavage surface; the color is lemon 
yellow. Orpiment is found in Romania, Peru, Japan, and the Soviet Union. In 
the United States it occurs at Mercer, Utah; Manhattan, Nevada; and in 
deposits from geyser waters in Yellowstone National Park.  See  ARSENIC. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sOrthida   �21m �33s 
    An order of articulate brachiopods which includes the oldest known 
representatives of the class. They first appeared in Early Cambrian times and 
became especially prolific and diverse in the Ordovician. Subsequently, the 
group diminished in relative abundance during Silurian and Late Paleozoic 
time, finally becoming extinct during the Permian. The majority of the order 
belong to the suborder Orthidina. Characteristically, these are biconvex, 
finely ribbed shells with a straight hinge line and well developed interareas 
on both valves. #See  ARTICULATA (BRACHIOPODA); BRACHIOPODA. 
 �34s[A. J. Rowell]
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�6v�2v�32sOrthoclase   �21m �33s 
    A name for potassium feldspar, KAlSi3O8, that usually contains some sodium 
feldspar (up to about 50 mole % NaAlSi3O8) either in solid solution or 
exsolved as relatively pure NaAlSi3O8. The latter can be present as albite or 
analbite or in structural states intermediate between albite and analbite.  
See  ALBITE; FELDSPAR. 
 �34s[Fritz H. Laves]
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�6v�2v�32sOrthoester   �21m �33s 
    A trialkyl derivative of a nonexistent orthoacid, RC(OH)3, with general 
formula RC(OR)3. The structure of the orthocarbonates is C(OR)4. Orthoesters 
are liquids with ethereal odors and are stable to aqueous alkalies but 
sensitive to acid hydrolysis. By far the most common and most important is 
ethyl orthoformate, HC(OC2H5)3, although ethyl orthoacetate and higher members 
are known. Orthoformate esters are frequently used in synthetic organic 
chemistry, for example, in preparing aldehydes from Grignard reagents.  See  
ESTER; GRIGNARD REACTION. 
 �34s[Evans B. Reid]
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�6v�2v�32sOrthogonal polynomials �21m �33s
    A special case of orthogonal functions that arise in many physical
problems (often as the solutions of differential equations), in the study of
distribution functions, and in certain other situations where one approximates
fairly general functions by polynomials.  See  PROBABILITY.
    Each set of orthogonal polynomials is defined with respect to a particular
averaging procedure. The average value of a suitable function f is denoted by
E{f}. An example is shown in Eq. (1). In general an averaging procedure has

	     E{f} = (1/2) INTEGRATION (-1 to 1)  f(x) dx		     (1)


	 E{f} = INTEGRATION (-INFINITY to INFINITY)  f(x) dsigma (x)	     (2)

the form shown in Eq. (2), a Stieltjes integral, where sigma is a distribution
function, that is, an increasing function with sigma(-INFINITY)=0 and
sigma(+INFINITY)=1.
    Two functions f and g are said to be orthogonal with respect to a given
averaging procedure if E{fBARg}=0 where the bar denotes complex conjugation. By
the system of orthogonal polynomials associated with the averaging procedure
is meant a sequence P0, P1, P2, . . .of polynomials Psubn having exact degree
n, which are mutually orthogonal, that is, E{PsubmBARPsubn}=0 for m NOT EQUAL n.
This last condition is equivalent to the statement that each Psubn is orthogonal
to all polynomials of degree less than n.  Thus Psubn has the form Psubn(x) = a0
+ a1x + a2xsup2 + .  .	.  + asubnxsupn where asubn = 0 and is subject to the n
conditions E{xsupk Psubn}=0 for k=0, 1, .  .  .  , n-1.  This gives n linear
equations in the n+1 coefficients of Psubn, leaving one more condition, called a
normalization, to be imposed.  The method of normalization differs in different
references.  See POLYNOMIAL SYSTEMS OF EQUATIONS.�34s[Carl S. Herz]
�33s

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�6v�2v�32sOrthonectida   �21m �33s 
    An order of Mesozoa. The orthonectids parasitize various marine 
invertebrates as multinucleate plasmodia. The plasmodia multiply by 
fragmentation. Eventually they give rise asexually, by polyembryony, to sexual 
males and females. Commonly only one sex arises from a given plasmodium. 
    These sexually mature forms escape as minute ciliated organisms. 
Structurally they are composed of a single layer of ciliated epithelial cells 
surrounding an inner mass of sex cells. The ciliated cells are disposed in 
rings around the body. 
    After insemination the eggs develop in the female and form ciliated 
larvae. When liberated, these larvae invade new individuals of their host and 
then disaggregate, liberating germinal cells which give rise to new plasmodia. 
  See  MESOZOA. 
 �34s[Bayard H. McConnaughey]
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�6v�2v�32sOrthoptera   �21m �33s 
    A heterogeneous order of generalized insects with gradual metamorphosis, 
chewing mouthparts, and four wings. The front pair of wings is thickened and 
leathery, while the second pair is thin, membranous, and folded fanwise 
beneath the first. Many forms have reduced wings or none at all. Most 
Orthoptera are plant eaters. Some live in marshy areas, but they are not 
aquatic in the true sense. 
    Orthoptera are among the noisiest of insects, possessing both 
well-developed sound-producing and sound-perceiving organs. The Locustidae 
produce sound by rubbing a series of tiny pegs on the inner faces of the 
femora against the outer wings. The auditory organs are on either side of the 
base of the abdomen. In most of the remaining Orthoptera sound is produced by 
rapidly rubbing the wings together, and the auditory organs are located at the 
bases of the front tibiae. 
    Habitats of the Orthoptera are variable. Representatives of most families 
are found on the surface of the ground. However, the Grylloblattidae occur 
under stones or in loose soil, the Tridactilidae in sand or mud, the Mantidae 
in grasses or herbaceous cover. Members of these families are not of great 
economic importance. 
    The number of families included in this order varies because the species 
differ so in shape and form. Some of the families are described below. 
    The walkingsticks and leaf insects are examples of the family Phasmidae. 
Many are remarkable mimics of dead twigs, being long, slender, wingless, and 
brownish in color (illustration a). Occasionally they become so abundant that 
they defoliate trees. 
    The family Blattidae includes the cockroaches and are flattened insects 
with a large, scalelike pronotum which overhangs the head. Some cockroaches 
are wingless, but most are winged. A few species live in human dwellings and 
are among the most abundant and persistent of household pests.  See  COCKROACH. 
    The family Mantidae includes the praying mantids, characterized by a long 
slender prothorax bearing a pair of large grasping legs, and a freely moving 
head with large eyes (illustration b). Mantids are predacious, clutching the 
prey with the large forelegs and bringing it to the mouth.  See  MANTIS. 
    The true grasshoppers and the migratory locusts are members of the family 
Locustidae. The antennae are usually less than half the length of the body. 
The hindlegs are adapted for jumping (illustration c). To this group belong 
the majority of crop-destroying Orthoptera. 
    Members of the family Tetrigidae are tiny grasshoppers, called grouse 
locusts or pygmy grasshoppers, which superficially resemble the Locustidae. 
The front wings, however, are reduced to small scalelike structures, and the 
pronotum extends backward over the whole body. 
  
    The family Tettigoniidae consists of the longhorn or green grasshoppers. 
The antennae are usually as long as, or longer than, the body, and most of the 
common species are green. The hindlegs are fitted for jumping. Most of the 
Tettigoniidae are active at night. To this group belong the katydids, meadow 
grasshoppers, cave or camel crickets, and the Mormon cricket. 
    The true crickets belong to the family Gryllidae. These are usually rather 
chunky, dark-colored insects with long antennae.  
    The mole crickets, insects highly specialized for fossorial existence, are 
in the family Gryllotalpidae. They burrow just beneath the soil, pushing up 
long ridges as do moles.  See  INSECTA. 
 �34s[Harold B. Maynard]
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�6v�2v�32sOrthoquartzite   �21m �33s 
    A rock (also known as quartzose sandstone) composed almost entirely (over 
95%) of detrital quartz grains. 
    The minor accessory minerals present in orthoquartzite are only the most 
stable ones, zircon and tourmaline. The texture of orthoquartzites tends to be 
distinctive. Typically, the grains are extremely well rounded and tend to have 
high sphericity as well. The particles of orthoquartzites are bound together 
by precipitated mineral cements. The most abundant mineral cement is quartz. 
In some orthoquartzites the silica cement is chert or opal. Not all 
orthoquartzites are well cemented; many are friable and almost unindurated. 
Carbonate minerals are also important as cementing agents in orthoquartzites. 
    Typically, orthoquartzites are associated with fossiliferous limestone and 
calcareous shale beds. Orthoquartzites are most abundant in thin sedimentary 
sections, primarily in the interior of continents, but may also be found as 
the early deposits in some geosynclines.  See  QUARTZ; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sOrthorhombic pyroxene   �21m �33s 
    A group of minerals having the general chemical formula XYSi2O6, in which 
the Y site contains Fe or Mg and the X site contains Fe, Mg, Mn, or a small 
amount of Ca (up to about 3%). The end members of this solid solution series 
are enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6). Names used for 
intermediate members of the series are enstatite, bronzite, hypersthene, and 
orthoferrosilite.  
    Many of the physical and optical properties of orthopyroxene are strongly 
dependent upon composition, and especially upon the Fe-Mg ratio. In hand 
specimens, orthopyroxene can be distinguished from amphibole by its 
characteristic 88DG cleavage angles, and from augite by color - augite is 
typically green to black, while orthopyroxene is more commonly brown, 
especially on slightly weathered surfaces. 
    Orthopyroxene is a widespread mineral in metamorphic rocks. It is 
characteristic of granulite facies metamorphism, in both mafic and feldspathic 
gneisses. Orthopyroxene occurs in many basalts and gabbros, particularly those 
of tholeiitic composition, and in many meteorites, but is notably absent from 
most alkaline igneous rocks. The greatest abundance of orthopyroxene is in 
ultramafic rocks, especially those in large layered intrusions.  See  PYROXENE. 
 �34s[Robert J. Tracy]
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�6v�2v�32sOscillation   �21m �33s 
    Any effect that varies in a back-and-forth or reciprocating manner. 
Examples of oscillation include the variations of pressure in a sound wave and 
the fluctuations in a mathematical function whose value repeatedly alternates 
above and below some mean value. 
    The term oscillation is for most purposes synonymous with vibration, 
although the latter sometimes implies primarily a mechanical motion. The 
alternating current and the associated electric and magnetic fields are 
referred to as electric (or electromagnetic) oscillations. 
    If a system is set into oscillation by some initial disturbance and then 
left alone, the effect is called a free oscillation. A forced oscillation is 
one in which the oscillation is in response to a steadily applied periodic 
disturbance.  
    Any oscillation that continually decreases in amplitude, usually because 
the oscillating system is sending out energy, is spoken of as a damped 
oscillation. An oscillation that maintains a steady amplitude, usually because 
of an outside source of energy, is undamped.  See  ANHARMONIC OSCILLATOR; 
DAMPING; FORCED OSCILLATION; HARMONIC OSCILLATOR; MECHANICAL VIBRATION; 
OSCILLATOR ; VIBRATION. 
 �34s[Joseph M. Keller]
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�6v�2v�32sOscillator �21m �33s
    An electronic circuit that converts energy from a direct-current source
into a periodically varying electrical output. If the output voltage is a
sine-wave function of time, the generator is called a sinusoidal, or harmonic,
oscillator. Only sinusoidal oscillators are discussed in this article. If the
output waveform contains abrupt changes in voltage, such as occur in a pulse
or square wave, the device is called a relaxation oscillator.  See  RELAXATION
OSCILLATOR; WAVE-SHAPING CIRCUITS.
    The fundamental laws governing sinusoidal oscillators are the same for all
oscillator circuits. These basic concepts are illustrated in Fig. 1. The
amplifier provides an output voltage vsubo as a consequence of an external
input signal voltage vsubs. The voltage vsubo is applied to a circuit called a
feedback network whose output is vsubf. If the feedback voltage vsubf were
made identically equal to the input voltage vsubs, and if the external input
were disconnected and the feedback voltage connected to the amplifier input
terminals 1 and 2, the amplifier would continue to provide the same output
voltage vsubo as before. This requires that the instantaneous values of vsubf
and vsubs be exactly equal at all times. Since no restriction was made on the
waveform, it need not be sinusoidal.
    If the entire circuit operates linearly and the amplifier or feedback
network or both contain reactive elements, the only periodic wave that will
preserve its form is the sinusoidal waveform, and such a circuit will be a
sinusoidal oscillator. For sinusoidal oscillators the condition where vsubs
equals vsubf requires that amplitude, phase, and frequency of vsubs and vsubf
be identical. The phase shift introduced in a signal while being transmitted
through a reactive network is invariably a function of the frequency, and
there is usually only one frequency at which vsubf and vsubs are in phase.
Therefore, a sinusoidal oscillator operates at the frequency for which the
total phase shift of the amplifier and feedback network is precisely zero (or
an integral multiple of 2PI). The frequency of a sinusoidal oscillator,
provided the circuit oscillates at all, is therefore determined by the
condition that the loop phase shift is zero.
    Another condition, which must clearly be met if the oscillator is to
function, is that the magnitude of vsubs and vsubf must be identical. If the
amplifier has a voltage amplification, or gain, A, then vsubo equals Avsubs.
The fraction of the voltage vsubo applied to the input through the feedback
network is called the feedback factor BETA. Therefore, the equations below are
obtained. If vsubf is to equal vsubs, then BETAA must equal 1. BETAA is called
the loop gain.

     v (sub f) = BETA v (sub 0)       v (sub f) = BETA A v (sub s)

    An oscillator will not function if, at the oscillator frequency, the
magnitude of the product of the gain of the amplifier and the feedback factor
of the feedback network is less than unity. The condition of unity loop gain
is called the Barkhausen criterion.
    If BETAA is less than unity, the removal of the external generator will
immediately result in a cessation of oscillations. An oscillator in which the
loop gain is exactly unity is an abstraction that is completely unrealizable
in practice. A practical oscillator always has a BETAA somewhat larger than
unity (say 5%) to ensure that, with incidental variations in transistor and
circuit parameters, BETAA does not fall below unity.
    The phase-shift oscillator (Fig. 2) exemplifies the principles set forth
above. A field-effect transistor (FET) amplifier of conventional design is
followed by three cascaded arrangements of a capacitor C and a resistor R, the
output of the last RC combination being returned to the gate. The phase of the
signal is shifted 180DG by the amplifier, and the network of resistors and
capacitors shifts the phase by an additional amount. At some frequency the
phase shift introduced by the RC network is precisely 180DG, and the total
phase shift around the circuit is exactly zero. At this particular frequency
the circuit will oscillate, provided that the magnitude of the amplification
is sufficiently large. The phase-shift oscillator is particularly suited to
frequencies from several hertz to several hundred kilohertz and so includes
the range of audio frequencies.
 �34s[Edward S. Yang]
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�6v�2v�32sOscillograph �21m �33s
    A measurement device for determining waveform by recording the
instantaneous values of a quantity such as voltage as a function of time. It
consists of three major components: (1) a primary detector for sensing the
instantaneous values of the quantity; (2) the timing system for introducing a
time scale on the record; and (3) some means for recording the waveform.  See
WAVEFORM.
    The electromagnetic oscillograph and the cathode-ray oscillograph are the
two basic forms of oscillographs in common use.
    Electromagnetic oscillographs are either direct-writing oscillographs or
light-beam oscillographs. The best known of the direct-writing forms is the
electrocardiograph. In this device the primary detector is a galvanometer with
a multiturn moving coil. Attached to the moving coil is a pen arm, which traces
an ink record on a continuously moving paper chart. The heart beats at about
one cycle per second (1 Hz) and at this low frequency the pen follows the
vibrations nicely and draws a graph in ink that can be read immediately.
See  GALVANOMETER.
    The light-beam oscillograph, usable up to 500 Hz, is a more accurate and
more commonly used type of oscillograph.  The essential components of a
light-beam-type magnetic oscillograph are shown in the illustration.  The
galvanometer has a moving coil, usually with a single U-shaped turn (bifilar
type), but multiturn coils are sometimes used.	A tiny mirror attached to the
moving coil is the only mass added to the moving system for recording purposes.
A beam of light, focused to a point of light by the lens, is reflected from the
mirror onto a photographic film that moves at constant speed.  The lateral
position of the point where the light impinges on the film is a function of the
position of the mirror and, therefore, the instantaneous value of the current
flowing in the moving coil.  The low-frequency limitations (500 Hz) of the
relatively large masses of the electromagnetic oscillograph element are
eliminated in the cathode-ray oscillograph.  This device is a cathode-ray
oscilloscope combined with a camera that makes a photographic record of the
screen image.  The terms cathode-ray oscillograph and cathode-ray oscilloscope
are commonly interchanged.  Strictly speaking, the oscillograph contains means
for producing records, whereas the oscilloscope does not.  See OSCILLOSCOPE.
 �34s[Isaac F.  Kinnard /Edward C.  Stevenson]
�33s

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�6v�2v�32sOscilloscope   �21m �33s 
    An electronic instrument which produces a luminous plot on a fluorescent 
screen showing the relationship of two or more variables. In most cases it is 
an orthogonal (x,y) plot with the horizontal axis being a linear function of 
time. The vertical axis is normally a linear function of voltage at the signal 
input terminal of the instrument. 
    The primary advantage of a cathode-ray oscilloscope over other forms of 
plotting devices is its speed of response. Commercially available instruments 
in the general-purpose category can display frequencies as high as 100 MHz 
while special high-speed oscilloscopes can respond as high as 2000 MHz. The 
horizontal linear time axis of one general-purpose oscilloscope may be varied 
in 25 calibrated ranges from a slow speed of 10 cm in 50 seconds to a high of 
10 cm in 0.2 microseconds. The same oscilloscope can record on photographic 
film a single trace at a rate of 250 cm/microsecond. 
    In its normal form the cathode-ray oscilloscope is made up of five basic 
elements:  
    1. The cathode-ray tube and associated controls in focus, intensity or 
brightness, and astigmatism.  See  CATHODE-RAY TUBE. 
    2. The vertical or signal amplifier and its associated devices such as 
input terminal, attenuators, position control, and ac or dc amplifier 
operation selector.  See  AMPLIFIER; VOLTAGE AMPLIFIER. 
    3. Horizontal-axis time-base circuits, frequently called the sweep 
generator. Included in this group are the sweep-base control, trigger or 
synchronizing circuits, and usually a circuit for turning on the cathode-ray 
tube beam only when the sweep is going in the left-to-right direction on the 
screen.  See  SWEEP GENERATOR. 
    4. Auxiliary facilities, such as amplitude or time calibrators and 
repetition rate generators. 
    5. Power supplies furnishing the correct operating voltages for the above 
circuits.  See  OSCILLOGRAPH. 
 �34s[Howard Vollum]
�33s 
  
  
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�6v�2v�32sOsmium   �21m �33s 
    A chemical element, Os, atomic number 76, and atomic weight 190.2. Osmium 
is a hard white metal of rare natural occurrence. 
    Osmium, like the other platinum metals, is catalytically active. Osmium 
tetroxide is used as an organic reagent and as a stain for tissue in 
microscopy. Osmium alloyed with rhodium, ruthenium, iridium, or platinum is 
used in fountain-pen nibs, phonograph needles, electrical contacts, and 
instrument pivots.  See  PLATINUM. 
    The chemistry of osmium is very complicated because of the many valences 
exhibited by the element and the tendency of each of these to form numerous 
complex ions. Osmium is a very hard metal; it and its alloys are 
wear-resistant. Pure osmium and alloys in which it predominates are 
unworkable, so that they must either be used in the cast form or fabricated by 
powder metallurgy. The table gives values of some physical properties of 
osmium.  
    Osmium tetrachloride, OsCl4, is a black solid which is insoluble in 
nonoxidizing acids. Osmium tetroxide, OsO4, is a very-pale-yellow crystalline 
solid with a melting point of 40DGC and a boiling point of 130DGC; it is the 
most important osmium compound. This very poisonous compound is soluble in 
water and carbon tetrachloride. It is a powerful oxidizing agent. 
 �34s[Henry J. Albert]
�33s 
  
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�6v�2v�32sOsmoregulatory mechanisms �21m �33s
    Physiological mechanisms for the maintenance of an optimal and constant
level of osmotic activity of the fluid within and around the cells, considered
to be most favorable for the initiation and maintenance of vital reactions in
the cell and for maximal survival and efficient functioning of the entire
organism.
    The actions of osmoregulatory mechanisms are, first, to impose constraints
upon the passage of water and solute between the organism and its surroundings
and, second, to accelerate passage of water and solute between organism and
surroundings. The first effect requires a change of architecture of membranes
in that they become selectively permeable and achieve their purpose without
expenditure of energy. The accelerating effect, apart from requiring a change
of architecture of cell membranes, requires expenditure of energy and
performance of useful osmotic work. Thus substances may be moved from a region
of low to a region of higher chemical activity. Such movement can occur in
opposition to the forces of diffusion of an electric field and of a pressure
gradient, all of which may act across the cell membrane. It follows that there
must be an energy source derived from the chemical reactions of cellular
metabolism and that part of the free energy so generated must be stored in
molecules driven across the membrane barrier. Active transport is the modern
term for such processes.  See  CELL MEMBRANES; OSMOSIS.
 �34s[William A. Brodsky ;Theodore P. Schib]
�33s

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�6v�2v�32sOsmosis   �21m �33s 
    The transport of solvent through a semipermeable membrane separating two 
solutions of different solute concentration. The solvent diffuses from the 
solution that is dilute in solute to the solution that is concentrated. 
    The flow of liquid through such a barrier may be stopped by applying 
pressure to the liquid on the side of higher solute concentration. The applied 
pressure required to prevent the flow of solvent across a perfectly 
semipermeable membrane is called the osmotic pressure and is a characteristic 
of the solution. The walls of cells in living organisms permit the passage of 
water and certain solutes, while preventing the passage of other solutes, 
usually of relatively high molecular weight. These walls act as selectively 
permeable membranes, and allow osmosis to occur between the interior of the 
cell and the surrounding media.  See  EDEMA; OSMOREGULATORY MECHANISMS; 
SOLUTION.  
 �34s[Francis J. Johnston]
�33s 
  
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�6v�2v�32sOstariophysi �21m �33s
    A superorder of actinopterygian fishes comprising two orders, Siluriformes
and Cypriniformes. The characters of the Ostariophysi agree in the main with
the Salmoniformes, the generalized basal order from which most teleosts have
evolved, but with one notable difference: The anterior four or five vertebrae
are much modified as a protective encasement for a series of bony ossicles
that form a chain connecting the swim bladder with the inner ear, the
so-called Weberian apparatus. Other characters include an abdominal pelvic
fin, if present, usually with many rays and a more or less horizontal pectoral
fin placed low on the side. Usually there are no fin spines, but one or two
spines derived from consolidated soft rays may be found in dorsal, anal, or
pectoral fins, and a spinelike structure is present in the adipose fin of
certain catfishes. The superorder Ostariophysi consists of 2 well-marked
orders, about 34 families, and approximately 5000 species. See CYPRINIFORMES;
SILURIFORMES.
 �34s[Reeve M. Bailey]
�33s

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�6v�2v�32sOsteichthyes �21m �33s
    The bony fishes, one of the three classes of Recent fishlike vertebrates.
It includes most of the familiar fishes. The Osteichthyes are similar to the
Chondrichthyes, or cartilaginous fishes, and contrast with the living Agnatha
in having jaws, paired nostrils, true teeth, paired pelvic and pectoral fins
and girdles (unless lost secondarily), three semicircular canals, and bony
scales (unless lost or modified). Separation of the Osteichthyes from the
Paleozoic Placodermi and Acanthodii is more difficult because these groups
agree in most basic vertebrate features. The Osteichthyes contrast with the
Chondrichthyes in having a bony skeleton (some Recent bony fishes possess a
largely cartilaginous skeleton), a swim bladder (at least primitively), a true
gill cover, and mesodermal ganoid, cycloid, or ctenoid scales (sharks possess
dermal denticles, or placoid scales) which are sometimes modified or lost.
Fertilization is usually external, but if it is internal, the intromittent
organ is not derived from pelvic-fin claspers. Most often a modified anal fin
or a fleshy tube or sheath functions in sperm transfer. See AGNATHA;
CHONDRICHTHYES; COPULATORY ORGAN; SCALE (ZOOLOGY); SWIM BLADDER.
    The Recent fauna of the class Osteichthyes includes 3 subclasses, 32
orders, about 357 families, roughly 3570 genera, and probably about 17,600
species. A classification of the Osteichthyes follows. Equivalent names are
given in parentheses; orders known only as fossils are preceded by asterisks.
For more detailed information see separate articles on each group (the order
Dipteriformes is discussed in the article Dipnoi).

Class Osteichthyes
    Subclass Actinopterygii
	Infraclass Chondrostei
	  Order: *Palaeonisciformes
		 Polypteriformes (Cladistia)
		 Acipenseriformes
	Infraclass Holostei
	  Order: Semionotiformes (Protospondyli,
		   Ginglymodi, and Lepisostei)
		 *Pyconodontiformes
		 Amiiformes (Halecomorphi)
		 *Aspidorhynchiformes
		 *Pholidophoriformes (Halecostomi)
	Infraclass Teleostei
	  Order: *Leptolepiformes
	  Order: Elopiformes (Isospondyli in part)
		 Anguilliformes (Apodes and
		   Saccopharyngiformes or
		   Lyomeri)
		 Notacanthiformes (Lyopomi and
		   Heteromi)
		 Clupeiformes
		 Osteoglossiformes (Isospondyli in
		   part and Mormyriformes)
		 Salmoniformes (Myctophiformes,
		   or Iniomi, and Haplomi)
		 Cetomimiformes (Cetunculi)
		 Ctenothrissiformes
		 Gonorynchiformes
		 Cypriniformes (Heterognathi,
		   Eventognathi, and Gymnonoti)
		 Siluriformes (Nematognathi)
		 Percopsiformes (Salmopercae and
		   Amblyopsiformes)
		 Batrachoidiformes (Haplodoci)
		 Gobiesociformes (Xenopterygii)
		 Lophiiformes (Pediculati)
		 Gadiformes (Anacanthini)
		 Atheriniformes (Beloniformes or
		   Synentognathi,
		   Cyprinodontiformes or
		   Microcyprini, and
		   Phallostethiformes)
		 Beryciformes (Berycomorphi)
		 Zeiformes (Zeomorphi)
		 Lampridiformes (Allotriognathi)
		 Gasterosteiformes (Thoracostei
		   and Solenichthyes)
		 Pegasiformes (Hypostomides)
		 Synbranchiformes (Synbranchii)
		 Perciformes (Acanthopterygii or
		   Percomorphi)
		 Pleuronectiformes (Heterosomata)
		 Tetradontiformes (Plectognathi)
    Subclass Crossopterygii
	  Order: *Osteolepiformes (Rhipidistia)
		 Coelacanthiformes (Coelacanthini)
    Subclass Dipnoi (Dipneusti)
	  Order: Dipteriformes
 �34s[Reeve M. Bailey]
�33s

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�6v�2v�32sOsteoglossiformes   �21m �33s 
    An order of soft-rayed, actinopterygian fishes that includes the mooneyes, 
featherbacks, mormyrids or elephantfishes, and bonytongues. 
    Osteoglossiforms have the primary bite between the well-toothed tongue and 
the roof of the mouth, usually the strongly toothed parasphenoid but 
occasionally the endopterygoids. The mouth is bordered by the premaxilla, 
which in some forms is fused to its mate from the opposite side, and the 
maxilla. A unique feature is the presence of paired, usually bony rods at the 
base of the second gill arch. 
    Osteoglossiforms represent one of the basal stocks among the teleosts. The 
Ichthyodectidae and two related families are well represented in Cretaceous 
marine deposits, and Allothrissops and Pachythrissops of the Jurassic and 
Cretaceous probably belong here. Recent members may be classified in 2 
suborders, 6 families, 22 genera, and about 135 species. All inhabit fresh 
water, and all are tropical except for the two species of Hiodontidae or 
mooneyes of North America, where they have a history to the Eocene. 
  
    Numerically the Osteoglossiformes are dominated by the Mormyridae, African 
river and lake fishes of varied size and form. In some the snout is very blunt 
and rounded, in others it is elongated (see illustration), the source of the 
vernacular name elephantfish. The related family Gymnarchidae consists of a 
single species. Scientifically, mormyrids and gymnarchids are of especial 
interest in that all are electrogenic. Modified muscles in the caudal peduncle 
generate and emit a continuous electric pulse. The discharge frequency varies, 
being low at rest and high when the fish is under stress. The mechanism 
operates like a radar device, since the fish is alerted whenever an electrical 
conductor enters the electromagnetic field surrounding it.  See  
ACTINOPTERYGII; ELECTRIC ORGAN (BIOLOGY). 
 �34s[Reeve M. Bailey]
�33s 
  
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�6v�2v�32sOsteolepiformes   �21m �33s 
    A primitive order of lobefin fishes, subclass Crossopterygii, which gave 
rise in Late Devonian time to the other crossopterygian order, 
Coelacanthiformes, and more importantly, at about the same time but on a 
different line produced the earliest amphibians, ancestors of the land 
vertebrate animals. Osteolepiformes are classified in 5 families and about 28 
genera.  See  COELACANTHIFORMES. 
    Osteolepiforms, known also as Rhipidistia, first appeared in the Lower 
Devonian. They were well diversified and common inhabitants of fresh waters 
through the Middle and Upper Devonian; they persisted through the 
Carboniferous but died out in the Lower Permian. Apparently, some Devonian 
forms successfully invaded marine waters.  See  CROSSOPTERYGII; GEOLOGICAL 
TIME SCALE. 
    Generalized osteolepiforms of the Devonian had thick, usually rhombic bony 
scales with a thin surface of enamel. They were usually fusiform fishes, 
commonly 1 - 2 ft (0.3 - 0.6 m) in length, but some Carboniferous types 
reached large size. Two dorsal fins were placed well back on the body, and the 
caudal fin was primitively heterocercal (upturned) but became diphycercal 
(symmetrical) in more advanced types (see illustration). The well-ossified 
head was covered with large dermal plating bones and had internal 
cartilage-replacement bones.  See  OSTEICHTHYES. 
 �34s[Reeve M. Bailey]
�33s 
  
  
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�6v�2v�32sOsteomyelitis �21m �33s
    Inflammation of a bone and its adjacent bone marrow, usually resulting
from infection. Pyogenic (pus-forming) organisms, such as the hemolytic
staphylococci, are the most frequent agents in acute cases. Chronic forms
occur and may be induced by tuberculosis, syphilis, and other microorganisms.
    The long tubular bones of the arms and legs are most often involved. Bone
destruction can become extensive if prompt diagnosis and treatment is not
initiated. The acute phase is marked by a pus-forming inflammatory reaction. A
persistent osteomyelitis tends to become chronic and shows combined reparative
attempts mingled with continued tissue breakdown. The acute clinical course is
marked by fever, chills, leukocytosis, and local symptoms of inflammation such
as pain, swelling, and redness of the affected part.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sOsteostraci   �21m �33s 
    An order of extinct jawless vertebrate fishes, also called Cephalaspida, 
of the class Agnatha, known from the Middle Silurian to Upper Devonian of 
Europe, Asia, and North America. They were mostly small, about 2 in. to 2 ft 
(5 to 60 cm) in length. The head and part of the body were encased in a solid 
armor of bone, and the posterior part of the body and the tail were covered 
with thick scales. Some early forms lacked paired fins, though most possessed 
flaplike pectoral fins. One or two dorsal fins were present. Their depressed 
shape and the position of the eyes on the top of the head suggest that 
Osteostraci were bottom dwellers (see illustration). The underside of the 
throat region was covered by small plates and there was a small mouth in 
front.  See  AGNATHA. 
 �34s[Robert H. Denison]
�33s 
  
  
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�6v�2v�32sOstracoda   �21m �33s 
    A subclass of the class Crustacea containing small bivalved animals from 
0.4 to 30 mm long, with most between 1 and 2 mm. They inhabit aquatic 
environments in nearly all parts of the world; the only terrestrial species 
known, Mesocypris terrestris Harding, lives in damp humus in Africa. Of the 
nearly 2000 species extant, none is truly parasitic and most are free-living. 
However, a few fresh-water and marine forms live commensally on other animals. 
Most ostracods are scavengers, some are herbivorous, and a few are predacious 
carnivores. Exceptional biological features are known; there are myodocopine 
ostracods which produce bioluminescence, and some species of podocopines form 
a secretion from spinning glands to enable them to climb polished surfaces. 
    Ostracods are classified in five orders: Archaeocopida, Leperditicopida, 
Palaeocopida, Podocopida, and Myodocopida. All representatives of the first 
three orders are extinct. The Podocopida is divided into suborders Podocopina, 
Metacopina (extinct), and Platycopina; the Myodocopida is divided into the 
suborders Myodocopina and Cladocopina. All fresh-water Ostracoda belong to the 
suborder Podocopina. 
    The two valves of the carapace, sufficient to enclose the rest of the 
animal, are joined dorsally along a hinge, which may vary from a simple 
juncture to a complex series of teeth and sockets. From the dorsal part of the 
carapace, the elongate body is suspended as a pliable sac, with lateral flaps 
of hypodermis extending between the lamellae of the valves. It is unsegmented, 
but reinforced by chitinous processes for rigidity in the vicinity of the 
appendages. There is no true abdominal region (see illustration). 
    In some species males are unknown and the eggs develop without 
fertilization (parthenogenesis); in other species males are produced and 
fertilize the eggs (syngamy). Most ostracods lay their eggs on the substrate 
or on vegetation, but some transfer them to the posterior space within the 
carapace, where they hatch and the young brood is retained for a time.  See  
CLADOCOPA; CRUSTACEA; MYODOCOPA; PALEOCOPA; PLATYCOPA; PODOCOPA. 
 �34s[Robert V. Kesling]
�33s 
  
  
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�6v�2v�32sOtter   �21m �33s 
    Members of the family Mustelidae, a large group of carnivores which also 
includes the martens, weasels, badgers, and skunks. Otters are more completely 
adapted to aquatic life than other members of the family, having a long thin 
body, short legs, and somewhat flattened head. The small ears possess a 
membrane which closes the ear canal when the animal dives. The feet have five 
webbed toes with nonretractile claws. The tail is broad and flattened, being 
used along with body movements for swimming. The short thick fur is impervious 
to water. Like other members of the family all have well-developed perianal 
scent glands.  See  CARNIVORA; MAMMALIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sOtto cycle   �21m �33s 
    The basic thermodynamic cycle for the prevalent automotive type of 
internal combustion engine. The engine uses a volatile liquid fuel (gasoline) 
or a gaseous fuel to carry out the theroretic cycle shown in the illustration. 
The cycle consists of two isentropic (reversible adiabatic) phases 
interspersed between two constant-volume phases. The theoretic cycle should 
not be confused with the actual engine built for such service as automobiles, 
motor boats, aircraft, lawn mowers, and other small self-contained power 
plants.  
    The thermodynamic working fluid in the cycle is subjected to isentropic 
compression, phase 1 - 2; constant-volume heat addition, phase 2 - 3; 
isentropic expansion, phase 3 - 4; and constant-volume heat rejection 
(cooling), phase 4 - 1. 
    The Otto cycle is represented in many millions of engines utilizing either 
the four-stroke principle or the two-stroke principle. Evidence indicates that 
actual Otto engines offer peak efficiencies (25PLUS/MINUS%) at compression 
ratios of 15PLUS/MINUS. Above this ratio, efficiency falls. The most probable 
explanation is that the extreme pressures associated with high compression 
cause increasing amounts of dissociation of the combusion products. This 
dissociation, near the beginning of the expansion stroke, exerts a more 
deleterious effect on efficiency than the corresponding gain from increasing 
compression ratio.  See BRAYTON CYCLE; CARNOT CYCLE; DIESEL CYCLE; INTERNAL 
COMBUSTION ENGINE; THERMODYNAMIC CYCLE. 
 �34s[Theodore Baumeister]
�33s 
  
  
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�6v�2v�32sOvarian disorders   �21m �33s 
    Ovarian disorders range from the fairly common physiological cystic 
enlargements of the ovaries to neoplasms of malignant character which may be 
life-threatening and require more urgent attention. 
    Developmental abnormalities are expressed either as sexual infantilism or 
as the increasingly complex variety of morphological, hormonal, and 
chromosomal parameters associated with sexual identity. Gonadal agenesis 
refers to bilateral rudimentary ``streak'' gonads and infantile external 
genitalia as seen in, for example, Turner's syndrome. The ovaries contain 
stroma only, without germ cells (ova). Ovarian ``dysgenesis'' includes 
chromosomal mosaics, true hermaphrodites who possess both ovarian and 
testicular tissue (ovotestis), and pseudohermaphrodites. 
    The ovary normally contains numerous microscopic cysts (Graafian 
follicles). After ovulation the cystic space may become filled with blood (a 
hemorrhagic corpus luteum). Not infrequently these physiological cysts enlarge 
which may result in a surgical procedure and excision of the cyst to exclude a 
neoplastic process. 
    The polycystic ovary syndrome includes bilateral polycystic ovaries, 
primary amenorrhea or oligomenorrhea, infertility, obesity, and hirsutism. 
There is a wide spectrum of clinical and pathological abnormality in this type 
of disorder. Amenorrhea or anovulatory bleeding and infertility are 
characteristic symptoms. The clinical and pathologic changes are due to a 
prolonged anovulatory state and unopposed estrogen stimulation. The ovaries 
are typically enlarged, with numerous subcapsular follicular cysts. 
    Endometriosis (ectopic endometrial glandular and stromal tissue) can also 
produce a cystic mass in the ovary (endometrioma) filled with old, brown, 
clotted blood (``chocolate cyst''). Spontaneous rupture of endometrial cysts 
and other hemorrhagic physiological cysts of the ovary may produce peritoneal 
irritation and acute abdominal symptoms. 
    The large number of often bewildering patterns of ovarian neoplasms have 
resulted in many different classifications. Those based on hormone secretion 
have become particularly imprecise. Unfortunately, most ovarian malignancies 
are discovered at an advanced stage, since ovarian neoplasms in general tend 
to be asymptomatic until they produce a palpable mass. Malignant neoplasms are 
``staged'' I - IV to determine the extent of growth beyond the ovary. The 
stage establishes a prognosis and most effective mode of treatment. This can 
be done accurately only by surgical examination of the abdominal cavity 
(laparotomy).  See  CANCER (MEDICINE); OVARY. 
 �34s[Marit Ek]
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�6v�2v�32sOvary   �21m �33s 
    A part of the reproductive system of all female vertebrates. Although not 
vital to individual survival, the ovary is vital to perpetuation of the 
species. The function of the ovary is to produce the female germ cells or ova, 
and in some species to elaborate hormones that assist in regulating the 
reproductive cycle. 
    The ovaries develop as bilateral structures in all vertebrates, but adult 
asymmetry is found in certain species of all vertebrates from the 
elasmobranchs to the mammals. 
    The ovary of all vertebrates functions in essentially the same manner. 
However, ovarian histology of the various groups differs considerably. Even 
such a fundamental element as the ovum exhibits differences in various groups. 
  See  OVUM. 
    The mammalian ovary is attached to the dorsal body wall. The free surface 
of the ovary is covered by a modified peritoneum called the germinal 
epithelium. Just beneath the germinal epithelium is a layer of fibrous 
connective tissue. Most of the rest of the ovary is made up of a more cellular 
and more loosely arranged connective tissue (stroma) in which are embedded the 
germinal, endocrine, vascular, and nervous elements. 
    The most obvious ovarian structures are the follicles and the corpora 
lutea. The smallest, or primary, follicle consists of an oocyte surrounded by 
a layer of follicle (nurse) cells. Follicular growth results from an increase 
in oocyte size, multiplication of the follicle cells, and differentiaion of 
the perifollicular stroma to form a fibrocellular envelope called the theca 
interna. Finally, a fluid-filled antrum develops in the granulosa layer, 
resulting in a vesicular follicle. 
    The cells of the theca interna hypertrophy during follicular growth and 
many capillaries invade the layer thus forming the endocrine element that is 
thought to secrete estrogen. The other known endocrine structure is the corpus 
luteum, which is primarily the product of hypertrophy of the granulosa cells 
remaining after the follicular wall ruptures to release the ovum. Ingrowths of 
connective tissue from the theca interna deliver capillaries to vascularize 
the hypertrophied follicle cells of this new corpus luteum; progesterone is 
secreted here.  See  ESTROGEN; ESTRUS; MENSTRUATION; PROGESTERONE. 
 �34s[Kenneth L. Duke]
�33s 
  
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�6v�2v�32sOverdrive   �21m �33s 
    An automotive device supplied as special equipment and containing a 
step-up planetary gear arrangement located between the transmission and 
propeller shaft. Overdrive permits the propeller shaft to be driven at 
transmission output shaft speed, or faster than transmission output shaft 
speed when the overdrive comes into operation. For a given forward speed of 
the car, overdrive gives lower engine speed, quieter operation, and reduced 
gasoline consumption. 
    The overdrive contains two major components, a planetary gear system and a 
free-wheeling device (or overrunning clutch). The overrunning clutch has two 
functions. It locks the transmission mainshaft and the overdrive output shaft 
together when the planetary gear system is inactive, providing direct drive 
through the overdrive. The second function is to permit the output shaft to 
overrun the transmission mainshaft when the planetary gear train is in action, 
providing overdrive. The overrunning clutch consists of an outer shell 
attached to the output shaft (see illustration). A circular inner member 
attached to the transmission shaft carries a series of cams or flats; a 
hardened steel roller rides on each cam. When the clutch is in action, the 
rollers wedge between the cams and outer shell so that the cams drive the 
outer shell. During overrunning, the outer shell rotates faster than the cams 
and the rollers move from loaded contact with the cams to disengage the 
clutch.  See  AUTOMOTIVE TRANSMISSION; CLUTCH; PLANETARY GEAR TRAIN. 
 �34s[Forest R. McFarland]
�33s 
  
  
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�6v�2v�32sOvertone (music and acoustics)   �21m �33s 
    An upper partial tone. As a consequence of the distinction ``upper,'' a 
sixth overtone, for example, is the same as the seventh partial tone, so that 
numbering tends to become confused. The word overtone has been used as a 
synonym for partial, harmonic, or mode of vibration, and the word tone has 
still other meanings in music. It seems desirable, therefore, not to use 
overtone at all, but to select one of the other words that has a more specific 
meaning.  See  HARMONIC (PERIODIC PHENOMENA); PARTIAL TONE; TONE (MUSIC AND 
ACOUSTICS).  
 �34s[Robert W. Young]
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�6v�2v�32sOvervoltage   �21m �33s 
    The difference between electrode potential under electrolysis conditions 
and the thermodynamic value of the electrode potential in the absence of 
electrolysis for the same experimental conditions; sometimes called 
overpotential. It is expressed in volts and is often quoted in absolute value. 
Understanding of overvoltage phenomena is essential in industrial 
electrochemistry, since they are often a major factor in determining the 
efficiency of electrode processes.  See  DECOMPOSITION POTENTIAL; 
ELECTROCHEMICAL PROCESS; ELECTRODE POTENTIAL; ELECTROLYSIS. 
    The overvoltage for a given electrode reaction depends on electrolysis 
conditions such as current density (current per unit area), concentration of 
electrolyzed substance, temperature, presence of foreign electrolytes, nature 
of the solvent, and absorption of foreign substances on the electrode. The 
overvoltage for given electrolysis conditions varies greatly from one 
electrolyzed substance to another - from less than 1 mV to a few volts. 
    Overvoltage values are determined from the variations of electrode 
potential with current density by the following method. The electrolytic cell 
is connected to a direct-current power supply, and the potential of the 
electrode being studied is measured against a reference electrode for 
different current densities. Results are plotted as a current-potential curve 
(see illustration), and the overvoltage is read on this curve. 
    Concentration overvoltage and activation overvoltage are the two 
components into which the overvoltage can generally be decomposed. 
Concentration overvoltage arises from local variations of the concentration of 
reactants or products of electrolysis or both near the electrode. The 
thermodynamic potential calculated for the concentrations at the electrode 
surface under electrolysis conditions is different from the potential in the 
absence of electrolysis. The difference between these two potentials is the 
concentration overvoltage. Activation overvoltage results from the relative 
slowness of electrode reactions. An energy barrier must be overcome by the 
substances involved in an electrode reaction just as for purely chemical 
reactions. Activation overvoltage corresponds to this additional consumption 
of electrical energy. 
 �34s[Paul Delahay]
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�6v�2v�32sOvum �21m �33s
    The egg or female sex cell. Strictly speaking, the term refers to this
cell when it is ready for fertilization, but it is often applied to earlier or
later stages. Confusion is avoided by using qualifying adjectives such as
immature, ripe, mature, fertilized, or developing ova. The mature ova are
generally spheroidal and large. The number of ova produced at one time varies
in different animals, from millions in many marine animals that spawn into the
surrounding sea water to about a dozen or less in mammals in which adaptations
for internal nourishment of the developing embryo and care of the young are
highly developed.
    In the ovary the immature ovum is associated with follicle cells through
which it receives material for growth. In mammals, as the egg matures, these
cells arrange themselves into a structure known as the Graafian, or vesicular,
follicle, consisting of a large fluid-filled cavity into which the ovum,
surrounded by several layers of cells, projects from the layer of follicle
cells that constitutes the inner wall (see illustration). The fluid contains
estrogenic female sex hormone secreted by cells in an intermediate layer of
the follicular wall.

    Yolk, or deutoplasm, is essentially a food reserve in the form of small
spherules, present to a greater or lesser extent in all eggs. It accounts
largely for the differences in size of eggs. Eggs are classified according to
the distribution of yolk. In the isolecithal type there is a nearly uniform
distribution through the cytoplasm, as in most small eggs. The yolk in
telolecithal eggs is increasingly concentrated toward one pole, as in the
large eggs of fish, amphibians, reptiles, and birds. Centrolecithal, or
centrally located, yolk occurs in eggs of insects and cephalopod mollusks.
See  GAMETOGENESIS; OOGENESIS.
 �34s[Albert Tyler /Howard L. Hamilton]
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�6v�2v�32sOxalate �21m �33s
    A salt or ester of oxalic acid with the formula

			  O==C--O--R
			     |
			  O==C--O--R'

where R and R' are alkyl groups or metallic ions attached to the acid to
produce a series of salts and esters. In general, alkali-metal salts are
water-soluble; others are insoluble. Many salts have practical applications,
for example, in pyrotechnics (sodium), blueprinting (potassium-iron), and
analytical procedures (ammonium). Diethyl oxalate is used as a solvent, as a
dyestuff intermediate, and in plastics.  See  ESTER; OXALIC ACID.
 �34s[Elbert H. Hadley]
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�6v�2v�32sOxalic acid �21m �33s
    A white solid acid melting with decomposition at 189.5DGC (373.1DGF).
Oxalic acid (ethanedioic acid), with the structure

			  O==C--O--H
			     |
			  O==C--O--H

is the first of a series of dicarboxylic acids. Its salts (oxalates) are
prevalent in nature, for example, KHC2O4 in plants of the oxalis family (wood
sorrel) and CaC2O4 in eucalyptus bark.	See  OXALATE.
    The acid is used as a bleaching agent for rust and ink stains, in textile
and leather production, and as monoglyceryl oxalate in the manufacture of
allyl alcohol and formic acid.
 �34s[Evans B. Reid]
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�6v�2v�32sOxazole �21m �33s
    One of a group of organic heterocyclic compounds containing oxygen and
nitrogen in the 1 and 3 positions of a five-membered diunsaturated ring. The
formula shows the structure and numbering system for a typical member of the
group, 1,3-oxazole, considered the parent compound. This compound is a
colorless, volatile, weakly basic liquid (bp 69 - 70DGC or 156-158DGF), with
an odor resembling that of pyridine. Oxazole is miscible with water and
organic solvents. Mineral acids form salts that tend to dissociate in water.
Oxazoles show appreciable resistance to disruption by heat, by acid, and by
alkali. The nucleus is susceptible to oxidation, the 4,5 position being the
usual point of attack. Hydrogenation over a platinum catalyst or with sodium
and alcohol gives tetrahydro derivatives (oxazolidines) or ring-cleavage
products.  See	AZOLE; HETEROCYCLIC COMPOUNDS.
 �34s[Walter J. Gensler]
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�6v�2v�32sOxidation process   �21m �33s 
    A process in which oxygen is caused to combine with other molecules. The 
oxygen may be used as elemental oxygen, as in air, or in the form of an 
oxygen-containing molecule which is capable of giving up all or part of its 
oxygen. Oxidation in its broadest sense, that is, an increase in positive 
valence or removal of electrons, is not considered here if oxygen itself is 
not involved.  See  OXIDATION-REDUCTION. 
    Most oxidations occur with the liberation of large amounts of energy in 
the form of either heat, light, or electricity. The stable ultimate products 
of oxidation are oxides of the elements involved. These oxidations occur in 
nature as corrosion, decay, and respiration and in the deliberate burning of 
matter such as wood, petroleum, sulfur, or phosphorus to oxides of the 
constituent elements. 
    The principal variables to be considered and controlled in any partial 
oxidation are temperature, pressure, reaction time (or contact time), nature 
of catalyst, if any, mole ratio of oxidizing agent, and whether the substance 
to be oxidized is to be kept in the liquid or vapor phase. Only a narrow range 
of conditions unique to each substance being oxidized and each product desired 
will give satisfactory yields. It is also essential to maintain conditions 
outside the range of spontaneous ignition, to avoid explosive mixtures or the 
accidental accumulation of unstable peroxides, and to choose materials which 
not only can resist the environmental conditions but also which do not have 
adverse catalytic effects or otherwise interfere with the desired reaction.  
See  COMBUSTION. 
 �34s[I. E. Levine]
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�6v�2v�32sOxidation-reduction �21m �33s
    An important concept of chemical reactions which is useful in
systematizing the chemistry of many substances. Oxidation can be represented
as involving a loss of electrons by one molecule and reduction as involving an
absorption of electrons by another. Both oxidation and reduction occur
simultaneously and in equivalent amounts during any reaction involving either
process.

    Oxidation number.  The oxidation state is a concept which describes some
important aspects of the state of combination of the elements. An element in a
given substance is characterized by a number, the oxidation number, which
specifies whether the element in question is combined with elements which are
more electropositive or more electronegative than it is. It further specifies
the combining capacity which the element exhibits in a particular combination.
A scale of oxidation numbers is defined by assigning to an oxygen atom in an
ion such as SO*EQN* the value of 2-. That for sulfur as 6+ then follows from
the requirement that the sum of the oxidation numbers of all the atoms add up
to the net charge on the species. The value of 2- for oxygen is not chosen
arbitrarily. It recognizes that oxygen is more electronegative than sulfur,
and that when it reacts with other elements it seeks to acquire two more
electrons, by sharing or outright transfer from the electropositive partner,
so as to complete a stable valence shell of eight electrons.
    Although oxidation number is in some respects similar to valence, the two
concepts have distinct meanings. In the substance H2, the valence of hydrogen
is 1 because each H makes a single bond to another H, but the oxidation number
is 0, because the hydrogen is not combined with a different element.  See
VALENCE.
    When the oxidation number of an atom in a species is increased, the
process is described as oxidation, no matter what reagent produces it; when a
decrease in oxidation number takes place, the process is described as
reduction, again without regard to the identity of the reducing agent. The
term oxidation has been generalized to imply combination of an element with an
element more electronegative than itself.  See	OXIDATION.

    Reactions.	 In an oxidation-reduction reaction, some element decreases in
oxidation state and some element increases in oxidation state. The substances
containing these elements are defined as the oxidizing agents and reducing
agents, and they are said to be reduced and oxidized, respectively. The
processes in question can always be represented formally as involving electron
absorption by the oxidizing agent and electron donation by the reducing agent.
For example, reaction (1) can be regarded as the sum of the two partial
processes, or half-reactions, (2) and (3). Similarly, reaction (4) consists of
the two half-reactions (5) and (6), with half-reaction (5) being taken five
times to balance the electron flow from reducing agent to oxidizing agent.

       2FE(sup 3+) + 2I(sup -) --> 2Fe(sup 2+)				     (1)

       2l(sup -) --> I(sub 2) + 2e(sup -)				     (2)

       2Fe(sup 3+) + 2e(sup -) --> 2Fe(sup 2+)				     (3)

16H(sup +) + 2MNO4(sup -) + 10I(sup -) --> 8H20 + 2Mn(sup 2+) + 5I(sub 2)    (4)

       2I(sup -) --> I(sub 2) + 2e(sup -)				     (5)

16H(sup +) + 2MnO4(sup -) + 10e(sup -) --> 2Mn(sup 2+) + 8H20		     (6)

    Each half-reaction consists of an oxidation-reduction couple; thus, in
half-reaction (6) the reducing agent and oxidizing agent making up the couple
are manganous ion, Mn(sup 2+), and permanganate ion, MnO4-, respectively; in
half-reaction (5) the reducing agent is I- and the oxidizing agent is I2. The
fact that MnO4- reacts with I- to produce I2 means that MnO4- in acid solution
is a stronger oxidizing agent than is I2. Because of the reciprocal relation
between the oxidizing agent and reducing agent comprising a couple, this
statement is equivalent to saying that I- is a stronger reducing agent than
Mn(sup 2+) in acid solution. Reducing agents may be ranked in order of
tendency to react, and this ranking immediately implies an opposite order of
tendency to react for the oxidizing agents which complete the couples. In the
list below some common oxidation-reduction couples are ranked in this fashion:


STRONG REDUCING AGENTS and WEAK OXIDIZING AGENTS

Mg = Mg(sup 2+) + 2e(sup -)
ZN = Zn(sup 2) + 2e(sup -)
H2 = 2H(sup +) + 2e(sup -)
Cu = 2Cu(sup 2) +2e(sup -)
I(sup -) = 1/2 I(sub 2) + e(sup -)
Fe(sup 2+) = Fe(sup 3+) + e(sup -)
Br(sup -) = 1/2 Br(sub 2) + e(sup -)
Cl(sup -) = 1/2 Cl(sub 2) + e(sup -)
4H20 + Mn(sup 2+) = MnO4(sup -) + 8H(sup +) + 5e(sup -)

WEAL REDUCING AGENTS and STRONG OXIDIZING AGENTS

See ELECTROCHEMICAL SERIES;
ELECTRONEGATIVITY; OXIDIZING AGENT.
 �34s[Henry Taube]
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�6v�2v�32sOxide   �21m �33s 
    A binary compound of oxygen with another element. Oxides have been 
prepared for essentially all the elements except the noble gases. Often, 
several different oxides of a given element can be prepared; a number exist 
naturally in the Earth's crust and atmosphere: silicon dioxide (SiO2) in 
quartz; aluminum oxide (Al2O3) in corundum; iron oxide (Fe2O3) in hematite; 
carbon dioxide (CO2) gas; and water (H2O). 
    Most elements will react with oxygen at appropriate temperature and oxygen 
pressure conditions, and many oxides may thus be directly prepared. Most 
metals in massive form react with oxygen only slowly at room temperatures 
because the first thin oxide coat formed protects the metal. The oxides of the 
alkali and alkaline-earth metals, except for beryllium and magnesium, are 
porous when formed on the metal surface, and they provide only limited 
protection to the continuation of oxidation, even at room temperatures. Gold 
is exceptional in its resistance to oxygen, and its oxide (Au2O3) must be 
prepared by indirect means. The other noble metals, although ordinarily 
resistant to oxygen, will react at high temperatures to form gaseous oxides. 
    Oxides may be classified as acidic or basic according to the character of 
the solution resulting from their reactions with water. The nonmetal oxides 
generally form acid solutions and the metal oxides generally form alkaline 
solutions.  See  ACID AND BASE; EQUIVALENT WEIGHT; OXYGEN. 
 �34s[Russell K. Edwards]
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�6v�2v�32sOxidizing agent   �21m �33s 
    A participant in a chemical reaction that absorbs electrons from another 
reactant. In the process a component atom of this substance undergoes a 
decrease in oxidation number. In this action as an oxidizing agent, the 
substance undergoes reduction. 
    A measure of the effectiveness of a reagent as an oxidizing agent is its 
reduction potential. This is, in electrochemical terms, the equivalent of the 
free-energy change for the reduction process. The element with the highest 
reduction potential (and, therefore, the strongest oxidizing agent) is 
fluorine, F2. The practical effectiveness of a given oxidizing (or reducing) 
agent will depend upon both the thermodynamics and the available kinetic 
pathway for the reaction process.  See  CHEMICAL THERMODYNAMICS. 
    Substances that are widely used as oxidizing agents in chemistry include 
ozone (O3), permanganate ion (MnO-4), nitric acid (HNO3), as well as oxygen 
itself. Organic chemists have empirically developed combinations of reagents 
to carry out specific oxidation steps in synthetic processes. The action of 
molecular oxygen as an oxidizing agent may be made more specific by 
photochemical excitation to an excited singlet electronic state.  See  
BIOLOGICAL OXIDATION; OXIDATION. 
 �34s[Francis J. Johnston]
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�6v�2v�32sOxime �21m �33s
    One of a group of chemicals derived from aldehydes (RCH==NOH,
aldoximes) or ketones (RR'C==NOH, ketoximes) used for isolation and
identification of carbonyl compounds. In general, they are easily purified and
have characteristic melting points. The properties of certain oximes have made
them industrially important. Oximes have received considerable attention
because of their stereochemistry and their participation in the Beckmann
rearrangement.	See  ALDEHYDE; KETONE; STEREOCHEMISTRY.
 �34s[Leallyn B. Clapp]
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�6v�2v�32sOximetry   �21m �33s 
    A technique that employs an oximeter, a photoelectric photometer, to 
measure the oxygenated fraction of the hemoglobin. This fraction is usually 
expressed in percent, which is referred to as the ``oxygen saturation of 
blood.''  
    One type of oximeter is designed to measure the oxygen saturation of blood 
circulating in a particular tissue of an intact animal or human. The tissue 
most commonly studied is the cartilaginous pinna of the ear, and the 
instrument used for this purpose is called an ear oximeter. 
    Another type of oximeter is designed to measure the oxygen saturation of 
blood during, or shortly after, its withdrawal from various sites in the 
vascular system. Such a device usually is designated a cuvette oximeter. 
 �34s[Earl H. Wood]
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�6v�2v�32sOxygen   �21m �33s 
    A gaseous chemical element, O, atomic number 8, and atomic weight 15.9994. 
Oxygen is of great interest because it is the essential element both in the 
respiration process in most living cells and in combustion processes. It is 
the most abundant element in the Earth's crust. About one-fifth (by volume) of 
the air is oxygen. Uncombined gaseous oxygen usually exists in the form of 
diatomic molecules, O2, but oxygen also exists in a unique triatomic form, O3, 
called ozone.  See  OZONE. 
    Oxygen is separated from air by liquefaction and fractional distillation. 
The chief uses of oxygen in order of their importance are (1) smelting, 
refining, and fabrication of steel and other metals; (2) manufacture of 
chemical products by controlled oxidation; (3) rocket propulsion; (4) 
biological life support and medicine; and (5) mining, production, and 
fabrication of stone and glass products. 
    Under ordinary conditions oxygen is a colorless, odorless, and tasteless 
gas; it condenses to a pale blue liquid. Oxygen is one of a small group of 
slightly paramagnetic gases, and it is the most paramagnetic of its group. 
Liquid oxygen is also slightly paramagnetic. Some data on oxygen and some 
properties of its ordinary form, O2, are listed in the table. 
    Practically all chemical elements except the inert gases form compounds 
with oxygen. Among the most abundant binary compounds of oxygen are water, 
H2O, and silica, SiO2, the latter being the chief ingredient of sand. Among 
compounds containing more than two elements, the most abundant are the 
silicates, which constitute most of the rocks and soil. Other widely occurring 
compounds are calcium carbonate (limestone and marble), calcium sulfate 
(gypsum), aluminum oxide (bauxite), and the various oxides of iron which are 
mined as a source of iron.  See  OXIDATION; OXIDE. 
 �34s[Arthur W. Francis]
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�6v�2v�32sOxymonadida   �21m �33s 
    An order of class Zoomastigophorea in the phylum Protozoa. These are 
colorless flagellate symbionts in the digestive tract of the roach 
Cryptocercus and of certain termites. They are xylophagous; that is, they 
ingest wood particles taken in by the host. Seven or more genera of medium or 
large size have been identified, the organisms varying from pyriform to ovoid 
in shape. At the anterior end a pliable necklike rostrum attaches the organism 
to the host intestinal wall, but they are sometimes free. They can be either 
uni- or multinucleate.  See  ZOOMASTIGOPHOREA. 
 �34s[James B. Lackey]
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�6v�2v�32sOxytetracycline   �21m �33s 
    A crystalline, amphoteric, broad-spectrum antibiotic elaborated by the 
actinomycete Streptomyces rimosus. The antibiotic is known commercially as 
Terramycin and is produced by fermentation processes. 
    Oxytetracycline is used against infectious diseases in both human and 
veterinary medicine. It has broad-spectrum activity against bacteria, 
spirochetes, rickettsiae, large viruses, and certain protozoa and metazoa. 
Oxytetracycline has agricultural application as an animal growth stimulant, in 
poultry feed to increase egg production, as a food preservative, and as a 
crop-spray ingredient.  See  ANTIBIOTIC. 
 �34s[Charles R. Stephens]
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�6v�2v�32sOxyurina   �21m �33s 
    A major group of the Nematoda, also known as the Oxyurata. It is one of 
the two suborders of Ascaridida. In one modern system of classification, it 
has essentially the composition and characteristics outlined under Oxyuroidea. 
In another, some of the component groups such as the Heterakidae, 
Cosmocercidae, and Kathlaniidae are assigned instead to the suborder 
Ascaridina.  See  ASCARIDIDA; NEMATA; OXYUROIDEA. 
 �34s[John T. Lucker]
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�6v�2v�32sOxyuroidea   �21m �33s 
    A large and prevalent major group of the class Nematoda. For convenience, 
they are treated here as a superfamily containing the families Oxyuridae, 
Atractidae, Thelastomatidae, Rhigonematidae, Kathlaniidae, Cosmocercidae, and 
Heterakidae or Subuluridae. There are about 135 genera. Hosts include 
terrestrial mammals, birds, reptiles, amphibians, fishes, insects, and other 
arthropods.  
    The species are small to medium-sized, thin-bodied Ascaridida. There are 
usually three or six lips, if present. Lateroventral cephalic papillae of the 
external circle may be present or absent. Usually the esophagus is grossly 
divided into a corpus, an isthmus, and a posterior bulb which often contains a 
valvular apparatus. The excretory system, in most instances, is H-type, with 
the terminal duct often short and in the form of a reservoir. Females are 
mostly oviparous, sometimes viviparous. Sexual dimorphism is often pronounced. 
    With one exception, known life cycles are direct. Typically the eggs pass 
out of the host's alimentary tract onto the ground. There they become fully 
embryonated and infective. Normally the infective egg does not hatch until a 
susceptible animal ingests it. The cecum and colon of the host are the typical 
locations of these parasites.  See  NEMATA. 
 �34s[John T. Lucker]
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�6v�2v�32sOzone �21m �33s
    A powerfully oxidizing allotropic form of the element oxygen. The ozone
molecule contains three atoms (O3). Ozone gas is decidedly blue, and both
liquid and solid ozone are an opaque blue-black color, similar to that of ink.
  See  OXYGEN.
    Some properties of ozone are given in the table. Ozone has a
characteristic, pungent odor familiar to most persons because ozone is formed
when electrical apparatus produces sparks in air. Ozone is irritating to
mucous membranes and toxic to human beings and lower animals.
    Ozone is a more powerful oxidizing agent than oxygen, and oxidation with
ozone takes place with evolution of more heat and usually starts at a lower
temperature than when oxygen is used. In the presence of water, ozone is a
powerful bleaching agent, acting more rapidly than hydrogen peroxide,
chlorine, or sulfur dioxide.  See  OXIDIZING AGENT.
    Ozone is utilized in the treatment of drinking-water supplies. Odor- and
taste-producing hydrocarbons are effectively eliminated by ozone oxidation.
Iron and manganese compounds which discolor water are diminished by ozone
treatment. Compared to chlorine, bacterial and viral disinfection with ozone
is up to 5000 times more rapid.
    Ozone occurs to a variable extent in the Earth's atmosphere. Near the
Earth's surface the concentration is usually 0.02 - 0.03 ppm in country air,
and less in cities except when there is smog. At vertical elevations above 13
mi (20 km), ozone is formed by photochemical action on atmospheric oxygen.
Maximum concentration of 5 x 10(sup 12)molecules/cm(sup 3) (more than 1000 times
the normal peak concentration at Earth's surface) occurs at an elevation of 19
mi (30 km).  See  ATMOSPHERIC OZONE.
 �34s[Arthur W. Francis]
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�6v�2v�32sOzonolysis   �21m �33s 
    A process which uses ozone to cleave unsaturated organic bonds. Generally, 
ozonolysis is conducted by bubbling ozone-rich oxygen or air into a solution 
of the reactant. The reaction is fast at moderate temperatures. Intermediates 
are usually not isolated but are subjected to further oxidizing conditions to 
produce acids or to reducing conditions to form alcohols or aldehydes. An 
unsymmetrical olefin is capable of yielding two different products whose 
structures are related to the groups substituted on the olefin and the 
position of the double bond. 
    Before World War I, ozonolysis was applied commercially to the preparation 
of vanillin from isoeugenol. The only modern application of the technique in 
the United States is in the manufacture of azelaic and pelargonic acids from 
oleic acid.  See  ALKENE; OZONE. 
 �34s[Robert K. Barnes]
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�6v�2v�32sPacific islands �21m �33s
    All of the islands of the open Pacific Ocean plus the Philippines on the
west. As thus defined, the islands number nearly 10,000, but they are
scattered over an area of 64,000,000 mi(sup 2)(166,000,000 km[sup 2]), roughly
one-third of the Earth's surface. The total island land area is 170,000
mi(sup 2) (440,000 km[sup 2]), or about one-quarter of 1% of the Pacific Ocean.
    Most Pacific islands rise from the deep sea. All the large islands and
many small ones bear volcanic high points rising hundreds, or even thousands,
of feet above sea level. The great majority of the islands, however, are low,
many being tiny islets on or close to surface reefs. Islands of all sizes are
concentrated in the southwest Pacific, particularly in the tropical latitudes.
    The typical Pacific islands lie far from continental shores and are of
volcanic origin. Some are not as isolated as they appear on maps, because
bathymetric charts show that they are high points on submarine ridges of one
sort or another. Many other islands are in linear groups that are independent
of midoceanic ridges. Still other islands are arranged in arcuate groups,
either continental arcs such as the Philippines or oceanic arcs such as the
Marianas and Palau.  See  OCEANIC ISLANDS.
 �34s[Harry S. Ladd]
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�6v�2v�32sPacific Ocean �21m �33s
    The Pacific Ocean has an area of 6.37 x 10 sup 7 mi sup 2 (1.65 x 10 sup 8
km sup 2) and a mean depth of 14,000 ft (4282 m). It covers 32% of the Earth's
surface and 46% of the surface of all oceans and seas, and its area is greater
than that of all land areas combined. Its mean depth is the greatest of the
three oceans and its volume is 53% of the total of all oceans. Its greatest
depths in the Marianas and Japan trenches are the world's deepest, more than
10  km.
    The two major wind systems driving the waters of the ocean are the
westerlies which lie about 40 - 50DG lat in both hemispheres (the ``roaring
forties'') and the trade winds from the east which dominate in the region
between 20DGN and 20DGS. These give momentum directly to the west wind drift
(flow to the east) in high latitudes and to the equatorial currents which flow
to the west. At the continents there is flow of water from one system to the
other and huge circulatory systems result.  See  OCEAN CURRENTS; SOUTHEAST
ASIAN WATERS.
    The swiftest flow (greater than 2 knots) is found in the Kuroshio Current
near Japan. It forms the northwestern part of a huge clockwise gyre whose
north edge lies in the west wind drift centered at about 40DGN, whose eastern
part is the south-flowing California Current, and whose southern part is the
North Equatorial Current.  See	OCEAN CURRENTS.
    Equatorward of 30DG lat heat received from the Sun exceeds that lost by
reflection and back radiation, and surface waters flowing into these latitudes
from higher latitudes (California and Peru currents) increase in temperature
as they flow equatorward and turn west with the Equatorial Current System.
They carry heat poleward and transfer part of it to the high-latitude cyclones
along the west wind drift. The temperature of the equatorward currents along
the eastern boundaries of the subtropical anticyclones is thus much lower than
that of the currents of their western boundaries at the same latitudes. The
highest temperatures (more than 82DGF or 28DGC) are found at the western end
of the equatorial region. Along the Equator itself somewhat lower temperatures
are found. The cold Peru Current contributes to its eastern end, and there is
apparent upwelling of deeper, colder water at the Equator.
    Upwelling also occurs at the edge of the eastern boundary currents of the
subtropical anticyclones. When the winds blow strongly equatorward (in summer)
the surface waters are driven offshore, and the deeper colder waters rise to
the surface and further reduce the low temperatures of these
equatorward-flowing currents.  See  UPWELLING.
    The limiting temperature in high latitudes is that of freezing. Ice is
formed at the surface at temperatures slightly less than 30DGF (-1DGC)
depending upon the salinity; further loss of heat is retarded by its
insulating effect. The ice field covers the northern and eastern parts of the
Bering Sea in winter, and most of the Sea of Okhotsk, including that part
adjacent to Hokkaido (the north island of Japan). Summer temperatures,
however, reach as high as 43DGF (6DGC) in the northern Bering Sea and as high
as 50DGF (10DGC) in the northern part of the Sea of Okhotsk.  See  BERING SEA.
    Pack ice reaches to about 62DGS from Antarctica in October and to about
70DGS in March, with icebergs reaching as far as 50DGS.  See  ICEBERG; SEA ICE.
    Surface waters in high latitudes are colder and heavier than those in low
latitudes. As a result, some of the high-latitude waters sink below the
surface and spread equatorward, mixing mostly with water of their own density
as they move, and eventually become the dominant water type in terms of
salinity and temperature of that density over vast regions.
    The most conspicuous water masses formed in the Pacific are the
Intermediate Waters of the North and of the South Pacific, which on the
vertical sections include the two huge tongues of low salinity extending
equatorward beneath the surface from about 55DGS and from about 45DGN. The
southern tongue is higher in salinity and density and lies at a greater depth.
 �34s[Joseph L. Reid]
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�6v�2v�32sPacking   �21m �33s 
    A seal usually used for high pressure as in steam and hydraulic 
applications. The motion between parts may be infrequent as in valve stems, or 
continual as in pump or engine piston rods. There is no sharp dividing line 
between seals and packing; both are dynamic pressure resistors under motion. 
Such diverse materials are used for packing as impregnated fiber, rubber, 
cork, or asbestos compounds. In packings, it is necessary that the surface 
finish of the contacting metal part be smooth for long life of the material.  
See  PRESSURE SEAL. 
 �34s[Paul H. Black]
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�6v�2v�32sPain   �21m �33s 
    Pain, especially in its acute form, is usually a reflection of a 
tissue-damaging or potentially tissue-damaging stimulus. There is a 
transmission system that conveys this information to the central nervous 
system. This phenomenon is called nociception. Pain is more complex than other 
sensory systems such as vision or hearing because it not only involves the 
transfer of sensory information to the nervous system, but produces suffering 
which then leads to aversive corrective behavior. In certain disease states, 
defects in the transmission system can of themselves generate false 
information to the nervous system, as though tissue damage were occurring in 
the periphery. An example of this is phantom limb pain, in which the 
individual often has a crushing type of pain in a foot that has been amputated. 
    Acute pain such as occurs with broken bones and other significant injuries 
is almost inevitably accounted for by the phenomenon of nociception and is 
probably a purely neurophysiological event. However, the more pain becomes a 
chronic phenomenon, the more such influences as psychological factors and 
behavior become part of the expression of pain. 
    Acute pain is a useful warning system. There are specific nerve paths for 
conducting this sensation (see illustration). Pain receptors in the skin and 
other tissues are nerve terminals which lack any special characteristics, and 
they are probably triggered by a chemical stimulus when potential tissue 
damage occurs. There appear to be two types of terminals: one which responds 
to many types of painful stimuli, whereas the other specifically responds to 
either mechanical or thermal energy. When the terminals are stimulated, the 
pain (that is, nociception message) is carried along specific small sensory 
fibers called A-delta and C fibers. The A-delta fibers are larger and transmit 
the ``first pain'' or ``fast pain.'' The smaller C fibers transmit a secondary 
dull continuous pain. These nerve fibers were traditionally believed to enter 
the spinal cord through the dorsal root, but it now seems that many also enter 
through the ventral root into the spinal cord. 
  
    Having entered the spinal cord, these fibers relay in the dorsal horn of 
the spinal gray matter, an area of considerable regulation and modulation of 
the incoming pain stimulus which is influenced by other incoming sensory 
stimuli; that is, touch or pressure sensations can suppress the transmission 
of signals in the small pain fibers. This helps to explain why when a person 
is hurting, the pain can be reduced by rubbing the affected part, and this 
phenomenon forms the basis of some of the treatment strategies of 
stimulation-produced analgesia. In addition, the incoming pain signal in the 
spinal cord is also modulated by descending signals from the brain. At times 
of anxiety, these pain signals may be augmented. From these relay stations in 
the dorsal horn, the pain signal is carried by two nerve paths up to the 
brain. The classical pathway is the spinothalamic tract, on the side of the 
spinal cord opposite to the incoming stimulus, and this leads to the posterior 
part of the thalamus in the brainstem, and from there nerve paths radiate the 
pain sensation to many parts of the cerebral cortex, where the pain is 
appreciated. In addition to this direct path, there is also a diffuse 
ascending path known as the spinoreticular tract which relays to many of the 
basal ganglia in the brain, and from there to areas of the brain connected 
with motivational and affective behavior such as the hippocampus and the 
cingulate gyrus. It is possible that narcotic analgesics exert some of their 
action on this ascending spinoreticular tract because these drugs tend to 
reduce the suffering aspects of pain, but still preserve many of the 
discriminative qualities so that individuals can still feel the pain, but it 
does not bother them so much.  See  ANALGESIC; NARCOTIC. 
    Certain parts of the brainstem around the central canal appear to exert a 
strong inhibitory effect on incoming pain signals. Stimulation of these areas 
probably releases endorphins, which are morphinelike substances produced by 
the body and liberated at various sites on the incoming pain path to suppress 
these signals.  See  ENDORPHINS. 
 �34s[Terence M. Murphy]
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�6v�2v�32sPaint   �21m �33s 
    A fluid, with viscosity, drying time, and flowing properties dictated by 
formulation, normally consisting of a vehicle or binder, a pigment, a solvent 
or thinner, and a drier, which may be applied in relatively thin layers and 
which changes to a solid in time. The change to a solid may or may not be 
reversible, and may occur by evaporation of the solvent, by chemical reaction, 
or by a combination of the two. 
    In modern technology, paint is classified in three major categories 
because of differing performance requirements: architectural paints, 
commercial finishes, and industrial coatings. A fourth category is artistic 
media.  
    Architectural paints are air-drying materials applied by brush or spray to 
architectural and structural surfaces and forms for decorative and protective 
purposes. Materials are classified by formulation type as solvent-thinned and 
water-thinned. The drying mechanism of solvent-thinned paints predominantly 
may be by solvent evaporation, oxidation, or a combination of the two. 
Solvent-thinned paints which dry essentially by solvent evaporation rely on a 
fairly hard resin as the vehicle. Resins include shellac, cellulose 
derivatives, acrylic resins, vinyl resins, and bitumens. In paints that dry by 
oxidation, the vehicle is usually an oil or an oil-based varnish. 
Water-thinned paints may be subdivided into those in which the vehicle is 
dissolved in water and those in which it is dispersed in emulsion form. Paints 
with water-soluble vehicles include the calcimines, in which the vehicle is 
glue, and casein paints, in which the vehicle is casein or soybean protein. 
Materials formed by emulsion polymerization are described as a latex, and 
products are called latex paints.  See  DRIER (PAINT); DRYING OIL; ENAMEL; 
POLYMER.  
    Commercial finishes include air-drying or baking-cured materials applied 
by brush, spray, or magnetic agglomeration to kitchen and laundry appliances, 
automobiles, machinery, and furniture and used as highway marking materials. 
Industrial coatings are subdivided by their intended service: 
corrosion-resistant coatings, high-temperature coatings, and coatings for 
immersion service.  See  PIGMENT; SURFACE COATING. 
 �34s[C. R. Martinson ;C. W. Sisler]
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�6v�2v�32sPalaeacanthocephala   �21m �33s 
    An order of the Acanthocephala, the adults of which are parasitic worms 
found in fishes, aquatic birds, and mammals. They have the following 
characteristics. The nuclei of the hypodermis are fragmented and the chief 
lacunar vessels are lateral. The males have usually 2 - 7 cement glands. The 
ligament sac in the female breaks down so that the eggs develop in the body 
cavity. Proboscis hooks occur in long rows and spines are present on the body 
of some species. Species which commonly occur in vertebrates are 
Leptorhynchoides thecatus and Corynosoma (see illustration). See 
ACANTHOCEPHALA.  
 �34s[Donald V. Moore]
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�6v�2v�32sPalaechinoida   �21m �33s 
    An extinct order of Perischoechinoidea with a rigid test in which the 
ambulacra bevel over the adjoining interambulacra. There are two or more 
columns of plates in the ambulacra, three or more in the interambulacra. (An 
exception is one only in Cravenechinus.) The order is known only from the 
Lower Carboniferous.  See  ECHINOIDEA; PERISCHOECHINOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sPalaeonemertini   �21m �33s 
    A rarer order of the class Anopla in the phylum Rhynchocoela, 
characterized by an unarmed proboscis, a thin gelatinous dermis, and either a 
two-layered or three-layered body musculature. Many members show primitive 
features, such as a peripherally located nervous system and the absence of 
ocelli, ciliated grooves, and intestinal diverticula. Cerebral organs, if 
present, are generally simple.  See  ANOPLA; RHYNCHOCOELA. 
 �34s[J. B. Jennings]
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�6v�2v�32sPalaeonisciformes   �21m �33s 
    A large order of chondrostean fish which includes the earliest known and 
most primitive ray-finned fishes and has a long history from the Lower 
Devonian, or earlier, to the Cretaceous. Many genera and species, both marine 
and fresh-water, have been described, but the majority are poorly understood. 
There are two principal Paleozoic sub-orders: the fusiform Palaeoniscoidei and 
their deep-bodied derivatives, the Platysomoidei. 
    Palaeoniscoids are small to medium-sized fishes with a heavily ossified 
exoskeleton. The body is usually covered with thick rhombic scales surfaced 
with enamel and articulated in oblique rows by a peg and socket mechanism. 
Posteriorly the body is tapered to a graceful point. This slender tapered 
portion is turned upward in heterocercal fashion to form the dorsal or axial 
lobe of the caudal fin. 
    Typical platysomids have a laterally compressed, rhombic-shaped body with 
an angle at the front of the long dorsal and anal fins, small pelvic fins or 
none, a short lower jaw, and an upright jaw suspension. The flank scales are 
much deeper than long; the tail is heterocercal and forked; the facial profile 
is very steep (see illustration).  See  ACANTHODII; CHONDROSTEI; 
CROSSOPTERYGII; OSTEICHTHYES. 
 �34s[Ted M. Cavender]
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�6v�2v�32sPalaeospondyloidea   �21m �33s 
    An ordinal name assigned to the single, tiny, problematic fish 
Palaeospondylus, which is known only from Middle Devonian shales of restricted 
extent in Caithness, Scotland. This animal seldom exceeded 2 in. (5 cm) in 
length. The skeleton consists of a well-calcified skull, vertebral column, 
caudal fin, and occasional traces of paired pelvic fins. The relationships of 
Palaeospondylus are uncertain. 
 �34s[Robert H. Denison]
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�6v�2v�32sPalate   �21m �33s 
    The roof of the mouth in those vertebrates whose mouth cavity and nasal 
passages are wholly or partially separate. 
    The palate of mammals consists of two portions. The hard palate, more 
anterior in position, underlies the nasal cavity, whereas the soft palate 
hangs like a curtain between the mouth and nasal pharynx. The hard palate has 
an intermediate layer of bone. The oral surface of the hard palate is a mucous 
membrane, covered with a stratified squamous epithelium. 
    The soft palate is a backward continuation from the hard palate. Its free 
margin connects on each side with two folds of mucous membrane, the palatine 
arches, enclosing a palatine tonsil. In the midline the margin extends into a 
fingerlike projection named the uvula. The oral side of the soft palate 
continues as the covering of the hard palate, and the submucosa contains pure 
mucous glands. 
    Besides separating the nasal passages from the mouth, the hard palate is a 
firm plate, against which the tongue crushes and manipulates food. The soft 
palate, at rest, is pendant. In sucking, swallowing, or vomiting it is raised 
to separate the oral from the nasal portion of the pharynx. The closing action 
also occurs in speech, except for certain consonants requiring nasal 
resonance.  See  SPEECH. 
 �34s[Leslie B. Arey]
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�6v�2v�32sPaleobiochemistry   �21m �33s 
    The study of chemical processes used by organisms that lived in the 
geological past. Most information on the nature of life in the geological past 
comes from the study of fossils; a record of biochemical processes that 
occurred can be found in the organic molecules of sedimentary rocks and 
fossils. The organic matter in fossil fuel deposits and finely dispersed in 
shales and limestones represents the debris of cells which have been 
chemically altered to a more stable form. A comparison of the molecular 
structure of these preserved organic compounds with that of components of 
living cells enables the researcher to identify similarities and 
dissimilarities between past and present biochemistry.  See  PETROLEUM, ORIGIN 
OF.  
    Paleobiochemical studies have shown that a number of the common chemical 
processes used by living organisms today have been in use for a very great 
length of time. Paleobiochemical techniques are also used on materials 
returned from extraterrestrial sources to determine whether life exists 
outside of Earth.  See  PALEOECOLOGY; PALEONTOLOGY. 
 �34s[Thomas C. Hoering]
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�6v�2v�32sPaleobotany   �21m �33s 
    The study of fossil plants and vegetation of the geologic past. As a 
branch of paleontology, it combines a knowledge of both geology and botany. 
Its materials are the fossilized remains of prehistoric plants preserved in 
the rocks, including fossil leaves, seeds, pollen, spores, and wood, and 
occasionally fruits and flowers. From such materials the paleobotanist 
attempts to reconstruct whole plants as they actually grew, as well as the 
ancient forests of which they were a living part. The fossil plant record also 
shows the succession of plants which have inhabited the Earth. Paleobotany 
involves not only collecting, describing, and naming fossil plants, but also 
aims at their interpretation in terms of the evolutionary history of plant 
groups, the relationships between extinct and living forms, the reconstruction 
of the environmental conditions under which the ancient forests lived, and the 
relative geologic ages of the rocks in which the fossils were found.  See  
FOSSIL SEEDS AND FRUITS; PALEONTOLOGY; PALYNOLOGY. 
    Most fossil plants are found in shales and fine-grained siltstones, which 
are the lithified muds of ancient deposits. The best collections are obtained 
from rocks originally laid down as deposits in floodplains, lakes, swamps or 
bogs, and coastal lagoons or estuaries. Volcanic ash, the lithified form of 
which is known as tuff, is also an excellent medium for the rapid burial and 
subsequent preservation of plant remains. Much knowledge of ancient plants is 
derived from a study of coal itself and of the fossil plants that occur both 
in coal and in the roof shales and underclays associated with coal beds.  See  
COAL; COAL BALLS; COAL PALEOBOTANY; PETRIFACTION. 
 �34s[Erling Dorf]
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�6v�2v�32sPaleoceanography   �21m �33s 
    The study of the history of the circulation, chemistry, biogeography, 
fertility, and sedimentation of the oceans. It is a quantitative field that 
uses the geologic record to construct and test rigorous models of the behavior 
of the past ocean. It is this emphasis that distinguishes the field from the 
study of marine facies which, as old as geology itself, has made use of the 
sparse oceanic record preserved on the continents. 
    Paleoceanography relies on three separate data sets: reconstruction of the 
configuration of continents and oceans and of the topography of the ocean 
floor, a temporal framework usually biostratigraphic in nature, and sediment 
data bearing on ocean chemistry, fertility, circulation, sediment input, and 
so on. The geographic reconstruction is furnished by plate tectonics from 
paleomagnetic and sea-floor spreading data. See BIOSTRATIGRAPHY; PLATE 
TECTONICS.  
    Environmental parameters are derived from the mineralogical, textural, 
chemical, and paleontological properties of ocean sediments. Calcium carbonate 
content is a key to the nature of the bottom waters; organic matter content 
informs about the level of oxygenation; silica and phosphate hold clues to the 
fertility of the surface water. Floral and faunal assemblages can, with 
careful statistical work, yield valuable numerical data on salinity, 
temperature, and a few other variables for times not too far in the past. 
Oxygen and carbon isotopes similarly provide salinity and temperature data. 
See MARINE SEDIMENTS; INDEX MINERALS; PALEONTOLOGY. 
 �34s[Tjeerd H. Van Andel]
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�6v�2v�32sPaleocene   �21m �33s 
    The oldest of five major worldwide divisions (epochs) of the Tertiary 
Period (Cenozoic Era); the epoch of geologic time extending from the end of 
the Cretaceous Period (Mesozoic Era) to the Eocene Epoch. See TERTIARY. 
    Paleocene time encompassed two intervals of regional uplift and marine 
regression, separated by an interval of marked changes in the distribution of 
temporarily expanding seas. The preceding Cretaceous was brought to a close by 
a shrinking of the world's previous seaways, giving rise to local fresh-water 
deposition, to extensive terrestrial deposits, and to widespread erosion 
through regional uplifts. True mountain-making movements (orogenies) were few 
though locally intense. Areas of known continuous marine deposition from 
Cretaceous to Paleocene are scattered and restricted. 
    Paleocene marine life was characterized by the spread of pelecypods, 
gastropods, echinoids, and foraminifers already present but rare in earlier 
seas. Great limestone-forming foraminifers dominated topical shallow clear 
marine waters. At a rather sharply defined biostratigraphic horizon (end of 
the Cretaceous) termed the time of ``the Great Dying,'' previously dominant 
cephalopod mollusks (true belemnites and am-mo-nites) and marine reptiles 
(ichthyosaurs, plesiosaurs, and mososaurs) became suddenly extinct, as did the 
huge dinosaur reptiles on land. The outstanding feature of Paleocene land life 
was the appearance, proliferation, and explosive evolution of small archaic 
placental mammals (insectivores, creodont carnivores, condylarths, ancestral 
rodents, and primates), while an older marsupial mammal fauna became isolated 
in Australia. In the closing phases of the Paleocene, mammalian faunas 
indicate reestablishment of some intercontinental land connections. See 
PALEONTOLOGY.  
 �34s[Robert M. Kleinpell]
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�6v�2v�32sPaleoclimatology �21m �33s
    That aspect of the study of climates which deals with ancient, long-term
patterns of weather, usually treated on a global or regional scale.
Paleoclimatology uses ``climatic indicators'' that are identified and analyzed
by geologists; this is one aspect of paleogeography, the study of past
environments. The science of paleoclimatology thus requires knowledge of both
geology and atmospheric physics. See CLIMATOLOGY; PALEOGEOGRAPHY.
    Examples of paleoclimatic indicators are: fossil palm leaves discovered in
Spitsbergen and Antarctica suggested tropical conditions there at one time;
the distinctive grooving of rock surfaces by glacier ice in India and the
Sahara Desert; bones of mammoths found in New York State. Continental drift
and plate tectonics explain some of the anomalies, but not all.
    Both modern and ancient climates are dependent upon two variables,
planetary conditions, and extraplanetary controls. The extraterrestrial
irregularities are due, on the one hand, to the slight changes in the
relationship of the Earth to the Sun, and, on the other, to extremely subtle
variations of the Sun's emissions. Variables within the planet itself depend
on such things as the amount of dust thrown into the atmosphere by volcanic
eruptions, changes in the height of mountains, and changes in the
oceanographic circulation.
    The Earth's environments have passed through both repetitive events and
evolutionary, noncyclic stages of development. The principal cyclic control is
probably the rotation of the Galaxy, which takes place about once every 200 -
250 x 10(sup 6) years. Its effects are not yet well understood, but it almost
certainly affects the Earth's gravitational field and many dependent factors,
not the least of which is climate.
    It is known that although there have been very large climatic changes in
the past, they could not have surpassed certain limits because the fossil
record of extinct life shows that favorable metabolic environments (about
25 PLUS/MINUS 10 DG C or 77 PLUS/MINUS 18 DG F) persisted in spite of all
revolutionary events. The geologic record shows that major extinctions and new
speciation tend to coincide in time with major events in paleogeography and
paleomagnetic and climatic history. The specific mechanisms of all these
changes constitute one of the major challenges of science. See EXTINCTION
(BIOLOGY); SPECIATION.
 �34s[Rhodes W. Fairbridge]
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�6v�2v�32sPaleocopa   �21m �33s 
    An order of extinct ostracods, also called Palaeocopida. The outstanding 
characteristic is a long, straight hinge (see illustration). These ostracods 
lack a frontal opening and a duplicature. Lobes and sulci, vertically 
elongate, are conspicuous in many genera and are conventionally designated 
from the anterior end of the valve as L1, S1, L2, S2. Of these, S2 is the most 
developed sulcus and is present in all lobate forms; it marks the position of 
the internal adductor muscle scars. Soft parts have never been found 
preserved, making comparison with living groups uncertain. The order is 
divided into suborders Beyrichicopina and Kloedenellocopina. See OSTRACODA. 
 �34s[Robert V. Kesling]
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�6v�2v�32sPaleoecology   �21m �33s 
    The ecology of the geologic past, a study of the relations of fossil 
organisms to each other and to the environments in which they lived. 
Paleoecology is a study based on inferences and interpretations and on the 
basic assumption that the animals and plants of the past lived under 
essentially the same environmental conditions as do living relatives.  In 
studying the record as preserved in the rocks, the paleoecologist attempts, as 
does the ecologist, to make a complete list of the animals and plants that 
inhabited a particular area and to obtain an estimate of their relative 
abundance. See ECOLOGY. 
    Some paleoecological interpretations are based upon data obtained from the 
fossils themselves, others on features of the rocks that contain the fossils; 
in many cases data from both sources are used. The shape of a cephalopod shell 
or other swimming or crawling form may suggest a mode of locomotion; the thin 
and fragile shells of certain foraminifers or gastropods may point clearly to 
a pelagic existence. Corals and sedentary or burrowing pelecypods and 
brachiopods may be found in position of growth, indicating that they probably 
were buried in the place where they lived. When the members of a fossil 
assemblage are compared with living relatives whose life habits are known, it 
may become apparent that the fossil assemblage is a mixed one with 
representatives from more than one environment. Such an interpretation may be 
confirmed if some members of the group exhibit signs of breakage or wear or if 
only the lighter unattached valves of sedentary forms are present. Studies of 
the isotopic composition of the shells may indicate the temperature or some 
other aspect of the past environment. 
    The rock containing the fossils may indeed be the major source of 
interpretative data. The texture of the enclosing sediments may reveal much 
about the site of deposition. In water-laid sediments the occurrence of graded 
beds, that is, beds in which the texture grades upward from coarse to fine, 
may point to landslides or some form of turbidity current that threw the 
sediment into suspension before permitting final settlement. The presence of 
certain types of ripple marks, mud cracks, rain prints, or other sedimentary 
structures may aid in determining the nature of the original environment. 
Other interpretations may be based on a high content of organic matter or lime 
carbonate, or on the presence of oolites, glauconite, or nodules of 
phosphorite or manganese oxides. See SEDIMENTARY ROCKS. 
    Combinations of the above-mentioned types of data may give a suggestion or 
a clear indication, in the case of water-laid sediments, about such features 
as type of bottom, nearness to land, depth, agitation, and turbidity. See 
PALEONTOLOGY.  
 �34s[Harry S. Ladd]
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�6v�2v�32sPaleogeography   �21m �33s 
    The geography of the ancient past. Because all the events of geologic 
history have taken place in the context of a geography different from that of 
the modern world, paleogeography has always been of central concern in 
studying geology. It encompasses all aspects of geographic information. Since 
the advent of plate tectonics, it has become apparent that certain geologic 
features are related to the shift of continental blocks and the opening and 
closing of ocean basins. Geography changes constantly through time. The 
correct interpretation of geologic history depends on the compilation of 
paleogeographic reconstructions of the geologic past. See CONTINENTAL DRIFT; 
PLATE TECTONICS. 
    Paleogeography provides the framework for all the geologic events that 
have affected the surface of the Earth. It is also vital in specialized 
studies, such as the distribution of organisms and community patterns in 
paleontology. Paleogeography also has practical value in the search for 
resources by providing reconstructions that can be used to locate promising 
regions in which to search for undiscovered resources. 
    Paleogeography incorporates information from many geologic disciplines. 
Geophysics, petrology, sedimentology, stratigraphy, and paleontology each 
provide different types of data. Past continental positions are determined 
through the use of paleomagnetism. The azimuth direction of the remanent 
magnetism of a rock indicates the direction to the magnetic north or south 
pole at the time the rock formed and therefore the original orientation of the 
area, which may have moved or rotated since the rocks formed. The inclination 
of the remanent magnetic field can be used to determine the ancient latitude. 
Latitude can also be inferred from the presence of climatically sensitive 
sediments. Longitude in the geologic past is difficult to determine because 
there are no preserved measures of absolute longitude as there are for 
latitude. However, reasonably accurate relative longitudes have been 
determined for most continents through time by a combination of simple spatial 
constraints, biogeographic relationships, and the apparent pattern of plate 
motion. See PALEOCLIMATOLOGY; PALEOGEOLOGY; PALEOMAGNETISM. 
 �34s[Richard A. Bambach]
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�6v�2v�32sPaleogeology   �21m �33s 
    The geology of an area at a specific time in the past. A paleogeologic map 
displays the distribution of rocks below an ancient landscape (erosion 
surface) which was subsequently buried by further sedimentation. The map is 
made by figuratively removing the covering rocks to reveal the old land 
surface. These maps depict structural features such as faults and folds, as 
well as ancient topography, especially stream drainage patterns. Maps also 
provide information on the geologic history of the region: the extent of 
ancient seas, the source areas of sediments, and the timing of deformation 
events. Paleogeologic maps have been most useful in identifying areas of 
buried topography or geologic structure which serve to trap petroleum below 
the overlying strata. 
 �34s[Charles W. Byers]
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�6v�2v�32sPaleomagnetism   �21m �33s 
    The study of the direction and intensity of the Earth's magnetic field 
through geologic time. Paleomagnetism has been, and continues to be, an 
important tool in unraveling the past movements of the Earth's tectonic 
plates. By studying the records of the ancient magnetic field left in rocks, 
earth scientists are able to learn how the continental and oceanic plates have 
moved relative to the Earth's spin axis and relative to one another. In 
addition, the global reference frame of the Earth's magnetic field provides a 
very useful basis for temporal correlation of rocks on a local or global 
geographic scale (magnetostratigraphy). See GEOMAGNETISM; PLATE TECTONICS. 
    Many rocks acquire remanent magnetizations at or about the time they are 
formed. These magnetizations are nearly always parallel to the direction of 
the Earth's magnetic field at the locality where the rock formed.  See  ROCK 
MAGNETISM.  
    In paleomagnetic studies a suite of carefully oriented samples spanning a 
time interval long enough to average magnetic secular variations is collected. 
For magnetostratigraphy, an ordered suite of samples spanning the 
stratigraphic section of interest are collected. The samples are taken to the 
laboratory, where they are cut into small upright cylinders and their 
magnetization is measured by using a sensitive magnetometer. 
    The end product of the laboratory experiments is a suite of magnetization 
vector directions from the collected samples. These directions are specified 
by the inclination I, the angle that the magnetization vector makes with the 
horizontal, and the declination D, the angle that the projection of the 
magnetization vector upon a horizontal plane makes with true north, reckoned 
positive clockwise from north. Provided that the sample collection represents 
a sufficiently long time span to average out secular variation, representative 
mean D and I values and an associated uncertainty in direction may be 
calculated by using statistical techniques. The mean declination and 
inclination, together with the inclination-latitude relationship mentioned 
earlier and some elementary spherical trigonometry, allow the calculation of a 
representative paleomagnetic pole from the rock unit. By connecting 
paleomagnetic poles of different ages in an ordered time sequence, an apparent 
polar wander path (APWP) may be constructed for a particular tectonic plate 
(see illustration). The APWP specifies the displacement history of a plate or 
continent with respect to the spin axis, and can be directly compared with 
APWPs from other plates or continents to determine whether relative movements 
have occurred. 
    The end product of a magnetostratigraphic study is a set of normal (N) and 
reversed (R) magnetizations from the stratigraphic section under 
investigation. The positioning and frequency of occurrence of these N-to-R and 
R-to-N transitions is highly diagnostic in many cases, and by using these data 
together with other local geologic information, such as the position of major 
unconformities, one stratigraphic section can be correlated with another over 
considerable distances. The method can also be used over intracontinental and 
intercontinental distances. However, because the field has only two possible 
states (N or R), correlation over longer distances where tectonics and 
sedimentation rates may vary is correspondingly less accurate. 
 �34s[Michael O. McWilliams]
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�6v�2v�32sPaleontology �21m �33s
    The study of animal history as recorded by fossil remains. Paleontology
lies on the boundary between two disciplines, biology and geology.
    The fossil record includes a very diverse class of objects ranging from
molds of microscopic bacteria in rocks more than 3 x 10(sup 9) years old to
unaltered bones of human fossils in ice-age gravel beds formed only a few
thousand years ago. Quality of preservation ranges from the occasional
occurrence of soft parts (skin and feathers, for example) to barely
decipherable impressions made by shells in soft mud that later hardened to
rock.  See  FOSSIL; MICROPALEONTOLOGY.
    The most common fossils are hard parts of various animal groups. Thus the
fossil record is not an accurate account of the complete spectrum of ancient
life but is biased in overrepresenting those forms with shells or skeletons.
Fossilized worms are extremely rare, but it is not valid to make the
supposition that worms were any less common in the geologic past than they are
now.  See  EDIACARAN FAUNA.
    The data of paleontology consist not only of the parts of organisms but
also of records of their activities: tracks, trails, and burrows. Dinosaur
footprints, for example, are very common in the Connecticut Valley. Even
chemical compounds formed only by organisms can, if extracted from ancient
rocks, be considered as part of the fossil record. Artifacts made by humans,
however, are not termed fossils, for these constitute the data of the related
science of archeology, the study of human civilizations.  See  ARCHEOLOGY;
PALEOBIOCHEMISTRY.
    Although life was not present on the Earth when it first formed, the raw
materials necessary for its natural development were available. Electrical
discharges and ultraviolet radiation induced the components of the original
atmosphere to form complex organic compounds which later combined to form the
prototype of a living cell. The oldest recognizable organisms so far
discovered are bacterialike cells from South African cherts dated at 3.1 x
10(sup 9) years old.  By the time the Gunflint cherts of Canada were deposited 2
x 10(sup 9) years ago, complex algae had evolved.  With the exception of a few
forms found in rocks just slightly older than 6 x 10(sup 8) years, invertebrate
animals are not found until the base of the Cambrian Period, 6 x 10(sup 8) years
ago.  It is not known why so many groups of invertebrates made their first
appearance as fossils in rocks of the same age; perhaps the base of the Cambrian
marks a time during which animals first developed hard parts.  The first
vertebrate fossils, primitive fishes, occur in Lower Ordovician rocks.
Vertebrates invaded the land with the evolution of amphibians in the Late
Devonian, about 3.5 x 10(sup 8) years ago.  Reptiles evolved soon afterward in
Mississippian times; the first true mammals are Jurassic in age.  Humans are
newcomers, a product of the last few million years.  Humans belong to a single
species, a natural product of an unplanned evolution; yet from a geological
perspective, humans have altered the Earth far more than it had changed in any
comparable time since life evolved.  See ANIMAL EVOLUTION; PALEOBOTANY.
 �34s[Stephen J.  Gould]
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�6v�2v�32sPaleosol   �21m �33s 
    A soil that formed or began forming on a landscape in the geologic past, 
that is, an ancient soil. There are three kinds of paleosols: relict, buried, 
and exhumed. 
    Relict soils formed on preexisting landscapes but subsequently were not 
buried under younger sediments. Buried soils are soils formed on preexisting 
landscapes and subsequently buried by younger sediment or rock. These soils 
crop out in excavations, such as stream banks or road cuts. Exhumed paleosols 
are soils that were buried but have been reexposed on the present land surface 
by erosion of the covering mantle. These soils occupy appreciable parts of the 
present land surface and are juxtaposed geographically to other soils more in 
harmony with the present environment. 
    Most paleosols are similar in their morphologies and properties to soils 
on the present land surface but are not necessarily analogs of the soils of 
the same area or region. Analyses of paleosols and comparison with present 
surface soils and their environments aid in reconstructing possible 
environments of landscapes of the geologic past.  See  SOIL. 
 �34s[Robert V. Ruhe]
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�6v�2v�32sPaleozoic �21m �33s
    A major segment of geologic time, representing the time of deposition of
the oldest sedimentary rocks on Earth which contain identifiable fossils and
whose strata can be correlated worldwide. Paleozoic strata commonly occur
across all continents as thousands of feet of limestone or dolomite with less
shale and sandstone, and in long, narrow geosynclinal troughs around the
continents filled with even thicker silty sandstones, dark shale, and chert.
    The era is divided into systems: Cambrian (beginning almost 6 x 10(sup 8)
years ago), Ordovician, Silurian, Devonian, the Carboniferous (Mississippian
and Pennsylvanian in North America), and the Permian, ending about 2.25 x
10(sup 8) years ago. These European-defined rock systems were based on changes
in fossil life and were conceived to mark events in geologic history such as
major rises and falls of sea level over the continents, mountain-building
episodes, and times of intrusion of molten magma (granite) in folded belts
peripheral to the continental plates. These major events of Earth history are
now believed to have been caused by extensive lateral movements of continental
plates opening and closing great oceans on the globe's surface.  See  PLATE
TECTONICS.
    Although a proto-Atlantic Ocean was present early in the Paleozoic, it had
essentially disappeared by the end of the Paleozoic as the Baltic-Russian and
African continents moved westward relative to North America and impinged on
this continent. Late in the Paleozoic the North American plate was completely
surrounded by uplifted mountain chains formed as the earlier subsiding troughs
were squeezed against it.
    Paleozoic life was quite different from that represented by the fossils of
later strata. Major groups of dominant Paleozoic invertebrate fossils such as
brachiopods, trilobites, nautiloid cephalopods, and bryozoans which dwelt in
the tropical seas of that time are now either extinct or very rare. The middle
of the Paleozoic saw the rise of fishes, followed in Carboniferous time by
amphibians. The humid equatorial climate of the Northern Hemisphere
Carboniferous resulted in vast coal-producing swamps, and much oil and gas
were produced by dark marine basinal shales and collected in limestone
bordering shallow continental seas. Reptiles up to several meters in length
dominated vertebrate life in the Permian. During this immense span of time the
most ancient forms of animal and plant life had evolved or become extinct,
setting the stage for modern plants, reptiles, mammals, and shellfish.
    In Permian time, climate all over the world became more extreme,
fluctuating between humid and dry, and in more arid times permitting
deposition of redbeds and gypsum and rock salt which precipitated as vast
inland saline seas evaporated. In marked contrast, a great southern continent
composed of a united Africa, South America, Antarctica, Australia, and India
was mostly ice-covered toward the end of the Paleozoic. The Paleozoic thus
closed after 3.5 x 10(sup 8) years with major changes in world climate, intense
and widespread movement of continental plates, and attendant episodes of
mountain building.  See  CAMBRIAN; CARBONIFEROUS; DEVONIAN; ORDOVICIAN;
SILURIAN.
 �34s[James L. Wilson]
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�6v�2v�32sPalladium �21m �33s
    A chemical element, Pd, atomic number 46, and atomic weight 106.4.
Palladium is a white and very ductile metal which resembles platinum and
follows it in importance and abundance.  See  PLATINUM.
    Palladium supported on carbon or alumina is used as a catalyst in a number
of chemical processes including hydrogenation in liquid and gas-phase
reactions. Perhaps the largest single use of pure palladium is for low-current
electrical contacts. Palladium is unique in the number of metals with which it
can be alloyed, and it generally produces ductile solid solutions.
    Palladium is soft and ductile and may be fabricated into fine wire and
thin sheet. On heating at temperatures up to about 800 DGC (1472 DGF), palladium
forms a visible oxide tarnish, PdO. This oxide is thin and adherent, not
tending to scale or flake off. Above 800 DGC (1472 DGF), the oxide dissociates
and the metal is bright if cooled quickly to room temperature. The values of
some important properties are given in the table. Hydrogen is readily absorbed
by palladium and will diffuse through heated palladium at a relatively rapid
rate. This property is used in hydrogen purifiers, which pass hydrogen but no
other gas. Palladium in ordinary atmospheres is resistant to tarnish but will
tarnish slightly in outdoor exposure to sulfur-contaminated atmospheres. At
room temperature palladium is resistant to hydrofluoric, phosphoric,
perchloric, acetic, hydrochloric, and sulfuric acids as gases, but may be
attacked by some of them at 100 DGC (212 DGF).
    Palladium chlorides and related compounds are the most important.
Palladium chloride, PdCl2, is used in electrodeposition; it and related
chlorides are used in refining cycles and as sources of pure palladium sponge
in thermal decomposition processes. Palladium monoxide, PdO, and dihydroxide,
Pd(OH)2, are used as sources of palladium catalysts. Sodium
tetranitropalladate, Na2Pd(NO2)4, and other complex salts are used as the
basis of plating baths.
 �34s[Henry J. Albert]
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�6v�2v�32sPalm   �21m �33s 
    Any member of the family Arecaceae (Palmae) a large family of 
monocotyledons (Liliopsida), with about 3500 species. Most palms are trees 
with a slender, unbranched trunk and a terminal crown of large leaves that are 
folded between the veins, like a fan. As with other monocotyledonous trees, 
the woodiness of the stem derives not from the presence of great amounts of 
xylem but from the lignification of other tissues. A few palms have climbing 
stems with scattered leaves, and some others have very short stems so that the 
leaves arise at or near the surface of the ground. 
    It is said that there are more than 1000 uses for palms and their 
products. The coconut palm (Cocos nucifera) and the date palm (Phoenix 
dactylifera) are the two most important palms.  See  ARECALES; COCONUT; 
LILIOPSIDA.  
 �34s[Arthur Cronquist]
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�6v�2v�32sPalmitate �21m �33s
    A salt (soap) or ester of palmitic acid,

			   O
			  //
		   C15H31C-O-H

in which the acidic hydrogen has been replaced by a metal or an organic
radical. Palmitates occur in nature, chiefly as the glyceryl esters, found in
substantial amounts in animal and vegetable fats. The esters of long-chain
alcohols are known as waxes. Simple esters have limited uses, chiefly in
plastics. Alkali metal palmitates are soluble in water and, with the similar
stearates and oleates, are the major components of toilet and laundry soaps.
Other metal soaps are used in lubrication greases, pharmaceuticals, and
cosmetics, and as waterproofing agents and fungicides.	See DETERGENT; ESTER;
FAT AND OIL; SOAP.
 �34s[Elbert H. Hadley]
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�6v�2v�32sPalpigradida   �21m �33s 
    An order of rare arachnids comprising 21 known species from tropical and 
warm temperate regions. American species occur in Texas and California. All 
are mi-nute, whitish, eyeless animals, varying from 0.027 to 0.112 in. (0.68 
to 2.8 mm) in length, that live under stones, in caves, and in other moist, 
dark places. The elongate body terminates in a slender, multisegmented 
flagellum set with setae. In a curious reversal of function, the pedipalps, 
the second pair of head appendages, serve as walking legs. The first pair of 
true legs, longer than the others and set with sensory setae, has been 
converted to tactile appendages which are vibrated constantly to test the 
substratum.  See  ARACHNIDA. 
 �34s[Willis J. Gertsch]
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�6v�2v�32sPalynology   �21m �33s 
    The scope of palynology has enlarged from the study of spores and pollen 
to include other microscopic organisms and microscopic fragments of megascopic 
plants and animals that are observed in sediments. The modern palynologist is 
concerned with advanced techniques of the compound microscope, transmission, 
and scanning electron microscopy, and with infrared, fluorescent, and 
ultraviolet microscopy.  See  MICROSCOPE. 
    Palynology can be divided into two fields, neopalynology and 
paleopalynology. The former is concerned with extant microorganisms and 
disassociated microscopic parts of megaorganisms; the latter is concerned with 
their fossil counterparts.  See  MICROPALEONTOLOGY; POLLEN. 
    Fossil spores and pollen occur in many types of rock. They are most 
abundant in organic sediments such as peat and coal. They are absent or rare 
only in the coarse elastic sediments, in some of the chemically precipitated 
rocks, and in rocks or soils that have been oxidized or metamorphosed after 
lithification.  
 �34s[Leonard R. Wilson]
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�6v�2v�32sPancarida   �21m �33s 
    A superorder of the subclass Malacostraca. It is composed of crustaceans 
of small size, measuring 0.04 - 0.12 or 0.16 in. (1 - 3 or 4 mm), which carry 
their eggs and embryos in a brood pouch, or marsupium, that is dorsally 
located and formed by the carapace. 
    The body is cylindrical and eruciform and lacks an external division 
between the thorax and pleon. Eyes are absent and the cephalon is united with 
the first thoracomere; the carapace covers the first to third thoracomeres. 
The antennule is biramous and the antenna has an exopodite. The maxillipeds 
usually have two endites and an epipodite which is respiratory in function. 
Nephridia are lacking. 
    The group includes a unique order, Thermosbaenacea, with two families, 
Thermosbaenidae and Monodellidae. In the first, there is one genus and one 
species, Thermosbaena mirabilis; the second contains one genus, Monodella, and 
five species.  See  MALACOSTRACA. 
 �34s[Theodore Monod]
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�6v�2v�32sPancreas   �21m �33s 
    A composite gland in most vertebrates, containing both exocrine cells - 
which produce and secrete enzymes involved in digestion - and endocrine cells, 
arranged in separate islets which elaborate at least two distinct hormones, 
insulin and glucagon, both of which play a role in the regulation of 
metabolism, and particularly of carbohydrate metabolism.  See  CARBOHYDRATE 
METABOLISM.  
  
    Anatomy.  The pancreas of mammals shows large variations. The extremes are 
the unique, massive pancreas of humans, and the richly branched organ of the 
rabbit. Usually, the main duct, the duct of Wirsung, opens into the duodenum 
very close to the hepatic duct. In humans, the pancreas weighs about 2.5 oz 
(70 g). It can be divided into head, body, and tail. Accessory pancreases are 
frequently found anywhere along the small intestine, in the wall of the 
stomach, and in Meckel's diverticulum. 
    The exocrine portion of the pancreas shows tubuloalveolar glands. Each 
terminal alveolus is called an acinus. The various acini have central 
cavities, which open into intralobular ducts through narrow intercalated 
tubes. The interlobular ducts anastomose and ultimately form the main duct of 
Wirsung. The activity of the acini is stimulated by secretin as well as by 
pilocarpine.  
    The endocrine portion shows cellular masses called islands or islets of 
Langerhans, in which the cellular cords or masses are more or less isolated by 
irregular spaces filled with connective tissue and blood capillaries. The two 
main types of cells are the alpha and the beta cells. 
    Between the grapelike exocrine portion with its ducts and the islands of 
Langerhans, it is possible to observe connective tissue septa, numerous blood 
vessels, and nerves. 
  
    Physiology.  The pancreatic juice carried to the duodenum is a slightly 
alkaline liquid containing trypsinogen, which, when activated, causes the 
hydrolysis of the proteins into amino acids, amylase, and maltase, which act 
on the glucides, and lipase, which causes the hydrolysis of fatty substances. 
The intense stimulation of the pancreatic secretion after ingestion of food is 
considered to be the result of a nervous reflex originating in the mouth and 
also by direct introduction of acids and fats into the duodenum, causing the 
liberation of a hormone called secretin into the bloodstream to stimulate the 
exocrine secretion. 
  
    In 1889 J. Von Mering and O. Minkowsky were the first to remove completely 
the pancreas of a dog, thereby causing true diabetes. F. Banting and C. Best 
(1922) prepared pancreatic extracts which were able to prevent the lethal 
effects of pancreatectomy. The same effect was obtained with extracts from 
pancreas in which, after ligature of the duct of Wirsung, the exocrine portion 
of the gland had disappeared.  See  DIABETES. 
    The alpha cells and beta cells in the islets are the sources of two 
hormones, insulin from the beta, and glucagon, also known as the hyperglycemic 
factor, from the alpha. The former is a hormone which influences carbohydrate 
metabolism, enabling the organism to utilize sugar. The latter accelerates the 
conversion of liver glycogen into glucose. Glucagon elevates the blood sugar 
level, and its effects are the opposite of those of insulin, so that the two 
hormones together maintain the sugar metabolism of the body in balance. When 
the level of sugar in the blood becomes too low, the secretion of glucagon is 
stimulated. See GLUCAGON: INSULIN. 
 �34s[Ernest L. Van Campenhout]
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�6v�2v�32sPancreas disorders   �21m �33s 
    The pancreas is affected by a variety of congenital and acquired diseases. 
Because of its dual functional role, diseases may develop in the exocrine 
portion of the pancreas or in the endocrine portion. 
    The most frequent congenital lesion of the pancreas is more appropriately 
designated as a developmental abnormality ectopic or aberrant pancreas. 
Ectopic pancreas can be found anywhere within the gastrointestinal tract, but 
is more frequent in the stomach and duodenum.  
    Cystic fibrosis (mucoviscidosis) is a systemic disease in which mucus 
secretion is altered so that a viscid mucus is produced. The disease is 
inherited as a mendelian recessive. Cystic fibrosis affects all exocrine 
glands, including the acinar portion of the pancreas. Production of altered 
mucus leads to dilation of the exocrine ducts (cystic), destruction of acinar 
tissue, and replacement of the destroyed tissue by fibrous connective tissue 
(fibrosis). The islets are not affected by this disease. Elevation in 
secretion of sodium and chloride in sweat is also common. 
    Acute hemorrhagic pancreatitis is a serious disease of unknown etiology 
which causes sudden liberation of activated pancreatic enzymes that digest the 
pancreatic parenchyma. The digestive process leads to dissolution of fat and 
production of calcium soaps. In addition, rupture of pancreatic vessels occurs 
with resultant hemorrhage and shock. This disease is associated with biliary 
tract disease, especially gallstones (cholelithiasis), and with alcoholism. 
    Diabetes mellitus is the principal disease associated with the endocrine 
portion of the pancreas and is the fifth leading cause of death in the United 
States.  See  DIABETES. 
 �34s[H. Thomas Norris]
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�6v�2v�32sPanda   �21m �33s 
    The name for two species of carnivores in the raccoon family, Procyonidae, 
both of which are found in Asia. 
    The red panda (Ailurus fulgens) inhabits the forested regions of the 
Himalayas and western China. It has thick fur which gives it a heavy stocky 
appearance. Actually, it weighs an average of 10 - 12 lb (4.5 - 5.5 kg). The 
fur is black on the lower parts and red on the back. The face is white with a 
dark stripe from each eye to the corner of the mouth. The red panda is 
primarily a vegetarian, feeding on lichens, roots, and bamboo shoots. It is 
arboreal, nesting in a hollow tree or in the fork of a tree. 
    The giant panda (Ailuropoda melanoleuca) superficially resembles a bear 
and may be a true member of the Ursidae. Little is known about the natural 
history of this animal; its diet of bamboo restricts it to the bamboo forests 
located in cold, damp mountainous areas of central China. The adults weigh 200 
- 300 lb (90 - 135 kg) and are somewhat smaller than the brown bear.  See  
CARNIVORA.  
 �34s[Charles B. Curtin]
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�6v�2v�32sPandanales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Arecidae of the class Liliopsida (monocotyledons). The order 
contains only the single pantropical family Pandanaceae, with nearly 900 
species. They are more or less arborescent, sparingly branched monocotyledons 
with numerous, long, firm, narrow, parallel-veined leaves that usually have 
spiny margins. The flowers are small, very numerous, unisexual, and usually 
crowded into a dense spadix.  See  ARECIDAE; LILIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPanel heating and cooling   �21m �33s 
    A system in which the heat-emitting and heat-absorbing means is the 
surface of the ceiling, floor, or wall panels of the space which is to be 
environmentally conditioned. The heating or cooling medium may be air, water, 
or other fluid circulated in air spaces, conduits, or pipes within or attached 
to the panel structure. For heating only, electric current may flow through 
resistors in or on the panels.  See  ELECTRIC HEATING. 
    Heat energy is transmitted from a warmer to a cooler mass by conduction, 
convection, and radiation. The output from heating surfaces comprises both 
radiation and convection components in varying proportions. In panel heating 
systems, especially the ceiling type, the radiation component predominates.  
See  RADIANT HEATING. 
    When a panel system is used for cooling, the dew-point temperature of the 
ambient air must remain below the surface temperature of the heat-absorbing 
panels to avoid condensation of moisture on the panels. Panel cooling 
effectively prevents the disagreeable feeling of cold air blown against the 
body and minimizes the occurrence of summer colds.  See  COMFORT HEATING; 
HOT-WATER HEATING SYSTEMS. 
 �34s[Erwin L. Weber /Richard Koral]
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�6v�2v�32sPantodonta   �21m �33s 
    An extinct order of mammals which included the first large land animals of 
the Tertiary of the Northern Hemisphere. They filled some of the ecologic 
niches left open by the extinction of the dinosaurs and were well adapted to a 
life in swamps. Their feet had five toes, usually with broad hooves, but one 
genus (Titanoides) had powerful digging claws. Pantolambda, from the middle 
Paleocene of North America, is the most primitive pantodont. See MAMMALIA. 
 �34s[Malcolm C. McKenna]
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�6v�2v�32sPantograph �21m �33s
    A four-bar parallel linkage, with no links fixed, used as a copying device
for generating geometrically similar figures, larger or smaller in size,
within the limits of the mechanism. In the illustration the curve traced by
point T will be similar to that generated by point S. This similarity results
because points T and S will always lie on the straight line BAR OTS; triangles
OBS and TCS are always similar because lengths BAR OB, BAR BS, BAR CT, and BAR
CS are constant and BAR OB is always parallel to BAR CT.  Distance BAR OT always
maintains a constant proportion to distance BAR OS because of the similarity of
the above triangles.  Numerous modifications of the pantograph as a copying
device have been made.	See FOUR-BAR LINKAGE.
   A second use of the pantograph geometry is seen in the collapsible parallel
linkage used on electric locomotives and rail cars to keep a current-collector
bar or wheel in contact with an overhead wire.	Two such congruent linkages in
planes parallel to the train's motion are affixed securely on the top of the
locomotive with joining horizontal members perpendicular to each other.  The
uppermost member collects the current, and powerful springs thrust the
configuration upward with sufficient pressure normally to make low-resistance
contact from wire to collector.
 �34s[Douglas P.  Adams]
�33s


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�6v�2v�32sPantothenic acid �21m �33s
    A member of the B vitamin group with the structural formula



		     H CH3  OH	O  H H H
		     | |    |	|| | | |
		  HO-C-C----C---C--N-C-C-COOH
		     | |    |	     | |
		     H CH3  H	     H H


It is a light-yellow, viscous oil which is readily soluble in H2O. Pantothenic
acid is widely distributed, and liver, kidneys, fresh green vegetables, and
egg yolks are among its best sources. Losses of the vitamin during cooking are
minimal, as it is present in stable conjugated form in food.
    No definite pathologic lesions due to a specific pantothenic acid
deficiency have been reported in humans. The factors affecting the requirement
for pantothenic acid are probably similar to those altering the needs for the
other B vitamins. Pantothenic acid has been reported to improve the reactions
of young men to stress. The widespread occurrence of pantothenic acid assures
protection against the deficiency state under most conditions. Most Americans
eat 10 mg of pantothenic acid per 2500 cal of good diet. The daily requirement
is probably about 3 - 5 mg.  See  VITAMIN.
 �34s[Stanley N. Gershoff]
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�6v�2v�32sPantotheria   �21m �33s 
    The most primitive of the three infraclasses of the mammalian subclass 
Theria. The early members of this infraclass retained many reptilian features 
of the jaws, but by the end of the Jurassic they were fully mammalian and gave 
rise to the marsupials and placentals at about the beginning of the 
Cretaceous.  See  EUTHERIA; MAETATHERIA; THERIA. 
 �34s[Malcolm C. McKenna]
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�6v�2v�32sPapaverales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), 
subclass Magnoliidae of the class Magnoliopsida (dicotyledons). The order 
consists of only 2 families, Papaveraceae and Fumariaceae, with about 300 
species each. Within its subclass, the order is marked by its syncarpous 
gynoceium, parietal placentation, and only two (seldom three) sepals. Most of 
the species are herbaceous, and many of them contain isoquinoline alkaloids 
similar to those in the order Ranunculales. The Papaveraceae, with regular 
flowers, numerous stamens, and a well-developed latex system, include the 
poppies  (Papaver and related genera; see illustration), bloodroot, 
(Sanguinaria), and celandine (Chelidonium). The Fumariaceae, with four or six 
stamens, irregular flowers that usually have some of the petals spurred or 
saccate, and no latex system, include the bleeding heart (Dicentra 
spectabilis) and some other common ornamentals.  See  MAGNOLIIDAE; 
MAGNOLIOPSIDA; POPPY; RANUNCULALES. 
 �34s[Arthur Cronquist]
�33s 
  
  
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�6v�2v�32sPaper   �21m �33s 
    The term ``paper'' is traditionally applied to felted or matted sheets of 
cellulose fibers, formed on a fine wire screen from a dilute water suspension, 
and bonded together as the water is removed and the sheet is dried. In 
present-day usage, paper may also include sheet materials produced from other 
types of fibers (particularly mineral or synthetic) formed and bonded by other 
means.  
  
    Raw materials.  Of the many raw materials used by the paper industry, 
cellulose fibers occupy the dominant position although other fibrous 
materials, particularly some of the synthetic high polymers, are assuming 
increasing importance.  
    Wood is the primary source of cellulose fibers for papermaking. Before 
use, the wood must be reduced to the fibrous state; this operation is called 
pulping. Commercial pulping operations are of three principal types: 
mechanical, full chemical, and semichemical. 
    Mechanical pulping involves the reduction of wood to the fibrous state by 
purely mechanical means. Logs, of either 2- or 4-ft (0.6- or 1.2-m) length, 
are ground into pulp by means of large revolving grindstones. Except for a few 
water-soluble components, all the constituents of the wood are present in the 
ground wood pulp.  
    Full chemical pulping employs chemical reagents to effect a separation of 
the cellulose fibers from the other wood components. Wood chips are cooked 
with suitable chemicals, usually at elevated temperatures and pressures. The 
object is to dissolve the lignin and other extraneous compounds, leaving the 
cellulose intact and in fibrous form. The kraft or sulfate process is the most 
extensively employed. Here the active pulping ingredients are sodium hydroxide 
and sodium sulfide, in a strongly alkaline solution. Almost any wood species 
can be pulped by this process. 
    Semichemical pulping includes several processes in which mild chemical 
action, followed by mechanical attrition, is used.  These processes are 
employed largely, but not exclusively, on deciduous wood species. 
    Other sources of cellulose fibers are significant, although of secondary 
importance. Cotton rags were for many years the principal raw material of the 
industry. Grasses, reeds, and agricultural residues find considerable use 
principally in Europe and the Far East. The reuse of waste papers is common, 
particularly for the lower grades of paper products. 
  
    Manufacture.  Paper manufacture is largely a mechanical operation, 
although the chemical and physicochemical aspects are all-important in 
determining the final sheet properties. The tendency of cellulose fibers to 
bond together, when dried from a water suspension, is basic to papermaking 
technology.  
    Unmodified cellulose fibers, as obtained from pulping and bleaching 
operations, are generally unsuited for papermaking. They must first be 
refined; the refining operation is conducted mechanically in beaters or 
refiners.  
    During refining, the pulp fibers are separated, crushed, frayed, 
fibrillated, and cut. They imbibe water and swell, becoming more flexible and 
more pliable. Their capacity to bond with one another on drying is greatly 
enhanced, partly through modification of the fiber surfaces and partly because 
of the creation of new surface area. 
    A variety of additive materials are introduced to the papermaking pulps 
during stock preparation. Fillers, such as clays, titanium dioxide, or calcium 
carbonate are used for the control of sheet opacity and for other secondary 
reasons. Many paper grades are sized to impart a degree of water resistance; 
rosin is the most commonly used sizing agent. Dyestuffs are used extensively 
for color control, even with white sheets. Other additives, such as 
wet-strength agents, deflocculating agents, and defoamers, are used as they 
are needed. 
    The continuous paper machine converts a very dilute aqueous suspension of 
fibers and other ingredients into a dry sheet of paper at speeds which may 
vary from a few feet to over ** mi per minute. The machine may be one city 
block in length; even though the operations are complex, the principles 
involved are relatively few and straightforward. 
    The Fourdrinier machine is one of the two major sheetforming devices in 
widespread use. It consists in essence of a continuously moving wire belt or 
screen, to which the dilute papermaking slurry is fed and from which the wet, 
formed sheet is removed continuously. The slurry issues through the slice onto 
the wire, as a jet of uniform and proper thickness, speed, and consistency.  
    The wet sheet at about 20% consistency is removed from the wire and 
transferred to a supporting woolen felt, which carries it through a series of 
press rolls. Here more water is expressed mechanically, and the sheet 
progresses to the drier section at about 33% consistency. The remaining water 
is removed by evaporation. 
    The second major type of paper machine, the cylinder machine, differs from 
the Fourdrinier only in the forming part. Here, in place of the moving wire, 
one or a series of rotary cylindrical filters is used. Each screen-covered 
cylinder is mounted in a vat and operates partially submerged in the dilute 
papermaking slurry being supplied to it. As the cylinder revolves, water 
drains through the screen to the interior of the cylinder, and the wet sheet 
is formed. This sheet is removed at the top of the cylinder.  
    Many grades of paper receive some type of treatment after formation and 
drying in order to enhance certain desirable characteristics. Machine 
calendering is one of the most common operations. Here the sheet is passed 
through the nips formed by a series of steel rolls, one held on top of the 
other. Machine coating is practiced extensively on book and magazine papers. 
The coating is often clay, contained in a high-solids aqueous suspension which 
includes suitable adhesives and other ingredients. 
  
    Paper products.  The products of the paper industry are extremely varied. 
Throughout the centuries paper has retained its traditional uses, but the 
phenomenal growth of the industry during the past few decades has been due 
largely to new applications and uses of paper and paper-based materials. These 
new applications for the most part involve converted products. Two common 
machine converting operations were mentioned above. Supercalendering, 
embossing, coating, waxing, laminating, impregnating, saturating, corrugating, 
and printing are but a few of the common operations. The corrugated paperboard 
shipping container is used widely, and represents one of the largest single 
uses of paper. Multiwall sacks are used extensively for handling granular 
materials. Food packaging, with its trend toward the small unit packages, has 
led to extensive paper utilization, with the paper often coated, waxed, 
resin-impregnated, or combined with other foils and films. Building papers and 
boards constitute a sizable fraction of the industry output.  See  CELLULOSE; 
WOOD CHEMICALS. 
 �34s[Roy P. Whitney]
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�6v�2v�32sPaprika   �21m �33s 
    A type of pepper, Capsicum annuum, (order Polemoniales), with nonpungent 
flesh, grown for its long red fruit. It is of American origin, but is most 
popular in Hungary and adjacent countries. Seeds are removed from the mature 
fruit, and the flesh is dried and ground to prepare the dry condiment commonly 
referred to as paprika. California is the only important producing state in 
the United States.  See  PEPPER; POLEMONIALES. 
 �34s[H. John Carew]
�33s 
  
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�6v�2v�32spara-Aminobenzoic acid   �21m �33s 
    A compound also known as PABA, often considered to be a water-soluble 
vitamin. p-Aminobenzoic acid, with the structure below, is widely distributed 
in foods and has been isolated from liver, yeast, and other sources rich in 
vitamin B. p-Aminobenzoic acid is a part of the folic acid molecule, and its 
presence in a folic acid - deficient diet results in increased intestinal 
synthesis of the folic acid. p-Aminobenzoic acid antagonizes the 
bacteriostatic action of sulfonamides. It is an effective antirickettsial 
agent and has been used to treat typhus, scrub typhus, and Rocky Mountain 
spotted fever. See FOLIC ACID. 
 �34s[Stanley N. Gershoff]
�33s 
  
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�6v�2v�32spara-Aminosalicylic acid �21m �33s
    A chemical compound (PAS) used in the treatment of tuberculosis. The drug,
because of its ability to delay the emergence of resistant tubercle bacilli
when used in combination with either isonicotinic acid hydrazide or
streptomycin, is one of the mainstays in the combined therapy of tuberculosis.
The emergence of strains of organisms resistant to PAS, occasional allergic
reactions, and especially gastric irritation from the large quantities
necessary in the therapy of tuberculosis, limit its range of usefulness. See
CHEMOTHERAPY.
 �34s[Nicholas J. Giarman /Emanuel Grunberg]
�33s

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�6v�2v�32sParabola �21m �33s
    A member of the class of curves that are intersections of a plane with a
cone of revolution. It is obtained (see illustration) when the cutting plane
is parallel to an element of the cone.	See  CONIC SECTION.
    In analytic geometry the parabola is defined at the locus of points (in a
plane) equally distant from the fixed point F (focus) and a fixed line
(directrix) not through the point. It is symmetric about the line through F
perpendicular to the directrix.
    The curve has numerous other properties of interest in both pure and
applied mathematics. For example, the trajectory of an artillery shell,
assumed to be acted upon only by the force of gravity, is a parabola.
    Archimedes found that area bounded by an arc of a parabola and its chord;
for example, the area bounded by the parabola y(sup 2)	= 2cx and its latus
rectum (the chord through F perpendicular to the axis) is 2/3 c(sup 2).  See
PARABOLOID.
 �34s[Leonard M. Blumenthal]
�33s


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�6v�2v�32sParaboloid   �21m �33s 
    A quadric surface (a surface having an equation of second degree in three 
variables) which has an axis such that every section of the surface by a plane 
parallel to the axis is a parabola or straight line, whereas every section by 
a plane perpendicular to the axis is a central conic with center on the axis. 
If these central conics are all ellipses (or circles or a single point), the 
paraboloid is called an elliptic paraboloid (see illustration); and in 
particular if the ellipses are circles, the paraboloid is a paraboloid of 
revolution. Otherwise, these sections normal to the axis are hyperbolas (or a 
pair of lines through the axis), and the paraboloid is a hyperbolic paraboloid. 
    Paraboloids of revolution are used as reflecting surfaces in automobile 
headlights and reflecting telescopes. Rays parallel to the axis are all 
reflected through a single point, called the focus, and rays from the focus 
are reflected parallel to the axis.  See  ANALYTIC GEOMETRY; MIRROR OPTICS; 
PARABOLA; QUADRIC SURFACE; SURFACE AND SOLID OF REVOLUTION. 
 �34s[James S. Frame]
�33s 
  
  
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�6v�2v�32sParachute   �21m �33s 
    A flexible, lightweight structure, generally intended to retard the 
passage of an object within or through atmosphere by materially increasing the 
resistive surface. A parachute is a decelerator or air-braking device in the 
general form of an oblate hemisphere. The parachute is the only suitably 
demonstrated device for emergency descent from aircraft. It comprises a canopy 
and cords, which form the suspension and attachment between canopy and object. 
A parachute canopy is a membrane which relies upon pressure differential 
across it to maintain its inflated shape. The differential is created by 
entrapment of an air mass on the inside and movement of the air on the outside. 
 �34s[S. E. Weaver]
�33s 
  
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�6v�2v�32sParacrinoidea   �21m �33s 
    A class of extinct Crinozoa characterized by a theca of numerous plates 
irregularly arranged, uniserial armlike appendages, and no clear distinction 
between adoral and aboral surfaces. Their affinities seem to lie with both 
cystoids and crinoids. The known forms are of Ordovician age.  See  CRINOIDEA; 
CRINOZOA; CYSTOIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sParaffin   �21m �33s 
    A term used variously to describe either a waxlike substance or a group of 
compounds. The former use pertains to the high-boiling residue obtained from 
certain petroleum crudes. It is recovered by freezing out on a cold drum and 
is purified by crystallization from methyl ethyl ketone. Paraffin wax is a 
mixture of 26- to 30-carbon alkane hydrocarbons; it melts at 52 - 57DGC 
(126-135DGF). Microcrystalline wax contains compounds of higher molecular 
weight and has a melting point as high as 90DGC (190DGF). The name paraffin 
was formerly used to designate a group of hydrocarbons - now known as alkanes. 
  See  ALKANE; WAX, PETROLEUM. 
 �34s[Allen L. Hanson]
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�6v�2v�32sParagonimiasis   �21m �33s 
    Presence of the fluke Paragonimus westermani encapsulated in the lungs or 
other tissues of humans. The Orient is the classic endemic focus, but cases of 
human infection have occurred in the Americas. The disease is connected with 
eating raw or pickled fresh-water crustaceans. Eggs of the fluke are passed in 
sputum or feces, embryonate in water, and hatch into the ciliated miracidium, 
which penetrates into snails (Melania sp.). Cercariae finally emerge and 
encyst in the crustacean. When the crustacean is ingested by humans, the 
metacercariae migrate by the way of the abdominal cavity and the diaphragm to 
the lungs. Symptoms resemble those of various lung diseases, such as 
tuberculosis, bronchiectasis, and cancer.  See  DIGENEA. 
 �34s[Jose F. Maldonado]
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�6v�2v�32sParainfluenza virus   �21m �33s 
    A member of the paramyxovirus subgroup of the myxoviruses which are 
associated with a variety of respiratory illnesses. The virus particles range 
in size from 90 to 200 nanometers, agglutinate red blood cells, and (like the 
influenza viruses) contain a receptor-destroying enzyme. They differ from the 
influenza viruses in their large size, their possession of the larger 
ribonucleoprotein helix characteristic of the paramyxoviruses, their tendency 
to lyse as well as agglutinate erythrocytes, and their generally poor growth 
in eggs. In culture, these viruses grow well in primary monkey or human 
epithelial cells.  See  ANIMAL VIRUS; INFLUENZA; MYXOVIRUS; PARAMYXOVIRUS. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sParallax (astronomy) �21m �33s
    The difference in direction to a celestial object from two separated
points of observation. The distance between observational points is the base
line. The base line may be provided by the diurnal motion of Earth around its
axis, in which case the parallax is diurnal or geocentric; it may be provided
by the motion of the Earth in its orbit about the Sun, in which case the
parallax is annual or heliocentric; or it may be provided by the motion of the
solar system within the local galaxy, in which case the parallax is secular,
as illustrated.
    The size of Earth produces a geocentric parallax noticeable in
observations of objects within the solar system. Horizontal parallax p(sub h) is
defined as the angle between the directions to the object S at the horizon as
seen from the center C of Earth and from the observer's location O on the
surface of Earth. The mean equatorial horizontal parallax refers to Earth's
equatorial radius (3963 mi or 6378 km) as seen from the mean distance of the
celestial object.
    The size of the Earth's orbit produces a heliocentric parallax noticeable
in observations of the nearer stars. This parallax plays a fundamental role in
the determination of the distances to stars and is of basic importance in the
study of the physical properties of stars. Stellar parallax or annual parallax
is the maximal angle subtended by one astronomical unit at the star's location.
    The motion of the solar system seems attractive for the purpose of
measuring stellar distances, because time alone provides an indefinite
extension of the base line. However, there are severe limitations because of
the motions of the stars themselves; hence only values of the average parallax
for groups of stars, called mean secular parallaxes, may be measured. The
method has proved important to extending geometric knowledge of the Milky Way
system, in a statistical fashion, by measuring average distances up to several
thousand light-years for groups of stars.
 �34s[Peter van de Kamp]
�33s


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�6v�2v�32sParallel circuit   �21m �33s 
    An electric circuit in which the elements, branches (elements in series), 
or components are connected between two points with one of the two ends of 
each component connected to each point. The illustration shows a simple 
parallel circuit. In more complicated electric networks one or more branches 
of the network may be made up of various combinations of series or 
series-parallel elements.  See  CIRCUIT (ELECTRICITY). 
    In a parallel circuit the potential difference (voltage) across each 
component is the same. However, the current through each branch of the 
parallel circuit may be different. For example, the lights and outlets in a 
house are connected in parallel so that each load will have the same voltage 
(120 volts) but each load may draw a different current (0.50 ampere in a 
60-watt lamp and 10 amperes in a toaster). 
 �34s[Clarence F. Goodheart]
�33s 
  
  
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�6v�2v�32sParallelepiped   �21m �33s 
    A polyhedron having six faces that are parallel in pairs. Each face is a 
parallelogram (see illustration). If adjacent edges are perpendicular, the 
parallelepiped is called a rectangular parallelepiped - or in common speech, a 
rectangular box. The formula V = Bh means that the volume of any 
parallelepiped is equal to the product of its base (that is, the area of a 
face) times the altitude (that is, the distance from the base to the parallel 
face). For a rectangular parallelepiped, the volume is equal to the product of 
the lengths of three edges that meet at a vertex.  See  POLYHEDRON; PRISMATOID 
AND PRISMOID; SOLID (GEOMETRY). 
 �34s[James S. Frame]
�33s 
  
  
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�6v�2v�32sParallelogram �21m �33s
    A quadrilateral, or four-sided polygon, having (1) opposite sides parallel
(definition), or (2) opposite sides equal and coplanar, or (3) a pair of
opposite sides equal and parallel.  Its opposite angles are equal, its adjacent
angles are supplementary, and its diagonals bisect each other.	Its area is the
product of its base and altitude.  The parallelogram law for the addition of
vectors states that, if two vectors are represented by the directed segments
RIGHT ARROW AB and RIGHT ARROW AD, their vector sum is represented by the
directed diagonal RIGHT ARROW AC of the parallelogram ABCD (see illustration).
See QUADRILATERAL; RECTANGLE; SQUARE.

 �34s[James S. Frame]
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�6v�2v�32sParalysis agitans   �21m �33s 
    A syndrome of the central nervous system caused by diffuse pathology of 
the brain, resulting in tremor, rigidity, and other symptoms. 
    From an etiological viewpoint, the syndrome may be divided into paralysis 
agitans (Parkinson's disease), postencephalitic parkinsonism, and symptomatic 
paralysis agitans in combination with liver disease (Wilson's disease) and 
arteriosclerosis. The pathological lesions of all these disorders are similar 
and involve degenerative lesions in the basal ganglia of the cerebral 
hemispheres and in the cerebral cortex.  See  PARKINSON`S DISEASE. 
    The course is progressive, but decades may pass before patients are 
severely disabled. Therapy is symptomatic and includes drugs such as 
belladonna root and its various components, hyoscine, atropine, stramonium, 
and scopalamine.  
 �34s[Fredrick C. Redlich]
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�6v�2v�32sParamagnetic resonance �21m �33s
    Magnetic resonance arising from electrons in paramagnetic substances or
from electrons in paramagnetic centers in diamagnetic substances. For applied
fields of several thousand gauss (several tenths of a tesla) the electron
paramagnetic resonance (EPR) experiments are done at microwave frequencies,
commonly at 3-cm or at 1-cm wavelengths. In some instances, nuclear resonance
apparatus has been used with correspondingly lower applied fields. The most
sensitive apparatus detects approximately 10(sup 12) electron spins for a line
1 gauss (10-(sup 4) tesla) broad, a sensitivity far greater than obtained by
nonresonant methods, such as those utilizing paramagnetic susceptibility.  See
ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY; MAGNETIC RESONANCE.
 �34s[Charles P. Slichter]
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�6v�2v�32sParamagnetism �21m �33s
    A property exhibited by substances which, when placed in a magnetic field,
are magnetized parallel to the field to an extent proportional to the field
(except at very low temperatures or in extremely large magnetic fields).
Paramagnetic materials always have permeabilities greater than 1, but the
values are in general not nearly so great as those of ferromagnetic materials.
Paramagnetism is of two types, electronic and nuclear.
    The following types of substances are paramagnetic:
    1. All atoms and molecules which have an odd number of electrons.
According to quantum mechanics, such a system cannot have a total spin equal
to zero; therefore, each atom or molecule has a net magnetic moment which
arises from the electron spin angular momentum. Examples are organic free
radicals and gaseous nitric oxide.
    2. All free atoms and ions with unfilled inner electron shells and many of
these ions when in solids or in solution. Examples are transition, rare-earth,
and actinide elements and many of their salts. This includes ferromagnetic and
antiferromagnetic materials above their transition temperatures. For a
discussion of these materials  see  ANTIFERROMAGNETISM; FERRIMAGNETISM;
FERROMAGNETISM.
    3. Several miscellaneous compounds including molecular oxygen and organic
biradicals.
    4. Metals. In this case, the paramagnetism arises from the magnetic
moments associated with the spins of the conduction electrons and is called
Pauli paramagnetism.
    Relatively few substances are paramagnetic. Aside from the Pauli
paramagnetism found in metals,the most important paramagnetic effects are
found in the compounds of the transition and rare-earth elements which have
partially filled 3d and 4f electron shells respectively.
    Electronic paramagnetism arises in a substance if its atoms or molecules
possess a net electronic magnetic moment. The magnetization arises because of
the tendency of a magnetic field to orient the electronic magnetic moments
parallel to itself.  See  ELECTRON SPIN.
    Nuclear paramagnetism arises when there is a net magnetic moment due to
the magnetic moments of the nuclei in a substance. Nuclear magnetic moments
are about 10(sup 3) times smaller than electron magnetic moments. As a result,
nuclear paramagnetism produces effects 10(sup 6) times smaller than electron
paramagnetic or diamagnetic effects.  See  DIAMAGNETISM; MAGNETIC RESONANCE;
NUCLEAR MOMENTS.
 �34s[Elijah Adams ;Frederic Keffer]
�33s

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�6v�2v�32sParamecium   �21m �33s 
    A single-celled protozoan belonging to family Parameciidae of the order 
Holotrichia which have cilia that usually cover the entire body surface. There 
are about nine species. Some species are large and have a cosmopolitan 
distribution. The majority of species occur in fresh water, especially in 
stagnant pools, and are divided into two morphologically distinct groups: 
aurelia and bursaria. Included in the first group are elongate, cigar-shaped 
organisms which appear to be nearly circular in cross section, and which have 
a rounded posterior end, and a temporary structure known as the cytopyge or 
anal opening located on the side. The bursaria group contains organisms which 
are short and broad, flat in cross section, with a blunt-posterior end and a 
nearly terminal cytopyge. The beating cilia are the locomotor organelles which 
propel paramecia in a spirally rotating manner.  
    Asexual reproduction by transverse binary fission is characteristic. In 
conjugation, there is a temporary fusion of cells with an exchange of nuclear 
material. Conjugation alters the genetics of the exconjugants since there is 
an exchange of nuclear material involving two different physiological variants 
or mating types. Conjugation only occurs between different strains.  See  
HOLOTRICHIA; REPRODUCTION (ANIMAL). 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sParameter �21m �33s
    An auxiliary variable, functions of which give the coordinates of a curve
or surface. The coordinates of a curve are functions of one parameter. A curve
in 3-space has parametric equations (1).

	       x = f(t)  y = g(t)   z = h(t)				     (1)

    The coordinates of a surface are functions of two parameters, shown in
Eqs. (2).

	    x = f(u,v)	y = g(u,v)   z = h(u,v) 			     (2)

    An arbitrary constant in an equation is also called a parameter.
Variations in the values of the parameter generate a system of equations which
may represent a family of curves or surfaces. Such families are called
one-parameter, two-parameter, and so on, according to the number of
independent parameters.  See  PARAMETRIC EQUATION.
 �34s[Louis Brand]
�33s

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�6v�2v�32sParametric amplifier   �21m �33s 
    A highly sensitive low-noise amplifier for ultrahigh-frequency and 
microwave radio signals, utilizing as the active element an inductor or 
capacitor whose reactance is varied periodically at another microwave or 
ultrahigh frequency. A varactor diode is most commonly used as the variable 
reactor. Amplification of weak signal waves occurs through a nonlinear 
modulation or signal-mixing process which produces additional signal waves at 
other frequencies. This process may provide negative-resistance amplification 
for the applied signal wave and increased power in one or more of the new 
frequencies which are generated.  See  VARACTOR. 
    There are several possible circuit arrangements for obtaining useful 
parametric amplification. The two most common are the up-converter and the 
negative-resistance amplifier. In both types, the pump frequency is normally 
much higher than the input-signal frequency. In the up-converter, a new signal 
wave is generated at a higher power than the input wave. In the 
negative-resistance device, negative resistance is obtained for the 
input-signal frequency, causing an enhancement of signal power at the same 
frequency.  See  NEGATIVE-RESISTANCE CIRCUITS. 
    The most important advantage of the parametric amplifier is its low level 
of noise generation. The parametric amplifier finds its greatest use as the 
first stage at the input of microwave receivers where the utmost sensitivity 
is required. Its noise performance has been exceeded only by the maser. Maser 
amplifiers are normally operated under extreme refrigeration using liquid 
helium at about 4 K above absolute zero (-452DGF). The parametric amplifier 
does not require such refrigeration but in some cases cooling to very low 
temperatures has been used to give improved noise performance that is only 
slightly poorer than the maser.  See  AMPLIFIER; MASER. 
 �34s[M. E. Hines]
�33s 
  
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�6v�2v�32sParametric arrays   �21m �33s 
    Arrays of sources (or receivers) of sound formed by variation of 
appropriate parameters of the propagation medium. Normally, these parameters 
are the local sound speed and the particle velocity which vary because of the 
presence of large-amplitude pump, or primary, sound waves. 
    The usual parametric source configuration simply consists of a directional 
transducer (often a plane piston or planar array) driven at two frequencies 
near the transducer resonance, forming a dual-frequency sound beam called the 
primary beam. Because sound-wave propagation is not a completely linear 
process, signals at new frequencies are formed effectively through the 
interaction of sound with sound as the beam progresses and are generated along 
the length of the primary beam. The lowest of these new frequencies is the 
difference of the two primary frequencies, and so the primary beam acts as an 
end-fire array of sources at the difference frequency. The effective length of 
the array will be determined by the attenuation of the primary beam, which 
occurs either as a result of small-signal absorption or, for sufficiently high 
primary amplitudes, as a result of nonlinear losses due to the generation of 
harmonics of the primary frequencies and other intermodulation components, 
such as the sum-frequency component. 
    Most applications of parametric sources have been to underwater acoustics, 
but their use in air, as well as in other media, may be expected. Because the 
effective length of a parametric source can be made quite long in practice, it 
is possible to generate highly directional difference-frequency beams, and 
because the primary amplitude is shaded very gradually along the length of the 
array, these beams can be made practically side-lobe-free, in contrast to the 
beams from conventional acoustic sources. As a result, echoes from a 
parametric source exhibit practically no reverberation, whereas conventional 
echoes may be obscured by reverberation from reflection of the side lobes. 
Thus, the parametric source may be expected to be useful in 
reverberation-limited situations where one desires a narrow beam from a small 
projector. Such applications include precision fathometry, subbottom 
profiling, echo ranging, communications, and Doppler navigation logs. In order 
to obtain the advantages of a parametric source, however, one must be willing 
to tolerate low efficiency and low search rate.  See  DIRECTIVITY; ECHO 
SOUNDER; SONAR; SOUND; UNDERWATER SOUND; UNDERWATER TRANSDUCER. 
 �34s[Mark B. Moffett]
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�6v�2v�32sParametric equation �21m �33s
    A type of mathematical equation used, typically, to represent curves in a
plane or in space of three diminesions. In principle, however, there is no
limitation to any particular number of dimensions. A parameter is actually an
independent variable.  In elementary analytic geometry a curve in the xy plane
is often studied, in the first instance, as the locus of an equation y = F(x) or
G(x,y) = 0.  The form y = F(x) is not adequate for the complete representation
of certain curves, whereas the form G(x,y) = 0 may be adequate.  The circle
x(sup 2) + y(sup 2) - 16 = 0 affords an example.  But the form G(x,y) = 0 is not
always convenient.  The parametric form x = f(t), y = g(t) is often the most
convenient; moreover, it is often the naturally occurring form of representation
of the curve.  For the circle x(sup 2) + y(sup 2) - 16 = 0, one possible
parametric representation is x = 4 cos t and y = 4 sin t.
  A pair of equations x = f(t), y = g(t), where f and g are continuous functions
defined for some interval of values of t, for example, a </= t </= b, is said to
define a parametric curve.  If one thinks of t as time, the equations define the
motion of the point (x,y) as t increases from a to b.  Clearly the path can
cross itself, double back on itself, or the point may even remain motionless.
  A parametric surface in space of three dimensions is defined by x = f(u,v),
y = g(u,v), z = h(u,v), where f, g, h are continuous functions of the two
parameters, u,v.  See ANALYTIC GEOMETRY; CALCULUS; PARTIAL DIFFERENTIATION.
 �34s[Angus E.  Taylor]
�33s

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�6v�2v�32sParamo   �21m �33s 
    A biological community, essentially a grassland, covering extensive high 
areas in equatorial mountains of the Western Hemisphere. Geographically, 
paramos are limited to the Northern Andes and adjacent mountains. Paramos 
occur in alpine regions above timberline and are controlled by a complex of 
climatic and soil factors peculiar to mountains near the Equator. The richly 
diverse flora and the fauna of the paramos are adapted to severely cold, 
mostly wet conditions. Humans have found some paramos suitable for living and 
use. Since paramos were defined originally in Spanish-speaking countries, no 
English equivalent has been given to the term.  See  GRASSLAND. 
 �34s[Harriet G. Barclay]
�33s 
  
  
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�6v�2v�32sParamyxovirus   �21m �33s 
    A subgroup of myxoviruses that includes the viruses of mumps, measles, 
parainfluenza, respiratory syncytial (RS) disease, and Newcastle disease. Like 
influenza viruses, the paramyxoviruses are ribonucleic acid (RNA) - containing 
viruses and possess an ether-sensitive lipoprotein envelope.  See  ANIMAL 
VIRUS; MEASLES; MUMPS; MYXOVIRUS; NEWCASTLE DISEASE; PARAINFLUENZA. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sParanoia   �21m �33s 
    A mental disease characterized by logically systematized delusions of 
persecution. The chief symptoms are systematic, consistent, and persistent 
delusions of persecution, far exceeding character traits of hypersensitivity, 
suspicions and querulousness, irascibility, and pettiness which are found in 
paranoid personalities. The hostility, seclusiveness, and cynicism of 
paranoiacs make contact with them difficult. The cause of paranoia is not 
clearly established.  See  PARANOID STATE; PSYCHOSIS. 
 �34s[Fredrick C. Redlich]
�33s 
  
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�6v�2v�32sParanoid state   �21m �33s 
    A severe and chronic psychosis with predominant delusions of persecution. 
The delusions, in contrast to those of paranoia, are inconsistent, illogical, 
and often accompanied by hallucinations. Otherwise, the personality does not 
show the disintegration which occurs in schizophrenia. However, some schools 
of psychiatry do not separate this mental disorder from the schizophrenias.  
See  PARANOIA; SCHIZOPHRENIA.  
 �34s[Fredrick C. Redlich]
�33s 
  
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�6v�2v�32sParapertussis   �21m �33s 
    An infection of the human respiratory tract caused by the bacillus 
Bordetella parapertussis. It resembles mild pertussis, from which it can be 
distinguished only  by bacteriological methods. The disease is widespread and 
is often confused with mild whooping cough. The course of the disease is about 
3 weeks and complications are rare. Only symptomatic care is usually necessary. 
 �34s[William L. Bradford]
�33s 
  
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�6v�2v�32sParasitic castration   �21m �33s 
    Destruction of the reproductive organs by parasites. Direct castration 
occurs when parasites penetrate the sex organs and feed on them, as trematodes 
do in the gonads of mollusks. Indirect castration results when parasites cause 
the gonads to atrophy. Some parasitic barnacles cause the regression of gonads 
of crabs but do not actually invade the gonads. Changes in the secondary sex 
characters commonly result. 
    In plants, the term parasitic castration has been employed to describe the 
action of smut fungi in the grain of wheat and the conversion of stamens and 
carpels into petals in plants infested by nematodes.  See  RHIZOCEPHALA; SMUT 
(MICROBIOLOGY).  
 �34s[Placidus G. Reischmann]
�33s 
  
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�6v�2v�32sParasitic oscillation   �21m �33s 
    An undesired oscillation which may occur in any type of circuit, such as 
an audio-, video-, or radio-frequency amplifier, oscillator, modulator, or 
pulse waveform generating circuit. For example, it often happens that with no 
apparent input signal to an amplifier an output voltage of considerable 
magnitude is obtained. The amplifier may be oscillating because some part of 
the output is inadvertently being fed back into the input. This feedback may 
result from the output impedance of the power supply. If feedback does occur 
through the power supply impedance, the oscillations can usually be stopped by 
the use of appropriately placed decoupling networks.  See  AMPLIFIER; FEEDBACK 
CIRCUIT; OSCILLATOR. 
    Feedback may also occur through the interelectrode capacitance from grid 
to plate of a tube, through lead inductances, stray wiring, and other paths, 
which are often difficult to determine exactly. Parasitic oscillations 
represent a waste of power, a distortion of the desired waveform, or a 
complete malfunctioning of the circuit. Hence these oscillations must be 
eliminated. This can usually be accomplished by a change in circuit 
parameters, a rearrangement of wiring, some additional bypassing or shielding, 
a change of tube or transistor, the use of an rf (radio-frequency) inductor in 
the plate circuit, the use of rf chokes in series with filament lead, and 
other methods.  See  ELECTRICAL NOISE; ELECTRICAL SHIELDING; GROUNDING. 
 �34s[Jacob Millman]
�33s 
  
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�6v�2v�32sParasitology   �21m �33s 
    A branch of biology which deals with those organisms, plant or animal, 
which have become dependent on other living creatures. The essential criterion 
of parasitism is dependency, the loss of freedom to live an independent 
existence; all degrees of dependency obtain, from transitory symbiosis to 
complete helplessness. There is a direct correlation between the degree of 
dependency and the extent of parasitism. Plants are primarily free-living and 
consequently parasitism is far more prevalent in the animal kingdom. Here it 
is both extensive and intensive, since few, if any, species are free from 
attack and since the parasites are often present in enormous numbers. 
Parasitism has appeared in every phylum of animals, where the individuals 
themselves are parasites or serve as hosts to parasitic forms. Some parasites 
are temporary or facultative, but most are obligatory, that is, unable to 
survive apart from their hosts. Parasitism involves a gradual and progressive 
adaptation on the part of the parasite, and recovery of an independent status 
becomes increasingly difficult.  
    Host animals are more or less seriously affected by parasitic infestation, 
but if well nourished, they may not manifest patent illness. If 
undernourished, the host becomes increasingly debilitated and the development 
or activity of the sexual organs is inhibited; parasitic castration is the 
ultimate result. Parasites may occlude ducts, produce lesions that permit 
bacterial invasion, or produce substances toxic to the host.  See  MEDICAL 
PARASITOLOGY; PARASITIC CASTRATION. 
 �34s[Horace W. Stunkard]
�33s 
  
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�6v�2v�32sParasympathetic nervous system   �21m �33s 
    A portion of the autonomic system. It consists of two neuron chains, but 
differs from the sympathetic nervous system in that the first neuron has a 
long axon and synapses with the second neuron near or in the organ innervated. 
In general, its action is in opposition to that of the sympathetic nervous 
system, which is the other part of the autonomic system. It cannot be said 
that one system, the sympathetic, always has a excitatory role and the other, 
the parasympathetic, an inhibitory role; the situation depends on the organ in 
question. However, it may be said that the sympathetic system, by altering the 
level at which various organs function, enables the body to rise to emergency 
demands encountered in flight, combat, pursuit, and pain. The parasympathetic 
system appears to be in control during such pleasant periods as digestion and 
rest. The alkaloid pilocarpine excites parasympathetic activity while atropine 
inhibits it.  See  AUTONOMIC NERVOUS SYSTEM; SYMPATHETIC NERVOUS SYSTEM. 
 �34s[Douglas B. Webster]
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�6v�2v�32sParathyroid gland   �21m �33s 
    An endocrine organ usually associated with the thyroid gland in the neck 
region and possessed by all vertebrates except the fishes. In response to 
lowered serum calcium concentration, a hormone is produced which promotes bone 
destruction and inhibits the phosphorus-conserving activity of the kidneys. 
    In humans, there are typically four glands situated as shown in the 
illustration; however, the number varies between three and six, with four 
appearing about 80% of the time. Variations in the positioning of the glands 
along the craniocaudal axis occur but, excepting parathyroid III which may 
occasionally be found upon the anterior surface of the trachea, the relation 
to the posterior surface of the thyroid is rarely lost. 
 �34s[William E. Dossel]
�33s 
    The site of action of the parathyroid hormone has not been finally 
settled. Three viewpoints have been proposed: the renal (phosphaturic) theory, 
the bone theory, and the citrate theory. 
    The renal theory is based on the finding that hypoparathyroid subjects 
respond to parathyroid hormone by a prompt urinary excretion of phosphate. By 
directly acting on the kidney, the hormone promotes the elimination of 
phosphate, resulting in a lowering of serum phosphate, withdrawal of 
phosphorus from the skeleton, and mobilization of calcium, thus elevating the 
levels of serum calcium. 
    The bone theory proposes that the primary effect of parathyroid hormone is 
stimulation of osteoclastic activity of bone. The preponderance of evidence 
supports the view that the parathyroid glands function in calcium homeostasis 
through a primary effect of the hormone on bone. 
    According to this theory, the hormone causes an increase of the 
concentration of citrate ion in the serum which, in turn, exerts a powerful 
solubilizing action on hydroxylapatite and on bone minerals. None of these 
theories has been established definitively. It appears likely that several or 
all of these mechanisms may be operative simultaneously.  See  ENDOCRINE 
SYSTEM (VERTEBRATE); PARATHYROID HORMONE. 
 �34s[C. Donnell Turner]
�33s 
  
  
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�6v�2v�32sParathyroid hormone   �21m �33s 
    The only known secretory product of the parathyroid glands. It is a 
polypeptide of 76 amino acids. The hormone is of prime importance in 
regulating calcium and phosphate metabolism, and thereby bone turnover. It is 
released into the bloodstream whenever the calcium concentration in blood 
plasma falls below normal levels. The released hormone acting upon three 
organs, kidney, bone, and intestinal tract, brings about the restoration of a 
normal blood calcium concentration. The hormone increases the absorption of 
calcium and phosphate in the intestine, retention of calcium and excretion of 
phosphate by the kidney, and resorption of both calcium and phosphate from 
bone; and inhibits the formation of new bone. The net result of all these  
changes is to elevate plasma calcium and depress plasma phosphate. Thus a 
negative feedback relationship exists between the action of parathyroid 
hormone, the blood calcium level, and the rate of parathyroid hormone 
secretion.  See  BONE; CALCIUM METABOLISM; PARATHYROID GLAND; PHOSPHATE 
METABOLISM.  
 �34s[Howard Rasmussen]
�33s 
  
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�6v�2v�32sParazoa   �21m �33s 
    A name proposed for a subkingdom of animals which includes the sponges. 
Erection of a separate subkingdom for the sponges implies that they originated 
from protozoan ancestors independently of all other Metazoa. This theory is 
supported by the uniqueness of the sponge body plan and by peculiarities of 
fertilization and development. Much importance is given to the fact that 
during the development of sponges with parenchymella larvae, the flagellated 
external cells of the larva take up an internal position as choanocytes after 
metamorphosis, whereas the epidermal and mesenchymal cells arise from what was 
an internal mass of cells in the larva. These facts suggest that either the 
germ layers of sponges are reversed in comparison with those of other Metazoa 
or the choanocytes cannot be homologized with the endoderm of other animals. 
Either interpretation supports the wide separation of sponges from all other 
Metazoa to form the subkingdom Parazoa or Enantiozoa.  See  METAZOA; PORIFERA. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sParenchyma   �21m �33s 
    A ground tissue chiefly concerned with the manufacture and storage of 
food. The primary functions of plants, such as photosynthesis, assimilation, 
respiration, storage, secretion, and excretion - those associated with living 
protoplasm - proceed mainly in parenchymal cells. Parenchyma is frequently 
found as a homogeneous tissue in stems, roots, leaves, and flower parts. Other 
tissues, such as sclerenchyma, xylem, and phloem, seem to be embedded in a 
matrix of paranchyma; hence the use of the term ground tissue with regard to 
parenchyma is derived. The parenchymal cell is one of the most frequently 
occurring cell types in the plant kingdom.  See  PLANT ANATOMY; PLANT 
PHYSIOLOGY.  
    Typical parenchyma occurs in pith and cortex of roots and stems as a 
relatively undifferentiated tissue composed of polyhedral cells that may be 
more or less compactly arranged and show little variation in size or shape. 
The mesophyll, that is, the tissue located between the upper and lower 
epidermis of leaves, is a specially differentiated parenchyma called 
chlorenchyma because its cells contain chlorophyll in distinct chloroplastids. 
    This chlorenchymatous tissue is the major locus of photosynthetic activity 
and consequently is one of the more important variants of parenchyma. 
Specialized secretory parenchymal cells are found lining resin ducts and other 
secretory structures.  See  PHOTOSYNTHESIS; SECRETORY STRUCTURES (PLANT). 
 �34s[Robert L. Hulbary]
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�6v�2v�32sParesis, general   �21m �33s 
    An inflammatory and degenerative disease of the brain caused by an 
infection with Treponema pallidum. It is also called syphilitic 
meningoencephalitis. The disease is characterized by psychotic behavior with a 
profound and progressive intellectual and emotional deterioration and 
neurological changes. The disease, unless treated, takes a rapid downhill 
course and results in death.  See  PSYCHOSIS; SYPHILIS. 
 �34s[Fredrick C. Redlich]
�33s 
  
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�6v�2v�32sParesthesia   �21m �33s 
    One of several designations given to abnormally intense and disagreeable 
pain arising from apparently trivial stimulation. Other terms used to identify 
the same symptom are spontaneous pain, overreaction or overresponse, paradoxic 
pain, hyperpathia, protopathic pain, dysesthesia, and central pain. Indeed, 
there are more different names available than there are good descriptions of 
the pain.  See  PAIN. 
 �34s[Frank A. Geldard]
�33s 
  
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�6v�2v�32sParity (quantum mechanics) �21m �33s
    A physical property of a wave function which specifies the wave function's
behavior under simultaneous reflection of all spatial coordinates of the wave
function through the origin, that is, when x is replaced by -x, y by -y, and z
by -z. If the wave function PSI satisfies Eq. (1), it is said to have even
parity. If, on the other hand, Eq.(2) holds, the wave function is said to have
odd parity. These two expressions can be combined in Eq. (3), where P =
PLUS/MINUS 1 is a quantum number having only the two values +1 (designated as
even parity) and -1 (odd parity). The physical property defined by P is
quantized and is called parity. More precisely, parity is defined as the
eigenvalue of the operation of space inversion. Parity is a concept that has
meaning only for waves and therefore has no meaning in classical particle
physics.

	    PSI (x,y,x) = PSI(-x,-y,-z) 				     (1)


	    PSI (x,y,z) = -PSI(-x,-y,-z)				     (2)


	    PSI(x,y,z) = P PSI(-x,-y,-z)				     (3)

    Parity does have a meaning for the Schroedinger wave function of a
particle in quantum mechanics. It likewise has meaning for the wave function
of any system.	See  QUANTUM MECHANICS.
    The conservation of parity is a consequence of the inversion symmetry of
space. Thus, parity would be conserved if the statement of physical laws were
independent of the handedness of the coordinate system used. Of course, the
fact that most people are right-handed is not a physical law but an accident
of evolution; there is nothing in the laws of physics which favors a
right-handed over a left-handed human. The same holds for optically active
organic compounds, such as the amino acids. However, the statement that the
neutrino is left-handed is a physical law.
    All the strong interactions between elementary particles (for example,
nuclear forces) and the electromagnetic interactions are symmetrical to
inversion, so that parity is conserved by these interactions. As far as is
known, only the BETA-interactions (which involve neutrinos) and the other weak
interactions are not symmetrical to inversion and do not conserve parity. The
weak interactions contribute little to all processes except the decays of
elementary particles (including BETA-decay of nuclei), so that in all other
processes parity is very nearly conserved.  See  ELEMENTARY PARTICLE;
FUNDAMENTAL INTERACTIONS.
    Atomic and nuclear energy states are characterized by a definite parity
(which may be different for different energy states of the same nucleus), and
the conservation of parity has an important bearing on atomic and nuclear
reactions.
    One of the selection rules which derives from parity conservation is the
following:  The same spin zero boson cannot decay both into two PI-mesons and
three PI-mesons, because these final states have opposite parities, even and odd
respectively.  But the positive K-meson is observed to do just this:  It has
both the K(PI 2) and K(PI 3) decay modes, and its spin is zero as deduced from
the distribution of momenta in the K(PI 3) mode.  Thus the conclusion is that
parity is not conserved in this decay.	In 1956, T.  D.  Lee and C.  N.  Yang
made the bold hypothesis that parity also is not conserved in BETA-decay.  They
reasoned that the magnitude of the BETA-decay coupling is about the same as the
coupling which leads to decay of the K-meson, so these decay processes may be
manifestations of a single kind of coupling.  Also, there is a very natural way
to introduce parity nonconservation in BETA-decay, namely by assuming a
restriction on the possible states of the neutrino (two-component theory).  See
NEUTRINO.

 �34s[Charles J. Goebel]
�33s


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�6v�2v�32sParkinson's disease   �21m �33s 
    A chronic, progressive disorder of the central nervous system, marked by 
slow movement and muscluar rigidity, weakness, and tremor at rest. 
    The typical patient is wide-eyed, staring, has a masklike face, walks with 
short shuffling steps, the body bent forward and the arms held stiffly at the 
sides. Tremor is marked when a limb is at rest and may become so pronounced 
that involuntary ``pill-rolling'' movements of the fingers and thumb appear. 
The tremor usually disappears upon movement, to be replaced by a cogwheel type 
of motion. Unless other brain areas have been affected by the precipitating 
agent, no mental changes are seen. 
    The disorder is slowly progressive and leads to increasing incapacity. 
Treatment has been largely symptomatic and directed toward the relief of pain 
and cramps and the development of the best mental and occupational situation 
possible under the circumstances. Developments in neurosurgery and 
chemotherapy have produced dramatic results in certain cases. Excellent 
reversal of the Parkinson state has been achieved by treatment with L-dopa. 
 �34s[N. Karle Mottet]
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�6v�2v�32sParrot   �21m �33s 
    Any member of the single family Psittacidae, which comprises the order 
Psittaciformes. There are about 316 species in 82 genera and all are 
essentially similar, with the exception of a few extreme forms. In addition to 
the forms commonly known as parrots, the family includes the parakeets, 
cockatoos, macaws, and lories. 
    One of the outstanding features of this group is the bill, which is short, 
stout, and strongly hooked and is used, along with the feet, in climbing. The 
degree of mobility of the parrots' upper jaw exceeds that seen in most other 
birds. These birds are fruit and seed eaters and have a well-developed crop. 
The wings are strong and rounded and while most species can fly rapidly for 
short distances, a few species have almost lost the ability to fly. Parrots 
are typically birds of forested areas and are distributed in large numbers 
throughout the tropics in Africa, Asia, South America, and Australia. 
Australian parakeets live on the ground principally, but perch and nest in 
trees. Plumage is brightly colored and most commonly green, but many species 
are multicolored. 
    Many species of this family are an important source of an infection of 
humans known as psittacosis, which has a high incidence among bird breeders 
and handlers.  See  PSITTACIFORMES; PSITTACOSIS. 
 �34s[Charles B. Curtin]
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�6v�2v�32sParsec �21m �33s
    A unit of measure of astronomical distances. One parsec is equivalent to
3.084 x 10(sup 13) kilometers, or 1.916 x 10(sup 13) miles. There are 3.26
light-years in 1 parsec. The parsec is defined as the distance at which the
semimajor axis of Earth's orbit around the Sun (1 astronomical unit) subtends
1 second of arc. Thus, because the angle is small, the equation below holds. A
parsec is then 206,265 astronomical units. At a distance of 1 parsec, the
parallax is 1 second of arc. The nearest star is about 1.3 parsecs distant;
the farthest known galaxy is several billion parsecs.  See  PARALLAX
(ASTRONOMY).

	   1 astronomical unit/ 1 parsec  = 1 second =	1/206,265


 �34s[Jesse L. Greenstein]
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�6v�2v�32sParsley   �21m �33s 
    A biennial, Petroselinum crispum, of European origin belonging to the 
plant order Umbellales. Parsley is grown for its foliage and is used to 
garnish and flavor foods. It contains large quantities of vitamins A and C. 
Two types, plain-leafed and curled, are grown for their foliage; Hamburg 
parsley (P. crispum var. tuberosum), also called turnip-rooted parsley, is 
grown for its edible parsniplike root.  See  UMBELLALES. 
 �34s[H. John Carew]
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�6v�2v�32sParsnip   �21m �33s 
    A hardy biennial, Pastinaca sativa, of Mediterranean origin belonging to 
the plant order Umbellales. The parsnip is grown for its thickened taproot and 
is used primarily as a cooked vegetable. Exposure of mature roots to low 
temperatures, not necessarily freezing, improves the quality of the root by 
favoring the conversion of starch to sugar.  See  UMBELLALES. 
 �34s[H. John Carew]
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�6v�2v�32sParthenocarpy   �21m �33s 
    The development of fruit without fertilization of the ovule. There are two 
types of natural parthenocarpy. Vegetative parthenocarpy is fruit set and 
development without the stimulation of either pollination or fertilization, so 
that it is endogenously controlled. Stimulative parthenocarpy requires 
pollination, but not fertilization, of the ovule for fruiting to occur. 
Examples of fruit in the first category are oriental persimmons, cultivated 
banana, Washington Navel orange, and pineapple; and in the second category, 
Black Corinth grape. 
    Three generic classes of chemical regulators are involved in 
parthenocarpy: auxins, cytokinins, and gibberellins. Depending on the stage of 
fruiting, each chemical regulator may have an independent action or may be 
dependent on another regulator for a fruit to respond and proceed through a 
normal developmental sequence.  See  AUXIN; CYTOKININS; FRUIT; GIBBERELLIN. 
 �34s[Robert H. Biggs]
�33s 
  
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�6v�2v�32sParthenogenesis   �21m �33s 
    A special type of sexual reproduction in which an egg develops without 
entrance of a sperm. It is common among rotifers, plant lice or aphids, 
thrips, many ants, bees, and wasps, and some crustaceans. Males are unknown in 
certain thrips and rotifers. Queen honeybees produce drones, or males, by 
parthenogenesis but also lay fertilized eggs that yield females, the workers, 
and queens. Aphids have successive generations of parthenogenetic females in 
spring and summer, then produce both sexes by parthenogenesis. These later 
mate; the females lay fertilized eggs that hatch in spring as females, and 
parthenogenesis begins. 
 �34s[Tracy I. Storer]
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�6v�2v�32sPartial differentiation �21m �33s
    A mathematical operation performed on functions of more than one variable.
In this article only two or three variables are considered; however, the
principles apply to functions of n variables, for any positive integer n > 1.
If z = f(x,y), the partial derivative  dz/dx is defined as the derivative of
f(x,y) with respect to x, y being regarded as fixed; that is,

	    CURLY Dz/CURLY Dx = lim (h --> 0) [f(x+h,y)-f(x,y)]/h

Another notation for CURLY Dz/CURLY Dx is f1(x,y). The other first partial
derivative is CURLY Dz/CURLY Dy, also written f2(x,y).	For values at particular
points the notation is

	      (CURLY Dz/CURLY Dy) (in terms of a,b) = f1(a,b)

In the case of a function of three variables, f(x,y,z), the expression is

		     CURLY Dz/CURLY Dy = f3(x,y,z)

    The second derivatives of f(x,y) are given by

	    f11(x,y) = (CURLY D/CURLY Dx)(CURLY Df/CURLY Dx)
	    f12(x,y) = (CURLY D/CURLY Dy)(CURLY Df/CURLY Dx)
	    f21(x,y) = (CURLY D/CURLY Dy)(CURLY Df/CURLY Dy)
	    f22(x,y) = (CURLY D/CURLY Dy)(CURLY Df/CURLY Dy)


It can happen that f12(x,y) NOT EQUAL f21(x,y), but this will not happen in
common practice, especially with elementary functions.	If f1, f2, f12, f21 are
defined in neighborhood of (a,b), and if f12, f21 are continuous at (a,b), then
f12(a,b) = f21(a,b).  In addition, there are more delicate theorems relating to
this matter.  The notion of the differentiability of a function is fundamental
in the theory of partial differentiation.  The requirement that f(x,y) be
differentiable is not the same as the requirement that f1(x,y) and f2(x,y) both
exist; it is a more inclusive requirement.  The geometric meaning of f being
differentiable at (a,b) is that the surface defined by z = f(x,y) has a tangent
plane not parallel to the z axis when x = a, y = b.  In analytic terms the
condition is that if

	  epsilon = f(ath, btk) - f(a,b) - f1(a,b)h - f2(a,b)k

then

	    lim [(h,k) -> (0,0)] epsilon/(|h| + |k|) = 0

A sufficient condition that f be differentiable at (a,b) is that the partial
derivatives f1, f2 be defined at all points near (a,b), and continuous at (a,b).
    The prime importance of the differentiability concept is that the
differentiability property is needed in proving the chain rule for functions
of several variables. This rule asserts that a differentiable function of a
differentiable function is differentiable, and the rule tells how to compute
partial derivatives of the composite function. For example, if x = f(s,t), y =
g(s,t), where f and g are differentiable, and if z = F(x,y), where F is
differentiable, then the composite function is G(s,t)= F[f(s,t),g(s,t)]. Then
z = G(s,t) is differentiable as a function of s and t, and

	    CURLY DG/CURLY Ds = (CURLY DF/CURLY Dx)(CURLY Df/CURLY Ds) +
	    (CURLY DF/CURLY Dy)(CURLY Dg/CURLY Ds)

These equations, expressing the formal part of the chain rule, are often
written in the form

	    CURLY Dz/CURLY Ds = (CURLY Dz/CURLY Dx)(CURLY Dx/CURLY Ds) +
	    (CURLY Dz/CURLY Dy)(CURLY Dy/CURLY Ds)

	    CURLY Dz/CURLY Dt = (CURLY Dz/CURLY Dx)(CURLY Dx/CURLY Dt) +
	    (CURLY Dz/CURLY Dy)(CURLY Dy/CURLY Dt)

See CALCULUS; DIFFERENTIATION; PARAMETRIC EQUATION.
 �34s[Angus E. Taylor]
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�6v�2v�32sPartial tone   �21m �33s 
    A simple sinusoidal component of a complex tone. It may be part of a 
complex physical oscillation; alternatively, a partial tone (or partial) is a 
component of a sound sensation, distinguished as a simple tone that cannot be 
further analyzed by the ear and that contributes to the timbre of the complex 
sound.  See  HEARING (HUMAN); TONE (MUSIC AND ACOUSTICS). 
    The physical sound from a violin string, for example, is usually composed 
of a number of partials. Each of these partials of the airborne sound results 
from vibration in a number of equal parts that takes place when the string 
vibrates as a whole. Such characteristic vibrations by aliquot parts are also 
called partials; they are components of the complex motion of the string. Each 
such characteristic vibration pattern can also occur individually, however. 
Thus it seems preferable to call these characteristic motions modes of 
vibration rather than partials, to make it clear that they can exist 
independently without being parts of a complex motion.  See  MODE OF 
VIBRATION; VIBRATION. 
    If a string is bowed steadily, the frequencies of the partials of the 
resulting complex tone will be integral multiples of the lowest (fundamental) 
frequency, and the partials may properly be called harmonics. If, however, the 
same string is struck or plucked and then allowed to vibrate freely, the 
frequencies of the corresponding modes of vibration are, in general, no longer 
exactly in the ratios of integers, and the partials and modes of vibration are 
inharmonic.  See  HARMONIC (PERIODIC PHENOMENA). 
 �34s[Robert W. Young]
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�6v�2v�32sParticle   �21m �33s 
    In classical mechanics, a body having finite mass but negligible 
extension. A particle has inertia and possesses gravitational properties. 
Because of its negligible extension, forces acting on a particle cannot cause 
rotational acceleration; therefore, the motion of a particle is regarded as 
one of pure translation. An extended body is composed of particles.  See  
RIDID BODY; RIGID-BODY DYNAMICS. 
    The term particle is also used in physics as a synonym for ``elementary 
particle.''  See  ELEMENTARY PARTICLE. 
 �34s[Dudley Williams]
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�6v�2v�32sParticle accelerator   �21m �33s 
    An electrical device which accelerates charged atomic or subatomic 
particles to high energies. The particles may be charged either positively or 
negatively. If subatomic, the particles are usually electrons or protons and, 
if atomic, are charged ions of various elements and their isotopes throughout 
the entire periodic table of the elements. 
    Accelerators that produce various subatomic particles at high intensity 
have many practical applications in industry and medicine as well as in basic 
research. Electrostatic generators, pulse transformer sets, cyclotrons, and 
electron linear accelerators are used to produce high levels of various kinds 
of radiation that in turn can be used to polymerize plastics, provide 
bacterial sterilization without heating, and manufacture radioisotopes which 
are utilized in industry and medicine for direct treatment of some illnesses 
as well as research. They can also be used to provide high-intensity beams of 
x-rays with extreme penetrating power that can be used for cancer therapy, as 
well as for x-ray radiographic determination of flaws and structural problems 
in heavy industrial steel castings and other types of structures.  See  
ISOTOPIC IRRADIATION; NONDESTRUCTIVE TESTING; RADIATION BIOLOGY; RADIATION 
CHEMISTRY; RADIOACTIVITY AND RADIATION APPLICATIONS; RADIOGRAPHY; RADIOISOTOPE 
PRODUCTION; RADIOLOGY. 
    Particle accelerators fall into two general classes - electrostatic 
accelerators that provide a steady dc potential, and varieties of accelerators 
that employ various combinations of time-varying electric and magnetic fields. 
  
    Electrostatic accelerators.  Electrostatic accelerators in the simplest 
form either accelerate the charged particle from the source of high voltage to 
ground potential or from ground potential to the source of high voltage. The 
maximum energy available from this kind of accelerator is limited by the 
ability of the voltage generator to provide some maximum high voltage.  See  
ELECTROSTATIC ACCELERATOR. 
  
    Time-varying field accelerators.  In contrast to the high-voltage-type 
accelerator which accelerates particles in a continuous stream through a 
continuously maintained potential, the time-varying accelerators must 
necessarily accelerate particles in small discrete groups or bunches. 
    An accelerator that varies only in electric field and does not use any 
magnetic guide or turning field is customarily referred to as a linear 
accelerator or linac. In the simplest version of this kind of accelerator, the 
electrodes that are used to attract and accelerate the particles are connected 
to a radio-frequency (rf) power supply or oscillator in such a way that 
alternate electrodes are of opposite polarity. In this way each successive gap 
between adjacent electrodes is alternately accelerating and decelerating. If 
these acceleration gaps are appropriately spaced to accommodate the increasing 
velocity of the accelerated particle, the frequency can be adjusted so that 
the particle bunches are always experiencing an accelerating electric field as 
they cross each successive gap. In this way modest voltages can be used to 
accelerate bunches of particles indefinitely, limited only by the physical 
length of the accelerator construction. Some of the larger linacs are over a 
mile in length.  See  LINEAR ACCELERATOR. 
    As accelerators are carried to a higher and higher energy, a linac 
eventually reaches some practical construction limit because of length. This 
problem of extreme length can be circumvented conveniently by accelerating the 
particles in a circular path maintained by either static or time-varying 
magnetic fields. Accelerators utilizing steady magnetic fields. Accelerators 
utilizing steady magnetic fields as guide paths are usually referred to as 
cyclotrons or synchrocyclotrons, and are arranged to provide a steady magnetic 
field over realtively large areas that allow the particles to travel in a 
circular orbit of gradually increasing diameter as they increase in energy.  
See  CYCLOTRON; MAGNETIC FIELD. 
    Practical limitations of magnet construction have kept the size of 
circular proton accelerators with static magnetic fields to the vicinity of 
100 to 1000 MeV. For even higher energies, up to 400 GeV per nucleon in the 
maximum-size proton accelerator in operation, it is necessary to vary the 
magnetic field as well as the electric field in time. In this way the magnetic 
field can be of a minimal practical size, which is still quite extensive for a 
400-GeV accelerator. This circular magnetic containment region or ``race 
track'' is injected with relatively low-energy particles that can coast around 
the magnetic ring when it is at minimum field strength. The magnetic field is 
then gradually increased to stay in step with the higher magnetic rigidity of 
the particles as they are gradually accelerated with a time-varying electric 
field.  See  SYNCHROTRON. 
  
    Superconducting magnets.  The study of the fundamental structure of nature 
and all associated basic research require an ever increasing energy in order 
to allow finer and finer measurements on the basic structure of matter. Since 
the voltage-varying and magnetic-field-varying accelerators also have limits 
to their maximum size in terms of cost and practical construction problems, 
the only way to increase particle energies even further is to provide 
higher-varying magnetic fields through superconducting magnet technology, 
which can extend electromagnetic capability by a factor of  4 to 5.  See  
SUPERCONDUCTING DEVICES. 
  
    Storage rings.  Beyond this limit the only other possibility is to 
accelerate particles in opposite directions and arrange for them to collide at 
certain selected intersection regions around the accelerator. The main 
technical problem is to provide adequate numbers of particles in the two 
colliding beams so that the probability of a collision is moderately high. 
Such storage ring facilities are in operation for both electrons and protons, 
and design and construction of much larger ones have been undertaken.  See  
STORAGE RINGS. 
  
    Collective accelerators.  Development of a completely different kind of 
accelerator, the collective-effect accelerator, has been undertaken in several 
countries. The idea of a collective accelerator is to accelerate a cloud or 
bunch of electrons containing within the electrostatic well established by its 
cloud structure one or a few protons or heavier atoms in such a way that the 
electrical forces providing the containment are sufficiently strong to 
literally drag the heavy particle along.  See  ELECTRON RING ACCELERATOR. 
 �34s[Harvey E. Wegner]
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�6v�2v�32sParticle detector   �21m �33s 
    A device used to detect and measure radiations characteristically emitted 
in nuclear processes, including GAMMA- or x-rays, lightweight charged 
particles (electrons or positrons), nuclear constituents (neutrons, protons, 
and heavier ions), and subnuclear constituents such as mesons. The device is 
also known as a radiation detector. Detectors are essential tools for the 
discovery of radioactive minerals, for all studies of the structure of matter 
at the atomic, nuclear, and subnuclear levels, and for protection from the 
effects of radiation. They have also become important practical tools in the 
analysis of materials using the techniques of neutron activation and x-ray 
fluorescence analysis. 
    A convenient way to classify radiation detectors is according to their 
mode of use: (1) For detailed observation of individual photons or particles, 
a pulse detector is used to convert each such event (that is, photon or 
particle) into an electrical signal. (2) To measure the average rate of 
events, a mean current detector, such as an ion chamber, is often used. (3) 
Position-sensitive detectors are used to provide information on the location 
of particles or photons in the plane of the detector. (4) Track-imaging 
detectors image the whole three-dimensional structure of a particle's track. 
(5) The time when a particle passes through a detector or a photon interacts 
in it is measured by a timing detector. Such information is used to determine 
the velocity of particles and when observing the time relationship between 
events in more than one detector. 
    The ionization produced by a charged particle is the effect most commonly 
employed in a particle detector. In the basic type of gas ionization detector, 
an electric field applied between two electrodes separates and collects the 
electrons and positive ions produced in the gas by the radiation to be 
measured. Track-imaging detectors rely on a secondary effect of the ionization 
along a particle's track to reveal its structure. 
    In semiconductor detectors, a solid replaces the gas. The ``insulating'' 
region (depletion layer) of a reverse-biased pn junction in a semiconductor is 
employed.  See  BUBBLE CHAMBER; CLOUD CHAMBER; IONIZATION CHAMBER; JUNCTION 
DETECTOR.  
    In addition to producing free electrons and ions, the passage of a charged 
particle through matter temporarily raises electrons in the material into 
excited states. When these electrons fall back into their normal state, light 
may be emitted and detected as in the scintillation detector.  See  LIQUID 
SCINTILLATION DETECTOR; SCINTILLATION COUNTER. 
    Neutral particles, such as neutrons, cannot be detected directly by 
ionization. Consequently, they must be converted into charged particles by a 
suitable process and then observed by detecting the ionization caused by these 
particles.  
    Although ionization detectors dominate the field, a number of detector 
types based on other radiation-induced effects are used. Notable examples are: 
(1) transition radiation detectors, which depend on the x-rays and light 
emitted when a particle passes through the interface between two media of 
different refractive indices; (2) track detectors, in which the damage caused 
by charged particles in plastic films and in minerals is revealed by etching 
procedures; (3) thermo- and radiophoto-luminescent detectors, which rely on 
the latent effects of radiation in creating traps in a material or in creating 
trapped charge; and (4) Cerenkov detectors, which depend on measurement of the 
light produced by passage of a particle whose velocity is greater than the 
velocity of light in the detector medium.  See  CERENKOV RADIATION. 
 �34s[Fred S. Goulding]
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�6v�2v�32sParticle properties   �21m �33s 
    Particles are solids or liquids in a subdivided state. Because of this 
subdivision, particles exhibit many special characteristics which are 
negligible in the bulk material. A powder is a bulk assemblage of fine, dry, 
solid particles. A dispersoid is a suspension of particles in a fluid. An 
aerosol is a suspension of particles, either liquid or solid, in a gas; the 
term usually refers to a dilute suspension of fine particles. Correspondingly, 
a hydrosol is a suspension of particles in a liquid. 
    The characteristics of particles are primarily related to their size, 
although other factors, such as shape, density, and composition, may also be 
important. Size is generally expressed in terms of some representative, 
average, or effective dimension of the particle. The most widely used unit of 
particle size is the micrometer. Another common method is to designate the 
screen mesh that has an aperture corresponding to the particle size. The 
screen mesh usually refers to the number of screen openings per unit length or 
area; several screen standards are in general use. 
    Natural and synthetic particles rarely consist of particles of only one 
size; a range of particle size is usually encountered. Thus, to describe a 
particulate material it is necessary to specify not only the size and shape of 
individual particles but also the relative amounts of each size. This is known 
as a size distribution, obtained by means of a particle-size analysis. A wide 
variety of particle-size analysis methods is available. Most methods of size 
analysis do not measure particle size. Instead, they measure some other 
properties of the particles which are then converted to an apparent or 
effective size by means of analytical or empirical relationships. 
    Mechanical dispersoids are formed by comminution, decrepitation, or 
disintegration of larger masses of material, as by grinding of solids or 
spraying of liquids, and usually involve a wide particle-size distribution. 
Condensed dispersoids are formed by condensation of the vapor phase or as the 
product of a vapor-phase reaction; these are usually very fine and often 
relatively uniform in size. Condensed dispersoids and fine mechanical 
dispersoids generally tend to flocculate or agglomerate; this action forms 
loose clusters of larger particle size. 
    If a particle suspended in a fluid is acted upon by a force, it will 
accelerate to a terminal velocity at which the resisting force due to fluid 
friction just balances the applied force. If a particle falls under the action 
of gravity, this velocity is known as the terminal gravitational settling 
velocity.  
    Particles suspended in a fluid partake of the molecular motion of the 
suspending fluid and hence acquire diffusional characteristics analogous to 
those of the fluid molecules. This random zigzag motion of the particles, 
commonly known as Brownian motion, is obvious under the microscope for 
particles smaller than 1 micrometer.  See  AIR FILTER; ATOMIZATION; BROWNIAN 
MOVEMENT; COLLOID; FLUIDIZATION; SIZE REDUCTION. 
 �34s[Charles E. Lapple]
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�6v�2v�32sParticle track etching �21m �33s
    A technique of selective chemical etching to reveal tracks of heavy
nuclear particles in a wide variety of solid substances. Developed in order to
see fossil particle tracks in extraterrestrial materials, the technique finds
application in many fields of science and technology.
    An etchable track is produced if the charged particle has a sufficiently
high radiation-damage rate and if the damaged region in the solid is
permanently localized. Thus only highly ionizing particles are detectable;
only nonconductors record tracks; and radiation-sensitive plastics can detect
lighter particles than can radiation-insensitive minerals and glasses. The
conical shape of the etched track depends on the ratio of the rate of etching
along the track to the bulk etching rate of the solid.
    The lunar surface, meteorites, and other objects exposed in space have
been irradiated by charged particles from a variety of sources in the Sun and
the Galaxy. Comparison of fossil particle tracks in lunar rocks and meteorites
with spacecraft measurements of present-day radiations has established that
solar flares and galactic cosmic rays have not changed over the last 2 x
10(sup 7) years - the typical time a lunar rock exists before being shattered by
impacting interplanetary debris.
    Studies of tracks in a piece of glass from the Surveyor 3 spacecraft after
a 2.6-year exposure on the lunar surface, and of tracks in plastic detectors
exposed briefly above the Earth's atmosphere in rockets, have led to the
surprising discovery that the Sun preferentially ejects heavy elements in its
flares rather than an unbiased sample of its atmosphere. The existence of
galactic cosmic rays with atomic number greater than 30 was discovered in 1966
when fossil particle tracks were first studied in meteorites. Several
particles heavier than uranium have been detected, indicating that cosmic rays
originate in sources where synthesis has proceeded explosively beyond uranium.
  See  COSMIC RAYS.
    Unique advantages of etched-track detectors in nuclear and elementary
particle physics are their ability to distinguish heavy-particle events in a
large background of lightly ionizing radiation and their ability to detect
individual rare events by a specialized technique such as electric-spark
scanning or ammonia penetration through etched holes. These advantages have
permitted such advances as the measurement of very long fission half-lives and
the discovery of ternary fission.  See	NUCLEAR FISSION; SUPERTRANSURANICS;
TRANSURANIUM ELEMENTS.
    The spontaneous fission (sup 238)U, present as a trace-element purity,
gives tracks that can be used to date terrestrial samples ranging from rocks
to human artifacts. Because fission tracks are erased in a particular mineral
at a well-defined temperature, one can use the apparent fission-track ages as
a function of distance from the heat source to measure the thermal (tectonic)
history of regions.  See  FISSION TRACK DATING.
    Filters are produced by irradiating thin plastic sheets with fission
fragments and then etching holes to the desired size. Uses include biological
research, wine filtration, and virus sizing.  A uranium exploration method
relies on a survey of radon emanation, as measured by alpha-particle tracks in
plastic detectors, to locate promising locations in which to drill. Plastic
detectors are also used in conjunction with a beam of high-energy heavy ions
to take radiographs of cancer patients that reveal details not detectable in
x-rays.
 �34s[P. Buford Price, Jr.]
�33s

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�6v�2v�32sPascal's law   �21m �33s 
    A law of physics which states that a confined fluid transmits externally 
applied pressure uniformly in all directions. More exactly, in a static fluid, 
force is transmitted at the velocity of sound throughout the fluid. The force 
acts normal to any surface. This natural phenomenon is the basis of the 
pneumatic tire, balloon, hydraulic jack, and related devices.  See  
HYDROSTATICS.  
 �34s[Karl Arnstein ;Robert S. Ross]
�33s 
  
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�6v�2v�32sPaschen-Back effect �21m �33s
    An effect on spectral lines obtained when the light source is placed in a
very strong magnetic field, first explained by F. Paschen and E. Back in 1921.
In such a field the anomalous Zeeman effect, which is obtained with weaker
fields, changes over to what is, in a first approximation, the normal Zeeman
effect. The term ``very strong field'' is a relative one, since the field
strength required depends on the particular lines being investigated. It must
be strong enough to produce a magnetic splitting that is large compared to the
separation of the components of the spin-orbit multiplet.  See	ATOMIC
STRUCTURE AND SPECTRA; ZEEMAN EFFECT.
 �34s[Francis A. Jenkins /William Watson]
�33s

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�6v�2v�32sPasseriformes   �21m �33s 
    The order of perching birds (an inadequate name used for lack of a better 
inclusive English name), containing about 5300 species, or more than half of 
the living species of birds. It comprises two major divisions: a somewhat more 
primitive group collectively known as suboscines, usually divided into three 
suborders (Eurylaimi, Tyranni, and Menurae); and the large suborder Oscines or 
Passeres, the songbirds. Separation of the Oscines from the suboscines is 
based chiefly on the structure of the syrinx and its attached muscles.  See  
AVES; PERCHING BIRDS. 
 �34s[Kenneth C. Parkes]
�33s 
  
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�6v�2v�32sPassive radar   �21m �33s 
    A technique for surveillance, mapping, navigation, and guidance that 
employs the reception of microwave-frequency energy radiated by warm bodies, 
or reflected from other sources.  See  MICROWAVE. 
    Passive radar has some similarity to both passive infrared systems and 
active (radiating) radar systems. Like infrared systems, passive radar emits 
no energy of its own, and therefore does not give away its position or 
existence. Target resolution is inferior to that of active radar and infrared 
systems. However, the ability of a passive radar system to discriminate 
between different targets and backgrounds can be better than that of either 
active radar or infrared. 
    The ability of a passive radar to discriminate between different objects 
depends primarily on (1) the apparent temperature differential between the 
objects (this takes into account emissivity and reflectivity), (2) the grazing 
angle between the antenna beam and the object, (3) antenna polarization, (4) 
antenna beam width, and (5) the minimum detectable signal level of the 
receiver. The grazing angle, or angle of incidence, is a factor when viewing 
smooth objects, such as bodies of water. Polarization is most pronounced at 
small grazing angles. The effect of polarization varies with the object, 
providing an additional method of discriminating between objects at the same 
temperature.  See  HEAT RADIATION; RADAR; REFLECTION OF ELECTROMAGNETIC 
RADIATION.  
 �34s[Philip J. Klass]
�33s 
  
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�6v�2v�32sPasteurization   �21m �33s 
    The application of mild heat for a specified time to a liquid food or 
beverage to enhance its keeping properties and to destroy any harmful 
microorganisms present.  For milk, the times and temperatures employed are 
based upon the thermal tolerance of Mycobacterium tuberculosis, one of the 
most heat-resistant of non-spore-forming pathogens. A temperature of either 
143DGF (61.7DGC) for 30 min or of 161DGF (71.7DGC) for 15 s is employed. 
Vegetative cells of most bacteria are killed by the heat treatment, but 
endospores are unaffected. 
 �34s[Charles F. Niven, Jr.]
�33s 
  
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�6v�2v�32sPatent   �21m �33s 
    Common designation for letters patent, which is a certificate of grant by 
a government of an exclusive right with respect to an invention for a limited 
period of time. A United States patent confers the right to exclude others 
from making, using, or selling the patented subject matter in the United 
States and its territories. Portions of those rights deriving naturally from 
it may be licensed separately, as the rights to use, to make, to have made, 
and to lease. Any violation of this right is an infringement. 
    An essential substantive condition which must be satisfied before a patent 
will be granted is the presence of patentable invention or discovery. To be 
patentable, an invention or discovery must relate to a prescribed category of 
contribution, such as process, machine, manufacture, composition of matter, 
plant, or design. In the United States there are different classes of patents 
for different members of these categories. 
 �34s[Donald W. Banner]
�33s 
  
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�6v�2v�32sPaterinida   �21m �33s 
    A small order of inarticulated brachiopods that is relatively common in 
Cambrian rocks and ranges into beds of Middle Ordovician age. Paterinids 
characteristically have a thin shell of calcium phosphate. Both valves are 
convex, the pedicle valve usually being the higher, and both typically have 
well-developed propareas. Internally, the lateral margins of the delthyrium 
are commonly thickened, but other features are poorly developed and the muscle 
scars are extremely faint.  See  BRACHIOPODA; INARTICULATA. 
 �34s[A. J. Rowell]
�33s 
  
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�6v�2v�32sPathogen   �21m �33s 
    Any agent capable of causing disease. The term pathogen is usually 
restricted to living agents, which include viruses, rickettsia, bacteria, 
fungi, yeasts, protozoa, helminths, and certain insect larval stages. See 
DISEASE.  
    Pathogenicity is the ability of an organism to enter a host and cause 
disease. The degree of pathogenicity, that is, the comparative ability to 
cause disease, is known as virulence. The terms pathogenic and nonpathogenic 
refer to the relative virulence of the organism or its ability to cause 
disease under certain conditions. This ability depends not only upon the 
properties of the organism but also upon the ability of the host to defend 
itself (its immunity) and prevent injury. The concept of pathogenicity and 
virulence has no meaning without reference to a specific host. For example, 
gonococcus is capable of causing gonorrhea in humans but not in lower animals. 
  See  MEDICAL MYCOLOGY; MEDICAL PARASITOLOGY; PLANT PATHOLOGY; PLANT VIRUSES 
AND VIROIDS; VIRULENCE. 
 �34s[Daniel N. Lapedes]
�33s 
  
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�6v�2v�32sPathology   �21m �33s 
    The study of the etiologies, mechanisms, and manifestations of disease. 
Techniques and knowledge gained from other disciplines, including anatomy, 
physiology, microbiology, biochemistry, and histology, are utilized. The 
information obtained from the study of pathology is necessary prior to 
developing methods with which to control and prevent disease. 
    With the light microscope it became possible to correlate the observed 
signs and symptoms in an individual with cellular changes. In its early stages 
pathology was very descriptive. Diseases were understood and categorized in 
part, by how gross and microscopic anatomy was altered. In the last half of 
the 19th century, by using this approach to pathology, coupled with 
microbiological techniques, it was learned that the major causes of human 
death were biotic agents: protozoans, bacteria, viruses, and fungi. Infectious 
diseases took a heavy toll in human lives. Better sanitation and public health 
measures were instrumental in controlling these diseases, and the production 
of antibiotics and immunization procedures further reduced their importance. 
It is now apparent that all diseases reflect changes at the molecular level. 
Scientists are beginning to understand what these biochemical alterations are 
in some diseases. 
    There are many branches of pathology. Divisions are made depending upon 
focus of interest. Clinical pathology is concerned with diagnosis of disease. 
As medicine has expanded, subspecialties such as surgical pathology and 
neuropathology have developed. Experimental pathology attempts to study 
disease mechanisms under controlled conditions. General pathology covers all 
areas, but in less detail, and serves in medical education. 
    A relatively new area of pathology is environmental pathology, which deals 
with disease processes resulting from physical and chemical agents. At 
present, the leading causes of death have environmental agents as the known or 
suspected major etiologic factors; these diseases include heart disease, 
atherosclerosis, and cancer. It is believed that with understanding, many such 
disease, like those produced in response to biotic agents, can be brought 
under control.  See  DISEASE. 
 �34s[N. Karle Mottet ;Carol Quaife]
�33s 
  
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�6v�2v�32sPathotoxin   �21m �33s 
    A chemical of biological origin, other than an enzyme, that plays an 
important causal role in a plant disease. Most pathotoxins are produced by 
plant pathogenic fungi or bacteria, but some are produced by higher plants, 
and one has been reported to be the product of an interaction between a plant 
and a bacterial pathogen. Some pathogen-produced pathotoxins are highly 
selective in that they cause severe damage and typical disease symptoms only 
on plants susceptible to the pathogens that produce them. Others are 
nonselective and are equally toxic to plants susceptible or resistant to the 
pathogen involved. A few pathotoxins are species-selective, and are damaging 
to many but not all plant species. In these instances, some plants resistant 
to the pathogen are sensitive to its toxic product.  See  PLANT PATHOLOGY; 
TOXIN.  
 �34s[Harry Wheeler]
�33s 
  
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�6v�2v�32sPattern recognition   �21m �33s 
    The construction of algorithms capable of processing the range of input 
patterns of interest to yield a set of output patterns such that the 
occurrence of each input causes a uniquely identifiable output pattern to 
occur.  
    It is essentially a decoding process in which discriminations are made. In 
simpler cases tree methods of serial segmentation find value. Ideally, such 
measures enable one to divide remaining patterns at each decision node into 
two disjoint patterns, similar in principle to the basis for binary decoding. 
    Another approach makes use of as many measures as possible that appear to 
be related to the desired discriminations. A search is then made for an 
appropriate differential weighting of these measures so as to obtain maximal 
spacing between patterns. This approach is sometimes referred to as factor 
analysis. See AUTOMATA THEORY 
 �34s[Norman B. Reilly]
�33s 
  
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�6v�2v�32sPauropoda   �21m �33s 
    A class, and perhaps the most obscure group, of the Myriapoda. They are 
pale creatures, no more than 0.04 - 0.08 in. (1 - 2 mm) in length, inhabiting 
damp situations in leaf litter, under bark, stones and debris, and in humus 
and similar detritus. Apparently very widely distributed as a class, they have 
been undiscovered only in deserts and in the arctic and antarctic regions. 
    Like millipedes, they are progoneate and have one pair of maxillae, and 
their trunk segments display a certain degree of amalgamation. Their peculiar 
bifurcate antennae and adult complement of 12 trunk segments with 9 pairs of 
functional legs are distinctive within the myriapod complex (see 
illustration). All pauropods lack eyes, spiracles, tracheae, and a circulatory 
system.  
    The class currently consists of 2 families with less than 10 genera; there 
are probably fewer than 60 species known.  See  MYRIAPODA. 
 �34s[Ralph E. Crabill, Jr.]
�33s 
  
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�6v�2v�32sPavement �21m �33s
    An artificial surface laid over the ground to facilitate travel. A
pavement's ability to support loads depends primarily upon the magnitude of
the load, how often it is applied, the supporting power of the soil
underneath, and the type and thickness of the pavement structure. Before the
necessary thickness of a pavement can be calculated, the volume, type, and
weight of the traffic (the traffic load) and the physical characteristics of
the underlying soil must be determined.
    Once the grading operation has been completed and the subgrade compacted,
construction of the pavement can begin. Pavements are either flexible or
rigid. Flexible pavements, which are composed of aggregate (sand, gravel, or
crushed stone) and bituminous material (see illustration), have less
resistance to bending than do rigid pavements, which are made of concrete.
Both types can be designed to withstand heavy traffic. Selection of the type
of pavement depends, among other things, upon (1) estimated construction
costs; (2) experience of the highway agency doing the work with each of the
two types; (3) availability of contractors experienced in building each type;
(4) anticipated yearly maintenance costs; and (5) experience of the owner in
maintenance of each type.  See	CONCRETE; HIGHWAY ENGINEERING.
 �34s[Archie N. Carter]
�33s

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�6v�2v�32sPawl   �21m �33s 
    The driving link or holding link of a ratchet mechanism, also called a 
click or detent. In the illustration the driving pawl at A, forced upward by 
lever B, engages the teeth of the ratchet wheel and rotates it 
counterclockwise. Holding pawl C prevents clockwise rotation of the wheel when 
the pawl at A is making its return stroke. Pawl and ratchet are an open, upper 
pair.  
    Driving and holding pawls likewise engage rack teeth on the plunger of a 
ratchet lifting jack, such as those supplied with automobiles. A ratchet wheel 
with a holding pawl only, acting as a safety brake, is fastened to the drum of 
a capstan, winch, or other powered hoisting device. 
    A double pawl can drive in either direction or be easily reversed in 
holding. A cam pawl prevents the wheel from turning clockwise by a wedging 
action while permitting free counterclockwise rotation. This technique is used 
in the automobile hill-holder to prevent the vehicle's rolling backward.  See  
ESCAPEMENT; RATCHET. 
 �34s[Douglas P. Adams]
�33s 
  
  
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�6v�2v�32sPaxillosina   �21m �33s 
    A suborder of Phanerozonida with pointed tube feet which lack suckers and 
with upper marginals directly over the lower ones when both series are 
present. Paxillae usually cover the upper surface of the body. Of the two 
included families the Porcellanasteridae are notable as essentially deep-water 
forms. The Astropectinidae occur in all seas from tidal level downward.  See  
PHANEROZONIDA.  
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sPea   �21m �33s 
    The pea is one of the oldest cultivated crops. It is a native to western 
Asia from the Mediterranean Sea to the Himalaya Mountains. It appears to have 
been carried to Europe as early as the time of the lake dwellers of 
prehistoric times. Peas were introduced into China from Persia about A.D. 400; 
they were introduced into the United States in very early Colonial days. 
        Garden peas (Pisum sativum) have wrinkled seed coats at maturity when 
dry; field peas (P. arvense) have a smooth seed coat. Both types are annual 
leafy plants. Each leaf bears three pairs of leaflets and ends in a slender 
tendril. Five to nine round seeds are enclosed in a pod about 3 in. (7.5 cm) 
long. Seed color varies from white to cream, green, yellow, or brown. 
Smooth-seeded varieties may be harvested fresh for freezing or canning, or 
harvested dry as edible peas. Dry peas may be split or ground and prepared in 
various ways, such as for split-pea soup. 
    Wisconsin, Washington, Minnesota, Oregon, Illinois, New York, 
Pennsylvania, Utah, and Idaho lead in the production of peas harvested green. 
 �34s[Kenneth J. Morrison]
�33s 
  
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�6v�2v�32sPeach   �21m �33s 
    A native of China, the peach (Prunus persica) is, adapted to the temperate 
zone where winter temperature does not go below -15DGF (-26DGC). Therefore, 
commercial culture is confined to the less rigorous parts of the temperate 
zone and to areas where nearby bodies of water may exert a modifying influence 
on climate. In Europe the peach is adapted to Italy, Spain, and southern 
France. Principal peach-growing areas in North America are (1) central 
California on the Pacific Coast, with some production in Washington, Oregon, 
Utah, and Colorado; (2) Maryland, Delaware, and southern New Jersey; (3) 
Georgia and the Carolinas; (4) Tennessee, Kentucky, and southern Illinois and 
Indiana; and (5) southern New England, the Niagara region of Canada, western 
New York, and western Michigan.  See  FRUIT; FRUIT, TREE. 
 �34s[Harold B. Tukey]
�33s 
  
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�6v�2v�32sPeanut   �21m �33s 
    A self-pollinated, one- to six-seeded legume which is cultivated 
throughout the tropical and temperate climates of the world. The oil, 
expressed from the seed, is of high quality, and a large percentage of the 
annual world production is used for this purpose. In the United States some 
65% goes into the cleaned and shelled trade, the end products of which are 
roasted or salted peanuts, peanut butter, and confections.  See  ROSALES. 
    Botanically, peanuts may be divided into three main types, Virginia, 
Spanish, and Valencia, based on branching order and pattern and the number of 
seeds per pod. The USDA Marketing Standards includes an additional type, 
Runner, which refers to the small-seeded Virginia type produced in Georgia and 
Alabama.  See  SEED. 
    The peanut's most distinguishing characteristic is the yellow flower, 
which resembles a butterfly (papilionaceous) and is borne above ground. The 
pod, a one-loculed legume, splits under pressure along a longitudinal ventral 
suture. Pod size varies, and seed weight varies from 0.007 to 0.175 oz (1/5 to 
5 g). The number of seeds per pod usually is two in the Virginia type, two or 
three in the Spanish, and three to six in the Valencia.  See  LEGUME. 
 �34s[Astor Perry]
�33s 
  
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�6v�2v�32sPear   �21m �33s 
    A fruit native to western Asia or nearby Europe and of very old culture, 
having been known nearly 1000 years B.C. It spread across Europe and was 
extremely popular in Belgium and France during the 18th and 19th centuries. 
Early settlers throughout North America made extensive efforts to grow pears, 
but it does best in an equable, dry climate with sufficient summer heat to 
develop good quality. Over 95% of United States commercial production is in 
the interior valleys of California, Oregon, and Washington. 
    Nearly all United States pear production is of the European pear (Pyrus 
communis) which includes the varieties Bartlett (known as Williams in Europe), 
d'Anjou, Bosc, and Comice. Other major types are P. serotina, the Oriental 
sand pear, with roundish shape and gritty flesh; hybrids between P. communis 
and P. serotina, represented by Kieffer and Leconte; and P. nivalis, the snow 
pear, grown in Europe for cider or perry (a fermented liquor).  See  FRUIT; 
FRUIT, TREE. 
 �34s[R. Paul Larsen]
�33s 
  
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�6v�2v�32sPearl   �21m �33s 
    Any mollusk-formed calcareous concretion that displays an orient and is 
lustrous. There are two major groups of bivalved mollusks in which gem pearls 
may form: the salt-water pearl oyster (Pinctada), and a number of genera of 
fresh-water clams. Usually, jewelers refer to salt-water pearls as Oriental 
pearls, regardless of their place of discovery, and to those from fresh-water 
bivalves as fresh-water pearls. 
    Between the body mass and the valves of the mollusk extends a curtainlike 
tissue called the mantle. In order for a pearl to form, a tiny object such as 
a parasite or a grain of sand must work through the mantle. When this happens, 
secretion of nacre around the invading object builds a pearl within the body 
of the mollusk. Whole pearls form within the body mass of the mollusk, in 
contrast to blister pearls, which form as protrusions on the inner surface of 
the shell. Edible oysters produce lusterless concretions, but never pearls. 
    The substitute for natural pearls, to which the name cultured pearl has 
been given, is usually made by inserting a large bead into a mollusk to be 
coated with nacre. 
 �34s[Richard T. Liddicoat, Jr.]
�33s 
  
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�6v�2v�32sPeat   �21m �33s 
    A dark-brown or black residuum produced by the partial decomposition and 
disintegration of mosses, sedges, trees, and other plants that grow in marshes 
and other wet places. Forest-type peat, when buried and subjected to 
geological influences of pressure and heat, is the natural forerunner of most 
coal. Moor peat is formed in relatively elevated, poorly drained moss-covered 
areas, as in parts of Northern Europe.  See  COAL; HUMUS. 
 �34s[Gilbert H. Cady]
�33s 
  
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�6v�2v�32sPebble mill   �21m �33s 
    A tumbling mill that grinds or pulverizes materials without contaminating 
them with iron. Because the pebbles have lower specific gravity than steel 
balls, the capacity of a given size shell with pebbles is considerably lower 
than with steel balls. The lower capacity results in lower power consumption. 
The shell has a nonmetallic lining to further prevent iron contamination, as 
in pulverizing ceramics or pigments (see illustration). Selected hard pieces 
of the material being ground can be used as pebbles to further prevent 
contamination.  See  TUMBLING MILL. 
 �34s[Ralph M. Hardgrove]
�33s 
  
  
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�6v�2v�32sPecan   �21m �33s 
    A large tree (Carya illinoensis) of the family Juglandaceae, and the nut 
from this tree. Native to valleys of the Mississippi River and tributaries as 
far north as Iowa, to other streams of Texas, Oklahoma, and northern and 
central Mexico, this nut tree has become commercially important throughout the 
southern and southwestern United States and northern Mexico. 
 �34s[Ralph H. Sharpe]
�33s 
  
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�6v�2v�32sPectin �21m �33s
    A group of polysaccharides occurring in the cell walls and intercellular
layers of all land plants. They are extractable with hot water, dilute acid,
or ammonium oxalate solutions. Pectins are precipitated from aqueous solution
by alcohol and are commercially used for their excellent gel-forming ability.
    Commercially, the primary source of pectin is the peel of citrus fruits
such as lemon and lime, although orange and grapefruit may be used. A
secondary source is apple pomace and sunflower heads.
    Pectin is widely used in the food industry, principally in the preparation
of gels. It is used as a base for jelly and as a stabilizer in some dairy
products and frozen desserts, such as sherbet, and also as edible protective
coatings for sausages, almonds, candied dried fruit, and soft dates.  See
GEL; POLYSACCHARIDE.
 �34s[Roy L. Whistler ;James R. Daniel]
�33s

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�6v�2v�32sPectinibranchia   �21m �33s 
    An order of gastropods, also called Mesogastropoda, which contains many 
important families of snails (see illustration). Respiration is by means of 
ctenidia which are composed of several gill leaves in the mantle cavity. The 
nervous system is not concentrated, an operculum is usually present, and the 
sexes are separate. Various families are found in the sea, in fresh water, and 
on land. 
  
    The family Littorinidae is of wide distribution,, and the various species 
live at the high-tide line or, more usually, in the intertidal zone. The 
common periwinkle (Littorina littorea L.) is a species in this family. Another 
important family, the Naticidae, contains the moon shells, with several 
species of economic importance because they are predatory on edible species of 
clams.  See  GASTROPODA; PROSOBRANCHIA. 
 �34s[William J. Clench]
�33s 
  
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�6v�2v�32sPectobothridia   �21m �33s 
    One of two subclasses of worms erected by Burmeister (1856) in the class 
Trematoda. They are characterized by caudal hooks or hard, posterior suckers 
or both, often situated on a haptorial portion of the body. Typically they are 
monogenetic and are parasites on the skin and gills of aquatic vertebrates. 
There are two orders, Monopisthocotylea and Polyopisthocotylea.  See  
TREMATODA.  
 �34s[Horace W. Stunkard]
�33s 
  
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�6v�2v�32sPectolite   �21m �33s 
    A mineral inosilicate with composition Ca2NaSi3O8(OH). The hardness is 5 
on Mohs scale, and the specific gravity is 2.75. The mineral is colorless, 
white, or gray with a vitreous to silky luster. Pectolite is found in the 
United States at Paterson, Bergen Hill, and Great Notch, New Jersey.  See  
SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sPediculosis   �21m �33s 
    Human infestation with lice. There are two biological varieties of the 
human louse, Pediculus humanus, var. capitis and var. corporis, each showing a 
strong preference for a specific location on the human body. Pediculus humanus 
capitis colonizes the head and P. h. corporis lives in the body-trunk region. 
    These lice are wingless insects which are ectoparasites. Their mouthparts 
are modified for piercing skin and sucking blood. The terminal segments of 
their legs are modified into clawlike structures which are utilized to grasp 
hairs and clothing fibers. 
    Lice are important vectors of human diseases. Their habit of sucking blood 
and their ability to crawl rapidly from one human to another transmit such 
diseases as typhus (rickettsial) and epidemic relapsing fever (spirochetal). 
The body fluids and feces of infected lice transmit these diseases.  See  
EPIDEMIC TYPHUS FEVER; LOUSE. 
 �34s[Richard Sudds]
�33s 
  
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�6v�2v�32sPedinoida   �21m �33s 
    An order of Diadematacea, making up those genera which possess solid 
spines and a rigid test. The ambulacra show typical diadematoid structure, and 
the tubercles are noncrenulate. The single known family, Pedinidae, includes 
15 genera, ranging from the Late Triassic onward, through only one genus, 
Caenopedina, survives today.  See  DIADEMATACEA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sPedogenesis   �21m �33s 
    Reproduction by larval or immature animals, especially the parthenogenetic 
reproduction by larvae of certain gall midges. Miastor is a typical example in 
which this phenomenon occurs.  See  NEOTENY. 
 �34s[Tracy I. Storer]
�33s 
  
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�6v�2v�32sPedology   �21m �33s 
    Defined narrowly, a science that is concerned with the nature and 
arrangement of horizons in soil profiles; the physical constitution and 
chemical composition of soils; the occurrence of soils in relation to one 
another and to other elements of the environment such as climate, natural 
vegetation, topography, and rocks; and the modes of origin of soils. Pedology 
so defined does not include soil technology, which is concerned with uses of 
soils.  
    Broadly, pedology is the science of the nature, properties, formation, 
distribution, and function of soils, and of their response to use, management, 
and manipulation. The first definition is widely used in the United States and 
less so in other countries. The second definition is worldwide.  See  SOIL; 
SOIL MECHANICS. 
 �34s[Roy W. Simonson]
�33s 
  
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�6v�2v�32sPeening   �21m �33s 
    A metal-finishing operation, also called shot peening, in which small 
steel shot is thrown against a piece such as a cutting tool. The impact of the 
shot on the work plastically deforms the surface to a depth of a few 
thousandths of an inch, producing residual compressive stress. The material is 
thus made more resistant to fatigue failure. Surface hardness of the material 
is also increased slightly by the cold working produced by the shot. The shot 
is hurdled at high velocity upon the work by centrifugal force or by an air 
blast.  See  MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sPegasiformes   �21m �33s 
    The sea moths or sea dragons, a small order of peculiar actinopterygian 
fishes also known as the Hypostomides. The body is encased in a broad, bony 
framework anteriorly and has bony rings posteriorly, simulating the seahorses 
and pipefishes (see illustration). Unlike those fishes, which have the mouth 
at the tip of the produced snout, sea moths have enlarged nasal bones which 
form a rostrum that projects well forward of the small, toothless mouth. The 
greatly expanded horizontal pectoral fin belies its appearance and does not 
function in aerial gliding. The pelvic fin is abdominal and consists of a 
slender spine and one or two long rays. The short, opposed dorsal and anal 
fins have no spines. There is no swim bladder.  See  GASTEROSTEIFORMES. 
    There is a single family, Pegasidae, with one genus, Pegasus, and four or 
five species. They live amidst vegetation on Indo-Pacific shores from East 
Africa to Japan, Australia, and Hawaii. There is no fossil record.  See  
ACTINOPTERYGII.  
 �34s[Reeve M. Bailey]
�33s 
  
  
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�6v�2v�32sPegasus   �21m �33s 
    The Winged Horse, in astronomy, an autumnal constellation. Pegasus is 
usually identified by the four bright stars ALPHA, BETA, GAMMA, and ALPHA 
(Alpha Andromedae) situated on the corners of a large square known as the 
Great Square in Pegasus (see illustration). The star Alpheratz at the 
northeastern corner of the square is really in the constellation Andromeda.  
See  CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sPegmatite   �21m �33s 
    Exceptionally coarse-grained and relatively light-colored crystalline rock 
composed chiefly of minerals found in ordinary igneous rocks. Extreme 
variations in grain size also are characteristic, and close associations with 
dominantly fine-grained aplites are common. Pegmatites are widespread and very 
abundant where they occur, especially in host rocks of Precambrian age, but 
their aggregate volume in the Earth's crust is small. Many pegmatites have 
been economically valuable as sources of clays, feldspars, gem materials, 
industrial crystals, micas, silica, and special fluxes, as well as beryllium, 
bismuth, lithium, molybdenum, rare-earth, tantalum-niobium, thorium, tin, 
tungsten, and uranium minerals.  See  APLITE; IGNEOUS ROCKS. 
    Essential minerals (1) in granitic pegmatities are quartz, potash 
feldspar, and sodic plagioclase; (2) in syenitic pegmatites, alkali feldspars 
with or without feldspathoids; and (3) in diorite and gabbro pegmatities, 
soda-lime or lime-soda plagioclase. Varietal minerals such as micas, 
amphiboles, pyroxenes, black tourmaline, fluorite, and calcite further 
characterize the pegmatites of specific districts. Accessory minerals include 
allanite, apatite, beryl, garnet, magnetite, monazite, tantalite-columbite, 
lithium tourmaline, zircon, and a host of rarer species. 
 �34s[Richard H. Jahns]
�33s 
  
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�6v�2v�32sPeking man   �21m �33s 
    The East Asian representatives of Homo erectus. The name is in reference 
to the site of Choukoutien near Peking (Beijing), China, where most of the 
fossils have been found. The fossils depict hominids similar to the other H. 
erectus samples. They had brains, on the average, about two-thirds of modern 
human size, large faces with massive brows and jaws (see illustration), and 
limbs functionally modern but exceptionally strongly built.  See  FOSSIL MAN. 
 �34s[Erik Trinkaus]
�33s 
  
  
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�6v�2v�32sPelecaniformes   �21m �33s 
    An order of aquatic birds characterized by having all four toes joined by 
webs. Several fossil and six living families are usually recognized. The 
latter include the pelicans (Pelecanidae), cormorants (Phalacrocoracidae), 
anhingas or snakebirds (Anhingidae), frigate birds or man-o'-war birds 
(Fregatidae), gannets and boobies (Sulidae), and tropic birds (Phaethontidae). 
All are primarily fish eaters, but methods of catching prey vary widely.  See  
AVES.  
 �34s[Kenneth C. Parkes]
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�6v�2v�32sPellagra   �21m �33s 
    A disease resulting from severe deficiency of the vitamin B complex. 
Niacin deficiency is the principal dietary defect in pellagra; the low 
tryptophan content of some foods also plays a role in this disease. 
    Skin, gastrointestinal, and neurologic symptoms may occur. Redness, 
scaling, and brownish discoloration of the skin, particularly in areas exposed 
to sunlight, are common lesions. Marked enlargement of the tongue and drooling 
are prominent symptoms. The mucous membranes of the mouth, eyes, urethra, and 
vagina may show swelling and have a bright red, smooth appearance. 
Gastrointestinal symptoms may be vague or may take the form of severe nausea, 
vomiting, and bloody diarrhea. Early nervous system involvement is displayed 
by the neurasthenic syndrome characterized by restlessness, anxiety, and 
insommia. Organic psychoses may follow continued deficiency and are marked by 
memory loss, confusion, and a variable pattern of affective behavior, such as 
depression or paranoia. Actual delirium, limb rigidity, and certain 
uncontrolled reflexes mark the severe case.  See  NIACIN; VITAMIN. 
 �34s[Edward G. Stuart /N. Karle Mo]
�33s 
  
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�6v�2v�32sPelletron accelerator �21m �33s
    An electrostatic particle accelerator that utilizes charging chains
consisting of steel cylinders joined by links of solid insulating material
such as nylon (see illustration). The metal cylinders are charged as they
leave a pulley at ground potential, and the charge is removed as they pass
over a pulley in a high-potential terminal.
    Pelletrons range in maximum operating voltages from 200 keV to 25 MV and
have beam current capabilities ranging from a few microamperes up to 0.8
milliampere. Machines to give 1 MV or more are enclosed in pressure tanks and
are insulated by sulfur hexafluoride (SF6) gas at pressures up to about 8 atm
(800 kilopascals).
    A large number of electrostatic accelerators of the belt-charged Van de
Graaff type are in operation throughout the world with voltage capabilities of
10 MV and below. Many of the 10-MV Van de Graaff accelerators have been
upgraded to 13 MV. Because of belt trouble above 10 MV, most of the upgraded
machines have been converted to Pelletron charging.  See  VAN DE GRAAFF
GENERATOR.
    Commonly, two-stage (tandem) accelerators have utilized a double-ended
column extending through the enclosing tank from one end to the other, with
the high-potential terminal at the center. Negative ions enter at one end, are
stripped of two or more electrons at the terminal, and receive a second
acceleration as they proceed in a straight line through the high-energy
accelerating tube to the other end of the accelerator. In a folded tandem
accelerator the column extends in only one direction from the terminal, and a
180 DG magnet directs ions through a second accelerating tube.	See PARTICLE
ACCELERATOR.
 �34s[Raymond G. Herb]
�33s


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�6v�2v�32sPelmatozoa   �21m �33s 
    A division of the Echinodermata made up of those forms which are anchored 
to the substrate during at least a part of the life history. Formerly treated 
as a formal unit of classification with the rank of subphylum, pelmatozoans 
are now realized to be a heterogeneous assemblage of forms with similar habits 
but dissimilar ancestry, their common features having arisen by convergent 
evolution. Most pelmatozoan echinoderms are members of the subphylum Crinozoa, 
but some echinozoans also exhibit a sedentary, anchored life, with 
modifications for such existence.  See  CRINOZOA; ECHINODERMATA; ECHINOZOA; 
ELEUTHEROZOA.  
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sPeltier effect   �21m �33s 
    A phenomenon discovered in 1834 by J. C. A. Peltier, who found that at the 
junction of two dissimilar metals carrying a small current the temperature 
rises or falls, depending upon the direction of the current. In view of 
experiments, which establish that the rate of intake or output of heat is 
proportional to the magnitude of the current, it can be shown that an 
electromotive force resides at a junction. Electromotive forces of this type 
are called Peltier emf's.  See  SEEBECK EFFECT; THERMOELECTRICITY; THOMSON 
EFFECT.  
 �34s[John W. Stewart]
�33s 
  
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�6v�2v�32sPelton wheel   �21m �33s 
    An impulse type of hydraulic turbine. In the impulse turbine, pressure of 
the water supply is converted into velocity by a nozzle. The water jet then 
impinges on the buckets of the turbine wheel or runner. In a Pelton wheel the 
buckets have a splitter in the middle to divide the water jet and cause it to 
flow across the cupped faces of the buckets and emerge at the sides (see 
illustration). The water jet thus imparts its kinetic energy to the buckets. 
Pelton wheels are usually operated from high-head sources.  See  HYDRAULIC 
TURBINE.  
 �34s[Frank H. Rockett]
�33s 
  
  
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�6v�2v�32sPelycosauria   �21m �33s 
    An extinct order of primitive, mammallike reptiles of the subclass 
Synapsida. They are characterized by a temporal fossa that lies low on the 
side of the skull. The group is known from rocks of the Upper Carboniferous 
and lower and middle Permian. Three suborders are included: Ophiacodonta, 
Edaphosauria, and Sphenacodontia. Late in the early Permian the sphenacodonts 
gave rise to more advanced mammallike reptiles, the therapsids.  See  
SYNAPSIDA; THERAPSIDA. 
 �34s[Everett C. Olson]
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�6v�2v�32sPendulum �21m �33s
    A rigid body mounted on a fixed horizontal axis, about which it is free to
rotate under the influence of gravity. The period of the motion of a pendulum
is virtually independent of its amplitude and depends primarily on the
geometry of the pendulum and on the local value of g, the acceleration of
gravity. Pendulums have therefore been used as the control elements in clocks,
or inversely as instruments to measure g.

    Motion.  In the schematic representation of a pendulum shown in the
figure, O represents the axis and C the center of mass. The line OC makes an
instantaneous angle THETA with the vertical. In rotary motion of any rigid
body about a fixed axis, the angular acceleration is equal to the torque about
the axis divided by the moment of inertia I about the axis. If m represents
the mass of the pendulum, the force of gravity can be considered as the weight
mg acting at the center of mass C.
    If the amplitude of motion is small, the motion is simple harmonic. The
period T, time for a complete vibration (for example, from the extreme
displacement right to the next extreme displacement right), is given by Eq.
(1).  See  HARMONIC MOTION.

	   T = 2 PI SQUARE ROOT OF [I/(mgh)]				  (1)

    The actual form of a pendulum often consists of a long, light bar or a
cord that serves as a support for a small, massive bob. The idealization of
this form into a point mass on the end of a weightless rod of length L is
known as a simple pendulum. An actual pendulum is sometimes called a physical
or compound pendulum. In a simple pendulum the lengths h and L become
identical, and the moment of inertia I equals mL(sup 2). Equation (1) for the
period becomes Eq. (2).

	   T = 2 PI SQUARE ROOT OF [L/G]				 (2)



    Center of oscillation.  Equation (2) can be used to define the equivalent
length of a physical pendulum. Comparison with Eq. (1) shows that Eq. (3)
holds. The point P on line OC of the figure, whose distance from the axis O
equals L, is called the center of oscillation. Points O and P are reciprocally
related to each other in the sense that if the pendulum were suspended at P, O
would be the center of oscillation.

	    L = I/mh							 (3)


    Types.  Kater's reversible pendulum is designed to measure g, the
acceleration of gravity. It consists of a body with two knife-edge supports on
opposite sides of the center of mass as at O and P (and with at least one
adjustable knife-edge). If the pendulum has the same period when suspended
from either knife-edge, then each is located at the center of oscillation of
the other, and the distance between them must be L, the length of the
equivalent simple pendulum. The value for g follows from Eq. (2).
    The ballistic pendulum is a device to measure the momentum of a bullet.
The pendulum bob is a block of wood into which the bullet is fired. The bullet
is stopped within the block and its momentum transferred to the pendulum. This
momentum is determined from the amplitude of the pendulum swing.  See
BALLISTICS.
    The spherical pendulum is a simple pendulum mounted on a pivot so that its
motion is not confined to a plane. The bob then moves over a spherical
surface. A Foucault pendulum is a spherical pendulum suspended so that its
plane of oscillation is free to rotate. Its purpose is to demonstrate the
rotation of the Earth.	See  FOUCAULT PENDULUM; SCHULER PENDULUM.
 �34s[Joseph M. Keller]
�33s

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�6v�2v�32sPenguin   �21m �33s 
    Any member of the avian order Sphenisciformes which contains the single 
family Spheniscidae. There are about 18 species, some of these having definite 
geographical races, included in 6 genera (fossil forms known since the 
Tertiary from Antarctic remains are excluded). 
    These flightless marine birds range throughout the Southern Hemisphere and 
are more highly adapted for their marine existence than any other group of 
birds. The feathers, unlike those of other birds, cover the body densely. The 
structurally modified wings do not fold, and function as flippers. The tail is 
short, the bill is variable in shape and form, the feet are short and webbed, 
and the legs are set far back on the body. The different species vary from 1 
to 4 ft (0.3 to 1.2 m) in height. Penguins move through the water by using the 
flippers for propulsion and the tail and feet for direction; they swim as fast 
as seals underwater. On shore these bipedal animals move in a rapid but 
ungainly manner. Their diet consists principally of crustaceans, fish, and 
mollusks, such as squid. Penguins are gregarious and nest in large colonies. 
They are monogamous, pairing for life. The number of eggs varies from one to 
three, and incubation behavior is variable with the species.  See  
SPHENISCIFORMES.  
 �34s[Charles B. Curtin]
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�6v�2v�32sPenicillin   �21m �33s 
    An antibiotic that is produced by a number of Penicillium and Aspergillus 
species and is active against gram-positive bacteria and a few gram-negative 
ones. Penicillin, discovered in 1928 by Alexander Fleming, was the first 
antibiotic to be widely and successfully used in the systemic therapy of acute 
bacterial infections in humans.  See  ANTIBIOTIC. 
    Penicillin is active in general against the gram-positive bacteria, 
including (except for resistant strains) most species of such genera as 
Streptococcus, Staphylococcus, Micrococcus, Clostridium, Borrelia, 
Corynebacterium, and Bacillus. The genera Treponema, Neisseria, and 
Actinomyces are also sensitive. The outstanding pharmacological characteristic 
of penicillin is its virtual nontoxicity (except for occasional allergic 
reactions, which can be fatal) to humans and most animals. 
 �34s[Ralph E. Bennett]
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�6v�2v�32sPenis   �21m �33s 
    The male organ of copulation, or phallus. In mammals the penis consists 
basically of three elongated masses of erectile tissue. The central corpus 
spongiosum (corpus urethrae) lies ventral to the paired corpora cavernosa. The 
urethra runs along the underside of the spongiosum and then normally rises to 
open at the expanded, cone-shaped tip, the glans penis, which fits like a cap 
over the end of the penis. Loose skin encloses the penis and also forms the 
retractable foreskin, or prepuce. 
    Erection of the penis is caused by nervous stimulation resulting in 
engorgement of the spiral helicine arteries and the plentiful venous sinuses 
of the organ. In most mammals other than Primates the penis is retracted into 
a sheath when not in use. 
    In submammalian forms the penis is not as well developed. Crocodilians, 
turtles, and some birds have a penis basically like that of mammals, although 
not as well developed, lying in the floor of the cloaca. When erected, it 
protrudes from the cloaca and functions in copulation. In most birds the penis 
is rudimentary. Other vertebrates lack a penis, although various functionally 
comparable organs may be developed such as the claspers on the pelvic fins of 
sharks and the gonopodia on the anal fins of certain teleost fishes.  See 
COPULATORY ORGAN. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sPennatulacea   �21m �33s 
    An order of the subclass Alcyonaria, commonly called the sea pens. These 
animals lack stolons and live with their bases embedded in the soft substratum 
of the sea. The colony consists of a distal rachis bearing many polyps and a 
polypless proximal peduncle, whose terminal end sometimes expands to form a 
bladder. The colony of Pennatula looks like a feather being formed of numerous 
secondary polyps which arise from leaf-shaped lateral expansions of the very 
elongated primary axial or terminal polyp. In the other form of colony the 
polyps arise directly from the primary one, as in Veretillum (see 
illustration) and Renilla and Cavernulaira. The colony has a horny unbranched 
axial skeleton composed of pennatulin and some calcium carbonate and 
phosphate, but Cavernularia, including the luminous  species, has a 
rudimentary one and Renilla lacks a skeleton.  See  ALCYONARIA. 
 �34s[Kenji Atoda]
�33s 
  
  
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�6v�2v�32sPennsylvanian   �21m �33s 
    A major division of late Paleozoic time, considered either as an 
independent period or as the younger subperiod of the Carboniferous. In North 
America the Pennsylvanian has been widely recognized as a geologic period and 
derives its name from a thick succession of mostly nonmarine, coal-bearing 
strata in Pennsylvania. Radiometric ages place the beginning of the period at 
approximately 320,000,000 years ago and its end at about 280,000,000 years 
ago. In northwestern Europe, strata of nearly equivalent age are commonly 
designated as Upper Carboniferous and in eastern Europe as Middle and Upper 
Carboniferous.  See  CARBONIFEROUS. 
    During Pennsylvanian time the paleoequator extended across North America 
from southern California to Newfoundland, through northwestern Europe into the 
Ukrainian region of eastern Europe, and across parts of northern China. This 
was a time of extensive coal deposition in a tropical belt that appears to 
have included areas from 15 to 20DG north and south of the paleoequator. Coal 
of this age is abundant and relatively widespread and has great economic 
importance.  
    Petroleum is commonly trapped in nearshore marine deposits of 
Pennsylvanian age, particularly in carbonate banks near the edge of shelves, 
in longshore bars and beaches, in reefs and mounds, and at unconformities 
associated with transgressive-regressive shore lines. Many of these traps 
contribute significantly to petroleum production. 
    Pennsylvanian paleogeography changed significantly during the period as 
the supercontinent Pangaea gradually was formed by the joining together of 
Gondwana and Laurasia. The result was an extensive orogeny, or 
mountain-building episode, which supplied the vast amounts of sediments that 
make up most of the Pennsylvanian strata in the eastern and midwestern part of 
the United States. 
    Evidence in the form of well-developed tree rings, less diverse fossil 
floras and faunas, and glacial deposits indicates that temperate and glacial 
conditions were common in nonequatorial climatic belts during Pennsylvanian 
time. Climatic fluctuations during the period caused significant increases and 
decreases in the amount of water that was temporarily stored in the glaciers 
in Gondwana and contributed to eustatic changes of sea level. 
 �34s[Charles A. Ross]
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�6v�2v�32sPentagon   �21m �33s 
    A geometric figure formed by the five line segments, or sides, that join 
in order five ordered points, or vertices, of a plane. In elementary geometry 
it is supposed that the sides do not cross, and even that the figure bounds a 
convex region of the plane (convex pentagon).  See  POLYGON; REGULAR POLYTOPES. 
 �34s[Leonard M. Blumenthal]
�33s 
  
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�6v�2v�32sPentamerida   �21m �33s 
    An order of articulate brachiopods whose genera are first known in rocks 
of Middle Cambrian age and which became extinct in the Devonian.  See  
ARTICULATA (BRACHIOPODA). 
    Prior to the Late Ordovician the order was represented only by the 
suborder Syntrophiidina, a group that was never abundant but which continued 
into Early Devonian times. The early syntrophiidines differed little from the 
orthaceans, their presumed ancestors, although a strong dorsal median fold was 
characteristic. In later forms the posterior margin was curved and interareas 
became reduced; internally one of the most typical features of the order was 
developed, a spondylium. This spoon-shaped structure is formed by coalescence 
of the dental plates and carried some or all of the ventral musculature. 
Changes also occurred in the brachial valve, notably in improved definition of 
the sockets and elaboration of the brachiophore bases, which commonly extended 
to the floor of the valve.  See  BRACHIOPODA; ORTHIDA. 
 �34s[A. J. Rowell]
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�6v�2v�32sPentastomida   �21m �33s 
    A class of bloodsucking arthropods, parasitic in the respiratory organs of 
vertebrates, that frequently are referred to as the Linguatulida or tongue 
worms. The adults are vermiform, with a short cephalothorax and an elongate, 
annulate abdomen that may be cylindrical or flattened. 
    The class is divided into two orders: the Cephalobaenida, a more primitive 
group and the Porocephalida, a more specialized one. The first has six-legged 
larvae and the other, four-legged larvae. The mitelike form of the larvae, 
with short stumpy legs, demonstrates relationship to the arthropods. 
Characteristic arthropod features include the presence of (1) jointed 
appendages in the larvae; (2) stigmata or breathing pores in the body wall; 
(3) specialized reproductive organs, especially those of the male; and (4) 
ecdysis or molting of larvae and nymphs. More than 50 species have been 
described.  
    Human infection occurs frequently in Africa and the Orient, where humans 
are an accidental intermediate host of the nymphal form. The liver is a common 
site of infection, and large numbers of larvae may produce serious and even 
fatal effects.  See  ARTHROPODA; CEPHALOBAENIDA; POROCEPHALIDA. 
 �34s[Horace W. Stunkard]
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�6v�2v�32sPentlandite   �21m �33s 
    A mineral having composition (Fe,Ni)9S8. Pentlandite is the major ore of 
nickel. It is usually massive, showing a well-defined octahedral parting. The 
hardness is 3.5 - 4 (Mohs scale) and the specific gravity varies from 4.6 to 
5.0, depending on the ratio of iron to nickel; greater amounts of iron cause 
an increase in the specific gravity. The luster is metallic and the color 
yellowish bronze. Pentlandite is found at many localities in small amounts, 
but its chief occurrence is at Sudbury, Ontario, where it is mined on a large 
scale.  See  NICKEL. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sPepper   �21m �33s 
    The garden pepper, Capsicum annuum (family Solanaceae), is a warm-season 
crop originally domesticated in Mexico. It is usually grown as an annual, 
although in warm climates it may be perennial. This species includes all 
peppers grown in the United States except for the ``Tabasco'' pepper (C. 
frutescens), grown in Louisiana. Other cultivated species, C. chinense, C. 
baccatum, and C. pubescens, are grown primarily in South America. Some 10 - 12 
strictly wild species also occur in South America. Peppers are grown 
worldwide, especially in the more tropical areas, where the pepper is an 
important condiment. 
    Sweet (nonpungent) peppers, harvested fully developed but still green, are 
widely used in salads or cooked with other foods. Perfection pimento, 
harvested red ripe, is used for canning. Paprika is made from ripe red pods of 
several distinct varieties; the pods are dried and ground.  See  PAPRIKA; 
PIMENTO.  
    The ripe color of most varieties is red, a few varieties are 
orange-yellow, and in Latin America brown-fruited varieties are common. 
Nutritionally, the mature pepper fruit has three to four times the vitamin C 
content of an orange, and is an excellent source of vitamin A.  See  ASCORBIC 
ACID; VITAMIN A. 
 �34s[Paul G. Smith]
�33s 
  
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�6v�2v�32sPeppermint   �21m �33s 
    The mint species Mentha piperita (family Lamiaceae), a sterile 
interspecific hybrid believed to have occurred in nature from the 
hybridization of fertile M. spicata. Peppermint oil is obtained by steam 
distillation from the partially dried hay. The main uses of peppermint oil are 
to flavor chewing gum, confectionary products, toothpaste, mouthwashes, 
medicines, and as a carminative in certain medical preparations for the 
alleviation of digestive disturbances. 
 �34s[Merritt J. Murray]
�33s 
  
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�6v�2v�32sPepsin   �21m �33s 
    A proteolytic enzyme found in the gastric juice of mammals, birds, 
reptiles, and fish. It is formed from a precursor, pepsinogen, which is found 
in the stomach mucosa. Pepsinogen is converted to pepsin either by 
hydrochloric acid, naturally present in the stomach, or by pepsin itself.  See 
  ENZYME. 
    Pepsin is prepared commercially from the glandular layer of fresh hog 
stomachs. It is a part of the crude preparation known as rennet, which is used 
to curdle milk in preparation for cheese manufacture. Pepsin is also used for 
a variety of other applications in food manufacturing; to modify soy protein 
and gelatin, thereby providing whipping qualities; to modify vegetable 
proteins for use in nondairy snack items; to make precooked cereals into 
instant hot cereals; and to prepare animal and vegetable protein hydrolysates 
for use in flavoring foods and beverages. 
 �34s[Myron Solberg]
�33s 
  
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�6v�2v�32sPeptic ulcer �21m �33s
    A sharply defined ulceration of the upper gastrointestinal tract,
characterized by loss of the mucous lining and variable penetration into or
through the organ wall. The loss of the mucosa, ordinarily covered by a mucous
secretion, lays bare the musculomembranous wall. The crater formed is covered
with an exudate lying over the raw tissue, except in chronic ulcers where the
pit consists largely of scar tissue. The exact causes are obscure, but
emotional tension and certain psychological patterns are frequently present in
affected individuals.
    Most ulcer patients show an increased acidity of the stomach or an
abnormal production of acid during times when the stomach is usually
quiescent, as at night. Both the action of the acid on raw surfaces and the
resulting muscular spasms are thought to be associated with the typical ulcer
pain. Scarring, constriction, perforation, and hemorrhage are common
complications.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sPeptide �21m �33s
    A compound that is made up of two or more amino acids joined by covalent
bonds which are formed by the elimination of a molecule of H2O from the amino
group of one amino acid and the carboxyl group of the next amino acid.
Peptides larger than about 50 amino acid residues are usually classified as
proteins. Glutathione is the most abundant peptide in mammalian tissue.
Hormones such as oxytocin (8), vasopressin (8), glucagon (29), and
adrenocorticotropic hormone (39) are peptides whose structures have been
deduced; in parentheses are the numbers of amino acid residues for each
peptide.
    For each step in the biological synthesis of a peptide or protein there is
a specific enzyme or enzyme complex that catalyzes each reaction in an ordered
fashion along the biosynthetic route. However, it is noteworthy that, although
the biological synthesis of proteins is directed by messenger RNA on cellular
structures called ribosomes, the biological synthesis of peptides does not
require either messenger RNA or ribosomes.  See  AMINO ACIDS; PROTEIN;
RIBONUCLEIC ACID (RNA); RIBOSOME.
 �34s[James M. Manning]
�33s

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�6v�2v�32sPeracarida   �21m �33s 
    A superorder of the class Crustacea, subclass Malacostraca. Common 
examples of the orders are the opossum shrimps, aquatic sow bugs, and 
side-swimmers. The Peracarida includes the orders Amphipoda, Cumacea, Isopoda, 
Mysidacea, Spelaeogriphacea, and Tanaidacea. These orders share a number of 
characters, the most notable being that the young develop within a thoracic 
marsupium, which they leave at a late stage of development. 
    Other features which distinguish the Peracarida from other groups of 
Malacostraca are the following: The protopodite of the second antenna usually 
consists of three segments. The thoracic legs are flexed between the fifth and 
sixth segments. The heart is generally elongate, extending through the greater 
part of the thoracic region; in the isopods, it may extend into, or lie 
entirely in, the abdomen, where respiratory exchange occurs. Spermatozoa are 
usually filiform, in contrast to their spherical or vesicular form in the 
Eucarida and Hoplocarida.  See  AMPHIPODA; CRUSTACEA; CUMACEA; ISOPODA; 
MYSIDACEA; SPELAEOGRIPHACEA; TANAIDACEA. 
 �34s[Thomas E. Bowman]
�33s 
  
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�6v�2v�32sPerbromate   �21m �33s 
    The negative ion BrO4-. The perbromate ion is tetrahedral, like the 
perchlorate and meta periodate ions. Perbromic acid is a very strong acid, and 
its potassium, rubidium, and cesium salts have relatively low solubilities in 
water. Aqueous solutions of the alkali perbromates are completely stable, 
while perbromic acid itself is stable up to concentrations of 6 M. Under 
ordinary conditions perbromates are very sluggish oxidizing agents, being 
among the least reactive of all the oxyhalogen compounds.  See  PERCHLORATE. 
 �34s[Evan H. Appelman]
�33s 
  
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�6v�2v�32sPerception   �21m �33s 
    Those subjective experiences of objects or events that ordinarily result 
from stimulation of the receptor organs of the body. This stimulation is 
transformed or encoded into neural activity (by specialized receptor 
mechanisms) and is relayed to more central regions of the nervous system where 
further neural processing occurs. Most likely, it is the final neural 
processing in the brain that underlies or causes perceptual experience, and so 
perceptionlike experiences can sometimes occur without external stimulation of 
the receptor organs, as in dreams. 
    In contemporary psychology, interest generally focuses on perception or 
the apprehension of objects or events, rather than simply on sensation or 
sensory process (such as vision or hearing). While no sharp line of 
demarcation between these topics exists, it is fair to say that sensory 
qualities are generally explicable on the basis of mechanisms within the 
receptor organ, whereas object and event perception entails higher-level 
activity of the brain.  See  HEARING (HUMAN); SENSATION; VISION. 
    Since objects or events are not experienced only through vision, the term 
perception obviously applies to other sense modalities as well. Certainly 
things and their movement may be experienced through the sense of touch. Such 
experiences derive from receptors in the skin (tactile perception), but more 
importantly, from the positioning of the fingers with respect to one another 
when an object is grasped, the latter information arising from receptors in 
the muscles and joints (haptic or tactual perception). The position of the 
parts of the body are also perceived with respect to one another whether they 
are stationary (proprioception) or in motion (kinesthesis), and the position 
of the body is experienced with respect to the environment through receptors 
sensitive to gravity such as those in the vestibular apparatus in the inner 
ear. Auditory perception yields recognition of the location of sound sources 
and of structures such as melodies and speech. Other sense modalities such as 
taste (gustation), smell (olfaction), pain, and temperature provide sensory 
qualities but not perceptual structures as do vision, audition, and touch, and 
thus are usually dealt with as sensory processes.  See  OLFACTION; PAIN; 
PROPRIOCEPTION; TEMPERATURE SENSES; TOUCH. 
  
    Constancy.  By and large, these perceptual properties of objects remain 
remarkably constant despite variations in distance, slant, and retinal locus 
caused by movements of the observer. This fact, referred to as perceptual 
constancy, is perhaps the hallmark of perception and more than any other, 
serves to characterize the field of perception. 
    Examples of perceptual constancy are: size (except at very great 
distances, objects appear the same size whether seen nearby or far away, 
although the size of its image on the retina can be very different); shape (a 
circle seen from the side is perceived as a circle, although it appears as an 
ellipse on the retina); orientation (objects appear to keep the same 
orientation in space, independently of the orientation of the observer's 
head); and position (a fixed object remains perceived as stationary even when 
its image on the retina moves because of eye or head movements). 
  
    Motion perception.  Perception is not only of objects but of events as 
well. The kind of event that has been investigated in great detail is the 
perception of motion. As already noted, perceived movement cannot simply be 
explained by the motion of an object's retinal image since image motion caused 
by observer or eye movement does not lead to perceived object movement. 
Moreover, an object tracked by smooth-pursuit eye movements will appear to 
move, although in that case there is essentially no motion of the object's 
image over the retina. Similarly, an afterimage will appear to move during eye 
movement even in a completely darkened room. This illusory effect is analogous 
to Emmert's law. Where ordinarily the movement of the retinal image caused by 
the moving eye is computed to signify ``no object motion,'' thus yielding 
position constancy (since the image motion and eye motion are equal in 
magnitude), the same computational rule must signify ``object motion'' in the 
case of the afterimage. 
  
    Form perception.  One question requiring study is why a triangular object 
appears triangular when a triangular image of the object is present on the 
retina. Before attempting to answer this question, it is important first to 
make certain distinctions. Form perception means the experience of a shaped 
region in the field. Recognition means the experience that the shape is 
familiar. Identification means that the function or meaning or category of the 
shape is known. For those who have never seen the shape before, it will be 
perceived but not recognized or identified. For those who have, it will be 
perceived as a certain familiar shape and also identified. Recognition and 
identification obviously must be based on past experience, which means that 
through certain unknown processes, memory contributes to the immediate 
experience that one has of the triangle, giving the qualities of familiarity 
and meaning. 
    The figure of a 4 in Fig. 1a is seen as one unit, separate from other 
units in the field, even if these units overlap. This means that the parts of 
the figure are grouped together by the perceptual system into a whole, and 
these parts are not grouped with the parts of other objects. This effect is 
called perceptional organization. There are other problems about form 
perception that remain to be unraveled. For example, the size of a figure can 
vary, as can its locus on the retina or even its color or type of contour, 
without affecting its perceived shape (Fig. 2). 
  
    A further fact about form perception is that it is dependent upon 
orientation. It is a commonplace observation that printed or written words are 
difficult to read when inverted, and faces look very odd or become 
unrecognizable when upside down. Simple figures also look different when their 
orientation is changed: a square looks like a diamond when tilted by 45DG 
(Fig. 3). 
  
    Geometrical illusions.  Related to the topic of form perception is the 
misperception of the size or direction of parts of figures that constitutes 
many of the geometric illusions. In an illusion figure, one particular part is 
perceived to be either longer or shorter than another part, although they are 
objectively equal (Fig. 4a); or the direction of a contour is perceived to be 
different from that of another contour although they are the same (Fig. 4b). 
For reasons still not understood, the background or context of the rest of the 
figure affects these parts. 
  
    Innate or learned?  A central problem is whether the perception of 
properties such as form and depth or the achievement of veridical perception 
as in the constancies is innately determined or is based on past experience. 
By ``innate'' it is meant that the perception is the result of evolutionary 
adaptation and thus is present at birth or when the necessary neural 
maturation has occurred. By ``past experience'' it is meant that the 
perception in question is the end result of prior exposure to certain relevant 
patterns or conditions, a kind of learning process. Despite centuries of 
discussion of this problem, and considerable experimental work, there is still 
no final answer to the question. It now seems clear that certain kinds of 
perception are innate, but equally clear that past experience also is a 
determining factor.  See  CONSCIOUSNESS; EXTRASENSORY PERCEPTION (ESP); 
INTELLIGENCE.  
 �34s[Irvin Rock]
�33s 
  
  
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�6v�2v�32sPerching birds   �21m �33s 
    Members of the order Passeriformes, which includes about 5300 species, 
approximately half of the living species of birds. There are two major 
divisions of the order, Oscines and Suboscines. 
    The foot is not webbed and has four toes, with the first toe or hallux 
pointing backward, enabling the bird to firmly grasp a branch or twig and 
perch. The resonating chamber of the syrinx has a number of membranes whose 
vibrations are controlled by muscles. The form of the bill varies with the 
type of food (seed, insects, nectar, or prey) used by the bird. 
    Species that live on insects frequently breed in high latitudes and 
migrate longer distances than other members of the order. There are some 
species that do not migrate at all. Some species are found in regions which 
are practically treeless, such as desert or mountain regions, while other 
species have adapted to cultivated areas, such as orchards and farms. 
    The members of the order have a cosmopolitan distribution and, except in 
polar regions, make up the predominant avifauna of a region. They exhibit a 
correspondingly wide variety of breeding habits, nesting sites, number of 
eggs, and types of nests. 
  
    Suboscines.  This primitive group is divided into the suborders Eurylaimi, 
Tyranni, and Menurae. The broadbills comprise the single family Eurylaimidae 
of the suborder Eurylaimi. Examples are the long-tailed broadbill (Psarisomus 
dalhousiae), indigenous in Asia; the green broadbill (Calystomena viridis) of 
Borneo, and the wattled broadbill (Eurylaimus sleerii) of the Philippines. The 
woodcreepers, wedgebills, and woodhewers make up the family Dendrocolaptidae 
of the suborder Tyranni. They range throughout Mexico into South America. In 
the same range are the ovenbirds of the family Furnariidae in the suborder 
Tyranni. The small antbirds of the family Formicariidae, suborder Tyranni, are 
confined to Central and South America. 
    The 400 species of flycatchers are placed in two families: Muscicapidae 
(members of the Oscines) which includes the Old World flycatchers or fantails; 
and Tyrannidae, the tyrant flycatchers. This latter group is confined to the 
Americas. The manakins, family Pipridae, of South and Central America are 
known for their elaborate courtship behavior. 
    Menurae is a small suborder that includes only four species of Australian 
birds, two species of lyrebirds (Menuridae), and two species of scrubbirds 
(Atrichornithidae).  
  
    Oscines.  This large suborder is composed of about 40 families commonly 
called songbirds. Among these are the common species found in the United 
States: larks, swallows, wrens, martins, waxwings, crows, warblers, thrushes, 
finches, vireos, blackbirds, orioles, and sparrows.  See  PASSERIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sPerchlorate   �21m �33s 
    A compound which contains chlorine in the 7+ oxidation state and which is 
derived from perchloric acid, HClO4. Perchlorates are more stable than 
chlorates, chlorites, or hypochlorites but are nevertheless excellent 
oxidizing agents. On heating, perchlorates decompose into potassium chloride, 
KCl, and oxygen gas. Because of their oxidizing properties, perchlorates find 
use in explosives and as oxidizing agents in the laboratory.  See  CHLORINE; 
HYPOCHLORITE; OXIDIZING AGENT; PERBROMATE. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sPerciformes   �21m �33s 
    The typical spiny-rayed fishes, also known by the ordinal names 
Acanthopteri and Percomorphi. This is the largest order of vertebrates; the 
approximately 7500 species include 41% of all fishes. Perciformes include a 
diversity of structural types and sizes. The characters of the Perciformes 
include fin spines, usually present; a pelvic fin which, if present, is 
usually thoracic or jugular in position; the pelvic girdle usually attached to 
the cleithra, sometimes connected by ligaments; the pelvic fin usually with a 
spine and 5 soft rays, the latter occasionally reduced; the pectoral fin base 
more or less vertical, usually placed well up on the side; a swim bladder 
without a duct; scales usually ctenoid, sometimes secondarily cycloid, absent, 
or variously modified; and caudal fin with 17 principal rays (15 branched) or 
fewer.  
    Perciform fishes dominate the modern vertebrate life of the oceans and 
have done so throughout the Cenozoic. The group first appeared in the Upper 
Cretaceous, after which it underwent a rapid adaptive radiation; many of the 
basic structural types, as well as most major perciform derivatives such as 
the Pleuronectiformes and Tetraodontiformes, were present in the Eocene. A few 
families of perciforms have been notably successful in fresh water. Other 
families have effectively adapted to life in the deep seas, and still others 
have become specialized for pelagic existence. It is in the shore areas, the 
offshore banks, the coral reefs, the coastal beaches and lagoons, and the 
intertidal zone, however, that the perciforms have attained their ultimate 
achievement. Here the enormous variety attests the adaptive effectiveness of 
the group.  See  ACTINOPTERYGII; OSTEICHTHYES. 
 �34s[Reeve M. Bailey]
�33s 
  
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�6v�2v�32sPercopsiformes   �21m �33s 
    A small order of actinopterygian fishes that is also known as the 
Salmopercae and Amblyopsiformes. The order is thought to be remotely related 
to the codfishes (Gadiformes) and toadfishes (Batrachoidiformes). The 
characters (see illustration) include single, ray-supported dorsal and anal 
fins, each usually with one to four anterior spines; pelvic fin, if present, 
subabdominal in position, with three to eight soft rays; pelvic girdle, if 
present, attached to the postcleithra; swim bladder without a duct; and body 
covered with cycloid or ctenoid scales.  See  BATRACHOIDIFORMES; GADIFORMES. 
    The order comprises three families, five genera, and eight Recent species 
of North American fresh-water fishes.  See  ACTINOPTERYGII; TELEOSTEI. 
 �34s[Reeve M. Bailey]
�33s 
  
  
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�6v�2v�32sPerformance rating �21m �33s
    A procedure for determining the value for a factor which will adjust the
measured time for an observed task performance to a task time that one would
expect of a trained operator performing the task, utilizing the approved
method and performing at normal pace under specified workplace conditions.
Normal time (ultimately subjectively based) is the time that a trained worker
requires to perform the specified task under defined workplace conditions,
employing the assumed philosophy of ``a fair day's work for a fair day's pay.''
    The performance rating process is concerned with determining normal pace
during the work portion of an average day and must, therefore, consider the
fatigue recovery aspects of allowance (nonwork) times occurring during the
day. The following two equations relate factors in determining how much time a
worker will be allowed per unit of output:

Standard time = normal time X allowances
  Normal time = observed time X rating factor

If the observed time for a task is adjusted by the performance rating factor
to determine normal time, and allowance time is added for nonwork time, the
standard time will represent the allowed time per unit of production.
    The most commonly employed rating technique throughout the history of
stopwatch time study, including the present, is referred to as pace rating. A
properly trained employee of average skill is time-studied while performing
the approved task method under specified work conditions. Rating consists only
of determining the relative pace (speed) of the operator in relation to the
observer's concept of what normal pace should be for the observed task,
including consideration of expected allowances to be applied to the standard.
See  HUMAN-FACTORS ENGINEERING; METHODS ENGINEERING; WORK MEASUREMENT.
 �34s[Philip E. Hicks]
�33s

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�6v�2v�32sPericarditis �21m �33s
     An inflammation of the pericardium. This occurs secondary to many general
clinical diseases, but is usually minor in degree and is not the more
significant part of the disease process. Primary pericarditis occurs when the
inflammation of the pericardium dominates the disease process. Most primary
pericarditis is of unknown cause. Known causes of paricarditis include
bacterial, tubercular, viral, and mycotic infections, as well as deranged
metabolic processes such as uremia, hypercholesterolemia, or neoplastic
diseases. Pericarditis can accompany acute myocardial infarction, or it may
occur associated with hypersensitivity diseases, (rheumatic fever, rheumatoid
arthritis, polyarteritis, and so on).  See  HEART; HEART DISORDERS.
 �34s[N. Karle Mottet]
�33s


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�6v�2v�32sPericycle   �21m �33s 
    As commonly defined, the outer boundary of the stele of plants. Originally 
it was interpreted as a band of cells between the phloem and the innermost 
layer (endodermis) of the cortex. Such pericycle is commonly found in roots 
and, in lower vascular plants, also in stems. In higher vascular plants, 
however, a distinct layer of cells may not be present between the phloem and 
the cortex. The pericycle, if present, may be composed of parenchyma or 
sclerenchyma cells with relatively thin or heavily thickened walls. It may be 
one to several layers in radial dimensions. 
    Primordia of branch roots commonly arise in the pericycle in seed plants, 
most frequently outside the xylem ridges (see illustration). The first cork 
cambium may also arise in the pericycle of those roots that have secondary 
vascular tissues. In roots, a part of the vascular cambium itself (that 
outside the primary xylem ridges) originates from pericycle cells.  See  
CORTEX (PLANT); ENDODERMIS; LATERAL MERISTEM; PARENCHYMA; PHLOEM; ROOT; 
SCLERENCHYMA; STEM; XYLEM. 
 �34s[Vernon I. Cheadle]
�33s 
  
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�6v�2v�32sPeriderm   �21m �33s 
    A group of tissues which replaces the epidermis in the plant body. Its 
main function is to protect the underlying tissues from desiccation, freezing, 
heat injury, mechanical destruction, and disease. Although periderm may 
develop in leaves and fruits, its main function is to protect stems and roots. 
The fundamental tissues which compose the periderm are the phellogen, 
phelloderm, and phellem. 
    The phellogen is the meristematic portion of the periderm and consists of 
one layer of initials. These exhibit little variation in form, appearing 
rectangular and somewhat flat in cross and radial sections, and polygonal in 
tangential sections. 
    The phelloderm cells are phellogen derivatives formed inward. The number 
of phelloderm layers varies with species, season, and age of the periderm. In 
some species, the periderm lacks the phelloderm altogether. The phelloderm 
consists of living cells with photosynthesizing chloroplasts and cellulosic 
walls.  
  
    The phellem, or cork, cells are phellogen derivatives formed outward. 
These cells are arranged in tiers with almost no intercellular spaces except 
in the lenticel regions. After completion of their differentiation, the 
phellem cells die and their protoplasts disintegrate. The cell lumens remain 
empty, excluding a few species in which various crystals can be found. The 
remarkable impermeability of the suberized cell walls is largely due to their 
impregnation with waxes, tannins, cerin, friedelin, and phellonic and 
phellogenic acids. 
    Lenticels are loose-structured openings that develop usually beneath the 
stomata and that facilitate gas transport through the otherwise impermeable 
layers of phellem.  See  BARK; SCLERENCHYMA. 
 �34s[Y. Waisel ;Hugh Wilcox]
�33s 
  
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�6v�2v�32sPeridotite   �21m �33s 
     An igneous rock consisting of more than 90% of millimeter- to 
centimeter-sized crystals of olivine, pyroxene, and hornblende, with olivine 
predominant. Other minerals are mainly plagioclase, chromite, and garnet. Most 
of the volume of the Earth's mantle probably is peridotite.  See  OLIVINE; 
PYROXENE.  
    Peridotites have three principal modes of occurrence corresponding 
approximately to their textures: (1) Peridotites with well-formed olivine 
crystals occur mainly as layers in gabbroic complexes. (2) Peridotite nodules 
in alkaline basalts and diamond pipes generally have equigranular textures, 
but some have irregular grains. (3) Peridotite also occurs on the walls of 
rifts in the deep sea floor and as hills on the sea floor, some of which reach 
the surface. 
    Peridotite is an important rock economically. Pure olivine rock (dunite) 
is quarried for use as refractory foundry sand and refractory bricks used in 
steelmaking. The sulfides associated with peridotites are common ores of 
nickel and platinoid metals. The chromite bands commonly associated with 
peridotites are the world's major ores of chromium.  See  IGNEOUS ROCKS. 
 �34s[Alfred T. Anderson, Jr.]
�33s 
  
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�6v�2v�32sPerigee   �21m �33s 
    The point nearest the Earth in the orbit of the Moon or of an artificial 
satellite. At perigee the Moon is 5% closer to Earth than at its mean 
distance. Because, on the average, the Moon and Sun subtend nearly equal 
angles, a solar eclipse near perigee lasts about 5 min; an eclipse near apogee 
is annular.  See  ECLIPSE; MOON; PERIHELION. 
 �34s[Gerard P. Kuiper]
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�6v�2v�32sPerihelion   �21m �33s 
    In astronomy, that point at one extremity of the major axis of the 
elliptical, parabolic, or hyperbolic orbit of a planet or comet about the Sun 
where the planet or comet is closest to the Sun. The instant when a planet or 
comet is at perihelion is referred to as the time of perihelion passage. For 
Earth this occurs about January 3.  See  ORBITAL MOTION. 
 �34s[Raynor L. Duncombe]
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�6v�2v�32sPeriodate   �21m �33s 
    A salt which contains iodine in the 7+ oxidation state and which is 
derived from periodic acid. Periodic acid is known in three forms: 
metaperiodic acid, HIO4; dimesoperiodic acid, H4I2O9; and paraperiodic acid, 
H5IO6. The corresponding salts are also known. Only the periodates of sodium 
and potassium are important.  See  IODINE. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sPeriodic motion �21m �33s
    Any motion that repeats itself identically at regular intervals. If x(t)
represents the displacement of any coordinate of the system at time t, a
periodic motion has the property defined by the following equation for every
value of the variable time t. The fixed time interval T between repetitions,
or the duration of a cycle, is known as the period of the motion.  See	PERIOD
(PERIODIC PHENOMENA).

	 x(t + T) = x(t)

    The vibration of a piano string after it is struck is a damped periodic
motion, not strictly periodic according to the definition. Although the motion
very nearly repeats itself, and with a fixed repetition time, each successive
cycle has a slightly smaller amplitude.  See  DAMPING; HARMONIC MOTION;
VIBRATION; WAVE MOTION.

 �34s[Joseph M. Keller]
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�6v�2v�32sPeriodic table   �21m �33s 
    A table of the elements, written in sequence in the order of atomic number 
or atomic weight and arranged in horizontal rows (periods) and vertical 
columns (groups) to illustrate the occurrence of similarities in the 
properties of the elements as a periodic function of the sequence. The concise 
table used in the element articles in this encyclopedia is as shown. 
    Each element, represented by its symbol and atomic number, occupies a 
separate square, and the sequential arrangement is in the order of atomic 
number. The table divides the elements into nine groups, designated by 
numerical column headings, and seven periods. Seven of the nine groups have 
frequently been further divided into a and b categories, the a elements often 
being classified as main-group and the b as sub-group elements. Two rows 
(lanthanide, or rare-earth, series and actinide series) occupy special 
positions outside the main body of the table, as they cannot be included 
conveniently in periods six and seven. The elements beyond lawrencium (atomic 
number 103) presumably should include a third group, the superactinide group, 
outside the main body of the table.  See  ACTINIDE ELEMENTS; TRANSURANIUM 
ELEMENTS.  
    In general, elements in the same group display a similar valence. The 
periodic table also effects a natural division of the elements in their 
elemental or uncombined state into metals and nonmentals. Between the area 
occupied by the typically metallic elements and that occupied by the  
nonmentals there is a somewhat ill-defined borderland of elements whose 
properties are transitional between the metallic and nonmetallic elements. 
These borderline elements are sometimes called metalloids. There are many 
other examples of the immense correlative power of the periodic arrangement 
including oxidation potential, heat of formation of type compounds, electrical 
conductivity, melting point, boiling point, ionic radius, ionization 
potential, electron affinity, optical spectrum, and magnetic behavior.  See  
METAL; METALLOID; NONMETAL; VALENCE. 
    Few systemizations in the history of science can rival the periodic 
concept as a broad revelation of the order of the physical world. In the 
rhythmic pattern of the properties of the elements, the architectural units of 
the universe, no aspect of their behavior changes in a capricious or wholly 
novel way. Whatever new elements may be discovered in the future, it is 
certain they will find a place in the periodic system, conforming to its order 
and exhibiting the proper familial characteristics.  See  ATOMIC STRUCTURE AND 
SPECTRA.  
 �34s[Glenn T. Seaborg]
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�6v�2v�32sPeriodontal disease �21m �33s
    An inflammatory lesion caused by bacteria affecting the tissues housing
the roots of the teeth. The disease, sometimes called pyorrhea, increases in
prevalence and severity with increasing age, and it is the principal cause of
tooth loss in adult humans throughout the world. When only the gum tissue or
gingiva is affected, the disease is called gingivitis, but when the process
extends into the deeper structures it is known as periodontis. The diseased
tissues appear abnormally red and slightly swollen, and they tend to bleed,
sometimes profusely, when the teeth are brushed. In some cases the gums may
become thickened and scarred, and they may recede, exposing the root surface.
As the disease advances, the attachment of the gum to the tooth is lost,
creating a periodontal pocket, a large portion of the gum is destroyed, and
the bone surrounding the roots is resorbed. The teeth become loose, abscesses
form, and extraction is required.
    The basic ingredients of successful treatment include debridement,
stabilization of the teeth, and thorough oral cleanliness. Using scalers and
curettes, all of the calculus and plaque are removed from the crown and root
surfaces, and the individual is trained intensively in techniques of oral
hygiene. This leads to resolution of the inflammation and to some shrinkage in
the gums, although the periodontal pockets remain. In advanced cases, surgical
intervention aimed at reduction in the pocket depth may be undertaken.	See
TOOTH DISORDERS.
 �34s[Roy C. Page]
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�6v�2v�32sPeriod (periodic phenomena)   �21m �33s 
    The time interval of a single repetition of a varying quantity of a motion 
or phenomenon which repeats itself regularly. The period is reciprocal of the 
frequency. Waves which have regularly repeated time-varying quantities are 
termed periodic.  See  FREQUENCY (WAVE MOTION); PERIODIC MOTION; WAVE MOTION. 
 �34s[William J. Galloway]
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�6v�2v�32sPerischoechinoidea   �21m �33s 
    A subclass of Echinodea lacking stability in the number of columns of 
plates that make up the ambulacra and interambulacra. The ambulacral columns 
vary from 2 to 20, the interambulacral from 1 to 14. Of the four included 
orders, three are exclusively Paleozoic; the other, Cidaroida, includes both 
Paleozoic and extant members and is probably ancestral to all other surviving 
echinoids.  See  BOTHRIOCIDARIODA; CIDAROIDA; ECHINOCYSTITOIDA; ECHINOIDEA; 
PALAECHINOIDA.  
 �34s[Howard B. Fell]
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�6v�2v�32sPeriscope   �21m �33s 
    An optical instrument that permits viewing along a displaced or deflected 
axis, providing an observer with the view from a position which may be 
inaccessible or dangerous. Periscopes range in complexity from the simple 
unit-power tank periscope to the complex multielement submarine periscope. 
    The tank periscope, intended to protect the user from bullets, employs a 
pair of plane, parallel, reflecting surfaces (either mirrors or prisms), so 
arranged in a mount that the path of light through the instrument forms a 
crude letter Z. If powers greater than unity are desired or if the periscope 
is to be used for sighting, a terrestrial telescope can be added to the 
periscope.  See  TELESCOPE. 
    In the submarine periscope, it is necessary to employ a telescope system 
having a wide field of view and uniform illumination across a field which can 
be fitted into a long, narrow tube whose length-to-diameter ratio may be 50 or 
greater. This is achieved by utilizing a plurality of lenses so spaced along 
the length of the tube as to cause the incoming principal rays from the edge 
of the field to be deviated from side to side within the tube (see 
illustration). In general, the greater the number of lenses, the wider the 
field of view. 
    Various modifications of the basic optical systems described here are 
employed as viewing periscopes in military aircraft and as viewing devices in 
particle accelerators and nuclear reactors. The cystoscope and endoscope are 
slender, sometimes mechanically flexible periscopes used for visual 
examination and photography of body cavities inaccessible to direct 
observation; an entirely different basis for the design of such instruments is 
in the use of bundles of optical fibers. See  OPTICAL FIBERS. 
 �34s[Edward K. Kaprelian]
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�6v�2v�32sPerissodactyla   �21m �33s 
    An order of exclusively herbivorous mammals, including the living horses, 
zebras, asses, tapirs, and rhinoceroses, and their diverse and abundant 
relatives. The feet are mesaxonic, that is, with the axis going through the 
third toe; the femur has a third trochanter for muscle attachment; and the 
astragalus has a characteristic flat head and short neck. Perissodactyls 
probably derive from a phenacodontid condylarth. The earliest known forms, the 
primitive horse Hyracotherium (formerly called Eohippus, the ``dawn horse'') 
and the tapir Homogalax, occur in early Eocene sediments of North America and 
Europe.  
    The explanation for the very successful radiation of early perissodactyls 
probably lies in their adaptations for fast running. Many later 
perissodactyls, however, particularly some of the rhinoceroses and 
bronthotheres, developed elephantine, columnar legs, in accord with their 
large to gigantic size. 
    The order is divided into 3 suborders with 13 families. A classification 
scheme for the order follows: 
 �34s[Frederick S. Szalay]
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�6v�2v�32sPeritoneum   �21m �33s 
    The membranous lining of the coelomic, especially the abdominal, cavity, 
and which surrounds most of the organs. It is composed mainly of flattened 
epithelial cells that produce a small amount of watery, or serous, fluid. In 
the embryo the coelomic wall is lined by this membrane, which continues over 
the developing viscera so that they are suspended and supported by the 
reflected peritoneum, principally from the dorsal body wall, but also from the 
ventral body wall in the region of the liver.  See  EPITHELIUM; FETAL MEMBRANE. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sPeritonitis �21m �33s
    Inflammation of the peritoneum. The condition may be caused by infectious
organisms or foreign substances introduced into the abdominal cavity. The
small amount of serous fluid normally present as a lubricant acts as an
excellent culture medium for bacterial growth and also as a means of spreading
invading materials. The source of such substances or organisms is commonly a
gastrointestinal inflammation, especially if perforation has occurred.
Appendicitis, peptic ulcer, cancer of the bowel, gallbladder disease, and
dysentery are common sources of infection that may produce peritonitis, as
well as blood-borne forms of tuberculosis and pneumonia.  See  PERITONEUM.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sPeritrichia   �21m �33s 
    A specialized subclass of the class Ciliatea composed of a large group of 
unusual-looking ciliate protozoans. Many are sessile and stalked, while some 
form colonies which may reach a large size. A number are attached as 
ectocommensals to a variety of animals and plants. A free-swimming stage in 
the life cycle, indispensable for distribution, is known as the telotroch. It 
is a small, mouthless form equipped with a single girdle of posteriorly 
located locomotor cilia. This is quite unlike the morphology of the mature, 
sedentary form, which is an inverted bell form atop a long stalk. 
    Vorticella (illustration a) and Epistylis are probably the best-known 
stalked forms. The former is a solitary ciliate, the latter a colony builder. 
Trichodina (illustration b) belongs to the group of mobile peritrichs.  See  
CILIATEA; HYMENOSTOMATIDA; THIGMOTRICHIDA. 
 �34s[John O. Corliss]
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�6v�2v�32sPerlite   �21m �33s 
    A natural glass with abundant spherical or convolute cracks that cause it 
to break into small pearllike masses or ``pebbles,'' usually less than a 
centimeter across. It is commonly gray or green with a pearly luster due to 
reflections from the thin air films formed along the perlitic fractures. 
    The water content of perlite is commonly 3 - 4% by weight. Under heat 
treatment (about 1500 - 2000DGF or 820 - 1100DGC) the contained moisture forms 
tiny steam bubbles in the softened glass, and the perlite is ``popped'' or 
exploded to roughly 15 or 20 times its original volume. Thus, the material is 
excellent for lightweight aggregate, insulation, fillers, and filters. Notable 
deposits are worked in California and New Mexico.  See  IGNEOUS ROCKS; 
VOLCANIC GLASS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sPermafrost   �21m �33s 
    Perennially frozen ground, occurring wherever the temperature remains 
below 32DGF (0DGC) for several years, whether the ground is actually 
consolidated by ice or not and regardless of the nature of the rock and soil 
particles of which the earth is composed. Perhaps 25% of the total land area 
of the Earth contains permafrost; it is continuous in the polar regions and 
becomes discontinuous and sporadic toward the Equator. During glacial times 
permafrost extended hundreds of miles south of its present limits in the 
Northern Hemisphere. 
    Temperature of permafrost at the depth of no annual change, about 30 - 100 
ft (10 - 30 m), crudely approximates mean annual air temperature. It is below 
23DGF (-5DGC) in the continuous zone, between 23 - 30DGF (-5 and -1DGC) in the 
discontinuous zone, and above 30DGF (-1DGC) in the sporadic zone. Temperature 
gradients vary horizontally and vertically from place to place and from time 
to time. 
    Ice is one of the most important components of permafrost, being 
especially important where it exceeds pore space. Physical properties of 
permafrost vary widely from those of ice to those of normal rock types and 
soil. The cold reserve, that is, the number of calories required to bring the 
material to the melting point and melt the contained ice, is determined 
largely by moisture content. 
    Permafrost develops today where the net heat balance of the surface of the 
Earth is negative for several years. Much permafrost was formed thousands of 
years ago but remains in equilibrium with present climates. Permafrost 
eliminates most groundwater movement, preserves organic remains, restricts or 
inhibits plant growth, and aids frost action. It is one of the primary factors 
in engineering and transportation in the polar regions. 
 �34s[Robert F. Black]
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�6v�2v�32sPermanent-magnet movable-coil ammeter   �21m �33s 
    An ammeter in which a small coil rotates in the air gap of a permanent 
magnet. The magnet produces a uniform radial magnetic field across the air 
gap. This type of ammeter is used universally for the measurement of direct 
currents. The illustration shows the general arrangement of the mechanism. 
    It is important that the magnetic system be of such materials and 
proportions that a constant flux density is maintained in the air gap. The 
coil is usually wound on a metal form and swung on highly polished steel or 
hard-alloy pivots between V-cup jewels or bearings of sapphire or hard glass. 
Bronze springs serve to carry the current in and out of the coil, as well as 
to oppose the electrical torque, resulting in a deflection proportional to the 
coil current. 
    The lower limit of useful torque is that which will overcome the residual 
bearing friction by a factor of several times the expected accuracy in terms 
of deflection. This sets a definite lower limit to a pivoted instrument, which 
is usually in the order of 10 - 20 microamperes full scale in a coil of 
several thousand ohms' resistance. 
    The high limit of current through the coil is set by thermal limits; about 
0.03 ampere is the maximum current in a moving coil. For higher values, the 
instrument is shunted; that is, the bulk of the current in bypassed around the 
moving coil through a shunt, with only a definite fraction of the current 
passing through the moving coil itself. See AMMETER.  
 �34s[John H. Miller /Jack Bass]
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�6v�2v�32sPermanganate �21m �33s
    The deep purple anion, MnO4-, which is derived from permanganic acid,
HMnO4. Although the parent acid is stable only in dilute aqueous solution, the
salts are well characterized. The permanganates resemble the perchlorates in
their oxidizing properties and solubility of both the heavy metal and
alkaline-earth metal salts. Permanganates are used as disinfectants, oxidizing
agents, wood preservatives, and bleaching agents.  See	OXIDIZING AGENT;
PERCHLORATE.
 �34s[E. Eugene Weaver]
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�6v�2v�32sPermian �21m �33s
    The name applied to the last period of geologic time in the Paleozoic Era
and to the corresponding system of rock formations that originated during that
period. The Permian Period commenced approximately 280,000,000 years ago and
ceased about 220,000,000 years ago. The system of rocks that originated during
this interval of time is widely distributed on all the continents of the
world. The Permian Period was a time of variable and changing climates, and
during much of this time latitudinal climatic belts were well developed.
During the later half of Permian time, many long-established lineages of
marine invertebrates became extinct and were not immediately replaced by new
fossil-forming lineages. Rocks of Permian age contain many resources,
including petroleum, coal, salts, and metallic ores.
    During the Permian Period, several important changes took place in the
paleogeography of the world. The joining of Gondwana to western Laurasia,
which had started during the Carboniferous, was completed during Wolfcampian
time (earliest Permian). The process of adding eastern Laurasia (Angara) to
the eastern edge of western Laurasia was completed during Artinskian time
(middle to latest Early Permian), and the supercontinent of Pangaea was
formed. One very large world ocean, Panthalassa, occupied the remaining 75% of
the Earth's surface, with a few small cratonic blocks, island arcs, and
atolls. See CONTINENTAL DRIFT.
    Most marine invertebrates of the Early Permian were continuations of
well-established phylogenetic lines of Middle and Late Carboniferous ancestry.
During Early Permian time, these faunas, dominated by brachiopods, bryozoans,
corals, fusulinaceans, and ammonoids, gradually evolved into a number of
specialized lineages. The tropical shallow-water faunas of southwestern North
America evolved almost in isolation from those of the Tethyan region, because
faunal exchanges had to cross either the cooler waters of a temperate shelf or
deep waters of Panthalassa. Fluctuating climates permitted rare dispersals of
some faunas between these two tropical faunal realms during the early part of
Early Permian time. The closure of the Uralian seaway during Artinskian time,
however, extended that dispersal path around Angara and into cold boreal
waters through which the tropical species could not disperse. With the
extension of this dispersal path, the faunas of the two tropical realms
evolved independently, with only extremely rare dispersals between them.
    Terrestrial faunas included insects which showed great advances over those
of the Coal Measures. Several modern orders emerged, among them the Mecoptera,
Odonata, Hemiptera, Trichoptera, Hymenoptera, and Coleoptera.
    Land plants changed during the Permian from those adapted to the moist
conditions of low-lying coal swamps, such as lepidodendrons and cordaites, to
those adapted to the drier, well-drained conditions of mountains and alluvial
plains, particularly conifers.
    Of the vertebrates, the labyrinthodont amphibians were common and varied;
however, the reptiles showed the most significant advances. Of several orders,
the most significant were the Theriodonta, or mammallike reptiles, that evolved
in the Triassic into mammals. Most of the known theriodonts are from South
Africa and the Soviet Union.  See  PALEOZOIC. 
�34s[Charles A. Ross ;June R. P. Ross]
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�6v�2v�32sPermittivity �21m �33s
    The permittivity EPSILON of a material medium is related to the
permittivity EPSILON(sub 0) of empty space by the equation EPSILON = k
EPSILON(sub 0), where k is the relative dielectric constant of the medium.  The
permittivity of empty space EPSILON(sup 0) has the value 8.85 x 10(sup -12)
coulomb(sup 2)/newton-meter(sup 2).  See CAPACITOR; COULOMB'S LAW; DIELECTRIC
CONSTANT; ELECTRICAL UNITS AND STANDARDS.

 �34s[Ralph P. Winch]
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�6v�2v�32sPerovskite   �21m �33s 
    A natural mineral and a structure type which includes no less than 150 
synthetic compounds. The mineral perovskite is ideally Ca[TiO3] but extensive 
substitutions occur. The perovskite structure type is of great technical 
interest, since slight distortions away from cubic symmetry result in 
noncentrosymmetric (polar) arrangements which may have ferroelectric and 
antiferroelectric properties. 
    The mineral occurs as rounded cubes modified by the octahedral and 
dodecahedral forms. Hardness is 5J (Mohs scale), specific gravity 4.0, luster 
subadamantine to submetallic. The color ranges from yellow, brownish yellow, 
reddish, and dark brown to black.  See  BARIUM TITANATE. 
 �34s[Paul B. Moore]
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�6v�2v�32sPeroxide   �21m �33s 
    A chemical compound which contains the peroxy (--O--O--) group, which may 
be considered to be a derivative of hydrogen peroxide (HOOH). An organic (or 
inorganic) peroxide is one in which some organic (or inorganic) substituent 
has replaced one or both hydrogens. Peroxides are used in such diverse 
reactions as oxidation, synthesis, polymerization, and oxygen generation. 
Inorganic peroxides include persulfates, hydrogen peroxide (H2O2), sodium 
peroxide, bivalent metal peroxides, and H2O2 addition compounds. Organic 
peroxides include peroxyacetic acid, dibenzoyl peroxide, and cumene peroxide.  
See  HYDROGEN PEROXIDE; OXIDIZING AGENT; OXYGEN. 
 �34s[Samuel S. Naistat]
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�6v�2v�32sPerpetual motion   �21m �33s 
    The expression perpetual motion, or perpetuum mobile, arose historically 
in connection with the quest for a mechanism which, once set in motion, would 
continue to do useful work without an external source of energy or which would 
produce more energy than it absorbed in a cycle of operation. This type of 
motion, now called perpetual motion of the first kind, involves only one of 
the three distinct concepts presently associated with the idea of perpetual 
motion.  
    Perpetual motion of the first kind refers to a mechanism whose efficiency 
exceeds 100%. Clearly such a mechanism violates the now firmly established 
principle of conservation of energy, in particular that statement of the 
principle of conservation of energy embodied in the first law of 
thermodynamics. (Indeed, the first law of thermodynamics is sometimes stated 
as ``A perpetuum mobile of the first kind cannot exist.'').  See  CONSERVATION 
OF ENERGY. 
    Perpetual motion of the second kind refers to a device that extracts heat 
from a source and then converts this heat completely into other forms of 
energy, a process which satisfies the principle of conservation of energy. A 
dramatic scheme of this type would be an ocean liner, which extracts heat from 
the nearly limitless oceanic source and then uses this heat for propulsion. 
This type of perpetual motion is, however, precluded by the second law of 
thermodynamics which is sometimes stated as ``A perpetuum mobile of the second 
kind cannot exist.'' 
    The third type of perpetual motion is, in contrast to the two types 
described above wherein useful output was the goal, merely a device which can 
continue moving forever. It could result in actual systems if all mechanisms 
by which energy is dissipated could be eliminated. Since experience indicates 
that dissipative effects in mechanical systems can be reduced, by lubrication 
in the case of friction, for example, but not eliminated, mechanical perpetual 
motion of the third kind can be approximated but never achieved. An example of 
a genuine case of this kind occurs in a superconductor. If a direct current is 
caused to flow in a superconducting ring, this current will continue to flow 
undiminished in time without application of any external force.  See  
SUPERCONDUCTIVITY; THERMODYNAMIC PRINCIPLES. 
 �34s[K. L. Kliewer]
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�6v�2v�32sPerseus   �21m �33s 
    A compact circumpolar constellation of the northern sky, like its 
neighbor, Cassiopeia, on the east. Both constellations lie in a brilliant part 
of the Milky Way. The prominent stars in Perseus form the capital script 
letter A (see illustration). This group is represented by the figure of the 
hero Perseus. Mirfak, a navigational star, lies in the right shoulder. The 
constellation is noted for its clusters of stars. Just above the head are the 
famous double clusters h and CHI in Perseus. Algol, the Demon Star, which is 
an eclipsing variable, is located in this constellation.  See  CASSIOPEIA; 
CONSTELLATION.  
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sPersian melon   �21m �33s 
    A long-season cultivar of muskmelon, Cucumis melo, of the gourd family, 
Cucurbitaceae. The fruit is round and without sutures; it has dark-green skin 
and thin, abundant netting. The flesh is deep orange, very thick and firm, and 
distinctly sweet in flavor. The flesh is very rich in potassium, vitamin A, 
and vitamin C.  See  MUSKMELON; VIOLALES. 
 �34s[Oscar A. Lorenz]
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�6v�2v�32sPersonality theory   �21m �33s 
    A branch of psychology concerned with developing a scientifically 
defensible model or view of human nature - in the modern parlance, a general 
theory of behavior. 
    Most personality theories can be classified in terms of two broad 
categories, depending on their underlying assumptions about human nature. On 
the one hand, there are a group of theories that see human nature as fixed, 
unchanging, deeply perverse, and self-defeating. These theories emphasize 
self-understanding and resignation; in the cases of Freudian psychoanalysis 
and existentialism, they also reflect a distinctly tragic view of life - the 
sources of human misery are so various that the best that can be hoped for is 
to control some of the causes of suffering. On the other hand, there are a 
group of theories that see human nature as plastic, flexible, and always 
capable of growth, change, and development. Human nature is basically 
benevolent; therefore bad societies are the source of personal misery. Social 
reform will produce human happiness if not actual perfection. These theories 
emphasize self-expression and self-actualization - in the cases of Carl Rogers 
and Abraham Maslow, they reflect a distinctly optimistic and romantic view of 
life.  
 �34s[Robert Hogan]
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�6v�2v�32sPersulfate   �21m �33s 
    A group of compounds more correctly known as peroxysulfates. Persulfates 
are salts of the two peroxy acids of sulfur, peroxymonosulfuric acid (H2SO5) 
and peroxydisulfuric acid (H2S2O8). All these compounds contain sulfur in an 
oxidation state of 6+, the same as in sulfates. The unusual thing about their 
structure is the presence of the peroxy group, --O--O--. 
    Ammonium peroxydisulfate is used as an oxidizing agent and a bleach, and 
is converted to other salts, such as potassium peroxydisulfate which is used 
as a polymerization promoter.  See  PEROXIDE; SULFUR. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sPERT �21m �33s
    An acronym for program evaluation and review technique; a planning,
scheduling, and control procedure based upon the use of time-oriented networks
which reflect the interrelationships and dependencies among the project tasks
(activities). The major objectives of PERT are to give management improved
ability to develop a project plan and to properly allocate resources within
overall program time and cost limitations; and to control the time and cost
performance of the project, and to replan when significant departures from
budget occur.
    The basic requirements of PERT, in its time or schedule form of
application, are:
    1. All individual tasks required to complete a given program must be
visualized in a clear enough manner to be put down in a network composed of
events and activities. An event denotes a specified program accomplishment at
a particular instant in time. An activity represents the time and resources
that are necessary to progress from one event to the next.
    2. Events and activities must be sequenced on the network under a logical
set of ground rules.
    3. Time estimates can be made for each activity of the network on a
three-way basis. Optimistic (minimum), most likely (modal), and pessimistic
(maximum) performance time figures are estimated by the person or persons most
familiar with the activity involved. The three-time estimates are used as a
measure of uncertainty of the eventual activity duration.
    4. Finally, critical path and slack times are computed. The critical path
is that sequence of activities and events on the network that will require the
greatest expected time to accomplish. Slack time is the difference between the
earliest time that an activity may start (or finish) and its latest allowable
start (or finish) time, as required to complete the project on schedule.  See
CRITICAL PATH METHOD (CPM).
    5. The difference between the pessimistic (b) and optimistic (a) activity
performance times is used to compute the standard deviation (STANDARD DEVIATION)
of the hypothetical distribution of activity performance times (STANDARD
DEVIATION = (b - a)/6).  The PERT procedure employs these expected times and
standard deviations (STANDARD DEVIATION SQUARED is called variance) to compute
the probability that an event will be on schedule, that is, will occur on or
before its scheduled occurrence time. 
  In the actual utilization of PERT, review and action by responsible managers
is required, generally on a biweekly basis, concentrating on important critical
path activities.  A major advantage of PERT is the kind of planning required to
create an initial network.  Network development and critical path analysis
reveal interdependencies and problem areas before the program begins that are
often not obvious or well defined by conventional planning methods.
 �34s[Joseph J.  Moder]
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�6v�2v�32sPerthite   �21m �33s 
    A parallel-to-subparallel intergrowth of potassium and sodium feldspar. 
Usually the intergrowth occurs in a lamellar fashion. In most cases the 
lamellae are approximately parallel. With increasing size of the intergrowths, 
cryptoperthites, microperthites, and macroperthites may be distinguished.  
    It is generally agreed that most - if not all - perthites are formed by 
unmixing of originally monoclinic (K,Na)AlSi3O8 into potassium- and 
sodium-rich feldspar domains. The unmixing starts with the formation of 
cryptoperthite, at any temperature below approximately 600DGC (1100DGF). 
Depending on the cooling rate, different unmixing textures can be expected. 
With decreasing temperature or with time, or both, the early formed domains 
grow continuously until they may become visible with a microscope or even to 
the naked eye.  See  FELDSPAR; MICROCLINE; ORTHOCLASE. 
 �34s[Fritz Laves]
�33s 
  
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�6v�2v�32sPerturbation (astronomy)   �21m �33s 
    Departure of a celestial body from the trajectory it would follow if 
moving only under the action of a single central force. Perturbations may be 
caused by either gravitational or nongravitational forces. 
    Planetary orbits are subject to two classes of disturbances: secular, or 
long-term, perturbations. Secular perturbations, so called because they are 
either progressive or have excessively long periods, arise because of the 
relative orientation of the orbits in space. They cause slow oscillatory 
changes of eccentricities and inclinations about their mean values with 
accompanying changes in the motions of the nodes and perihelia. Periodic 
perturbations arise from the relative positions of the planets in their 
orbits. When the disturbed and disturbing planets are aligned on the same side 
of the Sun, the perturbation reaches a maximum, and reduces to minimum when 
alignment is reached on opposite sides of the Sun. 
    The motions of planetary satellites, natural and artificial, reflect both 
gravitational and nongravitational perturbations. The centrifugal force 
arising from the rotation of a planet causes a deformation or oblateness of 
figure. In such a case the central mass does not attract as if it were 
concentrated at its center. For a close satellite the principal perturbation 
arises from the attraction of this equatorial bulge. 
 �34s[Raynor L. Duncombe]
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�6v�2v�32sPerturbation (mathematics) �21m �33s
    A modification in the mathematical structure of a problem changing the
problem from one that can be solved exactly, the unperturbed problem, to one,
the perturbed problem, for which it is usually possible to obtain only an
approximate solution. The methods employed for this purpose form perturbation
theory. These methods attempt to express the solution of the perturbed problem
in terms of the properties of the solutions of the unperturbed problem.
    Examples of perturbation problems can be found in nearly every branch of
mathematics and physics, and in astronomy. The simplest case occurs in
ordinary algebra. Suppose that the roots of the equation f(x) = 0 are known
(the unperturbed problem), and that the roots of the equation f(x) +
EPSILON g(x) = 0 are to be found (the perturbed problem). The parameter EPSILON
measures the size of the perturbation. Another set of examples occurs in
linear differential equations and in particle dynamics. Possible perturbations
include changes in the forces considered to be acting on the particle as well
as changes in initial conditions.  See	PERTURBATION (ASTRONOMY).
    Several examples occur in partial differential equations. One physical
realization occurs in the theory of wave propagation where the perturbations
can be changes in the index of refraction, changes in initial conditions, or
changes in the nature or shape of the surfaces encountered by the waves. All
of these changes can occur separately or concurrently. The first of these
changes is called a volume perturbation, the second a perturbation of initial
conditions, and the third a perturbation of boundary conditions. Similar
examples can be taken from quantum mechanics, where the volume perturbation
corresponds to a change in the hamiltonian, and perturbation of initial
conditions to quantum mechanical time-dependent perturbation theory. Other
partial differential equations of physics, such as the Laplace equation, the
diffusion equation, and the equations of hydrodynamics, furnish further
examples.  See	PERTURBATION (QUANTUM MECHANICS).
    All of these problems are linear and can therefore be cast into an
equation of the form A PSI = LAMBDA PSI, where PSI is the unknown quantity,
LAMBDA is a constant, and A is an operator involving among other possibilities
differentiation and integration. The quantity PSI may be a scalar, a vector,
or more generally a matrix quantity. When solutions can be obtained for only
special values of LAMBDA, the eigenvalues, the equation is called the
eigenvalue equation, and the associated problem is called the eigenvalue
problem. The operator A contains the perturbation: that is, A equals A(sub 0) +
EPSILON A(sub 1), where A(sub 0) is the unperturbed operator and EPSILON A(sub
1), the perturbing term.

 �34s[Herman Feshbach]
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�6v�2v�32sPerturbation (quantum mechanics) �21m �33s
    An expansion technique useful for solving complicated quantum-mechanical
problems in terms of solutions for simple problems. Perturbation theory in
quantum mechanics provides an approximation scheme whereby the physical
properties of a system, modeled mathematically by a quantum-mechanical
description, can be estimated to a required degree of accuracy. Such a scheme
is useful because very few problems occurring in quantum mechanics can be
solved analytically. Consequently an approximation technique must be employed
in order to give an approximate analytic solution or to provide suitable
algorithms for a numerical solution. Even for problems which admit an exact
analytic solution, the exact solution may be of such mathematical complexity
that its physical interpretation is not apparent. For these situations,
perturbation techniques are also desirable.
    Here the discussion of the application of perturbation techniques to
quantum mechanics is limited to the domain of nonrelativistic quantum theory.
Applications, of a similar but mathematically more intricate nature, have also
been made in quantum electrodynamics and quantum field theory.	See
NONRELATIVISTIC QUANTUM THEORY; QUANTUM ELECTRODYNAMICS; QUANTUM FIELD THEORY;
QUANTUM MECHANICS.
    Perturbation theory is applied to the Schroedinger equation, H PSI = (H(sub
0) + LAMBDA V)PSI = i PLANCK'S CONSTANT(CURLY d/CURLY dt)PSI [where PLANCK'S
CONSTANT is Planck's constant h divided by 2PI, and (d/dt) represents partial
differentiation with respect to the time variable t], for which the exact
hamiltonian H is split into two parts: the approximate (unperturbed)
time-independent hamiltonian H(sub 0) whose solutions of the corresponding
Schroedinger equation are known analytically, and the perturbing potential
LAMBDAV. The basic idea is to expand the exact solution PSI in terms of the
solution set of the unperturbed hamiltonian H0 by means of a power series in
the coupling constant LAMBDA. Such a procedure is expected to be successful if
the system characterized by the unperturbed hamiltonian closely resembles that
characterized by the exact hamiltonian. Supposedly the differences are not
singular in character, but change as a continuous function of the parameter
LAMBDA. 
    Perturbation theory is used in two contexts to provide information about
the state of the system, which in quantum mechanics is determined by the wave
function PSI.  If LAMBDA V is time-independent, an objective may be to find the
stationary states of the system PSI(sub n) whose time dependence is given by exp
(-iE(sub n)t/PLANCK'S CONSTANT), where i = SQUARE ROOT OF -1 and E(sub n)
represents the energy of the stationary state labeled by n.  If LAMBDA V is
either time-independent or time-dependent, an objective may be to find the time
evolution of a state which at some specified time was a stationary state of the
unperturbed hamiltonian.  The perturbing potential is then considered as causing
transitions from the original state to other states of the unperturbed
hamiltonian, and application of time-dependent perturbation theory provides the
probability of such transitions.  See PERTURBATION (MATHEMATICS).

 �34s[David M.  Fradkin]
�33s

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�6v�2v�32sPesticide   �21m �33s 
    A material useful for the mitigation, control, or elimination of plants or 
animals detrimental to human health or economy. Algaecides, defoliants, 
desiccants, herbicides, plant growth regulators, and fungicides are used to 
regulate populations of undesirable plants which compete with or parasitize 
crop or ornamental plants. Attractants, insecticides, miticides, acaricides, 
molluscicides, nematocides, repellants, and rodenticides are used principally 
to reduce parasitism and disease transmission in domestic animals, the loss of 
crop plants, the destruction of processed food, textile, and wood products, 
and parasitism and disease transmission in humans. 
    Materials used to control or alleviate disease conditions produced in 
humans and animals by plants or by animal pests are usually designated as 
drugs. For example, herbicides are used to control the ragweed plant, while 
drugs are used to alleviate the symptoms of hay fever produced in humans by 
ragweed pollen. Similarly, insecticides are used to control malaria 
mosquitoes, while drugs are used to control the malaria parasites transmitted 
to humans by the mosquito. 
    Some pesticides are obtained from plants and minerals. Examples include 
the insecticides cryolite, a mineral, and nicotine, rotenone, and the 
pyrethrins which are extracted from plants. A few pesticides are obrained by 
the mass culture of microorganisms. Most pesticides, however, are products of 
chemical manufacture. Two outstanding examples are the insecticide DDT and the 
herbicide 2,4-D. 
    By the mid-1960s, it became apparent that the benefits stemming from the 
ability of the new pesiticides, particularly DDT, to control insect pests 
could be counterbalanced by adverse effects on other elements of the 
environment. Detailed reviews of the properties, stability, persistence, and 
impact upon all facets of the environment were carried out not only with DDT, 
but with other chlorinated, organic insecticides as well. Concern over the 
undesirable effects of pesticides culminated in the amendment of the Federal 
Insecticide, Fungicide, and Rodenticide Act (FIFRA) by Public Law 92-516, the 
Federal Environmental Pesticide Control Act (FEPCA) of 1972. The purpose 
behind this strengthening of earlier laws was to prevent exposure of either 
humans or the environment to unreasonable hazard from pesticides. 
    Pesticides must be selected and applied with care. Recommendations as to 
the product and method of choice for control of any pest problem - weed, 
insect, or varmint - are best obtained from county or state agricultural 
extension specialists.  See  AGRICULTURAL CHEMISTRY; FUNGISTAT AND FUNGICIDE; 
HERBICIDE; INSECTICIDE; RODENTICIDE. 
 �34s[George F. Ludvik]
�33s 
  
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�6v�2v�32sPetalichthyida   �21m �33s 
    A small order of dorsoventrally flattened fishes belonging to the class 
Placodermi. These fishes range from the Lower to the Upper Devonian - although 
they are never common - and are an exclusively marine group. 
    The external armor is in two shields of large plates (see illustration). 
The head is characterized by the dorsal position of the orbits, which are 
completely enclosed in the shield, and the long occipital region which has a 
marked effect on the bone pattern in the hind end. The braincase has a long, 
narrow occipital region, but is otherwise broad and flat with prominent 
lateral processes in the postorbital and occipital regions. The jaws and tooth 
plates are unknown. The trunk armor is short, with long spines in front of the 
pectoral fins and a poorly differentiated articulation with the head shield.  
See  ARTHRODIRA; PLACODERMI. 
 �34s[Robert H. Denison]
�33s 
  
  
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�6v�2v�32sPetalite �21m �33s
    A rare pegmatitic mineral with composition LiAlSi4O10. Its economic
significance is markedly disproportionate to the number of its occurrences. It
is the only basic raw material suitable for production of a group of materials
known as crystallized glass ceramics (melt-formed ceramics). These extremely
fine-grained substances are based on a keatite-type structure (stuffed silica
derivative). Among the desirable properties of such submicroscopic aggregates
are their exceedingly low thermal expansion and high strength, making them
suitable for use in cooking utensils and telescopic mirror blanks.
    The color is white, pink, pale green, or gray to black. Hardness is 6 1/2.
Petalite is mined from a large lithium-rich pegmatite at Bikita, Zimbabwe, the
only major world source.
 �34s[E. William Heinrich]
�33s

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�6v�2v�32sPetrifaction   �21m �33s 
    A mechanism by which the remains of extinct organisms are preserved in the 
fossil record. In petrifactions (though chiefly in plants rather than animals) 
the original shape and topography of the tissues, and occasionally even minute 
cytological details, are retained relatively undeformed. 
    The term petrifaction was adopted as a scientific term before knowledge 
existed of the geochemical mechanism or processes involved. It was formerly 
widely believed that in the formation of a petrifaction the organic matter of 
the organism or tissue was replaced molecule by molecule with mineral material 
entering in solution in percolating groundwater. It is now evident that what 
actually happens is that the mineral fills cell lumena and the intermicellar 
interstices of cell walls with insoluble salts depositing from solution. 
Petrifaction is hence a form of mineral emplacement or embedding, by which the 
organic residues are filled with solid substance which infiltrates in 
solution. The most common substances involved in petrifactions are silica, 
SiO2, and calcium carbonate, CaCO3 (calcite). Occasionally phosphate minerals, 
pyrite, hematite, and other less common minerals make up all or part of the 
petrifaction matrix.  See  FOSSIL; PALEOBOTANY; PETRIFIED FORESTS. 
 �34s[Elso S. Barghoorn]
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�6v�2v�32sPetrified forests   �21m �33s 
    Exposures containing appreciable numbers of petrified tree trunks, either 
standing upright or lying prostrate in the enclosing sedimentary rocks; 
sometimes called fossil forests. The best-known examples are the Petrified 
Forest of Arizona, and the fossil forests near Cairo, Egypt; near Calistoga, 
California; near Vantage Bridge, Washington; and in Yellowstone National Park, 
Wyoming.  See  PALEOBOTANY; PETRIFACTION. 
 �34s[Erling Dorf]
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�6v�2v�32sPetrochemical �21m �33s
    One of a large number of substantially pure chemical substances produced
commercially from petroleum or natural gas. Ordinarily the term does not
include hydrocarbon fuels and lubricants nor chemicals produced by other than
the processor handling the petroleum raw material. Organic compounds make up
the great bulk, as well as number, of petrochemicals, but several inorganic
compounds (ammonia, carbon black, sulfur, and hydrogen peroxide) also are
produced in large amounts.  See  PETROLEUM PRODUCTS.
    Petrochemicals should not be regarded as a particular type or class of
chemical, since all of them have been, and many still are, made from other raw
materials. However, a majority of the chemicals once made from other raw
materials came to be made entirely, or almost entirely, from petroleum or
natural gas because of the abundance of supplies and relatively low cost. It
is anticipated that processes using other raw materials will become
increasingly important.  See  ALKYLATION; CATALYTIC REFORMING; CRACKING;
NATURAL GAS; ORGANIC CHEMICAL SYNTHESIS; PETROLEUM.
 �34s[Julius D. Heldman ;W. W. Reynolds]
�33s

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�6v�2v�32sPetrofabric analysis   �21m �33s 
    The systematic study of the fabrics of rocks, generally involving 
statistical study of the orientations and distribution of large numbers of 
fabric elements. The term fabric denotes collectively all the structural or 
spatial characteristics of a rock mass. The fabric elements are classified 
into two groups: (1) megascopic features, including bedding, schistosity, 
foliation, cleavage, faults, joints, folds, and mineral lineations; and (2) 
microscopic features, including the shapes, orientations, and mutual 
arrangement of the constituent mineral crystals (texture) and of internal 
structures (twin lamellae, deformation bands, and so on) inside the crystals. 
    The aim of fabric analysis is to obtain as complete and accurate a 
description as possible of the structural makeup of the rock mass with a view 
to elucidating its kinematic history. The fabric of a sedimentary rock, for 
example, may retain evidence of the mode of transport, deposition, and 
compaction of the sediment in the size, shape, and disposition of the 
particles; similarly, that of an igneous rock may reflect the nature of the 
flow or of gravitational segregation of crystals and melt during 
crystallization. The fabrics of deformed metamorphic rocks (tectonites) have 
been most extensively studied by petrofabric techniques with the objective of 
determining the details of the history of deformation and recrystallization.  
See  STRUCTURAL GEOLOGY; STRUCTURAL PETROLOGY. 
 �34s[John M. Christie]
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�6v�2v�32sPetrographic province   �21m �33s 
    A region in which the igneous rocks, formed during a limited period, show 
a certain community of character (chemical, mineralogical, or petrological) 
that distinguishes them from other rocks in the area. Rocks of a petrographic 
province are consanguineous in that they are, theoretically at least, derived 
from a common parental magma. A classsic province is represented by the high 
potassic rocks around Rome and Naples. 
    If the chemical analyses of rocks representing a genetically related 
series are plotted on a variation diagram, many characteristics of the series 
may be brought out. The illustration shows the percentages of oxides in each 
rock analysis as plotted against the silica content of that analysis. 
    All points lie on or close to smooth curves which represent the variation 
in composition with silica content (or roughly with time, because younger 
rocks tend to be more silicic). Noteworthy are the positive slopes for 
potassium oxide (K2O) and sodium oxide (Na2O), the arched curve for aluminum 
oxide (Al2O3), and the negative slopes for magnesium oxide (MgO), calcium 
oxide (CaO), ferrous oxide (FeO), and ferric oxide (Fe2O3). The combined 
alkali (Na2O + K2O) curve is seen to cross the CaO curve at 56.5% silica. This 
silica value is known as the lime-alkali index for the rock series. On the 
basis of lime-alkali indices, the numerous rock series are arbitrarily divided 
into groups as follows: alkalic < 51, alkali-calcic > 51 < 56, calc-alkalic > 
56 < 61, and calcic > 61. 
    Certain rock series appear related to certain types of geological 
environments. In general, the more alkalic types occur in regions subjected to 
tension and vertical movement (faulting and subsidence), whereas more calcic 
series are found in compressional regions (fold-mountain belts). 
 �34s[Carleton A. Chapman]
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�6v�2v�32sPetrography   �21m �33s 
    The description of rocks with goals of classification and interpretation 
of origin. Most schemes for the classification of rocks are based on the size 
of grains and the proportions of various minerals. Interpretations of origin 
rely on field relations, structure, texture, and chemical composition as well 
as sizes and proportions of different kinds of grains. The names of rocks are 
based on the sizes and relative proportions of different minerals; boundaries 
between the names are arbitrary. The conditions of formation of a rock can be 
estimated from the types and textures of its constituent minerals. 
    The description of rocks begins in the field with observation of the shape 
and structure of bodies of rock at the scale of centimeters to kilometers. The 
geometrical relations between and structures within mappable rock units are 
generally the domain of field geology, but are simply rock descriptions at a 
reduced scale. 
    A petrographer can correctly name most rocks in which most crystals are 
larger than about 0.04 in. (1 mm) simply by examining the rock with a 10-power 
magnifying lens. Rocks with smaller grains require either microscopical 
examination or chemical analysis for proper classification. 
    Sizes and shapes of grains and voids are the most important features of a 
rock relevant to its origin. The same features also affect density, porosity, 
permeability, strength, and magnetic behavior. It is also essential to know 
the identity, abundance, and compositions of minerals constituting the grains 
in order to name a rock and infer its conditions of formation.  See  
MINERALOGY; PETROLOGY. 
 �34s[Alfred T. Anderson, Jr.]
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�6v�2v�32sPetrolatum   �21m �33s 
    A smooth, semisolid blend of mineral oil with waxes crystallized from the 
residual type of petroleum lubricating oil. The wax molecules are microneedles 
and hold a large amount of oil in a gel. Petrolatums are useful because they 
cling, lubricate, and resist both moisture and oxidation. They serve as 
lubricants in baking and candymaking; as carriers in polishes, cosmetics, and 
ointments; as rust preventives; as waterproofing agents for paper; and in 
other uses calling for an inert greaselike material. 
 �34s[J. K. Roberts]
�33s 
  
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�6v�2v�32sPetroleum   �21m �33s 
    A naturally occurring, oily, flammable liquid composed principally of 
hydrocarbons, and occasionally found in springs or pools but usually obtained  
from beneath the Earth's surface by drilling wells. Formerly called rock oil, 
unrefined petroleum is now usually termed crude oil. 
  
    Composition.  Petroleum is separated by distillation into fractions 
designed as (1) straight-run gasoline, boiling at up to about 390DGF (200DGC); 
(2) middle distillate, boiling at about 365 - 653DGF (185 - 345DGC), from 
which are obtained kerosine, heating oils, and diesel, jet, rocket, and gas 
turbine fuels; (3) wide-cut gas oil, which boils at about 653 - 1000DGF (345 - 
540DGC), and from which are obtained waxes, lubricating oils, and feed stock 
for catalytic cracking to gasoline; and (4) residual oil, which may be 
asphaltic.  
    The physical properties and chemical composition of petroleum vary 
markedly, depending on its source. As it comes from the earth, it ranges from 
an occasional nearly colorless liquid consisting chiefly of gasoline to a 
heavy black tarry material high in asphalt content. Most crudes are black, 
many are amber, red, or brown by transmitted light and show a greenish 
fluorescence by reflected light and have a specific gravity in the range about 
0.82 - 0.95. 
    Hydrocarbons constitute 50 - 98% of petroleum, and the remainder is 
composed chiefly of organic compounds containing oxygen, nitrogen, or sulfur, 
and trace amounts of organometallic compounds. The hydrocarbon types found in 
petroleum are paraffins (alkanes), cycloparaffins (naphthenes or 
cycloalkanes), and aromatics. Olefins (alkenes) and other unsaturated 
hydrocarbons are usually absent.  See  ALICYCLIC HYDROCARBON; ALKANE; AROMATIC 
HYDROCARBON.  
    The number of carbon atoms in hydrocarbons of a given boiling range 
depends on the hydrocarbon type. In general, gasoline will include 
hydrocarbons having 4 - 12 carbon atoms; kerosine, 10 - 14; middle distillate, 
12 - 20; and wide-cut gas oil, 20 - 36. Five main classes of compounds are 
present in the gasoline fraction: straight-chain paraffins, branched-chain 
paraffins, alkylcyclopentanes, alkylcyclohexanes, and alkylbenzenes. 
    Asphalt is a dark-brown to black solid or semisolid consisting of carbon, 
hydrogen, oxygen, sulfur, and sometimes nitrogen. It is made up of three 
components: (1) asphaltene, a hard, friable, infusible powder; (2) resin, a 
semisolid to solid ductile and adhesive material; and (3) oil, which is 
structurally similar to the lubricating oil fraction from which it is derived. 
  See ASPHALT AND ASPHALTITE; PETROCHEMICAL; PETROLEUM ENGINEERING; PETROLEUM 
PROCESSING; PETROLEUM PRODUCTS; PETROLEUM PROSPECTING. 
 �34s[Louis Schmerling]
�33s 
  
    Origin.  Accumulation of petroleum is believed to involve three steps: 
generation of oil, primary migration (the movement of oil from source to 
reservoir rock), and secondary migration (the redistribution of oil within the 
reservoir rock to form a pool). 
    Oil is generated in sedimentary basins. These basins are shallow 
depressions on the continents that have intermittently been covered with sea 
water, or offshore basins on continental shelves. They are hundreds of square 
kilometers in area and contain sediments of three types: (1) rock particles 
varying from sands to clay muds, which were eroded from hills and mountains 
and were carried to the basins by streams; (2) biochemical and chemical 
precipitates such as limestone, gypsum, anhydrite, and chert; and (3) organic 
matter from the plants and animals that lived in the sea or were carried in by 
rivers. The third type of sediment, the organic matter, is the source of 
petroleum.  
    It is believed that oil is generated from organic matter in two ways. A 
small amount, probably less than 10%, comes directly from the hydrocarbons 
that marine organisms form as part of their living cells. The second process, 
by which about 90% of the oil is formed, involves the formation of 
hydrocarbons from the decay and alteration of buried organic matter. Nearly 
all of the hydrocarbons containing up to 10 carbon atoms and probably over 
half of those of higher molecular weight are formed in this manner. 
    Primary migration of petroleum from source to reservoir, the second step 
in accumulation, is caused by the movement of water, which carries oil or its 
precursor out of compacting sediments. When the source muds are deposited, 
they contain 70 - 80% water. The remainder is solids, mostly clay minerals or 
carbonate particles; as they build up to great thicknesses in a sedimentary 
basin, water is squeezed out by the weight of the overlying sediments. The 
water and oil move in the direction of least resistance (lowest hydrostatic 
pressure) at rates of 0.8 - 3.9 in. (2 - 10 cm) per year. Early in compaction, 
the direction of fluid movement is straight up. As compaction progresses, 
there is lateral as well as vertical movement of the fluids. 
    In secondary migration, the last stage in the origin of an oil 
accumulation, the oil droplets are moved about within the reservoir to form 
the pool. Secondary migration includes in some instances a second step, during 
which crustal movements of the earth shift the position of the pool within the 
reservoir rock. 
    The composition and character of the oil ultimately formed is controlled 
by all phases of its origin and migration. The organic structures from which 
the oil originated, the increasing formation of light hydrocarbons with depth, 
the selectivity of the migration phase in carrying only part of the 
hydrocarbons from the source rock, and the alteration of the oil within the 
reservoir by subsurface water and ground movements, are believed important in 
determining the composition of petroleum.  See  PETROLEUM GEOLOGY; 
SEDIMENTATION (GEOLOGY). 
 �34s[John M. Hunt]
�33s 
  
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�6v�2v�32sPetroleum engineering   �21m �33s 
    An eclectic discipline comprising the technologies used for the 
exploitation of crude oil and natural gas reservoirs. It is usually subdivided 
into the branches of petrophysical, geological, reservoir drilling, 
production, and construction engineering. After an oil or gas accumulation is 
discovered, technical supervision of the reservoir is transferred to the 
petroleum engineering group, although in the exploration phase the drilling 
and petrophysical engineers have played a role in the completion and 
evaluation of the discovery. 
    By the use of down-hole logging tools and of laboratory analysis of cores 
made during the drilling operation, the petrophysical engineer estimates the 
porosity, permeability, and oil content of the reservoir rock that has been 
sampled at the drill site.  See  WELL LOGGING. 
    The geological engineer, using the petrophysical data, the seismic surveys 
conducted during the exploration operations, and an analysis of the regional 
and environmental geology, develops inferences concerning the lateral 
continuity and extent of the reservoir.  See  PETROLEUM GEOLOGY; SEISMIC 
EXPLORATION FOR OIL AND GAS. 
    The reservoir engineer, using the initial studies of the petrophysicist 
and geological engineers together with the early performance of the wells 
drilled into the reservoir, attempts to assess the producing rates (barrels of 
oil or millions of cubic feet of gas per day) that individual wells and the 
entire reservoir are capable of sustaining. One of the major assignments of 
the reservoir engineer is to estimate the ultimate production that can be 
anticipated from both primary and enhanced recovery from the reservoir.  See  
PETROLEUM ENHANCED RECOVERY; PETROLEUM RESERVES; PETROLEUM RESERVOIR 
ENGINEERING; PETROLEUM RESERVOIR MODELS. 
    The drilling engineer has the responsibility for the efficient penetration 
of the earth by a well bore, and for cementing of the steel casing from the 
surface to a depth usually just above the target reservoir. The drilling 
engineer or another specialist, the mud engineer, is in charge of the fluid 
that is continuously circulated through the drill pipe and back up to surface 
in the annulus between the drill pipe and the bore hole.  See   DRILL-HOLE 
LOGGING; ROTARY TOOL DRILL. 
    The production engineer, upon consultation with the petrophysical and 
reservoir engineers, plans the completion procedure for the well. This 
involves a choice of setting a liner across the formation or perforating a 
casing that has been extended and cemented across the reservoir, selecting 
appropriate pumping techniques, and choosing the surface collection, 
dehydration, and storage facilities.  See  OIL AND GAS WELL COMPLETION. 
    Major construction projects, such as the design and erection of offshore 
platforms, require the addition of civil engineers to the staff of petroleum 
engineering departments, and the design and implementation of natural gasoline 
and gas processing plants require the addition of chemical engineers.  See  
OIL AND GAS, OFFSHORE; PETROLEUM. 
 �34s[Todd M. Doscher]
�33s 
  
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�6v�2v�32sPetroleum enhanced recovery �21m �33s
    Novel technology designed to enhance the fraction of the original oil in
place in a reservoir. Interest in developing enhanced recovery technology
accelerated as it became more certain that over two-thirds of the oil
discovered in the United States, and a still greater percentage in the rest of
the world, would remain unrecovered through the application of conventional
primary and secondary (waterflood) operations.	See  OIL AND GAS FIELD
EXPLOITATION; PETROLEUM RESERVES.
    The problems encountered in developing such technology are very great
because of the nature of fluid flow within a subsurface reservoir, and because
of the inability of engineers to exercise any intimate degree of control over
the flow and distribution of fluids within the reservoir. The only points of
contact with the reservoir are at the surface of the producing and injection
wells that lead down to the reservoir sands.
    A subsurface reservoir comprises the interconnected pores of sandstone or
porous carbonate rocks. There is a great tendency for oil being displaced by
encroaching water or an expanding gas to be bypassed by the displacing phase.
The tendency to bypass is exacerbated by the differences in wettability of the
reservoir minerals by water and oil. If the encroaching water succeeds in
bypassing oil, then pressure gradients, above an attainable level, will be
required to effect displacement of the trapped ganglia.  See  PETROLEUM
GEOLOGY.
    There are certain shallow reservoirs at a low temperature and a relatively
low pressure which have a high porosity that is saturated with medium- to
high-viscosity oils. The viscosity is so high and the pressure so low that
production rates are virtually uneconomic. Because of the much lower mobility
of the viscous oils, injected water will readily finger through the reservoir
without displacing a significant quantity of oil. Thermal technology has been
developed by which the viscosity of the crude within the reservoir can be
lowered and a suitable pressure gradient applied to achieve significant
recovery of crude from such reservoirs. The methods are the steam soak, or
cyclic stimulation, in which oil is produced from the same well into which
steam had earlier been injected; the steam drive, which is much like a
waterflood but with the substitution of steam for injected water; and wet, or
quenched, in-situ combustion in which a steam drive is generated in situ by
the combustion of a limited fraction of the oil in place. See OIL MINING; OIL
SAND; OIL SHALE.
 �34s[Todd M. Doscher]
�33s

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�6v�2v�32sPetroleum geology   �21m �33s 
    The application of geological concepts to finding and producing petroleum. 
Petroleum deposits are usually found in sedimentary rocks. With a few notable 
exceptions, economically important hydrocarbon deposits are found in the 
subsurface at depths ranging from several feet to several miles.  See  
PETROLEUM.  
  
    Subsurface deposits.  Petroleum occurs in porous rocks such as sandstones, 
limestones, and dolomites. In the subsurface, porosity, the space between the 
particles, is usually filled with water, but under certain conditions the 
fluid is a mixture of water and hydrocarbons, either oil or gas or both. The 
conditions necessary for the accumulation of hydrocarbons are a source and a 
trap to collect them. There are many circumstances which provide the right 
conditions to form an oil or gas field. 
    There are two basic types of hydrocarbon traps, stratigraphic and 
structural, but these often occur in combination with each other. The lens and 
the unconformity are examples of stratigraphic traps. The anticline, fault, 
and salt dome are examples of structural traps. 
    The quality of a reservoir rock is controlled primarily by porosity and 
permeability. Porosity is measured as a percentage of total rock volume. 
Permeability, the ability of the reservoir rock to transmit or allow movement 
of fluids, is provided by the interconnection of pores in the rock and allows 
oil and gas to flow from the reservoir into wells for production. 
    The voids or pore space in all reservoir rocks are filled with fluid, 
which may be water or various combinations of water, oil, and gas. In traps 
that contain oil, water, and free gas, the fluids occur in distinct zones: 
gas, the lightest fluid, occurring at the top of the trap, followed by oil, 
and then water. Where there is no oil, the gas is immediately above the water. 
  
    Surface occurrences.  Petroleum deposits encountered on the surface are 
often outcrops of former subsurface deposits which have been exposed by 
erosion; others are leakages of hydrocarbons from subsurface accumulations. In 
some instances, particles of hydrocarbons (that is, heavy oil, asphalt, or 
tar) have been mixed with sediments and laid down as a primary part of the 
deposit. The hydrocarbons in surface deposits are normally heavy tars and 
asphalts which are residues from normal subsurface oils that have been latered 
by the escape of volatile fractions and oxidation.  See  ASPHALT AND 
ASPHALTITE.  
    Most oil or tar sand deposits are former subsurface deposits which have 
been exhumed by erosion. A few are primary deposits in which particles of tar 
or asphalt have been deposited along with conventional sediments. Only a few 
surface deposits are economically important.  See  OIL SAND. 
    Vast quantities of oil shale are exposed on the surface, with still 
greater quantities present at only modest depths of burial.  See  OIL AND GAS 
FIELD EXPLORATION; PETROLEUM PROSPECTING; SEDIMENTARY ROCKS. 
 �34s[F. E. Foss]
�33s 
  
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�6v�2v�32sPetroleum processing �21m �33s
    The recovery and processing of various usable fractions from the complex
crude oils. The usable fractions include gasoline, jet fuel, kerosine, fuel
oil, asphalt, lubricating oils, and many others.
    Crude oil is a mixture of many different hydrocarbon compounds of the
paraffin type (wax compounds) and of the naphthene type (asphalt compounds),
making the chemistry of petroleum refining extremely complex. The refining
processes can be grouped under three main headings: (1) separating (by
distillation, treating, and blending) the crude oils to isolate the desired
products; (2) breaking the remaining large chemical compounds into smaller
chemical compounds by cracking; (3) building desired product properties by
chemical reactions, such as reforming, alkylation, and isomerization.  See
ALKYLATION (PETROLEUM); CATALYTIC REFORMING; CRACKING; DISTILLATION;
ISOMERIZATION; PETROLEUM PRODUCTS.
 �34s[John J. McKetta ;Harold L. Hoffman]
�33s

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�6v�2v�32sPetroleum products   �21m �33s 
    Crude petroleum is the starting material not only for the fuels used for 
transportation and energy production but also for petrochemical feedstocks, 
solvents, lubricants, asphalts, and many other specialities (see table). The 
more complex the family of petroleum products, the more energy is needed to 
refine the crude. 
    Liquid products such as gasoline, jet fuel, diesel fuel, and marine fuel, 
which together account for more than half the volume of output, serve the 
transportation industry. Hydrocarbons from butane to C12 appear in gasoline 
blends; higher-boiling-fraction C8 to C15 hydrocarbons are the blend stocks 
for jet fuels for the air-transport industry. Most specialized 
energy-consuming process units in refineries, such as catalytic reformers and 
cracking units, exist to convert crude fractions to gasoline of a quality that 
meets the needs of the modern automobile engine. Added to the usual engine 
requirements for antiknock performance and volatility of fuel are 
environmental demands for emission standards in exhaust.  See  ANTIKNOCK 
AGENTS; CRACKING; GASOLINE; JET FUEL. 
    Diesel fuels are blended from both distilled and cracked fractions - up to 
C22-type hydrocarbons. They range in volatility from kerosine for lightweight 
automotive diesel engines to gas oil for large, lower-speed industrial or 
marine diesel engines. Marine fuels are made from the highest-boiling crude 
fraction - the residuum from crude distillation blended with heavy cracked gas 
oils.  See  DIESEL FUEL; KEROSINE. 
    Petroleum fuels heat homes, factories, and offices and also generate 
electric power. Home-heating oils are similar to diesel fuels in boiling range 
but must exhibit storage stability and good atomization and combustion 
performance in small oil burners.  See  FUEL OIL. 
    From each of the fractions distilled from crude or its cracked or 
processed products, valuable and indispensable nonfuel materials are made. 
Some of the major products are: ethylene, propylene, butylene, and other 
reactive gases for the petrochemical industry; liquefied petroleum gases for 
heating, cooking, and drying, solvents for the paint and dry-cleaning 
industries and as vehicles for aerosol products; lubricants of all types; 
specialty oils for hydraulic fluids, transformers, emulsions, cutting fluids, 
pharmaceuticals, inks, preservatives, and so on; and wax for a host of 
applications.  See  LIQUEFIED PETROLEUM GAS (LPG); LUBRICANT; PETROCHEMICAL; 
SOLVENT; WAX, PETROLEUM. 
    The residues from processing crude can also yield such familiar products 
as asphalt, the major road-building material, and coke, burned as fuel or made 
into electrodes.  See  ASPHALT AND ASPHALTITE; COKE; PETROLEUM; PETROLEUM 
PROCESSING.  
 �34s[W. G. Dukek]
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�6v�2v�32sPetroleum prospecting   �21m �33s 
    The search for commercially valuable accumulations of petroleum. This 
search at the one extreme may be carried out in a completely haphazard manner 
with entire dependence on luck for success or, at the other extreme, it may be 
a highly organized procedure involving the use of complex precision 
instruments, skilled and experienced personnel, and advanced scientific 
reasoning. In either case the final and critical step is always the drilling 
of an exploratory hole. 
    Scientific petroleum prospecting consists of (1) determination of 
generally favorable regions with respect to source, reservoir, trap, timing, 
and hydrodynamic conditions; (2) finding local geological features 
(anticlines, fault traps, and pinch-outs) within these regions, believed to be 
suited to the trapping of petroleum; (3) location and programming of 
exploratory holes to test the presence or absence of commercially significant 
petroleum accumulations on these local features; and (4) after initial 
discovery, determination of the extent and character of the accumulation. 
Prospecting methods are the means employed to gain the information called for 
in these four steps. They are commonly classified as geological and 
geophysical, but there is no sharp distinction between the two; all involve 
geological reasoning and interpretation. See GEOCHEMICAL PROSPECTING; 
GEOPHYSICAL EXPLORATION; PROSPECTING; WELL LOGGING. 
 �34s[Hollis D. Hedberg]
�33s 
  
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�6v�2v�32sPetroleum reserves   �21m �33s 
    Proved reserves are the estimated quantities of crude oil liquids which 
with reasonable certainty can be recovered in future years from delineated 
reservoirs under existing economic and operating conditions. Thus, estimates 
of crude oil reserves do not include synthetic liquids which at some time in 
the future may be produced by converting coal or oil shale, nor do reserves 
include fluids which may be recovered following the future implementation of a 
supplementary or enhanced recovery scheme. 
    Indicated reserves are those quantities of petroleum which are believed to 
be recoverable by already implemented but unproved enhanced oil recovery 
processes or by the application of enhanced recovery processes to reservoirs 
similar to those in which such recovery processes have been proved to increase 
recovery.  
    Thus, crude oil reserves can be called upon in the future with a high 
degree of certainty, subject of course to the limitations placed on production 
rate by fluid flow within the reservoir and the capacity of the individual 
producing wells and surface facilities to handle the produced fluids. It is 
important  to bear in the mind the distinction between resources and reserves. 
The former term refers to the total amount of oil that has been discovered in 
the subsurface, whereas the latter refers to the amount of oil that can be 
economically recovered in the future. The ratio of the ultimate recovery (the 
sum of currently proved reserves and past production) to the resource or 
original oil in place is the anticipated recovery efficiency. See NATURAL GAS; 
PETROLEUM; PETROLEUM ENHANCED RECOVERY. 
 �34s[Todd M. Doscher]
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�6v�2v�32sPetroleum reservoir engineering   �21m �33s 
    The technology concerned with the prediction of the optimum economic 
recovery of oil or gas from hydrocarbon-bearing reservoirs. It is an eclectic 
technology requiring coordinated application of many disciplines: physics, 
chemistry, mathematics, geology, and chemical engineering. 
    Originally, the role of reservoir engineering was exclusively that of 
counting oil and natural gas reserves. The scope of reservoir engineering has 
broadened to include the analysis of optimum ways for recovering oil and 
natural gas, and the study and implementation of enhanced recovery techniques 
for increasing the recovery above that which can be expected from the use of 
conventional technology. See PETROLEUM RESERVES. 
    The overall recovery of crude oil from a reservoir is a function of the 
production mechanism, the reservoir and fluid parameters, and the 
implementation of supplementary recovery techniques. In general, recovery 
efficiency is not dependent upon the rate of production except for those 
reservoirs where gravity segregation is sufficient to permit segregation of 
the gas, oil, and water. Where gravity drainage is the producing mechanism, 
which occurs when the oil column in the reservoir is quite thick and the 
vertical permeability is high and a gas cap is initially present or is 
developed on producing, the reservoir will also show a significant effect of 
rate on the production efficiency. See PETROLEUM ENHANCED RECOVERY. 
 �34s[Todd M. Doscher]
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�6v�2v�32sPetroleum reservoir models   �21m �33s 
    Physical and computational systems whose behavior is designed to resemble 
that of actual reservoirs. Early in the petroleum industry's history, there 
was a great deal of effort invested in the development of very complex 
physical (laboratory) models, while the computational models were quite 
simple. Since the explosive evolution of computers, the computational models 
have become more and more complex, and there has been a diminishing emphasis 
on the physical models. Today the emphasis is on combining the best features 
of the physical and computational models to permit evaluation of the complex 
behavior of enhanced oil recovery processes. 
    There are three basic requirements for a useful model: (1) a quantitative 
description of the relevant properties of a reservoir; (2) a means of 
describing the mechanics of fluids movements in a reservoir; and (3) a means 
of combining 1 and 2 by constructing analogous physical systems or by 
constructing mathematical systems which can be solved by computational 
techniques.  
    The purpose of a petroleum reservoir model is to estimate field 
performance (for instance, oil recovery) under a variety of operating 
strategies. Whereas the reservoir can be produced only once - and at 
considerable expense - a model can be produced many times. If this production 
can be accomplished at low expense and over a short period of time, then 
observation of the model's performance can give great insight into how to 
optimally produce a reservoir. Petroleum reservoir models can range in 
complexity from the intuition and judgment of an engineer to complex 
mathematical or physical systems describing enhanced oil recovery processes. 
See OIL FIELD MODEL; PETROLEUM RESERVOIR ENGINEERING. 
 �34s[Harvey S. Price]
�33s 
  
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�6v�2v�32sPetrology   �21m �33s 
    The study of rocks, their occurrence, composition, and origin. Petrography 
is concerned primarily with the detailed description and classification of 
rocks, whereas petrology deals primarily with rock formation, or petrogenesis. 
Experimental petrology reproduces in the laboratory the conditions of high 
pressure and temperature which existed at various depths in the Earth where 
minerals and rocks were formed. A petrological description includes definition 
of the unit in which the rock occurs, its attitude and structure, its 
mineralogy and chemical composition, and conclusions regarding its origin. For 
a discussion of mineral identification, petrographic analysis, and the 
classification of rocks see MINERALOGY; PETROGRAPHY; ROCK. See also IGNEOUS 
ROCKS; METAMORPHIC ROCKS; SEDIMENTARY ROCKS. 
 �34s[William Ingersoll Rose, Jr.]
�33s 
  
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�6v�2v�32sPetromyzonida   �21m �33s 
    The order comprising the lampreys, sometimes called Petromyzontiformes, 
which are eellike, jawless vertebrates (class Agnatha) often grouped with the 
Myxinoidea as Cyclostomata. They differ from Myxinoidea as follows: The single 
nasal opening is on the dorsal side of the head and ends internally as a blind 
sac; the mouth is surrounded by a circular oral disk and provided with a 
rasping tongue (both disk and tongue are set with horny teeth); seven pairs of 
gill pouches open separately to the exterior; adults have well-developed eyes; 
dorsal and caudal fins are separate, and both are supported by fin rays; there 
are separate sexes and a distinct larval stage. See AGNATHA; CYCLOSTOMATA 
(CHORDATA); HYPEROARTII; MYXINOIDEA; OSTEOSTRACI. 
 �34s[Robert H. Denison]
�33s 
  
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�6v�2v�32sPewter   �21m �33s 
    A tarnish-resistant alloy of lead and tin always containing appreciably 
more than 63% tin. Other metals are sometimes used with or in place of the 
lead; among them are copper, antimony, and zinc. Pewter is commonly worked by 
spinning and it polishes to a characteristic luster. Because pewter work 
hardens only slightly, pewter products can be finished without intermediate 
annealing. Early pewter, with high lead content, darkened with age. With less 
than 35% lead, pewter was used for decanters, mugs, tankards, bowls, dishes, 
candlesticks, and canisters. The lead remained in solid solution with the tin 
so that the alloy was resistant to the weak acids in foods. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32spH �21m �33s
    A term used to describe the hydrogen-ion activity of a system. It is
defined by the expression pH =- log a(sub H)+. Here a(sub H)+ is the activity
of the hydrogen ion. In dilute solutions, the activity is essentially equal to
the concentration, and the pH may be approximately defined as pH =-
log 10 [H+], where [H+] is the concentration of the hydrogen ion in moles
per liter. The use of the pH makes negative exponents unnecessary in
describing the hydrogen-ion activity. In a system in which the hydrogen-ion
activity is 10(sup -3) mole/liter, the pH is 3.  See  ACID AND BASE;
HYDROGEN ION.
 �34s[Francis J. Johnston]
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�6v�2v�32sPhaeophyta   �21m �33s 
    A division of algae, also called brown algae, so characteristically marine 
that the larger members are commonly known as brown seaweeds or kelps. They 
are widely distributed but are most numerous on the rocky coasts of the colder 
oceans. However, they may occur in the open ocean, on muddy salt marshes, or 
as epiphytes or endophytes. Many are able to withstand the desiccation of the 
intertidal zone, whereas others must remain continuously submerged. A few 
genera may be found at considerable depth. Both annual and perennial species 
have been discovered. They are distinguished from other algae by their brown 
or olive-green color, by their reproduction, and by the structure of the plant 
body.  
    All of the Phaeophyta have an alternation of generations, that is, a 
definite alternation of sexual and asexual individuals. The plant which 
develops the gametes (sex cells) is called the gametophyte, and that which 
produces the asexual spores is termed the sporophyte. Sexual reproduction is 
isogamous or anisogamous, involving a small, active male gamete and a large, 
nonmotile egg. Asexual reproduction is accomplished by fragmentation of the 
thallus or in most species by zoospores (swimming spores). 
    The plant body is always multicellular and may vary from a simple 
filamentous form to a complex body having a basal attachment structure and a 
stemlike main plant body. The latter may be branched or unbranched and of 
lengths varying up to about 165 ft (50 m). They may be broad and ribbonlike or 
have leaflike branches, and they are often provided with air bladders. The 
largest forms may have a relatively complex internal structure. 
    The Phaeophyta are divided into three classes on the basis of their life 
cycles, Isogeneratae, Heterogeneratae, and Cyclosporeae.  See  ALGAE. 
 �34s[Paul A. Vestal]
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�6v�2v�32sPhagocytosis   �21m �33s 
    A mechanism by which single cells of the animal kingdom, such as smaller 
protozoa, engulf and carry particles into the cytoplasm. It differs from 
endocytosis primarily in the size of the particle rather than in the 
mechanism; as particles approach the dimensions and solubility of 
macromolecules, cells take them up by the process of endocytosis. See 
EDOCYTOSIS.  
    Cells such as the free-living amebas or the wandering cells of the metazoa 
often can ``sense'' the direction of a potential food source and move toward 
it (chemotaxis). If, when the cell contacts the particle, the particle has the 
appropriate chemical composition, or surface charge, it adheres to the cell. 
The cell responds by forming a hollow, conelike cytoplasmic process around the 
particle, eventually surrounding it completely. Although the particle is 
internalized by this sequence of events, it is still enclosed in a portion of 
the cell's surface membrane and thus isolated from the cell's cytoplasm. The 
combined particle and membrane package is referred to as a food or phagocytic 
vacuole. See VACUOLE. 
    Ameboid cells of the metazoa also selectively remove foreign particles, 
bacteria, and other pathogens by phagocytosis. After the foreign particle or 
microorganism is trapped in a vacuole inside the macrophage, it is usually 
digested. To accomplish this, small packets (lysosomes) of lytic proenzymes 
are introduced into the phagocytic vacuole, where the enzymes are then 
dissolved and activated.  See  LYSOSOME. 
 �34s[Philip W. Brandt]
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�6v�2v�32sPhalangida �21m �33s
    An order of the class Arachnida. Individual members of the Phalangida or
Opiliones, popularly known as harvestmen or daddy-longlegs, typically possess
an unsegmented cephalothorax broadly joined to a segmented abdomen, paired
chelate chelicerae, paired palpi, four pairs of segmented legs, and a pair of
simple eyes. In length the bodies range from less than 0.04 to over 0.6 in. (1
to over 15 mm); respiration is by tracheae; some forms possess scent glands at
the anteriolateral portions of the cephalothorax.
    Worldwide in distribution, phalangids vary greatly in appearance. The
majority of temperate forms have relatively small bodies and enormously
elongated legs. Tropical forms are often somewhat flattened and have
relatively shorter legs.
    Three suborders are recognized: the Cyphophthalmi, small mitelike forms;
the Laniatores, often with flattened bodies, mainly tropical in distribution
with some adapted to cave life; and the Palpatores, typically the long-legged
forms of temperate areas.  See	ARACHNIDA.
 �34s[Clarence J. Goodnight ;Marie L. Goodnight]
�33s

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�6v�2v�32sPhanerozonida   �21m �33s 
    An order of Asteroidea in which pedicellariae may occur, but are not the 
crossed type, and in which the margins of the body are defined by two 
conspicuous series of marginal plates, one placed vertically above the other. 
The marginals constitute a buttressing skeleton which is usually more robust 
than the ambulacral skeleton. Pentamerous symmetry is more constant than in 
the other orders of Asteroidea. The order is represented by Paleozoic fossils 
(suborder Pustulosa) and by three extant suborders.  See  ASTEROIDEA; 
NOTOMYOTINA; PAXILLOSINA; VALVATIDA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sPharetronidia   �21m �33s 
    A subclass of sponges of the class Calcarea in which the skeleton is 
formed of quadriradite spicules cemented together into a compact network, or 
includes an aspiculous massive basal skeleton formed of irregular calcitic 
spherulites. Spicules shaped like tuning forks are characteristically present, 
and free triradiates, quadriradiates, and diactinal spicules may occur.  See  
CALCAREA.  
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sPharmaceutical chemistry   �21m �33s 
    The chemistry of drugs and of medicinal and pharmaceutical products. The 
important aspects of pharmaceutical chemistry are as follows: 
    1. Isolation, purification, and characterization of medicinally active 
agents and materials from natural sources (mineral, vegetable, 
microbiological, or animal) used in treatment of disease and in compounding 
prescriptions.  
    2. Synthesis of medicinal agents not known from natural sources, or the 
synthetic duplication, for reasons of economy, purity, or adequate supply, of 
substances first known from natural sources. 
    3. Semisynthesis of drugs, whereby natural substances are transformed by 
means of comparatively simple steps into products which possess more favorable 
therapeutic or pharmaceutical properties. 
    4. Determination of the derivative or form of a medicinal agent which 
exhibits optimum medicinal activity and at the same time lends itself to 
stable formulation and elegant dispensing. 
    5. Determination of incompatibilities, chemical and biological, between 
the various ingredients of a prescription. 
    6. Establishment of safe and practical standards, with respect to both 
dosage and quality, to assure uniform and therapeutically reliable forms for 
all medication. 
    7. Improvement and promotion of the use of chemical agents for prevention 
of illness, alleviation of pain, cure of disease, and search for new 
therapeutic agents, particularly where no satisfactory remedy now exists.  See 
  PHARMACOGNOSY; PHARMACOLOGY; PHARMACY. 
 �34s[Walter H. Hartung]
�33s 
  
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�6v�2v�32sPharmacognosy   �21m �33s 
    The general biology, biochemistry, and economics of nonfood natural 
products of value in medicine, pharmacy, and other health professions. The 
products studied are of biologic origin, either plant or animal. Pharmacognosy 
literally means knowledge of drugs, as do pharmacology and pharmacy. The 
center of interest in pharmacology, however, is on the mode of action of 
drugs. In pharmacy major attention is directed toward provision of suitable 
dosage forms, their production and distribution. Pharmacognosy is restricted 
to natural products with attention centered on sources of drugs, plant and 
animal, and on the biosynthesis and identity of their pharmacodymanic 
constitutents.  
    Organs, or occasionally entire plants or animals, are dried or frozen for 
preservation and are termed crude drugs. They may be used medicinally in 
essentially this form or as sources of mixtures or of chemicals obtained by 
processes of extraction. Mixtures obtained by exudation from living plants 
include such drugs as opium, turpentine, and acacia. Processes of extraction 
are required to obtain such mixtures as peppermint oil (steam distillation), 
podophyllum resin (percolation), and parathyroid extract (solution). 
    Pure chemicals may be extracted from a crude drug (for example, the 
glycoside digitoxin from digitalis or the hormone insulin from pancreas), from 
a mixture obtained by exudation (for example, the alkaloid morphine from 
opium), or from an extracted mixture (for example, the terpene menthol from 
peppermint oil). 
    Vitamins as a class of natural products are within the scope of 
pharmacognosy, although many are obtained commercially by laboratory 
synthesis. Included also are antibiotics and biologicals (serums, vaccines, 
and diagnostic biological products). 
    Development of synthetic drugs related chemically to the active 
constituent of a natural product has frequently followed investigation of 
native use of the natural product as drug or poison. The objective of such 
development is usually to produce a drug having fewer undesirable side effects 
while retaining the useful therapeutic action.  See  PHARMACEUTICAL CHEMISTRY; 
PHARMACOLOGY; PHARMACY. 
 �34s[Richard A. Deno]
�33s 
  
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�6v�2v�32sPharmacology   �21m �33s 
    The science of detection and measurement of the effects of drugs or other 
chemicals on biological systems. The effect of chemicals may be beneficial 
(therapeutic) or harmful (toxic). The pure chemicals or mixtures may be of 
natural origin (plant, animal, or mineral) or may be synthetic compounds. 
    The broad area covered may be conveniently divided into a number of 
categories: chemotherapy, the use of chemicals to destroy invading organisms 
such as bacteria and molds in or on the host; pharmacotherapy, the use of 
drugs to restore or replace normal function in various tissue cells, organs, 
or integrated units; pharmacodynamics, studies on the mechanism of action of 
drugs which may utilize physiological, biochemical, or electrical techniques; 
toxicology, the study of the poisonous effects of chemicals; 
psychopharmacology, the study of the effects of chemicals on the behavior of 
humans or animals; biochemical pharmacology, the effects of chemicals on 
biochemical reactions in living systems, and the effects of these systems on 
the chemicals, that is, their metabolism; structure-activity relationship, 
relationship of biological activity to chemical structure and molecular 
properties; and clinical pharmacology, the study and evaluation of the effects 
of drugs in humans.  See  CHEMOTHERAPY; PATHOLOGY; TOXICOLOGY. 
 �34s[Charles J. Kensler]
�33s 
  
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�6v�2v�32sPharmacy   �21m �33s 
    The health profession concerned with the discovery, development, 
production, and distribution of drugs. Drugs are substances (other than 
devices) used to diagnose, prevent, cure, or relieve the symptoms of disease. 
For relations to closely allied fields  see  MEDICINE; PHARMACEUTICAL 
CHEMISTRY; PHARMACOGNOSY; PHARMACOLOGY. 
    General practice is carried on in exclusive prescription pharmacies, 
semiprofessional pharmacies, and drug stores. It consists of compounding and 
dispensing drugs on order of the physician, dentist, or veterinarian; serving 
as consultant on drugs to the health professions and to the public; and 
selling other health supplies such as antiseptics, bandages, and home remedies. 
    A hospital pharmacy includes special administrative features, provision of 
drugs for nursing stations, manufacturing of pharmaceutical preparations, 
teaching of nurses and medical and pharmacy interns, service to the hospital 
committee on pharmacy and therapeutics, preparation and revision of a hospital 
formulary, and monitoring the drug regimen of the individual patient (clinical 
pharmacy). The pharmacist may have charge of investigational drugs, 
radioactive pharmaceuticals, medical and surgical sterile supplies, and 
gaseous drugs for inhalation therapy. 
 �34s[Richard A. Deno]
�33s 
  
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�6v�2v�32sPharynx   �21m �33s 
    A chamber at the oral end of the vertebrate alimentary canal, leading to 
the esophagus. In adult humans it is divided anteriorly by the soft palate 
into a nasopharynx and an oropharynx, lying behind the tongue but anterior to 
the epiglottis; there is also a retropharyngeal compartment, posterior to both 
epiglottis and soft palate. The nasopharynx receives the nasal passages and 
communicates with the two middle ears through auditory tubes. The retropharynx 
leads to the esophagus and to the larynx, and the paths of breathing and 
swallowing cross within it.  See  ESOPHAGUS; LARYNX; PALATE. 
 �34s[William W. Ballard]
�33s 
  
A700000001555�254s�5v�1v1284
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�6v�2v�32sPhase-angle measurement �21m �33s
    The determination of the relative times at which alternating currents and
voltages in a circuit take on zero values. If two voltages v1 and v2 are zero
at the same instant, they are in phase, with zero phase difference (or out of
phase with 180DG difference). If one voltage v1 passes through zero 1/8 cycle
before a second voltage v2, it leads by 360DG/8 or 45DG. The common phase
meter, a commercial device for determining the angle between current and
voltage, can be used when its presence will not disturb the circuits under
measurement. When phase angles to be measured are in high-impedance or
low-power circuits, this device is unsatisfactory and other measurement
methods must be used.  See  PHASE (PERIODIC PHENOMENA); PHASE METER.
 �34s[Harry Sohon /Edward C. Stevenson]
�33s

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�6v�2v�32sPhase-contrast microscope   �21m �33s 
    A microscope used for making visible differences in phase or optical path 
in transparent or reflecting specimens. It is one of the most important 
instruments available for studying living cells and is widely used in 
biological and medical research. 
    The essential features of a phase-contrast microscope are shown in the 
illustration. The practical problem is to find some way of separating the 
incident or direct light from that diffracted by the object. This is done by 
placing a diaphragm D of easily recognizable shape, such as an annulus, at the 
front focal plane of the substage condenser C. Light from each point of the 
focal plane passes as a parallel pencil of rays through the specimen S and is 
brought to a focus at the rear focal plane P of the objective O. Thus, on 
removing the eyepiece, an image of the annulus will be seen at the back of the 
objective lens. This image corresponds to the incident light. In addition, 
when a specimen is present, some light is diffracted by it and spreads out to 
fill the whole of the back lens of the objective. Thus, apart from the small 
area of overlap over the image of the annulus, the direct and diffracted waves 
are essentially separated at the plane P. A phase plate is now inserted at 
this level. This can be a transparent disk with an annular groove of such 
dimensions that it coincides exactly with the image of the diaphragm D. All 
the direct light now passes through the groove in the phase plate, whereas the 
diffracted light passes mainly outside the groove. Since the diffracted light 
has to pass through a greater thickness of transparent material than the 
direct light, a phase difference, depending on the refractive index of the 
phase-plate material and on the thickness of the groove, is introduced between 
them. If this phase difference is about one-quarter of a wavelength, the basic 
conditions for phase contrast will have been achieved. If the phase plate is 
made to retard the incident wave by a quarter of a wavelength, the crests and 
troughs of the two waves will coincide, giving a resultant of greater 
amplitude. Refractile details will appear bright (negative contrast) instead 
of dark (positive contrast). 
  
    The phase-contrast microscope is the routine instrument for the 
examination of living cells because it is possible to study the cell structure 
under excellent optical conditions and with no loss in resolving power. The 
method is also useful for the study of unstained tissue sections and has found 
considerable use for the comparison of material in the electron and optical 
microscopes.  See  ELECTRON MICROSCOPE; MICROSCOPE; OPTICAL MICROSCOPE. 
 �34s[Robert Barer]
�33s 
  
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�6v�2v�32sPhase equilibrium �21m �33s
    A general field of physical chemistry dealing with the various situations
in which two or more phases (or states of aggregation) can coexist in
thermodynamic equilibrium with each other, with the nature of the transitions
between phases, and with the effects of temperature and pressure upon these
equilibria. Many superficial aspects of the subject are largely qualitative,
for example, the empirical classification of types of phase diagrams; but the
basic problems always are susceptible to quantitative thermodynamic treatment,
and in many cases, statistical thermodynamic methods can be applied to simple
molecular models.
    Thermodynamics requires that when two phases, ALPHA and BETA, are free to
exchange heat, mechanical work, and matter (chemical species), the temperature
T, the pressure P, and the chemical potential (partial molar free energy)
MU(sub i) of each particular component i must be equal in both phases at
equilibrium.  Algebraically, equilibrium exists when T(sub ALPHA) = T(sub BETA),
P(sub ALPHA) = P(sub BETA), MU(sub i,ALPHA) = MU(sub i, BETA), and MU(sub j,
ALPHA) = MU(sub j, BETA).
  These conditions of thermal, mechanical, and material equilibrium need not
all be present if the equilibrium between phases is subject to inhibiting
restrictions.  Thus, for a solution of a nonvolatile solute in equilibrium with
the solvent vapor, the condition of equality of solute chemical potentials
MU(sub 2,ALPHA) = MU(sub 2,BETA) need not apply, since there can be no solute
molecules in the vapor phase.  Similarly, in osmotic equilibria, in which
solvent molecules can pass through a semipermeable membrane, whereas solute
molecules cannot, MU(sub 1,ALPHA) = MU( sub 1,BETA) and T(sub 1,ALPHA) = T(sub
2,BETA), but the solute chemical potentials MU(sub 2) are unequal, as are the
pressures on opposite sides of the membrane.  See OSMOSIS; SOLUTION.
   If a system consists of P phases and C distinguishable components, there are
C + 2 thermodynamic variables (C chemical potentials MU(sub i), plus the
temperature and pressure) which are interrelated by an equation for each phase.
Since there are P independent equations relating the C + 2 variables, only F = C
+ 2 - P variables need be fixed to define completely the state of the system at
equilibrium; the other variables are then beyond control.  This relation for the
number of degrees of freedom F, or variance, is called the phase rule.	It has
proved to be a powerful tool in interpreting and classifying types of phase
equilibria.
  When chemical changes may occur in the system, the number of components C is
the number of independent components whose amounts can be varied by the
experimenter; this is equal to the total number of chemical species present less
the number of independent chemical equilibria between them.
  An invariant system has no degrees of freedom (F = 0), for which the number
of phases P = C + 2.  For a one-component system, such an invariant point is a
triple point at which three phases coexist at a single temperature and pressure
only; for a two-component system, a quadruple point (four phases) would be
invariant.  See TRIPLE POINT.
 �34s[Robert L.  Scott]
�33s

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�6v�2v�32sPhase inverter   �21m �33s 
    A circuit having the primary function of changing the phase of a signal by 
180DG. The phase inverter is most commonly employed as the input stage for a 
push-pull amplifier. Therefore, the phase inverter must supply two voltages of 
equal magnitude and 180DG phase difference. A variety of circuits are 
available for the phase inversion.  See  PUSH-PULL AMPLIFIER. 
    Overall fidelity of a phase inverter and push-pull amplifier can be 
adversely affected by improper design of the phase inverter. The principal 
design requirement is that frequency response of one input channel to the 
push-pull amplifier be identical to the frequency response of the other 
channel.  
    The simplest form of phase-inverter circuit is a transformer with a 
center-tapped secondary. Careful design of the transformer assures that the 
secondary voltages are equal. The transformer forms a good inverter when the 
inverter must supply power to the input of the push-pull amplifier. However, 
it usually costs more, occupies more space, and weighs more than a transistor 
or vacuum-tube circuit, and its frequency response may not be as uniform as 
that which can be obtained from solid-state or vacuum-tube circuits. 
    An amplifier that provides two equal output signals 180DG out of phase is 
called a paraphase amplifier. If coupling capacitors can be omitted, the 
simplest paraphase amplifier is shown in the illustration. Approximately the 
same current flows through RsubL and RsubE, and therefore if RsubL and RsubE 
are equal, the ac output voltages from the collector and from the emitter are 
equal in magnitude and 180DG out of phase.  See  PHASE (PERIODIC PHENOMENA); 
PHASE-ANGLE MEASUREMENT. 
 �34s[Harold F. Klock]
�33s 
  
  
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�6v�2v�32sPhase-locked loops   �21m �33s 
    Electronic circuits for locking an oscillator in phase with an arbitrary 
input signal. A phase-locked loop (PLL) is used in two fundamentally different 
ways: (1) as a demodulator, where it is employed to follow (and demodulate) 
frequency or phase modulation, and (2) to track a carrier or synchronizing 
signal which may vary in frequency with time. When operating as a demodulator, 
the PLL may be thought of as a matched filter operating as a coherent 
detector. When used to track a carrier, it may be thought of as a narrow-band 
filter for removing noise from the signal and regenerating a clean replica of 
the signal.  See  DEMODULATOR; ELECTRIC FILTER. 
    The basic components of a phase-locked loop are shown in the illustration. 
The input signal is a sine or square wave of arbitrary frequency. The 
voltage-controlled oscillator (VCO) output signal is a sine or square wave of 
the same frequency as the input, but the phase angle between the two is 
arbitrary. The output of the phase detector consists of a direct-current (dc) 
term, and components of the input frequency and its harmonics. The low-pass 
filter removes all alternating-current (ac) components, leaving the dc 
component, the magnitude of which is a function of the phase angle between the 
VCO signal and the input signal. If the frequency of the input signal changes, 
a change in phase angle between these signals will produce a change in the dc 
control voltage in such a manner as to vary the frequency of the VCO to track 
the frequency of the input signal. 
    The most widespread use of phase-locked loops is undoubtedly in television 
receivers. Synchronization of the horizontal oscillator to the transmitted 
sync pulses is universally accomplished with a PLL. The color reference 
oscillator is often synchronized with a phase-locked loop. Phase-locked loops 
are also used as frequency demodulators. They have been applied to stereo 
decoders made on silicon monolithic integrated circuits. High-performance 
amplitude demodulators may be built using phase-lock techniques.  See  
AMPLITUDE-MODULATION DETECTOR. 
 �34s[Thomas B. Mills]
�33s 
  
  
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�6v�2v�32sPhase meter �21m �33s
    An instrument for the measurement of electrical phase angles. It is
sometimes called the crossed-coil meter or the Tuma phase meter and is the
basic element of power-factor meters and synchroscopes. When used for
power-factor indication, it contains two movable coils A and B on a common
shaft, as shown in the illustration. The two coils move as a unit, the angle
BETA between them remaining fixed. There is no restraining spring acting on
the shaft and the system will turn as long as currents produce an average
nonzero torque.
    The meter contains a fixed coil C, which carries the load current. The
fixed coil is made in two sections so that its magnetic field will be nearly
uniform in the neighborhood of the movable coils.
    Suppose the current in coil B leads the current in fixed coil C by the
angle THETA, and the current in coil B lags the current in coil A by the angle
PHI. It can be shown that if BETA equals PHI, then the shaft will turn (ALPHA
will vary) until ALPHA equals THETA and the phase meter indicates directly the
phase angle between the sources supplying coil C and the crossed coils. A
change of 5DG in phase angle produces a deflection of 5DG on the phase meter.
See  PHASE-ANGLE MEASUREMENT; POWER-FACTOR METER.
 �34s[Harry Sohon /Edward C. Stevenson]
�33s

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�6v�2v�32sPhase modulation �21m �33s
    A special kind of angle modulation in which the linearly increasing angle
of a sine-wave carrier has added to it a phase angle that is proportional to
the instantaneous value of the modulating wave (message to be communicated).
Phase modulation (PM) is a scheme for impressing the message to be
communicated upon a high-frequency sine-wave carrier. There is a direct
proportionality between the message to be communicated and the phase
variations imparted to the modulated wave propagated to the receiver. For
basic concepts, technical terms, and supplementary information	see
MODULATION.  See also  ANGLE MODULATION.
    Like other forms of angle modulation, PM reduces noise at the cost of
extra bandwidth occupancy, transmits constant average signal power, transmits
constant peak power equal to twice the average signal power, and has a
channel-grabbing property whereby, if two signals reach the PM detector, the
larger signal is accepted to the near exclusion of the smaller.
    Important applications include certain types of telegraph, telemetering,
and data-processing systems. PM is used in certain microwave radio relay
systems, some of which carry telephone conversations and television programs
simultaneously. PM techniques are used in many types of measuring and control
systems.
    The sharp limitation of range (channel grabbing) is especially important
for some services. Typical examples using PM include mobile and fixed radio
systems for police, airway, and military applications.	See  PHASE-MODULATION
DETECTOR; PHASE MODULATOR.
 �34s[Howard S. Black]
�33s

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�6v�2v�32sPhase-modulation detector   �21m �33s 
    A device which recovers or detects the modulating signal from a 
phase-modulated carrier. Any frequency-modulation (FM) detector with minor 
modifications will detect phase-modulated waves.  See  FREQUENCY-MODULATION 
DETECTOR; PHASE MODULATION. 
    The only difference between FM and phase modulation (PM) is the manner in 
which the modulation index varies with the modulating frequency. The 
modulation index is independent of the modulating frequency in PM but is 
inversely proportional to the modulating frequency in FM. Therefore an FM 
detector, when used to detect a phase-modulated wave, produces an output 
voltage which is proportional to the modulating frequency, assuming the 
original modulating signal to be of constant amplitude. Consequently, a 
low-pass filter with a single reactive element, such as an RC 
(resistance-capacitance) filter, is needed in the output of the FM detector 
which is used to detect a phase-modulated wave. 
 �34s[Charles L. Alley]
�33s 
  
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�6v�2v�32sPhase modulator   �21m �33s 
    An electronic circuit that causes the phase angle of the modulated wave to 
vary (with respect to the unmodulated carrier) in accordance with the 
modulating signal. Since frequency is the rate of change of phase, a phase 
modulator will produce the characteristics of frequency modulation (FM) if the 
frequency characteristics of the modulating signal are so altered that the 
modulating voltage is inversely proportional  to frequency. Commercial FM 
transmitters normally employ a phase modulator because a crystal-controlled 
oscillator can then be used to meet the strict carrier-frequency control 
requirements of the Federal Communications Commission. The chief disadvantage 
of phase modulators is that they generally produce insufficient 
frequency-deviation ratios, or modulation index, for satisfactory noise 
suppression. Frequency multiplication can be used, however, to increase the 
modulation index to the desired value, since the frequency deviation is 
multiplied along with the carrier frequency.  See  FREQUENCY MODULATION; PHASE 
MODULATION; PHASE-MODULATION DETECTOR. 
  
    Many types of phase modulators have been devised. A simple modulator is 
shown in the illustration. In this circuit the modulating voltage changes the 
capacitance of the varactor diode. The phase shift depends upon the relative 
magnitudes of the capacitive reactance of the varactor diode and the load 
resistance R. Therefore the phase shift varies with the modulating voltage and 
phase modulation (PM) is accomplished. However, the phase shift is not 
linearly related to the capacitance and the capacitance of the varactor diode 
is not linearly related to the modulating voltage.  See  VARACTOR. 
    In one system of phase modulation the phase shift of the carrier is 
obtained by means of a special electron tube, the phasitron. The principal 
difficulty with normal phase modulators is a result of the relatively small 
amount of phase shift that can be produced without introducing nonlinearities 
and the resultant large amount of frequency multiplication which is usually 
needed. This difficulty is alleviated to a large degree by means of the 
phasitron in which a phase swing of as much as PLUS/MINUS200DG can be obtained 
with low distortion. 
 �34s[Charles L. Alley]
�33s 
  
  
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�6v�2v�32sPhase rule �21m �33s
    A relationship used to determine the number of state variables F, usually
chosen from among temperature, pressure, and species compositions in each
phase, which must be specified to fix the thermodynamic state of a system in
equilibrium. It was derived by J. Willard Gibbs. The phase rule (in the
absence of electric, magnetic, and gravitational phenomena) is given by the
equation below, where C is the number of chemical species present at
equilibrium, P is the number of phases, and M is the number of independent
chemical reactions. Here phase is used to indicate a homogeneous, mechanically
separable portion of the system, and the term independent reactions refers to
the smallest number of chemical reactions which, upon forming various linear
combinations, includes all reactions which occur among the species present.
The number of independent state variables F is referred to as the degrees of
freedom or variance of the system.

    F = C - P - M + 2

See  CHEMICAL EQUILIBRIUM; CHEMICAL
THERMODYNAMICS; PHASE EQUILIBRIUM; THERMODYNAMIC PROCESSES.
 �34s[Stanley I. Sandler]
�33s


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�6v�2v�32sPhase transitions �21m �33s
    Changes of state brought about by a change in an intensive variable (for
example, temperature or pressure) of a system. Some familiar examples of phase
transitions are the gas-liquid transition (condensation), the liquid-solid
transition (freezing), the normal-to-superconducting transition in electrical
conductors, the paramagnet-to-ferromagnet transition in magnetic materials,
and the superfluid transition in liquid helium. Further examples include
transitions involving amorphous or glassy structures, spin glasses,
charge-density waves, and spin-density waves.  See  AMORPHOUS SOLID;
CHARGE-DENSITY WAVE; METALLIC GLASSES; SPIN-DENSITY WAVE; SPIN GLASS;
SUPERCONDUCTIVITY; SUPERFLUIDITY.
    Typically the phase transition is brought about by a change in the
temperature of the system. The temperature at which the change of state occurs
is called transition temperature (usually denoted by T[sub c]). For example, the
liquid-solid transition occurs at the freezing point.
    The two phases above and below the phase transition can be distinguished
from each other in terms of some ordering that takes place in the phase below
the transition temperature. For example, in the liquid-solid transition, the
molecules of the liquid get ``ordered'' in space when they form the solid
phase. In a paramagnet, the magnetic moments on the individual atoms can point
in any direction (in the absence of an internal magnetic field), but in the
ferromagnetic phase the moments are lined up along a particular direction,
which is then the direction of ordering. Thus in the phase above the
transition, the degree of ordering is smaller than in the phase below the
transition. One measure of the amount of disorder in a system is its entropy,
which is the negative of the first derivative of the thermodynamic free energy
with respect to temperature. When a system possesses more order, the entropy
is lower. Thus at the transition temperature the entropy of the system changes
from a higher value above the transition to some lower value below the
transition.  See  ENTROPY; FERROMAGNETISM; PARAMAGNETISM.
    This change in entropy can be continuous or discontinuous at the
transition temperature. In other words, the development of order in the system
at the transition temperature can be gradual or abrupt. This leads to a
convenient classification of phase transitions into two types, namely,
discontinuous and continuous.
    Discontinuous transitions involve a discontinuous change in the entropy at
the transition temperature. A familiar example of this type of transition is
the freezing of water into ice. As water reaches the freezing point, order
develops without any change in temperature. Thus there is a discontinuous
decrease in the entropy at the freezing point. This is characterized by the
amount of latent heat that must be extracted from the water for it to be
``ordered'' into the solid phase (ice). Discontinuous transitions are also
called first-order transitions.
    In a continuous transition, entropy changes continuously, and hence the
growth of order below T(sub c) is also continuous. There is no latent heat
involved in a continuous transition. Continuous transitions are also called
second-order transitions. The paramagnet-to-ferromagnet transition in magnetic
materials is an example of such a transition.  See  SECOND-ORDER TRANSITION.
    The degree of ordering in a system undergoing a phase transition can be
made quantitative in terms of an order parameter. At temperatures above the
transition temperature the order parameter has a value zero, and below the
transition it acquires some nonzero value. For example, in a ferromagnet the
order parameter is the magnetic moment per unit volume (in the absence of an
externally applied magnetic field). It is zero in the paramagnetic state since
the individual magnetic moments in the solid may point in any random
direction. Below the transition temperature, however, there exists a preferred
direction of ordering, and as the temperature  is lowered below T(sub c), more
and more individual magnetic moments start to align along the preferred
direction of ordering, leading to a continuous growth of the magnetization or
the macroscopic magnetic moment per unit volume in the ferromagnetic state.
Thus the order parameter changes continuously from zero above to some nonzero
value below the transition temperature. In a first-order transition, the order
parameter would change discontinuously at the transition temperature.  See
CRITICAL PHENOMENA.
 �34s[David J. Sellmyer ;S. Jafarey]
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�6v�2v�32sPhase velocity �21m �33s
    The velocity of propagation of a pure sine wave of infinite extent. In one
dimension, for example, the form of the disturbance for such a wave is given
by y(x,t)=A sin [2PI(x/LAMBDA-t/T)]. Here x is the position at which the
disturbance y(x,t) exists at time t, LAMBDA is the wavelength, T is the period
which is related to the wave frequency by T=1/f, and A is the disturbance
amplitude. The argument of the sine function is called the phase. The phase
velocity is the speed with which a point of constant phase can be said to
move. Thus x/LAMBDA-ft=constant, so the phase velocity vsubp is given by
dx/dt=v(sub p)=LAMBDA f. This is the basic relationship connecting phase
velocity, wavelength, and frequency.  See  PHASE (PERIODIC PHENOMENA); SINE
WAVE; WAVE MOTION. 
    The phase velocity for waves in a medium is determined in part by
intrinsic properties of the medium. For all mechanical waves in elastic media,
the square of the phase velocity is proportional to the ratio of the
appropriate elastic property of the medium to the appropriate inertia
property. The phase velocity of electromagnetic waves depends upon the medium
as well. In vacuum, the phase velocity c is given by c(sup 2) = 1/EPSILON(sub
0)MU(sub 0) APPROX EQUAL 9x10(sup 16) m(sup 2)/s(sup 2), where EPSILON(sub 0)
and MU(sub 0) are respectively the permitivity and permeability of the vacuum.
Phase velocity may also depend upon the mode of wave propagation - in general,
upon the frequency of the wave. Waves of different frequencies will travel at
different speeds, resulting in a phenomenon called dispersion. See 
ELECTROMAGNETIC RADIATION; LIGHT; WAVE EQUATION; YOUNG'S MODULUS. 
 �34s[S. A. Williams]
�33s

A700000001627�254s�5v�1v1283
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�6v�2v�32sPhase (astronomy)   �21m �33s 
    In astronomy, the changing appearance of the Moon, inner planets, and Mars 
due to the angular difference between the incident light from the Sun and the 
viewing direction of the observer. Phases of the Moon are a familiar sight. 
During a lunar month (29.53 Earth days), the Moon completes a cycle of 
appearances or phases: dark of the moon, or new moon, during which the Moon is 
nearer the Sun than it is the Earth; crescent until first quarter (about a 
week after new moon); half an illuminated disk; the Moon continues to wax, 
being gibbous, until it is full; it then wanes through third quarter and 
completes the cycle as illustrated. 
    Mercury and Venus show phases like those of the Moon. Mars varies in 
appearance from full to gibbous.  See  MOON. 
 �34s[Gerard P. Kuiper]
�33s 
  
  
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�6v�2v�32sPhase (periodic phenomena)   �21m �33s 
    The fractional part of a period through which the time variable of a 
periodic quantity (alternating electric current, vibration) has moved, as 
measured at any point in time from an arbitrary time origin. In the case of a 
sinusoidally varying quantity, the time origin isusually assumed to be the 
last point at which the quantity passed through a zero position from a 
negative to a positive direction. 
    In comparing the phase relationships at a given instant between two 
time-varying quantities, the phase of one is usually assumed to be zero, and 
the phase of the other is described, with respect to the first, as the 
fractional part of a period through which the second quantity must vary to 
achieve a zero of its own (see illustration). In this case, the fractional 
part of the period is usually expressed in terms of angular measure, with one 
period being equal to 360DG or 2PI radians.  See  PHASE-ANGLE MEASUREMENT; 
SINE WAVE. 
 �34s[William J. Galloway]
�33s 
  
  
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�6v�2v�32sPhenanthrene �21m �33s
    Colorless, crystalline hydrocarbon (C12H10) which melts at about 100DGC
(212DGF) and boils at 332DGC (630DGF). Phenanthrene (I) is usually obtained
from coal tar, but it may also be produced by the hydrogenation of coal.
    Phenanthrene has little commercial importance, except that it may be
hydrgoenated to symmetrical octahydrophenanthrene, which in turn may be
rearranged and dehydrogenated to anthracene.  Phenanthrene is of interest
because the nucleus is found in some resin acids and is produced by the
degradation of certain alkaloids.  Reduction product of
1,2-cyclopentenophenanthrene (III) may be regarded as forming the skelton of the
steroids.  See AROMATIC HYDROCARBON; POLYNUCLEAR HYDROCARBON; STEROL.

 �34s[Charles K. Bradsher]
�33s


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�6v�2v�32sPhenocryst   �21m �33s 
    A relatively large crystal embedded in a finer-grained or glassy igneous 
rock. The presence of phenocrysts gives the rock a porphyritic texture. 
Phenocrysts are represented most commonly by feldspar, quartz, biotite, 
hornblende, pyroxene, and olivine. Strictly speaking, phenocrysts crystallize 
from molten rock material (lava or magma). They commonly represent an earlier 
and slower stage of crystallization than does the matrix in which they are 
embedded.  See  IGNEOUS ROCKS; PORPHYROBLAST. 
 �34s[Carleton A. Chapman]
�33s 
  
A700000003350�254s�5v�1v1288
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�6v�2v�32sPhenol �21m �33s
    The simplest member of a class of organic compounds possessing a hydroxyl
group attached to a benzene ring or to a more complex aromatic ring system.
Phenol itself, C6H5OH, may also be called hydroxybenzene or carbolic acid.
Pure phenol is a colorless solid melting at 42DGC (108DGF), moderately soluble
in water, and weakly acidic (pK 9.9).
    Phenol has broad biocidal properties, and dilute aqueous solutions have
long been used as an antiseptic. At higher concentrations phenol causes severe
skin burns; it is a violent systemic poison.  See  ANTISEPTIC.
    Phenol has the structure shown. Simple substituted phenols, such as the
three isomeric chlorophenols, are named as indicated, using the ortho (o),
meta (m), and para (p) prefixes. In more highly substituted phenols the
positions of substitution are indicated by numbers (as in 2,
4-dichlorophenol). Compounds with more than one hydroxyl group per aromatic
ring are known as polyhydric phenols, and include catechol, resorcinol,
hydroquinone, phloroglucinol, and pyrogallol.
    Phenol is one of the most versatile and important industrial organic
chemicals. It is the starting point for many diverse products used in the home
and industry. A partial list includes: nylon, epoxy resins, surface active
agents, synthetic detergents, plasticizers, antioxidants, lube oil additives,
phenolic resins (with formaldehyde, furfural, and so on), polyurethanes,
aspirin, dyes, wood preservatives, herbicides, drugs, fungicides, gasoline
additives, inhibitors, explosives, and pesticides.  See  PARA-AMINOPHENOL;
CATECHOL; CRESOL; HYDROQUINONE; NAPHTHOL; PHENOLIC RESIN; PICRIC ACID;
RESORCINOL.
 �34s[Robert I. Stirton ;Martin Stiles]
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�6v�2v�32sPhenolic resin   �21m �33s 
    One of the condensation products of phenols or phenolic derivatives with 
aldehydes such as formaldehyde and furfural. The phenol-formaldehyde resins, 
developed commerically between 1905 and 1910, were the first truly synthetic 
polymers and have found wide usage. They are characterized by low cost, 
dimensional stability, high strength, and resistance to aging. 
    Phenolic resins can be cast from syrupy intermediates or molded from 
B-stage solid resins. Laminated products can be produced by impregnating 
fiber, cloth, wood, and other materials with the resin. An important type of 
phenolic resin product is rigid foam. Cured phenolic plastics are rigid, hard, 
and resistant to chemicals (except strong alkali) and to heat. 
    Some of the uses for phenolic resins are for making precisely molded 
articles, such as telephone parts, for manufacturing strong and durable 
laminated boards, or for impregnating fabrics, wood, or paper. Phenolic resins 
are also widely used as adhesives, as the binder for grinding wheels, as 
thermal insulation panels, as ion-exchange resins, and in paints and 
varnishes.  See  ADHESIVE; EPOXY RESIN; ION EXCHANGE; PHENOL; PLASTICS 
PROCESSES; POLYMERIZATION. 
 �34s[John A. Manson]
�33s 
  
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�6v�2v�32sPhenylalanine   �21m �33s 
    An amino acid considered essential for normal growth of animals. Dietary 
phenylalanine is the source of tyrosine in animal tissues. Phenylalanine 
originates biosynthetically from phosphoenolpyruvic acid and 
D-erythrose-4-phosphate by way of shikimic acid and prephenic acid.  See  
AMINO ACIDS; TYROSINE. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sPhenylpyruvic oligophrenia   �21m �33s 
    A type of mental deficiency caused by an inherited defect in the 
metabolism of the amino acid phenylalanine caused by the absence of the liver 
enzyme responsible for the metabolism of phenylalanine. The condition, also 
known as phenylketonuria, is rare with an incidence of 1 in 25,000 in the 
general population and 3 in 2300 mental deficients. The concentration of 
unmetabolized phenylalanine is associated with dimunition of activity in the 
higher mental centers and in permanent intellectual retardation.  See  
PHENYLALANINE.  
    It is possible to alter the levels of such abnormal metabolites in the 
body by changing the diet; phenylalanine-free diets have been developed. There 
are many indications that if diet control can be applied in early infancy, 
damage to the nervous system can be halted and more normal maturation 
anticipated.  See  METABOLIC DISORDERS. 
 �34s[Maurice G. Kott]
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�6v�2v�32sPhlebitis   �21m �33s 
    An inflammatory process in the wall of a vein. Infection may come from a 
distance and gain a foothold in the vessel wall, or probably more commonly, 
veins become infected secondarily from continuous inflamed tissues. When a 
larger vein is infected, thrombosis or intravascular clotting is likely to 
occur which results in partial or complete occlusion of the vessel. Inflamed 
veins predispose to thrombosis because the damaged walls lose their smooth 
lining and become roughened, a condition which allows the deposition of clot 
material from the blood. Thrombi either become organized and recanalized or 
are dislodged to form emboli. Dislodged venous thrombi or emboli pass through 
the right side of the heart and on to the lungs. Small emboli plug pulmonary 
arteries, cause anoxia, and may result in necrosis or infarction of a region 
of the lung. Large emboli may result in massive pulmonary embolism with 
obstruction of several major pulmonary arteries, causing sudden death. See 
EMBOLUS; THROMBOSIS. 
 �34s[Frank A. Carone]
�33s 
  
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�6v�2v�32sPhlebotomus fever   �21m �33s 
    A mild, insect-borne virus disease of humans occurring commonly in 
Mediterranean countries and in the Soviet Union, China, and India. It is also 
known as sandfly fever. 
    After the bite of an infected female sandfly (in the case of the Middle 
East members of the group), the patient develops headache, malaise, 
conjunctivitis, nausea, pain, and stiffness. All patients recover. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sPhloem   �21m �33s 
    The principal food-conducting tissue in vascular plants. Its conducting 
cells are known as sieve elements, but phloem may also include companion 
cells, parenchyma cells, fibers, sclereids, rays, and certain other cells. As 
a vascular tissue, phloem is spatially associated with xylem, and the two 
together form the vascular system.  See  XYLEM. 
    Sieve elements differ from phloem parenchyma cells in the structure of 
their walls and to some extent in the character of their protoplasts. Sieve 
areas, distinctive structures in sieve element walls, are specialized primary 
pit fields in which there may be numerous modified plasmodesmata. 
Plasmodesmata are strands of cytoplasm connecting the protoplasts of two 
contiguous cells. These strands are often surrounded by callose, a 
carbohydrate material, that appears to form rapidly in plants when they are 
placed under stress. 
    Companion cells are specialized parenchyma cells that occur in close 
ontogenetic and physiologic association with sieve tube members. Some 
sieve-tube members lack companion cells. The precise functional relationship 
between these two kinds of cells is unknown. 
    Parenchyma cells in the phloem occur singly or in strands of two or more 
cells. They store starch, frequently contain tannins or crystals, commonly 
enlarge as the sieve elements become obliterated, or may be transformed into 
sclereids or cork cambium cells. 
    Phloem fibers vary greatly in length (from less than 0.04 in. or 1 mm in 
some plants to 20 in. or 50 cm in the ramie plant). The secondary walls are 
commonly thick and typically have simple pits, but may or may not be lignified. 
 �34s[Michael A. Walsh]
�33s 
  
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�6v�2v�32sPhlogopite   �21m �33s 
    A mineral of the mica group, also called bronze mica. Its composition is 
K2[Mg,Fe(II)]6(Si6,Al2)O20(OH)4, including minor amounts of sodium, Na, that 
substitute for potassium, K, and containing small amounts of Mn, Fe(III), and 
Ti. With an increase in Fe(II), it grades into biotite from which there is no 
sharp distinction. It is the Mg-rich half of the biotite-phlogopite series 
known to the mica industry as amber mica. Phlogopite is widely used as an 
electrical insulator. 
    It occurs in disseminated flakes, foliated masses, or large crystals. The 
specific gravity is 2.8 - 3.0, and hardness is 2.5 - 3.0 on Mohs scale. Thin 
sheets are transparent in shades of light brown and green. Phlogopite occurs 
chiefly in certain peridotites (kimberlites), in carbonatites, in 
serpentinized periodotites, in marbles derived from impure dolomitic 
limestones, and as very large crystals of commercial importance in 
coarse-grained plagioclase-apatite-calcite-pyroxene rocks of pegmatitic 
affinity. It alters to vermiculite.  See  MICA; SILICATE MINERALS. 
 �34s[E. William Heinrich]
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�6v�2v�32sPhobic reaction �21m �33s
    A type of neurosis. The specific forms which phobic reactions, or intense
irrational fears whose irrationality the individual may realize without being
able to dispel the fear itself, may take are varied. These have often been
labeled to render them into a medically acceptable terminology, for example,
nyctophobia, which is morbid fear of darkness; ocholophobia, fear of crowds;
zoophobia, fear of animals; claustrophobia, fear of being in a confined space;
agorophobia, fear of open places; and hydrophobia, fear of water. These
specific fears may be considered neurotic symptoms rather than relatively
discrete patterns related to particular psychodynamic foundations and may
occur within the context of broader patterns of maladaptive reaction.
    The origins of phobic reactions also tend to be quite varied; sometimes
they originate in a generalized fear of a class of objects, one member of
which initially caused a pain reaction. For example, when a child is bitten by
a dog it may learn morbidly to fear all dogs. Phobias sometimes result in a
more complex, symbolic, and distorted displacement, as when a fear of sexual
penetration leads to a fear of all sharp objects.  See	NEUROSIS.
 �34s[Jerome S. Bruner ;Walter Mischel]
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�6v�2v�32sPholidophoriformes   �21m �33s 
    An extinct actinopterygian group composed of mostly small fusiform fishes 
of an advanced holostean level that are found in both marine and fresh-water 
deposits and range from the Middle Triassic to the Lower Cretaceous. 
    The best-known representative is Pholidophorus bechei (see illustration), 
which exhibits holostean features in its enamel-covered ganoid scales, fin 
rays, and head bones; in the fulcra bordering all the fins with a strong 
series on the upper caudal lobe; and in the structure of its caudal skeleton.  
See  ACTINOPTERYGII; HOLOSTEI; TELEOSTEI. 
 �34s[Ted M. Cavender]
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�6v�2v�32sPholidota   �21m �33s 
    An order of mammals comprising the living pangolins, or scaly anteaters, 
and their poorly known fossil predecessors. All living pangolins are assigned 
to the genus Manis. They are found in Africa south of the Sahara and in 
southeastern Asia, including certain islands of the East Indies. 
    Pangolins feed principally on termites and ants. The elongate tubular 
skull without teeth, long protrusive tongue, small eyes with heavy eyelids, 
thick skin, strong legs, five-toed feet with large claws, and large tail 
enable these unique animals to rip open ant nests and termite dens and devour 
the animals therein. The greatest peculiarity of animals in the genus Manis is 
a covering of all but the undersides of the body by an armor of large 
imbricating dermal horny scales. Living pangolins are frequently characterized 
as being animated pine cones. The position and number of hairs in relation to 
the scales are peculiar to each modern species.  See  MAMMALIA. 
 �34s[Donald E. Savage]
�33s 
  
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�6v�2v�32sPhonetics   �21m �33s 
    The science that deals with the production, transmission, and perception 
of spoken language. At each level, phonetics overlaps with some other 
sciences, such as anatomy, physiology, acoustics, psychology, and linguistics. 
In each case, phonetics focuses on phenomena relevant to the study of spoken 
language.  
    Speech is normally produced by exhaling air from the lungs through the 
vocal tract. The vocal tract extends from the larynx through the pharynx and 
the oral cavity to the lips. If the velum (soft palate) is not raised, the air 
also passes through the nasal cavities. The shape and size of the oral cavity 
can be varied by the movement of active articulators: tongue, lips, and velum. 
  See  PALATE. 
    Phoneticians usually describe speech sounds with reference to their point 
(or place) of articulation and their manner of articulation. The point of 
articulation of a sound is the place of maximum constriction within the vocal 
tract. The great majority of sounds are produced by moving some part of the 
tongue toward some region on the roof of the mouth. Exceptions are 
articulations involving lips and those sounds in which the vocal folds serve 
as articulators. 
    At most of these points of articulation, sounds can be produced with 
several manners of articulation. One way to classify manners of articulation 
refers to the degree of stricture employed in producing the sound. Sounds 
produced with complete constriction of the vocal tract are stops, or plosives. 
If the closure is incomplete, but the articulators are brought close enough so 
that the air passing between them is set into turbulent motion, the resultant 
sounds are fricatives or spirants. If the articulators are approximated, but 
the constriction remains large enough so that air can pass through without 
friction, the sounds are called approximants - vowellike sounds functioning as 
consonants. Most of these consonant sounds can be voiced or voiceless; vowels 
are normally voiced. The terms ``voiced'' and ``voiceless'' refer to the 
presence and the absence of vocal fold vibration. 
    Acoustic phonetics deals with the manner in which the spoken message is 
encoded in the sound waves. According to the generally accepted source-filter 
theory of speech acoustics, sound is generated at a source (which for phonated 
speech is consituted by the vibrating vocal folds) and passed through the 
vocal tract. The opening and closing of the vocal folds create a succession of 
condensations and rarefactions of air molecules - variations in air pressure - 
and transform kinetic energy into acoustic energy. The sound wave generated at 
the glottis can be considered, for practical purposes, a complex periodic 
wave, and as such it contains energy at frequencies that are multiples of the 
fundamental frequency (harmonics). 
    The vocal tract acts as a filter, transmitting more energy at those 
frequencies that correspond to the resonances of the vocal tract than at other 
frequencies. Energy concentrations at the resonance frequencies of the vocal 
tract are referred to as formants. 
    In principle, the source and filter are independent of each other; 
consider the fact that the same vowel can be sung at different fundamental 
frequencies (pitches), and different vowels can be produced at the same pitch. 
The sound wave can be described by specifying its fundamental frequency, 
amplitude, and spectrum. 
    The subject matter of phonetics is not limited to the production and 
perception of vowels and consonants; of equal importance are such prosodic and 
suprasegmental aspects of spoken language as duration, fundamental frequency, 
and intensity, as they determine such linguistically relevant phenomena as 
tone and intonation, stress and emphasis, and the signaling of various 
boundaries - boundaries of morphemes and words, phrases, clauses, and 
sentences.  See  SPEECH. 
 �34s[Ilse Lehiste]
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�6v�2v�32sPhonocardiography   �21m �33s 
    The science of graphic visualization and interpretation of sound 
vibrations associated with each heartbeat. The phonocardiogram is the record 
obtained, the phonocardiograph the recording instrument. The latter generally 
consists of an audio amplifier coupled to an carbon crystal or capacitance 
microphone which is placed in turn over several areas of the chest of interest 
to the observer. 
    The technique is used in general to verify auscultatory findings obtained 
by the use of the stethoscope. However, since the various microphones in use 
do not resemble the frequency characteristics of the human ear, the records 
obtained are not accurate representations of what is being heard. But 
phonocardiography serves well to time auscultatory events, particularly if the 
heart rate is rapid or irregular. 
    For timing purposes phonocardiograms are generally recorded simultaneously 
with a pulse record, or an electrocardiogram (see illustration). For the 
purpose of a more detailed analysis, phonocardiograms have been obtained from 
other areas, such as the esophagus, and from the cavities of the heart and the 
large blood vessels by means of special phonocatheters advanced into the heart 
through the venous system (intracardiac phonocardiography). The miniature 
microphones placed on the tip of the catheter are related to hydrophones used 
in solar application. Spectral phonocardiography is an adaptation of the 
Potter sound spectrograph whereby frequency, time, and loudness of heart 
noises can be displayed in graphic form.  See  ELECTROCARDIOGRAM. 
 �34s[Hans H. Hecht]
�33s 
  
  
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�6v�2v�32sPhonolite   �21m �33s 
    A light-colored, aphanitic (not visibly crystalline) rock of volcanic 
origin, composed largely of alkali feld spar, feldspathoids (nepheline, 
leucite, sodalite), and smaller amounts of dark-colored (mafic) minerals 
(biotite, soda amphibole, and soda pyroxene). Phonolite is chemically the 
effusive equivalent of nepheline syenite and similar rocks. Rocks in which 
plagioclase (oligoclase or andesine) exceeds alkali feldspar are rare and may 
be called feldspathoidal latite.  See  FELDSPATHOID; MAGMA. 
    Phonolites are rare and highly variable rocks. They occur as volcanic 
flows and tuffs and as small intrusive bodies (dikes and sills). They are 
associated with trachytes and a wide variety of feldspathoidal rocks.  See  
IGNEOUS ROCKS; TRACHYTE. 
 �34s[Carleton A. Chapman]
�33s 
  
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�6v�2v�32sPhonon   �21m �33s 
    A sound quantum. The energy of a phonon is hv, where h is Planck's 
constant and v the frequency of vibration of the sound wave. The phonon is 
thus analogous to the photon, a light quantum. 
    The concept of a phonon as a packet of sound waves, the wave packet having 
particlelike aspects, is particularly convenient in the theory of the thermal 
conductivity of insulators, where one may speak of a phonon gas, collisions 
between phonons, and a phonon mean free path. In the theory of the properties 
of superfluid helium, the quanta of longitudinal sound waves in the liquid 
helium are called phonons.  See  CONDUCTION (HEAT). 
 �34s[Jules de Launay]
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�6v�2v�32sPhonoreception   �21m �33s 
    The perception of sound by animals through specialized sense organs. A 
sense of hearing is possessed by animals belonging to two divisions of the 
animal kingdom: the vertebrates and the insects. The sense is mediated by the 
ear, a specialized organ for the reception of vibratory stimuli. Such an organ 
is found in all except the most primitive vertebrates, but only in some of the 
many species of insects. The vertebrate and insect types of ear differ in 
evolutionary origin and in their modes of operation, but both have attained 
high levels of performance in the reception and discrimination of sounds.  See 
  SOUND. 
  
    Vertebrates.   The vertebrate ear is a part of the labyrinth, located deep 
in the bone or cartilage of the head, one ear on either side of the brain. A 
complex assembly of tubes and chambers contains a membranous structure which 
bears within it a number of sensory endings of different kinds.  See  EAR. 
    The membranous labyrinth is shown in a generalized schematic form in Fig. 
1. It is convenient to recognize two divisions, a superior division, which 
includes the three semicircular canals and the utricle, and an inferior 
division, which includes the saccule and its appendages, the lagena and the 
cochlea. The superior division is remarkably uniform in character from the 
higher fishes upward, but the inferior division shows many variations. The 
saccule is always present. The lagena is present in all classes except the 
mammals, although it is missing in occasional species. The cochlea is found in 
reptiles, birds, and mammals. 
    The sensory endings within these parts of the labyrinth also vary in the 
vertebrate series. Again there is uniformity for the superior division. There 
is a crista in each ampulla of the three semicircular canals and a utricular 
macula. In all but the mammals (with a few individual exceptions), there is a 
macula neglecta, usually located on the floor of the utricle or close to the 
junction of utricle and saccule. All forms have a saccular macula. All those 
with a lagena (in general, all except the mammals) have a lagenar macula. All 
the amphibians have a papilla amphibiorum, but it is found in no other forms. 
A basilar papilla appears in certain amphibians, is continued in the reptiles, 
and then is developed in a more elaborate form as the cochlea of higher 
reptiles, birds, and mammals. 
    These endings contain ciliated cells (hair cells) which are supplied by 
fibers of the eighth cranial (auditory) nerve. In the cristae the cilia of the 
hair cells are particularly long and are embedded in a gelatinous substance 
that forms a cap or cupola. In the maculae the cilia are surmounted by a flat 
plate of gelatinous material in which numerous granules of calcium carbonate 
(otoliths) are usually embedded. The ciliated cells in the papillae lie on a 
movable membrane (basilar membrane) and have a membranous covering, the 
tectorial membrane. 
    The superior part of the labyrinth generally serves for bodily posture and 
equilibrium, whereas the saccule and its appendages (lagena and cochlea) serve 
for hearing. However, there are exceptions to this rule, the most important of 
which is that in the higher vertebrates, including mammals and probably birds 
and reptiles, the saccule serves only for equilibrium. 
    Beginning with the amphibians, which are the earliest vertebrates to spend 
a considerable portion of their lives on land, there appears a special 
mechanism, the middle ear, whose function is the transmission of aerial 
vibrations to the endings of the inner ear. All the vertebrates above the 
fishes, and certain of the fishes as well, have some type of 
sound-facilitative mechanism.  See  EAR; HEARING (HUMAN). 
  
    Invertebrates.   The group of invertebrates which has received the most 
attention has been the insects. Other arthropods, such as certain crustaceans 
and spiders, have also been found to be sensitive to sound waves. 
    The insect ear consists of a superficial membrane of thin chitin with an 
associated group of sensilla called scolophores. Such an apparatus is shown in 
simplified form in Fig. 2. These ears are found in most species of katydids, 
crickets, grasshoppers, cicadas, waterboatmen, mosquitoes, and nocturnal and 
spinner moths. The occur in different places in the body: on the antennae of 
mosquitoes, on the forelegs of katydids and crickets, on the metathorax of 
cicadas and waterboatmen, and on the abdomen of grasshoppers. Probably these 
differently situated organs represent separate evolutionary developments, 
through the association of a thinned-out region of the body wall with sensilla 
that are found extensively in the bodies of insects and that by themselves 
seem to serve for movement perception. 
    The insects mentioned above are noted for their production of stridulatory 
sounds made by rubbing the edges of the wings together, or  a leg against a 
wing, or by other means. These sounds are produced by the males and serve for 
enticing the females in mating. A most striking adaptation is that shown by 
mosquitoes: The ear of the male mosquito is sensitive only to a narrow range 
of frequencies around 380 Hz, and this frequency is the one which is produced 
by the wings of the female in flight.  If the ear of the male mosquito is made 
nonfunctional, the mosquito fails to find a mate. 
 �34s[Ernest G. Wever]
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�6v�2v�32sPhoresy   �21m �33s 
    A relationship between two different species of organisms in which the 
larger, or host, organism transports a smaller organism, the guest. It is 
regarded as a type of commensalism in which the relationship is limited to 
transportation of the guest.  See  ECOLOGICAL INTERACTIONS. 
 �34s[Charles B. Curtin]
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�6v�2v�32sPhoronida   �21m �33s 
    A small, relatively homogeneous group of animals now generally considered 
to constitute a separate animal phylum. Two genera, Phoronis and Phoronopsis, 
and about 16 species are recognized. 
    Phoronids may occur in vertical tubes placed just below the surface in 
intertidal or subtidal mud flats, or as feltlike masses of intertwined tubes 
attached to rocks, pilings, or old logs in shallow water. In both cases the 
tubes, composed basically of a secreted, parchmentlike material, are encrusted 
with small particles of sand or shell. A third living habit concerns those 
phoronids found inside channels, probably self-made, in limestone rock or the 
shells of dead pelecypod mollusks. 
    The geographical distribution of phoronids appears to be worldwide in 
temperate and tropical seas. There are no records of phoronids from the polar 
regions.  
  
    The body is more or less elongate, ranging in length from about 1.6 to 8 
in. (4 to 20 cm), and bears a crown of tentacles arranged in a double row 
surrounding the mouth which is usually crescent-shaped (see illustration). The 
anus occurs at the level of the mouth and is borne on a papilla immediately 
outside the double row of tentacles. The digestive tract is therefore 
U-shaped, the mouth and anus opening close together at one end of the animal. 
The tentacles rest on a connective tissue base known as the lophophore. 
Associated with the mouth is a ciliated flap of tissue known as the epistome.  
See  LOPHOPHORE. 
    The phylum includes both dioecious animals and hermaphrodites. All 
phoronids may reproduce sexually, and in most cases the life history includes 
the pelagic actinotroch larva. Some species reproduce asexually by transverse 
fission.  
 �34s[Joan R. Marsden]
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A700000001411�254s�5v�1v1293
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�6v�2v�32sPhosphate �21m �33s
    A negative ion having the formula PO4sup3-. Phosphates are derived from
phosphoric acid, H3PO4.
    The term phosphate is a broad term which encompasses all anions derived
from acids containing phosphorus in the 5+ oxidation state, as indicated in
the following list (all of those listed are obtained from P4O10 and water):

(HPO3)(sub n)	  Metaphosphoric acid
H5P3O10 	  Triphosphoric or tripolyphosphoric acid
H4P3O7		  Pyrophosphoric acid
H3PO4		  Orthophosphoric acid

    Phosphates are important ingredients in commercial fertilizers. Certain
organic phosphates have been used as insecticides and nerve gases.  See
FERTILIZER; ORGANOPHOSPHORUS COMPOUND; PHOSPHORUS.
 �34s[E. Eugene Weaver]
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�6v�2v�32sPhosphate metabolism   �21m �33s 
    Organic phosphate compounds are present in the structural units of every 
animal cell, and inorganic phosphate is associated with calcium in bone and 
teeth. The total phosphorus in the adult human body is about 1.2% by weight, 
with only 0.14% by weight present in the soft tissues and the remainder in 
mineralized tissue in the form of apatite crystals. Blood phosphate plays an 
important role in regulating neutrality, and it is in equilibrium with both 
bone and cellular organic phosphates. The blood level is held relatively 
constant by regulating phosphate excretion by the kidney. This control is 
primarily mediated by action of parathyroid hormone. Vitamin D enhances the 
entry of phosphate into bone. Phosphate plays an important role in absorption 
of sugar from the intestine and reabsorption of glucose from the kidney.  See  
PARATHYROID HORMONE; VITAMIN D. 
    The central role of phosphates in life processes is indicated by their 
occurrence in ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Through 
the formation of lecithins, phosphates are involved in fat metabolism. 
Phosphates play a major role in the conservation and transfer of energy, 
particularly of the energy produced in the tricarboxylic acid cycle (Krebs 
cycle), in glycolysis, and in the pentose shunt. They do so by participating 
in many phosphorylation and transphosphorylation reactions involving sugars 
and other organic compounds.  See  CARBOHYDRATE METABOLISM; CHROMOSOME; KREBS 
CYCLE; LIPID METABOLISM; NUCLEIC ACID. 
    Phosphorus-containing coenzyme systems include the pyridine (nicotinamide) 
and the riboflavin nucleotide systems concerned with oxidation-reduction 
reactions; coenzyme A, the functional form of pantothenic acid, concerned with 
transacetylation, acylation, and condensation reactions; the diphosphothiamine 
system concerned with decarboxylation; and pyridoxal phosphate concerned with 
transamination.  See  BIOCHEMISTRY; BIOLOGICAL OXIDATION; COENZYME; ENERGY 
METABOLISM.  
 �34s[Morton K. Schwartz]
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�6v�2v�32sPhosphate minerals   �21m �33s 
    Any naturally occurring inorganic salts of phosphoric acid, H3[PO4]. All 
known phosphate minerals are orthophosphates. There are over 150 species of 
phosphate minerals, and their crystal chemistry is often very complicated. 
Phosphate mineral paragenesis can be divided into three categories: primary 
phosphates (crystallized directly from a melt or fluid), secondary phosphates 
(derived from the primary phosphates by hydrothermal activity), and rock 
phosphates (derived from the action of water upon buried bone material, 
skeletons of small organisms, and so forth).  See  MINERAL; PHOSPHATE. 
 �34s[Paul B. Moore]
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�6v�2v�32sPhosphatide �21m �33s
    A complex lipid containing phosphorus. The phosphatides, also known as
phospholipids, are ususally divided into groups on the basis of compounds from
which they are derived. For example, glycerophosphatides are derived from
glycerophosphoric acid, sphingophosphatides are derived from sphingosine
phosphate, and inositol phosphatides are derived from inosital phosphates.

			 CH2OR1
			 |
		     R2O-C-H
			 |	O
			 |	||
			 CH2O-P-O2
				|
				OH

    Phosphatidyl ethanolamine, lecithin, phosphatidyl inositol, and the
plasmalogens are present in both plant and animal tissues; phytoglycolipids
have been found only in plants; sphingomyelin has been found only in animal
tissues. The phosphatides are important components of biological membranes.
    Since an individual phosphatide may contain a variety of fatty acid
residues, it may be described as pure only with this limitation in mind. Most
of the highly unsaturated fatty acids of animal tissue lipids occur in the
phosphatides. Phosphatides can act as protective colloids, as wetting and
emulsifying agents, and as antioxidants, and are therefore used considerably
in the food and petroleum industries. The chief source of commercial
phosphatides is soybean.  See  LIPID.
 �34s[Herbert E. Carter ;Roy H. Gigg]
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�6v�2v�32sPhosphorescence �21m �33s
    A delayed luminescence, that is, a luminescence that persists after
removal of the exciting source. It is sometimes called afterglow.
    This original definition is rather imprecise, because the properties of
the detector used will determine whether or not there is an observable
persistence. There is no generally accepted rigorous definition or uniform
usage of the term phosphorescence. In the literature of inorganic luminescent
systems, some authors define phosphorescence as delayed luminescence whose
persistence time decreases with increasing temperature. According to this
usage, luminescence whose persistence time is independent of temperature is
called fluorescence regardless of the length of the afterglow; a
temperature-independent afterglow of long duration is called simply a slow
fluorescence, which implies that the atomic or molecular transition involved
is forbidden to a greater or lesser degree by the spectroscopic selection
rules. The most common mechanism of phosphorescence in photoconductive
inorganic systems, however, occurs when electrons or holes, set free by the
excitation process and trapped at lattice defects, are expelled from their
traps by the thermal energy in the system and recombine with oppositely
charged carriers with the emission of light.  See  HOLES IN SOLIDS; SELECTION
RULES (PHYSICS).
    In the organic literature the term phosphorescence is reserved for the
forbidden luminescent transition from a metastable energy state M to the
ground state G, while the afterglow corresponding to the M->E->G process
(where E is a higher energy state) is called delayed fluorescence.  See
FLUORESCENCE; LIGHT; LUMINESCENCE.
 �34s[Clifford C. Klick ;James H. Schulman]
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�6v�2v�32sPhosphorus �21m �33s
    A chemical element, P, atomic number 15, atomic weight 30.9738. Phosphorus
forms the basis of a very large number of compounds, the most important class
of which are the phosphates. For every form of life, phosphates play an
essential role in all energy-transfer processes such as metabolism,
photosynthesis, nerve function, and muscle action. The nucleic acids which
among other things make up the hereditary material (the chromosomes) are
phosphates, as are a number of coenzymes. Animal skeletons consist of a
calcium phosphate.  See  PHOSPHATE.
    About three-quarters of the total phosphorus (in all of its chemical
forms) used in the United States goes into fertilizers. Other important uses
are as builders for detergents, nutrient supplements for animal feeds, water
softeners, additives for foods and pharmaceuticals, coating agents for
metal-surface treatment, additives in metallurgy, plasticizers, insecticides,
and additives for petroleum products.  See  DETERGENT; WATER SOFTENING.
    Of the nearly 200 different phosphate minerals, only one, fluorapatite,
Ca5F(PO4)3, is mined chiefly from large secondary deposits originating from
the bones of dead creatures deposited on the bottom of prehistoric seas and
from bird droppings on ancient rookeries.  See	PHOSPHATE MINERALS.
    Research in phosphorus chemistry indicates that there may be as many
compounds based on phosphorus as on carbon. In organic chemistry it has been
customary to group the various chemical compounds based on carbon into
families which are called homologous series. This can also be done in the
chemistry of phosphorus compounds, even though many phosphorus-based families
are incomplete. The best known of the families of compounds based on
phosphorus is the group of chain phosphates. Phosphate salts consist of
cations, such as sodium, along with chain anions, such as
(P[sub n]O[sub 3n+1)(sup [n+2]-),  which may have 1 - 1,000,000 phosphorus
atoms per anion.
    The phosphates are based on phosphorus atoms tetrahedrally surrounded by
oxygen atoms, with the lowest member of the series being the simple PO4(sup 3-)
anion (the orthophosphate ion). The family of chain phosphates is based on a
row of alternating phosphorus and oxygen atoms in which each phosphorus atom
remains in the center of a tetrahedron of four oxygen atoms. There is also a
closely related family of ring phosphates, a member of which, the
trimetaphosphate, is shown in Fig. 1.
    An interesting structural characteristic of many known phosphorus
compounds is the formation of cagelike structures. Such cagelike molecules are
exemplified by white phosphorus, P4, and one of the phosphorus pentoxides,
P4O10 (Fig 2). Network structures are also common; for example, black
phosphorus crystals in which the atoms are bonded together in the form of
vast, corrugated planes (Fig. 3).
    In the majority of its compounds, phosphorus is chemically bonded to four
neighboring atoms. There is a large number of compounds in which one of the
four neighboring atoms is absent, and in which its place is taken by an
unshared pair of electrons. There are also a few compounds in which there are
five or six neighboring atoms bonded to the phosphorus. These compounds are
very reactive and tend	to be unstable.
    During the 1960s and 1970s a large number of organic-phosphorus compounds
were prepared. Most of these chemical structures involve three or four
neighboring atoms bonded to the phosphorus, but stable structures having two,
five, or six neighboring atoms per phosphorus are also known.  See
ORGANOPHOSPHORUS COMPOUND; PHOSPHATE.
    Essentially all of the phosphorus used in commerce is in the form of
phosphates. The majority of phosphatic fertilizers consist of highly impure
monocalcium or dicalcium orthophosphate, Ca(H2PO4)2 and CaHPO4. These
phosphates are salts of orthophosphoric acid.  See  FERTILIZER.
    The phosphorus compound of major biological importance is
adenosinetriphosphate (ATP), which is an ester of the salt, sodium
tripolyphosphate, widely employed in detergents and water-softening compounds.
Practically every reaction in metabolism and photosynthesis involves the
hydrolysis of this tripolyphosphate to its pyrophosphate derivative, called
adenosinediphosphate (ADP).  See  ADENOSINETRIPHOSPHATE (ATP).
 �34s[John R. Van Wazer]
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�6v�2v�32sPhotoacoustic spectroscopy �21m �33s
    A technique for measuring small absorption coefficients in gaseous and
condensed media, involving the sensing of optical absorption by detection of
sound. It is frequently called optoacoustic spectroscopy.
    During the transmission of optical radiation through a sample (gas,
liquid, or solid), the absorption of radiation by the sample can be measured
by several techniques. The straightforward detection technique requires a
measurement of the optical radiation level with and without the sample in the
optical path. The transmitted power Pout and the incident power Pin are
related through the equation below, where ALPHA is the absoprtion
coefficient and l is the length of the absorber. With this technique,
the minimum measurable value of ALPHAl is of the order of 10(sup -4) unless
special precautions have been taken to stabilize the source of radiation.

P(sub out) = P(sub in)e(sup -ALPHA l)

    Optoacoustic detection is a calorimetric method where no direct detection
of optical radiation is carried out but, instead, a measurement is made of the
power absorbed by the medium from the incident radiation. The optoacoustic
signal is proportional to the incident power and the absorption-length product
ALPHAl. Thus, for given sources of noise from the detection transducers, the
signal-to-noise ratio improves as the incident energy is increased.
    If optical radiation is amplitude-modulated at an audio frequency, the
absorption of such radiation by a gaseous medium that has been confined in a
cell with appropriate optical windows for the entrance and exit of the
radiation, and nonradiative relaxation of the medium, will cause a periodic
variation in the temperature of the column of the irradiated gas. Such a
periodic rise and fall in temperature gives rise to a corresponding periodic
variation in the gas pressure at the audio frequency. The audio-frequency
pressure fluctuations (that is, sound) are efficiently detected using a
sensitive gas-phase microphone.
    The capability of measuring extremely small absorption coefficients and
correspondingly small concentrations of the absorption gases has many
applications, including high-resolution spectroscopy of isotopically
substituted gases, excited states of molecules and forbidden transitions, and
pollution detection. The pollution measurements have demonstrated that the
optoacoustic spectroscopy technique in conjunction with tunable lasers can be
routinely used for on-line real-time in-place detection of undesirable gaseous
constituents at sub-parts-per-billion levels.  See  LASER; LASER SPECTROSCOPY.
    A very sensitive calorimetric spectroscopic technique has been developed
for the study of weak absorption in liquids and solids. This technique uses a
pulsed tunable laser for excitation and a submerged piezoelectric transducer,
in the case of a liquid, or a contacted piezoelectric transducer, in the case
of a solid, for the detection of the ultrasonic signal generated due to the
absorption of the radiation and its subsequent conversion into a transient
ultrasonic signal (see illustration). Because of the capability of measuring
very small fractional absorptions, the technique is clearly applicable to the
area of monitoring water pollution, impurity detection in thin semiconductor
wafers, transmission studies of ultra-pure glasses (used in optical fibers for
optical communications), and so forth.	See  ABSORPTION OF ELECTROMAGNETIC
RADIATION.
 �34s[C. K. N. Patel]
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�6v�2v�32sPhotochemistry �21m �33s
    The branch of chemistry concerned with reactions of excited molecules
produced by the absorption of light. Chemical reactions involve the breaking
(and formation) of chemical bonds which requires energies in the range of 200
- 600 kilojoules/mole; this corresponds to the energy of light quanta in the
ultraviolet (100 - 400 nanometers), visible (400 - 700 nm) and near-infrared
(700 - 1000 nm) regions of the elctromagnetic spectrum. Light of shorter
wavelengths (x-rays, GAMMA-rays) has sufficient energy to ionize and to
dissociate molecules. The effects of this light constitute the field of
radiation chemistry, while light of longer wavelengths (infrared) is not
sufficiently energetic to produce electronic excitation in single quanta
excitations.  See  INORGANIC PHOTOCHEMISTRY; LASER PHOTOCHEMISTRY; RADIATION
CHEMISTRY.
    Electronic excitation of molecules reduces the ionization potential and
increases the electron affinity of a molecule by an increment equal to the
excitation energy, and therefore promotes electron transfer reactions. The
change in electronic configuration produced by light absorption leads to
dramatic changes in chemical properties. Since the Gibbs free energy is also
increased by light absorption, electronically excited molecules may undergo
spontaneous reactions to products which are thermodynamically inaccessible
from the ground state. This is	exemplified by the photosynthetic reaction in
which the conversion of carbon dioxide (CO2) and water (H2O) to carbohydrate
(CH2O)(sub n) and oxygen (O2) is mediated by excited chlorophyll molecules. The
reverse process of respiration leads to a decrease in Gibbs free energy and
takes place spontaneously in the absence of light.  See  PHOTOSYNTHESIS.

    Quantum yield.   The rate of a photochemical reaction is proporational to
the rate at which electronically excited molecules M* are produced; since each
absorbed photon (energy hv) of light produces one excited molecule, this
excitation rate is equal to the rate of light absorption or the intensity of
light absorbed, I(sub a).  The reaction rate is therefore proportional to I(sub
a), or rate =GAMMA I(sub a), where the proportionality constant, GAMMA =
rate/I(sub a), known as the quantum yield (or efficiency), is characteristic of
the reaction under the conditions of examination.  The quantum yield of the
primary process (that involving the excited molecule) cannot exceed unity and is
usually much lower than this since electronically excited molecules revert to
the ground state (with or without the emission of luminescence) within a period
of APPROXIMATELY 10(sup -3) s (triplet state) to 10(sup -8) s (singlet states).
These so-called photophysical processes, summarized in the illustration,
complete with photochemical reactions, effectively reducing the quantum yield.
Additionally, the excited molecule may be quenched by other molecules Q in a
process of electron transfer [reaction (1)] or energy transfer [reaction (2)],
which leave the molecule M chemically unchanged.

     M* + Q --> (M+Q-) or (M-Q+) --> M + Q				    (1)


     M* + Q --> M + Q							    (2)

    On the other hand, if the primary photochemical products are atoms or free
radicals, these may undergo secondary nonphotochemical chain reactions which
amplify the primary process and lead to overall photochemical quantum yields
much greater than unity. An example is reaction (3a), in which the primary
process (3b) is followed by the chain propagating steps (3c and 3d) to produce
APPROXIMATELY 10(sup 6) molecules of HCl for each light photon absorbed by Cl2,
that is, an overall quantum yield of 10(sup 6).  See  CHAIN REACTION
(CHEMISTRY). 

	       H2 + Cl2 (REACTION CONDITON = LIGHT) -->  2HCl		   (3a)


	       Cl2 + light -->	2Cl					   (3b)


	       Cl + H2 --> H + HCL					   (3c)


	       H + Cl2 --> Cl + HCl					   (3d)


    Mechanisms.   Photochemical reaction mechanisms are deduced from
measurements of the dependence of quantum yields on such different reaction
variables as reactant concentration and the concentration of added substances.
Primarily it is necessary to identify the nature of the electronically excited
state responsible for the reaction, which is most often singlet ([sup 1]M*) or
triplet ([sup 3]M*) following which a theoretical examination of the orbital
transformations along the reaction coordinate may be attempted.
    If the quantum yield is reduced by a selective triplet-state quencher (Q)
which introduces the competing process [reaction (4)], then the triplet state
(sup 3)M* may be regarded as the

       (sup 3)M* + Q --> M + (sup 3)Q*					     (4)

reactive state; otherwise the singlet state (sup 1)M* is assigned the role of
reactive intermediate. The rate constant of the primary process may then be
obtained by monitoring the decay of intermediate (sup 1,3)M* in absorption or
emission following flash or laser pulse excitation.  See  EXCITED STATE; FREE
RADICAL; QUANTUM CHEMISTRY; TRIPLET STATE.
    Photochemical reactions are classified as unimolecular, in which the
excited molecule itself undergoes chemical change, or bimolecular if the
excited molecule reacts with another molecule present.	See  CHEMICAL DYNAMICS.
 �34s[Brian Stevens]
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�6v�2v�32sPhotoclinometer   �21m �33s 
    A term applied to directional surveying instruments which record 
photographically the direction and magnitude of well deviations from the 
vertical. Two instruments of this type are in wide use, the Schlumberger 
photoclinometer and the Surwell clinograph. Both instruments record a series 
of deviation measurements on one trip into and out of the well. From this 
series of data it is possible to plot quite accurately the course of the well. 
    In the Schlumberger photoclinometer (see illustration) the deviation from 
the vertical is indicated by a small metal ball which rolls in a transparent 
glass bowl graduated in circular degrees. The direction of the deviation in 
azimuth is indicated by a magnetic compass. With the instrument suspended by 
an electrical cable, the  positions of the compass and steel ball are 
photographed on a 35-mm film by operation of electrical controls at the 
surface. Correlation of the pictures with the depths at which they are taken 
yields a measure of the magnitude and direction of deviation of the hole as a 
function of depth. 
    The Surwell clinograph also operates electrically but is powered by 
batteries contained in the instrument. The deviation from the vertical is 
indicated by a box level gage and the direction in azimuth by a gyroscopic 
compass, permitting its use inside steel pipe. See SURVEYING. 
 �34s[Holbrook G. Botset]
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�6v�2v�32sPhotoconductive cell   �21m �33s 
    A device for detecting electromagnetic radiation (photons) by variation of 
the electrical conductivity of a substance (a photoconductor) upon absorption 
of the radiation by this substance. During operation the cell is connected in 
series with an electrical source and current-sensitive meter, or in series 
with an electrical source and resistor. Current in the cell, as indicated by 
the meter, is a measure of the photon intensity, as is the voltage drop across 
the series resistor. Photoconductive cells are made from a variety of 
semiconducting materials in the single-crystal or polycrystalline form.  See  
PHOTOCONDUCTIVITY; PHOTOELECTRIC DEVICES. 
 �34s[Sebastian R. Borrello]
�33s 
  
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�6v�2v�32sPhotoconductivity   �21m �33s 
    The increase in electrical conductivity caused by the excitation of 
additional free charge carriers by light of sufficiently high energy in 
semiconductors and insulators. Effectively a radiation-controlled electrical 
resistance, a photoconductor can be used for a variety of light- and 
particle-detection applications, as well as a light-controlled switch. Other 
major applications in which photoconductivity plays a central role are 
television cameras (vidicons), normal silver halide emulsion photography, and 
the very large field of electrophotographic reproduction.  See  OPTICAL 
DETECTORS; OPTICAL MODULATORS; PARTICLE DETECTOR; PHOTOGRAPHY; TELEVISION 
CAMERA TUBE. 
    Although all insulators and semiconductors may be said to be 
photoconductive, that is, they show some increase in electrical conductivity 
when illuminated by light of sufficiently high energy to create free carriers, 
only a few materials show a large enough change, that is, show a large enough 
photosensitivity, to be practically useful in applications of photoconductors. 
    Since the electrical conductivity SIGMA of a material is given by the 
product of the carrier density, its charge, and its mobility, an increase in 
the conductivity can be formally due to either an increase in carrier density 
or an increase in mobility. Although cases are found in which both types of 
effects are observable, photoconductivity in single-crystal materials is due 
primarily to an increase in carrier density. In polycrystalline materials, on 
the other hand, where transport may be limited by potential barriers between 
the crystalline grains, an increase in mobility due to photoexcitation effects 
on these intergrain barriers may dominate the photoconductivity. 
    The variation of photoconductivity with photon energy is called the 
spectral response of the photoconductor. Spectral response curves typically 
show a fairly well-defined maximum at a photon energy close to that of the 
bandgap of the material, that is, the minimum energy required to excite an 
electron from a bond in the material into a higher-lying conduction band where 
it is free to contribute to the conductivity. This energy ranges from 3.7 eV, 
in the ultraviolet, for zinc sulfide (ZnS) to 0.2 eV, in the infrared, for 
cooled lead selenide (PbSe). 
    Another major characteristic of a photoconductor of practical concern is 
the rate at which the conductivity changes with changes in photoexcitation 
intensity. If a steady photoexcitation is turned off at some time, for 
example, the length of time required for the current to decrease to 1/e of its 
initial value is called the decay time of photoconductivity, tsubd. The 
magnitude of the decay time is determined by the lifetime PI and by the 
density of carriers trapped in imperfections as a result of the previous 
photoexcitation, which must now also be released in order to return to the 
thermal equilibrium situation.  See  PHOTOCONDUCTIVE CELL. 
 �34s[Richard H. Bube]
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�6v�2v�32sPhotocopying processes   �21m �33s 
    Those means by which a copy is created on a sensitized surface (generally 
paper, film, or metal plate) by the action of radiant energy. The term is 
generally applied only to documentary reproduction. The document to be 
photocopied must already have been prepared by other applications of 
photography, by manuscript, or by typewriter. A document in this case is 
classed as either a line drawing or continuous tone illustration or a 
combination. Some photocopying processes do not handle tone satisfactorily. 
Photocopying offers practical printing methods for the production of a single 
copy or a limited number of copies or for the production of a stencil or 
master from which to run off larger numbers by use of diazo paper or offset 
lithography.  See  PHOTOGRAPHIC MATERIALS; PHOTOGRAPHY. 
    Advantages of photocopying are its photographic accuracy except for 
occasional problems with color; reduction and enlargement ability of some 
processes; speed in most instances; space saving in the case of microfilm; 
economy of labor and materials over any other short-run copying process; 
convenience of handling the thin flexible material as opposed to letterpress 
type or electrolytic plates; simplicity of machine operation by untrained 
staff for certain processes; and flexibility as achieved by the combination of 
photocopying processes with other printing processes. 
    Photocopying processes may be somewhat arbitrarily divided into seven 
classes: silver halide photocopying, transfer processes, thermography, plan 
copying, electrostatic processes, the electrolytic process, and microfilming. 
 �34s[David C. Weber]
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�6v�2v�32sPhotodiode   �21m �33s 
    A semiconductor two-terminal component with electrical characteristics 
that are light-sensitive. All semiconductor diodes are light-sensitive to some 
degree, unless enclosed in opaque packages, but only those designed 
specifically to enhance the light sensitivity are called photodiodes. 
    Most photodiodes consist of semiconductor pn junctions housed in a 
container designed to collect and focus the ambient light close to the 
junction. They are normally biased in the reverse, or blocking, direction; the 
current therefore is quite small in the dark. When they are illuminated, the 
current is proportional to the amount of light falling on the photodiode.  See 
  JUNCTION DIODE. 
    Photodiodes are used both to detect the presence of light and to measure 
light intensity.  See  PHOTOELECTRIC DEVICES. 
 �34s[W. Rex Sittner]
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�6v�2v�32sPhotodissociation laser �21m �33s
    A device which uses intense pulses of ultraviolet light to dissociate
molecules in such a way as to leave one constituent in an excited state
capable of sustaining laser action. The most notable examples are iodine
compounds, which have given peak 1.3-micrometer pulse powers above 10(sup 9) W
from the excited iodide atoms.	See  LASER.
 �34s[Stephen F. Jacobs; A.L. Ponikaur]
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�6v�2v�32sPhotoelasticity   �21m �33s 
    An experimental technique for the measurement of stresses and strains in 
material objects by means of the phenomenon of mechanical birefringence. 
Photoelasticity is especially useful for the study of objects with irregular 
boundaries and stress concentrations, such as pieces of machinery with notches 
or curves, structural components with slits or holes, and materials with 
cracks. The method provides a visual means of observing overall stress 
characteristics of an object by means of light patterns projected on a screen 
or photographic film. Photoelasticity is generally used to study objects 
stressed in two planar directions (biaxial), but with refinements it can be 
used for objects stressed in three spatial directions (triaxial).  See  
BIREFRINGENCE.  
    When a stressed model is subjected to monochromatic polarized light in a 
polariscope the birefringence of the model causes the light to emerge 
refracted into two orthogonal planes. Because the velocities of light 
propagation are different in each direction, there occurs a phase shifting of 
the light waves. When the waves are recombined with the polariscope, regions 
of stress where the wave phases cancel appear black, and regions of stress 
where the wave phases combine appear light. Therefore, in models of complex 
stress distribution, light and dark fringe patterns (isochromatic fringes) are 
projected from the model (see illustration). These fringes are related to the 
stresses.  See  POLARIZED LIGHT. 
    When white light is used in place of monochromatic light, the relative 
retardation of the model causes the fringes to appear in colors of the 
spectrum. White light is often used for demonstration, and monochromatic light 
is used for precise measurements.  See  STRESS AND STRAIN. 
 �34s[William Zuk]
�33s 
  
  
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�6v�2v�32sPhotoelectric devices   �21m �33s 
    Devices which give an electrical signal in response to visible, infrared, 
or ultraviolet radiation. They are often used in systems which sense objects 
or encoded data by a change in transmitted or reflected light. Photoelectric 
devices which generate a voltage can be used as solar cells to produce useful 
electric power. The operation of photoelectric devices is based on any of the 
several photoelectric effects in which the absorption of light quanta 
liberates electrons in or from the absorbing material.  See  PHOTOVOLTAIC 
CELL; SOLAR CELL. 
    Photoconductive devices are photoelectric devices which utilize the 
photoinduced change in electrical conductivity to provide an electrical 
signal. Photoemissive systems have also been used in photoelectric 
applications. These vacuum-tube devices utilize the photoemission of electrons 
from a photocathode and collection at an anode.  See  PHOTOCONDUCTIVE CELL; 
PHOTOEMISSION.  
    Many photoelectric systems now utilize silicon photodiodes or 
phototransistors. These devices utilize the photovoltaic effect, which 
generates a voltage due to the photoabsorption of light quanta near a pn 
junction. Modern solid-state integrated-circuit fabrication techniques can be 
used to create arrays of photodiodes which can be used to read printed 
information.  See  PHOTODIODE; PHOTOELECTRICITY; PHOTOTRANSISTOR. 
 �34s[Richard A. Chapman]
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�6v�2v�32sPhotoelectricity   �21m �33s 
    The process by which electromagnetic radiation incident on a solid, 
liquid, or gas liberates electrical charge, which is detectable in an electric 
field. The process is strictly quantum in nature. 
    The quantum nature of electromagnetic radiation also manifests itself in 
the liberation of electrons and positive holes within the interior of a solid, 
giving rise to photoconductive and photovoltaic effects. These phenomena are 
readily observable in the class of solids known as semiconductors.  See  
PHOTOCONDUCTIVITY; PHOTOELECTRIC DEVICES; PHOTOEMISSION; PHOTOVOLTAIC EFFECT. 
 �34s[Michael A. Kinch]
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�6v�2v�32sPhotoemission   �21m �33s 
    The ejection of electrons from a solid (or less commonly, a liquid) by 
incident electromagnetic radiation. Photoemission is also called the external 
photoelectric effect. The visible and ultraviolet regions of the 
electromagnetic spectrum are most often involved, although the infrared and 
x-ray regions are also of interest. For important practical applications of 
photoemission  See  PHOTOTUBE; TELEVISION CAMERA TUBE.  See also  
PHOTOELECTRICITY.  
    The salient experimental features of photoemission are the following: (1) 
There is no detectable time lag between irradiation of an emitter and the 
ejection of photoelectrons. (2) At a given frequency the number of 
photoelectrons ejected per second is proportional to the intensity of the 
incident radiation. (3) The photoelectrons have kinetic energies ranging from 
zero up to a well-defined maximum, which is proportional to the frequency of 
the incident radiation and independent of the intensity. 
    In 1905 Albert Einstein made the clarifying assumption that 
electromagnetic radiation had characteristics like those of particles when it 
delivered energy to electrons in the emitter. In Einstein's approach the light 
beam behaves like a stream of photons, each of energy hNU, where h is Planck's 
constant, and NU is the frequency of the photon. The energy required to eject 
an electron from the emitter has a well-defined minimum value SIGMA PHI called 
the photoelectric threshold energy. When a photon interacts with an electron, 
the latter absorbs the entire photon energy.  See  PHOTON. 
    For hNU values below the threshold, photoelectrons are not ejected. Even 
though the electrons absorb photon energy, they do not receive enough to 
surmount the potential barrier at the surface, which normally holds the 
electrons in the solid. For photon energies above SIGMA PHI, the kinetic 
energies of photoelectrons range from zero up to a maximum value, E = hNU - 
SIGMA PHI. This is the Einstein photoelectric law, and E is commonly termed 
the Einstein maximum energy.  See  HEAT RADIATION; SCHOTTKY EFFECT. 
 �34s[L. Apker]
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�6v�2v�32sPhotoferroelectric imaging   �21m �33s 
    The process of storing an image in a ferroelectric material by utilizing 
either the intrinsic or extrinsic photosensitivity in conjunction with the 
ferroelectric properties of the material. Specifically, photoferroelectric 
(PFE) imaging is a process of storing photographic images or other optical 
information in transparent lead lanthanum zirconate titanate (PLZT) ceramics. 
The photoferroelectric imaging device consists simply of a thin flat plate 
(about 0.01 inch or 0.2 - 0.3 mm thick) of optically polished PLZT ceramic 
with transparent conductive indium - tin oxide (ITO) electrodes 
sputter-deposited on the two major surfaces. The image to be stored is exposed 
onto one of the ITO electroded surfaces by using near-ultraviolet illumination 
in the intrinsic photosensitivity region (corresponding to a bandgap energy of 
approximately 3.35 eV) of the PLZT. Simultaneously, a voltage pulse is applied 
across the electrodes to switch the ferroelectric polarization from one stable 
remanent state to another. Images are stored both as spatial distributions of 
light-scattering centers in the bulk of the PLZT and as surface deformation 
strains which form a relief pattern of the image on the exposed surface. Both 
the light scattering and surface strains are related to spatial distributions 
of ferroelectic domain orientations introduced during the image-storage 
process. These spatial distributions correspond to brightness variations in 
the image to which the PLZT is exposed. The stored image may be viewed 
directly or it may be projected onto a screen by using either transmitted or 
reflected light. 
    Important potential applications of photoferroelectric imaging are 
temporary image storage and display. Various types of image processing, 
including image contrast enhancement, are also offered by the capability of 
switching from a positive to a negative stored image in discrete steps.  See  
ELECTRONIC DISPLAY; FERROELECTRICS; PHOTOCONDUCTIVITY. 
 �34s[Cecil E. Land]
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�6v�2v�32sPhotoflash lamp   �21m �33s 
    A combustion lamp that burns with a burst of high-intensity light of short 
time duration and with definitely regulated time characteristics. The 
illustration shows the lumen-time characteristics of some typical lamps. 
    A photoflash lamp has a glass bulb filled with finely shredded aluminum 
foil in an atmosphere of oxygen. The foil is ignited by a low-voltage dry cell. 
    Photoflash lamps are used for taking ordinary photographs under poor 
ambient-light conditions, for high-speed photographs requiring fast shutter 
speed and high-intensity light, and for photographs requiring special 
lighting.  See  PHOTOGRAPHY. 
 �34s[John O. Kraehenbuehl]
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�6v�2v�32sPhotogrammetry   �21m �33s 
    The practice of obtaining surveys by means of photography. The camera 
commonly is airborne with its axis vertical, but oblique and horizontal 
(ground-based) photographs also are applicable. Data reduction is accomplished 
by stereoscopic line-of-sight geometry with use of both analytical and analog 
methods.  See  AERIAL PHOTOGRAPH; REMOTE SENSING; SURVEYING. 
    In vertical aerial surveys adjacent photos are overlapped. The two images 
of the same terrain are then superimposed for three-dimensional viewing by 
human operators or automated sensors. 
    In a widely used analog procedure the two photos are placed in the 
projectors of a stereoplotting instrument. With the aid of visible 
ground-control points the photos are oriented to the relative positions they 
had at the instants of exposure. In a typical automated stereoplotting system 
(see illustration) scanning devices substitute for human eyes to sense 
model-surface slope and thus to control servomechanisms that raise and lower 
the plotting table and translate it along a succession of closely spaced 
parallel horizontal dimensions, or ground profiles. 
 �34s[Richard H. Dalitz]
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�6v�2v�32sPhotographic materials   �21m �33s 
    The common sensitive materials of photography, that is, plates, films, and 
papers. They consist of a support of glass, plastic sheet, or paper, 
respectively, coated with an emulsion, which usually is a suspension of silver 
halide crystals in gelatin and provides the light-sensitive layer in which the 
picture will be formed (see illustration).  See  PHOTOLYSIS. 
  
  
    Spectral sensitivity.  The silver halides are normally sensitive only to 
the ultraviolet, violet and blue wavelengths, but they can be made sensitive 
to longer wavelengths by adding special dyes to the emulsion; these dyes are 
usually of the carbocyanine and merocyanine class, and the process of adding 
them to the emulsion is known as optical sensitizing. To reproduce tone values 
as seen by the eye, emulsions must respond to wavelengths to which the eye is 
sensitive, that is, approximately 400 - 700 nanometers, or from violet to red. 
    Nonsensitized emulsions are also termed blue-sensitive, color-blind, or 
ordinary. Emulsions that have been treated with dyes to extend the sensitivity 
through the green are known as orthochromatic; when the sensitivity is 
extended through the red they are known as panchromatic. With ordinary 
emulsions, blues are reproduced light and greens and reds dark. Orthochromatic 
emulsions reproduce blues and greens as light and reds as dark, and they are 
used extensively in portraiture, commercial and industrial photography, and 
the graphic arts. Panchromatic emulsions give reasonably good reproduction in 
black and white of the tone values of colored subjects and are particularly 
useful with incandescent light sources. The furthest extension of sensitizing 
by dyes is to about 1300 nm, which is in the near-infrared region. Ultraviolet 
sensitivity is limited by the strong optical absorption by gelatin below 28 
nm, but emulsions sensitive to much shorter wavelengths can be prepared with 
the gelatin greatly reduced. 
  
    Photographic products.  Thousands of types and sizes of plate, film, and 
paper are available for a wide variety of applications. Generally, each field 
of use requires special properties. Amateur, professional, commercial, 
motion-picture, and industrial products for camera use range from an exposure 
index of less than 10 to over 1000 and from very fine grain and optimum 
sharpness to fairly coarse grain with corresponding loss in definition. They 
can be obtained in a wide range of contrasts and spectral sensitivity, 
especially for scientific and industrial photography. Films of extremely high 
contrast and density are used in graphic reproduction in industry and the 
printing trade to give high-contrast line and halftone negatives. 
Blue-sensitive emulsions are used for copying and making duplicate negatives. 
Many types of x-ray film are made, some coated on both sides and used with or 
without intensifying screens. Special multiple coatings are used for color 
photography.  
  
    Nonsilver processes.  There are many special-purpose photographic 
processes that use compounds other than silver salts. In general, they have 
neither the sensitivity to light possible with silver compounds nor the 
amplification factor of development. They are largely used in the so-called 
reprography methods. They include inorganic compounds which are 
photosensitive, especially iron salts and dichromates in colloid layers such 
as gelatin, glue, albumen, shellac, polyvinyl alcohol, and other synthetic 
resins which become insoluble on exposure. Many important processes are based 
on light-sensitive organic compounds, especially diazo compounds; unsaturated 
compounds such as cinnamic acid derivatives, which are insolubilized by 
cross-linking on exposure; systems in which polymerization occurs as a result 
of free-radical formation on exposure; dyes which are formed or destroyed 
directly by light action; and thermographic systems, in which a thermal 
pattern produces an image by means of melting a composition or initiating a 
chemical reaction, or by producing a deformation in an electrostatically 
charged layer. Electrophotographic systems rely on the elimination of a charge 
by exposure of a charged layer and the subsequent development of an 
electrostatic image by charged powder, or by electrolytic deposition of metal 
in an image-sensitive photoconducting system. 
 �34s[Walter Clark]
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�6v�2v�32sPhotography   �21m �33s 
    The process of forming visible images directly or indirectly by the action 
of light or other forms of radiation on sensitive surfaces. In the traditional 
sense, photography utilizes the action of light to bring about changes in 
silver halides. These changes may be invisible, necessitating a developer to 
reveal the image, or they may be a directly visible darkening (print out). 
Most photography is of the first kind and the function of the developer is to 
convert the exposed silver halide to silver. The bright parts of the subject 
give more exposure than the dark parts so that a negative results; that is, 
the brighter parts of the subject correspond to the darker parts of the 
reproduction. A positive, in which the relation between light and dark areas 
corresponds to that of the subject, is obtained when a negative is printed 
onto a sheet of similar material so that the negative tones are reversed. In 
the reversal process, direct production of the positive occurs if the 
developed negative silver is removed chemically and the remaining silver 
halide is then redeveloped; direct positive images can also be obtained 
directly by using special materials.  See  PHOTOGRAPHIC MATERIALS. 
    The major branches of photography include infrared, ultraviolet, 
high-speed, and stereoscopic photography, photographic photometry, 
nuclear-particle recording, and microphotography (including microfilming).  
See  AERIAL PHOTOGRAPH; ASTRONOMICAL PHOTOGRAPHY; CAMERA; CINEMATOGRAPHY; 
LANTERN SLIDES; LENS (OPTICS); MICRORADIOGRAPHY; OPTICAL MICROSCOPE; 
OSCILLOSCOPE; PHOTOELASTICITY; PHOTOGRAMMETRY; RADIOGRAPHY; SCHLIEREN 
PHOTOGRAPHY; SHOCK-WAVE DISPLAY; SPECTROGRAPHY; STROBOSCOPIC PHOTOGRAPHY; 
UNDERWATER PHOTOGRAPHY. 
  
    Infrared photography.  Emulsions with special sensitizing dyes can respond 
to radiation up to about 1300 nanometers. Photographs can thus be made of 
subjects associated with radiation in the near-infrared, such as spectra, 
stars, and hot objects. Subjects which selectively transmit, emit, or reflect 
near-infrared radiation, especially in a manner different from visible 
radiation, can also be photographed. Infrared photographs taken from long 
distances or from high altitudes show improved clarity of detail because the 
atmosphere may selectively transmit the near-infrared and also because the 
contrast of ground objects may be higher as a result of their different 
reflectivities in the near infrared. Grass and foliage appear white because 
chlorophyll is transparent to the near-infrared. 
    Infrared photography is used to record the distribution of temperature at 
the surface of heated objects; for photography in total darkness (the object 
being illuminated only by infrared); in criminology, for deciphering altered 
or deteriorated documents and other objects; and for photographing textiles 
where dark dyes interfere with visual examination. It is used in medicine 
because the skin is somewhat transparent to infrared and the subcutaneous 
veins may be revealed for diagnosis.  See  INFRARED RADIATION. 
  
    Ultraviolet photography.  This is used in the printing field, in 
spectrography, and in photomicrography and by mineralogists, police 
investigators, museums, and art galleries. The two methods of ultraviolet 
photography are (1) the fluorescence method, in which the subject is 
illuminated by ultraviolet and a filter is used on the camera to absorb the 
reflected ultraviolet and permit only the visible fluorescence to reach the 
film; and (2) the reflected ultraviolet method, in which an ultraviolet source 
is used and the camera is provided with a filter which permits only 
ultraviolet to reach the film.  See  ULTRAVIOLET RADIATION. 
  
    High-speed photography.  High-speed photography deals with photography at 
exposure times shorter than those given by normal shutters, and at picture 
(frame) frequencies faster than those given by motion-picture cameras with 
intermittent film movement. It serves a great variety of purposes in 
technological studies. 
    The best normal still-camera mechanical shutters give exposures not much 
shorter than 1/1000 s. Very short exposures are given by magnetooptical 
shutters (Faraday effect), electrooptical shutters (Kerr effect), and pulsed 
electron image tubes. So-called pulsed light sources are used to give very 
intense illumination of short duration. They include the xenon 
gaseous-discharge tubes, an electric spark in air, exploding wires, the argon 
flash bomb, high-voltage flying-spot cathode-ray tubes, and lasers. High-speed 
single x-ray pictures have been taken by discharging a short-duration high 
potential through the x-ray tube. High-intensity repetitive flashes can be 
obtained with xenon flash tubes for so-called stroboscopic photography with 
frequencies up to 10sup5 per second. 
    The classical camera for serial frame separation is the motion-picture 
camera. Normal cameras with intermittent movement of film are usually limited 
to 128 frames/s (standard rates are 16 or 24). For higher rates (up to 10,000 
frames/s or more), continuous film movement is used with optical compensation 
for image motion, such as a rotating plane-parallel glass block or a ring of 
mirrors or lenses. 
    There is a mechanical limit to the rate at which film can be moved. Higher 
speeds of movement are obtained by using stationary film and moving the image 
by means of a rapidly rotating mirror. 
  
    Photomicrography.  The apparatus for photography of the image formed by 
the microscope consists essentially of an optical bench on which are aligned 
an illuminating system, a compound microscope, and a lensless camera connected 
to the microscope eyepiece by a light-tight extension. Good photomicrographs 
can be made by attaching a simple camera to a microscope. Resolving power is 
more important than mere magnification. Maximum resolution is given by the 
electron microscope.  See  METALLOGRAPHY; MICROSCOPE. 
  
    Stereoscopic photography.  Stereoscopic photography is possible with a 
single camera if two separate photographs are made, one after the other, from 
viewpoints separated by the interocular or other appropriate distance, by 
displacing the lens of a single camera for the two exposures, or by rotating 
the object or the camera so as to give a pair of exposures on one film. Most 
stereoscopic photography is done either by the simultaneous method, in which 
two photographs are made at the same time with two separate cameras; with a 
stereoscopic camera, essentially two cameras in one body with matched optical 
systems and coupled focusing movements; or with single cameras using beam 
splitters to give two photographs side by side on the film.  See  STEREOSCOPY. 
  
    Photographic photometry.  The intensity of radiation, or the spectral 
distribution of intensity, can be measured by photography. The radiation whose 
intensity is to be measured is compared with that from a standard source by 
matching the photographic densities produced by both. The method is capable of 
high precision if the characteristics of photographic materials are accurately 
known and the results are interpreted intelligently.  See  PHOTOMETRY. 
  
    Microphotography.  The process of making photographs on a greatly reduced 
scale is termed microphotography. Microfilming is the special technique of 
copying documents to reduced size on film, usually 16- and 35-mm film, but 70- 
and 102-mm widths are used in rolls, strips, and assemblages of strips, as 
well as sheet film. The negatives, or contact positive films or paper prints 
made from them, can be read in enlarging readers. Special projection equipment 
for reading the films may give an enlargement onto a table or a diffusing 
rear-projection screen, with provision for rapidly winding the film and 
framing any desired page. In reader-printers the image may be viewed and, if 
desired, a paper print may be made in the reader by stabilization processing 
or electrophotographic means.  See  COLOR PHOTOGRAPHY. 
 �34s[Walter Clark]
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�6v�2v�32sPhotoluminescence �21m �33s
    A luminescence excited in a body by some form of electromagnetic radiation
incident on the body. The term photoluminescence is generally limited to cases
in which the incident radiation is in the ultraviolet, visible, or infrared
regions of the electromagnetic spectrum.
    Photoluminescence may be either a fluorescence or a phosphorescence, or
both. Energy can be stored in certain luminescent materials by subjecting them
to light or some other exciting agent, and can be released by subsequent
illumination of the material with light of certain wavelengths. This type of
photoluminescence is called stimulated photoluminescence.  See	FLUORESCENCE;
LUMINESCENCE; PHOSPHORESCENCE.
 �34s[Clifford C. Klick ;James H. Schulman]
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�6v�2v�32sPhotolysis   �21m �33s 
    Chemical decomposition by the action of radiant electromagnetic energy, 
especially light. Photolysis occurs in certain crystals, notably the silver 
halides, when they are exposed to radiation. When this occurs, the effect of 
the radiation is to produce a definite chemical change resulting in the 
separation of photolytic silver. Photolysis occurs in many other materials, 
such as the lead and thallium halides, zinc oxide, the metallic azides, and in 
organic compounds such as the oxalates, styphnates, and fulminates. 
    A photographic emulsion consists of microcrystalline grains of silver 
bromide, AgBr, or silver chloride, AgCl, embedded in gelatin. Upon prolonged 
exposure to light, so-called print-out specks of silver form within and on the 
surface of the grains. Much shorter exposures produce a latent image which can 
be made visible by the process of development. Experiments have shown that the 
latent image consists of only a very few atoms of silver in each grain. The 
high sensitivity of the photographic system comes about because of the 
enormous gain (10sup9) which can be achieved by reduction of each exposed 
grain that occurs during development. 
    The Gurney-Mott theory of the photographic process proposes a two-stage 
mechanism. In the first stage a light quantum is absorbed at a point within 
the silver halide grain, releasing a mobile electron and a positive hole. 
These mobile defects diffuse to trapping sites (sensitivity centers) within 
the volume or on the surface of the grain. In the second stage, the trapped 
(negatively charged) electron is neutralized by an interstitial (positively 
charged) silver ion, which combines with the electron to form a silver atom. 
The silver atom at the sensitivity center is capable of trapping a second 
electron, after which the process repeats itself, causing the silver speck to 
grow. The positive holes are assumed to diffuse to the surface without 
recombining with electrons, where they escape or react with the gelatin. 
    The early Gurney-Mott theory has been criticized, especially because of 
the assumed lack of electron-hole recombination. The essential idea of an 
electronic process linking the initial absorption of light quanta with the 
formation of the image speck is nevertheless well founded.  See  
PHOTOCHEMISTRY; PHOTOGRAPHIC MATERIALS. 
 �34s[Frederick C. Brown]
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�6v�2v�32sPhotometer   �21m �33s 
    An instrument used for making measurements of light, or electromagnetic 
radiation, in the visible range. In general, photometers may be divided into 
two classifications: laboratory photometers, which are usually fixed in 
position and yield results of high accuracy; and portable photometers, which 
are used in the field or outside the laboratory and yield results of lower 
accuracy. Each class may be subdivided into visual (subjective) photometers 
and photoelectric (objective or physical) photometers. These in turn may be 
grouped according to function, such as photometers to measure luminous 
intensity (candelas or candlepower), luminous flux, illumination 
(illuminance), luminance (photometric brightness), light distribution, light 
reflectance and transmittance, color, spectral distribution, and visibility. 
Visual photometric methods have largely been supplanted commercially by 
physical methods, but because of their simplicity, visual methods are still 
used in educational laboratories to demonstrate photometric principles.  See  
ILLUMINANCE; LUMINANCE; LUMINOUS FLUX; LUMINOUS INTENSITY. 
 �34s[G. A. Horton]
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�6v�2v�32sPhotometry �21m �33s
    That branch of science which deals with the calculation and measurement of
light or of its time rate of flow. The term light is usually restricted to
electromagnetic radiations of wavelengths that are capable of affecting the
human eye. However, in some cases photometry has come to mean also the
measurement of radiations in the nearby ultraviolet and infrared regions.
    Photometry is usually concerned with measurements of luminous intensity,
luminous flux, luminous flux density, luminance, light distribution, color,
and the reflectance and transmittance of light; by extension it may even
include visibility measurements. Photometric tests are made of light sources,
lighting fixtures (luminaires), lighting materials, and lighting
installations.	See  ILLUMINANCE; LUMINANCE; LUMINOUS FLUX; LUMINOUS INTENSITY.
    Since light is defined as radiant energy that is capable of producing
visual sensation, photometric measurements either are made by the human eye or
are based upon its visual responses. Because the spectral response of the
human eye differs with the individual, the International Commission on
Illumination (ICI) has adopted a standard spectral luminous effeciency curve
which has been accepted as being that of photopic vision of the normal eye.
See  VISION.
    Modernization of photometric measurements using photoelectric solid-state
cells rather than visual observers has removed the problem of the visual
individuality of the observer. Such cells generate electric currents when
irradiated and cover the range of the visual spectrum. Although their spectral
response is not the same as the standard ICI spectral luminous efficiency
curve, their response can be modified by filters to match the ICI curve. Also,
geometric corrections can be accomplished by properly shaped translucent
shields to correct for deviations from Lambert's cosine law of incidence
caused by shadows or reflections.  See	PHOTOCONDUCTIVE CELL.
    Although visual photometry has been largely supplanted by physical
photometry, photometric measurements in both cases are eventually based on the
primary standard of luminous intensity, the candela, cd. In October 1979 the
General Conference on Weights and Measures redefined the base SI unit candela
as the luminous intensity, in a given direction, of a source that emits
monochromatic radiation of frequency 540 x 10(sup 12) hertz and of which the
radiant intensity in that direction is 1/683 watt per steradian. The specified
frequency corresponds to a wavelength of 555 nanometers in free space, which
is at the maximum visual response of the human eye.  See  ILLUMINATION.
 �34s[Warren B. Boast]
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�6v�2v�32sPhotomorphogenesis   �21m �33s 
    The regulatory effect of light on plant form, involving growth, 
development, and differentiation of cells, tissues, and organs. Morphogenic 
influences of light on plant form are quite different from light effects that 
nourish the plant through photosynthesis, since the former usually occur at 
much lower energy levels than are necessary for photosynthesis. Light serves 
as a trigger in photomorphogenesis, frequently resulting in energy expenditure 
orders of magnitude larger than the amount required to induce a given 
response. Photomorphogenic processes determine the nature and direction of a 
plant's growth and thus play a key role in its ecological adaptations to 
various environmental changes.  See  PHOTOSYNTHESIS. 
    Morphogenically active radiation is known to control seed and spore 
germination, growth and development of stems and leaves, lateral root 
initiation, opening of the hypocotyl or epicotyl hook in seedlings, 
differentiation of the epidermis, formation of epidermal hairs, onset of 
flowering, formation of tracheary elements in the stem, and form changes in 
the gametophytic phase of ferns, to mention but a few of such known phenomena. 
Many nonmorphogenic processes in plants are also basically controlled by light 
independent of photosynthesis. Among these are chloroplast movement, 
biochemical reactions involved in the synthesis of flavonoids, anthocyanins, 
chlorophyll, and carotenoids, and leaf movements in certain legumes. 
 �34s[Winslow R. Briggs]
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�6v�2v�32sPhotomultiplier   �21m �33s 
    A very sensitive vacuum-tube detector of light or radiant flux containing 
a photocathode which converts the light to photoelectrons; one or more 
secondary-electron-emitting electrodes or dynodes which amplify the number of 
photoelectrons; and an output electrode or anode which collects the secondary 
electrons and provides the electrical output signal. It is also known as a 
multiplier phototube. Because of the very large amplification provided by the 
secondary-emission mechanism, and the very short time variation associated 
with the passage of the electrons within the device, the photomultiplier is 
applied to the detection and measurement of very low light levels, especially 
if very high speed of response is required. 
    The illustration is a schematic of a typical photomultiplier and shows its 
operation. Light incident on a semitransparent photocathode located inside an 
evacuated envelope causes photoelectron emission from the opposite side of the 
photocathode. The efficiency of the photoemission process is called the 
quantum efficiency, and is the ratio of emitted photoelectrons to incident 
photons (light particles). Photoelectrons are directed by an accelerating 
electric field to the first dynode, where from 3 to 30 secondary electrons are 
emitted for each incedent electron, depending upon the dynode material and the 
applied voltage. These secondaries are directed to the second dynode, where 
the process is repeated and so on until the multiplied electrons from the last 
dynode are collected by the anode. 
    A typical photomultiplier may have 10 stages of secondary emission and may 
be operated with an overall applied voltage of 2000 V. In most 
photomultipliers the focusing of the electron streams is done by electrostatic 
fields shaped by the design of the electrodes. Some special photomultipliers 
designed for very high speed utilize crossed electrostatic and magnetic fields 
which direct the electrons in approximate cycloidal paths between electrodes. 
    Because there is very little noise associated with the amplification 
process in a photomultiplier, detection limits are primarily determined by the 
quantum efficiency of the photocathode, by the thermionic or dark emission of 
electrons from the photocathode, and by the bandwidth of the observation. 
    Ever since their invention, photomultipliers have been found useful in 
low-level photometry and spectrometry. The most important applications of 
photomultipliers are related to scintillation counting, and these form the 
basis of a whole science of tracer chemistry that has been applied to 
agriculture, medicine, and industrial problems. In medicine, 
photomultiplier-scintillator combinations are used in the gamma-ray camera, 
computerized tomography, and the positron scanner.  See  COMPUTERIZED 
TOMOGRAPHY; GAMMA-RAY DETECTORS; NUCLEAR MEDICINE; RADIOLOGY; SCINTILLATION 
COUNTER.  
 �34s[Ralph W. Engstrom]
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�6v�2v�32sPhoton �21m �33s
    A quantum of a single mode (that is, single wavelength, direction, and
polarization) of the electromagnetic field. There are also two other
definitions of photon in use, not entirely consistent with the first
definition or each other: an elementary light particle or ``fuzzy ball,'' and
an informal unit of light energy. The fuzzy-ball definition emphasizes a
particle character of light suggested, for example, by momentum exhibited in
the Compton effect and light levitation phenomena. However, the fuzzy-ball
picture lacks a rigorous foundation and is not required for the explanation of
any fundamental phenomenon. As an informal unit of energy, the photon equals
hv, where h is Planck's constant (= 6.626 x 10(sup -34) joule-second), and v
is the frequency of the light in hertz.  See  COMPTON EFFECT; PHOTOEMISSION;
QUANTUM (PHYSICS).
 �34s[Murray Sargent, III]
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�6v�2v�32sPhotoperiodism   �21m �33s 
    The growth, development, or other responses of organisms to the length of 
night or day or both. Photoperiodism has been observed in plants and animals, 
but not in bacteria (prokaryotic organisms), other single-celled organisms, or 
fungi.  
    A true photoperiodism response is a response to the changing day or night. 
Some species respond to increasing day lengths and decreasing night lengths 
(for example, by forming flowers or developing larger gonads); this is called 
a long-day response. Other species may exhibit the same response, or the same 
species may respond in some different way, to decreasing days and increasing 
nights; this is a short-day response. Sometimes a response is independent or 
nearly independent of day length, and is said to be day-neutral. There are 
many plant responses to photoperiod. These include development of reproductive 
structures in lower plants (mosses) and in flowering plants; rate of flower 
and fruit development; stem elongation in many herbaceous species as well as 
coniferous and deciduous trees (usually a long-day response and possibly the 
most widespread photoperiodism response in higher plants); autumn leaf drop 
and formation of winter dormant buds (short days); development of frost 
hardiness (short days); formation of roots on cuttings; formation of many 
underground storage organs such as bulbs (onions, long days), tubers (potato, 
short days), and storage roots (radish, short days); runner development 
(strawberry, short day); balance of male to female flowers or flower parts 
(especially in cucumbers); aging of leaves and other plant parts; and even 
such obscure responses as the formation of foliar plantlets (such as the 
minute plants formed on edges of Bryophyllum leaves), and the quality and 
quantity of essential oils (such as those produced by jasmine plants). Note 
that a single plant, for example, the strawberry, might be a short-day plant 
for one response and a long-day plant for another response. 
  
    Animal responses.   There are also many responses to photoperiod in 
animals, including control of several stages in the life cycle of insects (for 
example, diapause) and the long-day promotion in birds of molting, development 
of gonads, deposition of body fat, and migratory behavior. Even feather color 
may be influenced by photoperiod (as in the ptarmigan). In several mammals the 
induction of estrus and spermatogenic activity is controlled by photoperiod 
(sheep, goat, snowshoe hare), as is fur color in certain species (snowshoe 
hare). Growth of antlers in American elk and deer can be controlled by 
controlling day length. Increasing day length causes antlers to grow, whereas 
decreasing day length causes them to fall off. By changing day lengths 
rapidly, a cycle of antler growth can be completed in as little as 4 months; 
slow changes can extend the cycle to as long as 2 years. When attempts are 
made to shorten or extend these limits even more, the cycle slips out of 
photoperiodic control and reverts to a 10 - 12-month cycle, apparently 
controlled by an internal annual ``clock.'' 
  
    Seasonal responses.  Response to photoperiod means that a given 
manifestation will occur at some specific time during the year. Response to 
long days (shortening nights) normally occurs during the spring, and response 
to short days (lengthening nights) usually occurs in late summer or autumn. 
Since day length is accurately determined by the Earth's rotation on its 
tilted axis as it revolves in its orbit around the Sun, detection of day 
length provides an extremely accurate means of determining the season at a 
given latitude. Such other environmental factors as temperature and light 
levels also vary with the seasons but are clearly much less dependable from 
year to year. 
  
    Mechanisms.  It has long been the goal of researchers on photoperiodism to 
understand the plant or animal mechanisms that account for the responses. 
Light must be detected, the duration of light or darkness must be measured, 
and this time measurement must be metabolically translated into the observed 
response: flowering, stem elongation, gonad development, fur color, and so 
forth. In spite of many years of intensive research, relatively little is 
known about these mechanisms. Enough is know, however, to suggest that the 
basic mechanisms differ not only between plants and animals but among 
different species as well. The roles (synchronization, anticipation, and so 
on) are similar in all organisms that exhibit photoperiodism, but the 
mechanisms through which these roles are achieved are apparently quite varied. 
    Strongest inhibition of flowering in short-day plants comes when the light 
interruption occurs around the time of the critical night (about 7 - 9 h for 
cocklebur plants), but actual effectiveness also depends on the length of the 
dark period. With short-day cockleburs, the shorter the night, the less the 
flowering and the longer the time that light inhibits flowering. 
    Orange-red wavelengths used as a night interruption are by far the most 
effective part of the spectrum in inhibition of short-day responses and 
promotion of long-day responses (flowering in most studies), and effects of 
orange-red light can be completely reversed by subsequent exposure of plants 
to light of somewhat longer wavelengths, called far-red light. These 
observations led in the early 1950s to discovery of the phytochrome pigment 
system, which is apparently the molecular machinery that detects the light 
effective in photoperiodism of higher plants.  See  PHYTOCHROME. 
    In photoperiodism of short-day plants, an optimum response is usually 
obtained when phytochrome is in the far-red receptive form during the day and 
the red-receptive form during the night. Although normal daylight contains a 
balance of red and far-red wavelengths, the red-receptive form is most 
sensitive, so the pigment under normal daylight conditions is driven mostly to 
the far-red receptive form. At dusk this form is changed metabolically, and 
the red-receptive form builds up. It is apparently this shift in the form of 
phytochrome that initiates measurement of the dark period. This is how a plant 
``sees'': when the far-red-sensitive form of the pigment is abundant, the 
plant ``knows'' it is in the light; the red-sensitive form (or lack of far-red 
form) indicates to the plant's biochemistry that it is in the dark. 
    The measurement of time - the durations of the day or night - is the very 
essence of photoperiodism. The discovery of a biological clock in living 
organisms was made in the late 1920s. It was shown that the movement of leaves 
on a bean plant (from horizontal at noon to vertical at midnight) continued 
uninterruptedly for several days, even when plants were placed in total 
darkess and at a constant temperature, and that the time between given points 
in the cyle (such as the most vertical leaf position) was almost but not 
exactly 24 h. In the case of bean leaves, it was about 25.4 h. Many other 
cycles have now been found with similar characteristics in virtually all 
groups of plants and animals. There is strong evidence that the clocks are 
internal and not driven by some daily change in the environment. Such rhythms 
are called circadian. 
    Circadian rhythms usually have period lengths that are remarkably 
temperature-insensitive, which is also true of time measurement in 
photoperiodism. Furthermore, the rhythms are normally highly sensitive to 
light, which may shift the cycle to some extent. Thus, daily rhythms in nature 
are normally synchronized with the daily cycle as the Sun rises and sets each 
day. Their circadian nature appears only when they are allowed to manifest 
themselves under constant conditions of light (or darkness) and temperature, 
so that their free-running periods can appear.  See  PERIODICITY IN ORGANISMS. 
 �34s[Frank B. Salisbury]
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�6v�2v�32sPhotophore gland   �21m �33s 
    A highly modified integumentary gland which arises from an epithelial 
invagination into the dermis. It becomes cut off from its site of origin and 
develops into a luminous organ composed of a lens and a light-emitting gland, 
at the back of which is a pigmented reflector of probably dermal-cell origin. 
These luminous bodies occur in deep-sea teleosts and elasmobranchs which live 
in areas of total darkness.  See  EPITHELIUM; GLAND. 
 �34s[Olin E. Nelsen]
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�6v�2v�32sPhotoreception   �21m �33s 
    The process of absorption of light energy by plants and animals and its 
utilization for biologically important purposes. In plants photoreception 
plays an essential role in photosynthesis and an important role in 
orientation. Photoreception in animals is the initial process in vision.  See  
PHOTOSYNTHESIS; TAXIS; VISION. 
    The photoreceptors of animals are highly specialized cells or cell groups 
which are light-sensitive because they contain pigments which are unstable in 
the presence of light of appropriate wavelengths. These light-sensitive 
receptor pigments absorb radiant energy and then undergo  physicochemical 
changes, which lead to the initiation of nerve impulses that are conducted to 
the central nervous system.  See  EYE (INVERTEBRATE); EYE (VERTEBRATE). 
 �34s[Verner J. Wulff]
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�6v�2v�32sPhotorespiration   �21m �33s 
    Plant respiration (especially CO2 evolution) during exposure to light; it 
differs biochemically from normal respiration occurring in the dark and is 
specifically associated with the oxidation of compounds produced during 
photosynthesis. The rate of photorespiratory CO2 release is often three to 
five times greater than the rate of dark respiration, and the biochemical 
reactions responsible for photorespiration are distinct from those of dark 
respiration. Strong evidence suggests that by blocking photorespiration, the 
net photosynthetic incorporation of CO2 in many species could be increased by 
at least 50%.  See  PHOTOSYNTHESIS. 
    The rate of glycolic acid synthesis, which occurs largely in leaf 
chloroplasts, is probably the most important factor controlling 
photorespiration. A number of the biochemical reactions responsible for the 
synthesis of glycolic acid in any given photosynthetic tissue are known. At 
least several of these may occur simultaneously. The CO2 produced during 
photorespiration arises from one or more reactions of the glycolate pathway. 
Photorespiratory CO2 is released from the oxidative decarboxylation of 
glyoxylic acid and during the conversion of glycine to serine.  See  CELL 
PLASTIDS.  
    It has often been suggested that a function of photorespiration is to 
protect the chloroplast against photooxidative destruction in an environment 
of high irradiance and low CO2 concentration. This hypothesis implies that 
photorespiration consumes toxic oxidants, thereby converting compounds to CO2 
and protecting the photochemical apparatus. Photosynthetic tissues possess a 
number of mechanisms for destroying toxic oxidants, however, and the mesophyll 
chloroplasts of C4 species are protected against photooxidative damage without 
having an active photorespiratory system. The hypothesis also fails to 
consider that species occur that are intermediate between C3 and C4 species in 
their photorespiration. Some C3 plants also have lower-than-usual rates of 
photorespiration.  See  PLANT RESPIRATION. 
 �34s[Israel Zelitch]
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�6v�2v�32sPhotosphere   �21m �33s 
    The visible surface of the Sun or other stars. The photosphere of the Sun 
is a gaseous layer a few hundred kilometers thick with an average effective 
temperature of 5780 K (9940DGF), determined from the total radiation per 
square centimeter.  See  SUN. 
 �34s[John W. Evans]
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�6v�2v�32sPhotosynthesis �21m �33s
    The light-dependent conversion of radiant energy into chemical energy as
adenosinetriphosphate (ATP) and reduced nicotinamide adenine dinucleotide
phosphate (NADP) serves as a prelude to the utilization of these compounds for
the reductive fixation of CO2 into organic molecules. Such molecules, broadly
designated as photosynthates, are usually but not invariably in the form of
carbohydrates such as glucose, and form the base for the nutrition of all
living things, as well as serving as the starting materials for fuel, fiber,
animal feed, oil, and other compounds used by people. Collectively, the
biochemical processes by which CO2 is assimilated into organic molecules are
known as the photosynthetic dark reactions, not because they must occur in
darkness, but because light - in contrast to the photosynthetic light
reactions - is not required.

    Csub3 photosynthesis.  The essential details of C3 (the reductive pentose
cycle) photosynthesis can be seen in Fig. 1. Three molecules of CO2 combine
with three molecules of the five-carbon compound ribulose bisphosphate (RuBP)
in a reaction catalyzed by RuBP carboxylase to form three molecules of an
enzyme-bound six-carbon compound. These are hydrolyzed into six molecules of
the three-carbon compound phosphoglyceric acid (PGA), which are phosphorylated
by the conversion of six molecules of ATP (releasing adenosinediphosphate,
ADP, for photophosphorylation via the light reactions). The resulting
compounds are reduced by the NADP formed in photosynthetic light reactions to
form six molecules of the three-carbon compound phosphoglyceraldehyde (PGAL).
One molecule of PGAL is made available for combination with another PGAL
(requiring a second ``turn'' of the Calvin-cycle wheel) to form a six-carbon
sugar, and the other five PGAL molecules, through a complex series of
enzymatic reactions, are rearranged into three molecules of RuBP, which can
again be carboxylated with CO2 to start the cycle turning again. The resulting
sugar is usually glucose-6-phosphate, which can be enzymatically converted
into sucrose or other compounds that may remain in the leaf or may be
transported from the leaf to other cells.

   Csub4 photosynthesis. Initially, the Csub3 cycle was thought to be the only
route for CO2 assimilation, although it was recognized by plant anatomists that
some rapidly growing plants (such as maize, sugarcane, and sorghum) possessed an
unusual organization of the photosynthetic tissues in their leaves (Kranz
morphology). Plants having the Kranz anatomy utilize an additional CO2
assimilation route now known as the C4-dicarboxylic acid pathway. The essential
features of C4 photosynthesis are seen in Fig. 2. Carbon dioxide enters a
mesophyll cell, where it combines with the three-carbon compound
phosphoenolpyruvate (PEP) to form a four-carbon acid, oxaloacetic acid, which is
reduced to malic acid, or transmitted to aspartic acid. The malic acid moves
into bundle sheath cells where the acid is decarboxylated, the CO2 is
assimilated via the C3 cycle, and the resulting three-carbon compound, pyruvic
acid, moves back into the mesophyll cell and is transformed into PEP, which can
be carboxylated again. The two cell types, mesophyll and bundle sheath, are not
necessarily adjacent (sedges are an example), but in all documented cases of C4
photosynthesis the organism had two distinct types of green cells.
    C4 metabolism is classified into three types, depending on the
decarboxylation reaction used with the four-carbon acid in the bundle sheath
cells.

		***BELOW HAS REACTION CONDITION INFORMATION**

			   NADP-malic enzyme
NADP(sup +) + malic acid --------------------->

			pyruvic acid + CO2 + NADPH

  2. NAD-ME type (Atriplex species):

			  NADP-malic enzyme
NAD(sup +) + malic acid ---------------------->
			pyruvic acid + CO2 + NADH

  3. PCK type (Panicum species):


			 Phosphoenolpyruvatecarboxykinase
Oxaloacetic acid + ATP ------------------------------------>
							PEP + CO2 + ADP


    CAM photosynthesis. Under arid and desert conditions, where soil water is in
short supply, transpiration during the day when temperatures are high and
humidity is low may rapidly deplete the plant of water, leading to desiccation
and death. By keeping stomates closed during the day, water can be conserved,
but the uptake of CO2, which occurs entirely through the stomata, is prevented.
Desert plants in the Crassulaceae, Cactaceae, Euphorbiaceae, and 15 other
families evolved, apparently independently of C4 plants, an almost identical
strategy of assimilating CO2 by which the CO2 is taken in at night when the
stomata open; water loss is low because of the reduced temperatures and
correspondingly higher humidities. First studied in plants of the Crassulaceae,
the process has been called crassulacean acid metabolism (CAM).
    In contrast to C4, where two cell types cooperate, the entire process occurs
within an individual cell. At night, CO2 combines with PEP through the action of
PEP carboxylase, resulting in the formation of oxaloacetic acid and its
conversion into malic acid. Th PEP is formed from starch or sugar via the
glycolytic route of respiration. Thus, there is a daily reciprocal relationship
between starch	(a storage product of C3 photosynthesis) and the accumulation of
malic acid (the terminal product of nighttime CO2 assimilation).
			   [Martin Gibbs ; Yoke Wah Kow ; Clanton C. Black, Jr.]
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�6v�2v�32sPhototransistor   �21m �33s 
    A semiconductor device with electrical characteristics that are 
light-sensitive. Phototransistors differ from photodiodes in that the primary 
photoelectric current is multiplied internally in the device, thus increasing 
the sensitivity to light.  See  PHOTODIODE; TRANSISTOR. 
    Some types of phototransistors are supplied with a third, or base, lead. 
This lead enables the phototransistor to be used as a switching, or bistable, 
device. The application of a small amount of light causes the device to switch 
from a low current to a high current condition.  See  PHOTOELECTRIC DEVICES. 
 �34s[W. Rex Sittner]
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�6v�2v�32sPhototrophic bacteria   �21m �33s 
    Green and purple bacteria which thrive under anaerobic conditions and 
carry out photosynthesis without oxygen production, or anoxygenic 
photosynthesis. For the photoassimilation of CO2, molecular nitrogen, and 
simple organic substrates, phototrophic bacteria require external hydrogen 
donors of a lower redox potential than water, such as hydrogen sulfide, 
elemental sulfur, thiosulfate, molecular hydrogen, or highly reduced organic 
compounds. The photosynthetic pigments of these bacteria are different from 
those of all other phototrophic organisms; they consist of the 
bacteriochlorophylls a, b, c, d, and e. In addition, a great variety of 
carotenoids are present which give phototrophic bacteria their characteristic 
colors. Most species multiply by binary fission. Under anaerobic conditions in 
the dark all species perform a fermentative metabolism. 
    The green and purple bacteria share their basic prokaryotic cellular 
organization with a third group of phototrophic bacteria, the cyanobacteria or 
blue-green algae. These bacteria perform the same type of oxygen-evolving 
(oxygenic) photosynthesis as the eukaryotic algae and higher plants; they are, 
therefore, conventionally classified with these groups of organisms.  See  
PHOTOSYNTHESIS.  
    Two physiological-ecological groups are distinguished among both the green 
and the purple bacteria and taxonomically established in four families: 
    1. Chlorobiaceae, mesophilic green sulfur bacteria 
    2. Chloroflexaceae, thermophilic gliding green bacteria 
Phototrophic purple bacteria (Rhodospirillineae) 
    3. Chromatiaceae, purple sulfur bacteria 
    4. Rhodospirillaceae, purple nonsulfur bacteria 
    The Chlorobiaceae constitute a small and physiologically uniform group of 
nonmotile bacteria. All species are strictly anaerobic and depend on both 
hydrogen sulfide and light for their development. In nature the Chlorobiaceae 
thrive in the anaerobic, sulfide-containing parts of all kinds of aquatic 
environments exposed to light, for example, ditches, ponds, lakes, rivers, 
sulfur springs, estuaries, and muddy marine habitats. 
    The only species of the family Chloroflexaceae is Chloroflexus 
aurantiacus. All strains are thermophilic and grow best between 122 and 140DGF 
(50 and 60DGC). The cells occur in long filaments and exhibit gliding motility 
on solid surfaces. Chloroflexus grows under anaerobic conditions with simple 
organic compounds; the color of the cells is dull green. The natural habitats 
of Chloroflexus are hot springs all over the world. 
    Like the Chlorobiaceae, the Chromatiaceae are strictly anaerobic and 
obligately phototrophic; their CO2 assimilation is linked to the oxidation of 
sulfide and sulfur to sulfate. In nature, blooms are often visible as pink to 
purple-red layers on sulfide-containing mud in ditches, ponds, sulfur springs, 
estuaries, and marine tide pools. They also form ``red water'' in stratified 
lakes and sewage lagoons. 
    The Rhodospirillaceae are the group of phototrophic bacteria with the 
greatest structural and metabolic diversity. Most species have the capacity to 
thrive either as anaerobic phototrophs or as aerobic respiring 
chemoorganotrophs. In nature, the Rhodospirillaceae occur in all kinds of 
aquatic habitats, particularly where the decomposition of organic matter by 
chemoorganotrophic bacteria results in anaerobic conditions and the liberation 
of low-molecular-weight breakdown products; they regularly flourish in sewage 
lagoons.  
 �34s[Norbert Pfennig]
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�6v�2v�32sPhototube   �21m �33s 
    An electron tube containing a photocathode from which electrons are 
emitted when it is exposed to light or other electromagnetic radiation. An 
elementary vacuum phototube consists of a photocathode, an anode or electron 
collector, and an evacuated envelope through which radiation is transmitted to 
the photocathode. A gas phototube contains, in addition, an inert gas which 
may be ionized by electron current from the photocathode. For a description of 
a phototube in which the electron current is amplified by means of a 
secondary-emission electron multiplier  see  PHOTOMULTIPLIER. 
    Phototubes serve as sensing elements in the detection and measurement of 
light and ultraviolet or infrared radiation. Phototubes also convert 
variations in intensity of incident radiation into corresponding variations in 
electron output current, as in light-controlled relay circuits, and in the 
conversion of sound modulation of photographic film into audio-frequency 
currents, as in the sound tracks on motion-picture film. 
    The fundamental characteristics of a phototube are its spectral 
sensitivity characteristic, or output current, expressed as a function of the 
wavelength of incident radiation at constant anode voltage, and its 
anode-current characteristics, which show the dependence of anode current on 
applied voltage and radiant flux input. 
    The anode current of a vacuum phototube is directly proportional to the 
intensity of radiation incident on its photocathode. The anode is normally 
connected to a positive potential of at least 20 volts relative to the 
photocathode in order that the anode current be limited by photoelectric 
emission rather than by space charge or electron emission velocity.  See  
ELECTRON MOTION IN VACUUM. 
    In a gas phototube the photoelectric current is amplified by partial 
ionization of a gas contained in the tube at low pressure. An inert gas such 
as neon or argon is used because photocathodes react chemically with other 
gases. At low anode voltages the anode current of a gas phototube is 
emission-limited like that of a vacuum phototube. At anode potentials greater 
than 25 volts, electrons emitted from the photocathode acquire sufficient 
energy to ionize some of the gas atoms. The total current is then the sum of 
the free-electron current, the positive-ion current, and the current due to 
secondary electrons which are produced by ion bombardment of the photocathode. 
The ratio of anode current, at an anode voltage sufficient to cause 
ionization, to the emission-limited current measured at a lower voltage is the 
gas amplification factor of a gas phototube. This factor ranges between 3 and 
10 in commercial gas phototubes.  See  ELECTRICAL CONDUCTION IN GASES. 
    Photocathodes of practical importance contain one or more of the alkali 
metals lithium, sodium, potassium, rubidium, or cesium in complex combination 
with other metals or with oxides of certain metals. Because of their high 
reflectivity and conductivity, the pure alkali metals have lower quantum 
efficiencies than do the more complex photocathodes, which invariably are 
semiconductors.  
 �34s[James L. Weaver]
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�6v�2v�32sPhotovoltaic effect   �21m �33s 
    A term most commonly used to mean the production of a voltage in a 
nonhomogeneous semiconductor, such as silicon, by the absorption of light or 
other electromagnetic radiation. In its simplest form, the photovoltaic effect 
occurs in the common photovoltaic cell, used, for example, in solar batteries 
and exposure meters. The photovoltaic cell consists of an np junction between 
two different semiconductors (see illustration), an n-type material in which 
conduction is due to electrons, and a p-type material in which conduction is 
due to positive holes. When light is absorbed near such a junction, new mobile 
electrons and holes are released, as in photoconduction. An additional feature 
of a photovoltaic cell, however, is that there is an electric field in the 
junction region between the two semiconductor types. The released charge moves 
in this field. This current flows in an external circuit without the need for 
a battery as required in photoconduction.  See  EXPOSURE METER; 
PHOTOCONDUCTIVITY; PHOTOELECTRICITY; SEMICONDUCTOR; SOLAR CELL. 
 �34s[L. Apker]
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�6v�2v�32sPhreatoicoidea   �21m �33s 
    A suborder of the Isopoda and class Crustacea. The body is subcylindrical, 
appearing laterally compressed, mainly because of the downward development of 
the pleura of the pleon (see illustration). The first and occasionally the 
second thoracic segment is fused with the head. Antennules are shorter than 
the antennae. The eyes may be large, small, or absent. Mouthparts are 
primitive.  
    The suborder is divided into two families, the Amphisopidae, and the 
Phreatoicidae. The suborder is an ancient one and includes a fossil, 
Protamphisopus wianamattensis, from the Triassic beds of New South Wales. 
Three extant species are recorded from Australia, Tasmania, New Zealand, and 
South Africa and one that is subterranean from India. Most species occur in 
fresh water. Several are blind, subterranean forms and one occurs in hot water 
from deep artesian bores. A few are semiterrestrial, burrowing forms. 
 �34s[Edith M. Sheppard]
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�6v�2v�32sPhrynophiurida   �21m �33s 
    An order of Ophiuroidea in which the vertebrae usually articulate by means 
of hourglass-shaped surfaces and the arms are able to coil upward or downward 
in the vertical plane. There is usually a leathery integument, in which 
calcareous granules or platelets are embedded. Most species are found in deep 
water. Of the families, the Gorgonocephalidae often have branched arms, the 
Asteronychidae have a large disk and slender arms, and the Asteroschematidae 
have a small disk and stout arms. 
    These families share a number of characteristics and are grouped in one 
suborder, Euryalina. One remaining family, the Ophiomyxidae, differs in having 
a soft, unprotected integument.  See  OPHIUROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sPhthalic acid   �21m �33s 
    One of the three benzenedicarboxylic acids of formula C6H4(COOH)2. 
Phthalic (or o-phthalic) acid, melting point 191DGC (376DGF), is the 1,2 
isomer; isophthalic (or m-phthalic) acid, melting point 347 - 348DGC (657 - 
658DGF), is the 1,3 isomer; terephthalic acid, melting point 425DGC (797DGF), 
is the 1,4 isomer. Phthalic acids are the most important of the three 
commercially. Phthalic acids are used in chemical analysis, in preparation of 
esters, and in preparation of phthaloyl chlorides. Phthalic anhydride, 
C6H4(CO)2O, can be decarboxylated to benzoic acid and can be used in the 
synthesis of indigo and derivatives of anthraquinone.  See  ACID ANHYDRIDE; 
CARBOXYLIC ACID; PHTHALIMIDE; POLYESTER RESINS; XYLENE. 
 �34s[Evans B. Reid]
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�6v�2v�32sPhthalimide   �21m �33s 
    The imide of o-phthalic acid, also called 1,3-isoindoledione. The melting 
point of phthalimide is 238DGC (460DGF), and it is only slightly soluble in 
water. It is a weak acid. In the form of its sodium or potassium salt it is 
widely used in the synthesis of both primary amines and amino acids. It is 
combined with the malonic ester in synthesis of complex amino acids. 
Phthalimide is used as starting material in the synthesis of methyl 
anthranilate, the active principle in jasmine and orange oils.  See  AMIDE; 
AMINE; PHTHALIC ACID. 
 �34s[Evans B. Reid]
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�6v�2v�32sPhycobilin   �21m �33s 
    A protein-bound pigment which occurs in some groups of algae, namely 
Cyanophyceae, Rhodophyta, and Cryptophyceae. In conjugated proteins (generic 
name, biliproteins), phycobilins act as light absorbers in photosynthesis. 
There are two types of algal biliproteins, the red phycoerythrins and the blue 
phycocyanins. The phycobilins are open-chain tetrapyrroles and are chemically 
related to the mammalian bile pigments such as biliverdin and bilirubin. They 
are attached to proteins by covalent bonds which may be cleaved by acid, 
alkali, or methanol. Two phycobilins have been characterized, phycocyanobilin 
and phycoerythrobilin; and there is spectral evidence for at least one more, 
termed phycourobilin. 
    Phycoerythrins and phycocyanins serve as accessory pigments to chlorophyll 
a in photosynthesis. The light energy they absorb is passed on to chlorphyll a 
with an efficiency approaching 100% and is preferentially used in that system 
of photosynthesis in which cytochrome reduction and oxygen production from 
water take place. In addition, phycobilin-absorbed light is essential for 
efficient photosynthetic utilization of the light energy absorbed by 
chlorophyll a itself.  See  CHLOROPHYLL; PHOTOSYNTHESIS. 
 �34s[Colm O hEocha]
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�6v�2v�32sPhylactolaemata   �21m �33s 
    A class of ectoproct bryozoans. Phylactolaemates have lophophores which 
usually are markedly U-shaped (or rarely nearly circular but still 
kidney-shaped) in basal outline, and relatively short, wide zooecia; these 
animals dwell only in fresh waters.  See  BRYOZOA; LOPHOPHORE. 
    Phylactolaemate colonies are either encrusting threadlike networks of 
relatively isolated zooecia with solid chitinous uncalcified walls, or small 
to large masses of gelatinous material in which the individual zooids are 
embedded side-by-side without definite separating zooecial walls. Stolons are 
not present. 
    Only a few (about 50) phylactolaemate species exist, all classified in a 
single order, the Plumatellida (or Plumatellina). Exclusively fresh-water, the 
phylactolaemates may have evolved relatively recently from ctenostomes, 
although some workers have suggested that phylactolaemates might be very 
primitive ectoprocts surviving as evolutionary relics.  See  CTENOSTOMATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sPhyllite   �21m �33s 
    A large group of regional metamorphic rocks derived from argillaceous 
sediments and recrystallized in the greenschist facies (low degree of 
metamorphism), essentially composed of white mica and quartz. Phyllites are 
fine-grained, strongly schistose rocks, and the schistosity surfaces exhibit a 
glittery sheen given off by mica. With very low metamorphism the phyllites 
pass into slates, and with increasing metamorphism they pass into mica 
schists.  See  METAMORPHIC ROCKS; SCHIST; SLATE. 
    Phyllite has a wide distribution in the crystalline mountain ranges of the 
world: the Highlands of Scotland, mountains of Norway, Harz Mountains and 
mountains of Saxony, Alps, Appalachians, and Great Lakes district of the 
United States. 
 �34s[T. F. W. Barth]
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�6v�2v�32sPhyllolepida   �21m �33s 
    One of six orders of the class Placodermi. These are highly specialized, 
benthic fish, about 1 ft (30 cm) long, making up the single, late Upper 
Devonian genus Phyllolepis (see illustration). 
    As in other placoderms the plate armor forms separate head and trunk 
shields. The armor is broad and low; the component plates are easily 
recognized by their characteristic ornament of concentric and transverse 
ridges. There is no sign of the orbits in the head shield, so unless these 
fishes were blind the eyes must have lain anterior or lateral to the bony 
shield. The braincase, tooth plates, and jaws are not known. One specimen from 
Scotland has elements of the vertebral column. 
    Phyllolepis was adapted for life on the bottom in streams and floodplains, 
where it probably fed on organic detritus. Its ancestry is unknown and its 
sudden appearance in the late Devonian something of a mystery.  See 
ARTHRODIRA; PLACODERMI. 
 �34s[Robert H. Denison]
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�6v�2v�32sPhyllonite   �21m �33s 
    A metamorphic rock - the name is a combination of phyllite and mylonite; 
the phyllonites occupy a position between the rock types representing the 
component parts of the name. There are two distinct stages of their 
development. In the first stage the original rock is granulated by extreme 
deformation and pulverized to a mylonite. In the second stage, but frequently 
overlapping the first stage in time, new minerals recrystallize and grow (this 
is called crystalloblastesis).  See  METAMORPHIC ROCKS; MYLONITE; PHYLLITE. 
 �34s[T. F. W. Barth]
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�6v�2v�32sPhylogeny   �21m �33s 
    The history of living organisms. Although the concepts of organic 
evolution and phylogeny are closely interlocked, the terms are not synonyms as 
frequently given in dictionaries and used by biologists. Considerable 
confusion in theoretical discussions has resulted from misuse of ``phylogeny'' 
and ``evolution.'' Evolution is change in organisms with respect to time. The 
study of evolution, or of evolutionary principles, is the investigation of the 
mechanisms of evolutionary modifications in organisms, be they large or small 
changes. Phylogeny is the history of living organisms - their lineages 
resulting from their evolution. Organisms evolve and leave a trail of 
phylogeny behind them. The study of phylogeny is the description of lineages 
of animals and plants from the origin of life to the present, the events being 
placed in a proper time scale. Phylogenetic studies may be purely descriptive 
or they may be explanatory, using evolutionary and other pertinent principles. 
Clearly, the study of phylogeny is a historical science and must be studied 
with the approaches used in any historical study. But the study of evolution 
is not a historical science, even though some evolutionary mechanisms, for 
example, speciation, require long periods of time for their completion. 
    Phylogenetic study began with the acceptance of the theory of organic 
evolution, and its origins can be traced to the publication of Charles 
Darwin's Origin of Species (1859). Because phylogeny is defined in terms of 
organic evolution, all phylogenetic theory and methodology must be based on 
evolutionary theory. Evolutionary mechanisms can be summarized into two basic 
ones that are fundamental to phylogenetic study. These are phyletic evolution 
or change in a single phyletic lineage with respect to time. A single phyletic 
lineage is a species reproducing generation after generation in time. The 
second is speciation or the multiplication of phyletic lineages, giving rise 
to two or more species from a single ancestral one. Phyletic evolution can 
occur without speciation, but speciation must involve phyletic evolution. A 
third evolutionary mechanism - extinction - is important to phylogenetic 
studies because of the resulting gaps produced between groups. The consequence 
of these evolutionary mechanisms is that an adaptive radiation of species will 
evolve from a single common ancestor. The goal of phylogenetic studies is to 
start with the species of this radiation and to ascertain what their 
phylogenetic history has been, what evolutionary mechanims were involved, and 
finally to arrange the taxa into a classification and a phylogenetic diagram 
(dendrogram) that express this history. 
    It must be emphasized that the same methods of phylogenetic analysis must 
be used by neontologists studying recent species and by paleontologists 
studying fossils. Contrary to common belief, paleontologists cannot observe 
phylogeny directly, but must offer hypotheses of phylogenetic relationships 
and then test them with empirical evidence as must neontologists. 
    The basic method in phylogenetic study is determination of homologous 
features. Homology is defined in terms of phylogeny, usually in the following 
form: Homologous features or conditions in two or more organisms are ones that 
can be traced phylogenetically back (or stem phylogenetically from) the same 
feature or condition in the immediate common ancestor of these organisms. 
Homology is defined in terms of phylogeny, and phylogeny is defined in terms 
of evolution; hence these definitions are strictly noncircular. 
    After hierarchical series of homologies are established and the homologies 
are analyzed further to ascertain their relative taxonomic value, or to 
distinguish between pleisomorphs (primitive homologies), convergent apomorphs 
(advanced homologies), and synapomorphs (uniquely derived features), 
phylogenies can be constructed using this information. Thus the procedure of 
working methods is first to recognize homologs using similarity as the 
recognizing criterion, and second to recognize phylogenies using homologs. 
    Determination of phylogenies and the establishment of classifications are 
associated, but are not the same. Classifications are based on phylogenetic 
studies and attempt to arrange organisms into taxa which reflect the entire 
evolutionary history of the group. Concepts of the best classification vary 
considerably, but most workers accept the idea that classifications should 
reflect two major aspects of evolutionary history, namely: (1) the degree of 
genetical similarity as ascertained by the degree of phenotypical similarity; 
and (2) the phylogenetic series of events, including branching of lineages, in 
the history of the group.  See  ANIMAL EVOLUTION; ANIMAL SYSTEMATICS; 
EXTINCTION (BIOLOGY); SPECIATION; TAXONOMIC CATEGORIES. 
 �34s[Walter J. Bock]
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�6v�2v�32sPhymosomatoida   �21m �33s 
    An order of Echinacea with a stirodont lantern and diademoid ambulacral 
plates; each interambulacral plate typically carried more than one primary 
tubercle. There are three families: the Pseudodiadematidae, of the Jurassic 
and Cretaceous; the Phymosomatidae, which arose in the Jurassic and has one 
surviving genus, Glyptocidaris, that occurs off Japan; and the 
Stomopneustidae, which also range from the Jurassic, with one surviving genus, 
Stomopneustes, in the Indo-Pacific region.  See  ECHINACEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sPhysical anthropology   �21m �33s 
    The science which treats the biological aspects of humans and their 
relation to their historical or cultural aspects. Psychology and the medical 
sciences deal intensively with particular areas of human nature; physical 
anthropology integrates these and other approaches into the broadest view 
possible. This view regards humans as a species of animal, encompassing 
evolution, the functioning of various organ systems, humans' adaptive 
qualities in relation to environment, genetics, and variation in individuals, 
populations, or racial groups.  See  ANATOMY, REGIONAL; GENETICS; MEDICINE; 
PSYCHOLOGY, PHYSIOLOGICAL AND EXPERIMENTAL. 
    The main distinction of the evolution of humans from the higher primates 
(monkeys and apes) is the emergence of upright walking and an enlarged brain. 
Speech and the making of tools are often cited as characteristics of humans, 
and may be regarded as the functional equivalent of the anatomically expanding 
cerebral cortex.  See  FOSSIL MAN; PRIMATES. 
    The modern reason for the study of human regional variation, which one 
recognizes as race, lies not in classification but in the fact that this is 
the most important zoological and evolutionary information available for the 
human species. The local variation and intergrading is seen, not as the result 
of the hybridizing of ``pure'' races, but as the result of the basic processes 
of small-scale evolution, for example, adaptation, selection, genetic drift, 
and continuous gene exchange. Adaptation may be expressed in body size 
(especially weight), which is correlated inversely with mean annual 
temperature according to Bergmann's rule; that is, body bulk tends to be 
greater, for greater heat conservation, in colder areas. Thus some bodily 
traits are not essentially racial, but are environmental in origin. 
    Individual variation has been at all times the central field of research 
in anthropology. Early studies in racial classification were concerned more 
with the existence than with the explanation of racial differences, and with 
testing such differences by standard measurements, means, and probability 
statistics. Later work was addressed to the traits themselves, that is, to 
organization of body form or of physiological response. This called for the 
addition, to traditional methods, of hypotheses based on theoretical 
considerations, experimental design (difficult in the case of humans), 
selection of nonstandard measures, a variety of techniques, and more refined 
statistics dealing with variation and correlation.  See  BIOMETRICS; HUMAN 
GENETICS; POPULATION GENETICS; SEROLOGY. 
 �34s[William W. Howells]
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�6v�2v�32sPhysical chemistry   �21m �33s 
    The branch of chemistry that deals with the interpretation of chemical 
phenomena and properties in terms of the underlying physical processes, and 
with the development of techniques for their investigation. The term chemical 
physics is often employed to denote a branch of physical chemistry where the 
emphasis is on the interpretation and analysis of the physical properties of 
individual molecules and bulk systems, instead of their reactions. Theoretical 
chemistry is another major branch, where the emphasis is on the calculation of 
the properties of molecules and systems, and which used the techniques of 
quantum mechanics and statistical thermodynamics. It is convenient to regard 
physical chemistry as dealing with three aspects of matter: its equilibrium 
properties, structure, and ability to change. 
  
    Equilibrium properties.  The study of matter in a state of equilibrium 
constitutes the field of chemical thermodynamics. In particular, chemical 
thermodynamics provides a technique for discussing the response of a system to 
a change in the external conditions (such as the shift in the boiling and 
freezing point of either a pure substance or a mixture when the applied 
pressure is changed, or when the composition of the mixture is modified), and 
for rationalizing the energy changes that occur in the course of a chemical 
reaction. The branch of thermodynamics dealing with the latter is called 
thermochemistry. Chemical thermodynamics also provides a framework for the 
determination of the maximum amount of work that may be generated by a system 
undergoing a specified change, and it therefore provides a way of establishing 
bounds for the efficiencies of a variety of devices, including engines, 
refrigerators, and electrochemical cells. Thermodynamics is used in chemistry 
to assess the position of equilibrium of a chemical reaction (that is, how far 
it will proceed), and to determine what conditions are necessary in order to 
optimize the yield of a particular product. The branch of chemical 
thermodynamics dealing with ionic reactions occurring in the presence of 
electrodes constitutes the field of equilibrium electrochemistry.  See  
CHEMICAL EQUILIBRIUM; CHEMICAL THERMODYNAMICS; ELECTROCHEMISTRY; ENTHALPY; 
ENTROPY; FREE ENERGY; GIBBS FUNCTION; THERMOCHEMISTRY. 
  
    Structure.  The principal role of quantum mechanics in chemistry is in the 
discussion of atomic and molecular structure, and in the interpretation of 
spectroscopic data. In the branch of physical chemistry known as computational 
quantum chemistry, interest centers on the numerical solution of the 
Schroedinger equation in order to obtain wave functions and geometries of 
molecules. Computational quantum chemistry is so developed that it is capable 
of being used to map the changes in the structures of molecules while they are 
in the course of reaction, when atoms and groups of atoms are being 
transferred from one molecule to another.  See  QUANTUM CHEMISTRY; 
SCHROEDINGER'S WAVE EQUATION. 
    Spectroscopic techniques are used not only to identify molecules present 
in a sample, but also to determine their shape, size, and electron 
distribution. The techniques fall into four categories: absorption 
spectroscopy, emission spectroscopy, Raman spectroscopy, and resonance 
techniques.  See  ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY; ELECTRON 
SPECTROSCOPY; MOLECULAR STRUCTURE AND SPECTRA; MOESSBAUER EFFECT; NUCLEAR 
MAGNETIC RESONANCE (NMR); PHOTOCHEMISTRY; RAMAN EFFECT; SPECTROSCOPY. 
        Techniques for the investigation of molecular structure based on 
diffraction depend on the observation of the direction through which radiation 
and particles are scattered when they impinge on a sample. Other techniques 
for investigating structure include the electric and magnetic properties of 
molecules, in particular, the determination of electric polarizabilities and 
dipole moments, magnetic properties, and the properties based on optical 
birefringence, such as optical activity and the Faraday effect.  See  X-RAY 
DIFFRACTION.  
    Structural properties and thermodynamic properties are brought together by 
statistical thermodynamics. This major theoretical procedure gives a way of 
predicting the thermodynamic properties of assemblies of molecules in terms of 
their individual energy levels. 
  
    Physical and chemical change.  The third major branch of physical 
chemistry is concerned with change: physical change and chemical change. In 
particular, it is concerned with the rate of change. Physical change includes 
the diffusion of one substance into another, or the migration of ions in an 
electrode solution. The application of thermodynamics to change in general 
constitutes the field of nonequilibrium thermodynamics.  See  DIFFUSION IN 
GASES AND LIQUIDS; GAS; TRANSPORT PROCESSES. 
    Chemical change may be studied at a variety of levels. Empirical chemical 
kinetics is the study of reactions in order to determine how their rates 
depend on the concentrations of the participants in the reaction and on the 
conditions, mainly the temperature. Investigation of the time dependence of 
reactions yields a detailed picture of the sequence of molecular 
transformations involved in a complex chemical reaction.  See  CHEMICAL 
DYNAMICS; PICOSECOND MOLECULAR PROCESSES; SHOCK TUBE. 
    An important extension of chemical kinetics is to the reactions that occur 
on surfaces; these are the processes involved in heterogeneous catalysis. A 
special application of surface chemistry is to the stability of colloidal 
suspensions of species in fluids, and another is to the processes that occur 
at the interface between an electrode and the solution in which it is 
immersed.  See  ADSORPTION; COLLOID; HETEROGENEOUS CATALYSIS; SURFACE PHYSICS. 
 �34s[P. W. Atkins]
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�6v�2v�32sPhysical geography   �21m �33s 
    The description, analysis, classification, and genetic interpretation of 
the natural features and phenomena of the Earth's surface. Numerous 
specialized but overlapping disciplines have developed within the broad field 
of physical geography. Among these are geomorphology, hydrology, meteorology 
and climatology; and biogeography. See BIOGEOGRAPHY; CLIMATOLOGY; 
GEOMORPHOLOGY; HYDROLOGY; METEOROLOGY. 
    Further considerations of physical geography appear in various topical and 
regional articles. For examples of principal topical discussions  see  
CARTOGRAPHY; CONTINENT; HILL AND MOUNTAIN TERRAIN; MATHEMATICAL GEOGRAPHY; 
PLAINS; TERRAIN AREAS. For physical geography of the continents and major 
island areas  see  AFRICA; ANTARCTICA; ARCTIC AND SUBARCTIC ISLANDS; ASIA; 
AUSTRALIA AND NEW ZEALAND; EAST INDIES; EUROPE; NORTH AMERICA; PACIFIC 
ISLANDS; SOUTH AMERICA; WEST INDIES. 
 �34s[Donald Lee Johnson]
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�6v�2v�32sPhysical law �21m �33s
    A physical phenomenon is said to be controlled or governed by a physical
law when the phenomenon is one of a broad class of phenomena such that it is
possible to formulate some regularity which applies to all members of the
class. If the phenomenon is one which can be described in terms of numerical
measurement, the law is often formulated in terms of mathematical relations
between the numbers obtained by measurement, as in the inverse square law of
universal gravitation. However, a mathematical formulation of a natural law is
by no means necessary.
 �34s[Percy W. Bridgman /Gerald Holton]
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�6v�2v�32sPhysical measurement �21m �33s
    Quantitative information on physical conditions, properties, or relations
essential for coordination of activities, efficiency of communication, and
understanding of the nature of things in science and engineering and in much
of everyday life. Time, distance, mass, temperature, force, power, and all
other physical quantities (or parameters or variables), as well as the
properties of matter, materials, and devices, must be described and measured
in terms which have the same meaning for everyone. The measuring device or
instrument is calibrated (that is, the functional relationship between its
indication and the magnitude of the measured quantity is determined) by direct
or indirect comparison with a standard which embodies, possesses, or generates
a fixed or reproducible magnitude of the physical quantity which is taken as
the unit or some multiple or fraction of the unit. Any measured quantity may
thus be expressed by a number (the magnitude ratio) and the name of the unit,
for example, a length of 1.54 meters. The general area of scientific activity
relating to standards and units and the accuracy of measurement is called
metrology.  See  UNITS OF MEASUREMENT.

    Standards.	In 1793 the French government adopted the decimal metric
system wherein the basic unit of length was defined as one ten-millionth of
the Earth's polar quadrant (as determined from latitude surveys), to be called
the meter. The basic unit for mass was defined as the mass of a cubic
decimeter of water, to be called the meter. The basic unit for mass was
defined as the mass of a cubic decimeter of water, to be called the kilogram.
For working standards, a platinum bar was marked with fine lines a meter
apart, and a platinum iridium cylinder was constructed equal in mass to a
cubic decimeter of water. When later refinements in measurement showed that
neither of these standards exactly realized the units as originally defined,
the discrepancies were eliminated by redefining the units in terms of the
material standards which had been constructed.	See  METRIC SYSTEM.
    In 1975, the United States adopted the Metric Conversion Act, declaring
that ``the policy of the U.S. shall be to coordinate and plan the increasing
use of the metric system in the United States,'' and established the U.S.
Metric Board ``to coordinate the voluntary conversion to the metric system.''
However, English units have become almost universal in some worldwide
industries - for example, dimensions of oil-drilling equipment, or altitude
measurement in aviation. Thus it is likely that there will always be
exceptions to uniformity, requiring special knowledge of special units for at
least some people even if the whole world ``goes metric'' in principle.
    At present the International System of Units (abbreviated SI, from the
French Systeme International d'Unites) is constructed from seven base units
for independent quantities plus two supplementary units for plane angle and
solid angle (Table 1). Units for all other quantities are derived from these
nine units. In Table 2 are listed 19 SI derived units with special names.
These units are derived from the base and supplementary units in a coherent
manner, which means they are expressed as products and quotients of the nine
base and supplementary units without numerical factors. All other SI derived
units are similarly derived in a coherent manner from the 28 base,
supplementary, and special-name SI units. For use with the SI units there is a
set of 16 prefixes (Table 3) to form multiples and submultiples of these
units. For mass, the prefixes are to be applied to the gram instead of to the
SI unit, the kilogram.
    The SI units together with the SI prefixes provide a logical and
interconnected framework for measurements in science, industry, and commerce.

    Units.  In some cases, quantities are commonly expressed in terms of
fundamental constants of nature, and use of these constants or ``natural
units'' is acceptable. Typical examples of natural units, with their symbols,
are:

		elementary charge			e
		electron mass				m(sub e)
		proton mass				m(sub p)
		Bohr radius				a(sub o)
		electron radius 			r(sub e)
		Compton wavelength of electron		lambda(sub c)
		Bohr magneton				mu(sub B)
		nuclear magneton			mu(sub N)
		speed of light				c
		Planck constant 			h

    Certain units which are not part of the SI are used so widely that it is
impractical to abandon them. The units that are accepted for continued use
with the International System are listed in Table 4. It is likewise necessary
to recognize, outside the International System, the following units which are
used in specialized fields:

		electronvolt				eV
		unified atomic mass unit		u
		astronomical unit			AU
		parsec					pc

Logarithmic measures such as pH, dB (decibel), and Np (neper) are acceptable.
    The internationally accepted definitions for the seven base units follow.

    Mass. The kilogram (kg) is equal to the mass of the International
Prototype Kilogram. The International Prototype is a platinum-iridium cylinder
preserved at the International Bureau of Weights and Measures at Sevres,
France. Mass is the only one of the base quantities for which the standard is
an arbitrarily defined object. No basic property of matter involving mass can
be measured with more precision than is possible in comparing kilogram masses
by weighing, about 1 part in 10sup8.  See  BALANCE; MASS; WEIGHT MEASUREMENT.

    Length. The meter (m) is the length equal to 1|650|763.73 wavelengths in
vacuum of the radiation corresponding to the transition between the levels
2p10 and 5d5 of the krypton-86 atom (an orange-red line).
    Two monochromatic radiations, one in the visible, the other in the
infrared spectral region, produced by helium-neon lasers stabilized on a
saturated absorption line of iodine or of methane, are recommended as wavelength
standards:  iodine 127, R(127), band 11 - 5, component i [wavelength in a
vacuum, 632|991.399 x 10-(sup 12) m]; methane, P(7), band v3 [wavelength in a
vacuum 3|393|231.40 x 10-(sup 12) m].  These lines are reproducible with an
uncertainty of the order of 1 in 10(sup 10) the value of their wavelength in
meters is subject to the uncertainty of the standard (the wavelength of the (sup
86)Kr line) estimated to be 4 in 10(sup 9).
    The wavelength of the methane line mentioned above multiplied by its
frequency (measured by comparison with the (sup 133)Cs transition of the
definition of the second) yields the speed of propagation of electromagnetic
waves in vacuum c = 299|792|458 m/s. This value of c will probably be kept
unaltered in the future and will permit the meter to be defined as the
distance electromagnetic radiation travels in a vacuum in 1/299|792|458 s.
See  ELECTROMAGNETIC RADIATION.
    Time interval. The second (s) is the duration of 9 192 631 770 periods of
the radiation corresponding to the transition betweeen the two hyperfine levels
of the ground state of the cesium-133 atom. In the best equipments the stability
and accuracy of the cesium frequency generator correspond to an uncertainty of 1
in 10-(sup 12) or even 1 in 10(sup 13).
    The second was long defined, for physical measurements as well as for
civil affairs, as 1/86,400 of the time required for an average complete
rotation of the Earth on its axis with respect to the Sun. Because of the
slight slowing of the Earth's rotation rate, the universal second thus defined
is not a constant. A time scale called Coordinated Universal Time (UTC)
recommended by the General Conference of Weights and Measures (CGPM) in 1975
is defined in such a manner that it differs from international atomic time
(TAI) by an exact whole number of seconds. This difference is adjusted
occasionally by the use of a positive or negative leap second at the end of
certain months to keep UTC in agreement with the time defined by the rotation
of the Earth with an approximation better than 9/10 second.  See  ATOMIC
CLOCK; ATOMIC TIME; EPHEMERIS TIME; TIME.
    Temperature. The kelvin (K), the unit of thermodynamic temperature, is the
fraction 1/273.16 of the thermodynamic temperature of the triple point of
water. The unit kelvin and its symbol K should also be used to express an
interval or differences of temperature.
    To provide convenient and adequately accurate means for practical
realization and measurement of temperature, the International Practical
Temperature Scale is used, based on the assigned values of the temperatures of a
number of reproducible equilibrium states (defining fixed points) and on
standard instruments calibrated at those temperatures.	Interpolation between
the fixed-point temperatures is provided by formulas used to establish the
relation between indications of the standard instruments and values of
International Practical Temperature.

    Electric current. The ampere (A) is that constant current which, if
maintained in two straight parallel conductors of infinite length and of
negligible circular sections, and placed 1 meter apart in a vacuum, would
produce between these conductors a force equal to 2 x 10-(sup 7) newton per
meter of length.  See  ELECTRIC CURRENT.

    Luminous intensity. The CGPM, in 1979, redefined the base SI unit candela
as the luminous intensity, in a given direction, of a source that emits
monochromatic radiation of frequency 540 x 10(sup 12) hertz and of which the
radiant intensity in that direction is 1/683 watt per steradian.  See
ILLUMINATION; LIGHT; LUMINOUS EFFICACY; LUMINOUS EFFICIENCY; LUMINOUS
INTENSITY; PHOTOMETRY; RADIOMETRY.

    Amount of substance. The mole is the amount of substance of a system which
contains as many elementary entities as there are atoms in 0.012 kilogram of
carbon-12. When the mole is used, the elementary entities must be specified,
and may be atoms, molecules, ions, electrons, other particles, or specified
groups of such particles.  See	ATOMIC MASS UNIT; ATOMIC WEIGHT;
GRAM-MOLECULAR WEIGHT; MOLECULAR WEIGHT; RELATIVE ATOMIC MASS; RELATIVE
MOLECULAR MASS.
 �34s[William A. Wildhack]
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�6v�2v�32sPhysical optics   �21m �33s 
    The study of the interaction of electromagnetic waves in the optical range 
with material systems. The optical range of wavelengths may be taken as the 
range from about 1 nanometer to about 1 mm. 
    The explanation of the absorption, reflection, scattering, polarization, 
and dispersion of light by a material medium in terms of the properties of the 
atoms and molecules making up the medium is the objective of physical optics. 
In the course of seeking this objective, physicists have found that optical 
investigations are powerful methods of determining the structures of atoms and 
molecules and of large systems composed thereof.  See ABSORPTION; ATOMIC 
STRUCTURE AND SPECTRA; CRYSTAL OPTICS; DIFFRACTION; DISPERSION (RADIATION); 
ELECTROMAGNETIC RADIATION; ELECTROOPTICS; FARADAY EFFECT; FLUORESCENCE; 
INTERFERENCE OF WAVES; LASER; LIGHT; MAGNETOOPTICS; MOLECULAR STRUCTURE AND 
SPECTRA; POLARIZED LIGHT; REFLECTION OF ELECTROMAGNETIC RADIATION; REFRACTION 
OF WAVES; SCATTERING OF ELECTROMAGNETIC RADIATION; SPECTROSCOPY. 
 �34s[Richard C. Lord]
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�6v�2v�32sPhysical science   �21m �33s 
    The fields of inquiry to which the general designation science may be 
appropriately applied are broadly divided into social science and natural 
science. The latter is further subdivided into biology and physical science. 
Physical science is generally considered to include astronomy, chemistry, 
geology, mineralogy, meteorology, and physics. These overlap more or less, as 
illustrated by astrophysics, chemical physics, physical chemistry, and 
geophysics. There is overlap, likewise, between the physical and biological 
sciences, as seen in biochemistry, biophysics, virology, and the close 
relation between geology and paleontology. The boundaries implied in all such 
classifications are artificial and consist of regions where one field shades 
into another.  See  ASTRONOMY; BIOLOGY; CHEMISTRY; GEOLOGY; METEOROLOGY; 
MINERALOGY; PHYSICS; SCIENCE. 
 �34s[Joel H. Hildebrand]
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�6v�2v�32sPhysical theory �21m �33s
    A physical theory usually involves the attempt to explain a certain class
of physical phenomena by deducing them as necessary consequences of other
phenomena regarded as more primitive and less in need of explanation. The
value of a theory depends on both the success with which it coordinates a wide
range of presently known facts and its fertility in suggesting places to look
for presently unknown phenomena.  See  SCIENTIFIC METHODS.
 �34s[Percy W. Bridgman /Gerald Holton]
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�6v�2v�32sPhysics   �21m �33s 
    Formerly called natural philosophy, physics is concerned with those 
aspects of nature which can be understood in a fundamental way in terms of 
elementary principles and laws. In the course of time, various specialized 
sciences broke away from physics to form autonomous fields of investigation. 
In this process physics retained its original aim of understanding the 
structure of the natural world and explaining natural phenomena. 
    The most basic parts of physics are mechanics and field theory. Mechanics 
is concerned with the motion of particles or bodies under the action of given 
forces. The physics of fields is concerned with the origin, nature, and 
properties of gravitational, electromagnetic, nuclear, and other force fields. 
Taken together, mechanics and field theory constitute the most fundamental 
approach to an understanding of natural phenomena which science offers. The 
ultimate aim is to understand all natural phenomena in these terms.  See  
CLASSICAL FIELD THEORY; MECHANICS; QUANTUM FIELD THEORY. 
    The older, or classical, divisions of physics were based on certain 
general classes of natural phenomena to which the methods of physics had been 
found particularly applicable. The divisions are all still current, but many 
of them tend more and more to designate branches of applied physics or 
technology, and less and less inherent divisions in physics itself. The 
divisions or branches, of modern physics are made in accordance with 
particular types of structures in nature with which each branch is concerned. 
    In every area physics is characterized not so much by its subject-matter 
content as by the precision and depth of understanding which it seeks. The aim 
of physics is the construction of a unified theoretical scheme in mathematical 
terms whose structure and behavior duplicates that of the whole natural world 
in the most comprehensive manner possible. Where other sciences are content to 
describe and relate phenomena in terms of restricted concepts peculiar to 
their own disciplines, physics always seeks to understand the same phenomena 
as a special manifestation of the underlying uniform structure of nature as a 
whole. In line with this objective, physics is characterized by accurate 
instrumentation, precision of measurement, and the expression of its results 
in mathematical terms. 
    For the major areas of physics and for additional listings of articles in 
physics  see  ACOUSTICS; ASTROPHYSICS; ATOMIC PHYSICS; BIOPHYSICS; CLASSICAL 
MECHANICS; ELECTRICITY; ELECTROMAGNETISM; HEAT; LOW-TEMPERATURE PHYSICS; 
MOLECULAR PHYSICS; NUCLEAR PHYSICS; OPTICS; SOLID-STATE PHYSICS; THEORETICAL 
PHYSICS.  
 �34s[William G. Pollard]
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�6v�2v�32sPhysiological acoustics   �21m �33s 
    The study of specific responses that may occur in the ear or elsewhere 
along the central auditory pathways, following presentation of an appropriate 
stimulus at any level of the auditory system. Such responses may be recorded 
with the aid of various techniques which may be mechanical, electrical, 
optical, and so forth. The specific stimulus for the ear is acoustic energy. 
Experimentally, signals with well-defined parameters are used. The approach 
employed by physiological acoustics thus is purely analytical. This is in 
contrast to the holistic approach employed by psychoacoustics, which lends 
itself well to experiments on human subjects. Systematic physiological 
experiments can be performed only in animals, but differences between humans 
and other mammals are mainly in degree, not in principle.  See  EAR; HEARING 
(HUMAN); PSYCHOACOUSTICS. 
 �34s[Juergen Tonndorf]
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�6v�2v�32sPhysiological ecology (plant)   �21m �33s 
    The study of growth, biological processes, and reproduction within plant 
populations in natural or controlled environments. The five principal growth 
forms are unicellular, thallose, and herbaceous forms, and shrubs and trees. 
Life cycle processes include (1) germination, (2) growth, (3) absorption and 
loss of water, gases, and mineral nutrients, (4) photosynthesis and 
respiration, and (5) production of reproductive structures (flowers, cones, 
fruits, seeds, and spores). Each process is the result of a complex series of 
physical and chemical reactions within and between cells. All cellular 
processes are within the context of the whole plant as governed by the 
interaction of its genes and its environment through space and time. 
    Plant physiological ecology is part of the attempt to answer the question: 
Why does a plant grow where it does? An answer requires a knowledge of the 
genetic structure of the population, the population's physiological processes 
and their rates, its growth forms, the populational demography including seed 
sources, and the population's niche in the operation and structure of the 
ecosystem.  
    Actual and potential geographic ranges of populations or species seldom 
coincide. Environments change. Populations evolve, migrate, and become extinct 
in response to environmental changes. The potential range of a species or a 
local population depends upon the degree and nature of genetic variation among 
its individuals, the range of physiological and morphological adaptability 
within each individual (acclimatization and phenotypic plasticity), and the 
frequency, extent of occurrence, availability, and stability of suitable 
environments. These complex variables, operating together, determine the kinds 
and rates of physiological processes, and thus the degree of vegetative and 
reproductive success of a population within an ecosystem or in the biosphere. 
See  BIOSPHERE; ECOSYSTEM. 
 �34s[William D. Billings]
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�6v�2v�32sPhysiology, general   �21m �33s 
    Physiology is the science of vital functions, the basic activities of 
living organisms and of their parts considered in relation to the activity of 
the whole. General physiology comprises that approach to the subject which 
emphasizes those functions or properties common to all forms of life, and 
interprets these in terms of principles of physical science. The cell 
principle asserts that all living organisms have a cellular state in the life 
cycle, and that cells are the units of structure and function in organisms. 
General physiology therefore emphasizes the functions of cells in relation to 
those of the whole organism.  See  CELL (BIOLOGY). 
    Traditionally, general physiology has been concerned with problems such as 
those of absorption (the accumulation or transport of certain materials by 
cells), the production of electricity by living organisms, the nature and 
functions of enzymes, the process of secretion (production and release of 
certain materials by cells), and the phenomena of movement and of sensitivity. 
    However, the control of the structure and function of a cell originates in 
the control, by the genetic material (DNA), of the synthesis of enzymes and 
other proteins. The specialized field of research known as molecular biology 
is the study of DNA and other macromolecules of cells in relation to 
inheritance and to cellular function, and may be considered as a part of 
general physiology.  See  MOLECULAR BIOLOGY. 
 �34s[Bradley T. Scheer]
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�6v�2v�32sPhytamastigophorea   �21m �33s 
    A class of the subphylum Sarcomastigophora, also known as the 
Phytomastigina. These are the plant flagellates which contain chlorophyll and 
other pigments, but colorless forms are also included. Encystment is frequent 
among phytoflagellates, cyst composition being one method of determining 
relationships for some colorless species. 
    The Phytamastigophorea include 10 orders: Chrysomonadida, 
Silicoflagellida, Coccolithophora, Heterochlorida, Cryptomonadida, 
Dinoflagellida, Ebriida, Euglenida, Chloromonadida, and Volvocida. See 
articles on these groups. See SARCOMASTIGOPHORA. 
 �34s[James B. Lackey]
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�6v�2v�32sPhytochrome   �21m �33s 
    A plant pigment which regulates plant growth and development in response 
to presence or absence of light, day length, or light quality. It is found 
either in a highly stable inactive form or a relatively less stable active 
form in plant tissues, depending upon their previous illumination history. 
Absorption of light by inactive phytochrome transforms it to the active form, 
in which conformation it may regulate phenomena such as stem elongation, leaf 
expansion, flowering, seed germination, the relative activity of different 
metabolic pathways, and a host of other physiological and biochemical 
processes.  
    The control of plant development by light, acting through phytochrome, 
enables a plant to respond most favorably to its own immediate environment. 
Thus it can be protected from squandering resources by expanding leaves in 
darkness; it can maximize elongation under conditions in which light for 
photosynthesis is absent or limiting, in order to reach illumination with 
highest efficiency; and it can regulate its flowering or dormancy in response 
to appropriate season by measuring the length of days and nights. Phytochrome 
thus provides a continuous monitor of the light environment. 
    Phytochrome consists of a protein bound to a group (chromophore) which 
absorbs visible light. The color and reactivity of phytochrome to visible 
light are strictly determined by the chromophore's properties. The chromophore 
has been identified as an open-chain tetrapyrrole closely related to the 
blue-green and red algal photosynthetic accessory pigments phycocyanin and 
phycoerythrin, which are also open-chain tetrapyrroles. It is somewhat less 
closely related to the chlorophylls, which are closed tetrapyrrole rings.  See 
   CHLOROPHYLL; PHYCOBILIN. 
 �34s[Winslow R. Briggs]
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�6v�2v�32sPhytoplankton   �21m �33s 
    Mostly autotrophic microscopic algae which inhabit the illuminated surface 
waters of the sea, estuaries, lakes, and ponds. Many are motile. Some perform 
diel (diurnal) vertical migrations, others do not. Some nonmotile forms 
regulate their buoyancy. However, their locomotor abilities are limited, and 
they are largely transported by horizontal and vertical water motions. 
    A great variety of algae make up the phytoplankton. Diatoms (class 
Bacillariophyceae) are often conspicuous members of marine, estuarine, and 
fresh-water plankton. Dinoflagellates (class Dinophyceae) occur in both marine 
and fresh-water environments and are important primary producers in marine and 
estuarine environments. Coccolithophorids (class Haptophyceae) are also marine 
primary producers of some importance. They do not occur in fresh water. 
    Even though marine and fresh-water phytoplankton communities contain a 
number of algal classes in common, phytoplankton samples from these two 
environments will appear quite different. These habitats support different 
genera and species and groups of higher rank in these classes. Furthermore, 
fresh-water plankton contains algae belonging to additional algal classes 
either absent or rarely common in open ocean environments. These include the 
green algae (class Chlorophyceae), the euglenoid flagellates (class 
Euglenophyceae), and members of the Prasinophyceae. 
    The phytoplankton in aquatic environments which have not been too 
drastically affected by human activity exhibit rather regular and predictable 
seasonal cycles. Coastal upwelling and divergences, zones where deeper water 
rises to the surface, are examples of naturally occurring phenomena which 
enrich the mixed layer with needed nutrients and greatly increase 
phytoplankton production. In the ocean these are the sites of the world's most 
productive fisheries.  See  EUTROPHICATION. 
 �34s[Robert W. Holmes]
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�6v�2v�32sPhytotronics   �21m �33s 
    Research using whole plants and conducted under controlled environmental 
conditions to determine responses to a single or known combination of 
environmental elements. Originally, the term phytotronics was used to identify 
research conducted specifically in phytotrons where controlled plant growth 
units are available for simultaneous use. The name phytotronics is also often 
applied to any research conducted with whole plants in a controlled 
environment plant growth chamber or room. 
 �34s[Henry Hellmers]
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�6v�2v�32sPiciformes   �21m �33s 
    An order of birds containing six families, which have in common some 
anatomical characters, including a peculiar arrangement of the tendons of the 
toes. The order is generally considered to be most closely related to the 
Passeriformes. All members of the Piciformes nest in holes, and young birds 
spend a long period in the nest. 
    The largest family is the Picidae, the woodpeckers, worldwide in 
distribution except for Madagascar, Australia, and most of Oceania. Three 
families are confined to the neotropical region: The jacamars (Galbulidae), 
many of which are highly iridescent and resemble giant hummingbirds; the 
puffbirds (Bucconidae), most of which have a peculiar notch at the tip of the 
bill; and the toucans (Ramphastidae), noted for large and often highly 
colorful bills. Barbets (Capitonidae) are pantropical, whereas honey guides 
(Indicatoridae) are confined to Africa and India.  See  AVES; PASSERIFORMES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sPicornavirus   �21m �33s 
    A viral group made up of the small (18 - 30 nanometer) ether-sensitive 
viruses that lack an envelope and have a ribonucleic acid (RNA) genome. The 
name is derived from ``pico'' meaning very small, and RNA for the nucleic acid 
type. Picornaviruses of human origin include the following subgroups: 
enteroviruses (polioviruses, coxsackieviruses, and echoviruses) and 
rhinoviruses. There are also picornaviruses of lower animals (for example, 
bovine foot-and-mouth disease, a rhinovirus).  See  ANIMAL VIRUS; 
COXSACKIEVIRUS; ECHOVIRUS; ENTEROVIRUS; FOOT-AND-MOUTH DISEASE; POLIOMYELITIS; 
RHINOVIRUS.  
 �34s[Joseph L. Melnick]
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�6v�2v�32sPicosecond molecular processes �21m �33s
    A branch of photophysics/photochemistry/photobiology dealing with various
types of molecular energy exchange and chemical reactions occurring on the
10-(sup 12) to 10-(sup 9) s time scale. The study of picosecond processes
overlaps the subpicosecond regime (less than 10-(sup 12) s) on one side and
the nanosecond regime (about 10-(sup 9) to 10-(sup 8) s) on the other.
    A typical experiment is initiated with an ultrashort pulse of energy -
radiation (light, produced by lasers) or particles (electrons, produced from
microwave linear accelerators). The study of picosecond molecular processes
deals with the rapid changes brought about by the absorption of these
ultrashort energy pulses by a system of molecules - gas, liquid, or solid.
These changes may be of either a chemical or a physical nature and can be
measured by ultrafast detection methods, which are usually based on some type
of spectroscopic monitoring of the system. The excited molecules may change
their structure internally, they may gain or lose electrons or protons, they
may chemically react, they may exchange their energy with other nearby
molecules, or they may simply lose memory of the initial dipole direction in
which they were excited (dephasing). The important aspect of picosecond
processes lies in the fact that elementary motion, such as rotation,
vibrational exchange, or charge transfer, takes place on this time scale.
These types of studies have therefore provided for the first time a direct
look at the relationship between such motion and overall chemical or
biological changes.  An interesting experimental application of picosecond
spectroscopy is in the discovery of the early ultrafast processes in the
complex chain of events responsible for animal vision.	See  CHEMICAL DYNAMICS.
 �34s[G. Wilse Robinson]
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�6v�2v�32sPicric acid �21m �33s
    A phenol in which nitro (NO2) groups are present in the 2, 4, and 6
positions on the ring of carbon atoms. The formula for picric acid is shown
below. It approaches the mineral acids in acid strength (pKsuba = 0.80).
    Picric acid was widely used as a yellow dye for silk before being replaced
by modern dyes of greater fastness. It has also been used as an explosive, and
can be detonated by both heat and shock.  See  PHENOL.
 �34s[Robert I. Stirton /Martin Stiles]
�33s

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�6v�2v�32sPicrite   �21m �33s 
    The term picrite has been used with several different meanings. It is 
generally considered to include certain medium- to fine-grained igneous rocks 
composed chiefly of olivine with smaller amounts of pyroxene, hornblende, and 
plagioclase feldspar (labradorite). Its feldspar content is slightly higher 
than that of peridotite and lower than that of gabbro. Certain 
analcite-bearing types, associated with teschenite, have also been included 
under the term picrite. Picrite is rare and is found in small intrusives 
(sills and dikes).  See  GABBRO; IGNEOUS ROCKS; PERIDOTITE. 
 �34s[Carleton A. Chapman]
�33s 
  
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�6v�2v�32sPictorial drawing   �21m �33s 
    A view of an object (actual or imagined) as it would be seen by an 
observer who looks at the object either in a chosen direction or from a 
selected point of view. Pictorial sketches often are more readily made and 
more clearly understood than are front, top, and side views of an object. 
Pictorial drawings, either sketched freehand or made with drawing instruments, 
are frequently used by engineers and architects to convey ideas to their 
assistants and clients.  See  DESCRIPTIVE GEOMETRY; ENGINEERING DRAWING. 
    In making a pictorial drawing, the viewing direction that shows the object 
and its details to the best advantage is chosen. The resultant drawing is 
orthographic if the viewing rays are considered as parallel, or perspective if 
the rays are considered as meeting at the eye of the observer. Perspective 
drawings provide the most realistic, and usually the most pleasing, likeness 
when compared with other types of pictorial views. 
    Several types of nonperspective pictorial views can be sketched, or drawn 
with instruments. In the isometric pictorial, the direction of its axes and 
all measurements along these axes are made with one scale (Fig. 1). Oblique 
pictorial drawings, while not true orthographic views, offer a convenient 
method for drawing circles and other curves in their true shape (Fig. 2). In 
order to reduce the distortion in an oblique drawing, measurements along the 
receding axis may be foreshortened. When they are halved, the method is called 
cabinet drawing. 
 �34s[Charles J. Baer]
�33s 
  
  
  
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�6v�2v�32sPicture tube   �21m �33s 
    A cathode-ray tube used as a television picture reproducer. Modern 
television picture tubes usually have large glass envelopes (see illustration) 
on the inner face of which a light-emitting layer of luminescent materials is 
deposited. A modulated stream of high-velocity electrons is made to scan this 
layer of luminescent materials in a series of horizontal lines so that it 
recreates the picture elements (light and dark areas).  See  CATHODE-RAY TUBE. 
    The number of electrons in the stream at any instant of time is varied by 
electrical impulses corresponding to the signal sent out by the television 
transmitter. These electrical impulses (picture information) were originally 
generated by a studio television camera. The home television receiver picks up 
the signal, and after suitable amplification and detection the picture 
information is supplied to the picture tube so that it recreates the original 
picture.  See  TELEVISION. 
    Picture tubes are defined by the method used to focus the stream of 
electrons to a small spot. Electrostatic focus, accomplished by applying 
voltage differences to the electron-gun electrodes, has become universal. The 
electron beam, produced by an electron gun, is deflected electromagnetically 
to cause it to scan the picture area. This deflection is accomplished by a 
deflecting yoke, made up of two pairs of shaped coils which fit around the 
small neck of the picture tube. When pulsating electric currents of proper 
wave shape and phase are supplied to these coils, they generate magnetic 
fields which cause the electron beam to bend as it passes through them. By 
changing the magnitude and direction of the magnetic field, the electron beam 
can be made to arrive at any point on the face of the picture tube. 
    The glass envelope is made of a special composition to minimize optical 
defects and to provide electrical insulation for high voltages. The structural 
design of the glass bulb is made to withstand 3 - 6 times the force of 
atmospheric pressure. 
    The luminescent screen is made of a thin layer of phosphors. The phosphor 
materials are primarily zinc cadmium sulfide (emits yellow light) and zinc 
sulfide (emits blue light). The luminescent screen is aluminized by vacuum 
evaporation from a small molten aluminum pellet. In the operation of the 
completed tube, the high-velocity electron beam penetrates the aluminum film, 
and its energy is transferred primarily to the luminescent screen. 
    A color television picture tube is similar to the black-and-white picture 
tube but differs in two ways. (1) The light-emitting screen is made up of 
small elemental areas, each capable of emitting light in one of the three 
primary colors (red, green, or blue) laid in interlaced arrays. (2) Means are 
provided for selectively exciting any one or more of these phosphor primaries. 
  See  COLOR TELEVISION. 
 �34s[C. Price Smith]
�33s 
  
  
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�6v�2v�32sPier   �21m �33s 
    A fingerlike structure projecting from shore and providing berths for 
ships to load and discharge passengers and cargo. Construction usually takes 
the form of a pile-supported platform using steel, timber, and concrete 
materials, thus allowing free water flow underneath. Sometimes bulkhead 
construction is used around the periphery with earth fill inside and pavement 
topping it. Piers may be housed over with sheds and may have special 
cargo-handling equipment. Uses of piers sometimes extend to recreational 
purposes, such as fishing piers, or to community purposes, such as car 
parking. Spaces between adjacent piers are called slips.  See  COASTAL 
ENGINEERING; WHARF. 
 �34s[Edward J. Quirin]
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�6v�2v�32sPiezoelectricity   �21m �33s 
    Electricity, or  electric polarity, resulting from the application of 
mechanical pressure on a dielectric crystal. The application of a mechanical 
stress produces in certain dielectric (electrically nonconducting) crystals an 
electric polarization (electric dipole moment per cubic meter) which is 
proportional to this stress. If the crystal is isolated, this polarization 
manifests itself as a voltage across the crystal, and if the crystal is 
short-circuited, a flow of charge can be observed during loading. Conversely, 
application of a voltage between certain faces of the crystal produces a 
mechanical distortion of the material. This reciprocal relationship is 
referred to as the piezoelectric effect. The phenomenon of generation of a 
voltage under mechanical stress is referred to as the direct piezoelectric 
effect, and the mechanical strain produced in the crystal under electric 
stress is called the converse piezoelectric effect.  See  POLARIZATION OF 
DIELECTRICS.  
    The necessary conditon for the piezoelectric effect is the absence of a 
center of symmetry in the crystal structure. Of the 32 crystal classes, 21 
lack a center of symmetry, and with the exception of one class, all of these 
are piezoelectric. Hydrostatic pressure produces a piezoelectric polarization 
in the crystals of those 10 classes that show pyroelectricity in addition to 
piezoelectricity.  See  CRYSTALLOGRAPHY; PYROELECTRICITY. 
  
    Molecular theory.  Quantitative theories based on the detailed crystal 
structure are very involved. Qualitatively, however, the piezoelectric effect 
is readily understood for simple crystal structures. The illustration shows 
this for a particular cubic crystal, zincblende (ZnS). Every Zn ion is 
positively charged and is located in the center of a regular tetrahedron ABCD, 
the corners of which are the centers of sulfur ions, which are negatively 
charged. When this system is subjected to a shear stress in the xy plane, the 
edge AB, for example, is elongated, and the edge CD of the tetrahedron becomes 
shorter. Consequently, these edges are no longer equivalent, and the Zn ion 
will be displaced along the z axis, thus giving rise to an electric dipole 
moment. The dipole moments arising from different octahedrons sum up because 
they all have the same orientation with respect to the axes x, y, and z. 
  
    Applications.  The sharp resonance curve of a piezoelectric resonator 
makes it useful in the stabilization of the frequency of radio oscillators. 
Quartz crystals are used almost exclusively in this application. In 
vacuum-tube oscillators, the crystal generally is part of the feedback 
circuit. Selective band-pass filters with low losses can be built by using 
piezoelectric resonators as circuit elements. A synthetic piezoelectric 
crystal which is often substituted for quartz in this application is ethylene 
diamine tartrate.  See  QUARTZ CLOCK. 
    Piezoelectric materials are used extensively in transducers for converting 
a mechanical strain into an electrical signal. Such devices include 
microphones, phonograph pickups, vibration-sensing elements, and the like. The 
converse effect, in which a mechanical output is derived from an electrical 
signal input, is also widely used in such devices as sonic and ultrasonic  
transducers, headphones, loudspeakers, and cutting heads for disk recording.  
See  DISK RECORDING; MICROPHONE; ULTRASONICS; UNDERWATER TRANSDUCER. 
 �34s[Harold Goodglass]
�33s 
  
    Piezoelectric materials.  The principal piezoelectric materials used 
commercially are crystalline quartz and rochelle salt, although the latter is 
being superseded by other materials, such as barium titanate. Quartz has the 
important qualities of being a completely oxidized compound (silicon dioxide), 
and is almost insoluble in water. Therefore, it is chemically stable against 
changes occurring with time. It also has low internal losses when used as a 
vibrator. Rochelle salt has a large piezoelectric effect, and is thus useful 
in acoustical and vibrational devices where sensitivity is necessary, but it 
decomposes at high temperatures (131DGF or 55DGC) and requires protection 
against moisture. Barium titanate provides lower sensitivity, but greater 
immunity to temperature and humidity effects. Other crystals that have been 
used for piezoelectric devices include tourmaline, ammonium dihydrogen 
phosphate (ADP), and ethylenediamine tartrate (EDT).  See  BARIUM TITANATE; 
QUARTZ; ROCHELLE SALT. 
 �34s[Frank D. Lewis]
�33s 
  
  
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�6v�2v�32sPig   �21m �33s 
    A mammal which is a member of the superfamily Suoidea in the order 
Artiodactyla, the even-toed ungulates. There are two families, the Suidae or 
Old World pigs (including wild and domestic species), and the Tayassuidae or 
peccaries of the Americas. The toes terminate in nails which are modified into 
hooves, the tail is short, and the body is covered sparsely with hair which is 
frequently bristlelike. These bristles are used in the manufacture of the 
highest-quality brushes. These animals are herbivorous. Tusklike canines are 
present in some species.  See  ARTIODACTYLA; TRICHINOSIS. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sPigeonite   �21m �33s 
    Monoclinic pyroxenes of the general formula (Mg,Fe)SiO3 having some augite 
in solid solution. Pigeonite bears the same relation to the orthorhombic 
pyroxenes as augite does to the diopside-hedenbergite series. Pigeonite is the 
orthorhombic pyroxene equivalent in the volcanic rocks.  See  AUGITE; 
DIOPSIDE; ORTHORHOMBIC PYROXENE; PYROXENE. 
 �34s[George W. DeVore]
�33s 
  
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�6v�2v�32sPigment   �21m �33s 
    A finely divided material which contributes to optical and other 
properties of paint, finishes, and coatings. Pigments are insoluble in the 
coating material, whereas dyes dissolve in and color the coating. Pigments are 
mechanically mixed with the coating and are deposited when the coating dries. 
Their physical properties generally are not changed by incorporation in and 
deposition from the vehicle. Pigments may be classified according to 
composition (inorganic or organic) or by source (natural or synthetic). 
However, the most useful classification is by color (white, transparent, or 
colored) and by function. Special pigments include anticorrosive, metallic, 
and luminous pigments.  See  DYE; LUMINOUS PAINT; PAINT. 
 �34s[C. R. Martinson ;C. W. Sisler]
�33s 
  
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�6v�2v�32sPigmentation   �21m �33s 
    The unique pigmentation of a given biological material is an expression of 
the quantity and quality of reflected incoming visible light. The light 
returning from the material is perceived as colored. The color is determined 
by the chemical composition and the physical properties of the light-exposed 
material. Consequently, a coloration may be due to pigments or may be of 
structural origin. 
    Pigments are essential constituents of the living world. Their 
contribution to the evolution and maintenance of life, and its manifold 
expressions, is most evident in the role of chlorophylls and the associated 
carotenoids of certain bacteria and most plants. These pigments harvest solar 
light energy for utilization in the photosynthesis of organic material from 
inorganic precursors.  See  CAROTENOID; CHLOROPHYLL; PHOTOSYNTHESIS. 
    The outermost structures on the animal skin are pigmented for many 
reasons, for example, to reduce the animal's visibility against a colored 
background, or to provide optical signals to the other sex or to other 
species. Conspicuously pigmented flowers attract pollinators, and colored 
fruits are easily found by animals, which eat them and then help disperse the 
seeds.  See  CHROMATOPHORE; PROTECTIVE COLORATION. 
    The role of pigments in communication depends on the ability of organisms 
to discriminate between different regions of the solar spectrum. In animals 
with eyes, this is accomplished by differently colored visual pigments 
contained in specialized receptor cells. Microorganisms, fungi, and plants 
also have special pigment systems that permit these organisms to move or grow 
toward, or away from, light (positive and negative phototaxis and 
phototropism, respectively).  See  PLANT MOVEMENTS. 
    Since organisms are totally dependent on light - at least indirectly - 
elaborate pigment systems have evolved which tune metabolic and activity 
patterns to the daily pattern of light and dark, and to the changes in the 
relative lengths of day and night occurring in the course of a year. The 
visual pigments of animals (the pigments of the extraretinal photoreceptor 
organs of many vertebrates and invertebrates) and the phytochromes of plants 
are typical representatives of pigments that correlate biological activity 
with light-dark cycles (photoperiodism). Colored coenzymes, such as flavins, 
may function as on-off switches of light-regulated metabolic processes.  See  
COLOR VISION; PHOTOPERIODISM; PHOTORECEPTION; PHYTOCHROME. 
    In the examples listed above, pigments mediate, in various ways,the  
beneficial actions of light. Absorbed solar light energy may, however, also 
have detrimental effects by causing undesirable, or even destructive, 
reactions. The pigmentation of the human skin by melanin provides a 
light-absorbing layer which protects the tissue layers below from such 
potentially damaging radiation of the Sun.  See  INTEGUMENT; SKIN. 
 �34s[Peter H. Homann]
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�6v�2v�32sPika   �21m �33s 
    Any member of the family Ochotonidae in the order Lagomorpha, which 
includes 14 species of small mammals restricted to northern Asia and northern 
North America. They may occur as high as 17,000 ft (5.2 km) in the Himalayas; 
all species inhabit rocky areas, dwelling deep under the rocks away from their 
natural enemies, hawks and foxes. 
    These mammals resemble rabbits, and have a vestigial tail; however, they 
have short, rounded ears (see illustration). Since they do not hibernate, they 
have evolved an interesting food-storage system to provide reserves for the 
winter. They cut and carry green herbage to the rocky areas they inhabit and 
spread it in the sun to dry. When the vegetation is dried, it is piled like a 
haystack. Common species include the Alaskan pika (Ochotona collaris) and the 
American pika (O. princeps).  See  LAGOMORPHA. 
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sPike   �21m �33s 
    Any of about five species of fish which compose the family Esocidae in the 
order Clupeiformes, known by a variety of names such as pickerel and 
muskellunge. These fish are voracious predators with an elongated beaklike 
snout and sharp teeth. The head is partly scaled and the body is covered with 
cycloid scales that have deeply scalloped edges. The body is cylindrical and 
compressed; thus, these fish are well adapted for rapid movements as they dart 
after prey. They prey upon each other, as well as other fish, amphibians, 
small aquatic birds and mammals, and rats. All species are edible but are 
considered second-rate game fish.  See  CLUPEIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sPile foundation   �21m �33s 
    A part of a foundation system which transfers loads from a superstructure 
to the soil or rock below the surface by means of structural elements called 
piles. A pile foundation is used when the upper layers of soil are too soft 
and compressible to support the loads of the superstructure in a safe and 
economical manner by means of footings or mat foundations. A pile foundation 
is often used to support structures over water or adjacent to water, in 
addition to supporting structures which are constructed over soft soils. 
    Piles, the structural elements which are installed into the soil or rock, 
may be wood, concrete, or steel; or concrete and wood or concrete and steel my 
be combined to form composite piles. Occasionally, wood piles may be 
reinforced with steel bands. 
    Piles obtain supporting capacity from the earth through end bearing or 
side friction or more generally through a combination of both (Figs. 1 and 2). 
Piles installed through very soft soils to rock or another firm stratum derive 
their capacity to support structural loads from the strength of the firm 
stratum on which the end of the pile is bearing. In such a situation the shaft 
of the pile acts as a column, and must have sufficient strength to transfer 
the imposed load from the top to the bottom of the pile.  
    Soil is not always underlain by rock or another firm stratum within a 
depth which can be reached economically by piles. In this condition, the soil 
in which the pile is embedded and the soil below the pile tip provide the 
support for the pile. As the pile is subjected to load, it tends to move 
downward. The tendency to move downward is resisted by the friction between 
the sides of the pile and the soil in which the pile is embedded, or the 
shearing resistance of the soil immediately adjacent to the sides of the pile. 
In addition, as the tip of the pile tends to move downward, the soil below the 
tip resists this tendency due to its bearing strength.  See  SOIL MECHANICS. 
  
    Most piles are installed by driving the piles into the soil. The equipment 
used consists of a steel-pile-driving hammer which operates between vertical 
leads. The leads are  attached to a mobile crane. The simplest type of hammer 
is a steel weight raised above the top of the pile and allowed to fall by 
gravity. Another type of hammer used is a diesel hammer. In this type, the ram 
is lifted by an explosion of diesel fuel in a combustion chamber. When the ram 
reaches a predetermined height above the top of the pile, it drops by gravity; 
the next fuel explosion for lifting occurs at the moment of impact. A second 
type of diesel hammer is one which has an air chamber above the ram. As the 
ram is raised by the explosion in the combustion chamber, the air in the air 
chamber is compressed. As the ram drops, the compressed air adds downward 
force to the ram. Methods of installing piles other than using pile-driving 
equipment involve drilling a hole into the soil with an auger or a rotating 
bucket and filling the hole with concrete.  See  CAISSON FOUNDATION; 
COFFERDAM; FOUNDATIONS; RETAINING WALL. 
 �34s[Gardner M. Reynolds]
�33s 
  
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�6v�2v�32sPilotage   �21m �33s 
    One of four procedures used in navigating an aircraft. The other three are 
position fixing, homing, and dead reckoning. Pilotage is the procedure of 
using landmarks, such as cities, towns, rivers, railroads, and prominent 
highways, to guide an aircraft to a destination. The installation of lights at 
airports and prominent spots across the country enhanced the ability of the 
pilot to direct the aircraft.  See  DEAD RECKONING; POSITION FIXING. 
    The introduction of radar brought a new dimension to pilotage. Airborne 
radar operating at microwaves produces very sharp maps of the terrain over 
which the aircraft is flying. Radar pilotage has also been adapted to missile 
guidance.  
 �34s[Peter C. Sandretto]
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�6v�2v�32sPiloting   �21m �33s 
    The form of navigation in which position is determined relative to 
external reference points, usually fixed points on the Earth. It is the oldest 
form of navigation. With the development of electronic aids, piloting 
techniques were extended far from shore. However, the term ``piloting'' is 
generally associated with nearness of land, where tidal and other currents may 
be strong, shoals and other underwater obstructions may be in near proximity, 
and maneuvering room is limited when other vessels are encountered. Thus it is 
not unusual for ships to employ the services of a local expert, called a 
pilot, to assist in the navigation of the vessel while it enters or leaves 
port. Whether a pilot is used or navigation is performed by ship's officers, 
the period during which piloting is performed is a critical one, requiring 
frequent and urgent exercise of judgment and attention to detail. 
    A conspicuous object, structure, or light that serves as an indicator for 
establishing the position of a craft or otherwise assisting in its safe 
navigation is called a mark. To be useful, not only must a mark be identified, 
but its position must be known accurately. Artificial marks designed and 
erected specifically to assist the navigator are called aids to navigation and 
include beacons, both lighted and unlighted, lighthouses, buoys, both lighted 
and unlighted, and lightships. Unlighted aids are called daymarks.  See  BUOY; 
LIGHTHOUSE.  
    In addition to visible aids to navigation, bottom topography can be of 
assistance in locating the position of a vessel. Sound signals transmitted 
through water or air may be used for navigation. Electronic beacons and 
positioning systems have been established at a number of places to assist in 
navigation.  See  ELECTRONIC NAVIGATION SYSTEMS. 
    The measurements made for piloting purposes are of direction, distance, 
differential distance between two points, and distance to the bottom.  
    Bearings are usually measured (1) by noting when two objects are in range 
(directly in line); (2) by means of a suitable attachment to a compass or 
compass repeater; (3) by pelorus, a compasslike instrument without directive 
properties; or (4) electronically, by radio direction finder, radar, or by the 
indication of the receiver-indicator of an electronic system of navigation. 
Distance is generally measured by radar. The difference in distance from the 
ship to two points is usually measured electronically by means of the 
receiver-indicator of a hyperbolic navigation system. Depth measurement is 
usually made by an echo sounder.  See  DIRECTION-FINDING EQUIPMENT; ECHO 
SOUNDER; HYPERBOLIC NAVIGATION SYSTEM; RADAR. 
    Position is generally determined by means of lines of positions, each 
indicating a series of possible positions of the craft at the time of 
measurement. A measured bearing provides a straight line of position (actually 
part of a great circle) passing through the object sighted. A measured 
distance provides a circular line of position with the object as the center 
and the distance as the radius. A measured differential distance provides a 
hyperbolic line of position. A position, called a fix, is usually determined 
by crossing two or more lines of position taken simultaneously or nearly so.  
See  CELESTIAL NAVIGATION; DEAD RECKONING; NAVIGATION; POLAR NAVIGATION. 
 �34s[Alton B Moody]
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�6v�2v�32sPilot production   �21m �33s 
    The production of a product, process, or piece of equipment on a simulated 
factory basis. In mass-production industries where complicated products, 
processes, or equipment are being developed, a pilot plant often leads to the 
presentation of a better product to the customer, lower development and 
manufacturing costs, more efficient factory operations, and earlier 
introduction of the product. Following the engineering development of a 
product, process, or complicated piece of equipment and its one-of-a-kind 
fabrication in the model shop, it becomes desirable and necessary to ``prove 
out'' the development on a simulated factory basis.  See  INDUSTRIAL COST 
CONTROL; MASS PRODUCTION; PRODUCT DESIGN; PRODUCTION ENGINEERING; QUALITY 
CONTROL.  
 �34s[James E. Woodall]
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�6v�2v�32sPimento   �21m �33s 
    A type of pepper, Capsicum annuum, grown for its thick, sweet-fleshed red 
fruit. A member of the plant order Polemoniales, pimento is of American 
origin, and gets its name from the Spanish word designating all sweet peppers. 
In the United States, however, the term pimento generally refers to the 
heart-shaped varieties (cultivars) grown for canning and used for stuffing 
olives and flavoring foods. Georgia is the only important pimento-producing 
state.  See  PEPPER; POLEMONIALES. 
 �34s[H. John Carew]
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�6v�2v�32sPina   �21m �33s 
    A fiber, also known as pineapple fiber, obtained from the large leaves of 
the pineapple plant grown in tropical countries. This natural fiber is white 
and especially soft and lustrous. In the Philippine Islands, it is woven into 
pina cloth, which is soft, durable, and resistant to moisture. Pina is also 
used in making coarse grass cloth and for mats, bags, and clothing.  See  
NATURAL FIBER; PINEAPPLE. 
 �34s[M. David Potter]
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�6v�2v�32sPinales   �21m �33s 
    An order of the plant class Pinopsida, subdivision Pinicae, in the 
division Pinophyta (Gymnospermae), with more than 50 genera and over 540 
species - the dominant forest-makers of the world. Here belong the pine, 
spruce, hemlock, cedar, fir, larch, juniper, cypress, yew, redwood, big tree, 
kauri, podocarpus, and many others. All are woody, being either shrubs or 
trees.  
    The conifers appeared in the Carboniferous, attained their zenith and 
widest distribution in the Jurassic, and then began to wane. However, the 
persistent species form some of the most extensive forests of modern times. 
Characteristically, they are mountain dwellers, ranging from the Arctic to the 
Antarctic circles, with the greatest display in the temperate zones and a good 
representation in the tropics. 
    The cones are unisexual. In some species, both male and female cones are 
borne on the same plant (monoecious); in others, on separate plants 
(dioecious). Usually, a year intervenes between pollination and fertilization, 
and another year between fertilization and embryo formation. 
    The conifers are used in landscaping and as the source of valuable lumber 
and such by-products as turpentine, tar, resin, and oils of various kinds. See 
separate articles on most of the major conifers by common names.  See  PINE; 
PINOPSIDA.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sPinch effect �21m �33s
    A name given to manifestations of the magnetic self-attraction of parallel
electric currents having the same direction. The effect at modest current
levels of a few amperes can usually be neglected, but when current levels
approach a million amperes such as occur in electrochemistry, the effect can
be damaging and must be taken into account by electrical engineers. The pinch
effect in a gas discharge has been the subject of intensive study, since it
presents a possible way of achieving the magnetic confinement of a hot plasma
(a highly ionized gas) necessary for the successful operation of a
thermonuclear or fusion reactor.
    The law of attraction which describes the interaction between parallel
electric currents was discovered by A. M. Ampere in 1820. For a cylindrical
wire of radius r meters carrying a total surface current of I amperes, it
manifests itself as an inward pressure on the surface (Fig. 1) given by
I(sup 2/2) x 10(sup 7) PIr(sup 2) pascals. For the electric currents of normal
experience, this force is small and passes unnoticed, but it is significant
that the pressure increases with the square of the current, I(sup 2). For
example, at 25,000 amperes the pressure amounts to about 1 atm (100
kilopascals) for a wire of 1-cm radius, but at 10(sup 6) amperes the pressure is
about 1600 atm or about 12 tons in.-(sup 2)(160 megapascals).
    There are a number of ways in which the magnetic field of a fusion reactor
can be arranged around the plasma to hold it together, and one of these
methods is the pinch effect. A fusion reactor using this type of confinement
would ideally be a toroidal tube in which the confined plasma would carry a
large electric current induced in it by magnetic induction from a transformer
core passing through the major axis of the torus. The current would have the
double function of ohmically heating the plasma and compressing the plasma
toward the center of the tube.
    Characteristically, as can be shown by high-speed photography, the pinch
forms at the inner surface of a discharge tube wall and contracts radially
inward, forming an intense line, the pinch, on the axis; the pinch rebounds
slightly; the contracted discharge rapidly develops necks and kinks; and in a
few microseconds all structure is lost in an apparently turbulent glowing gas
which fills the tube. Thus, the pinch turns out to be unstable, and plasma
confinement is soon lost by contact with the wall. The cause of the
instability is easily seen qualitatively: The pinch confinement can be
described as being caused by the magnetic field lines encircling the pinch
which are stretched longitudinally but which are in compression transversely
(Fig. 2). For a uniform cylindrical pinch, the magnetic pinch pressure is
everywhere equal to the outward plasma pressure, but at a neck or on the
inward side of a kink, the magnetic field lines crowd together, creating a
higher magnetic pressure than the outward gas pressure. Consequently, the neck
contracts still further, the kink cuts in on the concave side and bulges out
on the convex side, and both perturbations grow. The instability has a
disastrous effect on the confinement time.
    The term theta pinch has come into wide usage to denote an important
plasma confinement system which relies on the repulsion of oppositely directed
currents and which is thus not in accord with the original definition of the
pinch effect (self-attraction of currents in the same direction). Plasma
confinement systems based on the original pinch effect are known as Z pinches.
    Tokamak is essentially a low-density, slow Z pinch in a torus with a very
strong longitudinal field. The helical magnetic field lines, resultant from
the externally applied field and that of the pinch, do not close, that is,
complete one revolution of the minor axis in going around the major axis of
the torus once. This is known theoretically to prevent the growth of certain
helical distortions of the plasma.
    The performance of tokamak experiments has raised the possibility of
achieving a net power balance; construction of several large tokamak
installations has been undertaken.  See  NUCLEAR FUSION.
 �34s[James A. Phillips]
�33s


A700000001237�254s�5v�1v1319
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18057216
�6v�2v�32sPine �21m �33s
    The genus Pinus, of the pine family, characterized by evergreen leaves,
usually in tight clusters (fascicles) of two to five, rarely single. There are
about 80 known species distributed throughout the Northern Hemisphere.
Botanically the leaves are of two kinds: (1) a scalelike form, the primary
leaf, which subtends a much shortened and eventually deciduous shoot bearing
(2) the secondary leaves or needles. The wood of pines is easily recognized by
the numerous resin ducts and by the characteristic resinous odor.  See
PINALES; PINE NUT; PINE OIL.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

A700000004713�254s�5v�1v1320
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18235392
�6v�2v�32sPineal body   �21m �33s 
    The pineal body, or epiphysis as it is sometimes named, is an outgrowth 
from the roof of the diencephalon of the brain. It is widespread throughout 
the vertebrate animal kingdom. In lampreys and some lower bony fish it serves 
as a third eye. In other vertebrates it serves as an endocrine organ that is 
regulated by light entering the body via the paired eyes. Its role in humans 
remains to be clarified. 
    The pineal organ is an unpaired, elongate, club-shaped, knoblike, or 
occasionally threadlike organ attached by a stalk to the roof of the 
forebrain. When it serves as a light receptor, it projects upward through the 
skull to lie under the skin; in other vertebrates it is buried beneath the 
roof of the skull. It is small in birds and some mammals, and relatively large 
in cocks, echidnas, marsupials, rodents, ungulates, and humans. Nervous 
elements predominate in lower vertebrates, whereas glandular cells predominate 
in higher ones. 
 �34s[George C. Kennedy]
�33s 
    One of the most important contributions to the understanding of pineal 
physiology was the demonstration of an indole compound in the gland. This 
active principle is known as melatonin. In addition to melatonin, the pineal 
gland has been shown to contain physiologically active substances such as 
histamine, serotonin, and catecholamines in fairly high concentrations. In 
addition, some enzymes involved in the formation and metabolism of these 
animals are also present in this organ. The melatonin enzyme was found to be 
present in the pineal gland of many animal species, including the cat, rat, 
cow, monkey, bird, and human. Studies of indolalkylamine metabolism indicate 
that the pineal may not function continuously and that its function may vary 
in a circadian fashion. See CIRCADIAN RHYTHMS. 
    The physiological significance of these amines and indoles and their 
cyclic variations is not precisely known. Much attention has been given to the 
possible extrapineal function of melatonin because it appears to alter the 
state of the reproductive systems, the estrous cycle, and the brain and causes 
a change in the pigment cells in the integument of amphibians. Furthermore, 
removal of the gland produces changes in the reproductive system which are in 
contrast to those resulting from melatonin treatment in addition to affecting 
the pituitary, adrenal, and thyroid glands. 
 �34s[Harry J. Clausen]
�33s 
  
A700000002212�254s�5v�1v1320
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0
�6v�2v�32sPineapple   �21m �33s 
    A low-growing perennial plant, indigenous to the Americas. The cultivated 
varieties (cultivars) belong to the species Ananas sativus of the plant order 
Bromeliales.  
    The edible portion of the pineapple develops from a mass of ovaries on a 
fleshy flower stock having persistent bracts (see illustration). On the 
cultivated types, the flowers are usually abortive. The leaves are long and 
swordlike and usually rough-edged. Commercial plantings bear fruit at the age 
of 12 - 20 months, and may continue to be productive for as much as 8 - 10 
years.  See  BROMELIALES. 
    The major producing area is Hawaii, where special methods of culture and 
harvesting have been developed. Pineapples are also grown in the West Indies 
and other tropical areas, and to a limited extent in southern Florida. 
    Pineapples are consumed fresh in considerable quantity, but because of 
distance from markets and the problems of transporting fresh fruit, most of 
the crop is canned as sliced pineapple or as juice.  See  FRUIT. 
 �34s[Joseph H. Camin]
�33s 
  
  
A700000002002�254s�5v�1v1320
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�6v�2v�32sPinene   �21m �33s 
    One of the most important terpenes. ALPHA-Pinene (I) is the chief 
constituent of turpentine and is a component of many essential oils. 
BETA-Pinene (II) often accompanies it and is usually present in smaller 
quantities. Both exist as dextro, levo, and racemic forms of optical isomers. 
Physical properties are listed in the table.  See  TERPENE. 
    Both pinenes are colorless oils which resinify on exposure to air. They 
undergo polymerization in the presence of anhydrous aluminum chloride or boron 
trifluoride. The polymers are used in the formulation of rubbers, paints, and 
varnishes.  
    The chemistry of the pinenes is intricate and complicated by their easy 
transformation into compounds with different ring structures. These molecular 
rearrangements permit the pinenes to be used as a convenient starting point 
for the synthesis of other important chemicals.  See  TURPENTINE. 
 �34s[Frank Wagner]
�33s 
  
A700000001263�254s�5v�1v1319
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�6v�2v�32sPine nut   �21m �33s 
    The edible seed of more than a dozen species of evergreen cone-bearing 
trees in the genus Pinus, native to the temperate zone of the Northern 
Hemisphere. The important nut-producing species are the stone pine (P. pinea) 
of southern Europe; the Swiss stone pine (P. cembra), native to the Swiss Alps 
and eastward through Siberia to Mongolia; and the pinon pine (P. cembroides 
var. edulis) of the arid regions of the southwestern United States. The seeds 
or nuts, variable in size according to species, are borne in cones which take 
3 - 4 years to develop.  See  PINE. 
 �34s[Laurence H. MacDaniels]
�33s 
  
A700000002011�254s�5v�1v1319
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18057216
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\IMG\CI1201.IMG
18235392
�6v�2v�32sPine oil   �21m �33s 
    A material fractioned from oils recovered from longleaf pine wood. The 
pine is either destructively distilled or solvent-extracted to yield 
turpentine, rosin, charcoal, and other useful commercial products. The 
recovered oils are further refined, and pine oil results from a definite cut 
in the fractionation. Good grades of pine oil consist largely of terpineol.  
See  WOOD CHEMICALS. 
    The textile industry uses pine oil as a penetrant, dispersing agent, 
wetting agent, and inhibitor of bacterial growth in practically all wet 
processing of cotton, silk, rayon, and woolen goods. In the paper industry it 
is used as a wetting and leveling agent in coating and as a preservative of 
casein. It also finds extensive use for the preparation of high-grade 
disinfectants, liniments, odorant blocks, dog soaps, cattle sprays, and other 
insecticidal preparations.  See  ESSENTIAL OILS. 
 �34s[William Mosher]
�33s 
  
A700000001135�254s�5v�1v1321
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�6v�2v�32sPinicae   �21m �33s 
    The largest and most important subdivision of the division Pinophyta 
(Gymnospermae), the other two subdivisions being the Cycadicae and Gneticae. 
The Pinicae are all woody plants, most of them trees with a central trunk and 
excurrent branches. They have simple, alternate or less often opposite or 
whorled leaves which are mostly rather small, often needlelike or scalelike. 
The wood lacks vessels and usually has resin canals.  See  CYCADICAE; 
GINKGOALES; GNETICAE; PINALES; PINOPHYTA; TAXALES. 
 �34s[Arthur Cronquist]
�33s 
  
A700000000703�254s�5v�1v1321
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�6v�2v�32sPinkeye   �21m �33s 
    An infection of the inner aspect of the eyelids caused by the Koch-Weeks 
bacillus. It is also known as epidemic conjunctivitis. The organism is 
regarded as identical with the nontypable form of Hemophilus influenzae. 
Pinkeye is widespread, occurs in epidemic outbreaks, and is said to be 
especially prevalent in Egypt.  See  CONJUNCTIVITIS. 
 �34s[William L. Bradford]
�33s 
  
A700000003241�254s�5v�1v1321
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0
�6v�2v�32sPinophyta   �21m �33s 
    A division of seed plants consisting of some 600 or 700 species, widely 
distributed on all continents, the most common and familiar ones being the 
cone-bearing evergreen trees of the order Pinales. Because the ovules (young 
seeds) are exposed directly to the air at the time of pollination, the 
Pinophyta are commonly called gymnosperms, in contrast to the angiosperms 
(division Magnoliophyta), which have the ovules enclosed in an ovary. The 
division Pinophyta consists of three subdivisions, the Pinicae, the Cycadicae, 
and the Gneticae. 
    The gymnosperms may be characterized as vascular plants with roots, stems, 
and leaves, and with seeds that are not enclosed in an ovary. The seeds are 
borne on cone scales in simple or compound cones, or exposed at the end of a 
stalk.  
    The gymnosperms evidently originated from Paleozoic vascular cryptogams at 
a time when the ferns (division Polypodiophyta) had not become fully 
differentiated from the ancestral rhyniophytes. Which of these two groups 
should be considered ancestral to the gymnosperms is a matter of convenience 
and arbitrary definition. The gymnosperms proliferated during the 
Carboniferous Period and became the dominant land plants by the beginning of 
the Mesozoic Era. During the Cretaceous Period they were progressively 
replaced by the angiosperms, and they now have the appearnace of a relict 
group. Only the order Pinales now has fairly numerous and diversified species 
represented by many individuals over large areas.  See CYCADICAE; GNETICAE; 
MAGNOLIOPHYTA; PINALES; PINICAE; POLYPODIOPHYTA. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001330�254s�5v�1v1321
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�6v�2v�32sPinopsida   �21m �33s 
    One of the two classes (the other being the Ginkgoopsida) which 
collectively make up the subdivision Pinicae of the division Pinophyta 
(Gymnospermae). There are two living orders, the Pinales and Taxales, plus the 
fossil order Cordaitales. The Pinopsida are characterized by entire-margined 
or slightly toothed leaves that are usually narrow and never fan-shaped, in 
contrast to the fan-shaped, often bilobed leaves of the Ginkgoopsida. All 
modern species of Pinopsida have naked sperms, in contrast to the flagellated, 
swimming sperms of the Ginkgoopsida.  See  CORDAITALES; GINKGOALES; PINALES; 
PINICAE; TAXALES. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001315�254s�5v�1v1321
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�6v�2v�32sPinta   �21m �33s 
    One of the treponematoses, which include yaws and syphilis, occurring 
principally in certain rural tropical areas of Central and South America: The 
infectious agent is a spirochete, Treponema carateum. The early lesions and 
the general evolution of pinta are similar to, but generally milder than, 
those of syphilis and yaws. Generalized lesions develop a reddish-purplish hue 
which is followed by atrophy and spotty achromia of the skin. Treponemes are 
present in the skin lesions. Serious late complications seem not to occur. 
Transmission is presumably by direct contact.  See  SPIROCHETE; SYPHILIS; YAWS. 
 �34s[Thomas B. Turner]
�33s 
  
A700000001360�254s�5v�1v1321
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�6v�2v�32sPinworm infection   �21m �33s 
    An infection, also known as enterobiasis, of the human intestinal tract by 
the pinworm (Enterobius vermicularis). Both male and female nematodes inhabit 
the large intestine but the vagina may also be invaded. The female produces 
several thousand eggs which accumulate in its uteri. Pregnant worms, 2 - 4 
weeks old, are expelled in the feces, or crawl out of the anus to lay the eggs 
and die. Their movements cause itching, and nervous disorders may ensue. The 
disease is diagnosed by observing the worm upon extrusion from the anus and by 
microscopical detection of eggs in anal swabs.  See  MEDICAL PARASITOLOGY; 
STRONGYLOIDEA.  
 �34s[Jose F. Maldonado]
�33s 
  
A700000001422�254s�5v�1v1321
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�6v�2v�32sPionium   �21m �33s 
    An exotic atom, also called the pi-mu atom, which is similar in structure 
to the hydrogen atom but with the proton replaced by a pion and the electron 
replaced by a muon. Pionium is unique among atoms that have been observed in 
the laboratory in that all of its constitutents are unstable particles not 
found in ordinary matter. Pionium is formed during the decay of a certain 
heavier particle called the neutral kaon. A kaon has many modes of decay, one 
of which results in the formation of a pion, a muon, and a neutrino.  See  
ELEMENTARY PARTICLE. 
    The nomenclature of exotic atoms is not well established, and the name 
pionium may also refer to the pion-electron atom. 
 �34s[Paul A. Souder]
�33s 
  
A700000003110�254s�5v�1v1321
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0
1322
\IMG\CI1321.IMG
118784
�6v�2v�32sPipeline   �21m �33s 
    A line of piping and the associated pumps, valves, and equipment necessary 
for the transportation of a fluid. Major uses of pipelines are for the 
transportation of petroleum, water (including sewage), chemicals, foodstuffs, 
pulverized coal, and gases such as natural gas, steam, and compressed air. 
Pipelines must be leakproof and must permit the application of whatever 
pressure is required to force conveyed substances through the lines. Pipe is 
made of a variety of materials and in diameters from a fraction of an inch up 
to 30 ft (9 m). Principal materials are steel, wrought and cast iron, 
concrete, clay products, aluminum, copper, brass, cement and asbestos (called 
cement-asbestos), plastics, and wood. 
    Pipe is described as pressure and nonpressure pipe. In many pressure 
lines, such as long oil and gas lines, pumps force substances through the 
pipelines at required velocities. Pressure may be developed also by gravity 
head, as for example in city water mains fed from elevated tanks or reservoirs. 
    Nonpressure pipe is used for gravity flow where the gradient is nominal 
and without major irregularities, as in sewer lines, culverts, and certain 
types of irrigation distribution systems. 
    Design of pipelines considers such factors as required capacity, internal 
and external pressures, water- or airtightness, expansion characteristics of 
the pipe material, chemical activity of the liquid or gas being conveyed, and 
corrosion.  
 �34s[Leslie N. McClellan]
�33s 
  
A700000001335�254s�5v�1v1322
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118784
�6v�2v�32sPiperales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae) in the 
subclass Magnoliidae of the class Magnoliopsida (dicotyledons). The order 
includes only 3 families and about 1500 species, more than 1400 of them 
belonging to the family Piperaceae. Within its subclass the order is marked by 
ethereal oil cells (like those of the Magnoliales), a usually more or less 
herbaceous habit, and especially by reduced, crowded flowers with orthotropous 
ovules (that is, the ovules are straight, with the micropyle and funicules at 
opposite ends).  See  BETEL NUT; BLACK PEPPER; CUBEB; MAGNOLIIDAE; 
MAGNOLIOPSIDA.  
 �34s[Arthur Cronquist]
�33s 
  
A700000001763�254s�5v�1v1322
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118784
�6v�2v�32sPipet   �21m �33s 
    A tube, usually made of glass or plastic, used almost exclusively to 
deliver accurately known volumes of liquids or solutions. There are two 
general categories of pipets: volumetric or transfer pipets and the graduated 
measuring type (see illustration). With volumetric pipets the liquid is sucked 
up above the mark on the stem above the bulb and the upper end is quickly 
closed with the index finger; the level of liquid in the stem is allowed to 
fall slowly, by regulating the pressure on the finger, until the bottom of the 
meniscus is tangent to the mark. 
    A graduated measuring pipet is used in the same general way except that 
the volume of liquid delivered can be varied by allowing the liquid to drain 
from one calibration mark to another. These pipets are usually not as accurate 
in delivering volumes of liquid as are the volumetric type.  See  TITRATION; 
VOLUMETRIC ANALYSIS. 
 �34s[Clark E. Bricker]
�33s 
  
  
A700000001615�254s�5v�1v1322
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�6v�2v�32sPirani gage �21m �33s
    A type of instrument used to measure vacuum by utilizing a resistance
change due to a temperature change in a filament. This fine-wire filament, one
of the four electrical resistances forming a Wheatstone bridge circuit, is
exposed to the vacuum to be measured. Electric current heats the wire; the
surrounding gas (in the vacuum) conducts heat away from the wire. At a stable
vacuum, the wire quickly reaches equilibrium temperature. If the pressure
rises, the gas carries away more heat, and the temperature of the wire
decreases. Since the resistance of the filament is a function of temperature,
the electrical balance of the Wheatstone bridge is changed. Pressure
measurement range of this type of gage is usually 1 torr to 10-(sup 4) torr
(10(sup 2) to 10-(sup 2) pascals).  See  VACUUM MEASUREMENT.
 �34s[Richard Comeau]
�33s

A700000001007�254s�5v�1v1322
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�6v�2v�32sPisces (constellation)   �21m �33s 
    The Fishes, in astronomy, a zodiacal constellation appearing in the autumn 
evening sky. Pisces is the twelfth and last sign of the zodiac. It is 
inconspicuous, having no star brighter than the fourth magnitude. But it is an 
important constellation because the vernal equinox, which marks the beginning 
of the astronomical year, is now located in it (see illustration).  See  
CONSTELLATION.  
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sPisces (zoology)   �21m �33s 
    A term that embraces all fishes and fishlike vertebrates. The Pisces 
include four well-defined groups that merit recognition as classes: the 
Agnatha or jawless fishes, the most primitive; the Placodermi or armored 
fishes, known only as Paleozoic fossils; the Chondrichthyes or cartilaginous 
fishes; and the Osteichthyes or bony fishes.  See  AGNATHA; CHONDRICHTHYES; 
OSTEICHTHYES; PLACODERMI. 
 �34s[Reeve M. Bailey]
�33s 
  
A700000001477�254s�5v�1v1322
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�6v�2v�32sPistachio   �21m �33s 
    A small spreading evergreen tree, Pistacia vera, native to the dry parts 
of Asia and Asia Minor and now grown in the Mediterranean region, Turkey, 
Iran, and Afghanistan as a nut crop. The green or reddish oval fruits are 
borne in clusters along the smaller branches. Classified botanically as a 
drupe, the fruit consists of an outer fleshy husk within which is a thin tough 
shell enclosing the usually green kernel which is covered with brown seed 
coats. The husk splits at maturity and is separated easily from the shell. 
    Pistachio nuts are used in the baking, ice cream and confectionary trades, 
and are an important food in central Asia, where much of the crop is used 
locally.  See  SAPINDALES. 
 �34s[Laurence H. MacDaniels]
�33s 
  
  
A700000002505�254s�5v�1v1322
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\IMG\CI1321.IMG
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�6v�2v�32sPitch   �21m �33s 
    That psychological property of sound characterized by highness or lowness. 
Pitch varies most directly with the frequency of sound waves (see 
illustration), and pitch discriminations can be made throughout the frequency 
range of normal hearing, from about 16 to 20,000 hertz (Hz). 
    A numerical scale of pitch has been constructed by the method of 
fractionation, selecting pitches that appear to be half as high as reference 
tones, and the method of bisection, selecting pitches that appear to fall 
halfway between two reference tones. The pitch unit was named the mel, and a 
value of 1000 mels was arbitrarily assigned to a tone of 1000 Hz. 
    The critical features of pitch perception are the detection and analysis 
of specific tonal values within a broad spectrum of frequency change. The 
ability to focus attention on specific tone patterns within larger patterns of 
sound is an outcome of several levels of receptor control including: neural 
feedback from the primary afferent centers of the brain; neuromotor control of 
frequency input by the middle ear muscles; and possibly dynamic tuning of the 
basilar membrane through middle ear tension on the spiral ligament.  See  EAR; 
HEARING (HUMAN). 
 �34s[Karl U. Smith]
�33s 
  
A700000001157�254s�5v�1v1323
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�6v�2v�32sPitcher plant �21m �33s
    Any member of the families Sarraceniaceae and Nepenthaceae. In these
insectivorous plants the leaves form deep cups or pitchers in which water
collects. Visiting insects, falling into this water, are drowned and digested
by the action of enzymes secreted by cells located in the walls of the
pitcherlike structures of these plants. Often these plants climb by tendrils.
The end of a tendril may develop into a pitcher, which captures and digests
insects.  See  INSECTIVOROUS PLANTS; SARRACENIALES
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s


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�6v�2v�32sPitchstone   �21m �33s 
    A natural glass with dull or pitchy luster and generally brown, green or 
gray color. It is extremely rich in microscopic, embryonic crystal growths 
(crystallites) which may cause its dull appearance. The water content of 
pitchstone is high and generally ranges from 4 to 10% by weight. Pitchstone is 
formed by rapid cooling of molten rock material (lava or magma) and occurs 
most commonly as small dikes or as marginal portions of larger dikes.  See  
IGNEOUS ROCKS; VOLCANIC GLASS. 
 �34s[Carleton A. Chapman]
�33s 
  
A700000001357�254s�5v�1v1323
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�6v�2v�32sPith   �21m �33s 
    The central zone of tissue of an axis in which the vascular tissue is 
arranged as a hollow cylinder. Pith is present in most stems and in some 
roots. Stems without pith rarely occur in angiosperms but are characteristic 
of psilopsids, lycopsids, Sphenophyllum, and some ferns. Roots of some ferns, 
many monocotyledons, and some dicotyledons include a pith although most roots 
have xylem tissue in the center. 
    Pith is composed usually of parenchyma cells often arranged in 
longitudinal files. This arrangement results from predominantly transverse 
division of pith mother cells near the apical meristem.  See  PARENCHYMA; 
ROOT; STEM. 
 �34s[H. Weston Blaser]
�33s 
  
A700000002404�254s�5v�1v1323
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310272
�6v�2v�32sPitot tube �21m �33s
    An instrument that measures the stagnation pressure of a flowing fluid;
also called an impact tube. Stagnation (also called impact or total) pressure
is the pressure that would be obtained if the fluid were brought to rest
isentropically. When total pressure is being measured, the system consists of
a primary sensing element mounted on a suitable support, pressure connecting
lines, and a pressure-indicating device. Normally, the connecting lines and
indicating devices are considered secondary elements and are not treated as
part of the pitot tube.  See  MANOMETER.
    The pitot tube is used primarily to obtain fluid velocity, total energy as
measured being composed of impact or stagnation pressure P2, static pressure
P1, and velocity v1 of the fluid of density RHO. Then for incompressible
(low-speed) flow, the equation shown below holds. For incompressible flow, the

	   [P(sub 2)/rho] = [P(sub 1)/rho] + [v(sub 1)(sup 2)/2]

total energy is expressed by Bernoulli's theorem. For compressible flow, total
energy can be expressed in terms of impact pressure, static pressure, and Mach
number, which is related to velocity.  See  BERNOULLI'S THEOREM; FLOW
MEASUREMENT.
 �34s[Lloyd N. Krause]
�33s


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�6v�2v�32sPituitary gland   �21m �33s 
    The most important single endocrine organ, because its secretions regulate 
many essential metabolic processes. The gland, also known as the hypophysis, 
is present in all vertebrates and is initimately related to the hypothalamus, 
a portion of the brain from which its main innervation is received. The human 
gland is lodged in the sella turcica, a deep depression in the sphenoid bone 
(see illustration). Although considerable anatomical variation is encountered 
among the vertebrate groups, the gland is roughly divisible into anterior, 
intermediate, and posterior lobes. The anterior and intermediate lobes 
collectively are referred to as the adenohypophysis. The posterior lobe, the 
pituitary stalk, and the median eminence of the floor of the brain are 
referred to collectively as the neurohypophysis. 
 �34s[George C. Kent, Jr.]
�33s 
  
    The pituitary gland secretes several distinct hormones which may be 
grouped into three general classifications according to their composition: (1) 
hormones consisting of a large number of amino acids linked together, called 
protein hormones; (2) hormones consisting of a large number of amino acids 
linked together and associated with various types of sugar molecules, called 
glycoprotein hormones; and (3) hormones consisting of only a relatively few 
amino acids linked together, called peptide hormones. 
    The protein hormones secreted by the pituitary gland (growth hormone and 
prolactin) are all elaborated from the anterior lobe, called the 
adenohypophysis. The pituitary glycoprotein hormones - thyroid-stimulating 
hormone, follicle-stimulating hormones, and luteinizing-hormone - are also 
secreted by the anterior lobe. Peptide hormones are secreted by the anterior 
lobe, as adrenocorticotrophin (ACTH) and by the posterior lobe, as oxytocin 
and vasopressin (or the antidiuretic hormone, ADH). Another peptide hormone, 
the melanocyte-stimulating hormone (MSH), is secreted by the pars intermedia 
of the hypophysis.  See  ADENOHYPOPHYSIS HORMONE; NEURO-HYPOPHYSIS HORMONE. 
    Among the effects of removal of the pituitary gland (hypophysectomy), if 
performed on an immature individual, are: (1) a severe retardation of somatal 
(body) growth; (2) failure of sex organs to achieve mature development, with a 
consequential disruption of normal reproductive function in the adult; (3) 
atrophy of the adrenal cortex, with a concomitant supression of the many 
biological processes dependent on that gland's steroid hormone production; and 
(4) emaciation of the thyroid gland, with accompanying depression of the 
body's metabolic rate, a bleaching effect on the skin (in some species), and a 
general adverse effect on body chemistry, leading to a variety of metabolic 
disorders. These phenomena are all attributable to deficiencies of anterior 
pituitary (adenohypophysial) hormones. Among the biological alterations 
associated with loss of posterior lobe function, the most serious is diabetes 
insipidus, or excessive loss of water through the kidney (diuresis). The 
hormones found in the posterior pituitary (neurohypophysis) are believed to be 
synthesized in the hypothalamus, and then transported to the posterior lobe 
through association with a carrier protein. Thus, it is probable that even 
excising the posterior lobe would not completely abolish the elaboration of 
the posterior lobe hormones. 
 �34s[Leo E. Reichert, Jr.]
�33s 
  
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�6v�2v�32sPituitary gland disorders �21m �33s
    Disorders resulting from deranged secretion of hormones by the pituitary
gland cells.

    Anterior lobe malfunction.	Growth hormone, unlike other anterior
pituitary hormones, does not have a specific target organ but affects the
tissues and organs of the body in general. Hypersecretion of growth hormone
usually results from a tumorous overgrowth of the cells. The hyperfunction is
usually manifest by increased bone growth. When this arises before puberty, it
results in abnormal tallness, or gigantism. When the hyperfunction has its
onset after the age of 20, the bone growth is manifest by a general increase
in the thickness of bones, a condition known as acromegaly.
    Cushing's disease is a clinical disorder of anterior pituitary cells
associated with enlargement of the cortex of the adrenal gland. This disease
is characterized by obesity of the trunk of the body, high blood pressure,
fatigability, weakness, hirsutism, edema, thinning of the bones, and in women,
amenorrhea.  See  ADRENAL GLAND.
    Simmond's disease is produced by extreme hypofunction of the anterior lobe,
usually as a result of a vascular or tumorous destructive proces. The vascular
changes are due to infarction of the gland which can be associated with
intrapartum or postpartum hemorrhage. The neoplasms that can cause Simmond's
disease may be primary in the pituitary gland or metastatic from other sites in
the body. With this disease there is a marked weight loss, weakness, and signs
of hypothyroidism and hypoadrenalism.

    Intermediate region. The intermediate region of the pituitary gland is the
probable site of production of the melanocyte-stimulating hormone. This hormone
is associated with the increase of pigmentation of the skin.

    Posterior lobe malfunction. Diabetes insipidus is produced by the
hypofunction of the posterior lobe of the pituitary gland. This desease is
manifested by increased thirst and increased urine production. Deficiency of the
antidiuretic hormone causes the tubules of the kidneys to lose their capacity to
concentrate urine. The basic lesion is often in the posterior lobe of the
pituitary or in the hypothalamus. Although oxytocin is highly important in the
reproductive process, the identification of the specific syndromes or disorders
associated with excess or insufficient oxytocin have not been described. See
DIABETES; LACTATION; PITUITARY GLAND.			   [N. Karle Mottet]

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�6v�2v�32spK �21m �33s
    The logarithm (to the base 10) of the reciprocal of the equilibrium
constant for a specified reaction under specified conditions (for example,
solvent and temperature). pK values are often more convenient to tabulate and
use than the equilibrium constants themselves. The value of K for the
dissociation of the HSO4- ion in aqueous solution at 25DGC (77DGF) is 0.0102
mole/liter. The logarithm is 0.008(sub 6) - 2 = -1.991(sub 4). pK is therefore
+1.991(sub 4). The choice of algebraic sign, although arbitrary, results in
positive values for most dissociation constants applicable to aqueous solutions.
The concept of pK is especially valuable in the study of solutions. See CHEMICAL
EQUILIBRIUM; IONIC EQUILIBRIUM; PH.
 �34s[Thomas F. Young]
�33s

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�6v�2v�32sPlacentation �21m �33s
    The intimate association or fusion of a tissue or organ of the embryonic
stage of an animal to its parent for physiological exchange to promote the
growth and development of the young. It enables the young, retained within the
body or tissues of the mother, to respire, acquire nourishment, and eliminate
wastes by bringing the bloodstreams of mother and young into close association
but never in direct connection (see illustration). Placentation characterizes
the early development of all mammals except the egg-laying duckbill platypus
and spiny anteater. It occurs in some species of all other orders of
vertebrates except the birds. In fact, in certain sharks and reptiles it is
almost as well developed as in mammals. A few examples are also known among
invertebrates (Peripatus, certain tunicates, and insects). See FETAL MEMBRANE.
 �34s[Harland W. Mossman]
�33s


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�6v�2v�32sPlacer mining �21m �33s
    The working of deposits of sand, gravel, and other alluvium and eluvium
containing concentrations of metals or minerals of economic importance. For
many years gold has been the most important product obtained, although
considerable platinum, cassiterite (tin mineral), phosphate, monazite,
columbite, ilmenite, zircon, diamond, sapphire, and other gems have been
produced. Other valuables recovered include native bismuth, native copper,
native silver, cinnabar, and other heavy weather-resistant metals or minerals.
A form of sluice box is the type of unit most often used to separate and
recover the valuable metals or minerals.
    A number of mining methods exist, but all have elements of preparing the
ground, excavating the pay material, separating and recovering the valuable
products, and stacking the tailings. A general classification of present
placer mining methods is small-scale hand methods, hydraulicking, mechanical
methods, dredging, and drift mining.
    Small-scale hand methods include the use of pans, rockers, long toms,
shoveling into boxes, ground sluicing, booming, and the use of dip boxes,
puddling boxes, dry washers, surf washers, and small-scale mechanical placer
machines.
    Hydraulic mining, used if water supplies are adequate and streams not
objectionably polluted, utilizes water under pressure, forced through nozzles,
to break and transport the placer gravel to the recovery plant (often sluice
box), where it is washed. The valuable material is separated and retained in
the sluice, and the tailings pass through and are stacked by water under
pressure.
    Mechanical methods include the use of equipment such as bulldozers,
draglines, front-end loaders, pumps, pipelines, hydraulic elevators, and
recovery units which are used in a number of different combinations depending
on the physical characteristics of placer deposits.
    Large flat-lying areas are best suited for dredging because huge volumes
of gravel can be handled fairly cheaply. Prior to dredging a deposit, the
ground must be prepared. This preparation may consist of removing trees and
other vegetation and overburden such as the frozen loess (muck) found in many
parts of Alaska by use of equipment and water under pressure (stripping).
    Because of relatively high costs, drift mining of placer deposits is not
used to a large extent. It consists of sinking a shaft to bedrock, driving
drifts up- and downstream in the valley deposits for 250-300 ft (75-90 m),
and then extending crosscuts the width of the pay streak.
 �34s[Earl H. Beistline]
�33s

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�6v�2v�32sPlacodermi   �21m �33s 
    A class of fishes (Pisces) known only from the Devonian Period, except for 
a few that survived into the base of the Carboniferous. They were true fishes 
or gnathostomes and can be distinguished from other fishes by the following 
characters: the gill chamber extends far under the cranium and is covered 
laterally by opercula; there is a neck joint between the cranium and the fused 
anterior vertebrae; the head and shoulder girdle are more or less covered with 
dermal bones composed typically of cellular bone and superficially of 
semidentine instead of true dentine; the bones are commonly ornamented with 
tubercles or ridges; the endoskeleton is cartilage and is sometimes calcified 
in a globular fashion or perichondrally ossified; the notochord is persistent, 
and the vertebrae consist only of neural and hemal arches; the tail is 
diphycercal or slightly heterocercal, and an anal fin is lacking. 
    Most placoderms were bottom-dwelling fishes, with the head and trunk 
dorsoventrally depressed; only one group had laterally compressed and deepened 
bodies, suggesting a more nectic manner of life. Most of them were small or 
moderate-sized, but a few were, for their time, gigantic, reaching a length of 
as much as 20 ft (6 m). They were the dominant fishes of the Devonian, and are 
found in both marine and fresh-water deposits. 
    The Placodermi are classified in nine orders, each with its own distinct 
specializations. The evolution and interrelations of these orders are matters 
of disagreement.  See  ACANTHOTHORACI; ANTIARCHA; ARTHRODIRA; PETALICHTHYIDA; 
PHYLLOLEPIDA; PSEUDOPETALICHTHYIDA; PTYCTODONTIDA; RHENANIDA; STENSIOELLIDA. 
 �34s[Robert H. Denison]
�33s 
  
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�6v�2v�32sPlacodontia   �21m �33s 
    A small but interesting order of marine reptiles of the subclass 
Euryapsida that are known only from deposits of Triassic age of Europe and the 
Near East (Israel). As the name implies, they are reptiles with grossly 
specialized dentitions - flat-crowned teeth are located in both the upper and 
lower jaws and on the palate - that probably functioned as crushing devices 
for hard-shelled prey. 
    The modification of the dentition had its effect on the entire skull, 
which became massive in relation to the body; and the coronoid region of the 
mandible, to which most of the masticatory muscles were attached, became 
greatly extended upward, giving it a superficial similarity to a mammalian 
jaw. The postcranial skeleton shows a number of aquatic adaptations. The 
thorax was box-shaped and the digits of hand and foot were probably webbed; 
the joint surfaces of the limb bones suggest that the animals did little if 
any walking on land. The advanced placodonts resemble sea turtles in overall 
appearance and in the fact that their body is encased in an armor of dermal 
bones similar to that of the dermochelyid sea turtle Psephophorus. The more 
generalized genera lack such an armor and possess a long tail.  See  
EURYAPSIDA; REPTILIA. 
 �34s[Rainer Zangerl]
�33s 
  
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�6v�2v�32sPlague   �21m �33s 
    An infectious disease of rodents and humans, occuring in bubonic or 
pneumonic form and caused by the bacillus Pasteurella pestis. This deadly 
malady has been notorious since the 3d century for its pandemics, most notably 
in 1332 - 1371 (the Black Death), 1630 (Milan), 1665 (London), and 1720 - 1721 
(Marseilles and Provence). Domestic plague is transmitted from rat to rat and 
from rat to human by the efficient vector flea Xenopsylla cheopis. However, 
wild rodents serve as true reservoirs of plague and are frequently directly 
implicated as a source of human infection (sylvatic plague). 
    In warm climates plague is usually bubonic, so called because of the 
characteristic swollen lymph nodes or buboes. Bubonic plague is usually 
transmitted to humans by rat fleas. In cold climates it is more likely to take 
the much more fatal and contagious pneumonic, or tonsillar, form. Pneumonic 
plague spreads from patients with primary pneumonic plague or from patients 
with bubonic plague and secondary pneumonic infection. 
    Human infection is rare in the United States and arises from exposure to 
fleas of wild rodents. Plague was often a serious health problem in cities in 
India, Burma, and Indonesia, although it is presently in decline. It persists 
there, however, in the wild rodents. It is fairly common in Vietnam. 
 �34s[Karl F. Meyer]
�33s 
  
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�6v�2v�32sPlains   �21m �33s 
    The relatively smooth sections of the continental surfaces, occupied 
largely by gentle rather than steep slopes and exhibiting only small local 
differences in elevation. Because of their smoothness, plains lands, if other 
conditions are favorable, are especially amenable to many human activities. 
Thus it is not surprising that the majority of the world's principal 
agricultural regions, close-meshed transportation networks, and concentrations 
of population are found on plains. Large parts of the Earth's plains, however, 
are hindered for human use by dryness, shortness of frost-free season, 
infertile soils, or poor drainage. Because of the absence of major differences 
in elevation or exposure or of obstacles to the free movement of air masses, 
extensive plains usually exhibit broad uniformity or gradual transition of 
climatic characteristics. 
    Somewhat more than one-third of the Earth's land area is occupied by 
plains. With the exception of ice-sheathed Antarctica, each continent contains 
at least one major expanse of smooth land in addition to numerous smaller 
areas. The largest plains of North America, South America, and Eurasia lie in 
the continental interiors, with broad extensions reaching to the Atlantic (and 
Arctic) Coast. The most extensive plains of Africa occupy much of the Sahara 
and reach south into the Congo and Kalahari basins. Much of Australia is 
smooth, with only the eastern margin lacking extensive plains.  See  TERRAIN 
AREAS.  
    Surfaces that approach true flatness, while not rare, constitute a minor 
portion of the world's plains. Most commonly they occur along low-lying 
coastal margins, the lower sections of major river systems, or the floors of 
inland basins. Nearly all are the products of extensive deposition by streams 
or in lakes or shallow seas. The majority of plains, however, are distinctly 
irregular in surface form, as a result of valley-cutting by streams or of 
irregular erosion and deposition by continental glaciers. 
 �34s[Edwin H. Hammond]
�33s 
  
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�6v�2v�32sPlanaria   �21m �33s 
    A flatworm of the turbellarian order Tricladida. The majority of species 
are free-living, fresh-water forms. One commonly studied species is Dugesia 
tigrina, formerly known as Planaria maculata, a typical example of the order. 
    Dugesia reaches a length of 0.6 - 0.7 in. (15 - 18 mm). The animal is 
broad and blunt anteriorly with lateral projections (see illustration), the 
auricles, which have a tactile function. A pair of eyespots, present on the 
anterior dorsal surface, function as photoreceptors. The ventral surface of 
Dugesia is ciliated. A muscular pharynx may be protruded from the 
posterioventrally located mouth. The genital pore is located posterior to the 
mouth. Circulatory and respiratory systems are lacking but the muscular system 
is well developed with circular, longitudinal, and oblique fibers. There is no 
body cavity or coelom, and consequently these animals are categorized as 
acoelomates. The digestive system is incomplete; that is, there is no anal 
opening, so the mouth serves both for ingestion of food and egestion of solid 
waste.  
    When cut in half transversely, planarians regenerate a tail on the 
posterior of the anterior portion and a head on the anterior part of the 
posterior part. If a transverse section is removed from the middle of the 
animal, the anterior of the section develops a head and the posterior, a tail. 
In addition, parts of one animal have been successfully grafted onto another 
animal.  See  REGENERATION (BIOLOGY). 
    One of the more unusual experimental observations on planaria was reported 
during behavioral studies in which ribonucleic acid (RNA), in the form of 
minced bodies of planarians that were trained to respond to certain stimuli, 
was fed to untrained planarians. The untrained planarians responded to that 
stimulus in a manner comparable to the response of the trained group.  See  
TRICLADIDA.  
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sPlanck's constant �21m �33s
    A fundamental physical constant which represents the elementary quantum of
action, action being defined as energy multiplied by time. Introduced by Max
Planck in 1900, it has the value h = 6.6261 x 10-(sup 27) erg-second or 6.6261
x 10-(sup 34) joule-second. The symbol PLANCK'S CONSTANT, sometimes called the
Dirac h, is often used for convenience in physics to denote the quantity
h/2PI, where PI = 3.1416. . . .
    As used by Planck in deriving his radiation law, h multiplied by the
frequency of radiation represented a bundle of energy, that is, a quantum of
energy. Radiant energy at any wavelength can occur only as multiples of this
energy; thus energy is quantized.  See	ATOMIC CONSTANTS; COMPTON EFFECT; HEAT
RADIATION; QUANTUM MECHANICS.
 �34s[Heinz G. Sell /Peter J. Walsh]
�33s

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�6v�2v�32sPlanck's radiation law   �21m �33s 
    A law of physics which gives the spectral energy distribution of the heat 
radiation emitted from a so-called blackbody at any temperature. Discovered by 
Max Planck, this law laid the foundation for the advent of the quantum theory 
because it was the first physical law to postulate that electromagnetic energy 
exists in discrete bundles, or quanta.  See  HEAT RADIATION; QUANTUM MECHANICS. 
 �34s[Heinz G. Sell ;Peter J. Walsh]
�33s 
  
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�6v�2v�32sPlane   �21m �33s 
    In the euclidean definition, a surface is ``that which has length and 
breadth only,'' and a plane surface is ``a surface which lies evenly with the 
straight lines on itself.'' A definition of a plane that avoids this somewhat 
loosely defined concept of a surface is as follows: ``A plane is a set of at 
least three points, not all collinear, such that if any two points A, B of the 
set are given, all points of the line AB are in the set, and such that if any 
four points A, B, C, D of the set are given, then at least one of the pairs of 
lines AB and CD, or AC and BD, or AD and BC have a common point.'' (This last 
restriction prevents a plane from occupying all of space.) Still another 
definition is this: ``A plane is the locus of points each equidistant from two 
points in space.'' Points or lines in the same plane are called coplanar. 
Three or more planes that have a line in common are called coaxial planes.  
See  ANALYTIC GEOMETRY; EUCLIDEAN GEOMETRY; LINE; POINT. 
 �34s[James S. Frame]
�33s 
  
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�6v�2v�32sPlaner   �21m �33s 
    A machine for the shaping of long, flat, or flat contoured surfaces by 
reciprocating the workpiece under a stationary single-point tool or tools. 
Usually the workpiece is too large to be handled on a shaper. 
    Planers are built in two general types, open-side or double-housing. The 
former is constructed with one upright or housing to support the crossrail and 
tools. The double-housing type has an upright on either side of the 
reciprocating table connected by an arch at the top.  See  MACHINING 
OPERATIONS; SHAPER; WOODWORKING. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sPlanet   �21m �33s 
    A relatively small, solid celestial body moving in orbit around a star, in 
particular the Sun. Besides Earth, the eight known planets of the solar system 
are Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto; in 
addition, over 2000 minor planets, or asteroids, mostly located between the 
orbits of Mars and Jupiter, are known (see illustration). 
    There are two basic groups of planets: the small, dense, terrestrial 
planets - Mercury, Venus, Earth, Mars, and Pluto - and the giant or Jovian 
planets - Jupiter, Saturn, Uranus, and Neptune. With the exception of Pluto, 
the terrestrial planets are all located within the inner solar system. The 
low-density Jovian planets extend outward from Jupiter to the remote outer 
reaches of the solar system. This distribution is not accidental, but is 
related to the fractionation of rocky, icy, and gaseous materials during the 
early stages of formation of the solar system. 
    The planets may also be divided into inferior planets, Mercury and Venus, 
located inside the Earth's orbit, and superior planets, from Mars to Pluto, 
circulating outside the Earth's orbit. 
    The motions of the planets in their orbits around the Sun are governed by 
Kepler's laws. Kepler's laws are true only when the mutual perturbations of 
the motions of the planets by the others are neglected. Table 1 gives the mean 
distance of the planets from the Sun and the sidereal period of revolution. 
    In the course of their motions around the Sun, Earth and other planets 
occupy a variety of relative positions or configurations. The inferior planets 
are in conjunction with the Sun when closest to the Earth-Sun direction, 
either between the Earth and the Sun (inferior conjunction) or beyond the Sun 
(superior conjunction). Between conjunctions, the geocentric angular distance 
from the planet to the Sun, or the elongation, varies up to a maximum value. 
The superior planets are not so limited, and their elongations can reach up to 
180DG when they are in opposition with the Sun. 
    The combinations of the orbital motions of Earth and of any other planet 
give rise to complicated apparent motions of the planets as seen from the 
Earth. Because the orbits of the main planets are, except for Pluto, only 
slightly inclined to the plane of the orbit of Earth, the apparent paths of 
the planets (except Pluto) are restricted to the zodiac, a belt 16DG wide 
centered on the ecliptic. The ecliptic is the path in the sky traced out by 
the Sun in its apparent annual journey as the Earth revolves around it. 
    The apparent motions with respect to the celestial sphere, that is, to the 
fixed stars, appear for the inferior planets as oscillations back and forth 
about the position of the Sun steadily moving eastward among the stars. For 
the superior planets, the apparent motion is generally eastward or direct, but 
for short periods near the time of opposition it is westward or retrograde. 
    The size, mass, density, and rotation period of each of the planets is 
given in Table 2. 
  
    Minute perturbations in the motion of some nearby stars have indicated the 
existence of minor components of small mass. The masses of these satellite 
bodies, although larger than planetary masses in the solar system, are 
considered to be too small to be self-luminous; they are consequently more 
like planets than dwarf stars. From this evidence it is inferred that 
planetary systems are not as uncommon in the universe as was previously 
believed, and studies of multiple-star systems suggest that one star in every 
three may be attended by planetary companions.  See  ASTEROID; JUPITER; MARS; 
MERCURY (PLANET); NEPTUNE; PLANETARY PHYSICS; PLUTO; SATURN; SOLAR SYSTEM; 
URANUS; VENUS. 
 �34s[J. Kelly Beatty]
�33s 
  
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�6v�2v�32sPlane table   �21m �33s 
    A tripod-mounted drawing board on which topographic survey details are 
compiled in the field. The table and affixed manuscript sheet are set up and 
oriented over a ground point represented on the sheet. Stadia sightings with 
an alidade locate additional points to be plotted by direction-distance 
observations. Points also are located by intersections of the sightings from 
two different plane-table setups. Sufficient additional points are observed to 
permit sketching planimetric map details. Elevation differences also are 
observed for sighted points to allow for contour plotting.  See  ALIDADE; 
STADIA; TOPOGRAPHIC SURVEYING AND MAPPING. 
 �34s[B. Austin Barry]
�33s 
  
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�6v�2v�32sPlanetarium   �21m �33s 
    A projection device which accurately portrays the stars and planets at any 
time in the past, present, or future from any point on the Earth or the near 
region of space. The modern planetarium instrument is a mechanical-electrical 
analog of space. 
    Planetarium projectors are basically designed around three major 
components: a star lamp that has the intrinsic brilliance of stars in nature; 
a planet-projection system that allows for freedom of motion on or off the 
Earth; and a computer that solves all of the mechanical and mathematical 
problems incurred with  modern planetarium programming. The planetarium dome 
is constructed of perforated aluminum sheets. With the large number of 
perforations, the planetarium dome can function as a theatrical scrim. This 
allows the instrument to project the stars on the surface of the dome and also 
allows for projection through the dome. 
    The star lamp is a high-pressure xenon arc which produces a tiny, 
intense-point source of light. This light is focused through thousands of 
individual lenses and pinholes and is projected to the dome. When the star 
projector is put in motion, the sky appears to move. There are two star balls, 
one for the northern hemisphere and one for the southern. 
 �34s[David M. Larsen]
�33s 
  
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�6v�2v�32sPlanetary gear train   �21m �33s 
    An assembly of meshed gears consisting of a central gear, a coaxial 
internal or ring gear, and one or more intermediate pinions supported on a 
revolving carrier. Sometimes the term planetary gear train is used broadly as 
a synonym for epicyclic gear train, or narrowly to indicate that the ring gear 
is the fixed member. In a simple planetary gear train the pinions mesh 
simultaneously with the two coaxial gears. With the central gear fixed, a 
pinion rotates about it as a planet rotates about its sun, and the gears are 
named accordingly: The central gear is the sun, the pinions the planets. The 
illustration shows a construction typical in industrial planetary trains. 
    In operation, input power drives one member of a planetary gear train, the 
second member is driven to provide the output, and the third member is fixed. 
If the third member is not fixed, no power is delivered. This characteristic 
provides a convenient clutch action. A brake band about the intermediate 
member and fixed to the gear housing serves to lock or free the third member; 
the band itself does not enter into the power path.  See  GEAR TRAIN. 
 �34s[John R. Zimmerman]
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�6v�2v�32sPlanetary nebula   �21m �33s 
    A gaseous shell thrown off by a star during the later stages of its 
evolution. Planetary nebulae are so called because many display small greenish 
disks, akin to those of the planets Uranus and Neptune in small telescopes. 
They have nothing to do with planets; rather they are associated with late 
stages of stellar evolution, a planetary nebula being the shell thrown off by 
a dying star just before it settles down to become a degenerate white dwarf. 
Well over a thousand have been cataloged, and the total number in the Galaxy 
must be about 20,000. 
    Although a planetary nebula is often symmetrical about the central star, 
which is usually seen, a great variety of forms appear. Delicate filamentary 
wisps and knotlike concentrations are the rule, indicating that the ejected 
shells must be usually very inhomogeneous. Spectra of planetary nebulae are 
dominated by emission lines of abundant elements, in various ionization 
stages, all excited because of the rich ultraviolet radiation emitted by the 
central star. 
    Possibly most stars with masses between one and five times that of the Sun 
produce planetary nebulae. When hydrogen becomes exhausted in the core of a 
solar-type main-sequence star, it evolves into a red giant whose outer 
envelope is eventually ejected. As the shell density declines, the shell is 
exposed to energetic photons of the hot residual core. Hence, the 
characteristic bright-line nebular spectrum is produced. As the nebula expands 
further, this spectrum gradually fades away. The hot, blue-white core, 
deprived of all nuclear fuel, slowly evolves to a white dwarf.  See  STELLAR 
EVOLUTION; WHITE DWARF STAR. 
    Planetary nebulae appear to be important suppliers of dust and gas to the 
interstellar medium.  See  INTERSTELLAR MATTER; NEBULA. 
 �34s[Lawrence H. Allen]
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�6v�2v�32sPlanetary physics �21m �33s
    The study of the structure, composition, and physical and chemical
properties of the planets of the solar system, including their atmospheres and
their immediate cosmic environment.
    Planetary scientists attempt to synthesize their information about the
structure and properties of each of the planets by constructing models of
them. The most obvious gross properties of the planet are its mass and its
radius. In constructing a model, it is required that the model be in
hydrostatic equilibrium. This means that at any interior point in the model,
the pressure must be great enough to sustain the weight of the overlying mass
of material. Thus, given the mass and the radius, estimating the interior
pressures through the principles of hydrostatic equilibrium, and knowing
something about the compressibilities of materials, it is generally possible
to place constraints upon the interior composition of the planets. Knowledge
of the equatorial bulge and the rate of spin provides additional information
about the distribution of mass in the interior of the planet.

    Classes of chemical composition.  The planets in the solar system have an
extremely wide range of properties. This distribution of characteristics can
be understood in part from a knowledge of the more abundant elements in nature
and their volatility properties. Approximately 98% of matter in the Sun, and
therefore also presumably in the matter from which the Sun and the solar
system were formed, consists of the gases hydrogen and helium. Most of the
remaining material consists of carbon, nitrogen, and oxygen, which in the
presence of very large amounts of hydrogen tends to form methane, ammonia, and
water. These substances are collectively called ices, and they evaporate at
relatively low temperatures. Both the light gases hydrogen and helium and the
ices are of quite low abundance on the Earth and the other inner planets in
the solar system. What comprises the bulk of the material in these planets is
the rocky material, constituting only about 3 parts in 1000 of the solar mix
of elements.
    The differences in the volatilities of these materials, which correlate
with the properties of the planetary bodies in the solar system, give
information about the properties of the environment in which the planets
formed in the solar system. The inner planets, composed predominantly of
rocks, evidently formed in a rather hot environment, so that the volatile
gases and ices were not condensed and did not collect along with the rocky
material, which presumably was condensed. The comets, residing at very large
distances from the Sun in the solar system, appear to be mixtures of rocky
materials and of the ices. The outer giant planets, Uranus and Neptune, appear
to be primarily composed of materials heavier than hydrogen and helium,
probably mixtures of rocky and icy materials. The two largest planets in the
solar system, Jupiter and Saturn, are much closer in composition to that of
the Sun itself, although studies of their interior structures tend to indicate
that there is some degree of enrichment in the heavier elements. These
differences in composition thus indicate that the tendency to collect hydrogen
and helium depends upon the size of the body which has formed, the larger
bodies being more successful in gravitationally capturing the elusive hydrogen
and helium.  See  COMET.
    These compositional classes provide a natural means for dividing the
planetary objects within the solar system into separate groups.

    Giant planets. The giant planets are Jupiter, Saturn, Uranus, and Neptune.
Nowhere in the interiors of the giant planets can anything resembling a solid
surface be expected. The temperatures in the interiors tend to be thousands to
tens of thousands of degrees Celsius. The pressures range up to tens of
millions of bars (10(sup 7) bars = 10(sup 12) pascals) and higher. Under these
circumstances all materials behave like fluids. There may be a certain amount
of compositional stratification, with denser fluids underlying lighter ones.

    Terrestrial planets. The terrestrial planets include Mercury, Venus,
Earth, and Mars. The Earth's Moon may also be considered a terrestrial planet.
The Earth consists of a thin upper crust composed of rocks of relatively low
density and low melting points, overlying a much thicker mantle composed
predominantly of metallic silicates and oxides, which in turn overlies a
substantial core, which is composed of much denser materials, believed
predominantly to be iron with other elements, either alloyed or in solution.
    It is conjectured that the interior of Venus is probably much like that of
the Earth, with a core, a mantle, and a crust. Major structural features on
the surface of the planet suggest extensive tectonic activity. The extent to
which the crust of Venus is subject to extensive continental drift motions is
quite unknown.	See  VENUS.
    The mass of Mars is approximately one-tenth that of the Earth, and hence
significant differences in the internal structure are to be expected. There
appears to be less of a density contrast between the core of Mars and that of
its mantle. Because the planet is smaller, the temperature increases less
rapidly with depth than in the case of the Earth, and hence Mars should have a
somewhat more rigid outer mantle and crust than the Earth. There is no
indication that large amounts of continental drift have taken place on Mars.
On the other hand, tectonic activity has clearly played a large role in the
history of Mars.  See  MARS.
    Mercury has only about half the mass of Mars. The mean density of Mercury
is very high, indicating that Mercury probably has an abnormally large core
predominantly composed of metallic iron. There is much evidence of extensive
tectonic activity, although, like Mars, the increase of temperature below the
surface of Mercury probably occurs sufficiently slowly so that the crust and
upper mantle are relatively rigid, and nothing resembling continental drift
has probably taken place.  See	MERCURY (PLANET).
    The Moon has a history which includes extensive episodes of melting and
differentiation. The upper layers of the Moon, which is only just over 1% of
the mass of the Earth, are quite rigid, and there is no evidence for extensive
horizontal motions of the structural units. The Moon is unique in the solar
system in having a relatively low density among the inner planets, and at best
a very small core, indicating that the planet is practically devoid of
metallic iron.	See  MOON.

    Galilean satellites. The four Galilean satellites of Jupiter - Io, Europa,
Ganymede, and Callisto - have masses which are all roughly comparable to the
mass of the Earth's Moon. The Galilean satellites appear to represent a
composition class which is slightly more volatile-rich than the pure rocky
materials characteristic of the inner solar system.  See  JUPITER.

    Atmospheres.  The temperature at the surface of a planetary body depends
in a complex manner on the properties of the overlying atmosphere, as well as
upon the distance of the planet from the Sun. The atmosphere of Venus is very
much hotter relative to the Earth than would be expected purely on the basis
of the relative distances from the Sun. The difference appears to arise from
the extensive operation of the greenhouse effect within the very thick
atmosphere of Venus.
    The only terrestrial planets with atmospheres are Venus, Earth, and Mars.
Both Mars and Venus have atmospheres composed predominantly of carbon dioxide.
The element of next greatest abundance in the atmospheres of Mars and Venus is
nitrogen, which is also the predominant element in the atmosphere of the
Earth. The next most abundant element in the terrestrial atmosphere is oxygen,
which is maintained there predominantly as the result of the operation of life.

    Magnetospheres.  Some of the planets contain substantial magnetic fields;
others do not. Within the inner solar system, the Earth possesses a relatively
strong field, Mercury a relatively weak one, and if Venus and Mars contain
significant intrinsic fields, they are sufficiently weak that they have not
been confirmed. On the other hand, Jupiter and Saturn have very strong
magnetic fields. The generation of planetary magnetic fields appears to depend
upon a combination of planetary rotation with an inner convecting layer having
significant electrical conductivity.  See  MAGNETOSPHERE; PLANET; VAN ALLEN
RADIATION.
 �34s[A. G. W. Cameron]
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�6v�2v�32sPlantaginales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of only the family Plantaginaceae, with about 250 species. Within its 
subclass the order is marked by its small, chiefly windpollinated flowers that 
have a persistent regular corolla. The perianth and stamens of the flowers are 
attached directly to the receptacle (hypogynous) and there are typically four 
petals. The plants are herbs or seldom half-shrubs with mostly basal, 
alternate leaves. The common plantain (Plantago major) is a familiar lawn weed 
of this order.  See  ASTERIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPlant anatomy   �21m �33s 
    The area of plant science concerned with the internal structure of plants. 
It deals both with mature structures and with their origin and development. 
    The plant anatomist dissects the plant and studies it from different 
planes and at various levels of magnification. At the level of the cell, 
anatomy overlaps plant cytology, which deals exclusively with the cell and its 
contents. Sometimes the name plant histology is applied to the area of plant 
anatomy directed toward the study of cellular details of tissues.  See  PLANT 
CELL; PLANT ORGANS; PLANT TISSUE SYSTEMS. 
 �34s[Katherine Esau]
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�6v�2v�32sPlant cell   �21m �33s 
    The basic unit of structure and function in nearly all plants. Although 
plant cells are variously modified in structure and function, they have many 
common features. The most distinctive feature of all plant cells is the rigid 
cell wall, which is absent in animal cells. The range of specialization and 
the character of association of plant cells is very wide. In the simplest 
plant forms a single cell constitutes a whole organism and carries out all the 
life functions. In just slightly more complex forms, cells are associated 
structurally, but each cell appears to carry out the fundamental life 
functions, although certain ones may be specialized for participation in 
reproductive processes. In the most advanced plants, cells are associated in 
functionally specialized tissues, and associated tissues make up organs such 
as the leaves, stem, and root.  See  CELL WALLS (PLANT). 
    Plant and animal cells are composed of the same fundamental constituents - 
nucleic acids, proteins, carbohydrates, lipids, and various inorganic 
substances - and are organized in the same fundamental manner. A 
characteristic of their organization is the presence of unit membranes 
composed of phospholipids and associated proteins and in some instances 
nucleic acids. 
    Perhaps the most conspicuous and certainly the most studied of the 
features peculiar to plant cells is the presence of plastids. Among the 
organisms generally classified as plants, only the bacteria, the blue-green 
algae, and some fungi lack plastids. The plastids are membrane-bound 
organelles with an inner membrane system. Chlorophylls and other pigments are 
associated with the inner membrane system.  See  CELL (BIOLOGY); CELL 
PLASTIDS; CELL PROTOPLAST; CHLOROPHYLL. 
 �34s[W. Gordon Whaley]
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�6v�2v�32sPlant evolution   �21m �33s 
    That phase of evolution dealing with the origin and development of the 
plant kingdom. 
  
    Evolutionary processes.  The processes of plant evolution - mutation, 
genetic recombination, natural selection, and reproductive isolation - are the 
same as in all forms of life, but profound differences between animals and 
plants in their mode of life, basic structure, and individual development 
bring about corresponding modifications in the mode of action and relative 
importance of these four processes. The developmental cycle of any particular 
organ in plants is shorter, less complex, and less well integrated with other 
organs than in animals. Consequently, plant organs can be modified much more 
profoundly by both environmental and genetic effects than can animal 
structures without producing inviability. 
    Because of their general lack of motility, plants evolved special devices 
for the dissemination of propagules, such as spores and seeds. In addition, 
flowering plants evolved elaborate structures which promote cross-pollination 
through the aid of animals. Together with the structures which protect the 
spores and seeds, the progressive elaboration of these methods of 
cross-pollination and dispersal of spores and seeds forms the principal thread 
around which the evolution of land plants is centered. A secondary thread is 
formed by the vascular system, which serves for conduction and support. On the 
other hand, the modifications of the outward form of leaves and stems as well 
as of the inner physiological conditions of their cells which adapt plants to 
different habitats and modes of life are of such a general nature that they 
are repeated in an almost parallel fashion many times in unrelated groups of 
plants. They are consequently less reliable as signposts for pathways of 
evolution than are corresponding structures in animals.  See  POPULATION 
DISPERSAL.  
    In addition, the dioecious condition (that is, individuals distinct as to 
sex - male or female) is far less widespread in plants than in animals. In 
some plants, such as mosses, monoecious types (having individuals both male 
and female) were derived secondarily from dioecious ancestors. 
    All of the above-mentioned characteristics of plants facilitated evolution 
through recombination of genetic characteristics derived from different 
adaptive systems. Hybridization between subspecies and species is particularly 
common in plants, and the hybrid derivatives are often vigorous and well 
adapted to new habitats. They can become stabilized in three different ways: 
by segregation of relatively homozygous intermediate types, by introgression, 
and by polyploidy. In the ancestry of most present-day plant species one of 
these processes has occurred. 
  
    Evolutionary history.  In the evolutionary history of the plant kingdom, 
five levels of advancement are recognized, each based upon an adaptive system 
offering new opportunities for adaptive radiation. These five stages include 
Procaryota, Eucaryota, Embryobionta, vascular plants, and seed plants. Each 
level was first reached by a group with generalized or primitive 
characteristics. Through adaptive radiation, this generalized group gave rise 
in each case to a number of evolutionary lines or phylads, the modern 
representatives of which are very different from one another.  See  PLANT 
KINGDOM.   
 �34s[G. Ledyard Stebbins]
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�6v�2v�32sPlant geography   �21m �33s 
    For botanists, the major subdivision concerned with all aspects of the 
spatial distribution of plants. This subdivision is also known as 
phytogeography, phytochorology, geographical botany, and geobotany, and is 
here restricted to terrestrial landscapes. By custom, it often involves some 
aspects of the distribution of plants in time, called historical plant 
geography, and is thus allied to paleoecology and paleobotany. 
    The function of plant geography is to record the observed empirical facts 
of plant distribution, and also to understand and interpret these facts. 
Attention is directed at species, population, community, and ecosystem levels 
of integration. Where possible, the study includes the prediction and control 
of distributional phenomena, especially as these relate to plant pests, 
parasites, and diseases, and to the introduction and spread of desirable 
species and vegetation types. All such knowledge is pertinent to forestry, 
agriculture, range and pasture management, wildlife habitat management, 
horticulture, and soil and water conservation. Except mainly for genetical 
research on speciation and subsequent dispersal and survival into island and 
continental areas, plant geography is not an experimental or quantitative 
science, and in general does not involve laboratory procedures and technologic 
equipment.  
    There are two major subdivisions of plant geography, focused upon the two 
most commonly employed levels of integration of plant life. Floristic plant 
geography embraces the spatial distribution of flora, while vegetational plant 
geography is the spatial distribution of vegetation. 
    Flora is a scientific term with no common usage. The flora of an area or 
period of time is the totality of all species within that geographical unit, 
independent of their relative abundances and their relationships to one 
another. The technical term ``population,'' in this connection, refers 
collectively to all individuals of any one species within a locality.  See  
SPECIES POPULATION. 
    Vegetation is a term of popular origin and refers to the mosaic of plant 
life that forms the natural or seminatural landscape. The vegetation of a 
region is the tapestry or carpet of plant life, developed by differential and 
varying combinations and growths of the numerous elements of the local flora. 
Technically, it is an organized and integrated whole, at a higher level of 
integration than the separate species, composed of those species and their 
populations. Vegetation possesses emergent properties not necessarily found in 
the species themselves, and is thus a holistic system in its own right. In 
turn, vegetation is a component which together with all factors of the 
environment becomes the ecosystem.  See  ECOSYSTEM; VEGETATION. 
 �34s[Frank E. Egler]
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�6v�2v�32sPlant growth   �21m �33s 
    An irreversible increase in the size of the plant. As plants, like other 
organisms, are made up of cells, growth involves an increase in cell numbers 
by cell division and an increase in cell size. Cell division itself is not 
growth, as each new cell is exactly half the size of the cell from which it 
was formed. Only when it grows to the same size as its progenitor has growth 
been realized. Nonetheless, as each cell has a maximum size, cell division is 
considered as providing the potential for growth.  See  CELL (BIOLOGY); CELL 
DIVISION.  
    While growth in plants consists of an increase in both cell number and 
cell size, animal growth is almost wholly the result of an increase in cell 
numbers. Another important difference in growth between plants and animals is 
that animals are determinate in growth and reach a final size before they are 
mature and start to reproduce. Plants have indeterminate growth and, as long 
as they live, continue to add new organs and tissues. In a plant new cells are 
produced all the time, and some parts such as leaves and flowers may die, 
while the main body of the plant persists and continues to grow. The basic 
processes of cell division are similar in plants and animals, though the 
presence of a cell wall and vacuole in plant cells means that there are 
certain important differences. This is particularly true in plant cell 
enlargement, as plant cells, being restrained in size by a cellulose cell 
wall, cannot grow without an increase in the wall. Plant cell growth is thus 
largely a property of the cell wall.  See  CELL WALLS (PLANT). 
  
    Sites of cell division.  Cell division in plants takes place in discrete 
zones called meristems. The stem and root apical meristems produce all the 
primary (or initial) tissues of the stem and root. The cylindrical vascular 
cambium produces more conducting cells at the time when  secondary thickening 
(the acquisition of a woody nature) begins. The vascular cambium is a sheet of 
elongated cells which divide to produce xylem or water-conducting cells on the 
inside, and phloem or sugar-conducting cells on the outside. Unlike the apical 
meristems whose cell division eventually leads to an increase in length of the 
stem and root, divisions of the vascular cambium occur when that part of the 
plant has reached a fixed length, and lead only to an increase in girth, not 
in length. The final meristematic zone, the cork cambium is another cylindrical
sheet of cells on the outer edge of older stems and roots of woody plants. It
produces new outer cells only, and these cells differentiate into the corky
layers of the bark so that new protective layers are produced as the tree
increases in circumference.  See  APICAL MERISTEM; BUD; LATERAL MERISTEM;
PERIDERM; ROOT; STEM. 
  
    Controls.  Plant growth is affected by internal and external factors. The 
internal controls are all the product of the genetic instructions carried in 
the plant. These influence the extent and timing of growth and are mediated by 
signals of various types transmitted within the cell, between cells, or all 
around the plant. Intercellular communication in plants may take place via 
hormones (or chemical messengers) or by other forms of communication not well 
understood. There are several hormones (or groups of hormones), each of which 
may be produced in a different location, that have a different target tissue 
and act in a different manner.  See  ABSCISIC ACID; AUXIN; CYTOKININS; 
GIBBERELLIN; PLANT HORMONES. 
    The external environments of the root and shoot place constraints on the 
extent to which the internal controls can permit the plant to grow and 
develop. Prime among these are the water and nutrient supply available in the 
soil. Because cell expansion is controlled by cell turgor, which depends on 
water, any deficit in the water supply of the plant reduces cell turgor and 
limits cell elongation, resulting in a smaller plant.  See  PLANT-WATER 
RELATIONS.  
    Mineral nutrients are needed for the biochemical processes of the plant. 
When these are in insufficient supply, growth will be less vigorous, or in 
extreme cases it will cease altogether.  See  PLANT MINERAL NUTRITION. 
    An optimal temperature is needed for plant growth. The actual temperature 
range depends on the species. In general, metabolic reactions and growth 
increase with temperature, though high temperature becomes damaging. Most 
plants grow slowly at low temperatures 32 - 50DGF (0 - 10DGC), and some 
tropical plants are damaged or even killed at low but above-freezing 
temperatures.  
    Light is important in the control of plant growth. It drives the process 
of photosynthesis which produces the carbohydrates that are needed to 
osmotically retain water in the cell for growth.  See  PHOTOSYNTHESIS. 
  
    Fruits and seeds.  Fruits and seeds are rich sources of hormones. Initial 
hormone production starts upon pollination and is further promoted by ovule 
fertilization. These hormones promote the growth of both seed and fruit 
tissue. Fruits grow initially by cell division, then by cell enlargement, and 
finally sometimes by an increase in air spaces. 
    The growth of a seed starts at fertilization. A small undifferentiated 
cell mass is produced from the single-celled zygote. This proceeds to form a 
small embryo consisting of a stem tip bearing two or more leaf primordia at 
one end and a root primordium at the other. Either the endosperm or the 
cotyledons enlarge as a food store.  See  FRUIT; SEED; SEED GERMINATION. 
  
    Flowering.  At a certain time a vegetative plant ceases producing leaves 
and instead produces flowers. This often occurs at a particular season of the 
year. The determining factor for this event is day length (or photoperiod). 
Different species of plants respond to different photoperiods.  See  FLOWER; 
PHOTOPERIODISM.  
    The light signal for flowering is recieved by the leaves, but it is the 
stem apex that responds. Exposing even a single leaf to the correct 
photoperiod can induce flowering. Clearly, then, a signal must travel from the 
leaf to the apex. Grafting a plant that has been photoinduced to flower to one 
not so induced can cause the noninduced plant to flower. It has been proposed 
that a flower-inducing hormone travels from the leaf to the stem apex and 
there induces changes in the development of the cells such that the floral 
morphology results. 
  
    Dormancy.  At certain stages of the life cycle, most perennial plants 
cease growth and become dormant. Plants may cease growth at any time if the 
environmental conditions are unfavorable. When dormant, however, a plant will 
not grow even if the conditions are favorable.  See  DORMANCY. 
  
    Leaf abscission.  As a perennial plant grows, new leaves are continuously 
or seasonally produced. At the same time the older leaves are shed because 
newer leaves are metabolically more efficient in the production of 
photosynthates. A total shedding of tender leaves may enable the plant to 
withstand a cold period or drought. In temperate deciduous trees, leaf 
abscission is brought about by declining photoperiods and temperatures.  See  
ABSCISSION; PLANT MORPHOGENESIS. 
�34s[Peter J. Davies]
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�6v�2v�32sPlant hormones   �21m �33s 
    A plant hormone, or phytohormone, is an organic compound that is 
synthesized in minute quantities in one part of a plant and translocated to 
another part, where it influences a specific physiological process.  
    Plants contain many chemically and physiologically distinct types of 
plant-regulatory substances, some of which properly can be called hormones. 
For example, auxins, gibberellins, cytokinins, and growth inhibitors are 
classes of plant regulatory chemicals, some of which satisfy the criterion of 
being hormonal in nature. 
    Other plant regulatory substances, for example, abscisic acid, ethylene, 
and vitamins, can also be classified as hormones; and still others, for 
example, growth retardants, may prove to be hormonal in nature for some 
plants.  See  ABSCISIC ACID; AUXIN; CYTOKININS; GIBBERELLIN; PLANT GROWTH; 
PLANT METABOLISM; PLANT MOVEMENTS; PLANT PHYSIOLOGY. 
 �34s[Robert H. Biggs]
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�6v�2v�32sPlant kingdom �21m �33s
    The worldwide array of plant life, including plants that have roots in the
soil, plants that live on or within other plants and animals, plants that
float on or swim about in water, and plants that are carried on dust particles
in the air. There is great variation in body form and size, ranging from
microscopic unicells such as bacteria and certain algae to the giant sequoia
trees of California. There are more than 300,000 known species of plants, and
discoveries add hundreds of ``new'' species every year.
    This large number of forms necessitates some systematic method of
classification. The species are therefore grouped into successively larger and
more inclusive categories to make a formal hierarchy of classification. The
white oak, for example, may be classified as follows:
    Subkingdom Embryobionta
	Division Magnoliophyta
	Class Magnoliopsida
	Subclass Hamamelidae
	Order Fagales
	Family Fagaceae
	Genus Quercus
	Species alba
    One system of classification of the entire plant kingdom is shown below
(the asterisk indicates groups known only as fossils). In the subkingdom
Thallobionta and in the division Bryophyta of the subkingdom Embryobionta the
scheme is shown only to the level of the class, but in the remainder of the
Embryobionta it is shown down to the level of the order. Common names are
inserted in parentheses after some of the scientific names. It should also be
noted that the distinction between the plant and animal kingdom is not
absolutely sharp among the unicellular flagellates; some of the groups, such
as the euglenoids, that are here classified as plants are treated as protozoan
animals by many zoologists. See separate articles on names marked by daggers.
Kingdom PlantaeSubkingdom Thallobionta (thallophytes)supd
    Division Schizophyta (prokaryotes)
	Class Schizomycetes (bacteria)supd
	Class Cyanophyceae (blue-green algae)supd
    Division Rhodophyta (red algae)supd
	Class Bangiophyceae
	Class Rhodophyceae
    Division Chlorophyta (green algae)supd
	Class Chlorophyceaesupd
	Class Charophyceaesupd
    Division Euglenophyta (euglenoids)supd
	Class Euglenophyceae
    Division Pyrrhophytasupd
	Class Cryptophyceae (cryptomonads)
	Class Desmophyceae
	Class Dinophyceae (dinoflagellates)
    Division Chrysophytasupd
	Class Chloromonadophyceae (chloromonads)
	Class Xanthophyceae (yellow-green algae)
	Class Chrysophyceae (golden algae)
	Class Bacillariophyceae (diatoms)
    Division Phaeophyta (brown algae)supd
	Class Laminariophyceae
	Class Fucophyceae
    Division Fungisupd
	Class Myxomycetes (slime molds)supd
	Class Oomycetes
	Class Chytridiomycetes (chytrids)
	Class Hyphochytridiomycetes
	Class Zygomycetes
	Class Ascomycotina (sac fungi)supd
	Class Basidiomycotina (club fungi)supd
Subkingdom Embryobionta (embryophytes)supd
    Division Rhyniophyta*supd
	Class Rhyniopsidasupd
    Division Bryophytasupd
	Class Anthocerotae (horned liverworts)supd
	Class Marchantiopsida (liverworts)supd
	Class Bryopsida (mosses)supd
    Division Psilotophytasupd
	Class Psilotopsida
    Division Lycopodiophytasupd
	Class Lycopodiopsida (club mosses)supd
	Class Isoetopsidasupd
    Division Equisetophytasupd
	Class Hyeniopsida*
	Class Sphenophyllopsida*
	Class Equisetopsida (horsetails)supd
    Division Polypodiophyta (ferns)supd
	Class Polypodiopsidasupd
	Order Protopteridales*
	Order Archaeopteridales*supd
	Order Ophioglossales
	Order Noeggerathiales*supd
	Order Marattialessupd
	Order Polypodialessupd
	Order Marsilealessupd
	Order Salvinialessupd
    Division Pinophyta (gymnosperms)supd
	Subdivision Cycadicaesupd
	Class Lyginopteridopsida*
8m]Order Lyginopteridales (seed ferns)
	Order Caytonialessupd
	Class Cycadopsida (cycads)
	Order Cycadalessupd
	Class Bennettitopsida (cycadeoids)*
	Order Bennettitales
	Subdivision Pinicaesupd
6m]Class Ginkgoopsida (maidenhair tree)
	Order Ginkgoalessupd
	Class Pinopsidasupd
	Order Pinales (conifers)supd
	Order Taxalessupd
	Subdivision Gneticaesupd
	Class Gnetopsida
	Order Ephedrales
	Order Welwitschiales
	Order Gnetales
2m]Division Magnoliophyta (angiosperms; flowering plants)supd
	Class Magnoliopsida (dicotyledons)supd
	Subclass Magnoliidaesupd
	Order Magnolialessupd
	Order Piperalessupd
	Order Aristolochialessupd
	Order Nymphaealessupd
	Order Ranunculalessupd
	Order Papaveralessupd
	Subclass Hamamelidaesupd
	Order Trochodendralessupd
	Order Hamamelidalessupd
	Order Eucommialessupd
	Order Urticalessupd
	Order Leitnerialessupd
	Order Juglandalessupd
	Order Myricalessupd
	Order Fagalessupd
	Order Casuarinalessupd
	Subclass Caryophyllidaesupd
	Order Caryophyllalessupd
	Order Batalessupd
	Order Polygonalessupd
	Order Plumbaginalessupd
	Subclass Dilleniidaesupd
	Order Dillenialessupd
	Order Thealessupd
	Order Malvalessupd
	Order Lecythidalessupd
	Order Sarracenialessupd
	Order Violalessupd
	Order Salicalessupd
	Order Capparalessupd
	Order Ericalessupd
	Order Diapensialessupd
	Order Ebenalessupd
	Order Primulalessupd
	Subclass Rosidaesupd
	Order Rosalessupd
	Order Podostemalessupd
	Order Haloragalessupd
	Order Myrtalessupd
	Order Protealessupd
	Order Cornalessupd
	Order Santalalessupd
	Order Rafflesialessupd
	Order Celastralessupd
	Order Euphorbialessupd
	Order Rhamnalessupd
	Order Sapindalessupd
	Order Geranialessupd
	Order Linalessupd
	Order Polygalalessupd
	Order Umbellalessupd
	Subclass Asteridaesupd
	Order Gentianalessupd
	Order Polemonialessupd
	Order Lamialessupd
	Order Plantaginalessupd
	Order Scrophularialessupd
	Order Campanulalessupd
	Order Rubialessupd
	Order Dipsacalessupd
	Order Asteralessupd
	Class Liliopsida (monocotyledons)supd
	Subclass Alismatidaesupd
	Order Alismatalessupd
	Order Hydrocharitalessupd
	Order Najadalessupd
	Order Triuridalessupd
	Subclass Commelinidaesupd
	Order Commelinalessupd
	Order Eriocaulalessupd
	Order Restionalessupd
	Order Juncalessupd
	Order Cyperalessupd
	Order Typhalessupd
	Order Bromelialessupd
	Order Zingiberalessupd
	Subclass Arecidaesupd
	Order Arecalessupd
	Order Cyclanthalessupd
	Order Pandanalessupd
	Order Aralessupd
	Subclass Liliidaesupd
	Order Lilialessupd
	Order Orchidalessupd
See  PLANT TAXONOMY.
 �34s[Arthur Cronquist]
�33s

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�6v�2v�32sPlant metabolism   �21m �33s 
    The complex of physical and chemical events of photosynthesis, 
respiration, and the synthesis and degradation of organic compounds. 
Photosynthesis produces the substrates for respiration and the starting 
organic compounds used as building blocks for subsequent biosyntheses of 
nucleic acids, amino acids, and proteins, carbohydrates and organic acids, 
lipids, and natural products.  See  PHOTORESPIRATION; PHOTOSYNTHESIS; PLANT 
RESPIRATION.  
 �34s[Irwin P. Ting]
�33s 
  
A700000010437�254s�5v�1v1333
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�6v�2v�32sPlant mineral nutrition   �21m �33s 
    The relationship between plants and all chemical elements other than 
carbon, hydrogen, and oxygen in the environment. Plants obtain most of their 
mineral nutrients by extracting them from solution in the soil or the aquatic 
environment. Mineral nutrients are so called because most of them have been 
derived from the weathering of minerals of the Earth's crust. Nitrogen is 
exceptional in that little occurs in minerals; its primary source is gaseous 
nitrogen of the atmosphere. 
    Some of the mineral nutrients are essential for plant growth; others are 
toxic, and some absorbed by plants may play no role in metabolism. Many are 
also essential or toxic for the health and growth of animals using plants as 
food. Six basic facts concerning nutrients have been established: plants do 
not need any of the solid materials in the soil - they cannot even take them 
up; plants do not need soil microorganisms; plant roots must have a supply of 
oxygen; all plants require at least 13 mineral nutrients; all of the essential 
mineral nutrients may be supplied to plants as simple ions of inorganic salts 
in solution; and all of the essential nutrients must be supplied in adequate 
but nontoxic quantities. 
    A mineral nutrient is regarded as essential if, in its absence, a plant 
cannot complete its life cycle. The essential nutrients are: nitrogen, sulfur, 
phosphorus, calcium, potassium, magnesium, boron, manganese, zinc, copper, 
molybdenum, and chlorine. In addition, sodium and silicon had been shown to be 
essential for some plants, beneficial to some, and possibly of no benefit to 
others. Cobalt was also shown to be essential for the growth of legumes when 
relying upon fixating atmospheric nitrogen. 
    Increasing salt concentrations progressively depress the water potential 
of solutions, thus making it more difficult for plants to absorb water. Plants 
can offset this effect by accumulating organic molecules or salts within their 
own cells. The ability of plants to accumulate soluble salts in their leaves 
without damage varies widely, from most common species which can accumulate 
only 1 - 2% of their dry weight, to some desert plants growing in saline soils 
which accumulate 20 - 50%.  See  PLANT-WATER RELATIONS. 
    A number of nutrients, sometimes at quite low concentrations, interfere 
directly with other aspects of plant metabolism. The ions of the heavy metals 
cobalt, nickel, chromium, manganese, copper, and zinc are particularly toxic 
in low concentrations. So is the aluminum ion, although this can exist only in 
significant quantities in quite acid solutions. The toxicities of these ions 
may result from their multivalent properties which permit them to interfere 
with the function of proteins and other large molecules by forming 
cross-linkages through adsorption to negatively charged sites. 
    Plants are particularly sensitive to aluminum and heavy-metal toxicities 
when the concentration of calcium in solution is low: increasing calcium 
increases the plant's tolerance. Tolerance to heavy metals also varies with 
plant species and cultivars. In some cases, tolerance may be achieved by 
excluding the toxic element from the plant. In other cases, tolerance is 
accompanied by an unusually high accumulation of the normally toxic element in 
plant tops, possibly by synthesis of specific chelators to remove it from cell 
metabolism.  
    The nutrient composition of plants is important to the health and 
productivity of animals which graze them. With the exception of boron, all 
elements which are essential for plant growth are also essential for 
herbivorous mammals. Animals also require sodium, iodine, and selenium and, in 
the case of ruminant herbivores, cobalt. 
    Animal requirements for mineral nutrients may also differ quantitatively 
from plants. For example, plants require large amounts of potassium and little 
or no sodium, whereas animals require large quantities of sodium and little 
potassium.  
    Plants and animals differ also in their tolerance of high levels of 
nutrients, sometimes with unfortunate results for the grazing animals. For 
example, high levels of selenium in some species do not harm the plant but may 
kill animals which graze them. 
 �34s[J. F. Loneragan]
�33s 
  
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�6v�2v�32sPlant morphogenesis   �21m �33s 
    A biological science concerned with the origin and development of the form 
and structure of plants. It deals with those phenomena, both internal and 
external, by which form is determined. Form is the result of orderly growth in 
various dimensions. The form of the body and of such structures as leaves, 
flowers, hairs, and cells are examples of this controlled development. Form 
and structure in living things are visible expressions of the organizing 
character of life at every level. 
    In almost all organisms an axis appears during development and its two 
ends become different, as root and shoot in higher plants. This polarity may 
be seen very early, even in the fertilized egg itself. It is evident in 
regeneration and also appears in physiological processes. The arrangement of 
lateral structures around the main polar axis is not irregular, but shows 
patterns of symmetry of various sorts. Most plant axes are vertical, and the 
distribution of leaves around them (phyllotaxy) is radial and precise. The 
points of attachment of leaves may be opposite, whorled, or dispersed in a 
spiral (alternate). Such spirals fall into definite classes which have simple 
mathematical relationships to one another. 
    A conspicuous fact of development is that during its course the parts of 
the organism become different from one another, a process called 
differentiation. An example of such differentiation is that between the two 
ends of the polar axis. Development in higher plants is accompanied by the 
progressive appearance of distinct organs - roots, stems, leaves, flowers, 
fruits, and seeds - the outward sign of division of labor within the plant. 
Growth and differentiation generally proceed together but there are cases 
where one occurs without the other. In the course of the life cycle of an 
organism, differential changes occur in a definite series of stages. Internal 
differences that arise among tissues and among cells during development are as 
marked as is external form. 
    Physiological differences between parts of the plant are common. Cells of 
the root are unable to synthesize sugar and certain other necessary substances 
and must get them from the shoots. Many localized physiological differences 
are maintained because of the impermeability of the cell membranes to 
substances produced in the cells.  
 �34s[Richard H. Goodwin]
�33s 
  
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�6v�2v�32sPlant movements   �21m �33s 
    The wide range of movements that allow plants to reorient themselves in 
relation to changed surroundings, to facilitate spore or seed dispersal, or, 
in the case of small free-floating aquatic plants, to migrate to regions 
optimal for their activities. There are two types of plant movement: abiogenic 
movements, which arise purely from the physical properties of the cells and 
therefore take place in nonliving tissues or organs; and biogenic movements, 
which occur in living cells or organs and require an energy input from 
metabolism.  
  
    Abiogenic movements.  Drying or moistening of certain structures causes 
differential contractions or expansions on the two sides of cells and  hence 
causes movements of curvature. Such movements are called hygroscopic and are 
usually associated with seed and spore liberation and dispersal. Examples of 
such movement occur in the ``parachute'' hairs of the fruit of dandelion 
(Taraxacum officinale), which are closed when damp but open when the air is 
dry to induce release from the heads and give buoyancy for wind dispersal. 
    Another type of abiogenic movement is due to changes in volume of dead 
water-containing cells. In the absence of a gas phase, water will adhere to 
lignocellulose cell walls. As water is lost by evaporation from the surface of 
these cells, considerable tensions can build up inside, causing them to 
decrease in volume while remaining full of water. The effect is most commonly 
seen in some grasses of dry habitats, such as sand dunes, where longitudinal 
rows of cells on one side of the leaf act as spring hinges, contracting in a 
dry atmosphere and causing the leaf to roll up into a tight cylinder, thus 
minimizing water loss by transpiration. 
  
    Biogenic movements.  There are two types of biogenic movement. One of 
these is locomotion of the whole organism and is thus confined to small, 
simply organized units in an aqueous environment. The other involves the 
change in shape and orientation of whole organs of complex plants, usually in 
response to specific stimuli. 
  
    Locomotion. In most live plant cells the cytoplasm can move by a streaming 
process known as cyclosis. Energy for cyclosis is derived from the respiratory 
metabolism of the cell. The mechanism probably involves contractile proteins 
very similar to the actomyosin of animal muscles. 
    Cell locomotion is a characteristic of many simple plants and of the 
gametes of more highly organized ones. Motility in such cells is produced by 
cilia anchored in the peripheral layers of the cell and projecting into the 
surrounding medium.  See  CILIA AND FLAGELLA. 
    Cell locomotion is usually not random but is directed by some 
environmental gradient. Thus locomotion may be in response to specific 
chemicals, in which case it is called chemotaxis. Light gradients induce 
phototaxis; temperature gradients induce thermotaxis; and gravity induces 
geotaxis. One or more of these environmental factors may operate to control 
movement to optimal living conditions. 
  
    Movement of organs. In higher plants, organs may change shape and position 
in relation to the plant body. When bending or twisting of the organ is evoked 
spontaneously by some internal stimulus, it is termed autonomous movement. The 
most common movements, however, are those initiated by external stimuli such 
as light and the force of gravity. Of these there are two kinds. In nastic 
movements (nasties), the stimulus usually has no directional qualities (such 
as a change in temperature), and the movement is therefore not related to the 
direction from which the stimulus comes. In tropisms, the stimulus has a 
direction (for instance, gravitational pull), and the plant movement direction 
is related to it. 
    The most common autonomous movement is circumnutation, a slow, circular, 
sometimes waving movement of the tips of shoots, roots, and tendrils as they 
grow; one complete cycle usually takes from 1 to 3 h. These movements are due 
to differential growth, but some may be caused by turgor changes in the cells 
of special hinge organs and are thus reversible. 
    1. Nastic movements. There are two kinds of nastic movements, due either 
to differential growth or to differential changes in the turgidity of cells. 
They can be triggered by a wide variety of external stimuli. 
    Photonastic (light/dark trigger) movements are characteristic of many 
flowers and inflorescences, which usually open in the light and close in the 
dark. Thermonasty (temperature-change trigger) is seen in the tulip and crocus 
flowers, which open in a warm room and close again when cooled. The most 
striking nastic movements are seen in the sensitive plant (Mimosa pudica). Its 
multipinnate leaves are very sensitive to touch or slight injury. Leaflets 
fold together, pinnae collapse downward, and the whole leaf sinks to hang 
limply.  
    Epinasty and hyponasty occur in leaves as upward and downward curvatures 
respectively. They arise either spontaneously or as the result of an external 
stimulus, such as exposure to the gas ethylene in the case of epinasty; they 
are not induced by gravity. 
    2. Tropisms. Of these the most universal and important are geotropism (or 
more properly gravitropism) and phototropism; others include thigmotropism and 
chemotropism.  
    In geotropism, the stimulus is gravity. The main axes of most plants grow 
in the direction of the plumb line with shoots upward (negative geotropism) 
and roots downward (positive geotropism).  
    In phototropism the stimulus is a light gradient, and unilateral light 
induces similar curvatures; those toward the source are positively 
phototropic; those away from the source are negatively phototropic. Main axes 
of shoots are usually positively phototropic, while the vast majority of roots 
are insensitive. 
    In thigmotropism (sometimes called haptotropism), the stimulus is touch; 
it occurs in climbing organs and is responsible for tendrils curling around a 
support. In many tendrils the response may spread from the contact area, 
causing the tight coiling of the basal part of the tendril into an elaborate 
and elastic spring. 
    Chemotropism is induced by a chemical substance. Examples are the 
incurling of the stalked digestive glands of the insectivorous plant Drosera 
and incurling of the whole leaf of Pinguicula in response to the nitrogenous 
compounds in the insect prey. A special case of chemotropism concerns response 
to moisture gradients; for example, under artificial conditions in air, the 
primary roots of some plants will curve toward and grow along a moist surface. 
This is called hydrotropism and may be of importance under natural soil 
conditions in directing roots toward water sources. See PLANT HORMONES; PLANT 
PHYSIOLOGY.  
 �34s[Leslie J. Audus]
�33s 
  
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�6v�2v�32sPlant organs   �21m �33s 
    Plant parts having rather distinct form, structure, and function. Organs, 
however, are interrelated through both evolution and development and are 
similar in many ways. 
    Roots, stems, and leaves are vegetative, or asexual, plant organs. They do 
not produce sex cells or play a direct role in sexual reproduction. In many 
species, nevertheless, these organs or parts of them (cuttings), may produce 
new plants asexually (vegetative reproduction). Sex organs are formed during 
the reproductive stage of plant development. In flowering plants, sex cells 
are produced in certain floral organs. The flower as a whole is sometimes 
called an organ, although it is more appropriate to consider it an assemblage 
of organs.  See  FLOWER; FRUIT; LEAF; REPRODUCTION (PLANT); ROOT; STEM. 
 �34s[Katherine Esau]
�33s 
  
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�6v�2v�32sPlant pathology   �21m �33s 
    The study of disease in plants; it is an integration of many biological 
disciplines and bridges the basic and applied sciences. 
  
    Kinds of plant diseases.  Diseases were first classified on the basis of 
symptoms. Three major categories of symptoms were recognized long before the 
causes of disease were known; necroses, destruction of cell protoplasts, 
(rots, spots, wilts); hypoplases, failure in plant development (chlorosis, 
stunting); and hyperplases, overdevelopment in cell number and size, 
(witches'-brooms, galls). This scheme remains useful for recognition and 
diagnosis.  
    When fungi, and then bacteria, nematodes, and viruses, were recognized as 
causes of disease, it became convenient to classify diseases according to the 
responsible agent. If the agents were infectious (biotic), the diseases were 
classified as being ``caused by bacteria,'' ``caused by nematodes,'' or 
``caused by viruses.'' To this list were added phanerogams and protozoans, and 
later mollicutes (mycoplasmas, spiroplasmas), rickettsias, and viroids. In a 
second group were those diseases caused by such noninfectious (abiotic) agents 
as air pollutants, inadequate oxygen, and nutrient excesses and deficiencies. 
  
    Symptoms of plant diseases.  Symptoms are expressions of pathological 
activity in plants. They are visible manifestations of changes in color, form, 
and structure: leaves may become spotted, turn yellow, and die; fruits may rot 
on the plants or in storage; cankers may form on stems; and plants may blight 
and wilt. 
    Those symptoms that are external and readily visible are considered 
morphological. Others are internal and primarily histological, for example, 
vascular discoloration of the xylem of wilting plants. Microscopic examination 
of diseased plants may reveal additional symptoms at the cytological level, 
such as the formation of tyloses (extrusion of living parenchyma cells of the 
xylem of wilted tissues into vessel elements). 
    It is important to make a distinction between the visible expression of 
the diseased condition in the plant, the symptom, and the visible 
manifestation of the agent which is responsible for that condition, the sign. 
The sign is the structure of the pathogen, and when present it is most helpful 
in diagnosis of the disease. 
    All symptoms may be conveniently classified into three major types because 
of the manner in which pathogens affect plants. Most pathogens produce dead 
and dying tissues, and the symptoms expressed are categorized as necroses. 
Early stages of necrosis are evident in such conditions as hydrosis, wilting, 
and yellowing. As cells and tissues die, the appearance of the plant or plant 
part is changed, and is recognizable in such common conditions as blight, 
canker, rot, and spot. 
    Many pathogens do not cause necrosis, but interfere with cell growth or 
development. Plants thus affected may eventually become necrotic, but the 
activity of the pathogen is primarily inhibitory or stimulatory. If there is 
decrease in cell number or size, the expressions of pathological activity are 
classified as hypoplases; if cell number or size is increased, the symptoms 
are grouped as hyperplases. These activities are very specific and most 
helpful in diagnosis. In the former group are such symptoms as mosaic, 
rosetting, and stunting, with obvious reduction in plant color, structure, and 
size. In the latter group are gall, scab, and witches'-broom, all visible 
evidence of stimulation of growth and development of plant tissues.  See  
CROWN GALL. 
 �34s[Carl W. Boothroyd]
�33s 
  
    Causative agents of disease.  The primary agents of plant disease are 
fungi, bacteria, viruses and viroids, nematodes, parasitic seed plants, and a 
variety of noninfectious agents. 
    Plants with symptoms caused by noninfectious agents cannot serve as 
sources of further spread of the same disorder. Such noninfectious agents may 
be deficiencies or excesses of nutrients, anthropogenic pollutants, or 
biological effects by organisms external to the affected plants. 
  
    Fungi. More plant diseases are caused by fungi than by any other agent. 
The fungi that cause plant disease derive their food from the plant (host) and 
are called parasites. Those that can live and grow only in association with 
living plant tissues are obligate parasites. Some fungi obtain their food from 
dead organic matter and are known as saprobes or saprophytes. Still others can 
utilize food from either dead organic matter or from living plant cells, and 
are referred to as either facultative parasites or facultative saprophytes.  
See  FUNGI. 
 �34s[C. Wayne Ellett]
�33s 
  
    Bacteria. Over 100 species of bacteria mainly in five genera cause disease 
in hundreds of different species of flowering plants. Destructive bacterial 
diseases affect the major cereal, vegetable, and fruit crops. None of the 
bacterial pathogens of plants causes serious diseases of humans or animals, 
and certain groups of green plants (mosses, ferns, conifers, and hardwood 
trees) have few or no major bacterial diseases.  See  BACTERIA. 
    Each species of bacterium produces a distinctive pattern of symptoms on 
those hosts that it attacks. With a few exceptions, most of the bacteria that 
cause disease in plants are non-spore-forming, rod-shaped, gram-negative 
cells. Some species give a positive reaction with the Gram stain; these 
include species of Streptomyces and Corynebacterium, a few species of Bacillus 
that cause soft rots, and those species of Clostridium that can decay plant 
storage organs under anaerobic conditions. Rickettsialike bacteria have been 
observed in diseased plants; unlike other plant pathogenic bacteria, these 
have been extremely difficult to grow in culture or have not been isolated. A 
number of yellows-type diseases previously considered to be caused by viruses 
have now been shown to be caused by mycoplasmalike organisms (MPLO),  members 
of the class Mollicutes. Pleomorphic MPLOs lacking cell walls have been 
observed in vascular tissues of yellows-affected plants; however, some have 
not as yet been characterized through isolation and cultivation techniques. 
 �34s[Arthur Kelman]
�33s 
  
    Viruses and viroids. Viruses and viroids are the simplest of the various 
causative agents of plant disease. The essential element of each of these two 
pathogens is an infective nucleic acid. The nucleic acid of viruses is covered 
by an exterior shell (coat) of protein, but that of viroids is not.  See  
PLANT VIRUSES AND VIROIDS. 
    The control or prevention of virus diseases involves breeding for 
resistance, propagation of virus-free plants, use of virus-free seed, 
practices designed to reduce the spread by vectors and, in some cases, the 
deliberate inoculation of plants with mild strains of a virus to protect them 
from the deleterious effects of severe strains. 
 �34s[R. I. Hamilton]
�33s 
  
    Nematodes. Nematodes (roundworms) are one of the most common groups of 
invertebrate animals. All soils that support plant life contain nematodes 
living in the water films that surround soil particles. Only a relatively 
small group of nematodes parasitize plants.  See  NEMATA. 
    Plant injury is of three general types and is related to feeding habits. 
Migratory endoparasites destroy tissues as they feed, producing necrotic 
lesions in the root cortex. Other migratory endoparasites invade leaf tissues 
and produce extensive brown spots. Sedentary endoparasites do not kill host 
cells, but induce changes in host tissues, which lead to an elaborate feeding 
site or gall. The third general type of symptom is produced by certain 
migratory ectoparasites, where root tips are devitalized and cease to grow 
without any associated swelling or necrosis. 
    In addition to the plant injury that they cause directly, nematodes are 
important factors in disease complexes. Lesions and galls provide entrance 
courts for soil fungi and bacteria, and many diseases caused by soil-borne 
pathogens are more severe when nematodes are present. Plant varieties 
resistant to soil-borne fungi sometimes lose this resistance when previously 
parasitized by nematodes. 
 �34s[Richard A. Rohde]
�33s 
  
    Seed plants. Many parasitic seed plants (estimated at nearly 3000) attack 
other higher plants. They occur in at least 15 different families. In some 
families (for example, the mistletoes Loranthaceae and Viscaceae) all members 
are parasitic; in others, only a single genus is parasitic in an otherwise 
autotrophic family (for example, the dodder, genus Cuscuta, of the morning 
glory family Convolvulaceae). 
    Most parasitic plants are terrestrial; that is, the parasitic connection 
with the host plant is through the roots. Other parasitic plants grow on the 
aboveground parts of the host. Some plants are classed as semiparasites 
because they can live in the soil as independent plants for a time, but are 
not vigorous or may not flower if they do not become attached to a suitable 
host.  
    The nutritional status of parasitic plants ranges from total parasites 
with no chlorophyll (for example, the broomrapes, Orobanchaceae) to plants 
that are well supplied with chlorophyll and obtain primarily water and 
minerals from their hosts (many mistletoes; see illustration). 
 �34s[Frank G. Hawksworth]
�33s 
  
  
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�6v�2v�32sPlant phylogeny   �21m �33s 
    The determination of probable phylogeny among plants is based partly on 
the study of fossils and partly on detailed comparisons among modern groups. 
In general, the fossil record is less useful to plant than to animal 
phylogenists because the critically important parts of plants (especially the 
reproductive organs) are seldom adequately preserved, and there are great gaps 
in the record. The morphological integration which permits animal taxonomists 
to make numerous sound deductions from fragmentary remains is much less 
evident in plants, and a given type of vegetative structure may be compatible 
with several different sorts of reproductive structures. 
    Despite these difficulties, a broad outline of plant phylogeny is becoming 
reasonably clear. At the level of families and orders, on the other hand, 
there are still many major points of disagreement. Although there is a general 
consensus about evolution among the angiosperms in terms of many individual 
characteristics and general trends, an agreement on the outline of the 
phylogeny of families and orders is only beginning to be evident.  See  
EMBRYOBIONTA; PALEOBOTANY; PLANT EVOLUTION; PLANT KINGDOM; THALLOBIONTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPlant physiology   �21m �33s 
    That branch of plant sciences that aims to understand how plants live and 
function. Its ultimate objective is to explain all life processes of plants by 
a minimal number of comprehensive principles founded in chemistry, physics, 
and mathematics. 
    Plant physiology seeks to understand all the aspects and manifestations of 
plant life. In agreement with the major characteristics of organisms, it is 
usually divided into three major parts: (1) the physiology of nutrition and 
metabolism, which deals with the uptake, transformations, and release of 
materials, and also their movement within and between the cells and organs of 
the plant; (2) the physiology of growth, development, and reproduction, which 
is concerned with these aspects of plant function; and (3) environmental 
physiology, which seeks to understand the manifold responses of plants to the 
environment. The part of environmental physiology which deals with effects of 
and adaptations to adverse conditions - and which is receiving increasing 
attention - is called stress physiology. 
    Plant physiological research is carried out at various levels of 
organization and by using various methods. The main organizational levels are 
the molecular or subcellular, the cellular, the organismal or whole-plant, and 
the population level. Work at the molecular level is aimed at understanding 
metabolic processes and their regulation, and also the localization of 
molecules in particular structures of the cell but with little if any 
consideration of other processes and other structures of the same cell. Work 
at the cellular level often deals with the same processes but is concerned 
with their integration in the cell as a whole. Research at the organismal 
level is concerned with the function of the plant as a whole and its different 
organs, and with the relationships between the latter. 
    Research at the population level, which merges with experimental ecology, 
deals with physiological phenomena in plant associations which may consist 
either of one dominant species (like a field of corn) or of numerous diverse 
species (like a forest). Work at the organismal and to some extent the 
population level is carried out in facilities permitting maintenance of 
controlled environmental conditions (light, temperature, water and nutrient 
supply, and so on).  See  PLANT METABOLISM; PLANT RESPIRATION; PHYSIOLOGICAL 
ECOLOGY (PLANT); PHYTOTRONICS. 
 �34s[Anton Lang]
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�6v�2v�32sPlant respiration   �21m �33s 
    A biochemical process whereby specific substrates are oxidized with a 
subsequent release of carbon dioxide, CO2. There is usually conservation of 
energy accompanying the oxidation which is coupled to the synthesis of 
energy-rich compounds, such as adenosinetriphosphate (ATP), whose free energy 
is then used to drive otherwise unfavorable reactions that are essential for 
physiological processes such as growth. Respiration is carried out by specific 
proteins, called enzymes, and it is necessary for the synthesis of essential 
metabolites, including carbohydrates, amino acids, and fatty acids, and for 
the transport of minerals and other solutes between cells. Thus respiration is 
an essential characteristic of life itself in plants as well as in other 
organisms.  
    Overall aerobic respiration is the end result of a sequence of many 
biochemical reactions that ultimately lead to O2 uptake and CO2 evolution. In 
the absence of O2, as may occur in bulky plant tissues such as the potato 
tuber and carrot root and in submerged plants such as germinating rice 
seedlings, the breakdown of hexose does not go to completion. The end products 
are either lactic acid or ethanol, which are produced by anaerobic glycolysis 
or fermentation. 
    The sequence of reactions of anaerobic glycolysis or fermentation is shown 
in the illustration. The enzymes associated with anerobic glycolysis have been 
isolated from many plant tissues, but more often ethanol and not lactic acid 
is the final product.  
    In aerobic tissues, pyruvic acid produced during glycolysis is completely 
oxidized with the accompanying synthesis of much more ATP than in anaerobic 
glycolysis.  Pyruvic acid oxidation takes place in the mitochondria by means 
of a cyclic sequence of reactions, the Krebs cycle, which begins when the 
first product of pyruvate oxidation, acetyl coenzyme A, reacts with 
oxaloacetic acid to produce citric acid. Oxaloacetic acid is eventually 
regenerated. Thus the cycle (also known as the citric acid cycle) can be 
repeated. In terms of conservation of chemical energy, the Krebs cycle is 
about 12 times more efficient than anaerobic glycolysis per mole of glucose 
oxidized.  See  KREBS CYCLE. 
    In addition to anaerobic glycolysis and the Krebs cycle, there are two 
other sequences of biochemical reactions related to respiration that are 
important in plant tissues: (1) The pentose phosphate pathway permits an 
alternate mechanism for converting hexose phosphate to pyruvate, and (2) in 
germinating fatty seeds the reactions of the Krebs cycle are modified so that 
acetyl coenzyme A is converted to succinic acid and then to hexose by a 
pathway called the glyoxylate cycle.  See  PHOTORESPIRATION; PHOTOSYNTHESIS; 
PLANT GROWTH; PLANT METABOLISM. 
 �34s[Israel Zelitch]
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�6v�2v�32sPlant taxonomy   �21m �33s 
    A study aimed at producing a system of classification of plants which best 
reflects the totality of their similarities and differences. The system is 
hierarchical, with the basic units being grouped into progressively larger and 
more inclusive units. 
    Most taxonomists take the species to be the basic unit of classification. 
In general, a species may be described as a self-perpetuating natural 
population which is permanently (in terms of human time) distinct and 
distinguishable from other such populations. In a typical species all 
individuals are capable of interbreeding to produce fertile offspring either 
directly or through other individuals of the same group, whereas such 
interbreeding between members of different species is restricted or prevented 
entirely by internal or external natural factors.  See  SPECIES CONCEPT. 
    A group of similar species is called a genus, a group of genera a family, 
a group of families an order, a group of orders a class, and a group of 
classes a division. Taxonomists sometimes find it useful to introduce 
intermediate categories, such as subgenus or subfamily, but these are optional 
groupings not required in the formal hierarchy. Conversely, it is sometimes 
useful to have an order with a single family or a family with a single genus. 
    The taxonomic system is a general-purpose system which seeks to balance 
the needs of all users by putting together those things that are most alike in 
all respects. This truly natural system is the ultimate but never fully 
attainable goal of taxonomists. Even if all the information that might be 
desired were available, taxonomists would often differ about how best to 
devise a natural system, but the range of choice would be limited and small. 
Artificial systems, on the other hand, based on a few arbitrarily chosen 
characters, are easily produced. There can be as many artificial systems as 
there are systematists and ends to be served. Modern taxonomists always prefer 
the natural system, falling back on an artificial system only when they feel 
unable to produce a reasonably natural one. 
    In an effort to assure the naturalness of the classification, taxonomists 
commonly rely heavily on the phylogenetic concept. An attempt is made to set 
up the system in such a way that every taxon (taxonomic group) has a unified 
ancestry. Such a group can be expected to be more homogeneous in features as 
yet unstudied than one which originated from diverse ancestors. The fossil 
record of plants is too fragmentary, however, and the critically important 
features are too seldom preserved to permit the kind of phylogenetic certitude 
that might be desired. In practice, if the taxonomist can devise a scheme that 
permits a logically possible phylogeny, without internal contradictions, the 
phylogenetic criterion is considered to be satisfied. The development of a 
natural classification and a phylogenetic scheme thus go hand in hand, each 
influencing the other. For many taxonomists the determination of probable 
phylogenetic relationships by this indirect method provides the chief stimulus 
and intellectual satisfaction for the work.  See  PLANT EVOLUTION; PLANT 
KINGDOM; PLANT PHYLOGENY. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPlant viruses and viroids   �21m �33s 
    Plant viruses are pathogens which are composed mainly of a nucleic acid 
(genome) normally surrounded by a protein shell (coat); they replicate only in 
compatible cells, usually with the induction of symptoms in the affected 
plant. Viroids are the smallest pathogens known, consisting of a 
single-stranded RNA with a molecular weight of about 120,000. 
    About 400 plant viruses are known. The genomes of most plant viruses, such 
as the tobacco mosaic virus (TMV) or the potato virus Y, are single-stranded 
RNAs; some RNA viruses (for instance, the wound tumor virus) have 
double-stranded RNA genomes. Cauliflower mosaic virus and bean golden mosaic 
virus are examples of viruses having double-stranded and single-stranded DNA. 
The genome of many plant viruses is a single piece and it is contained in a 
single particle whereas those of brome mosaic and some other viruses are 
segmented and distributed between several particles. There are also several 
low-molecular-weight RNAs (satellite RNAs) which depend on helper viruses for 
their replication.  See  RIBONUCLEIC ACID (RNA). 
    Less than a dozen viroids are known, but they cause very serious diseases 
in such diverse plants as chrysanthemum, citrus, coconut, and potato. Their 
replication is poorly understood; the available evidence does not allow 
discrimination between synthesis on an RNA (that is, viroid) template or 
synthesis on a DNA (that is, host genome) template.  See  PLANT PATHOLOGY; 
VIROIDS; VIRUS. 
 �34s[R. I. Hamilton]
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�6v�2v�32sPlant-water relations   �21m �33s 
    Water is the most abundant constituent of all physiologically active plant 
cells. Leaves, for example, have water contents which lie mostly within a 
range of 55 - 85% of their fresh weight. Other relatively succulent parts of 
plants contain approximately the same proportion of water, and even such 
largely nonliving tissues as wood may be 30 - 60% water on a fresh-weight 
basis. The smallest water contents in living parts of plants occur mostly in 
dormant structures, such as mature seeds and spores. The great bulk of the 
water in any plant constitutes a unit system. This water is not in a static 
condition. Rather it is part of a hydrodynamic system, which in terrestrial 
plants involves absorption of water from the soil, its translocation 
throughout the plant, and its loss to the environment, principally in the 
process known as transpiration. 
  
    Cellular water relations.  The typical mature, vacuolate plant cell 
constitutes a tiny osmotic system, and this idea is central to any concept of 
cellular water dynamics. Although the cell walls of most living plant cells 
are quite freely permeable to water and solutes, the cytoplasmic layer that 
lines the cell wall is more permeable to some substances than to others.  
    If a plant cell in a flaccid condition - one in which the cell sap exerts 
no pressure against the encompassing cytoplasm and cell wall - is immersed in 
pure water, inward osmosis of water into the cell sap ensues. This gain of 
water results in the exertion of a turgor pressure against the protoplasm, 
which in turn is transmitted to the cell wall. This pressure also prevails 
throughout the mass of solution within the cell. If the cell wall is elastic, 
some expansion in the volume of the cell occurs as a result of this pressure, 
although in many kinds of cells this is relatively small. 
    If a turgid or partially turgid plant cell is immersed in a solution with 
a greater osmotic pressure than the cell sap, a gradual shrinkage in the 
volume of the cell ensues; the amount of shrinkage depends upon the kind of 
cell and its initial degree of turgidity. When the lower limit of cell wall 
elasticity is reached and there is continued loss of water from the cell sap, 
the protoplasmic layer begins to recede from the inner surface of the cell 
wall. Retreat of the protoplasm from the cell wall often continues until it 
has shrunk toward the center of the cell, the space between the protoplasm and 
the cell wall becoming occupied by the bathing solution. This phenomenon is 
called plasmolysis.   See  OSMOREGULATORY MECHANISMS. 
    In some kinds of plant cells movement of water occurs principally by the 
process of imbibition rather than osmosis. The swelling of dry seeds when 
immersed in water is a familiar example of this process.  
  
    Stomatal mechanism.  Various gases diffuse into and out of physiologically 
active plants. Those gases of greatest physiological significance are carbon 
dioxide, oxygen, and water vapor. The great bulk of the gaseous exchanges 
between a plant and its environment occurs through tiny pores in the epidermis 
called stomates. Although stomates occur on many aerial parts of plants, they 
are most characteristic of, and occur in greatest abundance in, leaves.  See  
EPIDERMIS (PLANT); LEAF. 
  
    Transpiration process.  The term transpiration is used to designate the 
process whereby water vapor is lost from plants. Although basically an 
evaporation process, transpiration is complicated by other physical and 
physiological conditions prevailing in the plant. Whereas loss of water vapor 
can occur from any part of the plant which is exposed to the atmosphere, the 
great bulk of all transpiration occurs from the leaves. There are two kinds of 
foliar transpiration: (1) stomatal transpiration, in which water vapor loss 
occurs through the stomates, and (2) cuticular transpiration, which occurs 
directly from the outside surface of epidermal walls through the cuticle. In 
most species 90% or more of all foliar transpiration is of the stomatal type.  
See  EVAPOTRANSPIRATION. 
    Transpiration is a necessary consequence of the relation of water to the 
anatomy of the plant, and especially to the anatomy of the leaves. Terrestrial 
green plants are dependent upon atmospheric carbon dioxide for their survival. 
In terrestrial vascular plants the principal carbon dioxide - absorbing 
surfaces are the moist mesophyll cells walls which bound the intercellular 
spaces in leaves. Ingress of carbon dioxide into these spaces occurs mostly by 
diffusion through open stomates. When the stomates are open, outward diffusion 
of water vapor unavoidably occurs, and such stomatal transpiration accounts 
for most of the water vapor loss from plants. Although transpiration is thus, 
in effect, an incidental phenomenon, it frequently has marked indirect effects 
on other physiological processes which occur in the plant because of its 
effects on the internal water relations of the plant. 
  
    Water translocation.  In terrestrial rooted plants practically all of the 
water which enters a plant is absorbed from the soil by the roots. The water 
thus absorbed is translocated to all parts of the plant. The mechanism of the 
``ascent of sap'' (all translocated water contains at least traces of solutes) 
in plants, especially tall trees, was one of the first processes to excite the 
interest of plant physiologists. 
    The upward movement of water in plants occurs in the xylem, which, in the 
larger roots, trunks, and branches of trees and shrubs, is identical with the 
wood. In the trunks or larger branches of most kinds of trees, however, sap 
movement is restricted to a few of the outermost annual layers of wood.  See  
XYLEM.  
    Root pressure is generally considered to be one of the mechanisms of 
upward transport of water in plants. While it is undoubtedly true that root 
pressure does account for some upward movement of water in certain species of 
plants at some seasons, various considerations indicate that it can be only a 
secondary mechanism of water transport. 
    Upward translocation of water (actually a very dilute sap) is engendered 
by an increase in the negativity of water potential in the cells of apical 
organs of plants. Such increases in the negativity of water potentials occur 
most commonly in the mesophyll cells of leaves as a result of transpiration. 
  
    Water absorption.  The successively smaller branches of the root system of 
any plant terminate ultimately in the root tips, of which there may be 
thousands and often millions on a single plant. Most absorption of water 
occurs in the root tip regions, and especially in the root hair zone. Older 
portions of most roots become covered with cutinized or suberized layers 
through which only very limited quantities of water can pass.  See  ROOT. 
    Whenever the water potential in the peripheral root cells is less than 
that of the soil water, movement of water from the soil into the root cells 
occurs. There is some evidence that, under conditions of marked internal water 
stress, the tension generated in the xylem ducts will be propagated across the 
root to the peripheral cells. If this occurs, water potentials of greater 
negativity could develop in peripheral root cells than would otherwise be 
possible. The absorption mechanism would operate in fundamentally the same way 
whether or not the water in the root cells passed into a state of tension. The 
process just described, often called passive absorption, accounts for most of 
the absorption of water by terrestrial plants. 
    The phenomenon of root pressure represents another mechanism of the 
absorption of water. This mechanism is localized in the roots and is often 
called active absorption. Water absorption of this type only occurs when the 
rate of transpiration is low and the soil is relatively moist. Although the 
xylem sap is a relatively dilute solution, its osmotic pressure is usually 
great enough to engender a more negative water potential than usually exists 
in the soil water when the soil is relatively moist. A gradient of water 
potentials can thus be established, increasing in negativity across the 
epidermis, cortex, and other root tissues, along which the water can move 
laterally from the soil to the xylem.  See  PLANT MINERAL NUTRITION. 
 �34s[Bernard S. Meyer]
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�6v�2v�32sPlasma chromatography   �21m �33s 
    A trace analysis technique in which trace molecules in a gas are detected 
by subjecting them to an ion-molecule reation and measuring the ion-molecules 
produced. It is capable of detecting and identifying a substance present in 
concentrations far below the parts-per-billion range. The instrumentation has 
been applied to a wide range of fundamental and analytical studies. Upper 
atmospheric ionmolecule reactions have been studied as well as many aspects of 
chemical analysis, such as the identification of specific organic molecules, 
biomedical analysis of large molecules, and pesticide analysis. 
    The great sensitivity of the method to trace compounds arises because the 
ion-molecule reaction occurs at atmospheric pressure, where millions of 
collisions of reactant ions with trace molecules are possible. This all occurs 
in the plasma chromatograph, where ion-molecules are created and then 
separated by drifting them through an electrical field toward a detector. The 
detector output of ion current versus time presents a plasmagram of the 
reactant ion and the different ion-molecules formed. Plasmagrams, both 
positive and negative, resemble chromatograms with a millisecond time scale. 
This technique uses a shutter grid to inject a discrete ion-molecule pulse 
into the drift region and a gating grid to produce a recordable scan of the 
plasmagram (see illustration).  See  CHROMATOGRAPHY; GAS CHROMATOGRAPHY; MASS 
SPECTROMETRY; TRACE ANALYSIS. 
 �34s[F. W. Karasek]
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�6v�2v�32sPlasma diagnostics   �21m �33s 
    The branch of science dealing with the experimental investigation of 
plasmas. Plasmas may appear in a stationary or in a flowing state, the latter 
being either in a steady or an unsteady motion. 
    Diagnostics deals with the determination of various physical properties, 
which are subdivided into microscopic properties, such as collision frequency, 
and macroscopic properties. The macroscopic properties are either static (such 
as density, pressure,and temperature); or transport (such as viscosity, 
diffusivity, heat conductivity, and electrical conductivity).  See  PLASMA 
PHYSICS.  
    In addition, there are many other variables, for example, electrical 
(current, voltage,and power) and dynamic (mass flow, velocity, thrust, and 
others).  
    Very few methods of measurement used in the other branches of fluid 
dynamics, for instance, aerodynamics, can be used in plasmas.  See  
AERODYNAMICS.  
    The two basic types of diagnostic methods are those measuring global 
quantities and those measuring local ones. The local quantities are measured 
as functions of both space and time. This is not always easy and sometimes 
mean values may be obtained as a compromise. 
    Methods of measurement lie between two extremes, namely, those interfering 
with the flow and those not interfering with it. The former group employs 
solid probes; the latter uses microwaves, optical spectroscopy, particle 
beams, and lasers. Methods for measuring global electrical quantities are well 
known from electrical engineering.  See  ELECTRICAL MEASUREMENTS. 
    Many diverse methods can be used in plasma diagnostics, including probe, 
optical, spectroscopic, microwave, laser, and energetic particle beam methods. 
    Probe methods have a limited applicability, since they usually disturb the 
very quantities intended to be measured. The various types of probes represent 
different degrees of disturbances to the flow and also possess different 
ranges of operation. Probes constitute the most common method used to obtain 
detailed information of the internal structure of plasmas. Sometimes the 
introduction of probes into a plasma is not permissible because the probe may 
be damaged by sputtering. Impurities introduced by the evaporation of probe 
material contaminate and cool the flow. If the probe interferes with the 
plasma flow, sometimes it may be used outside the plasma, that is, between the 
flow and the solid wall. 
    Spectroscopic methods may be applied in the optical, ultraviolet, and 
x-ray regions of the spectrum. These methods are based on emission spectra. 
They employ photographic and photoelectric recording equipment. Microwave 
methods use radiation which may be considered as close to equilibrium 
radiation. These methods utilize instruments called microwave interferometers. 
Thomson scattering of laser light may serve as a diagnostic method. In 
concept, the idea is similar to radar.  See  INTERFEROMETRY; LASER; MICROWAVE 
SPECTROSCOPY.  
 �34s[Demetrios G. Samaras]
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�6v�2v�32sPlasmal �21m �33s
    Those aldehydic components of lipids which give positive color tests with
reagents used for detecting aldehydes in tissues. The Feulgen test for
plasmalogens depends on the liberation of plasmal, principally derivatives of
palmitaldehyde and stearaldehyde, from these lipids by the action of mercuric
chloride and acetic acid.  See	LIPID; PHOSPHATIDE.
 �34s[Roy H. Gigg ;Herbert E. Carter]
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�6v�2v�32sPlasma physics   �21m �33s 
    That field of physics which relates to the study of highly ionized gases. 
A gas which is composed of a nearly equal number of positive and negative free 
charges (positive ions and electrons) is called a plasma. Because it is 
composed of charged particles, a plasma exhibits many phenomena not 
encountered in ordinary gases. In addition to their importance in many new 
areas of applied science, these effects are evident in astrophysical phenomena 
(most of the matter in this universe exists in the plasma state), both in 
stellar atmospheres and in interstellar space. Plasma phenomena have also been 
observed in the tenuous ionized gases of the Earth's outer atmosphere.  See  
COSMIC ELECTRODYNAMICS. 
    Practical interest in plasma physics arises from various applications of 
gas discharges, from the study of electron beams in electron tubes, and from 
the new research fields of ultra-high-temperature processes and controlled 
fusion. These latter experiments require millions or even hundreds of millions 
of degrees kinetic temperature (to be defined later). To obtain a 
comprehensive physical picture of plasma it is necessary to consider its 
behavior from two different aspects: 
    1. The microscopic picture,  relating to its particlelike properties, such 
as the effects of interparticle collisions in producing diffusion and other 
transport phenomena, ionization, x-radiation, and other particulate processes. 
In this picture a plasma exhibits properties some of which are much like those 
of any gas. 
    2. The macroscopic picture, where the collective or fluidlike properties 
are most evident. These properties include conduction of electricity, 
propagation of various kinds of waves, and ability to support classes of 
unstable and turbulent behavior peculiar to conducting fluids. Many of the 
macroscopic behavioral properties of plasma are related to the general field 
of magnetohydrodynamics.  See  MAGNETOHYDRODYNAMICS. 
    The charged particles of a plasma interact with each other through the 
electrostatic or Coulomb field with which each is surrounded. On the 
microscopic scale, these electrostatic fields give rise to localized 
attractive or repulsive forces between the particles as they pass near to each 
other, resulting in mutual deflection. On the macroscopic scale, the summation 
of the many infinitesimal electrostatic and magnetic fields produced by the 
moving plasma particles results in a smeared-out or averaged electromagnetic 
field. The plasma then reacts collectively, that is, as a conducting fluid, to 
the total electromagnetic field in which it is immersed. This field consists 
of the combination of the plasma electromagnetic field and any externally 
imposed fields. The coupled nature of the plasma motion and the 
electromagnetic field in which it moves causes most of the complexity of 
plasma behavior.  See  NUCLEAR FUSION; PINCH EFFECT. 
 �34s[Richard F. Post]
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�6v�2v�32sPlasmin   �21m �33s 
    A proteolytic enzyme which can digest many proteins through the process of 
hydrolysis. Plasmin (fibrinolysin) is found in plasma in the form of the inert 
precursor, or zymogen, plasminogen (profibrinolysin); its site of synthesis in 
the body is unknown. Plasma also contains several inhibitors which limit the 
action of plasmin. 
    Plasma itself has the potentiality of activating plasminogen, perhaps 
because it contains intrinsic activators. Plasmin activator is produced in 
blood vessel walls, from which it is released following vascular injury. 
Further, the process of blood coagulation may foster the activation of 
plasminogen. Moreover, the presence of a fibrin clot enhances activation of 
plasminogen by many agents, perhaps because plasminogen is adsorbed to the 
fibrin and thus separated from its inhibitors in the plasma. 
    Plasmin can act on many protein substrates. It liquefies coagulated blood 
by digesting fibrin (fibrinolysis), the insoluble meshwork of the clot; it 
also digests fibrinogen, the precursor of fibrin, rendering it incoagulable. 
The digestion products of fibrinogen and fibrin are anticoagulant substances 
which further interfere with the clotting process. Plasmin also inactivates 
several other protein procoagulant factors, particularly proaccelerin (factor 
V) and antihemophilic factor (factor VIII). Other substrates attacked by 
plasmin include gamma globulin and several protein or polypeptide hormones 
found in plasma. 
    The action of plasmin may be related to certain body defenses. It can 
convert the first component of complement to a proteolytic enzyme. It can also 
liberate polypeptide kinins (for example, bradykinin) from plasma precursors. 
The kinins can reproduce such elements of the inflammatory process as pain, 
dilatation and increased permeability of small blood vessels, and the 
migration of leukocytes. Besides these physiological actions, plasmin also 
hydrolyzes casein, gelatin, denatured hemoglobin, and certain synthetic esters 
of arginine and lysine, for example, p-toluenesulfonylarginine methyl ester 
(TAME).  See  BLOOD; COMPLEMENT; FIBRINOGEN; IMMUNOLOGY; INFLAMMATION. 
 �34s[Oscar D. Ratnoff]
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�6v�2v�32sPlasmodiophorida   �21m �33s 
    Protozoa composing an order of Mycetozoia. They are endoparasitic in 
plants, primarily causing, for example, club root of cabbage, powdery scab of 
potatoes, and galls in Ruppia. Underground portions of host plants are invaded 
by young parasites, often a flagellate which sometimes arises from a freshly 
excysted ameba. Becoming intracellular, the young parasite grows and develops 
into a plasmodium. At maturity, the plasmodium produces uninucleate cysts 
(spores) which are released upon degeneration of the damaged cell. Under 
favorable conditions, the released spores hatch into uninucleate stages which 
become the infective forms. In the invaded area, the host's tissue commonly 
undergoes hypertrophy to form a gall.  See  MYCETOZOIA. 
 �34s[Richard P. Hall]
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�6v�2v�32sPlasmodiophoromycetes   �21m �33s 
    A class of slime molds (Myxomycota) with one order, Plasmodiophorales, and 
one family, Plasmodiophoraceae. These fungi are microscopic and occur in 
aquatic or semiaquatic habitats. They are obligate parasites of algae, 
vascular plants, and aquatic fungi. They are characterized by the formation of 
a naked multinucleate mass of protoplasm called a plasmodium. The plasmodium 
can migrate by amoeboid movement from cell to cell in the host, increasing its 
volume as nuclear division occurs. At a certain size it cleaves into several, 
usually multinucleate parts. Each part forms a membrane-bound reproductive 
structure (sporangium). When the sporangium is mature, motile spores 
(zoospores) are formed within it. Subsequently zoospores are discharged 
outside the host through pores in the host cell that are formed by the fungus. 
The zoospores have two flagella of unequal length, both inserted anteriorly 
and of the whiplash type. The flagella and an amoeboid motion carry the 
zoospores to the host plant, where they encyst. Subsequently the encysted 
zoospore produces a germ tube that penetrates a host cell, forming a 
plasmodium within it. Sexual reproduction is known in some species. 
    Serious diseases caused by members of the class are club root of crucifers 
and powdery scab of potato. All members of the class cause enlargement of the 
infected host tissue. Their distribution is worldwide.  See  MYXOMYCOTA. 
 �34s[Robert A. Paterson]
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�6v�2v�32sPlaster   �21m �33s 
    A plastic mixture of solids and water which sets to a hard, coherent solid 
and which is used to line the interiors of buildings. A similar material of 
different composition, used to line the exteriors of buildings, is known as 
stucco. The term plaster is also used in the industry to designate plaster of 
paris.  
    Plaster is usually applied in one or more base (rough or scratch) coats up 
to ** in. (1.9 cm) thick, and also in a smooth, white, finish coat about ** 
in. (0.16 cm) thick. The solids in the base coats are hydrated (or slaked) 
lime, sand, fiber or hair (for bonding), and portland cement (the last may be 
omitted in some plasters). The finish coat consists of hydrated lime and 
gypsum plaster (in addition to the water).  See  LIME (INDUSTRY); MORTAR; 
PLASTER OF PARIS. 
 �34s[John F. McMahon]
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�6v�2v�32sPlaster of paris   �21m �33s 
    The hemihydrate of calcium sulfate, composition CaSO4 PRODUCT **H2O, made 
by calcining the mineral gypsum, composition CaSO4 PRODUCT 2H2O, at 
temperatures up to 480DGF (250DGC). It is used for making plasters, molds, and 
models.  
    When the powdered hemihydrate is mixed with water to form a paste or 
slurry, the calcining reaction is reversed and a solid mass of interlocking 
gypsum crystals with moderate strength is formed. Upon setting there is very 
little (a slight contraction) dimensional change, making the material suitable 
for accurate molds and models. 
    Diverse types of plaster, varying in the time taken to set, the amount of 
water needed to make a pourable slip, and the final hardness, are made for 
different applications. These characteristics are controlled by the 
calcination conditions (temperature and pressure) and by additions to the 
plaster.  See  GYPSUM; PLASTER. 
 �34s[John F. McMahon]
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�6v�2v�32sPlasticity   �21m �33s 
    The property of a solid body whereby it undergoes a permanent change in 
shape or size when subjected to a stress exceeding a particular value, called 
the yield value. Many solid materials obey Hooke's law at low stress. As the 
stress is increased, however, departures from Hooke's law occur, and some 
plastic flow takes place; that is, the material does not completely recover 
its original shape or size when the stress is released. 
    Plastic behavior is often accompanied by time-dependent effects such as 
creep (the increase in strain with time at constant stress), stress relaxation 
(the decay of stress with time at constant strain), and elastic aftereffect or 
recovery (the gradual decrease to a limiting permanent strain when the stress 
is removed). The study of these phenomena in all their manifestations is the 
science of rheology. For the influence of dislocations on plastic flow in 
crystals  see  CRYSTAL.  See also  CREEP (MATERIALS); ELASTICITY; HOOKE'S LAW; 
RHEOLOGY; STRESS AND STRAIN. 
 �34s[R. F. S. Hearmon]
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�6v�2v�32sPlastics processing   �21m �33s 
    Those methods and techniques used to convert plastics materials in the 
form of pellets, granules, powders, sheets, fluids, or preforms into formed 
shapes or parts. The plastic materials may contain a variety of additives 
which influence the properties as well as the processability of the plastics. 
After forming, the part may be subjected to a variety of ancillary operations 
such as welding, adhesive bonding, machining, and surface decorating 
(painting, metallizing). 
  
    Injection moldings.  Injection molding consists of heating and 
homogenizing plastics granules in a cylinder until they are sufficiently fluid 
to allow for pressure injection into a relatively cold mold where they 
solidify and take the shape of the mold cavity. Solid particles, in the form 
of pellets or granules, constitute the main feed for injection moldable 
plastics. The major advantages of the injection-molding process are the speed 
of production, minimal requirements for postmolding operations, and 
simultaneous multipart molding. 
  
    Extrusion.  In this process, plastic pellets or granules are fluidized, 
homogenized, and continuously formed. Products made this way include tubing, 
pipe, sheet, wire and substrate coatings, and profile shapes. The process is 
used to form very long shapes or a large number of small shapes which can be 
cut from the long shapes. Pellets used for other processing methods, such as 
injection  molding, are made by chopping long filaments of extruded plastic.  
See  EXTRUSION. 
  
    Blow molding.  This process consists of forming a tube (called a parison) 
and introducing air or other gas to cause the tube to expand into a free-blown 
hollow object or against a mold for forming into a hollow object with a 
definite size and shape. The parison is traditionally made by extrusion, 
although injection-molded tubes have been introduced.  
  
    Thermoforming.  Thermoforming is the forming of plastics sheets into parts 
through the application of heat and pressure. Tooling for this process is the 
most inexpensive compared to other plastics processess. However, it can 
accommodate very large parts as well as small parts. 
  
    Rotational molding.  In this process, finely ground powders are heated in 
a rotating mold until melting or fusion occurs. If liquid materials are used, 
the process is often called slush molding. The melted or fused resin uniformly 
coats the inner surface of the mold. When cooled, a hollow finished part is 
removed.  
  
    Compression and transfer molding.  Compression molding consists of 
charging a plastics powder or preformed plug into a mold cavity, closing a 
mating mold half, and applying pressure to compress, heat, and cause flow of 
the plastic to conform to the cavity shape. 
  
    Foam processes.  Foamed plastics materials have achieved a high degree of 
importance in the plastics industry. Foams can be made soft and flexible to 
hard and rigid. There are three types of cellular plastics: blown (expanded 
matrix, such as a natural sponge), syntactic (the encapsulation of hollow 
organic or inorganic microspheres in the matrix), and structural (dense outer 
skin surrounding a foamed core). 
    There are seven basic processes used to generate plastics foams. They 
include: the incorporation of a chemical blowing agent that generates gas 
(through thermal decomposition) in the polymer liquid or melt; gas injection 
into the melt which expands during pressure relief; generation of gas as a 
by-product of a chemical condensation reaction during cross-linking; 
volatilization of a low-boiling liquid (for example, Freon) through the 
exothermic heat of reaction; mechanical dispersion of air by mechanical means 
(whipped cream); incorporation of nonchemical gas-liberating agents (adsorbed 
gas on finely divided carbon) into the resin mix which is released by heating; 
and expansion of small beads of thermoplastic resin containing a blowing agent 
through the external application of heat. 
  
    Casting and encapsulation.  Casting is a low-pressure process requiring 
nothing more than a container in the shape of the desired part. For 
thermoplastics, liquid monomer is poured into the mold and, with heat, allowed 
to polymerize in place to a solid mass. For vinyl plastisols, the liquid is 
fused with heat. Thermosets are poured into a heated mold wherein the 
cross-linking reaction completes the conversion to a solid. Encapsulation and 
potting are terms for casting processes in which a unit or assembly is encased 
or impregnated, respectively, with a liquid plastic which is subsequently 
hardened by fusion or chemical reaction. 
  
    Calendering.  In the calendering process, a plastic is masticated between 
two rolls that squeeze it out into a film which then passes around one or more 
additional rolls before being stripped off as a continuous film. Fabric or 
paper may be fed through the latter rolls so that they become impregnated with 
the plastic.  See  POLYMER. 
 �34s[Sidney H. Goodman]
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�6v�2v�32sPlateau   �21m �33s 
    Any elevated area of relatively smooth land. Usually the term is used more 
specifically to denote an upland of subdued relief that on at least one side 
drops off abruptly to adjacent lower lands. In most instances the upland is 
cut by deep but widely separated valleys or canyons. Small plateaus that stand 
above their surroundings on all sides are often called tables, tablelands, or 
mesas. The abrupt edge of a plateau is an escarpment or, especially in the 
western United States, a rim. 
 �34s[Edwin H. Hammond]
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�6v�2v�32sPlate girder   �21m �33s 
    A beam built up of steel plates and shapes which may be welded or bolted 
together to form a deep beam larger than can be produced by a rolling mill 
(see illustration). As such, it is capable of supporting greater loads on 
longer spans. The typical welded plate girder consists of flange plates welded 
to a deep web plate. A bolted configuration consists of flanges built of 
angles and cover plates bolted to the web plate. Both types may have vertical 
stiffeners connected to the web plate, and both may have additional cover 
plates on the flanges to increase the load capacity of the member. Box girders 
consist of common flanges connected to two web plates, forming a closed 
section.  
    In general, the depth of plate girders is one-tenth to one-twelfth of the 
span length, varying slightly for heavier or lighter loads. On occasion, the 
depth may be controlled by architectural considerations. 
  
    Stiffeners, plates or angles, may be attached to the girder web by welding 
or bolting to increase the buckling resistance of the web. Stiffeners are also 
required to transfer the concentrated forces of applied loads and reactions to 
the web without producing local buckling. 
    Splices are required for webs and flanges when full lengths of plates are 
not available from the mills or when shorter lengths are more readily 
fabricated. Splices provide the necessary continuity required in the web and 
flanges.  
 �34s[John B. Scalzi]
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�6v�2v�32sPlate tectonics �21m �33s
    Plate tectonics theory provides an explanation for the present-day
tectonic behavior of the Earth, particularly the global distribution of
mountain building, earthquake activity, and volcanism in a series of linear
belts. Numerous other geological phenomena such as lateral variations in
surface heat flow, the physiography and geology of ocean basins, and various
associations of igneous, metamorphic, and sedimentary rocks can also be
logically related by plate tectonics theory.
    The theory is based on a simple model of the Earth in which a rigid outer
shell 30 - 93 mi (50 - 150 km) thick, the lithosphere, consisting of both
oceanic and continental crust as well as the upper mantle, is considered to
lie above a hotter, weaker semiplastic asthenosphere. The asthenosphere, or
low-velocity zone, extends from the base of the lithosphere to a depth of
about 430 mi (700 km). The brittle lithosphere is broken into a mosaic of
internally rigid plates which move horizontally across the Earth's surface
relative to one another. Only a small number of major lithospheric plates
exist, which grind and scrape against each other as they move independently
like rafts of ice on water. Most dynamic activity such as seismicity,
deformation, and the generation of magma occur only along plate boundaries,
and it is on the basis of the global distribution of such tectonic phenomena
that plates are delineated.  See  ASTHENOSPHERE; LITHOSPHERE.
    The plate tectonics model for the Earth is consistent with the occurrence
of sea-floor spreading and continental drift. Convincing evidence exists that
both these processes have been occurring for at least the last 6 x 10(sup 8)
years. This evidence includes the magnetic anomaly patterns of the sea floor,
the paucity and youthful age of marine sediment in the ocean basins, the
topographic features of the sea floor, and the indications of shifts in the
position of continental blocks which can be inferred from paleomagnetic data
on paleopole positions, paleontological and paleoclimatological observations,
the match-up of continental margins and geological provinces across
present-day oceans, and the structural style and rock types found in ancient
mountain belts.
    Geological observations, geophysical data, and theoretical considerations
support the existence of three fundamentally distinct types of plate
boundaries, named and classified on the basis of whether immediately adjacent
plates move apart from one another (divergent plate margins), toward one
another (convergent plate margins), or slip past one another in a direction
parallel to their common boundary (transform plate margins). The boundaries of
plates can, but need not, coincide with the contact between continental and
oceanic crust. The velocity at which plates move varies from plate to plate
and within portions of the same plate, ranging between 0.8 and 8 in. (2 and 20
cm) per year.
    Not only does plate tectonics theory explain the present-day distribution
of seismic and volcanic activity around the globe and physiographic features
of the ocean basins such as trenches and mid-oceanic rises, but most Mesozoic
and Cenozoic mountain belts appear to be related to the convergence of
lithospheric plates. Two different varieties of modern mobile belts have been
recognized, cordilleran type and collision type. The Cordilleran range, which
forms the western rim of North and South America (the Rocky Mountains, Pacific
Coast ranges, and the Andes) have for the most part been created by the
underthrusting of an ocean lithospheric plate beneath a continental plate.
Underthrusting along the Pacific margin of South America is causing the
continued formation of the Andes. The Alpine-Himalayan belt, formed where the
collision of continental blocks buckled intervening volcanic belts and
sedimentary strata into tight folds and faults, is an analog of the present
tectonic situation in the Mediterranean, where the collision of Africa and
Europe has begun.
    Plate tectonics is considered to have been operative as far back as 2.5 x
10(sup 9) years. Prior to that interval, evidence suggests that plate tectonics
may have occurred, although in a markedly different manner, with higher rates
of global heat flow producing smaller convective cells or more densely
distributed mantle plumes which fragmented the Earth's surface into numerous
small, rapidly moving plates.  See  CONTINENTAL DRIFT; OROGENY.
 �34s[Walter C. Pitman, III]
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�6v�2v�32sPlatinum   �21m �33s 
    A chemical element, Pt, atomic number 78, and atomic weight 195.09. 
Platinum is a soft, ductile, white noble metal. The platinum-group metals - 
platinum, palladium, iridium, rhodium, osmium, and ruthenium - are found 
widely distributed over the Earth. Their extreme dilution, however, precludes 
their recovery, except in special circumstances. 
    The platinum-group metals have wide chemical use because of their 
catalytic activity and chemical inertness. As a catalyst, platinum is used in 
hydrogenation, dehydrogenation, isomerization, cyclization, dehydration, 
dehalogenation, and oxidation reactions.  See  CATALYSIS; ELECTROCHEMICAL 
PROCESS.  
    Platinum is not affected by atmospheric exposure, even in sulfur-bearing 
industrial atmospheres. Platinum remains bright and does not visually exhibit 
an oxide film when heated, although a thin, adherent film forms below 842DGF 
(450DGC). Hydrogen or other reducing atmospheres are not directly harmful to 
platinum at elevated temperatures. Platinum may be worked to fine wire and 
thin sheet, and by special processes, to extremely fine wire. Important 
physical properties are given in the table. 
    Platinum can be made into a spongy form by thermally decomposing ammonium 
chloroplatinate or by reducing it from an aqueous solution. In this form it 
exhibits a high absorptive power for gases, especially oxygen, hydrogen, and 
carbon monoxide. The high catalytic activity of platinum is related directly 
to this property. Platinum strongly tends to form coordination compounds.  See 
  CRACKING; HYDROGENATION. 
    Platinum dioxide, PtO2, is a dark-brown insoluble compound, commonly known 
as Adams catalyst. Platinum(II) chloride, PtCl2, is an olive-green 
water-insoluble solid. Chloroplatinic acid, H2PtCl6, is the most important 
platinum compound.  See  IRIDIUM; OSMIUM; PALLADIUM; RHODIUM; RUTHENIUM. 
 �34s[Henry J. Albert]
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�6v�2v�32sPlatyasterida   �21m �33s 
    An order of Asteroidea (starfishes) in which the living representatives 
have five to many flat straplike arms. Both the fossil forms and the living 
representatives have a well-developed ambulacral groove like other starfishes. 
One row of marginal plates is developed around the arm. Fossil platyasterids 
appeared first in the Ordovician, but only one family survives today.  See  
ASTEROZOA; ECHINODERMATA. 
 �34s[Andrew C. Campbell]
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�6v�2v�32sPlatycopa   �21m �33s 
    A suborder of the order Podocopida (crustaceans of the subclass Ostracoda) 
including marine forms with two pairs of thoracic legs. It is also called 
Platycopina. Both fossil and living platycopines have the inner lamella of 
each valve calcified only in the distal rim, if at all; hence, if a narrow 
duplicature is developed, it is continuous with the outer lamella and does not 
form a vestibule (troughlike cavity) between the lamellae. Platycopines are 
limited to one living family (Cytherellidae). 
    All living species are benthonic, and fossil forms probably were also. 
Some species live at great depths in the Atlantic Ocean, but others have been 
found in the Arabian, Caribbean, and Mediterranean seas in shallower water.  
See  OSTRACODA. 
 �34s[Robert V. Kesling]
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�6v�2v�32sPlatyctenea   �21m �33s 
    An order of the ctenophores whose members are sedentary or parasitic. They 
often lack ribs in the adult stage. They are flattened due to the shortening 
of the main axis and by the extension of the pharynx into a creeping sole (see 
illustration). These organisms are beautifully colored. The primary tentacles 
are well developed and the canals branch profusely to form a network. Some are 
viviparous with the cydippid embryos developing in brood chambers formed by 
the expansion of the canals.  See  CTENOPHORA; TENTACULATA. 
 �34s[Taku Komai]
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�6v�2v�32sPlatyhelminthes   �21m �33s 
    A phylum of the invertebrates, commonly called the flatworms. They are 
bilaterally symmetrical, nonsegmented, dorsoventrally flattened worms 
characterized by lack of coelom, anus, circulatory and respiratory systems, 
and exo- or endoskeleton. They possess a protonephridial excretory system, a 
complicated hermaphroditic reproductive system, and a solid mesenchyme which 
fills the interior of the body (see illustration). Three classes occur in the 
phylum: (1) the Turbellaria, mainly free-living, predacious worms; (2) the 
Trematoda, or flukes, holozoic ecto- or endoparasites; and (3) the Cestoda, or 
tapeworms, saprozoic endoparasites in the enteron of vertebrates, whose larvae 
are found in the tissues of invertebrates or vertebrates. 
    Turbellaria are widespread in fresh water and the littoral zones of the 
sea, while one group of triclads occurs on land in moist habitats. Adult 
trematodes occur on, or in, practically all tissues and cavities of the 
vertebrates on which they feed. They are responsible for troublesome diseases 
in humans and animals. Larval flukes are frequent in mollusks, mainly 
gastropods, and occasionally occur in pelecypods. Vector hosts, such as 
insects and fish, are often interpolated between mollusk and vertebrate. Adult 
tapeworms, living in the enteron or the biliary ducts, compete with the host 
for food and accessory food factors such as vitamins. Larval tapeworms reside 
chiefly in arthropods, but larvae of one group, the Cyclophyllidea, develop in 
mammals, which may be severely impaired, or even killed, by the infection.  
See  CESTODA; TAPEWORM DISEASE; TREMATODA; TURBELLARIA. 
 �34s[Chauncey G. Goodchild]
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�6v�2v�32sPlatypus   �21m �33s 
    The single species Ornithorhynchus anatinus of the family 
Ornithorhynchidae in the order Monotremata, which occurs in eastern Australia 
and Tasmania. This mammal is a monotreme, which lays eggs and incubates them 
in a manner similar to birds. The platypus is also known as the duckbill, 
duckmole, and water mole. One of the most primitive mammals, it retains some 
reptilian characteristics. The female lacks a marsupium; however, marsupial or 
epipubic bones are well developed. 
    Ornithorhynchus is well adapted to aquatic life. It ranges from the 
tropical, sea-level streams to cold lakes at altitudes of 6000 ft (1800 m). It 
is covered with short dense fur, typical of many aquatic mammals; the external 
ears are lacking; and the tail is broad and flattened, resembling that of the 
beaver (see illustration). The rubbery bill is covered with a highly sensitive 
skin. There are no teeth, but horny ridges are used for crushing food and 
grubbing on the bottom of rivers.  See  MONOTREMATA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sPlaya   �21m �33s 
    The low, essentially flat part of a basin or other undrained area in an 
arid region. In heavy rains the playa may be temporarily covered with a 
shallow sheet of water and is then a playa lake. 
    Five principal types of playa have been recognized. The dry playa develops 
where the water table is below capillary reach of surface; its surface is 
hard, flat, and smooth, composed of silt and clay. The moist playa or salina 
occurs where the water table is within capillary reach of surface; this type 
may be further subdivided into (1) salt-encrusted playa, where the water table 
is at the surface or so near it that salt water evaporating on the surface 
leaves a salt crust (see illustration); and (2) clay-encrusted playa, where 
the water table is near the limit of capillary action and salt brought to the 
surface is mixed with silt and clay, forming a puffy surface (self-rising 
ground). The crystal-body playa is essentially a massive body of crystalline 
salt at or very close to the surface. The compound playa results from a water 
table at different levels in different parts; these exhibit characteristics of 
dry playa in one part and moist playa in another. The lime-pan playa is formed 
in basins receiving drainage from limestone terrain and has a floor of hard 
travertine.  See  SALINE EVAPORITE. 
 �34s[Thomas Clements]
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�6v�2v�32sPlecoptera   �21m �33s 
    An order of primitive insects known as the stoneflies. Except for wings 
and tracheal gills, there are relatively slight differences between aquatic 
immature stages and adult mature ones. The soft and somewhat flattened body, 
strong legs, paired tarsal claws, chewing mouthparts, and rusty blacks, dull 
yellows, and browns are characteristic of both immature and adult stages (see 
illustration).  
    Immature stoneflies live in the rapid, stony parts of clean, swift 
streams, although a few species occur along the rocky shores of large 
temperate lakes. 
    Adult stoneflies hold the wings close to the backs when at rest or 
walking. The hindwings are pleated and hidden. The name Plecoptera mean 
pleated wings. 
    In various parts of North America stonefly adults emerge in every month of 
the year. Successions of species reach their peaks of abundance from November 
to March, with some extending to August. Nymphs and adults of some species are 
plant feeders; others are carnivorous.  See  INSECTA. 
 �34s[Lewis Berner]
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�6v�2v�32sPlectoidea   �21m �33s 
    A superfamily of small free-living nematodes. They are characterized by 
simple spiral amphids or variants thereof (mushroom-shaped, 
hand-mirror-shaped, question-mark-shaped), elongate, cylindro-conoid stoma, an 
esophagus with a corpus, isthmus and valvulated basal bulb, and reflexed 
ovaries. They are found mainly in terrestrial habitats, sometimes in fresh 
water, and rarely marine.  See  NEMATA. 
 �34s[Bruce Hopper]
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�6v�2v�32sPleiades �21m �33s
    A beautiful group of stars resembling a little dipper, in the
constellation of Taurus, known since earliest records. The Pleiades is a
typical open cluster (see illustration). Its distance is 420 light-years (1 ly
= 9.46 x 10(sup 12) km), its linear diameter about 15 light-years, its age 5 x
10(sup 7) years. The cluster is permeated with diffuse nebulosity. Though early
accounts refer to the Pleiades in terms of seven stars, only six are now
conspicuous to the unaided eye, which raises a theory that one, the lost
Pleiad, has faded.
 �34s[Helen S. Hogg]
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�6v�2v�32sPleistocene   �21m �33s 
    A time-stratigraphic term referring to the sequence of geologic deposits 
(the Pleistocene Series) and a geologic-time term (the Pleistocene Epoch of 
the Quaternary Period) for the interval of time during which these sediments 
were deposited. 
    The lack of an agreed-upon definition has led to a concerted international 
effort to establish a stratotype section with a fixed lower boundary based on 
biostratigraphical data. The section will be located in the area of marine 
sediments in the province of Calabria in southern Italy. 
    The Pleistocene was characterized by numerous (at least 17), cyclic, 
worldwide changes of climate. These changes had a profound effect on the fauna 
and flora of the time, and on surficial geologic processes. Surficial 
sediments and landforms in many areas of the world date primarily from this 
interval of time.  See  GLACIAL EPOCH; QUATERNARY. 
 �34s[W. Hilton Johnson]
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�6v�2v�32sPleochroic halos   �21m �33s 
    Small halos of color or color differences that are sometimes observed 
around inclusions in minerals. If halos occur in doubly refracting substances, 
they may show pleochroic discoloration, and the term pleochroic halos is 
loosely applied to such small colored halos generally.  See  PLEOCHROISM. 
    The halos are found only around minute inclusions of certain minerals, 
especially zircon, allanite, monazite, and others known to contain minor 
amounts of uranium or thorium. Pleochroic halos are most widespread in 
minerals such as biotite, which is never known in the metamict state. Halos 
have been reported in many rock-making minerals, including amphiboles, 
pyroxenes, and micas. See RADIOACTIVE MINERALS. 
    The halos are usually spheroidal (circular, as seen in microscopic 
section), sometimes consisting of several concentric rings, and are fairly 
sharply bounded. 
 �34s[Patrick M. Hurley]
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�6v�2v�32sPleochroism   �21m �33s 
    In some colored transparent crystals, the effect wherein the color is 
quite different in different directions through the crystals. In such a 
crystal the absorption of light is different for different polarization 
directions. In colored transparent tourmaline the effect may be so strong that 
one polarized component of a light beam is wholly absorbed, and the crystal 
can be used as a polarizer.  See  DICHROISM; TRICHROISM. 
 �34s[Bruce H. Billings]
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�6v�2v�32sPleuracanthodii   �21m �33s 
    An order of Paleozoic sharklike fishes abundant in fresh-water deposits of 
the Carboniferous and Early Permian. Although primitive in cranial structures, 
the pleuracanths differ notably from other sharks in several regards (see 
illustration). The teeth are two-pronged; there is a long spine projecting 
upward and backward from the posterior part of the braincase; the tail extends 
directly backward, in contrast with the upturned heterocercal caudal fin of 
other sharks; and the paired fins have the archipterygial skeletal pattern of 
a central axis and side branches, in contrast with a fan-shaped arrangement in 
typical sharks.  See  ELASMOBRANCHII. 
 �34s[Alfred S. Romer]
�33s 
  
  
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�6v�2v�32sPleural diseases   �21m �33s 
    Primary pleural diseases are relatively infrequent. However, involvement 
of the pleura following or in conjunction with some other underlying disease, 
most often pulmonary, is common. Pleural adhesions may be found in as many as 
two-thirds of autopsies in a general hospital. Types of pleural disorders are 
inflammatory conditions, noninflammatory conditions, and tumors. 
    Pleuritis, or pleurisy, will have different characteristics depending on 
the underlying cause and the stage of the disease. Most instances of pleuritis 
accompany or follow inflammatory diseases of the lungs, including 
tuberculosis, bacterial pneumonia, lung infarcts, septicemia, rheumatic fever, 
and uremia. An important reaction is the appearance of fluid in the pleural 
cavity, but it is important to differentiate between the fluid secondary to 
true inflammation of the pleura (exudate) and that following cardiac failure 
(transudate).  
    Hemorrhagic pleuritis is characterized by the presence of bloody fluid and 
may be encountered in cases of hemorrhagic diseases or tumor involvement. 
    Hydrothorax is the accumulation of serous fluid in the pleural cavity, 
often bilaterally. Most cases are related to heart failure. Hemothorax is the 
accumulation of bloody fluid in the pleural cavity. Whether caused by 
traumatic injury or spontaneous rupture of an aneurysm, this is a serious 
emergency. Chylothorax is the accumulation of milky fluid, usually of 
lymphatic origin. The condition may be due to trauma, tumor obstruction of 
lymphatics, or congenital anomalies. Pneumothorax is the accumulation of air 
or gas in the pleural cavity. Spontaneous pneumothorax may follow the rupture 
of alveoli or bullae and is usually associated with emphysema, asthma, or 
tuberculosis. Traumatic pneumothorax is a feared complication of penetrating 
chest wounds. 
 �34s[Victor E. Goldenberg]
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�6v�2v�32sPleuromeiales   �21m �33s 
    An order of Early Triassic lycopods consisting of the genus Pleuromeia. 
The upright unbranched stem of the plant was about 3 ft (1 m) tall with 
grasslike leaves and a single terminal strobilus of overlapping, scalelike, 
round sporophylls that bore one sporangium each. From its four-lobed base 
sprang regularly spaced rootlets.  See  LYCOPODIOPSIDA. 
 �34s[Chester A. Arnold]
�33s 
  
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�6v�2v�32sPleuronectiformes   �21m �33s 
    One of the most distinctive orders of actinopterygian fishes, also called 
Heterosomata, composed of the flatfishes: halibut, plaice, flounders, soles, 
tonguesoles, and their allies. The striking feature of the group is the loss 
of bilateral symmetry (see illustration), a characteristic of almost all 
vertebrates. Like the ancestors of pleuronectiforms, young flatfishes are 
symmetrical and swim upright; early in life, however, one eye migrates across 
the top of the skull to lie on the same side as the other eye. This 
transformation is associated with deformation of the skull bones and nerves, a 
change in position so that the fish lies on one (the blind) side, partial or 
complete depigmentation of the blind surface, and sometimes modification and 
development of asymmetry in paired fins, dentition, squamation, visceral 
anatomy, and other structures.  See  ACTINOPTERYGII; FLATFISH. 
 �34s[Reeve M. Bailey]
�33s 
  
  
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�6v�2v�32sPleuropneumonia-like organism (PPLO)   �21m �33s 
    The nature of these organisms and their classification are uncertain. They 
may represent a special growth form of bacteria, which they resemble in many 
respects. At present they are classified in the separate order Mycoplasmatales 
in the class Mollicutes.  See  MOLLICUTES. 
 �34s[Louis L. Dienes]
�33s 
  
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�6v�2v�32sPliocene   �21m �33s 
    The youngest of the five major worldwide divisions (epochs) of the 
Tertiary Period (Cenozoic Era), extending from the end of the Miocene to the 
beginning of the Pleistocene (Quarternary). Pliocene also denotes the series 
of rocks and their fossils formed during this epoch. Pliocene rocks include 
most normal sedimentary types. Those of a mechanical (clastic) rather than 
chemical or biogenetic origin are the more common. High surface relief was 
characteristic of Pliocene times. Marine deposits are patchy though often 
thick.  See  CENOZOIC; MIOCENE; PLEISTOCENE; TERTIARY. 
    In its broadest usage, Pliocene time encompassed two intervals of 
widespread orogenic movements separated by an interval of general marine 
expansion. During Pliocene time the ocean withdrew from eastern Europe and 
Turkestan, never to return. Remnants were left such as the Aral, Caspian, and 
Black Seas. A later Pliocene North Sea retreated progressively northward. 
    Through the Pliocene worldwide temperatures dropped steadily. By 
mid-Pliocene time world climates had attained temperatures essentially those 
of today though generally more arid. Since then climates have become colder 
(peaks of Ice Age glaciation), then again warmer than those prevalent during 
historic time. 
    Pliocene marine shellfish look modern. Equatorward expansion of bivalves, 
gastropods, and foraminiferids (especially cassidulinids) having cold-water 
environmental optima was general. Pliocene mammals were larger than those in 
preceding epochs. Grasslands were widespread. Mio-Pliocene land emergences 
permitted intercontinental interchanges of mammals before endemic evolution on 
land set in. Afro-Asian Pliocene seaways (Zanzibar, Red Sea, Persia, Mekran 
coast) semi-isolated much of the earlier Pliocene fauna in Africa. Late 
Pliocene and early Pleistocene land reconnections permitted further 
intercontinental mammal interchanges; northern forms went south, others 
migrated to the newly reconnected South America, while many distinctive South 
American mammals died out. Much of the climacteric Plio-Pleistocene mammal 
fauna remained in the modern Oriental Region, but north and northwest of the 
Himalayas selective scouring during the subsequent Ice Age was rigorous, and 
this fauna was modified both in Old World and New. 
 �34s[Robert M. Kleinpell]
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�6v�2v�32sPlowing   �21m �33s 
    Cutting and turning a furrow with a plow is usually the first and most 
effective operation of tillage. There are two types of plow, the moldboard 
(see illustration) and the disk. See TILLAGE. 
    The working part of the moldboard plow is the plow bottom. The shape of 
the moldboard may be described as a section cut from a warped cylinder, on the 
inside of which the soil spirals. Moldboard plows have largely been adapted to 
power farming by the selection of bottoms suitable for certain soils and 
combination of these bottoms into gangs sufficiently large to use the power of 
the tractor at the approximate speed for which the bottoms were designed. 
    The effectiveness of plowing may be materially increased by attachments. A 
jointer, or a jointer and coulter combination, which cuts a small furrow in 
front of the plow shin, permits complete coverage of sod. Where there are 
large amounts of rubbish or crop residue, a weed chain or wire can be used to 
drag the debris into the furrow and hold it there until covered. A furrow 
wheel may reduce land-side friction, and a depth gage on the beam helps secure 
uniform depth. A modified form of plow bottom, called a lister, is in effect a 
right and a left moldboard joined at the shin so as to throw soil both to the 
right and to the left. This produces a furrow or trough, called a list, in 
which seed is planted. Because it concentrates rainfall in the furrow, this 
method is used largely in areas of light rainfall. 
    The disk plow consists of a number of disk blades attached to one axle or 
gang bolt. This plow is used for rapid, shallow plowing. In fields where 
numerous rocks and roots are present, the disk plow, which rolls over 
obstacles, is substituted for the moldboard. The disk is also used for sticky 
soils that will not scour on a moldboard. 
 �34s[Mark L. Nichols]
�33s 
  
  
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�6v�2v�32sPLT-Bedsonia group   �21m �33s 
    Once called viruses, the psittacosis-lymphogranuloma-trachoma group (PLT), 
now known as Bedsonia or Chlamydia, belonging to the Chlamydozoaceae, are 
actually bacteria. They have a complex cell wall containing the amino sugar 
muramic acid; exhibit a two-type nucleic acid (deoxyribonucleic acid and 
ribonucleic acid) cycle; grow and divide by binary fission; and are 
susceptible to certain antibiotics. The feature Bedsonia have in common with 
true viruses is the obligatory intracellular nature of their parasitism, which 
appears to be due to a limited capability for production of high-energy 
compounds. They are simply parasites depending upon the energy-producing 
systems of the host cells for their multiplication. 
    The agents belonging to the Bedsonia group are responsible for many 
diseases in avian, mammalian, and marsupial reservoirs in enzootic or 
epizootic form. Psittacosis and ornithosis are the most familiar Bedsonia 
infections. Among the diseases of mammals which are caused by Bedsonia are 
abortion, arthritis, conjunctivitis, encephalomyelitis, enteritis, and 
pneumonitis.  
 �34s[Karl F. Meyer]
�33s 
  
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�6v�2v�32sPlug gage   �21m �33s 
    A cylinder of steel whose diameter is ground, then honed or lapped to gage 
tolerance, a plug gage is used for checking the diameter of a cylindrical 
cavity such as a bolt hole or bearing bore. Plug gages are comparable to those 
of ring gages. A single plug gage controls one limit of tolerance of an inside 
diameter.  
 �34s[Richard A. Bambach]
�33s 
  
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�6v�2v�32sPlum   �21m �33s 
    Any of the smooth-skinned stone fruits grown on shrubs or small trees. 
Plums are widely distributed in all land areas of the North Temperate Zone, 
where many species and varieties are adapted to different climatic and soil 
conditions.  
    There are four principal groups: (1) Domestica (Prunus domestica) of 
European or Southwest Asian origin, (2) Japanese or Salicina (P. salicina) of 
Chinese origin, (3) Insititia or Damson (P. insititia) of Eurasian origin, and 
(4) American (P. americana and P. hortulana). The Domesticas are large, meaty, 
prune-type plums. A prune is a plum which dries without spoiling. 
 �34s[R. Paul Larsen]
�33s 
  
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�6v�2v�32sPlumbaginales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Caryophyllidae of the class Magnoliopsida (dicotyledons). The 
order consists of only the family Plumbaginaceae, with about 350 species. The 
plants are herbs or less often shrubs. The flowers are strictly pentamerous; 
that is, each floral whorl has five members; the petals are fused and all 
alike in shape and size. The family contains a few garden ornamentals, such as 
species of Armeria, known as thrift or sea-pink.  See  CARYOPHYLLIDEA; 
MAGNOLIOPSIDA.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sPluto �21m �33s
    The most distant known planet in the solar system. Pluto was discovered in
1930.
    Pluto is visible only through fairly large telescopes. Periodic variations
in its brightness demonstrate that the surface of Pluto is covered with bright
and dark markings and indicate a period of rotation of 6.3 days. This long
period and the high inclination and eccentricity of Pluto's orbit around the
Sun have led several scientists to propose that Pluto was originally a
satellite of Neptune.
    Pluto has a small satellite, provisionally named Charon, discovered in
1978, in a synchronous orbit; that is, the satellite's period is identical to
the rotational period of the planet. This discovery made possible a
determination of Pluto's mass, which was found to be about one-seventh that of
the Earth's Moon. With a diameter in the range of 1900 - 2500 mi (3000 - 4000
km) Pluto must have a mean density close to 1 g/cmsup3. Pluto must be composed
largely of ices, a conclusion that gains support from the detection of an
infrared absorption band of solid methane in its spectrum. The average
temperature of Pluto must be close to 50 K (-370DGF), which is consistent with
the presence of solid methane on the planet's surface, in equilibrium with a
small amount of methane gas.  See  PLANETARY PHYSICS.
 �34s[Tobias C. Owen]
�33s

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�6v�2v�32sPluton   �21m �33s 
    A rock body formed by the consolidation of magma (molten rock) below the 
surface of the Earth, but now exposed due to erosion. A rather elaborate 
terminology based on the supposed three-dimensional shape of plutons has 
evolved. But because of the difficulty in determining the real shape, the 
modern tendency is to use the general term pluton, except for relatively small 
bodies.  See  BATHOLITH; LACCOLITH. 
 �34s[Marland P. Billings]
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�6v�2v�32sPlutonium �21m �33s
    A chemical element, Pu, atomic number 94. Plutonium is a reactive, silvery
metal in the actinide series of elements. The principal isotope of chemical
interest is (sup 239)Pu, with a half-life of 24,131 years. It is formed in
nuclear reactors by the process shown in the following reaction. Plutonium-239
is fissionable, but may also capture neutrons to form higher plutonium
isotopes.

		**BELOW HAS REACTION CONDITION INFORMATION**

					BETA-			 BETA-
(sup 238)U + n ---------> (sup 239)U -----------> (sup 239)Np ------------->
				      23.5 min		       2.33 days

					(sup 239)Pu

    Plutonium-238, with a half-life of 87.7 years, is utilized in heat sources
for space application, and has been used for heart pacemakers. Plutonium-239
is used as a nuclear fuel, in the production of radioactive isotopes for
research, and as the fissile agent in nuclear weapons.	See  NUCLEAR FUELS;
NUCLEAR REACTOR.
    Plutonium exhibits a variety of valence states in solution and in the
solid state. Plutonium metal is highly electropositive. Numerous alloys of
plutonium have been prepared, and a large number of intermetallic compounds
have been characterized.
    Reaction of the metal with hydrogen yields two hydrides. The hydrides are
formed at temperatures as low as 150DGC (300DGF). Their decomposition above
750DGC (1400DGF) may be used to prepare reactive plutonium powder. The most
common oxide is PuO2, which is formed by ignition of hydroxides, oxalates,
peroxides, and nitrates of any oxidation state in air of 870 - 1200DGC (1600 -
2200DGF). A very important class of plutonium compounds are the halides and
oxyhalides. Plutonium hexafluoride, the most volatile plutonium compound
known, is a strong fluorinating agent. A number of other binary compounds are
known. Among these are the carbides, silicides, sulfides, and selenides, which
are of particular interest because of their refractory nature.
    Because of its radiotoxicity, plutonium and its compounds require special
handling techniques to prevent ingestion or inhalation. Therefore, all work
with plutonium and its compounds must be carried out inside glove boxes. For
work with plutonium and its alloys, which are attacked by moisture and by
atmospheric gases, these boxes may be filled with helium or argon.  See
ACTINIDE ELEMENTS; NEPTUNIUM; NUCLEAR CHEMISTRY; TRANSURANIUM ELEMENTS;
URANIUM.
 �34s[Fritz Weigel]
�33s

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�6v�2v�32sPlywood   �21m �33s 
    A wood product in which thin sheets of wood are glued together, grains of 
adjacent sheets being at right angles to each other in the principal plane. 
Because of this cross-grained orientation, mechanical properties of plywood 
are less directional than are those of natural lumber. Originally developed to 
replace wide wood boards that had been sawed directly from large logs, plywood 
was manufactured from logs not suitable for other purposes. It has since found 
such widespread use that tree farms are now cultivated specifically to yield 
logs suitable for processing into sheets for plywood. 
    Plywood with waterproof glue is designated exterior type; it is also used 
interiorly where moisture is present. Plywood with moisture-resistant glue is 
designated interior type; it can withstand an occasional soaking but neither 
repeated soakings nor continuous high humidity. 
    Most commonly used plywoods are H-in. (6.35 mm) sanded interior paneling 
or J-in. (12.70 mm) exterior grade plywood sheeting. Other standard 
thicknesses extend to 1 in. (25.40 mm) for interior types and to 1G in. (28.58 
mm) for exterior types. The most common panel size is 4 x 8 ft (122 x 244 mm); 
larger sizes are manufactured for such special purposes as boat hulls.  See  
LUMBER MANUFACTURE; WOOD PRODUCTS. 
 �34s[Frank H. Rockett]
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�6v�2v�32sPneumatolysis   �21m �33s 
    The alteration of rocks by the action of magmatic gases. The gases 
accompany magmatic intrusions and permeate the intruded rocks along fissures 
and other lines of least resistance.  See  METAMORPHISM; METASOMATISM. 
    Primary magmatic gases are acid and therefore react readily with limestone 
to form skarn rocks. Appreciable amounts of heavy metals (usually present as 
chlorides, fluorides, or sulfides) are associated with the magmatic gases. 
These are captured by the limestone and retained as deposits of skarn rocks or 
ores. In other places the heavy metals are deposited in granite or schist, 
causing the formation of greisen.  See  GREISEN; SKARN. 
 �34s[T. F. W. Barth]
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�6v�2v�32sPneumococcus   �21m �33s 
    The major causative microorganism (Streptococcus pneumoniae) of lobar 
pneumonia. Pneumococci occur singly or as pairs or short chains of oval or 
lancet-shaped cocci, flattened at proximal sides and pointed at distal ends. A 
capsule of polysaccharide envelops each cell or pair of cells. The organism is 
nonmotile and stains gram-positive unless degenerating. 
    Pneumococci have been isolated from the upper respiratory tract of healthy 
humans, monkeys, calves, horses, and dogs. Epizootics of pneumococcal 
infection have been described in monkeys, guinea pigs, and rats but are not 
the source of human infection. In humans, pneumococci may be found in the 
upper respiratory tract of nearly all individuals at one time or another. 
Following damage to the epithelium lining the respiratory tract, they may 
invade the lungs. They are the principal cause of lobar pneumonia in humans 
and may cause also pleural empyema, pericarditis, endocarditis, meningitis, 
arthritis, peritonitis, and infection of the middle ear. Approximately one of 
four cases of pneumococcal pneumonia is accompanied by invasion of the 
bloodstream by pneumococci, producing bacteremia. The mortality of untreated 
pneumococcal infection is high but has been reduced significantly by the use 
of antibiotics. A polyvalent vaccine of 14 pneumococcal capsular 
polysaccharides is available for the prevention of pneumococcal infection 
caused by the more virulent pneumococcal types in those segments of the 
population at high risk of fatal infection. 
 �34s[Robert Austrian]
�33s 
  
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�6v�2v�32sPneumocystosis   �21m �33s 
    A disease resulting from infection with protozoa of the genus 
Pneumocystis. The organisms grow in the lungs and form cysts. Pneumocystosis 
became important when outbreaks of a disease were observed in premature 
infants who developed cough, progressive difficulty in breathing, and x-ray 
changes in both lung fields. 
    The organism appears to be transmitted from person to person. With 
refinements in management of patients, the number of cases has declined. 
 �34s[Jacob K. Frenkel]
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�6v�2v�32sPneumonia   �21m �33s 
    An acute or chronic inflammation of the lung tissues, occurring in humans 
and in many animals. Pneumonia in humans may be caused by numerous microbial, 
immunological, physical, or chemical agents. It may also be associated with 
many systemic diseases. Any or all parts of the lung may be involved, with the 
inflammatory exudate filling the alveolar air spaces of one or more lobes, as 
in lobar pneumonia, or the smaller segments, as in lobular pneumonia. It may 
be disposed in and around the bronchi, as in bronchopneumonia, or may be 
limited predominantly to the interalveolar areas, as in interstitial 
pneumonia. Pneumonia may be a primary disease or a secondary event in other 
diseases.  
    The bacterial pneumonias may sometimes cause destruction of parts of the 
lung by abscess formation or may involve the pleura in an empyema or sterile 
effusions. Inflammation and dilation of bronchi (bronchiectasis) and spread of 
infection to remote organs is not unusual. Fortunately, however, most common 
pneumonias heal by resolution, leaving lung structure and function little 
changed. Viral pneumonias may have superimposed bacterial infection. 
 �34s[Maxwell Finland]
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�6v�2v�32sPneumonitis   �21m �33s 
    An acute infectious disease of the respiratory tract in which localized 
air-sac infiltration of varying degree with fluid and a diversity of cells is 
a prominent feature. The most common causative agent in humans is a Mycoplasma 
pneumonia - the so-called Eaton agent of pneumonia. The disease has come to be 
known as primary atypical pneumonia.  See  PNEUMONIA. 
 �34s[Karl F. Meyer]
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�6v�2v�32sPodicipediformes   �21m �33s 
    The order of birds (sometimes Podicipitiformes) containing the single 
family Podicipedidae, the grebes. Their legs are set far back on the body, 
with compressed, bladelike tarsi. The toes of grebes are not webbed, however, 
but are individually broadened and lobed. The plumage is dense and silky, and 
the tail is rudimentary. The larger species feed principally on fish, while 
the smaller ones eat a high proportion of aquatic invertebrates. Many grebes 
have elaborate and often spectacular aquatic courtship displays. The nests are 
built in water, and may be built up from the bottom, anchored to emergent 
vegetation, or even floating. The family as a whole is virtually cosmopolitan. 
  See  AVES; DIVING BIRDS. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sPodocopa   �21m �33s 
    A suborder, also called Podocopina, of the order Podocopida (subclass 
Ostracoda) in which the inner lamella has a calcified rim, the duplicature, 
joining the outer lamella along a chitinous zone of concrescence that is wider 
than either lamella. Radial pore canals, leading to sensory setae, lie in the 
zone of concrescence. The two valves fit firmly, hermetically sealing the 
animal inside when closed. Although variously sculptured, none of the 
podocopines have strong vertical lobation. 
    Appendages include antennules, antennae, mandibles, maxillae, and three 
pairs of thoracic legs. Antennules and antennae are both used in locomotion; 
swimming forms are provided with long, feathered setae, whereas crawling forms 
have only short, stout setae. Mandibles are strongly constructed and, with 
rare exceptions, used for mastication. If eyes are present, the two lateral 
eyes and the median eye are joined into one central structure. No heart is 
developed.  
    Five living superfamilies can be distinguished: Cypridacea, Macrocypracea, 
Bairdiacea, Cytheracea, and Darwinulacea. The Podocopa (Ordovician to Recent) 
have many fossil representatives, both marine and fresh-water. The superfamily 
Cytheracea, in particular, is abundant in the Mesozoic and Cenozoic, and has 
many divisions. Nearly half of all fossil species described belong to this 
taxon.  See  OSTRACODA. 
 �34s[Robert V. Kesling]
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�6v�2v�32sPodostemales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of only the family Podostemaceae, with about 200 species, the 
greatest number occurring in tropical America. They are submerged aquatics 
with modified, branching, often thalluslike shoots, and small, perfect flowers 
with a much reduced perianth.  See  MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sPoecilosclerida   �21m �33s 
    An order of sponges of the class Demospongiae in which the skeleton 
includes two or more types of megascleres, each localized in a particular part 
of the sponge colony. Frequently one type of megasclere is restricted to the 
dermis and another type occurs in the interior of the sponge. Sometimes one 
category is embedded in spongin fibers while a second category, usually 
spinose, protrudes from the fibers at right angles. Spongin is always present 
but varies in amount from species to species. Microscleres are usually 
present; often several types occur in one species. A wide variety of 
microsclere categories is found in the order but asters are never present.  
See  DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
�33s 
  
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�6v�2v�32sPogonophora   �21m �33s 
    The beard worms - a phylum of sedentary marine worms living in cool waters 
of all the world's oceans, generally at depths between 330 and 13,200 ft (100 
and 4000 m), shallower at higher latitudes and deeper in trenches. Often 
described as ``the twentieth-century phylum,'' they were first dredged early 
in the century and thoroughly investigated only in the 1950s. 
    Pogonophorans construct a tube, and are the only nonparasitic metazoans to 
have no trace of a mouth, gut, or anus. These are long, slender worms, the 
diameter in most being less than 0.04 in. (1 mm) and the length being over 100 
times the diameter. Superficially the tubes remind one of corn silk or coarse 
thread, but most have a characteristic banding pattern of brown or yellow 
pigments. The larger tubes are sometimes rigid, thicker, and dark-colored. 
    These worms absorb their nutrients through the pinnules and microvilli of 
tentacles without the aid of digestive enzymes. This is accomplished against 
concentration gradients and may be supplemented by limited pinocytosis and 
phagocytosis by tentacular surfaces in a few species. 
    The sexes are separate, with the gonopore location being the only sexual 
dimorphism. Fertilization has not been observed but must occur within the 
maternal tube, as fertilized eggs and developing larvae have been found there. 
 �34s[Edward B. Cutler]
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�6v�2v�32sPoint   �21m �33s 
    In axiomatic geometry, usually a completely undefined (primitive) element, 
although there are axiomatizations of geometry in which those properties of 
``point'' that are desired are given by postulates. To illustrate: In a 
geometry in which line is a primitive element, points may be defined as 
classes of lines that conform to certain requirements (postulates). In 
n-dimensional metric analytic point geometry, a point may be defined as an 
ordered n-tuple of numbers.  See  ANALYTIC GEOMETRY; EUCLIDEAN GEOMETRY. 
 �34s[Leonard M. Blumenthal]
�33s 
  
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�6v�2v�32sPoint-contact diode   �21m �33s 
    A semiconductor rectifier using the barrier formed between a specially 
prepared semiconductor surface and a metal point to produce the rectifying 
action. The contact is usually maintained by mechanical pressure, but in some 
instances it may be welded or bonded. The rectifying action implies that the 
resistance of the contact is significantly greater for one direction of 
applied voltage (reverse direction) than for the other (forward direction).  
See  SEMICONDUCTOR RECTIFIER.  
    Point-contact diodes have been widely used in radio and television, and 
most notably in computers, microwave detectors, and ultrahigh-frequency 
mixers. Today their use is very small, having been usurped by junction diodes 
and Schottky barrier diodes. 
 �34s[Lloyd P. Hunter]
�33s 
  
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�6v�2v�32sPoint-contact transistor   �21m �33s 
    A transistor in which the emitter and collector consist of metal point 
contacts closely spaced on the surface of a block of semiconductor. The usual 
configuration is with both points on the same surface and about 2 mils (50 mm) 
apart, although good devices have also been made with the points on opposite 
sides of a thin wafer of semiconductor. This type of transistor was the first 
transistorlike device invented. Today point-contact transistors are almost 
unknown in practical use. 
 �34s[Lloyd P. Hunter]
�33s 
  
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�6v�2v�32sPoint source �21m �33s
    A source having definite position but no extension in space. In discussing
radiation, it is convenient to define the concept of point source. If the
radiation propagates in radially straight lines (or, which is the same thing,
in spherical waves) from the point source, conservation of energy demands that
the intensity of the radiation decrease in any direction inversely as the
square of the distance from the source. No physical source is actually a
mathematical point, but for distances sufficiently large compared to
dimensions of the source, the inverse-square law may be a good approximation.
See  INVERSE-SQUARE LAW.
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sPoison   �21m �33s 
    A substance which by chemical action and at low dosage can kill or injure 
living organisms. Broadly defined, poisons include chemicals toxic for any 
living form: microbes, plants, or animals. For example, antibiotics such as 
penicillin, although nontoxic for mammals, are poisons for bacteria. In common 
usage the word is limited to substances toxic for humans and mammals, 
particularly where toxicity is a substance's major property of medical 
interest. Because of their diversity in origin, chemistry, and toxic action, 
poisons defy any simple classification. Almost all chemicals with recognized 
physiological effects are toxic at sufficient dosage. 
  
    Origin and chemistry.  Many poisons are of natural origin. Some bacteria 
secrete toxic proteins (for example, botulinus, diphtheria, and tetanus 
toxins), among the most poisonous compounds known. Lower plants notorious for 
poisonous properties are ergot (Claviceps purpurea) and a variety of toxic 
mushrooms.  See  ERGOT; TOXIN. 
    Higher plants, which constitute the major natural source of drugs, contain 
a great variety of poisonous substances. Many of the plant alkaloids double as 
drugs or poisons, depending on dose. These include curare, quinine, atropine, 
mescaline, morphine, nicotine, cocaine, picrotoxin, strychnine, lysergic acid, 
and many others.  See  ATROPINE; COCAINE; MORPHINE; QUININE. 
    Poisons of animal origin (venoms) are similarly diverse. Toxic marine 
animals alone include examples of every phylum. Insects and snakes represent 
the best-known venomous land animals, but on land, too, all phyla include 
poison-producing species. Among mammalian examples are certain shrews with 
poison-producing salivary glands. 
    Poisons of nonliving origin vary in chemical complexity from the toxic 
elements, for example, the heavy metals, to complex synthetic organic 
molecules. Most of the heavy metals (gold, silver, mercury, arsenic, and lead) 
are poisons of high potency in the form of their soluble salts. Strong acids 
or bases are toxic largely because of corrosive local tissue injury. 
    The chemically reactive gases hydrogen sulfide, hydrocyanic acid, 
chlorine, bromine, and ammonia are also toxic, even at low concentration, both 
because of their corrosiveness and because of more subtle chemical interaction 
with enzymes or other cell constituents. 
    Many organic substances of synthetic origin are highly toxic and represent 
a major source of industrial hazard. Most organic solvents are more or less 
toxic on ingestion or inhalation. Many alcohols, such as methanol, are toxic. 
Many solvents (for example, carbon tetrachloride, tetrachloroethane, dioxane, 
and ethylene glycol) produce severe chemical injury to the liver and other 
viscera, sometimes from rather low dosage. 
  
    Actions.  The action of poisons is generally described by the 
physiological or biochemical changes which they produce. For most poisons, a 
descriptive account can be given which indicates what organic system (for 
example, heart, kidney, liver, brain, and bone marrow) appears to be most 
critically involved and contributes most to seriously disordered body function 
or death. In many cases, however, organ effects are multiple, or functional 
derangements so generalized that a cause of death cannot be localized. 
    More precise understanding of the mechanism of poisons requires detailed 
knowledge of their action in chemical terms. Information of this kind is 
available for only a few compounds, and then in only fragmentary detail. 
Poisons that inhibit acetylcholinesterase have toxic actions traceable to a 
single blocked enzyme reaction, hydrolysis of normally secreted acetylcholine. 
Understanding of the mechanism of chemical inhibition of cholinesterase is not 
complete, but allows some prediction of chemical structures likely to act as 
inhibitors.  
    Carbon monoxide toxicity is also partly understood in chemical terms, 
since formation of carboxyhemoglobin, a form incapable of oxygen transport, is 
sufficient to explain the anoxic features of toxicity. 
    Heavy metal poisoning in many cases is thought to involve inhibition of 
enzymes by formation of metal mercaptides with enzyme sulfhydryl groups, the 
unsubstituted form of which is necessary for enzyme action. This is a general 
reaction that may occur with a variety of sulfhydryl-containing enzymes in the 
body. Specific susceptible enzymes whose inhibition explains toxicity have not 
yet been well documented.  See  METALS, PATHOLOGY OF. 
    Metabolic antagonists active as poisons function by competitive blocking 
of normal metabolic reactions. Some antagonists may act directly as enzyme 
inhibitors, others may be enzymatically altered to form derivatives which are 
even more potent inhibitors at a later metabolic step.  See  ENZYME INHIBITION. 
    Where poison mechanisms are relatively well understood it has sometimes 
been possible to employ rationally selected antidotes. 
  
    Health aspects.  The environment of civilized people abounds in toxic 
chemicals: drugs, industrial products, fuels, pesticides, and many common 
household products such as cleaning agents and paint products. Acute and 
chronic poisoning represent an appreciable public health hazard. 
    The multiplication of toxic chemicals in everyday use led, in 1953, to the 
establishment of information centers known as Poison Control Centers, which 
provide physicians with emergency telephone information concerning toxicity 
and antidotes for common poisons as well as for brand-named chemical 
formulations.  See  TOXICOLOGY. 
 �34s[Elijah Adams]
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�6v�2v�32sPoison gland   �21m �33s 
    The specialized gland of certain fishes, as well as the granular and some 
mucous glands of many aquatic and terrestrial Amphibia. The poison glands of 
fishes are simple or slightly branched acinous structures which use the 
holocrine method of secreting a mucuslike substance. The poison glands of 
snakes are modified oral or salivary glands. Amphibian glands are simple, 
acinous, holocrine, with granular secretion. In some cases these amphibian 
poison glands produce mucus by a merocrine method of secretion. These glands 
function as protective devices.  See  GLAND. 
 �34s[Olin E. Nelsen]
�33s 
  
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�6v�2v�32sPoisonous plants �21m �33s
    Almost 400 species of vascular plants, representing 70 families, have been
listed as poisonous plants occurring in the United States. About 100 of these
species cause dermatitis. Among these are such common plants as poison ivy
(Rhus radicans), poison sumac (R. vernix), and viper's bugloss (Echium
vulgare). Plants representing nearly a score of species possess spines,
thorns, or fruits having sharp awns, which often cause injury to animals. In
addition to these are many other species more appropriately cataloged as
poisonous plants because they contain toxic principles which initiate
pathological conditions in animals.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sPolar-coordinate navigation systems   �21m �33s 
    Systems in which one or more signals are emitted from a facility (or 
colocated facilities) to produce simultaneous indication of bearing and 
distance; also called rho-theta systems. Since a bearing is a radial line of 
position and a distance is a circular line of position, the polar-coordinate 
system always ensures a position fix produced by the intersection of two lines 
of position which are at right angles to each other. Since the reference for 
both lines of position is at a common origin, computation for any course 
referred to this origin is simplified.  See  ELECTRONIC NAVIGATION SYSTEMS. 
    For discussions of specific systems  see  DOPPLER VOR; RADIO RANGE; TACAN; 
VORTAC.  
 �34s[Peter C. Sandretto]
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�6v�2v�32sPolarimetric analysis �21m �33s
    A method of chemical analysis based on the optical activity of the
substance being determined. Optically active materials are asymmetric; that
is, their molecules or crystals have no plane or center of symmetry.
Asymmetric substances possess the power of rotating the plane of polarization
of plane-polarized light. Measurement of the extent of this rotation,
polarimetry, is performed by an instrument known as a polarimeter. Polarimetry
is applied to both organic and inorganic materials.  See  OPTICAL ACTIVITY;
POLARIZED LIGHT.
    Polarimetry may be used for either qualitative or quantitative analytical
work. In qualitative applications, the presence of an optically active
material is shown, and then a calculation of specific rotation often leads to
the identification of the unknown. In quantitative work, the concentration of
a given optically active material is determined either from a calibration
curve of percentage of the constituent versus angular rotation or from the
specific rotation, assuming the angular rotation to be a linear function of
concentration.
    Polarimetry is widely used in carbohydrate chemistry, especially in the
analysis of sugar solutions. Also, since there is great difference between the
biological activities of the different optical forms of organic compounds,
polarimetry is widely used in biochemical research to identify the molecular
configurations.
    Optical rotary dispersion is the measurement of the specific rotation as a
function of wavelength. The information obtained by this method has shown that
minor changes in configuration of a molecule have a marked effect on its
dispersion properties. By using the properties of compounds of known
configuration, it has been possible to determine the absolute configurations
of many other molecules and to identify various isomers.  See  COTTON EFFECT;
OPTICAL METHODS OF CHEMICAL ANALYSIS; ROTATORY DISPERSION.
 �34s[Robert F. Goddu ;James N. Little]
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�6v�2v�32sPolaris   �21m �33s 
    Alpha Ursae Minoris, the North Star, or polestar. Polaris is located 
approximately 1DG of arc from the north celestial pole, the point where the 
Earth's axis of rotation now pierces the celestial sphere. Two bright stars at 
the end of the bowl of the Big Dipper point toward Polaris. Polaris has a 
faint, main-sequence companion.  See  STAR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sPolarity (electricity)   �21m �33s 
    The designation of positive and negative terminals in an electric circuit 
carrying a current. The choice of positive charge, and thereby of positive 
potential, is a purely arbitrary one which had been made long before the 
discovery of the electron. It is now known that the primary current carriers 
in most circuit elements are electrons, which possess negative charge. 
    For a seat of electromotive force, such as a battery or a generator, the 
positive terminal is by convention the one at which electrons enter from the 
external circuit; the negative terminal is the one from which they leave. For 
a ``passive'' element, such as an ammeter or a resistor, the positive terminal 
is the one through which the electrons leave the element; the negative 
terminal is the one through which they enter from the circuit.  See  
POTENTIALS.  
 �34s[John W. Stewart]
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�6v�2v�32sPolarization of dielectrics   �21m �33s 
    A vector quantity representing the electric dipole moment per unit volume 
of a dielectric material.  See  DIELECTRICS; DIPOLE MOMENT. 
    Dielectric polarization arises from the electrical response of individual 
molecules of a medium and may be classified as electronic, atomic, 
orientation, and space-charge or interfacial polarization, according to the 
mechanism involved. 
    Electronic polarization represents the distortion of the electron 
distribution or motion about the nuclei in an electric field. 
    Atomic polarization arises from the change in dipole moment accompanying 
the stretching of chemical bonds between unlike atoms in molecules.  See  
MOLECULAR STRUCTURE AND SPECTRA. 
    Orientation polarization is caused by the partial alignment of polar 
molecules, that is, molecules possessing permanent dipole moments, in an 
electric field. This mechanism leads to a temperature-dependent component of 
polarization at lower frequencies. 
    Space-charge or interfacial polarization occurs when charge carriers are 
present which can migrate an appreciable distance through a dielectric but 
which become trapped or cannot discharge at an electrode. This process always 
results in a distortion of the macroscopic field and is important only at low 
frequencies.  See  DIELECTRIC CONSTANT; ELECTRIC FIELD; ELECTRIC 
SUSCEPTIBILITY.  
 �34s[Robert D. Waldron]
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�6v�2v�32sPolarization of waves   �21m �33s 
    Polarization is the phenomenon which is exhibited when a transverse wave 
is polarized. The term polarization is also used to describe the process of 
polarizing a wave. 
    In an unpolarized wave, the vibrations in a plane perpendicular to the ray 
appear to be oriented in all directions with equal probability. In a polarized 
wave the displacement direction of the vibrations is completely predictable. 
For certain disturbances, such as the transverse acoustic wave produced when a 
steel bar is struck, the polarization is complete. Electromagnetic radiation 
is normally unpolarized if it is generated by atomic processes. Thus 
ultraviolet, visible, and infrared radiations produced by heated bodies or 
electrical discharges are generally unpolarized. Radiation generated by 
vacuum-tube oscillators or transistor oscillators is always polarized. The 
probability waves (matter waves) associated with atomic or nuclear particles 
are generally unpolarized.  See  ELECTROMAGNETIC RADIATION; POLARIZED LIGHT; 
QUANTUM MECHANICS. 
 �34s[Bruce H. Billings]
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�6v�2v�32sPolarized light   �21m �33s 
    Light which has its electric vector oriented in a predictable fashion with 
respect to the propagation direction. In unpolarized light, the vector is 
oriented in a random, unpredictable fashion. Even in short time intervals, it 
appears to be oriented in all directions with equal probability. Most light 
sources seem to be partially polarized so that some fraction of the light is 
polarized and the remainder unpolarized. 
    According to all available theoretical and experimental evidence, it is 
the electric vector rather than the magnetic vector of a light wave that is 
responsible for all the effects of polarization and other observed phenomena 
associated with light. Therefore, the electric vector of a light wave, for all 
practical purposes, can be identified as the light vector.  See  CRYSTAL 
OPTICS; ELECTROMAGNETIC RADIATION; LIGHT; POLARIZATION OF WAVES. 
    One of the simplest ways of producing linearly polarized light is by 
reflection from a dielectric surface. At a particular angle of incidence, 
known as Brewster's angle, the reflectivity for light whose electric vector is 
in the plane of incidence becomes zero. The reflected light is thus linearly 
polarized at right angles to the plane of incidence. 
  
    Linear polarizing devices.  The first polarizers were glass plates 
inclined so that the incident light was at Brewster's angle. Such polarizers 
are quite inefficient since only a small percentage of the incident light is 
reflected as polarized light. 
    Certain natural materials absorb linearly polarized light of one vibration 
direction much more strongly than light vibrating at right angles. Such 
materials are termed dichroic. Tourmaline is one of the best-known dichroic 
crystals, and tourmaline plates were used as polarizers for many years.  See  
DICHROISM.  
    Other natural materials exist in which the velocity of light depends on 
the vibration direction. These materials are called birefringent. One of the 
best-known of these birefringent crystals is transparent calcite (Iceland 
spar). The Nicol prism is made of two pieces of calcite cemented together 
(Fig. 1). The cement is Canada balsam, in which the wave velocity is 
intermediate between the velocity in calcite for the fast and the slow ray. 
The angle at which the light strikes the boundary is such that for one ray the 
angle of incidence is greater than the critical angle for total reflection. 
Thus the rhomb is transparent for only one polarization direction.  See  
BIREFRINGENCE; CRYSTAL; OPTICS. 
    A different type of polarizer, made of quartz, is shown in Fig. 2. Here 
the vibration directions are different in the two pieces so that the two rays 
are deviated as they pass through the material. The incoming light beam is 
thus separated into two oppositely linearly polarized beams which have an 
angular separation between them, and it is possible to select either beam. 
    A third mechanism for obtaining polarized light is the Polaroid sheet 
polarizer, of which there are three types. The first is a microcrystalline 
polarizer in which small crystals of a dichroic material are oriented parallel 
to each other in a plastic medium. The second type depends for its dichroism 
on a property of an iodine-in-water solution. The iodine appears to form a 
linear high polymer. If the iodine is put on a transparent oriented sheet of 
material such as polyvinyl alcohol (PVA), the iodine chains apparently line 
themselves parallel to the PVA molecules and the resulting dyed sheet is 
strongly dichroic. A third type of sheet polarizer depends for its dichroism 
directly on the molecules of the plastic itself. This plastic consists of 
oriented polyvinylene. 
  
    Polarization by scattering.  When an unpolarized light beam is scattered 
by molecules or small particles, the light observed at right angles to the 
original beam is polarized. The best-known example of polarization by 
scattering is the light of the north sky.  See  SCATTERING OF ELECTROMAGNETIC 
RADIATION.  
  
  
    Types.  Polarized light is classified according to the orientation of the 
electric vector. In linearly polarized light, the electric vector remains in a 
plane containing the propagation direction. For monochromatic light, the 
amplitude of the vector changes sinusoidally with time. In circularly 
polarized light, the tip of the electric vector describes a circular helix 
about the propagation direction. The amplitude of the vector is constant. The 
frequency of rotation is equal to the frequency of the light. In elliptically 
polarized light, the vector also rotates about the propagation direction, but 
the amplitude of the vector changes so that the projection of the vector on a 
lane at right angles to the propagation direction describes an ellipse. 
  
    Circular and elliptical polarizing devices.  Circularly and elliptically 
polarized light are normally produced by combining a linear polarizer with a 
wave plate. A Fresnel rhomb can be used to produce circularly polarized light. 
    A plate of material (quartz, calcite, or other birefringent crystals) 
which is linearly birefringent is called a wave plate or retardation sheet. 
Wave plates have a pair of orthogonal axes which are designated fast and slow. 
Polarized light with its electric vector parallel to the fast axis travels 
faster than light polarized parallel to the slow axis. The thickness of the 
material can be chosen so that for light traversing the plate, there is a 
definite phase shift between the fast component and the slow component. A 
plate with a 90DG phase shift is termed a quarter-wave plate. 
    If linearly polarized light is incident normally on a quarter-wave plate 
and oriented at 45DG to the fast axis, the transmitted light will be 
circularly polarized. If the linearly polarized light is at an angle other 
than 45DG to the fast axis, the transmitted radiation will be elliptically 
polarized.  
  
    Analyzing devices.  Polarized light is one of the most useful tools for 
studying the characteristics of materials. The absorption constant and 
refractive index of a metal can be calculated by measuring the effect of the 
metal on polarized light reflected from its surface.  See  REFLECTION OF 
ELECTROMAGNETIC RADIATION. 
    The analysis of polarized light can be performed with a variety of 
different devices. If the light is linearly polarized, it can be extinguished 
by a linear polarizer and the direction of polarization of the light 
determined directly from the orientation of the polarizer. If the light is 
elliptically polarized, it can be analyzed with the combination of a 
quarter-wave plate and a linear polarizer. Any such combination of polarizer 
and analyzer is called a polariscope. 
 �34s[Bruce H. Billings]
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�6v�2v�32sPolarized-vane ammeter   �21m �33s 
    An ammeter in which the current to be measured passes through a small 
coil, distorting the field of a circular permanent magnet, and an iron vane 
aligns itself with the axis of the distorted field, the deflection being 
roughly proportional to the current (see illustration). Reversed electrical 
polarity reverses the direction of motion; the instrument thus indicates 
current flowing into or out of a battery. This instrument is of only moderate 
accuracy. It is used in large numbers in battery chargers and automobiles 
because of its low cost. See AMMETER. 
 �34s[John H. Miller /Jack Bass]
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�6v�2v�32sPolar meteorology   �21m �33s 
    A regional branch of atmospheric science, taking special account of 
circumstances and phenomena prevailing in the northern and southern polar 
regions. The atmosphere in these regions has a distinctive character brought 
about principally by low solar elevation, as well as the given geographical 
configuration of oceans and land masses. 
 �34s[Norbert Untersteiner]
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�6v�2v�32sPolar molecule   �21m �33s 
    A molecule possessing a permanent electric dipole moment. Molecules 
containing atoms of more than one element are polar except where forbidden by 
symmetry; molecules formed from atoms of a single element are nonpolar (except 
ozone). The dipole moments of polar molecules result in stronger 
intermolecular attraction, increased viscosities, higher melting and boiling 
points, and greater solubility in polar solvents than in nonpolar molecules.  
See  DIPOLE MOMENT. 
 �34s[Robert D. Waldron]
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�6v�2v�32sPolar navigation   �21m �33s 
    The complex of navigational techniques modified from those used in other 
areas to suit the distinctive regional character of polar areas. Although 
polar navigation has become routine to a rising number of navigators operating 
in and through such high-latitude parts of the world, their success continues 
to be based on a sound grasp of the regional differences and the developing 
adaptations of navigational principles and aids to suit these peculiar area 
needs. For example, in polar regions the meridians radiate outward from the 
poles, and parallels are concentric circles. Thus the rectangular coordinates 
familiar to the navigator accustomed to using the Mercator projection are 
replaced by polar coordinates. 
  
    Limitations.  Piloting in polar regions is strongly affected by the 
absence of any great number of aids to navigation. Also, natural landmarks may 
not be shown on the chart, or may be difficult to identify. The appearance of 
some landmarks changes markedly under different ice conditions. When snow 
covers both the land and a wide ice foot attached to the shore and extending 
for miles seaward, even the shore line is difficult to locate.  See  PILOTING. 
    Charts of polar regions are less reliable than those of other regions, 
because relatively little surveying has been done in the polar areas. Because 
relatively few soundings are shown on charts, ships entering harbors often 
send small boats ahead to determine the depth of water available. 
    The availability of electronic aids in polar regions is somewhat limited. 
Radar is useful, but experience in interpretation of the scope in polar 
regions is essential for reliable results. This is particularly true in 
aircraft, where the relative appearance of water and land areas often reverse 
in winter and summer. A radio direction finder is useful, when radio signals 
are available. The use of electronics in polar regions is further restricted 
by magnetic storms, which are particularly severe in the auroral zones.  See  
ELECTRONIC NAVIGATION SYSTEMS; RADAR. 
    Reliable dead reckoning depends upon the availability of accurate 
measurement of direction and distance (or speed and time). There are 
difficulties in meeting these requirements in polar regions. Direction is 
measured largely by a compass. The magnetic compass becomes unreliable in the 
vicinity of the magnetic poles of the Earth, and the north-seeking gyrocompass 
becomes unreliable in the vicinity of the geographical poles of the Earth. The 
directional gyro is widely used in aircraft operating in polar regions but is 
not in general use aboard ship. Distance or speed measurement in polar regions 
presents no problems in aircraft. When ships operate in ice, however, the 
sensing element in the water may be adversely affected or damaged by the ice. 
At best, dead reckoning is difficult aboard a ship operating in the ice, not 
only because of the difficulty encountered in measuring course and speed, but 
also because neither of these may be constant for very long.  See  AIRCRAFT 
COMPASS SYSTEM; DEAD RECKONING; GYROSCOPE. 
    Celestial navigation is of great importance in polar regions, sometimes 
providing the only means of determining position accurately, or establishing a 
directional reference. When operating in lower latitudes, navigators generally 
avoid observation of bodies near the horizon because of the uncertainty of the 
refraction correction there. In polar regions, even though refraction is more 
uncertain, navigators often have no choice. Near the equinoxes the Sun may be 
the only body available for several weeks, and it remains close to the 
horizon. During the polar summer the Sun is often the only celestial body 
available. When only one body is available, it is observed at frequent 
intervals, perhaps hourly, and a series of running fixes is plotted.  See  
CELESTIAL NAVIGATION. 
  
    Modern technology.  Advances in technology have contributed significantly 
to the safety and reliability of navigation in polar regions. The availability 
of modern echo sounders has made possible a continuous plot of the bottom 
profile beneath a vessel while under way. Sonar indicates the approach of a 
vessel to an underwater obstruction. Reliable inertial navigators are 
available to make aircraft navigation in polar regions almost routine, 
providing both positional and directional data. Ship inertial navigation 
systems are available and have made practicable submarine operations under sea 
ice.  See  INERTIAL GUIDANCE SYSTEM; SONAR. 
    The U.S. Navy Navigation Satellite System (NNSS) provides means for 
intermittent position determination for craft equipped with suitable 
instrumentation. The NAVSTAR Global Positioning System (GPS) will virtually 
eliminate the limitations for aircraft of the Navy Navigation Satellite 
System.  See  SATELLITE NAVIGATION SYSTEMS. 
 �34s[Alton B Moody]
�33s 
  
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�6v�2v�32sPolarographic analysis �21m �33s
    An electrochemical technique used in analytical chemistry. Polarography
involves measurements of current-voltage curves, obtained when voltage is
applied to electrodes (usually two) immersed in the solution being
investigated. One of these electrodes is a reference electrode: its potential
remains constant during the measurement. The second electrode is an indicator,
and its potential varies in the course of measurement of the current-voltage
curve, because of the change of the applied voltage. In the simplest version,
so-called dc polarography, the indicator electrode is a dropping mercury
electrode, consisting of a mercury drop hanging at the orifice of a fine-bore
glass capillary. The capillary is connected to a mercury reservoir so that
mercury flows through the capillary at the rate of a few milligrams per
second. The outflowing mercury forms a drop at the orifice, which grows until
it falls off. The lifetime of each drop is several seconds (usually 2 to 5).
Each drop forms a new electrode; its surface is practically unaffected by
processes taking place on the previous drop. Hence each drop represents a
well-reproducible electrode with fresh, clean surface.	See  ELECTROCHEMICAL
TECHNIQUES; ELECTRODE.
    The dropping-mercury electrode is immersed in the solution to be
investigated and placed in a cell containing the reference electrode.
Polarographic current-voltage curves can be recorded with a simple instrument
consisting of a potentiometer or another source of voltage and a
current-measuring device (such as a sensitive galvanometer). The voltage can
be varied by manually changing the applied voltage in finite increments,
measuring current at each, and plotting current as a function of the voltage.
Alternatively, commercial instruments are available in which voltage is
increased linearly with time (a voltage ramp), and current variations are
recorded automatically (see illustration).
    Polarographic studies can be applied to investigation of electrochemical
problems, to elucidation of some fundamental problems of inorganic and organic
chemistry, and to solution of practical problems. In electrochemistry
polarography allows measurement of potentials, and yields information about
the rate of the electrode process, adsorption-desorption phenomena, and fast
chemical reactions accompanying the electron transfer. In fundamental
applications, polarography allows determination of the form and charge of the
species (for example, inorganic complex or organic ion) in the solution.
Polarography also permits the study of equilibria (complex formation,
acid-base, tautomeric), rates, and mechanisms.
 �34s[Peter Zuman]
�33s

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�6v�2v�32sPolaron   �21m �33s 
    The object that results when an electron in the conduction band of a 
crystalline insulator or semiconductor polarizes or otherwise deforms the 
lattice in its vicinity. The polaron comprises the electron plus its 
surrounding lattice deformation. (Polarons can also be formed from holes in 
the valence band.) If the deformation extends over many lattice sites, the 
polaron is ``large,'' and the lattice can be treated as a continuum. Charge 
carriers inducing strongly localized lattice distortions form ``small'' 
polarons.  See  BAND THEORY OF SOLIDS; HOLES IN SOLIDS; INSULATOR; 
SEMICONDUCTOR.  
 �34s[David M. Larsen]
�33s 
  
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�6v�2v�32sPolar triangle   �21m �33s 
    The poles of a great circle on a sphere are the points of the sphere where 
a perpendicular to the plane of the great circle at its center cuts the 
sphere. To obtain the polar triangle of a spherical triangle ABC, construct on 
its sphere three great circles having respective poles A, B, and C. Two great 
circles, one having B and the other C as poles, intersect in two points on 
opposite hemispheres defined by the great circle through BC. Denote by A' the 
point lying on the same hemisphere as A. Locate B' and C' by a like procedure. 
The spherical triangle A' B' C' is the polar triangle of ABC. In geometry it 
is shown that, if one spherical triangle is the polar of a second, then the 
second is the polar of the first.  See  TRIGONOMETRY. 
 �34s[Lyman M. Kells]
�33s 
  
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�6v�2v�32sPolemoniales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of 7 families with about 4600 species. The Solanaceae, having about 
2300 species, are the largest family of the order, followed by the 
Convolvulaceae (about 1400 species), Hydrophyllaceae and Polemoniaceae (about 
300 species each), and Cuscutaceae (about 150 species). In general, the 
flowers of the Polemoniales are sympetalous, that is, the petals are fused by 
their margins; they have a regular or nearly regular, usually five-lobed 
corolla; and the stamens, being equal in number with the petals or corolla 
lobes, alternate with the lobes. The fruit is usually capsular or softly 
fleshy, only seldom drupaceous. Except for many members of the Polemoniaceae, 
the leaves are nearly always alternate. 
    Petunias (in the Solanaceae), potatoes (Solanum tuberosum), sweet potatoes 
(Ipomoea batatas, in the Convolvulaceae), morning glory (Ipomoea pupurea), 
dodder (Cuscuta, a nongreen, twining parasite), and phlox (family 
Polemoniaceae) are familiar members of the Polemoniales.  See  ASTERIDAE; 
EGGPLANT; FRUIT; MAGNOLIOPSIDA; PAPRIKA; PIMENTO; POTATO, IRISH; POTATO, 
SWEET; TOBACCO; TOMATO. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sPoliomyelitis   �21m �33s 
    An acute infectious viral disease which in its serious form affects the 
central nervous system and, by destruction of motor neurons in the spinal 
cord, produces flaccid paralysis. However, about 99% of infections are either 
inapparent or very mild.  See   CENTRAL NERVOUS SYSTEM; VIRUS. 
    The virus probably enters the body through the mouth; primary 
multiplication occurs in the throat and intestines. The blood seems to be the 
most likely route to the central nervous system. The severity of the infection 
may range from a completely inapparent through minor influenzalike illness, or 
an aseptic meningitis syndrome (nonparalytic poliomyelitis) with stiff and 
painful back and neck, to the severe forms of paralytic and bulbar 
poliomyelitis. In all clinical types virus is regularly present in the enteric 
tract.  
    Poliomyelitis occurs throughout the world. In temperate zones it appears 
chiefly in summer and fall although winter outbreaks have been known. It 
occurs in all age groups, but less frequently in adults because of their 
acquired immunity. 
    Salk vaccine (formalin-killed), prepared from virus grown in monkey kidney 
cultures, was first developed and used in the United States. Oral poliovirus 
vaccine (Sabin) is also now generally used throughout the world as an 
effective immunizing agent. The vaccine is a living, attenuated virus. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sPolishing   �21m �33s 
    The smoothing of a surface by the cutting action of abrasive grit either 
glued to or impregnated in a flexible wheel or belt. Polishing, not a 
precision process, removes stock until the desired surface condition is 
obtained.  
    When a considerable amount of stock must be removed, operations may start 
with a coarse grit. Then a finer-grit wheel may be used for finishing. Common 
polishing machines usually are lathe-type machines with wheels at either end 
of a power spindle, or one end may drive an abrasive belt. 
    For a mirrorlike surface, a precision abrading process called 
superfinishing is used. Superfinishing removes minute flaws or inequalities. 
Superfinishing is performed with an extremely fine-grit abrasive stone, shaped 
to match and cover a large portion of the work surface.  See  LAPPING; 
MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sPollen   �21m �33s 
    The small male reproductive bodies produced in the pollen sacs of seed 
plants (gymnosperms and angiosperms). 
    On maturation in the pollen sac, a pollen grain may reach 0.00007 mg as in 
spruce, or less than 1/20 of this weight. A grain usually has two waxy, 
durable outer walls, the exine, and an inner fragile wall, the intine. These 
walls surround the contents with their nuclei and reserves of starch and oil. 
    Pollen identification depends on interpretation of morphological features. 
Exine and aperture patterns are especially varied in the more highly evolved 
dicots, so that recognition at family, genus, or even species level may be 
possible despite the small surface area available on a grain (Figs. 1 and 2). 
Since the morphological characters are conservative in the extreme, usually 
changing very slowly through geologic time, studies of fine detail serve to 
establish the lineal descent of many plants living today.  See  PALYNOLOGY. 
    Extreme variations in size may occur within a family, but pollen grains 
range mainly from 24 to 50 micrometers, with the dicot range being from 2 MUm 
in Myosotis to 250 MUm in Mirabilis; the monocots range from about 15 to 150 
MUm or more in the ginger family, with eelgrass (Zostera) having pollen 
measuring 2550 x 3.7 MUm in a class of its own; living gymnosperms range from 
15 MUm in Gnetum to about 180 MUm in Abies (including sacs), while fossil 
types range from about 11 MUm (one-furrowed) to 300 MUm. 
    Most grains are free (monads) though often loosely grouped because of the 
spines or sticky oils and viscin threads. Compound grains (polyads) are richly 
developed in some angiosperm families, commonly occurring in four (tetrads), 
or in multiples of four up to 64 or more. 
    In most microspores (pollen and spore) the polar axis runs from the inner 
(proximal) face to the outer (distal) face, as oriented during tetrad 
formation. The equator crosses it at right angles. Bilateral grains dominate 
in the gymnosperms and monocots, the polar axis usually being the shorter one, 
with the single aperture on the distal side. On the other hand, almost all 
dicot pollen is symmetrical around the polar axis (usually the long axis), 
with shapes ranging mainly from spheroidal to ellipsoidal, with rounded 
equatorial outlines, and sometimes ``waisted'' as in the Umbelliferae. 
Three-pored grains often have strikingly triangular outlines in polar view. 
  
    Rarely lacking, the flexible membranes of apertures are sometimes covered 
only by endexine. They allow for sudden volume changes, as for the emergence 
of the germ tube. They are classified as furrows, with elongate outlines, and 
pores usually more or less circular in shape. Short slitlike intermediate 
forms occur. A few families have no apertures, some as a result of reduction 
of exine as an adaptation to wind- or water-pollination. Gymnosperms may also 
lack openings, or have one small papilla;  most have one furrow, or a long 
weak area. 
    Few pollen grains are completely smooth (psilate) at ordinary 
magnifications; most have both sculpture on the surface and structure below 
it.  See  FLOWER; POLLINATION; REPRODUCTION (PLANT). 
 �34s[Lucy M. Cranwell]
�33s 
  
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�6v�2v�32sPollination   �21m �33s 
    The transport of pollen grains from the plant parts that produce them to 
the ovule-bearing organs, or to the ovules (seed precursors) themselves. In 
gymnosperms, the pollen, usually dispersed by the wind, is simply caught by a 
drop of fluid excreted by each freely exposed ovule. In angiosperms, where the 
ovules are contained in the pistil, the pollen is deposited on the pistil's 
receptive end (the stigma), where it germinates.  See  FLOWER. 
    Without pollination, there would be no fertilization; it is thus of 
crucial importance for the production of fruit crops and seed crops. 
Pollination also plays an important part in plant breeding experiments aimed 
at increasing crop production through the creation of genetically superior 
types.  See  BREEDING (PLANT); REPRODUCTION (PLANT). 
  
    Self- and cross-pollination.  In most plants, self-pollination is 
difficult or impossible, and there are various mechanisms which are 
responsible. For example, in dichogamous flowers, the pistils and stamens 
reach maturity at different times; in protogyny, the pistils mature first, and 
in protandry, the stamens mature before the pistils. Selfing is also 
impossible in dioecious species, where some plants bear flowers that have only 
pistils (pistillate or female flowers), while other individuals have flowers 
that produce only pollen (staminate or male flowers). In monoecious species, 
where pistillate and staminate flowers are found in the same plant, 
self-breeding is at least reduced. Heterostyly is another device that promotes 
outbreeding. Here some flowers (pins) possess a long pistil and short stamens, 
while others (thrums) exhibit the reverse condition; each plant individual 
bears only pins or only thrums. 
  
    Flower attractants.  As immobile organisms, plants normally need external 
agents for pollen transport. These can be insects, wind, birds, mammals, or 
water, roughly in that order of importance. In some plants the pollinators are 
simply trapped; in the large majority of cases, however, the flowers offer one 
or more rewards, such as sugary nectar, oil, solid food bodies, perfume, sex, 
an opportunity to breed, a place to sleep, or some of the pollen itself. For 
the attraction of pollinators, flowers provide either visual or olfactory 
signals. Color includes ultraviolet, which is perceived as a color by most 
insects and at least some hummingbird species. Fragrance is characteristic of 
flowers pollinated by bees, butterflies, or hawkmoths, while carrion or dung 
odors are produced by flowers catering to certain beetles and flies. A few 
orchids, using a combination of olfactory and visual signals, mimic the 
females of certain bees or wasps so successfully that the corresponding male 
insects will try to mate with them, thus achieving pollination 
(pseudocopulation).  
    While some flowers are ``generalists,'' catering to a whole array of 
different animals, others are highly specialized, being pollinated by a single 
species of insect only. Extreme pollinator specificity is an important factor 
in maintaining the purity of plant species in the field, even in those cases 
where hybridization can easily be achieved artifically in a greeenhouse or 
laboratory, as in most orchids. The almost incredible mutual adaptation 
between pollinating animal and flower which can frequently be observed 
exemplifies the idea of coevolution.  See  POLLEN. 
 �34s[Bastiaan J. D. Meeuse]
�33s 
  
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�6v�2v�32sPollucite   �21m �33s 
    A mineral tectosilicate having the composition Cs4Al4Si9O26PRODUCTH2O. 
Well-formed crystals are usually cubic, but are rare; it is most commonly 
massive. Hardness is 6**2 on Mohs scale, and specific gravity is 2.90. The 
mineral is colorless to white with a vitreous luster. Pollucite is rare. It 
has been found in large masses in the Varutraesk pegmatite in Sweden and in 
the lithium-rich pegmatite at Bakita, Rhodesia. In the United States it was at 
one time worked as an ore of cesium at Newry, Maine, and in the Black Hills of 
South Dakota.  See  SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sPolonium �21m �33s
    A chemical element, Po, atomic number 84. Marie Curie discovered the
radioisotope (sup 21)0Po in pitchblende. This isotope is the penultimate
member of the radium decay series. All polonium isotopes are radioactive, and
all are short-lived except the three ALPHA-emitters, artificially produced
(sup 208)Po (2.9 years) and (sup 209)Po (100 years), and natural (sup 210)Po
(138.4 days).
    Polonium (sup 210)Po) is used mainly for the production of neutron
sources. It can also be used in static eliminators and, when incorporated in
the electrode alloy of spark plugs, is said to improve the cold-starting
properties of internal combustion engines.
    Most of the chemistry of polonium has been determined using
(sup 210)Po, 1 curie of which weighs 222.2 micrograms; work with weighable
amounts is hazardous, requiring special techniques. Polonium is more metallic
than its lower homolog, tellurium. The metal is chemically similar to
tellurium, forming the bright red compounds SPoO3 and SePoO3. The metal is
soft, and its physical properties resemble those of thallium, lead, and
bismuth. Valences of 2 and 4 are well established; there is some evidence of
hexavalency. Polonium is positioned between silver and tellurium in the
electrochemical series.
    Two forms of the dioxide are known: low-temperature, yellow, face-centered
cubic (UO2 type), and high-temperature, red, tetragonal. The halides are
covalent, volatile compounds, resembling their tellurium analogs.  See
RADIOACTIVITY; TELLURIUM.
 �34s[Kenneth W. Bagnall]
�33s

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�6v�2v�32sPolyacrylate resin �21m �33s
    Polymers obtained from a variety of acrylic monomers, such as acrylic and
methacrylic acids, their salts, esters, and amides, and the corresponding
nitriles. The most important monomers are shown here.

		  CH3				  H
		   |				  |
	      CH2==C--COOCH3		     CH2==C--COOC2H5
	      Methyl methacrylate	      Ethyl acrylate

				       H
				       |
				  CH2==C--CN
				 Acrylonitrile

    Polymethyl methacrylate is a hard, transparent polymer with high optical
clarity, high refractive index, and good resistance to the effects of light
and aging. It and its copolymers are useful for lenses, signs, indirect
lighting fixtures, transparent domes and skylights, dentures, and protective
coatings.
    Solutions of polymethyl methacrylate and its copolymers are useful as
lacquers. Aqueous latexes formed by the emulsion polymerization of methyl
methacrylate with other monomers are useful as water-based paints and in the
treating of textiles and leather.
    Polyethyl acrylate is a tough, somewhat rubbery product. The monomer is
used mainly as a plasticizing or softening component of copolymers.
    Methyl methacrylate is of interest as a polymerizable binder for sand or
other aggregates, and as a polymerizable impregnant for concrete; usually a
cross-linking acrylic monomer is also incorporated. The binder systems
(polymer concrete) are used as overlays for bridge decks as well as for
castings, while impregnation is used to restore concrete structures and
protect bridge decks against corrosion by deicing salts.  See  ACRYLONITRILE;
PLASTICS PROCESSES; POLYACRYONITRILE RESINS; POLYMERIZATION.
 �34s[John A. Manson]
�33s

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�6v�2v�32sPolyacrylonitrile resins   �21m �33s 
    Hard, horny, relatively insoluble, and high-melting materials produced by 
the polymerization of acrylonitrile. Polyacrylonitrile (polyvinyl cyanide) is 
used almost entirely in copolymers. The copolymers fall into three groups: 
fibers, plastics, and rubbers. The presence of acrylonitrile in a polymeric 
composition tends to increase its resistance to temperature, chemicals, 
impact, and flexing. 
    The major use of acrylonitrile is in the form of fibers. By definition an 
acrylic fiber must contain at least 85% acrylonitrile; a modacrylic fiber may 
contain less, from 35 to 85% acrylonitrile. The high strength, high softening 
temperature, resistance to aging, chemicals, water, and cleaning solvents, and 
the soft woollike feel of fabrics have made the product popular for many uses 
such as sails, cordage, blankets, and various types of clothing. 
    Copolymers of vinylidene chloride with small proportions of acrylonitrile 
are useful as tough, impermeable, and heat-sealable packaging films. 
    Use has been made of copolymers of acrylonitrile with butadiene, often 
called NBR (formerly Buna N) rubbers, which contain 15 - 40% acrylonitrile. 
The NBR rubbers resist hydrocarbon solvents such as gasoline, abrasion, and in 
some cases show high flexibility at low temperatures.  See  RUBBER. 
    The development of blends and interpolymers of acrylonitrile-containing 
resins and rubbers represented a significant advance in polymer technology. 
The products, usually called ABS resins, typically are made by blending 
acrylonitrilestyrene copolymers with a butadiene-acrylonitrile rubber, or by 
interpolymerizing polybutadiene with styrene and acrylonitrile. 
    The combination of low cost, good mechanical properties, and ease of 
fabrication led to the rapid development of various uses for ABS resins. 
Applications include products requiring high impact strength, such as pipe, 
and sheets for structural uses, such as industrial duct work and components of 
automobile bodies.  See  ACRYLONITRILE; PLASTICS PROCESSES; RUBBER; STYRENE. 
 �34s[John A. Manson]
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�6v�2v�32sPolyamide resins   �21m �33s 
    Horny, whitish, translucent, high-melting polymers. They are used for 
fibers, bristles, bearings, gears, molded objects, coatings, and adhesives. 
The term nylon formerly referred specifically to synthetic polyamides as a 
class. Because of many applications in mechanical engineering, nylons are 
considered engineering plastics. 
    The most common commercial aliphatic polamides are nylons-6,6; -6; -6,10; 
-11; and -12. Nylon-6,6, nylon-6,10, nylon-6,12, and nylon-6 are the most 
commonly used polyamides for general applications as molded or extruded parts; 
nylon-6,6 and nylon-6 find general application as fibers. 
    As a group, nylons are strong and tough. Mechanical properties depend in 
detail on the degree and distribution of crystallinity, and may be varied by 
appropriate thermal treatment or by nucleation techniques. Because of their 
generally good mechanical properties and adaptability to both molding and 
extrusion, certain nylons are often used for gears, bearings, and electrical 
mountings. Nylon bearings and gears perform quietly and need little or no 
lubrication. Nylon resins are also used extensively as filaments, bristles, 
wire insulation, appliance parts, and film. Properties can also be modified by 
copolymerization. Reinforcement of nylons with glass fibers results in 
increased stiffness, lower creep and improved resistance to elevated 
temperatures.  See  HETEROCYCLIC POLYMER; NYLON; PLASTICS PROCESSES; POLYETHER 
RESINS; POLYMERIZATION. 
 �34s[John A. Manson]
�33s 
  
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�6v�2v�32sPolychaeta   �21m �33s 
    The largest class of the phylum Annelida, containing 68 - 70 families. 
About 1600 genera and 10,000 species have been named from worldwide areas; 
about one-fourth of this number may be synonymous. Polychaeta (meaning ``many 
setae'') is conveniently, though not clearly divisible into the Errantia, or 
free-moving annelids, and Sedentaria, or tubicolous families.  See  ERRANTIA; 
SEDENTARIA.  
    The body may be long, cylindrical, and multisegmented, or short and 
compact, with a limited number of segments. It consists of prostomium (Fig. 
1), or head; peristomium, or first segment around the mouth; trunk, or body 
proper; and tail region, or pygidium. Most segments have highly diagnostic 
paired, lateral fleshy appendages called parapodia. These are provided with 
secreted supporting rods and spreading fascicles of setae, or hooks, which 
display remarkable specificity. 
    The anterior end, or prostomium, may be a simple lobe derived from the 
larval trochophore, modified as a pseudoannulated cone, or covered by 
peristomial structures so as to be invisible. Oral tentacles for food 
gathering may be eversible from the buccal cavity; they may be long, slender, 
or thick and their surface smooth or papillated. 
    The anterior, preoral end may be developed as a thick, fleshy papillated, 
nonretractile proboscis (Fig. 2), or the prostomium may be completely 
retractile into the first several segments and protected by a cage formed of 
setae directed forward, or concealed by a compact operculum formed of setae of 
the first several segments. The anterior end of the alimentary tract is 
muscular or epithelial; it may be covered with soft papillae or hard 
structures. These structures function for secretion, food gathering, and 
maintaining traction; they are named for their form or function. 
    The trunk is the main body region and is composed of metameres numbering 
few to many. They may be similar to one another (homonomous) as in Errantia, 
or different (heteronomous) resulting in anterior thoracic and posterior 
abdominal regions. 
    Reproduction is highly evolved and diversified; it can be sexual or 
asexual. Sexual reproduction is usually dioecious, with the two sexes similar. 
In rare cases it is dimorphic.  
    Polychaetes range in length from a fraction of 0.04 in. (1 mm) to more 
than 144 in. (360 cm). Colors and patterns are varied and specific, due to 
pigment and refraction of light. Littoral, warm-water species may be brilliant 
and multicolored, whereas polar and deep-water species tend to be drab or 
sometimes melanistic to almost black. 
    Most polychaetes are free-living; some of the remaining members are 
commensal with another animal for attachment surface, for food, or for 
protection. Polychaetes are distributed in all marine habitats and show 
remarkable specificity according to latitude, depth, and kinds of substrata. 
Most of the families tend to be represented in any major geographic area, 
although taxa may differ with place.  See  ANNELIDA. 
 �34s[Olga Hartman]
�33s 
  
  
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�6v�2v�32sPolychlorinated biphenyls   �21m �33s 
    A generic term covering a family of partially or wholly chlorinated 
isomers of biphenyl. The commercial mixtures generally contain 40 - 60% 
chlorine with as many as 50 different detectable isomers present (see 
illustration). The polychlorinated biphenyl (PCB) mixture is a colorless, 
viscous fluid, relatively insoluble in water, that can withstand very high 
temperatures without degradation. PCBs do not conduct electricity and the more 
highly chlorinated isomers are not readily degraded in the environment. 
    The major uses of PCBs can be grouped in three major categories: open 
uses, partially closed systems, and closed systems. Examples of uses in the 
first category are in paints, inks, plastics, and paper coatings. The PCBs in 
all these products are in direct contact with the environment and can be 
leached out by water or vaporize into the atmosphere. The so-called carbonless 
carbon paper contains PCBs in the encapsulated ink and has been claimed to be 
responsible for the PCBs found extensively in recycled paper. They have also 
been used as plasticizers in polyvinyl chloride (PVC) and chlorinated rubbers. 
Uses of PCBs regarded as partially closed systems include the working fluid in 
heat exchangers and hydraulic systems. 
    It is not known exactly how PCBs are released into the environment or in 
what quantities. However, sewage outfalls, industrial and municipal disposal, 
leaking from dumps, and burning of refuse are certainly important sources. 
PCBs are found universally in the sewage of major cities, although the 
atmosphere is the major pathway of global transport. 
    The U.S. Food and Drug Administration has placed strict limits on PCBs in 
food products. Studies of the toxic properties of PCBs have revealed some 
serious effects on humans. There is evidence that one class, the 
polybrominated biphenyls (PBBs), have caused abnormalities in the blood of 
exposed humans that indicate damage to the immune system. 
 �34s[George Harvey]
�33s 
  
                             ******FORMULA****** 
  
  
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�6v�2v�32sPolycladida   �21m �33s 
    A class of marine Turbellaria which are several millimeters to several 
centimeters in length and whose leaflike bodies have a central intestine with 
radiating branches. Most species live in the littoral zone on the bottom, on 
seaweed or on other objects, or as commensals in the shells of mollusks and 
hermit crabs. None are parasitic. Except in warm waters, they are seldom 
brightly colored.  See  TURBELLARIA. 
 �34s[E. Ruffin Jones]
�33s 
  
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�6v�2v�32sPolycythemia �21m �33s
    Any disorder characterized by an increase in the numbers of circulating
red cells and usually by a concomitant increase in hemoglobin. Relative
polycythemia is really the result of hemoconcentration in which there is fluid
loss from the blood, with corresponding increases in proportions of cellular
elements. It is seen in excessive vomiting, diarrhea, dehydration, and similar
conditions. See ERYTHROCYTOSIS.
    Secondary polycythemias result from a compensatory increase in the
formation of red cells following hypoxia of the bone marrow. This condition is
most often associated with chronic conditions, such as heart or lung diseases
and prolonged exposure to high altitudes.
    Primary polycythemia or polycythemia vera is a chronic and ultimately
fatal disease of middle and old age in which there is a gradual increase in
the number of red cells and usually an increase in the number of platelets and
leukocytes. The viscosity of the blood is increased, which tends to slow the
rate of blood flow; this factor, together with the high platelet count,
predisposes to the thromboses commonly seen in the condition. Paradoxically
there is also a hemorrhagic tendency.
 �34s[Cecil Hougie]
�33s

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�6v�2v�32sPolyester resins   �21m �33s 
    Polymeric materials in which ester groups are in the main chains. The 
aliphatic polyesters tend to be relatively soft, and the aromatic derivatives 
are usually hard and brittle, or tough. The properties of either group may be 
modified by cross linking, crystallization, plasticizers, or fillers.  See  
ESTER.  
    The commercial products are: alkyds, which are used in paints, enamels, 
and in molding compounds; unsaturated polyesters or unsaturated alkyds which 
are used extensively with fiber glass for boat hulls and panels; aliphatic 
saturated polyesters; aromatic polyesters, such as polyethylene terephthalate 
which is used in the form of fibers and films; and the aromatic polycarbonates. 
    This article is devoted mainly to these five products. The polydiallyl 
esters, while frequently listed with the polyesters, are not true polyesters 
as defined above. For a discussion of polyvinyl esters  see  POLYVINYL RESINS. 
    The alkyds are commonly used as coatings. Combinations of conventional 
vegetable drying oils and alkyd resins represent the basis of most of the 
oil-soluble paints. The drying oil - alkyd may be further modified by the 
inclusion of a vinyl monomer, such as styrene. Some of the styrene 
polymerizes, probably as a graft polymer, and the remainder polymerizes and 
copolymerizes in the final drying or curing of the paint.  See  DRYING OIL; 
PAINT; POLYMERIZATION. 
    The unsaturated polyesters, in combination with glass fiber, have found 
applications as panels, roofing, radar domes, boat hulls, and protective armor 
for soldiers. The compositions are distinguished by ease of fabrication and 
high impact resistance.  See  POLYMERIC COMPOSITE. 
    Saturated aliphatic polyesters have long been frequently used as 
intermediates in the preparation of prepolymers for making segmented 
polyurethanes. Lactone rings can also be opened to yield linear polyesters. 
    The aromatic polyesters which have achieved general importance are the 
polyethylene terephthalates, which yield very strong and chemically resistant 
fibers and films. Polyethylene terephthalate is the principal ingredient of 
polyester fibers. Other aromatic polyesters include polybutylene 
terephthalate, which may be molded or extruded to yield materials that can 
replace metals or thermoset resins in some automotive, electrical, and 
specialty applications, especially when reinforced with glass fibers or 
mineral fillers. 
    Aromatic polycarbonates are a strong, tough group of thermoplastic 
polymers formed most frequently from bisphenol A and phosgene. The products, 
polycarbonates, are noted for high softening temperatures, and high impact 
resistance, clarity, and resistance to creep. Polycarbonate is usually 
available as a molding compound. Because of its high strength, toughness, and 
softening point, the resin, both by itself and as a glass-reinforced material, 
has found many electrical domestic and engineering applications. It is often 
used to replace glass and metals. Examples include bottles, unbreakable 
windows, appliance parts, electrical housings, marine propellers, and shotgun 
shells. Flame-retardant grades are of interest because of low toxicity and 
smoke emission on burning. 
    Polydiallyl esters are polymers of diallyl esters. Thermosetting molding 
compounds may be produced by careful limitation of the initial polymerization 
to yield a product which is fusible. Major applications are in electronic 
components, sealants, coatings, and glass-fiber composites.  See  PLASTICS 
PROCESSES.  
 �34s[John A. Manson]
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�6v�2v�32sPolyether resins   �21m �33s 
    Thermoplastic or thermosetting materials which contain ether-oxygen 
linkages, --C--O--C--, in the polymer chain. Depending upon the nature of the 
reactants and reaction conditions, a large number of polyethers with a wide 
range of properties may be prepared. The main groups of polyethers in use are 
epoxy resins, phenoxy resins, polyethylene oxide and polypropylene oxide 
resins, polyoxymethylene, and polyphenylene oxides. 
    The epoxy resins form an important and versatile class of cross-linked 
polyethers characterized by excellent chemical resistance, adhesion to glass 
and metals, electrical insulating properties, and ease and precision of 
fabrication. Various fillers such as calcium carbonate, metal fibers and 
powders, and glass fibers are commonly used in epoxy formulations in order to 
improve such properties as the strength and resistance to abrasion and high 
temperatures. Some reactive plasticizers act as curing agents, become 
permanently bound to the epoxy groups, and are usually called flexibilizers. 
Rubbery polymers are added to improve toughness and impact strength. Epoxies 
are commonly used in protective coatings. They are used as potting or 
encapsulating compositions for the protection of delicate electronic 
assemblies from the thermal and mechanical shock of rocket flight, and as dies 
for stamping metal forms. 
    Polyethylene oxide and polypropylene oxide are thermoplastic products 
whose properties are greatly influenced by molecular weight. Low-to-moderate 
molecular-weight polyethylene oxides vary in form from oils to waxlike solids. 
They are relatively nonvolatile, are soluble in a variety of solvents, and 
have found many uses as thickening agents, plasticizers, lubricants for 
textile fibers, and components of various sizing, coating, and cosmetic 
preparations. The polypropylene oxides of similar molecular weight have 
somewhat similar properties, but tend to be more oil-soluble (hydrophobic) and 
less water-soluble (hydrophilic). While polyalkylene oxides are not of 
interest as such in structural materials, polypropylene oxides are used 
extensively in the preparation of polyurethane foams. 
    Phenoxy resins are transparent, strong, ductile, and resistant to creep, 
and, in general, resemble polycarbonates in their behavior. The major 
application is as a component in protective coatings, especially in metal 
primers.  See  POLYESTER RESINS. 
    Polyphenylene oxide (PPO) is the basis for an engineering plastic 
characterized by chemical, thermal, and dimensional stability. Polyphenylene 
oxide is outstanding in its resistance to water. Uses include medical 
instruments, pump parts, and insulation. 
    Polyoxymethylene, or polyacetal, resins are polymers of formaldehyde. 
Having high molecular weights and high degrees of crystallinity, they are 
strong and tough and are established in the general class of engineering 
thermoplastics. Polyacetals are typically resistant to fatigue, creep, organic 
chemicals (but not strong acids or bases), and have low coefficients of 
friction. Electrical properties are also good. The combination of properties 
has led to many uses such as plumbing fittings, pump and valve components, 
bearings and gears, computer hardware, automobile body parts, and appliance 
housings.  See  EPOXIDATION; PLASTICS PROCESSING; POLYMERIZATION. 
 �34s[John A. Manson]
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�6v�2v�32sPolyethylene glycol   �21m �33s 
    Any of a series of water-soluble polymers with the general formula 
HO--(CH2--CH2--O)subn--H. These colorless, odorless compounds range in 
appearance from viscous liquids up to waxy solids. 
    The low-molecular-weight members, diethylene glycol (n = 2) through 
tetraethylene glycol (n = 4), are produced as pure compounds and find use as 
humectants, dehydrating solvents for natural gas, textile lubricants, 
heat-transfer fluids, solvents for aromatic hydrocarbon extractions, and 
intermediates for polyester resins and plasticizers. 
    The intermediate members of the series with average molecular weights of 
200 to 20,000 are used commercially in ceramic, metal-forming, and 
rubber-processing operations; as drug suppository bases and in cosmetic 
creams, lotions, and deodorants; as lubricants; as dispersants for casein, 
gelatins, and inks; and as antistatic agents. 
    The highest members of the series have molecular weights from 100,000 to 
10,000,000. They are of interest because of their ability at very low 
concentrations to reduce friction of flowing water.  See  ETHYLENE OXIDE; 
GLYCOL; POLYOXYETHYLATION OF ALCOHOL. 
 �34s[Robert K. Barnes]
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�6v�2v�32sPolyfluoroolefin resins   �21m �33s 
    Resins distinguished by their resistance to heat and chemicals and by the 
ability to crystallize to a high degree. Several main products are based on 
tetrafluoroethylene, F2CDOUBLE BARCF2 (TFE), hexafluoropropylene, F2CDOUBLE 
BARCFCF3 (HFP), and monochlorotrifluoroethylene, FClCDOUBLE BARCF2 (CFTE).  
See  POLYVINYL RESINS. 
    The polymer polytetrafluoroethylene is insoluble, resistant to heat and 
chemical attack, and in addition, has the lowest coefficient of friction of 
any solid. Aqueous suspensions of the polymer can also be used for coating 
various articles. However, special surface treatments are required to ensure 
adhesion because polytetrafluoroethylene does not adhere well to anything. 
Polytetrafluoroethylene (TFE resin) is used for bearings, valve seats, 
packings, gaskets, coatings, and tubing, and can withstand relatively severe 
conditions. Because of its excellent electrical properties, 
polytetrafluoroethylene is useful when a dielectric material is required for 
service at a high temperature. The nonadhesive quality is turned to advantage 
in use of polytetrafluoroethylene to coat articles such as rolls and cookware 
to which materials might otherwise adhere. 
    The properties of polymonochlorotrifluoroethylene (CTFE resin) are 
generally similar to those of polytetrafluoroethylene; however, the presence 
of the chlorine atoms in the former causes the polymer to be a little less 
resistant to heat and to chemicals. Thus the applications of 
polychlorotrifluoroethylene are in general similar to those for 
polytetrafluoroethylene.  
    Polyvinylidene fluoride has properties generally similar to those of other 
fluorinated resins: relative inertness, low dielectric constant, and thermal 
stability (up to about 150DGC or 300DGF). The resins (PVF2 resins) are, 
however, stronger and less susceptible to creep and abrasion than TFE and CTFE 
resins. Applications of polyvinylidene fluoride are mainly as electrical 
insulation, piping, process equipment, and as a protective coating in the form 
of a liquid dispersion. 
    Several types of fluorinated, noncrystallizing elastomers have been 
developed in order to meet needs (usually military) for rubbers which possess 
good low-temperature behavior with a high degree of resistance to oils and to 
heat, radiation, and weathering.  See  HALOGENATED HYDROCARBON; PLASTICS 
PROCESSING; POLYMERIZATION. 
 �34s[John A. Manson]
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�6v�2v�32sPolygalales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 7 families and nearly 1900 species, mostly of tropical and 
subtropical regions. The vast majority of the species belong to only 3 
families, the Malpighiaceae (about 800 species), Polygalaceae (about 750 
species), and Vochysiaceae (about 200 species). Within its subclass the order 
is distinguished by its simple leaves and usually irregular flowers, which 
have the perianth and stamens attached directly to the receptacle 
(hypogynous), and often have the anthers opening by terminal pores instead of 
longitudinal slits.  See  MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPolygon   �21m �33s 
    A polygon of n sides (n > 2) is a figure formed by joining an ordered set 
of n points called vertices by line segments called sides that connect each 
point to its immediate successor (where the first point is considered the 
successor of the last). Euclid considered a polygon to be this figure and also 
the plane region bounded by these vertices and sides. Modern usage considers 
the polygon to be any set of line segments and vertices forming a closed 
broken line. A polygon is simple if each side intersects only its two 
neighbors. It is plane or skew according as its sides do or do not lie in a 
plane. A simple plane polygon is convex if the polygonal region that it bounds 
lies wholly on one side of the (extended) line of each side. A polygon of 3, 
4, 5, 6, 7, 8, 10, or 12 sides is called a triangle (3), quadrilateral (4), 
pentagon (5), hexagon (6), heptagon (7), octagon (8), decagon (10), or 
dodecagon (12).  See  HEXAGON; OCTAGON; PENTAGON; QUADRILATERAL; REGULAR 
POLYTOPES; SQUARE; TRAPEZOID; TRIANGLE. 
 �34s[James S. Frame]
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�6v�2v�32sPolygonales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Caryophyllidae of the class Magnoliopsida (dicotyledons). The 
order consists only of the family Polygonaceae, with about 800 species, most 
abundant in north temperate regions. Within its subclass the order is 
characterized by its well-developed endosperm; an ovary with but one chamber 
(unilocular), mostly tricarpellate with a single basal ovule which is usually 
straight and has the micropyle at the opposite end from the stalk 
(orthotropous); the flowers are often trimerous, that is, with floral parts in 
sets of three, and usually two or three sets of stamens. Rhubarb (Rheum 
rhaponticum) and buckwheat (Fagopyrum esculentum) are familiar members of the 
Polygonales.  See  BUCKWHEAT; CARYOPHYLLIDAE; MAGNOLIOPSIDA; RHUBARB. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPolyhedron   �21m �33s 
    A solid, all of whose boundary points lie in a finite number of planes (at 
least four). The bounding sections formed by these planes are plane polygonal 
regions called faces of the polyhedron, whose sides are called edges of the 
polyhedron, and whose vertices are called vertices of the polyhedron. 
According to the number of faces, a polyhedron is called a tetrahedron (4 
faces), pentahedron (5), hexahedron (6), octahedron (8), dodecahedron (12), or 
icosahedron (20). For any convex polyhedron (but not for all polyhedrons) the 
numbers of vertices V, edges E, and faces F are related by the equation V - E 
+ F = 2. 
    Regular polyhedrons (see illustration) are convex polyhedrons whose faces 
are congruent regular polygons, forming equal dihedral angles at each edge. 
There are only five such regular solids, called platonic solids.  See  CUBE; 
EUCLIDEAN GEOMETRY; OCTAHEDRON; PARALLELEPIPED; PRISM; PRISMATOID AND 
PRISMOID; PYRAMID AND FRUSTUM; REGULAR POLYTOPES; SOLID (GEOMETRY); 
TETRAHEDRON.  
 �34s[James S. Frame]
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�6v�2v�32sPolyhydric alcohol   �21m �33s 
    Polyhydric alcohols, or, as they are preferably called, polyols, are 
compounds containing more than one hydroxyl group (--OH). Each hydroxyl is 
attached to separate carbon atoms of an aliphatic skeleton. This group 
includes glycols, glycerol, and pentaerythritol and also such products as 
trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, sorbitol, inositol, 
and polyvinyl alcohol. Polyols are obtained from many plant and animal sources 
and are synthesized by a variety of methods. 
    Polyols such as glycerol, pentaerythritol, trimethylolethane, and 
trimethylolpropane are used in making alkyd resins for decorative and 
protective coatings. Glycols, glycerol, 1,2,6-hexanetriol, and sorbitol find 
application as humectants and plasticizers for gelatin, glue, and cork. 
Explosives are made by the nitration of glycols, glycerol, and pentaerythritol. 
    The polymeric polyols used in manufacture of the urethane foams represent 
a series of synthetic polyols. These polyols are generally polyoxyethylene or 
polyoxypropylene adducts of di- to octahydric alcohols.  See  GLYCEROL; 
GLYCOL; PENTAERYTHRITOL. 
 �34s[Philip C. Johnson]
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�6v�2v�32sPolymer �21m �33s
    The terms polymer, high polymer, macromolecule, and giant molecule are
used to designate high-molecular-weight materials of either synthetic or
natural origin. Plastics are relatively stiff at room temperature, rubbers or
elastomers are flexible and retract quickly after stretching, and fibers are
especially strong filamentary materials. Coatings are generally either
plastics or rubbers which have been applied as a thin layer on a substrate. In
practice, plastics, rubbers, fibers, and coatings are used as formulations of
the polymers with other ingredients such as fillers, pigments, plasticizers,
flow improvers, and stabilizers against aging and degradation.
    For a discussion of the general methods of preparation  see
POLYMERIZATION. For descriptions of typical synthetic polymers of the
condensation type  see	CELLULOSE; PHENOLIC RESIN; POLYAMIDE RESINS; POLYESTER
RESINS; POLYETHER RESINS; POLYSULFIDE RESINS; POLYURETHANE RESINS; SILICONE
RESINS; UREA-FORMALDEHYDE-TYPE RESINS. For descriptions of some important
members of the addition-type synthetic polymers  see  HYDROCARBON RESIN;
POLYACRYLATE RESIN; POLYACRYLONITRILE RESINS; POLYFLUOROOLEFIN RESINS;
POLYOLEFIN RESINS; POLYSTYRENE RESIN; POLYVINYL RESINS.
    Natural products, rubbers, fibers, and paints are treated in other
articles. Biological polymers, as in enzymes and living tissues, are treated
in the appropriate articles.  See  MANUFACTURED FIBER; NATURAL FIBER; PROTEIN;
RUBBER; SURFACE COATING; TERPENE.

    Properties.  The properties of polymeric materials are determined by the
molecular properties of the macromolecules, the morphology, and the type of
formulation involving plasticizers or fillers. The morphology in turn depends
on the conditions of fabrication in which molecular orientation or
crystallization may be induced. Properties also depend on the temperature and
elapsed time of the measurement.
    Molecular properties include molecular size and weight, molecular
structure or architecture, molecular-weight distribution, polarity, and
flexibility of the polymeric chains (or chain segments between cross-links in
cured or vulcanized polymers). Taken together, molecular properties determine
the attractive forces between the molecules, the morphology or arrangement of
masses of molecules, and the general behavior of the polymer.
    The desirable properties of high polymers (strength and resistance to
solvents) increase rapidly with increasing molecular weight in the low ranges
of molecular weight, and more slowly in the high ranges. On the other hand,
the melt viscosity increases with molecular weight in the opposite manner,
slowly at first and rapidly in the higher-molecular-weight range. The ease of
fabrication (molding, extrusion, and shaping) of polymeric compositions varies
inversely with the melt viscosity; that is, the materials become increasingly
difficult to mold or extrude at very high values of molecular weight. The
optimum molecular weight of a polymer frequently varies for different
applications and different methods of fabrication.
    The atoms in the chains are held together by primary valence bonds. If it
were possible to apply a force of tension only to the primary valence bonds,
then a tensile strength of more than 2,000,000 psi (1.4 x 10(sup 10) pascals)
would be observed. In reality, however, under tension the molecules slip past
one another and the resistance to that slippage is due to the effective
attractive forces (van der Waals forces) between the molecules. Also, flaws
reduce strength by initiating cracks.
    The flexibility of linear polymer chains and of the segments between
cross-links in cured products is decreased by the presence of polar groups and
regularity in the molecular structure. A nonpolar, irregular chain should be
the most flexible. Products containing highly flexible chains are rubbery,
soft, and nonbrittle, with relatively high resistance to impact and to tear.
    When a close fit between chains is possible, crystallization can take
place spontaneously or upon drawing or cooling. The presence of polar
attracting groups enhances the tendency to crystallize. It is also possible to
form extended-chain crystallites in which molecules are packed together
longitudinally in fibrillar arrays. This may be achieved by the use of special
extrusion or spinning techniques. Such highly anisotropic crystallites confer
high levels of strength and stiffness in the chain direction.
    The illustration shows responses of typical rubbers, plastics, and fibers
to large stresses. A reinforced or crystallizable rubber exhibits a relatively
low value of breaking stress, but high elongation. A ductile plastic such as
polyethylene exhibits yielding, drawing, and, at high elongations, some
strengthening due to orientation. A brittle plastic such as polystyrene does
not yield much, and breaks at a low elongation; a fiber exhibits the highest
strength, high stiffness, and low elongation. In all cases, the shape and
position of the stress-strain curve are determined by the balance struck
between cohesion and segmental mobility at the temperature and testing rate
concerned.
    As plastics are used extensively in engineering applications, the
toughness or the ability to resist fracture assumes considerable importance.
Much research has been directed to the understanding of fracture processes, on
one hand, and to the improvement of toughness, on the other. In general,
toughness appears to require some combination of high modulus with the ability
to dissipate applied energy by relaxation of molecular segments or of added
materials.


    Compounding.  Plastic masses, rubber formulations, coatings, and other
polymeric compositions may contain age inhibitors, strengthening and coloring
pigments, flow improvers, and plasticizing or softening agents.
    The addition of plasticizers to a polymeric material causes it to be
softer and more rubbery in character. Plasticizers are held in association
with the polymer chains by secondary valence forces. They separate the
molecules, thus reducing the effective intermolecular attractive forces.
Finely divided polar substances may act as strengthening fillers for plastics.
Age inhibitors, such as free-radical inhibitors and light-masking agents, are
almost always incorporated in polymeric compositions.
    Polyblends are produced by the addition of small amounts of a rubbery
polymer to a polymeric glass. The impact strength of the glass thus is
substantially increased. The rubbery polymer is not truly compatible and
exists as a finely dispersed separate phase. Interpenetrating polymer
networks, in which two different networks are intertwined, constitute an
interesting special case of polyblends. The phenomenon has somewhat similar
counterparts in inorganic glass technology and in physical metallurgy.	See
PLASTIC PROCESSING; POLYMERIC COMPOSITE.
 �34s[John A. Manson]
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�6v�2v�32sPolymeric composite   �21m �33s 
    Any of the combinations or compositions that comprise two or more 
materials as separate phases, at least one of which is a polymer. By combining 
a polymer with another material, such as glass, carbon, or another polymer, it 
is often possible to obtain unique combinations or levels of properties. 
Typical examples of synthetic polymeric composites include glass-, carbon-, or 
polymerfiber-reinforced thermoplastic or thermosetting resins, 
carbon-reinforced rubber, polymer blends, silica- or mica-reinforced resins, 
and polymer-bonded or mica-reinforced resins, and polymer-bonded or 
-impregnated concrete or wood. It is also often useful to consider as 
composites such materials as coatings (pigment-binder combinations) and 
crystalline polymers (crystallites in a polymer matrix). Typical naturally 
occurring composites include wood (cellulosic fibers bonded with lignin) and 
bone (minerals bonded with collagen). On the other hand, polymeric 
compositions compounded with a plasticizer or very low proportions of pigments 
or processing aids are not ordinarily considered as composites. 
    Typically, the goal is to improve strength, stiffness, or toughness, or 
dimensional stability by embedding particles or fibers in a matrix or binding 
phase. A second goal is to use inexpensive, readily available fillers to 
extend a more expensive or scarce resin. Still other applications include the 
use of some fillers such as glass spheres to improve processability, the 
incorporation of dry-lubricant particles such as molybdenum sulfide to make a 
self-lubricating bearing, and the use of fillers to reduce permeability. 
    For discussion of common thermosetting matrixes  see  POLYESTER RESINS; 
POLYETHER RESINS. For details on glass and carbon fibers  see  GLASS; 
GRAPHITE. For information on common thermoplastic matrixes, polyblends, 
polymer concretes, and reinforced rubber  see  POLYACRYLATE RESIN; POLYAMIDE 
RESINS; POLYMER; POLYSTYRENE RESINS; POLYVINYL RESINS; RUBBER.  See also  
COMPOSITE MATERIALS. 
 �34s[John A. Manson]
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�6v�2v�32sPolymerization �21m �33s
    The linking of small molecules (monomers) to make larger molecules.
Polymerization requires that each small molecule have at least two reaction
points or functional groups. There are two distinct major types of
polymerization processes, condensation polymerization, in which the chain
growth is accompanied by elminination of small molecules such as H2O or CH3OH,
and addition polymerization, in which the polymer is formed without the loss
of other materials. There are many variants and subclasses of polymerization
reactions.

	      O
	      ||       Catalyst
nH2N--(CH2)5--C--OH ------------->

  epsilon-Aminocaproic acid

		    H	       O
		    ||	       ||
	H BRACKET --N--(CH2)5--C-- BRACKET nOH + (n - 1)H2O		   (1)

    An example of the condensation process is the reaction of
EPSILON-aminocaproic acid in the presence of a catalyst to form the polyamide,
nylon-6. The repeating structural unit is equivalent to the starting material
minus H and OH, the elements of water. The molecules formed are linear because
the total functionality of the reaction system (functional groups per molecule)
is always two. However, if a trifunctional material, such as a tricarboxylic
acid, were added to the nylon-6,6 polymerizing mixture, a branched polymeric
structure would result, because two of the carboxylic groups would participate
in one polymer chain, and the third carboxylic group would start the growth of
another. Under appropriate conditions, these chains can become bridges between
linear chains and the polymer becomes cross-linked. The arrangements of the
chains are shown in Fig. 1.  See  CONDENSATION REACTION.

    An example of addition polymerization is reaction (2). The

		 Catalyst or
	nH2C==O  ------------> [--CH2--O--]sub n			   (2)
		  Initiator

	Formal- 	       Polyoxymethylene
	dehyde

structure of the repeating unit is the difunctional monomeric unit, or
``mer.'' In the presence of catalysts or initiators, the monomer yields a
polymer by the joining together of n mers. If n is a small number, 2 - 10, the
products are dimers, trimers, tetramers, or oligomers, and the materials are
usually gases, liquids, oils, or brittle solids. In most solid polymers, n has
values ranging from a few score to several hundred thousand, and the
corresponding molecular weights range from a few thousand to several million.
The end groups of this example of addition polymers are shown to be fragments
of the initiator.
    If only one monomer is polymerized, the product is called a homopolymer.
The polymerization of a mixture of two monomers of suitable reactivity leads
to the formation of a copolymer, a polymer in which the two types of mer units
have entered the chain in a more or less random fashion. If chains of one
homopolymer are chemically joined to chains of another, the product is called
a block or graft copolymer:


			A--B--A--A--B--B--B
			  Random copolymer

		     A--A--A--A--B--B--B--B--B
			   Block copolymer

			    A--A--A--A--A
				  |
				  B
				  |
				  B
				  |
				  B
			    Graft polymer

    Isotactic and syndiotactic (stereoregular) polymers are formed in the
presence of complex catalysts, or by changing polymerization conditions, for
example, by lowering the temperature. The groups attached to the chain in a
stereoregular polymer are in a spatially ordered arrangement. The
configuration of these ordered polymers and the disordered, atactic form is
shown in Fig. 2. The regular structures of the isotactic and syndiotactic
forms make them often capable of crystallization. The crystalline melting
points of isotactic polymers are often substantially higher than the softening
points of the atactic product.
    In Fig. 2 each carbon atom to which a phenyl group is attached is
asymmetrically substituted. For illustration, the heavily marked bonds are
assumed to project up from the paper, and the dotted bonds down. Thus in a
fully syndiotactic polymer, asymmetric carbons alternate in their left- or
right-handedness (alternating d, l configurations), while in an isotactic
polymer, successive carbons have the same steric configuration (d or l).

    Among the several kinds of polymerization catalysis, free-radical
initiation has been most thoroughly studied and is most widely employed.
Atactic polymers are readily formed by free-radical polymerization, at
moderate temperatures, of vinyl and diene monomers and some of their
derivatives.  See  CATALYSIS; FREE RADICAL.
    Some polymerizations can be initiated by materials, often called ionic
catalysts, that contain highly polar reactive sites or complexes. The term
heterogeneous catalyst is often applicable to these materials because many of
the catalyst systems are insoluble in monomers and other solvents. These
polymerizations are usually carried out in solution from which the polymer can
be obtained by evaporation of the solvent or by precipitation on the addition
of a nonsolvent. A distinguishing feature of complex catalysts is the ability
of some representatives of each type to initiate streoregular polymerization
at ordinary temperatures or to cause the formation of polymers which can be
crystallized.  See  CHAIN REACTION (CHEMISTRY); CHEMICAL DYNAMICS;
HETEROGENEOUS CATALYSIS; INHIBITOR (CHEMISTRY); INORGANIC POLYMER; ORGANIC
REACTION MECHANISM; PLASTICS PROCESSING; POLYMER.
 �34s[John A. Manson]
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�6v�2v�32sPolymorphism (crystallography)   �21m �33s 
    The property of crystallizing in two or more forms. The term is applied to 
crystals of the same substance having a different structure. Substances such 
as CaCO3 which exist in two crystal forms are said to be dimorphous, while 
substances such as TiO2 which appear in three forms are termed trimorphous. 
The modifications of these substances are stable at normal temperature and in 
a comparable temperature range. Other polymorphic forms, such as those of SiO2 
(quartz, cristoballite, and trydimite), have a specific nonoverlapping 
stability range. Many polymorphic modifications are only stable under high 
pressure. The polymorphic forms of the elements are called allotropic 
modifications.  See  CRYSTAL. 
 �34s[Willy C. Dekeyser]
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�6v�2v�32sPolymorphism (genetics)   �21m �33s 
    A form of genetic variation, specifically a discontinuous variation, 
occurring within plant and animal species in which distinct forms exist 
together in the same population, even the rarest of them being too common to 
be maintained solely by mutation. Thus the human blood groups are examples of 
polymorphism, while geographical races are not; nor is the diversity of height 
among humans because height is ``continuous,'' and does not fall into distinct 
tall, medium, and short types.  See  BLOOD GROUPS; MUTATION. 
    Distinct forms must be controlled by some switch which can produce one 
form or the other without intermediates such as those arising from 
environmental differences. This clear-cut control is provided by the 
recombination of the genes. Each gene may have numerous effects and, in 
consequence, all genes are nearly always of importance to the organism by 
possessing an overall advantage or disadvantage. They are very seldom of 
neutral survival value as minor individual variations in appearance often are. 
Thus a minute extra spot on the hindwings of a tiger moth is in itself 
unlikely to be of importance to the survival of the insect, but the gene 
controlling this spot is far from negligible since it also affects fertility.  
See  RECOMBINATION (GENETICS). 
    Genes having considerable and discontinuous effects tend to be eliminated 
if harmful, and each gene of this kind is therefore rare. On the other hand, 
those that are advantageous and retain their advantage spread through the 
population so that the population becomes uniform with respect to these genes. 
Evidently, neither of these types of genes can provide the switch mechanism 
necessary to maintain a polymorphism. That can only be achieved by a gene 
which has an advantage when rare, yet loses that advantage as it becomes 
commoner.  
    Occasionally there is an evironmental need for diversity within a species, 
as in butterfly mimicry. Mimicry is the resemblance of different species to 
one another for protective purposes, chiefly to avoid predation by birds. 
Sexual dimorphism also falls within the definition of genetic polymorphism. In 
any species, males and females are balanced at optimum proportions which are 
generally near equality. Any tendency for one sex to increase relative to the 
other would be opposed by selection. Batesian mimicry and sexual dimorphism, 
however, are rather special instances.  See  GENETICS. 
 �34s[E. B. Ford]
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�6v�2v�32sPolymyxin   �21m �33s 
    A basic polypeptide antibiotic produced as a fermentation product by 
certain strains of the bacterium Bacillus polymyxa. The antibacterial action 
of the polymyxins is uniquely directed toward gram-negative bacteria. Cautious 
therapeutic applications have saved many lives in the treatment of urinary 
tract infections, bacterial meningitis, bacteremias, and burn and wound 
infections. The polymyxins are strongly bactericidal, and resistance toward 
them is not readily developed by susceptible microorganisms.  See  ANTIBIOTIC. 
 �34s[Robert G. Benedict]
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�6v�2v�32sPolynomial systems of equations �21m �33s
    Systems of mathematical equations of the form of system (1). Each

		    f1(x1,x2, . . . , x[sub n]) = 0
		    f2(x1,x2, . . . , x[sub n]) = 0
		    . . . . . . . . . . . . . . .
		    f(sub m) (x1,x2, . . . , x[sub n]) = 0		     (1)

f(sub i)(x1,x2, . . . ,x[sub n]), i = 1, 2, . . . , m, is a sum of terms of the
form shown in expression (2), where the coefficient (a[sub i1])(sup i2) . . .
(sub in) is
									     (2)
a(sub i1)(sup i2) . . . (sub in)x1(sup i1)  x2(sup i2) . . . x(sub n)(sup in)


a constant, or fixed number, and the exponent isubj of the variable xsubj is a
nonnegative whole number. An example of such a system in two variables is
system (3). The expressions f(sub i)(x1,x2, . . . ,x[sub n]) are called
polynomials in

		    x(sup 2) - xy + y(sup 2) - 1 = 0
		    x(sup 2) + xy - 3y(sup 2) - 2x + 2y + 1 = 0 	    (3)


several variables.  The problem posed by system (1) is to find necessary and
sufficient conditions that there exist values of the variables x1 = a1, x2 = a2,
.  .  .  , x(sub n) = a(sub n) which simultaneously satisfy each equation of the
system, and to find all such sets of values, which are called solutions of the
system.  In example (3), a complete set of solutions is given by x = 1, y = 0; x
= 0, y = 1; x = 1; y = 1; and x = -1, y = -1.  See EQUATIONS, THEORY OF; LINEAR
SYSTEMS OF EQUATIONS.
 �34s[Ross A.  Beaumont]
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�6v�2v�32sPolynuclear hydrocarbon   �21m �33s 
    One of a class of hydrocarbons possessing more than one ring. The aromatic 
polynuclear hydrocarbons may be divided into two groups. In the first, the 
rings are fused, which means that at least two carbon atoms are shared between 
adjacent rings. Examples are naphthalene (I), which has two six-membered 
rings, and acenaphthene (II), which has two six-membered rings and one 
five-membered ring. 
    In the second group of polynuclear hydrocarbons, the aromatic rings are 
joined either directly, as in the case of biphenyl (III), or through a chain 
of one or more carbon atoms, as in 1,2-diphenylethane (IV). 
                             ******FORMULA****** 
  
    The higher-boiling polynuclear hydrocarbons found in coal tar or in tars 
produced by the pyrolysis or incomplete combustion of carbon compounds are 
frequently fused ring hydrocarbons, some of which may be carcinogenic. See 
ANTHRACENE; AROMATIC HYDROCARBON;  BIPHENYL; DIPHENYLMETHANE; 
HEXAPHENYLETHANE; INDENE; NAPHTHALENE; PHENANTHRENE; STEROID; TRIPHENYLMETHANE. 
 �34s[Charles K. Bradsher]
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�6v�2v�32sPolyolefin resins   �21m �33s 
    Polymers derived from unsaturated hydrocarbons containing the ethylene or 
diene groups. Broadly, polyolefin resins may include virtually all addition 
polymers; however, the term polyolefin is specifically used for polymers of 
ethylene, the alkyl derivatives of ethylene (the ALPHA-olefins), and the 
dienes.  
    The polyvinyl resins, the fluorocarbon polymers, and other addition 
polymers are covered in other articles.  See  POLYFLUOROOLEFIN RESINS; 
POLYMERIZATION; POLYVINYL RESINS. 
    Polyethylene is a whitish, transulucent polymer of moderate strength and 
high toughness. The physical properties vary markedly with the degree of 
crystallinity and with the size and distribution of the crystalline regions. 
With increasing crystallinity or density, the products generally become 
stiffer and stronger, and have higher softening temperatures and higher 
resistance to penetration by liquids and gases; at the same time, they lose 
some of their resistance to tear, impact, and stress cracking, and higher 
temperatures and pressures are needed for molding. The major uses of 
polyethylene are as packaging films, containers and bottles, molded articles, 
electrical insulation, coatings, and pipe.  See  ETHYLENE. 
    High-molecular-weight, isotactic, highly crystalline polypropylene is 
generally similar in properties to high-density polyethylene. In comparison 
with the latter, isotactic polypropylene is harder and stronger. The use of 
both unoriented and oriented films of polyproylene homopolymers and copolymers 
has grown rapidly in such applications as packaging for textiles and food, 
liners for bags, and heat-shrinkable wraps for records and other articles.  
See  PROPYLENE. 
    The polyisobutylene polymers vary in properties from low-molecular-weight 
oils to high-molecular-weight rubbery solids. The main use of polyisobutylene 
is in the form of the copolymer with isoprene. The copolymer, known as butyl 
rubber, is distinguished by its impermeability to gases and its resistance to 
aging; it is used in automobile tires and tubes. 
    The discovery of the stereospecific catalysts has made possible the 
formation of high-molecular-weight, isotactic, crystalline polymers of other 
ALPHA-olefins, such as ALPHA-butene, ALPHA-octene, and ALPHA-dodecene. Typical 
melting temperatures of the crystalline, isotactic polymers of some of the 
ALPHA-olefins are relatively high. The high-melting polymers are of 
considerable interest because relatively few thermoplastic polymers are 
available which have high softening temperatures and at the same time can be 
easily fabricated. 
    Butadiene, isoprene, and 2-chlorobutadiene have gained the widest use of 
dienes employed in the polymer field. Butadiene and 2-chlorobutadiene have 
long been used in the production of synthetic rubbers by free-radical 
catalysis. The isoprene structure has long been recognized to correspond to 
the repeating unit in natural rubber.  See  POLYACRYLONITRILE RESINS; POLYMER; 
POLYSTYRENE RESIN. 
 �34s[John A. Manson]
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�6v�2v�32sPolyoma virus   �21m �33s 
    A virus which is capable of producing a number of tumors in newborn mice, 
rats, guinea pigs, hamsters, and rabbits. The virus grows readily in mouse 
embryo tissue culture and destroys the cells with the production of a 
hemagglutinin. It transforms hamster cells growing in tissue culture, which 
are then capable of producing sarcomas in hamsters. During the transformation 
process the infectious virus disappears, although an antigen to the virus can 
be demonstrated in the tumors.  See  ANIMAL VIRUS; TISSUE CULTURE. 
 �34s[Alice E. Moore]
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�6v�2v�32sPolyoxyethylation of alcohol �21m �33s
    The process of reacting an alcohol with ethylene oxide to produce a
polyether, as in the reaction below, in which R of the alcohol may be
aliphatic or aromatic.

			 O
			/  \
		ROH + nCH2--CH2 --> RO--(CH2CH2O)sub n H


    Polyoxyethylation of alcohols can be carried out to produce
very-high-molecular-weight products. Depending on the size of the alcohol and
the moles of ethylene oxide added, these high-molecular-weight products begin
to resemble in properties the simple, high polymers of ethylene oxide,
polyethylene glycols.  See  ETHYLENE OXIDE; POLYETHYLENE GLYCOL;
POLYMERIZATION.
 �34s[Robert K. Barnes]
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�6v�2v�32sPolyplacophora   �21m �33s 
    A class of the phylum Mollusca, sometimes termed Loricata. Commonly called 
chitons, polyplacophorans form a relatively stereotyped class of nearly 600 
marine species placed in 43 genera, almost all living on wave-swept rocks of 
the lower littoral and upper sublittoral. The class is divided into two 
orders, the Lepidopleurida and Chitonida.  See  CHITON. 
    Adhering by a sucker foot, as in gastropod limpets, the flattened 
elliptical chiton body bears eight articulated shell plates. The detailed 
structure of the plates, and of the mantle tissues which secrete them, are 
consistent with shell-mantle anatomy throughout the other molluscan classes.  
See  MOLLUSCA. 
 �34s[W. D. Russell-Hunter]
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�6v�2v�32sPolyploidy   �21m �33s 
    The occurrence of related forms possessing chromosome numbers which are 
multiples of a basic number (n), the haploid number. Forms having 3n 
chromosomes are triploids; 4n, tetraploids; 5n, pentaploids, and so on. 
Autopolyploids are forms derived by the multiplication of chromosomes from a 
single diploid organism. As a result the homologous chromosomes come from the 
same source. These are distinguished from allopolyploids, which are forms 
derived from a hybrid between two diploid organisms. As a result, the 
homologous chromosomes come from different sources. 
    About one-third of the species of vascular plants have originated at least 
partly by polyploidy, and as many more appear to have ancestries which involve 
ancient occurrences of polyploidy. The condition can be induced artificially 
with the drug colchicine. 
    In animals, most examples of polyploidy occur in groups which are 
parthenogenetic, such as crustaceans of the genus Artemia, certain earthworms, 
weevils of the family Curculionidae, moths of the genus Solenobia, sawflies of 
the genus Diprion, lizards of the genus Cnemidophorus, and fishes of the 
genera Carassius and Poecilopsis; or in species which reproduce asexually by 
fission, such as the flatworm Dendrocoelum infernale. Sexual polyploids have 
been found in the frog genera Odontophrynus, Ceratophrys, and Phyllomedusa.  
See  CHROMOSOME ABERRATION; GENE; GENETICS. 
 �34s[G. Ledyard Stebbins]
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�6v�2v�32sPolypodiales   �21m �33s 
    The largest order of modern ferns, commonly called the true ferns, with 
approximately 250 genera and 9000 species; also known as Filicales. Although 
well represented in the temperate regions, they reach their greatest 
development in the moist tropics. They vary in habit from small filmy 
structures to large treelike plants. Many are epiphytic (live perched on other 
plants) and a number are climbing species. A few are aquatic. Perhaps the most 
striking species are the tropical tree ferns with their upright, unbranched 
stems and terminal clusters of large graceful leaves. 
    The Polypodiales differ from the other fern orders in being 
leptosporangiate - that is, their sporangium, or spore sac, arises from a 
single surface cell - and in having small sporangia with a definite number of 
spores. The wall of the sporangium is almost encircled with a ring of cells 
having unevenly thickened walls. This ring is called the annulus. When the 
sporangium is mature, the annulus, acting as a spring, causes the sporangium 
wall to rupture, thus discharging the spores. These plants are valued for 
their beauty and for the clues they give to the evolutionary history of the 
Polypodiales which extends back through the coal measures of the Paleozoic.  
See  PALEOBOTANY. 
    The sporophyte is the conspicuous phase of the true ferns, and like other 
vascular plants it has true roots, stems, and leaves (Fig. 1). In most ferns, 
especially those of the temperate regions, the mature stem is usually a 
creeping rhizome (underground stem) without aerial branches. However, in 
several species the stems are branched, and in some they are erect. Whereas in 
the tropics the leaves are usually persistent and evergreen, in temperate 
regions the leaves of most species die back each year and are replaced by new 
ones the next growing season. Characteristic of this order is the apparent 
uncoiling of the leaves from the base toward the apex. 
    The internal structure of the blade of the leaf and of the root is very 
similar to that of these organs in the seed plants. The main difference is the 
presence of large intercellular spaces in the fern leaf and the frequent lack 
of apparent distinction between the spongy and palisade cells of the 
mesophyll, possibly because most ferns grow in the shade. 
    The life cycle of the fern consists of two independent (self-sustaining) 
alternating generations. The common leafy fern plant is the sporophytic 
(spore-producing) generation. When the mature spores are discharged and reach 
a suitable substrate, they germinate and produce a small, flat, green, 
heart-shaped structure known as the prothallium or gametophytic 
(gamete-producing) generation. The gametophyte produces the sex organs 
antheridia (male) and archegonia (female). The gametes (sperm and egg) unite 
in fertilization and the resultant cell, or zygote, develops into the 
spore-bearing (sporophytic) fern plant. 
    In all ferns, the spores are produced in special multicellular organs 
known as sporangia. Except for a few genera, the sporangia are arranged in 
groups or clusters called sori (Fig. 2). These are on the lower surface of the 
leaves or fertile fronds, either along the midrib of the pinnae, near the leaf 
margins, or scattered. Usually each sorus is covered by a flaplike structure 
called the indusium, which may be of various shapes and sizes. However, a few 
ferns have naked sori, and others have a false indusium formed by the folding 
or inrolling of the leaf margin.  See  POLYPODIOPHYTA; PSILOTOPHYTA. 
 �34s[Paul A. Vestal]
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�6v�2v�32sPolypodiophyta �21m �33s
    A division of the plant kingdom, commonly called the ferns, which is
widely distributed throughout the world but is most abundant and varied in
moist, tropical regions. The Polypodiophyta are sometimes treated as a class
Polypodiopsida of a broadly defined division Tracheophyta (vascular plants).
The group consists of five living orders (Ophioglossales, Marattiales,
Polypodiales, Marsileales, and Salviniales), plus several orders represented
only by Paleozoic fossils. The vast majority of the nearly 10,000 species
belong to the single order Polypodiales, sometimes also called Filicales.  See
POLYPODIOPSIDA.
    The Polypodiophyta ordinarily have well-developed roots, stems, and leaves
that contain xylem and phloem as conducting tissues. The central cylinder of
vascular tissue in the stem usually has well-defined parenchymatous leaf gaps
where the leaf traces depart from it. The leaves are spirally arranged on the
stem and are usually relatively large, with an evidently branching vascular
system. In most kinds of ferns the leaves, called fronds, are compound or
dissected.  See  LEAF; PHLOEM; ROOT; STEM; XYLEM.
    The Polypodiophyta show a well-developed alternation of generations, both
the sporophyte and the gametophyte generation being detached and
physiologically independent of each other at maturity. The sporophyte is much
the more conspicuous, and is the generally recognized fern plant. On some or
all of its leaves it produces tiny sporangia which in turn contain spores.
See  COENOPTERIDALES; MARATTIALES; OPHIOGLOSSIDAE; POLYPODIALES; REPRODUCTION
(PLANT).
 �34s[Arthur Cronquist]
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�6v�2v�32sPolypodiopsida   �21m �33s 
    A class (also known as Filicineae) of the plant division Polypodiophyta 
containing a large group of plants commonly called ferns. They are widely 
distributed throughout the world with their greatest development in the moist 
tropics. Polypodiopsida is an old class with a good representation in the 
Paleozoic flora. Some of the plant fossils resemble contemporary living 
species.  See  COENOPTERIDALES; MARATTIALES; OPHIOGLOSSIDAE; POLYPODIALES; 
POLYPODIOPHYTA.  
 �34s[Paul A. Vestal]
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�6v�2v�32sPolypteriformes   �21m �33s 
    A distinctive and apparently ancient order of actinopterygian fishes, also 
called the Cladistia or the bichirs. Their characters include thick, rhombic, 
ganoid scales with an enamellike covering; a slitlike spiracle behind the eye; 
a well-ossified internal skeleton; a symmetrical caudal fin, basically 
heterocercal, with the upper part continuous with the dorsal fin (see 
illustration); a dorsal series of free, spinelike finlets, each supported by a 
radial bone; a distinctive pectoral fin base with three enlarged radial bones; 
and paired ventral lungs. 
    This order consists of a single family, the Polypteridae, that is known 
from the Eocene. The two Recent genera, Polypterus, with about 10 species, and 
Erpetoichthys, with 1 species, are confined to fresh waters of tropical 
Africa.  See  ACTINOPTERYGII; CHONDROSTEI. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sPoly-p-xylylene resins   �21m �33s 
    Linear, crystallizable resins based on an unusual polymerization of 
p-xylene and derivatives. The polymers are tough and chemically resistant, and 
may be deposited as adherent coatings by a vacuum process. The vapor 
deposition process makes it possible to coat small microelectronic parts with 
a thin layer of the polymer.  See  XYLENE. 
 �34s[John A. Manson]
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�6v�2v�32sPolysaccharide �21m �33s
    A class of high-molecular-weight carbohydrates, colloidal complexes, which
break down on hydrolysis to monosaccharides containing five or six carbon
atoms. The polysaccharides are considered to be polymers in which
monosaccharides have been glycosidically joined with the elimination of water.
A polysaccharide consisting of hexose monosaccharide units may be represented
by the reaction below.

		nC6H12O6 --> (C6H10O5)sub n + (n - 1)H2O

    The term polysaccharide is limited to those polymers which contain 10 or
more monosaccharide residues. Polysaccharides such as starch, glycogen, and
dextran consist of several thousand D-glucose units. Polymers of relatively
low molecular weight, consisting of two to nine monosaccharide residues, are
referred to as oligosaccharides.  See  DEXTRAN; GLUCOSE; GLYCOGEN;
MONOSACCHARIDE; STARCH.
    Polysaccharides are often classified on the basis of the number of
monosaccharide types present in the molecule. Polysaccharides, such as
cellulose or starch, that produce only one monosaccharide type (D-glucose) on
complete hydrolysis are termed homopolysaccharides. On the other hand,
polysaccharides, such as hyaluronic acid, which produce on hydrolysis more
than one monosaccharide type (N-acetylglucosamine and D-glucuronic acid) are
named heteropolysaccharides.  See CARBOHYDRATE.
 �34s[William Z. Hassid]
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�6v�2v�32sPolystyrene resin   �21m �33s 
    A hard, transparent, glasslike thermoplastic resin. Polystyrene is 
characterized by excellent electrical insulation properties, relatively high 
resistance to water, high refractive index, clarity, and low softening 
temperature.  
    Besides the many applications of styrene in combination with other 
materials as in rubber and paints, large quantities of the homopolymer and the 
polyblends are employed in the injection molding of toys, panels, and novelty 
items, and also in the extrusion of sheets. The sheets are used for panels, or 
they may be further shaped by vacuum forming for uses such as liners for 
refrigerator doors.  See  ACRYLONITRILE; POLYMER; POLYMERIZATION; RUBBER; 
STYRENE.  
 �34s[John A. Manson]
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�6v�2v�32sPolysulfide resins   �21m �33s 
    Resins that vary in properties from viscous liquids to rubberlike solids. 
Organic polysulfide resins are prepared by the condensation of organic 
dihalides with a polysulfide. 
    Compounding and fabrication of the rubbery polymers can be handled on 
conventional rubber machinery. The polysulfide rubbers are distinguished by 
their resistance to solvents such as gasoline, and to oxygen and ozone. The 
polymers are relatively impermeable to gases. The products are used to form 
coatings which are chemically resistant and special rubber articles, such as 
gasoline bags. The polysulfide rubbers were among the first polymers to be 
used in solid-fuel compositions for rockets.  See  ORGANOSULFUR COMPOUND; 
POLYMERIZATION; RUBBER. 
 �34s[John A. Manson]
�33s 
  
  
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�6v�2v�32sPolysulfone resins   �21m �33s 
    Engineering plastics whose molecules contain sulfone groups in the main 
chain. Polysulfones are characterized by high strength, stiffness, toughness, 
and thermal and oxidation stability. The aromatic structure and the presence 
of the sulfone groups are responsible for the resistance to heat and 
oxidation. The polymers can be used over a wide temperature range, and in the 
presence of acids, alkalies, aqueous media, and nonpolar organic solvents; 
resistance to creep and dimensional changes due to aging is high. Uses include 
electronic and automotive parts, hardware and houseware items, and plumbing 
parts.  See  SULFONE. 
 �34s[John A. Manson]
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�6v�2v�32sPolytrichales   �21m �33s 
    An order of acrocarpous perennial mosses. They vary in size from small to 
large and short to tall. The stems are usually rigid, simple, or slightly 
branched. They arise from a prostrate subterranean rhizome. The stems are 
regarded as being highly developed, with a specialized central axis. The 
leaves are mostly lanceolate from a sheathing base. The inflorescence is 
usually dioecious. The male flower is terminal, large, and discoid. The female 
flower is terminal and budlike.  See  BRYOPSIDA; EUBRYA. 
 �34s[Winona H. Welch]
�33s 
  
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�6v�2v�32sPolytropic process �21m �33s
    A process which occurs with an interchange of both heat and work between
the system and its surroundings. The nonadiabatic expansion or compression of
a fluid is an example of a polytropic process. The interrelationships between
the pressure (P) and volume (V) and pressure and temperature (T) for a gas
undergoing a polytropic process are given by Eqs. (1) and (2), where a and b
are the polytropic constants for the process of interest. These constants,
which are usually determined from experiment, depend upon the equation of
state of the gas, the amount of heat transferred, and the extent of
irreversibility in the process.  See  GAS; ISENTROPIC PROCESS; ISOTHERMAL
PROCESS; THERMODYNAMIC PROCESSES.
 �34s[Stanley I. Sandler]
�33s

			     PV(sup a) = constant			     (1)

			     P(sup b)/T = constant			     (2)


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�6v�2v�32sPolyurethane resins   �21m �33s 
    Resins that can be produced in forms varying from hard, glossy, 
solvent-resistant coatings, to abrasion- and solvent-resistant rubbers, 
fibers, and flexible-to-rigid foams. The foams have found the widest use. The 
more flexible foams are employed as upholstery material for furniture, as rug 
backing, insulation, and crash pads. The more rigid foams are employed as the 
core in structural and insulating laminates and as insulation in refrigerated 
appliances and vehicles. Polyurethanes are also used as adhesives, for 
example, in the bonding of rubber and of nylon. 
    Polyurethane (or polyisocyanate) resins are produced by the reaction of a 
diisocyanate with a compound containing at least two active hydrogen atoms, 
such as a diol or diamine. Linear, fiber-forming polymers are formed by the 
addition of diisocyanates to diols, while cross-linking is made possible by 
the use of polyols or isocyanates having more than two functional groups. Much 
urethane technology is based on the use of prepolymers in which a 
low-molecular-weight polyester or polyether is prepared. Some of these have 
terminal hydroxyl groups, while those reacted with an excess of diisocyanate 
have terminal isocyanate groups. Thus segments of a rubbery block alternate 
with segments of rigid urethane units. This growth process is called chain 
extension.  See  PLASTICS PROCESSING; POLYESTER RESINS; POLYETHER RESINS; 
POLYMERIZATION; URETHANE. 
 �34s[John A. Manson]
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�6v�2v�32sPolyvinyl resins �21m �33s
    Polymeric materials generally considered to include polymers derived from
monomers having the structure

			       R1
			      /
			CH2==C
			      \
			       R2

in which R1 and R2 represent hydrogen, alkyl, halogen, or other groups. This
article refers to polymers whose names include the term vinyl. For discussions
of other vinyl-type polymers  see  POLYACRYLATE RESIN; POLYACRYLONITRILE
RESINS; POLYFLUOROOLEFIN RESINS; POLYOLEFIN RESINS; POLYSTYRENE RESIN.
    Many of the monomers can be prepared by addition of the appropriate
compound to acetylene. For example, vinyl chloride, vinyl fluoride, vinyl
acetate, and vinyl methyl ether may be formed by the reactions of acetylene
with HCl, HF, CH3OOH, and CH3OH, respectively. Processes based on ethylene as
a raw material have also become common for the preparation of vinyl chloride
and vinyl acetate.
    The polyvinyl resins may be characterized as a group of thermoplastics
which, in many cases, are inexpensive and capable of being handled by
solution, dispersion, injection molding, and extrusion techniques. The
properties vary with chemical structure, crystallinity, and molecular weight.
    Polyvinyl acetals are relatively soft, water-insoluble thermoplastic
products obtained by the reaction of polyvinyl alcohol with aldehydes.
Properties depend on the extent to which alcohol groups are reacted. Polyvinyl
butyral is rubbery and tough and is used primarily in plasticized form as the
inner layer and binder for safety glass. Polyvinyl formal is the hardest of
the group; it is used mainly in adhesive, primer, and wire-coating
formulations, especially when blended with a phenolic resin.
    Polyvinyl acetate is a leathery, colorless thermoplastic material which
softens at relatively low temperatures and which is relatively stable to light
and oxygen. The polymers are clear and noncrystalline. The chief applications
are as adhesives and binders for water-based or emulsion paints.  See  PAINT.
    Polyvinyl alcohol is a tough, whitish polymer which can be formed into
strong films, tubes and fibers that are highly resistant to hydrocarbon
solvents. Although polyvinyl alcohol is one of the few water-soluble polymers,
it can be rendered insoluble in water by drawing or by the use of
cross-linking agents. Two groups of products are available, those formed by
the essentially complete hydrolysis of polyvinyl acetate, and those formed by
incomplete hydrolysis.
    The former may be plasticized with water or glycols and molded or extruded
into films, tubes, and filaments which are resistant to hydrocarbons. These
products are used for liners in gasoline hoses, for grease-resistant coating
and paper adhesives, for treating paper and textiles, and as emulsifiers and
thickeners.
    Polyvinyl carbazole is a tough, glassy thermoplastic with excellent
electrical properties and a relatively high softening temperature. Uses of the
product has been limited to small-scale electrical applications requiring
resistance to high temperatures.
    Polyvinyl chloride (PVC) is a tough, strong thermoplastic material which
has an excellent combination of physical and electrical properties. The
products are usually characterized as plasticized or rigid types. Polyvinyl
chloride (and copolymers) is the second most commonly used polyvinyl resin and
one of the most versatile plastics. The plasticized types are somewhat elastic
materials which are familiar in the form of shower curtains, floor coverings,
raincoats, dishpans, dolls, bottle-top sealers, prosthetic forms, wire
insulation, and films, among others. Rigid products, which may consist of the
homopolymer, copolymer, or polyblends, are commonly used in the manufacture of
phonograph records, pipe, chemically resistant liners for chemical-reaction
vessels, and siding and window sashes.
    Polyvinylidene chloride is a tough, horny thermoplastic with properties
generally similar to those of polyvinyl chloride. Because of its relatively
low solubility and decomposition temperature, the material is most widely used
in the form of copolymers with other vinyl monomers, such as vinyl chloride.
The copolymers are employed as packaging film, rigid pipe, and as filaments
for upholstery and window screens.
    Polyvinyl ethers exist in several forms varying from soft, balsamlike
semisolids to tough, rubbery masses, all of which are readily soluble in
organic solvents. Polymers of the alkyl vinyl ethers are used in adhesive
formulations and as softening or flexibilizing agents for other polymers.
    Polyvinyl fluoride is a tough, partially crystalline thermoplastic
material which has a higher softening temperature than polyvinyl chloride.
Films and sheets are characterized by high resistance to impact and cracking
caused by flexing and temperature and by resistance to weathering.
    Polyvinyl pyrrolidone is a water-soluble polymer of basic nature which has
film-forming properties, strong absorptive or complexing qualities for various
reagents, and the ability to form water-soluble salts which are
polyelectrolytes. The main uses are as a water-solubilizing agent for
medicinal agents such as iodine, and as a semipermanent setting agent in hair
sprays. Certain synthetic textile fibers containing small amounts of
vinylpyrrolidone as a copolymer have improved affinity for dyes.  See
PLASTICS PROCESSING; POLYMER; POLYMERIZATION.
 �34s[John A. Manson]
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�6v�2v�32sPomegranate   �21m �33s 
    A small deciduous tree, Punica granatum, belonging to the plant order 
Myrtales. Pomegranate is grown as an ornamental as well as for its fruit. The 
pomegranate is a native of Asia. It was originally known for its medicinal 
qualities, and cures for various ills were attributed to the fruit juice, the 
rind, and the bark of the roots. The fruit is a reddish, pomelike berry, 
containing numerous seeds imbedded in crimson pulp, from which an acid, 
reddish juice may be obtained. Limited quantities are grown in California and 
the Gulf states.  See  FRUIT, TREE; MYRTALES. 
 �34s[J. Harold Clarke]
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�6v�2v�32sPoplar �21m �33s
    Any tree of the genus Populus, family Salicaceae, marked by simple,
alternate leaves which are usually broader than those of the willow, the other
American representative of this family. Poplars have scaly buds, bitter bark,
flowers and fruit in catkins, and a five-angled pith.  See  SALICALES; WILLOW.
    Some species are commonly called cottonwood because of the cottony hairs
attached to the seeds. Other species, called aspens, have weak, flattened leaf
stalks which cause the leaves to flutter in the slightest breeze. One of the
important species in the United States is the quaking, or trembling, aspen (P.
tremuloides). The soft wood of this species is used for paper pulp. The
European aspen (P. nigra), which is similar to the quaking aspen, is sometimes
planted, and its variety, italica, the Lombardy poplar of erect columnar
habit, is used in landscape planting. The black cottonwood (P. trichocarpa) is
the largest American poplar and is also the largest broad-leaved tree in the
forests of the Pacific Northwest. The cottonwood or necklace poplar (P.
deltoides) is native in the eastern half of the United States. In the balsam
or tacamahac poplar (P. balsamifera), the resin is used in medicine as an
expectorant. The wood is used for veneer, boxes, crates, furniture, paper
pulp, and excelsior.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sPoppy �21m �33s
    A plant, Papaver somniferum (Papaveraceae), which is probably a native of
Asia Minor. It is cultivated extensively in China, India, and elsewhere. This
plant is the source of opium, obtained by cutting into the fruits (capsules)
soon after the petals have fallen. The white latex (juice) flows from the cuts
and hardens when exposed to the air. This solidified latex is collected,
shaped into balls or wafers, and often wrapped in the flower petals. This is
the crude opium, which contains at least 20 alkaloids, including morphine and
codeine.  See  DRUG ADDICTION; MORPHINE; PAPAVERALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sPopulation dispersal �21m �33s
    The process by which groups of living organisms expand the space or range
within which they live. Because of their reproductive capacity, all
populations have a natural tendency to expand. As increased area supports more
individuals, dispersal and reproduction are intimately correlated.
    Distinction should be made between dispersal and seasonal migration.
Birds, butterflies, salmon, and others migrate regularly without necessarily
expanding their geographic range, since they usually return to their original
areas or die out.  See	MIGRATORY BEHAVIOR.
    Dispersal consists of several phases: (1) the production of units, that
is, of individuals or parts of individuals (disseminules) fit or adapted for
dispersal; (2) the transportation of individuals or disseminules to the new
habitat; and (3) ecesis, the process of becoming established through
germination, rooting, or physiological and psychological adjustment.
    Certain disseminules (propagules or diaspores) may represent various
stages of the life cycle of the individual. Many free-living animals do not
produce special dispersal structures but rely upon the ability of the entire
organism to move about (vagility). Organisms attached to a substratum, such as
most plants and certain animals, produce disseminules adapted to certain
agents of dispersal (see table).
    Individuals or disseminules are transported in five general ways:
self-dispersal (autochory), water dispersal (hydrochory), wind dispersal
(anemochory), animal dispersal (zoochory), and dispersal by humans
(anthropochory).
    Success in population dispersal depends upon three factors: fitness of the
new habitat, fitness of the migrating individuals, and the chance
juxtaposition of these two which, in the long run, depends on the number of
individuals invading the new habitat. The probability for a new habitat to be
favorable is greatest close to the parent population. Spores blown over great
distances have less chance of landing in spots suited for germination than
have seeds falling close to the parent plant. In wide-range dispersal larger
numbers of disseminules are usually necessary than at close range, to ensure
ecesis.  See  PARTHENOGENESIS; SPECIATION.
 �34s[Kornelius Lems /Herbert G. Baker]
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�6v�2v�32sPopulation genetics   �21m �33s 
    The study of both experimental and theoretical consequences of mendelian 
heredity on the population level, in contradistinction to classical genetics 
which deals with the offspring of specified parents on the familial level. The 
genetics of populations studies the frequencies of genes, genotypes, and 
phenotypes, and the mating systems. It also studies the forces that may alter 
the genetic composition of a population in time, such as recurrent mutation, 
migration, and intermixture between groups, selection resulting from genotypic 
differential fertility, and the random changes incurred by the sampling 
process in reproduction from generation to generation. This type of study 
contributes to an understanding of the elementary step in biological 
evolution. The principles of population genetics may be applied to plants and 
to other animals as well as humans.  See  GENETICS; MENDELISM. 
 �34s[Ching Chun Li]
�33s 
  
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�6v�2v�32sPorcelain   �21m �33s 
    A high-grade ceramic ware characterized by high strength, a white color 
(under the glaze), very low absorption, good translucency, and a hard glaze. 
Equivalent terms are European porcelain, hard porcelain, true porcelain, and 
hard paste porcelain.  See  GLAZING; POTTERY. 
    Porcelain is distinguished from other fine ceramic ware, such as china, by 
the fact that the firing of the unglazed ware (the bisque firing) is done at a 
lower temperature (1800 - 2200DGF or 1000 - 1200DGC) than the final or glost 
firing, which may be as high as 2700DG F (1500DGC). In other words, the ware 
reaches its final state of maturity at the maturing temperatures of the glaze. 
    The white color is obtained by using very pure white-firing kaolin or 
china clay and other pure materials, the low absorption results from the high 
firing temperature, and the translucency results from the glass phase.  See  
CERAMICS.  
 �34s[John F. McMahon]
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�6v�2v�32sPorcupine   �21m �33s 
    Any of about 26 species of rodents which have spines or quills in addition 
to regular hair. These mammals are included in two families, the Hystricidae, 
or Old World porcupines, and the Erethizontidae, or New World porcupines. The 
spines are sharply pointed, erectile hairs which serve the animal as defensive 
structures and are controlled by powerful skin muscles. These animals have 
short limbs which terminate in five functional digits in the Old World species 
and four in the New World species.  See  MAMMALIA; RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sPorifera   �21m �33s 
    The sponges, a phylum of the animal kingdom which includes about 5000 
described species. The body plan of sponges is unique among animals. Currents 
of water are drawn through small pores or ostia in the sponge body and leave 
by way of larger openings called oscula. The beating of flagella on collar 
cells or choanocytes, localized in chambers in  the interior of the sponge, 
maintains the water current. Support for the sponge tissues is provided by 
calcareous or siliceous spicules, or by organic fibers, or by a combination of 
organic fibers and siliceous spicules. Some species have a compound skeleton 
of organic fibers, siliceous spicules, and a basal mass of aragonite or 
calcite. The skeletons of species with supporting networks of organic fibers 
have long been used for bathing and cleaning purposes. Because of their 
primitive organization, sponges are of interest to zoologists as an aid in 
understanding the origin of multicellular animals.  See  ANIMAL KINGDOM; 
PARAZOA.  
    The Porifera are divided into the classes Hexactinellida, Calcarea, 
Demospongiae, and Sclerospongiae on the basis of the skeletal structure. A 
taxonomic scheme of the Porifera follows. See separate articles on each group, 
except those with an asterisk. 
                                                                               
  
Class Hexactinellida 
    Subclass Amphidiscophora 
        Order: Amphidiscosa 
               Hemidiscosa 
    Subclass Hexasterophora 
        Order: Hexactinosa 
               Lychniscosa 
               Lyssacinosa 
               Reticulosa 
Class Calcarea 
    Subclass Calcinea 
        Order: Clathrinida 
               Leucettida 
    Subclass Calcaronea 
        Order: Leucosoleniida 
               Sycettida 
    Subclass Pharetronidia 
Class Demospongiae 
    Subclass Tetractinomorpha* 
        Order Homosclerophorida 
              Choristida 
              Spirophorida 
              Hadromerida 
              Axinellida 
    Subclass Ceractinomorpha* 
        Order Dendroceratida 
              Dictyoceratida 
              Halichondrida 
              Haplosclerida 
              Poecilosclerida 
Class Sclerospongiae 
    The Porifera have a fossil record extending from the Cambrian to Recent 
times. More than 1000 genera of fossil sponges have been described from the 
Paleozoic, Mesozoic, and Cenozoic eras. 
 �34s[Willard D. Hartman]
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�6v�2v�32sPorocephalida   �21m �33s 
    One of two orders in the class Pentastomida of the phylum Arthropoda. In 
this order the larvae have four legs. The hooks on the adult articulate on a 
chitinous base or fulcrum and are arranged in a flattened trapezoidal pattern, 
or in a curved line or straight line. There are no podial or parapodial lobes. 
The two suborders are Porocephaloidea and Linguatuloidea. 
    The Porocephaloidea comprise five families - Porocephalidae, Sebekidae, 
Armilliferidae, Sambonidae, and Subtriquetridae - and include most of the 
pentastomid species. These animals are cylindrical, with rounded ends. The 
hooks are sessile; the mouth is anterior between the hooks. Adults occur in 
the respiratory passages of snakes, lizards, turtles, and crocodiles; the 
larvae are encysted in fishes, snakes, lizards, crocodiles, and mammals. 
    Linguatuloidea contains a single family, with a single genus, Linguatula. 
Adults live in the nasal cavities of carnivorous mammals; the larvae are 
encysted in herbivorous mammals. The body is elongate, flattened ventrally, 
annulate, and attenuated posteriorly.  See  PENTASTOMIDA. 
 �34s[Horace W. Stunkard]
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�6v�2v�32sPorphyrin �21m �33s
    A class of red-pigmented compounds with a cyclic tetrapyrrolic structure
in which the four pyrrole rings are joined through their ALPHA-carbon atoms by
four methene bridges (DOUBLE BARCH--). The pyrrole formula is shown here.
    The porphyrins form part of the active nucleus of chlorophylls a and b,
hemoglobin, myohemoglobin, cytochromes, and the enzymes catalase and
peroxidase.  The parent sub stance is synthetic porphin in which the hydrogen

			    CHEMICAL FORMULA

(H) atoms in the eight BETA-positions on the pyrrole rings are unsubstituted.
Naturally occuring porphyrins differ from porphin and from each other by various
side chains in these eight BETA-positions.  See CHLOROPHYLL; CYTOCHROME; ENZYME;
HEMOGLOBIN; IRON METABOLISM; SPECTROPHOTOMETRIC ANALYSIS.
 �34s[Rudolf Schmid]
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�6v�2v�32sPorphyroblast   �21m �33s 
    A relatively large crystal formed in a metamorphic rock. The presence of 
abundant porphyroblasts gives the rock a porphyroblastic texture. Minerals 
found commonly as porphyroblasts include biotite, garnet, chloritoid, 
staurolite, kyanite, sillimanite, andalusite, cordierite, and feldspar. 
Porphyroblasts are generally a few millimeters or centimeters across, but some 
attain a diameter of over 1 ft (30 cm). They may bebounded by well-defined 
crystal faces, or their outlines may be highly irregular or ragged. Very 
commonly they are crowded with tiny grains of other minerals that occur in the 
rock.  
    Most commonly, porphyroblasts develop in schist and gneiss during the late 
stages of recrystallization. As the rock becomes reconstituted, certain 
components migrate to favored sites and combine there to develop the large 
crystals.  See  GNEISS; METAMORPHIC ROCKS; SCHIST. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sPorphyry �21m �33s
    An igneous rock characterized by porphyritic texture, in which large
crystals (phenocrysts) are enclosed in a matrix of very fine-grained to
aphanitic (not visibly crystalline) material. Porphyries are generally
distinguished from other porphyritic rocks by their abundance of phenocrysts
and by their occurrence in small intrusive bodies (dikes and sills) formed at
shallow depth within the earth. In this sense porphyries are hypabyssal rocks.
See  IGNEOUS ROCKS; PHENOCRYST.
    Porphyries occur as marginal phases of medium-sized igneous bodies
(stocks, laccoliths) or as apophyses (offshoots) projecting from such bodies
into the surrounding rocks. They are also abundant as dikes cutting
compositionally equivalent plutonic rock, or as dikes, sills, and laccoliths
injected into the adjacent older rocks.  See  LACCOLITH.
 �34s[William Ingersoll Rose, Jr.]
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�6v�2v�32sPorpoise   �21m �33s 
    Any one of several species of marine mammal of the family Phocaenidae 
found in tropical and temperate oceans of the world and in several large river 
systems in South America and Asia. The common porpoise (Phocaena phocaena) is 
the smallest member of the order Cetacea and is found in the North Atlantic 
Ocean, the North Sea, and the Baltic  Sea. 
    The porpoise is an excellent swimmer, although the flippers are relatively 
small. They are social animals with a well developed brain and use an 
elaborate system of behavior and sounds to communicate between individuals. 
Moreover, they are able to echolocate with a highly developed sonar system. 
Hair is lacking and there is usually an insulating layer of fat beneath the 
smooth thick skin. Porpoises must surface periodically to breathe, but they 
are otherwise well adapted for their marine environment. During the summer, 
after a gestation period of 1 year, a single young is born. 
    The Pacific harbor porpoise (P. vomerina) is a common sight in waters 
along the West Coast of the United States. The black finless porpoise 
(Neomeris phocaenoides) of the Pacific and Indian oceans lacks a dorsal fin, 
but is similar to the common porpoise in its habits and has been found as far 
as 1000 mi (1600 km) up the Yellow River of China. The river porpoise of South 
America is blind.  See  CETACEA; DOLPHIN. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sPortland cement   �21m �33s 
    A generic name for a hydraulic cement. Portland cement is composed mainly 
of high-limed calcium silicates with lesser amounts of high-limed aluminates 
and ferrites which are ground together with a small amount of gypsum to a fine 
powder. During hardening, the calcium compounds chemically combine with water 
to produce the hard, finished product. The portland cement-water hydration 
product becomes the binder which joins sand and stone in mortar and concrete 
to produce a monolithic product.  See  CONCRETE. 
    Special cements include white cement, expansive cements, very rapid early 
setting cement, and oil well cements. White cement, used for architectural 
purposes, is made from materials which contain no iron compounds. Expansive 
cements expand during the hardening process and are used to induce stress in 
the embedded steel. The very rapid setting cement is used where strength is 
required within an hour. Oil well cements are used for sealing very deep and 
high-temperature well shafts. All of these variations in cements are made by 
adjusting the chemical and physical composition.  See  AIR-ENTRAINING PORTLAND 
CEMENT; CEMENT. 
 �34s[N. R. Greening]
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�6v�2v�32sPorulosida �21m �33s
    An order of Radiolaria. The central capsule shows many pores, more or less
uniformly distributed over the surface of the thick and usually globular
layer. The development of skeletal structures ranges from none at all through
scattered spines to well-developed latticework shells. Colonial aggregates, in
which the capsules of a number of organisms are embedded in a matrix, occur in
several genera.  See  RADIOLARIA.
 �34s[Richard P. Hall]
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�6v�2v�32sPosition fixing   �21m �33s 
    The process of determining a craft's present position without reference to 
a previous position. Two technologies are commonly used for applying this 
procedure. One is radio (electronic) and the other is astronomical 
(celestial). Position fixing by the use of radio technology is sometimes 
called radio location. Some experts consider celestial navigation a procedure 
apart from any other. This article deals only with radio location.  See  
CELESTIAL NAVIGATION. 
    Many radio position-fixing systems were developed during World War II. 
Some of these employed continuous-wave technology, while others used pulse 
technology. Hyperbolic, radial, and polar geometry were all used. Four 
hyperbolic systems were developed: loran, Decca, loran C, and Omega.  See  
DECCA; LORAN; OMEGA. 
    A special system developed for aircraft was vhf omnidirectional radio 
range (VOR), but this system gave only radial guidance. DME, a 
distance-measuring system, was combined with VOR to form a circular/radial 
position system. Tacan, a military radial/distance system, supplied distance 
to the VOR to form VORTAC.  See  POLAR-COORDINATE NAVIGATION SYSTEMS; RADIO 
RANGE; TACAN; VORTAC. 
    Since 1972 the Navy Navigation Satellite system (formerly called Transit) 
has been in use by the military and private industry. It is basically a 
hyperbolic system, with various positions of the satellite serving as the 
equivalent of hyperbolic radio transmitting stations. The accuracy of the 
system can be of the order of feet.  
 �34s[Peter C. Sandretto]
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�6v�2v�32sPositive-displacement flowmeter �21m �33s
    A quantity meter in which the flowing stream is separated into discrete
quantities. These meters capture definite volumes one after another and pass
them downstream.
    In the nutating-disk meter a circular disk attached to a spherical center
is restrained from rotation by a vertical partition through a slot in the disk
(illustration  a ). It is allowed to nutate so that its shaft is in contact
with a conical guide. The liquid entering the inlet port moves the disk in a
nutating motion until the liquid discharges from the outlet port. The double
conical shape of the measuring chamber makes a seal with the disk as it goes
through the nutating motion. Normally a mechanical counter indicates the
number of cycles as the fluid flows through smoothly with no pulsations.
    In the rotary-vane meter an eccentrically mounted drum carries radial
spring-loaded vanes. These vanes slide in and out, and seal against the meter
casing to form pockets that carry a measured amount of fluid on each cycle.
    In the oval-shaped gear meter two carefully formed gears provide their own
synchronization, and with close-fitting teeth to the outer circular-shaped
chambers they deliver a fixed quantity of liquid for each rotation. The lobed
impeller is similar to the oval gear except that the synchronization is done
by external circular gears and therefore the rotation is smooth.
    In the rotary-abutment meter two displacement rotating vanes interleave
with cavities on an abutment rotor (illustration  b ). The three elements are
geared together, and the meter depends upon close clearances rather than
rubbing surfaces to provide the seals.
    In the liquid-sealed drum-type gas meter a cylindrical chamber is filled
more than half full with water and divided into four rotating compartments
formed by trailing vanes. Gas entering through the center shaft from one
compartment to another forces rotation that allows the gas then to exhaust out
the top as it is displaced by the water.  See  FLOW MEASUREMENT; QUANTITY
FLOWMETER.
 �34s[Mead Bradner ;Lewis P. Emerson]
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�6v�2v�32sPositron �21m �33s
    An elementary particle with mass equal to that of the electron, and
positive charge equal in magnitude to the electron's negative charge. The
positron is thus the antiparticle (charge-conjugate particle) to the electron.
The positron has the same spin and statistics as the electron. Positrons, like
electrons, appear as decay products of many heavier particles;
electron-positron pairs are produced by high-energy photons in matter.	See
ELECTRON; ELECTRON-POSITRON PAIR PRODUCTION; ELEMENTARY PARTICLE.
    A positron is, in itself, stable, but cannot exist indefinitely in the
presence of matter, for it will ultimately collide with an electron. The two
particles will be annihilated as a result of this collision, and photons will
be created. However, a positron can first become bound to an electron to form
a short-lived ``atom'' termed positronium.  See  POSITRONIUM.
 �34s[Charles J. Goebel; Jack Greenberg]
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�6v�2v�32sPositronium   �21m �33s 
    The bound state of an electron and a positron. Positronium was discovered 
by studies of the so-called annihilation radiation from positrons stopped in 
gases. It is formed in a collision between a positron and a gas atom which 
results in the capture of an atomic electron by the positron.  See  POSITRON. 
    Positronium is of particular interest because it is the two-body system to 
which quantum electrodynamics is applicable, and its study has served as an 
important confirmation of the theory of quantum electrodynamics.  See  QUANTUM 
ELECTRODYNAMICS.  
    No states of positronium other than the ground n = 1 state (n = 1,2,3, . . 
. , being the principal quantum number) have been found. Studies of positron 
annihilation in solids and liquids indicate that a perturbed form of 
positronium exists under certain conditions. 
    Positronium is an unstable atom and annihilates with the emission of 
photons.  
 �34s[Vernon Hughes]
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�6v�2v�32sPostulate �21m �33s
    In a formal deductive system, a proposition accepted without proof, from
which other propositions are deduced by the conventional methods of formal
logic. There is a certain arbitrariness as to which propositions are to be
treated as postulates, because when certain proved propositions are treated as
such, other propositions which were originally postulates often become proved
propositions. In strict usage, the term postulate is nearly equivalent to
axiom, although axiom is often loosely used to denote a truth supposed to be
self-evident.  See  LOGIC.
 �34s[Percy W. Bridgman /Henry Margenau]
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�6v�2v�32sPotassium �21m �33s
    A chemical element, K, atomic number 19, and atomic weight 39.102. It
stands in the middle of the alkali metal family, below sodium and above
rubidium. This lightweight, soft, low-melting, reactive metal melts at 63.7DGC
(147DGF), boils at 760DGC  (1400DGF) and has a density of 0.8199/ cm(sup 3)
(0.4739 oz/in.sup3) at 100DGC (212DGF). It is very similar to sodium in its
behavior in metallic forms.  See  ALKALI METALS; SODIUM.
    Potassium chloride finds its main use in fertilizer mixtures. It also
serves as the raw material for the manufacture of other potassium compounds.
Potassium hydroxide is used in the manufacture of liquid soaps, and potassium
carbonate in making soft soaps. Potassium carbonate is also an important raw
material for the glass industry. Potassium nitrate is used in matches, in
pyrotechnics, and in similar items which require an oxidizing agent.
    Potassium is a very abundant element, ranking seventh among all the
elements in the Earth's crust, 2.59% of which is potassium in combined form.
Sea water contains 380 parts per million, making potassium the sixth most
plentiful element in solution.
    Potassium is even more reactive than sodium. It reacts vigorously with the
oxygen in air to form the monoxide, K2O, and the peroxide, K2O2. In the
presence of excess oxygen, it readily forms the superoxide, KO2.
    Potassium does not react with nitrogen to form a nitride, even at elevated
temperatures. With hydrogen, potassium reacts slowly at 200DGC (390DGF) and
rapidly at 350 - 400DGC (660 - 752DGF). It forms the least stable hydride of
all the alkali metals.
    The reaction between potassium and water or ice is violent, even at
temperatures as low as -100DGC (-148DGF). The hydrogen evolved is usually
ignited in reaction at room temperature. Reactions with aqueous acids are even
more violent and verge on being explosive.
 �34s[Marshall Sittig]
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�6v�2v�32sPotato, Irish   �21m �33s 
    The plant Solanum tuberosum of the nightshade family, Solanaceae. The 
origin of the potato was in the South American continent. Many wild 
tuber-bearing species are found in Central and South America. Potatoes have 
been under cultivation for several thousand years in these areas, supplying a 
large portion of the food for the inhabitants. They were introduced into 
Europe early in the 16th century and into the United States from Ireland in 
1719.  
    The potato is an annual, herbaceous, dicotyledonous plant. It is sometimes 
regarded as a potential perennial because of its ability to reproduce 
vegetatively by means of tubers. Tubers, which are shortened, thickened 
underground stems, form on the ends of lateral stems (stolons) underground. 
The principal areas in the mature tubers from the exterior inward are the 
periderm, cortex, vascular ring, perimedullary zone, and central pith. Lateral 
buds (eyes) on the mature tuber are the growing points for the new crop when 
whole tubers or pieces with at least one eye are planted.  See  POLEMONIALES. 
    Potatoes are grown commercially in every state in the United States. The 
number of farms on which potatoes are grown is decreasing at a rather rapid 
rate, accompanied by more potato-planted acres per farm. With more efficient 
methods of growing and harvesting potatoes, higher yields and better methods 
of storing have resulted. Most of the production is concentrated in 15 states: 
Idaho, Washington, Maine, Oregon, California, North Dakota, Wisconsin, New 
York, Minnesota, Colorado, Michigan, Pennsylvania, Florida, Nevada, and Texas. 
    Chemical composition varies with variety, area of growth, cultural 
practices, maturity at harvest, subsequent storage history, and other factors. 
Starch calorically is the most important nutritional component. The 
nutritional or biological value of protein in potatoes is rather high. Potato 
protein contains substantially more of all the essential amino acids, except 
histidine, than does that of whole wheat. Potatoes contain appreciable amounts 
of the B vitamins: niacin, thiamine, riboflavin, and pyridoxine. Potatoes are 
an excellent source of vitamin C. Inorganic constituents or minerals of 
potatoes are predominantly potassium, phosphorus, sulfur, chlorine, magnesium, 
and calcium. Iron is present in sufficient quantity for potatoes to be a good 
nutritional source of this mineral. 
 �34s[O. Siddiqi]
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�6v�2v�32sPotato, sweet   �21m �33s 
    The fleshy root of the plant Ipomoea batatas. The sweet potato was 
mentioned as being grown in Virginia as early as 1648. Louisiana is the 
leading state for commercial production of both the canned and fresh products. 
    There are two principal types of sweet potato, the kind erroneously called 
yam and the Jersey type. The chief difference between the two is that in 
cooking or baking the yam, much of the starch is broken down into simple 
sugars (glucose and fructose) and an intermediate product, dextrin. This gives 
it a moist, syrupy consistency somewhat sweeter than that of the dry (Jersey) 
type. On cooking, the sugar in the dry type remains as sucrose. 
    The true yam (Dioscorea) includes both edible and medicinal varieties. The 
latter are grown primarily for their cortisone, a steroid. Cortisone yams can 
be grown in the southern United States, particularly in Louisiana.  See  
POTATO, IRISH; STEROID. 
 �34s[Julian C. Miller]
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�6v�2v�32sPotential barrier   �21m �33s 
    A region including a maximum of potential energy which prevents a particle 
on one side of the region from passing to the other side. According to 
classical physics, a particle must possess an energy exceeding the height of 
the potential barrier to surmount it. However, quantum mechanics shows that a 
particle with less energy has a finite probability for penetrating the 
barrier. The probability decreases rapidly as the particle energy decreases. 
Such barriers are exemplified by the negative field of the electrons 
surrounding an atom, by the positive charge in the nucleus which repels a 
positively charged particle, or by barriers to electrons or holes created in 
solid-state devices as a result of fixed charge distributions.  See  NUCLEAR 
STRUCTURE; SEMICONDUCTOR. 
 �34s[Robert Stratton]
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�6v�2v�32sPotential energy �21m �33s
    The energy due to position or configuration. The electric field RIGHT ARROW
E and the magnetic field RIGHT ARROW B in a region of space can be calculated
from an electrostatic potential and a magnetic vector potential.  Then the force
on a charged particle can be calculated, and the work done on the particle along
a specific path by the force can be obtained.  Thus, macroscopic changes in
potential energy that result from pushing an object from one place to another
with a stick or pulling it with a string have their microscopic origin in the
electromagnetic potential energy which holds the atoms in place in the stick or
string together.  However, there are macroscopic forces which produce different
potential energy functions, such as the elastic spring.
    Each potential energy function specifies the dependence of the force on
position, and each one contains a constant, which is a reference potential
energy. This constant part of the potential energy simply specifies a
reference point and does not contribute to the force. See ENERGY; HOOKE'S LAW;
POTENTIALS.
 �34s[Brian DeFacio]
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�6v�2v�32sPotential flow   �21m �33s 
    Fluid flow which can be specified by a velocity potential. In contrast to 
creeping flow, it represents a condition where inertia is controlling and 
viscous forces are negligible. When the effect of viscosity of a fluid is 
negligible, in most cases the fluid flow will be irrotational at all times if 
it starts from rest. For irrotational flow, there exists a velocity potential 
such that the velocity vector is the gradient of the velocity potential. For 
an incompressible fluid, the velocity potential satisfies the Laplace 
equation, which has been investigated very thoroughly from a mathematical 
point of view. Potential flow applies to most subsonic flow problems of a 
fluid of small viscosity, such as air and water, far away from a solid wall 
but not in the boundary-layer flow near the wall.  See  BOUNDARY-LAYER FLOW; 
FLUID FLOW; LAPLACE`S IRROTATIONAL MOTION; POTENTIAL. 
 �34s[Shih-I Pai]
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�6v�2v�32sPotentials   �21m �33s 
    Functions or sets of functions from whose first derivatives a vector can 
be formed. A vector is a quantity which has a magnitude and a direction, such 
as force. 
    A single function, the scalar potential, is used in gravitation theory, 
electricity and magnetism, fluid mechanics, and other areas. The vectors 
obtained from it by partial differentiation are in these cases the 
gravitational, electric and magnetic field strengths, and the velocity, 
respectively. The vector potential is a set of three functions whose first 
derivatives give the magnetic induction.  See  ELECTRIC FIELD; GRAVITATION; 
LAPLACE`S IRROTATIONAL MOTION: MAGNETIC FIELD; MAGNETIC INDUCTION. 
 �34s[F. Rohrlich]
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�6v�2v�32sPotentiometer (variable resistor)   �21m �33s 
    A variable-resistance device with three terminals used in electric 
circuits. As shown schematically in the illustration, the three terminals are 
the two ends of a resistor (or series combination of resistors) and a movable 
connection, which allows adjustment of the resistance between this movable 
connection and either end connection. The movable connection often consists of 
a sliding contact which moves along the actual resistor element. The size or 
rating of a potentiometer is specified by giving its total resistance in ohms 
and the permissible losses in watts. By using only the movable and one fixed 
connection, a potentiometer may be used as a rheostat.  See  RESISTOR; 
RHEOSTAT.  
    A potentiometer is used to adjust and control the electric potential 
difference (voltage) applied to some device or part of a circuit. The output 
voltage may be varied from zero to the value of the input voltage. Examples of 
its use are as a field-current control on an electric generator and as a 
volume control on a radio. Since the resistance between the input terminals is 
fixed (assuming the load device takes little current), potentiometers with 
precision resistors are used as range selector switches on vacuum-tube 
voltmeters and other precision electronic measuring equipment. Other uses are 
to divide a voltage into two parts, to compare two voltages, and to divide the 
total resistance between two parts of a circuit. 
 �34s[Clarence F. Goodheart]
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�6v�2v�32sPotentiometer (voltage meter)   �21m �33s 
    A device for the measurement of an electromotive force (emf) by comparison 
with a known potential difference. The known potential difference is 
established by the flow of a definite current through a known resistance, 
using a standard cell as a reference. The principal potentiometer circuits are 
(1) the constant-current dc potentiometer (historically known as Poggendorff's 
first method); (2) the Brooks deflectional dc potentiometer (a variant of the 
basic constant-current potentiometer); (3) the constant-resistance dc 
potentiometer (known as Poggendorff's second method); (4) the Drysdale ac 
potentiometer; and (5) the Tinsley-Gall ac potentiometer.  See  ELECTRICAL 
MEASUREMENTS; ELECTROMOTIVE FORCE (EMF); POTENTIALS; VOLTAGE MEASUREMENT; 
VOLTMETER.  
 �34s[Almon J. Corson]
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�6v�2v�32sPottery   �21m �33s 
    Vessels made entirely or partly of clay, and fired to a strong, hard 
product; occasionally, the term refers to just the lower grades of such ware. 
Pottery may be glazed or unglazed.  See  CERAMICS; GLAZING. 
    Grades of pottery (such as china, stoneware, earthenware, and other 
special types) are distinguished by their color, strength, absorption (the 
weight of water soaked up when the piece is submerged, expressed as a 
percentage of the original weight), and translucency (ability to transmit 
light). All these properties refer to the material or ``body'' under any glaze 
present.  See  PORCELAIN. 
    Absorption is due to the presence of open pores or voids in the fired 
material into which water can penetrate; in general, the higher the firing 
temperature, the lower the absorption. Body color is determined mainly by 
raw-material purity. Strength depends on the porosity and also on the amount 
and type of glass and crystals developed in the body on firing. Translucency 
is obtained in products in which there is low porosity and little differences 
in index of refraction between the glass and crystals in the body.  See  CLAY. 
 �34s[John F. McMahon]
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�6v�2v�32sPottiales   �21m �33s 
    An order of mosses. The plants vary in height from minute (up to 0.8 in. 
or 5 mm) to large (up to 4 in. or 10 cm). The stems are erect, simple, or with 
dichotomous or fasciculate branching. The leaves are crowded, in several rows, 
and rarely three-ranked. They vary in shape, but the majority are lanceolate 
to broadly ovate or obovate. The costa is strong, mostly percurrent or 
excurrent.  See  BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sPoultry production   �21m �33s 
    Poultry production can be divided into two major categories, meat 
production and egg production. Heavy-breed chickens, turkeys, ducks, and geese 
are used for meat, while light-breed chickens are used primarily for egg 
production.  
    Trends in both broiler and egg production have been toward vertical 
integration. This means that the various aspects of production and marketing 
(production of hatching eggs, incubation, feeding, growing, processing, and 
marketing) are being merged within the framework of a single organization. 
 �34s[Glenn O. Bressler]
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�6v�2v�32sPour point   �21m �33s 
    The lowest temperature at which an oil will pour when cooled under 
prescribed test conditions. Because petroleum oils are complex mixtures which 
become plastic solids when cooked, several solidification temperatures are 
used, each defined by a definite test procedure. The cloud point is the 
temperature at which a solid phase separates from solution. For a grease, the 
dropping point is the temperature at which the plastic solid becomes 
sufficiently fluid to flow through an orifice. For waxes, the melting point is 
the temperature at which the material becomes fluid enough to drop from the 
test thermometer; the congealing point is the temperature at which a sample 
wetting the thermometer appears to congeal and hence rotate with the 
thermometer.  
 �34s[Mott Souders]
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�6v�2v�32sPowder metallurgy   �21m �33s 
    A process for making a wide range of components and shapes from a variety 
of metals and alloys in the form of powder. The process is automated and uses 
pressure and heat to form precision metal parts into net or near-net shapes, 
requiring a minimum amount of secondary finishing.  
    The most common metals available in powder form are iron, tin, nickel, 
copper, aluminum, and titanium, and refractory metals such as tungsten, 
molybdenum, tantalum, and niobium (columbium). Prealloyed powders such as 
low-alloy steels, bronze, brass, and stainless steel are produced in which 
each particle is itself an alloy. Also available in powder form are 
nickel-cobalt-base superalloys and tool steels. 
    Although most powder metallurgy parts and products are pressed in 
mechanical or hydraulic compacting presses, other techniques such as cold and 
hot isostatic pressing, powder metallurgy forging, and direct powder rolling 
are used to make a variety of important end products. 
    Powder metallurgy has environmental advantages in terms of both materials 
and energy conservation. Instead of generating scrap, the process utilizes 
scrap from other metal processes and recycles it into useful powders. Energy 
savings as high as 50% can be realized using lower-density powder metallurgy 
parts in place of steel parts machined from bar stock that require extensive 
machining. Only the amount of raw material needed for the finished part is 
used.  See  METALLURGY. 
 �34s[Peter K. Johnson]
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�6v�2v�32sPower   �21m �33s 
    The time rate of doing work. Like work, power is a scalar quantity, that 
is, a quantity which has magnitude but no direction. Some units often used for 
the measurement of power are the watt (1 joule of work per second) and the 
horsepower (550 foot-pounds of work per second).  See  WORK. 
    Power is a concept which can be used to describe the operation of any 
system or device in which a flow of energy occurs. In many problems of 
apparatus design, the power, rather than the total work to be done, determines 
the size of the component used. Any device can do a large amount of work by 
performing for a long time at a low rate of power, that is, by doing work 
slowly. However, if a large amount of work must be done rapidly, a high-power 
device is needed. High-power machines are usually larger, more complicated, 
and more expensive than equipment which need operate only at low power. A 
motor which must lift a certain weight will have to be larger and more 
powerful if it lifts the weight rapidly than if it raises it slowly. An 
electrical resistor must be large in size if it is to convert electrical 
energy into heat at a high rate without being damaged. 
 �34s[Paul W. Schmidt]
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�6v�2v�32sPower amplifier   �21m �33s 
    The final stage in multistage amplifiers, such as audio amplifiers and 
radio transmitters, designed to deliver appreciable power to the load. 
    Power amplifiers may be called upon to supply power ranging from a few 
watts in an audio amplifier to many thousands of watts in a radio transmitter. 
In audio amplifiers the load is usually the dynamic impedance presented to the 
amplifier by a loudspeaker, and the problem is to maximize the power delivered 
to the load over a wide range of frequencies. The power amplifier in a radio 
transmitter operates over a relatively narrow band of frequencies with the 
load essentially a constant impedance.  See  AMPLIFIER. 
 �34s[Harold F. Klock]
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�6v�2v�32sPower factor �21m �33s
    The ratio of watts average (or active) power to the apparent power of an
alternating-current circuit. By definition, and of general application, the

     Power factor (pf) = (watts average power/rms volts x rms amperes)

equation below holds, which is the ratio of instrument readings. A watt-meter
indicates average power and electrodynamometer or iron-vane instruments show
root-mean-square (rms) voltage and current. For the steady-state ac circuit
under sinusoidal voltage and current, pf = cos THETA, where THETA is the phase
angle between the voltage and current. This definition is restricted to sine
waves of the same frequency.  See  ALTERNATING-CURRENT CIRCUIT THEORY;
NONSINUSOIDAL WAVEFORM; POWER-FACTOR METER.
 �34s[Bernard L. Segal]
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�6v�2v�32sPower-factor meter �21m �33s
    An instrument used to indicate whether load currents and voltages are in
time-phase with one another.  See  POWER FACTOR.
    The single-phase meter contains a fixed coil that carries the load
current, and crossed coils that are connected to the load voltage. There is no
spring to restrain the moving system, which takes a position to indicate the
angle between the current and voltage. The scale can be marked in degrees or
in power factor.  See  PHASE METER.
    The angle between the currents in the crossed coils is a function of
frequency, and consequently each power-factor meter is designed for a single
frequency and will be in error at all other frequencies.
 �34s[Harry Sohon /Edward C. Stevenson]
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�6v�2v�32sPower plant �21m �33s
    A means for converting stored energy into work. Stationary power plants
such as electric generating stations are located near sources of stored
energy, such as coal fields or river dams, or are located near the places
where the work is to be performed, as in cities or industrial sites. Mobile
power plants for transportation service are located in vehicles, as the
gasoline engines in automobiles and diesel locomotives for railroads.
    Most power plants convert part of the stored raw energy of fossil fuels
into kinetic energy of a spinning shaft. Some power plants harness nuclear
energy. Elevated water supply or run-of-the river energy is used in
hydroelectric power plants. For transportation, the plant may produce a
propulsive jet, as in some aircraft, instead of the rotary motion of a shaft.
Other sources of energy, such as winds, tides, waves, geothermal sources,
ocean thermal, nuclear fusion, and solar radiation, have been of negligible
commercial significance in the generation of power despite their magnitudes.
See  ELECTRIC POWER GENERATION; ENERGY SOURCES.
 �34s[Theodore Baumeister ;Leonard M. Olmsted; Kenneth A. Roe]
�33s

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�6v�2v�32sPower shovel   �21m �33s 
    A digging machine, usually self-propelled on crawler, rubber tire, or 
sometimes rail mountings. It is equipped with a shovel boom, dipper attached 
to the front end of a dipper stick, dipper trip mechanism, padlock (dipper 
sheave block), crowding mechanism, and cables all carried on a fully revolving 
superstructure. See BULK-HANDLING MACHINES; CONSTRUCTION EQUIPMENT. 
 �34s[Arthur M. Perrin]
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�6v�2v�32sPower steering   �21m �33s 
    A steering control system for a propelled vehicle in which an auxiliary 
power source assists the driver by providing the major force required to 
direct the road wheels. The term power steering commonly applies to any system 
with a variety of power booster arrangements. The principal components of the 
power booster are the control valve, the power actuator, and the power source. 
Dependent upon the type of system, these components normally operate in 
conjunction with a hand steering wheel, linkage system, and steered wheels. 
(Most systems provide for manual steering gear backup.) As a vehicle follows a 
driver-steered course, the valve senses any deviation between the course-set 
position of the steering wheel and the corresponding position of the steered 
wheels and releases power to the actuator until the error is corrected.  See  
AUTOMOTIVE STEERING. 
    The power steering control system is a hydraulic servomechanism. As the 
driver's desired course is set at the steering wheel, the system (servoloop) 
compares the desired course with a measurement of the actual and ``signals'' a 
measure of the error. The error signal proportionately actuates the power to 
assist the steering maneuver. This servoloop essentially controls the power 
assist function of the steering system. Normally the actual servoloop is added 
to a manual steering arrangement; however, manual backup steering is not 
mandatory for off-highway vehicles. The main components of the system that 
maintain the servo function are the control valve and actuator. This 
combination can be introduced in various arrangements. The illustration shows 
an integral power steering assembly. 
    In each system an engine-driven pump with a reservoir acts as the power 
source. The pump delivers hydraulic fluid oil under a working pressure to the 
controller valve. The controller valve meters the working pressure or pump 
delivery to the actuator, a hydraulic cylinder. The controlling valve responds 
to a torque from the steering wheel and to reactive shock from the road wheels 
and directs oil as required to the actuator. In the stroking action the 
hydraulic cylinder repositions the steered wheels to maintain the vehicle 
course set by the driver at the steering wheel. In order for the driver to 
``feel'' the road at the steering wheel, the control valve is desensitized to 
low steering torque efforts, and then manual steer results. 
 �34s[James L. Rau]
�33s 
  
  
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�6v�2v�32sPoynting's vector �21m �33s
    A vector, the outward normal component of which, when integrated over a
closed surface in an electromagnetic field, represents the outward flow of

	      PI = BOLDE x BOLDH = mu(sup -1)BOLDE x BOLDB

energy through that surface. It is given by the equation below, where BOLDE is
the electric field strength, BOLDH the magnetic field strength, BOLDB the
magnetic flux density, and mu the permeability.
    When an electromagnetic wave is incident on a conducting or absorbing
surface, theory predicts that it should exert a force on the surface in the
direction of the difference between the incident and the reflected Poynting's
vector.  See  ELECTRIC FIELD; ELECTROMAGNETIC RADIATION; MAGNETIC FIELD;
MAXWELL`S EQUATIONS; RADIATION PRESSURE.
 �34s[William R. Smythe]
�33s

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�6v�2v�32sPrairie dog   �21m �33s 
    A stout, fossorial rodent belonging to the family Sciuridae. Three species 
are recognized; the black-tailed prairie dog (Cynomys ludovicianus), once 
abundant on the prairies of the western United States, but now reduced in 
numbers; the white-tailed prairie dog (C. gunnisoni), discontinuously 
distributed in mountainous and high plains areas of Colorado, New Mexico, 
Utah, and Montana; and C. mexicanus, found in northern Mexico. 
    All species have the same general features. The tail is short and flat, 
the ears are small, and the limbs are short and terminate in long claws which 
are used for burrowing. Prairie dogs are social animals and live in colonies. 
Their burrows below the ground are constructed with the main entrance 
surrounded by a  mound of hard-packed dirt to protect against flooding.  See  
RODENTIA; SOCIAL ANIMALS. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sPrandtl-Meyer expansion fan   �21m �33s 
    A steady planar compressible fluid flow that occurs only at supersonic 
speeds. The Prandtl-Meyer expansion fan is essentially the isentropic flow 
around a corner from a uniform supersonic flow. The illustration shows a 
typical expansion fan, starting from the uniform flow at Mach number M1 along 
a flat wall, and turning a corner through an angle THETA. The straight lines 
in the fan are Mach waves, which  are standing sound waves in the fluid. The 
flow behind the fan is also uniform, but the velocity is higher and the 
pressure is lower. A centered Prandtl-Meyer expansion fan develops at a sharp 
corner in a supersonic flow. All flow quantities are constant on a given Mach 
wave in the fan and depend only on M1 and the angle through which the flow has 
turned.  See  FLUID FLOW. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
  
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�6v�2v�32sPraseodymium   �21m �33s 
    A chemical element, Pr, atomic number 59, and atomic weight 140.91. 
Praseodymium is a metallic element of the rare-earth group. The stable isotope 
140.907 makes up 100% of the naturally occurring element. The oxide is a black 
powder, the composition of which varies according to the method of 
preparation. If oxidized under a high pressure of oxygen it can approach the 
composition PrO2. The black oxide dissolves in acid with the liberation of 
oxygen to give green solutions or green salts which have found application in 
the ceramic industry for coloring glass and for glazes. For properties of the 
metal  see  RARE-EARTH ELEMENTS. 
 �34s[Frank H. Spedding]
�33s 
  
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�6v�2v�32sPreamplifier   �21m �33s 
    A voltage amplifier suitable for operation with a low-level input signal. 
It is intended to be connected to another amplifier with a higher input level. 
Preamplifiers are necessary when an audio amplifier is to be used with 
low-output transducers such as magnetic phonograph pickups. A preamplifier may 
incorporate frequency-correcting networks to compensate for the frequency 
characteristics of a given input transducer and to make the frequency response 
of the preamplifier-amplifier combination uniform.  See  AMPLIFIER; VOLTAGE 
AMPLIFIER.  
 �34s[Harold F. Klock]
�33s 
  
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�6v�2v�32sPrecambrian   �21m �33s 
    The term used to designate rocks and time older than the Cambrian, which 
is the oldest geologic period from which abundant fossils have been recovered. 
Age determinations by means of radioactive elements place the upper boundary 
of the Precambrian at about 600,000,000 years ago. The oldest dated rocks 
appear to be 3,800,000,000 - 4,000,000,000 years old. All evidence indicates 
that these rocks are not remnants of the original Earth crust,and it follows 
that the Precambrian interval of Earth history lasted well over 3,500,000,000 
years.  See  CAMBRIAN; RADIOACTIVE MINERALS; ROCK AGE DETERMINATION. 
    The variety of rocks found in Precambrian terranes is great, but one 
continent differs little from another in this regard. They are represented by 
all kinds of sedimentary rocks (conglomerates, sandstones, shales, and 
limestones) and extensive lava flows. In general, the older rocks exhibit 
higher degrees of metamorphism, but this is not invariably true. Precambrian 
rocks underlie all other rocks of continents. They occur beneath a great 
blanket of flat or deformed sedimentary and other rocks, are near the surface 
covered by a thin veneer of strata or unconsolidated material, or are exposed. 
    Precambrian rocks are important sources of metalliferous ores, 
particularly iron, nickel, gold, uranium, and copper. Tremendous stores of 
iron ore have been mined and are still being mined from the Canadian shield, 
the Baltic shield, the Ukraine, and the Brazilian and Guianan shields. The 
ores are mostly sedimentary, but large masses of magnetite of probably igneous 
origin are also productive.  
    Life was probably abundant in the seas of late Precambrian time, since 
Early Cambrian formations contain abundant fossils, but forms with shells 
capable of being preserved were still few. 
    The atmosphere undoubtedly underwent great change in Precambrian time, but 
evidence from the rocks indicates that climatic variation from dry to wet and 
from hot to cold took place then as later. Glaciation was widespread. 
 �34s[J. Paul Fitzsimmons]
�33s 
  
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�6v�2v�32sPrecast concrete   �21m �33s 
    Concrete that has been cast into a form which is later incorporated into a 
structure. A concrete structure may be constructed by casting the concrete in 
place on the site, by building it of components cast elsewhere, or by a 
combination of the two. Concrete cast in other than its final position is 
called precast. 
    In contrast with cast-in-place concrete construction, in which columns, 
beams, girders, and slabs are cast integrally or bonded together by successive 
pours, precast concrete requires field connections to tie the structure 
together. These connections can be a major design problem. 
    Precast units can be standardized. Savings can then result from repeated 
reuse of forms and assembly-line production. Furthermore, high quality can be 
maintained because of the controls that can be kept on production under plant 
conditions. However, there is always the possibility that transportation, 
handling, and erection costs for the precast units will offset the savings.  
See  CONCRETE; CONCRETE SLAB; PRESTRESSED CONCRETE. 
 �34s[Frederick S. Merritt]
�33s 
  
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�6v�2v�32sPrecession   �21m �33s 
    An angular velocity of the axis of spin of a spinning rigid body, which 
arises as a result of external torques acting on the body. Examples are the 
precession of a spinning top, the Earth (precession of the equinoxes), an 
airplane propeller, or a gyroscope.  See  PRECESSION OF EQUINOXES. 
    The uniform precession of a charged spinning body in a uniform magnetic 
field is called Larmor precession. This motion, similar to that of a rapidly 
spinning top, is of great importance in atomic physics.  See  LARMOR 
PRECESSION.  
    Consider a top spinning rapidly about its z axis of symmetry and placed 
horizontally on a point support at O (see illustration). The top does not fall 
as a result of the pull of gravity (its weight, W), as might be supposed; 
rather, the z axis of the top rotates slowly about the vertical y axis while 
maintaining its position in the horizontal xz plane. This rotation about the y 
axis is called precession. 
 �34s[Ray E. Bolz]
�33s 
  
  
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�6v�2v�32sPrecession of equinoxes   �21m �33s 
    A slow change in the direction of orientation of Earth's axis of rotation 
which results in a gradual westward motion of the equinoxes. There are two 
types, known as lunisolar precession and planetary precession. 
    The lunisolar precession of the equinoxes is caused by the gravitational 
attraction of the Sun and the Moon which, as a result of the polar flattening 
of Earth and the inclination of Earth's axis, gives rise to a small turning 
moment, or torque, on the Earth in its orbit. As a result of this torque, 
Earth's axis describes a cone about the normal to the plane of its orbit (see 
illustration); the period of this precession is approximately 26,000 years in 
a direction opposite to that of Earth's rotation. 
    The planetary precession is a comparatively small eastward motion of the 
equinoxes caused by the action of the other planets in altering the plane of  
Earth's orbit. 
    As a result of precession, Polaris will not always be the pole star. Vega 
will be nearer to the north celestial pole in about 12,000 years; ALPHA 
Draconis was the pole star about 4600 years ago. Another effect of precession 
is that the signs of the zodiac no longer correspond to their respective 
constellations.  See  PRECESSION. 
 �34s[J. Allen Hynek]
�33s 
  
  
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�6v�2v�32sPrecious stones   �21m �33s 
    The materials found in nature that are used frequently as gemstones, 
including amber, beryl (emerald and aquamarine), chrysoberyl (cat's-eye and 
alexandrite), coral, corundum (ruby and sapphire), diamond, feldspar 
(moonstone and amazonite), garnet (almandite, demantoid, and pyrope), jade 
(jadeite and nephrite), jet, lapis lazuli, malachite, opal, pearl, peridot, 
quartz (amethyst, citrine, and agate), spinel, spodumene (kunzite), topaz, 
tourmaline, turquois and zircon.  See  GEM. 
    The terms precious and semiprecious have been used to differentiate 
between gemstones on a basis of relative value. Because there is a continuous 
gradation of values from materials sold by the pound to those valued at many 
thousands of dollars per carat, and because the same mineral may furnish both, 
a division is essentially meaningless. 
 �34s[Richard T. Liddicoat, Jr.]
�33s 
  
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�6v�2v�32sPrecipitation gages   �21m �33s 
    Instruments used to measure the amount of rainfall or snowfall expressed 
as inches or millimeters of depth of water which falls on a level surface. 
When precipitation gages are equipped with a recorder, the time of occurrence 
and the rate or intensity are available as well. Other forms of precipitation, 
such as dew, frost, and moisture absorbed by the soil, are also of 
considerable interest, but they are not measured by the precipitation gages 
routinely used by the meteorological services. 
    The gage is basically an open container and funnel constructed to minimize 
any splashing out and mounted 1 - 3 ft (0.3 - 0.9 m) above the surface to 
prevent splashing into the container, yet not so high that its catch is 
affected by the wind which normally increases in velocity with height. From 
the volume of water calibrated and the area of the opening, the depth of water 
is easily obtained. A continuous type of recording rain gage uses a spring 
balance to record on a clock-driven chart the weight of the precipitation 
calibrated.  
    When a suitable wind screen is used, snowfall can be measured by melting 
the snow collected in a rain gage (the funnel must be removed). Fallen snow is 
also measured by taking sample cores.  See  SNOW GAGE. 
 �34s[Verner E. Suomi]
�33s 
  
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�6v�2v�32sPrecipitation (chemistry)   �21m �33s 
    The process of producing a separable solid phase within a liquid medium. 
In analytical chemistry, precipitation is widely used to effect the separation 
of a solid phase in an aqueous solution. For example, the addition of a water 
solution of silver nitrate to a water solution of sodium chloride results in 
the formation of insoluble silver chloride. Quite often, one of the components 
in the solution is thus virtually completely separated in a relatively pure 
form. It can then be isolated from the solution phase by filtration or 
centrifugation, and the substance determined by weighing. This procedure is 
known as gravimetric analysis. Precipitation may also be used merely to effect 
partial or complete separation of a substance for purposes other than that of 
gravimetric analysis. Such purposes might involve either the isolation of a 
relatively pure substance or the removal of undesirable components of the 
solution.  See  GRAVIMETRIC ANALYSIS. 
    The extent to which a component can be separated from solution can be 
determined from the solubility-product constant obtained by determining the 
quantity of dissolved substance present in a known amount of saturated 
solution. This value is known as the solubility. The solubility can be 
drastically altered merely by adding to the solution any of the ions that make 
up the precipitate, for example, by adding varying quantities of either silver 
nitrate or sodium chloride to a saturated solution of silver chloride. 
Although solubility can be altered over a wide range, the solubility product 
itself remains practically constant over this same range.  See  SOLUBILITY 
PRODUCT CONSTANT. 
    Various techniques may be employed in order to reduce contamination by 
foreign ions. Precipitation from dilute solution is often effective. Heating 
the reaction mixture speeds recrystallization processes by which incorporated 
foreign ions may be returned to the solution phase. Precipitation from 
homogeneous solution results in the slow formation of large crystals of small 
surface area and hence lessens coprecipitation. If all these methods fail to 
reduce adequately the quantity of foreign ions incorporated in the solid 
phase, the precipitate is dissolved and reprecipitated by the previous 
procedure.  See  CHEMICAL SEPARATION TECHNIQUES; CRYSTALLIZATION; NUCLEATION. 
 �34s[Louis Gordon /Royce W. Murray]
�33s 
  
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�6v�2v�32sPrecipitation (meteorology)   �21m �33s 
    The fallout of water drops or frozen particles from the atmosphere. Liquid 
types are rain or drizzle, and frozen types are snow, hail, small hail, ice 
pellets (also called ice grains; in the United States, sleet), snow pellets 
(graupel, soft hail), snow grains, ice needles, and ice crystals. In England 
sleet is defined as a mixture of rain and snow, or melting snow. Deposits of 
dew, frost, or rime, and moisture collected from fog are occasionally also 
classed as precipitation.  See  HAIL; SNOW. 
    All precipitation types are called hydrometeors, of which additional forms 
are clouds, fog, wet haze, mist, blowing snow, and spray. Whenever rain or 
drizzle freezes on contact with the ground to form a solid coating of ice, it 
is called freezing rain, freezing drizzle, or glazed frost; it is also called 
an ice storm or a glaze storm, and sometimes is popularly known as silver thaw 
or erroneously as a sleet storm.  See  CLOUD; FOG. 
    Rain, snow, or ice pellets may fall steadily or in showers. Steady 
precipitation may be intermittent though lacking sudden bursts of intensity. 
Hail, small hail, and snow pellets occur only in showers; drizzle, snow 
grains, and ice crystals occur as steady precipitation. Showers originate from 
instability clouds of the cumulus family, whereas steady precipitation 
originates from stratiform clouds. 
    The amount of precipitation, often referred to as precipitation or simply 
as rainfall, is measured in a collection gage. It is the actual depth of 
liquid water which has fallen on the ground, after frozen forms have been 
melted, and is recorded in millimeters or inches and hundredths. A separate 
measurement is made of the depth of unmelted snow, hail, or other frozen 
forms.  See  PRECIPITATION GAGES; SNOW GAGE. For discussions of other topics 
related to precipitation  see  ATMOSPHERIC GENERAL CIRCULATION; CLOUD PHYSICS; 
DEW; DEW POINT; HUMIDITY; HYDROLOGY; HYDROMETEOROLOGY; RAIN SHADOW; VAPOR 
PRESSURE; WEATHER MODIFICATION. 
 �34s[J. R. Fulks]
�33s 
  
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�6v�2v�32sPrecipitin   �21m �33s 
    The visible result of the chemical interaction of antigen and antibody. 
Not all antibodies will result in precipitation, yet they may participate in 
agglutination reactions or add onto particulate antigens, and evidence for 
their occurrence together with precipitating antibody can be obtained for most 
sera. Precipitins may be noted qualitatively or be quantified by noting the 
end-point dilution (titer) of serum required to give a precipitate at the 
threshold of visibility, or the amount of antibody may be determined in 
milligrams or micrograms by analysis of the precipitate with correction for 
the antigen contained therein.  See  ANTIBODY; ANTIGEN. 
 �34s[Donald Raum]
�33s 
  
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�6v�2v�32sPrecision approach radar (PAR)   �21m �33s 
    A radar system located on an airfield for observation of the position of 
an aircraft with respect to an approach path and specifically intended to 
provide guidance to the aircraft during its approach to the field. 
    The PAR system consists of a ground radar equipment which is alternately 
connected to two antenna systems. One of these antenna systems sweeps a narrow 
beam over a 20DG sector in the horizontal plane. The second antenna system 
sweeps a narrow beam over a 7DG sector in the vertical plane. Since the 
reflected energy from the aircraft produces information on the location of the 
aircraft in terms of angles and distance, the two sweeps give the 
three-dimensional location of the aircraft (see illustration). See AIRCRAFT 
LOW-APPROACH SYSTEMS; ELECTRONIC NAVIGATION SYSTEMS. 
 �34s[Peter C. Sandretto]
�33s 
  
  
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�6v�2v�32sPregnancy �21m �33s
    In humans, the period of about 280 days required for the normal
intrauterine development of a child. Changes in the mother are most marked in
the reproductive organs, including the uterus itself. A slight enlargement and
softening of the uterus may be detectable on examination by the end of the
second month. As further enlargement occurs, the muscular wall becomes
hypertrophied and then stretched, so that it is soft and somewhat flexible.
During the fourth month the uterus ordinarily rises above the pelvis and
becomes abdominal. By the end of pregnancy the upper portion of the uterus
lies just beneath the anterior rib margins.
    Changes in the breasts begin early in pregnancy with enlargement and
tenderness being common. As term approaches, the nipples and areolae increase
in size and deepen in pigmentation, reflecting the proliferation of the
glandular elements of the breasts. A watery or slightly turbid discharge,
called colostrum, may be expressed from the nipples, especially in women who
have had previous pregnancies.
    Changes in the vagina, ovaries, and cervix accompany the progression of
pregnancy. Systemic alterations in renal function, cardiovascular adaptation,
blood volume, and cell counts, as well as in chemical constituents of blood
and tissue all reflect the complex physiologic changes required for successful
pregnancy.
    Pregnancies may proceed to term or may be concluded by natural or
artifical means prior to normal delivery. The term abortion is used to
indicate the passage of a fetus of less than 2 lb (1000 g), or before the
twenty-eighth week of pregnancy. This may occur spontaneously as a result of
any one of many causes, including induced methods.  See  MENSTRUATION;
PREGNANCY DISORDERS.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sPregnancy disorders   �21m �33s 
    Pregnancy in humans leads to the relatively common minor complaints of 
nausea, vomiting, fatigue, and constipation but also to more serious disorders. 
    Hemorrhage in pregnancy may be due to many causes but by far the most 
common is abortion; less common causes are ectopic pregnancy, premature labor, 
and premature separation of the placenta from the uterus. 
    In ectopic pregnancy, a not infrequent occurrence, a fertilized ovum 
becomes implanted in a tissue other than the uterine mucosa. Most often the 
site of implantation is some part of the Fallopian tube which becomes 
stretched and inflamed as growth proceeds; rupture is not uncommon. In other 
cases, the fertilized ovum may be implanted on some part of the peritoneum or 
mesentery and produce some symptoms if growth proceeds. There are occasional 
reports of the delivery of a full-term, normal infant by abdominal operation 
from such a site. In most cases, however, it is believed that the embryo does 
not survive and is gradually absorbed by the body. 
    Two important tumors of the placenta occur. They are the hydatidiform mole 
and choriocarcinoma. These growths are grouped together because in about 
one-half of the cases choriocarcinoma is preceded by a hydatidiform mole. They 
should be considered separate, however, because the mole in most cases is a 
benign lesion, whereas choriocarcinoma may be fatal.  
    It is estimated that of every 100 fertilizations, only 70% will develop 
into term infants. Of the remainder, one-third are abnormal  so that 
implantation cannot occur, one-third result in abnormal embryos which are 
aborted, and one-third are lost by spontaneous abortion from other causes.  
See  FERTILIZATION; PLACENTATION; PREGNANCY. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sPrehnite �21m �33s
    A mineral with the formula Ca2(Al,Fe(sup 3+)(OH)2-[Si3AlO10], with Al in
parentheses in octahedral and Al in brackets in tetrahedral coordination by
oxygens. The mineral usually occurs as stalactitic aggregates or as curved
crystals, has a vitreous luster, and is yellowish green to pale green in
color. Hardness is 6 - 6** on Mohs scale; specific gravity 2.8 - 2.9. Common
occurrences include vesicular basalts such as the Keweenaw basalts in the
Upper Peninsula of Michigan, and the Watchung basalts in New Jersey.  See
SILICATE MINERALS.
 �34s[Paul B. Moore]
�33s

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�6v�2v�32sPress fit   �21m �33s 
    A force fit that has negative allowance; that is, the bore in the fitted 
member is smaller than the shaft which is pressed into the bore. Tight fits 
have slight negative allowance so that light pressure is required to assemble 
the parts; they are used for gears, pulleys, cranks, and rocker arms. Medium 
force fits have somewhat greater negative allowance and require considerable 
pressure for assembly; they are used for fastening locomotive wheels, car 
wheels, and motor armatures.  See  ALLOWANCE; FORCE FIT; SHRINK FIT. 
 �34s[Paul H. Black]
�33s 
  
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�6v�2v�32sPressure �21m �33s
    The ratio of force to area. The force per unit area at the interior of the
Sun is estimated to be 3 x 10(sup 17) dynes/cm(sup 2) (3 x 10(sup 16)Pa). In
interstellar space, pressure approaches zero. Atmospheric pressure at the
surface of Earth is in the vicinity of 14 lb/in.(sup 2) (100 kPa). Pressures in
enclosed containers less than this value are spoken of as vacuum pressures.
This is absolute pressure measured above zero pressure as a reference level.
Inside a steam boiler, the pressure may be 800 lb/in.(sup 2) (5.5 MPa) or
higher. Such pressure, measured above atmospheric pressure as a reference
level, is gage pressure, designated psig.  Differential pressure is the
difference between any two pressures, neither of which is atmospheric.  See 
PRESSURE MEASUREMENT. 
 �34s[Frank H. Rockett]
�33s

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�6v�2v�32sPressure altimeter   �21m �33s 
    A device that precisely measures the pressure of the air at the level an 
aircraft is flying and converts the pressure measurement to an indication of 
height above sea level according to a standard pressure-altitude relationship. 
In essence, a pressure altimeter is a highly refined aneroid barometer since 
it utilizes an evacuated capsule whose movement or force is directly related 
to the pressure on the outside of the capsule. Various methods are used to 
sense the capsule function and cause a display to respond such that the pilot 
sees the altitude level much as one looks at a watch. 
    Because altitude measured in this manner is also subject to changes in 
local barometric pressure, altimeters are provided with a barosetting that 
allows the pilot to compensate for these weather changes, the sea-level air 
pressure to which the altimeter is adjusted appearing in a window of the dial. 
Flights below 18,000 ft (5486 m) must constantly contact the nearest traffic 
center to keep the altimeters so updated. Flights above 18,000 ft and over 
international waters utilize a constant setting of 29.92 in. Hg, or 1013.2 
millibars (101.32 kPa), so that all highflying aircraft have the same 
reference and will be interrelated, thus providing an extra margin of safety. 
 �34s[James W. Angus]
�33s 
  
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�6v�2v�32sPressure measurement   �21m �33s 
    The determination of the magnitude of a fluid force applied to a unit 
area. Pressure measurements are generally classified as gage pressure, 
absolute pressure, or differential pressure.  See  PRESSURE. 
    Pressure gages generally fall in one of three categories, based on the 
principle of operation: liquid columns, expansible-element gages, and 
electrical pressure transducers. 
    Liquid-column gages include barometers and manometers. They consist of a 
U-shaped tube partly filled with a nonvolatile liquid. Water and mercury are 
the two most common liquids used in this type of gage.  See  BAROMETER; 
MANOMETER.  
    There are three classes of expansible metallic-element gages: bourdon, 
diaphragm, and bellows. Bourdon-spring gages, in which pressure acts on a 
shaped, flattened, elastic tube, are by far the most widely used type of 
instrument. These gages are simple, rugged, and inexpensive. In 
diaphragm-element gages, pressure applied to one or more contoured diaphragm 
disks acts against a spring or against the spring rate of the diaphragms, 
producing a measurable motion. In bellows-element gages, pressure in or around 
the bellows moves the end plate of the bellows against a calibrated spring, 
producing a measurable motion.  See  BOURDON-SPRING PRESSURE GAGE. 
    Electrical pressure transducers convert a pressure to an electrical signal 
which may be used to indicate a pressure or to control a process. Such devices 
as strain gages, resistive, magnetic, crystal, and capacitive pressure 
transducers are commonly used to convert the measured pressure to an 
electrical signal.  See  PRESSURE TRANSDUCER; STRAIN GAGE. 
 �34s[John H. Zifcak]
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�6v�2v�32sPressure seal   �21m �33s 
    A seal used to make pressure-proof the interface (contacting surfaces) 
between two parts that have frequent or continual relative rotational or 
translational motion; such seals are known as dynamic seals, as compared with 
static seals. While the pressure in seals is lower than that in gaskets, the 
motion hinders their effectiveness so that there are more types of seals than 
gaskets, each type attempting to serve its environment. The materials are 
leather, rubber, cotton, and flax, and for piston rings, cast iron. The forms 
of nonmetallic seals are rectangular, V-ring, and O-ring. Cartridge seals are 
available for rolling-contact bearings. Special seals include carbon ring and 
labyrinth seals for turbines and mechanical seals for pumps.  See  GASKET. 
 �34s[Paul H. Black]
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�6v�2v�32sPressure transducer   �21m �33s 
    An instrument component which detects a fluid pressure and produces an 
electrical, mechanical, or pneumatic signal related to the pressure.  See  
TRANSDUCER.  
    In general, the complete instrument system comprises a pressure-sensing 
element such as a bourdon tube, bellows, or diaphragm element: a device which 
converts motion or force produced by the sensing element to a change of an 
electrical, mechanical, or pneumatic parameter; and an indicating or recording 
instrument. Frequently the instrument is used in an autocontrol loop to 
maintain a desired pressure.  See  PROCESS CONTROL. 
    Although pneumatic and mechanical transducers are commonly used, 
electrical measurement of pressure is often preferred because of a need for 
long-distance transmission, higher accuracy requirements, more favorable 
economics, or quicker response. Electrical pressure transducers may be 
classified by the operating principle as resistive transducers, strain gages, 
magnetic transducers, crystal transducers, capacitive transducers, and 
resonant transducers. 
    In resistive pressure transducers, pressure is measured by an element that 
changes its electrical resistance as a function of pressure. Many types of 
resistive pressure transducers use a movable contact, positioned by the 
pressure-sensing element. One form is a contact sliding along a continuous 
resistor, which may be straight-wire, wire-wound, or nonmetallic such as 
carbon.  
    Strain-gage pressure transducers might be considered to be resistive 
transducers, but are usually classified separately. They convert a physical 
displacement into an electrical signal. When a wire is placed in tension, its 
electrical resistance increases. The change in resistance is a measure of the 
displacement, hence of the pressure. Another variety of strain gage transducer 
uses integrated circuit technology. Resistors are diffused onto the surface of 
a silicon crystal within the boundaries of an area which is etched to form a 
thin diaphragm.  See  INTEGRATED CIRCUITS; STRAIN GAGE. 
    In magnetic pressure transducers, a change of pressure is converted into 
change of magnetic reluctance or inductance when one part of a magnetic 
circuit is moved by a pressure-sensing element - bourdon tube, bellows, or 
diaphragm.  
    Piezoelectric crystals produce an electric potential when placed under 
stress by a pressure-sensing element. Crystal transducers offer a high speed 
of response and are widely used for dynamic pressure measurements in such 
applications as ballistics and engine pressures.  See  PIEZOELECTRICITY. 
    Capacitive pressure transducers almost invariably sense pressure by means 
of a metallic diaphragm, which is also used as one plate of a capacitor.  
    The resonant transducer consists of a wire or tube fixed at one end and 
attached at the other (under tension) to a pressure-sensing element. The wire 
is placed in a magnetic field and allowed to oscillate. As the pressure is 
increased, the element increases the tension in the wire or tube, thus raising 
its resonant frequency.  See  PRESSURE MEASUREMENT. 
 �34s[John H. Zifcak]
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�6v�2v�32sPressure vessel   �21m �33s 
    A cylindrical or spherical metal container capable of withstanding 
pressures exerted by the material enclosed. Pressure vessels are important 
because many liquids and gases must be stored under high pressure. Special 
emphasis is placed upon the strength of the vessel to prevent explosions as a 
result of rupture. Codes for the safety of such vessels have been developed 
that specify the design of the container for specified conditions. 
    Most pressure vessels are required to carry only low pressures and thus 
are constructed of tubes and sheets rolled to form cylinders. Some pressure 
vessels must carry high pressures, however, and the thickness of the vessel 
walls must increase in order to provide adequate strength. Hydraulic and 
pneumatic cylinders are machine elements that are forms of pressure vessels. 
 �34s[James J. Ryan]
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�6v�2v�32sPressurized blast furnace   �21m �33s 
    A blast furnace operated under higher than normal pressure. The pressure 
is obtained by throttling the off-gas line, which permits a greater volume of 
air to be passed through the furnace at lower velocity and results in an 
increasing smelting rate. The process permits large increases in the weight of 
high-temperature air blown into the bottom of the furnace at lower gas 
velocities, thus increasing the rate of smelting and decreasing the rate of 
coke consumption, and also permitting smoother operation with less flue dust 
production through decreased pressure drop between bottom and top pressures.  
See  FURNACE. 
 �34s[Bruce S. Old]
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�6v�2v�32sPrestressed concrete   �21m �33s 
    Concrete with stresses induced in it before use so as to counteract 
stresses that will be produced by loads. Prestress is most effective with 
concrete, which is weak in tension, when the stresses induced are compressive. 
One way to produce compressive prestress is to place a concrete member between 
two abutments, with jacks between its ends and the abutments, and to apply 
pressure with the jacks. The most common way is to stretch steel bars or 
wires, called tendons, and to anchor them to the concrete; when they try to 
regain their initial length, the concrete resists and is pre-stressed. The 
tendons may be stretched with jacks or by electrical heating. 
    Prestressed concrete is particularly advantageous for beams. It permits 
steel to be used at stresses several times larger than those permitted for 
reinforcing bars. It permits high-strength concrete to be used economically, 
for in designing a member with reinforced concrete, all concrete below the 
neutral axis is considered to be in tension and cracked, and therefore 
ineffective, whereas the full cross section of a prestressed concrete beam is 
effective in bending.  See  CONCRETE BEAM; REINFORCED CONCRETE; STRESS AND 
STRAIN.  
 �34s[Frederick S. Merritt]
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�6v�2v�32sPriapulida   �21m �33s 
    One of the minor groups of wormlike marine animals, now regarded as a 
separate phylum of the animal kingdom with uncertain zoological affinities. 
The phylum is a small one with only two genera, Priapulus and Halicryptus. 
    Priapulida inhabit the colder waters of both hemispheres. They burrow in 
mud and sand of the sea floor, from the intertidal region to depths of 14,850 
ft (4500 m). 
    Priapulids are small to medium-sized animals, the largest specimen 
attaining 6 in. (15 cm) in length. The body of Priapulus is made up of three 
distinct portions: proboscis, trunk, and caudal appendage (see illustration). 
Separated by a constriction from the trunk, the bulbous, introversible 
proboscis usually constitutes the anterior third of the body and is marked by 
25 longitudinal ridges of papillae or spines. The mouth is located at the 
anterior end of the proboscis and is surrounded by concentric rows of teeth. 
The cylindrical trunk is annulated, but not segmented, and is often covered 
with irregularly dispersed spines and tubercles. At the posterior end of the 
trunk there are three openings: the anus and two urogenital apertures. 
 �34s[Mary E. Rice]
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�6v�2v�32sPrilling   �21m �33s 
    A combination spray-drying and crystallizing technique used to produce 
agglomerates (prills) of ammonium nitrate for fertilizer. In prilling, a hot 
concentrated solution of ammonium nitrate is sprayed into a tower in which it 
crystallizes as it descends through a rising current of atmospheric air.  See  
CRYSTALLIZATION; DRYING. 
 �34s[Marvin Yelles]
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�6v�2v�32sPrimary battery   �21m �33s 
    An electric battery designed to deliver only one continuous or 
intermittent discharge. It cannot be recharged efficiently. Primary batteries 
are designed to deliver limited amounts of electric energy, determined by the 
materials used and the size of the cell. When the available energy drops to 
zero, the battery is usually discarded. Primary batteries may be classified by 
the type of electrolyte used. 
    Aqueous-electrolyte batteries use solutions of acids, bases,  or salts in 
water as the electrolyte. These solutions have ionic conductivities and 
practically no electronic conductivity. Disadvantages of such cells include 
corrosion of the electrode materials by the electrolyte, a relatively high 
evaporation rate of water vapor which can cause call failure, and the 
difficulties of preventing leakage. For examples of cells with aqueous 
electrolytes  see  BATTERY; DRY CELL; MERCURY BATTERY; RESERVE BATTERY. 
    Solid-electrolyte batteries use electrolytes of solid crystalline salts 
which have predominantly ionic conductivity. Solid electrolyte batteries may 
be classified in two broad categories: (1) cells with solid crystalline salt, 
such as silver iodide, as the electrolyte; (2) cells with ion-exchange 
membrane as the electrolyte.  See  ION-SELECTIVE MEMBRANES AND ELECTRODES. 
    Waxy-electrolyte batteries use waxy materials, such as polyethylene 
glycol, in which a small amount of a salt is dissolved in the molten wax. At 
room temperatures these materials are solid. The conductivity is small and the 
current output is limited. 
    Fused-electrolyte batteries use crystalline salts or bases which are solid 
at room temperature. In use, the cell is heated and maintained at a 
temperature above the melting point of the electrolyte.  See  FUSED SALT 
SOLUTION.  
 �34s[Jack Davis]
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�6v�2v�32sPrimary vascular system (plant)   �21m �33s 
    The arrangement of conducting elements which serves for two-way 
transportation of substances between different parts of a plant. The 
conducting elements are of two principal kinds: xylem, which is mainly 
responsible for the conduction of water together with dissolved inorganic 
substances upward from the root to other plant organs; and phloem, which is 
mainly responsible for the conduction of food materials (assimilates), a flow 
which may take place in either direction. In the shoot region of the plant, 
xylem and phloem are usually associated into vascular bundles. In the root, 
however, they usually alternate with one another on different radii.  See  
PHLOEM; XYLEM. 
 �34s[William R. Philipson]
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�6v�2v�32sPrimates �21m �33s
    The order of mammals to which humans belong. The great diversity of niches
occupied by primates within a mainly arboreal ecological zone renders the
order difficult to define in terms of diagnostic morphological
characteristics. Primates have been characterized in terms of a number of
evolutionary trends, among which are retention of a generalized limb structure
and dentition; increasing digital mobility; replacement of claws by flat
nails; development of steroscopic vision, with concomitant reduction in the
olfactory apparatus, leading to shortening of the face; and progressive
development of the brain, notably in the cerebral cortex. Not all these trends
are observable in every primate line, but the list does serve to characterize
the order as a whole.

    Living forms.  One of the most striking peculiarities of Primates is that
the order contains a gradational series of living forms which reflect to some
extent extinct stages in its evolution. Primates divide naturally into two
suborders, Prosimii and Anthropoidea, designated, respectively, the ``lower''
and ``higher'' primates. Each suborder contains six families, and these are
individually discussed below.
    Tupaiidae comprises the five genera of tree shrews; it is sometimes
referred to the menotyphylan insectivores but is here considered,
provisionally, the most primitive prosimian family. Whatever their correct
classification, these small, slender, clawed, rather squirrellike creatures
provide a model of the very earliest primates.
    In the absence of competition, the prosimians of Madagascar were able to
realize their full evolutionary potential. There are three living families,
Lemuridae, Indriidae, and Daubentoniidae. Lemuridae contains six genera, all
arboreal forest livers. The diurnal forms, Lemur and Hapalemur, are
gregarious, while the nocturnal Lepilemur, Cheirogaleus, Microcebus, and
Phaner tend to a solitary existence. All, apart from the vertical clinger and
leaper Lepilemur and, to some extent, Hapalemur, are quadrupedal. Indriidae
are all wholly arboreal vertical clingers and leapers. Indri and Propithecus
are diurnal whereas Avahi is nocturnal, but all live in similar family groups
of around two to five individuals. The single aberrant genus of Daubentoniidae
is Daubentonia. With rodentlike anterior teeth, this solitary nocturnal animal
is primarily insectivorous.  See  AYE-AYE.
    Included in the family Lorisidae are the lorises of Asia and the galagos
of Africa. Both Asian forms are nocturnal and probably solitary. Locomotion is
by slow, deliberate climbing. All African forms are nocturnal.	See  LORIS AND
ALLIES.
    Tarsiidae contains only Tarsius of Southeast Asia. The name Tarsius
derives from the greatly elongated ankle region, a specialization associated
with a vertical clinging and leaping way of life. Usually found solitary or in
pairs, Tarsius is highly territorial.  See  TARSIER.
    New World monkeys are characterized by the possession of three premolar
teeth; there are two families, Callithricidae and Cebidae. Callithricidae
includes the most primitive forms which make up the marmosets and the
tamarins. These diurnal animals live in small family groups in the high forest
canopy, where they adopt a springing form of locomotion. Except on the big
toe, marmosets have claws, not nails. Cebidae is a very diverse family in
which locomotion varies from arm-swinging quadrupedalism in forms such as
Ateles, to quadrupedalism at all levels of the forest in Cebus and its
relatives. All cebids are diurnal, apart from the large-eyed Aotus. The basic
social unit is the small family group, although in forms such as the very
vocal howler monkey, troops may be much larger. In four genera, Ateles,
Brachyteles, Lagothrix, and Alouatta, the tail is prehensile and tactile, and
is used as an extra ``hand'' in locomotion.  See  MARMOSET; MONKEY.
    The family Cercopithecidae includes all of the Old World monkeys and is
divided into two subfamilies, Cercopithecinae and Colobinae. Geographically,
there are also two separate groups, African and Asian, with only Macaca common
to both, but the geographical and taxonomic divisions do not exactly coincide.
The Colobinae, for example, Colobus, the guereza, and Presbytis, the langur,
are arboreal browsers, as are some cercopithecines, for example, Cercocebus
albigena. Macaca is arboreal or terrestrial with equal facility. The baboons,
for example, Papio and Theropithecus, are primarily terrestrial. Social
organization ranges from loose groups among the langurs to highly organized
troops in some species of Papio.
    The superfamily of apes and humans, Hominoidea contains three families,
Hylobatidae, Pongidae, and Hominidae. Hylobatidae includes Hylobates (the
gibbon) and the larger Symphalangus (the siamang), both restricted to
Southeast Asia, and both highly arboreal brachiators (arm swingers). Pan (the
chimpanzee), Gorilla of Africa, and Pongo (the orangutan) of Sumatra and
Borneo are members of the Pongidae. Pan is both a forest and woodland dweller;
Gorilla and Pongo inhabit dense forests. Pongo is a deliberate climber, while
Gorilla and Pan, though classed as semibrachiators, are secondarily adapted to
quadrupedal knuckle walking. Hominidae contains one living species, Homo
sapiens, a large, erect, ommivorous terrestrial biped. Humans' anterior
dentition is greatly reduced and the brain greatly enlarged, features
correlating with the highly developed manual dexterity.  See  CHIMPANZEE;
GORILLA; ORANGUTAN.
 �34s[Ian M. Tattersall]
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    Fossils.  In evolution of primates, natural selection tended to favor
those with larger brains, better sight, and more dexterous front feet that
functioned as hands. The incisors are reduced to two in each of the jaws in
the monkeys and apes.
    Small prosimian primates were common in the middle latitudes of North
America and Europe from the Paleocene until the middle Eocene. Apparently,
faunal interchange between the two regions was greatly reduced or ceased
during the middle Eocene. Then as aridity increased in the late Eocene and
early Oligocene, the distribution of the primates shifted southward in both
hemispheres. Some of these prosimians were extremely close to primitive stocks
of other orders, such as the Insectivora and the Dermoptera. Even in the
groups that were surely primates, infraordinal affinities are obscured by the
presence of both lemuroid and tarsioid characters.
    New World monkeys (Cebidae) appeared for the first time in the late
Oligocene and early Miocene of Argentina. They were already typical ceboids at
that time. Thus, no clue is provided concerning their ancestral relationships.
Others have been found in the late Miocene of Columbia. It seems likely that
the Cebidae arose from an early prosimian group from North America, probably
the Omomyidae.
    Old World monkeys (Cercopithecidae) and apes (Pongidae) seem to have
descended from different groups of Eocene prosimians. The earliest
cercopithecoid, Moeripithecus, occurs in the early Oligocene of Egypt. Other
more advanced genera such as Ankarapithecus, Mesopithecus, and Dolichopithecus
have been found in Pliocene faunas, mostly in central and southern Europe.
Some of the living genera also extend back into the Pliocene. There are many
divergent lineages of apes and apelike creatures, all restricted to the
Eastern Hemisphere.  See  FOSSIL MAN.
 �34s[Malcolm C. McKenna ;Ruben A. Stiron]
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�6v�2v�32sPrime mover   �21m �33s 
    The component of a power plant that transforms energy from the thermal or 
the pressure form to the mechanical form. Mechanical energy may be in the form 
of a rotating or a reciprocating shaft, or a jet for thrust or propulsion. The 
prime mover is frequently called an engine or turbine and is represented by 
such machines as waterwheels, hydraulic turbines, steam engines, steam 
turbines, windmills, gas turbines, internal combustion engines, and jet 
engines. These prime movers operate by either of two principles: (1) balanced 
expansion, positive displacement, intermittent flow of a working fluid into 
and out of a piston and cylinder mechanism so that by pressure difference on 
the opposite sides of the piston, or its equivalent, there is relative motion 
of the machine parts; or (2) free continuous flow through a nozzle where fluid 
acceleration in a jet (and vane) mechanism gives relative motion to the 
machine parts by impulse, reaction, or both.  See  GAS TURBINE; HYDRAULIC 
TURBINE; IMPULSE TURBINE; INTERNAL COMBUSTION ENGINE; POWER PLANT; REACTION 
TURBINE; STEAM ENGINE; STEAM TURBINE; TURBINE. 
 �34s[Theodore Baumeister]
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�6v�2v�32sPrimer (explosive)   �21m �33s 
    An agent used with explosives, propellants, and pyrotechnics to produce 
the initial fire. The primer itself may be initiated by percussion, stab 
action, friction, or heat. The gases and hot particles from the primer serve 
in turn to ignite a large mass of material, for example, the gunpowder in a 
cartridge, the primary explosive in a detonator cap, or the powder train in a 
fuse. The term primer is sometimes loosely applied to a detonator. 
    Although primer compositions deflagrate rapidly, they do not detonate. In 
fact, a detonating action would be most undesirable because it would tend to 
blow away rather than to ignite the charge.  See  DETONATOR; MATCH; 
PYROTECHNICS.  
 �34s[William E. Gordon]
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�6v�2v�32sPrimer (surface coating)   �21m �33s 
    A material used for the first coat of paint or as the prime coat in a 
protective coating system. Primers are designed to promote adhesion of the 
coating system to the substrate, to furnish a good base for further coatings, 
and to prevent attack on the substrate by air, water, or other materials. 
    Wood primers are formulated to give maximum adhesion and to provide 
adequate flexibility for adjustment to dimensional changes that occur when 
wood swells or shrinks because of changes in moisture content. Primers used as 
the undercoat for enamels must provide a smooth film and permit  easy sanding. 
    Metal primers must provide excellent adhesion by mechanical anchorage or 
chemical bonding. Modern corrosion-resistant coating systems applied over 
steel contain inorganic or organic zinc dispersed in a suitable vehicle. Zinc 
is attacked by the electrochemical corrosion process preferentially before 
steel, and protects the steel substrate by this sacrificial property. Prime 
coat materials for nonferrous metallic substrates usually are generically 
related to materials used in subsequent coats. 
    Primers for porous surfaces such as concrete must seal the surface 
adequately to provide a uniform base for future coatings. 
    Primers are always pigmented. In clear finishes the coat which performs 
this function is described as a sealer, an undercoater, or a wash coat.  See  
CORROSION; PAINT; PIGMENT; SURFACE COATING. 
 �34s[C. R. Martinson ;C. W. Sisler]
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�6v�2v�32sPrimulales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of three families: the Myrsinaceae, with about 1000 species; the 
Primulaceae, with about 800 species; and the Theophrastaceae, with a little 
more than 100 species. These are plants with sympetalous flowers; that is, the 
petals are fused by their margins to form a corolla with a basal tube and 
terminal lobes. The functional stamens are opposite the corolla lobes, and 
there is a compound ovary that has a single style and two to numerous ovules.  
See  DILLENIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPrinted circuit   �21m �33s 
    A generic term applied to circuits fabricated by any of several graphic 
art processes. Printed circuits greatly simplify mass production and increase 
equipment reliability. Their most important contribution, however, is the 
tremendous reduction achieved in size and weight of electronic devices. 
  
    Technology.  Printed circuit technology may be divided into three basic 
parts: engineering, photography, and manufacturing. 
    The configuration in which circuit elements are located and the routing of 
conductor paths establish the precise circuit pattern. Using this pattern, an 
artwork master normally several times larger than the final size of the 
printed circuit is prepared. Scale depends on the type of circuit that is to 
be fabricated. 
    Enlarged masters are photographically reduced to the required size within 
the allowable tolerance, thereby reducing any dimensional inaccuracies which 
have occurred in the master. When a number of circuits are to be fabricated 
simultaneously, precision step-and-repeat equipment provides a composite 
master, with each circuit precisely positioned relative to key indexing 
locations, and all identical. 
    The circuit pattern masters are used to fabricate the screens and masks 
for the application of photoresistive materials in the actual formation of the 
required patterns on the finished parts. The masters are also used in  the  
preparation of numerous types of tooling, for example, drill templates, tapes 
for operation of numerical-tape-controlled drilling equipment (especially 
common where high precision is required), routing templates and dicing 
fixtures for trimming printed circuits to final configuration, and laminating 
and holding fixtures. Numerous processes including etching, screening, 
plating, laminating, vacuum deposition, diffusion, and application of 
protective coatings, are used in combination to produce various types of 
printed circuits. 
  
    Applications.  Printed circuits can be divided, in terms of application, 
into printed wiring, thick and thin films, hybrid circuits, and integrated 
circuits.  
  
    Printed wiring. The printed wiring board is a copper-clad dielectric 
material with conductors etched on one or both sides. Single-sided boards are 
commonly used in ground-support equipment in which space and weight are not 
critical.  
    Two-sided boards may use plated-through holes or eyelets to provide 
electrical continuity between the sides. They are used in those applications 
in which the maximum number of interconnections (conductors) in a given area 
are required for minimum cost. 
    Multilayer printed wiring boards capitalize on the reduced size of 
miniaturized and microelectronic parts. The surface area is required for 
mounting of these subminiature parts and integrated circuit packages has 
decreased significantly while the number of interconnections has increased 
manyfold. In a multilayer board, conductors are located on several insulated 
internal layers. Common multilayer circuits consist of a number of two-sided 
etched copper foil boards, separated by an insulating layer (the prepreg) and 
laminated together under controlled temperature, pressure, and time. 
  
    Thick-film circuits. Thick-film circuits consist of such passive elements 
as resistors, capacitors, and inductors deposited on wafers or substrates of 
such dielectric materials as ceramic, glass, porcelain-coated metal, and the 
like. They are used commercially for mass fabrication of passive networks for 
use in linear microcircuits and large-signal digital modules. A large-scale 
electronic computer may contain millions of these circuits. Thick-film design 
and manufacture are usually based on film thickness of approximately 0.001 in. 
(25 micrometers). 
  
    Thin-film circuits. The deposition of thin films was the first application 
of printed circuit technology to microelectronics. Like thick-film circuits, 
thin-film circuits consist primarily of passive elements; however, such active 
elements as field-effect transistors and diode devices have become available. 
The most important operational difference between thick- and thin-film 
circuits is that thin films have greater precision and stability. 
  
    Hybrid circuits. Thick thin-film hybrid circuits consist of one or more 
substrates upon which the passive elements of the circuit (resistor networks 
and inductances) have been deposited and to which discrete components (such as 
diodes, transistors, integrated circuits, capacitors, and inductors) have been 
attached by split-tip series resistance welding or by thermocompression 
bonding, or by soldering (thick-film hybrids only). After components and 
external leads are attached, the circuit is normally encapsulated in a resin 
compound. Hybrid circuits allow advantageous use of thick- and thin-film 
fabrication techniques for such applications as complex switching circuits and 
differential amplifiers in the form of functional electronic building blocks. 
  
    Integrated circuits. An integrated circuit is a semiconductor device with 
both active and passive elements diffused into a silicon wafer to form a 
functional circuit. Many of the fabrication processes for integrated circuits 
such as photomasking, selective etching, and photoreduction are similar to 
those used in the manufacture of other printed circuits.  See  INTEGRATED 
CIRCUITS.  
 �34s[Low K. Lee]
�33s 
  
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�6v�2v�32sPrinting �21m �33s
    A process in which an image is reproduced on a surface, such as paper.
Although photocopying was not originally considered a printing process, the
demarcation between the two has become blurred.

    Printing processes.  There are five general printing processes: relief,
planographic, intaglio, screen printing, and electrostatic.
    Relief printing is a process in which the printing element consists of a
raised surface of type, lines, and points which can be inked and pressed onto
the paper. The nonprinting areas are below the printing surface.
    Planographic printing is a process in which the printing areas of the
plate are on the same surface as the nonprinting areas, and printing is
accomplished by using the principle that grease and water do not mix.
Commercial lithography is a planographic process that uses thin metal plates
made photomechanically and mounted on a press which prints indirectly by a
process commonly called offset. The inked image on the plate is first
transferred to an intermediate rubber-covered offset cylinder which then
prints the image on the paper; thus the name offset. Relief and intaglio
printing can also be printed by the offset principle.
    Intaglio printing, also known as gravure printing, is accomplished by
cutting or engraving and etching various sizes of minute cells (or wells)
below the surface of a plate or cylinder to hold the ink. After the cells are
flooded and loaded with ink, the surface of the plate is scraped of excess ink
by a doctor blade. Therefore, the depth and size of each cell determine the
amount of ink that is available to be transferred to the printed surface.
    Screen printing, also called porous printing, is a process in which ink is
brushed or squeegeed through a stencil image on a fine screen onto paper or
other surface such as metal or glass. The screen holds the image area and may
carry either pictorial or typographic material.
    Electrostatic processes were once considered as copying rather than
printing processes. However, copying equipment is becoming faster, and
so-called printing equipment is being developed for much of the short-run
printing required for offices and many in-plant operations. Xerography has
been combined with electronic imaging to produce electronic printing systems.
 �34s[Michael H. Bruno]
�33s

    Letterpress printing. The relief process of printing is the oldest of the
major printing processes. The distinguishing characteristic of the relief
process of printing is that the image areas of the image carrier stands out in
relief from the body of the carrier itself (Fig. 1).
    By this definition, the relief printing process can be broken down further
into two relief printing methods, each characterized by the nature of the
image carrier. One method has been identified, by tradition, as letterpress
printing, with the image carrier being rigid. The second relief printing
method was originally identified as rubber-plate printing, whose image carrier
is soft, flexible, and resilient. However, the application of this method has
become so widespread, although highly specialized, that it has achieved
recognition as a printing process in its own right, and is now identified as
the flexographic process. However, the line between letterpress and
flexographic printing has become somewhat blurred because of a development in
flexible and resilient image carriers. Relief printing plates made of
photopolymeric materials have been developed. These plates are not quite as
flexible or resilient as rubber, but more importantly with respect to printing
processes and methods, they are used in a letterpress printing application,
namely, book presses. The distinguishing characteristics here are the nature
of the ink used and the inking system of the book press.
    A frequently heard term is wraparound. This is often incorrectly described
as a process or method. Wraparound refers to a letterpress metal image carrier
which is thin enough and flexible so that it can be mounted on (wrapped
around) a cylinder. A press whose image-carrying segment is cylindrical is
identified as a rotary press. With the exception of screen printing, all the
major printing processes utilize rotary presses almost exclusively. The
wraparound technique was developed in an attempt to provide lower-cost
photoengraving techniques and to adapt the inherent speed advantage of a
rotary press to job and commercial letterpress printing. The wraparound plate
should be distinguished from the curved (rigid) plates used on letterpress
rotary presses.
    Letterpresses are generally classified by using the printing unit
configuration as the criterion. The platen press (Fig. 2a) has a flat platen
and bed. The flatbed cylinder press (Fig. 2b) has a flat bed, with an
impression cylinder in place of a platen. The rotary press (Fig. 2c) has two
cylindrical printing units, the flatbed being replaced by a plate cylinder,
and both members are cylindrical.

    The platen and flatbed cylinder presses of letterpress printing are
inherently sheet-fed. For some special printing applications both have been
adapted to roll feeding or, in the case of the platen press, to imprint
continuous fan-folded business forms. Flatbed cylinder presses were built to
print from rolls of paper to meet the needs of the so-called country newspaper.

    Lithographic printing. Originally this was a term that signified printing
from stone. Stone lithography has ceased to exist except as an art medium. The
term offset is almost universally used as synonymous with lithography.
Technically this is incorrect because the offset technique has been applied to
both gravure and letterpress printing. Direct lithography - pressing the inked
lithographic plate directly against the substrate - predated modern offset
lithography. While direct lithography virtually disappeared many years ago,
there are applications in conjunction with offset lithography on
blanket-to-blanket web (lithographic) offset presses (Fig. 3).
    Lithographic presses, direct or indirect, are rotary presses - the plate
is mounted on a cylinder which rotates during printing. For the sheet-fed
configuration of a lithographic press (Fig. 4), the plate cylinder is provided
with the means to rotate it independent of its gear drive. This feature is
provided to make possible the desired front-to-back position of the image on
the sheet. When the dampening system is disengaged and a shallow-etched
flexible relief image carrier is mounted on the plate cylinder, printing is
accomplished by indirect (offset) letterpress printing.
    In offset printing, primarily lithographic, the basic configuration of
rotary printing is universal. However, many adaptations of the rotary
configuration have been designed. Offset makes it possible to print both sides
(sheet or web) in the same printing nip (see Fig. 5). A printing unit in this
configuration comprises two plate/blanket cylinder couples so assembled that
the blanket cylinders run against each other. Each blanket cylinder transfers
the ink for its image and, at the same time, acts as the impression cylinder
for the opposing blanket cylinder. These presses are called blanket-to-blanket
presses. See  PRINTING PRESS.




    Gravure printing. This process is used for printing and coating
applications which fall into three major categories: publication, packaging,
and specialties. The publication field includes magazines, mail-order
catalogs, brochures, newspaper supplements, comics, and other types of
commercial printing. Packaging consists of folding cartons and flexible
packages. The specialty field represents items such as wall and floor
coverings, decorated household paper products, cigarette filter tips, and a
wide variety of other products. Gravure is also used for applying accurately
metered quantities of coatings to paper and other substrates.
    The gravure image carrier is a copper-plated cylinder or copper plate on
which the image is in the form of cells or cups engraved in the surface. A
printing unit, or a gravure press, consists of a gravure cylinder and an
impression roll, with attached inking and doctoring mechanisms. A gravure
press may consist of from one to eight or more units that are coupled together
with paper-feeding and roll- or folder-delivery arrangements.
    The basic gravure printing or coating unit consists of the engraved
gravure image carrier cylinder at the bottom (Fig. 6) with a rubber-covered
impression roll at the top. Gravure ink transfer is accomplished when the
paper is pressed into contact with the liquid ink in the gravure cell.
 �34s[Harvey F. George; M. H. Bruno]
�33s

    Flexographic printing. Flexography is a rotary letterpress printing
process, traditionally using flexible rubber printed plates and fluid,
fast-drying ink. Since the development of photopolymer flexo printing plates,
the original definition of exclusively rubber flexo printing plates is no
longer accurate. The flexo industry refers to its printing plates as being
rubber or elastomeric in composition. In addition the inks used in flexography
are no longer always limited to the original fluid, fast-drying variety. A
typical flexographic printing ink distribution system is shown in Fig. 7.

    As the result of the accuracy, versatility, speed, economy, and print
quality of modern flexographic printing, it has become the major method for
printing flexible packaging materials, including paper and plastic films, in
addition to multiwall bags, corrugated containers, envelopes,
pressure-sensitive labels, and paperback books.
 �34s[Douglas D. Tuttle]
�33s

    Screen printing. A printing process that differs markedly from the other
printing methods in that the printing plate is actually a porous stencil,
which is supported by a meshed screen of fabric or metal. Screen printing
technology has, to a high degree, refined a basic stenciling process into a
commercial and industrial imaging system that is virtually limitless in its
application. The diverse range of substrates which may be printed in this way
include paper, metal, plastics, textiles, glass, ceramic, and wood.
    The basic screen printing method may be simply described as follows. A
woven fabric generally of polyester, nylon, or stainless steel (although silk
is still used occasionally) is stretched tightly upon a wood or aluminum
rectangular frame and affixed. A stencil is then prepared either separately
from the fabric (indirect method), or directly onto the mesh using a liquid,
photosensitive emulsion (direct and direct/indirect methods).
    The image, if photographic or consisting of fine, detailed lines, is
developed within the stencil medium by using light-exposure techniques. An
alternative method is to hand-cut the stencil by using paper or plastic film.
    The fully prepared printing screen is mounted onto any of several
automatic or semiautomatic types of press. A squeegee is attached in a
position which permits a regular sweeping motion, and ink is introduced into
the screen (Fig. 8). By means of a constant pressure exerted from one side of
the screen to the other, the squeegee presses the ink through the mesh at
every point which is not blocked off by the stencil. As the flexible screen is
pressed downward, it contacts the substrate, and the fluid ink transfers,
forming the printed image.
    The foremost advantage of screen printing is in flexibility of
application. Through the selection of different fabrics of varying mesh
fineness, the amount of printing ink applied may range from a very thick wet
film of 150 micrometers (0.006 in.) to a relatively thin dry ink deposit of 4
or 5 micrometers (0.00015 or 0.00020 in.).
    The range of color reproduction by screen printing is actually limitless.
In most instances, each color is applied singly and allowed to dry before the
next color is printed.
    Refinements such as the control of ink deposits, ultraviolet
polymerization, and automatic substrate handling equipment have contributed
greatly to significant increases in the speed of screen printing production
and improved quality of the printed image.
 �34s[Tim McSweeney]
�33s


    Screenless printing. Processes involving screenless printing are
considered essentially specialty processes because of the difficulty of
controlling them. These are collotype and screenless lithography. Other means
are being developed to simulate the effects of screenless printing and these
may well lift this type of printing out of the specialty area.
    Collotype printing is known in different countries by different names, for
example, photogelatin, phototype, heliotype, heliogravure, albertype, and
artotype. The process, although not literally planographic, belongs in the
planographic category. It has the distinguishing characteristics of using no
screen and comes the closest of any process to simulating a true photographic
gradation.
    The plate, usually of aluminum when used on a rotary press (when flatbed
presses are used, the base is heavy plate glass), is coated with a layer of
light-sensitive (bichromated) gelatin, over which is laid an unscreened
(continuous-tone) photographic negative of the copy to be reproduced. When the
two are exposed to light, the gelatin is affected in varying degrees in
proportion to the amount of light admitted to it through the dark, medium, and
light portions of the negative. The plate is developed in water to yield a
printing image which varies not only in thickness (in terms of thousandths of
an inch) but also in moisture content and consequent ability to repel ink or
hold it in varying amounts. The more hardened areas accept less moisture and
swell less, thus accepting more ink.
    Because of its screenlessness it has been used to great advantage in
reproducing ancient manuscripts which either were falling to pieces from age
or were likely to suffer from handling and exposure to the air. Of all
processes, collotype is capable of giving the most faithful facsimile
reproductions.
    Screenless lithography is a means of printing lithographic plates made
from continuous-tone positives or negatives on an offset press. Best results
have been obtained with special photopolymer plates exposed from positives.
Unlike collotype, the plates are run conventionally on the press with regular
inks and fountain solutions.
 �34s[Michael H. Bruno]
�33s

    Color printing.  The basic principle of color reproduction through the
media of modern color printing of pictures lies in the use of only three
primary colors to create the illusion of seeing in the reproduction of the
hundreds or more colors the eye sees in the original.
    Modern color printing, in which only three colored printing inks were
used, came about by bringing together the invention of photography, the
halftone screen, coal tar, and synthetic dyes to make transpararent color inks
and color separation filters. All of these technologies were coupled with a
better understanding of color vision.  See  COLOR VISION.
    Practically all commercial color printing also uses a fourth printing of
black ink, and therefore could be considered a four-color process.
    Color printing with inks is the controlled proportioning of red, green,
and blue light, and this is accomplished in an indirect way called the
subtractive process. Printing is done with yellow, magenta, and cyan inks
because these colors ideally are complementary to blue, green, and red
respectively, and each can individually shut off the reflectance of red,
green, and blue light from the paper surface.
    The first step in any color reproduction system is called color
separation. In this process three individual photographic picture images are
exposed separately through red, green, and blue filters. Each filtered
separation image must also be exposed through a halftone screen to create
different-size dots that control the hues and tone values. When the filtered
color separation and halftone screen exposures are made simultaneously to the
same photographic film, it is called the direct screen process. In the
indirect process, the filtered color-separation negatives are normal
continuous-tone photographic images, and the halftone dot screening is
introduced in a second photographic step with a high-contrast film to create
positive dot images.The direct screen method is more economical and thus more
popular.

    Electronic color scanning.	The more sophisticated, contemporary
electronic scanners are capable of producing, in one operation, completely
color-corrected, enlarged or reduced, cropped, and properly screened sets of
separations from original copy in a matter of minutes. In most cases, the
quality of these separations exceeds that achieved by conventional
photo-mechanical techniques. This capability, coupled with speed, makes
electronic scanners highly cost-effective pieces of production equipment.
    The primary function of an electronic color scanner is to produce the four
black-and-white separations representing the yellow, magenta, cyan, and black
images from an original full-color image. These final separation films may be
either continuous-tone or halftone images. They must be produced in such a
manner that the printing plates made from them are capable of producing a
full-color image which is of optimum sharpness, color content, and contrast
ratio.
    The original copy is a transparent color-positive photograph, mounted on
the transparent scanning drum. Prior to mounting the transparency, certain of
its characteristics must be measured. Both the highlight (brightest) and
shadow (darkest) areas of interest must be measured on an accurate
densitometer in order to determine their optical densities. The
neutral-colored area (true gray area) of the picture must also be determined
and measured for its color content. These densities, along with information as
to which printing process and inks will be used to print the final images, and
information about the characteristics of the paper that the image is to be
printed on, will be used by a trained scanner operator to set up the scanner's
color computer.
    After the operator has set up the color computer, the first piece of
unexposed film is placed on the recording or output drum. The machine is now
ready to scan and produce the first of the four output separations. The
machine must make four consecutive runs in order to produce the four images. A
fresh piece of film is loaded on the recording drum for each successive
exposure.
    The scanning light beam passes through the transparent color photo and
into the optical color head. At any given moment, the scanning light beam
emitted from the lamp, after being filtered through the transparent color
photo, contains all of the colors contained in the transparency at the exact
spot the beam is focused on. This filtered light beam is then broken down in
the color head into the three additive primary colors of red, green, and blue.
    Color masking is a process by which several deficiencies in the color
printing process are corrected or compensated for. The biggest of these
deficiencies is the result of imperfectly colored printing inks.
    In addition to color masking, image enhancement can be performed.
Enhancement is a general term for picture detail improvement which results in
an apparent sharpening of the picture.
    After all of this, the electronic signal representing the picture must be
modified to compensate for the characteristics of the film being exposed on
the recording drum. On most modern scanners, this is simply a matter of
entering the output film's exposure characteristics into the scanner's
computer.
    The electronic output signal now represents the desired image to be
exposed on the separation film. Next it is necessary to convert the electronic
signal into light. This is done in the recording head. There are three
commonly used light sources employed in recording heads: glow lamps, lasers,
and xenon arc lamps.
    Electronic prepress systems are capable of handling full-color images to
make up full-color pages. These systems employ electronic scanners, color
video terminals, and contemporary interactive computer technology.
    There are systems that are totally interactive; that is, the operator can
manipulate images on a computer-controlled color video terminal, in real time,
changing position, cropping, and modifying colors.
 �34s[Brian M. Chapman]
�33s

    Platemaking.  Platemaking is the hub around which the conventional
printing processes revolve. Printing plates or cylinders are made in any one
or a combination of six different ways: (1) manually: by hand tools,
engravers, knives, and so forth, to produce plates for the relief, intaglio,
and stencil processes; greasy crayon or tusche can be used for making
hand-drawn lithographic plates; (2) mechanically: by engraving and geometric
lathes, ruling machines, pantographs, and so on, for the relief and intaglio
processes; by hand transferring and benday machines for lithographic plates;
(3) electrochemically: by electrodeposition of metals to produce
longer-wearing images; (4) electronically: by the use of electronic engravers
to make relief and intaglio image carriers; (5) electrostatically: by using
xerographic or Electrofax principles to make relief and lithographic plates;
(6) photomechanically: by combining the photographic process to produce images
and a mechanical process to make plates. The photomechanical method is the
most important and most widely used, as it has not only speeded up the
reproduction of pictorial and text matter, but has improved their quality.

    Photoengraving. This is the photomechanical process used to produce relief
plates in line, halftone, and color for letterpress, letterset, and
flexographic printing. There are two general types of engravings, line and
halftone, which are divided into five groups: one-color line engravings;
one-color halftone engravings; one-color combination line and halftone
engravings; multicolor sets of engravings in line or halftone, or in line and
half-tone combinations; and four-color process engravings. In photoengraving,
the metal in the nonprinting areas is removed by chemical etching or
mechanical routing.

    Gravure platemaking and cylinder making. Gravure is printed principally on
web presses from etched cylinders and is generally called rotogravure. In
sheet-fed gravure, printing is done from a thin copper plate wrapped around
the plate cylinder.
    Lithographic platemaking. Before describing the lithographic platemaking
process, the chemistry of the lithographic principle should be considered. A
lithographic plate must have image areas that are ink-receptive and nonwettable
by water, and the nonimage areas must be water-receptive and nonwettable by
ink. The chemistry of the production of the images is related to the changes
that occur to the photosensitive coatings on exposure to light and the
development process. Some become insoluble and oleophilic (ink-receptive) and
are used as the printing areas. Others serve as stencils, and other means of
making the images oleophilic are used. Chemistry is also involved in making the
nonimage areas water-receptive and desensitized to ink.
    Stencil process image carriers. There are two stencil processes in general
use: screen printing and stencil duplicating.
    Screen printing image carriers are produced manually or by photomechanical
means. The printing screen is made by attaching the screen material to a rigid
frame and stretching it so it is level and smooth. The stencil is applied to
the bottom side of the screen which is in contact with the surface to be
printed; and ink with a consistency similar to thick paint is transferred by
rubbing it over the screen surface with a rubber squeegee.
    Manual stencils are made by knife-cutting special film stencil materials
which consist of two plastic layers. They can also be produced by artists
drawing directly on the screens with special materials.    [Michael H. Bruno]


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�6v�2v�32sPrinting press   �21m �33s 
    There are four major types of printing presses or printing systems: 
letterpress, offset lithography, intaglio, and screen printing. In letterpress 
the image to be reproduced is in relief. Ink is applied to the raised areas, 
and when a sheet of paper is pressed against them, the film of ink is split, 
the image from the plate being left on the paper surface. In offset 
lithography, a planographic process, the image is on the same plane as the 
nonimage areas. The printing or transfer of the image is from a flat surface 
which is chemically treated so that ink is accepted in the image areas and 
repelled in the nonimage areas of the plate. In the intaglio printing process 
the image to be reproduced is recessed below the surface of the printing plate 
or image carrier. Included in this category are engraving and gravure 
processes. In screen printing (serigraphy) the design to be reproduced is an 
open area in a finely woven screen, the nonprinting areas of the screen being 
blocked off; the ink is squeegeed through the open areas onto the surface 
which is to be printed. 
  
    Offset presses.  There are several varieties of offset presses available: 
sheet-fed, single-color and multicolor, perfecting, web, and common impression 
cylinder presses. 
    Sheet-fed single-color offset presses have a thin, flexible printing plate 
wrapped around a plate cylinder. The plates may be made of zinc, aluminum, 
chrome-plated steel, plastic, or paper. Short- to medium-run work is usually 
done on a single-color press, and even two- to four-color may be printed on 
such a press by running the sheet through the required number of times. For 
longer runs and higher-quality work, two-color and multi-color presses are 
available.  
    In most web offset presses, two units print the top and bottom of the web 
simultaneously. The units are arranged in opposition to each other so that 
each blanket not only prints an image on the web of paper, but also acts as 
the impression cylinder for the opposing blanket cylinder.  
    The common impression cylinder press consists of a number of plate and 
blanket cylinders grouped around one large impression cylinder. The claimed 
chief advantage is better control of register, or accurate positioning of one 
color image over another. 
  
    Letterpress.  Letterpress is the oldest and second in use of the  printing 
processes. There are three basic designs of letterpresses: platen, flat-bed 
cylinder, and rotary. 
    A platen press is one in which the impression of the image is obtained by 
bringing two flat surfaces together. The form, or relief printing plate, is 
placed in a vertical position against the bed of the press, and by means of a 
clamshell or sliding action the form is pressed against the paper to be 
printed against a platen covered with a sheet of heavy oiled paper.  
    The flat-bed cylinder press utilizes a moving flat bed to hold the form or 
image carrier while a fixed cylinder provides the impression pressure. The 
paper which is held by grippers in the cylinder is rolled over the form, and 
the image is transferred as the bed passes under the cylinder. It prints while 
one revolution of the cylinder is made, and is raised during the second 
revolution of the cylinder to permit the bed to return to its original 
position.  
    The rotary letterpress is the fastest of the letterpress types and has the 
widest use. On rotary letterpresses, both the image-carrying (plate) surface 
and the impression surface are cylindrical. Any kind of plate that can be 
curved to fit the cylinder and has sufficient rigidity may be used, such as 
electrotypes, stereotypes, or wrap-around metal or plastic. 
  
    Intaglio printing processes.  Intaglio processes include engraving, 
gravure, and rotogravure. 
    In the engraving process, the design is etched or mechanically cut in a 
copper or steel plate. An impression member presses the paper down on the 
inked plate with considerable pressure, producing very fine detail. 
    In sheet-fed gravure, the flat, flexible etched sheet of copper is clamped 
around the plate cylinder of a press. The sheets of paper are then pressed 
firmly against the plate by a rubber-covered impression roll. Sheet-fed 
gravure is used for short runs, for art reproductions and very high-quality 
printing.  
    A rotogravure unit consists of a printing or engraved cylinder, an 
impression cylinder, and an inking device. The image in the form of tiny cells 
etched into the copper-plated engraved cylinder has ink applied. The 
impression cylinder is covered with a resilient rubber composition that 
presses the paper into contact with the ink in tiny cells of the printing 
surface. Gravure inks are quite volatile and dry rapidly; however, a dryer 
after each printing unit dries the ink before the next color is applied. This 
permits color to be printed over dry color, resulting in the longest range of 
density of any process.  Multicolor rotogravure presses have as many as twelve 
printing units, six colors on each side of the web, and modern presses have 
automatic register controls, which assure exact color register. Rotogravure is 
used for package printing, printing flexible films and foil, long-run 
mail-order catalogs, and high-quality magazines. 
  
    Screen printing.  Although much screen printing is done by a hand method 
with very simple equipment consisting of a table, a frame, and a squeegee, 
most commercial screen printing is done on mechanical or power-operated 
presses. Screen printing is used to print both line and halftone on most any 
material. It has been used for art prints (where it is referred to as 
serigraphy), decalcomanias, posters, greeting cards, wallpaper, and many items 
which would be difficult to print by other processes.  See  INK; PRINTING. 
 �34s[William A. Rocap, Jr.]
�33s 
  
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�6v�2v�32sPrism   �21m �33s 
    A polyhedron of which two faces are congruent polygons in parallel planes, 
and the other faces are parallelograms (see illustration). The bases B are the 
congruent polygons; the lateral faces are the parallelograms; the lateral 
edges are the edges not lying in the bases; and the perpendicular distance 
between the bases is the altitude l. Sections parallel to the bases are 
congruent to the bases. A prism is a right prism if its  lateral edges are 
perpendicular to the bases; an oblique prism otherwise. A prism is called a 
triangular prism if its bases are triangles; a pentagonal prism if its bases 
are pentagons; and a parallelepiped if its bases are parallelograms. The 
volume of any prism is equal to the area of its base times its altitude (V = 
Bh).  See  POLYHEDRON; PRISMATOID AND PRISMOID. 
 �34s[James S. Frame]
�33s 
  
  
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�6v�2v�32sPrismatic astrolabe   �21m �33s 
    A surveying instrument used to make the celestial observations needed in 
establishing an astronomical position. The instrument (see illustration) 
consists of an accurate prism, a small pan of mercury to serve as an 
artificial horizon, an observing telescope with two eyepieces of different 
power, level bubbles and leveling screws, a magnetic compass and azimuth 
circle, adjusting screws, flashlight-battery power source, light, and a 
rheostat to control the intensity of illumination. 
    By using a fixed prism, the instrument measures a fixed altitude, usually 
45DG. As a rising star increases altitude past that for which the instrument 
was constructed, the direct image appears to move upward from the bottom of 
the field of vision to the top. The image reflected by the mercury horizon 
appears to move downward from top to bottom. At the established altitude the 
rays produce images at the center of the field of view. A fixed altitude is 
used to minimize error due to variations from standard atmospheric refraction. 
Each accurately timed observation provides one line of position. 
 �34s[Alton B Moody]
�33s 
  
  
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�6v�2v�32sPrismatoid and prismoid   �21m �33s 
    A prismatoid (see illustration) is a polyhedron all of whose vertices lie 
in two parallel base planes. If the two base polygons have the same number of 
sides, the prismatoid is called a prismoid. Among the prismatoids are included 
pyramids, frustums of pyramids, wedges, parallelepipeds, and other prisms. The 
area of a section of a prismatoid by a plane parallel to and between its bases 
is a quadratic function (possibly linear or constant) of the distance from the 
plane to one of the bases. A generalized prismoid is any solid for which the 
area of a section parallel to a fixed base is represented by a polynomial of 
degree less than or equal to 3 in terms of the distance from that base. 
Frustums of cones, segments of spheres, and many other solids satisfy this 
condition.  See  PARALLELEPIPED; POLYHEDRON; PRISM; PYRAMID AND FRUSTUM. 
 �34s[James S. Frame]
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�6v�2v�32sPrivacy systems (scrambling) �21m �33s
    Devices and methods for ensuring the privacy of overseas telephone
conversations. Such privacy can be accomplished in a variety of ways; two are
particularly widely used at circuit terminals. The first method employs
equipment for inverting speech, making high frequencies low and low
frequencies high, for example, to make overseas telephone conversations
unintelligible to the casual listener. A more complicated method employs
filters to separate the transmitted speech signal into several
narrow-frequency bands. Each of these is then treated differently in a
prearranged manner, the relative positions of individual portions of the
signal being interchanged in the frequency spectrum by inversion and
transposition.	See  ELECTRIC FILTER; TELEPHONY.
 �34s[Charles C. Duncan ;Richard B. Nichols]
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�6v�2v�32sProbability �21m �33s
    Although probability theory derives its notion and terminology from
intuition, a vague statement such as ``John will probably come'' is as remote
from it as the statement ``John is forceful and energetic'' is remote from
mechanics. Probability theory constructs abstract models, mostly of a
qualitative nature, and only experience can show whether these reasonably
describe laws of nature or life. As always in mathematics, only logical
relations and implications enter the theory, and the notion of probability is
just as undefinable (and as intuitive) as are the notions of point, line, or
mass.

    The sample space.  One speaks of probabilities only in connection with
conceptual (not necessarily performable) experiments and must first define the
possible outcomes. It is necessary to distinguish between elementary
(indivisible) and compound outcomes or events. Each elementary outcome is
called sample point; their aggregate is the sample space. The conceptual
experiment is defined by the sample space, and it must be introduced and
established at the outset.

    Events.  In examining a bridge hand, one may ask whether it contains an
ace or satisfies some other condition. In principle each such event may be
described by specifying the sample points which do satisfy the stipulated
condition. Thus every compound event is represented by an aggregate of sample
points, and in probability theory these terms are synonymous. The standard
notations of set theory are used to describe relations among events.  See  SET
THEORY.
    Given an event A one may consider the case that A does not occur. This is
the negation or complement of A, denoted by A'; it consists of those sample
points that do not belong to A. Given two events A and B, the event C that
either A or B or both occur is the union of A and B and denoted by C= A UNION
B. In particular A UNION A' is the whole sample space SCRIPT S which therefore
represents certainty. The event D, both A and B occur, is the intersection of
A and B and written D = A INTERSECTION B. It consists of the points common to
A and B.

    Probabilities in finite spaces.  If the sample space SCRIPT S contains only
N points E1, . . . , EsubN their probabilities may be any numbers such that P{E
sub j} > or = 0 and P{E1} + . . . + P{E sub N} = 1. The probability P{A} of an
event A is the sum of the probabilities of all points contained in A; thus
P{SCRIPT S} = 1. 
    Frequently considerations of symmetry lead one to consider all Esubj as
equally likely; that is, to set P(E sub j) = 1/N. In this case P(A) = n/N where
n is the number of points in A; for a gambler betting on A, these represent
the ``favorable cases.'' For example, in throwing a pair of ``perfect'' dice,
one naturally assumes that the 36 possible outcomes are equally likely. This
model does not lose its justification or usefulness by the fact that actual
dice do not live up to it. The assumption of perfect randomness in games, card
shuffling, industrial quality control, or sampling is rarely realized, and the
true usefulness of the model stems from the experience that noticeable
departures from the ideal scheme lead to the detection of assignable causes
and thus to theoretical or experimental improvements.

    Conditional probability--independence.  Suppose that a population of N
people includes NsubA color-blind persons and NsubH females.  To the event A ``a
randomly chosen person is color-blind'' can be ascribed probability P{A} = N(sub
A)/N, and similarly for the event H that a person be female one has P{B} = N(sub
H)/N.  If N(sub A)H is the number of color-blind females, the ratio N(sub
AH)/N(sub H) may be interpreted as probability that a randomly chosen female be
color-blind; here the experiment ``random choice in the population'' is replaced
by a selection from the female subpopulation.  In the original experiment, N(sub
AH)/N is the probability of the simultaneous occurrence of both A and H, so that
N(sub AH)/N(sub H) = P{A INTERSECTION H}/P{H}.	Similar situations occur so
frequently that it is convenient to define the conditional probability of the
event A relative to H by Eq.  (1).  This concept is useful whenever it is
desired to restrict the consideration to those cases where the event H occurs
(or where the hypothesis H is fulfilled).  Thus, in betting on an event A the
knowledge that H occurred would induce one to replace P{A} by P{A|H}.

		  P{A|H} =-P{A INTERSECTION H}/P{H}.			     (1)

    Independent trials.  The intuitive frequency interpretation of probability
is based on the concept of experiments repeated under identical conditions; a
theoretical model for this concept can be developed.
    Consider an experiment described by a sample space SCRIPT S; for simplicity
of language it can be assumed that SCRIPT S consists of finitely many sample
points E1, .  .  .  , E(sub N).  When the same experiment is performed twice in
succession, the thinkable outcomes are the N(sup 2) pairs of sample points
(E1,E1), (E1,E2), .  .	.  , (E[sub N], E[sub N]), and these now constitute the
new sample space.  It is called the combinatorial product of SCRIPT S by itself
and denoted by SCRIPT S x SCRIPT S.
    Probabilities must be assigned to the events in SCRIPT S x SCRIPT S. If the
second trial is independent of the first, the probabilities in SCRIPT S x SCRIPT
S follow the productive rule P{E[sub i],E[sub j]} = P{E sub i}P{E sub j}.  In
the case of n tossings of a coin, this rule leads to the probability 2(sup -n)
for each sample point in agreement with the requirement of equally likely cases.
In the more general case of Bernoulli trials, each trial results in success S or
failure F, and P{S} = p, P{F} = q where p + q = 1.  (This may be considered as
the model of a skew coin.) A succession of n independent trials of this kind
leads to the sample space of n-tuples ( .  .  .  FS), and the probability of
such a point is the product (pqqp .  .	.  qp) obtained on replacing each S by p
and each F by q.

    Markov chains.  Markov chains represent an important scheme for dependent
trials.  Suppose that at each trial the possible outcomes are E1, .  .	.  ,
EsubN and that whenever Esubi occurs the conditional probability of Esubj at the
next trial is psubij, independently of what happened at the preceding trials.
Here, of course, p(sub ij) > or = 0 and p(sub i1) + p(sub i2) + .  .  .  + p(sub
iN) = 1 for each i.  The p(sub ij) are called transition probabilities.  The
whole process is now determined if the initial probabilities, PI(sub i), at the
first trial are known.	For example, P{E(sub a) E(sub b) E(sub c)} = PI(sub a)
p(sub ab) p(sub bc).  The probability of the event ``E(sub c) at the third
trial'' is obtained by summation over all a and b, and so on.  Markov chains,
and their analog with continuous time, represent the simplest type of stochastic
process.  See STOCHASTIC PROCESS.

    Random variables and their distributions.  The theory of probability
traces its origin to gambling, and the gambler's gain may still serve as the
simplest example of a random variable. With every possible outcome (sample
point) there is associated a number, namely, the corresponding gain. In other
words, the gain is a function on the sample space, and such functions are
called random variables. With the same experiment, one may associate many
random variables.
    Every random variable X has a distribution function F(t) = P{X < or = t}. If
X assumes only finitely many values, then F(t) is a step function.  The notion
of independence carries over:  Two random variables X and Y are independent if
P{X < or = x, Y < or = t} = P{X < or = s} P{Y < or = t}.

    Expectations.  Given a random variable X one may interpret its
distribution function F(t) as describing the distribution of a unit mass along
the real axis such that the interval a < x < or = b carries mass F(b) - F(a). In
the case of a discrete variable assuming the values x1, x2, . . . with
probabilities p1, p2, .  .  .  the entire mass is concentrated at the points
xsubi; if F'(x) = f(x) exists, it represents the ordinary mass density as
defined in mechanics.  The center of gravity of this mass distribution is called
the expectation of X; the usual symbol for it is E(X), but physicists and
engineers use notations such as <X>, <X>(sub Av), or X.  In the cases mentioned,
E(X) is given by Eqs.  (2) and (3).

	    E(X) = SUMMATION p(sub i) x(sub i)				 (2)


	   E(X) = INTEGRATION (-INFINITY to +INFINITY) xf(x) dx 	     (3)

    Before discussing the significance of the new concept, a few frequently
used definitions are appropriate. Put m = E(X). Then (X - m)(sup 2) is, of
course, a random variable. In mechanics, its expectation represents the moment
of inertia of the mass distribution. In probability, it is called variance of
X, given by Eq. (4). Its positive root is the standard deviation.

      Var(X) = E(X - m)(sup 2) = E(X sup 2) - m(sup 2)			    (4)

The variance is a measure of spread: It is zero only if the entire mass is
concentrated at the point m, and it increases as the mass is moved away from m.
In the case of two variables X1 and X2 with expectations m1 and m2 it is
necessary to consider not only the two variances (s[sub i])(sup 2) = E[(X[sub i]
- m[sub i])(sup 2)] but also the covariance Cov(X1,X2) = E[(X - m1)(X2 - m2)] =
E(X1X2) = m1m2.  The covariance divided by s1s2 is called the correlation
coefficient of X1 and X2.  If it vanishes, X1 and X2 are called uncorrelated.
Every pair of independent variables is uncorrelated, but the converse is not
true.

    Laws of large numbers.  To explain the meaning of the expectation and, at
the same time, to justify the intuitive frequency interpretation of probability,
consider a gambler who at each trial may gain the amounts x1, x2, .  .	.  ,
xsubn with probabilities p1, p2, .  .  .  , p(sub n).  The gains at the first
and second trials are independent random variables X1, X2 with the indicated
distribution and the common expectation m = SUMMATION p(sub i) x(sub i).  The
event that an individual gain equals xsubi has probability p(sub i), and the
frequency interpretation of probability leads one to expect that in a large
number n of trials this event should happen approximately npsubi times.  If this
is true, the total gain Ssubn = X1 + X2 + .  .	.  + X(sub n) should be
approximately nm; that is, the average gain (1/n)S(sub n) should be close to m.
The law of large numbers in its simplest form asserts this to be true.	See GAME
THEORY; STATISTICS.

 �34s[William Feller]
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�6v�2v�32sProblem solving (psychology)   �21m �33s 
    The voluntary ideational behavior sustained by an organism in order to 
attain a goal which is not immediately accessible. Nearly identical processes 
have been distinguished by different terms, depending upon the circumstances 
under which the problem solving takes place. Usually the term problem solving 
has been restricted to those studies which require the subject to manipulate a 
tool, assemble a device, or discover the significance of a detour. If, on the 
other hand, the solution calls for the recognition of a relationship, that is, 
the ideational classification of stimuli, the process has been called concept 
formation or discrimination learning, when observed in infrahuman animals. 
When the problem to be solved requires the correction of a malfunction of 
equipment, the process has been called troubleshooting. When the problem 
requires a choice between two or more alternative courses of action, the 
process has been called decision making. 
    Heuristics is defined as the study of the mental processes involved in 
problem solving. The various stages in the process of solving a problem for 
which there is no immediate solution appear to be as follows: 
    Initially the individual must comprehend the nature of the problem. The 
gestalt psychologists argue for the importance of this first stage since they 
believe that the manner in which a problem is ``seen'' will determine, at 
least in part, the likelihood of attaining a successful solution. 
    The second stage, sometimes called the preparation stage, is devoted to 
obtaining information which appears to be relevant to the final solution. This 
is also a crucial stage since if the individual establishes an inappropriate 
set or bias, he may reject as irrelevant information which is in fact 
relevant. A bias in favor of irrelevant information is almost as undesirable. 
During this stage the problem solver tries to examine all of the relevant 
information and its implications for his problem. Discovery of 
interrelationships is in itself additional information which may be relevant. 
    The third stage, sometimes called the incubation stage, is characterized 
by the termination of active seeking for the desired solution and the turning 
of attention to other matters. It is usually described as a passive period of 
hopeful waiting for insight. Successful problem solving may follow a period of 
apparent inattention to the problem because the problem solver has avoided 
inappropriate information-handling biases during this so-called incubation 
period.  
    The final stage, when the problem solver achieves the solution and 
comprehends its significance, is called insight. 
    Clearly, the process of problem solving depends upon memory of information 
as well as imaginative and creative handling of the information.  See  
LEARNING THEORIES; MEMORY. 
 �34s[E. James Archer]
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�6v�2v�32sProboscidea   �21m �33s 
    An order of placental mammals derived from unknown Paleocene or Eocene 
condylarths of Africa and known from the late Eocene to the Recent. Although 
fossil representatives such as the deinotheres of the Old World and the 
mastodonts of both the Old and New worlds were once abundant, only the two 
species of living elephants survive. 
    As a rule, the order is characterized by predominantly herbivorous, large 
to gigantic animals. Either the upper or lower second incisors, or both, 
enlarge to become tusks. Each foot on the pillarlike legs has five toes which 
are bound together in a broad, rounded pad. Elephants are proboscis-bearing 
(hence the name of the order) and in the living species the trunks are 
sensitive and work together with the tusks during feeding. The skull, because 
of the effects of trunk development, has undergone profound morphological 
modifications in the course of elephant evolution. 
    Proboscideans were confined to Africa well into the Miocene, when 
different groups spread to Europe, Asia, and North America. From North America 
various proboscideans entered South America during the Pleistocene.  See  
ELEPHANT; MAMMALIA. 
 �34s[Frederick S. Szalay]
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�6v�2v�32sProcellariiformes   �21m �33s 
    An order of oceanic birds characterized by tubelike nostril openings, 
webbed feet, dense plumage, compound horny sheath of the bill and, often, a 
peculiar musky odor. Included families are Diomedeidae (albatrosses), 
Procellariidae (petrels, fulmars, shearwaters), Hydrobatidae (storm petrels), 
and Pelecanoididae (diving petrels). The order encompasses extremes in size 
paralleled only by the Falconiformes, ranging from the wandering albatross 
(Diomedea exulans), with a wingspread of almost 12 ft (3.7 m), to the 
sparrow-sized least petrel (Halocyptena microsoma). 
    All of the families except the diving petrels are widely distributed; the 
latter are confined to the Southern Hemisphere, mostly in higher altitudes, 
although one species follows cold currents off western South America almost to 
the Equator. The diving petrels are convergent in anatomy and habits toward 
the auks (Alcidae) of the Northern Hemisphere.  See  AVES; OCEANIC BIRDS. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sProcess control �21m �33s
    A field of engineering dealing with ways and means by which conditions of
processes are brought to and maintained at desired values, and undesirable
conditions are avoided as much as possible. In general, a process is
understood to mean any system where material and energy streams are made to
interact and to transform each other. Examples are the generation of steam in
a boiler; the separation of crude oil by fractional distillation into gas,
gasoline, kerosine, gas-oil and residue; the sintering of iron ore particles
into pellets; and the polymerization of propylene molecules for the
manufacture of polypropylene. In the wide sense, process control also
encompasses determining the desired values.
    Process control in the wide sense includes a number of functions, which
can be arranged in a hierarchy, as follows:

			       Scheduling
			      Mode Setting
			     Quality control
		   Regulatory control/Sequence control
			    Coping with faults

    Computerized instrumentation has revolutionized the interaction with plant
personnel, in particular the process operators. Traditionally, the central
control room was provided with long panels or consoles, on which alarm lights,
indicators, and recorders were mounted. Costs were rather high, and
surveyability was poor. In computerized instrumentation, visual display units
can provide information in a concise and flexible way, adapted to human needs
and capabilities.  See	AUTOMATION; CONTROL SYSTEMS.
 �34s[Johannes E. Rijnsdorp]
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�6v�2v�32sProcess engineering   �21m �33s 
    A branch of engineering in which a process effects chemical and mechanical 
transformations of matter, conducted continuously or repeatedly on a 
substantial scale. Process engineering constitutes the specification, 
optimization, realization, and adjustment of the process applied to 
manufacture of bulk products or discrete products. Bulk products are those 
which are homogeneous throughout and uniform in properties, are in gaseous, 
liquid, or solid form, and are made in separate batches or continuously. 
Examples of bulk product processes include petroleum refining, municipal water 
purification, the manufacture of penicillin by fermentation or synthesis, the 
forming of paper from wood pulp, and the manufacture of paint, whiskey, 
plastic resin, and so on. Discrete products are those which are separate and 
individual, although they may be identical or very nearly so. Examples of 
discrete product processes include the casting, molding, forging, shaping, 
forming, joining, and surface finishing of the component piece parts of end 
products or of the end products themselves. Processes are chemical when one or 
more essential steps involve chemical reaction. Almost no chemical process 
occurs without many accompanying mechanical steps such as pumping and 
conveying, size reduction of particles, classification of particles and their 
separation from fluid streams, evaporation and distillation with attendant 
boiling and condensation, absorption, extraction, membrane separations, and 
mixing.  
 �34s[Edward F. Leonard]
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�6v�2v�32sProchirality   �21m �33s 
    The property displayed by a prochiral molecule or a prochiral atom 
(prostereoisomerism). A molecule or atom is prochiral if it contains, or is 
bonded to, two constitutionally identical ligands (atoms or groups), 
replacement of one of which by a different ligand makes the molecule or atom 
chiral. Examples are shown in the illustration. 
  
    None of molecules I - IV is chiral, but if one of the underlined pair of 
hydrogens is replaced, say, by deuterium, chirality results in all four cases. 
In compound I, ethanol, a prochiral atom or center can be discerned (CALPHA : 
CH2); upon replacement of H by D, a chiral atom or center is generated, whose 
configuration depends on which of the two pertinent atoms (H1 or H2) is 
replaced. Molecule V is chiral to begin with, but separate replacement of H1 
and H2 (say, by bromine) creates a new chiral atom at CALPHA and thus gives 
rise to a pair of chiral diastereomers. No specific prochiral atom can be 
discerned in molecules II - IV, which are nevertheless prochiral (III has a 
prochiral axis). 
    Faces of double bonds may also be prochiral (and give rise to prochiral 
molecules), namely, when addition to one or other of the two faces of a double 
bond gives chiral products. 
    Although the term prochirality is widely used, especially by biochemists, 
a preferred term is prostereoisomerism. This is because replacement of one or 
other of the two corresponding ligands (called heterotopic ligands) or 
addition to the two heterotopic facrs often gives rise to achiral 
diastereomers without generation of chirality. Thus not all compounds which 
display prostereoisomerism, also display prochirality. See MOLECULAR 
ISOMERISM; STEREOCHEMISTRY. 
 �34s[Ernest L. Eliel]
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�6v�2v�32sProcyon   �21m �33s 
    Alpha Canis Minoris, a nearby bright star, 0.3 magnitude, of spectral type 
F5; Procyon is slightly brighter than a normal main-sequence F5 star. Thus it 
may be an old star that has begun to evolve off the main sequence. Its great 
age is indicated by a close faint companion of absolute magnitude +13, 
yellowish in color, that is, a white dwarf star, which is nearing the end of 
its evolution.  See  STAR. 
 �34s[Jesse L. Greenstein]
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�6v�2v�32sProduct design   �21m �33s 
    The determination and specification of the parts of a product and their 
interrelationship so that they become a unified whole. The design must satisfy 
a broad array of requirements in a condition of balanced effectiveness. A 
product is designed to perform a particular function or set of functions 
effectively and reliably, to be economically manufacturable, to be profitably 
salable, to suit the purposes and the attitudes of the consumer, and to be 
durable, safe, and economical to operate. For instance, the design must take 
into consideration the particular manufacturing facilities, available 
materials, know-how, and economic resources of the manufacturer. The product 
may need to be packaged; usually it will also need to be shipped so that it 
should be light in weight and sturdy of construction. The product should 
appear significant, effective, compatible with the culture, and appear to be 
worth more than the price.  See  PRODUCTION ENGINEERING; PRODUCTION PLANNING. 
 �34s[Richard I. Felver]
�33s 
  
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�6v�2v�32sProduction engineering   �21m �33s 
    The planning and control of the mechanical means of changing the shape, 
condition, and relationship of materials within industry toward greater 
effectiveness and value. Production engineering is a term applied to certain 
aspects of planning and control of manufacturing; it is a service function to 
the production department. 
    As industry and technology evolve to greater levels of sophistication, 
complexity, and specialization, the broad area of figuring out what to do 
becomes more involved and at the same time better understood. By this process 
some of what had been originally performed by either the production department 
or the industrial engineer becomes a separate activity with its own background 
of knowledge, principles, and techniques. 
    Production engineering as a planning activity takes place between product 
design and the planning of the overall manufacturing process. Overall 
manufacturing planning is usually considered within the profession of 
industrial engineering. The purpose of production engineering is to refine and 
adjust the design of the product to the problems involved in its proposed 
manufacture. Conversely it should solve certain problems, mainly mechanical, 
such as those involved in processing, tools, dies, and new or special 
equipment necessary to manufacture the product efficiently and according to 
the established specifications.  See  INDUSTRIAL ENGINEERING; PRODUCT DESIGN; 
PRODUCTION METHODS. 
 �34s[Richard I. Felver]
�33s 
  
  
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�6v�2v�32sProduction methods   �21m �33s 
    In product engineering, the processes that are used to obtain a given 
product. Basically all production processes and methods can be classified as 
one of two types. Analytic industrial processes break down a given material 
into several products, as in an ore-reducing plant. Synthetic processes create 
one product from several different materials, as in a blast furnace or an 
automobile assembly plant. Processes may be a combination analytic-synthetic 
(wood-furniture factory) or synthetic-analytic (feed mill). 
    Industry frequently classifies processes into those that (1) change the 
shape, called forming, including cutting, molding, bending, dissolving, and 
machining; (2) change the chemical or internal characteristics, called 
treating, including mixing, blending, heat treating, and refining; (3) change 
the external surface, called finishing, including rinsing, coating, drying, 
and painting; and (4) add other pieces, called assembling, including 
attaching, joining, packaging, fitting, and fastening. 
    Most industries use a combination of at least two basic processes. In 
fact, some processes can be placed logically in more than one class, for 
example, metal plating. 
    Processes have also been classified into continuous or process-type 
operations, as in an oil refinery, and intermittent (or repetitive) or 
manufacturing-type operations. 
    Almost all production processes or methods change the form or condition of 
some material, or add or deduct other materials, aided by workers or 
machinery, or both, with the end objective being a product which has greater 
utility by nature of its new form or characteristics than the initial material. 
    The end product and the start material are of fundamental importance. 
Together, these are termed the product-material factor; this factor is the 
chief influencing feature in the choice of production methods. A change in the 
end product or in the characteristics of the start material may cause or allow 
significant changes in the production processes or methods.  See  PROCESS 
ENGINEERING; PRODUCT DESIGN; PRODUCTION ENGINEERING. 
 �34s[Richard Muther]
�33s 
  
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�6v�2v�32sProduction planning   �21m �33s 
    The function of a manufacturing enterprise responsible for the efficient 
planning, scheduling, and coordination of all production activities. The 
planning phase involves forecasting demand and translating the demand forecast 
into a production plan that optimizes the company's objective, which is 
usually to maximize profit while in some way optimizing customer satisfaction. 
These twin objectives are not always synonomous. During the scheduling phase 
the production plan is translated into a detailed, usually day-by-day, 
schedule of products to be made. During the coordination phase actual product 
output is compared with scheduled product output, and this information is used 
to adjust production plans and production schedules.  See  INDUSTRIAL COST 
CONTROL; OPTIMIZATION. 
    If the production or manufacturing process is viewed as an input-output 
process, then the production planning function can be viewed as a control 
process with feedback (see illustration). The control is in the form of 
schedules and plans, while the feedback results from the comparison of the 
production reports with the production schedules.  See  CONTROL SYSTEMS; 
INVENTORY CONTROL; PRODUCTION ENGINEERING; PRODUCTION METHODS. 
 �34s[John E. Biegel]
�33s 
  
  
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�6v�2v�32sProductivity   �21m �33s 
    Output in relation to input. Financial measures such as profit-and-loss 
statements characteristically state the relation of money-valued outputs of an 
enterprise in relation to costs, which are money-valued inputs. However, 
productivity refers to measures of output and input that are cast primarily in 
physical units, for example, the number of units of product in relation to an 
input factor such as production worker labor-hours. 
    Productivity measures do not have universal validity. The units to be used 
for measuring output are characteristically a function of the problem which is 
being addressed. The output of a single product is of interest to the 
industrial engineer, whose task is to gage the change in efficiency that is 
traceable to the installation of a new piece of equipment or a new set of 
operations for a complete production system. By contrast, the economist is 
typically interested in output measures that require the calculation of an 
index number which is a composite representation of output change. 
    The problems in relation to gaging productivity are usually concentrated 
on the measure of output, owing to the requirement of measuring a ``mix'' of 
products that not only vary in physical characteristics at a given moment, but 
also vary in proportion through time. In that case it is necessary to 
``weight'' the measure of output, thereby assigning differential importance to 
its components.  See  INDUSTRIAL COST CONTROL; PRODUCTION PLANNING. 
 �34s[Seymour Melman]
�33s 
  
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�6v�2v�32sProduct of inertia   �21m �33s 
    The product of inertia of area A relative to the indicated XY rectangular 
axes is IsubXsubY = INTEGRAL xy dA (see illustration). The product of inertia 
of the mass contained in volume V relative to the XY axes is IsubXsubY = 
INTEGRAL xy RHO dV - similarly for IsubYsubZ and IsubZsubX. 
    Relative to principal axes of inertia, the product of inertia of a figure 
is zero. If a figure is mirror symmetrical about a YZ plane, IsubZsubX = 
IsubXsubY = 0.  See  MOMENT OF INERTIA. 
 �34s[Nelson S. Fisk]
�33s 
  
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�6v�2v�32sProgesterone   �21m �33s 
    A steroid hormone produced in the corpus luteum and placenta. The hormone 
has an important physiological role in the luteal phase of the menstrual cycle 
and in the maintenance of pregnancy. In addition, progesterone produced in the 
testis and adrenals has a key role as an intermediate in the biosynthesis of 
androgens, estrogens, and the corticoids (adrenal cortex steroids).  See  
ANDROGEN; CHOLESTEROL; ESTROGEN; MENSTRUATION; PREGNANCY; STEROID; STEROL. 
 �34s[Ralph I. Dorfman]
�33s 
  
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�6v�2v�32sProgrammable controllers   �21m �33s 
    Control devices, normally used in industrial control applications, that 
employ the hardware architecture of a computer and a relay ladder diagram 
language. Programmable controllers (PCs), also referred to as programmable 
logic controllers (PLCs), were developed in the late 1960s for the automotive 
industry as replacements for large relay control panels. They are now widely 
accepted as an industrial control device in many industries, such as 
automotive, chemical, petroleum, pipelines, pulp and paper, food, rubber, 
off-road vehicles, power utilities, tobacco, mining, textiles, and machinery. 
  
    Architecture.  The relations of the basic components of a programmable 
controller are diagrammed in the illustration. A programmer is temporarily 
connected to the programmable controller for the purpose of entering, 
examining, or editing the ladder diagram logic. Once the logic is established 
as desired by the control engineer, the programmer can be disconnected and 
used on another programmable controller. Thus one programmer can service many 
(10 - 50) programmable controllers of the same manufacturer. 
    A central processor unit (CPU) and its associated input/output (I/O) form 
the permanent part of the programmable controller. Each unit normally has its 
own power supply or supplies that accept ac power and produce a well-regulated 
dc voltage. The central processor unit includes a processor, a logic memory, 
and a storage memory. The processor operates upon instructions stored in the 
logic memory. See MICROPROCESSOR. 
    The logic memory stores the instructions that result from the ladder 
diagram display entered by the control engineer with the programmer. These 
instructions indicate how the processor is to react to various changes in 
inputs and what outputs to activate or deenergize. The storage memory is used 
to retain numerical values, such as those used with timing or counting or 
arithmetic operations. The storage memory may be a physically separate memory 
or a portion of the logic memory set aside specifically for numerical storage. 
  See  COMPUTER STORAGE TECHNOLOGY; DIGITAL COMPUTER. 
    The input/output section isolates the central processor unit from the 
outside world represented by customer-supplied field devices. This isolation 
protects the central processor unit from electrical noise and voltage 
variations that can occur in industrial environments. Without this protection 
the central processor unit would operate in an unreliable and erratic manner. 
The field devices are inputs providing data to the programmable controller, 
and outputs to be controlled by the programmable controller. The input/output 
structure can be local (up to 80 ft or 25 m) or remoted up to 10,000 ft (3000 
m) from the central processor unit. Four wires (central processor unit to 
input/output connection) replace hundreds of field device wires over long 
distances in large installations. 
  
  
    Programming.  A relay ladder diagram format of programming was selected to 
be as compatible as possible with the previous control devices (relays, 
timers, and counters) that the programmable controller was designed to 
replace. Use of the relay ladder diagram eliminates the need to introduce a 
new language at this level of control and greatly enhances the acceptance of 
programmable controllers by all levels of industrial control personnel.  See  
CONTROL SYSTEMS. 
 �34s[Robert A. Whitehouse]
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�6v�2v�32sProgramming languages �21m �33s
    Notations with which people can communicate algorithms to computers and to
one another. To accomplish this, programmers specify a sequence of operations
to be applied to data objects.	See  ALGORITHMS.
    Computers process a rather low-level machine language, which has simple
instructions (load, add, and so forth) and data (for example, words or partial
words including those containing instructions). The following example shows
machine code that assigns the absolute value of B - C to A.

			   Machine
Location		    code			Meaning
040003			100020040001		load B into register A1
040004			250320040002		subtract C from register A1
040005			741020040007		GOTO 040007 if A1 posi-
						   tive
040006			110020000000		negate A1
040007			010020040000		store contents of A1 into A

    Writing in machine code places a tremendous burden on the programmers.
They must write in an unusual number base (for example, eight), and transfer
data from memory to registers in order to perform operations. They must know
the machine's operation codes for the operations. Finally, they must know the
addresses of the operands (both registers and data words) and instructions (to
be able to skip some operations with branches).
    The introduction of assembler languages eased communication between
programmers and machines by permitting symbolic rather than numeric or
positional references for operation codes (opcodes) and addresses. Additional
data and instructions could therefore be added to a program without changes to
existing instructions. Thus, in the example below, which shows assembler code
that assigns the absolute value of B - C to A, instructions could be added
between the branch positive, BP, and its destination.

		 Op-
Label		code		Operands		  Comments
		L		A1,B
		AN		A1,C		    CTR DOT A1 : = B - C
		BP		A1, NEXT
		N		A1		    CTR DOT A1 : = - A1
NEXT		S		A1,A		    CTR DOT A : = |B - C|

    Unfortunately, assembler language does not go very far in improving
programmers' ability to communicate algorithms. A programmer must still be
concerned with expressing algorithms in a form suitable to particular
machines. For instance, if an expression requires more registers than are
available, the programmer must create temporary variables to hold partial
results of expressions. High-level languages allow programmers to communicate
more easily with one another by providing structures that hide the
architecture of the machine. Expressions in high-level languages can be
written almost as they are in algebra so that the programmer need not worry
about which register contains a particular computation or which registers are
available.  See  DIGITAL COMPUTERS; DIGITALCOMPUTER PROGRAMMING.

    Structure of programming languages.  The components of a program and their
relationships are shown in the illustration. The basic executable unit of a
programming language is a program. Programs contain declarations of data and
procedures, and a sequence of statements to be executed. Programming languages
manipulate objects or values like integers or files. Objects are referred to
by their names, called identifiers. Constant objects, such as the number 2
(one of whose names is the literal ``2''), do not change. Variable objects may
contain either other objects or the names of other objects.
    Expressions. Expressions combine objects and operators to yield new
objects. They may be written in one of several forms. Infix operators appear
between their operands; prefix operators precede their opernds; and postfix
operators follow their operands. Most languages adopt infix notation for
binary operators and prefix notation for unary operators.
    Statements. Important types of statements include assignment statements,
compound statements, conditional statements, repetitive statements, and GOTO
statements.
    Assignment statements assign values to variables. In an assignment of form
(1), the value in the object denoted by B is

				A: = B					   (1)

assigned to the object denoted by A. The order of evaluation of the two sides
of an assignment statement differs among languages - some insist on
left-to-right evaluation, while others leave the order of evaluation
undefined. Some languages permit the same value to be assigned to several
variables in a single statement. Other languages treat assignment just like
any other operator in an expression.

		  BEGIN stmt1; stmt2;. . . stmtn END			   (2)

    A compound statement (2) groups a sequence of statements into a single
statement. Execution of statements within a sequence of statements is in the
order in which the statements appear in the sequence.

		     IF expression THEN statement			   (3)

    The IF statement (3) allows a programmer to indicate that a statement is
to be executed if a stated condition is true. If the expression is true, the
statement following the THEN is executed followed by the statement after the
IF statement. If the expression is false, the statement is skipped. Many
languages permit execution of one of two alternate statements with the form of

		IF expression THEN statement1 ELSE statement2		     (4)

IF statement (4). If the expression is true, statement 1
is executed; otherwise statement 2 is executed. In either case, execution
continues with the statement after the IF.
    One of the advantages offered by computers is performing the same
operations repetitively.Two statements in programming languages support this
kind of execution: FOR and WHILE. The FOR statement (5) is generally used when
a programmer knows how many times a statement is to be executed.

FOR variable-identifier: =
		   expression TO expression DO statement		     (5)

		      WHILE expression DO statement			     (6)


    The WHILE statement (6) is used when continued execution depends on a
condition that is true for an unknown number of repetitions. The statement is
executed as long as the expression is true. When the expression becomes false,
execution continues with the statement after the WHILE.
    The lowest-level control statement in most languages is the GOTO statement

				GOTO identifier 			     (7)

(7). This statement provides for the direct transfer of control to a statement
labeled with the identifier that is the argument of the GOTO statement.

    Procedures. As problems grow more complex, they must be divided into
subproblems that can be solved independently. This approach to problem solving
is supported by procedure declarations, which group sequences of statements
together and associate identifiers with the statements. A procedure identifier
abstracts the details of the computation carried out by the statements of the
procedure by encapsulating them. Procedures also help reduce the size of a
program by permitting similar code to be written only once.

    Data types.  Data types are sets of values and the operations that can be
performed on them. Data types offer programmers two principal advantages:
abstraction and authentication. Data types hide representational details from
programmers so they do not have to be concerned whether characters are
represented by ASCII or EBCDIC codes, or whether they are left-aligned and
blank-filled or right-aligned and zero-filled in machine words. The concept of
data type also provides useful redundancy. The appearance of a variable as an
operand in an expression implies a particular type for the variable (for
example, A + B implies that A and B are either integer or real) that may be
checked against the type of its current value.
    Scalars are indivisible units of data; they are treated as single complete
entities. Most languages contain several scalar types. The most common are
integer, real, boolean, and character.
    While variables usually hold values of objects, they may also hold the
name of some object. Variables that can hold such values are called references
or pointers.
    Collections of related values can be represented by aggregate types.
Aggregate types can be characterized by the types of the objects that are
their elements and the manner in which the elements are accessed. Elements of
a collection can have homogeneous or heterogeneous types, and can be selected
by their position in a collection or by a name associated with them.

    Binding attributes to variables.  Both constant and variable objects have
values and attributes. Among the most important attributes of an object are
its type, scope, and extent.
    In most programming languages, the same identifier can name distinct
objects in different parts of a program. The scope of an identifier is that
part of a program in which the identifier has the same meaning. Generally that
section of program is called a scope unit and corresponds to a procedure or
compound statement. The set of objects that can be referenced in a scope unit
is called the environment.
    Data types may be associated with objects statically via declarations or
dynamically via assignment statements. In statically typed languages, the type
associated with an object remains the same throughout the scope of the object.
    The extent or lifetime of a variable is the period of program execution
during which the storage is allocated. Static storage is created at the
beginning of the program execution and not deallocated until program
termination. Local storage for variables is created on entry to the block or
procedure containing the variable's declaration and destroyed on exit. Dynamic
storage is created and destroyed by execution of special statements in the
programming language.

    Parameters.  Parameters communicate information between the calling
environment and the environment of the called procedure. Although this can be
accomplished by referencing nonlocal variables, communicating with parameters
permits a procedure to work on different variables each time it is called.
Formal parameters are identifiers that are locally declared in a procedure
definition; actual parameters are expressions in a procedure call statement.

    Functions.	Function declarations are similar to procedure declarations.
But unlike a procedure, which communicates its results by assigning values to
its parameters, a function communicates a distinguished value known as the
result of the function. This result of the function is often indicated by
assigning a value to the name of the function.
    When a statement in a procedure or function alters the value of a nonlocal
variable or reference or name parameter, the procedure is said to have a side
effect. It may not be quite accurate to call such assignments in procedures
side effects, because procedures must include such statements in order to
communicate their results. Therefore, they are really main effects of
procedures. Side effects in functions are less desirable because they make the
results of expression evaluation unpredictable. Thus, it is good programming
practice to avoid side effects in functions and to use procedures instead when
side effects are desired.

    Applicative and object-oriented languages.	Most programming languages are
imperative languages, because their programs largely consist of a series of
commands to assign values to objects. ALGOL, PL/1, FORTRAN, ADA, COBOL and
PASCAL are imperative languages. Two other types of languages exist:
applicative and object-oriented. No commonly used language is purely
imperative, applicative, or objectoriented, so that every language has some
characteristics of each type of language.
    A purely applicative language would be one in which there are no
statements, only expressions without side effects. The name ``applicative''
refers to the repeated application of functions to the results of other
functions to specify an algorithm. LISP, APL, and VAL are three languages in
which programs are largely applicative.
    Programs written in object-oriented languages consist of sequences of
commands directed at objects. An object receiving a command invokes an
internal procedure to respond to the command. Assignment statements are
replaced by commands to objects requesting that they change their values.

    Defining new data types.  Procedure and function declarations define new
operations that operate on existing objects. Type declarations add new objects
and operations to a programming language. Designing programs with user-defined
types allows solutions to be stated in problem-oriented rather than
machine-oriented terms. ALGOL 68- and PASCAL-type declarations allow
programmers to define the representation of new objects of the data types and
ALGOL 68 permits the definition of operations.

    Particular languages.  The more widely used languages include FORTRAN,
ALGOL, COBOL, BASIC, PL/1, APL, SNOBOL, LISP, PASCAL, C, and ADA.
    FORTRAN was designed in the mid-1950s for efficient scientific
applications. To achieve this goal, the data and control components of the
language are very simple, providing programmers with low-level hardware
operations. FORTRAN control structures are particularly primitive; neither
compound statements nor unbound repetitive statements are part of the
language. Two conditional statements (arithmetic and logical IF's) and a
bounded iteration statement (DO) provide the only relief from GOTO'S and
labels.
    ALGOL was designed in the late 1950s and early 1960s as a language for
scientific computation. The elegance of ALGOL's structure has made it the
basis for several subsequent language design efforts, most notably ALGOL 68
and PASCAL, and its control structures have even been adopted by PL/1 and
later versions of FORTRAN. ALGOL was also the first language whose syntax was
defined with a formal grammar (called Backus-Naur Form or BNF after two of the
principal designers of ALGOL). ALGOL control statements are very much like
statements (2) - (5). However, all repetitive statements are variants of the
FOR statement, and compound statements are units of scope that may contain
declarations of their own local variables.
    COBOL was designed in 1959 and 1960 as a language to support business
applications on computers. As was the case with FORTRAN, efficiency was a
primary concern. Because of its intended application area, COBOL has
relatively poor features for specifying computations (such as expressions,
functions, and parameters), but strong features for data description and
input/output. Two of the most noticeable features of COBOL programs are the
English-like syntax and the division of programs into four parts.
    BASIC was developed at Dartmouth College in the early 1960s to provide
nonscience majors with a simple, interactive language. BASIC supports real and
string data as well as one- or two-dimensional arrays of one of these
primitive types. Dimension statements permit string lengths and array
subscripts to be specified. Variable names are restricted to single letters,
followed by an optional digit if the variable is numeric or $ if the variable
is a string. Thus, types are statically associated with variables.
    PL/1 was designed in the mid-1960s to meet the needs of a broad range of
applications: scientific, business applications, and systems. PL/1 combines
and extends many of the features of FORTRAN (parameter binding and formated
input/output), ALGOL (scope rules, variable extents, recursive procedures, and
control structures), and COBOL (record structures and picture data types). By
providing redundant features from different languages, the designers have made
the language difficult to learn and encouraged users to stay in one of the
FORTRAN, COBOL, or ALGOL subsets of PL/1.
    APL, designed during the mid-1950s to the mid-1960s, is an interactive
language whose operators accept and produce arrays with homogeneous elements
of type number or character. The only control construct is a GOTO statement,
but this is inconsequential since many APL programs have a strong applicative
flavor. Although they often consist of an imperative sequence of assignements,
APL programs generally operate on array objects to produce new array objects.
Thus repetitive constructs are not needed as frequently as they are in
ALGOL-like languages.
    SNOBOL was developed during the 1960s as a string processing language.
SNOBOL has no real control constructs. Instead, every statement either
``succeeds'' or ``fails'' and may optionally GOTO a new statement depending on
success or failure. SNOBOL statements provide extremely powerful operators for
string manipulation.
    LISP, invented in the late 1950s, is a very widely used language in the
artificial intelligence community. The basic type in LISP is the list, usually
denoted by a sequence of items in parentheses. The power of LISP comes from
the representation of everything by lists, which leads to very powerful
environments of tools and extensions to LISP. LISP is a good language to write
experimental programs, since its flexibility is useful for dealing with the
unexpected. However, it executes slowly and uses a lot of memory.  See
ARTIFICIAL INTELLIGENCE.
    PASCAL was developed in 1969 to obtain a language which would be suitable
to teach programming as a systematic activity with constructs that could be
implemented reliably and efficiently. The control structures of PASCAL are
similar to statements (2) - (5) and (6). However, a CASE statement was added
as an additional selection statement. This statement permits execution of one
alternative among several statements based on the value of a selection
expression.
    C was designed in 1972 to implement the Unix operating system. Similar to
PASCAL in many ways, C has enumerated types, records (called structures), the
CASE statement, and reference variables. The main difference between
programming in C and other languages is the extensive use of reference
variables, which are defined to be equivalent to arrays.  See  OPERATING
SYSTEM.
    ADA, designed in the mid-1970s, won a competition sponsored by the
Department of Defense to design a new language to support development of
embedded systems. ADA was designed using PASCAL as a base, to meet the
reliability and efficiency requirements imposed by these applications.	See
DIGITAL COMPUTER PROGRAMMING.
 �34s[J. D. Gannon ;M. D. Weiser]
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�6v�2v�32sProgression (mathematics) �21m �33s
    Ordered, countable sets of numbers, x1, x2, x3, . . . , not necessarily
all different.	In general such sets are called sequences, whereas the term
progression is usually confined to the special types:  the arithmetic, in which
the difference x(sub k) - x(sub k-1) between successive terms is constant; the
geometric, in which the ratio x(sub k)/x(sub k-1) is constant; and the harmonic,
in which the reciprocals of the terms are in arithmetic progression.  If the
first term of an arithmetic progression is a and the common difference b, then
the terms of the progression are given by Eqs.	(1).

x1 = a, x2 = a + b, x3 - a + 2b, .  .  .  , x(sub n) = a + (n -1)b, .  .  .  (1)

The sum of the first n terms S(sub n) is given by Eq. (2).

S(sub n) = n(x1 + x[sub n])/2 = n{a + [(n - 1)/2]b}			     (2)

    If the first term of a geometric progression is a and the common ratio r,
then the terms of the progression are given by Eqs. (3).

x1 = a, x2 - ar, x3 = ar(sup 2), .  .  .  , x(sub n) = ar(sup n-1), .  .  .  (3)

Excluding the case r = 1 (when all terms are the same), the sum S(sub n) of the
first n terms is given by Eq. (4).

		  S(sub n) = a(1 - r[sup n])/(1 - r)			     (4)

    The arithmetic mean A and the geometric mean G of n positive numbers are
defined by Eqs. (5) and (6).

		  A = (x1 + x2 + . . . + x(sub n))/n			     (5)


	       G = (x1x2 . . . x[sub n])(sup 1)/n			     (6)

    The reciprocals of sequence (1) form a harmonic progression. There is no
compact expression for the sum of n terms.

				x2 = (2 x1 x3)/(x1 + x3)		     (7)

If x1, x2, x3 are in harmonic progression, then Eq. (7) is called their
harmonic mean.
 �34s[Lane Barksdale]
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�6v�2v�32sProjective geometry   �21m �33s 
    A geometry that investigates those properties of figures that are 
unchanged (invariant) when the figures are projected from a point to a line or 
plane.  
    Two features of plane projective geometry are (1) introduction of an ideal 
line that each ordinary line g intersects (the intersection being common to 
all lines parallel to g), and (2) the principle of duality, according to which 
any statement that is obtained from a valid one (theorem) by substituting for 
each concept involved, its dual, is also valid. (``Line'' and ``point'' are 
dual, ``connecting two points by a line'' is dual to ``intersecting two 
lines,'' and so on.) The subject has been developed both synthetically (as a 
logical consequence of a set of postulates) and analytically (by the 
introduction of coordinates and the application of algebraic processes).  See  
CONFORMAL MAPPING; EUCLIDEAN GEOMETRY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sProlamine   �21m �33s 
    A protein that is soluble in 70 - 80% aqueous ethanol, but insoluble in 
absolute ethanol, water, and other neutral solvents. Prolamines have a high 
content of proline and amide nitrogen and only small amounts of basic amino 
acids, but very large amounts (up to 40%) of glutamic acid, mostly present as 
glutamine. Prolamines are found in plant products, for example, gliadin from 
wheat kernel, hordein from barley, and zein from corn.  See  AMINO ACIDS; 
GLUTAMIC ACID; PROLINE; PROTEIN. 
 �34s[Gertrude E. Perlmann]
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�6v�2v�32sProline   �21m �33s 
    A common amino acid of proteins occurring in varying amounts in 
essentially all individual proteins, and in mixtures of proteins at about 4% 
by weight. It is a major amino acid (10 - 15%) in the collagen proteins. 
    Nutritionally, proline is not required in the diet of humans or other 
mammals, since it is formed freely from glutamic acid, which in turn is 
derived from products of carbohydrate metabolism. See AMINO ACIDS; 
CARBOHYDRATE METABOLISM; COLLAGEN; GLUTAMIC ACID. 
 �34s[Elijah Adams]
�33s 
  
  
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�6v�2v�32sPromethium   �21m �33s 
    A chemical element, Pm, atomic number 61. Promethium is the ``missing'' 
element of the lanthanide rare-earth series. The atomic weight of the most 
abundant separated radioisotope is 147. 
    Although a number of scientists have claimed to have discovered this 
element in nature as a result of observing certain spectral lines, no one has 
succeeded in isolating element 61 from naturally occurring materials. It is 
produced artificially in nuclear reactors, since it is one of the products 
that results from the fission of uranium, thorium, and plutonium. 
    All the known isotopes are radioactive. Its principal uses are for 
research involving tracers. Its main application is in the phosphor industry. 
It has also been used to manufacture thickness gages and as a nuclear-powered 
battery in space applications.  See  RARE-EARTH ELEMENTS. 
 �34s[Frank H. Spedding]
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�6v�2v�32sPronghorn   �21m �33s 
    An antelopelike animal, Antilocapra americana, the sole representative of 
the family Antilocapridae, and of uncertain taxonomic affinities. This animal 
is reputed to be the fastest ungulate in North America and is the only 
hollow-horned ungulate with branched horns present in both sexes. Like the 
deer, the pronghorn sheds its horns each fall; the new growth is complete by 
midsummer.  
    The pronghorn lives in small herds in rather wild, rocky desert country. 
They feed on cactus, sagebrush, and other vegetation. In late summer, a buck 
begins to accumulate a harem of about 10 - 15 does. Two young are usually born 
in the spring after a gestation period of 35 weeks. Average life-span for a 
pronghorn is about 8 years.  See  ARTIODACTYLA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sProny brake �21m �33s
    An absorption dynamometer that applies a friction load to the output shaft
by means of wood blocks, flexible band, or other friction surface. The prony
brake (see illustration) provides means for measuring the torque T, developed
as in the equation below, where L is the distance

			     T = L(W - W(sub 0))

shown on the drawing, W is the scale weight with the brake operating, and W0
is the scale weight with the brake free. Small prony brakes are air-cooled;
the drum of larger ones may be filled with water to absorb the heat during
operation.  See  DYNAMOMETER.
 �34s[Frank H. Rockett]
�33s


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�6v�2v�32sPropane   �21m �33s 
    A member of the alkane or paraffin series of hydrocarbons, formula 
CH3CH2CH3. It makes up 3 - 18% of natural gas. It is readily liquefied 
(melting point, -187.7DGC or -305.9DGF; boiling point, -42.1DGC or -43.8DGF), 
and mixtures with liquefied butane are sold as liquefied petroleum gas (LPG) 
in cylinders under moderate pressure for domestic fuel.  See  LIQUEFIED 
PETROLEUM GAS (LPG). 
    At temperatures above about 650DGC (1200DGF), propane undergoes cracking 
to ethylene and methane. This reaction is the basis of an important commercial 
source of ethylene. 
    In the petroleum industry, propane is used as a combined solvent and 
refrigerant for the refining of lubricants and other products.  See  ALKANE; 
CRACKING; PETROLEUM PROCESSING. 
 �34s[Louis Schmerling]
�33s 
  
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�6v�2v�32sPropanol   �21m �33s 
    One of the three-carbon saturated, aliphatic alcohols (alkanols). Normal 
propanol (also known as propyl alcohol, 1-propanol, or ethyl carbinol), 
CH3CH2CH2OH, has chemical properties similar to other primary aliphatic 
alcohols. It is a colorless, mobile liquid of pungent odor and taste and has 
the following physical properties: molecular weight, 60.09; boiling point, 
97.2DGC (207DGF); melting point, -126.2DGC (-195.2DGF); specific gravity, 
0.8036 at 20DGC (28DGF). The compound is miscible with water and is soluble in 
most organic liquids. Normal propanol is toxic and flammable. 
    Isopropanol, CH3CHOHCH3 (also known as 2-propanol, isopropyl alcohol, and 
dimethyl carbinol), is the simplest of the secondary alcohols and is a major 
industrial organic chemical. It is a colorless, mobile, and toxic liquid 
(molecular weight, 60.09; boiling point, 82.3DGC or 180DGF; melting point, 
-88.5DGC or -127.3DGF; specific gravity, 0.786 at 20DGC or 28DGF) and has a 
pungent odor and taste. It is miscible with water and soluble in most organic 
liquids. Isopropanol is used primarily for the manufacture of acetone by 
catalytic dehydrogenation. It is also used as a solvent, an extractant, an 
antifreeze, and as a rubbing alcohol.  See  ALCOHOL. 
 �34s[P. Dwight Sherman, Jr.]
�33s 
  
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�6v�2v�32sPropellant   �21m �33s 
    Usually, a combustible substance that produces heat and supplies ejection 
particles, as in a rocket engine. A propellant is both a source of energy and 
a working substance; a fuel is chiefly a source of energy, and a working 
substance is chiefly a means for expending energy. Because the distinction is 
more decisive in rocket engines, the term propellant is used primarily to 
describe chemicals carried by rockets for propulsive purposes.  See  AIRCRAFT 
FUEL; FUEL; ROCKET ENGINE; THERMODYNAMIC CYCLE. 
    Propellants are classified as liquid or as solid. Even if a propellant is 
burned as a gas, it may be carried under pressure as a cryogenic liquid to 
save space. For example, liquid oxygen and liquid hydrogen are important 
high-energy liquid bipropellants. 
  
    Liquid propellants.  A liquid propellant releases energy by chemical 
action to supply motive power for jet propulsion. The three principal types of 
propellants are monopropellant, bipropellant, and hybrid propellant. 
Monopropellants are single liquids, either compounds or solutions. 
Bipropellants consist of fuel and oxidizer carried separately in the vehicle 
and brought together in the engine. Hybrid propellants use a combination of 
liquid and solid materials to provide propulsion energy and working substance. 
Typical liquid propellants are listed in the table.  See  METAL-BASE FUEL. 
            The availability of large quantities and their high performance 
led to selection of liquefied gases such as oxygen for early liquid-propellant 
rocket vehicles. Liquids of higher density with low vapor pressure (see table) 
are advantageous for the practical requirements of rocket operation under 
ordinary handling conditions. Such liquids can be retained in rockets for long 
periods ready for use and are convenient for vehicles that are to be used 
several times. The high impulse of the cryogenic systems is desirable for 
rocket flights demanding maximum capabilities, however, such as space 
exploration or transportation of great weights for long distances. 
 �34s[Stanley Singer]
�33s 
  
    Solid propellants.  A solid propellant is a mixture of oxidizing and 
reducing materials that can coexist in the solid state at ordinary 
temperatures. When ignited, a propellant burns and generates hot gas. Although 
gun powders are sometimes called propellants, the term solid propellant 
ordinarily refers to materials used to furnish energy for rocket propulsion. 
    A solid propellant normally contains three essential components: oxidizer, 
fuel, and additives. Oxidizers commonly used in solid propellants are ammonium 
and potassium perchlorates, ammonium and potassium nitrates, and various 
organic nitrates, such as glyceryl trinitrate (nitroglycerin). Common fuels 
are hydrocarbons or hydrocarbon derivatives, such as synthetic rubbers, 
synthetic resins, and cellulose or cellulose derivatives. The additives, 
usually present in small amounts, are chosen from a wide variety of materials 
and serve a variety of purposes. Catalysts or suppressors are used to increase 
or decrease the rate of burning; ballistic modifiers may be used for a variety 
of reasons, as to provide less change in burning rate with pressure 
(platinizing agent); stabilizers may be used to slow down undesirable changes 
that may occur in long-term storage. 
    Solid propellants are classified as composite or double base. The 
composite types consist of an oxidizer of inorganic salt in a matrix of 
organic fuels, such as ammonium perchlorate suspended in a synthetic rubber. 
The double-base types are usually high-strength, high-modulus gels of 
cellulose nitrate (guncotton) in glyceryl trinitrate or a similar solvent. 
 �34s[H. W. Ritchey]
�33s 
  
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�6v�2v�32sPropeller (aircraft)   �21m �33s 
    A hub-and-multiblade device for changing rotational power of an aircraft 
engine into thrust power for the purpose of propelling an aircraft through the 
air (see illustration). An air propeller operates in a relatively thin medium 
compared to a marine propeller, and is therefore characterized by a relatively 
large diameter and a fairly high rotational speed. It is usually mounted 
directly on the engine drive shaft in front of or behind the engine housing.  
See  PROPELLER (MARINE CRAFT). 
    Usually propellers have two, three, or four blades; for high-speed or 
high-powered airplanes, six or more blades are used. In some cases these 
propellers have an equal number of opposite rotating blades on the same shaft, 
and are known as dual-rotation propellers. 
    A propeller blade advances through the air along an approximate helical 
path which is the result of its forward and rotational velocity components. 
This action is similar to a screw being turned in a solid surface, except that 
in the case of the propeller a slippage occurs because air is a fluid. Because 
of the similarity to the action of a screw, a propeller is also known as an 
airscrew. To rotate the propeller blade, the engine exerts a torque force. 
This force is reacted on by the blade in terms of lift and drag force 
components produced by the blade sections in the opposite direction. As a 
result of the rational forces reacting on the air, a rotational velocity 
remains in the propeller wake with the same rotational direction as the 
propeller. This rotational velocity times the mass of the air is proportional 
to the power input. The sum of all the lift and drag components of the blade 
sections in the direction of flight are equal to the thrust produced. These 
forces react on the air, giving an axial velocity component opposite to the 
direction of flight. By the momentum theory, this velocity times the mass of 
the air going through the propeller is equal to the thrust. 
    A propeller blade must be designed to withstand very high centrifugal 
forces. The blade also must withstand the thrust force produced plus any 
vibratory forces generated, such as those due to uneven flow fields. To 
withstand the high stresses due to rotation, propeller blades have been made 
from a number of materials, including wood, aluminum, hollow steel, and 
plastic composites. The most common material used has been solid aluminum. 
However, the composite blade constructions are being used for new turboprop 
installations because of their very light weight and high strength 
characteristics.  
  
    For a small, low-power airplane, very simple, fixed-pitch, single-piece, 
two-blade propellers are used. The rotational speed of these propellers 
depends directly on the power input and forward speed of the airplane. Because 
of the fixed-blade angle of this type of propeller, it operates near peak 
efficiency only at one condition. To overcome the limitations of the simple 
fixed-pitch propeller, configurations that provide for variable blade angles 
are used. The blades of these propellers are retained in their hub so that 
they can be rotated about their centerline while the propeller rotates. For 
the normal range of operation, the blade angle varies from the low blade angle 
needed for takeoff to the high blade angle needed for the maximum speed of the 
airplane.  See  AIRCRAFT PROPULSION; AIRPLANE; HELICOPTER. 
 �34s[Henry V. Borst]
�33s 
  
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�6v�2v�32sPropeller (marine craft)   �21m �33s 
    A component of a ship-propulsion power plant which converts engine torque 
into propulsive force or thrust, thus overcoming a ship's resistance to 
forward motion by creating a sternward accelerated column of water. Since 1860 
the screw propeller has been the only propeller type used in ocean transport, 
mainly because of the evolution of the marine engine toward higher rotative 
speed.  
    The advantages of a screw propeller include light weight, flexibility of 
application, good efficiency at high rotative speed, and relative 
insensitivity to ship motion. The fundamental theory of screw propellers is 
applicable to all forms of marine propellers. In its present form a screw 
propeller consists of a streamlined hub attached outboard to a rotating engine 
shaft, on which are mounted two to seven blades. The blades are either solid 
with the hub, detachable, or movable. The screw propeller has the 
characteristic motion of a screw; it revolves about the axis along which it 
advances. The screw blades are approximately elliptical in outline. 
    One or more screw propellers are usually fitted as low as possible at the 
ship's stern to act as thrust-producing devices (see illustration). The low 
position of the propellers affords good protection and sufficient immersion 
during the pitching movements of the ship. The choice of the number of 
propellers to incorporate into a vessel design is based upon several factors. 
In general, a single-screw arrangement yields a higher propulsive efficiency 
than multiple screws, particularly when most of the propeller is operating in 
the boundary layer of the ship and can recover some of the energy loss. In 
addition, single-screw propulsion systems generally result in savings in 
machinery cost and weight in comparison to multiple-screw arrangements. 
  
    The formation and collapse of vapor-filled bubbles, or cavities, causes 
noise, vibration, and often rapid erosion of the propeller material, 
especially in fast, high-powered vessels. This phenomenon is known as 
cavitation. As long as the rotational and translational speeds of the 
propeller are not too high, the onset of cavitation can be delayed or limited 
to an acceptable amount by clever design of blade sections.  See  CAVITATION. 
    Supercavitating and superventilated propellers are designed to have fully 
developed blade cavities which spring from the leading edge of the blade, 
cover the entire back of the blade, and collapse well downstream of the blade 
trailing edge. The blade of such propellers has unique sections which usually 
are wedge-shaped with a sharp leading edge, blunt trailing edge, and concave 
face. Supercavitating propellers have cavities filled with water vapor and 
small amounts of gases dissolved in the fluid media. Superventilated 
propellers have cavities filled primarily with air from the water surface or 
gases other than water vapor from a gas supply system through the propeller 
shaft.  
    For ships which normally operate at widely varying speeds and propeller 
loadings (towboats, rescue vessels, trawlers, and ferryboats), the application 
of controllable-pitch (rotatable-blade) propellers permits the use of full 
engine power at rated rpm under all operational conditions, ensuring maximum 
thrust production, utmost flexibility, and maneuverability. Since these 
propellers are also reversible, they permit the use of nonreversible machinery 
(gas turbines).  See  MARINE ENGINE; MARINE MACHINERY. 
 �34s[Jacques B. Hadler]
�33s 
  
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�6v�2v�32sProperdin �21m �33s
    A serum protein which acts only in conjunction with complement and
Mg(sup 2+) and participates in diverse activities such as the destruction of
bacteria, neutralization of viruses, and the lysis of certain red cells.
    Properdin activity has been found in the serum of germ-free rats and
humans with agammaglobulinemia, and its level varies little with a variety of
disease states, although transient changes in the serum activity have been
noted in pneumococcal pneumonia, in meningococcemia, and following total body
irradiation.  See  SERUM.
 �34s[Donald Raum]
�33s

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�6v�2v�32sPropionibacteriaceae   �21m �33s 
    A family of anaerobic bacteria. This family contains relatively anaerobic, 
nonmotile, nonsporulating, gram-positive, predominantly rod-shaped bacteria 
that ferment glucose and certain other substrates. Some of the 
Propionibacteria are involved in the production of Swiss cheese and others in 
the production of vitamin B12. Two genera are included in the family, 
Propionibacterium and Eubacterium. 
 �34s[Horace A. Barker]
�33s 
  
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�6v�2v�32sPropositional calculus   �21m �33s 
    A branch of automata theory that concerns itself with the manipulation of 
linguistic statements. Rules of symbolic logic are employed to reduce compound 
statements into simplified component statements. Of central interest is 
whether a particular statement is ``true'' or ``false.'' For example, consider 
the following statements: ``New York is a city'' is true; ``St. Louis is the 
mayor of New York'' is false; ``Cats are mammals'' is true. Suppose now that 
the symbols X, Y, and Z are assigned to these statements, respectively. New 
statements can be made using the symbols. The new statement ``Both X and Z'' 
is true, since the first and last statements are both true. The statement 
``Both X and Y'' is false, since the second statement is false. The statement 
``Either X or Y'' is true, since at least one of them is true. 
    When the statements considered are limited to those that can unambiguously 
be said to be true (T) or false (F), or, more succinctly, when the sets A, S, 
and Z each have two members, {T,F}, then the language is referred to as 
switching algebra, switching theory, or Boolean algebra. The study of 
switching functions is of particular importance in computing machines where 
binary sequences are used to represent numbers. See AUTOMATA THEORY; BOOLIAN 
ALGEBRA; SWITCHING THEORY. 
 �34s[Norman B. Reilly]
�33s 
  
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�6v�2v�32sProprioception   �21m �33s 
    The sense of position and movement of the limbs and the sense of muscular 
tension. The awareness of the orientation of the body in space and the 
direction, extent, and rate of movement of the limbs depend in part upon 
information derived from sensory receptors in the joints, tendons, and 
muscles. Information from these receptors, called proprioceptors, is normally 
integrated with that arising from vestibular receptors (which signal 
gravitational acceleration and changes in velocity of movements of the head), 
as well as from visual, auditory, and tactile receptors. Sensory information 
from certain proprioceptors, particularly those in muscles and tendons, need 
not reach consciousness, but can be used by the motor system as feedback to 
guide postural adjustments and control of well-practiced or semiautomatic 
movements such as those involved in walking. 
    Receptors for proprioception are the endings of peripheral nerve fibers 
within the capsule or ligaments of the joints or within muscle. These endings 
are associated with specialized end organs such as Pacinian corpuscles, 
Ruffini's cylinders, and Golgi organs (the latter resembling histologic Golgi 
structures in the skin), and muscle spindles.  See  CUTANEOUS SENSATION; 
SENSATION; SOMESTHESIS. 
 �34s[Robert LaMotte]
�33s 
  
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�6v�2v�32sPropulsion �21m �33s
    The process of causing a body to move by exerting a force against it.
Propulsion is based on the reaction principle, stated qualitatively in
Newton's third law, that for every action there is an equal and opposite
reaction. A quantitative description of the propulsive force exerted on a body
is given by Newton's second law, which states that the force applied to any
body is equal to the rate of change of momentum of that body, and is exerted
in the same direction as the momentum change.  See  NEWTON'S LAWS OF MOTION.
    In the case of a vehicle moving in a fluid medium, such as an airplane or
a ship, the required change in momentum is generally produced by changing the
velocity of the fluid (air or water) passing through the propulsive device or
engine. In other cases, such as that of a rocket-propelled vehicle, the
propulsion system must be capable of operating without the presence of a fluid
medium; that is, it must be able to operate in the vacuum of space. The
required momentum change is then produced by using up some of the propulsive
device's own mass, which is called the propellant.  See  AERODYNAMIC FORCE;
AIRFOIL; FLUID DYNAMICS; FLUID FLOW; FLUID MECHANICS; PROPELLANT.
    The two terms most generally used to describe propulsion efficiency are
thrust specific fuel consumption for engines using the ambient fluid (air or
water), and specific impulse for engines which carry all propulsive media on
board.	See  SPECIFIC FUEL CONSUMPTION; SPECIFIC IMPULSE.
    The energy source for most propulsion devices is the heat generated by the
combustion of exothermic chemical mixtures composed of a fuel and an oxidizer.
An air-breathing chemical propulsion system generally uses a hydrocarbon such
as coal, oil, gasoline, or kerosine as the fuel, and atmospheric air as the
oxidizer. A non-air-breathing engine, such as a rocket, almost always utilizes
propellants that also provide the energy source by their own combustion.
    Where nuclear energy is the source of propulsive power, the heat developed
by nuclear fission in a reactor is transferred to a working fluid, which
either passes through a turbine to drive the propulsive element such as a
propeller, or serves as the propellant itself. Nuclear-powered ships and
submarines are accepted forms of transportation. Nuclear-powered airplanes and
rockets are still in the developmental stages.	See  AIRPLANE; ELECTROMAGNETIC
PROPULSION; JET PROPULSION; SHIP POWERING AND STEERING; TURBINE PROPULSION.
 �34s[Jerry Grey]
�33s

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�6v�2v�32sPropylene   �21m �33s 
    A gas, CH3--HCDOUBLE BARCH2, with boiling point -48DGC (-54DGF) and 
melting point -185DGC (-301DGF). All processes for thermal or catalytic 
cracking of hydrocarbons yield propylene. Preparation outside of normal 
refinery operations is usually by catalytic dehydrogenation of propane. 
    Major uses for propylene include the production of isopropyl alcohol, from 
which acetone is obtained; polypropylene plastics; tripropylene for the 
alkylation of phenol to produce alkyl phenols, from which are derived nonionic 
detergents and lubricating oil additives; and propylene chlorohydrin for 
producing propylene oxide from which are obtained detergents, hydraulic 
fluids, and lubricants.  See  ALKENE; ETHYLENE; POLYOLEFIN RESINS. 
 �34s[Charles A. Cohen]
�33s 
  
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�6v�2v�32sProsobranchia   �21m �33s 
    The largest subclass in the class Gastropoda, containing most marine 
snails, the limpets and whelks, a limited number of nonpulmonate land snails, 
and nearly all of the nonpulmonate fresh-water snails. Three orders are 
commonly recognized, the Archaeogastropoda (Aspidobranchia), Mesogastropoda 
(Pectinibranchia), and Neogastropoda. 
    Respiration is usually by means of ctenidia, or gills, located in front of 
the heart. When gills are absent, respiration is carried on by means of a 
pallial outgrowth or a pulmonary chamber. An operculum is usually present, 
there is generally only one pair of tentacles, and the sexes are usually 
separate. The shells produced have evolved many shapes. Certain of the 
primitive forms have nacreous shells, such as Haliotis, the sea ear or 
abalone, and Trochus, the top shell. The shells are used for jewelry, inlay 
work, and buttons.  See  ARCHAEOGASTROPODA; GASTROPODA; NEOGASTROPODA; 
PECTINIBRANCHIA.  
 �34s[William J. Clench]
�33s 
  
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�6v�2v�32sProspecting   �21m �33s 
    The search for mineral deposits that can be worked at a profit. A prospect 
is an occurrence of minerals of potential value before that value has been 
determined by exploration and development. Mineral deposits include those 
containing metallic elements, such as gold, copper, lead, zinc, or iron; 
nonmetallic materials, such as asbestos, clay, phosphates, potash, or sulfur; 
and mineral fuels, such as coal or petroleum. Deposits worked for their 
aggregate of materials, such as sand, gravel, or dimension stone, are usually 
considered deposits of the rock itself. 
    Prospecting is generally done by groups of specialists. Such a group may 
consist of a geologist, a geophysicist, a mining engineer, and assistants as 
required for traversing and sampling. Specialists in photogeology and in 
geophysical and geochemical prospecting may be called in as needed. 
    Mineral prospecting normally proceeds from the general to the specific, 
from consideration of large regions to smaller favorable areas within the 
region, and finally to individual prospects. Following a preliminary 
investigation, including a library search and a study of available maps and 
reports, prospecting may often be concentrated on smaller areas immediately. 
Study of the geology will indicate what minerals might be found in a given 
area. Prospecting methods may be subdivided into direct and indirect methods. 
  
    Direct methods.  A map of some sort is required to prospect large areas 
systematically. Published geologic and topographical maps are suitable bases 
for plotting mineral occurrence or guides to minerals. Aerial photographs are 
excellent for such purposes, though ground control is necessary to produce 
maps from them. Photogeologic studies combined with ground checking may 
indicate such guides as soil staining, alteration, or structures in deeply 
weathered areas that might otherwise be missed. 
    Ore guides are mostly associations of geologic and other regional factors. 
The ore guides that are significant depend on the characteristic rock 
associations and distribution of the ore in any given region or locality. For 
example, mineral stream gravel or placer deposits generally indicate veins or 
lodes in the country rock of the backland. Local ore guides include rock 
alteration and channel ways such as fractures, faults, contacts between 
dissimilar rocks, and breccia pipes. Ore shoots are likely to occur where 
mineralizing solutions encounter easily replaced rock such as limestone, where 
veins cross contacts, at intersections of veins and faults, at rolls in 
faults, and along veins wherever there is a change in physical or chemical 
environment.  
    Any departure from normal structure, topography, rock color, or vegetation 
may prove significant. Old mine workings, dumps, prospect pits, and burrows of 
animals may yield information on mineralization and on the size and trend of 
deposits. See ORE AND MINERAL DEPOSITS. 
    At the point where the mineral indications go beneath the surface, 
trenches or test pits must be dug or bulldozed. Ground sluicing or 
hydraulicking may be used to advantage where water is abundant, slopes are 
moderately steep, and stream pollution is unobjectionable. Drilling is 
sometimes necessary in prospecting areas covered by swamp, water, or rock or 
where deep overburden makes trenching or pitting unduly expensive.  See  
BORING AND DRILLING (MINERAL). 
    Prospects are sampled to determine their composition. Methods for 
representative sampling depend on the size, character, and accessibility of 
the project. 
  
    Indirect methods.  In a large proportion of indirect prospecting 
geophysical methods are used, involving the measurement of physical quantities 
associated with buried mineral deposits and geological structures. The 
measurements are interpreted in terms of geology. Plausible assumptions are 
made about the subsurface, and the physical effects of assumed structures or 
deposits are computed or estimated and compared with the geophysical 
measurements. The assumptions are modified until there is reasonable agreement 
between the computations and observed data. 
    Selection of a suitable geophysical method is facilitated by information 
on the geologic habits and environment of the deposits sought and on their 
physical properties compared with the surrounding rocks. Magnetic methods and 
the various electrical methods are commonly used in prospecting for metallic 
minerals, because these minerals usually have magnetic and electrical 
properties that contrast with those of the surrounding rocks. Seismic and 
gravitational methods have been less used because measurable contrasts in 
elasticity and density are less common, but they have been indirectly useful 
in locating buried structures. Little information on depth is theoretically 
obtainable with magnetic, gravitational, self-potential, and thermal methods, 
where physical effects are produced by the bodies or structures themselves. 
Information on depth can usually be obtained with resistivity or seismic 
methods, where effects are produced by transmitting electrical or seismic 
energy through the ground. The use of airborne magnetic, electromagnetic, 
spectral, and radioactivity methods makes it possible to cover large and 
otherwise inaccessible areas at comparatively low cost. All three types of 
surveys may be made simultaneously from the same aircraft, at little more than 
the cost of a survey by a single method.  See  GEOPHYSICAL EXPLORATION; 
MAGNETOMETER; REMOTE SENSING. 
  
    Geochemical and botanical methods.  Geochemical prospecting involves the 
analysis of elements in the soils, rocks, surface and underground waters, 
organisms, and vegetation for the purpose of defining areas where the 
anomalous distribution of the elements indicates the presence of ore. 
Botanical prospecting involves analysis of elements found in deep-rooted 
plants or plants commonly associated with certain elements or mineral deposits 
and comparative study of morphological differences in plants growing in 
mineralized and nonmineralized areas.  See  GEOBOTANICAL INDICATORS; 
GEOCHEMICAL PROSPECTING; PETROLEUM PROSPECTING. 
 �34s[A. F. Banfield]
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�6v�2v�32sProstate gland   �21m �33s 
    A triangular body in men, the size and shape of a chestnut, that lies 
immediately in front of the bladder with its apex directed down and forward. 
It is found only in the male, having no female counterpart. The prostatic 
portion of the urethra extends through it, passing from the bladder to the 
penis. This organ contains 15 - 20 branched, tubular glands which form 
lobules. The gland ducts open into the urethra. Between the gland clusters, or 
alveoli, there is a dense, fibrous, connecting tissue, the stroma, which also 
forms a tough capsule around the gland, continuous with the bladder wall. 
Penetrating the prostate to empty into the urethra are the ejaculatory ducts 
from the seminal vesicles which are located above and behind the organ (see 
illustration). The prostatic gland secretes a viscid, alkaline fluid which 
aids in sperm motility and in neutralizing the acidity of the vagina, thus 
enhancing fertilization. 
    After middle age, the prostate is sometimes subject to new tissue growth, 
usually benign, that may result in interference with urine flow through the 
compressed urethra. 
 �34s[Walter J. Bock]
�33s 
  
  
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�6v�2v�32sProsthesis   �21m �33s 
    A device designed and constructed to fill space on or within the living 
body, to restore function on or within the body, to achieve a cosmetic 
reconstruction of a superficial feature of the body, or combinations of these. 
Because of trauma or disease, it is frequently necessary or appropriate 
treatment to remove surgically portions of the body, such as a whole limb, a 
segment of long bone, interfaces of a joint, blood vessels, or a valve within 
the heart. In many cases such body features and components can be replaced by 
components or devices which restore substantially appearance and function. The 
variety of prostheses which are designed and successfully implanted increases 
each year. Prostheses of proved efficacy are manufactured in standard ranges 
of dimensions and also to custom dimensions. 
 �34s[William Rostoker]
�33s 
  
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�6v�2v�32sProtactinium �21m �33s
    A chemical element, symbol Pa, atomic number 91. Isotopes of mass numbers
216, 217, and 222 - 238 are known, all of them radioactive. Only (sup 231)Pa
(the parent of actinium), (sup 234)Pa, and (sup 234)(sup m)Pa occur in nature.
The most important of these is (sup 231)Pa, an ALPHA-emitter with a half-life of
32,500 years. The artificial isotope, (sup 233)Pa, is important as an
intermediary in the production of fissile (sup 233)U. Both (sup 231)Pa and
(sup 233)Pa can be synthesized by neutron irradiation of thorium.  See
ACTINIUM; RADIOACTIVITY; URANIUM.
    Protactinium is, formally, the third member of the actinide series of
elements and the first in which a 5f electron appears, but its chemical
behavior in aqueous solution resembles that of tantalum and niobium more
closely than that of the other actinides.  See	NIOBIUM; TANTALUM.
    Metallic protactinium is silver in color, malleable, and ductile. Samples
exposed to air at room temperature show little or no tarnishing over a period
of several months. The numerous compounds of protractinium that have been
prepared and characterized include binary and polynary oxides, halides,
oxyhalides, sulfates, oxysulfates, double sulfates, oxynitrates, selenates,
carbides, organometallic compounds, and noble metal alloys.  See  ACTINIDE
ELEMENTS.
 �34s[Harold W. Kirby]
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�6v�2v�32sProtandry   �21m �33s 
    That condition in which an animal is first a male and then becomes a 
female. It occurs in many groups, including oysters and cyclostomes. The 
reverse condition is protogyny.  See  PROTOGYNY. 
 �34s[Tracy I. Storer]
�33s 
  
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�6v�2v�32sProteales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of only two families: the Proteaceae, with about 1400 species, and 
the Elaeagnaceae, with about 50. Within its subclass the order is marked by 
its flowers, which are strongly perigynous; that is, the perianth and stamens 
are united into a basal cup that is distinct from the ovary. The calyx is 
four-lobed and often corolla-like, and the true petals are much reduced or 
absent.  See  MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sProtective coloration   �21m �33s 
    When the color pattern of an animal increases the probability of its 
survival, the animal is said to be protectively colored. There are three types 
of protective coloration: cryptic or concealing coloration, which renders the 
animal inconspicuous; aposematic or warning coloration, which advertises the 
presence of an otherwise well-protected animal; and mimicry, whereby a species 
imitates an aposematic species. Batesian mimics are edible species which gain 
protection by mimicking genuine aposematics, whereas Muellerian mimicry 
involves the sharing of a pattern by two aposematic species, a type of double 
protection.  
    Cryptic coloration is widespread. The most common form is countershading, 
that is, dark pigmentation on the dorsal surface, light on the ventral 
surface. An example will explain its utility. When a small fish swims near the 
surface, a gull sees the dark dorsal surface blending into the dark waters, 
whereas a larger fish from below sees the light belly blending into the bright 
surface water. Background matching, however, may be much more precise, and 
correspondingly more difficult to interpret except as adaptation. It may 
involve morphology and behavior as well as color. Thus the walking stick 
insects of the family Phasmidae closely resemble twigs of the plants upon 
which they live, even to the angle at which they rest upon a branch. Numerous 
insects, when at rest, resemble the leaves of the plants upon which they feed. 
Kallima, the dead-leaf butterfly, exemplifies this (see illustration). 
    Aposematic coloration is displayed by animals that are distasteful to 
predators, such as some butterflies; those animals equipped with poison 
glands, such as many salamanders; and those animals, such as bees, which have 
stings or other formidable defenses. These may be in sharp contrast to 
patterns of related animals which are more subject to predation. See 
CHROMATOPHORE.  
 �34s[Edward O. Dodson]
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�6v�2v�32sProtein �21m �33s
    A polymeric compound made up of various amino acids as the monomeric
units. Proteins generally contain from 50 to 1000 amino acid residues per
polypeptide chain. The amino acids are joined by peptide bonds between the
ALPHA-carboxyl groups and the ALPHA-amino groups of adjacent amino acids in
which the ALPHA-amino group  of the first amino acid residue is usually free,
as shown below.  See  AMINO ACIDS; PEPTIDE.

		       R	   R'          R"
		       |	   |	       |
		  H2N--CH--CO--NH--CH--CO--NH--CH--CO

    Occurrence.  Proteins are of importance in all biological systems, playing
a wide variety of structural and functional roles. They form the primary
organic basis of structures such as hair, tendons, muscle, skin, and
cartilage. All of the enzymes, the catalysts in biochemical transformations,
are protein in nature. Many hormones, such as insulin and growth hormone, are
proteins. The substances responsible for oxygen and electron transport
(hemoglobin and the cytochromes, respectively) are conjugated proteins that
contain a metalloporphyrin as the prosthetic group. The chromosomes are highly
complex nucleoproteins, that is, proteins conjugated with nucleic acid. The
viruses are also nucleoprotein in nature. Thus, proteins play a fundamental
role in the processes of life.	See  NUCLEOPROTEIN.

    Specificity.  The linear arrangement of the amino acid residues in a
protein is termed its sequence (primary structure). The sequence in which the
different amino acids are linked in any given protein is highly specific and
characteristic for that particular protein.
    This specificity of sequence is one of the most remarkable aspects of
protein chemistry. The number of possible permutations of sequence in even so
small a protein as insulin, of molecular weight 5732 and with 51 amino acid
residues, is astronomic: 10(sup 51) permutations. Yet it has been established
that the pancreatic cell of a given species has only one of these possible
sequences. The elucidation of the mechanism conferring such a high degree of
specificity on the biosynthetic reactions by which proteins are built up from
free amino acids has been one of the key problems of modern biochemistry.  See
MOLECULAR BIOLOGY.

    Structure.	Finding the structure of proteins required the development of
many new techniques in protein chemistry. The strategy in the sequencing of a
protein has depended upon coupling of the free ALPHA-amino residue of the
polypeptide with a chemical, particularly phenyl isothiocyanate, that will
remove that residue under appropriate conditions and leave the remainder of
the polypeptide chain intact. At this point in the process it is possible to
identify the cleaved derivative of the first amino acid residue or the
remaining amino acids by a variety of techniques such as chromatography. The
stepwise process is then repeated in this manner as far as possible.
    From the proteins that have been sequenced thus far, it is clear that
proteins of similar function from different species and those with common
genetic origins have similar sequences. When there are differences in the
sequence, the changes are usually conservative, for example, the replacement
of one aliphatic amino acid by a different aliphatic amino acid.
    Proteins are not stretched polymers; rather, the polypeptide backbone of
the molecule can fold in several ways by means of hydrogen bonds between the
carbonyl oxygen and the amide nitrogen.
    The long polypeptide chains of proteins, particularly those of the fibrous
proteins, are held together in a rather well-defined configuration. The
backbone is coiled in a regular fashion, forming an extended helix. As a
result of this coiling, peptide bonds separated from one another by several
amino acid residues are brought into close spatial approximation. The
stability of the helical configuration can be attributed to hydrogen bonds
between these peptide bonds. The particular helical configuration which best
fits the available x-ray data is that proposed by L. Pauling and R. B. Corey
(see illustration). In this structure, each --NH group in peptide bond is
			O
			||
hydrogenbonded in the --C-- group three residues removed from it. #See
FIBROUS PROTEIN.
    In addition to the ALPHA-helical sections of proteins, there are also
segments that contain BETA-structures in which there are hydrogen bonds
between two polypeptide chains that run in parallel or antiparallel fashion.
    The third level of folding in a protein (that is, tertiary structure)
comes about through various interactions between different parts of the
molecule. The disulfide bridge formed between  two cysteine residues at
different linear locations in the molecule can stabilize parts of a
three-dimensional structure by introducing a primary valence bond as a
cross-link. Hydrogen bonds between different segments of the protein,
hydrophobic bonds between nonpolar side chains of amino acids such as
phenylalanine and leucine, and salt bridges such as those between positively
charged lysyl side chains and negatively charged aspartyl side chains all
contribute to the individual tertiary structure of a protein.
    Finally, for those proteins that contain more than one polypeptide chain
per molecule, there is usually a high degree of interaction between each
subunit, for example, between the ALPHA- and BETA-polypeptide chains of
hemoglobin. This feature of the protein structure is termed its quarternary
structure.  See  GLOBULAR PROTEIN.

    Properties.  The properties of proteins are determined in part by their
amino acid composition. For example, the net charge on the macromolecule at
any given hydrogen-ion concentration is largely a function of the relative
number of basic (lysine, histidine, and arginine) and dicarboxylic amino acids
(aspartic and glutamic acids). This net charge strongly influences the
solubility of the protein at different pH values since the solubility depends
in part on the proportion of polar groupings on the macromolecule. When the
hydrogen-ion concentration is high (low pH), the net charge is positive; when
the hydrogen-ion concentration is low (high pH), the net charge is negative.
The pH at which the net charge of the protein is zero is defined as the
isoelectric point (pI).
    As macromolecules that contain many side chains that can be protonated and
unprotonated depending upon the pH of the medium, proteins are excellent
buffers. The fact that the pH of blood varies only very slightly in spite of
the numerous metabolic processes in which it participates is due to the very
large buffering capacity of the blood proteins.
    As mentioned above, it is the interaction of the side chains of amino
acids that provides some proteins (enzymes) with their ability to catalyze
essential reactions in body metabolism. Likewise, the specific binding of
small ions to the side chains of proteins permits the transport of essential
metal ions into and out of cells, as in the action of the sodium pump within
the membrane of some cells.


    Biosynthesis.  Although a few proteins such as collagen are stable
indefinitely in adulthood, most body proteins are in a continual process of
degradation and synthesis (turnover). Thus, the half-life of serum proteins in
humans is about 10 days. The sequence of amino acid residues in a protein is
controlled by the sequence of the DNA as expressed in a molecule called
messenger RNA.	See  DEOXYRIBONUCLEIC ACID (DNA); RIBONUCLEIC ACID (RNA);
RIBOSOMES.
 �34s[Gertrude E. Perlmann ;James M. Manning]
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�6v�2v�32sProtein metabolism   �21m �33s 
    The transformation and fate of food proteins from their ingestion to the 
elimination of their excretion products. Proteins are of exceptional 
importance to organisms because they are the chief constituents, aside from 
water, of all the soft tissue of the body. Special proteins have unique roles 
as structural and functional elements of cells and tissues. Examples are 
keratin of skin, collagen of tendons, actin and myosin of muscle, the blood 
proteins, enzymes in all tissues, and portein hormones of the hypophysis.  See 
  BLOOD; ENZYME; HORMONE; MUSCLE; PROTEIN. 
    Protein is digested to amino acids in the gastrointestinal tract. These 
are absorbed and distributed among the different tissues, where they form a 
series of amino acid pools that are kept equilibrated with each other through 
the medium of the circulating blood. The needs for protein synthesis of the 
different organs are supplied from these pools. Excess amino acids in the 
tissue pools lose their nitrogen by a combination of transamination and 
deamination. The nitrogen is largely converted to urea and excreted in the 
urine. The residual carbon products are then further metabolized by pathways 
common to the other major foodstuffs - carbohydrates and fats.  See  AMINO 
ACIDS; CARBOHYDRATE; LIPID. 
    The recommended daily protein intake is 0.016 oz/lb (1 g/kg) body weight 
for adults, for example, 2.45 oz (70 g) for a 165-lb (74-kg) man. This is 
increased to 0.056 oz/lb (3.5 g/kg) for infants up to 1 year, and from 0.024 
to 0.032 oz/lb (1.5 to 2 g/kg) for growing children and for women during the 
latter half of pregnancy and during nursing. 
  
    Nutritive value.  All proteins are not equally nutritious. Animal proteins 
are generally superior to vegetable proteins. Rarely, this may result from 
resistance to digestion, which usually is counteracted by cooking or heating. 
A well-known example is soybean meal. The nutritive value of its protein is 
improved by heating because this destroys a substance in the meal which 
inhibits digestion by trypsin. Overheating lowers the nutritional value of 
proteins by making lysine unavailable. This is a problem of some concern in 
connection with the manufacture of prepared breakfast cereals. The major cause 
of poor nutritional value, however, is a low content or unavailability of one 
or more of the indispensable amino acids. Vegetable proteins tend to be 
lacking in lysine and tryptophan. 
  
    Digestion.  This occurs to a limited extent in the stomach and is 
completed in the duodenum of the small intestine. The main proteolytic enzyme 
of the stomach is pepsin, which is secreted in an inactive form, pepsinogen. 
Its transformation to the active pepsin is initiated by the acidity of the 
gastric juice and accelerated and completed by pepsin. Pepsin preferentially 
hydrolyzes peptide bonds containing an aromatic amino acid, and it requires an 
acid medium to function.  See  PEPSIN. 
    A second proteinase in the stomach, rennin, present only in infancy, is 
particularly adapted to the digestion of milk protein. Digestion is initiated 
by the well-known milk-clotting reaction used in cheese manufacture. Rennin 
requires less acid than pepsin to be active. In infancy, hydrochloric acid 
secretion by the stomach is not fully developed.  See  RENNIN. 
    The acid chyme is discharged from the stomach, containing partially 
degraded proteins, into a slightly alkaline fluid in the small intestine. This 
fluid is composed of pancreatic juice and succus entericus, the intestinal 
secretion. The pancreas secretes three known proteinases, trypsin, 
chymotrypsin, and carboxypeptidase. Trypsin and chymotrypsin are 
endopeptidases; that is, they cleave internal peptide bonds. The so-called 
peptidases are exopeptidases. They cleave terminal peptide bonds. Trypsin has 
a predilection for those containing the basic amino acid residues of lysine 
and arginine. These two proteinases perform the major share in hydrolyzing 
proteins to small peptides. Digestion to amino acids is completed by the 
exopeptidases. Carboxypeptidase acts on peptides from the free carboxyl end; 
aminopeptidases from the free amino end. Other peptidases act on di- or 
tripeptides, or peptides containing such special amino acids as proline. 
    The absorbed amino acids are carried by the portal blood system to the 
liver. From there, they are distributed to the rest of the body. 
 �34s[David M. Greenberg]
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�6v�2v�32sProteocephaloidea   �21m �33s 
    An order of tapeworms of the subclass Cestoda. With one exception, these 
worms are intestinal parasites of fresh-water fishes, amphibians, and 
reptiles. The holdfast organ bears four suckers and, frequently, an apical 
organ which may be suckerlike. The segmental anatomy is very similar to that 
of the Tetraphyllidea. Most authorities recognize two families, the 
Proteocephalidae and the Monticellidae.  See  CESTODA; TETRAPHYLLIDEA. 
 �34s[Clark P. Read]
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�6v�2v�32sProteomyxidia   �21m �33s 
    A subclass of Actinopodea including protozoan organisms without protective 
coverings (tests) or skeletal elements. Pseudopodia are fine, branched, and 
interconnected (reticulopodia), or filamentous (filopodia). Many are invaders 
of algae and other plants. There is one order, Proteomyxida, which contains 
two families, Pseudosporidae and Vampyrellidae.  See  ACTINOPODEA. 
 �34s[Richard P. Hall]
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�6v�2v�32sProterozoic   �21m �33s 
    A term, more or less synonymous with Algonkian, widely used for later 
Precambrian time in the twofold subdivision of the Precambrian. Many students 
of the Precambrian find this division unsuitable and no longer use it. The 
Geological Society of Canada has divided it into Lower, Middle, and Upper, and 
has endeavored to fit later Precambrian rocks and history into this framework. 
Many more recent investigators have used the term Proterozoic and this 
threefold subdivision for the youngest three divisions in the fourfold 
subdivision of the Precambrian.  See  PRECAMBRIAN. 
 �34s[J. Paul Fitzsimmons]
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�6v�2v�32sProtista   �21m �33s 
    A kingdom of organisms proposed by E. H. Haeckel in 1866 to include all 
unicellular organisms previously grouped under the phylum Thallobionta of the 
plant kingdom and under the phylum Protozoa of the animal kingdom. Also 
included are 12 groups of problematical single-celled organisms whose 
affinities with the plants or animals are not readily apparent. Organisms of 
the kingdom Protista lack definite cellular arrangement. Although they may be 
colonial in habit, all cells are identical, and there is no differentiation of 
cells for specific purposes or functions. Representative organisms classed 
under the Protista include the bacteria, molds, fungi, algae, diatoms, 
infusorians, foraminifers, and radiolarians.  See  PROTOZOA; THALLOBIONTA. 
 �34s[Daniel J. Jones]
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�6v�2v�32sProtobranchia   �21m �33s 
    An order of bivalves containing three marine superfamilies, of ancient 
origin, with primitive, solely respiratory ctenidia. The little-known 
edentulous Solemyacea may constitute a separate order or subclass. In the 
Nuculacea and Nuculanacea, the taxodont hinge bears two series of similar 
vertical teeth separated by the amphidetic ligament. Deep-sea exploration has 
revealed that, in certain abyssal faunas, protobranch species (Nuculanacea) 
may outnumber conventional lamellibranch bivalves.  See  BIVALVIA; MOLLUSCA. 
 �34s[Richard D. Purchon]
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�6v�2v�32sProtococcida   �21m �33s 
    An order of the protozoan subclass Coccidia in the class Telosporea, 
subphylum Sporozoa. This is a very small group containing perhaps six species 
in perhaps three genera. All are parasites of marine invertebrates. Only 
sexual reproduction is known.  See  COCCIDIA. 
 �34s[Norman D. Levine]
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�6v�2v�32sProtogyny   �21m �33s 
    A condition in hermaphroditic or dioecious animals in which the female 
reproductive structures mature before the male structures. It is of rare 
occurrence. Botanically, protogyny occurs in some plant species in which the 
stigma develops, withers, and dies before the anthers mature.  See  PROTANDRY. 
 �34s[Tracy I. Storer]
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�6v�2v�32sProton �21m �33s
    A particle that is the positively charged constituent of ordinary matter.
Together with the neutron, the proton is the building stone of all atomic
nuclei; a single proton constitutes the nucleus of the hydrogen atom. The most
important properties that characterize the proton are its charge, which is
identical in magnitude but of opposite sign to that of the electron (the
negatively charged constituent of ordinary matter) and has the value of 4.8033
x 10-(sup 10) esu = 1.6022 x 10-(sup 19) coulomb; its mass, 1.6726 x 10-(sup 24)
g = 1836.1msube (msube is the mass of the electron); its spin, 1/2PLANCK'S
CONSTANT = (1/2) 1.0546 x 10-(sup 27) erg-sec (PLANCK'S CONSTANT is Planck's
constant h divided by 2PI); its magnetic moment, 1.4106 x 10-(sup 23) erg/gauss;
its lifetime, which, according to all available evidence, is infinite; and the
fact that it obeys the Pauli exclusion principle, that is, it is a fermion
(obeys Fermi-Dirac statistics).  See  ELEMENTARY PARTICLE; NUCLEON.
 �34s[Emilio G. Segre]
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�6v�2v�32sProton-proton chain   �21m �33s 
    A group of nuclear reactions involving fusion of light nuclei that 
converts hydrogen into helium. It is believed to be the principal source of 
energy in main sequence stars of a little more than a solar mass and of 
less-massive stars. Completion of a chain results in the consumption of four 
protons, the synthesis of a helium nucleus and two neutrinos, and the release 
of 26.73 MeV of energy. The energy E reflects the mass difference between the 
four protons and the helium nucleus, and is calculated on the basis of the 
Einstein mass-energy equation E = mcsup2, where m is the mass difference and 
csup2 is the square of the velocity of light. Because hydrogen is the fuel 
consumed in the process, it is referred to as hydrogen burning by means of the 
proton-proton chain.  See  NUCLEAR FUSION; NUCLEAR REACTION; STELLAR EVOLUTION. 
 �34s[Georgeanne R. Caughlan]
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�6v�2v�32sPrototheria �21m �33s
    One of the four subclasses of the class Mammalia. Prototheria contains a
single order, the Monotremata. No ancestral genera of fossil monotremes are
known, and the structure of the living monotremes is so specialized that the
affinities of the Prototheria are largely conjectural. Most mammalogists
believe that the prototheres arose from a different stock of therapsid
reptiles than the one that gave rise to the Theria.
    No fossils earlier than the Pleistocene are known, and these come from
Australia. The duckbilled platypus and several species of the echidnas are
living representatives of this group. Everything indicates that the
Prototheria represent a very small and relatively unsuccessful group that has
miraculously survived in an isolated corner of the Earth.  See	MAMMALIA;
MONOTREMATA; THERAPSIDA.
 �34s[D. Dwight Davis /Frederick S. Szalay]
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�6v�2v�32sPrototype   �21m �33s 
    The term used at that stage in the design process when the component has 
been realized physically in a form that satisfies the functional, 
environmental, reliability, maintainability, packaging, and other 
requirements, but when the design does not necessarily reflect the techniques 
of manufacture by which it will be ultimately produced in quantity. Although 
considerations of the methods of manufacture play a role in the preliminary 
stages of the evolution of a component, early developmental versions, since 
they are produced in small quantities (one to several units), may not be 
designed to take advantage of the methods of fabrication peculiar to 
large-scale production.  See  PILOT PRODUCTION. 
 �34s[Robert W. Mann]
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�6v�2v�32sProtozoa �21m �33s
    A group of eukaryotic microorganisms traditionally classified in the
animal kingdom. Although the name signifies primitive animals, some Protozoa
(phytoflagellates and slime molds) show enough plantlike characteristics to
justify claims that they are plants.
    Protozoa are almost as widely distributed as bacteria. Free-living types
occur in soil, wet sand, and in fresh, brackish, and salt waters. Protozoa of
the soil and sand live in films of moisture on the particles. Habitats of
endoparasites very. Some are intracellular, such as malarial parasites in
vertebrates, which are typical Coccidia in most of the cycle. Other parasites,
such as Entamoeba histolytica, invade tissues but not individual cells. Most
trypanosomes live in the blood plasma of vertebrate hosts. Many other
parasites live in the lumen of the digestive tract or sometimes in coelomic
cavities of invertebrates, as do certain gregarines.  See  COCCIDIA;
GREGARINIA; TRYPANOSOMATIDAE.
    Many Protozoa are uninucleate, others are binucleate or multinucleate, and
the number also may vary at different stages in a life cycle. Protozoa range
in size from 1 to 10(sup 6) micrometers. Colonies are known in flagellates,
ciliates, and Sarcodina. Although marked differentiation of reproductive and
somatic zooids characterizes certain colonies, such as Volvox, Protozoa have
not developed tissues and organs.

    Morphology.  A protozoan may be a plastic organism (ameboid type) but
changes in form are often restricted by the pellicle. A protective layer is
often secreted outside the pellicle, although the pellicle itself may be
strengthened by incorporation of minerals. Secreted coverings may fit closely,
for example, the cellulose-containing theca of Phytomonadida and
Dinoflagellida, analogous to the cell wall in higher plants. The
dinoflagellate theca (Fig. 1a) may be composed of plates arranged in a
specific pattern. Tests, as seen in Rhizopodea (Arcellinida, Gromiida,
Foraminiferida), may be composed mostly of inorganic material, although
organic (chitinous) tests occur in certain species. Siliceous skeletons, often
elaborate, characterize the Radiolaria (Figs. 1d and 2c). A vase-shaped
lorica, from which the anterior part of the organism or its appendages may be
extended, occurs in certain flagellates (Fig. 1b) and ciliates (Fig. 1c).
Certain marine ciliates (Tintinnida) are actively swimming loricate forms.
    Flagella occur in active stages of Mastigophora and flagellated stages of
certain Sarcodina and Sporozoa. A flagellum consists of a sheath enclosing a
matrix in which an axoneme extends from the cytoplasm to the flagellar tip. In
certain groups the sheath shows lateral fibrils (mastigonemes) which increase
the surface area and also may modify direction of the thrust effecting
locomotion. Although typically shorter than flagella, cilia are similar in
structure.  See  CILIA AND FLAGELLA.
    Two major types of pseudopodia have been described, the
contraction-hydraulic and the two-way flow types. The first are lobopodia with
rounded tips and ectoplasm denser than endoplasm. The larger ones commonly
contain granular endoplasm and clear ectoplasm. Two-way flow pseudopodia
include reticulopodia of Foraminiferida and related types, filoreticulopodia
of Radiolaria, and axopodia of certain Heliozoia.
    In addition to nuclei, food vacuoles (gastrioles) in phagotrophs,
chromatophores and stigma in many phytoflagellates, water-elimination vesicles
in many Protozoa, and sometimes other organelles, the cytoplasm may contain
mitochondria, Golgi material, pinocytotic vacuoles, stored food materials,
endoplasmic reticulum, and sometimes pigments of various kinds.

    Nutrition. In protozoan feeding, either phagotropic (holozoic) or saprozoic
(osmotrophic) methods predominate in particular species. In addition,
chlorophyll-bearing flagellates profit from photosynthesis; in fact, certain
species have not been grown in darkness and may be obligate phototrophs.
    Phagotropic ingestion of food, followed by digestion in vacuoles, is
characteristic of Sarcodina, ciliates, and many flagellates. Digestion follows
synthesis of appropriate enzymes and their transportation to the food vacuole.
Details of ingestion vary. Formation of food cups, or gulletlike invaginations
to enclose prey is common in more or less ameboid organisms, such as various
Sarcodina, many flagellates, and at least a few Sporozoa. Entrapment in a
sticky reticulopodial net occurs in Foraminiferida and certain other
Sarcodina. A persistent cytostome and gullet are involved in phagotrophic
ciliates and a few flagellates. Many ciliates have buccal organelles
(membranes, membranelles, and closely set rows of cilia) arranged to drive
particles to the cytostome. Particles pass through the cytostome into the
cytopharynx (gullet), at the base of which food vacuoles (gastrioles) are
formed. Digestion occurs in such vacuoles.
    By definition saprozoic feeding involves passage of dissolved foods through
the cortex. It is uncertain to what extent diffusion is responsible, but
enzymatic activities presumably are involved in uptake of various simple
sugars, acetate and butyrate. In addition, external factors, for example, the
pH of the medium, may strongly influence uptake of fatty acids and phosphates.

    Reproduction.  Reproduction occurs after a period of growth which ranges,
in different species, from less than half a day to several months (certain
Foraminiferida). General methods include binary fission, budding, plasmotomy,
and schizogony. Fission, involving nuclear division and replication of
organelles, yields two organisms similar in size. Budding produces two
organisms, one smaller than the other. In plasmotomy, a multinucleate organism
divides into several, each containing a number of nuclei. Schizogony,
characteristic of Sporozoa, follows repeated nuclear division, yielding many
uninucleate buds.
    Simple life cycles include a cyst and an active (trophic) stage undergoing
growth and reproduction. In certain free-living and parasitic species, no cyst
is developed. Dimorphic cycles show two active stages; polymorphic show
several. The former include adult and larva (Suctoria); flagellate and ameba
(certain Mastigophora and Sarcodina); flagellate and palmella (nonflagellated;
certain Phytomonadida); and ameba and plasmodium (Mycetozoia especially).

    Parasitic protozoa.  Parasites occur in all major groups. Sporozoa are
exclusively parasitic, as are some flagellate orders (Trichomonadida,
Hypermastigida, and Oxymonadida), the Opalinata, Piroplasmea, and several
ciliate orders (Apostomatida, Astomatida, and Entodiniomorphida). Various
other groups contain both parasitic and free-living types. Protozoa also serve
as hosts of other Protozoa, certain bacteria, fungi, and algae.
    Relatively few parasites are distinctly pathogenic, causing amebiasis,
visceral leishmaniasis (kala azar), sleeping sickness, Chagas' disease,
malaria, tick fever of cattle, dourine of horses, and other diseases.  See
AMEBIASIS; CHAGAS' DISEASE; CILIOPHORA; CNIDOSPORA; LEISHMANIASIS; MALARIA;
SARCOMASTIGOPHORA; SLEEPING SICKNESS; SPOROZOA.
 �34s[Richard P. Hall]
�33s

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�6v�2v�32sProtractor   �21m �33s 
    An instrument used to construct and measure angles formed by the lines of 
a plane (see illustration). In its simplest form it consists of half a 
circular disk of metal or transparent material, with the bounding semicircle 
graduated in degrees, usually from 0 to 180DG. The midpoint of the diameter of 
the semicircle is marked. It serves as the vertex of angles constructed or 
measured.  See  ANGLE. 
 �34s[Leonard M. Blumenthal]
�33s 
  
  
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�6v�2v�32sProtura   �21m �33s 
    An order of primitive wingless insects belonging to the subclass 
Apterygota. These insects are less than 0.08 in. (2 mm) in length, elongate, 
fragile, eyeless, and from pale amber to white in color. Their metamorphosis 
is called anamorphosis; that is, a segment is added to the abdomen at each of 
the three molts, a condition found in some of the lower Arthropoda. Further, 
they are the only insects in which antennae are absent. The prothoracic legs 
functionally replace these structures. The Protura are found to inhabit moist 
and decaying vegetation or moss.  See  APTERYGOTA; INSECTA. 
 �34s[Harlow B. Mills]
�33s 
  
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�6v�2v�32sProustite   �21m �33s 
    A mineral having composition Ag3AsS3. It occurs in prismatic crystals 
terminated by steep ditrigonal pyramids, but is more commonly massive or in 
disseminated grains. Hardness is 2 - 2.5 (Mohs scale) and specific gravity is 
5.55. The luster is adamantine and the color ruby red. It is called light ruby 
silver in contrast to pyrargyrite, dark ruby silver. Proustite and pyrargyrite 
are found together in silver veins. Noted localities are at Chanarcillo, 
Chile; Freiberg, Germany; Guanajuato, Mexico; and Cobalt, Ontario, Canada.  
See  PYRARGYRITE. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sPseudoborniales   �21m �33s 
    An order of fossil plants found in Middle and Upper Devonian rocks. The 
group is related to Sphenophyllales and includes a single family and two 
monotypic genera. Pseudobornia ursina is known from Bear Island (north of 
Norway), Alaska, and Germany. Prosseria grandis is found in New York State. 
Sphenopsid characters are more firmly established in this order than in 
Hyeniales.  See  HYENIALES; SPHENOPHYLLALES; SPHENOPSIDA. 
 �34s[Harlan P. Banks]
�33s 
  
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�6v�2v�32sPseudomonadaceae   �21m �33s 
    A family of gram-negative, aerobic non-spore-forming bacteria. There are 
four genera: Pseudomonas, Xanthomonas, Zoogloea, and Gluconobacter. A few 
species in the genus Pseudomonas are pathogenic for plants, animals, and 
humans; nearly all Xanthomonas species are pathogenic for plants; and species 
in the genus Gluconobacter are used in industrial microbiological processes 
for the production of vinegar, gluconic acid, and L-sorbose. Motile species 
invariably possess one or more polar flagella. Photosynthetic pigments are not 
found, but other types of pigments such as pyocyanin and fluorescein are 
common.  
 �34s[William C. Haynes]
�33s 
  
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�6v�2v�32sPseudopetalichthyida   �21m �33s 
    An order of class Placodermi based on two genera, Pseudopetalichthys (see 
illustration) and Paraplesiobatis, from the Lower Devonian Hunsrueckschiefer 
of Germany. They were slender fishes, 4 to 6 in. (10 to 15 cm) long, with 
prominent, dorsally placed orbits, a ventral mouth, and a skull covered with 
many small, bony plates and large, lateral gill covers. There is a 
well-developed articulation between the cranium and fused anterior vertebrae. 
The shoulder girdle is short and not expanded into a trunk shield, the 
pectoral fins are narrow-based, and the pelvic fins are relatively long-based. 
The trunk and tail may be covered with scales, and there is a series of 
crested, median dorsal scales.  See  PLACODERMI. 
 �34s[Robert H. Denison]
�33s 
  
  
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�6v�2v�32sPseudophyllidea   �21m �33s 
    An order of tapeworms of the subclass Cestoda, parasitic in the intestine 
of all classes of vertebrates. Typically, the head is simple in structure with 
two groovelike attachment organs, the bothria. Most pseudophyllideans are 
segmented and polyzoic with replication of the reproductive systems, although 
there are a number which do not show such replication and are monozoic. 
    Dibothriocephalus latus, the broad or fish tapeworm of humans and certain 
piscivorous mammals, is a pseudophyllidean. In humans, this worm sometimes 
precipitates a pernicious anemia by competing with the host for vitamin B12. 
Larval pseudophyllideans are occasionally found as parasites in the 
extraintestinal tissues of humans, producing a condition known as sparganosis. 
  See  CESTODA; SPARGANOSIS; TAPEWORM DISEASE. 
 �34s[Clark P. Read]
�33s 
  
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�6v�2v�32sPseudoscorpionida   �21m �33s 
    An order of terrestrial Arachnida having the general appearance of 
miniature scorpions without the postabdomen and sting. The body length is 
seldom greater than 0.2 in. (5.0 mm). Typically, each finger of the anterior 
appendages, or chelicerae, has a serrula composed of a row of ligulate plates. 
Ducts of silk glands open near the end of the movable finger, often in 
connection with a simple or branched spinneret. The second pair of appendages, 
or palpi, are large and conspicuous, usually with glands that discharge venom 
through a terminal tooth on one or both of the chelal fingers. The four pairs 
of legs are ambulatory. 
    Pseudoscorpions feed chiefly on small arthropods and, although frequently 
found on birds, mammals, and insects, are considered nonparasitic. 
Pseudoscorpions are common in the nests of mammals, birds, and social insects, 
in woody debris and forest litter, under stones, and in crevices in the bark 
of trees. About 2000 species have been described.  See  ARACHNIDA. 
 �34s[C. Clayton Hoff]
�33s 
  
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�6v�2v�32sPseudosphaeriales (lichenized)   �21m �33s 
    An order of the class Ascolichenes. The order is also called the 
Pleosporales. They resemble the typical pyrenomycetous lichens except for the 
structure of the ascocarp, which is not a true perithecium. It is flask-shaped 
and lined with a layer of interwoven, branched pseudoparaphyses. The asci, 
with bitunicate walls, are located in scattered locules. 
    There are two major families: the larger one, Arthophyreniaceae, is a 
widespread family with at least 5 genera; the Mycoporaceae is a small family 
with two well-known genera, Dermatina and Mycoporellum. All of the species in 
this order are crustose and many lack a well-defined thallus.  See  
ASCOLICHENES.  
 �34s[Mason E. Hale, Jr.]
�33s 
  
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�6v�2v�32sPseudotuberculosis   �21m �33s 
    A disease of rodents and birds caused by the bacterium Pasteurella 
pseudotuberculosis. The disease is occasionally transmitted to humans. 
    This sporadic, or epizootic, plaguelike disease in rodents and animals 
causes small abscesses in the liver, spleen, and intestinal wall. In humans, 
the infection has been reported as an acute fatal septicemia in some cases. 
The organisms may localize in mesenteric lymph nodes of the ileocecal region 
and cause acute appendicitis and gastrointestinal symptoms. 
 �34s[Karl F. Meyer]
�33s 
  
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�6v�2v�32sPsilomelane   �21m �33s 
    A basic oxide of barium and manganese with the idealized composition 
BaMnMn8O16(OH)4. The mineral is not well crystallized, and typically occurs as 
fine-grained masses and crusts. The color is iron black to dark steel gray. 
The hardness is about 5.5 on Mohs scale, and the specific gravity is 4.71. 
Psilomelane often occurs admixed with other manganese oxides, chiefly 
pyrolusite, and with clay and hydrated iron oxides. 
 �34s[Clifford Frondel]
�33s 
  
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�6v�2v�32sPsilophytales   �21m �33s 
    A group long recognized as an order of fossil plants (subdivision 
Psilopsida) collected in rocks of Late Silurian and Devonian age. It has been 
subdivided into three categories whose descriptions include the chief kinds of 
plants formerly included in Psilophytales. They are given in this Encyclopedia 
as three classes of the division Rhyniophyta: Rhyniopsida, Zosterophyllopsida, 
and Trimerophytopsida.  See EMBRYOBIONTA; PALEOBOTANY; RHYNIOPHYTA; 
RHYNIOPSIDA; TRIMEROPHYTOPSIDA; ZOSTEROPHYLLOPSIDA. 
 �34s[Harlan P. Banks]
�33s 
  
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�6v�2v�32sPsilotophyta   �21m �33s 
    A division of the plant kingdom consisting of only two genera with three 
living species, Psilotum nudum, P. complanatum, and Tmesipteris tannensis. 
Psilotum is widespread in tropical and subtropical regions of both 
hemispheres, but Tmesipteris is confined to Australia and some of the Pacific 
islands. The Psilotophyta have the typical life cycle of vascular cryptograms, 
with an alteration of sporophyte and gametophyte generations, the sporophyte 
being much the larger and more complex. The Psilotophyta have no economic 
importance, but they are interesting as possible remnants of an ancient 
(Silurian and Devonian) group of plants, the Rhyniophyta, which is regarded as 
ancestral to all other vascular plants.  See  LYCOPODIOPHYTA; POLYPODIOPHYTA; 
PSILOPHYTALES; RHYNIOPHYTA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sPsittaciformes   �21m �33s 
    An order of birds containing the single family Psittacidae, the parrots. 
This group, with over 300 species, is predominantly tropical, but has extended 
as far north as the central United States (the extinct Carolina parakeet, 
Conuropsis carolinensis) and as far south as Tierra del Fuego (the Austral 
parakeet, Enicognathus ferrugineus). Only one species, the monk parakeet 
(Myiopsitta monachus) of southern South America, is known to build its own 
nest of sticks. All other parrots nest in holes or cavities. 
    Parrots are highly popular as cage birds; many have brilliantly colored 
plumage, and some have exceptional powers of vocal mimicry. The order is a 
strongly differentiated one, relatively uniform anatomically in spite of 
superficial diversity in size and form. Its relationships to other orders are 
uncertain.  See  AVES; PARROT. 
 �34s[Kenneth C. Parkes]
�33s 
  
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�6v�2v�32sPsittacosis   �21m �33s 
    An infection of humans derived from contact with birds, and a frequent 
infection in psittacines caused by the bacterium Chlamydia psittaci. Of the 
130 bird species involved, 71 are psittacine; parrots, parakeets, turkeys, 
ducks, and pigeons are the important sources. Inhalation of dust from infected 
droppings or handling of feathers, viscera, or waste contaminated by infected 
birds may cause human disease, which is usually respiratory and may range from 
fatal to subclinical. 
 �34s[Julius Schachter]
�33s 
  
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�6v�2v�32sPsocoptera   �21m �33s 
    An order of insects commonly referred to as book lice, bark lice, and 
psocids, the latter a general term for all members of the order. They are 
usually less than 0.25 in. (0.6 cm) long, though rarely some may reach about 
0.5 in. (1.2 cm). Wings may be absent, and when present are of differing 
distinctive venational types. 
    Book lice are most common among old papers on dusty shelves, in cereals, 
or other domestic situations. They are usually pale, wingless types of 
insects. Many bark lice, the majority winged, occur on the bark or foliage of 
trees, and some are found under dead bark or beneath stones. Nymphs of a few 
species occur on tree trunks as clusters of gregarious individuals, but 
disperse when mature. 
    Psocoptera are worldwide, especially in warm countries, and some 1300 
species are known. Current classification lists about 37 families for this 
group.  See  INSECTA. 
 �34s[Ashley B. Gurney]
�33s 
  
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�6v�2v�32sPsoriasis �21m �33s
    An inflammatory skin disease of unknown cause, usually chronic or
recurrent but often acute in nature.
    The local lesions occur predominantly at certain sites, such as elbows,
knees, and scalp. They consist of dull red, well-defined plaques or patches,
usually covered by distinctive silvery scales which when removed disclose tiny
capillary bleeding points. The lesions spread by peripheral extension and may
involve huge areas of the body. The plaques are inconstant in size, shape, and
location during the course of the disease, but they have a tendency to recur.
Tension, fatigue, and overtreatment often produce flareups.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sPsychoacoustics   �21m �33s 
    All of the psychological interactions between humans (and animals) and the 
world of sound. It encompasses all studies of the perception of sound, as well 
as the production of speech.  See  HEARING (HUMAN); SPEECH. 
 �34s[Lawrence E. Marks]
�33s 
  
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�6v�2v�32sPsychoanalysis   �21m �33s 
    In the narrow sense, a form of psychotherapy invented and described by 
Sigmund Freud. In a broader (and incorrect) sense, psychoanalysis refers to 
almost any form of psychotherapy other than behavior modification, but 
especially those forms that emphasize the therapist-client relationship and 
the view that the client may be unaware of the reasons for his or her actions. 
Included in this second category are viewpoints (for example, existential 
psychoanalysis) that are radically inconsistent with the spirit of Freud's 
ideas.  See  PSYCHOANALYTIC THEORY; PSYCHOTHERAPY. 
 �34s[Robert Hogan]
�33s 
  
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�6v�2v�32sPsychoanalytic theory   �21m �33s 
    A perspective on human nature - especially on the origins of neurosis - 
developed by Sigmund Freud. 
    Three primary assumptions characterize psychoanalytic theory. First, 
everything of importance in adult mental life is assumed to take place in the 
unconscious part of the psyche. Sexual and aggressive impulses make up most of 
the content of the unconscious mind; these unconscious sexual and aggressive 
thoughts, wishes, and impulses are a potential source of adult neurosis, and 
the material that must be analyzed during psychoanalysis. The second key 
assumption of psychoanalytic theory is that of psychic determinism - every 
mental event has a cause. Hence the term Freudian slip - lapses of memory, 
slips of the tongue, and other apparently trivial mistakes - are often subtle 
expressions of sex and aggression. But more importantly, psychoanalysis 
concentrates on these seemingly trivial events because, when suitably 
interpreted, they are thought to provide direct access to the unconscious 
mind. The third assumption is that adult personality is formed in childhood.  
See  NEUROSIS; PSYCHOANALYSIS; PSYCHOSIS; PSYCHOTHERAPY. 
 �34s[Robert Hogan]
�33s 
  
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�6v�2v�32sPsycholinguistics   �21m �33s 
    An area of study which draws from linguistics and psychology and focuses 
upon the comprehension and production of language. Although psychologists have 
long been interested in language, and the field of linguistics is an older 
science than psychology, scientists in the two fields have had little contact 
until the work of Noam Chomsky was published in the late 1950s. Chomsky's 
writing had the effect of making psychologists acutely aware of their lack of 
knowledge about the structure of language, and the futility of focusing 
attention exclusively upon the surface structure of language. As a result, 
psycholinguists, who have a background of training in both linguistics and 
psychology, have been attempting since the early 1960s to gain a better 
understanding of how the abstract rules which determine human language are 
acquired and used to communicate appropriately created meaningful messages 
from one person to another via the vocal-auditory medium. Research has been 
directed to the evolutionary development of language, the biological bases of 
language, the nature of the sound system, the rules of syntax, the nature of 
meaning, and the process of language acquisition. 
 �34s[David S. Palermo]
�33s 
  
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�6v�2v�32sPsychology, physiological and experimental   �21m �33s 
    Experimental and physiological psychology are aspects of the discipline of 
psychology. That discipline comprises a large number of subareas which include 
the study of individual differences, developmental factors, learning, 
motivation, perception, and social influences and applied areas such as 
clinical, counseling, and industrial psychology. The common core of psychology 
is its concern with the understanding and utilization of the systematic 
determinants of behavior. Because behavior is determined by the social world 
and by the biological nature of people, psychology is both a social science 
and a biological science. 
    Experimental psychology refers to both a method and a broad subject-matter 
area. As a method, experimental psychology uses certain procedures to obtain 
controlled observations which permit an accurate assessment of the 
relationships between the variables being studied. In the prototype design, a 
single variable (say, number of presentations) is varied, and the effect on a 
second variable (such as recall) is measured with all other conditions being 
held constant. More complex designs permit the control of two or more 
variables. Such experiments are most readily conducted under controlled 
laboratory conditions. However, the attempt to approximate the basic 
procedures of controlled observations underlie psychological research 
regardless of the setting or area. 
    Physiological psychology refers to the study of the interrelationships 
between the biology and the behavior of humans and animals. The predominant 
focus has been on the roles of sensory mechanisms, neurophysiology, and 
neurochemistry in mediating and modulating behavior. The procedures typically 
involve the modification of specific physiological systems and a determination 
of the effect on behaviors such as perception, learning, motivation, or 
performance. Because of the highly technical character of physiological 
psychology and its close relationship to other developed and developing 
disciplines such as biochemistry, neuroanatomy, pharmacology, and physiology, 
it is highly interdisciplinary. 
 �34s[Wilse B. Webb]
�33s 
  
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�6v�2v�32sPsychoneurosis   �21m �33s 
    Any of a number of seemingly unrelated mental illnesses; such as anxiety 
neurosis, phobic states, obsessive and compulsive states, and hysteria. 
Anxiety neurosis and neurocirculatory asthenia (its mild chronic form) are 
closely related to manic-depressive disease. Hysteria, which is essentially a 
condition found in adolescent girls and young women, is characterized by 
unconsciously motivated trances, seizurelike states, pseudoparalyses, and so 
forth. It is believed to have a particular type of psychopathology, as do the 
phobic and obsessive-compulsive states.  See  HYSTERIA; MENTAL ILLNESS; 
OBSESSIVE-COMPULSIVE REACTION; PHOBIC REACTION. 
 �34s[Raymond D. Adams]
�33s 
  
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�6v�2v�32sPsychopathy   �21m �33s 
    The most disabling and protracted of the social maladjustments; also 
referred to as sociopathy. Individuals suffering from such disturbances 
exhibit obscure types of disorder of character and personality. Afflicted 
individuals find it impossible to curb their impulses and conform to the 
demands of school, employment, and society. Some of these psychopathies may be 
conditioned by the environment in which the child is brought up, and others 
probably represent delays in the maturation or occult diseases of the brain. 
Psychiatry and mental health workers have found no means of correcting or 
effectively coping with these disorders. Many tend to disappear once adulthood 
is reached.  See  ABNORMAL BEHAVIOR; MENTAL ILLNESS. 
 �34s[Raymond D. Adams]
�33s 
  
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�6v�2v�32sPsychopharmacologic drugs   �21m �33s 
    Drugs that significantly influence behavior, the way a person thinks, 
feels, and acts. Psychopharmacologic drugs were first discovered accidentally 
through incidental behavioral effects noted in drug treatment of physical 
disease. They then were developed through chemical rearrangements of the 
parent compound and were tested on animals. Psychopharmacology, a science for 
the systematic study and synthesis of such drugs, was stimulated by the 
appearance of new drugs and their application to mental disorders, new 
laboratory methods, and extensive governmental financial support. 
    Psychopharmacologic drugs are used to treat mental and physical disorders 
by influencing behavioral excesses and imbalances. They are tools for research 
into social and biological processes which influence the organization of 
behavior. In humans, research is applied to normal or disturbed behavior, its 
subjective aspects (feeling, thought, and states of consciousness), objective 
aspects (sensorimotor and intellectual performance), and theoretical aspects 
(distribution of psychic energies, personality structure, and so on). Research 
on animals deals with drug effects on learning, maturation, drive (fear, 
hunger, sex, punishment, and reward), adaptation to stress and stimuli, motor 
activities, and associated neurochemistry. 
    Psychopharmacologic drugs are usually classified into seven groups: 
antipsychotics (major tranquilizers, neuroleptics); antidepressants (psychic 
energizers); anxiolytics (minor tranquilizers); sedative-hypnotics (sleeping 
medications); psychostimulants; psychodysleptics (psychotomimetics); and 
others (lithium, piracetam, and so on).  See  AMPHETAMINE; BARBITURATES; 
NARCOTIC; PSYCHOTOMIMETIC DRUG; TRANQUILIZER. 
 �34s[Daniel X. Freedman]
�33s 
  
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�6v�2v�32sPsychosis   �21m �33s 
    A term that designates severe psychiatric disorders. Psychosis refers 
usually to psychiatric conditions which are characterized, among other 
symptoms, by serious impairment of judgment and power of willing or 
determining (volition). The rigidity of the symptoms and the marked incapacity 
for corrective learning are outstanding characteristics of the term. Although 
Sigmund Freud stressed that psychotic patients suffer from ``a break with 
reality,'' neurotics suffer from an intrapsychic, unconscious conflict between 
instinctual and controlling forces. In practice there is no qualitative 
difference between psychosis and neurosis; it is rather one of degree. From a 
forensic (legal) viewpoint, the psychotic patient is not considered 
responsible for his acts or competent to enter legally binding agreements.  
See  NEUROSIS; PARANOIA; PARANOID STATE; SCHIZOPHRENIA. 
 �34s[Fredrick C. Redlich]
�33s 
  
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�6v�2v�32sPsychosurgery   �21m �33s 
    Any surgical procedure on the brain, such as prefrontal lobotomy, 
leukotomy, transorbital lobotomy, and topectomy, that has been recommended as 
treatment for various psychoses (particularly schizophrenia and psychotic 
depression), severe neuroses, and chronic painful conditions. Most 
psychiatrists will not agree to the use of psychosurgery in neurosis and 
depressions.  
    The techniques vary according to the specific procedure. In most cases the 
operation consists of severing bilaterally the white projection fibers to the 
prefrontal areas. It is used sparingly, even for the treatment of psychoses, 
and has been largely replaced by tranquilizing drugs.  See  NEUROSIS; 
PSYCHOSIS; TRANQUILIZER. 
 �34s[Fredrick C. Redlich]
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�6v�2v�32sPsychotherapy   �21m �33s 
    Any treatment or therapy that is psychological in nature. Although the 
term is popularly used as if there were one process or set of procedures that 
constitutes psychotherapy, this is true only in a very general way; in 
actuality, there are many (over 130) psychotherapies. In general, 
psychotherapeutic treatment has been perceived primarily as a treatment for 
the so-called psychoneurotic disorders.  See  NEUROSIS; PSYCHOSIS. 
    In addition to designating psychotherapies in terms of their theoretical 
or procedural differences, for example, psychoanalysis, client-centered 
therapy, or behavior therapy, the psychotherapies can also be classified in 
terms of the number or category of clients receiving therapy, or in terms of 
the length of psychotherapy. Thus therapies have been categorized as 
individual or group psychotherapy, family or marital therapy, child or adult 
psychotherapy, and brief or time-limited versus long-term psychotherapy. 
    Perhaps best known of all forms of psychotherapy is psychoanalysis. The 
goal of the therapy is to gradually overcome the patient's defenses and to 
help the patient attain insight into the respressed conflicts which are the 
source of difficulty. Apart from the historic departures of Alfred Adler and 
Carl Jung from the fold of psychoanalysis, and the subsequent theoretical 
modifications offered by Karen Horney and Harry Stack Sullivan, there has been 
the development of what is loosely termed psychoanalytic psychotherapy.  See  
PSYCHOANALYSIS; PSYCHOANALYTIC THEORY. 
    There are so many other psychotherapies that only a few can be mentioned. 
Client-centered therapy was developed by Carl Rogers during the 1940s. A major 
emphasis of this therapy is placed on the inherent capacity of the client to 
change as long as the therapist is emphatic and genuine and conveys 
nonpossessive warmth. 
    Another therapy which has received much attention is behavior therapy, 
which differs in noticeable ways from the psychoanalytic therapies. The focus 
is on ridding the patient of symptoms and in changing behavior. There is no 
concern with hypothesized inner conflicts or unconscious problems responsible 
for the symptoms; the therapist is generally more directive or active; and the 
length of therapy is much shorter. 
    Cognitive therapies have emerged which tend to emphasize faulty 
preceptions and cognitions (thoughts) as possible causes of personal 
difficulties. In these therapies attempts are made to change the client's 
cognitions so that they are more realistic and more positive. Also, there has 
been some integration of cognitive and behavioral emphases, so that some 
therapists refer to themselves as cognitive behaviorists. 
 �34s[Sol L. Garfield]
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�6v�2v�32sPsychotomimetic drug   �21m �33s 
    A class of drugs reliably inducing temporary states of altered perception, 
often with symptoms similar to those of psychosis. The drug experiences are 
clearly perceived, vivid, and remembered. Sensory input is heightened and 
subjective experience is intensified, but control is diminished. The subject's 
feelings and momentary perceptions gain an independence from the normal 
corrections of logic; whatever stray item occupies the attention (a sensation 
or an unguarded and unevaluated memory or thought) becomes at the moment 
compellingly significant. Thinking and perceiving of this order coexist with 
the capacity for, but not an interest in, normal thought and function. 
    These drugs have been called psychedelic (or mind-manifesting) because of 
the convincing clarity with which a normally suppressed, more sensory, 
plastic, and primitive part of the mind is revealed to the drugged subject, 
contrasting sharply with the capacity to focus, discriminate, and judge. 
    The two drugs of chief interest, mescaline and LSD-25 (lysergic acid 
diethylamide), are of ancient lineage. Mescaline, used in highly structured 
Amerindian tribal religious rituals, is derived from peyote buttons, which are 
the dried tops of a cactus found in the southwestern United States. LSD has 
been synthesized with compounds derived from ergot, which infests such grasses 
as rye. (Some ergot derivatives are useful in the treatment of migraine and in 
obstetrics for the postpartum constriction of the uterus and blood vessels.)  
See  ERGOT. 
    There are a number of other psychotomimetics (less potent than LSD but 
similar), such as those from mushrooms (psilocybin and psilocin) and those 
synthesized in the laboratory. Ring-substituted amphetamines resembling 
mescaline's structure (for example, ``STP'') have been produced in profusion. 
There are subtle differences among these newer compounds, some producing more 
or less euphoria and more or less dyscontrol and anxiety. 
 �34s[Daniel X. Freedman]
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�6v�2v�32sPsychrometer �21m �33s
    An instrument consisting of two thermometers which is used in the
measurement of the moisture content of air or other gases. The bulb or sensing
area of one of the thermometers either is covered by a thin piece of clean
muslin cloth wetted uniformly with distilled water or is otherwise coated with
a film of distilled water. The temperatures of both the bulb and the air
contacting the bulb are lowered by the evaporation which takes place when
unsaturated air moves past the wetted bulb. An equilibrium temperature, termed
the wet-bulb temperature (T sub W), will be reached; it closely approaches the
lowest temperature to which air can be cooled by the evaporation of water into
that air. The water-vapor content of the air surrounding the wet bulb can be
determined from this wet-bulb temperature and from the air temperature
measured by the thermometer with the dry bulb (T sub D) by using an expression
of the form e = e(sub SW)- aP [T(sub D) - T(sub W]. Here e is the water-vapor
pressure of the air, e(sub SW) is the saturation water-vapor pressure at the
wet-bulb temperature, P is atmospheric pressure, and a is the psychrometric
constant, which depends upon properties of air and water, as well as on speed
of ventilation of air passing the wet bulb.  See  PSYCHROMETRICS.
 �34s[Robert M. Schultz]
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�6v�2v�32sPsychrometrics   �21m �33s 
    A study of the physical and thermodynamic properties of the atmosphere. 
The properties of primary concern in air conditioning are (1) dry-bulb 
temperature, (2) wet-bulb temperature, (3) dew-point temperature, (4) absolute 
humidity, (5) percent humidity, (6) sensible heat, (7) latent heat, (8) total 
heat, (9) density, and (10) pressure. 
    The dry-bulb temperature is the ambient temperature of the air and water 
vapor as measured by a thermometer or other temperature-measuring device in 
which the thermal element is dry and shielded from radiation.  See  AIR 
TEMPERATURE; TEMPERATURE. 
    If the bulb of a dry-bulb thermometer is covered with a silk or cotton 
wick saturated with distilled water and the air is drawn over it at a velocity 
not less than 1000 ft/min (5 m/s), the resultant temperature will be the 
wet-bulb temperature. Where the dry-bulb and wet-bulb temperatures are the 
same, the atmosphere is saturated. 
    The dew-point temperature is the temperature at which the water vapor in 
the atmosphere begins to condense. This is also the temperature of saturation 
at which the dry-bulb, wet-bulb, and dew-point temperatures are all the same.  
See  DEW POINT. 
    The actual quantity of water vapor in the atmosphere is designated as the 
absolute humidity. Percentage or relative humidity is the ratio of the actual 
water vapor in the atmosphere to the quantity of water vapor the atmosphere 
could hold if it were saturated at the same temperature.  See  HUMIDITY. 
    Sensible heat, or enthalpy of dry air, is heat which manifests itself as a 
change in temperature.  See  ENTHALPY. 
    Latent heat, or enthalpy of vaporization, is the heat required to change a 
liquid into a vapor without change in temperature. Latent heat is sometimes 
referred to as the latent heat of vaporization and varies inversely as the 
pressure.  
    The total heat, or enthalpy, of the atmosphere is the sum of the sensible 
heat, latent heat, and superheat of the vapor above the saturation or 
dew-point temperature. Total heat is relatively constant for a constant 
wet-bulb temperature, deviating only about 1.5 - 2% low at relative humidities 
below 30%. 
    The density of the atmosphere varies with both altitude and percentage 
humidity. The higher the altitude the lower the density, and the higher the 
moisture content the lower the density.  See  DENSITY. 
    Atmospheric pressure is usually referred to as barometric pressure. 
Pressure varies inversely as elevation, as temperature, and as percentage 
saturation.  See  MOISTURE-CONTENT MEASUREMENT; PSYCHROMETER. 
 �34s[John Everetts, Jr.]
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�6v�2v�32sPteridospermae   �21m �33s 
    Seed ferns, extinct plants characterized by naked seeds borne on large 
fernlike fronds. Fossil evidence indicates that these plants reached their 
greatest abundance during the Pennsylvanian Period. Subsequently they 
declined, becoming extinct sometime during the Jurassic. 
    Because some seed ferns had rather long, slender stems with relatively 
small amounts of supporting tissue, it has been surmised that they were 
vinelike and required the support of more sturdy neighboring plants. Others 
grew erect without the aid of other plants and in their general appearance 
they superficially resembled modern tree ferns. Stems of pteridosperms were 
mostly unbranched. Near the apex of their stems they produced a crown of 
large, spirally arranged fronds (see illustration). Both the fronds of tree 
ferns and the pteridosperm leaves were compound. Slender adventitious roots 
were borne on the lower parts of the stem.  See  PALEOBOTANY. 
 �34s[Wilson N. Stewart]
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�6v�2v�32sPterobranchia   �21m �33s 
    A group of animals regarded as a class of the Hemichordata. They are all 
marine, widely distributed but not common, benthonic (chiefly deep-sea) 
inhabitants. All are small or microscopic, sessile, tube-dwelling (tubicolous) 
organisms. They may be colonial or pseudocolonial with a cuticular skeleton. A 
U-shaped gut is present. A lophophore of one or several pairs of arms arises 
from the collar or middle of the three body segments.  See  HEMICHORDATA. 
 �34s[Theodore H. Bullock]
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�6v�2v�32sPteropsida   �21m �33s 
    A large group of vascular plants characterized by having parenchymatous 
leaf gaps in the stele and by having leaves which are thought to have 
originated in the distant past as branched stem systems. Some botanists regard 
the Pteropsida as a natural group which they recognize as a class, 
subdivision, or division. Others regard it as an artificial assemblage of 
plants that have undergone certain similar changes from a rhyniophyte 
ancestry. The various components of the Pteropsida are here treated as three 
separate divisions under the names Magnoliophyta, Pinophyta, and 
Polypodiophyta. See MAGNOLIOPHYTA; PINOPHYTA; POLYPODIOPHYTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sPterosauria   �21m �33s 
    An extinct order of flying reptiles (Archosauria) of the Mesozoic Era. 
Impressions show that a thin membrane of skin extended from the side of the 
body and arm to the enormously elongated fourth finger of each hand and formed 
a wing analogous to that of a bat (see illustration). The first three fingers 
were short and clawed; the fifth was absent. A large keeled sternum supported 
strong wing muscles, and a slender coracoid bone braced the shoulder joint 
against the sternum in a fashion similar to that of birds. Pterosaurs had 
large, lightly constructed skulls; small bodies; light, hollow bones; and 
small, five-toed rear feet.  See  ARCHOSAURIA; REPTILIA. 
 �34s[Joseph T. Gregory]
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�6v�2v�32sPterygota   �21m �33s 
    The larger of two subclasses of Insecta. All have wings in the adult stage 
or have been derived from winged ancestors; that is, if wingless, they are 
secondarily so. The primitive pterygotes make up the section Paleoptera; the 
mayflies, dragonflies, and damselflies are in this category. All other 
pterygotes constitute the section Neoptera. The more primitive Neoptera have 
exopterygote development, as in the grasshoppers, true bugs, and others, 
whereas the more specialized Neoptera have endopterygote development, as in 
the butterflies, moths, beetles, wasps, and so on.  See  APTERYGOTA; 
ENDOPTERYGOTA; EXOPTERYGOTA; INSECTA. 
 �34s[Frank M. Carpenter]
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�6v�2v�32sPtychodactiaria   �21m �33s 
    An order of the zoantharian anthozoans of the phylum Coelenterata. This 
group of solitary sea anemones is known only from two genera, Ptychodactis and 
Dactylanthus from the Arctic and Antarctic.  See  ANTHOZOA; COELENTERATA. 
 �34s[Cadet Hand]
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�6v�2v�32sPtyctodontida   �21m �33s 
    One of nine orders of the class Placodermi. These are small extinct fishes 
of the Devonian and lowest Mississippian of North America, Eurasia, Africa, 
and Australia. They include both marine and fresh-water species, and although 
usually only the stout tooth plates are preserved, the ossified internal 
skeleton is found in two genera.  See  PLACODERMI. 
 �34s[Robert H. Denison]
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�6v�2v�32sPublic health   �21m �33s 
    Organized social action for the maintenance and promotion of health. This 
broad concept has evolved from much more restricted definitions which limited 
public health to the actions of governmental health agencies directed at the 
prevention of infectious diseases. The advances of medical science and the 
growing complexity of society have made more imperative the need for 
extensions of organized social action for the preservation of the individual's 
health.  
    In broad terms, public health is concerned with determining the health 
status of the population (vital statistics), the genetic and bionomic factors 
in health and disease (epidemiology), and with building upon this knowledge 
the structure and function of organized health programs and services (public 
health administration). 
 �34s[Edward M. Cohart]
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�6v�2v�32sPulley   �21m �33s 
    A wheel with a flat, crowned, or grooved rim used with a flat belt, 
V-belt, or a rope to transmit motion and energy. Pulleys for use with V-belt 
and rope drives have grooved surfaces and are usually called sheaves. A 
combination of ropes, pulleys, and pulley blocks arranged to gain a mechanical 
advantage, as for hoisting a load, is referred to as block and tackle.  See  
BELT DRIVE; BLOCK AND TACKLE. 
    Pulleys for flat belts are made of cast iron, fabricated steel, wood, and 
paper. A particular pulley design must be based on such considerations as the 
ability to resist shock, to conduct heat, and to resist corrosive 
environments. The face must be smooth enough to minimize belt wear; yet there 
must be adequate friction between belt and pulley face to carry the load. 
    The two common types of V-belt pulley are pressed-steel and cast-iron. The 
pressed-steel pulleys are suitable for single-belt drives. For multiple-belt 
drives, or in single-belt drives where pulley mass should be high to get a 
flywheel effect, cast-iron pulleys are used. 
 �34s[John R. Zimmerman]
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�6v�2v�32sPulmonata   �21m �33s 
    A subclass of the molluscan class Gastropoda, containing about 23,500 
species of snails that are grouped into three superorders: Systellommatophora, 
Basommatophora, and Stylommatophora. Pulmonates include most of the common 
snails found on land or in freshwaters, although a few prosobranchs have 
invaded both habitats. Certain pulmonates are intertidal to subtidal on rocky 
shores of tropical and temperate regions.  See  BASOMMATOPHORA; 
STYLOMMATOPHORA; SYSTELLOMATOPHORA. 
    Fresh-water pulmonates belong to the superorder Basommatophora and are 
adapted to make use of accidental transport on birds or insects from one 
isolated body of water to another. Mostly they have short life-spans, and 
their numbers vary dramatically with the seasons. 
    Land pulmonates belong to the superorder Stylommatophora and are 
marginally terrestrial in that they can be active only when the humidity in 
their microhabitat is 90% or above. Thus many are cryptic or nocturnal in both 
feeding and reproduction. They can survive in deserts by estivating for up to 
6 years.  See  GASTROPODA. 
 �34s[G. Alan Solem]
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�6v�2v�32sPulsar   �21m �33s 
    A celestial radio source producing intense short bursts of radio emission. 
A total of about 330 pulsars have been found, and hundreds of thousands of 
pulsars must exist in the Milky Way Galaxy - most of them too distant to be 
detected with existing radio telescopes.  See  RADIO ASTRONOMY; RADIO SOURCES 
(ASTRONOMY).  
    Pulsars are distinguished from all other types of celestial radio sources 
in that their emission, instead of being constant over time scales of years or 
longer, consists of periodic sequences of brief pulses. The interval between 
pulses, or pulse period, is nearly constant for a given pulsar, but for 
different sources ranges from approximately 1.5 ms to 4 s. The bursts of 
emission are generally confined to a window whose width is a few percent of 
the interpulse period. 
    Compelling evidence exists that pulsars are neutron stars, the collapsed 
cores left behind when certain types of moderate- to high-mass stars become 
unstable and undergo supernova explosions. A newly formed neutron star spins 
rapidly (perhaps many times per second) and has an extremely strong magnetic 
field strength. The neutron star's spin causes the beamed emission to sweep 
around the sky like a rotating searchlight, thereby causing the periodic 
pulses observed from a particular location such as the Earth. 
    Pulsars have provided astronomers with a unique set of probes for the 
investigation of the diffuse gas and magnetic fields in interstellar space. 
Measurement of absorption at 1420 MHz, the frequency of the hyperfine 
transition in ground-state neutral hydrogen atoms, gives information on the 
structure of gas clouds, and in many cases provides an estimate of the pulsar 
distance. The broadband, pulsed nature of pulsar signals makes them ideal for 
measurements of dispersion by ionized gas. 
 �34s[Joseph H. Taylor, Jr.]
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�6v�2v�32sPulsed gas laser �21m �33s
    A device which uses pulsed gas discharges to achieve the population
inversion required for laser action. Pulsed gas discharges permit a further
departure from equilibrium than is possible in a continuous discharge. Thus
pulsed laser action can be obtained in some additional gases which could not
be made to lase continuously. In some of them the length of the laser pulse is
limited when the lower state is filled by stimulated transitions from the
upper level, and so introduces absorption at the laser wavelength. An example
of such a self-terminating laser is the nitrogen laser, which gives pulses of
several nanoseconds duration at a wavelength of 337 nanometers in the
ultraviolet. Very powerful laser radiation in the vacuum ultraviolet region,
between 100 and 200 nm, can be obtained from short-pulse discharges in
hydrogen, and in rare gases such as xenon at high pressure. (High pressure
leads to higher power.) When the gas pressure is too high to permit an
electric discharge, excitation may be provided by an intense burst of fast
electrons from a small accelerator, the so-called E beam.
    Some gases, notably carbon dioxide, which can provide continuous laser
action, also can be used to generate intense pulses of microsecond duration.
For this purpose, gas pressures of about 1 atm 10(sup 5) Pa are used, and the
electrical discharge takes place across the diameter of the laser column,
hence the name transverse-electrical-atmospheric (TEA) laser.  See
GAS-DISCHARGE LASER; LASER.
 �34s[Stephen F. Jacobs ;Arthur L. Schawlow]
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�6v�2v�32sPulse generator   �21m �33s 
    An electronic circuit capable of producing a waveform that rises abruptly, 
maintains a relatively flat top for an extremely short interval, and then 
rapidly falls to zero. A relaxation oscillator, such as a multivibrator, may 
be adjusted to generate a rectangular waveform having an extremely short 
duration, and as such it is referred to as a pulse generator. However, there 
is a class of circuits whose exclusive function is generating short-duration, 
rectangular waveforms. These circuits are usually specifically identified as 
pulse generators. An example of such a pulse generator is the triggered 
blocking oscillator, which is a single relaxation oscillator having 
transformer-coupled feedback from output to input.  See  BLOCKING OSCILLATOR; 
MULTIVIBRATOR; RELAXATION OSCILLATOR. 
    Pulse generators sometimes include, but are usually distinguished from, 
trigger circuits. Trigger circuits, generate a short-duration, fast-rising 
waveform for initiating or triggering an event or a series of events in other 
circuits. In the pulse generator, the pulse duration and shape are of equal 
importance to the rise and fall times.  See  TRIGGER CIRCUIT. 
    The term pulse generator is often applied not only to an electronic 
circuit generating prescribed pulse sequences but to an electronic instrument 
designed to generate sequences of pulses with variable delays, pulse widths, 
and pulse train combinations, programmable in a predetermined manner, often 
microprocessor-controlled.  
    A network, formed in such a way as to simulate the delay characteristics 
of a lossless transmission line, and appropriate switching elements to control 
the duration of a pulse form the basis for a variety of types of pulse 
generators. Some delay-line-controlled pulse generators are capable of 
generating pulses containing considerable amounts of power for such 
applications as modulators in radar transmitters.  See  DELAY LINE; 
WAVE-SHAPING CIRCUITS. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sPulse jet   �21m �33s 
    A type of jet engine characterized by periodic surges of thrust. The pulse 
jet engine was widely known for its use during World War II on the German V-1 
missile (see illustration). The basic engine cycle was invented in 1908. The 
inlet end of the engine is provided with a grid to which are attached flap 
valves. These valves are normally held by spring tension against the grid face 
and block the flow of air back out of the front of the engine. They can be 
sucked inward by a negative differential pressure to allow air to flow into 
the engine. Downstream from the flap valves is the combustion chamber. A fuel 
injection system is located at the entrance to the combustion chamber. The 
chamber is also fitted with a spark plug. Following the combustion chamber is 
a long exhaust duct which provides an inertial gas column. 
    Pulse jets have also been used to propel radio-controlled target drones 
and experimental helicopters. In the latter case, they were mounted on the 
blade tips for directly driving the rotor. The high fuel consumption, noise, 
and vibrations generated by the pulse jet limit its scope of applications.  
See  PROPULSION. 
 �34s[Benjamin Pinkel]
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�6v�2v�32sPulse modulation �21m �33s
    A system of modulation in which the amplitude, duration, position, or mere
presence of discrete pulses may be so controlled as to represent the message
to be communicated. These several forms of pulse modulation are commonly
called, respectively, pulse-amplitude modulation (PAM), pulse-duration
modulation (PDM), pulse-position modulation (PPM), and pulse-code modulation
(PCM).	See  AMPLITUDE MODULATION; FREQUENCY MODULATION; MODULATION; PHASE
MODULATION.
    Ease of multiplexing channels by time division is one of the important
economic advantages of all forms of pulse modulation. Circuits for
accomplishing this are simple and low in cost, and because this low cost is
shared by a number of channels, the cost per channel is even lower. When pulse
modulation is applied to long-distance communications, these time-division
techniques also permit substantial simplification at way stations and
branching points. Because each information-bearing pulse keeps its
individuality in journeying from the first transmitter to the last receiver,
it is comparatively easy to drop and add message channels at various
intermediate points along the way.

    All forms of pulse modulation may be defined in terms of pulse carrier,
and all involve sampling; in addition, PCM implies quantization and coding.
    A pulse carrier (Fig. 1) consists of a series of regularly recurrent
pulses. In general, the power associated with each pulse differs essentially
from zero only during a limited interval of time, which is the pulse width.
Sampling is a process of extracting successive portions of predetermined
duration taken at regular intervals from a continuously varying magnitude-time
wave.

    Pulse-amplitude modulation.  Pulse-amplitude modulation is amplitude
modulation of a pulse carrier. The modulated wave (Fig. 1) is linearly
proportional to equally spaced samples of the modulating wave. Chief interest
in PAM lies in its application to time-division multiplexing.  See
MULTIPLEXING.

    Pulse-duration modulation.	Pulse-duration modulation is modulation of a
pulse carrier wherein the value of each instantaneous sample of a modulating
wave produces a pulse of proportional duration (Fig. 1) by varying the
leading, trailing, or both edges of the pulse. PDM is also termed pulse-length
modulation or pulse-width modulation. In constrast to PAM, PDM is able to
trade extra bandwidth for noise reduction. Only the position of the modulated
edge or edges conveys information, and the part of each PDM pulse that conveys
no information represents wasted pulse power. When this wasted power is
subtracted from PDM the result is PPM.

    Pulse-position modulation.	Pulse-position modulation is modulation of a
pulse carrier wherein the value of each instantaneous sample of a modulating
wave varies the position in time of a pulse relative to its unmodulated time
of occurrence (Fig. 1). Channel pulses, one for each channel, are transmitted
in turn and are preceded by a synchronizing pulse called a marker. This array
of marker-plus-channel pulses repeats itself every 125 microseconds and is
called a frame.
    In practical applications, even though PPM is more efficient than PDM,
both are highly inefficient when used for certain purposes, for example, when
used for multiplexing ordinary telephone channels. Consequently, communication
engineers have shown considerable interest in PCM, which not only is more
efficient but also possesses many other very important advantages.

    Pulse-code modulation.  Pulse-code modulation is a method of transmitting
continuously varying message waves in which (1) the message wave is sampled,
(2) the value of each sample is replaced by the closest one of a finite set of
permitted values, and (3) these permitted values are then each unambiguously
represented by some one of the possible patterns of N on-or-off pulses. These
three operations are known as sampling, quantizing, and coding, respectively.
    Quantization changes a continuously varying signal to a stepped signal.
Graphically quantization means that the straight line representing the
relationship between input and output has been replaced by a stair-step
function. Clearly, by using small enough steps, errors may be cut to an
arbitrary minimum (Fig. 2).

    When speech samples are quantized, the steps are tapered as shown in Fig.
2b. By tapering most of the steps logarithmically, nearly uniform percentage
precision is obtained over most of the range, and far fewer steps are
required. To reproduce telephonic speech to a high degree of fidelity, 128
logarithmic steps will suffice.
    In PCM these 128 values are represented by a 7-pulse code. Each voice
sample is represented with adequate accuracy by some one of the possible
patterns of 7 on-or-off pulses. Channels are multiplexed by time division.
    Similarly, other forms of communication may also be represented by
on-or-off pulses. In practice, a single PCM system may process many kinds of
communications and may provide many channels of each kind.
    Ordinarily, PCM systems are called upon to transmit large numbers of
on-or-off pulses per second. For example, for each telephone conversation, PCM
transmits 56,000 on-or-off pulses per second in each direction of
transmission. A one-way television signal might be represented by 70,000,000
on-or-off pulses per second.
    By having to make only on-or-off distinctions, PCM is able to deliver a
high-quality signal even when noise and interference are so bad that it is
barely possible to recognize the pulses. At each regenerative repeater, just
so long as incoming pulses are correctly identified, a new pulse generator may
be caused to generate new pulses of correct magnitude, waveform, and timing.
 �34s[Harold S. Black]
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�6v�2v�32sPulse modulator �21m �33s
    A device for the pulse modulation of a radio-frequency carrier signal.
See  PULSE MODULATION.

    Pulse-amplitude scheme.  Pulse-amplitude modulation (PAM) requires least
complexity of any of the pulse-modulation schemes and is usually used in the
initial stages of other pulse-modulation methods. The amplitude of the input
signal is sampled periodically at a fixed frequency, producing a sequence of
pulses whose amplitude varies in accordance with the modulating signal. In a
common practical system the trains of pulses representing several input
signals are interlaced to form a time-division multiplex system.
    Illustration a is a block diagram of the essential parts of a four-channel
time-division multiplex transmitter that uses PAM. Illustration b depicts some
of the representative waveforms. The principal elements of this system are the
distributor, or commutator, which can be either electromechanical or
electronic; a source of square waves at the sampling frequency fsubs; and a
source of square waves of one-half the sampling frequency fsubs/2 obtained
from a bistable circuit arranged to provide both phases of the square wave.
    In a practical PAM system, it is necessary to provide guard time between
pulses to reduce interchannel crosstalk. Illustration a shows the distributor
output pulses passing through a synchronous gate operating at twice the
sampling frequency. This gate selects the center portion of each pulse in the
train. The resultant output is a series of amplitude-modulated pulses having
only one-half of their original width with blank time between each pulse. The
waveform in the output line with signals due to channel A alone is shown in
illustration b.

    Pulse-duration scheme.  Pulse-duration modulation (PDM) can be produced by
converting a train of pulses from a PAM system. If the signals generated in a
PAM system are added to constant-amplitude sampling-frequency pulses, a train
of pulses of varying amplitude, but with a finite minimum value, is produced
at the output of the adder. The resulting pulses can be differentiated and
clipped, resulting in a series of reverse sawtooth shapes in which the width
of each sawtooth becomes proportional to the amplitude of the particular
pulse. When passed through a limiter, the desired sequence of pulses, having
constant amplitude and varying in width in accordance with the signal, is
generated.

    Pulse-position scheme.  Pulse-position modulation (PPM), can be derived
from PDM. In one method, the variable-width pulses are first differentiated
and then rectified to permit only the negative peaks to trigger a monostable
multivibrator. In the latter, each negative trigger pulse causes a pulse of
constant amplitude and width to be generated, whose time position is
proportional to the amplitude of the modulating signal (input).

    Pulse-code scheme.	The sampling techniques needed for pulse-code
modulation (PCM) can be similar to those employed in PAM. To quantize a
signal, the amplitude sample is first matched with a series of amplitude
standards and then converted to the one with the nearest standard value. One
method of achieving this is to utilize a multiplicity of regenerative devices,
each of which changes between its OFF and ON state within a very small input
voltage range. By proper biasing, these devices can be set to operate at any
given input voltage level. When the inputs and outputs of, for example, eight
of these level selectors work in parallel, it is possible to convert input
signals of varying amplitudes to outputs of eight discrete amplitudes.	See
ANALOG-TO-DIGITAL CONVERTER.
    Coding can be accomplished with the aid of a special beam code tube,
consisting of a cathode-ray tube with conventional gun and electrostatic
deflection plates, a special aperture plate placed perpendicularly to the
electron beam and an anode. The aperture plate has horizontal slots in eight
vertical rows in accordance with the pulse code.
 �34s[Edward L. Ginzton /Charles L. Alley]
�33s


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�6v�2v�32sPulse transformers   �21m �33s 
    Iron-cored devices which are used in the transmission and shaping of 
low-power pulses whose widths range from a fraction of a microsecond to about 
25 microseconds. Among the extensive applications of pulse transformers are 
the following: (1) to couple between the stages of pulse amplifiers; (2) to 
invert the polarity of a pulse; (3) to change the amplitude and impedance 
level of a pulse; (4) to differentiate a pulse; (5) to effect ``dc isolation'' 
between a source and a load; (6) to act as coupling element in certain 
pulse-generating circuits. 
    In many cases the functions listed above can be accomplished as well or 
better with transistor circuits. However, the pulse transformer, being a 
passive element, has none of the instability associated with active circuits. 
    A pulse transformer behaves as a reasonable approximation to a perfect 
transformer when used in connection with the fast waveforms it is intended to 
handle. The core of a pulse transformer is usually molded from a magnetic 
ceramic such as sintered manganese-zinc ferrite.  See  TRANSFORMER. 
 �34s[Christos C. Halkias]
�33s 
  
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�6v�2v�32sPumice   �21m �33s 
    A rock froth, formed by the extreme puffing up of liquid lava by expanding 
gases liberated from solution in the lava prior to and during solidification. 
Some varieties will float in water for many weeks before becoming waterlogged. 
Typical pumice is siliceous (rhyolite or dacite) in composition, but the 
lightest and most vesicular pumice (known also as reticulite and thread-lace 
scoria) is of basaltic composition.  See  LAVA; VOLCANIC GLASS. 
 �34s[Gordon A. Macdonald]
�33s 
  
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�6v�2v�32sPump   �21m �33s 
    A machine that draws a fluid into itself through an entrance port and 
forces the fluid out through an exhaust port (see illustration). A pump may 
serve to move liquid, as in a cross-country pipeline; to lift liquid, as from 
a well or to the top of a tall building; or to put fluid under pressure, as in 
a hydraulic brake. These applications depend predominantly upon the discharge 
characteristic of the pump. A pump may also serve to empty a container, as in 
a vacuum pump or a sump pump, in which case the application depends primarily 
on its intake characteristic.  See  CENTRIFUGAL PUMP; COMPRESSOR; DISPLACEMENT 
PUMP; FAN; FUEL PUMP; PUMPING MACHINERY; VACUUM PUMP. 
 �34s[Elliott F. Wright]
�33s 
  
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�6v�2v�32sPumping machinery   �21m �33s 
    Devices which convey fluids, chiefly liquids, from a lower to a higher 
elevation or from a region of lower pressure to one of higher pressure. 
Pumping machinery may be broadly classified as mechanical or as 
electromagnetic.  
    In mechanical pumps the fluid is conveyed by direct contact with a moving 
part of the pumping machinery. The two basic types are (1) velocity machines, 
centrifugal or turbine pumps, which impart energy to the fluid primarily by 
increasing its velocity, then converting part of this energy into pressure or 
head, and (2) displacement machines with plungers, pistons, cams, or other 
confining forms which act directly on the fluid, forcing it to flow against a 
higher pressure.  See  CENTRIFUGAL PUMP; DISPLACEMENT PUMP. 
    Where direct contact between the fluid and the pumping machinery is 
undesirable, as in atomic energy power plants for circulating liquid metals 
used as reactor coolants or as solvents for reactor fuels, electromagnetic 
pumps are used. There are no moving parts in these pumps; no shaft seals are 
required. The liquid metal passing through the pump becomes, in effect, the 
rotor circuit of an electric motor.  See  ELECTROMAGNETIC PUMPS. 
 �34s[Elliott F. Wright]
�33s 
  
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�6v�2v�32sPumpkin   �21m �33s 
    The term commonly applied to the larger, orange-colored fruit of the 
Cucurbita species, used when ripe as a table vegetable, in pies, or for autumn 
decoration. Although some taxonomists would restrict the term pumpkin to the 
species Cucurbita pepper and C. moschata, it is also used in referring to C. 
mixta. New Jersey, Illinois, and California are important producing states.  
See  VIOLALES. 
 �34s[H. John Carew]
�33s 
  
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�6v�2v�32sPuna   �21m �33s 
    An alpine biological community in the central portion of the Andes 
Mountains of South America. Sparsely vegetated, treeless stretches cover high 
plateau country (altiplano) and slopes of central and southern Peru, Bolivia, 
northern Chile, and northwestern Argentina. The poor vegetative cover and the 
puna animals are limited by short seasonal precipitation as well as by the low 
temperatures of high altitudes. 
    Like the paramos of the Northern Andes, punas occur above timberline, and 
extend upward, in modified form, to perpetual snow. Due to greater heights of 
the Central Andean peaks, aeolian regions, that is, regions supplied with 
airborne nutrients above the upper limit of vascular plants, generally the 
snowline, are more extensive here than above the paramos.  See  PARAMO. 
 �34s[Harriet G. Barclay]
�33s 
  
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�6v�2v�32sPure culture �21m �33s
    A culture that contains cells of one kind, all progeny of a single cell.
The concept of a pure culture is an important one in microbiology because most
considerations of microorganisms require dealing with only one kind of a cell
at a time. Many techniques have been evolved for the isolation and maintenance
of pure cultures.  See	MICROBIOLOGICAL METHODS.
 �34s[Jackson W. Foster /R. E. Kallio]
�33s

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�6v�2v�32sPurine   �21m �33s 
    A heterocyclic organic compound containing fused pyrimidine and imidazole 
rings; the structural formula is shown below. A large number of substituted 
purine derivatives occur in nature, some of which, as components of nucleic 
acids and coenzymes, play vital roles in the genetic and metabolic processes 
of all living organisms.  See  COENZYME; NUCLEIC ACID. 
                             ******FORMULA****** 
  
    The purine bases, adenine and guanine (see formulas), together with 
pyrimidines, are fundamental components of all nucleic acids. Certain 
methylated derivatives of adenine and  guanine are also present in some 
nucleic acids in low amounts. In biological systems, hypoxanthine, adenine, 
and guanine occur mainly as their 9-glycosides, the sugar being either ribose 
or 2-deoxyribose, although other pentoses are found in rare cases. Such 
compounds are termed nucleosides generically, and inosine (hypoxanthine 
nucleoside), adenosine, or guanosine specifically. The name nucleotide refers 
to phosphorylated derivatives of nucleosides, the principal ones containing 
5'-phosphate groups, as in guanosine 5'-phosphate and adenosine 
5'-triphosphate (ATP).  See  ADENOSINETRIPHOSPHATE (ATP); PYRIMIDINE. 
 �34s[Standish C. Hartman]
�33s 
  
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�6v�2v�32sPush-pull amplifier   �21m �33s 
    A two-transistor or two-tube amplifier circuit often used as the 
power-output stage of a multistage amplifier. The power-output stage in an 
audio amplifier is normally expected to furnish from 5 to 50 watts or more. 
Use of one active device in this stage is not feasible, because the transistor 
or tube would have to operate class A with a low conversion efficiency, on the 
order of 10%. Use of two or more active devices in parallel does not improve 
efficiency. However, with two active devices operating in push-pull, it is 
possible to supply the required amount of power with conversion efficiency on 
the order of 50%. This higher efficiency means that the amplifier does not 
require as much power from the power supply, because less power is dissipated 
as heat by the power amplifier active devices.  See  POWER AMPLIFIER. 
    A simplified circuit for a push-pull amplifier is shown in the 
illustration. The input signals must be of equal magnitude but 180DG out of 
phase. The collector current in the transistor with the positive signal is 
increasing while the transistor current in the other transistor is decreasing. 
This relation between the collector currents gave rise to the name push-pull. 
Because of their phase relationship, the two currents flowing in the two 
halves of the primary winding of the transformer produce an output similar to 
that of a single source connected to the transformer. 
    Although the active devices are indicated as junction transistors in the 
illustration, other devices such as triodes, beam power tubes, or field-effect 
transistors (FET) can also be used in this push-pull arrangement.  See  
AMPLIFIER.  
 �34s[Harold F. Klock]
�33s 
  
  
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�6v�2v�32sPycnodontiformes   �21m �33s 
    An order of very specialized deep-bodied fishes near the holostean level 
of organization that are known only from the fossil record. They were a 
widespread group that first appeared in the Upper Triassic of Europe, 
flourished during the Jurassic and Cretaceous, and persisted to the Upper 
Eocene. Pycnodontiforms are a closely interrelated group most commonly found 
preserved in marine limestone and associated with coraliferous facies. 
    They are characterized (see illustration) by a laterally compressed, 
disk-shaped body; long dorsal and anal fins, with each fin ray supported by 
its own endoskeletal element; an externally symmetrical tail; and an axial 
skeleton having greatly extended neural and hemal spines that are expanded in 
some forms, and neural and hemal arches which are well ossified - sometimes 
interdigitating around the notochord - but with no vertebral centra. 
 �34s[Ted M. Cavender]
�33s 
                                                                               
  
  
  
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�6v�2v�32sPycnogonida   �21m �33s 
    A subphylum of marine arthropods, consisting of about 600 Recent and 
perhaps 1 Devonian species. The Pycnogonida, or Pantopoda, are commonly called 
sea spiders. This name refers to the appearance of the adults and does not 
indicate affinity with terrestrial spiders. 
    The pycnogonids are characterized by reduction of the body to a series of 
cylindrical trunk somites supporting the appendages, a large specialized 
feeding apparatus called the proboscis, and a reduced abdomen. In many genera 
there are seven pairs of appendages, of which the first four are on the first 
or cephalic segment. This segment also bears a dorsal tubercle containing four 
simple eyes. Each of the remaining three trunk segments bears a single pair of 
legs.  
    Pycnogonids are found in all seas except the inner Baltic and Caspian, 
from intertidal regions to depths of 21,500 ft (6500 m), and one species is 
bathypelagic at about 3300 ft (1000 m). They are especially common in polar 
seas. Most of the intertidal species spend their lives in association with 
some coelenterate as encysted, parasitic larval and juvenile stages, or are 
ectoparasitic as adults, being attached to anemones and hydroids by their 
claws and proboscides. A few have been found riding in hydromedusae, and some 
occur in the mantle cavity of bivalves or on nudibranchs and holothurians. 
Most of the deep-sea species are known only as adults, and their mode of life 
is a mystery.  See  ARTHROPODA. 
 �34s[Joel W. Hedgpeth]
�33s 
  
  
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�6v�2v�32sPyelonephritis   �21m �33s 
    A bacterial infection of the kidney and the kidney pelvis which occurs in 
two forms, acute and chronic. It is very frequently associated with some 
disturbance of urine flow in the lower urinary tract. Examples of common 
causes of obstruction are stones in the ureter, cancer, or an enlarged 
prostate. The causative bacteria commonly are those that normally inhabit the 
large intestine. The symptoms are shaking chills, fever, pain in the flank, 
and frequent passage of small amounts of urine which has a burning quality. 
Antibiotic therapy brings prompt relief of symptoms and eradicates the 
infection. However, if the interference to urine flow is not relieved, 
reinfection often occurs.  See  KIDNEY; NEPHRITIS. 
 �34s[Gary E. Striker]
�33s 
  
  
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�6v�2v�32sPygasteroida   �21m �33s 
    An order of Diadematacea which exhibits various stages in the backward 
migration of the anus out of the apical system. They have four genital pores 
(instead of five), noncrenulate tubercles, and simple ambulacral plates. All 
members are referred to a single family, the Pygasteridae.  See  DIADEMATACEA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sPyramid and frustum   �21m �33s 
    A pyramid is a polyhedron of which one face is called the base, and the 
other faces (called lateral faces) are triangles having a common vertex which 
is called the vertex of the pyramid. The distance from the vertex to the base 
is called the altitude. The volume of a pyramid is one-third the product of 
its base times its altitude (V = N Bh). Sections of a pyramid formed by planes 
parallel to the base are similar to the base. A pyramid with a triangular base 
is a triangular pyramid, or tetrahedron. If the base of a pyramid is a regular 
polygon and the lateral edges are equal, the pyramid is called a regular 
pyramid; if the polygon is a square, the pyramid is called a square pyramid. A 
frustum of a pyramid is a segment of a pyramid included between two parallel 
planes.  See  POLYHEDRON; PRISMATOID AND PRISMOID. 
 �34s[James S. Frame]
�33s 
  
  
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�6v�2v�32sPyran �21m �33s
    One of a group of organic compounds containing a heterocyclic six-membered
ring made up of one oxygen and five carbon atoms. Formulas (I) and (II) show
pyrans of various kinds.  See  HETEROCYCLIC COMPOUNDS.
*EQN*
    All pyranosidic carbohydrates are tetrahydropyran derivatives. Glycals,
for example, glucal, are DELTA(sup 2,3)-dihydropyrans. The ALPHA-pyrone system
is present in cardiac-active steroid derivatives, such as scillaren A,
isolated from squill and from a variety of toad.
    Formulas (III) and (IV) show some benzopyran types. Vitamin E, or
ALPHA-tocopherol, is a chroman (III) derivative. Coumarin, or ALPHA-chromone
(IV), the fragrant body in clover, was formerly used in flavors and perfumes.
 �34s[Walter J. Gensler]
�33s
			     ******FORMULA******


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�6v�2v�32sPyrargyrite   �21m �33s 
    A mineral having composition Ag3SbS3. The mineral occurs as prismatic 
crystals and in massive form and in disseminated grains. The hardness is 2.5 
on Mohs scale and specific gravity is 5.85. The luster is adamantine and the 
color a deep ruby red to black, giving it the name dark ruby silver. 
Pyrargyrite is an important silver ore when it is found in veins associated 
with proustite and other silver minerals. It has been mined as silver ore at 
Chanarcillo, Chile; Freiberg, Germany; Guanajuato, Mexico; and Cobalt, 
Ontario, Canada.  See  PROUSTITE. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sPyrazole   �21m �33s 
    One of a group of organic heterocyclic compounds with two nitrogen atoms 
occupying adjacent positions in a doubly unsaturated five-membered ring, as in 
the structure below. The pyrazole system is aromatic in character. 
Electrophilic substitution occurs preferentially at the 4 position. The 
nucleus is resistant to disruption by oxidation. Certain drugs and dyes are 
pyrazole derivatives.  See  AZOLE; HETEROCYCLIC COMPOUNDS. 
                             ******FORMULA****** 
  
    The parent compound shown above is a water-soluble, colorless solid, mp 
70DGC (158DGF), bp 187DGC (360DGF), with an odor resembling that of pyridine. 
Pyrazole is both a weak base and a weak acid. 
    Antipyrine and aminopyrine are pyrazole derivatives of considerable value 
as analgesics and antipyretics, respectively. Azo coupling of 
3-methyl-1-phenyl-5-pyrazolone or of related compounds gives 4-azopyrazolone 
derivatives, which are of interest as wool, food, and photographic dyes.  See  
DYE.  
 �34s[Walter J. Gensler]
�33s 
  
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�6v�2v�32sPyrenulales   �21m �33s 
    An order of the class Ascolichenes, also known as the Pyrenolichenes. The 
flask-shaped perithecia are uniformly immersed in the medulla of the thalli 
with a small ostiole opening at the surface. The asci and paraphyses arise 
from a blackened hypothecium and line the walls of the perithecium. The spores 
eventually burst the ascal walls and ooze out through the ostiole in a jelly 
matrix.  
    There are about 10 families, 50 genera, and more than 1500 species in the 
Pyrenulales. The major taxonomic criteria for separating genera and species 
are the septation and color of spores, since vegetative characters are so 
poorly developed.  See  ASCOLICHENES. 
 �34s[Mason E. Hale, Jr.]
�33s 
  
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�6v�2v�32sPyridine �21m �33s
    An organic heterocyclic compound containing a triunsaturated six-membered
ring of five carbon atoms and one nitrogen atom. Pyridine (I) and pyridine
homologs are obtained by extraction of coal tar or by synthesis. The pyridine
system is found in natural products, for example, in nicotine (II) from
tobacco, in ricinine (III) from castor bean, in pyridoxine or vitamin B6 (IV),
in nicotinamide or niacinamide or vitamin P (V), and in several groups of
alkaloids.  See  HETEROCYCLIC COMPOUNDS.

			     ******FORMULA******

    Pyridine (I) is a colorless, hygroscopic liquid with a pungent, unpleasant
odor. When anhydrous it boils at 115.2 - 115.3DGC (239.4 - 239.5DGF). Pyridine
is miscible with organic solvents as well as with water. The pyridine system
is aromatic. It is stable to heat, to acid, and to alkali. Pyridine is used as
a solvent for organic and inorganic compounds, as an acid binder, as a basic
catalyst, and as a reaction intermediate.
    Pyridine is an irritant to skin (eczema) and other tissues
(conjunctivitis), and chronic exposure has been known to cause liver and
kidney damage. Repeated exposure to atmospheric levels greater than 5 parts
per million is considered hazardous.
 �34s[Walter J. Gensler]
�33s
			     ******FORMULA******


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�6v�2v�32sPyrimidine   �21m �33s 
    A heterocyclic organic compound (I) containing nitrogen atoms at positions 
1 and 3. Naturally occurring derivatives of the parent compound are of 
considerable biological importance as components of nucleic acids and 
coenzymes and, in addition, synthetic members of this group have found use as 
pharmaceuticals.  See  COENZYME; NUCLEIC ACID. 
                             ******FORMULA****** 
  
    Pyrimidine compounds which are found universally in living organisms 
include uracil (II), cytosine (III), and thymine (IV). Together with purines 
these substances make up the ``bases'' of nucleic acids, uracil and cytosine 
being found characteristically in ribonucleic acids, with thymine replacing 
uracil in deoxyribonucleic acids. A number of related pyrimidines also occur 
in lesser amounts in certain nucleic acids. Other pyrimidines of general 
natural occurrence are orotic acid and thiamine (vitamin B1).  See  
DEOXYRIBONUCLEIC ACID (DNA); PURINE; RIBONUCLEIC ACID (RNA). 
                             ******FORMULA****** 
  
    Among the sulfa drugs, the pyrimidine derivatives, sulfadiazine, 
sulfamerazine, and sulfamethazine, have general formula (V). These agents are 
inhibitors of folic acid biosynthesis in microorganisms. The barbiturates are 
pyrimidine derivatives which possess potent depressant action on the central 
nervous system.  See  BARBITURATES; SULFA DRUGS. 
 �34s[Standish C. Hartman]
�33s 
                             ******FORMULA****** 
  
  
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�6v�2v�32sPyrite   �21m �33s 
    A mineral having composition FeS2. Pyrite has a Mohs hardness of 6 - 6.5 
and a density of 5.02. The luster is metallic, the color brass yellow, and the 
streak greenish black or brownish black. 
    Pyrite, or iron pyrites, is the most common ``fool's gold,'' but it is 
hard and brittle whereas gold is soft and sectile. Its hardness also 
distinguishes it from softer chalcopyrite. Marcasite was once thought to be 
polymorphous with pyrite, but precise analyses show that marcasite contains 
excess iron, whereas pyrite is stoichiometric FeS2.  See  MARCASITE. 
    Pyrite is the most common and most widespread sulfide mineral. It forms 
under almost all known conditions of mineral deposition. Under oxidizing 
conditions, pyrite readily alters to iron sulfates and eventually to limonite, 
forming gossan, the surface expression of pyrite-rich mineral deposits.  See 
#LIMONITE.  
    Because of its high sulfur content (53.4%), pyrite has become a source of 
sulfur for the production of sulfuric acid. In some places, it is mined for 
sulfur alone.  See  SULFUR. 
 �34s[Lawrence Grossman]
�33s 
  
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�6v�2v�32sPyroclastic rocks   �21m �33s 
    Rocks of extrusive (volcanic) origin, composed of rock fragments produced 
directly by explosive eruptions. Pyroclastic fragments may represent shattered 
and comminuted older rocks (volcanic, plutonic, sedimentary, or metamorphic) 
or solidified lava droplets formed by violent explosion.  See  TUFF; VOLCANO. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sPyroelectricity �21m �33s
    The property of certain crystals to produce a state of electric polarity
by a change of temperature. Certain dielectric (electrically nonconducting)
crystals develop an electric polarization (dipole moment per unit volume) when
they are subjected to a uniform temperature change. This pyroelectric effect
occurs only in crystals which lack a center of symmetry and also have polar
directions (that is, a polar axis). These conditions are fulfilled for 10 of
the 32 crystal classes. Typical examples of pyroelectric crystals are
tourmaline, lithium sulfate monohydrate, cane sugar, and ferroelectric barium
titanate.
    Pyroelectric crystals can be regarded as having a built-in or permanent
electric polarization. When the crystal is held at constant temperature, this
polarization does not manifest itself because it is compensated by free charge
carriers that have reached the surface of the crystal by conduction through
the crystal and from the surroundings. However, when the temperature of the
crystal is raised or lowered, the permanent polarization changes, and this
change manifests itself as pyroelectricity.
    The magnitude of the pyroelectric effect depends upon whether the thermal
expansion of the crystal is prevented by clamping or whether the crystal is
mechanically unconstrained. In the clamped crystal, the primary pyroelectric
effect is observed, whereas in the free crystal, a secondary pyroelectric
effect is superposed upon the primary effect. The secondary effect may be
regarded as the piezoelectric polarization arising from thermal expansion, and
is generally much larger than the primary effect.  See	PIEZOELECTRICITY.
 �34s[H. Graenicher]
�33s

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�6v�2v�32sPyrolusite   �21m �33s 
    A mineral having composition MnO2. Well-developed crystals (polianite) are 
rare; it is usually in radiating fibers or reniform coatings. The hardness is 
1 - 2 on the Mohs scale (often soiling the fingers) and the specific gravity 
is 4.75. The luster is metallic and the color iron-black. It frequently forms 
pseudomorphs after other manganese minerals, notably manganite. 
    Pyrolusite is extensively mined as a manganese ore in many countries, 
chiefly in the Soviet Union, Ghana, India, the Republic of South Africa, 
Morocco, Brazil, and Cuba.  See  MANGANESE; MANGANITE. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sPyrolysis �21m �33s
    The chemical change of a substance by means of heat alone. Thermal
rearrangements into isomers, thermal polymerizations, and thermal
decompsitions are all included in the term pyrolysis, but it does not include
thermal changes that require catalysts or changes that are initiated by other
forms of energy. A transformation promoted by ultraviolet radiation, for
example, would be photolytic, not pyrolytic.  See  CATALYSIS; PHOTOCHEMISTRY.
    An inorganic example of pyrolysis is shown in reaction (1).

	      2NaHCO3  -->  Na2CO3  +  H2O  +  CO2			 (1)
	      Sodium	    Sodium    Water   Carbon
	   bicarbonate	   carbonate	      dioxide

    Prominent industrial examples include cracking of petroleum; destructive
distillation of coal or wood; pyrolysis of methane into hydrogen and carbon
black; and syntheses of biphenyl from benzene, or styrene from ethylbenzene,
or ethylene from ethane.  See  CARBON BLACK; CHARCOAL; COAL CHEMICALS; COKE;
CRACKING; DESTRUCTIVE DISTILLATION; LIME (INDUSTRY).
    Many pyrolytic reactions involve the production of radicals as
intermediates. Propane and butane are representative paraffins that have been
carefully studied for their pyrolytic behavior. Propane changes at 500 -
600DGC (930 - 1110DGF) bidirectionally into methane and ethylene or into
hydrogen and propylene. Butane gives rise to methane plus propylene, ethane
plus ethylene, and to a lesser extent hydrogen plus butylene. In stable
organic molecules each carbon is surrounded by eight electrons, as in the
structures for methane and propane shown here, wherein each bond depicts an
electron pair.

		 H			 H  H  H
		 |			 |  |  |
	      H--C--H		      H--C--C--C--H
		 |			 |  |  |
		 H			 H  H  H
	      Methane			 Propane

    The bond energies holding a molecule together become smaller with
increasing temperature. If, eventually, one of the bonds comes to zero energy,
the molecule falls apart into radicals. One of the carbons in a radical has
only seven electrons around it, for example, the methyl radical,

				H
				|
			     H--C CTR DOT
				|
				H

The dot represents one electron. Methylene represents a carbon with only six
electrons around it, as in methylene itself,

				       H
				       |
			     CH2 or H--C:

    The products from propane or butane mentioned above are the result of
initial thermal cleavage of the alkane into radicals, for example, methyl and
ethyl from propane, as shown by reaction (2), followed by collision of these
radicals (RSMALL BULLET) with undecomposed hydrocarbon to form RH and create a
new radical, as shown by reaction (3). The C3H7SMALL BULLET radical then
breaks into an unsaturated hydrocarbon and a smaller radical or atom, as seen
in reaction (4). These new radicals take the place of the original RSMALL
BULLET and cause the breakdown to continue. This ``chain reaction'' is
interrupted when the radicals (RSMALL BULLET or HSMALL BULLET) combine with
each other to form R--R, R--H, or H--H.  See  CHAIN REACTION (CHEMISTRY); FREE
RADICAL.

		   C3H8 -> CH3 CTR DOT + C2H5 CTR DOT			     (2)


		  C3H8 + R CTR DOT -> RH + C3H7 CTR DOT 		     (3)


			    CHEMICAL FORMULA				     (4)

			    CHEMICAL FORMULA				     (5)

    There are compounds which pyrolyze by mechanisms not involving bond
rupture into radicals. Instead, pairs of electrons stay together and move as a
unit, usually because of some structural feature within the molecule that
makes it possible. Lower temperatures are usually observed for such processes
than for the ones involving scission into radicals. For example, ethyl acetate
decomposes at 500DGC (930DGF) into acetic acid and ethylene, isopropyl acetate
at 450DGC (1000DGF) into acetic acid and propylene, and tertiary butyl acetate
at 350DGC (660DGF) into acetic acid and isobutylene. It is thus evident that
esters of primary alcohols are more stable than those of secondary alcohols
which, in turn, are more stable than esters of tertiary alcohols. All three
types pyrolyze by a mechanism involving a quasi six-membered ring in the
transition state and realignment of electron pairs, as in reaction (5). The
term R represents a hydrogen or a methyl group. This cyclic transition state
mechanism has been adapted to countless other reactions of organic chemistry.
See  ORGANIC REACTION MECHANISM.
 �34s[Charles D. Hurd]
�33s

			     ******FORMULA******


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�6v�2v�32sPyrometallurgy   �21m �33s 
    Processes employing chemical reactions at elevated temperatures for the 
extraction of metals from ores and concentrates. Pyrometallurgy is the 
principal means of metal production. 
    The processes of pyrometallurgy may be divided into preparation processes 
which convert the raw material to a form suitable for further processing (for 
example, roasting to convert sulfides to oxides), reduction processes which 
reduce metallic compounds to metal (the blast furnace which reduces iron oxide 
to pig iron), and refining processes which remove impurities from crude metal 
(fractional distillation to remove iron, lead, and cadmium from crude zinc). 
    The complete production scheme, from ore to refined metal, may employ 
pyrometallurgical processes (steel, lead, tin, zinc), or only the primary 
extraction processes may be pyrometallurgical, with other methods used for 
refining (copper, nickel). In some cases (uranium, tungsten, molybdenum), 
isolated pyrometallurgical processes are used in a treatment scheme which is 
predominately nonpyrometallurgical.  See  IRON METALLURGY; METALLURGY. 
 �34s[Herbert H. Kellogg]
�33s 
  
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�6v�2v�32sPyrometer   �21m �33s 
    A temperature-measuring device, originally an instrument that measures 
temperatures beyond the range of thermometers, but now in addition a device 
that measures thermal radiation in any temperature range. This article 
discusses radiation pyrometers; for other temperature-measuring devices  see  
BOLOMETER; THERMISTOR; THERMOCOUPLE. 
    The illustration shows a very simple type of radiation pyrometer. Part of 
the thermal radiation emitted by a hot object is intercepted by a lens and 
focused onto a thermopile. The resultant heating of the thermopile causes it 
to generate an electrical signal (proportional to the thermal radiation) which 
can be displayed on a recorder. 
    Unfortunately, the thermal radiation emitted by the object depends not 
only on its temperature but also on its surface characteristics. The radiation 
existing inside hot, opaque objects is so-called blackbody radiation, which is 
a unique function of temperature and wavelength and is the same for all opaque 
materials. However, such radiation, when it attempts to escape from the 
object, is partly reflected at the surface. In order to use the output of the 
pyrometer as a measure of target temperature, the effect of the surface 
characteristics must be eliminated. A cavity can be formed in an opaque 
material and the pyrometer sighted on a small opening extending from the 
cavity to the surface. The opening has no surface reflection, since the 
surface has been eliminated. Such a source is called a blackbody source, and 
is said to have an emittance of 1.00. By attaching thermocouples to the 
blackbody source, a curve of pyrometer output voltage versus blackbody 
temperature can be constructed.  See  BLACKBODY; HEAT RADIATION. 
    Pyrometers can be classified generally into types requiring that the field 
of view be filled, such as narrow-band and total-radiation pyrometers; and 
types not requiring that the field of view be filled, such as optical and 
ratio pyrometers. The latter depend upon making some sort of comparison 
between two or more signals. 
    The optical pyrometer should more strictly be called the 
disappearing-filament pyrometer. In operation, an image of the target is 
focused in the plane of a wire that can be heated electrically. A rheostat is 
used to adjust the current through the wire until the wire blends into the 
image of the target (equal brightness condition), and the temperature is then 
read from a calibrated dial on the rheostat. 
    The ratio, or ``two-color,'' pyrometer makes measurements in two 
wavelength regions and electronically takes the ratio of these measurements. 
If the emittance is the same for both wavelengths, the emittance cancels out 
of the result, and the true temperature of the target is obtained. This 
so-called graybody assumption is sufficiently valid in some cases so that the 
``color temperature'' measured by a ratio pyrometer is close to the true 
temperature.  See  TEMPERATURE MEASUREMENT; THERMOMETER. 
 �34s[Thomas P. Murray]
�33s 
  
  
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�6v�2v�32sPyrometric cone   �21m �33s 
    One of a numbered series of ceramic compositions formed into triangular 
pyramids of height about four times the base (illustration a), and designed so 
that each will soften and bend under its own weight after a certain heat 
treatment (illustration b). 
    The cones are used to indicate when ceramic ware has been adequately 
fired, to check on heat uniformity in a kiln, and in determining the 
pyrometric cone equivalent of refractories.  See  REFRACTORY. 
    Cones are most useful for firing ware made of materials similar to those 
in the cones (clay, feldspar, quartz), in other words, the classical clay 
products. For radically different materials, such as the magnetic ceramic 
ferrites, cones are less useful as indicators of maturity.  See  CERAMICS; 
KILN; SINTERING.  
 �34s[John F. McMahon]
�33s 
  
  
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�6v�2v�32sPyromorphite   �21m �33s 
    A mineral series in the apatite group, or in the larger grouping of 
phosphate, arsenate, and vanadate-type minerals. In this series lead (Pb) 
substitutes for calcium (Ca) of the apatite formula Ca5(PO4)3(F,OH,Cl), and 
little fluorine (F) or hydroxide (OH) is present.  See  APATITE. 
    The pyromorphite series crystallizes in the hexagonal system. Crystals are 
prismatic. Other forms are granular, globular, and botryoidal. Pyromorphite 
colors range through green, yellow, and brown; vanadinite occurs in shades of 
yellow, brown, and red. 
    Pyromorphites are widely distributed as secondary minerals in oxidized 
lead deposits. Pyromorphite is a minor ore of lead; vanadinite is a source of 
vanadium and minor ore of lead.  See  LEAD; VANADIUM. 
 �34s[Wayne R. Lowell]
�33s 
  
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�6v�2v�32sPyrophyllite   �21m �33s 
    A hydrated aluminum silicate with composition Al2Si4O10(OH)2. The mineral 
is commonly white, grayish, greenish, or brownish, with a pearly to waxy 
appearance and greasy feel. It occurs as compact masses, as radiating 
aggregates (see illustration), and as foliated masses. Pyrophyllite belongs to 
the layer silicate (phyllosilicate) group of minerals. The mineral is soft 
(hardness 1 - 1** on the Mohs scale) and has easy cleavage parallel to the 
structural layers. The mineral is highly stable to acids. 
    Pyrophyllite is used principally for refractory materials, used at high 
temperatures, and in other ceramic applications. The main sources for 
pyrophyllite in the United States are in North Carolina. An unusual form from 
the Transvaal is called African wonderstone.  See  SILICATE MINERALS. 
 �34s[George W. Brindley]
�33s 
  
  
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�6v�2v�32sPyrotechnics   �21m �33s 
    Fireworks. The ingredients of fireworks are basically the familiar 
incendiary components. Chlorates and nitrates are the oxidizers, and sulfur, 
charcoal, antimony  sulfide, and sometimes powdered metals are the fuels. 
Colors are produced by the metals present in the salts; sodium gives yellow; 
calcium, brick red; strontium, scarlet; barium, green; and copper, blue-green. 
Iron filings and aluminum powder give sparks and brilliant effusions. 
    Highway warning flares are usually made with strontium nitrate to impart a 
brilliant scarlet hue and employ a slow-burning fuel of wax and sawdust. 
    Colored smokes are used for military signaling and for daylight displays. 
They incorporate a variety of vaporizable organic dyes in the usual 
pyrotechnic compositions.  See  INCENDIARY. 
 �34s[William E. Gordon]
�33s 
  
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�6v�2v�32sPyrotheria   �21m �33s 
    An extinct order of primitive, mastodonlike, herbivorous, hoofed mammals 
restricted to the Eocene and Oligocene deposits of South America. There is 
only one family (Pyrotheriidae) in the order and four genera in the family. 
    The characters of this group superficially resembling those in early 
proboscideans are nasal openings over orbits indicating the presence of a 
trunk, strong neck musculature, and six upper and four lower bilophodont cheek 
teeth. Pyrotheres are distantly related to the members of the superorder 
Paenungulata, including Proboscidea, Xenungulata, and others.  See  
PROBOSCIDEA; XENUNGULATA. 
 �34s[Gideon T. James]
�33s 
  
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�6v�2v�32sPyroxene   �21m �33s 
    A family of diverse and important rock-forming minerals crystallizing over 
a wide range of temperature, pressure, and composition and occurring as 
constituents of a host of rock types. The table gives the formulas and space 
groups of the pyroxene family. The crystal chemistry of the pyroxene family is 
very complicated. Since the pyroxenes are so ubiquitous and since their 
compositions range extensively, research on these compounds reveals important 
information about the origin and thermal history of certain rocks. 
    Common pyroxenes range in color from white (pure diopside and enstatite) 
through shades of yellow and green to deep brown and greenish-black 
(ferrosilite, hedenbergite, and aegirine). Specific gravity ranges from 3.2 
(enstatite) to 4.0 (ferrosilite). The hardness on Mohs scale ranges from 5J to 
6.  
    Pyroxenes occur in most rock types of medium to high grade, particularly 
in basalts, pyroxenites, charnockites, and ultrabasic rocks derived from 
gravity settling of ferromagnesian minerals. As constituents in ultrabasic 
rocks, they occur often with olivine and spinel. Diopside and hedenbergite are 
typical metamorphic and skarn products. Augites occur as constituents of 
basalts and charnockites, and as common metamorphic products, often associated 
with hornblende amphibole. 
 �34s[Paul B. Moore]
�33s 
  
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�6v�2v�32sPyroxenite   �21m �33s 
    A heavy, dark-colored, phaneritic (visibly crystalline) igneous rock 
composed largely of pyroxene with smaller amounts of olivine or hornblende. 
Pyroxenite composed largely of orthopyroxene occurs with anorthosite and 
peridotite in large, banded gabbro bodies. Some of these pyroxenite masses are 
rich sources of chromium. Certain pyroxenites composed largely of 
clinopyroxene are also of magmatic origin, but many probably represent 
products of reaction between magma and limestone. Other pyroxene-rich rocks 
have formed through the processes of metamorphism and metasomatism.  See  
GABBRO; IGNEOUS ROCKS; PERIDOTITE; PYROXENE. 
 �34s[Carleton A. Chapman]
�33s 
  
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�6v�2v�32sPyrrhophyta   �21m �33s 
    A small division of algae characterized by the presence of yellowish-green 
to golden-brown plastids; by the storage of food reserves in the form of 
starch or starchlike compounds, and sometimes oils; and by the general absence 
of cell walls (but if cell walls are present, they usually contain cellulose). 
Most members are unicellular and biflagellate. A few are without flagella. The 
usual method of reproduction is by cell division, although some reproduce by 
means of zoospores (swimming spores) or nonmotile spores, and sexual 
reproduction has been observed in a few. 
    Pyrrophyta are divided into two classes: the Desmophyceae (Desmokontae) 
and the Dinophyceae. Other researchers include a third class, Cryptophyceae.  
See  ALGAE. 
 �34s[Paul A. Vestal]
�33s 
  
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�6v�2v�32sPyrrhotite   �21m �33s 
    A mineral with composition Fe1-subxS (x = 0 to 0.2). Eskebornite, 
Fe1-subxSe, is the selenium analog. The iron-deficient pyrrhotites are 
ferrimagnetic at room temperature, but at some higher temperature they become 
paramagnetic, presumably because of vacancy disorder. 
    The mineral occurs as rounded grains to large masses, more rarely as 
tabular pseudohexagonal crystals and rosettes. Color is brownish bronze-yellow 
with dark grayish-black streaks. Hardness is 4 on Mohs scale and specific 
gravity 4.6 (for the composition Fe7S8). 
    Pyrrhotite occurs in basic igneous rocks as a late-stage fractional 
differentiate, particularly in norites and gabbros, and sufficient quantities 
may constitute an ore of iron. Pyrrhotite also occurs with magnetite and 
chondrodite in contact metamorphic marbles, and in low-temperature veins with 
calcite and other sulfides and sulfosalts. 
 �34s[Paul B. Moore]
�33s 
  
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�6v�2v�32sPyrrole �21m �33s
    One of a group of organic compounds containing a doubly-unsaturated
five-membered ring in which nitrogen occupies one of the ring positions.
Pyrrole (I) is a representative compound. The pyrrole system is found in the
green leaf pigment, chlorophyll, in the red blood pigment, hemoglobin, and in
the blue dye, indigo. Interest in these colored bodies has been largely
responsible for the intensive study of pyrroles. Tetrahydropyrrole, or
pyrrolidine (II), is part of the structures of two protein amino acids,
proline and hydroxyproline, and of hygrine, which is an alkaloid from Peruvian
coca.  See  HETEROCYCLIC COMPOUNDS; HYDROXYPROLINE; INDOLE; PORPHYRIN; PROLINE.
 �34s[Walter J. Gensler]
�33s

			 ******CHEMICAL FORMULA******


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�6v�2v�32sPythagorean theorem �21m �33s
    This theorem states that in any right triangle the square on the
hypotenuse is equal to the sum of the squares on the other two sides: r(sup 2) =
x(sup 2) + y(sup 2). More than 100 different proofs have been given for this
extremely important theorem of euclidean plane geometry.
    The three-dimensional Pythagorean theorem may be phrased ``the square of
the diagonal of a rectangular box is equal to the sum of the squares of three
adjacent edges that meet at a vertex: r(sup 2) = x(sup 2) + y(sup 2) + z(sup
2).''  See  SQUARE; TRIANGLE.
 �34s[James S. Frame]
�33s

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�6v�2v�32sPython   �21m �33s 
    A reptile in the subfamily Pythoninae of the family Boidae. There are 9 
species, all occurring in Africa, Southeast Asia, New Guinea, and Australia. 
The entire subfamily includes 7 genera and 21 species that are called pythons, 
but the genus Python is more commonly considered and will be discussed here. 
The pythons are distinguished from the closely related boas on the basis of 
skull structure and methods of reproduction.  See  BOA. 
    Pythons are large, muscular constrictors that have long, powerful teeth, 
but no poison apparatus. Pythons kill their prey by constriction and eventual 
suffocation. In this process, bones are rarely broken. Most pythons are 
semiarboreal, occurring in forest regions; however, some are found among 
shrubs and in grassy areas.  See  REPTILIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sQ fever   �21m �33s 
    An acute, febrile, infectious disease in humans, characterized by sudden 
onset, patchy pneumonitis, absence of rash, and low mortality, and caused by a 
bacterialike microorganism, Coxiella burneti. 
    A primary cycle between certain small animals, such as bandicoots in 
Australia, and their tick parasites, occurs in nature. Secondary cycles of 
obscure relationship to the primary cycle occur in domestic animals. Placentas 
of domestic animals are highly infectious and become, on disintegration, the 
chief source of infection for humans and animals. Vaccination of valuable 
dairy herds gives some promise of reduction of incidence in humans and 
domestic animals.  See  RICKETTSIOSES. 
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sQ meter �21m �33s
    A direct-reading instrument widely used for measuring the Q of an electric
circuit at radio frequencies. Originally designed to measure the Q of coils,
the Q meter has been developed into a flexible, general-purpose instrument for
determining many other quantities such as (1) the distributed capacity,
effective inductance, and self-resonant frequency of coils; (2) the
capacitance, Q or power factor, and self-resonant frequency of capacitors; (3)
the effective resistance, inductance or capacitance, and the Q of resistors;
(4) characteristics of intermediate- and radio-frequency transformers; and (5)
the dielectric constant, dissipation factor, and power factor of insulating
materials.  See  ELECTRICAL MEASUREMENTS; HIGH-FREQUENCY IMPEDANCE
MEASUREMENTS; Q (ELECTRICITY).
 �34s[Isaac F. Kinnard /Edward C. Stevenson]
�33s

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�6v�2v�32sQuadraphonic sound system   �21m �33s 
    A sound reproduction system in which four discrete channels of program are 
fed to four loudspeakers located at various points around the listener. The 
listener's enjoyment of the reproduced sound is heightened through being 
immersed in the sound field. The usual arrangement of loudspeakers in the 
listening room is depicted in the illustration. The loudspeakers are situated 
near the corners of the room and are designated as left-front (LF   ), 
left-back (LB   ), right-back (RB   ), and right-front (RF   ). 
  
    The program producer may apportion segments of the total musical program 
among the four channels to achieve a variety of effects in reproduction. Thus, 
the sound heard in a concert hall may be simulated by feeding the direct sound 
from the orchestra to the LF    and RF    loudspeakers while feeding the 
concert hall reverberation sounds primarily to the LB    and RB    
loudspeakers. Or the orchestral sounds may be distributed to all four 
loudspeakers, thereby giving a sense of intimacy as though the listener were 
seated among the musicians. Moreover, an illusion of moving sound sources may 
be achieved by sweeping the source gradually from one channel to another. 
    The variety of effects employed in quadraphonic sound reproduction are 
possible because of the psychoacoustic effect termed localization, that is, 
the ability of a listener to ascertain, through the sense of hearing, the 
location of a sound. In persons having normal hearing, the ability is highly 
developed for most sounds within the audible range of frequencies. 
    In 4-2-4 matrixing, the original four channels are combined, or matrixed, 
in electronic circuits to compress the signals into two channels that can be 
recorded or transmitted as conventional stereophonic signals. In a receiver or 
phonograph playback system, the two-channel signals are processed, or 
dematrixed, to yield four output signals that more or less resemble the four 
input signals. The term ``matrix'' comes from the mathematical terminology 
applied to an array of algebraic equations such as those used to describe the 
4-2-4 encode-decode processes. 
    A quadraphonic sound reproduction system, needing twice as many 
loudspeakers and amplifiers, is clearly more complex and costly than a 
stereophonic system. However, the availability of relatively inexpensive and 
compact integrated circuits and solid-state amplifiers makes four-channel 
systems economically viable. Because of their ability to use existing 
two-channel records and FM transmitters, 4-2-4 quadraphonic systems entered 
the marketplace quickly. The technical problems initially encountered in true 
four-channel phonograph records and players have been solved, so that reliable 
equipment of this type is available.  See  AMPLIFIER; INTEGRATED CIRCUITS; 
SOUND-REPRODUCING SYSTEMS. 
 �34s[J. Guy Woodward]
�33s 
  
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�6v�2v�32sQuadrature   �21m �33s 
    The condition in which the phase angle between two alternating quantities 
is 90DG, corresponding to one-quarter of an electrical cycle. The electric and 
magnetic fields of electromagnetic radiation are in space quadrature, which 
means that they are at right angles in space.  See  ELECTROMAGNETIC RADIATION. 
    The current and voltage of a perfect coil are in quadrature because the 
coil current lags behind the coil voltage by exactly 90DG. The current and 
voltage of a perfect capacitor are also in quadrature, but here the current 
leads the voltage by 90DG. In these last two cases the current and voltage are 
in time quadrature.  See  ALTERNATING-CURRENT CIRCUIT THEORY. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sQuadric surface �21m �33s
    A surface defined analytically by an equation of the second degree in
three variables. If these variables are x, y, z, such an equation has the form:
 �34s[James S. Frame]
�33s

ax(sup 2) + by(sup 2) + cz(sup 2) + 2exy + 2fxz
		+ 2gyz + 2px + 2qy + 2rz + d = 0

Every plane section of such a surface is a conic. See CYLINDER; ELLIPSOID
AND SPHEROID; HYPERBOLOID; PARABOLOID; SURFACE AND SOLID OF REVOLUTION.
[J.S.F.]

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�6v�2v�32sQuadrilateral   �21m �33s 
    In the euclidean sense, the quadrilateral is a geometric figure bounded by 
four straight-line segments called sides, each of which intersects each of two 
adjacent sides in points called vertices, but fails to intersect the opposite 
side (considered as a finite segment). The four vertices are connected in 
pairs by the four sides and by two other segments called diagonals. If the 
latter intersect each other, the quadrilateral is a plane quadrilateral; if 
not, it is a skew quadrilateral. A plane quadrilateral is called a rhombus if 
its four sides are equal, or a kite if it has two pairs of equal adjacent 
sides. A quadrilateral with one pair of sides parallel is a trapezoid; if the 
other pair of sides are equal, the trapezoid is called isosceles. A 
quadrilateral with two pairs of parallel sides is a parallelogram.  See  
PARALLELOGRAM; RECTANGLE; SQUARE; TRAPEZOID. 
 �34s[James S. Frame]
�33s 
  
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�6v�2v�32sQualification test   �21m �33s 
    A formally defined series of tests by which the functional, environmental, 
and reliability performance of a component or system may be evaluated in order 
to satisfy the contractor as to its satisfactory design and construction prior 
to approval and acceptance. In all but the simplest of systems it is 
impossible to anticipate the wide variety of circumstances under which the 
component and the system will be expected to perform satisfactorily. Therefore 
it is essential that the contractor and the contractee for a component or 
system have a mutual understanding as to how the component or system must 
perform under specified circumstances in order to be satisfactory.  See  
ENVIRONMENTAL TEST; INSPECTION AND TESTING; RELIABILITY OF EQUIPMENT; SYSTEMS 
ENGINEERING.  
 �34s[Robert W. Mann]
�33s 
  
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�6v�2v�32sQualitative chemical analysis �21m �33s
    The branch of chemistry concerned with identifying the elements and
compounds present in a sample of matter. Inorganic qualitative analysis
traditionally used classical ``wet'' methods to detect elements or groups of
chemically similar elements, but instrumental methods have largely superseded
the test-tube methods. Methods for the detection of organic compounds or
classes of compounds have become increasingly available and important in
organic, forensic, and clinical chemistry. Once it is known which elements and
compounds are present, the role of quantitative analysis is to determine the
composition of the sample.  See  ANALYTICAL CHEMISTRY; QUANTITATIVE CHEMICAL
ANALYSIS.

    Inorganic analysis.  The operating principles of all systematic inorganic
qualitative analysis schemes for the elements are similar: separation into
groups by reagents producing a phase change; isolation of individual elements
within a group by selective reactions; and confirmation of the presence of
individual elements by specific tests.
    Through usage and tradition, descriptive terms for sample sizes have been:

		macro			0.1 gram or more
		semimicro		0.01 to 0.1 gram
		micro			1 milligram (1 mg or 10-(sup 3) g)
		ultramicro		1 microgram (1 MUg or 10-(sup 6) g)
		submicrogram		less than 1 microgram

For defining the smallest amount of a substance that can be detected by a
given method, the term ``limit of identification'' is used. Under favorable
conditions, an extremely sensitive method can detect as little as 10-sup1sup5g.
    Spot tests are selective or specific single qualitative chemical tests
carried out on a spot plate (a glass or porcelain plate with small depressions
in which drop-size reactions can be carried out), on paper, or on a microscope
slide. On paper or specially prepared adsorbent surfaces, spot tests become
one- or two-dimensional through the use of solvent migration and differential
adsorption (thin-layer chromatography). Containing selected indicator dyes, pH
indicator paper strips are widely used for pH estimation. Solid reagent
monitoring devices and indicator tubes are used for the detection and
estimation of pollutant gases in air.  See  CHROMATOGRAPHY; PH.
    By use of the microscope, crystal size and habit can be used for
qualitative identification. With the addition of polarized light, chemical
microscopy becomes a versatile method. See CHEMICAL MICROSCOPY.
    Any instrumental method of quantitative analysis can be adapted to
qualitative analysis. Some, such as electrochemical methods, are not often
used in qualitative analysis (an exception would be the ubiquitous pH meter).
Others, such as column chromatography (gas and liquid) and mass spectrometry,
are costly and uncomplicated but capable of providing unique results. Emission
spectroscopy is important in establishing the presence or absence of a
suspected element in forensic analysis. The simple qualitative flame test
developed into flame photometry and subsequently into atomic absorption
spectrometry.  See  ATOMIC SPECTROSCOPY; FLAME PHOTOMETRY; SPECTROCHEMICAL
ANALYSIS.
    Bombardment of surfaces by x-rays, electrons, and positive ions has given
rise to a number of methods useful in analytical chemistry. X-ray diffraction
is used to determine crystal structure and to identify crystalline substances
by means of their diffraction patterns. X-ray fluorescence analysis employs
x-rays to excite emission of characteristic x-rays by elements. The electron
microprobe uses electron bombardment in a similar manner to excite x-ray
emission.  See	SECONDARY ION MASS SPECTROMETRY (SIMS); X-RAY DIFFRACTION;
X-RAY FLUORESCENCE ANALYSIS.
    Neutron activation analysis, wherein neutron bombardment induces
radioactivity in isotopes of elements not naturally radioactive, involves
measurement of the characteristic gamma radiation and modes of decay produced
in the target atoms.  See  ACTIVATION ANALYSIS.
 �34s[Jack L. Lambert]
�33s

    Organic analysis.  Qualitative analysis of an organic compound is the
process by which the characterization of its class and structure is
determined. Due to the numerous classes of organic compounds and the
complexity of their molecular structures, a systematic analytical procedure is
often required. A typical procedure entails an initial assignment of compound
classification, followed by a complete identification of the molecular
structure.
    The initial step is an examination of the physical characteristics. Color,
odor, and physical state can be valuable clues. The physical constants of an
unknown compound provide pertinent data for the analyst. Constants such as
melting point, boiling point, specific gravity, and refractive index are
commonly measured.
    The preliminary chemical tests which are applied are elemental analysis
procedures. The elements generally associated with carbon, hydrogen, and
oxygen are sulfur, nitrogen, and the halogens. The analyst is usually
interested in the latter group. These elements are converted to water-soluble
ionic compounds via sodium fusion. The resulting products are then detected
with wet chemical tests. The solubility of a compound in various liquids
provides information concerning the molecular weight and functional groups
present in the compound. In order to indicate the presence or absence of a
functional group, specific classification reactions are tested. The reactions
are simple, are rapid, and require a small quantity of sample. No single test
is conclusive evidence. A judicious choice of reactions can confirm or negate
the presence of a functional group.
    Instrumental methods are commonly applied for functional group
determination and structure identification. Absorption spectroscopy and
infrared absorption spectroscopy are among the most important techniques.
Whenever a molecule is exposed to electromagnetic radiation, certain
wavelengths cause vibrational, rotational, or electronic effects within the
molecule. The radiation required to cause these effects is absorbed. The
nature and configuration of the atoms determine which specific wavelengths are
absorbed. Raman spectroscopy is slightly different from the other
spectroscopic techniques. A sample is irradiated by a monochromatic source.
Depending upon the vibrational and rotational energies of the functional
groups in the sample, light is scattered from the sample in a way which is
characteristic of the functional groups. Instrumental techniques are also
valuable for structure identification. In addition to infrared spectroscopy,
nuclear magnetic resonance and mass spectrometry are widely used. Other, less
common techniques are electron spin resonance, x-ray diffraction, and nuclear
quadrupole spectroscopy.  See  INFRARED SPECTROSCOPY; MASS SPECTROMETRY;
NUCLEAR MAGNETIC RESONANCE (NMR); RAMAN EFFECT.
    In addition to chemical structure, the physical structure of a sample can
be important. Thermal analysis is a useful procedure for examining structural
characteristics. Among the most popular methods are thermogravimetry,
differential thermal analysis, differential scanning calorimetry, and thermal
mechanical analysis.  See  CALORIMETRY; THERMAL ANALYSIS.
    Although the implementation of instrumental methods has greatly simplified
qualitative analytical procedures, no individual instrument is capable of
complete identification for all samples. A complement of instrumental and wet
chemical techniques is generally required for adequate proof of
identification.  See  SPECTROSCOPY.
 �34s[Sidney Siggia ;Kenneth Longmore]
�33s

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�6v�2v�32sQuality control   �21m �33s 
    A system of inspection, analysis, and action applied to a manufacturing 
operation so that, by inspecting a small portion of the product currently 
produced, an estimate of the overall quality of the product can be made to 
determine what, if any, changes must be made in the operation in order to 
achieve or maintain the required level of quality. 
    The term is usually applied in the context of manufacturing operations. 
The manufacturer makes one or more items, each having a set of specifications 
for various characteristics, which must be met to satisfy the needs of the 
customer. The specifications may be set either by the customer in view of the 
intended use of the product of by the manufacturing organization in view of 
its understanding of the customer's intended use, or by both. Quality control 
is the set of activities in a manufacturing operation which undertakes to 
ensure that the specifications of the finished product are satisfied. More 
general terms such as quality assurance and total quality control embrace all 
the technical and management aspects of product quality and safety during the 
design, specification, development, manufacturing, and use stages. Periodic 
reviews of quality standards and quality operations are also undertaken to 
ensure that products remain satisfactory to the consumer and competitive in 
the market place. This can involve comparative testing of competitive products 
and the undertaking of quality improvement programs. Since most manufacturing 
organizations have a profit objective, quality control performs its tasks with 
the objective of minimizing costs, that is, with obtaining the specified 
product quality at the lowest possible cost.  See  INDUSTRIAL COST CONTROL; 
INSPECTION AND TESTING. 
 �34s[John A. Clements]
�33s 
  
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�6v�2v�32sQuantitative chemical analysis   �21m �33s 
    That branch of analytical chemistry which deals with the determination of 
the relative proportions of constituents in a compound or of components in a 
mixture.  
    In an ultimate analysis, the amount of a single element or compound is 
determined. In a proximate analysis, certain constituents are determined as a 
group of indefinite relative composition, for example, the determination of 
ash in a sample of coal. 
    Methods of analysis may be direct or indirect. In a direct gravimetric 
method, the desired constituent is converted to a compound of definite, known 
composition and this compound is weighed. In a direct volumetric method, the 
desired constituent is determined by measuring the volume of reagent of known 
concentration required to react completely with the constituent. 
    In an indirect gravimetric method, a mixture of substances which includes 
the desired constituent is weighed and then wholly, or in part, converted to 
some other substance, or mixture of substances, of known composition and 
weighed. The amount of desired constituent can then be calculated.  See  
GRAVIMETRIC ANALYSIS. 
    In an indirect volumetric method, a measured quantity of reagent is added 
in excess of the amount required to react with the desired constituent. The 
excess reactant is then determined by titration, and the amount of it which 
reacts with the desired constituent is determined by difference.  See  
TITRATION; VOLUMETRIC ANALYSIS. 
    Methods of quantitative analysis vary with the amount of sample taken and 
of constituent being determined. A macroanalysis (decigram analysis) uses a 
sample of about 0.1 - 0.5 gram; a semimicroanalysis (centigram analysis) uses 
a sample of about 0.01 - 0.1 g; a microanalysis (milligram analysis) uses a 
sample of about 1 - 10 mg; and an ultramicroanalysis (microgram analysis) uses 
a sample of approximately 0.001 mg.  See  BALANCE. 
    Since the accuracy of a quantitative analysis depends in part on the 
nature of the material and on the nature and quantities of foreign 
constituents present, calibration of methods of unknown applicability is often 
necessary.  
    In the correction factor method, the procedure is applied to a sample 
containing all the constituents in the given sample except the desired one. 
Any numerical result obtained is applied as a correction factor to the value 
obtained on the unknown. 
    In the synthetic sample method, the procedure of analysis is applied in 
parallel to the unknown and to a sample containing constituents of the same 
nature and approximate amounts as those in the unknown, and with the desired 
constituent added in known amount. Any discrepancy in the value obtained on 
the synthetic sample is applied to correct the analysis of the unknown. See 
ANALYTICAL CHEMISTRY; SPECTROCHEMICAL ANALYSIS; SPECTROPHOTOMETRIC ANALYSIS; 
VOLUMETRIC ANALYSIS. 
 �34s[Stephen G. Simpson]
�33s 
  
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�6v�2v�32sQuantity flowmeter �21m �33s
    A device that measures fluid flow by measuring or passing fixed amounts of
fluid. By introducing a timing element, all quantity meters can be converted
to volume flow rate meters. Quantity meters may be classified as
positive-displacement meters or rotating impeller-type meters.	See
POSITIVE-DISPLACEMENT FLOWMETER; ROTATING IMPELLER FLOWMETER.
    The accuracy of quantity meters is dependent to varying degrees on the
physical properties of the fluid being measured. Changes from the reference
(calibration) conditions of viscosity and density must be considered if
accurate measurement is required.  See	FLOW MEASUREMENT.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

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�6v�2v�32sQuantization   �21m �33s 
    A term applied to the transition from a description of a system of 
particles or fields in the classical approximation where canonically conjugate 
variables commute to a description where these variables are treated as 
noncommuting operators. In this transition, Poisson bracket relations among 
dynamical variables are replaced by commutation relations. So-called second 
quantization is the analogous treatment of a field, such as the 
electromagnetic field solution of Maxwell's equations, the wave-function 
solution of SchrACCENT UMLAUTodinger's equation, or the solution of Dirac's 
equation, with commutation relations imposed upon the field as the canonical 
coordinate and with the generalized momentum defined through the Lagrangian. 
The wave equation in the latter two cases resulted from the first 
quantization.  See  CANONICAL TRANSFORMATIONS; LAGRANGE'S EQUATIONS; 
NONRELATIVISTIC QUANTUM THEORY; QUANTUM ELECTRODYNAMICS; QUANTUM FIELD THEORY. 
    A result of treating a system through quantum mechanics is that some 
observable quantities become quantized, or assume discrete values. 
Independently of the above operator discussion, the process leading to 
discrete values for an observable is also called quantization. For example, 
energy and angular momentum historically were first treated this way through 
quantization postulates hypothesizing discrete eigenvalue spectra.  See  
ATOMIC STRUCTURE AND SPECTRA; QUANTUM MECHANICS. 
 �34s[Kenneth E. Lassila]
�33s 
  
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�6v�2v�32sQuantized vortices �21m �33s
    A type of flow pattern exhibited by superfluids, such as liquid (sup 4)He
below 2.17 K (-455.76DGF). The term vortex designates the familiar whirlpool
pattern where the fluid moves circularly around a central line and the
velocity diminishes inversely proportionally to the distance from the center.
The strength of a vortex is determined by the circulation, which is the line
integral of the velocity around any path enclosing the central line.  See
VORTEX.
    A superfluid is believed to be characterized by a macroscopic (that is,
large-scale) quantum-mechanical wave function PSI. This wave function locks
the superfluid into a coherent state. Since the velocity around the vortex
increases without limit as the center is approached, the superfluid density
and thus PSI must vanish at the center in order to avoid an infinite energy.
Thus the central core of the vortex marks the zeros, or nodal lines, in the
macroscopic wave function.  See  NONRELATIVISTIC QUANTUM THEORY; QUANTUM
MECHANICS.
    Quantized vortex lines are usually produced by rotating a vessel
containing superfluid helium. At very low rotation speeds, no vortices exist:
the superfluid remains at rest while the vessel rotates. At a certain speed
the first vortex appears and corresponds to the first excited rotational state
of the system. If the container continues to accelerate, additional quantized
vortices will appear. At any given speed the vortices form a regular array
which rotates with the vessel.
    Quantized vortex lines were first detected in the mid-1950s by their
influences on superfluid thermal waves traveling across the lines. In the late
1950s it was discovered that electrons in liquid helium form tiny charged
bubbles which can become trapped on the vortex core but can move quite freely
along the line. These electron bubbles (often referred to as ions) have been
one of the most useful probes of quantized vortices. Researchers have been
able to use ions to detect single quantized vortex lines. In one experiment
the trapped ions are pulled out at the top of the vortex lines, accelerated,
and focused onto a phosphor screen. The pattern of light thus produced on the
phosphor is a map of the position of the vortices where they contact the
liquid meniscus (see illustration).  See  LIQUID HELIUM; SUPERFLUIDITY.
 �34s[Raymond E. Peck]
�33s


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�6v�2v�32sQuantum chemistry �21m �33s
    A branch of chemistry concerned with the application of quantum mechanics
to chemical problems. More specifically, it is concerned with the electronic
structure of molecules. Methods developed since 1960 permit the quantum
chemist to obtain reliable approximate solutions to the nonrelativistic
Schroedinger equation. The method which dominates the field of quantum
chemistry is the Hartree-Fock or self-consistent-field approximation.  See
HAMILTON'S EQUATIONS OF MOTION; NONRELATIVISTIC QUANTUM THEORY; QUANTUM
MECHANICS.
    For closed-shell molecules, the form of the Hartree-Fock wave function is

    PSI(sub HF) = A(n)PHI(sub 1)(1)PHI(sub 2)(2) . . . PHI(sub n)(n)	    (1)

given by Eq. (1), in which A(n), the antisymmetrizer for n electrons, has the
effect of making a Slater determinant out of the orbital product on which it
operates. ThePHI's are spin orbitals, products of a spatial orbital CHI and a
one-electron spin function ALPHA or BETA. For any given molecular system,
there are an infinite number of wave functions of form (1), but the
Hartree-Fock wave function is the one for which the orbitals PHI have been
varied to yield the lowest possible energy [Eq. (2)].

	  E = INTEGRATION (PSI*(sub HF)HPSI(sub HF)dtau )		     (2)

    The resulting Hartree-Fock equations are relatively tractable due to the
simple form of the energy E for single determinant wave functions [Eq. (3)].

E(sub HF) = SUMMATION (i) I(i VERTICAL BAR i) + SUMMATION (i) SUMMATION      (3)
	    [(ij VERTICAL BAR ij) - (ij VERTICAL BAR ji)]
    To solve the Hartree-Fock equations exactly, either the orbitals PHI must
be expanded in a complete set of analytic basis functions or strictly
numerical (that is, tabulated) orbitals must be obtained. The former approach
is impossible from a practical point of view for systems with more than two
electrons, and the latter has been accomplished only for atoms and for a few
diatomic molecules. Therefore, the exact solution of the Hartree-Fock
equations is abandoned for polyatomic molecules. Instead an incomplete (but
reasonable) set of analytic basis functions is adopted and solved for the best
variational [that is, lowest energy given by Eq. (2)] wave function of form
(1). Such a wave function is referred to as being of self-consistent-field
(SCF) quality. For very large basis sets, then, it is reasonable to refer to
the resulting SCF wave function as near-Hartree-Fock.
    For large chemical systems, only minimum basis sets (MBS) can be used in
ab initio theoretical studies. The term ``large'' includes molecular systems,
with 100 or more electrons.
    Ab initio theoretical methods have had the greatest impact on chemistry in
the area of structural predictions. The most encouraging aspect of ab initio
geometry predictions is their reliability. Essentially all molecular
structures appear to be reliably predicted at the Hartree-Fock level of
theory. Even more encouraging, many structures are accurately reproduced by
using only minimum-basis-set self-consistent-field methods. A fairly typical
example is methylenecyclopropane (see illustration), with its
minimum-basis-set self-consistent-field structure compared with experiment in
the table. Carbon-carbon bond distances differ typically by 0.002 nm from
experiment, and angles are rarely in error by more than a few degrees. Thus,
for many purposes, theory may be considered complementary to experiment in the
area of structure prediction.
    The Hartree-Fock formalism has very powerful predictive capabilities.
However, large classes of chemical problems cannot be reasonably described,
and a good deal of discretion is required on the part of the theoretician.
Although state-of-the-art quantum chemistry has gone well beyond the
Hartree-Fock model, the model is certain to remain the cornerstone of
electronic structure theory for many years to come.  See  CHEMICAL BONDING;
CHEMICAL STRUCTURES; MOLECULAR ORBITAL THEORY; MOLECULAR STRUCTURE AND
SPECTRA; RESONANCE (MOLECULAR STRUCTURE).
 �34s[Henry F. Schaefer, III]
�33s

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�6v�2v�32sQuantum chromodynamics   �21m �33s 
    A theory of the strong (``nuclear'') interactions among quarks, which are 
regarded as fundamental constituents of matter. Quantum chromodynamics (QCD) 
seeks to explain why quarks combine in certain configurations to form the 
observed patterns of subnuclear (or ``elementary'') particles, such as the 
proton and pi meson. According to this picture, the strong interactions among 
quarks are mediated by a set of force particles known as gluons. Strong 
interactions among gluons may lead to new structures that correspond to as yet 
undiscovered particles. The long-studied nuclear force which binds protons and 
neutrons together in atomic nuclei is regarded as a collective effect of the 
elementary interactions among constituents of the composite protons and 
neutrons.  See  NUCLEAR STRUCTURE. 
    Quantum chromodynamics has not yet been subjected to rigorous experimental 
tests. Several qualitative predictions of quantum chromodynamics do seem to 
have been borne out. Part of the esthetic appeal of the theory is due to the 
fact that quantum chromodynamics is nearly identical in mathematical structure 
to quantum electrodynamics (QED) and to the unified theory of weak and 
electromagnetic interactions put forward by S. Weinberg and A. Salam. This 
resemblance encourages the hope that a unified description of the strong, 
weak, and electromagnetic interactions may be at hand.  See  QUANTUM 
ELECTRODYNAMICS; WEAK NUCLEAR INTERACTIONS. 
  
    Gauge theories.  At the heart of current theories of the fundamental 
interactions is the idea of gauge invariance, which draws its name from some 
early investigations by H. Weyl into a possible connection between scale 
changes and the equations of electrodynamics. Weyl attempted to deduce 
electromagnetism from a symmetry principle, the conjectured invariance of 
physical laws under a change of length scale chosen independently at every 
position of space and time. This specific undertaking ran afoul of quantum 
mechanics, but the general strategy and the name have survived. It is widely 
believed that gauge theories constructed to embody various symmetry principles 
represent the correct quantum-mechanical descriptions of the strong, weak, and 
electromagnetic interactions.  See  SYMMETRY LAWS (PHYSICS). 
  
    Color.  Although the quark model reproduces the properties of the observed 
hadron states, a theoretical inconsistency arises from the fact that the 
quarks must be spin-** particles. According to the Pauli exclusion principle, 
identical spin-** particles cannot occupy the same quantum state. The 
properties of several of the baryon resonances can be reproduced only by 
having three quarks of the same flavor in a configuration which is symmetric 
under the interchange of any pair of quarks, contrary to the exclusion 
principle. It was shown that the quark model could be made to conform with the 
Pauli principle if a new distinguishing property was imputed to the quarks. 
Each quark flavor is thus said to come in three colors, arbitrarily labeled 
red, blue, and green. Color may be regarded as the strong-interaction analog 
of the electric charge. Color cannot be created or destroyed by any of the 
known interactions. Like electric charge, it is said to be conserved.  See  
COLOR (QUANTUM MECHANICS); EXCLUSION PRINCIPLE. 
    In the face of evidence that color could be regarded as the conserved 
charge of the strong interactions, it was natural to seek a gauge symmetry 
which would have color conservation as its consequence. An obvious candidate 
for the gauge symmetry group is the unitary group SU(3), now to be applied to 
color rather than flavor. The theory of strong interactions among quarks which 
is prescribed by local color gauge symmetry is known as quantum 
chromodynamics. The mediators of the strong interactions are eight massless 
spin-1 bosons, one for each generator of the symmetry group. These 
strong-force particles are named gluons because they make up the ``glue'' 
which binds quarks together into hadrons. Gluons also carry color, and hence 
have strong interactions among themselves. 
  
    Asymptotic freedom.  The theoretical description of the strong 
interactions has historically been inhibited by the very strength of the 
interaction, which renders low-order perturbative calculations untrustworthy. 
In 1973 it was found that, in many gauge theories, the effective strength of 
the interaction becomes increasingly feeble at short distances. For quantum 
chromodynamics, this implies that the interaction between quarks becomes weak 
at small separations, a property known as asymptotic freedom. This discovery 
raises the hope that some aspects of the strong interactions might be treated 
by using familiar computational techniques that are predicated upon the 
smallness of the interaction strength. 
  
    Quarkonium.  It was suggested in 1974 that the bound system of an 
extremely massive quark with its antiquark would be so small that the strong 
force would be extremely feeble. In this case, the binding between quark and 
antiquark is mediated by the exchange of a single massless gluon, and the 
spectrum of bound states resembles that of an exotic atom composed of an 
electron and an antielectron (positron) bound electromagnetically in a Coulomb 
potential generated by the exchange of a massless photon. Since the 
electron-positron atom is known as positronium, the heavy quark-antiquark atom 
has been called quarkonium. Two families of heavy quark-antiquark bound 
states, the  PSI/J system composed of charmed quarks and the Y system made up 
of b quarks, have been discovered. Both have level schemes characteristic of 
atomic spectra, which have been analyzed by using tools of nonrelativistic 
quantum mechanics developed for ordinary atoms.  See  CHARM; ELEMENTARY 
PARTICLE; FUNDAMENTAL INTERACTIONS; GLUONS; J PARTICLE; POSITRONIUM; QUANTUM 
FIELD THEORY; QUARKS; STRONG NUCLEAR INTERACTIONS. 
 �34s[Chris Quigg]
�33s 
  
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�6v�2v�32sQuantum electrodynamics   �21m �33s 
    The study of the properties of electromagnetic radiation and its 
interaction with electrically charged matter; in particular, with atoms and 
their constituent electrons. The fundamental equations governing quantum 
electrodynamics actually are believed to encompass all of atomic physics, 
chemistry, properties of bulk matter, and classical electromagnetic theory. 
Almost all the phenomena readily perceived by the senses are believed to be 
ultimately understandable in terms of the laws of quantum electrodynamics. In 
the scope of its application and implications, and in the simplicity of the 
underlying assumptions, quantum electrodynamics is rivaled only by 
gravitational theory. In practice, the term quantum electrodynamics refers to 
those phenomena specific to the quantum nature of electromagnetic radiation. 
This includes the study of emission and absorption of light by atoms and the 
basic interactions of light with electrons and other fundamental particles.  
See  ELECTROMAGNETIC RADIATION; GRAVITATION; QUANTUM FIELD THEORY. 
    Quantum electrodynamics was formulated by P. A. M. Dirac, W. Heisenberg, 
and W. Pauli shortly after the foundations of quantum mechanics were laid down 
in 1925. This formulation gave a satisfactory interpretation of the 
wave-particle duality of light, in which it is possible for light to manifest 
itself both in ways appropriate to a particle (photon) description or a 
wavelike description, in accordance with the experimental foundations of 
quantum mechanics. About the same time, Dirac discovered an equation 
describing the motion of electrons which incorporated both the requirements of 
quantum theory and of relativity. However, introduction of electromagnetic 
interaction into Dirac's equation, while successfully describing the magnetic 
properties of the electron and the existence of its oppositely charged 
counterpart, or antiparticle, the positron, led to mathematical disasters 
which inhibited the further development of the theory until after World War 
II. Stimulated by experiments using microwave techniques developed during 
World War II, S. Tomonaga, R. P. Feynman, J. S. Schwinger, F. J. Dyson, and 
many others found a way to bypass, but not solve, these difficulties. Since 
that time the theory has been confronted by very sophisticated experimental 
tests and has passed them satisfactorily.  See  QUANTUM MECHANICS. 
 �34s[J. D. Bjorken]
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�6v�2v�32sQuantum electronics   �21m �33s 
    A loosely defined field concerned with the interaction of radiation and 
matter, particularly those interactions involving quantum energy levels and 
resonance phenomena, and especially those involving lasers and masers. Quantum 
electronics encompasses useful devices such as lasers and masers and their 
practical applications; related phenomena and techniques, such as nonlinear 
optics and light modulation and detection; and related scientific problems and 
applications, such as quantum noise processes, laser spectroscopy, picosecond 
spectroscopy, and laser-induced optical breakdown. 
    In one sense any electronic device, even one as thoroughly classical in 
nature as a vacuum tube, may be considered a quantum electronic device, since 
quantum theory is accepted to be the basic theory underlying all physical 
devices. In practice, however, quantum electronics is usually understood to 
refer to only those devices such as lasers and atomic clocks in which 
stimulated transitions between discrete quantum energy levels are important, 
together with related devices and physical phenomena which are excited or 
explored using lasers. Other devices such as transistors or superconducting 
devices which may be equally quantum-mechanical in nature are not usually 
included in the domain of quantum electronics.  See  ATOMIC CLOCK; LASER; 
LASER SPECTROSCOPY; MASER; NONLINEAR OPTICS; OPTICAL DETECTORS; OPTICAL 
MODULATORS; OPTICAL PULSES; QUANTUM MECHANICS. 
 �34s[A. E. Siegman]
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�6v�2v�32sQuantum field theory �21m �33s
    Quantum theory of physical systems possessing an infinite number of
degrees of freedom, such as the electromagnetic field, gravitational field, or
wave fields in a medium. Its major applications lie in the attempted
description of fundamental particles and their associated wave fields under
circumstances in which both the effects of quantum mechanics and of special
relativity are important. Quantum field theory is also used in nonrelativistic
quantum theory for systems of many particles such as electrons in a metal, or
for sound waves in liquids and solids.	See  NONRELATIVISTIC QUANTUM THEORY;
QUANTUM MECHANICS; RELATIVISTIC QUANTUM THEORY; RELATIVITY.

    The classical electromagnetic field is a dynamical system which must be
subjected to the rules of quantum mechanics. The consequence of this
requirement is the emergence of a particle (photon) interpretation from the
classical Maxwell equations, manifesting the particle-wave duality of quantum
mechanics in its most perfect form. The experimental success of this theory
for electrodynamics has led to its imitation for the more complex interactions
of other fundamental particles. For example, in 1935 H. Yukawa tried to
interpret the strong force between protons and neutrons responsible for the
existence of nuclei in the same way. This force is only effective when the
protons and neutrons are within a distance on the order of 10(sup -13) cm
(4x10(sup -14) in.) of each other. Yukawa supposed the nuclear force was due
to the exchange of a particle, called a meson, between the nucleons, just as
the electrostatic force between charged particles is interpreted in terms of
exchange of a photon, as illustrated.  See  ELEMENTARY PARTICLE; MESON;
QUANTUM ELECTRODYNAMICS.
    The short range of the force can be understood as a consequence of a
finite rest mass m for the meson.  When the nucleon emits the meson, the energy
cost is approximately mc(sup 2).  According to the uncertainty principle, if the
energy of the system is observed to the accuracy DELTAE, the observation must be
made over an interval of time DELTAt longer than hVERTICAL BARDELTAE.  Thus, an
energy fluctuation of a magnitude of SIMILAR mc(sup 2) can only take place over
a time interval DELTA t<h VERTICAL BAR mc(sup 2), consistent with the
uncertainty principle.	In this time interval, the meson can travel no further
than c DELTA t<h VERTICAL BAR mc(sup 2), a distance identified with the range of
the force.  In this way Yukawa was led to predict the mass of the meson (m
NEARLY EQUAL 270 m(sub electron)) intermediate between the electron and proton
mass.  To describe this Yukawa meson, now known as PI-meson, one writes an
analog of the Maxwell field equations and then passes to the quantum description
in a way similar to that described for the electromagnetic field.  A
satisfactory particle description is thereby obtained, again embodying the
characteristic wave-particle duality of the quantum theory.  See UNCERTAINTY
PRINCIPLE.
    However, although the theory of isolated mesons is satisfactory, when
interactions such as the couplings to the nucleons are introduced, chaos
emerges. The field equations have not been solved. There are grave doubts as
to whether solutions exist. There is, aside from the electromagnetic
interactions, no small parameter in terms of which there might exist a series
expansion.
    Faced with this dilemma, theoretical physicists have concentrated their
efforts on finding general properties of any relativistic quantum field
theory, rather than detailed consequences of a specific set of equations.
These efforts have been fruitful in three different areas: (1) the quantum
theory of symmetries and their connection with conservation laws such as
conservation of energy, momentum, and angular momentum; (2) the study of
certain exact properties; in particular the mathematical property of
analyticity of the functions used to describe collision processes; (3) the use
of the diagrammatic method of R. P. Feynman, developed for quantum
electrodynamics and appropriately modified for the study of more general
interactions, to interpret qualitatively many of the features of collision
processes between relativistic particles.  See	SYMMETRY LAWS (PHYSICS).
 �34s[J. D. Bjorken]
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�6v�2v�32sQuantum mechanics �21m �33s
    The modern theory of matter, of electromagnetic radiation, and of the
interaction between matter and radiation; also, the mechanics of phenomena to
which this theory may be applied. Quantum mechanics, also termed wave
mechanics, generalizes and supersedes the older classical mechanics and
Maxwell's electromagnetic theory. Atomic and subatomic phenomena provide the
most striking evidence for the correctness of quantum mechanics and best
illustrate the differences between quantum mechanics and the older classical
physical theories. Quantum mechanics is needed to explain many properties of
bulk matter, for instance, the temperature dependence of the specific heats of
solids.
    The formalism of quantum mechanics is not the same in all domains of
applicability. In approximate order of increasing conceptual difficulty,
mathematical complexity, and likelihood of future fundamental revision, these
domains are the following: (i) Nonrelativistic quantum mechanics, applicable
to systems in which particles are neither created nor destroyed, and in which
the particles are moving slowly compared to the velocity of light. Here a
particle is defined as a material entity having mass, whose internal structure
either does not change or is irrelevant to the description of the system. (ii)
Relativistic quantum mechanics, applicable in practice to a single
relativistic particle (one whose speed equals or nearly equals c); here the
particle may have zero rest mass, in which event, its speed must equal c.
(iii) Quantum field theory, applicable to systems in which particle creation
and destruction can occur; the particles may have zero or nonzero rest mass.
This article is concerned mainly with nonrelativistic quantum mechanics, which
apparently applies to all atomic and molecular phenomena, with the exception
of the finer details of atomic spectra. Nonrelativistic quantum mechanics also
is well established in the realm of low-energy nuclear physics.  See  ATOMIC
STRUCTURE AND SPECTRA; NUCLEAR PHYSICS; QUANTUM FIELD THEORY; RELATIVISTIC
QUANTUM THEORY.

    Planck's constant.  The quantity 6.61 x 10(sup -34) joule-second, first
introduced into physical theory by Max Planck in 1901, is a basic ingredient
of the formalism of quantum mechanics. Planck's constant commonly is denoted
by the letter h; the notation PLANCK'S CONSTANT=h/2pi also is standard.

    Uncertainty principle.  In classical physics the observables
characterizing a given system are assumed to be simultaneously measurable (in
principle) with arbitrarily small error. For instance, it is thought possible
to observe the initial position and velocity of a particle and therewith,
using Newton's laws, to predict exactly its future path in any assigned force
field. According to the uncertainty principle, accurate measurement of an
observable quantity necessarily produces uncertainties in one's knowledge of
the values of other observables. In particular, for a single particle relation
(1a) holds, where  DELTAx represents the uncertainty (error) in the location

	   DELTA x DELTA p(sub x) > PLANCK'S CONSTANT                      (1a)

of the x coordinate of the particle at any instant, and DELTA p(sub x) is the
simultaneous uncertainty in the x component of the particle momentum.  Relation
(1a) asserts that under the best circumstances, the product DELTA x DELTA p(sub
x) of the uncertainties cannot be less than about 10(sup -34) joule-second.
    The uncertainty relation (1b) is derived and interpreted somewhat

	   DELTA t DELTA E UNION PLANCK'S CONSTANT                     (1B)

differently than relation (1a); it asserts that for any  system, an energy
measurement with error DELTAE must be performed in a time not less than
DELTA t APPROXIMATELY EQUAL TO PLANCK'S CONSTANT/DELTA E. If a system endures
for only DELTA t sec, any measurement of its energy must be uncertain by at
least DELTA E APPROXIMATELY EQUAL TO PLANCK'S CONSTANT DELTA t.  See UNCERTAINTY
PRINCIPLE.

    Wave-particle duality.  It is natural to identify such fundamental
constituents of matter as protons and electrons with the mass points or
particles of classical mechanics. According to quantum mechanics, however,
these particles, in fact all material systems, necessarily have wavelike
properties. Conversely, the propagation of light, which, by Maxwell's
electromagnetic theory, is understood to be a wave phenomenon, is associated
in quantum mechanics with massless energetic and momentum-transporting
particles called photons. The quantum-mechanical synthesis of wave and
particle concepts is embodied in the de Broglie relations, given by Eqs. (2a)

				lambda = h/p				   (2a)


				   f = E/h				   (2b)

and (2b). These give the wavelength lambda and wave frequency f associated
with a free particle (a particle moving freely under no forces) whose momentum
is p and energy is E; the same relations give the photon momentum p and energy
E associated with an electromagnetic wave in free space (that is, in a vacuum)
whose wavelength is lambda and frequency is f.	See  PHOTON.
    The wave properties of matter have been demonstrated conclusively for
beams of electrons, neutrons, atoms (hydrogen, H, and helium, He), and
molecules (H2). When incident upon crystals, these beams are reflected into
certain directions, forming diffraction patterns. Diffraction patterns are
difficult to explain on a particle picture; they are readily understood on a
wave picture, in which wavelets scattered from regularly spaced atoms in the
crystal lattice interfere constructively along certain directions only.  See
ELECTRON DIFFRACTION; NEUTRON DIFFRACTION.
    The particle properties of light waves are observed in the photoelectric
effect and the compton effect.	See  COMPTON EFFECT; PHOTOEMISSION.

    Complementarity.  Wave-particle duality and the uncertainty principle are
thought to be examples of the more profound principle of complementarity,
first enunciated by Niels Bohr (1928). According to the principle of
complementarity, nature has ``complementary'' aspects; an experiment which
illuminates one of these aspects necessarily simultaneously obscures the
complementary aspect. To put it differently, each experiment or sequence of
experiments yields only a limited amount of information about the system under
investigation; as this information is gained, other equally interesting
information (which could have been obtained from another sequence of
experiments) is lost. Of course, the experimenter does not forget the results
of previous experiments, but at any instant, only a limited amount of
information is usable for predicting the future course of the system.

    Quantization.  In classical physics the possible numerical values of each
observable, meaning the possible results of exact measurement of the
observable, generally form a continuous set. For example, the x coordinate of
the position of a particle may have any value between -INFINITY and +INFINITY.
In quantum mechanics the possible numerical values of an observable need not
form a continuous set, however. For some observables, the possible results of
exact measurement form a discrete set; for other observables, the possible
numerical values are partly discrete, partly continuous; for example, the
total energy of an electron in the field of a proton may have any positive
value between 0 and +INFINITY, but may have only a discrete set of negative
values, namely, -13.6, -13.6/4, -13.6/9, -13.6/16 eV, . . . . Such observables
are said to be quantized; often there are simple quantization rules
determining the quantum numbers which specify the allowable discrete values.
Spectroscopy, especially the study of atomic spectra, probably provides the
most detailed quantitative confirmation of quantization. See QUANTIZATION.

    Probability considerations.  The uncertainty and complementarity
principles, which limit the experimenter's ability to describe a physical
system, must limit equally the experimenter's ability to predict the results of
measurement on that system.  Suppose, for instance, that a very careful
measurement determines that the x coordinate of a particle is precisely x =
x(sub 0).  This is permissible in nonrelativistic quantum mechanics.  Then
formally, the particle is known to be in the eigenstate corresponding to the
eigenvalue x = x(sub 0) of the x operator.  Under these circumstances, an
immediate repetition of the position measurement again will indicate that the
particle lies at x = x(sub 0).	Knowing that the particle lies at x = x(sub 0)
makes the momentum psubx of the particle completely uncertain, however,
according to relation (1a).  A measurement of p(sub x) immediately after the
particle is located at x = x(sub )0 could yield any value of p(sub x) from
-INFINITY to +INFINITY.
    More generally, suppose the system is known to be in the eigenstate
corresponding to the eigenvalue alpha of the observable A.  Then for any
observable B, which is to some extent complementary to A, that is, for which an
uncertainty relation of the form of relations (1) limits the accuracy with which
A and B can simultaneously be measured, it is not possible to predict which of
the many possible values B = beta will be observed.  However, it is possible to
predict the relative probabilities P(sub alpha)(beta) of immediately thereafter
finding the observable B equal to beta, that is, of finding the system in the
eigenstate corresponding to the eigenvalue B = beta.
    To the eigenvalues correspond eigenfunctions, in terms of which P(sub alpha)
(beta) can be computed.  In particular, when alpha is a discrete eigenvalue of
A, and the operators depend only on x and p(sub x), the probability P(sub alpha)
(beta) is postulated as in Eq.	(3), where u(x,alpha) is the eigenfunction
corresponding to A = alpha; v(x,beta) is the eigenfunction corresponding to B =
beta; and the asterisk denotes the complex conjugate.  The integral in Eq.  (3)
is called the projection of u(x,alpha) on v (x,beta).  The quantity
|u(x,alpha)|(sup 2)dx is the probability that the system, known to be in the
eigenstate A = alpha, will be found in the interval x to x + dx.  See
EIGENFUNCTION; EIGENVALUE (QUANTUM MECHANICS).

  P(sub alpha) (beta) = |INTEGRATION (-INFINITY to INFINITY)
	 dx v*(x,beta)u(x,alpha)|(sup 2)				     (3)


    Wave function.  When the system is known to be in the eigenstate
corresponding to A = alpha, the eigenfunction u(x,alpha) is the wave function;
that is, it is the function whose projection on an eigenfunction v(x,beta) of
any observable B gives the probability of measuring B = beta. The wave
function psi(x) may be known exactly; in other words, the state of the system
may be known as exactly as possible (within the limitations of uncertainty and
complementarity), even though psi(x) is not the eigenfunction of a known
operator. This circumstance arises because the wave function obeys
Schroedinger's wave equation. Knowing the value of psi(x) at time t = 0, the
wave equation completely determines psi(x) at all future times. In general,
however, if psi(x,0) = u(x,alpha), that is, if psi(x,t) is an eigenfunction of
A at t = 0, then psi(x,t) will not be an eigenfunction of A at later times t >
0.
    A system described by a wave function is said to be in a pure state. Not
all systems are described by wave functions, however. For example, a beam of
hydrogen atoms streaming out of a small hole in a hydrogen discharge tube can
be regarded as a statistical ensemble or mixture of pure states oriented with
equal probability in all directions.

    Schroedinger equation.  Equation (4) describes a plane wave of frequency
f, wavelength lambda, and amplitude A(lambda), propagating in the positive x
direction. The previous discussion concerning wave-particle duality suggests
that this is the form of the wave function for a beam of free particles moving
in the x direction with momentum p = p(sub x), with Eq. (2) specifying the
connections between f, lambda and E, p. Differentiating Eq. (4), it is seen
that Eqs. (5) hold. Since for a free particle E = (p sup 2)/2m, it follows also
that Eq. (6) is valid.	See  WAVE MOTION.

	     psi(x,t) = A(lambda) exp {2pii[(x/lambda) - ft]}		     (4)


p(sub x)psi = (h/lambda)psi = (PLANCK'S CONSTANT/i)(CURLY D psi)/(CURLY Dx) (5a)


E psi = hf psi = -(PLANCK'S CONSTANT/i) (CURLY D psi/CURLY Dt)              (5b)


{[(-PLANCK'S CONSTANT)sup 2]/2m}[(CURLY D(sup 2)psi)/CURLY D x(sup 2)] +
V(x)psi = - (PLANCK'S CONSTANT/i) (CURLY D psi/CURLY Dt)                    (6)

    Equation (6) holds for a plane wave of arbitrary lambda, and therefore for
any superposition of waves of arbitrary lambda, that is, arbitrary p(sub x).
Consequently, Eq.  (6) should be the wave equation obeyed by the wave function
of any particle moving under no forces, whatever the projections of the wave
function on the eigenfunctions of p(sub x).  Equations (5) and (6) further
suggest that for a particle whose potential energy V(x) changes, in other words,
for a particle in a conservative force field, psi(x,t) obeys Eq.  (7).

   {[(-PLANCK'S CONSTANT)sup2]/2m}[(CURLYDsup2 psi)/CURLYDxsup2] + V(x)psi =
		-(PLANCK'S CONSTANT/i) (CURLYDpsi/CURLYDt)                   (7)

    Equation (7) is the time-dependent Schroedinger equation for a
one-dimensional (along x), spinless particle. Noting Eq. (5b), and observing
that Eq. (7) has a solution for the form of Eq. (8), it is inferred that
psi(x) of Eq. (8) obeys the time-independent Schroedinger equation, Eq. (9).
See  FORCE.

	      psi(x,t) = psi(x) exp (-iEt/PLANCK'S CONSTANT)                 (8)


   {[(-PLANCK'S CONSTANT)sup2]/2m}[(CURLYDsup2 psi)/CURLYDxsup2] + V(x)psi =
				Epsi					     (9)

    Equation (9) is solved subject to reasonable boundary conditions, for
example, that psi must be continuous and must not become infinite as x
approaches PLUS/MINUS INFINITY. These boundary conditions restrict the values
of E for which there exist acceptable solutions psi(x) to Eq. (9), the allowed
values of E depending on V(x). In this manner, the allowed energies of atomic
hydrogen listed in the earlier discussion of quantization are obtained.
    The forms of Eqs. (5a), (7), and (9) suggest that the classical observable
psubx must be replaced by the operator (PLANCK'S CONSTANT-i) (CURLYD/CURLYDx).
With this replacement, Eq. (10) holds.	In other words, whereas the classical
canonically conjugate variables x and psubx are numbers, obeying the commutative
law in Eq. (11a), the quantum-mechanical quantities x and psubx are
noncommuting operators, obeying Eq. (11b).

	    (xpsubx - psubx x)psi = i(PLANCK'S CONSTANT)psi                 (10)


			  xpsubx - psubx x = 0				   (11a)


		xpsubx - psubx x = i(PLANCK'S CONSTANT)                    (11b)


    Correspondence principle.  Since classical mechanics and Maxwell's
electromagnetic theory accurately describe macroscopic phenomena, quantum
mechanics must have a classical limit in which it is equivalent to the older
classical theories. Although there is no rigorous proof of this principle for
arbitrarily complicated quantum-mechanical systems, its validity is well
established by numerous illustrations.
 �34s[Edward Gerjuoy]
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�6v�2v�32sQuantum numbers   �21m �33s 
    The quantities, usually discrete with integer or half-integer values, 
which are needed to characterize a physical system of one or more atomic or 
subatomic particles. Specification of the set of quantum numbers serves to 
define such a system or, in other words, to label the possible states the 
system may have. In general, quantum numbers are obtained from conserved 
quantities determinable by performing symmetry transformations consisting of 
arbitrary variations of the system which leave the system unchanged. For 
example, since the behavior of a set of particles should be independent of the 
location of the origin in space and time (that is, the symmetry operation is 
translation in space-time), it follows that momentum and energy are rigorously 
conserved.  See  SYMMETRY LAWS (PHYSICS). 
    In general, each physical system must be studied individually to find the 
symmetry transformations, and thus the conserved quantities and possible 
quantum numbers. The quantum numbers themselves, that is, the actual state 
labels, are usually the eigenvalues of the physical operators corresponding to 
the conserved quantities for the system in question. #See  EIGENVALUE (QUANTUM 
MECHANICS); ELEMENTARY PARTICLE; NONRELATIVISTIC QUANTUM THEORY; PARITY 
(QUANTUM MECHANICS). 
    It is not necessary that the conserved quantity be ``quantized'' in order 
to be regarded as a quantum number; for example, a free particle possesses 
energy and momentum, both of which can have values from a continuum but which 
are used to specify the state of the particle. 
 �34s[Kenneth E. Lassila]
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�6v�2v�32sQuantum solids �21m �33s
    A class of solids whose atoms or molecules undergo large zero-point motion
even in the quantum ground state (at temperature T = 0 K = -459.67DGF) as a
result of their small mass and the weak attractive part of their interaction
potential. The most striking examples are the isotopes of helium, sup3He and
sup4He, which have a root-mean-square displacement from their lattice sites of
approximately 25%. Further examples are the molecular hydrogens, H2, D2, and
HD, as well as some heavier molecular solids.  See  INTERMOLECULAR FORCES;
NONRELATIVISTIC QUANTUM THEORY; QUANTUM MECHANICS.
    These materials display quantum effects in their bulk properties when
cooled to temperatures near absolute zero so that the chaotic thermal motion
is reduced. Both of the helium isotopes remain liquid all the way to absolute
zero, unless external pressure (SIMILAR 3 megapascals APPROX EQUAL 30 atm) is
applied. This is because the atoms are not at rest at 0 K; the zero-point
motion acts as an internal pressure which must be overcome in order to bring
the atoms close enough together for solidification. All other substances,
including the hydrogens, freeze under their own vapor pressure above 10 K.
    The two melting curves are quite different in detail because of the
different types of quantum statistics which the particles obey. There is a
pronounced minimum in the (sup 3)He melting pressure which is unique, and can
be understood by considering the entropies of the liquid and the solid. The
pressure minimum leads to the bizarre situation of the addition of heat
causing freezing. The inverse of this process, the adiabatic formation of the
solid by compression, is an important process which has been used extensively
to cool liquid and solid sup3He to temperatures of approximately 1 mK.	See
CRYOGENICS; LIQUID HELIUM.
 �34s[E. Dwight Adams]
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�6v�2v�32sQuantum  statistics   �21m �33s 
    The statistical description of particles or systems of particles whose 
behavior must be described by quantum mechanics rather than by classical 
mechanics. As in classical, that is, Boltzmann statistics, the interest 
centers on the construction of appropriate distribution functions. However, 
whereas these distribution functions in classical statistical mechanics 
describe the number of particles in given (in fact, finite) momentum and 
positional ranges, in quantum statistics the distribution functions give the 
number of particles in a group of discrete energy levels. In an individual 
energy level there may be, according to quantum mechanics, either a single 
particle or any number of particles. This is determined by the symmetry 
character of the wave functions. For antisymmetric wave functions only one 
particle (without spin) may occupy a state; for symmetric wave functions, any 
number is possible. Based on this distinction, there are two separate 
distributions, the Fermi-Dirac distribution for systems described by anti 
symmetric wave functions and the Bose-Einstein distribution for systems 
described by symmetric wave functions.  See  BOLTZMANN STATISTICS; 
BOSE-EINSTEIN STATISTICS; EXCLUSION PRINCIPLE; FERMI-DIRAC STATISTICS; KINETIC 
THEORY OF MATTER; QUANTUM MECHANICS; STATISTICAL MECHANICS. 
 �34s[Max Dresden]
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�6v�2v�32sQuantum (physics)   �21m �33s 
    A term characterizing an excitation in a wave or field, connoting 
fundamental particlelike properties such as energy or mass, momentum, and 
angular momentum for this excitation. In general, any field or wave equation 
that is quantized, including systems already treated in quantum mechanics that 
are second-quantized, leads to a particle interpretation for the excitations 
which are called quanta of the field. This term historically was first applied 
to indivisible amounts of electromagnetic, or light, energy usually referred 
to as photons. The photon, or quantum of the electromagnetic field, is a 
massless particle, best interpreted as such by quantizing Maxwell's equations. 
Analogously, the electron can be said to be the quantum of the Dirac field 
through second quantization of the Dirac equation, which also leads to the 
prediction of the existence of the positron as another quantum of this field 
with the same mass but with a charge opposite to that of the electron. In 
similar fashion, quantization of the gravitational field equations suggests 
the existence of the graviton. The pi meson or pion was theoretically 
predicted as the quantum of the nuclear force field. Another quantum is the 
quantized lattice vibration, or phonon, which can be interpreted as a 
quantized sound wave since it travels through a quantum solid or fluid, or 
through nuclear matter, in the same manner as sound goes through air. 
    The use of the word, quantum, as an adjective (quantum mechanics, quantum 
electrodynamics) implies that the particular subject is to be treated 
according to the modern rules that have evolved for quantized systems.  See  
ELEMENTARY PARTICLE; GRAVITATION; GRAVITON; MAXWELL'S EQUATIONS; MESON; 
NONRELATIVISTIC QUANTUM THEORY; PHONON; PHOTON; QUANTIZATION; QUANTUM 
ELECTRODYNAMICS; QUANTUM FIELD THEORY; QUANTUM MECHANICS. 
 �34s[Kenneth E. Lassila]
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�6v�2v�32sQuarks �21m �33s
    The basic constituent particles, of which ``elementary'' particles are now
believed to be composed. Theoretical models built on the quark concept have
been very successful in understanding and predicting many phenomena in
particle physics. However, the experimental observation of free quarks remains
ambiguous.
    The evolution in the late 1950s and early 1960s of hadron spectroscopy
revealed an order and symmetry among the states of hadronic matter that could
be interpreted in terms of representations of the SU(3) symmetry group. This
in turn was interpreted by M. Gell-Mann, and independently by G. Zweig, as a
consequence of the grouping of elementary constituents of fractional electric
charge (christened quarks by Gell-Mann), in pairs and triplets to form the
observed hadrons. The general features of the quark model of hadrons have
withstood the tests of time, and many of the static properties of hadrons are
consistent with predictions of this model.  See  SYMMETRY LAWS (PHYSICS);
UNITARY SYMMETRY.
    The deep inelastic scattering of electrons on protons revealed form
factors corresponding to pointlike constituents of the proton. J. D. Bjorken
referred to these proton constituents as partons, although from the beginning
it was recognized the partons and quarks might be merely different
manifestations of the same entities.
    Physicists now believe that the proton and neutron are not fundamental
constituents of matter, but that they are made of quarks, very much as the
nuclei of (sup 3)H and (sup 3)He are made of protons and neutrons and as the
molecules of NO2 and N2O are made of oxygen and nitrogen atoms.
    Until 1974 only three flavors of quarks were known; two of very nearly
equal mass, of which the proton, neutron, and pi mesons are composed, and a
third, more massive quark which is a constituent of strange particles such as
the K mesons and hyperons such as LAMBDADG. The names attached to these quarks
are the up quark (u), the down quark (d), and the strange quark (s). Baryons
are presumed to be composed of three quarks, such as the proton (uud), neutron
(udd), LAMBDA DG(uds),and XI-(dss). Mesons are composed of a quark-antiquark
pair, such as the PI+(u BAR d), PI-(ud), K+(u BAR s), and K-(BARu s).
Antiparticles such as the antiprotons are formed by the antiquarks of those
forming the particle, for example, the antiproton p (BAR uud).	See  BARYON;
MESON.
    The quantum numbers of quarks are simply added to give the quantum numbers
of the elementary particle which they form on combination. The natural unit of
electric charge of a quark is +2/3 or -1/3 of the charge on a proton (1.6 x
10-(sup 19) coulomb), and the baryon number of each quark is +1/3; the charge,
baryon number, and so forth, of each antiquark is just the negative of that
for each quark.
    The properties of quarks, to the extent that they are now understood, are
presented in the table, where additional quarks discussed below are also
included.
    In 1974 two very different experiments found evidence for unusual
elementary particles. These particles (labeled the J or PSI) were the first
evidence for the existence of a new quark, the c (charm) quark. The J/PSI
meson is understood to be the vector meson composed of the cc quark pair, and
its decay into mesons is strongly inhibited, so that its lifetime is about
1000 times typical heavy, unstable mesons. This new quark had been predicted
earlier by S. Glashow. Subsequently mesons and baryons containing the c quark
were found and studied	See  CHARM; J PARTICLE.
    In 1977 a new resonance at about 9.4 GeV was reported; subsequent detailed
study confirmed that this is the lowest-lying state of a new quark system. The
quarks which make up this UPSILON (upsilon) particle have been called b for
``bottom'' (or ``beauty'').
    Theoretical physicists expect that there is a heavier quark, as yet
undiscovered, which may have the same relationship to b that the charm quark
does to the strange quark. This quark, together with its presumed quantum
numbers, is indicated in the table as the t quark, where t stands for ``top''
or ``truth.''  See  UPSILON PARTICLE.
    In spite of the dramatic progress in the understanding of elementary
particles which has accompanied the quark concept and the corresponding
revolution in the concept of matter at its most fundamental level, at least
three major questions remain. First, it has not been determined whether or not
there are free quarks. Second, the number of flavors (kinds) of quarks has not
been determined. The predicted t quark has not been found, and it is not known
whether there are an infinite number of quarks, or whether the flavor spectrum
stops at five or six. Third, it is not known whether quarks are truly
fundamental entities, or whether quarks in turn have their own internal
substructure.  See  ELEMENTARY PARTICLE.
 �34s[Lawrence W. Jones]
�33s

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�6v�2v�32sQuarrying   �21m �33s 
    The process of extracting stone for commercial use from natural rock 
deposits. The industry has two major branches: a dimension-stone branch, 
involving preparation of blocks of various sizes and shapes for use as 
building stone, monumental stone, paving stone, curbing, and flagging; and a 
crushed-stone branch, involving preparation of crushed and broken stone for 
use as a basic construction, chemical, and metallurgical raw material. 
 �34s[Stefan H. Boshkov]
�33s 
  
A700000002756�254s�5v�1v1435
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�6v�2v�32sQuartz   �21m �33s 
    The most abundant and widespread of all minerals. Quartz is an important 
rock-forming mineral that occurs as a subordinate constituent of many igneous, 
metamorphic, and sedimentary rocks; it is the principal constituent of 
sandstone and quartzite and of unconsolidated sands and gravels. 
    Quartz does not vary much in its chemical composition (SiO2, silicon 
dioxide). The hardness is arbitrarily designated as 7 on Mohs scale, and the 
specific gravity is 2.650. Ordinarily, quartz is colorless and transparent, 
and the luster is vitreous. Amethyst is a purple or bluish-violet gem variety, 
and citrine is an orange-brown gem variety. Rose quartz is a massive type 
found in pegmatites. Quartz also may have a smoky yellow to dark smoky brown 
color, varying to brownish-black and almost opaque.  See  AMETHYST. 
    Quartz is enantiomorphous; crystals are either right-handed or 
left-handed, and correspondingly rotate the plane of polarization of 
transmitted light. It also is piezoelectric. Twinning is common in quartz. 
Large flawless crystals of quartz are employed for the manufacture of quartz 
oscillator plates, as prisms in optical spectrographs, and as other optical 
devices. Quartz also is employed as an abrasive and in the manufacture of 
firebrick and other refractories and of glass.  See  CRYSTAL OPTICS; 
PIEZOELECTRIC CRYSTAL; PIEZOELECTRICITY; POLARIZED LIGHT; SPECTROSCOPY. 
 �34s[Clifford Frondel]
�33s 
  
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�6v�2v�32sQuartz clock   �21m �33s 
    A clock that uses the piezoelectric property of a quartz crystal. When a 
quartz crystal vibrates, a difference of electric potential is produced 
between two of its faces. The crystal has a natural frequency of vibration, 
depending on its size and shape, and if it is introduced into an oscillating 
electric circuit having nearly the same frequency as the crystal, two effects 
take place simultaneously: The crystal is caused to vibrate at its natural 
frequency, and the frequency of the entire circuit becomes the same as the 
natural frequency of the crystal. 
    In the clock the alternating current from the oscillating circuit is 
amplified, and the frequency is subdivided in steps such as from 100 kHz to 1 
kHz. This setup finally drives a synchronous motor and gear train to display 
time by hands on a clock face. The best crystals run for a year with 
accumulated errors of less than 0.1 s.  See  CLOCK; OSCILLATOR. 
 �34s[Gerald M. Clemence]
�33s 
  
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�6v�2v�32sQuartzite   �21m �33s 
    A metamorphic rock consisting largely or entirely of quartz. Most 
quartzites are formed by metamorphism of sandstone; but some have developed by 
metasomatic introduction of quartz, SiO2, often accompanied by other chemical 
elements, for example, metals and sulfur (ore quartzites).  See  METAMORPHIC 
ROCKS; METASOMATISM; SANDSTONE. 
    Pure sandstones yield pure quartzites. Impure sandstones yield a variety 
of quartzite types. The cement of the original sandstone is in quartzite 
recrystallized into characteristic silicate minerals, whose composition often 
reflects the mode of development. Even the Precambrian quartzites correspond 
to types that are parallel to present-day deposits.  See  QUARTZ. 
 �34s[T. F. W. Barth]
�33s 
  
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�6v�2v�32sQuasars �21m �33s
    A class of astronomical objects that appear starlike on a photographic
plate but possess many other characteristics, such as a large red shift, that
prove they are not stars. The name ``quasar'' is a contraction of the term
``quasistellar object'' (QSO) which was originally given to these objects for
their photographic appearance. The objects appear starlike because their
angular diameters are less than about 1 second of arc, which is the resolution
limit of ground-based optical telescopes imposed by atmospheric effects.
    The color of quasars is generally much bluer than that of most stars with
the exception of white dwarf stars. The blueness of quasars as an identifying
characteristic led to the discovery that many blue starlike objects have a
large red shift and are therefore quasars. The quasistellar objects (QSOs)
discovered this way turned out to emit little or no radio radiation and to be
about 100 times more numerous than the radio-emitting quasistellar radio
sources (QSSs). Why some should be strong radio emitters and most others not
is unknown. The orbiting Einstein x-ray observatory has found that most
quasars also emit strongly at x-ray frequencies. The optical continuum
emission is highly variable, with time scales ranging from fractions of a day
to several years. The rapid fluctuations indicate that there are some
components in quasars that have diameters less than a light-day or about
10sup1sup0 km, the size of the solar system. The radiation mechanism
responsible for the optical and x-ray continuum emission is not known, but the
most likely possibilities are either synchrotron emission or inverse Compton
radiation.  See  GALAXY; GALAXY, EXTERNAL.
    The many similarities of the observed characteristic of quasars with radio
galaxies, Seyfert galaxies, and BL Lacertae objects strongly suggest that
quasars are also active nuclei of galaxies. There is good statistical evidence
which shows that quasars with large red shifts are spatially much more
numerous than those with small red shifts. Because high-red-shift objects are
very distant and emitted their radiation at an earlier epoch, quasars must
have been much more common in the universe about 10(sup 10) years ago. Since
this is about the same epoch when galaxies are thought to form, it is possible
that quasars may be associated with the birth of some galaxies.
    The prodigious amount of energy released by quasars over their lifetime
must involve a quantity of matter more than 10(sup 9) times the mass of the Sun,
and contained within a volume similar in size to the solar system. The source
of this energy and the mechanism by which particles are accelerated to
velocities very near the speed of light are not known.	See  ASTRONOMICAL
SPECTROSCOPY; BLACK HOLE; INFRARED ASTRONOMY; NEUTRON STAR; RADIOASTRONOMY;
SUPERMASSIVE STARS.
 �34s[William Dent]
�33s

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�6v�2v�32sQuasiatom   �21m �33s 
    A transient electronic structure, formed in an atom-atom or ion-atom 
collision, which closely approximates the characteristics usually identified 
with a stable atom whose atomic number equals the combined charge carried by 
the colliding nuclei. Such short-lived atomic states can be formed in 
energetic atomic collisions, when the distance of closest approach between 
nuclear charge centers shrinks beyond the mean classical orbiting radius of 
the most-bound electrons, so that even these innermost electrons see a nearly 
monopolar electric field generated by the single charge center momentarily 
formed by the two nuclear collision partners. Thus, for the short time of this 
close nuclear promixity, all the surrounding electrons behave as if their 
motion were governed by this single charge center, leading to the formation of 
an electronic energy-level structure and electronic shells associated with a 
united atom having atomic number Z = Z1 + Z2, where Z1 and Z2 are the 
individual atomic numbers of the colliding system.  See  ATOMIC STRUCTURE AND 
SPECTRA.  
    An especially important potential utilization of quasiatom formation was 
suggested in 1969 as a possible vehicle for investigating superheavy atoms, 
considerably beyond the heaviest stable systems available. Such atomic systems 
are of intrinsic interest in quantum field theory, and particularly in the 
quantum electrodynamics of the very strong fields which can be generated in 
atoms when an electron is bound by a charge center whose value Z exceeds 
1/ALPHA CONGRUENT 137, where ALPHA is the fine-structure constant. Under the 
latter conditions the usual perturbative expansion of all 
quantum-electrodynamic effects in terms of the coupling constant ZALPHA is of 
doubtful validity, and any new theoretical approaches require experimental 
verification.  See  QUANTUM ELECTRODYNAMICS; QUANTUM FIELD THEORY. 
 �34s[Jack S. Greenberg]
�33s 
  
A700000004352�254s�5v�1v1436
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�6v�2v�32sQuasielastic light scattering �21m �33s
    Small frequency shifts or broadening from the frequency of the incident
radiation in the light scattered from a liquid, gas, or solid. The term
quasielastic arises since the frequency changes are usually so small that,
without instrumentation specifically designed for their detection, they would
not be observed and the scattering process would appear to occur with no
frequency changes at all, that is, elastically. The technique is used by
chemists, biologists, and physicists to study the dynamics of molecules in
fluids, mainly liquids and liquid solutions.
    Several distinct experimental techniques are grouped under the heading of
quasielastic light scattering (QLS). Intensity fluctuation spectroscopy (IFS)
is the technique most often used to study such systems as macromolecules in
solution and critical phenomena where the molecular motions to be studied are
rather slow (see illustration). This technique, also called photon correlation
spectroscopy and, less frequently, optical mixing spectroscopy, is used to
measure the dynamical constants of processes with relaxation time scales
slower than about 10(sup -6) s. For faster processes, dynamical constants are
obtained by utilizing techniques known as filter methods, which obtain direct
measurements of the frequency changes of the scattered light by utilizing a
monochromator or filter much as in Raman spectroscopy.	See  INTERFEROMETRY;
RAMAN EFFECT; SCATTERING OF ELECTROMAGNETIC RADIATION.
    If light is scattered by a collection of scatterers, the scattered
intensity at a point far from the scattering volume is the result of
interference between the wavelets scattered from each of the scatterers and,
consequently, will depend on the relative positions and orientations of the
scatterers, the scattering angle THETA, and the wavelength LAMBDA of the light
used. The structure of scatterers in solution whose size is comparable to
(4PI LAMBDA) sin THETA/2 (IDENTICAL q) where q is the length of the scattering
vector, may be studied by a technique variously called static light
scattering, integrated intensity light scattering, or in the older literature
simply light scattering.
 �34s[Robert Pecora]
�33s


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�6v�2v�32sQuaternary   �21m �33s 
    A period that encompasses at least the last 3,000,000 years of the 
Cenozoic Era, and is concerned with major worldwide glaciations and their 
effect on land and sea, on worldwide climate, and on the plants and animals 
that lived then. The Quaternary is divided into the Pleistocene Epoch and 
Holocene. The universal term Pleistocene is gradually replacing Quaternary; 
Holocene involves the last 7000 years since the Pleistocene.  See  CENOZOIC; 
GLACIAL EPOCH; HOLOCENE; PLEISTOCENE. 
 �34s[Sidney E. White]
�33s 
  
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�6v�2v�32sQuaternary ammonium salts �21m �33s
    Analogs of ammonium salts in which organic radicals have been substituted
for all four hydrogens of the original ammonium cation. Substituents may be
alkyl, aryl, or aralkyl, or the nitrogen may be part of a ring system. Such
compounds are usually prepared by treatment of an amine with an alkylating
reagent under suitable conditions. They are typically crystalline solids which
are soluble in water and are strong electrolytes. Treatment of the salts with
silver oxide, potassium hydroxide, or an ion-exchange resin converts them to
quaternary ammonium hydroxides, which are very strong bases, as shown in the
reaction below.

		    R(sup 1)			       R(sup 1)
		    | + 	-		       | +	    -
	 R(sup 4)---N---R(sup 2)X --------> R(sup 4)---N---R(sup 2)OH
		    |				       |
		    R(sup 3)			       R(sup 3)

		Quaternary			   Quaternary
	       ammonium salt		       ammonium hydroxide

    Some quaternary ammonium salts have found use as water repellents,
fungicides, emulsifiers, paper softeners, antistatic agents, and corrosion
inhibitors.  See  AMINE; AMMONIUM SALT; SURFACE-ACTIVE AGENT.
 �34s[Paul E. Fanta]
�33s

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�6v�2v�32sQuaternions �21m �33s
    An associative, noncommutative algebra based on four linearly independent
units or basal elements. Quaternions were originated in 1843, by W. R.
Hamilton.
    The four linearly independent units in quarternion algebra are commonly
denoted by 1, i, j, k, where 1 commutes with i, j, k and is called the
principal unit or modulus. These four units are assumed to have the following
multiplication table:

	1(sup2) = 1   i(sup2) = j(sup2) = k(sup2) = ijk = -1
	i(jk) = (ij)k = ijk
	1i = i1    ij = j1    1k = k1

The i, j, k do not commute with each other in multiplication, that is, ij NOT
EQUAL ji, jk NOT EQUAL kj, ik NOT EQUAL ki, etc. But all real and complex
numbers do commute with i, j, k; thus if c is a real number, then ic = ci,
jc = cj, and kc = ck. On multiplying ijk = -1 on the left by i, so that iijk =
i(-1) = -i, it is found, since isup2 = -1, that jk = i. Similarly jjk = ji =
-k; when exhausted, this process leads to all the simple noncommutative
relations for i, j, k, namely,

		  ij = -ji = k	 jk = -kj = i	 ki = -ik = j

More complicated products, for example, jikjk = -kki = i, are evaluated by
substituting for any adjoined pair the value given in the preceding series of
relations and then proceeding similarly to any other adjoined pair in the new
product, and so on until the product is reduced to PLUS/MINUS 1, PLUS/MINUS i,
PLUS/MINUSj, or PLUS/MINUSk. Multiplication on the right is also permissible;
thus from ij = k, one has ijj = kj, or -i = kj. Products such as jj and jjj
may be written j(sup 2) and j(sup 3).
    All the laws and operations of ordinary algebra are assumed to be valid in
the definition of quaternion algebra, except the commutative law of
multiplication for the units i, j, k. Thus the associative and distributive
laws of addition and multiplication apply without restriction throughout.
Addition is also commutative, for example, i + j = j + i.
 �34s[Don M. Yost]
�33s

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�6v�2v�32sQuebracho �21m �33s
    Any of a number of trees belonging to different genera but having similar
qualities, all indigenous to South America and valuable for both wood and
bark. The heartwood of one South American tree, Schinopsis lorentzii (family
Anacardiaceae), is called quebracho (meaning ax-breaker) in reference to the
exceedingly hard wood, one of the hardest known. Quebracho is the world's most
important source of tannin.  See  SAPINDALES; TANNIN.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sQueensland tick typhus   �21m �33s 
    A benign infectious disease found in rural northeastern Australia, caused 
by bacterialike microorganisms, Rickettsia australis, and presumed to be 
carried by the tick Ixodes holocyclus. The symptomatology, including eschar, 
is similar to rickettsialpox.  See  RICKETTSIOSES. 
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sQuellung reaction   �21m �33s 
    Swelling of the capsule of a bacterial cell, caused by contact with serum 
containing antibodies capable of reacting with polysaccharide material in the 
capsule. The swelling is thought to be caused mainly by incorporation of 
fluid. The quellung phenomenon is sometimes employed as a method for rapid 
identification of bacteria seen in sputum or spinal fluid, being applicable to 
the Pneumococcus, Klebsiella, and Hemophilus.  See  HEMOPHILIC BACTERIA; 
IMMUNOLOGY; KLEBSIELLA; MEDICAL BACTERIOLOGY; PNEUMOCOCCUS. 
 �34s[Paul B. Beeson]
�33s 
  
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�6v�2v�32sQueueing theory   �21m �33s 
    The mathematical theory of the formation and behavior of queues or waiting 
lines. The name is also applied loosely to the mathematical study of a wide 
variety of problems connected with traffic congestion and storage systems. 
Uneven flow through a service point, with fluctuating arrivals and service 
times, constitutes a major topic of operations research. For the 
mathematician, queueing theory is particularly interesting because it is 
concerned with relatively simple stochastic processes, which are in general 
non-Markovian and possibly stationary.  See  OPERATIONS RESEARCH; STOCHASTIC 
PROCESS.  
    The principal pioneer of queueing theory, A. K. Erlang, began in 1908 to 
study problems of telephone congestion. It is of interest to study the waiting 
times of subscribers in a manually operated system - for example, the average 
waiting time and the chance that a subscriber will obtain service immediately 
without waiting - and to examine how much the waiting times will be affected 
if the number of operators is altered, or conditions are changed in any other 
way. If there are more operators or if service can be speeded up, subscribers 
will be pleased because waiting will be reduced, but the improved facility 
will be more expensive to maintain; therefore, a reasonable balance must be 
struck.  
    Related problems in the use of automatic telephone exchanges and of 
long-distance lines able to carry only a limited number of messages 
simultaneously have resulted in much mathematical study of telephone traffic 
problems. Similar problems arise in other contexts. In a factory a number of 
machines, such as looms, may be under the care of one or more repairer. If a 
machine breaks down, it must stand idle until a repairer is free from 
repairing other machines. Machines here correspond to telephone subscribers, 
breakdown corresponds to attempts to make a call, and repair corresponds to 
connection. Other examples of congestion situations are aircraft flying around 
in circles waiting to use an airport landing strip, automobiles lining up at a 
turnpike toll booth, and customers lining up at the counter of a retail shop, 
waiting for service. 
    What is most interesting to investigate varies with the circumstances. 
Sometimes it is the mean waiting time of customers, sometimes the frequency 
with which the queue length exceeds a given limit, sometimes the proportion of 
the servers' time that is idle, and sometimes the average duration of a period 
during which a server is continuously occupied. In the study of stocking a 
warehouse or retail shop, known generally as the theory of inventories, the 
frequency with which the stock will be exhausted is considered under various 
reordering policies. Similar considerations apply in the theory of dams and 
water storage.  See  LINEAR PROGRAMMING; SYSTEMS ENGINEERING. 
 �34s[Francis J. Anscombe]
�33s 
  
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�6v�2v�32sQuince   �21m �33s 
    The deciduous tree Cydonia oblonga, originally from Asia, grown for its 
edible fruit. The fruit is a pear-shaped or apple-shaped pome, 
characteristically tomentose, aromatic, sour, astringent, and green, turning 
clear yellow at maturity. Used mostly for jam and jelly or as a stewed fruit, 
the fruit of the quince develops a pink color in cooking.  See  DECIDUOUS 
PLANTS; FRUIT; FRUIT, TREE; ROSALES. 
 �34s[Harold B. Tukey]
�33s 
  
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�6v�2v�32sQuinine   �21m �33s 
    The chief alkaloid of the bark of the cinchona tree, which is indigenous 
to certain regions of South America. The structure of quinine is shown below. 
    Until the 1920s quinine was the best chemotherapeutic agent for the 
treatment of malaria. However, clinical studies definitely established the 
superiority of the newer synthetic antimalarials such as primaquine, 
chloroquine, and chloroguanide.  See  ALKALOID; CINCHONA; MALARIA. 
 �34s[S. Morris Kupchan]
�33s 
                             ******FORMULA****** 
  
  
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�6v�2v�32sQuinoa �21m �33s
    An annual herb, Chenopodium quinoa (family Chenopodiaceae), a native of
Peru, and the staple food of many people in South America. These plants, grown
at high altitudes, produce large quantities of highly nutritious seeds used
whole in soups or ground into flour, which is made into bread or cakes. The
seeds are also used as poultry feed, in medicine, and in making beer. In the
United States the leaves are sometimes used as a substitute for spinach.  See
CARYOPHYLLALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sQuinoline �21m �33s
    One of a group of organic compounds containing a benzene ring fused to the
2,3 positions of pyridine. Positions ALPHA, BETA, and GAMMA are the same,
respectively, as positions 2, 3, and 4. Quinoline and some of its homologs are
obtained as coal tar extractives. The quinoline ring system appears in
synthetic chemotherapeutic agents and in dyes. Quinine and other natural
alkaloids also contain the quinoline ring. Quinoline itself is useful as a
solvent, a source of nicotinic acid, an acid acceptor and dehydrohalogenating
agent, and the starting point in organic syntheses.  See  HETEROCYCLIC
COMPOUNDS; PYRIDINE.

			     *****EQUATION***** 			     (1)

    Quinoline (I) is a colorless, steam-volatile liquid, with bp 237.1DGC
(458.8DGF), mp -15DGC (59DGF), specific gravity (15DGC) 1.09771. Quinoline is
a weakly basic tertiary amine that forms simple salts with acids, and
quaternary salts with alkylating agents. The molecule is aromatic in character.
    Quinoline is oxidized by nitric acid and other reagents to quinolinic acid
(II). In contrast, quinolinium quaternary salts are oxidized by alkaline
permanganate to give products of pyridine ring disruption.  See  ISOQUINOLINE;
QUININE.
 �34s[Walter J. Gensler ;Maurice Shamma]
�33s
			     ******FORMULA******


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�6v�2v�32sQuinone   �21m �33s 
    One of a class of aromatic diketones in which the carbon atoms of the 
carbonyl groups are part of the ring structure. The name quinone is applied to 
the whole group, but it is often used specifically to refer to p-benzoquinone 
(I). o-Benzoquinone (II) is also known but the meta isomer does not exist. 
    Quinones are prepared by oxidation of the corresponding aromatic ring 
systems containing amino (--NH2) or hydroxyl (--OH) groups on one or both of 
the carbon atoms being converted to the carbonyl group. 
                             ******FORMULA****** 
  
    Three of the several possible quinones derived from naphthalene are known: 
1,4-napthoquinone (III), 1,2-naphthoquinone, and 2,6-naphthoquinone (IV). 
                             ******FORMULA****** 
  
    Important naturally occurring naphthoquinones are vitamins K1 and K2 which 
are found in blood and are responsible for proper blood clotting reaction. A 
number of quinone pigments have been isolated from plants and animals. 
Illustrative of these are juglone found in unripe walnut shells and spinulosin 
from the mold Penicillium spinulosum. 9,10-Anthraquinone derivatives form an 
important class of dyes of which alizarin is the parent type. p-Benzoquinone 
is manufactured for use as a photographic developer.  See  ANTHRAQUINONE 
PIGMENTS; AROMATIC HYDROCARBON; DYE; HYDROQUINONE; KETONE; 
OXIDATION-REDUCTION; VITAMIN K. 
 �34s[David A. Shirley]
�33s 
  
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�6v�2v�32sQ (electricity) �21m �33s
    Often called the quality factor of a circuit, Q is defined in various
ways, depending upon the particular application. In the simple RL and RC
series circuits, Q is the ratio of reactance to resistance, as in Eqs. (1),
where X(sub L) is the inductive reactance, X(sub C) is the capacitive
reactance, and

     Q = X(sub L)/R    Q  =  X(sub C)/R    (a numerical value)		   (1)

R is the resistance. An important application lies in the dissipation factor
or loss angle when the constants of a coil or capacitor are measured by means
of the alternating-current bridge.
    Q has greater practical significance with respect to the resonant circuit,
and a basic definition is given by Eq. (2), where Q(sub 0) means evaluation at
resonance. For certain circuits, such as cavity resonators, this is the only
meaning Q can have.

Q(sub O) = 2PI (max stored energy per cycle/energy lost per cycle)	     (2)

For the RLC series resonant circuit with resonant frequency f(sub 0), Eq. (3)

      Q(sub O) = ePIf(sub O)L/R = 1/2PIf(sub O)CR			     (3)

holds, where R is the total circuit resistance, L is the inductance, and C is
the capacitance. Q(sub 0) is the Q of the coil if it contains practically the
total resistance R. The greater the value of Q(sub 0), the sharper will be the
resonance peak.

			Q(sub O) = f(sub O)/(f(sub 2) - f(sub 1))	     (4)

In terms of the resonance curve, Eq. (4) holds, where f(sub 0) is the
frequency at resonance, and f(sub 1) and f(sub 2) are the frequencies at the
half-power points.  See  RESONANCE (ALTERNATING-CURRENT CIRCUITS).
 �34s[Burtis L. Robertson]
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�6v�2v�32sRabbit   �21m �33s 
    Any of the mammals which make up the family Leporidae in the order 
Lagomorpha. This family contains about 52 species of rabbits and hares, which 
are essentially grassland animals. They are found in all regions of the world, 
from intensely cold to very hot climates, and are absent only in Antarctica. 
In some regions, such as Australia and New Zealand, they are not native but 
have been introduced by humans and have become pests. There are 28 Old World 
species and 24 New World species, most of which fall into two genera, Lepus 
(hares) and Sylvilagus. All are short-tailed animals with long ears and 
elongated hindlimbs. 
    Unlike the hares, rabbits are social animals, and the wild European rabbit 
lives in large colonies. Rabbits use burrows but the hare rests in a 
depression in grass which is called a form. While both are active during the 
cool of the morning and early evening, the hare is more active nocturnally. 
These animals have many predatory enemies such as birds, foxes, weasels, and 
various species of cats. Their swiftness and their keen senses of hearing and 
smell afford them some protection.  See  LAGOMORPHA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sRabies   �21m �33s 
    An acute, encephalitic viral infection. In humans it is almost invariably 
fatal. Human beings are infected from the bite of a rabid animal, usually a 
dog. Canine rabies can infect all warm-blooded animals, and death usually 
results. Some animals show chiefly paralytic signs, whereas others manifest 
encephalitic hyperexcitability and viciousness. However, the vampire bat may 
transmit virus for months while apparently not infected, and in other wild 
animals infection may take other courses than the fatal encephalitis. 
    The virus is believed to move from the saliva-infected wound through 
sensory nerves to the central nervous system, multiply there with destruction 
of brain cells, and thus produce encephalitis, with severe excitement, throat 
spasm upon swallowing (hence hydrophobia, or fear of water), convulsions, and 
death - with paralysis sometimes intervening before death. 
    Diagnosis in the human is made by observation of Negri bodies in brains of 
animal inoculated with the patient's saliva, or in the patient's brain after 
death. Diagnosis in dogs is essential for guidance concerning human 
vaccination. A dog which has bitten a person is isolated and watched for 7 
days for signs of rabies; if none occur, rabies was absent. If signs do 
appear, the animal is killed and the brain examined for Negri bodies, or for 
rabies antigen by testing with fluorescent antibodies. 
    The wound is cleansed immediately and sometimes cauterized with nitric 
acid. The patient is vaccinated. Indiscriminate human vaccination is 
inadvisable, because there is risk of vaccination encephalitis. Vaccination is 
avoided if the biting animal is found not to be rabid. Human vaccination is 
done with a series of doses of fixed virus. The hope is that antibodies will 
develop in the patient as a result of vaccination before the infecting street 
virus has had sufficient time to multiply. 
    Rabies is controlled by compulsory vaccination of dogs and destruction of 
stray dogs; cattle may also be vaccinated. A living attenuated virus vaccine 
is available, and has been suggested for use in persons subjected to a high 
risk, for example, those who work with stray dogs in an area where canine 
vaccination is not compulsory. 
 �34s[Joseph L. Melnick]
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�6v�2v�32sRaccoon   �21m �33s 
    Any of the carnivorous mammals which belong to the family Procyonidae, of 
which there are 16 species, found only in the New World. The robust body is 
covered with long gray-brown hair, the face is marked by a black mask across 
the eyes, and the bushy tail has five to seven black rings. The raccoon is 
plantigrade when standing but walks with its heels off the ground. The digits 
are long and terminate in nonretractile claws. This animal is found in 
woodland areas near forest edges, and is a good climber. Primarily 
terrestrial, it is also a good swimmer, although it cannot pursue prey 
underwater. A litter of five or six is born each year, and the animals live 
and travel in family groups.  See  CARNIVORA; MAMMALIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sRacemization   �21m �33s 
    The formation of a racemate from a pure enantiomer. Alternatively stated, 
racemization is the conversion of one enantiomer in a 50:50 mixture of the two 
enantiomers (+ and -, or R and S) of a substance. Racemization is normally 
associated with the loss of optical activity over a period of time since 50:50 
mixtures of enantiomers are optically inactive.  See  OPTICAL ACTIVITY. 
    Racemization is an energetically favored process since it reflects a 
change from a more ordered to a more random state. But the rate at which 
enantiomers racemize is typically quite slow unless a suitable mechanistic 
pathway is available, since racemization usually, but not always, requires 
that a chemical bond at the chiral center of an enantiomer be broken. 
Racemization of enantiomers possessing more than one chiral center requires 
that all chiral centers of half of the molecules invert their configurations.  
See  ENTROPY. 
    The observation and study of racemization have important implications for 
the understanding of the mechanisms of chemical reactions and for the 
synthesis and analysis of chiral natural products such as peptides. Moreover, 
racemization is of economic importance since it provides a way of converting 
an unwanted enantiomer into a useful one. Synthetic medicinal agents are often 
produced industrially as racemates. After resolution and isolation of the 
desired enantiomer, half of the product would have to be discarded were it not 
for the possibility of racemizing the unwanted isomer and of recycling the 
resultant racemate. 
 �34s[Samuel H. Wilen]
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�6v�2v�32sRadar   �21m �33s 
    An acronym for radio detection and ranging, the original and still 
principal application of radar. The name is applied to both the technique and 
the equipment used. 
    Radar is a sensor; its purpose is to provide estimates of certain 
characteristics of its surroundings of interest to a user, most commonly the 
presence, position, and motion of such objects as aircraft, ships, or other 
vehicles in its vicinity. In other uses, radars provide information about the 
Earth's surface (or that of other astronomical bodies) or about meteorological 
conditions. To provide the user with a full range of sensor capability, radars 
are often used in combinations or with other elements of more complete systems. 
    Radar operates by transmitting electromagnetic energy into the 
surroundings and detecting energy reflected by objects. If a narrow beam of 
this energy is transmitted by the directive antenna, the direction from which 
reflections come and hence the bearing of the object may be estimated. The 
distance to the reflecting object is estimated by measuring the period between 
the transmission of the radar pulse and reception of the echo. In most radar 
applications this period will be very short since electromagnetic energy 
travels with the velocity of light. 
  
    Kinds of radar.  The physical nature of radars varies greatly. Several 
radars are available for use on small boats as a safety and navigation aid, 
some so small as to be carried by an operator. Another radar seen in a 
hand-held form is that used by police to measure the speed of automobiles.  
See  MARINE NAVIGATION. 
    Perhaps the largest radars are those covering acres of land, long arrays 
of antennas all operating together to monitor the flight of space vehicles or 
astronomical bodies. Other very large radars are designed to monitor flight 
activity at substantial distances. These are large mainly because they must 
use longer-than-usual radio wavelengths associated with ionospheric 
containment of the signal for over-the-horizon operations. 
    More common in size are those radars seen at airports, with rotating 
antennas 10 - 30 ft (3 - 9 m) wide. Radars intended for mobile use, 
particularly airborne radars, are quite compact.  See  AIRBORNE RADAR. 
    Radars intended principally to determine the presence and position of 
reflecting targets in a region around the radar are called search radars. 
Other radars examine further the targets detected: examples are height finders 
with antennas that scan vertically in the direction of an assigned target, and 
tracking radars that are aimed continuously at an assigned target to obtain 
great accuracy in estimating target motion. In some modern radars, these 
search and track functions are combined, usually with some computer control. 
Surveillance radar connotes operation of this sort, somewhat more than just 
search alone. There are also very complex and versatile radars with 
considerable computer control, with which many functions are performed and 
which are therefore called multifunction radars. Very accurate tracking radars 
intended for use at missile test sites or similar test ranges are called 
instrumentation radars. Radars designed to detect clouds and precipitation are 
called meteorological or weather radars.  See  RADAR METEOROLOGY; SURVEILLANCE 
RADAR.  
    Some radars have separate transmit and receive antennas sometimes located 
miles apart. These are called bistatic radars, the more conventional 
single-antenna radar being monostatic. Some useful systems have no transmitter 
at all and are equipped to measure, for radarlike purposes, signals from the 
targets themselves. Such systems are often called passive radars, but the 
terms radiometers or signal intercept systems are generally more appropriate.  
See  PASSIVE RADAR. 
    The terms primary and secondary are used to describe, respectively, radars 
in which the signal received is reflected by the target and radars in which 
the transmission causes a transponder (transmitter-responder) carried aboard 
the target to transmit a signal back to the radar.  See  AIR-TRAFFIC CONTROL; 
ELECTRONIC NAVIGATION SYSTEMS. 
  
    Operation.  It is convenient to consider radars composed of four principal 
parts: the transmitter, antenna, receiver, and display (see illustration). 
    The transmitter provides the rf signal in sufficient strength (power) for 
the radar sensitivity desired and sends it to the antenna, which causes the 
signal to be radiated into space in a desired direction. The signal propagates 
(radiates) in space, and some of it is intercepted by reflecting bodies. These 
reflections, in part at least, are radiated back to the antenna. The antenna 
collects them and routes all such received signals to the receiver, where they 
are amplified and detected. The presence of an echo of the transmitted signal 
in the received signal reveals the presence of a target. The echo is indicated 
by a sudden rise in the output of the detector, which produces a voltage 
(video) proportional to the sum of the rf signals being received and the rf 
noise inherent in the receiver itself. The time between the transmission and 
the receipt of the echo discloses the range to the target. The direction or 
bearing of the target is disclosed by the direction the antenna is pointing 
when an echo is received. 
  
    A duplexer permits the same antenna to be used on both transmit and 
receive, and is equipped with protective devices to block the very strong 
transmit signal from going to the sensitive receiver and damaging it. The 
antenna forms a beam, usually quite directive, and, in the search example, 
rotates throughout the region to be searched.  See  ANTENNA (ELECTROMAGNETISM). 
    The radar reflections are among the signals received by the antenna in the 
period between transmissions. Most search radars have a pulse repetition 
frequency (prf), antenna beamwidth, and rotation rate such that several pulses 
are transmitted (perhaps 20 to 40) while the antenna scans past a target. This 
allows a buildup of the echo being received. Some radars are equipped with 
low-noise rf preamplifiers to improve sensitivity. Almost all radars use some 
form of mixing of the received signal with a local oscillator signal to 
produce an intermediate frequency (i-f) signal, commonly at 30 or 60 MHz, more 
conveniently amplified and processed. The local oscillator signal, offset from 
the transmitted rf by precisely the i-f, can be supplied by a waveform 
generator. The mixer is sometimes called the first detector. The i-f signal is 
then fed to a detector (sometimes called the second detector) which produces a 
video signal, a voltage proportional to the strength of the i-f at its imput. 
This video may be amplified and used in the cathode-ray tube (CRT) of the 
radar display to cause a bright spot to be formed on the face of the tube when 
an echo is received, in a manner indicating the range and bearing of the 
target.  See  CATHODE-RAY TUBE; DETECTOR; ELECTRONIC DISPLAY; MIXER; RADIO 
RECEIVER.  
    Radar carrier frequencies are broadly identified by a nomenclature that 
originated in wartime secrecy and has since been found very convenient and 
widely accepted. The spectrum is divided into bands, the frequencies and 
wavelengths of which are given in the table. The charged layers of the 
ionosphere present a highly refractive shell at radio frequencies well below 
the microwave frequencies of most radars. Consequently, over-the-horizon 
radars have been built in the 10-MHz area to exploit this skip path.  See  
CONTINUOUS-WAVE RADAR; MONOPULSE RADAR; RADIO SPECTRUM ALLOCATIONS. 
 �34s[Robert T. Hill]
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�6v�2v�32sRadar meteorology   �21m �33s 
    The study of the scattering of radar waves by all types of atmospheric 
phenomena and the use of radar for making weather observations and forecasts. 
In general, radar is useful in meteorology because it is capable of detecting 
water and ice particles, but it can also be used to observe lightning and 
regions of the atmosphere which have large gradients of temperature and water 
vapor. Important applications of radar treated in meteorology include use of 
radar for rainfall measurement; study of cloud and precipitation formation; 
observations of tornadoes and hurricanes; and radar echoes from targets other 
than water or ice particles.  See  DOPPLER RADAR; METEOROLOGY; RADAR; STORM 
DETECTION.  
 �34s[Louis J. Battan]
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�6v�2v�32sRadiance �21m �33s
    The physical quantity that corresponds closely to the visual brightness of
a surface. A simple radiometer for measuring the (average) radiance of an
incident beam of optical radiation (light, including invisible infrared and
ultraviolet radiation) consists of a cylindrical tube, with a hole in each end
cap to define the beam cross section there, and with a photocell against one
end to measure the total radiated power in the beam of all rays that reach it
through both holes (see illustration). If A1 and A2 are the respective areas
of the two holes, D is the length of the tube (distance between holes), and
PHI is the radiant flux or power measured by the photocell, then the (average)
radiance is approximately given by the equation L = PHI/(A1 MULT A2/D sup 2) W
MULT m(sup -2) MULT  sr(sup -1)
 �34s[Fred E. Nicodemus]
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�6v�2v�32sRadian measure �21m �33s
    A radian is the angle subtended at the center of a circle by an arc of the
circle equal in length to its radius. It is proved in geometry that equal
central angles of two circles subtend arcs proportional to their radii; and
the converse is true. Hence the radian is independent of the length of the
radius. The figure represents two circles of radius r. Arc AB of length r
subtends 1 radian (rad) at the center O of the circle, and arc A'B' of length
s subtends THETA rad at its center. Since arcs on equal circles are
proportional to their subtended central angles, s/r = THETA/1, or the formula
s = r THETA holds. If THETA = 2PI, s = 2PIr, the circumference of the circle.
Therefore 2PI rad is the complete angle about a point or 360 DG, and 2PI rad =
360 DG, 1 rad = 360 DG/2PI = 57.2958 DG.
    The degree as a unit of angle has come down from antiquity. However, its
use in various theories involves clumsy constants. The use of the radian
avoids these constants. The radian is employed generally as a measure of angle
in theoretical discussions; when no unit of angle is mentioned, the radian is
understood.
 �34s[Lyman M. Kells]
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�6v�2v�32sRadiant heating   �21m �33s 
    Any system of space heating in which the heat-producing means is a surface 
that emits heat to the surroundings by radiation rather than by conduction or 
convection. The surfaces may be radiators such as baseboard radiators or 
convectors, or they may be the panel surfaces of the space to be heated.  See  
PANEL HEATING AND COOLING. 
    The heat derived from the Sun is radiant energy. Radiant rays pass through 
gases without warming them appreciably, but they increase the sensible 
temperature of liquid or solid objects upon which they impinge. The same 
principle applies to all forms of radiant-heating systems, except that 
convection currents are established in enclosed spaces and a portion of the 
space heating is produced by convection. Any radiantheating system using a 
fluid heat conveyor may be employed as a cooling system by substituting cold 
water or other cold fluid. However, the technique is not practical on the 
scale required for comfort control of an occupied space. 
 �34s[Erwin L. Weber /Richard Koral]
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�6v�2v�32sRadiata   �21m �33s 
    Members of the Eumetazoa which have a primary radial symmetry. The group 
includes the Coelenterata and Ctenophora, although historically such groups as 
Porifera and Echinodermata were included. 
    The radiate animals are, in general, of the tissue grade of construction 
and do not possess organs, although simply constructed nervous, digestive, and 
muscular systems are present. The only body cavity is the digestive cavity 
(gastrovascular cavity) and, although primitively a simple sac, it may become 
complex through branching and the development of compartments. Typically there 
is a single opening into the digestive cavity, namely, the mouth; this opening 
serves for the egestion of materials as well as for food intake. 
 �34s[Cadet Hand]
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�6v�2v�32sRadiation   �21m �33s 
    The emission and propagation of energy; also, the emitted energy itself. 
The etymology of the word implies that the energy propagates rectilinearly, 
and in a limited sense, this holds for the many different types of radiation 
encountered.  
    The major types of radiation may be described as electromagnetic, 
acoustic, and particle, and within these major divisions there are many 
subdivisions. Electromagnetic radiation is classified roughly in order of 
decreasing wavelength as radio, microwave, visible, ultraviolet, x-rays, and 
GAMMA-rays. Acoustic or sound radiation may be classified by frequency as 
infrasonic, sonic, or ultrasonic in order of increasing frequency, with sonic 
being between about 16 and 20,000 Hz. The traditional examples of particle 
radiation are the ALPHA- and BETA-rays of radioactivity.  See  ELECTROMAGNETIC 
RADIATION; RADIOACTIVITY; SOUND. 
 �34s[McAllister H. Hull, Jr.]
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�6v�2v�32sRadiation biology   �21m �33s 
    A study of the influence of light or ionizing radiation, such as x-rays or 
fast particles, on living systems. Radiation biology ranges from a 
consideration of the effects of visible light on metabolism to the effects of 
cosmic rays on whole organisms. Because of the breadth of the subject, it is 
studied to a large extent in separate areas. The first division separates the 
fields of ionizing radiation effects and photon effects. The term photon 
effects means the action of ultraviolet, visible, and infrared light in the 
region where ionization does not occur.  See  RADIATION; X-RAYS. 
  
    Ionizing radiation.  Ionizing radiation produces random energy releases of 
great size and generally of great disruptive effect. When such a disruptive 
effect is on functioning units of the organism, it is termed direct action. It 
is termed indirect action when it occurs on the water moiety through formation 
of active radicals, notably OH and H, which exert a gentler action, and which, 
because of radical diffusion, can act at some distance from the original 
ionization. To measure any effect of ionizing radiation the amount of 
radiation must be measured. Such measurement is called dosimetry; the amount 
given is the dose.  See  DOSIMETRY. 
    A considerable variety of effects exists. A brief sampling follows. 
    1. Survival after whole-body irradiation. A whole animal, such as a mouse, 
which has been irradiated, shows no marked immediate effect. After a few days, 
however, a definite increase in mortality occurs if exposure exceeded 400 
roentgens, with a mean lethal dose at 500 r and a rapid increase in mortality 
rising to 100% at 1000 r. In such a process death is due to effects on a 
variety of organs, the blood-forming organs and any rapidly dividing tissue 
being the most sensitive. 
    2. Production of mutations. Ionizing radiation produces mutations in any 
living organism, with the possible exception of viruses. The majority of such 
mutations are lethal.  See  MUTATION. 
    3. Production of chromosome breaks. All kinds of chromosome abnormalities, 
including chromosome breaks, are produced by ionizing radiation. Chromosome 
breaks can restitute and probably the majority do so.  See  CHROMOSOME 
ABERRATION.  
    4. Delay of cell division. Ionizing radiation inhibits cell division to a 
marked degree. The degree of delay increases with the dose.  See  CELL 
DIVISION.  
    5. Formation of giant cells. Cells in which division is delayed may grow 
into giant cells many times the normal size in the case of mammalian cells, or 
into long filaments in the case of bacteria. 
    6. Reduction of survival. If means for studying the ability of a cell to 
divide are available, it is found that animal, plant, and bacterial cells and 
viruses are reduced in survival. 
    7. Action on biological macromolecules. All important biological 
macromolecules - deoxyribonucleic acid (DNA), ribonucleic acid (RNA), enzymes, 
and antigens - are destructively affected by ionizing radiation. Roughly 
speaking, if one ionization occurs within the molecule, or if one active 
radical in the case of nucleic acid, or 1 - 10 radicals in the case of protein 
reach the molecule, it loses its function.  See  DEOXYRIBONUCLEIC ACID (DNA); 
RIBONUCLEIC ACID (RNA). 
    8. Action on metabolism and protein synthesis. Metabolism is reduced by 
ionizing radiation but much less than is division or the formation of lethal 
mutations. Also reduced are protein synthesis and formation of microsomal 
particles, or ribosomes, which are agents in protein synthesis.  See  
RIBOSOMES.  
    9. There is evidence that ionizing radiation can precipitate the 
destructive action of enzymes already in the cell. One is a nuclease which 
rapidly degrades the DNA, drastically damaging the cell. 
    10. The phenomenon of recovery from the effects of ionizing radiation has 
been established. Cellular DNA which has become fragmented by ionizing 
radiation is found to be restored to much larger size if the cells in which 
damage has occurred are held for 20 min (bacteria) or several hours (mammalian 
cells).  
    11. Ionizing radiation, in rather high doses, acts to depress the immune 
system.  
  
    Ultraviolet light.  Ultraviolet light differs from ionizing radiation in 
that it is less energetic and much more specific. Before any of its effects 
can occur, ultraviolet light must first be absorbed, a process which is 
wavelength-dependent. The most significant absorption of ultraviolet light is 
by nucleic acids. There is also absorption in protein, notably by aromatic 
amino acids and cystine. 
    Light of high intensity (such as sunlight), though not readily absorbed, 
may yet produce marked biological action, the high intensity compensating for 
the lack of absorption. Light in the near-ultraviolet range, where absorption 
by nucleic acids is inefficient, has been found to have significant actions. 
    Because it must be absorbed, ultraviolet light rarely exerts lethal action 
on whole animals. The effects produced by ultraviolet radiation are discussed 
below.  
  
    Burns. Ultraviolet light produces drastic action on the skin, familiar as 
sunburn. Prolonged exposure to ultraviolet light or sunlight can cause skin 
cancer, particularly in individuals with fair skin. 
  
    Chromosome breaks. Ultraviolet light produces chromosome breaks, though of 
a less drastic kind than ionizing radiation, being more commonly in one of two 
chromatids rather than the whole chromosome. 
  
    Delay of cell division and giant-cell formation. Cell division is 
inhibited by ultraviolet light, and such cells can grow to giant size. 
  
    Reduction of survival. Ultraviolet light reduces survival, though not so 
simply, in general, as ionizing radiation, there being a tendency for a 
fraction of the cells to show less sensitivity. 
  
    Photoreactivation. If cells or virus-infected cells are subjected to 
illumination by light in the blue, or near-ultraviolet, range, the degree of 
survival is markedly increased, a phenomenon known as photoreactivation. 
  
    Excision repair. There is also a process whereby dimers and cross-links in 
the DNA can be enzymatically removed and the DNA then accurately patched. This 
occurs in the dark. Mutant cells lacking one or more of the enzymes needed 
have been isolated. 
  
    Action on metabolism and protein synthesis. These are affected by 
ultraviolet light, though not in the same way as by ionizing radiation. More 
effect takes place on the DNA-synthetic process, which is less affected by 
ionizing radiation. 
  
    Recombination repair. A still only partly understood mechanism exists by 
which one intact genome can be assembled from two damaged genomes. This is 
related to genetic recombination, and the enzymes needed have not yet been 
characterized.  
  
    Error-prone repair. It has been found that in some cells the effect of 
radiation is to induce a new repair system which is less specific in its 
action than those described above. 
  
    Mutations. Ultraviolet light, including near ultraviolet, produces 
mutation in all cells, including bacteria. It does so by a relatively small 
chemical alteration in the DNA. 
 �34s[Ernest C. Pollard]
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�6v�2v�32sRadiation chemistry �21m �33s
    The study of the chemical effects of the absorption of high-energy
radiation in matter. High-energy radiation includes the emanation associated
with radioactive decay and fission (that is, ALPHA-particles, electrons,
GAMMA-rays, and neutrons), together with their related atom and fission
recoils; and the artificial analogs of such emanations produced by
accelerating electrons, protons, deuterons, and helium nuclei, as well as
charged nuclei of higher atomic number and x-rays.
    Sources of high-energy radiations in the laboratory and industry include
radioactive nuclides (for example, (sup 60)Co, (sup 90)Sr, and (sup 3)H and
instruments such as x-ray tubes, Van de Graaff generators, the betatron, the
cyclotron, and the synchroton. The linear electron accelerator (linac) has
proved particularly valuable for the study of transient species which have
lifetimes as short as 16 picoseconds. A variety of fast reaction techniques
are used to study the reactions of short-lived species produced by such short
pulses. These include optical absorption, fluorescence, ionic conductivity,
electron spin resonance, nuclear magnetic resonance, resonance Raman
scattering, and polarography.  See  CERENKOV RADIATION; ELECTRON PARAMAGNETIC
RESONANCE (EPR) SPECTROSCOPY; FLUORESCENCE; IONIC CRYSTALS; NUCLEAR MAGNETIC
RESONANCE (NMR); POLAROGRAPHIC ANALYSIS; RAMAN EFFECT.
    Energy is transmitted to irradiated material by momentum transfer and by
excitation and ionization. The latter two always accompany the former.
Momentum transfer is characteristic of processes involving neutrons; it is
always involved to some extent in particle effects and is an important
contribution to heavy-particle (for example, proton) effects. In a momentum
transfer interaction, the usual effect is the ejection of a nucleus from its
molecular or crystalline structure. In the case of solids, this is known as
the Wigner effect. When crystalline material is involved, the process is
called discomposition. Discomposition results not only from neutron impact,
but also secondarily from impacts involving high-energy displaced nuclei.
    For other than momentum transfer, the principal effects are due to
fast-moving charged particles. A 1-MeV charged particle, unlike a parent
photon which produces only one ionization (as in a Compton process), may
produce a total of 10(sup 5) ions (and electrons) and excited molecules. The
distribution of such primarily produced entities is inhomogeneous, and is
affected greatly by the nature of the radiation and by the state of
aggregation of the material irradiated. It is this inhomogeneity of the
distribution of activated species which is an important fact differentiating
radiation chemistry from photochemistry and thermally induced chemistry.  See
PHOTOCHEMISTRY.
    Chemical effects of high-energy radiation must be guarded against in
nuclear reactors (where effects include radiation corrosion, water
decomposition, and the Wigner effect) and in living systems (because of
mutations, cancer production, and so on). Such effects may be deliberately
employed to induce polymerization of special kinds: to cross-link, to graft,
and to thermally stabilize polymers; to sterilize foods, medicinals, and
surgical materials; to change the properties of catalysts; and to induce
reactions not possible by other means or to induce them under unusual
environmental conditions, such as under extremely low temperature, in very
thick layers, and in heavy-walled (pressure) vessels.  See  RADIATION BIOLOGY;
RADIATION DAMAGE TO MATERIALS;	RADIOACTIVITY AND RADIATION APPLICATIONS.
 �34s[Sheffield Gordon]
�33s

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�6v�2v�32sRadiation damage to materials �21m �33s
    Harmful changes in the properties of liquids, gases, and solids, caused by
interaction with nuclear radiations. For a discussion of radiation damage in
minerals  see  METAMICT STATE. For a description of damage caused to
biological systems by radiation  see  RADIATION BIOLOGY.
    Radiation damage is usually associated with materials of construction that
must function in an environment of intense high-energy radiation from a
nuclear reactor. Materials that are an integral part of the fuel element or
cladding and nearby structural components are subject to such intense nuclear
radiation that a decrease in the useful lifetime of these components can
result. Radiation damage will also be a factor in thermonuclear reactors.  See
NUCLEAR FUSION; NUCLEAR REACTOR.
    Electronic components are extremely sensitive to even moderate radiation
fields. Transistors malfunction because of defect trapping of charge carriers.
Ferroelectrics such as BaTiO3 fail because of induced isotrophy; quartz
oscillators change frequency and ultimately become amorphous.
High-permeability magnetic materials deteriorate because of hardening.
Plastics used for electrical insulation rapidly deteriorate.
    There are several mechanisms that function on an atomic and nuclear scale
to produce radiation damage in a material if the radiation is sufficiently
energetic, whether it be electrons, protons, neutrons, x-rays, fission
fragments, or other charged particles.
    Electronic excitation and ionization is most severe in liquids and organic
compounds and appears in a variety of forms such as gassing, decomposition,
viscosity changes, and polymerization in liquids. Rapid deterioration of the
mechanical properties of plastics takes place either by softening or by
embrittlement, while rubber suffers severe elasticity changes at low fluxes.
Cross-linking, scission, free-radical formation, and polymerization are the
most important reactions.  See	RADIATION CHEMISTRY.
    In an environment of neutrons, transmutation effects may be important.
Even materials that have a low cross section such as aluminum can show an
appreciable accumulation of impurity atoms from transmutations. The elements
boron and europium have very large cross sections and are used in control
rods. Damage to the rods is severe in boron-containing materials.  See
TRANSMUTATION.
    Displaced atoms are the most important source of radiation damage in
nuclear reactors outside the fuel element. It is a consequence of the ability
of the energetic neutrons born in the fission process to knock atoms from
their equilibrium position in their crystal lattice, displacing them many
atomic distances away into interstitial positions and leaving behind vacant
lattice sites.
    Nuclear irradiations performed at low temperatures (4 K) result in the
maximum retention of radiation-produced defects. As the temperature of
irradiation is raised, many of the defects are mobile and some annihilation
may take place. The increased mobility, particularly of vacancies and vacancy
agglomerates, may lead to acceleration of solid-state reactions, such as
precipitation, short- and long-range ordering, and phase changes. These
reactions may lead to undesirable property changes.
    The presence of small amounts of impurities may profoundly affect the
behavior of engineering alloys in a radiation field. It has been observed that
helium concentrations as low as 10(sup -9) seriously reduce the high-temperature
ductility of a stainless steel. Small amounts of copper, phosphorus, and
nitrogen have a strong influence on the increase in the ductile-brittle
transition temperature of pressure vessel steels under irradiation. Heat
treatment prior to irradiation determines the retention of both major alloying
components and impurities in solid solution in metastable alloys. It also
affects the number and disposition of dislocations. Thus heat treatment is an
important variable in determining subsequent radiation behavior.
 �34s[Douglas S. Billington]
�33s

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�6v�2v�32sRadiation microbiology   �21m �33s 
    A field of basic and applied radiobiology concerned chiefly with the 
damaging effects of radiations on microorganisms. The basic studies are 
usually designed to elucidate the mechanism by which radiations produce their 
biological damage. The applied field, radiation food sterilization, has as its 
ultimate goal the ``cold'' sterilization of food and food products by 
subjecting them to large doses of ionizing radiation.  See  FOOD PRESERVATION; 
RADIATION BIOLOGY. 
 �34s[George E. Stapleton]
�33s 
  
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�6v�2v�32sRadiation pressure   �21m �33s 
    Pressure exerted by electromagnetic radiation on objects on which it 
impinges. This pressure is caused by the fact that electromagnetic radiation 
transmits energy and possesses momentum. These pressures are very small. The 
effect is conspicuous in the case of a comet near the Sun, where the radiation 
pressure from the Sun forces the lighter cometary constituents away from the 
Sun.  See  ELECTROMAGNETIC RADIATION. 
 �34s[William R. Smythe]
�33s 
  
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�6v�2v�32sRadiation shielding   �21m �33s 
    Physical barriers designed to provide protection from the effects of 
ionizing radiation; also, the technology of providing such protection. Major 
sources of radiation are nuclear reactors and associated facilities, medical 
and industrial x-ray and radioisotope facilities, charged-particle 
accelerators, and cosmic rays. Types of radiation are directly ionizing 
(charged particles) and indirectly ionizing (neutrons, gamma rays, and 
x-rays). In most instances, protection of human life is the goal of radiation 
shielding. In other instances, protection may be required for structural 
materials which would otherwise be exposed to high-intensity radiation, or for 
radiation-sensitive materials such as photographic film and certain electronic 
components.  
    Charged particles lose energy and are thus attenuated and stopped 
primarily as a result of coulombic interactions with electrons of the stopping 
medium. Gamma-ray and x-ray photons lose energy principally by three types of 
interactions: photoemission, Compton scattering, and pair production. Neutrons 
lose energy in shields by elastic or inelastic scattering. Elastic scattering 
is more effective with shield materials of low atomic mass, notably 
hydrogenous materials, but both processes are important, and an efficient 
neutron shield is made of materials of both high and low atomic mass.  See  
COMPTON EFFECT; ELECTRON-POSITRON PAIR PRODUCTION; PHOTOEMISSION. 
    The most common criteria for selecting shielding materials are radiation 
attenuation, ease of heat removal, resistance to radiation damage, economy, 
and structural strength. 
    For neutron attenuation, the lightest shields are usually hydrogenous, and 
the thinnest shields contain a high proportion of iron or other dense 
material. For gamma-ray attenuation, the high-atomic-number elements are 
generally the best. For heat removal, particularly from the inner layers of a 
shield, there may be a requirement for external cooling with the attendant 
requirement for shielding the coolant to provide protection from induced 
radioactivity.  
    Metals are resistant to radiation damage, although there is some change in 
their mechanical properties. Concretes, frequently used because of their 
relatively low cost, hold up well; however, if heated they lose water of 
crystallization, becoming somewhat weaker and less effective in neutron 
attenuation.  
    If shielding cost is important, cost of materials must be balanced against 
the effect of shield size on other parts of the facility, for example, 
building size and support structure. If conditions warrant, concrete can be 
loaded with locally available material such as natural minerals (magnetite or 
barytes), scrap steel, water, or even earth. 
    Radiation shields vary with application. The overall thickness of material 
is chosen to reduce radiation intensities outside the shield to levels well 
within prescribed limits for occupational exposure or for exposure of the 
general public. The reactor shield is usually considered to consist of two 
regions, the biological shield and the thermal shield. The thermal shield, 
located next to the reactor core, is designed to absorb most of the energy of 
the escaping radiation and thus to protect the steel reactor vessel from 
radiation damage. It is often made of steel and is cooled by the primary 
coolant. The biological shield is added outside to reduce the external dose 
rate to a tolerable level. See  RADIATION BIOLOGY; RADIATION DAMAGE TO 
MATERIALS.  
 �34s[Richard E. Faw]
�33s 
  
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�6v�2v�32sRadiator   �21m �33s 
    Any of numerous devices, units, or surfaces that emit heat, mainly by 
radiation, to objects in the space in which they are installed. Because their 
heating is usually radiant, radiators are of necessity exposed to view. They 
often also heat by conduction to the adjacent thermally circulated air. 
    Radiators are usually classified as cast-iron (or steel) or nonferrous. 
They may be directly fired by wood, coal, charcoal, oil, or gas (such as 
stoves, ranges, and unit space heaters). The heating medium may be steam, 
derived from a steam boiler, or hot water, derived from a water heater, 
circulated through the heat-emitting units. 
    Electric heating elements may be substituted for fluid heating elements in 
all types of radiators, convectors, and unit ventilators.  See  HOT-WATER 
HEATING SYSTEM; RADIANT HEATING; STEAM HEATING. 
 �34s[Erwin L. Weber /Richard Koral]
�33s 
  
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�6v�2v�32sRadio   �21m �33s 
    Communication between two or more points, employing electromagnetic waves 
as the transmission medium. 
    Radio waves transmitted continuously, with each cycle an exact duplicate 
of all others, indicate only that a carrier is present. The message must cause 
changes in the carrier which can be detected at a distant receiver. The method 
used for the transmission of the information is determined by the nature of 
the information which is to be transmitted as well as by the purpose of the 
communication system. 
    In code telegraphy the carrier is keyed on and off to form dots and 
dashes. The technique, often used in ship-to-shore and amateur communications, 
has been largely superseded in many other point-to-point services by more 
efficient methods. 
    In frequency-shift transmission the carrier frequency is shifted a fixed 
amount to correspond with telegraphic dots and dashes or with combinations of 
pulse signals identified with the characters on a typewriter. This technique 
is widely used in handling the large volume of public message traffic on long 
circuits, principally by the use of teletypewriters.  See  TELETYPEWRITER. 
    In amplitude modulation the amplitude of the carrier is made to fluctuate, 
to conform to the fluctuations of a sound wave. This technique is used in AM 
broadcasting, television picture transmission, and many other services.  See  
AMPLITUDE-MODULATION RADIO. 
    In frequency modulation the frequency of the carrier is made to fluctuate 
around an average axis, to correspond to the fluctuations of the modulating 
wave. This technique is used in FM broadcasting, television sound 
transmission, and microwave relaying.  See  FREQUENCY-MODULATION RADIO. 
    In pulse transmission the carrier is transmitted in short pulses, which 
change in repetition rate, width, or amplitude, or in complex groups of pulses 
which vary from group to succeeding group in accordance with the message 
information. These forms of pulse transmission are identified as pulse-code, 
pulse-time, pulse-position, pulse-amplitude, pulse-width, or pulse-frequency 
modulation. Such techniques are complex and are employed principally in 
microwave relay systems.  See  PULSE MODULATION. 
    In radar the carrier is normally transmitted as short pulses in a narrow 
beam, similar to that of a searchlight. When a wave pulse strikes an object, 
such as an aircraft, energy is reflected back to the station, which measures 
the round-trip time and converts it to distance. A radar can display varying 
reflections in a maplike presentation on a cathode-ray tube.  See  RADAR. 
    Hundreds of thousands of radio transmitters exist, each requiring a 
carrier at some radio frequency. To prevent interference, different carrier 
frequencies are used for stations whose service areas overlap and receivers 
are built to select only the carrier signal of the desired station. Resonant 
electric circuits in the receiver are adjusted, or tuned, to accept one 
frequency and reject others. 
    All nations have a sovereign right to use freely any or all parts of the 
radio spectrum. But a growing list of international agreements and treaties 
divides the spectrum and specifies sharing among nations for their mutual 
benefit and protection. Each nation designates its own regulatory agency. In 
the United States all nongovernmental radio communications are regulated by 
the Federal Communications Commission (FCC).  See  AMATEUR RADIO; RADIO 
BROADCASTING; RADIO BROADCASTING NETWORKS; RADIO SPECTRUM ALLOCATIONS. 
 �34s[John D. Singleton]
�33s 
  
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�6v�2v�32sRadioactive fallout   �21m �33s 
    The radioactive material which results from a nuclear explosion in the 
atmosphere. In particular the term radioactive fallout applies to the debris 
which is deposited on the ground, but common usage has extended its coverage 
to include airborne material as well. 
    In a nuclear explosion, the device itself and the material immediately 
surrounding it are vaporized into a fireball which then rises in the 
atmosphere. The altitude at which the fireball cools sufficiently to 
stablilize depends very much on the yield of the explosion. In general, an 
atomic explosion fireball stablizes as a cloud high in the troposphere, while 
a thermonuclear fireball tends to break through into the stratosphere and 
stabilize at altitudes above 10 mi (16 km). 
    As the fireball cools, the vaporized materials condense to fine particles. 
If the explosion has taken place high above the Earth, the only material 
present is that of the device itself, and the fine particulates are carried by 
the winds and distributed over a wide area before they descend to Earth. 
Bursts at or near the surface carry large amounts of inert material up with 
the fireball. A large fraction of the radioactive debris condenses out onto 
the large inert particles of soil and other material, and many small 
radioactive particulates attach themselves to the larger inert particles. The 
larger particles settle rapidly by gravity, and large percentages of the 
radioactivity may be deposited in a few hours near the site of the explosion. 
In contrast, the radioactivity from a thermonuclear explosion at high altitude 
takes months or years to reach the surface. 
    Local fallout is the deposition of large radioactive particles near the 
site of the explosion. It may present a hazard near the test site or in the 
case of nuclear warfare. The smaller particulates, which are spread more 
widely, constitute the worldwide fallout. The radioactivity comes from the 
isotopes (radionuclides) of perhaps 60 different elements formed in fission, 
plus a few others formed by activation and any unfissioned uranium or 
plutonium. Each of the radioactive isotopes produced in the fission process 
goes through three or four succes-sive radioactive decays before becoming a 
stable nuclide. The radioactive half-lives of these various fission products 
range from fractions of a second to about 100 years. Thus decay is very rapid 
at first and then slows down as the short-lived isotopes disappear. 
    The debris in the troposphere circles the Earth in about 2 weeks, while 
material injected into the stratosphere may travel much faster. The horizontal 
distribution in the north-south direction is much slower, and it may take 
several months for radioactivity to cover the Northern Hemisphere, for 
example, following a test in the tropics. Radioactive debris in the 
troposphere is brought down to the surface of the Earth mainly in 
precipitation. Radioactive material remains about 30 days in the troposphere. 
    Once the fallout has actually been deposited, its fate depends largely on 
the chemical nature of the radionuclides involved. Since the level of gamma 
radiation from worldwide fallout is negligible, attention has been paid to 
those nuclides which might possibly present some hazard when they enter the 
biosphere. Plants, for example, may be contaminated directly by foliar 
deposition or indirectly by deposition on the soil that is followed by root 
uptake. Animals may be contaminated by eating the plants, and humans by eating 
plant or animal foods. 
    The three radionuclides of greatest interest are strontium-90, cesium-137, 
and iodine-131. Strontium-90 is a beta emitter which follows calcium 
metabolically. and tends to deposit in the bone. The radiation emitted could 
result in bone cancer or leukemia if the levels were to become sufficiently 
high. Cesium-137 is distributed throughout soft tissues and emits both beta 
and gamma radiation. It is considered to be a possible genetic hazard but not 
as dangerous to the individual as strontium-90. Iodine-131 is relatively 
short-lived and probably is a hazard only as it appears in tropospheric 
fallout.  See RADIATION BIOLOGY. 
 �34s[John H. Harley]
�33s 
  
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�6v�2v�32sRadioactive minerals   �21m �33s 
    The so-called radioactive minerals loosely comprise species that contain 
uranium or thorium as an essential part of their chemical composition, 
together with minerals in which these elements are sometimes present in solid 
solution, usually in small and variable amounts. About 150 minerals fall into 
the first category, including many that are rare or that are imperfectly 
known. The principal uranium minerals from the point of view of their economic 
importance are the oxide uraninite and its variety pitchblende, the vanadates 
carnotite and tyuyamunite, the silicate coffinite, the phosphates autunite and 
torbernite, and the complex oxides brannerite and davidite. The chief thorium 
minerals of economic interest are the silicates thorite and thorogummite and 
the oxide thorianite.  See RADIOACTIVITY; RADIUM; THORIUM; URANIUM. 
 �34s[Clifford Frondel]
�33s 
  
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�6v�2v�32sRadioactive tracer   �21m �33s 
    A radioactive isotope which, when injected into a chemically similar 
substance or artificially attached to a biological or physical system, can be 
traced by radiation detection devices. Many problems in biology, medicine, and 
industrial engineering not amenable to other approaches can be solved by the 
use of these tracers.  See  RADIOACTIVITY; RADIOACTIVITY AND RADIATION 
APPLICATIONS; RADIOISOTOPE; RADIOISOTOPIC ASSAY. 
    The simplest radioactive tracer studies consist of the tagging of a 
biological entity with a radioactive isotope (radioisotope). The entity is 
then tracked by following the radiation from the isotope. The operation 
becomes more complex when a large number of biological particles are labeled, 
for example, in the tagging of red blood cells or bacteria. When the labeled 
substance is injected into an animal, it is impossible to follow the 
individual labled particles, but their average movement can be tracked by 
observations of the radiation. 
    A radioisotope may also be used to tag its own element. Phosphorus-32 can 
be introduced into the soil where a plant is growing, and the amount of 
phosphorus absorbed and its distribution throughout the plant can be studied. 
    In most biological tracer experiments, the radioisotope is introduced into 
the system and its radiation subsequently measured with Geiger-Mueller 
counters or scintillation detectors. Extremely soft (low-intensity) radiations 
may be detected by the use of photographic film.  See  AUTORADIOGRAPHY; 
GEIGER-MUELLER COUNTER; SCINTILLATION COUNTER. 
    Many medical tracer techniques have achieved the status of important and 
widely used diagnostic tests. For example, radioiodine has found wide use in 
determining the status of the human thyroid gland. 
    There are various scanning techniques which enable one to locate a 
radioisotope within the body and consequently to delineate an organ or an 
abnormality within an organ. Scanners have been devised which can accurately 
fix the size of the thyroid gland and determine the presence of nodules or 
abnormalities in the shape of the gland. Other scanning techniques have been 
highly successful in locating tumors within the brain. 
    Radioactive tracers have become increasingly useful in industrial 
applications, such as the testing of wear and corrosion in mechanical 
components. On a larger scale, the entire operation of an industrial plant can 
be tested and controlled by tracer techniques.  See  NONDESTRUCTIVE TESTING. 
 �34s[Gordon L. Brownell]
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�6v�2v�32sRadioactive waste management   �21m �33s 
    The treatment and containment of radioactive wastes. Radioactive waste 
management is required to some degree in all operations associated with the 
use of nuclear energy for national defense or peaceful purposes. Liquid, 
solid, and gaseous radioactive wastes are produced in the mining of ore, 
production of reactor fuel materials, reactor operation, processing of 
irradiated reactor fuels, and numerous related operations. Wastes also result 
from the use of radioactive materials, for example, in research laboratories, 
industrial operations, and medical treatment. 
  
    Aspects of management.  The chief aim in the safe handling and containment 
of radioactive wastes is the prevention of radiation damage to humans and the 
environment by controlling the dispersion of radioactive materials. Liquid or 
solid waste products containing significant quantities of the more toxic 
radioactive materials require isolation and permanent containment in media 
from which any potential escape into the human environment would be at 
tolerable levels. The radioactive materials of major concern are those that 
may be readily incorporated into the body and those that have relatively long 
half-lives, ranging from a few years to thousands of years.  See  RADIATION 
BIOLOGY; RADIATION DAMAGE TO MATERIALS. 
    Waste management is focused on those radioisotopes which originate in 
nuclear reactors. Here the fission products (chemical elements formed by 
nuclear fragmentation of actinide elements such as uranium or plutonium) 
accumulate in the nuclear fuel, along with plutonium and other transuranic 
nuclides.  See  NUCLEAR REACTOR; PLUTONIUM; TRANSURANIUM ELEMENTS; URANIUM. 
    If fuel discharged from the nuclear reactor is reprocessed, uranium and 
plutonium are recovered after chemical dissolution. During recovery, the 
favored treatment processes produce high-level waste in the form of an acidic 
aqueous stream. Regulations require that the high-level wastes from nuclear 
fuel reprocessing plants be solidified within 5 years after reprocessing and 
shipped to a Federal repository within 10 years after reprocessing.  See  
NUCLEAR FUELS REPROCESSING. 
    Mined cavity disposal and sub-seabed geologic disposal are the chief 
options under investigation for final storage and disposal of radioactive 
waste. The basic requirement for acceptable final storage or disposal of 
radioactive waste is the capability to contain and isolate the waste safely 
until decay has reduced the radioactivity to nonhazardous levels - or at least 
to levels found in nature. 
    Exactly how long near-total containment and isolation of nuclear waste 
must be maintained is much debated. However, two characteristics of waste 
change radically in a few hundred years. First, the heat generation rate 
decreases an order of magnitude in the period of 10 to 100 years and another 
order of magnitude in the period of 100 to 1000 years. Second, the toxicity of 
the high-level waste decreases about three orders of magnitude in the first 
300 to 400 years due to decay of short-lived fission products. It is then at 
levels comparable with those of an equal volume of average ores of common 
toxic elements. 
 �34s[Allison M. Platt]
�33s 
  
    Decommissioning of a nuclear facility.  Because of the buildup of 
radioactive contamination in a nuclear facility such as a nuclear reactor or 
reprocessing plant, detailed plans must be made, beginning with the design and 
construction stages of the facility, to provide adequate protection from 
radiation exposure during the final decommissioning stage. 
    There are four principal methods of decommissioning in use: layaway, 
mothballing, dismantlement, and entombment. Layaway and mothballing are 
temporary measures to postpone final action until some of the radioactivity 
has decayed. They involve removing the easily removable high-level radioactive 
objects. Entombment, as the name implies, is making the radioactive 
contamination inaccessible by the use of demolition techniques and covering 
the residue with reinforced concrete.  See  DECONTAMINATION OF RADIOACTIVE 
MATERIALS; HEALTH PHYSICS; RADIOACTIVITY. 
 �34s[Karl Z. Morgan]
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�6v�2v�32sRadioactivity �21m �33s
    A phenomenon resulting from an instability of the atomic nucleus in
certain atoms whereby the nucleus experiences a spontaneous but measurably
delayed nuclear transition or transformation with the resulting emission of
radiation. The discovery of radioactivity by H. Becquerel in 1896 marked the
birth of nuclear physics.
    All chemical elements may be rendered radioactive by adding or by
subtacting (except for hydrogen and helium) neutrons from the nucleus of the
stable ones. The availability of this wide variety of radioactive isotopes has
stimulated their use in science and technology in an enormous number of
applications. For a discussion of some of the more important applications  see
RADIOACTIVITY AND RADIATION APPLICATIONS.  See also ALPHA RAYS; BETA RAYS;
GAMMA RAYS.
    A particular radioactive transition may be delayed by less than a
microsecond or by more than a billion years, but the existence of a measurable
delay or lifetime distinguishes a radioactive nuclear transition from a
so-called prompt nuclear transition, such as is involved in the emission of
most gamma rays. The delay is expressed quantitatively by the radioactive
decay constant, or by the mean life, or by the half-period for each type of
radioactive atom, discussed below.
    There are five types of radioactivity commonly found in nearly all
elements, each characterized by the particular type of nuclear radiation which
is emitted by the transforming parent nucleus: alpha-ray decay, negatron beta
decay, positron beta decay, electron capture, and gamma-ray decay. In addition,
there are several other decay modes that are observed more rarely in specific
regions of the periodic table. 

    Transition rates and decay laws.  The rate of radioactive transformation,
or the activity, of a source equals the number A of identical radioactive
atoms present in the source, multiplied by their characteristic radioactive
decay constant LAMBDA. Thus Eq. (1) holds, where the decay constant LAMBDA has
dimensions of second(sup -1). The numerical value of LAMBDA expresses the
statistical probability of decay of each radioactive atom in a group of
identical atoms, per unit time. For example, if LAMBDA = 0.01 s(sup -1) for a
particular radioactive species, then each atom has a chance of 0.01 (1%) of
decaying in 1 s, and a chance of 0.99 (99%) of not decaying in any given 1-s
interval. The constant LAMBDA is one of the most important characteristics of
each radioactive nuclide: LAMBDA is essentially independent of all physical
and chemical conditions such as temperature, pressure, concentration, chemical
combination, or age of the radioactive atoms.

		   Activity = A LAMBDA disintegrations per second	     (1)

    Many radioactive nuclides have two or more independent and alternative
modes of decay. For example, (sup 238)U can decay either by alpha-ray
emission or by spontaneous fission. When two or more independent modes of
decay are possible, the nuclide is said to exhibit dual decay. The competing
modes of decay of any nuclide have independent partial decay constants given
by the probabilities LAMBDA1, LAMBDA2, LAMBDA3, . . . , per second, and the
total probability of decay is represented by the total decay constant LAMBDA,
defined by Eq. (2).

		   LAMBDA = LAMBDA1 + LAMBDA2 + LAMBDA3 + . . . 	     (2)

    The actual life of any particular atom can have any value between zero and
infinity. The average or mean life of a large number of identical radioactive
atoms is, however, a definite and important quantity. The total L of the
life-spans of all the A0 atoms initially present is given by Eq. (3). Then the
average lifetime L/A(sub 0), which is called the mean life TAU, is given
by Eq. (4).

			  L = A(sub 0)/LAMBDA				     (3)


			  TAU = 1/LAMBDA				     (4)

    The time interval over which the chance of survival of a particular
radioactive atom is exactly one-half is called half-period T. The half-period
T is related to the total radioactive decay constant LAMBDA, and to the mean
life TAU, by Eq. (5). For mnemonic reasons, the half-period T is much more
frequently employed than the total decay constant LAMBDA or the mean life TAU.

			T = 0.693/LAMBDA = 0.693 TAU			      (5)


    Radioactive series decay.  In a number of cases a radioactive nuclide A
decays into a nuclide B which is also radioactive; the nuclide B decays into C
which is also radioactive, and so on. For example, (sup 232)Th decays into a
series of 10 successive radioactive nuclides. Substantially all the primary
products of nuclear fission are negatron beta-ray emitters which decay through
a chain or series of two to six successive beta-ray emitters before a stable
nuclide is reached as an end product.

    Alpha-ray decay.  Alpha-ray decay is that type of radioactivity in which
the parent nucleus expels an alpha ray (a helium nucleus). The atomic number,
or nuclear charge, of the decay product is 2 units less than that of the
parent, and the nuclear mass of the product is 4 atomic mass units less than
that of the parent, because the emitted alpha particle carries away this
amount of nuclear charge and mass. This decrease of 2 units of atomic number
or nuclear charge between parent and product means that the decay product will
be a different chemical element, displaced by 2 units to the left in a
periodic table of the elements.
    In the simplest case of alpha decay, every alpha ray would be emitted with
exactly the same velocity and hence the same kinetic energy. However, in most
cases there are two or more discrete energy groups called lines. For example,
in the alpha decay of a large group of (sup 238)U atoms, 77% of the alpha
decays will be by emission of alpha rays whose kinetic energy is 4.20 MeV,
while 23% will be by emission of 4.15-MeV alpha rays. When the 4.20-MeV alpha
ray is emitted, the decay product nucleus is formed in its ground (lowest
energy) level. When a 4.15-MeV alpha ray is emitted, the decay product is
produced in an excited level, 0.05 MeV above the ground level. This nucleus
promptly transforms to its ground level by the emission of a 0.05-MeV gamma
ray or alternatively by the emission of the same amount of energy in the form
of a conversion electron and the associated spectrum of characteristic x-rays.
Thus in all alpha-ray spectra, the alpha rays are emitted in one or more
discrete and homogeneous energy groups, and alpha-ray spectra are accompanied
by gamma-ray and conversion electron spectra whenever there are two or more
alpha-ray groups in the spectrum.
    Among all the known alpha-ray emitters, most alpha-ray energy spectra lie
in the domain of 4 - 6 MeV, although a few extend as low as 2 MeV and as high
as 10 MeV. There is a systematic relationship between the kinetic energy of
the emitted alpha rays and the half-period of the alpha emitter. The
highest-energy alpha rays are emitted by short-lived nuclides, and the
lowest-energy alpha rays are emitted by the very-long-lived alpha-ray
emitters. H. Geiger and J. M. Nuttall showed that there is a linear
relationship between log LAMBDA and the energy of the alpha ray.
    The Geiger-Nuttall rule is inexplicable by classical physics, but emerges
clearly from quantum, or wave, mechanics. In 1928 the hypothesis of
transmission through nuclear potential barriers was shown to give a
satisfactory account of the alpha-decay data, and it has been altered
subsequently only in details.  See  POTENTIAL BARRIER.

    Beta-ray decay.  Beta-ray decay is a type of radioactivity in which the
parent nucleus emits a beta ray. There are two types of beta decay
established: in negatron beta decay (BETA-) the emitted beta ray is a
negatively charged electron (negatron); in positron beta decay (BETA+) the
emitted beta ray is a positively charged electron (positron). In beta decay
the atomic number shifts by one unit of charge, while the mass number remains
unchanged. In contrast to alpha decay, when beta decay takes place between two
nuclei which have a definite energy difference, the beta rays from a large
number of atoms will have a continuous distribution of energy.
    For each beta-ray emitter, there is a definite maximum or upper limit to
the energy spectrum of beta rays. This maximum energy, E(sub max), corresponds
to the change in nuclear energy in the beta decay. As in the case of alpha
decay, most beta-ray spectra include additional continuous spectra which have
less maximum energy and which leave the product nucleus in an excited level
from which gamma rays are then emitted. 
    For nuclei very far from stability, the energies of these excited states
populated in beta decay are so large that the excited states may decay by
proton, neutron, two-neutron or alpha emission, or spontaneous fission and, it
is predicted theoretically, by two-proton emission.
    The continuous spectrum of beta-ray energies implies the simultaneous
emission of a second particle besides the beta ray, in order to conserve
energy and angular momentum for each decaying nucleus. This particle is the
neutrino. The neutrino has zero charge and presumably zero rest mass, travels
at the same speed as light (3 x 10[sup 10] cm/s), and is emitted as a
companion particle with each beta ray.	See  NEUTRINO.

    Gamma-ray decay.  Gamma-ray decay is a transition between two excited
levels of a nucleus, or between an excited level and the ground level. A
nucleus in its ground level cannot emit any gamma radiation. Therefore
gamma-ray decay occurs only as a sequel of another radioactive decay process
or of some other process whereby the product nucleus is left in an excited
state. Such additional processes include the fusion of two nuclei, Coulomb
excitation, and induced nuclear fission.  See  COULOMB EXCITATION; NUCLEAR
FISSION; NUCLEAR FUSION.
    A gamma ray is high-frequency electromagnetic radiation (a photon) in the
same family with radiowaves, visible light, and x-rays. The energy of a gamma
ray is given by hv, where h is Planck's constant and v is the frequency of
oscillation of the wave in hertz. The gamma-ray or photon energy hv lies
between 0.05 and 3 MeV for the majority of known nuclear transitions.
Higher-energy gamma rays are seen in neutron capture and some reactions.  See
ELECTROMAGNETIC RADIATION.
    Gamma rays carry away energy, linear momentum, and angular momentum, and
account for changes of angular momentum, parity, and energy between excited
levels in a given nucleus. This leads to a set of gamma-ray selection rules
for nuclear decay and a classification of gamma-ray transitions as
``electric'' or as ``magnetic'' multipole radiation of multipole order 2(sup l),
where l = 1 is called dipole radiation, l = 2 is quadrupole radiation, and l =
3 is octupole, l being the vector change in nuclear angular momentum. The most
common type of gamma-ray transition in nuclei is the electric quadrupole (E2).
There are cases where several hundred gamma rays with different energies are
emitted in the decays of atoms of only one isotope.  See  MULTIPOLE RADIATION.

    Spontaneous fission.  This involves the spontaneous breakup of a nucleus
into two heavy fragments and neutrons. After the discovery of fission in 1939,
it was subsequently discovered that isotopes like (sup 238)U had very weak
decay branches for spontaneous fission, with branching ratios on the order of
10(sup -6). New isotopes subsequently identified like (sup 252)Cf have large
(3.1%) spontaneous fission branching.
 �34s[Joseph H. Hamilton]
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�6v�2v�32sRadioactivity and radiation applications   �21m �33s 
    The field in which the subatomic fragments emitted in radioactive decay 
(alpha-, beta-, gamma-rays) or produced by high-voltage accelerators 
(electrons, protons, x-rays) are applied to the problems of science, 
engineering, industry, and medicine. The techniques are extraordinarily 
versatile and sensitive and are basically inexpensive. A disadvantage that 
limits the range and extent of these applications is the health hazard that 
may be involved.  See  RADIOACTIVITY; RADIOISOTOPE. 
    Tracer applications are based on two principles. First is the chemical 
similarity of radioactive atoms and other atoms of the same element. 
Periodically a few of the radioactive atoms decay, emitting some penetrating 
subatomic fragments that can be detected one by one, usually through their 
ability to cause ionization. Thus the movement of a particular element can be 
followed through various chemical, physical, and biological steps. The second 
principle involves the characteristic half-life and nature of the emitted 
fragments. This makes a radioactive species unique and thereby detectable 
above a background of radioactive emitters associated with elements. For 
discussions of radioisotope techniques relating to tracer methodology  see  
ACTIVATION ANALYSIS; ISOTOPE DILUTION TECHNIQUES.  See also  NONDESTRUCTIVE 
TESTING; RADIOACTIVE TRACER; RADIOECOLOGY; RADIOISOTOPIC ASSAY; RADIOMETRIC 
ASSAY.  
    Penetration and scattering applications arise from the fact that subatomic 
fragments can penetrate a thick section of a material, and yet a small 
fraction of the incident particles can be backscattered by a relatively thin 
section. The oldest application of the penetrating properties of energetic 
ionizing photons is radiography. An extension of this technique is 
autoradiography. Since World War II the penetration and scattering properties 
of beta- and gamma-rays have been applied in industry in the form of thickness 
gages.  See  AUTORADIOGRAPHY; RADIOGRAPHY. 
    The absorption of small amounts of energy from ionizing particles and 
ionizing photons has chemical effects that have been the basis of several 
practical applications. The oldest application of this principle is radiation 
therapy. For example, in cancer therapy the local affected areas are 
irradiated by external beams of gammas from cobalt-60 or of radiation from 
accelerators. Radioactive sources have also been administered internally to 
induce beneficial biochemical reactions in patients afflicted with various 
ailments.  See  ISOTOPIC IRRADIATION; RADIOLOGY. 
    A related area is the radiation sterilization of biomedical supplies. The 
advantages to this method of biochemical destruction of microscopic life are 
that (1) unlike steam sterilization, it can be performed at low temperatures 
on plastics and other thermally unstable materials, and (2) unlike germicidal 
gases, ionizing radiation can reach every point in the treated product. 
Radiation-sterilized objects are not radioactive. 
    The radiation preservation of food is an area of considerable promise. 
Small doses can inhibit sprouting in potatoes, kill insects in wheat, and 
sterilize pork products but practical applications have been sharply limited 
due to a cautious role by regulatory authorities in approving such procedures. 
    Kinetic energy of emissions in radioactive decay can be converted to 
useful forms of light, heat, and electricity.  See  LUMINOUS PAINT; NUCLEAR 
BATTERY.  
 �34s[Joseph Silverman]
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�6v�2v�32sRadio altimeter   �21m �33s 
    A low-power radar which measures the distance of an aircraft (or other 
aerospace vehicle) above the ground. Radio altimeters are often used in 
aircraft during bad-weather landings. They are an essential part of many 
blind-landing and automatic navigation systems and are used over mountains to 
indicate terrain clearance. Special types are used in surveying for quick 
determination of profiles. Radio altimeters are used in bombs, missiles, and 
shells as proximity fuses to cause detonation or to initiate other functions 
at predetermined altitudes. Similar devices are used as landing aids on 
spacecraft.  
    Like other radar devices, the altimeter measures distance by determining 
the time required for a radio wave to travel to and from a target, in this 
case the ground. If the ground were a perfectly flat horizontal plane, 
specular (mirror-type) reflection would occur and the distance measured would 
be exactly the altitude. Actually, the ground is not smooth, and energy is 
scattered back to the radar from all parts of the ground illuminated by the 
transmitter. For the radar to measure distance to the ground accurately, it 
must distinguish between the energy from points near the vertical and that 
from more distant points. Most radio altimeters use either pulse or frequency 
modulation, the former being more popular for high altitudes, and the latter 
for low altitudes. 
    In a pulse altimeter the radio-frequency carrier is modulated with short 
pulses (<0.25 MUs). The short pulse permits measurement, even at low 
altitudes, of the time delay between the leading edge of the transmitted pulse 
and that of the pulse returned from the ground. The simplest pulse altimeter 
displays the received signal on a cathode-ray tube with circular sweep, 
allowing the pilot to make a determination of the leading edge position for 
the echo signal. 
    In an fm altimeter, a continuous carrier is swept in frequency in some 
manner, usually to give a triangular or sinusoidal frequency-time curve. The 
difference in frequency between that received from the ground (but transmitted 
earlier) and that being transmitted is a measure of the time delay. 
 �34s[Raman K. Mehra]
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�6v�2v�32sRadio astronomy �21m �33s
    The study of celestial objects by measurement and analysis of the
electromagnetic radiation they emit in the wavelength range from 1 mm to 30 m
(0.04 in. to 100 ft). Radio astronomers study an entire range of celestial
objects, including the normal stars, planets, galaxies, and the exotic
quasars, pulsars, and x-ray sources. The universe itself is also a source of
microwave radiations. The cosmic microwave radiation and radio emission from
pulsars and quasars are discussed in separate articles.  See  COSMIC MICROWAVE
RADIATION; PULSAR; QUASARS.

    Radio universe.  Since the late 1940s, radio telescopes have been used to
map the skies and determine the positions and intensities, or fluxes, of
individual sources of radio emission. Such maps have been made with increasing
sensitivity and angular resolution; the latter property enables astronomers to
determine the position of the radio sources accurately. Knowing the position,
astronomers can refer to optical photographs of the sky and establish
precisely which object is emitting radio waves. This procedure has led to the
identification of radio sources with many bright galaxies and even with the
most distant objects in the universe, the quasistellar objects (or quasars).
However, nearly one-fifth of all radio sources are unidentified, that is,
excellent photographs taken at the radio source positions show no object at
all from which the radiation could arise. One concludes from this that these
unidentified sources are ``normal'' galaxies and quasars at such great
distances that they cannot be seen optically.  See  RADIO TELESCOPE.
    It appears that there are more sources at great distances per unit volume
of space than there are nearby, a result that means the universe is expanding,
and that those distant sources are, in general, stronger than ones nearby.
These two conclusions are fundamentally linked, and together they mean that
further work on cosmological problems must await a clearer understanding of
the nature and evolution of individual radio sources.  See  COSMOLOGY.

    Galaxies and radio galaxies.  Spiral galaxies, such as the Milky Way
Galaxy, are often radio sources, although most are quite weak. Elliptical
galaxies are usually not radio sources. However, the few elliptical galaxies
that are radio sources are very spectacular ones, being among the most
energetic radio objects in the sky. These are known as the radio galaxies. The
radio emission of these galaxies emanates from the whole volume of the galaxy
and, in most cases, a huge volume outside the galaxy. Frequently there are two
large regions on opposite sides of the galaxy from which most of the radio
energy is emitted. The radio emission from galaxies and radio galaxies is
almost certainly generated by the electron synchrotron process, in which
relativistic electrons spiral around magnetic field lines and emit a
continuous radiation spectrum throughout the band accessible to radio
astronomers.  See  GALAXY, EXTERNAL.

    Solar system astronomy.  The Sun is an intense radio source, but only
because it is so close to Earth. If it were at the distance of the nearest
stars, its radio emission could not be detected. Solar radio emission tends to
be intermittent: solar flares that produce cosmic rays and plasma streams that
interact with Earth are visible as radio bursts; these are most frequent
during the peak of the 11-year solar cycle.  S ee COSMIC RAYS; SUN.
    Jupiter is a much stronger radio source than had been expected from
estimates of its surface temperature by optical astronomers. Most of its radio
emission is caused by electron synchrotron emission in its very strong
magnetic field. The very-long-wavelength emission of Jupiter is impulsive, and
its strength depends upon the position of Io, one of Jupiter's moons. Similar
impulsive long-wave length bursts have also been discovered from Saturn.  See
JUPITER; SATURN.

    Radio stars.  Several nearby, apparently normal stars are detectable at
radio wavelengths. Such stars as Algol, BETA Persei, and AR Lacerta are
multiple star systems that have been found to be radio sources. The radio
emission from these stars is dominated by radio bursts in which the radio
fluxes may increase by a factor of 100 or more. It is clear that the radio
bursts are initiated by or are a product of mass exchange processes going on
between (at least) two closely bound stars.
    An extreme example of radio emission from stars comes from stars that are
also x-ray sources. Again, these objects are usually binary systems in which
mass exchange plays a deciding role in their continuing evolution, but with
x-ray sources one of the component stars appears to be a star that has
exhausted its reservoir of nuclear fuel and is collapsing to its final state.
See  BINARY STAR; X-RAY ASTRONOMY.

    Supernova remnants.  During the formation of a supernova, the atmospheric
envelope of a star is ejected and in this process becomes a rapidly expanding
cloud of relativistic particles, magnetic field, and filaments of ionized gas.
These conditions are precisely those necessary for the generation of radio
emission through electron synchrotron radiation, and very intense radio
sources indeed exist at the positions of old supernovae chronicled by ancient
astronomers. One of the most interesting of the radio sources associated with
a supernova remnant is the Crab Nebula.  See  CRAB NEBULA; SUPERNOVA.

    HII regions.  An HII region (a region of ionized hydrogen) is a large
cloud of interstellar gas that has been ionized and heated by one or more
bright, hot stars located within. These nebulae are sources of both continuum
and line energy at radio and optical wavelengths. Since cosmic matter consists
mostly of hydrogen, the ionized gas consists mainly of protons and electrons
that emit continuum energy by the bremsstrahlung process.  See	NEBULA.

    Hydrogen line.  Study of the 21-cm line of neutral atomic hydrogen has
been exceptionally rewarding in its contribution to the knowledge of galactic
structure and of the physical characteristics of interstellar gas. Line
intensity normally reflects the amount of gas in the line of sight; line
wavelength and width indicate the line-of-sight velocity of the gas and the
state of internal motion, just as with recombination lines in HII regions. If
the gas overlies a strong radio source, the gas temperature can be inferred by
observing the 21-cm line in absorption.
    The structure of the Galaxy has been elucidated by the study of the amount
and velocity of the hydrogen within it. A prime advantage of this method is
that the very distant gas is just as visible as nearby gas, whereas optical
studies of the whole  Galaxy are impossible because the very distant stars are
made invisible by intervening clouds of dust. The results of these radio
studies indicate that the Milky Way Galaxy is a spiral.  See  GALAXY.

    Molecular lines.  It has long been believed that simple molecules could
not exist in the tenuous gas between stars, because the starlight radiation
field would be sufficiently intense to break apart even the simplest molecular
species. In spite of these arguments, by 1968 radio astronomers had found
rotational transitions of three simple molecules, OH, H2O, and NH3. These
molecules were found in dark clouds of gas and dust usually associated with
HII regions; such dark clouds are believed to be the sites of recent and
continuing star formation. Since 1968 dozens of molecular species have been
detected in interstellar space. Observations of the molecular species may make
it possible to establish the chemistry and thermodynamics in the interstellar
clouds from which, ultimately, stars, planets, and life itself must form.  See
MOLECULAR STRUCTURE AND SPECTRA; RADIO TELESCOPE.
 �34s[Robert L. Brown]
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�6v�2v�32sRadio broadcasting   �21m �33s 
    The transmission of audible program material for direct reception by the 
general public.  See  RADIO. 
  
    Frequency allocations.  By international agreement, certain portions of 
the radio-frequency (rf) spectrum are set aside for radio broadcasting. In the 
United States over 4400 stations occupy the standard broadcast band of 107 
channels, extending from 535 to 1605 kHz, each channel being 10 kHz wide. In 
addition, there are over 3300 stations on 100 channels utilizing the 
frequency-modulation (FM) band from 88 to 108 MHz, each channel being 200 kHz 
wide. These stations are all licensed to provide domestic service. There are 
also some shortwave broadcast stations operating in high-frequency bands 
between 5950 and 26,100 kHz that provide international service. In Europe and 
Asia the low-frequency band 150 - 290 kHz is also used for broadcasting. All 
radio stations of any kind in the United States must be licensed by the 
Federal Communications Commission (FCC).  See  RADIO SPECTRUM ALLOCATION. 
  
    AM medium-frequency band.  The power of the rf carrier is specified in the 
FCC construction permit and licence. Nine power levels are recognized, ranging 
from 100 W for small stations serving local areas to 50,000 W for the largest 
stations serving large areas and long distances. In other countries much 
higher powers are sometimes employed. 
    It is often possible to avoid interfering with other stations by the use 
of a directional antenna, which radiates low power in the directions of other 
stations assigned to the same or adjacent channels and concentrates its power 
over the areas to be served. The majority of stations in the United States and 
Canada are directional. 
    Signals from standard broadcast stations arrive at the receiving points in 
two ways, by ground wave and by sky, or ionospheric, wave. In the daytime, 
only ground waves are received, but at night both ground and sky waves are 
effective.  See  RADIO-WAVE PROPAGATION. 
    In the United States the geographical area over which a standard broadcast 
station projects its signals is divided by the FCC into two categories: A 
primary service area is served by ground-wave propagation with sufficient 
field intensity to provide consistent day and night service. A secondary 
service area is serviced by ground waves of insufficient strength to provide 
consistent day and night service, or is served by sky waves at night. 
    Except for a limited number of channels reserved for service over large 
areas, each broadcast channel is used by a large number of geographically 
separated stations. All these stations do not have the same priority status. A 
dominant station is permitted to broadcast full-time. Some secondary stations 
are permitted to operate only between the hours of sunrise and sunset, so that 
they will not interfere with other stations having higher priority status on 
the channel. 
  
    VHF-FM band.  FM broadcasting has full provision for high-fidelity 
reproduction, is ordinarily free of static, substantially suppresses noise and 
interference, and is relatively free of fading. The service range is 
ordinarily 40 - 80 mi (65 - 130 km) line of sight, but stations with high 
power and high antenna elevation are often receivable at distances up to 150 
mi (250 km).  See  FREQUENCY MODULATION; FREQUENCY-MODULATION DETECTOR; 
FREQUENCY-MODULATION RADIO; FREQUENCY MODULATOR. 
    Receiving systems for AM and FM radio signals are quite different, but 
combination receivers and tuners are now commonplace. 
    In the United States and many countries, FM broadcasting is assigned 
frequencies between 88 and 108 MHz. The channel spacing is 200 kHz. This 
facilitates the broadcasting of high-fidelity broad-band stereo. 
  
    Shortwave broadcasting.  Shortly after World War I, experimental 
transmissions on radio frequencies between about 4000 and 20,000 kHz proved 
that communication was possible over great distances by reflection from layers 
of ionized air, which vary in altitude from about 150 and 250 mi (250 and 400 
km). Radio waves from the transmitting antennas follow a path at low angles 
above Earth until they arrive at the reflecting ionized layers of air where 
they are reflected back to Earth. A number of successive reflections between 
the layers of the ionosphere and Earth make transmission for many thousands of 
miles possible and, at times, completely around the world.  See  AMATEUR RADIO. 
  
    Satellite broadcasting.  Intercontinental communications by satellite are 
now commonplace. The satellites in use are capable of handling several 
thousand narrow-channel telephone, telegraph, and teletype channels and many 
wide-channel television channels, all simultaneously. 
    Experimental broadcast and communications satellites, in geosynchronous 
orbit, are received throughout much of North America on small earth terminals 
in the 4- or 12-GHz bands, by broadcasters, cable television operators, 
hotels, apartment complexes, camps, and individuals in isolated areas. All of 
them provide a choice of television channels and some also offer FM radio. The 
proliferation of small TVRO (television receive only) earth stations will 
promote the global village concept by adding a new dimension to international 
broadcasting.  See  COMMUNICATIONS SATELLITE. 
 �34s[J. Gordon Elder]
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�6v�2v�32sRadio broadcasting networks   �21m �33s 
    A group of broadcast stations interconnected by leased channels on wire, 
microwave, or satellite to one or more central feed points for the purpose of 
receiving and rebroadcasting program material of a timely nature. Networks 
make it possible to broadcast live programs simultaneously to the public 
through affiliated radio stations; they make national and regional markets 
available to advertisers and offer stations quality entertainment and public 
service programs. 
    For over 40 years, all domestic radio broadcast networks utilized 
telephone company facilities exclusively in the AT&T toll system for their 
program distribution. In 1971 the Federal Communications Commission (FCC) 
regulations allowed other common carriers to begin offering program channel 
service to radio networks in direct competition with AT&T. 
    In 1972 Microwave Communications Incorporated (MCI) began offering 
high-quality program channel service between New York City and Washington on 
its terrestrial microwave system. Other common carriers were quick to follow 
suit, picking up program distribution contracts in areas of the country where 
this service was not cost-effective for AT&T. 
    A satellite relay system is basically a microwave technology using 
high-power amplifiers at the transmit end coupled to high-gain antennas which 
beam a wide-band signal to domestic communications satellites in geostationary 
orbit over the Earth. These ``fixed-position'' satellites, operated and 
maintained in their orbits by the common carriers, repeat the signal on 
assigned frequencies, beaming a broad pattern, or footprint, which can cover 
an entire continent. The signal may be received at any location in the 
coverage area using high-technology, low-noise amplifiers (LNAs) coupled to 
standard microwave reflectors aimed toward the satellite.  See  COMMUNICATIONS 
SATELLITE.  
    Domestic radio networks first began leasing point-to-point microwave and 
satellite channels in the early 1970s to supplement their AT&T distribution 
networks. Satellite channels in particular offered a higher-quality audio 
service at lower rates due to the decreased number of radio systems and audio 
amplifiers in the chain from transmitting station to final receive point. 
Several radio networks have taken advantage of this new technology, and many 
new networks have entered into the field as a direct result of the 
availability of high-quality audio transmission systems via satellite. 
Developments in satellite technology have vastly improved the signal quality 
that networks can provide to stations, and have reopened programming 
possibilities which had been all but lost to radio.  See  RADIO; RADIO 
BROADCASTING.  
 �34s[William J. Wisniewski]
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�6v�2v�32sRadiocarbon dating �21m �33s
    A method of obtaining age estimates on organic materials which has been
used to date samples as old as 70,000 years. A date is obtained based on
radiocarbon ([sup 14]C) which gives a temporal index of when a specific sample
was removed from its reservoir.
    Radiocarbon determinations can be obtained on wood, charcoal, marine and
fresh-water shell, bone and antler, peat and organic-bearing sediments,
carbonate deposits such as tufa, caliche, and marl, and dissolved CO2 and
carbonates in ocean, lake, and groundwater sources. Each sample type has
specific problems associated with its use for dating purposes, including
contamination and special environmental effects. While the impact of (sup 14)C
dating has been most profound in archeological research and particularly in
prehistoric studies, extremely significant contributions have also been made
in hydrology and oceanography.
    Carbon has three naturally occurring isotopes. Two ([sup 12]C and
[sup 13]C) are stable, but the third, (sup 14)C, decays by very weak beta
decay (electron emission) to (sup 14)N with a half-life of approximately 5700
years. Naturally occurring (sup 14)C is produced as a secondary effect of
cosmic-ray bombardment of the upper atmosphere. As (sup 14)CO2, it is
distributed on a worldwide basis into various atmospheric, biospheric, and
hydrospheric reservoirs on a time scale much less than its half-life.
Metabolic processes in living organisms and relatively rapid turnover of
carbonates in surface ocean waters maintain (sup 14)C levels at approximately
constant levels in most of the biosphere.  See	ISOTOPE.
    To the degree that (sup 14)C production has proceeded long enough without
significant variation to produce an equilibrium or steady-state condition,
sup1sup4C levels observed in contemporary materials may be used to
characterize the original (sup 14)C activity in the corresponding carbon
reservoirs. Once a sample has been removed from exchange with its reservoir,
as at the death of an organism, the amount of sup1sup4C begins to decrease as
a function of its half-life. A (sup 14)C age determination is based on a
measurement of the residual (sup 14)C activity in a sample compared to the
activity of a sample of assumed zero age (a contemporary standard) from the
same reservoir.
    A measurement of the sup1sup4C content of an organic sample will provide
an accurate determination of the sample's age if it is assumed that (1) the
production of (sup 14)C by cosmic rays has remained essentially constant long
enough to establish a steady state in the (sup 14)C/(sup 12)C ratio in the
atmosphere; (2) there has been a complete and rapid mixing of (sup 14)C
throughout the various carbon reservoirs; (3) the carbon isotope ratio in the
sample has not been altered except by sup1sup4C decay; and (4) the total
amount of carbon in any reservoir has not been altered. In addition, the
half-life of (sup 14)C must be known with sufficient accuracy, and it must be
possible to measure natural levels of sup1sup4C to appropriate levels of
accuracy and precision. Studies have shown that the primary assumptions on
which the method rests have been violated both systematically and to varying
degrees for particular sample types. Methods for calibration and corrections
of conventional (sup 14)C values have been developed.
 �34s[R. E. Taylor ;Richard A. Mueller]
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�6v�2v�32sRadiochemical laboratory   �21m �33s 
    An installation to provide a safe environment for handling and 
investigating radioactive materials. Common features of radiochemical 
laboratories include readily decontaminated surfaces, good ventilation, 
cleanliness, good lighting, special arrangements for waste disposal, and 
filtration of both supply and exhaust air. Specific features of these 
laboratories depend primarily on the type and amount of radioactivity handled. 
 �34s[Stephen Lawroski]
�33s 
  
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�6v�2v�32sRadiochemistry   �21m �33s 
    A subject which embraces all applications of radioactive isotopes to 
chemistry. It is not precisely defined and is closely linked to nuclear 
chemistry.  See  NUCLEAR CHEMISTRY; RADIOISOTOPE. 
    In general, radiochemical studies can be classified according to whether 
the use of isotopes represents a convenient or a unique solution to a problem. 
Convenient applications usually exploit the high sensitivity of tracer 
techniques because alternative analytical procedures are slower and often less 
accurate. The efficiencies of chemical separations are studied by labeling the 
desired compound and following the radioactivity during the separations. 
 �34s[Donald R. Stranks]
�33s 
  
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�6v�2v�32sRadio compass   �21m �33s 
    A popular term for an automatic radio direction finder used for navigation 
purposes on ships and aircraft. It is not strictly a compass, because it 
indicates direction with respect to the radio station to which it is tuned, 
rather than to the north magnetic pole. The modern radio compass uses a 
nondirectional antenna in combination with a bidirectional loop antenna to 
provide a unidirectional bearing indication. Navigators thus know at all times 
whether they are traveling toward or away from the radio station used as a 
reference. The antennas are used with a special radio receiver that provides a 
visual indication of direction on a meter or cathode-ray indicator.  See  
DIRECTION-FINDING EQUIPMENT. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sRadioecology   �21m �33s 
    The study of the interaction of radioisotopes or ionizing radiation with 
populations, ecological communities, or ecosystems. Radioecology began with 
attempts to predict the impact of the developing nuclear industry on 
environmental systems: the fates of radioactive substances released into the 
environment, and the effects of ionizing radiation on ecological processes. 
The value of radioactive tracers for ecological studies emerged rapidly, as 
did the utility of ionizing radiation effects as a tool in ecological 
research. Radioecology now encompasses the use of radioactive tracers, as well 
as fates and effects of radioactive substances in the environment.  See  
RADIOACTIVE TRACER; RADIOISOTOPE. 
 �34s[Richard G. Wiegert]
�33s 
  
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�6v�2v�32sRadio-frequency amplifier   �21m �33s 
    A tuned amplifier that amplifies the high-frequency signals commonly used 
in radio communications. The frequency at which maximum gain occurs in a 
radio-frequency (rf) amplifier is made variable by changing either the 
capacitance or the inductance of the tuned circuit. A typical application is 
the amplification of the signal received from an antenna before it is mixed 
with a local oscillator signal in the first detector of a radio receiver. The 
amplifier that follows the first detector is a special type of rf amplifier 
known as an intermediate-frequency (i-f) amplifier.  See  AMPLIFIER; 
INTERMEDIATE-FREQUENCY AMPLIFIER. 
    An rf amplifier is distinguished by its ability to tune over the desired 
range of input frequencies. The shunt capacitance, which adversely affects the 
gain of a resistance-capacitance coupled amplifier, becomes a part of the 
tuning capacitance in the rf amplifier, thus permitting high gain at radio 
frequencies. The power gain of an rf amplifier is always limited at high radio 
frequencies, however. 
    Two typical rf amplifier circuits are shown in the illustration. The 
conventional bipolar transistor amplifier of illustration a uses tapped coils 
in the tuned circuits to provide optimum gain-bandwidth characteristics 
consistent with the desirable value of tuning capacitance. Inductive coupling 
provides the desired impedance transformation in the input and output 
circuits. The tuning capacitors are usually ganged so as to rotate on a single 
shaft, providing tuning by a single knob. Sometimes varactor diodes are used 
to tune the circuits, in which case the tuning control is a potentiometer that 
controls the diode voltage. Automatic gain control (AGC) is frequently used on 
the rf amplifier, as shown. AGC voltage controls the bias and hence the 
transconductance of the amplifier. In the field-effect transistor (FET) 
circuit (illustration b), tapped coils are not required because of the very 
high input and output resistances of the FET. Characteristics of the FET are 
similar to those of a pentode tube except for the lower drain voltage and 
higher capacitance between the input and the output. Thus the FET circuit is 
similar to a pentode circuit except that it is much simpler because it has no 
heater, screen grid, or suppressor grid equivalents.  See  AUTOMATIC GAIN 
CONTROL (AGC); SEMICONDUCTOR; TRANSISTOR. 
 �34s[Charles L. Alley]
�33s 
  
  
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�6v�2v�32sRadio-frequency spectroscopy �21m �33s
    The branch of spectroscopy concerned with the measurement of the intervals
between atomic or molecular energy levels that are separated by frequencies from
about 100 kHz to 1000 MHz (10[sup 5] - 10[sup 9] sec[sup -1]).	The importance
of radio-frequency spectroscopy lies in the fact that certain specific
properties of the nucleus, such as spin, magnetic dipole moment, and electric
quadrupole moment, play a relatively major role in determining the intervals
between closely lying energy levels; the results of this branch of spectroscopy
have been of great importance in determining nuclear properties.  For a
discussion of radio-frequency spectroscopy by the molecular-beam
magnetic-resonance method see MOLECULAR BEAMS.	See also MAGNETIC RESONANCE;
MICROWAVE SPECTROSCOPY; NUCLEAR MOMENTS; SPECTROSCOPY.

 �34s[Polykarp Kusch]
�33s

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�6v�2v�32sRadiography �21m �33s
    The technique of producing a photographic image of an opaque specimen by
the penetration of radiation such as gamma rays, x-rays, neutrons, or charged
particles. When a beam of radiation is transmitted through any heterogeneous
object, it is differentially absorbed, depending upon the varying thickness,
density, and chemical composition of this object. The image registered by the
emergent rays on a photographic film adjacent to the specimen under
examination constitutes a shadowgraph or radiograph of its interior.
Radiography is the general term applied to this nondestructive technique of
testing the gross internal structure of any object, whether it be of the chest
of a human patient for evidence of tuberculosis, silicosis, heart pathology,
or embedded foreign objects; of bones in case of fractures, arthritis or other
bone diseases; or of a weld in a pipe to observe cracks, inclusions, or voids.
    The term roentgenography specifically applies to the use of x-radiation,
and is derived from the name of its discoverer, W. C. Roentgen.
Microradiography is simply the extension of radiography to small specimens and
the photographic enlargement of these images (up to 500x). When the image is
registered on a fluorescent screen and visually observed, the technique is
called fluoroscopy. The photography of the fluorescent image, as in mass chest
examinations, is called photofluoroscopy. The radiation-sensitive silver
halide emulsion of photographic film may be replaced by the dry-plate
electrostatically formed image of xerography, and the technique becomes
xeroradiography. High-speed radiography, for instance, of fast-moving objects,
can be achieved by specially designed x-ray apparatus with exposure times as
short as 0.2x10(sup -6) sec.  See  COMPUTERIZED TOMOGRAPHY; MICRORADIOGRAPHY;
RADIOLOGY.
    A newer development in the field of radiography is the use of neutron
beams. These neutrons can penetrate matter with relative ease since they are
not electrically charged. There are low attenuations of thermal neutrons in
most of the heavy elements; therefore, large thicknesses of heavy materials
can be inspected by neutrons in less time than would be required by
x-radiography or gamma radiography. On the other hand, the neutron absorption
coefficients of elements with low atomic numbers are high; hydrogen, lithium,
and boron are particularly attenuating. With neutrons, it is possible to
visualize materials such as liquids, adhesives, rubber, plastic, or explosives
even when they are in metal assemblies.
    Proton radiography employs beams of protons. A monoenergetic charged
particle has a well-defined distance it will travel in a given material before
it is stopped; the distance is called the range. Since most of the attenuation
of the charged particles occurs near the end of the range, a very small change
in material thickness will result in a large change in radiation transmission.
Therefore, the sensitivity of this method to small changes in object thickness
is very great, if the total path for the radiation approximates the range.
See  CHARGED PARTICLE BEAMS.
 �34s[Gerold H. Tenney; George L. Clark; Howard Berger]
�33s
    In heavy-ion radiography, accelerated beams of heavy charged particles are
used to produce detailed radiographs that can be used for diagnostic
applications to cancer research and therapy. These beams have special physical
qualities that make them particularly well-suited to imaging research: they
have an exact range-energy relationship, and they scatter less than electrons
or x-rays. To make a radiograph, the beam passes through the tissue or organ
to be imaged, and is collected in a stack of approximately 50 thin plastic
sheets. If the particle passes through dense tissue, it stops in the front
part of the detector stack; conversely, if the tissue is less dense, the
particles travel farther to the back of the stack. The data can be viewed in
oblique light and photographed, but for quantitative reconstruction a light
beam scattered off the plastic sheets is digitized with a television-type
vidicon camera and a computer, and the integrated data are stored on magnetic
tapes or disks. Heavy-ion radiography thus provides a more precise resolution
of density and composition of tissue than does conventional x-ray radiography.
 �34s[Cornelius A. Tobias]
�33s

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�6v�2v�32sRadioimmunoassay   �21m �33s 
    A general method employing the reaction of antigen with specific antibody, 
permitting measurement of the concentration of virtually any substance of 
biologic interest, often with unparalleled sensitivity. The basis of the 
method is summarized in the competing reactions shown in the illustration. The 
unknown concentration of the antigenic substance in a sample is obtained by 
comparing its inhibitory effect on the binding of radioactively labeled 
antigen to a limited amount of specific antibody with the inhibitory effect of 
known standards. 
    A typical radioimmunoassay is performed by the simultaneous preparation of 
a series of standard and unknown mixtures in test tubes, each containing 
identical concentrations of labeled antigen and specific antibody. After an 
appropriate reaction time the antibody-bound (B) and free (F) fractions of the 
labeled antigen are separated by one of a variety of techniques. The B/F 
ratios in the standards are plotted as a function of the concentration of 
unlabeled antigen (standard curve), and the unknown concentration of antigen 
is determined by comparing the observed B/F ratio with the standard curve. 
    The radioimmunoassay principle has found wide application in the 
measurement of a large and diverse group of substances in a variety of 
problems of clinical and biological interest. It is therefore not unexpected 
that there are differences in the specific methods employed for the assay of a 
particular substance. The full potential of the method has yet to be 
exploited. It seems that virtually any substance of biologic interest can be 
measured, the method being modified according to the characteristics of the 
particular substance.  See  ANTIBODY; ANTIGEN; ANTIGEN-ANTIBODY REACTION; 
IMMUNOLOGY.  
 �34s[Rosalyn S. Yalow]
�33s 
  
  
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�6v�2v�32sRadioisotope   �21m �33s 
    A radioactive isotope (as distinguished from a stable isotope) of an 
element. Atomic nuclei are of two types, unstable and stable. Those in the 
former category are said to be radioactive and eventually are transformed, by 
radioactive decay, into the latter. One of the three types of radioactive ray 
(ALPHA-, BETA-, and GAMMA-rays) is emitted during each stage of the decay. 
    The term radioisotope is also loosely used to refer to any radioactive 
atomic species. Whereas approximately a dozen radioisotopes are found in 
nature in appreciable amounts, hundreds of different radioisotopes have been 
artifically produced by bombarding stable nuclei with various atomic 
projectiles.  See  ISOTOPE; RADIOACTIVITY.  
 �34s[Henry E. Duckworth]
�33s 
  
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�6v�2v�32sRadioisotopic assay   �21m �33s 
    An analytical technique including procedures for separating and 
reproducibly measuring a radioactive tracer. Separations for the assay may be 
made carrier-free (without stable atoms added) or with a few milligrams of 
added carrier by techniques such as ion exchange, solvent extraction, 
distillation, precipitation, electrodeposition, and isotopic exchange. Once 
separated, the radioisotopes may be measured as gases, liquids, or solids in 
Geiger-Mueller, proportional, or scintillation counters, or in ionization 
chambers.  
    Geiger and proportional counters are used primarily for measuring beta 
rays, whereas solid scintillation detectors are used for gamma rays. Liquid 
scintillation counters eliminate many self-absorption problems with weak beta 
emitters.  See  ACTIVATION ANALYSIS; PARTICLE DETECTOR; RADIOACTIVE TRACER; 
RADIOCHEMISTRY; RADIOMETRIC ASSAY. 
 �34s[W. Wayne Meinke]
�33s 
  
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�6v�2v�32sRadiolaria   �21m �33s 
    A subclass of Actinopodea. These marine protozoan types are noted for the 
siliceous skeletons found in most species (see illustration). A characteristic 
feature is the capsule which separates the outer from the inner cytoplasm and 
is composed of one or two membranes, apparently organic (chitin or tectin). 
The capsule, which encloses the nuclei, contains pores whose distribution and 
relative number are taxonomic features. Presence of true axopodia (permanent 
pseudopods with axial filaments) is doubtful; the pseudopodia are mostly 
filamentous (filopodia), less commonly filamentous, branched, and 
interconnected (filoreticulopodia). This is an old group, as indicated by 
Lower Silurian fossils of specialized types. 
    In the order Porulosida (sometimes known as Peripylina) the capsule is a 
fairly thick membrane with more or less uniformly distributed pores. In the 
order Oculosida, which is subdivided into the Monopylina and the Tripylina 
groups, capsular pores are restricted to certain areas.  See  ACTINOPODEA; 
OCULOSIDA; PORULOSIDA. 
 �34s[Richard P. Hall]
�33s 
  
  
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�6v�2v�32sRadiolarian earth   �21m �33s 
    A porous, earthy, unconsolidated sediment formed from the opaline silica 
skeletal remains of Radiolaria. It is formed from radiolarian oozes that 
accumulate on the deep ocean floor. The indurated equivalent with pores filled 
by silica is radiolarite. The radiolarian earths and radiolarites are white 
and cream-colored. Radiolarites are known from Devonian to Tertiary age.  See  
CHERT; RADIOLARIA; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
�33s 
  
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�6v�2v�32sRadiology   �21m �33s 
    The medical science concerned with x-rays, radioactive materials, and 
other ionizing radiations, and the application of the principles of this 
science to diagnosis and treatment of disease. Nonionizing radiations of 
infrared and ultrasound are also used for diagnosis. 
    Diagnostic radiology uses radiation, usually x-rays, to study the 
configuration of anatomical structures or the function of body organ systems.  
See  RADIOGRAPHY. 
    Radioactive isotopes are used to obtain images of organ systems and 
functions. The accumulation of isotope in a tumor or an organ such as the 
thyroid is recorded by a suitable GAMMA-ray detector attached to an electronic 
amplifier and recording equipment. The image of the radioactivity concentrated 
in an organ is viewed on a television-type screen and recorded on a 
photographic print.  See  RADIOACTIVE TRACER. 
    Sound waves of 1 - 10 MHz are transmitted from a crystal transducer, and 
after amplification are displayed on an oscilloscope and recorded on a 
photographic print. The ultrasound pulses demonstrate organ structures such as 
the heart, liver, and spleen. Although the resolution is less fine than that 
obtained with x-ray, there is an advantage in that the ultrasound is 
nonionizing radiation. Ultrasound is particularly useful, therefore, in 
determining the size and degree of development of the human fetus.  See  
ULTRASONICS.  
    Infrared radiation from the human body is used to detect tumors such as 
breast tumors, which are near the body surface. The technique, thermography, 
is based on the idea that tumors are warmer than the surrounding normal 
tissue. This increase in temperature is detected by an infrared device, and 
the ``hot spot'' scan is displayed on a television-type screen, with permanent 
records kept on photographic prints. 
    Radiation therapy deals with the treatment of disease with ionizing 
radiation. The diseases most commonly treated are cancer and allied diseases. 
Radiation therapy has been found useful in the management of some diseases 
such as ringworm of the scalp and bursitis, but because of possible serious 
complications occurring many years later, the use of ionizing radiation is 
generally avoided if alternative methods of treatment are available. 
    In cancer therapy the objective is to destroy a tumor without causing 
irreparable radiation damage in normal body tissues that must of necessity be 
irradiated in the process of delivering a lethal dose to the tumor. This 
applies particularly to important normal structures in the vicinity of the 
tumor. The relative radiosensitivity of the tumor with respect to these normal 
structures is the chief factor determining the success of the treatment. 
 �34s[Lee B. Lusted]
�33s 
  
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�6v�2v�32sRadiometric assay   �21m �33s 
    An analytical technique that includes procedures for measuring, by tracer 
methods, elements which are not themselves radioactive. It is particularly 
useful for small quantities of inorganic ions or complex organic compounds. 
    In radiometric titrations a radioactive tracer is used to determine the 
equivalence point in a volumetric determination, where a highly insoluble 
compound of definite composition is formed. At least one of the solutions used 
in the titration must be radioactive, because progress of the titration is 
followed by plotting the radioactivity of the solution versus volume of 
reagent added. The equivalence point is determined by the intersection of two 
straight lines on this plot, because a definite change in solution activity 
appears after equivalent amounts of reagents have reacted.  See  RADIOACTIVE 
TRACER; RADIOCHEMISTRY; RADIOISOTOPIC ASSAY; TITRATION. 
 �34s[W. Wayne Meinke]
�33s 
  
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�6v�2v�32sRadiometry �21m �33s
    The detection and measurement of radiant electromagnetic energy.
Conventionally, radiometry is concerned with infrared radiation. Generally,
the devices used in radiometry can be, and are, used with visible light.
However, the use of devices applicable only to visible light is commonly
excluded from the term radiometry.  See  ELECTROMAGNETIC RADIATION; INFRARED
RADIATION; PHOTOMETRY.
    Detectors such as the thermometer which respond to the increase in
temperature resulting from the absorption or radiant energy are termed thermal
detectors. Improvement in thermal detectors has been concerned with securing a
large and rapid rise in temperature and high sensitivity in the detection of
changes in temperature. To secure the largest rise for a given radiation, the
detector should absorb it as completely as possible; the loss of heat must be
as small as possible. The heat capacity of a thermal detector should be small,
so that a rapid rise in temperature will occur. The area of the absorbing
surface should be small to permit the measurement of narrow beams of radiation.
    The thermocouple, one of the oldest and still one of the best radiation
detectors, produces an electromotive force (emf) when heated. The sensitive
portion of a bolometer undergoes a change in resistance when heated.  See
BOLOMETER; THERMOCOUPLE.
    The photographic effect does not depend upon a rise in temperature but
upon the freeing of a bound electron by the absorption of a single quantum of
radiation. Detectors utilizing this principle are termed quantum detectors, or
photodetectors. Photoemissive cells and photomultipliers are of interest for
visible radiation, but have limited application in the near infrared to about
1 MUm. Photoconductive cells using materials such as lead sulfide, lead
selenide, and lead telluride are useful to 5 MUm. Photoconductive cells of
single crystals of gold-doped germanium, cooled with liquid helium, have
sensitivity extending to much longer wavelengths.  See	PHOTOCONDUCTIVE CELL;
PHOTODIODE; PHOTOMULTIPLIER; PHOTOTRANSISTOR; PHOTOTUBE.
 �34s[Howard W. Russell /George R. Harrison]
�33s

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�6v�2v�32sRadio range   �21m �33s 
    A radio facility emitting signals which, when received by appropriate 
companion equipment, provide a direct indication of the bearing of the 
facility from the vehicle. 
    The vhf omnidirectional radio range (VOR) operates at very high 
frequencies and provides radial lines of position in any direction. It is the 
most important type of radio range in civil air operations, and several 
hundred VOR ranges are in use throughout the world. 
    This facility operates in the 112 - 118-MHz range and employs a 
directional and a nondirectional antenna. The emission from the directional 
antenna, when combined with radiation from the nondirectional antenna, results 
in a cardioid space pattern. This space pattern is rotated at a speed of 30 Hz 
either physically or through the use of a goniometer. No other modulation 
appears on the rotating (variable phase) pattern. The rotation is synchronous 
with a frequency modulation which is imposed on a small amount of rf energy 
derived from the source that feeds the variable phase antenna and radiated by 
the nondirectional antenna. The output of the nondirectional antenna is 
therefore a carrier frequency that is amplitude-modulated with frequencies 
that vary cyclically from 9480 to 10,440 Hz at a 30-Hz rate. This signal 
appears everywhere in space within the coverage of the station. A special 
receiver is necessary to derive navigational information from the VOR.  See  
DOPPLER  VOR; ELECTRONIC NAVIGATION SYSTEMS. 
 �34s[Peter C. Sandretto]
�33s 
  
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�6v�2v�32sRadio receiver �21m �33s
    That part of a radio communication system which abstracts the desired
information from the radio-frequency (rf) energy collected by the antenna. All
radio receivers must perform three basic functions: selectivity,
amplification, and detection. For basic discussion of radio principles	see
RADIO.

    Functions.	Many radio signals transmitted at the same time are available
at the antenna. From these many signals the receiver must select the single
one desired. This is done by tuning the receiver to the frequency of the
desired carrier. The tuning circuit contains a combination of inductances L
and capacitances C, one or more of which are variable. The frequency f
selected is determined by the relation below, where f is in hertz (Hz), L is
in henries, and C is in farads. Tuning the receiver, therefore, consists in
changing the inductance or capacitance, usually the latter. When so tuned the
inductance-capacitance circuit accepts the desired frequency and rejects other
frequencies. By using several such circuits in series, a high degree of
selectivity may be obtained.

		f = 1/2 PI [SQUARE ROOT of LC]

    Because the incoming signal may be weak and because a certain minimum
energy is required to operate the loudspeaker, the headphones, or the
television picture tube, considerable amplification must take place between
the input of the receiver and its output. This is usually called the gain of
the receiver. It may amount to 10,000,000 times in voltage or 140 decibels
(dB).  See  AMPLIFIER.
    The energy collected by the antenna and presented to the input of the
receiver is in the form of rf waves which act as a carrier for the information
to be transmitted. The purpose of the detector in a receiver is to remove the
desired communication from this carrier and to convert it into a form that
will actuate the output device, such as a loudspeaker;	See
AMPLITUDE-MODULATION DETECTOR; DETECTOR; FREQUENCY-MODULATION DETECTOR.

    Types.  Two general types of receiver are in use today, the
tuned-radio-frequency (TRF) and the superheterodyne. Both of these can be used
for amplitude-modulated (AM) signals. Frequency-modulation (FM) receivers are
almost always superheterodyne.	See  AMPLITUDE MODULATION;
AMPLITUDE-MODULATION RADIO; FREQUENCY MODULATION; FREQUENCY-MODULATION RADIO.
    In a TRF receiver all amplification up to the detector takes place at the
frequency of the incoming signal. This usually requires several stages of
tuned amplification. Each stage is tuned to the same frequency and the tuning
elements are ganged together for convenience when tuning. TRF receivers are
especially applicable to the low-frequency and very-low-frequency bands from
about 10,000 to 300,000 Hz. The illustration shows a block diagram of a TRF
receiver compared to a superheterodyne receiver.
    With the increased use of higher frequencies for broadcasting and
communications, a high degree of selectivity became necessary. The TRF
receiver was inadequate in this respect. This selectivity was obtained by
circuit techniques involving frequency changing. The superheterodyne circuit
changes the frequency by heterodyning, or beating, two frequencies together to
get a third. When two signals of small difference in frequency are
superimposed on a nonlinear device, the output consists of energy at two major
combination frequencies which are the sum and difference of the original
frequencies. The sum of the two beating frequencies is usually eliminated by
tuned circuits following the heterodyne process. The difference frequency,
referred to as the intermediate frequency, is passed on for further
amplification and, what is more important, through stages of high selectivity
because selectivity at lower frequencies is more easily obtained than at high
frequencies.


    FM and SSB receivers.  In an FM system the carrier is modulated by the
desired message by varying the frequency of the carrier instead of varying its
amplitude. The receiver for such a system must have some means of producing a
varying voltage amplitude to correspond to the varying frequency. In other
words, the FM signal must be converted to an AM signal. A limiter is employed
to eliminate all amplitude variations of the carrier and to deliver to the
final detector a signal which is free of noise. Several types of FM detectors,
usually called discriminators, are employed.
    The single-sideband (SSB) receiver is designed for the reception of but
one sideband of intelligence on a reduced carrier. The independent-sideband
(ISB) receiver is used for the reception of two channels of intelligence, one
on an upper and one on a lower sideband. To effect this type of reception
without undue distortion, the carrier is exalted or reinforced, and in some
instances a local carrier synchronized by the incoming reduced carrier is
employed for demodulation, sometimes referred to as product detection. To
separate the sidebands and carrier, carefully designed filters are employoed.
See  SINGLE SIDEBAND.
 �34s[Walter Lyons]
�33s

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�6v�2v�32sRadio spectrum allocations �21m �33s
    The specification of the frequency bands of the radio spectrum which are
made available for use by the various radio services. The radio spectrum is
the part of the natural spectrum of electromagnetic radiation lying between
the frequency limits of 10(sup 4)  and 3 x 10(sup 11) hertz. Since the
characteristics of the radiation do not change abruptly, these limits are not
sharply defined and may change, depending upon the demands for changes in
service and upon changes in technology.
    For purposes of identification the radio spectrum is divided into bands
differing from adjacent bands by frequency ratios of 10. These bands are
identified by the metric wavelength of the shortest waves in each band
(kilometric band, and so forth) or by adjective or numerical designators (for
example, very-low-frequency band or band 4). The wavelength LAMBDA in meters
is related to the frequency f in hertz by the relationship c = fLAMBDA, where
c, the velocity of propagation of radio waves in space, is about 3 x 10(sup 8)
m/s.
    The major classes of radio services are provided with international
allocations within the various frequency bands. Within the United States there
is a national allocation which conforms in general to the international
allocation. The Radio Regulations divide the Earth into three regions and
contain a detailed table of allocations, with some differences between
regions, together with exceptions to the listed allocations and conditions for
frequency use. Regional and national allocations can be made within the
worldwide framework, but any departures by signatories of the Regulations must
be accomplished on the basis of noninterference with services operating in
accordance with the agreed allocations.
    The characteristics of radio waves, due to natural phenomena such as
atmospherics, vary greatly with frequency. Attempts are made to take advantage
of propagation chracteristics when making allocations, and normally the
frequencies allocated have characteristics satisfying the operational
requirements of the service with regard to distance, bandwidth, and other
technique factors. Allocations are sometimes made for a new service which has
already begun to use a particular band. Since many services have varied and
continually growing requirements, allocations to them will be found in several
frequency bands. In making specific frequency assignments to individual radio
stations, the separation between assignable frequencies must be such as to
avoid harmful interference in view of the bandwidth requirements, the
frequency stability of the transmitters, the frequency selectivity of the
receivers, the transmitter power and its distance separation from receivers
operating on adjacent channels, and other factors. Thus the separation between
adjacent assigned frequencies varies widely between services and between
different locations in the frequency spectrum.	See  BANDWIDTH REQUIREMENTS
(COMMUNICATIONS).
    In most of the frequency bands it has been found practical to make block
allocations of frequencies to given services in each region. Individual
frequency assignments can then be made to radio stations operating in that
service without the need for coordination with frequency assignments to
stations in other services. Interference problems are more easily resolved, as
they occur between compatible users. Some of these allocations are exclusive
to a particular service, and some are shared between specified services to
varying degrees. Similarly, in some services the stations have individual
frequency assignments which are protected from interference from other
stations, whereas the stations in other services may share time or geographic
coverage on frequencies common to several stations. These arrangements are
based upon the nature and the needs of the particular services being provided.
 �34s[Lawrence M. Palmer ;Jack M. Linthicum]
�33s

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�6v�2v�32sRadio telescope   �21m �33s 
    An instrument used in astronomical research to detect and measure the 
radio-frequency power coming from various directions in the sky. It consists 
of three complementary parts: the large reflecting surface that collects and 
focuses the incident radiation; the electronic receiver that amplifies and 
detects cosmic radio signals; and a data display device. From the ground, 
observations with radio telescopes must be made at wavelengths shorter than 
100 ft (30 m), because of ionospheric attenuation, and longer than 0.04 in (1 
mm), because the very-short-wavelength radio radiation is absorbed by 
atmospheric H2O, CO2, and O3. 
    The fundamental principle of a radio telescope is identical to that of a 
reflecting telescope used at visual wavelenths. The incoming waves (radio or 
optical) are intercepted by a precise mirror and reflected to a common focal 
point. The shape of the reflecting surface or ``dish'' is important: the radio 
waves must arrive ``in phase'' at the focal point following their reflection 
from the dish; that is, the path length from the point of reflection to the 
focus must be exactly the same for all points on the dish. This restriction 
can be most simply satisifed if the shape of the reflecting surface is made 
paraboloidal; consequently, most modern radio telescopes have this shape (see 
illustration).  See OPTICAL TELESCOPE; TELESCOPE. 
    Once the radio waves are collected and brought together at the focal point 
of the telescope, they are in general still extremely weak. The incoming 
radio-frequency (rf) signals are first amplified at the focus 10 to 1000 times 
and then converted to a lower frequency, the intermediate frequency (i - f), 
that can be easily transmitted by cables from the focal point to the 
telescope-control building. There the i-f is further amplified, and the signal 
is detected and displayed in the manner the astronomer finds most suited to 
the particular investigation. 
    The types of astronomical objects that emit radio-frequency radiation and 
hence can be studied by radio astronomers are of such a diverse nature that a 
variety of radio telescopes and receiving equipment are necessary for a modern 
radio observatory. Two general astronomical considerations dictate what 
instruments are needed: first, radio telescopes should have the highest 
possible angular resolution so that the small-scale details of radio sources 
can be studied; second, the radio receivers should be extremely sensitive to 
the very weak signals emitted by cosmic radio sources. 
    The ultimate in angular resolution is achieved by the technique of 
very-long-baseline interferometry (VLBI), in which one simultaneously utilizes 
radio telescopes separated by thousands of miles. Data are acquired 
independently at each telescope and recorded on video tape. Precise time 
markings are made on the tape using hydrogen maser clocks. After the data are 
recorded, the video tapes from the separate telescopes are brought together; 
the time markings on the individual tapes are aligned, and data taken at 
precisely the same times can be compared and analyzed. Such VLBI techniques 
have achieved angular resolutions of about 0.0003 second of arc. See ASTRONOMY. 
 �34s[Robert L. Brown]
�33s 
  
  
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�6v�2v�32sRadio transmitter   �21m �33s 
    A generator of high-frequency electric current whose characteristics of 
amplitude, frequency, or phase angle may be altered, or modulated, in 
accordance with the intelligence to be transmitted. A radio transmitter 
consists of several distinct major components to accomplish the objectives of 
a particular design for a particular requirement. 
    The power a transmitter delivers to the antenna may vary from a fraction 
of a watt to 1,000,000 watts. Lower powers are used mainly for portable or 
mobile services, while higher powers are required for broadcasting over large 
areas and in point-to-point communications. Transmitters may be classified by 
the type of modulation used. 
    Amplitude-modulation (AM) transmitters have two principal design types, 
either low-level modulated or high-level modulated. The low-level modulated 
transmitter is modulated at its low power stages, requiring little modulation 
power. The high-level modulated transmitter requires modulation power to be 
equal to about 50% of the carrier's power. In order to amplify faithfully and 
reproduce the modulation at low levels, the power amplifiers must be linear. 
These are usually class B linear amplifiers. Because it operates over a large 
band of frequencies, the high-power modulator must have linear power 
amplification to achieve low distortion.  See  AMPLIFIER. 
    Frequency modulation (FM) transmitters are basically similar to AM 
transmitters, except that the AM modulation amplifying system is dispensed 
with and the exciter must be a variable frequency source. One method of 
modulating the frequency at the exciter utilizes a reactance in the 
frequency-determining section of the oscillator in the exciter. The reactance 
value is changed electronically or electrically in accordance with the 
low-frequency modulating signal. The remainder of the transmitter, as in AM 
transmitters, is made up of frequency doubling and tripling stages of power 
amplification and a power output stage. 
    The use of single sideband/independent sideband (SSB/ISB) transmission for 
communications has become very popular because of several peculiar system 
characteristics. Its ability to transmit intelligence occupying only half the 
bandwidth, as compared to AM and FM modes, and its adaptability for modulating 
by many low-frequency subcarriers are principal advantages. 
    Typical equipment arrangement for SSB transmission comprises an SSB 
generator (exciter), which makes the SSB or ISB signal (using a system of 
balanced modulators usually) and filters to separate out the carrier for 
reinsertion in the desired amount, select the desired sideband, and reject the 
undesired sideband. The output, at low levels of several watts, is then 
amplified without frequency changing in linear class B amplifiers to the final 
power output for coupling to the antenna. This method is commonly referred to 
as linear SSB transmission. A principal design objective is the maximizing of 
power output in order to minimize spurious radiation. 
 �34s[Walter Lyons]
�33s 
  
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�6v�2v�32sRadio-wave propagation   �21m �33s 
    The means by which radio signals are transported through space from a 
transmitting antenna to a receiving antenna.  See  RADIO. 
    The frequencies around 20 kHz can be received reliably at distances of 
thousands of miles but are limited to telegraph-type signals and require very 
large transmitting antennas. Higher frequencies are needed for voice, and 
still higher frequencies for television transmission. As the frequency 
increases, the transmission range tends to decrease. Frequencies above 100 MHz 
can transmit wide-band signals, but they are limited to approximately 
line-of-sight distances with the usual type of equipment. However, distances 
of 200 mi (320 km) or more are possible by the use of high power and large 
antennas to provide narrow ``searchlight'' beams. 
    Reflections from the ionosphere (ionized layers 50 - 250 mi or 80 - 400 km 
above the Earth's surface) provide a useful but variable long-distance service 
at frequencies less than about 30 MHz. These reflections account for the 
long-range broadcast coverage at night and for the shortwave intercontinental 
communication.  See  RADIO BROADCASTING. 
    The principal components of the received radio signal are shown 
symbolically in the illustration. The vector sum of the direct, reflected, and 
surface waves has been called the space wave, ground wave, or tropospheric 
transmission to differentiate it from the ionospheric reflections. The 
ionospheric and surface waves are the principal components at frequencies 
below 10 - 30 MHz. The direct and reflected rays are the principal factors at 
frequencies above 30 - 50 MHz. Although the ionospheric, direct, and 
ground-reflected waves can be easily visualized as rays, the surface wave is 
more difficult to understand; it originates at the air-Earth boundary because 
the Earth is not a perfect reflector. 
    Variations in signal level with time are caused by changing atmospheric 
conditions. The severity of the fading usually increases as either the 
frequency or path length increases. Most  fading is temporary diversion of 
energy to some direction other than the intended location, associated with 
refraction or interference, but absorption effects are important in the 
microwave region. 
    The dielectric constant of the atmosphere normally decreases gradually 
with increasing altitude. The result is that on the average the radio ray is 
bent or refracted toward the Earth so that the distance to the radio horizon 
is slightly greater than to the optical horizon. The amount of refraction is 
variable, and exceptionally long-range transmission may occur occasionally. 
Conversely, when the radio energy is bent away from the Earth (upward 
bending), the transmission loss is increased.  See  REFRACTION OF WAVES. 
 �34s[Kenneth Bullington]
�33s 
  
  
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�6v�2v�32sRadish   �21m �33s 
    A cool-season annual or biennial crucifer, Raphanus sativus, of Chinese 
origin belonging to the plant order Capparales. The radish is grown for its 
thickened hypocotyl, which is eaten uncooked as a salad vegetable. Colors 
include red, yellow, white, black, pink, and red-white combinations.  See  
CAPPARALES.  
 �34s[H. John Carew]
�33s 
  
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�6v�2v�32sRadium �21m �33s
    A chemical element, Ra, with atomic number 88. Radium is a rare
radioactive element found in uranium minerals to the extent of 1 part for
about every 3,000,000 parts of uranium. Chemically, radium is an
alkaline-earth metal having properties quite similar to those of barium.
Biologically, radium concentrates in bones by replacing calcium and, as a
result of prolonged irradiation, causes anemia and cancerous growths. Because
radiations from radium and its decay products preferentially destroy malignant
tissue, radium has been used to check the growth of cancer. For this use pure
radium compounds are sealed in tubes or needles; or radon, the gaseous decay
product of radium, is pumped into small tubes. The use of radium in
luminescent paints for watch, clock, and meter dials, and signs visible in the
dark depends on its ALPHA-radiation striking a scintillator such as zinc
sulfide.
    Thirteen isotopes of radium are known; all are radioactive; four occur
naturally; the rest are produced synthetically. Only (sup 226)Ra is
technologically important. It is distributed widely in nature, usually in
exceedingly small quantities. The most concentrated source is pitchblende.
    When first prepared, nearly all radium compounds are white, but they
discolor on standing because of intense radiation. Radium salts ionize the
surrounding atmosphere, thereby appearing to emit a blue glow. Radium
compounds will discharge an electroscope, fog a light-shielded photographic
plate, and produce phosphorescence and fluorescence in certain inorganic
compounds such as zinc sulfide. The emission spectrum of radium compounds is
similar to those of the other alkaline earths; radium halide imparts a
carmine-red color to a flame.  See  ALKALINE-EARTH METALS; RADIOACTIVITY;
RADON.
 �34s[Murrell L. Salutsky]
�33s

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�6v�2v�32sRadius �21m �33s
    A line segment that joins the center of a circle with any one of its
points. If r denotes the length of a radius, the length (circumference) of a
circle is 2PIr, and the area enclosed is PIr(sup 2). The radius of a circle may
be marked off on the circle six times as a chord, and a regular hexagon
obtained.  See	CIRCLE.
 �34s[Leonard M. Blumenthal]
�33s

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�6v�2v�32sRadius of gyration �21m �33s
    A relation of the area or mass of a figure to its moment of inertia. If I
is the moment of inertia about a line of a figure whose area is A, the figure's
radius of gyration with respect to that line is k = + SQUARE ROOT OF (I/A)
Accordingly, I = k(sup 2)A.  For a figure of mass M, k = + SQUARE ROOT OF (I/M);
I = k(sup 2)M.	In these equations, k is measured in length units such as feet.
Geometrically similar figures have equal radii of gyration about corresponding
centroidal axes.  See MOMENT OF INERTIA.
 �34s[Nelson S. Fisk]
�33s

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�6v�2v�32sRadome   �21m �33s 
    A strong, but electrically transparent, thin shell used to house a radar 
antenna, or a space-communications antenna of similar structure. The shell 
must be large enough not to interfere with the scanning motion of the antenna. 
In airborne radar the radome prevents the antenna from upsetting the 
aerodynamic characteristics of the airplane or missle and protects the antenna 
against aerodynamic forces. Shipboard radars frequently require radomes to 
protect them against wind and water damage and blast pressures from nearby 
guns. Large land-based radars are usually shielded by radomes, especially in 
severe climatic conditions.  See   ANTENNA (ELECTROMAGNETISM); RADAR. 
 �34s[Robert I. Bernstein]
�33s 
  
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�6v�2v�32sRadon �21m �33s
    A chemical element, symbol Rn, atomic number 86. Radon is produced as a
gaseous emanation from the radioactive decay of radium. The element is highly
radioactive and decays by the emission of energetic alpha particles. It is the
heaviest of the noble, or inert, gas group and thus is characterized by
chemical inertness. All isotopes are radioactive with short half-lives.  See
INERT GASES; RADIOACTIVITY; RADIUM.
    In addition to the three natural isotopes, 22 isotopes have been
synthesized by nuclear reactions of artificial transmutation in cyclotrons and
linear accelerators, but none of these is as long-lived as (sup 222)Rn.
See  NUCLEAR REACTION.
    Any surface exposed to (sup 222)Rn becomes coated with an active
deposit which consists of a group of short-lived daughter products. The
radiations of this active deposit include energetic alpha, beta, and gamma
rays.
    Radon possesses a particularly stable electronic configuration, which
gives it the chemical properties characteristic of noble-gas elements. It has
a boiling point of -65 DG C (-85 DG F) and a melting point variously reported as
-71 DG C and -113 DG C (-96 DG F and -171 DG F). The spectrum of radon has been
extensively studied, and resembles that of the other inert gases.
 �34s[Earl K. Hyde]
�33s

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�6v�2v�32sRaffinose   �21m �33s 
    The best-known trisaccharide (oligosaccharide), consisting of one 
melibiose and two sucrose units, widely distributed in higher plants. 
Sugarbeets contain about 0.5% of this sugar. The best-known sources are 
cottonseed meal and the manna of Eucalyptus. It is also known as melitose, 
melitriose, gossypose, and O-ALPHA-D-galactopyranosyl-(1 -> 
6)-O-ALPHA-D-glucopyranosyl-(1 -> 2)-BETA-D-fructofuranoside.  See  
OLIGOSACCHARIDE.  
    Raffinose was found to be enzymically synthesized in plants from uridine 
diphosphate D-galactose and sucrose by an enzyme which transfers the 
D-galactose moiety of this sugar nucleotide to sucrose, resulting in the 
formation of raffinose. 
 �34s[William Z. Hassid]
�33s 
  
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�6v�2v�32sRafflesiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of three families and fewer than a hundred species, all tropical or 
subtropical. The plants are highly specialized, nongreen, rootless parasites 
which grow from the roots of the host. They have few or solitary, rather large 
to very large flowers with numerous ovules and a single set of tepals that are 
commonly united into a conspicuous, corolloid calyx.  See  MAGNOLIOPSIDA; 
ROSIDAE.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sRailroad engineering   �21m �33s 
    That part of transportation engineering involved in the planning, design, 
development, construction, maintenance, and use of facilities for the 
transportation of goods and people in wheeled units of rolling stock running 
on, and guided by, rails normally supported on cross ties and held to fixed 
alignment.  
  
    Freight handling systems.  In the United States and in most other 
countries the part of railroad engineering concerned with fixed properties, 
including tracks, roadbeds, bridges, buildings, yards, and terminals, is 
considered to be a part of the broad discipline of civil engineering. In a 
like manner the part of railroad engineering concerned with rolling stock, 
including locomotives, cars, car ferries, and other such equipment, is 
considered to be a part of the broad discipline of mechanical engineering. In 
addition, the staffs of railroad organizations include many architects and 
chemical, electrical, and industrial engineers. 
    Probably the most important transformation that has taken place on 
railroads in the United States in recent times was the change from steam to 
diesel power. Electric locomotives are used on some 700 mi (1100 km) of 
Conrail lines, with only a scattering of such power at other locations.  See  
DIESEL ENGINE; LOCOMOTIVE. 
    Many different kinds of freight cars have been developed. Specially 
equipped long flat cars with tie-down devices are used for handling trailers 
or containers in piggyback service; similar cars are equipped to carry 
containers (see illustration). Other innovations include designs to carry 
double stacks of containers and low-profile piggyback cars which carry 
trailers in a well to allow clearance through eastern tunnels. Other long flat 
cars are equipped with fully enclosed bilevel and trilevel racks for handling 
assembled automobiles and trucks. Covered hopper cars, some of large capacity, 
are used for shipment of cement, grain, flour, and other dry bulk commodities. 
Large-capacity tank cars are in demand for liquid bulk shipments. Most 
refrigerator cars are equipped with gasoline engine - driven mechanical 
refrigeration units. 
  
    Main lines are equipped with signal systems to aid in the expedition and 
safety of train movements. Increasingly the signaling facilities are augmented 
by two-way communication with train crews. 
    Electronic computers are widely used by railroads for data processing and 
for storage and retrieval of information. A widely used automation device is 
the hotbox detector, which locates and records overheated car axle bearings in 
moving trains, thus permitting the defective car to be set out before complete 
journal failure and possible derailment. 
 �34s[William J. Hedley]
�33s 
  
    Passenger systems.  Experience has shown that, as a country develops, its 
need for a reliable and efficient railway system grows. There are many 
distinct characteristics and functions of the guided rail transport system 
that cannot be achieved as efficiently, as economically, or as acceptably 
within environmental constraints by other modes of transport. This is 
particularly true of the passenger services. 
    Transportation systems in industrialized nations are subject to increasing 
pressures from population expansion, congestion and overcrowding, energy 
shortages, and rising costs. 
    The post - World War II Japanese experience is considered a model of 
development. The high-speed rail passenger system, Shinkansen, is considered 
one of the finest technological achievements of the 20th century. It is very 
profitable and reliable, operating comfortable trains that run frequently. It 
has an excellent safety record and has transported more than 1,600,000,000 
passengers during its years of service without a single casualty. 
    TGV (Tres Grande Vitesse), the French high-speed passenger system serving 
the Paris-Lyon route, has a roadbed that takes advantage of flat or rolling 
terrain by following the natural contours, with no tunnels. This roadbed has 
the general appearance of a slowly curving motorway. However, while it is less 
than half the width of a four-lane highway, it has three times the 
passenger-carrying capacity per hour. 
    After World War II the United States railroads emphasized freight services 
to the great detriment of passenger operations. With the availability of more 
powerful locomotives and the invention of the Janney coupler, along with the 
air brake, it became possible to make up trains of great length. As trains get 
longer and heavier, they must get slower. The average freight train speed in 
the United States (calculated by dividing total train miles by total train 
hours) is 20.1 mi/h (32.3 km/h). Such a train must have a flat track, which is 
unsuitable for passenger operations. 
    As passenger trains were forced to slow for flattened curves, they ran 
late consistently, and their schedules were lengthened as a consequence. 
Slower track carrying heavier freight became uncomfortable and unreliable. The 
passenger services were all but immobilized by the heavy freight, slow speed, 
and flat track. Orders for new passenger cars were discontinued. 
    In 1970 Congress passed the Rail Passenger Service Act, and in compliance 
with the provisions of this act the National Railroad Passenger Corporation 
(Amtrak) was created on May 1, 1971. In the early 1980s Amtrak's network 
served all but 4 of the top 50 metropolitan areas. However, in many of them 
the amount of service was not more than one train a day, with many scheduled 
times at inconvenient hours. With over 60% of its system running no more than 
once a day, Amtrak cannot even offer a reasonable round trip connection to a 
very large segment of its prospective passengers. 
 �34s[L. Fletcher Prouty]
�33s 
  
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�6v�2v�32sRainbow   �21m �33s 
    Colored arcs seen in the skies when the Sun or Moon is illuminating large 
numbers of falling raindrops. Such arcs are centered around the antisolar 
point (180DG from the Sun). Among the parallel rays striking a water droplet 
there is one ray which after one internal reflection leaves the drop at the 
smallest angle of deviation from the direction of the incident rays. All other 
rays emerge from the drop, after an internal reflection, at larger angles of 
deviation. The ray of minimum deviation, also called Descartes ray, has the 
greatest intensity and thus is more visible than the others. Since the angle 
of deviation for such a ray of minimum deviation is smaller for the red light 
and larger for the blue, the rays of minimum deviation from a large number of 
water drops of the same size are seen as arcs of different colors, red on the 
upper part, and blue on the lower part of the bow. The angular distance of 
these arcs from the antisolar point is greater than 42DG, and the system of 
arcs is called the primary rainbow. The rays of minimum deviations, after two 
internal reflections, form similar colored arcs at the angular distance of 
51DG from the antisolar point to make the secondary rainbow. In this rainbow 
the color sequence is reversed, with the red arcs visible on the lower part, 
the blue on the upper part of the bow. 
 �34s[Zdenek Sekera]
�33s 
  
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�6v�2v�32sRainforest   �21m �33s 
    A term used loosely in plant geography for forests of broad-leaved 
(dicotyledonous), mainly evergreen trees found in continually moist climates 
in the tropics, subtropics, and some parts of the temperate zones. Sometimes 
the term is unjustifiably extended to include other very wet forests such as 
the Olympic Rain Forest of the state of Washington in which the trees are 
mostly conifers. 
    The tropical rainforest includes the vast Amazon forest as well as large 
areas in western and central Africa, Malaysia, Indonesia, and New Guinea. It 
is the home of an enormous number of plant and animal species. Tropical 
rainforests are usually mixed in composition, no one species forming a large 
proportion of the whole stand, but in some parts of the tropics there are 
rainforests dominated by a single species. 
    Certain structural features are characteristic of tropical rainforests, 
such as the thin, flangelike buttresses of the larger trees, and flowers and 
fruit that are produced, as in the cacao (Theobroma cacao), on the trunk 
(cauliflorous) instead of on the branches. 
    Orchids and other epiphytes and woody vines (lianes) are common. In a 
rainforest that has not been culled for timber or recently disturbed, the 
undergrowth is generally rather thin, and visibility is about 60 ft (18 m) or 
more on the ground. 
    In the rainforest, animals of most groups, like plants, show great species 
diversity. For example, there are several times as many species of birds as in 
an equal area of North American broad-leaved forest. Insects are far more 
numerous than other animals, in terms of species and sheet numbers. 
Interactions between plants and animals are extremely complex, and many very 
specialized relationships have evolved, such as those between certain types of 
ants and the trees in whose hollow twigs they live. In the tropical rainforest 
an unusually large variety of organisms live together in a state of balance, 
so that it is one of the most stable as well as most complex ecosystems on 
Earth.  
 �34s[Paul W. Richards]
�33s 
  
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�6v�2v�32sRain shadow   �21m �33s 
    An area of diminished precipitation on the lee side of mountains. There 
are marked rain shadows, for example, east of the coastal ranges of 
Washington, Oregon, and California, and over a larger region, much of it arid, 
east of the Cascade Range and Sierra Nevadas. All mountains decrease 
precipitation on their lee; but rain shadows are sometimes not marked if moist 
air often comes from diferent directions, as in the Appalachian region. 
    The causes of rain shadow are (1) precipitation of much of the moisture 
when air is forced upward on the windward side of the mountains, (2) 
deflection or damming of moist air flow, and (3) downward flow on the lee 
slopes, which warms the air and lowers its relative humidity. 
 �34s[J. R. Fulks]
�33s 
  
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�6v�2v�32sRaman effect   �21m �33s 
    A phenomenon observed in the scattering of light as it passes through a 
material medium, whereby the light suffers a change in frequency and a random 
alteration in phase. Raman scattering differs in both these respects from 
Rayleigh and Tyndall scattering, in which the scattered light has the same 
frequency as the unscattered and bears a definite phase relation to it. The 
intensity of normal Raman scattering is roughly one-thousandth that of 
Rayleigh scattering in liquids and smaller still in gases.  See  SCATTERING OF 
ELECTROMAGNETIC RADIATION; TYNDALL EFFECT. 
    Because of its low intensity, the Raman effect was not discovered until 
1928, although the scattering of light by transparent solids, liquids, and 
gases had been investigated for many years before. The development of the 
laser has led to a resurgence of interest in the Raman effect and to the 
discovery of a number of related phenomena.  See  LASER. 
    When the exciting radiation falls within the frequency range of a 
molecule's absorption band in the visible or ultraviolet spectrum, the 
radiation may be scattered by two different processes, resonance fluorescence 
or the resonance Raman effect. Both these processes give much more intense 
scattering than the normal nonresonant Raman effect. The absolute frequencies 
of the resonance Raman effect shift by exactly the amount of any shift in the 
exciting frequency, just as do those of the normal Raman effect. Thus the main 
characteristic of the resonance as compared to the normal Raman effect is its 
intensity, which may be greater by two or three orders of magnitude.  See  
FLUORESCENCE.  
    Raman scattering is analyzed by spectroscopic means. The collection of new 
frequencies in the spectrum of monochromatic radiation scattered by a 
substance is characteristic of the substance and is called its Raman spectrum. 
Although the Raman effect can be made to occur in the scattering of radiation 
by atoms, it is of greatest interest in the spectroscopy of molecules and 
crystals. In a typical experiment monochromatic radiation from a laser 
impinges on the sample in an appropriate transparent cell. Raman scattering is 
approximately uniform in all directions and is usually studied at right 
angles. In this way the intense radiation of the laser beam interferes least 
with the observation of the weak scattered light. 
    Raman spectroscopy is of considerable value in determining molecular 
structure and in chemical analysis. Molecular rotational and vibrational 
frequencies can be determined directly, and from these frequencies it is 
sometimes possible to evaluate the molecular geometry, or at least to find the 
molecular symmetry. Even when a precise determination of structure is not 
possible, much can often be said about the arrangement of atoms in a molecule 
from empirical information about the characteristic Raman frequencies of 
groups of atoms. This kind of information is closely similar to that provided 
by infrared spectroscopy; in fact, Raman and infrared spectra often provide 
complementary data about molecular structure. Raman spectra also provide 
information for solid-state physicists, particularly with respect to lattice 
dynamics but also concerning the electronic structures of solids.  See  
INFRARED SPECTROSCOPY; LATTICE VIBRATIONS; MOLECULAR STRUCTURE AND SPECTRA. 
 �34s[Richard C. Lord]
�33s 
  
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�6v�2v�32sRamapithecus   �21m �33s 
    The genus name given by G. E. Lewis in 1934 to a fragmentary upper jaw of 
an extinct hominoid from the early Pliocene of the Siwalik Hills, northern 
India. Ramapithecus, which includes at least two species, is now represented 
by jaws and teeth from the later Miocene of eastern Africa and eastern Europe 
and from the early Pliocene of Asia. The fossils exhibit characteristics which 
suggest that they may be the earliest known hominids, direct ancestors of 
modern humans.  See  FOSSIL MAN. 
 �34s[Erik Trinkaus]
�33s 
  
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�6v�2v�32sRamie   �21m �33s 
    The plant Boehmeria nivea, a stingless member of the nettle family; the 
only member of the family used commercially for fiber. This herbaceous 
perennial is erect, usually nonbranching, and 3 to 9 ft (1 to 2 m) tall at 
maturity. The fiber, which comes from the inner bark, is exceptionally strong 
and has uses similar to those for fiber flax. Ramie is grown mostly in the 
tropics and subtropics of the Far East and Brazil.  See  NATURAL FIBER. 
 �34s[Elton G. Nelson]
�33s 
  
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�6v�2v�32sRamjet �21m �33s
    The simplest of the air-breathing propulsion engines (see illustration).
In flight, air enters the front of the diffuser at high velocity. The diffuser
is shaped to reduce the airspeed and hence its kinetic energy as it passes
through. With an efficient diffuser, the reduction in kinetic energy results
in a nearly equal increase in potential energy, in the form of an increase in
air pressure. This higher-pressure air enters the combustion chamber, where
fuel is continuously injected and burned. The hot gas is then ejected
rearwardly through the discharge nozzle at velocity VsubJ greater than flight
speed V0. To a first approximation, thrust F is given in the equation below,
where M is the mass of air per second flowing through the engine.  See
AIRCRAFT PROPULSION; JET PROPULSION.

		    F = M (V[sub j] - V[sub 0])

    The efficiency of a ramjet in converting the chemical energy in the fuel
into kinetic energy of the airstream depends upon the ratio of the pressure in
the combustion chamber to the ambient air pressure. This pressure ratio in
turn depends upon the flight speed or, more exactly, upon flight Mach number.
Ramjet engines are usually considered for applications in the range of flight
Mach numbers between 2.5 and 8.
    The scramjet (supersonic combustion ramjet) is essentially a ramjet engine
intended for flight at hypersonic speeds (that is, above Mach 6), with the
gases flowing through the combustion chamber and burning at supersonic speeds.
By designing the engine inlet diffuser to provide supersonic speeds in the
combustion chamber for flight at hypersonic speeds, the problem of extreme air
temperature and dissociation in the combustion chamber, which limits the
practical flight speed of ramjet engines, is avoided in the scramjet. Thus
while ramjets cease to be practical at Mach numbers above 8, scramjets, on the
other hand, are believed to be feasible up to Mach 25.
 �34s[Benjamin Pinkel]
�33s


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�6v�2v�32sRangefinder (optics) �21m �33s
    An optical instrument for measuring distance, usually from its position to
a target point. Light from the target enters the optical system through two
windows spaced apart, the distance between the windows being termed the base
length of the rangefinder. The rangefinder operates as an angle-measuring
device for solving the triangle comprising the rangefinder base length and the
line from each window to the target point. Rangefinders can be classified in
general as being of either the coincidence or the stereoscopic type.
    In coincidence rangefinders, one-eyed viewing through a single eyepiece
provides the basis for manipulation of the rangefinder adjustment to cause two
images or parts of each to match or coincide. This type of device is used, in
its simpler forms, in photographic cameras. The basic optical arrangement is
shown in the illustration, where M1 and M2 are a semitransparent mirror and a
reflecting mirror, respectively. When coincidence is obtained, that is, when
the target T is seen in the same apparent position along either path, the
rangefinder equation is satisfied.

    Stereoscopic rangefinders are entirely different, although externally they
resemble coincidence rangefinders except for the fact that they possess two
eyepieces. It is essentially a large stereobinocular fitted with special
reticles which allow a skilled user to superimpose the stereo image formed by
the pair of reticles over the images of the target seen in the eyepieces, so
that the reticle marks appear to be suspended over the target and at the same
apparent distance.
 �34s[Edward K. Kaprelian]
�33s

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�6v�2v�32sRankine cycle   �21m �33s 
    A thermodynamic cycle used as an ideal standard for the comparative 
performance of heat-engine and heat-pump installations operating with a 
condensable vapor as the working fluid. Applied typically to a steam power 
plant, as shown in the illustration, the cycle has four phases: (1) heat 
addition bcde in a boiler at constant pressure p1 changing water at b to 
superheated steam at e, (2) isentropic expansion ef in a prime mover from 
initial pressure p1 to back pressure p2, (3) heat rejection fa in a condenser 
at constant pressure p2 with wet steam at f converted to saturated liquid at 
a, and (4) isentropic compression ab of water in a feed pump from pressure p2 
to pressure p1. 
  
    This cycle more closely approximates the operations in a real steam power 
plant than does the Carnot cycle. Between given temperature limits it offers a 
lower ideal thermal efficiency for the conversion of heat into work than does 
the Carnot standard. Losses from irreversibility, in turn, make the conversion 
efficiency in an actual plant less than the Rankine cycle standard.  See  
CARNOT CYCLE; REFRIGERATION CYCLE; THERMODYNAMIC CYCLE; VAPOR CYCLE. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sRanunculales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Magnoliidae of the class Magnoliopsida (dicotyledons). The order 
consists of 8 families and about 3200 species. The vast majority of the 
species belong to only 3 families, the Ranunculaceae (2000), Berberidaceae 
(650), and Menispermaceae (425). Within its subclass, the order is 
characterized by its mostly separate carpels, triaperturate pollen, herbaceous 
or only secondarily woody habit, frequently numerous stamens, generally more 
than two sepals, and lack of ethereal oil cells. Many members of the order 
contain isoquinoline alkaloids, most notably berberin. The barberry 
(Berberis), in the family Berberidaceae, and the buttercup (Ranunculus), 
columbine (Aquilegia), larkspur (Delphinium), and wind flower (Anemone), in 
the family Ranunculaceae, are familiar genera of the Ranunculales.  See  
MAGNOLIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sRapakivi granites �21m �33s
    Granites containing large, pink, usually ellipsoidal, alkali feldspars
(orthoclase or microcline), which are rimmed by mantles of white plagioclase
(albite or oligoclase). Typically these mantled feldspar phenocrysts occur in
a finer-grained matrix of quartz, plagioclase, alkali feldspar, and biotite or
hornblende.  See  GRANITE.
 �34s[Wilfred A. Elders]
�33s

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�6v�2v�32sRape   �21m �33s 
    Rape (Brassica napus) and turnip rape (B. campestris) plants are members 
of the Cruciferae family. The name is derived from the Latin rapum, meaning 
``turnip,'' to which these plants are closely related. The aerial portions of 
rape plants have been bred to produce oilseeds, fodder, and vegetable crops. 
Rape seed is small, round, and usually black, although varieties with yellow 
seed coats are also grown. 
    The seeds contain over 40% oil. Rapeseed contributes approximately 10 - 
12% of the world's total edible vegetable oil supply and is one of the few 
edible oilseed crops that can be produced in northern Canada, Europe, and 
Asia, or, as a cool season crop, in subtropical areas. Rapeseed meal, the 
by-product of oilseed extraction, is high-quality protein feed supplement for 
livestock and poultry. In some Asian countries it is also used as a fertilizer 
and soil conditioner for specialty crops such as tobacco and citrus fruits. 
Rape is widely used for forage. In some countries the whole plant is cut and 
fed to cattle.  See  CAPPARALES; TURNIP. 
 �34s[R. K. Downey]
�33s 
  
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�6v�2v�32sRare-earth elements   �21m �33s 
    The group of 17 chemical elements with atomic numbers 21, 39, and 57 - 71; 
the name lanthanides is reserved for the elements 58 - 71. The name rare 
earths is a misnomer, because they are neither rare nor earths.  See  ACTINIDE 
ELEMENTS; LANTHANIDE CONTRACTION; PERIODIC TABLE. 
    Most of the early uses of the rare earths took advantage of their common 
properties and were centered principally in the glass, ceramic, lighting, and 
metallurgical industries. Today these applications use a very substantial 
amount of the mixed rare earths just as they are obtained from the minerals, 
although sometimes these mixtures are supplemented by the addition of extra 
cerium or have some of their lanthanum and cerium fractions removed. 
    The elements exhibit very complex spectra, and the mixed oxides, when 
heated, give off an intense white light which resembles sunlight, a property 
finding application in cored carbon arcs, such as those employed in the movie 
industry.  
    The rare-earth metals have a great affinity for the nonmetallic elements, 
as, for example, hydrogen, carbon, nitrogen, oxygen, sulfur, phosphorus, and 
the halides. Considerable amounts of the mixed rare earths are reduced to 
metals, such as misch metal, and these alloys are used in the metallurgical 
industry. Alloys made of cerium and the mixed rare earths are used in the 
manufacture of lighter flints. Rare earths are also used in the petroleum 
industry as catalysts. Yttrium aluminum garnets (YAG) are used in the jewelry 
trade as artificial diamonds. 
    Although the rare earths are widely distributed in nature, they generally 
occur in low concentrations. They are found in high concentrations as mixtures 
in a number of minerals. The relative abundance of the different rare earths 
in various rocks, geological formations, and the stars is of great interest to 
the geophysicist, astrophysicist, and cosmologist. 
    The rare-earth elements are metals possessing distinct individual 
properties. Many of the properties of the rare-earth metals and alloys are 
quite sensitive to temperature and pressure. They are also different when 
measured along different crystal axes of the metal; for example, electrical 
conductivity, elastic constants, and so on. The rare earths form organic salts 
with certain organic chelate compounds. These chelates, which have replaced 
some of the water around the ions, enhance the differences in properties among 
the individual rare earths. Advantage is taken of this technique in the modern 
ion-exchange methods of separation.  See  CHELATION; ION EXCHANGE; TRANSITION 
ELEMENTS.  
 �34s[Frank H. Spedding]
�33s 
  
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�6v�2v�32sRarefied gas dynamics �21m �33s
    That branch of gas dynamics dealing with the flow of gases at such low
density that the molecular mean free path is not negligibly small. Under these
conditions the gas no longer behaves as a continuous fluid. Important
modifications in flow phenomena occur which are ascribable to the discrete
molecular structure of the gas. The subject is also called superaerodynamics.
See  GAS DYNAMICS.
    It is convenient to divide rarefied gas dynamics into three flow regimes.
These are called free molecule flow, transition flow, and slip flow,
corresponding respectively to highly rarefied, moderately rarefied, and only
slightly rarefied flow conditions. Phenomena in the three regimes are quite
dissimilar, so the subdivision is useful. The term ``rarefied'' is relative,
but can be made quantitative in terms of a dimensionless number, called the
Knudsen number K, defined as the ratio of the mean free path LAMBDA divided by
a characteristic dimension L of the flow field, for example, the diameter of a
duct or the length of a high-altitude rocket (K = LAMBDA/L). Free molecule
flow corresponds to K > 1, slip flow to K < 1, and transition to intermediate
values of K.
    The regime of highly rarefied flow corresponds in the aerodynamic case to
flight at altitudes of 100 mi (160 km) or more. The mean free path is long
compared to dimension L. It follows that molecules, which impinge on a surface
and are then reemitted, travel very far before colliding with another
molecule. The essential simplification of free molecule flow is that such
collisions can be neglected. For the aerodynamic case of flow past a convex
body the incident molecules are thus not disturbed by the presence of the body.
    Slip flow is the flow regime of only slight rarefaction, corresponding in
the aerodynamic case to flight at altitudes of the order of 20 - 50 mi (32 -
80 km). Noncontinuum effects can be thought of in terms of small corrections
to ordinary continuum flow. The term slip flow arises from the phenomenon of
slip, according to which a rarefied gas adjacent to a surface does not adhere
rigidly to it but rather has a finite velocity, known as the slip velocity,
determined by the local stress and temperature gradients.
    No simple formulation for the transition regime has yet been developed,
except for the fundamental Maxwell-Boltzmann equation itself. Under some
situations this equation can be solved for arbitrary values of LAMBDA/L,
including those corresponding to transition flow conditions. These special
solutions, together with a large number of experimental results, indicate that
simple interpolation between slip flow on the one hand, and free molecule flow
on the other hand, will usually suffice for the transition flow regime.
 �34s[Samuel A. Schaaf]
�33s

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�6v�2v�32sRaspberry   �21m �33s 
    The horitcultural name for certain species of the genus Rubus, plant order 
Rosales. In these species the fruit, when ripe (unlike the blackberry), 
separates thimblelike from the receptacle. Raspberry plants are upright shrubs 
with perennial roots and prickly, biennial canes (stems). There are several 
species, both American and European, from which the cultivated raspberries 
have been developed. Varieties are grouped as to color of fruit - black, red, 
and purple, the last being hybrids between the red and black types. Leading 
states in commercial production are Michigan, Oregon, New York, Washington, 
Ohio, Pennsylvania, New Jersey, and Minnesota. The fruit is sold fresh for 
dessert purposes, is canned, and is made into jelly or jam, but quick freezing 
is the most important processing method.  See  FRUIT; ROSALES. 
 �34s[J. Harold Clarke]
�33s 
  
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�6v�2v�32sRat   �21m �33s 
    The name for over 650 species of mammals in a number of different families 
of the order Rodentia. These animals are of great economic importance to 
humans. In addition to harboring many diseases transmissible to humans, such 
as bubonic plague, endemic typhus, rat-bite fever, and infectious jaundice, 
ingested rats can transmit trichinosis to swine.  See  MURINE TYPHUS FEVER; 
PLAGUE; RATBITE FEVER; TRICHINOSIS. 
    Rats are usually active at night, feeding on nearly every type of food. 
The teeth are modified for gnawing, with sharp and chisellike incisors that 
grow continually and have heavily enameled fronts. There is a large space 
between the one pair of incisors and the molars into which the flesh of the 
cheeks is drawn during gnawing. This prevents the cheek teeth from meeting and 
wearing down and also prevents the swallowing of dirt and debris.  See  
RODENTIA.  
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sRat-bite fever   �21m �33s 
    Either of two diseases transmitted by the bite of a rat; each has a 
distinct etiology and symptomatology. One is due to a spiral organism, 
Spirillum minus, and the other to Streptobacillus moniliformis. 
    Following a bite and infection with Spirillum, the wound heals promptly, 
but after a period of 5 - 28 days there is sudden onset of symptoms, including 
a flare-up of the wound site which may subsequently ulcerate, regional 
lymphangitis and lymphadenitis, chills, relapsing type of fever, macular skin 
rash, malaise, and headache. Fever may continue for weeks in untreated cases. 
    Most cases of streptobacillary fever have followed a rat bite, although 
sporadic cases have been reported in which no direct contact with rats could 
be established. Following a bite, the local wound ordinarily heals, but 
occasionally an abscess develops. The onset of the disease after an incubation 
period of 1 - 5 days is abrupt, with chills, fever, vomiting, headache, severe 
pains in the back and joints, a maculopapular rash, and leukocytosis. Acute 
arthritis is one of the most prominent and persistent symptoms. 
 �34s[Thomas B. Turner]
�33s 
  
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�6v�2v�32sRatchet   �21m �33s 
    A wheel, usually toothed, operating with a catch or a pawl so as to rotate 
in a single direction (see illustration). A ratchet and pawl mechanism locks a 
machine such as a hoisting winch so that it does not slip. The locking action 
may serve to produce rotation in a desired direction and to disengage in the 
undesired direction as in a drill brace. The catch or pawl may be of various 
shapes such as an eccentrically mounted disk or ball bearing. Gravity, a 
spring, or centrifugal force (with the catch mounted internal to the ratchet) 
are commonly used to hold the pawl against the ratchet. A ratchet and pawl 
provides an arresting action.  See  BRAKE; ESCAPEMENT; PAWL. 
 �34s[Frank H. Rockett]
�33s 
  
  
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�6v�2v�32sRate-of-climb indicator   �21m �33s 
    A device used to indicate changes in the vertical position of an aircraft 
by comparing the actual outside air pressure to a reference volume that lags 
the outside pressure because a calibrated restrictor imposes a lag-time 
constant to the reference pressure volume. The faster the aircraft changes its 
height, the greater the pressure differential which will exist across a 
differential pressure capsule. As the capsule expands or contracts, a very 
delicate mechanism converts the motion to a readout usually graduated in feet 
per minute. 
 �34s[James W. Angus]
�33s 
  
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�6v�2v�32sRatites   �21m �33s 
    A group of flightless (except for tinamous), mostly large, running birds 
formerly segregated as a superorder of birds, the Palaeognathae, but whose 
interrelationships have been a long-standing controversy. The ratites 
represent two or three phyletic lines (the emus, cassowaries, moos and kiwis, 
ostriches and elephant birds, and rheas and tinamous - the last two groups may 
be very closely related) which evolved from a common ancestral stock, possibly 
much like the still volant tinamous of Central and South America.  See  
AEPYORNITHIFORMES; APTERYGIFORMES; AVES; CASUARIIFORMES; DINORNITHIFORMES; 
NEOGNATHAE; STRUTHIONIFORMES; TINAMIFORMES. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sRauwolfia �21m �33s
    A genus of mostly poisonous, tropical trees and shrubs of the dogbane
family (Apocynaceae). Certain species are the source of valuable emetics and
cathartics. The species Rauwolfia serpentina has received special attention as
the source of tranquilizing drugs. Among the purified alkaloids obtained from
R. serpentina, reserpine is perhaps the one most used as a tranquilizing
agent.	See  GENTIANALES; TRANQUILIZER.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sRaw water   �21m �33s 
    Water obtained from natural sources such as streams, reservoirs, and 
wells. Natural water always contains impurities in the form of suspended or 
dissolved mineral or organic matter and as dissolved gases acquired from 
contact with earth and atmosphere. Industrial or municipal wastes may also 
contaminate raw water.  See  WATER POLLUTION. 
    If admitted to a steam-generating unit, such contaminations may corrode 
metals or form insulating deposits of sediments or scale on heat-transfer 
surfaces, with resultant overheating and possible failure of pressure parts. 
    Raw water can be treated to remove objectionable impurities or to convert 
them to forms that can be tolerated. For steam generation, suspended solids 
are removed by settling or filtration. Scale-forming hardness is diminished by 
chemical treatment to produce insoluble precipitates that are removable by 
filtration, or soluble compounds that do not form scale. Essentially complete 
purification is achieved by demineralizing treatment or evaporation.  See  
WATER SOFTENING; WATER TREATMENT. 
 �34s[F. G. Ely]
�33s 
  
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�6v�2v�32sRay   �21m �33s 
    Any of about 350 species of the order Batoidea, subclass Elasmobranchii, 
which also contains the skates, sawfishes, guitar fishes, electric rays, and 
sting rays. Their bodies are flattened with large pectoral fins attached to 
the side of the head, and ventral gill slits. Spiracles are located behind the 
eyes of the upper surface of the head. These openings lead to the pharynx and 
serve for the intake of water in respiration. The rays are characterized by a 
long spikelike tail. Some males have well-developed claspers, copulatory 
structures. The manta ray (Manta birostris) is a familiar example; it is noted 
for its ability to leap above the surface of the water to a height of 6 ft 
(1.8 m), using the pectoral fins for propulsion and balance.  See  BATOIDEA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sReactance   �21m �33s 
    The opposition that inductance and capacitance offer to alternating 
current through the effect of frequency. Reactance alters the magnitude of 
current and also changes the circuit phase angle. 
    Inductive reactance XsubL equals 2PIfL, where f is the frequency in hertz 
and L is the self-inductance in henrys. The voltage E across an inductance 
reaches its peak 90DG before the current I reaches its peak, and I = E/XsubL 
amperes. Capacitive reactance XC    equals 1/(2PIfC), where C is the 
capacitance in farads. The voltage E across a capacitance reaches its peak 
90DG after the current reaches its peak, and I = E/XsubC amperes. Reactances 
are components of impedance which, in general, includes resistance R and 
reactance.  
 �34s[Burtis L. Robertson]
�33s 
  
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�6v�2v�32sReaction turbine   �21m �33s 
    A power-generation prime mover utilizing the steady-flow principle of 
fluid acceleration, where nozzles are mounted on the moving element. The rotor 
is turned by the reaction of the issuing fluid jet and is utilized in varying 
degrees in steam, gas, and hydraulic turbines. All turbines contain nozzles; 
the distinction between the impulse and reaction principles rests in the fact 
that impulse turbines use only stationary nozzles, while reaction turbines 
must incorporate moving nozzles.  See  IMPULSE TURBINE; PRIME MOVER. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sReactive intermediates �21m �33s
    Unstable compounds which are formed as necessary intermediate stages
during a chemical reaction. Thus, if a reaction in which A is converted to B
requires that A first be converted to C, then C is an intermediate in the
reaction (A -> C -> B). The term reactive further implies a certain degree of
instability of the intermediate; reactive intermediates are typically isolable
only under special conditions, and most of the information regarding the
structure and properties of reactive intermediates comes from indirect
experimental evidence.
    In organic reactions the most common types of reactive intermediates are
those arising from dissociative reactions, in which carbon has a decreased
valence. Associative reactions can also give rise to some of the same
intermediates, and to others in which carbon has an increased valence.	See
VALENCE.
    Carbocations are compounds in which carbon bears a positive charge.
Classical carbocations (also called carbenium ions) are trivalent, and have
only six valence electrons. Nonclassical carbocations (also called carbonium
ions) are tetra- or pentavalent, and have eight valence electrons. Examples
are the methyl cation (classical) CH3+, and the methonium ion
(nonclassical) CH5+.
    Carbanions are compounds in which carbon bears a negative charge. A
carbanion will always have a positive counterion in association with it;
depending upon the particular cation and the stability of the carbanion, the
association may be ionic, covalent, or some intermediate combination of ionic
and covalent bonding. Carbanions are trivalent, with eight valence electrons.

			   |	     |
			 --C--M<-->--C:(sup -)	(sup +)M
			   |	     |
		       Covalent       Ionic

    Free radicals are neutral compounds having an odd number of electrons and
therefore one unpaired electron. Carbon free radicals are trivalent, with
seven valence electrons, and typically assume a planar structure. Free
radicals are primarily electron-deficient species and are stabilized by
structural features which donate electron density or delocalize the odd
electron by resonance.	See  FREE RADICAL; RESONANCE (MOLECULAR STRUCTURE).
    Radical ions are charged compounds with an unpaired electron, and are
either radical cations (positively charged) or radical anions (negatively
charged). In many cases a radical ion is derived from a stable neutral
molecule by addition of one electron (radical anion) or removal of one
electron (radical cation).
    Carbenes are compounds which have a divalent carbon. The divalent carbon
also has two nonbonded electrons, for a total of six valence electrons. The
two nonbonded electrons may have either the same spin quantum number, which is
a triplet state, or an opposite spin quantum number, which is a singlet state.
Generation of carbenes is most commonly by photolysis or thermolysis of diazo
compounds or ketenes, or by alpha-elimination reactions.
    There are many other kinds of reactive intermediates which do not fit into
the previous classifications. Some are simply compounds which are unstable for
a variety of possible reasons, such as structural strain or an unusual
oxidation state.  See  CHEMICAL DYNAMICS; ELECTROPHILIC AND NUCLEOPHILIC
REAGENTS; MOLECULAR ORBITAL THEORY.
 �34s[Carl C. Wamser]
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�6v�2v�32sReactor physics �21m �33s
    The science of the interaction of the elementary particles and radiations
characteristic of nuclear reactors with matter in bulk. These particles and
radiations include neutrons, beta rays, and gamma rays of energies between
zero and about 10(sups7) electronvolts.  See  BETA RAYS; GAMMA RAYS.
    The study of the interaction beta and gamma radiations with matter is,
within the field of reactor physics, undertaken primarily to understand the
absorption and penetration of energy through reactor structures and shields.
See  RADIATION SHIELDING.
    With this exception, reactor physics is the study of those processes
pertinent to the chain reaction involving neutron-induced nuclear fission with
consequent neutron generation. Reactor physics is differentiated from nuclear
physics, which is concerned primarily with nuclear structure. Reactor physics
makes direct use of the phenomenology of nuclear reactions. Neutron physics is
concerned primarily with interactions between neutrons and individual nuclei
or with the use of neutron beams as analytical devices, whereas reactor
physics considers neutrons primarily as fission-producing agents. In the
hierarchy of professional classification, neutron physics and reactor physics
are both ranked as subfields of the more generalized area of nuclear physics.
See  CHAIN REACTION (PHYSICS); NEUTRON; NUCLEAR FISSION; NUCLEAR PHYSICS.

    Concepts.  Reactor physics borrows most of its basic concepts from other
fields. From nuclear physics comes the concept of the nuclear cross section
for neutron interaction, defined as the effective target area of a nucleus for
interaction with a neutron beam. The total interaction is the sum of
interactions by a number of potential processes, and the probability of each
of them multiplied by the total cross section is designated as a partial cross
section. An outgrowth of this is the definition of macroscopic cross section,
which is the product of cross section (termed microscopic, for specificity)
with atomic density of the nuclear species involved.
    Cross sections vary with energy according to the laws of nuclear
structure. In reactor physics this variation is accepted as input data to be
assimilated into a description of neutron behavior. Common aspects of cross
section dependence, such as variation of absorption cross section inversely as
the square root of neutron energy, or the approximate regularity of resonance
structure, form the basis of most simplified descriptions of reactor processes
in terms of mathematical or logical models.
    The concept of neutron flux is related to that of macroscopic cross
section. This may be defined as the product of neutron density and neutron
speed, or as the rate at which neutrons will traverse the outer surface of a
sphere embedded in the medium, per unit of spherical cross-sectional area. The
product of flux and macroscopic cross section yields the reaction rate per
unit volume and time.

    Criticality.  The critical condition is what occurs when the arrangement
of materials in a reactor allows, on the average, exactly one neutron of those
liberated in one nuclear fission to cause one additional nuclear fission. If a
reactor is critical, it will have fissions occurring in it at a steady rate.
This desirable condition is achieved by balancing the probability of
occurrence of three competing events: fission, neutron capture which does not
cause fission, and leakage of neutrons from the system. If v is the average
number of neutrons liberated per fission, then criticality is the condition
under which the probability of a neutron causing fission is 1/v. Generally,
the degree of approach to criticality is evaluated by computing k(sub eff), the
ratio of fissions in successive links of the chain, as a product of
probabilities of successive processes.

    Reactivity.  Reactivity is a measure of the deviation of a reactor from
the critical state at any frozen instant of time. The term reactivity is
qualitative, because several sets of units are in current use to describe it.

    Reflectors.  Reflectors are bodies of material placed beyond the
chain-reacting zone of a reactor, whose function is to return to the active
zone (or core) neutrons which might otherwise leak. Reflector worth can be
crudely measured in terms of the albedo, or probability that a neutron passing
from core to reflector will return again to the core.
    Good reflectors are materials with high scattering cross sections and low
absorption cross sections. The first requirement ensures that neutrons will
not easily diffuse through the reflector, and the second, that they will not
easily be captured in diffusing back to the core.
    Beryllium is the outstanding reflector material in terms of neutronic
performance. Water, graphite, D2O, iron, lead, and (sup 238)U are also good
reflectors.

    Reactor dynamics.  Reactor dynamics is concerned with the temporal
sequence of events when neutron flux, power, or reactivity varies. The
inclusive term takes into account sequential events, not necessarily concerned
with nuclear processes, which may affect these parameters. There are basically
three ways in which a reactor may be affected so as to change reactivity. A
control element, absorbing rod, or piece of fuel may be externally actuated to
start up, shut down, or change reactivity or power level; depletion of fuel
and poison, buildup of neutron-absorbing fission fragments, and production of
new fissionable material from the fertile isotopes (sup 232)Th,
(sup 234)U, (sup 238)U, and (sup 240)Pu make reactivity depend upon
the irradiation history of the system; and changes in power level may produce
temperature changes in the system, leading to thermal expansion, changes in
neutron cross sections, and mechanical changes with consequent change of
reactivity.

    Reactor control physics.  Reactor control physics is the study of the
effect of control devices on reactivity and power level. As such, it includes
a number of problems in reactor statics, because the primary question is to
determine the absorption of the control elements in competition with the other
neutronic processes. It is, however, a problem in dynamics, given the above
information, to determine what motions of the control devices will lead to
stable changes in reactor output.

    Reactivity changes.  Long-term reactivity changes may represent a limiting
factor in the burning of nuclear fuel without costly reprocessing and
refabrication. As the chain reaction proceeds, the original fissionable
material is depleted, and the system would become subcritical if some form of
slow addition of reactivity were not available. This is the function of shim
rods in a typical reactor. The reactor is originally loaded with enough fuel
to be critical with the rods completely inserted. As the fuel burns out, the
rods are withdrawn to compensate.  See	NUCLEAR FUELS.

    Reactor kinetics.  This is the study of the short-term aspects of reactor
dynamics with respect to stability, safety against power excursion, and design
of the control system. Control is possible because increases in reactor power
often reduce reactivity to zero (the critical value) and also because there is
a time lapse between successive fissions in a chain resulting from the finite
velocity of the neutrons and the number of scattering and moderating events
intervening, and because a fraction of the neutrons is delayed.  See  DELAYED
NEUTRON; NUCLEAR REACTOR.
 �34s[Bernard I. Spinrad]
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�6v�2v�32sReactor (electricity)   �21m �33s 
    A device for introducing an inductive reactance into a circuit. Inductive 
reactance x is a function of the product of frequency f and inductance L; 
thus, x = 2PIfL. For this reason, a reactor is also called an inductor. Since 
a voltage drop across a reactor increases with frequency of applied currents, 
a reactor is sometimes called a choke. All three terms describe a coil of 
insulated wire.  See  CHOKE (ELECTRICITY); INDUCTOR. 
    According to their construction, reactors can be divided into those that 
employ iron cores and those where no magnetic material is used within the 
windings. The first type consists of a coil encircling a circuit of iron which 
usually contains an air gap or a series of air gaps. The air gaps are used to 
attenuate the effects of saturation of the iron core. The second type, called 
an air-core reactor, is a simple circular coil, wound around a cylinder 
constructed of nonmagnetic material for greater mechanical strength. This 
strength is necessary for the coil to withstand the electromagnetic forces 
acting on each conductor. These forces become very large with heavy current 
flow, and their direction tends to compress the coil into less space: radial 
forces tend to elongate internal conductors in the coil and to compress the 
external ones while the axial forces press the end sections toward the center 
of the coil. 
    Both iron-core and air-core reactors may be of the air-cooled dry type or 
immersed in oil or a similar cooling fluid. Both types of reactors are 
normally wound with stranded wire in order to reduce losses due to eddy 
currents and skin effect. In addition, it is important to avoid formation of 
short-circuited metal loops when building supporting structures for air-core 
reactors since these reactors usually produce large magnetic fields external 
to the coil. If these fields penetrate through closed-loop metal structures, 
induced currents will flow, causing both losses and heating of the structures. 
Which of these two reactor types should be used depends on the particular 
application.  See  EDDY CURRENT; SKIN EFFECT (ELECTRICITY). 
 �34s[Victor R. Stefanovic]
�33s 
  
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�6v�2v�32sReagent chemicals �21m �33s
    High-purity chemicals used for analytical reactions, for the testing of
new reactions where the effects of impurities are unknown, and in general for
chemical work where impurities must either be absent or at known
concentrations.
    Chemicals are purified by a variety of methods, the most common being
recrystallization from solution. If the desired chemical is volatile and the
impurities are not volatile, sublimation is an effective method of
purification. For liquid chemicals, distillation is an effective procedure.
Finally, the simplest procedure may be to synthesize the desired reagent from
pure materials.  See  CHEMICAL SEPARATION TECHNIQUES; CRYSTALLIZATION;
DISTILLATION.
    Commercial chemicals are available at several levels of purity. Chemicals
labeled ``technical'' or ``commercial'' are usually quite impure. The grade
``USP'' indicates only that the chemical meets the requirements of the United
States Pharmacopeia. The term ``CP'' means only that the chemical is purer
than ``technical.'' Chemicals designated ``reagent grade'' or ``analyzed
reagent'' are specially purified materials which usually have been analyzed to
establish the levels of impurities. The American Chemical Society has
established specifications and tests for purity for some chemicals. Materials
which meet these specifications are labeled ``Meets ACS Specifications.''
 �34s[Kenneth G. Stone /Charles Rulf]
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�6v�2v�32sRealgar   �21m �33s 
    A mineral having composition AsS and crystallizing in the monoclinic 
system. Realgar can occur in short, vertically striated crystals, but more 
frequently is granular and in crusts. The hardness is 1.5 - 2 (Mohs scale) and 
the specific gravity is 3.48. The luster is resinous and the color red to 
orange. Realgar is found in ores of lead, silver, and gold associated with 
orpiment and stibnite. It occurs with the silver and lead ores in Hungary, 
Czechoslovakia, and Germany. Good crystals have come from Binnenthal, 
Switzerland, and Allchar, Macedonia. In the United States it is found at 
Manhattan, Nevada; Mercer, Utah; and as deposits from geyser waters in 
Yellowstone National Park.  See  ARSENIC; ORPIMENT; STIBNITE. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sReal-time systems   �21m �33s 
    Computer systems in which the computer is required to perform its tasks 
within the time restraints of some process or simultaneously with the system 
it is assisting. Usually the computer must operate faster than the system 
assisted in order to be ready to intervene appropriately. 
    Real-time computer systems and applications span a number of different 
types.  
    In real-time control and real-time process control the computer is 
required to process systems data (inputs) from sensors for the purpose of 
monitoring and computing system control parameters (outputs) required for the 
correct operation of a system or process. The type of monitoring and control 
functions provided by the computer for subsystem units ranges over a wide 
variety of tasks, such as turn-on and turn-off signals to switches; feedback 
signals to controllers (such as motors, servos, and potentiometers) to provide 
adjustments or corrections; steering signals; alarms; monitoring, evaluation, 
supervision, and management calculations; error detection, and 
out-of-tolerance and critical parameter detection operations; and processing 
of displays and outputs. 
    In real-time assistance the computer is required to do its work fast 
enough to keep up with a person interacting with it (usually at a computer 
terminal device of some sort, for example, a screen and keyboard). The 
computer supports the person or persons interacting with it and provides 
access, retrieval, and storage functions, usually through some sort of 
data-base management system, as well as data processing and computational 
power. System access allows the individual to intervene in the system's 
operation. The real-time computer also often provides monitoring or display 
information, or both.  See  DATA-BASE MANAGEMENT SYSTEM; MULTIACCESS COMPUTER. 
    In real-time robotics the computer is a part of a robotic or 
self-contained machine. Often the computer is embedded in the machine, which 
then becomes a smart machine. If the smart machine also has access to, or has 
embedded within it, artificial intelligence functions (for example, a 
knowledge base and knowledge processing in an expert system fashion), it 
becomes an intelligent machine.  See  ARTIFICIAL INTELLIGENCE; COMPUTER; 
DIGITAL COMPUTER; EMBEDDED SYSTEMS; EXPERT SYSTEMS; ROBOTICS. 
 �34s[Earl C. Joseph]
�33s 
  
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�6v�2v�32sReal variable   �21m �33s 
    A variable whose range is a subset of the real numbers. By extension the 
term is also used to refer to the theory of functions of one or more real 
variables. This theory has to do with properties of broad classes of 
functions, such as continuity, types of discontinuities, differentiability of 
functions, oscillation and variation of functions, and the various kinds of 
integrals.  See  INTEGRATION; CALCULUS. 
    Real numbers are those commonly used in the geometric theory of 
measurement. The integers and fractions, also called rational numbers, are 
included among the real numbers. In practice an irrational number x is 
specified by telling which rational numbers are less than x and which are 
greater than x. Such a division of the rational numbers into two classes was 
used by J. W. R. Dedekind as the formal definition of a real number and is 
called a Dedekind cut. 
 �34s[Lawrence M. Graves]
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�6v�2v�32sReamer   �21m �33s 
    A multiple-cutting-edge tool designed to enlarge or accurately size and 
finish an existing hole in solid material by removal of a small amount of 
stock. The cutting edges may be ground on the apexes between longitudinal 
flutes or grooves, or cutting may take place on chamfered edges at the end of 
the reamer. Reaming is performed either manually or by machine. See MACHINING 
OPERATIONS.  
 �34s[Alan H. Tuttle]
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�6v�2v�32sReceiving gage   �21m �33s 
    A fixed gage designed to inspect a number of dimensions and also their 
relation to each other. It is termed a receiving gage because of its 
similarity to the cavity which receives the part in actual service. The gage 
checks on ``go'' limits. A typical example is a chamber gage which resembles 
the chamber of a rifle but which is made to the largest limits of size of a 
cartridge. Thus any cartridge which fits the receiving gage will fit any rifle 
chamber of that caliber. 
 �34s[Rush A. Bowman]
�33s 
  
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�6v�2v�32sReciprocating aircraft engine   �21m �33s 
    A fuel-burning internal combustion piston engine specially designed and 
built for minimum fuel consumption and light weight in proportion to developed 
shaft power. The rotating output shaft of the engine may be connected to a 
propeller, ducted fan, or helicopter rotor. 
    Reciprocating aircraft engines are used in about 86% of all powered 
aircraft flying in the United States. Most of the aircraft powered by these 
engines belong to the general aviation segment of the domestic aviation fleet. 
The reciprocating aircraft engine is used to power single-engine and 
multiengine airplanes, helicopters, and airships. It is the principal engine 
used in aircraft for air taxi, pilot training, business, personal, and sport 
flying as well as aerial application of seed, fertilizer, herbicides, and 
pesticides for farming. See AGRICULTURAL AIRCRAFT; GENERAL AVIATION. 
    Predominantly, reciprocating aircraft engines operate on a four-stroke 
cycle, where each piston travels from one end of its stroke to the other four 
times in two crankshaft revolutions to complete one cycle. The cycle is 
composed of four distinguishable events called intake, compression, expansion 
(or power), and exhaust, with ignition taking place late in the compression 
stroke and combustion of the fuel-air charge occurring early in the expansion 
stroke. These spark-ignition engines burn specially formulated aviation 
gasolines. See INTERNAL COMBUSTION ENGINE. 
    Most modern aircraft using engines with up to 336 kW (450 hp) output are 
powered by air-cooled, horizontally opposed, reciprocating engines. The trend 
in modern reciprocating engine development is toward lower engine weight and 
improved fuel economy rather than increased power. 
 �34s[Kenneth J. Stuckas]
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�6v�2v�32sReciprocity principle   �21m �33s 
    In the scientific sense, a theory that expresses various reciprocal 
relations for the behavior of some physical systems. Reciprocity applies to a 
physical system whose input and output can be interchanged without altering 
the response of the system to a given excitation. Optical, acoustical, 
electrical, and mechanical devices that operate equally well in either 
direction are reciprocal systems, whereas unidirectional devices violate 
reciprocity. The theory of reciprocity facilitates the evaluation of the 
performance of a physical system. If a system must operate equally well in two 
directions, there is no need to consider any nonreciprocal components when 
designing it. 
    Some systems that obey the reciprocity principle are any electrical 
network composed of resistances, inductances, capacitances, and ideal 
transformers; systems of antennas, which obey certain restrictions; mechanical 
gear systems; and light sources, lenses, and reflectors. 
    Devices that violate the theory of reciprocity are transistors, vacuum 
tubes, gyrators, and gyroscopic couplers. Any system that contains the above 
devices as components must also violate the reciprocity theory. The gyrator 
differs from the transistor and vacuum tube in that it is linear and passive, 
as opposed to the active and nonlinear character of the other two devices.  
See  GYRATOR; TRANSISTOR. 
 �34s[Hugh S. Landes]
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�6v�2v�32sRecombination (genetics)   �21m �33s 
    The formation of new combinations of genes by the replacement of a portion 
of the genetic material from one cell lineage with its counterpart derived 
from another lineage. Genetic recombination is a normal consequence of sexual 
reproduction, and the increased adaptability conferred on organisms by 
recombination is believed to be an evolutionary advantage that is responsible 
for the widespread occurrence of sex in diverse forms of life. Genetic 
recombination is also known to occur by processes such as transduction in 
microorganisms that more nearly resemble infection than sexual union. 
Recombination has provided geneticists with their most powerful experimental 
tool for resolving the hereditary material into its genic elements and for 
investigating chromosome behavior and structure.  See  CHROMOSOME. 
    Two cells from separate lineages are brought together in sexual organisms 
by fertilization, followed by fusion of their haploid nuclei, each containing 
a single set of chromosomes, to form a single diploid nucleus which contains 
two sets. Reassortment of genes and chromosomes of the diploid into new 
combinations occurs when single sets of chromosomes are segregated into 
haploid eggs, sperm, or spores during nuclear divisions during meiosis.  See  
MEIOSIS; SYNGAMY. 
    An inherited difference referable to corresponding places (loci) in 
homologous chromosomes is said to be due to homologous (allelic) genes. When 
an allelic difference is present the corresponding locus is said to be 
genetically marked. Genes at nonhomologous loci are termed nonallelic. Genes 
at different loci can recombine with one another. 
    Differences at different gene loci within a pair of homologous chromosomes 
do not ordinarily behave independently of one another during meiosis, but 
remain in their original combinations unless a mutual exchange of equivalent 
segments occurs between paired homologs through a process known as 
crossing-over. Genes are said to be linked, and therefore located in the same 
chromosome pair, when parental gene combinations occur more frequently than 
recombinations among the products of meiosis.  See  LINKAGE (GENETICS). 
 �34s[David D. Perkins]
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�6v�2v�32sRecording   �21m �33s 
    Any process for preserving signals, sounds, data, or other information for 
future reference or reproduction. Recording systems in common use include 
magnetic tape and wire recording, disk recording as on phonograph records, 
photographic recording of waveforms, and facsimile recording of photographic 
and other material. The term recording is also used to denote the end product 
of a recording process, such as a magnetic tape, disk recording, or paper 
chart from a graphic-level recorder.  See  DATA-PROCESSING SYSTEMS; DISK 
RECORDING; FACSIMILE; GRAPHIC RECORDING INSTRUMENTS; MAGNETIC RECORDING; 
OPTICAL RECORDING; SOUND RECORDING. 
 �34s[Harry F. Olson]
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�6v�2v�32sRectangle   �21m �33s 
    A parallelogram whose adjacent sides are perpendicular. A rhombus is a 
plane quadrilateral having four equal sides. A square is a rectangle with four 
equal sides and is therefore also a rhombus. The area of either the rectangle 
or the rhombus is equal to the product of the base times the altitude (A = 
bh).  See  QUADRILATERAL; SQUARE. 
 �34s[James S. Frame]
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�6v�2v�32sRectifier �21m �33s
    A nonlinear circuit component that allows more current to flow in one
direction than in the other. An ideal rectifier is one that allows current to
flow in one (forward) direction unimpeded but allows no current to flow in the
other (reverse) direction. Thus, ideal rectification might be thought of as a
switching action, with the switch closed for current in one direction and open
for current in the other direction. Rectifiers are used primarily for the
conversion of alternating current (ac) to direct current (dc).	See
ELECTRONIC POWER SUPPLY.
    A variety of rectifier elements are in use. The vacuum-tube rectifier can
efficiently provide moderate power. Its resistance to current flow in the
reverse direction is essentially infinite because the tube does not conduct
when its plate is negative with respect to its cathode. In the forward
direction, its resistance is small and almost constant. Gas tubes, used
primarily for higher power requirements, also have a high resistance in the
reverse direction. The semiconductor rectifier has the advantage of not
requiring a filament or heater supply. This type of rectifier has
approximately constant forward and reverse resistances, with the forward
resistance being much smaller. Mechanical rectifiers can also be used. The
most common is the vibrator, but other devices are also used.  See  GAS TUBE;
MECHANICAL RECTIFIER; SEMICONDUCTOR RECTIFIER; VIBRATOR.
    If the average current is subtracted from the current flowing in the
rectifier, an alternating current results. This ripple current flowing through
a load produces a ripple voltage which is often undesirable. Filter and
regulator circuits are used to reduce it to as low a value as is required.
See  ELECTRIC FILTER; VOLTAGE REGULATOR.



    A half-wave rectifier circuit is shown in Fig. 1. The rectifier, a diode,
is practically ideal. The ac input is applied to the primary of the
transformer; secondary voltage e supplies the rectifier and load resistor
RsubL. The rectifying action of the diode is shown in Fig. 2, in which the
current i of the rectifier is plotted against the voltage esubd across the
diode. The applied sinusoidal voltage from the transformer secondary is shown
under the voltage axis; the resulting current i flowing through the diode is
shown at the right to be half-sine loops.
    A full-wave rectifier circuit uses two separate diodes. The resulting
current wave shape is shown in Fig. 3. A more continuous flow of direct
current is produced because the first diode conducts for the positive
half-cycle and the second diode conducts for the negative half-cycle.
    When high dc power is required by an electronic circuit, a polyphase
rectifier circuit may be used. It is also desirable when expensive filters
must be used. This is particularly true of power supplies for the final
radio-frequency and audio-frequency stages of large radio and television
transmitters.
 �34s[Donald L. Waidelich]
�33s

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�6v�2v�32sRectilinear motion   �21m �33s 
    Motion is defined as continuous change of position of a body. If the body 
moves so that every particle of the body follows a straight-line path, then 
the motion of the body is said to be rectilinear.  See  MOTION. 
    When a body moves from one position to another, the effect may be 
described in terms of motion of the center of mass of the body from a point A 
to a point B (see illustration). If the center of mass of the body moves along 
a straight line connecting the points A and B, then the motion of the center 
of mass of the body is rectilinear. If the body as a whole does not rotate 
while it is moving, then the path of every particle of which the body is 
composed is a straight line parallel to or coinciding with the path of the 
center of mass, and the body as a whole executes rectilinear motion. This is 
shown by the straight line connecting points P1 and P2 in the illustration.  
See  CENTER OF MASS. 
    Rectilinear motion is an idealized form of motion which rarely, if ever, 
occurs in actual experience, but it is the simplest imaginable type of motion 
and thus forms the basis for the analysis of more complicated motions. 
However, many actual motions are approximately rectilinear and may be treated 
as such without appreciable error. For example, a ball thrown directly upward 
may follow, for all practical purposes, a straight-line path. The motion of a 
high-speed rifle bullet fired horizontally may be essentially rectilinear for 
a short length of path, even though in its larger aspects the ideal path is a 
parabola. The motion of an automobile traveling over a straight section of 
roadway is essentially rectilinear if minor variations of path are neglected. 
The motion of a single wheel of the car is not rectilinear, although the 
motion of the center of mass of the wheel may be essentially so.  See  
BALLISTICS.  
 �34s[Rogers D. Rusk]
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�6v�2v�32sRed alder �21m �33s
    A deciduous tree, Alnus rubra, which grows from Alaska to northern
California and eastern Idaho. It is recognized by its stalked buds, simple
leaves, and dry, conelike, ellipsoid fruit. With the big-leaf maple it shares
the role of principal hardwood tree in the Pacific Northwest, where most of
the commercially important trees are conifers. The wood is used in furniture.
See  FAGALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sRedbeds   �21m �33s 
    Detrital sedimentary rocks pigmented by red ferric oxide that coats 
grains, fills pores as cement, or is dispersed in a muddy matrix. These 
conspicuously colored rocks commonly constitute thick sequences of nonmarine 
to shallow marine deposits. Detrital redbeds accumulated in many parts of the 
globe during the past 1,000,000,000 years of Earth history. Development of red 
pigment in most redbeds was complex and is difficult to decipher. Ferric 
oxides also pigment marine chert, limestone, and iron formations, but these 
chemical deposits are not usually included among redbeds. 
    Some redbeds contain abundant grains of sedimentary and low-grade 
metamorphic rocks and relatively few grains of iron-bearing minerals. Most of 
them, however, contain feldspar and relatively abundant grains of opaque black 
oxides derived from igneous and high-grade metamorphic source rocks. Clay 
minerals in redbeds, as in most other ancient detrital deposits, are 
predominantly illite and chlorite, thus providing no specific clue to the 
climate in the source area or at the place of deposition. 
    As one tectonic end member, extensive uniform redbed-evaporite sequences 
accumulated on stable cratons, whereas some thick wedges of desert redbeds 
with border conglomerates accumulated in rift valleys. On a global scale, 
paleomagnetic evidence of the distribution of redbeds relative to their pole 
position corroborates paleogeographic data, suggesting that most redbeds, 
evaporites, and eolian sandstones accumulated less than 30DG north and south 
of a paleoequator where hot, dry climate generally prevailed. Moreover, 
continental drift reconstructions reveal that the most widespread redbeds in 
the geologic record developed near the Equator in late Paleozoic and early 
Mesozoic time when the continents were assembled in a great landmass, Pangea.  
See  CONTINENTAL DRIFT; PALEOMAGNETISM; SEDIMENTARY ROCKS. 
 �34s[Franklyn B. Van Houten]
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�6v�2v�32sRed dwarf star �21m �33s
    A red star of low luminosity. Dwarf stars are commonly those main-sequence
stars fainter than an absolute magnitude of about +1. Red dwarfs are the
faintest and coldest of the dwarfs. They are present among both old and young
stars. Red dwarfs are the most numerous class of stars in space, although they
are so faint that their presence in remote parts of the Galaxy must be
inferred from their frequency near the Sun and their contribution to the total
mass of the Galaxy. They have a low-energy output per unit mass and a long
nuclear lifetime.  See	DWARF STAR; STAR.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sRed Sea �21m �33s
    The Red Sea lies between Arabia and northeastern Africa. It is about 1200
mi (2000 km) long and 180 mi (300 km) wide, and has a maximum depth of about
7600 ft (2300 m). Details of its bathymetry remain incomplete.
    Geophysical and geological findings have indicated that the Red Sea basin
is a complex system of rifts formed during several periods of crustal movement
and accompanying vulcanism. These movements have resulted in the separation of
the ancient crystalline rock masses of Africa and Arabia. The Red Sea basin is
a segment of the African rift system.
    The water that enters the Red Sea basin is surface water from the Gulf of
Aden, which flows through the straits over a sill. No permanent rivers
discharge into the Red Sea, and the Suez Canal affords no appreciable
interchange with the Mediterranean.
    Because of a peculiar thermohaline situation set up by the cooling and
evaporation of northward-flowing surface water, a shallow density-driven
outflow is produced that carries saline water back to the straits, where it
spills over the sill and into the Gulf of Aden.
    The well-mixed water that fills the Red Sea basin from 660 ft (200 m) to
over 6600 ft (2000 m) exhibits remarkably uniform properties. Its salinity is
nearly constant at 40.6 parts per thousand and its temperature remains near
22DGC (72DGF).
 �34s[A. Conrad Neumann ;David A. Ross]
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�6v�2v�32sRedshift �21m �33s
    A systematic displacement toward longer wavelengths of lines in the
spectra of distant galaxies, and also of the continuous part of the spectrum.
First studied systematically by E. Hubble, redshift is central to
observational cosmology, in which it provides the basis for the modern picture
of an expanding universe.
    There are two fundamental properties of redshifts. First, the fractional
redshift DELTA LAMBDA/LAMBDA is independent of wavelength. This rule has been
verified from 21 cm (radio radiation from neutral hydrogen atoms) to about 6 x
10(supe -5)cm (the visible region of the electromagnetic spectrum) and leads to
the interpretation of redshift as resulting from a recession of distant
galaxies. Though this interpretation has been questioned, no other mechanism
is known that would explain the observed effect.
    Second, redshift is correlated with apparent magnitude in such a way that
when redshift is translated into recession speed and apparent magnitude into
distance, the recession speed is found to be nearly proportional to the
distance. This rule was formulated by Hubble in 1929, and the constant of
proportionality bears his name.  See  HUBBLE CONSTANT; MAGNITUDE (ASTRONOMY).
 �34s[David Layzer]
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�6v�2v�32sRedtop grass   �21m �33s 
    One of the bent grasses, Agrostis alba and its relatives, which occur in 
cooler, more humid regions of the United States on a wide variety of soils. 
Redtop tolerates both wet and dry lands and acid and infertile soils, and it 
is used where other species of grasses do not thrive. Redtop is a perennial 
and makes a coarse, loose turf. The inflorescence is a reddish open panicle. 
Redtop is used for pasture and hay and is fairly nutritious if harvested 
promptly when heading occurs. It is effective in preventing erosion by holding 
banks of drainage ditches, waterways, and terrace channels.  See  CYPERALES. 
 �34s[Howard B. Sprague]
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�6v�2v�32sReduced-pressure distillation   �21m �33s 
    A technique used to distill high-molecular-weight substances. Materials 
that have large molecules require high temperatures for distillation. There is 
a limit to this temperature increase, for above 750DGF (400DGC), thermal 
decomposition begins. Therefore, to distill a material such as a motor oil 
whose distillation temperature is well over 750DGF (400DGC), some other 
distillation technique must be used. If a distillation vessel is put under a 
vacuum, thus removing atmospheric pressure from the surface of the boiling 
liquid, the vapor can escape more easily from the liquid surface and at an 
appreciably lower temperature. 
    Distillation at reduced pressure is widely used as a separation process. 
Equipment ranges in size from that which will handle micro quantities in the 
laboratory to plant installations that process thousands of barrels per day. 
One of the largest uses of this process has evolved in the petroleum industry. 
The same types of distillation equipment are used to purify and separate fatty 
acids, a major ingredient in soap and in lubricating greases. Another large 
user of vacuum-steam distillation is the edible fat and oil industry. The 
process is called deodori-zation, which removes objectionable taste and odor 
from cottonseed and soybean oils, which are the main constituents in 
shortenings, and in salad and cooking oils.  See  DISTILLATION. 
 �34s[Edward S. Barnitz]
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�6v�2v�32sRedwood �21m �33s
    A member of the pine family, Sequoia sempervirens, is the tallest tree in
the Americas, attaining a height of 350 ft (107 m) and a diameter of 27 ft
(8.2 m). Its present range is limited to a strip along the Pacific Coast,
extending from southwest Oregon to south of San Francisco. The leaves are
evergreen, sharply pointed, small, disposed in two vertical rows on short
branches, and scalelike on the main stem. The cones are egg-shaped. The bark
is a dull red-brown, on old trees sometimes 1 ft (0.3 m) thick, densely
fibrous, and highly resistant to fire. The tree gets its common name from the
color of the bark as well as that of the heartwood.
    The wood holds paint well and is used for bridge timbers, tanks, flumes,
silos, posts, shingles, paneling, doors, caskets, furniture, siding, and many
other building purposes.  See  PINALES; PINE.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sReed-type frequency meter �21m �33s
    An instrument for measuring frequency using the principle of vibrating
reeds. The reeds for such instruments, assembled into a so-called reed comb,
are made of specially selected and properly tempered steel. They have bent
tips which are enameled white for visibility. When the supply voltage is
applied to the instrument, all of the reeds receive vibrational impulses; the
effect is visible only in the reed or reeds which are in resonance. The reed
in resonance behaves somewhat like a whip, the tip swinging through a readily
visible arc, the natural period of vibration depending upon length and
thickness. The reeds in the comb are vibrated by an electromagnet energized
from the source of which the frequency is to be measured. See FREQUENCY
MEASUREMENT.
 �34s[Edward B. Curdts]
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�6v�2v�32sReef   �21m �33s 
    A mass or ridge of rock or rock-forming organisms in a water body, a rock 
trend on land or in a mine, or a rocky trend in soil. Usually the term reef 
means a rocky menace to navigation, within 6 fathoms (11 m) of the water 
surface. Various kinds of calcium carbonate - secreting animals and plants 
create biogenic, or organic, reefs throughout the warmer seas. Most biogenic 
reefs are made of corals and associated organisms, but some entire reefs and 
important parts of others consist mainly of lime-secreting algae, hydrozoans, 
annelids, oysters, or sponges.  See  ALGAE; CORALLINE ALGAE; SCLERACTINIA. 
    The term fringing reef refers to a coral or other biogenic reef that 
fringes the edge of the land. A barrier reef ordinarily made of corals or 
other organisms, parallels the shore at the seaward side of a natural lagoon. 
An atoll is an annular coral reef that surrounds a lagoon.  See  ATOLL. 
 �34s[Preston Cloud]
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�6v�2v�32sReflecting microscope   �21m �33s 
    A microscope whose objective is composed of two mirrors, one convex and 
the other concave (see illustration). The imaging properties are independent 
of the wavelength of light, and this freedom from chromatic aberration allows 
the objective to be used even for infrared and ultraviolet radiation. Although 
the reflecting microscope is simple in appearance, the construction tolerances 
are so small and so difficult to achieve that the system is used only when 
refracting objectives are unsuitable. The distance from the objective to the 
specimen can be made very large; this large working distance is useful in 
special applications, such as examining objects situated within metallurgical 
furnaces. Reflecting microscopes have been mainly used for microspectrometry 
in the infrared and the ultraviolet, and for ultraviolet microphotography.  
See  MICROSCOPE; OPTICAL MICROSCOPE. 
 �34s[D. S. Grosch]
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�6v�2v�32sReflection of electromagnetic radiation �21m �33s
    The returning or throwing back of electromagnetic radiation such as light,
ultraviolet rays, radio waves, or microwaves by a surface upon which the
radiation is incident. In general, a reflecting surface is the boundary
between two materials of different electromagnetic properties, such as the
boundary between air and glass, air and water, or air and metal. Devices
designed to reflect radiation are called reflectors or mirrors.
    The simplest reflection laws are those that govern plane waves of
radiation. The law of reflection concerns the incident and reflected rays (as
in the case of a beam from a flashlight striking a mirror) or, more precisely,
the wave normals of the incident and reflected waves. The law states that the
incident and reflected rays and the normal to the reflecting surface all lie
in one plane, called the plane of incidence, and that the reflection angle
THETA(sub refl) equals the angle of incidence THETA(sub inc) as in Eq.
(1) [see illustration]. The angles THETA(sub inc) and THETA(sub refl)
are measured between the surface normal and the incident and reflected rays,
respectively. The surface (in the above example, that of the mirror) is
assumed to be smooth, with surface irregularities small compared to the
wavelength of the radiation. This results in so-called specular reflection. In
contrast, when the surface is rough, the reflection is diffuse. An example of
this is the diffuse scattering of light from a screen or from a white wall
where light is returned through a whole range of different angles.

		      THETA(sub refl) = THETA(sub inc)			     (1)

    The reflectivity of a surface is a measure of the amount of reflected
radiation. It is defined as the ratio of the intensities of the reflected and
incident radiation. The reflectivity depends on the angle of incidence, the
polarization of the radiation, and the electromagnetic properties of the
materials forming the boundary surface. These properties usually change with
the wavelength of the radiation. Reflecting materials are divided into two
groups: transparent materials also called dielectrics, and opaque conducting
materials, usually metals.
    The reflectivity of polished metal surfaces is usually quite high. Silver
and aluminum, for example, reflect more than 90% of visible light. In ordinary
mirrors the reflecting surface is the interface between metal and glass, which
is thus protected from oxidation, dirt, and other forms of deterioration. When
it is not permissible to use this protection for technical reasons, one uses
``front-surface'' mirrors, which are usually coated with evaporated aluminum.
    The material property that determines the amount of radiation reflected
from an interface between two dielectric media is the phase velocity v of the
electromagnetic radiation in the two materials. In optics one uses as a
measure for this velocity the refractive index n of the material, which is
defined by Eq. (2) as the ratio of the velocity of light c in vacuum and the
phase velocity in the material. For visible light, for example, the refractive
index of air is about n = 1, the index of water is about n = 1.33, and the
index of glass is about n = 1.5.  See  PHASE VELOCITY; REFRACTION OF WAVES.

				n = c/v 				     (2)


	   R = ([V(sub 1) - V(sub 2)]/[V(sub 1) + V(sub 2)]) (sup 2)	     (3)
	     = ([n(sub 2) - n(sub 1)]/[n 4(sub 2) + n(sub 1)]) (sup 2)

    For normal incidence (THETA[sub inc] = 0) the reflectivity R of the
interface is given by Eq. (3), in which the material constants are labeled 1
and 2, where the radiation is incident in material 1. The reflectivity of an
air-water interface is about 2% (R = 0.02) and that of an air-glass interface
about 4% (R = 0.04); the other 98% or 96% are transmitted through the water or
glass, respectively.  See  ALBEDO; GEOMETRICAL OPTICS; MIRROR OPTICS;
REFLECTION OF SOUND.
 �34s[Herwig Kogelnik]
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�6v�2v�32sReflection of sound   �21m �33s 
    The return of sound waves from surfaces on which they are incident. The 
geometrical laws for reflection of sound waves are the same as those for light 
waves. The apparent differences involve only questions of scale, because the 
average wavelength of sound is about 100,000 times that of light. For example, 
a mirror or lens used to produce a beam of sound waves must be enormously 
large compared to mirrors and lenses used in optical systems.  See  REFLECTION 
OF ELECTROMAGNETIC RADIATION. 
    A concave surface tends to concentrate the reflected sound waves. Convex 
reflectors tend to spread the reflected waves. Therefore, when placed at the 
boundaries of a room, they tend to diffuse the sound throughout the room. For 
this reason, some radio-broadcasting studios employ cylindrical convex panels 
as part of their wall construction to promote diffusion.  See  ARCHITECTURAL 
ACOUSTICS; ECHO; SOUND. 
 �34s[Cyril M. Harris]
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�6v�2v�32sRefraction of waves �21m �33s
    The change of direction of propagation of any wave phenomenon which occurs
when the wave velocity changes. The term is most frequently applied to visible
light, but it also applies to all other electromagnetic waves, as well as to
sound and water waves.
    The physical basis for refraction can be readily understood with the aid
of the illustration. Consider a succession of equally spaced wavefronts
approaching a boundary surface obliquely. The direction of propagation is in
ordinary cases perpendicular to the wavefronts. In the case shown, the
velocity of propagation is less in medium 2 than in medium 1, so that the
waves are slowed down as they enter the second medium. Thus, the direction of
travel is bent toward the perpendicular to the boundary surface (that is,
THETA2 < THETA1). If the waves enter a medium in which the velocity of
propagation is faster than in their original medium, they are refracted away
from the normal.

    Snell's law.  The simple mathematical relation governing refraction is
known as Snell's law. If waves traveling through a medium at speed v1 are
incident on a boundary surface at angle THETA1 (with respect to the normal),
and after refraction enter the second medium at angle THETA2 (with the normal)
while traveling at speed v(sub 2), then Eq. (1) holds. The index of refraction
n of a medium is defined as the ratio of the speed of waves in vacuum c to their
speed in the medium. Thus c = n(sub 1)v(sub 1) = n(sub 2)v(sub 2), and
therefore Eq. (2) holds. The refracted ray, the normal to the surface, and the
incident ray always lie in the same place. 

	 V(sub 1)/V(sub 2) = sin THETA(sub 1)/sin THETA(sub 2)		     (1)


	     n(sub 1) sin THETA(sub 1) = n(sub 2) sin THETA(sub 2)	     (2)

    The relative index of refraction of medium 2 with respect to that of
medium 1 may be defined as n = n(sub 1)/n(sub 2). Snell's law then becomes
Eq. (3). For sound and other elastic waves which require a medium in which to
propagate, only this last form has meaning. Equation (3) is frequently used for
light when one medium is air, whose index of refraction is very nearly unity.

	    sin THETA(sub 1) = n sin THETA(sub 2)			     (3)

    When the wave travels from a region of low velocity (high index) to one of
high velocity (low index), refraction occurs only if (n[sub 1]/n[sub 2]) sin
THETA1 < 1.  If THETA1 is too large for this relation to hold, then sin THETA2
> 1, which is meaningless. In this case the waves are totally reflected from
the surface back into the first medium. The largest value that THETA1 can have
without total internal reflection taking place is known as the critical angle
THETA(sub c). Thus sin THETA(sub c) = n(sub 2)/n(sub 1).

    Atmospheric refraction.  The index of refraction of the Earth's atmosphere
increases continuously from 1.000000 at the edge of space to 1.000293 (yellow
light) at 0DGC and 760 mmHg (101.3 kilopascals) pressure. Thus celestial
bodies as seen in the sky are actually nearer to the horizon than they appear
to be. The effect decreases from a maximum of about 35 minutes of arc for an
object on the horizon to zero at the zenith, where the light enters the
atmosphere at perpendicular incidence.
    Other manifestations of atmospheric refraction are the mirages and
``looming'' of distant objects which occur over oceans or deserts, where the
vertical density gradient of the air is quite uniform over a large area.  See
MIRAGE.

    Sound waves.  The velocity of sound in a gas is proportional to the square
root of the absolute temperature. Because of the vertical temperature
gradients in the atmosphere, refraction of sound can be quite pronounced. As
in mirage formation, to allow large-scale refraction the temperature at a
given height must be uniform over a rather large horizontal area.  See
ATMOSPHERIC ACOUSTICS; SOUND.

    Seismic waves.  The velocity of elastic waves in a solid depends upon the
modulus of elasticity and upon the density of the material. Waves propagating
through solid earth are refracted by changes of material or changes of
density. Worldwide observations of earthquake waves enable scientists to draw
conclusions on the distribution of density within the Earth.  See  SEISMOLOGY.

    Water waves.  As the waves enter shallower water they travel more slowly.
As a train of waves approaches a coastline obliquely, its direction of travel
becomes more nearly perpendicular to the shore because of refraction.  See
WAVE MOTION IN LIQUIDS.
 �34s[John W. Stewart]
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�6v�2v�32sRefractometric analysis �21m �33s
    A method of chemical analysis based on the measurement of the index of
refraction of a substance. The most common type of refractometer is the Abbe
refractometer. It is simple to use, requiring but a drop or two of sample and
allowing a measurement of refractive index to be made in 1 - 2 min, with a
precision of 0.0001. More precise measurements of refractive indices may be
made by using a dipping or immersion refractometer, the prism of which is
completely immersed in the sample. The most precise measurements of the
refractive indices of gases or solutions containing small traces of impurities
are made with an interferometer. For measurements in flowing systems,
differential refractometers are used.
    The measurement of refractive index is used to identify compounds whose
other physical constants are quite similar. Because minute amounts of
impurities often cause a measurable change in the refractive index of a pure
material, refractive index is often used as a criterion for purity. A
measurement of refractive index gives information as to the gross amount of
impurity; it does not serve to identify the impurity.  See  OPTICAL METHODS OF
CHEMICAL ANALYSIS; REFRACTION OF WAVES.
 �34s[Robert F. Goddu ;James N. Little]
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�6v�2v�32sRefractory   �21m �33s 
    One of a number of ceramic materials for use in high-temperature 
structures or equipment. The term high temperatures is somewhat indefinite but 
usually means above about 1830DGF (1000DGC), or temperatures at which, because 
of melting or oxidation, the common metals cannot be used. In some special 
high-temperature applications, the so-called refractory metals such as 
tungsten, molybdenum, and tantalum are used. See CERAMICS. 
    The greatest use of refractories is in the steel industry, where they are 
used for construction of linings of equipment such as blast furnaces, hot 
stoves, and open-hearth furnaces. Other important uses of refractories are for 
cement kilns, glass tanks, nonferrous metallurgical furnaces, ceramic kilns, 
steam boilers, and paper plants. Special types of refractories are used in 
rockets, jets, and nuclear power plants. Many refractory materials, such as 
aluminum oxide and silicon carbide, are also very hard and are used as 
abrasives; some applications, for example, aircraft brake linings, make use of 
both characteristics. 
    Refractory materials are commonly grouped into (1) those containing mainly 
aluminosilicates; (2) those made predominantly of silica; (3) those made of 
magnesite, dolomite, or chrome ore, termed basic refractories (because of 
their chemical behavior); and (4) a miscellaneous category usually referred to 
as special refractories. 
 �34s[John F. McMahon]
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�6v�2v�32sRefrigeration   �21m �33s 
    The cooling of a space or substance below the environmental temperature. 
Mechanical refrigeration is primarily an application of thermodynamics wherein 
the cooling medium, or refrigerant, goes through a cycle so that it can be 
recovered for reuse. The commonly used basic cycles, in order of importance, 
are vapor-compression, absorption, steam-jet or steam-ejector, and air. Each 
cycle operates between two pressure levels, and all except the air cycle use a 
two-phase working medium which alternates cyclically between the liquid and 
vapor phases.  See  REFRIGERATION CYCLE. 
  
    Vapor-compression cycle.  The vapor-compression cycle consists of an 
evaporator in which the liquid refrigerant boils at low temperature to produce 
cooling, a compressor to raise the pressure and temperature of the gaseous 
refrigerant, a condenser in which the refrigerant discharges its heat to the 
environment, usually a receiver for storing the liquid condensed in the 
condenser, and an expansion valve through which the liquid expands from the 
high-pressure level in the condenser to the low-pressure level in the 
evaporator. This cycle may also be used for heating if the useful energy is 
taken off at the condenser level instead of at the evaporator level.  See  
HEAT PUMP. 
  
    Absorption cycle.  The absorption cycle accomplishes compression by using 
a secondary fluid to absorb the refrigerant gas, which leaves the evaporator 
at low temperature and pressure. Heat is applied, by means such as steam or 
gas flame, to distill the refrigerant at high temperature and pressure. The 
most-used refrigerant in the basic cycle is ammonia; the secondary fluid is 
then water. The condenser, receiver, expansion valve, and evaporator are 
essentially the same as in any vapor-compression cycle. The compressor is 
replaced by an absorber, generator, pump, heat exchanger, and 
controlling-pressure reducing valve. 
  
    Steam-jet cycle.  The steam-jet cycle uses water as the refrigerant. 
High-velocity steam jets provide a high vacuum in the evaporator, causing the 
water to boil at low temperature and at the same time compressing the flashed 
vapor up to the condenser pressure level. Its use is limited to air 
conditioning and other applications for temperatures above 32DGF (0DGC). 
  
    Air cycle.  The air cycle, used primarily in airplane air conditioning, 
differs from the other cycles in that the working fluid, air, remains as a gas 
throughout the cycle. Air coolers replace the condenser, and the useful 
cooling effect is obtained by a refrigerator instead of by an evaporator. A 
compressor is used, but the expansion valve is replaced by an expansion engine 
or turbine which recovers the work of expansion. Systems may be open or 
closed. In the closed system, the refrigerant air is completely contained 
within the piping and components, and is continuously reused. In the open 
system, the refrigerator is replaced by the space to be cooled, the 
refrigerant air being expanded directly into the space rather than through a 
cooling coil. 
  
    Refrigerants.  The working fluid in a two-phse refrigeration cycle is 
called a refrigerant. Ammonia and Freon-22 are most important for industrial 
refrigeration, Freon-11 and Freon-12 being used most often for commercial and 
air conditioning work in which nontoxic refrigerants are necessary. A 
secondary cooling liquid that does not chage from the liquid phase is called a 
brine. Solutions of sodium chloride or calcium chloride in water are 
frequently used as circulating brines in refrigeration systems.  See  AIR 
CONDITIONING; AIR COOLING; AUTOMOTIVE AIR CONDITIONING; COOLING TOWER; DRY 
ICE; MARINE REFRIGERATION; REFRIGERATOR. 
 �34s[Carl F. Kayan]
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�6v�2v�32sRefrigeration cycle   �21m �33s 
    A sequence of thermodynamic processes whereby heat is withdrawn from a 
cold body and expelled to a hot body. Theoretical thermodynamic cycles consist 
of nondissipative and frictionless processes. For this reason, a thermodynamic 
cycle can be operated in the forward direction to produce mechanical power 
from heat energy, or it can be operated in the reverse direction to produce 
heat energy from mechanical power. 
    One of the first cycles used for mechanical refrigeration was the reverse 
Brayton cycle. With the development of Freon and other condensable fluids for 
the vapor-compression cycle, the reverse Carnot cycle became more important. 
The reversed cycle is used primarily for the cooling effect that it produces 
during a portion of the cycle and so is called a refrigeration cycle. It may 
also be used for the heating effect, as in the comfort warming of space during 
the cold season of the year.  See  HEAT PUMP; THERMODYNAMIC PROCESSES. 
    In the refrigeration cycle a substance, called the refrigerant, is 
compressed, cooled, and then expanded. In expanding, the refrigerant absorbs 
heat from its surroundings to provide refrigeration. After the refrigerant 
absorbs heat from such a source, the cycle is repeated. Compression raises the 
temperature of the refrigerant above that of its natural surroundings so that 
it can give up its heat in a heat exchanger to a heat sink such as air or 
water. Expansion lowers the refrigerant temperature below the temperature that 
is to be produced inside the cold compartment or refrigerator. The sequence of 
processes performed by the refrigerant constitutes the refrigeration cycle. 
When the refrigerant is compressed mechanically, the refrigerative action is 
called mechanical refrigeration.  See  BRAYTON CYCLE; CARNOT CYCLE; 
REFRIGERATION; THERMODYNAMIC CYCLE.   
 �34s[Theodore Baumeister]
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�6v�2v�32sRefrigerator   �21m �33s 
    An insulated, cooled compartment. If it is large enough for the entry of a 
person, it is termed a walk-in box; otherwise it is called a reach-in 
refrigerator. Cooling may be by mechanical or gas refrigeration, by water or 
dry ice, or by brine circulation. Temperatures maintained depend upon the 
requirements of the product stored, generally varying from 55DGF (13DGC) down 
to 0DGF (-18DGC), and sometimes lower. 
    A household or domestic refrigerator is a factory-built, self-contained 
cabinet. The range of storage capacities is wide and varies among 
manufacturers. Modern designs have a main compartment for holding food above 
freezing, a second compartment for storage below freezing, and trays for the 
freezing of ice cubes. Low-temperature household refrigerators, or home 
freezers, for the storage of frozen foods are manufactured in both the chest 
and the upright, or vertical, types. 
    A commercial refrigerator is any factory-built refrigerated fixture, 
cabinet, or room that can be assembled and disassembled readily. Commercial or 
built-in refrigerators are used in restaurants, markets, hospitals, hotels, 
and schools for the storage of food and other perishables.  See  REFRIGERATION. 
 �34s[Carl F. Kayan]
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�6v�2v�32sRegeneration (biology)   �21m �33s 
    The replacement by an organism of parts of the body which have been lost 
or severely injured. The term is comprehensive and covers a wide range of 
restorative activities in a variety of organisms. Some authors prefer to use 
the term reconstitution rather than regeneration. The capacity for 
regeneration varies greatly among different groups of organisms. Among the 
invertebrates, many of the hydroids, flatworms, annelids, echinoderms, and 
arthropods can replace major portions of the body. In certain instances, 
particularly in sponges, a few cells or a small fragment of the original 
organism is capable of reconstituting a completely new individual. In the 
vertebrates, the highest capacity for regeneration is found in the Amphibia, 
of which many species can regenerate a complete limb, a tail, portions of the 
eye, the lower jaw, and a number of other highly organized structures. 
    As a rule, the structures formed as a result of regeneration are 
duplicates of the original structures and possess all their functional 
characteristics. Under some circumstances, however, the regenerate may be of a 
different type than the original structure. After removal, the eye of a 
crustacean may be replaced by an antenna. A head, instead of a tail, may be 
formed at the posterior end of a flatworm. An amphibian, through regenerative 
activities, may establish supernumerary limbs that were not previously present. 
    Many organisms, both invertebrate and vertebrate, although they may be 
incapable of regenerating complex organs, or major portions of the body, have 
the capacity for reconstituting various types of tissues. Examples are the 
continual or periodic replacement by various animals of skin, scales, 
feathers, teeth, antlers, the lining of the alimentary canal, and some 
components of the reproductive tract. Such activities, which represent a phase 
on the normal life cycle of an individual, are often referred to as repetitive 
or physiological regeneration. Wound healing and the repair of bone fractures 
can likewise be regarded as types of regenerative activity. 
    Among the major problems are the sources of the cells which enter into 
regenerative activities and the manner in which they achieve the requisite 
potentialities for forming specialized tissues, new organs, or discrete 
portions of the body. Also of primary importance are problems of polarity and 
the relationship of local and organismic factors in the establishment of 
growth patterns. 
 �34s[Elmer G. Butler]
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�6v�2v�32sRegeneration (engineering)   �21m �33s 
    The process of feeding back a portion of the output signal of an amplifier 
to its input in such a way that the input signal is reinforced. The result is 
greatly increased amplification. The feedback must be positive; that is, the 
two signals must be in phase, and it must be limited in magnitude to prevent 
the circuit from going into oscillation.  See  FEEDBACK CIRCUIT. 
 �34s[John Markus]
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�6v�2v�32sRegenerative braking   �21m �33s 
    A system of dynamic braking in which the electric drive motors are used as 
generators and return the kinetic energy of the motor armature and load to the 
electric supply system. This method is employed when the load is losing a 
large amount of potential energy, as in the case of an electric train 
descending a long grade or a pumped storage reservoir being emptied. The 
potential energy of the load is first converted to kinetic energy in the motor 
armature and load and then to electrical energy.  See  DYNAMIC BRAKING. 
 �34s[Arthur R. Eckels]
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�6v�2v�32sRegolith   �21m �33s 
    The mantle or blanket of unconsolidated or loose rock material that 
overlies the intact bedrock and nearly everywhere forms the land surface. The 
regolith may be residual (weathered in place), or it may have been transported 
to its present site. The undisturbed residual regolith may grade from 
agricultural soil at the surface, through fresher and coarser weathering 
products, to solid bedrock several feet or more beneath the surface. The 
transported regolith includes the alluvium of rivers, sand dunes, glacial 
deposits, volcanic ash, coastal deposits, and the various mass-wasting 
deposits that occur on hillslopes. The lunar surface also has a regolith. This 
layer of fragmental debris is believed to derive from prolonged meteoritic and 
secondary fragment impact.  See  BASEMENT ROCK; SOIL; WEATHERING PROCESSES. 
 �34s[Victor R. Baker]
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�6v�2v�32sRegularia   �21m �33s 
    The name given by G. Cuvier in 1817 to an assemblage of echinoids in which 
the anus and periproct lie within the apical system. The test is globular and 
preponderantly radially symmetrical, and the ambulacral plates are commonly 
compound. The group included, in effect, all those echinoids which did not 
fall in the Irregularia.  See  ECHINOIDEA; IRREGULARIA. 
 �34s[Howard B. Fell]
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�6v�2v�32sRegular polytopes �21m �33s
    The n-dimensional analogs of the regular polygons (n = 2) and platonic
solids (n = 3). They are conveniently denoted by their Schlaefli symbols
{p,q,...}; for instance, the pentagon, hexagon, octagon, tetrahedron,
octahedron are denoted by {5}, {6}, {8}, {3,3}, {3,4}. The cube is {4,3}
because its faces are squares {4} and there are three of them at each vertex.
The five platonic solids {p,q}, are determined by the inequality below. The
numbers of vertices, edges, faces (V,E,F) can be deduced from the obvious
relations pF = 2E = qV with the help of Euler's formula V - E + F = 2.

		       (p - 2) (q - 2) < 4

    The general polytope (sometimes loosely called a ``polyhedron'' [see
illustration] regardless of the number of dimensions) is a finite region of
n-dimensional space enclosed by a finite number of hyperplanes. When any
redundant hyperplanes have been discarded, those that remain contain (n -
1)-dimensional polytopes called cells or ``facets.'' For instance, the cells
of a polygon are its sides, those of a polyhedron are its faces, and those of
a four-dimensional polytope are solids. See ANALYTIC GEOMETRY; EUCLIDEAN
GEOMETRY.
 �34s[H. S. M. Coxeter]
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�6v�2v�32sRegulation   �21m �33s 
    The process of maintaining a quantity or condition essentially constant 
despite variations in such factors as line voltage and load. In an industrial 
process-control system, the speed, temperature, voltage, or position of a 
critical element can be kept constant by measuring the condition being 
regulated and feeding back into the system a signal representing the 
difference between the actual and the desired quantities.  See  CONTROL 
SYSTEMS; REGULATOR. 
    The term regulation is also used in the opposite sense to indicate the 
difference between the maximum and minimum voltages at the terminals of a 
tube, transformer, generator, or other device over the range of normal 
operating conditions.  See  SERVOMECHANISM; VOLTAGE REGULATOR. 
 �34s[Frank H. Rockett]
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�6v�2v�32sRegulator   �21m �33s 
    A control device designed to maintain the value of some quantity 
substantially constant. The value to be maintained can usually be established 
at any value within the range of the regulator by making an appropriate 
setting. A regulated system is a feedback control system employing a regulator 
to maintain some quantity of the system at a constant value.  See  CONTROL 
SYSTEMS; FEEDBACK CONTROL SYSTEM. 
 �34s[John A. Hrones]
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�6v�2v�32sReheating �21m �33s
    The addition of heat to steam of reduced pressure after the steam has
given up some of its energy by expansion through the high-pressure stages of a
turbine. The reheater tube banks are arranged within the setting of the
steam-generating unit in such relation to the gas flow that the steam is
restored to a high temperature. Under suitable conditions of initially high
steam pressure and superheat, one or two stages of reheat can be
advantageously employed to improve thermodynamic efficiency of the cycle.  See
STEAM-GENERATING UNIT; STEAM TURBINE; SUPERHEATER; VAPOR CYCLE.
 �34s[R. A. Miller]
�33s

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�6v�2v�32sReindeer   �21m �33s 
    A ruminant, Rangifer tarandus, of the deer family, Cervidae. Reindeer 
inhabit the Arctic region and have a circumpolar distribution. They have been 
domesticated for centuries and are economically important to the Laplanders, 
who use them as draft animals as well as for their skins, flesh, and milk. 
    Both sexes have antlers, a characteristic peculiar to this deer species. 
Although their senses of sight and hearing are not sharp, their sense of smell 
is quite keen. The hooves are round and broad, giving the animal stable 
footing in snow and on ice. In the wild, they are the most migratory of all 
deer, traveling in large herds in search of food during the winter months. The 
rutting season begins during the autumn migrations; fawns are born during the 
spring.  See  ARTIODACTYLA; DEER. 
 �34s[Charles B. Curtin]
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�6v�2v�32sReinforced concrete   �21m �33s 
    Portland-cement concrete with steel embedded in it to assist in carrying 
loads. Steel in several of the following forms may be used: bars or rods, 
wire, pipe, and structural shapes such as wide-flange beams. 
    Loads are transferred between concrete and steel by the bond along the 
surface of intersection. The bond may be substantially improved mechanically 
by giving reinforcing bars raised surfaces, in which case the bars are called 
deformed bars.  See  CONCRETE. 
 �34s[Frederick S. Merritt]
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�6v�2v�32sRelapsing fever   �21m �33s 
    An acute infectious disease caused by various species of the genus 
Borrelia, a spirochete. The disease is characterized by episodes of fever 
which subside spontaneously and recur over a period of weeks. 
Epidemiologically, two types are recognized, a louse-borne type which often 
occurs in epidemics, and a tick-borne type which usually is endemic. The 
clinical characteristics of the two are similar. 
 �34s[Thomas B. Turner]
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�6v�2v�32sRelative atomic mass �21m �33s
    The ratio of the average mass per atom of the natural nuclidic composition
of an element to 1/12 of the mass of an atom of nuclide (sup 12)C. For example,
MU(Cl) = 35.453. Relative atomic mass replaces the concept of atomic weight.
It is also known as relative nuclidic mass.  See  NUCLIDE.
 �34s[Thomas C. Waddington]
�33s

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�6v�2v�32sRelative molecular mass �21m �33s
    The ratio of the average mass per formula unit of the natural nuclidic
composition of a substance to 1/12 of the mass of an atom of nuclide (sup 12)C.
For example, MU(KCl) = 74.555. Relative molecular mass replaces the concept of
molecular weight.  See	NUCLIDE.
 �34s[Thomas C. Waddington]
�33s

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�6v�2v�32sRelative motion   �21m �33s 
    All motion is relative to some frame of reference. The simplest laboratory 
frame of reference is three mutually perpendicular axes at rest with respect 
to an observer. In terms of the frame of reference of an observer some 
distance from Earth, the laboratory frame of reference would be moving with 
Earth as it rotates on its axis and as it revolves about the Sun. What would 
be a simple form of motion in the laboratory frame of reference would appear 
to be a much more complicated motion in the frame of reference of the distant 
observer.  See  FRAME OF REFERENCE. 
    Motion means continuous change of position of an object with respect to an 
observer. To another observer in a different frame of reference the object may 
not be moving at all, or it may be moving in an entirely different manner. The 
motions of the planets were found in ancient times to appear quite complicated 
in the laboratory frame of reference of an observer on Earth. By transferring 
to the frame of reference of an imaginary observer on the Sun, Johannes Kepler 
showed that the relative motion of the planets could be simply described in 
terms of elliptical orbits. The validity of one description is no greater than 
the other, but the latter description is far more convenient. 
 �34s[Rogers D. Rusk]
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�6v�2v�32sRelativistic electrodynamics �21m �33s
    The study of the interaction between charged particles and electric and
magnetic fields when the velocities of the particles approach that of light.
Relativistic electrodynamics, which can be considered as an extension of the
everyday laws of electricity and magnetism, is an important consideration in
high-energy particle accelerators, in high-current, high-voltage vacuum tubes,
and in electromagnetic radiation.
    The laws which relate the electric and magnetic fields to the charges and
currents which produce them are known as Maxwell's equations. Charged
particles or current elements in such fields experience a force which is
called a Lorentz force. The motion of these charged particles and current
elements in such fields is then determined by Newton's laws, appropriately
generalized for relativistic velocities.  See  ELECTRON MOTION IN VACUUM;
MAXWELL'S EQUATIONS; RELATIVISTIC MECHANICS; RELATIVITY.
    J. C. Maxwell's contribution to relativistic electrodynamics was to
formulate his four equations and to introduce the concept of displacement
current. Although each equation was originally deduced for static or other
restrictive conditions, Maxwell implied the validity of each for fields which
varied in arbitrary ways with time.  See  DISPLACEMENT CURRENT.
    One direct consequence of Maxwell's equations comes from the fact that the
fields E and B, in the absence of charges and currents, satisfy a wave
equation with wave velocity given by the equation below. The fact that this is
the same as the velocity of light led Maxwell to infer that light, like
electricity, magnetism, and optics, is an electromagnetic wave phenomenon.

	    c = 1 / SQUARE ROOT OF [EPSILON(sub 0) MU(sub 0)]
		APPROXIMATELY EQUALS  3 X 10(sup 8) m/s

    The property of the Maxwell equations that makes them applicable to
problems in relativistic electrodynamics is their relativistic invariance.
Specifically, this means that Maxwell's equations will seem correct to an
observer traveling with a constant velocity as well as to an observer at rest.
One must realize, however, that a magnetic field in the rest frame will appear
to be both an electric and a magnetic field in the moving frame.  See  FRAME
OF REFERENCE.
    One interesting consequence of the way in which the fields transform is
the diminution of the repulsive force between charges moving with high
velocity in parallel paths. An important application of this effect is to
relativistic beams of particles which might be expected to disperse as a
result of space-charge forces. In linear electron accelerators, for example,
this transverse divergence can be neglected. Another way of looking at the
phenomenon is that in the rest system of the electrons, the accelerator
appears extremely short because of the Lorentz contraction, and the
space-charge forces have little time to spread the beam apart.
    The tendency for relativistically charged beams not to diverge is of
course not restricted to accelerators. It is important in consideration of
electron optics in high-current, high-voltage vacuum tubes, and may even be
important in controlled fusion devices.
    Most electron linear accelerators produce extremely relativistic electrons
whose longitudinal motion is also inhibited by their large relativistic mass.
Indeed, the phase oscillations which are responsible for longitudinal
stability become extremely slow; since all electrons are effectively traveling
at the same velocity c, they neither fall behind nor overtake one another and
are therefore less inclined to get out of phase.
    For relativistic effects in the motion of charged particles in circular
particle accelerators see CYCLOTRON; SYNCHROTRON.
 �34s[Robert L. Gluckstern]
�33s
    The tendency for relativistically charged beams not to diverge is of
course not restricted to accelerators. It is important in consideration of
electron optics in high-current, high-voltage vacuum tubes, and may even be
important in controlled fusion devices.
    Most electron linear accelerators produce extremely relativistic electrons
whose longitudinal motion is also inhibited by their large relativistic mass.
Indeed, the phase oscillations which are responsible for longitudinal
stability become extremely slow; since all electrons are effectively traveling
at the same velocity c, they neither fall behind nor overtake one another and
are therefore less inclined to get out of phase.
    For relativistic effects in the motion of charged particles in circular
particle accelerators see CYCLOTRON; SYNCHROTRON.
 �34s[Robert L. Gluckstern]
�33s

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�6v�2v�32sRelativity �21m �33s
    A general theory of physics, primarily conceived by Albert Einstein, which
involves a profound analysis of time and space, leading to a generalization of
physical laws, with far-reaching implications in important branches of physics
and in cosmology. Historically, the theory developed in two stages. Einstein's
initial formulation in 1905 (now known as the special, or restricted, theory
of relativity) does not treat gravitation; and one of the two principles on
which it is based, the principle of relativity (the other being the principle
of the constancy of the speed of light), stipulates the form invariance of
physical laws only for inertial reference systems. Both restrictions were
removed by Einstein in his general theory of relativity developed in 1915,
which exploits a deep-seated equivalence between inertial and gravitational
effects, and leads to a successful ``relativistic'' generalization of Isaac
Newton's theory of gravitation. Because of the extreme weakness of gravitation
compared with all other known forces, the non-newtonian consequences of the
general theory have been verified only in a few instances, and their
applications are largely confined to cosmology, the structure of neutron stars
and black holes, and the motion of bodies in the solar system. For the same
reason, where it can be applied, the special theory suffices in the treatment
of relativistic effects in atomic, nuclear, and high-energy physics, and its
confirmation in numerous instances of this kind renders it among the most
securely established of modern scientific theories.

    Special (restricted) theory.  The scientific developments that led to the
birth of special relativity arose from a dilemma confronting physicists in the
latter part of the 19th century in their attempts to reconcile Maxwell's
electromagnetic equations with the principles of newtonian mechanics. The
latter were then widely held to have universal validity, but Maxwell's
equations do not preserve their form under galilean transformations, such as
that given by Eqs. (1) [see Fig. 1], that is, they do not satisfy the galilean
principle of relativity. Hence, it appeared necessary to assume that these
equations apply only in one special inertial frame, one in which the
electromagnetic, or luminiferous, ether is at rest, and which, it was also
natural to suppose, served to define Newton's absolute space and absolute
velocity. In particular, the absolute velocity of the Earth in its orbital
motion around the Sun could be expected to reveal itself in appropriate
electromagnetic or optical experiments. However, all such experiments yielded
negative results. The most famous of these, the interferometer experiment of
A. A. Michelson and E. W. Morley in 1881, presented the greatest challenge.
See FRAME OF REFERENCE; MAXWELL'S EQUATIONS.

	      x' = x - vt     y' = y     z' = z     t' = t                 (1)

    H. A. Lorentz was led in 1892 to postulate that a body whose absolute
speed of translational motion is v is contracted in the direction of motion by
the factor SQUARE ROOT[1 - v(sup 2)/c(sup 2)]. This hypothesis was independently
suggested by G. F. FitzGerald, and is thus known as the Lorentz-FitzGerald
contraction hypothesis. Other experiments, as well as critical methodological
considerations, led Lorentz to arrive gradually, in 1899 and 1904, at a
general solution of the problem posed by all the unexpected negative
ether-drift results. This solution involved replacing galilean transformations
(1) by space-time transformation equations (2), and introducing associated
transformation equations for the electromagnetic field quantities. Henri
Poincare corrected and completed Lorentz's theory in 1905 and 1906, and named
Eqs. (2) Lorentz transformations. See LIGHT; LORENTZ TRANSFORMATIONS.

	x' = (x - vt)/{SQUARE ROOT [1 - v(sup 2)/c(sup 2)]}
	y' = y
	z' = z
	t' = (t - vx/c(sup 2))/{SQUARE ROOT[1 - v(sup 2)/c(sup 2)]}          (2)

    Although the mathematical structure of the Lorentz-Poincare theory is the
same as that of the theory advanced independently by Einstein in 1905, there
is nevertheless a vast difference in the underlying conceptual framework.
Einstein started with the idea that the absoluteness of time and space is
imposed not by nature but by custom, and that therefore, in particular, the
notion of simultaneity must be reexamined critically.


    Einstein's formulation. The theory is erected on two fundamental
postulates and two definitions. The postulates, which Einstein called the
principle of relativity and the principle of the constancy of the velocity of
light, can be stated respectively as follows: (I) The analytical form of
physical laws is the same in all inertial reference systems. (II) The speed of
light in vacuum is a universal constant.
    Postulate I is a generalization of the galilean principle of relativity to
embrace all phenomena rather than only those of mechanics (excepting, however,
gravitational phenomena). It is II that, taken together with I, clearly
represents a radical breaking away from traditional thinking. The new
formulation of simultaneity is also intimately connected with II. Two
spatially separated localized occurrences (or events) are simultaneous when
the readings of two identical clocks adjacent to the events are the same, and
it is known that the clocks are synchronized. However, when the clocks are not
near each other, their synchronism must be defined.
    The definition given by Einstein is tied to II, and can be stated as
follows: (A) Two identical clocks C, C', situated at two distant points P, P'
fixed in a given inertial frame S, synchronize in S, when the respective C and
C' times t1, t'1 of the sending of a light signal at P and its arrival at P'
are connected by the formula t'1 - t1 = t2 - t'1 with the C time t2 of its
return to P after reflection at P' back to P.
    Two events that are simultaneous in terms of clocks synchronized in S
according to definition A are not simultaneous when referred to identical
clocks synchronized according to A in a reference frame that is moving
relative to S. In other words, simultaneity is a relative concept: it depends
on the reference system under consideration. This relativity is at the heart
of many relativistic phenomena.
    Einstein's second definition clarifies the operational meaning of the
length of a moving rod, and can be stated as follows: (B) The length, as
measured in a given inertial frame S, of a rigid rod that is in uniform motion
relative to S is the length between the instantaneous positions of the ends of
the rod, where the instantaneousness and the length are determined
respectively by clocks synchronized in S and by meter sticks at rest in S.
    With the aid of these postulates and definitions, Einstein succeeded in
deriving the Lorentz transformations and deduced the Lorentz-FitzGerald
contraction.

    Time dilation. Another remarkable result concerns the relativity of time.
As measured in S', the time interval between a given pair of events which have
the same coordinates in S' is smaller by the factor SQUARE ROOT[1 - v(sup
2)/c(sup 2)] than it is as measured in S.  In this respect, too, there is
complete reciprocity, as there must be, between S and S':  the time interval
between two events is a relative concept.  Einstein pointed out also the
following corollary of this time dilation effect.  If one of two identical
clocks, initially synchronized and adjacently at rest in an inertial frame,
makes a round trip, it will lag behind the other upon its return.  Convincing
verification of time ilation is provided by the measured lifetimes of fast
mesons and by more precise experiments employing the Moessbauer effect.  See
CLOCK PARADOX; MOESSBAUER EFFECT.

    Minkowski's formulation. Hermann Minkowski's geometric approach,
formulated in 1908, which was an important link in the development of general
relativity, centers on absolute quantities in the special theory, and provides
it with the efficient formalism of tensors in space-time.
    Minkowski's space-time, his ``world,'' is the continuum of all events. It
is a four-dimensional, quasieuclidean space, with the line element dssup2 (the
distance squared between two neighboring events) given by Eq. (3), defining
the metric of the space the ``Minkowski metric.'' See SPACE-TIME.

ds(sup 2) = SUMMATION (alpha, beta = 0 to 3) eta(sub alpha beta) dx(sup alpha)
	     dx(sup beta) IS IDENTICAL TO (dx sup 0)sup 2
	      - SUMMATION (i=1 to 3) (dx sup i)sup 2			     (3)

			     x(sup 0) = ct
			     x(sup 1) = x
			     x(sup 2) = y
			     x(sup 3) = z

	   eta(sub alpha beta) = 0 (if alpha NOT EQUAL beta)

	eta(sub 00) = -eta(sub 11) = -eta(sub 22) = eta(sub 33) = 1

    The tensors of special relativity represent absolute quantities, which are
determined by their components, relative quantities, obeying the appropriate
transformation rules associated with the Lorentz group.
    One may replace dx(sup alpha) in Eqs. (3) by finite DELTA x(sup alpha), and
according as DELTA s(sup 2) is greater than 0, less than 0, or equal to 0, one
has a timelike, spacelike, or null interval (displacement four-vector).  The
first and last cases correspond to possible displacements of a particle with
m(sub 0) NOT EQUAL 0 or m(sub 0) = 0.  The integral INTEGRAL ds/c along a
particle's space-time path (or ``world line'') gives the time as measured by a
clock accompanying the particle:  the ``proper time'' on the world line.

    Applications. The branches of classical physics can usually be
relativistically generalized by applying postulate I in Minkowski's
formulation of expressing the physical laws in tensor form. Minkowski's method
is successful in the case of a particle moving under the action of an external
field of force, and important but limited results exist for systems of
interacting particles. A dramatic consequence is Einstein's mass-energy
equivalence, which has implications ranging from nuclear reactor energy
production to atomic bombs.
 �34s[Herman M. Schwartz]
�33s

    General theory.  One of the basic tenets of special relativity is that no
physical effect can propagate with a velocity greater than the speed of light,
and so c represents a universal speed limit. On the other hand, classical
gravitational theory describes the gravitational field of a body throughout
space as a function of its instantaneous position, which is equivalent to the
assumption that gravitational effects propagate with an infinite velocity;
that is, classical gravitational theory is an action-at-a-distance theory.
Thus, special relativity and classical gravitational theory are inconsistent,
and a modified theory of gravity is necessary. This is the theory developed by
Einstein in 1905 - 1915, following his discovery of special relativity.

    Principle of equivalence. It had long been considered a fundamental and
puzzling question why bodies of different mass fall with the same acceleration
in a gravitational field, or equivalently why the trajectory of a test body is
independent of its mass. This situation was explained by Newton with the
statement that both the gravitational force on a body and its inertial
resistance to acceleration are proportional to its mass.
    The explanation by Newton is not very profound and is more in the nature
of an ad hoc description. A deeper and more natural explanation occurred to
Einstein. In physics there are numerous examples of forces other than
gravitation which are mass-proportional; these generally arise due to the use
of accelerated coordinate systems to describe the motion. One well-known
example is the centrifugal force encountered in a rotating coordinate system.
Consider one observer in the gravitational field of the Earth and another in
an accelerating elevator or rocket in free space (Fig. 2). If both drop a test
body, they will observe it to accelerate relative to the floor. According to
classical theory, the Earth-based observer would attribute this to a
gravitational force and the elevator-based observer would attribute it to the
accelerated floor overtaking the uniformly moving body. However, Einstein
reasoned that the effects are identical and the theory of gravity should
provide an equivalent description of the two systems. This is the famous
principle of equivalence that Einstein made the physical cornerstone of
general relativity; it states that on a local scale the physical effects of a
gravitational field are indistinguishable from the physical effects of an
accelerated coordinate system. From the point of view of the principle of
equivalence, it is evident why the motion of a test body in a gravitational
field is independent of its mass. See CENTRIFUGAL FORCE.


    Tensor field equations. The close connection between gravity and
accelerating coordinate systems suggests that the equations describing the
gravitational field be cast in a form that is manifestly independent of the
coordinate system in order to achieve the maximum simplicity and elegance. The
study of systems of equations that are independent of coordinate systems was
begun by K. Gauss in connection with his study of geometry on curved
two-dimensional surfaces, and was carried to a high state of development by B.
Riemann and T. Levi-Civita in the tensor calculus. Tensors are a basically
simple generalization of vectors. See CALCULUS OF TENSORS.
    The space-time of relativity contains one covariant second-rank tensor of
particularly great importance, called the metric tensor gsupmunu, which is a
generalization of the Lorentz metric of special relativity etasubalphabeta,
introduced in Eqs. (3). Nearby points in space-time, known as events, which
are separated by coordinate distances dxsupmu have an invariant physical
separation whose square, the line element, is given by Eq. (4). This quantity
is a generalization of the line element in special relativity and has the same
relation to the concept of proper time. See RIEMANNIAN GEOMETRY.

		  dssup2 = gsubmunu dxsupmu dxsupnu			   (4)

    Tensor equations are equations in which one tensor of a given rank and
type (contravariant or covariant) is set equal to another of the same type.
The field equations of general relativity are tensor equations for the metric
tensor, which completely describes the geometry of the space. The Riemann
tensor (or curvature tensor), Rsubmubetanusupalpha, plays a central role in the
geometric structure of a space; if it is zero, the space is termed flat and has
no gravitational field; if nonzero, the space is termed curved. In terms of the
contracted Riemann tensor, a Riemann tensor summed over alpha = beta, the
Einstein field equations for free space are given by Eq. (5). In a region of
space containing matter or energy, the zero on the right side of this equation
is replaced by a tensor representing the energy content of space.

	       Rsubmualphanusupalpha IS IDENTICAL TO Rsubmunu = 0	    (5)

    The field equations are a set of 10 second-order partial differential
equations since the four-by-four symmetric tensor Rsubmunu has 10 independent
components; they are to be solved for the metric tensor. A solution in a given
coordinate system defines an Einstein space-time. The curvature of this space
corresponds to the intrinsic presence of a gravitational field. That is, the
concept of a field of mechanical force in classical gravitational theory is
replaced by the geometric concept of curved space in relativity theory. See
DIFFERENTIAL EQUATION.

    Motion of test bodies. The path of a test body is a generalization of a
straight line in euclidean space; it is the shortest ``distance'' (in terms of
intervals ds) between points in space-time, known as a geodesic. General
relativity theory possesses an extraordinary property; because the field
equations are nonlinear, unlike newtonian theory, the motion of a test body in
a gravitational field is not arbitrary since the body itself has mass and
contributes to the field. Indeed, it turns out that the field equations are so
restrictive that the geodesic equation of motion is a necessary consequence
and need not be treated as a separate postulate.

    Schwarzschild solution. A very important solution of the field equations,
obtained by K. Schwarzschild in 1916, represents the field in free space
around a spherically symmetric body such as the Sun. It is the basis for a
relativistic description of the solar system and all of the experimental tests
of general relativity which have been carried out.

    Gravitational redshift. There are a number of ways to show that
electromagnetic radiation of a given frequency emitted in a gravitational
field will appear to an outside observer to have a lower frequency; that is,
it will be redshifted. Accurate and dependable measurements involve the use of
the Moessbauer effect in terrestrial experiments. With this technique the
redshift has been measured by R. Pound and G. Rebka to be within about 1% of
the value predicted by general relativity theory.

    Perihelion shift. The equations of motion can be solved for a planet
considered as a test body in the Schwarzschild field of the Sun. As should be
expected, the orbits obtained are very similar to the ellipses of classical
theory. Small differences occur, however, the most interesting of which is
that the ellipse rotates very slowly in the plane of the orbit so that the
perihelion, the point of closest approach of the planet to the Sun, is at a
slightly different angular position on each orbit. This shift is extremely
small. It is greatest in the case of the planet Mercury, whose perihelion
advance is predicted to be only 43 seconds of arc in a century. This is in
excellent agreement with the value for the discrepancy between classical
theory and observation, which was well known and unaccountable for many years
before the discovery of general relativity.

    Deflection of star light. The principle of equivalence suggests an
extraordinary phenomenon of gravity. Light or other electromagnetic radiation
crossing the Einstein elevator horizontally will appear to be deflected
downward in a parabolic arc because of the upward acceleration of the
elevator. The same phenomenon must occur for light in the gravitational field
of the Sun; it must be deflected toward the Sun. A calculation of this
deflection gives 1.75 seconds for the net deflection of starlight grazing the
edge of the Sun. Modern measurements are made by tracking quasars as they pass
near or behind the Sun. With these techniques, the deflection has been
measured to be within 1% of the value predicted by general relativity. See
QUASARS; RADIO ASTRONOMY.

    Radio time delay. In the curved space around the Sun the distance between
points in space, for example between two planets, is not the same as it would
be in flat space. In particular, the round-trip travel time of a radar signal
sent between the Earth and Venus will be measurably increased by the curvature
effect when the Earth, the Sun, and Venus are approximately lined up. Although
the maximum time delay is only a few hundred microseconds, it has been
accurately measured. Subsequent experiments using the Viking spacecraft were
able to achieve an accuracy of about one-half of 1%, and agree with the
predictions of general relativity.
    For the applications of general relativity to the study of black holes, to
cosmology, and to gravitational radiation see BLACK HOLE; COSMOLOGY;
GRAVITATION; PULSAR.
 �34s[Ronald J. Adler]
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�6v�2v�32sRelativistic quantum theory �21m �33s
    The quantum theory of particles which is consistent with the special
theory of relativity, and thus can describe particles moving arbitrarily close
to the speed of light. It is now realized that the only satisfactory
relativistic quantum theory is quantum field theory; the attempt to relativize
the Schroedinger equation for the wave function of a single particle fails, as
shown below. However, with a change of interpretation, relativistic wave
equations do correctly describe some aspects of the motions of particles in an
electromagnetic field. See QUANTUM FIELD THEORY; QUANTUM MECHANICS; RELATIVITY.
    The Schroedinger equation for the wave function psi(BOLD r,t) of a particle
is
		    E psi = H(BOLD p, BOLD r)psi			     (1)

Eq. (1), where E is the energy operator i PLANCK'S CONSTANT(CURLY D/CURLY Dt),
p is the momentum operator -i PLANCK'S CONSTANT DEL, and H(p,r) is the
classical hamiltonian. For a nonrelativistic particle in free space, H =
BOLD p(sup 2)/2m. The naive way to relativize Eq. (1) would be to use the
relativistic hamiltonian, Eq.  (2).  However, this equation is not
relativistically invariant.

   H = SQUARE ROOT [(mc sup 2) sup 2 + BOLD p(sup 2) c(sup 2)]		    (2)


   E(sup 2) phi = [(mc sup 2) sup 2 + BOLD p(sup 2) c(sup 2)]phi	    (3)

    The so-called Klein-Gordon equation, Eq. (3), is relativistically invariant.
However, the only possible density of a conserved quantity

	       rho PROPORTIONAL TO phi*E phi - phi E phi*		    (4)

formed from phi is of the form shown in (4).  But this cannot be a probability
density, because it is not positive definite (it changes sign when phi is
replaced by phi*).
    But rho, in (4), can be interpreted as a charge density (when multiplied
by a unit charge e); phi is then to be interpreted as a matrix element
of a field operator PHI of a quantized field whose quanta are particles with
mass m and charge e and no spin.
    P. A. M. Dirac found a relativized form of Eq. (1) which is both linear in
E and has a positive definite density form given by Eq. (5), where beta and
BOLDalpha are constants which obey Eq. (6).

	 E psi = [beta mc(sup 2) + BOLD alpha DOT BOLD p c]psi

		     (rho PROPORTIONAL TO psi*psi)			     (5)



   alpha(sub i) alpha(sub j) + alpha(sub j) alpha(sub i) = 0, i NOT EQUAL j

		      alpha beta + beta alpha = 0,

		       (alpha[sub i]) (sup 2) = 1,

			    beta(sup 2) = 1,

			      i,j = 1,2,3				     (6)

Obviously the four constants beta and alphasubi cannot be numbers; however,
they can be 4 x 4 matrices, and psi is then a four-component object called a
Dirac spinor.  Taking Eq.  (5) as an eigenequation for E, one finds four
eigenstates (because H is a 4 x 4 matrix), two with E = + SQUARE ROOT [(mc sup
2)sup 2 + p(sup 2) c(sup 2)] and two with E = - SQUARE ROOT [(mc sup 2)sup 2 +
p(sup 2) c(sup 2)].  The interpretation of the two positive energy states is
that they are the two spin states of a particle with spin 1/2 PLANCK'S CONSTANT.
But the two negative energy states are an embarrassment; even if one started
with a particle in a positive energy state, it would quickly make radiative
transitions down through the negative energy states.  Dirac's solution was to
observe that if the particle described by psi obeyed the Pauli principle, then
one can suppose that all the negative energy states are already filled with
particles, thus excluding any more.  See SPINOR.

    There are still four single-particle states for a given momentum BOLD p: the
two spin states of a particle with positive energy, and the two states
obtained by subtracting a negative energy particle (of momentum -BOLD p). These
last states (``hole states'') have positive energy and a charge opposite the
charge of the particle. The hole is in fact the antiparticle; if the particle
is an electron, the hole is a positron. However, with the filling up of the
negative energy states, one no longer has a single-particle system, and psi,
just as in the Klein-Gordon case, no longer can be interpreted as a wave
function but as a matrix element of a field operator PSI. See POSITRON.
 �34s[Charles J. Goebel]
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�6v�2v�32sRelaxation oscillator   �21m �33s 
    An electronic circuit which has two stable states, resulting in two 
distinct output levels, and which switches between the two states at a rate 
determined by the rate of rise or decay of voltage across the storage element 
in an RC or RL circuit. The output waveform is usually nonsinusoidal and may 
be approximately a square wave, a sawtooth wave, or a series of short 
repeating pulses. See WAVESHAPING CIRCUITS. 
    In one of the most widely used forms of relaxation oscillator, the astable 
multivibrator, two devices are connected so that they are alternately ON and 
OFF. The blocking oscillator is representative of a class of circuits in which 
a single device has two stable conditions. There are other relaxation 
oscillators using single transistors or transistorlike devices with a current 
gain greater than unity, such as the point-contact transistor and the 
unijunction transistor, sometimes called a double-base diode. 
Negative-resistance devices such as tunnel diodes can also be used as a 
relaxation oscillator. See BLOCKING OSCILLATOR; MULTIVIBRATOR; 
NEGATIVE-RESISTANCE CIRCUITS; TUNNEL DIODE. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sRelaxation time of electrons �21m �33s
    The characteristic time for a distribution of electrons in a solid to
approach or ``relax'' to equilibrium after a disturbance is removed. The most
familiar example is a transport property such as electrical resistivity, in
which the disturbance is the electric field, equilibrium is the state of zero
current, and relaxation time is inversely proportional to the resistivity.
Closely related is the concept of a lifetime, which is related to equilibrium
properties rather than transport properties. The lifetime is the mean time
that an electron will remain in a given quantum state before changing its
state as a result of a collision. A related concept is the electron mean free
path, the average distance traveled before a collision. Although
characteristic collision times are quite short (of the order of 10(sup -14) s
at room temperature), mean free paths are surprisingly long, ranging from
about 100 atom distances at room temperature to 1,000,000 atom distances in a
pure metal near absolute zero. This is surprising because atoms in a solid are
very densely packed. It corresponds classically to the unlikely event that a
rifle bullet might travel for miles through a dense forest without hitting a
tree. From a practical point of view, that is what makes metals useful as
electrical conductors. See FREE-ELECTRON THEORY OF METALS.
    The conduction process may be viewed as the steady-state balance between
the accelerating effect of an electric field and the decelerating effect of
collisions. This process is best viewed in terms of the probability
distribution function for the electron gas, f(k,r,t), where k specifies the
momentum, r the position, and t the time. Viewed in k-space, the entire
distribution shifts from its equilibrium distribution f0 under the influence
of a perturbation such as an electric field (see illustration). A steady state
is reached due to collisions which tend to restore the distribution to f0. The
relaxation time TAU is most simply defined in terms of the rate at which the
perturbed distribution approaches its equilibrium value once the perturbation
is removed, as given in the equation below.

	      (Delta f/Delta t)(sub coll.) = (f - f[sub 0])/TAU

    Electron mean free paths in solids are extremely long because the electron
wave (viewed quantum-mechanically) readjusts in a perfectly periodic crystal
lattice to avoid the atom cores and spend most of its time in the spaces
between. In the analogy of the rifle in the forest, the bullet will not travel
very far, but the sound of the gun can, because sound waves bend around the
trees. In a perfectly periodic lattice, the scattering vanishes because the
electron waves scattered from the ion cores interfere in a coherent and
constructive way. Any disturbance to the lattice periodicity tends to destroy
this wave pattern, resulting in less transmission of the electron wave. The
conductivity of solids at room temperature, for example, is limited by the
scattering produced by atomic vibrations. At lower temperatures, when the
vibrations are reduced, the conductivity is limited by scattering from
impurities and lattice imperfections. See CRYSTAL; ELECTRICAL RESISTIVITY;
LATTICE VIBRATIONS.
 �34s[Richard J. Higgins]
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�6v�2v�32sRelay   �21m �33s 
    An electromechanical or solid-state device operated by variations in the 
input which, in turn, operate or control other devices connected to the 
output. They are used in a wide variety of applications throughout industry, 
such as in telephone exchanges, digital computers, motor and sequencing 
controls, and automation systems. Highly sophisticated relays are utilized to 
protect electric power systems against trouble and power blackouts as well as 
to regulate and control the generation and distribution of power. In the home, 
relays are used in refrigerators, automatic washers and dishwashers, and heat 
and air-conditioning controls. Although relays are generally associated with 
electrical circuitry, there are many other types, such as pneumatic and 
hydraulic. Input may be electrical and output directly mechanical, or 
vice-versa. See RELAY CONTROL SYSTEM. 
    Relays using discrete solid-state components, operational amplifiers, or 
microprocessors can provide more sophisticated designs. Their use is 
increasing, particularly in applications where the relay and associated 
equipment are packaged together. See AMPLIFIER; MICROPROCESSOR. 
 �34s[J. Lewis Blackburn]
�33s 
  
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�6v�2v�32sRelay control systems   �21m �33s 
    Feedback systems in which the actuator input applied to the plant is 
either at its maximum or minimum allowed value as a result of a preceding 
relay. The use of relays is motivated by their low cost and high reliability 
as well as the fact that for many systems they represent the realization of an 
optimal control policy. For example, if it is required to transfer the state 
of a linear system from one point to another in a minimum amount of time, 
under the constraint that the input does not exceed specified bounds, the 
solution can be shown to consist of a control that uses only its minimum and 
maximum values and thus dictates a feedback structure that would be 
implemented using relays as the primary actuators. 
    Relay control systems are generally characterized by a set of regions in 
state space (the vector space of state vectors) in which the control assumes 
either its maximum or minimum values. The surfaces separating these regions 
are called switching surfaces. Each point on a switching surface may be 
identified as being normal, if a trajectory enters that point from one side of 
the surface and exists from the other, or as being an end point, if 
trajectories on both sides of the surface at that point are directed toward 
the surface. Trajectories entering a switching surface at an end point do not 
leave the surface but ``chatter'' along it (the relay switches back and forth 
infinitely fast) until a normal point is encountered and the trajectory exits 
from the switching surface. Depending on the controlled system, the existence 
of the chattering mode may or may not be acceptable.  See  CONTROL SYSTEMS; 
NONLINEAR CONTROL THEORY.  
 �34s[Patrick E. Barry ;C. T. Chen]
�33s 
  
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�6v�2v�32sReliability of equipment   �21m �33s 
    The probability that a component part, equipment, or system will 
satisfactorily perform its intended function under given circumstances, such 
as environmental conditions, limitations as to operating time, and frequency 
and thoroughness of maintenance. 
    Reliability is influenced by all aspects of an engineering effort; the 
ultimate reliability of a component or a system depends upon the quality of 
research involved in its conception, its design, the manner in which it is 
manufactured, the external influences on its operation, maintenance 
considerations, and other factors. See SYSTEMS ENGINEERING. 
    In specific circumstances reliability can mean a number of different 
things. Reliability can be considered as a function of time, the time that a 
piece of equipment operates satisfactorily compared with the total time over 
which it was expected to operate. A second specific definition of reliability 
is the average number of hours of maintenance for each hour of satisfactory 
performance. A third definition is the fraction of the total number of 
identical equipments which operate satisfactorily over a given period of time. 
A fourth measure of reliability compares the actual performance of a device 
with its ideal performance. This fourth definition of reliability permits a 
direct comparison between the performance of devices under idealized 
conditions and under actual conditions, thus introducing the influence of 
environment on performance, and it also considers the realization of a 
performance ratio rather than a good-bad or operate-nonoperate evaluation. 
 �34s[Robert W. Mann]
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�6v�2v�32sReluctance �21m �33s
    A property of a magnetic circuit analogous to resistance in an electric
circuit.
    Every line of magnetic flux is a closed path. Whenever the flux is largely
confined to a well-defined closed path, there is a magnetic circuit. That part
of the flux that departs from the path is called flux leakage. See MAGNETIC
CIRCUITS; MAGNETIC FLUX.
    For any closed path of length l in a magnetic field H, the line integral
of H cos alpha dl around the path is the magnetomotive force (mmf) of the
path, as in Eq. (1), where alpha is the angle between H and the path. If the
path encloses N conductors, each with current I, Eq. (2) holds. See
MAGNETOMOTIVE FORCE.

		    mmf = LINE INTEGRAL H cos alpha dl			     (1)


		 mmf = LINE INTEGRAL H cos alpha dl = NI		     (2)

    Consider the closely wound toroid shown in the illustration. For this
arrangement of currents, the magnetic field is almost entirely within the
toroidal coil, and there the flux density or magnetic induction B is given by
Eq. (3), where l is the mean circumference of the toroid and mu is the
permeability. The flux PHI within the toroid of cross-sectional area A is
given by either form of Eqs. (4), which is similar in form to the equation for
the electric circuit, although nothing actually flows in the magnetic circuit.
The factor l/mu A is called the reluctance SCRIPT R of the magnetic circuit. The
reluctance is not constant because the permeability mu varies with changing
flux density. See MAGNETIC INDUCTANCE; MAGNETIC PERMEABILITY.
 �34s[Kenneth V. Manning]
�33s

			     B = mu (NI/l)				     (3)


			  PHI = BA = (mu A/l)NI 			     (4)

	     PHI = NI/(//mu A) = mmf/(l/mu A) = mmf/(SCRIPT R)

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�6v�2v�32sReluctance motor   �21m �33s 
    A synchronous motor which starts as an induction motor and, upon nearing 
full speed, locks into step with the rotating field and runs at synchronous 
speed. The stator and rotor windings are similar to those of an induction 
motor. The rotor is of squirrel-cage construction, to allow induction-motor 
starting, and has salient-pole projections which provide synchronous operation 
at full speed. The reluctance motor is built only in small sizes and for 
situations in which low cost and simplicity are mandatory and efficiency is of 
little concern. See INDUCTION MOTOR; SYNCHRONOUS MOTOR. 
 �34s[Loyal V. Bewley]
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�6v�2v�32sRemote-control system   �21m �33s 
    A system in which there is an appreciable distance separating the 
controlled quantity and the controlling quantity. There are three essential 
components in a remote-control system: a controlling quantity, a transmission 
medium, and a controlled quantity. 
    A familiar example of a remote-control system is the telephone system. A 
voice at one end of the telephone cable controls the motion of a 
sound-producing diaphragm at the other end of the cable. The voice is the 
controlling quantity, and the sound produced by the diaphragm is the 
controlled quantity. Three other examples of remote-control systems are a 
masterslave manipulator, a telemetering system, and television. See CONTROL 
SYSTEMS; REMOTE MANIPULATORS; TELEMETERING; TELEVISION. 
 �34s[John G. Truxal]
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�6v�2v�32sRemote manipulators   �21m �33s 
    Mechanical, electromechanical, or hydromechanical devices which enable a 
person to perform manual operations while separated from the site of the work. 
Manipulators are often equipped with touch and vision sensors, and thus can 
extend human hands and eyes over distances from a few inches to many miles. 
Reasons which keep a person from performing the work directly include a 
hazardous work environment or one which is difficult or impossible to gain 
access to because it is blocked, too large, too small, or too far away. 
    Manipulators are controlled directly by a person who moves control handles 
or operates switches similar to those on a derrick, thereby causing the 
manipulator to move about on wheels or tracks, or to reach out and grasp 
objects with a mechanical arm. Manipulators are different from industrial 
robots, which operate without direct and continuous manual control. See 
ROBOTICS.  
    The table lists some typical manipulator applications, together with the 
typical distance between operator and work. 
    A manipulator usually has several links and joints arranged to make an 
arm. On the end is a gripper or tool which permits objects to be grasped, 
handles turned, force exerted, buttons pushed, doors opened, and so on. If the 
operator can be located nearby, he or she can operate the arm like a large 
puppet, but electric or hydraulic power is more common. 
    The typical elements of a manipulator system are shown pictorially in the 
illustration which is a radioactive hot lab situation in which the operator 
views the work through a lead-glass window. Manipulator systems have control 
stations, transmission links for command and sensing signals, an effector 
system at the remote site with various sensor systems, and display systems for 
helping the operator see and feel what the effector system is doing. 
    Visual sensing is provided by direct view or television. Large 
manipulators such as that on the space shuttle may have television cameras 
mounted at the shoulder, elbow, and wrist to give the operator a choice of 
views. Touch or force sensing is provided either by direct mechanical 
connection or by electric motors which drive the control handle back against 
the operator's hand so as to imitate the force being exerted by the distant 
gripper. See CONTROL SYSTEMS; HUMAN-MACHINE SYSTEMS. 
 �34s[Daniel E. Whitney]
�33s 
  
  
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�6v�2v�32sRemote sensing   �21m �33s 
    The gathering and recording of information about terrain and ocean 
surfaces without actual contact with the object or area being investigated. 
Remote sensing uses the visual, infrared, and microwave portions of the 
electromagnetic spectrum. Remote sensing is generally conducted by means of 
remote sensors installed in aircraft and satellites. 
  
    Photography.  Photography is probably the most useful remote sensing 
system. Much of the experience gained over the years from photographs of the 
terrain taken from aircraft is being drawn upon for use in space. 
    Multispectral photography isolates the reflected energy from a surface in 
a number of given wavelength bands and records each spectral band separately 
on film. This technique allows selection of the significant bandwidths in 
which a given area of terrain displays maximum tonal contrast and, hence, 
increases the effective spectral resolution of the system over conventional 
black-and-white or color systems. Because of its spectral selectivity 
capabilities, the multispectral approach provides a means of collecting a 
great amount of specific information. 
  
    Multispectral imagery.  Multispectral scanning systems record the spectral 
reflectance by photoelectric means (rather than by photochemical means as in 
multispectral photography) simultaneously in several individual wavelengths 
within the visual and near-infrared portions of the electromagnetic spectrum. 
    In satellite applications, optical energy is sensed by an array of 
detectors simultaneously in four spectral bands from 0.47 to 1.1 micrometers. 
As the optical sensors for the various frequency bands sweep across the 
underlying terrain in a plane perpendicular to the flight direction of the 
satellite, they record energy from individual areas on the ground. The 
smallest individual area distinguished by the scanner is called a picture 
element or pixel, and a separate spectral reflectance is recorded in analog or 
digital form for each pixel. The spectral reflectance values for each pixel 
can be transmitted electronically to ground receiving stations in near-real 
time, or stored on magnetic tape in the satellite until it is over a receiving 
station. When the signal intensities are received on the ground, they can be 
reconstructed almost instantaneously into the virtual equivalent of 
conventional aerial photographs. 
  
    Infrared.  Thermal infrared radiation is mapped by means of infrared 
scanners similar to multispectral scanners, but in this case radiated energy 
is recorded generally in the 8 - 14-MUm portion of the electromagnetic 
spectrum. The imagery provided by an infrared scanning system gives 
information that is not available from ordinary photography or from 
multispectral scanners operating in the visual portion of the electromagnetic 
spectrum. See EMISSIVITY; INFRARED RADIATION. 
    In the past, thermal infrared images were generally recorded on 
photographic film. Videotape records are replacing film as the primary 
recording medium and permit better imagery to be produced and greater 
versatility in interpretation of data. 
    Thermal infrared mapping (thermography) from satellite altitudes is 
proving to be useful for a number of purposes, one of which is the mapping of 
thermal currents in the ocean. Thermal infrared mapping from aircraft and 
satellite altitudes has many other uses also, including the mapping of 
volcanic activity and geothermal sites, location of groundwater discharge into 
surface and marine waters, and regional pollution monitoring. 
  
    Microwave radar.  This type of remote sensing utilizes both active and 
passive sensors. The active sensors such as radar supply their own 
illumination and record the reflected energy. The passive microwave sensors 
record the natural radiation. A variety of sensor types are involved. These 
include imaging radars, radar scatterometers and altimeters, and 
over-the-horizon radars using large ground-based antenna arrays, as well as 
passive microwave radiometers and imagers. One of the most significant 
advantages of these instruments is their all-weather capability, both day and 
night. See MICROWAVE; RADAR. 
  
    High-frequency (hf) radar.  Such radars utilize frequencies in the 3 - 
30-MHz portion of the electromagnetic spectrum (median wavelength of about 20 
m) and are thus not within the microwave part of the spectrum. The energy is 
transmitted by ground-based antennas in either a sky-wave or surface-wave 
mode. In the sky-wave mode, the energy is refracted by the various ionospheric 
layers back down to the Earth's surface some 480 - 1800 mi or 800 - 3000 km 
(on a single-hop basis) away from the hf radar antenna site. The incident 
waves are reflected from such surface features as sea waves. 
 �34s[Peter C. Badgley]
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�6v�2v�32sRennin   �21m �33s 
    An enzymatic protein used for coagulating milk casein in cheese making. It 
is also related to the cheese-ripening process through its proteolytic 
activity. See CHEESE; ENZYME. 
    Rennin is secured from rennet. In preparing rennet extract, only milk-fed 
calves are used. After butchering, the fourth stomach, or abomasum, is removed 
and freed of its food content. At the rennet factory, the stomachs are 
processed to extract the rennin. 
 �34s[Paul H. Tracy]
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�6v�2v�32sRenormalization   �21m �33s 
    A program in quantum field theory consisting of a set of rules for 
calculating S-matrix amplitudes which are free of ultraviolet (or 
short-distance) divergences, order by order in perturbative calculations in an 
expansion with respect to coupling constants. See QUANTUM FIELD THEORY; 
SCATTERING MATRIX. 
 �34s[Itzhak Bars]
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�6v�2v�32sRepulsion motor   �21m �33s 
    An alternating-current (ac) commutator motor designed for single-phase 
operation. The chief distinction between the repulsion motor and the 
single-phase series motors is the way in which the armature receives its 
power. In the series motor the armature power is supplied by conduction from 
the line power supply. In the repulsion motor, however, armature power is 
supplied by induction (transformer action) from the field of the stator 
winding. For discussion of the ac series motor  see  UNIVERSAL MOTOR.  See 
also  ALTERNATING-CURRENT MOTOR. 
    The repulsion motor primary or stationary field winding is connected to 
the power supply. The secondary or armature winding is mounted on the motor 
shaft and rotates with it. The terminals of the armature winding are 
short-circuited through a commutator and brushes. There is no electrical 
contact between the stationary field and rotating armature (see illustration). 
  See  WINDINGS IN ELECTRIC MACHINERY. 
 �34s[Irving L. Kosow]
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�6v�2v�32sReptilia   �21m �33s 
    A class of vertebrates composed of four living orders, the turtles or 
Chelonia, the tuatara or Rhynchocephalia, the lizards and snakes or Squamata, 
and the crocodilians or Crocodilia. Numerous extinct orders are also known. 
The group first appeared in the Carboniferous and underwent a culminating 
evolutionary radiation in the Mesozoic, often called the age of reptiles. 
Although the major portion of the class is not extinct, several Recent groups, 
particularly the Squamata, are very successful, and there are approximately 
5000 living species of reptiles as compared to about 4000 living mammals. A 
classification for the class is given here; see separate articles on each of 
the groups listed. 
    Subclass Anapsida 
        Orders: Cotylosauria 
                Mesosauria 
                Chelonia 
    Subclass Ichthyopterygia 
        Order: Ichthyosauria 
    Subclass Euryapsida 
        Orders: Sauropterygia 
                Placodontia 
                Araeoscelidia 
    Subclass Lepidosauria 
        Orders: Eosuchia 
                Rhynchocephalia 
                Squamata 
    Subclass Archosauria 
        Orders: Thecodontia 
                Crocodilia 
                Pterosauria 
                Saurischia 
                Ornithischia 
    Subclass Synapsida 
        Orders: Pelycosauria 
                Theropsida 
                Ictidosauria 
    The reptiles are the most primitive of the completely terrestrial 
vertebrates and are consequently the first to exhibit amniote features. In the 
reptiles the eggs are covered by a complex series of protective layers, 
including a leathery or calcareous shell. A rich supply of food material in 
the form of yolk is deposited inside the ovum to furnish food for the 
developing embryo. A series of protective extraembryonic membranes, the serosa 
and amnion, appears later in embryogenesis to protect the embryo from water 
loss and shock. A third such membrane, the allantois, functions as a storage 
sac for nitrogenous wastes. The serosa and allantois usually fuse to form a 
respiratory structure. Gaseous exchanges take place across the shell and 
seroallantoic membrane between the outside air and the blood vessels of the 
allantois. All of these adaptations made it possible for the reptile egg to be 
deposited on land, undergo its development there, and hatch into a fully 
developed form without a gilled larval stage. Most reptilian eggs are buried 
in the soil or in rotting vegetation out of the direct sunlight.  See  AMNIOTA. 
    If all intermediate fossil forms are taken into account, the above 
features of the life history are the only ones that serve to distinguish the 
reptiles from the amphibians. As a matter of fact, several fossil groups near 
the evolutionary transition between the two classes, known only from bony 
remains, cannot be satisfactorily placed in one or the other class because 
information on their life histories is not available.  See  AMPHIBIA. 
    In recent reptiles, discrete dry horny scales cover the body; mucous 
glands are lacking in the skin; there is a single occipital condyle; mammary 
glands are lacking; the middle ear region has a single ossicle (stapes), if 
functional, transmitting vibrations to inner ear; the heart is composed of two 
auricles and a single partially divided ventricle (fully divided in 
Crocodilia); and pleural cavities and peritoneal cavities are continuous 
(separated in crocodilians). Although primarily oviparous, some lizards and 
snakes are ovoviviparous or viviparous. 
    The Mammalia are descended from one reptilian as is the class Aves. Both 
groups differ markedly from the Reptilia. 
 �34s[Jay M. Savage]
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�6v�2v�32sReproductive system disorders   �21m �33s 
    Those disorders which involve the structures of the female and male 
reproductive systems. 
  
    Female system.  Failure of the ovaries to form normally results in short 
stature, sterility, lack of development of female secondary sex 
characteristics, such as breast growth, fat deposition in buttocks, thighs, 
and mons pubis, and female escutcheon. Destruction of the ovaries after 
puberty results in loss of fertility, cessation of menses, and the loss of 
secondary sex characteristics. 
    Neoplastic enlargement of the ovary can be cystic or solid, benign or 
malignant. Malignant ovarian tumors are commonly asymptomatic in their early 
stages, and may be quite widely spread in the pelvic and abdominal cavity 
before they are discovered.  See  OVARY. 
    Inflammation  is the most common disorder of the Fallopian tubes, and if 
it is repeated or severe, destruction of the tubal lining with closure of the 
outer ends of the tubes can occur. This inflammation can be caused by several 
bacterial organisms. Sterility commonly results because the tube is 
permanently closed to passage of eggs and sperm. 
    The second most common problem involving the tube is pregnancy. The egg is 
fertilized in the outer portion of the tube and descends to implant in the 
uterus, but in some cases the passage is delayed and the conceptus attaches to 
the wall of the tube. The tube has a small lumen and thin wall, and the 
growing pregnancy quickly enlarges and grows through the tube, leading usually 
to rupture and hemorrhage into the peritoneal cavity. 
    The muscle (myometrium) and lining (endometrium) of the uterus are 
susceptible to various problems, including tumors, infections, and hormonal 
derangements. Benign tumors of the myometrium (fibroids) are a common 
disorder, producing irregular enlargement of the uterus and sometimes causing 
pain, obstruction of the urinary tract, and heavy vaginal bleeding. 
    Infection of the endocervical glands with gonococcus or chlamydia 
trachomatis agent can occur. This may be asymptomatic, except for producing a 
mucopurulent discharge, or it may cause pain when the cervix is manipulated, 
particularly during intercourse. The infection can ascend from this area into 
the internal genital organs and adjacent structures.  See  GONORRHEA. 
    Cancer of the surface tissue of the cervix is a relatively common, 
potentially lethal problem. Early malignant changes can be detected by 
examining properly stained exfoliated cells in the manner of G. N. 
Papanicolaou. This cancer produces few symptoms, except for irregular bleeding 
(usually postcoital).  
    Inflammation of the lining of the vagina can be due to several common 
organisms: Monilia, a fungus; Trichomonas, a protozoan; and Haemophilus 
vaginalis, a bacterium. Infection of the female external genitalia can be due 
to Monilia, bacterial agents, and viruses. This infection can be diffuse and 
widespread, or localized to hair follicles, Bartholin's glands, or lesions 
such as warts or ulcers. Inflammation can be caused by allergic reactions to 
soap, powders, semen, lubricant, or even clothing. 
  
    Male system.  During fetal life the testicles form in the abdominal cavity 
near the kidneys and migrate down into the scrotum. Failure of descent 
permanently damages the sperm-producing cells causing infertility but allows 
the interstition cells which produce hormones to survive. Other causes of male 
infertility are organic problems which block passage of seminal fluid or 
interfere with sperm production. 
    The testes are sites of malignant tumors which carry a high mortality 
rate. Undescended testicles are more susceptible to malignant transformation, 
and for that reason should be removed when discovered, and appropriate 
hormonal replacement instituted.  See  TESTIS. 
    The major disorder of the epididymis is infection and inflammation, which 
can lead to scarring and permanent blockage of the ducts. The vas deferens and 
seminal vesicles are rarely afflicted by disease, although the network of 
veins surrounding the vas can become engorged and tortuous, and is called a 
variococoele. Inflammation is the chief disorder of the urethra causing 
dysuria and a discharge. 
    Inflammation and development of small calculi are relatively minor 
ailments of the prostate. Benign overgrowth of this gland is the most common 
and troublesome complaint. This enlargement results in constriction of the 
urethra, which obstructs urinary outflow and leads to an increasing residual 
of urine in the bladder. Cance of the prostate is another relatively frequent 
problem in older males. This tumor spreads primarily to bones, where it is 
markedly painful. 
    Inflammation (balanitis) or narrowing (phimosis) of the foreskin which 
covers the glans penis can occur and may interfere with urination. These 
conditions are unknown in parts of the world where the foreskin is removed for 
social or medical reasons. Viral infections can produce warty growths 
(condyloma acuminata) or painful ulcers (herpetic lesions) of the glans or 
shaft of the penis, and a syphilitic infection can result in the firm, 
painless ulcer known as a chancre. 
    Disorders of erectile capability range from tumescence unaccompanied by 
sexual desire (Peyronie's disease, priapism) to impotence, both psychologic 
and secondary to old age. Carcinoma can arise from the surface epithelium of 
the penis and spread both locally and via the lymphatic system to the nodes of 
the groin. This disease is virtually unknown in populations where males are 
circumcised at birth. 
 �34s[Gay M. Guzinski]
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�6v�2v�32sReproductive system   �21m �33s 
    The structures concerned with the production of sex cells (gametes) and 
perpetuation of the species. The reproductive function constitutes the only 
vertebrate physiological function that necessitates the existence of two 
morphologically different kinds of individuals in each animal species, the 
males and the females (sexual dimorphism). The purpose of the reproductive 
function is fertilization, that is, the fusion of a male and a female sex cell 
produced by two distinct individuals. 
  
    Anatomy.  Egg cells, or ova, and sperm cells, or spermatozoa, are formed 
in the primary reproductive organs, which are collectively known as gonads. 
Those of the male are called testes; those of the female are ovaries. The 
gonads are paired structures, although in some forms what appears to be an 
unpaired gonad is the result either of fusion of paired structures or of 
unilateral degeneration. 
    The reproductive elements formed in the gonads must be transported to the 
outside of the body. In most vertebrates, ducts are utilized for this purpose. 
These ducts, together with the structures that serve to bring the gametes of 
both sexes together, are known as accessory sex organs. The structures used to 
transport the reproductive cells in the male are known as deferent ducts and 
those of the female as oviducts. In a few forms no ducts are present in either 
sex, and eggs and sperm escape from the body cavity through genital or 
abdominal pores. 
    Oviducts, except in teleosts and a few other fishes, are modifications of 
Muellerian ducts formed during early embryonic development. In all mammals, 
each differentiates into an anterior, nondistensible Fallopian tube and a 
posterior, expanded uterus. In all mammals except monotremes the uterus leads 
to a terminal vagina which serves for the reception of the penis of the male 
during copulation. The lower part, or neck, of the uterus is usually 
telescoped into the vagina to a slight degree. This portion is referred to as 
the cervix. 
    In most vertebrates the reproductive ducts in both sexes open posteriorly 
into the cloaca. In some, modifications of the cloacal region occur and the 
ducts open separately to the outside or, in the male, join the excretory ducts 
to emerge by a common orifice.  See  COPULATORY ORGAN; OVARY; PENIS; 
REPRODUCTION (ANIMAL); TESTIS. 
 �34s[Charles K. Weichert]
�33s 
  
    Physiology.  The physiological process by which a living being gives rise 
to another of its kind is considered one of the outstanding characteristics of 
plants and animals. It is one of the two great drives of all animals; 
self-preservation and racial perpetuation. 
  
    Estrous and menstrual cycles. The cyclic changes of reproductive 
activities in mammalian females are known as estrous or menstrual cycles. 
    Most mammalian females accept males only at estrus (heat). Estrus in 
mammals can occur several times in one breeding season; the mare, ewe, and rat 
come to estrus every 21, 16, and 5 days respectively if breeding does not take 
place. This condition is called polyestrus. The bitch is monestrous; she has 
only one heat, or estrus, to the breeding season and if not served then, she 
does not come into heat again for a prolonged interval, 4 - 6 months according 
to different breeds. In monestrous and seasonally polyestrous species the 
period of sexual quiescence between seasons is called anestrus.  See  ESTRUS. 
    The reproductive cycle of the female in the primate and human is well 
marked by menstruation, the period of vaginal blood flow. Menstruation does 
not correspond to estrus but occurs between the periods of ovulation at the 
time the corpus luteum declines precipitously.  See  MENSTRUATION. 
  
    Mating. Mating, also called copulation or coitus, is the synchronized 
bodily activity of the two sexes which enables them to deposit their gametes 
in close contact. It is essential for successful fertilization because sperm 
and ovum have a very limited life span. 
    The logistics of sperm transport to the site of fertilization in the 
oviduct present many interesting features in mammals, but it is important to 
distinguish between passive transport of sperm cells in the female genital 
tract, and sperm migration, which clearly attributes significance to the 
intrinsic motility of the cell. Viable spermatozoa are actively motile, and 
although myometrial contractions play a major role in sperm transport through 
the uterus, progressive motility does contribute to migration into and within 
the oviducts. Even though a specific attractant substance for spermatozoa has 
not yet been demonstrated to be released from mammalian eggs or their 
investments, some form of chemotaxis may contribute to the final phase of 
sperm transport and orientation toward the egg surface. 
    Although in most mammalian species the oocyte is shed from the Graafian 
follicle in a condition suitable for fertilization, ejaculated spermatozoa 
must undergo some form of physiological change in the female reproductive 
tract before they can penetrate the egg membranes. The interval required for 
this change varies according to species, and the process is referred to as 
capacitation. The precise changes that constitute capacitation remain unknown, 
although there is strong evidence that they are - at least in part - 
membrane-associated phenomena, particularly in the region of the sperm head, 
that permit release of the lytic acrosomal enzymes with which the spermatozoon 
gains access to the vitelline surface of the egg. 
    Fertilization takes place in the oviducts of mammals and the fertilized 
eggs or embryos do not descend to the uterus for some 3 to 4 days in most 
species. During this interval, the embryo undergoes a series of mitotic 
divisions until it comprises a sphere of 8 or 16 cells and is termed a morula. 
Formation of a blastocyst occurs when the cells of the morula rearrange 
themselves around a central, fluid-filled cavity, the blastocoele. As the 
blactocyst develops within the uterine environment, it sheds its protective 
coat and undergoes further differentiation before developing an intimate 
association with the endometrium, which represents the commencement of 
implantation or nidation. 
    Association of the embryo with the uterine epithelium, either by 
superficial attachment or specific embedding in or beneath the endometrium, 
leads in due course to the formation of a placenta and complete dependence of 
the differentiating embryo upon metabolic support from the mother. 
Implantation and placentation exhibit a variety of forms, but in all instances 
the hormonal status of the mother is of great importance in determining 
whether or not implantation can proceed.  See  PREGNANCY. 
  
    Endocrine function. The endocrine glands secrete certain substances 
(hormones) which are necessary for growth, metabolism, reproduction, response 
to stress, and various other physiological processes. The endocrine glands 
most concerned with the process of reproduction are the pituitary and the 
gonads.  
    The posterior lobe of the pituitary gland secretes two neurohumoral 
agents, vasopressin and oxytocin. These are involved in reproduction only 
indirectly, through their effect on uterine contractility in labor and on the 
release of milk from the mammary gland when a suckling stimulus is applied. 
The anterior lobe secretes a variety of trophic hormones, including two 
gonadotrophic hormones, the follicle-stimulating hormone (FSH) and the 
luteinizing or interstitial-cell stimulating hormone (LH or ICSH). These 
hormones act directly on both ovaries and testes.  See  ADENOHYPOPHYSIS 
HORMONE; PITUITARY GLAND. 
    The gonadal (steroidal) hormones control the secretion of gonadotrophins 
by acting on the hypothalamus. It has been suggested that steroids act by 
means of a ``negative feedback''; that is, high levels of circulating gonadal 
hormones stop further release of gonadotropins. However, although this is true 
for experiments involving pharmacological doses of such hormones, it may not 
be the case with endogenous physiological levels. It is certainly true that 
less steroid is required to inhibit pituitary function in the female than in 
the male. Under certain circumstances small doses of gonadal hormones can 
stimulate release of gonadotropic hormones from the pituitary. Estrogen can 
stimulate the release of LH; hence the occurrence of ovulation in rats, 
rabbits, sheep, and women. Progesterone can also facilitate ovulation in 
persistently estrous rats, in chickens, and in estrous sheep and monkeys.  See 
  ESTROGEN; PROGESTERONE. 
    The formation of gametes (spermatogenesis and oogenesis) is controlled by 
anterior pituitary hormones. The differentiation of male and female 
reproductive tracts is influenced, and mating behavior and estrous cycles are 
controlled, by male or female hormones. The occurrence of the breeding season 
is mainly dependent upon the activity of the anterior lobe of the pituitary, 
which is influenced through the nervous system by external factors, such as 
light and temperature. The  transportation of ova from the ovary to the 
Fallopian tube and their subsequent transportation, development, and 
implantation in the uterus are controlled by a balanced ratio between estrogen 
and progesterone. Furthermore, it is known that estrogens, androgens, and 
progesterone can all have the effect of inhibiting the production or the 
secretion, or both, of gonadotrophic hormones, permitting the cyclic changes 
of reproductive activity among different animals. 
    Mammary glands are essential for the nursing of young. Their growth, 
differentation, and secretion of milk, and in fact the whole process of 
lactation, are controlled by pituitary hormones as well as by estrogen and 
progesterone. Other glands and physiological activities also influence 
lactation, although this is largely via the trophic support of other pituitary 
hormones.  
 �34s[M. C. Chang ;Michael J. K. Har]
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�6v�2v�32sReproductive behavior   �21m �33s 
    The systems of behavior pattern by means of which, in different types of 
animals, the sperm is brought to the egg and the parental care of the 
resulting young is ensured. The mere existence of eggs and sperm and of 
reproductive organs is not enough to guarantee that the eggs will reach the 
sperm. There must also be, for each species, a set of behavior patterns 
through which the male and female approach each other and, by their mutual 
reactions to each other, facilitate the movement of the sperm toward the egg. 
Similarly, there must exist patterns of parental behavior, appropriate for 
each type of animal, by which the parent approaches the young and provides the 
necessary care. The various patterns of reproductive behavior thus constitute 
one of the ways in which animals are adapted to their environment. These 
patterns have evolved under the influence of natural selections just as have 
other aspects of structure, function, and behavior. 
 �34s[Daniel S. Lehrman]
�33s 
    In birds, both auditory and visual stimulations are effective in 
courtship. In some species of bird the male reacts differently and 
appropriately to stuffed and mounted specimens of males or of females of his 
own species. Female birds of many species are attracted by the courtship songs 
of the males. Visual, auditory, tactual, and chemical stimuli have all been 
shown to be effective in various of the lower vertebrates (fish, amphibians, 
and reptiles). 
  
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�6v�2v�32sReproduction (plant)   �21m �33s 
    Plants produce new individuals by either asexual or sexual means. 
  
    Asexual reproduction.  In simple one-celled plants, asexual reproduction 
is accomplished by division of the vegetative cell and the subsequent growth 
of the two nearly equal daughter cells which form new individuals. This is 
known as simple (binary) fission. Another method, known as budding or 
gemmation, occurs in the unicellular yeast plant. In this plant a small 
protuberance, which later becomes detached by constriction, emerges from the 
cell. The new cell thus formed may develop directly into a new plant, or it 
may undergo division and give rise to more new individuals. In some liverworts 
and mosses, the plants produce small multicellular structures called gemmae, 
which, when detached, may grow, forming new plants. Still another method 
occurring in many kinds of plants is effected by means of spores, which are 
usually formed by multiple division of an ordinary vegetative cell or in a 
specialized organ (sporangium). 
    Asexual reproduction also takes place by fragmentation, in which a part of 
the vegetative plant becomes detached and develops into a new plant. Many of 
the higher plants have underground stems, such as rhizomes, tubers, bulbs, or 
corms, which are efficient agents in vegetative or asexual propagation.  See  
BUDDING; GRAFTING OF PLANTS; STEM CUTTINGS. 
  
    Sexual reproduction.  Sexual reproduction appeared early in the evolution 
of the plant kingdom. In this kind of reproduction, specialized sex cells 
called gametes fuse, forming a zygote which has the potential to produce a new 
individual. In some lower plants such as the alga Spirogyra, two protoplasts 
of ordinary vegetative cells may serve as gametes and unite, but in others the 
protoplast undergoes multiple fission, producing many motile cells. If these 
are relatively large and few in number, they will function as spores, 
germinating and producing new plants asexually. However, if these motile cells 
are quite small and numerous, they will function as gametes, fusing in pairs 
to form zygotes. When these gametes are similar in size, appearance, and 
structure they are called isogametes; when morphologically different, they are 
heterogametes. Therefore, fusion of isogametes is known as isogamy, and that 
of heterogametes as heterogamy. In heterogamy, the large, nonmotile gamete 
(female) is the egg and the smaller, motile gamete (male) is the sperm. In 
thalloid plants, if the plant (thallus) produces both kinds of gametes, it is 
homothallic; if only one kind, it is heterothallic.  See  THALLOBIONTA. 
    Early in the course of evolution, certain cells of plants became 
specialized as sex organs to produce differentiated gametes. The male organ 
producing numerous motile gametes (sperms) is called the antheridium, and the 
female organ usually producing only one egg is the oogonium. In more highly 
organized plants, the one-celled oogonium is replaced by a more complicated, 
multicellular archegonium containing one egg. The plant bearing antheridia 
alone is the male gametophyte, and the one with archegonia is the female 
gametophyte. Eventually, the seed plants appeared at the top of the 
evolutionary scale, bearing flowers with stamens and pistils. The stamens bear 
anthers (microsporangia) which produce the pollen grains (microspores) 
comparable to the asexual spores found in lower plants. In the same flower, or 
in another, there is a pistil composed usually of an ovary, a style, and a 
stigma. The ovary contains one or more ovules (megasporangia), in which the 
megaspores are formed. One of these will develop into the embryo sac (female 
gametophyte). The pollen grain will germinate only on the stigma of a flower. 
The transfer of pollen to the stigma (pollination) is accomplished by various 
agencies; some of these agencies are gravitation, wind, insects, birds, and 
other animals.  See  EMBRYOBIONTA; FLOWER; POLLINATION. 
    After pollination the pollen tube penetrates the ovule and enters the 
embryo sac, where it swells and bursts, releasing the two sperms. The tube 
nucleus degenerates. One of the sperms unites with the fusion nucleus, forming 
the so-called triple fusion (endosperm) nucleus which divides and grows into 
the endosperm, a food-accumulating tissue. The other sperm unites with the 
egg, forming a zygote which may soon begin to grow at the expense of the 
endosperm, giving rise to the embryo. 
    Pollination and fertilization may affect structures outside the embryo and 
endosperm, resulting in abscission of the pistil, or they may affect chemical 
composition, color, and time of ripening of the fruit. Such influence is 
called ectogony. In triple fusion, genetic factors which are responsible for 
certain characteristics of the endosperm such as color, form, shape, and 
chemical composition may be introduced by the male gamete. Appearance of these 
characteristics is known as xenia. 
    As the embryo and endosperm are developing, the ovule enlarges, its 
integuments become modified into protective coverings (seed coats) and the 
seed is fully formed. 
    Seed development is accompanied by changes in the ovary, which enlarges 
and becomes the fruit. The fruits of plants display a wide range in form, 
size, texture, design, and chemical composition, but in all cases fruits aid 
in dissemination of seeds.  See  FRUIT; SEED; SEED GERMINATION. 
 �34s[Perry D. Strausbaugh /Earl L.]
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�6v�2v�32sReproduction (animal)   �21m �33s 
    The formation of new individuals, which may occur by asexual or sexual 
methods. In the asexual methods, which occur mainly among the lower animals, 
the offspring are derived from a single individual. Sexual methods are general 
throughout the animal kingdom, with offspring ordinarily derived from the 
paired union of special cells, the gametes, from two individuals. Basic to all 
processes of reproduction is the origin of the new individual from one or more 
living cells of the parent or parents. 
  
    Asexual reproduction.  Asexual processes of reproduction include binary 
fission, multiple fission, fragmentation, budding, and polyembryony. Among the 
protozoa and lower metazoa, these are common methods of reproduction. However, 
the last-mentioned process can occur in mammals, including humans. 
    Binary fission involves an equal, or nearly equal, longitudinal or 
transverse splitting of the body of the parent into two parts, each of which 
grows to parental size and form. This method of reproduction occurs regularly 
among protozoans. 
    Multiple fission, schizogony, or sporulation produces several new 
individuals from a single parent. It is common among the Sporozoa, such as the 
malarial parasite, which form cystlike structures containing many cells, each 
of which gives rise to a new individual. 
    Fragmentation is a form of fission occurring in some metazoans, especially 
the Platyhelminthes, or flatworms; the Nemertinea, or ribbon worms; and the 
Annelida, or segmented worms. The parent worm breaks up into a number of 
parts, each of which regenerates missing structures to form a whole organism. 
    Budding is a form of asexual reproduction in which the new individual 
arises from a relatively small mass of cells that initially forms a growth or 
bud on the parental body. The bud may assume parental form either before 
separation from the body of the parent as in external budding, or afterward, 
as in internal budding. External budding is common among sponges, 
coelenterates, bryozoans, flatworms, and tunicates. Internal budding occurs 
among fresh-water sponges and bryozoans. In the sponges the internal buds, 
termed gemmules, consist of groups of primitive cells surrounded by a dense 
capsule formed by the body wall. If the parent animal dies as a result of 
desiccation or low temperature, the cells of the gemmules can later be 
released and form new sponges. In the bryozoans the similarly functioning buds 
are known as statoblasts. 
    Polyembryony is a form of asexual reproduction, occurring at an early 
developmental stage of a sexually produced embryo, in which two or more 
offspring are derived from a single egg. Examples are found scattered 
throughout the animal kingdom, including humans; in humans it is represented 
by identical twins, triplets, or quadruplets. 
  
    Sexual reproduction.  Sexual reproduction in animals assumes various forms 
which may be classified under conjugation, autogamy, fertilization (syngamy), 
and parthenogenesis. Basically, the various processes all involve the 
occurrence of certain special nuclear changes, termed meiotic divisions, 
preliminary to the production of the new individual. See GAMETOGENESIS; 
MEIOSIS.  
    Conjugation occurs principally among the ciliate protozoans, such as 
Paramecium, and involves a temporary union of two individuals during which 
each is ``fertilized'' by a micronucleus from the other. 
    In autogamy the nuclear changes described for conjugation take place, but 
since there is no mating, there is no transfer of micronuclei. Instead, the 
prospective migratory micronucleus reunites with the stationary one. The 
process may be considered related to parthenogenesis. 
    Fertilization, or syngamy, comprises a series of events in which two 
cells, the gametes, fuse and their nuclei, which had previously undergone 
meiotic divisions, fuse. In metazoans, the gametes are of two morphologically 
distinct types: spermatozoa, or microgametes, and eggs, also called ova or 
macrogametes. These types are produced by male and female animals, 
respectively, but in some cases both may be produced by a single, 
hermaphroditic, individual. The nucleus of the spermatozoon has half the 
number of chromosomes characteristic of the ordinary (somatic) cells of the 
animal. The nucleus of the ripe egg in some animals, for instance, 
coelenterates and echinoderms, also has attained this haploid condition, but 
in most species of animals it is at an early stage of the meiotic divisions 
when ready for fertilization. In the latter situation, the meiotic divisions 
of the egg, characterized by formation of small, nonfunctional cells termed 
polar bodies, are completed after the sperm enters, whereupon the haploid egg 
nucleus fuses with the haploid sperm nucleus. Fertilization thus produces a 
zygote with the diploid chromosome number typical of the somatic cells of the 
species (23 pairs in humans) and this is maintained during the ensuing cell 
divisions.  
    Parthenogenesis is the development of the egg without fertilization by a 
spermatozoon. It is listed as a form of sexual reproduction because it 
involves development from a gamete. Rotifers, crustaceans, and insects are the 
principal groups in which it occurs naturally. It has also been induced 
(artificial parthenogenesis) in species from all the major phyla by various 
kinds of chemical or physical treatment of the unfertilized egg. Even in 
mammals, several adult rabbits have reportedly been thus produced.  See  
ESTRUS; OOGENESIS; OVUM; SPERM CELL; SPERMATOGENESIS. 
 �34s[Albert Tyler /Howard L. Hamil]
�33s 
  
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�6v�2v�32sReproduction (animal) �21m �33s
    The formation of new individuals, which may occur by asexual or sexual
methods. In the asexual methods, which occur mainly among the lower animals,
the offspring are derived from a single individual. Sexual methods are general
throughout the animal kingdom, with offspring ordinarily derived from the
paired union of special cells, the gametes, from two individuals. Basic to all
processes of reproduction is the origin of the new individual from one or more
living cells of the parent or parents.

    Asexual reproduction.  Asexual processes of reproduction include binary
fission, multiple fission, fragmentation, budding, and polyembryony. Among the
protozoa and lower metazoa, these are common methods of reproduction. However,
the last-mentioned process can occur in mammals, including humans.
    Binary fission involves an equal, or nearly equal, longitudinal or
transverse splitting of the body of the parent into two parts, each of which
grows to parental size and form. This method of reproduction occurs regularly
among protozoans.
    Multiple fission, schizogony, or sporulation produces several new
individuals from a single parent. It is common among the Sporozoa, such as the
malarial parasite, which form cystlike structures containing many cells, each
of which gives rise to a new individual.
    Fragmentation is a form of fission occurring in some metazoans, especially
the Platyhelminthes, or flatworms; the Nemertinea, or ribbon worms; and the
Annelida, or segmented worms. The parent worm breaks up into a number of
parts, each of which regenerates missing structures to form a whole organism.
    Budding is a form of asexual reproduction in which the new individual
arises from a relatively small mass of cells that initially forms a growth or
bud on the parental body. The bud may assume parental form either before
separation from the body of the parent as in external budding, or afterward,
as in internal budding. External budding is common among sponges,
coelenterates, bryozoans, flatworms, and tunicates. Internal budding occurs
among fresh-water sponges and bryozoans. In the sponges the internal buds,
termed gemmules, consist of groups of primitive cells surrounded by a dense
capsule formed by the body wall. If the parent animal dies as a result of
desiccation or low temperature, the cells of the gemmules can later be
released and form new sponges. In the bryozoans the similarly functioning buds
are known as statoblasts.
    Polyembryony is a form of asexual reproduction, occurring at an early
developmental stage of a sexually produced embryo, in which two or more
offspring are derived from a single egg. Examples are found scattered
throughout the animal kingdom, including humans; in humans it is represented
by identical twins, triplets, or quadruplets.

    Sexual reproduction.  Sexual reproduction in animals assumes various forms
which may be classified under conjugation, autogamy, fertilization (syngamy),
and parthenogenesis. Basically, the various processes all involve the
occurrence of certain special nuclear changes, termed meiotic divisions,
preliminary to the production of the new individual. See GAMETOGENESIS;
MEIOSIS.
    Conjugation occurs principally among the ciliate protozoans, such as
Paramecium, and involves a temporary union of two individuals during which
each is ``fertilized'' by a micronucleus from the other.
    In autogamy the nuclear changes described for conjugation take place, but
since there is no mating, there is no transfer of micronuclei. Instead, the
prospective migratory micronucleus reunites with the stationary one. The
process may be considered related to parthenogenesis.
    Fertilization, or syngamy, comprises a series of events in which two
cells, the gametes, fuse and their nuclei, which had previously undergone
meiotic divisions, fuse. In metazoans, the gametes are of two morphologically
distinct types: spermatozoa, or microgametes, and eggs, also called ova or
macrogametes. These types are produced by male and female animals,
respectively, but in some cases both may be produced by a single,
hermaphroditic, individual. The nucleus of the spermatozoon has half the
number of chromosomes characteristic of the ordinary (somatic) cells of the
animal. The nucleus of the ripe egg in some animals, for instance,
coelenterates and echinoderms, also has attained this haploid condition, but
in most species of animals it is at an early stage of the meiotic divisions
when ready for fertilization. In the latter situation, the meiotic divisions
of the egg, characterized by formation of small, nonfunctional cells termed
polar bodies, are completed after the sperm enters, whereupon the haploid egg
nucleus fuses with the haploid sperm nucleus. Fertilization thus produces a
zygote with the diploid chromosome number typical of the somatic cells of the
species (23 pairs in humans) and this is maintained during the ensuing cell
divisions.
    Parthenogenesis is the development of the egg without fertilization by a
spermatozoon. It is listed as a form of sexual reproduction because it
involves development from a gamete. Rotifers, crustaceans, and insects are the
principal groups in which it occurs naturally. It has also been induced
(artificial parthenogenesis) in species from all the major phyla by various
kinds of chemical or physical treatment of the unfertilized egg. Even in
mammals, several adult rabbits have reportedly been thus produced.  See
ESTRUS; OOGENESIS; OVUM; SPERM CELL; SPERMATOGENESIS.
 �34s[Albert Tyler /Howard L. Hamilton]
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�6v�2v�32sReproduction (plant) �21m �33s
    Plants produce new individuals by either asexual or sexual means.

    Asexual reproduction.  In simple one-celled plants, asexual reproduction
is accomplished by division of the vegetative cell and the subsequent growth
of the two nearly equal daughter cells which form new individuals. This is
known as simple (binary) fission. Another method, known as budding or
gemmation, occurs in the unicellular yeast plant. In this plant a small
protuberance, which later becomes detached by constriction, emerges from the
cell. The new cell thus formed may develop directly into a new plant, or it
may undergo division and give rise to more new individuals. In some liverworts
and mosses, the plants produce small multicellular structures called gemmae,
which, when detached, may grow, forming new plants. Still another method
occurring in many kinds of plants is effected by means of spores, which are
usually formed by multiple division of an ordinary vegetative cell or in a
specialized organ (sporangium).
    Asexual reproduction also takes place by fragmentation, in which a part of
the vegetative plant becomes detached and develops into a new plant. Many of
the higher plants have underground stems, such as rhizomes, tubers, bulbs, or
corms, which are efficient agents in vegetative or asexual propagation.  See
BUDDING; GRAFTING OF PLANTS; STEM CUTTINGS.

    Sexual reproduction.  Sexual reproduction appeared early in the evolution
of the plant kingdom. In this kind of reproduction, specialized sex cells
called gametes fuse, forming a zygote which has the potential to produce a new
individual. In some lower plants such as the alga Spirogyra, two protoplasts
of ordinary vegetative cells may serve as gametes and unite, but in others the
protoplast undergoes multiple fission, producing many motile cells. If these
are relatively large and few in number, they will function as spores,
germinating and producing new plants asexually. However, if these motile cells
are quite small and numerous, they will function as gametes, fusing in pairs
to form zygotes. When these gametes are similar in size, appearance, and
structure they are called isogametes; when morphologically different, they are
heterogametes. Therefore, fusion of isogametes is known as isogamy, and that
of heterogametes as heterogamy. In heterogamy, the large, nonmotile gamete
(female) is the egg and the smaller, motile gamete (male) is the sperm. In
thalloid plants, if the plant (thallus) produces both kinds of gametes, it is
homothallic; if only one kind, it is heterothallic.  See  THALLOBIONTA.
    Early in the course of evolution, certain cells of plants became
specialized as sex organs to produce differentiated gametes. The male organ
producing numerous motile gametes (sperms) is called the antheridium, and the
female organ usually producing only one egg is the oogonium. In more highly
organized plants, the one-celled oogonium is replaced by a more complicated,
multicellular archegonium containing one egg. The plant bearing antheridia
alone is the male gametophyte, and the one with archegonia is the female
gametophyte. Eventually, the seed plants appeared at the top of the
evolutionary scale, bearing flowers with stamens and pistils. The stamens bear
anthers (microsporangia) which produce the pollen grains (microspores)
comparable to the asexual spores found in lower plants. In the same flower, or
in another, there is a pistil composed usually of an ovary, a style, and a
stigma. The ovary contains one or more ovules (megasporangia), in which the
megaspores are formed. One of these will develop into the embryo sac (female
gametophyte). The pollen grain will germinate only on the stigma of a flower.
The transfer of pollen to the stigma (pollination) is accomplished by various
agencies; some of these agencies are gravitation, wind, insects, birds, and
other animals.	See  EMBRYOBIONTA; FLOWER; POLLINATION.
    After pollination the pollen tube penetrates the ovule and enters the
embryo sac, where it swells and bursts, releasing the two sperms. The tube
nucleus degenerates. One of the sperms unites with the fusion nucleus, forming
the so-called triple fusion (endosperm) nucleus which divides and grows into
the endosperm, a food-accumulating tissue. The other sperm unites with the
egg, forming a zygote which may soon begin to grow at the expense of the
endosperm, giving rise to the embryo.
    Pollination and fertilization may affect structures outside the embryo and
endosperm, resulting in abscission of the pistil, or they may affect chemical
composition, color, and time of ripening of the fruit. Such influence is
called ectogony. In triple fusion, genetic factors which are responsible for
certain characteristics of the endosperm such as color, form, shape, and
chemical composition may be introduced by the male gamete. Appearance of these
characteristics is known as xenia.
    As the embryo and endosperm are developing, the ovule enlarges, its
integuments become modified into protective coverings (seed coats) and the
seed is fully formed.
    Seed development is accompanied by changes in the ovary, which enlarges
and becomes the fruit. The fruits of plants display a wide range in form,
size, texture, design, and chemical composition, but in all cases fruits aid
in dissemination of seeds.  See  FRUIT; SEED; SEED GERMINATION.
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sReproductive behavior �21m �33s
    The systems of behavior pattern by means of which, in different types of
animals, the sperm is brought to the egg and the parental care of the
resulting young is ensured. The mere existence of eggs and sperm and of
reproductive organs is not enough to guarantee that the eggs will reach the
sperm. There must also be, for each species, a set of behavior patterns
through which the male and female approach each other and, by their mutual
reactions to each other, facilitate the movement of the sperm toward the egg.
Similarly, there must exist patterns of parental behavior, appropriate for
each type of animal, by which the parent approaches the young and provides the
necessary care. The various patterns of reproductive behavior thus constitute
one of the ways in which animals are adapted to their environment. These
patterns have evolved under the influence of natural selections just as have
other aspects of structure, function, and behavior.
 �34s[Daniel S. Lehrman]
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�6v�2v�32sReproductive system �21m �33s
    The structures concerned with the production of sex cells (gametes) and
perpetuation of the species. The reproductive function constitutes the only
vertebrate physiological function that necessitates the existence of two
morphologically different kinds of individuals in each animal species, the
males and the females (sexual dimorphism). The purpose of the reproductive
function is fertilization, that is, the fusion of a male and a female sex cell
produced by two distinct individuals.

    Anatomy.  Egg cells, or ova, and sperm cells, or spermatozoa, are formed
in the primary reproductive organs, which are collectively known as gonads.
Those of the male are called testes; those of the female are ovaries. The
gonads are paired structures, although in some forms what appears to be an
unpaired gonad is the result either of fusion of paired structures or of
unilateral degeneration.
    The reproductive elements formed in the gonads must be transported to the
outside of the body. In most vertebrates, ducts are utilized for this purpose.
These ducts, together with the structures that serve to bring the gametes of
both sexes together, are known as accessory sex organs. The structures used to
transport the reproductive cells in the male are known as deferent ducts and
those of the female as oviducts. In a few forms no ducts are present in either
sex, and eggs and sperm escape from the body cavity through genital or
abdominal pores.
    Oviducts, except in teleosts and a few other fishes, are modifications of
Muellerian ducts formed during early embryonic development. In all mammals,
each differentiates into an anterior, nondistensible Fallopian tube and a
posterior, expanded uterus. In all mammals except monotremes the uterus leads
to a terminal vagina which serves for the reception of the penis of the male
during copulation. The lower part, or neck, of the uterus is usually
telescoped into the vagina to a slight degree. This portion is referred to as
the cervix.
    In most vertebrates the reproductive ducts in both sexes open posteriorly
into the cloaca. In some, modifications of the cloacal region occur and the
ducts open separately to the outside or, in the male, join the excretory ducts
to emerge by a common orifice.	See  COPULATORY ORGAN; OVARY; PENIS;
REPRODUCTION (ANIMAL); TESTIS.
 �34s[Charles K. Weichert]
�33s

    Physiology.  The physiological process by which a living being gives rise
to another of its kind is considered one of the outstanding characteristics of
plants and animals. It is one of the two great drives of all animals;
self-preservation and racial perpetuation.

    Estrous and menstrual cycles. The cyclic changes of reproductive
activities in mammalian females are known as estrous or menstrual cycles.
    Most mammalian females accept males only at estrus (heat). Estrus in
mammals can occur several times in one breeding season; the mare, ewe, and rat
come to estrus every 21, 16, and 5 days respectively if breeding does not take
place. This condition is called polyestrus. The bitch is monestrous; she has
only one heat, or estrus, to the breeding season and if not served then, she
does not come into heat again for a prolonged interval, 4 - 6 months according
to different breeds. In monestrous and seasonally polyestrous species the
period of sexual quiescence between seasons is called anestrus.  See  ESTRUS.
    The reproductive cycle of the female in the primate and human is well
marked by menstruation, the period of vaginal blood flow. Menstruation does
not correspond to estrus but occurs between the periods of ovulation at the
time the corpus luteum declines precipitously.	See  MENSTRUATION.

    Mating. Mating, also called copulation or coitus, is the synchronized
bodily activity of the two sexes which enables them to deposit their gametes
in close contact. It is essential for successful fertilization because sperm
and ovum have a very limited life span.
    The logistics of sperm transport to the site of fertilization in the
oviduct present many interesting features in mammals, but it is important to
distinguish between passive transport of sperm cells in the female genital
tract, and sperm migration, which clearly attributes significance to the
intrinsic motility of the cell. Viable spermatozoa are actively motile, and
although myometrial contractions play a major role in sperm transport through
the uterus, progressive motility does contribute to migration into and within
the oviducts. Even though a specific attractant substance for spermatozoa has
not yet been demonstrated to be released from mammalian eggs or their
investments, some form of chemotaxis may contribute to the final phase of
sperm transport and orientation toward the egg surface.
    Although in most mammalian species the oocyte is shed from the Graafian
follicle in a condition suitable for fertilization, ejaculated spermatozoa
must undergo some form of physiological change in the female reproductive
tract before they can penetrate the egg membranes. The interval required for
this change varies according to species, and the process is referred to as
capacitation. The precise changes that constitute capacitation remain unknown,
although there is strong evidence that they are - at least in part -
membrane-associated phenomena, particularly in the region of the sperm head,
that permit release of the lytic acrosomal enzymes with which the spermatozoon
gains access to the vitelline surface of the egg.
    Fertilization takes place in the oviducts of mammals and the fertilized
eggs or embryos do not descend to the uterus for some 3 to 4 days in most
species. During this interval, the embryo undergoes a series of mitotic
divisions until it comprises a sphere of 8 or 16 cells and is termed a morula.
Formation of a blastocyst occurs when the cells of the morula rearrange
themselves around a central, fluid-filled cavity, the blastocoele. As the
blactocyst develops within the uterine environment, it sheds its protective
coat and undergoes further differentiation before developing an intimate
association with the endometrium, which represents the commencement of
implantation or nidation.
    Association of the embryo with the uterine epithelium, either by
superficial attachment or specific embedding in or beneath the endometrium,
leads in due course to the formation of a placenta and complete dependence of
the differentiating embryo upon metabolic support from the mother.
Implantation and placentation exhibit a variety of forms, but in all instances
the hormonal status of the mother is of great importance in determining
whether or not implantation can proceed.  See  PREGNANCY.

    Endocrine function. The endocrine glands secrete certain substances
(hormones) which are necessary for growth, metabolism, reproduction, response
to stress, and various other physiological processes. The endocrine glands
most concerned with the process of reproduction are the pituitary and the
gonads.
    The posterior lobe of the pituitary gland secretes two neurohumoral
agents, vasopressin and oxytocin. These are involved in reproduction only
indirectly, through their effect on uterine contractility in labor and on the
release of milk from the mammary gland when a suckling stimulus is applied.
The anterior lobe secretes a variety of trophic hormones, including two
gonadotrophic hormones, the follicle-stimulating hormone (FSH) and the
luteinizing or interstitial-cell stimulating hormone (LH or ICSH). These
hormones act directly on both ovaries and testes.  See	ADENOHYPOPHYSIS
HORMONE; PITUITARY GLAND.
    The gonadal (steroidal) hormones control the secretion of gonadotrophins
by acting on the hypothalamus. It has been suggested that steroids act by
means of a ``negative feedback''; that is, high levels of circulating gonadal
hormones stop further release of gonadotropins. However, although this is true
for experiments involving pharmacological doses of such hormones, it may not
be the case with endogenous physiological levels. It is certainly true that
less steroid is required to inhibit pituitary function in the female than in
the male. Under certain circumstances small doses of gonadal hormones can
stimulate release of gonadotropic hormones from the pituitary. Estrogen can
stimulate the release of LH; hence the occurrence of ovulation in rats,
rabbits, sheep, and women. Progesterone can also facilitate ovulation in
persistently estrous rats, in chickens, and in estrous sheep and monkeys.  See
  ESTROGEN; PROGESTERONE.
    The formation of gametes (spermatogenesis and oogenesis) is controlled by
anterior pituitary hormones. The differentiation of male and female
reproductive tracts is influenced, and mating behavior and estrous cycles are
controlled, by male or female hormones. The occurrence of the breeding season
is mainly dependent upon the activity of the anterior lobe of the pituitary,
which is influenced through the nervous system by external factors, such as
light and temperature. The  transportation of ova from the ovary to the
Fallopian tube and their subsequent transportation, development, and
implantation in the uterus are controlled by a balanced ratio between estrogen
and progesterone. Furthermore, it is known that estrogens, androgens, and
progesterone can all have the effect of inhibiting the production or the
secretion, or both, of gonadotrophic hormones, permitting the cyclic changes
of reproductive activity among different animals.
    Mammary glands are essential for the nursing of young. Their growth,
differentation, and secretion of milk, and in fact the whole process of
lactation, are controlled by pituitary hormones as well as by estrogen and
progesterone. Other glands and physiological activities also influence
lactation, although this is largely via the trophic support of other pituitary
hormones.
 �34s[M. C. Chang ;Michael J. K. Harper]
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�6v�2v�32sReproductive system disorders   �21m �33s 
    Those disorders which involve the structures of the female and male 
reproductive systems. 
  
    Female system.  Failure of the ovaries to form normally results in short 
stature, sterility, lack of development of female secondary sex 
characteristics, such as breast growth, fat deposition in buttocks, thighs, 
and mons pubis, and female escutcheon. Destruction of the ovaries after 
puberty results in loss of fertility, cessation of menses, and the loss of 
secondary sex characteristics. 
    Neoplastic enlargement of the ovary can be cystic or solid, benign or 
malignant. Malignant ovarian tumors are commonly asymptomatic in their early 
stages, and may be quite widely spread in the pelvic and abdominal cavity 
before they are discovered.  See  OVARY. 
    Inflammation  is the most common disorder of the Fallopian tubes, and if 
it is repeated or severe, destruction of the tubal lining with closure of the 
outer ends of the tubes can occur. This inflammation can be caused by several 
bacterial organisms. Sterility commonly results because the tube is 
permanently closed to passage of eggs and sperm. 
    The second most common problem involving the tube is pregnancy. The egg is 
fertilized in the outer portion of the tube and descends to implant in the 
uterus, but in some cases the passage is delayed and the conceptus attaches to 
the wall of the tube. The tube has a small lumen and thin wall, and the 
growing pregnancy quickly enlarges and grows through the tube, leading usually 
to rupture and hemorrhage into the peritoneal cavity. 
    The muscle (myometrium) and lining (endometrium) of the uterus are 
susceptible to various problems, including tumors, infections, and hormonal 
derangements. Benign tumors of the myometrium (fibroids) are a common 
disorder, producing irregular enlargement of the uterus and sometimes causing 
pain, obstruction of the urinary tract, and heavy vaginal bleeding. 
    Infection of the endocervical glands with gonococcus or chlamydia 
trachomatis agent can occur. This may be asymptomatic, except for producing a 
mucopurulent discharge, or it may cause pain when the cervix is manipulated, 
particularly during intercourse. The infection can ascend from this area into 
the internal genital organs and adjacent structures.  See  GONORRHEA. 
    Cancer of the surface tissue of the cervix is a relatively common, 
potentially lethal problem. Early malignant changes can be detected by 
examining properly stained exfoliated cells in the manner of G. N. 
Papanicolaou. This cancer produces few symptoms, except for irregular bleeding 
(usually postcoital).  
    Inflammation of the lining of the vagina can be due to several common 
organisms: Monilia, a fungus; Trichomonas, a protozoan; and Haemophilus 
vaginalis, a bacterium. Infection of the female external genitalia can be due 
to Monilia, bacterial agents, and viruses. This infection can be diffuse and 
widespread, or localized to hair follicles, Bartholin's glands, or lesions 
such as warts or ulcers. Inflammation can be caused by allergic reactions to 
soap, powders, semen, lubricant, or even clothing. 
  
    Male system.  During fetal life the testicles form in the abdominal cavity 
near the kidneys and migrate down into the scrotum. Failure of descent 
permanently damages the sperm-producing cells causing infertility but allows 
the interstition cells which produce hormones to survive. Other causes of male 
infertility are organic problems which block passage of seminal fluid or 
interfere with sperm production. 
    The testes are sites of malignant tumors which carry a high mortality 
rate. Undescended testicles are more susceptible to malignant transformation, 
and for that reason should be removed when discovered, and appropriate 
hormonal replacement instituted.  See  TESTIS. 
    The major disorder of the epididymis is infection and inflammation, which 
can lead to scarring and permanent blockage of the ducts. The vas deferens and 
seminal vesicles are rarely afflicted by disease, although the network of 
veins surrounding the vas can become engorged and tortuous, and is called a 
variococoele. Inflammation is the chief disorder of the urethra causing 
dysuria and a discharge. 
    Inflammation and development of small calculi are relatively minor 
ailments of the prostate. Benign overgrowth of this gland is the most common 
and troublesome complaint. This enlargement results in constriction of the 
urethra, which obstructs urinary outflow and leads to an increasing residual 
of urine in the bladder. Cance of the prostate is another relatively frequent 
problem in older males. This tumor spreads primarily to bones, where it is 
markedly painful. 
    Inflammation (balanitis) or narrowing (phimosis) of the foreskin which 
covers the glans penis can occur and may interfere with urination. These 
conditions are unknown in parts of the world where the foreskin is removed for 
social or medical reasons. Viral infections can produce warty growths 
(condyloma acuminata) or painful ulcers (herpetic lesions) of the glans or 
shaft of the penis, and a syphilitic infection can result in the firm, 
painless ulcer known as a chancre. 
    Disorders of erectile capability range from tumescence unaccompanied by 
sexual desire (Peyronie's disease, priapism) to impotence, both psychologic 
and secondary to old age. Carcinoma can arise from the surface epithelium of 
the penis and spread both locally and via the lymphatic system to the nodes of 
the groin. This disease is virtually unknown in populations where males are 
circumcised at birth. 
 �34s[Gay M. Guzinski]
�33s 
  
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�6v�2v�32sReptilia �21m �33s
    A class of vertebrates composed of four living orders, the turtles or
Chelonia, the tuatara or Rhynchocephalia, the lizards and snakes or Squamata,
and the crocodilians or Crocodilia. Numerous extinct orders are also known.
The group first appeared in the Carboniferous and underwent a culminating
evolutionary radiation in the Mesozoic, often called the age of reptiles.
Although the major portion of the class is not extinct, several Recent groups,
particularly the Squamata, are very successful, and there are approximately
5000 living species of reptiles as compared to about 4000 living mammals. A
classification for the class is given here; see separate articles on each of
the groups listed.
Class Reptilia
    Subclass Anapsida
	Orders: Cotylosauria
		Mesosauria
		Chelonia
    Subclass Ichthyopterygia
	Order: Ichthyosauria
    Subclass Euryapsida
	Orders: Sauropterygia
		Placodontia
		Araeoscelidia
    Subclass Lepidosauria
	Orders: Eosuchia
		Rhynchocephalia
		Squamata
    Subclass Archosauria
	Orders: Thecodontia
		Crocodilia
		Pterosauria
		Saurischia
		Ornithischia
    Subclass Synapsida
	Orders: Pelycosauria
		Theropsida
		Ictidosauria
    The reptiles are the most primitive of the completely terrestrial
vertebrates and are consequently the first to exhibit amniote features. In the
reptiles the eggs are covered by a complex series of protective layers,
including a leathery or calcareous shell. A rich supply of food material in
the form of yolk is deposited inside the ovum to furnish food for the
developing embryo. A series of protective extraembryonic membranes, the serosa
and amnion, appears later in embryogenesis to protect the embryo from water
loss and shock. A third such membrane, the allantois, functions as a storage
sac for nitrogenous wastes. The serosa and allantois usually fuse to form a
respiratory structure. Gaseous exchanges take place across the shell and
seroallantoic membrane between the outside air and the blood vessels of the
allantois. All of these adaptations made it possible for the reptile egg to be
deposited on land, undergo its development there, and hatch into a fully
developed form without a gilled larval stage. Most reptilian eggs are buried
in the soil or in rotting vegetation out of the direct sunlight.  See  AMNIOTA.
    If all intermediate fossil forms are taken into account, the above
features of the life history are the only ones that serve to distinguish the
reptiles from the amphibians. As a matter of fact, several fossil groups near
the evolutionary transition between the two classes, known only from bony
remains, cannot be satisfactorily placed in one or the other class because
information on their life histories is not available.  See  AMPHIBIA.
    In recent reptiles, discrete dry horny scales cover the body; mucous
glands are lacking in the skin; there is a single occipital condyle; mammary
glands are lacking; the middle ear region has a single ossicle (stapes), if
functional, transmitting vibrations to inner ear; the heart is composed of two
auricles and a single partially divided ventricle (fully divided in
Crocodilia); and pleural cavities and peritoneal cavities are continuous
(separated in crocodilians). Although primarily oviparous, some lizards and
snakes are ovoviviparous or viviparous.
    The Mammalia are descended from one reptilian as is the class Aves. Both
groups differ markedly from the Reptilia.
 �34s[Jay M. Savage]
�33s

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�6v�2v�32sRepulsion motor �21m �33s
    An alternating-current (ac) commutator motor designed for single-phase
operation. The chief distinction between the repulsion motor and the
single-phase series motors is the way in which the armature receives its
power. In the series motor the armature power is supplied by conduction from
the line power supply. In the repulsion motor, however, armature power is
supplied by induction (transformer action) from the field of the stator
winding. For discussion of the ac series motor	see  UNIVERSAL MOTOR.  See
also  ALTERNATING-CURRENT MOTOR.
    The repulsion motor primary or stationary field winding is connected to
the power supply. The secondary or armature winding is mounted on the motor
shaft and rotates with it. The terminals of the armature winding are
short-circuited through a commutator and brushes. There is no electrical
contact between the stationary field and rotating armature (see illustration).
See  WINDINGS IN ELECTRIC MACHINERY.
 �34s[Irving L. Kosow]
�33s


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�6v�2v�32sReserve battery   �21m �33s 
    A battery which is inert until an operation is performed which brings all 
the cell components into the proper state and location to become active.  See  
BATTERY.  
    There are several types of reserve batteries. In water-activated or 
electrolyte-activated batteries the water or electrolyte component is not 
present during storage. It is added just before the cell is put into use. In 
thermal batteries the electrolyte is a solid at room temperature and has very 
low conductivity. If the temperature is raised above the melting point, the 
conductivity of the electrolyte becomes excellent and the cell is capable of 
delivering significant power. 
    Practical, water-activated battery systems have been developed using 
magnesium anodes against silver chloride or cuprous chloride cathodes. The 
most important design factor is to ensure that the electrolyte is delivered as 
quickly as possible at the time of activation, at the same time avoiding 
chemical short-circuiting of the cells. The dry elements of water-activated 
batteries are stored in a sealed container and are capable of indefinite 
storage life. 
    Any cell can be made as a reserve-electrolyte cell. If the electrodes are 
in place, it is necessary only to add the electrolyte to make a complete cell. 
In practice, however, the separation of the electrolyte is done only when 
excessive deterioration would occur during wet storage prior to use. 
    The liquid-activated batteries have disadvantages which are difficult to 
overcome; for example, automatic electrolyte-charging equipment may cause 
intercell shorting. An alternative approach is to introduce a gas which reacts 
with the spacer material to form a conducting electrolyte. Such gas-activated 
batteries operate over a wide temperature range. 
    Thermal batteries are also known as heat-activated or fused-electrolyte 
batteries. Some compounds, such as sodium chloride and potassium hydroxide, 
show very low conductivity in the solid state of room temperature but very 
good conductivity in the molten state. For example, a mixture of sodium 
hydroxide and potassium hydroxide becomes an excellent ionic conductor when 
heated about 338DGF (170DGC). If a zinc anode and a silver oxide cathode are 
combined with solid pads of the eutectic mixture, all the elements of a cell 
are present. Thermal batteries are capable of operation at very low ambient 
temperatures, provided that a suitable heat source is available to melt the 
electrolyte.  
 �34s[Jack Davis]
�33s 
  
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�6v�2v�32sReservoir   �21m �33s 
    A pond or lake built for the storage of water, usually by the construction 
of a dam across a river. The size of reservoir needed is a function of the 
water demands, the natural flows of the river impounded, and the extent of 
droughts to be encountered. Water supplies may be taken directly from the 
reservoirs, or water may be released from the reservoirs to augment river 
flows past a water supply intake downstream.  See  DAM; WATER SUPPLY 
ENGINEERING.  
 �34s[Richard Hazen]
�33s 
  
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�6v�2v�32sResin   �21m �33s 
    Originally a category of vegetable substances soluble in ethanol but 
insoluble in water, but generally in modern technology an organic polymer of 
indeterminate molecular weight. The class of flammable, amorphous secretions 
of conifers or legumes are considered true resins. Water-swellable secretions 
of various plants, especially the Burseraceae, are called gum resins. The 
natural vegetable resins are largely polyterpenes and their acid derivatives, 
which find application in the manufacture of lacquers, adhesives, varnishes, 
and inks. 
    The synthetic resins, originally viewed as substitutes for certain natural 
resins, have a large place of their own in industry and commerce. 
Phenol-formaldehyde, phenol-urea, and phenol-melamine resins are important 
commercially. Any unplasticized organic polymer is considered a resin, thus 
nearly any one of the common plastics may be viewed as a synthetic resin. 
Water-soluble resins are marketed chiefly as substitutes for vegetable gums 
and in their own right for highly specialized applications. 
Carboxymethylcellulose, hydroxyalkylated cellulose derivatives, modified 
starches, polyvinyl alcohol, polyvinylpyrrolindone, and polyacrylamides are 
widely used as thickening agents for foods, paints, and drilling muds, as 
fiber sizings, in various kinds of protective coatings, and as encapsulating 
substances.  See  POLYMER. 
 �34s[Frank Wagner]
�33s 
  
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�6v�2v�32sResistance heating   �21m �33s 
    The generation of heat by electric conductors carrying current. The degree 
of heating for a given current is proportional to the electrical resistance of 
the conductor. If the resistance is high, a large amount of heat is generated, 
and the material is used as a resistor rather than as a conductor.  See  
ELECTRICAL RESISTANCE. 
    In addition to having high resistivity, heating elements must be able to 
withstand high temperatures without deteriorating or sagging. Other desirable 
characteristics are low temperature coefficient of resistance, low cost, 
formability, and availability of materials. Most commercial resistance alloys 
contain chromium or aluminum or both, since a protective coating of chrome 
oxide or aluminum oxide forms on the surface upon heating and inhibits or 
retards further oxidation. 
    Since heat is transmitted by radiation, convection, or conduction or 
combinations of these, the form of element is designed for the major mode of 
transmission. The simplest form is the helix, using a round wire resistor, 
with the pitch of the helix approximately three wire diameters. This form is 
adapted to radiation and convection  and is generally used for room or air 
heating. It is also used in industrial furnaces, utilizing forced convection 
up to about 1200DGF (650DGC). Such helixes are stretched over grooved 
high-alumina refractory insulators and are otherwise open and unrestricted. 
    The electrical resistance of molten salts between immersed electrodes can 
be used to generate heat. Limiting temperatures are dependent on decomposition 
or evaporization temperatures of the salt. Parts to be heated are immersed in 
the salt. Heating is rapid and, since there is no exposure to air, oxidation 
is largely prevented. Disadvantages are the personnel hazards and discomfort 
of working close to molten salts. 
    A major application of resistance heating is in electric home appliances, 
including electric ranges, clothes dryers, water heaters, coffee percolators, 
portable radiant heaters, and hair dryers. Resistance heating also has 
application in home or space heating.  
    If the resistor is located in a thermally insulated chamber, most of the 
heat generated is conserved and can be applied to a wide variety of heating 
processes. Such insulated chambers are called ovens or furnaces, depending on 
the temperature range and use. The term oven is generally applied to units 
which operate up to approximately 800DGF (430DGC). Typical uses are for baking 
or roasting foods, drying paints and organic enamels, baking foundry cores, 
and low-temperature treatments of metals. The term furnace generally applies 
to units operating above 1200DGF (650DGC). Typical uses of furnaces are for 
heat treatment or melting of metals, for vitrification and glazing of ceramic 
wares, for annealing of glass, and for roasting and calcining of ores.  See  
ELECTRIC HEATING; FURNACE.   
 �34s[Willard Roth]
�33s 
  
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�6v�2v�32sResistance measurement �21m �33s
    The quantitative determination of that property of an electrically
conductive material, component, or circuit called electrical resistance. The
unit of measurement is called the ohm. Measurements for engineering
applications usually range from 0.1 microhm (10(sup -7)ohm) to 1000 teraohms
(10(sup 15)ohms). Measurements often required for materials development, basic
research, or experiments under extreme environmental conditions extend this
span to include zero ohms (superconductors) and 10(sup 18) ohms.  See
ELECTRICAL RESISTANCE; ELECTRICAL RESISTIVITY; OHM`S LAW; SUPERCONDUCTIVITY.
    Resistance may be measured by using either direct or alternating current
(dc or ac). If dc is used, the true resistance of the conductor is measured.
When the measurement is made with ac, the result is usually called the
effective resistance. The effect of leakage can usually be eliminated by
suitably guarding one terminal or portion of the resistance to be measured.
Physically a guard consists of a low-resistance conductor, electrically
insulated from the guarded terminal and located to intercept the leakage
current.
    Commonly used methods of measuring resistance may be classified as either
deflection methods or comparison methods. As implied by the names, deflection
methods utilize the deflection of ammeters or voltmeters, which may be
calibrated in terms of resistance under specific operating conditons, while
comparison methods are based upon the use of a calibrated resistor, which can
be compared to an unknown resistor. The fundamental deflection method is known
as the voltmeter-ammeter method. Basic comparison methods are by potential
drop, Wheatstone bridge, and Kelvin bridge.

			     R(sub x) = V/A				     (1)

    The voltmeter-ammeter method of measuring resistance is illustrated in
Fig. 1. Simultaneous readings of the voltmeter and ammeter are taken, and the
unknown resistance Rsubx is calculated from Ohm's law, as shown in Eq. (1).
For the voltmeter connection shown in Fig. 1a, the true resistance Rsubx is
slightly larger than the calculated value, because the ammeter measures the
sum of the currents in Rsubx and the voltmeter. For the connection of Fig. 1b,
Rsubx will be slightly smaller than that calculated, since the voltmeter
reading is larger than the potential drop across Rsubx The circuit in Fig. 1a
causes the least error when Rsubx is low; the circuit in Fig. 1b causes the
least error when Rsubx is high. A practical and widely used simplification
based on this method is used in the ohmmeter.  see OHMMETER.
    Comparison by potential drop, accomplished with the circuit of Fig. 2, is
a logical development from the voltmeter-ammeter method. Only one meter is
used for the measurement. The unknown resistance Rsubx is connected in series
with a standard resistance Rsubx, which may be either fixed or adjustable.
Since the same current flows in both resistors, by Ohm's law Eqs. (2) hold.

	   I = V(sub s)/R(sub s) = V(sub x)/R(sub x)			     (2)
	   R(sub x) = R(sub s)V(sub x)/V(sub s)

This method is seldom used commercially, but it is capable of high accuracy
for comparing like-value resistors, using a potentiometer for the potential
difference measurement.  See  WHEATSTONE BRIDGE.
    High-quality standard resistors are used in comparison methods of
resistance measurement. The unknown resistance is measured by comparing it to
the accurately known value of the standard resistor. These standards are
specially constructed and treated to achieve (1) constancy over long periods
of time, (2) a low temperature coefficient, (3) a low internal thermal
voltage, and (4) stability under varying humidity conditons. When intended for
use in ac circuits, an additional requirement is to reduce the
self-inductance, the distributed capacitance, and the skin effect of the
resistor to the lowest practical values so that the standard will have a
minimum frequency coefficient.	See  ELECTRICAL UNITS AND STANDARDS.
 �34s[Charles E. Applegate]
�33s


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�6v�2v�32sResistance welding   �21m �33s 
    A process in which the heat for producing the weld is generated by the 
resistance to the flow of current through the parts to be joined. The 
application of external force is required; however, no fluxes, filler metals, 
or external heat sources are necessary. Most metals and their alloys can be 
successfully joined by resistance welding processes. Several methods are 
classified as resistance welding processes: spot, roll-spot, seam, projection, 
upset, flash, and percussion. 
    In resistance spot welding, coalescence at the faying surfaces is produced 
in one spot by the heat obtained from the resistance to electric current 
through the work parts held together under pressure by electrodes. The size 
and shape of the individually formed welds are limited primarily by the size 
and contour of the electrodes.  See  SPOT WELDING. 
    In roll resistance spot welding, separated resistance spot welds are made 
with one or more rotating circular electrodes. The rotation of the electrodes 
may or may not be stopped during the making of a weld. 
    In resistance seam welding, coalescence at the faying surfaces is produced 
by the heat obtained from resistance to electric current through the work 
parts held together under pressure by electrodes. The resulting weld is a 
series of overlapping resistance spot welds made progressively along a joint 
by rotating the electrodes. 
    In projection welding, coalescence is produced by the heat obtained from 
resistance to electric current through the work parts held together under 
pressure by electrodes. The resulting welds are localized at predetermined 
points by projections, embossments, or intersections. 
    In upset welding, coalescence is produced simultaneously over the entire 
area of abutting surfaces or progressively along a joint, by the heat obtained 
from resistance to electric current  through the area of contact of those 
surfaces. Pressure is applied before heating is started and is maintained 
throughout the heating period. 
    In flash welding, coalescence is produced simultaneously over the entire 
area of abutting surfaces by the heat obtained from resistance to electric 
current between the two surfaces and by the application of pressure after 
heating is substantially completed. Flash and upsetting are accompanied by 
expulsion of the metal from the joint.  See  FLASH WELDING. 
    In percussion welding, coalescence is produced simultaneously over the 
entire abutting surfaces by the heat obtained from an arc produced by a rapid 
discharge of electrical energy with pressure percussively applied during or 
immediately following the electrical discharge. 
    Most metals and alloys can be resistance-welded to themselves and to each 
other. The weld properties are determined by the metal and by the resultant 
alloys which form during the welding process. Stronger metals and alloys 
require higher electrode forces, and poor electrical conductors require less 
current. Copper, silver, and gold, which are excellent electrical conductors, 
are very difficult to weld because they require high current densities to 
compensate for their low resistance. Medium- and high-carbon steels, which are 
hardened and embrittled during the normal welding process, must be tempered by 
multiple impulses.  See  WELDING AND CUTTING OF METALS.   
 �34s[Ernest F. Nipples]
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�6v�2v�32sResistor   �21m �33s 
    A component of an electric circuit that produces heat while offering 
opposition, or resistance, to the flow of electric current. All conductors 
exhibit resistance in varying degrees; however, the term resistor is generally 
used only to describe a device specifically used to introduce resistance into 
an electric circuit. The unit of resistance measurement is the ohm. Resistors 
are described by stating their total resistance in ohms along with their safe 
power-dissipating ability in watts. A more detailed description would specify 
the residual inductance and stray shunt capacitance of the resistor.  See  
ELECTRICAL RESISTANCE; ELECTRICAL RESISTIVITY. 
    Resistors may be classified according to the general field of engineering 
in which they are used. Power resistors range in size from about 5 watts to 
many kilowatts and may be cooled by air convection, air blast, or water. 
Direct-current (dc) ammeters employ resistors as meter shunts to bypass the 
major portion of the current around the low-current elements.  
    Voltmeters of both the dc and the ac types employ scale-multiplying 
resistors designed for accuracy and stability. By far the greatest number of 
resistors manufactured are intended for use in the electronics field. 
    Resistors are also classified according to their construction, which may 
be composition, film-type, or wire-wound. Further classification may be made 
according to whether the resistor has a fixed or adjustable resistance. 
Adjustable resistors may be further classified as adjustable-slide, rheostat, 
and potentiometer types.  See  POTENTIOMETER (VARIABLE RESISTOR); RHEOSTAT. 
    The composition resistor is by far the most widely manufactured type 
because of its low cost, reliability, and small size. Basically it is a 
mixture of resistive materials, usually carbon, and a suitable binder molded 
into a cylinder. Copper wire leads are attached to the ends of the cylinder, 
and the entire resistor is molded into a plastic or ceramic jacket. 
    The film-type resistor is rapidly replacing the composition resistor in 
applications in which greater stability of resistance with voltage, 
temperature, and humidity is demanded. Basically this resistor consists of a 
conducting film of carbon, metal, or metal oxide deposited upon a ceramic 
cylinder. The value of the resistance is controlled by controlling the 
thickness and length of the film. 
    Wire remains the most stable form of resistance material available; 
therefore all high-precision instruments rely upon wire-wound resistors. 
    For discussion of nonlinear resistors  see  THERMISTOR; VARISTOR. 
 �34s[Robert L. Ramey]
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�6v�2v�32sResolving power (optics) �21m �33s
    A quantitative measure of the ability of an optical instrument to produce
separable images. The images to be resolved may differ in position because
they represent (1) different points on the object, as in telescopes and
microscopes, or (2) images of the same object in light of two different
wavelengths, as in prism and grating spectroscopes. For the former class of
instruments, the resolving limit is usually quoted as the smallest angular or
linear separation of two object points, and for the latter class, as the
smallest difference in wavelength or wave number that will produce separate
images. Since these quantities are inversely proportional to the power of the
instrument to resolve, the term resolving power has generally fallen into
disfavor. It is still commonly applied to spectroscopes, however, for which
the term chromatic resolving power is used, signifying the ratio of the
wavelength itself to the smallest wavelength interval resolved. The figure
quoted as the resolving power or resolving limit of an instrument may be the
theoretical value that would be obtained if all optical parts were perfect, or
it may be the actual value found experimentally. Aberrations of lenses or
defects in the ruling of gratings usually cause the actual resolution to fall
below the theoretical value, which therefore represents the maximum that could
be obtained with the given dimensions of the instrument in question. This
maximum is fixed by the wave nature of light and may be calculated for given
conditions by diffraction theory.  See	DIFFRACTION.
 �34s[Francis A. Jenkins /George R. Harrison]
�33s

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�6v�2v�32sResonance ionization spectroscopy   �21m �33s 
    A form of atomic and molecular spectroscopy in which wavelength tunable 
light sources are used to remove electrons from  (that is, ionize) a given 
kind of atom or molecule. Laser-based resonance ionization spectroscopy (RIS) 
techniques have been developed and used with ionization detectors such as the 
proportional counter to show that single atoms can be detected. Both RIS and 
one-atom detectors find a wide range of applications in physics, chemistry, 
and oceanography, and in the environmental sciences.  
 �34s[G. S. Hurst]
�33s 
  
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�6v�2v�32sResonance transformer   �21m �33s 
    An electrostatic particle accelerator in which the high-voltage terminal 
oscillates over the voltage range PLUS/MINUSV. These machines are used 
principally for acceleration of electrons. Electron current is allowed to pass 
only when the terminal voltage is near its peak value of -V. Thus the energy 
spread of the electrons can be held to a moderate value. 
    The resonant transformer (see illustration) consists of a low-voltage 
primary winding which surrounds the lower end of a high-voltage coil stack. 
The stack is made up of a number of thin flat windings called pancake coils, 
in which the multisection vacuum tube is coaxially mounted. The inductance and 
capacitance of the high-voltage secondary have values such that the resonant 
frequency of this circuit is equal to the frequency at which primary power is 
supplied.  See  PARTICLE ACCELERATOR. 
    Resonance transformers are used for industrial radiography, for x-ray 
therapy, for food and drug sterilization, and for the processing of plastics.  
See  COCKCROFT-WALTON ACCELERATOR. 
 �34s[Raymond G. Herb]
�33s 
  
  
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�6v�2v�32sResonance (acoustics and mechanics)   �21m �33s 
    When a mechanical or acoustical system is acted upon by an external 
periodic driving force whose frequency equals a natural free oscillation 
frequency of the system, the amplitude of oscillation becomes large and the 
system is said to be in a state of resonance. 
    A knowledge of both the resonance frequency and the sharpness of resonance 
is essential to any discussion of driven vibrating systems. When a vibrating 
system is sharply resonant, careful tuning is required to obtain the resonance 
condition. Mechanical standards of frequency must be sharply resonant so that 
their peak response can easily be determined. In other circumstances, 
resonance is undesirable. For example, in the faithful recording and 
reproduction of musical sounds, it is  necessary either to have all 
vibrational resonances of the system outside the band of frequencies being 
reproduced or to employ heavily damped systems.  See  ACOUSTIC RESONATOR; 
SYMPATHETIC VIBRATION; VIBRATION. 
 �34s[Lawrence E. Kinsler]
�33s 
  
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�6v�2v�32sResonance (alternating-current circuits)   �21m �33s 
    A condition in a circuit characterized by relatively unimpeded oscillation 
of energy from a potential to a kinetic form. In an electrical network there 
is oscillation between the potential energy of charge on capacitance and the 
kinetic energy of current in inductance. This is analogous to the mechanical 
resonance seen in a pendulum. 
    Three kinds of resonant frequency in circuits are officially defined. 
Phase resonance is the frequency at which the phase angle between sinusoidal 
current entering a circuit and sinusoidal voltage applied to the terminals of 
the circuit is zero. Amplitude resonance is the frequency at which a given 
sinusoidal excitation (voltage or current) produces the maximum oscillation of 
electric charge in the resonant circuit. Natural resonance is the natural 
frequency of oscillation of the resonant circuit in the absence of any forcing 
excitation. These three frequencies are so nearly equal in low-loss circuits 
that they do not often have to be distinguished. 
    Resonance is of great importance in communications, permitting certain 
frequencies to be passed and others to be rejected. Thus a pair of telephone 
wires can carry many messages at the same time, each modulating a different 
carrier frequency, and each being separated from the others at the receiving 
end of the line by an appropriate arrangement of resonant filters. A radio or 
television receiver uses much the same principle to accept a desired signal 
and to reject all the undesired signals that arrive concurrently at its 
antenna; tuning a receiver means adjusting a circuit to be resonant at a 
desired frequency. 
 �34s[Hugh H. Skilling]
�33s 
  
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�6v�2v�32sResonance (molecular structure) �21m �33s
    A feature of the valence-bond method, which is a mathematical procedure to
obtain approximate solutions to the Schroedinger equation for molecules. The
valence-bond method is based on the theorem that if two or more solutions to
the Schroedinger equation are available, certain linear combinations of them
will also be solutions. It has this basis in common with its rival, the
molecular orbital method. The valence-bond and molecular orbital approaches
are both approximations and, if carried out to their logical and exact
extremes, must yield identical results; nevertheless, both are often described
as theories. In the valence-bond theory, combinations of solutions represent
hypothetical structures of the molecule in question. These structures are said
to be resonance (or contributing) structures, and the real molecule is said to
be the resonance hybrid (or just simply the hybrid) of these structures.  See
MOLECULAR ORBITAL THEORY; SCHROEDINGER'S WAVE EQUATION.
    The resonance theory provided a solution for a molecule which had baffled
and preoccupied chemists for a century - benzene. The principal use of
resonance still lies in  the qualitative description of molecules whose
properties would otherwise be difficult to understand.	See  BENZENE.
    Until the beginning of the 20th century, benzene posed a baffling
challenge to organic chemists. In spite of its relatively simple formula,
C6H6, they were unable to conceive of a suitable structure for it. While a
great many structures were proposed, the properties of benzene corresponded to
none of them.

			    H			 H
			    |			 |
		       H    C	H	     H	 C   H
			\ // \	/	      \ / \\ /
			 C     C	       C    C
			 |     ||	       ||   |
			 C     C	       C    C
			/ \\  / \	      / \ // \
		       H    C	 H	     H	 C    H
			    |			 |
			    H			 H

			   (I)			(II)

    In the early 1870s F. A. Kekule proposed a revolutionary idea; benzene
must be represented by two structures, (I) and (II), rather than one, and all
compounds containing the benzene skeleton must be subject to a rapid
equilibration (oscillation) between the two. Kekule's description of benzene
was not completely satisfactory. While it accounted for the number of
substituted benzene isomers, it did not explain why the compound failed to
exhibit reactivity indicating the presence of multiple bonds. The problem was
resolved with the advent of quantum mechanics in the early part of this
century. In a sense, this solution is an expansion of Kekule's oscillating
pair; the so-called activation energy (the energy which must be imparted to a
molecule in order to make it overcome the barrier that keeps it from being
converted into another molecule) is negative in the case of benzene with
respect to the oscillation, and this molecule therefore exists neither as (I)
nor as (II) at any time, but it is an intermediate form (III) all the time.
This intermediate structure of benzene is described in terms of Kekule's
structures with the symbol =D between them; this is intended to signify that
benzene has neither structure, but in fact is a hybrid of the two. The
properties of benzene are thereby indicated to be those of neither (I) nor
(II), but to be intermediate between the two.
    The only property of the hybrid which is not intermediate between those of
the hypothetical contributing structures is the energy: the energy of a
resonance hybrid is by definition always at a minimum. This fact is
responsible for the abnormal reluctance of benzene to undergo addition
reactions; such reactions would lead to products that no longer have the
resonance energy.
			     ******CHEMICAL STRUCTURE******

    Although benzene is the classical example of resonance, the phenomenon is
certainly not limited to it. Furthermore, the properties of all compounds are
affected by resonance to some degree.
    Although the molecular orbital approach has largely supplanted the
valence-bond method, the resonance language remains so convenient that it is
still used.  See  CHEMICAL BONDING; CONJUGATION AND HYPERCONJUGATION;
MOLECULAR STRUCTURE AND SPECTRA; QUANTUM CHEMISTRY.
 �34s[William J. Le Noble]
�33s

A700000001666�254s�5v�1v1489
\IMG\CI1441.IMG
6983168
�6v�2v�32sResorcinol �21m �33s
    A dihydric phenol (C6H6O2), also called m-dihydroxybenzene. Resorcinol,
with the structure shown below, is a water-soluble, crystalline compound (m.p.
110DGC or 230DGF) that is colorless when pure. Aqueous solutions of resorcinol
are mildly acidic; (pK 9.2); in this and other respects its properties are
similar to those of phenol. Resorcinol is irritating to the skin, particularly
that of individuals who have become sensitized, and it is a severe systemic
poison.

			       OH
				|
			      // \
			      |  ||
			      \\ / \
				   OH

    The largest industrial use for resorcinol is in the manufacture of
adhesives and resins. Adhesives based on resorcinol are exceptionally
water-resistant, and are used extensively in marine plywood and other products
which are constantly exposed to water.	See  CATECHOL; CRESOL; HYDROQUINONE;
PHENOL.
 �34s[Martin Stiles]
�33s

A700000005220�254s�5v�1v1489
\IMG\CI1441.IMG
6983168
1490
\IMG\CI1441.IMG
7081472
�6v�2v�32sRespiration   �21m �33s 
    The various processes associated with the biochemical transformation of 
the energy available in the organic substrates derived from foodstuffs, to 
energy usable for synthetic and transport processes, external work, and, 
eventually, heat. This transformation, generally identified as metabolism, 
most commonly requires the presence of oxygen and involves the complete 
oxidation of organic substrates to carbon dioxide and water (aerobic 
respiration). If the oxidation is incomplete, resulting in organic compounds 
as end products, oxygen is typically not involved, and the process is then 
identified as anaerobic respiration.  See  BIOLOGICAL OXIDATION; METABOLISM. 
    The term ``external respiration'' is more appropriate for describing the 
exchange of O2 and CO2 between the organism and its environment. In most 
multicellular organisms, and nearly all vertebrates (with the exception of a 
few salamanders lacking both lungs and gills), external respiration takes 
place in specialized structures termed respiratory organs, such as gills and 
lungs.  See  LUNG; RESPIRATORY SYSTEM. 
    The ultimate physical process causing movement of gases across living 
tissues is simple passive diffusion. Respiratory gas exchange also depends on 
two convective fluid movements. The first is the bulk transport of the 
external medium, air or water, to and across the external respiratory exchange 
surfaces. The second is the transport of coelomic fluid or blood across the 
internal surfaces of the respiratory organ. These two convective transports 
are referred to as ventilation and circulation (or perfusion). They are active 
processes, powered by ciliary or muscular pumps. 
    In all vertebrates and many invertebrates, the circulating internal medium 
(coelomic fluid, hemolymph, or blood) contains a respiratory pigment, for 
example, hemocyanin or hemoglobin, which binds reversibly with O2, CO2, and 
protons. Respiratory pigments augment respiratory gas exchange, both by 
increasing the capacity for bulk transport of the gases, and by influencing 
gas partial pressure (concentration) gradients across tissue exchange 
surfaces.  See  BLOOD; HEMOGLOBIN; RESPIRATORY PIGMENTS (INVERTEBRATE). 
    The physiological adjustment of organisms to variations in their need for 
aerobic energy production involves regulated changes in the exchange and 
transport of respiratory gases. The adjustments are effected by rapid 
alterations in the ventilatory and circulatory pumps and by longer-term 
modifications in the respiratory properties of blood. 
 �34s[Kjell Johansen]
�33s 
  
A700000001375�254s�5v�1v1490
\IMG\CI1441.IMG
7081472
�6v�2v�32sRespirator   �21m �33s 
    A device to protect the eyes and respiratory tract from noxious gases, 
vapors, and aerosols. Three general types of respirators are in common use, 
depending on the principle of operation utilized. The military protective mask 
(see illustration) and the commercial gas mask furnish clean air to the wearer 
by removing the contamination from the ambient atmosphere by means of a filter 
and a bed of adsorbent material. Rebreathing masks use closed systems which 
absorb the carbon dioxide and water from exhaled air and generate oxygen for 
inhalation. Air-supply masks furnish clean air from a source independent of 
the surrounding atmosphere. 
 �34s[Seymour D. Silver]
�33s 
  
  
A700000004660�254s�5v�1v1490
\IMG\CI1441.IMG
7081472
�6v�2v�32sRespiratory pigments (invertebrate)   �21m �33s 
    Colored, metal-containing proteins which combine reversibly with oxygen, 
and which are found in the body fluids or tissues of invertebrate animals. The 
role of these pigments is primarily to aid in the transport of molecular 
oxygen. Thus they are distinguished from respiratory enzymes, which are 
concerned with the metabolic consumption of oxygen. Four distinctly colored 
groups of respiratory pigments exist among invertebrates: hemoglobins (purple, 
become orange-red with oxygen), chlorocruorins (green, become red with 
oxygen), hemocyanins (colorless, become blue with oxygen), and hemerythrins 
(colorlesss, become red with oxygen). 
    Each of the pigments is composed of two parts, a large protein molecule to 
which is bound one or more small moieties called prosthetic groups, each of 
which is or contains a metal. The metal binds the oxygen, and this binding 
imparts the characteristic color to the pigment. In hemoglobins the prosthetic 
group is an iron porphyrin compound called heme. Chlorocruorin contains a 
similar iron porphyrin which differs from heme only in that a vinyl group in 
the molecule is replaced by formyl. The prosthetic group of hemerythrin 
consists of two adjacent iron atoms which bind an oxygen molecule between 
them. The prosthetic group of hemocyanin is analogous and consists of two 
adjacent copper atoms. Pigments containing vanadium have been found in 
tunicates, but these substances do not combine reversibly with oxygen and so 
cannot be considered respiratory pigments. 
    The protein part of the pigment confers reversibility upon the combination 
of the metal with oxygen. In the absence of protein, the prosthetic groups 
lose their capacity to combine with oxygen reversibly. Instead, the metals are 
irreversibly oxidized: Electrons are transferred from metal to oxygen. The 
bonds between metal and protein so alter the electronic energy levels of the 
metal that this transfer, if it occurs, is reversible. For this reason, the 
combination of hemoglobin with oxygen is described as oxygenation rather than 
oxidation. The protein is also responsible for certain physiological 
adaptations  of the pigment to the environment. Thus the affinity of the 
pigment for oxygen is often highest in animals which inhabit environments with 
the lowest oxygen content.  See  HEMOGLOBIN. 
 �34s[Austen F. Riggs]
�33s 
  
A700000006022�254s�5v�1v1490
\IMG\CI1441.IMG
7081472
1491
\IMG\CI1441.IMG
7208448
�6v�2v�32sRespiratory system �21m �33s
    The system of organs involved in the acquisition of oxygen and the
elimination of carbon dioxide by an organism. The lungs and gills are the two
most important structures of vertebrates involved in the phase known as
external respiration, or gaseous exchanges, between the blood and environment.
Internal respiration refers to the gaseous exchanges which occur between the
blood and cells. Certain other structures in some species of vertebrates serve
as respiratory organs; among these are the integument or skin of fishes and
amphibians. The moist, highly vascular skin of anuran amphibians is important
in respiration. Certain species of fish have a vascular rectum which is
utilized as a respiratory structure, water being taken in and ejected
regularly by the animal. Saclike cloacal structures occur in some aquatic
species of turtles. These are vascular and are intermittently filled with, and
emptied of, water. It is thought that they may function in respiration. During
embryonic life the yolk sac and allantois are important respiratory organs in
certain vertebrates.  See  ALLANTOIS; GILLS; YOLK SAC.
    Structurally, respiratory organs usually present a vascular surface that
is sufficiently extensive to provide an adequate area of absorption for
gaseous exchange. This surface is moist and thin enough to allow for the
passage of gases.
 �34s[Robert S. McEwen /Thomas S. Parson]
�33s


    The shape and volume of the lung, because of its pliability, conforms
almost completely to that of its cavity. The lungs are conical; each has
an apex and a base, two surfaces, two borders, and a hilum. The apex extends
into the superior limit of the thoracic cavity. The base is the disphragmatic
surface. The coastal surface may show bulgings into the intercostal spaces. The
medial surface has a part lying in the space beside the vertebral column and a
part imprinted by the form of structures bulging outward beneath the mediastinal
pleura. The cardiac impression is deeper on the left lung because of the
position of the heart.
    For convenience the lung may be divided into anatomical areas. The bronchial
tree branches mainly by dichotomy. The ultimate generations, that is, the
respiratory bronchioles, alveolar ducts, and alveoli constitutes all of the
respiratory portion of the lung. The trachea and extrapulmonary bronchi are kept
open by C-shaped bars of hyaline cartilage. When in their branching the bronchi
and bronchioles are reduced to a diameter of 1 mm or less, they are then free of
cartilage and are called terminal bronchioles. One of the terminal bronchioles
enters the apex of a secondary lobule of the lung. These secondary lobules are
anatomic units of the lung, whose hexagonal bases rest on the pleura or next to
a bronchiole or blood vessel. Finer lines divide the bases of the secondary
lobules into smaller areas. These are the bases of primary lobules, each served
by a respiratory bronchiole. See LUNG; RESPIRATION. [Leo P. Clements]

A700000004005�254s�5v�1v1491
\IMG\CI1441.IMG
7208448
�6v�2v�32sRespiratory system disorders   �21m �33s 
    Any disease of the respiratory system, airways or blood circulation is apt 
to reduce the respiratory function. It is well known, however, that less than 
one-third of the lung is needed to sustain life at rest. The other two-thirds 
more or less represent reserve volume which is used when more oxygen is 
needed, as when the body is working. Hence, slight reduction of the 
respiratory function manifests itself in shortness of breath, dyspnea, at 
work. Dyspnea at rest is a sign of severe disease of the respiratory system. 
In extreme conditions this is accompanied by incomplete oxygenation of the 
blood in the lung. The blood cannot get rid of all carbon dioxide and cannot 
take up enough oxygen. It becomes darker and has a bluish appearance where it 
shines through the skin, as on the lips or nails. This condition is called 
cyanosis. If the respiratory insufficiency continues to increase, basic 
functions of the organism are disturbed by the lack of oxygen. The patient 
loses consciousness, since the brain is most sensitive to lack of oxygen, and 
finally dies if no relief can be given. 
    Diseases of the respiratory system can be located in the lung tissue 
itself, interfering directly with the gas exchange; in the airways, thus 
reducing the air brought into the lung; or in the blood vessels, disturbing 
the blood circulation through the lung. Under rare conditions the respiratory 
musculature can be paralyzed, as in poliomyelitis, or injured by an accident.  
See  BRONCHIAL DISORDERS; LUNG DISORDERS; NOSE DISORDERS. 
    Among the diseases of pulmonary circulation, congenital malformations of 
the heart and pulmonary artery account for many cases of respiratory 
insufficiency of newborn and younger children, called blue babies because of 
the cyanosis. In adults, acquired heart diseases greatly influence the 
pulmonary circulation.  See  CARDIOVASCULAR SYSTEM; CIRCULATION DISORDERS. 
 �34s[Samuel P. Hammar]
�33s 
  
A700000001411�254s�5v�1v1491
\IMG\CI1441.IMG
7208448
�6v�2v�32sResponse   �21m �33s 
    A quantitative expression of the manner in which a microphone, amplifier, 
loudspeaker, or other component or system performs its intended function. A 
linear response means that the output signal is exactly proportional to the 
input signal for the entire range of frequencies over which the device is 
intended to operate. A logarithmic response means that the output signal is a 
logarithmic function of the input signal. The frequency response of a device, 
often presented as a curve on a graph, is the deviation over the frequency 
range from the response at some selected frequency, such as 1000 Hz.  See  
AMPLIFIER; CHARACTERISTIC CURVE; LOUDSPEAKER; MICROPHONE. 
 �34s[John Markus]
�33s 
  
A700000001137�254s�5v�1v1491
\IMG\CI1441.IMG
7208448
�6v�2v�32sRestionales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Commelinidae of the class Liliopsida (monocotyledons). The order 
consists of 5 families and nearly 500 species, some 400 of them belonging to 
the Restionaceae. The vast majority of the species grow in temperate regions 
of the Southern Hemisphere. The Restionales have reduced flowers and a single, 
pendulous, orthotropous ovule in each of the 1 - 3 locules of the ovary.  See  
COMMELINIDAE; LILIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001634�254s�5v�1v1491
\IMG\CI1441.IMG
7208448
�6v�2v�32sRest mass �21m �33s
    A constant associated with a material body which determines its inertial
properties and its internal energy content. It is sometimes called the
inertial mass.
    For a particle of rest mass m(sub o), Newton's second law of motion is given
by Eq. (1), where BOLD a = d BOLD v/dt is the acceleration of the particle and
BOLD F is the force applied to it. For a particle moving with a speed near that
of light Eq. (1)


	    m(sub o) d BOLD v/dt = BOLD F				    (1)


 (d/dt){(m[sub o] BOLD v)/SQUARE ROOT [1 - v(sup 2)/c(sup 2)]} = BOLD F     (2)

must be replaced by the relativistic equation of motion, Eq. (2), where c
is the speed of light. These equations are used for the measurement of the
rest masses of particles by deflection in suitable force fields.  See  INERTIA
OF ENERGY; RELATIVISTIC MECHANICS; RELATIVITY.
 �34s[Edward L. Hill]
�33s

A700000002500�254s�5v�1v1491
\IMG\CI1441.IMG
7208448
1492
\IMG\CI1441.IMG
7322624
�6v�2v�32sResultant of forces   �21m �33s 
    A system of at most a single force and a single couple whose external 
effects on a rigid body are identical with the effects of the several actual 
forces that act on the body. For analytic purposes, forces are grouped and 
replaced by their resultant. Forces can be added graphically (see 
illustration) or analytically. The sum of more than two vector forces can be 
found by extending the method of illustration c to a three-dimensional vector 
polygon in which one force is drawn from the tip of the previous one until all 
are laid out. The resultant force is the force vector required to close the 
polygon directed from the tail of the first force vector to the tip of the 
last. A force system has a zero force resultant if its vector polygon closes.  
See  CALCULUS OF VECTORS. 
    Two force systems are equivalent if their resultant forces, as described 
above, are equal and if their total vector moments about the same point are 
also equal. Vector moments are combined in the same manner as forces, that is, 
by parallelograms, triangles, or polygons. A resultant is the equivalent force 
system having the fewest possible forces and couples.  See  COUPLE; FORCE; 
STATICS.  
 �34s[Nelson S. Fisk]
�33s 
  
  
  
A700000002162�254s�5v�1v1492
\IMG\CI1441.IMG
7322624
�6v�2v�32sRetaining wall   �21m �33s 
    A wall designed to maintain differences in ground elevations by holding 
back a bank of material. Sometimes a retaining wall also serves as a 
foundation wall. The illustration shows the major types of retaining wall. 
    Material that exerts pressure on the back of the wall is called backfill. 
It includes material taken from the excavation and placed behind the wall. The 
load applied on backfill above the top of the wall level is called surcharge. 
    Earth pressure against the back of a wall is known as active pressure and 
acts at an oblique angle; the precise angle depends upon the character of the 
backfill, slope of the ground surface, presence of surcharge, and groundwater 
level. Earth pressure against the front of a wall is known as passive 
pressure, although the soil must be somewhat compressed before this force 
develops.  
    External stability is achieved when a wall is proportioned so that it will 
neither rotate nor slide under all dead-load and applied forces.  See  
FOUNDATIONS.  
 �34s[Robert D. Chellis]
�33s 
  
  
A700000000777�254s�5v�1v1492
\IMG\CI1441.IMG
7322624
�6v�2v�32sReticular formation   �21m �33s 
    Characteristic clusters of nerve cell bodies (gray matter) and their 
meshwork or reticulum, of fibers which are found in the brainstem and the 
diencephalon. The reticular formation is thought to be a complex, highly 
integrated mechanism which exerts both inhibition and facilitation on almost 
every type of activity of the central nervous system.  See  NERVOUS SYSTEM 
(VERTEBRATE).  
 �34s[Douglas B. Webster]
�33s 
  
A700000000551�254s�5v�1v1492
\IMG\CI1441.IMG
7322624
�6v�2v�32sReticulosa   �21m �33s 
    An order of the subclass Hexasterophora in the class Hexactinellida. This 
is a group of Paleozoic hexactinellids with a branching form. Each branch is 
provided with dermal, parenchymal, and gastral spicule reticulations.  See  
HEXASTEROPHORA.  
 �34s[Willard D. Hartman]
�33s 
  
A700000001420�254s�5v�1v1492
\IMG\CI1441.IMG
7322624
�6v�2v�32sRetinitis   �21m �33s 
    Any inflammatory condition involving the retina. In the majority of cases 
the adjacent middle layer of the eyeball (the choroid layer which contains the 
blood vessels of the eyeball) is also involved, in which case the condition is 
known as chorioretinitis. 
    Retinitis may be due to infectious agents (usually bacteria, less commonly 
fungi, and rarely viruses), mechanical injury of the eyeball (contusion), or 
intense light (photoretinitis). Chronic progressive damage to the retina 
(degenerative retinopathy) is frequently associated with conditions such as 
diabetes, high blood pressure, arteriosclerosis, senility, leukemia, and 
anemia.  See  EYE DISORDERS. 
 �34s[W. Robert Adams]
�33s 
  
A700000001156�254s�5v�1v1492
\IMG\CI1441.IMG
7322624
�6v�2v�32sRetortamonadida   �21m �33s 
    An order of parasitic flagellate protozoa belonging to the class 
Zoomastigophora. All retortamonads are medium to large in size and have a 
complicated blepharoplast-centrosome-axostyle apparatus (see illustration). 
Retortamonadida have two or four flagella, one turned ventrally into a 
cytostomal depression. The nucleus is located at the anterior tip. The body is 
twisted. These organisms are actually symbionts, ingesting bacteria in the 
digestive tracts of their hosts.  See  ZOOMASTIGOPHOREA. 
 �34s[James B. Lackey]
�33s 
  
  
A700000001773�254s�5v�1v1492
\IMG\CI1441.IMG
7322624
1493
\IMG\CI1441.IMG
7419392
�6v�2v�32sRetrograde motion (astronomy)   �21m �33s 
    In astronomy, either an apparent east-to-west motion of a planet or comet 
with respect to the background stars or a real east-to-west orbital motion of 
a comet about the Sun or of a satellite about its primary. The majority of the 
objects in the solar system revolve from west to east about their primaries. 
However, near the time of closest approach of Earth and a superior planet, 
such as Jupiter, because of their relative motion, the superior planet appears 
to move from east to west with respect to the background stars. The same 
apparent motion occurs for an inferior planet, such as Venus, near the time of 
closest approach to Earth. 
    Actual, rather than apparent, retrograde motion occurs among the 
satellites and comets; the eighth and ninth satellites of Jupiter and the 
ninth satellite of Saturn are examples.  See  ORBITAL MOTION. 
 �34s[Raynor L. Duncombe]
�33s 
  
A700000001614�254s�5v�1v1493
\IMG\CI1441.IMG
7419392
�6v�2v�32sReverberation   �21m �33s 
    After sound has been produced in, or enters, an enclosed space, it is 
reflected repeatedly by the boundaries of the enclosure, even after the source 
ceases to emit sound. This prolongation of sound after the original source has 
stopped is called reverberation. A certain amount of reverberation adds a 
pleasing characteristic to the acoustical qualities of a room. However, 
excessive reverberation can ruin the acoustical properties of an otherwise 
well-designed room. A typical record representing the sound-pressure level at 
a given point in a room plotted against time, after a sound source has been 
turned off, is given in the decay curve shown in the illustration. The rate of 
sound decay is not uniform but fluctuates about an average slope. See 
ARCHITECTURAL ACOUSTICS; SOUND. 
 �34s[Cyril M. Harris]
�33s 
  
A700000001160�254s�5v�1v1493
\IMG\CI1441.IMG
7419392
�6v�2v�32sReverse transcriptase   �21m �33s 
    A viral deoxyribonucleic acid (DNA) polymerase capable of effecting 
reverse transcription, that is, DNA synthesis directed by a ribonucleic acid 
(RNA) template. Reverse transcription is the opposite of the usual 
transcription, that is, RNA synthesis using a DNA template. Reverse 
transcriptases are present in tumor viruses and are related to retroviruses. 
They are used in genetic engineering to obtain DNA copies from purified RNA.  
See  DEOXYRIBONUCLEIC ACID (DNA); RIBONUCLEIC ACID (RNA). 
 �34s[Howard M. Temin]
�33s 
  
  
A700000002330�254s�5v�1v1493
\IMG\CI1441.IMG
7419392
�6v�2v�32sRevetment   �21m �33s 
    A means of protecting river banks against erosion. In some cases the 
revetment must act as a solid barrier; however, as a basic principle, it 
should be designed to induce the high-velocity thread of the current to move 
away from the threatened area. This is most readily accomplished when (1) the 
revetment is initiated at a point upstream from the point of attack and at an 
angle of not more than about 15DG to the current, and (2) the surface of the 
revetment is rough. 
    Revetments may consist of the following types: (1) rock paving with a 
heavy rock toe base; (2) rock, asphaltic, or concrete paving with brush, 
lumber, or concrete mattress for the underwater portions; and (3) pile dikes 
or other permeable fences. Rock paving has the advantage of flexibility in 
settling to heal breaches and permits easy repair. 
    Groins are short dikes, approximately perpendicular to the bank, which are 
sometimes used in lieu of revetment, particularly in areas where the bed is 
relatively stable. For other information on control of rivers  see  RIVER 
ENGINEERING.  See also  GROIN (ENGINEERING); RETAINING WALL. 
 �34s[Wendell E. Johnson]
�33s 
  
A700000001475�254s�5v�1v1493
\IMG\CI1441.IMG
7419392
�6v�2v�32sReynolds number   �21m �33s 
    In fluid mechanics, the ratio RHOvd/MU, where RHO is fluid density, v is 
velocity, d is a characteristic length, and MU is fluid viscosity. The 
Reynolds number is significant in the design of a model of any system in which 
the effect of viscosity is important in controlling the velocities or the flow 
pattern. In the evaluation of drag on a body submerged in a fluid and moving 
with respect to the fluids, the Reynolds number is important. 
    The Reynolds number also serves as a criterion of type of fluid motion. In 
a pipe, for example, laminar flow normally exists at Reynolds numbers less 
than 2000, and turbulent flow at Reynolds numbers above about 3000.  See  
DYNAMIC SIMILARITY; FLUID; MECHANICS. 
 �34s[Glenn Murphy]
�33s 
  
A700000001136�254s�5v�1v1493
\IMG\CI1441.IMG
7419392
�6v�2v�32sRhabdiasoidea   �21m �33s 
    A group of parasitic nematodes set apart by their characteristic 
morphology and by their life cycles, which may include a parasitic generation 
alternating with one or more free-living generations. One species, 
Strongyloides stercoralis, causes an important diarrheal disease of humans; 
others parasitize domestic animals and cause great economic loss; still others 
are of great biological interest because of their varied methods of 
reproduction and complicated life cycles. See NEMATA. 
 �34s[J. Allen Scott]
�33s 
  
A700000001154�254s�5v�1v1493
\IMG\CI1441.IMG
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�6v�2v�32sRhabditoidea   �21m �33s 
    A superfamily of small to moderate-sized nematodes with small, porelike, 
anteriorly located amphids and esophagus with corpus (with metacorpal 
swelling), isthmus, and valvulated basal bulb. Mainly terrestrial, they range 
in habits from saprophagous to parasitic. The life cycle is direct but 
sometimes includes an infective juvenile state that must be transported by an 
invertebrate. Most species live in decaying plant and animal matter and 
utilize saprophytic insects for transportation.  See  NEMATA. 
 �34s[Bruce Hopper]
�33s 
  
A700000001770�254s�5v�1v1493
\IMG\CI1441.IMG
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�6v�2v�32sRhabdocoela   �21m �33s 
    Formerly considered an order of the Turbellaria, the group Rhabdocoela is 
now usually divided into three orders, Catenulida, Macrostomida, and 
Neorhabdocoela. The temocephalids are a group of rhabdocoeles closely related 
to the Dalyelloida. They are sometimes considered a distinct order but are 
usually classified under the Neorhabdocoela. They differ from other 
rhabdocoeles chiefly in the possession of tentacles and adhesive organs and in 
the absence of epidermal cilia in most species. 
    The rhabdocoeles have a simple, unbranched intestine, with little if any 
diverticulations. The pharynx is simple or bulbous, the gonads are usually 
compact, a cuticular apparatus is associated with the copulatory organ, and 
there are two main longitudinal nerves. This large group includes most small 
fresh-water Turbellaria as well as many marine and some terrestrial species.  
See  TURBELLARIA. 
 �34s[E. Ruffin Jones]
�33s 
  
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�6v�2v�32sRhabdomesonata   �21m �33s 
    An extinct order of ectoproct bryozoans in the class Stenolaemata. Small 
and twiglike, a rhabdomesome branch consists of moderately short, tubular, 
solidwalled, moderately complex zooecia fused together and tightly packed 
radially around the central axis of the cylindrical branch.  See  BRYOZOA; 
STENOLAEMATA.  
 �34s[Roger J. Cuffey]
�33s 
  
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�6v�2v�32sRhamnales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 3 families; the Rhamnaceae with about 900 species, the Vitaceae 
with about 700, and the Leeaceae with about 70. The Rhamnales have a single 
set of stamens, opposite the petals, and usually a well-developed 
intrastaminal disk. The ovary has two or more locules, with one or two erect, 
bitegmic ovules from the base in each locule. The seeds have a well-developed 
endosperm.  See  MAGNOLIOPSIDA; ROSIDAE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sRheiformes �21m �33s
    An order of birds consisting of the single family Rheidae, containing the
two living species of rheas.
    The larger and better-known species, the common or greater rhea (Rhea
americana), inhabits grasslands of Brazil, Uruguay, Paraguay, Argentina, and
Bolivia. A smaller species, the Darwin's or lesser rhea (Pterocnemia pennata),
is found from southeastern Peru to the Straits of Magellan. Often called
American ostriches, the rheas differ from the true ostrich of Africa in their
smaller size, feathered head and neck, three- instead of two-toed feet, and
several other anatomical characters. Rheas have long been hunted for their
flesh and their plumage (from which feather dusters are made).	See  RATITES.
 �34s[Kenneth C. Parkes]
�33s



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�6v�2v�32sRhenanida   �21m �33s 
    An order of extinct marine fishes of the class Placodermi. They are 
flattened, raylike fishes with large, semicircular pectoral fins, dorsal eyes 
and nostrils, and a terminal mouth. The head shield consists of a few large 
plates separated by mosaics of small tesserae. The trunk shield is formed of 
only a single ring of plates, projecting ventrally far under the head.  See  
PLACODERMI.  
 �34s[Robert H. Denison]
�33s 
  
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�6v�2v�32sRhenium   �21m �33s 
    A chemical element, Re, with atomic number 75 and atomic weight 186.2. 
Rhenium is a transition element. It is a dense metal (21.04) with the very 
high melting point of 3440DGC (6220DGF). 
    Rhenium is similar to its homolog technetium in that it may be oxidized at 
elevated temperatures by oxygen to form the volatile heptoxide, Re2O7; this in 
turn may be reduced to a lower oxide, ReO2. The compounds ReO3, Re2O3, and 
Re2O, are well known. Perrhenic acid, HReO4, is a strong monobasic acid and is 
only a very weak oxidizing agent. Complex perrhenates, such as cobalt 
hexammine perrhenate, [Co(NH3)6(ReO4)3], are also known. 
    The halogen compounds of rhenium are very complicated, and a large series 
of halides and oxyhalides have been reported. Rhenium forms two 
well-characterized sulfides, Re2S7 and ReS2, as well as two selenides, Re2Se7 
and ReSe2. The sulfides have their counterparts in the technetium compounds, 
Tc2S7 and TcS2.  See  TECHNETIUM; TRANSITION ELEMENTS. 
 �34s[Sherman Fried]
�33s 
  
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�6v�2v�32sRheology �21m �33s
    The study of the deformation and flow of matter. The states of matter
differ strikingly in their density and in the ease with which they can be
deformed. The less dense the state of matter, the more easily deformable it
ordinarily is. The viscosity of a gas arises from the crossing-over of
molecules from a fast-moving layer into a neighboring more slowly moving
layer, and vice versa. Because this crossing-over increases with temperature
rise, the viscosity of a gas increases with temperature. Solids and liquids,
however, become more fluid with temperature rise.  See	VISCOSITY.

    Viscosity and structure.  The extra fluidity of liquids, as compared with
solids, arises from the great increase in the number of imperfections
introduced with the 10% expansion in melting shown by normal liquids. Ice is
an exception since it contracts by about 10% on melting. As a result, water
has a viscosity of about 17 millipoises at the melting point, which is close
to the normal value for ordinary liquids. (A poise is a unit of viscosity
equal to 1 dyne s/cm sup 2.) A solid melts at that temperature at which the
entropy from imperfections multiplied by the temperature equals the heat of
introducing them.
    That the viscosity of a system closely reflects structure is exemplified
by the fact that for nearly all simple substances the viscosity of the liquid
at the melting point is approximately 2 centipoises. Also, the reciprocal of
the viscosity, the fluidity, is a linear function of the molecular volume.
    Because holes are necessary to permit viscous flow and since pressure
tends to decrease the number of holes in a system, it follows that pressure
normally increases viscosity. The pressure coefficient of viscosity indicates
that the empty space a molecule requires in order to flow viscously is about
one-seventh its normal volume. To provide such vacant sites, an activation
energy of about one-third the heat of vaporization is required, as shown by
the temperature coefficient of viscosity. The normal effect of pressure rise
and temperature drop in increasing viscosity is modified in the rare cases
where such changes promote significant modifications in structure.
    When linear high polymers change their state, they introduce holes only in
directions normal to the length of the chain. Thus a linear polymer melts in
two directions but retains its solidlike properties along the chain length.
The result is that high polymers progress by wriggling a segment at a time, as
shown by the fact that they exhibit the expected pressure and temperature
coefficients of a molecule the size of a segment.

    Significant structures in liquids.	To obtain quantitative expressions for
the viscosity, a model of the liquid state is important. Upon melting, liquids
ordinarily expand and become orders of magnitude more fluid. Since melting
does not materially change nearest-neighbor distances, the expansion must be
due to holes in the liquid. These holes should approximate molecular size,
since smaller holes would not be as effective in facilitating molecular
motion, that is, are wasteful of entropy, and larger holes are unduly wasteful
of energy. Melting occurs because a thermodynamically more stable state
appears at the melting temperature where the kinetic energy can support the
fluidized vacancies, characteristic of the liquid state, against the potential
energy of melting, which tends to collapse the holes. Such considerations lead
to a model of the liquid in which fluidized vacancies in the liquid closely
mirror the vapor molecules in their size, motion, and concentration.

    High polymer viscosity.  Liquid high polymers shear as a result of
individual kinetic units jumping in a direction to reduce the stress. If the
high polymer is long enough, there will be entanglements and a given molecule
will be obligated to ``slalom'' around a neighbor with which it is entangled.

    Non-newtonian viscosity.  Frequently, systems are found which do not obey
Newton's equation for viscosity; instead, a small increase in stress is found
to disproportionately increase the rate of flow.  See  NON-NEWTONIAN FLUID.
 �34s[Henry Eyring]
�33s

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�6v�2v�32sRheostat   �21m �33s 
    A variable resistor constructed so that its resistance value may be 
changed without interrupting the circuit to which it is connected. It is used 
to vary the current in a circuit. The resistive element of a rheostat may be a 
metal wire or ribbon, carbon disks, or a conducting liquid.  See  
POTENTIOMETER (VARIABLE RESISTOR); RESISTOR. 
    The metallic rheostat is the most common. The wire or ribbon is 
constructed in a coil or a grid, and taps are brought out from different 
sections of the element to a multicontact switch which can short-circuit any 
section of the resistor or switch it out of the circuit. For more continuous 
control, as is needed for laboratory rheostats, a sliding-contact finger bears 
directly on closely wound coils of resistive wire. 
    Typical applications of rheostats are for starting or controlling the 
speed of motors, for adjusting generator characteristics, for controlling 
storage-battery charging, for dimming lights, and for imposing artificial 
loads on electrical equipment during tests. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sRheostatic braking   �21m �33s 
    A system of dynamic braking in which direct-current drive motors are used 
as generators and convert the kinetic energy of the motor rotor and connected 
load to electrical energy, which in turn is dissipated as heat in a braking 
rheostat connected to the armature. This is accomplished by disconnecting the 
motor armature from the supply lines and connecting them to a braking 
rheostat, while the field circuit remains connected to the supply line. The 
polarity of the armature remains unchanged, but the direction of the armature 
current reverses, resulting in a negative torque. The torque can be controlled 
by means of the braking rheostat.  See  DIRECT-CURRENT MOTOR; DYNAMIC BRAKING. 
 �34s[Arthur R. Eckels]
�33s 
  
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�6v�2v�32sRheumatic fever   �21m �33s 
    A childhood illness which follows an infection by Streptococcus 
hemolyticus, group A, by about 3 weeks. It is characterized by arthritis 
(redness and swelling of joints) and carditis (inflammation of heart tissue). 
Typically the polyarthritis is migratory with involvement of two or more 
larger joints. Other symptoms include nosebleeds and chorea, also known as St. 
Vitus dance. Fever, rapid pulse, paleness, and indisposition are present. 
Rheumatic fever is important because it can cause heart disease. The 
likelihood of this occurring and of permanent heart damage are increased by 
rheumatic fever's natural tendency to recur. Heart damage can show up later, 
though the active illness was decidedly mild.   
 �34s[Paul L. Boisvert]
�33s 
  
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�6v�2v�32sRheumatism   �21m �33s 
    Any combination of muscle or joint pain, stiffness, or discomfort arising 
from nonspecific disorders. It is generally used as a lay expression to 
indicate a chronic or recurrent condition affecting a certain area and 
precipitated by cold, dampness, or emotional stress. 
    Lumbago, wryneck, charleyhorse and shinsplint are commonly used 
expressions included under the catchall category of rheumatism. 
 �34s[Robert Searles]
�33s 
  
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�6v�2v�32sRhinoceros   �21m �33s 
    Odd-toed ungulates (order Perissodactyla) which are members of the family 
Rhinoceratidae. The bodies and limbs of these mammals are massive and 
thick-skinned. Also characteristic of the family are the horns (epidermal 
derivatives), either one or two, which are composed of a solid mass of hairs 
attached to a bony prominence on the skull. The feet are tridactyl, with three 
hooves; the middle one is the most developed. There are five species, found in 
Asia and Africa. 
    Sight is poorly developed in these animals; however, hearing is quite good 
and the sense of smell is excellent. Most species are solitary and aggressive 
and are extremely dangerous since they are unpredictable in behavior. All 
species are vegetarians. See PERISSODACTYLA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sRhinovirus   �21m �33s 
    A subgroup of the picornavirus group, including at least 70 antigenically 
distinct types. Like the enteroviruses, the rhinoviruses are small (17 - 30 
nanometers), contain ribonucleic acid (RNA), and are not inactivated by ether. 
Unlike the enteroviruses, they are isolated from the nose and throat rather 
than from the enteric tract, and are unstable if kept under acid conditions.  
See  ENTEROVIRUS. 
    Rhinoviruses have been recovered chiefly from adults with colds and only 
rarely from patients with more severe respiratory diseases.  See  COMMON COLD; 
PICORNAVIRUS.  
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sRhizobiaceae   �21m �33s 
    A family of aerobic, rod-shaped, gram-negative, soil-inhabiting bacteria 
consisting of two genera, Rhizobium and Agrobacterium. The bacteria that make 
up the two genera are chemoorganotrophs; metabolism is respiratory and 
molecular oxygen is the terminal receptor. Carbohydrates are utilized by all 
strains without appreciable acid formation; that is, gas is not produced. 
 �34s[Oscar N. Allen]
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�6v�2v�32sRhizocephala   �21m �33s 
    An order of crustacean parasites related to the barnacles. Worldwide in 
distribution, they prey on other crustaceans, principally Decapoda, such as 
crabs, shrimp, and their allies. Rhizocephala produce modifications affecting 
the abdomen of the crab, making males resemble females and causing immature 
females to acquire precociously the adult form. These parasites have become so 
modified by their mode of life that, as adults, they are no longer 
recognizable as barnacles, or even as crustaceans.  See  PARASITIC CASTRATION. 
    An adult rhizocephalan is a thin-walled sac enclosing a visceral mass, 
composed chiefly of ovaries and testes. It shows no trace of segmentation, 
appendages, or sense organs. Even an alimentary tract is missing. Instead, it 
possesses a threadlike root system which penetrates the interior of the host 
in all directions and absorbs nourishment from the body fluids of the crab. 
 �34s[Placidus G. Reischmann]
�33s 
  
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�6v�2v�32sRhizomastigida   �21m �33s 
    An order of the class Zoomastigophorea, also known as the Rhizomastigina. 
All Rhizomastigida species are microscopic and ameboid, and have one or two 
flagella. Rhizomastigida are small, colorless, and rarely abundant. They 
generally occur at the mud - water interface and are holozoic, saprozoic, or 
parasitic. Life histories are poorly known, except for Mastigella. The group 
is small, but species occur in both fresh and salt water.  See  
ZOOMASTIGOPHOREA.  
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sRhizopodea   �21m �33s 
    A class of Sarcodina including both parasitic and free-living species 
found in fresh and salt water and the soil. No species forms true axopodia; 
instead, pseudopodia may be filopodia, lobopodia, or reticulopodia; or there 
may be no pseudopodia in some cases. Rhizopodea include five subclasses: 
Lobosia, Filosia, Granuloreticulosia, Mycetozoia, and Labyrinthulia.  See  
FILOSIA; GRANULORETICULOSIA; LABYRINTHULIA; LOBOSIA; MYCETOZOIA; SARCODINA. 
 �34s[Richard P. Hall]
�33s 
  
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�6v�2v�32sRhizosphere   �21m �33s 
    The soil region subject to the influence of plant roots. It is 
characterized by a zone of increased microbiological activity and is an 
example of the relationship of soil microbes to higher plants. 
    A sharp boundary cannot be drawn between the rhizosphere and the soil 
unaffected by the plant (edaphosphere). At the root surface the rhizosphere 
effect is most intense, falling off sharply with increasing distance. 
    Growth of a plant markedly changes the microbial population of soil within 
its influence. In the rhizosphere there are more microorganisms than in soil 
distant from the plant. This increase is most pronounced with bacteria but is 
evident with other groups. The rhizosphere effect is seen in seedling plants; 
it increases with the age of the plant and usually reaches a maximum at the 
stage of greatest vegetative growth. Upon death of the plant the microbial 
population reverts to the level of the surrounding soil. Leguminous plants 
support higher rhizosphere populations than nonlegumes. The stimulation of 
microorganism growth in the rhizosphere results chiefly from the liberation of 
readily available organic substances by the growing plant.  See  SOIL 
MICROBIOLOGY.  
 �34s[Allan G. Lochhead]
�33s 
  
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�6v�2v�32sRhizostomeae   �21m �33s 
    An order of the class Scyphozoa with the most highly organized features of 
this class. The umbrella is generally higher than it is wide. The margin of 
the umbrella is divided into many lappets but is not provided with tentacles. 
Many radial canals, which are connected with each other to form a complicated 
network, issue from the cruciform stomach. The oral part is eight-sided, with 
many suctorial mouths surrounded by numerous small tentacles (see 
illustration). Usually there is no central mouth. No species of this group is 
injurious to the human skin. Some large forms, such as Rhopilema, are used as 
food in China and Japan. 
    A fair number of fossils of this order were found in the strata of the 
Jurassic Period.  See  SCYPHOZOA. 
 �34s[Tohru Uchida]
�33s 
  
  
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�6v�2v�32sRhodesian man   �21m �33s 
    The name frequently given to a group of archaic Homo sapiens found in 
eastern and southern Africa during the middle and the early upper Pleistocene, 
probably between 200,000 and 100,000 years ago. The best-known specimens are 
from Broken Hill, Northern Rhodesia, now Kabwe, Zambia. They include a nearly 
complete cranium (see illustration), the upper jaw of another individual, a 
child's skull fragment, and limb bones of at least four adults.  See  FOSSIL 
MAN.  
 �34s[Erik Trinkaus]
�33s 
  
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�6v�2v�32sRhodium �21m �33s
    A chemical element, Rh, atomic number 45, and atomic weight 102.905.
Rhodium is a hard, white metal, considerably less ductile than either platinum
or palladium, but much more ductile than any of the other platinum metals.
    The principal use of rhodium is as an alloying element for platinum.
Rhodium is an excellent hydrogenation catalyst and is active in catalytic
reforming of hydrocarbons. Rhodium is also used in electrical contact
applications. It is easily electroplated to form a hard, wear-resistant,
permanently bright surface, used on both stationary and sliding electrical
contacts, for mirrors and reflectors, and as a finish on jewelry.  See
ELECTROPLATING OF METALS; PLATINUM.
    Rhodium is resistant to most common acids, including aqua regia, even at
moderate temperatures. It is attacked by hot sulfuric acid, hot hydrobromic
acid, sodium hypochlorite, and free halogens at 200 - 600DGC (390 - 1110DGF).
The values of some important physical properties are given in the table.
    Rhodium trichloride, RhCl3, is a red compound which is insoluble in water.
Rhodium trihydroxide is soluble in some acids and may be used to produce
rhodium salts. Rhodium sulfate, Rh2(SO4)3.XH2O, is red or yellow and
soluble in water.
 �34s[Henry J. Albert]
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�6v�2v�32sRhodochrosite   �21m �33s 
    The mineral form of manganese carbonate. Calcium, iron, magnesium, and 
zinc have all been reported to replace some of the manganese. The equilibrium 
replacement of manganese by calcium increases with the temperature of 
crystallization.  
    Rhodochrosite occurs more often in massive or columnar form than in 
distinct crystals. The color ranges from pale pink to brownish pink. Hardness 
is 3.5 - 4 on Mohs scale, and specific gravity is 3.70. 
    Well-known occurrences of rhodochrosite are in Europe, Asia, and South 
America. In the United States large quantities occur at Butte, Montana. As a 
source of manganese, rhodochrosite is also important at Chamberlain, South 
Dakota, and in Aroostook County, Maine.  See  CARBONATE MINERALS; MANGANESE. 
 �34s[Robert I. Harker]
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�6v�2v�32sRhodonite   �21m �33s 
    A mineral inosilicate with composition MnSiO3. Hardness is 5.5 - 6 on Mohs 
scale, and specific gravity is 3.4 - 3.7. The luster is vitreous and the color 
is rose red, pink, or brown. Rhodonite is similar in color to rhodochrosite, 
manganese carbonate, but it may be distinguished by its greater hardness and 
insolubility in hydrochloric acid. It has been found at Langban, Sweden; near 
Sverdlovsk in the Ural Mountains; and at Broken Hill, Australia. Fine crystals 
of a zinc-bearing variety, fowlerite, are found at Franklin, New Jersey.  See  
SILICATE MINERALS. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sRhodophyta   �21m �33s 
    A large phylum of small to medium-sized plants, which are also called red 
algae. The phylum contains 2500 species, all of which are marine except about 
50 that grow in fresh water. They are most abundant in the tropics, where in 
clear water they are found growing at a depth of 600 ft (180 m). The majority 
of the species grow on rocks or other solid substrata, although there are a 
number of species which grow on other algae as epiphytes, endophytes, or 
partial parasites. There is a wide variation in the structure and appearance 
of the plants of this group, some being beautiful in either the fresh or the 
dried state (see illustration). 
    The red algae have a wide range of uses. Porphyra, highly esteemed in the 
Orient, is widely cultivated in some of the quieter estuaries and tidelands of 
Japan, where the sun-dried product is used both as a condiment and in soups. 
The most important contribution of members of this phylum is the production of 
agar, which is obtained from various species of Gelidium and other red algae. 
The red algae also include coralline or calcareous forms which contribute 
appreciably to the building up of coral reefs. 
    Phycoerythrin is usually present in such abundance that it masks the 
chlorophylls and carotenoids and gives the plants a range of shades of red 
which account for their common name, red algae. Sometimes phycocyanin is also 
present, giving the plants a bluish to almost black color. The variation in 
the amounts of these different pigments accounts for the wide diversity of 
colors to be found in the group. 
 �34s[Paul A. Vestal]
�33s 
  
  
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�6v�2v�32sRhombifera   �21m �33s 
    An extinct class of Echinodermata in which the thecal canals crossed the 
sutures at the edges of the plates, so that one-half of any canal lay in one 
plate and the other half on an adjoining plate. The canals sometimes occurred 
in lozenge-shaped clusters, called rhombs (see illustration). The theca was 
ovoid and sessile in earlier forms and comprised numerous irregular plates. 
The ambulacral grooves were trimerous and were restricted to the brachioles. 
In later forms the thecal plates became fewer and larger, and were arranged in 
five horizontal rows, termed cycles. A stem developed aborally and the 
ambulacra became pentamerous, traversing part of the theca before ascending 
the brachioles. Rhombifera probably died out in the Devonian.  See  CYSTOIDEA; 
ECHINODERMATA.  
 �34s[Howard B. Fell]
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�6v�2v�32sRhubarb   �21m �33s 
    A herbaceous perennial, Rheum rhaponticum, of Mediterranean origin, 
belonging to the plant order Polygonales. Rhubarb is grown for its thick 
petioles which are used mainly as a cooked dessert; it is frequently called 
the pieplant. The leaves, which are high in oxalic acid content, are not 
commonly considered edible. Outdoor rhubarb is a common garden vegetable in 
most areas of the United States except the South. Michigan and Washington are 
important centers for forced or hothouse rhubarb.  See  POLYGONALES. 
 �34s[H. John Carew]
�33s 
  
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�6v�2v�32sRhynchobdellae   �21m �33s 
    An order of the class Hirudinea. These leeches possess an eversible 
proboscis and lack hemoglobin in the blood. They may be divided into two 
families, the Glossiphoniidae and the Ichthyobdellidae. Glossiphoniidae are 
flattened, mostly small leeches occurring chiefly in fresh water. 
Ichthyobdellidae typically have cylindrical bodies with conspicuous, powerful 
suckers used to attach themselves to passing fish. They frequently have 
lateral appendages which aid in respiration.  See  HIRUDINEA. 
 �34s[Kenneth H. Mann]
�33s 
  
  
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�6v�2v�32sRhynchocephalia   �21m �33s 
    One of the two surviving orders of lepidosaurian reptiles; they are 
represented today, however, by a single species, Sphenodon punctatus. They 
have a typical diapsid skull, and, also typically, the teeth are fused to the 
edges of the jaws; their only characteristic specialization is that the upper 
jaw forms an overhanging beak. 
    The Rhynchocephalia appear to have originated from the Eosuchia in the 
Lower Triassic. The fairly numerous fossil genera are classified into five 
families.  See  LEPIDOSAURIA; REPTILIA. 
 �34s[Alan J. Charig]
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�6v�2v�32sRhynchocoela �21m �33s
    A phylum of bilaterally symmetrical, unsegmented, ribbonlike worms,
frequently referred to as the Nemertinea. They have an eversible proboscis and
a complete digestive tract with an anus. There is no coelom or body cavity,
and the mesenchyme or parenchyma and the muscle fibers fill the area between
the ciliated epidermis and the cellular lining of the digestive tract. Many
species are brightly colored, sometimes having stripes or transverse bars.
    The nemertineans are the simplest animals with a circulatory system. There
are two lateral blood vessels and in some a third, unpaired dorsal vessel. The
blood consists of a colorless fluid which may contain blood cells of several
types. In species in which the blood is colored, the pigment is present in the
cells. There is no heart, but the walls of the principal vessels may be
contractile.
    The nervous system has a pair of cerebral ganglia forming the brain as
well as two longitudinal nerve cords and many smaller nerves. The ganglia and
lateral cords may contain unusually large neurochord cells. In the epidermis
there are scattered sensory nerve cells, probably tactile. A few to many
simple eyes, or ocelli, may be present in front of the cerebral ganglia. There
are no special respiratory organs; respiration occurs through the body
surface. Nemertineans are usually either male or female, but a few individuals
have both sex organs. Fertilization occurs outside the body in many species
but may be internal in certain forms.
    The nemertineans are mostly marine, bottom-dwelling worms, found in
greatest numbers along the coasts of northern temperate regions. They live
under stones, among the tangled masses of plants, in sand, mud, or gravel, and
sometimes form mucus-lined tubes. A few are pelagic, fresh-water, or
terrestrial. Certain species are commensal with other animals, but none can be
regarded as parasitic in a strict sense.
    That the Rhynchocoela represent the most highly organized acoelomate
animals is indicated by the circulatory system, the presence of an anus, and
the specialization of the epidermis. All groups of animals more complex than
the nemertineans have some kind of cavity, a pseudocoele or coelom, between
the body wall and the gut, instead of solid mesenchyme.  See  COELOM.
    The phylum Rhynchocoela, containing about 550 known species, is divided
into two classes, Anopla and Enopla.  See  ANOPLA; ENOPLA.
 �34s[Arthur G. Humes ;J. B. Jennings]
�33s

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�6v�2v�32sRhynchonellida   �21m �33s 
    An order of articulate brachiopods, representatives of which are first 
known in beds of Middle Ordovician age and which is still extant.  See  
ARTICULATA (BRACHIOPODA). 
    Typical forms are dorsibiconvex, with a dorsal fold and ventral sulcus; 
the posterior margin is curved, the dorsal interarea absent, and the ventral 
one greatly reduced. The majority of forms are ornamented by ribs. The 
majority of the order are impunctate.  See  BRACHIOPODA. 
 �34s[A. J. Rowell]
�33s 
  
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�6v�2v�32sRhyniophyta   �21m �33s 
    A division of the subkingdom Embryobionta. The bryophytes and vascular 
plants are included in this subkingdom. The category Rhyniophyta was devised 
for the relatively simple Silurian-Devonian vascular plants long held to be 
ancestral to other groups of vascular plants and usually referred to as 
Psilophytales. These plants have leafless stems and lack roots; their general 
morphological structure is not complex. The three classes of Rhyniophyta 
currently recognized are Rhyniopsida, Zosterophyllopsida, and 
Trimerophytopsida.  See  EMBRYOBIONTA; PSILOPHYTALES; RHYNIOPSIDA; 
TRIMEROPHYTOPSIDA; ZOSTEROPHYLLOPSIDA. 
 �34s[Harlan P. Banks]
�33s 
  
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�6v�2v�32sRhyniopsida   �21m �33s 
    The earliest demonstrable vascular land plants, appearing in Silurian 
(mid-Ludlovian) time. Their small, leafless axes were usually branched 
dichotomously in three planes; adventitious and perhaps pseudomonopodial 
branching also occurred. Sporangia were usually terminal on the main axes. 
Some terminated in lateral branches, and some were subtended by adventitious 
branches.  See  RHYNIOPHYTA.   
 �34s[Harlan P. Banks]
�33s 
  
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�6v�2v�32sRhyolite   �21m �33s 
    A light-colored, aphanitic (not visibly crystalline) or porphyritic rock 
of volcanic origin. If completely crystallized, rhyolite consists largely of 
alkali feldspar and quartz (or another polymorph of SiO2), and subordinate 
oligoclase, and minor amounts of mafic minerals (biotite, hornblende, or 
pyroxene).  See  IGNEOUS ROCKS. 
    Rhyolites composed almost entirely of glass are called obsidian, 
pitchstone, perlite, or pumice. Typical rhyolites are mixtures of 
microcrystalline aggregates and glassy material, but many are entirely 
crystalline, and some grade into microgranite. Porphyritic varieties are those 
in which numerous large crystals (phenocrysts) are disseminated throughout an 
aphanitic matrix.  See  OBSIDIAN; PHENOCRYST; PORPHYRY; VOLCANIC GLASS. 
    Next to basalt and andesite, rhyolite is considered the most common 
volcanic rock. It occurs most frequently as a fragmented rock (tuff) which 
originated from a pyroclastic eruption, either as an ash flow or an air-fall 
deposit. The rock is widespread and occurs in large quantities associated with 
andesite, basalt, and small amounts of dacite and quartz latite.  See  MAGMA; 
PETROGRAPHIC PROVINCE; TUFF. 
 �34s[William Ingersoll Rose, Jr.]
�33s 
  
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�6v�2v�32sRiboflavin   �21m �33s 
    Also known as vitamin B2, riboflavin is widely distributed in nature, and 
is found mostly in milk, egg white, liver, and leafy vegetables. It is a 
water-soluble yellow-orange fluorescent pigment. 
    Riboflavin deficiency results in poor growth and other pathologic changes 
in the skin, eyes, liver, and nerves. Riboflavin deficiency in humans is 
usually associated with a cracking at the corners of the mouth called 
cheilosis; inflammation of the tongue, which appears red and glistening 
(glossitis); corneal vascularization accompanied by itching; and a scaly, 
greasy dermatitis about the corners of the nose, eyes, and ears.  See  VITAMIN. 
 �34s[Stanley N. Gershoff]
�33s 
  
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�6v�2v�32sRibonucleic acid (RNA)   �21m �33s 
    A class of long, unbranched nucleic acid macromolecules mainly involved in 
translating into proteins the genetic information that is carried in DNA. 
There are two main functions of RNAs: that of acting as tools in the synthesis 
of any protein and that of acting specifically in the syntheses of one 
protein. The major classes of RNA of the first kind include transfer RNA 
(tRNA) and ribosomal RNA (rRNA), and of the second kind messenger RNA (mRNA); 
viral RNA may belong to both classes. RNA is the intermediate of the pathway 
DNA -> RNA -> protein. The information in DNA is first transcribed into RNA 
which then directs the synthesis of specific proteins.  See  DEOXYRIBONUCLEIC 
ACID (DNA). 
    RNA is a long chain of repeating units called nucleotides. Each nucleotide 
is composed of three parts: a five-carbon sugar called ribose; a phosphate 
group; and most important, attached to each ribose, is one of four bases - 
adenine (A), cytosine (C), guanine (G), or uracil (U). Thus there are four 
different nucleotides, depending on which base is present. The structure and 
function of the RNA depend on the sequence and the number of nucleotides of 
which it is composed. 
    Transfer RNA is the smallest class of RNA known that has a biological 
function, which is to carry an amino acid to the ribosome during the process 
of protein synthesis. Transfer RNAs have a molecular weight of 25,000 daltons 
and contain from 73 to 90 nucleotides. 
    Ribosomes are particles made up of RNA and protein, which act as 
workbenches on which the process of linking amino acids together to form 
proteins occurs. Each ribosome consists of two subunits, a large one and a 
small one. Ribosomal RNAs, like tRNAs, are stable molecules, but, unlike 
tRNAs, they contain very few unusual bases.  See  RIBOSOMES. 
    The concept of a messenger (m) RNA that was involved in the expression of 
the genetic information in DNA was a big step forward in understanding the 
mechanisms involved in molecular genetics. Messenger RNA is synthesized by the 
enzyme RNA polymerase from one of the two strands of DNA. It is made up of a 
sequence of ribose nucleotides with bases complementary to those of that 
strand of DNA.  See  PROTEIN. 
    While most organisms carry their genetic information in the form of DNA, 
certain viruses have RNA as their genetic material. In RNA viruses, the RNA 
plays the role of DNA (since it contains the genetic information) and often 
acts as mRNA as well. There are several different types of viral RNA. In some 
RNA viruses the RNA is present as a long, single-stranded piece (such as in 
polio virus), while in other cases the RNA is in several small single-stranded 
pieces (such as in influenza virus, which contains about 10 different pieces 
of RNA in each virus).  See VIRUS. 
 �34s[Bernard S. Dudock]
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�6v�2v�32sRibose �21m �33s
    A water-soluble pentose, also known as D-ribose, which, together with
2-deoxy-D-ribose, makes up the carbohydrate constituents of nucleic adics,
which are found in all living organisms. The structural formula is shown.

HCOH---
 |    |
HCOH  O
 |    |
HCOH  |
 |    |
HC----
 |
CH2OH
D-Ribose

    The universal occurrence of nucleic acids in all living cells makes this
pentose highly interesting to biochemists and biologists. The type of nucleic
acid that yields D-ribose is referred to as ribonucleic acid (RNA). D-Ribose
is a constituent not only of the nucleic acids, but also of several vitamins
and coenzymes. As in the nucleic acids, this sugar occurs in the furanose
configuration in these natural products.  See  NUCLEIC ACID.
 �34s[William Z. Hassid]
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�6v�2v�32sRibosomes   �21m �33s 
    Complex particles, about 25 nanometers in diameter, which synthesize 
proteins within the living cell. They are composed of many different proteins 
in addition to three molecules of ribonucleic acid (RNA). Each ribosome 
consists of two sub-units, one approximately twice the size of the other. The 
small subunit contains one large molecule of RNA, whereas the large subunit 
has two RNA molecules, one twice as large as that in the small subunit, and 
the other is considerably smaller (see illustration). The ribosomal proteins 
make up the remainder of the ribosome particle. There are about 20 different 
proteins in the small subunit and 40 in the large one; as far as has been 
determined, each kind of protein molecule is present in just a single copy in 
the entire ribosome. The ribosomes in higher organisms (with a total mass of 
4,000,000 molecular weight units) are one-third to one-half larger than the 
ribosomes in bacteria and other microorganisms (which have a mass of about 
2,700,000). A considerably greater mass of proteins accompanies the somewhat 
larger RNA molecules in higher organisms. Fully one-half of the mass is 
represented by proteins in these ribosomes. 
    Ribosomes play an important part in protein synthesis. Part of the 
deoxyribonucleic acid (DNA) in the nucleus serves as a template for the 
synthesis of a messenger RNA (mRNA) molecule, whose sequence of nucleotides is 
complementary with that of the DNA. This mRNA strand then passes along a 
ribosome, where it mediates the production of a specific protein molecule as 
follows. When each successive set of three nucleotides arrive at the 
appropriate site on the ribosome, a specific amino acid is added to the 
polypeptide. The mechanism of this ``translation'' depends upon each kind of 
the 20 amino acids being coupled enzymatically with a particular transfer RNA 
(tRNA) molecule. If a particular set of three nucleotides (triplet) on this 
coupled tRNA is complementary with the mRNA triplet in position, the coupled 
amino acid is added to the growing polypeptide chain by the ribosome.  See  
GENETIC CODE; RIBONUCLEIC ACID (RNA). 
    Several ribosomes may be spaced along the length of the mRNA molecule, 
simultaneously utilizing different parts of its coded structure. This complex 
is called a polyribosome or polysome. Polysomes are not formed by a simple 
association between intact ribosomes and the mRNA strand. Instead, a union is 
first established between the small subunit and the messenger. A large subunit 
is then added to the assembly, resulting in the complete protein-synthesizing 
machine. Further ribosomes are joined together in this sequential manner at 
the leading end of the messenger, thus forming the polysome. In the cells of 
higher organisms, polysomes may exist free in the cytoplasm, or may be 
attached to membranes of the endoplasmic reticulum (ER), making proteins to be 
contained within a membranous enclosure.  See  ENDOPLASMIC RETICULUM.   
 �34s[Breck E. Byers]
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�6v�2v�32sRibulose �21m �33s
    A pentose sugar, also known as D-riboketose and D-erythropetulose; it has
never been prepared in crystalline form, and exists only as a sirup. The
structural formula of ribulose is shown below.

				   CH2OH
				   |
				   C==O
				   |
				  HCOH
				   |
				  HCOH
				   |
				   CH2OH

    Ribulose-5-phosphate occurs in aminal and plant tissues. It can be
converted to ribulose-1,5-diphosphate by a phosphokinase enzyme acting in the
presence of adenosinetriphosphate (ATP). The ribulose-5-phosphate is also a
significant intermediate in the carbohydrate metabolism through the pentose
phosphate pathway.  See  ADENOSINETRIPHOSPHATE (ATP); CARBOHYDRATE METABOLISM;
MONOSACCHARIDE.
 �34s[William Z. Hassid]
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�6v�2v�32sRice �21m �33s
    The plant Oryza sativa is the major source of food for nearly one-half of
the world's population. In China, Japan, Korea, the Philippines, India, and
other countries of Asia, rice is far more important than wheat as a source of
carbohydrates. The most important rice-producing countries are mainland China,
India, and Indonesia, but in many smaller countries rice is the leading food
crop. In the United States rice production is largely concentrated in selected
areas of Arkansas, California, Louisiana, and Texas. Although this represents
less than 1% of the world rice acreage, the United States often is the world's
largest exporter of rice, since most of the world rice crop is consumed in the
countries where it is produced.
    Over 95% of the world rice crop is used for human food. Although most rice
is boiled, a considerable amount is consumed as breakfast cereals. Rice starch
also has many uses. Broken rice is used as a livestock feed and for the
production of alcoholic beverages. The bran from polished rice is used for
livestock feed; the hulls are used for fuel and cellulose. The straw is used
for thatching roofs in the Orient and for making paper, mats, hats, and
baskets. Cultivated rices are classified as upland and lowland. Upland types,
which can be grown in high-rainfall areas without irrigation, produce
relatively low yields. The lowland types, which are grown submerged in water
for the greater part of the season, produce higher yields. In contrast to most
plants, rice can thrive when submerged because oxygen is transported from the
leaves to the roots. Rice varieties are also classified as long- or
short-grain. Most longgrain rices have high amylose content and are dry or
fluffy when cooked, while most short-grain rices have lower amylose content
and are sticky when cooked.  See  CYPERALES.
 �34s[John A. Shellenberger]
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�6v�2v�32sRicinuleida   �21m �33s 
    An order of extremely rare arachnids, also known as the Podogona, with a 
body less than 1 in. (2.5 cm) in length. Superficially, they resemble ticks in 
general appearance and movement, and are found only in tropical Africa and in 
the Americas, from the Amazon to Texas. The two anterior pairs of appendages 
are chelate. Less than 25 modern species are known. The occurrence of several 
fossils from Carboniferous time suggests that the group was formerly more 
common. See ARACHNIDA. 
 �34s[C. Clayton Hoff]
�33s 
  
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�6v�2v�32sRickets �21m �33s
    A disorder of calcium and phosphorus metabolism, primarily affecting bony
structures, due to vitamin D deficiency. Precursor substances from the diet
are normally converted to vitamin D by the action of ultraviolet light
(sunlight) on the skin. Therefore, infants are affected, especially in winter.
Dark-skinned peoples require additional vitamin D as dietary supplement
because their skin pigmentations interfere with natural production.  See
VITAMIN D.
    In children, rickets consists of defective calcification and excessive
production of cartilage at the ends of growing bones. Restlessness, constant
movement often with hair loss from pillow contact, and defects of the ribs and
long bones are typical symptoms. Bowlegs and pigeon breast may result, in
addition to craniotabes, or softening of the flat skull bones, which later
develop abnormally to form a square, boxlike skull. There is an increased
susceptibility to fracture, delayed tooth eruption and enamel defects, and
other indications of faulty mineral deposition.  See  TOOTH DISORDERS.
    In adults, vitamin D deficiency produces osteomalacia, or demineralization
of bones. In addition, low calcium levels in the blood may be present with
resulting irritability, spasms, and convulsions, particularly of the hands,
face, and larynx.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sRickettsiales   �21m �33s 
    An order of very small bacteria (0.2 - 0.5 micrometer in diameter) which 
are mostly obligate intracellular parasites of animals. The order is divided 
into three families, Rickettsiaceae, Bartonellaceae, and Anaplasmaceae. Only 
the species causing trench fever in humans, among mammalian parasites, has 
been grown on artificial, cell-free media. Another, the Q-fever agent, is 
filterable.  See  CARRION'S DISEASE; EPIDEMIC TYPHUS FEVER; Q FEVER. 
 �34s[Cornelius B. Philip]
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�6v�2v�32sRickettsialpox   �21m �33s 
    A benign, infectious disease, similar to typhus, caused by bacterialike 
microorganisms, Rickettsia akari. It is transmitted by the mite 
Allodermanyssus sanguineus. In addition to a rash, patients exhibit a primary 
eschar, or ulcer, and often show a swelling of lymph glands similar to that in 
scrub typhus and fievre boutonneuse.  See  RICKETTSIOSES.   
 �34s[Cornelius B. Philip]
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�6v�2v�32sRickettsioses �21m �33s
    Infectious diseases affecting humans and some animals, caused by certain
bacterial microorganisms; the human pathogens are members of the genera
Rickettsia, Rochalimaea, and Coxiella, which are classified in the family
Rickettsiaceae. These rickettsial agents make up a restricted group found in
parasitic arthropods, such as lice, fleas, ticks, and mites, which act as
vectors and often as the primary hosts.
    Rickettsia species are the etiologic agents of such human diseases as
epidemic and endemic, or murine, typhus, the Rocky Mountain spotted fever group
of diseases, including fievre boutonneuse of the Old World, and several tick
typhuses, as well as the rickettsialpox of the Atlantic coast of the United
States and of the Soviet Union, and tsutsugamushi disease, or scrub typhus, of
the Far East.
    Clinically all are characterized by fever and a cutaneous eruption which is
absent only in related Q fever.  In addition, a primary ulcer, or eschar, at the
site of vector attachment and swollen glands which drain this area are
associated with the fievre boutonneuse subgroup, north Asian and Queensland tick
typhuses, rickettsialpox, and scrub typhus.  Orchitis and swelling of the
scrotum customarily occur in male guinea pigs infected with murine typhus and
the spotted fever group.  See EPIDEMIC TYPHUS FEVER; FIEVRE BOUTONNEUSE; MURINE
TYPHUS FEVER; Q FEVER; RICKETTSIALPOX; SCRUB TYPHUS.
 �34s[Cornelius B.  Philip]
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�6v�2v�32sRiemannian geometry �21m �33s
    The geometry of an N-dimensional space which bears a coordinate system (x)
and has associated with it and the given coordinate system a set of Nsup2
functions gsubasubb(x) which are involved in the determination of certain
fundamental geometric magnitudes, including lengths of arcs in the space,
lengths of vectors, angles between vectors, the measures of parts of the space
(areas, volumes, and hypervolumes), and curvatures. The arc length s, for
example, of a parameterized arc xsupa = xsupa(t) of class C' is given by the

		DELTA s =  s(t[sub 2]) - s(t[sub 1]) = INTEGRATION
       (FROM T(sub 1) TO t(sub 2)) SQUARE ROOT [g(sub ab)X'(sup a)X'(sup b)dt]

formula below. Here, the prime (') indicates differentiation with
respect to t so that x'supa = dxsupa/dt and Einstein's summation convention is
in force. The quantities gsubasubb are, as indicated, functions of the N
coordinate variables xsup1, xsup2, . . . , xsupN denoted briefly by x. The
general subject may be classified into subspecies by means of the value of N
and the conditions imposed on the fundamental quantities g(sub ab) either
directly or through derived quantities.
    Riemannian geometry was initiated in 1854 by G. F. B. Riemann (1826 -
1866), with the quadratic forms g(sub ab)x'(sup a)x'(sup b) of course limited to
the positive definite case. In later developments this restriction is removed.
    The importance of this subject is due to several facts. First, it includes
both euclidean geometry and the geometry of surfaces in euclidean space and,
in addition, provides their most natural extension, a generalization of great
richness and scope. Second, it provides a geometrical realization or
application for certain of the major abstractions of tensor analysis and
together with that discipline, with which it merges, forms the pattern and
much of the motivation for general relativity. Third, most of the work on the
incorporation of electricity and magnetism into the mold of general relativity
(unified field theory) almost necessarily involves a riemannian geometry of
one type or another (nonsymmetric g[sub ab]'s, symmetric g[sub ab] with N = 5,
g[sub ab]'s in projective coordinates). Fourth, riemannian geometry provides
the steppingstone to a variety of generalized differential geometries.
    Finally, in the absence of contrary evidence, the success of the general
theory of relativity suggests that actual physical space may correspond more
closely to a noneuclidean riemannian geometry than to the euclidean variety.
See  RELATIVITY.
 �34s[Homer V. Craig]
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�6v�2v�32sRift valley   �21m �33s 
    An elongated, relatively narrow depression caused by the subsidence of a 
crustal block between two or more faults. The boundary faults are steeply 
inclined down toward the downthrown block, and where the direction of 
displacement has been ascertained, it indicates that the dislocations are 
gravity faults. Thus, rift valleys are the surface expression of large graben 
(see illustration).  See  FAULT AND FAULT STRUCTURES; GRABEN. 
    Rift valleys have been interpreted to be a part of the major tectonic 
pattern of the Earth and to be the result of deep-seated deforming pressures. 
They are associated with many mid-oceanic ridges, where they are interpreted 
to be graben in an uplifted segment of oceanic crust over a rising column of 
mantle material. These rift valleys occupy positions where the oceanic crust 
has split apart and new crust is forming. Smaller, less extensive rift valleys 
occur in regions underlain by continental crust. 
 �34s[Philip H. Osberg]
�33s 
  
  
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�6v�2v�32sRigel   �21m �33s 
    Beta Orionis, a blue supergiant of spectral type B8. Although Rigel is one 
of the apparently brightest stars in the sky, it is too distant to have a 
measurable parallax or proper motion. It has a faint companion with which it 
is probably physically connected. Having a luminosity about 60,000 times that 
of the Sun means that Rigel is an exceptionally young star in rapid evolution, 
with a life span of only a few million years.  See  STAR. 
 �34s[Jesse L. Greenstein]
�33s 
  
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�6v�2v�32sRigid body   �21m �33s 
    An idealized extended solid whose size and shape are definitely fixed and 
remain unaltered when forces are applied. Treatment of the motion of a rigid 
body in terms of Newton's laws of motion leads to an understanding of certain 
important aspects of the translational and rotational motion of real bodies 
without the necessity of considering the complications involved when changes 
in size and shape occur. Many of the principles used to treat the motion of 
rigid bodies apply in good approximation to the motion of real elastic solids. 
  See  RIGID-BODY DYNAMICS. 
 �34s[Dudley Williams]
�33s 
  
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�6v�2v�32sRigid-body dynamics �21m �33s
    A rigid body is defined as an assemblage or system of mass particles that
are located rigidly with respect to one another and therefore can have no
motion relative to each other. Motion of a rigid body can occur by movement of
all points of the body in a parallel direction through equal distances during
a given interval of time, called translation, or by movement of all points in
circles about a common axis with a common angular velocity, called rotation,
or by combined translation and rotation.

    Center of mass.  The center of mass of a rigid body is a single point
located within the body such that any force acting externally on the body
along a line of action which passes through this point will result in pure
translation, that is, no rotation.  See  CENTER OF MASS.
    To calculate the position of the center of mass, consider a rigid body
divided into elemental volumes (or particles) labeled dV in the illustration.
Each such elemental volume is located from a fixed point O by means of a
position vector BOLD r and has a mass density rho.
    The total mass m of the rigid body can then be expressed by the volume
integral in Eq. (1).

		  m = INTEGRATION (in terms of V) rho dV		     (1)

    The center of mass of the rigid body is then defined as the point within
the rigid body whose position vector BOLD r(sub c) is given by Eq. (2).

     BOLD r(sub c) = (1/m) INTEGRATION (in terms of V) BOLD r rho dV	     (2)


    Translational motion.  The motion of the center of mass of a rigid body
can be derived directly from Newton's second law applied to each particle of
the rigid body and summed for all such particles. All internal forces sum to
zero because such forces between particles of the body occur in equal and
opposite pairs (Newton's third law). The motion of the center of mass is the
same as would be the case if all of the mass of the body were concentrated at
this mass center and all external forces were applied there. The motion of
translation of the center of mass of a rigid body is therefore determined by
the same methods as those used to determine the motion of a particle. The one
significant difference from particle dynamics, however, is that the forces are
those acting on the actual rigid body and as such often depend upon the shape
of the body, its orientation in space, and sometimes upon the time rate of
change of this orientation in space. Examples are the motion of an airplane or
projectile through air or of a submarine through water.
    The motion in translation may also be expressed in terms of a vector
quantity called linear momentum.  The quantity BOLD P = m BOLD v(sub c) where
BOLD v(sub c) is the velocity of the center of mass is defined as the linear
momentum of the rigid body.  The resultant external force BOLD F acting on a
rigid body is equal to the time rate of change of the linear momentum of the
body.

    If no external forces are acting, the linear momentum of the body remains
constant - a statement of the law of conservation of linear momentum.  See
CONSERVATION OF MOMENTUM; MOMENTUM.


    Rotational motion.	The equations that describe the rotational or angular
motion of a rigid body are again derivable from Newton's second law. In the
illustration, a rigid body of arbitrary shape is located and moving in an
inertial system. This body may be subdivided into mass particles dV of mass
m(sub i) and Newton's law applied to each particle, resulting in Eq. (3).

     m(sub i) (d[sup 2] BOLD r)/(dt[sup 2]) = BOLD F			  (3)

    If both sides of Eq. (3) are multiplied by the position vector BOLD r (by
means

      m(sub i) {BOLD r MULT (d(sup 2) BOLD r)/(dt(sup 2))} =		     (4)
		   (BOLD r MULT BOLD F)

of a cross product), Eq. (4) is derived. Here the vector product (BOLDr CROSS
CTR DOT BOLDF) is defined as the moment BOLDM or torque about the point O of the
resultant force BOLDF acting on dV.  See  TORQUE.

m(sub i) {BOLD r MULT [(d(sup 2) BOLD r)/(dt(sup 2))]} = m(sub 1) (d/dt)[BOLD r
MULT (d BOLD r/dt)] = (d/dt) [BOLD r MULT (m(sub i) BOLD r/dt)] 	     (5)

    Equation (4) may be written as Eq. (5) where BOLD r, BOLD v, and m(sub i)
are the position vector, velocity, and mass, respectively, of the element dV.
Equation (5) is valid for each elemental particle of the rigid body, and by
summing the equation for all such particles of the body, the equation of
angular motion is obtained from Eq. (6a), where m(sub i) has been replaced by
rho dv as before.

SUMMATION BOLD M(sub o) = (BOLD M[sub o])(sub ext) = (d/dt) INTEGRATION
(in terms of V) (BOLD r MULT BOLD v) rho dV = [d BOLD H(sub o)]/dt	    (6a)


INTEGRATION (in terms of V) [BOLD r MULT (m[sub i] BOLD v)] =
INTEGRATION (in terms of V) (BOLD r MULT BOLD v) rho dV = BOLD H(sub o)  (6b)

Equation (6b) expresses the moment of linear momentum or the
angular momentum about a point O of a rigid body. On the left-hand side of Eq.
(6a), the summation process results in zero for all equal and opposite
collinear forces between particles (internal forces), and only the moment
M(sub o) of the external forces exists.
    Equation (6a) expresses the principle of angular momentum, that the time
rate of change of angular momentum of a rigid body about any point O fixed in
an inertial system is equal to the resultant moment of external forces about
the same point O. If the rigid body is not acted upon by any external moment,
the angular momentum must remain constant. This is the principle of
conservation of angular momentum.  See	ANGULAR MOMENTUM.
    An important extension of Eq. (6a), the angular momentum equation, is as
follows. Equation (6a) is precisely valid for the case in which the angular
momenta and the moments of external forces are taken about the center of mass
of a rigid body, even though the center of mass does not remain at rest in an
inertial system. In this case the equation is that given as Eq. (7), where c

	       BOLD M(sub c) = d BOLD H(sub c)/dt			  (7)

denotes the center of mass. Equation (7) is useful, for example, in predicting
the motion of a projectile moving through air under accelerated conditions.
The fact that the angular equation can be written about the moving center of
mass fixed within the projectile, instead of about some fixed point located
exterior to the projectile, results in great simplification in the analysis of
the motion.  See  BALLISTICS.

    Work and energy.  The change in the kinetic energy T of a rigid body in
going from locations 1 to 2 in space is equal to the work done on the rigid
body during this period. Or, the rate of change of kinetic energy of the body
is equal to the rate at which work is done by all external forces acting on
the body. If the forces acting are a function of the coordinates only and the
work done is therefore independent of the path the body follows (gravitational
forces, for example), the system is said to be conservative (frictionless,
without energy dissipation). Then the work done on the body by the forces
acting on it, when this body moves in a conservative force field from
locations 1 to 2 in space, is called the potential energy of the body at 2
with respect to 1, or is equal to the negative of the change in potential
energy V of the body in passage from 1 to 2. The sum of kinetic and potential
energies therefore remains constant throughout the motion. This is the
principle of conservation of energy for a conservative system.	See
CONSERVATION OF ENERGY; ENERGY; WORK.
 �34s[Ray E. Bolz]
�33s

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�6v�2v�32sRing �21m �33s
    A tie member or chain link. Tension or compression applied through the
center of a ring produces bending moment, shear, and normal force on radial
sections. Because shear stress is zero at the boundaries of the section where
bending stress is maximum, it is usually neglected.
 �34s[William J. Krefeld /Waldo G. Bowman]
�33s

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�6v�2v�32sRing gages   �21m �33s 
    Cylindrical rings of steel whose inside diameters are ground and then 
honed or lapped to gage tolerance; used for checking the diameter of a 
cylindrical object such as a shaft. The gaging surface is sometimes 
chrome-plated or made of carbide for wear resistance. A single gage controls 
one limit of tolerance of the diameter. The ``go'' gage will pass over the 
largest acceptable part but will not pass over any larger part. The ``not go'' 
gage will not pass over any part within tolerance but will pass over an 
undersize part. A ring gage is not preferred as a ``not go'' gage; an 
out-of-round or bent part could have many diameters undersize but still be 
``accepted'' because the gage would ``not go'' if the part were bent or if 
even one diameter were up to the low limit. 
 �34s[Rush A. Bowman]
�33s 
  
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�6v�2v�32sRing theory   �21m �33s 
    The mathematical term ring is used to designate a type of algebraic system 
with two compositions satisfying most but not all the properties of addition 
and multiplication in the system of integers, 0, PLUS/MINUS1, PLUS/MINUS2, . . 
. . In precise terms a ring is a set R with two binary compositions called 
addition and multiplication whose results on an ordered pair (a,b), a,b in R, 
are denoted by a + b and ab, respectively. These compositions must satisfy the 
following conditions: 
    C. a + b and ab belong to R (closure). 
    A1. a + b = b + a (commutative law). 
    A2. (a + b) + c = a + (b + c), (associative law). 
    A3. There exists an element 0 (called zero) in R satisfying a + 0 = a for 
every a in R. 
    A4. For each a in R there exists an element -a (called the negative of a) 
in R such that a + (-a) = 0. 
    M1. (ab)c = a(bc). 
    D. a(b + c) = ab + ac; (b + c)a = ba + ca (distributive laws). 
    The importance of the concept of a ring stems from the fact that it 
embraces many special cases which are fundamental in all branches of 
mathematics. Thus it includes the ring I of integers, the ring R0 of rational 
numbers, the ring R# of real numbers, the ring C of complex numbers, various 
rings of functions, rings of matrices, and so on. 
    The conditions A1 - A4 on the addition composition are exactly equivalent 
to the statement that any ring is a commutative group relative to its addition 
composition. This group is called the additive group of the ring. The 
algebraic system consisting of the set of elements of a ring together with its 
multiplication composition is called the multiplicative semigroup of the ring. 
  See  GROUP THEORY. 
    Various classes of rings are singled out by imposing conditions on the 
multiplicative semigroup. Thus integral domains are rings in which the product 
of nonzero elements is nonzero. Division rings are rings whose sets of nonzero 
elements are groups relative to the multiplication composition, and fields are 
division rings satisfying the commutative law of multiplication. 
 �34s[Nathan Jacobson]
�33s 
  
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�6v�2v�32sRipple tank   �21m �33s 
    A shallow tray containing a liquid and equipped with a means for 
generating surface waves. Ripple tanks are used to study a number of physical 
phenomena which can be described in terms of wave mechanics. Water, acoustic, 
light, and electromagnetic waves can be investigated with equal facility. 
    Control of water depth in the ripple tank by contouring the bottom permits 
the simulation of a variable or discontinuous medium for wave propagation. For 
example, a step change in the depth results in a wave diffraction in accord 
with Snell's law. Acoustical and optical wave diffractions have been studied 
in ripple tanks, and phase fronts near two-dimensional models of antenna 
structures and radomes have also been investigated. Ripple-tank models of 
large-scale water-wave motions in harbors and along seacoasts can be useful.  
See  WAVE MOTION IN LIQUIDS. 
 �34s[Donald R. F. Harleman]
�33s 
  
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�6v�2v�32sRipple voltage   �21m �33s 
    The total voltage across the load resistor of a rectifier minus the 
average voltage across the same resistor. The ripple can be expressed as a 
Fourier series. To reduce the ripple voltage, a low-pass filter is usually 
placed between the rectifier and the load. The filter is more efficient in 
reducing the ripple voltage if the fundamental frequency of the ripple voltage 
is high. See ELECTRIC FILTER; RECTIFIER. 
 �34s[Donald L. Waidelich]
�33s 
  
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�6v�2v�32sRitz's combination principle   �21m �33s 
    The empirical rule, formulated by W. Ritz in 1905, that sums and 
differences of the frequencies of spectral lines often equal other observed 
frequencies. The rule is an immediate consequence of the quantum-mechanical 
formula hf = Esubi - Esubf relating the energy hf of an emitted photon to the 
initial energy Esubi and final energy Esubf of the radiating system; h is 
Planck's constant and f is the frequency of the emitted light. See ATOMIC 
STRUCTURE AND SPECTRA; ENERGY LEVEL (QUANTUM MECHANICS); QUANTUM MECHANICS. 
 �34s[Edward Gerjuoy]
�33s 
  
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�6v�2v�32sRiver �21m �33s
    A natural, fresh-water surface stream that has considerable volume
compared with its smaller tributaries. The tributaries are known as brooks,
creeks, branches, or forks. Rivers are usually the main stems and larger
tributaries of the drainage systems that convey surface runoff from the land.
Rivers flow from headwater areas of small tributaries to their mouths, where
they may discharge into the ocean, a major lake, or a desert basin. See RIVER
TIDES.
    Rivers flowing to the ocean drain about 68% of the Earth's land surface.
The remainder of the land either is covered by ice or drains to closed basins
(common in desert regions). Regions draining to the sea are termed exoreic,
while those draining to interior closed basins are endoreic. Areic regions are
those which lack surface streams because of low rainfall or lithologic
conditions.
    Sixteen of the largest rivers account for nearly half of the total world
river flow of water. The Amazon River alone carries nearly 20% of all the
water annually discharged by the world's rivers. Rivers also carry large loads
of sediment. The total sediment load for all the world's rivers averages about
22 x 10(sup 9) tons (20 x 10[sup 9] metric tons) brought to the sea each year.
Sediment loads for individual rivers vary considerably. The Yellow River of
northern China is the most prolific transporter of sediment. Draining an
agricultural region of easily eroded loess, this river averages about 2.2 x
10(sup 9) tons (2 x 10[sup 9] metric tons) of sediment per year, one-tenth of
the world average.  See  STREAM TRANSPORT AND DEPOSITION.
    Perfectly straight rivers are uncommon. Meandering is the most common
river pattern. Braided rivers have channels that are divided into anastomosing
branches by alluvial islands and bars. Braided rivers often have steeper
gradients, more variable discharges, coarser sediment loads, and lower
sinuosity than meandering rivers.
    River discharge varies over a broad range, depending on many climatic and
geologic factors. The low flows of the river influence water supply and
navigation. The high flows are a concern as threats to life and property.
However, floods are also beneficial in providing new soil and moisture for
crops.
    Floods are but one attribute of rivers that affect human society. Many of
the world's rivers are managed to conserve the natural flow for release at
times required by human activity, to confine flood flows to the channel and to
planned areas of floodwater storage, and to maintain water quality at optimum
levels.  See  RIVER ENGINEERING.
 �34s[Victor R. Baker]
�33s

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�6v�2v�32sRiver engineering   �21m �33s 
    A branch of civil engineering concerned with the improvement and 
stabilization of the channels of rivers (particularly channels in erodible 
alluvium) to better serve the needs of people. In many major river systems 
there is a need to improve channels in order to provide greater flood flow 
capacity, provide navigable waterways, improve water supplies, and stabilize 
the channels to permit development of adjacent valley ares. This has led to 
development of extensive and costly river control or management schemes.  See  
RIVER.  
    Channel improvement and stabilization works have been integral parts of 
river control plans where meandering, shifting channels or inappropriate 
channel alignments and cross sections are problems. Large-scale channel 
improvement and stabilization plans have been carried out on many of the 
world's major rivers. In the United States one of the most extensive examples 
of river engineering is found in the Mississippi Basin. 
 �34s[Homer B. Willis]
�33s 
  
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�6v�2v�32sRiver tides   �21m �33s 
    Tides that occur in rivers emptying directly into tidal seas. These tides 
show three characteristic modifications of ocean tides. (1) The speed at which 
the tide travels upstream depends on the depth of the channel. (2) The further 
upstream, the longer the duration of the falling tide and the shorter the 
duration of the rising tide. (3) The range of the tide decreases with distance 
upstream.  See  TIDE. 
    In a river the difference between the depths of water at high and low 
tides may be relatively large, leading to a marked difference between the 
speeds at which high and low tides move. The difference in depth between 
various points on the river also partially explains the second modification, 
or duration of fall and rise. In addition, the river flow, which may fluctuate 
widely, helps a falling tide but hinders a rising tide, increasing the 
difference in duration. 
    The third modification or decrease in tidal range upstream may be 
accounted for by loss of energy of the water through friction with the sides 
and bottom of the channel. Although friction always saps energy from the tide, 
if the channel becomes constricted within a short distance, the water may be 
forced into a smaller space, thus producing a larger tidal range. 
 �34s[Blair Kinsman]
�33s 
  
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�6v�2v�32sRivet   �21m �33s 
    A short rod with a head formed on one end. A rivet is inserted through 
aligned holes in two or more parts to be joined; then by pressing the 
protruding end, a second head is formed to hold the parts together 
permanently. The first head is called the manufactured head and the second one 
the point. In forming the point, a hold-on or dolly bar is used to back up the 
manufactured head and the rivet is driven, preferably by a machine riveter. 
    Blind rivets are special rivets that can be set without access to the 
point. They are available in many designs but are of three general types: 
screw, mandrel, and explosive (see illustration). In the mandrel type the 
rivet is set as the mandrel is pulled through. In the explosive type an 
explosive charge in the point is set off by a special hot iron; the explosion 
expands the point and sets the rivet.  See  RIVETED JOINT. 
 �34s[Paul H. Black]
�33s 
  
  
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�6v�2v�32sRiveted joint   �21m �33s 
    The permanent joining of two or more machine or structural members, 
usually plates, by means of rivets. The plates may be lapped or butted. In the 
butt joint one or more cover plates must be used to accomplish the joining. 
One of these cover plates is often made wider than the other. The rivets in 
the joint may be disposed in several ways to form single or multiple rows in a 
regular or staggered arrangement.  See  RIVET. 
 �34s[L. Sigfred Linderoth, Jr.]
�33s 
  
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�6v�2v�32sRobotics   �21m �33s 
    The study of problems associated with the design, application, and control 
and sensory systems of robots. Historically, the term robot has been used 
loosely, and has been applied to almost any feedback-controlled mechanical 
system. While the exact usage of the term is a matter of preference, the 
computer-controlled mechanical arm used in industrial applications probably 
represents a reasonable middle ground for definition by example. Most concerns 
of practitioners of robotics are involved in such devices, and so much of the 
work in robotics relates directly to these devices. 
  
    Design.   Some robots have very simple mechanical designs, involving only 
a few degrees of freedom of movement. However, the design of robot 
manipulators can also be quite complex. In a typical industrial robot arm, six 
degrees of freedom of movement (exclusive of gripper closure) are required to 
enable the gripper to approach an object from any orientation. 
    The problem of powering the robot's joints is made more difficult by the 
complex mechanical articulation. One approach is to place a prime mover 
(electrical, hydraulic, or pneumatic) at the joint itself. However, the weight 
and bulk of such motors and their associated gearing place constraints on the 
performance and mechanical design of the arm. A second approach is to place 
the prime movers in the immobile base of the robot and to transmit motion to 
the joints through mechanical linkages such as shafts, belts and cables, or 
gearing. This overcomes many of the problems associated with the first 
approach, but introduces a new set of problems in designing intricate, 
backlash-free mechanical linkages which can transmit power effectively through 
the complex articulations of the arm in all of its positions. No single 
approach has clearly dominated the field. 
  
    Control systems.  A robot control system is the apparatus (usually 
electronic) which directs the activities of the mechanical parts. This may 
consist of only a sequencing device and a set of mechanical stops, so that the 
mechanism moves in a repetitive pattern between selected positions. However, 
more sophisticated systems employ servo-controlled positioning of the joints, 
and a measure of the actual joint position is obtained from a transducer, such 
as an optical shaft encoder, and is compared with the position specified for 
the joint. If the desired position and the measured position differ, the 
circuitry applies a correcting drive signal to the joint motor.  See  
SERVOMECHANISM.  
    More sophisticated systems generate the robot's trajectory automatically 
by computer. Such computations may be based on mathematical descriptions of 
work objects or tasks contained in the computer's data base. The computer may 
also generate trajectories for the robot which are not fixed but vary with the 
state of the external world as reported by the robot's sensory system. 
    The most advanced robot control systems make use of hierarchical control. 
Each level of a hierarchy of control stages accepts, from its superior level, 
a statement of a goal to be achieved. Higher levels attempt to achieve their 
current goal by issuing sequences of commands (subgoals) to their subordinate 
levels. In selecting these subgoals, each level takes into account its own 
goal, its sensory input describing the state of the external world, and the 
status of its subordinate level. These systems are sensory-interactive and 
constitute a task-decomposition hierarchy similar to that of many functions in 
the human nervous system. They allow the robot to be instructed with very 
general commands at the highest level. See CONTROL SYSTEMS. 
  
    Sensory systems.   The purpose of a robot sensory system is to gather 
specific information needed by the control system and, in more advanced 
systems, to maintain an internal, predictive model of the environment. Visual, 
proximity, tactile, acoustic, and force or torque senses are all used. 
    The most commonly used means of sensing objects at a distance is some form 
of visual sense. Usually this is done by computer analysis of an image from a 
television camera. Two important approaches are ambient light systems and 
structured light systems. Ambient light systems rely on normal sources of 
scene illumination, while structured light systems provide special patterns of 
illumination whose shape and orientation are known to the sensory system. 
    The most advanced robot sensory systems are multimodal (based on several 
senses) and are also hierarchically structured to generate successive levels 
of description, at increasing degrees of complexity and decreasing rates, 
suitable for use by the successive levels of a hierarchical control system.  
See  ARTIFICIAL INTELLIGENCE; COMPUTER-AIDED DESIGN AND MANUFACTURING; DIGITAL 
CONTROL.  
 �34s[Ernest Kent]
�33s 
  
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�6v�2v�32sRoche limit   �21m �33s 
    The closest distance which a satellite, revolving around a parent body, 
can approach the parent without being pulled apart tidally. The simplest 
formal definition is that the Roche limit is the minimum distance at which a 
satellite can be in equilibrium under the influence of its own gravitation and 
that of the central mass about which it is describing a circular orbit. If the 
satellite is in a circular orbit and has negligible mass, the same density as 
the primary, and zero tensile strength, the Roche limit is 2.46 times the 
radius of the primary. 
    When a star has exhausted the supply of hydrogen in its core, its radius 
will increase by a factor of 10 to 100. A star in a binary system may then 
exceed its Roche limit, material will thus escape from that star, and its 
companion will receive the excess material.  See  BINARY STAR. 
 �34s[P. K. Seidelmann]
�33s 
  
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�6v�2v�32sRochelle salt   �21m �33s 
    The sodium potassium salt of the d-tartaric acid NaKC4H4O6PRODUCT4H2O, 
also called Seignette salt, the first crystalline solid discovered to possess 
the properties of ferroelectricity. Such crystals have been widely used, for 
example, in microphones and phonograph pickup cartridges, because of their 
large piezoelectric effect.  See  FERROELECTRICS; PIEZOELECTRICITY. 
 �34s[H. Graenicher]
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�6v�2v�32sRock �21m �33s
    A relatively common aggregate of mineral grains. Some rocks consist
essentially of but one mineral species (monomineralic, such as quartzite,
composed of quartz); others consist of two or more minerals (polymineralic,
such as granite, composed of quartz, feldspar, and biotite). Rock names are
not given for those rare combinations of minerals that constitute ore
deposits, such as quartz, pyrite, and gold. In the popular sense rock is
considered also to denote a compact substance, one with some coherence; but
geologically, friable volcanic ash also is a rock. A genetic classification of
rocks is shown in the following list.

Igneous
    Intrusive
	Plutonic (deep)
	Hypabyssal (shallow)
    Extrusive
	Flow
	Pyroclastic (explosive)
Sedimentary
    Clastic (mechanical or detrital)
    Chemical (crystalline or precipitated)
    Organic (biogenic)
Metamorphic
    Cataclastic
    Contact metamorphic and pyrometasomatic
    Regional metamorphic (dynamothermal)
Hybrid
    Metasomatic
    Migmatitic
    Exceptions to the requirement that rocks consist of minerals are obsidian,
a volcanic rock consisting of glass; and coal, a sedimentary rock which is a
mixture of organic compounds.  See  COAL; IGNEOUS ROCKS; METAMORPHIC ROCKS;
OBSIDIAN; ROCK MECHANICS; SEDIMENTARY ROCKS; VOLCANIC GLASS.
 �34s[E. William Heinrich]
�33s

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�6v�2v�32sRock age determination �21m �33s
    Age determinations of rocks and minerals based on the radioactivity of
certain chemical elements that occur within them. Measurements of rock ages
have enabled geologists to reconstruct the geologic history of the Earth from
the time of its formation to the present. Age determinations of rocks from the
Moon have also contributed to knowledge of the history of the Earth.  See
RADIOACTIVITY.
    Many rocks and minerals contain radioactive atoms that decay spontaneously
to form stable atoms of other elements. Under certain conditions these
radiogenic daughter atoms accumulate within the mineral crystals so that the
ratio of the daughter atoms divided by the parent atoms increases with time.
This ratio can be measured very accurately with a mass spectrometer, and is
then used to calculate the age of the rock by means of an equation based on
the law of radioactivity. The radioactive atoms used for dating rocks and
minerals have very long half-lives, measured in billions of years. They occur
in nature only because they decay very slowly. The pairs of parents and
daughters used for dating are listed in the table.  See  DATING METHODS.
    The rubidium - strontium (Rb - Sr) method is based on the nuclide
rubidium-87, which decays to stable strontium-87 by emitting a beta particle
from its nucleus. The abundance of the radiogenic strontium-87 therefore
increases with time at a rate that is proportional to the Rb/Sr ratio of the
rock or mineral. This method has been used to date not only igneous but also
metamorphic rocks. The method is particularly well suited to the dating of
very old rocks such as the ancient gniesses near Godthaab in Greenland, which
are almost 3.8 x 10(sup 9) years old.
    The potassium-argon-method is based on the assumption that all of the
atoms of radiogenic argon-40 that form within a postassium-bearing mineral
from the decay of potassium-40 accumulate within it. This assumption is
satisfied only by a few minerals and rocks because argon is an inert gas that
does not readily form bonds with other atoms. These conditions are satisfied
only by the micas, sanidine feldspar, hornblende, and the clay minerals
glauconite and illite. The K-Ar method of dating has been used to establish a
chronology of mountain building events in North America beginning about 2.8 x
10(sup 9) years and continuing to the present. In addition, the method has been
used to date reversals of the polarity of the Earth's magnetic field during
the past 1.3 x 10(sup 7) years.
    All of the atoms of uranium and thorium are radioactive and decay through
a series of daughters to stable atoms of lead. Minerals that contain both
elements can be dated by three separate methods based on the decay of
uranium-238 to lead-206, uranium-235 to lead-207, and thorium-232 to lead-208.
    In fission-track dating, the trail of damage from particles emitted by the
decay of uranium-238 is enlarged by etching a polished surface of the solid
with a suitable reagent. The tracks can then be counted visually by use of a
microscope. The number of spontaneous fission tracks per square centimeter is
proportional to the concentration of uranium and to the age of the sample.
See  FISSION TRACK DATING; RADIOCARBON DATING.
 �34s[Gunter Faure]
�33s

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�6v�2v�32sRock burst   �21m �33s 
    A sudden and violent rock failure around a mining excavation on a 
sufficiently large scale to be considered a hazard endangering the existence 
of mine openings, equipment, and personnel. It has been estimated that the 
energy released in some big bursts was equivalent to that released in 
exploding 200 tons (180 metric tons) of TNT. Such bursts resemble small 
earthquakes and may be detected several hundred miles away. 
    Rock bursts are related to the fracture of rock in place and require two 
conditions for their occurrence: a stress in the rock mass sufficiently high 
to exceed its strength, and physical characteristics of the rock which enable 
it to store energy up to the threshold value for sudden rupture. Rocks which 
yield gradually in plastic strain when under load usually do not generate rock 
bursts.  See  ROCK MECHANICS. 
 �34s[Simon H. Bauer]
�33s 
  
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�6v�2v�32sRock cleavage   �21m �33s 
    A secondary foliation produced in a rock by deformation. Since there 
exists no general agreement on the terminology or the mechanical significance 
of the various types of cleavage, it is described as a special type of 
foliation. Foliation is a descriptive term that includes all types of parallel 
or subparallel primary and secondary planar structures that pervade a rock. 
Such surfaces usually exist as planes of weakness along which a rock may split. 
    Rock cleavage is usually associated with folding, but may also occur in 
response to faulting. It is generally best developed in fine-grained rocks and 
in rocks with a high content of platy minerals such as mica. 
    Most rock cleavage is associated with dynamically metamorphosed rocks. The 
processes most commonly recognized in the generation of cleavage include 
shear, flow, extension and rotation, flattening, solution, and 
recrystallization. However, rock cleavage also occurs in deformed 
unmetamorphosed rocks. 
 �34s[Paul J. Roper]
�33s 
  
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�6v�2v�32sRock control (geomorphology)   �21m �33s 
    The influence of differences in earth materials on development of 
landforms. The materials out of which landforms are carved are not 
homogeneous; even in a small area they differ from one another in composition, 
physicochemical properties, mechanical behavior, and various other ways. These 
differences and variations are visibly reflected in landforms. Examples of the 
problems of rock control include such instances as cuestas, structural 
benches, knickpoints, dike ridges, mesas, hogbacks, structural plains, and 
karst and inversion of topography. 
 �34s[Eiju Yatsu]
�33s 
  
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�6v�2v�32sRocket   �21m �33s 
    Either a propulsion system or a complete vehicle driven by such a 
propulsive engine. A rocket engine provides the means whereby chemical matter 
is burned to release the energy stored in it and the energy is expended, by 
ejection at high velocity of the products of combustion (the working fluid). 
The ejection imparts motion to the vehicle in a direction opposite to that of 
the ejected matter. A rocket vehicle is propelled by rocket reaction and 
includes all components necessary for such propulsion, and a payload such as 
an explosive charge, scientific instruments, or a crew. A rocket vehicle also 
includes guidance and control equipment mounted in a structural airframe or 
spaceframe.  See  ROCKET ENGINE; ROCKET STAGING; SATELLITE (SPACECRAFT); SPACE 
FLIGHT; SPACE PROBE. 
 �34s[George P. Sutton]
�33s 
  
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�6v�2v�32sRocket astronomy   �21m �33s 
    The discipline comprising measurements of the electromagnetic radiation 
from the Sun and other celestial bodies of wavelengths that are almost 
completely absorbed between the 150-mi (250-km) level reached by sounding 
rockets and 25-mi (40-km) level attained by balloons. 
    Rocket astronomy's principal subdiscipline is the Sun. The results from 
solar rocket astonomy have changed and refined the model of the solar 
atmosphere, have thrown new light on the role of magnetic fields in the Sun's 
atmosphere, and have led to a new understanding of solar flares. Rocket 
astronomy is more difficult for the study of celestial bodies other than the 
Sun because of the low intensity of their emissions. 
    Closely connected with and forming a part of rocket astronomy is the study 
of the ionosphere, the airglow, and the composition of the Earth's outer 
atmosphere by measuring the attenuation of solar extreme ultraviolet radiation 
with rocket-borne instrumentation. This work is often placed in the 
interdisciplinary fields of solar-terrestrial relationships, aeronomy, and 
ionospheric physics.  See  IONOSPHERE. 
    Astronomical research from space vehicles is conducted now to a greater 
degree from orbiting observations than from rockets. Nevertheless, rockets 
will continue to provide means for making new discoveries and for proving 
experiments for satellites at costs that are modest.  See  SATELLITE ASTRONOMY. 
 �34s[Richard Tousey]
�33s 
  
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�6v�2v�32sRocket engine   �21m �33s 
    An engine that provides the power to drive a vehicle in the form of a 
reaction force imparted to the vehicle by the momentum of ejected matter. This 
matter, called the propellant, is stored within the rocket vehicle. In a 
chemical rocket the propellant consists of chemical compounds which, when 
reacting with each other, release energy. The chemical rocket is thus 
differentiated from other types of rocket engines, which use nuclear or 
electric energy as their power source.  See  ELECTROMAGNETIC PROPULSION; ION 
PROPULSION; INTERPLANETARY PROPULSION; PROPELLANT; PROPULSION; SPACECRAFT 
PROPULSION.  
    The performance of a missile or space vehicle propelled by a rocket engine 
is usually expressed in terms of such parameters as range, maximum velocity 
increase of the flight, maximum altitude, or time to reach a given target. 
Engine performance parameters (such as exhaust velocity, specific impulse, 
thrust, or engine weight) are used in computing these vehicle performance 
criteria. The table gives typical performance values.  See  SPECIFIC IMPULSE; 
THRUST.  
    Liquid propellants usually consist of a separate oxidizer, for example, 
liquid oxygen or liquid fluorine; and a separate fuel, for example, gasoline, 
alcohol, or hydrazine. The propellants are fed under pressure from tanks in 
the vehicle into a thrust chamber where they are injected, mixed, and burned 
at high pressures and high temperatures to form the gaseous reaction products, 
which in turn are accelerated and ejected at high velocities. An injector 
introduces and meters the flow of the liquid propellants and also atomizes and 
mixes them in the correct proportions so that they can be readily vaporized 
and burned. 
    In a solid-propellant rocket engine the propellant is contained in solid 
form within the combustion chamber. Solid propellants themselves usually have 
a plastic, cakelike appearance and burn on their exposed surface to form hot 
exhaust gases that exhaust through the nozzle. The physical mass or body of 
the propellant is called the grain. In some rockets there is more than one 
grain inside the same combustion chamber. 
    The most severe heating of the hardware takes place at the exhaust nozzle. 
Here high-velocity gas at high-combustion temperature oxidizes, softens, 
wears, and erodes the nozzle material. If the nozzle throat is eroded 
unsymmetrically, then the thrust vector direction may shift and cause the 
flight to become erratic. For this reason it is often desirable to put special 
heat-resistant ceramic or graphite inserts into the nozzle throat region to 
minimize unsymmetrical enlargement of the nozzle area. Multiple layers of 
filament-reinforced plastic ablative materials have also been used in nozzle 
throats and insulating liners, particularly in large solid-rocket units. 
    Jetevators (elevator-type control surfaces for jet deflection) and 
individually gimbaled nozzles have been used to control the thrust vector in 
solid-propellant rockets. An accurate alignment of the nozzle axis with the 
center of gravity of the flying vehicle is essential to minimize flight 
errors, particularly in unguided vehicles with fixed single nozzles. 
    The igniters used for solid-propellant rocket units have been almost 
exclusively the pyrotechnic type. A pyrotechnic igniter usually consists of an 
electrically heated wire surrounded by a small amount of sensitive primer 
powder charge which ignites a larger main igniter charge. 
    A hybrid rocket uses a liquid propellant together with a solid propellant 
in the same rocket engine. The arrangement of the solid fuel is similar to 
that of the grain of a solid-propellant rocket; however, no burning takes 
place directly on the surface of the grain because it contains little or no 
oxidizer. Instead, the fuel on the grain surface is heated, decomposed, and 
vaporized, and the vapors burn with the oxidizer some distance away from the 
surface. Many different propellant combinations can be used, ranging from 
cryogenic oxidizers, such as fluorine, or conventional storable oxidizers, 
through metallic fuels, to ordinary simple fuels such as rubbers or plastics. 
    Because flights of rocket-propelled vehicles are usually fairly expensive 
and because it is difficult to obtain sufficient and accurate data from 
fast-moving flight vehicles, it is accepted practice to test rocket engines 
and components extensively on the ground under simulated flight conditions. 
Components such as an igniter or a turbine are tested separately. Complete 
engines are tested in static engine test stands; the complete vehicle is also 
tested statically. In the latter two tests the engine and vehicle are 
adequately secured by suitable structures. Only in flight tests are they 
allowed to leave the ground.  See  GUIDED MISSILE; LAUNCH COMPLEX; SPACE PROBE. 
 �34s[George P. Sutton]
�33s 
  
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�6v�2v�32sRocket-sled testing   �21m �33s 
    A method of subjecting structures and devices to high accelerations or 
decelerations and aerodynamic flow phenomena under controlled conditions. The 
test object is mounted on a sled chassis running on precision steel rails and 
accelerated by rockets and decelerated by water scoops. This captive track 
testing of full-scale aircraft and missile components makes possible the 
recovery of expensive test sections, facilitates the instrumentation of the 
test and the reception and recording of the data, and provides a degree of 
repeatability not normally achieved under conditions of free flight. The 
components tested may be the rocket engines themselves, sections of flight 
vehicles, or equipment intended to operate under conditions of high 
acceleration or other specific environments such as structural vibration, 
acoustic vibration, and related aerodynamic conditions. 
    The test vehicle takes many diversified forms, ranging from the highly 
faired, aerodynamically clean type used to explore the transonic and 
supersonic speed ranges to the relatively simple all-purpose utility type used 
for testing in the subsonic and low transonic ranges. All sleds are carried on 
shoes or slippers that grip the railhead in order to prevent derailing. They 
may be powered by either liquid- or solid-propellant rocket engines. 
    Tests conducted on the track provide initial design data, developmental 
evaluation, and performance measurements of completed components and 
equipments.  
 �34s[Ernest Coleal]
�33s 
  
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�6v�2v�32sRocket staging   �21m �33s 
    The use of successive rocket sections, each having its own engine or 
engines. One way to minimize the weight of large missiles, or space vehicles, 
is to use multiple stages. The first or initial stage is usually the heaviest 
and biggest and often called the booster; the next few stages are successively 
smaller and are generally called sustainers. Each stage is a complete vehicle 
in itself and carries its own propellant (either solid or liquid; both fuel 
and oxidizer), its own propulsion system, and has its own tankage and control 
system.  
    Once the propellant of a given stage is expended, the dead weight of that 
stage including empty tanks, rocket engine, and controls is no longer useful 
in contributing additional kinetic energy to the succeeding stages. By 
dropping off this useless weight, the mass that remains to be accelerated is 
made smaller; therefore it is possible to accelerate the payload to higher 
velocity than would be attainable if multiple staging were not used. 
    It is quite possible to employ different types of power plants, different 
types of propellants, and entirely different configurations in successive 
stages of any one multistage vehicle (see illustration). Because staging adds 
complications, it is impractical to have more than four to seven stages in any 
one vehicle.  See  ROCKET ENGINE. 
 �34s[George P. Sutton]
�33s 
  
  
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�6v�2v�32sRock exfoliation   �21m �33s 
    The splitting off of concentric thin sheets or shells from the surface of 
massive rocks, a common weathering process in moist climates. Both above and 
beneath the ground surface, hydration of complex silicate minerals through the 
action of percolating water carrying minute amounts of carbon dioxide causes 
peripheral expansion in rocks. The process is particularly effective at edges 
and corners of joint blocks where penetration can be from several directions. 
Separation of successive curved shells tends to produce increasingly rounded 
forms. Chemical exfoliation is distinct from the spalling of rocks by fire or 
sheeting of rocks in response to release of load.  See  WEATHERING PROCESSES. 
 �34s[C. F. Stewart Sharpe]
�33s 
  
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�6v�2v�32sRock magnetism   �21m �33s 
    The natural remanent magnetization (NRM) of igneous, metamorphic, and 
sedimentary rocks has enabled inferences to be made concerning changes in the 
direction and intensity of the geomagnetic field in past geological times and 
the relative movements of the poles and the continents. 
    The magnetic properties of rocks result from the iron oxide minerals 
present to a few percent in many rocks. Two groups are especially important: 
the magnetite (Fe3O4) - ulvospinel (Fe2TiO4) solid solution series and the 
hematite (Fe2O3) - ilmenite (FeTiO3) solid solution series. Intergrowths 
between magnetic minerals are of frequent occurrence. 
    Magnetization induced by the present geomagnetic field is also important 
in the interpretation of air or ground geomagnetic surveys. Laboratory 
experiments, applying magnetic fields to rocks at different temperatures, are 
important not only in the interpretation of NRM but also as a petrological and 
mineralogical tool.  See  PALEOMAGNETISM. 
 �34s[Stanley K. Runcorn]
�33s 
  
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�6v�2v�32sRock mechanics �21m �33s
    Application of the principles of mechanics and geology to quantify the
response of rock when it is acted upon by environmental forces, particularly
when human-induced factors alter the original ambient conditions. Rock
mechanics is an interdisciplinary engineering science that requires
interaction between physics, mathematics, and geology, and civil, petroleum,
and mining engineering. The present state of knowledge permits only limited
correlations between theoretical predictions and empirical results. Therefore,
the most useful principles are based upon data obtained from laboratory and
in-place measurements and from prototype behavior (behavior of the completed
engineering works). Increasing emphasis is upon in-place measurements because
rock properties are regarded as site-specific; that is, the properties of the
rock system at one site probably will be significantly different from those at
another site, even if geologic environments are similar.  See  ENGINEERING
GEOLOGY; TECTONOPHYSICS.
    Because of the interdisciplinary aspects, there is no standardization of
rock mechanics terminology. However, the following terms and definitions are
useful.
    Environmental factors are the natural factors and human influences that
require consideration in engineering problems in rock mechanics. The major
natural factors are geology, ambient stresses, and hydrology. The human
influences derive from the application of chemical, electrical, mechanical, or
thermal energy during construction (or destruction) processes.
    The ambient stress field is the distribution and numerical value of the
stresses in the environment prior to its distrubance by humans.
    The term rock system includes the complete environment that can influence
the behavior of that portion of the Earth's crust that will become part of an
engineering structure. Generally, all natural environmental factors are
included.
    A rock element is the coherent, intact piece of rock that is the basic
constitutent of the rock system and which has physical, mechanical, and
petrographic properties that can be described or measured by laboratory tests
on each such element. The concepts of rock system and rock element enable the
concomitant engineering design to be optimized according to the principles of
system engineering.  See  SYSTEMS ENGINEERING.
    ``Rock failure'' occurs when a rock system or element no longer can
perform its intended engineering function. Failure may be evidenced by
fractures, distortion of shape, or reduction in strength. ``Failure
mechanism'' includes the causes for the manner of rock failure.  See  ROCK
BURST; SOIL MECHANICS; TUNNEL; UNDERGROUND MINING.
 �34s[William R. Judd]
�33s

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�6v�2v�32sRocky Mountain spotted fever   �21m �33s 
    An acute, infectious, typhuslike disease of humans caused by Rickettsia 
rickettsi and transmitted by species of ixodid, or hard-shelled, ticks. The 
disease is also called American spotted fever. The primary cycle in nature 
involves chiefly rodents, hares and rabbits, and dogs in some areas plus the 
opossum and cavy in Brazil together with the appropriate ticks which infest 
them.  See  ACARINA; RICKETTSIALES. 
    Incubation in areas with highly virulent strains may be as short as 2 - 3 
days after tick attachment. Onset is sudden with headache, chills, high fever 
to 105DGF (40.6DGC), prostration, and appearance in 3 - 4 days of a 
measleslike rash. The rash, appearing initially on the forearms and ankles, 
becomes maculopapular over the entire body, including the palms and soles. The 
temperature may remain high for 2 weeks, falling gradually during the third 
week if death has not supervened. In milder disease, incubation may be 1 - 2 
weeks and all symptoms are reduced in severity; ambulatory cases have been 
recognized. According to virulence and to age of the victim, mortality has 
varied in different areas from about 5 to 80%. Lasting immunity follows 
recovery.  
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sRodentia   �21m �33s 
    An order of mammals. Members are characterized by (1) a single pair of 
ever-growing upper and lower incisors, the enamel being limited to the front 
of the teeth so that wear produces and maintains a permanent chisel edge; (2) 
posterior cheek teeth that are reduced to a maximum of five upper and four 
lower teeth on each side; and (3) modification of the lower jaw articulation 
to permit free movement of the lower jaw in an anteroposterior direction, 
separating the cheek teeth when the incisors are in use, and vice versa. The 
most striking feature of the rodents is their ability to gnaw, as well 
illustrated by a beaver cutting trees for dams. Rodents range in size from the 
South American capybara (Hydrochoerus), with a skull 10 - 12 in. (25 - 31 cm) 
long and weighing more than 100 lb (45 kg), down to mouse-sized rodents, among 
the smallest of all mammals, weighting less than 3 - 4 oz (85 - 114 g). Most 
rodents are the size of a squirrel or a rat.  See  MAMMALIA. 
    The gestation period is short (20 days in the house mouse) and litters are 
large. The generation time is also short (sexual maturity at 6 weeks in the 
house mouse), so that several generations may occur per year. There can thus 
be a rapid increase of numbers under favorable conditions. The resultant 
population densities lead to mass migrations, legendary among Scandinavian 
lemmings but known among other rodents, such as gray squirrels in New York 
State.  
    Some rodents (rats and mice of the family Muridae) associate closely with 
humans, living on stored food (especially grains) or refuse. Rodents serve as 
vectors for disease. Both wild and domestic rodents are an important source of 
furs.  
 �34s[Albert E. Wood]
�33s 
  
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�6v�2v�32sRodenticide   �21m �33s 
    One of several groups of materials commonly referred to as pesticides. 
Rodenticides are commonly thought of in terms of lethal chemical agents used 
to kill rodents whether applied as baits or dusts or used as fumigants. 
Rodenticides are frequently used to control other mammals that are not 
rodents, such as moles, rabbits, and hares. 
    Several common forms of rodenticides include: anticoagulants, which cause 
death by internal bleeding in rats and mice but are relatively safe for humans 
and pets; toxicants, or acute rodenticides, which cause death when a single 
dose is ingested and are usually restricted by law to use by trained 
individuals; lethal gases, used to kill rodents in their burrows or other 
confined areas; and dusts, which are used as contact poisons or which may 
adhere to the animal's body and be ingested during grooming. There are laws 
and regulations governing the sale and use of rodenticides, as there are for 
other types of pesticides. No rodenticide is universally effective; therefore, 
the proper rodenticide must be selected that is most satisfactory for a 
particular problem.  See  PESTICIDE; RODENTIA. 
 �34s[William E. Howell]
�33s 
  
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�6v�2v�32sRolling contact   �21m �33s 
    Contact between bodies such that the relative velocity of the two 
contacting surfaces at the point of contact is zero. Common applications of 
rolling contact are the friction gearing of phonograph turntables, speed 
changers, and wheels on roadways. Rolling contact mechanisms are, generally 
speaking, a special variety of cam mechanisms. The concepts of rolling contact 
are used in the study of antifriction bearings and in the study of the 
behavior of toothed gearing.  See  ANTIFRICTION BEARING; CAM MECHANISM; GEAR. 
    Pure rolling contact can exist between two cylinders rotating about their 
centers, with either external or internal contact. Two friction disks (see 
illustration) have external rolling contact if no slipping occurs between 
them. The rotational speeds of the disks are then inversely proportional to 
their radii. 
 �34s[John R. Zimmerman]
�33s 
  
  
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�6v�2v�32sRoll mill   �21m �33s 
    A series of rolls operating at different speeds and used to grind paint or 
to mill flour. In paint grinding, a paste is fed between two low-speed rolls 
running toward each other at different speeds. Because the next roll in the 
mill is turning faster, it develops shear in the paste and draws the paste 
through the mill. The film is scraped from the last high-speed roll. For 
grinding flour, rolls are operated in pairs, rolls in each pair running toward 
each other at different speeds. Grooved rolls crush the grain; smooth rolls 
mill the flour to the desired fineness.  See  GRINDING MILL. 
 �34s[Ralph M. Hardgrove]
�33s 
  
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�6v�2v�32sRoof construction   �21m �33s 
    The structural form used to provide the cover for homes, buildings, 
arenas, stadiums, and other activity areas. Whatever form the roof 
construction takes, it must be capable of resisting the loads to which it will 
be subjected, carrying them satisfactorily to the walls or other supports, and 
providing protection from the elements. Principal roof loads are created by 
high winds and, in the northern climates, by snow and ice. 
    The most common type of roof is encountered in the construction of 
wood-framed structures used as dwelling units. This type of construction 
supports the roof surface, which consists of composition shingles or tile 
placed on sheathing or plywood on rafters running from the peak of the roof to 
its eaves. Industrial buildings use essentially the same construction, except 
that trusses, longitudinal members, and roofing itself are generally 
fabricated from steel. See TRUSS. 
    Concrete, reinforced plastics, and steel are used in roofing large and 
small areas. Thin reinforced concrete sections have been used for shell roofs 
of cylindrical or spherical shape, or in the form of hyperbolic paraboloids 
(see illustration). Another form of equilibrium roof is a series of flat 
plates meeting at various angles to provide support one for the other. 
    A radically different type of structure has provided  support by 
differential air pressure. These structures consist of a durable fabric or 
film supported by cables or, alternatively, make use of a double membrane 
which provides its own support. In either case, the interior of the structure 
is maintained at a slightly higher air pressure than the ambient atmospheric 
pressure, and in this way the shape of the structure is maintained. See 
BUILDINGS.  
 �34s[Charles M. Antoni]
�33s 
  
  
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�6v�2v�32sRoot   �21m �33s 
    The absorbing and anchoring organ of vascular plants. Roots are simple 
axial organs that produce lateral roots, and sometimes buds, but bear neither 
leaves nor flowers. Elongation occurs in the root tip. The older portion of 
the root, behind the root tip, may thicken through cambial activity. Some 
roots, grass for example, scarcely thicken, but tree roots can become 4 in. 
(10 cm) or more in diameter near the stem. Roots may be very long. The longest 
maple (Acer) roots are usually as long as the tree is tall, but the majority 
of roots are only a few inches long. The longest roots may live for many 
years, while small roots may live for only a few weeks or months. 
    Root tips and the root hairs on their surface take up water and minerals 
from the soil. They also synthesize amino acids and growth regulators 
(gibberellins and cytokinins). These materials move up through the woody, 
basal portion of the root to the stem. The thickened, basal portion of the 
root anchors the plant in the soil. Thickened roots, such as carrots, can 
store food that is later used in stem growth. See CYTOKININ; GIBBERELLIN. 
    Roots usually grow in soil where: it is not too dense to stop root tip 
elongation; there is enough water and oxygen for root growth; and temperatures 
are high enough (above 39DGF or 4DGC) to permit root growth, but not so high 
that the roots are killed (above 104DGF or 40DGC). In temperate zones most 
roots are in the uppermost 4 in. (10 cm) of the soil; root numbers decrease so 
rapidly with increasing depth that few roots are found more than 6 ft (2 m) 
below the surface. Roots grow deeper in areas where the soil is hot and dry; 
roots from desert shrubs have been found in mines more than 230 ft (70 m) 
below the surface. In swamps with high water tables the lack of oxygen 
restricts roots to the uppermost soil layers. Roots may also grow in the air. 
Poison ivy vines form many small aerial roots that anchor them to bark or 
other surfaces. 
    The primary root originates in the seed as part of the embryo, normally 
being the first organ to grow. It grows downward into the soil and produces 
lateral second-order roots that emerge at right angles behind the root tip. 
Sometimes it persists and thickens to form a taproot. The second-order 
laterals produce third-order laterals and so on until there are millions of 
roots in a mature tree root system. In contrast to the primary root, most 
lateral roots grow horizontally or even upward. In many plants a few 
horizontal lateral roots thicken more than the primary, so no taproot is 
present in the mature root system. 
    Adventitious roots originate from stems or leaves rather than the embryo 
or other roots. They may form at the base of cut stems, as seen in the 
horticultural practice of rooting cuttings. 
 �34s[B. F. Wilson]
�33s 
  
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�6v�2v�32sRoot-mean-square �21m �33s
    The square root of the arithmetic mean of the squares of a set of numbers
is called their root-mean-square. If the numbers are x1, x2, x3, . . . ,
xsubn, the root-mean-square is equal to

 SQUARE ROOT OF  [(x1(sup 2) + x2(sup 2) + x3(sup 2) . . . + x(subn))/n]

It is valuable as an average of the magntidues of quantities, and it is not
affected by the signs of the quantities.
    Among applications of root-mean-square the most important is the standard
deviation from the arithmetic mean. If

	 BAR x = arithmetic mean = [x1 + x2 + x3 . . . + x(sub n)/n]

then the standard deviation = s =
     SQUARE ROOT OF
     [(x1 - BAR x)(sup 2) + (x2 - BAR x)(sup 2) + (x3 - BAR x)(sup 2) +
		. . . (x (sub n) - BAR x)(sup 2))n]

    Thus standard deviation from the mean is the root-mean-square of the
deviations from the mean.  See	STATISTICS.
 �34s[Hollis R. Cooley]
�33s

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�6v�2v�32sRoot (mathematics) �21m �33s
    If a function f(x) has the value 0 for x=a, a is a root of the equation
f(x)= 0.  The fundamental theorem of algebra states that any algebraic equation
of the form a(sub 0)x(sup n) + a(sub 1)x(sup n(sup -1) + .  .  .  a(sub n-1)x +
a(sub n) = 0, where the a(sub k)'s are real numbers, has at least one root.
From this it follows readily that such an equation has roots, real or complex,
in number equal to the index (here n) of the highest power of x.

    Furthermore, if a + ib (where i = SQUARE ROOT OF -1) is a complex root of
the given equation, so is a - ib, the conjugate of a + ib.  Equations of degrees
up to four may be solved algebraically.  This statement means that the roots may
be expressed as functions of the coefficients, the functions involving the
elementary arithmetical processes of addition, multiplication, raising a number
to a power, or extracting the root of a certain order of a given number.  It was
proved by H.  Abel and by E.  Galois that it is not possible to solve
algebraically the general algebraic equation of degree higher than four.
However, it is possible to determine the real roots of an algebraic equation to
any desired degree of approximation.  See CALCULUS; EQUATIONS, THEORY OF;
NUMERICAL ANALYSIS.
 �34s[Arnold N.  Lowan /Salomon Bochner]
�33s

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�6v�2v�32sRope   �21m �33s 
    A long flexible structure consisting of many strands of wire, plastic, or 
vegetable fiber such as manila. Rope is classifed as a flexible connector and 
is used generally for hoisting, conveying, or transporting loads; transmitting 
motion; and occasionally transmitting power. For flexibility and to reduce 
stresses as the rope bends over the sheave (pulley), a rope is made of many 
small strands.  See  PULLEY. 
 �34s[Paul H. Black]
�33s 
  
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�6v�2v�32sRosales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), which 
gives its name to the subclass Rosidae in the class Magnoliopsida 
(dicotyledons). The order consists of 17 families and nearly 20,000 species; 
some 13,000 of these belong to the Leguminosae, 3000 to the Rosaceae, 1400 to 
the Crassulaceae, and 700 to the Saxifragaceae.  See  MAGNOLIOPSIDA. 
    The legume family (Leguminosae) generally has stipulate, compound leaves; 
10 to many stamens which are often united (connate) by their filaments; and a 
single carpel which typically ripens into a dry fruit that opens at maturity 
down both sutures, releasing the nonendospermous seeds. Many or most members 
of the family harbor symbiotic nitrogen-fixing bacteria in their roots.  See  
ALFALFA; BEAN; CLOVER; COWPEA; KENTUCKY COFFEE TREE; KUDZU; LESPEDEZA; LOCUST 
(FORESTRY); LUPINE; PEA; PEANUT; SOYBEAN; VETCH. 
    The ill-defined family Rosaceae typically has stipulate leaves and 
perigynous flowers; that is, the perianth and stamens are united into a basal 
cup distinct from the ovary. The important genus Prunus, includes cherries, 
plums, apricots, peaches, and almonds. Pyrus includes apples and pears. Some 
other familiar members of the Rosaceae are roses (Rosa), strawberries 
(Fragaria), raspberries and blackberries (Rubus), and hawthorn (Crataegus).  
See  ALMOND; APPLE; APRICOT; BLACKBERRY; CHERRY; CRABAPPLE; CURRANT; 
GOOSEBERRY; NECTARINE; PEACH; PEAR; PLUM; QUINCE; RASPBERRY; STRAWBERRY; WILD 
BLACK CHERRY. 
    Members of the family Crassulaceae are notable for their very succulent 
leaves and resistance to desiccation. They have more or less separate carpels, 
which are often of the same number as the petals. Species of Sedum, Kalanchoe, 
and Sempervivum are often cultivated as ornamentals.  See  ROSIDAE.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sRose curve   �21m �33s 
    A type of plane curve that consists of loops (leaves, petals) emanating 
from a common point and that has a roselike appearance. Taking the common 
point O as the pole of a polar coordinate system (see illustration), these 
curves have equations of the form RHO = a . sin nTHETA, where a > 0 and n is a 
positive integer (also RHO = a . cos nTHETA, with a different choice of the 
initial line of the coordinate system). The curve is a circle of diameter a 
for n = 1. It has n or 2n leaves, according as n is an odd or even integer, 
respectively. The lemniscate is sometimes called a two-leaved rose, though its 
equation RHOsup2 = asup2 cos 2THETA is not of the form given above.  See  
LEMNISCATE OF BERNOULLI. 
 �34s[Leonard M. Blumenthal]
�33s 
  
  
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�6v�2v�32sRosidae   �21m �33s 
    A large subclass of the class Magnoliopsida (dicotyledons) in the division 
Magnoliophyta (Angiospermae, the flowering plants), consisting of 16 orders, 
108 families, and about 60,000 species. Most of them have a well-developed 
corolla with the petals separate from each other. A well-developed nectary 
disk is very often present. The largest orders in the subclass are the Rosales 
(20,000 species), Myrtales (9000 species), Euphorbiales (7500 species), and 
Sapindales (6700 species).  See  CELASTRALES; CORNALES; EUPHORBIALES; FLOWER; 
GERANIALES; HALORAGALES; LINALES; MAGNOLIOPSIDA; MYRTALES; PODOSTEMALES; 
POLYGALALES; PROTEALES; RAFFLESIALES; RHAMNALES; ROSALES; SANTALALES; 
SAPINDALES; UMBELLALES. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sRosin   �21m �33s 
    A brittle resin ranging in color from dark brown to pale lemon yellow and 
derived from the oleoresin of pine trees. Rosin is insoluble in water, but 
soluble in most organic solvents. It softens at about 180 - 190DGF (80 - 
90DGC). Rosin consists of about 90% resin acids and about 10% neutral 
materials such as anhydrides, sterols, and diterpene aldehydes and alcohols. 
    Rosin is obtained by wounding living trees and collecting the exudate (gum 
rosin), by extraction of pine stumps (wood rosin), and as a by-product from 
the kraft pulping process (sulfate or tall oil rosin). 
    The largest single use of rosin is in sizing paper to control water 
absorption, an application in which fortified rosin is important. Rosin soaps 
are used as emulsifying and tackifying agents in synthetic rubber manufacture. 
Other rosin uses include adhesives, printing inks, and chewing gum. 
 �34s[Irving S. Goldstein]
�33s 
  
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�6v�2v�32sRotary engine   �21m �33s 
    Internal combustion engine that duplicates in some fashion the 
intermittent cycle of the piston engine, consisting of the 
intake-compression-power-exhaust cycle, wherein the form of the power output 
is directly rotational. 
    Four general categories of rotary engines can be considered: (1) 
cat-and-mouse (or scissor) engines, which are analogs of the reciprocating 
piston engine, except that the pistons travel in a circular path; (2) 
eccentric-rotor engines, wherein motion is imparted to a shaft by a principal 
rotating part, or rotor, that is eccentric to the shaft; (3) multiple-rotor 
engines, which are based on simple rotary motion of two or more rotors; and 
(4) revolving-block engines, which combine reciprocating piston and rotary 
motion. See AUTOMOBILE; COMBUSTION CHAMBER; DIESEL CYCLE; DISEL ENGINE; GAS 
TURBINE; INTERNAL COMBUSTION ENGINE; OTTO CYCLE. 
 �34s[Wallace Chinitz]
�33s 
  
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�6v�2v�32sRotary tool drill   �21m �33s 
    A bit and shaft used for drilling wells. A turntable on the derrick floor 
rotates a string of hollow steel drill pipe at the bottom of which is a steel 
bit. The bit grinds the rock. A drilling fluid is pumped down through the 
drill pipe; the fluid flushes out the rock cuttings and returns up the space 
between drill string and hole side. 
    The drilling fluid may be air, water, or, most  commonly, mud (a mixture 
of various clays and chemicals, each having a special function). The mud cools 
and lubricates the bit, removes cuttings from the hole, and cakes the wall of 
the hole to prevent caving before steel casing is set. The hydrostatic 
pressure exerted by the column of mud in the hole prevents blowouts which may 
result when the bit penetrates a high-pressure oil or gas zone. 
 �34s[A. L. Ponikvar]
�33s 
  
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�6v�2v�32sRotating impeller flowmeter �21m �33s
    A quantity meter that converts velocity to a proportional rotational
speed. They include various types of turbine meters, propellers, helical
impellers, and vortex cage meters.
    The gas turbine meter is characterized by a large central hub that
increases the velocity of the gas and also puts it through the tips of the
rotor to increase the torque. Readout can be by a directly connected
mechanical register or by a proximity sensor counting the blade tips as they
pass by. Most of the available types of liquid-turbine meters are designed to
cause a fluid flow or hydraulic lift to reduce end thrust, and several depend
upon the flowing fluid to lubricate the bearings.
    The elements in the propeller, helical impeller, and vortex cage meters
(see illustration) rotate at a speed that is linear with fluid velocity. The
useful rangeability is large and limited mostly by the bearings.  See  FLOW
MEASUREMENT; QUANTITY FLOWMETER.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

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�6v�2v�32sRotating reference frame �21m �33s
    A frame of reference whose coordinate axes are rotating with respect to
some inertial system. A point P fixed in the rotating frame moves uniformly
about a circle in the inertial frame and is accelerated toward the axis of
rotation. There is an additional acceleration (with respect to the inertial
system) if P moves in the rotating system. For example, if the point moves
away from the axis along a moving radius, it gains circular speed also. See
CENTRIPETAL FORCE; CORIOLIS ACCELERATION AND FORCE.
    Newton's second law can be modified to take account of these two
accelerations that are not observed in the rotating system. Let A(sub Ac),
A(sub C), A(sub Co) be the observed, centripetal, and Coriolis accelerations,
respectively. The total acceleration in the inertial frame is given by Eq.
(1), where the accelerations must be added vectorially. Defining the

	       A = A(sub Ac) + A(sub C) + A(sub Co)

centrifugal force, Fc = -mAc, and the Coriolis force, F(sub Co) = -mA(sub Co),
permits Newton's second law to be written as Eq. (2). In rotating systems a
consistent mechanical theory is obtained if the two additional forces are

	       mA(sub Ac) = F + F(sub C) + F(sub Co)

added to the impressed forces. Conversely, the presence of such forces will
enable the observer to determine the motion of the reference frame. See FRAME
OF REFERENCE; INERTIAL REFERENCE FRAME.
 �34s[Bernard Goodman]
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�6v�2v�32sRotational flow   �21m �33s 
    Fluid flow in which each minute particle of fluid rotates about its own 
axis. Rotational flow is possible only with a real or viscous fluid. A 
specific case is a forced vortex in which a fluid rotates like a solid body. 
Such a case might be obtained with a fluid in a cylindrical vessel rotating 
about its central axis, assuming no motion of the fluid relative to the 
container. A more common case is that in which the container is fitted with 
vanes, as the impeller of a centrifugal pump. As a small element rotates about 
the central axis, it also rotates about its own axis. See FLUID FLOW; VORTEX. 
 �34s[Robert L. Daugherty]
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�6v�2v�32sRotational motion �21m �33s
    The motion of a rigid body which takes place in such a way that all of its
particles move in circles about an axis with a common angular velocity; also,
the rotation of a particle about a fixed point in space. Rotational motion is
illustrated by (1) the fixed speed of rotation of the Earth about its axis;
(2) the varying speed of rotation of the flywheel of a sewing machine; (3) the
rotation of a satellite about a planet; (4) the motion of an ion in a
cyclotron; and (5) the motion of a pendulum.  Circular motion is a rotational
motion in which each particle of the rotating body moves in a circular path
about an axis.	Such motion is exhibited by the first and second examples.  For
information concerning the other examples see HARMONIC MOTION; PARTICLE
ACCELERATOR; PENDULUM.
     The speed of rotation, or angular velocity, remains constant in uniform
circular motion.  In this case, the angular displacement THETA experienced by
the particle or rotating body in a time t is THETA = OMEGA t, where OMEGA is the
constant angular velocity.  A special case of circular motion occurs when the
rotating body moves with constant angular acceleration.  If a body is moving in
a circle with an angular acceleration of ALPHA radians/s(sup2), and if at a
certain instant it has an angular velocity OMEGA(sub 0), then at a time t
seconds later, the angular velocity may be expressed as OMEGA = OMEGA(sub 0) +
ALPHA t, and the angular displacement as THETA = OMEGA(sub 0)t + 1/2 ALPHA
t(sup2).  See ACCELERATION; VELOCITY.

    A rotating body posesses kinetic energy of rotation which may be expressed
as T(sub rot) = 1/2 I OMEGA(sup 2), where OMEGA is the magnitude of the angular
velocity of the rotating body and I is the moment of inertia, which is a measure
of the opposition of the body to angular acceleration.	The moment of inertia of
a body depends on the mass of a body and the distribution of the mass relative
to the axis of rotation.  For example, the moment of inertia of a solid cylinder
of mass M and radius R about its axis of symmetry is 1/2 MR(sup2).
    The action of a torque L is to produce an angular acceleration ALPHA
according to the equation below, where I OMEGA, the product of moment of inertia
and angular velocity, is called the angular momentum of the rotating body.  This
equation points out that the angular momentum I OMEGA of a rotating body, and
hence its angular velocity OMEGA, remains constant unless the rotating body is
acted upon by a torque.  Both L and IOMEGA may be represented by vectors.

	      L = I ALPHA = I [ d OMEGA/dt] = [d/dt](I OMEGA)

    It is readily shown that the work done by the torque L acting through an
angle THETA on a rotating body originally at rest is exactly equal to the
kinetic energy of rotation.  See  ANGULAR MOMENTUM; MOMENT OF INERTIA;
RIGID-BODY DYNAMICS; TORQUE; WORK.

 �34s[Carl E. Howe /R. J. Stephenson]
�33s

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�6v�2v�32sRotational viscometers   �21m �33s 
    Instruments, such as the concentric cylinder device designed by M. M. 
Couette in 1890 for examining non-newtonian fluids. The basic concept is to 
rotate one cylinder while the shear stress (converted to torque) is measured 
on the other. The easiest way to measure torque is by the deflection of a 
pointer attached to a torsion wire, but far more sophisticated systems have 
been developed including a unit designed for very low shear stress 
measurements. In this design, friction is reduced by means of an air bearing, 
and the torque is measured with an electronic null-balance system. The average 
shear rate is readily controlled by means of a variable-speed drive. 
    In addition to variable shear rate, the Couette design has the advantage 
of a small dispersion of shear rates within the sample if the annular gap is 
small compared to the radii.  
 �34s[Henry L. Gabelnick]
�33s 
  
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�6v�2v�32sRotatory dispersion   �21m �33s 
    A term used to describe the change in rotation as a function of wavelength 
experienced by linearly polarized light as it passes through an optically 
active substance.  See  OPTICAL ACTIVITY. 
    In all materials the rotation varies with wavelength. The variation is 
caused by two quite different phenomena. The first accounts in most cases for 
the majority of the variation in rotation and should not strictly be termed 
rotatory dispersion. It depends on the fact that optical activity is actually 
circular birefringence. In other words, a substance which is optically active 
transmits right circularly polarized light with a different velocity from left 
circularly polarized light. 
    In addition to this pseudodispersion which depends on the material 
thickness, there is a true rotatory dispersion which depends on the variation 
with wavelength of nsubr and nsubl. See POLARIZED LIGHT. 
 �34s[Bruce H. Billings]
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�6v�2v�32sRotifera   �21m �33s 
    A phylum of pseudocoelomate, microscopic, mainly free-living aquatic 
animals, characterized by an anterior ciliary apparatus, the corona, whose 
cilia when in motion have the appearance of a pair of rapidly rotating wheels. 
This structure is implicit in the phyletic name (literally ``wheel bearers'') 
and the older popular name wheel animalcules. 
    The Rotifera show considerable diversity  in form and structure, but all 
are bilaterally symmetrical, pseudocoelomate animals possessing complete 
digestive, excretory, nervous, and reproductive systems, but lacking separate 
respiratory and circulatory systems. They possess two features unique to their 
phylum: the corona, which is a retractile trochal disk, and the mastax, which 
is a gizzardlike structure derived from the modified pharynx. 
    Rotifers are dioecious and sexually dimorphic; females are commoner than 
males, some of which are degenerate organisms lacking a mouth and digestive 
organs. Males, when produced in the life cycle, are short-lived and survive 
for only hours or at the most a few days. 
    The three major subdivisions of the Rotifera, now given class status, are 
the Seisonacea, Bdelloidea, and Monogononta.  See  BDELLOIDEA; MONOGONONTA; 
SEISONACEA.  
 �34s[J. B. Jennings]
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�6v�2v�32sRous sarcoma   �21m �33s 
    The first filterable agent (virus) known to cause a solid tumor in 
chickens. It was discovered in 1911 by P. F. Rous, who won the Nobel prize in 
1967 for his discovery. Certain strains of the virus cause tumors in hamsters, 
rabbits, monkeys, and other species. The Rous virus is known as a 
``defective'' virus in that it is incapable of producing tumors by itself. It 
is a ribonucleic acid (RNA) virus and belongs to the avian leukosis group.  
See  ANIMAL VIRUS; AVIAN LEUKOSIS. 
 �34s[Alice E. Moore]
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�6v�2v�32sRubber   �21m �33s 
    Originally, a natural or tree rubber, which is a hydrocarbon polymer of 
isoprene units. With the development of synthetic rubbers having some rubbery 
characteristics but differing in chemical structure as well as properties, a 
more general designation was needed to cover both natural and synthetic 
rubbers. The term elastomer, a contraction of the words elastic and polymer, 
was introduced, and defined as a substance that can be stretched at room 
temperature to at least twice its original length and, after having been 
stretched and the stress removed, returns with force to approximately its 
original length in a short time. 
    Three requirements must be met for rubbery properties to be present in 
both natural and synthetic rubbers: long threadlike molecules, flexibility in 
the molecular chain to allow flexing and coiling, and some mechanical or 
chemical bonds between molecules. 
    Natural rubber and most synthetic rubbers are also commercially available 
in the form of latex, a colloidal suspension of polymers in an aqueous medium. 
Natural rubber comes from trees in this form; many synthetic rubbers are 
polymerized in this form; some other solid polymers can be dispersed in water. 
  See  POLYMER. 
    Latexes are the basis for a technology and production methods completely 
different from the conventional methods used with solid rubbers. 
    In the crude state, natural and synthetic rubbers possess certain physical 
properties which must be modified to obtain useful end products. The raw or 
ummodified forms are weak and adhesive. They lose their elasticity with use, 
change markedly in physical properties with temperature, and are degraded by 
air and sunlight. Consequently, it is necessary to transform the crude rubbers 
by compounding and vulcanization procedures into products which can better 
fulfill a specific function. 
    Smoke sheet and pale crepe represent the forms in which the major portion 
of natural rubber is commercially available. The smoked sheet is obtained by 
acid coagulation of the tree latex, sheeting the coagulum, and then drying and 
smoking the sheets of rubber. The chemicals in the smoke preserve the rubber 
from molds and other organisms. Pale crepe is obtained by treating the latex, 
before or after coagulation, with sodium bisulfite and then washing, drying, 
and sheeting. Synthetic rubbers are usually available as bales, pellets, or 
flakes made from the polymerization process. 
    Although natural rubber may be obtained from hundreds of different plant 
species, the most important source is the rubber tree (Hevea brasiliensis). 
Natural rubber is cis-1,4-polyisoprene, containing approximately 5000 isoprene 
units in the average polymer chain.  See  RUBBER TREE. 
    Butadiene-styrene rubber (SBR) is the most important synthetic rubber and 
the most widely used rubber in the entire world. Formerly designated GR-S, 
SBRs are obtained by the emulsion polymerization of butadiene and styrene in 
varying ratios. However, in the most commonly used type, the ratio of 
butadiene to styrene is approximately 78:22. Unlike natural rubber, SBR does 
not crystallize on stretching and thus has low tensile strength unless 
reinforced. The major use for SBR is in tires and tire products. Other uses 
include belting, hose, wire and cable coatings, flooring, shoe products, 
sponge, insulation, and molded goods. 
    Butyl rubber is essentially a polyisobutylene except for the presence of 
diolefin, usually isoprene, to provide the unsaturation necessary for 
vulcanization. Butyl rubbers have excellent resistance to oxygen, ozone, and 
weathering. In addition, these rubbers exhibit good electrical properties and 
high impermeability to gases. The high impermeability to gases results in use 
of butyl as an inner liner in tubeless tires. Other widespread uses are for 
wire and cable products, injection-molded and extruded products, hose, 
gaskets, and sealants, and where good damping characteristics are needed. 
    Ethylene-propylene polymers are produced by the copolymerization of 
ethylene and propylene. These copolymers exhibit outstanding resistance to 
heat, oxygen, ozone, and other aging and degrading agents. Abrasion resistance 
in tire treads is excellent. The mechanical properties of their vulcanizates 
are generally approximately equivalent to those of SBR. 
    One of the first synthetic rubbers used commercially in the rubber 
industry is neoprene, a polymer of chloroprene, 2-chlorobutadiene-1,3. The 
neoprenes have exceptional resistance to weather, sun, ozone, and abrasion. 
They are good in resilience, gas impermeability, and resistance to heat, oil, 
and flame. They are fairly good in low temperature and electrical properties. 
This versatility makes them useful in many applications requiring oil, 
weather, abrasion, or electrical resistance or combinations of these 
properties, such as wire and cable, hose, belts, molded and extruded goods, 
soles and heels, and adhesives. 
    Nitrile-type rubbers are copolymers of acrylonitrile and a diene, usually 
butadiene. The nitrile rubbers can be blended with natural rubber, polysulfide 
rubbers, and various resins to provide characteristics such as increased 
tensile strength, better solvent resistance, and improved weathering 
resistance.  
    There are other specialty rubbers available which are important because of 
specific properties, but in the aggregate they make up only approximately 2% 
of the world production of synthetic rubbers. One of these, silicone rubber, 
is a linear condensation polymer based on dimethyl siloxane. 
    In general, the silicone rubbers have relatively poor physical properties 
and are difficult to process. They are unaffected by ozone, are resistant to 
hot oils, and have excellent electrical properties.  See  SILICON; SILICONE 
RESINS.  
    Proper choice of catalyst and order of procedure in polymerization have 
led to development of thermoplastic elastomers. The leading commercial types 
are styrene block copolymers having a structure which consists of polystyrene 
segments or blocks connected by rubbery polymers such as polybutadiene, 
polyisoprene, or ethylene-butylene polymer. Thermoplastic elastomers are very 
useful in providing a fast and economical method of producting a variety of 
products, including molded goods, toys, sporting goods, and footwear. One of 
the disadvantages for many applications is the low softening point of the 
thermoplastic elastomers.  See  POLYSULFIDE RESINS; POLYURETHANE RESINS. 
 �34s[Edward G. Partridge]
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�6v�2v�32sRubber tree �21m �33s
    Hevea brasiliensis, a member of the spurge family (Euphorbiaceae) and a
native of the Amazon valley. It is the natural source of commercial rubber.
    It has been introduced into all the tropical countries supporting the
rainforest type of vegetation, and is grown extensively in established
plantations, especially in Malaysia. The latex from the trees is collected and
coagulated. The coagulated latex is treated in different ways to produce the
kind of rubber desired. Rubber is made from the latex of a number of other
plants, but Hevea is the rubber plant of major importance.  See  EUPHORBIALES;
RUBBER.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sRubella   �21m �33s 
    A benign, infectious virus disease of humans characterized by coldlike 
symptoms and transient, generalized rash. This disease, also known as German 
measles, is primarily a disease of childhood. However, maternal infection 
during early pregnancy may result in infection of the fetus, giving rise to 
serious abnormalities and malformations. The congenital infection persists in 
the infant, who harbors and sheds virus for many months after birth. 
    In rubella infection acquired by ordinary person-to-person contact, the 
virus is believed to enter the body through respiratory pathways. Antibodies 
against the virus develop as the rash fades, increase rapidly over a 2 - 3 
week period, and then fall during the following months to levels that are 
maintained for life. One attack confers life-long immunity, since only one 
antigenic type of the virus exists. Immune mothers transfer antibodies to 
their offspring, who are then protected for approximately 4 - 6 months after 
birth.  See  IMMUNITY. 
    Gamma globulin has been used for pregnant women exposed to rubella, but 
there is no convincing evidence that it offers any protection to the mother or 
to the fetus. A number of live, attenuated strains have been developed for use 
in vaccines; several are commercially available. 
 �34s[Joseph L. Melnick]
�33s 
  
A700000001164�254s�5v�1v1513
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�6v�2v�32sRubellite   �21m �33s 
    The red to red-violet variety of the gem mineral tourmaline. Perhaps the 
most sought-for of the many colors in which tourmaline occurs, it was named 
for its resemblance to ruby. The color is thought to be caused by the presence 
of lithium. It has a hardness of 7 - 7.5 on Mohs scale, a specific gravity 
near 3.04, and refractive indices of 1.624 and 1.644. Fine gem-quality 
material is found in Brazil, Madagascar, Maine, southern California, the Ural 
Mountains, and elsewhere.  See  GEM; TOURMALINE. 
 �34s[Richard T. Liddicoat, Jr.]
�33s 
  
A700000001262�254s�5v�1v1513
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�6v�2v�32sRubiales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of the single family Rubiaceae, with about 6500 species. The Rubiales 
are marked by their inferior ovary; regular or nearly regular corolla with the 
petals grown together by their margins; stamens (equal in number to the 
petals) which are attached to the corolla tube alternate with the lobes; and 
opposite leaves with interpetiolar stipules or whorled leaves without 
stipules.  See  COFFEE; IPECAC; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
A700000003476�254s�5v�1v1513
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1514
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�6v�2v�32sRubidium   �21m �33s 
    A chemical element, Rb, atomic number 37, and atomic weight 85.47. 
Rubidium is an alkali metal. It is a light, low-melting, reactive metal. 
    Most uses of rubidium metal and rubidium compounds are the same as those 
of cesium and its compounds. The metal is used in the manufacture of electron 
tubes, and the salts in glass and ceramic production. 
    Rubidium is a fairly abundant element in the Earth's crust, being present 
to the extent of 310 parts per million (ppm). This places it just below carbon 
and chlorine and just above fluorine and strontium in abundance. Sea water 
contains 0.2 ppm of rubidium, which (although low) is twice the concentration 
of lithium. Rubidium is like lithium and cesium in that it is tied up in 
complex minerals; it is not available in nature as simple halide salts as are 
sodium and potassium. 
    Rubidium has a density of 1.53 g/cmsup3 (95.5 lb/ftsup3), a melting point 
of 39DGC (102DGF), and a boiling point of 688DGC (1270DGF). 
    Rubidium is so reactive with oxygen that it will ignite spontaneously in 
pure oxygen. The metal tarnishes very rapidly in air to form an oxide coating, 
and it may ignite. The oxides formed are a mixture of Rb2O, Rb2O2, and RbO2. 
The molten metal is spontaneously flammable in air. 
    Rubidium reacts violently with water or ice at temperatures down to 
-100DGC (-148DGF). It reacts with hydrogen to form a hydride which is one of 
the least stable of the alkali hydrides. Rubidium does not react with 
nitrogen. With bromine or chlorine, rubidium reacts vigorously with flame 
formation. Organorubidium compounds can be prepared by techniques similar to 
those used for sodium and potassium.  See  ALKALI METALS; CESIUM. 
 �34s[Marshall Sittig]
�33s 
  
A700000003111�254s�5v�1v1514
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�6v�2v�32sRubidium-gas cell clock   �21m �33s 
    A type of atomic clock which makes use of microwave transitions of 
rubidium atoms that are mixed with a buffer gas in a glass cell. In an 
atmosphere consisting of an inert buffer gas mixture of suitable pressure, 
atoms can be confined by collisions with the buffer gas atoms which are not 
perturbing and thus can have a long interaction time with the exciting 
radiation, leading to narrow lines. This principle is used in passive and 
active rubidium-gas cell clocks. A small amount of rubidium and the buffer gas 
are placed in a glass cell inside a microwave cavity and kept at the proper 
temperature for optimum rubidium vapor pressure. Light from a rubidium lamp is 
passed through a filter and then through the cell through windows in the 
cavity. In the passive clock, under proper conditions the light transmitted 
through the cell is reduced in intensity, if the exciting radiation in the 
cavity is at the hyperfine resonance frequency (6834 megahertz). This decrease 
in intensity is detected with a photocell and is usd to control the frequency 
of the exciting radiation. In an active clock, the light causes a population 
excess in the upper hyperfine level of the rubidium atoms, and maser 
oscillator action takes place. In both rubidium clocks the collisions with the 
buffer gas atoms change the resonance frequency by a nonnegligible amount, and 
therefore the clocks must be calibrated initially and are subject to drifts in 
rate. See ATOMIC CLOCK; MASER. 
 �34s[Leonard S. Cutler]
�33s 
  
A700000002025�254s�5v�1v1514
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�6v�2v�32sRuby �21m �33s
    The red variety of the mineral corundum, in its finest quality the most
valuable of gemstones. Only medium to dark tones of red to slightly violet-red
or very slightly orange-red are called ruby; light reds, purples, and other
colors are properly called sapphires. In its pure form the mineral corundum,
with composition Al2O3, is colorless. The rich red of fine-quality ruby is the
result of the presence of a minute amount of chromic oxide. The chromium
presence permits rubies to be used for lasers producing red light.  See
CORUNDUM; LASER; SAPPHIRE.
    The finest ruby is the transparent type with a medium tone and a high
intensity of slightly violet-red, which has been likened to the color of
pigeon's blood. Star rubies do not command comparable prices, but they, too,
are in great demand. The ruby was among the first of the gemstones to be
duplicated synthetically and the first to be used extensively in jewelry.  See
GEM.
 �34s[Richard T. Liddicoat, Jr.]
�33s

A700000001116�254s�5v�1v1514
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�6v�2v�32sRugosa �21m �33s
    An order of extinct corals which flourished during the Paleozoic Era. The
Rugosa, or Tetracorallia, first appeared in the Ordovician and became extinct
in the Permian. Nothing is known of the soft parts. Both simple and compound
skeletons are common, the simple ones having typically a curved, horn-shaped
appearance, and the compound ones forming groups of cylindrical stems or
polygonal columns. The simple form is called a corallite, the compound one a
corallum.  See	COELENTERATA.
 �34s[Erwin Charles Stumm]
�33s

A700000001650�254s�5v�1v1514
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�6v�2v�32sRunge vector �21m �33s
    The Runge vector describes certain unchanging features of a
nonrelativistic two-body interaction for which the potential energy is
inversely proportional to the distance r between the bodies or, alternatively,
in which each body exerts a force on the other that is directed along the line
between them and proportional to r(sup -2). Two basic interactions in nature are
of this type: the gravitational interaction between two masses (called the
classical Kepler problem), and the Coulomb interaction between like or unlike
charges (as in the hydrogen atom). Both at the classical level and the
quantum-mechanical level, the existence of a Runge vector is a reflection of
the symmetry inherent in the interaction.  See	COULOMB'S LAW; KEPLER'S
EQUATION; NONRELATIVISTIC QUANTUM THEORY; QUANTUM MECHANICS; SYMMETRY LAWS
(PHYSICS).
 �34s[David M. Fradkin]
�33s

A700000001104�254s�5v�1v1514
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�6v�2v�32sRunning birds   �21m �33s 
    Any of 10 living and a number of recently extinct species of flightless, 
mostly large, heavy birds which are included in six orders, Struthioniformes, 
Rheiformes, Casuariiformes, Aepyornithiformes, Dinornithiformes, and 
Apterygiformes, usually categorized as the Ratites. They are characterized by 
a flat, keelless sternum and reduced wings.  See  AEPYORNITHIFORMES; 
APTERYGIFORMES; CASUARIIFORMES; DINORNITHIFORMES; RATITES; RHEIFORMES; 
STRUTHIONIFORMES.  
 �34s[Charles B. Curtin]
�33s 
  
A700000001034�254s�5v�1v1514
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�6v�2v�32sRunning fit   �21m �33s 
    The intentional difference in dimensions of mating mechanical parts that 
permits them to move relative to each other. A free running fit has liberal 
allowance; it is used on high-speed rotating journals or shafts. A medium fit 
has less allowance; it is used on low-speed rotating shafts and for sliding 
parts. Running fits are affected markedly by their surface finish and the 
effectiveness of lubrication.  See  ALLOWANCE. 
 �34s[Paul H. Black]
�33s 
  
A700000002540�254s�5v�1v1514
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�6v�2v�32sRunway   �21m �33s 
    A strip of land surfaced for the takeoff and landing of aircraft. The 
design and construction of runways for airports have engaged the attention of 
engineers for a long time. Not only is it necessary to provide a level, 
obstruction-free landing and approach area, it is also necessary to select the 
type of pavement, location, and construction of runways and taxiways which 
permits maximum use of the landing area. Further, it is necessary to develop 
durable surfaces which require minimum maintenance and long life. In the 
design of runways, consideration must be given to the type of lighting which 
will be used and how it can be installed so as to minimize possible 
interference with aircraft movements.  See  AIRPORT ENGINEERING. 
    A runway design for a given location is based on the traffic requirements, 
the type of aircraft using the field, and the relative positions of the 
terminals, taxiways, and runways. For most major airports, an alignment is 
sought which gives minimum noise interference with adjacent communities. All 
these elements must be compatible with runway orientation, which ideally 
provides that the major or instrument runway is in the direction of maximum 
wind conditions (see illustration). 
 �34s[John M. Kyle]
�33s 
  
  
A700000001772�254s�5v�1v1515
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�6v�2v�32sRust (microbiology)   �21m �33s 
    Fungi of the order Uredinales, or diseases caused by these fungi. Rusts 
are important plant pathogens of wheat and flax: other rusts, such as those of 
oats, barley, rye, white pines, apple, and snapdragon are also of economic 
importance. Rusts vary in specificity of parasitism. 
    Microcyclic rusts, such as Puccinia malvacearum (hollyhock rust), produce 
only teleutospores and basidiospores. Macrocyclic rusts produce other types of 
spores in addition. Autoecious rusts, such as P. asparagi (asparagus rust), 
complete their life cycle on one species of host. Heteroecious rusts require 
two species of host. 
    Control of rusts by dusting with fungicides is practical for ornamentals, 
but breeding resistant varieties of hosts offers the greatest promise for 
control of all rusts. Eradication of alternate hosts is effective in 
controlling white pine blister rust.  See  UREDINALES. 
 �34s[Robert M. Page]
�33s 
  
A700000001112�254s�5v�1v1515
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�6v�2v�32sRutabaga   �21m �33s 
    The plant Brassica napobrassica, a cool-season, hardy biennial crucifer of 
European origin, belonging to the order Capparales and probably resulting from 
the natural crossing of cabbage and turnip. The fleshy roots are cooked and 
usually eaten mashed as a vegetable. Rutabagas have been widely grown as a 
livestock feed in northern Europe and eastern Canada. Commercial production is 
limited to Canada and the northern part of the United States.  See  
CAPPARALES; TURNIP. 
 �34s[H. John Carew]
�33s 
  
A700000002347�254s�5v�1v1515
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�6v�2v�32sRuthenium   �21m �33s 
    A chemical element, Ru, atomic number 44, and atomic weight 101.07. 
Ruthenium is a hard, white metal, workable only at high temperatures and then 
only with difficulty. 
    Ruthenium is an excellent catalyst, useful in reactions involving 
hydrogenation, isomerization, oxidation, and reforming. Uses of pure ruthenium 
metal are minor. Ruthenium is an effective hardener for platinum and 
palladium. Alloys with large percentages (30 - 70%) of ruthenium containing 
other precious or base metals have been used for electrical contacts and 
applications, where extreme wear resistance and corrosion resistance are 
required, as in fountain pen nibs and instrument pivots. 
    Ruthenium is resistant to the common acids, including aqua regia, at 
temperatures up to 100DGC (212DGF), and up to 300DGC (570DGF) in the case of 
sulfuric acid. It also resists hydrofluoric and phosphoric acids at 100DGC 
(212DGF).  
    Potassium ruthenate, KRuO2.H2O, is soluble in water and is useful in 
purifying ruthenium. Ruthenium trichloride, RuCl3, is soluble in water but 
decomposes in hot water. Ruthenium tetroxide is highly volatile and poisonous. 
 �34s[Henry J. Albert]
�33s 
  
A700000001770�254s�5v�1v1515
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�6v�2v�32sRutile   �21m �33s 
    The most frequent of the three polymorphs of titania, TiO2; the two other 
polymorphs are brookite and anatase. 
    The mineral occurs as striated tetragonal prisms and needles, commonly 
repeatedly twinned. The color is deep blood red, reddish brown, to black, 
rarely violet or yellow. Specific gravity is 4.2, and hardness 6.5 on Mohs 
scale. Melting point is 1825DGC (3317DGF). 
    Rutile occurs as an accessory in many rock types, ranging from plutonic to 
metamorphic rocks, and even as detrital material in sediments and placers 
because of its resistance to weathering. Large crystals have been found in 
some granite pegmatities. Other rocks include contact metamorphic limestones, 
alpine gneisses and schists, and pyrophyllite-lazulite schists. Rutile is 
commonly associated with apatite in high-temperature veins. In sufficient 
quantities, it is marketed as an ore of titanium.  See  TITANIUM. 
 �34s[Paul B. Moore]
�33s 
  
A700000000643�254s�5v�1v1515
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�6v�2v�32sRydberg atom   �21m �33s 
    An atom which possesses one valence electron orbiting about an atomic 
nucleus within an electron shell well outside all the other electrons in the 
atom. Such an atom approximates the hydrogen atom in that a single electron is 
interacting with a positively charged core.  See  ELECTRON CONFIGURATION. 
 �34s[James E. Bayfield]
�33s 
  
A700000003115�254s�5v�1v1515
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1516
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�6v�2v�32sRydberg constant �21m �33s
    An atomic constant describing the binding energy between electron and
atomic nucleus.  It is connected with the other universal physical constants by
the relation, in cgs electrostatic units, R INFINITY = 2PI(sup 2)me(sup
4)/ch(sup 3).  Here, m and e are the mass and charge of the electron, c the
velocity of light, and h is Planck's constant.  (In SI units it is given by R
INFINITY =MU(sub 0)(sup 2)me(sup 4)c(sup 3)/8h(sup 3) where MU(sub 0) = 4PI x
10(sup -7) henry/meter is the permeability of vacuum.) The Rydberg constant is
determined from measurements of the wavelength of spectral lines of hydrogenlike
atoms, which permit accurate theoretical calculations.	The relationship between
the wavelength and the Rydberg constant is given by Bohr's formula, corrected
for finite nuclear mass, relativistic effects, and Lamb shifts.  The symbol R
INFINITY indicates that the constant refers to a hypothetical atom of infinite
mass.  To correct for the finite nuclear mass M of a real atom, one replaces the
electron mass m in the preceding formula by the reduced mass of electron and
nucleus, that is, by mM/m + M), and one obtains a somewhat smaller Rydberg R = R
INFINITY/(1 + mM).  Because the Rydberg constant can be determined with high
precision, it has traditionally been a cornerstone in the evaluation of the
other fundamental constants; it gives a precise relation between these
constants, which must be satisfied by any set of constants that may be adopted.

 �34s[Theo W. Hansch]
�33s

A700000000751�254s�5v�1v1516
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�6v�2v�32sRye   �21m �33s 
    A winter-hardy and drought-resistant cereal plant, Secale cereale, in the 
grass family (Graminae). Rye grain is used for animal feed, human food, and 
production of spirits. Ground rye is mixed with other feeds for livestock. It 
is often fall-sown to provide soil cover and important pasturage for 
livestock. Next to wheat, rye has the most desirable gluten for breadmaking. 
 �34s[John A. Shellenberger]
�33s 
  
A700000001304�254s�5v�1v1517
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�6v�2v�32sSable   �21m �33s 
    A carnivore, Martes zibellina, of the family Mustelidae, found in northern 
Asia from the Urals to Japan; it is a valuable fur-bearing animal and quite 
similar to the American marten. The sable obtains food by preying upon other 
arboreal animals, principally birds and small mammals. It is well adapted to 
these activities, with its slender, supple body and short limbs with five 
short toes on each foot that terminate in sharp curved claws. The long bushy 
tail serves as a balancing organ. Mating occurs in summer, and three or four 
young are born the following spring.  See  CARNIVORA; MARTEN. 
 �34s[Charles B. Curtin]
�33s 
A700000001257�254s�5v�1v1517
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�6v�2v�32sSaccharin�21m�33s
    An organosulfur compound, also called o-sulfobenzoic imide. The material
used as a sweetening agent (about 500-700 times as seet as cane sugar) is the
sodium salt, which passes largely unchanged through the body and is excreted in
the urine. The slight bitter aftertase of sodium saccharin is mainly that of
impurities from the conventional sythesis and can be avoided by syntehses
startingfrom anthranilic acid or benzothiophene. Saccharin is used in food
preparation for low-caloric diets and in diabetes therapy, where normal sugars
cannot be tolerated. See ORGANOSULFUR COMPOUND. 
�34s[Norman Kharasch]
�33s
  
A700000002055�254s�5v�1v1517
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�6v�2v�32sSacoglossa   �21m �33s 
    An order of the gastropod subclass Opisthobranchia containing a thousand 
living species of herbivorous sea slugs; sometimes called Ascoglossa. They 
occur in all the oceans, at shallow depths, reaching their greatest size and 
diversity in tropical seas. 
    Members of this abundant order have two common features: the herbivorous 
habit (except for Olea and a species of Stiliger which eat the eggs of other 
mollusks) and the possession of a uniseriate radula in which the oldest, often 
broken, teeth are usually retained within the body, not discarded. 
    Sacoglossans possess varied shells; there may be a single shell, capacious 
and coiled (Volvatella, illustration a; Cylindrobulla), a flattened open 
dorsal shell (Lobiger), or two lateral shells (the bivalved gastropods, 
Berthelinia, illustration b). In the highest sacoglossans (Elysia, Limapontia) 
the true shell is completely lost after larval metamorphosis.  See  
OPISTHOBRANCHIA.  
 �34s[T. E. Thompson]
�33s 
  
  
A700000001560�254s�5v�1v1517
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�6v�2v�32sSafety factor �21m �33s
    An empirical number by which the strength of a material is divided to
obtain a conservative design stress. A safety factor is used because of
uncertainties in operating conditions that may be encountered, nonuniformities
in materials, simplifying design assumptions, effects of aging such as
corrosion, and strains introduced inadvertently during fabrication and
transportation and because of the seriousness of failure. Safety factors vary
widely depending on the material, consequences of failure, and operating
conditions. For ductile materials, safety factors applied to yield strength
are often 1.5 - 4. For brittle materials that fracture with no prior evidence
of incipient failure, factors of 5 - 8 may be appropriate.  See  STRESS AND
STRAIN.
 �34s[William J. Krefeld /Waldo G. Bowman]
�33s

A700000001162�254s�5v�1v1517
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�6v�2v�32sSafety glass �21m �33s
    A unitary structure formed of two or more sheets of glass between each of
which is interposed a sheet of plastic, usually polyvinyl butyral. In usual
manufacture, two clean and dry sheets of plate glass and a sheet of plastic
are preliminarily assembled as a sandwich under slight pressure to produce a
void-free bond. The laminate is then pressed under heat long enough to unite.
For use in surface vehicles the finished laminated glass is approximately 1/4
in. (6 mm) thick; for aircraft it is thicker.  See  GLASS.
 �34s[Frank H. Rockett]
�33s

A700000001263�254s�5v�1v1517
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�6v�2v�32sSafety valve   �21m �33s 
    A relief valve set to open at a pressure safely below the bursting 
pressure of a container, such as a boiler or compressed air receiver. 
Typically, a disk is held against a seat by a spring; excessive pressure 
forces the disk open (see illustration). Construction is such that when the 
valve opens slightly, the opening force builds up to open it fully and to hold 
the valve open until the pressure drops a predetermined amount. This 
differential or blow-down pressure and the initial relieving pressure are 
adjustable.  See  VALVE. 
 �34s[Theodore Baumeister]
�33s 
  
  
A700000002547�254s�5v�1v1518
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�6v�2v�32sSafflower   �21m �33s 
    An oilseed crop (Carthamus tinctorius) that is a member of the thistle 
(Compositae) family and produces its seed in heads. Flowers vary in color from 
white through shades of yellow and orange to red. The seed is shaped like a 
small sunflower seed, and is covered with a hull that may be white with a 
smooth surface or off-white to dark gray with a ridged surface. Depending on 
hull thickness, the oil content varies from 25 to 45%.  Safflower is grown 
commercially in Mexico, Australia, and California.  See  ASTERALES. 
    There are two types of safflower oil, both with 6 - 8% palmitic acid and 1 
- 2% stearic acid. One type, the standard or polyunsaturated type, has 76 - 
79% linoleic acid and 11 - 17% oleic acid. This high-linoleic type has been 
used in soft margarines, in salad oils, and in the manufacture of paints and 
varnishes. It has had limited use for frying foods, because heat causes it to 
polymerize and form a tough film on the cooking vessel. The second type of 
oil, called the high - oleic-acid or monounsaturated type, has 76 - 79% oleic 
acid and 11 - 17% linoleic acid. Its fatty-acid composition is similar to that 
of olive oil, but the flavor is bland. High-oleic safflower oil is a premium 
frying oil.  See  FAT AND OIL (FOOD). 
 �34s[Paulden F. Knowles]
�33s 
  
A700000000712�254s�5v�1v1518
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�6v�2v�32sSaffron �21m �33s
    The plant Crocus sativus, a member of the iris family (Iridaceae). A
native of Greece and Asia Minor, it is now cultivated in various parts of
Europe, India, and China. This crocus is the source of a potent yellow dye
used for coloring foods and medicine. The dye is extracted from the styles and
stigmas of the flowers.  See  LILIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

A700000000725�254s�5v�1v1518
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�6v�2v�32sSage �21m �33s
    The plant Salvia officinalis, a member of the mint family (Labiatae), the
leaves of which yield a spice and an aromatic oil. It is a half-shrub native
to the Mediterranean region but is now widely cultivated. It is much used as a
flavoring in stuffing for fowl and in meats, especially sausage. Oil of sage
is used in making perfumes.  See  LAMIALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sSagittarius   �21m �33s 
    The Archer, in astronomy, a zodiacal and summer constellation, the major 
portion of which lies directly in the Milky Way. Sagittarius is the ninth sign 
and the southernmost constellation of the zodiac (see illustration). The Milky 
Way in Sagittarius is very bright, containing rich star fields and clusters, 
because its direction lies in the center of the Milky Way stellar system.  See 
  CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sSailplane   �21m �33s 
    A high-performance glider. Sailplane construction traditionally has been 
of wood and plywood, although the use of aluminum alloy has become common. The 
use of fiber glass as primary structure has also come into prominence, since 
it is possible to produce the external shapes in accurate molds with greater 
precision, resulting in improved performance.  See  GLIDER. 
 �34s[Ernest Schweizer]
�33s 
  
A700000000664�254s�5v�1v1518
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�6v�2v�32sSaint Elmo's fire   �21m �33s 
    A type of corona discharge observed on ships under conditions approaching 
those of an electrical storm. The charge in the atmosphere induces a charge on 
the masts and other elevated structures. The result of this is a corona 
discharge which causes a spectacular glow around these points.  See  CORONA 
DISCHARGE.  
 �34s[Glenn H. Miller]
�33s 
  
A700000001007�254s�5v�1v1518
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�6v�2v�32sSalenioida   �21m �33s 
    An order of Echinacea in which the apical system includes one or several 
large angular plates covering the periproct, with the other characters similar 
to those of the hemicidaroid urchins. There are two families: the 
Acrosaleniidae, an extinct group confined to the Jurassic and Cretaceous; and 
the Saleniidae, which ranged from the Jurassic onward, with two surviving 
deep-sea genera. See  ECHINACEA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sSalicales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of the single family Salicaceae, with about 350 species. There are 
only two genera, Salix (willow) and Populus (poplar and cottonwood), the 
former by far the larger. The Salicales are dioecious, woody plants, with 
alternate, simple, stipulate leaves and much reduced flowers that are 
aggregated into catkins. The mature seeds are plumose-hairy and are 
distributed by the wind.  See  DILLENIIDAE; MAGNOLIOPSIDA; POPLAR; WILLOW. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sSalicylate �21m �33s
    A salt or ester of salicylic acid having the general formula shown below
and formed by replacing the carboxylic hydrogen of the acid by a metal (M) to
give a salt or by an organic radical (R) to give an ester. Alkali-metal salts

				     O
				   //
		       (o)-C6H4 (OH)C-O-M  or -R

are water-soluble; the others, insoluble. Sodium salicylate is used in
medicines as an antirheumatic and antiseptic, in the manufacture of dyes, and
as a preservative (illegal in foods). Salicylic acid is used in the
preparation of aspirin. The methyl ester is the chief component of oil of
wintergreen. This ester is used in pharmaceuticals as a component of rubbing
liniment. It is also used as a flavoring agent and an odorant.	See  ASPIRIN.
 �34s[Elbert H. Hadley]
�33s



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�6v�2v�32sSaline evaporite   �21m �33s 
    A sedimentary deposit of soluble salts resulting from the evaporation of a 
standing body of water. Quantitatively the most important evaporites are 
anhydrite, CaSO4; gypsum, CaSO4PRODUCT2H2O; and halite (rock salt), NaCl. 
Other evaporites, of much more limited distribution and volume but of economic 
significance, include potassium chlorides, sodium carbonates, borates, and 
nitrates.  See  ANHYDRITE; GYPSUM; HALITE. 
    Evaporites, being soluble, are rarely exposed at the surface except in 
arid regions. Gypsum is common near the surface but is invariably replaced by 
anhydrite at greater depths, the latter mineral being vastly more abundant. 
Other evaporite minerals are not related to depth of burial, except for the 
effects of near-surface solution. 
 �34s[L. L. Sloss]
�33s 
  
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�6v�2v�32sSaline water reclamation   �21m �33s 
    The partial or almost complete demineralization of sea and brackish 
waters, geothermal brines, wastewaters, and industrial effluents to make fresh 
water suitable for human or animal consumption, diverse industrial uses, 
irrigation, recreation, or aquifer recharge - broadly referred to as 
desalination.  
    A diversity of approaches are used for the separation of water from saline 
solutions. Thermal processes effect separation by means of phase changes and 
include distillation and freezing processes. In the membrane processes, one or 
more suitably designed organic membranes accomplish the separation process. In 
a single membrane design, reverse osmosis pressure forces the fresh water 
through the membrane. In electrodialysis, a system using multiple membranes, 
direct current leads to formation of pure water and brine streams and drives 
the salt ions toward the electrodes through charge-selective membranes. There 
are also chemical processes. In the ion exchange process, substances are added 
to exchange the ions in the solution or to precipitate the salts. In the 
solvent extraction process, chemicals with greater affinity for water can 
remove the wastes from solutions.  See  ION EXCHANGE; OSMOSIS; WATER TREATMENT. 
    There is no optimum or universal process for demineralization of all water 
streams. Each so-called contaminated or modified water stream must be 
evaluated for its peculiarities and an optimal process for purification for a 
specific end use recommended. Among the criteria to be considered are: 
salinity, identity of the ions present, presence of other contaminants, nature 
of the water, availability, energy costs, quantity, and quality. 
 �34s[Robert Bakish]
�33s 
  
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�6v�2v�32sSalmonella �21m �33s
    A genus of the bacterial family Enterobacteriaceae, causing enteric
infections with or without blood invasion (septicemia). Most species are
motile, utilize citrate, decarboxylate ornithine, form gas from glucose, and
produce hydrogen sulfide; but notable exceptions exist, for example, S. typhi
and S. paratyphi A. Salmonellae do not ferment lactose, produce indole, or
split urea; the Voges-Proskauer reaction is negative.  See  ENTEROBACTERIACEAE.
    The habitats of the salmonellae are many animal species and humans - which
may be carriers or may suffer from Salmonella infection. Salmonellae are
frequently found in chickens, ducks, turkeys, pigs, rodents, and turtles; they
are not uncommon in cattle. Animal infections are often septicemic. Although
the present nomenclature suggests otherwise, there is no animal specificity
for a serotype. Some serotypes, however, are found in humans only (for
example, S. typhi and S. paratyphi A).	See  SALMONELLOSES.
 �34s[Alexander W.C. vonGraevenitz]
�33s

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�6v�2v�32sSalmonelloses �21m �33s
    Diseases caused by Salmonella. These include enteritis and septicemia with
or without enteritis (5 - 10%). S. typhi, S. paratyphi A, B, and C, and
occasionally S. cholerae suis, cause particular types of septicemia called
typhoid and paratyphoid fever, respectively; while all other types may cause
enteritis or septicemia, or both together.  See  SALMONELLA.
    Typhoid fever has an incubation period of 5 - 14 days. It is typified by a
slow onset with initial bronchitis, diarrhea or constipation, a characteristic
fever pattern (increase for 1 week, plateau for 2 weeks, and decrease for 2 -
3 weeks), a slow pulse rate, development of rose spots, swelling of the
spleen, and often an altered consciousness; complications include perforation
of the bowel and osteomyelitis. Typhoid fever leaves the individual with a
high degree of immunity. Paratyphoid fever has a shorter course and is
generally less severe than typhoid fever.
    Enteric fevers, that is, septicemias due to types of Salmonella other than
those previously mentioned, are more frequent in the United States than
typhoid and paratyphoid fever but much less frequent than Salmonella
enteritis. In children and in previously healthy adults, enteric fevers are
most often combined with enteritis and have a favorable outlook. The organisms
involved are the same as those causing Salmonella enteritis.
    Inflammation of the small bowel, enteritis, due to Salmonella is one of
the most important bacterial zoonoses. The most frequent agents are S.
typhimurium, S. enteritidis, S. newport, S. heidelberg, S. infantis, and S.
derby. The incubation period varies from 6 hr to several days. Diarrhea and
fever are the main symptoms; the intestinal epithelium is invaded, and early
bacteremia is probable.
 �34s[Alexander W.C. vonGraevenitz]
�33s

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�6v�2v�32sSalmoniformes   �21m �33s 
    An order of soft-rayed fishes comprising salmon and their allies. They 
make up the stem group from which most higher teleostean fishes have evolved. 
    Salmoniformes are generalized fishes characterized by soft or articulated 
fin rays; adipose dorsal fin usually present; cycloid (smooth) scales; pelvic 
fin in an abdominal position (well back on the trunk), usually with more than 
six rays, and with pelvic girdle free from pectoral girdle; pectoral fin 
placed low on the side and more or less horizontal; upper jaw usually bordered 
by premaxillae and maxillae; premaxillae not protractile; no weberian 
apparatus connecting the swim bladder with the inner ear, but a duct joining 
the bladder with the gut; and branchiostegal rays variable in number and 
arrangement.  
    Salmoniform fishes have an imperfect fossil record; they were present in 
the Cretaceous and likely earlier. The order consists of 8 suborders, 37 
families, some 212 genera, and about 950 species. Most are marine fishes, and 
they are common in all seas. A few are adapted to pelagic or shore waters, 
most live in the bathypelagic zone, some tolerate arctic seas, and many 
inhabit warm or cold fresh water, or are anadromous; that is, they enter fresh 
water to reproduce. 
    The suborder Salmonoidei includes many of the most important and best 
known of all fishes, especially in the family Salmonidae, or salmon, trouts, 
whitefishes, and graylings. The Argentinoidei (argentines, deep-sea smelts, 
and related fishes), Alepocephaloidei (slickheads), and Bathylaconoidei are 
found chiefly in deep marine waters. The Galaxioidei are mostly small, 
fresh-water fishes of south temperate lands, especially Australia and New 
Zealand. Esocoidei are a small group, the pikes and mudminnows of northern 
fresh waters. The Stomiatoidei, lightfishes and allies, inhabit the middle 
depths of the oceans. They are of small size and often grotesque form, and are 
equipped with photophores. The Myctophoidei is the largest  of the suborders. 
All species are marine; some, including lizardfishes, inhabit shore waters, 
but the majority are oceanic. Those that live at moderate to considerable 
depths, such as the lantern fishes, commonly have photophores on the head and 
body.  See  PHOTOPHORE GLAND; TELEOSTEI. 
 �34s[Reeve M. Bailey]
�33s 
  
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�6v�2v�32sSalt dome   �21m �33s 
    An intrusive body of rock salt which has penetrated large thicknesses of 
overlying sedimentary rock. Salt domes are distinguished from other geological 
deformations involving salt in being roughly circular or elliptical in cross 
section and in having horizontal dimensions of the same order of magnitude or 
less than their vertical dimensions.  See  SALINE EVAPORITE. 
    Salt domes are best known along the Gulf Coast of the United States, both 
onshore and offshore, where they are economically important because of their 
association with deposits of oil and sulfur. 
 �34s[Lewis L. Nettleton]
�33s 
  
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�6v�2v�32sSalt gland   �21m �33s 
    A specialized gland located around the eyes and nasal passages in marine 
turtles, snakes, and lizards, and in birds such as the petrels, gulls, and 
albatrosses, which spend much time at sea. In the marine turtle it is an 
accessory lacrimal gland which opens into the conjunctival sac. In seagoing 
birds and in marine lizards it opens into the nasal passageway. Salt glands 
copiously secrete a watery fluid containing a high percentage of salt, higher 
than the salt content of urine in these species. As a consequence, these 
animals are able to drink salt-laden sea water without experiencing the 
dehydration necessary to eliminate the excess salt via the kidney route.  See  
EXCRETION; GLAND. 
 �34s[Olin E. Nelsen]
�33s 
  
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�6v�2v�32sSaltmarsh   �21m �33s 
    Generally, a maritime habitat characterized by special plant communities, 
which occur primarily in the temperate regions of the world; however, typical 
saltmarsh communities can be formed in association with mangrove swamps in the 
tropics and subtropics. In inland areas where saline springs emerge or where 
there are salt lakes, typical saltmarsh communities can be found, dominated 
sometimes by the same species that occur on maritime saltmarsh. The extensive 
areas of inland salt desert, while exhibiting some features of similarity with 
saltmarsh, are nevertheless best regarded as a separate entity. Excess sodium 
chloride is the predominant environmental feature of saltmarsh (maritime or 
inland). In the case of salt deserts sodium chloride is only one of the alkali 
salts that may occur in excess. 
    Saltmarsh can form on mud, muddy sand, or sandy mud, but not on pure sand 
because mobile sand does not provide a sufficiently stable substrate. Typical 
physiographic features associated with saltmarsh are the creeks, which serve 
as drainage channels, and pools, which are known as pans. 
    The plants that grow on saltmarshes must tolerate the excess sodium 
chloride and are termed halophytes. These plants possess features associated 
with the halophytic environment, for example, development of succulence, waxy 
cuticle, and salt-excreting glands; overall there is a tendency for the 
vegetation to exhibit a drab grayness. The predominant life forms on 
saltmarshes are the hemicryptophytes and therophytes. There are also 
characteristic algae associated with saltmarsh vegetation. Particular 
communities of green, red, and blue-green algae are common, and in the 
Atlantic - North Sea area there may be extensive communities of free-living 
brown fucoids. 
 �34s[Valentine J. Chapman]
�33s 
  

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�6v�2v�32sSalt (chemistry) �21m �33s
    A compound formed when one or more of the hydrogen atoms of an acid are
replaced by one or more cations of the base. The common example is sodium
chloride in which the hydrogen ions of hydrochloric acid are replaced by the
sodium ions (cations) of sodium hydroxide. There is a great variety of salts
because of the large number of acids and bases which has become known.
    Salts are classified in several ways. One method - normal, acid, and basic
salts - depends upon whether all the hydrogen ions of the acid or all the
hydroxide ions of the base have been replaced:

      Class			   Examples
Normal salts		NaCl,NH4Cl, Na2SO4, Na2CO3, Na3PO4
			Ca3 (PO4)2
Acid salts		NaHCO3, NaH2PO4, Na2,HPO4, NaHSO4
Basic salts		Pb(OH)Cl, Sn(OH)Cl

The other method-simple salts, double salts (including alums), and complex
salts-depends upon the character of completeness of the ionization:

      Class			    Examples
Simples salts		NaCl, NaHCO3, Pb(OH)Cl
Double salts		KCl.MgCl2
Alums			Kal(SO4)2, NaFe(SO4)2, NH4,Cr(SO4)2
Complex salts		K3Fe(CN)6, Cu(NH3)4 Cl2, K2Cr2O7

See ACID AND BASE; CHEMICAL BONDING.
 �34s[Alfred B. Garrett]
�33s

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�6v�2v�32sSalt (food) �21m �33s
    The chemical compound sodium chloride. It is used extensively in the food
industry as a preservative and flavoring, as well as in the chemical industry
to make chlorine and sodium.  See  CHLORINE; FOOD PRESERVATION; SALT
(CHEMISTRY); SODIUM.
    Salt was originally made by evaporating sea water (solar salt). This
method is still in common usage; however, impurities in solar salt make it
unsatisfactory for most commercial uses, and these impurities also lead to
clumping. Salt, freshly produced from sea-water evaporation ponds, may contain
large numbers of halophilic (salt-loving) microorganisms. In the United States
refined salt is obtained from underground mines located in Michigan and
Louisiana. Salt is usually handled during the refining processes as brine.
    Salt is liable to clumping during periods of high humidity, so preventives
are added. Materials used include magnesium carbonate and certain silicates.
Iodides are added in those areas where iodine deficiencies exist.
 �34s[Roy Earl Morse]
�33s




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�6v�2v�32sSalviniales   �21m �33s 
    A small order of heterosporous, leptosporangiate ferns (division 
Polypodiophyta) which float on the surface of the water. The delicate, 
branching stem is provided with small, simple to bifid or more or less 
dissected leaves. The sporangia are enclosed in specialized appendages of the 
leaves, called sporocarps. The order contains only a single family, with two 
widely distributed genera, Salvinia and Azolla, with only about 20 species in 
all.  See  POLYPODIOPHYTA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sSamarium �21m �33s
    A chemical element, Sm, atomic number 62, belonging to the rare-earth
group. Its atomic weight is 150.35, and there are 7 naturally occurring
isotopes;  (sup 147)Sm, (sup 148)Sm, and (sup 149)Sm are radioactive
and emit ALPHA particles.
    Samarium oxide is pale yellow, is readily soluble in most acids, and gives
topaz-yellow salts in solutions. Samarium has found rather limited use in the
ceramic industry, and it is used as a catalyst for certain organic reactions.
One of its isotopes has a very high cross section for the capture of neutrons,
and therefore there has been some interest in samarium in the atomic industry
for use as control rods and nuclear poisons.  See  LANTHANUM; RARE-EARTH
ELEMENTS.
 �34s[Frank H. Spedding]
�33s

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�6v�2v�32sSampled-data control system   �21m �33s 
    An information system for controlling a process, or for transforming 
information to meet desired requirements, in which the signals or data in one 
or more components of the system appear in the form of a sequence of pulses or 
codes. A control system which incorporates sampling and usually a digital 
controller or digital processor is a sampled-data control system. Sampled-data 
controls find a wide range of both military and civilian applications, such as 
missile guidance, ground-controlled interception, satellite launch, energy 
processing, machine tool control, assembly-line automation, and laboratory 
instrumentation. With the advent of inexpensive micro- and minicomputers, most 
control systems designed today are various versions of sampled-data control 
systems.  
    The operation which converts continuous data into discrete and 
intermittent form is often referred to as sampling. Usually the sampled 
information is expressed in digital form for digital control computation. In 
this case the sampling takes the form of analog-to-digital conversion. Such 
operation may be performed by an electronic scanning device, a digital data 
link, or a mechanical switching device. The digital controller can be realized 
as a computer program describing a control algorithm if a digital computer is 
available for on-line operation, or a microprocessor implementing a control 
procedure if a microcomputer is incorporated into the control system. The 
output of the digital controller is a digital signal which is converted to a 
continuous control signal by a digital-to-analog converter. In some 
applications the digital-to-analog converter is an integral part of the 
process. The main purpose of the digital controller is to generate data or 
codes for controlling the process so that the performance requirements of the 
control system may be fulfilled.  See  ANALOG-TO-DIGITAL CONVERTER; DIGITAL 
COMPUTER; DIGITAL-TO-ANALOG CONVERTER; MICROCOMPUTER; MICROPROCESSOR. 
 �34s[Julius T. Tou]
�33s 
  
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�6v�2v�32sSampling techniques   �21m �33s 
    In analytical chemistry, the operations required to obtain a laboratory 
sample from a large quantity of raw material. Most commercial materials are 
not homogeneous; that is, their compositions vary from portion to portion. If 
the analysis for a constituent is to be significant, the portion used in the 
laboratory must have the same composition as that of the original material. 
The problem of obtaining a representative sample may be more difficult than 
the problems of analysis. 
    Sampling of a gas is difficult. Special equipment and procedures are 
required to obtain a representative, homogeneous portion for analysis. 
    Homogeneous liquids in tanks or barrels are sampled by dipping several 
portions from different locations or by drawing off portions from several 
containers. Liquids flowing in open rivers or in pipes are sampled by dipping 
from the open stream or by using a bypass system on a pipe. 
    Segregation is a major problem for solids, so usually 0.5 - 2.0% of the 
material is taken as the gross sample to be certain of a representative 
portion. Metallic materials such as steels, brasses, sheet metals, and wires 
are sampled by drilling, cutting, milling, or filing if reasonably 
homogeneous, or by taking very fine drillings from several locations if 
segregation of components is known to exist. Nonmetallic materials, such as 
coal, ores, rocks, ore veins, and soils, consist of particles of varying size 
as well as of varying composition. The gross sample comprises particles from 
various portions of the material that have been ground and mixed together. 
    Since the final laboratory sample may still be somewhat heterogeneous, a 
portion several times the amount needed for an analysis is dissolved in a 
suitable solvent, and the sample solution is diluted to a definite volume. A 
fraction of this solution is taken for analysis. It is called an aliquot.  See 
  ANALYTICAL CHEMISTRY. 
 �34s[Charles Rulfs]
�33s 
  
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�6v�2v�32sSampling voltmeter   �21m �33s 
    A special class of voltmeter that uses sampling techniques and is 
particularly effective in measuring high-frequency signals or signals mixed 
with noise. High-frequency sampling instruments can operate on signals with 
frequencies as high as 12 gigahertz and amplitudes as small as 1 millivolt. 
Sampling voltmeters, as conventional voltmeters, may use scale and pointer 
meters, graphic recorders, cathode-ray tubes, or digital-type indicators for 
readout of measured quantities.  See  VOLTAGE MEASUREMENT. 
    The sampling technique detects the instantaneous value of an input signal 
at prescribed times by means of an electronic switch connecting the signal to 
a memory capacitor. At the prescribed times, the switch is closed for a very 
short interval and the memory capacitor is thereby briefly connected to the 
signal. The connection generates a sample; that is, the capacitor charges to a 
voltage proportional to the input voltage at the instant of sampling. After a 
sample the capacitor voltage remains constant until the next sample is taken. 
The sampled voltage from this type of sample-and-hold circuit is an 
approximate replica of the signal waveform assembled bit by bit and therefore 
at a much lower frequency. By proper adjustment of frequencies and by 
filtering, the sampled signal waveform may be made to be an extremely exact 
waveform replica of the input signal. Since the reproduced waveform is on a 
different time scale than the original, the type of sampling described above 
is sometimes called equivalent-time sampling. 
 �34s[James K. Skilling]
�33s 
  
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�6v�2v�32sSand   �21m �33s 
    A loose material consisting of small mineral particles, or rock and 
mineral particles, distinguishable by the naked eye. Most sands are formed by 
natural agencies. Many deposits of such sands contain clay and silt in varying 
amounts; some deposits contain pebbles. The mineral composition of sands 
varies as does the size of the grains composing them. Sands are widely 
distributed and have many industrial uses. 
    The term sand also is applied, especially commercially, to small mineral 
or rock particles produced by crushing larger materials; for example, 
limestone sand made by crushing limestone, slag sand from slag, or sand made 
by crushing quartzite. Various granular materials, not necessarily of 
inorganic composition, likewise may be called sand because they consist of 
sand-size particles. 
    Natural sands result primarily from the disintegration of rocks by 
weathering or erosion. Streams and the waves and currents of lakes and oceans 
are major agencies eroding rock into sand. The grinding action of glaciers is 
another important sand-producing agency. 
    Some sand deposits are formed in place by the weathering of rocks, such as 
sandstone. Others are the result of the sorting out and concentration of sand 
from particulate material (composed of particles of various sizes) by the 
running water of streams or by the waves and currents of lakes and seas. Wind 
also concentrates sand from certain materials. Deposits of sand accumulate as 
bars in rivers and streams, in river deltas, as beaches and bars of lakes or 
seas, and as dunes built up by wind. See DUNE. 
 �34s[John E. Lamar]
�33s 
  
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�6v�2v�32sSandalwood �21m �33s
    The name applied to any species of the genus Santalum of the sandalwood
family (Santalaceae). However, the true sandalwood is the hard, close-grained,
aromatic heartwood of a parasitic tree, S. album, of the Indo-Malayan region.
This fragrant wood is used in ornamental carving, cabinet work, and as a
source of certain perfumes. The odor of the wood is an insect repellent, and
on this account the wood is much used in making boxes and chests. The fragrant
wood of a number of species in other families bears the same name, but none of
these is the real sandalwood.  See  SANTALALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sSand dollar   �21m �33s 
    An echinoderm belonging to the order Clypeasteroida in the class 
Echinoidea. Sand dollars have a flat, disk-shaped body, with the mouth in a 
mid-ventral position and with the anus also on the ventral surface. There are 
several species. 
    Sand dollars live in sand, on the surface or partly buried, from the 
low-tide mark to depths of 4800 ft (1460 m). Burrowing and locomotion are 
assisted by the short spines which cover the body. Sand dollars ingest sand 
grains covered with diatoms or other algae.  See  CLYPEASTEROIDA. 
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sSandstone   �21m �33s 
    A variety of sedimentary rocks. There are three major groups of sandstone: 
terrigenous, carbonate, and pyroclastic. Terrigenous sandstones consist of 
clasts derived from the erosion of preexisting rocks located outside the 
depositional basin. They are transported by moving currents of wind and water 
and are deposited in a wide variety of environments. Carbonate sandstones 
consist mainly of fragments of calcium carbonate in the form of skeletal 
debris, oolites or other layered carbonate pellets, and locally derived 
(intraformational) carbonate rock debris (intraclasts). Most carbonate sands 
are deposited in marine environments and are conventionally classified and 
analyzed as limestones. Pyroclastic sandstones are composed of rock fragments 
which were produced directly by explosive volcanic activity. They are 
volumetrically the least important sandstone variety.  See  LIMESTONE. 
    Sandstones consist of two major components: the framework fraction 
composed of all sand-sized clasts, regardless of composition, and the 
interstitial areas between the grains. In both modern sands and ancient 
sandstones, these interstices may be empty voids. However, in most ancient 
sandstones the interstices are commonly filled with either of two essentially 
mutually exclusive materials: crystalline chemical cement (usually silica or 
calcium carbonate) or fine-grained matrix. This dichotomy produces the two 
principal sandstone groups: arenites and wackes. 
    Sandstones represent between 10 and 20% of the total volume of sedimentary 
rocks in the Earth's crust. Most sandstone is found within the continental 
blocks rather than in ocean basins, and most sandstone in continental blocks 
occurs within the mobile mountain belts rather than in the stable cratons.  
See  SEDIMENTARY ROCKS. 
 �34s[Frederic L. Schwab]
�33s 
  
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�6v�2v�32sSanitary engineering �21m �33s
    A specialty field generally developed in civil engineering but not limited
to that branch. The sanitary engineer has the responsibility (1) to conceive,
design, appraise, direct and manage engineering works and projects developed,
as a whole or in part for the protection and promotion of the public health,
particularly as it relates to improvement of the environment, and (2) to
investigate and correct engineering works and other projects that are capable
of injury to the public health by being or becoming faulty in conception,
design, direction, or management.
    Sanitary engineering practice includes surveys, reports, designs, reviews,
management, operation and investigation of works or programs for (1) water
supply, treatment and distribution; (2) sewage collection, treatment and
disposal; (3) control of pollution in surface and underground waters; (4)
collection, treatment, and disposal of refuse; (5) sanitary handling of milk
and food; (6) housing and institutional sanitation; (7) rodent and insect
control; (8) recreational place sanitation; (9) control of atmospheric
pollution and air quality in both the general air of communities and in
industrial work spaces; (10) control of radiation hazards exposure; and (11)
other environmental factors affecting health, comfort, safety, and well-being
of people.  See  AIR POLLUTION; RADIATION BIOLOGY; SEWAGE; SEWAGE COLLECTION
SYSTEMS; SEWAGE DISPOSAL; SEWAGE TREATMENT; WATER POLLUTION; WATER SUPPLY
ENGINEERING.
 �34s[William T. Ingram]
�33s

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�6v�2v�32sSantalales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 10 families and about 2200 species. The largest families are the 
Loranthaceae (about 1400 species), Santalaceae (about 400 species), Olacaceae 
(about 230 species), and Balanophoraceae (about 100 species). A few of the 
more primitive members of the Santalales are autotrophic, but otherwise the 
order is characterized by progressive adaptation to parasitism, accompanied by 
progressive simplification of the ovules. Some members of the Santalales, such 
as sandalwood, are rooted in the ground and produce small branch roots which 
invade and parasitize the roots of other plants. Others, such as mistletoe 
(Viscum and other genera of the Loranthaceae), grow on trees, well above the 
ground.  See  MAGNOLIOPSIDA; MISTLETOE; ROSIDAE; SANDALWOOD. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sSapindales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of 17 families and about 6700 species. The largest families are the 
Rutaceae (about 1600 species), Sapindaceae (1500 species), Meliaceae (about 
1400 species), Anacardiaceae (about 600 species), and Burseraceae (about 600 
species). Most of the Sapindales are woody plants, with compound or lobed 
leaves and polypetalous, hypogynous to perigynous flowers with one or two sets 
of stamens and only one or two ovules in each locule of the ovary.  See  
BUCKEYE; CASHEW; CITRON; GRAPEFRUIT; KUMQUAT; LEMON; LIGNUMVITAE; LIME 
(BOTANY); MAGNOLIOPSIDA; MAHOGANY; MANDARIN; MAPLE; ORANGE; PISTACHIO; POISON 
IVY; QUEBRACHO; ROSIDAE; TANGERINE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sSapphire   �21m �33s 
    The name given to all gem varieties of the mineral corundum, except those 
that have medium to dark tones of red that characterize ruby. Although the 
name sapphire is most commonly associated with the blue variety, there are 
many other colors of gen corundum to which sapphire is applied correctly; 
these include yellow, brown, green, pink, orange, purple, colorless, and 
black. Sapphire has a hardness of 9, a specific gravity near 4.00, and 
refractive indices of 1.76 - 1.77. Asterism, the star effect, is the result of 
reflections from tiny, lustrous, needlelike inclusions of the mineral rutile, 
plus a domed form of cutting.  See  CORUNDUM; GEM; RUBY; RUTILE. 
 �34s[Richard T. Liddicoat, Jr.]
�33s 
  
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�6v�2v�32sSapropel   �21m �33s 
    A mud, slime, or ooze deposited in more or less open water. Sapropel may 
vary widely in composition depending upon relative contributions from 
decomposing substances derived from plants and animals. Hydrogen sulfide, 
produced during the initial biochemical degradation of these substances, 
promotes preservation of the more resistant parts of the organisms. Most 
marine sapropelic deposits contain no appreciable contribution from humic 
substances of terrestrial origin, but certain carbonaceous marine shales 
appear to contain some humic matter that was part of the original sapropelic 
deposit. Metamorphism of a sapropelic deposit leads to such products as 
torbanite, oil shales, and asphaltites.  See  ASPHALT AND ASPHALTITE; OIL 
SHALE; TORBANITE. 
 �34s[Irving A. Breger]
�33s 
  
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�6v�2v�32sSarcodina   �21m �33s 
    A superclass of Protozoa (in the subphylum Sarcomastigophora) in which 
movement involves protoplasmic flow, often with recognizable pseudopodia. 
Gametes may be flagellated, as in certain Foraminiferida. Most species are 
floating or creeping; a few are sessile. The pellicle is relatively thin, and 
the body is apt to be plastic unless restrained by skeletal structures. 
Sarcodina live in fresh, brackish, or salt waters; soil or sand; and as 
endoparasites in animals and plants. A group may be limited to a specific 
habitat, but many have a rather wide range. Sarcodina include two major 
classes: Rhizopodea and Actinopodea.  See  ACTINOPODEA; RHIZOPODEA; 
SARCOMASTIGOPHORA.  
 �34s[Richard P. Hall]
�33s 
  
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�6v�2v�32sSarcomastigophora   �21m �33s 
    A subphylum of Protozoa, including those forms that possess flagella or 
pseudopodia or both. Organisms have a single type of nucleus, except the 
developmental stages of some Foraminiferida. Sexuality, if present, is 
syngamy, the fusion of of two gametes. Spores typically are not formed. 
Flagella may be permanent or transient or confined to a certain stage in the 
life history; this is true also of pseudopodia. Both flagella and pseudopodia 
may be present at the same time. 
    Three superclasses are included: (1) Mastigophora, commonly flagellates, 
contains 19 orders. (2) Opalinata includes 1 order; these organisms were once 
considered as ciliates, but further research has indicated flagellate 
kinships. (3) Sarcodina comprises organisms which normally possess pseudopodia 
and are flagellated only in the developmental stages; 13 orders possess 
irregularly distributed lobose or filose and branching pseudopodia, while 7 
orders have radially distributed axopodia, often with axial filaments.  See  
MASTIGOPHORA; OPALINATA; SARCODINA. 
    Most of the plant flagellates will live in either fresh-water or in both 
fresh and salt water. The zooflagellates are small and are not sufficiently 
abundant to enter markedly into the food chain. But the parasites and 
symbionts are of considerable interest economically and theoretically, for 
example, the trypanosomes and the peculiar xylophagous (wood-eating) symbionts 
of termites. In the termites these parasites actually digest the wood eaten by 
the host. Conspicuous in the ecology of marine waters are the dinoflagellates, 
radiolarians, and acantharians, especially in tropical waters of otherwise low 
productivity.  See  PROTOZOA. 
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sSarcopterygii   �21m �33s 
    A name often employed to unite the lobefin, or crossopterygian, fishes and 
the lungfishes as a subclass of the Osteichthyes. The older names Amphibioidei 
and Choanichthyes are equivalent to Sarcopterygii. The structural differences 
between lobefin fishes and lungfishes are great, and their common ancestry, if 
any, lies in the Lower Devonian, long antedating the origin of the earliest 
tetrapods which have since diverged into four classes. It seems best to rank 
the Crossopterygii (lobefins) and Dipnoi (lungfishes), each equivalent in rank 
to the subclass Actinopterygii (rayfin fishes), in which the vast majority of 
fishes are classified.  See  ACTINOPTERYGII; CROSSOPTERYGII; DIPNOI; 
OSTEICHTHYES.  
 �34s[Reeve M. Bailey]
�33s 
  
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�6v�2v�32sSarcoptiformes   �21m �33s 
    A suborder of the order Acarina. These are minute (0.01 - 0.06 in. or 0.3 
- 1.5 mm long) globular mites without stigmata, but there may be a tracheal 
system.The legs may be simple, or enlarged in the male, and may terminate in 
suckers, claws, or in a modification of both. The chelicerae are normally 
pincerlike. In normal development these creatures pass through four stages 
(larva and first, second, and third nymphal stages) before becoming adult. 
Under certain conditions a dispersal form, the hypopus, may develop between 
the first and second nymphal stages. 
    This group can be separated rather easily into the pale, weakly 
sclerotized Acaridiae and the dark, heavily sclerotized Oribatei. In the 
Acaridiae are found some of the most serious pests of stored food products; 
these destroy large quantities of wheat and other grains. The Sarcoptids are 
skin parasites of warm-blooded vertebrates. Sarcoptes scabiei and its 
varieties burrow in the skin of humans, cattle, sheep, goats, camels, horses, 
and dogs under crowded winter conditions and produce an itching that can lead 
to secondary infections. 
    The oribatid mites were at one time regarded as free-living soil or 
compost organisms of no economic significance. Investigations, however, have 
shown that they play an important part in the breakdown of organic matter, and 
that they act as an intermediate host of tapeworms. 
 �34s[Herbert H. J. Nesbitt]
�33s 
  
  
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�6v�2v�32sSarcosporida   �21m �33s 
    An order of Protozoa of the class Haplosporea which comprises parasites in 
skeletal and cardiac muscle of vertebrates. The organisms have a very wide 
distribution both geographically and in host species, infecting reptiles, 
birds, and mammals (including marsupials). Humans are occasionally infected by 
the parasite referred to as Sarcocystis lindemanni. It is doubtful whether 
Sarcosporida found in different hosts are themselves always different, though 
this has often been assumed. Host specificity is known not to be strict; 
however, it is unlikely that there is only one species - S. miescheriana - as 
proposed by some investigators. 
    The life cycle of Sarcocystis is quite typical of the Sporozoa, since 
sexual stages are lacking and schizogony (multiple fission), though sometimes 
claimed, apparently does not occur. Instead, reproduction is by binary 
fission, with the eventual development of cysts (Miescher's tubules) in the 
muscles. These cysts are relatively large structures, easily visible with the 
unaided eye, and contain myriads of the minute crescentic spores. Infection of 
a new host is believed to be by the ingestion of these spores with food and 
water contaminated by feces from an infected animal. 
    Cyst morphology is probably more stable than anything else about the 
parasite. In general, the cyst wall is said to have one or two layers, the 
outer one being either smooth or provided with spines or villi. The genesis of 
the wall is in some dispute; some think it is formed by the host as a reaction 
to the parasite, but the majority opinion is that the parasite itself forms 
the cyst. The cyst is divided internally into compartments and the outer wall 
is smooth or rough, depending on the species. The spores are crescent-shaped.  
See  HAPLOSPOREA; PROTOZOA; SPOROZOA. 
 �34s[Reginald D. Manwell]
�33s 
  
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�6v�2v�32sSargasso Sea   �21m �33s 
    A region of the west-central North Atlantic Ocean surrounded entirely by 
water. The western and northern boundaries of the Sargasso are formed by the 
Gulf Stream. The eastern and southern boundaries are more vague, but generally 
extend east to about 30DGW and south to about 20DGN (see illustration). The 
Sargasso is a clockwise gyre; that is, the currents revolve anticyclonically 
about its center. The Sargasso supplies much of the deep circulation of the 
Gulf Stream. The Bermuda Islands rise in the center of the Sargasso.  See  
GULF STREAM. 
    The Sargasso gets its name from yellow-brown sargassum weed, which 
collects in the center of the gyre supported by translucent amber floats the 
size of small grapes scattered over the plants. The color contrast between 
weeds and water is emphasized by the electric blue color of the water caused 
by the extremely low plankton productivity. Sargassum is a brown alga which 
grows attached in shallow water in the Caribbean. Some breaks loose and is 
transported north to the Sargasso, where it is caught in the gyre, and this is 
presumably the original source for the floating weed. 
 �34s[John M. Teal]
�33s 
  
  
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�6v�2v�32sSarraceniales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of 3 well-marked small families: the Droseraceae, with about 90 
species; the Nepenthaceae, with about 80 species; and the Sarraceniaceae, with 
only about 16. The plants characteristically grow in waterlogged soils which 
are deficient in available nitrogen. They are herbs or shrubs with alternate, 
simple leaves that are modified for catching insects, from which they absorb 
nitrogenous nutrients. The pitcher plants (Sarracenia), sundew (Drosera), and 
Venus' flytrap (Dionaea muscipula) are well-known members of the order.  See  
DILLENIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sSassafras �21m �33s
    A medium-sized tree, Sassafras albidum, of the eastern United States,
extending north as far as southern Maine. Sometimes it is only a shrub in the
north, but from Pennsylvania southward heights of 90 ft (27 m) or more with
diameters of 4 - 7 ft (1.2 - 2.1 m) have been reported for this plant.
Sassafras is said to live from 700 to 1000 years. It can be recognized by the
bright-green color and aromatic odor of the twigs and leaves. The leaves are
simple or mitten-shaped (hence a common name ``mitten-tree''), or they may
have lobes on both sides of the leaf blade.  See  MAGNOLIALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sSatellite astronomy   �21m �33s 
    The study of astronomical objects using detectors mounted on 
Earth-orbiting satellites or deep-space probes so that observations 
unobstructed by the Earth's atmosphere can be made. Many astronomical 
satellites carrying detectors to record electromagnetic radiation at 
wavelengths shorter than visible light (ultraviolet, x-, and gamma rays) have 
been launched. In general, one or two wavelength regions have been observed 
from each satellite. Successive satellites incorporated significant 
developments in operating modes and improvements in detector sensitivity and 
resolution such that new discoveries were possible. 
  
    Ultraviolet astronomy.  Since the Earth's atmosphere is opaque to 
ultraviolet light with wavelengths shorter than about 310 nm, ultraviolet 
astronomy had to await the space age. The Orbiting Solar Observatory (OSO) 
satellites were the first to be devoted primarily to astronomical ultraviolet 
and x-ray observations. Eight satellites (OSO 1 - 8) were launched over a 
12-year period beginning in 1963. The solar ultraviolet experiments included 
both low- and high-spectral-resolution spectrometers. The resulting detailed 
maps of the Sun greatly increased understanding of the temperature versus 
height profile of the solar atmosphere. Time-resolved spectra and images were 
also recorded so that the first ultraviolet observations of solar flares were 
conducted.  
    Extensive ultraviolet observations of stars and nebulae in the Galaxy but 
outside the solar system were first conducted with ultraviolet telescopes on 
the Orbiting Astronomical Observatory (OAO) satellites. Two of these 
spacecraft were successfully launched. 
    The International Ultraviolet Explorer (IUE), launched in 1978 into a 
geosynchronous orbit, carried a 45-cm-aperture telescope. IUE has been used 
for a great many observations of objects within the Galaxy, such as galactic 
x-ray binaries, hot stars, and globular clusters. However, its greater 
sensitivity has also permitted for the first time ultraviolet observations of 
extragalactic objects. Active galaxies and nearby bright quasars such as 3C273 
have been some of the primary targets.  See  GALAXY, EXTERNAL; QUASARS; 
ULTRAVIOLET ASTRONOMY. 
  
    X-ray astronomy.  X-ray astronomy which can only be done from above the 
Earth's atmosphere, really came of age with the launch of the first Small 
Astronomy Satellite (SAS 1) in 1970, designated Uhuru. It carried two 
proportional-counter x-ray detectors in which a pulse of electric charge 
proportional to the energy of the incident x-ray photon is detected. Uhuru 
enabled the key discovery of x-ray binary systems in which a collapsed object 
(neutron star or black hole) accretes gas from the atmosphere of a ``normal'' 
companion star. 
    The OSO satellites also carried cosmic x-ray detectors (proportional 
counters similar to Uhuru) and contributed much to the detailed understanding 
of individual sources. Qualitatively different cosmic x-ray satellite 
experiments, however, were launched in 1974 and 1975 with the Astronomical 
Netherlands Satellite (ANS) and SAS 3 satellite respectively. ANS was the 
first x-ray observatory: it was a pointed instrument carrying a variety of 
x-ray detectors that could be operated by an on-board computer in a variety of 
modes. The most significant discovery made with ANS was that of the x-ray 
burst sources. The SAS 3 satellite contained a variety of x-ray detectors. It 
obtained precise source positions which allowed many x-ray sources to be 
identified optically for the first time. 
    A major increase in sensitivity for x-ray astronomy was achieved with the 
High Energy Astronomical Observatory (HEAO) satellite. The total number of 
x-ray sources known was quadrupled to some 1500 as a result of HEAO 1, and 
high-quality continuum spectra were obtained for many sources. The increased 
sensitivity meant that fainter objects (primarily extragalactic) such as 
active galaxies and quasars could be well observed for the first time. The 
HEAO 2 satellite, or Einstein Observatory, launched in 1978, provided the 
first x-ray images of celestial objects and detected thousands of new sources. 
Perhaps foremost of its discoveries is the finding that even the most distant 
quasars known (and many previously unknown) are detected as strong x-ray 
sources and that collectively these may contribute most of the mysterious 
cosmic x-ray background. 
    Gamma rays, which are more energetic than x-rays, still do not penetrate 
the Earth's atmosphere. The SAS 2, launched in 1972, first established the 
existence of gamma-ray point sources (such as the Crab and Vela pulsars) at 
energies of about 100 MeV. The detector used was a digitized spark chamber in 
which gamma rays are detected by the secondary electron pair they produce upon 
interacting in the detector. A similar but much longer-lived (and slightly 
more sensitive) spark chamber gamma-ray telescope was launched in 1975 by the 
Europeans as the COS-B mission.  See  GAMMA-RAY ASTRONOMY. 
 �34s[Jonathan E. Grindlay]
�33s 
  
    Space Telescope.  The Space Telescope is a large telescope to be operated 
in orbit above the disturbing effects of the Earth's atmosphere. The telescope 
provides broad wavelength coverage and can be used to collect light with 
wavelengths from 115 nanometers in the ultraviolet, through the optical (or 
visible) portion of the spectrum, and on to wavelengths of 1 millimeter in the 
far infrared. With its 8-ft (2.4-m) primary mirror diameter, it will be the 
largest astronomical telescope ever placed above the atmosphere. The Space 
Telescope is a long-lived mission. After a shuttle launch, periodic visits by 
the shuttle will allow in-orbit replacement of failed or obsolete components. 
    The telescope optics are of the Ritchey-Chretien type. Light enters the 
telescope through the aperture door, strikes the 8-ft-diameter (2.4-m) primary 
mirror, and is reflected to the secondary mirror where it is reflected again, 
back through a hole, to a focus behind the primary mirror. At the focal plane, 
the light is shared among at most eight instruments, and only four may be 
operating simultaneously. The initial set of scientific instruments includes 
two cameras, two spectrographs, and a photometer, thus forming a powerful 
complement of instruments capable of carrying out most types of observations 
performed by ground-based observatories. 
    Above the atmosphere, the Space Telescope can observe at wavelengths where 
the atmosphere is opaque or only partially transparent, as in the ultraviolet 
and the infrared. But by far the most compelling reason to place a telescope 
above the atmosphere is the greatly improved angular resolution that can be 
achieved. Under good conditions, images with a diameter of about 1 arc-second 
may be formed with ground-based telescopes. At visible wavelengths, the Space 
Telescope will form images with a diameter of 0.07 arc-second. When the Space 
Telescope observes a faint star, the area of the image occupied by the star 
will be 200 times smaller and include 400 times less background light (because 
the sky is twice as dark in space) than if the star is observed by the 
same-sized ground-based telescope. This means that the Space Telescope can 
detect stars 20 times fainter than the faintest stars that can be detected by 
a similar ground-based telescope and 10 times fainter than can be detected 
with the Hale Telescope.  See  OPTICAL TELESCOPE; SCIENTIFIC SATELLITES; 
TELESCOPE.  
 �34s[Edward J. Groth]
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�6v�2v�32sSatellite navigation systems   �21m �33s 
    Electronic navigation systems employing artificial satellites as signal 
sources, signal relays, or position references. Earth satellites can provide 
the advantages of rectilinear (straight-line) propagation between distant 
points on the surface of the Earth. The satellites' position in space can be 
determined so accurately that they may be used as position references in 
systems for precision navigation.  See  ELECTRONIC NAVIGATION SYSTEMS. 
  
    NNSS.  There is only one satellite navigation system now in operation, the 
U.S. Navy's Navigation Satellite System (NNSS). Developed initially for 
ballistic missile submarines for use in updating their inertial navigation 
system, NNSS has been extended for use by ships. The Navy's system provides 
high-accuracy fixes several times a day at every point on the Earth. It is 
passive and uses low-orbit satellites and the Doppler technique.  See  DOPPLER 
EFFECT.  
    NNSS satellites are at an altitude of about 500 nautical miles (900 km), 
where they circle the world in 100 min. Their orbit planes are inclined 70DG 
from the Earth's equatorial plane, so that worldwide service is available. 
Accuracy of NNSS is approximately 0.1 nautical mile (0.2 km) when ionospheric 
corrections are applied. Accuracy is reduced for fast-moving vehicles such as 
aircraft unless their velocity is known accurately, because their own motion 
introduces Doppler shift on the received signals. 
  
    Global Positioning System.  The Global Positioning System (GPS) when fully 
implemented will be a universal positioning or navigation system that will 
provide for the first time three-dimensional position accuracies to 10 m (30 
ft), velocity to an accuracy of 0.03 m/s (0.1 ft/s), and time to atomic clock 
accuracy. This information will be available anywhere on Earth. A total of 18 
- 24 satellites, called NAVSTAR, will be placed in orbit in the fully 
implemented system. Master stations and satellite monitor stations control the 
system and transmit data to the satellites. It is anticipated that user 
equipment will be installed on airplanes, ships, and ground vehicles and will 
also be available as a portable navigation set. 
    The basis for the GPS accurate position (and time) determination is the 
precise measurement of the transit time of the radiated signals from 4 
satellites from the constellation. Since accurate time is essential, the 
satellites carry atomic clocks. To use the GPS and obtain the highest 
accuracy, great care must be taken to model and correct any errors in the 
received time delay such as clock drift and propagation delay. The satellites 
are continuously monitored, and error corrections are transmitted to the 
individual NAVSTAR satellites for inclusion in the satellite-generated signal 
so that the receiver can automatically correct for position and time. 
    The GPS is designed to provide precision velocity measurements to the user 
by measuring the Doppler shift in the satellite-transmitted carrier frequency. 
The accuracy of this measure is primarily influenced by the dynamics of the 
host vehicle of the receiver. 
    Satellites will be in 12 h orbits. The orbital inclination is 63DG, and 
the satellites will be divided into three groups of eight, each group in a 
separate orbital plane, staggered by 120DG. This arrangement permits 
continuous four-satellite coverage of the entire Earth. New satellites will be 
added as older satellites wear out after 5 - 7 years of service life. 
 �34s[Louis Feit]
�33s 
  
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�6v�2v�32sSatellite (astronomy)   �21m �33s 
    A relatively small celestial body moving in orbit about a planet. The Moon 
is the only natural satellite of Earth. In all, the solar system contains 47 
known satellites distributed among the nine planets as follows: Jupiter, 15; 
Saturn, 21; Uranus, 5; Mars, 2; Neptune, 2; Earth, 1; and (probably) Pluto, 1. 
Venus and Mercury have no satellites. 
    The largest satellites reach planetary dimensions, with four (Ganymede and 
Callisto of Jupiter, Titan of Saturn, and possibly Triton of Neptune) having 
diameters greater than 3000 mi (5000 km), that is, exceeding that of the 
planet Mercury. The smallest known satellites, found around Jupiter, Saturn, 
and Mars, are less than 30 mi (50 km) in diameter, and some are as small as 6 
mi (10 km). 
    The surface gravity of most satellites is too feeble to retain an 
atomsphere having a density comparable to Earth's. An exception is Titan, with 
an atmosphere composed primarily of hydrogen and methane and having a density 
about equal to Earth's.  See  JUPITER; MARS; MOON; NEPTUNE; PLANET; PLUTO; 
SATURN; URANUS. 
 �34s[J. Kelly Beatty]
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�6v�2v�32sSatellite (spacecraft)   �21m �33s 
    A spacecraft that is in orbit about a planet (usually the Earth). 
Spacecraft are devices intended for observation, research, or communications 
in space. Even those spacecraft which are destined to probe the outer reaches 
of the solar system usually complete at least a fractional revolution around 
the Earth before being placed into the transfer orbit. Devices such as 
military missiles which usually follow ballistic paths between the launching 
site and the target do not orbit the Earth and thus are not referred to as 
satellites.  
    The path of rotation of a satellite might be either a circle or an 
ellipse, but the center of the Earth is at its center or focal point. Unless 
the satellite is disturbed by the gravitational pull of another body, it 
remains in a flat plane known as the orbit plane. The Earth revolves through 
this plane with a period of approximately 24 h. The orbit plane also rotates 
very slowly.  See  ORBITAL MOTION. 
    Satellites in low Earth orbits are located several hundred miles above the 
surface of the Earth. They have typical revolution times (also called periods) 
of about 90 min. These orbits have fairly short lifetimes (of the order of 
days, weeks, or sometimes months). Fairly large masses (weights) can be placed 
into these orbits by simple launch vehicles. Satellites in these orbits are 
primarily used for observation and military  functions. Additional uses for 
these orbits include aeronomy, as well as the study of radiation belts and 
other natural phenomena.  See  MILITARY SATELLITES; SCIENTIFIC SATELLITES. 
    With the proper choice of orbit parameters, the Sunsynchronous (or 
heliosynchronous) orbit plane requires 1 entire Earth year to make a full 
revolution. During this time the Earth makes a full revolution about the Sun. 
As observed from a position on the Sun, the satellite orbit plane remains in 
the same apparent orientation throughout the year. Since the shadows on the 
Earth and the illumination of clouds remain more or less constant throughout 
the year (varying with the normal seasonal changes), this is very valuable in 
terms of making weather forecasts or observations.  See  METEOROLOGICAL 
SATELLITES.  
    Earth-synchronous or geosynchronous orbits have a period that exactly 
matches the Earth's rotation. However, they may have an inclination, in which 
case the orbit plane does not lie in the Earth's equatorial plane. While the 
satellite appears to hover at a fixed longitude over the Earth, it moves up 
and down along a latitude line in a narrow figure-eight motion. 
    A geostationary satellite orbit is a special type of geosynchronous orbit 
in which the inclination is 0DG. Since the satellite plane lies in the 
equatorial plane of the Earth, a perfectly positioned satellite appears to 
hover directly over the Equator at one particular longitude. An observer 
positioned on a fardistant planet and looking back at the Earth would see the 
geostationary satellite rotating with the Earth through space. The 
geosynchronous and geostationary satellites are typically 22,238 mi (35,788 
km) above the surface of the planet or approximately 26,000 mi (42,000 km) 
from its center. They are used primarily for communications and observation of 
weather systems.  See  COMMUNICATIONS SATELLITE. 
    In addition to the research, communications, observation, meteorological 
(weather), and military satellites, there are radio navigation and Earth 
survey satellites. The radio navigation satellites provide position 
information to ships, airplanes, land vehicles, and even backpack receivers. 
Earth resources satellites provide observation information on potential 
mineral and petroleum reserves on the Earth. They also help to monitor land 
usage and pollution.  See  APPLICATIONS SATELLITES; SATELLITE NAVIGATION 
SYSTEMS.  
 �34s[Walter L. Morgan]
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�6v�2v�32sSaturable reactor   �21m �33s 
    An iron-core inductor in which the effective inductance is changed by 
varying the permeability of the core. Saturable-core reactors are used to 
control large alternating currents where rheostats are impractical. Theater 
light dimmers often employ saturable reactors. 
    In the illustration of two types of saturable-core reactors, illustration 
a shows two separate cores, while in illustration b a three-legged core is 
formed by placing two two-legged cores together. The load winding, connected 
in series with the load, carries the alternating current and acts as an 
inductive element. The control winding carries a direct current of adjustable 
magnitude, which can saturate the magnetic core. 
    Reducing the magnitude of the control current reduces the intensity of 
saturation. This increases the reactance of the load winding. As the reactance 
increases, the voltage drop in the load winding increases and causes a 
reduction in the magnitude of the voltage applied to the load.  See  
INDUCTANCE; MAGNETIC PERMEABILITY; MAGNETIZATION. 
 �34s[Wilson S. Pritchett]
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�6v�2v�32sSaturation   �21m �33s 
    The condition in which, after a sufficient increase in a causal force, 
further increase in the force produces no additional increase in the resultant 
effect. Many natural phenomena display saturation. For example, after a 
magnetizing force becomes sufficiently strong, further increase in the force 
produces no additional magnetization in a magnetic circuit; all the magnetic 
domains have been aligned, and the magnetic material is saturated.  See  
MAGNETIC MATERIALS. 
    After a sponge has absorbed all the liquid it can hold, it is saturated. 
In thermionic vacuum tubes thermal saturation is reached when further increase 
in cathode temperature produces no (or negligible) increase in cathode 
current; anode saturation is reached when further increase in plate voltage 
produces substantially no increase in anode current.  See  DISTORTION 
(ELECTRONIC CIRCUITS): SATURATION CURRENT; VACUUM TUBE. 
    In colorimetry the purer a color is, the higher its saturation. Radiation 
from a color of low saturation contains frequencies throughout much of the 
visible spectrum.  See  COLORIMETRY. 
 �34s[Frank H. Rockett]
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�6v�2v�32sSaturation current   �21m �33s 
    A term having a variety of specific applications but generally meaning the 
maximum current which can be obtained under certain conditions. 
    In a simple two-element vacuum tube, it refers to either the 
space-charge-limited current on one hand or the temperature-limited current on 
the other. In the first case, further increase in filament temperature 
produces no significant increase in anode current, whereas in the latter a 
further increase in voltage produces only a relatively small increase in 
current.  See  VACUUM TUBE. 
    In a gaseous-discharge device, the saturation current is the maximum 
current which can be obtained for a given mode of discharge. Attempts to 
increase the current result in a different type of discharge.  See  ELECTRICAL 
CONDUCTION IN GASES. 
    A third case is that of a semiconductor. Here again the saturation current 
is that maximum current which just precedes a change in conduction mode.  See  
SEMICONDUCTOR.  
 �34s[Glenn H. Miller]
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�6v�2v�32sSaturn �21m �33s
    The second-largest planet in the solar system and the sixth in order of
distance to the Sun. The outermost planet known prior to 1781, Saturn is
surrounded by a beautiful system of rings. Saturn is also the only planet that
has a satellite (Titan) with a dense atmosphere. This distant planetary system
has been visited by three NASA spacecraft: a preliminary survey by Pioneer 11
in September 1979, and a more sophisticated reconnaissance by Voyager 1 in
November 1980 and Voyager 2 in August 1981.
    Saturn makes one revolution about the Sun in 29.46 years. The equatorial
diameter of Saturn is about 75,000 mi (120,800 km), and the polar diameter
about 67,700 mi (109,000 km). The volume is 769 (Earth = 1) with a few percent
uncertainty. The mass is about 95.2 (Earth = 1) or 1/3500 (Sun = 1). The mean
density is 0.68 g/cm(sup 3), the lowest mean density of all the planets. The
rotation axis of both the planet and the rings is inclined 26 DG 45' to the
perpendicular to the orbital plane. The disk of Saturn is much more
homogeneous than that of Jupiter. There is no feature comparable to the Great
Red Spot, and the contrast of the features that are visible is very low (Fig.
1).

    The optical spectrum of Saturn is characterized by strong absorption bands
of methane (CH4) and by much weaker bands of ammonia (NH3). Absorption lines
of molecular hydrogen (H2) have also been detected.
    The temperature the planet should assume in response to solar heating is
calculated to be about -323DGF (76 K), somewhat lower than the measured value
of -294DGF (92 K). This suggests that Saturn has an internal heat source of
roughly the same magnitude as that on Jupiter. As in the case of Jupiter, a
thermal inversion exists in the upper atmosphere. The inversion region is well
above the main cloud layer, which is thought to consist primarily of frozen
ammonia crystals, with an admixture of some other substances to provide the
yellowish color sometimes observed in the equatorial zone.
    The theoretical models for the internal structure of Saturn are similar to
those for Jupiter, that is, a dense core surrounded by hydrogen compressed to
a metallic state which gradually merges into an extremely deep atmosphere. The
fact that the two planets radiate comparable amounts of energy despite their
difference in size means that smaller Saturn must have some additional energy
source besides gravitational contraction. The existence of a magnetic field
and belts of trapped electrons has been deduced from observations of
nonthermal radiation and mapped out in detail by the Pioneer and Voyager
spacecraft.
    Jupiter and Saturn are relatively similar bodies. Both seem to have
compositions virtually identical with that of the Sun and the other stars -
rich in hydrogen and helium. In that sense, they may represent the primitive
material from which the entire solar system was formed, whereas the other
planets have undergone fractionation processes resulting in the loss of most
of the light gases.  See  PLANETARY PHYSICS.
    The most remarkable feature associated with Saturn is the complex ring
system that surrounds the planet (Fig. 2). The system is divided into six main
regions, designated A through F. Both theory and observations prove that the
ring system is made up of myriad separate particles that move independently in
flat, mostly circular orbits in Saturn's equatorial plane. Periodic
perturbations by the major satellites are responsible, in part, for the main
divisions of Saturn's rings.

    Saturn has 21 known satellites. The largest and brightest, Titan, was
discovered by C. Huygens in 1655 and is visible with small telescopes; the
other satellites are much fainter. Titan's mean apparent diameter corresponds
to a linear diameter of approximately 3600 mi (5800 km). But this diameter
refers to the satellites's atmosphere, which is filled with a dense aerosol
produced photochemically by incident sunlight. The solid surface of Titan has
a diameter of 3182 mi (5120 km), making this satellite larger than Mercury but
smaller than Jupiter's giant Ganymede. This object contains a large fraction
of icy material and is thus quite different from the Moon or the inner planets
in composition. Furthermore, it is large and cold enough to retain an
atmosphere of gases with relatively high molecular weights. The main
constituent of this atmosphere is molecular nitrogen. The surface of Titan is
so cold (-292DGF or 93 K) that methane can liquefy. Hence, this substance may
play the same role on Titan as water does on Earth. See PLANET; SATELLITE
(ASTRONOMY).
 �34s[Tobias C. Owen]
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�6v�2v�32sSaurischia   �21m �33s 
    One of two orders of extinct reptiles popularly known as dinosaurs, the 
other being the Ornithischia. The order is distinguished by the triradiate 
shape of the pelvis. The order originated from thecodont reptiles by Middle 
Triassic times and is classified in the subclass Archosauria. Saurischian 
bones have been found on all continents except Antarctica, and in rock strata 
ranging from Middle Triassic to latest Cretaceous in age. See REPTILIA. 
    Most recent classifications recognize two suborders, the Sauropodomorpha 
and the Theropoda. The latter includes all carnivorous dinosaurs, and the 
former consits of the Brontosaurus-like animals (sauropods), together with 
their ancestral stock, the prosauropods. The Prosauropoda includes the oldest 
known saurischians, ranging from Middle Triassic time to the close of the 
Triassic Period. Like their ancestral thecodont stock, many show pronounced 
anatomical tendencies toward bipedal posture. Most, like Plateosaurus, appear 
to have been herbivores, but fragmentary evidence indicates that some may have 
been carnivorous. In general, prosauropods were of moderate size, up to 15 - 
20 ft (4.5 - 6 m) long, and less than a ton (1 short ton = 0.9 metric ton) in 
weight.  
    The saurischians became extinct at the end of the Cretaceous, due to 
unknown causes. Most saurischians died out, leaving no descendants, but 
studies indicate that birds, via Archaeopteryx, probably are direct 
descendents of a small theropod dinosaur.  See  ARCHAEORNITHES; ARCHOSAURIA; 
DINOSAUR; ORNITHISCHIA; THECODONTIA. 
 �34s[John H. Ostrom]
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�6v�2v�32sSauropterygia   �21m �33s 
    An order of Mesozoic reptiles (subclass Euryapsida) that are, without 
exception, adapted to the marine environment. The order includes the closely 
related nothosaurs and plesiosaurs. These reptiles, along with the 
ichthyosaurs, played a significant role as predators within the marine animal 
community of the Mesozoic Era.  See  ICHTHYOSAURIA; REPTILIA. 
    The Nothosauria are the relatively generalized stem group from which the 
plesiosaurs evolved. With the exception of a single New World species and a 
record from Japan, nothosaurs are known primarily from Europe and the Near 
East (Israel) in rocks of Triassic age. The nothosaurs are notably diverse in 
the mode and degree of secondary aquatic modification. The directions of 
aquatic specialization involve shortening, or more often lengthening, of the 
neck; enlargement of the orbits or the temporal fenestrae; and reduction, or 
more commonly increase, in the number of phalanges in manus (hand), pes 
(foot), or both. A feature of considerable evolutionary significance in the 
light of plesiosaurian differentiation is the great individual variability in 
the number of presacral vertebrae (32 - 42) in Pachypleurosaurus edwardsi. See 
SKELETAL SYSTEM. 
    The Plesiosauria are the successful, compact, and highly specialized 
offshoot of the nothosaurs that attained worldwide distribution. Two major 
trends of plesiosaur evolution may be discerned since the Early Jurassic: In 
the one group there was a tendency toward a shortening of the neck from 27 to 
13 vertebrae and an increase in skull size: in the other group the opposite 
trend led to forms of bizarre body proportions, for example, Elasmosaurus, 
with a neck containing 76 vertebrae. The plesiosaurs were carnivorous.  See  
EURYAPSIDA; REPTILIA. 
 �34s[Rainer Zangerl]
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�6v�2v�32sSavanna   �21m �33s 
    The term savanna was originally used to describe a tropical grassland with 
more or less scattered dense tree areas. This vegetation type is very abundant 
in tropical and subtropical areas, primarily because of climatic factors. The 
modern definition of savanna includes a variety of physiognomically or 
environmentally similar vegetation types in tropical and extratropical 
regions. The physiognomically savannalike extratropical vegetation types 
(forest tundra, forest steppe, and everglades) differ greatly in environment 
and species composition. 
    In the widest sense savanna includes a range of vegetation zones from 
tropical savannas with vegetation types such as the savanna woodlands to 
tropical grassland and thornbush. In the extratropical regions it includes the 
``temperate'' and ``cold savanna'' vegetation types known under such names as 
taiga, forest tundra, or glades.  See  GRASSLAND; TAIGA; TUNDRA. 
 �34s[Helmut Lieth]
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�6v�2v�32sSawing   �21m �33s 
    The parting of material by using metal disks, blades, bands, or abrasive 
disks as the cutting tools. Sawing a piece from stock for further machining is 
called cutoff sawing, while shaping or forming a piece is referred to as 
contour sawing. 
    Machine sawing of metal is performed by five types of saws or processes: 
hacksawing, band sawing, cold sawing, friction sawing, and abrasive sawing. 
Hacksaws are used principally as cutoff tools. The toothed blade, held in 
tension, is reciprocated across the workpiece. Band saws cut rapidly and are 
suited for either cutoff or contour sawing. They are basically a flexible 
endless band of steel running over pulleys or wheels. Cold sawing is 
principally a cutoff operation. The blade is a circular disk with cutting 
teeth on its periphery. Friction sawing is a rapid process used to cut steel 
as well as certain plastics. Cutting is done as the high-speed blade wipes the 
metal from the kerf after softening it with frictional heat. Abrasive sawing 
is a cutoff process using thin rubber or bakelite bonded abrasive disks. In 
addition to steel, other materials such as nonferrous metals, ceramics, glass, 
certain plastics, and hard rubber are cut by this method.  See  MACHINING 
OPERATIONS; WOODWORKING. 
 �34s[Alan H. Tuttle]
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�6v�2v�32sSawtooth-wave generator   �21m �33s 
    A device which generates a current or voltage waveform whose magnitude is 
a continuously increasing function of time for a fixed interval, and then 
repeats the sequence periodically. The most widely used sawtooth waveform is 
ideally a linear function of time during the forward or rising interval and 
appears as shown in Fig. 1a, with the total period T made up of the active 
forward interval Tsubf, a retrace interval Tsubr, and an inactive interval 
Tsubi. Often it is desirable to make the two intervals Tsubr and Tsubi as 
small as possible, with Tsubi often being zero. 
    Electronic circuits can generate only an approximation of the idealized 
waveform which, as a result, tends to have deviations which are often of the 
type shown in Fig. 1b. 
    Sawtooth waveforms are used as time-base elements or sweep generators and 
in time-delay and time-measuring equipment.  See  SWEEP GENERATOR; TIME-DELAY 
CIRCUITS.  
    The complexity of electronic circuitry required to generate linear 
sawtooth voltage waves depends upon the accuracy to which such generation is 
specified. An approximate linear sawtooth may be generated by a dc voltage 
source, a series RC circuit, and a switch which has a small but nonzero 
resistance when closed. This elementary sawtooth generator is shown 
schematically in Fig. 2. 
    A periodic sawtooth waveform can be generated by using an astable 
relaxation oscillator as a switch in the above circuit. When used as switches 
in sawtooth generators, relaxation oscillators may be synchronized with 
external pulses to maintain an accurately controlled period.  See  RELAXATION 
OSCILLATOR.  
    If the switch of Fig. 2 is the switch in a synchronous or keyed clamp each 
interval of the total period of the waveform may be controlled directly from 
an external source of pulses.  See  CLAMPING CIRCUIT. 
    Either a positive-going or a negative-going sawtooth may be generated by 
making VB    either positive or negative and replacing the clamp with a 
bidirectional clamp. If the potential VsubB is variable at a rate which is 
slow compared to the periodicity of the waveform, a succession of sawtooth 
waveforms of varying amplitude will be generated. One particular application 
of such a circuit would be to generate one of the components of the rotating 
radial sweep if VsubB were to be made to vary sinusoidally with the desired 
angular modulation. 
 �34s[Glenn M. Glasford]
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�6v�2v�32sScabies   �21m �33s 
    A severe skin condition caused by sarcoptic mites. Sarcoptes scabiei 
infects a wide range of mammals, including humans, horses, hogs, sheep, dogs, 
cattle, foxes, and goats. Infection may begin with a few mites and may rapidly 
increase to a heavy infection within a few weeks. As the mite population 
becomes large, a hypersensitive condition develops, called dermatitis, which 
results in intense itching. 
 �34s[Richard Sudds]
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�6v�2v�32sScalar   �21m �33s 
    A term synonymous in mathematics with ``real'' in real number or real 
function. The magnitude of a vector two units in length is the real number of 
scalar 2. The dot or scalar product of two vectors is the product of three 
real numbers associated with them (the magnitude of the first vector times the 
magnitude of the second, times the cosine of the angle between them) and is 
therefore a scalar. If in the functional relationships S DOUBLE BAR S(s,y,z), 
F = F(x,y,z), S is a real number and F is a vector, then S(s,y,z) is a scalar 
function but F(x,y,z) is a vector function.  See  CALCULUS OF VECTORS. 
 �34s[Homer V. Craig]
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�6v�2v�32sScale (music)   �21m �33s 
    A series of notes arranged from low to high by a specified scheme of 
intervals, suitable for musical purposes. A musical scale is an arrangement of 
intervals evolved in the making of music. When a given note of a musical scale 
is repeated, it need not be given exactly the same frequency; the frequency 
may be modified significantly in accordance with the artistic requirements of 
the musical context. Thus it is difficult to be very specific about the 
``specified scheme of intervals.'' 
    The number of musical scales and the intervals they contain are myriad, 
but most of them have in common the interval called the octave. The musical 
import of two notes an octave apart is so often the same that the two notes 
are given the same letter name.  See  OCTAVE. 
    A diatonic scale is one in which the octave is divided into intervals of 
two different sizes, five of one and two of the other. If the size of the 
smaller interval (often called a half-step) is taken as one unit, the sequence 
of intervals in the major diatonic scale is 2, 2, 1, 2, 2, 2, 1; in the minor 
diatonic scale (melodic ascending), the sequence is 2, 1, 2, 2, 2, 2, 1. The 
relative sizes shown by these numbers are approximate; only in equal 
temperament are the sizes exactly as indicated here. 
 �34s[Robert W. Young]
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�6v�2v�32sScale (zoology)   �21m �33s 
    The fundamental unit of the primary scaled integument of vertebrates, of 
which the epidermal or dermal component may be the more conspicuous or 
elaborated. Although there is great diversity in detailed structure among 
vertebrate groups, scaled integuments may be interpreted as the most effective 
way of producing a physically strong external body surface, with maximum 
flexibility necessitated by the fundamental pattern of vertebrate locomotion 
relying on lateral sinusoidal movements of the body. 
    In the majority of extant fish the epidermis is extremely thin. The term 
``scale'' when used in an ichthyological context usually refers to the 
relatively large, prominent, dermal ossification, on the outer surface of 
which lie so-called dental tissues. The latter - enamel or enamellike 
mineralizations and dentine - are therefore present at the dermoepidermal 
boundary. The scaled integument of fish shows a definite pattern which is 
governed by the orientation of the underlying myomeres and by changes in body 
depth and length. The general evolutionary tendency toward thinning, or size 
reduction, of integumentary sclerifications seen in various fish lineages is 
probably associated with weight reduction and increased locomotor efficiency. 
    Scaled integuments are absent from modern amphibians, although 
nonoverlapping dermal ossifications are seen in some apodans. It seems 
probable that the absence of scales in other modern species is associated with 
the secondary utilization of the integument as a respiratory surface. In most 
reptilian lineages, there is a ubiquity of scaled integuments, with or without 
dermal ossifications, but always with elaboration of epidermal tissues. The 
distribution of keratinaceous protein types varies in different groups of 
reptiles, and in this respect modern crocodilian scales exactly resemble the 
leg scales of birds. Scaled integuments prompted the subclass name for 
lepidosaurs - the tuatara, lizards, and snakes, the last two constituting the 
order Squamata. Lepidosaurian scales may or may not possess dermal 
ossifications. Among extant reptiles the extraordinary development of the 
dermal skeleton in turtles forms the characteristic carapace. 
 �34s[Paul F. Maderson]
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�6v�2v�32sScandium �21m �33s
    A chemical element, Sc, atomic number 21, and atomic weight 44.956. It is
the first transition element of the first long period. Some physical
properties of scandium are listed in the table.
    Isotopes of scandium include (sup 40)Sc and (sup 51)Sc and one
corresponding to every intermediate value. Except for (sup 45)Sc, which occurs
naturally, the isotopes occur only when generated by nuclear reactions.
    The oxide and other compounds of scandium are used as catalysts in the
conversion of acetic acid to acetone, in making propanol, and in converting
dicarboxylic acids to ketones and cyclic compounds. Treatment with scandium
sulfate solution is an economical means of improving the germination of the
seeds of many species of plants.
    Scandium-47 has a satisfactory half-life for use as a tracer, and it can
be prepared carrier-free. The presence of 2.5 - 25 atom % of scandium in the
anode increases the voltage, the voltage stability, and the service life of a
nickel alkaline storage battery.
    The chief scandium ore mineral is thortveitite, which is found in granite
pegmatities, and in some ores of tin, of tungsten, and of the rare earths.
Scandium is widely distributed in many parts of the world.  See  RARE-EARTH
ELEMENTS; TRANSITION ELEMENTS.
 �34s[W. D. Wilkinson]
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�6v�2v�32sScanning acoustic microscope   �21m �33s 
    A type of microscope based on a sapphire pellet with a thin-film 
piezoelectric transducer on the back side. This serves to convert 
electromagnetic energy into acoustic energy in the form of a collimated beam 
which propagates through the pellet to the front surface. There a spherical 
cavity filled with the coupling fluid forms an acoustic lens. This lens is 
ideal - it does not suffer from aberrations - and it brings the acoustic beam 
into a diffraction-limited focus near the center of curvature of the spherical 
surface. The wave velocity decreases by a factor of 7.5 in passing through 
this solid-liquid interface. The strong refraction at the lens surface reduces 
the spherical aberration below the diffraction limit. The object under 
examination is placed near the focus. It is then scanned by mechanical 
techniques, point by point and line by line, in a raster pattern familiar in 
television display. The cathode-ray tube used to monitor the image has a scan 
synchronized with the mechanical motion of the object. Thus each point on the 
monitor screen corresponds to a given point on the object. An electronic 
signal proportional to the acoustic power reflected from or transmitted 
through the object is used to modulate the intensity of the electron beam in 
the monitor. The image on screen which reproduces the ``image of acoustic 
reflectivity'' is recorded photographically. See ACOUSTIC MICROSCOPE. 
 �34s[C. F. Quate]
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�6v�2v�32sScanning electron microscope   �21m �33s 
    An electron microscope that builds up its image as a time sequence of 
points in a manner similar to that employed in television. 
    The imaging method of the scanning electron microscope (SEM) allows 
separation of the two functions of a microscope, localization and information 
transfer. The SEM utilizes a very fine probing beam of electrons which sweeps 
over the specimen to emit a variety of radiations. The signal, which is 
proportional to the amount of radiation leaving an individual point of the 
specimen at any instant, can be used to modulate the brightness of the beam of 
the display cathode-ray tube as it rests on the corresponding point of the 
image. In practice, the points follow one another with great rapidity so that 
the image of each point becomes an image of a line, and the line in turn can 
move down the screen so rapidly that the human eye sees a complete image as in 
television. The image can also be recorded in its entirety by allowing the 
point-by-point information to build up in sequence on a photographic film (see 
illustration).  
    As with all microscopy research, it is very often the preparative 
methodology that determines the success or failure of the research. A specific 
requirement of scanning electron microscope preparation is that the material 
be dried. This preparative step must be done very carefully to minimize 
surface tension effects. The two methods most often used - critical-point 
drying and freeze drying - have yielded excellent results even for very 
fragile biological tissue. Alternatively, the drying step can be avoided 
completely by observing the specimen while it is still frozen. Interior 
structure can be revealed by using techniques in which the specimen is broken 
at a low temperature.  See  ELECTRON MICROSCOPE; MICROTECHNIQUE. 
 �34s[Thomas L. Hayes]
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�6v�2v�32sScaphopoda   �21m �33s 
    A minor class in the phylum Mollusca, comprising two families, Dentaliidae 
and Siphonodentaliidae, each with few and little-known genera. It is 
exclusively marine and widely distributed at moderate depths in temperate and 
tropical seas. 
    Dentalium, the ``elephant's tusk shell,'' is the best-known example. The 
tubular cylindrical shell is slightly concave on its dorsal side; it tapers 
gently posteriorward. The animal lives embedded almost vertically in sand, 
shell gravel, or gravel, the posterior tip of the shell protruding slightly. 
    Dentalium burrows through the substratum with its muscular, protrusible 
foot, the firm pointed tip easily penetrating the deposits. The extended foot 
becomes anchored in the bottom by anterolateral folds which are inflated by 
blood pressure. Contraction of the foot then draws the animal forward, and 
these actions are associated with pulses of water expelled through the 
posterior mantle orifice. There is a spacious hemocoel, but no obvious heart 
and no pericardium. The sexes are separate.  See  MOLLUSCA. 
 �34s[Richard D. Purchon]
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�6v�2v�32sScapolite   �21m �33s 
    An aluminosilicate mineral. It is commonly found as light-colored, 
translucent tetragonal prisms. Scapolite is normally white, but many other 
colors are known, including some used as semiprecious gems resembling amethyst 
and citrine. The mineral has a Mohs hardness of 5 - 6. The formula of 
scapolite is (Na,Ca)4(Al,Si)6SI6O24(Cl,CO3,SO4). In nature significant amounts 
of K and SO4 substitute for Na and CO3. 
    Scapolite is a common mineral in metamorphic rocks, particularly in those 
which contain calcite. It is found in marbles, gneisses, skarns, and schists. 
Scapolite probably forms about 0.1% of the Earth's upper crust. It is commonly 
found as inclusions in igneous rocks derived from deep within the Earth's 
crust, and probably makes up several percent of the lower crust.  See  
SILICATE MINERALS. 
 �34s[David E. Ellis]
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�6v�2v�32sScarlet fever   �21m �33s 
    An acute contagious disease which is the classic example of infection by 
the microorganism Streptococcus hemolyticus. Fever, sore throat, rash, 
headache, and vomiting follow 2 - 7 days after contact with a carrier, whose 
illness was probably not scarlet fever. The bright-red rash over the body is 
papular, or rough, with sandpaperlike quality. It blanches on pressure to 
leave a transient image, as in sunburn. A deeply flushed face with circumoral 
pallor, or paleness around the mouth, completes the patient's outward 
appearance. The tongue is aptly described as ``strawberry,'' sometimes 
``raspberry.'' There is a firey redness of the throat, and the swollen tonsils 
are covered with patches of white exudate. The rash fades in a few days to be 
followed by desquamation, or peeling, while symptoms abate. 
    Early complications such as visibly swollen neck glands and draining ears 
mean bacterial spread from the nose and throat. Late complications, such as 
deafness, rheumatic fever, and nephritis, or kidney disease, appear during the 
convalescence.  See  RHEUMATIC FEVER. 
 �34s[Paul L. Boisvert]
�33s 
  
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�6v�2v�32sScattering experiments (atoms and molecules)    �21m �33s 
        Experiments in which an incident particle or system of particles, such 
as an electron, atom, or molecule, is deflected by collision with an atom or a 
molecule. Such experiments are useful for many reasons: They provide checks on 
the theory of scattering and yield information on the nature of atomic and 
molecular forces. They are also important since the experiments can be 
designed to simulate conditions in planetary atmospheres, to provide 
information on electric discharges, to assist in the development of gaseous 
lasers, and to aid in the theory of stellar absorption and of planetary 
nebulae.  See  ELECTRICAL CONDUCTION IN GASES; LASER; PLANETARY NEBULA; STAR. 
 �34s[Hans W. Meissner]
�33s 
  
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�6v�2v�32sScattering experiments (nuclei)   �21m �33s 
    Experiments in which beams of particles such as electrons, nucleons, alpha 
particles and other atomic nuclei, and mesons are deflected by elastic 
collisions with atomic nuclei. Much is learned from such experiments about the 
nature of the scattered particle, the scattering center, and the forces acting 
between them. Scattering experiments, made possible by the construction of 
high-energy particle accelerators and the development of specialized 
techniques for detecting the scattered particles, are one of the main sources 
of information regarding the structure of matter.  See  NUCLEAR STRUCTURE; 
PARTICLE ACCELERATOR; PARTICLE DETECTOR; SCATTERING MATRIX. 
 �34s[K. A. Erb]
�33s 
  
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�6v�2v�32sScattering layer   �21m �33s 
    A layer of organisms in the sea which causes sound to scatter and returns 
echoes. Recordings by sonic devices of echoes from sound scatterers indicate 
that the scattering organisms are arranged in approximately horizontal layers 
in the water, usually well above the bottom. The layers are found in both 
shallow and deep water. 
 �34s[Jacques B. Hadler]
�33s 
  
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�6v�2v�32sScattering matrix   �21m �33s 
    A matrix which expresses the initial state in a scattering experiment in 
terms of the possible final states, and hence enters the calculation of the 
probabilities that certain reactions will occur in a collision of two or more 
particles. The problem is compounded in the relativistic domain, in which 
particles may be created or destroyed in a collision and the forces between 
elementary particles are known only approximately. It was therefore suggested 
by W. Heisenberg in 1943 that the S matrix should play a fundamental rather 
than a subsidiary role in relativistic quantum mechanics.  See  ELEMENTARY 
PARTICLE; MATRIX MECHANICS; NUCLEAR REACTION; QUANTUM MECHANICS; SCATTERING 
EXPERIMENTS (NUCLEI). 
    A Lorentz covariant perturbation theory for the direction calculation of S 
on the basis of quantum field theory was developed in 1948 - 1949 by J. 
Schwinger, R. P. Feynman, and F. J. Dyson. The covariant perturbation 
techniques have been remarkably successful in quantum electrodynamics; the 
fine structure constant is small, and provides a natural expansion parameter 
for the perturbation series. In contrast, the strength of the specifically 
nuclear forces between elementary particles prevents a meaningful perturbation 
expansion of S for particle reactions. Much emphasis has consequently been 
given to Heisenberg's original conjecture that Lorentz invariance and 
unitarity could be used in part to determine S. It has in fact been possible 
to develop at least a partial dynamical theory of the S matrix by 
supplementing the requirements of Lorentz invariance and unitarity by 
analyticity conditions derived from perturbation theory. Although this 
dispersion-relation, or S-matrix, theory of strong interactions has been quite 
successful in some instances, particularly in the study of low-energy 
baryon-meson scattering, the theoretical basis of the approach remains 
obscure.  See  DISPERSION RELATIONS; PERTURBATION (QUANTUM MECHANICS); QUANTUM 
ELECTRODYNAMICS; QUANTUM FIELD THEORY. 
 �34s[Loyal Durand, III]
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�6v�2v�32sScattering of electromagnetic radiation �21m �33s
    The process in which energy is removed from a beam of electromagnetic
radiation and reemitted with a change in direction, phase, or wavelength. All
electromagnetic radiation is subject to scattering by the medium (gas, liquid,
or solid) through which it passes.
    It has been known since the work of J. Maxwell in the nineteenth century
that accelerating electric charges radiate energy and, conversely, that
electromagnetic radiation consists of fields which accelerate charged
particles. Light in the visible, infrared, or ultraviolet region interacts
primarily with the electrons in gases, liquids, and solids - not the nuclei.
The scattering process in these wavelength regions consists of acceleration of
the electrons by the incident beam, followed by reradiation from the
accelerating charges.  See  ELECTROMAGNETIC RADIATION.
    Scattering processes may be divided according to the time between the
absorption of energy from the incident beam and the subsequent reradiation.
True ``scattering'' refers only to those processes which are essentially
instantaneous. Mechanisms in which there is a measurable delay between
absorption and reemission are usually termed luminescence.  See  LUMINESCENCE.
    Instantaneous scattering processes may be further categorized according to
the wavelength shifts involved. Some scattering is ``elastic''; there is no
wavelength change, only a phase shift. In 1928 C. V. Raman discovered the
process in which light was inelastically scattered and its energy was shifted
by an amount equal to the vibrational energy of a molecule or crystal.
    In liquids or gases two distinct processes generate inelastic scattering
with small wavelength shifts. The first is Brillouin scattering from pressure
waves. When a sound wave propagates through a medium, it produces alternate
regions of high compression (high density) and low compression (or
rarefaction). Brillouin scattering of light to higher (or lower) frequencies
occurs because the medium is moving toward (or away from) the light source.
This is an optical Doppler effect.  See  DOPPLER EFFECT.
    The second kind of inelastic scattering studied in fluids is due to
entropy and temperature fluctuations, and is known as Rayleigh scattering.
These entropy fluctuations produce a broadening in the scattered radiation
centered about the exciting wavelength, rather than sharp, well-defined
wavelength shifts. Under the assumption that the scattering in fluids is from
particles much smaller than the wavelength of the exciting light, Lord
Rayleigh derived in 1871 an equation, shown below, for such scattering. The

r(sup 2)I(THETA)/I(sub o) =PI d GAMMA(sup -4) v(sup 2)
			   [1 + cos(sup 2)THETA](n - 1)(sup 2)

dependence of scattering intensity upon the inverse fourth power of the
wavelength given in Rayleigh's equation is responsible for the fact that
daytime sky looks blue and sunsets red: blue light is scattered out of the
sunlight by the air molecules more strongly than red; at sunset, more red
light passes directly to the eyes without being scattered.  See  ENTROPY.
    Rayleigh's derivation of his scattering equation relies on the assumption
of small, independent particles. Under some circumstances of interest, both of
these assumptions fail. Colloidal suspensions provide systems in which the
scattering particles are comparable to or larger than the exciting
wavelengths. Such scattering is called the Tyndall effect and results in a
nearly wavelength-independent (that is, white) scattering spectrum. The
Tyndall effect is the reason clouds are white (the water droplets become
larger than the wavelengths of visible light).	See  TYNDALL EFFECT.
    The breakdown of Rayleigh's second assumption - that of independent
particles - occurs in all liquids. There is strong correlation between the
motion of neighboring particles. This leads to fixed phase relations and
destructive interference for most of the scattered light. The remaining
scattering arises from fluctuations in particle density discussed above.
 �34s[James F. Scott]
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�6v�2v�32sScent gland   �21m �33s 
    A specialized skin gland of the tubuloalveolar or acinous variety found in 
many mammals. These glands produce substances having peculiar odors. In some 
instances they are large, in others small. Examples of large glands are the 
civet gland in the civet cat, the musk gland in the musk deer, and the 
castoreum gland in the beaver. The civet gland is an anal gland, whereas the 
musk and castoreum are preputial. Examples of small scent glands are the 
preputial or Tyson's glands in the human male which secrete the smegma, and 
the vulval glands in the female. The secretions in all of the above glands are 
sebaceous.  See  GLAND. 
 �34s[Olin E. Nelsen]
�33s 
  
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�6v�2v�32sScheelite   �21m �33s 
    A mineral consisting of calcium tungstate, CaWO4. Sheelite occurs in 
colorless to white, tetragonal crystals; it may also be massive and granular. 
Its fracture is uneven, and its luster is vitreous to adamantine. Scheelite 
has a hardness of 4.5 - 5 on Mohs scale and a specific gravity of 6.1. Its 
streak is white. The mineral is transparent and fluoresces bright bluish-white 
under ultraviolet light. 
    Scheelite is an important tungsten mineral and occurs in small amounts in 
vein deposits. The most important scheelite deposit in the United States is 
near Mill City, Nevada.  See  TUNGSTEN. 
 �34s[Edward C. T. Chao]
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�6v�2v�32sSchematic drawing   �21m �33s 
    Concise, graphical symbolism whereby the engineer communicates to others 
the functional relationship of the parts in a component and, in turn, of the 
components in a system. The symbols do not attempt to describe in complete 
detail the characteristics or physical form of the elements, but they do 
suggest the functional form which the ensemble of elements will take in 
satisfying the functional requirements of the component. They are different 
from a block diagram in that schematics describe more specifically the 
physical process by which the functional specifications of a block diagram are 
satisfied.  
    An electrical schematic is a functional schematic which defines the 
interrelationship of the electrical elements in a circuit, equipment, or 
system. The symbols describing the electrical elements are stylized, 
simplified, and standardized to the point of universal acceptance (Fig. 1). 
    In a mechanical schematic, the graphical descriptions of elements of a 
mechanical system are more complex and more intimately interrelated than the 
symbolism of an electrical system and so the graphical characterizations are 
not nearly as well standardized or simplified (Fig. 2). However, a mechanical 
schematic illustrates such features as components, acceleration, velocity, 
position force sensing, and viscous damping devices.  See  DRAFTING; 
ENGINEERING DRAWING. 
 �34s[Robert W. Mann]
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�6v�2v�32sSchist   �21m �33s 
    A large group of rocks which by deformation during regional metamorphism 
have acquired a schistosity; that is, they show a more or less perfect 
cleavage along which they easily split up in flaggy slabs. There are 
orthoschists derived from igneous rocks and paraschists derived from 
sedimentary rocks. Crystalline schist is a common designation for all rocks 
which have recrystallized during regional metamorphism.  See  METAMORPHISM; 
SCHISTOSITY.  
    The chemical composition of a schist is similar to that of most silicate 
rocks, but mineralogically all schists have one feature in common, namely, 
that one or more of the major mineral constituents are flaky or fibrous 
(having the crystal structure of phyllosilicates or inosilicates). 
 �34s[T. F. W. Barth]
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�6v�2v�32sSchistosity   �21m �33s 
    A type of cleavage characteristic of metamorphic rocks, notably schists 
and phyllites. The rocks tend to split along parallel planes defined by the 
distribution and parallel arrangement (preferred orientation) of platy mineral 
crystals such as muscovite, biotite, chlorite, talc, and graphite or, less 
commonly, rodlike crystals such as tremolite, actinolite, and hornblende. The 
preferrred orientation of these minerals is produced during deformation and 
recrystallization (metamorphism) of the rocks. The schistosity may have a 
consistent attitude over large areas but local folding is also common.  See  
PHYLLITE; ROCK CLEAVAGE; SCHIST. 
 �34s[John M. Christie]
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�6v�2v�32sSchistosomiasis   �21m �33s 
    A disease in which humans are parasitized by any of three species of blood 
flukes: Schistosoma mansoni, S. haematobium, and S. japonicum. Adult S. 
mansoni prefer the veins of the hemorrhoidal plexus, S. haematobium those of 
the vesical plexus, and S. japonicum those of the small intestine. The disease 
is also known as bilharziasis. See DIGENEA. 
    An embryonated egg passed in feces or urine hatches in fresh water, 
liberating a miracidium larva which penetrates into specific gastropod snails. 
The larval cycle in the snail lasts for about 1 month. The cercaria emerges 
from the mollusk, swims in the water, and penetrates the skin of the final 
host upon coming in contact with it. 
    Schistosomiasis is an agricultural hazard for all ages in irrigated lands 
or swamps. Elsewhere fluvial waters are the main source of infection, in which 
case incidence is marked in human beings who are less than 15 years old and is 
higher among boys than among girls. 
 �34s[Jose F. Maldonado]
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�6v�2v�32sSchistostegiales   �21m �33s 
    An order of mosses composed of one monogeneric family and one species. The 
plants are small, slender, and glaucous. They are green when young and turn 
reddish-brown with age. The species grows in loose mats on sandstone in 
crevices and caves, on granite, on limestone slopes of old quarries, and in 
rabbit burrows, mine shafts, and damp cellars. This moss attracts special 
attention because of the luminous protonema.  See  BIOLUMINESCENCE; BRYOPSIDA. 
 �34s[Winona H. Welch]
�33s 
  
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�6v�2v�32sSchizogoniales   �21m �33s 
    A small, unique order of the Chlorophyta containing algae that are 
submicroscopic filaments or macroscopic ribbons and sheets a few centimeters 
wide. They are attached by rhizoids to rocks in salt or fresh water, usually 
occurring in alpine streams, on highly nitrogenous soil, or on bones, 
especially in the Arctic. There is one stellate chloroplast and a central 
pyrenoid in subrectangular cells; in Prasiola the cells are often grouped in 
fours.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
�33s 
  
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�6v�2v�32sSchizophrenia   �21m �33s 
    A group of mental disorders which are characterized by withdrawal from 
reality and by disturbances in thinking and feeling. It is also called 
dementia praecox. In many cases it leads to disorganization of the personality 
but not necessarily to mental deterioration. One of the outstanding symptoms 
of schizophrenia is a lack of rapport and serious misjudgment of reality 
processes. Certain types of schizophrenics are refractory to influence and 
suggestion; this behavior in its extreme form is called negativism. 
    The main symptom of schizophrenia is a rather severe dissociation in 
thinking and feeling. The patients think and speak incoherently and 
illogically. There is a blocking of the thinking process; ideas become very 
vague. Emotions become flattened and also extremely ambivalent or 
contradictory at the same time. 
    The course of schizophrenia is often progressive, leading to a terminal 
state of deterioration; however, there are many cases which do not reach such 
a terminal state and many even recover spontaneously.  See  PSYCHOSIS. 
 �34s[Fredrick C. Redlich]
�33s 
  
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�6v�2v�32sSchlieren photography   �21m �33s 
    An optical technique that detects density gradients occurring in a gas 
flow. The schlieren system is used particularly in supersonic wind tunnels 
because it clearly shows the density gradients created by the shock and 
expansion waves of the airflow around the wind tunnel model. 
    A simple schlieren system operates as shown in the illustration. A source 
of light is shielded so that only a small rectangular slit emits light. A lens 
is placed at its focal distance from the slit so that the light is bent into a 
parallel beam. A second lens collects the parallel beam into an image of the 
slit and forms an inverted image on the screen or photographic plate. If a 
knife-edge is moved into the light stream near the slit image, the image at 
the screen darkens uniformly. Consider the system just described to be 
oriented so that the parallel light beam crosses a wind tunnel section. A 
light ray a, bent from the parallel path by density gradients in the test 
section, cannot be brought to focus at the slit image and is interrupted by 
the knife-edge so that a dark spot occurs at a' on the screen. Light ray b, 
deflected the opposite way by a different density gradient, escapes the 
knife-edge and appears as a light spot at b' on the screen. Thus a picture of 
the density gradients appears on the screen.  See  SHOCK-WAVE DISPLAY; WIND 
TUNNEL.  
 �34s[D. Philp Ankeney]
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�6v�2v�32sSchmidt camera   �21m �33s 
    An optical system consisting of a spherical mirror with a corrector plate 
near its focus. Cameras of the Schmidt type are of great value in direct 
astronomical photography and also as parts of spectroscopes. 
    The corrector plate is a figured plate with one plane surface, its second 
surface being aspherical, slightly deviating from a plane, such that the 
aperture aberration of the mirror is balanced. The curvature of the plate at 
the axis is zero.  See  OPTICAL SURFACES. 
    The Schmidt camera is a catadioptric system which is free from aperture 
aberration, asymmetry, and chromatic aberrations. It can therefore be used at 
the very large aperture of f/2.0 or more. The image, although sharp, has 
curvature of field, but the field can be flattened by adding a second mirror 
to produce what could be called a Schmidt-Cassegrain system. 
    The success of the Schmidt camera, which at first was used mostly for 
astronomical purposes, has led to numerous designs of catadioptric systems for 
other purposes, such as microscope optics and camera lenses. See ASTRONOMICAL 
PHOTOGRAPHY.  
 �34s[Max J. Herzberger]
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�6v�2v�32sSchottky anomaly   �21m �33s 
    A contribution to the heat capacity of a solid arising from the thermal 
population of discrete energy levels as the temperature is raised. The effect 
is particularly prominent at low temperatures, where other contributions to 
the heat capacity are generally small.  See  SPECIFIC HEAT OF SOLIDS. 
    Discrete energy levels may arise from a variety of causes, including the 
removal of orbital or spin degeneracy by magnetic fields, crystalline electric 
fields, and spin orbit coupling, or from the magnetic hyperfine interaction. 
Such effects commonly occur in paramagnetic ions.  See  LOW-TEMPERATURE 
THERMOMETRY.  
    Corresponding to the Schottky heat capacity, there is a contribution to 
the entropy. This can act as a barrier to the attainment of low temperatures 
if the substance is to be cooled either by adiabatic demagnetization of by 
contact with another cooled substance. Conversely, a substance with a Schottky 
anomaly can be used as a heat sink in experiments at low temperatures 
(generally below 1 K or -457.9DGF) to reduce temperature changes resulting 
from the influx or generation of heat.  See  ADIABATIC DEMAGNETIZATION; 
LOW-TEMPERATURE PHYSICS. 
 �34s[W. P. Wolf]
�33s 
  
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�6v�2v�32sSchottky effect   �21m �33s 
    The enhancement of the thermionic emission of a conductor resulting from 
an electric field at the conductor surface. Since the thermionic emission 
current is given by the Richardson formula, an increase in the current at a 
given temperature implies a reduction in the work function of the emitter. The 
reduction in work function can be calculated by considering the effect of a 
constant externally applied field on the potential energy of an electron near 
the conductor surface, and is found to be proportional to the square root of 
the field.  See  ELECTRONIC WORK FUNCTION; THERMIONIC EMISSION. 
    A plot of the logarithm of the current versus the square root of the anode 
voltage should yield a straight line. An example is given in the illustration 
for tungsten; the deviation from the straight line for low anode voltages is 
due to space-charge effects. See SPACE CHARGE. 
 �34s[Adrianus J. Dekker]
�33s 
  
  
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�6v�2v�32sSchroedinger's wave equation   �21m �33s 
    The differential equation of quantum mechanics (first proposed by Erwin 
Schroedinger in 1926) whose solution determines the average result, also 
termed expectation value, of every conceivable experiment on the physical 
system under examination. When solved, the Schroedinger equation yields the 
wave function; from the wave function, expectation values are computed. The 
term wave equation comes from the resemblance of the Schroedinger equation to 
those differential equations describing acoustic and electromagnetic waves. 
Also, its consequences and mode of derivation are consistent with the tenet 
that electrons and other particulate constituents of matter have wavelike 
properties.  See  QUANTUM MECHANICS; WAVE EQUATION; WAVE MOTION. 
 �34s[Edward Gerjuoy]
�33s 
  
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�6v�2v�32sSchuler pendulum   �21m �33s 
    Any apparatus which swings, because of gravity, with a natural period of 
84.4 min, that is, with the same period as a hypothetical simple pendulum 
whose length is the Earth's radius. In 1923 Max Schuler showed that such an 
apparatus has the unique property that the pendulum arm will remain vertical 
despite any motions of its pivot. It is therefore useful as a base for 
navigational instruments. Schuler also showed how gyroscopes can be used to 
increase the period of a physical pendulum to the desired 84.4 min. 
    Gyrocompasses employ the Schuler principle to avoid errors due to ship 
accelerations. The principle has become the foundation of the science of 
inertial navigation.  See  GYROCOMPASS; INERTIAL GUIDANCE SYSTEM. 
 �34s[Robert H. Cannon, Jr.]
�33s 
  
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�6v�2v�32sSciatica �21m �33s
    Pain in the lower extremities, hips, and back caused by irritation of the
sciatic nerves. The location of the specific pain and the causes producing it
are quite varied.
    Sciatica may result from mechanical pressure on the nerves or their roots
in the cord. Trauma, herniated intervertebral disks, pregnancy, inflammation,
or tumors may cause compression. Toxic or metabolic disorders, such as lead
poisoning, diabetes mellitus, alcoholism, and vitamin-B deficiency may induce
sciatic pain by producing changes in the nerves. Inflammations, both local and
systemic, may also cause temporary or permanent nerve injury. Certain viral
diseases, syphilis, and local infections act in this manner.  See  SYPHILIS;
VIRUS; VITAMIN.
    In addition, lesions in the anal region and the prostate may induce
sciatica through reflex stimulation. Joint diseases, pelvic strain, and injury
most often precipitate an attack. The pain of sciatica may begin suddenly and
violently or gradually, as a nagging discomfort. The pain is usually along the
leg, with later extension to the thigh and back. Numbness of the outside of
the foot may occur. See PAIN.
 �34s[Edward G. Stuart /N. Karle Mottett]
�33s


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�6v�2v�32sScience �21m �33s
    In common usage the word science is applied to a variety of disciplines or
intellectual activities which have certain features in common. Usually a
science is characterized by the possibility of making precise statements which
are susceptible of some sort of check or proof. This often implies that the
situations with which the special science is concerned can be made to recur in
order to submit themselves to check, although this is by no means always the
case. There are observational sciences such as astronomy or geology in which
repetition of a situation at will is intrinsically impossible, and the
possible precision is limited to precision of description.
    A common method of classifying sciences is to refer to them as either
exact sciences or descriptive sciences. Examples of the former are physics
and, to a lesser degree, chemistry; and of the latter, taxonomical botany or
zoology. The exact sciences are in general characterized by the possibility of
exact measurement. One of the most important tasks of a descriptive science is
to develop a method of description or classification that will permit
precision of reference to the subject matter.  See  PHYSICAL SCIENCE.
 �34s[Percy W. Bridgman /Gerald Holton]
�33s

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�6v�2v�32sScientific satellites   �21m �33s 
    Satellites used to gain scientific knowledge. They may be satellites of 
Earth or of other planets. Satellites used to apply space knowledge and 
techniques to practical purposes are called applications satellites. Many 
spacecraft are used for multiple purposes, combining space exploration, 
science, and applications in various ways. Scientific satellites are often 
called research satellites, and applications satellites are commonly 
designated by the application field, for example, navigation satellites.  See  
SATELLITE (SPACECRAFT). 
    The space environment was one of the first areas investigated in 
scientific satellites. In this environment are cosmic rays, dust, magnetic 
fields, and various radiations from the Sun and galaxies. The Van Allen 
radiation belts were discovered by the first United States satellites. Soon 
thereafter satellites and space probes confirmed the existence of the solar 
wind. The continued investigation of these phenomena by satellites and space 
probes has led to the knowledge that a planet with a strong magnetic field, 
like Earth, is surrounded by a complex region, called a magnetosphere. 
    Numerous investigations are made of the effect of the space environment on 
materials and processes, including effects of radiation damage, meteoric 
erosion processes, and extremely high vacuum for very long periods. The 
influence of prolonged weightlessness on welding, alloying metals, growing 
crystals, and biological processes constitutes a promising field for future 
practical applications. 
    Scientific satellites, sounding rockets, and space probes are having a 
profound effect on earth science. The Earth and its atmosphere can now be 
studied in comparison with the other planets and their atmospheres, providing 
greater insight into the formation and evolution of the solar system. 
Satellites also make possible precision measurements of the size and shape of 
Earth and its gravitational field, and even measurements of the slow drifting 
of continents relative to each other. 
    Satellites are also having a great influence on astronomy by making it 
possible to observe celestial objects in all the wavelengths that reach the 
vicinity of Earth, whereas the ionosphere and atmosphere prevent most of these 
radiations from reaching telescopes on the ground. Moreover, even in the 
visible wavelengths, the small-scale turbulence and continuously varying 
refraction in the lower atmosphere distorts the image of a stellar object 
viewed at the ground, so that there is a limit to the improvement that can be 
achieved by increasing the size of a ground-based telescope.  See  SATELLITE 
ASTRONOMY.  
    The applications satellites emphasize the continuing day-to-day practical 
utilization of the satellite. They are operational in nature, although some of 
them, either directly or as a by-product of their operational output, 
contribute significantly to research, for example, with meteorological or 
Earth survey satellites. An applied research program generally precedes 
establishment of an applications satellite system. Thus, Tiros and Nimbus laid 
the groundwork for operational meteorological satellites. Likewise, Syncom 
satellites preceded the Intelsat communications system.  See  APPLICATIONS 
SATELLITES; COMMUNICATIONS SATELLITE; METEOROLOGICAL SATELLITES; MILITARY 
SATELLITES; SATELLITE NAVIGATION SYSTEMS; SPACE FLIGHT. 
 �34s[Homer E. Newell]
�33s 
  
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�6v�2v�32sScintillation counter   �21m �33s 
    A particle or radiation detector which operates through emission of light 
flashes that are detected by a photosensitive device, usually a 
photomultiplier. The scintillation counter not only can detect the presence of 
a particle, gamma ray, or x-ray, but can measure the energy, or the energy 
loss, of the particle or radiation in the scintillating medium. The sensitive 
medium may be solid, liquid, or gaseous, but is usually one of the first two. 
The scintillation counter is one of the most versatile particle detectors, and 
is widely used in industry, scientific research, and radiation monitoring, as 
well as in exploration for petroleum and radioactive minerals that emit gamma 
rays. Many low-level radioactivity measurements are made with scintillation 
counters.  See  LOW-LEVEL COUNTING; PHOTOMULTIPLIER. 
    Scintillation counters are made of transparent crystalline materials or 
liquids or plastics. In order to be an efficient detector, the bulk 
scintillating medium must be transparent to its own luminescent radiation, and 
since most detectors are quite extensive, covering meters in length, the 
transparency must be of a high order. One face of the scintillator is placed 
in optical contact with the photosensitive surface of the photomultiplier, as 
shown in the illustration. In order to direct as much as possible of the light 
flash to the photosensitive surface, reflecting material is placed between the 
scintillator and the inside surface of the container. 
    Scintillation counters are often used in temporal coincidence with other 
particle or radiation detectors. The coincidence technique eliminates or 
reduces the problem of local background or false events.  See  PARTICLE 
DETECTOR.  
 �34s[Robert Hofstadter]
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�6v�2v�32sScleractinia   �21m �33s 
    An order of the subclass Zoantharia which comprises the true or stony 
corals (see illustration). These are solitary or colonial anthozoans which 
attach to a firm substrate. They are profuse in tropical and subtropical 
waters and contribute to the formation of coral reefs or islands. Some species 
are free and unattached. 
    Most of the polyp is impregnated with a hard calcareous skeleton. The 
polyps increase rapidly by budding, and the skeletons of polyps which settle 
in groups may fuse to form a colony. The pyriform, ciliated planula larva 
swims with its aboral extremity, which is composed of an ectodermal sensory 
layer, directed anteriorly. Planulation occurs periodically in conformity with 
lunar phases in many tropical species.  See  ZOANTHARIA. 
 �34s[Kenji Atoda]
�33s 
  
  
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�6v�2v�32sSclerenchyma   �21m �33s 
    Single cells or aggregates of cells whose principal function is thought to 
be mechanical support of plants or plant parts. Sclerenchyma cells have thick 
secondary walls and may or may not remain alive when mature. They vary greatly 
in form and are of widespread occurrence in vascular plants. Two general 
types, sclereids and fibers, are widely recognized, but since these 
intergrade, the distinction is sometimes arbitrary. 
 �34s[Norman H. Boke]
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�6v�2v�32sSclerospongiae   �21m �33s 
    A class of sponges that lay down a compound skeleton including an 
external, basal mass of calcium carbonate, either aragonite or calcite, as 
well as internal siliceous spicules and protein fibers. The living tissue 
forms a thin layer over the basal calcareous skeleton and extends into surface 
depressions of the skeleton; the organization of the tissue is similar to that 
of encrusting demosponges. Sclerosponges are common inhabitants of cryptic 
habitats on coral reefs in both the Caribbean and Indo-Pacific biogeographic 
regions.  See  DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
�33s 
  
  
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�6v�2v�32sScorpionida   �21m �33s 
    An order of the Arachnida which have chelate pedipalps and chelicera, a 
terminal caudal sting, and abdominal pectines (see illustration). The body is 
divided into a cephalothorax, which is covered by the unsegmented carapace, 
and a segmented abdomen. This latter division is differentiated into an 
anterior preabdomen and a postabdomen which, plus the terminal sting, 
constitutes the ``tail'' or cauda. The cephalothorax bears the chelicera, the 
pedipalps, and four pairs of walking legs. The preabdomen contains seven 
segments, while the postabdomen has five. The carapace bears three groups of 
small, simple eyes, a median pair and two groups of anterolateral eyes. True 
cave scorpions lack eyes. 
    Approximately 1000 species are known. Scorpions are widely distributed 
throughout the tropical zone and the warmer areas of the temperate zone. About 
56 species are found in the United States, 22 of which are in Arizona. 
California and Florida also have a rich scorpion fauna. Scorpions are found 
over three-fourths of the United States; they are absent from the New England 
states, Iowa, and areas immediately surrounding the Great Lakes. 
    The only species in the United States that are known to be lethal are 
Centruroides sculpturatus and C. gertschi, found in southern and central 
Arizona and the adjacent areas of California, New Mexico, and Mexico. The 
venom of these scorpions has proved fatal to healthy children up to 16 years 
of age and to adults suffering from hypertension and general debility.  See  
ARACHNIDA.  
 �34s[Herbert L. Stahnke]
�33s 
  
  
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�6v�2v�32sScorpius   �21m �33s 
    The Scorpion, in astronomy, one of the most beautiful and vivid 
constellations in the sky. Scorpius is the eighth sign of the zodiac (see 
illustration). The bright red star Antares is situated at the heart. Antares 
is one of the largest stars known, having a diameter over 450 times that of 
the Sun. As in Sagittarius, the Milky Way in Scorpius is bright and rich in 
star clouds and clusters.  See  CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sScouring   �21m �33s 
    A process that removes natural encrustants which are of a resistant nature 
from fibers. In natural fibers such as cotton or linen, very resistant and 
water-repellent waxes are the main concern. These waxes require relatively 
severe treatment with caustic soda and other auxiliaries at high temperatures 
to form a wax soap (saponification). Thus the waxes are emulsified prior to 
being rinsed or washed out. With protein fibers such as wool, the repellent is 
a natural grease. This also must be removed by emulsification with a slightly 
aklalinized soap or detergent, but with much lower temperature and alkaline 
content. Synthetic fibers, including rayons and synthetics, contain no natural 
encrustants but have so-called producer oils added to them as protective 
agents. These are self-emulsifiable oils and are removed concurrently with any 
sizing agent used in the desizing process, thus eliminating the need for a 
scour. See DESIZING. 
 �34s[John J. McDonald]
�33s 
  
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�6v�2v�32sScrapie   �21m �33s 
    A transmissible, usually fatal disease of adult sheep characterized by 
degeneration of the central nervous system. (It is classed as a slow virus 
disease.) The disease is known in Great Britain, France, Belgium, Iceland, the 
United States, Canada, and northern India. Scrapie has certain similarities 
with kuru, a human disease in New Guinea, and mink encephalopathy. 
    Scrapie affects both sexes and is insidious in its onset, starting with 
hyperexcitability and progressive itch. Later, loss of wool occurs when the 
animal rubs against fixed objects or bites and nibbles its skin. Some animals 
do not rub but are either nervous and tremble when approached or appear 
sleepy. Incoordination of gait is constant and usually more evident in the 
hindquarters. In the final stages the sheep, being unable to stand, lie down, 
become emaciated, and die.  See  VIRUS INFECTION, LATENT, PERSISTENT, SLOW. 
 �34s[I. Zlotnik]
�33s 
  
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�6v�2v�32sScreening   �21m �33s 
    A mechanical method of separating a mixture of solid particles into 
fractions by size. The mixture to be separated, called the feed, is passed 
over a screen surface containing openings of definte size. Particles smaller 
than the openings fall through the screen and are collected as undersize. 
Particles larger than the openings slide off the screen and are caught as 
oversize. A single screen separates the feed into only two fractions. Two or 
more screens may be operated in series to give additional fractions. Screening 
occasionally is done wet, but most commonly it is done dry. 
    Industrial screens may be constructed of metal bars, perforated or slotted 
metal plates, woven wire cloth, or bolting cloth. The openings are usually 
square but may be circular or rectangular.  See  MECHANICAL CLASSIFICATION; 
MECHANICAL SEPARATION TECHNIQUES; SEDIMENTATION (INDUSTRY). 
 �34s[Warren L. McCabe]
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�6v�2v�32sScrew   �21m �33s 
    A cylindrical body with a helical groove cut into its surface. For 
practical purposes a screw may be considered to be a wedge wound in the form 
of a helix so that the input motion is a rotation while the output remains 
translation. The screw is to the wedge much the same as the wheel and axle is 
to the lever in that it permits the exertion of force through a greatly 
increased distance. 
    The screw is by far the most useful form of inclined plane or wedge and 
finds application in the bolts and nuts used to fasten parts together; in lead 
and feed screws used to advance cutting tools or parts in machine tools; in 
screw jacks used to lift such objects as automobiles, houses, and heavy 
machinery; in screw-type conveyors used to move bulk materials; and in 
propellers for airplanes and ships.  See  PROPELLER (AIRCRAFT); PROPELLER 
(SHIP); SCREW FASTENER; SCREW JACK; SCREW THREADS; SIMPLE MACHINE. 
 �34s[Richard M. Phelan]
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�6v�2v�32sScrew fastener   �21m �33s 
    A threaded machine part used to join parts of a machine or structure. 
Screw fasteners are used when a connection that can be disassembled and 
reconnected and that must resist tension and shear is required. A nut and bolt 
is a common screw fastener. Bolt material is chosen to have an extended 
stress-strain characteristic free from a pronounced yield point. Nut material 
is chosen for slight plastic flow. 
    The nut is tightened on the bolt to produce a preload tension in the bolt, 
as illustrated. This preload has several advantageous effects. It places the 
bolt under sufficient tension so that during vibration the relative stress 
change is slight with consequent improved fatigue resistance and locking of 
the nut. Preloading also increases the friction between bearing surfaces of 
the joined members so that shear loads are carried by the friction forces 
rather than by the bolt.  See  BOLT; JOINT (STRUCTURES); NUT (ENGINEERING); 
SCREW.  
 �34s[Frank H. Rockett]
�33s 
  
  
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�6v�2v�32sScrew jack   �21m �33s 
    A mechanism for lifting and supporting loads, usually of large size. A 
screw jack mechanism consists of a thrust collar and a nut which rides on a 
bolt; the threads between the nut and bolt normally have a square shape. A 
standard form of screw jack has a heavy metal base with a central threaded 
hole into which fits a bolt capable of rotation under a collar thrusting 
against the load.  See  SCREW; SIMPLE MACHINE. 
 �34s[James J. Ryan]
�33s 
  
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�6v�2v�32sScrew-thread gage   �21m �33s 
    This type of gage may be used to inspect the pitch diameter, major 
diameter, minor diameter, lead, straightness, and thread angle of a screw 
thread (see illustration). Screw-thread gages may be plug gages,ring gages, 
snap gages, or indicating gages. A ``go'' plug or ring gage inspects all these 
characteristics. A ``not go'' plug or ring gage checks only pitch diameter and 
lead. See PLUG GAGE; RING GAGE; SNAP GAGE. 
 �34s[Rush A. Bowman]
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�6v�2v�32sScrew threads �21m �33s
    Continuous helical ribs on a cylindrical shank. Screw threads are used
principally for fastening, adjusting, and transmitting power. To perform these
specific functions, various thread forms have been developed. A thread on the
outside of a cylinder or cone is an external (male) thread; a thread on the
inside of a member is an internal (female) thread (Fig. 1).  See  SCREW.
     Types of thread.  A thread may be either right-hand or left-hand.	A
right-hand thread on an external member advances into an internal thread
when turned clockwise; a left-hand thread advances when turned counterclock-
wise.  If a single helical groove is cut or formed on a cylinder, it is called
a single-thread screw.	Should the helix angle be increased sufficiently for a
second thread to be cut between the grooves of the first thread, a double
thread will be formed on the screw.  Double, triple, and even quadruple threads
are used whenever a rapid advance is desired, as on valves..
     Pitch and major diameter designate a thread.  Lead is the distance advanced
parallel to the axis when the screw is turned one revolution.  For a single
thread, lead is equal to the pitch; for a double thread, lead is twice the
pitch. For a straight thread, the pitch diameter is the diameter of an
imaginary coaxial cylinder that would cut the thread forms at a height where
the width of the thread and groove would be equal. 
     Thread forms have been developed to satisfy particular requirements.
Where strength is required for the transmission of power and motion, a
thread having faces that are more nearly perpendicular to the axis is
preferred.  These threads, with their strong thread sections, transmit
power nearly parallel to the axis of the screw.
     Thread fastener.  Most threaded fasteners are a threaded cylindrical rod
with some form of head on one end.  Of the many forms available, five types
meet most requirements for threaded fasteners and are used for the bulk of
production work:  bolt, stud, cap screw, and set screw (Fig. 2).  Bolts
and screws can be obtained with varied heads and points.
     A bolt is generally used for drawing two parts together.  A stud is
a rod threaded on both ends.  Studs are used for parts that must be
removed frequently and for applications where bolts would be impractical.
     Cap screws (plated or unplated) are widely used in machine tools for
assembling parts in automotive and aeronautical equipment.  They are available
in four standard heads:  hexagon, flat, round, and fillister.  Flathead and
roundhead screws have slotted heads (Fig. 3).  Machine screws are similar to
cap screws and fulfill the same purpose, being employed principally on small
work.  Set screws made of hardened steel are used to hold parts in a position
relative to one another.  Wood screws, lag screws, and hanger bolts are used
in wood.


[Warren J. Luzadder]
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�6v�2v�32sScrophulariales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Asteridae of the class Magnoliopsida (dicotyledons). The order 
consists of 12 families and nearly 10,000 species. The largest families are 
the Scrophulariaceae (about 2700 species), Acanthaceae (about 2600 species), 
Gesneriaceae (about 1800 species), Bignoniaceae (about 800 species), and 
Oleaceae (about 600 species). The Scrophulariales usually have a superior 
ovary and generally either an irregular corolla or one with fewer stamens than 
corolla lobes, or commonly both. They uniformly lack stipules.  See  
ASTERIDAE; MAGNOLIOPSIDA; OLIVE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sScrub typhus   �21m �33s 
    An acute mite-borne infection due to the bacterialike microorganism 
Rickettsia tsutsugamushi. It was first called tsutsugamushi disease in Japan. 
It is characterized in humans by high fever, a macular rash, usually a primary 
eschar or ulcer, and swelling of the regional lymph glands. The vectors are 
larval trombiculid mites whose species vary according to location and season, 
but in most areas are chiefly Trombicula akamushi and T. deliensis. The 
primary disease cycle is between these mites, which are parasitic on 
vertebrates only in the larval stage, and field mice and rats. Transmission to 
humans occurs from the next generation of larval mites.  See  ACARINA; 
RICKETTSIOSES.  
 �34s[Cornelius B. Philip]
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�6v�2v�32sSculpin   �21m �33s 
    Any of several species of small fish of the family Cottidae, known as sea 
scorpions in Europe. There are 24 species in North America, many being common 
throughout the fresh waters of the United States, and most marine species 
occurring along the Pacific Coast. These fish have bony plates under the eyes, 
spiny pelvic and anal fins, and sometimes spines on the head. All the 
fresh-water species are small, and many spawn under rocks or logs. These fish 
are not of economic importance since many, especially most of the large 
species, are coarse and tasteless; they are used as bait.  See  PERCIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sScurvy   �21m �33s 
    An acute or chronic disease due to vitamin-C (ascorbic acid) deficiency. 
In adults scurvy is rarely due to dietary deficiency in normally fed 
populations, since 3 - 12 months of severe deficiency are required to produce 
symptoms. These include weakness, weight loss, irritability, and vague pains. 
Clinical signs include such things as delayed wound healing, tooth changes, 
gingivitis, petechial hemorrhages, and nosebleeds. Stress, infection, and 
hemorrhage may aggravate the situation. The disease is often related to 
chronic or severe gastrointestinal disease or to food idiosyncrasies. Any type 
of healing requires increased vitamin C supply, and infection similarly 
increases body demand.  See  ASCORBIC ACID. 
 �34s[N. Karle Mottet]
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�6v�2v�32sScyphozoa   �21m �33s 
    A class of the phylum Coelenterata, containing five living orders - 
Stauromedusae, Cubomedusae, Coronatae, Semaeostomeae, and Rhizostomeae - and a 
fossil order, Stromatoporoidea. They are all marine and usually take two 
forms, the polyp, or scyphopolyp, and the medusa, or scyphomedusa. However, 
some are polyplike and sessile throughout their lives, while others are always 
pelagic and lack the sessile polyp stage. Among the Coelenterata, the 
Scyphozoa are characterized by having well-developed medusae of large size and 
fairly well-organized polyps of small size. 
    The scyphomedusae are generally found near the coast. Exceptions are 
certain pelagic forms and the Coronatae, which are abyssal. Most of the 
Stauromedusae have a circumpolar distribution, while the Cubomedusae and 
Rhizostomeae are found mostly in warm and tropical seas. Scyphomedusae are 
carnivorous, except the Rhizostomeae which are plankton eaters. Some 
Semaeostomeae and Cubomedusae are injurious to humans because of their 
nematocysts. On the other hand, some Rhizostomeae are used as food in the 
Orient.  See  COELENTERATA. 
 �34s[Tohru Uchida]
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�6v�2v�32sSea anemone   �21m �33s 
    Any polyp of the nearly 1000 anthozoan coelenterates belonging to the 
order Actiniaria. They occur intertidally and subtidally in marine and 
estuarine habitats, attached to solid substrates or burrowing into soft 
sediments. No fresh-water or truly planktonic species are known. Anemones may 
be very small, a fraction of an inch long, to large individuals more than 3 ft 
(90 cm) in length or diameter.  See  ACTINIARIA. 
    Sea anemones enter into a number of interesting symbiotic partnerships. 
Some are host to single-celled marine algae, which grow within the cells of 
the anemone. The algae provide organic materials which aid in the nutrition of 
the anemone. Other anemones live on gastropod shells inhabited by hermit 
crabs.  See  ANTHOZOA; COELENTERATA. 
 �34s[Cadet Hand]
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�6v�2v�32sSea horse   �21m �33s 
    Any of the 50 species of marine fishes which occur principally in tropical 
and subtropical waters and are members of the genus Hippocampus in the order 
Gasterosteiformes. These fishes, together with the pipefishes, make up the 
family Syngnathidae. The sea horses vary in length from about 2 in. to 1 ft (5 
to 30 cm). As with other members of the order, they have an elongate, tubelike 
snout.  
    The short compressed body is encompassed by bony rings which afford 
rigidity (see illustration). The head is bent ventrally, and the mouth is 
located at the end of the tubiform snout. Small dorsal and pectoral fins are 
present, but a caudal fin is lacking. These fishes have a prehensile tapering 
tail, which is used for clinging to seaweed.  See  GASTEROSTEIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSea ice   �21m �33s 
    Ice formed by the freezing of sea water. Ice in the sea includes sea ice, 
river ice, and land ice. Land ice is principally icebergs. River ice is 
carried into the sea during spring breakup and is important only near river 
mouths. The greatest part, probably 99% of ice in the sea, is sea ice.  See  
ICEBERG; TERRESTRIAL FROZEN WATER. 
    The freezing point temperature and the temperature of maximum density of 
sea water vary with salinity. When freezing occurs, small flat plates of pure 
ice freeze out of solution to form a network which entraps brine in layers of 
cells. As the temperature decreases more water freezes out of the brine cells, 
further concentrating the remaining brine so that the freezing point of the 
brine equals the temperature of the surrounding pure ice structure. The brine 
is a complex solution of many ions. 
    The brine cells migrate and change size with changes in temperature and 
pressure. The general downward migration of brine cells through the ice sheet 
leads to freshening of the top layers to near zero salinity by late summer. 
During winter the top surface temperature closely follows the air temperature, 
whereas the temperature of the underside remains at feezing point, 
corresponding to the salinity of water in contact. 
    The sea ice in any locality is commonly a mixture of recently formed ice 
and old ice which has survived one or more summers. Except in sheltered bays, 
sea ice is continually in motion because of wind and current. 
 �34s[Waldo Lyon]
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�6v�2v�32sSea-level datum planes   �21m �33s 
    Sea level is the elevation of the sea surface measured as the vertical 
distance between the surface and some fixed point on land - a rocky outcrop on 
a beach, a mountain peak, or a reference point installed by humans. 
    Mean sea level is the average elevation and is frequently used as a 
reference level in describing the elevation of points on land, or of depths in 
the sea. The surface of the sea is by no means a stationary spheroidal 
surface, so that the accurate determination of its average elevation requires 
a long series of observations.  See  SEA-LEVEL FLUCTUATIONS. 
    Other datum planes are used for special purposes. Half-tide level is the 
average of all the highest and lowest readings during the period (called high 
and low waters). Mean low water and mean high water are the averages of all of 
the low waters and high waters, respectively; half-tide level lies midway 
between these two means.  See  TIDE. 
 �34s[June G. Pattullo]
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�6v�2v�32sSea-level fluctuations   �21m �33s 
    The height of sea level is constantly changing and its average value (mean 
sea level) will depend critically on the length of a series of observations 
and the period in time during which they are made. Furthermore, the sea 
surface is not even level.  See  SEA-LEVEL DATUM PLANES. 
    Sea level stands about 10 in. (25 cm) higher along the coast of Maine than 
it does off Florida, and the steepest slope occurs off Cape Hatteras, North 
Carolina. This occurs because the water in the sea is always in motion. The 
motion of any fluid over the surface of the Earth is influenced by the 
rotation of the Earth itself; fluids are deflected to the right (looking 
downstream) in the Northern Hemisphere, to the left in the Southern. This 
effect results from the Coriolis acceleration.  See  CORIOLIS ACCELERATION AND 
FORCE; OCEAN CURRENTS. 
    Sea level also deviates from the mean under the influence of differential 
atmospheric pressure. If pressure is high over one area of the ocean and low 
somewhere else, the water will tend to flow toward the low-pressure area. Thus 
the sea surface behaves as an inverted barometer; it stands high where air 
pressure is low, and low where air pressure is high.  See  AIR PRESSURE. 
    A third effect can be observed where parts of the sea surface are 
separated from each other by a land barrier. Precise leveling across the 
continent of North America shows that sea level is about 20 in. (50 cm) higher 
on the Pacific than on the Atlantic side of the continent. Currents and winds 
probably cause part of this difference in level, but another important factor 
is the difference in the density of the sea water. 
 �34s[June G. Pattullo]
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�6v�2v�32sSeals and allies   �21m �33s 
    Carnivorous mammals of the suborder Pinnipedia, which includes 32 species 
of seals, sea lions, and walrus in 3 families. All species of the order are 
found along coastal areas, from the Antarctic to the Arctic regions. Although 
many species have a restricted distribution the group as a whole has worldwide 
distribution.  
    Pinnipeds are less modified both anatomically and behaviorally for marine 
life than are other marine mammals: Each year they return to land to breed, 
and they have retained their hindlimbs. They are primarily carnivorous 
mammals, with fish supplying the basic diet. They also eat crustaceans, 
mollusks, and, in some instances, sea birds. Their body is covered with a 
heavy coat of fur, the limbs are modified as flippers, the eyes are large, the 
external ear is small or lacking, and the tail is absent or very short. 
    The single species of the family Odobenidae is the walrus (Odobenus 
rosmarus); it grows to 10 ft (3 m) and weighs 3000 lb (1350 kg). It has no 
external ears but does have a distinct neck region. The upper canines of both 
sexes are prolonged as tusks which can be used defensively . The walrus feeds 
on marine invertebrates and fish. 
    The family Otariidae, the eared seals, includes the sea lions and fur 
seals, which are characterized by an external ear. The neck is longer and more 
clearly defined than that of true seals, and the digits lack nails. The 
California sea lion (Zalophus californianus) occurs along the Pacific coast, 
and is commonly seen in zoos and performing in circuses. The southern sea lion 
(Otaria byronia) is found around the Galapagos Islands and along the South 
American coast. Stellar's sea lion (Eumetopias jubatus) is a large species 
found along the Pacific coast. Minor species are the Australian sea lion 
(Neophoca cinerea) and the New Zealand species (Phocarctos hookeri). 
    Phocidae, the largest family of pinnipeds, comprises the true seals. It 
includes the monk seals, elephant seals, common seals, and other less 
well-known forms. The family is unique in that the digits have nails, the 
soles and palms are covered with hair, and the necks are very short. Most 
species live in marine habitats; however, the Caspian seal (Pusa caspica) 
lives in brackish water. The only fresh-water seal is the Baikal seal (P. 
sibirica). One of the best-known species is the Atlantic gray seal 
(Halichoreus grypus), a species found in the North Atlantic along the coasts 
of Europe, Iceland, and Greenland.  See  CARNIVORA; MAMMALIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSeamount and guyot �21m �33s
    A seamount is an isolated submarine mountain rising 3000 ft (900 m) or
more above the ocean floor. In the Pacific Basin there are at least 10,000
such mountains, which occur as volcanic peaks on ridges, or rises, or as
individual peaks.
    Flat-topped seamounts are called guyots, or tablemounts. They are present
on all ocean floors but are most common in the Pacific. Bottom samples of
coral and volcanic erosion debris indicate that the flattops were once at sea
level though they are now 1000 - 7000 ft (300 - 2100 m) below the ocean
surface. Thus guyots are ancient islands which were truncated to sea level by
erosion.  See  OCEANIC ISLANDS.
 �34s[Edwin L. Hamilton ;Henry W. Menard Jr.]
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�6v�2v�32sSeaplane   �21m �33s 
    An airplane capable of navigating on, taking off from, and alighting upon 
the surface of water. Seaplanes are grouped into two main types: flying boats 
and float planes. In the flying boat, the hull, which provides buoyancy and 
planing area, is an integral part of the airframe, a specially designed 
fuselage which supports the wings and tail surfaces and houses the crew, 
equipment, and cargo. Although multihull flying boats have been built, modern 
use is confined to single-hull boats with lateral stability on the water 
provided by small floats or pontoons attached to the wings (Fig. 1). The float 
plane is a standard landplane made capable of water operation by the addition 
of floats which are attached to the airframe by struts. In practice the twin 
float is used exclusively, lateral stability on the water being provided by 
the separation of the two identical floats (Fig. 2). A seaplane with 
retracting wheels which permit either land or water operation is known as an 
amphibian.  See  AIRPLANE. 
 �34s[Richard A. Hoffman]
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�6v�2v�32sSearch coil   �21m �33s 
    A device used for measuring the flux density in a small region of a 
magnetic field. The apparatus consists of a small coil connected by flexible 
leads to a ballistic galvanometer. If the coil is placed with its plane 
perpendicular to the magnetic field, the flux threading the coil is PHI = BA, 
where A is the area of the coil and B the magnetic induction.  See  MAGNETIC 
FLUX; MAGNETIC INDUCTION. 
    When the coil is quickly turned through a quarter turn or is withdrawn 
from the field to a place where B is zero, the flux through the coil is 
changed from BA to zero. During this change, there is an electromotive force 
(emf) and a current in the closed circuit whose instantaneous values are 
proportional to the rate of change of flux through the coil. The deflection of 
the ballistic galvanometer is proportional to the charge q, which is the 
integral of the current over time, and is thus proportional to the total flux 
change BA. However, if the instrument is properly calibrated, the value of B 
can be determined.  See  FLUXMETER; GALVANOMETER. 
 �34s[Kenneth V. Manning]
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�6v�2v�32sSea squirt   �21m �33s 
    A sessile tunicate of the class Ascidiacea (phylum Chordata), so named 
because water is squirted from two openings when the animal is touched. Sea 
squirts may be solitary, colonial, or compound, but always have a permanent 
enclosing structure, the test or tunic, composed of polysaccharide material 
structurally similar to cellulose. In colonies, individuals are loosely 
attached together; in compound ascidians the individual zooids are embedded in 
a common test. 
    The individual is roughly cylindrical in shape and attached at the base 
(see illustration). The mantle encloses a space or atrial cavity. Suspended 
within the atrial cavity is a pharynx or branchial sac perforated by numerous 
oval slits (stigmata), each one bearing a ring of marginal cilia. Constant 
beating of the cilia draws in water through the inhalant or oral aperture into 
the branchial sac. Water passes through the stigmata into the atrial cavity 
and is discharged at the exhalant or atrial aperture. In the branchial sac, 
food particles from the water are filtered on a mucous sheet which is then 
rolled up and passed to the esophagus. Sea squirts are dioecious, and 
fertilization may be internal or external. 
    Sea squirts occur in all seas and at all depths in the ocean. Most require 
a hard substratum on which to settle, but a few live in sand or mud. They are 
particularly common in shallow-water environments such as rocky shores, piers, 
and boat hulls, and in mangrove lagoons.  See  ASCIDIACEA; TUNICATA. 
 �34s[Ivan Goodbody]
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�6v�2v�32sSea state   �21m �33s 
    The description of the ocean surface or state of the sea surface with 
regard to wave action. Wind waves in the sea are of two types: Those still 
growing under the force of the wind are called sea: those no longer under the 
influence of the wind that produced them are called swell. Differences between 
the two types are important in forecasting ocean wave conditions. 
    Those waves which are still growing under the force of the wind have 
irregular, chaotic, and unpredictable forms. The unconnected wave crests are 
only two to three times as long as the distance between crests and commonly 
appear to be traveling in different directions. As the waves grow, they form 
regular series of connected troughs and crests with wave lengths commonly 
ranging from 12 to 35 times the wave heights. The appearance of the sea 
surface is termed state of the sea (Table 1). The height of a sea is dependent 
on the strength of the wind, the duration of time the wind has blown, and the 
fetch (distance of sea surface over which the wind has blown).  See  OCEAN 
WAVES.  
    As sea waves move out of the generating area into a region of weaker 
winds, a calm, or opposing winds, their height decreases as they advance, 
their crests become rounded, and their surface is smoothed. These waves are 
more regular and more predictable than sea waves and, in a series, tend to 
show the same form or the same trend in characteristics. Wave lengths 
generally range from 35 to 200 times wave heights. A descriptive 
classification of swell waves is given in Table 2. 
 �34s[Neil A. Benfer]
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�6v�2v�32sSea urchin   �21m �33s 
    A marine echinoderm of the class Echinoidea. These invertebrates are found 
commonly in shallow waters. The soft internal organs are enclosed in and 
protected by a test or shell (see illustration) consisting of a number of 
plates which fit closely together and are located under the skin. The oral 
surface is in contact with the substratum. Five teeth, located in the mouth, 
form part of Aristotle's lantern, a complex chewing structure. Like many other 
species of echinoderms, sea urchins use tube feet for locomotion and 
pincerlike structures called pedicellariae to keep the shell clean. Sea 
urchins feed on most available animal and vegetable materials. 
    Many species burrow into the sand, while others move into rock crevices to 
protect themselves from severe tidal action.  See  ECHINOIDEA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSea wall   �21m �33s 
    A structure at the water's edge to resist encroachment of the sea and to 
retain the natural soil or deposited fill behind the wall. A usual type of sea 
wall has a sloped or inclined face toward the sea to deflect and dissipate the 
wave forces. Sometimes it resembles a breakwater. Timber, steel, and concrete 
are used in a variety of forms.  See  COASTAL ENGINEERING. 
 �34s[Edward J. Quirin]
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�6v�2v�32sSea water �21m �33s
    Water is most often found in nature as sea water (APPROXIMENT 98%). Sea
water is an aqueous solution of salts of a rather constant composition of
elements. Its presence determines the climate and makes life possible on the
Earth. The boundaries of sea water are the boundaries of the oceans, the
mediterranean seas, and their embayments.  See	HYDROSPHERE.

    Physical properties.   Sea water is basically a concentrated electrolyte
solution containing many dissolved salts. The ratio of water molecules to salt
molecules is about 100 to 1. Since nearly all the salt exists as electrically
conducting ions, the ratio of water molecules to ions is about 50 to 1. In the
neighborhood of ions, extremely high electric fields exist and water molecules
near them become aligned; water molecules that remain in the vicinity of the
ions for a long time constitute a hydration shell, and the ions are said to be
solvated.  See	SOLVATION.
    As a consequence of the salts in sea water, its physical properties differ
from those of pure water, the difference being closely proportional to the
concentration of the salts or the salinity. Salinity measurements, along with
pressure and temperature data, are used to differentiate water masses. In
studying the movement of water masses in the oceans and their small- and
large-scale circulation patterns, including geostrophic flow, properties such
as density, compressibility, thermal expansion coefficients, and specific
heats need to be known as functions of temperature, pressure, and salinity.
    The large value of the osmotic pressure of sea water is of great
significance to biology and desalination by reverse osmosis. Related to
osmotic pressure and very important to the formation of ice is the reversal of
the freezing point and temperature of maximum density of sea water compared to
pure water: the freezing point temperature is lowered to 28.6DGF (-1.9DGC),
and the temperature of maximum density is decreased from just below 39DGF
(4DGC) for pure water to about 25.7DGF (-3.5DGC) for 35% salinity sea water.
    Since the greatest ocean depths exceed 33,000 ft (10,000 m) and more than
54% of the oceans' area is at pressure above 400 bars (40 megapascals), it is
necessary to consider the effect of pressure, as well as temperature, on the
physical properties of sea water. The pressure corresponding to the maximum
ocean depth is about 1100 bars (110 MPa). The usual pressure dependence of
viscosity is a consequence of the open structure of water which is altered by
pressure, temperature, and solutes.
    Because electromagnetic radiation can propagate in the ocean for only
limited distances, sound waves are the principal means of communication in
this medium.  See  UNDERWATER SOUND.
 �34s[Frederick H. Fisher]
�33s

    Composition of sea water.	The concentrations of the various components
of sea water are regulated by numerous chemical, physical, and biochemical
reactions. The present-day compositions of sea waters (see table) are
controlled both by the makeup of the ultimate source materials and by the
large number of reactions, of chemical and physical natures, occurring in the
oceans.
    Only calcium, among the major cations of sea water, is present in a state
of saturation, and such a situation generally occurs only in surface waters.
Here its concentration is governed by the solubility of calcium carbonate.
Barium concentrations in deep waters can be limited by the precipitation of
barium sulfate. The noble gases and dissolved gaseous nitrogen have their
marine concentrations determined by the temperature at which their water mass
was in contact with the atmosphere and are in states of saturation or very
nearly so.
    The formation and alteration of minerals on the sea floor apparently are
responsible for controlling the concentrations of the major cations Na, K, Mg,
and Ca. Such clay minerals as illite, chlorite, and montmorillonite are
presumably synthesized from these dissolved species and the river-transported
weathered solids (aluminosilicates, such as kaolinite). The SiO2 is introduced
in part as diatom frustules.
    The formation of ferromanganese minerals, sea-floor precipitates of iron
and manganese oxides, may govern the concentrations of a suite of trace
metals, including zinc, manganese, copper, nickel, and cobalt. These elements
are in highly undersaturated states in sea water but are highly enriched in
these marine ores, the so-called manganese nodules.  See  AUTHIGENIC MINERALS.
    Cation-exchange reactions between positively charged species in sea water
and such minerals as the marine clays and zeolites appear to regulate, at
least in part, the amounts of sodium, potassium, and magnesium, as well as
other members of the alkali and alkaline-earth metals which are not major
participants in mineral formations.
    Superimposed upon the chemical processes are changes in the chemical
makeup of sea water by the melting of ice, evaporation, mixing with runoff
waters from the continents, and upwelling of deeper waters. The net effects of
the first three processes are changes in the absolute concentrations of all of
the elements but with no major changes in the relative amounts of the
dissolved species.
 �34s[Edward D. Goldberg]
�33s
    In the open sea, all the organic matter is produced by the photosynthesis
and growth of unicellular planktonic forms. During this growth, all the
elements essential for living matter are obtained from the sea water. Some
elements are present in great excess, such as the carbon of CO2, the
potassium, and the sulfur (as sulfate). Other elements - for example,
phosphorus, nitrogen, and silicon - are present in small enough quantities so
that plant growth removes virtually all of the supply from the water. During
photosynthesis, as these elements are being removed from the water, oxygen is
released.  See	PHOTOSYNTHESIS.
    The organic matter formed by photosynthesis and growth of the unicellular
plants may be largely eaten by the zooplankton, and these in turn form the
food for larger organisms. At each step of the food chain a large proportion
of the eaten material is digested and excreted, and this, along with dead
organisms, is decomposed by bacterial action. The decomposition process
removes oxygen from the water and returns to the water those elements
previously absorbed by the phytoplankton.
    The distribution of oxygen and essential nutrient elements in the sea is
modified by the spatial separation of these biological processes.
Photosynthesis is limited to the surface layers of the ocean, generally no
more than 330 ft (100 m) or so of depth, but the decomposition of organic
material may take place at any depth. Reflecting this separation of processes,
the concentration of nutrient elements in the surface is low, rises to maximum
values at intermediate depths 990 - 2600 ft (300 - 800 m), and decreases
slightly to fairly constant values which extend nearly to the bottom.  See
SEA-WATER FERTILITY.
 �34s[Bostwick H. Ketchum]
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�6v�2v�32sSea-water fertility   �21m �33s 
    A measure of the potential ability of sea water to support life. Fertility 
is distinguished from productivity, which is the actual production of living 
material by various trophic levels of the food web. Fertility is a broader and 
more general description of the biological activity of a region of the sea, 
while primary production, secondary production, and so on, is a quantitative 
description of the biological growth at a specified time and place by a 
certain trophic level.  See  BIOLOGICAL PRODUCTIVITY. 
    The potential of the sea to support growth of living organisms is 
determined by the fertilizer elements that marine plants need for growth. 
Fertilizers, or inorganic nutrients as they are called in oceanography, are 
required only by the first trophic level in the food web, the primary 
producers; but the supply of inorganic nutrients is a fertility-regulating 
process whose effect reaches throughout the food web. When there is an 
abundant supply to the surface layer of the ocean that is taken up by marine 
plants and converted into organic matter through photosynthesis, the entire 
food web is enriched, including zooplankton, fish, birds, whales, benthic 
invertebrates, protozoa, and bacteria. 
    The elements needed by marine plants for growth are divided into two 
categories depending on the quantities required: The major nutrient elements 
that appear to determine variations in ocean fertility are nitrogen, 
phosphorus, and silicon. The micronutrients are elements required in extremely 
small, or trace, quantities including essential metals such as iron, 
manganese, zinc, cobalt, magnesium, and copper, as well as vitamins and 
specific organic growth factors such as chelators. The overall pattern of 
ocean fertility is set by the major fertilizer elements - nitrogen, 
phosphorus, and silicon - and not by micronutrients. 
    The regions of the world's oceans differ dramatically in overall 
fertility. Extremes exist because the overall pattern of fertility is 
determined by the processes that transport nutrients to the sunlit upper layer 
of the ocean where there is energy for photosynthesis.  See  SEA WATER. 
 �34s[Richard T. Barber]
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�6v�2v�32sSebaceous gland   �21m �33s 
    A gland which produces and liberates sebum, a mixture composed of fat, 
cellular debris, and keratin. When the gland arises in association with a hair 
follicle, it forms a thickened outpushing from the side of the developing 
follicle near the epidermis. Central cells in these sebaceous glands form oil 
droplets within the cytoplasm. These cells disintegrate to liberate the 
sebaceous substance and are therefore of the holocrine type. The Meibomian or 
tarsal glands, within the tarsus or supporting plate at the edge of the 
eyelids, are sebaceous and complex tubuloacinous structures. The numerous 
separate glands open along the entire edge of the upper and lower lids. 
Retained secretions of the tarsal glands produce a chalozion or Meibomian 
cyst.  See  GLAND. 
 �34s[Olin E. Nelsen]
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�6v�2v�32sSecondary emission   �21m �33s 
    The emission of electrons from the surface of a solid into vacuum caused 
by bombardment with charged particles, in particular with electrons. The 
mechanism of secondary emission under ion bombardment is quite different from 
that under electron bombardment; it is only in the latter case that the term 
secondary emission is generally used. 
    The bombarding electrons and the emitted electrons are referred to, 
respectively, as primaries and secondaries. Secondary emission has important 
practical applications because the secondary yield, that is, the number of 
secondaries emitted per incident primary, may exceed unity. Thus, secondary 
emitters are used in electron multipliers, especially in photomultipliers, and 
in other electronic devices such as television pickup tubes, storage tubes for 
electronic computers, and so on. 
    The emission of secondary electrons can be described as the result of 
three processes: (1) excitation of electrons in the solid into high-energy 
states by the impact of high-energy primary electrons, (2) transport of these 
secondary electrons to the solid-vacuum interface, and (3) escape of the 
electrons over the surface barrier into the vacuum. The efficiency of each of 
these three processes, and hence the magnitude of the secondary emission yield 
DELTA, varies greatly for different materials. 
    Most of the materials used in practical devices are semiconductors or 
insulators whose band-gap energies are much larger than their electron 
affinities. Examples are magnesium oxide (MgO), beryllium oxide (BeO), cesium 
antimonide (Cs3Sb), and potassium chloride (KCl). Maximum DELTA values in the 
8 - 15 range are typically obtained at primary energies of several hundred 
volts.  
    In certain semiconductors the bands are bent downward to such an extent 
that the vacuum level lies below the bottom of the conduction band in the 
bulk. A material with this characteristic is said to have negative effective 
electron affinity. The most important material in this category is 
cesium-activated gallium phosphide, GaP(Cs). The illustration shows the curve 
of yield DELTA versus primary energy Esubp for GaP(Cs) by comparison with MgO. 
Values of DELTA exceeding 100 are readily obtained, with maximum yields at 
energies in the 5 - 10-kV region. See BAND THEORY OF SOLIDS; SEMICONDUCTOR. 
 �34s[Alfred H. Sommer]
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�6v�2v�32sSecondary ion mass spectrometry (SIMS)   �21m �33s 
    A technique for microchemical analysis that permits the visualization of a 
three-dimensional chemical image of the solid state using primary ions. It is 
also referred to as ion probing. 
    The ion probing of a material is accomplished by bombarding the sample 
with a 5 - 20-keV beam of primary ions. This process sputters off the surface 
layers of the sample, producing a variety of secondary species including 
neutral atoms and molecules, secondary electrons, photons, and positive and 
negative ions. Mass spectrometric analysis of the positive and negative 
sputtered secondary ions can provide microchemical characterization with 
lateral resolution better than that of the electron probe and can produce 
sampling depths similar to those of the ion scattering and Auger electron 
spectrometry techniques. In addition, mass spectrometric analysis provides 
full elemental coverage from hydrogen to uranium, including the capability of 
isotopic characterization. This combination of ion-bombardment excitation and 
mass spectrometric detection can provide detectabilities in the range of 
10-sup1sup5 to 10-sup1sup9 g.  See  ELECTRON SPECTROSCOPY; MASS SPECTROMETRY; 
MASS SPECTROSCOPE. 
    Like the electron microprobe, the ion probe is used to analyze samples on 
a microscale. Applications include the analysis of small mineral samples or 
patches in geological and lunar materials and the analysis of airborne 
particulates and semiconductor devices. The ion probe provides much better 
elemental sensitivity and broader elemental coverage than does the electron 
probe in these applications. In addition, the ion probe provides isotopic 
detection, which is unavailable with the electron probe. Another feature of 
the microanalytical capability of the ion probe is the ability to acquire 
secondary ion images of the elements present in the sample under study.  See  
ANALYTICAL CHEMISTRY. 
 �34s[C. A. Evans, Jr.]
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�6v�2v�32sSecond sound �21m �33s
    A type of wave propagated in the superfluid phase of liquid helium (helium
II) and in certain other substances under special conditions. The name is
misleading since second sound is not in any sense a sound wave, but a
temperature or entropy wave. In ordinary or first sound, pressure and density
variations propagate with very small accompanying variations in temperature;
in second sound, temperature variations propagate with no appreciable
variation in density or pressure.  See	LIQUID HELIUM; SUPERFLUIDITY.
    The two-fluid model of helium II provides further insight into the nature
of second sound. In this model the liquid can be described as consisting of
superfluid and normal components of densities RHO(sub s) and RHO(sub n),
respectively, such that the total density RHO = RHO(sub s) + RHO(sub n). The
superfluid component is frictionless and devoid of entropy; the normal
component has a normal viscosity and contains the entropy and thermal energy
of the system. In a temperature or second-sound wave, the normal and
superfluid flows are oppositely directed so that
 RHO(sub s)V(sub s) + RHO(sub n)V(sub n) = 0 where V(sub s) and V(sub n) are
the superfluid and normal flow velocities. Thus a variation in relative
densities of the two components, and hence a temperature fluctuation,
propagates with no change in total density or pressure. In a first-sound wave,
the two components move in phase, that is, V(sub n) NEARLY EQUAL V(sub s). 
    Theoretical predictions that second sound should exist in certain solid
dielectric crystals under suitable conditions have been confirmed
experimentally for solid helium single crystals at temperatures between 0.4
and 1.0 K (-459.0 and -457.9DGF).  See	DIELECTRICS.
    Another quite different class of materials can exhibit second sound. In
smectic A liquid crystals, when the wave vector is oblique with respect to the
layers of these ordered structures, a modulation of the interlayer spacing can
propagate at nearly constant density.  See  LIQUID CRYSTALS.
 �34s[Henry A. Fairbank]
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�6v�2v�32sSecretory structures (plant)   �21m �33s 
    Cells or organizations of cells which produce a variety of secretions. The 
secreted substance may remain deposited within the secretory cell itself or 
may be excreted, that is, released from the cell. Substances may be excreted 
to the surface of the plant or into intercellular cavities or canals. Some of 
the many substances contained in the secretions are not further utilized by 
the plant (resins, rubber, tannins, and various crystals), while others take 
part in the functions of the plant (enzymes and hormones). Secretory 
structures range from single cells scattered among other kinds of cells to 
complex structures involving many cells; the latter are often called glands. 
    Epidermal hairs of many plants are secretory or glandular. Such hairs 
commonly have a head composed of one or more secretory cells borne on a stalk. 
The hair of a stinging needle is bulbous below and extends into a long, fine 
process above. If one touches the hair, its tip breaks off, the sharp edge 
penetrates the skin, and the poisonous secretion is released. 
    Glands secreting a sugary liquid - the nectar - in flowers pollinated by 
insects are called nectaries. Nectaries may occur on the floral stalk or on 
any floral organ: sepal, petal, stamen, or ovary. 
    The hydathode structures discharge water - a phenomenon called guttation - 
through openings in margins or tips of leaves. The water flows through the 
xylem to its endings in the leaf and then through the intercellular spaces of 
the hydathode tissue toward the openings in the epidermis. Strictly speaking, 
such hydathodes are not glands because they are passive with regard to the 
flow of water. 
    Some carnivorous plants have glands that produce secretions capable of 
digesting insects and small animals. These glands occur on leaf parts modified 
as insect-trapping structures. In the sundews (Drosera) the traps bear stalked 
glands, called tentacles. When an insect lights on the leaf, the tentacles 
bend down and cover the victim with a mucilaginous secretion, the enzymes of 
which digest the insect.  See  INSECTIVOROUS PLANTS; VENUS' FLYTRAP. 
    Resin ducts are canals lined with secretory cells that release resins into 
the canal. Resin ducts are common in gymnosperms and occur in various tissues 
of roots, stems, leaves, and reproductive structures. 
    Gum ducts are similar to resin ducts and may contain resins, oils, and 
gums. Usually, the term gum duct is used with reference to the dicotyledons, 
although gum ducts also may occur in the gymnosperms. 
    Oil ducts are intercellular canals whose secretory cells produce oils or 
similar substances. Such ducts may be seen, for example, in various parts of 
the plant of the carrot family (Umbelliferae). 
    Laticifers are cells or systems of cells containing latex, a milky or 
clear, colored or colorless liquid. Latex occurs under pressure and exudes 
from the plant when the latter is cut. 
 �34s[Katherine Esau]
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�6v�2v�32sSedation   �21m �33s 
    A state of decreased nervous activity induced by a variety of drugs. A 
quiet environment, an unworried mind, and relaxed musculature enhance the drug 
effects. Most of the drugs used for sedation in the early days of pharmacology 
also caused drowsiness and were therefore called hypnotic sedatives.  See  
BARBITURATES; BELLADONNA; OPIATES. 
    The two most popular groups of currently used antianxiety drugs are the 
propamediol carbamates (meprobamate and its congeners) and the benzodiazepines 
(diazepam and its congeners). The barbiturates and the histamines, which also 
have sedative properties, continue to be used. All these drugs act on the 
central nervous system, but their sites and mode of action are not fully 
understood.  
    Sedatives and tranquilizers are widely used in medicine to allay 
apprehensiveness, anxiety, irritability, and strong emotional reactions such 
as occur to some degree during the menopause in women and, more prominently, 
in anxiety states.  See  TRANQUILIZER. 
 �34s[Raymond D. Adams]
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�6v�2v�32sSedentaria   �21m �33s 
    A group of 28 or more families of polychaete annelids in which the 
anterior, or cephalic, region is more or less completely concealed by 
overhanging peristomial structures, or the body is divided into an anterior 
thoracic and a posterior abdominal region; the pharynx or proboscis is usually 
soft and epithelial, lacking hard jaws or paragnaths.  See  POLYCHAETA. 
 �34s[Olga Hartman]
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�6v�2v�32sSedimentary rocks   �21m �33s 
    One of the three major groups of rocks that make up the crust of the 
Earth, the other two being igneous and metamorphic. Most sedimentary rocks are 
layered, and, as is implied by the name, have originated by the sedimentation, 
or settling, of particles. Thus layering, or stratification, is the most 
important single characteristic of sediments and sedimentary rocks, even 
though there are some igneous and metamorphic rocks that show some kind of 
stratification or pseudostratification.  See  IGNEOUS ROCKS; METAMORPHIC 
ROCKS; SEDIMENTATION (GEOLOGY). 
    The distinction between sedimentary and other rocks is understood best by 
considering their origin. Sediments are formed at or very near the surface of 
the Earth as a result of processes operating at the surface, at normal surface 
temperatures and pressures. Most igneous and metamorphic rocks, on the other 
hand, are formed as a result of conditions deep in the crust of the Earth, 
where temperatures and pressures may be very high. Some overlapping between 
the three rock families exists; for example, it is difficult to classify rocks 
that originate as volcanic ash falls (igneous) but are then transported and 
become interlayered with normal sediments. 
    Though sedimentary rocks account for only 5% of the Earth's outer crust, 
they make up 75% of the exposed rocks at the surface. In general, sediments 
are distributed as a rather thin layer at the surface. The thickness of this 
thin layer may vary greatly from place to place; the thickness of the total 
sedimentary volume may vary from a few tens of feet to well over 30,000 ft 
(9000 m) in some places where the crust is subsiding rapidly. Sedimentary 
rocks are important as the source of many major mineral resources, notably 
coal, oil and gas, iron ores, and limestone. 
    The three most abundant kinds of sedimentary rocks, shale, sandstone, and 
limestone, together account for over 95% of all sediments. 
  
    Origin.  Sedimentary rocks originate primarily as the result of the 
fragmentation and destruction of preexisting rocks. As rocks are weathered by 
the action of water, wind, frost, and organic decay, large masses become 
mechanically broken into finer sizes and some of the constituents dissolve in 
rain or soil water. The solid fragments, ions in solution, and colloids in 
suspension, are transported, primarily by running water and secondarily by 
wind and groundwater, from the site of weathering, the source area, to this 
site of deposition. Transportation of detritus may be temporarily interrupted 
by sedimentation in streams or lakes, resulting in river bars, alluvial fans, 
or lake deltas. Eventually, however, most of the material reaches the site of 
the lowest gravitational potential energy on the Earth's surface, the bottom 
of the sea. After final deposition the soft, water-saturated muds, silts, and 
sands become buried under successive layers of later sediment, the water is 
squeezed out, the sediments become compacted, and chemical changes result in 
cementing the original unconsolidated material to a rock.  See  DIAGENESIS. 
    Sedimentary petrologists commonly divide the sedimentary rocks into two 
large groups, the detrital and the chemical. The detrital (sometimes called 
clastic) rocks are formed by the sedimentation of mineral or rock fragments 
that were derived from the mechanical disintegration of preexisting rocks in 
the source area and have been transported, as solids, to the site of 
deposition. The chemical rocks originate as chemical precipitates at the site 
of deposition; they may be inorganic precipitates formed from supersaturated 
solutions or they may be formed by the biochemical action of organisms, as are 
the calcium carbonate shells of mollusks.  See  PETROLOGY; ROCK; SEDIMENTATION 
(GEOLOGY).  
  
    Textural characteristics.  Textural analysis has led to a fuller 
understanding of the genetic factors involved in the formation of sediment by 
settling of particles through a fluid (water) or gaseous (air) medium, for the 
textures are directly related to the hydrodynamics of the medium. 
    Perhaps the most important textural property is the size of individual 
particles. Size of particles is the basis for the division of the detrital 
rocks into lutite, or shale (fine); arenite, or sandstone (medium); and 
rudite, or conglomerate (coarse). 
    In addition to the size distribution, textural analysis includes the study 
of the shape and roundness of the particles. Shape is defined as the degree of 
sphericity, or approach to a sphere. Roundness is defined as the degree of 
sharpness of corners or edges of a particle. Another element of texture is the 
surface configuration of the grains. 
  
    Chemical composition.  Chemical composition of rocks is expressed in terms 
of the oxides of the elements. Determinations are made normally by wet 
chemical analysis and the assumption is made that the elements are present as 
oxides, the oxygen not being determined directly. The average sedimentary rock 
contains about 58% silicon dioxide (SiO2), 13% aluminum oxide (Al2O3), 6% 
calcium oxide (CaO), 5% ferrous oxide (FeO) and ferric oxide (Fe2O3), 5% 
carbon dioxide (CO2), and smaller amounts of magnesium oxide (MgO), potassium 
oxide (K2O), sodium oxide (Na2O), and many trace elements. This average 
sediment may be thought of as a combination of sandstone, shale, and limestone. 
  
    Sedimentary structures.  These are the larger textural features of 
sediments that were formed during or shortly after deposition, as distinct 
from the still larger elements of structure, folds and faults, which were 
produced much later than deposition. Sedimentary structures include the 
mechanical (such as bedding and ripple marks), made by the currents that 
transport sediment; the chemical (such as oolites, concretions, and nodules), 
produced by inhomogeneous precipitation; and the organic (such as fossils), 
produced by organisms living in the environment.  See  STRUCTURAL GEOLOGY. 
  
    Classification and nomenclature.  Sedimentary rocks have been classified 
on two bases, the purely descriptive, and the genetic. The disadvantage of the 
purely descriptive approach is that rocks of widely divergent origin might be 
lumped together. The disadvantage of the genetic approach is that the origin 
of the rock must be known before it can be classified. 
    The more fruitful approach has been to combine the two into a 
classification based on objective descriptive characteristics that have some 
genetic significance. Using this as a basis, geologists subdivide the 
sedimentary rocks into two broad groups, the clastic or detrital, and the 
nonclastic or chemically precipitated rocks. The clastic rocks are 
characterized by individual grains, often of heterogeneous composition, 
derived from the erosion of preexisting rocks; the grains may be more or less 
rounded by abrasion during transport from the erosional area to the site of 
deposition. The chemical precipitates are normally fairly homogeneous in 
composition and are characterized by an interlocking crystalline texture, 
where the crystal sizes may vary greatly.  See  ARENACEOUS ROCKS; ARGILLACEOUS 
ROCKS; CHERT; CONGLOMERATE; LIMESTONE; SANDSTONE; SHALE; WEATHERING PROCESSES. 
 �34s[Raymond Siever]
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�6v�2v�32sSedimentation (geology)   �21m �33s 
    The processes that operate at or near the Earth's surface to deposit 
rock-forming material, or sediment. Sedimentation includes the weathering 
processes that act mechanically and chemically to break up preexisting rocks, 
the processes of transportation (wind, ice, and water) by which the material 
is carried from its source to the depositional site, the processes of 
deposition in the sedimentary environment, and the postdepositional processes 
or diagenesis by which the sediment is compacted and hardened to rock.  See  
DIAGENESIS; STREAM TRANSPORT AND DEPOSITION; WEATHERING PROCESSES. 
 �34s[Raymond Siever]
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�6v�2v�32sSedimentation (industry)   �21m �33s 
    A process based on the settling of solid particles through a liquid. 
Sedimentation is used in several ways in industrial processes. It may be used 
to obtain a concentrated slurry from a dilute suspension of a solid in a 
liquid. This is called thickening. It may also be used to remove solid 
particles from a liquid to obtain a clear supernatant liquid. This is called 
clarification. The driving force for the process is the difference in density 
between the solid and liquid. Ordinarily, sedimentation is accomplished by the 
force of gravity, and the liquid is water or an aqueous solution. When the 
particles are fine, or when the density difference is small, gravity settling 
may be too slow to be practicable; then centrifugal force can be used in place 
of gravity. When centrifugal force is not adequate, the more positive method 
of filtration may be used. All these methods of treating solids and liquids 
fall into the generic group of mechanical separations.  See  CENTRIFUGATION. 
    Particles too small to be settled at practicable rates are often amenable 
to flocculation, accomplished by adding agents such as sodium silicate, alum, 
lime, and alumina. The particles agglomerate into coarse flocs, which act like 
single large particles, settle at a practicable speed, and leave a clear 
supernatant liquid behind.  See  CLARIFICATION; FLOCCULATION; MECHANICAL 
SEPARATION TECHNIQUES; THICKENING. 
 �34s[Warren L. McCabe]
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�6v�2v�32sSeebeck effect   �21m �33s 
    A thermoelectric phenomenon discovered in 1821 by T. J. Seebeck. He found 
that near a closed circuit composed of two linear conductors of different 
metals, a magnetic needle would be deflected if, and only if, the two 
junctions were at different temperatures; and, further, that if the cooler 
junction were to become the warmer, the direction of deflection would be 
reversed.  
    The Seebeck electromotive force which gives rise to the current is 
directly measurable with a voltmeter or potentiometer. It depends only upon 
the material and temperatures. 
 �34s[John W. Stewart]
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�6v�2v�32sSeed   �21m �33s 
    A fertilized ovule containing an embryo which forms a new plant upon 
germination. Seed-bearing characterizes the higher plants - the gymnosperms 
(conifers and allies) and the angiosperms (flowering plants). Gymnosperm 
(naked) seeds arise on the surface of a structure, as on a seed scale of a 
pine cone. Angiosperm (covered) seeds develop within a fruit, as the peas in a 
pod.  See  FLOWER; FRUIT. 
  
    Structure.  One or two tissue envelopes, or integuments, form the seed 
coat which encloses the seed except for a tiny pore, the micropyle (see 
illustration). The micropyle is near the funiculus (seed stalk) in angiosperm 
seeds. The hilum is the scar left when the seed is detached from the 
funiculus. The embryo consists of an axis and attached cotyledons (seed 
leaves). The part of the axis above the cotyledons is the epicotyl (plumule); 
that below, the hypocotyl, the lower end of which bears a more or less 
developed primordium of the root (radicle). The epicotyl, essentially a 
terminal bud, possesses an apical meristem (growing point) and, sometimes, 
leaf primordia. The seedling stem develops from the epicotyl. An apical 
meristem of the radicle produces the primary root of the seedling, and 
transition between root and stem occurs in the hypocotyl.  See  APICAL 
MERISTEM; MAGNOLIOPHYTA; PINOPHYTA; ROOT; STEM. 
    Two to many cotyledons occur in different gymnosperms. The angiosperms are 
divided into two major groups according to number of cotyledons: the 
monocotyledons and the dicotyledons. Mature gymnosperm seeds contain an 
endosperm which surrounds the embryo. In some mature dicotyledon seeds the 
endosperm persists, the cotyledons are flat and leaflike, and the epicotyl is 
simply an apical meristem. In other seeds, such as the bean, the growing 
embryo absorbs the endosperm, and food reserve for germination is stored in 
fleshy cotyledons. The endosperm persists in common monocotyledons, for 
example, corn and wheat, and the cotyledon, known as the scutellum, functions 
as an absorbing organ during germination. Grain embryos also possess a 
coleoptile and a coleorhiza sheathing the epicotyl and the radicle, 
respectively. The apical meristems of lateral seed roots also may be 
differentiated in the embryonic axis near the scutellum of some grains. 
  
    Dispersal.  Mechanisms for seed dispersal include parts of both fruit and 
seed. Some dry fruits have membranous air sacs which aid in seed dispersal by 
water. In cockelburrs and beggarticks the seeds are enclosed in spiny, barbed 
fruits which are readily carried in the fur of animals. Seed dispersal by wind 
is aided by hair tufts of dandelion achenes, wings of elm and maple fruits and 
of conifer seeds, and by seed-coat hairs of willow, cottonwood, and milkweed 
seeds. Cotton fibers consist of greatly elongated epidermal hairs of the seed 
coat as do also the kapok fibers of the silk-cotton tree (Ceiba pentandra). 
 �34s[Roger M. Reeve]
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�6v�2v�32sSeiche   �21m �33s 
    A short-period oscillation in an enclosed or semienclosed body of water, 
analogous to the free oscillation of water in a dish. The initial displacement 
of water from a level surface can arise from a variety of causes, and the 
restoring force is gravity, which always tends to maintain a level surface. 
Once formed, the oscillations are characteristic only of the geometry of the 
basin itself and may persist for many cycles before decaying under the 
influence of friction.  See  WAVE MOTION IN LIQUIDS. 
    The most straightforward example of a seiche is that occurring in a long, 
narrow lake of uniform depth; in this case the water surface elevations have a 
simple physical analogy with the vibrations of a plucked string. Seiches are 
also a common feature of semienclosed basins such as bays, gulfs, and harbors, 
the natural period for an open-end rectangular basin of uniform depth being 
twice that of the same sized basin closed at both ends. A problem of great 
practical significance is the oscillation of water in harbors, which can 
generate strong and unpredictable currents affecting the safety of ships 
entering or leaving the harbor; under extreme conditions these can also cause 
damage to moored vessels. 
    A seiche can be generated when the water is subject to changes in wind or 
atmospheric pressure gradients or, in the case of semienclosed basins, by the 
oscillation of adjacent connected water bodies having a periodicity close to 
that of the seiche or of one of its harmonics. Other, less frequent causes of 
seiches include heavy precipitation over a portion of the lake, flood 
discharge from rivers, seismic disturbances, and submarine mudslides or slumps. 
 �34s[David M. Farmer]
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�6v�2v�32sSeismic risk   �21m �33s 
    The probability that social or economic consequences of earthquakes will 
equal or exceed specified values at a site, at several sites, or in an area, 
during a specified exposure time. 
    Although the term seismic risk is occasionally used in a general sense to 
mean the potential for both the occurrence of natural phenomena and the 
economic and life loss associated with earthquakes, it is useful to 
differentiate between the concepts of seismic hazard and seismic risk. Seismic 
hazard may be defined as any physical phenomena that result either from 
surface faulting during shallow earthquakes or from the ground shaking 
resulting from an earthquake and that may produce adverse effects on human 
activities.  
    The exposure time is the time period of interest for seismic hazard or 
risk calculations. In practical applications, the exposure time may be 
considered to be the design lifetime of a building or the length of time over 
which the numbers of casualties will be estimated. 
 �34s[S. T. Algermissen]
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�6v�2v�32sSeismic stratigraphy   �21m �33s 
    Determination of the nature of sedimentary rocks and their fluid content 
from analysis of seismic data. Seismic stratigraphy is divided into 
seismic-sequence (facies) analysis and reflection-character analysis. 
    In seismic-sequence analysis the first step is to separate 
seismic-sequence units, also called seismic-facies units. This is usually done 
by mapping unconformities where they are shown by angularity. Angularity below 
an unconformity may be produced by erosion at an angle across the former 
bedding surfaces or by toplap (offlap), and angularity above an unconformity 
may be produced by onlap or downlap, the latter distinction being based on 
geometry. The unconformities are then followed along reflections from the 
points where they cannot be so identified, advantage being taken of the fact 
that the unconformity reflection is often relatively strong. The procedure 
often followed is to mark angularities in reflections by small arrows before 
drawing in the boundaries.  See  UNCONFORMITY. 
    Reflection-character analysis may be based on information from boreholes 
which suggests that a particular interval may change nearby in a manner which 
increases its likelihood to contain hydrocarbon accumulations. Lateral changes 
in the wave shape of individual reflection events may suggest where the 
stratigraphic changes or hydrocarbon accumulations may be located.  See  
BORING AND DRILLING (MINERAL); PETROLEUM PROSPECTING. 
    Where sufficient information is available to develop a reliable model, 
expected changes are postulated and their effects are calculated and compared 
to observed seismic data. The procedure is called synthetic seismogram 
manufacture; it usually involves calculating seismic data based on sonic and 
density logs from boreholes, sometimes based on a model derived in some other 
way. The sonic and density data are then changed in the manner expected for a 
postulated stratigraphic change, and if the synthetic seismogram matches the 
actual seismic data sufficiently well, it implies that the changes in earth 
layering are similar to those in the model.  See  GEOPHYSICAL EXPLORATION; 
SEDIMENTARY ROCKS; SEISMOGRAPHIC INSTRUMENTATION; SEISMOLOGY; STRATIGRAPHY. 
 �34s[Robert E. Sheriff]
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�6v�2v�32sSeismographic instrumentation �21m �33s
    A variety of devices or systems of devices for measuring the motion of the
Earth, generally as a result of the passing of seismic waves, but also as a
result of earth tides or deformation in response to tectonic forces. Typically
consisting of a sensor (seismometer), a signal conditioning element or
elements (galvanometer, mechanical or electronic amplifier, filters,
analog-to-digital conversion circuitry, telemetry modules, and so on), and a
recording system (analog visible or direct, FM, or digital magnetic tape or
disk), seismographs find a wide range of application, from industrial
vibration measurement through petroleum exploration and earthquake studies to
investigations of the Earth's gravity field.
    The seismograph records a measure of ground motion as a function of time.
The seismoscope is a simpler type of device which indicates only the
occurrence of relatively strong ground shaking, not its time of occurrence or
duration.
    The seismometer is the basic sensing element in seismographic instruments,
and is found in one of two fundamentally different types, termed inertial and
strain. The inertial seismometer (Fig. 1) generates an output signal
proportional to the relative motion between its frame (usually attached to the
ground or the point of interest) and an internal inertial reference mass. The
strain seismometer (Fig. 2), or, better, the linear extensometer, is an
instrument capable of providing a continuous precise measurement of the
distance between two points, spaced typically 3 ft to 0.6 mi (1 m to 1 km)
apart.
    A variation on the linear extensometer is the tiltmeter. Here change in
the relative elevations of two points is monitored, usually with respect to a
liquid-level surface. Horizontal distance between the reference points may be
as little as a fraction of an inch or as large as several hundred feet.
Tiltmeters are used primarily in monitoring long-term deformation of the
Earth's surface due to geological processes, with particular application to
investigations of possible precursory phenomena to earthquakes and volcanic
eruptions.
    The gravimeter is no more than a vertical-component accelerometer, that
is, a pendulum sensing vertical ground motion and equipped with a displacement
transducer, analogous to the above inertial tiltmeter. Gravimeters are widely
used in geophysical exploration, in the study of earth tides, and in the
recording of very-low-frequency (0.003 to 0.01 Hz) seismic waves from
earthquakes.
    The complete seismograph produces a record of the properly conditioned
signal from the seismometer, along with appropriate timing information. The
recording system may be as simple as a mechanical stylus scratching a line on
a smoke-covered chart in a portable microearthquake seismograph, or as complex
as a high-speed multichannel computer-controlled system handling 25,000 18-bit
digital words per second.  See	EARTHQUAKE; GEOPHYSICAL EXPLORATION;
SEISMOLOGY.
 �34s[Thomas U. McEvilly]
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�6v�2v�32sSeismology   �21m �33s 
    The science of strain-wave propagation in the planets and their naturally 
occurring satellites. Strain waves propagate in the interior from a source and 
are recorded by seismographs distributed over the surface. From wave analysis 
it is possible to infer the subsurface structure and sometimes the mechanism 
of the source. See SEISMOGRAPHIC INSTRUMENTATION. 
  
    Earthquake.  An earthquake is the abrupt release of strain waves through 
the action of natural forces within the Earth. The depth of occurrence of the 
earthquake focus (that is, the region of the Earth at which the earthquake 
originates) provides a basis for classification. Shallow-focus earthquakes 
occur at depths down to 36 mi (60 km), intermediate focus at 36 - 180 mi (60 - 
300 km), and deep focus at 180 - 420 mi (300 - 700 km). Most earthquakes have 
shallow foci. 
    The size of an earthquake can be viewed in two ways, either subjectively 
by considering the intensity of shaking at a given location, or objectively by 
considering the magnitude of the earthquake source. A commonly used intensity 
scale is that of Medvedev, Sponheuer, and Karnik (msk scale). Observed shaking 
levels are thereby characterized numerically from I (not felt, but 
instrumentally observed) to XII (landscape permanently changed, and almost all 
human-made structures totally destroyed). 
    The magnitude scale is instrumental and characterizes the earthquake 
magnitude quantitatively, independently of individual sensations, and in a 
manner roughly related to the total energy output from the earthquake focus. 
Its development was originally the work of C. F. Richter in 1935, and it is 
still usually associated with his name despite modifications and extensions by 
others which have taken place since.  See  EARTHQUAKE. 
  
    Propagation theory.  Understanding of seismic phenomena in the Earth 
relies heavily on the theory of strain propagation in solids. Observational 
experience indicates that the stress-strain relationship of the Earth for 
infinitesimal strains and seismic frequencies is highly elastic (with the 
possible exception of a surficial weathered layer less than 180 mi or 300 km 
thick) so that Hooke's law is applicable for seismic propagation theory to a 
high degree of approximation.  See  HOOKE'S LAW. 
    In a large solid body made up of regions which are individually 
homogeneous and isotropic, there can occur irrotational waves (sometimes 
called dilational or compressional waves) and equivoluminal waves. Since the 
velocity of the irrotational waves is always greater than that of the 
equivoluminal waves, irrotational waves arrive first and are designated by the 
symbol P (for primus), while equivoluminal waves coming in second are designed 
S (secundus). These are the basic body-wave types. Particle displacement in P 
is orthogonal to the wavefront whereas in S it is parallel (see illustration). 
    Considerations so far have been limited to the interior of an elastic 
solid. For seismic purposes the boundary of greatest interest is the free 
surface of the Earth. Special wave types called Rayleigh waves are 
characterized by particle trajectories which are retrograde ellipses; that is, 
when the particle is at the top of its ellipse, it is moving in a direction 
opposite to that of the propagation of the wave (see illustration). There is 
no component of particle motion which is both transverse to the ray path and 
parallel to the free surface of the solid supporting the Rayleigh-wave 
propagation. The speed of these waves is usually about 0.9 of the shear-wave 
velocity in the medium. 
    If the boundary value problem under consideration is further complicated 
by the introduction of a layer over the half space, an additional wave type 
will be found to propagate within the upper layer but with its particle motion 
horizontally polarized with respect to the bedding planes (see illustration). 
The Love wave is dispersive with phase velocities lying between the shear 
velocities for the two media involved. 
  
    Elastic waves incident upon an interface between two media having 
differing elastic properties undergo reflection and refraction analogously to 
electromagnetic and acoustic waves. In the elastic case the phenomena are more 
complicated because of the occurrence of wave conversions from P to S at the 
interface.  See  REFRACTION OF WAVES. 
    With the application of digital computers to seismic problems it has 
become feasible to study wave propagation in multiple horizontally layered 
structures with a large number of layers. Using available computational 
programs, it is possible to predict the seismogram which would result from an 
Earth model consisting of multiple horizontal layers or from an unlayered 
sphere, either liquid or solid. 
  
    Earth's interior.  The primary application of seismology has been to 
determine the internal structure of the Earth. In seismic prospecting the 
Earth is excited by explosives, dropping weights, eccentric wheels, and so 
forth, and the reflections and refractions of the seismic energy produced are 
observed and analyzed in terms of the subsurface structure. The motivation is 
usually the determination of sedimentary formations favorable for the 
occurrence of natural deposits of commercially valuable materials (usually 
petroleum).  See  GEOPHYSICAL EXPLORATION. 
    The Earth behaves like a low-pass filter to seismic waves so that at 
longer recording times, corresponding roughly to deeper penetration of the 
waves into the Earth, the waves will be longer than at shorter observation 
times, corresponding to closer proximity to the source and less penetration 
into the Earth. As a consequence, the structural detail available about the 
Earth from seismology is somewhat greater at shallower depths than nearer the 
Earth's center. 
  
    Monitoring nuclear explosions.  An important research goal of seismology 
has been the development of adequate techniques for distinguishing underground 
nuclear explosions from earthquakes. The problem of remote monitoring of 
underground nuclear explosions by the detection and analysis of the seismic 
signals they produce has been reduced essentially to the questions of how weak 
a signal can be detected with confidence and whether the discrimination 
criteria for separating explosions from earthquakes, which have been 
established for moderate and strong events, are applicable to those yielding 
low levels of radiated energy. Seismic monitoring consists of three steps: 
detection of the signal from an event, location of the source, and 
identification of the source as an earthquake or an explosion. 
 �34s[Ker Clive Thomson]
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�6v�2v�32sSeisonacea   �21m �33s 
    A class of the phylum Rotifera which comprises a group of little-known 
marine animals. They form a single family with about seven species and are 
found only in Europe. The Seisonacea are epizoic or possibly ectoparasitic on 
crustacea. They have a very elongated jointed body with a small head; a long, 
slender neck region; a thicker, fusiform trunk; and an elongated foot, 
terminating in a perforated disk (see illustration). The Seisonacea are larger 
than other rotifers, attaining sizes up to 0.12 in. (33 mm) in length.  See  
ROTIFERA.  
 �34s[Elbert H. Ahlstrom]
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�6v�2v�32sSelachii   �21m �33s 
    An order that includes all fossil and Recent sharks, with the possible 
exception of Devonian cladoselachians. The selachian endoskeleton is entirely 
cartilaginous, but it is frequently calcified superficially. The body is 
covered in minute scales, which in Recent sharks are simple and nongrowing, 
but which in earlier sharks often fused together and became enlarged during 
life and may have been more regularly arranged over the body. 
    Sharks periodically shed their teeth, sometimes replacing several thousand 
in a lifetime, and their teeth are common in the fossil record. It is 
therefore rather surprising that the earliest records of sharks are from the 
Late Devonian, when they were already highly diversified. 
    There are 5 groups of fossil and living sharks: cladoselachians, 
symmoriids, ctenacanthids, hybodontids, and neoselachians.  See  
CHONDRICHTHYES; SHARK. 
 �34s[John G. Maisey]
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�6v�2v�32sSelaginellales   �21m �33s 
    The plant order of small club mosses, containing only one living genus, 
Selaginella. Of the approximately 700 species, most live in the tropics, 
although a few species give the genus a worldwide distribution in moist, shady 
habitats. One species, the ``resurrection plant'' (S. lepidophylla), is 
tolerant of xeric (dry) conditions, reacting to the absence of water by 
contracting into a ball-shaped mass which unrolls when moisture becomes 
available. A few species are cultivated for their ornamental value. 
    The order has many characteristics of other members of the phylum 
Lycopsida. It differs mainly from the order Lycopodiales in having two sizes 
of spores and a special, small membranous outgrowth called a ligule which is 
borne on the upper or adaxial base of the leaf. The sex organs always develop 
on separate gametophytes, and the sporangia always occur on clusters of 
sporophylls collectively called strobili. Arising at the base of each leafless 
branch is a rootlike organ called a rhizophore which bears adventitious roots 
at its tip.  See  LYCOPODIALES; ROOT. 
 �34s[Paul A. Vestal]
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�6v�2v�32sSelection rules (physics) �21m �33s
    General rules concerning the transitions which may occur between the
states of a quantum-mechanical physical system. They derive in almost all
cases from the symmetry properties of the states and of the interaction which
gives rise to the transitions. The system may have a classical (nonquantum)
counterpart, and in this case the selection rules may often be related to the
classical conserved quantities. A first use of selection rules is in
determining the symmetry classes of the states; but in a great variety of ways
they may yield other information about the system and the conservation laws.
See  QUANTUM MECHANICS; SYMMETRY LAWS (PHYSICS).
    For an isolated system the total angular momentum is a conserved quantity;
this fact derives from a fundamental fact of nature, namely, that space is
isotropic. Each state is then classifiable by angular momentum J and its z
component M (= -J, -J + 1, . . . , +J). Angular momenta combine in a vectorial
fashion. Thus, if the system makes a particle-emitting transition J1,M1 ->
J2,M2, the emitted particles must carry away angular momentum (j, MU), where j
= J1 - J2. This implies that MU = M1 - M2 and that j takes on values J1 - J2,
J1 - J2 + 1, . . . , (J1 + J2). Thus in transitions (J = 4 i J = 2) the
possible j values comprise only 2, 3, 4, 5, 6, and, if it is also specified
that M1 - M2 = PLUS/MINUS 4, only 4, 5, 6. Observe that J2 is additive.  See
ANGULAR MOMENTUM; QUANTUM NUMBERS.
    Another fundamental symmetry, the parity, which determines the behavior of
a system (or of its description) under inversion of the coordinate axes, is
conserved by the strong and electromagnetic interactions, and gives a
classification of systems as even (PI =+1) or odd (PI =-1). Under combination
the parity combines multiplicatively. Thus, if the transition above is
4 PLUS/MINUS -> 2 PLUS/MINUS, it follows that jPI = 2-, . . . , 6-, while
4 PLUS/MINUS -> 2 PLUS/MINUS would give jPI = 2+, . . . , 6+. The angular
momentum j may be a combination of intrinsic spin s and orbital angular
momentum 1. Scalar, pseudoscalar, vector, and pseudovector particles are
respectively characterized by s(sup PIs) = 0+, 0-, 1-, 1+, where PI(sub s), is
the ``intrinsic'' parity, while l always carries PIsubl = (-1)supl.  See
PARITY; SPIN (QUANTUM MECHANICS).
    The isospin symmetry of the elementary particles is almost conserved,
being broken by electromagnetic and weak interactions. It is described by the
group SU(2), of unimodular unitary transformations in two dimensions. Since
the SU(2) algebra is identical with that of the angular momentum SO(3),
isospin behaves like angular momentum with its three generators T replacing J.
  See  ISOTOPIC SPIN.
    The isospin group is a subgroup of SU(3) which defines a more complex
fundamental symmetry of the elementary particles. Two of its eight generators
commute, giving two additive quantum numbers, Tsubz and strangeness S' (or,
equivalently, charge and hypercharge). The strangeness is conserved (DELTAS' =
0) for strong and electromagnetic, but not for weak, interactions. The
selection rules and combination laws for SU(3) and its many extensions, and
the quark-structure underlying them, correlate an enormous amount of
information and make many predictions about the elementary particles.  See
BARYON; ELEMENTARY PARTICLE; MESON; QUARKS; UNITARY SYMMETRY.
    A great variety of other groups have been introduced to define relevant
symmetries for atoms, molecules, nuclei, and elementary particles. They all
have their own selection rules, representing one aspect of the symmetries of
nature.
 �34s[J. Bruce French]
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�6v�2v�32sSelectivity   �21m �33s 
    The ability of a radio receiver to separate a desired signal frequency 
from other signal frequencies, some of which may differ only slightly from the 
desired value. Selectivity is achieved by using tuned circuits that are 
sharply peaked and by increasing the number of tuned circuits. With a sharply 
peaked circuit, the output voltage falls off rapidly for frequencies 
increasingly lower or higher than that to which the circuit is tuned.  See  Q 
(ELECTRICITY ); RADIO RECEIVER; RESONANCE (ALTERNATING-CURRENT CIRCUITS). 
 �34s[John Markus]
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�6v�2v�32sSelenium �21m �33s
    A chemical element, Se, atomic number 34, atomic weight 78.96. The
properties of this element are similar to those of tellurium.  See  TELLURIUM.
    The abundance of this widely distributed element in the Earth's crust is
estimated to be about 7 x 10 (sup -5)% by weight, occurring as the selenides of
heavy elements and to a limited extent as the free element in association with
elementary sulfur. Selenium minerals do not occur in sufficient quantity,
however, to be useful as commercial sources of the element; instead,
selenium-bearing copper sulfide ores represent the primary sources.
    Major uses of selenium include the photocopying process of xerography, the
decolorization of glasses tinted by the presence of iron compounds, and use as
a pigment in plastics, paints, enamels, glass, ceramics, and inks. The use of
selenium in rectifiers has declined with increased use of silicon and
germanium for this purpose. Selenium is also employed in photographic exposure
meters and as a metallurgical additive to improve the machinability of certain
steels.
    Selenium burns in air with a blue flame to give selenium dioxide, SeO2.
The element also reacts directly with a variety of metals and nonmetals,
including hydrogen and the halogens. Nonoxidizing acids fail to react with
selenium, but nitric acid, concentrated sulfuric acid, and strong alkali
hydroxides dissolve the element.
    The only important compound of selenium with hydrogen is hydrogen
selenide, H2Se, a colorless flammable gas possessing a distinctly unpleasant
odor, and a toxicity greater and a thermal stability less than that of
hydrogen sulfide. Dissolved in water, hydrogen selenide can precipitate many
heavy-metal ions as very slightly soluble selenides. Organic compounds
containing C-Se bonds are numerous and vary from the simple selenols, RSeH,
selenenic acids, RSeOH, organyl selenium halides, RSeX, and diorganyl
selenides and diselenides, R2Se and R2Se2, to those molecules exhibiting
biological activity such as selenoamino acids and selenopeptides.
 �34s[John W. George]
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�6v�2v�32sSemaeostomeae   �21m �33s 
    An order of the class Scyphozoa including most of the common medusae. The 
umbrella of these medusae is more flat than high and is usually domelike. The 
margin of the umbrella is divided into many lappets. Sensory organs are 
situated between the lappets. The tentacles are generally well developed and 
very long except in a few forms. 
    The life history of this group shows the typical alternation of 
generations, with forms passing through several larval stages. The 
Semaeostomeae are distributed mostly in temperate zones and are generally 
coastal forms with a few exceptions. Some of them are known to have violent 
poison on their tentacles. Several fossils of this group were found in the 
strata of the Jurassic period.  See  SCYPHOZOA. 
 �34s[Tohru Uchida]
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�6v�2v�32sSemiconductor �21m �33s
    A solid crystalline material whose electrical conductivity is intermediate
between that of a metal and an insulator. Semiconductors exhibit conduction
properties that may be temperature-dependent, permitting their use as
thermistors, or voltage-dependent, as in varistors. By making suitable
contacts to a semiconductor or by making the material suitably inhomogeneous,
electrical rectification and amplification can be obtained. Semiconductor
devices, rectifiers, and transistors have replaced vacuum tubes almost
completely in low-power electronics, making it possible to save volume and
power consumption by orders of magnitude. In the form of integrated circuits,
they are vital for complicated systems. The optical properties of a
semiconductor are important for the understanding and the application of the
material. Photodiodes, photoconductive detectors of radiation, injection
lasers, light-emitting diodes, solar-energy conversion cells, and so forth are
examples of the wide variety of optoelectronic devices.  See  CRYOSAR;
INTEGRATED CIRCUITS; LASER; LIGHT-EMITTING DIODE; PHOTODIODE; PHOTOELECTRIC
DEVICES; SEMICONDUCTOR RECTIFIER; THERMISTOR; TRANSISTOR; VARISTOR.

    Conduction in semiconductors.  The electrical conductivity of
semiconductors ranges from about 10sup3 to 10-sup9 ohm-sup1 cm-sup1, as
compared with a maximum conductivity of 10sup7 for good conductors and a
minimum conductivity of 10-sup1sup7 ohm-sup1 cm-sup1 for good insulators.  See
  ELECTRIC INSULATOR.
    The electric current is usually due only to the motion of electrons,
although under some conditions, such as very high temperatures, the motion of
ions may be important. The basic distinction between conduction in metals and
in semiconductors is made by considering the energy bands occupied by the
conduction electrons.  See  BAND THEORY OF SOLIDS; IONIC CRYSTALS.
    At absolute zero temperature, the electrons occupy the lowest possible
energy levels, with the restiction that at most two electrons may be in the
same energy level. In semiconductors and insulators, there are just enough
electrons to fill completely a number of energy bands, leaving the rest of the
energy bands empty. The highest filled energy band is called the valence band.
The next higher band, which is empty at absolute zero temperature, is called
the conduction band. The conduction band is separated from the valence band by
an energy gap which is an important characteristic of the semiconductor. In
metals, the highest energy band that is occupied by the electrons is only
partially filled. This condition exists either because the number of electrons
is not just right to fill an integral number of energy bands or because the
highest occupied energy band overlaps the next higher band without an
intervening energy gap. The electrons in a partially filled band may acquire a
small amount of energy from an applied electric field by going to the higher
levels in the same band. The electrons are accelerated in a direction opposite
to the field and thereby constitute an electric current. In semiconductors and
insulators, the electrons are found only in completely filled bands, at low
temperatures. In order to increase the energy of the electrons, it is
necessary to raise electrons from the valence band to the conduction band
across the energy gap. The electric fields normally encountered are not large
enough to accomplish this with appreciable probability. At sufficiently high
temperatures, depending on the magnitude of the energy gap, a significant
number of valence electrons gain enough energy thermally to be raised to the
conduction band. These electrons in an unfilled band can easily participate in
conduction. Furthermore, there is now a corresponding number of vacancies in
the electron population of the valence band. These vacancies, or holes as they
are called, have the effect of carriers of positive charge, by means of which
the valence band makes a contribution to the conduction of the crystal.  See
HOLES IN SOLIDS.
    The type of charge carrier, electron or hole, that is in largest
concentration in a material is sometimes called the majority carrier and the
type in smallest concentration the minority carrier. The majority carriers are
primarily responsible for the conduction properties of the material. Although
the minority carriers play a minor role in electrical conductivity, they can
be important in rectification and transistor actions in a semiconductor.

    Intrinsic semiconductors.  A semiconductor in which the concentration of
charge carriers is characteristic of the material itself rather than of the
content of impurities and structural defects of the crystal is called an
intrinsic semiconductor. Electrons in the conduction band and holes in the
valence band are created by thermal excitation of electrons from the valence
to the conduction band. Thus an intrinsic semiconductor has equal
concentrations of electrons and holes. The carrier concentration, and hence
the conductivity, is very sensitive to temperature and depends strongly on the
energy gap. The energy gap ranges from a fraction of 1 eV to several
electron-volts. A material must have a large energy gap to be an insulator.

    Impurity semiconductors.  Typical semiconductor crystals such as germanium
and silicon are formed by an ordered bonding of the individual atoms to form
the crystal structure. The bonding is attributed to the valence electrons
which pair up with valence electrons of adjacent atoms to form so-called
shared pair or covalent bonds. These materials are all of the quadrivalent
type; that is, each atom contains four valence electrons, all of which are
used in forming the crystal bonds.  See  CRYSTAL.
    Atoms having a valence of +3 or +5 can be added to a pure or intrinsic
semiconductor material with the result that the +3 atoms will give rise
to an unsatisfied bond with one of the valence electrons of the semiconductor
atoms, and +5 atoms will result in an extra or free electron that is not
required in the bond structure. Electrically, the +3 impurities add holes and
the +5 impurities add electrons. They are called acceptor and donor
impurities, respectively. Typical valence +3 impurities used are boron,
aluminum, indium, and gallium. Valence +5 impurities used are arsenic,
antimony, and phosphorus.
    Semiconductor material ``doped'' or ``poisoned'' by valence +3
acceptor impurities is termed p-type, whereas material doped by valence +5
donor material is termed n-type. The names are derived from the fact that the
holes introduced are considered to carry positive charges and the electrons
negative charges. When the carrier concentration is predominantly determined
by the impurity content, the conduction of the material is said to be
extrinsic.

    Materials.	 The group of chemical elements which are semiconductors
includes germanium, silicon, gray (crystalline) tin, selenium, tellurium, and
boron. Germanium, silicon, and gray tin belong to group IV of the periodic
table and have crystal structures similar to that of diamond. Germanium and
silicon are two of the best-known semiconductors. They are used extensively in
devices such as rectifiers and transistors.
    A large number of compounds are known to be semiconductors. A group of
semiconducting compounds of the simple type AB consists of elements from
columns symmetrically placed with respect to column IV of the periodic table.
Indium antimonide (InSb), cadmium telluride (CdTe), and silver iodide (AgI)
are examples of III-V, II-IV, and I-VI compounds, respectively. Many practical
applications have been found for the various III-V compounds. These compounds
have zincblende crystal structure which is geometrically similar to the
diamond structure possessed by the elemental semiconductors, germanium and
silicon, of column IV, except that the four nearest neighbors of each atom are
atoms of the other kind.
    The properties of semiconductors are extremely sensitive to the presence
of impurities. It is therefore desirable to start with the purest available
materials and to introduce a controlled amount of the desired impurity. The
zone refining method is often used for further purification of obtainable
materials.  See  ZONE REFINING.
    For basic studies as well as for many practical applications, it is
desirable to use single crystals. Various methods are used for growing
crystals of different materials. For many semiconductors the Czochralski
method is commonly used. The method of condensation from the vapor phase is
used to grow crystals of a number of semiconductors.
    The introduction of impurities, or doping, can be accomplished by simply
adding the desired quantity to the melt from which the crystal is grown. When
the amount to be added is very small, a preliminary ingot is often made with a
larger content of the doping agent; a small slice of the ingot is then used to
dope the next melt accurately. Impurities which have large diffusion constants
in the material can be introduced directly by holding the solid material at an
elevated temperature while this material is in contact with the doping agent
in the solid or the vapor phase.
    A doping technique, ion implantation, has been used extensively. The
impurity is introduced into a layer of semiconductor by causing a controlled
dose of highly accelerated impurity ions to impinge on the semiconductor.  See
  ION IMPLANTATION.
    A growing subject of scientific and technological interest is amorphous
semiconductors. The representative amorphous semiconductors are selenium,
germanium, and silicon in their amorphous states, and arsenic and germanium
chalcogenides.	See  AMORPHOUS SOLID.

    Rectification in semiconductors.  In semiconductors, narrow layers can be
produced which have abnormally high resistances. The resistance of such a
layer is nonohmic; it may depend on the direction of current, thus giving rise
to rectification. Rectification can also be obtained by putting a thin layer
of semiconductor or insulator material between two conductors of different
material.
    A narrow region in a semiconductor which has an abnormally high resistance
is called a barrier layer. A barrier may exist at the contact of the
semiconductor with another material, at a crystal boundary in the
semiconductor, or at a free surface of the semiconductor. In the bulk of a
semiconductor, even in a single crystal, barriers may be found as the result
of a nonuniform distribution of impurities. The thickness of a barrier layer
is small, usually 10-sup3 to 10-sup5 cm (4 x 10-sup4 to 4 x 10-sup6 in.).
    A barrier is usually associated with the existence of a space charge. In
an intrinsic semiconductor, a region is electrically neutral if the
concentration n of conduction electrons is equal to the concentration p of
holes. Any deviation in the balance gives a space charge equal to e(p - n),
where e is the charge on an electron. In an extrinsic semiconductor, ionized
donor atoms give a positive space charge and ionized acceptor atoms give a
negative space charge.
 �34s[Hsu Y. Fan]
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�6v�2v�32sSemiconductor diode   �21m �33s 
    A two-terminal electronic device that utilizes the properties of the 
semiconductor from which it is constructed. In a semiconductor diode without a 
pn junction, the bulk properties of the semiconductor itself are used to make 
a device whose characteristics may be sensitive to light, temperature, or 
electric field. In a diode with a pn junction, the properties of the pn 
junction are used. The most important property of a pn junction is that, under 
ordinary conditions, it will allow electric current to flow in only one 
direction. Under the proper circumstances, however, a pn junction may also be 
used as a voltage-variable capacitance, a switch, a light source, a voltage 
regulator, or a means to convert light into electrical power.  See  
SEMICONDUCTOR.  
    The conductivity of a semiconductor is proportional to the number of 
electrical carriers (electrons and holes) it contains. In a 
temperature-compensating diode, or thermistor, the number of carriers changes 
with temperature.  See  THERMISTOR. 
    In a photoconductor the semiconductor is packaged so that it may be 
exposed to light. Light photons whose energies are greater than the band gap 
can excite electrons from the valence band to the conduction band, increasing 
the number of electrical carriers in the semiconductor.  See  
PHOTOCONDUCTIVITY.  
    In some semiconductors the conduction band has more than one minimum. This 
results in a region of negative differential conductivity, and a device 
operated in this region is unstable. The current pulsates at microwave 
frequencies, and the device, a Gunn diode, may be used as a microwave power 
source.  See  MICROWAVE SOLID-STATE DEVICES. 
    A rectifying junction is formed whenever two materials of different 
conductivity types are brought into contact. Most commonly, the two materials 
are an n-type and a p-type semiconductor, and the device is called a junction 
diode. However, rectifying action also occurs at a boundary between a metal 
and a semiconductor of either type. If the metal contacts a large area of 
semiconductor, the device is known as a Schottky barrier diode; if the contact 
is a metal point, a point-contact diode is formed.  See  POINT-CONTACT DIODE; 
SCHOTTKY EFFECT. 
    The contact potential between the two materials in a diode creates a 
potential barrier which tends to keep electrons on the n side of the junction 
and holes on the p side. When the p side is made positive with respect to the 
n side by an applied field, the barrier height is lowered and the diode is 
forward biased. Majority electrons from the n side may flow easily to the p 
side, and majority holes from the p side may flow easily to the n side. When 
the p side is made negative, the barrier height is increased and the diode is 
reverse-biased. Then, only a small leakage current flows: Minority electrons 
from the p side flow into the n side, and minority holes from the n side flow 
into the p side. The current-voltage characteristic of a typical diode is 
shown in the illustration. Rectifying diodes can be made in a variety of 
sizes, and much practical use can be made of the fact that such a diode allows 
current to flow in essentially one direction only.  See  JUNCTION DIODE; 
SEMICONDUCTOR RECTIFIER; TUNNEL DIODE. 
 �34s[Stephen Nygren]
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�6v�2v�32sSemiconductor heterostructures �21m �33s
    Structures consisting of two different semiconductor materials in junction
contact, with unique electrical or electrooptical characteristics. A
heterojunction is a junction in a single crystal between two dissimilar
semiconductors. The most important differences between the two semiconductors
are generally in the energy gap and the refractive index. In semiconductor
heterostructures, differences in energy gap permit spatial confinement of
injected electrons and holes, while the differences in refractive index can be
used to form optical waveguides. Semiconductor heterostructures have been used
for diode lasers, light-emitting diodes, optical detector diodes, and solar
cells. In fact, heterostructures must be used to obtain continuous operation
of diode lasers at room temperature. Heterostructures also exhibit other
interesting properties such as the quantization of confined carrier motion in
ultrathin heterostructures and enhanced carrier mobility in modulation-doped
heterostructures. Structures of current interest utilize III - V and IV - VI
compounds having similar crystal structures and closely matched lattice
constants.  See  BAND THEORY OF SOLIDS; LASER; LIGHT-EMITTING DIODE; OPTICAL
DETECTORS; REFRACTION OF WAVES; SOLAR CELL.
    The most intensively studied and thoroughly documented materials for
heterostructures are GaAs and Al(subx)Ga(sub 1-x)As. Several other III - V and
IV - VI systems also are used for semiconductor heterostructures. A close
lattice match is necessary in heterostructures in order to obtain high-quality
crystal layers by epitaxial growth and thereby to prevent excessive carrier
recombination at the heterojunction interface.	See  CRYSTAL STRUCTURE. 
    When the narrow energy gap layer in heterostructures becomes a few tens of
nanometers or less in thickness, new effects that are associated with the
quantization of confined carriers are observed. These ultrathin
heterostructures are referred to as superlattices or quantum well structures,
and they consist of alternating layers of GaAs and Al(subx)Ga(sub 1-x)As. These
structures are generally prepared by molecular-beam epitaxy. Each layer is 5
to 40 nanometers thick.
    In the GaAs layers, the motion of the carriers is restricted in the
direction perpendicular to the heterojunction interfaces, while they are free
to move in the other two directions. The carriers can therefore be considered
as a two-dimensional gas. The Schroedinger wave equation shows that the
carriers moving in the confining direction can have only discrete bound
states.  See  QUANTUM MECHANICS.
    Another property of semiconductor heterostructures is illustrated by a
modulation doping technique that spatially separates conduction electrons in
the GaAs layer and their parent donor impurity atoms in the
Al(subx)Ga(sub 1-x)As layer.  Since the carrier mobility in semiconductors is
decreased by the presence of ionized and neutral impurities, the carrier
mobility in the modulation-doped GaAs is larger than for a GaAs layer doped with
impurities to give the same free electron concentration.  Higher carrier
mobilities should permit preparation of devices that operate at higher
frequencies than are possible with doped layers.  See SEMICONDUCTOR.
 �34s[H.  C.  Casey, Jr.]
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�6v�2v�32sSemiconductor laser �21m �33s
    A device in which laser action takes place through the stimulated
recombination of free electrons in the conduction band of a direct-gap
conductor with holes in the valence band. In recombining, the electrons give
up energy corresponding nearly to the band gap. This energy is radiated as a
light quantum. In direct-gap semiconductors, such as gallium arsenide,
recombination occurs directly without the emission or absorption of a quantum
of lattice vibrations. A flat junction between p-type and n-type material may
be used. When a current is passed through this junction in the forward
direction, a large number of holes and electrons are brought together. A light
wave passing along the plane of the junction can be amplified by stimulating
such recombination of electrons and holes. The ends of the semiconducting
crystal provide the mirrors to complete the laser structure.  See  LASER;
SEMICONDUCTOR.
 �34s[Stephen F. Jacobs ;Arthur L.Schawlow]
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�6v�2v�32sSemiconductor memories   �21m �33s 
    Devices for storing digital information that are fabricated by using 
integrated circuit technology. Semiconductor memories are widely used to store 
programs and data in almost every digital system. Initially developed as a 
replacement for magnetic core memories, which were used as the main computer 
storage memory, semiconductor memories started to appear in the early 1970s 
and have almost totally replaced core memories as the main computer memory 
elements.  
    Many different types of semiconductor memories are used in computer 
systems to perform various functions - bulk data storage, program storage, 
temporary storage, and cache (or intermediate) storage. Almost all of the 
memories are a form of random-access memory (RAM), where any storage location 
can be accessed in the same amount of time. However, there are many different 
types of RAMs, the most frequently used of which is the writable and readable 
memory that is simply referred to as a RAM. 
    Although the RAM acronym indicates the random-access capability, it is a 
misnomer since almost all semiconductor memories except for a few specialty 
types can be randomly accessed. A more appropriate name for the memory would 
be a read/write RAM to indicate that data can be written into the memory as 
well as be read out of it. 
    There are many other forms of semiconductor memories in use - 
mask-programmable read-only memories (ROMs), fuse-programmable read-only 
memories (PROMs), ultraviolet-erasable programmable read-only memories (UV 
EPROMs), electrically alterable read-only memories (EAROMs), electrically 
erasable programmable read-only memories (EE PROMs), and nonvolatile static 
RAMs (NV RAMs). All of these memory types are also randomly accessible, but 
their main distinguishing feature is that once information is loaded into the 
storage cells, it stays there even if the power is shut off. 
    The first of these memory types, the ROM, is programmed by the memory 
manufacturer during the actual device fabrication. Here, though, there are two 
types of ROMs; one is called last-mask or contact-mask programmable, and the 
other is often called a ground-up design. 
    As an alternative to the mask-programmable memories, semiconductor 
manufacturers developed all the other programmable memory types to permit the 
users to program the memories themselves. The first of the user-programmable 
devices was the fuse PROM. The fuse-programmable memories are one-time 
programmable memories - once the information is programmed in, it cannot be 
altered.  
    The birth of the microprocessor in the early 1970s brought with it new 
memory types that offered a feature never before available - reusability. 
Information stored in the memory could be erased - in the case of the UV 
EPROM, by an ultraviolet light, and in the case of the EAROM, EE PROM, or NV 
RAM, by an electrical signal - and then the circuit could be reprogrammed with 
new information that could be retained indefinitely. All of these memory types 
are starting to approach the ideal memory element for the computer, an element 
that combines the flexibility of the RAM with the permanence of the ROM when 
power is removed.  See  COMPUTER STORAGE TECHNOLOGY; INTEGRATED CIRCUITS; 
LOGIC CIRCUITS; MICROPROCESSOR. 
 �34s[Dave Bursky]
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�6v�2v�32sSemiconductor rectifier   �21m �33s 
    An electrical component which conducts current preferentially in one 
direction and inhibits the flow of current in the other direction by utilizing 
the properties of a semiconductor material such as silicon. A major use of the 
semiconductor rectifier is as a component in electrical equipment designed to 
convert electrical power from alternating current (ac) to direct current (dc). 
Such total equipment, consisting of not only the semiconductor components but 
also circuit breakers, bus work, fuses, sharing reactors, control circuits, 
and so forth, is often referred to as a semiconductor rectifier equipment. See 
CIRCUIT BREAKER; CONTROLLED RECTIFIER; RECTIFIER; SEMICONDUCTOR; SEMICONDUCTOR 
DIODE.  
    In 1957 the first commercial silicon-controlled rectifier (SCR) was 
developed in the United States. This electrical semiconductor component has, 
besides the anode and cathode of the rectifier diode, a third control 
electrode, or gate, which allows initiating conduction through the device by 
means of a small trigger pulse signal from a suitable control circuit. By the 
late 1960s the SCR had become the dominant power-control component. Its 
application ranges from adjustable speed control of fractional horsepower 
motors, to lighting and heating controls, to control of the largest dc motors. 
See THYRATRON. 
  
    Silicon rectifier diodes.  The electrical heart of the rectifier diode is 
the junction between a p-type region and an n-type region of conductivity 
within a monocrystalline slice of silicon semiconductor material called the 
pellet or the chip. The processing required to form such a pn junction is 
accomplished in commercially available silicon devices by either a diffusion 
or an alloying process.  See  SEMICONDUCTOR; SILICON DIODE. 
    Once the pn junction is formed, it exhibits its rectifying characteristic 
by allowing current to flow in the conventional sense from anode (p-type) to 
cathode (n-type) under a very small forward bias voltage; when the voltage 
polarity is reversed, the pn junction blocks the flow of current and only a 
very small reverse blocking or leakage current will flow under 
high-reverse-voltage bias. 
    Single-pellet silicon diodes are commercially available in current ratings 
from under 1 A to the order of 5000 A. Diodes may be connected in parallel for 
greater current capability. Voltage ratings of single-pellet silicon rectifier 
diodes commercially available cover a range from 50 to 5000 V. Individual 
diodes can be connected in series for greater voltage capability and diodes 
can also be obtained prepackaged in series strings for high-voltage 
applications.  
  
    Schottky diodes.  Unlike the pn junction of a regular silicon rectifier 
diode, the Schottky diode makes use of the rectification effect of a 
metal-to-silicon barrier. The Schottky diode, sometimes called Schottky 
barrier diode, overcomes a major limitation of the pn junction diode; being a 
majority-carrier device, it has both higher forward conductivity (lower 
forward voltage drop) and faster switching speeds than its minority-carrier pn 
junction counterpart. However, other factors confine its use to low-voltage 
power supply applications - chiefly its limited reverse blocking voltage, 
typically 45 V, with a maximum of 80 V available. Secondary shortcomings are 
high reverse blocking current and restricted temperature of operation, with 
commercial devices providing a maximum of 347DGF (175DGC), compared with 
392DGF (200DGC) for pn junction diodes. 
    Integrated circuits used in computer and instrument systems commonly 
require voltages less than 15 V and frequently as low as 5 V. Thus, the 
advantage of low forward voltage drop and faster switching favors the Schottky 
rectifier diode, due to its inherent efficiency. This is particularly true for 
the high-frequency-switching regulator supply applications where power at 
switching frequencies of 20 - 50 kHz has to be rectified. The higher reverse 
blocking current of the Schottky diode is more than compensated for by the 
Schottky's superior switching speeds. 
  
    Silicon-controlled rectifier.  The SCR is a triode reverse-blocking 
thyristor. Whereas diodes use two alternate layers of pn-type semiconductor 
material and transistors use three such layers, thyristor devices utilize four 
layers forming three or more junctions within a slice of silicon semiconductor 
material.  
    The most widely used of all thyristor devices for power control is the 
SCR. For specialized ac-switching power control, such as in lamp dimmers and 
heating controls, the bidirectional triode thyristor, popularly called the 
triac, has also come into widespread usage. 
  
    The illustration shows a typical arrangement of alternate p and n layers 
in an SCR structure. The thickness is exaggerated in proportion for clarity of 
illustration. With positive voltage on its cathode with respect to its anode, 
the SCR blocks the flow of reverse current in a manner similar to that of a 
conventional silicon rectifier diode. When the voltage is reversed, the SCR 
blocks forward current flow until a low-power trigger signal is applied 
between the gate terminal and the cathode, whereupon the SCR switches into a 
highly conductive state with a voltage drop of approximately 1 V between anode 
and cathode similar to that of the rectifier diode. Once in conduction, the 
SCR continues to conduct even after the gate signal is removed, provided the 
anode (or load) current remains above the holding current level, typically in 
the order of milliamperes. If anode current momentarily drops below the 
holding current level or if the anode voltage is momentarily reversed, the SCR 
reverts to its blocking state and the gate terminal regains control. 
  
    ASCRs, RCTs, and GTOs.  These devices are all in the thyristor family and 
are mainly used in place of SCRs in power circuits requiring operation from a 
dc source. The ASCR, asymmetrical silicon-controlled rectifier, and RCT, 
reverse-conducting thyristor, have the advantage of faster turnoff time than 
the SCR, and thus require a less costly auxiliary circuit to effect turnoff. 
The RCT has an added circuit advantage, as it has a built-in reverse rectifier 
diode in parallel with the device. Along with faster turnoff times, the ASCR 
and RCT devices  have lower forward-voltage drops for comparable 
forward-blocking voltage ratings and silicon area, thus increasing the 
device's efficiency. 
    The GTO, gate turnoff device, is also a thyristor. Like the SCR, it is a 
symmetrical reverse-blocking triode thyristor (unlike the ASCR and RCT, which 
cannot block reverse voltages), but it has the added advantage of being able 
to turn off current when a negative signal is applied to the gate. Thus the 
GTO does not require an auxiliary circuit to commutate it off as do the SCR, 
ASCR, and RCT devices. The added complexity of gate turnoff makes the GTO 
higher priced than similarly rated SCRs. 
 �34s[F. B. Golden]
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�6v�2v�32sSemionotiformes   �21m �33s 
    An order of actinopterygian fishes which appeared first in the upper 
Permian, reached maximum development in the Triassic and Jurassic, and 
persists in the Recent fauna as the gars. 
    In the Semionotiformes the body is encased in a heavy armor of 
interlocking ganoid scales, which are thick, are more or less rhomboidal, and 
have an enamellike surface. Modern forms have an elongate body and bony jaws 
provided with enlarged conical teeth (see illustration). 
    The single Recent family, Lepisosteidae, contains one genus, Lepisosteus, 
with seven species restricted to lowland fresh and brackish waters of North 
and Central America.  See  ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sSenescence   �21m �33s 
    The deteriorative biological changes relating to aging in all living 
forms, including humans. Maturation and development during infancy and 
childhood are also age-related but are usually set apart from senescence. 
    From the moment the ovum is fertilized and a new individual is created 
until death in old age, the processes of aging are at work. As organs are 
formed and molded into their normal shapes, degeneration of excesses of cells 
takes place. In other words, the processes of cell multiplication, organ 
formation, and growth overlap the degenerative processes of aging at every 
moment in the life cycle. However, it is only after the attainment of full 
maturation and growth in early adulthood that the deteriorative effects of 
aging predominate over those of maturation and grow. 
    It is known that there are human structural and physiological changes that 
appear to be due to the aging process in every organ system. The impaired 
efficiency of the cardiovascular, pulmonary, renal, neuromuscular, and other 
systems from the fourth to the eighth decades of human life have been 
documented in many scientific studies. The declines are as follows: brain 
weight, 10%; blood flow to brain, 20%; cardia output at rest, 30%; number of 
glomeruli in kidneys, 44%; glomerular filtration rate, 31%; number of fibers 
in nerves, 37%; nerve conduction velocities, 10%; number of taste buds, 64%; 
maximum pulmonary ventilation, 44%; basal metabolic rate, 16%. 
    The structural changes in all the various organs consist of: reduction in 
the size of the constituent cells of the various organs; loss of cells; 
increase in connective, supporting (interstitial) tissues; accumulation of 
lipofuscin (wear-and-tear pigment) in the cytoplasm of cells; and reduction in 
the activity of many enzymes. Collagen, the substance of which connective 
tissues are composed, and elastin undergo structural and chemical alterations, 
accounting for the age changes in skin and the stiffening of blood vessels. 
Fibroblasts, the cells that produce collagen, are no longer as capable of 
mitotic division as in earlier life, and this causes a slowness in healing of 
wounds.  
    Senescence, then, appears to be a diverse and complex process with 
functional manifestations referable to all structures of the human organism. 
Of course the declining forces of vitality result ultimately in death.  See  
DEATH; GERONTOLOGY. 
 �34s[Raymond D. Adams]
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�6v�2v�32sSensation   �21m �33s 
    A term commonly used to refer to the subjective experience resulting from 
stimulation of a sense organ, for instance, a sensation of warm, sour, or 
green. As a general scientific category, the study of sensation is the study 
of the operation of the senses. Sense receptors are the means by which 
information presented as one form of energy, for example, light, is converted 
to information in the form used by the nervous system, that is, impulses 
traveling along nerve fibers.  See  SENSE ORGAN. 
    Each sense has mechanisms and characteristics peculiar to itself, but all 
display the phenomena of absolute threshold, differential threshold, and 
adaptation. Not until sufficient stimulation impinges on a receptor can the 
presence of a stimulus be detected. The quantity of stimulation required is 
known as the absolute threshold. Not until a sufficient change occurs in some 
aspect of a stimulus can the change be detected. The magnitude of the change 
required is called the differential threshold. Under steady stimulation there 
is a decrease in sensitivity of the corresponding sense, as indicated by a 
shift in the absolute threshold and in the magnitude of sensation. After the 
stimulation ceases, sensitivity increases. An obvious example of visual 
adaptation occurs when one goes from bright to dim surroundings or vice versa. 
    With fairly good accuracy humans can localize visual objects, sounds, and 
cutaneous contacts and can discriminate the spatial orientation of the body 
and its members. With rather poor accuracy humans can localize many of the 
stimuli originating within the body. 
    With the exception of hearing, in which sense localization depends on 
differences in the acoustic stimuli reaching the two ears, there appears to be 
a common principle involved in giving spatially separated receptors their 
different local signs. Stimulation at different points on the receptive 
surface results in peaks of electrical activity at different loci in the 
brain. In no sense is there anything like a private wire from each sensory 
cell to a corresponding point in the brain. In fact, there are so many 
opportunities for a signal to go astray on its way from the receptor to the 
brain that it is surprising that spatial discrimination is as good as it is. 
Nevertheless, there is clear evidence that, by a combination of anatomical and 
functional arrangements, spatial differences at the receptor level are 
translated into topologically similar spatial differences in brain activity.  
See  HEARING (HUMAN). 
    The nerve fibers between receptor and brain do not serve merely as 
transmitters of sensory information. Their interconnections enable them to 
influence one another's sensitivity and to perform logical operations like 
those carried out inside computers. As a result the information arriving in 
the sensory areas of the brain is not merely a more or less faithful replica 
of that presented to the receptors but in addition has had certain aspects of 
the information selected for special signaling.  See  CHEMICAL SENSES; PAIN; 
PARESTHESIA; SOMESTHESIS; TASTE; TOUCH; VISION. 
 �34s[John F. Hahn]
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�6v�2v�32sSense organ   �21m �33s 
    A structure which is a receptor for external or internal stimulation. A 
sense organ is often referred to as a receptor organ. External stimuli affect 
the sensory structures which make up the general cutaneous surface of the 
body, the exteroceptive area, and the tissues of the body wall or the 
proprioceptive area. These somatic area receptors are known under the general 
term of exteroceptors. Internal stimuli which originate in various visceral 
organs such as the intestinal tract or heart affect the visceral sense organs 
or interoceptors. A receptor structure is not necessarily an organ; in many 
unicellular animals it is a specialized structure within the organism. 
Receptors are named on the basis of the stimulus which affects them, 
permitting the organism to be sensitive to changes in its environment. 
    Photoreceptors are structures which are sensitive to light and in some 
instances are also capable of perceiving form, that is, of forming images. 
Light-sensitive structures include the stigma of phytomonads, photoreceptor 
cells of some annelids, pigment cup ocelli and retinal cells in certain 
asteroids, the eyespot in many turbellarians, and the ocelli of arthropods. 
The compound eye of arthropods, mollusks, and chordates is capable of image 
formation and is also photosensitive.  See  PHOTORECEPTION. 
    Phonoreceptors are structures which are capable of detecting vibratory 
motion or sound waves in the environment. The most common phonoreceptor is the 
ear, which in the vertebrates has other functions in addition to sound 
perception.  See  EAR; PHONORECEPTION. 
    Statoreceptors are structures concerned primarily with equilibration, such 
as the statocysts found throughout the various phyla of invertebrates and the 
inner ear or membranous labyrinth filled with fluid. 
    The sense of smell is dependent upon the presence of olfactory neurons, 
called olfactoreceptors, in the olfactory epithelium of the nasal passages 
among the vertebrates.  See  OLFACTION. 
    The sense of taste is mediated by the taste buds, or gustatoreceptors. In 
most vertebrates these taste buds occur in the oral cavity, on the tongue, 
pharynx, and lining of the mouth; however, among certain species of fish, the 
body surface is supplied with taste buds as are the barbels of the catfish.  
See  TASTE. 
    The surface skin of vertebrates contains numerous varied receptors 
associated with sensations of touch, pain, heat, and cold.  See  CHEMICAL 
SENSES; CUTANEOUS SENSATION; SENSATION. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSensible temperature �21m �33s
    The temperature at which air with some standard humidity, motion, and
radiation would provide the same sensation of human comfort as existing
atmospheric conditions. Of the many sensible temperature formulas thus far
proposed, none is completely satisfactory or generally accepted. Some are
purely empirical, modifying the actual temperature according to the humidity;
others are theoretical, and express estimated heat loss rather than an
equivalent temperature.  See  AIR TEMPERATURE; HUMIDITY.
    Most used is effective temperature, represented by lines of equal comfort
on a chart of dry-bulb versus wet-bulb temperature; it relates existing
comfort to that in motionless, saturated air. Effective temperature is
approximated by the temperature-humidity index (THI), given in DGF by Eq. (1),

			    THI = 15 + 0.4t			     (1)

where t is the sum of the dry-bulb and wet-bulb temperatures. The THI is
tabulated at major Weather Bureau stations, and accumulated into so called
cooling degree days.  See  TEMPERATURE-HUMIDITY INDEX.
    Under cold conditions, atmospheric moisture is negligible and wind becomes
important in heat removal. Wind chill, given by Eq. (2), does not estimate

  Windchill = (10.45 + 10  (SQUARE ROOT v) - v) (33 - t)	      (2)

sensible temperature as such, but heat loss in kcal/msup2 hr, for wind speed v
in m/sec and air temperature t in DGC. It is used by the U.S. Army, government
agencies, construction contractors, and others in cold areas.
 �34s[Arnold Court]
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�6v�2v�32sSensitivity   �21m �33s 
    The ability of the output of a device or system to respond to an input 
stimulus. Mathematically, sensitivity is expressed as the ratio of the 
response or change induced in the output to a stimulus or change in the input. 
If the sensitivity varies with the level of the input signal, then sensitivity 
is usually expressed in terms of the derivative of the output with respect to 
the input at a specified input level. 
    The reciprocal of sensitivity is called the scale factor or figure of 
merit, and it represents the conversion factor by which the output indicator 
or scale reading must be multiplied to obtain the magnitude of the input. 
Occasionally the scale factor is called the sensitivity through loose usage. 
 �34s[John Markus]
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�6v�2v�32sSensory learning   �21m �33s 
    Learning in which an organism is trained to respond to changes in, or 
differences between, some aspects of the physical stimulus presented to one of 
the sense organs. Included are discrimination learning and perceptual 
learning, both of which are distinct from motor learning. Discrimination 
learning is a form of sensory learning in which the organism is required to 
respond to a difference between stimuli; reward or punishment or both are 
presented to provide differential reinforcement. Simple pavlovian 
conditionedresponse formation is excluded from sensory learning. This involves 
the learning of a given muscular or glandular response as it becomes reliably 
elicited when a particular stimulus or sensory event occurs, usually following 
a series of trials with an effective eliciting stimulus. 
    Sensory preconditioning is a particular experimental approach to the study 
of sensory learning in which two or more stimuli, such as tones, lights, or 
skin contacts, are paired a few times. One of these stimuli is then made a 
conditioned stimulus to elicit a certain response. The test for 
preconditioning is to present the other element or elements from the original 
combination in order to determine its power to elicit the response. A positive 
outcome is accepted if the original pairing has been effective, that is, if it 
has resulted in a stronger or more reliable response than would be the case 
without the original pairing of stimuli. Simple stimulus generalization is 
ruled out by testing control subjects.  See  CONDITIONED REFLEX. 
    Examples of sensory learning are numerous and familiar. Learning a melody 
or development of familiarity with a face are two of the most common. 
Cross-modality associations are also formed in everyday experiences. A face 
recalls a voice, and vice versa. A particular landmark recalls another that is 
nearby but not simultaneously present. Much of knowledge (cognition) is 
derived from sequential experience that is the essence of sensory learning.  
See  COGNITION; LEARNING THEORIES; MEMORY. 
 �34s[Austin H. Riesen]
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�6v�2v�32sSepioidea   �21m �33s 
    An order of the molluscan subclass Coleoidea. The members of this order 
are abundant as shore- and bottom-dwelling forms in marine environments. One 
pair of tentacles is elongate and capable of retraction into pits at their 
base. The eye is highly developed in this group, an internal shell is present 
which in some species is partly external, the fins are separated posteriorly, 
and chromatophores are present in the dermis. The shell of Sepia is the 
cuttlebone of commerce.  See  COLEOIDEA; DECAPODA (MOLLUSCA). 
 �34s[Charles B. Curtin]
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�6v�2v�32sSepiolite   �21m �33s 
    A complex hydrated magnesium silicate mineral named for its resemblance to 
cuttlefish bone, alternately named meerschaum (sea foam). The ideal 
composition, Mg8(H2O)4(OH)4Si12O30, is modified by some additional water of 
hydration, but is otherwise quite representative. Interlaced disoriented 
fibers aggregate into a massive stone so porous that it floats on water. These 
stones are easily carved, take a high polish with wax, and harden when warmed. 
  See  CLAY MINERALS; SILICATE MINERALS. 
 �34s[William F. Bradley]
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�6v�2v�32sSeptibranchia   �21m �33s 
    An order containing three marine genera in two families, Poromyidae and 
Cuspidariidae. Septibranchia show an extreme modification toward a scavenging 
habit. Small crustaceans that are trapped are ingested by the wide esophagus 
and crushed in the muscular stomach. Septibranchs can therefore be considered 
as predacious carnivores, unlike the majority of class Bivalvia which are 
microphagous filter feeders.  See  BIVALVIA; MOLLUSCA. 
 �34s[Richard D. Purchon]
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�6v�2v�32sSepticemia   �21m �33s 
    A condition in which infection is disseminated through the body in the 
bloodstream; this has also been referred to as blood poisoning. Bacteremia and 
viremia are related terms used when the presence of bacteria or viruses in the 
blood has been demonstrated or is postulated. Usually in these states there is 
an identifiable local focus of infection from which the infecting agents are 
discharged into the blood. The principal clinical manifestations are fever, 
rigors, generalized aching, prostration, and local signs of infection in the 
lung, kidney, joints, and so on. Without treatment the death rate is high in 
septicemia, but many lives can be saved by administration of appropriate 
antimicrobial drugs. 
 �34s[Paul B. Beeson]
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�6v�2v�32sSeptic tank   �21m �33s 
    A single-story settling tank in which settled sludge is in immediate 
contact with sewage flowing through the tank while solids are being decomposed 
by anaerobic bacterial action. Such tanks have limited use in municipal 
treatment, but are the primary resource for the treatment of sewage from 
individual residences which are not serviced by a sewage collection system. 
    Septic tanks have a capacity of approximately 1 day's flow. Since sludge 
is collected in the same unit, additional capacity is provided for sludge. 
About one-half of a 500-gal (1900-liter) tank is occupied by sludge in 5 years 
in an ordinary household installation. The majority of states require a 
minimum capacity of 500 gal (1900 liters) in a single tank. Home disposal 
units are buried in the ground and are not serviced until the system gives 
trouble from clogging or overflow. 
    Municipal and institutional units are designed to hold 12 - 14 hours' 
flow, with additional sludge capacity provided. Provision is made for sludge 
withdrawal about once a year. 
    Septic tank effluent is dangerous and odorous. It will contain pathogenic 
bacteria and sewage solids. Particles of sludge and scum are trapped in the 
flow and will cause nuisance at the point of discharge unless properly 
handled.  See  SEWAGE TREATMENT. 
 �34s[William T. Ingram]
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�6v�2v�32sSequoia �21m �33s
    The giant sequoia or big tree (Sequoia gigantea) occupies a limited area
in California and is said to be the oldest and most massive of all living
things. The leaves are evergreen, scalelike, and overlapping on the branches.
In height sequoia is a close second to the redwood (300 - 330 ft or 90 - 100
m) but the trunk is more massive. Sequoia trees may be 27 - 30 ft (8 - 9 m) in
diameter 10 ft (3 m) from the ground. The stump of one tree showed 3400 annual
rings. The red-brown bark is 1 - 2 ft (0.3 - 0.6 m) thick and spongy. Vertical
grooves in the trunk give it a fluted appearance. The heartwood is dull
purplish-brown and lighter and more brittle than that of the redwood. The wood
and bark contain much tannin, which is probably the cause of the great
resistance to insect and fungus attack. The most magnificent trees are within
the General Grant and Sequoia National Parks. See PINALES; REDWOOD.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sSeries �21m �33s
    The indicated sum of a succession of numbers or terms. Series are used to
obtain approximate values of infinite repeating decimals, to solve
transcendental equations, to obtain values of logarithms or trigonometric
functions, to evaluate integrals, and to solve boundary value problems.
    For a finite series, with only a limited number of terms, the sum is found
by addition. For an infinite series, with an unlimited number of terms, a sum
or value can be assigned only by some limiting process. When the simplest such
process yields a value, the infinite series is convergent. Many tests for
convergence enable one to learn whether a sum can be found without actually
finding it.
    If each term of an infinite series involves a variable x and the series
converges for each value of x in a certain range, the sum will be a function
of x. Often the sum is a given function of x, f(x), for which a series having
terms of some given form is desired. Thus the Taylor's series expansion

f(x) = SUMMATION (n = 0 to INFINITY) f(sup(n))	(a) [(x-a)(sup n)/n!]

can be found for a large class of functions, the analytic functions, and
represents such functions for sufficiently small values of VERTICAL BAR x -
a VERTICAL BAR. For a much less restricted type of function on the interval -pi
< x < pi, a Fourier series expansion of the form can be found.

(1/2)A(Sub O) + SUMMATION (n = 1 to INFINITY)(A[sub n] cos nx + B[sub n] sin nx)



    Finite series.  Here the problem of interest is to determine the sum of
the first n terms,

  S(sub n) = u(sub O) + u(sub 1) + u(sub 2) + . . . + u(sub n-1)

when u(sub n) is a given function of n. Examples are the arithmetic series, with
u(sub n) = a + nd and S(sub n) = (n/2)[2a + (n - 1)d], and the geometric series,
with u(sub n) = ar(sup n) and S(sub n) = a(1 - r[sup n])/ (1 - r).
See  PROGRESSION (MATHEMATICS).

    Convergence and divergence.  An infinite series is the indicated sum of an
unlimited number of terms

      u(sub O) + u(sub 1) + u(sub 2) + . . . + u(sub n) . . .

or more briefly

		  SUMMATION (n=0 to INFINITY) u(sub n)

or simply SUMMATION u(sub n), read ``sigma of u(sub n).'' The sum S(sub n) of
the first n terms is known as the nth partial sum.  Thus S(sub n) is the finite
sum

		    SUMMATION (k=0 to n-1) u(sub k)

If, as n increases indefinitely or becomes infinite, the partial sum S(sub n)
approaches a limit S, then the infinite series SUMMATION usubn is convergent. S
denotes the sum or value of the series. For example, if VERTICAL BAR r VERTICAL
BAR < 1,

		   S = SUMMATION ar(sup n) = a/(1-r)
since
		     S(sub n) = a(1 - r[sup n])/(1-4)

    If, as n becomes infinite, the partial sum S[sub n] does not approach a
finite limit, then the infinite series SUMMATION u(sub n) is divergent.  For
example, SUMMATION 1 diverges, since here S(sub n) = n becomes infinite with n.
Also SUMMATION(-1)(sup n) diverges, since here S(sub n) = (1/2)[1 - (-1)(sup n
+1)] which is alternately one and zero.
    Positive series are series each of whose terms is a positive number or
zero.  For such series, the partial sum Ssubn increases as n increases.  If for
some fixed number A no sum S(sub n) ever exceeds A, the sums are bounded and
admit A as an upper bound.  In this case, S(sub n) must approach a limit, and
the series is convergent.  If every fixed number is exceeded by some S(sub n),
the sums are unbounded.  In this case, S(sub n) must become positively infinite
and the series is divergent.  The tests for convergence of positive series are
tests for boundedness, and this is shown by a comparison of S(sub n) with the
partial sums of another series or with an integral.
    For any series SUMMATION u(sub n) which may have both positive and negative
terms, the series of absolute values, SUMMATION VERTICAL BAR u(sub n) VERTICAL
BAR, is a positive series whose convergence may be proved by one of the tests
for positive series.  If SUMMATION VERTICAL BAR u(sub n) VERTICAL BAR converges,
then SUMMATION u(sub n) necessarily converges and is said to converge
absolutely.  The sum of an absolutely convergent series is independent of the
order of the terms.
    A series which converges but which does not converge absolutely is said to
be conditionally convergent. For such a series, a change in the order of the
terms may change the sum or cause divergence.

    Power series.  There are series with u(sub n) = a(sub n) x(sup n). For such
a series, it may happen that

	   lim (n ->INFINITY)|(a[sub n+1])/a(sub n)| = A

If A = 0, the series converges for all values of x. If A = 0, the series
converges for all x of the interval -1/A < x < 1/A. It will diverge for all x
with VERTICAL BAR x VERTICAL BAR > 1/A. For any power series, the interval of
convergence is related in this way to a number A, which, however, in the
general case has to be given by the superior limit of

			  SQUARE ROOT [|a(sub n)|]

Similar remarks apply to the series with u(sub n) = a(sub n)(x - c)(sup n). Here
the interval of convergence is VERTICAL BAR x - c VERTICAL BAR < 1/A.  One of
the most important power series is the binomial series:

1 + mx + [m(m-1)/(1 TIMES 2)]x(sup 2) + . . . + [m(m-1)(m-2) . . .
		      (m-n+1)/n!]x(sup n) + . . .

When m is a positive integer, this is a finite sum of m + 1 terms which equals
(1 + x)(sup m) by the binomial theorem.  When m is not a positive integer, the
interval of convergence is -1 < x < 1, and for x in this interval, the sum of
the series is (1 + x)(sup m).  See BINOMIAL THEOREM.  Let the power series
SUMMATION a(sub n)(x - c)(sup n) have the sum function f(x).  Then a0 = f(c),
a(sub n) = f[sup(n)] (c)/n!, and the series is the Taylor series of f(x) at x =
c.  Thus, every power series whose interval of convergence has positive length
can be put in the form

f(x) = f(c) + f'(c)(x-c)/1! + . . . + f[sup(n)]  (c)(x-c) [sup(n)]/n! + . . .

where f(x) is the sum function.

    The Maclaurin series is the special case of Taylor series with c = 0:

  f(x) = f(O) + f'(O)x/1! + . . . + f[sup(n)](O)(x[sup n])/n! + . . .


    Uniform convergence.  Let each term of a series be a function of z,
u(sub n) = g(sub n)(z).  Let S(sub n) be the sum of the first n terms, and S the
sum to which the series converges for a particular value of z.	Then R(sub n) =
S - S(sub n) is the remainder after n terms, and for the particular value of z,
lim R(sub n) must equal zero.  If, for a given range of z, it is possible to
make R(sub n)(z) arbitrarily small for sufficiently large n without specifying
which z in the range is under consideration, the series converges uniformly.
See FOURIER SERIES AND INTEGRALS.
 �34s[Philip Franklin /Salomon  Bochner]
�33s
Series sometimes appear in disguised form in arithmetic.  Thus the approximation
of a rational number by an infinite repeating decimal is really a geometric
series.

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�6v�2v�32sSeries circuit   �21m �33s 
    An electric circuit in which the principal circuit elements have their 
terminals joined in sequence so that a common current flows through all the 
elements. The circuit may consist of any number of passive and active 
elements, such as resistors, inductors, capacitors, electron tubes, and 
transistors.  See  CIRCUIT (ELECTRICITY). 
 �34s[Robert L. Ramey]
�33s 
  
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�6v�2v�32sSerine   �21m �33s 
    An amino acid. Serine reacts with periodate to yield glyoxylate, ammonia, 
and formaldehyde. Serine is a biosynthetic precursor of several important 
metabolites: glycine, cysteine, choline (and hence betaine), and the side 
chain of tryptophan.  See  AMINO ACIDS; CYSTEINE; TRYPTOPHAN. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sSerology   �21m �33s 
    The division of biological science concerned with antigen-antibody 
reactions in serum. It properly encompasses any of these reactions, but is 
often used in a limited sense to denote laboratory diagnostic tests, 
especially for syphilis. The techniques of blood grouping have come from the 
study of antigen-antibody reactions in serum, as have techniques for 
identification of genetic polymorphism and quantitation of numerous serum 
proteins. With these advances came the means for developing transfusion 
therapy with cells and plasma. In addition, these techniques led to 
identification of antibodies involved in incompatibility reactions, such as in 
erythroblastosis fetalis, and the development of effective measures to prevent 
their occurrence. Further, extension of these techniques to identification of 
antigens on white cells led to effective methods of histocompatibility typing, 
facilitating organ transplantation.  See  ANTIGEN-ANTIBODY REACTION; 
TRANSPLANTATION BIOLOGY. 
 �34s[Donald Raum]
�33s 
  
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�6v�2v�32sSerotonin   �21m �33s 
    A compound, also known as 5-hydroxytryptamine, derived from tryptophan, an 
indole containing amino acid. It is widely distributed in the animal and 
vegetable kingdoms. In mammals it is found in gastrointestinal 
enterochromaffin cells, in blood platelets, and in brain and nerve tissue. 
    Serotonin is a local vasoconstrictor, plays a role in brain and nerve 
function and in regulation of gastric secretion and intestinal peristalsis, 
and has pharmacologic properties. Administration of serotonin produces a type 
of sedation and other depressant conditions of the nervous system. These facts 
suggest that in the brain serotonin may be a neurohormone. 
    The interaction of serotonin with alkaloids such as reserpine is of 
clinical importance. These drugs, which are used as tranquilizing agents in 
treatment of nervous and mental disorders, release serotonin from the brain. 
The hallucinogen D-lysergic acid diethylamide (LSD) may act by antagonism to 
serotonin.  See  TRANQUILIZER. 
 �34s[Morton K. Schwartz]
�33s 
  
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�6v�2v�32sSerpentine   �21m �33s 
    The name applied to the hydrous magnesium silicate mineral assemblage of 
the rock serpentinite. Serpentinization alters host rocks to compositions 
approximating 3MgO.2SiO2.2H2O, and the names serpentine, chrysotile, 
antigorite, and others are applied with regard to degree of crystallinity 
achieved locally in given serpentinized groundmasses.  See  SERPENTINITE; 
SILICATE MINERALS. 
    The most important serpentine mineral is the fibrous variety, chrysotile, 
which accounts for as much as 95% of the asbestos of commerce. Although 
chrysotile is not a crystal in the sense of having plane-bounding faces, it 
nevertheless has a crystal structure.  See  ASBESTOS. 
    Serpentine is easily cut and polished for ornamental stone. It may be 
blackish-green through leek green to nearly white, or brownish or yellowish. 
Serpentinized carbonate rocks sometimes show an attractive clouded green color 
and are then called verde antique, or serpentine marble. 
 �34s[William F. Bradley]
�33s 
  
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�6v�2v�32sSerpentinite   �21m �33s 
    An abundantly occurring, fine-grained massive rock, generally considered 
to have been derived by pneumatolytic or hydrothermal processes from 
preexisting basic and ultrabasic igneous intrusions. The inferred processes 
are called serpentinization and the product serpentine.  See  METAMORPHIC 
ROCKS; PNEUMATOLYSIS; SERPENTINE. 
    Serpentine differs from the more conventional rocks - those composed of 
separable grains of various minerals - for serpentine has no idiomorphic 
crystal grains of its own. When grains are apparent, they are relicts from the 
preexisting crystalline rock which was altered to serpentine while retaining 
the old grain boundaries (called pseudomorphs after the older grain). 
 �34s[T. F. W. Barth]
�33s 
  
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�6v�2v�32sSerum   �21m �33s 
    The liquid portion that remains when blood is allowed to clot 
spontaneously and is then centrifuged to remove the blood cells and clotting 
elements. It is approximately of the same volume (55%) as plasma and differs 
from it only by the absence of fibrinogen which, after conversion into fibrin, 
is incorporated in the separated clot.  See  FIBRINOGEN. 
    Blood serum contains 6 - 8% solids, including proteins, such as albumin 
and globulins; hormones; antibodies; and enzymes as well as certain nutritive 
organic materials in small amount, such as amino acids, glucose, and fats. 
Somewhat less than 1% of the serum consists of inorganic substances. Small 
amounts of waste material such as urea, uric acid, xanthine, creatinine, 
creatine, bile pigments, and ammonia are also present. Trace amounts of gases 
such as oxygen and carbon dioxide are dissolved in the serum.  See  BLOOD. 
    Certain types of sera, both human and animal, are used in clinical 
medicine. Immune serum and hyperimmune serum are developed either by naturally 
occurring disease or are deliberately prepared by repeated injection of 
antigens to increase antibody titer for either diagnostic tests or the 
treatment of active disease. These sera are referred to as antisera, since 
they have a specific antagonistic action against specific antigens.  See  
SEROLOGY.  
    By custom, the clear portion of any liquid material of animal origin 
separated from its solid or cellular elements is also referred to as sera, for 
example, the fluid in a blister and the fluids present in certain natural 
cavities of the body, such as the peritoneal, pleural, or pericardial sacs. 
These are more properly referred to, however, as effusions. 
 �34s[Reuben Straus]
�33s 
  
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�6v�2v�32sServomechanism   �21m �33s 
    An automatic feedback control system for mechanical motion. The name 
servomechanism, or ``servo'' for short, is often used interchangeably with 
feedback control system, but strictly speaking, it applies only to those 
feedback control systems in which the controlled quantity or output is 
mechanical position or one of its derivatives (velocity and acceleration).  
See  CLOSED-LOOP CONTROL SYSTEM; CONTROL SYSTEMS; CYBERNETICS; FEEDBACK 
CONTROL SYSTEM. 
    The purpose of a servomechanism is to provide one or more of the following 
objectives: (1) accurate control of the motion of a shaft or rod without the 
need for human attendants (automatic control); (2) maintenance of accuracy 
with mechanical load variations, changes in the environment, power supply 
fluctuations, aging and deterioration of components (regulation and 
self-calibration); (3) control of a high-power load from a low-power command 
signal (power amplification); (4) control of an output shaft from a remotely 
located input shaft, without the use of mechanical linkages (remote control, 
shaft repeater). 
    Many of these objectives can be attained by a nonfeedback system if 
accurately calibrated components are available. The salient advantage of the 
feedback method of control is that it permits economical design and production 
of accurate systems using individually inaccurate components. A typical 
servomechanism requires only a few of its components to have high accuracy.  
See  OPEN-LOOP CONTROL SYSTEM. 
    Historically, the first applications of servomechanisms were in speed 
governing of engines, automatic steering of ships, automatic control of guns, 
and electromechanical analog computers. Today servomechanisms are employed in 
almost every industrial field. Among the most significant applications are 
cutting tools for discrete parts manufacturing, rollers in sheet and web 
processes (such as steel rolling, paper calendering, high-speed printing), 
elevator control and vehicle drives, robots and remote manipulators in nuclear 
and other hazardous environments, positioning of cameras, telescopes, and 
antennas, stabilization and guidance of spacecraft, recording tape and disk 
drives, and mechanical knee and arm prostheses. For these and other 
applications  see  AUTOMATION; FLIGHT CONTROLS; GOVERNOR; GUIDANCE SYSTEMS; 
HUMAN-FACTORS ENGINEERING; HUMAN-MACHINE SYSTEMS; MEDICAL CONTROL SYSTEMS; 
PROCESS CONTROL; REGULATOR; REMOTE-CONTROL SYSTEM; ROBOTICS. 
 �34s[Gerald Weiss]
�33s 
  
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�6v�2v�32sSet theory   �21m �33s 
    A mathematical term referring to the study of collections or sets. 
Consider a collection of objects (such as points, dishes, equations, 
chemicals, numbers, or curves). This set may be denoted by some symbol, such 
as X. It is useful to know properties that the set X has, irrespective of what 
the elements of X are. The cardinality of X is such a property. 
    Two sets A and B are said to have the same cardinal written C(A) = C(B), 
provided there is a one-to-one correspondence between the elements of A and 
the elements of B. For finite sets this notion coincides with the phrase ``A 
has the same number of elements as B.'' However, for  infinite sets the above 
definition yields some interesting consequences. For example, let A denote the 
set of integers and B the set of odd integers. The function f(n) = 2n - 1 
shows that C(A) = C(B). Hence, an infinite set may have the same cardinal as a 
part or subset of itself. 
    A is called a subset of B if each element of A is an element of B, and it 
is expressed as A CONTAINED IN B. The collection of odd integers is a subset 
of itself. 
    One approved method of forming a set is to consider a property P possessed 
by certain elements of a given set X. The set of elements of X having property 
P may be considered as a set Y. The expression p EPSILON X is used to denote 
the fact that p is an element of X. Then Y = {p/p EPSILON X and p has property 
P}. Another approved method is to consider the set Z of all subsets of a given 
set X. Paradoxically, it is not permissible to regard the collection of all 
sets as a set. 
    In set theory, one is interested not only in the properties of sets but 
also in operations involving sets: addition, subtraction, multiplication, and 
mapping. The sum of A and B (A + B or A UNION B) is the set of all elements in 
either A or B; that is, A + B = {p/p EPSILON A or p EPSILON B}. The 
intersection of A and B (A . B, A INTERSECTION B, or AB) is the set of all 
elements in both A and B; that is, A . B = {p/p EPSILON A and p EPSILON B}. If 
there is no element which is in both A and B, one says that A does not 
intersect B and writes A . B = 0. The expression A - B is used to denote the 
collection of elements of A that do not belong to B; that is A - B = {p/p 
EPSILON A and p EPSILON B}.  
 �34s[R. H. Bing]
�33s 
  
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�6v�2v�32sSewage   �21m �33s 
    A combination of (1) liquid wastes conducted away from residences, 
institutions, and business buildings and (2) the liquid wastes from industrial 
establishments with (3) such surface, ground, and storm water as may find its 
way or be admitted into the sewers. Category 1 is known as sanitary or 
domestic sewage; 2 is usually referred to as industrial waste; 3 is known as 
storm sewage. 
    Sewage is the waste water reaching the sewer after use; hence it is 
related in quantity and in flow fluctuation to water use. The quantity of 
sewage is generally less than the water consumption since some portion of 
water used for fire fighting, lawn irrigation, street washing, industrial 
processing, and leakage does not reach the sewer. These losses are compensated 
for partly by the addition of water from private wells, groundwater 
infiltration, and illegal connections from roof drains. 
    Sewage is actually water with a small amount of impurity in it. 
Examination of sewage is required to know the effects of these impurities. 
Various tests are used to aid in determining the characteristics, composition, 
and condition of sewage. These include physical examination, solids 
determination, tests for determining the oxygen requirement of organic matter, 
chemical and bacteriological tests, and examination under the microscope.  See 
  SEWAGE COLLECTION SYSTEMS; SEWAGE DISPOSAL; SEWAGE SOLIDS; SEWAGE TREATMENT. 
 �34s[William T. Ingram]
�33s 
  
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�6v�2v�32sSewage collection systems   �21m �33s 
    Systems of pipes and conduits, together with control devices, pumping 
stations, and appurtenances used for the collection and transfer of waste 
waters.  See  SEWAGE. 
    Sewer pipe is manufactured from a number of materials, such as vitrified 
clay, concrete, asbestos cement, corrugated iron, cast iron, and steel. 
Plastics, bituminous wood fiber, and wood stave pipe also are used. All sewer 
pipes may be surcharged or filled at some time and must be capable of 
withstanding some hydraulic pressure. Sewers are laid underground and must be 
able to withstand external pressures such as those caused by soil, water, and 
the extra weight of traffic. Large-diameter reinforced concrete sewers or 
conduits may be constructed in place. 
    Sewer appurtenances are access holes, inlets, regulators, inverted 
siphons, and outfalls. An access hole is an opening at the ground or street 
surface permitting a worker to enter to make examinations and repairs. An 
inlet provides an opening for storm water and is usually placed at the curb 
line of the street. The structure below the inlet is called a catch basin. A 
regulator is a device designed to divert sewage flow from one sewer to another 
channel. It is most frequently used in combined sewer systems to regulate the 
amount of storm water permitted to flow to a sewage treatment plant. An 
inverted siphon is a length of sewer set below hydraulic grade line so that it 
flows under pressure between an inlet chamber and an outlet chamber. The 
outfall is a structure designed to admit treated or untreated sewage to a 
receiving body of water. 
    Major factors affecting the quantity and flow patterns of sanitary sewage 
are (1) population and population increase; (2) population density and density 
change; (3) water use, demand, and consumption; (4) industrial requirements, 
present and future; (5) commercial requirements, present and future; (6) 
expansion of service geographically; (7) groundwater geology of area; and (8) 
topography of area.  See  SEWAGE DISPOSAL. 
 �34s[William T. Ingram]
�33s 
  
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�6v�2v�32sSewage disposal   �21m �33s 
    The discharge of waste waters into surface-water or groundwater courses, 
which constitute the natural drainage of an area. Most waste waters contain 
offensive and potentially dangerous substances, which can cause pollution and 
contamination of the receiving water bodies. Contamination is defined as the 
impairment of water quality to a degree that creates a hazard to public 
health. Pollution refers to the adverse effects on water quality that 
interfere with its proper and beneficial use. The principal sources of 
pollution are domestic sewage and industrial wastes. 
    Water-quality criteria deal with the physical, chemical, and biological 
parameters of pollution. The most common standards are concerned with physical 
appearance, odor production, dissolved-oxygen concentration, pathogenic 
contamination, and potentially toxic or harmful chemicals. The allowable 
quantity and concentration of these characteristics and substances vary with 
the water usage.  See  SEWAGE TREATMENT; WATER ANALYSIS; WATER POLLUTION; 
WATER PURIFICATION. 
 �34s[Donald J. O'Connor]
�33s 
  
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�6v�2v�32sSewage solids   �21m �33s 
    A semiliquid mass, called sludge, removed from the liquid flow of sewage. 
Solids treatment depends on the source of the solid and its characteristics. 
Solids are removed as screenings, grit, primary sludge, secondary sludge, and 
scum. Sewage solids must be disposed of without nuisance or hazard to health. 
Burial, incineration, and drying for use as fertilizer are means of final 
disposal. Lagooning may be used; in seacoast cities sludge may be taken out to 
sea. Disposal of sewage solids on land has limited application. It has certain 
public-health dangers and must be closely supervised. 
 �34s[William T. Ingram]
�33s 
  
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�6v�2v�32sSewage treatment   �21m �33s 
    Any process to which sewage is subjected in order to remove or alter its 
objectionable constituents and thus render it less offensive or dangerous. 
These processes may be classified as preliminary, primary, secondary, or 
complete, depending on the degree of treatment accomplished. Preliminary 
treatment may be the conditioning of industrial waste prior to discharge to 
remove or to neutralize substances injurious to sewers and treatment 
processes, or it may be unit operations which prepare the water for major 
treatment. Primary treatment is the first and sometimes the only treatment of 
sewage. It is the removal of floating solids and coarse and fine suspended 
solids. Secondary treatment utilizes biological methods, that is, oxidation 
processes following primary treatment by sedimentation. Complete treatment 
removes much of the suspended, colloidal, and organic matter. 
    Septic tanks and Imhoff tanks are considered secondary treatment methods 
because sedimentation is combined with biological digestion of the sludge.  
See  IMHOFF TANK; SEPTIC TANK. 
 �34s[William T. Ingram]
�33s 
  
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�6v�2v�32sSewing machine   �21m �33s 
    A mechanism that stitches cloth, leather, book pages, and other material 
by means of a double-pointed needle or any eye-pointed needle. In ordinary 
two-threaded machines, a lock stitch is formed (see illustration). A presser 
foot held against the material with a yielding spring adjusts itself 
automatically to variations in thickness of material and allows the operator 
to turn the material as it feeds through the machine. A cluster of cams, any 
one of which can be selected to guide the needle arm, makes possible a variety 
of stitch patterns.  See  CAM MECHANISM. 
 �34s[Frank H. Rockett]
�33s 
  
  
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�6v�2v�32sSex-influenced inheritance   �21m �33s 
    That part of the inheritance pattern on which sex differences operate to 
promote character differences. These characters are expressed differently in 
the two sexes, even when their genotypes are identical. Some confusion exists 
between this term and sex-limited inheritance. Such characteristics as 
baldness in humans and some eye colors in Drosophila all show influence of 
sex. The effects frequently are attributed to hormones secreted by the sex 
cells, but often hormones cannot be demonstrated. The differences found within 
the individual body cells of the sexes may be the responsible factors. 
Sex-limited inheritance differs from sex-influenced in that the inheritance 
finds expression only in one sex. 
 �34s[John W. Gowen]
�33s 
  
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�6v�2v�32sSex-linked inheritance �21m �33s
    The transmission to successive generations of differences that are due to
genes located in the sex chromosomes. Because of their ready identification
and straightforward expression, sex-linked genes have frequently provided the
basis for crucial observations and experiments in genetics. The best-known
sex-linked traits in humans concern color vision and hemophilia.  See  COLOR
VISION.
    Sex-linked genes are distinguished as such not by the traits they control
but by characteristic patterns of transmission with respect to the sex
difference, paralleling in all respects the distribution of the
sex-determining X or Y chromosomes. Except for the sex-determinng factors
themselves, sex-linked genes are no more likely than nonsex-linked genes to be
concerned with differences in the primary or secondary sexual traits that
differentiate male from female.
    The following tabulation is an example of sex-linked inheritance in
Drosophila. In this example X is the X chromosome, Y the Y chromosome, and W,
w are sex-linked genes concerned with eye pigment:

      Parents:	X(sup w)X(sup w)     x			 X(sup W)Y
	      White-eyed			       Red-eyed
		female					 male
	    Eggs; All X(sup w)			     Sperm: 1/2 X(sup W), 1/2 Y

      Offspring: 1/2X(sup w)X(sup W),			1/2 X(sup w)Y
	       Red-eyed 			       White-eyed
		females 				  males

      Parents:	X(sup W)X(sup w)      x 		  X(sup w) Y
	       Red-eyed 			       White-eyed
		female					  male
	 Eggs: 1/2 X(sup W), 1/2 X(sub w)	      Sperm: 1/2 X(sup w), 1/2 Y

      Offspring:
1/4 X(sup W)X(sup w)	1/4 X(sup w)X(sup w)	1/4 X(sup W)Y	    1/4X(sup w)Y
      Red-eyed	       White-eyed		  Red-eyed	    White-eyed
       females		 females		   males	      males


     Sex linkage is termed complete in the case of genes located in the
differential segment of the X chromosome which is not represented by a
homologous counterpart in the Y. Sex linkage is termed incomplete or
partial if genes are located in homologous X- and Y-chromosome segments
that pair and exchange parts during meiosis. Cases of completely Y-
linked genes are rare or doubtful, and evidence for partially sex-linked
genes in humans is inconclusive. See GENETICS; HUMAN GENETICS; SEX-
INFLUENCED INHERITANCE.
 �34s[David D. Perkins]
�33s

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�6v�2v�32sSextant   �21m �33s 
    A navigation instrument used for measuring angles, primarily altitudes of 
celestial bodies. Originally, the sextant had an arc of 60DG, or 1/6 of a 
circle, from which the instrument derived its name. Because of the 
double-reflecting principle used, such an instrument could measure angles as 
large as 120DG. In modern practice, the name sextant is commonly applied to 
all instruments of this type regardless of the length of the arc, which is 
seldom exactly 60DG. The optical principles of the sextant are similar to 
those of the prismatic astrolabe.  See  PRISMATIC ASTROLABE. 
    Modern instruments may be grouped into two classes, marine sextants and 
air sextants. The marine sextant (see illustration) is designed for use by 
mariners. It utilizes the visible sea horizon as the horizontal reference. An 
instrument designed for use in aircraft is called an air sextant. Such 
sextants have built-in artificial horizons. Most modern air sextants are 
periscopic to permit observation of celestial bodies without need of an 
astrodome in the aircraft.  See  CELESTIAL NAVIGATION. 
 �34s[Alton B Moody]
�33s 
  
  
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�6v�2v�32sSexual dimorphism   �21m �33s 
    A fundamental phenomenon of life is bisexuality. In some organisms the 
functional separation of the sexes is not marked by obvious morphological 
differences. Commonly, however, the sexes can be diagnosed by differences of 
form or pattern. Such diagnostic differences between the sexes make up the 
phenomenon of sexual dimorphism. Sexual dimorphism among animals ranges from 
differences detectable only with care to differences so extreme that careful 
study has been required to assign the sexes to the same species (see 
illustration). In the great majority of flowering plants, a single flower 
includes both stamens and pistils. Just as in hermaphroditic animals, sexual 
dimorphism is out of the question here. However, staminate and pistillate 
flowers are separate in some species, and thus sexual dimorphism can occur. 
    Another type of sexual dimorphism, polymorphism, is exemplified by 
termites. In this group it appears to be one aspect of a complex polymorphism 
based upon the caste society of these insects.  See  SOCIAL INSECTS. 
 �34s[Edward O. Dodson]
�33s 
  
  
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�6v�2v�32sSferics   �21m �33s 
    The electromagnetic radiations originating in atmospheric electrical 
discharges. Thunderstorms are the principal sources of these discharges. These 
radiations are also known as atmospherics, but the contracted form sferics is 
more common.  See  ATMOSPHERIC ELECTRICITY; ELECTROMAGNETIC RADIATION; 
THUNDERSTORM.  
    Sferics are the major cause of static heard in amplitudemodulated radio 
receivers. This same noise can, however, be put to good use in the detection 
and tracking of severe storms such as squall lines, tornadoes, snowstorms, and 
hailstorms. Sferics are important in two major scientific areas: (1) the 
determination of the characteristics of the source of the radiations; and (2) 
the study of the propagation of these radiations within and beyond the Earth's 
atmosphere.  See  RADIO-WAVE PROPAGATION; STORM DETECTION. 
 �34s[Christos G. Stergis]
�33s 
  
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�6v�2v�32sShadow �21m �33s
    A region of darkness caused by the presence of an opaque object interposed
between such a region and a source of light. A shadow can be totally dark only
in that part called the umbra, in which all parts of the source are screened
off. With a point source, the entire shadow consists of an umbra, since there
can be no region in which only part of the source is eclipsed. If the source
has an appreciable extent, however, there exists a transition surrounding the
umbra, called the penumbra, which is illuminated by only part of the source.
Depending on what fraction of the source is exposed, the illumination in the
penumbra varies from zero at the edge of the full shadow to the maximum where
the entire source is exposed. The edge of the umbra is not perfectly sharp,
even with an ideal point source, because of the wave character of light.  See
DIFFRACTION; ECLIPSE.
 �34s[Francis A. Jenkins /William W. Watson]
�33s

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�6v�2v�32sShadowgraph of fluid flow   �21m �33s 
    A simple method of making visible the disturbances that occur in fluid 
flow at high velocity. Light passing through a flow field of varying density 
is retarded differently through the field, resulting in a turning of the 
wavefronts, that is, a refraction of the rays. This is the basis for 
shadowgraph flow visualization. 
    The application of the shadowgraph to a wind tunnel is shown in the 
illustration. The rays enter normal to the sidewall through the window at the 
left. As they pass through the test section, they encounter a change in 
density of the fluid in the tunnel and are deflected. The light rays then fall 
on a screen, where they are observed or photographed. Where the rays have 
crowded together, the screen is brightly illuminated, and where the rays 
diverge, the screen is dark. Where the spacing is unchanged, the illumination 
is normal, even though there has been a change of density along the path of 
the ray. 
    The light need not be parallel when it enters the test section, so a slit 
source and lens system are not required. This simplicity makes the shadowgraph 
system considerably less expensive than the other principal methods of flow 
visualization, that is, schlieren and interferometer. It is often used where 
the finer resolution of the other systems is not required or desirable.  See  
INTERFEROMETRY; SCHLIEREN PHOTOGRAPHY; SHOCK-WAVE DISPLAY. 
 �34s[Richard F. Chandler]
�33s 
  
  
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�6v�2v�32sShaft balancing   �21m �33s 
    The process (often referred to as rotor balancing) of redistributing the 
mass attached to a rotating body in order to reduce vibrations arising from 
centrifugal force. 
    A rotating shaft supported by coaxial bearings (for example, ball bearings 
together with any attached mass, such as a turbine disk or motor armature) is 
called a rotor. If the center of mass of a rotor is not located exactly on the 
bearing axis, a centrifugal force will be transmitted via the bearings to the 
foundation. The horizontal and vertical components of this force are periodic 
shaking forces that can travel through the foundation to create serious 
vibration problems in neighboring components. 
    Any rigid shaft may be dynamically balanced by adding or subtracting a 
definite amount of mass at any convenient radius in each of two arbitrary 
transverse cross sections of the rotor. The so-called balancing planes 
selected for this purpose are usually located near the ends of the rotor, 
where suitable shoulders or balancing rings have been machined to permit the 
convenient addition of mass (lead weights, calibrated bolts, and so on) or the 
removal of mass (by drilling or grinding). Long rotors, running at high 
speeds, may undergo appreciable elastic deformations. For such flexible rotors 
it is necessary to utilize more than two balancing planes.  See  MECHANICAL 
VIBRATION.  
 �34s[Burton Paul]
�33s 
  
  
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�6v�2v�32sShafting   �21m �33s 
    The machine element that supports a roller and wheel so that they can 
perform their basic functions of rotation. Shafting, made from round metal 
bars of various lengths and machined to dimension the surface, is used in a 
great variety of shapes and applications. Because shafts carry loads and 
transmit power, they are subject to the stresses and strains of operating 
machine parts. Most shafting is rigid and carries bending loads without 
appreciable deflection. Some shafting is highly flexible; it is used to 
transmit motion around corners. 
    Shafts used in special ways are given specific names, although 
fundamentally all applications involve transmission of torque. The primary 
shafting connection between a wheel and a housing is an axle. A short shaft is 
a spindle. A short stub shaft mounted as part of a motor or engine or 
extending directly therefrom is a head shaft. A secondary shaft that is driven 
by a main shaft and from which power is supplied to a machine part is called a 
countershaft.  See  BELT DRIVE; PULLEY. 
 �34s[James J. Ryan]
�33s 
  
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�6v�2v�32sShale   �21m �33s 
    A fine-grained laminated or fissile sedimentary rock made up of silt- and 
clay-sized particles. The average shale consists of about one-third quartz, 
one-third clay minerals, and one-third miscellaneous minerals, including 
carbonates, iron oxides, feldspars, and organic matter. The tendency for 
shales to split along bedding planes is called fissility. 
    No general agreement on a classification scheme for shales has been 
reached. On the basis of the composition of the silt fraction, shales have 
been divided into groups roughly paralleling sandstone types: quartzose shale 
(orthoquartzite), feldspathic shale (arkose), chloritic shale (graywacke), and 
micaceous shale (subgraywacke). Shales may also be subdivided on the basis of 
origin into residual (from reworked soils), transported, and hybrid. The 
hybrid shales are mixtures of clastic and nonclastic materials (carbonate, 
organic matter, iron oxides). In this group belong the black shales.  See  
BLACK SHALE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
�33s 
  
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�6v�2v�32sShaper   �21m �33s 
    A machine tool for cutting flat or flat, contoured surfaces by 
reciprocating a single-point tool across the workpiece. A shaper is usually 
used for small pieces requiring a short cutter stroke rather than for large 
pieces requiring longer strokes provided by a planer. Shapers are classified 
as either horizontal or vertical depending on the plane of motion of the 
reciprocating ram.  See  MACHINING OPERATIONS. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sShark   �21m �33s 
    Any of about 225 species of elasmobranchs, cartilaginous fishes which 
occur in all oceans but are most abundant in tropical and subtropical regions. 
Some species frequent shore waters while others occur near the surface in deep 
waters; some deep-sea forms may be luminescent. All species are carnivorous; 
Carcharhinus nicaraguensis, a fresh-water species, will eat humans. 
    Frilled and cow sharks are the most primitive modern sharks. The cow shark 
(Hexanchus griseum) is a European species with six gill slits, eyes that lack 
a nictitating membrane, primitive teeth, and a single dorsal fin. Heptranchias 
maculatus is one of several species known as the seven-gill sharks. The oil 
shark (H. deani) is found in Japanese waters. 
    Cat sharks and their allies, characterized by five gill slits, an anal 
fin, and two dorsal fins, are the most highly specialized sharks. The lesser 
spotted dogfish (Schyliorhinus caniculus) and the greater spotted dogfish (S. 
stellaris) are common European species. The most ferocious shark is the 
human-eating mackerel shark (Lamnia nasus). The largest sharks are the basking 
sharks, which may reach a length of 40 ft (12 m). 
    Angel and dogfish sharks are small, a foot or more in length. Squalus 
acanthias may reach a length of 3 ft (0.9 m). It is common on both sides of 
the Atlantic, the North Sea, and the Mediterranean. In front of each dorsal 
fin, there is a spine with attached poison glands that can cause painful 
wounds.  See  ELASMOBRANCHII; SELACHII. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sShear   �21m �33s 
    A straining action wherein applied forces produce a sliding or skewing 
type of deformation. A shearing force acts parallel to a plane as 
distinguished from tensile or compressive forces, which act normal to a plane. 
Examples of force systems producing shearing action are forces transmitted 
from one plate to another by a rivet that tend to shear the rivet, forces in a 
beam that tend to displace adjacent segments by transverse shear, and forces 
acting on the cross section of a bar that tend to twist it by torsional shear 
(see illustration). Shear forces are usually accompanied by normal forces 
produced by tension, thrust, or bending. Shearing stress is the intensity of 
distributed force expressed as force per unit area.  See  STRESS AND STRAIN. 
 �34s[John B. Scalzi]
�33s 
  
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�6v�2v�32sShear center   �21m �33s 
    The point in the plane through a section of a structural member at which a 
shear force can be applied without producing a twist of that section. The 
shear center of a section through a member, such as a rolled wide-flange beam, 
that has two planes of symmetry coincides with the geometric center or 
centroid of the section. When such a member is loaded transversely, it bends 
without twisting. However, the structural member may be unsymmetrical or may 
have but one plane of symmetry and may be loaded elsewhere than through that 
plane, as in an open section made of thin material for resisting bending in 
aircraft construction; then the load force and the reaction force of the 
structural member constitute a couple that, in general, causes the member to 
twist. The load can be applied through a unique point in the plane of the 
section so that the moments in the plane of the section are balanced and the 
beam will not twist. This unique point is called the shear center. In the 
illustration, in a the channel bends when the load is along the axis of 
symmetry; in b the channel twists, even with the load through the centroid if 
the load is not along the axis of symmetry; in c the channel resists twisting 
when the load is through the shear center.  See  LOADS, TRANSVERSE; SHEAR. 
 �34s[John B. Scalzi]
�33s 
  
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�6v�2v�32sSheathed bacteria   �21m �33s 
    Bacteria consisting of chains of cells, usually rod-shaped, enclosed in a 
hyaline envelope or sheath. Seven genera are recognized: Sphaerotilus, 
Leptothrix, Streptothrix, Lieskeella, Phragmadiothrix, Crenothrix, and 
Clonothrix. Sphaerotilus natans and S. discophorus oxidize ferrous and 
manganous ions and deposit the oxides on the sheaths in large amounts. Thus 
they play an important role in the cycles of iron and manganese in nature and 
may be responsible in part for the accumulation of such metal oxides. Also, S. 
natans is involved in water pollution. Massive growths of Sphaerotilus 
frequently develop in streams and rivers polluted by domestic sewage and 
industrial wastes, especially those from paper pulp mills. The extensive, 
white, cottony growths destroy the natural beauty of rivers and beaches and 
cause considerable financial losses to commercial fishermen by fouling lines, 
clogging and destroying nets, and even coating the fish. 
 �34s[Jacob L. Stokes]
�33s 
  
  
  
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�6v�2v�32sSheep   �21m �33s 
    Sheep are members of the family Bovidae in the order Artiodactyla, the 
even-toed, hoofed mammals. Sheep were possibly the first animals to be 
domesticated about 11,000 years ago in southwestern Asia.  See  ARTIODACTYLA. 
    Wild sheep range over the mountainous areas of the Northern Hemisphere. 
They have long, curved, massive horns and mixed coats with a hairy outercoat 
and a woolly undercoat. Although wild sheep vary in size, they are usually 
larger than domestic sheep and have shorter tails. 
    Sheep are called lambs until about 12 months of age, from which time to 24 
months they are referred to as yearlings, and thereafter as two-year-olds, and 
so on. The female sheep is called a ewe and the male, a ram or buck; the 
castrated male is called a wether. Sheep meat is called lamb or mutton 
depending on the age at slaughter. 
    A precise definition is difficult since sheep are so variable. Horns, if 
present, tend to curl in a spiral. If horns occur in both sexes, they are 
usually larger for the ram. In some breeds only the rams have horns but many 
breeds are hornless. The hairy outercoat common to wild sheep was eliminated 
by breeding in most domestic sheep. Wool may cover the entire sheep, or the 
face, head, legs, and part pf the underside may be bare; or wool may be absent 
as in some breeds (bred for meat) with a short hairy coat or a coat which 
constantly sheds. Although domestic sheep are mostly white, shades of brown, 
gray, and black occur, sometimes with spotting or patterns of color.  See  
WOOL.  
    Hundreds of breeds of sheep of all types, sizes, and colors are found over 
the world. Wool-type breeds, mostly of Merino origin, are important in the 
Southern Hemisphere, but both fine- and long-wool types are distributed all 
over the world. Sheep with fat tails or fat rumps are common in the desert 
areas of Africa and Asia. These usually produce carpet wool. Milk breeds are 
found mostly in central and southern Europe. Meat breeds from the British 
Isles are common over the world. 
 �34s[Clair E. Terrill]
�33s 
  
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�6v�2v�32sSheet-metal forming   �21m �33s 
    The shaping of thin sheets of metal (usually less than 1/4 in. or 6 mm) by 
applying pressure through male or female dies or both. Parts formed of sheet 
metal have such diverse geometries that it is difficult to classify them. 
Sheet forming is accomplished basically by processes such as stretching, 
bending, deep drawing, embossing, bulging, flanging, roll forming, and 
spinning. In most of these operations there are no intentional major changes 
in the thickness of the sheet metal.  See  METAL FORMING. 
    Stretch forming is a process in which the sheet metal is clamped between 
jaws and stretched over a form block. The process is used primarily in the 
aerospace industry to form large panels with varying curvatures. 
    Bending is one of the most common processes in sheet forming. The part may 
be bent not only along a straight line, but also along a curved path 
(stretching, flanging). In addition to male and female dies used in most 
bending operations, the female die can be replaced by a rubber pad (Fig. 1). 
The roll-forming process replaces the vertical motion of the dies by the 
rotary motion of rolls with various profiles. Each successive roll bends the 
strip a little further than the preceding roll. 
    While many sheet-forming processes are carried out in a press with male 
and female dies usually made of metal, there are some processes which utilize 
rubber to replace one of the dies. The simplest of these processes is the 
Guerin process (Fig. 2). 
    A great variety of parts are formed by the deep-drawing process (Fig. 3), 
the successful operation of which requires a careful control of factors such 
as blank-holder pressure, lubrication, clearance, material properties, and die 
geometry.  
    Many parts require one or more additional processes. Embossing consists of 
forming a pattern on the sheet by shallow drawing. Coining consists of putting 
impressions on the surface by a process that is essentially forging, the best 
example being the two faces of a coin. Shearing is separation of the material 
by the cutting action of a pair of sharp tools, similar to a pair of scissors. 
  See  COINING. 
    The spinning process forms parts with rotational symmetry over a mandrel 
with the use of a tool or roller. There are two basic types of spinning: 
conventional or manual spinning, and shear spinning. The conventional spinning 
process forms the material over a rotating mandrel with little or no change in 
the thickness of the original blank. In shear spinning (hydrospinning, 
floturning) the deformation is carried out with a roller in such a manner that 
the diameter of the original blank does not change but the thickness of the 
part decreases by an amount dependent on the mandrel angle. Shear spinning 
produces parts with various shapes (conical, curvilinear, and also tubular by 
tube spinning on a cylindrical mandrel) with good surface finish, close 
tolerances, and improved mechanical properties.  See  MACHINING OPERATIONS; 
METAL COATINGS; SPINNING (METALS). 
 �34s[Serope Kalpakjian]
�33s 
  
  
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�6v�2v�32sShellac   �21m �33s 
    The lac resin (secreted by the lac insect) when used in flake (or shell) 
form. Shellac varnish is a solution of shellac in denatured alcohol. 
    Shellac varnish is widely used in wood finishing where a fast-drying, 
light-colored, hard finish is desired. Shellac varnish is not water-resistant 
and is not suitable for exterior coatings, but is widely used for finishing 
floors and for sealing materials (for example, asphalt, coal tar, oil stains, 
and resinous knots) which are soluble in mineral spirits but not in alcohol.  
See  WOOD FINISHING. 
 �34s[C. R. Martinson ;C. W. Sisler]
�33s 
  
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�6v�2v�32sShielded thermocouple �21m �33s
    An instrument for obtaining the total temperature inside a pitot tube
using a thermocouple rather than a pressure-sensing device. The thermocouple
must be shielded to minimize losses due to radiation and conduction.
Calibration takes account of probe configurations, wall conductivity, and gas
stream conditions. The same instrument may be used for subsonic and supersonic
flows. See AIR-VELOCITY MEASUREMENT; THERMOCOUPLE.
 �34s[Herbert Fox]
�33s

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�6v�2v�32sShipbuilding   �21m �33s 
    The construction of large vessels which travel over seas, lakes, or 
rivers. Many different approaches have been used in the construction of ships. 
Sometimes a ship must be custom-built to suit the particular requirements of a 
low-volume trade route with unique cargo characteristics. On the other hand, 
there are many instances where a significant number of similar ships are 
constructed, providing an opportunity to employ procedures which take 
advantage of repetitive processes. 
    The building of a ship can be divided into seven phases: design, 
construction planning, work prior to keel laying, ship erection, launching, 
final outfitting, and sea trials.  See  SHIP DESIGN. 
    The construction planning process establishes the construction techniques 
to be used and the schedules which all of the shipbuilding activities must 
follow. Construction planners generally start with an erection diagram on 
which the ship is shown broken down into erection zones and units. To 
facilitate the fabrication of steel, insofar as possible, the erection units 
are designed to be identical. The size (or weight) of the erection units 
selected is usually limited by the amount of crane capacity available. Once 
the construction planners have established the manner in which the ship is to 
be erected and the sequence of construction, the schedules for construction 
can be developed. Working backward from the time an erection unit is required 
in the dock, with allowances made for the many processes involved, a schedule 
of working plans and for procurement of purchased equipment is prepared. 
    Before the keel of a ship is laid (or when the first erection unit is 
placed in position) a great deal of work must have been accomplished for work 
to proceed efficiently. The working drawings prepared by ship designers 
completely define a ship, but often not in a manner that can be used by the 
construction trades people. Structural drawings prescribe the geometry of the 
steel plates used in construction, but they cannot be used, in the form  
prepared, to cut steel plates. Instead, the detailed structural drawings must 
be translated into cutting sketches, or numerical-control cutting tapes, which 
are used to fabricate steel. Several organizations have developed 
sophisticated computer programs which readily translate detailed structural 
drawings into machine-sensible tapes which can be used to drive cutting 
torches.  
    If all of the preceding work has been accomplished properly and on 
schedule, the erection of a ship can proceed rapidly; however, problem areas 
invariably arise. When erecting a ship one plate at a time, there are no 
serious fitting problems; but when 900-metric-ton erection units do not fit 
(or align) properly, there are serious problems which tend to offset some of 
the advantages for this practice. 
    A ship is launched as soon as the hull structure is sufficiently complete 
to withstand the strain. Ships may be launched endwise, sidewise, or by 
in-place flotation (for example, graving docks). The use of a graving dock 
requires a greater investment in facilities than either of the other two 
methods, but in some cases there may be an overall advantage due to the 
improved access to the ship and the simplified launch procedure.  See  DRY 
DOCKING.  
    The final outfitting of a ship is the construction phase during which 
checks are made to ensure that all of the previous work has been accomplished 
in a satisfactory manner; and lastminute details, such as deck coverings and 
the top coat of paint, are completed. It is considered good practice to 
subject as much of the ship as possible to an intensive series of tests while 
at the dock, where corrections and final adjustments are more easily made than 
when at sea. As a part of this test program, the main propulsion machinery is 
subjected to a dock trial, during which the ship is secured to the dock and 
the main propulsion machinery is operated up to the highest power level 
permissible.  
    When a comprehensive program of dockside tests have been completed, the 
only capabilities which have not been demonstrated are the operation of the 
steering gear during rated-power conditions and the operation of the main 
propulsion machinery at rated power; these capabilities must be demonstrated 
during trials at sea. 
 �34s[Roy L. Harrington]
�33s 
  
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�6v�2v�32sShip design �21m �33s
    The design of a ship involves a selection of the features of form, size,
proportions, and other factors which are open to choice, in combination with
those features which are imposed by circumstances beyond the control of the
design naval architect. As ships have increased in size and complexity, plans
for building them have become more detailed and more varied. The intensive
research since the period just prior to World War II has brought about many
technical advances in the design of ships. These changes have been brought
about principally by the development of new welding techniques, developments
in main propulsion plants, advances in electronics, and changes in materials
and methods of construction.  See  SHIPBUILDING.
    Ships are designed, built, and operated to fulfill the requirements and
limitations specified by the operator and owner. These owner's requirements
denote the essential considerations which are to form the basis for the
design. They may be generally stated as (1) a specified minimum deadweight
carrying capacity, (2) a specified measurement tonnage limit, (3) a selected
speed at sea, or a maximum speed on trial, and (4) maximum draft combined with
other draft limitations. Apart from these requirements, the ship owner expects
the designer to provide a thoroughly efficient ship.
    All ships have many requirements which are common to all types, whether
they are naval, merchant, or special-purpose ships. The first of such
requirements is that the ship must be capable of floating when carrying the
load for which it was designed. One of the first things which a designer must
do is to determine the weight and size of the ship and decide upon a suitable
hull form to provide the necessary buoyancy to support the weight that has
been chosen.  See  BUOYANCY.
    From the specified requirements, an approach is made to the selection of
the dimensions, weight, and displacement of the new design. This is a detailed
operation, but some rather direct approximations can be made to start the
design process. This is usually done by analyzing data available from an
existing ship which is closely similar. Length and speed are related to the
hull form, the propulsion machinery, and the propeller design. Speed has an
important influence on the length selected for the ship. The speed of the ship
is related to the length in terms of the ratio *EQN*, where V is the speed in
knots and L is the effective waterline length of the ship. As the speed-length
ratio increases, the resistance of the ship increases.
    A ship must be stable under all normal conditions of loading and
performance at sea. This means that when the ship is inclined from the
vertical by some external force, it must return to the vertical when the
external force is removed. Stability may be considered in the transverse or in
the longitudinal direction. In surface ships, longitudinal stability is of
much less concern than transverse stability. Submarines, however, are
concerned with longitudinal stability in the submerged condition. The
transverse stability of a surface ship must be considered in two ways, first
at small angles of inclination, called initial stability, and second at large
angles of inclination. Initial stability depends upon two factors,  (1) the
height of the center of gravity of the ship above the base line and (2) the
underwater form of the ship.
    A ship at sea is subjected to many forces because of the action of the
waves, the motion of the ship, and the cargo and other weights which are
distributed throughout the length of the ship. These forces produce stresses
in the structure, and the structure must be of suitable strength to withstand
the action. The determination of the minimum amount of material required for
adaquate strength is essential to attaining the minimum weight of the hull.
The structure as a whole bends in a longitudinal plane, with the maximum
bending stresses being found in the bottom and top of the hull girder.
Therefore, depth is important because as it is increased, less material is
required in the deck and bottom shell. However, there are limits which control
the maximum depth in terms of practical arrangement and efficiency of design.
    The desired hydrostatic and hydrodynamic characteristics which the new
ship form should have are formulated by considering resistance and propulsion,
stability, volumetric requirements, steering and maneuverability, performance
in rough weather, and the freeboard necessary for seaworthiness and safety.
After these factors have been decided, the work on the drawing of the hull
form is started. The geometry of the ship is usually delineated on a drawing
which is commonly referred to as the lines drawing of the ship, or simply the
lines.
    After the lines have been completed, they are usually sent to a model
basin (test facility) for the construction of a scale model and for testing in
a basin. These tests are generally carried out in still water. The power
determined from these experiments represents the power required to drive the
ship under ideal sea conditions, and allowances must be made for many
departures from these ideals, such as probable rough water performance and
increased resistance as a result of fouling.
    After having established the principal dimensions, form, and general
arrangement of the ship, the designer undertakes the problem of providing a
structure capable of withstanding the forces which may be imposed upon it. The
hull of a steel merchant ship is a complex structure, unique in the field of
engineering structures in that it is primarily a plate structure, depending
for its major overall strength on the plating of the shell, decks, and in most
cases, also on the inner bottom and longitudinal bulkheads. The framing
members, each of which has its own function to perform, are designed primarily
to maintain the plate membranes to the planned contours and their positions
relative to each other when subjected to the external forces of water pressure
and breaking seas, as well as to the internal forces caused by the services
for which the ship is designed. Unlike most other large engineering
structures, the forces supporting the ship's hull as well as the loads which
may be imposed upon it vary considerably, and in many cases, cannot be
determined accurately. As a result, those responsible for the structural
design of ships must be guided by established standards.
 �34s[John C. Niedermair]
�33s

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�6v�2v�32sShip powering and steering �21m �33s
    The earliest propulsion of boats and vessels by paddles or oars was
succeeded or augmented by the use of sails. Power propulsion essentially began
in the early 1800s, when the steam engine was adapted to marine use. The first
screw-propelled steamer made the Atlantic crossing in 1845. The great
advantages of the screw propeller have made it almost the only form of
propulsion of marine craft since that time. There has been some use of pump
jets for propulsion of small craft and hydrofoil boats.  See  PROPELLER
(MARINE CRAFT).
    Steam reciprocating engines were the only form of power plant used until
about 1890, when both steam turbines and diesel engines were developed. These
latter plants have largely supplanted steam reciprocating engines. Nuclear
steam turbine units are now used for submarine and some surface ship
propulsion, mainly aboard naval ships. The relatively high cost of nuclear
power precludes its widespread use for merchant ships. Another power plant
which has been used in ships is the gas turbine.  See  MARINE ENGINE; SHIP
PROPULSION REACTOR.
     For steering and maneuvering, ships are normally fitted with one
or more rudders at the stern and in most cases located directly aft
of the propeller or propellers. Multiple rudders are commonly fitted to
warships where a high degree of maneuverability is required and where twin or
quadruple screws are employed. Rudders are controlled from the bridge and
other locations through an electrical or hydraulic system which actuates large
hydraulic steering engines.
    Because the gyrocompass is more sensitive in detecting initial deviations
from the course than is the human eye, automatic steering devices offer an
improvement over hand steering. The improvement may be greater than sometimes
realized, since reduction of rudder drag reduces the fuel used, reduction of
rudder angle required reduces the work of the steering engine, and reduction
of deviations from course reduces the distance traveled.  See  GYROCOMPASS.
    By international law, oceangoing ships for years have been required to
have sufficient power for astern operation to secure proper control of the
ship in all normal circumstances. In addition, maximum safe speed under
conditions of reduced visibility is an important operational consideration and
is closely related to stopping ability. The problem varies with type of power
plant. A system finding great favor with ship operation is the controllable
and reversible pitch propeller which avoids the necessity of reversing
machinery.
 �34s[Richard B. Couch]
�33s

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�6v�2v�32sShip propulsion reactor �21m �33s
    A nuclear reactor used to supply motive power to a ship. Nuclear reactors
for shipboard propulsion can, in theory, be of any type used for the
production of useful power.  See  NUCLEAR REACTOR.
    In all the shipboard nuclear power plants that have been built, energy
conversion is based on the steam-turbine cycle, and that portion of the plant
is more or less conventional. Gas-turbine applications are also possible, and
these have been studied.  See  MARINE ENGINE.
    There are four radical differences between shipboard reactors and similar
installations ashore, involving (1) problems of weight and space limitations,
(2) problems of plant reliability and on-board maintenance, (3) problems in
plant safety, and (4) problems inherent in location on a moving platform.
    Only two types of reactors - the pressurized-water reactor and the
sodium-cooled reactor - have actually been applied to operating vessels. The
pressurized-water plant has been the favorite because it can be more easily
shielded and maintained in working order.
    The U.S. Navy has in operation and under construction a large number of
nuclear-powered submarines and surface vessels. The first nonnaval marine
installations of nuclear power were on the Soviet icebreaker Lenin (1957) and
on the United States demonstration merchant ship NS Savannah (1959), both of
which used pressurized-water reactors.
 �34s[Louis H. Roddis, Jr. ;Michael G. Parsons]
�33s

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�6v�2v�32sShip routing   �21m �33s 
    The selection of the most favorable route for a ship's transit, based upon 
the environment conditions expected during the voyage, and the effect of these 
conditions upon the ship's performance. Ship routing is also referred to as 
weather routing, optimum track ship routing, and meteorological navigation. 
    Routes can be computed to minimize transit time, avoid waves over a 
specific height, or, more typically, provide a compromise between travel time 
and the roughness of the seaway to be encountered. Routing offices compute the 
initial route based on the long-range weather forecast, but maintain a daily 
plot of both the ship and the storm positions. If conditions warrant, an 
amended route is sent to the ship master based on the latest weather changes.  
See  OCEAN WAVES; WEATHER FORECASTING AND PREDICTION. 
    Although the effect of waves is the predominant factor considered, the 
ship router also utilizes knowledge of ocean currents, wind, and hazards to 
navigation such as ice fields or fog banks. On United States east coast runs, 
the currents are more important than the waves. Most major oil companies have 
their tankers follow the Gulf Stream on the northbound runs and avoid it on 
the return trip.  See  OCEAN CURRENTS. 
 �34s[Richard W. James]
�33s 
  
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�6v�2v�32sShip salvage   �21m �33s 
    Rendering aid to distressed property from a peril at sea or while on other 
navigable waters. The property in distress could be a vessel of any size or 
character, from a rowboat to a cruise ship, or the cargo aboard a ship. 
    Typical salvage situations involve, in order of frequency, rescue 
tourings, strandings, and sunken vessels. 
    Ships frequently are disabled at sea as a result of breakdown of engines, 
fire, collision, loss of propeller, or loss of steering control. The shipowner 
arranges for a salvage tug to tow the disabled vessel to a port of repair. 
    Crew error, failure or malfunction of a navigational aid, and storm 
conditions are often the cause of a vessel's running aground on a submerged 
reef and occasionally right up on a shoreline. If the vessel is not too hard 
aground, it can frequently be refloated by pulling of tugs on high tide. More 
serious strandings may require the employment of beach-gear ground tackles. 
    When a vessel is sunk with the main deck still out of water, a combination 
of patching any holes or openings and pumping out the water is all that is 
necessary to refloat. When the vessel's main deck is under water, more 
sophisiticated techniques are required. If the vessel's main deck is not 
deeply submerged, a waterproof wall may be built around the main deck to above 
the water surface and the vessel then patched and pumped. This wall or barrier 
is known as a cofferdam. Plastics technology has given birth to the so-called 
foam-in-salvage technique, whereby chemicals are combined by a mixing gun held 
by a diver in the sunken ship to form urethane foam. The foam hardens quickly, 
forcing the water out of the vessel and making it buoyant. Another similar 
technique entails pumping of particles of expanded plastic in bead or disk 
form into the sunken vessel to displace the water. A third technique involves 
the pumping of hollow polystyrene balls the size of basketballs into the 
sunken vessel. 
    One of the quickest and most convenient methods of raising sunken vessels, 
particularly smaller ones, is the use of heavy lift cranes. Divers attach 
slings to the sunken vessel, and floating cranes lift the vessel to the 
surface where it can be pumped out. 
    A similar technique involves the use of heavy lift craft. Wire cables are 
passed under the sunken vessel. The lift craft take on ballast water to 
increase their draft. At low tide, the wires are pulled tight on the deck of 
the lift craft. On the rising tide, the salvage crews pump out the lift craft 
to raise the wreck off the bottom. 
    Occasionally, sunken barges and smaller vessels can be refloated by use of 
pontoons. Pontoons can be metal tanks or large rubber bags. The pontoon is 
flooded to sink it alongside the sunken vessel. A diver then attaches the 
pontoon to the wreck with chain or wire cable. The water in the pontoon is 
forced out by pumping in compressed air, and this gives the necessary buoyancy 
to lift the vessel. 
 �34s[Peter S. Barracca]
�33s 
  
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�6v�2v�32sShipworm   �21m �33s 
    A wood-boring mollusk of the family Teredinidae. These borers, also called 
pileworms, are found around the world in tropical to boreal and marine to 
nearly fresh-water habitats, from the intertidal to depths of about 2000 ft 
(600 m). It was not until humans became navigators that shipworms became 
pests. Before the advent of steel ships, they were greatly feared, and even 
today hundreds of millions of dollars are spent annually on prevention and 
repair due to their activity.  See  BIVALVIA. 
 �34s[Ruth D. Turner]
�33s 
  
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�6v�2v�32sShock absorber   �21m �33s 
    Effectively a spring, a dashpot, or a combination of the two, arranged to 
minimize the acceleration of the mass of a mechanism or portion thereof with 
respect to its frame or support. 
    The spring type of shock absorber is generally used to protect delicate 
mechanisms, such as instruments, from direct impact or instantaneously applied 
loads. Such springs are often made of rubber or similar elastic material.  See 
  SHOCK ISOLATION. 
    An example of the dashpot type of shock absorber is the direct-acting 
shock absorber in an automotive spring suspension system (see illustration). 
Here the device is used to dampen and control a spring movement. The energy of 
the mass in motion is converted to heat by forcing a fluid through a 
restriction, and the heat is dissipated by radiation and conduction from the 
shock absorber.  See  VIBRATION DAMPING. 
    There are also devices available which combine springs and viscous damping 
(dashpots) in the same unit. They use elastic solids (such as rubber or 
metal), compressed gas (usually nitrogen), or both for the spring. A 
flat-viscosity hydraulic fluid is used for the viscous damping. 
 �34s[L. Sigfred Linderoth, Jr.]
�33s 
  
  
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�6v�2v�32sShock isolation   �21m �33s 
    The application of isolators to alleviate the effects of shock on a 
mechanical device or system. The term shock generally denotes suddenness, 
either in the application of a force or in the inception of a motion.   See  
SHOCK WAVE. 
    Shock isolation is accomplished by storing energy in a resilient medium 
(isolator, cushion, and so on) and releasing it at a slower rate. The 
effectiveness of an isolator depends upon the duration of the shock impact. 
    Rubber is the most common material used in commerical shock isolators. 
Rubber isolators are generally used where the shock forces are created through 
small displacements. For larger displacement shock forces, such as those 
experienced by shipping containers in rough handling conditions, thick 
cushions of felt, rubberized hair, sponge rubber, cork, or foam plastics are 
used.  
    The shock load must be divided between the case, the shock cushion, and 
the equipment. The case, since it must withstand effects of rough handling 
such as sliding and dropping, is by necessity rigid. The more rigid the case 
the closer to a 1:1 ratio will be the transfer of the shock from outside to 
inside. The absorption of the shock is primarily between the cushion and the 
equipment.  See  DAMPING; SHOCK ABSORBER; SPRING (MACHINES); VIBRATION DAMPING. 
 �34s[K. W. Johnson]
�33s 
  
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�6v�2v�32sShock syndrome   �21m �33s 
    A state characterized by prostration and hypotension due to diminished 
circulating blood volume, with reduced capillary blood flow and tissue hypoxia. 
    The five major varieties of shock include neurogenic, oligemic, vasogenic 
or endotoxic, cardiogenic, and anaphylactic shock. The most common types of 
shock are neurogenic and oligemic. Neurogenic  shock is synonymous with 
fainting and is produced by nervous or psychic stimuli such as sudden pain, 
fright, or fear. These stimuli result in a sudden outflow of autonomic 
impulses to the arterioles, producing vasodilatation and rapid pooling of 
blood in the splanchnic bed and voluntary muscles. Cerebral blood flow is thus 
reduced, and fainting occurs. Neurogenic shock is usually self-limiting, and 
rest in a recumbent position is usually sufficient to restore cerebral blood 
flow.  
    Oligemic shock is synonymous with hypovolemic or hemorrhagic shock. It is 
caused by such things as massive hemorrhage, severe crushing injuries, and 
burns. There is a true reduction in blood volume which may lead to hypotension 
and irreversible damage to vital centers. 
    ``Shock'' is also used, sometimes inaccurately, to denote superficially 
similar states, such as emotional, insulin, and electrical shock. Pallor, 
apprehension, cold skin, sweating, low blood pressure, and rapid, shallow 
pulse and breathing are typical findings. Any form of severe stress may induce 
this state; the most common causes are hemorrhage, physical injury with severe 
pain, and burns. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sShock tube   �21m �33s 
    A laboratory device for rapidly raising confined samples of gases to 
preselected high temperatures and densities. This is accomplished by a shock 
wave, generated by breaking a fragile diaphragm, which separates the 
low-pressure section (test) from the high-pressure section (driver) of the 
tube. Shock tubes can be circular, square, or rectangular in cross section. 
    Samples which attained thermodynamic equilibrium after passage through the 
shock front have thus been prepared under homogeneous conditions at high 
temperatures. Their optical and spectroscopic properties can be measured. 
Conversely, when the indices of refraction or the spectroscopic parameters of 
the various species suspected of being present in the shock-heated gases are 
known (or can be reliably estimated), their measurement provides information 
on the exact composition and hence of the thermodynamic state of the gas.  See 
  CHEMICAL THERMODYNAMICS; SPECTROSCOPY. 
 �34s[Simon H. Bauer]
�33s 
  
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�6v�2v�32sShock wave   �21m �33s 
    A fully developed compression wave of large amplitude. Shock waves arise 
from sharp and violent disturbances generated from a lightning stroke, bomb 
blast, or other form of intense explosion, and from steady supersonic flow 
over bodies. Measurements of fluid density, pressure, and temperature across 
the surfaces show that these quantities always increase along the direction of 
flow, and that the rates of change are usually so rapid as to be beyond the 
spatial resolution of most instruments. These surfaces of abrupt change in 
fluid properties are called shock waves or shock fronts.  See  WAVE MOTION IN 
FLUIDS.  
 �34s[Shao-Chi Lin]
�33s 
  
    Display.  Shock waves can be made visible for purposes of photography. 
Shock-wave display techniques are used most extensively in aerodynamic test 
facilities. The most widely used techniques are the schlieren and shadowgraph 
techniques; optical interferometry is also used. All three methods are based 
on the variation of the index of refraction with density in the medium 
traversed by light. The schlieren and shadowgraph methods have the advantage 
of simplicity of experimental set-up, and are usually preferred because they 
readily display even weak disturbances such as nearly sonic shocks.  See  
INTERFEROMETRY; SCHLIEREN PHOTOGRAPHY; SHADOWGRAPH OF FLUID FLOW. 
    The interferometric technique permits a direct measurement of the density 
change across the shock wave. The change is presented as displacement of 
interference fringes (see illustration). The method is best suited for 
obtaining quantitative information on shock waves and flow fields. 
    For the visualization of shock-wave phenomena in gases of low density, the 
above methods are too insensitive. Under such conditions, techniques utilizing 
the absorption of radiation or corpuscular rays provide better means for 
detection of shock waves. Best suited for the radiation absorption techniques 
are the spectral regions of strong continuous absorption, which for most gases 
are in the ultraviolet, or the region of soft x-rays. Corpuscular-ray 
absorption techniques use monoenergetic particles such as electrons, protons, 
or ALPHA particles.  
    Two other methods for visualization of especially low-density flows are 
the glow discharge and afterglow techniques. In these techniques the gas, 
prior to entering the test chamber, is subjected to a strong electrical 
discharge which excites it to luminescence. The light emission accompanying 
the discharge is used in the first method; the glow persisting after the 
exciting discharge has been cut off is used in the second method. Shock waves 
become visible because density changes affect the luminous intensity as well 
as the spectral distribution of the glow. 
 �34s[Eva M. Winkler]
�33s 
  
  
    Solids.  The term shock wave, with reference to solids, generally means a 
large-amplitude compressional stress wave or pressure pulse that produces an 
almost instantaneous increase in the value of stress or pressure in a material. 
    Large-amplitude stress waves occur in the Earth as a result of events such 
as underground explosions, earthquakes, and meteorite impacts. In the 
laboratory they can be produced under controlled conditions and have become a 
very useful tool in solid-state and geophysical research. 
    The passage of a shock wave through a solid produces changes in the 
physical state of the material. For example, it can redistribute atoms, change 
the levels of electron energy, and alter the internal energy balance. 
Measurements of the behavior of solids subjected to strong compressional 
pulses have provided new insight into the basic properties of solids and have 
led to the development of commercially valuable processes such as explosive 
cladding and welding, explosive cutting and forming, shock compaction of 
powders into solid parts, explosive excavation of the Earth, and production of 
new materials or those with special properties. 
    Experimenters usually produce shock waves in a material by the detonation 
of a high explosive in contact with the material or by the impact of a 
high-speed projectile fired from a gun or accelerated by a high explosive. 
 �34s[Ronald K. Linde]
�33s 
  
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�6v�2v�32sShore birds   �21m �33s 
    A general term, synonymous with waders, applied to members of the 19 
families in the suborders Charadrii, Pterocli, Lari, and Alcae of the order 
Charadriiformes. These include such common species as the plovers, curlews, 
jacanas, phalaropes, woodcocks, avocets, snipes, and oyster catchers. Most of 
these birds are always found near water, although their habitats are quite 
varied. Wide variations are noted for their feet, which may be webbed and may 
terminate in three or four long or short toes. See CHARADRIIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sShort circuit   �21m �33s 
    A term commonly used to describe an electrical connection of negligible 
impedance connected across a pair of terminals. There is implication of an 
undesirable condition arising from electrical insulation failure due to 
improper operation, mechanical damage, or damage by natural causes such as 
lightning and rain. Protection against such short circuits is a major field in 
electrical power engineering.  See  ELECTRIC PROTECTIVE DEVICES. 
    Short circuits are often used to advantage on high-frequency transmission 
lines. For instance, a stub transmission line, one-quarter wavelength long and 
short-circuited at one end, acts as an insulator at the opposite end and 
therefore is used as a support for a high-frequency transmission line.  See  
CIRCUIT (ELECTRICITY); TRANSMISSION LINES. 
 �34s[Robert L. Ramey]
�33s 
  
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�6v�2v�32sShort takeoff and landing (STOL)   �21m �33s 
    The term applied to heavier-than-air craft that cannot take off and land 
vertically, but can operate within areas substantially more confined than 
those normally required by aircraft of the same size. A pure STOL aircraft is 
a fixed-wing vehicle that derives lift primarily from free-stream airflow over 
the wing and its high lift system, sometimes with significant augmentation 
from the propulsion system. Although all vertical takeoff and landing (VTOL) 
machines, including helicopters, can lift greater loads by developing forward 
speed on the ground before liftoff, they are still regarded as VTOL (or V/STOL 
craft), operating in the STOL mode.  See  VERTICAL TAKEOFF AND LANDING (VTOL). 
    It has been customary to define STOL capability in terms of the runway 
length required to take off or land over a 50-ft (15-m) obstacle, the concept 
of ``short'' length being variously defined as from 500 to 2000 ft (150 to 600 
m), depending on the high-lift concept employed and on the mission of the 
aircraft. In addition to being able to operate from short runways, STOL 
aircraft are usually expected to be able to maneuver in confined airspace so 
as to minimize the required size of the terminal area. Such aircraft must 
therefore have unusually good slow-flight stability and control 
characteristics, especially in turbulence and under instrument flight 
conditions.  See  AIRPLANE. 
 �34s[R. E. Kallio]
�33s 
  
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�6v�2v�32sShrew   �21m �33s 
    An insectivorous mammal of the family Soricidae found in Asia, Africa, 
Europe, and North America. The family includes 265 species grouped into three 
subfamilies, the red-toothed, white-toothed, and armored shrews. Shrews are 
small, savage animals with extremely sharp teeth. The tail is moderately long, 
the eyes are minute, the snout is sharp-pointed, and the ears are small (see 
illustration).  
    Shrews are solitary individuals that establish distinct territories. 
Couples are found together only during mating. The female builds the nest, a 
hole in the ground lined with leaves and moss, where she deposits her litter 
of 5 - 10 young, born after a gestation period of between 2 - 3 weeks. The 
female mates at the age of 1 year; however, since the life-span of the shrew 
is only 15 months, the females always die during the fall after the litter is 
born.  See  INSECTIVORA. 
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sShrimp   �21m �33s 
    The common name applied to a number of crustaceans which are members of 
various orders or suborders; however, the edible and economically important 
species are decapod crustaceans of the suborder Natantia. The various ostracod 
orders, including small to microscopic forms, are referred to as the seed 
shrimp. The stomatopoda are known as the mantis shrimp, while the opossum 
shrimp are members of the order Mysidaceae. Mantis shrimps are marine 
crustaceans which inhabit burrows on the bottom of the sea. The body is 
elongate with a broad abdomen; the second thoracic limb, used to capture 
smaller crustaceans, is quite large and resembles the front leg of a praying 
mantis. The head is unique in that there are two movable segments bearing the 
antennules and eyes. Largest species is about 1 ft (30 cm) long. See DECAPODA 
(CRUSTACEA); OSTRACODA; STOMATOPODA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sShrink fit   �21m �33s 
    A fit that has considerable negative allowance so that the diameter of a 
hole is less than the diameter of a shaft that is to pass through the hole, 
also called a heavy force fit. Shrink fits are used for permanent assembly of 
steel external members, as on locomotive wheels. The difference between a 
shrink fit and a force fit is in method of assembly. In shrink fits, the outer 
member is heated, or the inner part is cooled, or both, as required. The parts 
are then assembled and returned to the same temperature.  See  ALLOWANCE; 
FORCE FIT. 
 �34s[Paul H. Black]
�33s 
  
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�6v�2v�32sShunting   �21m �33s 
    The act of connecting one device to the terminals of another so that the 
current is divided between the two devices in proportion to their respective 
admittances. Shunting is widely used in ammeters, galvanometers, and other 
current-measuring instruments to bypass part of the current around the 
instrument so as to change the measuring range. Resistors are frequently 
shunted across tuned circuits to broaden the tuning characteristics. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sSiberian tick typhus   �21m �33s 
    A relatively benign, rash- and eschar-producing, spotted feverlike disease 
in northern Asia, caused by the bacterialike microorganism Rickettsia 
sibirica. The disease is transmitted by four species of Dermacentor and two of 
Haemaphysalis. Clinically, Siberian tick typhus resembles most closely fievre 
boutonneuse, including low mortality, but epidemiologically, it is like 
American spotted fever, with the primary natural cycle between ground 
squirrels and other small rodents, and their immature tick parasites.  See  
FIEVRE BOUTONNEUSE; RICKETTSIALES; RICKETTSIOSES; ROCKY MOUNTAIN SPOTTED FEVER. 
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sSideband �21m �33s
    The frequency band located either above or below the carrier frequency
within which fall the frequency components of the wave produced by the process
of modulation. Apart from the carrier, all components of an
amplitude-modulated sinusoidal carrier, when taken together, form a pair of
sidebands extending on either side of the carrier frequency in mirror symmetry
and containing all the frequency components of the modulating wave. The
sidebands above and below the carrier frequency are called upper sideband and
lower sideband, respectively.  See  AMPLITUDE MODULATION; CARRIER; MODULATION;
SINGLE SIDEBAND.
 �34s[Harold S. Black]
�33s

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�6v�2v�32sSiderite   �21m �33s 
    The mineral form of ferrous carbonate, often containing appreciable 
amounts of magnesium and manganese substituting for iron. 
    Siderite has hexagonal (rhombohedral) symmetry and the same structure as 
calcite. Individual crystals are often rhombohedral in shape, sometimes with 
curved faces. Massive varieties also occur. Siderite is often brownish and 
sometimes gray or greenish. The specific gravity is 3.9 and the hardness is 4 
on Mohs scale.  See  CALCITE; CARBONATE MINERALS. 
 �34s[Robert I. Harker]
�33s 
  
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�6v�2v�32sSiderocapsaceae   �21m �33s 
    A family of chemolithotrophic bacteria found in iron-bearing waters. It is 
a heterogeneous group of gram-negative organisms, possessing in common the 
ability to deposit iron or manganese compounds around the cells. Cell 
morphology may be apparent only after treatment with dilute acid. The species 
are organized into four genera.  See  CHEMOLITHOTROPHIC BACTERIA. 
 �34s[Ralph S. Wolfe]
�33s 
  
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�6v�2v�32sSignal detection theory   �21m �33s 
    A theory in psychology which characterizes not only the acuity of an 
individual's discrimination but also the psychological factors that bias the 
individual's judgments. Failure to separate these two aspects of 
discrimination had tempered the success of theories based upon the classical 
concept of a sensory threshold. The theory provides a modern and more complete 
account of the process whereby an individual makes fine discriminations. 
    The theory of signal detection has two parts of quite different origins. 
The first comes from mathematical statistics and is a translation of the 
theory of statistical decisions. The major contribution of this part of the 
theory is that it permits a determination of the individual's discriminative 
capactiy, or sensitivity, that is independent of the judgmental bias or 
decision criterion the individual may have had when the discrimination was 
made. The second part of the theory comes from the study of electronic 
communications. It provides a means of calculating for simple signals, such as 
tones and lights, the best discrimination that can be attained. The prediction 
is based upon physical measurements of the signals and their interfering noise. 
    This opportunity to compare the sensitivity of human observers with the 
sensitivity of an ``ideal observer'' for a variety of signals is of 
considerable usefulness, and of growing interest, in sensory psychology. 
Signal detection theory has been applied to several topics in experimental 
psychology in which separation of intrinsic discriminability from decision 
factors is desirable. Included are attention, imagery, learning, conceptual 
judgment, personality, reaction time, manual control, and speech. 
    The analytical apparatus of the theory has been of value in the evaluation 
of the performance of systems that make decisions based on uncertain 
information. Such systems may involve only people, or people and machines 
together, or only machines. Examples come from medical diagnosis, where 
clinicians may base diagnostic decisions on a physical examination, or on an 
x-ray image, or where machines make diagnoses, perhaps by counting blood cells 
of various types. 
 �34s[John A. Swets]
�33s 
  
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�6v�2v�32sSignal generator   �21m �33s 
    A piece of electronic test equipment that delivers a sinusoidal output of 
accurately calibrated frequency. The frequency may be anywhere from audio to 
microwave, depending upon the intended use of the instrument. The frequency 
and the amplitude are adjustable over a wide range. The oscillator must have 
excellent frequency stability, and its amplitude must remain constant over the 
tuning range. 
    The Wien-bridge oscillator is commonly used for frequencies up to about 
200 kHz. For a radio-frequency signal generator up to about 200 MHz, a 
resonant circuit oscillator is used (such as a tuned-plate tuned-grid, 
Hartley, or Colpitts). Beyond this range vhf and microwave oscillators are 
used.  See  OSCILLATOR. 
 �34s[Jacob Millman]
�33s 
  
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�6v�2v�32sSignal-to-noise ratio   �21m �33s 
    The ratio, at some location, of some measure of the desired signal to the 
same measure of the total noise, abbreviated S/N. This is a primary 
consideration in the design of any communication system, and is a measure of 
the efficiency of the system. Distortion of the signal by noise causes errors. 
The objective of engineering is the maintenance of error-free communications 
over the smallest possible S/N value.  See  ELECTRICAL NOISE. 
 �34s[Walter Lyons]
�33s 
  
A700000002174�254s�5v�1v1570
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�6v�2v�32sSignificant figures �21m �33s
    The numbers considered here are real numbers in decimal form. All numbers
are considered as positive although the theory applies also to negative
numbers. Every digit in a number is significant unless its sole purpose is to
fix the position of the decimal point. Thus the significant figures in 31.50
are 3, 1, 5, and 0; in 0.0315 only 3, 1, and 5 are significant; and in 3005
both zeros, 3, and 5 are significant.
    A number having n significant figures has n-figure accuracy. Thus 31.50
and 3005 have four-figure accuracy but 0.00315 has three-figure accuracy.
Evidently the number 3150 may have four-figure or three-figure accuracy. The
so-called powers-of-ten notation is used to remove ambiguity. In the
powers-of-ten notation a number is written in the form a x 10(sup k) where k is
an integer and a equals 1 or lies between 1 and 10. In this notation all
figures in a are significant. Thus 3150 would be written 3.150 x 10(sup 3) to
indicate four-figure accuracy and written 3.15 x 10(sup 3) to indicate
three-figure accuracy.
 �34s[Lyman M. Kells]
�33s

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�6v�2v�32sSilica minerals �21m �33s
    Silica (SiO2) occurs naturally in at least nine different varieties
(polymorphs), which include tridymite, cristobalite, coesite, and stishovite,
in addition to high (BETA) and low (ALPHA) quartz. These forms are
characterized by distinctive crystallography, optical characteristics,
physical properties, pressure-temperature stability ranges, and occurrences.
    The crystal structures of all silica polymorphs except stishovite contain
silicon atoms surrounded by four oxygens, thus producing tetrahedral
coordination polyhedra. Each oxygen is bonded to two silicons, creating an
electrically neutral framework. Stishovite differs from the other silica
minerals in having silicon atoms surrounded by six oxygens (octahedral
coordination.) Ideal high tridymite is composed of sheets of SiO4 tetrahedra
oriented perpendicular to the c crystallographic axis (Fig. 1) with adjacent
tetrahedra in these sheets pointing in opposite directions. High cristobalite,
like tridymite, is composed of parallel sheets of SiO4 tetrahedra with
neighboring tetrahedra pointing in opposite directions. However, the hexagonal
rings are distorted and adjacent sheets are rotated 60 DG with respect to one
another, resulting in the geometry shown in Fig. 2. Coesite also contains
silicon atoms tetrahedrally coordinated by oxygen. These polyhedra share
corners to form chains composed of four-membered rings. Silicon in stishovite
is octahedrally coordinated by oxygen. These coordination polyhedra share
edges and corners to form chains of octahedra parallel to the c
crystallographic axis.
    Chemically, all silica polymorphs are ideally 100% SiO2. However, unlike
quartz which commonly contains few impurities, the compositions of tridymite
and cristobalite generally deviate significantly from pure silica. This
usually occurs because of a coupled substitution in which a trivalent ion such
as Al(sup 3+) or Fe(sup 3+) substitutes for Si(sup 4+), with electrical
neutrality being maintained by monovalent or divalent cations occupying
interstices in the relatively open structures of these two minerals.  See
COESITE; QUARTZ; STISHOVITE.
 �34s[John C. Drake]
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�6v�2v�32sSilicate minerals �21m �33s
    All silicates are built of a fundamental structural unit, the so-called
SiO4 tetrahedron. The crystal structure may be based on isolated SiO4 groups
or, since each of the four oxygen ions can bond to either one or two silicon
(Si) ions, on SiO4 groups shared in such a way as to form complex isolated
groups or indefinitely extending chains, sheets, or three-dimensional
networks. Mixed structures in which more than one type of shared tetrahedra
are present also are known.  See  SILICON.
    Silicates are classified according to the nature of the sharing mechanism,
as revealed by x-ray diffraction study. The sharing mechanism gives rise to a
characteristic ratio of Si to O, but it is possible for oxygen ions that are
not bonded to Si to be present in the structure, and sometimes some or all of
any aluminum present must be counted as equivalent to Si.
    The detailed crystallographic and physical properties of the various
silicates are broadly related to the type of silicate framework that they
possess. Thus, the phyllosilicates as a group typically have a platy crystal
habit, with a cleavage parallel to the plane of layering of the structure, and
are optically negative with rather high birefringence. The inosilicates, based
on an extended one-dimensional rather than two-dimensional linkage of the SiO4
tetrahedra, generally form crystals of prismatic habit; if cleavage is
present, it will be parallel to the direction of elongation. The
tectosilicates commonly are equant in habit, without marked preference for
cleavage direction, and tend to have a relatively low birefringence.
    Silicate minerals make up the bulk of the outer crust of the Earth and
form in a wide range of geologic environments. Many silicates are of economic
importance. For discussions of certain silicate mineral groups	see
AMPHIBOLE; ANDALUSITE; CHLORITE; CHLORITOID; EPIDOTE; FELDSPAR; FELDSPATHOID;
GARNET; HUMITE; MICA; OLIVINE; PYROXENE; SCAPOLITE; SERPENTINE; ZEOLITE.
 �34s[Clifford Frondel]
�33s

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�6v�2v�32sSilicate polymers   �21m �33s 
    Crystalline and amorphous inorganic polymers of silicates. Examples 
include the naturally occurring fiberlike asbestos and sheetlike mica. The 
industrially important water-soluble alkali metal silicates can give highly 
viscous polymeric solutions. Borosilicate glasses form another important group 
of silicate polymers. The Pyrex type is well known for its resistance to 
thermal shock; the leached Vycor type is porous and can be used for filtering 
bacteria and viruses. Asbestos occurs as ladder polymers, of which crocidolite 
is the most important, and as layer polymers exemplified by chrysotile. The 
zeolites, many of which have been found naturally or have been synthesized, 
are three-dimensional network polymers. Their uses as molecular sieves are 
well known.  See  ASBESTOS; INORGANIC POLYMER; MOLECULAR SIEVE; SILICATE; 
SILICATE MINERALS; ZEOLITE. 
 �34s[Robert A. Shaw]
�33s 
  
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�6v�2v�32sSiliceous sinter �21m �33s
    A porous silica deposit formed around hot springs. It is white to light
gray and sometimes friable. Geyserite is a variety of siliceous sinter formed
around geysers. The siliceous sinters are deposited as the hot subterranean
waters cool after issuing at the surface and become supersaturated with silica
that was picked up at depth. The sinters are frequently deposited on algae
that live in the pools around the hot springs. See GEYSER.
 �34s[Raymond Siever]
�33s

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�6v�2v�32sSilicoflagellata �21m �33s
    A class of unicellular flagellate microorganisms of the plant division
Chrysophyta, which are a part of the marine plankton. Their exoskeletons,
siliceous and coarsely perforate (see illustration), resemble those of the
Radiolaria, with which they have been grouped. They are usually subpyramidal
or hemispherical in shape, and delicately filigreed. Two families and 11
genera of silicoflagellates have been described from siliceous sedimentary
rocks ranging in age from Upper Cretaceous to Recent, in association with
abundant diatoms and siliceous sponge spicules. See CHRYSOPHYTA;
MICROPALEONTOLOGY; PHYTOPLANKTON.
 �34s[Daniel J. Jones]
�33s


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�6v�2v�32sSilicoflagellida   �21m �33s 
    An order of the phylum Protozoa, class Phytamastigophorea. These organisms 
are marine flagellates which have an intermal, siliceous, tubular skeleton; 
numerous small, discoid, yellow chromatophores; and a single flagellum. At 
times the organisms also put forth, from the ends of their skeletal tubes, 
long, rather threadlike pseudopodia. The skeleton forms a basket within which 
the moiety of protoplasm lies, but skeletal elements always have at least a 
thin covering. There is a single genus, Dictyocha, with four species. See 
PHYTAMASTIGOPHOREA.  
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sSilicon �21m �33s
    A chemical element, Si, atomic number 14, and atomic weight 28.086.
Silicon is the most abundant electropositive element in the Earth's crust. The
element is a metalloid with a decided metallic luster; it is quite brittle. It
has a specific gravity of 2.42 at 20DGC (68DGF), melts at 1420DGC (2588DGF),
and boils at 3280DGC (5936DGF). The element is usually tetravalent in its
compounds, although sometimes divalent, and is decidedly electropositive in
its chemical behavior. In addition, pentacoordinate and hexacoordinate
compounds of silicon are known. See METALLOID.
    Crude elementary silicon and its intermetallic compounds are used in
alloying constituents to strengthen aluminum, magnesium, copper, and other
metals. Metallurgical silicon of 98-99% purity is used as the starting
material for manufacturing organosilicon compounds and silicone resins,
elastomers, and oils. Silicon chips are used in integrated circuits.
Photovoltaic cells for direct conversion of solar energy to electricity use
wafers sliced from single crystals of electronic-grade silicon. Silicon
dioxide is used as the raw material for making elementary silicon and for
silicon carbide. Sizable crystals of it are used for piezoelectric crystals.
Fused quartz sand becomes silica glass, used in chemical laboratories and
plants as well as an electrical insulator. A colloidal dispersion of silica in
water is used as a coating agent and as an ingredient in certain polishes.
    Naturally occurring silicon contains 92.2% of the isotope of mass number
28, 4.7% of silicon-29, and 3.1% of silicon-30. In addition to these stable,
natural isotopes, several artificially radioactive isotopes are known.
Elementary silicon has the physical properties of a metalloid, resembling
germanium below it in group IV of the periodic table. In very pure form
silicon is an intrinsic semiconductor, although the extent of its
semiconduction is greatly increased by the introduction of minute amounts of
impurities. Silicon resembles the metals in its chemical behavior. It is about
as electropositive as tin, and decidedly more positive than germanium or lead.
In keeping with this rather metallic character, silicon forms tetrapositive
ions and a variety of covalent compounds; it appears as a negative ion in only
a few silicides and as a positive constituent of oxy acid or complex anions.
    Several series of hydrides are formed, a variety of halides (some of which
contain silicon-to-silicon bonds), and also many series of oxygen-containing
compounds which may be either ironic or covalent in their properties.
    Silicon occurs in many forms of the dioxide and as almost numberless
variations of the natural silicates. For a discussion of the structures and
compositions of the representative classes see SILICATE MINERALS.
    In abundance, silicon exceeds by far every other element except oxygen. It
constitutes 27.72% of the solid crust of the Earth, whereas oxygen constitutes
46.6%, and the next element after silicon, aluminum, accounts for 8.13%.
    Silicon is reported to form compounds with 64 of the 96 stable elements,
and it probably forms silicides with 18 other elements. Besides the metal
silicides, used in large quantities in metallurgy, silicon forms useful and
important compounds with hydrogen, carbon, the halogen elements, nitrogen
oxygen, and sulfur. In addition, useful organosilicon derivatives have been
prepared.
 �34s[Edwin P. Plueddemann]
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�6v�2v�32sSilk �21m �33s
    The lustrous fiber produced by the larvae of silkworms; also the thread or
cloth made from such fiber.
    The cocoons of the silkworm are delivered to a factory, called a filature,
where the silk is unwound from the cocoons and the strands are collected into
skeins. The process of unwinding the filament from the cocoon is called
reeling. As the filament of a single cocoon is too fine for commerical use, 3
-10 strands are usually reeled at a time to produce the desired diameter of
raw silk thread. The usable length of the reeled filament is from 1000 to 2000
ft (300 to 600 m). The remaining part of the filament is valuable raw material
for the manufacture of spun silk.
    The term reeled silk is applied to the raw silk strand that is formed by
combining several filaments from separate cocoons. It is reeled into skeins,
which are packed in small bundles called books. From the filature, the books
of reeled silk go to the throwster where they are transformed into silk yarn,
also called silk thread, by a process known as throwing. Silk throwing is
analogous to the spinning process that changes cotton, linen, or wool fibers
into yarn. The manufacture of those fibers, however, unlike that of silk yarn,
does not include carding, combing, and drawing out, the usual processes for
producing a continuous yarn.
 �34s[M. David Potter]
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�6v�2v�32sSillimanite �21m �33s
    A nesosilicate mineral with the composition Al2O[SiO4], crystallizing in
the orthorhombic system. It commonly occurs in slender crystals or parallel
groups, and is frequently fibrous, hence the synonym fibrolite. There is one
perfect cleavage, luster is vitreous, color is brown, pale green, or white,
hardness is 6-7 on Mohs scale, and the specific gravity is 3.23.
Sillimanite, andalusite, and kyanite are polymorphs of Al2O[SiO4]. See
ANDALUSITE; KYANITE; SILICATE MINERALS.
 �34s[Paul B. Moore]
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�6v�2v�32sSilurian �21m �33s
    A geologic period of time during which a system of rocks, the Silurian
System, was deposited. The strata are identified in terms of the principal
forms of life extant at that time as determined by means of the fossils.
Silurian rocks are present in many parts of the world, but three of the
best-known sections are in Great Britain, eastern United States, and central
Bohemia. See PALEOZOIC.
    Silurian deposits are represented by both terrestrial and marine strata,
but the latter are predominant. The marine strata are composed largely of
carbonate facies, either limestone or dolomite, but there are sandstones and
shales, including dark graptolite shales. The marine strata commonly contain a
large invertebrate fossil fauna.
    In Late Silurian times the seaways of eastern North America began to dry
up, leading to the formation of salt beds. Lower and Middle Silurian strata
bear extensive iron deposits, mostly in the form of hematite.
    Silurian rocks contain a prolific invertebrate fossil assemblage. Among
the best-represented groups were the brachiopods belonging to the class
Articulata (Pygocaulia). The coral faunas also were prolific. Crinoids
(echinoderms) became common for the first time. Primitive fish (Agnatha and
Placodermi) are the only known Silurian vertebrates.
 �34s[Thomas W. Amsden]
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�6v�2v�32sSiluriformes �21m �33s
    The catfishes, a highly distinctive order (also called Nematognathi) of
actinopterygian fishes. In the Siluriformes the Weberian apparatus is more
complex than in the related Cypriniformes. The body is usually naked or the
scales are enlarged and modified into large, overlapping bony plates. There
are one to four pairs of barbels, and both the pectoral and dorsal fins
usually have a strong spine (see illustration). This is a large group,
including about 26 familes and perhaps 2000 species, of which nearly 1200 live
in South America. Catfishes are known from Eocene times. See ACTINOPTERYGII;
CYPRINIFORMES.
 �34s[Reeve M. Bailey]
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�6v�2v�32sSilver �21m �33s
    A chemical element, Ag, atomic number 47, atomic mass 107.868. It is a
gray-white, lustrous metal. Chemically it is one of the heavy metals and one
of the noble metals; commercially it is a precious metal. There are
twenty-five isotopes of silver. Their atomic masses range from 102 to 117.
    In most of its uses, silver is alloyed with one or more other metals.
Silver, with the highest thermal and electrical conductivities of all the
metals, is used for electrical and electronic contact points. Silver also has
well-known uses in jewelry and silverware. Alloys in which silver is an
ingredient include dental amalgam and metals for engine pistons and bearings.
See AMALGAM; SILVER ALLOYS.
    Silver is a rather rare element. Sometimes it occurs in nature as the free
element (native silver) or alloyed with other metals. For the most part,
however, silver is found in ores containing silver compounds. The principal
silver ores are argentite, cerargyrite or horn silver, and several minerals in
which silver sulfide is combined with sulfides of other metals. About
three-fourths of the silver produced is a by-product of the extraction of
other metals, copper and lead in particular. See SILVER METALLURGY.
    Pure silver is a white, moderately soft metal (2.5-3 on Mohs hardness
scale), somewhat harder than gold. When polished, it has a brilliant luster
and reflects 95% of the light falling on it. Its density is 10.5 times that of
water. The quality of silver, its fineness, is expressed as parts of pure
silver per 1000 parts of total metal. Commercial silver is usually 999 fine.
    Although silver is the most active chemically of the noble metals, it is
not very active in comparison with most other elements. It does not oxidize at
all readily (as iron does when it rusts), but it reacts with sulfur or
hydrogen sulfide to form the familiar silver tarnish. Electroplating silver
with rhodium will prevent this discoloration. Silver itself does not react
with dilute nonoxidizing acids (hydrochloric or sulfuric acids) or strong
bases (sodium hydroxide). However, oxidizing acids (nitric or concentrated
sulfuric acids) dissolve it by reaction to form the unipositive silver ion,
Ag(sup +). This ion, which is present in solutions of all simple, soluble
compounds of silver, is rather easily reduced to the free metal, as in the
deposition of silver mirrors by organic reducing agents.
    Silver is almost always monovalent in its compounds, but an oxide,
fluoride, and sulfide of divalent silver are known. Some coordination
compounds of silver, also called silver complexes, contain divalent and
trivalent silver. Although silver does not oxidize when heated, it can be
oxidized chemically or electrolytically to form silver oxide or peroxide, a
strong oxidizing agent. Because of this activity, silver finds considerable
use as an oxidation catalyst in the production of certain organic materials.
See PHOTOGRAPHIC MATERIALS.
    Soluble silver salts, especially AgNo3, have proved lethal in doses as
small as 0.070 oz (2 g). Silver compounds may be slowly absorbed by the body
tissues, with a resulting bluish or blackish pigmentation of the skin
(argyria). See SILVER INTOXICATION.
 �34s[William E. Cooley]
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�6v�2v�32sSilver alloys �21m �33s
    Combinations of silver with one or more other metals. Pure silver is very
soft and ductile but can be hardened by alloying. Copper is the favorite
hardener and normally is employed in the production of sterling silver, which
must contain a minimum of 92.5% silver, and also in the production of coin
silver.
    Silver-copper eutectic and modifications containing other elements such as
zinc, tin, cadmium, phosphorus, or lithium are widely used for brazing
purposes, where strong joints having relatively good corrosion resistance are
required. Where higher strengths at elevated temperature are required,
silver-copper-palladium alloys and other silver-palladium alloys are suitable.
The addition of a small amount of silver to copper raises the recrystallizing
temperature without adverse effect upon the electrical conductivity.
    Silver may be alloyed with gold or palladium in any ratio, producing soft
and ductile alloys; certain of these intermediate alloys are useful for
electrical contacts, where resistance to sulfide formation must be achieved.
    Silver has proved to be a useful component for high-duty bearings in
aircraft engines, where it may be overlaid with a thin layer of lead and
finally with a minute coating of indium. Specially developed alloys of silver
with tin, plus small percentages of copper and zinc in the form of moderately
fine powder, can be mixed with mercury to yield a mass which is plastic for a
time and then hardens, developing relatively high strength despite the fact
that it contains about 50% mercury. This material was developed specifically
for dental use and is generally known as amalgam, although the term amalgam
actually includes all the alloys of mercury with other metals. See AMALGAM;
SILVER.
 �34s[Edmund M. Wise]
�33s
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�6v�2v�32sSilver chloride electrode �21m �33s
    An electrode made of silver, covered or intimately mixed with silver
chloride. One method of preparation consists of coating a silver wire or a
silver-plated noble metal with silver chloride by electrolysis as an anode in
a chloride solution. A second method consists of three steps: (1) pasting
silver oxide or oxalate on a platinum helix, (2) reducing the oxide or axalate
to silver by heating to 500DGC (930DGF), and (3) chlorodizing part of the
silver to silver chloride. A third method consists of pasting an intimate
mixture of silver oxide and silver chlorate on a platinum helix and heating to
500DGC (930DGF). Although these systems are frequently referred to as silver
chloride electrodes, they are, in reality, not electrodes until immersed in a
chloride solution. The standard potential of the silver chloride electrode is
- 0.2224 volt relative to the normal hydrogen electrode at 25DGC (77DGF).  See
CALOMEL ELECTRODE; ELECTRODE POTENTIAL; HYDROGEN ELECTRODE.
 �34s[Walter J. Hamer]
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�6v�2v�32sSilverfish   �21m �33s 
    An insect of the order Thysanura, also known as a bristle tail. There are 
over 350 cosmopolitan species, all considered among the most primitive 
insects. They are small, wingless insects with biting mouthparts (see 
illustration). Silverfish belong to two families, the Machilidae and 
Lepismatidae. These insects are active and can run rapidly. They are fond of 
paste and glue, frequently being destructive to book bindings, and also feed 
on starched fabrics and rayon. They can be serious pests in libraries.  See  
THYSANURA.  
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sSilver intoxication   �21m �33s 
    Accidental or suicidal ingestion of large oral doses of silver nitrate 
(lunar caustic) solution may cause a severe gastrointestinal irritation 
producing an acute gastroenteritis. Nausea, vomiting, and diarrhea may ensue, 
and death from shock may occur within a few hours. The mouth is usually 
stained black by the silver nitrate. 
    Argyria is a clinical condition produced by chronic occupational exposure 
to excess silver. It does not produce generalized constitutional symptoms but 
may lead to lifelong disfigurement. Local or generalized changes of the skin 
pigmentation characterized by slate gray patches of discoloration occur. The 
skin may become black with a metallic sheen that often appears most 
prominently over the exposed parts of the body. The risk of accidental 
industrial chronic silver intoxication is slight because of its limited 
absorption.  See  METALS, PATHOLOGY OF; SILVER. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sSilver metallurgy   �21m �33s 
    The art and science of extracting silver metal economically from various 
ores, and the reclamation of silver from the myriad types of industrial 
processes or scrap produced therefrom. It includes all processes of separating 
silver from its ores, alloys, and solutions, as well as the smelting, 
refining, and working of the metal and its alloys and compounds. It deals with 
the technical application of the chemical and physical properties of silver to 
its concentration, extraction, purification, alloying, working, and 
compounding to meet the requirements of technical needs. 
    Chloridization is an extractive process in which silver is precipitated 
from aqueous solutions. In chloridization methods the silver contained in ores 
or metallurgical products is first converted by means of a chloridizing roast 
into a compound which is soluble in water or in certain aqueous solutions. The 
silver is then precipitated as an insoluble compound by suitable reagents and 
the precipitate worked for the metal. 
    The cyanidation process differs only in minor details, such as strength of 
the solution and time of treatment, from the cyanide process used in the 
recovery of gold. No preliminary metallurgical treatment other than fine 
grinding is required.  See  GOLD METALLURGY. 
    The Parkes process is based on the greater affinity of silver for zinc 
than for lead. Slab zinc is added to an argentiferous lead bath, the 
temperature of which has been raised higher than the melting point of the 
zinc. When the zinc has been melted and thoroughly mixed into the lead bath, 
the bath temperature is lowered and a silver-zinc alloy separates and floats 
on the top of the kettle. This zinc crust, as it is now called, is pressed off 
to remove the excess lead. The resultant high silver-lead retort bullion is 
then cupeled to recover the silver and gold. 
    Cupellation is the oldest and most widely known method of separating and 
recovering gold and silver from lead. The silver-lead bullion is charged to a 
reverberatory-type cupellation furnace. After the charge has melted, air is 
blown across the top of the molten bath, causing oxidation of the lead and 
other impurities which separate as in impure litharge slag, leaving behind the 
silver and gold as a dore alloy. 
    By far the most important method of silver production results from the 
smelting and subsequent treatment of the silver-rich slimes resulting from the 
electrolytic refining of copper. Slimes which have been leached with sulfuric 
acid or air are filtered and charged to a reverberatory-type furnace with 
appropriate fluxes. The resulting melt slag contains most of the base-metal 
impurities. Upon completion of this step, the material remaining in the 
furnace consists of gold, silver, selenium, tellurium, and residual base 
metals, and is called matte. Treatment of the matte eventually yields dore 
which is then cast into anodes for subsequent electrolytic refining.  See 
#SILVER.  
 �34s[R. D. Mushlitz]
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�6v�2v�32sSilviculture   �21m �33s 
    The theory and practice of controlling the establishment, composition, and 
growth of stands of trees for any of the goods and benefits that they may be 
called upon to produce. The techniques proceed on the assumption that the 
natural vegetation of any site normally tends to extend itself to occupy all 
available growing space, making the fullest possible use of the available 
growth factors, such as light and moisture. The forester attempts, usually by 
cutting in the act of harvesting useful wood products, to create vacancies in 
the forest vegetation that will provide environments that are favorable either 
to the establishment of new, desirable trees (reproduction cuttings) or to the 
enhanced growth of those that remain (intermediate cuttings). 
    Integrated schedules of treatment for stands are called silvicultural 
systems. They cover both intermediate and reproduction cuttings but are 
classified and named in terms of the general method of reproduction cutting 
contemplated. Such programs are evolved for particular situations and kinds of 
stands with due regard for all the significant biological and economic 
considerations involved. These considerations include the desired uses of the 
land, kind of wood products sought, prospective costs and returns of the 
enterprise represented by management of the stand, funds available for 
long-term investment in stand treatments, harvesting techniques and equipment 
employed, reduction of losses from damaging agencies, and the natural 
requirements that must be met in reproducing the stand and fostering its 
growth.  See  FOREST AND FORESTRY. 
 �34s[David M. Smith]
�33s 
  
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�6v�2v�32sSimilitude   �21m �33s 
    The use in scientific studies and engineering designs of the corresponding 
behavior between large and small objects of similar nature. Two structures 
behave similarly if they are geometrically, kinematically, and dynamically 
similar. For geometric similarity, the ratios of critical dimensions, such as 
ratios of diameters to lengths, must be equal. For kinematic similarity, 
corresponding velocities and velocity gradients must be in the same ratios at 
corresponding locations. For dynamic similarity, ratios of forces acting 
within the two structures, such as viscosity and inertia, must be equal.  See  
DIMENSIONAL ANALYSIS; DYNAMIC SIMILARITY. 
    Model tests in wind and water tunnels, towing tanks, dynamometers, antenna 
test ranges, and plasma reacting with magnetic fields are predicated on 
similitude relations between the model being tested and the full-sized object 
being studied. The use of small models greatly increases the speed with which 
design changes can be explored, and considerable economies are achieved.  See  
TOWING TANK; WIND TUNNEL. 
 �34s[Frank H. Rockett]
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�6v�2v�32sSimple machine   �21m �33s 
    Any of several elementary machines, one or more of which is found in 
practically every machine. The group of simple machines usually includes only 
the lever, wheel and axle, pulley (or block and tackle), inclined plane, 
wedge, and screw. However, the gear drive and hydraulic press may also be 
considered as simple machines. The principles of operation and typical 
applications of simple machines depend on several closely related concepts.  
See  EFFICIENCY; FRICTION; MECHANICAL ADVANTAGE; POWER; WORK. 
    Two conditions for static equilibrium are used in analyzing the action of 
a simple machine. The first condition is that the sum of forces in any 
direction through their common point of action is zero. The second condition 
is that the summation of torques about a common axis of rotation is zero. 
Corresponding to these two conditions are two ways of measuring work. In 
machines with translation, work is the product of force and distance. In 
machines with rotation, work is the product of torque and angle of rotation.  
See  BLOCK AND TACKLE; GEAR DRIVE; HYDRAULIC PRESS; INCLINED PLANE; LEVER; 
SCREW; TORQUE; WEDGE; WHEEL AND AXLE. 
 �34s[Richard M. Phelan]
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�6v�2v�32sSimulation   �21m �33s 
    The development and use of computer models for the study of actual or 
postulated dynamic systems. The essential characteristic of simulation is the 
use of models for study and experimentation rather than the actual system 
modeled. In practice, it has come to mean the use of computer models because 
modern electronic computers are so much superior for most kinds of simulation 
that computer modeling dominates the field. ``Systems,'' as used in the 
definition, refers to an interrelated set of elements, components, or 
subsystems. ``Dynamic systems'' are specified because the study of static 
systems seldom justifies the sophistication inherent in computer simulation.  
See  ANALOG COMPUTER; COMPUTER; DIGITAL COMPUTER. 
    ``Postulated'' systems as well as ``actual'' ones are included in the 
definition because of the importance of simulation for testing hypotheses, as 
well as designs of systems not yet in existence. The ``development'' as well 
as ``use'' of models is included because, in the empirical approach to system 
simulation, a simplified simulation of a hypothesized model is used to check 
educated guesses, and thus to develop a more sophisticated and more realistic 
simulation of the simuland. The simuland is that which is simulated, whether 
real or postulated. 
    Among the systems which have been simulated are aerospace systems; 
chemical and other industrial processes; structural dynamics; physiological 
and biological systems; automobile, ship, and submarine dynamics; social, 
ecological, political, and economic systems; corporations and small 
businesses; traffic and transportation systems; electrical, electronic, 
optical, and acoustic systems; electrical energy systems and other energy 
related systems; and learning, thinking, and problem-solving systems.  See  
CHEMICAL ENGINEERING; CYBERNETICS; ECOLOGY; ELECTRICAL POWER SYSTEMS 
ENGINEERING; INDUSTRIAL ENGINEERING; LEARNING THEORIES; OPTIMIZATION; PROBLEM 
SOLVING (PSYCHOLOGY); PROCESS ENGINEERING; PRODUCTION ENGINEERING; STRUCTURAL 
ANALYSIS.  
        Mathematical modeling is a recognized and valuable adjunct, and 
usually a precursor, of computer simulation. Mathematical modeling does not 
necessarily precede simulation, however; sometimes the simuland is not well 
enough understood to permit rigorous mathematical description. In such cases 
it is often possible to postulate a functional relationship of the elements of 
the simuland without specifying mathematically what that relationship is. This 
is the building-block approach, for which analog computers are particularly 
well suited. Parameters related to the function of the blocks can be adjusted 
until some functional criteria are met. Thus the mathematical model can be 
developed as the result of, rather than as a requirement for, simulation. 
    Analog computers, in which signals are continuous and are processed in 
parallel, were originally the most popular for simulation. Their modular 
design made it natural to retain the simulation-simuland correspondence, and 
their parallel operation gave them the speed required for real-time operation. 
The result was unsurpassed human-machine rapport. However, block-oriented 
digital simulation languages were developed which allow a pseudo - 
simulation-simuland correspondence, and digital computer speeds have increased 
to a degree that allows real-time simulation of all but very fast or very 
complex systems.  See  DATA-PROCESSING SYSTEMS; DIGITAL COMPUTER PROGRAMMING. 
    Hybrid simulation, in which both continuous and discrete signals are 
processed, both in parallel and serially, is the result of a desire to combine 
the speed and human-machine rapport of the analog computer with the precision, 
logic capability, and memory capacity of the digital computer. Hybrid 
simulation now makes possible the simulation of a new array of systems which 
require combinations of computer characteristics unavailable in either 
all-analog or all-digital computers. 
 �34s[John H. McLeod, Jr.]
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�6v�2v�32sSine wave   �21m �33s 
    A wave having a form which, if plotted, would be the same as that of a 
trigonometric sine or cosine function. The sine wave may be thought of as the 
projection on a plane of the path of a point moving around a circle at uniform 
speed. It is characteristic of one-dimensional vibrations and one-dimensional 
waves having no dissipation.  See  HARMONIC MOTION. 
    The sine wave is the basic function employed in harmonic analysis. It can 
be shown that any complex motion in a one-dimensional system can be described 
as the superposition of sine waves having certain amplitude and phase 
relationships. The technique for determining these relationships is known as 
Fourier analysis.  See  FOURIER SERIES AND INTEGRALS; WAVE EQUATION; WAVE 
MOTION; WAVEFORM. 
 �34s[William J. Galloway]
�33s 
  
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�6v�2v�32sSingle crystal   �21m �33s 
    In crystalline solids the atoms or molecules are stacked in a regular 
manner, forming a three-dimensional pattern which may be obtained by a 
three-dimensional repetition of a certain pattern unit called a unit cell. 
When the periodicity of the pattern extends throughout a certain piece of 
material, one speaks of a single crystal. A single crystal is formed by the 
growth of a crystal nucleus without secondary nucleation or impingement on 
other crystals.  See  CRYSTAL; CRYSTALLOGRAPHY. 
    Ideally, single crystals are free from internal boundaries. They give rise 
to a characteristic x-ray diffraction pattern. Many types of single crystal 
exhibit anisotropy, that is, a variation of some of their physical properties 
according to the direction along which they are measured. For example, the 
electrical resistivity of a randomly oriented aggregate of graphite 
crystallites is the same in all directions. 
 �34s[David Turnbull]
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�6v�2v�32sSingle sideband   �21m �33s 
    An electronic signal-processing technique in which a spectrum of 
intelligence is translated from a zero reference frequency to a higher 
frequency without a change of frequency relationships within the translated 
spectrum. Single-sideband (SSB) signals have no appreciable carrier. 
    Amplitude-modulated (AM) signals have identical upper and lower sidebands 
symmetrically located on each side of the translation frequency, which is 
often called the carrier. The SSB spectrum differs from the AM spectrum in 
having little or no carrier and only one sideband.  See  AMPLITUDE MODULATION. 
    In the SSB signal-processing action, the intelligence spectrum to be 
translated is applied to the signal input port of a balanced modulator. A 
higher-frequency sinusoidal signal, often called a carrier, is applied to the 
other input port of this circuit. Its function is to translate the zero 
reference spectrum to the carrier frequency and to produce the upper and lower 
sidebands, which are symmetrically located on each side of the carrier. The 
carrier frequency power is suppressed to a negligible value by the balanced 
operation of the modulator and does not appear at the output. Generally, the 
balanced modulator operates at an intermediate frequency which is lower than 
the frequency of transmission. Following the balanced modulator is a sideband 
filter which is designed to remove the unwanted sideband signal power and to 
allow only the desired intelligence spectrum to pass.  See  AMPLITUDE 
MODULATOR; MODULATOR. 
    There are many advantages in the use of SSB techniques for communication 
systems. The two primary advantages are the reduction of transmission 
bandwidth and transmission power. The bandwidth required is not greater than 
the intelligence bandwidth and is one-half that used by amplitude modulation. 
The output power required to give equal energy in the intelligence bandwidth 
is one-sixth that of amplitude modulation. 
    Propagation of radio energy via ionospheric refraction provides the 
possibility for multiple paths of differing path length which can cause a 
selective cancellation of frequency components at regular frequency spacings. 
This produces in amplitude modulation a severe distortion of the intelligence 
because of the critically dependent carrier-to-sideband amplitude and phase 
relationships. SSB is much less affected under these conditions.  See  
RADIOWAVE PROPAGATION. 
 �34s[David M. Hodgin]
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�6v�2v�32sSink flow   �21m �33s 
    A point in three-dimensional flow, into which fluid is presumed to flow 
uniformly from all directions. The strength of a sink is defined as the volume 
per unit time flowing into the point. A sink may also be defined as a negative 
source.  See  SOURCE FLOW. 
    In two-dimensional flow, in which all flow occurs in parallel planes that 
have identical flow patterns, a sink is a straight line into which fluid flows 
uniformly from all directions at right angles to the line. It appears as a 
point on the customary two-dimensional flow diagram.  See  FLUID FLOW. 
 �34s[Victor L. Streeter]
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�6v�2v�32sSintering   �21m �33s 
    The welding together and growth of contact area between two or more 
initially distinct particles at temperatures below the melting point, but 
above one-half of the melting point in degrees Kelvin. Since the rate of 
sintering is greater with smaller than with larger particles, the process is 
most important with powders, as in powder metallurgy and in firing of ceramic 
oxides.  
    Although sintering does occur in loose powders, it is greatly enhanced by 
compacting the powder, and most commercial sintering is done on compacts. 
Compacting is generally done at room temperature, and the resulting compact is 
subsequently sintered at elevated temperature without application of pressure. 
For special applications, the powders may be compacted at elevated 
temperatures and therefore simultaneously pressed and sintered. This is called 
hot pressing or sintering under pressure. 
    Certain compacts from a mixture of different component powders may be 
sintered under conditions where a limited amount of liquid is formed at the 
sintering temperature. This is called liquid-phase sintering, important in 
certain powder-metallurgy and ceramic applications.  See  CERAMICS; POWDER 
METALLURGY.  
 �34s[F. V. Lenel]
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�6v�2v�32sSinus   �21m �33s 
    Any space in an organ, tissue, or bone, but usually referring to the 
paranasal sinuses of the face. In humans, four such sinuses, lined with 
ciliated, mucus-producing epithelium, communicate with each nasal passage 
through small apertures. The ethmoid and sphenoid sinuses are located 
centrally between and behind the eyes. The frontal sinuses lie above the nasal 
bridge, and the maxillary sinuses are contained in the upper jaw beneath the 
orbits. The mastoid portion of the temporal bone contains air cells lined with 
similar epithelium. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sSiphonales   �21m �33s 
    A large order of green algae which are coenocytic, nonseptate (tubular), 
and mostly marine. The tubular elements, essentially unicells, may be solitary 
and little-branched, or aggregated and intricately organized to form thalli of 
macroscopic size. There are 7 families and about 30 genera. 
    Just within the thin wall of the tube is a thick layer of cytoplasm 
containing numerous nuclei and discoid chloroplasts, usually with pyrenoids. A 
large vacuole within the cytoplasm extends the full length of the thallus. The 
tubes may be constricted but rarely septate except, with age, by ingrowths 
from the lateral wall. Reproductive structures are often delimited by a 
septum. Starch accumulates as a food.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sSiphonaptera   �21m �33s 
    An order of insects commonly known as the fleas. These animals are of 
importance because they are bloodsucking pests of humans and animals and 
transmit serious diseases from animals to humans. 
    Fleas in the adult stage are recognized with ease (see illustration). They 
are small, dark brown in color, laterally flattened, and with three pairs of 
legs modified for jumping. The body is more or less oval in shape and armed 
with spines and setae, adapting the flea for living among the hairs of 
animals. The head is provided with mouthparts modified for sucking blood. 
    Fleas have a complete metamorphosis; that is, the life history involves 
four distinct phases: the egg, larva, pupa, and adult. Each adult female flea 
lays a number of eggs, 400 or more in some species, over a long period of time. 
    The great majority of species are obscure ectoparasites of mammals, and to 
a lesser extent birds, and do not affect humans. Host specificity is exhibited 
in that, ordinarily, a certain taxonomic group of fleas will parasitize a 
specific taxonomic group of hosts. Nevertheless, some species may leave their 
favored hosts and attack humans, and these species are the pests and disease 
carriers. Bubonic plague, which was responsible for millions of deaths in 
India during the first two decades of this century, is a disease of rats. 
Murine or endemic typhus fever is another disease transmitted from domestic 
rats to humans by rat fleas. Flea allergy is the term applied to the severe 
reactions which sometimes result from flea bites.  See  INSECTA. 
 �34s[Irving Fox]
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�6v�2v�32sSiphonocladales   �21m �33s 
    An order of green algae in the division Chlorophytia. Four families are 
recognized in this order, all marine and mostly tropical: Valoniaceae, 
Siphonocladaceae, Boodleaceae, and Anadyomenaceae. 
    In the Valoniaceae the plants are essentially unicellular, coenocytic 
vesicles, and spherical or clavate (see illustration). The other families 
exhibit erect tufts of branched filaments, uni- or multiseriate, or expanded 
blades, some of which have the branch elements forming reticulations and 
anastomosing networks. Chloroplasts are usually close or loose networks, with 
pyrenoids. Starch accumulates as food.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sSiphonophora   �21m �33s 
    An order of the class Hydrozoa of the phylum Coelenterata, characterized 
by an extremely complex organization of components of several different types, 
some having the basic structure of a jellyfish, others of a polyp. The 
components may be connected by a stemlike region or may be more closely united 
into a compact organism. 
    Most siphonophores possess a float and are animals of the open seas. Best 
known is the Portuguese man-of-war, Physalia, with a float as much as 16 in. 
(40 cm) long and tentacles which extend downward for many feet. These animals 
may be swept shoreward and may make swimming not only unpleasant but 
dangerous.  See  COELENTERATA; HYDROZOA. 
 �34s[Sears Crowell]
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�6v�2v�32sSipunculida   �21m �33s 
    A phylum of marine worms which dwell in burrows, secreted tubes, or 
adopted shells. The body is greatly elongated in the dorsoventral axis (see 
illustration). As a result, both mouth and anus occur close together at one 
end of the body, and the alimentary tract is a long, twisted loop which 
extends ventrally and shows little differentiation along its course. The 
jawless mouth, surrounded by tentacles, is situated in an eversible proboscis, 
or introvert, which is projected by hydrostatic pressure generated by the 
strongly muscular body wall. The proboscis can be withdrawn by special 
retractor muscles which are anchored in the trunk wall and extend to its tip. 
    The body cavity is an extensive schizocoele which shows no evidence of 
segmentation but is filled with fluid and traversed by fibers. A pair of giant 
metanephridia whose ventral pores open to the exterior serve in excretion. The 
nervous system includes a dorsal brain that is continuous, by right and left 
connectives, with a nonsegmented ventral cord which extends to the posterior 
tip of the worm. The sexes are separate. 
    There are about a dozen genera with probably more than 200 species. 
Sipunculids are widely distributed. All are bottom dwellers, feeding by means 
of tentacles and cilia on the organic matter in the ingested mud and sand. 
 �34s[Frank A. Brown, Jr.]
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�6v�2v�32sSiren (acoustics)   �21m �33s 
    An apparatus for generating sound by the mechanical interruption of the 
flow of fluid (usually air) by a perforated rotating disk or cylinder. The 
disk may be so shaped that the fluid under pressure causes the disk to rotate, 
or conversely, the rotation of the disk may cause the flow of the fluid. The 
resulting frequency is the product of the speed of rotation of the disk and 
the number of perforations. 
    Before the advent of the jet engine, the siren was the most powerful of 
human-made steady sound sources, and as such it found application in fog 
signaling and as an emergency warning signal.  See  SOUND. 
 �34s[Robert W. Young]
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�6v�2v�32sSirenia   �21m �33s 
    An order of thoroughly aquatic placental mammals which include the living 
manatees and dugongs, the extinct (since the 18th century) Steller's sea cow, 
and their numerous fossil relatives. Included also is the peculiar, late 
Eocene - early Oligocene Moeritherium. Living sirenians have torpedo-shaped 
bodies ending in a horizontal tail fluke. These mammals are nearly hairless, 
thick-skinned, and without hindlimbs and with paddlelike forelimbs. The manus 
of living forms, although pentadactyl, shows no digits externally. The 
anterior parts of the palate and mandible are covered with rough, horny 
plates. The large head sits on a short neck, and the nostrils are on the 
dorsal surface of the snout. These animals have a small mouth with lips 
bearing hairs, the ears have no pinnae, and the small tongue is rugose and 
fixed.  See  MAMMALIA. 
 �34s[Frederick S. Szalay]
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�6v�2v�32sSirius �21m �33s
    Alpha Canis Majoris, the brightest star visible from Earth. From the
Northern Hemisphere, Sirius appears in the south in midwinter. It is the sixth
nearest star to the Sun. Absolute magnitude +1.4 and spectral type Al are
representative of early type A stars, with an effective temperature near
10,000 K (17,500DGF), strong hydrogen lines, and large Balmer continuous
absorption. Special interest attaches to the system of Sirius because of the
close companion, of the ninth magnitude [more than 10(sup 4) times fainter] -
the white dwarf ALPHA CMaB. The period of the binary system is 50 years.  See
BINARY STAR; STAR; WHITE DWARF STAR.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sSirocco   �21m �33s 
    A southerly or southeasterly wind current from the Sahara or from the 
deserts of Saudia Arabia which occurs in advance of cyclones moving eastward 
through the Mediterranean Sea. The sirocco is most pronounced in the spring, 
when the deserts are hot and the Mediterranean cyclones are vigorous. It is 
observed along the southern and eastern coasts of the Mediterranean Sea from 
Morocco to Syria as a hot, dry wind capable of carrying sand and dust great 
distances from the desert source. The sirocco is cooled and moistened in 
crossing the Mediterranean and produces an oppressive, muggy atmosphere when 
it extends to the southern coast of Europe.  See   AIR MASS; WIND. 
 �34s[Frederick Sanders]
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�6v�2v�32sSisal   �21m �33s 
    A fiber obtained from the leaves of Agave sisalana, produced in Brazil, 
Haiti, and several African countries, including Tanzania, Kenya, Angola, and 
Mozambique.  
    Sisal is used mainly for twine and rope, but some of the lower grades are 
used for upholstery padding and paper. The greatest quantity goes into farm 
twines, followed by industrial tying twine and rope. Most sisal-fiber ropes 
made in the United States are small to medium in size, intended for light 
duty. Sisal is sometimes used for marine cordage in Europe.  See  NATURAL 
FIBER.  
 �34s[Elton G. Nelson]
�33s 
  
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�6v�2v�32sSkarn   �21m �33s 
    A rock term generally reserved for rocks composed entirely, or almost so, 
of lime-bearing silicates, and derived from nearly pure limstones and 
dolomites into which large amounts of silicon, aluminum, iron, and magnesium 
have been introduced.  See  METAMORPHIC ROCKS; PNEUMATOLYSIS. 
    Skarn is an old term in mining. Now it is applied to a coarse-grained rock 
or association of minerals formed by reaction between hot silica-rich 
solutions or acid gases and a limestone or dolomite. The reaction rock is made 
up of coarse masses of minerals of various kinds, depending upon the 
temperature and the composition of the reacting gases or solutions. The most 
important skarn minerals are andradite-garnet, hedenbergite-diopside, 
iron-rich hornblende, and actinolite-tremolite. 
 �34s[T. F. W. Barth]
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�6v�2v�32sSkeletal system �21m �33s
    A structure composed of bone or cartilage or a combination of both which
provides a framework for the animal body and serve as attachment for muscles.
Relative movements of bony parts are permitted by the joints (articulations)
between them. Although the skeleton of vertebrates is characteristically
internal (endoskeleton), many species also possess external or exoskeletal
structures of bony or horny material to provide support or protection, as the
shell of the turtle. Such exoskeletal structures and their derivatives make up
the dermal skeleton. Invertebrates which possess skeletal structures have an
exoskeleton composed of chitin.  See  BONE; CARTILAGE.
    The vertebrate skeleton is primarily an endoskeleton of cartilage and
bone. There are two types of bone: cartilage (endochondral, or replacing bone)
and dermal (membrane, or investing bone). The first type is preformed in
cartilage and ossifies later, while the second type ossifies directly from
membrane without any cartilaginous predecessor. Otherwise they are similar.
Dermal bone occurs only in the skull and shoulder region. An exoskeleton of
hair, claws, nails, hoofs, horns, antlers, feathers, scales, and bony ossicles
in the skin may also be present.
    The skeleton consists of the axial skeleton, comprising the skull,
vertebral column, and associated structures, and the appendicular skeleton or
appendages (Fig. 1). The skull is subdivided into the neurocranium and the
branchiocranium, also known as the splanchnocranium or viscerocranium.
    Axial Skeleton.  Skeletal organization is quite uniform except for
variations in the number, size, and arrangement of the bones.
    Skull.  The neurocranium consists of the braincase and three pairs
of capsules associated with the organs of smell, sight, equilibrium, and
hearing.  The olfactory capsules develop around the olfactory lobes, the
optic capsules around the eyeballs, and the otic or auditory capsules
around the ear.  The olfactory and otic capsules united with the rest
of the neurocranium, but the optic capsules remain free to permit
eye movements.	The neurocranium has openings (foramina) for blood
vessels and nerves.
    The Chondrichthyes, that is, the sharks, rays, and chimeras, have
a cartilaginous skull, but adult Osteichthyes, or bony fishes, and
tetrapods have an ossified skull.  Some bony fishes have more than 150
separate skull bones, but during evolution the number was reduced to
approximately 27 bones in the human skull.
    In the living amphibians, all specialized, the number of skull bones
is reduced from 40 in Eryops to 18 in some of the legless Apoda (Ichthyophis)
and the bullfrog, Rana, and to 15 in the mudpuppy, Necturus.  Much of the
reduction in number of skull bones is due to fusion.
    The skull of some Permian cotylosaurs, the most primitive reptiles, is
very similar to that of labyrinthodont amphibians. The skull of the living
lizard Iguana has most of the neurocranial bones found in Eryops.
    The adult bird skull is fused into four complexes: (1) a thin, rigid,
continuous neurocranium; (2) a vertically hinged upper beak supported on a
basically reptilian upper jaw and palate; (3) a mandible of fused reptilian
elements; and (4) a jointed hyobranchial chain.
    Adult mammalian skulls typically lack paired prefrontal, postfrontal,
postorbital, quadratojugal, and septomaxillary bones. The reduction in total
number of bones is due to fusion. Paired frontal, jugal, lacrimal, nasal,
parietal, periotic, and squamosal bones plus unpaired basioccipital,
basisphenoid, dermosupraoccipital, ethmoid, presphenoid, and vomer form the
neurocranium. The paired premaxillary, maxillary, pterygoid and palatine bones
form the upper jaw, and the dentary, the lower jaw.
    In the adult human skull (Fig. 2) the pre- and basisphenoids fuse to form
the sphenoid; the basi-, ex-, and supraoccipitals to form the occipital; the
periotic, squamosal, and tympanic to form the temporal; and the premaxillary
and maxillary to form the maxilla. The upper and middle turbinates, or
conchae, fuse with the ethmoid. The basi-, cerato-, and thyrohyals form the
hyoid bone.

    Vertebrae. The term vertebrate refers to the vertebral column composed of
vertebrae. A typical vertebra consists of a body, or centrum, and a neural
arch surrounding the spinal cord lying dorsal to the centrum. The neural arch
has spinal and transverse processes for the attachment of muscles and
ligaments and articular processes for articulation with adjacent vertebrae.
Fish have little regional differentiation in the vertebral column, but
tetrapods typically have cervical, thoracic, lumbar, sacral, and caudal
vertebrae. Legless forms lack sacral vertebrae. The first two cervical
vertebrae, the atlas and axis, of amphibian fossils and amniotes are
specialized for support and movements of the head. Humans and some other
tetrapods have fibrocartilaginous intervertebral disks between the bodies of
most of the movable vertebrae. There are seven cervical vertebrae in most
mammals, including humans and giraffes.
    Two types of ribs, dorsal and ventral, occur in vertebrates. Dorsal ribs
lie in the septum dividing the trunk musculature into dorsal and ventral
masses; ventral or pleural ribs lie in the septa dividing the trunk
musculature into segments. Many fish have both types of ribs, but only dorsal
ribs occur in tetrapods. The earliest tetrapods had ribs on all of the
presacral vertebrae, but in living forms, especially birds and mammals,
movable ribs are usually restricted to the thoracic region. In tetrapods with
functional hindlegs, one or more sacral ribs articulate with the pelvis.
Cervical ribs, when present, are usually short, whereas thoracic ribs are
typically long and curved. In turtles the trunk vertebrae and ribs are
immovably fused to the carapace or upper shell. In certain mammals (living
armadilloes and fossil glyptodonts) there is considerable fusion of the trunk
vertebrae.
    Most tetrapods have a sternum, which in amniotes is connected with the
ventral ends of the ribs via costal cartilages. The sternum in living
amphibians is mostly cartilaginous and incorporated with the pectoral girdle;
it does not connect with the ribs, which are short and straight. The sternum
is absent in the legless living amphibians, turtles, legless lizards, and
snakes. The bird sternum is an ossified quadrangular bone with a prominent
median keel in flying forms and penguins. The keel is absent in ostriches and
related forms. The mammalian sternum is an ossified median structure of
several parts which may remain separate or fuse.


    Appendicular skeleton.  This consists of the skeleton of the pectoral
(shoulder) and the pelvic (hip) girdles and of the paired appendages. The
girdles provide attachment and support for the appendages, which may be fins,
flippers, legs, wings, or arms. The pectoral girdle differs from the pelvic
one in consisting of both dermal and cartilage bones and, except in skates and
pterodactyls, in having no cartilaginous or bony connection with the vertebral
column. The pectoral girdle is located close to the head; in bony fishes (and
some early tetrapods) it is actually attached to the skull, while the pelvic
girdle is associated with the outlets of the digestive and reproductory
systems. However, in some bony fishes the pelvic fins are in the throat region
anterior to the pectoral fins.
    Birds have a swordlike scapula, an enlarged coracoid, and clavicles joined
by an interclavicular element, the furcula, to form the wishbone. The avian
pelvis consists of expanded ilium, articulating with several sacral vertebrae,
and a slender pubis paralleling the expanded ischium. The pubic symphysis is
typically lacking, but ostriches and rheas have both a public and an ischiatic
symphysis.
    Mammals typically have a clavicle, a scapula, and no coracoid except in
monotremes (egg-laying mammals). Horses have only the scapula. The three
pelvic bones are usually fused into a single pelvic bone. Monotremes,
marsupials, insectivores, many rodents, hoofed mammals, and carnivores have
both a pubic and an ischiatic symphysis.
    The skeleton of the paired appendages (fins) of fish consists of
cartilaginous or bony fin rays. Tetrapod limbs are all modifications of a
single basic pattern. Most variation occurs in the number and shape of hand
and foot bones. The forearm and leg bones fuse in frogs, and the ulna and
fibula are small splints in hoofed mammals.
 �34s[F. Gaynor Evans]
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�6v�2v�32sSkeletal system disorders �21m �33s
    The skeletal system, because of its highly specialized structure and
function, reacts to injury and disease in distinctive ways.

    Fractures.	Trauma-induced fractures are the most common bone disorder.
Repair of traumatic bone injury is similar to that observed in other
connective tissues; however, the cells involved are highly specialized. The
repair of a fractured bone can be so complete that a few years after fracture
an x-ray may not reveal the presence of any residual change.  See  BONE.

    Osteoporosis.  The most common metabolic bone disease is osteoporosis. It
is a generalized loss of cortical bone substance associated with metabolic
changes in the body. Generally it is demineralized degeneration of the bones
that accompanies aging. Although both sexes are affected, it occurs earlier
and is more rapidly progressive in females. Osteoporosis is the result of
decreased bone formation, rather than increased bone destruction, and is
associated with a negative calcium balance in the body.

    Rickets and osteomalacia.  Vitamin D deficiency results in a bone
metabolic abnormality called rickets in children and osteomalacia in adults.
Children have an unusually high need for vitamin D and calcium because of
their rapidly growing bone structure. The precise role of vitamin D in this
process is not known. It appears to be essential for the normal mineralization
of the bone matrix.  See  CALCIUM METABOLISM; VITAMIN D.

    Osteitis deformans.  This disorder is of unknown cause and is
characterized by continuous destruction of bone and its simultaneous
replacement by an abnormally soft, poorly mineralized bone. As the normal bone
is resorbed, it is replaced by a light, bulky, porous osteoid matrix that is
unusually rich in blood vessels. Although thicker than the normal bone, it is
softer and prone to fracture.

    Fibrous dysplasia.	This condition is characterized by focal areas of
replacement of bone by fibrous connective tissue. The cause is unknown;
however, the mechanism of change suggests that it is an abnormality in the
process of remodeling bone.

    Neoplasms.	Although metastatic neoplasms to bone are common, primary
osteogenic tumors derived from bone-forming tissues are very rare. The three
principal osteogenic tumors are osteoma, osteoid osteoma, and osteogenic
sarcoma. Osteogenic sarcoma is the most important because of its high rate of
malignancy. Osteoma and osteoid osteoma are benign excessive proliferations or
scarring of the bone tissue similar to that which occurs following fracture.
They result in an excessive production of bone in a localized region of the
skeleton and do not undergo malignant change.
    A rare malignant neoplasm arising in the soft tissues within bones is
Ewing's sarcoma. The neoplasm is apparently derived from the endothelial cells
of blood vessels within marrow. It is most frequent in adolescents and young
adults, usually in long tubular bones or the pelvic bones, or both.  See
ONCOLOGY.

    Osteomyelitis.  This is an inflammatory disease of bone in which the
infectious organism enters the bone substance either by way of the bloodstream
or through direct penetration due to trauma.  The organisms are usually
pyogenic (staphylococci, streptococci, pneumococci, or gonococci), although
other organisms such as hemophilous influenzae, coliform bacilli, and
tuberculosis may rarely be involved.  Necrosis and spread of the infection
may ensue, with extensive bone destruction and pus formation.

    Arthritis.	Inflammatory joint lesions are termed arthritis.  When caused
by a specific organism, it is termed septic arthritis.	See ACHONDROPLASIA;
ARTHRITIS; DYSCHONDROPLASIA; JOINT (Anatomy) DISORDERS.

[N. Karle Mottet]
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�6v�2v�32sSkin   �21m �33s 
    The external covering, or integument, of the body. There are two layers, 
the outer epidermis, which is composed of flattened, ectodermal, epithelial 
cells, and the inner corium or dermis, which is formed by fibrous yet elastic 
mesodermal connective tissue containing a rich supply of blood vessels, 
nerves, and accessory structures. 
    The outer portion of the epidermis of higher vertebrates consists of many 
layers of dead cornified (horny) cells; the inner portion is usually a single 
layer of dividing cells, the stratum germinativum, which is in contact with 
the underlying dermis. In more primitive forms there are fewer layers of dead 
cells and little cornification. 
    The integument has many important functions: protection; pliability and 
extensibility, which allow movement; regulation of body fluids; excretion of 
wastes; regulation of body temperature; development of sense organs in the 
skin; and the formation of specialized accessory structures.  See  INTEGUMENT. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sSkin disorders �21m �33s
    The skin is subject to localized and generalized disorders, as well as
those of primary occurrence in the skin and those secondary to involvement of
other tissue.
    There are many rather common skin disorders that so far have offered
little evidence of their causes. Common examples include psoriasis,
parapsoriasis, and lichen planus. The skin is the major recipient of damage
from trauma of all kinds. Sunburn, frostbite, lacerations, hematomas, thermal
burns, and chemical irritations and burns are commonplace.
    Skin conditions occurring in regions which have a large number of oil, or
sebaceous, glands usually produce an increase in secretion so that an oily
skin is characteristic. Acne is the most common example and probably results
from endocrinologic imbalance and other factors. See ACNE.
    The skin is the site of direct and indirect involvement in many
infections. The most common diseases produced by the pyogenic, or pus-forming,
bacteria include erysipelas, impetigo, folliculitis, and furunculosis, or
boils.
    Viral infections account for many of the infectious exanthemas such as
measles, chickenpox, and smallpox, and also for other disorders. They are
responsible for the formation of warts and may be implicated in certain other
skin tumors. Viruses are also the agents of herpes simplex (fever blisters)
and herpes zoster (shingles).  See  CHICKENPOX AND SHINGLES; MEASLES; SMALLPOX.
    Fungus infections include the superficial types, such as tinea or
ringworm, moniliasis or athlete's foot, and similar disorders. The infrequent
but potentially dangerous systemic fungus infections such as actinomycosis,
blastomycosis, and histoplasmosis often are marked by skin lesions.  See
ACTINOMYCOSIS; BLASTOMYCOSIS; CANDIDIASIS; HISTOPLASMOSIS.
    The term eczema is generally used to include any noninfectious,
inflammatory lesion in which there are papules, blisters, and oozing of serum
from the affected parts. The erythemas are disorders in which the prominent
feature is involvement of the vasculature of the affected skin. Such
involvement often produces a characteristic swelling, or edema, in these
disorders and includes urticaria, or hives, which results from any of the
allergic agents.  See  ECZEMA.
    Skin lesions are common in many psychic disorders. These vary from simple
pruritus (itching) to widespread, compound lesions of great severity.
    Primary tumors of the skin are common and most often are of the benign
varieties. Skin cancer, however, has one of the highest rates of incidence of
any malignancy. Its superficial location and relative ease of treatment also
produce one of the highest rates of cure, if diagnosis is made early.  See
SKIN.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sSkin effect (electricity)   �21m �33s 
    The crowding of high-frequency electric current into a thin surface layer 
of a conductor. 
    For a steady unidirectional flow of electricity, the current is uniformly 
distributed over the cross section of a uniform conductor. For an alternating 
current, there is no longer this simple uniformity, but the current density is 
greater near the outer surface than at the center. The magnitude of the 
nonuniformity increases as the frequency rises. For low frequencies, the 
effect is very small, but at frequencies for which the wavelength within the 
conducting material is comparable with the dimensions of the conductor, or 
smaller, the entire current may be considered to be within a relatively thin 
surface layer. 
    The skin effect is largely due to self-induced electromotive forces 
(emf's) which are different for different paths within the conductor. These 
emf's increase with frequency, since they depend upon the rate of change of 
flux. The skin effect results in a resistance for a conductor that is greater 
for an alternating current than for a direct current, since the effective 
cross section of the conductor is decreased. 
 �34s[Kenneth V. Manning]
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�6v�2v�32sSkin friction (physics)   �21m �33s 
    A type of friction force which exists at the surface, or skin, of a solid 
body immersed in a much larger volume of fluid which is in motion with 
velocity u1 relative to the body, as illustrated. The magnitude of skin 
friction per unit surface area, the shear stress TAUsubw, was equated by Isaac 
Newton to the rate of deformation of an adjacent fluid element (du/dy)subw 
times a transport property of the fluid called the absolute or dynamic 
viscosity coefficient MU. Such flow distortion is significant only in a thin 
boundary layer, which may be laminar or turbulent and outside of which, with y 
greater than DELTA(x), the motion is essentially inviscid.  See  VISCOSITY. 
    The skin friction force contributes directly to the drag of the body; it 
also contributes indirectly because, by its action, the inviscid outer flow 
may be modified with effect upon the pressure distribution.  See  
BOUNDARY-LAYER FLOW. 
 �34s[Bernard M. Leadon]
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�6v�2v�32sSkink   �21m �33s 
    A cosmopolitan reptile belonging to the family Scincidae. They are small- 
to medium-sized, mostly terrestrial or fossorial lizards with a cylindrical 
body and short legs; in some instances the legs are vestigial. The body scales 
have cores of bone. In many species the eyelids are transparent. Skinks 
exhibit pleurodont dentition; that is, the teeth are attached on the side of 
the jaw. Most species are ovoviviparous. Three genera, Neoseps, Lygosoma, and 
Eumeces, occur in the United States. A number of species of Tiliqua, largest 
of the skinks, inhabit Malaysia and Australia.  See  REPTILIA; SQUAMATA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSkin test   �21m �33s 
    A procedure for evaluating the immunity status, involving the introduction 
of a reagent into or under the skin. Certain toxic antigens applied in minute 
doses will give a visible, but readily tolerated, lesion if less than a 
threshold amount of antibody is present in the skin. Examples are the Schick 
test for diphtheria antitoxin and the Dick test for scarlet fever antitoxin. 
If sufficient antibody is present, the toxin will be largely or completely 
neutralized, and the lesion will be minimal or absent. False positive 
reactions occur in allergies to the reagent toxin, but these can be controlled 
with inactivated toxins. Other tests involve substances that are not reactive 
with normal individuals, but give a visible skin reaction in the presence of 
antibodies acquired as a result of hypersensitivity or allergy to an infecting 
organism. A positive reaction is thus presumptive for previous contact with a 
specific infectious agent, for example, the tuberculin and Mantoux tests for 
tuberculosis, and the Mallein test for glanders. In a few instances, antibody 
may be injected as a reagent and the neutralization of toxins present in the 
skin observed, as in the Schultz-Charlton blanching test in scarlet fever. 
 �34s[Henry P. Treffers]
�33s 
  
  
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�6v�2v�32sSkull   �21m �33s 
    The bones or cartilages of the head which form the cranium and the face. 
The skull surrounds and protects the brain and major sense organs and includes 
the jaws. 
    The skull is formed from a large number of individual pieces. In humans 
the eight cranial bones which form a hollow, protective braincase are the 
occipital, sphenoid, ethmoid, and frontal, as well as the paired temporal and 
parietal bones. There are 14 facial bones: the vomer, mandible (lower jaw), 
and the paired nasal, lacrimal, zygomatic (cheek), palatine, maxillary (upper 
jaw), and inferior turbinate of the nasal passage. Several of these bones 
represent fusions of what are several bones in most other animals. Skull 
articulations are generally fixed, serrated sutures. The skull base contains 
openings, or foramina, for blood vessels and nerves. Certain parts of the 
skull bones are hollow and form air sinuses such as frontal, maxillary, and 
mastoid.  See  SINUS; SKELETAL SYSTEM. 
 �34s[Thomas S. Parsons]
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�6v�2v�32sSkunk   �21m �33s 
    Any one of a group of carnivores in the family Mustelidae, which also 
includes otters, weasels, badgers, and martens. Skunks are found only in the 
New World and range from North America to South America, with six species in 
the United States. They are characterized by their glossy black and white coat 
and two musk glands at the base of the tail. 
    The common striped skunk (Mephitis mephitis) has a white stripe on each 
side running into the tail. These animals are terrestrial and somewhat 
fossorial (burrowing). They are carnivores primarily but eat vegetation such 
as seeds, leaves, and nuts during the winter. During the summer they feed on 
insects, fruit, eggs, and rodents. The spotted skunks (Spilogale) range from 
the eastern United States to Colorado and Wyoming.  See  CARNIVORA; MAMMALIA; 
MARTEN; OTTER; WEASEL. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSkutterudite �21m �33s
    A mineral with composition (Co,Ni)AS3, an ore of cobalt and nickel.
Commonly the mineral is massive with metallic luster and tin-white color. The
hardness is 51/2 - 6 on Mohs scale and the specific gravity 6.6. Skutterudite
is found at Freiberg, Annaberg, and Schneeberg in Germany, and at Cobalt,
Ontario.  See  COBALT; NICKEL.
 �34s[Cornelius S. Hurlbut, Jr.]
�33s

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�6v�2v�32sSlate   �21m �33s 
    A group name for various very fine-grained rocks derived from mudstones, 
siltstones, and other clayey sediments as a result of low-degree regional 
metamorphism. Highly characteristic of slates is the perfect fissility or 
slaty cleaveage which is a regular and perfect planar schistosity, the slates 
themselves thus grading into phyllites. The chief minerals of slates are 
muscovite (as sericite), chlorite, and quartz. Common accessories are 
tourmaline, rutile, epidote, sphene, hematite, and ilmenite.  See  METAMORPHIC 
ROCKS; PHYLLITE; ROCK CLEAVAGE; SCHISTOSITY. 
    Slates are widely used for roofing purposes. They are easily prepared and 
fixed, are weatherproof and durable, and in many areas are cheaper and better 
than any other thatching materials. The making of slates is still performed by 
hand using chisel and mallet. Big slabs are split into separate slates, the 
thickness of which varies with the size required and the quality of the rock. 
The slates are afterward trimmed to size either by hand or by machine-driven 
rotating knives. 
 �34s[T. F. W. Barth]
�33s 
  
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�6v�2v�32sSleep and dreaming   �21m �33s 
    There has been extensive research relative to sleep and dreaming including 
studies of sleep patterns, sleep structure and physiology, as well as the 
cognitive aspects of sleep, which include dreaming. More recently, emphasis 
was placed on sleep disorders. 
  
    Patterns and structure of sleep.  Patterns of sleep refer to the 
occurrence or absence of sleep, usually described in a 24-hour context, and 
involve such indices as amount of sleep, number of sleep episodes, and 
placement of sleep, for example, nocturnal or diurnal sleep. The typical adult 
pattern is a single, nocturnal episode of about 7 to 8 h. 
    Sleep structures refer to the characteristics of the ongoing process of 
sleep and are generally indexed by the electroencephalogram (EEG). These 
measures are taken by pasting electrode cups to the head. Some electrodes pick 
up ``brain waves,'' others eye movements, and others muscle tonus. Frequently, 
heart rate, respiration, and temperature are also measured. 
    The EEG-defined sleep structure has been found to display five distinct 
patterns or ``stages'' of sleep in a night of sleep in human subjects. These 
have been designated stages 1 - 4 and stage 1-REM, which shows the presence of 
stage 1 (an EEG essentially identical with waking) plus a number of phasic 
occurrences, particularly rapid eye movements (REM). 
    Many studies of normal subjects have revealed consistent overall 
characteristics of adult human sleep. A typical night of sleep for a young 
adult is shown in the illustration. Summary aspects of the figure typify the 
overall structure of human sleep; about 50% of the night is spent in stage 2 
sleep, about 20% in slow-wave sleep (stages 3 and 4), about 25% in REM sleep, 
and about 5% in stage 1 (light sleep). Most of the slow-wave sleep occurs 
early in the sleep period, and REM sleep occurs episodically about every 90 to 
100 min, with REM periods increasing in duration across the night. 
  
    Experimental variations.  Sleep may be viewed as a biological rhythm - an 
endogenously determined tendency of a biological event to repeat itself on a 
time schedule. As such, it may be expected that temporal variations 
significantly influence sleep. Sleep schedules may be varied in three, often 
interacting ways: time awake prior to sleep may be short or long, sleep length 
may be reduced, and the placement of sleep may be varied. Sleep latency and 
prior wakefulness are inversely related, reaching an asymptote at about 60 h. 
Increased prior wakefulness increases the amount of slow-wave sleep and 
decreases the time spent in light stages of sleep (stage 0 after sleep onset 
and stage 1). Prior wakefulness has only limited effects on 1-REM. The time of 
day of sleep occurrence sharply affects the latency of stage 1-REM. Shortened 
sleep amounts down to about 3 - 4 h result in disproportionately less 1-REM 
sleep, but have little effect on stage 4. Subsequent sleep shows a sleep debt, 
evidenced by extended amounts of sleep. Displaced sleep, which results from 
such activities as shiftwork schedules or jet travel, is characterized by 
temporal variations in the sleep structure such as reduced 1-REM latencies and 
by increased awakening tendencies. 
    Deprivation is a major variable in sleep. The most certain finding of 
sleep deprivation studies is increased sleepiness, and many of the performance 
variations associated with sleep deprivation may be attributable to this 
factor. Under extreme deprivation, such as 10 days, biochemical changes are 
best described as subtle. The established neurological aftereffects of sleep 
deprivation are decreased pain thresholds, mild muscular tremors, and double 
vision. Reflexes and sensory systems seem unaffected. There is also some 
evidence for a short-term memory deficit. However, short and highly motivating 
tasks are remarkably resistant to sleep deprivation. 
    The sleep structure variable which has received the most intensive 
experimental attention is stage 1-REM deprivation. Clearly 1-REM sleep is 
strongly resistant to deprivation conditions, as evidenced by increased 
difficulty in maintaining deprivation and a ``rebound'' of 1-REM amounts after 
deprivation. However, its effects on waking behavior are less clear. Gross 
personality effects and simple learning, verbal memory, or performance effects 
are not present. Indeed, stage 1-REM deprivation appears facilitative in 
patients showing clinical depression. The most positive evidence suggests that 
1-REM plays a role in long-term memory storage of material which requires 
extensive adaptational changes (unexpected learning). 
  
  
    Sleep disorders.  Sleep disorders are deviations from normative sleep with 
associated clinical aspects. These include the typical childhood disorders of 
sleepwalking, night terrors, and enuresis, and the more general anomalies such 
as narcolepsy, hypersomnia, and insomnias. Also included are the secondary 
sleep disorders associated with other pathological conditions, for example, 
those typically present in depressions or associated with central nervous 
system pathology such as sleeping sickness or encephalitis lethargica. 
    While ``insomnia'' can be taken as a general term for all sleep disorders, 
it now specifically refers to a behavioral syndrome in which the person 
complains of an inability to initiate sleep or maintain sleep (awakening 
during sleep or early awakening). Because it is thought of as a symptom 
(rather than a disorder per se), it, like body temperature, may reflect a wide 
range of causes. The amount of insomnia clearly depends on definition of 
degree, chronicity, and the sex and age of the subject. It is likely that all 
persons have transient periods of disturbed sleep. 
    Because insomnia is a syndrome, the insomnia expressed by a particular 
person may require a detailed diagnosis and very different responses. A few 
``types'' of insomnia are illustrative: 
    Situational insomnias result from transient life stress such as work 
requirements, significant psychological changes such as loss of a loved one, 
location of occupational changes, or financial stress. They are typically 
transient, arising from situational changes and disappearing when adaptation 
has occurred. 
    In pseudo-insomnia, the individual may believe that he or she is getting 
less than is needed, and attributes feelings of inadequacy or stress to lack 
of sleep. 
    In rhythmic insomnia, the person's sleep period may be ``entrained'' by 
habit or circumstances to late hours of retiring; and because of work 
requirements the individual must be awakened early, resulting in inadequate 
amounts of sleep. 
    Psychopathological insomnias occur because sleep is sensitive to chronic 
psychological disturbances, such as anxiety and particularly depression. The 
insomnia may be reflecting this underlying psychological circumstance. 
    Aging insomnia is the type experienced by many individuals as nighttime 
awakening becomes prevalent as a part of the aging process. 
    All of these examples of insomnia require different approaches and 
treatment (or nontreatment). In the past there has been a tendency for a 
single mode of treatment, the sleeping pill. A great deal of sleep research 
has been focused on its role, and has added to knowledge of another insomnia, 
the drug-related insomnia, in which chronic use of sleeping pills may not 
yield a cure, but in fact may cause disturbed sleep. 
  
    Dreams.  The discovery in the 1950s of the relationship between the 
electrophysiological index of an ``active'' phase of sleep and the index's 
very close assocation with the subjective state of dreaming joined these 
essentially independent interests. Simply, the identification of stage 1-REM 
displayed a unique and significant part of sleep as an active and exciting 
biological state. This stage's close indexing of dreams indicated their 
presence across the night far beyond the morning recall to which dreams had 
been previously confined. 
    The REM stage of sleep occurs in bursts of 90-min cycles and occupies some 
20 - 25% of each night of all normal human sleep. The initial report that some 
85% of awakenings from this state yielding a recall readily judged to be 
dreaming emphasized the large amount of sleep time occupied by dreaming. 
Detailed studies of REM awakenings have shown that reported materials from REM 
periods across the night become progressively longer, more vivid, and more 
unrealistic with successive REM. 
    Awakenings from non-REM periods clearly established that all cognitive 
aspects of sleep are not confined to the REM period. While judges can 
distinguish between reports of REM awakenings and NREM awakenings with high 
accuracy, on the basis of those having ``dreamlike'' quality (REM reports) and 
those having ``thoughtlike'' characteristics (NREM reports), as many as 30 - 
50% of the NREM awakenings may contain some cognitive aspects. 
    A primary question which goes beyond the issue of dreams being reflective 
is whether dreams have a functional role. Many roles have been ascribed: At an 
affectional level, gratification of impulses, catharsis, synthesis, rehearsal, 
and self-education have been suggested. At a cognitive level, information 
processing and consolidation of acquired material have been attributed to the 
dream state. Few of the affective notions have received experimental tests, 
though clinical testimony had often been asserted. The more cognitive roles 
have been explored by REM deprivation experiments with limited success, 
although these experiments have seldom considered the dream per se as a 
critical variable. 
    In general, the modern era of sleep research has provided much information 
on the biological basis of the dream state and the substantial place and 
character of that state in the animal kingdom and human existence. 
 �34s[Wilse B. Webb]
�33s 
  
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�6v�2v�32sSleeping sickness �21m �33s
    A specific disease of humans confined to tropical Africa. It is caused by
Trypanosoma gambiense or T. rhodesiense, protozoa which are transmitted by
tsetse flies of the genus Glossina. The disease is also known as African
trypanosomiasis, maladie du sommeil, and Schlafkrankheit. See TRYPANOSOMATIDAE.
    The incubation period is 2-3 weeks.  The first signs are a local
reaction at the site of the bite, fever, enlargement of adjacent lymph
nodes, skins rash, edema, and cardiovascular disturbances.  Asthenia develops
early and is often very pronounced.  The meningoencephalitic stage is
accompanied by increasing lassitude and general indifference to surroundings.
Somnolence, indifference to food, and emaciation characterize the late phase
of the disease.
    Diagnosis can be made from the clinical signs of the primary trypanosome
chancre, the irregular fever, glandular enlargement, particularly in the neck,
coupled with the demonstration of trypanosomes in stained thick films and
smears from gland puncture, peripheral blood, and lumbar puncture.
    The organic arsenical compounds tryparsamide and melarsen BAL are
effective, especially when administered early in the disease.  Antrypol
(Bayer 205) and pentamidine have also been used in treatment but are more
valuable as prophylactic drugs.  Control against tsetse files is the main
objective.  See MEDICAL PARASITOLOGY.

[M. Yamaguchi]
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�6v�2v�32sSlider-crank mechanism �21m �33s
    A four-bar linkage with output crank and ground member of infinite length.
A slider crank (see illustration) is most widely used to convert reciprocating
to rotary motion (as in an engine) or to convert rotary to reciprocating
motion (as in pumps), but it has numerous other applications. Positions at
which slider motion reverses are called dead centers. When crank and
connecting rod are extended in a straight line and the slider is at its
maximum distance from the axis of the crankshaft, the position is top dead
center (TDC); when the slider is at its minimum distance from the axis of the
crankshaft, the position is bottom dead center (BDC).
    The conventional internal combustion engine employs a piston arrangement
in which the piston becomes the slider of the slider-crank mechanism. Radial
engines for aircraft employ a single master connecting rod to reduce the
length of the crankshaft. The master rod, which is connected to the wrist pin
in a piston, is part of a conventional slider-crank mechanism. The other
pistons are joined by their connecting rods to pins on the master connecting
rod.
    To convert rotary motion into reciprocating motion, the slider crank is
part of a wide range of machines, typically pumps and compressors. Another use
of the slider crank is in toggle mechanisms, also called knuckle joints. The
driving force is applied at the crankpin so that, at TDC, a much larger force
is developed at the slider.  See  FOUR-BAR LINKAGE.
 �34s[Douglas P. Adams]
�33s



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�6v�2v�32sSlide rule   �21m �33s 
    A mechanical analog computing aid which is used extensively for 
multiplication and division and to a lesser degree for looking up functions. 
In its most common form a slide rule consists of a body formed from two 
parallel members rigidly fastened together, a slide which can be moved left or 
right between the body members, and a transparent indicator which carries a 
hairline and can be moved left or right over the face of the body and the 
slide. Scales are provided on the body and the slide as shown in the 
illustration.  
    The C and D scales, used for multiplication and division, are graduated 
from 1 on the left to 10 on the right, with intermediate numbers distributed 
logarithmically. Multiplication on a slide rule is based upon the fact that 
the product of two numbers can be obtained by adding the logarithms of the two 
numbers and then taking the antilogarithm. Division, which is accomplished by 
subtraction of the logarithm of the divisor from that of the dividend, is 
performed in reverse sequence to multiplication.  See  LOGARITHM. 
    Many slide rules also carry scales from which sines, cosines, tangents, 
natural logarithms, logarithms to the base 10, squares, and cubes can be read. 
 �34s[William W. Seifert]
�33s 
  
  
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�6v�2v�32sSlip �21m �33s
    The difference between the operating speed of an induction motor and its
synchronous speed (the speed of the rotating field). Slip s is usually expressed
as a decimal fraction of synchronous speed nsubs as in the equation below,

   s = [n(sub s) - n]/n(sub s)



where n is the rotor, or operating, speed.  See  INDUCTION MOTOR.
 �34s[William W. Seifert]
�33s

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�6v�2v�32sSlip rings   �21m �33s 
    Electromechanical components which, in combination with brushes, provide a 
continuous electrical connection  between rotating and stationary conductors. 
Typical applications of slip rings are in electric rotating machinery, 
synchros, and gyroscopes. Slip rings are also employed in large assemblies 
where a number of circuits must be established between a rotating device, such 
as a radar antenna, and stationary equipment. 
    Slip rings are usually constructed of steel with the cylindrical outer 
surface concentric with the axis of rotation. Insulated mountings insulate the 
rings from the shaft and from each other. Conducting brushes are arranged 
about the circumference of the slip rings and held in contact with the surface 
of the rings by spring tension.  See  ELECTRIC ROTATING MACHINERY; MOTOR. 
 �34s[Arthur R. Eckels]
�33s 
  
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�6v�2v�32sSlope �21m �33s
    The trigonometric tangent of the angle ALPHA that a line makes with the x
axis. In the illustration the slope of a plane curve C at a point P of C is
the slope of the line that is tangent to C at P. If y = f(x) is an equation in
rectangular coordinates of curve C, the slope of C at P(x0, y0) is the value
of the derivative dy/dx = f'(x) at P, denoted by f'(x(sub 0)), and hence an
equation of the nonvertical tangent to C at P is y - y(sub 0) =
f'(x[sub 0]) (x - x[sub 0]).
  See  ANALYTIC GEOMETRY; CALCULUS.
 �34s[Leonard M. Blumenthal]
�33s


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�6v�2v�32sSloth   �21m �33s 
    A mammal, found exclusively in Central and South America, of the family 
Bradypodidae in the order Edentata. These animals are all slow-moving, 
arboreal species that cling to branches upside down by means of long, curved 
claws, and are difficult if not impossible to dislodge. The long hairs of the 
fur, which are a mixture of white, black, brown, and yellow or green, are 
unusual in that they are grooved. Algae inhabit the grooves, being green when 
conditions are damp and turning yellow in times of drought. The sloth has a 
short stumpy tail, round eyes, a small mouth, and horny lips which assist in 
eating leaves, a preferred food. The small pinnae of the ears are hidden in 
the fur.  See  EDENTATA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sSlug (zoology)   �21m �33s 
    A terrestrial pulmonate mollusk in which the shell is absent or reduced to 
a small internal or external rudiment. The slug form has evolved independently 
several times. The incorporation into the muscular foot region of the body 
organs (which are contained within the shell in other mollusks) results in a 
streamlined body shape (see illustration), enabling the animal to enter small 
holes or crevices. 
    Lung respiration occurs as in other pulmonates, but skin respiration is 
probably at least as important. 
    A few slugs are carnivorous, such as Testacella which eats earthworms, but 
the majority are herbivores and may become serious horicultural and 
agricultural pests.  See  PULMONATA. 
 �34s[Norman Runham]
�33s 
  
  
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�6v�2v�32sSmallpox   �21m �33s 
    An acute, infectious, viral disease characterized by severe systemic 
involvement and a single crop of skin lesions which proceeds through macular, 
papular, vesicular, and pustular stages. Smallpox is also known as variola 
major.  
    The virus enters through mucous membranes of the upper respiratory tract 
and propagates in lymphoid or other tissues. After a 12-day incubation period, 
the disease begins with 1 - 5 days of fever and malaise. During this period of 
time the virus multiplies, circulates in the bloodstream, localizes in the 
epidermis, and produces skin lesions. The lesions form crusts which fall off 2 
- 4 weeks afterward and leave pink scars which fade slowly. Permanent scarring 
occurs because there is involvement of all skin layers, including necrosis of 
the corium. 
    Smallpox has had worldwide distribution, but with vaccination it has been 
virtually eliminated from some countries. It is transmitted by contact with 
oral or nasal secretions of infected persons or with material from skin 
lesions. Because the virus survives drying, it also has been transmitted 
through infected bedclothes or on cotton imported from distant countries. 
    All human beings are susceptible. Children are born with maternal 
antibodies, but these are soon lost. Control measures in addition to 
vaccination include rigorous quarantine regulation of travelers and of those 
in contact with known or suspected cases, and strict isolation procedures with 
patients.  
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sSmithsonite �21m �33s
    The natural form of zinc carbonate. Smithsonite may contain small amounts
of iron, calcium, cobalt, copper, manganese, cadmium, and magnesium. It occurs
chiefly as a secondary mineral formed by the oxidation of primary zinc ores,
such as sphalerite.
    Smithsonite is rarely found in well-formed crystals and is more often in
compact or porous masses with a cauliflowerlike appearance. Its color is often
dirty white, but a variety of tints, including greens, blues, and browns, may
be found, depending on the impurities present. For pure smithsonite the
hardness is about 4 or 41/2 on Mohs scale and the specific gravity is 4.43.
See  CARBONATE MINERALS; ZINC.
 �34s[Robert I. Harker]
�33s

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�6v�2v�32sSmog   �21m �33s 
    The word smog can be defined in several ways. It was originally used to 
refer to a combination of coal smoke and fog, the particles of the smoke often 
serving as nuclei on which the water vapor condensed. Later, the term was used 
to refer to any dirty urban atmosphere. A unique type was recognized in the 
late 1940s and early 1950s and came to be known as photochemical smog, defined 
as that type of air pollution which owes many of its properties to the 
products of photochemical reactions involving the vapor of various organic 
substances, especially hydrocarbons, oxides of nitrogen, and atmospheric 
oxygen. Occasionally fog may also be involved in the formation of such smog. 
    Photochemical smog is particularly apparent in cities where little coal is 
burned, there is little industrial pollution, and there are large 
concentrations of automobiles. The intensity of photochemical smog at any time 
and place, like that of any type of air pollution, is markedly influenced by 
meteorological conditions and the geographical features of the region. 
    Usually the most obvious and immediately annoying aspect of the urban 
pollution is the murkiness of the air which it produces. This visibility 
decrease is mainly caused by the suspended particles. The median particle size 
in photochemical smog is so small that the particles preferentially scatter 
blue light, and depending upon the nature and angle of illumination, the air 
may appear blue or brown. The brown appearance is accentuated by the presence 
of nitrogen dioxide (NO2), which is itself brown in color and results from 
various combustion operations, including driving automobiles. 
    Another especially annoying feature of photochemical smog is the eye 
irritation and lacrimation it produces. This irritation seems to result from 
the combined effects of several gases, especially formaldehyde, acrolein, 
peroxyacetyl nitrate (PAN), and possibly peroxybenzoyl nitrate.  See  AIR 
POLLUTION.  
 �34s[Richard D. Cadle]
�33s 
  
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�6v�2v�32sSmoke �21m �33s
    Fumes and smoke are dispersions of finely divided solids or liquids in a
gaseous medium. The particle-size range is 0.01 - 5.0 micrometers. Typical
dispersions are smokes from incomplete combustion of organic matter such as
tobacco, wood, and coal; soot or carbon black; oil-vapor mists; chemical fumes
such as sulfur trioxide (SO3) and phosphorus pentoxide (P2O5) mists, ammonium
chloride (NH4Cl), and metal oxides; and the products of hydrolysis of metal
chlorides by moist air. Oil-vapor and P2O5 mists (formed by burning phosphorus
in moist air) have been extensively used in military operations to produce
screening smokes.  See	AIR POLLUTION; SCREENING SMOKE.
 �34s[Henry H. Storch /Aaron J. Teller]
�33s

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�6v�2v�32sSmooth manifold   �21m �33s 
    A manifold with an appropriate additional structure that allows one to 
speak of differentiability or smoothness of real-valued functions. Such 
manifolds are also called differentiable manifolds. In this setting one can 
define the notion of a smooth mapping between smooth manifolds and can 
introduce the differential of a smooth mapping, which is a generalization of 
derivative.  See  MANIFOLD (MATHEMATICS). 
 �34s[Anthony W. Knapp]
�33s 
  
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�6v�2v�32sSmut (microbiology)   �21m �33s 
    A term applied both to fungi of the order Ustilaginales of the subclass 
Heterobasidiomycetidae and to the diseases which they cause. Plants of several 
families are subject to smuts, but smuts of cereals are of greatest economic 
importance. The control measure most effective against all types of smuts is 
the development and planting of resistant varieties of grain. Bunt of wheat is 
also controlled by treating seed with organic mercurial fungicides to kill 
spores adhering to the seed coat.  See  BASIDIOMYCOTINA; USTILAGINALES. 
 �34s[Robert M. Page]
�33s 
  
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�6v�2v�32sSnail   �21m �33s 
    Any of the approximately 74,000 species in the class Gastropoda of the 
phylum Mollusca or, alternatively, any of the 12 or so species of land 
pulmonate gastropods used as human food. 
    The shell of snails is in one piece and typically turbinate (see 
illustration), but may be planospiral or limpet-shaped, or may be secondarily 
lost (as in land slugs and marine nudibranchs). In development, gastropods 
have undergone torsion (the visceral mass and the mantle-shell covering it 
have become twisted through 180DG in relation to the head and foot) so that 
the mantle cavity is placed anteriorly above the head. 
    In feeding, all snails use a characteristic rasping tongue or radula. This 
is a chitinous ribbon bearing teeth which is moved over a supporting 
protrusible ``tongue'' with a to-and-fro action. The radular apparatus has a 
twofold function: it serves both for rasping off food material (mechanically 
like an inverted version of the upper incisor teeth of a beaver) and for 
transporting the food back into the gut like a conveyor belt. 
    Both fresh-water and terrestrial snail species serve as vectors (first or 
sole intermediate hosts) in the transmission of flukes (Trematoda) infecting 
humans or domestic animals. For further discussion of these parasitic diseases 
  see CLONORCHIASIS; DIGENEA; FASCIOLIASIS; SCHISTOSOMIASIS.  See   also 
GASTROPODA; PULMONATA. 
 �34s[W. D. Russell-Hunter]
�33s 
  
  
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�6v�2v�32sSnake   �21m �33s 
    Any of the 3000 species of reptiles which belong to the 13 living families 
of the suborder Serpentes in the order Squamata. Except for Antarctica, they 
have a worldwide distribution but are most abundant in warm climates. The more 
primitive pythons and boas have external vestiges of hindlimbs, but most other 
snakes are limbless. The elongate body is covered with scales, external ear 
openings are lacking, and the eyes lack mobility. The eyelids are fused 
together to form a transparent membrane over the eyes which is cast off during 
the periodic shedding of the skin. 
    The kinetic skull, mandible-quadrate articulation, and ligamentous 
connection between the anterior ends of the two mandibular rami permit extreme 
opening of the mouth, thus enabling snakes to swallow animals several times 
their size. Poisonous snakes with canaliculate fangs are called 
proteroglyphous. These teeth are erectile and are used only for stabbing. 
    There are oviparous species, which produce shell-covered eggs shortly 
after being fertilized, as well as viviparous species, which give birth to 
well-developed young. The eggs are usually deposited in a hole in the ground 
where there is sufficient humidity and temperature to permit development.  See 
BOA; PYTHON; REPTILIA; SQUAMATA; VIPER. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sSnap gage   �21m �33s 
    A gage with two surfaces that are flat and parallel; these are spaced to 
control one limit of tolerance of an outside diameter or a length. Adjustable 
snap gages have adjustable gaging surfaces so that the distance between gaging 
surfaces can be set with gage blocks and used to gage any dimension falling 
within the limits of the frame size. Snap gaging is faster than ring gaging, 
but it checks only one diameter or length at a time. See RING GAGE. 
 �34s[Rush A. Bowman]
�33s 
  
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�6v�2v�32sSnap ring   �21m �33s 
    A form of spring used principally as a fastener. Piston rings are a form 
of snap ring used as seals. The ring is elastically deformed, put in place, 
and allowed to snap back toward its unstressed position into a groove or 
recess. The snap ring may be used externally to provide a shoulder that 
retains a wheel or a bearing race on a shaft, or it may be used internally to 
provide a construction that confines a bearing race in the bore of a machine 
frame, as illustrated. 
 �34s[L. Sigfred Linderoth, Jr.]
�33s 
  
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�6v�2v�32sSnow   �21m �33s 
    The most common form of frozen precipitation, usually flakes of starlike 
crystals, matted ice needles, or combinations, often rime-coated; also 
transitions between these forms, or combinations of them with ice columns, 
plate crystals, snow pellets, snow grains, or ice pellets.  See  PRECIPITATION 
(METEOROLOGY).  
  
    When newly fallen, snow has a gravity ratio commonly 1:10 but varying 
greatly. Wet snow is heavier, but dry, fluffy snow may have a specific gravity 
of 1:30 or less. The density of snow cover is increased by packing, melting, 
and refreezing, and also because snow changes to ice granules through transfer 
of water by sublimation from small to larger ice particles. Deep in glaciers 
entrapped air is compressed and density approaches that of pure ice. 
    Snow, especially when new, is a good insulator because air is entrapped by 
the flakes. Thus it often protects low vegetation, such as grains, from 
freezing. Air temperatures tend to be low over snow because it reflects 
sunshine, is a good radiator at night, and interferes with conduction of heat 
from the ground. 
 �34s[J. R. Fulks]
�33s 
  
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�6v�2v�32sSnowfield and neve   �21m �33s 
    The term snowfield is usually applied to mountain and glacial regions to 
refer to an area of snow-covered terrain with definable geographic margins. 
Where the connotation is very general and without regard to geographical 
limits, the term snow cover is more appropriate; but glaciology requires more 
precise terms with respect to snowfield areas. 
    These terms differentiate according to the physical character and age of 
the snow cover. Technically, a snowfield can embrace only new or old snow 
(material from the current accumulation year). Anything older is categorized 
as firn or ice. The term neve is a descriptive phrase used to refer to  
consolidated granular snow not yet changed to glacier ice. Because of the need 
for simple terms, however, it has become acceptable to use the term neve when 
specifically referring to a geographical area of snowfields on mountain slopes 
or glaciers (that is, an area covered with perennial ``snow'' and embracing 
the entire zone of annually retained accumulation).  See  GLACIOLOGY. 
 �34s[Maynard M. Miller]
�33s 
  
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�6v�2v�32sSnow gage   �21m �33s 
    An instrument for measuring the amount of water equivalent in snow; more 
commonly known as a snow sampler. Frequently snow samplers are made of a 
lightweight seamless aluminum tube consisting of easily coupled lengths. Other 
snow samplers have been developed from material such as fiber glass and 
plastic for use in shallow snow, deep dense snow, and so forth. 
    To obtain a measurement the sampler is pushed vertically through the snow 
to the ground surface. The sampler, together with its snow core, is withdrawn 
and weighed. The water equivalent of the snow layer is obtained by subtracting 
the weight of the sampler from the total. In addition to any error introduced 
by the scale, there is usually a 6 - 8% error in the weight of samples taken 
in this manner. 
    Automatic devices that permit the remote observation of the water 
equivalent of the snow have been developed. These devices also permit 
telemetering the data to a central location, eliminating the need for travel 
to the snowfields.  See  SNOW; SNOW SURVEYING. 
 �34s[Robert T. Beaumont]
�33s 
  
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�6v�2v�32sSnow line   �21m �33s 
    A term generally used to refer to the elevation of the lower edge of a 
snow field. In mountainous areas, it is not truly a line but rather an 
irregular, commonly patchy border zone, the position of which in any one 
sector has been determined by the amount of snowfall and ablation. These 
factors may vary considerably from one part to another. On glacier surfaces 
the snow line is sometimes referred to as the glacier snow line or neve line 
(the outer limit of retained winter snow cover on a glacier). 
    Year-to-year variation in the position of the orographical snow line is 
great. The mean position over many decades, however, is important as a factor 
in the development of nivation hollows and protalus ramparts in deglaciated 
cirque beds.  See  GLACIOLOGY; SNOWFIELD AND NEVE. 
 �34s[Maynard M. Miller]
�33s 
  
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�6v�2v�32sSnow surveying   �21m �33s 
    An inventory of the winter's accumulation of snow. A snow survey is made 
at the same location each year, and this site is referred to as the snow 
course. A snow course consists of a series of sampling points, usually 10 - 
20, to give a dependable sampling average. These points are situated at a 
measured spacing, 50 - 100 ft (15 - 30 m) apart, along a permanently marked 
and mapped route. The significant information obtained by means of a snow 
survey is the average water equivalent of all of the samples taken at the snow 
course. The snow courses may be sampled several times a year, year after year. 
  See  SNOW GAGE. 
    While snow surveys were initially made to forecast water supplies for 
irrigation, they are also used for water-management purposes, flood 
forecasting, power requirements, and municipal water supplies.  See  SURFACE 
WATER.  
 �34s[Robert T. Beaumont]
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�6v�2v�32sSoap �21m �33s
    A particular type of detergent in which the water-solubilizing group of
the 10 - 18-carbon chain is carboxylate, --COO-, and the positive ion is
usually sodium, Nasupp, or potassium, Ksupp. All similar compounds with
hydrophilic groups other than carboxylate are synthetic detergents or
surfactants.  See  DETERGENT.
    Because of the marked imbalance of polarity within the molecules of soaps,
they have unusual surface and solubility characteristics. The feature of their
molecular structure which causes these properties is the location of the
hydrophilic function at or near the end of a long hydrocarbon (hydrophobic)
chain. Thus one part of the molecule is water-seeking while the other portion
is oil-seeking. This explains why these molecules concentrate and orient
themselves at interfaces, such as an oil-solution interface where the
hydrophobic portion enters the oil and the hydrophilic portion stays in the
water.
    Soap was formerly manufactured by an alkaline hydrolysis reaction called
saponification, according to the reaction below (R represents the hydrocarbon

       C3H5(OOCR)3 + 3NaOH   3NaOOCR + C3Hb5(OH)3
	 Fat	    Caustic    Soap	 Glycerin
		    soda

chain). An important modern process for making soap is the direct hydrolysis
of fats by water at high temperatures. This permits fractionation of the fatty
acids, which are neutralized to soap in a continous process. Advantages for
this process are close control of the soap concentration, the preparation of
soaps of certain chain lengths for specific purposes, and easy recovery of the
by-product glycerin. After the soap is recovered, it is pumped into giant
mixers called crutchers where builders, perfumes, and other desired
ingredients are added. (Builders are alkaline compounds which improve the
cleaning performance of the detergent.) Finally, the soap is rolled into
flakes, cast or milled into bars, or spray-dried into soap powder.
    There is one serious disadvantage to the general use of soap which has
caused its replacement by synthetic detergents for home laundering. The
problem is that carboxylate ions of the soap react with the calcium and
magnesium ions in natural hard water to form insoluble materials called lime
soap. In areas having a hard water supply, this is the floating curd which
appears in the washbowl and as bathtub ring. The greatest market remaining for
soap is as bar soap for personal bathing.
    Metallic soaps are alkaline-earth or heavy-metal long-chain carboxylates
which are insoluble in water but soluble in nonaqueous solvents. They are used
as additives to lubricating oils, in greases, as rust inhibitors, and in
jellied fuels.
 �34s[Roy C. Mast]
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�6v�2v�32sSoapstone   �21m �33s 
    A soft talc-rich rock. Soapstones are rocks composed of serpentine, talc, 
and carbonates (magnesite, dolomite, or calcite). They represent original 
periodites which were altered at low temperatures by hydrothermal solutions 
containing silicon dioxide, carbon dioxide, and other dissolved materials 
(products of low-grade metasomatism). The whole group of rocks may loosely be 
referred to as soapstones because of their soft, soapy consistency. 
 �34s[T. F. W. Barth]
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�6v�2v�32sSocial animals   �21m �33s 
    Those animals which exhibit social behavior. This may be defined as any 
behavior stimulated by or acting upon another animal of the same species. In 
this broad sense, almost any animal which is capable of behavior is to some 
degree social. Even those animals which are completely sedentary, such as 
adult sponges and sea squirts, have a tendency to live in colonies and are 
social to that extent. Social reactions are occasionally given by other 
species than the animals's own, like the relations between domestic animals 
and humans. See SOCIAL INSECTS. 
    There are three general types of social animals: those which are found 
only in temporary aggregations, including all the lower invertebrates; those 
which live in permanent groups bound together by care-giving behavior, such as 
the insect societies; and those whose permanent groups are organized through 
mutual imitation and agonistic behavior (often in addition to care giving), 
such as many vertebrate societies. Humans belong to a fourth type in which a 
highly developed communication system has been added to the above social 
processes.  
  
    Fish.  The behavioral capacities of fish reflect their watery habitat. As 
to psychological capacities, fish are capable of rapid conditioning. Trout in 
hatcheries quickly learn to rise for food at any splashing noise. Fish can 
learn simple mazes and are able to find their way back to their home ranges 
quickly under natural conditions. On the other hand, much of their social 
behavior, particularly that connected with mating and nest building, is 
largely organized by instinct. 
    Compared with birds and mammals, fish lack some of the basic types of 
social behavior. As water-living animals they have no problem of waste 
disposal and consequently have no special patterns of eliminative behavior. 
Fish never feed their young after hatching, with the exception of certain 
species whose newly hatched young feed on mucus from the adult's body, and 
their care-giving behavior is limited to offering shelter and driving off 
predators. The connection between offspring and parents is brief at best, and 
the young show little care-soliciting behavior. 
    Much of the social life of fishes is concentrated around the school, in 
which mutual imitation or allelomimetic behavior coordinates the group and 
provides mutual safety. Other social life centers around mating and nest 
building. The school is perhaps the most typical part of the social life of 
fishes. It is usually an association between newly hatched young, but many 
species such as herring and mackerel spend most of their lives in such a group. 
  
    Birds.  Birds are highly social animals. They move so rapidly and freely 
that their social behavior is not always apparent to the casual observer, but 
when their actions are studied more carefully it is found that birds develop a 
great variety and complexity of social organization. Their biological 
differentiation of labor is simpler than that of insects since there are only 
three principal types of individuals: males, females, and young. However, they 
exhibit all the basic types of social behavior in well-developed forms and are 
capable of considerable differentiation of behavior on the basis of learning 
and experience. 
    With their great mobility, birds frequently encounter other birds of 
different species, which creates a problem of separation between different 
societies. The bright colors and plumage changes found in many birds are 
closely related to social behavior and serve to differentiate between species, 
the sexes, and adults and young. Display behavior, emphasizing the plumage, is 
a prominant part of sexual behavior in many species. 
    Display is a type of communication. In addition, birds have a wide variety 
of vocal signals; best known of these is the song of the perching birds, now 
shown to be a territorial signal. In some species the song is in part learned; 
in others it is developed even by birds reared in isolation. Certain species, 
such as crows and parrots, have powers of vocal imitation superior to those of 
mammals, but even when human words are learned, there is no evidence that the 
birds do more than learn a complicated trick. Such parroting is found in 
nature in the songs of mockingbirds. Vocal and visual signals are important 
mechanisms in the social organization of birds but are not used as a true 
language.  
    Social behavior is affected still more directly by aerial life. One major 
adaptation is rapid development by the young. This means that in many birds 
there is little opportunity for a parent-offspring relationship to develop. 
The most prominent social relationship of most birds is consequently the 
male-female relationship or pair bond. 
    Primary socialization or imprinting takes place very rapidly in precocious 
birds such as ducks and geese, but is spread over a longer period in birds 
which are hatched in an immature state. The parasitic cowbirds and cuckoos do 
not become imprinted by their foster parents but join their own kind after 
leaving the nest. 
    Another fundamental social process is that of becoming attached to 
particular places or home sites, or localization. Many species of birds have 
two homes, one during the breeding season and another at other times. 
Migrations between the two may cover thousands of miles and require 
extraordinary capacities for orientation, the basic nature of which is the 
subject of much research.  See  MIGRATORY BEHAVIOR. 
    Flocking behavior is typical of birds. In the smaller flying birds 
extraordinarily complex and rapid maneuvers based on allelomimetic behavior 
are seen; in some larger birds there appears considerable leadership. In 
contrast, the hawks and owls are almost entirely solitary in their movements. 
Territory is another important feature of bird life, ranging from the large 
breeding territories of the perching birds to the small nest area in gulls and 
other colonial sea birds. In still others there are no definite territories. 
  
    Mammals.  The postnatal development of each species of mammal is closely 
related to the social organization typical of the adults. Every highly social 
animal has a short period early in life when it readily forms attachments to 
any animal with which it has prolonged contact. This critical period for 
primary socialization can be demonstrated by removing young from their natural 
parents and rearing them by hand. Experience before this period has no effect; 
during the period the young animal transfers all its native social behavior to 
the human; thereafter this process becomes increasingly difficult, although 
captivity and hand feeding have some effect on adults. 
    The process of socialization begins almost immediately after birth in 
ungulates like the sheep, and the primary relationship is formed with the 
mother. In dogs and wolves the process does not begin until about 3 weeks of 
age, at a time when the mother is beginning to leave the pups. Consequently 
the strongest relationships are formed with litter mates, thus forming the 
foundation of a pack. Many rodents stay in the nest long after birth; primary 
relationships are therefore formed with nest mates. Young primates are 
typically surrounded by a group of their own kind, but because they are 
carried for long periods the first strong relationship tends to be with the 
mother.  
    Mammals may develop all types of social behavior to a high degree, but not 
necessarily in every species. Mammals have great capacities for learning and 
adaptation, which means that social relationships are often highly developed 
on the basis of learning and habit formation as well as on the basis of 
heredity and biological differences. The resulting societies tend to be 
malleable and variable within the same species and to show considerable 
evidence of cultural inheritance from one generation to the next. Mammalian 
societies have been completely described in relatively few forms, and new 
discoveries will probably reveal the existence of a greater variety of social 
organization.  
    Basic social organization and behavior of humans obviously differs from 
that of all other primates, although there is some relationship. At the same 
time, the range of variability of human societies as seen in the nuclear 
family does not approach that in mammals as a whole. Human societies are 
characterized by the presence of all fundamental types of social behavior and 
social relationships rather than by extreme specialization. 
 �34s[John P. Scott]
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�6v�2v�32sSocial hierarchy �21m �33s
    Social hierarchy was first adequately described in flocks of domestic
hens. The fighting behavior of each individual hen is organized so that in any
given pair hen A always pecks hen B and never the reverse. In this way an
entire flock can be described in terms of social rank, or a ``peck order.''
This type of social organization is an important general phenomenon among
higher animals.
    A social hierarchy is composed of many dominance-subordination
relationships. When two hens are brought together for the first time, they
usually fight. The next time they come together the fighting has a shorter
duration. One hen forms a habit of winning and the other of losing. Eventually
the relationship is reduced to a threat or peck by the dominant hen and
avoidance or submission to the peck by the subordinate one. The hens thus form
a social relationship based on agonistic behavior, which is defined as any
behavior connected with conflict between two individuals and including
fighting, escape, and submissive behavior.
    There are several possible arrangements of dominance hierarchies in a
group of three animals, assuming that each relationship shows either a
complete dominance or complete peacefulness. These groupings are: (1)
straight-line dominance order, commonly found in flocks of hens in aggressive
breeds; (2) triangular order, less common; (3) one dominant and two
subordinate, common in mouse groups; (4) two dominant animals and one
subordinate; (5) all animals peaceful, with no dominance order, frequently
found in young animals reared together.
     Since most of the lower invertebrates do not exhibit agonistic behavior,
social hierarchies have so far been found only in arthropods and vertebrates.
In arthropods, the best examples are found in crayfish and other Crustacea.
While fighting occurs in some insects, it is doubtful whether true dominance-
subordination relationships are formed. In the higher social insects, such as
ants, bees, wasps, or termites, no fighting takes place within the colony. On
the other hand, dominance-subordination relationships have been described in all
classes of vertebrates, although they have little importance among amphibia,
which are poorly equipped for fighting behavior. The social hierarchy is thus
a typical attribute of vertebrate societies, reaching its highest development
in birds and mammals.
    The dominance hierarchy can be either extremely important or completely
nonexistent, depending on the species of animal involved. Well-developed
dominance relationships tend to be relatively permanent, persisting over
periods of years in long-lived animals. Such social organization may have
important effects on the control of fighting within a species; on the division
of food, territory, and mates; and upon the selection of surviving
individuals. At the same time it should be remembered that this is only one
aspect of social organization and that important relationships which do not
involve fighting, such as care-dependency and leader-follower relationships,
also exist in animal societies.  See  SOCIAL ANIMALS; SOCIAL INSECTS.
 �34s[John P. Scott]
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�6v�2v�32sSocial insects   �21m �33s 
    Social organization has evolved independently several times in unrelated 
groups of the class Insecta. The best examples of these social insects are the 
termites of the order Isoptera and the social wasps, bees, and ants of the 
order Hymenoptera. Characteristically, social insects are differentiated and 
specialized in structure, function, and behavior into castes, which live in 
colonies and exhibit group integration and division of labor. The principal 
castes are the reproductives, which include the king and queen, and the 
steriles, the workers and soldiers. The reproductive caste performs the 
fundamental function of reproduction, distribution of the species accompanied 
by swarming or colonizing flight, choosing the site for the new colony, 
excavation of the first galleries, feeding, and care of the first young of the 
new colony. 
    The termite sterile castes consist of workers and soldiers represented by 
both sexes; but among social wasps, bees, and ponerine ants (Ponerinae), the 
workers are the only sterile caste, consisting of nonreproductive females. The 
function of the workers, which form a majority in the population, is that of 
tending the eggs, young nymphs, or larvae, feeding and cleaning other castes, 
constructing and repairing the nest, and gathering food. The soldiers are 
structurally modified to defend the community against predators.  See  
HYMENOPTERA; ISOPTERA. 
 �34s[Kumar Krishna]
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�6v�2v�32sSodalite   �21m �33s 
    A mineral of the feldspathoid group with chemical composition 
Na4Al3Si3O12Cl. Sodalite is usually massive or granular with poor cleavage. 
The Mohs hardness is 5.5 - 6.0, and the density 2.3. The luster is vitreous, 
and the color is usually blue but may also be white, gray, or green. Notable 
occurrences are at Mount Vesuvius; Bancroft, Ontario; and on the Kola 
Peninsula of the Soviet Union.  See  FELDSPATHOID; SILICATE MINERALS. 
 �34s[Lawrence Grossman]
�33s 
  
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�6v�2v�32sSoda niter �21m �33s
    A nitrate mineral having chemical composition NaNO3 (sodium nitrate). Soda
niter is by far the most abundant of the nitrate minerals. It is usually
massive granular. Its hardness is 1.5 to 2 on Mohs scale, and its specific
gravity is 2.266. It has a vitreous luster and is transparent. It is colorless
to white, but when tinted by impurities, it is reddish brown, gray, or lemon
yellow.  See  NITRATE MINERALS.
    Soda niter is a water-soluble salt found principally as a surface
efflorescence in arid regions, or in sheltered places in wetter climates. The
only large-scale commercial deposits of soda niter in the world occur in a
belt along the eastern slope of the coast ranges in the Atacama, Tarapaca, and
Antofagasta Deserts of northern Chile. Chilean nitrate had a monopoly of the
world's fertilizer market for many years, but now occupies a subordinate
position owing to the development of synthetic processes for nitrogen fixation
which permit the production of nitrogen from the air. This has led to the
commercial production of artificial nitrates and has reduced the need to
import Chilean nitrates for use in the manufacture of fertilizers and
explosives.  See  FERTILIZER; NITRATE.
 �34s[George Switzer]
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�6v�2v�32sSodium   �21m �33s 
    A chemical element, Na, atomic number 11, and atomic weight 22.9898. 
Sodium is between lithium and potassium in the periodic table. The element is 
a soft, reactive, low-melting metal with a specific gravity of 0.97 at 20DGC 
(68DGF). Sodium is commercially the most important alkali metal.  See  ALKALI 
METALS.  
    Sodium ranks sixth in abundance among all the elements in the Earth's 
crust, which contains 2.83% sodium in combined form. Sodium is, after 
chlorine, the second most abundant element in solution in sea water. The 
important sodium salts found in nature include sodium chloride (rock salt), 
sodium carbonate (soda and trona), sodium borate (borax), sodium nitrate 
(Chile saltpeter), and sodium sulfate. Sodium salts are found in sea water, 
salt lakes, alkaline lakes, and mineral springs. 
    Sodium reacts rapidly with water, and even with snow and ice, to give 
sodium hydroxide and hydrogen. When exposed to air, freshly cut sodium metal 
loses its silvery appearance and becomes dull gray because of the formation of 
a coating of sodium oxide. Sodium does not react with nitrogen, even at very 
high temperatures, but will react with ammonia to form sodium amide. Sodium 
and hydrogen react above about 200DGC (390DGF) to form sodium hydride. Sodium 
reacts with carbon with difficulty, if at all, but will react with the 
halogens. Sodium also reacts with various metal halides to give the metal plus 
sodium chloride. 
    Sodium does not react with paraffin hydrocarbons but does form addition 
compounds with naphthalene and other polycyclic aromatic compounds and with 
arylated alkenes. The reaction of sodium with alcohols is similar to, but less 
rapid than, the reaction of sodium with water. There are two general reactions 
with organic halides. One of these involves condensation of two organic, 
halogen-bearing compounds by removal of the halogen. The second type of 
reaction involves replacement of the halogen by sodium, giving an organosodium 
compound.  
 �34s[Marshall Sittig]
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�6v�2v�32sSodium-vapor lamp �21m �33s
    A low-pressure electric discharge lamp of monochromatic yellow light. The
arc tube contains a small amount of sodium and some neon gas to facilitate
starting. Sodium radiation is confined to a single band in the yellow region
of the spectrum. This results in high luminous efficiency, but extremely poor
color rendition limits low-pressure sodium lamps to tasks where color is
unimportant.
    The high-pressure sodium lamp that is used in the United States utilizes
an amalgam of sodium and mercury inside an arc tube surrounded by an evacuated
glass jacket. Operation of the arc at a higher temperature and pressure
produces a continuous radiation pattern rather than the line pattern of yellow
light associated with the earlier low-pressure sodium lamp. Its high luminous
efficiency (over 100 lumens per watt), combined with good color rendition, has
brought increasing use of the high-pressure sodium lamp in many general
lighting applications.	See  ILLUMINATION; LAMP; VAPOR LAMP.
 �34s[V.G. McClusky]
�33s

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�6v�2v�32sSofar   �21m �33s 
    The name associated with explosive signals which follow transmission paths 
through deep ocean layers. Sounds originating moderately deep in the ocean are 
refracted so that they propagate to large distances with small losses. For 
example, a small explosive charge set off at a 3000- or 4000-ft (900- or 
1200-m) depth is detectable many hundreds of miles away. If such an explosive 
signal is received at two or more points, the geographical position of the 
explosion can be calculated. This signaling technique has been used to locate 
aviators who have been forced down in the open ocean, and explains the origin 
of the word sofar, which stands for sound fixing and ranging.  See  UNDERWATER 
SOUND.  
 �34s[Robert W. Morse]
�33s 
  
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�6v�2v�32sSoft chancre   �21m �33s 
    An ulcerative venereal disease of the genitals or adjacent tissues of 
humans caused by the Ducrey bacillus (Hemophilus ducreyi). It can be 
distinguished from a syphilitic lesion by cultural methods. 
    The disease begins as a macule or pustule upon the prepuce or glans in the 
male or upon the labia minora or vestibule in the female a few hours after 
sexual intercourse. The pustule soon forms a painful, ragged, soft ulcer with 
undermined edges. The regional lymph nodes become swollen and tender, and 
lesions may become multiple by autoinoculation.  See  HEMOPHILIC BACTERIA; 
VENEREAL DISEASE. 
 �34s[William L. Bradford]
�33s 
  
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�6v�2v�32sSoft-iron ammeter   �21m �33s 
    An ammeter in which current in a coil causes two pieces of magnetic 
material within the coil, one fixed and one attached to a pointer, to become 
similarly magnetized and to repel each other, moving the pointer. These 
ammeters are widely used for alternating-current measurements. The 
illustration shows a repulsion-vane type with pointer, control spring, and 
damping vane on the moving element. 
    Requiring about 300 ampere-turn for full-scale deflection in the larger 
sizes, the common 5-ampere range will have a 60-turn coil of 0.06-in. diameter 
(1.5-mm) copper wire, ample to carry even considerable overloads. The iron 
vanes tend to saturate magnetically on overload and thereby act as further 
overload protection. Actuating forces are high, and only the limitations in 
the iron vanes in responding exactly to the coil current limit the accuracy. 
The most accurate instruments of this type are accurate to within 0.25%. See 
AMMETER.  
 �34s[John H. Miller /Jesse Beams]
�33s 
  
  
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�6v�2v�32sSoftware engineering �21m �33s
    The process of engineering software, that is, not only the programs that
run on a computer, but also the requirements definition, functional
specification, design description, program implementation, test methods, and
maintenance that lead to this code. Engineering implies the systematic
application of scientific and technological knowledge through the medium of
sound engineering principles to reach a practical goal. This goal becomes
practical through the process of trading off performance, reliability, cost,
and other characteristics of the resulting product in the light of the funds
and time available for development. In short, software engineering is based on
fundamental engineering principles and guided by economic considerations.

    Development stages.  A programmer tends to program, that is, to write a
series of instructions in a programming language, and to give less thought to
other stages of what is really, considered as a whole, a design process. On
the other hand, in terms of software engineering, the problem-solving steps
that have been learned in the other branches of engineering may be applied
also here.
    In general, these steps include formulating the problem, searching for and
developing solutions, evaluating the solutions in terms of the goals, and
refining and verifying the solution selected. In each branch of engineering
these general steps have been adapted to the needs of the branch. In software
engineering there is now considerable agreement on six stages: requirements
definition, functional specification, design, implementation, test, and
maintenance. As work proceeds through this series of stages, the findings of
later stages flow back to influence earlier stages.

    Requirements definition. In the first stage, requirements definition, the
basic question is what exactly the user expects the software to do. This
problem-oriented definition provides the guide to later development stages and
a checklist upon which the using and the development organizations can achieve
agreement on what must be done. It provides a base against which the resulting
software can eventually be compared for acceptance by the user.

    Functional specification. At the functional specification stage, the user
requirements are converted into a specification of the functions to be
performed by the proposed system. While the requirements definition may have
set values for some performance objectives that were closely related to the
problem circumstances, the functional specification carries the goal-setting
process further. Under constraints set by the current capabilities of computer
technology and the user's resources, the specification sets target numbers for
performance, reliability, compatibility, and other product objectives.

    Design. Software design is the process of going from what a system is to
do to how it is to do it. In this stage the designer transforms the functional
specification into a form from which the programmer can more easily code the
problem represented by the specification into a list of programming
instructions. While small, simple problems can sometimes be coded directly by
a programmer, experience shows that most real problems cannot be coded
directly and that an intervening design step is needed.
    Design includes the task of partitioning (or decomposing) the entire
problem area into subsystems, routines, subroutines, modules, and so forth,
which individually become easier to understand than the whole. Together these
elements may be thought of as a hierarchy, like an organization chart, or
alternatively, a flow diagram, like a strip road map. In either case, a key
task of the design process is determining the manner in which these elements
relate to one another - their interconnections. Developing the best possible
arrangement - best in terms of the constraints imposed by the product
specified and development resources - makes design an engineering activity.

    Implementation.  In its essentials the implementation stage tranforms
the output documents of the design stage into program instructions.  These
instructions are converted by a software support program called a compiler
or an interpreter into machine code, that is, the binary digits that
constitute the internal language of a computer.
    In practice, however, if the program is of more than trival size, the
probability of making errors in programming is high.  One of the most
effective ways of finding these erros is the program review, conducted
before the program is compiled.
    Errors can occur in the earlier stages, too.  In fact, studies of the
development process indicate that errors tend to be more frequent in the
early stages than in the later ones, and more expensive to correct in later
stages than in earlier ones.  Thus it is important to review the results
at each stage and to eliminate errors as early as possible.
    Testing.  After the program is complied, it is tested, that is, a set
of values is assigned to the inputs, the program is run, and the program
outputs are compared to the correct outputs.  Since the test is being
done by computer, a large number of input sets may be run.  Moreover,
the task of comparing the test output with the known correct output
can also be performed by computer.  When this match fails, the
circumstances are reported back to the programmer to find and correct
the errors.  Even at computer speeds, the test of all combinations and
paths might run up into years of test time.  So it becomes the task of
test personnel to devise test procedures that exercise the program
adequately in a reasonable time and at affordable cost.
    Maintenance.  The term maintenance, adopted from hardware terminology,
has a different meaning in software.  Software maintenance takes place in
response to finding a previously unknown error, or to a change in the
environment in which the system functions, or to an opportunity to
improve performance.  The "repair" does not return the software to its
original state, as hardware repair does.  It advances the system to a new
and sometimes poorly understood state.	One result is often the introduction
of further errors, leading to the need for further maintenance as soon as
they are discovered, and so on.  See DIGITAL COMPUTER; DIGITAL COMPUTER
PROGRAMMING.

    Software Management.  The methods of software engineering find
application in business, industry, and government.  The problems are
often very large.  The resulting projects employ many people and
sustantial resources over long time periods.  Personnel, money, and
time are the factors with which management works, so that software
development becomes subject to the methods of management:  planning,
organizing, scheduling, directing, monitoring, controlling, and so on.
But because software engineering is relatively new, managers lack working
experience in it, and software engineers have not had time to grow into
management.  Even when some of them do, the pace of ongoing technology
may outmode them.
    If management could confine itself to management and engineers to
engineering, the problems would be fewer and less severe.  In software,
however, there is reason to believe that management decisions, that is,
decisions properly within the management sphere, can influence design
matters, and vice versa.

[Ware Myers]
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�6v�2v�32sSoil   �21m �33s 
    Finely divided rock-derived material containing an admixture of organic 
matter and capable of supporting vegetation. Soils are independent natural 
bodies, each with a unique morphology resulting from a particular combination 
of climate, living plants and animals, parent rock materials, relief, the 
groundwaters, and age. Soils support plants, occupy large portions of the 
Earth's surface, and have shape, area, breadth, width, and depth. Soil, as 
used here, differs in meaning from the term as used by engineers, where the 
meaning is unconsolidated rock material.  See  PEDOLOGY. 
  
    Origin and classification.  Soil covers most of the land surface as a 
continuum. Each soil grades into the rock material below and into other soils 
at its margins, where changes occur in relief, groundwater, vegetation, kinds 
of rock, or other factors which influence the development of soils. Soils have 
horizons, or layers, more or less parallel to the surface and differing from 
those above and below in one or more properties, such as color, texture, 
structure, consistency, porosity, and reaction (see illustration). The 
succession of horizons is called the soil profile. 
    Soil formation proceeds in stages, but these stages may grade indistinctly 
from one into another. The first stage is the accumulation of unconsolidated 
rock fragments, the parent material. Parent material may be accumulated by 
deposition of rock fragments moved by glaciers, wind, gravity, or water, or it 
may accumulate more or less in place from physical and chemical weathering of 
hard rocks. The second stage is the formation of horizons. This stage may 
follow or go on simultaneously with the accumulation of parent material. Soil 
horizons are a result of dominance of one or more processes over others, 
producing a layer which differs from the layers above and below.  See  
WEATHERING PROCESSES. 
    Systems of soil classification are influenced by concepts prevalent at the 
time a system is developed. The earliest classifications were based on 
relative suitability for different crops, such as rice soils, wheat soils, and 
vineyard soils. Over the years, many systems of classification have been 
attempted but none has been found markedly superior. Two bases for 
classification have been tried. One basis has been the presumed genesis of the 
soil; climate and native vegetation were given major emphasis. The other basis 
has been the observable or measurable properties of the soil. 
    The Soil Survey staff of the U.S. Department of Agriculture and the 
land-grant colleges adopted the current classification scheme in 1965. This 
system differs from earlier systems in that it may be applied to either 
cultivated or virgin soils. Previous systems have been based on virgin 
profiles, and cultivated soils were classified on the presumed characteristics 
or genesis of the virgin soils. The new system has six categories, based on 
both physical and chemical properties. These categories are the order, 
suborder, great group, subgroup, family, and series, in decreasing rank. The 
orders and the general nature of the included soils are given in the table. 
The suborder narrows the ranges in soil moisture and temperature regimes, 
kinds of horizons, and composition, according to which of these is most 
important. The taxa (classes) in the great group category group soils that 
have the same kinds of horizons in the same sequence and have similar moisture 
and temperature regimes. The great groups are subdivided into subgroups that 
show the central properties of the great group, intergrade subgroups that show 
properties of more than one great group, and other subgroups for soils with 
atypical properties that are not characteristic of any great group. The 
families are defined largely on the basis of physical and mineralogic 
properties of importance to plant growth. The soil series is a group of soils 
having horizons similar in differentiating characteristics and arrangement in 
the soil profile, except for texture of the surface portion, and developed in 
a particular type of parent material. 
  
    Surveys.  Soil surveys include those researches necessary (1) to determine 
the important characteristics of soils, (2) to classify them into defined 
series and other units, (3) to establish and map the boundaries between kinds 
of soil, and (4) to correlate and predict adaptability of soils to various 
crops, grasses, and trees; behavior and productivity of soils under different 
management systems; and yields of adapted crops on soils under defined sets of 
management practices. Although the primary purpose of soil surveys has been to 
aid in agricultural interpretations, many other purposes have become 
important, ranging from suburban planning, rural zoning, and highway location, 
to tax assessment and location of pipelines and radio transmitters. This has 
happened because the soil properties important to the growth of plants are 
also important to its engineering uses. 
    Two kinds of soil maps are made. The common map is a detailed soil map, on 
which soil boundaries are plotted from direct observations throughout the 
surveyed area. Reconnaissance soil maps are made by plotting soil boundaries 
from observations made at intervals. The maps show soil and other differences 
that are of significance for present or foreseeable uses. 
 �34s[Guy D. Smith]
�33s 
  
    Physical properties.  Physical properties of soil have critical importance 
to growth of plants and to the stability of cultural structures such as roads 
and buildings. Such properties commonly are considered to be: size and size 
distribution of primary particles and of secondary particles, or aggregates, 
and the consequent size, distribution, quantity, and continuity of pores; the 
relative stability of the soil matrix against disruptive forces, both natural 
and cultural; color and textural properties, which affect absorption and 
radiation of energy; and the conductivity of the soil for water, gases, and 
heat. These usually would be considered as fixed properties of the soil 
matrix, but actually some are not fixed because of influence of water content. 
The additional property, water content - and its inverse, gas content - 
ordinarily is transient and is not thought of as a property in the same way as 
the others. However, water is an important constitutent, despite its transient 
nature, and the degree to which it occupies the pore space generally dominates 
the dynamic properties of soil. Additionally, the properties listed above 
suggest a macroscopic homogeneity for soil which it may not necessarily have. 
In a broad sense, a soil may consist of layers or horizons of roughly 
homogeneous soil materials of various types that impart dynamic properties 
which are highly dependent upon the nature of the layering. Thus, a discussion 
of dynamic soil properties must include a description of the intrinsic 
properties of small increments as well as properties it imparts to the system. 
    From a physical point of view it is primarily the dynamic properties of 
soil which affect plant growth and the strength of soil beneath roads and 
buildings. While these depend upon the chemical and mineralogical properties 
of particles, particle coatings, and other factors discussed above, water 
content usually is the dominant factor. Water content depends upon flow and 
retention properties, so that the relationship between water content and 
retentive forces associated with the matrix becomes a key physical property of 
a soil.  See  EROSION; GROUNDWATER; SOIL MECHANICS. 
 �34s[Walter H. Gardner]
�33s 
  
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�6v�2v�32sSoil chemistry   �21m �33s 
    The study of the composition and chemical properties of soil. Soil 
chemistry involves the detailed investigation of the nature of the solid 
matter from which soil is constituted and of the chemical processes that occur 
as a result of the action of hydrological, geological, and biological agents 
on the solid matter. Because of the broad diversity among soil components and 
the complexity of soil chemical processes, the application of a wide variety 
of concepts and methods employed in the chemistry of aqueous solutions, of 
amorphous and crystalline solids, and of solid surfaces is required. 
  
    Elemental composition.   The elemental composition of soil varies over a 
wide range, permitting only a few general statements to be made. Those soils 
that contain less than 12 - 20% organic carbon are termed mineral. All other 
soils are termed organic. Carbon, oxygen, hydrogen, nitrogen, phosphorus, and 
sulfur are the most important constituents of organic soils and of soil 
organic matter in general. Carbon, oxygen, and hydrogen are most abundant; the 
content of nitrogen is often about one-tenth that of carbon, while the content 
of phosphorus or sulfur is usually less than one-fifth that of nitrogen (Table 
1).  
    Besides oxygen, the most abundant elements found in mineral soils are 
silicon, aluminum, and iron. The distribution of chemical elements will vary 
considerably from soil to soil and, in general, will be different in a 
specific soil from the distribution of elements in the crustal rocks of the 
Earth. The most important micro or trace elements in soil are boron, copper, 
manganese, molybdenum, and zinc, since these elements are essential in the 
nutrition of green plants. Also important are cobalt, selenium, cadmium, and 
nickel. The average distribution of trace elements in soil is not greatly 
different from that in crustal rocks (Table 2). 
    The elemental composition of soil varies with depth below the surface 
because of pedochemical weathering. The principal processes of this type that 
result in the removal of chemical elements from a given soil horizon are: (1) 
soluviation (ordinary dissolution in water), (2) cheluviation (complexation by 
organic or inorganic ligands), (3) reduction, and (4) suspension. The 
principal effect of these four processes is the appearance of illuvial 
horizons in which compounds such as aluminum and iron oxides, 
aluminosilicates, or calcium carbonate have been precipitated from solution or 
deposited from suspension.  See  WEATHERING PROCESSES. 
    Minerals.  The minerals in soils are the products of physical, 
geochemical, and pedochemical weathering. Soil minerals may be either 
amorphous or crystalline. They may be classified further, approximately, as 
primary or secondary minerals, depending on whether they are inherited from 
parent rock or are produced by chemical weathering, respectively. 
    The bulk of the primary minerals that occur in soil are found in the 
silicate minerals. Chemical weathering of the silicate minerals is responsible 
for producing the most important secondary minerals in soil. These are found 
in the clay fraction and include aluminum and iron hydrous oxides (usually in 
the form of coatings on other minerals), carbonates, and aluminosilicates.  
See  CLAY MINERALS; SILICATE MINERALS. 
  
    Ion exchange.   A portion of the chemical elements in soil is in the form 
of cations that are not components of inorganic salts but that can be replaced 
reversibly by the cations of leaching salt solutions or acids. These cations 
are said to be exchangeable, and their total quantity is termed the cation 
exchange capacity (CEC) of the soil. The CEC of a soil generally will vary 
directly with the amounts of clay and organic matter present and with the 
distribution of clay minerals. 
    The stoichiometric exchange of the anions in soil for those in a leaching 
salt solution is a phenomenon of relatively small importance in the general 
scheme of anion reactions with soils. Under acid conditions (pH < 5) the 
exposed hydroxyl groups at the edges of the structural sheets or on the 
surfaces of clay-sized particles become protonated and thereby acquire a 
positive charge. The degree of protonation is a sensitive function of pH, the 
ionic strength of the leaching solution, and the nature of the clay-sized 
particle.  
  
    Soil solution.   The solution in the pore space of soil acquires its 
chemical properties through time-varying inputs and outputs of matter and 
energy that are mediated by the several parts of the hydrologic cycle and by 
processes originating in the biosphere (see illustration). The soil solution 
thus is a dynamic and open natural water system whose composition reflects the 
many reactions that can occur simultaneously between an aqueous solution and 
an assembly of mineral and organic solid phases that varies with both time and 
space.  See  SOIL. 
 �34s[Garrison Sposito]
�33s 
  
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�6v�2v�32sSoil conservation �21m �33s
    The practice of arresting and minimizing artificially accelerated soil
deterioration. Its importance has grown because cultivation of soils for
agricultural production, deforestation and forest cutting, grazing of natural
range, and other disturbances of the natural cover and position of the soil
have increased greatly in the last 100 years.

    Erosion and deterioration.	 The exact extent of accelerated soil erosion
in the world today is not known, particularly as far as the rate of soil
movement is concerned. However, it may be said that nearly every semiarid area
with cultivation or long-continued grazing, every hill land with moderate to
dense settlement in humid temperate and subtropical climates, and all
cultivated or grazed hill lands in the Mediterranean climate areas suffer to
some degree from such erosion. Recognized problems of erosion are found in
such culturally diverse areas as southern China, the Indian plateau, south
Australia, the South African native reserves, the Soviet Union, Spain, the
southeastern and midwestern United States, and Central America.
    Within the United States the most critical areas have been the hill lands
of the Piedmont and the interior Southeast, the Great Plains, the Palouse area
hills of the Pacific Northwest, southern California hills, and slope lands of
the Midwest. The high-intensity rainstorms of the Southeast, and the cyclical
droughts of the Plains have predisposed the two larger areas to erosion.  See
EROSION.
    Soil may deteriorate either by physical movement of soil particles from a
given site or by depletion of the water-soluble elements in the soil which
contribute to the nourishment of crop plants, grasses, trees, and other
economically usable vegetation. The physical movement generally is referred to
as erosion. Wind, water, glacial ice, animals, and tools used by humans may be
agents of erosion. For purposes of soil conservation, the two most important
agents of erosion are wind and water, especially as their effects are
intensified by the disturbance of natural cover or soil position.
    Depletion of soil nutrients obviously is a part of soil erosion. However,
such depletion may take place in the absence of any noticeable amount of
erosion. The disappearance of naturally stored nitrogen, potash, phosphate,
and some trace elements from the soil also affects the usability of the soil
for human purposes. The natural fertility of virgin soils always is depleted
over time as cultivation continues, but the rate of depletion is highly
dependent on management practices.  See  PLANT MINERAL NUTRITION; SOIL.
    Accelerated erosion may be induced by any land use practice which denudes
the soil surfaces of vegetative cover. For example, cultivation of any row
crop on a slope without soil-conserving practices is an invitation to
accelerated erosion. Cultivation of other crops, like the small grains, also
may induce accelerated erosion, especially where fields are kept bare between
crops to store moisture. Forest cutting, overgrazing, grading for highway use,
urban land use, or preparation for other large-scale engineering works also
may speed the natural erosion of soil.

    Causes of soil mismanagement.   One of the chief causes of
erosion-inducing agricultural practices in the United States has been
ignorance of their consequences. The cultivation methods of the settlers of
western European stock who set the pattern of land use in this country came
from a physical environment which was far less susceptible to erosion than
North America, because of the mild nature of rainstorms and the prevailing
soil textures in Europe. Corn, cotton, and tobacco, moreover, were crops
unfamiliar to European agriculture. In eastern North America the combination
of European cultivation methods and American intertilled crops resulted in
generations of soil mismanagement.
    On the Plains and in other susceptible western areas, small grain
monoculture, particularly of wheat, encouraged the exposure of the uncovered
soil surface so much of the time that water and wind inevitably took their
toll. On rangelands, lack of knowledge as to the precipitation cycle and range
capacity, and the urge to maximize profits every year contributed to a slower,
but equally sure denudation of cover.
    Finally, the United States has experienced extensive erosion in mountain
areas because of forest mismanagement. Clearcutting of steep slopes, forest
burning for grazing purposes, inadequate fire protection, and shifting
cultivation of forest lands have allowed vast quantities of soil to wash out
of the slope sites where they could have produced timber and other forest
values indefinitely.  See  FOREST MANAGEMENT AND ORGANIZATION.

    Effects on other resources.   Accelerated erosion may have consequences
which reach far beyond the lands on which the erosion takes place and the
community associated with them. During periods of heavy wind erosion, for
example, the dust fall may be of economic importance over a wide area beyond
that from which the soil cover has been removed. The most pervasive and
widespread effects, however, are those associated with water erosion. Removal
of upstream cover changes the regimen of streams below the eroding area.
    A long chain of other effects also ensues. Because of the extremes of low
water in denuded areas during dry seasons, water transportation is made
difficult or impossible without regulation, fish and wildlife support is
endangered or disappears, the capacity of streams to carry sewage and other
wastes safely may be seriously reduced, recreational values are destroyed, and
run-of-the-river hydroelectric generation reaches a very low level.  See
RANGELAND CONSERVATION; WATER CONSERVATION.
 �34s[Edward A. Ackerman ;Donald J. Patten]
�33s

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�6v�2v�32sSoil mechanics �21m �33s
    The study of the response of masses composed of soil, water, and air to
imposed loads. Because both water and air are able to move through the soil
pores, the discipline also involves the prediction of these transport
processes. Soil mechanics provides the analytical tools required for
foundation engineering, retaining wall design, highway and railway subbase
design, tunneling, earth dam design, mine excavation design, and so on.
Because the discipline relates to rock as well as soils, it is also known as
geotechnical engineering.  See	ENGINEERING GEOLOGY.
    Soil consists of a multiphase aggregation of solid particles, water, and
air. This fundamental composition gives rise to unique engineering properties,
and the description of the mechanical behavior of soils requires some of the
most sophisticated principles of engineering mechanics. The terms multiphase
and aggregation both imply unique properties. As a multiphase material, soil
exhibits mechanical properties that show the combined attributes of solids,
liquids, and gases. Individual soil particles behave as solids, and show
relatively little deformation when subjected to either normal or shearing
stresses. Water behaves as a liquid, exhibiting little deformation under
normal stresses, but deforming greatly when subjected to shear. Being a
viscous liquid, however, water exhibits a shear strain rate that is
proportional to the shearing stress. Air in the soil behaves as a gas, showing
appreciable deformation under both normal and shear stresses. When the three
phases are combined to form a soil mass, characteristics that are an outgrowth
of the interaction of the phases are manifest. Moreover, the particulate
nature of the solid particles contributes other unique attributes.  See
VISCOSITY.
    When dry soil is subjected to a compressive normal stress, the volume
decreases nonlinearly; that is, the more the soil is compressed, the less
compressible the mass becomes. Thus, the more tightly packed the particulate
mass becomes, the more it resists compression. The process, however, is only
partially reversible, and when the compressive stress is removed the soil does
not expand back to its initial state.
    When this dry particulate mass is subjected to shear stress, an especially
interesting behavior owing to the particulate nature of the soil solids
results. If the soil is initially dense (tightly packed), the mass will expand
because the particles must roll up and over each other in order for shear
deformation to occur. Conversely, if the mass is initially loose, it will
compress when subjected to a shear stress. Clearly, there must also exist a
specific initial density (the critical density) at which the material will
display zero volume change when subjected to shear stress. The term dilatancy
has been applied to the relationship between shear stress and volume change in
particulate materials. Soil is capable of resisting shear stress up to a
certain maximum value. Beyond this value, however, the material undergoes
large, uncontrolled shear deformation.	See  SHEAR.
    The other limiting case is saturated soil, that is, a soil whose voids are
entirely filled with water. When such a mass is initially loose and is
subjected to compressive normal stress, it tends to decrease in volume;
however, in order for this volume decrease to occur, water must be squeezed
from the soil pores. Because water exhibits a viscous resistance to flow in
the microscopic pores of fine-grained soils, this process can require
considerable time, during which the pore water is under increased pressure.
This excess pore pressure is at a minimum near the drainage face of the soil
mass and at a maximum near the center of the soil sample. It is this gradient
(or change in pore water pressure with change in position within the soil
mass) that causes the outflow of water and the corresponding decrease in
volume of the soil mass.
    Conversely, if an initially dense soil mass is subjected to shear stress,
it tends to expand. The expansion, however, may be time-dependent because of
the viscous resistance to water being drawn into the soil pores. During this
time the pore water will be under decreased pressure. Thus, in saturated soil
masses, changes in pore water pressure and time-dependent volume change can be
induced by either changes in normal stress or by changes in shear stress.  See
ROCK MECHANICS; SOIL.
 �34s[Charles A. Moore]
�33s

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�6v�2v�32sSoil microbiology   �21m �33s 
    A study of the microorganisms in soil, their functions, and the effect of 
their activities on the character of soil and the growth and health of plant 
life, particularly cultivated crops. It embraces the biology of microorganisms 
- their morphology, physiology, and taxonomy - as well as their biochemistry. 
It is related to soil chemistry and physics and, in its application, to 
agronomy, plant physiology, and plant pathology. 
    The soil microorganisms are viruses, Myxomycetes (slime fungi), protozoa, 
algae, yeasts, fungi, actinomycetes, and bacteria. Soil is distinguished from 
subsoil chiefly by the presence of organic matter in the soil. The organic 
matter is composed of dead plant and animal tissues and the products of their 
decomposition by microorganisms. The microorganisms derive their energy by 
oxidizing plant and animal residues. Plants depend upon nutrients made 
available by microorganisms which form an essential link in the cycle of food 
in nature. 
    The soil is the greatest natural reservoir of microorganisms. These take 
part in many reactions in the soil. Some microorganisms are specific, taking 
part only in certain reactions (Nitrosomonas, which oxidize ammonia to 
nitrite). Some are more general in their metabolism (heterotrophic types, 
which depend on organic material for energy source) and take part in many 
reactions. One group of organisms may use the products of another, for 
example, during the course of decomposition of complex nitrogenous material to 
nitrate. In addition to forming simpler degradation products, microorganisms 
synthesize many complex substances. These are bound up in microbial 
protoplasm, while excess amounts may be liberated to provide essential 
substances for other groups of microorganisms. In contrast to this associative 
action are antagonisms caused by competition for the same food or by the 
ability of many organisms to produce antibiotics. 
    Microorganisms are not uniformly distributed throughout the soil, because 
soil is not homogeneous but comprises a variety of microenvironments. The 
organisms congregate in colloidal films about the surface of soil particles 
and are particularly abundant around fragments of decaying plant and animal 
debris.  
    The majority of soil microorganisms are most active at pH 6.0 - 6.8. Some 
sulfur-oxidizing bacteria tolerate a pH as low as 2.0 and other organisms a pH 
as high as 10. A supply of free oxygen is important to most organisms. With 
abundant oxygen, decomposition proceeds rapidly to completion. However, in 
close-textured and wet soils where anaerobic conditions (molecular oxygen not 
available) prevail, organic matter is decomposed slowly and incompletely. The 
number of microorganisms is highest in spring and fall. Proximity to growing 
plants markedly increases the number of soil organisms. 
 �34s[Allan G. Lochhead]
�33s 
  
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�6v�2v�32sSolar cell �21m �33s
    A semiconductor electrical junction device which absorbs and converts the
radiant energy of sunlight directly and efficiently into electrical energy.
Solar cells may be used individually as light detectors, for example in
cameras, or connected in series and parallel to obtain the required values of
current and voltage for electric power generation.
    Most solar cells are made from single-crystal silicon and have been very
expensive for generating electricity, but have found application in space
satellites and remote areas where low-cost conventional power sources have
been unavailable.
    The conversion of sunlight into electrical energy in a solar cell involves
three major processes: absorption of the sunlight in the semiconductor
material; generation and separation of free positive and negative charges to
different regions of the solar cell, creating a voltage in the solar cell; and
transfer of these separated charges through electrical terminals to the
outside application in the form of electric current.
    When light is absorbed in the semiconductor, a negatively charged electron
and positively charged hole are created. The heart of the solar cell is the
electrical junction which separates these electrons and holes from one another
after they are created by the light. An electrical junction may be formed by
the contact of: a metal to a semiconductor (this junction is called a Schottky
barrier); a liquid to a semiconductor to form a photoelectrochemical cell; or
two semiconductor regions (called a pn junction).
    The fundamental principles of the electrical junction can be illustrated
with the silicon pn junction. Pure silicon to which a trace amount of a
fifth-column element such as phosphorus has been added is an n-type
semiconductor, where electric current is carried by free electrons. Each
phosphorus atom contributes one free electron, leaving behind the phosphorus
atom bound to the crystal structure with a unit positive charge. Similarly,
pure silicon to which a trace amount of a column-three element such as boron
has been added is a p-type semiconductor, where the electric current is
carried by free holes. The interface between the p- and n-type silicon is
called the pn junction. The fixed charges at the interface due to the bound
boron and phosphorus atoms create a permanent dipole charge layer with a high
electric field. When photons of light energy from the Sun produce
electron-hole pairs near the junction, the build-in electric field forces the
holes to the p side and the electrons to the n side. This displacement of free
charges results in a voltage difference between the two regions of the
crystal. When a load is connected at the terminals, an electron current flows
and useful electrical power is available at the load.  See  SEMICONDUCTOR;
SOLAR-ELECTRIC POWER GENERATION; SOLAR ENERGY.
 �34s[Donald G. Schueler]
�33s

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�6v�2v�32sSolar constant   �21m �33s 
    The rate at which energy is received from the Sun just outside Earth's 
atmosphere. At Earth's mean distance from the Sun, the solar constant is 1.36 
x 10sup3 watts per square meter. Depending on the Sun's distance from the 
zenith, up to a third of this energy may be scattered in Earth's atmosphere. 
From the measured solar constant, the total radiation of the Sun is 3.86 x 
10sup2sup6 watts.  See  SUN. 
 �34s[John W. Evans]
�33s 
  
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�6v�2v�32sSolar corona �21m �33s
    The outermost extension of the atmosphere of the Sun which is normally
visible only at a total solar eclipse. The corona consists of gas at a density
of about 10(sup - 15)g/cm(sup 3) (or 10(sup -12) atmospheric density) and a
normal kinetic temperature between 1 and 2 x 10(sup 6) K (between 2 and 4 x
10(sup 6) DGF).  It is easily observable at an eclipse to a height of 2 - 3
solar diameters.  The outer envelope often consists of beautiful radial
streamers composed of electrons and protons ejected by the Sun.  See SUN. 
 �34s[John W.  Evans]
�33s

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�6v�2v�32sSolar-electric power generation   �21m �33s 
    For solar-electric power plants, the annual sunshine hours should be as 
large as possible, and the humidity, which causes absorption and scattering, 
should be low. Four methods can be used to generate solar-electric power: the 
solar-thermal distributed receiver system; the solar-thermal central receiver 
system; the photovoltaic system; and photoelectrochemical conversion. In each 
case, the overall system may serve as backup, that is, operating only when the 
sun shines, equipped only with a minor energy storage capacity (for example, 
for 1 h full output) to bridge temporary cloud coverage; alternatively, the 
overall system may include a conventional fuel system to replace solar energy 
at night or during cloudy days; and finally, the independent solar-electric 
system includes a storage system to ensure continuous power-generating 
capacity based on solar energy only. 
    In the solar-thermal distributed system, solar radiation is absorbed over 
a large area covered with flat-plate (nonconcentrating) collectors or 
parabolic-trough concentrators, which focus the sunlight on a heat pipe 
carrying the working fluid (Fig. 1). The flat-plate collector operates at 
turbine inlet temperatures of 250 - 500DGF (121 - 260DGC). With the parabolic 
trough, temperatures between 550 - 1000DGF (288 - 538DGC) at turbine inlet are 
attainable. The higher the temperature, the higher the efficiency and the 
smaller the land area needed for a given power level, but the more expensive 
is the system. 
    In the solar-thermal central receiver system, sunlight is concentrated on 
a receiver by a large number of mirrors designed to follow the Sun 
(heliostats). The receiver is a heater located atop a tower (Fig. 2) which is 
served by a certain collector area. A given solar power plant may consist of 
an arbitrary number of these modules. 
    In the photovoltaic systems, electricity is produced by means of solar 
cells. The photovoltaic system offers the advantage that its performance and, 
to a large extent, its economy are independent of size over a wide range.  See 
  SOLAR CELL. 
    Energy generation and storage by photochemical conversion appears to have 
a great deal of potential. By immersing semiconductors in liquid electrolytes, 
solar energy can be changed by the same converter either to electricity or to 
chemical energy by generating chemical fuels, especially hydrogen. In a 
photoelectrochemical converter (Fig. 3), light replaces a conventional power 
source in driving the electrolytical decomposition of water. 
 �34s[Krafft A. Ehricke]
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�6v�2v�32sSolar energy �21m �33s
    The energy transmitted from the Sun. This energy is in the form of
electromagnetic radiation. The Earth receives about one-half of one-billionth
of the total solar energy output. In 1971, based on radiation measurements in
space, the National Aeronautics and Space Administration proposed a new space
solar constant of 1353 W/m(sup 2), and a standard spectral irradiance in W/m(sup
2) over a small range of wavelengths (bandwidth) centered at the wavelength (in
micrometers) shown in the illustration.  Accordingly, the solar radiation energy
in the ultraviolet is 105.8 W/m(sup 2) (7.82% of the solar constant), in the
visible 640.4 W/m(sup 2) (47.33%), and in the infrared 606.8 W/m(sup 2)
(44.85%).  The solar radiation energy output is essentially constant.  However,
because of the ellipticity of the Earth's orbit, the solar constant varies
between 1398 W/m(sup 2) about January 3 and 1308 W/m(sup 2) about July 4, or 3%
about the mean value.  Based on its cross-sectional area, the rotating Earth
receives therefore 751 x 10(sup 15) kWh annually.  Actually, passage through the
atmosphere splits the radiation reaching the surface in a direct and a diffuse
component, and reduces the total energy through selective absorption by dry air
molecules, dust, water molecules, and thin cloud layers, while heavy cloud
coverage eliminates all but the diffuse radiation.  On a global basis, about 50%
of the total annual incident radiation is reflected back into space by clouds,
15% by the surface, and about 5.3% is absorbed by bare soil.  Of the remaining
29.7%, only about 1.7% is absorbed by marine vegetation and 0.2% by land
vegetation.  By far the largest portion is used to evaporate water and lift it
into the atmosphere.  The evaporation energy is radiated into space by vapor
condensation to clouds.  The solar energy spent to lift the water can be partly
recovered in the form of waterpower (hydraulic energy).  Solar energy can be
utilized in the form of heat, organic chemical energy through photosynthesis,
and wind power, and also in the form of photovoltaic power (generating
electricity by means of solar cells).  The two greatest problems in utilizing
solar energy are its low concentration and its irregular availability due to the
diurnal cycle and to seasonal and climatic variations.	See SOLAR-ELECTRIC POWER
GENERATION; SOLAR HEATING AND COOLING.	Since solar energy is more time-variant
than its use, it is necessary to supply either nonsolar makeup energy or to
store solar energy.  Solar energy storage systems may be divided into two
groups:  controlled energy storage and environmental energy storage.  Controlled
energy storage systems store the energy in as concentrated form as possible in a
variety of forms - as thermal, chemical, mechanical, or potential
(gravitational) energy; environmental energy storage systems utilize the thermal
energy stored by solar irradiation in the ground, air, and water.  In contrast
to thermal controlled energy storage, environmental energy storage generally is
thermal energy storage at temperatures below the user level.  See ENERGY
STORAGE.�34s[Krafft A.  Ehricke]
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�6v�2v�32sSolar heating and cooling �21m �33s
    The use of solar energy to produce heating or cooling for technological
purposes. Beneficial uses include distillation of sea water to produce salt or
potable water; heating of swimming pools; space heating; heating of water for
domestic, commercial, and industrial purposes; cooling by absorption or
compression refrigeration; and cooking.  See  SOLAR ENERGY.

    Distillation.   Production of potable water from sea water by solar
distillation is accomplished in several parts of the world by use of
glass-roofed solar stills (see illustration). Production of salt from the sea
has been accomplished for hundreds of years by trapping ocean water in shallow
ponds at high tide and simply allowing the water to evaporate under the
influence of the Sun.  See  DISTILLATION.

    Swimming pool heating.  Swimming pool heating is a moderate-temperature
application which, under suitable weather conditions, can be accomplished with
a simple unglazed and uninsulated collector. For applications where a
significant temperature difference exists between the fluid within the
collector passages and the ambient air, both glazing and insulation are
essential.

    Space heating.   Space heating can be carried out by active systems which
use separate collection, distribution, and storage subsystems, or by passive
designs which use components of a building to admit, store, and distribute the
heat resulting from absorbing the incoming solar radiation within the building
itself.
    Passive systems can be classified as direct-gain when they admit solar
radiant energy directly into the structure through large south-facing windows,
or as indirect-gain when a wall or a roof absorbs the solar radiation, stores
the resulting heat, and then transfers it into the building. Passive systems
are generally effective where the number of hours of sunshine during the
winter months is relatively high, where moderate indoor temperature
fluctuations can be tolerated, and where the need for summer cooling and
dehumidification is moderate or nonexistent.
    Active systems may use either water or air to transport heat from
roof-mounted south-facing collectors to storage in rock beds or water tanks.
The stored heat may be withdrawn and used directly when air is the transfer
fluid. When the heat is collected and stored as hot water, fan-coil units are
generally used to transfer the heat to air which is then circulated through
the warmed space. Standby energy sources are included in designs for active
systems, since some method of providing warmth must be included for use when
the Sun's radiant energy is inadequate for long periods of time. The standby
heater may be something as simple as a wood-burning stove or fireplace, or as
complex as an electrically powered heat pump.  See  COMFORT HEATING; HEAT PUMP.

    Service water heating.   Solar water heating for domestic, commercial, or
industrial purposes is an old and successful application of solar-thermal
technology. The most widely used water heater, and one that is suitable for
use in relatively warm climates where freezing is a minor problem, is the
thermosiphon type. A flat-plate collector is generally used with a storage
tank which is mounted above the collector. A source of water is connected near
the bottom of the tank, and the hot water outlet is connected to its top. A
downcomer pipe leads from the bottom of the tank to the inlet of the
collector, and an insulated return line runs from the top of the collector to
the upper part of the storage tank which is also insulated.
    The system is filled with water, and when the Sun shines on the collector,
the water in the tubes is heated. It then becomes less dense than the water in
the downcomer, and the heated water rises by thermosiphon action into the
storage tank. It is replaced by cool water from the bottom of the tank, and
this action continues as long as the Sun shines on the collector with adequate
intensity.
    For applications where the elevated storage tank is undesirable or where
very large quantities of hot water are needed, the tank is placed at ground
level. A small pump circulates the water in response to a signal from a
controller which senses the temperatures of the collector and the water near
the bottom of the tank. Heat exchangers may also be used with water at
operating pressure within the tubes of the exchanger and the collector water
outside to eliminate the necessity of using high-pressure collectors.  See
HOT-WATER HEATING SYSTEM.

    Cooling.  Cooling can be provided by both active and passive systems.
    The two feasible types of active cooling systems are Rankine cycle and
absorption. The Rankine cycle system uses solar collectors to produce a vapor
(steam or one of the fluorocarbons generally known as Freon) to drive an
engine or turbine. A condenser must be used to condense the spent vapor so it
can be pumped back through the vaporizer. The engine or turbine drives a
conventional refrigeration compressor which produces cooling in the usual
manner.  See  RANKINE CYCLE; REFRIGERATION.
    Passive cooling systems make use of three natural processes: convection
cooling with night air; radiative cooling by heat rejection to the sky on
clear nights; and evaporative cooling from water surfaces exposed to the
atmosphere. The effectiveness of each of these processes depends upon local
climatic conditions.  See  ENERGY STORAGE; SOLAR CELL.
 �34s[John I. Yellott]
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�6v�2v�32sSolar magnetic field   �21m �33s 
    The magnetic field that pervades the ionized and highly conducting gas 
composing the Sun. The field is observable by optical means at the Sun's 
surface; subsurface fields are inferred from the laws of magnetohydrodynamics 
and from the behavior of sunspots; the effects of magnetic fields external to 
the Sun are seen in solar prominences, flares, and coronal streamers, and in 
relation to the solar wind, an outflow of tenuous plasma into interplanetary 
space.  See  SOLAR WIND; SUN. 
    Direct measurement of the strength and polarity of the complex patterns of 
magnetic fields at the Sun's surface is accomplished by means of an instrument 
known as the solar magnetograph that utilizes the Zeeman effect.  See  ZEEMAN 
EFFECT.  
 �34s[Horace W. Babcock]
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�6v�2v�32sSolar neutrinos   �21m �33s 
    Neutrinos produced in nuclear reactions inside the Sun. The first direct 
test of how the Sun produces its luminosity (observed most conspicuously on 
Earth as sunlight) was carried out by observing these particles. The results 
of this experiment were in diagreement with theoretical predictions based upon 
the supposedly well-established theory of stellar evolution. This discrepancy 
between theory and observation suggests that either the process by which the 
Sun shines is not understood as well as previously believed, or some 
modification in the classical physical theory of neutrinos is required.  See  
NEUTRINO; NUCLEAR REACTION; PROTON-PROTON CHAIN. 
 �34s[John N. Bahcall]
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�6v�2v�32sSolar radiation   �21m �33s 
    The electromagnetic radiation and particles (electrons, protons, and rarer 
heavy atomic nuclei) emitted by the Sun. Electromagnetic energy has been 
observed over the whole spectrum with wavelengths varying from 0.01 nanometer 
to 30 kilometers. The bulk of the energy is in the spectrum of visible light 
(400 - 800 nm). The solar spectrum doubtless extends far beyond the observed 
limits in both directions.  See  SOLAR ENERGY. 
    The Sun also emits a continuous stream of electrons with shorter bursts of 
electron and proton showers sufficiently intense to affect the ionization of 
the upper terrestrial atmosphere.  See  IONOSPHERE; SOLAR WIND; SUN. 
 �34s[John W. Evans]
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�6v�2v�32sSolar system �21m �33s
    The Sun and the bodies moving about it. There are more than 10(sup 8) stars
in the Galaxy, but only in the case of the solar system is a star with an
accompanying planetary system known. The other stars are all so far away that
there has been no means of detecting any possible accompanying planets. The
Sun is about 3 x 10(sup 7) light-years (3 x 10(sup 20 km) from the center of the
Galaxy, part of the flattened distribution of stars forming the disk of the
Galaxy. The Galaxy also has a substantial spheroidal content of stars which
are believed to have been the first to form when the Galaxy was formed. The
disk stars, such as the Sun, are believed to have formed later, and many
examples of the process of star formation are observed continuing within the
galactic disk today. By observing how such star formation occurs, it may be
possible to gain clues as to the way in which the Sun, and possibly its
accompanying planetary system, was formed.  See  GALAXY; STELLAR EVOLUTION;
SUN.
    The planetary system contains an inner group of four planets, Mercury,
Venus, Earth, and Mars, which are primarily composed of rocks and iron. A
large number of very small bodies, typically only a few kilometers in radius
or less, called asteroids, occur in orbits beyond that of Mars. These too are
primarily rocky in composition.  See  ASTEROID; EARTH; MARS; MERCURY (PLANET);
VENUS.
    Beyond the asteroids lies the largest planet in the solar system, Jupiter.
Jupiter is primarily composed of the light gases hydrogen and helium. Saturn,
the second most massive planet in the solar system, lying beyond Jupiter, is
basically similar in composition and structure.  See  JUPITER; SATURN.
    Beyond Saturn lie two intermediate-mass planets, Uranus and Neptune. While
these planets contain substantial amounts of hydrogen and helium, the majority
of their mass appears to be composed of heavier elements, probably some
combination of rocky materials plus ``ices,'' water, ammonia, and methane.
See  NEPTUNE; URANUS.
    Still farther out is a very small planet, Pluto, considerably less massive
than the Earth and apparently composed primarily of rocks and frozen ices.
See  PLANET; PLUTO.
    Well beyond the region of the planets lies a huge region of space, out to
about 100,000 astronomical units (1.5 x 10sup3 km) from the Sun, which appears
to contain a very large number of small, frozen bodies called comets. The
space within the solar system is also filled with much smaller bodies. Some of
these weigh many kilograms. These are the meteoroids, which become bright
meteors or fireballs after entering the Earth's atmosphere, and are called
meteorites when they are found on the ground.  See  COMET; METEOR; METEORITE.
    There is a long history of speculation about the origin of the solar
system. These theories can be classed as monistic, in which usually the Sun
and planets are thought to have formed as an isolated system, and dualistic,
in which the planets are generally thought to have been formed as a result of
an interaction between two stars. It is now generally believed that some form
of monistic theory is required to understand the origin of the solar system.
 �34s[A. G. W. Cameron]
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�6v�2v�32sSolar wind   �21m �33s 
    The continuous outward flow of ionized solar gas and a ``frozen-in'' 
remnant of the solar magnetic field through the solar system. This flow arises 
from strong outward pressure in the solar corona, becomes supersonic at a few 
solar radii (1 solar radius = 6.96 x 10sup5 km or 4.32 x 10sup5 mi) above the 
visible surface of the Sun (the photosphere), and attains speeds in the range 
250 - 750 km/s (155 - 465 mi/s) in interplanetary space. The solar wind is 
believed to remain supersonic out to a distance from the Sun of 50 - 100 
astronomical units (AU), where it is slowed by interaction with the 
interstellar gas and magnetic field.  See  INTERSTELLAR MATTER; SOLAR MAGNETIC 
FIELD.  
    In 1962, sampling instruments on the Mariner 2 spacecraft revealed an 
important pattern in the variations of solar wind speed with time. The 
observed speed rose systematically from low values to high values in 1 or 2 
days and then returned to low values during the next 3 to 5 days. Each of 
these high-speed streams tended to be seen at approximately 27-day intervals 
or to recur with the rotation period of the Sun as viewed from the spacecraft. 
A similar recurrence tendency had been noted in geomagnetic activity, and was 
widely interpreted as the effect of localized, long-lived streams of particles 
emitted from the Sun and swept past the Earth once during each solar rotation. 
The high-speed solar wind streams have been linked to recurrent geomagnetic 
activity and found to be prominent features of the solar wind much of the time 
since 1962.  See  GEOMAGNETISM; MAGNETOSPHERE. 
    X-ray and ultraviolet images of the Sun have brought attention to the 
features known as coronal holes. These are regions of abnormally low coronal 
density that appear dim in any radiation emitted (x-rays and the ultraviolet) 
or scattered (the white-light corona observed at eclipse) from the corona. 
Detailed examination of these images suggests that the holes are regions where 
the solar magnetic field is open, with field lines reaching from their visible 
roots in the photosphere out into interplanetary space. In contrast, the 
structure seen in the bright or dense corona suggests closed magnetic fields, 
with field lines connecting separated locations in the photosphere.  See  
SOLAR CORONA. 
    The largest and longest-lived coronal holes have been observed to occur in 
the polar regions of the Sun (as defined by its rotation axis); conspicuous 
polar holes existed for at least 8 years during the last 11-year sunspot 
cycle. These polar holes are related to the weak, dipolelike, general magnetic 
field of the Sun and are thus of opposite polarity in the two hemispheres.  
See  SUN. 
 �34s[A. J. Hundhausen]
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�6v�2v�32sSoldering   �21m �33s 
    The joining of metals by causing a lower-melting-point metal to wet or 
alloy with the joint surfaces and then freeze in place. A solder is defined as 
a joining material that melts below 427DGC (800DGF); brazing alloys melt above 
this temperature. Solders are used to establish reliable electrical 
connections, to make a liquid- or gas-tight joint, and to hold parts together 
physically. The usual practice is to rivet, crimp, or otherwise support the 
load and to seal the space with solder. 
    The most commonly used solders are alloys of tin and lead that melt below 
the melting point of tin. Antimony, bismuth, cadmium, silver, and arsenic are 
sometimes added to improve strength, wetting qualities, or grain size, or to 
produce alloys having desired melting ranges. 
    In order that surfaces will accept solder readily, they and the solder 
must be free from oxide or other obstructing films. When necessary, parts are 
cleaned chemically or by abrasion. Also, readily solderable coatings such as 
gold, silver, or tin may be applied. Even so, fluxes are usually used to aid 
the soldering process. Rosin, a pine product, is an effective and practically 
harmless flux used widely for electrical connections. When less stringent 
requirements exist and when less carefully prepared surfaces are to be 
soldered, rosin is mixed with chemically active agents that aid materially in 
soldering. The rosin-type fluxes may be incorporated as the core of wire 
solders or dissolved in various solvents for direct application to joints 
prior to soldering. Inorganic salts, such as zinc chloride and ammonium 
chloride, are widely used where stronger fluxes are needed. 
    In applying solders, joints are heated by soldering irons, torches, 
induction heaters, or furnaces, or by immersion in molten solder. In the first 
two instances, solder is fed to the joints by hand. In the third and fourth, 
preformed shapes are placed close to the joints before fluxing and heating.  
See  BRAZING. 
 �34s[George M. Bouton]
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�6v�2v�32sSolenoid (electricity)   �21m �33s 
    An electrically energized coil of insulated wire which produces a magnetic 
field within the coil. If the magnetic field produced by the coil is used to 
magnetize and thus attract a plunger or armature to a position within the 
coil, the device may be considered to be a special form of electromagnet and 
in this sense the words solenoid and electromagnet are synonymous. In a wider 
scientific sense the solenoid may be used to produce a uniform magnetic field 
for various investigations. So long as the length of the coil is much greater 
than its diameter (20 or more times), the magnetic field at the center of the 
coil is sensibly uniform, and the field intensity is almost exactly that given 
by the equation for a solenoid of infinite length. 
    When used as an electromagnet of the plunger type, the solenoid usually 
has an iron or steel casing. The casing increases the mechanical force on the 
plunger and also serves to constrain the magnetic field. The addition of a 
butt or stop at one end of the solenoid greatly increases the force on the 
plunger when the distance between the plunger and the stop is small. The 
illustration shows a steel-clad solenoid with plunger and plunger stop. The 
relation of force versus distance with and without the stop is also shown.  
See  ELECTROMAGNET. 
 �34s[Jerome Meisel]
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�6v�2v�32sSolenoid (meteorology)   �21m �33s 
    In meteorological usage, solenoids are hypothetical tubes formed in space 
by the intersection of a set of surfaces of constant pressure (isobaric 
surfaces) and a set of surfaces of constant specific volume of air (isoteric 
surfaces). The isobaric and isosteric surfaces are such that the values of 
pressure and specific volume, respectively, change by one unit from one 
surface to the next. The state of the atmosphere is said to be barotropic when 
there are no solenoids, that is, when isobaric and isosteric surfaces 
coincide. The number of solenoids cut by any plane surface element of unit 
area is a measure of the torque exerted by the pressure gradient force, 
tending to accelerate the circulation of air around the boundary of the area.  
See  BAROCLINIC FIELD; BAROTROPIC FIELD; ISOPYCNIC. 
 �34s[Frederick Sanders]
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�6v�2v�32sSolid solution   �21m �33s 
    The crystalline state, where at least one atomic position may accommodate 
more than one atomic species. In mineralogy and metallurgy, solid solution is 
a frequent and important phenomenon. In metallurgy, examples are the systems 
Au-Ag-Hg and Fe-Ni. In mineralogy, the most important examples are the systems 
CaO - MgO - CO2, Na2O - CaO - Al2O3 - SiO2, and MgO - Al2O3 - SiO2. 
    It is easiest to conceptualize the phenomenon by defining structure type, 
mineral species, and end-member composition. The structure type is a design of 
distinct-symmetry independent atomic positions possessing an asymmetric unit 
of atomic positions which must be specified along with the unit cell 
properties, including axial parameters, crystal system, and space group. 
Mineral species is defined on the basis of structure type and end-member 
composition, where each end member in principle is a distinct species.  See  
CRYSTAL; CRYSTALLOGRAPHY; MINERALOGY. 
 �34s[Paul B. Moore]
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�6v�2v�32sSolids pump   �21m �33s 
    A device used to move solids upward through a chamber or conduit. It is 
able to overcome the large dynamic forces at the base of a solids bed and 
cause the entire bed to move upward. 
    Solids pumps are used to cause motion of solids in process-type equipment 
in which treatment of solids under special conditions of temperature, 
oxidation, and reduction can be combined with upward motion and discharge of 
the spent solids overhead from the reacting vessel. The solids pump has found 
its principal application in the operation of oil-shale retorts. 
    Solids pumps are inherently of the positive displacement type. One 
practical method uses a reciprocating piston mounted on a trunnion permitting 
it to swing into an inclined position for filling and then to swing back into 
vertical position for discharge. The illustration shows a mechanically driven 
solids pump in four positions through its cycle of operation.  See  
BULK-HANDLING MACHINES. 
 �34s[Clyde Berg]
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�6v�2v�32sSolid-state battery   �21m �33s 
    A battery in which both the electrodes and the electrolyte are solid-state 
materials. Solid-state batteries have been developed as a logical application 
for a relatively new class of materials, the solid electrolytes, otherwise 
known as superionic conductors. These materials are very good conductors for 
ions, but they maintain a very high resistance toward electronic conduction. 
These properties are necessary for any electrolyte; the high ionic 
conductivity minimizes the internal resistance of the battery, while the high 
electronic resistance minimizes its self-discharge rate. Typical materials are 
BETA-Al2O3 for Nasupp conduction, Ag4RbI5 for Agsupp conduction, and 
Li-BETA-Al2O3 for Lisupp conduction. The polycrystalline electrolytes 
typically employed in batteries have ionic conductivities about one-fifth that 
of the corresponding single crystals. 
    Solid-state batteries generally fall into the low-energy-density category. 
This limitation arises from the polarization, at high current densities, at 
the solid-solid interfaces. However, in certain applications the inherent 
advantages of solid-state batteries are more important than the net energy 
content or the current output. From a manufacturing point of view, they are 
easier to miniaturize, and there is no problem with the electrolyte leaking 
out of the container. They have long shelf life and are resistant to shock and 
vibration. They have a wider operating temperature range, and there is no 
abrupt decrease in performance such as that which occurs when a liquid 
electrolyte freezes or boils.  See  BATTERY; LITHIUM PRIMARY CELL. 
 �34s[Gabriel G. Barna]
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�6v�2v�32sSolid-state chemistry   �21m �33s 
    The science of the elementary, atomic compositions of solids and the 
transformations that occur in and between solids and between solids and other 
phases to produce solids. Solid-state chemistry deals primarily with those 
microscopic features which are uniquely characteristic of solids and which are 
the causes for the macroscopic chemical properties and the chemical reactions 
of solids. As with other branches of the physical sciences, solid-state 
chemistry also includes related areas that furnish concepts and knowledge 
essential to understandings and explanations of those phenomena which are more 
characteristic of the subject itself. 
    The overlap of solid-state chemistry and solid-state physics is extensive. 
In general, solid-state physics treats properties, such as energy and entropy, 
which are continuously variable in the solid, whereas solid-state chemistry 
concerns those properties which are discontinuous because of chemical 
reactions; in a sense, by definition the properties are chemical because they 
are discontinuous. In another perspective, solid-state chemistry tends to be 
based on structure in configuration space, whereas solid-state physics tends 
to be based on momentum space.  See  SOLID-STATE PHYSICS. 
 �34s[Robert J. Thorn]
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�6v�2v�32sSolid-state physics   �21m �33s 
    The study of the physical properties of solids, such as electrical, 
dielectric, elastic, and thermal properties, and their understanding in terms 
of fundamental physical laws. Most problems in solid-state physics would be 
called solid-state chemistry if studied by scientists with chemical training, 
and vice versa. Solid-state physics emphasizes the properties common to large 
classes of compounds rather than the dependence of properties upon 
compositions, the latter receiving greater emphasis in solid-state chemistry. 
In addition, solid-state chemistry tends to be more descriptive, while 
solid-state physics focuses upon quantitative relationships between properties 
and the underlying electronic structure.  See  SOLID-STATE CHEMISTRY. 
    Many of the scientists who study the physics of liquids identify with 
solid-state physics, and the term ``condensed-matter physics'' has been used 
by some researchers to replace ``solid-state physics'' as a division of 
physics. It includes noncrystalline solids such as glass as well as 
crystalline solids.  See  AMORPHOUS SOLID; GLASS. 
    In solid-state physics it is generally assumed that the electronic states 
can be described as wavelike. The individual electronic states, called Bloch 
states, have energies which depend upon the wave number (a vector equal to the 
momentum divided by Planck's constant PLANCK'S CONSTANT), and the wave number 
is restricted to a domain called the Brillouin zone. This energy given as a 
function of the wave number is called the band structure. There are several 
curves, called bands, for each line in the Brillouin zone.  See  BRILLOUIN 
ZONE.  
    The total energy of a solid includes a sum of the energies of the occupied 
electronic states. Since the energy bands depend upon the positions of the 
atoms, so does the total energy, and the stable crystal structure is that 
which minimizes this energy. The theory has not proved adequate to really 
predict the crystal structure of various solids, but it is possible to predict 
the changes in energy under various distortions of the lattice. There are in 
fact three times as many independent distortions, called normal modes, as 
there are atoms in the solid. Each has a wave number, and the frequencies of 
the normal vibrational modes, as a function of wave number in the Brillouin 
zone, form vibrational bands in direct analogy with the electronic energy 
bands. These can be directly calculated from quantum theory or measured by 
using neutron or x-ray diffraction.  See  CRYSTAL; LATTICE VIBRATIONS; NEUTRON 
DIFFRACTION; X-RAY DIFFRACTION. 
 �34s[Walter A. Harrison]
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�6v�2v�32sSolid (geometry)   �21m �33s 
    A geometric solid, usually called a solid, is a set of points forming a 
finite portion of three-dimensional space, continuously joined together, and 
separated from the rest of space by a set of points called its boundary. An 
interior point of the solid is a point I such that all points of space within 
a sufficiently small positive distance d from I are points of the solid. A 
boundary point is a point B such that, no matter how small a positive distance 
d is chosen, there are points of the solid and points not of the solid within 
distance d of B. A plane section of the solid consists of the points common to 
the solid and to a given plane that intersects the solid. A bounding section 
contains boundary points but no interior points of the solid. Any two bounding 
sections cut by parallel planes may be called bases of the solid. The distance 
between these planes is then called the altitude, and the section by a plane 
halfway between the bases is called a midsection of the solid. A solid is 
called convex if every line segment joining a pair of its points belongs to 
the solid.  See  POLYHEDRON; SURFACE AND SOLID OF REVOLUTION. 
 �34s[James S. Frame]
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�6v�2v�32sSoliton   �21m �33s 
    An isolated wave which propagates without dispersing its energy over 
larger and larger regions of space. In most of the scientific literature, the 
requirement that two solitons emerge unchanged from a collision is also added 
to the definition; otherwise one speaks of a solitary wave. 
    There are many equations of mathematical physics which have solutions of 
the soliton type. Correspondingly, the phenomena which they describe, be it 
the motion of waves in shallow water or in an ionized plasma, exhibit 
solitions. The first observation of this kind of wave was made in 1834 by John 
Scott Russell, who followed on horseback a soliton propagating in the windings 
of a channel. In 1895, D. J. Korteweg and H. de Vries proposed an equation for 
the motion of waves in shallow waters which possesses soliton solutions, and 
thus established a mathematical basis for the study of the phenomenon. 
Interest in the subject, however, lay dormant for many years, and the major 
body of investigations began only in the 1950s. Researches done by analytical 
methods and by numerical methods made possible with the advent of computers 
gradually led to a complete understanding of solitons.  See  WAVE MOTION IN 
LIQUIDS.  
    Eventually, the fact that solitons exhibit particlelike properties, 
because the energy is at any instant confined to a limited region of space, 
received attention, and solitons were proposed as models for elementary 
particles. However, it is difficult to account for all of the properties of 
known particles in terms of solitons. It has been realized that some of the 
quantum fields which are used to describe particles and their interactions 
also have solutions of the soliton type. The solitons would then appear as 
additional particles, and may have escaped experimental detection because 
their masses are much larger than those of known particles. In this context 
the requirement that solitons emerge unchanged from a collision has been found 
too restrictive, and particle theorists have used the term soliton where 
traditionally one would speak of a solitary wave.  See  ELEMENTARY PARTICLE; 
QUANTUM FIELD THEORY. 
 �34s[Claudio Rebbi]
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�6v�2v�32sSolpugida   �21m �33s 
    An order of nonvenomous, spiderlike predatory arachnids found chiefly in 
arid and semiarid, tropical, and warm-temperate regions. They are also known 
as the Solifugae or sun spiders. The relatively large anterior appendages, or 
chelicerae, are used for holding and crushing prey. The solpugids are agile 
and usually stalk their prey during the night. A fossil form is known from 
Pennsylvanian time.  See  ARACHNIDA. 
 �34s[C. Clayton Hoff]
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�6v�2v�32sSolstice   �21m �33s 
    The two days during the year when the Earth is so located in its orbit 
that the inclination (about 23JDG, or 23DG45)At the time of the summer 
solstice every place north of the Arctic Circle will have 24 h of sunlight and 
the length of day at all places north of the Equator will be more than 12 h, 
increasing in length with increasing latitude. Identical conditions are found 
in the Southern Hemisphere at the time of the Northern Hemisphere's winter 
solstice.  See  MATHEMATICAL GEOGRAPHY. 
 �34s[Van H. English]
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�6v�2v�32sSolubility product constant �21m �33s
    A special type of simplified equilibrium constant (symbol K(sub sp)
defined for, and useful for, equilibria between solids (s) and their
respective ions in solution, for example, reaction (1). For this relatively
simple equilibrium, Eqs. (2) and (3) apply.

       AgCl(s)	REVERSIBLE REACTION  Ag+  + Cl- 		      (1)

	 [Ag+][Cl-] NEARLY EQUAL TO K(sub sp)			      (2)

		    (Ag+)(Cl-)/(AgCl) = K(sub sp)		      (3)

    It can be demonstrated experimentally that a small increase in the molar
concentration of chloride ion [Cl-] causes a reduction in the concentration of
silver present as Ag+. Similarly, an increase in [Agsupp] reduces [Cl-].
The product of the two concentrations is approximately constant as indicated
by Eq. (2) and equal to the Ksp of Eq. (3). Equation (3) is exact since
the variables are activities instead of concentrations. In accordance with the
choice of standard state usually made for a solid, the activity of solid AgCl
is unity, hence Eq. (4) holds.

(Ag+)(Cl-) = K(sub sp) = l.8 x l0(sub -10) mole(sup 2) liter(sup -2)	(4)

    In practice, various complications arise: addition of too much of either
ion produces more complicated ions and hence actually increases the apparent
concentration of the other ion. Addition of a salt without a common ion (that
is, a salt supplying neither Ag nor Cl-) either may react with Ag or
Cl- or may merely increase the concentration of both ions by a lowering of the
mean ionic activity coefficient.  See  IONIC EQUILIBRIUM; PRECIPITATION
(CHEMISTRY).
 �34s[Thomas F. Young]
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�6v�2v�32sSolubilizing of samples   �21m �33s 
    The process by which samples that do not dissolve easily are converted 
into different chemical compounds which are soluble. The sample may be heated 
in air to evolve volatile components or to oxidize a component to a volatile 
higher oxidation state with the formation of an acid-soluble form, as in the 
roasting of a sulfide to form the oxide and sulfur dioxide. Most frequently, 
the sample is treated with a solvent which reacts with one or more 
constituents of the sample. The choice of solvent is determined by the 
chemical reactions which are required. Reactions used include solvation, 
neutralization, complex formation, metathesis, displacement, 
oxidation-reduction, or combinations of these. 
    Many substances do not dissolve at temperatures obtainable in the presence 
of liquid water. Fused salt reactions employing temperatures of 750 - 2030DGF 
(400 - 1100DGC) are necessary for the attack and decomposition of many types 
of samples. The material used as the solvent is called a flux, and the process 
of melting the mixture of dry, solid flux with the sample is called a fusion. 
 �34s[Charles L. Rulfs]
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�6v�2v�32sSolution   �21m �33s 
    A homogeneous mixture of two or more components whose properties vary 
continuously with varying proportions of the components. A liquid solution can 
be distinguished experimentally from a pure liquid by the fact that during 
transfers into other single phases at equilibrium (freezing and vaporizing at 
constant pressure) the temperature and other properties vary continuously, 
whereas those of a pure liquid remain constant. For an apparent exception see 
AZEOTROPIC MIXTURE.  See also  SOLVENT. 
    Gases, unless highly compressed, are mutually soluble in all proportions. 
    A solid solution is, similarly, a single phase whose composition and other 
properties vary continuously with changing composition of the liquid phase 
with which it is in equilibrium.  See  SOLID SOLUTION. 
    The extent to which substances can form solutions depends upon the kind 
and strength of the attractive forces between the several molecular species 
involved. It is necessary to consider the attractive forces exerted by 
molecules of the following types: (1) nonpolar molecules; (2) polar molecules, 
that is, those containing electric dipoles; (3) ions; and (4) metallic atoms. 
    Actual solutions may be considered in terms of their departure from a 
simple idealized model - a mixture of components having the same attractive 
fields, which mix without change in volume or heat content. This is analogous 
to an ideal gas mixture, which is formed with no heat of mixing and in which 
the total pressure is the sum of the partial pressures. In such a solution the 
escaping tendency of the individual molecules is the same, whether they are 
surrounded by similar or by different molecules. 
 �34s[Joel H. Hildebrand]
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�6v�2v�32sSolvation   �21m �33s 
    The association or combination of a solute unit (ionic, molecular, or 
particulate) with solvent molecules. This association may involve chemical or 
physical forces, or both, and may vary in degree from a loose, indefinite 
complex to the formation of a distinct chemical compound. Such a compound 
contains a definite number of solvent molecules per solute molecule. Solvation 
occurring in aqueous solutions is referred to as hydration. In certain 
colloidal suspensions, solvation is, to a large extent, responsible for the 
stability of the sol.  See  COLLOID; HYDRATION; SOLUTION; SOLVENT. 
 �34s[Francis J. Johnston]
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�6v�2v�32sSolvent   �21m �33s 
    By convention, the component present in the greatest proportion in a 
homogeneous mixture of pure substances (solutions). Components of mixtures 
present in minor proportions are called solutes. Thus, technically, 
homogeneous mixtures are possible with liquids, solids, or gases dissolved in 
liquids; solids in solids; and gases in gases. In common practice this 
terminology is applied mostly to liquid mixtures for which the solvent is a 
liquid and the solute can be a liquid, solid, or gas.  See  SOLUTION. 
    Three broad classes of solvents are recognized - aqueous, nonaqueous, and 
organic. Formalistically, the nonaqueous and organic classifications are both 
not aqueous, but the term organic solvents is generally applied to a large 
body of carbon-based compounds that find use industrially and as media for 
chemical synthesis. Organic solvents are generally classified by the 
functional groups that are present in the molecule, for example, alcohols, 
halogenated hydrocarbons, or hydrocarbons; such groups give an indication of 
the types of physical or chemical interactions that can occur between solute 
and solvent. Nonaqueous solvents are generally taken to be inorganic 
substances and a few of the lower-molecular-weight, carbon-containing 
substances such as acetic acid, methanol, and dimethylsulfoxide. Nonaqueous 
solvents can be solids (for example, fused LiI), liquids (H2SO4), or gases 
(NH3) at ambient conditions; the solvent properties of fused sodium iodide 
(NaI) are manifested in the molten state, whereas hydrogen sulfate (H2SO4) and 
ammonia (NH3) must be liquefied to act as solvents. 
 �34s[J. J. Lagowski]
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�6v�2v�32sSolvent extraction   �21m �33s 
    A technique, also called liquid extraction, for separating the components 
of a liquid solution. This technique depends upon the selective dissolving of 
one or more constituents of the solution into a suitable immiscible liquid 
solvent. It is particularly useful industrially for separation of the 
constituents of a mixture according to chemical type, especially when methods 
that depend upon different physical properties, such as the separation by 
distillation of substances of different vapor pressures, either fail entirely 
or become too expensive. 
    Industrial plants using solvent extraction require equipment for carrying 
out the extraction itself (extractor) and for essentially complete recovery of 
the solvent for reuse, usually by distillation.  See  DISTILLATION. 
    The petroleum refining industry is the largest user of extraction. In 
refining virtually all automobile lubricating oil, the undesirable 
constituents such as aromatic hydrocarbons are extracted from the more 
desirable paraffinic and naphthenic hydrocarbons. By suitable catalytic 
treatment of lower boiling distillates, naphthas rich in aromatic hydrocarbons 
such as benzene, toluene, and the xylenes may be produced. The latter are 
separated from paraffinic hydrocarbons with suitable solvents to produce 
high-purity aromatic hydrocarbons and high-octane gasoline. Other industrial 
applications include so-called sweetening of gasoline by extraction of 
sulfur-containing compounds; separation of vegetable oils into relatively 
saturated and unsaturated glyceride esters; recovery of valuable chemicals in 
by-product coke oven plants; pharmaceutical refining processes; and purifying 
of uranium. 
    Solvent extraction is carried out regularly in the laboratory by the 
chemist as a commonplace purification procedure in organic synthesis, and in 
analytical separations in which the extraordinary ability of certain solvents 
preferentially to remove one or more constituents from a solution 
quantitatively is exploited. Batch extractions of this sort, on a small scale, 
are usually done in separatory funnels, where the mechanical agitation is 
supplied by handshaking of the funnel.  See  EXTRACTION. 
 �34s[Robert E. Treybal]
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�6v�2v�32sSomasteroidea   �21m �33s 
    A subclass of Asterozoa comprising sea stars of very generalized 
structure, the jaws often only partly developed. Somasteroids are sluggish sea 
stars, living on soft bottom, depending to a large extent upon ciliary 
currents and tube-foot mechanisms for obtaining small organisms as food. The 
oldest somasteroids are found as fossils in early Paleozoic deposits of Late 
Cambrian (Tremadoc) age. A living representative is Platasterias, found in 
western coastal areas of Mexico. Somasteroids are the precursors of modern sea 
stars.  See  ASTEROIDEA; ASTEROZOA; CRINOIDEA; ECHINODERMATA; OPHIUROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sSomatic cell genetics   �21m �33s 
    There are two approaches to somatic cell genetics: (1) the genetics of 
somatic cells, especially in culture, of higher organisms; and (2) the 
genetics of higher organisms by means of their somatic cells, especially in 
culture.  
    The first approach, pioneered by T. T. Puck, handles somatic cells in 
culture as cultures of unicellular organisms for the purpose of genetic 
research. It involves the same sorts of operations used, for example, in the 
genetics of bacteria, yeast, unicellular algae, and protozoa. It includes 
studies on colonies or cultures derived from single cells (clones) in respect 
of mutation; control of differentiation; metabolic cooperation; gene 
complementation; reactions to physical, chemical, or biological agents; cell 
fusion; uptake and integration of foreign genetic material, especially viral; 
and so on. 
    The second approach started with Curt Stern's discovery in 1936 of somatic 
gene segregation in the fruit fly Drosophila. In the mid-1950s the 
``parasexual cycle'' in the fungus Aspergillus nidulans (later, in many 
others) was discovered and harnessed to genetic analysis. This made it clear 
that mitotically multiplying cells in which loss of one or more chromosomes 
occurs or can be induced could provide an alternative to the germ cells for 
genetic analysis. How to obtain chromosome loss in mammalian cells came 
unexpectedly in 1967 from the new avenue of somatic cell hybridization. 
 �34s[Guido Pontecorvo]
�33s 
    Somatic cell hybridization, the experimental fusion of somatic cells, has 
provided an alternative to sex as a way of studying genetics in higher 
organisms. Particularly striking has been the impact of somatic cell 
hybridization in the field of human genetics, especially for human gene 
mapping.  See  GENETIC MAPPING; HUMAN GENETICS. 
    In somatic cells grown in tissue culture, fusion occurs spontaneously as a 
rare event, and the frequency with which this happens can be greatly increased 
by treating the cells with Sendai virus or polyethylene glycol. In a small 
proportion of cells, the nuclei also fuse. By making a suitable choice of 
parental cells, and growing the cells in culture media which will support only 
the growth of cells having some particular phenotype, it is possible to select 
mononuclear cell hybrids which contain chromosomes from each parent. 
Intraspecific (human-human) or interspecific (human-mouse) hybrids can be 
obtained.  
    An exciting use of intraspecific hybrids was first described by C. 
Milstein and coworkers. They immunized a mouse against a particular antigen 
and then fused spleen cells from this mouse with a cell line from a mouse 
myeloma (a tumor which is capable of producing immunoglobulin). A small 
proportion of the hybrid clones which were selected and grown were making a 
single specific antibody against the original antigen. Similar hybrids, 
usually known as hybridomas, have proved an invaluable source of highly 
specific monoclonal antibodies for use in research and in diagnosis.  See  
GENETIC ENGINEERING. 
 �34s[Sue Povey]
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�6v�2v�32sSomatization �21m �33s
    A type of neurosis. Perhaps the simplest and most diffuse form of neurotic
symptomatology are the so-called neurasthenias in which the principal
complaint is a general malaise involving a loss of energy and zest along with
a vague sense of aches, pains, and fatigue. Neurasthenic symptoms tend to be
centered about those activities in the person's life that have proved either
notably unrewarding or internally stressful.
    Hypochondriacal symptoms, in the form of exaggerated somatic complaints
accompanied by a strong but ambivalent fear of falling ill and with much
concern about diet, elimination, and health, represent another form of
somatization. A common origin of hypochondriacal symptoms is a family
background where parents were excessively concerned with the health of
children and in which sickness was rewarded with attention. Feelings of
inadequacy on the part of the patient later in life may then be translated
into reward-seeking somatic complaints.  See  HYSTERIA; NEUROSIS.
 �34s[Jerome S. Bruner ;Walter Mischel]
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�6v�2v�32sSomesthesis   �21m �33s 
    A general term for the somatic sensibilities aroused by stimulation of 
bodily tissues such as the skin, muscles, tendons, joints, and the viscera. 
Six primary qualities of somatic sensation are commonly recognized: 
touch-pressure (including temporal variations such as vibration), warmth, 
coolness, pain, itch, and the position and movement of the joints. These basic 
sensory qualities exist because each is served by a different set of sensory 
receptors (the sensory endings of certain peripheral nerve fibers) which 
differ not only in their sensitivites to different types of stimuli, but also 
in their connections to structures within the central nervous system. 
    The somatic sensory pathways are dual in nature. One major part, the 
lemniscal system, receives input from large-diameter myelinated peripheral 
nerve fibers (for example, those serving the sense of touch-pressure). The 
second major somatic pathway is called the anterolateral system. It receives 
input from small-diameter myelinated and unmyelinated peripheral nerve fibers 
carrying pain and temperature information.  See  CUTANEOUS SENSATION; PAIN; 
PARESTHESIA; PROPRIOCEPTION; TEMPERATURE SENSES. 
 �34s[Robert LaMotte]
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�6v�2v�32sSonar   �21m �33s 
    A term that refers both to the application of underwater sound to the 
detection and location of objects in the sea and to the apparatus used in such 
applications. The word is derived from ``sound navigation and ranging''; the 
British use the word asdic. Since electromagnetic radiations, such as visible 
light or radar, do not penetrate the sea significantly, sonar is the most 
successful method of underwater detection. Except for underwater telephones, 
sonar is also the only means by which a fully submerged submarine can apprise 
itself of what is happening around it. Thus sonar is used by submarines to 
locate surface vessels and other submarines and to navigate through a mine 
field or under the ice in the Arctic Ocean.  See  UNDERWATER TELEPHONE. 
    There are two general types of sonar methods, active and passive. In an 
active sonar system a pulse of sound is generated by the searcher and 
projected into the water. This sound is reflected back from the target and 
detected by the searcher as an echo. Since the speed of sound in sea water is 
known, the range and also the bearing of the target can be determined. This 
method is also called echo-ranging. In a passive system the searcher detects 
the noises emitted by the target. Unless more than one listening station is 
involved, passive sonar provides information only as to the existence of a 
noise source and its bearing from the searcher. Because the noise radiated 
from a ship is often characteristic of the type of ship, passive sonar, in 
contrast to active sonar, may help in classifying the target.  See  UNDERWATER 
SOUND.  
    Since active sonar systems involve the deliberate emission of sound, which 
discloses the searcher to easy detection by other vessels, they are used 
sparingly by submarines, which rely on concealment for their safety. The 
active systems are therefore used mainly by surface antisubmarine vessels, 
such as destroyers, since they generally operate in an environment which is 
too noisy for successful passive detection of a quiet submarine. 
    Sonar may also be used by aircraft for submarine detection by dropping 
buoys (called sonobuoys) in the water, by mine sweepers in locating mines, or 
by acoustic torpedoes. Sonar may be used for detection of schools of fish or 
for the location of submerged wrecks. An echo sounder, or depth indicator, is 
essentially an active sonar system which sends sound pulses to the sea bottom. 
  See  ACOUSTIC TORPEDO; ECHO SOUNDER; SONOBUOY. 
 �34s[Robert W. Morse]
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�6v�2v�32sSonic barrier   �21m �33s 
    An aeronautical term coined during World War II to symbolize the technical 
difficulties for crewed aircraft in accelerating through the speed of sound. 
In designing and testing subsonic airplanes to fly at increasingly higher 
speeds, aeronautical engineers found the following adverse effects as the 
speed of the airplane approached the speed of sound: a rapid rate of increase 
in drag, breakdown of lift, and loss of control and maneuverability of the 
airplane. Such experience suggested that these violent and uncontrollable 
aerodynamic phenomena might be inevitable in the transonic range of speeds 
(that is, in the immediate vicinity of the speed of sound). These adverse 
effects are collectively and loosely termed the sonic barrier. 
    As a result of intensive research the source of most of the difficulties 
has been traced to the formation of local shock waves, which in turn interact 
with the boundary layer and induce flow separation from the aerodynamic 
surfaces. By careful choice of geometrical shapes, such as use of thin wings 
and slim bodies, it has been found possible to design airplanes with 
relatively smooth flow characteristics through the transonic speed range.  See 
  AIRPLANE; TRANSONIC FLIGHT. 
 �34s[Shao-Chi Lin]
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�6v�2v�32sSonic boom �21m �33s
    Strong pressure waves (shock waves) generated by aircraft in supersonic
flight and heard along the ground as explosivelike sounds called booms or
bangs. Contrary to popular belief, a sonic boom does not occur at only one
location at the instant the aircraft exceeds the local speed of sound.
Instead, aircraft flying faster than the speed of sound (approximately 1100
ft/s or 330 m/s) generate shock waves that radiate away from and behind the
aircraft and are dragged with it as long as it is flying supersonic. Where
this trailing shock wave intercepts the ground, it is heard as an impulsive
type of sound. In some situations, depending on the shockwave pattern, an
observer may hear two distinct booms or a double boom rather than hear the
single sound.  See  SHOCK WAVE; SUPERSONIC FLIGHT.
    Some factors affecting the sonic boom signal heard on the ground, such as
speed, altitude, and route of the aircraft, can be reasonably well controlled.
Others, such as meteorological conditions, topography, and ground-level air
turbulence, cannot be modified. Consequently, the extent to which sonic booms
can be predicted and controlled for known flight profiles and flying
conditions is limited by those environmental factors beyond human control.
    Aircraft altitude is the primary contributor to the magnitude of the sonic
boom and is one of the factors most accessible to control measures. Current
U.S. Air Force aircraft are restricted from supersonic flight over inhabited
areas below an altitude of 30,000 ft (9 km) except for special missions or in
the event of emergency. It appears that there is no possibility to
significantly reduce or eliminate the sonic boom from current and
next-generation aircraft which fly at supersonic speeds. The operational
controls and flight regulations necessary to ensure acceptable levels of sonic
booms on the ground have not been achieved for present-day aircraft flying
over land. Commercial transports are prohibited by law in the United States
from supersonic operations over land or over water where the sonic booms may
be propagated to impact on land.
 �34s[Henning E. Von Gierke ;Charles W. Dixon]
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�6v�2v�32sSonobuoy   �21m �33s 
    A miniature broadcasting station placed in the sea to receive underwater 
sounds and transmit them to an aircraft or other remote point. Its original 
and still most common application is to provide an ``ear in the water'' to 
permit an aircraft to detect typical sounds made by submarines. 
    Aircraft have unique advantages in antisubmarine warfare because of their 
mobility and relative invulnerability to submarine counterattack. Without the 
ability to launch sonobuoys, however, the aircraft's capability against 
submerged submarines would be very limited, since underwater acoustics is 
probably the most effective means of detection.  See  UNDERWATER SOUND. 
    While the most common type of sonobuoy merely listens for underwater 
sounds arriving from all directions, there are designs which even operate as 
miniature sonars. Such active sonobuoys are needed if the submarine is masked 
by natural sea background.  See  SONAR. 
    Sonobuoys have been used as geophysical instruments, providing a 
convenient means of picking up underwater acoustic returns in ocean bottom 
profiling.  
 �34s[Irvin H. Gatzke]
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�6v�2v�32sSonoscan   �21m �33s 
    A type of acoustic microscope that uses unfocused beams with a wavelength 
of 15 micrometers. It does not require mechanical scanning. The acoustic beam 
deforms a liquid-solid interface in a pattern that represents the image. A 
laser beam reflected from this deformed surface is modulated in a way that 
corresponds to the image imprinted on the surface. The information on the 
laser beam is extracted with suitable electronics and presented for display on 
the face of a television monitor. See ACOUSTIC MICROSCOPE. 
 �34s[C. F. Quate]
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�6v�2v�32sSorghum �21m �33s
    Sorghum includes many widely cultivated grasses having a variety of names.
Sorghum is known as guinea corn in West Africa, Kafir corn in South Africa,
mtama in East Africa, and durra in the Sudan. In India sorghums are called
juar, jowar, or cholam; in China, kaoliang; and in America, milo. Cultivated
sorghums in the United States are classified as a single species, Sorghum
bicolor, although there are many varieties and hybrids. The two major types of
sorghum are the grain, or nonsaccharine, type, cultivated primarily for grain
production and to a lesser extent for forage, and the sweet, or saccharine,
type, used for forage production and for making syrup and sugar.
    Grain sorghum is grown in the United States chiefly in the Southwest and
the Great Plains. It is a warm-season crop which withstands heat and moisture
stress better than most other crops, but extremely high temperatures and
extended  drought may reduce yields. It is most extensively grown in Texas,
Kansas, Nebraska, Oklahoma, Missouri, Colorado, and South Dakota. This grain
production is fed to cattle, poultry, swine, and sheep primarily. Sorghum is
considered nearly equal to corn in feed value.
 �34s[Frederick R. Miller]
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�6v�2v�32sSound �21m �33s
    Subjectively, what is heard with the ears; objectively, a mechanical
disturbance from equilibrium in an elastic material medium, that is, a
material medium which will return to its original equilibrium state after the
disturbing influence has been removed. Most of the solids, liquids, and gases
of common experience possess such elasticity to a certain degree and can
therefore act as sources, transmitters, and receivers of sound.  See
ACOUSTICS.
    Originally sound referred entirely to what is heard. Later, however,
interest developed in sounds which human beings cannot hear, either because
the frequency is too low (infrasound) or too high (ultrasound). Audible sound
is usually described as audio or sonic. The characterization of sound by
frequency (associated with pitch in hearing - the higher pitch corresponding
to higher frequency) is well known, since most sounds of interest are produced
by the mechanical vibration of material media, for which the frequency means
the number of complete vibrations or cycles per second, now referred to as
hertz (Hz). Another important quantity characterizing sound is the sound
pressure, that is, the excess pressure in the medium being traversed by the
sound over the normal equilibrium pressure in the undisturbed medium.  See
PITCH; SOUND PRESSURE.

    Nature of sound propagation.  Sound is propagated through a medium such as
air by means of wave motion. In wave propagation the medium as a whole does
not move, but only the disturbance, for example, the hump on the water surface
in the case of a water wave. For a sound wave in a fluid like air, the
``hump'' is the excess pressure p in the medium constituting the disturbance.
Such an excess pressure produced at one point does not remain there but moves
through the fluid with velocity c, which is called the velocity of sound; in
the case of air at 20DGC this is 1129ft/s (344 m/s). See  WAVE (PHYSICS); WAVE
MOTION.

    Harmonic waves.  The most important type of sound wave in practice is that
called harmonic. For such a wave in the x direction with the velocity c, the
excess pressure is given by Eq. (1). The quantity p0 is called the amplitude

      p = p(sub 0) cos (2PI V [t - (x/c)])			 (1)

of the wave; it is the maximum value of the excess pressure as the wave
travels along. The minimum value is of course -p(sub 0). The quantity NU is the
frequency of the wave. At any given point the value of the excess pressure
repeats itself NU times a second or in the time 1/NU, which is called the
period of the wave, designated by T. Another important quantity connected with
the wave is the wavelength, designated by LAMBDA. At any given time the excess

		   GAMMA = c/v = cT				 (2)

pressure repeats itself after a space interval given by Eq. (2). This relation
between wave length, velocity, and frequency holds for harmonic waves of all
kinds and in all material media. Low frequencies are always associated with
long wavelengths, and vice versa. For an audible sound wave in air with
frequency 10(sup 3) Hz, the wavelength is about 13 in. (34 cm). On the other
hand, for an ultrasonic wave of frequency 10(sup 6) Hz, the wavelength is about
1.3 x 10(sup -2) in. (3.4 x 10 [sup -2] cm).
    Associated with the excess pressure p of the sound wave, there is also an
actual displacement of the medium from equilibrium in the x direction. This is
usually called the particle displacement.
    A very important quantity descriptive of a sound wave is the intensity I,
qualitatively a measure of what might be called its strength but precisely
defined as the time average rate of flow of energy per unit time through unit
area of the medium through which the wave is passing, with the understanding
that the area in question is perpendicular to the direction of wave
propagation. This is the time average power transported by the wave. From the
principles of mechanics it is the time average of the product of the particle
velocity u and the acoustic pressure p.
    A very important unit of intensity in practical sound problems is the
decibel (dB). A sound of intensity I is said to be a number of decibels above
or below a standard reference intensity I0, given by expression (3). Thus, if

			  10 log(sub 10) I/I(sub o)			    (3)

I = 2I0, I is about 3.01 dB above I0, or if I = I(sub 0)/2, I is about 3.01 dB
below I(sub 0).  For many problems in audio acoustics I(sub 0) is taken as the
threshold of hearing (the minimum audible intensity), which for the average
person with so-called normal hearing is approximately 10(sup -16) W/cm(sup 2).
In terms of this reference, normal conversational speech directly in front of
the mouth has an intensity of about 65 dB, whereas next to an aircraft jet
engine the intensity is about 160 dB.  For a normal human ear the intensity at
which sound ceases to be heard but produces pain in the ear and incipient damage
is about 120 dB.  See DECIBEL; LOUDNESS.  The harmonic sound wave considered so
far is called a plane wave since the particle displacement and pressure are
uniform over any plane surface perpendicular to the direction of propagation.
Such a plane is called a wavefront.  A type of sound wave of even greater
importance than the plane wave is the spherical wave.  Such a wave spreads
equally in all directions from a point source; it can be realized ideally by a
small sphere that pulsates radially.  The radiated sound then moves out so that
the particle velocity u and the sound pressure p are uniform over a sphere or
radius r with center at the source and are, of course, dependent on r.

    Velocity of sound.	The general theoretical expression for the velocity of
sound in a compressible fluid medium in which the instantaneous pressure and
density are respectively p and RHO is given by Eq. (4), where the ratio

c = SQUARE ROOT OF [(dp)/(d RHO)]					    (4)

dp/d RHO must be taken under adiabatic conditions, that is, such that no heat
enters or leaves the element of the medium as the sound wave passes through it.
For a gas, Eq. (4) gives Eq. (5), where GAMMA is the ratio of the specific

	 c = SQUARE ROOT OF [GAMMA P/RHO]				     (5)

heat at constant pressure to that at constant volume. For air, GAMMA = 1.4.
From the general gas equation of an ideal gas, Eq. (4) takes the form of Eq. (6)

	 c = SQUARE ROOT OF [GAMMA R T] 				     (6)

where R is the gas constant per unit mass, and T is the absolute temperature.
Denoting the velocity of sound in the gas at 0DGC by c0 permits Eq. (6) to
be put in the form of Eq. (7), where THETA is temperature in Celsius degrees.

	 c = c(sub 0) SQUARE ROOT OF [1 + (THETA/273)]			     (7)

See GAS; WAVE MOTION IN FLUIDS.
    Sound is propagated through the bulk of an elastic solid by means of a
longitudinal compressional wave. The word longitudinal means that the
displacement of the medium through which the sound is passing lies in the
direction of propagation. In addition to longitudinal waves, solids can also
propagate pure shear waves which are transverse in character, in which the
displacement of the medium is at right angles to the direction of propagation.
Though not strictly a sound wave in the usually accepted sense, this type of
wave is considered acoustical in character; it is exemplified as a component
of seismological or earthquake waves.
    The velocity of sound varies with the temperature [Eq. (7) for gases],
usually increasing as the temperature increases, and hence can have different
values in different parts of a medium. This is of particular importance in
sound propagation through the atmosphere. The same is true for sound
transmitted through the ocean. Even in an isothermal medium the velocity will
change with the frequency. This is known as sound dispersion. It is a very
small effect at low frequencies and becomes perceptible only at very high or
ultrasonic frequencies. See ULTRASONICS; UNDERWATER SOUND.

    Sound rays.   Though sound, like light, is propagated by waves, under
certain conditions its transmission may be described adequately by means of
rays, which are lines perpendicular to the wavefront in each case, giving the
direction of sound propagation. The necessary conditions for the use of rays
are: the wavefront must not change its direction materially over a distance of
one wavelength; the amplitude of the sound pressure must not change materially
over the same distance; finally the wavelength itself (in the case of
transmission through a nonhomogeneous medium) must change very little over a
distance of its own order of magnitude.

    Reflection and refraction.	 When a sound wave strikes a surface
separating media of different density and elasticity, reflection and
refraction can take place. The laws governing these phenomena are the same as
those for light.  See  REFLECTION OF SOUND; REFRACTION OF WAVES.

    Diffraction and scattering.   Like light waves, sound waves bend around
obstacles, that is, experience diffraction. Indeed, sound can be heard around
a corner on account of diffraction, which in the case of audible sound is a
much more obvious phenomenon than the diffraction of visible light because the
wavelength of audible sound is much greater than that of visible light. The
bending of sound waves due to diffraction becomes less marked as the
wavelength decreases relative to the spatial dimensions of the diffracting
obstacles. With ultrasonic radiation it is possible to produce sound shadows.
Another common name for sound diffraction is scattering, particularly if many
obstacles are involved.  See  DIFFRACTION; SCATTERING OF ELECTROMAGNETIC
RADIATION.

    Transducers.   Sound sources are referred to as transducers, since they
transfer energy of various forms into sound. The most important source of
sound in human experience is the human voice, which in ordinary conversation
and singing produces power outputs of from 30 to 60 microwatts. Other
important sources are mechanically vibrating solids, liquids, and gases. Solid
vibrators include strings, rods, plates, and membranes (as in musical
instruments). The organ pipe is an example of a sound source based on
vibrating air. The turbulent flow of air in the exhaust of a jet plane
produces intense sound. The break of the surf on the shore is a liquid source
of sound.  See	ACOUSTIC NOISE; VIBRATION.
    More sophisticated sound sources depend on the ability to produce
mechanical vibrations by means of magnetic and electrical effects. The
ordinary telephone receiver and the radio loudspeaker are illustrations. Still
more elaborate and valuable for many applications are piezoelectric and
magnetostrictive oscillators. These electroacoustic transducers have found
wide use as both sources and receivers, for example, in underwater sound, in
sound recording and reproduction, in radio and television, and in the
industrial applications of sound.  See	DISK RECORDING; EARPHONES;
LOUDSPEAKER; MAGNETIC RECORDING; MAGNETOSTRICTION; MICROPHONE; PIEZOELECTRIC
CRYSTAL; PIEZOELECTRICITY; SONAR; SOUND RECORDING; SOUND-REPRODUCING SYSTEMS;
STEREOPHONIC SOUND; TELEPHONE; TRANSDUCER; UNDERWATER TRANSDUCER. See also
ATMOSPHERIC ACOUSTICS; BIOACOUSTICS.

    Attenuation of sound.   When a sound wave from a given source passes
through any actual physical medium, its intensity decreases with distance from
the source. This is known as attenuation. Part of this is due to the
geometrical spreading of the wavefront, unless it is confined in a tube or
duct. The rest of the attenuation is due to the influence of the medium and
the surfaces over which or through which the sound passes. Reflection,
refraction, and scattering can all serve to increase the effective attenuation
of sound propagation in a given direction. The medium itself produces
attenuation by changing a part of the acoustic energy into heat, a kind of
attenuation known as sound absorption. Mechanisms responsible for this are
viscosity, heat conduction, and molecular relaxation processes.  See  SOUND
ABSORPTION.
 �34s[R. Bruce Lindsay]
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�6v�2v�32sSound absorption   �21m �33s 
    The process by which the intensity of sound is diminished due to the 
conversion of the energy of the sound wave into heat. The absorption of sound 
is a special but important case of sound attenuation.  See  SOUND. 
    In actual material media the geometrical attenuation is supplemented by 
absorption due to the interaction between the sound wave and the physical 
properties of the medium. This dissipates the sound energy by transforming it 
into heat and hence decreases the intensity of the wave. In all practical 
cases the attenuation due to such absorption is exponential in character. 
    The four so-called classical origins of dissipative sound absorption in 
material media are: shear viscosity, heat conduction, heat radiation, and 
diffusion. In the flow of a fluid, viscosity represents the tendency for the 
fluid flowing in one layer parallel to the direction of motion to retard the 
motion in adjoining layers. The role of heat conduction can be qualitatively 
understood as follows. The compression associated with the propagation of a 
sound wave in a fluid produces a local increase in temperature, which sets up 
a temperature gradient through which heat will flow by conduction before the 
succeeding rarefaction cools the gas. This is an irreversible phenomenon, 
leading to energy dissipation into heat. For ordinary gases in which the 
components differ little in molecular weight, the effect of diffusion on 
absorption is negligible compared with that of viscosity and heat conduction. 
 �34s[R. Bruce Lindsay]
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�6v�2v�32sSound pressure �21m �33s
    The incremental variation in the static pressure of a medium when a sound
wave is propagated through it. This incremental variation is also known as the
excess pressure.
    The unit of pressure commonly used in acoustics is the microbar (= 0.1
newton/m(sup 2) = 1 dyne/cm(sup 2)). One microbar is approximately 10(sup -6)
times the normal atmospheric pressure.
    Instantaneous sound pressure at a point is the incremental change from the
static pressure of the medium at a given instant caused by the presence of a
sound wave.
    Peak sound pressure for any specified time interval is the maximum
absolute value of the instantaneous sound pressure in that interval.
    Effective sound pressure at a point is the root-mean-square (rms) value of
the instantaneous sound pressure over a time interval at that point. The time
interval should be an integral number of periods if the sound pressure is
periodic. In the case of nonperiodic sound pressures, the interval should be
long enough to make the value obtained essentially independent of small
changes in the length of the interval.
 �34s[William J. Galloway]
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�6v�2v�32sSound recording   �21m �33s 
    The technique of entering sound, especially music, on a storage medium for 
playback at a subsequent time. The storage medium most widely used is magnetic 
tape.  See  MAGNETIC RECORDING. 
  
    Monophonic.   In the simplest form of sound recording, a single microphone 
picks up the composite sound of a musical ensemble, and the microphone's 
output is recorded on a reel of 1/4-in. (0.63-cm) magnetic tape. This 
single-track, or monophonic, recording suffers from a lack of dimension, since 
in playback the entire ensemble will be heard from a single point source: the 
loudspeaker.  
  
    Stereophonic.  An improved recording, with left-to-right perspective and 
an illusion of depth, may be realized by using two microphones which are 
spaced or angled appropriately and whose outputs are routed to separate tracks 
on the tape recorder. These two tracks are played back over two loudspeakers, 
one each on the listener's left and right. Under ideal conditions this 
stereophonic system will produce an impressive simulation of the actual 
ensemble.  See  STEREOPHONIC SOUND. 
  
    Binaural.   In this system, two microphones are placed on either side of a 
small acoustic baffle in an effort to duplicate the human listening condition. 
The recording is played back over headphones, so that the microphones are, in 
effect, an extension of the listener's hearing mechanism. 
  
    Multitrack.   To give the recording engineer more technical control over 
the recording medium, many recordings are now made using a multiple-microphone 
technique. In place of the stereo microphone, one or more microphones are 
located close to each instrument or group of instruments.  See  MICROPHONE. 
    In the control room the engineer mixes the outputs of all microphones to 
achieve the desired musical balance. As a logical extension of this technique, 
the microphone outputs may not be mixed at the time of the recording, but may 
be routed to 16 or more tracks on a tape recorder, for mixing at a later date. 
    When many microphones are so used, each instrument in the ensemble is 
recorded - for all practical purposes - simultaneously. The complex time delay 
and acoustic relationships within the room are lost in the recording process, 
and the listener hears the entire ensemble from one perspective only, as 
though he or she were as close to each instrument as is each microphone. 
Electronic signal processing devices may not be entirely successful in 
restoring the missing information, and the listener hears a recording that may 
be technically well executed, yet lacking the apparent depth and musical 
cohesiveness of the original. However, the multitrack technique becomes 
advantageous when it is impractical or impossible to record the entire 
ensemble at once stereophonically.  See  SOUND-REPRODUCING SYSTEMS. 
 �34s[John M. Woram]
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�6v�2v�32sSound-reinforcement system   �21m �33s 
    A sound-reinforcement system (public address system) is an electronic 
means for augmenting the sound output of a speaker, singer or musical 
instrument. The use of a sound-reinforcement system makes it possible to 
render speech intelligible where it was either too weak to be heard above the 
general noise or too reverberant. In certain instances, for example, in large 
halls or outdoors, the volume range of the orchestra at the listener's 
location is inadequate for full artistic appeal and the use of a 
sound-reinforcement system is required to augment the original sound. The 
basic elements of a sound-reinforcement system are microphones, amplifiers, 
volume controls, and loudspeakers.  See  AMPLIFIER; LOUDSPEAKER; MICROPHONE. 
    Sound-reinforcement systems for auditorium-sized rooms may use a single 
horn loudspeaker located at the ceiling above a stage; direct-radiator 
loudspeakers placed on the walls; or two column loudspeakers at each side of 
the stage. The directivity pattern of the microphone should be such that the 
region of least response is directed toward the loudspeakers and the audience. 
Sound-reinforcement systems for outdoor theaters may employ a single 
loudspeaker station located above the stage and directed into the audience 
area, or a number of loudspeakers, each supplying a portion of the audience 
area with sound. 
 �34s[Harry F. Olson]
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�6v�2v�32sSound-reproducing systems   �21m �33s 
    Units which pick up sound at one point and reproduce it at the same point 
or some other point at the same time or some subsequent time. 
Sound-reproducing systems may be classified as monaural, binaural, monophonic, 
stereophonic, or quadraphonic. 
    The purpose of the several types of sound-reproducing systems is to 
provide esthetic and pragmatic communication - namely, music and speech. A 
major requirement is that the sound-reproducing system have the capabilities 
for controlling and processing the audio signal so as to supply the sound 
characteristics that the listener feels is appropriate for the program. The 
impact of the music thus depends both upon engineering and subjective factors. 
Engineering factors include, in approximate order of importance, distortion 
that is low or appropriate; smoothness of response with frequency; full 
frequency range of response; low system internal noise; ability to produce 
sound levels at least 50 decibels (dB) above the background noise in the 
listening space; proper amount of auditory perspective; and appropriately 
coordinated room acoustics in originating and listening spaces. 
  
    Monaural system.  A monaural sound-reproducing system utilizes one or more 
microphones connected to a single transducing channel which, in turn, is 
coupled to one or two earphones worn by the listener. The most common example 
is the telephone in which there is, in general, a single source of sound, one 
microphone, a transducer, and one earphone coupled to the ear of the listener. 
  See  EARPHONES; MICROPHONE; TELEPHONE. 
  
    Binaural system.  A binaural sound-reproducing system is a closed-circuit 
type of sound-reproducing system in which two microphones, used to pick up the 
original sound, are each connected to two independent corresponding 
transducing channels which, in turn, are coupled to two independent 
corresponding earphones worn by the listener. Use of the binaural 
sound-reproducing system is limited to specific  applications, as in 
conference or court recording. The microphones are mounted in a dummy 
simulating the human head in shape and dimensions and at the locations 
corresponding to the ears of the human head.  See  BINAURAL SOUND SYSTEM. 
  
    Monophonic system.  A monophonic sound-reproducing system employs one or 
more microphones, used to pick up the original sound, which are coupled to a 
single transducing channel which, in turn, is coupled to one or more 
loudspeakers in reproduction. The monophonic sound-reproducing system is the 
most widely employed of all sound-reproducing systems. Examples are the disk 
phonograph, radio, sound motion picture, television, and magnetic tape 
reproducer sound systems. The monophonic sound-reproducing system is of the 
field type, in which the sound is picked up by a microphone in a field and 
reproduced by means of a loudspeaker into a field.  See  LOUDSPEAKER. 
                                                                               
  
  
    Stereophonic system.  A stereophonic sound-reproducing system is a 
field-type sound-reproducing system in which two or more microphones, used to 
pick up the original sound, are each coupled to a corresponding number of 
independent transducing channels which, in turn, are each coupled to a 
corresponding number of loudspeakers arranged in substantial geometrical 
correspondence to that of the microphones. Each ear of the listener receives 
sound from all sources. The relative amounts depend on the acoustics of the 
space of origination, the microphone array used, the mix-down process, the 
acoustics of the listening space, the placement and directionality of the 
loudspeakers, and the location and orientation of the listener.  See  
STEREOPHONIC SOUND. 
  
    Quadraphonic system.  A quadraphonic sound-reproducing system is a 
field-type system in which each of the four microphones used to pick up the 
original sound is coupled to four transducing channels which, in turn, are 
coupled to four loudspeakers arranged in substantial geometrical 
correspondence to the arrangement of the microphones.  See  QUADRAPHONIC SOUND 
SYSTEM.  
    High-quality sound-reproducing systems can reproduce the sounds of speech 
and music without frequency discrimination. Furthermore, these 
sound-reproducing systems cover the frequency range of normal hearing. These 
considerations show that high-quality sound-reproducing systems provide the 
capabilities of reproducing the entire audio frequency range. High-grade 
sound-reproducing systems also provide the capabilities of covering the 
amplitude range of sound reproduction in the home and can be built so that 
harmonic distortion is not perceptible. 
    The sounds of speech and music vary continuously in frequency and 
amplitude. Therefore, in order to reproduce these sounds the transient 
response of the reproducing system must exhibit a high order of excellence. 
The response of a part or the whole of a sound-reproducing system to a sudden 
change in the amplitude or frequency is a measure of the transient response. 
In general, transient response is good if system frequency response (including 
source and listening spaces) is smooth, with no significant, sudden peaks or 
dips. These deviations from a flat response are usually due to resonances, 
often in the loudspeaker. The effects are most marked at the extremes of the 
frequency range. 
 �34s[Vincent Salmon]
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�6v�2v�32sSource flow   �21m �33s 
    A source, in three-dimensional flow, is a point from which fluid issues at 
a uniform rate in all directions. The strength of a source is defined as the 
volume per unit time issuing from the point. Because the flow is outward and 
is uniform in all directions, velocity vsubr at distance r from the source is 
the strength m divided by the area of sphere through the point with center at 
the source, or vsubr = m/4PIrsup2. 
    A sink is a negative source, or a point into which fluid flows uniformly 
in all directions. A well point is the physical equivalent of a sink. It is a 
relatively small region in a porous medium into which fluid flows uniformly 
from all directions. Similarly, if fluid were injected into the porous medium 
through the well point, it would be equivalent to a source flow. 
    In two-dimensional flow, all flow occurs in a set of parallel planes, with 
no flow normal to them. The flow is identical in each of these parallel planes. 
    A source, in two-dimensional flow, is a line normal to the planes of flow, 
from which fluid is imagined to flow uniformly in all directions at right 
angles to the line. The source appears as a point on the customary 
two-dimensional flow diagram. 
    Sources and sinks are used as flow elements in conjunction with doublets, 
vortices, and uniform flow to develop complex flow situations. The combination 
of a source, an equal sink, and a uniform flow, properly placed, results in 
flow about a closed body in three-dimensional flow and about a cylinder in 
two-dimensional flow. The source and sink concepts are generally useful for 
frictionless flow situations, and have little meaning in frictional 
situations, such as flow in a pipe.  See  SINK FLOW; STREAMLINE FLOW. 
 �34s[Victor L. Streeter]
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�6v�2v�32sSourwood �21m �33s
    A deciduous tree, Oxydendrum arboreum, of the heath family, indigenous to
the southeastern section of the United States, and found from Pennsylvania to
Florida and west to Indiana and Louisiana. It is usually a small or
medium-sized tree. The wood is not used commercially. Sourwood is also known
as sorrel tree, and it is widely planted as an ornamental.  See  ERICALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
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�6v�2v�32sSouth African tick-bite fever   �21m �33s 
    An infectious tick-borne disease of humans which is similar to fievre 
boutonneuse but has some biological and ecological differences. A similar, or 
the same, disease occurs in East Africa, where it is known as Kenya tick 
typhus. The chief vector in Kenya is the dog tick (Haemaphysalis leachi), and 
in South Africa it is this vector plus Rhipicephalus evertsi.  See  FIEVRE 
BOUTONNEUSE; RICKETTSIOSES. 
 �34s[Cornelius B. Philip]
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�6v�2v�32sSouth America   �21m �33s 
    The southernmost of the New World or Western Hemisphere continents, 
three-fourths of which lies within the tropics. South America is approximately 
4500 mi (7200 km) long and at its greatest expanse 3000 mi (4800 km) wide. Its 
total area is estimated to be about 7,000,000 misup2 (18,000,000 kmsup2), more 
than two-thirds of which consists of plains. The land portions stand upon a 
continental platform somewhat larger than the land above sea level so that 
varying widths of shallow sea extend over the continental shelf to the steep 
scarp of the submarine continental slopes.  See  CONTINENT. 
    At least three-fourths of the South American coastline is composed of 
highland masses; consequently, the areas of maximum elevation on the continent 
generally lie not far from the ocean. This topography has an important 
influence on the continental climate and causes drainage of streams to be 
toward the interior more often than is true of any other continent. Moreover, 
there is a paucity of large independent rivers, for they have little chance of 
developing when the coasts throughout the well-watered parts are guarded by 
mountains and plateaus that overlook the seas, forcing the bulk of the 
drainage to find an ultimate outlet at a small number of well-marked gaps. 
Thus the Andes, the Brazilian and Guiana Highlands, and to some extent the 
Patagonian Plateau have determined the present form and configuration of the 
entire South American land mass. 
    Because of the vast extent of the Andes, a greater proportion of South 
America than of any other continent lies above 10,000 ft (3000 m). Throughout 
the Andean mountain mass there are high plateaus and deep longitudinal 
valleys, especially in the middle and northern parts. Although in general the 
Andes resulted from folding and faulting, volcanic formations are common in 
southern Colombia, Ecuador, central and southern Peru, and western Bolivia. 
Possibly nowhere else on Earth is there a more spectacular series of volcanic 
peaks than in western Bolivia and on each side of the structural depression in 
Ecuador.  
    The broadest lowland routes to the interior are gapways between the three 
great barriers separating coastal from interior South America: the Andes 
cordilleran belt, the Guiana Highlands, and the Brazilian Highlands. The 
breach between the Andes and the Guiana Highlands is some 300 mi (480 km) 
wide. The western part, near the Andes, is composed of rolling and irregular 
plains. The climate of the gap is warm to hot at all times. Rainforest 
vegetation covers the southern part, and tropical savanna and semideciduous 
forest predominate in sections of the northern region. The low-relief plain 
occurs between the hilly margins of the Brazilian Highlands and the Guiana 
Highlands. Through it courses the Amazon River. The climate is monotonously 
warm to hot. Vegetation is evergreen tropical rainforest along rivers and in 
low sections; semideciduous forest and areas of savanna are found on 
interstream uplands and the bordering hillier country. 
2556A q6    There are three great river systems in South America and a number 
of important rivers which are not a part of these systems. First of the river 
systems in size is the Amazon which, with its many tributaries, drains a basin 
covering 2,700,000 misup2 (7,000,000 kmsup2), or about 40% of the continent. 
Next is the system composed of the Paraguay, Parana, and La Plata rivers, the 
last being a huge estuary. The third river complex, located in southern 
Venezuela, is the Orinoco, which drains water from 365,000 misup2 (945,000 
kmsup2) of land, emptying into the Atlantic Ocean along the northeast edge of 
the continent. The major independent rivers are the Sao Francisco, arising on 
the Brazilian plateau, and the Magdalena-Cauca, which empties into the 
Caribbean.  
    South America is unique in having a west-coast desert that extends almost 
to the Equator, an east-coast desert poleward from latitude 40DG (the 
Patagonian), and a desert probably receiving less rain than any on Earth (the 
Atacama). Three major islands or island groups lie off the South American 
coast. Trinidad, in the north, contains a range of mountains exceeding at 
points 3000 ft (900 m) elevation. The Falklands stand on the shallow 
continental shelf off the rocky coast of Patagonia, near the southern tip of 
the continent. On the Equator and about 550 mi (885 km) west of Ecuador are 
the Galapagos, a group of 13 volcanic islands. 
 �34s[C. Langdon White]
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�6v�2v�32sSoutheast Asian waters �21m �33s
    All the seas between Asia and Australia and the Pacific and the Indian
oceans. They form a geographical and oceanographical unit because of their
special structure and position, and make up an area of 3,450,000 misup2
(8,940,000 kmsup2), or about 2.5% of the surface of all oceans.
    The surface circulation is completely reversed twice a year by the
changing monsoon winds. The subsurface circulation carries chiefly the
outrunners of the intermediate waters of the Pacific Ocean into these seas.
The tides are mostly of the mixed type. Diurnal tides are found in the Java
Sea, in the Gulf of Tonkin, and in the Gulf of Thailand. Semidiurnal tides
with high amplitudes occur in the Malacca Straits.  See  INDIAN OCEAN; PACIFIC
OCEAN.
 �34s[Klaus Wyrtki]
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    The Southeast Asian seas are characterized by the presence of numerous
major plate boundaries. In Southeast Asia the plate boundaries are identified,
respectively, by young, small ocean basins with their spreading systems and
associated high heat flow; deep-sea trenches and their associated earthquake
zones and volcanic chains; and major strike-slip faults such as the Philippine
Fault (similar to the San Andreas Fault in California). The Southeast Asian
seas are thus composed of a mosaic of about 10 small ocean basins whose
boundaries are defined mainly by trenches and volcanic arcs. The dimensions of
these basins are much smaller than the basins of the major oceans. The major
topographic features of the region are believed to represent the surface
expression of plate interactions, the scars left behind on the sea floor.  See
PLATE TECTONICS.
 �34s[Dennis E. Hayes]
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�6v�2v�32sSouth Pole   �21m �33s 
    That end of the Earth's rotational axis opposite the North Pole. It is the 
southernmost point on the Earth and one of the two points through which all 
meridians pass (the North Pole being the other point). This is the geographic 
pole and is not the same as the south magnetic pole. The South Pole lies 
inland from the Ross Sea, within the land mass of Antarctica, at an elevation 
of about 9200 ft (2800 m). 
    There is no natural way to determine local Sun time because there is no 
noon position of the Sun, and shadows point north at all times, there being no 
other direction from the South Pole.  See  MATHEMATICAL GEOGRAPHY; NORTH POLE. 
 �34s[Van H. English]
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�6v�2v�32sSoybean   �21m �33s 
    Glycine max, a legume native to China that has become a major source of 
vegetable protein and oil for human and animal consumption and for industrial 
usage. The valued portion of the plant is the seed, which contains about 40% 
protein and 21% oil. Illinois, Iowa, Arkansas, Missouri, Indiana, Mississippi, 
Minnesota, Ohio, Louisiana, and Tennessee are the major soybean producers in 
the United States.  See  FAT AND OIL (FOOD). 
 �34s[Walter R. Fehr]
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�6v�2v�32sSpace   �21m �33s 
    Physically, space is that property of the universe associated with 
extension in three mutually perpendicular directions. Space, from a newtonian 
point of view, may contain matter, but space exists apart from matter. Through 
usage, the term space has come to mean generally outer space or the region 
beyond Earth. Geophysically, space is that portion of the universe beyond the 
immediate influence of Earth and its atmosphere. From the point of view of 
flight, space is that region in which a vehicle cannot obtain oxygen for its 
engines or rely upon an atmospheric gas for support (either by buoyancy or by 
aerodynamic effects). Astronomically, space is a part of the space-time 
continuum by which all events are uniquely located. Relativistically, the 
space and time variables of uniformly moving (inertial) reference systems are 
connected by the Lorentz transformations. Gravitationally, one characteristic 
of space is that all bodies undergo the same acceleration in a gravitational 
field and therefore that inertial forces are equivalent to gravitational 
forces. Perceptually, space is sensed indirectly by the objects and events 
within it. Thus, a survey of space is more a survey of its contents.  See  
EUCLIDEAN GEOMETRY; LORENTZ TRANSFORMATIONS; RELATIVITY; SPACE-TIME. 
 �34s[S. F. Singer]
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�6v�2v�32sSpace biology   �21m �33s 
    The inclusive term for the various biological sciences concerned with the 
study of living things in the space environment. Medical studies that are 
intrinsically linked to the purposes of biology are termed biomedical; and 
exobiology is the search for and study of extraterrestrial life. Areas of 
space biology can also be identified by the inherent physical factors of space 
and space travel and their effects on living organisms: (1) biophysics - 
factors due to the environment of space, such as temperature, pressure, 
radiation, and weightlessness; (2) biodynamics - factors due to the flight 
dynamics of the spacecraft, including acceleration, vibration, and noise; and 
(3) aerospace medicine - factors created by the involvement of human beings in 
the artificial environment of the spacecraft, including breathing gases, 
nutrition, toxicology, and isolation.  See  AEROSPACE MEDICINE; EXOBIOLOGY. 
 �34s[Thora W. Halstead]
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�6v�2v�32sSpace charge   �21m �33s 
    The net electric charge within a given volume. If both positive and 
negative charges are present, the space charge represents the excess of the 
total positive charge diffused through the volume in question over the total 
negative charge. 
 �34s[Edward G. Ramberg]
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�6v�2v�32sSpace communications   �21m �33s 
    Communication between a vehicle in outer space and the Earth, using 
high-frequency electromagnetic radiation. Provision for such communication is 
an essential requirement in any space mission. The total communication system 
ordinarily includes (1) transmission of command signals to the spacecraft; (2) 
radio equipment for tracking by ground stations; and (3) transmission of 
scientific and applications data from the spacecraft to the Earth, sometimes 
singled out as telemetry. For space probes and satellites generally, telemetry 
is a key element for the success of the mission. A somewhat special 
communication system is the communications satellite system in which the 
spacecraft serves solely as a relay station between remote points on the 
Earth, and is here excluded from consideration.  See  COMMUNICATIONS 
SATELLITE; SATELLITE (SPACECRAFT); SPACE FLIGHT; SPACE PROBE. 
    Certain characteristic constraints distinguish space communication systems 
from their terrestrial counterparts. Although only line-of-sight propagation 
is required, both the transmitter and receiver are in motion. The movement of 
spacecraft in relation to the Earth, for instance, makes it necessary to 
provide geographically scattered stations for adequate continuous coverage. 
    Because enormously greater distances are involved (millions of kilometers 
in the case of space probes to Mars, Venus, and beyond), the signal received 
on Earth is so small that local interference, both artificial and natural, has 
to be drastically reduced. For this purpose, the transmitting frequency has to 
be made sufficiently high, and highly sensitive receivers (large antennas and 
special low-noise amplifiers such as masers) are used which eliminate all but 
the unavoidable emanations from space (galactic noise). For deep-space 
missions, large steerable antennas, such as at Goldstone, California (see 
illustration), are located as far from civilization as practical. 
Sophisticated data processing is required, and the ground-receiver complex 
includes several large, high-speed digital computers and associated processing 
equipment.  See  MASER; RADIO RECEIVER; RADIO TELESCOPE. 
    The spacecraft installations for communication are constrained by weight 
and space considerations, since there are practical limitations to these 
spacecraft parameters. An additional concern is reliability, since the 
equipment must operate, sometimes for years, unattended under the extremes of 
the space environment. To increase reliability, highly reliable components and 
equipment are developed, and redundancy is used, where practical, to eliminate 
single-point system failures. In some applications the spacecraft antenna must 
point toward Earth stations for acceptable transmission of data; hence angular 
pointing accuracies of 1DG or less are often required. Spacecraft power is 
always at a premium, and techniques must be employed to minimize the 
consumption by the communications system. The transmitter is usually a major 
consumer of power, and efforts must be made to increase the transmitter 
efficiency to reduce total power requirements. All aspects of data 
transmission must meet much closer tolerances than are common on terrestrial 
systems.  
    The Tracking and Data Relay Satellite System (TDRSS) consists of two 
geostationary relay satellites 130DG apart in longitude, and a ground terminal 
located at White Sands, New Mexico. The purpose of TDRSS is to provide 
telecommunication services between low-Earth-orbiting user spacecraft and user 
control centers. A principal advantage of the system is the elimination of 
many of the worldwide ground stations for tracking low-orbiting spacecraft. 
 �34s[David S. Hepler]
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�6v�2v�32sSpacecraft ground instrumentation   �21m �33s 
    Instrumentation located on the Earth for monitoring, tracking, and 
communicating with crewed spacecraft, satellites, and space probes. Radars, 
communication antennas, and optical instruments are classified as ground 
instrumentation. They are deployed in networks and, to a lesser extent, in 
ranges. Ranges are relatively narrow chains of ground instruments used to 
follow the flights of missiles, sounding rockets, and spacecraft ascending to 
orbit. Some ranges are a few miles long; others, such as the U.S. Air Force's 
Eastern Test Range, stretch for thousands of miles. Networks, in contrast, are 
dispersed over wide geographical areas so that their instruments can follow 
satellites in orbit as the Earth rotates under them at 15DG per hour, or space 
probes on their flights through deep space. 
    Networks are of two basic kinds: networks supporting satellites in Earth 
orbit, and networks supporting spacecraft in deep space far from Earth. A 
third concept will be added in the 1980s, when a series of telecommunications 
and data relay satellites (TDRS) will be operational to replace most of the 
network of the Earth-orbit stations. The TDRSs will be placed in 
geosynchronous orbits to relay signals to and from other orbiting spacecraft 
during more than 85% of each orbit, to and from a single ground station. 
    Ranges and networks have various technical functions: 
    1. Tracking: determination of the positions and velocities of space probes 
and satellites through radio and optical means. 
    2. Telemetry and acquisition: reception of telemetered signals from 
scientific instruments and spacecraft housekeeping functions.  See  
TELEMETERING.  
    3. Voice reception and transmission: provision for communication with the 
crew of a spacecraft, such as the space shuttle. 
    4. Command: transmission of coded commands to spacecraft equipment, 
including scientific instruments. 
    5. Television reception: provision for observation of the crew, spacecraft 
environment, and so on. 
    6. Ground communication: telemetry, voice, television, command, tracking 
data, and spacecraft acquisition data transmission between network sites and 
the central mission control center. 
    7. Computing: calculation of orbital elements and radar acquisition data 
prior to transmission to a central point; also, computation of the signals 
that drive visual displays at a mission control center.  See  SPACE 
COMMUNICATIONS; SPACE NAVIGATION AND GUIDANCE. 
 �34s[Tecwyn Roberts]
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�6v�2v�32sSpacecraft propulsion   �21m �33s 
    The movement of a space vehicle in space, from a planetary surface to 
space, or from space to a planetary surface. A number of spacecraft propulsion 
systems have been devised. Different spacecraft propulsion systems may exhibit 
considerable variation in such parameters as thrust, specific impulse, 
relative cost, state of development, and safety. All these variables must be 
considered when choosing a space propulsion system for a given mission. The 
more conventional propulsion systems are discussed in INTERPLANETARY 
PROPULSION.  See also  ASTRONAUTICAL ENGINEERING; SATELLITE (SPACECRAFT); 
SPACE FLIGHT; SPACE PROBE; SPECIFIC IMPULSE; THRUST. 
  
    Rocket engines.  The conventional chemical rocket imparts forward thrust 
to a spacecraft from the momentum of matter ejected as the result of a 
chemical reaction. A single propellant or a combination of propellants, which 
may be either liquid or solid, may be used. For a full discussion of 
conventional rocket engines  see  PROPELLANT; PROPULSION; ROCKET; ROCKET 
ENGINE; ROCKET STAGING; SPACECRAFT STRUCTURE. 
    Electrostatic propulsion may be accomplished by a high-speed beam of ions 
used to provide thrust to a spacecraft. In an electromagnetic propulsion 
engine the spacecraft obtains motive power by means of a high-speed discharge 
of a plasma fluid. At present, such engines are only in the experimental 
stage.  See  ELECTROMAGNETIC PROPULSION; ION PROPULSION; PLASMA PHYSICS. 
    Nuclear rockets use nuclear fission or fusion to heat a fluid for 
propulsive purposes. An experimental nuclear rocket engine, the Nerva, has 
undergone testing. 
  
    Solar engines.  Solar energy may possibly be used for spacecraft 
propulsion, either directly by photon pressure on huge solar sails or 
indirectly from solar cells or from a reflector-boiler combination used to 
heat a fluid. The useful application of solar energy apparently is restricted 
to the inner solar system.  See  SOLAR ENERGY. 
    One way to use solar energy in space is to collect it in a focal area, 
occupied by a boiler, to heat a working fluid. This fluid can be expelled to 
produce thrust directly, a technique called solar heat exchanger drive (SHED), 
or used to drive a turbine-generator system producing electrical energy: solar 
turboelectric drive (STED). 
  
    Nuclear pulse propulsion.  The nuclear pulse (NP) drive would use nuclear 
charges, pulse units (PU), to provide thrust. The energy released by the 
nuclear fuel in the PU is transferred to a bulk mass (propellant), turning it 
into a plasma. As the plasma is hurled toward the vehicle, part or all of it 
is intercepted at the vehicle base by an elastic, massive, and heat-resistant 
impulse absorber. The impulse absorber is designed to transfer the momentum of 
the plasma to the vehicle in a nondestructive manner, thereby driving the 
vehicle forward, like a gust of wind propelling a sailboat. The 
momentum-transfer process at the impulse-absorber level is violent, but it can 
be cushioned to tolerable levels at the payload level. The difference is 
comparable to that between the impact an automobile tire receives when hitting 
a hole in the road and its gentle reflection at the passenger level due to 
multiple shock absorption and cushioning. 
  
    Fusion propulsion.  Fusion propulsion is the ultimate in nuclear 
propulsion short of complete eradication of matter, as in a matter-antimatter 
reactor. The heart of the hypothetical fusion propulsion system is the 
controlled thermonuclear reactor (CTR). The CTR is based on the production of 
power from exothermal nuclear fusion reactions. Essentially, three reaction 
types can be considered: deuterium-deuterium, deuterium-tritium, and 
(helium-3)-deuterium reactions. The first two reactions release large amounts 
of energy in the form of neutrons. Neutrons penetrate magnetic fields and, 
therefore, cannot be contained within a fusion reaction chamber. A fusion 
reaction in which a major amount of released energy is deposited in neutrons 
is unsuitable for steady-state operation (in contrast to nonsteady pulse 
operation), because the outflux of neutrons constitutes an excessive energy 
leakage. Moreover, a strong neutron-producing fusion drive would require heavy 
shielding for crew and sensitive equipment. For this reason, the CTR system is 
based on the thermonuclear reaction of deuterium with a helium-3 isotope to 
produce helium-4 and protons. 
 �34s[Krafft A. Ehricke]
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�6v�2v�32sSpacecraft structure   �21m �33s 
    The supporting structure for systems capable of leaving the Earth and its 
atmosphere, performing a useful mission in space or around the Moon or the 
Earth, Sun, and other planets, sometimes returning to the Earth and sometimes 
landing on the bodies. Among the principal technologies that enter into the 
design of spacecraft structures are aerodynamics, aerothermodynamics, heat 
transfer, structural mechanics, dynamics, materials technology, and systems 
analysis. In applying these technologies to the structural design of a 
spacecraft, the significant factors influencing the design are analyzed and 
trade studies are made to arrive at a design which fulfills system 
requirements at a minimum weight with acceptable reliability, and is capable 
of being realized in a reasonable period of time. 
    The structural aspects of space flight can be divided into six broad 
regions or phases: (1) transportation, handling, and storage; (2) testing; (3) 
boosting; (4) Earth-orbiting; (5) reentry, landing, and recovery on the Earth; 
(6) interplanetary flight, with orbiting of or landing on other planets. Each 
phase has its own structural design criteria based upon the specific 
environment, the configuration existing during the phase, the mission 
objectives, and whether the craft is crewed or uncrewed. These design criteria 
require detailed consideration of heat, static loads, dynamic loads, rigidity, 
vacuum effects, radiation, meteoroids, acoustical loads, atmospheric pressure 
loads, foreign atmospheric composition, solar pressure, fabrication 
techniques, magnetic forces, sterilization requirements, accessibility for 
repair, and interrelation of one effect with the others. 
    The basic spacecraft structural design considerations apply equally well 
to both crewed and uncrewed spacecraft. The degree of reliability of the 
design required is, however, much greater for crewed missions. Also, the 
spacecraft structures in the case of crewed missions must include life-support 
systems, and reentry and recovery provisions. In the case of lunar or 
planetary missions where landing and leaving the foreign body is required, 
additional provisions for propulsion, guidance, control, spacecraft 
sterilization, and life-support systems must be realized and the structure 
must be designed to accommodate them. 
  
    Testing.  To ensure that spacecraft structures will meet mission 
requirements criteria in general requires testing to levels above the expected 
environmental conditions by a specific value. The test level must be set to 
provide for variations in materials, manufacture, and anticipated loads. In 
cases where structures are required to perform dynamic functions repeatedly, 
life testing is required to ensure proper operation over a given number of 
cycles of operation.  See  INSPECTION AND TESTING. 
  
    Boost.  The purpose of the boost phase is to lift the vehicle above the 
sensible atmosphere, to accelerate the vehicle to the velocity required, and 
to place the spacecraft at a point in space, heading in the direction required 
for the accomplishment of its mission. For space missions, the required 
velocities range from 26,000 ft/s (8 km/s) for nearly circular orbits to 
36,000 ft/s (11 km/s) for lunar or interplanetary missions. Achievement of 
these velocities requires boosters many times the size of the spacecraft 
itself. Generally, this boosting is accomplished by a chemically powered 
rocket propulsion system using liquid or solid propellants. Multiple stages 
are required to reach the velocities for space missions. Vertical launching 
from a surface stand is the method generally employed to boost a spacecraft on 
the mission trajectory. 
  
    Space phase.  The space phase begins after the boost phase and continues 
until reentry. In contrast with the boost phase, effects are repeated numerous 
times in various combinations. The spacecraft structure must be designed for 
the most severe combination of effects to be encountered during the entire 
mission.  
    The primary function of the structure in a spacecraft is to provide an 
enclosure containing an environment satisfactorily conditioned for the proper 
operation of the equipment and crew. This environment generally consists of 
active or passive temperature control, energetic particle radiation 
protection, an artificial atmosphere and other life-support systems, and, for 
the future spaceceaft and space stations, an artificial gravity.  See  SPACE 
FLIGHT; SPACE STATION. 
  
    Design considerations.  Spacecraft structures must accommodate numerous 
design considerations. The most significant loads that must be considered are 
propulsion, control system, pressure vessel loads, and reentry loads. Other 
subtle loads to be considered are gravity gradient loads, atmospheric drag in 
orbit, solar radiation pressure, meteoric particles, energetic particles, 
thermal extremes and thermal temperature gradients, inertial loads, sloshing, 
and acceleration loads.  See  SATELLITE (SPACECRAFT); SPACE PROBE. 
    Propulsion loads vary widely among spacecraft; however, the general loads 
to be considered are the same. These are shock, acceleration, deceleration, 
vibration, torsion, sloshing, pressure vibrations, and temperature. 
    The spacecraft control system imparts inertial loads throughout the 
structure. In the zero gravity environment every change in loading or 
orientation must be reacted through the structure. 
    Spacecraft structural design usually requires that part of the principal 
structure be a pressure vessel. Efficient pressure vessel design is therefore 
imperative. An important material property, especially in pressure vessel 
design, is notch sensitivity, referring to the material's apparent brittleness 
under biaxial strain. This apparent brittleness contributed to premature 
failure of some early boosters.  See  PRESSURE VESSEL. 
    A principal problem in the design of spacecraft arises from the presence 
of meteoric particles in space. These particles may have extremely high 
velocities relative to the spacecraft (up to 225,000 ft/s or 68 km/s). 
Probability of collision with larger particles is extremely low, but 
small-particle collisions will be frequent. Hence a protective structure of 
some type is required to prevent rupturing of pressurized compartments, damage 
to equipment, or erosion of surfaces needed for thermal protection in the 
space or reentry phases by collision with such particles.  See  METEOR. 
    Radiation shielding may be required for some vehicles, particularly those 
operating for extended times within the Earth's magnetically trapped radiation 
belts or during times of highspot activity. The shielding may or may not be an 
integral part of the structure. 
    Temperature extremes in the structure and the enclosed environment are 
controlled by several techniques. Passive control is accomplished by surface 
coatings which maintain a balance between absorbed and emitted thermal 
radiation. Another passive technique is multilayer thermal blankets which 
control the radiation transfer from the spacecraft to space and vice versa. 
Other techniques used to actively control spacecraft temperatures are thermal 
louvers and heat pipes. 
    Thermal gradients must be considered in spacecraft design, especially when 
the spacecraft has one surface facing the Sun continuously. In some cases it 
is desirable to slowly rotate the spacecraft to eliminate such gradients. 
  
    Reentry phase.  Although the atmospheric layer of the Earth is relatively 
thin, it is responsible for the reduction of vehicle velocity and the 
resulting deceleration loads, as well as for the severe heating experienced by 
reentering vehicles. A body entering the Earth's atmosphere possesses a large 
amount of energy. This energy must be dissipated in a manner which allows the 
reentering vehicle to survive. Most of the vehicle's original energy can be 
transformed into thermal energy in the air surrounding the vehicle, and only 
part of the original energy is retained in the vehicle as heat. The fraction 
that appears as heat in the vehicle depends upon the characteristics of the 
flow around the vehicle. In turn, the flow around the vehicle is a function of 
its geometry, attitude, velocity, and altitude. Several different approaches 
are used, such as the zero-lift, blunt, high-drag body (as exemplified by the 
intercontinental ballistic missiles and the man-in-space programs), and high 
lift-to-drag ratio glide vehicles.  See  ATMOSPHERIC ENTRY. 
    A third approach is to combine the rather compact ballistic shape with 
some of the lift capability of the glide vehicles. Figure 1 shows typical 
examples of the three shapes. 
    In crewed applications, vehicles employing aerodynamic lift during reentry 
have several advantages over zero-lift ballistic bodies. (1) The use of lift 
allows a more gradual descent, thus reducing the deceleration forces on both 
vehicle and occupants. (2) The vehicle's ability to glide and maneuver within 
the atmosphere gives it greater accuracy in either hitting a target or landing 
at a predetermined spot. (3) It can accommodate greater errors of guidance 
systems because for a given deceleration it can tolerate a greater range of 
entry angles. (4) Greater temperature control is afforded because aerodynamic 
lift may be varied to control altitude with velocity. 
  
  
    Spacecraft structures.  Spacecraft structures may be divided into two 
broad categories, erectable and rigid. 
    It is sometimes required to confine a spacecraft within a protective nose 
fairing which is part of the boost vehicle system. This fairing provides an 
efficient aerodynamic shape for the front end of the booster and also provides 
protection to the enclosed spacecraft from aerodynamic and other effects. This 
fairing, however, constrains the shape of the spacecraft to something usually 
resembling a cylinder. In order to meet some mission objectives, however, it 
is necessary to design erectable structures. 
    Erectable structures take many and varied shapes. They are sometimes 
relatively simple hinged booms, while on other occasions they become quite 
large and massive. 
    An example of a large rigid deployable spacecraft structure is the 3300-lb 
(1500-kg) Pegasus spacecraft with its 2000 ftsup2 (185 msup2) of aluminum 
target-sheet capacitor detectors. 
    Another type of unique erectable structure is the antennas on the radio 
astronomy explorer spacecraft. The four 750-ft (229-m) V antennas were erected 
in flight as were two 300-ft (91-m) damper tubes and two 60-ft (18-m) dipole 
antennas. Each of these antenna booms was stored as a 0.002-in.-thick 
(50-micrometer) beryllium copper alloy tape approximately 2 in. (5 cm) wide in 
a mechanism within the spacecraft. Upon commands from the Earth the booms were 
deployed from the mechanism to form J-in.-diameter (13-mm) tubes. In addition 
to serving the function of radio astronomy antennas, the long booms provided 
gravity stabilization so that one axis of the spacecraft always faced the 
Earth.  
    Another unique type of erectable structure is the large inflatable sphere, 
such as the Echo and Pegeos spacecraft. This type of structure provides a 
relatively large surface area for relatively little weight. 
    A spacecraft with a relatively large variety of elements to be erected was 
the orbiting geophysical observatory (OGO). Figure 2 shows an artist's 
conception of the OGO in orbit with the booms fully extended. 
    Many more spacecraft structures are rigid than erectable or inflatable. 
Some examples of rigid spacecraft structures are the orbiting astronomical 
observatory (OAO), the Nimbus weather spacecraft, the Telstar communications 
spacecraft, the Syncom communications spacecraft, the orbiting solar 
observatory (OSO), and the lunar and command service modules. 
    In cases such as the orbiting astronomical observatory, where the least 
distortion of the structure effects the alignment of the astronomical 
telescope, great care must be taken to eliminate thermal gradients. The 
passive temperature control techniques previously described are utilized to 
keep the thermal gradients to a minimum. 
    The Nimbus spacecraft structure illustrates yet another unique concept in 
design - the basic wheel section connected by trusses to the attitude control 
system housing. The solar array tracks the Sun during the sunlight portion of 
the spacecraft orbit. 
    The Telstar communications spacecraft is a classic example of the basic 
spherical-shaped rigid spacecraft. The solar cells on the surface supply power 
to the electronics. The white coating provides passive thermal control. 
    The Syncom spacecraft (Fig. 3) was one of the relatively early spacecraft 
to incorporate its own propulsion system. Its cylindrical sides are covered 
with solar cells. 
    The orbiting solar observatory (OSO 7) spacecraft (Fig. 4) features a sail 
which tracks the Sun while the main spacecraft body rotates. 
    An example of one of the largest rigid uncrewed spacecraft is the 
applications technology satellite (ATS 7). This spacecraft - over 50 ft (15 m) 
across - embodies structural trusses, a flexible erectable parabolic reflector 
30 ft (9 m) in diameter, active solar array, active controls, active thermal 
system, and a propulsion system. 
  
    Prospects.  The small uncrewed spacecraft have become more and more 
sophisticated electronically and have incorporated lightweight three-axis 
control systems. Graphite-fiber-reinforced plastics are universally used to 
reduce the structural weight factor, allowing a greater percentage of the 
available weight to go to useful payload. 
    The Space Transportation System (STS), whose first orbital flight was 
launched in April 1981, heralds a new era in spacecraft and structural design. 
Spacecraft can now rely on reaching orbit by using the space shuttle. The 
shuttle can place payloads up to 29,484 kg (65,000 lb) into orbit and return 
payloads up to 14,515 kilograms (32,000 lb). 
    Crewed missions will require that complex spacecraft structural assemblies 
be made in space. This will require unique designs, built on modular concepts 
that can be transported by the space shuttle and assembled in space. These 
large space stations will be the stepping stones for the interplanetary travel 
of the distant future.  See  SPACE SHUTTLE. 
 �34s[Elmer W. Travis]
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�6v�2v�32sSpace flight   �21m �33s 
    The penetration by humans into the reaches of the universe above the 
terrestrial atmosphere and investigation of these regions by automated, 
remote-controlled, and crewed vehicles. 
    The purpose of space flight is to provide significant contributions to the 
physical and mental needs of humanity on a national and global basis. Such 
contributions fall specifically in the areas of (1) earth resources of food, 
forestry, atmospheric environment, energy, minerals, water, and marine life; 
(2) earth and space sciences for research; (3) commercial materials 
processing, manufacturing in space, and public services. More general goals of 
space flight include expansion of knowledge; exploration of the unknown, 
providing a driving force for technology advancement and, hence, improved 
Earth-based productivity; development and occupation of new frontiers with 
access to extraterrestrial resources and unlimited energy; strengthening of 
national prestige, self-esteem, and security; and providing opportunity for 
international cooperation and understanding. 
    To conduct crewed space flight, the two leading space-faring nations, the 
United States and the Soviet Union, developed spacecraft systems and the 
necessary ground facilities, research and development base, operational 
know-how, planning experience, and management skills. In the United States, 
crewed space programs are conducted by the National Aeronautics and Space 
Administration (NASA), a federal agency established in 1958 for the peaceful 
exploration of space. In the Soviet Union, crewed space flights have been 
under the auspices of the U.S.S.R. Academy of Sciences. The first crewed 
spacecraft, the Vostok 1, piloted by Yuri A. Gagarin, was launched on April 
12, 1961, and returned after completing one revolution of the Earth. The first 
American crewed space flight took place 3 weeks later, when NASA launched Alan 
B. Shepard for a 15-minute suborbital test flight. Since then, a large number 
of crewed spacecraft have been put into orbit around the Earth by both 
nations, and nine have been launched toward the Moon by the United States (see 
table).  
  
    Crewed spacecraft.  A crewed spacecraft is a vehicle capable of sustaining 
humans above the terrestrial atmosphere. In a more limited sense, the term 
``crewed spacecraft'' is usually understood to apply to vehicles for 
transporting and sustaining human crews in space for time periods limited by 
prestored on-board supplies, as distinct from orbital space stations which 
support theoretically unlimited habitation of humans in space by autonomous 
systems, crewed maintenance, and periodic resupply.  See  SPACE STATION. 
    The basic requirements of crewed spacecraft are quite different from those 
of uncrewed space probes and satellites. The presence of humans on board 
necessitates a life-sustaining environment and the means to return safely to 
Earth. The major common feature of all crewed spacecraft, therefore, is the 
atmospheric-return element or reentry module. It consists basically of a 
pressure-tight cabin for the protection, comfort, and assistance of the crew, 
and surrounded by heat-resistant material, the thermal protection system, to 
cope with the high frictional and radiative heating accompanying the energy 
dissipation phase of the atmospheric-return flight to Earth. 
    The main system that distinguishes crewed spacecraft from other spacecraft 
is the environmental control and life support system. It provides the crew 
with a suitably controlled atmosphere. Because all atmospheric supplies must 
be carried into space, it is essentially to recirculate and purify the 
spacecraft atmosphere to keep the total weight of the vehicle within 
reasonable limits. 
    Space suits or pressure suits are mobile spacecraft or chambers that house 
the astronauts and protect them from the hostile environment of space. They 
provide atmosphere for breathing, pressurization, and thermal control; protect 
astronauts from heat, cold, glare, radiation, and micrometeorites; contain a 
communication link and hygiene equipment; and must have adequate mobility. The 
suit is worn during launch, docking, and other critical flight phases. 
    Because of the potential hazards of space flight to personnel, crewed 
spacecraft must meet stringent requirements of safety and hardware 
reliability. Reliability is associated with the probability that the 
spacecraft systems will operate properly for the required length of time and 
under the specified conditions. To assure survival and return in all 
foreseeable emergencies, the design of a ``crew-rated'' spacecraft includes 
standby systems (double or triple redundancy) and allows for launch escape, 
alternate and ``degraded'' modes of operation, contingency plans, emergency 
procedures, and abort trajectories to provide maximum probability of mission 
success. The priority order of this reliability requirement is: (1) crew 
safety, (2) minimum achievable mission fulfillment, (3) mission data return, 
and (4) minimal degradation. 
             
    Soviet programs.  After the first Sputnik launch on October 4, 1957, 
developments of Soviet crewed space flight capability followed in quick 
succession, leading from the first-generation Vostok (East) to the 
second-generation Voskhod (Ascent) and to the third-generation Soyuz (Union) 
spacecraft.  
    The Vostok, a single-seater for short-duration missions and ballistic 
reentry from Earth orbits, consisted of a near-spherical cabin covered 
entirely with TPS material, having three small viewports and external radio 
antennas. The approximately 7-ft (2-m) sphere of the cabin was attached to a 
service module. The second-generation Voskhod, essentially a greatly modified 
Vostok, was a short-duration multiperson craft and was designed to permit EVA 
or spacewalking by one of the crew. 
    The Soyuz is much heavier, larger, and more advanced in its orbital 
systems, permitting extended orbital stay times. It consists of three main 
sections or modules: a descent vehicle, an orbital module, and an 
instrument-assembly module. The three elements are joined together, with the 
descent vehicle in the middle. Shortly before atmospheric reentry, the two 
outer modules are jettisoned. Soyuz spacecraft carried originally up to three 
cosmonauts. However, when an accidental explosive decompression of the descent 
cabin during reentry caused the death of the Soyuz 11 crew in 1971, the third 
seat was removed to make room for the necessary additional life support 
equipment, and the Soyuz thereafter carried only two cosmonauts. An improved 
version, the Soyuz T (for ``Transport'') was introduced in 1980. 
  
    United States programs.  During the first decade after its inception in 
1961, the United States crewed space program was conducted in three major 
phases - Mercury, Gemini, and Apollo. 
    Mercury was, in its basic characteristics, similar to the Soviet Vostok, 
but it weighed only about a third as much, as necessitated by the smaller 
missiles of the United States at that time (the Redstone and Atlas). The 
one-person Mercury capsules used ballistic reentry and were designed to answer 
the basic questions about humans in space: how they were affected by 
weightlessness, how they withstood the gravitational forces of boost and 
entry, how well they could perform in space. 
    The second United States step into space was the Gemini Program. With the 
two-person Gemini capsule (Fig. 1), for the first time a crewed spacecraft had 
been given operational maneuverability in space. Its reentry module flew a 
lifting reentry trajectory for precise landing point control. In addition, its 
design permitted EVA by one of the crew. 
    The third-generation spacecraft, Apollo, had five distinct parts: the 
command module (CM), the service module (SM), the lunar module (LM), the 
launch escape system (LES), and the spacecraft/lunar module adapter SLA). The 
three modules made up the basic spacecraft; the LES and SLA were jettisoned 
early in the mission after they had outlived their function. The CM served as 
the control center for the spacecraft and provided living and working quarters 
for the three-member crew for the entire flight, except for the period when 
two persons entered the LM for the descent to the Moon and return. The CM was 
the only part of the spacecraft that returned to Earth, flying a lifting 
trajectory with computer-steered maneuvers. The LM carried two astronauts from 
the orbiting command/service module (CSM) down to the surface of the Moon, 
provided a base of operations there, and returned the two astronauts to a 
rendezvous with the CSM in orbit. On the last three lunar landings, Apollo 15 
- 17, lunar exploration was supported by the lunar roving vehicle.  See  MOON. 
2569A| 
    A more powerful booster was required to lift the Apollo spacecraft to 
Earth orbit and thence to the Moon. At the Army Ballistic Missile Agency 
(ABMA) in 1958, a team of engineers under Wernher von Braun set out to prove 
that vastly more powerful space rockets could be built from existing hardware 
by clustering engines and tanks. This project evolved into the Saturn Program 
of NASA. The Saturn 5, which became the Apollo lunar launch vehicle, had the 
capability to lift 250,000 lb (113 metric tons) into low Earth orbit and to 
send 100,000 lb (45 metric tons) to the Moon. 
  
    Skylab.  The experimental space station Skylab (Fig. 2) was the largest 
object ever placed in space, and the first crewed project in the U.S. Space 
Program with the specific purpose of developing the utility of space flight in 
order to expand and enhance humanity's well-being on Earth. To that end, 
Skylab's equipment included Earth resources remote sensing instruments and the 
first crewed solar telescopes in space. A total of three crews of three 
astronauts each carried out experiments and observations on Skylab. Skylab was 
launched uncrewed by Saturn 5 in 1973. Subsequently visited by three astronaut 
crews the space station underwent repair in orbit, a first for the space 
program.  
  
  
    Space transportation system.  After the joint American-Soviet space 
mission in 1975, attention began to focus on the routine application of newly 
acquired know-how, systems, and experience, specifically in form of the 
emerging space transportation system (STS), with the ability to transport 
inexpensively a variety of useful payloads to orbit, as the mainstay and 
``work horse'' of the United States space program in the 1980s and 1990s. The 
two major components of the STS are the Space Shuttle and the Spacelab. 
    The space shuttle is a reusable system. It has three major elements: the 
orbiter, an external tank (ET) containing the liquid propellants to be used by 
the orbiter main engines for ascent, and two solid-propellant rocket boosters 
(SRBs) ``strapped on'' to the external tank. The orbiter and SRB casings are 
reusable; the ET is expended on each launch.  See  SPACE SHUTTLE. 
    The Spacelab is a major adjunct of the shuttle. Developed and funded by 10 
member nations of the European Space Agency (ESA), the large pressurized 
Spacelab module with an external equipment pallet is designed to be the most 
important payload carrier during the space shuttle era. With its large 
transport capacity, the Spacelab is intended to support a wide spectrum of 
missions in science, applications, and technology by providing versatile and 
economical laboratory and observation facilities in space for many users. 
Another objective is to reduce significantly the time from experiment concept 
to experiment result, compared with previous space practice, and also to 
reduce the cost of space experimentation. It allows the direct participation 
of qualified scientists and engineers to operate their own equipment in orbit. 
  See  SPACE TECHNOLOGY; SPACECRAFT STRUCTURE. 
 �34s[Jesco Von Puttkamer]
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�6v�2v�32sSpace navigation and guidance   �21m �33s 
    The determination of the position and velocity of a space probe with 
respect to a target body or a known reference body such as the Earth 
(navigation) and, based upon this determination, the application of propulsive 
maneuvers to alter the subsequent path of the probe (guidance). 
    Space navigation can be viewed as determining the current position and 
velocity of the probe and then using that determination as the basis of 
predicting future motion. This determination is made by taking a series of 
measurements relating to the probe's motion and combining these measurements 
in such a manner as to make the most accurate estimate of the probe's current 
position and velocity, taking into account possible small errors or 
inaccuracies in the measurements themselves. One of the most powerful (and 
accurate) measurements which can be made is the relative velocity between an 
Earth tracking station and the space probe itself. This is accomplished by 
broadcasting an electromagnetic signal which consists of a single tone having 
a stable frequency to the probe. The probe will receive this signal shifted in 
frequency in exact proportion to the relative station-probe velocity. This 
frequency shift is known as the Doppler effect. It is also possible to measure 
the station-probe distance (or range) using electromagnetic signals. 
    For missions to the outer planets (Jupiter and beyond) or for a mission to 
a body such as a comet or asteroid, the position of the target may be 
sufficiently uncertain as to make a strictly Earth-relative navigation scheme 
inadequate. Here it is necessary to make measurements that directly involve 
the target. Typical of these is to obtain optical measurements of the location 
of the target relative to a star as seen from the probe itself. 
    In its simplest form, guidance consists of comparing the predicted future 
motion of the probe against that which is desired, and if these are 
sufficiently different, executing a propulsive maneuver to modify that future 
position. Typically, the probe will contain a small rocket motor which can be 
fired in any desired direction by first rotating the spacecraft away from its 
cruise orientation and then holding the new attitude fixed while the rocket 
motor is firing. 
    Multiplanet missions use an accurate delivery to the first target not only 
to scientifically explore that planet, but also to use the planet's 
gravitational attraction to change the course of velocity of the probe 
advantageously for the next leg of the mission toward the second and 
subsequent targets. The encounter with a planet becomes in itself a type of 
guidance correction as the planet changes the path of the probe.  See  
GUIDANCE SYSTEMS; SPACE PROBE. 
 �34s[David W. Curkendall]
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�6v�2v�32sSpace power systems   �21m �33s 
    On-board assemblages of equipment to generate, store, and distribute 
electrical energy on satellites and spacecraft. Present-generation spacecraft 
fly power systems from tens of watts to several kilowatts. 
    With few exceptions, the power systems for United States satellites have 
used photovoltaic generation for power, batteries for energy storage, and a 
host of electrical equipment for appropriate regulation, conversion, and 
distribution. Fuel cells have been limited primarily to use in the crewed 
space program, supplying power for Gemini, Apollo, and the space shuttle. 
Radioisotope thermoelectric generators (RTGs) have powered, or augmented solar 
power on, a few planetary missions and probes, powered lunar instruments left 
on the Moon by the Apollo missions, and augmented the solar-array battery 
power system on at least one Earth-orbiting spacecraft. 
 �34s[Floyd E. Ford]
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�6v�2v�32sSpace probe �21m �33s
    An instrumented vehicle, the payload of a rocket-launching system,
designed specifically for flight missions to other planets, the Moon, and into
deep space, as distinguished from Earth-orbiting satellites. Some of the
important space probes are given in the table.	See  SATELLITE (SPACECRAFT).
    The space probe is used primarily for scientific purposes, and the payload
is designed to explore planetary geology, geography, geodesy, atmospheric
physics, exobiology, other specialized aspects of planets, their satellites,
comets, and, lastly, the charged particle and electromagnetic field
environment about planets and in deep space. The task is usually designated as
the mission, which, for exploration to celestial bodies, is customarily
divided into the cruise and encounter phases. The instrumentation is usually
distinct for the two parts of the mission. Instruments carried are cosmic-ray
telescopes, plasma detectors, magnetometers, neutral gas detectors, television
cameras, gamma-ray telescopes, various specialized surface scanners, and
micrometeoroid detectors. Cruise-mode operation entails interplanetary physics
such as the measurement of solar-wind and cosmic-ray propagation and
diffusion, whereas the encounter mode is occupied with exploration and
measurements of the properties of the planet.
    Research on planetary surfaces usually includes instruments devoted to
chemical examination of the surface and atmosphere, as well as the surface
geology and meteorology. Exobiological instrumentation may be included.

    Design.  The spacecraft is a self-contained automatic device with a
self-sustaining power generation system deriving energy from the Sun or a
nuclear thermoelectric generator converting  this energy to usable form, and
distributing electricity to the various electronic subsystems. The other
subsystems of the spacecraft are: communication, which includes command
receivers, data encoder and transmitter, and antennas; attitude control, which
may be passive, using spin angular momentum or solar pressure vanes; a frame
or structure which must maintain its integrity throughout the mission; and a
thermal control system to provide the proper operating temperatures in the
spacecraft interior. Spacecraft which are designed to enter a planetary
atmosphere or land on a planetary surface must also include an aeroshield and
a landing system. Viking also included a bioshield to maintain a sterile
environment during Earth launch.
    Ideally, a spacecraft will have an extensive system of monitoring sensors
for determining the engineering status of the various subsystems during flight
and relaying this information back to Earth via the telemetry link. Propulsion
is provided in those spacecraft where midcourse trim of the velocity is
needed; such instances are required when a target planet is to be encountered.
Lastly, the payload comprises the package to be delivered to a specified point
in space in operational condition so measurements can be taken and signals
returned to Earth.  See  SPACE TECHNOLOGY.

    Mission.  Although some missions require only a trajectory into
interplanetary space, others are directed to specified bodies in the solar
system. Such trajectories invariably require that during the cruise mode
(midcourse) the velocity be corrected, since the launch booster rocket alone
seldom can attain sufficient accuracy of guidance to intercept the target
planet with acceptable precision. The correction is applied by firing a small
vernier rocket which is a part of the spacecraft. The thrust is in a direction
computed so that the correct velocity to be gained is attained by vectorially
adding the increment to the precorrection cruise velocity. To determine the
correction requires tracking from one to several days to ensure accuracy.  See
SPACE NAVIGATION AND GUIDANCE.
    Some planetary probes are designed for encounter only (flyby); others are
designed for capture into orbit as an artificial satellite; and, finally,
there are those which are decelerated to land. All these require special
on-board propulsion. In cases where an atmosphere is present, aerodynamic
devices are useful to reduce the entry speed. These can take the form of
ablating nose cones, balutes, retrorockets, pure parachutes, or some
combination of these.

    Spacecraft classification.	A number of families of spacecraft, with
similar missions and often similar designs, have arisen. Some of the important
types of space probes are listed below.

    Pioneer. This family of spacecraft, used mainly for interplanetary
physics, constitutes a series which has radically changed in appearance. All
Pioneer spacecraft are spin-stabilized using the angular momentum of the
spacecraft following spin motor firing some time in the launch sequence.
Perhaps the archetype of interplanetary spacecraft of the spinning type is the
Pioneer 5 (Fig. 1), which set a record early in 1960 for deepspace
communication and the first long-term measurement of various interplanetary
particle and field effects.
    Attempts using the Pioneer were revived in 1962 when a new Pioneer
configuration was planned using orientation of the spinning spacecraft after
insertion into interplanetary orbit. There are presently four such spacecraft
in interplanetary orbit about the Sun.
    Since its beginnings early in the space program, Pioneer has undergone
significant system changes, as has the Mariner spacecraft. The key differences
are still the use of spin angular momentum to stabilize the Pioneer and an
attitude-control system to maintain orientation of the Mariner. The
Pioneer-Jupiter spacecraft (Pioneers 10 and 11) encountered the planet Jupiter
on Dec. 3, 1973, and Dec. 3, 1974, respectively. Pioneer 10 was sent on a
solar system escape trajectory. Pioneer 11 encountered Saturn in September
1979.

    Ranger. This series of spacecraft began with an attempt to land a
single-axis seismometer upon the lunar surface. The technique included deboost
into lunar trajectory and a semisoft landing, where the residual impact was
removed using a balsa wood absorbing sphere. This design was abandoned in
favor of a hard landing using television coverage during the approach to
obtain photogeological information of the surface. The consequences of Ranger
program redirection was a primarily photogeological spacecraft boosted by an
Atlas-Agena rocket. The whole Ranger program included nine launches, of which
three provided extensive photographic coverage of the lunar surface.

    Surveyor. This spacecraft (Fig. 2) was a complicated system for delivering
a payload of exploration and scientific instruments to the surface of the Moon
and providing telemetry, power, and control after landing. The spacecraft
design restricted operation to the first lunar day. However, the electrical
system on some spacecraft survived the lunar night, and partial operation was
possible into the second day.
    The TV camera operated with a programmable mirror ahead of the optical
train so that panoramic coverage over 360DG in azimuth and from 40 to -60DG
could be obtained. The alpha backscatter experiment for elemental analysis of
the surface soil was a novel device which formed the basic chemical probe used
on Surveyor.


    Lunar orbiter. This was a spacecraft specifically designed to place an
instrument package in a stable orbit about the Moon. There were five flights
in the series, all of which produced high-quality photographs. Orbiter
returned 1950 photographs of the Moon which permitted selection of five prime
Apollo landing sites; the photographs have proved extremely useful in
interpretive studies of the geology of the lunar surface.

    Mariner. Mariner is the archetype of planetary explorers in the United
States space program. There were seven missions flown in the 1960s. Five were
successful, two to Venus and three to Mars. The Mariner spacecraft is
solar-powered, has an attitude-controlled stabilization system with Sun and
Canopus or Earth pointing, and uses two-way Doppler compatible with the Deep
Space Network for tracking and trajectory determination. A gradual evolution
of design has taken place since the first firing to Venus in 1962.
    Mariner 9 was designed to map 70% of the Martian surface within 90 days
with high-resolution cameras, and at the same time study the atmosphere and
surface via ultraviolet and infrared spectroscopy and infrared radiometry.
S-band occultation was also studied by using the spacecraft radio experiment,
and celestial mechanics by using orbital information.
    Mariner 10 was the first in the long series of Mariner spacecraft to
encounter the planet Mercury. The unique role of spacecraft in planetary
science was again demonstrated by Mariner 10 with its close view of the
dynamical structure of the upper cloud layer of Venus, the surface of Mercury,
and finally the quite unexpected discovery of a global magnetic field at
Mercury.

    Viking. This spacecraft (Fig. 3) was the most ambitious uncrewed probe
attempted. Two launches of a combined orbiter-lander took place in August and
September 1975, with arrival at Mars in June and August 1976. The principal
motivation for these missions was the search for biological activity on the
Martian surface. Besides these experiments, some additional instruments were
carried for geophysical and geochemical exploration, and comprehensive imaging
systems were borne on both the orbiter and lander. Sophisticated telemetry and
control systems were included in the lander along with an on-board computer.


    Voyager. Voyager is a dual-spacecraft mission to the outer planets. It
encountered Jupiter and several of the Jovian satellites, providing the first
close-up information on these objects, including images of the Jovian
satellites. Subsequent to the Saturn encounters, which took place respectively
in November 1980 (Voyager 1) and August 1981 (Voyager 2), these spacecraft
were turned to the outer parts of the solar system. Voyager 1 will eventually
move toward the apex of the heliosphere, while Voyager 2 is to be aimed first
to Uranus and finally reaching Neptune. Voyager belongs to the same general
class of spacecraft as Galileo, discussed below.

    Galileo. Galileo is a mission designed to launch two spacecraft toward
Jupiter, one to orbit the planet over a period of up to 20 months, together
with an entry probe transported to Jupiter on an independent spacecraft (bus).
There is to be an approach to Mars (possibly as close as 170 mi or 275 km) en
route to Jupiter. Galileo is a massive spacecraft system, totaling 8310 lb
(3770 kg) when fully loaded with propellant.
    The orbiter spacecraft is a dual-spin device with the spinning section or
rotor carrying booms and fuel tanks. This section is designed from the
scientific standpoint to provide for full spherical coverage for particle
experiments. The stationary section of the spacecraft carries an articulated
science platform for the various imaging experiments.
 �34s[Charles P. Sonett]
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�6v�2v�32sSpace processing   �21m �33s 
    The carrying out of various processes on materials aboard orbiting 
spacecraft. The low-gravity, high-vacuum environment associated with orbiting 
space vehicles offers unique opportunities to investigate various processes in 
ways that cannot be duplicated on Earth. Gravity-driven convective flows, 
sedimentation, and hydrostatic pressure are virtually eliminated. Materials 
can be melted, shaped, and solidified in the absence of a container. Processes 
that require an extremely high vacuum in conjunction with large heat loads or 
the evolution of large quantities of gas may be performed in the wake of an 
orbiting vehicle. Because orbital velocity is much greater than the molecular 
speed of the residual atmosphere, the wake of an orbiting vehicle is an almost 
perfect vacuum chamber with virtually unlimited pumping speed and a 4 K heat 
sink.  
    The absence of sedimentation allows materials with large density 
differences to be maintained in stable dispersion for long periods of time 
under diffusion-controlled conditions. This offers an excellent opportunity to 
study processes such as Ostwald ripening, bubble coalescence and dissolution, 
the formation and stability of flocs, and solidification of alloys and 
composites.  
    The absence of hydrostatic pressure eliminates the tendency for materials 
to sag and deform when melted. This offers new dimensions in the important 
process of float zone crystal growth, not only in the length of the zone that 
can be maintained (which provides better thermal control), but also in the 
extension of the float zone process to materials with such low surface tension 
that float zone processing is unfeasible in Earth's gravity.  See  CRYSTAL 
GROWTH.  
    Containerless processing eliminates problems such as container 
contamination and nucleation. Thermal physical measurements can be made at 
temperatures well above the melting points of any known material or on 
materials that are extremely reactive in the melt. Unique glasses and 
nonequilibrium metastable phases may be formed by allowing materials to cool 
well below their normal freezing points by denying them nucleation sites. 
Containerless melting or distillation in the high vacuum in the wake of an 
orbiting vehicle could produce metal samples of unprecedented purity. 
 �34s[Robert J. Naumann]
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�6v�2v�32sSpace shuttle   �21m �33s 
    A reusable orbital spacecraft. The Space Shuttle Transportation System 
(STS) was designed to be the mainstay of the United States space program. 
    The STS provides the ability to reduce the cost of space flight to low 
Earth orbit, providing more economical rapid access to space for 
government-sponsored public services and for commercial, scientific, and 
defense purposes. The space shuttle is designed to perform three basic 
categories of missions. The sortie missions are to last from one to several 
weeks, using one or more of the Spacelab modules being developed by the 
European Space Agency. These missions are to consist of a variety of 
experiments and observations. The second type of mission uses additional 
propulsion stages to carry spacecraft to orbits beyond the capabilities of the 
space shuttle alone. In the third type of mission, satellites are deployed 
directly into orbit or retrieved from orbit with the aid of the shuttle remote 
manipulator system.  See  REMOTE MANIPULATORS; SPACE STATION. 
    The space shuttle flight system (see illustration) consists of an orbiter, 
three space shuttle main engines (SSMEs), an external tank (ET), and a booster 
made up of two solid-fuel rocket boosters (SRBs). The orbiter, the SSME, and 
the SRBs are reusable; an ET is expended on each launch. The orbiter normally 
carries a crew of four, with provisions for a crew of as many as seven, and 
payloads. It can remain in orbit normally 7 days, return to Earth with 
personnel and payload, similar to a conventional airplane, and be refurbished 
for a subsequent flight in 14 days. 
    The space shuttle is launched with the three SSMEs and the two SRBs 
burning in parallel. The SRBs are separated at approximately 122 s after 
lift-off. After SRB separation, the orbiter continues to ascend, using the 
three SSMEs. The main engine cutoff (MECO) takes place 480 s after lift-off. 
The ET separation occurs immediately after MECO. The orbital maneuvering 
system (OMS) engines provide the additional velocity needed to insert the 
orbiter into an elliptical orbit having a minimum apogee of 150 n-mi (280 km). 
At first apogee, the orbiter initiates the first of two maneuvers to 
circularize the orbit at an altitude of 150 n-mi (280 km). 
    Following the completion of orbital operations, the orbiter is oriented to 
a tailfirst attitude. The OMS engine provides the deceleration thrust 
necessary for deorbiting, and the orbiter is reoriented with nose forward for 
entry. The orientation of the orbiter is established and maintained by the 
reaction control system (RCS) down to the altitude at which the atmospheric 
density is sufficient for the pitch and roll aerodynamic control surfaces to 
be effective. 
    During the final phases of descent during entry, flight path control is 
maintained by using aerodynamic surfaces. The proper vehicle approach to the 
runway with respect to position, energy, and heading must be maintained. 
    The thermal protection system represents an advance in the technology of 
atmospheric reentry. All previous spacecraft were designed for one mission 
life, and ablator-type exterior insulation was a proved technique for 
protection against high thermal heating. With the reusability requirements and 
the weight sensitivity of the space shuttle, it became necessary to move into 
other types of heat protection materials. Carbon-carbon protection was 
developed for the higher-temperature regions, that is, the nose and landing 
edges of the wing. However, due primarily to weight considerations this 
material was inappropriate as thermal protection for the other areas of the 
vehicle. The reusable surface insulation (RSI), better known as tile, is a 
low-density, porous, brittle, anisotropic material. The orbiter thermal 
protection system utilizes approximately 32,000 primarily 6 x 6 in. (15 x 15 
cm) tiles for thermal insulation. The tiles are numerically milled and are 
essentially nonconductors that prevent the passage of heat into the structure. 
See ATMOSPHERIC ENTRY; SPACE FLIGHT; SPACE TECHNOLOGY; SPACECRAFT STRUCTURE. 
 �34s[Aaron Cohen]
�33s 
  
  
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�6v�2v�32sSpace station   �21m �33s 
    An autonomous facility in space for the conduct of scientific and 
technological research, Earth-oriented applications, and support of other 
space activities. A space station provides the facilities, centralized 
computers and data management, and crew support to conduct a wide variety of 
space activities. 
    Initially, the space station would be placed in a low orbit a few hundred 
miles above Earth. Future possibilities, however, include stations in 24-h 
synchronous orbits at 23,000 mi (37,000 km) altitude, orbits of the Moon, and 
even orbits of other planets. 
    Skylab, the first United States space station, was launched on May 14, 
1973, and boarded by a three-man crew on May 25, 1973. Skylab was made up 
primarily from space systems developed for the Apollo lunar program and 
modified as necessary for a space station - type mission. Skylab had three 
separate flight crews for a total of 171 days in orbit. Skylab conducted a 
multidiscipline program of research and applications, including Earth 
observations, astronomy, and manufacturing in space. 
    The National Aeronautics and Space Administration (NASA) envisions the 
future space station as an assembly of shuttle transportable modules. 
Individual modules would be provided for living quarters, utilities such as 
power and stabilization, and various types of payload activities, for example, 
space manufacturing or astronomy. 
    The space station and the reusable space shuttle provide major new 
opportunities for cost-effective space operations. The reusable space shuttle 
not only will provide a lower-cost means of placing a spacecraft in orbit, but 
will also materially reduce the cost of each space mission by providing a 
capability to maintain or repair spacecraft in orbit and by returning the 
spacecraft to Earth for use in subsequent missions. The space station, as the 
companion element to the shuttle, will provide the long-duration, 
space-maintainable, crew-supported and -operated facility which will further 
reduce the cost of space operations.  See  SPACE SHUTTLE. 
    With the space station logistically supported by the low-cost reusable 
space shuttle, activities in Earth orbit will assume a routine character like 
those in laboratories and test facilities on Earth. Personnel will include 
scientists and others without specialized space flight training. 
 �34s[Thomas Hagler]
�33s 
  
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�6v�2v�32sSpace technology �21m �33s
    The systematic application of engineering and scientific disciplines to
the exploration and utilization of outer space. Space technology developed so
that spacecraft and humans could function in an environment quite different
from the Earth's surface. In outer space there is no atmosphere, no apparent
gravity, no means of mechanical support, no simple source of power, and no
easy access for repair. The lack of air pressure means that there is no
natural convection cooling of electronic components, that there is no oxygen
for burning of fuel or breathing, and that liquids quickly evaporate. For each
mission and each requirement, space technology provides an answer, always with
the goal of greatest reliability and lowest spacecraft mass.
    Some technology is fundamental to the functioning of almost all
spacecraft: structure and materials, thermal control, tracking and
positioning, attitude control, and electrical power. The spacecraft structure
must be designed to survive the forces encountered during testing, ground
handling, launching, and any deployment of appendages. It is usually made of a
metal (aluminum, magnesium, beryllium) or a composite (boron/epoxy or
graphite/epoxy). It not only must support all the spacecraft parts, but must
also be shaped to fit inside the available launch vehicle volume.  See
SPACECRAFT STRUCTURE.
    To maintain temperatures at acceptable levels, various active and passive
devices are used: surfaces with special absorptivities and emissivities,
numerous types of thermal insulation, mechanical louvers to vary the heat
radiated to space, heat pipes, and electrical heaters.
    The location of a spacecraft can be measured by determining its distance
from the transit time of radio signals or by measuring the direction of
received radio signals, or by both.  See  SPACE NAVIGATION AND GUIDANCE;
SPACECRAFT PROPULSION.
    Most spacecraft either have no appreciable rotation (three-axis
body-stabilized) or spin around a well-defined axis (spinner). A
body-stabilized spacecraft is simpler in overall concept than the spinner, and
may be necessary in many missions, but often requires more hardware than a
spinning spacecraft. The orientation of a spacecraft is measured with Sun
sensors (the simplest method), star trackers (the most accurate), and horizon
or radio-frequency (rf) sensors (usually to determine the direction toward
Earth). Attitude corrections are made by small thrusters or by reaction or
momentum wheels.  See  STAR TRACKER.
    Primary electrical power is most often provided by solar cells made from a
thin section of crystalline silicon, protected by a thin glass cover. When
sunlight is interrupted by an eclipse, power is often obtained from
nickel-cadmium batteries that serve as a secondary power source.  See  SOLAR
CELL; SPACE POWER SYSTEMS.
    The status and condition of a spacecraft are determined by telemetry.  See
SPACE COMMUNICATIONS; TELEMETERING.
    Many spacecraft missions have special requirements and hence necessitate
special equipment. Satellites that leave the Earth's gravitational field to
travel around the Sun and visit other planets have special requirements.
Spacecraft that return to Earth require special protection. In some missions
one spacecraft must find, approach, and make contact with another spacecraft.
See  SATELLITE (SPACECRAFT); SPACE PROBE.
 �34s[Gary D. Gordon]
�33s

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�6v�2v�32sSpace-time �21m �33s
    A term used to denote the geometry of the physical universe as suggested
by the theory of relativity. It is also called space-time continuum. Whereas
in Newtonian physics space and time had been considered quite separate
entities, A. Einstein and H. Minkowski showed that they are actually
intimately intertwined.
    Einstein showed that in general two observers, each using the same
techniques of observation but being in motion relative to each other, will
disagree concerning the simultaneity of distant events. But if they do
disagree, they are also unable to compare unequivocally the rates of clocks
moving in different ways, or the lengths of scales and measuring rods.
Instead, clock rates and scale lengths of different observers and different
frames of reference must be established so as to assure the principal observed
fact. Each observer, using his own clocks and scales, must measure the same
speed of propagation of light. This requirement leads to a set of
relationships known as the Lorentz transformations.  See  LORENTZ
TRANSFORMATIONS.
    In accordance with the Lorentz transformations, both the time interval and
the spatial distance between two events are relative quantities, depending on
the state of motion of the observer who carries out the measurements. There
is, however, a new absolute quantity that takes the place of the two former
quantities. It is known as the invariant, or proper, space-time interval TAU
and is defined by Eq. (1), where T is the


      TAU = T(sup 2) - [(1/c[sup 2])R(sup 2)			 (1)


ordinary time interval, R the distance between the two events, and c the speed
of light in empty space. Whereas T and R are different for different
observers, TAU has the same value. In the event that Eq. (1) would render TAU
imaginary, its place may be taken by SIGMA, defined by Eq. (2). If both TAU
and SIGMA are zero, then

	   SIGMA(sup 2) = R(sup 2) - c(sup 2)T(sup 2)		 (2)

a light signal leaving the location of one event while it is taking place will
reach the location of the other event precisely at the instant the signal from
the latter is coming forth.
    The existence of a single invariant interval led the mathematician
Minkowski to conceive of the totality of space and time as a single
four-dimensional continuum, which is often referred to as the Minkowski
universe. In this universe, the history of a single space point in the course
of time must be considered as a curve (or line), whereas an event, limited
both in space and time, represents a point. So that these geometric concepts
in the Minkowski universe may be distinguished from their analogs in ordinary
three-dimensional space, they are referred to as world curves (world lines)
and world points, respectively.  See  GRAVITATION; RELATIVITY.
 �34s[Peter G. Bergmann]
�33s

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�6v�2v�32sSpallation reaction   �21m �33s 
    A nuclear reaction in which the interacting nuclei are disintegrated into 
a large number of the constituent protons, neutrons, and other light 
particles. Compared to more conventional reactions involving the simple 
transfer of nucleons between the colliding nuclei, spallation is a violent 
process occurring at high incident energy. In extreme cases the nuclei are 
apparently completely shattered into individual nucleons. Spallation can be 
induced by lighter particles, such as high-energy protons; however, these 
reactions are usually less violent and produce only a few particles (typically 
3 to 10). Although the most abundant final products are protons, neutrons, and 
alpha particles, the spallation of larger fragments such as lithium and carbon 
also occurs. 
    The spallation reaction plays an important role in determining the 
abundance of elements in the universe. In particular, sup6Li, sup9Be, 
sup1sup0B, and sup1sup1B are created from spallation of interstellar gas by 
galactic cosmic rays.  See  NUCLEAR REACTION. 
 �34s[David K. Scott]
�33s 
  
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�6v�2v�32sSpark chamber   �21m �33s 
    A triggered electronic particle-detecting device whose purpose is to make 
visible and to locate accurately in space the tracks of charged particles. It 
is generally classified as wide-gap or narrow-gap; the most common form is a 
narrow-gap array of parallel-plate condensers, the plates of which are spaced 
about 3/8 in. (10 mm) apart, filled with a mixture of helium and neon at 
atmospheric pressure. A spark-chamber system requires an external initiating 
signal from an auxiliary particle-detection system, which triggers the 
application of a short-duration high-voltage pulse to the array of plates. The 
high-voltage pulse produces a spark discharge that follows or marks the path 
of the ionizing particles. If several tracks are present they are all visible, 
up to at least 20 (although the chamber will not resolve tracks less than 
about 0.04 in. or 1 mm apart). Stereo cameras usually provide accurate track 
location; electronic and digital methods of track location are also used. 
Spark chambers are characterized by somewhat lower resolution than bubble 
chambers, a high rate of data collection, and a unique triggering capability 
that selects only those events that have passed logical selection tests to be 
photographed.  See  BUBBLE CHAMBER; PARTICLE DETECTOR; SPARK COUNTER. 
 �34s[Arthur Roberts]
�33s 
  
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�6v�2v�32sSpark counter   �21m �33s 
    A particle detector which uses the ionization produced in a gas by 
high-speed charged particles to trigger a spark between two electrodes. Spark 
counters react in a very short time (about 10-sup9 s) to the particle, and 
thus can be used for fast timing. The spark is visible and can be 
photographed.  See  PARTICLE DETECTOR. 
    The principal components of a spark counter are two plane, parallel 
metallic electrodes, with a gas between the electrodes consisting of a mixture 
of argon and an organic gas such as xylene. A potential difference of about 
2000 volts is placed across the electrodes, which are spaced about 0.08 in. or 
2 mm apart.  See  SPARK CHAMBER. 
 �34s[William B. Fretter]
�33s 
  
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�6v�2v�32sSpark gap   �21m �33s 
    The region between two electrodes in which a disruptive electrical spark 
may take place. The gap should be taken to mean the electrodes as well as the 
intervening space. Such devices may have many uses. The ignition system in a 
gasoline engine furnishes a very important example. Another important case is 
the use of a spark gap as a protective device in electrical equipment. Here, 
surges in potential may be made to break down such a gap so that expensive 
equipment will not be damaged.  See  BREAKDOWN POTENTIAL; ELECTRIC SPARK. 
 �34s[Glenn H. Miller]
�33s 
  
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�6v�2v�32sSpark plug   �21m �33s 
    A device that screws into the cylinder of an internal combustion engine to 
provide a pair of electrodes between which an electrical discharge is passed 
to ignite the combustible mixture. It consists of an outer steel casing that 
is electrically grounded to the engine and a ceramic insulator, sealed into 
the casing, through which a central electrode passes (see illustration). The 
high-tension current jumps the gap between this electrode and a similar one 
fixed to the outer casing. The electrodes are made of alloys that resist 
electrical and chemical erosion. The parts exposed to the combustion gases are 
designed to operate at temperatures hot enough to prevent electrically 
conducting deposits, but cool enough to avoid ignition of the mixture before 
the spark occurs.  See  IGNITION SYSTEM. 
 �34s[Augustus R. Rogowski]
�33s 
  
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�6v�2v�32sSpatangoida   �21m �33s 
    An order of exocyclic Euechinoidea in which the posterior ambulacral 
plates form a shield-shaped area behind the mouth. The plates are arranged in 
two similar parallel longitudinal series. The apical system is compact. The 
families are defined mainly by reference to the fascioles, which are 
ribbonlike bands of minute, close-set uniform ciliated spinules on various 
parts of the test. The order reached its maximum in the mid-Tertiary, but is 
still richly represented today in most seas.  See  EUECHINOIDEA. 
 �34s[Howard B. Fell]
�33s 
  
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�6v�2v�32sSpearmint   �21m �33s 
    Either of two vegetatively propagated, clonal cultivar species (Mentha 
spicata and M. longifolia) of mints of the family Lamiaceae (Labiatae). They 
are grown primarily in Idaho, Indiana, Michigan, Washington, and Wisconsin as 
a source of essential oil of spearmint. 
    Principal uses of the oil are in flavoring gum, toothpaste, and candy. 
Chopped fresh leaves of M. spicata preserved in vinegar are used as a 
condiment served with lamb, especially in England, and dried or freeze-dried 
leaves of several strains are used in flavoring soups, stews, tea, or sauces. 
Sprigs of the decorative curly mint M. crispa (or M. spicata var. crispata) 
are often used in mixed drinks such as mint juleps.  See  LAMIALES. 
 �34s[Merritt J. Murray]
�33s 
  
  
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�6v�2v�32sSpecial functions �21m �33s
    Functions which occur often enough to acquire a name. Some of these, such
as the exponential, logarithmic, and the various trigonometric functions, are
extensively taught in school and occur so frequently that routines for
calculating them are built into many pocket calculators.  See
DIFFERENTIATION; LOGARITHM; TRIGONOMETRY.
    The more complicated special functions, or higher transcendental functions
as they are often called, have been extensively studied by many mathematicians
because they arose in the problems which were being studied. Among the more
useful functions are the following: the gamma function defined by Eq. (1),
which generalizes the factorial; the related beta function

 GAMMA(x) = INTEGRATION (0 TO INFINITY) t(sup [x-1] e(sup -t) dt	   (1)

defined by Eq. (2), which generalizes the binomial coefficient;

B(x,y) = INTEGRATION (0 TO 1) t(sup x-1)(1 - t)(sup y-1)  dt		   (2)

and elliptic integrals, which arose when mathematicians tried to
determine the arc length of an ellipse, and their inverses, the elliptic
functions. The hypergeometric function and its generalizations includes many
of the special functions which occur in mathematical physics, such as Bessel
functions, Legendre functions, error functions, and the classical orthogonal
polynomials of Jacobi, Laguerre, and Hermite. The zeta function defined by Eq.
(3) has many applications in number the

ZETA(s) = SUMMATION (n=1 TO INFINITY)n(sup -s)				   (3)

ory, and it also arises in M. Planck's work on radiation.  See  BESSEL
FUNCTIONS; ELLIPTIC FUNCTION AND INTEGRAL; GAMMA FUNCTION; HEAT RADIATION;
HYPERGEOMETRIC FUNCTIONS; LEGENDRE FUNCTIONS; NUMBER THEORY; ORTHOGONAL
POLYNOMIALS.
 �34s[Richard Askey]
�33s

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�6v�2v�32sSpeciation   �21m �33s 
    The process by which new species of organisms evolve from preexisting 
species. It is part of the whole process of organic evolution, and the modern 
period of its study began with the publication in 1858 of Charles Darwin's and 
Alfred Russel Wallace's theory of evolution by natural selection, and in 1859 
of Darwin's On the Origin of Species. 
    There was no problem of speciation during the period when it was believed 
that species were divinely created and immutable. Belief in the fixity of 
species was almost universal before the middle of the 19th century. Then it 
was gradually realized that all species continuously change, or evolve; 
however, the causative mechanism remained to be discovered. 
    Darwin proposed a mechanism. He argued that: (1) Within any species 
population there is always some heritable variation; the individuals differ 
among themselves in structure, physiology, and behavior. (2) Natural selection 
acts upon this variation by eliminating the less fit. Thus if two members of 
an animal population differ from each other in their ability to find a mate, 
obtain food, escape from predators, resist the ravages of parasites and 
pathogens, or survive the rigors of the climate, the more successful will be 
more likely than the less successful to leave descendants. The more successful 
is said to have greater fitness, to be better adapted, or to be selectively 
favored. Likewise among plants: one plant individual is fitter than another if 
its heritable characteristics make it more successful than the other in 
obtaining light, water, and nutrients, in protecting itself from herbivores 
and disease organisms, or in surviving adverse climatic conditions. Over the 
course of time, as the fitter members of a population leave more descendants 
than the less fit, their characteristics become commoner.  See  SPECIES 
CONCEPT.  
    This is the process of natural selection, which tends to preserve the well 
adapted at the expense of the ill adapted in a variable population. The 
genetic variability that must exist if natural selection is to act is 
generated by genetic mutations in the broad sense, including chromosomal 
rearrangements together with point mutations.  See  GENETICS; MUTATION. 
    If two separate populations of a species live in separate regions, exposed 
to different environments, natural selection will cause each population to 
accumulate characters adapting it to its own environment. The two populations 
will thus diverge from each other and, given time, will become so different 
that they are no longer interfertile. At this point, speciation has occurred: 
two species have come into existence in the place of one. This mode of 
speciation, speciation by splitting, is probably the most common mode. 
 �34s[E. C. Pielou]
�33s 
  
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�6v�2v�32sSpecies concept   �21m �33s 
    The idea that the diversity of nature is divisible into a finite number of 
definable kinds, or species. In biology the species concept is of threefold 
importance: as the basic unit of classification, as a step of irreversibility 
in evolution (origin of species), and as the unit of ecological diversity 
(niche utilization and so on). The idea of species is fundamental to all 
biological research. 
    The members of a species form: (1) a reproductive community in which the 
individuals of a species (of animals) recognize each other as potential mates 
and seek each other for the purpose of reproduction - a multitude of devices 
(isolating mechanisms) ensures intraspecific reproduction in all organisms; 
(2) an ecological unit which, regardless of the individuals composing it, 
interacts as a unit with other species with which it shares the environment - 
each species fills a particular ecological niche; and (3) a genetic unit 
consisting of a large, intercommunicating gene pool, whereas the individual is 
merely a temporary vessel holding a small portion of the contents of the gene 
pool for a short period of time. These three properties raise the species 
above the typological interpretation of a ``class of objects.'' 
    The species is a relational (rather than intrinsic) concept, because a 
species is better characterized by its property of reproductive isolation from 
other species (its relation of noninterbreeding) than by intrinsic properties. 
Derived from the observation of the reproductive gap between species that 
coexist at a given locality, species are most meaningful where the relation of 
two coexisting populations is described, that is, without the complications of 
the dimensions of area (longitude and latitude) and time. Such species are 
nondimensional for all practical purposes. Two or more species are called 
sympatric when their geographical ranges overlap. Species that occupy mutually 
exclusive geographical areas are allopatric.  See  SPECIATION. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sSpecific charge   �21m �33s 
    The ratio of charge to mass expressed as e/m, of a particle. The 
acceleration of a particle in electromagnetic fields is proportional to its 
specific charge. Specific charge can be determined by measuring the velocity 
which the particle acquires in falling through an electric potential; by 
measuring the frequency of revolution in a magnetic field; or by observing the 
orbit of the particles in combined electric and magnetic fields.  See  
ELECTRON MOTION IN VACUUM; ELEMENTARY PARTICLE. 
 �34s[Charles J. Goebel]
�33s 
  
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�6v�2v�32sSpecific fuel consumption   �21m �33s 
    Weight flow rate of fuel required to produce a unit of power or of thrust, 
often abbreviated as SFC. In reciprocating piston engines, SFC is usually 
expressed as pounds of fuel per hour required to deliver 1 shaft horsepower. 
For ram and turbojet engines it is given as pounds per hour for one pound of 
thrust. For turboprops, where both shaft and jet power are taken from the 
engine, SFC is given as fuel rate for equivalent shaft horsepower.  See  
RAMJET; TURBOJET. 
 �34s[Robert R. Hibbard]
�33s 
  
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�6v�2v�32sSpecific gravity   �21m �33s 
    The specific gravity of a material is defined as the ratio of its density 
to the density of some standard material, such as water at a specified 
temperature, for example, 60DGF (15DGC), or (for gases) air at standard 
conditions of temperature and pressure. Specific gravity is a convenient 
concept because it is usually easier to measure than density, and its value is 
the same in all systems of units.  See  DENSITY. 
 �34s[Leo Nedelsky]
�33s 
  
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�6v�2v�32sSpecific heat   �21m �33s 
    The ratio of the amount of heat required to raise unit mass of a material 
1 degree in temperature to the amount of heat required to raise the same mass 
of a reference substance 1 degree in temperature. Both measurements are made 
at a reference temperature and in nearly all cases at either constant volume 
or constant pressure. Water is usually the reference substance. Because the 
heat capacity of water is nearly unity, the value of specific heat for a 
material is nearly equal to its heat capacity. Specific heat, as defined here, 
is a ratio without units, although it is often defined differently. For 
clarity it is recommended that thermodynamic discussion be carried out in 
terms of heat capacity instead of specific heat. Also, it is desirable to 
define heat units in electrical terms.  See  CHEMICAL THERMODYNAMICS; HEAT 
CAPACITY.  
 �34s[Harold C. Weber]
�33s 
  
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�6v�2v�32sSpecific impulse   �21m �33s 
    The quantity used to define the amount of thrust potentially available 
from rocket propellant combinations; also called specific thrust. Specific 
impulse is the thrust that theoretically can be obtained when unit weight of 
the propellant reacts in unit time, that is, pound thrust per pound per second 
of propellant flow. This ratio for specific impulse has the dimension of time 
and can be expressed as seconds. For example, a propellant having an impulse 
of 300 will theoretically deliver 300 lb of thrust when the propellant is 
consumed at the rate of 1 lb/s. Alternatively, 1 lb of propellant will deliver 
1 lb of thrust for 300 s.   See  JET FUEL; PROPULSION; THRUST.  
 �34s[Robert R. Hibbard]
�33s 
  
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�6v�2v�32sSpeckle   �21m �33s 
    The generation of a random intensity distribution, called a speckle 
pattern, when light from a highly coherent source, such as a laser, is 
scattered by a rough surface or inhomogeneous medium. Although the speckle 
phenomenon has been known since the time of Isaac Newton, the development of 
the laser is responsible for the present-day interest in speckle. Speckle has 
proved to be a universal nuisance as far as most laser applications are 
concerned, and it was only in the mid-1970s that investigators turned from the 
unwanted aspects of speckle toward the uses of speckle patterns, in a wide 
variety of applications. The principle applications for speckle patterns fall 
into two areas: metrology, and stellar speckle interferometry.  See  LASER. 
    Objects viewed in coherent light acquire a granular appearance as shown in 
the illustration. The surfaces of most materials are extremely rough on the 
scale of an optical wavelength. When nearly monochromatic light is reflected 
from such a surface, the optical wave resulting at any moderately distant 
point consists of many coherent wavelets, each arising from a different 
microscopic element of the surface. Since the distances traveled by these 
various wavelets may differ by several wavelengths if the surface is truly 
rough, the interference of the wavelets of various phases results in the 
granular pattern of intensity called speckle. 
 �34s[James C. Wyant]
�33s 
  
  
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�6v�2v�32sSpectrochemical analysis   �21m �33s 
    Any of a number of techniques for the determination of the presence or the 
concentration of elemental or molecular constituents in a sample through the 
use of spectrometric measurements. Spectrometric measurements entail the 
monitoring of electromagnetic radiation as it is caused to emanate from, or 
interact with, the sample of interest. 
    The interaction of an analyte with electromagnetic radiation is based on 
changes in the level of some characteristic energy state of the analyte, for 
example, the oscillatory motion of a chemical bond, or the orbital location of 
a valence electron, or the rotational motion of the magnetic vector of an 
atomic nucleus. All of these types of characteristic states are quantized in 
energy by the principles of quantum mechanics. The change in energetic state 
of an analyte can be caused by the absorption of emission of energy of an 
amount exactly equal to the difference in energies of the two states.  See  
QUANTUM MECHANICS. 
    In the basic spectrometric experiment which is performed for 
spectrochemical analysis, the sample is put into a state in which it will 
interact in the manner desired with the radiation of choice (see 
illustration). For absorption and luminescence measurements, a source of the 
radiation of choice is focused onto the sample so that the radiation may be 
absorbed. For emission measurements the sample is supplied with external 
energy which is not radiative, for example, the heat from a flame, or an 
applied voltage, or momentum from mechanical motion. The sample then emits the 
gained energy as photons (packets, quanta) of radiation. 
    The next stage of the experiment separates all the wavelengths of 
radiation present so that they may be measured independently of all others. 
Finally, there is a device for detecting the amount of radiation present 
(emission or luminescence) or absent (absorption). The identity of the 
wavelengths gives qualitative information. The amount of radiation gives 
quantitative information.  See  GAMMA-RAY DETECTORS; GEIGER-MUELLER COUNTER; 
PHOTOTUBE; SPECTROSCOPY; TRANSDUCER. 
 �34s[Andrew T. Zander]
�33s 
  
  
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�6v�2v�32sSpectrography   �21m �33s 
    The use of photography to record the electromagnetic spectrum displayed in 
a spectroscope. The technique is used mainly in atomic and molecular physics, 
in analysis of the chemical composition of materials, and in astronomical 
photography.  See  ASTRONOMICAL PHOTOGRAPHY; SPECTROCHEMICAL ANALYSIS; 
SPECTROSCOPY.  
 �34s[Walter Clark]
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�6v�2v�32sSpectrohelioscope �21m �33s
    An instrument for the monochromatic visual observation of the Sun. A
telescope projects an image of the Sun on the first slit of a powerful
spectroscope. The resulting spectrum is imaged in the plane of a second slit
which permits only a single line element of the spectrum to emerge from the
instrument. The emergent line element is a monochromatic image of that part of
the Sun that falls on the first slit. When the two slits are vibrated
synchronously at high frequency, persistence of vision permits monochromatic
observation of an area of the solar surface. The slits may also be moved at a
slow rate and the image recorded photographically. This modification of the
spectrohelioscope is a simple form of the spectroheliograph.
 �34s[Robert R. McMath /John W. Evans]
�33s

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�6v�2v�32sSpectrophotometric analysis   �21m �33s 
    A method of chemical analysis based on the absorption or attenuation by 
matter of electromagnetic radiation of a specified wavelength or frequency. 
The region of the electromagnetic spectrum most useful for chemical analysis 
is that between 200 nanometers and 300 micrometers. Since the sample being 
analyzed absorbs the radiation, spectrophotometric analysis is sometimes 
referred to as absorptimetric analysis. 
    The instruments used in this work are referred to as spectrophotometers. A 
simple spectrophotometer consists of a source of radiation, such as a light 
bulb; a monochromator containing a prism or grating which disperses the light 
so that only a limited wavelength, or frequency, range is allowed to irradiate 
the sample; the sample itself; and a detector, such as a photocell, which 
measures the amount of light transmitted by the sample. 
    In most quantitative analytical work, a calibration or standard curve is 
prepared by measuring the absorption of known amounts of the absorbing 
material at the wavelength at which it strongly absorbs. Such a calibration 
curve is shown in Fig. 1 for the absorbing material whose absorption spectrum 
is shown in Fig. 2. The absorbance of the sample is read directly from the 
measuring circuit of the spectrophotometer. 
    When the transmittance or the absorbance of a sample is measured and 
plotted as a function of wavelength, an absorption spectrum is obtained. The 
spectrum shown in Fig. 2 indicates that the sample transmits the least light 
at 410 nm and transmits the most around 700 nm. 
  
    Infrared spectrophotometry.  The interaction with matter of 
electromagnetic radiation of wavelength between 1 and 300 MUm (infrared 
region) induces either rotational or vibrational energy level transitions, or 
both, within the molecules involved. The frequencies of infrared radiation 
absorbed by a molecule are determined by its rotational energy levels and by 
the force constants of the bonds in the molecule. Since these energy levels 
and force constants are usually unique for each molecule, so also the infrared 
spectrum of each molecule is usually unique. Because of their individuality, 
infrared spectra of organic compounds are considered equivalent to, or 
superior to, the preparation of chemical derivatives for the identification of 
species in organic chemistry. For this reason the infrared portion of the 
spectrum is often called the fingerprint region. 
  
    Attenuated total reflectance.  This is a technique in which the infrared 
spectrum of a surface can be obtained without any chemical treatment of the 
sample; its principle is based on the phenomenon of energy reflection at the 
interface of two media which have different refractive indexes and are in 
optical contact with each other. The spectrum obtained is similar to that of a 
transmission spectrum of a material. This technique is used extensively for 
analyzing coatings, pastes, paints, fibers, and fabrics. 
  
  
    Near-infrared spectrophotometry.  This designates work carried out between 
0.78 and 3 MUm. The absorption bands in this region are mainly overtones 
(harmonics) of bands in the infrared region. These bands are quite sharp and 
are of great value in quantitative analysis for various functional groups, 
particularly those containing hydrogen atoms. 
  
    Visible spectrophotometry.  The visible region of the spectrum covers the 
narrow range from about 380 to 780 nm. The spectrophotometers for this region 
use tungsten lamps as light sources, glass or quartz prisms or gratings in the 
monochromators, and photomultiplier cells as detectors. Within this narrow 
portion of the electromagnetic spectrum, a majority of the spectrophotometric 
analyses are made. Figure 2 shows a typical visible absorption spectrum. The 
substance, peroxytitanate ion, absorbs light in the region below 500 nm; that 
is, it absorbs violet, blue, and green light and transmits red, orange, and 
yellow. For analytical work, the wavelength of maximum absorption is usually 
used, in this case 410 nm. 
  
    Ultraviolet spectrophotometry.  The spectral region from 200 to 400 nm, 
called the near ultraviolet, is commonly used in chemical analysis. 
Ultraviolet spectrophotometers usually have a hydrogen lamp as a radiation 
source; a quartz prism, or a grating in the monochromator; and a 
photomultiplier tube as a detector. Simple inorganic ions and their complexes 
as well as organic molecules can be detected and determined in this region. 
  
    Filter photometry.  In filter photometry, the monochromator of the 
spectrophotometer is replaced by a filter. This filter passes a band of light 
of a much wider range of wavelengths than those passed by even the poorest 
monochromator. The filter is chosen so as to transmit best the light which the 
sample absorbs most. Filter photometers are generally much less expensive than 
spectrophotometers. By careful use of calibration curves, filter photometers 
can give sufficiently accurate and precise results for a wide variety of 
applications.  See  ANALYTICAL CHEMISTRY; MOLECULAR STRUCTURE AND SPECTRA; 
OPTICAL METHODS OF CHEMICAL ANALYSIS. 
 �34s[James N. Little]
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�6v�2v�32sSpectroscopy   �21m �33s 
    An analytic technique concerned with the measurement of the interaction 
(usually the absorption or the emission) of radiant energy with matter, with 
the instruments necessary to make such measurements, and with the 
interpretation of the interaction both at the fundamental level and for 
practical analysis. 
    A display of such data is called a spectrum, that is, a plot of the 
intensity of emitted or transmitted radiant energy (or some function of the 
intensity) versus the energy of that light. Spectra due to the emission of 
radiant energy are produced as energy is emitted from matter, after some form 
of excitation, then collimated by passage through a slit, then separated into 
components of different energy by transmission through a prism (refraction) or 
by reflection from a ruled grating or a crystalline solid (diffraction), and 
finally detected. Spectra due to the absorption of radiant energy are produced 
when radiant energy from a stable source, collimated and separated into its 
components in a monochromator, passes through the sample whose absorption 
spectrum is to be measured, and is detected. Instruments which produce spectra 
are variously called spectroscopes, spectrometers, spectrographs, and 
spectrophotometers.  See  SPECTRUM. 
    Interpretation of spectra provides fundamental information on atomic and 
molecular energy levels, the distribution of species within those levels, the 
nature of processes involving change from one level to another, molecular 
geometries, chemical bonding, and interaction of molecules in solution. At the 
practical level, comparisons of spectra provide a basis for the determination 
of qualitative chemical composition and chemical structure, and for 
quantitative chemical analysis. 
  
    Origin of spectra.  Atoms, ions, and molecules emit or absorb 
characteristically; only certain energies of these species are possible; the 
energy of the photon (quantum of radiant energy) emitted or absorbed 
corresponds to the difference between two permitted values of the energy of 
the species, or energy levels. (If the flux of photons incident upon the 
species is great enough, simultaneous absorption of two or more photons may 
occur.) Thus the energy levels may be studied by observing the differences 
between them. The absorption of radiant energy is accompanied by the promotion 
of the species from a lower to a higher energy level; the emission of radiant 
energy is accompanied by falling from a higher to a lower state; and if both 
processes occur together, the condition is called resonance. 
  
    Instruments.  Spectroscopic methods involve a number of instruments 
designed for specialized applications. 
    An optical instrument consisting of a slit, collimator lens, prism or 
grating, and a telescope or objective lens which produces a spectrum for 
visual observation is called a spectroscope. 
    If a spectroscope is provided with a photographic camera or other device 
for recording the spectrum, the instrument is called a spectrograph.  See  
SPECTROGRAPHY.  
    A spectroscope that is provided with a calibrated scale either for 
measurement of wavelength or for measurement of refractive indices of 
transparent prism materials is called a spectrometer. 
    A spectrophotometer consists basically of a radiant-energy source, 
monochromator, sample holder, and detector. It is used for measurement of 
radiant flux as a function of wavelength and for measurement of absorption 
spectra.  
    An interferometer is an optical device that measures differences of 
geometric path when two beams travel in the same medium, or the difference of 
refractive index when the geometric paths are equal. Interferometers are 
employed for high-resolution measurements and for precise determination of 
relative wavelengths.  See  INTERFEROMETRY. 
  
    Methods and applications.  Since the early methods of spectroscopy there 
has been a proliferation of techniques, often incorporating sophisticated 
technology.  
    Astronomical spectroscopy involves the study of radiant energy emitted by 
celestial objects by combined spectroscopic and telescopic techniques to 
obtain information about their chemical composition, temperature, pressure, 
density, magnetic fields, electric forces, and radial velocity.  See  
ASTRONOMICAL SPECTROSCOPY. 
    Atomic absorption and fluorescence spectroscopy is a branch of electronic 
spectroscopy that uses line spectra from atomized samples to give quantitative 
analysis for selected elements at levels down to parts per million, on the 
average.  See  FIRE ASSAYING; TRACE ANALYSIS. 
    Attenuated total reflectance spectroscopy is the study of spectra of 
substances in thin films or on surfaces obtained by the technique of 
attenuated total reflectance or by a closely related technique called 
frustrated multiple internal reflection. In either method the sample is 
penetrated by a radiant-energy beam one or more times. The technique is 
employed primarily in infrared spectroscopy for qualitative analysis of 
coatings and of opaque liquids. 
    Electron paramagnetic spectroscopy is a microwave technique, based on the 
splitting of electronic energy levels in a magnetic field and is used to 
establish structures of species containing unpaired electrons.  See  ELECTRON 
PARAMAGNETIC RESONANCE SPECTROSCOPY (EPR). 
    Electron spectroscopy includes a number of subdivisions, all of which are 
associated with electronic energy levels.  See  ELECTRON SPECTROSCOPY. 
    Fourier transform spectroscopy is a family of techniques consisting of a 
fundamentally different method of irradiation of the sample, in which all 
pertinent wavelengths simultaneously irradiate it for a short period of time 
and the absorption spectrum is obtained by mathematical manipulation of the 
cyclical power pattern so obtained. 
    Gamma-ray spectroscopy includes the techniques of activation analysis and 
Moessbauer spectroscopy.  See  ACTIVATION ANALYSIS; MOESSBAUER EFFECT; NEUTRON 
SPECTROMETRY.  
    Information on processes which occur on a picosecond time scale can be 
obtained by making use of the coherent properties of laser radiation, as in 
coherent anti-Stokes Raman spectroscopy.  See  LASER SPECTROSCOPY. 
    The source of a mass spectrometer produces ions, often from a gas, but 
also in some instruments from a liquid, a solid, or a material adsorbed on a 
surface. The dispersive unit provides either temporal or spatial dispersion of 
ions according to their mass-to-charge ratio.  See  MASS SPECTROMETRY; 
SECONDARY ION MASS SPECTROMETRY (SIMS). 
    In multiplex or frequency-modulated spectroscopy each optical wavelength 
exiting the spectrometer output is encoded with an audio frequency that 
contains the optical wavelength information. Use of a wavelength analyzer then 
allows recovery of the original optical spectrum. 
    Raman spectroscopy, in which Raman scattering of light by the molecules 
under study is analyzed by spectroscopic means, provides information of use in 
structural chemistry.  See  RAMAN EFFECT. 
    X-ray spectroscopy involves the excitation of inner electrons in atoms 
which is manifested as x-ray absorption; emission of a photon as an electron 
falls from a higher lvel into the vacancy thus created is x-ray fluorescence. 
The techniques are used for chemical analysis.  See  X-RAY FLUORESCENCE 
ANALYSIS; X-RAY SPECTROSCOPIC ANALYSIS. 
 �34s[Maurice M. Bursey]
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�6v�2v�32sSpectrum �21m �33s
    The term spectrum is applied to any class of similar entities or
properties strictly arrayed in order of increasing or decreasing magnitude. In
general, a spectrum is a display or plot of intensity of radiation (particles,
photons, or acoustic radiation) as a function of mass, momentum, wavelength,
frequency, or some other related quantity. For example, a BETA-ray spectrum
represents the distribution in energy or momentum of negative electrons
emitted spontaneously by certain radioactive nuclides, and when radionuclides
emit ALPHA-particles, they produce an ALPHA-particle spectrum of one or more
characteristic energies. A mass spectrum is produced when charged particles
(ionized atoms or molecules) are passed through a mass spectrograph in which
electric and magnetic fields deflect the particles according to their
charge-to-mass ratios. The distribution of sound-wave energy over a given
range of frequencies is also called a spectrum.  See  MASS SPECTROSCOPE; SOUND.
    In the domain of electromagnetic radiation, a spectrum is a series of
radiant energies arranged in order of wavelength or of frequency. The entire
range of frequencies is subdivided into wide intervals in which the waves have
some common characteristic of generation or detection, such as the
radio-frequency spectrum, infrared spectrum, visible spectrum, ultraviolet
spectrum, and x-ray spectrum.
    Spectra are also classified according to their origin or mechanism of
excitation, as emission, absorption, continuous, line, and band spectra. An
emission spectrum is produced whenever the radiations from an excited light
source are dispersed. An absorption spectrum is produced against a background
of continuous radiation by interposing matter that reduces the intensity of
radiation at certain wavelengths or spectral regions. The energies removed
from the continuous spectrum by the interposed absorbing medium are precisely
those that would be emitted by the medium if properly excited. A continuous
spectrum contains an unbroken sequence of waves or frequencies over a long
range. Line spectra are discontinuous spectra characteristic of excited atoms
and ions, whereas band spectra are characteristic of molecular gases or
chemical compounds.  See  ATOMIC STRUCTURE AND SPECTRA; BAND SPECTRUM;
ELECTROMAGNETIC RADIATION; LINE SPECTRUM; MOLECULAR STRUCTURE AND SPECTRA;
SPECTROSCOPY.
 �34s[William F. Meggers /William W. Watson]
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�6v�2v�32sSpectrum analyzer   �21m �33s 
    A device which sweeps over a portion of the radio-frequency spectrum, 
responds to signals whose frequencies lie within the swept band, and displays 
them in relative magnitude and frequency on a cathode-ray-tube screen. In 
essence, it is a superheterodyne receiver having a local oscillator whose 
frequency is varied cyclically, usually at the power-line frequency.  See  
RADIO RECEIVER. 
    Spectrum analyzers are used specifically to study the spectra of pulsed 
transmitters, such as radar, to make sure that they are operating properly, 
without spurious emissions. Spectrum analyzers are often built into test 
equipment and military identification receivers. The output display shown in 
the illustration can be used to monitor radio traffic in the frequency band 
being covered; transmitter activity which might be overlooked by a standard 
receiver groping about in its tuning range will show up on the panorama 
display with its carrier frequency identified.  See  ELECTRIC POWER 
MEASUREMENT; FREQUENCY MEASUREMENT. 
 �34s[Mark G. Foster]
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�6v�2v�32sSpeech �21m �33s
    A set of audible sounds produced by disturbing the air through the
integrated movements of certain groups of anatomical structures. Humans attach
symbolic values to these sounds for communication. There are many approaches
to the study of speech.
    The physiology of speech production may be described in terms of
respiration, phonation, and articulation. These interacting processes are
activated, coordinated, and monitored by acoustical and kinesthetic feedback
through the nervous system.

    Respiration.  Most speech sounds are formed during exhalation.
Consequently, during speech the period of exhalation is generally much longer
than that of inhalation. The aerodynamics of the breath stream influence the
rate and mode of the vibration of the vocal folds. This involves interactions
between the pressures initiated by thoracic movements and the position and
tension of the vocal folds.
    The pressure pattern of the sublaryngeal air is closely related to the
loudness of the voice and appears to be correlated with the perception of
stress. For example, the word ``permit' may be either a noun or a verb,
depending on the placement of stress. Various attempts have been made to
correlate units such as the syllable and the phrase to the contractions of
specific respiratory muscles.

    Phonation.	The phonatory and articulatory mechanisms of speech may be
regarded as an acoustical system whose properties are comparable to those of a
tube of varying cross-sectional dimensions. At the lower end of the tube, or
the vocal tract, is the larynx. The upper end of the vocal tract may terminate
at the lips, at the nose, or both. The length of the vocal tract averages 6.4
in. (16 cm) in men and may be increased by either pursing the lips or lowering
the larynx.
    The larynx is the primary mechanism for phonation, that is, the generation
of the glottal tone. The vocal folds attach anteriorly to the thyroid
cartilage and posteriorly to the vocal processes of the arytenoid cartilages.
The vibrating edge of the vocal folds measures about 0.92 - 1.08 in. (23 - 27
mm) in men and considerably less in women. The aperture between the vocal
folds is called the glottis. The tension and position of the vocal folds are
adjusted by the intrinsic laryngeal muscles, primarily through movement of the
two arytenoid cartilages. By contraction of groups of muscles, the arytenoid
cartilages may be pulled either anterior-posteriorly or laterally in opposite
directions. They may also be rotated about the vertical axis. During
whispering and unvoiced sounds, the glottis assumes the shape of a triangle,
with the apex directly behind the thyroid cartilage.  See  LARYNX.
    When the vocal folds are brought together and there is a balanced air
pressure to drive them, they vibrate laterally in opposite directions. During
phonation, the vocal folds control the energy by regulating the frequency and
amount of air passing through the glottis. Their rate and mode of opening and
closing are dependent upon the position and tension of the folds and the
pressure and velocity of airflow. The tones are produced by the recurrent
puffs of air passing through the glottis and striking into the supralaryngeal
cavities.
    As the air passes through the glottis with increasing velocity, the
pressure perpendicular to the direction of air flow is reduced. This allows
the tension of the folds to draw them together, either partly or completely,
until sufficient pressure is built up to drive them apart again.
    Speech sounds produced during phonation are called voiced. Almost all of
the vowel sounds of the major languages and some of the consonants are voiced.
In English, voiced consonants may be illustrated by the initial and final
sounds in the following words: bathe, dog, man, jail. The speech sounds
produced when the vocal folds are apart and are not vibrating are called
unvoiced; examples are the consonants in the words hat, cap, sash, faith.
During whispering all the sounds are unvoiced.
    The rate of vibration of the vocal folds is the fundamental frequency of
the voice and correlates well with the perception of pitch. The frequency
increases when the vocal folds are made taut. Relative differences in the
fundamental frequency of the voice are utilized in almost all languages to
signal some aspects of linguistic information.

    Articulation.  The activity of the structures above and including the
larynx in forming speech sound is called articulation. It involves some
muscles of the pharynx, palate, tongue, and face and of mastication.
    The primary types of speech sounds of the major languages may be
classified as vowels, nasals, plosives, and fricatives. They may be described
in terms of degree and place of constriction along the vocal tract.  See
PHONETICS.
    The only source of excitation for vowels is at the glottis. During vowel
production the vocal tract is relatively open and the air flows over the
center of the tongue, causing a minimum of turbulence. The phonetic value of
the vowel is determined by the resonances of the vocal tract, which are in
turn determined by the shape and position of the tongue and lips.
    The nasal cavities can be coupled onto the resonance system of the vocal
tract by lowering the velum and permitting airflow through the nose. Vowels
produced with the addition of nasal resonances are called nasalized vowels.
Nasalization may be used to distinguish meanings of words made up of otherwise
identical sounds, such as bas and banc in French. If the oral passage is
completely constricted and air flows only through the nose, the resulting
sounds are nasal consonants.
    Plosives are characterized by the complete interception of airflow at one
or more places along the vocal tract. The pressure which is built up behind
the intercepting mechanism may not be immediately released or may be released
through the oral or nasal orifice. The places of constriction and the manner
of the release are the primary determinants of the phonetic properties of the
plosives. The words par, bar, tar, car begin with plosives.
    Fricatives are produced by partial constriction along the vocal tract
which results in turbulence. Their properties are determined by the place or
places of constriction and the shape of the modifying cavities. The fricatives
in English may be illustrated by the initial and final consonants in the words
vase, this, faith, hash.  See  SPEECH DISORDERS.
 �34s[Ronald S. Tikofsky ;William S. Y. Wang]
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�6v�2v�32sSpeech disorders �21m �33s
    Speech disorders may be classified according to their causes or symptoms.
The major causes are organic, imitative environmental, and psychogenic.
Organic disorders may result from disease, impairment, or absence of the
organs of speech. Imitative disorders occur when the child imitates defective
speech. A speech disorder has a psychogenic origin when there is a
psychological basis for its presence. A classification includes disorders of
articulation, rhythm, voice, and symbolization.
    Disorders of articulation may be so severe that the resultant speech is
unintelligible. Specific sounds or groups of sounds may be omitted, added,
substituted, or distorted. The following are some examples of this type of
disorder: Lalling, involving misarticulated r, l, t, and d sounds, may be
caused by poor control of the tongue tip. In lisping, misarticulated sibilant
sounds, particularly s and z, are often substituted by the th sound. Delayed
speech, involving the absence of many consonants and poor intelligibility, is
often caused by slow physical or psychological maturation. Dysarthria,
generalized sound substitutions and distortions, is caused by lesions in the
peripheral or central nervous system.
    Disorders of rhythm are characterized by disruptions of the normal rate of
speech. Two common disorders of rhythm are stuttering or stammering and
cluttering.
    Voice disorders are usually described as defects of pitch, loudness, and
voice quality. Improper use of the voice may cause an injury to the vocal
folds and intensify an existing voice disorder. Disorders of pitch are
characterized as too high, too low, monotonous, and repeated pitch patterns. A
high-pitched voice is most often caused by psychological tension. The muscles
of the larynx are contracted so that the pitch is raised beyond its normal
range. Voice quality disorders are usually described as hoarseness, nasality,
denasality, and similar conditions. Hoarseness may be caused by pharyngeal or
laryngeal pathologies. Cleft palate speech is a striking example of nasality.
Nasal speech may also follow a paralysis of the palatal muscles. The nasal
speech results because the palate is unable to aid in achieving nasopharyngeal
closure.
    Symbolization disorders involve an impairment of language formulation and
expression. They may occur without a concomitant impairment of speech
production. Common types of these disorders are aphasia and delayed speech.
See  SPEECH.
 �34s[Ronald S. Tikofsky ;William S. Y. Wang]
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�6v�2v�32sSpeech recognition   �21m �33s 
    The process of analyzing an acoustic speech signal to identify the 
linguistic message that was intended, so that a machine can correctly respond 
to spoken commands. 
    Speaking commands to a computer is the ultimate in natural 
human-to-machine communications. Natural, spontaneous interactions are 
possible, with little or no user training, permitting direct access by the 
physically handicapped or those who cannot type well. Speech allows rapid data 
entry and frees the user's hands and eyes for other tasks. 
    Fluent conversation with a machine is difficult to achieve because of 
intrinsic variabilities and complexities of speech, as well as limitations in 
current practical capabilities. Recognition difficulty increases with 
vocabulary size, confusability of words, reduced frequency bandwidth, noise, 
frequency distortions, the population of speakers that must be understood, and 
the form of speech to be processed. To limit the problem, speech recognizers 
have been primarily confined to small vocabularies of words with large 
differences in sound structure, and isolated words, with pauses before and 
after each word. 
    Isolated word recognizers are thus the easiest to develop, followed in 
turn by recognizers of digit strings and strictly formatted word sequences. 
Sometimes the identification of every spoken word is not necessary, and 
detecting key words in context is enough to determine the topic of a 
conversation. Such a word-spotting system usually deals only with the 
important words, which are stressed and well articulated, easing the 
recognition, despite arbitrary contexts. Full understanding of the total 
sentence meaning and intended machine response involves cooperative use of 
many ``knowledge sources'' like acoustics, phonetics, prosodics, restricted 
syntax, semantics, and task-dictated discourse constraints. The term 
speech-understanding system is frequently used to describe such a 
sentence-understanding device. 
    Speech recognition requires the transformation of the continuous-speech 
signal into discrete representations which may be assigned proper meanings, 
and which, when comprehended, may be used to effect responsive behavior. Only 
some of the information in the speech signal is related to selecting the 
correct machine response, so that a critical task is to extract all, and only, 
those parts that convey the message. Recognizers primarily differ on how they 
reduce the data to message-distinguishing features, and how they classify 
utterances from that reduced data. 
    Since most devices must be trained to the specific pronunciation of each 
talker, a precompiling stage involves the user speaking sample pronunciations 
for each allowable utterance (word, phrase, or sentence), while identifying 
each with its typewritten form or a vocabulary item number. Later, when an 
unknown utterance is spoken, it is compared with all the lexicon of expected 
pronunciations to find which training sample it most closely resembles. For 
complex utterances, other linguistic and situational information can be used 
to help guide and confirm hypothesized word sequences. 
 �34s[Wayne A. Lea]
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�6v�2v�32sSpeed   �21m �33s 
    The time rate of change of position of a body without regard to direction. 
It is the numerical magnitude only of a velocity and hence is a scaler 
quantity. Linear speed is commonly measured in such units as meters per 
second, miles per hour, or feet per second. 
    Average linear speed is the ratio of the length of the path traversed by a 
body to the elapsed time during which the body moved through that path. 
Instantaneous speed is the limiting value of the foregoing ratio as the 
elapsed time approaches zero.  See  VELOCITY. 
 �34s[Rogers D. Rusk]
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�6v�2v�32sSpeed regulation   �21m �33s 
    The change in steady-state speed of a machine, expressed in percent of 
rated speed, when the machine load is reduced from rated load to zero. The 
definition of regulation is usually taken to mean the net change in a 
steady-state characteristic, and does not include any transient deviation or 
oscillation that may occur prior to reaching the new operation point. This 
same definition is used for stating the speed regulation of electric motors as 
well as for certain drive systems, such as steam turbines. 
 �34s[Paul M. Anderson]
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�6v�2v�32sSpelaeogriphacea   �21m �33s 
    A crustacean order within the Malacostraca: Peracarida, erected for a 
single living species, Spelaeogriphus lepidops. This shrimplike animal lives 
in a stream inside a cave on Table Mountain, South Africa. Spelaeogriphus has 
a slender, flexible body, seven pairs of walking legs, five pairs of 
swimmerets, and a very long flagellum on each second antenna. A short carapace 
is fused to the first thoracic segment, covers the second, and forms 
ventrolateral branchial chambers, within which are cuplike gills, attached to 
the first thoracic segment. The animals creep over the stream bottom with 
their walking legs and swim by undulations of the whole trunk. See  PERACARIDA. 
 �34s[Jean H. Langenheim]
�33s 
  
A700000002165�254s�5v�1v1636
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�6v�2v�32sSpermatogenesis   �21m �33s 
    The process by which spermatogonia become sperm through meiosis. 
Spermatocytes arise by mitotic divisions of spermatogonia. The spermatocytes 
undergo meiotic divisions to produce the spermatids with N (haploid) number of 
chromosomes.  See  GAMETOGENESIS. 
    Postmeiotically the spermatids undergo spermiogenesis, a transformation of 
nuclear and cytoplasmic constituents. The DNA within the nucleus condenses and 
assumes the shape characteristic of that species. Vesicles derived from the 
Golgi complex fuse to form the acrosomal vesicle anterior to the nucleus. The 
mitochondria fuse with each other. The distal centriole located posterior to 
the nucleus forms the point at which the nine doublet and two central 
microtubules of the axial filament in the flagellum arise. At its proximal 
end, the axial filament passes through the center of the mitochondrial helix. 
With maturation, all the other cytoplasmic structures are set aside in the 
cytoplasmic body, which is later lost.  See  MEIOSIS; SPERM CELL. 
 �34s[Gertrude W. Hinsch]
�33s 
  
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�6v�2v�32sSperm cell   �21m �33s 
    The male gamete. The typical sperm of most animals has a head containing 
the nucleus and acrosome, a middle piece with the mitochondria, and a tail 
(see illustration). Sperm, as well as the acrosome shape, varies with the 
species. The nucleus consists of condensed chromatin (deoxyribonucleic acid, 
DNA) and histone proteins. Each human sperm contains 22 autosomes and one sex 
chromosome, either an X or Y. Thus in many animals the sperm determines the 
genetic sex of the embryo. The acrosome, derived from the Golgi complex, 
contains hydrolytic enzymes, that is, hyaluronidase capable of lysing the egg 
coats at fertilization. Actin molecules which aid in sperm-egg interaction are 
found in the area between the acrosome and nucleus. The mitochondria in the 
middle piece apparently provide the energy necessary for the motility created 
by the tail. The tail has a central core, or axial filament, made up of nine 
double tubules and two central tubules (9+2 microtubule pattern).  See  CILIA 
AND FLAGELLA; SPERMATOGENESIS. 
 �34s[Gertrude W. Hinsch]
�33s 
  
  
A700000001143�254s�5v�1v1636
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�6v�2v�32sSperrylite   �21m �33s 
    A mineral with composition PtAs2. Crystals are usually cubes or 
cubo-octahedrons. Its hardness is 6 - 7 on Mohs scale, and its specific 
gravity 10.59. The luster is metallic, and the color tin-white. Sperrylite is 
a rare mineral, found originally in the Sudbury district of Ontario, Canada, 
where it is mined as an ore of platinum. It has also been found in the 
Bushveld Igneous Complex, Transvaal, South Africa, and in gravels of the 
Timpton River, eastern Siberia.  See  PLATINUM. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
A700000002135�254s�5v�1v1636
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�6v�2v�32sSphaeractinoidea   �21m �33s 
    An extinct group of fossil marine sponges closely related to modern 
Sclerospongiae and fossil Stromatoporoidea. They may have appeared as early as 
Late Carboniferous (Pennsylvanian Period) of the Late Paleozoic Era. They were 
definitely present during the Permian Period at the close of the Paleozoic and 
flourished during the Mesozoic Era, especially during the Jurassic and 
Cretaceous, declining rapidly during the late Cretaceous, and may have 
persisted until Eocene time, during the Cenozoic Era. 
    Like the stromatoporoids, they have a radial canal system; in vertical 
thin section the skeleton is calcareous and characterized by a latticelike 
pattern. The vertical structures are round, short to long rodlike pillars. 
However, unlike the stromatoporoids, the horizontal component of the reticulum 
is trabecular or rodlike rather than separated parallel or concentric plates, 
or they may be thin, flat, or broadly arched cyst plates.  See  
SCLEROSPONGIAE; STROMATOPOROIDEA. 
 �34s[Joseph St. Jean, Jr.]
�33s 
  
A700000001411�254s�5v�1v1636
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�6v�2v�32sSphaerioidaceae   �21m �33s 
    A family of fungi of the order Sphaeropsidales containing many plant 
pathogens (also called Sphaeropsidaceae or Phomaceae). There are 300 genera 
and 5000 species recognized by some authorities. 
    The pycnidia, or fruit bodies, containing the asexual spores (conidia) are 
either black or dark colored. They are flask-, cone-, or lens-shaped 
structures with thin walls and a round, relatively small pore. The conidia are 
discharged from the pycnidia in tendrils or cirrhi. 
    The genera are usually arranged into spore groups, depending on the number 
of cells, shape of spore cluster, and whether the spores are bright or dark.  
See  SPHAEROPSIDALES. 
 �34s[N. F. Buchwald]
�33s 
  
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�6v�2v�32sSphaeropsidales   �21m �33s 
    An order of fungi, also known as Phomales or Phyllostictales, of the class 
Fungi Imperfecti. Some members of this group are saprophytes and others are 
parasites causing diseases of plants. There are 520 genera with 5000-6000 
species known. 
    The asexual spores (pycnospores or conidia) are formed in globose or 
flasklike fruit bodies called pycnidia. The pycnidia, found with or without 
openings, may be separate or joined by vegetative hyphae. Conidiophores 
(hyphae bearing the conidia) are short or absent. The conidia are always slime 
spores; that is, they become separated with the slime from the cells producing 
them.  
    This order is divided into four families: Sphaerioidaceae, Zythiaceae, 
Leptostromataceae, and Discellaceae (Excipulaceae). See DISCELLACEAE; FUNGI 
IMPERFECTI; LEPTOSTROMATACEAE; SPHAERIOIDACEAE; ZYTHIACEAE. 
 �34s[N. F. Buchwald]
�33s 
  
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�6v�2v�32sSphagnales   �21m �33s 
    The single order of mosses in the subclass Sphagnobrya. The order is 
composed of one family, the Sphagnaceae, and the family contains one genus, 
Sphagnum, with numerous species. 
    The plants are large and grow in deep tufts or mats. Sphagnum plants are 
erect, usually light grayish-green but occasionally yellowish or reddish. As 
growth occurs in the younger portions, the older portions die. Only the young 
plants bear rhizoids. The stems are usually very slender and are erect because 
of the proximity of other plants. The branches are arranged spirally on the 
stem or in whorls. The leaves of divergent branches are spirally arranged, 
ecostate, and composed of a layer of two kinds of cells, forming a continuous 
cellular net. The sporophyte is borne upon a pseudopodium, the leafless, 
terminal, elongated stem portion of the gametophyte. The capsule is globose or 
elliptical and chestnut-colored or black. The operculum is small, flattish or 
convex, and separated from the urn by an annulus composed of a layer of 
thin-walled cells near the top of the capsule. No peristome is present. Spores 
are disseminated at maturity by an audible explosive discharge from the 
capsule, along with the operculum.  See #SPHAGNOBRYA. 
 �34s[Winona H. Welch]
�33s 
  
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�6v�2v�32sSphagnobrya   �21m �33s 
    A subclass of the Bryopsida which contains the single order Sphagnales. 
These plants grow in deep tufts or mats, commonly in bogs but also in other 
more or less wet habitats. They are grayish-green and have numerous branches 
spirally arranged on the threadlike stem (see illustration) and densely 
crowded near the apex. Usually some branches are divergent while others are 
appressed-pendent.  
    The leaves of the plants are ecostate and composed of a layer of two kinds 
of cells, large hyaline dead cells alternating with narrow chlorophyllose 
living cells, together forming a cellular net. The sporophyte has either no 
seta or a very short one. The foot is embedded in the upper end of the 
pseudopodium, which is an elongation of the gametophyte stem and substitutes 
for the seta in elevating the capsule beyond the leafy plant.  See  BRYOPSIDA; 
SPHAGNALES.  
 �34s[Winona H. Welch]
�33s 
  
  
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�6v�2v�32sSphalerite �21m �33s
    A mineral, BETA-ZnS, also called blende. It is the low-temperature form
and more common polymorph of ZnS. Pure BETA-ZnS on heating inverts to
wurtzite, ALPHA-ZnS, at 1868DGF (1020DGC).
    The mineral is most commonly in coarse to fine, granular, cleavable
masses. The luster is resinous to submetallic; the color is white when pure,
but is commonly yellow, brown, or black, darkening with increased percentage
of iron. There is perfect dodecahedral cleavage; the hardness is 3J on Mohs
scale; specific gravity is 4.1 for pure sphalerite.
    Sphalerite is a common and widely distributed mineral. It occurs both in
veins and in replacement deposits in limestones. As the chief ore mineral of
zinc, sphalerite is mined on every continent. The United States is the largest
producer, followed by Canada, Mexico, Soviet Union, Australia, Peru, the Congo
River area, and Poland.  See  WURTZITE; ZINC.
 �34s[Cornelius S. Hurlbut, Jr.; Paul B. Moore]
�33s

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�6v�2v�32sSphene �21m �33s
    A nesosilicate mineral, composition CaTiO(SiO4), also known as titanite.
Sphene usually occurs as well-formed crystals with a characteristic wedge
shape. There is prismatic cleavage and often a well-developed parting.
Hardness is 5 - 5J on Mohs scale; specific gravity, 3.4 - 3.5. Luster is
resinous to adamantine and the color brown, green, yellow, gray, or black.
Fine crystals are found at Binnental and St. Gothard, Switzerland; Arendal,
Norway; and in Ontario and Quebec, Canada. Sphene, associated with nepheline
and apatite, occurs in huge masses on the Kola Peninsula, U.S.S.R., where it
is mined as a source of titanium.  See	SILICATE MINERALS.
 �34s[Cornelius S. Hurlbut, Jr. ;Paul B. Moore]
�33s

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�6v�2v�32sSphenisciformes   �21m �33s 
    The order of penguins, the most completely aquatic of living birds, 
containing the single family Spheniscidae. The 16 living species breed on 
coasts and islands of the Southern Hemisphere, and all known fossils are from 
the southern half of the Earth. Although penguins are popularly associated 
with Antarctica, only two species, the Adelie (Pygoscelis adeliae) and the 
emperor (Aptenodytes forsteri), actually inhabit the Antarctic continent. The 
range of one species, the Galapagos penguin (Spheniscus mendiculus), reaches 
the Equator. The emperor is the largest living species. 
    Behavior patterns of penguins are complex and varied. Surface-nesting 
species tend to be communal in their social relationships, while the hole 
nesters usually have more strongly developed family bonds. Experiments with 
Adelie penguins in Antarctica indicate that these birds are able to navigate 
across the ice by utilizing solar position.  See  PENGUIN. 
 �34s[Kenneth C. Parkes]
�33s 
  
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�6v�2v�32sSphenophyllales   �21m �33s 
    An extinct group of articulate land plants, common during Late 
Pennsylvanian and Early Permian times. They are typified by Sphenophyllum a 
small, branching plant, probably of trailing habit. The long, jointed stems 
had superposed, longitudinal, surficial ribs between nodes. The vascular 
system contained a solid xylem core with triangular primary wood. The leaves 
were wedge-shaped and had toothed, notched, or rounded distal margins. Long, 
terminal cones, when found detached, contained sporangia and spores. Most 
species were homosporous (produced spores of a single type).  See  PALEOBOTANY. 
 �34s[Sergius H. Mamay]
�33s 
  
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�6v�2v�32sSphere   �21m �33s 
    Both in euclidean solid geometry and in common usage the word sphere 
denotes a solid of revolution obtained by revolving a semicircle of radius r 
about its diameter. Its total volume is V = (4/3)PIrsup3. 
    However, in analytic geometry, and more generally in modern mathematics, 
the word sphere denotes a spherical surface that bounds a solid sphere. In 
this sense a sphere is the locus of all points P in three-dimensional space 
whose distance from a fixed point O (called the center) is equal to a given 
number. The word radius may refer either to one of the segments OP, or to 
their common length r. A plane that intersects a sphere in just one point is 
called a tangent plane and is perpendicular to the radius drawn from the 
center of the sphere to that point. A plane that intersects a sphere in more 
than one point intersects it in a circle. The circle is called a great circle 
or a small circle of the sphere according to whether the plane does or does 
not pass through the center of the sphere. If two parallel planes intersect a 
sphere, the spherical surface between them is called a zone. 
    Any great circle of a sphere divides it into two hemispheres. A second 
great circle cuts a hemisphere into two lunes. A third great circle cuts each 
lune into two spherical triangles.  See  SURFACE AND SOLID OF REVOLUTION. 
 �34s[James S. Frame]
�33s 
  
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�6v�2v�32sSpherical aberration �21m �33s
    An optical aberration which has symmetry of rotation. Such aberrations
arise from the fact that the rays of different aperture generally do not come
to the same focus, and are also known as aperture aberrations. An axis point
of a system with symmetry of rotation has only aperture errors. See MIRROR
OPTICS.
different field angles. This procedure, again, enables the designer to balance
out aperture aberrations as functions of the field angle by eventually
introducing small aberrations of opposite order at the axis. See ABERRATION
(OPTICS).
 �34s[Max J. Herzberger]
�33s

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�6v�2v�32sSpherical harmonics �21m �33s
    A spherical harmonic or solid spherical harmonic of degree n is a
homogeneous function, R(sub n)(x,y,z), of degree n which satisfies Laplace's
equation below. Here n is any number, and (xsup2 + ysup2 +
zsup2)[sup (-n-1)/2] PRODUCT R(sub n)(x,y,z) is a spherical harmonic of degree
-n - 1. There are analogous definitions for spaces of any number of


DELTA R = [CURLYD(sup 2)R/CURLYD x(sup 2)] + [CURLYD(sup 2)R/CURLYD y(sup 2)] +
	  [CURLYD(sup 2)R/CURLYD z(sup 2)] = 0

dimensions. In the present article, n is a nonnegative integer and Rsubn a
polynomial in x, y, z (polynomial spherical harmonic). In terms of spherical
coordinates r, THETA, PHI, R(sub n)(x, y, z) = r(sup n)S(sub n)(THETA,PHI),
where Ssubn, a polynomial in cos THETA, sin THETA, cos PHI, sin PHI is a
spherical surface harmonic of degree n. There are 2n + 1 linearly independent
spherical surface harmonics of degree n; any spherical surface harmonic of
degree n is a linear combination of these, and conversely any linear
combination of spherical surface harmonics of degree n is again a spherical
surface harmonic of degree n. 
    Spherical harmonics occur in potential theory. They occur in connection
with Laplace's equation not only in spherical coordinates but also in
spheroidal coordinates (spheroidal harmonics) and confocal coordinates
(ellipsoidal surface harmonics). In spherical coordinates, spherical surface
harmonics occur in connection with Laplace's and Poisson's equations, the wave
equation, and the Schroedinger equation. In mathematical physics, spherical
harmonics appear in the theories of gravitation, electricity and magnetism,
hydrodynamics, and in other fields.
 �34s[A. Erdelyi]
�33s

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�6v�2v�32sSphingolipid �21m �33s
    A complex lipid which contains the amino alcohols sphingosine (I) and
dihydrosphingosine or phytosphingosine (II).

	      CH3(CH2)12CH==CH--CH--CH --CH2OH		   (I)
				|   |
				OH  NH2

		CH3(CH2)13CH--CH--CH--CH2OH		   (II)
			  |   |   |
			  OH  OH  NH2

    Ceramides are fatty acid amides of sphingosines. Ceramides of sphingosine
are the basic components of cerebrosides (glycosides of ceramides) and
sphingomyelins (phosphoryl choline esters of ceramides). Oligosaccharide
phosphate esters of the ceramides of phytosphingosine (phytoglycolipids) are
present in plant seeds.  See  GLYCOLIPID; LIPID; PHOSPHATIDE.
 �34s[Roy H. Gigg ;Herbert E. Carter]
�33s


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�6v�2v�32sSpica   �21m �33s 
    Alpha Virginis, one of the brightest and nearest of the hot, main-sequence 
stars, spectral type B1. Spica is 1500 times brighter than the Sun and has a 
temperature of nearly 20,000 K. Spica is a spectroscopic binary of 4-day 
period consisting of two nearly identical stars. Rapidly rotating, and 
relatively close, they are nearly unstable and are connected by streams of 
matter.  See  STAR. 
 �34s[Jesse L. Greenstein]
�33s 
  
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�6v�2v�32sSpice and flavoring   �21m �33s 
    Ingredients added to food to provide all or a part of the flavor. Spices 
are pungent or aromatic substances of vegetable origin used in foods at levels 
that yield no significant nutritive value. Flavor is the perception of those 
characteristics of a substance taken orally that affect the senses of taste 
and olfaction. The term flavoring refers to a substance which may be a single 
chemical species or a blend of natural or synthetic chemicals whose primary 
purpose is to provide all or part of the particular flavor effect to any food 
or other product taken orally. Flavorings are categorized by source: animal, 
vegetable, mineral, and synthetic.  See  OLFACTION; TASTE. 
    Other than by source, flavorings can be divided into two groups; one group 
affects primarily the sense of taste, and the other affects primarily the 
sense of olfaction. The members of the first group are called seasonings, the 
members of the second group are called flavors. The same terms can be used to 
divide flavorings in another way. The term flavors can be applied to those 
products which provide a characterizing flavor to a food or beverage. The term 
seasoning can then be applied to those products which modify or enhance the 
flavor of a food or beverage - spices added to meats, blends added to potato 
chips, lemon added to apple pie. 
    In the United States most flavorings are processed. Therefore another 
classification is by method of manufacture. Chemicals are produced by chemical 
synthesis or physical isolation and include such substances as vanillin, salt, 
monosodium glutamate, citric acid, and menthol. Concentrates are powders 
manufactured by dehydrating vegetables, such as onions and garlic, or 
concentrated fruit juices. Condiments are single ingredients or blends of 
flavorful foods, spices, and seasonings, some of which may have been derived 
by fermentation, enzyme action, roasting, or heating. They are usually 
designed to be added to prepared food at the table (for example, chutney, 
vinegar, soy sauce, prepared mustard). Other manufactured flavorings include 
essential oils, hydrolyzed plant proteins, process flavors, and compounded or 
blended flavorings.  See  ESSENTIAL OILS. 
    Spices are natural substances that have been dehydrated and consist of 
whole or ground aromatic and pungent parts of plants, for example, anise, 
cinnamon, dill, nutmeg, and pepper. Such spices are also classified as herbs 
if grown in temperate climates. 
    Extracts are natural substances produced by extraction from solutions of 
the sapid constituents of spices and other botanicals in food-grade solvents. 
They are also available as synthetics. 
 �34s[Earl J. Merwin]
�33s 
  
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�6v�2v�32sSpilite   �21m �33s 
    An aphanitic (microscopically crystalline) to very-fine-grained igneous 
rock, with more or less altered appearance, resembling basalt but composed of 
albite or oligoclase, chlorite, epidote, calcite, and actinolite. 
    In spite of the highly sodic plagioclase, spilites are generally classed 
with basalts because of the low silica content (about 50%). They also retain 
many textural and structural features characteristic of basalt. 
    Spilites are found most frequently as lava flows and more rarely as small 
dikes and sills. Spilitic lavas typically show pillow structure, in which the 
rock appears composed of closely packed, elongated, pillow-shaped masses up to 
a few feet across. Pillows are typical of subaqueous lava flows. Vesicles, 
commonly filled with various minerals, may give the rock an amygdaloidal 
structure.  See  AMYGDULE; BASALT; IGNEOUS ROCKS. 
 �34s[William Ingersoll Rose, Jr.]
�33s 
  
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�6v�2v�32sSpinach   �21m �33s 
    A cool-season annual of Asiatic origin, Spinacia oleracea, belonging to 
the plant order Caryophyllales. It is grown for its foliage and served as a 
cooked vegetable or as a salad. New Zealand spinach (Tetrogonia expansa) and 
Mountain spinach (Atriplex hortense) are also called spinach but are less 
commonly grown. Spinach plants are usually dioecious.  See  CARYOPHYLLALAES. 
 �34s[H. John Carew]
�33s 
  
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�6v�2v�32sSpinal cord   �21m �33s 
    The portion of the central nervous system within the spinal canal of the 
vertebral column, that is, the entire central nervous system except the brain. 
The spinal cord extends from the foramen magnum at the base of the skull to a 
variable level of the spinal canal; it terminates at the lumbar level in 
humans and extends well into the caudal region in fishes. 
    The outer portion of the spinal cord is made up of nerve fibers most of 
which are oriented longitudinally and carry information between parts of the 
spinal cord, between spinal cord and brain, and between brain and spinal cord. 
The outer white matter is divided into dorsal, lateral, and ventral columns. 
The interior of the spinal cord consists of gray matter and is divided into a 
dorsal sensory horn (or column) and a ventral motor horn (or column). In the 
thoracic and lumbar regions of the cord there is also a small lateral horn (or 
column) which contains preganglionic sympathetic neurons. In the very center 
of the bilaterally symmetrical spinal cord is a small central canal, 
containing cerebrospinal fluid.  See  SYMPATHETIC NERVOUS SYSTEM. 
    Paired spinal nerves enter the spinal canal between each pair of vertebrae 
and connect with the spinal cord. The number of spinal nerves varies widely in 
vertebrates; in humans there are 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 
5 sacral, and 1 coccygeal). Each spinal nerve divides into a dorsal sensory 
root and a ventral motor root before entering the spinal cord. The motor 
neurons of the ventral horn, in addition to receiving synapses from dorsal 
root axons, also receive synaptic endings from neurons in other parts of the 
spinal cord and from long axons coming from the brain. The axons of the 
ventral horn neurons leave the cord through the ventral root of the spinal 
nerve and run with peripheral nerves to innervate the muscles of the body. 
With this complex synaptic and fiber organization, the spinal cord can act as 
the integrating center for spinal reflexes (such as the knee jerk reflex), 
send sensory information from the brain, and receive information from the 
brain to initiate or inhibit muscular activity.  See  MOTOR SYSTEMS; NERVOUS 
SYSTEM (VERTEBRATE); SENSATION. 
 �34s[Douglas B. Webster]
�33s 
  
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�6v�2v�32sSpinal cord disorders �21m �33s
    In addition to those disorders common to the brain, the spinal cord is
subject to certain lesions because of its position or structure. A few of the
more important are mentioned.
    Spinal cord injury results from dislocation, fracture, or compression in
many cases, but a special form, called spinal shock, may result from a severe
blow without actual distortion of adjacent tissue. In this case, there is a
temporary paralysis which gradually clears. In direct damage, the cord may be
slightly, partially, or completely damaged at one or more levels. Typical
motor and sensory losses follow, with a poor prognosis for recovery if the
nerve tissue is severely injured.
    A fairly common type of potential cord injury is seen in a number of cases
of slipped disks, in which the inner, soft part of the vertebral column
extrudes into the spinal canal. If this compresses the cord, functional loss
of temporary or permanent degree can follow; more often, pressure is exerted
on spinal roots so that pain, numbness, and some type of muscle weakness
intervene.
    Inflammations may result from known or unknown agents and in meningitis
may involve primarily the coverings; in myelitis, the cord itself. The
meningococcus, pneumococcus, streptococcus, tubercle bacillus, and other
microorganisms frequently cause meningitis. The most widely known cause of
myelitis is the poliomyelitis virus group.  See  MENINGITIS; SPINAL CORD.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sSpin-density wave   �21m �33s 
    The ground state of a metal in which the conduction-electron spin density 
has a sinusoidal variation in space. The periodicity of the wave is unrelated 
to the lattice periodicity. Instead it is determined by the dimensions of the 
conduction-electron Fermi surface in momentum space. 
 �34s[Albert W. Overhauser]
�33s 
  
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�6v�2v�32sSpine   �21m �33s 
    The backbone, or vertebral column. The spine is one of the most 
characteristic structures of all vertebrates, but wide variations in numbers 
of vertebrae and development of particular regions are common. 
    Humans have 33 vertebrae (see illustration): 7 cervical (neck), 12 
thoracic, 5 lumbar, 5 sacral, and usually 4 coccygeal (caudal). The cervical 
segments curve forward, while the thoracic segments curve backward and also 
articulate with the 12 pairs of ribs. The heavy lumbar vertebrae of the loin 
are followed by the fused sacral bones which form the dorsal wall of the 
pelvis. The coccyx consists of a small, curved group that forms a semiflexible 
``tail'' at the base of the spine. Despite common general features, the 
vertebrae vary in shape and size. Their thick ventral bodies, or centra, form 
the main supporting portion of the vertebral column and are separated by 
cartilaginous intervertebral disks. Dorsal to the centra, neural arches cover 
and protect the spinal cord.  See  SKELETAL SYSTEM; SPINAL CORD. 
 �34s[Thomas S. Parsons]
�33s 
  
  
A700000002605�254s�5v�1v1640
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�6v�2v�32sSpinel �21m �33s
The mineral of ideal composition Al(sup VI)(sub 2)[Mg(sup IV)O(sub 4)].  The
spinel structure type is one of the most important and most comprehensively
studied of all structure types.  Its general formula can be written M(sup
VI)(sub 2)[T(sup IV)O(sub 4), where the Roman numerals indicate the coordination
numbers:  the M atoms are octahedrally coordinated and the T atoms are
tetrahedrally coordinated.  The oxygen atoms are in cubic closest-packed
arrangement, making the structure quite dense.

No less than 200 different spinels have been synthesized, usually by fusion of
the component oxides.  Synthetic spinels have many uses.  The ferrimagnetic
properties of some spinels are of great importance in the solid-state industry.
See FERRIMAGNETISM.

Mineralogically, the spinels are divided into three series:  the spinel (Al(sup
3+) series, the magnetite (Fe(sup 3+) series, and the chromite (Cr(sup 3+))
series.  Some members of these series, such as magnetite and chromite, are
valuable ores of their principal metals.  The Al2MgO4 spinel frequently occurs
as rounded grains.  The hardness is 8 on Mohs scale and the specific gravity
3.5, but extensive substitutions can significantly alter these values.	Thus
spinel may be blue, gray, green, yellow, red, brown, black, or colorless.  See
CHROMITE; MAGNETITE.
�34s[Paul B. Moore]
�33s

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�6v�2v�32sSpin glass   �21m �33s 
    One of a wide variety of materials which contain interacting atomic 
magnetic moments and also possess some form of disorder, in which the 
temperature variation of the magnetic susceptibility undergoes an abrupt 
change in slope, that is, a cusp, at a temperature generally referred to as 
the freezing temperature. At lower temperatures the spins have no long-range 
magnetic order, but instead are found to have static or quasistatic 
orientations which vary randomly over macroscopic distances. The latter state 
is referred to as spin-glass magnetic order. Spin-glass ordering is usually 
detected by means of magnetic susceptibility measurements, although additional 
data are required to demonstrate the absence of long-range order. Closely 
related susceptibility cusps can also be observed by using neutron 
diffraction. It is not generally agreed whether spin glasses undergo a phase 
transition or not.  See  MAGNETIC SUSCEPTIBILITY; NEUTRON DIFFRACTION; PHASE 
TRANSITIONS.  
 �34s[R. E. Walstedt]
�33s 
  
A700000001711�254s�5v�1v1639
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�6v�2v�32sSpin label   �21m �33s 
    A molecule which contains an unpaired electron spin which can be detected 
with electron spin resonance (ESR) spectroscopy. Molecules are labeled when an 
atom or group of atoms which exhibit some unique physical property is 
chemically bonded to a molecule of interest. Groups containing unpaired 
electrons include organic free radicals and a variety of types of 
transition-metal complexes (such as vanadium, copper, iron, and manganese). 
Through analysis of ESR spectra, rates of molecular motion and relative 
orientations of spin-labeled molecules whose motion is restrained by 
surrounding molecules can be determined. Measurement of rates of molecular 
motion and molecular orientation has proved to be very important in the study 
of a variety of types of biologically important problems.  See  ELECTRON 
PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY. 
 �34s[Robert Kreilik]
�33s 
  
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�6v�2v�32sSpinning (metals)   �21m �33s 
    A production technique for shaping and finishing metal. In the spinning of 
metal, a sheet is rotated and worked by a round-ended tool. The sheet is 
formed over a mandrel. Spinning may serve to smooth wrinkles in drawn parts, 
provide a fine finish, or complete a forming operation as in curling an edge 
of a deep-drawn part. Spun products range from precision reflectors and nose 
cones to kitchen utensils.  See  SHEET-METAL FORMING. 
 �34s[Ralph L. Freeman]
�33s 
  
A700000002240�254s�5v�1v1640
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�6v�2v�32sSpinning (textiles)   �21m �33s 
    The making of yarn either from fiber or from a spinning solution. In the 
drawing out, twisting, and winding of either natural or synthetic fibers into 
a continuous thread or yarn, several operations, collectively called spinning, 
are performed. The fiber is drawn in or drafted from an initial length per 
unit weight as it comes from the carding operation into a longer length per 
unit weight. The fiber is also twisted to the required number of turns per 
unit length, forming yarn. Finally, the yarn is wound as a cop on a paper tube 
or is delivered to a bobbin or spool. 
    In the production of synthetic fiber, the dope, melt, or other liquid from 
which the artificial filament is produced is filtered and pumped through small 
holes in a spinneret. Liquid synthetic material emerges from the spinneret 
into conditions necessary to convert it to a pliant solid: an acid bath, an 
evaporation column, or a cooling chamber. Where the filaments emerge, they are 
stretched, and then twisted and taken up on a bobbin.  See  SYNTHETIC FIBER; 
TEXTILE.  
 �34s[Frank H. Rockett]
�33s 
  
A700000001506�254s�5v�1v1640
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12241920
�6v�2v�32sSpinulosida   �21m �33s 
    An order of Asteroidea in which pedicellariae rarely occur, are never of 
the crossed type, and do not form rosettes around the spines. The marginal 
plates are small, and the mouth plates are conspicuous but unkeeled. The 
spines normally occur in groups; in the deep-water Pterasteridae they are 
webbed. The tube feet have suckers and usually lie in two longitudinal rows 
along each ambulacrum. The order is generally considered to include 13 
families, and is represented at all depths except the ultraabyssal. Best known 
are the sun stars (Solasteridae), the starlets (Asterinidae), and the genus 
Acanthaster (Acanthasteridae), notorious for damaging coral reefs.  See  
ASTEROIDEA.  
 �34s[Andrew C. Campbell]
�33s 
  
A700000002210�254s�5v�1v1639
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�6v�2v�32sSpin (quantum mechanics) �21m �33s
    The intrinsic angular momentum of a particle. It is that part of the
angular momentum of a particle which exists even when the particle is at rest,
as distinguished from the orbital angular momentum. The total angular momentum
of a particle is the sum of its spin and its orbital angular momentum
resulting from its translational motion. The general properties of angular
momentum in quantum mechanics imply that spin is quantized in half integral
multiples of SLASHED H (SLASHED H = h/2PI, where h is Planck's
constant); orbital angular momentum is restricted to half even integral
multiples of SLASHED H. A particle is said to have spin 3l, meaning
that its spin angular momentum is 3/2 SLASHED H.  See  ANGULAR MOMENTUM.
    A nucleus, atom, or molecule in a particular energy level, or a particular
elementary particle, has a definite spin. The spin is an intrinsic or internal
characteristic of a particle, along with its mass, charge, and isotopic spin.
See  ELECTRON SPIN; ISOTOPIC SPIN; QUANTUM MECHANICS; SYMMETRY LAWS (PHYSICS).
 �34s[Charles J. Goebel]
�33s

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�6v�2v�32sSpiral   �21m �33s 
    A term used generically to describe any geometrical entity that winds 
about a central point or axis while also receding from it. Spiral staircases, 
helices, nonplanar loxodromes (curves that intersect those of a given class at 
a constant angle, for example, rhumb lines, in case the curves are on a sphere 
whose meridians form the given class) are examples of spirals whose windings 
do not lie in a plane.  See  HELIX. 
 �34s[Leonard M. Blumenthal]
�33s 
  
A700000001371�254s�5v�1v1641
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12241920
�6v�2v�32sSpiriferida   �21m �33s 
    An order of fossil articulate brachiopods whose earliest members are found 
in rocks of Middle Ordovician age and whose latest representatives are known 
in Lower Jurassic sediments.  See  ARTICULATA (BRACHIOPODA). 
    Four quite distinct groups are regarded as suborders of the order: 
Athyrididina, Atrypidina, Spiriferidina, and Retziidina (see illustration). 
The principal feature shared by these groups is the spiralium, a pair of 
spirally coiled ribbons of calcite supported by the crura. The direction of 
the axes of coiling and the nature of any connection between the two spirals 
varies considerably within the order.  See  BRACHIODODA. 
 �34s[A. J. Rowell]
�33s 
  
A700000001163�254s�5v�1v1641
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�6v�2v�32sSpirillaceae   �21m �33s 
    A family of bacteria containing gram-negative, non-spore-forming, 
helically curved, relatively nonflexible rods which, if motile, have polar 
flagella. Members possess a respiratory type of metabolism (they obtain energy 
by oxidation of nutrients via an electron transport system, with molecular 
oxygen as the terminal electron acceptor). Organic compounds, usually amino 
acids or salts of organic acids, serve as carbon and energy sources. Two 
genera are assigned to this family, Spirillum and Campylobacter. 
 �34s[Noel R. Krieg]
�33s 
  
A700000002671�254s�5v�1v1641
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�6v�2v�32sSpirochaetales   �21m �33s 
    The spirochetes, an order of bacteria characterized by elongate cells 
twisted three-dimensionally into a spiral shape. Some members of the order are 
pathogens, causing syphilis (in humans), relapsing fever (in humans), and 
Weil's disease (in humans and other animals), while others are parasitic or 
free-living forms in water or sediments. The pitch of the spiral varies so 
that different species may appear as tightly coiled, almost closed springs; as 
regular open-coiled, forms; or as irregularly twisted cells. The spirochetes 
are unusually flexible, being able to elongate and contract or to superimpose 
secondary waves on the primary coils of the spiral.  See  RELAPSING FEVER; 
SYPHILIS; WEIL'S DISEASE. 
    All spirochetes are motile, usually swimming rapidly with a motion that 
involves both forward progress and spinning around the long axis. Flagella are 
absent in this group. Electron microscopy reveals axial filaments or bundles 
of axial filaments anchored near each pole of the cell, which are wound around 
the cell proper within an outer membrane and overlap in the center of the 
cell. It is presumed, without direct evidence, that the axial filaments are 
the organelle of motility. 
    There is a single family, Spirochaetaceae, with five genera (Spirochaeta, 
Cristispira, Treponema, Borrelia, and Leptospira). 
 �34s[Sydney C. Rittenberg]
�33s 
  
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�6v�2v�32sSpirophorida   �21m �33s 
    An order of sponges of the class Demospongiae, subclass Tetractinomorpha, 
with a globular shape and a skeleton of oxeas and triaenes as megascleres, and 
microspined, contorted, sigmalike microscleres. Members of this order range 
down to depths of at least 5900 ft (1800 m).  See  DEMOSPONGIAE. 
 �34s[Willard D. Hartman]
�33s 
  
  
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�6v�2v�32sSpirotrichia   �21m �33s 
    A major subclass of the class Ciliatea which contains those ciliate 
Protozoa that are typified by conspicuous, compound ciliary structures. The 
structures are the cirri, which occur on the ventral surface of the body, and 
the buccal organelles, which include the undulating membrane and the adoral 
zone of membranelles. This group of ciliates is classified into six orders. 
Separate articles appear on each of the six orders listed below: 
 �34s[John O. Corliss]
�33s 
  
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�6v�2v�32sSpiruroidea �21m �33s
    A superfamily of parasitic nematodes which inhabit the alimentary canal,
respiratory system, or orbital, nasal, and oral cavities of vertebrates. All
domestic animals are subject to parasitism by one or more members of this
group. A few species are accidental parasites of humans. In the alimentary
tract they cause gastroenteritis, peritonitis, and more or less serious
hemorrhage and inflammation. Some species cause tumors and nodules; others
infect the eyes of their hosts, sometimes causing blindness.  See  NEMATA.
    These worms may be slender and threadlike, large and heavy-bodied, or
short and thick. The sexes are dimorphic in some species. The mouth is usually
surrounded by a single pair of lateral pseudolabia, and interlabia are
sometimes present.
    The life cycle of all spiruroids thus far studied is indirect; that is, an
intermediate host is involved which is usually some kind of an arthropod. The
definitive host becomes infected by swallowing the intermediate host
containing the infective stage of the parasite.
 �34s[John L. Gardiner /Bruce Hopper]
�33s

A700000003036�254s�5v�1v1642
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�6v�2v�32sSpleen   �21m �33s 
    An organ of the circulatory system present in most vertebrates, lying in 
the abdominal cavity usually in close proximity to the left border of the 
stomach.  
    In humans the spleen normally measures about 1 by 3 by 5 in. (2.5 x 7.5 x 
12.5 cm) and weighs less than 1/2 lb (230 g). It is a firm organ with an oval 
shape and is indented on its inner surface to form the hilum, or stalk of 
attachment to the peritoneum. This mesentery fold also carries the splenic 
artery and vein to the organ. 
    The spleen is an important part of the blood-forming, or hematopoietic, 
system; it is also one of the largest lymphoid organs in the body and as such 
is involved in the defenses against disease attributed to the 
reticuloendothelial system. Although the chief functions of the spleen appear 
to be the production of lymphocytes, the probable formation of antibodies, and 
the destruction of worn-out red blood cells, other less well-understood 
activities are known. For example, in some animals it may act as a reservoir 
for red blood cells, contracting from time to time to return these cells to 
the bloodstream as they are needed. In the fetus and sometimes in later life, 
the spleen may be a primary center for the formation of red blood cells. 
Another function of the spleen is its role in biligensis. Because the spleen 
destroys erythrocytes, it is one of the sites where extrahepatic bilirubin is 
formed.  See  BILIRUBIN; SPLEEN DISORDERS. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sSpleen disorders �21m �33s
    The spleen is rarely the site of primary disorders except those of
vascular origin, but it is frequently involved in systemic inflammations,
metabolic diseases, and generalized blood disorders.
    Among vascular disturbances, acute and chronic congestion are prominent,
particularly chronic congestion caused by cardiac failure, cirrhosis of the
liver, and obstruction of the blood flow from the spleen by thrombi, scarring,
or tumor tissue. Obstruction of the splenic artery or its branches by thrombi
may result in an infarct caused by either cardiac or blood disease.
    Inflammations include acute and chronic forms. The characteristic
engorgement of blood often causes a marked enlargement of the organ.
Bacteremias frequently produce this enlargement, or splenomegaly, and
inflammation, but any severe infectious disease such as diphtheria or
pneumonia may do so.
    The leukemias, especially when of the lymphocytic or neutrophilic
varieties, cause some of the most prominent cases of splenomegaly as well as
other changes.
    Tumors originating in the spleen are rare and usually limited to such
benign growths as hemangiomas, lymphangiomas, and fibromas, but malignant
lymphomas and lymphosarcomas also occur. Secondary tumors, which originate
elsewhere and metastasize to the spleen, are not uncommon, particularly the
lymphoma group.  See  SPLEEN.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sSplines   �21m �33s 
    A series of projections and slots used instead of a key to prevent 
relative rotation of cylindrically fitted machine parts. Splines are several 
projections machined on the shaft; the shaft fits into a mating bore called a 
spline fitting. Splines are made in two forms, square and involute, as 
illustrated. Since there are several projections (integral keys) to share the 
force in transmitting power, the splines can be shallow, thereby not weakening 
the shaft as much as a standard key. 
    Three classes of fits are used for square splines: sliding (as for gear 
shifting) under load, sliding when not loaded, and permanent fit. Square 
splines have been used extensively for machine parts. In the automotive 
industry, square splines have been replaced generally by involute splines. 
    Involute splines are used to prevent relative rotation of cylindrically 
fitted machine parts and have the same functional characteristics as square 
splines. The involute spline, however, is like an involute gear, and the 
spline fitting (internal part) is like a mating internal gear.  See  MACHINE 
KEY.  
 �34s[Paul H. Black]
�33s 
  
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�6v�2v�32sSpodumene   �21m �33s 
    The name given to the monoclinic lithium pyroxene LiAl(SiO3)2. Spodumene 
commonly occurs as white to yellowish prismatic crystals, often with a 
``woody'' appearance.  
    Spodumene is usually found as a constituent in certain granitic 
pegmatites. The emerald-green variety, hiddenite, and a lilac variety, 
kunzite, are used as precious stones. Spodumene from pegmatites is used as an 
ore for lithium.  See  LITHIUM; PYROXENE. 
 �34s[George W. DeVore]
�33s 
  
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�6v�2v�32sSponge   �21m �33s 
    The name for all members of the phylum Porifera. The majority of these 
organisms are marine; however, one family of sponges, the Spongillidae, occurs 
in fresh water. Sponges of commercial value are collected in the 
Mediterranean, in the Gulf of Mexico, and near the West Indies.  See  PORIFERA. 
 �34s[Charles B. Curtin]
�33s 
  
  
A700000001330�254s�5v�1v1642
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�6v�2v�32sSpongiomorphida   �21m �33s 
    A small extinct order of Triassic and Jurassic marine invertebrate fossils 
of uncertain biologic affinities. The spongiomorphs had calcareous skeletons 
composed of small vertical rods called pillars and interconnecting horizontal 
rods called trabeculae or synapticulae. The pillars consist of upwardly 
inflected cone-in-cone layers of fibrous calcareous deposits. The trabeculae 
are formed from thickened zones at the base of the flare of the cones in the 
pillars. Spongiomorphs were sessile, benthonic, shallow marine organisms 
associated with calcareous deposits, especially reefs.  See HYDROZOA. 
 �34s[Joseph St. Jean, Jr.]
�33s 
  
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�6v�2v�32sSpontaneous combustion �21m �33s
    Combustion that occurs when certain materials are stored in bulk. The
oxidizing action of microorganisms often produces the initial heat. As the
temperature increases, the air trapped in the material takes over the
oxidation process, liberating more heat. Because the heat cannot be dissipated
to the surroundings, the temperature of the material rises still more and the
rate of oxidation increases. Eventually the material reaches an ignition point
and bursts into flame. For example, coal is subject to spontaneous combustion
and is generally stored in shallow piles to allow the heat of oxidation to
dissipate. See COMBUSTION.
    For gases at ordinary temperatures, molecular collisions do not usually
cause combustion. At elevated temperatures the collisions of the thermally
agitated molecules are more frequent. More important as a cause of chemical
reaction is the greater energy involved in the collisions. Moreover, there is
very little combustion attributable to direct reaction between the molecules.
Instead, a high-energy collision dissociates a molecule into atoms, or free
radicals. These molecular fragments react with greater ease, and the
combustion process proceeds generally by a chain reaction involving these
fragments. See CHAIN REACTION (CHEMISTRY).
    Under certain conditions, where the rate of chain branching equals or
exceeds the rate at which chains are terminated, the combustion process speeds
up to explosive proportions; because of the rapidity of molecular events, a
large number of chains are formed in a short time so that essentially all of
the gas undergoes reaction at the same time; that is, an explosion results.
Another cause of explosion in gaseous combustion arises when the rate at which
heat is liberated in the reaction is greater than the rate at which the heat
dissipates to the surroundings. See CHEMICAL DYNAMICS; EXPLOSION AND
EXPLOSIVE; FLAME.
 �34s[Bernard Lewis]
�33s

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�6v�2v�32sSporobolomycetales   �21m �33s 
    An order of fungi belonging to the class Basidiomycetes. The single 
family, Sporobolomycetaceae, contains six genera. The family is characterized 
by the formation of aerial sterigmata upon which the asexual ballistospores 
are formed. When mature these spores are discharged with great force by a 
drop-excretion mechanism. All species of the Sporobolomycetaceae are strictly 
oxidative.  See  BASIDIOMYCOTINA. 
 �34s[Herman J. Phaff]
�33s 
  
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�6v�2v�32sSporotrichosis   �21m �33s 
    A mycotic infection of humans caused by Sporothrix schenckii, a fungus 
that is worldwide in distribution. This organism has been isolated from 
timber, plants, and soil. The disease is most frequently seen in farmers and 
horticulturists. The primary lesion develops within 2 - 8 weeks following 
introduction of the fungus into the tissue, and appears as a hard, pink 
nodule. Gradually, the lesion becomes darker in color and undergoes necrosis 
with discharge of purulent material. As the infection progresses, numerous 
nodules develop along the lymphatic chain of the arm. Rarely does the disease 
progress beyond this form. Although the primary lesion may heal, the secondary 
nodules may persist for months to years if untreated. 
 �34s[Leanor D. Haley]
�33s 
  
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�6v�2v�32sSporozoa   �21m �33s 
    A subphylum of Protozoa, typically with spores. The spores are simple and 
have no polar filaments. There is a single type of nucleus. There are no cilia 
or flagella except for flagellated microgametes in some groups. In most 
Sporozoa there is an alternation of sexual and asexual stages in the life 
cycle. In the sexual stage, fertilization is by syngamy, that is, the union of 
male and female gametes. All Sporozoa are parasitic. The subphylum is divided 
into three classes - Telosporea, Toxoplasmea, and Haplosporea. See 
HAPLOSPOREA; PROTOZOA; TELOSPOREA; TOXOPLASMEA. 
 �34s[Norman D. Levine]
�33s 
  
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�6v�2v�32sSpotted fever group   �21m �33s 
    A term that includes several disease entities in humans in various parts 
of the world, caused by the Rickettsiae. The diseases have in common 
transmission to humans by various species of ticks or certain parasitic mites. 
Included in the group are Rocky Mountain spotted fever, fievre boutonneuse, 
several tick typhuses, and rickettsialpox.  See  FIEQVRE BOUTONNEUSE; 
QUEENSLAND TICK TYPHUS; RICKETTSIALES; RICKETTSIALPOX; ROCKY MOUNTAIN SPOTTED 
FEVER; SIBERIAN TICK TYPHUS; SOUTH AFRICAN TICK-BITE FEVER. 
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sSpot welding �21m �33s
    A resistance-welding process in which coalescence is produced by the flow
of electric current through the resistance of metals held together under
pressure. Usually the upper electrode moves and applies the clamping force.
Pressure must be maintained at all times during the heating cycle to prevent
flashing at the electrode faces. Electrodes are water-cooled and are made of
copper alloys because pure copper is soft and deforms under pressure. The
electric current flows through at least seven resistances connected in series
for any one weld. (see illustration). After the metals have been fused
together, the electrodes usually remain in place sufficiently long to cool the
weld. See RESISTANCE WELDING; WELDING AND CUTTING OF METALS.
 �34s[Eugene J. Limpel]
�33s

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�6v�2v�32sSpring (hydrology)   �21m �33s 
    A place where groundwater discharges upon the land surface because the 
natural flow of groundwater to the place exceeds the flow from it. Springs are 
ephemeral, discharging intermittently, or permanent, discharging constantly. 
Springs are usually at mean annual air temperatures. The less the discharge, 
the more the temperature reflects seasonal temperatures. Spring water usually 
originates as rain or snow (meteoric water). 
    Hot-spring water may differ in composition from meteoric water through 
exchange between the water and rocks. Common minerals consist of component 
oxides. Oxygen of minerals has more sup1sup8O than meteoric water. Upon 
exchange, the water is enriched in sup1sup8O. Most minerals contain little 
deuterium, so that slight deuterium changes occur. Some hot-spring waters are 
acid from the oxidation of hydrogen sulfide to sulfate. 
    Mineral spring waters have high concentrations of solutes and wide ranges 
in chemistry and temperatures; hot mineral springs may be classified as hot 
springs as well as mineral springs. Most mineral springs are high either in 
sodium chloride or sodium bicarbonate (soda springs) or both; other 
compositions are found, such as a high percentage of calcium sulfate from the 
solution of gypsum. 
    The chemical compositions of spring waters are seldom in chemical 
equilibrium with the air. Groundwaters whose recharge is through grasslands 
may contain a thousand times as much CO2 as would be in equilibrium with air, 
and those whose recharge is through forests may contain a hundred times as 
much as would be in equilibrium with air. Sulfate in groundwater may be 
reduced in the presence of organic matter to H2S, giving some springs the odor 
of rotten eggs.  See  GEYSER; GROUNDWATER HYDROLOGY. 
 �34s[Ivan Barnes]
�33s 
  
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�6v�2v�32sSpring (machines)   �21m �33s 
    A machine element for storing energy as a function of displacement. Force 
applied to a spring member causes it to deflect through a certain displacement 
thus absorbing energy. 
    A spring may have any shape and may be made from any elastic material. 
Even fluids can behave as compression springs and do so in fluid pressure 
systems. Most mechanical springs take on specific and familiar shapes such as 
helix, flat, or leaf springs. All mechanical elements behave to some extent as 
springs because of the elastic properties of engineering materials. 
    The most frequent use of springs is to supply motive power in a mechanism. 
Common examples are clock and watch springs, toy motors, and valve springs in 
auto engines. A special case of the spring as a source of motive power is its 
use for returning displaced mechanisms to their original positions, as in the 
door-closing device, the spring on the cam follower for an open cam, and the 
spring as a counterbalance. Frequently a spring in the form of a block of very 
elastic material such as rubber absorbs shock in a mechanism. Springs also 
serve an important function in vibration control.  See  SHOCK ABSORBER; SHOCK 
ISOLATION.  
    Springs may be classified into six major types according to their shape. 
These are flat or leaf, helical, spiral, torsion bar, disk, and constant force 
springs. A leaf spring is a beam of cantilever design with a deliberately 
large deflection under a load. The helical spring consists essentially of a 
bar or wire or uniform cross section wound into a helix. In a spiral spring, 
the spring bar or wire is wound in an Archimedes spiral in a plane. A spiral 
spring is unique in that it may be deflected in one of two ways or a 
combination of both of them (see illustration). A torsion bar spring consists 
essentially of a shaft or bar of uniform section. The disk spring consists 
essentially of a disk or washer supported at the outer periphery by one force 
and an opposing force on the center or hub of the disk. A constant force 
spring is used when a constant force must be applied regardless of 
displacement.  
 �34s[L. Sigfred Linderoth, Jr.]
�33s 
  
  
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�6v�2v�32sSpruce �21m �33s
    Evergreen tree belonging to the genus Picea of the pine family. The
needles are single, usually four-sided, and borne on little peglike
projections; the cones are pendulous. Resin ducts in the wood may be seen with
a magnifying lens, but they are fewer than in Pinus.
    The white spruce (P. glauca), ranging from northern New England to the
Lake States and Montana and northward into Alaska, is distinguished by the
somewhat bluish cast of its needles, small cylindrical cones, and gray or
pale-brown twigs without pubescence (hairs). Red spruce (P. rubens) is a
similar tree but with greener foliage; smaller, more oval cones; and more or
less pubescent twigs. Occurring naturally with white spruce in the
northeastern United States and adjacent Canada, red spruce extends southward
along the Appalachians into North Carolina. Black spruce (P. mariana) ranges
from northern New England and Newfoundland to Alaska. However, it occurs
sparingly in the Appalachians to West Virginia. The cones are smaller than in
the white and red species and are egg-shaped or nearly spherical and
persistent. The twigs are pubescent.

    Blue spruce (P. pungens), also known as Colorado blue spruce, is probably
the best known of the western species because of its wide use as an ornamental
tree. The twigs are glabrous (without pubescence). Engelmann spruce (P.
engelmanni) has needles usually of a deep blue-green color, sometimes much
like those of the blue spruce but the young twigs are slightly hairy. The
cones, although cylindrical, are smaller than in blue spruce. This species is
also a Rocky Mountain tree like the blue spruce, but it is more widely
distributed from British Columbia to Arizona and also in the mountains of
Oregon and Washington. Sitka spruce (P. sitchensis) is the largest spruce in
the Northern Hemisphere. The leaves have a pungent odor, are considerably
flattened, and stand out from the twig in all directions. It ranges from
Alaska to northern California. The Norway spruce (P. abies), the common spruce
of Europe, is much planted in the United States for timber, as well as for
ornamental purposes. It can be recognized by the dark-green color of the
leaves; glabrous, pendent, short branchlets; and large cones, usually near the
top of the tree.  See  PINALES.
 �34s[Arthur H. Graves /Kenneth P.Davis]
�33s

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�6v�2v�32sSpur gears   �21m �33s 
    Gears that transmit power between parallel shafts and have straight teeth 
parallel to the gear axis (Fig. 1). It is common practice, however, to group 
helical gears that have parallel shafts under the heading of spur gears. Spur 
gears are classified as external, internal, and rack and pinion. External spur 
gears, the most common, have teeth which point outward from the center of the 
gear. Internal or annular gears have teeth pointing inward toward the gear 
axis. A rack may be considered as a gear having an infinite pitch circle 
radius. Thus its pitch surface is a plane. A rack and pinion running together 
transform rectilinear motion into rotary motion, or vice versa. 
  
    Helical gears.  Gears running on parallel axes and with teeth twisted 
oblique to the gear axis are essentially spur gears (Fig. 2). Because of the 
twist, contact is progressive across the tooth surface, starting at one edge 
and proceeding across the face of the tooth. The action results in reduced 
impact and quieter operation, particularly at high speed. Herringbone gears 
are equivalent to two helical gears of opposite hand placed side by side. They 
are especially suited for high-speed operation and eliminate the axial thrust 
produced by single helical gears. 
  
  
    Crossed helical gears.  Where shafts cross obliquely, motion is 
transmitted by crossed helical gears. The teeth are helical but differ from 
the teeth of worm gears in that no one tooth (thread) makes a complete turn on 
the pitch circle. Pitch surfaces of crossed helical gears are cylindrical as 
with spur gears. However, with crossed shafts, the oblique teeth have point 
contact rather than the line contact that occurs with parallel shafts. Helical 
gears are referred to as right- or left-hand in the same manner as screw 
threads, a right-hand gear being one on which the teeth twist clockwise as 
they recede from an observer looking along the axis. See WORM GEARS. 
 �34s[John R. Zimmerman]
�33s 
  
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�6v�2v�32sSputtering   �21m �33s 
    The process by which atoms or groups of atoms are ejected from a metal 
surface as the result of heavy-ion impact. It generally takes place at the 
cathode of a self-maintained gaseous discharge, indicating that the important 
agent is the positive ion. Although sputtering is useful for certain 
processes, such as the generation of a clean surface, it is usually harmful. 
In the case of an oxide-coated thermionic cathode, sputtering by positive-ion 
bombardment may destroy the surface completely. 
 �34s[Glenn H. Miller]
�33s 
  
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�6v�2v�32sSquall   �21m �33s 
    A strong wind with sudden onset and more gradual decline, lasting for 
several minutes. Wind speeds in squalls commonly reach 30 - 60 mi/h (13 - 27 
m/s), with a succession of brief gusts of 80 - 100 mi/h (36 - 45 m/s) in the 
more violent squalls. Squalls may be local in nature, as with isolated 
thunderstorms, or may occur over a wide area in the vicinity of a 
well-developed cyclone, where the squalls locally reinforce already strong 
winds.  
    The most common type of squall is the thundersquall or rainsquall 
associated with heavy convective clouds, frequently of the cumulonimbus type. 
It is formed when cold air, descending in the core of the thunderstorm rain 
area, reaches the Earth's surface and spreads out. Particularly in desert 
areas, the thunderstorm rain may largely or wholly evaporate before reaching 
the ground, and the squall may be dry, often associated with dust storms.  See 
  DUST STORM; SQUALL LINE; THUNDERSTORM. 
    Squalls of a different type result from cold air drainage down steep 
slopes. The force of the squall is derived from gravity and depends on the 
descending air which is colder and more dense than the air it replaces. 
So-called fall winds of this kind are common on mountainous coasts of high 
latitudes, where cold air forms on elevated plateaus and drains down fiords or 
deep valleys. 
 �34s[Chester W. Newton]
�33s 
  
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�6v�2v�32sSquall line   �21m �33s 
    A line of thunderstorms, near whose advancing edge squalls occur along an 
extensive front. The thundery region, 12 - 30 mi (20 - 50 km) wide and up to 
1200 mi (2000 km) long, moves at a typical speed of 30 knots (15 m/s) for 6 - 
12 h or more and sweeps a broad area. In the United States, severe squall 
lines are most common in spring and early summer when northward incursions of 
maritime tropical air east of the Rockies interact with polar front cyclones. 
Ranking next to hurricanes in casualties and damage caused, squall lines also 
supply most of the beneficial rainfall in some regions.  See  FRONT; SQUALL; 
THUNDERSTORM.  
 �34s[Chester W. Newton]
�33s 
  
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�6v�2v�32sSquamata �21m �33s
    The dominant order of living reptiles composed of the lizards and snakes.
The group first appeared in Jurassic times and today is found in all but the
coldest regions. Various forms are adapted for arboreal, burrowing, or aquatic
lives, but most squamates are fundamentally terrestrial. There are about 4700
Recent species: 2200 lizards and 2500 snakes.
    The order is readily distinguished from all known reptiles by its highly
modified skull; an enlarged and movable quadrate; and a temporal opening that
is lost or reduced in many forms. No other reptiles show these modifications,
which allow for great kinesis in the lower jaw since it articulates with the
quadrate. In addition, the order is distinct from other living reptile groups
because its members have no shells or secondary palates and the males possess
paired penes.
    Traditionally the Squamata have been divided into two major subgroups, the
lizards, suborder Sauria, and the snakes, suborder Serpentes. The latter group
is basically a series of limbless lizards, and it is certain that snakes are
derived from some saurian ancestor. There are many different legless lizards,
and it has been suggested that more than one line has evolved to produce those
species currently grouped together as snakes.

    Sauria.  The majority of saurians are insectivorous, but a few feed on
plants while others, notably the Varanidae and allies, feed on larger prey
including birds and mammals. The largest living lizard is the Komodo dragon
(Varanus komodoensis).
    The majority of lizards are quadrupedal in locomotion and are usually
ambulatory scamperers or scansorial. Some forms are bipedal, at least when in
haste. Perhaps the most famous of the bipedal forms is the Jesus Cristo
lizard, Basiliscus, of Middle America which runs for some distance across the
surface of small streams when frightened.
    The coloration of each species of lizard is characteristic. Most forms
exhibit marked differences in coloration between the sexes, at least during
the breeding season, and frequently the young are markedly different from the
parents. Color changes occur in rapid fashion among some species, and all are
capable of metachrosis or changing color to a certain extent. Contrary to
popular legend, the changes are not made to match the color of the background;
they seem, rather, to be under neurohormonal control and to fluctuate with
temperature and light and with the activities of the lizard.  See
CHROMATOPHORE; SEXUAL DIMORPHISM.
    There are but two species of venomous lizards, both members of the genus
Heloderma, in the family Helodermatidae. The Gila monster (H. suspectum) is
found in western New Mexico, Arizona, extreme southern Nevada, southwestern
Utah, eastern California, and northwestern Mexico. The beaded lizard (H.
horridum) is a Mexican species. Venom is produced in glands along the lower
jaw and penetrates wounds inflicted by the anterior recurved teeth. No
mechanism for injection of the venom into the wound is present, contrary to
the situation in most snakes.
    The following list indicates the major evolutionary lines and families of
lizards. Families indicated by an asterisk contain limbless, snakelike
species. All members of the families Pygopodidae, Anelytropsidae, Dibamidae,
Amphisbaenidae, Feyliniidae, and Anniellidae are snakelike lizards.
    Family: Iguanidae
	    Agamidae
	    Chamaeleontidae
Gekkota line
    Family: Gekkonidae
	    Pygopodidae
Scincomorpha line
    Family: Xantusiidae
	    Teiidae
	    Lacertidae
	    Gerrhosauridae*
	    Scincidae*
	    Anelytropsidae*
	    Dibamidae*
Annulata line
    Family Amphisbaenidae*
Anguimorpha line
    Family: Anguidae*
	    Anniellidae*
	    Feyliniidae*
	    Xenosauridae
	    Helodermatidae
	    Varanidae
	    Lanthonotidae

    Serpentes.	Snakes are basically specialized, limbless lizards which were
probably evolved from burrowing forms but have now returned from subterranean
habitats to occupy terrestrial, arboreal, and aquatic situations. The
following characteristics are typical of all serpents. There is no temporal
arch so that the lower jaw and quadrate are very loosely attached to the
skull. This gives the jaw even greater motility than is the case in lizards.
The body is elongate with 100 - 200 or more vertebrae, and the internal organs
are elongate and reduced. A spectacle covers the eye.
    The largest living snake is the Indian python (Python reticulatus), which
reaches 30 ft (9 m) in length and a weight of 250 lb (113 kg). The largest
venomous snake is the king cobra (Ophiophagus hannah), of southern Asia, which
is known to attain a length of 18 ft (5.5 m).

    The senses of snakes are fundamentally similar to those of all terrestrial
vertebrates. Great dependence is placed upon olfaction and the Jacobson's
organs (olfactory canals in the nasal mucosa). The tongue of all snakes is
elongate and deeply bifurcated. When not in use it can be retracted into a
sheath located just anterior to the glottis, but it is protrusible and is
constantly being projected to pick up samples for the Jacobson's organs from
the surrounding environment. Snakes are deaf to airborne sounds and receive
auditory stimuli only through the substratum via the bones of the head. The
eyes are greatly modified from those in lizards, and there is no color vision.
Some groups are totally blind and have vestigial eyes covered by scales or
skin.
    Four basic patterns of locomotion are found in snakes, and several may be
used by a particular individual at different times (see illustration). The
most familiar type is curvilinear. In this pattern the snake moves forward by
throwing out lateral undulations of the body and pushing them against any
irregularity in the surface (illustration a). Snakes using rectilinear
locomotion move forward in a straight line, without any lateral undulations,
by producing wavelike movements in the belly plates (illustration b).
Laterolinear locomotion, or sidewinding, is used primarily on smooth or
yielding surfaces and is very complex. In essence, the snake anchors a portion
of the body in the substratum and lifts the rest out laterally to a new
position. When the lifted part is anchored again, the portions of the body
left behind are lifted to the new position (illustration c). By a constant
lifting and anchoring of alternate parts the snake moves in a lateral
direction. Concertina locomotion movement resembles the expansion and
contraction of that musical instrument (illustration d).
    The vast majority of living snakes are harmless to humans, although a
number are capable of inflicting serious injury with their venomous bites. The
venom apparatus has evolved principally as a method of obtaining food, but it
is also advantageous as a defense against attackers. Fangs are teeth modified
for the injection of venom into the victim, and the venom glands are modified
salivary glands connected to the grooved fangs by a duct. Special muscles are
present in all proglyphous snakes to force the venom into the wound. The venom
itself is a complex substance containing a number of enzymes. Certain of these
enzymes attack the blood, others the nervous system, and some are spreaders.
    The following list indicates the major groups of living snakes:

		   Family:  Typhlopidae
			    Leptotyphlopidae
			    Aniliidae
			    Boidae
			    Colubridae
			    Elapidae
			    Hydrophiidae
			    Viperidae
			    Crotalidae


 �34s[Jay M. Savage]
�33s

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�6v�2v�32sSquare �21m �33s
    A portion of a plane bounded by four equal line segments, or sides, each
two of which are either parallel or mutually perpendicular. The intersecting
sides determine four points, vertices of the square. The diagonals join pairs
of vertices that are not on a side (see illustration). They are equal and
mutually perpendicular. If a is the length of a side of a square, its area is
a x a or a(sup 2), and this is the motivation for calling a(sup 2) ``a
squared.'' See  QUADRILATERAL; RECTANGLE. 
 �34s[Leonard M. Blumenthal]
�33s

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�6v�2v�32sSquash   �21m �33s 
    The common name for edible fruits of several species of the genus 
Cucurbita: C. pepo, C. moschata, C. maxima, and C. mixta. Those species 
originated in the Americas but are now grown in most countries around the 
world. Within squash there is tremendous variation in size, shape, color, and 
usage.  
    The most clearly defined group is summer squash, fruit of any species of 
Cucurbita eaten as a vegetable when immature. It is most commonly C. pepo. 
Fruit color may be white, yellow, or light or dark green, and the green may be 
solid or striped. Shapes may be flattened disks as in Pattypan, cylindrical as 
in Zucchini and Cocozelle, or with necks as in the straightneck and crookneck 
types. Summer squash has mild flavor, high water content, and relatively low 
nutritional value. 
    Winter squash is fruit of Cucurbita eaten when mature and derives its name 
from its ability to be stored for several weeks or months before consumption. 
Varieties of winter squash are found in all four species. The Table Queen 
group, synonymous with Acorn, is C. pepo, Butternut belongs to C. moschata, 
Green-striped Cushaw is C. mixta, while C. maxima has the widest range of 
types, including Buttercup, Hubbards, and Delicious of various colors, Banana, 
and Boston Marrow. Flesh color varies from light yellow to dark orange, and 
the edible portion ranges from thin to very thick. 
 �34s[H. M. Munger]
�33s 
  
  
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�6v�2v�32sSquid   �21m �33s 
    The name for a number of marine cephalopod mollusks characterized by a 
reduced internal shell, either calcareous or chitinous in composition. There 
are 10 tentacles; one pair is elongate with suckers on the terminal end, while 
the other four pairs have suckers along their entire length. The ink sac, 
together with the protective coloration provided by chromatophores, serves to 
protect these animals. The nervous system and sensory organs are well 
developed.  
    Squid feed on small fish, crustaceans, and other marine organisms. They in 
turn are preyed upon by large fish, whales, and oceanic birds. The sexes are 
separate.  
    The giant squid (Architeuthis principes) is the largest invertebrate, with 
a body length that may exceed 12 ft (3.7 m).  See  CEPHALOPODA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sSQUID �21m �33s
    A device which, in its original form, consists of two Josephon tunnel
junctions connected in parallel on a superconducting loop (see illustration).
The term is an acronym for superconducting quantum interference device. A
small applied current I flows through the junctions as a supercurrent, without
developing a voltage, by means of Cooper pairs tunneling through the barriers.
However, when the applied current exceeds a certain critical value a voltage V
is generated.  See  INTERFERENCE OF WAVES; JOSEPHSON EFFECT; SUPERCONDUCTIVITY.
    The SQUID has important device applications. When each Josephson tunnel
junction is shunted with an external resistance to eliminate hysteresis on the
current-voltage characteristic and the SQUID is biased with a constant current
greater than the critical value, the voltage across the SQUID is also an
oscillatory function of the magnetic flux. If one measures the change in
voltage produced by the application of a flux equivalent to a small fraction
of one flux quantum, one has a very sensitive magnetometer. Another important
potential application is as a logic element or memory cell in high-speed
computers.  See  COMPUTER STORAGE TECHNOLOGY; LOGIC CIRCUITS; MAGNETOMETER.
    The rf SQUID, which consists of a single junction interrupting a
superconducting loop, is in fact misnamed, since no interference takes place.
See  SUPERCONDUCTING DEVICES.
 �34s[John C. Lane]
�33s


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�6v�2v�32sStability augmentation   �21m �33s 
    Modification of inherent aircraft handling qualities by automatic control 
devices which supplement the pilot's control inputs. These automatic control 
devices were first known as stabilizers, then stability augmentation systems 
(SAS), and subsequently command augmentation systems (CAS) in certain 
applications.  
    Stability augmentation systems use instruments that measure the aircraft 
motion (sensors) and automatically control the aerodynamic control surfaces to 
modify the motion in a desired manner. This feedback control is generally in 
series with the pilot's input so that the cockpit controllers do not move and 
the pilot is unaware of the feedback control inputs. On the other hand, fully 
automatic guidance and control systems, such as autopilots and automatic 
landing systems, are mechanized in parallel with the pilot's inputs. The 
stability augmentation is typically an inner loop of the fully automatic 
control systems.  See  AUTOMATIC LANDING SYSTEM; AUTOPILOT; CONTROL SYSTEMS. 
 �34s[Herman A. Rediess]
�33s 
  
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�6v�2v�32sStabilization   �21m �33s 
    Any process that minimizes or prevents fluctuations in a quantity or 
condition. In the navigation of ships and aircraft, a gyroscope provides the 
stabilization required for maintaining a desired direction of a navigational 
device, such as an autopilot, despite the motions and maneuvers of the ship or 
aircraft. More powerful stabilization equipment keeps a radar antenna aimed in 
a desired direction in space despite the roll, pitch, and turning maneuvers of 
a ship or aircraft.  See  GYROSCOPE; NAVIGATION. 
    Stabilization is introduced into vacuum-tube or transistor amplifier 
stages by means of feedback to reduce distortion by making the amplification 
substantially independent of electrode voltages and tube constants. With 
magnetic materials, aging and other treatments produce stabilization of 
magnetic characteristics.  See  FEEDBACK CIRCUIT. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sStabilizer   �21m �33s 
    The horizontal or vertical aerodynamic wing surfaces that provide aircraft 
stability and longitudinal balance in flight. Horizontal and vertical 
stabilizers (fins) are similar to the aircraft wing in structural design and 
function of providing lift at angle of attack to the wind. However, 
stabilizers are not required to supply lift to overcome aircraft weight during 
flight, and when the wing-fuselage center of pressure is behind the aircraft 
center of gravity, the aerodynamic load on the horizontal stabilizer may be 
downward. Various stabilizer arrangements are shown in the figure.  See  
FLIGHT CONTROLS. 
 �34s[Malcolm J. Abzug]
�33s 
  
  
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�6v�2v�32sStadia   �21m �33s 
    A surveying distance-measuring method in which the interval intercepted on 
a vertically held rod by two cross hairs in the alidade or transit telescope 
is convertible to the horizontal distance from instrument to rod. The rod is 
called a stadia rod; the cross hairs are called stadia hairs and are fixed 
equidistant above and below the horizontal cross hair. Their distance apart, 
i, is a fixed ratio (conveniently 1:100, or 0.3:100 in levels) of the 
telescope's focal length. Hence the distance D from the focal point to an 
object is basically proportional to the interval R observed on the rod. 
    For horizontal stadia sighting, the small distance from the focal point to 
the instrument center is added to the distance obtained by reading the rod 
intercept as shown in the illustration. This distance is about 1 ft (30 cm) 
for externally focusing instruments; it is negligible for internally focusing 
telescopes. In all of stadia instruments, the figures and graduations on the 
stadia rod are read through the instrument telescope.  See  SURVEYING. 
 �34s[B. Austin Barry]
�33s 
  
  
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�6v�2v�32sStained glass   �21m �33s 
    Glass colored by any of several means. It is frequently assembled to 
produce a varicolored mosaic or representation for decorative uses. The glass 
is colored by one of three means: fusing with metallic oxides, enameling, or 
painting. The addition of small percentages of metal compounds, usually 
oxides, to molten glass or pot metal produces throughout the glass a color 
characteristic of the compound. If the percentage of metal compound is low, 
the glass is tinted; if the percentage is high, the glass is saturated.  See  
SATURATION.  
    The glass can also be coated with transparent metallic oxides and fired to 
bond the enameling to the glass base. Transparent pigments can be painted onto 
the glass in solid colors, or in several colors to render a picture; the 
pigments are then baked or burned onto the glass. With a hot iron, the glazier 
cuts the glass to shape, joins it into the design with channeled lead strips, 
and solders the joints. Either the tracery of the window or iron frames 
support the finished glaziery.  See  GLASS. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sStainless steel   �21m �33s 
    The generic name commonly used for that entire group of iron-base alloys 
which exhibit phenomenal resistance to rusting and corrosion because of 
chromium (Cr) content. Contents of Cr exceeding 10%, with carbon (C) held 
suitably low, make iron effectively rustproof. 
    Other alloy elements, notably nickel (Ni) and molybdenum (Mo), can also be 
added to the basic stainless composition to produce both variety and 
improvement of properties. Over 100 different stainless steels are produced 
commercially, about half as standardized grades. Some are more properly 
classed as stainless irons since they do not harden as steel; others are true 
steels to which corrosion resistance becomes an added feature. Still others 
that are neither properly steels nor irons introduce totally new classes of 
materials, from both mechanical and chemical standpoints.  See  ALLOY; STEEL. 
 �34s[Carl A. Zapffe]
�33s 
  
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�6v�2v�32sStain (microbiology)   �21m �33s 
    Any colored, organic compound, usually called dye, used to stain tissues, 
cells, cell components, or cell contents. The dye may be natural or synthetic. 
The object stained is called the substrate. The small size and transparency of 
microorganisms make them difficult to see even with the aid of a high-power 
microscope. Staining facilitates the observation of a substrate by introducing 
differences in optical density or in light absorption between the substrate 
and its surrounding or between different parts of the same substrate. In 
electron microscopy, and sometimes in light microscopy (as in the silver 
impregnation technique of staining flagella or capsules), staining is 
accomplished by depositing on the substrate ultraphotoscopic particles of a 
metal such as chromium or gold (the so-called shadowing process); or staining 
is done by treating the substrate with solutions of metallic compounds such as 
uranyl acetate or phosphotungstic acid.  See  ACID-FAST STAIN; GRAM'S STAIN. 
 �34s[Georges Knaysi]
�33s 
  
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�6v�2v�32sStalactites and stalagmites   �21m �33s 
    Stalactites, stalagmites, dripstone, and flowstone are travertine deposits 
in limestone caverns, formed by the evaporation of waters bearing calcium 
carbonate. Stalactites grow down from the roofs of caves and tend to be long 
and thin, with hollow cores. The water moves down the core and precipitates at 
the bottom, slowly extending the length while keeping the core open for more 
water to move down. 
    Stalagmites grow from the floor up and are commonly found beneath 
stalactites; they are formed from the evaporation of the same drip of water 
that forms the stalactite. Stalagmites are thicker and shorter than 
stalactites and have no central hollow core.  See  CAVE; LIMESTONE. 
 �34s[Raymond Siever]
�33s 
  
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�6v�2v�32sStall-warning indicator   �21m �33s 
    A device that determines the critical angle of attack for a given 
aircraft, at which point the drag coefficient overpowers the lift coefficient 
and the aircraft will no longer sustain itself in steady-state condition 
(level flight or climb/descent). The indicator usually operates from vane 
sensors, airflow pressure sensors, tabs on the leading edge of the wings, and 
computing devices which include accelerometers, airspeed detectors, and 
vertical gyros. 
 �34s[James W. Angus]
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�6v�2v�32sStanding wave   �21m �33s 
    A disturbance which is oscillatory in time and which has an amplitude that 
varies in space between zero and a maximum value. Standing waves may be 
formed, for example, near an ideal boundary by the interaction of incident and 
perfectly reflected traveling waves. The points in space where the amplitudes 
are zero are called nodes. Antinodes are points where amplitude is a maximum. 
Standing waves are a limiting case of stationary waves.  See  WAVE (PHYSICS); 
WAVE MOTION. 
 �34s[William J. Galloway]
�33s 
  
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�6v�2v�32sStanding-wave detector   �21m �33s 
    An electric indicating instrument used for detecting standing waves along 
a transmission line or in a waveguide and measuring the resulting 
standing-wave ratio. It can also be used to measure the wavelength and hence 
the frequency of an electromagnetic wave in a line. The detecting device is 
usually a bolometer, thermocouple, or crystal, connected to an indicating 
meter directly or through an amplifier. The detecting device is moved along 
the line while observing the meter indication; the positions along the line at 
which maximum and minimum readings are obtained correspond to the nodes and 
antinodes of the standing wave.  See  STANDING WAVE; WAVEGUIDE; WAVELENGTH 
MEASUREMENT; WAVEMETER. 
 �34s[John Markus]
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�6v�2v�32sStaphylococcus   �21m �33s 
    A genus of the bacterial family Micrococcaceae. The staphylococci are 
parasites of humans, their usual habitat being the nasopharynx and skin. They 
may cause boils, osteomyelitis, wound infections, and some cases of 
septicemia, pneumonia, and kidney infection. They are gram-positive, 
spherical, pathogenic bacteria, and typically occur in grapelike clusters. 
Staphylococci grow readily on the usual laboratory culture media; 
meat-infusion agar, containing human or animal blood, is frequently employed 
for their isolation. Staphylococci are relatively resistant to adverse 
physical and chemical agents. Antibiotic-resistant strains of staphylococci 
have become increasingly numerous. Infections due to these resistant strains 
have raised serious problems of treatment and control. 
    Staphylococci produce several serologically distinct toxins and enzymes 
which are assumed to contribute to their capacity to produce disease. Among 
these substances, exotoxin produces intense destruction of the local tissues 
when injected into the skin of experimental animals and is rapidly fatal when 
injected intravenously. Three hemolysins are produced which dissolve red blood 
cells.  
    Enterotoxin is distinct from exotoxin and other staphylococcal products 
and is responsible for the acute gastrointestinal symptoms of staphylococcal 
food poisoning. Leukocidin is a toxin which destroys the white blood cells, 
thereby aiding the staphylococci in their attempt to counteract an important 
defense mechanism of the host. The production of the enzyme coagulase is 
confined essentially to pathogenic staphylococci. In the test tube, coagulase 
produces a clot in the blood plasma of some animal species. Hyaluronidase, an 
enzyme produced by most pathogenic staphylococci, breaks down certain 
constituents of the intracellular ground substance of many body tissues. 
 �34s[John E. Blair]
�33s 
  
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�6v�2v�32sStar �21m �33s
    A celestial body, consisting of a large, self-luminous mass of hot gas
held together by its own gravity. The composition by weight of the average
star is about 70% hydrogen, 28% helium, 1.5% carbon, nitrogen, oxygen, and
neon, and 0.5% iron group and heavier elements.
    The stars contain by far the largest fraction of the mass of the universe.
Stars are born, produce nuclear energy, evolve, and eventually die. Their
life-spans range from 10(sup 6) years, for a star of high luminosity, to
10(sup 10) for the Sun, and up to 10(sup 13) years for the faintest
main-sequence stars. The oldest known in the Milky Way Galaxy are over
10(sup 10 ) years old.
    The nomenclature for the identification of stars gives their general
location and brightness. The sky was subdivided into constellations and the
brighter stars were named; Greek alphabet letters generally describe them,
with ALPHA representing the brightest star, visually, in a given
constellation. Thus Betelgeuse, or ALPHA Orionis, is the visually brightest
star in Orion. Fundamentally a star is defined by its coordinates on the
celestial sphere, right ascension and declination, and its brightness, or
apparent magnitude. Because of the precession of the equinoxes, celestial
coordinates must be specified for a given epoch. About 6000 stars are visible
to the naked eye, but over 10sup1sup2 exist in the Milky Way Galaxy.
    Many stars have a total space motion of from 100 to 500 km/s (60 to 300
mi/s) with respect to the Sun. Such objects are called high-velocity stars and
belong, according to the nomenclature of Walter Baade, to population II, that
is, stars found in the spheroidal halo of a galaxy. The more slowly moving
stars are younger, and may be members of population I like the Sun; such stars
are found in the spiral regions or flattened disk of a galaxy.
    Figure 1, a Hertzsprung-Russell or H-R diagram, is a means for plotting
the relation between the luminosity and surface temperature or other
temperature-dependent parameter, such as color or spectral type of the stars.
Along the main sequence the stars are distinguished chiefly by mass;
luminosity is a steep function of the mass. Data for stars out to 20 parsecs
(6.2 x 10(sup 17)m or 65 light-years) are plotted in Figure 1. The use of
photoelectric color, like that of spectral type, is essentially an arrangement
by surface temperature and is convenient and accurate.	See
HERTZSPRUNG-RUSSELL DIAGRAM; PARALLAX (ASTRONOMY); PARSEC.


    Stellar spectra.  The spectrum of a star in a large majority of cases
shows absorption lines superposed on a continuous background. The interior of
the star is at high temperature and pressure; its spectrum is nearly that of a
blackbody. The star shades off into space through a reversing layer or stellar
atmosphere in which the continuous spectrum and absorption lines are formed.
In the Sun this reversing layer is about 150 mi (250 km) thick; its base is
about 9900DGF (5750 K), and its outer layer is almost 8100DGF (4200 K).
Outside the normal reversing layer in many stars there may be a temperature
inversion in the low-density chromosphere and corona. The temperature of the
corona eventually reaches 2,700,000DGF (1,500,000 K). The emission lines of
these outer layers affect only slightly the spectrum of the integrated light
of a star and carry about one-millionth of the total energy. The x-ray
radiation of the Sun is highly variable. Some stars have been found by their
strong (and variable) x-ray emission.  See  SUN; X-RAY ASTRONOMY; X-RAY STAR.

    Spectral classification.  The spectral classification of a star gives in a
simple symbolic form the essential features of its complex spectrum. By
inspection, many features are found to vary in a smooth way from one star to
another. This variation is correlated with the colors of the stars. As a
result, spectral classification includes a vast majority of the stars and
represents a sequence of decreasing reversing-layer temperature, from the left
to the right (Fig. 2). There are several side sequences whose temperatures
approximately correspond with those of normal stars, in which apparent
abundance differences exist. Both dwarf and giant stars exist over a wide
range of spectral type. Decimal subdivisions are used; in more refined
analyses prefixes and suffixes are added.
    A more refined system of spectral classification has been widely used. In
this system, a two-parameter set of criteria provides by inspection both
spectral type and estimated luminosity. The luminosity is indicated by a
suffix ranging from Ia, extremely bright supergiant, to III, normal giant, to
V, main-sequence or dwarf star. Thus the designation G2 V represents a star
like the Sun, G2 III a giant of nearly the same temperature, with certain
luminosity-sensitive features enhanced.

    Temperature and luminosity.  Most differences in appearance of stellar
spectra are caused by changes in the surface temperature. The degree of
ionization and excitation of the atoms at a given temperature dictates whether
an element will have an appreciable concentration of atoms in the lower atomic
level that produces the absorption line.
    Absolute magnitude effects occur because the ionization equation depends
on pressure, the electron concentration setting the recombination rate.
Consequently, stars of low surface gravity, which have lower pressures, will
show a given percentage of ionization at a temperature about 900DGF (500 K)
lower than those of the main sequence.

    Age, evolution, and mass.  A group of color-magnitude curves for clusters,
open (population I) and globular (very old population II), shows that,
although the fainter end of the main sequence is essentially the same in all
groups of stars, the brighter ends vary from one group to another in
accordance with differences in age and composition. Stellar evolution causes
such variations.

    These H-R diagrams are most significant for the study of the ages, nuclear
energy sources, and evolution of the stars. The location of a star in an H-R
diagram is completely determined by its mass and chemical composition, if the
latter is specified in detail throughout the star. A point in an H-R diagram,
specified by L and R, does not uniquely determine the mass, however, because
of the possible variation of composition with depth. (Such variation is caused
by the consumption of hydrogen and its conversion to helium by thermonuclear
processes.)
    The masses of the stars are determined for stars which are members of a
double or multiple system, either visual or spectroscopic binaries. The
application of Newton's laws provides the mass, although often with
considerable uncertainty.  See	BINARY STAR.
    The stars of the main sequence, for which stellar evolution has not yet
been a substantial factor in displacement from their normal positions in an
H-R diagram, obey a fairly well-established mass-luminosity relationship.
However, because of stellar evolution, stars move off the main sequence. If
they belong to population I, they essentially move horizontally in an H-R
diagram, so that they still obey the mass-luminosity relationship. Population
II stars with low metal abundances brighten by factors up to a hundred in the
subgiant and red-giant stage.  See  MASS-LUMINOSITY RELATION; STELLAR
EVOLUTION.

    Stellar rotation.  Another important property of stars is their rotation
on their axes. The prevalence of wide double stars and of close spectroscopic
binaries indicates that a large amount of angular momentum is often contained
in the material that condensed to form the stars. In close double systems,
revolution and rotation are often synchronous. In single stars, especially
those of early spectral type, rotation is rapid.

    Composition.  The chemical composition of a star can be deduced from the
spectrum of its atmosphere or from the theory of its internal structure. There
is evidence that stars need not be chemically homogeneous. Products of nuclear
reactions may concentrate in the center so that the ratio of helium to
hydrogen increases inward. Very slow mixing, however, counteracts the tendency
toward complete diffusive separation by gravity of heavy from light elements.
    Studies of the composition of stars can be made only in their atmospheres.
The ratio of hydrogen to heavy elements is about 8000 - 12,000 to 1, by number
of atoms, for the Sun and young population I stars. For extreme high-velocity
stars, old objects of population II, the abundance of the metals may be up to
400 times lower.

    Motions.  Galactic dynamics and kinematics are an important part of the
subject of stellar statistics and are intimately connected with the
distribution of the stars in space. The Milky Way galaxy has a mass of about 6
x 10sup1sup1 Suns, of which only a small fraction is visible from Earth. These
stars move in orbits around their common center of gravitation, located about
8500 parsecs (2.6 x 10(sup 20) m or 1.6 x 10(sup 17) mi) from Earth in the
constellation Sagittarius. The galaxy is highly flattened by its systematic
rotation, the linear rotational velocity being approximately 220 km/s (137
mi/s) at the Sun's distance from the galactic center; this corresponds to a
rotation period of 200,000,000 years.  See  GALAXY; GALAXY, EXTERNAL.
    Superposed on the systematic rotation of the galaxy are individual motions
of the stars. Each star moves in a somewhat elliptical orbit and therefore
shows a peculiar velocity with respect to the local standard of rest, the
standard moving in a circular orbit around the galactic center. The Sun has an
orbit of small ellipticity and inclination, so that solar motion with respect
to the mean of neighboring stars can be detected by analysis of either radial
velocities or proper motions of nearby stars.

    Spatial distribution.  The distribution of stars in space is the subject
of studies of stellar statistics and galactic structure. The Milky Way is the
dominant feature of the galaxy, even to the eye, and represents the mean plane
in which the stars are concentrated. It is nearly a great circle, indicating
that the Sun lies near the galactic plane.
    The number of stars at a given apparent magnitude is a function of
galactic latitude and longitude; in the galactic plane the complex structure
of interstellar clouds of dust that absorb light and the irregularities of
spiral structure produce a somewhat irregular, patchy appearance, with a
maximum number of faint stars in the direction of the galactic center in
Sagittarius, with subsidiary maxima in Cygnus and Carina. These maxima are
probably caused by spiral arms seen lengthwise.
 �34s[Jesse L. Greenstein]
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�6v�2v�32sStarch   �21m �33s 
    The reserve carbohydrate stored usually in the seeds, roots, or stems of 
plants. It is second in abundance only to cellulose as a source of 
carbohydrates. Although starch is widespread in plants, only a few sources are 
sufficiently abundant to make extraction commercially feasible. These sources 
are corn, tapioca, potato, sago, waxy maize, wheat, sorghum, rice, and 
arrowroot.  See  CARBOHYDRATE. 
    There are two basic types of starch molecules: the linear starch polymer 
and the branched starch polymer. Amylose molecules vary in size from about 100 
to over 1000 glucose units. Amylopectin molecules are three or more times 
larger. The sizes of the molecules and the amounts of each type present in the 
starch granule determine the specific properties of the individual starch. 
    Natural starches find a variety of uses as thickeners in foods and 
industrial processes. Many times, however, these starches are processed 
further to obtain viscosity or texture differences of even greater use. 
 �34s[T. A. White]
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�6v�2v�32sStar clouds   �21m �33s 
    Aggregations of thousands or millions of stars spread over hundreds or 
thousands of light-years in space. The Milky Way is composed of such star 
clouds, the heaviest clouds being in the richest parts, such as Cygnus, 
Sagittarius, Carina, and Scutum. The stars in such clouds may appear unevenly 
distributed because of the presence of obscuring interstellar dust and gas.  
See  GALAXY. 
 �34s[Helen S. Hogg]
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�6v�2v�32sStar clusters �21m �33s
    Groups of stars held together by gravitational attraction. The two chief
types are open clusters, formerly called galactic clusters, containing from a
dozen up to many hundreds of stars, and globular clusters, composed of
thousands to hundreds of thousands of stars. A relative of the star cluster is
the stellar association, a group of dozens or hundreds of relatively young
stars spread loosely over a large volume of space. Star clusters are important
in outlining the shape and extent of the Milky Way Galaxy and in deriving
theories of stellar evolution on the assumption that stars of a given cluster
were formed at the same time.
    Open clusters lie along the backbone of the Milky Way Galaxy, strongly
concentrated to the central plane of the Milky Way. A dozen are visible to the
unaided eye, over 1000 are cataloged, and many more must exist. Most open
clusters have an asymmetrical appearance. Most stars in open clusters fall
along the highly populated branch of the spectrum-luminosity diagram known as
the main sequence. The point where this sequence starts furnishes a criterion
of the age of the cluster. At one extreme are young clusters, formed in recent
geologic times, like the Orion Nebula cluster with an age of about 10(sup 6)
years. At the other extreme is NGC 188, with an age of 5 x 10(sup 9) years,
comparable with that of old systems like globular clusters.
    Groups of thousands to hundreds of thousands of stars in globular symmetry
constitute globular clusters. Though they are scattered widely in galactic
latitude, their strong concentration toward the region of Sagittarius-Scorpio
led Harlow Shapley in 1917 to postulate this as the center of the Milky Way
Galaxy. Several are visible to the unaided eye, like Messier 13, the great
cluster in Hercules (see illustration). A total of 131 have been cataloged in
the Milky Way Galaxy, including several so far distant that they are really
intergalactic. A few dozen more may be undetected. Color-magnitude diagrams
differ appreciably from those of open clusters. The spectra show low metal
content (ratio of iron to hydrogen), but the ratio varies from cluster to
cluster. Many globular clusters probably formed in various parts of the Galaxy
during its first 10sup9 years. Some globular cluster stars may be the oldest
stars in the Galaxy. Their ages, based on low-metallicity evolutinary models,
can be as great as 1.41 x 10( sup 11) years.  See  GALAXY; STAR; STELLAR
EVOLUTION.
 �34s[Helen S. Hogg]
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�6v�2v�32sStark effect �21m �33s
    The effect of an electric field on spectrum lines. The electric field may
be externally applied; but in many cases it is an internal field caused by the
presence of neighboring ions or atoms in a gas, liquid, or solid. Discovered
in 1913 by J. Stark, the effect is most easily studied in the spectra of
hydrogen and helium, by observing the light from the cathode dark space of an
electric discharge. Because of the large potential drop across this region,
the lines are split into several components. For observation perpendicular to
the field, the light of these components is linearly polarized.
    The linear Stark effect exhibits large, nearly symmetrical patterns. The
interpretation of the linear Stark effect was one of the first successes of
the quantum theory. According to this theory, the effect of the electric field
on the electron orbit is to split each energy level of the principal quantum
number n into 2n - 1 equidistant levels, of separation proportional to the
field strength.  See  ATOMIC STRUCTURE AND SPECTRA.
    The quadratic Stark effect occurs in lines resulting from the lower energy
states of many-electron atoms. The quadratic Stark effect is basic to the
explanation of the formation of molecules from atoms, of dielectric constants,
and of the broadening of spectral lines.
    The intermolecular Stark effect is produced by the action of the electric
field from surrounding atoms or ions on the emitting atom. The intermolecular
effect causes a shifting and broadening of spectrum lines. The molecules being
in motion, these fields are inhomogeneous in space and also in time. Hence the
line is not split into resolved components but is merely widened.
 �34s[Francis A. Jenkins /William W. Watson]
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�6v�2v�32sStar tracker   �21m �33s 
    A device for automatically following a star or a planet to measure the 
angles between specified reference directions and the direction from the 
celestial body's apparent place to the observer's position. With this data it 
is then possible by completion of geometric computations, using the difference 
between the measured angles and the expected angles, to update the observer's 
position.  
    Star trackers, sometimes referred to as astrotrackers, are used for 
celestial navigation of aircraft and spacecraft, for sensing of a space 
vehicle's angular attitude in flight, and for aiming large telescopes or 
tracking antennae at stars, planets, and other bodies of scientific interest. 
A star tracker is an essential part of any automatic celestial navigation 
system. Any tracker system solves the same celestial problem as a human 
navigator with a hand-held sextant.  See  CELESTIAL NAVIGATION. 
    The basic operation of one form of star tracker is shown in the 
illustration. Starlight enters the objective lens system to form an image at 
the focal plane. Vibrating reeds are located at the focal plane so as to 
periodically modulate the image before the light reaches a photodetector. The 
detector converts the modulated light into an electric voltage or current 
which is amplified and demodulated to develop the pointing error. In addition, 
the signal electronics develop a star presence signal to confirm that a star 
is present and to control the gain of the tracking servoloops. The 
pointing-error signal indicates the direction and magnitude of telescope 
error. Each signal (up-down and left-right) actuates a motor as part of a 
servoloop to redirect the optics to the two respective angles, the elevation 
angle e and the azimuth angle a; this step centers the star image and nulls 
the error signal. The photodetector commonly used for this form of star 
tracker is the photomultiplier.  See  PHOTOMULTIPLIER. 
    Space-borne star-sensing and -tracking systems have been developed as an 
important tool in the astrionics of rocket, satellite, and deep-space 
vehicles. Used primarily as a pointing and attitude reference for the vehicle 
or its instruments and for the generation of navigation and guidance data, 
these devices in the United States have been given a vital role in many 
missions of the National Aeronautics and Space Administration.  See  SPACE 
NAVIGATION AND GUIDANCE. 
 �34s[Elbridge P. Eaton]
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�6v�2v�32sStatic   �21m �33s 
    A hissing, crackling, or other sudden sharp noise that tends to interfere 
with the reception, utilization, or enjoyment of desired signals or sounds. 
Perhaps the commonest form of static is that heard in ordinary broadcast 
receivers during electrical storms. Interference in radio receivers caused by 
improperly operating electric devices in the vicinity is sometimes also called 
static.  See  SFERICS. 
    The crackling sounds heard when long-playing plastic phonograph records 
are played are also called static. These sounds are caused by sudden 
deflection of the phonograph needle by dust particles, which are attracted by 
the grooves of the record by surface electric charges caused by friction on 
dry days. Static appears as momentary white specks in a television picture.  
See  ELECTRICAL INTERFERENCE; ELECTRICAL NOISE. 
 �34s[John Markus]
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�6v�2v�32sStatic electricity   �21m �33s 
    The study of electric charges at rest and the fields they produce. The 
fundamental fact of static electricity (or electrostatics) is that similarly 
electrified bodies repel each other, whereas oppositely electrified bodies 
attract each other. Coulomb's law of force is the basic quantitative law of 
electrostatics.  See  COULOMB'S LAW; ELECTRIC CHARGE; ELECTROSTATICS. 
 �34s[Ralph P. Winch]
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�6v�2v�32sStatics   �21m �33s 
    The branch of mechanics that describes bodies which are acted upon by 
balanced forces and torques so that they remain at rest or in uniform motion. 
This includes point particles, rigid bodies, fluids, and deformable solids in 
general. Static point particles, however, are not very interesting, and 
special branches of mechanics are devoted to fluids and deformable solids. For 
example, hydrostatics is the study of static fluids, and elasticity and 
plasticity are two branches devoted to deformable bodies. Therefore this 
article will be limited to the discussion of the statics of rigid bodies in 
two- and three-space dimensions.  See  BUOYANCY; ELASTICITY; HYDROSTATICS; 
MECHANICS.  
    In statics the bodies being studied are in equilibrium. The equilibrium 
conditions are very similar in the planar, or two-dimensional, and the 
three-dimensional rigid body statics. These are that the vector sum of all 
forces acting upon the body must be zero; and the resultant of all torques 
about any point must be zero. Thus it is necessary to understand the vector 
sums of forces and torques. 
    In studying statics problems, two principles, superposition and 
transmissibility, are used repeatedly on force vectors. They are applicable to 
all vectors, but specifically to forces and torques (first moments of forces). 
The principle of superposition of vectors is that the sum of any two vectors 
is another vector. The principle of transmissibility of a force applied to a 
rigid body is that the same mechanical effect is produced by any shift of the 
application of the force along its line of action. To use the superposition 
principle to add two vectors, the principle of transmissibility is used to 
move some vectors along their line of action in order to add to their 
components.  
    The moment of a force about a directed line is a signed number whose value 
can be obtained by applying these two rules: (1) The moment of a force about a 
line parallel to the force is zero. (2) The moment of a force about a line 
normal to a plane containing the force is the product of the magnitude of the 
force and the least distance from the line to the line of the force.  See  
EQUILIBRIUM OF FORCES; FORCE; TORQUE. 
 �34s[Brian DeFacio]
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�6v�2v�32sStationary state   �21m �33s 
    In quantum mechanics, an energy state for which the probability of any 
observation is independent of time, that is, stationary. Because stationary 
states endure in time, they conveniently characterize physical systems. For 
example, at any instant, it is customary and useful to think that each atom in 
a discharge tube is in one of its stationary states, with most atoms in the 
ground state, and the radiation from the tube a measure of the populations of 
the excited states.  See  ENERGY LEVEL (QUANTUM MECHANICS); EXCITED STATE; 
GROUND STATE; QUANTUM MECHANICS. 
 �34s[Edward Gerjuoy]
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�6v�2v�32sStatistical mechanics �21m �33s
    That branch of physics which endeavors to explain the macroscopic
properties of a system on the basis of the properties of the microscopic
constituents of the system. Usually the number of constituents is very large.
All the characteristics of the constituents and their interactions are
presumed known; it is the task of statistical mechanics (often called
statistical physics) to deduce from this information the behavior of the
system as a whole.

    Scope.  Elements of statistical mechanical methods are present in many
widely separated areas in physics. For instance, in the classical Boltzmann
problem one attempts to explain the thermodynamic behavior of gases on the
basis of classical mechanics applied to the system of molecules.
    Statistical mechanics gives more than an explanation of already known
phenomena. By using statistical methods, it often becomes possible to obtain
expressions for empirically observed parameters, such as viscosity
coefficients, heat conduction coefficients, and virial coefficients, in terms
of the forces between molecules. Statistical considerations also play a
significant role in the description of the electric and magnetic properties of
materials. See BOLTZMANN STATISTICS; INTERMOLECULAR FORCES; KINETIC THEORY OF
MATTER.
    If the problem of molecular structure is attacked by statistical methods,
the contributions of internal rotation and vibration to thermodynamic
properties, such as heat capacity and entropy, can be calculated for models of
various proposed structures. Comparison with the known properties often
permits the selection of the correct molecular structure.
    Perhaps the most dramatic examples of phenomena requiring statistical
treatment are the cooperative phenomena or phase transitions. In these
processes, such as the condensation of a gas, the transition from a
paramagnetic to a ferromagnetic state, or the change from one crystallographic
form to another, a sudden and marked change of the whole system takes place.
See  PHASE TRANSITIONS.
    Statistical considerations of quite a different kind occur in the
discussion of problems such as the diffusion of neutrons through matter. In
this case, one knows the probability of the various events which affect the
neutron, such as the capture probability and scattering cross section. The
problem here is to describe the physical situation after a large number of
these individual events. The procedures used in the solution of these problems
are very similar to, and in some instances taken over from, kinetic
considerations. Similar problems occur in the theory of cosmic-ray showers.
    It happens in both low-energy and high-energy nuclear physics that a
considerable amount of energy is suddenly liberated. An incident particle may
be captured by a nucleus, or a high-energy proton may collide with another
proton. In either case, there is a large number of ways (a large number of
degrees of freedom) in which this energy may be utilized. To survey the
resulting processes, one can again invoke statistical considerations.  See
SCATTERING EXPERIMENTS (NUCLEI).
    Of considerable importance in statistical physics are the random
processes, also called stochastic processes or sometimes fluctuation
phenomena. The Brownian motion, the motion of a particle moving in an
irregular manner under the influence of molecular bombardment, affords a
typical example. The stochastic processes are in a sense intermediate between
purely statistical processes, where the existence of fluctuations may safely
be neglected, and the purely atomistic phenomena, where each particle requires
its individual description.  See  BROWNIAN MOVEMENT; STOCHASTIC PROCESS.
    All statistical considerations involve, directly or indirectly, ideas from
the theory of probability of widely different levels of sophistication. The
use of probability notions is, in fact, the distinguishing feature of all
statistical considerations.  See  PROBABILITY; PROBABILITY (PHYSICS);
STATISTICS.

    Methods.  Consider a system of N particles, each of the mass m, contained
in a volume V.	Call the positions of the particles x(sub 1), y(sub 1), z(sub
1), .  .  .  , x(sub N), y(sub N), z(sub N), their cartesian velocities v(sub
x1), .	.  .  , v(sub zN), and their momenta p(sub x1), .  .  .  , p(sub zN).
The simplest statistical description concentrates on a discussion of the
distribution function f(x,y,z;v(sub x),v(sub y),v(sub z);t).  The quantity
f(x,y,z;v(sub x),v)sub y),v(sub z);t) (dxdydzdv(sub x)dv(sub y)dv(sub z)) gives
the (probable) number of particles of the system in those positional and
velocity ranges where x lies between x and x + dx; v(sub )x between v(sub x) and
v(sub )x + dv(sub x), and so on.  These ranges are finite.  Observations made on
a system always require a finite time; during this time the microscopic details
of the system will generally change considerably as the phase point moves.  The
result of a measurement of a quantity Q will therefore yield the time average,
as in Eq.  (1).  The integral is along the


   BAR Q = (1/t) INTEGRATION (0 TO t) Q dt				 (l)


trajectory in phase space; Q depends on the variables x1, . . . , p(sub zN) and
t. To evaluate the integral, one must know the trajectory, which requires the
solution of the complete mechanical problem.
    Ensembles. J. Willard Gibbs first suggested that instead of calculating a
time average for a single dynamical system, one should instead consider a
collection of systems, all similar to the original one. Such an ensemble of
systems is to be constructed in harmony with the available knowledge of the
single system, and may be represented by an assembly of points in the phase
space, each point representing a single system. If, for example, one knows the
energy of a system precisely, but nothing else, the appropriate representative
example would be a uniform distribution of ensemble points over the energy
surface, and no ensemble points elsewhere.  An ensemble is characterized by a
density function RHO(x1, .  .  .  ,z(sub N); p(sub x1), .  .  .  ,p(sub zN);t)
IDENTICAL RHO(x,p,t).  The significance of this function is that the number of
ensemble systems dN(sub e) contained in the volume element dx(sub 1) .	.  .
d(sub zN; dp(sub x) .  .  .  dp(sub zN) of the phase space (this volume element
will be called dGAMMA), at time t is as given in Eq.  (2).

		   p(x,p,t) d GAMMA  =	dN(sub e)		     (2)


The ensemble average of any quantity Q is given by Eq. (3). The basic idea
now is to replace the time average of an individual system by the ensemble

BAR Q(sub ens) = (INTERGRAL Qp d GAMMA)/(INTERGRAL p d GAMMA)	     (3)

average, at a fixed time, of the representative ensemble. Stated formally,
one identifies BAR Q(sub t) defined by Eq. (1), in which no statistics is
involved, with BAR Q(sub ens) defined by Eq. (3), in which probability
assumptions are explicitly made.
    Relation to thermodynamics. It is certainly reasonable to assume that the
appropriate ensemble for a thermodynamic equilibrium state must be described
by a density function which is independent of the time, since all the
macroscopic averages which are to be computed as ensemble averages are
time-independent.
    The so-called microcanonical ensemble is defined by Eq. (4a), where c is a
constant, for the energy E between E(sub 0) and E(sub 0) + DELTA E; for other
energies Eq.  (4b) holds.  By using Eq.  (3), one may calculate any
microcanonical average.  The calculations, which involve integrations over
volumes bounded by two energy surfaces, are not trivial.  Still, one may obtain
in this way many of the results of classical Boltzmann statistics.  For
applications and for the interpretation of thermodynamics, the canonical
ensemble is much more preferable.  In this case one describes a system which is
not isolated but which is in thermal contact with a heat reservoir.

			       p(p,x) = c			      (4a)

			       p(p,x) = o			      (4b)

     There is yet another ensemble which is extremely useful and which is
particularly suitable for quantum-mechanical applications. Much work in
statistical mechanics is based on the use of this so-called grand canonical
ensemble.  In the grand ensemble one has a collection of systems; the number of
particles in each system is no longer the same, but varies from system to
system.  The density function RHO(N,p,x) dGAMMA(sub N) gives the probability
that there will be in the ensemble a system having N particles, and that this
3system, in its 6N-dimensional phase space GAMMA(sub N), will be in the region
of phase space dGAMMAsubN.�34s[Max Dresden]
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�6v�2v�32sStatistics �21m �33s
    The field of knowledge concerned with collecting, analyzing, and
presenting data. Not only workers in the physical, biological, and social
sciences, but also engineers, business managers, government officials, market
analysts, and many others regularly use statistical methods in their work. The
methods range from simple counting to complex mathematical systems designed to
extract the maximum amount of information from very expensive data.
    In an important sense statistics may be regarded as a field of application
of probability theory. The common problem faced by any worker who must
collect, analyze, and present data is that of random variation which prevents
repetition of exactly the same result when a measurement is repeated.
Statistical methods are employed to assess the magnitude of random variation,
to minimize it, to balance it out, to remove it by calculation procedures, and
to analyze it by suitably arranged patterns of observation. The theory of
probability is concerned with the properties of random variables and hence
furnishes the basis for developing techniques for controlling them.  See
PROBABILITY.
    Viewing statistics from another direction, it is the science of deriving
information about populations by observing only samples of those populations.
A population is any well-specified collection of elements. Thus, one may refer
to the population of adults in the continental United States viewing
television screens at 8:14 P.M. on Aug. 6, 1970. Populations may be finite or
infinite. An element of a univariate population is characterized by the value
of a random variable which measures some single attribute of interest in the
population. Thus, one may be interested in whether or not individuals of the
television audience were or were not viewing program A; with each individual
one may associate a random variable, let it be X, which takes on the value of
1 if the individual is watching A and 0 if he is not. If one were interested
in a second characteristic of the elements of the television audience (such as
age), he would be said to be dealing with a bivariate population; a third
characteristic (such as economic status) would make it a trivariate or, less
specifically, a multivariate population.

    Distributions.  In a univariate population, the population distribution is
a curve (function of the random variable which characterizes the elements of
the population) from which one can determine the proportion of the population
which has elements in a certain range of the random variable. For example, the
curve of Fig. 1 provides the distribution of annual incomes of family units in
the United States in 1954. The total area under the curve is 1. The area under
the curve between any two vertical lines gives the proportion of the families
having annual incomes between the two values marked on the horizontal scale by
the two vertical lines.
    The distribution is also referred to as the distribution function, the
density function, the frequency function, or the probability density.
    The total area under the distribution curve to the left of each point can
also be plotted to give a curve which starts at zero and reaches unity as the
variable becomes large; the resulting curve is sometimes called the cumulative
distribution function, the probability distribution, or simply the
distribution. The cumulative form of the curve of Fig. 1 is shown in Fig. 2;
the height of the curve at any point on the horizontal scale equals the area
to the left of that point under the curve of Fig. 1 and is the proportion of
the population having incomes less than the value at that point. The
distribution (in either frequency or cumulative form) gives complete
information about the way the characterizing variable is spread through the
population.

    Population parameters. Populations (or population distributions) are often
specified incompletely by certain population parameters. Some of these
parameters are location parameters or measures of central tendency; a second
class of important parameters consists of measures of dispersion or scale
parameters.
    The most widely used location parameters are the mean, the median, and the
mode. The mean is the average over all the population of the values of the
random variable. It is often represented by the Greek letter mu. In
mathematical terms, if x is the random variable, f(x) the frequency function
for a given population, and F(x) its cumulative form, then the mean is as
shown in Eq. (1).

mu = INTEGRATION (-INFINITY to INFINITY) xf(x) dx = INTEGRATION (-INFINITY to
			   INFINITY) x dF (x)				  (1)

    The median, often designated by Med, M, or X(sub.50), is a number such that,
at most, one-half the values of the variable associated with the elements of the
population fall above or below it. The mode is the most frequent value of the
random variable; if the frequency function has a unique maximum value, the
mode is the value of the random variable at which the frequency function
reaches its maximum. Location parameters are numbers near the center of the
range over which the random variable of the population varies.
    The extent to which a population is scattered on either side of its center
is roughly indicated by measures of dispersion such as the standard deviation,
the mean deviation, the interquartile range, the range, and sometimes others.
The standard deviation is the square root of the mean square of the deviations
from the mean; it is usually denoted by the Greek letter SIGMA; SIGMA(sup 2) is

SIGMA(sup 2) = INTEGRATION (-INFINITY to INFINITY) (x - mu)(sup 2) f(x) dx

= INTEGRATION (-INFINITY to INFINITY) (x - mu)(sup 2) dF (x)	      (2)

called the variance and is expressed by Eq. (2). The mean deviation, shown in
Eq. (3), is the average over the population of the deviations from the mean,
all taken to be positive.

Mean deviation = INTEGRATION (-INFINITY to INFINITY) |(x - mu)|f(x) dx

       = INTEGRATION (-INFINITY to INFINITY)|(x - mu)| dF (x)		    (3)


The interquartile range (often denoted by Q) is the difference X(sub.75) -
X(sub.25), where X(sub.75) is the value of the random variable such that
one-quarter of the population has values larger than X(sub.75), and X(sub.25)
is the number such that one-quarter of the population has values smaller than
X(sub.25). The three numbers, X(sub.25), X(sub.50), X(sub.75), are called
quartiles; these divide the population into quarters. The range is the
difference between the largest and the smallest of the population elements. 

    Sampling.  If one examines every element of a population and records the
value of the random variable for each, complete information is obtained about
the distribution of the random variable in the population, and there is no
statistical problem. It is usually impossible or uneconomical to make a
complete enumeration (or census) of a population, and one must therefore be
content to examine only a part or sample of the population. On the basis of
the sample, one draws conclusions about the entire population; the conclusions
thus drawn are not certain in the sense that they would likely have been
somewhat different if a different sample of the population had been examined.
The problem of drawing valid conclusions from samples and of specifying their
range of uncertainty is known as the problem of statistical inference.
    Simple random sampling is a method of selecting a sample of n elements out
of a population of N elements so that all such samples have an equal
probability of being drawn. This may be done by selecting a first element at
random from the population, then a second element at random from the remaining
population, and so on until the n elements are selected.
    The observations of a sample, besides providing estimates of population
parameters, can also be used to obtain an estimate of the population's
frequency function. This estimate is determined by dividing the range of the
sample observations into several intervals of equal length L and counting the
number of observations occurring in each interval; these numbers are then
divided by nL to determine fractions giving the relative density of the sample
occurring in each interval; then on a sheet of graph paper one lays out the
intervals on a horizontal axis and plots horizontal lines above each interval
at a height equal to the fraction corresponding to the interval; finally the
successive plotted horizontal lines are connected by vertical lines to form a
broken line curve known as a histogram (Fig. 3).

    When a population can be regarded as being made up of several
nonoverlapping subpopulations, one may draw a sample from it by drawing a
simple random sample from each subpopulation (or stratum); this procedure is
called stratified random sampling. Systematic sampling may be regarded as a
sampling of units at regular intervals in the population, for example, every
tenth unit. When the elements of a population are in mutually exclusive groups
called the primary units of the population, then a sample of these primary
units might be made, and then, from those selected, a sample of the individual
elements would be made. This procedure is called two-stage sampling or
subsampling. Multistage sampling can involve more than two stages of sampling.
    Many important sampling distributions are derived for random samples drawn
from a normal or Gaussian distribution, which is a bell-shaped symmetrical
distribution centered at its mean MU.

    Estimation.  In making an estimate of the value of a parameter of a
population from a sample, a function (called the estimator) of the
observations is used. For example, for estimating the mean of a normal
population the mean of the sample observations is usually taken as the
estimator. Another estimator is the average of the two most extreme
observations. In fact, there is an infinitude of estimators. The problem of
estimation is to find a ``good'' estimator.
    A good estimator may be regarded as one which results in a distribution of
estimates concentrated near the true value of the parameter and which can be
applied without excessive effort.

    Tests of hypotheses.  Besides estimation of parameters, another major area
of statistical inference is the testing of hypotheses. A hypothesis is merely
an assertion that a population has a specific property. The test consists of
drawing a sample from the population and determining whether or not it is
consistent with the assertion. Very often the hypothesis is a statement about
the mean of a population; that it has a given value, that it is the same as
that of another population, that it exceeds that of another population by at
least 10 units, and the like. Thus, one may be comparing a new blend of
gasoline with a current blend, a new drug with a standard one, a new
manufacturing process with an existing one.

    Experimental design.  An experiment is performed to obtain information
about the relations between several variables. For example, one may study the
effect of storage temperature and duration of storage on the flavor of a
frozen food. Three variables (flavor, temperature, and duration) are involved;
one (flavor) is called the subject of the experiment; the other two are called
factors which influence the subject. Sometimes the factors have intrinsic
value in themselves; sometimes they are merely nuisance variables which must
be taken into account because it is impossible to perform the experiment
without them.
    There exist in the statistical literature great numbers of specific
experimental designs. These are patterns for making experimental observations;
the actual construction of the designs requires quite advanced mathematics
based on group theory, finite geometries, and combinatorial analysis. The
mathematical problem is to find a pattern from which it is possible to extract
the desired information and yet minimize the number of observations.

    Regression and correlation.  The regression problem is that of estimating
certain unknown constants or parameters occurring in a function which relates
several variables; the variables may be random or not. By far the most easily
handled cases are those in which the function is linear in the unknown
parameters, and it is worth considerable effort to transform the function to
that form if at all possible.  See  ANALYSIS OF VARIANCE; BIOMETRICS;
DISTRIBUTION (PROBABILITY).
 �34s[Alexander M. Mood]
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�6v�2v�32sStaurolite �21m �33s
    A nesosilicate mineral, (Al,Mg)18(Fe,Al)4(Si,Al)8O48HSIMILAR 4. Staurolite
is frequently in crystals, usually a combination of the vertical prism with
the basal and side pinacoid. The hardness is 7 - 7** on Mohs scale; specific
gravity is 3.7. The luster is resinous to vitreous but may be dull when the
mineral is impure or altered; the color is reddish brown to black. At St.
Gothard, Switzerland, staurolite is found on kyanite in parallel orientation.
In the United States it is found in schists in many states, notably New
Hampshire, Massachusetts, North Carolina, Georgia, Virginia, and New Mexico.
See  SILICATE MINERALS.
 �34s[Cornelius S. Hurlbut, Jr. ;Paul B. Moore]
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�6v�2v�32sStauromedusae   �21m �33s 
    An order of the class Scyphozoa, usually found in circumpolar regions. The 
egg develops into a planula which can only creep since it lacks cilia. The 
planula changes into a polyp that metamorphoses directly into a combined polyp 
and medusa form. The medusa is composed of a cuplike bell called a calyx 
(medusan part) and a stem or stalk (polyp part) which terminates in a pedal 
disk (see illustration). The calyx is eight-sided and has eight groups of 
short, capped tentacles and eight sensory bodies, called anchors, on its 
margin. The mouth, situated at the center of the calyx, has four thin lips and 
leads to the stomach in which gastral filaments are arranged in a row on 
either side of each interradius. Though sessile, the medusa can move in a 
leechlike fashion by alternate attachment and release of the pedal disk, using 
the substratum as an anchor.  See  SCYPHOZOA. 
 �34s[Tohru Uchida]
�33s 
  
  
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�6v�2v�32sSteady flow   �21m �33s 
    Fluid flow in which all conditions at any one point are constant with 
respect to time. True steady flow is found only with laminar flow. In 
turbulent flow there are continual fluctuations in velocity and pressure at 
every point. However, if the values fluctuate on both sides of an average 
value that is constant, then the flow is mean steady flow. See FLUID FLOW; 
LAMINAR FLOW. 
 �34s[Robert L. Daugherty]
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�6v�2v�32sSteam   �21m �33s 
    Water vapor, or water in its gaseous state. Steam is the most widely used 
working fluid in external combustion engine cycles, where it will utilize 
practically any source of heat, that is, coal, oil, gas, nuclear fuel (uranium 
and thorium), waste fuel, and waste heat. It is also extensively used as a 
thermal transport fluid in the process industries and in the comfort heating 
and cooling of space. The universality of its availability and its highly 
acceptable, well-defined physical and chemical properties also contribute to 
the usefulness of steam.  See  BOILER; STEAM CONDENSER; STEAM ENGINE; 
STEAM-GENERATING UNIT; STEAM HEATING; STEAM TURBINE; THERMODYNAMIC CYCLE; 
THERMODYNAMIC PRINCIPLES; WATER. 
 �34s[Theodore Baumeister]
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�6v�2v�32sSteam condenser   �21m �33s 
    A heat-transfer device used for condensing steam to water by removal of 
the latent heat of steam and its subsequent absorption in a heat-receiving 
fluid, usually water, but on occasion air or a process fluid. Steam condensers 
may be classified as contact or surface condensers. 
    In the contact condenser, the condensing takes place in a chamber in which 
the steam and cooling water mix. The direct contact surface is provided by 
sprays, baffles, or void-effecting fill. In the surface condenser, the 
condensing takes place separated from the cooling water or other 
heat-receiving fluid (or heat sink). A metal wall, or walls, provides the 
means for separation and forms the condensing surface. 
    Both contact and surface condensers are used for process systems and for 
power generation serving engines and turbines. Modern practice has confined 
the use of contact condensers almost entirely to such process systems as those 
involving vacuum pans, evaporators, or dryers, and to condensing and 
dehumidification processes inherent in vacuum-producing equipment such as 
steam jet ejectors and vacuum pumps. The steam surface condenser is used 
chiefly in power generation but is also used in process systems, especially in 
those in which condensate recovery is important. Air-cooled surface condensers 
are used in process systems and in power generation when the availability of 
cooling water is limited.  See  STEAM; STEAM TURBINE;# VAPOR CONDENSER. 
 �34s[Joseph F. Sebald]
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�6v�2v�32sSteam engine �21m �33s
    A machine for converting the heat energy in steam to mechanical energy of
a moving mechanism, for example, a shaft. The steam engine can utilize any
source of heat in the form of steam from a boiler. Most modern machine
elements had their origin in the steam engine: cylinders, pistons, piston
rings, valves and valve gear crossheads, wrist pins, connecting rods,
crankshafts, governors, and reversing gears.  See  BOILER; STEAM.
    The 20th century saw the practical end of the steam engine. The steam
turbine replaced the steam engine as the major prime mover for electric
generating stations. The internal combustion engine, especially the high-speed
automotive types which burn volatile (gasoline) or nonvolatile (diesel) liquid
fuel, has completely displaced the steam locomotive with the diesel locomotive
and marine steam engines with the motorship and motorboat. Because of the
steam engine's weight and speed limitations, it was also excluded from the
aviation field.  See  DIESEL ENGINE; GAS TURBINE; INTERNAL COMBUSTION ENGINE;
STEAM TURBINE.
    A typical steam reciprocating engine consists of a cylinder fitted with a
piston (Fig. 1). A connecting rod and crankshaft convert the piston's
to-and-fro motion into rotary motion. A flywheel tends to maintain a
constant-output angular velocity in the presence of the cyclically changing
steam pressure on the piston face. A D slide valve admits high-pressure steam
to the cylinder and allows the spent steam to escape (Fig. 2). The power
developed by the engine depends upon the pressure and quantity of steam
admitted per unit time to the cylinder.
    Engines are classified as single- or double-acting, and as horizontal
(Fig. 1) or vertical depending on the direction of piston motion. If the steam
does not fully expand in one cylinder, it can be exhausted into a second,
larger cylinder to expand further and give up a greater part of its initial
energy. Thus, an engine can be compounded for double or triple expansion.
    Steam engines can also be classed by functions, and are built to optimize
the characteristics most desired in each application. Stationary engines drive
electric generators, in which constant speed is important, or pumps and
compressors, in which constant torque is important.
 �34s[Theodore Baumeister]
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�6v�2v�32sSteam-generating furnace   �21m �33s 
    An enclosed space provided for the combustion of fuel to generate steam. 
The closure confines the products of combustion and is capable of withstanding 
the high temperatures developed and the pressures used. Its dimensions and 
geometry are adapted to the rate of heat release, to the type of fuel, and to 
the method of firing so as to promote complete burning of the combustible and 
suitable disposal of ash. In water-cooled furnaces the heat absorbed 
materially affects the temperature of gases at the furnace outlet and 
contributes directly to the generation of steam.  See  FURNACE; 
STEAM-GENERATING UNIT. 
 �34s[George W. Kessler]
�33s 
  
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�6v�2v�32sSteam-generating unit   �21m �33s 
    The wide diversity of parts, appurtenances, and functions needed to 
release and utilize a source of heat for the practical production of steam at 
pressures to 5000 psi (34 megapascals) and temperatures to 1100DGF (600DGC); 
often referred to as a steam boiler for brevity. 
    The essential steps of the steam-generating process include (1) a furnace 
for the combustion of fuel, or a nuclear reactor for the release of heat by 
fission, or a waste heat system; (2) a pressure vessel in which feedwater is 
raised to the boiling temperature evaporated into steam, and generally 
superheated beyond the saturation temperature; and (3) in many modern central 
station units, a reheat section or sections for resuperheating steam after it 
has been partially expanded in a turbine. This aggregation of functions 
requires a wide assortment of components, which may be variously employed in 
the interests, primarily, of capacity and efficiency in the steam-production 
process. Their selection, design, operation, and maintenance constitute a 
complex process which, in the limit, calls for the highest technical skill.  
See  BOILER; SUPERHEATER; WATER-TUBE BOILER. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sSteam heating   �21m �33s 
    A heating system that uses steam generated from a boiler. The steam 
heating system conveys steam through pipes to heat exchangers, such as 
radiators, convectors, baseboard units, radiant panels, or fan-driven heaters, 
and returns the resulting condensed water to the boiler. Such systems normally 
operate at pressure not exceeding 15 pounds per square inch gage (psig) or 103 
kilopascals gage, and in many designs the condensed steam returns to the 
boiler by gravity because of the static head of water in the return piping. 
    In a one-pipe steam heating system, a single main serves the dual purpose 
of supplying steam to the heat exchanger and conveying condensate from it. 
Ordinarily, there is but one connection to the radiator or heat exchanger, and 
this connection serves as both the supply and return. A two-pipe system is 
provided with two connections from each heat exchanger, and in this system 
steam and condensate flow in separate mains and branches. When the steam 
condensate cannot be returned by gravity to the boiler in a two-pipe system, 
an alternating return lifting trap, condensate return pump, or vacuum return 
pump must be used to force the condensate back into the boiler.  See  BOILER; 
COMFORT HEATING. 
 �34s[John W. James]
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�6v�2v�32sSteam jet ejector   �21m �33s 
    A steam-actuated device for pumping compressible fluids, usually from 
subatmospheric suction pressure to atmospheric discharge pressure. A steam jet 
ejector is most frequently used for maintaining vacuum in process equipment in 
which evaporation or condensation takes place. Because of its simplicity, 
compactness, reliability, and generally low first cost, it is often preferred 
to a mechanical vacuum pump for removing air from condensers serving steam 
turbines, especially for marine service. 
    Two or more stages may be arranged in series, depending upon the total 
compression ratio required. Two or more sets of series stages may be arranged 
in parallel to accommodate variations in capacity. Vapor condensers are 
usually interposed between the compression stages of multistage steam jet 
ejectors to condense and remove a significant portion of the motive steam and 
other condensable vapors. 
 �34s[Joseph F. Sebald]
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�6v�2v�32sSteam separator   �21m �33s 
    A device for separating a mixture of the liquid and vapor phases of water. 
Steam separators are used in most boilers and may also be used in saturated 
steam lines to separate and remove the moisture formed because of heat loss. 
    Steam separators have many forms and may be as fundamental as a simple 
baffle that utilizes inertia caused by a change of direction. Most modern, 
high-capacity boilers use a combination of cyclone separators and steam 
dryers. Cyclone separators slice the steam-water mixture into thin streams so 
that the steam bubbles need travel only short distances through the mixture to 
become disengaged, and then they whirl the mixture in a circular path, 
creating a centrifugal force many times greater than the force of gravity. 
Steam dryers remove small droplets of water from the steam by providing a 
series of changes in direction and a large surface area to intercept the 
droplets.  See  BOILER WATER; STEAM; STEAM TURBINE. 
 �34s[Earl E. Coulter]
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�6v�2v�32sSteam turbine   �21m �33s 
    A machine for generating mechanical power in rotary motion from the energy 
of steam at temperature and pressure above that of an available sink. By far 
the most widely used and most powerful turbines are those driven by steam. 
Until the 1960s essentially all steam used in turbine cycles was raised in 
boilers burning fossil fuels (coal, oil, and gas) or, in minor quantities, 
certain waste products. However, modern turbine technology includes nuclear 
steam plants as well as production of steam supplies from other sources.  See  
NUCLEAR REACTOR. 
    The illustration shows a small, simple mechanical-drive turbine of a few 
horsepower. It illustrates the essential parts for all steam turbines 
regardless of rating or complexity: (1) a casing, or shell, usually divided at 
the horizontal center line, with the halves bolted together for ease of 
assembly and disassembly; it contains the stationary blade system; (2) a rotor 
carrying the moving buckets (blades or vanes) either on wheels or drums, with 
bearing journals on the ends of the rotor; (3) a set of bearings attached to 
the casing to support the shaft; (4) a governor and valve system for 
regulating the speed and power of the turbine by controlling the steam flow, 
and an oil system for lubrication of the bearings and, on all but the smallest 
machines, for operating the control valves by a relay system connected with 
the governor; (5) a coupling to connect with the driven machine; and (6) pipe 
connections to the steam supply at the inlet and to an exhaust system at the 
outlet of the casing or shell. 
    Steam turbines are ideal prime movers for driving machines requiring 
rotational mechanical input power. They can deliver constant or variable speed 
and are capable of close speed control. Drive applications include centrifugal 
pumps, compressors, ship propellers, and, most important, electric generators. 
    Steam turbines are classified (1) by mechanical arrangement, as 
single-casing, cross-compound (more than one shaft side by side), or 
tandem-compound (more than one casing with a single shaft); (2) by steam flow 
direction (axial for most, but radial for a few); (3) by steam cycle, whether 
condensing, noncondensing, automatic extraction, reheat, fossil fuel, or 
nuclear; and (4) by number of exhaust flows of a condensing unit, as single, 
double, triple flow, and so on. Units with as many as eight exhaust flows are 
in use.  See  TURBINE. 
 �34s[Frederick G. Baily]
�33s 
  
  
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�6v�2v�32sStearate �21m �33s
    A salt (soap) or ester of stearic acid having the general formula
			 O
			//
		  C17H35C -O-M	or -R

and formed by replacing the carboxylic hydrogen by a metal (M) to give a salt,
or by an organic radical (R) to give an ester. Stearates occur in substantial
amounts in animal and vegetable fats. The esters of long-chain alcohols are
known as waxes. Other esters of monohydric and polyhydric alcohols are used in
cosmetics, lacquers, nonionic surface-active agents, and plasticizers.
Alkali-metal salts are major components of toilet and laundry soaps. Other
metal salts are used in paints, waterproofing, pharmaceuticals, cosmetics,
fungicides, and lubricant and grease additives.  See  FAT AND OIL; FAT AND OIL
(FOOD); SOAP; WAX, ANIMAL AND VEGETABLE.
 �34s[Elbert H. Hadley]
�33s

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�6v�2v�32sSteel   �21m �33s 
    Any of a great number of alloys that contain the element iron as the major 
component and small amounts of carbon as the major alloying element. These 
alloys are more properly referred to as carbon steels, and make up well over 
90% of the tonnage of steels produced throughout the world. Small amounts, 
generally on the order of a few percent, of other elements such as manganese, 
silicon, chromium, molybdenum, and nickel may also be present in carbon 
steels. However, when large amounts of alloying elements are added to iron to 
achieve special properties, other designations are used to describe the 
alloys. For example, large amounts of chromium, over 12%, are added to produce 
the important groups of alloys known as stainless steels.  See  STAINLESS 
STEEL.  
    Low-carbon steels, sometimes referred to as mild steels, usually contain 
less than 0.25% carbon. These steels are easily hot-worked and are produced in 
large tonnages for beams and other structural applications. The medium-carbon 
steels contain between 0.25 and 0.70% carbon, and are most frequently used for 
machine components that require high strength and good fatigue resistance. 
High-carbon steels contain more than 0.7% carbon and are in a special category 
because of their high hardness and low toughness. This combination of 
properties makes the high-carbon steels ideal for bearing applications where 
wear resistance is important and the compressive loading minimizes brittle 
fracture that might develop on tensile loading.  See  FERROALLOY; HEAT 
TREATMENT (METALLURGY); IRON; IRON ALLOYS; STEEL MANUFACTURE. 
 �34s[George Krauss]
�33s 
  
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�6v�2v�32sSteel manufacture   �21m �33s 
    The sequence through which blast-furnace iron, scrap iron, and alloying 
additions are processed to combine and to separate them into desired 
compositions of steel and residual slag. The manufacture of steel is a 
tremendous industry with extensive ramifications resulting from the complex 
interrelation of technical and economic considerations. 
    Chemically, all steelmaking processes may be classified as acid or basic, 
depending upon the refractory and slag combination. Acid processes use silica 
(SiO2) refractories throughout and are able to accommodate slags that become 
saturated with this component under operating conditions. These acid systems 
can be used to eliminate C, Mn, and Si from the charge, but require select raw 
materials within final steel specifications for P and S. Basic processes use 
magnesite (MgO) or equivalent refractories in the portions of the furnace that 
contact molten slag and metal, and accommodate lower silica slags with 
compensating amounts of lime (CaO). These systems can eliminate C, Mn, and Si 
as effectively as can the acid systems; they will also eliminate P and 
appreciable amounts of S. 
  
    Technologically, processes can be grouped into three main types: (1) 
pneumatic, (2) open-hearth, and (3) electric. In pneumatic processes all heat 
is derived from the initial heat content of the charge materials, principally 
molten, and the thermochemical balance of the refining reactions; the 
selective oxidation of the refining is accomplished by blowing air or 
commercial oxygen. In open-hearth processes the major source of heat is the 
combustion of fuel (usually gas or oil); this in turn depends for its success 
on the regenerative principle of preheating air to attain steelmaking 
temperatures efficiently. In electric processes the major source of heat is 
electric current (arc or resistance or both). Because this heat can be 
produced in the presence or absence of oxygen, electric furnaces can operate 
in a neutral or nonoxidizing atmosphere or vacuum, and thus are either 
preferred or required for alloys with significant amounts of easily oxidized 
elements and for other special grades where improved control of gases is 
required.  
    To use available raw materials and heat sources effectively for particular 
grades of steel, steelmaking processes of the pneumatic, open-hearth, and 
electric types exist, and any of these may be either acid or basic in their 
chemistry. In many cases, there is an overlapping or combination of the above 
principles in a single process.  See  STEEL. 
 �34s[Gerhard Derge]
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�6v�2v�32sStellar evolution   �21m �33s 
    The large-scale, systematic, and irreversible changes with time of the 
structure and composition of a star. Stars are born, age, and die much like 
living beings, but the time scales involved are incomparably longer: for stars 
like the Sun, the lifetimes are on the order of 10sup1sup0 years, and even for 
the most massive stars, whose life expectancy is much shorter, the life span 
is still on the order of 10sup7 or 10sup6 years. As a consequence, stellar 
evolution cannot be observed directly, except for rare cases of objects 
``caught'' at stages of unusually fast structural changes. 
    Evidence on stellar evolution is therefore mainly indirect. Many stars are 
observed at various stages of evolution; the task is to identify these stages. 
It is useful to plot two of the most important stellar characteristics: 
luminosity (the total amount of energy emitted per second) and effective 
temperature (which represents the stars' surface conditions) against each 
other in a Hertzsprung-Russell diagram, as is done schematically in the 
illustration. Most stars will fall on the main sequence, which runs diagonally 
across the diagram from hot and luminous stars in the upper left corner to 
cool and faint stars in the lower right corner. In addition, there are two 
fairly distinct groups: the rather cool but very luminous giants (and even 
more luminous supergiants) and the fairly hot but quite faint white dwarfs. 
    Stars are believed to be formed from condensations in the interstellar 
medium. Once a reasonably massive initial condensation forms, it attracts 
surrounding particles by its force of gravitation. Although other forces may 
also assist, gravitation appears to be the force that builds stars. However, 
once they have formed, gravitation tends to destroy them.  See  GRAVITATION. 
    A star's life is a perpetual struggle against self-destruction, namely 
collapse under the effect of self-gravitation. At the outset, the object 
(called a protostar) is little more than a huge contracting cloud of cold 
material. Its contraction accelerates all the time, and outright would 
collapse into a black hole if forces were not generated to counteract the 
gravitational contraction. Such a force is the thermal pressure of the gas. As 
the particles fall closer together, their potential energy is converted into 
kinetic energy of motion, and redistributed in numerous collisions; the 
resulting random motions are perceived as rising temperature of the gas, and 
associated with it is the pressure which eventually begins to balance 
gravitation.  
    A temperature gradient is established in the star in the sense that 
temperature steadily decreases from the center to the surface. According to 
the second law of thermodynamics, energy must flow down the temperature 
gradient. A star must therefore shine simply because it is hotter inside, even 
if it does not possess adequate energy sources. Because of the stars' enormous 
masses, gravitational potential energy is a large source of radiative energy, 
and the Sun could shine at its present luminosity for about 10sup8 years 
without nuclear sources. 
  
    When the central temperature of a contracting star reaches about 5 x 
10sup6 to 1 x 10sup7 K (9 x 10sup6 to 1.8 x 10sup7|DGF), thermonuclear 
reactions are ignited in which hydrogen is converted into helium. The onset of 
thermonuclear reactions provides sufficient heat to keep the pressure 
sufficiently high to halt the star's contraction. The star enters a long and 
quiet period of its life, and is now a main-sequence star. Eventually, 
however, all the available hydrogen in its interior will be consumed. As the 
energy source at the star's center vanishes, gravitation prevails again, and 
contraction resumes. Contraction heats up the star everywhere, but most 
important is the rise in temperature in the layers immediately surrounding the 
helium-rich core in which hydrogen has been exhausted. In a relatively thin 
shell, which still contains hydrogen, the temperature will rise again high 
enough to ignite the thermonuclear conversion of hydrogen into helium. Instead 
of a nuclear-burning core, the star now has a nuclear-burning shell. This 
shell-burning stage is short, since the energetically inactive core cannot 
maintain a sufficiently high temperature to support the weight of the outer 
layers by gas pressure, and begins to contract again. However, this time the 
star is no longer a homogeneous body: the nuclear burning shell acts as a 
``watershed'' for internal motions, which are roughy speaking, reflected on 
it. Thus, while the core is contracting, the envelope outside the 
nuclear-burning shell begins to expand. The size of the star begins to 
increase, but at the same time the outer layers cool off as they expand, there 
is a drop in the surface temperature, and the color changes from white to 
yellow to red: the star evolves rapidly into a red giant.  See  GIANT STAR; 
THERMONUCLEAR REACTION. 
    If massive enough, the star reaches, in its contracting core, a 
temperature of or above 10sup8 K (1.8 x 10sup8|DGF). Then a new thermonuclear 
reaction starts in the core, namely, a transformation of three helium nuclei 
into one nucleus of carbon. This new energy source again temporarily 
stabilizes the star and halts core contraction and envelope expansion, and a 
second quiet stage ensues. As nuclear fuel, helium is much inferior to 
hydrogen, and is therefore consumed much faster. Another contraction of the 
core follows, accompanied eventually by an enormous expansion of the outer 
layers, and the star becomes a red supergiant, with a radius of 500 solar 
radii or so. 
    Sooner or later the star runs out of energy, and it may happen so abruptly 
that the inevitable gravitational contraction changes to a real collapse. In a 
matter of hours, the cool material of the envelope falls quite close to the 
center and heats up enormously, and thermonuclear reactions ignite 
instantaneously. The implosion suddenly changes into an explosion, which 
releases a large amount of luminous energy but also shatters the star to 
pieces. A supernova is observed.  See  GRAVITATIONAL COLLAPSE; SUPERNOVA. 
    The final evolutionary stage of a star of small mass is one of complete 
electron degeneracy. The star does not generate energy, and shines only by 
gradually releasing the heat energy accumulated at earlier stages in the 
random motions of the particles. At the early stages it is still hot enough to 
maintain a fairly high surface temperature, and it is observed as a white 
dwarf. The low luminosity, combined with high effective temperature, implies 
that white dwarfs are very small stars. Only when a solar mass is squeezed 
into the size of the Earth does the complete electron degeneracy set in. The 
radius of a degenerate dwarf is smaller for a larger mass of the dwarf, and 
theoretical calculations show that the radius tends to zero when the mass 
approaches about 1.4 solar masses (this value is known as the Chandrasekhar 
limit). Beyond that limit no stable configuration is possible for a degenerate 
dwarf, since the pressure of the degenerate electrons is not strong enough to 
counteract self-gravitation. By the sheer weight of the material, electrons 
are squeezed into atomic nuclei and combine with protons to form neutrons. 
This permits further drastic contraction of the star, which continues until 
the neutrons are squeezed to such a small volume per particle that the 
uncertainty principle begins to operate again, this time on the degenerate 
neutron gas, and generates a new strong pressure opposing further contraction. 
A star can stabilize again, and exist for a long time as a neutron star. At 
least some neutron stars are observed as pulsars.  See  NEUTRON STAR; PULSAR; 
WHITE DWARF STAR. 
    One way of producing neutron stars is the supernova explosion. The 
supernova observed in A.D. 1054 by Chinese astronomers in Taurus left behind a 
nebulosity (Crab Nebula) surrounding a pulsar. It is not clear if all 
supernovae leave a stellar remnant. If the mass of the remnant is larger than 
about 3 solar masses, not even the pressure of the degenerate neutrons can 
stop its collapse. Therefore it has been concluded that such an object would 
collapse without a limit and become a black hole.  See  BLACK HOLE; STAR. 
 �34s[Mirek Plavec]
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�6v�2v�32sStem   �21m �33s 
    The organ of vascular plants that usually develops branches and bears 
leaves and flowers. On woody stems a branch that is the current season's 
growth from a bud is called a twig. The stems of some species produce 
adventitious roots.  See  ROOT. 
  
    General characteristics.  While most stems are erect, aerial structures, 
some remain underground, others creep over or lie prostrate on the surface of 
the ground, and still others are so short and inconspicuous that the plants 
are said to be stemless, or acaulescent. When stems lie flattened immediately 
above but not on the ground, with tips curved upward, they are said to be 
decumbent, as in juniper. If stems lie flat on the ground but do not root at 
the nodes (joints), the stem is called procumbent or prostrate, as in 
purslane. If a stem creeps along the ground, rooting at the nodes, it is said 
to be repent or creeping, as in ground ivy. 
    Most stems are cylindrical and tapering, appearing circular in cross 
section; others may be quadrangular or triangular. 
    Herbaceous stems (annuals and herbaceous perennials) die to the ground 
after blooming or at the end of the growing season. They usually contain 
little woody tissue. Woody stems (perennials) have considerable woody 
supporting tissue and live from year to year. A woody plant with no main stem 
or trunk, but usually with several stems developed from a common base at or 
near the ground, is known as a shrub. 
 �34s[Nelle Ammons]
�33s 
  
    External features.  A shoot or branch usually consists of a stem, or axis, 
and leafy appendages. Stems have several distinguishing features. They arise 
either from the epicotyl of the embryo in a seed or from buds. The stem bears 
both leaves and buds at nodes, which are separated by leafless regions or 
internodes, and sometimes roots and flowers (see illustration). 
    The nodes are the regions of the primary stem where leaves and buds arise. 
The number of leaves at a node is usually specific for each plant species. In 
deciduous plants which are leafless during winter, the place of former 
attachment of a leaf is marked by the leaf scar. The scar is formed in part by 
the abscission zone formed at the base of the leaf petiole. The stem regions 
between nodes are called internodes. Internode length varies greatly among 
species, in different parts of the same stem, and under different growing 
conditions.  
    Lenticels are small, slightly raised or ridged regions of the stem surface 
that are composed of loosely arranged masses of cells in the bark. Their 
intercellular spaces are continuous with those in the interior of the stem, 
therefore permitting gas exchange similar to the stomata that are present 
before bark initiation. 
  
    There are three major types of stem branching: dichotomous, monopodial, 
and sympodial. Dichotomy occurs by a division of the apical meristem to form 
two new axes. If the terminal bud of an axis continues to grow and lateral 
buds grow out as branches, the branching is called monopodial. If the apical 
bud terminates growth in a flower or dies back and one or more axillary buds 
grow out, the branching is called sympodial. Often only one bud develops so 
that what appears to be single axis is in fact composed of a series of lateral 
branches arranged in linear sequence. 
    The large and conspicuous stems of trees and shrubs assume a wide variety 
of distinctive forms. Columnar stems are basically unbranched and form a 
terminal leaf cluster, as in palms, or lack obvious leaves, as in cacti. 
Branching stems have been classified either as excurrent, when there is a 
central trunk and a conical leaf crown, as in firs and other conifers, or as 
decurrent (or deliquescent), when the trunk quickly divides up into many 
separate axes so that the crown lacks a central trunk, as in elm.  See  TREE, 
 �34s[Jack B. Fisher]
�33s 
  
    Internal features.  The stem is composed of the three fundamental tissue 
systems that are found also in all other plant organs: the dermal (skin) 
system, consisting of epidermis in young stems and periderm in older stems of 
many species; the vascular (conducting) system, consisting of xylem (water 
conduction) and phloem (food conduction); and the fundamental or ground tissue 
system, consisting of parenchyma and sclerenchyma tissues in which the 
vascular tissues are embedded. The arrangement of the vascular tissues varies 
in stems of different groups of plants, but frequently these tissues form a 
hollow cylinder enclosing a region of ground tissue called pith and separated 
from the dermal tissue by another region of ground tissue called cortex.  See  
CORTEX (PLANT); EPIDERMIS (PLANT); PARENCHYMA; PHLOEM; PITH; SCLERENCHYMA; 
XYLEM.  
    Part of the growth of the stem results from the activity of the apical 
meristem located at the tip of the shoot. The derivatives of this meristem are 
the primary tissues: epidermis, primary vascular tissues, and the ground 
tissues of the cortex and pith. In many species, especially those having woody 
stems, secondary tissues are added to the primary. These tissues are derived 
from the lateral meristems, oriented parallel with the sides of the stem: cork 
cambium (phellogen), which gives rise to the secondary protective tissue 
periderm, which consists of phellum (cork), phellogen (cork cambium), and 
phelloderm (secondary cortex) and which replaces the epidermis; and vascular 
cambium, which is inserted between the primary xylem and phloem and forms 
secondary xylem (wood) and phloem.  See  APICAL MERISTEM; LATERAL MERISTEM. 
    The vascular tissues and the closely associated ground tissues - pericycle 
(on the outer boundary of vascular region), interfascicular regions (medullary 
or pith rays), and frequently also the pith - may be treated as a unit called 
the stele. The variations in the arrangement of the vascular tissues serve as 
a basis for distinguishing the stelar types. The word stele means column and 
thus characterizes the system of vascular and associated ground tissues as a 
column. This column is enclosed within the cortex, which is not part of the 
stele.  See  PERICYCLE. 
 �34s[James E. Gunckel]
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�6v�2v�32sStenolaemata   �21m �33s 
    A class of bryozoans. Entirely marine, stenolaemates have lophophores 
which are circular in basal outline, and zooecia which are tubular or 
prismatic, gradually tapering to their proximal ends, and are short and fat to 
long and slender.  See  BRYOZOA. 
    Stenolaemate colonies vary greatly in size and shape, some being small and 
delicate and others large and massive. Many colonies are regionated or 
divisible into distinct endozone and exozone portions. The individual zooids 
within a stenolaemate colony may be relatively isolated, loosely bundled, or 
compactly fused. 
    The Stenolaemata include several thousand species ranged among six orders, 
the Fenestrata, Cryptostomata, Rhabdomesonata, Trepostomata, Cystoporata, and 
Cyclostomata. First appearing late in the Early Ordovician, stenolaemates 
expanded quickly to dominate Paleozoic bryozoan assemblages from the 
mid-Ordovician through the Permian; much reduced since then, they were locally 
important during the Jurassic and Cretaceous, and some cyclostomes persist 
today as evolutionary remnants of this formerly dominant class.  See  
CRYPTOSTOMATA; CYCLOSTOMATA (BRYOZOA); CYSTOPORATA; FENESTRATA; 
RHABDOMESONATA; TREPOSTOMATA. 
 �34s[Roger J. Cuffey]
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�6v�2v�32sStensioellida   �21m �33s 
    An order of class Placodermi based on Stensioella heintzi from the Lower 
Devonian Hunsrueckschiefer of Germany. It is a slender, unarmored fish about 
12 in. (30 cm) long, quite different from other members of the class, but 
shown to be related by the presence of three small bones on its head, by the 
extension of the gill chamber far under the cranium, and by the articulation 
between the endocranium and the fused anterior vertebrae. The head is bluntly 
rounded; the shoulder girdle is short and not expanded into a trunk shield; 
the body is gradually tapering; and the pectoral fins are long, slender, and 
flaplike, and the pelvic fins are small. This may be the most primitve known 
placoderm, but its relationships cannot be determined with certainty from the 
single known specimen.  See  PLACODERMI. 
 �34s[Robert H. Denison]
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�6v�2v�32sStenurida   �21m �33s 
    An order of Ophiuroidea, comprising the most primitive brittlestars, known 
only from Paleozoic sediments. The general structure of the skeleton exhibits 
many features in common with the Somasteroidea, differing chiefly in having 
the virgalia reduced and the arm more clearly differentiated from the central 
disk. The order is believed to be ancestral to all other groups of 
Ophiuroidea.  See  ECHINODERMATA; OPHIUROIDEA; SOMASTEROIDEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sStepping motor   �21m �33s 
    A device which operates from electrical power to produce mechanical 
rotation and which, unlike a conventional motor, does not produce a speed 
dependent on the signals applied to its inputs but steps from one angular 
position to another as determined by which of its inputs are energized. This 
means that the stepping motor can serve as a link between any digital device 
and a world characterized by analog processes. The stepping motor has found 
extensive use as an interface between computers and mechanical devices. They 
are used extensively in equipment which couples to computers, such as 
printers, plotters, and magnetic memory disks. The main advantage of the 
stepping motor lies in the fact that it can provide very accurate mechanical 
positioning without any feedback. This means that, in general, a stepping 
motor and amplifier can be built more economically than other devices to 
perform the job of converting digital signals to mechanical rotation. For 
example, in addition to computer interfaces, stepping motors can be used in 
automobile control systems, home appliances, and even watches.  See  COMPUTER 
GRAPHICS; DIGITAL-TO-ANALOG CONVERTER. 
    There are three major types of stepping motors: the permanent-magnet 
stepping motor, the variable-reluctance motor, and a cam-operated motor. This 
last type relies on magnetic forces to move the output to mechanical detent 
positions. The first two types have the highest performance and are most 
commonly used.  See  MOTOR. 
 �34s[Charles K. Taft]
�33s 
  
  
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�6v�2v�32sStereochemistry   �21m �33s 
    The study of the three-dimensional arrangement of atoms or groups within 
molecules and the properties which follow from such arrangement. Molecules 
that have identical molecular structures but differ in the relative spatial 
arrangement of component parts are stereoisomers. Inorganic and organic 
compounds exhibit stereoisomerism. Examples are structures (I) - (VIII). 
    The nature of the stereochemistry of a molecule is determined by its 
symmetry. The symmetry elements to be considered are: planes of symmetry, axes 
of symmetry, centers of symmetry, and reflection or mirror symmetry. Two types 
of stereoisomers are known. Those such as (VII) and (VIII), which are devoid 
of reflection symmetry - which cannot be superimposed on their image in a 
mirror - are called enantiomers. All other stereoisomers, such as the pairs 
(I) - (II), (III) - (IV), and (V) - (VI), are called diastereomers. The 
configuration of a stereoisomer designates the relative position of the atoms 
associated with a specific structure. The structures of stereoisomers (I) and 
(II) differ only in configuration. The same is true for (III) and (IV), (V) 
and (VI), and (VII) and (VIII).  See  ENANTIOMORPH. 
 �34s[Samuel H. Wilen]
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�6v�2v�32sStereophonic radio transmission   �21m �33s 
    The standard method for stereo broadcast over frequency-modulation (FM) 
transmitters, utilizing means to transmit a signal which can be received 
without impairment, by the usual monaural FM receiver as well.  See  
STEREOPHONIC SOUND. 
    In order for a stereo FM receiver to separate the sounds between the right 
and left microphones, the output from the right microphone is inverted in 
phase and then added to that of the left microphone. The voltage thus obtained 
is the difference voltage between the two microphones and is used to 
amplitude-modulate a supersonic subcarrier (38 kHz) in double-sideband 
suppressed carrier mode. This audio signal is inserted together with the sum 
signal to modulate the transmission.  See  SINGLE SIDEBAND. 
    The final process in the receiver for retrieval of the tones from the left 
and right microphones is to again take the sum, this time of the left plus 
right and the difference left minus right, and the difference between these 
pairs.  See  FREQUENCY MODULATION; FREQUENCY MODULATION RADIO; RADIO 
BROADCASTING.  
 �34s[Walter Lyons]
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�6v�2v�32sStereophonic sound   �21m �33s 
    A system of sound recording or transmission in which two or more 
microphone channels are connected to corresponding loudspeakers in the 
listening room in order to create a sound field at the listener's ears which 
is perceived to be very much like that in the original space. Binaural is a 
special form of stereophonic sound in which two microphones replace the ears 
of a dummy head and are connected through an amplifier/recorder system to a 
pair of headphones worn by the listener. Quadraphonic is a special case of 
stereo in which four channels and loudspeakers are employed.  See  BINAURAL 
SOUND SYSTEM; QUADRAPHONIC SOUND SYSTEM. 
    There are three basic microphone techniques for stereophonic pickup: 
coincident, spaced apart, and individual instrument (also called close 
miking). The techniques are sometimes combined. 
    The coincident technique employs two microphones located very close 
together, often enclosed in the same case. Usually the two microphones will 
have cardioid pickup patterns, or one will be a cardioid and the other a 
figure-eight. The coincident technique relies only on differences in sound 
intensity to determine position of the phantom images.  See  MICROPHONE. 
    By placing the microphones several feet apart, it is possible to provide a 
good pickup of a large group without moving the microphones as far into the 
reverberant field. The spaced-apart technique provides both time and intensity 
cues.  
    Beginning in the late 1960s it became common for most popular music to be 
recorded in an acoustically dead studio using a large number of microphones, 
each located close to one instrument or small group of instruments. By 
proportionally mixing the microphone outputs between the two stereo channels, 
it is possible to provide the intensity cues necessary for perceiving phantom 
images. No time cues are produced.  See  SOUND RECORDING; SOUND-REPRODUCING 
SYSTEMS.  
 �34s[Rollins Brook]
�33s 
  
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�6v�2v�32sStereoscopy   �21m �33s 
    The phenomenon of simultaneous vision with two eyes, producing a visual 
experience of the third dimension, that is, a vivid perception of the relative 
distances of objects in space. In this experience the observer seems to see 
the space between the objects located at different distances from the eyes. 
    Stereopsis, or stereoscopic vision, is believed to have an innate origin 
in the anatomic and physiologic structures of the retinas of the eyes and the 
visual cortex. It is present in normal binocular vision because the two eyes 
view objects in space from two points, so that the retinal image patterns of 
the same object points in space are slightly different in the two eyes. The 
stereoscope, with which different pictures can be presented to each eye, 
demonstrates the fundamental difference between stereoscopic perception of 
depth and the conception of depth and distance from the monocular view.  See  
VISION.  
 �34s[Kenneth N. Ogle]
�33s 
  
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�6v�2v�32sStereospecific catalyst �21m �33s
    Stereospecific polymerization catalysts lead to the formation of
stereoregular (tactic) polymers, that is, polymers where the centers of steric
isomerism in the main chain are arranged in a regular fashion with respect to
their configurations. Three factors determine the tacticity of a polymeric
chain during its formation: (1) the kind of monomer approach to the growing
chain end, (2) the kind of attack of the growing chain end on the double bond
(cis or trans opening), and (3) the configuration in the initiation step. In
addition to a regular head-to-tail configuration and absence of branching,
reactions have to be assumed. The kind of monomer approach is strongly
affected by electrical and stereochemical forces, and therefore changes in
monomer structure and environment greatly influence polymer tacticity. The
intermediate radical or ion can be assumed to have a planar or near-planar
structure, and in an uncomplexed form it should be able to rotate freely
around its axis with cis and trans addition being equally possible. Upon
addition of the next monomer this carbon changes into a tetrahedral structure,
thereby creating two isomeric forms, isotactic or syndiotactic.  See
ASYMMETRIC SYNTHESIS; CATALYSIS; POLYMERIZATION; STEREOCHEMISTRY.
 �34s[Anton Schindler]
�33s

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�6v�2v�32sSteric effect (chemistry)   �21m �33s 
    The influence of the spatial configuration of reacting substances upon the 
rate, nature, and extent of reaction. The sizes and shapes of atoms and 
molecules, the electrical charge distribution, and the geometry of bond angles 
influence the courses of chemical reactions. The steric course of 
organochemical reactions is greatly dependent on the mode of bond cleavage and 
formation, the environment of the reaction site, and the nature of the 
reaction conditions (reagents, reaction time, and temperature). The effect of 
steric factors is best understood in ionic reactions in solution.  See  
STEREOCHEMISTRY.  
 �34s[Ernest Wenkert]
�33s 
  
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�6v�2v�32sSterility   �21m �33s 
    The incapability of reproduction; usually regarded as synonymous with 
infertility. The latter term refers to the inability to conceive during the 
course of normal sexual activity. The male deficiency most commonly 
responsible for sterility is the deficiency of sperm production in quantity or 
quality. There are numerous causes for impaired spermatogensis, which include 
hypoplasia of the testes, often associated with undescended testes, traumatic 
or infectious atrophy, environmental chemical toxicity, prolonged fever or 
other causes of increased testicular temperature, and endocrine disorders. It 
has become increasingly apparent that exposure to toxic chemicals and 
excessive consumption of alcoholic beverages markedly alters male fertility. 
Obstruction of the ducts of the seminal tracts also has been shown to produce 
sterility in the male. Defective delivery of the sperm into the vagina may 
result from surgery or congenital defects of the penis (hypospadias). 
    Female sterility may be related to defects in the production of ova or the 
interference with their union with the spermatozoa. As with the male, the 
female factors may be caused by defective anatomic development or by altered 
functional changes in the endocrine system and the reproductive organ response 
to the endocrine system. Chemical exposures can contribute to female 
infertility in a manner similar to that of males. Trace metals, pesticides, 
and so forth may also interfere with implantation of the ova in the uterine 
wall. Vaginal causes of infertility may be either organic or functional, and 
very often a combination of factors. Occlusion of the Fallopian tubes, often 
secondary to previous episodes of gonorrheal salpingitis, is a common cause of 
infertility. Endometriosis and chronic inflammatory disease of the pelvic 
region may also cause fertility problems.  See  REPRODUCTIVE SYSTEM DISORDERS. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sSterilization   �21m �33s 
    An act of destroying all forms of life on and in an object. A substance is 
sterile, from a microbiological point of view, when it is free of all living 
microorganisms. Sterilization is used principally to prevent spoilage of food 
and other substances and to prevent the transmission of diseases by destroying 
microbes that may cause them in humans and animals. Microorganisms can be 
killed either by physical agents, such as heat and irradiation, or by chemical 
substances.  
    Heat sterilization is the most common method of sterilizing 
bacteriological media, foods, hospital supplies, and many other substances. 
Either moist heat (hot water or steam) or dry heat can be employed, depending 
upon the nature of the substance to be sterilized. Moist heat is also used in 
pasteurization, which is not considered a true sterilization technique because 
all microorganisms are not killed; only certain pathogenic organisms and other 
undesirable bacteria are destroyed.  See  PASTEURIZATION. 
    Many kinds of radiations are lethal, not only to microorganisms but to 
other forms of life. These radiations include both high-energy particles as 
well as portions of the electromagnetic spectrum.  See  RADIATION BIOLOGY. 
    Filtration sterilization is the physical removal of microorganisms from 
liquids by filtering through materials having relatively small pores. 
Sterilization by filtration is employed with liquid that may be destroyed by 
heat, such as blood serum, enzyme solutions, antibiotics, and some 
bacteriological media and medium constituents. Examples of such filters are 
the Berkefeld filter (diatomaceous earth), Pasteur-Chamberland filter 
(porcelain), Seitz filter (asbestos pad), and the sintered glass filter. 
    Chemicals are used to sterilize solutions, air, or the surfaces of solids. 
Such chemicals are called bactericidal substances. In lower concentrations 
they become bacteriostatic rather than bactericidal; that is, they prevent the 
growth of bacteria but may not kill them. Other terms having similar meanings 
are employed. A disinfectant is a chemical that kills the vegetative cells of 
pathogenic microorganisms but not necessarily the endospores of spore-forming 
pathogens. An antiseptic is a chemical applied to living tissue that prevents 
or retards the growth of microorganisms, especially pathogenic bacteria, but 
which does not necessarily kill them. 
    The desirable features sought in a chemical sterilizer are toxicity to 
microorganisms but nontoxicity to humans and animals, stability, solubility, 
inability to react with extraneous organic materials, penetrative capacity, 
detergent capacity, noncorrosiveness, and minimal undesirable staining 
effects. Rarely does one chemical combine all these desirable features. Among 
chemicals that have been found useful as sterilizing agents are the phenols, 
alcohols, chlorine compounds, iodine, heavy metals and metal complexes, dyes, 
and synthetic detergents, including the quaternary ammonium compounds. 
 �34s[Charles F. Niven, Jr.]
�33s 
  
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�6v�2v�32sSteroid   �21m �33s 
    A compound, principally of plant or animal origin, composed of a series of 
four carbon rings joined together to form a structural unit called 
cyclopentenoperhydro-phenanthrene (see illustration). There are six centers of 
asymmetry so that 64 compounds (stereoisomers) with this structure are 
possible. Natural steroids are made by enzymes in living cells and are limited 
in number. The most abundant members of the class are the sterols; male and 
female sex hormones are steroids.  See  HORMONE; STEROL. 
 �34s[Thomas F. Gallagher]
�33s 
  
  
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�6v�2v�32sSterol   �21m �33s 
    Any one of the natural products derived from the steroid nucleus. Sterols 
are components of virtually all plant and animal cells with the notable 
exception of bacteria, where they are either totally absent or present in 
small amounts. They are waxy, colorless solids soluble in most organic 
solvents and virtually insoluble in water. They contain one alcohol functional 
group and may be either saturated (plant sterols; see illustration) or 
unsaturated (all animal sterols and many plant sterols). In nature they are 
often esterified with higher fatty acids. In living cells they often serve as 
part of cell membranes. In vertebrates cholesterol is the principal although 
not the sole sterol.  See  CHOLESTEROL; STEROID. 
 �34s[Thomas F. Gallagher]
�33s 
  
  
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�6v�2v�32sStibnite   �21m �33s 
    A mineral with composition Sb2S3 (antimony trisulfide), the chief ore of 
antimony. It crystallizes in slender, prismatic, vertically striated crystals 
which may be curved or bent. It is often in bladed, granular, or massive 
aggregates. The hardness is 2 on Mohs scale and the specific gravity 4.5 - 
4.6. The luster is metallic and the color lead-gray to black. It is one of few 
minerals that fuses easily in the match flame (525DGC or 977DGF). 
    Stibnite has been found in various mining districts in Germany, Rumania, 
France, Bolivia, Peru, and Mexico. In the United States the Yellow Pine mine 
at Stibnite, Idaho, is the largest producer. Other deposits are in Nevada and 
California. The finest crystals have come from the island of Shikoku, Japan.  
See  ANTIMONY. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sStickleback   �21m �33s 
    Any fish which is a member of the family Gasterosteidae in the order 
Gasterosteiformes. They have a variable number of free spines in front of the 
dorsal fin. These small, fresh-water and marine fishes are found in cold and 
temperate waters of the Northern Hemisphere. All species are of some economic 
importance since they feed largely on mosquito larvae.  See  GASTEROSTEIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sStilbellaceae   �21m �33s 
    A family of fungi of the order Moniliales. The conidiophores are 
aggregated in long bundles or fascicles generally having the conidia in a head 
at the top. The hyphae and conidia are hyaline or dark. There are about 80 
genera and 350 species known. All species are doubtless stilboid stages of 
Penicillium. See MONILIALES. 
 �34s[N. F. Buchwald]
�33s 
  
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�6v�2v�32sStilbite   �21m �33s 
    A mineral belonging to the zeolite family of silicates. It crystallizes in 
sheaflike aggregates of thin tabular crystals. There is perfect cleavage 
parallel to the side pinacoid, and here the mineral has a pearly luster; 
elsewhere the luster is vitreous. The color is usually white but may be brown, 
red, or yellow. Hardness is 3**2 - 4 on Mohs scale; specific gravity is 2.1 - 
2.2.  See  ZEOLITE. 
    Stilbite is found in Iceland, India, Scotland, Nova Scotia, and in the 
United States at Bergen Hill, New Jersey, and the Lake Superior copper 
district in Michigan. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sStirling engine   �21m �33s 
    An engine in which work is performed by the expansion of a gas at high 
temperature to which heat is supplied through a wall. Like the internal 
combustion engine, a Stirling engine provides work by means of a cycle in 
which a piston compresses gas at a low temperature and allows it to expand at 
a high temperature. In the former case the heat is provided by the internal 
combusion of fuel in the cylinder, but in the Stirling engine the heat 
(obtained from externally burning fuel) is supplied to the gas through the 
wall of the cylinder (see illustration). 
    The rapid changes desired in the gas temperature are achieved by means of 
a second piston in the cylinder, called a displacer, which in moving up and 
down transfers the gas back and forth between two spaces, one at a fixed high 
temperature and the other at a fixed low temperature. When the displacer is 
raised, the gas will flow from the hot space via the heater and cooler tubes 
into the cold space. When it is moved downward, the gas will return to the hot 
space along the same path. During the first transfer stroke the gas has to 
yield up a large amount of heat to the cooler; an equal quantity of heat has 
to be taken up from the heater during the second stroke.  See  INTERNAL 
COMBUSTION ENGINE. 
 �34s[Roelof J. Meijer]
�33s 
  
  
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�6v�2v�32sStirling numbers �21m �33s
    Numbers which arise in enumerating the ways of distributing the elements
of a set S into r indistinguishable cells so that no cell remains empty.  There
results a set-partition or partition of S into r blocks, and these are counted
by the Stirling number of the second kind S(n,r).  The Stirling numbers satisfy
the boundary conditions S(n,1) = S(n,n) = 1, and the recursion S(n,r) = S(n -
1,r - 1) + rS(n - 1,r) (which may be established by distinguishing within the
family of partitions between the distributions for which the first n - 1
elements of S leave a cell unoccupied which must be filled by the last element
and those for which the first n - 1 elements occupy every cell).  In addition,
they satisfy formula (1), where (x)(sub r) stands for the falling factorial x(x
- 1) (x - 2) .	.  .  (x - r + 1).  The Bell number, given by Eq.  (2), is the
total number of partitions of an n-element set.  These numbers also give the
number of rhyme schemes for a poem of n lines.	Thus, there are B(4) = 15 ways
to rhyme a quatrain:  abcd, aabc, abac, .  .  .  , aabb, .  .  .  , abbb, aaaa.
The Stirling numbers of the first kind, s(n,r), are defined as the coefficients
of the generating function of (x)(sub n).  See COMBINATORIAL THEORY; ENUMERATION
OF ARRANGEMENTS.
 �34s[T.  Balderes]
�33s

	  x(sup n) = SUMMATION (v=1 to n) S(n,r) (x)r			     (1)


	  B(n) = SUMMATION (r=1 to n) S(n)				     (2)


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�6v�2v�32sStochastic control theory �21m �33s
    A branch of control theory which aims at predicting and minimizing the
magnitudes and limits of the random deviations of a control system through
optimizing the design of the controller. Such deviations occur when random
noise and disturbance processes are present in a control system, so that the
system does not follow its prescribed course but deviates from the latter by a
randomly varying amount.
    In contrast to deterministic signals, random signals cannot be described
as given functions of time such as a step, a ramp, or a sine wave. The exact
function is unknown to the system designer; only some of its average
properties are known.
    A random signal may be generated by one of nature's processes, for
instance, radar noise and wind- or wave-induced forces and moments on a radar
antenna or a ship. Alternatively, it may be generated by human intelligence,
for instance, the bearing of a zigzagging aircraft, or the contour to be
followed by a duplicating machine.
    One outstanding experimental fact about nature's random processes is that
these signals are very closely Gaussian. The word ``Gaussian'' is a
mathematical concept which describes one or more signals i1, i2, . . . , isubn
having the following properties:
    1. The amplitude of each signal is normally distributed.
    2. The joint distribution function of any number of signals at the same or
different times taken from the set is a multivariate normal distribution. This
experimental fact is not surprising in view of the fact that a random process
of nature is usually the sum total of the effects of a large number of
independent contributing factors. For instance, thermal noise is due to the
thermal motions of billions of electrons and atoms. An ocean-wave height at
any particular time and place is the sum of wind-generated waves at previous
times over a large area.  See  DISTRIBUTION (PROBABILITY); ELECTRICAL NOISE;
STOCHASTIC PROCESS.
    The underlying mechanism that generates a random process can usually be
described in physical or mathematical terms. For instance, the underlying
mechanism that generates shot-effect noise is thermionic emission. If the
generating mechanism does not change with time, any measured average property
of the random process is independent of the time of measurement aside from
statistical fluctuations, and the random process is called stationary. If the
generating mechanism does change, the random process is called nonstationary.
See  CONTROL SYSTEMS.
 �34s[S. S. L. Chang]
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�6v�2v�32sStochastic process �21m �33s
    A physical stochastic process is any process governed by probabilistic
laws. Examples are (1) development of a population as controlled by Mendelian
genetics; (2) Brownian motion of microscopic particles subjected to molecular
impacts or, on a different scale, the motion of stars in space; (3) succession
of plays in a gambling house; and (4) passage of cars by a specified highway
point.
    In each case, a probabilistic system is evolving; that is, its state is
changing with time. Thus the state at time t depends on chance: It is a random
variable x(t). The parameter set of values of t involved is usually (and will
always be in this article) either an interval (continuous parameter stochastic
process) or a set of integers (discrete parameter stochastic process). Some
authors, however, apply the term stochastic process only to the continuous
parameter case.
    If the state of the system is described by a single number, x(t) is
numerical-valued. In other cases, x(t) may be vector-valued or even more
complicated. For the numerical case, as the state changes, its values
determine a function of time, the sample function, and the probability laws
governing the process determine the probabilities assigned to the various
possible properties of sample functions.
    A mathematical stochastic process is a mathematical structure inspired by
the concept of a physical stochastic process, and studied because it is a
mathematical model of a physical stochastic process or because of its
intrinsic mathematical interest and its applications both in and outside the
field of probability. The mathematical stochastic process is defined simply as
a family of random variables. That is, a parameter set is specified, and to
each parameter point t a random variable x(t) is specified. If one recalls
that a random variable is itself a function, if one denotes a point of the
domain of the random variable x(t) by OMEGA, and if one denotes the value of
this random variable at OMEGA by x(t,OMEGA), it results that the stochastic
process is completely specified by the function of the pair (t,OMEGA) just
defined, together with the assignment of probabilities. If t is fixed, this
function of two variables defines a function of OMEGA, namely, the random
variable denoted by x(t). If OMEGA is fixed, this function of two variables
defines a function of t, a sample function of the process.
    Probabilities are ordinarily assigned to a stochastic process by assigning
joint probability distributions to its random variables. These joint
distributions, together with the probabilities derived from them, can be
interpreted as probabilities of properties of sample functions. For example,
if t0 is a parameter value, the probability that a sample function is positive
at time t0 is the probability that the random variable x(t0) has a positive
value. The fundamental theorem at this level is that, to any self-consistent
assignment of joint probability distributions, there corresponds a stochastic
process.
    Stationary processes are the stochastic processes for which the joint
distribution of any finite number of the random variables is unaffected by
translations of the parameter; that is, the distribution of x(t1 + h), . . . ,
x(t[sub n] + h) does not depend on h.  See  PROBABILITY.
    A Markov process is a process for which, if the present is given, the
future and past are independent of each other.	More precisely, if t1 < .  .  .
< tsubn are parameter values, and if 1 < j < n, then the sets of random
variables [x(t1), .  .	.  ,x(t sub j-1)] and [x(t sub j+1), .	.  .  ,x(t sub
n)] are mutually independent for given x(t sub j).  Equivalently, the
conditional probability distribution of x(t sub n) for given x(t1), .  .  .
,x(t sub n-1) depends only on the specified value of x(t sub n-1) and is in fact
the conditional probability distribution of x(t sub n), given x(t sub n-1).  An
important and simple example is the Markov chain, in which the number of states
is finite or denumerably infinite.
    A martingale is a stochastic process with the property that, if t1 < . . .
< (t sub n) are parameter values, the expected value of x(t sub n) for given
x(t1), .  .  .	, x(t sub n-1) is equal to x(t sub n-1).  That is, the expected
future value, given present and past values, is equal to the present value.  The
interpretation that a martingale can be thought of as the fortune of a player
after the successive plays of a fair gambling game is obvious.
    A process with independent increments is a continuous-parameter process
with the property that, if t1 < . . . <(t sub n) are parameter values, the
successive increments in x(t2) - x(t1), . . . ,x(t sub n) - x(t sub n-1) are
mutually independent. If y(t) = x(t) - x(t0), where t0 is fixed, the y(t)
process is then a Markov process.  See	GAME THEORY; INFORMATION THEORY;
LINEAR PROGRAMMING; OPERATIONS RESEARCH.
 �34s[Joseph L. Doob]
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�6v�2v�32sStoichiometry �21m �33s
    The interpretation of mass and energy relationships indicated by chemical
reaction equations. It is also defined as those calculations used to find the
quantities of reactants and products in a chemical reaction.
    Such calculations fall into four groups. The first group includes the law
of conservation of matter, the law of chemical combining weights, and the law
of combining proportions. The second group is based on the law of conservation
of energy and includes heat of reaction, heat added, heat lost, and other
energy effects associated with the reaction. The third group comprises the
equilibrium relationships of the system and includes not only chemical
equilibria but also physical equilibria such as gas-liquid systems. The fourth
group includes the rate of reaction relationships of both chemical changes and
physical changes.
    In the chemical laboratory only the first group is usually considered. The
second and fourth groups are neglected, or the reactions are forced to
completion artificially. The third group is utilized in a limited number of
cases. The units used are grams, moles, milliliters, or liters. The types of
calculations include weight-weight, weight-volume (either gases or liquids),
and thermal relationships. For any calculation a knowledge of the reaction
under consideration and a balanced reaction equation are necessary.
    All four groups of principles must be used in industrial stoichiometry.
Flowing systems require the use of reaction rates and detailed considerations
of heat losses, purity of materials, effects of catalysts, and side reactions.
 �34s[Kenneth G. Stone; Charles L. Rulfs]
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�6v�2v�32sStoker   �21m �33s 
    A mechanical means for feeding coal into, and for burning coal in, a 
furnace. There are three basic types of stokers. Chain or traveling-grate 
stokers have a moving grate on which the coal burns; they carry the coal from 
a hopper into the furnace and move the ash out (see illustration). Spreader 
stokers mechanically or pneumatically distribute the coal from a hopper at the 
furnace front wall and move it onto the grate which usually moves continuously 
to dispose of the ash after the coal is burned. Underfeed stokers are arranged 
to force fresh coal from the hopper to the bottom of the burning coal bed, 
usually by means of a screw conveyor. The ash is forced off the edges of the 
retort peripherally to the ashpit or is removed by hand. 
 �34s[George W. Kessler]
�33s 
  
  
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�6v�2v�32sStokes stream function   �21m �33s 
    A degenerate (one-component) vector potential used in analyzing and 
describing axially symmetric fluid-flow fields. In a steady axially symmetric 
flow, the rotation of a streamline about the axis of symmetry generates a 
stream surface. A certain mass rate of flow exists inside this stream surface 
which is the same at every axial station because, by definition, there is no 
flow through the stream surface. The value of the Stokes stream function PSI 
at a point in the flow is equal to **2PI times mass rate of flow inside the 
stream surface passing through that point. If the fluid motion is slow enough 
to neglect compressibility, PSI also measures the volume rate of flow inside a 
stream surface.  See  FLUID FLOW. 
 �34s[Arthur E. Bryson, Jr.]
�33s 
  
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�6v�2v�32sStokes' theorem �21m �33s
    The assertion that under certain light restrictions the surface integral
of (INVERTED DELTA MULT F) . V over a surface patch S is equal to the line
integral of f . TAU taken around C, the boundary curve of S, provided the sense
of transcription of C is right-handed relative to o. This can be expressed as
the equation

DOUBLE INTERGRAL (INVERTED DELTA MULT F . V ds = INTERGRAL F . TAU ds

Here F is a vector function, V is one of the two unit normals to the two-sided
surface S, s is arc length measured positively in the sense which is
right-handed relative to V, and TAU is the unit tangent vector to C in the sense
of increasing s.  See  CALCULUS OF VECTORS.
 �34s[Homer V. Craig]
�33s

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�6v�2v�32sStolonifera   �21m �33s 
    An order of the Alcyonaria which lacks coenenchyme. They form either 
simple or rather complex colonies. The polyp has a cylindrical body with a 
retractile oral portion which can withdraw into a solid anthostele or calyx 
protected by many calcareous spicules. The base of the mature polyp is 
attached to a creeping stolon which is a ribbonlike network or thin flat mat 
from which daughter polyps arise. Daughter polyps never bud from the wall of 
the primary polyp. Each polyp is connected by solenial tubes of the stolons, 
or by transverse platforms.  See  ALCYONARIA. 
 �34s[Kenji Atoda]
�33s 
  
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�6v�2v�32sStomach   �21m �33s 
    The stomach is a pouchlike dilatation of the gut immediately beyond the 
esophagus, in which food is held for a time and subjected to vigorous 
digestive action. 
    The stomach of fishes is generally simple and saccular in contour. Some 
are more or less straight; others are curved or J-shaped. Entrance into the 
stomach is controlled by a cardiac sphincter, which also serves to prevent 
undue entrance of water. 
    Although reptiles display great variety in body form, they generally show 
no remarkable variations in stomach configuration other than those required to 
fit it within the available space. In crocodiles the anterior part of the 
stomach is enlarged and very muscular and has a structure resembling the 
gizzard of birds. 
    The stomach of birds is divided into two chambers, the relatively 
thin-walled, glandular proventriculus and the very muscular gizzard. Within 
the proventriculus the food is mixed with mucus and digestive juices produced 
by the glandular mucous membrane. The softened and partially digested mass is 
then passed on into the gizzard, where it is vigorously ground and comminuted 
as digestion continues. Grinding action of the gizzard, highly developed in 
seed-eating birds, is augmented by the presence of gizzard stones which these 
birds ingest. 
    The most elaborate mammalian stomachs are found among the cud-chewing 
artiodactyl (even-toed) ungulates, such as the cow, sheep, deer, goat, 
giraffe, and camel. Stomachs of the ruminants are not only complex but large; 
that of an ox has a capacity of up to 60 gal. Typically the stomach is divided 
into four chambers: rumen (paunch), reticulum (honeycomb), omasum (psalterium 
or manyplies), and abomasum. The first three are generally considered to be 
expansions of the lower end of the esophagus. Only the fourth has a glandular 
wall and corresponds to a true stomach. This arrangement makes it possible for 
these animals to ingest large amounts of food in a relatively short period of 
time, and then rework and digest it. 
    Carnivores have a stomach with general features typical of the majority of 
mammals, including humans (see illustration). The region of the stomach 
adjacent to the esophageal opening is known as the cardia, and a portion of 
the smooth muscle at this junction forms the relatively weak cardiac 
sphincter. The expansible muscular fundus and body curve downward and to the 
right, gradually narrowing to form the pyloric antrum and the somewhat more 
muscular pyloric canal. The distal part of the encircling muscle constitutes 
the pyloric sphincter, which controls the passage of material from stomach to 
duodenum. See DIGESTINE SYSTEM; ITESTINE. 
 �34s[Victor M. Emmel]
�33s 
  
  
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�6v�2v�32sStomach disorders   �21m �33s 
    Disorders of the stomach include congenital defects, inflammations, 
tumors, and reaction to toxic materials and foreign bodies. 
    One of the most common congenital lesions is hypertrophic pyloric 
stenosis, which is heralded by regurgitation or vomiting of food. The male 
infant is affected most frequently; however, the mode of genetic inheritance 
has not been fully established. 
    Acute gastritis, an inflammatory process, is an extremely frequent lesion, 
the result of irritation to the mucosa of the stomach by a variety of 
substances, including caffeine, alcohol, salicylates, other drugs, and acid 
and alkaline solutions. The mucosa responds with a wide variety of changes, 
ranging from edema to erosion and ulceration. Chronic gastritis, another 
inflammatory process, leads to a variety of changes in the stomach. Different 
areas of the stomach can be affected, depending on the type of injury. 
capacity of up to 60 gal (228 liters). Typically the stomach is divided into 
four chambers: rumen (paunch), reticulum (honeycomb), omasum (psalterium or 
manyplies), and abomasum. The first three are generally considered to be 
expansions of the lower end of the esophagus. Only the fourth has a glandular 
wall and corresponds to a true stomach. This arrangement makes it possible for 
these animals to ingest large amounts of food in a relatively shor period of 
time, and then rework and digest it. 
the encircling muscle constitutes the pyloric sphincter, which controls the 
passage of material from stomach to duodenum. See DIGESTIVE SYSTEM; INTESTINE. 
 �34s[Victor M. Emmel]
�33s 
    Stress can cause changes in the mucosa of the stomach. Peptic ulcers are 
found in 10% of the population of North America. The stomach is the second 
most frequent site of involvement after the duodenum.  See  PEPTIC ULCER. 
    Foreign bodies in the stomach are quite varied in their number and type. 
In most cases, small objects pass through into the bowel. Larger objects 
however are retained and may, if sharp, lacerate the stomach wall and cause 
perforation. Bezoars are accumulations of foreign material, often hair, which 
may reach immense size. 
    Tumors of the stomach are both benign and malignant. Of the two most 
frequent benign tumors, one is derived from the epithelium of the mucosa, an 
adenoma, and the other is derived from the smooth muscle of the underlying 
layers, a leiomyoma. Hemorrhage is frequent in persons with leiomyoma. The 
most frequent malignant tumor in the stomach arises from the epithelium of the 
mucosa, an adenocarcinoma. Extension and metastasis are common in this disease 
even when treated early in its course.  See  STOMACH. 
 �34s[H. Thomas Norris]
�33s 
  
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�6v�2v�32sStomatopoda   �21m �33s 
    The only order of the superorder Hoplocarida belonging to the subclass 
Malacostraca of the class Crustacea. This order of the mantis shrimps contains 
four families: Lysioquillidae, Bathysquillidae, Squillidae, and 
Gonodactylidae. Stomatopoda are among the larger Crustacea; the size of an 
adult ranges from about 0.5 in. (12 mm) to over 12 in. (300 mm). 
    The bodies of stomatopods are narrow and elongate, almost eruciform. Only 
part of the cephalic and thoracic somites are fused and covered by the dorsal 
shield, the carapace. Those cephalic somites which bear the antennulae and the 
eyes are free and visible anterior to the carapace, and the last four thoracic 
somites are similarly exposed. The tail fan consists of a well-developed, 
sometimes peculiarly sculptured or deformed, median plate, the telson, and the 
two uropods. Anteriorly, the carapace bears a flattened movable plate, the 
rostrum. The eyes are large, stalked, and movable. Of eight pairs of thoracic 
appendages the first is narrow, slender, and hairy, probably being used for 
cleaning purposes. The second thoracic leg is very strong and heavy. It has 
become a large raptorial claw that shows a great resemblance to that of the 
praying mantis, and for this reason the Stomatopoda are often given the name 
mantis shrimps. 
    The Stomatopoda are marine animals rarely found in brackish water. Most 
species are confined to tropical and subtropical areas, though some occur in 
the boreal and antiboreal regions. The majority of stomatopods live in the 
littoral and sublittoral zones, but a few species have been found in greater 
depths, down to 2500 ft (760 m).  See  MALACOSTRACA. 
 �34s[Lipke B. Holthuis]
�33s 
  
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�6v�2v�32sStone and stone products   �21m �33s 
    The term stone is applied to rock that is cut, shaped, broken, crushed, 
and otherwise physically modified for commercial use. The two main divisions 
are dimension stone and crushed stone. Other descriptive terms may be used, 
for example, building stone, roofing stone, or precious stone.  See  GEM; ROCK. 
    The term dimension stone is applied to blocks that are cut and milled to 
specified sizes, shapes, and surface finishes. The principal uses are for 
building and ornamental applications. Granites, limestones, sandstones, and 
marbles are widely used; basalts, diabases, and other dark igneous rocks are 
used less extensively. Soapstone is used to some extent. Rock suitable for use 
as dimension stone must be obtainable in large, sound blocks, free from 
incipient cracks, seams, and blemishes, and must be without mineral grains 
that might cause stains as a result of weathering. It must have an attractive 
color, and generally a uniform texture. 
    Slate differs from other dimension stone because it can be split into thin 
sheets of any desired thickness. Commercial slate must be uniform in quality 
and texture and reasonably free from knots, streaks, or other imperfections, 
and have good splitting properties. Roofing slates are important products of 
most slate quarries. However, the roofing-slate industry has declined 
considerably because of competition from other types of roofing. Slate is also 
used for milled products such as blackboards, electrical panels, window and 
door sills and caps, baseboards, stair treads, and floor tile.  See  SLATE. 
    Nearly all the principal types of stone - granite, diabase, basalt, 
limestone, dolomite, sandstone, and marble - may be used as sources of 
commercial crushed stone; limestone is by far the most important. Crushed 
stone is made from sound, hard stone, free from surface alteration by 
weathering. Stone that breaks in chunky, more or less cubical fragments is 
preferred. Commercial stone should be free from certain deleterious 
impurities, such as opalescent quartz, and free from clay or silt. Crushed 
stone is used principally as concrete aggregate, as road stone, or as railway 
ballast. Other uses for limestone are as a fluxing material to remove 
impurities from ores smelted in metallurgical furnaces, in the manufacture of 
alkali chemicals, calcium carbide, glass, paper, paint, and sugar, and for 
filter beds and for making mineral wool. 
 �34s[Robert L. Bates]
�33s 
  
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�6v�2v�32sStorage battery �21m �33s
    An assembly of identical voltaic cells in which the electrochemical action
is reversible so that the battery may be recharged by passing a current
through the cells in the opposite direction to that of discharge. While many
nonstorage batteries have a reversible process, only those that are
economically rechargeable are classified as storage batteries. Storage
batteries, sometimes known as electric accumulators or secondary batteries,
have two general classifications: lead-acid and alkaline.  See	BATTERY;
PRIMARY BATTERY.
    Some of the important uses of storage batteries are to start gasoline and
diesel engines; to operate communications circuits; switch tripping and
closing in power-generating and handling systems; emergency lighting;
emergency power both with and without conversion to alternating current;
railway car lighting and air conditioning; rapid transit car controls; marine
power systems; power for underwater exploratory vehicles and submarines; to
activate photographic and portable sound systems as well as portable TV and
radio; and various military applications.

    Lead-acid battery.	The lead-acid type of storage battery is so classified
because the electrolyte is an acid and the plates are largely lead. The
positive active material is lead peroxide and the negative active material is
lead sponge. The active materials are supported by grids made of lead alloys.
    A great many types of lead-acid cells are produced, but all have certain
features in common. One is the open-circuit cell electromotive force (emf),
which exists between a positive lead peroxide (PbO2) electrode and a negative
sponge lead (Pb) electrode when the two are immersed in sulfuric acid
electrolyte (H2SO4 + H2O). The emf value is independent of the quantities of
lead peroxide, lead, or electrolyte present but does vary with temperature and
sulfuric acid concentration. The relatively small variation with temperature
is given in millivolts/DGC over a range 0 - 40DGC, as 0.30 for 1.200 sp gr
electrolyte, 0.22 for 1.250 sp gr, 0.19 for 1.280 sp gr, and 0.18 for 1.300 sp
gr.
    Reaction (1) represents the cell reactions insofar as beginning and end

PbO2 + Pb + 2H2SO4  REVERSIBLE REACTION (CHARGE/DISCHARGE)  2PbSO4 + 2H2O    (1)

materials are concerned.  On discharge the overall effect is a reduction of
PbOsub2 at the positive electrode and an oxidation of Pb at the negative
electrode, accompanied by sulfation in both cases.  In charging, a counter
voltage is imposed on the cell terminals, and current is forced through the cell
in a direction opposite to that in which the cell discharges.  This reverses the
ionic movements in relation to the electrodes and, if effect, reverses the cell
reactions. On discharge the eletrolyte specific gravity decreases, and on
charge it increases.  Specific gravity serves as a measure of the sulfuric
acid concentration and thus as an index of state of charge.

    Alkaline-type battery.  The alkaline-type storage battery is so classified
because the electric energy is obtained from chemical action of an alkaline
solution. One type of battery has positive plates of some nickel compound and
negative plates of iron. Another type uses a nickel compound and cadmium. A
third uses silver oxide and zinc. The nickel-iron cell will serve as an
example, as described below.
    The nickel-iron alkaline battery is composed of cells having a hydrated
nickel oxide and iron in an alkaline solution. The positive active material in
this cell is a higher oxide or hydroxide of nickel. The negative material is
fine iron powder. The electrolyte is potassium hydroxide, to which a little
lithium hydroxide is sometimes added.
    The chemical behavior of the nickel-iron cell is shown in reaction (2).

2NiOOH.H2O+Fe  REVERSIBLE REACTION (CHARGE/DISCHARGE)	2Ni(OH)2+Fe(OH)     (2)

The KOH electrolyte supplies ions for conductivity, but unlike the lead-acid
battery, the concentration of electrolyte does not undergo any net change in
the chemical action of the cell. As a consequence, the specific gravity of
the electrolyte does not change and cannot be used to indicate the state of
charge of an alkaline battery, as in the case of the lead-acid battery.
However, it also means that the gravity stays up all times, and the battery is
much less susceptible to accidental damage from freezing than lead-acid.
 �34s[W. W. Smith]
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�6v�2v�32sStorage rings   �21m �33s 
    Annular vacuum chambers in which beams of high-energy charged particles 
can be stored and caused to collide nearly head-on. The motivation for using 
this technique is found in the following kinematic considerations. Unlike 
fixed-target systems (cyclotrons, synchrotrons, linear accelerators) in which 
a beam of accelerated particles traverses a fixed target, colliding-beam 
systems use the full energy of each particle to produce reactions. 
    A positron-electron (esuppe-) storage ring consists of a ring of bending 
and focusing magnets enclosing a doughnut-shaped vacuum chamber in which 
counterrotating beams of esupp and e- are stored for periods of several hours. 
The two beams are made to collide with each other about 10sup6 times per 
second in straight interaction sections of the vacuum chamber. These are 
surrounded with detectors for the observation of collision products. An 
alternate design consists of two separate but intersecting rings, one to store 
e-, the other to store e- or esupp, with collisions taking place at the 
intersection points. 
    Collisions between antiparticle and particle such as esuppe- are of 
particular interest since most of the resulting processes proceed through 
annihilation which forms a particularly simple final state with the quantum 
numbers of the photon. This is one of the main reasons why esuppe- collisions 
have proved to be particularly fruitful in deepening understanding of the 
fundamental structure of matter. 
    Because they have a small mass, esupp and e- in circular orbits radiate a 
substantial amount of energy in the form of synchrotron radiation, just as 
they do in electron synchrotrons. As in electron synchrotrons, the radiation 
itself constitutes a very intense source of x-rays which has proved very 
useful in various branches of biology, chemistry, and physics.  See  
SYNCHROTRON; SYNCHROTRON RADIATION. 
 �34s[Karl Strauch]
�33s 
    Proton storage rings consist of a pair of annular vacuum chambers in which 
high-energy protons can be stored and caused to collide nearly head-on. The 
motivation for using this technique is the same as for electron-positron 
storage rings. Namely, the colliding-beam storage ring permits studying 
reactions at energies that cannot be achieved with economically feasible 
conventional accelerators.  See  PARTICLE ACCELERATOR. 
 �34s[Mark Barton]
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�6v�2v�32sStorage tank   �21m �33s 
    A container for storing liquids or gases. A tank may be constructed of 
ferrous or nonferrous metals or alloys, reinforced concrete, wood, or 
filament-wound plastics, depending upon its use. Tanks resting on the ground 
have flat bottoms; those supported on towers have either flat or curved 
bottoms. Standpipes, which are usually cylindrical shells of steel or 
reinforced concrete resting on the ground, are frequently of great height and 
comparatively small diameter. They are built to contain water for a 
distribution system, and height is required to maintain pressure in the 
system. Tanks for other liquids and for gases, where storage is more important 
than pressure, are generally lower and of greater diameter. 
 �34s[Charles M. Antoni]
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�6v�2v�32sStorage tube   �21m �33s 
    An electron tube into which information can be introduced and then 
extracted at a later time; also called a memory tube.  See  ELECTRON TUBE. 
    The process of introducing information into a storage tube is known as 
writing, and that of extracting useful information is called reading. The 
deliberate removal of information from the storage surface is called erasing. 
In some tubes, reading automatically effects erasing; in others, a separate 
operation is required. A charge-storage tube is one in which information is 
retained on a surface in the form of electric charges. 
    The characteristics of a storage tube are largely governed by the nature 
of the storage surface. This surface is usually a deposit or sheet of 
insulating or semiconducting material. The point-by-point potential of this 
surface is varied, in a controlled way, by the dielectric charging processes 
associated with electron bombardment of an insulator. The low conductivity of 
the storage-surface material ensures that the charge (or potential) pattern 
will not be dissipated before the reading operation is initiated and completed. 
    Storage tubes utilize electron guns to address the storage surface and 
effect the operations of reading, writing, and erasing. The electron gun will 
produce a fine pencil beam of electrons which may be deflected, as in a 
cathode-ray tube, to any desired location on the storage surface, or it will 
produce a flood beam which uniformly and simultaneously floods the entire 
storage surface with electrons.  See  CATHODE-RAY TUBE. 
 �34s[Norman W. Patrick]
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�6v�2v�32sStorm   �21m �33s 
    An atmospheric disturbance involving perturbations of the prevailing 
pressure and wind fields on scales ranging from tornadoes (0.6 mi or 1 km 
across) to extratropical cyclones (1.2 - 1900 mi or 2 - 3000 km across); also, 
the associated weather (rain storm, blizzard, and the like). Storms influence 
human activity in such matters as agriculture, transportation, building 
construction, water impoundment and flood control, and the generation, 
transmission, and consumption of electric energy.  See  WIND. 
    The form assumed by a storm depends on the nature of its environment, 
especially the large-scale flow patterns and the horizontal and vertical 
variation of temperature; thus the storms most characteristic of a given 
region vary according to latitude, physiographic features, and season. 
Extratropical cyclones and anticyclones are the chief disturbances over 
roughly half the Earth's surface. Their circulations control the embedded 
smaller-scale storms. Large-scale disturbances of the tropics differ 
fundamentally from those of extratropical latitudes.  See  HURRICANE; SQUALL; 
THUNDERSTORM; TORNADO; TROPICAL METEOROLOGY. 
    Cyclones form mainly in close proximity to the jet stream, that is, in 
strongly baroclinic regions where there is a large increase of wind with 
height. Weather patterns in cyclones are highly variable, depending on 
moisture content and thermodynamic stability of air masses drawn into their 
circulations. Warm and occluded fronts, east of and extending into the cyclone 
center, are regions of gradual upgliding motions, with widespread cloud and 
precipitation but usually no pronounced concentration of stormy conditions. 
Extensive cloudiness also is often present in the warm sector. Passage of the 
cold front is marked by a sudden wind shift, often with the onset of gusty 
conditions, with a pronounced tendency for clearing because of general 
subsidence behind the front. Showers may be present in the cold air if it is 
moist and unstable because of heating from the surface. Thunderstorms, with 
accompanying squalls and heavy rain, are often set off by sudden lifting of 
warm, moist air at or near the cold front, and these frequently move eastward 
into the warm sector.  See  CYCLONE; JET STREAM; WEATHER. 
    Extratropical cyclones alternate with high-pressure systems or 
anticyclones, whose circulation is generally opposite to that of the cyclone. 
The circulations of highs are not so intense as in well-developed cyclones, 
and winds are weak near their centers. In low levels the air spirals outward 
from a high; descent in upper levels results in warming and drying aloft. 
Anticyclones fall into two main categories, the warm ``subtropical'' and the 
cold ``polar'' highs. 
    Between the scales of ordinary air turbulence and of cyclones, there exist 
a variety of circulations over a middle-scale or mesoscale range, loosely 
defined as from about one-half up to a few hundred miles. Alternatively, these 
are sometimes referred to as subsynoptic-scale disturbances because their 
dimensions are so small that they elude adequate description by the ordinary 
synoptic network of surface weather stations. Thus their detection often 
depends upon observation by indirect sensing systems.  See  METEOROLOGICAL 
SATELLITES; RADAR METEOROLOGY; STORM DETECTION. 
 �34s[Chester W. Newton]
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�6v�2v�32sStorm detection   �21m �33s 
    Microbarographs, radars, satellite-borne instruments, and sferics 
detectors (radio receivers) are used to detect storms and to assess their 
potential for destruction.  See  METEOROLOGY. 
    Certain severe thunderstorms emit an identifiable kind of infrasound, or 
ultralow-frequency acoustic wave. Traveling at the speed of sound and ducted 
between the ground and high-temperature layers in the upper atmosphere, these 
waves are often so powerful that they can be detected by sensitive pressure 
detectors, called microbarographs, more than 900 mi (1500 km) from the 
emitting storm. Accoustical detection is not used for storm warning, and its 
main value lies in storm research. 
 �34s[T. M. Georges]
�33s 
    Radar can penetrate storm clouds to provide a three-dimensional, inside 
view of the storm. Advanced weather radars provide accurate images of both 
precipitation intensity inside a storm's shield of clouds and precipitation 
velocity. Contoured reflectivity maps are routinely displayed by radars of the 
U.S. National Weather Service on widely used plan position indicator scopes 
giving range and azimuth to precipitation targets whose reflectivity 
(intensity) is indicated by a stepped brightness scale. While a storm's 
reflectivity image is valuable for rainfall assessment and severe weather 
warnings, it is not a highly reliable tornado indicator. Highest reflectivity 
areas often signify hail. Radar warnings are primarily based on reflectivity 
values, on storm top heights, and sometimes on circulatory features of hook 
echoes seen in the patterns of reflectivity.  See  RADAR. 
 �34s[R. J. Doviak]
�33s 
    Weather satellites provide photographs of nearly an entire hemisphere to 
users throughout the United States within 30 min of scan time. Scans are made 
routinely every 30 min. Intensity and development of weather systems are 
monitored by combining sequential pictures into motion picture loops. In 
addition, infrared data allow display of cloud systems at night, and 
indications of both temperature and water vapor by both night and day. 
Satellite photos reveal components of storm systems heretofore inaccessible. 
For example, frontal boundaries can be located precisely in most cases, since 
sharp cloud lines usually accompany fronts. The location and configuration of 
jet streams can often be identified through analysis of cirrus cloud streaks. 
Low-level moisture can be tracked via low clouds moving from the Gulf of 
Mexico into the central plains of the United States in advance of severe 
thunderstorm and tornado outbreaks. Developing low-pressure systems are 
identifiable as soon as they generate even the smallest of cloud bands. Two of 
the most destructive weather phenomena, hurricane and the severe thunderstorm, 
are efficiently monitored by using satellite technology. 
 �34s[John Weaver]
�33s 
    Lightning discharges produce a wide spectrum of electromagnetic signals 
that are detected by radio receivers and provide effective means for locating 
and tracking thunderstorms. Both the electric and the magnetic components of 
lightning-produced radio signals have been used to locate and track 
thunderstorms through triangulation methods from the directions of arrival at 
two or more spaced stations.  See  HURRICANE; LIGHTNING; SFERICS; STORM; 
THUNDERSTORM.  
 �34s[William L. Taylor]
�33s 
  
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�6v�2v�32sStorm surge   �21m �33s 
    A transient, localized disturbance at sea level, resulting from the action 
of a tropical cyclone, an extratropical cyclone, or a squall over the sea. 
Storm surges, or storm tides, are not to be confused with tsunamis, or tidal 
waves, which result from seismic or molar disturbances of the Earth.  See  
TSUNAMI.  
    The time history of the surge at a given location at shore is represented 
by the surge hydrograph. This is a time sequence of the difference between the 
measured tide and the predicted periodic tide. Maximum surge elevations of 15 
ft (4.6 m) above predicted tide are not uncommon. In the case of 
hurricane-induced surges, the peak water level seems to depend primarily upon 
the atmospheric pressure at the hurricane center. However, the horizontal 
scale, the direction and speed of propagation of the hurricane, and the 
coastal geometry and bottom topography are important influencing factors in 
the storm surge behavior. 
    A storm surge is essentially a forced inertiogravitational wave of great 
wavelength. This implies that the duration or speed of the storm determines 
the dynamic augmentation of the water level at shore above that which would 
occur if the storm were stationary. Also, the inertial character of surges can 
explain quasi-periodic resurgences that often follow the primary forced surge. 
 �34s[Robert O. Reid]
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�6v�2v�32sStraight-line mechanism   �21m �33s 
    A mechanism that produces a straight-line (or nearly so) output motion 
from an input element that rotates, oscillates, or moves in a straight line. 
Common machine elements, such as linkages, gears, and cams, are often used in 
ingenious ways to produce the required controlled motion. The more elegant 
designs use the properties of special points on one of the links of a four-bar 
linkage.  See  CAM MECHANISM; GEAR; FOUR-BAR LINKAGE; SLIDER-CRANK MECHANISM. 
 �34s[John A. Smith]
�33s 
  
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�6v�2v�32sStrain   �21m �33s 
    Deformation or change in shape of a material as a consequence of applied 
forces. Strain is directly measurable, and from such measurement, within the 
elastic limit, the internal stress that accompanied the strain can be 
determined. The strain-producing action sets up stresses in the material, and 
these stresses cause a deformation or strain; that is, an initial strain is 
always accompanied by a stress. For this reason, the two phenomena are usually 
dealt with together.  See  ELASTIC LIMIT; STRAIN GAGE; STRESS AND STRAIN. 
 �34s[Frank H. Rockett]
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�6v�2v�32sStrain gage   �21m �33s 
    A device which measures mechanical deformation (strain). Normally it is 
attached to a structural element, and uses the change of electrical resistance 
of a wire or semiconductor under tension. Capacity, inductance, and reluctance 
are also used. 
    The strain gage converts a small mechanical motion to an electrical signal 
by virtue of the fact that when a metal (wire or foil) or semiconductor is 
stretched, its resistance is increased. The change in resistance is a measure 
of the mechanical motion. In addition to their use in strain measurement, 
these gages are used in sensors for measuring the load on a mechanical member, 
forces due to acceleration on a mass, or stress on a diaphragm or bellows.  
See  STRAIN. 
 �34s[John H. Zifcak]
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�6v�2v�32sStrain rosette   �21m �33s 
    A pattern of intersecting gage lines on a surface along which linear 
strains are measured to determine stresses at a point. A rosette gage is an 
assembly of strain-measuring components arranged to measure strains in the 
directions of the respective gage lines. To facilitate computations, three 
lines are usually oriented to form a rectangular or 45DG rosette, or to form 
an equiangular rosette, also called the delta or 60DG rosette (see 
illustration).  
    Electrical resistance-type gages are usually employed. With suitable 
instrumentation, strains are recorded in microinches per inch. Strains in the 
selected directions can also be measured by mechanical gages. The three 
measured strains permit evaluation of strains in any direction.  See  STRAIN 
GAGE; STRESS AND STRAIN. 
 �34s[John B. Scalzi]
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�6v�2v�32sStrake   �21m �33s 
    The slender forward extension of the inboard region of the wing of a 
combat aircraft used to provide increased lift in the high angle-of-attack 
maneuvering condition. In contrast to the normal attached-flow design 
principles, these strakes are built to allow the flow to separate along the 
leading edge in the high angle-of-attack range and to roll up into strong 
leading-edge vortices. The illustration shows a typical strake installation 
and the vortices generated. These are highly stable, and their strong swirling 
motion creates a lower pressure area on the strake upper surface, resulting in 
a large incremental increase in lift force known as vortex lift. This vortex 
lift increases rapidly in the high angle-of-attack range and is less 
susceptible to the normal stalling characteristics encountered with 
conventional lifting surfaces.  See  AIRFOIL; SUBSONIC FLIGHT; VORTEX; WING. 
 �34s[Edward C. Polhamus]
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�6v�2v�32sStrand line   �21m �33s 
    The line which marks the boundary between land and a lake, sea, or ocean. 
The strand line is quite dynamic. It moves with the tides, it may move with 
the seasons, and it also moves as the sea level changes. Many varieties of 
organisms respond to shifts in the strand line, as to textural parameters of 
sediments and the coastal morphology. 
    Geologists are often concerned with locating the position of ancient 
strand lines in the rock record. The shifting of strand line positions through 
time as recorded in the stratigraphic record may be determined by analysis of 
fossils, rock textures, and structures. This enables the reconstruction of 
environments and helps to establish a paleogeography.  See  FACIES (GEOLOGY); 
PALEOGEOGRAPHY; SEA-LEVEL FLUCTUATIONS. 
 �34s[Richard A. Davis, Jr.]
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�6v�2v�32sStrange particles   �21m �33s 
    Particles possessing the attribute of strangeness, a quantum number 
associated with one of the several quarks which are thought to constitute 
their structure. The nomenclature arose from the fact that the production 
rates for strange particles, which are compatible with strong interaction 
times of the order of 10-sup2sup3 s, appeared to be inconsistent with their 
relatively long lifetimes (in the range 10-sup8 to 10-sup1sup0 s). This 
inconsistency was resolved by experimental observations that strange particles 
are invariably produced in pairs (associated production) of equal and opposite 
strangeness, while their decay channels are restricted to those in which the 
strangeness quantum number is not conserved. This nonconservation of 
strangeness greatly suppresses the decay rates relative to the production 
rates.  See  ELEMENTARY PARTICLE; QUARKS. 
 �34s[Horace D. Taft]
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�6v�2v�32sStratigraphic nomenclature   �21m �33s 
    A system of naming used by geologists in classifying the rock record of 
the geologic history of the Earth. Sedimentary rocks, laid down layer by layer 
in seas, lakes, river floodplains, and elsewhere, are the principal record of 
that history. The record is complex, for rocks vary greatly not only 
vertically from layer to layer but also, though over greater distances, 
horizontally along the layers. 
    It has been traditional in geology to consider that any succession of such 
layers represents a succession in time, the lower layers being the older (law 
of superposition). Stratigraphic nomenclature attempts to express the time 
relations of the various layers as well as their intrinsic physical character. 
Extension of such time relations laterally from one succession to another is 
called correlation. Within a local area the rocks can be correlated by tracing 
individual layers or groups of layers, but for great distances, especially 
from continent to continent, correlation depends on the proved generalization 
that the fossils in the layers of a given age differ from those in layers of 
all other ages (law of faunal succession).  See  STRATIGRAPHY. 
    Two main schemes of stratigraphic nomenclature are prevalent - a North 
American and a European - though considerable divergences remain in the usage 
on each continent. Continental European usage emphasizes time (as recorded by 
fossil content) as the chief basis for stratigraphic nomenclature (Table 1). 
The stratigraphic subdivisions recognized are intended to embrace all layers 
laid down during a given time interval; in practice they include all layers 
containing the fossils of one fauna in the succession of fossil faunas. North 
American usage, on the other hand, considers that units based directly on the 
characters of the rocks - rock-stratigraphic units - should have a separate 
formal status equal to that of the time-stratigraphic units (Table 2). There 
is a trend in North American usage to distinguish between time-stratigraphic 
units, based on time in the abstract, and biostratigraphic units, based only 
on fossil content; zone and related terms are placed in the latter category 
and stage, series, and system in the former. 
    The stratigraphic nomenclatures discussed above were worked out in and for 
the generally fossiliferous rocks deposited during the last 600,000,000 years 
(the Phanerozoic Eon, the time since the appearance of the 
olenellid-archeocyathid fauna at the beginning of the Cambrian Period). The 
rocks deposited in earlier times (Cryptozoic Eon or Precambrian time) contain 
few if any fossils that can be used in correlation. A broad division of 
Precambrian time into Proterozoic (younger than about 2,600,000,000 years) and 
Archean (older) is generally accepted, and a threefold subdivision of the 
Proterozoic (time or rocks) is also widespread, although full agreement on the 
ages to be assigned to the boundaries has not yet been reached. 
    Table 3 gives the subdivision of the Phanerozoic Eon into eras and periods 
as now accepted almost throughout the world (the principal divergences are 
noted). The subdivision of the periods or systems into epochs or series is 
less uniform. 
    The general principles of rock-stratigraphic nomenclature are also 
extended to igneous and metamorphic rocks. The units are in all ways 
comparable to rock-stratigraphic units in sedimentary rocks. Nonsedimentary 
rocks are also assigned to time-stratigraphic units (for metamorphic rocks, 
based on the time of their original formation, not their metamorphism). Except 
for the volcanic igneous rocks, however, it is rarely possible to assign them 
with any precision. 
 �34s[John Rodgers]
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�6v�2v�32sStratigraphy   �21m �33s 
    The branch of geology that studies layered or stratified rocks. Chiefly it 
concerns sedimentary rocks, but its principles may also be applied to layered 
igneous rocks, such as lavas and tuffs, and to metamorphic rocks that were 
formed from sedimentary or volcanic rocks. It deals with the observed 
interrelations of the layers of such rocks and with the historical conclusions 
that can be inferred from those interrelations. Other aspects of sedimentary 
geology are sedimentary petrography (the study of the materials composing 
sedimentary rocks) and sedimentation (the study of the processes by which 
sediments are formed at present). Stratigraphy is of great practical value 
with important application to petroleum exploration.  See  SEDIMENTARY ROCKS; 
SEDIMENTATION (GEOLOGY). 
    The first task of stratigraphy is the description of local sequences of 
strata; from these descriptions local geologic history can be inferred. The 
second task of stratigraphy is the correlation of these local sequences, that 
is, the determination of their mutual time relations and the integration of 
the local histories into a regional or worldwide chronologic framework. The 
third task of stratigraphy is interpretation of the geologic history of the 
Earth from the scattered data of local sequences and criteria of correlation. 
The conclusions of sedimentation, sedimentary petrology, and plant and animal 
ecology provide clues for this interpretation.  See  ROCK AGE DETERMINATION; 
STRATIGRAPHIC NOMENCLATURE. 
 �34s[John Rodgers]
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�6v�2v�32sStratosphere   �21m �33s 
    The stable layer of the atmosphere extending from the top of the 
troposphere (6 - 10 mi or 10 - 16 km) to the temperature maximum at 30 - 34 mi 
(50 - 55 km). In the lower stratosphere below about 19 mi (30 km), temperature 
slowly decreases (in winter high latitudes), remains constant (in 
mid-latitudes), or increases with height (in low latitudes). In the upper 
stratosphere (APPROX EQUAL 19 - 30 mi or 30 - 50 km) there is a general 
increase of temperature with height to above 62DGF (290 K) over the summer 
pole and -9DGF (250 K) over the winter pole. Winds in summer are steady 
moderate easterlies, but large-scale systems with a westerly ``polar night'' 
jet stream  form in winter. In late winter and spring these systems also 
affect the lower stratosphere, causing large temperature changes called 
``sudden warmings'' and also large increases in the tracer (ozone and 
radioactivity) content.  See  JET STREAM. 
    In composition the stratosphere is notable for its photochemically formed 
ozone layer and its low water vapor content. Its thermal structure can be 
broadly explained in terms of the radiation balance due to its minor 
constituents of ozone, carbon dioxide, and water vapor.  See  ATMOSPHERE; 
TROPOPAUSE; TROPOSPHERE. 
 �34s[R. J. Murgatroyd]
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�6v�2v�32sStrawberry   �21m �33s 
    Low-growing perennials, spreading by stolons, with fruit consisting of a 
fleshy receptacle, and ``seeds'' in pits or nearly superficial on the 
receptacle. The strawberry in the United States is derived from two species: 
Fragaria chiloensis, which grows along the Pacific Coast of North and South 
America, and F. virginiana, the eastern meadow strawberry, both members of the 
order Rosales.  See  ROSALES. 
    The strawberry is the most universally grown of the small fruits, both in 
the home garden and in commercial plantings. Home garden production is 
possible in nearly all of the states, provided water can be supplied where 
rainfall is insufficient. Commercial production is important in probably 
three-fourths of the states. The following states are large producers: Oregon, 
California, Tennessee, Michigan, Louisiana, Washington, Arkansas, Kentucky, 
and New York.  See  FRUIT. 
 �34s[Joseph H. Camin]
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�6v�2v�32sStream gaging   �21m �33s 
    The measurement of streamflow. A stream-gaging station is a site on a 
river where a record of streamflow is obtained. It usually consists of an 
instrument installation to record the fluctuating water level and also 
consists of facilities for making current-meter measurements of discharge. The 
discharge of a river is measured in the field by a direct observation of 
velocity and of the cross-sectional area through which this velocity is 
applicable. By definition, discharge is the product of mean velocity times 
cross-sectional area. 
    Direct measurement of velocity is made by a current meter, an instrument 
with vanes, blades, or rotor cups that turn a shaft at a rate depending on the 
speed of the water (see illustration). Each rotation of the shaft causes a 
click that is audible in an earphone worn by the observer. The number of 
clicks in a clocked period of time can be translated into velocity by a 
calibration table. Indirect measurements are made by a water-stage recorder, 
which makes a graphical record of water level in a well that is connected by 
an open pipe to the natural stream. Fluctuations of water surface are followed 
by a float connected by a wire to the recorder.  See  FLOW MEASUREMENT. 
 �34s[Luna B. Leopold]
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�6v�2v�32sStreaming potential   �21m �33s 
    The potential which is produced when a liquid is forced to flow through a 
capillary or a porous solid. The streaming potential is one of four related 
electrokinetic phenomena which depend upon the presence of an electrical 
double layer at a solid-liquid interface. This electrical double layer is made 
up of ions of one charge type which are fixed to the surface of the solid and 
an equal number of mobile ions of the opposite charge which are distributed 
through the neighboring region of the liquid phase. In such a system the 
movement of liquid over the surface of the solid produces an electric current, 
because the flow of liquid causes a displacement of the mobile counterions 
with respect to the fixed charges on the solid surface. The applied potential 
necessary to reduce the net flow of electricity to zero is the streaming 
potential.  
 �34s[Quentin Van Winkle]
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�6v�2v�32sStreamline flow   �21m �33s 
    A condition of fluid flow characterized by the absence of turbulence. 
Other designations employed are laminar flow or viscous flow. 
    Fluid-flow particles in streamline flow follow well-defined continuous 
paths or streamlines. At a fixed point in streamline flow, the flow velocity 
either remains constant (steady flow) or varies in a regular fashion with time 
(unsteady flow). In some instances, streamline flow can best be depicted as 
formed from thin layers of fluid which slip past each other (lamellar flow). 
As an illustration, streamline flow in a straight pipe might be considered as 
formed from layers in the shape of concentric annuli. If the flow can properly 
be represented by thin, plane layers (laminae) sliding past each other, the 
flow is commonly referred to as laminar flow, although this term is often used 
to designate streamline flow in general. 
    The persistence of streamline flow in a given system is largely dependent 
on the value of a nondimensional parameter called the Reynolds number. When 
the Reynolds number of a particular flow exceeds a certain value (the critical 
Reynolds number), transition from streamline flow to turbulent flow generally 
takes place.  See  BOUNDARY-LAYER FLOW; FLUID MECHANICS; LAMINAR FLOW; 
REYNOLDS NUMBER; STREAMLINING; TURBULENT FLOW. 
 �34s[Arthur G. Hansen]
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�6v�2v�32sStreamlining   �21m �33s 
    The contouring of a body to reduce its resistance to motion through a 
fluid. The resistance to motion is referred to as the drag of the body. 
    Streamlining of a body must take into account the nature of drag forces. 
Drag forces are of four types: induced drag, wave drag, pressure (or form) 
drag, and skin friction. Induced drag is usually important for finite lifting 
surfaces such as wings; wave drag is important for bodies moving at supersonic 
speeds. Bodies moving at subsonic speeds are influenced by pressure drag and 
skin friction. Pressure drag is caused by inequalities in pressure forces 
acting in the direction of motion of the body. The action of viscosity tends 
to cause the flow to separate from the surface of the body with the consequent 
formation of a region of swirling or eddy flow termed the body wake. This eddy 
formation leads to a reduction in the downstream pressure on the body and 
hence gives rise to a force opposite to the body motion. 
    In general, the streamlining of a body in subsonic flow is the contouring 
of the body in such a manner that the wake is reduced to a minimum. Drag is 
then mainly the result of skin friction. Because one of the main causes of 
flow separation, and hence wake formation, is rapid deceleration of flow along 
the body, the contouring must make deceleration gradual. These considerations 
lead to the following general rules for streamlining: (1) The forward portion 
of the body should be well rounded, and (2) the body should curve back 
gradually from the forward section to a tapering after-section with the 
avoidance of sharp corners along the body surface. These conditions are well 
illustrated by teardrop shapes, as in the illustration.  See  STREAMLINE FLOW. 
  
    At supersonic speeds the airflow can accommodate sudden changes in 
direction by being compressed or expanded. Where this change in direction 
first occurs, a compression shock wave is created. The flow changes direction 
again at the midpoint of the body, creating an expansion shock wave. At the 
tail of the body the direction changes again, creating a compression shock 
wave. At each of these shock waves, changes in pressure, density, and velocity 
occur, and in this process energy is lost. This energy loss results in wave 
drag. Bodies which are streamlined for supersonic speeds are characterized by 
a sharp pointed nose, a sharp pointed tail, and a minimum number of direction 
changes between the nose and the tail.  See  AIRPLANE; SHOCK WAVE. 
 �34s[Arthur G. Hansen]
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�6v�2v�32sStream transport and deposition   �21m �33s 
    The sediment debris load of streams is a natural corollary to the 
degradation of the landscape by weathering and erosion. Eroded material 
reaches stream channels through rills and minor tributaries, being carried by 
the transporting power of running water and by mass movement, that is, by 
slippage, slides, or creep. The size represented may vary from clay to 
boulders. At any place in the stream system the material furnished from places 
upstream either is carried away or, if there is insufficient transporting 
ability, is accumulated as a depositional feature. The accumulation of 
deposited debris tends toward increased ease of movement, and this tends 
eventually to bring into balance the transporting ability of the stream and 
the debris load to be transported. 
 �34s[Luna B. Leopold]
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�6v�2v�32sStrength of materials   �21m �33s 
    A branch of applied mechanics concerned with the behavior of materials 
under load, relationships between externally applied loads and internal 
resisting forces, and associated deformations. 
    A material offers resistance to external load only insofar as the 
component elements can furnish cohesive strength, resistance to compaction, 
and resistance to sliding. The relations developed in strength of materials 
analysis evaluate the tensile, compressive, and shear stresses that a material 
is called upon to resist. The most important factors in determining the 
suitability of a structural or machine element for a particular application 
are strength and stiffness. 
 �34s[John B. Scalzi]
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�6v�2v�32sStreptococcus   �21m �33s 
    A genus of bacterium in the family Streptococcaceae. Streptococcus is a 
spherical bacterium that grows characteristically in chains (that is, 
individual cells adhere end to end in the same plane). When viewed through a 
microscope, the chains of individual streptococcal cells have the appearance 
of a string of pearls. They are gram-positive. Streptococci grow aerobically 
but usually best under reduced oxygen tension. The genus contains numerous 
species, some of which are highly pathogenic for humans or animals. Others are 
opportunistic pathogens. Others are saprophytic or are actually used to 
benefit humans. One example is S. lactis, which is used as a starter culture 
in preparing fermented milk products.  See  LANCEFIELD DIFFERENTIATION SCHEME. 
 �34s[Hazel W. Wilkinson]
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�6v�2v�32sStreptomycetaceae   �21m �33s 
    A family of bacteria comprising aerobic actinomycetes that usually produce 
a typical vegetative or substrate and aerial mycelium. They occur abundantly 
in soil and in other natural substrates. They produce a variety of enzymes 
(proteolytic, diastatic, and oxidative), vitamins (B12), and antibiotics. Most 
of the important antibiotics isolated since the discovery of penicillin are 
produced by members of this family. It is sufficient to mention streptomycin, 
chloramphenicol, the tetracyclines, neomycin, erythromycin, novobiocin, 
nystatin, candicidin, and oleandomycin.  See  ANTIBIOTIC; ENZYME. 
 �34s[Selman A. Waksman]
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�6v�2v�32sStreptomycin   �21m �33s 
    A colorless antibiotic substance produced by certain species of 
Streptomyces, mainly S. griseus. Streptomycin is reduced chemically to 
dihydrostreptomycin, giving a preparation that is frequently used in clinical 
practice in preference to streptomycin itself. Streptomycin and 
dihydrostreptomycin are active against gram-positive, gram-negative, and 
acid-fast bacteria, such as mycobacteria. These compounds have been used 
widely to treat infections of humans, animals, and plants. 
 �34s[Selman A. Waksman]
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�6v�2v�32sStress and strain   �21m �33s 
    Related terms used to define the intensity of internal reactive forces in 
a deformed body and associated unit changes of dimension, shape, or volume 
caused by externally applied forces. 
    Stress is a measure of the internal reaction between elementary particles 
of a material in resisting separation, compaction, or sliding that tend to be 
induced by external forces. Total internal resisting forces are resultants of 
continuously distributed normal and parallel forces that are of varying 
magnitude and direction and are acting on elementary areas throughout the 
material. These forces may be distributed uniformly or nonuniformly. Stresses 
are identified as tensile, compressive, or shearing, according to the 
straining action. 
    Strain is a measure of deformation such as (1) linear strain, the change 
of length per unit of linear dimensions; (2) shear strain, the angular 
rotation in radians of an element undergoing change of shape by shearing 
forces; or (3) volumetric strain, the change of volume per unit of volume. The 
strains associated with stress are characteristic of the material. Strains 
completely recoverable on removal of stress are called elastic strains. Above 
a critical stress, both elastic and plastic strains exist, and that part 
remaining after unloading represents plastic deformation called inelastic 
strain. Inelastic strain reflects internal changes in the crystalline 
structure of the metal. Increase of resistance to continued plastic 
deformation due to more favorable rearrangement of the atomic structure is 
called strain hardening. 
    A stress-strain diagram is a graphical representation of simultaneous 
values of a stress and strain observed in tests and indicates material 
properties associated with both elastic and inelastic behavior (see 
illustration). It indicates significant values of stress-accompanying changes 
produced in the internal structure.  See  ELASTICITY; STRENGTH OF MATERIALS. 
 �34s[John B. Scalzi]
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�6v�2v�32sStress concentration   �21m �33s 
    A condition in which a stress distribution has high localized stresses. A 
stress concentration is usually induced by an abrupt change in shape of a 
member. In the vicinity of notches, holes, changes in diameter of a shaft, or 
application points of concentrated loads, maximum stress is several times 
greater than where there is no geometrical discontinuity. Local stress 
disturbance is rapidly dissipated and effectively disappears at distances from 
the discontinuity equal to the major dimension of the section.  See  STRESS 
AND STRAIN. 
 �34s[John B. Scalzi]
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�6v�2v�32sStress (psychology)   �21m �33s 
    A term used to designate both environmental events and the body's response 
to such events. Of particular concern has been the relationship between stress 
and disease. Body response has been thought to extend from behavioral 
aberrations to a host of other bodily problems referred to in some circles as 
psychosomatic disorders, that is, diseases resulting from an individual's 
interaction with the environment. 
  
    Designation of stressor.  A specific event can be considered a stressor 
only if it evokes some aberrant bodily response. For example, if homeostasis 
of the electrolyte balance or body temperature is disrupted, there are 
immediate and serious consequences. However, most diseases that are believed 
to be stressor-related involve regulatory processes of the body, such as 
cardiovascular and gastrointestinal functioning. In these cases it is not as 
apparent when one is dealing with an aberrant stress response, particularly in 
the early phases of the disease process. An approach to the problem is 
suggested by the following, dealing specifically with cardiovascular 
functioning.  See  HOMEOSTASIS. 
    The cardiovascular system is a dynamic one, capable of altering its 
function over a wide range of values as metabolic demands vary. For example, 
the amount of blood pumped by the heart can increase four- or fivefold as one 
goes from sleep to strenuous exercise in order to meet the tissues' increasing 
needs for oxygen, nutrients, and the elimination of metabolic waste products. 
Since exercise in the healthy individual is not generally considered to be a 
potential disease-causing event, change in bodily functioning in itself cannot 
be considered to indicate the presence of a stressor. Rather, what must be 
determined is whether a bodily response to this event deviates in some manner 
from the immediate metabolic demands of the situation in such a way that the 
response is excessive and inefficient. 
    This point is illustrated by the following two observations. With exercise 
as the reference point, the cardiac output has been observed to increase 
disproportionately more to a threatening environmental event than the 
resultant increase in O2 consumption. For example, exercising on a treadmill 
increased the cardiac output by approximately 100% and the O2 consumption by 
160%. On the other hand, exposure to the threatening event, while having a 
similar effect on the cardiac output, increases the O2 consumption only half 
as much. A similar impairment of kidney function has also been reported under 
these circumstances. Although the kidney is not part of the cardiovascular 
system, it is intimately associated with cardiovascular functioning because it 
can control the blood pressure through the regulation of sodium excretion. 
Sodium excretion was observed to be impeded (that is, sodium was retained in 
the plasma) when a particularly threatening event was encountered, but not in 
the absence of the event. The net result is an acute elevation of the blood 
pressure, which may in the long run result in hypertension and its various 
sequelae, such as heart disease and stroke. 
  
    Designating the individual at risk.  This is a particularly difficult 
research problem because it takes long periods of time to ascertain whether a 
given bodily response to environmental events is the precursor of an eventual 
disease state. The problem can be illustrated by research dealing with the 
causes of high blood pressure, a condition present in about 20% of the 
population by the age 60. The issue is to determine how to identify in early 
adulthood those individuals who will eventually develop high blood pressure, 
and then to try to prevent it or at least impede its course of development. 
One approach is suggested by the observation of appreciable individual 
differences in cardiovascular reactivity, such as heart rate, cardiac force, 
and blood pressure, to a standardized laboratory task inducing some pain or 
fright in young adult college men. For example, when measuring heart rate, 
some individuals do not react at all, some react modestly with increases of 10 
- 25 beats per minute, while others react appreciably, in excess of 50 bpm. 
    Based on this observation, it may be asked whether those who react most 
are more apt to become hypertensive later in life. While this question cannot 
be answered with absolute certainty, there are other observations suggestive 
of this likelihood. First, the stress response of high heart rate appears to 
be a stable characteristic of some individuals. That is, individuals who are 
particularly reactive to one challenging event usually remain so to the other 
quite different challenges. Similarly, the less reactive subjects remain so 
under these varying circumstances. Second, particularly reactive subjects are 
more apt to have a hypertensive parent than the less reactive. Together, these 
data suggest that there is some sort of predisposition to overreact 
cardiovascularly to environmental events. These observations reinforce the 
point that events which are stressors for some are not so for others, although 
there are other bodily responses and possible disease states which may be 
influenced by these events in the cardiovascularly nonreactive individuals. 
  
    Other views.  There are viewpoints about the nature of stress which differ 
somewhat from those discussed above. A most popular one is that of H. Selye, 
who viewed any environmental event as a stressor if it increases the demand on 
the body to readjust itself, with the intensity of the demand being the 
critical quality, not the nature of the demand. Another approach is 
illustrated by J. W. Mason, whose research has detailed the involvement of 
various psychoendocrines (hormones) to stressors. One facet of his work 
suggests that these various hormones prepare the body for action so that the 
individual can cope with the stress. This is very similar to Cannon's 
viewpoint in regard to epinephrine, the hormone released by the adrenal gland 
which not only stimulates the cardiovascular system, but as importantly, 
facilitates metabolism. Finally, a position offered by S. R. Burchfield looks 
at the stress response as maladaptive only when the individual uses coping 
styles which after numerous exposures to stress fail to conserve bodily 
resources and promote homeostasis. This view emphasizes the importance of 
behavioral adjustments to stressors.  See  EMOTION; HORMONE. 
 �34s[Paul A. Obrist]
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�6v�2v�32sStrigiformes   �21m �33s 
    The order of birds containing the owls. Two families are usually 
recognized: the true owls (Strigidae) and the barn owls (Tytonidae). The bay 
owl (Phodilus badius) of southern Asia is in some respects intermediate, but 
has been found to have behavioral and anatomical characteristics of its own 
and has been separated as a third family, Phodilidae. Although the Tytonidae 
and Strigidae are generally similar in superficial appearance, there are many 
anatomical differences between them. Both families are widely distributed; 
each family includes both diurnal and nocturnal hunters. Some of the latter 
have developed amazing adaptations for locating prey in total darkness, 
employing only hearing; involved are the disk of stiffened feathers around the 
face, which acts as a sound-gathering parabolic reflector, and a marked 
asymmetry of the ear openings, permitting a form of triangulation on the prey. 
All owls lay the white eggs typical of hole-nesting birds, but a few species 
nest on the ground or in old hawk or crow nests. All are wholly carnivorous or 
insectivorous, but the ecological spectrum occupied by owls is extremely 
broad.  See  AVES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sStrip mining �21m �33s
    A surface method of mining by removing the material overlying the bed and
loading the uncovered mineral, usually coal. It is safer than underground
mining because neither the workers nor the equipment is subjected to such
hazards as roof falls and explosions caused by gas or dust ignitions. Coal
near the outcrop or at shallow depth can be stripped not only more cheaply but
more completely than by deep mining, and the need for leaving pillars of coal
to support the mine roof is eliminated. The roof over coal at shallow depth is
weak and difficult to support in underground workings by conventional methods,
yet this same weakness, of cover and of coal seam, makes stripping less
difficult.
    Power shovels, draglines, bulldozers, and other types of earth-moving
equipment slice a cut through the overburden down to the coal. The stripped
overburden (spoil) is stacked in a long ridge (spoil bank) parallel with the
cut and as far as possible from undisturbed overburden (high wall). The slope
of a spoil bank is approximately 1.4:1 and that of a high wall under average
conditions is 0.3:1. The uncovered coal (berm) is then fragmented, loaded, and
transported from the pit. Spoil from each succeeding cut is stacked
overlapping and parallel with the previous ridge and also fills the space left
by the coal removed (see illustration).  See  LAND RECLAMATION; MINING.
 �34s[Orus L. Bennett]
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�6v�2v�32sStripping   �21m �33s 
    The removal of a volatile component from a liquid by vaporization. The 
stripping operation is an important step in many industrial processes which 
employ absorption to purify gases and to recover valuable components from the 
vapor phase. In such processes, the rich solution from the absorption step 
must be stripped in order to permit recovery of the absorbed solute and 
recycle of the solvent.  See  GAS ABSORPTION OPERATIONS. 
    Stripping may be accomplished by pressure reduction, the application of 
heat, or the use of an inert gas (stripping vapor). Many processes employ a 
combination of all three; that is, after absorption at elevated pressure, the 
solvent is flashed to atmospheric pressure, heated, and admitted into a 
stripping column which is provided with a bottom heater (reboiler). Solvent 
vapor generated in the reboiler or inert gas injected at the bottom of the 
column serves as stripping vapor which rises countercurrently to the 
downflowing solvent. When steam is used as stripping vapor for a system not 
miscible with water, the process is called steam stripping. 
    In addition to its use in conjunction with gas absorption, the term 
stripping is also used quite generally in technical fields to denote the 
removal of one or more components from a mixed system. Such usage covers (1) 
the distillation operation which takes place in a distilling column in the 
zone below the feed point, (2) the extraction of one or more components from a 
liquid by contact with a solvent liquid, (3) the removal of organic or metal 
coatings from solid surfaces, and (4) the removal of color from dyed fabrics.  
See  DISTILLATION; ELECTROPLATING OF METALS; SOLVENT EXTRACTION. 
 �34s[Arthur L. Kohl]
�33s 
  
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�6v�2v�32sStroboscope   �21m �33s 
    An instrument for observing moving bodies by making them visible 
intermittently and thereby giving them the optical illusion of being 
stationary. A stroboscope may operate by illuminating the object with 
brilliant flashes of light or by imposing an intermittent shutter between the 
viewer and the object. 
    Stroboscopes are used to measure the speed of rotation or frequency of 
vibration of a mechanical part or system. They have the advantage over other 
instruments of not loading or disturbing the equipment under test. Mechanical 
equipment may be observed under actual operating conditions with the aid of 
stroboscopes. Parasitic oscillations, flaws, and unwanted distortion at high 
speeds are readily detected. 
    The flashing-light stroboscopes employ gas discharge tubes to provide a 
brilliant light source of very short duration. Tubes may vary from neon glow 
lamps, when very little light output is required, to special stroboscope tubes 
capable of producing flashes of several hundred thousand candlepower with a 
duration of only a few millionths of a second.  See  NEON GLOW LAMP; VAPOR 
LAMP.  
 �34s[Arthur R. Eckels]
�33s 
  
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�6v�2v�32sStroboscopic photography   �21m �33s 
    Stroboscopic or ``strobe'' photography generally refers to pictures of 
both single and multiple exposure taken by flashes of light from electrical 
discharges. Originally the term referred to multiple-exposed photographs made 
with a stroboscopic disk as a shutter. One essential feature of modern 
stroboscopic photography is a short exposure time, usually much shorter than 
can be obtained by a mechanical shutter. 
    High-speed photography with stroboscopic light has proved to be one of the 
most powerful research tools for observing fast motions in engineering and in 
science. Likewise, the electrical system of producing flashes of light in 
xenon-filled flash lamps is of great utility for studio, candid, and press 
photography.  See  PHOTOGRAPHY; STROBOSCOPE. 
 �34s[Harold E. Edgerton]
�33s 
  
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�6v�2v�32sStromatolite   �21m �33s 
    Any of the minute finely laminated structures mostly preserved in 
carbonate rock. Stromatolites may be planar, but are usually undulose or built 
up into finely layered domes or hemispherical masses. Such structures form 
today in marine intertidal zones and on certain lake shores by the growth of 
abundant mucilaginous filamentous blue-green algae which trap carbonate muds 
or clay. The very fine layers result from periodic influx of fine-grain 
sediment carried over shoreward flats by rising tides or winds. The 
filamentous algae glue this material in place, and during periods of 
nondeposition grow up through it, preparing the surface for the next ingress 
of sediment. 
    Algally induced stromatolites occur throughout the geologic column. Many 
distinctive and very large forms are known from the North American and 
Siberian middle Precambrian through Cambrian strata - the earliest organic 
structures known in the geologic record. The prevalence, size, and diversity 
of form seen in the Precambrian and Cambrian have been proposed to result from 
the lack of the ubiquitous hordes of browsing gastropods seen today living and 
devouring the algal scum responsible for the mats.  See  BIOHERM; BIOSTROME. 
 �34s[James L. Wilson]
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�6v�2v�32sStromatoporoidea   �21m �33s 
    An extinct order thought to be sponges because of its close similarity to 
the class Sclerospongiae. Generally accepted stromatoporoids first appeared at 
the beginning of the Middle Ordovician and died out at the end of the 
Devonian. The stromatoporoidea is worldwide in distribution, most commonly 
associated with bedded limestones and fossil reefs. 
    The stromatoporoid skeleton is called a coenosteum, which begins 
development as a thin, encrusting layer of irregular rods and plates called 
the peritheca. Most coenostea are hemispherical, or thick laminar or 
lens-shaped structures. However, some stromatoporoids are thin and irregularly 
undulatory and others are cylindrical, cone-shaped, branching, or 
anastomosing.  See  SCLEROSPONGIAE. 
 �34s[Joseph St. Jean, Jr.]
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�6v�2v�32sStrong nuclear interactions �21m �33s
    One of the fundamental physical interactions, which acts between a pair of
hadrons. Hadrons include the nucleons, that is, neutrons and protons; the
strange baryons, such a lambda (LAMBDA) and sigma (SIGMA); the mesons, such as
pion (PI) and rho (RHO); and the strange meson, kaon (K). The nature of the
interaction is determined principally through observation of the collision of
a hadron pair. From this one learns that the interaction has a short range of
about 10-sup1sup5 m and is by far the dominant force within this range, being
much larger than the electromagnetic interaction, which is next in magnitude.
The strong interaction conserves parity and is time-reversal-invariant.  See
BARYON; HADRON; MESON; NUCLEON; PARITY (QUANTUM MECHANICS); STRANGE PARTICLES;
SYMMETRY LAWS (PHYSICS).
    The interaction between the baryons, including the nucleons and the
strange baryons, is thought to arise from the exchange of mesons. The
interaction for relatively large distances between nucleons is generated by
the exchange of single pions (illustration a). At shorter separation distances
the exchange of two-pion systems, such as the RHO (illustration b), dominates.
An important contribution is furnished by the exchange of two separate pions
with (illustration c) or without (illustration d) the formation of an excited
state of of the nucleon. The interaction between the strange baryons, and
between the strange baryons and the nucleons, also arises in part from the
exchange of pions, but the exchange of kaons can be equally important.
    The range of the interaction generated by the exchanges can be calculated
by using the simple formula below, where m is the mass of the exchange

			     Range =   /mc

particles, PLANCK'S CONSTANT is Planck's constant divided by 2PI, and c is the
speed of light. According to the above equation, the range of the interaction
developed when a single pion is exchanged (illustration a) is equal to 1.4 x
10-sup1sup5 m, while that due to illustration c is 0.7 x 10-sup1sup5 m.
    The internal structure and finite size of the baryons (they consist of
three quarks enclosed in a ``bag,'' the quarks interacting through the
exchange of gluons) must be considered when the baryons are very close
together, that is, for separation distances less than approximately 0.7 x
10-sup1sup5 m. The consequences for nuclear forces are being actively
investigated.  See  ELEMENTARY PARTICLE; FUNDAMENTAL INTERACTIONS; GLUONS;
QUARKS.
 �34s[Herman Feshbach]
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�6v�2v�32sStrongyloidea   �21m �33s 
    A group of roundworms. As adults they inhabit the gastrointestinal tract, 
kidney, or respiratory tract of amphibians, reptiles, birds, and a wide 
variety of mammals, including humans. Taxonomically, they are considered by 
some authorities to constitute an order; by others, a superfamily of the class 
Nematoda.  
    Strongyloids are usually oviparous, occasionally viviparous. Each female 
hookworm (the common strongyloid parasite of humans) passes 5000 - 20,000 eggs 
per day. Larval stages are usually passed in the soil, but only rarely are 
they parasites of earthworms and snails. 
    Infection occurs through the penetration of the host's skin by the larvae, 
through the ingestion of larvae in water or on grass, or by the ingestion of 
an intermediate host harboring the larvae. The larvae migrate to their adult 
habitats directly through the tissues or by way of the bloodstream. The adult 
worms live varying lengths of time, up to 14 years. 
    Some of the most important parasites of humans and domestic animals belong 
to this group, and the morbidity, mortality, and economic loss due to them are 
extensive. The hookworms of humans, Necator and Ancylostoma, are estimated to 
infect more than 700,000,000 persons in the tropical and subtropical areas of 
the world. The damage to the host depends upon the species and numbers of 
worms and the age and resistance of the host.  See  HOOKWORM DISEASE; NEMATA. 
 �34s[Harold W. Brown]
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�6v�2v�32sStrontianite   �21m �33s 
    The mineral form of strontium carbonate, usually with some calcium 
replacing strontium. It characteristically occurs in veins with barite or 
celestite or as masses in certain sedimentary rocks. Strontianite is normally 
prismatic, but it may also be massive. It may be colorless or gray with 
yellow, green, or brownish tints. The hardness is 3J on Mohs scale, and the 
specific gravity is 3.76. It occurs at Strontian, Scotland, and in Germany, 
Austria, Mexico, and India and, in the United States, in the Strontium Hills 
of California.  See  CARBONATE MINERALS; STRONTIUM. 
 �34s[Robert I. Harker]
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�6v�2v�32sStrontium   �21m �33s 
    A chemical element, Sr, atomic number 38, and atomic weight 87.62. 
Strontium is the least abundant of the alkaline-earth metals. The crust of the 
Earth is 0.042% strontium, making this element as abundant as chlorine and 
sulfur. The main ores are celestite, SrSO4, and strontianite, SrCO3.  See  
ALKALINE-EARTH METALS; STRONTIANITE. 
    Strontium nitrate is used in pyrotechnics, railroad flares, and tracer 
bullet formulations. Strontium hydroxide forms soaps and greases with a number 
of organic acids which are structurally stable, resistant to oxidation and 
breakdown over a wide temperature range. 
    Strontium is divalent in all its compounds which are, aside from the 
hydroxide, fluoride, and sulfate, quite soluble. Strontium is a weaker complex 
former than calcium, giving a few weak oxy complexes with tartrates, citrates, 
and so on. Some physical properties of the element are given in the table.  
 �34s[Reed F. Riley]
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�6v�2v�32sStrophomenida   �21m �33s 
    The largest order of articulate brachiopods which first appeared in Lower 
Ordovician times and ultimately became extinct in the Late Triassic. The order 
is diverse and shows an extraordinary degree of morphological variation. Many 
of the more bizarre forms adopted an attached mode of life, either anchored by 
spines or cemented by part of the pedicle valve; such a change is accompanied 
or preceded by atrophy of the pedicle. Four suborders are recognized: 
Strophomenidina, Chonetidina, Productidina, and Oldhaminidina.  See  
ARTICULATA (BRACHIOPODA); BRACHIOPODA. 
 �34s[A. J. Rowell]
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�6v�2v�32sStructural adhesive   �21m �33s 
    An adhesive composition capable of bearing loads of considerable 
magnitude. For the aerospace industry a structural adhesive is precisely 
defined as an adhesive compound which will meet or exceed the requirements for 
the various types called out in either the United States Military 
Specification, MM-A-132 (formerly MIL-A-5090D), for metal-to-metal adhesives, 
or MIL-A-25463(AGS) for honeycomb adhesives. This definition is accepted in 
the airframe manufacturing industry of North America, Japan, and western 
Europe. In other industries, namely, transportation, appliance, communication 
equipment, and lumber product manufacturing, adhesives which may not meet all 
the rigorous requirements of the aerospace industry are still used in a 
load-carrying function. In these cases the definition is not as precise, but 
in general it is expected that the structural or high-strength adhesive will 
not fail when a bonded joint, prepared from the thickness of metal or other 
material typical for that industry, is stressed to its yield point. It is also 
intended that this high strength will remain at an acceptable level over the 
temperature range and through the other environmental conditions which a 
finished unit could be exposed to in actual use.  See  ADHESIVE. 
    Structural adhesives are classified first by performance, that is, by 
their minimum mechanical strengths over a certain temperature range. Other 
classifications are by their intended end use (that is, metal to metal or 
sandwich adhesive) and by the process for bond formation (that is the 
temperature required to achieve a good bond). Lastly, there is the chemical 
composition of the basic polymers in the blend. Chemically, structural 
adhesives are blends of high modulus and brittle phenolic or epoxy resins 
modified by lower modulus and tougher elastomer or thermoplastic resins. They 
are called two-polymer adhesives, polymer alloys, or adhesive polyblends. 
 �34s[Albert F. Martin]
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�6v�2v�32sStructural analysis   �21m �33s 
    The determination of stresses and strains in a given structure. All 
structures must be designed to carry loads without danger of overall collapse 
or failure of their components. One way to assure structural safety is to 
ascertain that stresses and strains produced by loads are less than those 
allowed by established design codes. This determination of stresses and 
strains in proposed new structures is a primary objective of structural 
analysis.  See  STRESS AND STRAIN. 
    Generally, analysis begins with a check of the overall stability of a 
structure. This involves first the determination of the forces exerted by the 
structure against its supports. If the supports are adequate to withstand 
these forces, they in turn react with equal but opposite forces against the 
structure.  
    If computation shows that the reactions balance the loads (weight of 
structure, occupants, stored materials, vehicles, wind, and earthquake 
forces), the structure is in static equilibrium.  See  STATICS. 
    The next step is determination of internal forces and unit stresses in the 
components of the structure. Finally, if necessary, the deformation of the 
structure as a whole and of its components may be calculated. Many of these 
tools are based on the assumption that the structure is elastic under loads; 
that is, stress is proportional to strain.  See  STRUCTURAL CONNECTIONS. 
 �34s[John B. Scalzi]
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�6v�2v�32sStructural connections   �21m �33s 
    Methods of joining the individual members of a structure to form a 
complete assembly. The connnections furnish supporting reactions and transfer 
loads from one member to another. Loads are transferred by fasteners (rivets, 
bolts) or welding supplemented by suitable arrangements of plates, angles, or 
other structural shapes. When the end of a member must be free to rotate, a 
pinned connection is used. 
    The suitability of a connection depends on its deformational 
characteristics as well as its strength. Rotational flexibility or complete 
rigidity must be provided according to the degree of end restraint assumed in 
the design. A rigid connection maintains the original angles between connected 
members virtually unchanged after loading. Flexible or nonrestraining 
connections permit rotation approximately equal to that at the ends of a 
simply supported beam. Intermediate degrees of restraint are called semirigid. 
    A commonly used form of connection for rolled-beam sections, called a web 
connection, consists of two angles attached to opposite sides of a member and 
which are in turn connected to the web of a supporting beam, girder, column, 
or framing at right angles. A shelf angle may be added to facilitate erection 
(Fig. 1). 
    A bracket or seat on which the end of the beam rests is a seat connection; 
it is intended to furnish the end reaction of the supported beam. Two general 
types are used: The unstiffened seat provides bearing for the beam by a 
projecting plate or angle leg which offers resistance only by its own flexural 
strength (Fig. 2); the stiffened seat is supported by a vertical plate or 
angle which transfers the reaction force to the supporting member without 
flexural distortion of the outstanding seat. 
  
    When the action line of a transferred force does not pass through the 
centroid of the connecting fastener group or welds, the connection is 
subjected to rotational moment which produces additional shearing stresses in 
the connectors. The load transmitted by diagonal bracing to a supporting 
column flange through a gusset plate is eccentric with reference to the 
connecting fastener group. 
    In beam-to-column connections and stiffened seat connections or when 
members transfer loads to columns by a gusset plate or a bracket, the 
fasteners are subjected to tension forces caused by the eccentric connection. 
Although there are initial tensions in the fasteners, the final tension is not 
appreciably greater than the initial tension. 
    Rigidity and moment resistance are necessary at the ends of beams forming 
part of a continuous framework which must resist lateral and vertical loads. 
Wind pressures tend to distort a building frame, producing bending in the 
beams and columns which must be suitably connected to transfer moment and 
shear. The resisting moment can be furnished by various forms of angle T for 
fasteners or welded or bracket connections. 
    Where appreciably angular change between members is expected, and in 
special cases where a hinge support without moment resistance is desired, 
connections are pinned. Many bridge trusses and large girder spans have pin 
supports.  See  BOLT; JOINT (STRUCTURES); RIVET. 
 �34s[John B. Scalzi]
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�6v�2v�32sStructural geology   �21m �33s 
    The branch of geology which, in the broadest sense, deals with the 
description and analysis of the forms and interrelations of rock bodies. 
Structural features that came into being during the formation of the rocks are 
termed primary; those that were imposed upon the rocks after their formation, 
secondary. Although it also considers primary structures such as bedding and 
igneous features, the major role of structural geology is the description and 
analysis of secondary structures. Tectonics is synonymous with structural 
geology. Investigations of tectonic features involve their descriptions and 
interrelations, the analysis of the mechanics by which they were formed, and 
finally the synthesis of all structural data.  See  FAULT AND FAULT 
STRUCTURES; FOLD AND FOLD STYSTEMS; GEOLOGY; TECTONOPHYSICS. 
 �34s[Philip H. Osberg]
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�6v�2v�32sStructural materials   �21m �33s 
    Construction materials which, because of their ability to withstand 
external forces, are considered in the design of a structural framework. 
Materials used primarily for decoration, insulation, or other than structural 
purposes are not included in this group. 
    Clay products include the solid masonry units such as brick and the hollow 
masonry units such as clay tile or terra-cotta. Building stones generally used 
are limestone, sandstone, granite, and marble. Until the advent of steel and 
concrete, stone was the most important building material. Concrete is a 
mixture of cement, mineral aggregate, and water, which, if combined in proper 
proportions, form a plastic mixture capable of being placed in forms and of 
hardening through the hydration of the cement. Wood is unique among the common 
structural materials. Its tensile strength is generally greater than its 
compressive strength. The principal structural metals are the structural 
steels, steel castings, aluminum alloys, magnesium alloys, and cast and 
wrought iron. Composite materials have made important contributions to the 
design of naval, air, and space vehicles and structures. The most familiar 
composite is formed using filaments or woven cloth of fiber glass embedded in 
a polyester or epoxy resin base.  See  BRICK; COMPOSITE MATERIAL; CONCRETE; 
STEEL; STONE AND STONE PRODUCTS; STRUCTURAL STEEL; TERRA-COTTA; TILE. 
 �34s[Charles M. Antoni]
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�6v�2v�32sStructural petrology   �21m �33s 
    The study of the structural aspects of rocks, as distinct from the purely 
chemical and mineralogical studies that are generally emphasized in other 
branches of petrology. The term was originally used synonymously with 
petrofabric analysis, but is sometimes restricted to denote the analysis of 
only microscopic structural and textural features.  See  PETROFABRIC ANALYSIS; 
PETROGRAPHY.  
 �34s[John M. Christie]
�33s 
  
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�6v�2v�32sStructural plate   �21m �33s 
    A flat plate or slab which is supported along the edges, such as the 
bottom or cover of a tank, cylinder head, bulkhead, or floor panel. A 
structural plate bends when subjected to forces applied normal to its surface. 
The bending produces curvature in all planes normal to the plate; this dishing 
action differs from bending of a beam where curvature occurs in a single plane 
under symmetrical loads. The analysis of stresses and deflections in a plate, 
while based on assumptions essentially the same as those used for a beam, is  
more complicated because of simultaneous bending in all planes. Simplified 
approximate analyses predict stresses and deflections useful in design.  See  
BEAM.  
 �34s[John B. Scalzi]
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�6v�2v�32sStructural steel   �21m �33s 
    Steel used in engineering structures, usually manufactured by either the 
open-hearth or the electric-furnace process. The exception is carbon-steel 
plates and shapes whose thickness is 7/16 in. (11 mm) or less and which are 
used in structures subject to static loads only. Thse products may be made 
from acid-Bessemer steel. The physical properties and chemical composition are 
governed by standard specifications of the American Society for Testing and 
Materials (ASTM). Structural steel can be fabricated into numerous shapes for 
various construction purposes.  See  STEEL. 
 �34s[Waldo G. Bowman]
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�6v�2v�32sStructures (engineering)   �21m �33s 
    Definite arrangements of related elements or members joined together to 
enable given sets of loads to be supported in a prescribed position. The 
principal structures designed by civil engineers are bridges, buildings, dams, 
docks, retaining walls, tanks and bins for storage, transmission towers, radio 
and television towers, highway pavements, and aircraft landing strips. As 
exploration of space and of the ocean depths is expanded, engineering 
structures differing in type and function from those now in use will be 
necessary.  See  AIRPORT ENGINEERING; BRIDGE; BUILDINGS; COASTAL ENGINEERING; 
DAM; FOUNDATIONS; HIGHWAY ENGINEERING; PAVEMENT; STORAGE TANK; TOWER. 
    A structure should be useful, safe, economical, and esthetically 
attractive. Primary emphasis must be placed upon safety, and attention once 
given to economy as the secondary design consideration is giving way to 
concern with structural appearance and esthetics. 
 �34s[Charles M. Antoni]
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�6v�2v�32sStruthioniformes   �21m �33s 
    An order of birds traditionally including but one living family, 
Struthionidae, which contains the single living species of ostrich (Struthio 
camelus). The living ostrich is now found in a wild state only in portions of 
Africa, but it formerly occupied open areas of most of Africa and Arabia, and 
parts of western and central Asia. At one time it was widely raised in 
capitivity for its ornamental wing and tail plumage. 
    The ostrich is the largest living bird and differs from all others in 
possessing a pubic symphysis and two-toed feet. It is highly adapted for 
running, having powerful legs and reduced wings; speeds of 30 - 37 mi/h (13.4 
- 16.5 m/s) have been recorded. The breastbone is of the ratite or unkeeled 
type. The head, neck, and thighs are almost featherless and are brightly 
colored in breeding males. The female may lay as many as 20 eggs; although 
these weigh about 2.9 lb (1.3 kg) each, they are small compared to the size of 
the parent.  See  AVES; RATITES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sStrychnine   �21m �33s 
    The principal alkaloid present in nux vomica, the seeds of a tree native 
to India, Strychnos nux-vomica. The complex structure is shown in the 
illustration. In the medical practice of an earlier day strychnine had a 
reputation as a cardiovascular stimulant, respiratory stimulant, and bitter 
tonic. The modern view is that the therapeutically desirable effects are 
obtainable only with doses bordering on the toxic. Pharmacological studies 
have shown that many of the therapeutic applications of strychnine have little 
or no rationale.  See  ALKALOID. 
 �34s[S. Morris Kupchan]
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�6v�2v�32sSturgeon   �21m �33s 
    Any of 10 species of large fish which comprise the family Acipenseridae in 
the order Acipenseriformes. These fish are found in North Temperate Zone 
waters, where they are almost exclusively bottom-living and feed on organisms 
such as mollusks, worms, and larvae. The body has five rows of bony plates, of 
which one is situated dorsally, two laterally, and two ventrally. The snout is 
elongate and there are four barbels on its lower surface; the mouth is 
ventrally located, and in the adult the jaws lack teeth (see illustration). 
The skeleton is mainly cartilaginous. 
    Sturgeons are known primarily for the roe, which is processed as caviar. A 
single female can produce millions of eggs, which may be removed from dead 
fish or stripped from living fish. The smoked flesh of the sturgeon is a 
delicacy.  See  ACIPENSERIFORMES. 
 �34s[Charles B. Curtin]
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�6v�2v�32sSty �21m �33s
    An acute inflammation of one of the glands of the eyelids. A painful
swelling appears at the root of an eyelash, usually an infection caused by a
staphylococcus organism. The swelling results from pus formation, local edema,
and obstruction of the gland duct. Drainage is usually spontaneous. In certain
cases, the inner glands may become chronically obstructed and the inflammation
will change to a small hard scar or nodule called a chalazion.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sStylasterina   �21m �33s 
    An order of the class Hydrozoa of the phylum Coelenterata, including 
several brightly colored branching or encrusting ``corals'' of warm seas. 
(True corals belong to a different class, the Anthozoa.) The calcareous 
skeleton is covered by living tissue and is penetrated by ramifying tubes. 
Nutritive polyps, the gastrozooids, lie in cups on one surface or along 
certain edges of the skeletal substance. A spine, or style, at the base of 
each cup gives the order its name. 
    Some authorities combine the Stylasterina with the Milleporina in a single 
order, the Hydrocorallina.  See  ANTHOZOA; HYDROZOA. 
 �34s[Sears Crowell]
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�6v�2v�32sStylolites   �21m �33s 
    Irregular surfaces occurring in certain rocks, mostly parallel to bedding 
planes, in which small toothlike projections on one side of the surface fit 
into cavities of like shape on the other side (see illustration). Stylolites 
are most common in limestones and dolomites but are also present in many other 
kinds of rock, including sandstones, gypsum beds, and cherts.  See  DOLOMITE; 
LIMESTONE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sStylommatophora   �21m �33s 
    A superorder of the molluscan subclass Pulmonata containing about 20,500 
species that are grouped into 56 families. Nearly all land snails without an 
operculum are stylommatophorans. They have eyespots on the tips of a pair of 
retractile tentacles, hermaphroditic reproduction with partial fusion of the 
male and female system, and, normally, a second pair of retractile tentacles 
that function as chemoreceptors. All are air-breathing and most are truly 
terrestrial. Three orders, based on excretory structures, are recognized. The 
more primitive Orthurethra and Measurethra have less efficient water 
conservation devices than do the more specialized members of the order 
Sigmurethra, in which a closed ureter functions in water conservation. The 
latter specialization apparently is a prerequisite for evolution toward a 
sluglike structure, since all 16 families with slugs or sluglike taxa belong 
to the Sigmurethra.  See  PULMONATA. 
 �34s[G. Alan Solem]
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�6v�2v�32sStyrene �21m �33s
    A colorless, liquid hydrocarbon with the formula C6H5CH==CH2. It
boils at 145.2DGC (293.4DGF) and freezes at -30.6DGC(-23.1DGF). The ethylenic
linkage of styrene readily undergoes addition reactions and under the
influence of light, heat, or catalysts undergoes self-addition or
polymerization to yield polystyrene.
    The majority of the styrene used is converted into polystyrene, but other
thermoplastic or even thermosetting resins are prepared from styrene by
copolymerization with suitable comonomers. A smaller quantity of styrene goes
into the manufacture of elastomers or synthetic rubbers.
    Styrene is a skin irritant. Prolonged breathing of air containing more
than 400 ppm of styrene vapor may be injurious to health.  See
POLYMERIZATION; POLYSTYRENE RESIN.
 �34s[Charles K. Bradsher]
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�6v�2v�32sSubgiant star   �21m �33s 
    A member of the family of stars intermediate between giants and the main 
sequence in the Hertzsprung-Russell (H-R) diagram. The mean luminosity of a 
subgiant is about 10 times the Sun; the surface temperature lies between 4000 
and 7000 K (6700 and 12,100DGF). The masses are about 1.4 times that of the 
Sun.  See  HERTZSPRUNG-RUSSELL DIAGRAM; STAR. 
    The subgiants are of particular importance in current theories of stellar 
evolution. If a main-sequence star has exhausted about 12% of its mass of 
hydrogen, the star begins to evolve, expanding, cooling at the surface, and 
brightening. Old stars of population II, of masses about 1.35 times the Sun, 
are now evolving into the subgiant region of the H-R diagram. The age of the 
oldest known stellar systems can be determined from the luminosities of the 
subgiants to be between 5,000,000,000 and 12,000,000,000 years.  See  STELLAR 
EVOLUTION.  
 �34s[Jesse L. Greenstein]
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�6v�2v�32sSubgraywacke   �21m �33s 
    An argillaceous sandstone with a composition intermediate between 
graywacke and orthoquartzite. Precise definitions of the boundaries of this 
group vary among sedimentary petrologists, but there is general agreement that 
these rocks contain moderate to large amounts of rock fragments, some clay 
matrix, and at least a small amount of feldspar. 
    The rock fragments in subgraywackes may be dominated by chert and other 
sedimentary species rather than metamorphic or igneous rocks. The pore spaces 
are filled with a combination of clay matrix and mineral cement, usually 
quartz and carbonate. The clay matrix is mainly muscovite (illite) with 
smaller amounts of kaolinite and, in some few cases, biotite and chlorite. 
    Subgraywackes are probably the most abundant sandstone type and are found 
in deposits of all ages. They occur in moderately thick stratigraphic sections 
and are of wide lateral extent.  See  GRAYWACKE; SANDSTONE; SEDIMENTARY ROCKS. 
 �34s[Raymond Siever]
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�6v�2v�32sSublimation   �21m �33s 
    The process by which solids are transformed directly to the vapor state 
without passing through the liquid phase. Sublimation is of considerable 
importance in the purification of certain substances such as iodine, 
naphthalene, and sulfur. 
    Sublimation is a universal phenomenon exhibited by all solids at 
temperatures below their triple points. For example, it is a common experience 
to observe the disappearance of snow from the ground even though the 
temperature is below the freezing point and liquid water is never present. The 
rate of disappearance is low, of course, because the vapor pressure of ice is 
low below its triple point. Sublimation is a scientifically and technically 
useful phenomenon, therefore, only when the vapor pressure of the solid phase 
is high enough for the rate of vaporization to be rapid.  See  PHASE 
EQUILIBRIUM; TRIPLE POINT; VAPOR PRESSURE. 
 �34s[Norman H. Nachtrieb]
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�6v�2v�32sSubmarine �21m �33s
    A ship which can operate completely submerged in the water. The term
formerly was applied to any ship or vessel capable of operating completely
underwater, but is now usually restricted to a ship built for military
purposes. The term submersible usually is applied to smaller underwater
vehicles built for commercial work or pleasure.
    Throughout World Wars I and II, submarines operated primarily on the
surface and submerged only in the final stage of an attack or to evade
detection. Since high surface speed was essential in this type of naval
warfare, submarine hulls were designed for minimum surface resistance,
sacrificing submerged performance. The increasing effectiveness of radar and
air patrols in locating and attacking surfaced submarines eventually forced
them to remain submerged for longer periods. In the late stages of World War
II the snorkel, a breathing tube permitting the submarine to get air for its
diesel engines without surfacing, was developed. However, even a snorkeling
submarine may be detected visually, especially by aircraft, because it is
submerged only slightly, and the snorkel tube leaves a wake.  See
ANTISUBMARINE WARFARE; NAVAL SHIP.
    The nuclear reactor, developed after World War II, does not require air
for its fuel, and enhanced the undersea capabilities of the submarine. As the
submarine began to operate more extensively beneath the sea, its hull was
modified in successive designs to give minimum submerged resistance, which
necessitated sacrifice of surfaced performance.  See  SHIP POWERING AND
STEERING; SHIP PROPULSION REACTOR.
    Submarines can be classified by their primary military missions. Attack
submarines are fast, long-range ships used primarily with torpedoes, but they
may also carry mines which can be laid from the torpedo tubes. They carry
sensitive underwater sound receivers and transmitters (sonar) in order to
fulfill their mission. They may be armed with acoustic homing torpedoes and
cruise missiles.
    Ballistic-missile submarines carry long-range, solid-fueled missiles that
can be launched while submerged and may be fitted with nuclear warheads. The
submarine can remain submerged for many days and, undetected, launch missiles
on any target within range. The missiles are stowed in and launched from
vertical tubes.  See  MISSILE.
    Experimental submarines are occasionally built to test new designs of hull
shape, deeper depth capability, power plants, or controls.
    Submersibles are usually small, deep-diving vehicles of limited operation.
Their primary use is for exploration and oceanographic study of the ocean
depths, development of equipment, or commercial work. Some designs take
advantage of the natural forces of gravity and buoyancy for vertical motion.
Other designs use vertically oriented propellers to propel the craft up and
down. Movement is restricted to short distances and slow speed because of
small size and small battery capacity. Submersibles are usually carried or
towed to their sites of operation.  See  OCEANOGRAPHIC SUBMERSIBLES.
    There are many similarities between a surface ship and a submarine.
However, the submarine has some unique features that enable it to submerge and
resist great sea pressure. Most submarines have two hulls, a pressure hull and
a nonpressure hull. The pressure hull is the watertight, pressure-proof
envelope in which equipment operates and the officers and crew live. The
structure of this pressure hull is very strong to resist the forces created by
the weight of water above the ship when it is submerged. In certain areas of
the submarine there is a nonpressure hull of lighter structure, forming the
main ballast tanks. A nonwatertight superstructure provides a smooth, fair
envelope to cover pipes, valves, and fittings on top of the hull. Above the
superstructure the fairwater similarly encloses the bridge, the periscope,
mast supports, and, if one is provided, the conning tower.
 �34s[Millard S. Firebaugh ;James H. Webber]
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�6v�2v�32sSubmarine cable   �21m �33s 
    A cable, mainly for communications usage, laid on the ocean floor to 
provide overseas links. This term is sometimes applied to power cables in 
water, but these are not usually of great length. 
    The first successful submarine telegraph cable was laid across the North 
Atlantic in 1866. The technology of submarine telephone cables was developed 
over a period of many years, partly in response to the limitations of overseas 
radio telephone service. Submarine telephone cables over large distances 
require periodically spaced amplifiers, called repeaters, to restore 
transmission losses in the cables. In May 1950, the first two repeatered 
submarine telephone cables (SA type) were laid between Key West and Havana. 
These were the prototype for the first transatlantic telephone cable (TAT-1, 
SB type) which was laid in 1955 - 1956.  See  TELEPHONE SERVICE. 
    The SA and SB systems used twin directional armored cables with long, 
flexible ``sausage-link'' type repeaters having the same appearance as the 
cable except for larger diameter. In 1954, before TAT-1 went in service, 
traffic volume studies showed submarine cable systems of greater capacity 
would be required to handle future growth. To obtain wider-band systems, a new 
approach to cable and repeater design was necessary. In the Bell System's SD 
and subsequent systems, flexible repeaters were replaced with repeaters in 
rigid beryllium-copper cylinders, and instead of armored deep-sea cable the 
strength members were placed in the center of the coaxial structure (see 
illustration). Coaxial cable used in submarine cable systems differs from that 
used on land in that the ocean type uses only a single coaxial tube rather 
than multiple tubes for transmission.  See  COAXIAL CABLE; COMMUNICATION 
CABLES; TRANSMISSION LINES. 
    Undersea telecommunications cables offer an efficient use for optical 
fiber technology since the potential cost-per-channel advantage over present 
systems is very high. Special cable design is required for light-wave 
communication. Mechanically the cable must protect the optical fibers from 
excessive microbending and tensile stress during laying and recovery. It must 
also be heavy enough to sink to the bottom, and provide a suitable power 
conductor to transmit the required system current and voltage.  See  OPTICAL 
COMMUNICATIONS.  
 �34s[Charles H. Sprankle]
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�6v�2v�32sSubmarine canyon   �21m �33s 
    A relatively narrow, deep depression in the sea floor with steep slopes, 
the bottom of which grades continuously downward. Great submarine canyons are 
cut into the continental slopes off most coasts of the world. Many of these 
have rocky walls thousands of feet high. They have narrow inner gorges, 
winding courses, numerous tributaries and are, in fact, quite comparable to 
the great canyons of the land (see illustration). Some of the canyons are 
direct continuations of land canyons, and others occur off large rivers which 
flow through broad flat-floored valleys on land. The submarine canyons extend 
outward down the slope virtually to the deep ocean floor. The outer portions 
of these sea valleys are of modest dimensions and extend across broad, gently 
sloping fans, comparable to the piedmont fans along the fronts of mountain 
ranges in arid regions.  See  CONTINENTAL MARGINS; MARINE GEOLOGY. 
 �34s[Francis P. Shepard]
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�6v�2v�32sSubsonic flight   �21m �33s 
    Movement of a vehicle through the atmosphere at a speed appreciably below 
that of sound waves. Subsonic flight extends from zero (hovering) to a speed 
approximately 85% of sonic speed corresponding to the ambient temperature. At 
higher vehicle velocities the local velocity of air passing over the vehicle 
surface may exceed sonic speed, and true subsonic flight no longer exists. 
    Vehicle type may range from a small helicopter, which operates at all 
times in the lower range of the velocity scale, to an intercontinental 
ballistic missile, which is operative throughout this and other velocity 
regimes, but is in subsonic flight for only a few seconds. The design of each 
is affected by the same principles of subsonic aerodynamics. Subsonic flow of 
a fluid such as air may be subdivided into a range of velocities in which the 
flow may be considered incompressible without appreciable error (below a 
velocity of approximately 300 mi/h or 135 m/s), and a higher range in which 
the compressible nature of the fluid becomes significant. In both cases the 
viscosity of the fluid is important. The theories which apply to compressible, 
inviscid fluids may be used almost without modification in some low-subsonic 
problems, and in other cases the results offered by these theories may be 
modified to account for the effects of viscosity and compressibility.  See  
COMPRESSIBLE FLOW; TRANSONIC FLIGHT; VISCOSITY. 
    A typical subsonic wing cross section (airfoil) has a rounded front 
portion (leading edge) and a sharp rear portion (trailing edge). Air 
approaching the leading edge comes to rest at some point on the leading edge, 
with flow above this point proceeding around the upper airfoil surface to the 
trailing edge, and flow below passing along the lower surface to the same 
point, where the flow again theoretically has zero velocity. The two points of 
zero local velocity are known as stagnation points. If the path from front to 
rear stagnation point is longer along the upper surface than along the lower 
surface, the mean velocity of flow along the upper surface must be greater 
than that along the lower surface. Thus, in accordance with the principle of 
conservation of energy, the mean static pressure must be less on the upper 
surface than on the lower surface. This pressure difference, applied to the 
surface area with proper regard to force direction, gives a net lifting force. 
Lift is defined as a force perpendicular to the direction of fluid flow 
relative to the body, or more clearly, perpendicular to the free-stream 
velocity vector.  See  AIRFOIL PROFILE; BERNOULLI'S THEOREM. 
    The wing, as the lifting device, whether fixed, as in the airplane, or 
rotating, as in the helicopter, is probably the most important aerodynamic 
part of an aircraft. However, stability and control characteristics of the 
subsonic airplane depend on the complete structure. Control is the ability of 
the airplane to rotate about any of the three mutually perpendicular axes 
meeting at its center of gravity. Static stability is the tendency of the 
airplane to return to its original flight attitude when disturbed by a moment 
about any of the axes. 
 �34s[James E. May]
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�6v�2v�32sSubstitution reaction   �21m �33s 
    One of a class of chemical reactions in which one atom or group (of atoms) 
replaces another atom or group in the structure of a molecule or ion. Usually, 
the new group takes the same structural position that was occupied by the 
group replaced. 
    Substitution reactions involve the attack of a reagent, which is the 
source of the new atom or group, on the substrate, the molecule or ion in 
which the replacement occurs. They involve the formation of a new bond and the 
breaking of an old bond. Substitution reactions are classified according to 
the nature of the reagent (electrophilic, nucleophilic, or radical) and 
according to the nature of the site of substitution (saturated carbon atom or 
aromatic carbon atom).  See  ELECTROPHILIC AND NUCLEOPHILIC REAGENTS. 
    Systematic names for substitution reactions are composed of the parts: 
name of group introduced + de + name of group replaced + ation, with suitable 
elision or change of vowels for euphony. Thus, the replacement of bromine by a 
methoxy group is called methoxydebromination.  See  ORGANIC REACTION MECHANISM. 
 �34s[Joseph F. Bunnett]
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�6v�2v�32sSubtraction   �21m �33s 
    One of the four fundamental operations of arithmetic and algebra. 
Subtraction is often regarded as an operation inverse to addition, that is, if 
a and b are numbers, the number a - b is defined as that number which added to 
b gives a. The more modern viewpoint eliminates subtraction completely by 
considering the number a - b as the sum of a and that number (denoted by -b) 
which added to b gives 0. The number symbolized by -b is called the inverse of 
b (with respect to addition). Every real number has a unique inverse (the 
number 0 is its own inverse). In this sense, ``subtraction'' may be performed 
on objects of many different kinds, and the original numerical operation 
greatly extended.  See  ADDITION; ALGEBRA; DIVISION; MULTIPLICATION; NUMBER 
THEORY.  
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sSucrose   �21m �33s 
    An oligosaccharide, ALPHA-D-glucopyranosyl-BETA-D-fructofuranoside, also 
known as saccharose, cane sugar, or beet sugar. The structure is shown in the 
illustration. Sucrose is very soluble in water and crystallizes from the 
medium in the anhydrous form. The sugar occurs universally throughout the 
plant kingdom in fruits, seeds, flowers, and roots of plants. Honey consists 
principally of sucrose and its hydrolysis products. Sugarcane and sugarbeets 
are the chief sources for the preparation of sucrose on a large scale. Another 
source of commercial interest is the sap of maple trees.  See  
OLIGOSACCHARIDE; SUGAR; SUGARBEET; SUGARCANE. 
 �34s[William Z. Hassid]
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�6v�2v�32sSuctoria   �21m �33s 
    A small specialized subclass of the protozoan class Ciliatea whose members 
were long considered entirely separate from the ``true'' ciliates. The sole 
order of this subclass is Suctorida. These forms show a number of highly 
specialized features. Most conspicuous are their tentacles, often numerous, 
which serve as mouths. These multiple organelles of ingestion fasten to the 
pellicle of prey organisms, generally passing ciliates. By forces not entirely 
understood, the tentacles are used to suck out the prey's protoplasm to 
provide sustenance for the suctorian. Nearly all species are stalked, and the 
sedentary, mature forms are devoid of any external ciliature. Young larval 
forms are produced by both endogenous and exogenous budding. These forms bear 
locomotor cilia and serve, as in the case of species of the Peritrichia, for 
dissemination (see illustration).  See  CILIATEA; PERITRICHIA. 
 �34s[John O. Corliss]
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�6v�2v�32sSudangrass   �21m �33s 
    An annual, warm-season grass of tropical origin said to have been grown in 
Egypt since early times, though its value was first recognized in Sudan only 
in 1909. In that same year it was introduced into the United States as a 
replacement for johnsongrass, which had become a noxious weed in many southern 
states.  
    Sudangrass (Sorghum bicolor var. sudanense, also called S. sudanense and 
S. vulgare var. sudanense) and its hybrids are commonly used as pasture, 
greenchop, silage, or hay. They fill an important need in many regions of the 
United States, because they produce high-quality forage for cattle and sheep 
during the summer, when other pasture is in short supply or of low quality. 
Many of the varieties and hybrids produce forage until frost.  See  GRASS 
CROPS.  
 �34s[Melvin R. George]
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�6v�2v�32sSugarbeet   �21m �33s 
    The plant Beta vulgaris, developed in modern times to fill the need for a 
sugar crop that could be grown in temperate climates. The sugarbeet was the 
source of only 5% of the world's commercial sugar in 1840, but by 1890 it 
supplied almost 50%, and since about 1920 this has dropped to about 40% with 
60% derived from sugarcane. European countries produce most of their sugar 
from sugarbeet, with some countries having surplus sugar for export.  See  
SUGARCANE.  
    The sugarbeet, fodder beet, garden beet, and leaf beet (Swiss chard) are 
cultivars of B. vulgaris and are genetically related. In addition to B. 
vulgaris, which includes all cultivated forms of beet, there are 12 species of 
wild beet in the Mediterranean and Middle Eastern regions. These wild beets 
are of interest in sugarbeet improvement as sources of disease resistance. 
Most varieties of sugarbeet in the United States are hybrid. 
 �34s[Dewey Stewart]
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�6v�2v�32sSugarcane   �21m �33s 
    Saccharum officinarum, a member of the grass family. This crop originated 
in New Guinea about 15,000 - 8000 B.C., and was later moved by primitive 
peoples westward into Southeast Asia and India and eastward into Polynesia. 
Most current commercial varieties are interspecific hybrids involving 
primarily two or more of the following species: S. officinarum, S. robustum, 
and S. spontaneum. 
    Sugar is generally removed from sugarcane in large factories by either the 
milling or the diffusion process. Milling crushes the stalks as they pass 
between a series of large metal rollers separating the fiber (bagasse) from 
the juice laden with sugar. The diffusion process separates sugar from finely 
cut stalks by dissolving it in hot water or hot juice. Processing of the juice 
is completed by clarification to remove nonsugar components, by evaporation to 
remove water, and by removal of molasses in high-speed centrifuges to produce 
centrifugal sugar. Bagasse and molasses are the principal by-products from 
processing sugarcane. 
 �34s[Robert E. Coleman]
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�6v�2v�32sSulfa drugs �21m �33s
    A family of drugs of the sulfonamide type with marked ability to halt the
growth of bacteria.
    With the discovery, by G. Domagk in 1932, of the chemotherapeutic activity
of the azo dye Prontosil (I), a new therapeutic era started. For the first
time it was possible to cure, by treatment with synthetic chemicals, such
frequently fatal bacterial infections as pneumonia, meningitis, and
septicemia. The active component of Prontosil is the colorless
p-aminobenzenesulfonamide (sulfanilamide) moiety (II), formed in the body as
the result of metabolic reduction.
    Sulfonamides are used today mostly as auxiliary drugs or in combination
with antibiotics. In certain infectious diseases, however, for instance, in
meningococcal infections and most infections of the urinary tract,
sulfonamides deserve preference over antibiotics.  See	ANTIBACTERIAL AGENTS;
ANTIBIOTICS; DRUG RESISTANCE; SULFONAMIDE.
 �34s[Rudolph L. Mayer /Emanuel Grunberg]
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�6v�2v�32sSulfate �21m �33s
   A negative ion having the formula SO4(sup 2-) and derived from sulfuric acid,
H2SO4. Because sulfuric acid contains two hydrogens, both normal sulfates and
bisulfates are known. Of the normal sulfates, most are quite soluble in water,
with the exception of those of silver, mercury(I), lead, strontium, barium,
and calcium. Sulfate is determined both qualitatively and quantitatively by
precipitation as barium sulfate, BaSO4, which is the most insoluble sulfate.
See  SULFUR; SULFURIC ACID.
 �34s[E. Eugene Weaver]
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�6v�2v�32sSulfide   �21m �33s 
    A negative ion having the formula Ssup2- and derived from hydrogen sulfide 
or hydrosulfuric acid, H2S. Because hydrosulfuric is a dibasic acid, sulfides, 
M2S, and bisulfides, MHS, are formed. Although most of the metal sulfides are 
insoluble in water, they dissolve in acids with the evolution of H2S gas. This 
escaping gas causes paper moistened with a solution of lead acetate to turn 
back, a test that can be used to detect the presence of sulfide ion in the 
original solution.  See  SULFUR. 
 �34s[E. Eugene Weaver]
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�6v�2v�32sSulfite �21m �33s
    A negative ion having the formula SO3(sup 2-) and derived from unstable
sulfurous acid, H2SO3.
    Because the sulfur in the sulfite ion has a 4+ oxidation state, it
undergoes oxidation to the sulfate ion or reduction to free sulfur or to the
sulfide ion. The sulfite ion can be identified by the fact that SO2 gas is
liberated when solutions are acidified, as represented by the reaction below.

2H+ + SO3(sup 2-) REVERSIBLE REACTION [H2SO3] REVERSIBLE REACTION  H2O + SO2


    Because sulfurous acid is dibasic, two series of salts, the normal
M2SO3 and acid MHSO3, are formed.  Most of these salts, with the
exception of the alkali metal and ammonium salts, are only slightly
soluble.  See SULFUR.
 �34s[E. Eugene Weaver]
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�6v�2v�32sSulfonamide �21m �33s
    One of a group of organosulfur compounds, RSO2NH2, that are readily
prepared by the reaction of sulfonyl chlorides and ammonia, as in the reaction
below. There is great interest in these substances because of the therapeutic

      RSOsub2Cl + 2NHsub3      RSOsub2NHsub2 + NHsub4Cl

sulfa drugs, but they are also of chemical value.  See	AMINE; ORGANOSULFUR
COMPOUND; SULFA DRUGS; SULFONYL CHLORIDE.
 �34s[Norman Kharasch]
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�6v�2v�32sSulfonation and sulfation �21m �33s
    Sulfonation is a chemical reaction in which a sulfonic acid group, --SO3H,
is introduced into the structure of a molecule or ion in place of a hydrogen
atom.
    Sulfation involves the attachment of the --OSO2OH group to carbon,
yielding an acid sulfate, ROSO2OH, or of the --SO4-- group between two
carbons, forming the sulfate, ROSO2OR.
    Sulfonation of aromatic compounds is the most important type of
sulfonation. This is accomplished by treating the aromatic compound with
sulfuric acid, as in the reaction below. The product of sulfonation is a
sulfonic acid.

					      l60DGC
			     + Hsub2SOsub4

	   Naphthalene


						      SOsub3H

								  +Hsub2O


					    Naththalene-
					    sulfonic acid

    Sulfonation may also be defined as any chemical process by which the
sulfonic acid group, --SO2OH, or the corresponding salt or sulfonyl halide
group, for example, --SO2Cl, is introduced into an organic compound. These
groups may be bonded to either a carbon or a nitrogen atom. The latter
compounds are designated N-sulfonates or sulfamates.
    Most sulfonates are employed as such in acid or salt form for applications
where the strongly polar hydrophilic --SO2OH group confers needed properties
on a comparatively hydrophobic nonpolar organic molecule. Some sulfonates,
such as methanesulfonic and toluenesulfonic acids, are used as catalysts. The
major quantity of sulfonates and sulfates is both marketed and used in salt
form. This category includes detergents; emulsifying, demulsifying, wetting,
and solubilizing agents; lubricant additives; and rust inhibitors.  See
ORGANO-SULFUR COMPOUND; SUBSTITUTION REACTION.
 �34s[P. H. Groggins /Robert S. Kapner]
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�6v�2v�32sSulfone   �21m �33s 
    One of a group of well-known, stable, and generally crystalline 
organosulfur compounds, RSO2R'. Sulfones are best prepared by the oxidation of 
sulfides, and also by the reaction of sulfonyl chlorides, RSO2Cl, with 
aromatic compounds in the presence of Friedel-Crafts catalysts, or by reaction 
of sodium sulfinates and alkyl bromides. Sulfones resist further oxidation, 
but can be reduced, under special conditions, to sulfides. Pyrolysis of 
sulfones frequently involves elimination of sulfur dioxide in a synthetically 
useful way. For example, alkenyl aryl sulfones smoothly yield alkenes. The 
sulfone group exerts a strong inductive effect and increases the acidity of 
ALPHA-hydrogen atoms. In (CH3SO2)3CH, the acidity becomes very high. The 
--SO2-- group also deactivates the benzene ring to attack by electrophilic 
reagents.  See  ORGANOSULFUR COMPOUND; SULFIDE; SULFOXIDE. 
 �34s[Norman Kharasch]
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�6v�2v�32sSulfur �21m �33s
    A chemical element, S, atomic number 16, and atomic weight 32.064. The
known stable isotopes and approximate percent abundances in natural sulfur are
(sup 32)S (95.1%); (sup 32)S (0.74%); (sup 34)S (4.2%); and (sup 36)S
(0.016%). The abundance of sulfur in the Earth's crust is 0.03 - 0.1%. It is
often found as the free element near volcanic regions (impure deposits).

    Properties.  The allotropes (different crystalline forms) of sulfur have
been studied intensively, but as yet the many modifications which exist for
every state (gas, liquid, and solid) of elemental sulfur are not fully
understood.
    Rhombic sulfur, also called brimstone and alpha sulfur (ALPHA-sulfur), is
the stable modification of the element below 95.5DGC (204DGF, the transition
point), and most of the other forms revert to this modification if allowed to
stand below this temperature. Rhombic sulfur is lemon-yellow, insoluble in
water, slightly soluble in ethyl alcohol, diethyl ether, and benzene, and very
soluble in carbon disulfide. Its density is 2.06 g/cm(sup 3) (1.19 oz/in(sup 3))
and its hardness is 2.5 on the Mohs scale. Its molecular formula is S8.
    Monoclinic sulfur, also called prismatic sulfur and beta sulfur
(BETA-sulfur), is the stable modification of the element above the transition
temperature and below the melting point. It crystallizes from molten sulfur in
needlelike prisms which are almost colorless. It has a density of 1.96
g/cm(sup 3) (1.13 oz/in(sup 3)) and a melting point of 119.3DGC (246.7DGF). Its
molecular formula is also S8.
    Plastic sulfur, also called gamma sulfur (GAMMA-sulfur), is formed when
molten sulfur at or near its normal boiling point is quenched to the solid
state (such as by pouring it into cold water). This form of sulfur is
amorphous and is only partially soluble in carbon disulfide.
    Liquid sulfur exhibits the remarkable property of increasing in viscosity
as its temperature is raised. Its color becomes dark reddish-black as its
viscosity increases, and both color darkening and viscosity reach a maximum at
200DGC (392DGF). Above this temperature, the color lightens and the viscosity
diminishes.
    At the normal boiling point of the element (444.60DGC or 832.28DGF)
gaseous sulfur is orange-yellow in color. As the temperature is raised, the
color becomes deep red and then becomes lighter until, at 650DGC (202DGF), it
is straw-yellow.
    Sulfur is an active element which combines directly with most of the known
elements. It can exist in both positive and negative oxidation states, and can
form ionic as well as covalent and coordinate covalent compounds. The uses of
sulfur are limited primarily to the manufacture of sulfur compounds. However,
large quantities of elemental sulfur are used in the vulcanization of rubber,
in lime-sulfur sprays to destroy plant parasites, in the manufacture of
artificial fertilizer and certain types of cements and electric insulators, in
certain ointments and medicinals, and in the manufacture of gunpowder and
matches. Sulfur compounds are used in the manufacture of chemicals, textiles,
soaps, fertilizers, leather, plastics, refrigerants, bleaching agents, drugs,
dyes, paints, paper, and other products.

    Principal compounds.  Hydrogen sulfide (H2S) is the most important
compound containing only hydrogen and sulfur. It is a colorless gas having a
foul odor (similar to that of rotten eggs) and is considerably more poisonous
than carbon monoxide, but warning of its presence (odor) is usually given
before its concentration in the atmosphere is considered dangerous.
    Metal sulfides can be classified into three categories: acid sulfides
(hydrosulfides, MHS, where M = a univalent metal ion), normal sulfides (M2S),
and polysulfides (M2S3). Other sulfides include carbon-sulfur compounds and
compounds containing the carbon-sulfur bond. Some important compounds are:
carbon disulfide, CS2, a liquid which is an excellent solvent for elemental
sulfur and phosphorus: carbon monosulfide, CS, and unstable gas formed by
passing an electric discharge through carbon disulfide; and carbon oxysulfide,
SCO, formed from carbon monoxide and free sulfur at an elevated temperature.
    Nitrogen-sulfur compounds which have been characterized are sulfur
nitride, N4S4 (also called tetranitrogen tetrasulfide), nitrogen disulfide,
NS2, and nitrogen pentasulfide, N2S5. They should properly be referred to as
nitrides because of the greater electronegativity of nitrogen, although in the
literature they are usually called sulfides.
    Phosphorus-sulfur compounds which have been characterized are P4S3, P4S5,
P4S7, and P4S10. All four are yellow crystalline materials that are used in
converting organic oxygen compounds (for example, alcohols) into the
corresponding sulfur analogs.
    The oxides of sulfur which have been characterized have the formulas SO,
S2O3, SO2, SO3, S2O7, and SO4. Sulfur dioxide, SO2 and sulfur trioxide, SO3,
are of far greater importance than the others. Sulfur dioxide can act as an
oxidizing agent and as a reducing agent. It reacts with water giving an acidic
solution (often called sulfurous acid) and bisulfite (HSO3-) and sulfite
(SO3[sup 2-]) ions. The dioxide is used as a refrigerant gas, as a disinfectant
and preservative, as a bleaching agent, and in the refining of petroleum
products. Its major use, however, is in the manufacture of sulfur trioxide and
sulfuric acid. Sulfur trioxide is used primarily in the preparation of
sulfuric acid and sulfonic acids.
    Although salts (or esters) of all the oxy acids are known, in many cases
the free acids themselves have not been isolated because of their instability.
Sulfurous acid is not actually known as a pure substance. Sulfuric acid
(H2SO4) is a colorless, viscous liquid with melting point 10.31DGC (50.56DGF).
It is a strong acid in water and reacts with most metals in either the dilute
or concentrated form. The concentrated acid is a strong oxidizing agent,
especially at elevated temperatures. Pyrosulfuric acid (H2S2O7) is an
excellent sulfonating agent and loses sulfur trioxide on being heated. It also
reacts vigorously with water, liberating a considerable quantity of heat. The
persulfuric acids (peroxymonosulfuric acid, H2SO5, called Caro's acid, and
peroxydisulfuric acid, H2S2O8, called Marshall's acid) are known as the acids
and salts. Sulfenic acids are known as the esters and halides. Sulfinic acids
are formed by the reduction of the chlorides of sulfonic acids with zinc or by
the reaction of Grignard reagents on sulfur dioxide in ether solution.
Sulfonic acids (alkyl) are prepared by oxidizing mercaptans (RSH) or alkyl
sulfides with concentrated nitric acid, by treatment of sulfites with alkyl
halides, or by the oxidation of sulfinic acids. Other important organic
oxygen-sulfur-containing compounds include the sulfoxides, R2SO (which may be
considered as being derived from sulfurous acid), and the sulfones, R2SO2
(from sulfuric acid).
    Important halogen derivatives of sulfuric acid are the organic sulfonyl
halides and the halosulfonic acids. Halogen-sulfur compounds which have been
well characterized are S2F2 (sulfur monofluoride), SF2, SF4, SF6, S2F10, S2Cl2
(sulfur monochloride), SCl2, SCl4, and S2Br2 (sulfur monobromide). The sulfur
chlorides are used in the commercial manufacture of rubber; and the
monochloride, which is a liquid at room temperature, is also used as a solvent
for organic compounds, sulfur, iodine, and certain metal compounds.
 �34s[Stanley Kirschner]
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�6v�2v�32sSulfuric acid   �21m �33s 
    A strong mineral acid with the chemical formula H2SO4. It is a colorless, 
oily liquid, sometimes called oil of vitriol or vitriolic acid. The pure acid 
has a density of 1.834 at 25DGC (77DGF) and freezes at 10.5DGC (50.9DGF). It 
is an important industrial commodity, used extensively in petroleum refining 
and in the manufacture of fertilizers, paints, pigments, dyes, and explosives. 
    Sulfuric acid is produced on a large scale by two commercial processes, 
the contact process and the lead-chamber process. In the contact process, 
sulfur dioxide, SO2, dioxide is converted to sulfur trioxide, SO3, by reaction 
with oxygen in the presence of a catalyst. Sulfuric acid is produced by the 
reaction of the sulfur trioxide with water. The lead-chamber process depends 
upon the oxidation of sulfur dioxide by nitric acid in the presence of water, 
the reaction being carried out in large lead rooms. 
    Sulfuric acid reacts vigorously with water to form several hydrates. The 
concentrated acid, therefore, acts as an efficient drying agent, taking up 
moisture from the air and even abstracting the elements of water from such 
compounds as sugar and starch. The concentrated acid also acts as a strong 
oxidizing agent. It reacts with most metals upon heating to produce sulfur 
dioxide.  See  SULFUR. 
 �34s[Francis J. Johnston]
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�6v�2v�32sSumatra �21m �33s
    The largest western island of Indonesia: about 1600 mi (2575 km) long and
250 mi (400 km) across at the widest portion. It has an area of 166,800 mi(sup2)
(432,000 km[sup2]) and extends from 6DGN to 6DGS latitude. The island is made up
of three distinct physiographic sections. Tertiary fold mountains on the west
rise against the Sunda Platform. Peaks of these mountains reach from 3000 to
8000 ft (900 to 2400 m). East of these mountains there is a belt of low hills
of folded Tertiary formations. The eastern half of the island is composed of
extensive, swampy, alluvial lowlands, which form part of the Sunda Platform.
Important petroleum deposits are found near Palembang and Medan. Small islands
to the east of Sumatra - Lingga, Bangka, and Billiton - contain major tin
deposits associated with granites of the Sunda Platform.
    Most of Sumatra has an annual average rainfall of over 150 in. (3.8 m).
The western slopes of the mountains may have as much as 30 in. (75 cm) more,
but the intermontane basins receive less. This results in a tropical
rainforest in the uplands and a tropical swamp forest in the lowlands. See
EAST INDIES.
 �34s[M. G. Audley-Charles]
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�6v�2v�32sSun �21m �33s
    The star around which the Earth revolves, and the planet's source of light
and heat. The Sun is a globe of gas, 8.65 x 10(sup 5) mi (1.4 x 10[sup 6] km) in
diameter, held together by its own gravity. Because of the weight of the outer
layers, the density and temperature increase inward, until a central
temperature of over 27,000,000DGF (15,000,000 K) and density more than 90
times that of water is reached. At these great temperatures and densities,
thermonuclear reactions converting hydrogen into helium take place, releasing
the energy which streams outward.
    The surface temperature of the Sun is about 10,000DGF (6000 K); since
solids and liquids do not exist at these temperatures, the Sun is entirely
gaseous. Almost all the gas is in atomic form, although a few molecules exist
in the coolest regions at the surface.
    The Sun is a typical member of a numerous class of stars, the spectral
type dG2 (d indicates dwarf).

    Solar structure.  The interior of the Sun can be studied only by inference
from the observed properties of the entire star. A number of theoretical
models using different assumptions have led to more or less similar results. A
central density of near 3.25 lb/in(sup 3) (90 g/cm[sup 3]) has been found,
decreasing to 4 x 10(sup -9)lb/in(sup 3) (10[sup -7]g/cm[sup 3]) at the
surface. The central temperature is about 27,000,000DGF (15,000,000 K),
decreasing to 8500DGF (5000 K) at the surface. Since this takes place over
430,000 mi (700,000 km), the temperature gradient is only 60DGF per mile (20 K
per kilometer). The radiation produced at the center by nuclear interactions
flows outward rapidly. 
    The energy of the Sun is produced by the conversion of hydrogen into
helium. For each hydrogen atom converted, one neutrino is produced. These
neutrinos cannot be detected; only higher-energy neutrinos produced by
subordinate processes can be observed. These have been detected at the Earth,
but in smaller quantities than expected. However, the neutrino emission theory
has sufficient uncertainties so that the theory of nuclear burning is still
generally accepted.  See  SOLAR NEUTRINOS.
    Although the material at the center of the Sun is dense, the photons
created by nuclear reactions are continually absorbed and reemitted and thus
make their way to the surface. The atoms in the center of the Sun are entirely
stripped of their electrons by the high temperatures, and most of the
absorption is by continuous processes, such as scattering of light by
electrons.
    In the outer regions of the solar interior, the temperature is low enough
for ions and even neutral atoms to form and, as a result, atomic absorption
becomes very important. The high opacity makes it very difficult for the
radiation to continue outward; steep temperature gradients are established
which result in convective currents. Most of the outer envelope of the Sun is
in such convective equilibrium. These large-scale mass motions produce many
interesting phenomena at the surface, including sunspots and solar activity.

    Radiation. Electromagnetic energy is produced by the Sun in essentially
all wavelengths. However, more than 95% of the energy is concentrated in the
relatively narrow band between 290 and 2500 nanometers and is accessible to
routine observation from ground stations on Earth. The maximum radiation is in
the green region, and the eyes of human beings have naturally evolved to be
sensitive to this range of the spectrum. The total radiation received from the
Sun is termed the solar constant.  See	ELECTROMAGNETIC RADIATION; SOLAR
RADIATION.

    Atmosphere. Although the Sun is gaseous, it is seen as a discrete surface
from which practically all the heat and light are radiated. One sees through
the gas to the point at which the density is so high that the material becomes
opaque. This layer, the visible surface of the Sun, is termed the photosphere.
Light from farther down reaches the Earth by repeated absorption and emission
by the atoms, but the deepest layers cannot be seen directly. The surface is
actually not sharp; the density drops off by about a factor of 3 for each 90
mi (150 km). However, the Sun is so far away that the smallest distance that
can be resolved with the best telescope is about 300 km, and thus the edge
appears sharp.	See  SOLAR CORONA; PHOTOSPHERE.
    Looking at the Sun in isolated wavelengths absorbed by its atmospheric
gases, one can no longer see down to the photosphere, but instead one sees the
higher levels of the atmosphere, known as the chromosphere. This name results
from the rosy color seen in this region at a solar eclipse. The chromosphere
is a rapidly fluctuating region of jets and waves coming up from the surface.
When all the convected energy coming up from below reaches the surface, it is
concentrated in the thin material and produces considerable activity. As a
result, this outer region is considerably hotter than the photosphere, with
temperatures up to 54,000DGF (30,000 K).  See  CHROMOSPHERE.
    When the Moon obscures the Sun at a total solar eclipse, the vast extended
atmosphere of the Sun called the corona can be seen (Fig. 1). The corona is
transparent and is visible only when seen against the dark sky of an eclipse.
Its density is low, but its temperature is high. The hot gas evaporating out
from the corona flows steadily to the Earth and farther in what is called the
solar wind.  See  SOLAR WIND.
    The photospheric features of the Sun (sunspots and faculae) are readily
observed with a small telescope by projection of the solar image through the
eyepiece onto a shaded white card. This is the only safe method for observing
the Sun without a specially designed solar eyepiece. Observations of
chromospheric phenomena require special filters to exclude all light except
that emitted by the atmospheric gases. Observations of the corona require
either a total eclipse or a coronagraph.

    Coronal holes. Early coronal observations showed that the corona was
occasionally not visible over certain regions. In particular, most of the time
it was quite weak over the poles. X-ray pictures revealed great bands of the
solar surface essentially devoid of corona for many months. These proved to be
regions where the local magnetic fields were connected to quite distant
places, so the fields actually reached out to heights from which the solar
wind could sweep the gas outward. Analysis of solar wind data showed that
equatorial coronal holes were associated with high-velocity streams in the
solar wind, and recurrent geomagnetic storms were associated with the return
of these holes. Thus the great intensity of the corona over sunspot regions is
partly due to their strong, closed magnetic fields which trap the coronal gas.
  See  SOLAR MAGNETIC FIELD.



    Solar activity.  The transient phenomena, known collectively as solar
activity, are all connected with sunspots or their remnants, and wax and wane
in a remarkable cycle of activity. The sunspot cycle consists of variations in
the sizes, numbers, and positions of the sunspots, expressed quantitatively as
the sunspot number. The number of sunspots peaks soon after the beginning of
each cycle and decays to a minimum in 11 years. The magnetic polarity of the
sunspot groups reverses in each successive cycle so that the complete cycle
lasts 22 years. The first spots of a cycle always occur at higher latitudes,
between 20 and 35DG, and as the spots increase in size they occur closer to
the equator. Almost no spots are observed outside the latitude range of 5 -
35DG. The average duration of the cycle from the first appearance of the last
low-latitude spots is nearly 14 years. The difference between these two
figures is the result of overlap of consecutive cycles.

    Sunspots. Sunspots are the most conspicuous features of solar activity and
are easily seen through a small telescope. They are dark areas on the Sun
produced by the most intense magnetic fields (see Fig. 2). The magnetic fields
produce a cooling of the surface, most likely by suppression of the normal
convection which transports energy from the lower levels. Deprived of this
supply of heat, the area cools by radiation and becomes a dark sunspot.
Curiously enough, the region close to the cool sunspot is the scene of the
hottest and most intense activity because of the great magnetic energy present
in the sunspot.  See  SUNSPOT.

    Prominences. Although prominences appear dark against the disk, they
appear bright against the dark sky. They occur only in regions of horizontal
magnetic fields, because these fields support them against the solar gravity.
Thus filaments on the disk are good markers of the transition from one
magnetic polarity to the opposite. Prominences are among the most beautiful of
solar phenomena.

    Plages. Just as prominences occur when the magnetic field changes from one
sign to the other, plages occur whenever the magnetic field is vertical and
relatively strong. They are bright regions visible in any strong spectrum
line. It is now believed that the field is, in fact, stable only when it
exceeds 1000 gauss (0.1 tesla), and the difference between apparently weaker
field regions, such as the chromospheric network, and plages is only the
density of clumps of strong field. Plages are normally associated with
sunspots; a typical active region will have the preceding magnetic field
clumped in a sunspot and the following field spread out in a plage.

    Flares. The most spectacular activity associated with sunspots is the
solar flare (Fig. 3). A flare is defined as an abrupt increase in the H-ALPHA
emission from the sunspot region. The brightness of the flare may be five
times that of the associated plage; the rise time is seldom longer than a few
minutes, sometimes only 10 s. Of course, the hydrogen brightening is only a
symptom; observations by other means show that there is a tremendous energy
release, of as much as 10(sup 33) ergs (10[sup 26] J) in a large flare. An
active sunspot group will show many small flares the size of a large sunspot.
Once or twice in the lifetime of a large group, a great flare will occur
covering the entire region. The energy released in the three or four great
flares each year equals that in all the small flares put together.
    Solar flares occur only as the result of sunspot activity. Practically all
sunspots produce some flares, but certain spots are prodigiously active. The
source of the flare energy is the magnetic fields surrounding the sunspots.
Every magnetic field has a certain minimum energy form, normally that where no
currents flow, a so-called potential field. But if the fields have been
strongly twisted, currents are set up which stabilize the twisted fields until
they become unstable and jump to lower energy states, producing flares.
 �34s[Harold Zirin]
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�6v�2v�32sSundew �21m �33s
    Any plant of the genus Drosera (90 species) of the family Droseraceae.
Sundews are small, herbaceous, insectivorous plants that grow on all the
continents, especially Australia. Numerous glandular hairs (tentacles) on the
leaf secrete a viscous fluid which traps a visiting insect. The tentacles then
bend inward about the victim, bringing it into contact with the surface of the
leaf where it is digested. The droplets secreted by the glands on the leaves
glitter like dewdrops in the morning sunlight; hence, the name, sundew.  See
INSECTIVOROUS PLANTS; SARRACENIALES; SECRETORY STRUCTURES (PLANT).
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sSundial   �21m �33s 
    An instrument for telling time by the Sun. It is composed of a style that 
casts a shadow and a dial plate, which is the surface upon which hour lines 
are marked and upon which the shadow falls. The style lies parallel to Earth's 
axis. The construction of the hour lines is based on the assumption that the 
apparent motion of the Sun is always on the celestial equator. The most widely 
used form is the horizontal dial that indicates local apparent time (Sun 
time). Other forms of the sundial indicate local mean time, and standard time. 
  See  TIME. 
 �34s[Robert N. Mayall]
�33s 
  
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�6v�2v�32sSundog   �21m �33s 
    One of two bright spots (or parhelia) which are seen on both sides of the 
Sun or Moon, usually with red coloring on the part closer to the Sun. They are 
produced by prismatic refraction on the alternate sides of the hexagonal ice 
crystals floating in the air with vertical axes of symmetry. For the Sun at 
the horizon they are situated in the inner halo; with higher Sun they approach 
the outer halo. The parhelia are the second most frequently observed halo 
phenomena.  See  HALO. 
 �34s[Zdenek Sekera]
�33s 
  
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�6v�2v�32sSunfish   �21m �33s 
    A name given to certain members of the fresh-water family Centrarchidae as 
well as to members of the marine family Molidae. There are two species of 
oceanic sunfishes in the genus Mola; both are large, stout, deep-bodied fishes 
with rough skin, and are found in warm seas, where they are frequently seen 
near the surface. These fishes appear to be tailless. 
    The fresh-water sunfishes are grouped together with the crappies. The 
warmouth sunfish (Chaenobryttus gulosus) is the only species found in the 
United States, where it occurs in the Mississippi River drainage and in 
streams along the Atlantic coast. It differs from other sunfishes in having 
teeth on the tongue. 
    The crappies are game fish of considerable importance. Crappies are a 
favorite species for pond culture. They can be readily transplanted and, under 
favorable conditions, multiply prodigiously.  See  PERCIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sSunflower   �21m �33s 
    Helianthus annuus, the most widely distributed of the 50 native North 
American species of this genus of the family Compositae. It is an extremely 
variable species, with two main divisions. The first involves wild weedy 
plants found along roadways and other recently disturbed areas; the second, 
domesticated plants grown in fields and gardens.  See  ASTERALES. 
    Within the domestic type there are two categories of plants: the 
ornamental, which has a few branches with larger heads than the wild, and the 
crop type, which has only a single stem and the largest head of all sunflowers 
(see illustration). Crop types are either oil or nonoil. Plant breeders have 
modified the plant for adaptation to modern, large-scale farming and have 
increased the oil content of the seeds. The present worldwide interest in 
growing sunflowers as a crop is due to the increased yield of the new 
commercially available oilseed hybrids. 
    Sunflowers are grown on all the continents and in many countries 
throughout the world. The Soviet Union is the major producer, followed by 
Argentina, the United States, and Canada. Sunflower oil (sunoil) is the second 
most important vegetable crop oil. It is a high-quality oil and is high in 
linoleic fatty acid. The oil is used in cooking, salad dressing, mayonnaise, 
margarine, and soap. 
 �34s[Benjamin H. Beard]
�33s 
  
  
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�6v�2v�32sSunlamp   �21m �33s 
    A special form of mercury arc discharge lamp designed to produce 
ultraviolet radiation. These lamps also produce some radiant energy in the 
visible region of the spectrum, thus having a light output as well as an 
ultraviolet output. The lamps are principally used for producing a skin tan on 
the human body. The less common uses include therapeutically producing vitamin 
D in the body for the treatment of rickets and causing fluorescence or 
photochemical reactions. 
    Sunlamps have ultraviolet radiation at wavelengths above 280 nanometers. 
The lower limit is set by the fact that the quartz and high-silica glass used 
for lamp envelopes do not transmit below 280 nm. The two lamps designed for 
tanning are the FS-40 and the RS. The FS-40 tubular fluorescent lamp 
(illustration a) operates with a low-pressure mercury arc that causes a 
special chemical phosphor coating inside the tube to radiate ultraviolet 
energy. The RS sunlamp (illustration b) is a reflector unit containing a 
high-pressure mercury arc tube for generating ultraviolet energy plus a 
tungsten filament (similar to incandescent lamp filaments) in series with the 
arc tube to serve as a ballast.  See  MERCURY-VAPOR LAMP; ULTRAVIOLET LAMP. 
 �34s[G. R. Peirce]
�33s 
  
  
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�6v�2v�32sSunspot   �21m �33s 
    A dark area in the photosphere of the Sun caused by a lowered surface 
temperature. The temperature at the center of a spot is about 4000 K 
(7200DGF), and the surface brightness is one-fifth that of the normal 
photosphere. The sizes and numbers of sunspots vary in the 11-year sunspot 
cycle, which is shared by all other forms of solar activity.  See  SUN. 
 �34s[John W. Evans]
�33s 
  
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�6v�2v�32sSuperacid �21m �33s
    An acid which has an extremely great proton-donating ability. It has
proved convenient to define a superacid somewhat arbitrarily as an acid, or
more generally, an acidic medium, which has a proton-donating ability equal to
or greater than that of anhydrous (100%) sulfuric acid.
    Superacids belong to the general class of proton or Broensted acids. A
proton acid is defined as any species which can act as a source of protons and
which will therefore protonate a suitable base, as in reaction (1).

		HA + B REVERSIBLE REACTION   BH+ + A-			  (1)

    The strengths of acids are often compared by measuring the extent of their
ionization in water, that is, the extent to which they can protonate the base
water, as in reaction (2).

		  HA + H2O REVERSIBLE REACTION	H3O+ A- 		   (2)

    However, all strong acids are fully ionized in dilute aqueous solution,
and they therefore appear to have the same strength. Their strengths are said
to be reduced or leveled to that of the hydronium ion (H3O+), which is the
most highly acidic species that can exist in water. In any case, many of the
superacids react with and are destroyed by water. For these reasons, the
strengths of superacids cannot be measured by the conventional means of
utilizing their aqueous solutions. The acidities of superacids can, however,
be conveniently measured in terms of the Hammett acidity function.  See  ACID
AND BASE.
    The Hammett acidity function is a method of measuring acidity based on the
determination of the ionization ratios of suitable weak bases (indicators),
usually by means of the change in absorption spectrum that occurs on
protonation of the base, although the nuclear magnetic resonance (NMR)
spectrum has also been used. The Hammett acidity function (H0) is defined by
Eq. (3), where K(sub BH+) is the dissociation constant of the acid form of the


	H(sub 0) = pK(sub BH+) - log ([BH+]/[B])			 (3)


indicator and [BH+]/[B] is the ionization ratio of the indicator. Hammett
acidity function (H sub 0) values for a number of superacids are given in the
table.	In each case the value refers to the 100% (anhydrous) acid. Each of the
superacids in the table is a liquid at room temperature, and each forms the
basis of a solvent system.  See  IONIC EQUILIBRIUM; SOLUTION.
 �34s[Ronald J. Gillespie]
�33s

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�6v�2v�32sSupercavitating propeller   �21m �33s 
    A propeller designed to overcome the limitation on ship speed caused by 
loss of thrust when cavitation is severe. In this design, cavitation on the 
backs of the blades (forward side) is induced by special blade sections at 
relatively low forward speed so that, when revolutions and engine power are 
increased, the whole back of each blade becomes enveloped by a sheet of 
cavitation. When this is completed, further increase in thrust at still higher 
engine power and rpm is obtained by the increase in positive pressure on the 
blade face (rear side); erosion is avoided because the collapse of the 
cavitation bubbles occurs some distance behind the trailing edges of the 
blades. A photograph of such a supercavitating propeller in action is shown in 
the illustration. The supercavitating propeller is no replacement for the 
conventional propeller, being suitable only for very high ship and engine 
speeds. See CAVITATION; PROPELLER (MARINE CRAFT). 
 �34s[Karl E. Schaefer]
�33s 
  
  
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�6v�2v�32sSupercharger   �21m �33s 
    An air pump or blower in the intake system of an internal combustion 
engine. Its purpose is to increase the air charge weight and power output from 
a given engine size. In an aircraft engine, the supercharger counteracts the 
power loss resulting from decreasing atmospheric pressure with increase of 
altitude.  See  INTERNAL COMBUSTION ENGINE. 
    In the case of the diesel engine, supercharging makes ignition of the fuel 
easier, and it does not require a fuel of better quality, as in the 
spark-ignited engine. Two-stroke-cycle engines always require some form of 
blower or air compressor to assist in charging the cylinder. However, only if 
the air pressure in the cylinder at the beginning of compression is 
substantially above atmospheric is the engine said to be supercharged. Large 
diesel engines all employ the two-stroke cycle, and are supercharged in order 
to be commercially competitive.  See  DIESEL ENGINE. 
    To enable a piston-type aircraft engine to develop its rated sea-level 
horsepower at altitude, a supercharger must be used to increase the pressure 
and weight of the intake air charge. Centrifugal compressors are used for this 
duty because of their relatively small size for a given capacity, and are 
driven either by a gear drive from the crankshaft or by a gas turbine powered 
from the engine exhaust.  See  RECIPROCATING AIRCRAFT ENGINE. 
 �34s[Frank C. Mock /J. A. Bolt]
�33s 
  
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�6v�2v�32sSuperclusters �21m �33s
    Associations of galaxy clusters and groups, typically composed of a few
rich clusters and many poorer groups and isolated galaxies. Most investigators
find superclusters to be arranged in broad sheets and chains, and some believe
that they are connected, forming cells which enclose nearly empty volumes of
space. If so, the large-scale structure of luminous matter in the universe is
therefore apparently best visualized as spongelike, at least on scales up to
about 10(sup 9) light-years (9.46 x 10(sup 24) m), which is about as far as
currently available data reliably extend.
    In 1784 William Herschel noted that most of the ``nebulae'' he had
discovered seemed to be concentrated in a wide belt across the northern sky,
oriented roughly perpendicularly to the Milky Way. However, it was 1953 before
Gerard de Vaucouleurs realized that observers on Earth are simply seeing from
the inside a great flattened system of groups and clusters of galaxies, with a
few individual objects scattered between. De Vaucouleurs called this the Local
Supercluster.
    The illustration shows the apparent positions of all galaxies in the
northern galactic hemisphere with known radial velocities less than 2000 km/s
(1243 mi/s). These objects are the Galaxy's nearest neighbors in extragalactic
space, and most are members of the Local Supercluster, which shows well as the
band of objects running diagonally across the illustration. The Virgo Cluster
is the concentration of points in this band to the upper left of center. The
cross shows the point toward which de Vaucouleurs' analysis suggests the
Galaxy is moving.
    Besides the Local Supercluster, there are perhaps 50 other superclusters
that have been noted. These have been found primarily through examination of
the various surveys of faint galaxies and of clusters of galaxies.  See
GALAXY, EXTERNAL.
 �34s[Harold G. Corwin, Jr.]
�33s


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�6v�2v�32sSuperconducting computers   �21m �33s 
    High-performance computers whose circuits employ superconductivity and the 
Josephson effect to reduce cycle time. Presently, high-performance, 
synchronous digital computers have operating cycle times in the range of 12 to 
60 nanoseconds. Josephson technology and superconductivity are being explored 
as ways of reducing the cycle time further; initially the goal is to reduce 
the cycle time below 4 ns, and ultimately it is thought that a subnanosecond 
cycle time will be possible. 
  
    Cycle time.   There are principally two components to the cycle time of a 
computer: the circuit delay and the package delay. Usually, for a given 
technology, higher circuit speed can be obtained by increasing dissipation or 
reducing circuit dimensions. However, there are two principal limitations to 
available circuit speed, both of which are related to power requirements.  See 
  INTEGRATED CIRCUITS. 
    The first is associated with the wasted power: the power dissipated in the 
form of heat. This has to be removed by a cooling medium, and the level of 
heat removal from circuit chips is limited on the one hand by the cost and 
complexity of the heat removal structures, and on the other hand by the 
allowable rise in temperature that the circuits can sustain while still 
operating correctly. Heat removal structures usually also limit the volumetric 
efficiency of packing circuit chips. 
    The second limitation to the power comes from the circuit engineers' 
ability to provide regulated power to the circuit chips. A large fraction of 
the chip input/output connections is devoted to power input - to carry the 
current, but even more importantly to reduce the power voltage swings caused 
by varying current drain on the chip as the circuits change their 
binary-encoded state under program control. This inductive variation of chip 
voltage becomes an increasingly difficult problem to solve as the circuit chip 
performance is improved. Of the two principal power limitations to circuit 
performance, this one appears the most demanding and severe. 
    The package delay component of the cycle time depends on the way the 
integrated circuit chips are arranged. The ultimate goal of high-performance 
computer packaging schemes is to achieve the highest possible density of 
circuits in a given volume, and this not only means high circuit density on 
each chip, but also high-density packing of chips. It is necessary to approach 
as closely as possible the ideal of a three-dimensional arrangement, and a 
good approximation is the card-on-board package, where chips are attached to 
both sides of the card, and the cards in turn are plugged closely together 
into the board. 
  
    Josephson technology.   The Josephson effect was quickly identified as 
being the basis of a very promising computer technology. Initially, the very 
high switching speed and low power dissipation of this device were the main 
attractions, but it soon developed that many other aspects of the physics and 
engineering were very favorably inclined toward superconducting computer 
technology. Principal among these was the essentially 
lossless-transmission-line characteristics of all superconducting circuit 
interconnection lines.  See  JOSEPHSON EFFECT. 
    The power removal problem is no longer a concern for such circuits. This 
has enabled particularly attractive and novel techniques to be applied to 
power supply and regulation concerns faced by the circuit designer; active 
power supply and regulation circuits can be incorporated into the on-chip 
design, thereby removing the problem of inductive variation of chip voltage. 
    These advantages naturally accrue from the basic physics of the 
superconducting state and the Josephson effect. The energy gap that 
characterizes these devices has shrunk from the hundreds of meV of the 
semiconductor electronic devices to just a few meV with the superconducting 
Josephson devices. 
    With little or no cooling constraint, simple attachment of 
high-performance chips to the package is feasible; there are no heat removal 
structures required, and the package can be fabricated in a high-density 
three-dimensional card-on-board arrangement. Josephson technology also makes 
possible the high-density superconducting interconnection lines and the 
ability, for the first time, to make a monolithic package in the sense that 
the package components are all made from the same substrate material: silicon. 
This has significant importance in that it allows the engineer to work with 
little concern for differential thermal expansion in the package design. 
    Much detailed engineering and materials work remains to be done before a 
superconducting computer becomes a reality. Josephson devices are not simple 
extensions of transistor circuits. They involve a much more significant break 
with the past than there was in going from the triode vacuum tube to the 
transistor. The biggest challenge is perhaps the sheer magnitude of the tasks 
involved in introducing a fundamentally new technology: chip technology, 
circuit design, package technology, design techniques, testing, and so forth. 
    There are also obvious difficulties associated with the use of liquid 
helium. Cooling the computer from 300 to 4.2 K (80 to -452DGF) will 
undoubtedly produce stresses. These have to be understood and taken care of; 
they affect the choice of materials both for the devices and for the package.  
See  SUPERCONDUCTING DEVICES. 
 �34s[Dennis J. Herrell]
�33s 
  
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�6v�2v�32sSuperconducting devices �21m �33s
    Devices that perform functions in the superconducting state that would be
difficult or impossible at room temperature, or that contain components which
perform such functions.  See  SUPERCONDUCTIVITY.

    Small-scale devices.  The unique properties of superconductors have led to
the development of tiny measuring and computing devices that are superior in
performance to their nonsuperconducting counterparts. Most of the devices
operate at or below 4.2 K (-452.0DGF) and involve Josephson tunneling. The
initial development of these devices took place in the second half of the
1960s. Fabrication of the devices has increasingly involved photolithography
and electron-beam lithography, and superconducting junctions and circuits are
now at the forefront of electronic ultraminiaturization.
    There are two types of superconducting quantum interference device (SQUID)
for detecting changes in magnetic flux: the dc SQUID and the rf SQUID. The dc
SQUID, which operates with a dc bias current, consists of two Josephson
junctions incorporated into a superconducting loop. The rf SQUID consists of a
single Josephson junction incorporated into a superconducting loop and
operates with a rf bias. Although rf SQUIDS have been used more widely than dc
SQUIDs, their sensitivity is considerably lower, and it seems unlikely that
their performance will be able to match that of the dc SQUID.  See  SQUID.
    Typical high-speed digital computers, using large-scale integration of
semiconductor devices, have cycle times of 30 to 50 nanoseconds. To achieve
the goal of further reducing this time to 1 ns, both the device switching time
and the time required to transmit a signal between different parts of the
computer must be less than 1 ns. Because the signal propagation speed is
typically 10sup8 m.s-sup1, the second requirement implies that the largest
dimension of the computer must be no greater than 0.1 m. Although
semiconducting devices with switching times substantially less than 1 ns are
available, they dissipate considerable amounts of power. Because of the
difficulty of extracting this power from a small volume, it does not appear
feasible to reduce the physical size of a main-frame computer sufficiently to
achieve a cycle time of 1 ns. On the other hand, preliminary studies of
Josephson junction computer elements established that these devices had the
combined requirements of very high switching speed and very low dissipation.
See  INTEGRATED CIRCUITS.
    The Josephson effects have an established role in standards laboratories.
The most important applications are the measurement of the fundamental
constant ratio e/h, and maintaining the standard volt. When a Josephson
junction is irradiated with microwaves of frequency f, constant-voltage steps
are induced on the I-V characteristic at voltages nhf/2e, where n is an
integer. Precise measurements of the voltages at which the steps are induced
by a known frequency have led to the most accurate determination available of
e/h. Furthermore, the standard volt at the United States National Bureau of
Standards and at a number of other national laboratories is maintained (but
not defined) by these voltage steps.  See  ATOMIC CONSTANTS; ELECTRICAL
MEASUREMENTS.
    The superconductor-insulator-superconductor (SIS) quasiparticle mixer is a
superconducting detector of microwave radiation. The device consists of a
small-area tunnel junction, fabricated from lead alloys, with the I-V
characteristic shown in the illustration. The mixer is operated near the sharp
onset in the current, where the characteristics are highly nonlinear. The
nonlinearity is used to mix the signal frequency with the local oscillator
frequency to produce an intermediate frequency that is coupled out of the
junction into a low-noise preamplifier. Such receivers are likely to have a
major impact on radio astronomy, and could be of great importance in such
applications as space communications.  See  RADIO ASTRONOMY; SPACE
COMMUNICATIONS.
 �34s[John H. Clarke]
�33s

    Large-scale devices.  Large-scale superconducting devices comprise
magnets, motors, generators, and cables using zero resistance superconducting
windings. Typical values of current densities used in superconducting coils
are 10(sup 9) A/m(sup 2) or more - about two orders of magnitude higher than
values used in normal conductors, resulting in a large reduction in size and
weight as well as in power consumption. 
    The most commonly used superconducting alloy is the niobium-titanium alloy
NbTi. This consists of titanium impurities in solution in niobium, the effect
ot the titanium impurities being to transform the niobium into a strong
superconductor. Commercial niobium-titanium wires are composed of thin
niobium-titanium filaments, typically 25 micrometers in diameter, embedded in
a copper matrix. The purpose of the copper matrix is to stabilize the
superconducting wire: it provides a parallel low-resistance conducting path
for the case where a normal spot would appear accidentally along one of the
niobium-titanium filaments, thus avoiding an avalanche effect that could
quench the whole superconducting coil.
    Magnetic fields are used extensively in high-energy physics to accelerate,
bend, focus, and store particle beams and to detect and identify elementary
particles. Superconducting magnets allow the utilization of higher fields in
larger volumes at a lower cost in capital investment and energy expenses. As
an example, the bubble chamber magnet at CERN would require a power of 70 MW
with conventional coils, while the superconducting version consumes less than
1 MW.
    Electric power generation has emerged as one of the most promising areas
of application of superconductivity. Because of the losses occurring in a
superconductor submitted to a high-intensity ac field, superconducting ac
generators have a superconducting rotor and a normal armature. The advantages
of the superconducting machine over conventional generators derive from the
absence of electrical losses in the rotor and from the higher magnetic field
intensity provided by the superconducting coils.  See  ALTERNATING-CURRENT
GENERATOR; GENERATOR.
    Short sections of flexible niobium-based superconducting ac cables have
been successfully tested. However, due to large refrigeration costs, it is
only at ratings of the order of 5 GVA that they could compete with
conventional forced-cooling cables.  See  DIRECT-CURRENT TRANSMISSION;
ELECTRIC POWER TRANSMISSION; TRANSMISSION LINES.
    Very large superconducting magnets with stored energies of the order of 10
gigajoules or more have several areas of application, including MHD power
plants, controlled fusion reactors and energy storage.
    Among the most promising applications for superconducting magnets for the
short term are motors for marine propulsion. The much smaller weight of the
superconducting motor makes electrical propulsion practical.
    One of the most spectacular applications of superconducting magnets is for
levitated trains for high-speed transportation. Their principle of operation
is quite simple: When a magnet moves over an electrically conducting medium
(such as a metallic sheet), the excited eddy currents are equivalent to an
image magnet of opposite polarity located beneath the conducting sheet.
Repulsion between the magnet and its image results in levitation of the
magnet, provided the field intensity and the speed of motion of the magnet are
high enough.
 �34s[Guy Deutscher]
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�6v�2v�32sSuperconductivity �21m �33s
    A phenomenon occurring at very low temperatures in many electric
conductors, in which the electrons responsible for conduction undergo a
collective transition to an ordered state with many unique and remarkable
properties. These include the vanishing of resistance to the flow of electric
current, the appearance of a large diamagnetism and other unusual magnetic
effects, substantial alteration of many thermal properties, and the occurrence
of quantum effects otherwise observable only at the atomic and subatomic level.
    Superconductivity was discovered by H. Kamerlingh Onnes in 1911, while
studying the variation with temperature of the electrical resistance of
mercury within a few degrees of absolute zero. He observed that the resistance
dropped sharply to an unmeasurably small value at a temperature of 4.2 K
(-452.0DGF). The temperature at which the transition occurs is called the
transition or critical temperature T(sub c).
    In 1933 W. Meissner and R. Ochsenfeld discovered that a metal cooled into
the superconducting state in a not-too-large magnetic field expels the field
from its interior. This discovery demonstrated that superconductivity involves
more than simply very high or infinite electrical conductivity, remarkable as
that alone is.	See  MEISSNER EFFECT.
    In 1957, J. Bardeen, L. N. Cooper, and J. R. Schrieffer reported the first
successful microscopic theory of superconductivity. It described how and why
the electrons in a conductor may form an ordered superconducting state, and
makes predictions about many properties of superconductors which are in good
agreement with experimental information.

    Occurrence.  Some 26 of the metallic elements are known to be
superconductors in their normal forms, and another 10 become superconducting
under pressure or when prepared in the form of highly disordered thin films
(Fig. 1). The number of known superconducting compounds and alloys runs into
the thousands. Superconductivity is thus a rather common characteristic of
metallic conductors.
    Despite the existence of a successful microscopic theory of
superconductivity, there are no completely reliable rules for predicting
whether a metal will be a superconductor. Certain trends and correlations are
apparent among the known superconductors, however - some with obvious bases in
the theory - and these provide empirical guidelines in the search for new
superconductors.
    The ordered superconducting state appears to be incompatible with any
long-range-ordered magnetic state: None of the ferromagnetic or
antiferromagnetic metals are also superconducting. The presence of nonmagnetic
impurities in a superconductor usually has very little effect on the
superconductivity, but the presence of impurity atoms which have localized
magnetic moments can markedly depress the transition temperature even in
concentrations as low as a few parts per million.
    The highest transition temperature observed is 23 K (-418DGF) for a
specially prepared alloy of niobium, aluminum, and germanium. There is no
widely accepted theoretical proof that superconductivity is necessarily
restricted to low temperatures.

    Magnetic properties.  The existence of the Meissner-Ochsenfeld effect, the
exclusion of a magnetic field from the interior of a superconductor, is direct
evidence that the superconducting state is not simply one of infinite
electrical conductivity. Instead, the Meissner-Ochsenfeld effect implies that
the superconducting state is a true thermodynamic equilibrium state, a new
phase which has lower free energy than the normal state of temperatures below
the transition temperature and which somehow requires the absence of magnetic
flux.
    The exclusion of magnetic flux by a superconductor costs some magnetic
energy. So long as this cost is less than the condensation energy gained by
going from the normal to the superconducting phase, the superconductor will
remain completely superconducting in an applied magnetic field. If the applied
field becomes too large, the cost in magnetic energy will outweigh the gain in
condensation energy, and the superconductor will become partially or totally
normal. The manner in which this occurs depends on the geometry and the
material of the superconductor. Consider first the simplest geometry, a very
long cylinder with field applied parallel to its axis. Distinct types of
behavior may then occur, depending on the type of superconductor.

    Thermal properties.  The appearance of the superconducting state is
accompanied by quite drastic changes in both the thermodynamic equilibrium and
thermal transport properties of a superconductor.
    Figure 2 shows the heat capacity of an aluminum specimen in both the
normal and superconducting states. In the normal state (produced at
temperatures below the transition temperature by applying a magnetic field
greater than the critical field), the heat capacity is determined primarily by
the normal electrons and is nearly proportional to the temperature. In zero
applied magnetic field, there appears a discontinuity in the heat capacity at
the transition temperature. At temperatures just below the transition
temperature, the heat capacity is larger than in the normal state. It
decreases more rapidly with decreasing temperature, however, and at
temperatures well below the transition temperature varies exponentially as
e(sup -DELTA/kT), where DELTA is a constant and k is Boltzmann's constant.
    Ordinarily a large electrical conductivity is accompanied by a large
thermal conductivity. However, the thermal conductivity of a pure
superconductor is less in the superconducting state than in the normal state,
and at very low temperatures approaches zero. Crudely speaking, the
explanation for the association of infinite electrical conductivity with
vanishing thermal conductivity is that the transport of heat requires the
transport of disorder (entropy). The superconducting state is one of perfect
order (zero entropy), and so there is no disorder to transport and therefore
no thermal conductivity.  See  ENTROPY.

    Two-fluid model.  C. J. Gorter and H. B. G. Casimir introduced in 1934 a
phenomenological theory of superconductivity based on the assumption that in
the superconducting state there are two components of the conduction electron
``fluid'' (hence the name given this theory, the ``two-fluid model''). One,
called the superfluid component, is an ordered condensed state with zero
entropy, hence is incapable of transporting heat. It does not interact with
the background crystal lattice, its imperfections, or the other conduction
electron component and exhibits no resistance to flow. The other component,
the normal component, is composed of electrons which behave exactly as they do
in the normal state. It is further assumed that the superconducting transition
is a reversible thermodynamic phase transition between two thermodynamically
stable phases, the normal state and the superconducting state, similar to the
transition between the liquid and vapor phases of any substance. The validity
of this assumption is strongly supported by the existence of the
Meissner-Ochsenfeld effect and by other experimental evidence. This assumption
permits the application of all the powerful and general machinery of the
theory of equilibrium thermodynamics. The results tie together the observed
thermodynamic properties of superconductors in a very satisfying way.

    Microscopic (BCS) theory.  The key to the basic interaction between
electrons which gives rise to superconductivity was provided by the isotope
effect. It is an interaction mediated by the background crystal lattice and
can crudely be pictured as follows: An electron tends to create a slight
distortion of the elastic lattice as it moves, because of the Coulomb
attraction between the negatively charged electron and the positively charged
lattice. If the distortion persists for a brief time (the lattice may ring
like a struck bell), a second passing electron will see the distortion and be
affected by it. Under certain circumstances, this can give rise to a weak
indirect attractive interaction between the two electrons which may more than
compensate their Coulomb repulsion.
    The first step toward a theory based on the interaction was taken by
Cooper in 1956, when he showed that two electrons with an attractive
interaction can bind together to form a ``bound pair'' (often called a Cooper
pair) if they are in the presence of a high-density fluid of other electrons,
no matter how weak the interaction is. The two partners of a Cooper pair have
opposite momenta and spin angular momenta. Then, in 1957, Bardeen, Cooper, and
Schrieffer showed how to construct a wave function in which all of the
electrons (at least, all of the important ones) are paired. Once this wave
function is adjusted to minimize the free energy, it can be used as the basis
for a complete microscopic theory of superconductivity.
As
    The successes of the BCS theory and its subsequent elaborations are
manifold. One of its key features is the prediction of an energy gap.
Excitations called quasiparticles (which are something like normal electrons)
can be created out of the superconducting ground state by breaking up pairs,
but only at the expense of a minimum energy of DELTA per excitation; DELTA is
called the gap parameter. The original BCS theory predicted that DELTA is
related to T(sub c) by DELTA = 1.76 kT(sub c) at T = 0 for all superconductors.
This turns out to be nearly true, and where deviations occur they are understood
in terms of modification of the BCS theory. The manifestations of the energy gap
in the low-temperature heat capacity and in electromagnetic absorption provide
strong confirmation of the theory.
 �34s[D. N. Langenberg]
�33s

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�6v�2v�32sSupercritical wing   �21m �33s 
    A wing with special streamwise sections, or airfoils, which provide 
substantial delays in the onset of the adverse aerodynamic effects which 
usually occur at high subsonic flight speeds. 
    When the speed of an aircraft approaches the speed of sound, the local 
airflow about the airplane, particularly above the upper surface of the wing, 
may exceed the speed of sound. Such a condition is called supercritical flow. 
On previous aircraft, this supercritical flow resulted in the onset of a 
strong local shock wave above the upper surface of the wing (illustration a). 
This local wave caused an abrupt increase in the pressure on the surface of 
the wing, which may cause the surface boundary-layer flow to separate from the 
surface, with a resulting severe increase in the turbulence of the flow. The 
increased turbulence leads to a severe increase in drag and loss in lift, with 
a resulting decrease in flight efficiency. The severe turbulence also caused 
buffet or shaking of the aircraft and substantially changed its stability or 
flying qualities.  See  AERODYNAMIC FORCE; AERODYNAMIC WAVE DRAG; LOW-PRESSURE 
FLUID FLOW; SHOCK WAVE; TRANSONIC FLIGHT. 
    Supercritical airfoils are shaped to substantially reduce the strength of 
the shock wave and to delay the associated boundary-layer separation 
(illustration b). Since the airfoil shape allows efficient flight at 
supercritical flight speeds, a wing of such design is called a supercritical 
wing.  See  AIRPLANE; WING. 
 �34s[Richard T. Whitcomb]
�33s 
  
  
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�6v�2v�32sSuperfluidity   �21m �33s 
    The frictionless flow of liquid helium at low temperature; also, the flow 
of electric current without resistance in certain solids at low temperature 
(superconductivity).  
    Both helium isotopes have a superfluid transition, but the detailed 
properties of their superfluid states differ considerably because they obey 
different statistics. Hesup4, with an intrinsic spin of 0, is subject to 
Bose-Einstein statistics, and Hesup3, with a spin of 1/2, to Fermi-Dirac 
statistics. There are two distinct superfluid states in Hesup3 called A and B. 
    The term ``superfluidity'' usually implies He II or the A and B phases of 
Hesup3, but the basic similarity between these and the ``fluid'' consisting of 
pairs of electrons in superconductors is sufficiently strong to designate the 
latter as a charged superfluid. Besides flow without resistance, superfluid 
helium and superconducting electrons display quantized circulating flow 
patterns in the form of microscopic vortices.  See  BOSE-EINSTEIN STATISTICS; 
LIQUID HELIUM; QUANTIZED VORTICES; SECOND SOUND; SUPERCONDUCTIVITY. 
 �34s[Lawrence J. Campbell]
�33s 
  
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�6v�2v�32sSupergiant star   �21m �33s 
    A member of the family containing the intrinsically brightest stars, 
populating the top of the Hertz-sprung-Russell diagram. Supergiant stars occur 
at all temperatures, from 54,000 to 5000DGF (30,000 to 3000 K), and have 
luminosities ranging from 10sup4 to 10sup6 times that of the Sun. The hot 
supergiants have radii 20 times that of the Sun. The cool supergiants are the 
largest known stars, reaching several thousand solar radii. Among the bright 
supergiants are Deneb (ALPHA-Cygni), Rigel (BETA-Orionis), and Betelgeuse 
(ALPHA-Orionis). Because few have masses exceeding 20 - 40 Suns, the ratio of 
luminosity to mass is high. Thermonuclear energy sources will therefore be 
rapidly exhausted, hydrogen being consumed in a time scale of only 10sup6 to 
10sup7 years. The supergiants are thus young but rapidly evolving massive 
stars. Nearly all are slightly variable in light and radial velocity, have 
highly turbulent and extended atmospheres, and show broadened spectral lines. 
See HERTZSPRUNG-RUSSELL DIAGRAM; STAR; STELLAR EVOLUTION. 
 �34s[Jesse L. Greenstein]
�33s 
  
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�6v�2v�32sSuperheater   �21m �33s 
    A component of a steam-generating unit in which steam, after it has left 
the boiler drum, is heated above its saturation temperature. The amount of 
superheat added to the steam is influenced by the location, arrangement, and 
amount of superheater surface installed, as well as the rating of the boiler. 
The superheater may consist of one or more stages of tube banks arranged to 
effectively transfer heat from the products of combustion.  See  
STEAM-GENERATING UNIT. 
 �34s[George W. Kessler]
�33s 
  
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�6v�2v�32sSuperheterodyne receiver   �21m �33s 
    A receiver that uses the heterodyne principle to convert the incoming 
modulated radio-frequency signal to a predetermined lower carrier frequency, 
the intermediate-frequency value. This is done by using a local oscillator 
tuned simultaneously with the input stage of the receiver, so that the 
oscillator frequency always differs from that of the incoming carrier by the 
i-f value. 
    With a fixed and favorably chosen i-f value, the i-f amplifier can 
efficiently provide the major portion of the amplification and selectivity 
required by the receiver. After amplification, the i-f signal is demodulated 
by the second detector to obtain the desired audio output signal. A similar 
circuit arrangement is used in television and radar receivers to obtain the 
desired video output signal.  See  HETERODYNE PRINCIPLE; RADIO RECEIVER. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sSuperinfection   �21m �33s 
    Either of two types of infection: a new infection occurring as a 
consequence of treatment of another infection, often because of the use of 
excessive numbers or amounts of antibiotics; or infection arising in 
individuals with specific underlying mechanisms promoting infections but not 
related to treatment of a preexisting infection. In most instances the 
superinfection is much more difficult to treat than the original infection. 
    An example of the first type is an individual who acquires virus influenza 
and may develop a complicating pneumococcal pneumonia. Then, if treated with 
two antibiotics, staphylococcal pneumonia may supervene as a superinfection. 
    The second type of superinfection can be subdivided into five categories: 
(1) Infection may arise solely as a consequence of an underlying disease. (2) 
Infections may arise as a consequence of the underlying disease plus therapy. 
(3) Infections may be due solely to treatment or procedures. (4) Infections 
may be increased in severity in compromised hosts, but not in frequency. (5) 
There are also certain societally related infections, including infections 
consequent to use of illicit drugs.  See  HOSPITAL INFECTIONS; INFECTION. 
 �34s[Donald Louria]
�33s 
  
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�6v�2v�32sSupermassive stars �21m �33s
    Hypothetical objects with masses exceeding 60 solar masses, the mass of
the largest known ordinary stars (1 solar mass = 2 x 10(sup 30) kg). The term
is most often used in connection with objects larger than 10sup4 solar masses
that might be the energy source in quasars and active galaxies. Since these
objects have not been observed and are hypothetical at present, very little is
known about their nature and behavior. About the only thing known for certain
about supermassive stars is that they are unstable.
 �34s[Harry L. Shipman]
�33s

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�6v�2v�32sSupermultiplet �21m �33s
    A generalization of the concept of a multiplet. A multiplet is a set of
quantum-mechanical states each of which has the same value of some fundamental
quantum numbers and differs from the other members of the set by other quantum
numbers, which take values from a range of numbers dictated by the fundamental
quantum numbers. There are as many states in the set as there are different
values of the varying quantum numbers. The number of states in the set is the
multiplicity or dimension of the multiplet. For example, there are 2S + 1
orientations or projections of a spin S, and these are labeled by M(sub S) with
values from -S to +S in integer steps. Each of the 2S + 1 members of this
multiplet, which has the dimension 2S + 1, is labeled by S and M(sub S), where S
labels the multiplet, and MsubS a state within the multiplet. Suppose that T
denotes another spinlike quantity but refers to a different physical property.
Then the 2T + 1 states also constitute a multiplet labeled by T with members
labeled by, say, T3. If sets of these multiplets are grouped together, then
one speaks of a supermultiplet. For example, one supermultiplet might be
denoted by a set of fundamental quantum numbers (LAMBDA1LAMBDA2LAMBDA3), and S
and T would range over a set of values dictated by the values of LAMBDA1,
LAMBDA2, and LAMBDA3. For a fixed value of S, MsubS would have its usual 2S +
1 values, and for fixed value of T, T3 would have 2T + 1 different values,
ranging from -T to +T. The dimension of a supermultiplet in this case would be
the sum over all values of S and T of the product (2S + 1)(2T + 1). A
supermultiplet then is a multiplet of multiplets.  See	QUANTUM MECHANICS;
QUANTUM NUMBERS.
    The term supermultiplet was first used by Eugene P. Wigner in a nuclear
theory in which protons and neutrons are treated equivalently, and the
dependence of the nuclear force on their differences and intrinsic spins is
ignored. When used in this connection, one speaks of Wigner supermultiplets.
See  SYMMETRY LAWS (PHYSICS).
 �34s[S. A. Williams]
�33s

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�6v�2v�32sSupernova   �21m �33s 
    The sudden brightening of a star, by a factor of up to 10sup1sup0, so that 
it may briefly outshine the entire galaxy in which it is located. Fading 
occurs slowly, over several years or more. The supernova phenomenon was first 
distinguished from that of ordinary novae by Walter Baade and Fritz Zwicky in 
1934. They also suggested that the most likely source of the enormous energy 
required was the formation of a neutron star, and that a certain star in the 
Crab Nebula, remnant of the 1054 supernova, was probably such a neutron star. 
Recent observations and calculations have essentially confirmed their 
suggestions.  See  CRAB NEBULA; NEUTRON STAR; NOVA; PULSAR. 
    A large galaxy like the Milky Way probably has about one supernova 
explosion every 30 years. Because dust blocks the view of much of the Milky 
Way, only six have been recorded in the last 1000 years (in 1006, 1054, 1572, 
1604, and probably 1181 and 1680). More than 100 supernovae have been found in 
other galaxies since 1885. 
    In all cases the spectra show large amounts of gas being thrown off at 
speeds of thousands of miles per second. The blown-off gas travels rapidly out 
into space, sweeping up more (interstellar) gas as it goes, and remains 
visible as a bright nebula for thousands of years. Such supernova remnants are 
also characteristically copious sources of radio and x-rays (indicating the 
presence of magnetic fields, relativistic electrons, and shock waves). Some 
dozens of them have thereby been identified in the galaxy.  See  ASTRONOMICAL 
SPECTROSCOPY; RADIO ASTRONOMY; X-RAY ASTRONOMY. 
 �34s[Virginia Trimble]
�33s 
  
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�6v�2v�32sSuperplasticity   �21m �33s 
    The unusual ability of some metals and alloys to elongate uniformly 
thousands of percent at elevated temperatures, much like hot polymers or 
glasses. Under normal creep conditions, conventional alloys do not stretch 
uniformly, but form a necked-down region and then fracture after elongations 
of only 100% or less. The most important requirements for obtaining 
superplastic behavior include a very small metal grain size, a well-rounded 
(equiaxed) grain shape, a deformation temperature greater than one-half the 
melting point, and a slow deformation rate.  See  ALLOY; CREEP (MATERIALS); 
EUTECTICS.  
    Superplasticity is important to technology primarily because large amounts 
of deformation can be produced under low loads. Thus, conventional 
metal-shaping processes (for example, rolling, forging, and extrusion) can be 
conducted with smaller, and cheaper, equipment. Nonconventional forming 
methods can also be used; for instance, vacuum-forming techniques, borrowed 
from the plastics industry, have been applied to sheet metal to form car 
panels, refrigerator door linings, and TV chassis parts.  See  METAL FORMING; 
PLASTICITY.  
 �34s[Erwin E. Underwood]
�33s 
  
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�6v�2v�32sSuperposition principle �21m �33s
    In classical wave theories (optics, acoustics), as in all theories
characterized by linear homogeneous differential equations, the sum of any
number of solutions to the equations is another solution. Thus (assuming
one-dimensional waves for simplicity), the amplitude of the resultant wave at
a point x and time t is the linear superposition of the amplitudes of all
waves reaching x at time t. This fact, known as the principle of
superposition, often also is taken to mean that if each of PSI1(x) and PSI2(x)
are possible waveforms at t = 0, then any linear combination PSI(x) = c1PSI1 +
c2PSI2 is a possible waveform at t = 0. This latter version is not a
consequence of the linearity of the differential equations, but rather is an
affirmation of the belief that the waveform can be chosen arbitrarily at any
initial instant.  See  WAVE MOTION.
The preceding paragraph holds equally well for quantum theory, where the wave
function PSI(x,t) obeys the linear homogenous Schroedinger equation.  In quantum
theory, however, because the wave function represents physical states, the
principle of superposition has a profound significance.  In particular, if the
observable A is certain to have value ALPHA1 in the state represented by u1, and
is certain to have value ALPHA2 in the state represented by u2, then
c(sup1)u(sup 1) + c(sup 2)u(sup 2) represents a state in which measurement of
the observable A is certain to yield either precisely ALPHA1, or else precisely
ALPHA2.  Thus, because probabilities are proportional to the ABSOLUTE VALUE PSI
SQUARED, superposition accounts for phenomena which are difficult to understand
from a classical viewpoint.  See QUANTUM MECHANICS.�34s[Edward Gerjuoy]
�33s

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�6v�2v�32sSuperposition theorem (electric networks)   �21m �33s 
    Essentially, that it is permissible, if there are two or more sources of 
electromotive force in a linear electrical network, to compute at any element 
of the network the response of voltage or of current that results from one 
source alone, and then the response resulting from another source alone, and 
so on for all sources, and finally to compute the total response to all 
sources acting together by adding these individual responses. 
    Thus, if a load of constant resistance is supplied with electrical energy 
from a linear network containing two batteries, two generators, or one battery 
and one generator, it would be correct to find the current that would be 
supplied to the load by one source (the other being reduced to zero), then to 
find the current that would be supplied to the load by the second source (the 
first source now being reduced to zero), and finally to add the two currents 
so computed to find the total current that would be produced in the load by 
the two sources acting simultaneously.  See  NETWORK THEORY. 
    By means of the principle of superposition, effects are added instead of 
causes. This principle seems so intuitively valid that there is far greater 
danger of applying superposition where it is incorrect than of failing to 
apply it where it is correct. It must be recognized that for superposition to 
be correct the relation between cause and effect must be linear. 
 �34s[Hugh H. Skilling]
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�6v�2v�32sSuperpressure balloon   �21m �33s 
    A nonextensible (constant-volume) free balloon which converts its free 
lift into superpressure by the time that float level is reached. As long as 
the balloon is superpressurized, it will continue to float at a constant 
density level. The main advantage of the superpressure balloon is the 
possibility of long-duration flights which can circumnavigate the globe. Small 
superpressure balloons 4 m (13 ft) in diameter have been released in the 
southern hemisphere, and some have remained aloft for over a year. Balloons 
having diameters of over 30 m (100 ft) have been flown with limited results. 
These balloons are relatively new, and the objective of the research has been 
to produce a reliable superpressure balloon capable of carrying 300 kg (660 
lb) to an altitude of 42 km (26 mi). 
 �34s[Walter R. Nagel]
�33s 
  
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�6v�2v�32sSupersaturation   �21m �33s 
    A solution is at the saturation point when dissolved solute in it 
crystallizes from it at the same rate at which it dissolves. Under prescribed 
experimental conditions of temperature and pressure, a solution can contain at 
saturation only one fixed amount of dissolved solute. However, it is possible 
to prepare relatively stable solutions which contain a quantity of a dissolved 
solute greater than that of the saturation value provided solute phase is 
absent. Such solutions are said to be supersaturated. They can be prepared by 
changing the experimental conditions of a system so that greater solubility is 
obtained, perhaps by heating the solution, and then carefully returning the 
system to or near its original state. The addition of solute phase will 
immediately relieve supersaturation. Solutions in which there is no 
spontaneous formation of solute phase for extended periods of time are said to 
be metastable. There is no sharp line of demarcation between an unstable and 
metastable solution. The process whereby initial aggregates within a 
supersaturated solution develop spontaneously into particles of new stable 
phase is known as nucleation. The greater the degree of supersaturation, the 
greater will be the number of nuclei formed.  See  NUCLEATION; PHASE 
EQUILIBRIUM; PRECIPITATION (CHEMISTRY). 
 �34s[Louis Gordon /Royce W. Murray]
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�6v�2v�32sSupersonic diffuser �21m �33s
    A passive compressor (or shaped duct) in which gas enters at a velocity
greater than the speed of sound, is decelerated in a contracting section, and
reaches sonic speed at a throat.
    Supersonic compression systems can be categorized to three basic types
(see illustration). External-compression inlets have the supersonic diffusion
taking place at or ahead of the cowl lip (or throat station) and generally
employ one or more oblique waves ahead of the normal shock.
Internal-compression inlets accomplish supersonic diffusion internally
downstream of the cowl lip. Deceleration of the flow is produced by a number
of weak reflecting waves in a gradually convergent channel. The third system
is a combination of external and internal compression and appears to represent
an effective compromise.
 �34s[James F. Connors ;Leonard J. Obery]
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�6v�2v�32sSupersonic flight   �21m �33s 
    Relative motion of a solid body and a gas at a velocity greater than that 
of sound propagation under the same conditions. The general characteristics of 
supersonic flight can be understood by considering the laws of propagation of 
a disturbance, or pressure impulse, in a compressible fluid. 
    If the fluid is at rest, the pressure impulse propagates uniformly with 
the velocity of sound in all directions, the effect always acting along an 
ever-increasing spherical surface. If, however, the source of the impulse is 
placed in a uniform stream, the impulse will be carried by the stream 
simultaneously with its propagation at sonic velocity relative to the stream. 
Hence the resulting propagation is faster in the direction of the stream and 
slower against the stream. If the velocity of the stream past the source of 
disturbance is supersonic, the effect of the impulse is restricted to a cone 
whose vertex is the source of the impulse and whose vertex angle decreases 
from 90DG (corresponding to Mach number equal to 1) to smaller and smaller 
values as the Mach number of the stream increases (see illustration). If the 
source of the pressure impulse travels through the air at rest, the conditions 
are analogous.  See  MACH NUMBER. 
    Consider the supersonic motion of a wing moving into air at rest. Because 
signals cannot propagate ahead of the wing, the presence of the wing has no 
effect on the undisturbed air until the wing passes through it. Hence there 
must be an abrupt change in the properties of the undisturbed air as it begins 
to flow over the wing. This abrupt change takes place in a shock wave which is 
attached to the leading edge of the wing, provided that the leading edge is 
sharp and the flight Mach number is sufficiently large. As the air passes 
through the shock wave, its pressure, temperature, and density are markedly 
increased.  
    Further aft of the leading edge, the pressure of the air is decreased as 
the air expands over the surface of the wing. Hence the pressure acting on the 
front part of the wing is higher than the ambient pressure, and the pressure 
acting on the rear part of the wing is lower than the ambient pressure. The 
pressure difference between front and rear parts produces a drag, even in the 
absence of skin friction and flow separation. The wing produces a system of 
compression and expansion waves which move with it. This phenomenon is similar 
to that of a speedboat moving with a velocity greater than the velocity of the 
surface waves. Because of this analogy, supersonic drag is called wave drag. 
It is peculiar to supersonic flight, and it may represent the major portion of 
the total drag of a body.  See  HYPERSONIC FLIGHT; SUBSONIC FLIGHT; TRANSONIC 
FLIGHT.  
 �34s[John E. Scott, Jr.]
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�6v�2v�32sSupertransuranics   �21m �33s 
    A group of predicted elements beyond the present periodic table of known 
elements, with atomic numbers around 114, expected to possess half-lives of 
the order of a year or longer. These are also referred to as the superheavy 
elements. Although they have not been discovered experimentally, it is 
generally believed that the difficulty lies in the synthesis of these 
elements, rather than in their stability once they are made. 
    The underlying physics responsible for the limited extent of the periodic 
table is the competition between attractive ``nuclear forces'' among the 
nucleons (that is, protons and neutrons) and the repulsive electrostatic 
forces among all the positively charged protons. The limit of the periodic 
table at an atomic number Z of approximately 109 is then set by the process of 
nuclear fission, which takes place when the disruptive effect of electrostatic 
forces overcomes the cohesive effect of the nuclear forces.  See  NUCLEAR 
FISSION.  
    The picture began to change when, in 1964, a clearer understanding was 
reached on the relation between fission half-lives and a well-known property 
of the nulceus called the magic numbers. If the number of protons (or 
neutrons) in a nucleus is equal to the proton magic number (or neutron magic 
number), then such a nucleus displays some special features; one example is 
that it is spherical in shape. Such a nucleus also displays an extra stability 
against fission. This means that this nucleus would have a longer half-life 
than would be expected otherwise. The superheavy elements are expected to 
occur with the center of the island at the proton magic number 114 and the 
neutron magic number 184.  See MAGIC NUMBERS; NUCLEAR STRUCTURE; RADIOACTIVITY. 
 �34s[C. F. Tsang]
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�6v�2v�32sSuppression   �21m �33s 
    The elimination of some undesired component of a signal. In automobile 
radio installations, suppression techniques are essential to prevent ignition 
interference from reaching the radio circuits. Radio receivers themselves 
often contain special noise-suppression circuits and devices; this is 
particularly true for communication receivers that operate in crowded and 
noisy shortwave bands. In many radar installations, suppression circuits and 
techniques are used to reduce clutter caused by the ground and by fixed 
objects close to the antenna.  See  ELECTRIC FILTER; ELECTRICAL INTERFERENCE; 
ELECTRICAL NOISE; SUPPRESSOR. 
 �34s[John Markus]
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�6v�2v�32sSuppressor   �21m �33s 
    A device used to reduce or eliminate noise or other signals that interfere 
with the operation of a communication system. The term may be applied to a 
noise filter in a radio receiver, but it is more frequently used to describe a 
device applied at the noise source, such as a resistor used in series with 
spark plugs of a gasoline engine, or a capacitor across the terminals of a 
commutator motor or other sparking device that acts as a noise generator. The 
term suppressor may also be applied to a filter used in power leads of an 
electronic device to eliminate unwanted signals of noise.  See  ELECTRIC 
FILTER; ELECTRICAL INTERFERENCE; ELECTRICAL NOISE; NOISE FILTER (RADIO); 
SUPPRESSION.  
 �34s[W. R. Le Page]
�33s 
  
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�6v�2v�32sSurface acoustic-wave devices �21m �33s
    Devices which employ surface acoustic waves (SAW) in the analog processing
of electronic signals with frequencies in the range 10sup7 - 10sup9 Hz.
    Surface acoustic waves which contain both compressional and shear
components in phase quadrature, propagating nondispersively along and bound to
solid surfaces, were discovered by Lord Rayleigh in the 1880s. As an example,
earthquakes furnish sources for propagating these waves on the Earth's
surface. It is of importance for electronic applications that if the solid is
a piezoelectric material, the surface acoustic energy is complemented by a
small amount of electric energy. This electric energy provides the physical
mechanism for the coupling between conventional electromagnetic signals and
propagating SAW. The coupling is attained by means of interdigital transducers
(IDT). SAW devices have led to a versatile microminiature technology for
analog signal processing in the frequency range 10(sup 7) - 10(sup 9) Hz.
Notable  devices include bandpass filters, resonators, oscillators, pulse
compression filters, and fast Fourier transform processors. Application areas
include the color television consumer market, radar, sonar, communication
systems, and nondestructive testing.  See  PIEZOELECTRIC CRYSTAL;
PIEZOELECTRICITY. 
    In the basic arrangement, a piezoelectric substrate, often crystalline
quartz, has a polished upper surface on which two transducers are deposited.
The input transducer is connected, via fine bonded leads, to the electric
source through an electrical matching network. The output transducer drives
the load, usually 50 ohms, through another electrical matching network.
Because these transducers are bidirectional, they lead to devices with at
least 6 decibels (dB) loss even in the passband. The unwanted acoustic waves
are absorbed by terminations at the ends of the piezoelectric substrate. A
metal baffle serves to isolate electromagnetically the two transducers.
    The transducers consist of a set of metal interdigital electrodes, each a
few hundred nanometers thick, fed from two bus-bars (see illustration). For
this transducer arrangement the period p of the interdigital electrode
structure is constant and equals one surface acoustic wavelength LAMBDA0 at
the center of frequency f0 of the response. The width of the metal electrodes
is typically p/4, being 100 micrometers at 10(sup 7) Hz and 1 MUm at 10(sup 9)
Hz.  The electrode overlap distance w is also constant and defines the acoustic
beam width, which is typically 40 wavelengths.	See  TRANSDUCER.
    The 100-micrometer electrodes are readily fabricate by using techniques
standard to the semiconductor integrated circuit industry of metallization:
photoresist, masking, and chemical etching. The 1-MUm electrodes require more
sophisticated processing techniques. These include conformable optical masks
and x-ray lithography coupled with sputter etching by radio-frequency and
ion-beam methods for even finer resolutions.  See  INTEGRATED CIRCUITS.
 �34s[J. H. Collins]
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�6v�2v�32sSurface-active agent   �21m �33s 
    A substance that even though present in small amounts, exerts a marked 
effect on the surface behavior of a system. These agents are essentially 
responsible for producing great changes in the surface energy of liquid or 
solid surfaces, and their ability to cause these changes is associated with 
their tendency to migrate to the interface between two phases. Consequently, 
surface-active agents are of potential interest wherever there are 
solid-solid, solid-liquid, solid-gas, liquid-liquid, or liquid-gas interfaces, 
and of particular interest at liquid-gas interfaces at which the 
surface-active agent is a solute whose presence makes the surface properties 
of the solution greatly different from those of the solvent.  See  INTERFACE 
OF PHASES. 
    Soap, for example, when dissolved in small quantities in water, is 
responsible for greatly decreasing the surface tension of water, and it is 
this property of soap that accounts for its ability to act as a detergent. In 
contrast to soap and other related substances that lower the surface energy of 
a liquid, other solutes, such as inorganic salts, acids, and bases may 
increase the surface tension of a liquid (see illustration), but their effect 
in increasing the surface tension is not nearly so great as the effect of 
those agents that decrease the surface tension. Occasionally the term 
surface-inactive solutes is applied to these substances whose presence causes 
an increase in surface tension.  See  DETERGENT; MONOMOLECULAR FILMS; SOAP. 
    Surface-active agents are usually classified in three groups: anionic, 
cationic, and nonionic types. Anionic types include carboxylate ions such as 
occur in sodium oleate. Cationic surface-active agents are usually derived 
from the amino group where, through either primary, secondary, or tertiary 
amine salts, the hydrophilic character may be achieved by aliphatic and 
aromatic groups that may be altered by substituents of varying polarity. The 
third class of surface-active agents, the nonionic type, are organic 
substances which contain groups of varying polarity and which render parts of 
the molecule lyophilic, whereas other parts of the molecule are lyophobic. In 
this class are often included certain colloidal substances, such as graphite 
and clays.  See  LUBRICANT; SURFACE TENSION. 
 �34s[Wendell H. Slabaugh]
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�6v�2v�32sSurface and solid of revolution   �21m �33s 
    A surface of revolution is generated by revolving a plane curve about a 
line in its plane. The generating curve may consist of one or more arcs or 
line segments connected together. A solid of revolution is generated by 
revolving a connected plane region about a line in its plane not cutting the 
region. The boundary of a solid of revolution is a surface of revolution. The 
line about which the generating plane curve or region is revolved is called 
the axis of revolution, or simply the axis. 
    In euclidean solid geometry the words cylinder, cone, and sphere commonly 
refer to solids of revolution, and the corresponding surfaces of revolution 
are called a cylindrical surface, a conical surface, or a spherical surface. 
However, in analytic geometry and more generally in modern usage, the words 
cylinder, cone, and sphere refer to surfaces, not solids. A circular cylinder 
is a surface generated by revolving an infinite line about a parallel axis, 
and a circular cone is a surface generated by revolving an infinite line about 
an intersecting axis. A spheroid is a surface obtained by revolving an ellipse 
about one of its axes. Paraboloids and hyperboloids of revolution are obtained 
by revolving parabolas and hyperbolas about their axes.  See  CONE; CYLINDER; 
ELLIPSOID AND SPHEROID; HYPERBOLOID; PARABOLOID; QUADRIC SURFACE; TORUS. 
 �34s[James S. Frame]
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�6v�2v�32sSurface coating   �21m �33s 
    A substance applied to other materials to change the surface properties, 
such as color, gloss, resistance to wear or chemical attack, or permeability, 
without changing the bulk properties. The term includes such materials as 
paints, varnishes, enamels, oils, greases, waxes, concrete, lacquers, 
asbestos, and fire retardant formulations of several varieties. In general, 
organic coatings are based on a vehicle, usually an oil or resin, which, after 
being spread out in a relatively thin film, changes to a solid. This change, 
called drying, may be due entirely to the evaporation of a volatile solvent, 
or it may be caused by a chemical reaction, such as oxidation or 
polymerization. Opaque materials called pigments, dispersed in the vehicle, 
contribute color, opacity, and increased durability and resistance.  See  
DRIER (PAINT); DRYING OIL; ELECTROPLATING OF METALS; ENAMEL; JAPANNING; 
LACQUER; METAL COATINGS; PAINT; PIGMENT; PRIMER (SURFACE COATING); SHELLAC; 
THINNER; VARNISH. 
 �34s[C. R. Martinson ;C. W. Sisler]
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�6v�2v�32sSurface condenser   �21m �33s 
    A heat-transfer device used to condense a vapor, usually steam, by 
absorbing its latent heat in a cooling fluid, ordinarily water. Most surface 
condensers consist of a chamber containing a large number of 
corrosion-resisting alloy tubes through which cooling water flows. The vapor 
contacts the outside surface of the tubes and is condensed on them. The tubes 
are arranged so that the cooling water passes through the vapor space one or 
more times. Air coolers are normally an integral part of the condenser but may 
be separate and external to it. The condensate is removed by a condensate pump 
and the noncondensables by a vacuum pump.  See  STEAM CONDENSER; VAPOR 
CONDENSER.  
 �34s[Joseph F. Sebald]
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�6v�2v�32sSurface hardening of steel �21m �33s
    The selective hardening of the surface layer of a steel product by one of
several processes which involve changes in microstructure with or without
changes in composition. Surface hardening imparts a combination of properties
to the finished product not produced by bulk heat treatment alone. Among these
properties are high wear resistance and good toughness or impact properties,
increased resistance to failure by fatigue resulting from cyclic loading, and
resistance to surface indentation by localized loads. The use of surface
hardening frequently is also favored by lower costs and greater flexibility in
manufacturing.
    The principal surface hardening processes are: (1) carburizing, (2) the
modified carburizing processes of carbonitriding, cyaniding, and liquid
carburizing, (3) nitriding, (4) flame hardening and induction hardening, and
(5) surface working. Carburizing introduces carbon into the surface layer of
low-carbon steel parts and converts that layer into high-carbon steel, which
can be quench-hardened by appropriate heat treatment. Carbonitriding,
cyaniding, and liquid carburizing, in addition to supplying carbon, introduce
nitrogen into the surface layer; this element permits lower case-hardening
temperatures and has a beneficial effect on the subsequent heat treatment. In
nitriding, only nitrogen is supplied, and reacts with special alloy elements
present in the steel. Whereas the foregoing processes change the composition
of the surface layer, flame hardening and induction hardening depend on a heat
treatment applied selectively to the surface layer of a medium-carbon steel.
Surface working by shot peening, surface rolling, or prestressing improved
fatigue resistance by producing a stronger case, compressive stresses, and a
smoother surface.  See	HEAT TREATMENT (METALLURGY); STEEL.
 �34s[Michael B. Bever ;Carl F. Floe]
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�6v�2v�32sSurface physics �21m �33s
    The study of the structure and dynamics of atoms and their associated
electron clouds in the vicinity of a surface, usually at the boundary between
a solid and a low-density gas. Thus, surface physics may be regarded as a
branch of solid-state physics which deals with those regions of large and
rapid variations of atomic and electron density that occur in the vicinity of
an interface between the two ``bulk'' components of a two-phase system. In
conventional usage, surface physics is distinguished from interface physics by
the restriction of the scope of the former to interfaces between a solid (or
liquid) and a low-density gas, often at ultra-high-vacuum pressures of p =
10(sup -10) to (10[sup -8] pascal or 10[sup -13] atm).	See  SOLID-STATE PHYSICS.
    More specifically, surface physics is concerned with two separate but
complementary areas of investigation into the properties of such
solid-``vacuum'' interfaces. Ultimately, interest centers on the specification
and theoretical prediction of surface composition and structure (that is, the
masses, charges, and positions of surface species), of the dynamics of surface
atoms (such as surface diffusion and vibrational motion), and of the
energetics and dynamics of electrons in the vicinity of a surface (such as
electron density profiles and localized electronic surface states). As a
practical matter, however, the nature and dynamics of surface species must be
determined experimentally by scattering and emission measurements involving
particles or electromagnetic fields (or both) external to the surface itself.
Thus, a second major interest in surface physics is the study of the
interaction of external entities (that is, atoms, ions, electrons, and
electromagnetic fields) with solids at their vacuum interfaces. It is this
aspect of surface physics which most clearly distinguishes it from
conventional solid-state physics, because quite different scattering and
emission experiments are utilized to examine surface as opposed to bulk
properties of a given sample.
    Techniques for characterizing the solid-vacuum interface are based on one
of two simple physical mechanisms for achieving surface sensitivity. The
first, which is the basis for field-emission and field-ionization microscopy,
is the achievement of surface sensitivity by utilizing electron tunneling
through the potential-energy barrier at a surface. The second mechanism for
achieving surface sensitivity is the examination of the elastic scattering or
emission of particles which interact strongly with the constituents of matter,
for example, ``low-energy'' (E <d 10(sup 3) eV) electrons, thermal atoms and
molecules, or ``slow'' (300 eV < E =d 10(sup 3) eV) ions. Since such entities
lose appreciable (DELTA E SIMILAR  10 eV) energy in distances of the order of
tenths of nanometers, typical electron analyzers with resolutions of tenths of
an electronvolt are readily capable of identifying scattering and emission
processes which occur in the upper few atomic layers of a solid.  See
ELECTRON DIFFRACTION; ELECTRON SPECTROSCOPY; FIELD EMISSION MICROSCOPY;
PHOTOEMISSION; X-RAY CRYSTALLOGRAPHY.
    A major reason for the renaissance in surface physics is the capacity to
generate in a vacuum chamber special surfaces which approximate the ideal of
being atomically flat. These surfaces are prepared by cycles of fast-ion
bombardment, thermal outgassing, and thermal annealing for bulk samples or
field evaporation of etched tips for field-ion microscopes. In this fashion,
reasonable facsimiles of uncontaminated, atomically flat solid-vacuum
interfaces of many simple metals and semiconductors have been prepared and
subsequently characterized by various spectroscopic techniques.
 �34s[C. B. Duke]
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�6v�2v�32sSurface tension   �21m �33s 
    The force acting in the surface of a liquid, tending to minimize the area 
of the surface. Surface forces, or more generally, interfacial forces, govern 
such phenomena as the wetting or nonwetting of solids by liquids, the 
capillary rise of liquids in fine tubes and wicks, and the curvature of 
free-liquid surfaces. The action of detergents and antifrothing agents and the 
flotation separation of minerals depend upon the surface tensions of liquids. 
    In the body of a liquid, the time-averaged force exerted on any given 
molecule by its neighbors is zero. Even though such a molecule may undergo 
diffusive displacements because of random collisions with other molecules, 
there exist no directed forces upon it of long duration. It is equally likely 
to be momentarily displaced in one direction as in any other. In the surface 
of a liquid, the situation is quite different; beyond the free surface, there 
exist no molecules to counteract the forces of attraction exerted by molecules 
in the interior for molecules in the surface. In consequence, molecules in the 
surface of a liquid experience a net attraction toward the interior of a drop. 
These centrally directed forces cause the droplet to assume a spherical shape, 
thereby minimizing both the free energy and surface area. 
    Liquids which wet the walls of fine capillary tubes rise to a height which 
depends upon the tube radius, the surface tension, the liquid density, and the 
contact angle between the solid and the liquid (measured through the liquid). 
In the illustration a liquid of a certain density is shown as having risen to 
a height h in a capillary whose radius is r. A balance exists between the 
force exerted by gravity on the mass of liquid raised in the capillary and the 
opposing force caused by surface tension. 
    Detergents, soaps, and flotation agents owe their usefulness to their 
ability to lower the surface tension of water, thereby stabilizing the 
formation of small bubbles of air. At the same time, the interfacial tension 
between solid particles and the liquid phase is lowered, so that the particles 
are more readily wetted and floated after attachment to air bubbles.  See  
FLOTATION; INTERFACE OF PHASES; SURFACE-ACTIVE AGENT. 
 �34s[Norman H. Nachtrieb]
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�6v�2v�32sSurface water   �21m �33s 
    A term commonly used to designate the water flowing in stream channels. 
The term is sometimes used in a broader sense as opposed to ``subsurface 
water.'' In this sense, surface water includes water in lakes, marshes, 
glaciers, and reservoirs as well as that flowing in streams. In the broadest 
sense, surface water is all the water on the surface of the Earth and thus 
includes the water of the oceans. Subsurface water includes water in the root 
zone of the soil and groundwater flowing or stored in the rock mantle of the 
Earth. Subsurface water differs from surface water in the mechanics of its 
movement as well as in its location. Surface and subsurface water are two 
stages of the movement of the Earth's water through the hydrologic cycle. The 
world's ocean and atmospheric moisture are two other main stages of the grand 
water cycle of the Earth.  See  HYDROLOGY. 
    The table gives estimates of the amounts of water in various parts of the 
hydrologic cycle and their detention periods. It may be noted that surface 
water on the continents is but a small part of the world's water and that the 
bulk of that is in fresh-water lakes. However, the detention period is also 
short. This means that the surface-water part, and especially the water in the 
streams, is rapidly discharged and replenished. That is why surface water, as 
well as the shallower groundwater, is called a renewable resource. Water that 
has a detention period of more than a generation is not renewed within 
sufficient time to be so considered.  See  GROUNDWATER HYDROLOGY; RIVER; 
TERRESTRIAL FROZEN WATER. 
  
            Precipitation that reaches the Earth is subdivided by processes of 
evaporation and infiltration into various routes of subsequent travel. 
Evaporation from wet land surfaces and from vegetation returns some of the 
water to the atmosphere immediately. Precipitation that falls at rates less 
than the local rate of infiltration enters the soil. Some of the infiltrated 
water is retained in the soil, sustaining plant life, and some reaches the 
groundwater. The precipitation that exceeds the capacity of the soil to absorb 
water flows overland in the direction of the steepest slope and concentrates 
in rills and minor channels. During storms most of the water in surface 
streams is derived from that portion of the precipitation which fails to 
infiltrate the soil.  See  PRECIPITATION (METEOROLOGY). 
    The distinction between surface and subsurface water, though useful, 
should not obscure the fact that water on the surface and water underground is 
physically connected through pores, cracks, and joints in rock and soil 
material. In many areas, particularly in humid regions, surface water in 
stream channels is the visible part of a reservoir, which is partly 
underground; the water surface of a river is the visible extension of the 
surface of the groundwater. 
 �34s[Luna B. Leopold]
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�6v�2v�32sSurge arrester   �21m �33s 
    A protective device designed primarily for connection between a conductor 
of an electrical system and ground to limit the magnitude of transient 
overvoltages on equipment. A lightning arrester is really a voltage-surge 
arrester.  
    The valve arrester consists of a single gap or multiple gaps in series 
with current-limiting elements. The gaps between spaced electrodes prevent the 
flow of current through the arrester except when the voltage across them 
exceeds the critical gap flashover. They reseal by developing voltage in the 
gaps sufficiently to interrupt current flow during normal system voltage, and 
also for temporary overvoltages. The current-limiting element is a nonlinear 
resistor whose resistance decreases substantially as the voltage across it 
increases.  
    Other types of arresters, called expulsion arresters, have spaced 
electrodes in an interrupting chamber which contains gas-evolving material.  
See  LIGHTNING; LIGHTNING AND SURGE PROTECTION. 
 �34s[Glenn D. Breuer]
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�6v�2v�32sSurgery   �21m �33s 
    That branch of medicine which generally treats diseases by operative 
intervention. Surgical procedures may involve relieving mechanical obstruction 
of a tubular organ, such as the intestine; or removing a diseased organ, which 
cannot be salvaged by medical treatment, such as a gangrenous appendix or 
inflamed gallbladder; or removing a malignant tumor with a margin of normal 
tissue; or repairing an injured organ, or removing it if the organ is 
irreparable and its absence is compatible with survival. 
    The field of surgery has become increasingly specialized, primarily by 
organ system, so that surgical diseases of the kidney, bladder, and other 
components of the urinary tract are treated by surgeons called urologists; 
surgery of the central nervous system, including the brain and spinal cord, is 
done by neurosurgeons; reconstructive and cosmetic surgery is done primarily 
by plastic surgeons; general surgeons continue to do most abdominal surgery, 
some head and neck surgery, and surgery of the soft tissues of the 
extremities; and surgical diseases of the bones and joints are treated by 
orthopedic surgeons. Some specialties within surgery have also developed into 
specialties that are not limited to one organ system, such as surgical 
oncology (cancer surgery), so that cancers in most parts of the body may be 
treated by surgeons with special training in malignant diseases.  See  
MEDICINE.  
 �34s[Robert F. Jones]
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�6v�2v�32sSurging   �21m �33s 
    A sudden and momentary change of voltage or current in a circuit. It can 
be due to a sudden change in the applied input signal, a sudden change in the 
load placed on the circuit, or to the action of a relay, switch, or other 
device that changes operating conditions within the circuit. The resulting 
surges or transients are often called pulses or impulses when they have only 
one polarity. An oscillatory surge includes both positive and negative 
polarity values.  See  ELECTRIC TRANSIENT. 
 �34s[John Markus]
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�6v�2v�32sSurveillance radar   �21m �33s 
    Ground radar used for traffic control purposes in the approach and landing 
zone. This radar is used primarily to assist controllers in converting random 
arrivals to regular landings and in positioning such aircraft so that they may 
make low approaches by the use of a fixed-beam, low-approach system or a 
precision radar low-approach system.  See  AIR-TRAFFIC CONTROL. 
 �34s[Peter C. Sandretto]
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�6v�2v�32sSurveying   �21m �33s 
    The measurement of dimensional relationships among points, lines, and 
physical features on or near the Earth's surface. Basically, surveying 
determines horizontal distances, elevation differences, directions, and 
angles. These basic determinations are applied further to the computation of 
areas and volumes and to the establishment of locations with respect to some 
coordinate system. 
    Surveying is typically used to locate and measure property lines; to lay 
out buildings, bridges, channels, highways, sewers, and pipelines for 
construction; to locate stations for launching and tracking satellites; and to 
obtain topographic information for mapping and charting. 
    Horizontal distances are usually assumed to be parallel to a common plane. 
Each measurement has both length and direction. Length is expressed in feet or 
in meters. Direction is expressed as a bearing of the azimuthal angle 
relationship to a reference meridian, which is the north-south direction. It 
can be the true meridian, a grid meridian, or some other assumed meridian. The 
degree-minute-second system of angular expression is standard in the United 
States.  See  AZIMUTH. 
    Reference, or control, is a concept that applies to the positions of lines 
as well as to their directions. In its simplest form, the position control is 
an identifiable or understood point of origin for the lines of a survey. 
Conveniently, most coordinate systems have the origin placed west and south of 
the area to be surveyed so that all coordinates are positive and in the 
northeast quadrant. 
    Vertical measurement adds the third dimension to an object's position. 
This dimension is expressed as the distance above some reference surface, 
usually mean sea level, called a datum. Mean sea level is determined by 
averaging high and low tides during a lunar month, but for certainty this must 
be carried out for as long as 19+ years.  See  SEA-LEVEL DATUM PLANES; 
SEA-LEVEL FLUCTUATIONS. 
  
    Horizontal control.  The main framework, or control, of a survey is laid 
out by traverse, triangulation, or trilateration. Some success has been 
achieved in locating control points from Doppler measurements of passing 
satellites, from aerial phototriangulation, from satellites photographed 
against a star background, and from inertial guidance systems. In traverse, 
adopted for most ordinary surveying, a line or series of lines is established 
by directly measuring lengths and angles. In triangulation, used mainly for 
large areas, angles are again directly measured, but distances are computed 
trigonometrically. This necessitates triangular patterns of lines connecting 
intervisible points and starting from a baseline of known length. New 
baselines are measured at intervals. Trigonometric methods are also used in 
trilateration, but lengths, rather than angles, are measured. The development 
of electronic distance measurement (EDM) instruments brought trilateration 
into significant use. 
  
    Distance measurement.  Traverse distances are usually measured with a 
surveyor's tape or by EDM, but also may sometimes be measured by stadia, 
subtense, or trig-traverse. 
    Whether on sloping or level ground, it is horizontal distances that must 
be measured. In taping, horizontal components of hillside distances are 
measured by raising the downhill end of the tape to the level of the uphill 
end. On steep ground this technique is used with shorter sections of the tape. 
The raised end is positioned over the ground point with the aid of a plumb 
bob. Where slope distances are taped along the ground, the slope angle can be 
measured with the clinometer. The desired horizontal distance can then be 
computed.  See  CLINOMETER. 
    In EDM the time a signal requires to travel from an emitter to a receiver 
or reflector and back to the sender is converted to a distance readout. The 
great advantage of electronic distance measuring is its unprecedented 
precision, speed, and convenience. Further, if mounted directly onto a 
theodolite, and especially if incorporated into it and electronically coupled 
to it, the EDM instrument with an internal computer can in seconds measure 
distance (even slope distance) and direction, then compute the coordinates of 
the sighted point with all the accuracy required for high-order surveying. 
    In the stadia technique, a graduated stadia rod is held upright on a point 
and sighted through a transit telescope set up over another point. The 
distance between the two points is determined from the length of rod 
intercepted between two horizontal wires in the telescope.  See  STADIA; 
TRANSIT (ENGINEERING). 
    In the subtense technique the transit angle subtended by a horizontal bar 
of fixed length enables computation of the transit-to-bar distance (Fig. 1). 
In trig-traverse the subtense bar is replaced by a measured baseline extending 
at a right angle from the survey line whose distance is desired. The distance 
calculated in either subtense or trig-traverse is automatically the horizontal 
distance and needs no correction. 
  
    Angular measurement.  The most common instrument for measuring angles is 
the transit or theodolite. It is essentially a telescope that can be rotated a 
measurable amount about a vertical axis and a horizontal axis. Carefully 
graduated metal or glass circles concentric with each axis are used to measure 
the angles. The transit is centered over a point with the aid of either a 
plumb bob suspended by a string from the vertical axis or (on some 
theodolites) an optical plummet, which enables the operator to sight along the 
instrument's vertical axis to the ground through a right-angle prism.  See  
THEODOLITE.  
  
    Elevation differences.  Elevations may be measured trigonometrically in 
conjunction with reduction of slope measurements to horizontal distances, but 
the resulting elevation differences are of low precision. 
    Most third-order and all second- and first-order measurements are made by 
differential leveling, wherein a horizontal line of sight of known elevation 
is sighted on a graduated rod held vertically on the point being checked (Fig. 
2). The transit telescope, leveled, may establish the sight line, but more 
often a specialized leveling instrument is used. For approximate results a 
hand level may be used.  See  LEVEL (SURVEYING). 
    Other methods of measuring elevation include trigonometric leveling which 
involves calculating height from measurements of horizontal distance and 
vertical angle; barometric leveling, a method of determining approximate 
elevation difference with aid of a barometer; and airborne profiling, in which 
a radar altimeter on an aircraft is used to obtain ground elevations. 
  
    Astronomical observations.  To determine meridian direction and geographic 
latitude, observations are made by a theodolite or transit on Polaris, the 
Sun, or other stars. Direction of the meridian (geographic north-south line) 
is needed for direction control purposes; latitude is needed where maps and 
other sources are insufficient. The simplest meridian determination is made by 
sighting Polaris at its elongation, as the star is rounding the easterly or 
westerly extremity of its apparent orbit. An angular correction is applied to 
the direction of sighting, which is referenced to a line on the ground. The 
correction value is found in an ephemeris.  See  EPHEMERIS; TOPOGRAPHIC 
SURVEYING AND MAPPING. 
 �34s[B. Austin Barry]
�33s 
  
  
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4165632
�6v�2v�32sSusceptance �21m �33s
    The imaginary part of the complex representation for the admittance Y,
defined by Eq. (1), of a circuit,

		     Y = G PLUS/MINUS jB			    (1)

where G is the real part, called the conductance, and B is the susceptance.
Since Y = 1/Z = 1/(R + jX) where X is the total reactance, X(sub L) - X(sub C),
and  R is the resistance, then Eq. (2) holds


Y = (R/[R(sup 2) + X(sup 2)]) - j(x/[R(sup 2) + X(sup 2)])	     (2)


and susceptance B = X/(R(sup 2) + X(sup 2)). This is the general expression for
susceptance which shows that susceptance is a function involving both
resistance and reactance.
    These functions find application chiefly in computation of parallel
circuits. See ADMITTANCE.
 �34s[Burtis L. Robertson]
�33s

A700000002064�254s�5v�1v1702
\IMG\CI1681.IMG
4165632
�6v�2v�32sSwaging   �21m �33s 
    A forging method, also called rotary swaging, that consists of two or four 
dies which are activated radially by blocks in contact with a series of 
rollers (see illustration). The rotation of the block-die assembly causes the 
curved ends of the blocks to be in contact with the rollers; relative motion 
between the blocks and the roller housing then gives a reciprocating motion to 
the dies. The hammering action on the outer surface of the stock reduces its 
diameter; the stock is generally prevented from rotating. 
    A variety of geometries is obtained in this process: (1) reduction of wall 
thickness of tubes; (2) sinking of tubes, whereby both the outside and inside 
diameters are reduced, the final wall thickness being controlled by a mandrel; 
(3) tapered or stepped sections; and (4) a variety of internal geometries, 
such as fins or hexagonal or spiral grooves, each with the use of a specially 
designed mandrel. See FORGING; METAL FORMING. 
 �34s[Serope Kalpakjian]
�33s 
  
  
A700000001246�254s�5v�1v1702
\IMG\CI1681.IMG
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�6v�2v�32sSwamp, marsh, and bog   �21m �33s 
    Wet flatlands, where mesophytic vegetation is areally more important than 
open water, are commonly developed in filled lakes, glacial pits and potholes, 
or poorly drained coastal plains or floodplains. Swamp is a term usually 
applied to a wet land where trees and shrubs are an important part of the 
vegetative association, and bog implies lack of solid foundation. Some bogs 
consist of a thick zone of vegetation floating on water. Coastal marshes 
covered with vegetation adapted to saline water are common on all continents.  
See  MANGROVE SWAMP. 
 �34s[Luna B. Leopold]
�33s 
  
A700000001437�254s�5v�1v1702
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4165632
�6v�2v�32sSweat gland   �21m �33s 
    A coiled, tubular gland found in mammals. There are two kinds, merocrine 
or eccrine, and apocrine. The latter are generally associated with hair 
follicles (see illustration). Merocrine glands are distributed extensively 
over the body in the human, whereas the apocrine variety is restricted to the 
scalp, nipples, axilla, external auditory meatus, external genitals, and 
perianal areas. Apocrine sweat glands are more numerous in mammals, with the 
exception of the chimpanzee and human, in which the merocrine variety 
predominates. The mammary glands probably represent modified apocrine sweat 
glands which grow inward and increase in complexity.  See  GLAND; MAMMARY 
GLAND.  
 �34s[Olin E. Nelsen]
�33s 
  
  
A700000003641�254s�5v�1v1702
\IMG\CI1681.IMG
4165632
�6v�2v�32sSweep generator   �21m �33s 
    An electronic circuit that generates a voltage or current, usually 
recurrent, as a prescribed function of time. The resulting waveform is used as 
a time base to be applied to the deflection system of an electron-beam device, 
such as a cathode-ray tube. Sweep generators are classified as linear, 
circular, rotating radial, or hyperbolic. 
    A linear sweep generator provides a current or voltage that is a linear 
function of time. The waveform is usually recurrent at uniform periods of time. 
    A circular sweep is generated by applying a constant-frequency sinusoid to 
the horizontal component of a deflection system and another of the same 
frequency but shifted in phase by 90DG to the vertical component. A spiral 
scan is generated if, in addition to the sinusoidal x and y components, a 
linear sawtooth waveform (v=kt) is used to modulate equally both the 
horizontal and voltage deflection waveform. 
    A rotating radial sweep may be generated by applying a linear sawtooth of 
current to the deflection system of a magnetically deflected cathode-ray 
device and rotating the deflection coil producing the magnetic field at a 
constant angular velocity. Such a sweep may also be generated by using a fixed 
position deflection system having separate x and y components of deflection. 
The same combination of linear and sinusoidal modulation as that of the spiral 
sweep may be used, except that the period of the linear modulation component 
to the x-y deflection system must be short compared to the period of the 
angular deflection. 
    There are applications, particularly in airborne radar systems, where 
hyperbolic sawtooth sweeps are preferred to linear sawtooth sweeps. Such a 
hyperbolic sweep used in a rotating radial sweep system provides for true 
ground range radar mapping in an airborne system. 
 �34s[Glenn M. Glasford]
�33s 
  
A700000001660�254s�5v�1v1703
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4281856
�6v�2v�32sSweetgum �21m �33s
    The tree Liquidambar styraciflua, also called redgum, a deciduous tree of
the southeastern United States. It is found northward as far as southwestern
Connecticut, and also grows in Central America. Sweetgum is readily
distinguished by its five-lobed, or star-shaped, leaves and by the corky wings
or ridges usually developed on the twigs. The erect trunk is a dark gray, but
the branches are lighter in color. In winter the persistent, spiny seedballs
are an excellent diagnostic feature.
    Sweetgum is used for furniture, interior trim, railroad ties, cigar boxes,
crates, flooring, barrels, woodenware, and wood pulp, and it is one of the
most important materials for plywood manufacture. Sweetgum is one of the most
desirable ornamental trees, chiefly because of its brilliant autumn
coloration.  See  HAMAMELIDALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

A700000001141�254s�5v�1v1703
\IMG\CI1681.IMG
4281856
�6v�2v�32sSwim bladder   �21m �33s 
    A gas-filled sac found in the body cavities of most bony fishes 
(Osteichthyes). The swim bladder has various functions in different fishes, 
acting as a float which gives the fish buoyancy, as a lung, as a hearing aid, 
and as a sound-producing organ. In many fishes it serves two or three of these 
functions, and in the African and Asiatic knife fishes (Notopteridae) it may 
serve all four. The swim bladder contains the same gases that make up air, but 
often in different proportions. 
 �34s[R. McNeill Alexander]
�33s 
  
A700000001446�254s�5v�1v1703
\IMG\CI1681.IMG
4281856
�6v�2v�32sSwimming birds   �21m �33s 
    Birds belonging to the orders Charadriiformes and Pelecaniformes. There 
are three important families of long-winged swimming birds (Charadriiformes): 
Stercorariidae, all of which occur in North America, comprising skuas and 
jaegers; Laridae, the gulls and terns; and Rynchopidae, which contains three 
species of skimmer. In the six families of pelicaniform birds all four toes 
are connected by webs. Included are the pelicans, gannets, cormorants, and 
frigate birds; all are rather large fish-eating birds. The order has a 
cosmopolitan distribution. All are found near water, some being entirely 
marine, others inland, fresh-water species.  See  CHARADRIIFORMES; 
PELECANIFORMES.  
 �34s[Charles B. Curtin]
�33s 
  
A700000001744�254s�5v�1v1703
\IMG\CI1681.IMG
4281856
�6v�2v�32sSwine production �21m �33s
    An agricultural business whose primary products are pork, lard, hides, and
innumerable by-products of a pharmaceutical nature. Pork and lard supply
approximately 15% of the total calories consumed as food in the United States.
Pork is more successfully cured and stored than many other meats, and it is
estimated that about 60% of the swine carcass is cured by various methods.
    Market hogs are produced with emphasis on red meat production while
minimizing fat or lard production. Breeders of purebred, hybrid, and crossbred
swine provide the seed stock for commercial hog production and are entrusted
with the responsibility of changing the carcass to better meet the demands of
the consumer. The purebred breeds of major significance include the following:
Berkshire, Chester White, Duroc, Hampshire, Landrace, Poland China, Spotted
Poland China, and Yorkshire.
 �34s[Aldon H. Jensen ;Donald E. Becker]
�33s

A700000002070�254s�5v�1v1703
\IMG\CI1681.IMG
4281856
�6v�2v�32sSwitched capacitor   �21m �33s 
    A module consisting of a capacitor with two metal oxide semiconductor 
(MOS) switches connected as shown in illustration a. These elements in the 
module are easily realized as an integrated circuit on a silicon chip by using 
MOS technology. The switched capacitor module is approximately equivalent to a 
resistor, as shown in illustration b. The fact that resistors are relatively 
difficult to implement gives the switched capacitor a great advantage in 
integrated-circuit applications requiring resistors. Some of the advantages 
are that the cost is significantly reduced, the chip area needed is reduced, 
and precision is increased. Although the switched capacitor can be used for 
any analog circuit realization such as analog-to-digital or digital-to-analog 
converters, the most notable application has been to voice-frequency 
filtering.  See  ANALOG-TO-DIGITAL CONVERTER; DIGITAL-TO-ANALOG CONVERTER; 
ELECTRIC FILTER; INTEGRATED CIRCUIT. 
 �34s[M. E. Van Valkenburg]
�33s 
  
  
A700000015142�254s�5v�1v1703
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\IMG\CI1681.IMG
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�6v�2v�32sSwitching circuit   �21m �33s 
    A constituent electric circuit of switching or digital data-processing 
systems. Well-known examples of such systems are digital computers, dial 
telephone systems, automatic accounting and inventory systems. In these and 
other switching systems the component circuit units receive, store, and 
manipulate information in coded (digital) form to accomplish the specified 
objectives of the system.  See  SWITCHING SYSTEMS (COMMUNICATIONS); SWITCHING 
THEORY.  
    Physically, switching circuits consist of conducting paths interconnecting 
discrete-valued electrical devices. The most generally used switching circuit 
devices are two-valued or binary, such as switches and relays in which manual 
or electromagnetic actuation opens and closes electric contacts; vacuum and 
gas-filled electronic tubes, semiconductor rectifiers and transistors, which 
do or do not conduct current; and magnetic structures, which can be saturated 
in either one of two directions. 
    The electrical conditions controlling these switching circuit devices are 
also generally two-valued or binary, such as open versus closed path, full 
voltage versus no voltage, large current versus small current, and high 
resistance versus low resistance. Such two-valued electrical conditions, as 
applied to the input of a switching circuit, represent either (1) a 
combination of events or situations which exist or do not exist; (2) a 
sequence of events or situations which occur in a certain order; or (3) both 
combinations and sequences of events or situations. The switching circuit 
responds to such inputs by delivering at its output, also in two-valued terms, 
new information which is functionally related to the input information. 
    The two fundamental characteristics of switching circuits are logic and 
memory. A switching circuit embodies such logical relationships as output X is 
to exist only if inputs A and B occur simultaneously; and output Y is to exist 
if either input A or input B occurs. The factor of memory, in turn, enables a 
switching circuit to hold or retain a given state after the condition that 
produced the state has passed. 
  
    Basic combinational circuits.  A combinational switching circuit is one in 
which a particular set of input conditions always establishes the same output, 
irrespective of the past history of the circuit. 
    In electronic switching circuits, so-called gates are used to perform 
logical functions equivalent to the series-parallel networks of switch 
contacts. In this sense, an electronic gate is an elementary combinational 
circuit. Gates do not function by physical rearrangement of interconnecting 
paths, as do switch or relay contacts. Instead, they function by control of 
voltage or current levels at their output. 
    The most commonly encountered gates are the AND and the OR gates. The AND 
gate produces an output only if all its inputs are concurrently present; an OR 
gate produces an output if any one or any combination of its inputs is present. 
  
    Basic sequential circuits.  A sequential switching circuit is one whose 
output depends not only upon the present state of its input, but also on what 
its input conditions have been in the past. Sequential circuits, therefore, 
require memory elements. A typical electronic memory element used in 
sequential circuits is a simple circuit called a flip-flop (Fig. 1). A 
flip-flop consists of two amplifiers connected so that the output of one 
amplifier is the input of the other. A voltage pulse will set the flip-flop 
into one of two states, and that state remains until another voltage pulse 
resets, or returns the flip-flop to its original state. It can therefore be 
used to remember that an event has taken place. 
    Relays, flip-flop, and similar memory elements provide static, or fixed, 
memory; they hold the stored information indefinitely, or until they are told 
to ``forget.'' In contrast, a delay line provides transient memory. A delay 
line has the property that an electrical signal applied to its input is 
delayed on its way to the output. 
  
    In switching systems, situations often arise where several similar circuit 
units are ready at the same instant to request collaboration with another type 
of functional circuit. Mutual interference among the requesting circuits is 
prevented by the lockout circuit. In response to concurrent inputs from a 
number of external circuits, a lockout circuit provides an output indication 
corresponding to one, and only one, of these circuits at any time. Lockout 
circuits are sometimes referred to as hunting or finding circuits. 
    Switching systems process information in coded form; the information they 
manipulate is generally in the form of numbers. Numerical codes are many and 
varied, each with its own characteristics and more or less distinct advantages 
for different switching circuit situations. Therefore, one of the common 
functional circuits in switching systems is the translating circuit, which 
translates information received in one code into the same information 
expressed in another code. These translating circuits are combinational 
circuits; a given input signal combination representing a code to be 
translated always produces the same output signals, which represent the 
desired code. 
ms pgs414-4186:50 
    Information received by a switching system is not always used immediately. 
It must be stored in register circuits for future use. 
    In a register circuit the coded information to be stored is applied as 
input, is retained by memory elements of the circuit and, when needed, the 
registered information is taken as output in the same code or in a different 
code. Register circuits are devised with a great variety of memory elements, 
and have capacities to store from a few to millions of information bits. A 
frequently encountered form of register circuit is the shift register. This 
type of register has the ability to shift its stored digital information 
internally to positions representing higher or lower numerical values in the 
code employed. For example, in decimal code registration a digit may be 
shifted from the units to the tens position. An obvious use of such registers 
is in digital computers when, for example, partial multiplication products 
have to be lined up for addition. 
    One of the most frequently encountered circuits in switching systems is 
the counting circuit whose function, in general, is to detect and count 
repeated current or voltage pulses which represent incoming information.  See  
COUNTING CIRCUIT; LOGIC CIRCUITS. 
 �34s[John Meszar]
�33s 
  
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�6v�2v�32sSwitching systems (communications)   �21m �33s 
    The assemblies of switching and control devices provided so that any 
station in a communications system may be connected as desired with any other 
station. A telecommunications network consists of transmission systems, 
switching systems, and stations. Transmission systems carry messages from an 
originating station to one or more distant stations. To enable the 
transmission facilities to be shared, stations are connected to and reached 
through switching system nodes that are part of most telecommunications 
networks. Switching systems act under built-in control to direct messages 
toward their ultimate destination or address. Most switching systems, known as 
central or end offices, are used to serve stations. A small number of systems 
serve as tandem (intermediate) switching offices for large urban areas or toll 
(long-distance) offices for interurban switching. These end and intermediate 
office functions are sometimes combined in the same switching system. 
  
    Switching system fundamentals.  Telecommunications switching systems 
generally perform three basic functions; they transmit signals over the 
connection or over separate channels to convey the identity of the called (and 
sometimes the calling) address (for example, the telephone number), and alert 
the called station; they establish connections through a switching network for 
conversational use during the entire call; they process the signal information 
to control and supervise establishment and disconnection of the switching 
network connection. 
    In some data or message switching when realtime communication is not 
needed, the switching network is replaced by a temporary memory for data 
storage. This type of switching is known as store-and-forward switching. 
  
    Signaling and control.  The control of switching systems is accomplished 
remotely by a specific form of data communications known as signaling. 
Switching systems are connected with one another by telecommunication channels 
known as trunks. They are connected with the served stations or terminals by 
lines. Until 1976 most signals were sent by direct- or alternating-current 
pulses over the lines and trunks. Then a new form of signaling was introduced 
for use between toll offices in the United States. It is known as common 
channel interoffice signaling (CCIS). As its name implies, a network of 
separate data communication paths is used for transmitting all signaling 
information between offices. 
    In some switching systems the signals for a call directly control the 
switching devices through which the transmission path is established. This 
direct control was the earliest and is still the most prevalent form of 
automatic switching around the world. 
    For most modern switching systems the signals for identifying or 
addressing the called station are received by a central control that processes 
calls on a time-shared basis. Central controls receive and interpret signals, 
select and establish communication paths, and prepare signals for 
transmission. These signals include addresses for use at succeeding nodes or 
for alerting (ringing) the called station. 
    Until the introduction of electronics into switching, the central controls 
employed complex relay logic circuits known in many systems as markers. Most 
electronic controls are now designed to process calls not only by complex 
logic, but also by the use of logic tables or a program of instructions stored 
in bulk electronic memory. The tabular technique is known as action translator 
(AT). The latter arrangement is now the most accepted and is known as stored 
program control (SPC). Either type of control may be distributed among the 
switching devices rather than residing centrally.  See  COMPUTER STORAGE 
TECHNOLOGY.  
    Common channel signaling became practical as a result of processor 
control. To reduce the number of data channels between all switching nodes, a 
signaling network with separate switching of signaling data is introduced. 
These signal switching nodes are fully interconnected and duplicated and are 
known as signal transfer points (STPs). 
  
    Switching connectives.  Space and time division are the two basic 
techniques used in establishing connections. When an individual conductor path 
is established through a switch for the duration of a call, the system is 
known as space division. When the transmitted speech signals are sampled and 
the samples multiplexed in time so that high-speed electronic devices may be 
used simultaneously by several calls, the switch is known as time division. 
    Most switching systems now in service are space division and employ some 
form of electromechanical switch. These switching systems, unless controlled 
electronically (for example, SPC) are referred to by the name of the 
electromechanical device (for example, a step-by-step, panel, crossbar, EMD, 
codebar, XY, or rotary system). These devices are connected together in 
successive stages to concentrate, distribute, and select idle paths as 
required to establish the required connections. The simplest arrangement is 
the one in which, as each digit is received, the call progresses from one 
stage to the next by making a selection and finding an idle link at each stage 
until the selection process is completed. 
  
    Electronic switching.  The invention of the transistor spurred the 
introduction of electronics and semiconductor technology into switching system 
design. Stored-program-control switching systems have been placed in service 
in the United States, serving more than 40,000,000 lines. Stored-program 
control has become the principal type of control for all types of new 
switching systems throughout the world, including toll, private branch, data, 
and Telex systems. About 1200 small systems were earlier placed in service in 
Canada, Great Britain, and France using action translator (AT) electronic 
logic and memory controls.  See  TELEPHONE; TELEPHONE SERVICE. 
 �34s[Amos E. Joel, Jr.]
�33s 
  
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�6v�2v�32sSycamore �21m �33s
    American sycamore (Platanus occidentalis) a member of the plane tree
family, known also as American plane tree, buttonball, or buttonwood, and
ranging from southern Maine to Nebraska and south into Texas and northern
Florida. It has the most massive trunk of any American hardwood.
Characteristic are the white patches which are exposed when outer layers of
the bark slough off; the simple, large, lobed leaves whose stalks completely
cover the conical winter buds; and the spherical fruit heads that are always
borne singly in the American species and persist throughout the winter. The
tough, coarse-grained wood is difficult to work, but is useful for butchers'
blocks, saddle trees, vehicles, tobacco and cigar boxes, crates, and slack
cooperage.  See  ROSALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

A700000000601�254s�5v�1v1705
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�6v�2v�32sSycettida   �21m �33s 
    An order of calcareous sponges of the subclass Calcaronea. This order 
includes the families Sycettidae, Heteropiidae, Grantiidae, Amphoriscidae, and 
Lelapiidae. Choanocytes occur in flagellated chambers, and the spongocoel is 
not lined with these cells.  See  CALCAREA. 
 �34s[Charles B. Curtin]
�33s 
  
A700000001521�254s�5v�1v1705
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�6v�2v�32sSyenite   �21m �33s 
    A phaneritic (visibly crystalline) plutonic rock with granular texture 
composed largely of alkali feldspar (orthoclase, microcline, usually 
perthitic) with subordinate plagioclase (oligoclase) and dark-colored (mafic) 
minerals (biotite, amphibole, and pyroxene). If sodic plagioclase (oligoclase 
or andesine) exceeds the quantity of alkali feldspar, the rock is called 
monzonite. Monzonites are generally light to medium gray, but syenites are 
found in a wide variety of colors (gray, green, pink, red), some of which make 
the material ideal for use as ornamental stone. Syenite is an uncommon 
plutonic rock and usually occurs in relatively small bodies (dikes, sills, 
stocks, and small irregular plutons).  See  IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
�33s 
  
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�6v�2v�32sSymmetrodonta   �21m �33s 
    An order of the extinct mammalian infraclass Pantotheria of Mesozoic 
times. Long considered to be aberrant triconodonts, symmetrodonts are not 
recognized as a discrete order of small carnivorous or insectivorous mammals 
whose earliest representatives were close to the ancestry of all theria and 
especially close to the origin of the order Pantotheria. See MAMMALIA; 
PANTOTHERIA.  
 �34s[Malcolm C. McKenna]
�33s 
  
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�6v�2v�32sSymmetry laws (physics) �21m �33s
    The physical laws which are the expressions of the existing symmetries. A
conservation law results from each such symmetry; that is, from each symmetry
the existence of a quantity which is conserved (a constant of the motion) can
be deduced. Selection rules result from conservation laws.  See  SELECTION
RULES.

    Space-time symmetries.  A symmetry (or invariance) of the world exists
whenever the description of the laws of physics is unaffected by a change in
the frame of reference. For instance, the position of the origin of a space
coordinate system is quite arbitrary; changing it makes no difference in the
description of the motion of bodies because the forces between bodies depend
only on their relative positions and not on any absolute position.
Equivalently, a system of bodies behaves the same if translated to another
place. This symmetry of space to translation implies the conservation of
momentum.
    Other symmetries of space-time are the irrelevance of (1) the origin of
the time coordinate, (2) the orientation of a coordinate system in space, and
(3) the velocity of a coordinate system (Lorentz invariance). Each of these
implies a conservation law, as shown in the table. All these symmetries are
termed continuous because the changes can be arbitrarily small; that is, a
finite change can be made bit by bit. The resulting constants of the motion
are classical quantities and are additive.  See  FRAME OF REFERENCE; LORENTZ
TRANSFORMATION; RELATIVITY; SPACE-TIME.
    Discrete symmetries (reflections) also exist, for which the irrelevant
change is not arbitrarily small. They imply constants of the motion (parities)
in quantum mechanics. These parities are multiplicative. For instance, the
direction of increasing time is irrelevant; the world is invariant to time
reversal (microscopic reversibility). Although, macroscopically, future and
past seem distinct, this is merely a result of the disposition of matter (a
state of anomalously small entropy at some time in the past) in the same way
that a point of space seems distinct by having a particular piece of matter
there. Space is also symmetrical to reflection of space or to inversion, the
reflection of all three directions of space; it is irrelevant which is the
positive direction of a space axis or of all three axes. This amounts to the
irrelevance of whether a right-handed or a left-handed coordinate system is
used. The resulting conserved quantity (eigenvalue of space inversion) is
(space) parity.  See  FUNDAMENTAL INTERACTIONS; PARITY (QUANTUM MECHANICS);
STRONG NUCLEAR INTERACTIONS; WEAK NUCLEAR INTERACTIONS.

    Internal symmetries.  Further symmetries, which are not space-time
symmetries, are called internal symmetries. For example, the zero of both
scalar and vector electromagnetic potentials is irrelevant; the addition of a
constant to an electromagnetic potential is of no consequence (so-called gauge
invariance of the first kind). In quantum mechanics this symmetry implies the
conservation of charge. There is also a discrete symmetry: It is (nearly)
irrelevant which sign of charge is called positive and which is called
negative. The qualification ``nearly'' is necessary here, just as in space
inversion, because the weak interactions do not observe the symmetry. Thus the
world is (nearly) invariant to charge reversal, the interchange of positive
and negative charge. A world with (nearly) the same properties as Earth's
would result if all electrons were replaced with positrons and all protons by
antiprotons and, in general, if all particles were replaced by their
antiparticles. The resulting (nearly) conserved quantity is termed charge
conjugation parity or charge parity, C.
    Although the weak interactions are not invariant under charge reversal or
space inversion, they are invariant (with one observed exception, Ksup0 decay)
to the combination of these reflections; they are also invariant under time
reversal. Equivalently stated, all interactions appear to very nearly conserve
CP and also T (C = charge conjugation, P = space inversion, T = time reversal).
    CP is not exactly conserved in Ksup0 decay. The smallness of the violation
contrasts with the maximal violation in weak interactions of C and P
separately. No other system is as sensitive to CP violation as the neutral K
mesons and no violation of either CP or T has been observed elsewhere.
    The reflection symmetries are correlated by the CPT theorem of G. Lueders.
This theorem states that a Lorentz invariant field theory is necessarily
invariant to the product of the three reflections: charge conjugation C, space
inversion P, and time reversal T. Elementary particle theory would be
enormously challenged if CPT invariance were found not to hold.  See MESON.
    The nuclear force between two protons is found to be identical to the
force between two neutrons. This is called the charge symmetry of the nuclear
force. More generally, the motion of a system composed of nucleous and pions
(the lowest mass quanta of the nuclear force field) is invariant to the
operation p  i n, PIsupp i PI-, PIsup0 i PIsup0. The combination of charge
conjugation and the charge symmetry operation is called isotopic inversion, G,
and carries each PI-meson into itself. The PI-meson thus has a G parity.
    Further, the nuclear force between a neutron and a proton is identical to
the force between two protons or two neutrons in the same orbital and spin
state (charge independence).  See  NUCLEAR STRUCTURE; SCATTERING EXPERIMENTS
(NUCLEI).
    The foregoing symmetry can be expressed as the isotropy of a
three-dimensional ``isotopic space,'' which implies the conservation of
``angular momentum,'' or isotopic spin I, in this space. The component of a
particle's isotopic spin along the ``third'' axis I3 is related linearly to
the charge of the particle. The consequences of charge independence are
formally very similar to the consequences of the conservation of angular
momentum.
    Charge independence can also be described as symmetry with respect to
arbitrary unimodular unitary transformations of the proton and neutron. The
group of such transformations is called SU2. The analogous group of
transformations on n particles is SUsubn.  See	SPINOR; UNITARY SYMMETRY.
    It appears that hadrons are well described as compounds of so-called
quarks, of which there are n kinds (flavors): u, d, s, c, b, . . . . The net
number of quarks of each flavor is changed only in weak interactions. Further,
the quarks of different flavors interact in the same way with the fundamental
strong interaction between them (the ``glue'' which binds them together to
make hadrons). This means that the strong interactions would have a flavor
SUsubn internal symmetry, were it not for the fact that the quarks of
different flavors have different masses; their masses seem to form roughly a
geometrical progression. In addition to this flavor SUsubn symmetry of strong
interactions, it is believed that there is also a color SU3 symmetry which is
exact but hidden.  See	COLOR (QUANTUM MECHANICS); FLAVOR; QUARKS.
 �34s[Charles J. Goebel]
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�6v�2v�32sSympathetic nervous system   �21m �33s 
    The portion of the autonomic nervous system concerned with nonvolitional 
preparation of the organism for emergency situations.  See  AUTONOMIC NERVOUS 
SYSTEM.  
    The sympathetic nervous system is best understood in mammals. It consists 
of two neuron chains from the thoracic and lumbar regions of the spinal cord 
to viscera and blood vessels. The first or preganglionic neuron has its cell 
body in the spinal cord and sends its axon to synapse with a postganglionic 
sympathetic neuron, which lies either in a chain of sympathetic ganglia 
paralleling the spinal cord or in a sympathetic ganglion near the base of the 
large blood vessels vascularizing the alimentary viscera. The postganglionic 
axons are longer than the preganglionic axons and extend to glands or smooth 
muscles of viscera and blood vessels. Sensory visceral nerve fibers innervate 
blood vessels and viscera and carry sensory information to the spinal cord, 
thus providing a visceral reflex (see illustration).  See  NERVOUS SYSTEM 
(VERTEBRATE); PARASYMPATHETIC NERVOUS SYSTEM. 
 �34s[Douglas B. Webster]
�33s 
  
  
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�6v�2v�32sSympathetic vibration   �21m �33s 
    The driving of a mechanical or acoustical system at its resonant frequency 
by energy from an adjacent system vibrating at this same frequency. Examples 
include the vibration of wall panels by sounds issuing from a loudspeaker, 
vibration of machinery components at specific frequencies as the speed of a 
motor increases, and the use of tuned air resonators under the bars of a 
xylophone to enhance the acoustic output. Increasing the damping of a 
vibrating system will decrease the amplitude of its sympathetic vibration but 
at the same time widen the band of frequencies over which it will partake of 
sympathetic vibration.  See  RESONANCE (ACOUSTICS AND MECHANICS); VIBRATION. 
 �34s[Lawrence E. Kinsler]
�33s 
  
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�6v�2v�32sSymphyla   �21m �33s 
    A class of the Myriapoda. The symphylans, like the pauropods, are tiny, 
pale, centipedelike creatures that inhabit humans or soil, or live under 
debris; in general, they live wherever there is sufficient moisture to 
preclude excessive water loss. They are similar to the Pauropoda and Diplopoda 
in being progoneate and anamorphic. Each of their mandibles, like those of 
millipedes, bears a movable gnathal lobe; at the same time their two pairs of 
maxillae are more reminiscent of the chilopods and lower insects than of the 
singly maxillate millipedes and pauropods. 
    The class consists of three families to which not more than 60 species 
have been assigned.  See  MYRIAPODA. 
 �34s[Ralph E. Crabill, Jr.]
�33s 
  
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�6v�2v�32sSynapsida   �21m �33s 
    A subclass of extinct reptiles, in general characterized by a temporal 
fenestra that lies below the junction of the postorbital and squamosal bones, 
a so-called lower temporal opening. In advanced forms, however, the 
postorbital-squamosal bridge is absent. Mammals arose from this group of 
reptiles during the Triassic Period.  See  REPTILIA. 
    Synapsids first appear in the geological record in the Upper 
Carboniferous. They flourished during the Late Paleozoic and Early Mesozoic 
but became extinct at about the end of the Triassic. During this span of time 
they underwent a broad adaptive radiation on land and made minor invasions of 
aquatic habitats. There were two major phases of this radiation: one early, by 
pelycosaurs, and the other later (Late Permian and Triassic), by therapsids.  
See  PELYCOSAURIA; REPTILIA; THERAPSIDA. 
 �34s[Everett C. Olson]
�33s 
  
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�6v�2v�32sSynaptic transmission   �21m �33s 
    A term applied to the physiological mechanisms by which one nerve cell 
(neuron) influences the activity of an anatomically adjacent neuron with which 
it is functionally coupled. The term synapse refers to the region of contact 
between two neurons which is characterized by unique local morphological 
specializations. Neurons also make anatomical and functional connection with 
various kinds of effector cells (muscle, gland, and sensory cells), and here 
the region of contact is called a neuroeffector junction. Neuroeffector 
transmission provides the means by which a neuron can exert control over the 
effector cell to which it is connected. Neuronal coupling by synaptic 
transmission makes possible the joining of many neurons to form extended 
pathways or complex networks over which information in the form of nerve 
impulses can be conducted, processed, and stored. Over such anatomical arrays, 
information is collected and carried from one region of the nervous systems to 
another, and it can be passed to various other parts of the organism supplied 
with neuronal connections. 
    Typically, as a nerve impulse arrives in presynaptic terminals it 
initiates a series of events which ultimately produce a change in the 
postsynaptic membranes of the adjacent neuron. This sequence of steps known as 
synaptic transmission takes place very rapidly, requiring as little as 0.6 
millisecond for its completion. It represents a unidirectional process which 
flows from pre- to postsynaptic elements, but not in the reverse direction. 
Thus one important characteristic of a synapse is that it exerts a valvelike 
action on the transmission of information. 
    The coupling mechanism which operates at various synapses can be 
essentially chemical or electrical in nature, but some synapses are known in 
which both types of transmissions are effective and operate in parallel. 
    In chemical transmission a nerve impulse which arrives at the presynaptic 
terminals initiates the rapid release of a chemical intermediary (transmitter) 
which diffuses through the fluid filling the very narrow space (synaptic 
cleft) between the pre- and postsynaptic elements. When the transmitter 
arrives at the postjunctional membrane of the postsynaptic cell it approaches 
and combines with receptor molecules. When the receptor molecule combines with 
the transmitter, it undergoes a change in shape, allowing ions to enter the 
membrane channel. Thus the transmitter can be thought of as a chemical ``key'' 
which opens a specific receptor protein gate at the mouth of an ionic channel 
in the postsynaptic membrane. Examples of relatively well-established 
transmitters are acetylcholine, morepinephrine, dopamine, serotonin, gamma 
aminobutyric acid, glutamic acid, and glycine.  See  ACETYLCHOLINE. 
    In addition to chemical transmission, another type of synaptic coupling 
exists which is fundamentally electrical in nature. By appropriate anatomical 
arrangements, the electric currents which accompany the propagated action 
potentials appearing in presynaptic structures can be channeled to flow across 
and stimulate postsynaptic membranes, and thereby they may initiate action 
potentials in the postsynaptic elements. This arrangement, known as electrical 
transmission, is relatively simple, and since synaptic delay is minimal, it is 
extremely rapid in its action. For this reason electrical transmission is 
utilized in animals, such as electric fish, which require virtually 
synchronous activation of electric organs distributed along the length of the 
body. The muscle cells which make up the ventricular muscle of the hearts of 
higher animals are also coupled electrically, but these junctions are not 
classified as synaptic.  See  ELECTRIC ORGAN (BIOLOGY); NERVOUS SYSTEM 
(VERTEBRATE).  
 �34s[William L. Nastuk]
�33s 
  
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�6v�2v�32sSynbranchiformes   �21m �33s 
    An order of eellike fishes that, unlike true eels, has the premaxillae 
present as distinct bones. The small gill apertures are often confluent across 
the breast. The gills are poorly developed, and in some species respiration is 
accomplished in part by highly vascularized buccopharyngeal pouches. There are 
no fin spines or pectoral fins, and the median fins, if developed, are 
continuous. Pelvic fins, if present, are small and located on the throat. The 
body may be naked or scaled. There is no swim bladder. The group, which has no 
fossil record, is classified into 2 suborders, 3 families, 7 genera, and 12 
species. These serpentine fishes inhabit swamps, caves, and sluggish fresh and 
brackish waters of tropical America, Australia, eastern and southeastern Asia, 
the East Indies, and tropical Africa.  See  ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
�33s 
  
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�6v�2v�32sSyncarida   �21m �33s 
    A superorder of the class Crustacea, subclass Malacostraca. There are only 
a few species of these higher crustaceans. They inhabit special regions such 
as subterranean wells and springs, as well as mountain lakes. Two orders are 
recognized, the Anaspidacea and Bathynellacea. A carapace and oostegites are 
lacking. Eyes may be pedunculate, sessile, or lacking. All thoracic limbs have 
exopodites, but none are chelate or subchelate.  See  ANASPIDACEA; 
BATHYNELLACEA; MALACOSTRACA. 
 �34s[Hans Jakobi]
�33s 
  
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�6v�2v�32sSynchro   �21m �33s 
    The name applied to a wide variety of rotary transducers, which are 
actually synchronous ac motors (or generators) adapted to serve as variable 
transformers in the measurement of angular position. The construction of a 
synchro is similar to that of a miniature three-phase synchronous motor or 
generator. The stator, which contains the three-phase winding, is a slotted 
cylindrical structure made up of punched steel laminations, and the rotor 
contains a single winding, which may be of the salient-pole, umbrella, or 
slotted-cylinder type.  See  ALTERNATING-CURRENT GENERATOR. 
    Instead of exciting the rotor field with direct current and driving the 
shaft at a constant velocity as in an alternator, the rotor field is excited 
with a constant single-phase ac voltage, and the shaft moves at only low 
speeds, often standing completely still. This unit, called a synchro 
generator, is basically a transformer with one primary (the rotor winding) and 
three secondaries (the Y-connected windings of the stator). The voltages 
induced in the secondary windings are proportional to the cosines of the 
angles between each stator coil and the rotor. Thus an electrical reference 
frame is formed which may be read at a remote point by means of another 
synchro, called a synchro control transformer, operating in reverse fashion as 
shown in the illustration. 
 �34s[J. Lowen Shearer]
�33s 
  
  
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�6v�2v�32sSynchronization   �21m �33s 
    The process of maintaining one operation in step with another. The 
commonest example is the electric clock, whose motor rotates at some integral 
multiple or submultiple of the speed of the alternator in the power station. 
In television, synchronization is essential in order that the electron beams 
of receiver picture tubes are at exactly the same spot on the screen at each 
instant as is the beam in the television camera tube at the transmitter.  See  
TELEVISION.  
 �34s[John Markus]
�33s 
  
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�6v�2v�32sSynchronous capacitor   �21m �33s 
    A rotating ac machine running with no mechanical load. It is often called 
a synchronous condenser. These machines generally have salient poles, eight 
poles being typical. However, two-pole generators are sometimes disconnected 
from the turbine and operated as synchronous condensers. The purpose of these 
machines is to provide reactive power to the system or to receive some 
reactive power. The amount and sign of the reactive power depends on the field 
current supplied to the rotor. This is an effective method of controlling the 
voltage at that point in the power system.  See  ELECTRIC POWER MEASUREMENT. 
    Rotating synchronous condensers are sometimes used at the terminals of dc 
transmission lines where it has been found necessary to have a connected 
rotating capacity about 30% of the converter terminal rating.  See  
DIRECT-CURRENT TRANSMISSIONS; SYNCHRONOUS MOTOR. 
 �34s[Lee A. Kilgore]
�33s 
  
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�6v�2v�32sSynchronous converter   �21m �33s 
    A synchronous machine used to convert alternating current (ac) to direct 
current (dc), or vice versa. The ac-to-dc converter has been superseded by the 
mercury arc rectifier (for reasons of efficiency, lower maintenance costs, and 
less trouble) or by motor-generator sets. Converters are no longer 
manufactured, but there are converters still in use.  See  DIRECT-CURRENT 
GENERATOR; SYNCHRONOUS MOTOR. 
 �34s[Loyal V. Bewley]
�33s 
  
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�6v�2v�32sSynchronous generator �21m �33s
    An alternating-current generator whose operating speed is proportional to
system frequency. These generators usually have the field winding mounted on
the rotor and a stationary armature winding mounted on the stator. In small
ratings where high reliability is imperative, the magnetic field may be
created by permanent magnets. Small alternators are also being built with
stationary field windings and rotating armatures with their leads brought out
through collector rings or fed directly to rotating rectifiers, which supply
direct current to the rotating field of a much larger alternator, as in the
brushless excitation system. Still another type, the inductor alternator, has
both its field and armature windings in the stator. See INDUCTOR ALTERNATOR.
    Although synchronous generators may be single-phase, they are usually two-
or three-phase; most are three-phase. Singlephase generators are rare because
they are larger than polyphase machines of the same kilovolt-ampere ratings,
because they have a pulsating torque and are noisy, and because single-phase
power is not well suited to self-starting ac motors other than those of
fractional horsepower sizes. For theory of synchronous machines see
SYNCHRONOUS MOTOR.
    In the usual type with rotating field windings, a pair of field poles must
pass a given point on the armature in 1 cycle. Hence the number of poles
required is determined from the frequency f in hertz (Hz) and speed by the
equation. The speed selected is that best suited to the prime mover. Steam

		       Number of poles = 120f/rpm

turbines operate most economically at high speed, hence two- or four-pole
generators are used for this service, running at 3600 or 1800 rpm,
respectively, for an output frequency of 60 Hz. For hydraulic turbine or
engine drive, slow-speed machines having many poles are customary. See
ALTERNATING-CURRENT GENERATOR.
 �34s[Leon T. Rosenberg]
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�6v�2v�32sSynchronous motor   �21m �33s 
    An alternating-current (ac) motor which operates at a fixed synchronous 
speed proportional to the frequency of the applied ac power. A synchronous 
machine may operate as a generator, motor, or capacitor depending only on its 
applied shaft torque (whether positive, negative, or zero) and its excitation. 
There is no fundamental difference in the theory, design, or construction of a 
machine intended for any of these roles, although certain design features are 
stressed for each of them. In use, the machine may change its role from 
instant to instant. For these reasons it is preferable not to set up separate 
theories for synchronous generators, motors, and capacitors. It is better to 
establish a general theory which is applicable to all three and in which the 
distinction between them is merely a difference in the direction of the 
currents and the sign of the torque angles.  See  ALTERNATING-CURRENT 
GENERATOR; ALTERNATING-CURRENT MOTORS; SYNCHRONOUS CAPACITOR. For special 
types of synchronous motors  see  HYSTERESIS MOTOR; RELUCTANCE MOTOR. 
 �34s[Richard T. Smith]
�33s 
  
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�6v�2v�32sSynchroscope   �21m �33s 
    An instrument used for indicating whether two alternating-current (ac) 
generators or other ac voltage sources are synchronized in time phase with 
each other. In one type, for example, the position of a continuously rotatable 
pointer indicates the instantaneous phase difference between the two sources 
at each instant. In more modern synchroscopes, a cathode-ray tube serves as 
the indicating means. 
    The term synchroscope is also applied to a special type of cathode-ray 
oscilloscope designed for observing extremely short pulses, using fast sweeps 
synchronized with the signal to be observed.  See  ELECTRIC POWER GENERATION; 
OSCILLOSCOPE.  
 �34s[John Markus]
�33s 
  
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�6v�2v�32sSynchrotron �21m �33s
    A device for accelerating charged particles (electrons or protons) by
directing them along a roughly circular path of fixed radius in a magnetic
guide field which varies cyclically and is perpendicular to the plane of the
orbit. As the particles pass through rf (radio frequency) accelerating
cavities placed along the orbit, their kinetic energy is increased. In this
way the accelerating action can be applied repetitively to the same particles,
thus multiplying their initial energy by factors of hundreds or thousands.

    Electron synchrotron.  Electron synchrotrons are used primarily for
research in elementary particle physics, but have found increasing use as
intense sources of ultraviolet and x-radiation in solid-state and materials
science.
    For normal injection energies, as a consequence of their low mass, the
velocity of the electrons injected into a synchrotron is usually so close to
the velocity of light that the change in velocity during acceleration is
insignificant. This is in contrast to the situation in proton synchrotrons or
cyclotrons where the velocity of the particles being accelerated may vary by a
factor of more than 10 during acceleration. As a result, the frequency of
revolution of electrons in the synchrotron is a constant, as is the frequency
of the power applied to the accelerating cavities. Injection into electron
synchrotrons is usually done from a preaccelerator such as a linac or
microtron having an output energy of tens or hundreds of MeV.
    The orbit radii of machines in use range from a few meters (1 m = 0.3 ft)
for a synchrotron which achieves a few hundred MeV peak energy, to 100 m for
the largest electron synchrotron, the 12-GeV instrument at Cornell University.
Most modern synchrotrons employ the alternating-gradient or strong-focusing
principle which minimizes the volume of the magnetic guide field, so that
while the circumference of the orbit may be almost a kilometer, the
cross-sectional dimensions of the evacuated volume surrounding the beam need
be only a few centimeters.
    In addition to the energy which must be supplied to the beam by the
cavities in order to match the rising strength of the guide field, energy must
also be supplied to replenish that which is emitted by the beam as
``synchrotron radiation.'' This electromagnetic radiation is given off by all
charged particles which follow a curved path, and is directed in a narrow beam
tangent to the path. It is so intense that, for many experiments in atomic and
solid-state physics and biology, synchrotron radiation is superior to that
produced by ultraviolet lamps and x-ray tubes and is being exploited
extensively for these purposes. Serious planning has been undertaken for
synchrotrons totally devoted to such studies.  See  SYNCHROTRON RADIATION.
 �34s[M. Tigner]
�33s

    Proton synchrotron.  The proton synchrotron developed from the cyclotron
by way of the synchrocyclotron. This progression has resulted in proton
accelerators with higher and higher beam energies. A 400-GeV accelerator is in
operation in the United States, while several European countries have
constructed a similar facility near Geneva, Switzerland.  See  CYCLOTRON.
    The chief limitation on the energy of the synchrotron is  the size and
cost of the large magnet within which all the beam acceleration takes place.
The incorporation of time-varying magnetic fields made it possible to overcome
this limitation. The use of numerous but small magnets whose fields could
track the proton beam energy led to the major breakthrough that made possible
higher proton beam energies at reasonable cost.
    The essential feature of a proton synchrotron is that the average radius
of the accelerated beam is a constant. This is accomplished by a careful
synchronization of the magnetic field and the accelerating radio frequency.
The magnets are arranged in an approximate circle and are powered in such a
way that the excitation current can be varied from low to high values during
acceleration. As the momentum of the proton increases during acceleration, the
magnetic field is increased in a manner that keeps the guide path of the
protons nearly constant. This permits the bundle of protons to be contained
within a relatively small vacuum chamber located within the magnets.
    In the proton synchrotron the ion source and initial accelerator system
are located away from the accelerator. As seen in the illustration, the proton
beam is injected into the ring of magnets at a low energy. At the peak of
energy it can be used on internal targets or it can be deflected and extracted
from the accelerator to be used externally.
    In order to hold the plasma of protons together, it is necessary to have
some restoration force that keeps the protons within the vacuum chamber. On
older ``weak-focusing'' accelerators, focusing of the beam and its maintenance
in a circular path are accomplished simultaneously by the gentle shaping of
the iron pole faces of the guide field magnets.  In order to keep the volume
of the magnetic field as small as possible, stronger focusing is needed to
reach higher energies. At Fermilab this is accomplished by using quadrupole
(four-pole) magnets that act like lenses in keeping particles together. Since
their effect is not combined with that of the guide field dipoles which
determine the mean orbit shape, the structure is called a separated-function
strong-focusing accelerator.  See  PARTICLE ACCELERATOR.
 �34s[James R. Sanford]
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�6v�2v�32sSynchrotron radiation �21m �33s
    Electromagnetic radiation emitted by charged particles in circular motion
at relativistic energies. Such emission of light by electrons, guided by
celestial magnetic fields, has long been known to astronomers and is
responsible, for example, for the beautiful background light in the Crab
Nebula. With the construction of high-energy electron synchrotrons and storage
rings for high-energy physics research, very powerful sources of synchrotron
radiation in the ultraviolet and x-ray parts of the spectrum became available
in many countries.  See  CRAB NEBULA; PARTICLE ACCELERATOR; SYNCHROTRON.
    As a continuum source, synchrotron radiation produces five or more orders
of magnitude higher intensity on an experimental sample than powerful
rotating-anode x-ray tubes. The power is radiated in a smooth, featureless
continuum without the spikes and structure associated with other sources. For
most regions of the ultraviolet and x-ray spectrum, no other source of intense
continuum radiation exists, and experiments must work only at the
characteristic lines of gas-discharge lamps and x-ray tubes. The radiation is
produced in a train of pulses which is different for each source. The pulse
duration can be as short as 50 picoseconds, and the interval between pulses
can be 1 microsecond or longer.
 �34s[Herman Winick ;Arthur Bienenstock]
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�6v�2v�32sSyncline   �21m �33s 
    A fold in which the beds are inclined down and toward the axis. Synclines 
may be symmetrical, asymmetrical, overturned, or recumbent. Most have elongate 
trends, with axes that plunge from the extremities toward interior points 
along the axes. Others, called basins, have no distinct trend. In general, the 
stratigraphically younger beds are found toward the center of curvature, but 
in complexly deformed regions such simple concepts may not apply. 
    Stratigraphic synclines are those folds which, regardless of their 
observed forms, are inferred from stratigraphic data to have been synclines 
originally. Structural synclines (see illustration) are those which have 
synclinal form regardless of their stratigraphic relations.  See  ANTICLINE; 
FOLD AND FOLD SYSTEMS. 
 �34s[Philip H. Osberg]
�33s 
  
  
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�6v�2v�32sSyngamy   �21m �33s 
    A process involving the union of sexual cells, or gametes. Customarily 
these are eggs (female) and spermatozoa (male), and syngamy involves the 
fusion of a single spermatozoon with a single egg. As such, syngamy is 
synonymous with fertilization. Accompanying the union of sexual cells are 
rather profound physiological and structural changes within the egg, but the 
process is considered terminated when a fusion of the gametic nuclei takes 
place.  
    Syngamy is essentially the same in all sexual organisms, although the 
length of the diploid and haploid phases can vary widely in relation to the 
entire life cycle. In many lower forms, however, it is not always possible to 
speak of syngamy as being the union of a spermatozoon and an egg, since the 
gametes are similar in appearance and undifferentiated as to sex.  See  CELL 
DIVISION.  
 �34s[Carl P. Swanson]
�33s 
  
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�6v�2v�32sSyphilis   �21m �33s 
    A subacute and chronic infectious disease of humans caused by a 
spirochete, Treponema pallidum, and transmitted principally by sexual 
intercourse, but occasionally by direct nonsexual contact or from a pregnant 
woman to her fetus as in congenital syphilis. With the introduction of 
penicillin, the prevalence of syphilis declined, but in no country is it a 
negligible health problem. 
    The disease evolves in a stepwise manner. The primary or initial lesion 
develops at the site of implantation of treponemes within 1 - 6 weeks. 
Characteristically, this lesion, also referred to as the hard chancre, is 
indurated, often like cartilage, and the lymph node draining the area is 
enlarged. The primary lesion is most often located on the genitalia; however, 
in about 10% of cases the primary lesion is extragenital, most commonly on the 
lips or fingers.  
    Treponemes are soon widely disseminated by the lymphatics and blood, 
giving rise after an interval of 3 - 8 weeks to generalized lesions 
characteristic of the secondary stage of the disease. Most prominent in this 
stage are treponeme-containing lesions of the skin and mucous membranes of the 
lips, mouth, and genitalia. Foci of spirochetal multiplication are present in 
many organs and may give rise to symptoms of arthritis, iritis, meningitis, 
nephritis, and hepatitis. Low-grade fever, loss of appetite, and general 
malaise are common. 
    Even without treatment, lesions of the secondary stage eventually subside, 
and the disease enters a latent phase in which infection persists but no 
lesions are detectable. The latent phase may persist for the rest of the 
individual's life, but without treatment it is probable that lesions of the 
tertiary stage will develop in the skin, central nervous system, heart, and 
great vessels or liver. Formerly, syphilis was an important cause of insanity 
and cardiovascular disease.  See  VENEREAL DISEASE. 
 �34s[Thomas B. Turner]
�33s 
  
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�6v�2v�32sSystellommatophora   �21m �33s 
    A superorder in the subclass Pulmonata containing three families 
(Rathouisiidae, Veronicellidae, and Onchidiidae) of sluglike mollusks that 
lack any trace of a shell, have separate external male and female orifices, 
lack a mantle cavity, have a posterior anus and excretory pore, and bear eyes 
on the tops of two contractile, but not retractile, tentacles.  See  PULMONATA. 
 �34s[G. Alan Solem]
�33s 
  
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�6v�2v�32sSystems analysis   �21m �33s 
    The application of mathematics to the study of systems. The term 
``operations research'' is reserved for the study of part of a system.  See  
OPERATIONS RESEARCH. 
    The basic idea is that a mathematical model of the system under study is 
constructed, a mathematical analysis is done of the mathematical model, and 
the results of this analysis are applied to the original system. 
    A great deal of experience is needed to construct the mathematical model 
and to interpret the results of the analysis. The mathematical analysis, 
usually involving a computer, is seldom routine. The procedures are so complex 
that often an individual systems analysis is required for the systems analysis 
of a particular system. Often particular parts of the process are carried out 
by different people. Ideally, there should be an interaction between the 
mathematical analysis, the construction of the mathematical model, and the 
interpretation of the results. 
    Systems analysis is thus a part of applied mathematics. What makes the 
difference between this and conventional applied mathematics is that the 
systems studied often involve human beings. The presence of humans and the 
application of the results to human systems introduce a great deal of 
complication.  
  
    There are three principal difficulties in the application of conventional 
mathematical techniques: dimensionality, the presence of both ``hard'' and 
``soft'' variables, and conflicting objectives. Dimensionality refers to the 
possibility that the description of a system may involve many state variables. 
This feature presents  serious difficulties, but they may be circumvented by 
means of various techniques. Much more serious is the presence of hard and 
soft variables and various combinations. Thus, for example, in a study of a 
business, the number of employees is a hard variable, while the quality of 
management is a soft variable. Hard and soft variables may be handled by the 
theory of fuzzy systems. In most situations, there are many objectives. In 
many cases, these objectives are in partial or complete conflict. Thus, it is 
impossible to write down a single criterion for performance. Fortunately, 
these questions can be studied by means of simulation. Simulation allows 
various possibilities to be studied in electronic time without disturbing the 
system or the people affected by the system.  See  DECISION THEORY; LINEAR 
SYSTEM ANALYSIS; OPTIMIZATION; SIMULATION; SYSTEMS ENGINEERING. 
 �34s[Richard Bellman]
�33s 
  
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�6v�2v�32sSystems ecology   �21m �33s 
    The combined approaches of systems analysis and the ecology of whole 
ecosystems and subsystems, when viewed as interdependent and functionally 
interacting components. Among the organisms and organs in any subdivision of 
vegetation or animal communities or microorganisms, there are many relations 
from one part to another and from each to the local environment. Physiological 
functions and other relations can sometimes be expressed in terms of 
mathematical functions. These need not be quantitative functions (one-to-one 
correspondence for real numbers) but may express logical alternatives 
(all-or-nothing) in the direction of flow of material or energy; they may 
express also alternative pathways of development or direction of change in the 
organic or inorganic parts of the system.  See  ECOLOGY; ECOSYSTEM; 
PHYSIOLOGICAL ECOLOGY (PLANT); SYSTEMS ANALYSIS. 
 �34s[Jerry S. Olson]
�33s 
  
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�6v�2v�32sSystems engineering   �21m �33s 
    The design of a complex interconnection of many elements (a system) to 
maximize an agreed-upon measure of system performance. Systems engineering, 
also referred to as system engineering, includes two parts: modeling, in which 
each element of the system and the criterion for measuring performance are 
described; and optimization in which adjustable elements are set at values 
that give the best possible performance. 
    The systems approach can be applied to problems ranging from the very 
simple to those so complex that the human mind is unable to comprehend the 
reasons for system behavior. A simple problem is the scheduling of the 
preparation of a family meal. A much more complex system problem is the 
control of the timing of hundreds or thousands of traffic lights in a city. 
    The techniques of systems engineering have been applied to a tremendous 
range of current problems, from industrial automation to control of weapons 
and space vehicles. 
    Modeling refers to the determination of a quantitative picture of the 
important system characteristics. This model may be in the form of collected 
data or analytical studies, or it may be a representation of the important 
system characteristics in a laboratory or computer simulation of the actual 
system.  
    A specific and rather narrow systems problem is the control of traffic 
flow through a tunnel. The designer has to maximize the number of cars 
permitted per hour through the tunnel. Tm do this, the designer can vary the 
speed limit through the tunnel and control the number of cars within the 
tunnel (or the rate at which cars enter). Before a system design can be 
decided on, measurements are made of the way in which cars move under various 
speeds and degrees of congestion. These data are combined with the known 
behavior characteristics of a driver. On the basis of this quantitative 
information, a computer simulation of the system can be constructed from which 
the engineer can study the effects of different operating rules, the results 
of a breakdown in one lane of the tunnel, or the effects of a driver who stays 
below the minimum speed limit.  See  SIMULATION. 
    The engineer attempts to design a system that is optimum according to a 
quantitative criterion which measures the quality of performance. For example, 
in the tunnel traffic problem, the entry and speed of cars are controlled in 
such a way that the tunnel handles the maximum possible number of cars per 
hour. In many situations, the criterion is probabilistic, and the engineer 
must optimize on the basis of probable behavior  of the system. The routing of 
emergency vehicles through a network of city streets is a typical example.  
See  OPTIMIZATION. 
    Three technological developments have enormously broadened the scope of 
problems amenable to the systems approach. The first of these is the 
understanding of the principles of automatic control (or the principles 
underlying automation). Systems can be designed in which the desired automatic 
control is realized by the appropriate choice of input signals and system 
configuration (including the use of feedback, with the actual system response 
compared automatically with the desired response and the error used to modify 
the response toward the desired value). In other words, technology has 
developed automatic, goal-seeking systems to replace the human role. The 
automatic control of elevators in a busy office building is a familiar 
example.  The second is the revolution in communications. Enormous quantities 
of data can be transmitted over great distances with nearly perfect fidelity. 
The third fundamental technological development has been the high-speed, 
electronic digital computer. The computer lies at the core of modern systems 
engineering, since it permits both the modeling and the optimization of 
systems vastly more complex than the human being alone can handle.  See  
AUTOMATION; CONTROL SYSTEMS; DIGITAL COMPUTER. 
 �34s[John G. Truxal]
�33s 
  
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�6v�2v�32sTabulata   �21m �33s 
    An extinct Paleozoic order of corals of the subclass Zoantharia in the 
class Anthozoa. The Tabulata are characterized by their exclusively colonial 
mode of growth (see illustration) and by secretion of a calcareous exoskeleton 
of slender tubes crossed by many transverse partitions called tabulae. Septa 
are of one order and are inconspicuous, represented by a row of spines, or 
even absent; 12 is a common number. Colony form varies widely, dependent in 
part on the ecological niche.  See  ZOANTHARIA. 
 �34s[Dorothy Hill]
�33s 
  
  
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�6v�2v�32sTacan   �21m �33s 
    A military rho-theta navigation system which allows the distance-measuring 
equipment (DME) to also provide bearing service without the large antennas or 
site errors characteristic of the civil very-high-frequency omnidirectional 
range (VOR). Range and accuracy are the same as DME (300 mi or 480 km, and 0.1 
mi or 0.16 km, respectively), with a bearing accuracy of 1DG. 
    To provide the added bearing service, the DME transponder is first 
arranged to operate at constant duty cycle. This means that the number of 
output pulses is held constant, whether the beacon is being interrogated by 
one or a hundred aircraft. 
    The total output of the transponder is amplitude-modulated by the rotating 
directional antenna system (see illustration). At the center of this system is 
the central radiator connected to the DME transponder, just as in the 
conventional DME. However, rotating around this radiation at 15 revolutions 
per second are two concentric dielectric cylinders. The inside one contains a 
single parasitic reflector which imparts a 15-Hz amplitude modulation to the 
DME replies, and the outside one contains nine parasitic elements which impart 
a 135-Hz amplitude modulation. On the same rotating shaft are mounted 
reference pulse generators which additionally modulate the transmitter with 
coded pulses, once per revolution for the 15-Hz signal (called the north 
reference burst) and nine times per revolution for the 135-Hz signal (called 
the auxiliary reference bursts).  
    In the airborne receiver the 15- and 135-Hz sine waves are detected and 
filtered and compared with the decoded reference bursts to provide a two-speed 
or fine-coarse bearing display whose accuracy and site freedom are superior to 
those of conventional VOR, yet with a ground antenna system which is small 
enough to mount on a ship's mast.  See  DISTANCE-MEASURING EQUIPMENT; 
ELECTRONIC NAVIGATION SYSTEMS; POLAR-COORDINATE NAVIGATION SYSTEM; RADIO RANGE. 
 �34s[Sven H. Dodington]
�33s 
  
  
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�6v�2v�32sTachometer   �21m �33s 
    An instrument that measures angular speed, as that of a rotating shaft. 
The measurement may be in revolutions over an independently measured time 
interval, as in a revolution counter, or it may be directly in revolutions per 
minute. The instrument may also indicate the average speed over a time 
interval or the instantaneous speed. Tachometers are used for direct 
measurement of angular speed and as elements of control systems to furnish a 
signal as a function of angular speed. 
 �34s[Alfred H. Wolferz]
�33s 
  
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�6v�2v�32sTachyon �21m �33s
    A hypothetical faster-than-light particle consistent with the special
theory of relativity. According to this theory, a free particle has an energy
E and a momentum p which form a Lorentz four-vector. The length of this vector
is a scalar, having the same value in all inertial reference frames. One
writes Eq. (1), where c is the speed of light and the parameter m (sup2) is

	  E (sub 2) - c(sup 2)p(sup2) = m(sup2) c(sup4) 	   (1)

a property of the particle, independent of its momentum and energy. Three
cases may be considered: m(sup2) may be positive, zero, or negative. The case
m(sup2)> 0 applies for atoms, nuclei, and the macroscopic objects of everyday
experience. The positive root m is called the rest-mass. If m(sup2)= 0, the
particle is called massless. A few of these are known: the electron neutrino,
the muon neutrino, the photon, and the graviton. The name tachyons (after a
Greek word for swift) has been given to particles with m(sup2) < 0.
    In general, the particle speed is given by Eq. (2). If m(sup2) < 0,


			      v =  ((cp)/E)c			      (2)

Eq. (1) implies E > cp and Eq. (2) gives v < c. If m(sup2) = 0, then E = cp and
v = c. In case m(sup2) < 0, one finds E < cp and v > c. Tachyons exist only  at
faster-than light speeds.  See	ELEMENTARY PARTICLE; RELATIVITY.
 �34s[Roland H. Good]
�33s

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�6v�2v�32sTaconite   �21m �33s 
    The name given to the siliceous iron formation from which the high-grade 
iron ores of the Lake Superior district have been derived. It consists chiefly 
of fine-grained silica mixed with magnetite and hematite. As the richer iron 
ores approach exhaustion in the United States, taconite becomes more important 
as a source of iron. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sTaeniodonta   �21m �33s 
    An order of extinct quadrupedal land mammals known from early Cenozoic 
deposits in North America and Asia. The stylinodontine taeniodonts were 
adaptively convergent with ground sloths. Stylinodontines developed into 
cow-sized animals with deep massive lower jaws, a reduced number of peglike 
upper and lower teeth, and large flattened claws. 
    Conoryctine taeniodonts were cat-sized to sheep-sized and had less 
specialization in dentition and skeleton. Conoryctines developed enlarged 
canines, but the lower jaws were unspecialized and the check teeth, though 
simplified, were low-crowned, enamel-enclosed, and cuspidate.  See  MAMMALIA. 
 �34s[Donald E. Savage]
�33s 
  
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�6v�2v�32sTaiga   �21m �33s 
    A zone of forest vegetation encircling the Northern Hemisphere between the 
arctic-subarctic tundras in the north and the steppes, hardwood forests, and 
prairies in the south. The chief characteristic of the taiga is the prevalence 
of forests dominated by conifers. The dominant trees are particular species of 
spruce, pine, fir, and larch. Other conifers, such as hemlock, white cedar, 
and juniper, occur locally, and the broad-leaved deciduous trees, birch and 
poplar, are common associates in the southern taiga regions. Taiga is a 
Siberian word, equivalent to ``boreal forest.''  See  TUNDRA. 
    The northern and southern boundaries of the taiga are determined by 
climatic factors, of which temperature is most important. However, aridity 
controls the forest-steppe boundary in central Canada and western Siberia. In 
the taiga the average temperature in the warmest month, July, is greater than 
50DGF (10DGC), distinguishing it from the forest-tundra and tundra to the 
north; however, less than four of the summer months have averages above 50DGF 
(10DGC), in contrast to the summers of the deciduous forest further south, 
which are longer and warmer. Taiga winters are long, snowy, and cold - the 
coldest month has an average temperature below 32DGF (0DGC). Permafrost occurs 
in the northern taiga. It is important to note that climate is as significant 
as vegetation in defining taiga. Thus, many of the world's conifer forests, 
such as those of the American Pacific Northwest, are excluded from the taiga 
by their high precipitation and mild winters. 
 �34s[J. C. Ritchie]
�33s 
  
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�6v�2v�32sTail assembly   �21m �33s 
    The assembly of the vertical tail, the horizontal tail, and a small 
section of the rear of the fuselage (see illustration). 
    The vertical tail consists of a fin which is fixed to the fuselage or body 
and a rudder which is movable by the pilot. The fin (fixed portion) is a 
symmetrical airfoil which is in line with the center line of the fuselage. In 
steady flight the rudder is stationary and approximately in line with the fin. 
The purpose of the vertical tail is to keep the airplane in line with the 
direction of motion or relative wind velocity. 
    The span of the horizontal tail lies in a horizontal plane and usually 
consists of a stabilizer and elevator. The stabilizer and elevator are 
symmetrical airfoil sections whose usual position is not in line with the 
fuselage center line. The principal purpose of the horizontal tail is to 
provide a balancing tail load so that the weight of the airplane, the lift, 
and the horizontal tail load are in equilibrium for a specific airspeed.  See  
AIRPLANE; ELEVATOR (AIRCRAFT); STABILIZER. 
 �34s[Richard G. Bowman]
�33s 
  
  
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�6v�2v�32sTalc �21m �33s
    A hydrated magnesium silicate with composition close to Mg3Si4O10(OH)2.
The mineral is commonly pale green, white, or grayish, with a greasy feel and
pearly luster. It occurs in foliated masses, fibrous forms, and also in
massive form, when it is commonly called soapstone. The mineral is
mechanically soft (hardness 1 - 1 1/2 on Mohs scale).
    The major talc-producing states in the United States are Vermont, Texas,
Montana, and New York, and the major talc-producing countries are Japan, the
United States, the Soviet Union, Korea, China, France, and Brazil.
    Talc is widely used in ceramics, paints, plastics, cosmetics, paper,
rubber, and in a variety of other applications. Massive talc, cut into slabs,
is used for laboratory tables, sinks, sanitary appliances, acid tanks,
electrical switchboards, mantels, and hearthstones. It is also used in
house-insulating materials.  See  SILICATE MINERALS.
 �34s[George W. Brindley]
�33s

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�6v�2v�32sTalus   �21m �33s 
    A heap or sloping sheet of loose rock waste at the base of a cliff or 
steep slope. Talus and its English equivalent, scree, are terms properly 
applied to the entire form and not to the fragmental material itself, which is 
rock waste or sliderock. Where the supply of rock waste is funneled downward 
through a notch or channel, a conelike talus usually develops. 
    The upper part of a talus is characteristically steep, consists of coarse, 
angular rock waste, and is easily set in motion by falling blocks or by 
climbers. In cold climates its interstices may contain snow or ice for much of 
the year. The lower part is usually less steep and may be partly filled with 
finer rock debris and soil on which vegetation can gain a foothold.  See  
LANDSLIDE.  
 �34s[C. F. Stewart Sharpe]
�33s 
  
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�6v�2v�32sTanaidacea   �21m �33s 
    An order of the eumalacostracans of the superorder Peracarida, derived 
from the genus Tanais. These animals have a worldwide distribution and with 
few exceptions are marine. They occur from the shore down to abyssal depths. 
They are free-living and benthonic. The order is divided into 2 suborders with 
5 families, 44 genera, and about 350 species. The body is linear, more or less 
cylindrical or dorsoventrally depressed. Thoracic segments 1 and 2 are fused 
with the head, and the last abdominal segment is fused with the telson. Eight 
pairs of thoracic legs are present, of which the first pair are maxillipeds, 
the second pair chelipeds (the first pair of pereiopods), and the following 
six pairs pereiopods.  See  PERACARIDA. 
 �34s[Karl Lang]
�33s 
  
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�6v�2v�32sTangent   �21m �33s 
    A term describing a relationship of two figures (usually of the same 
dimension) in the neighborhood of a common point. The figures are tangent at a 
point P if they touch at P but do not intersect in a sufficiently small 
neighborhood of P. To be more precise, if P denotes a point of a curve C (see 
illustration), a line L is a tangent to C at P provided L is the limit of 
lines joining P to a variable point Q of C, as Q approaches P along C (that 
is, for Q sufficiently close to P, the line PQ is arbitrarily close to L). 
 �34s[Leonard M. Blumenthal]
�33s 
  
  
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�6v�2v�32sTangerine �21m �33s
    A name applied to certain varieties of a variable group of loose-skinned
citrus fruits belonging to the species Citrus reticulata. Although mandarin
and tangerine are often used interchangeably to designate the whole group,
tangerine is applied more strictly to those varieties (cultivars) having
deep-orange or scarlet rinds, whereas the term mandarin is more properly used
to include all members of this quite variable group of citrus fruits.  See
MANDARIN.
    The fruits are deep orange, loose-skinned, and small to medium-sized, and
possess small seeds with green cotyledons. Tangerines are easily peeled and
eaten out of hand as fresh fruit. About one-third of the crop is utilized in
juice, sherbets, and canned sections.
 �34s[Frank E. Gardner ;C. Jack Hearn]
�33s

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�6v�2v�32sTank circuit   �21m �33s 
    An inductor and capacitor in parallel. The term is quite often used to 
denote the parallel resonant circuit in the output stage of a radio 
transmitter, but it has been applied to any parallel resonant circuit. In many 
cases the inductance in the tank circuit is one winding of a two-winding, 
air-core transformer. The secondary is connected to some load, such as an 
antenna. Power is delivered from the source to the load through the tank 
circuit, with an effort usually made to adjust the parameters for maximum 
power transfer.  See  IMPEDANCE MATCHING; RESONANCE (ALTERNATING-CURRENT 
CIRCUITS); TRANSFORMER. 
 �34s[Harold F. Klock]
�33s 
  
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�6v�2v�32sTannin   �21m �33s 
    A generic term for a widely occurring group of substances of vegetable 
origin, capable of rendering raw hides into leather. Common tannin (tannic 
acid) occurs in oak gallnuts; tannins are also present in tea, sumac, oak 
bark, and mangrove bark.  
    Substances capable of tanning, and hence called tannins, are often of 
greatly different chemical structure; all tannins, however, have the property 
of converting the gelatin of hides into insoluble nonputrefying material, thus 
changing the hide into leather.  
    Tannins may be divided into three main classes: (1) condensed tannins that 
cannot be hydrolyzed either by acids or enzymes; (2) hydrolyzable tannins; and 
(3) tannins of unclassified nature. 
 �34s[Evans B. Reid]
�33s 
  
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�6v�2v�32sTantalite   �21m �33s 
    A mineral with composition (Fe,Mn)Ta2O6, an oxide of iron, manganese, and 
tantalum. Niobium substitutes for tantalum in all proportions; a complete 
series extends to columbite (Fe,Mn)Nb2O6. Pure tantalite is rare. Tantalite is 
common in short prismatic crystals. The hardness is 6 on Mohs scale, and the 
specific gravity 7.95 (pure tantalite). The luster is submetallic and the 
color iron black. Tantalite is the principal ore of tantalum. The chief 
producing areas are the Congo and Nigeria.  See  NIOBIUM; TANTALUM. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sTantalum   �21m �33s 
    A chemical element, symbol Ta, atomic number 73, and atomic weight 
180.948. It is a member of the fifth group of the periodic table and is in the 
5d transitional series. Oxidation states of IV, III, and II are also known.  
See  TRANSITION ELEMENTS. 
    Tantalum metal is used in the manufacture of capacitors for electronic 
equipment, including citizen band radios, smoke detectors, heart pacemakers, 
and automobiles. It is also used for heat-transfer surfaces in chemical 
production equipment, especially where extraordinarily corrosive conditions 
exist. Its chemical inertness has led to dental and surgical applications. 
Tantalum forms alloys with a large number of metals. Of special importance is 
ferrotantalum, which is added to austentitic steels to reduce intergranular 
corrosion.  
    The metal is quite inert to acid attack except by hydrofluoric acid. It is 
very slowly oxidized in alkaline solutions. The halogens and oxygen react with 
it on heating to form the oxidation-state-V halides and oxide. At high 
temperature it absorbs hydrogen and combines with nitrogen, phosphorus, 
arsenic, antimony, silicon, carbon, and boron. Tantalum also forms compounds 
by direct reaction with sulfur, selenium, and tellurium at elevated 
temperatures.  
 �34s[Edwin M. Larsen]
�33s 
  
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�6v�2v�32sTaper pin �21m �33s
    A tapered self-holding pin used to connect parts together. Standard taper
pins have a diametral tape 1/4 in. to 12 in. (0.6 cm to 30 cm) and are driven
in holes drilled and  reamed to fit. The pins are made of soft steel or are
cyanide-hardened. They are sometimes used to connect a hub or collar to a
shaft. Taper pins are frequently used to maintain the location of one surface
with respect to another. A disadvantage of the taper pin is that the holes
must be drilled and reamed after assembly of the connected parts; hence they
are not interchangeable.  See  COTTER PIN.
 �34s[Paul H. Black]
�33s

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�6v�2v�32sTaper pipe thread gage   �21m �33s 
    A type of gage whereby size is determined by measuring the distance that a 
plug gage will screw into a coupling or that a ring gage will screw onto a 
pipe. A manufacturer of pipe or couplings has a master plug and master ring 
gage. Standards for pitch diameter and taper of a pipe thread were first set 
to control threads used in petroleum service, where safety depended on 
accuracy of the thread dimensions. See RING GAGE. 
 �34s[Rush A. Bowman]
�33s 
  
  
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�6v�2v�32sTapeworm disease   �21m �33s 
    The presence in the body of humans of either the larval or adult stages of 
any of the following cestodes: Hymenolepis nana, Taenia saginata, T. solium, 
Dibothriocephalus latus,  or Echinococcus granulosus.  See  CESTOIDEA. 
    Hymenolepiasis nana is a cosmopolitan parasite that occurs in rodents. The 
cycle is direct, human to human or rodent to human. Ingestion of the egg 
results in development of larvae, called cysticercoids, in cysts in the 
duodenum. After the larvae burst out of the cyst, they attach to the small 
intestine and become adults. The entire cycle takes a month. Heavy infection 
may result in nervous disorders. 
    Taenia saginata, the beef tapeworm, is acquired by ingesting the larval 
form of the parasite Cysticercus bovis, which grows in the intramuscular 
connective tissues of herbivores. The adult, attached to the small intestine, 
may grow to a length of many feet. The gravid segments containing the eggs are 
motile and disengage from the rest of the tapeworm to pass actively through 
the anus. No symptoms may be exhibited by parasitized individuals, although 
nervous disorders may arise.  
    The epidemiology of T. solium, the pork tapeworm, is essentially the same 
as that described for T. saginata, except that the parasite uses the hog as 
intermediate host. The larva of T. solium is called C. cellulosae. 
    Dibothriocephalus latus, the fish tapeworm, inhabits the small intestine 
of humans. While the Baltic and Scandinavian countries are the classical 
focus, other cold regions are also important ones. The eggs of the worm 
embryonate and hatch in water. The ciliated embryo is ingested by water fleas 
which are ingested by many species of fish. Larvae mature in the mammal. Heavy 
infection may cause intestinal obstruction and possibly toxemia. 
    Echinococcosis is also known as hydatid disease and is an infection by the 
larval forms of E. granulosus, Alveococcus multilocularis, and E. oligarthus 
in humans or other intermediate hosts. Adult E. granulosus are minute 
parasites of the small intestine of carnivores. The carnivore passes eggs in 
feces so that grazing sheep and other herbivores or rodents become parasitized 
with the hydatid larva. The cycle is completed by carnivores preying on them. 
Humans become exposed by petting dogs or ingesting contaminated grass.  See  
ECHINOCOCCOSIS.  
 �34s[Jose F. Maldonado]
�33s 
  
A700000001145�254s�5v�1v1716
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�6v�2v�32sTapir   �21m �33s 
    An odd-toed ungulate of the family Tapiridae. These animals, with four 
species, have a discontinuous distribution in South America and Asia and 
consequently have a theoretical importance biologically. 
    These animals have a prehensile, trunklike muzzle, a short tail, and small 
eyes. The forefoot has four toes, the hindfoot three. The female gives birth 
to a single young after a gestation period of 13 months. Tapirs are nocturnal, 
timid animals; they spend the daytime hours in dense thickets. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sTardigrada   �21m �33s 
    A phylum of microscopic, bilaterally symmetrical invertebrates which are 
generally less than 0.04 in. (1 mm) in length. About 400 species are known. 
Commonly called water bears, bear animalcules, or urslets, they are worldwide 
in distribution and are found in all habitats. 
    The tardigrade body consists of an anterior prostomium and five segments. 
The mouth is located in the prostomium in a centroterminal position. A soft, 
nonchitinous cuticle surrounds the body and lines the fore- and hindgut. The 
cuticle may be smooth or sculptured and forms innervated cephalic appendages 
and spines on the trunk and legs. Four pairs of ventrolateral legs arise from 
the trunk and terminate in claws or other modified structures. The digestive 
tract is tubular and more or less lobed due to the presence of diverticular 
dilations. The sexes are separate and the gonads are unpaired dorsal sacs with 
paired gonoducts in the male and in theory also in the female. 
    Food storage cells float in the spacious body cavity, the coelom, which 
lacks a parietal or visceral peritoneum in the adult. Circulatory and 
respiratory structures are lacking. 
    During their active life of 18 months, tardigrades molt about 12 times. 
They are unable to feed in the 5 - 10 days of molting; the buccal cavity 
requires at least 5 days for renewal. 
    Tardigrada live as active forms, without encystment, only when surrounded 
by a pellicle of water. When the surrounding medium dries up, most  
tardigrades continue to live as inactive or anabiotic barrel-shaped structures 
called cysts without any protective cover (see illustration). Desiccation 
begins when there is a loss of oxygen from the water. The animal responds by 
contraction and loss of body water. Dried eggs also survive. Moistened animals 
usually revive, but anabiosis and revival cannot be repeated indefinitely. 
    Most species are widely distributed. Dissemination may be by wind, birds, 
and certain terrestrial animals which transport tardigrade eggs and barrels. 
The tropics have few species, and members of the family Scutechiniscidae are 
rare on the Antarctic continent. Most tardigrades are terrestrial. They are 
found among lichens, liverworts, densely growing soft-leaved mosses, and also 
in rather hard-leaved Pottiacea and Grimmiacea.  See  EUTARDIGRADA; 
HETEROTARDIGRADA.  
 �34s[Eveline Marcus]
�33s 
  
  
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�6v�2v�32sTarget flowmeter �21m �33s
    A volume flow rate meter that measures the force on a circular obstruction
(see illustration). The target-type meter, besides being particularly suitable
for dirty fluids, has the important advantage of a built-in secondary device
and is without the disadvantage of fluid-filled connection pipes to the
secondary device that is characterist of many volume flow rate meters.	See
FLOW MEASUREMENT; VOLUME FLOW RATE METER.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s


A700000001307�254s�5v�1v1717
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�6v�2v�32sTarpon   �21m �33s 
    A species of fish, Megalops atlantica, which is included together with the 
ladyfish and machete in the family Elopidae in the order Elopiformes. These 
fish are widely distributed in tropical and subtropical seas and enter 
fresh-water streams of the United States. 
    The tarpon is the largest of the herringlike fishes, weighing up to 300 lb 
(135 kg) and reaching a length of nearly 8 ft (2.4 m). The body is covered 
with large scales, and there is a single, soft, rayed dorsal fin, strong jaws, 
a bony plate under the mouth, and numerous small teeth (see illustration).  
See  ELOPIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
  
A700000001652�254s�5v�1v1717
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�6v�2v�32sTarsier   �21m �33s 
    A primate of the family Tarsiidae. These mammals are found in the Far 
East, the Philippines, Malaya, and the Celebes. They exhibit characteristics 
that relate them to the true monkeys. 
    The tarsier is insectivorous, thus maintaining what is considered to be a 
primitive diet. Advanced characteristics are seen in the skull structure in 
that it is round with a flattened face,  and the large eyes are separated from 
the temporal fossae in the orbital depression. The tail is long and ratlike 
but not prehensile. The bones of the hindlegs are long, especially the tarsal 
bones, allowing tarsiers to jump and to leap like frogs. The tarsier is 
nocturnal and the enormous eyes dilate at night for better vision. Breeding is 
continuous throughout the year, with a single young born at a time.  See  
MAMMALIA; PRIMATES. 
 �34s[Charles B. Curtin]
�33s 
  
A700000002105�254s�5v�1v1717
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�6v�2v�32sTartaric acid �21m �33s
    A compound that possesses two similar asymmetric carbon atoms and occurs
in four isometric forms: (1) a dextrorotary form, which rotates
plane-polarized light to the right; (2) a levorotary form, which rotates
plane-polarized light to the left; (3) a racemate form, which is an optically
inactive equimolecular mixture of separate crystals of the dextro and levo
forms; and (4) a meso form, which is optically inactive because of internal
compensation. Structural formulas of the dextro, levo, and meso forms are
shown.
    The dl-tartaric acid is known as the racemic acid.

		    COOH	       COOH	       COOH
		    |		       |	       |
		 H--C--OH	   HO--C--H	    H--C--OH
		    |		       |	       |
		HO--C--H	    H--C--OH	    H--C--OH
		    |		       |	       |
		    COOH	       C00H	       C00H

	   d-Tartaric acid	l-Tartaric acid    m-Tartaric acid

    The principal uses of tartaric acid are in cream of tartar, Rochelle salt,
and tartar emetic.  See CARBOXYLIC ACID; TARTRATE.
 �34s[Evans B. Reid]
�33s

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�6v�2v�32sTartrate   �21m �33s 
    A chemical compound that is a salt or ester of tartaric acid, which is 
formed by the replacement reaction of the carboxylic hydrogens of tartaric 
acid by a metal (salt) or organic radical (ester). One or both of the 
carboxylic hydrogens can be replaced to produce an extensive series of salts, 
esters, and mixed (double) salts. Many salts have practical applications, as 
in cream of tartar (monopotassium salt), Rochelle salts (sodium-potassium 
salt), tartar emetic (potassium-antimony salt), medicines, and textile dyeing 
(calcium salt).  See  ROCHELLE SALT; TARTARIC ACID. 
 �34s[Elbert H. Hadley]
�33s 
  
A700000002737�254s�5v�1v1717
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�6v�2v�32sTaste   �21m �33s 
    One of the chemical senses. The so-called taste of food is not a simple 
sensory pattern. It depends not only on the specific taste receptors of the 
tongue but also upon other sensations supplied by the chemical, tactile, warm, 
and cold receptors in the mouth, and the smell receptors in the nose. Taste 
and odor discrimination is reduced if the sense of smell is suppressed, for 
example, by a cold.  See  OLFACTION. 
    In humans and mammals, taste buds are located on the tongue in the 
fungiform, foliate, and circumvallate papillae and in adjacent structures of 
the throat. Taste buds are goblet-shaped clusters of longer slender cells from 
the distal end of which a cluster of microvilli protrude into the taste pores 
that open toward the tongue surface. Individual sensory nerve fibers branch 
profusely within the taste bud.  See  TONGUE. 
    No simple relation exists between chemical stimuli and taste quality 
except, perhaps, for the sourness of acids. The taste qualities of inorganic 
salts are complex, only sodium chloride giving the purely saline taste. Sweet 
and bitter tastes occur in many chemical classes. The middle top portion of 
the tongue surface is insensitive to all tastes. Sensitivity to sweet is 
greatest at the tip, to sour at the sides, and to bitter at the back; salt 
sensitivity is relatively more homogeneous around the edges.  See  CHEMICAL 
SENSES.  
 �34s[Carl Pfaffman]
�33s 
  
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�6v�2v�32sTaurus   �21m �33s 
    The Bull, in astronomy, a winter constellation. Taurus is the second sign 
of the zodiac. The group contains two notable star clusters, the Hyades and 
the Pleiades. The Hyades is a V-shaped cluster, the V forming the head of the 
charging bull, with the fiery bright star Aldebaran in the right eye (see 
illustration). This star has long been used in navigation. Farther west lies 
the compact, beautiful cluster of six stars, the famous Pleiades.  See  
CONSTELLATION; HYADES; PLEIADES. 
 �34s[Ching-Sung Yu]
�33s 
  
A700000003320�254s�5v�1v1718
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�6v�2v�32sTautomerism �21m �33s
    The reversible interconversion of structural isomers of organic chemical
compounds. Such inter-conversions usually involve transfer of a proton, but
anionotropic rearrangements may be reversible and so be classed as tautomeric
interconversions.
    A cyclic system containing the grouping --CONH-- is called a lactam, and
the isomeric form, --COH==N--, a lactim. These terms have been extended
to include the same structures in open-chain compounds when considering
the shift of the hydrogen from nitrogen to oxygen.

    The molecular grouping --COCH< may in certain substances exist partly or
wholly as --COH==C<. The former constitutes the keto form and the
latter the enol form. The existence of an enol in an acyclic system requires
that a second carbonyl group (or its equivalent, for example, `C==N--)
be attached to the same --CN< as an aldehyde or ketone carbonyl. Thus, ethyl
acetoacetate tautomerizes demonstrably, but ethyl malonate does not.  Where
the enol form includes an aromatic ring such as phenol, the existence of the
keto form is often not demonstrable, although in some substances there may be
either chemical or spectroscopic evidence for both forms. Closely related to
keto-enol tautomerism is the prototropic interconversion of nitro and aci
forms of aliphatic nitro compounds such as nitromethane.
    In general, tautomeric forms exist in substances possessing functional
groups which can interact additively and which are so placed that
intramolecular reaction leads to a stable cyclic system. The cyclic form
usually predominates (especially if it contains five or six members).  See
MOLECULAR ISOMERISM.
 �34s[Wyman R. Vaughan]
�33s
sh

A700000001155�254s�5v�1v1718
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�6v�2v�32sTaxales   �21m �33s 
    A small order of gymnosperms, division Pinophyta, in the class Pinopsida. 
The plants are trees or shrubs with evergreen, often needlelike leaves, 
vegetatively much like the related order Pinales. They differ from the Pinales 
in having a well-developed fleshy covering surrounding the individual seeds, 
which are terminal or subterminal on short shoots instead of being in definite 
cones. The most familiar genus of the Taxales in the Northern Hemisphere is 
the yew (Taxus).  See  PINALES; PINOPHYTA;  YEW. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001774�254s�5v�1v1718
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�6v�2v�32sTaxis   �21m �33s 
    A mechanism of orientation by means of which an animal moves in a 
direction related to a source of stimulation. There exists a widely accepted 
terminology in which the nature of the stimulus is indicated by a prefix such 
as phototaxis (light), chemotaxis (chemical compounds), geotaxis (gravity), 
thigmotaxis (contact), rheotaxis (water current), and anemotaxis (air 
current). The directions toward or away from the stimulus are expressed as 
positive or negative, respectively. Finally, the sensory and locomotory 
mechanisms by means of which the orientation is achieved are denoted by a 
second type of prefix forming a compound noun with taxis. Positive 
phototropotaxis thus describes a mechanism by means of which an animal carries 
out a directed movement toward a source of light along a path which permits 
the animal's paired eyes to receive equal intensities of light throughout the 
movement.  
 �34s[Otto E. Lowenstein]
�33s 
  
A700000000770�254s�5v�1v1718
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�6v�2v�32sTaxodonta �21m �33s
    A subclass of pelecypod mollusks in which the hinge is of the taxodont
type; that is, the dentition is a series of similar alternating teeth and
sockets along the hinge margin. Geologically, these marine mollusks range from
the Ordovician to the Recent. The adductor muscles are approximately of equal
size, and gills are present for both respiratory and feeding purposes.	See
MOLLUSCA; PELECYPODA.
 �34s[Charles B. Curtin]
�33s

A700000002622�254s�5v�1v1718
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�6v�2v�32sTaxon   �21m �33s 
    A monophyletic group of organisms at any level in the taxonomic hierarchy 
is a taxon (plural, taxa). A clear distinction must be made between the 
category, which is any one of the levels in the hierarchy, and the taxon, 
which is the actual group at that categorial level. Thus the genus is a 
category and Turdus is a taxon of passerine birds at the generic level. 
Categories are words that are defined. Taxa or taxonomic groups are real 
entities in nature that can be recognized, delimited, and described. Taxa at 
any level consist of one or more taxa of the next lower level. Members of a 
taxon are recognized by the joint possession of similar features and are 
distinguished from other taxa by differences in characteristics. These 
differences correspond to the presumed gap in evolutionary relationships 
between the taxa. Members of any taxon are presumed to share a monophyletic 
origin, and most taxa, but not all, contain more than one taxon of the 
next-lower rank. See TAXONOMIC CATEGORIES. 
    Species taxa are given special recognition because these groups can be 
ascertained by criteria other than features shared in common. Members of the 
same species interbreed with one another and are reproductivity isolated from 
other such species. See ANIMAL SYSTEMATICS; PLANT TAXONOMY; SPECIES CONCEPT. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sTaxonomic categories �21m �33s
    Any one of the levels in the hierarchy of biological classification of
organisms. The categories - for example, species, genus, family - are words
that can be defined clearly and definitely. The species category is regarded
as the most important because its definition has real biological meaning -
that is, there is no breeding between species - and because the categories at
higher and lower levels are defined in terms of the species.  See  SPECIES
CONCEPT.
    A clear distinction must be made between the category and groups of
organisms or taxa which are described at that categorial level.  See  TAXON.
    Definitions of genera and higher categories are in the following form: A
genus is a monophyletic group of one or more species. It is used to indicate
the phylogenetic origin of the group and the amount of evolutionary change
that has occurred. Groups of similar species are placed together in a genus,
similar genera in a family, and so forth, with all similar phyla in a kingdom.
All living organisms are divided into a small number of kingdoms, two to five,
depending on the authority.  See  ANIMAL SYSTEMATICS.
    The respective position of two different animals in this system may be
expressed as follows:

    CATEGORIES		     TAXA (Lion)	   TAXA(Housefly)

    Kingdom		     Animalia		   Animalia
    Phylum		     Chordata		   Arthropoda
    Class		     Mammalia		   Insecta
    Order		     Carnivora		   Diptera
    Family		     Felidae		   Muscidae
    Genus		     Felis		   Musca
    Species		     leo		   domestica

    Note that the binominal (generic and trivial) name is italicized. These
seven taxonomic categories are the minimum used to classify animals. In large
groups or those with a complex evolutionary history, more categories are
utilized. Those which are generally accepted are as follows:

		     Kingdom		 Suborder
		     Phylum		 Superfamily (-oidea)
		     Subphylum		 Family (-idae)
		     Superclass 	 Subfamily (-inae)
		     Class		 Tribe (-ini)
		     Subclass		 Genus
		     Cohort		 Subgenus
		     Superorder 	 Species
		     Order		 Subspecies


    Standardized endings are used for superfamilies, families, subfamilies,
and tribes. Names for taxa above the level of the superfamily are not
standardized (although -iformes is used for orders in some groups), but
usually vary from one group of animals to another without uniform endings.
See  PLANT TAXONOMY; ZOOLOGICAL NOMENCLATURE.
 �34s[Walter J. Bock]
�33s

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�6v�2v�32sTaxonomy   �21m �33s 
    A study aimed at producing a hierarchical system of classification of 
organisms which best reflects the totality of their similarities and 
differences. Most taxonomists believe that such a classification can be 
achieved only by heavy reliance on the phylogenetic concept, so that each 
taxon (a taxonomic group, at whatever rank) represents a unified evolutionary 
group.  
    It is customary to recognize two kingdoms of organisms, Plantae (plants) 
and Animalia (animals), but various other arrangements of two, three, four, or 
even five kingdoms have been proposed.  See  ANIMAL SYSTEMATICS; PLANT 
KINGDOM; PLANT PHYLOGENY; PLANT TAXONOMY; TAXON; TAXONOMIC CATEGORIES. 
 �34s[Arthur Cronquist]
�33s 
  
A700000001542�254s�5v�1v1719
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�6v�2v�32sTea   �21m �33s 
    A small tree, Camellia (Thea) sinensis. A preparation of its leaves dried 
and cured by various processes; and a beverage made from these leaves. The 
plant is an evergreen tree of the Theaceae family, native to southeastern 
Asia, and does best in a warm climate where the rainfall averages 90 - 200 in. 
(2200 - 5000 mm). China, Japan, Taiwan, India, Sri Lanka, and Indonesia are 
among the leading tea-producing countries, with China contributing about 
one-half of the world's supply. 
    Tea leaves contain caffeine, various tannins, aromatic substances 
attributed to an essential oil, and other materials of a minor nature, 
including proteins, gums, and sugars. The tannins provide the astringency, the 
caffeine the stimulating properties.  See  THEALES. 
 �34s[Earl L. Core]
�33s 
  
A700000001173�254s�5v�1v1719
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�6v�2v�32sTechnetium �21m �33s
    A chemical element, Tc, atomic number 43. The element was the first made
artificially in a cyclotron. It is also produced as a major constituent of
nuclear reactor fission products or, alternatively, by action of neutrons on
(sup98)Mo. The isotope (sup99)Tc is most suitable for chemical investigation
because of its long half-life, 2 x 10(sup5) years. The chemistry of technetium
is very similar to that of rhenium, and corresponding compounds have been
prepared in many cases.  See  NUCLEAR REACTION; RHENIUM; TRANSITION ELEMENTS.
 �34s[Sherman Fried]
�33s

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�6v�2v�32sTechnology �21m �33s
    Systematic knowledge and action, usually of industrial processes but
applicable to any recurrent activity. Technology is closely related to science
and to engineering. Science deals with humans' understanding of the real world
about them - the inherent properties of space, matter, energy, and their
interactions. Engineering is the application of objective knowledge to the
creation of plans, designs, and means for achieving desired objectives.
Technology deals with the tools and techniques for carrying out the plans.
 �34s[Robert S. Sherwood /Harold B. Maynard]
�33s

A700000002603�254s�5v�1v1719
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�6v�2v�32sTectonophysics   �21m �33s 
    A branch of geophysics that deals with the physical processes of inelastic 
deformations - flow and fracture - of the Earth on all scales, from those of 
plate tectonics and associated mountain building to crystal-lattice defects. 
It encompasses some 16 orders of magnitude. High-pressure, high-temperature 
rheological properties of rocks - elasticity, plasticity, and viscosity - are 
measured in the laboratory under conditions simulating the natural environment 
as realistically as possible. The tectonophysicist then uses these 
measurements to construct analytical and numerical models of the deformations 
of the crust and upper mantle and test them against field observations by 
structural geologists, structural petrologists, and geophysicists. 
    In dealing with such phenomena as folding, faulting, and compaction in the 
relatively brittle crust, tectonophysics becomes intimately associated with 
engineering rock mechanics. To treat deep-seated processes, the 
tectonophysicist teams with geochemists, geodesists, and seismologists to 
investigate flow in the relatively ductile mantle and coupling of the driving 
forces in the asthenospheric with the lithospheric plates.  See  ENGINEERING 
GEOLOGY; GEOPHYSICS; PLATE TECTONICS; ROCK MECHANICS; SEISMOLOGY; STRUCTURAL 
GEOLOGY.  
 �34s[John Handin]
�33s 
  
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�6v�2v�32sTektite   �21m �33s 
    A member of one of several groups of objects that are composed almost 
entirely of natural glass formed from the melting and rapid cooling of 
terrestrial rocks by the energy accompanying impacts of large extraterrestrial 
bodies. Tektites are dark brown to green, show laminar to highly contorted 
flow structure on weathered surfaces and in thin slices, are brittle with 
excellent conchoidal fracture, and occur in large masses but are mostly small 
to microscopic in size. 
    Five major groups of tektites are known: North American, 34,000,000 years 
old; Czechoslovakian (moldavites), 15,000,000 years; Ivory Coast, 1,300,000 
years old; Russian (irgizites) 1,100,000 years old; and (5) Australasian, 
700,000 years old. The North American, Ivory Coast, and Australasian tektites 
also occur as microtektites in oceanic sediment cores near the areas of their 
land occurrences. In the land occurrences, virtually all of the tektites are 
found mixed with surface gravels and recent sediments that are younger than 
their formation ages. 
    The chemical compositions of tektites differ from those of ordinary 
terrestrial rocks principally in that they contain less water and have a 
greater ratio of ferrous to ferric iron, both of which are almost certainly a 
result of their very-high-temperature history. 
 �34s[Elbert A. King]
�33s 
  
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�6v�2v�32sTelecast   �21m �33s 
    A television broadcast, involving the transmission of the picture and 
sound portions of the program by separate transmitters at assigned carrier 
frequencies within the channel assigned to a television station. A telecast is 
intended for reception by the general public, just as is a radio broadcast.  
See  TELEVISION; TELEVISION TRANSMITTER. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sTelegraphy   �21m �33s 
    A method of communication employing electrical signaling impulses produced 
and received manually or by machines. Telegraph signals are transmitted over 
open wire or cable land lines, submarine cables, or radio. Telegraphy as a 
communication technique uses essentially a narrow frequency band and a 
transmission rate adapted to machine operations.  See  ELECTRICAL 
COMMUNICATIONS.  
    Early equipment devised by Samuel F. B. Morse consisted of a mechanical 
transmitter and receiver or register. Operators soon learned to handle 
messages faster using simple manual keys and audible sounders. Subsequently, 
telegraph transmission and reception again became mechanized.  See  
TELETYPEWRITER EXCHANGE (TWX) SERVICE; TELEX. For other ways in which 
telegraphy may be used,  see  FACSIMILE; TELEPHOTOGRAPHY; TELETYPESETTER. 
    Telegraph facilities for use by the general public to transmit messages 
both domestically and internationally are provided by communication companies 
and government administrations. Special telegraph facilities include those for 
news services, distribution of market prices of securities and commodities, 
and private lines between such points as the factories and offices of a 
company for the exchange of messages, orders, payroll data, and inventories. 
Fire and police alarms are a special form of telegraphy. The armed forces have 
extensive fixed and mobile telegraph systems. 
    For manual operation, the telegraph code consists of short dot and long 
dash signals (see illustration). Most automatic printing telegraph circuits, 
including American cable operation, use a code of five equally spaced signals 
or units per letter or other symbol perforated into a paper tape. Machines 
translate automatic teleprinter code to cable code, cable code to automatic 
code, or, for special applications, make other translations. For stock 
quotation systems and teletypesetter operation, a six-unit code is used to 
provide control of machine action. For data transmission or machine control, a 
seven- or eight-unit code may be used. 
    A single circuit provides transmission in only one direction at a time. 
For transmission in both directions simultaneously, a duplex circuit is used. 
Multiplex (time-division) apparatus provides two, three, or more channels 
operable in both directions simultaneously over a single circuit. 
Carrier-current techniques enable several circuits, each comprising one or 
more communication channels, to operate through the same wide-band wire, 
cable, or radio facility.  See  MULTIPLEXING. 
    In automatic transmission, an operator at a manual keyboard, operated like 
a typewriter, perforates a tape. The tape is fed through an electromechanical 
or photocell tape reader that drives the tape at a rapid and uniform rate and 
transmits the electrical code. Interconnecting wire lines or other 
communication channels carry these code pulses to the receiver. The received 
impulses may automatically actuate a reperforator to produce a duplicate 
punched tape for retransmission or later transcription; the impulses may 
actuate a teletypewriter (also called a teleprinter) to retype the original 
message; or they may actuate other terminal equipment, such as an accounting 
machine.  
    Alternatively, the equipment at both terminals may be teletypewriters, in 
which keyboard and printing mechanisms are combined in one machine. Such 
machines use a five-unit code but operate without perforated tape. Business 
firms that originate and receive numerous messages have such equipment 
installed at their offices for direct service. Also widely used are facsimile 
instruments that transmit or reproduce a typed or handwritten message as a 
picture on electrosensitive paper. 
 �34s[Grosvenor Hotchkiss]
�33s 
  
    To achieve speed and accuracy in handling messages en route, direct 
circuits may be set up from the originating keyboard to the distant 
teleprinter. This is done in TWX and Telex services. This mode of system 
operation is known as circuit switching. If direct channels are not 
immediately available, or if the volume of traffic calls for a storage 
interval, the message is transferred by means of perforated tape at switching 
centers. This mode is known as message switching or store-and-forward 
switching. Message switching has the advantage of using the channel capacities 
of trunk lines more fully than do direct connections. It is used in the United 
States for public telegram service and on leased-wire switched networks.  See  
TELEX.  
 �34s[W. E. Girardin]
�33s 
    In overseas communications, telegraph messages or telegrams are 
transmitted over high-frequency radio (3 - 30 MHz), transoceanic submarine 
cables, or satellite communication channels. A decreasing number of messages 
are carried on high-frequency radio. These types of overseas channels are also 
used for private-line service, which is referred to as leased-channel service, 
and customer-to-customer teletypewriter service, which is known as Telex in 
the international service. 
 �34s[Eugene D. Becken]
�33s 
  
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�6v�2v�32sTelemetering   �21m �33s 
    The branch of engineering concerned with the presentation of measured data 
at a location remote from the source of the data; also called telemetry. An 
example of a highly complex telemetering system is the equipment utilized to 
measure the temperature in a space vehicle in flight, radio this temperature 
to a ground observing station, and provide a meter or visual display to the 
observers. Telemetry encompasses all remote metering, whether the distances 
involved be many miles, as in space flights, or only a few feet, as in the 
measurement of reactivity in the core of a nuclear reactor.  See  OPEN-LOOP 
CONTROL SYSTEM. 
    Telemetry involves three separate functions: (1) generation of a signal 
(electrical or otherwise), which measures the pertinent physical variable and 
is in suitable form for transmission; (2) transmission of the information to 
the remote location; and (3) conversion of the data into a form appropriate 
for display, recording, or application to further data-processing equipment. 
    Telemetry systems are conventionally classified according to the 
transmission medium. Although the information can be transmitted in a variety 
of forms (such as the brightness of a modulated light beam), the great 
majority of systems fall into three categories: mechanical, electrical, and 
radio telemetry. 
    Mechanical telemetry, involving mechanical coupling between the points of 
measurement and of data utilization, is generally restricted to short 
distances because of the high attenuation of mechanical media, the low 
velocity of propagation (of sound, for example), the difficulty of 
constructing efficient and simple mechanical amplifiers, and the requirement 
of a continuous mechanical coupling medium throughout the transmission path. 
    The term electrical telemetry is utilized in contrast to radio telemetry 
and refers to wired telemetry systems in which the information is transmitted 
by variations of a voltage or current in the electric circuit. Modern 
electrical telemetry systems often involve complex electronic equipment, as 
when the information is transmitted (over wires) in the form of a television 
picture.  
    Radio telemetry, originally used to telemeter information from weather 
balloons to ground observers, employs electromagnetic radiation as the 
transmission means. 
 �34s[John G. Truxal]
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�6v�2v�32sTeleostei   �21m �33s 
    The youngest, largest, and most successful of three organizational levels 
(infraclasses) of the subclass Actinopterygii or rayfin fishes. The 
teleosteans are derived, through one or perhaps more evolutionary lines, from 
the Holostei, which in turn are descendants of the Chondrostei.  See  
ACTINOPTERYGII; CHONDROSTEI; HOLOSTEI. 
    The transition from holostean to teleostean involves attainment of paired 
bracing bones (uroneurals) in the supporting skeleton of the caudal fin; 
development of a homocercal caudal fin in which two or more lowermost hypural 
bones articulate with a single centrum; loss of fulcral scales at the upper 
and lower base of the caudal fin; development of intermuscular bones; increase 
from one to two pairs of hypohyal bones; reduction in thickness of scales and 
gradual loss of the enamel surface (ganoin); development of cycloid scales; 
development of a supraoccipital bone; fusion of the paired prevomers into a 
single median bone; reduction in number of bones in the mandible; increased 
ossification of the vertebral column; and assumption by the swim bladder of a 
hydrostatic function instead of a respiratory one. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sTeleostomi   �21m �33s 
    This name was originally proposed by R. Owen to include bony fishes with a 
terminal mouth, excluding the lungfishes or Dipnoi. Later, it was employed in 
a more inclusive sense to mean a class of fishes, including the bony fishes or 
subclasses Osteichthyes, and the extinct subclass Acanthodii. Together with 
class Chondrichthyes and class Placodermi, it forms the group of jawed fishes 
or Gnathostomata.  See  ACANTHODII; CHONDRICHTHYES; OSTEICHTHYES; PLACODERMI. 
 �34s[Robert H. Denison]
�33s 
  
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�6v�2v�32sTelephone   �21m �33s 
    The term telephone formerly referred to the telephone receiver, the 
instrument originally invented by Alexander Graham Bell; it is now commonly 
applied to the telephone set, which includes a transmitter and an electric 
network in addition to the receiver. The transmitter and receiver are housed 
together in a handset. A cord connects the electrical components of the 
handset to the network in the telephone set. 
    The transmitter (see illustration) is a transducer which converts 
acoustical energy into electric energy. In most transmitters an electric 
current is modulated by the variations in contact resistances of carbon 
granules. Sound waves impinge on the diaphragm of the transmitter, causing the 
carbon granules to move closer together, making more contacts and decreasing 
resistance, or to move further apart, making fewer contacts and higher 
resistance. The transmitter has a frequency-response range from 250 to 5000 
Hz. The frequency response rises uniformly to a broad maximum in the region of 
2500 Hz. Because of these charcteristics and those of the telephone receiver, 
the speech heard by the listener resembles closely that of direct mouth-to-ear 
speech as heard by a listener a few feet from the person speaking. 
    The receiver transducer operates on the relatively low power used in the 
telephone circuit to convert electric energy into acoustical energy. Unlike a 
loudspeaker, the telephone receiver is designed for close coupling to the ear. 
There are two types of receiver units, the bipolar receiver and the ring 
armature receiver. The bipolar receiver is used with operator's head telephone 
sets. The ring armature receiver is used with the telephone set in which high 
efficiency is particularly important. The advantages of the ring armature 
receiver are its low acoustic impedance and high available power response over 
a wide frequency range (350 - 3500 Hz). These advantages are achieved by the 
piston action of a thin, nonmagnetic, light-weight, dome-shaped diaphragm, 
which is attached to a ring-shaped armature of magnetic material driven at its 
periphery by a ring-shaped magnetic coil associated with a ring-shaped 
permanent magnet.  See  TELEPHONE SERVICE. 
 �34s[Rowland F. Davis]
�33s 
  
  
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�6v�2v�32sTelephone civil defense system   �21m �33s 
    A telephone system specially designed to assist in protecting civilian 
population, limiting casualties and property damage, and preserving maximum 
civilian support of military effort. Civil defense (CD) requirements for 
communications fall into two broad categories: preattack warning, and 
administrative and control communications to alleviate disaster after an 
attack. Both requirements are usually best met with private-line telephone 
systems furnished by telephone companies, supplemented by local and interurban 
telephone service arranged to assure continuity of service. 
    In the United States the basic facility for attack warning is a 
private-line telephone system known as the National Warning System (NAWAS), 
which connects over 1000 warning points in the United States with three Office 
of Civil Defense (OCD) warning centers. 
    OCD's primary system of operational communications is the National 
Communications System (NACOM I). NACOM I network consists of approximately 
30,000 mi (48,000 km) of cable and radio facilities for both voice and data 
communications. Terminal equipment for this network is in place and is 
exercised daily for administrative purposes. The major function of NACOM I is 
to furnish the means for coordinating emergency government operation.  See  
TELEPHONE SERVICE; TELEPHONE SYSTEMS CONSTRUCTION. 
 �34s[John A. Osterman]
�33s 
  
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�6v�2v�32sTelephone equipment, customer   �21m �33s 
    The apparatus used by the individual telephone subscriber. The equipment 
used by residential and business customers is the telephone set. The customer 
may be provided a set as part of telephone service or may purchase a set. 
Various premium types of service, telephone sets, and accessories can be 
obtained to supplement or replace the basic ones. 
    When the serving central office is equipped, the customer can obtain a 
premium service in which dialing a telephone number is accomplished by pushing 
buttons on the telephone rather than turning a circular wheel as in 
rotary-dial service. Each depressed button on the dial generates two audio 
tones, and each button has its own pair of tones which are recognized by the 
central-office (or PBX) switching equipment as individual digits of the 
telephone number being called. Pushbutton telephones can be arranged to send 
such pairs of tones over the circuit after a telephone connection has been 
established. This feature permits users to send information to a remote device 
such as a computer. 
    Telephone sets used to provide basic service are the desk type and wall 
type, with a dial and a handset for talking and listening. Sets used for 
business are generally connected to more than one telephone line and are 
equipped with pickup keys to terminate these lines and a hold key. They are 
called key telephone sets and may be equipped with 6, 10, 12, 18, 20, 24, or 
30 buttons. 
    An automatic dialer is a device which stores telephone numbers in a memory 
medium and automatically dials the number when the user performs the proper 
interface function. Automatic dialers are available as an integral part of a 
telephone set or as adjuncts to existing sets. The memory medium for number 
storage usually consists of either a perforated plastic card or an integrated 
circuit memory in which the digits are stored in binary form.  See  BUSINESS 
COMMUNICATIONS SYSTEM; TELEPHONE; TELEPHONE SERVICE; TELEPHONE SIGNALING; 
TELEPHONE SYSTEMS CONSTRUCTION; TELEPHONY; VIDEO TELEPHONE. 
 �34s[William Schiavoni]
�33s 
  
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�6v�2v�32sTelephone service �21m �33s
    The technology of supplying the many types of services offered to the
telephone subscriber. These services include local, long-distance, and
overseas connections for voice and data, information services, answering
services, and the leasing of private lines. Advances in telephony have created
a growing number of communications features. These include multipoint (party)
telephone conversations (or conference calls) under customer control,
temporary transfer of telephone number while visiting friends, abbreviated
dialing for commonly used numbers, and an indication to the telephone user
when someone else is trying to reach the subscriber's line.

    Local service.  A local telephone service operates within an exchange, or
within a connected system of exchanges in the same locality, and furnishes to
subscribers an intercommunicating service. An exchange is a unit established
by a telephone company for the administration of telephone service in a
specified area, usually a town, a city, or a village and its environs. The
exchange consists of one or more central offices together with the associated
plant used in furnishing telephone service in that area. An exchange area is
the territory served by an exchange.
    To provide this intercommunicating service, centrally located switching
equipment is used to establish a communication connection between any two
telephone subscribers. This switching equipment also provides for connections
to distant exchanges.
    Local exchange transmission circuits are usually two-wire cable pairs
operated at voice frequency, although short-haul analog and digital carrier
systems are being more widely used, especially in large metropolitan areas. In
the longer transmission circuits, carrier systems are normally employed.
    The design of a telephone system involves a consideration of telephone
sets, subscriber loops, telephone central offices, interoffice trunks, and
numbering plans.
    The telephone set converts acoustic energy into the electrical analog
signal which is transmitted. In addition, it generates the supervisory
(on-hook and off-hook) and the address information used by switching and
signaling systems to establish connections. The main telephone set, together
with extension telephones, may have exclusive use of a telephone line. This is
individual-line service. Many telephone lines terminate on private branch
exchanges to service a number of extension telephones.	See  TELEPHONE
EQUIPMENT, CUSTOMER; TELEPHONE SYSTEMS CONSTRUCTION.
    Subscriber loops are the relatively short circuits, or lines, connecting
the customers' telephone sets to a central office. These lines usually employ
copper wire.
    The telephone central office houses the switching equipment that serves
the telphones of its immediate community. The telephone building, housing one
or more local central offices, is the ``wire center'' at which the loops of
customers in the vicinity converge and are terminated. The primary function of
the central office is to initiate and control the switching and signaling
action which, on demand, permits the connection of any local telephone to any
other telephone served by the same central office, in the same exchange, in
nearby exchanges, and in distant cities. For this purpose, each central office
has direct interoffice trunks radiating from it to many of the other local
central offices in the same area, to tandem offices, and to long-distance
offices for intermediate connection to circuits to more distant places.
    A tandem office is an intermediate switching location whose primary
function is to interconnect trunk circuits to and from the local central
offices in the area. In this manner trunking economies are achieved,
particularly in those instances where traffic volumes between two local
central offices are not sufficient to justify a large number of direct
interoffice trunks between the two local offices.
    Interoffice trunks are the speech- and data-carrying telephone circuits
between the switching systems in the exchange and its environs. Carrier
systems usually provide the transmission medium for the longer trunk circuits.
For shorter trunk circuits, under about 5 - 10 mi (8 - 16 km), the two-wire
circuit operating at voice frequencies continues to be the basic transmission
medium. Some carrier systems use the digital transmission technology of
time-division multiplex (TDM) and pulse-code modulation (PCM). Time-division
multiplex (TDM) techniques have also been introduced into switching systems.

    Intercity service.	Intercity service provides and operates the equipment
for switching and transmitting telephone communications between exchange areas.

    Long-distance office. A centralized location in an exchange area is
provided to handle long-distance service for that area and often for other
nearby exchange areas. Large urban areas may have more than one subsidiary
long-distance office when it becomes necessary because of physical limitations
at a single office or to protect service by decentralization.
    Calls between exchange areas served by two different long-distance offices
are often too few in number to justify economically a direct connection
between the two long-distance offices. Such calls are routed through a third
long-distance office, which can connect directly to the first two offices. At
the third long-distance office, these calls are termed through-switched
traffic.
    Virtually all of the intercity trunks in the United States are equipped
for the dialing of calls to completion by either operators or customers.
Mechanized offices are equipped with dial-switching systems designed for
interurban service. The type of switching system used depends on the number of
calls to be handled and the distribution of traffic. Long-distance offices are
also provided with the equipment needed for trunks and signaling.  See
TELEPHONE SIGNALING.

    Intercity trunks. These are the telephone channels between exchange areas.
The distortion and attenuation limits are kept within a narrow range, since
these effects are cumulative for the trunks that constitute a connection. At
the minimum, there is a connecting circuit between the long-distance office
and a local office at each end of an intercity trunk. If an interurban call is
between two small towns 3000 mi (5000 km) apart, several intercity trunks in
tandem may be used for the connection.
    Transmission systems for intercity trunks include voice-frequency circuits
on wire cable or open-wire lines, and carrier systems for wire cable,
open-wire lines, and carrier systems for wire cable, open-wire lines, coaxial
cable, and radio relay. Transmission systems for intercity trunks, for the
most part, utilize long-haul analog carrier systems and microwave radio relay
systems. Development of long-haul digital systems has been completed, and they
are being utilized in the network as they become economical.

    Electronic switching systems. Many telephone companies are finding it
economical to replace existing electromechanical switching systems with
electronic ones. Especially in metropolitan areas an electronic switching
system has lowered costs, increased productivity, and improved flexibility for
tandem switching offices.  See	SWITCHING SYSTEMS (COMMUNICATIONS).

    Direct distance dialing (DDD). This service permits customers to dial
their own interurban calls. DDD service requires dial-switching equipment for
the local office at the originating end and all long-distance offices through
which a DDD call may be routed. Automatic equipment is required to record the
calling and called telephone numbers and the duration of the conversation. A
comprehensive numbering plan is required to give each telephone a unique
number to indicate the destination of a call to the dial-switching systems.
Finally, a standard routing pattern and a general switching plan are required.
The standard routing pattern provides a basis for the selection of important
switching centers, called control switching points (CSPs). The general
switching plan provides an orderly method of interconnection over the most
direct and economical route available.

    Satellite communications. Satellites add circuit capacity and diversity to
terrestrial facilities and submarine cable (for overseas circuits). Having
diverse transmission systems makes it possible to maintain alternate
communications paths to ensure that at least one will be available in the
event of facility failures, outages, or natural disaster. In some areas,
satellites provide 50 - 100% of available facilities. For overseas circuits,
the cost per channel on satellites is considerably higher than on submarine
cable facilities.  See	COMMUNICATIONS SATELLITE.

    Light-wave cable. Cable systems that transmit calls on pulses of light,
rather than as electrical signals, are coming into use in the
telecommunications industry. These systems use hair-thin strands of glass to
carry signals translated into pulses of laser light. The progression to
commercial feasibility was made possible by the discovery of an efficient
means of generating light in microscopic semiconductors and the fact that this
light has extremely low attentuation when transmitted through glass fibers,
even over many kilometers.  See  OPTICAL COMMUNICATIONS.

    Overseas service.  Overseas and international telephone service links the
telephone systems of about 245 countries and territories throughout the world.
Overseas telephone circuits are provided mainly by three transmission systems:
tropospheric scatter radio, submarine cable, and space satellites. Each has a
definite field of use determined by its own characteristics and limitations.
    Tropospheric scatter systems normally operate in the 300 - 3000-MHz
frequency range and provide about 60 - 100 telephone circuits. Although radio
waves at these frequencies are not reflected by the ionosphere, it has been
found that, if large amounts of power are radiated, scattered energy will be
received over relatively long distances beyond the horizon. By the use of
high-powered transmitters (up to 10 kW) and large, high-gain antennas with
diameters of 30 - 60 ft (9 - 18 m), signals of strength adequate for a
multi-channel telephone service can be provided over distances of about 200 mi
(320 km).  See	TROPOSPHERIC SCATTER.
    Submarine cables generally consist of a single conductor and a grounded
coaxial return conductor. Submarine cables over long distances require
underwater amplifiers to restore the transmission losses in the cable. These
amplifiers, called repeaters, must be capable of operation at 3 or 4 nautical
miles (6 - 7 km) below the surface and must have a long life.  See  SUBMARINE
CABLE.
    Customer international direct distance dialing (IDDD) from the United
States was first introduced in March 1970. It has been expanded to include
over 75 countries and has been implemented in certain exchanges in many United
States cities. Many foreign countries also permit customer dialing to the
United States. Where IDDD service is not available, the international calls
are established by the operator.
 �34s[Richard B. Nichols ;Charles H. Sprankle]
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�6v�2v�32sTelephone signaling   �21m �33s 
    A system supplementing or replacing the usual telephone bell, to increase 
the area covered or otherwise provide more comprehensive means for notifying 
subscribers that they are wanted at the telephone. 
    Functionally, these systems are of two types: those which give extended 
coverage without indicating which individual is wanted, and those which 
indicate that a particular individual is wanted. The first type of system uses 
audible or visual signaling systems, generally operated in synchronism with 
the ringing cycle of the central office or private branch exchange (PBX) 
ringing source. 
    Special systems which give an indication that a particular individual is 
wanted include code calling systems, loudspeaker paging systems, and radio 
paging systems. Code calling systems sound a prearranged code number 
throughout the premises, usually by signals on single-stroke musical gongs. 
Loudspeaker paging systems may use a special microphone or the transmitter of 
the exchange attendant's regular telephone headset, transferred temporarily to 
the loudspeaker system by operating a key. Radio paging systems comprise 
miniature radio receivers which can be carried in a pocket. The receiver gives 
the wearer an audible signal in response to a radio-frequency signal actuated 
by a telephone operator at a nearby long-distance switchboard.  See  TELEPHONE 
SERVICE.  
 �34s[Rowland F. Davis]
�33s 
  
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�6v�2v�32sTelephone systems construction   �21m �33s 
    The process of physically constructing the outside plant portion of a 
complete telephone system. Principal components consist of (1) subscriber loop 
facilities from the central office to the customer's premises, (2) interoffice 
trunks, and (3) interurban cable or wire facilities. This construction 
includes new construction and reinforcement or replacement of existing 
facilities. The construction process includes the pole lines and conduit 
systems, as well as the wires and cables. It also includes joining and testing 
the circuits to ensure electrical integrity. Circuits are provided over cable 
or wire.  See  COMMUNICATION CABLES. 
    Telephone cables consist of conductors that are individually insulated 
with strip paper, paper pulp, or plastic; the conductors are then twisted into 
pairs, and the pairs are stranded to produce a cable core. The cores are 
protected by an outer sheath of plastic or, occasionally for special purposes, 
of lead. When plastic is used, a metallic shield, generally of aluminum but 
occasionally of copper, must be placed between the core and the sheath to 
provide electrical protection. 
    Wire circuits are open wire, multiple wire, or buried wire. Open wires are 
individual, bare, spaced wires supported by crossarms or bracket-mounted 
insulators on poles. Multiple wire consists of insulated conductors that are 
stranded around an insulated high-strength steel support wire, but does not 
have the outer sheath of a cable. 
    There are two predominant types of underground wire. One is composed of 
two pairs of 20-gage copper steel conductors with 40% conductivity. It is 
provided with an aluminum shield, protected by a plastic sheath, and is used 
for service connections from terminal to house in urban situations. The second 
type is a one-pair facility with 19-gage copper conductors protected by 
helix-wound steel armor wires covered by a plastic sheath. This facility is 
used predominantly for long, rural service connections. 
    Pole line design is based upon the aerial structures to be supported and 
the average winter weather conditions typical for the location. For economic 
reasons, communication lines and power lines often share the same poles. This 
is also done to avoid the objections to two adjacent parallel pole lines. 
    Underground lines are placed in conduit, which runs between manholes 
placed at 400- to 1000-ft (100- to 300-m) intervals. Manholes are usually of 
concrete cast in position; however, they also may be precast. Conduit is 
available in a number of materials, including concrete, fiber, fiber cement, 
iron pipe, and vitrified clay.  See  TELEPHONE SERVICE. 
 �34s[Guy L. Childberg]
�33s 
  
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�6v�2v�32sTelephony   �21m �33s 
    The transmission of speech to a distant point by means of electric waves. 
There are three elements basic to telephonic communication: (1) telephone sets 
to change sound energy into electric waves and back again; (2) transmission 
systems to carry electric waves over any distance within acceptable limits of 
distortion and attenuation; and (3) switching systems to connect any two 
telephone sets.  See  SWITCHING SYSTEMS (COMMUNICATIONS); TELEPHONE; TELEPHONE 
SERVICE.  
    The telephone was invented by Alexander Graham Bell and patented in the 
United States in 1876. Service was inaugurated by renting pairs of telephone 
sets to individuals for local service. Increasing demands for telephone 
service led to the development of a switchboard in 1878, a central switching 
office where a number of telephone sets can be interconnected manually by an 
operator. Efforts to interconnect any two telephone sets automatically were 
first successful in 1889 with the invention by Almon B. Strowger of a 
practicable mechanism called a step-by-step switch. Thus, a verbal request for 
a connection to a distant telephone, formerly handled by an operator, was 
replaced by dial signals initiated by the customer. Subsequent research and 
development improved the step-by-step switch, led to more advanced 
electromechanical switching systems, and made possible the electronic 
switching system with complex memory and logic functions analogous to computer 
systems. Scientific research also led to the development of push-button 
telephones, introduced in the United States in 1963. 
    To connect any two of millions of telephones automatically, memory is 
needed to retain the called number and logic is needed to decide the proper 
action to be taken from a number of possible alternatives which vary from call 
to call. As the number of telephones to be interconnected grew, larger cities 
needed several and then many central offices. Central offices are 
interconnected by interoffice circuits, sometimes directly. For connections 
between cities or urban areas, a combination of direct and switched circuits 
is used. 
    Both cables and radio relay systems are employed in international 
telephone communications. Water barriers between countries are bridged by 
underseas cables, high-frequency radio systems, and satellites in synchronous 
orbit. The International Telecommunication Union (ITU), established in 1932, 
is recognized by the United Nations as the specialized agency responsible for 
international telecommunications. 
 �34s[Clifton R. Williamson]
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�6v�2v�32sTelephotography   �21m �33s 
    The transmission of photographs over electrical communication channels. 
The channels are generally those provided by common carrier communication 
companies. Individual users furnish telephotograph or facsimile machines to 
suit their requirements. In the United States such services range from simple 
arrangements connecting two sets of machines within a city to large networks 
involving many cities and types of communication facilities. Coordination must 
thus be effected between the communication companies and the manufacturers of 
equipment used by customers for their stations to ensure compatibility of 
equipment and optimum system performance. Unattended reception is made 
possible by starting and synchronizing receiving machines with pulses or tones 
generated at the transmitting location. 
    The transmission of news photographs and weather maps is one example of a 
use with contrasting requirements involving different types of machines and 
networks. Normally, news pictures require half-tone reproductions with a 
number of shades of gray. At the receiving locations photographic reproducing 
devices are used. In contrast, weather maps and other black-and-white 
nonshaded material can readily be reproduced by direct recording processes.  
See  FACSIMILE. 
 �34s[Charles C. Duncan]
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�6v�2v�32sTelephoto lens   �21m �33s 
    A photographic lens system specially designed to give a large image of a 
distant object in a camera of relatively short focal length. A telephoto lens 
generally consists of a positive lens system and a negative lens system, 
separated by a considerable distance.  See  CAMERA; LENS (OPTICS). 
 �34s[Max J. Herzberger]
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�6v�2v�32sTelescope   �21m �33s 
    An instrument used to collect, measure, or analyze radiation from a 
distant object. Most commonly, telescope refers to an assemblage of lenses or 
mirrors, or both, that enhances the ability of the eye either to see objects 
more distinctly or to see fainter objects. In its most general meaning, 
telescope refers to a device that collects radiation, which may be in the form 
of electromagnetic radiation or particle radiation, from a limited direction 
in space. 
    Optical telescopes may be classified as refracting telescopes, which use 
only lenses; reflecting telescopes, which use mirrors to focus light; and 
catadioptric telescopes, which use both lenses and mirrors. Special types of 
reflecting telescopes which depart radically from conventional ground-based 
systems are the multimirror telescope and the space telescope.  See  OPTICAL 
TELESCOPE.  
    As collectors of radiation from a specific direction, telescopes may be 
classified as focusing and nonfocusing. Nonfocusing telescopes are used for 
radiation with energies of x-rays and above. Focusing telescopes, intended for 
nonvisible wavelengths, are similar to optical ones, but they differ in the 
details of construction.  See  GAMMA-RAY ASTRONOMY; INFRARED ASTRONOMY; RADIO 
TELESCOPE; X-RAY TELESCOPE. 
 �34s[William M. Sinton]
�33s 
  
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7319552
�6v�2v�32sTelestacea   �21m �33s 
    An order of the subclass Alcyonaria. Telestacea are typified by Telesto, 
which forms an erect branching colony by lateral budding from the body wall of 
an elongated primary or axial polyp. The stolon is bandlike or membranous. 
Sclerites are scattered singly, partly fused, or entirely fused to form a 
rigid tube.  See  ALCYONARIA. 
 �34s[Kenji Atoda]
�33s 
  
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7319552
�6v�2v�32sTeletypesetter   �21m �33s 
    A system for automatically operating a linecasting machine (Linotype or 
Intertype) to produce lines of type at high speed under the control of 
perforated tape. Basically, this system consists of separating the complex 
manual operation of a linecasting machine into two simple operations and 
making one of them fully automatic under the control of perforated tape. 
    The perforated tape is produced on a Teletypesetter perforator, which has 
a keyboard layout similar to that of a typewriter, plus additional keys to 
control the various functions of the linecasting machine. The perforated tape 
controls the Teletypesetter operating unit, which is attached to the 
linecasting machine keyboard. Thus the linecasting machine is fully automatic 
under the control of perforated tape produced on the perforator. 
    Teletypesetter equipment has been used by most daily and weekly newspapers 
and by many commercial shops throughout the world. By supplementing 
Teletypesetting equipment with Teletype sending and receiving equipment 
operating over special long-distance data telegraph circuits, the perforated 
tape could be used to produce type automatically in hundreds of printing 
plants throughout the country. 
 �34s[Virginius N. Vaughan, Jr.]
�33s 
  
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�6v�2v�32sTeletypewriter �21m �33s
    An electromechanical device, also called a teleprinter, for transmitting
and receiving messages over a telegraph circuit. A sending and receiving
teletypewriter performs two functions: The keyboard transmitter generates
coded electrical signals for transmission over a telegraph circuit; and the
typing unit converts such signals into a printed message. A page
teletypewriter (see illustration) prints a message in page form, usually on a
continuous roll of paper 8J in. (21.6 cm) wide.
    The Baudot code, once widely used in printing telegraph devices, consists
of five code pulses, any one of which may be either marking or spacing. In
single-current signals, a marking pulse is an interval of time during which
current flows through the circuit, and a spacing pulse is an interval during
which no current flows. In polar signals, frequently used over long lines, a
marking pulse is an interval during which negative current flows, and a
spacing pulse is an interval during which positive current flows. There are 32
possible combinations of the five code pulses. One combination is assigned to
each of the 26 letters of the alphabet. Because only 26 code combinations are
available for printing in each of the two shift positions, both capital and
lowercase letters cannot be used. The uppercase position is reserved for the
10 digits, punctuation marks, and commonly used symbols.
    On June 17, 1963, the American Standards Association (now the American
National Standards Institute) adopted a new American Standard Code for
Information Interchange (ASCII). The code consists of seven code pulses, or
bits, instead of five as in the Baudot code. There are thus 2(sup 7), or 128,
discrete permutations of the seven bits in the code. Sixty-four of these
permutations were assigned to printing characters, including the capital
letters of the alphabet, the ten digits, punctuation marks, and special
symbols, Thirty-five permutations were assigned to control characters, and the
remainder were unassigned. A revised code was approved in 1967.
 �34s[Fred W. Smith]
�33s


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�6v�2v�32sTeletypewriter exchange (TWX) service   �21m �33s 
    A teletypewriter communication service provided by Western Union for 
approximately 53,000 subscribers in the continental United States, except 
Alaska. Direct interconnection is available to the Canadian TWX network. 
Canadian Telex subscribers are able to make southbound calls to United States 
TWX subscribers. Access to Western Union Telex subscribers is on a 
message-switched basis.  See  TELETYPEWRITER; TELEX. 
    Any TWX subscriber can communicate directly with any other TWX subscriber. 
Connections are established in a manner similar to that of the telephone and 
Telex networks. After a connection is established between two TWX subscribers, 
communication is carried on in a conversational mode, using the teletypewriter 
keyboard and printer. TWX subscribers may also have teletypewriters equipped 
with tape sending and receiving units. This type of teletypewriter allows the 
subscriber to prepare messages in advance for later transmission.   See   
TELEGRAPHY; TELEX. 
 �34s[Earl C. Mansfield]
�33s 
  
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�6v�2v�32sTelevision   �21m �33s 
    The electrical transmission and reception of transient visual images. Like 
motion pictures, television consists of a series of successive images, which 
are registered on the brain as a continuous picture because of the persistence 
of vision. Each visual image impressed on the eye persists for a fraction of a 
second. In television in the United States, 30 complete pictures are 
transmitted each second, which with the use of interlaced scanning is fast 
enough to avoid evident flicker. 
    At the television transmitter, minute portions of a scene are sampled 
individually for brightness (and color for color television), and the 
information for each portion is transmitted consecutively. At the receiver, 
each portion is reproduced in its proper position and with correct brightness 
(and color) to reproduce the original scene.  See COLOR TELEVISION. 
    The scene is focused on a photoelectric screen of a camera tube. Each 
portion of the screen is changed by the photoelectrons to a degree depending 
upon the brightness of the particular portion. The screen is scanned by an 
electron beam just as a reader scans a page of printed type, character by 
character, line by line. When so scanned, an electric current flows with an 
instantaneous magnitude proportional to the brightness of the portion scanned. 
  See  TELEVISION CAMERA; TELEVISION CAMERA TUBE. 
    Variations in the current are transmitted to the receiver, where the 
process is reversed. An electron beam in the picture tube is varied in 
intensity (modulated) by the incoming signals as it scans the picture-tube 
screen in synchronism with the scanning at the transmitter. The photoelectric 
surface of the picture tube produces light in proportion to the intensity of 
the electron beam which strikes it. In this way the minute portions of the 
original scene are re-created in their proper positions, brightness, and (for 
color transmission) color values.  See  TELEVISION RECEIVER; TELEVISION 
TRANSMITTER.  
    In the United States an individual picture (frame) is considered to be 
made up of 525 lines, each line containing several hundred picture elements. 
All these lines are scanned and the light values are sent to the receiver so 
that each second 30 pictures are received. 
    The bandwidth required for any information transmission system is a 
function of the number of bits of information, or the detail, to be 
transmitted per second. The standard of 525 lines sets the vertical detail, 
and the standard aspect ratio (picture width to  height) of 4:3 requires 700 
horizontal picture elements for equal horizontal and vertical resolution, or 
350 sets of alternate black-and-white squares. The picture is reproduced 30 
times a second, for a total of 525 x 350 x 30 or 5,512,500 complete cycles per 
second. Less detail than this is actually transmitted and received; the 
highest video frequency actually transmitted is 4.2 MHz.  See  BANDWIDTH 
REQUIREMENTS (COMMUNICATIONS). 
    The band of frequencies assigned to a television station for the 
transmission of synchronized picture and sound signals is called a television 
channel. In the United States a television channel is 6 MHz wide, with the 
visual carrier frequency 1.25 MHz above the lower edge of the band and the 
aural carrier 0.25 MHz below the upper edge of the band. 
    The sound portion of the program is transmitted by frequency modulation. 
Maximum frequency deviation (bandwidth) of the sound signals is 25 kHz. 
Transmission of visual signals uses amplitude modulation and vestigial 
sideband transmission.  See  AMPLITUDE MODULATION; FREQUENCY MODULATION. 
    Television programs are recorded for rebroadcast at a later time and for 
many other reasons. The principal method is video tape recording (VTR) in 
which television sound and pictures are recorded on magnetic tapes.  See  
CLOSED-CIRCUIT TELEVISION; TELETEXT AND VIDEOTEXT; TELEVISION STANDARDS. 
 �34s[Steve deSatnick]
�33s 
  
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�6v�2v�32sTelevision camera   �21m �33s 
    An electrooptical system used to pick up and convert a visual image or 
scene into an electrical signal called video. The video may be transmitted by 
cable or wireless means to a suitable receiver or monitor some distance from 
the actual scene. 
    A television camera may fall within one of several categories: studio, 
portable, or telecine  (Fig. 1). It may also be one of several highly 
specialized cameras used for remote viewing of inaccessible places, such as 
the ocean bottom or inside nuclear power reactors. the camera may be capable 
of producing color or monochrome (black and white) pictures. Modern cameras 
are entirely solid-state, except for the actual pickup tube. Even the pickup 
tube may be replaced by solid-state semiconductors called charge-coupled 
devices.  See  TELEVISION CAMERA TUBE. 
  
    Essential elements.  Every camera shares certain essential elements: an 
optical system, one or more picture pickup devices, preamplifiers, scanning 
circuits, blanking and synchronizing circuits, and video processing circuits 
and control circuits. Color cameras also include some kind of color-encoding 
circuit.  
    A television camera may be one-piece, with all components contained in one 
assembly, or it may consist of a head and a camera control unit (CCU) 
connected by a multiple-conductor cable, and be called a camera chain. The 
head unit comprises the optical system with lens, the picture pickup tubes, 
and a minimum of electronics necessary to generate and amplify the minute 
signals from the pickup tubes. The CCU may contain electronics and controls 
which allow a skilled operator to adjust brightness (luminance), color 
(balance, saturation, and hue), and certain correction circuits (registration, 
gamma, and aperture) which improve the picture. A specially calibrated 
oscilloscope, called a waveform monitor, is generally provided to the CCU 
operator so that accurate voltage levels may be set. Modern cameras provide 
for automatic as well as manual setup of some or all of the above adjustments. 
Some cameras have a built-in microprocessor which enables complete setup of 
all parameters by simply pushing one button. 
  
  
    Typical configurations.  Live studio cameras are equipped with several 
ancillary systems to enhance their operation. An electronic viewfinder, 
actually a small television monitor, shows the camera operator what the camera 
is seeing, making it possible to frame and focus the picture. The tally system 
consists of one or more red lights which illuminate when the camera's picture 
is ``on the line'' so that production and on-camera personnel know which 
camera is active. Generally an intercom system is built into the camera so 
that the director can communicate with the camera operator. The camera itself 
may be mounted upon a tripod, but more often it is on a dolly and pedestal, 
which allows the camera to be moved around on the studio floor and raised or 
lowered as desired. A pan head permits the camera to be rotated to the left or 
right and furnishes the actual mounting plate for the camera. The lens zoom 
and focus controls are mounted on a panning handle convenient to the operator. 
A common accessory on studio cameras is the videoprompter. This is a 
television monitor on which the program script may be displayed and read by 
the on-camera personnel. 
    Telecine cameras are used in conjunction with film or slide projectors to 
televise motion pictures and still images. They generally employ vidicon 
tubes, one for monochrome and three for color, and many of the usual controls 
are automatic so as to require less operator attention. 
    Portable cameras usually combine all of the basic elements into one 
package and may be used for a multitude of purposes. The units are 
lightweight, often have built-in microphones for sound pickup, and can be 
handled by one person. Used in conjunction with modern battery-powered 
portable videocassette recorders, an entire video and audio pickup system can 
be carried and operated by one or two people (Fig. 2).  See  COLOR TELEVISION; 
TELEVISION; TELEVISION RECEIVER; TELEVISION STANDARDS; TELEVISION TRANSMITTER. 
 �34s[Earl F. Arbuckle, III]
�33s 
  
  
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�6v�2v�32sTelevision camera tube   �21m �33s 
    An electron tube having a light-sensitive receptor that converts an 
optical image into an electrical television video signal. The tube is used in 
a television camera to generate a train of electrical pulses representing the 
light intensities present in an optical image focused on the tube. Each point 
of this image is interrogated in its proper turn by the tube, and an 
electrical impulse corresponding to the amount of light at that point of the 
optical image is generated by the tube. This signal represents the video or 
picture portion of a television signal.  See  TELEVISION CAMERA. 
  
    Image orthicon.  The image orthicon made broadcast television practical. 
It was used for more than 20 years as the primary studio and field camera tube 
for black and white and color television programming because of its high 
sensitivity and its ability to handle a wide range of scene contrast and to 
operate at very low light levels. It is one of the most complicated camera 
tubes. The image orthicon is divided into an image section, a scanning 
section, and a multiplier section, within a single vacuum envelope. The image 
isocon is a further development of the image orthicon. 
  
    Photoconductive tubes.  These types have a photoconductor as the 
light-sensitive portion. The name vidicon was applied to the first 
photoconductive camera tube developed by RCA. It is loosely applied to all 
photoconductive camera tubes, although some manufacturers adopt their own 
brand names. The vidicon tube is a small tube that was first developed as a 
closed-circuit or industrial surveillance television camera tube. The 
development of new photoconductors has improved its performance to the point 
where it is now utilized in one form or another in most television cameras. 
Its small size and simplicity of operation make it well suited for use in 
systems to be operated by relatively unskilled people. 
    The vidicon is a simply constructed storage type of camera tube (see 
illustration). The signal output is developed directly from the target of the 
tube and is generated by a low-velocity scanning beam from an electron gun. 
The target generally consists of a transparent signal electrode deposited on 
the face-plate of the tube and a thin layer of photoconductive material, which 
is deposited over the electrode. The photoconductive layer serves two 
purposes. It is the light-sensitive element, and it forms the storage surface 
for the electrical charge pattern that corresponds to the light image falling 
on the signal electrode. 
    Photoconductor properties determine to a large extent the performance of 
the different types of vidicon tubes. The first and still most widely used 
photoconductor is porous antimony trisulfide. The latest photoconductors are 
the lead oxide, selenium-arsenic-tellurium, cadmium selenide, zinc-cadmium 
telluride, and silicon diode arrays. 
  
    Silicon intensifier.  The silicon intensifier camera tube utilizes a 
silicon diode target, but bombards it with a focused image of high-velocity 
electrons. Each high-energy electron can free thousands of electron carriers 
in the silicon wafer (compared to one carrier per photon of light on a silicon 
diode vidicon). This high amplification allows the camera to operate at light 
levels below that of the dark-adapted eye. The silicon intensifier tube is 
utilized for nighttime surveillance and other extremely low-light-level 
television uses in industrial, scientific, and military applications. 
  
    Solid-state imagers.  These are solid-state devices in which the optical 
image is projected onto a large-scale integrated-circuit device which detects 
the light image and develops a television picture signal. Typical of these is 
the charge-coupled-device imager. The term charge-coupled device (CCD) refers 
to the action of the device which detects, stores, and then reads out an 
accumulated electrical charge representing the light on each portion of the 
image. The device detects light by absorbing it in a photoconductive 
substrate, such as silicon. The charge carriers generated by the light are 
accumulated in isolated wells on the surface of the silicon that are formed by 
voltages applied to an array of electrodes on top of an oxide insulator formed 
on the surface of the silicon. A practical CCD imager consists of a structure 
that forms several hundred thousand individual wells or pixels, and transfers 
the charges accumulated in these pixel wells out to an output amplifier in the 
proper sequence.  See  CHARGE-COUPLED DEVICES. 
 �34s[Robert G. Neuhauser]
�33s 
  
  
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�6v�2v�32sTelevision networks   �21m �33s 
    Arrangements of communications channels, suitable for transmission of 
video and accompanying audio signals, which link together groups of television 
broadcasting stations or closed-circuit television users in different cities 
so that programs originating at one point can be fed simultaneously to all 
others.  
    In the United States, television network service is furnished by the 
telephone companies. The facilities consist of intercity channels, which 
interconnect the principal long-distance telephone offices in various cities, 
and local channels, which connect the telephone offices with the broadcasters' 
studios or other user locations in each city. In the telephone offices the 
intercity and local channels are brought together in a television operating 
center (TOC) where means are provided for testing, monitoring, and connecting 
the channels in various patterns as required for service. 
  
    Use of networks.  The principal users of television services are the 
broadcasting companies. Each of the broadcasting companies contracts on a 
monthly basis for daily service from its principal originating points to the 
various broadcasting stations affiliated with its network. 
    The broadcasting companies frequently order ``occasional'' channels in 
addition to their normal networks in order to pick up programs from other than 
the usual originating points, or to supplement the normal layout with 
additional channels when it is desired to split the network into several 
parts, each receiving a different program. 
    Television networks are also provided, both on a regular daily and 
occasional basis, for educational television (ETV) users. These are generally 
state and regional networks interconnecting schools, hospitals, or government 
agencies as well as ETV broadcasting stations. To keep charges down, these 
channels frequently run from one user location directly to another, without 
passing through telephone company operating centers. 
    Television networks are also used for transmitting programs which are not 
broadcast but are displayed on screens at the receiving locations. These are 
called closed-circuit television networks. Their principal use is to 
distribute sports events to the theaters and hotels and for industrial 
conferences and displays. These networks are generally temporary.  See  
CLOSED-CIRCUIT TELEVISION. 
  
    Intercity channels.  The facilities for intercity video channels are 
largely TD-2 radio relay routes which are also used to furnish intercity 
telephone circuits. TD-2 is a frequency-modulated (FM) microwave radio system 
capable of providing 12 wide-band channels in each direction in the frequency 
band from 3700 to 4200 MHz. Each channel is capable of carrying one video 
signal or up to 1200 voice channels. The relay stations are spaced an average 
of 25 - 30 mi (40 - 48 km) apart. 
  
    Local channels.  The local channels, which connect the telephone company 
office with the broadcasters' studios and transmitters and with closed-circuit 
users' locations, are largely composed of cable facilities. Local channels can 
also be provided by microwave radio facilities. Economic considerations 
usually determine whether radio or cable should be used, but the decision may 
be influenced by the need for a speedy installation, such as for one-time news 
or sports events. 
  
    Satellites.  Commercial television service is available over both the 
Atlantic and Pacific oceans using satellites furnished by the International 
Satellite Consortium managed by Comsat. Both the American Telephone and 
Telegraph Co., and the United States international telegraph carriers furnish 
such services, renting the overseas channels from Comsat. The overseas 
channels are connected to the television networks in the United States at the 
TOCs in New York or San Francisco by terrestrial links to the Earth stations 
furnished by the telephone companies.  See  COMMUNICATIONS SATELLITE; 
TELEVISION.  
 �34s[James R. Rae]
�33s 
  
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�6v�2v�32sTelevision receiver �21m �33s
    The equipment used to receive the transmitted modulated radio-frequency
signals and produce synchronized visual images and sound for entertainment or
educational purposes. The radio-frequency portion operates on the
superheterodyne principle.  See  AMPLITUDE MODULATION; FREQUENCY MODULATION;
RADIO RECEIVER.
    The first television receivers to be mass-produced were monochrome; that
is, they provided pictures in black and white only. Later, color receivers,
which produce pictures in full color as well as black-and-white, became
available.  See  TELEVISION.

    Monochrome.  Since most broadcast television transmissions in the United
States are horizontally polarized, the most basic type of television-receiving
antenna is the horizontally mounted half-wave dipole. Because the stations
serving a given area may operate on widely different frequencies, however, the
dipole dimensions must be a compromise that permits reasonable performance on
all the desired channels. More complex antennas combine several dipole
elements of various lengths, and passive reflectors may be used to achieve
some degree of horizontal directivity.	See  ANTENNA (ELECTROMAGNETISM).
    The tuner selects the desired channel and converts the frequencies
received to lower frequencies within the passband of an intermediate-frequency
amplifier. The output from the tuner is applied to this amplifier. Several
stages of amplification are required to obtain the desired output signal level
and selectivity.
    The output of the i-f amplifier consists of two modulated rf signals. One
of these, which is amplitude-modulated, provides a varying signal
corresponding to the black-and-white portions of the picture, a blanking
signal to render the return trace invisible on the picture tube, horizontal
sync pulses to initiate the retrace of the beam at the end of each line, and
vertical sync pulses to initiate the retrace of the beam at the end of each
picture field. The other signal is frequency-modulated and contains the
transmitted sound information. These two rf signals are applied to a diode
which produces a rectified output that follows the instantaneous peak value of
the amplitude-modulated picture carrier.
    Coincidentally a 4.5-MHz signal results from the heterodyne beat of the
picture and sound carriers. This signal contains the frequency-modulated sound
information, which can be further amplified and detected in the sound channel.
This is known as the intercarrier sound (ICS) system. For sound reproduction,
the intercarrier sound signal formed at the detector is passed through a
4.5-MHz i-f amplifier, through some form of amplitude limiting, and to an FM
detector, which converts the sound carrier modulation to an audio-frequency
signal. This signal is then passed through an audio amplifier to a loudspeaker
and converted to acoustic output.
    Since television receivers, like radio receivers, may be subjected to
widely varying incoming signal strengths, some form of automatic gain control
(AGC) is necessary.  See  AUTOMATIC GAIN CONTROL (AGC).
    Picture synchronizing information is obtained from the video signal by
means of sync separation circuits. In addition, these circuits must separate
this information from noise and interference during the reception of weak
signals, particularly if impulse noise is present.
    Two independent sweep systems are employed in the vertical and horizontal
sweep circuits. Each employs a timing generator, generally of the oscillatory
type, controlled by the synchronizing information obtained from the sync
separators. The oscillators are followed by drive and waveform-shaping
circuits. These are followed by power amplifier stages capable of providing
the currents required by the deflection coils of the yoke for picture-tube
beam deflection. Substantially different techniques are required for vertical
and horizontal scanning.
    The display device for a television receiver is a cathode-ray tube,
consisting of an evacuated bulb containing an electron gun and a phosphor
screen, which emits light when excited by an electron beam. The intensity of
the electron beam is controlled by the video signal, which is applied either
to the grid or the cathode of the electron gun. The position of the electron
beam is controlled by electromagnetic fields produced by the deflection yoke
placed around the neck of the tube.  See  CATHODE-RAY TUBE.

    Color receivers.  Television receivers designed to produce images in full
color are necessarily more complex than those designed to produce monochrome
images only, because additional information must be handled to produce color.
In monochrome systems, the video signal controls only the luminance of the
various areas of the image. In color systems, it is necessary to control both
the luminance and chrominance of the picture elements.
    For color television purposes, chrominance is most frequently expressed
quantitatively in terms of the amounts of two hypothetical, zero-luminance
primary colors (usually designated I and Q), which must be added to or
subtracted from a neutral color of a given luminance to produce the color in
question. As a practical matter, color television receivers produce full-color
images as additive combinations of red, green, and blue primary-color images,
and it is necessary to process the luminance and chrominance information
contained in a color signal in such a way as to make it usable by a practical
reproducing device.
    Color television broadcasts in the United States employ signal
specifications that are fully compatible with those used for monochrome,
making it possible for color programs to be received on monochrome receivers
and monochrome programs to be received on color receivers. Compatibility is
achieved by encoding the color information at the transmitting end of a color
television system in such a way that the transmitted signal consists
essentially of a normal monochrome signal (conveying luminance information)
supplemented by an additional modulated wave conveying chrominance information.
    Special decoding circuits are necessary in a color receiver to process the
luminance and chrominance information in a color signal so that it can be used
for the control of a practical color kinescope utilizing red, green, and blue
primary colors.
    The great majority of color television receivers employ the shadow-mask
color kinescope, in which color images are produced in the form of closely
intermingled red, green, and blue dots. The primary-color phosphor dots are
excited by three separate electron beams, which are prevented from striking
dots of the wrong color by the shadowing effect of an aperture mask located
about 1/2 in. (1.25 cm) behind the special phosphor screen. The three beams in
such a kinescope are all deflected simultaneously by the fields produced by a
single deflection yoke placed in the conventional position around the neck of
the tube. It is necessary, however, to provide an auxiliary deflection system
to maintain convergence of the three beams in all areas of the viewing screen,
so that the primary-color images are properly registered. A special
convergence yoke is commonly placed around the neck of the kinescope, just
ahead of the electron guns; this yoke has separate coils for electromagnetic
control of the positions of the three beams.  See  COLOR TELEVISION.
 �34s[Chester M. Sinnett /John W. Wentworth]
�33s

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�6v�2v�32sTelevision scanning   �21m �33s 
    The process of scrutinizing the brightness of each element of detail 
contained in the image of a scene to be transmitted by television. In 
monochrome television, the process is instrumental in converting the 
brightness of each individual element so scrutinized into a unique 
voltage-time response suitable for transmission. In color television, the 
brightness variations of each scene are first separated by red, green, and 
blue filters, after which the conversion from brightness to voltage on a time 
basis occurs separately for each of the three colors. Scanning also takes 
place in the receiver in exact synchronism with the camera-tube scanning, and 
synchronizing signals are transmitted for that purpose.  See  COLOR 
TELEVISION; TELEVISION; TELEVISION RECEIVER. 
    An image is analyzed by scanning it according to a fine structure of 
parallel, nearly horizontal lines called a scanning raster. The complete 
raster is rectangular in shape. If all the lines are scanned in sequence, the 
process is called sequential scanning. 
    A variation of this kind of scanning, called interlaced scanning, is used 
to conserve bandwidth in the transmission system without introducing 
intolerable flicker. Flicker is a function of the frequency of repetition of 
coverage of the raster. With interlaced scanning, alternate (odd-numbered) 
lines are scanned first, and the remaining (even-numbered) lines are scanned 
next. The entire raster area is covered or scanned twice. Therefore, the 
picture repetition rate for interlaced scanning is twice that of sequential 
scanning, which is at the same velocity along each line, and a corresponding 
reduction in the sensation of flicker is obtained. This is called double 
interlacing and is standard in all broadcast television systems. The entire 
raster is covered 30 times per second; therefore, the picture area is scanned 
60 times a second. 
 �34s[Donald G. Fink]
�33s 
  
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�6v�2v�32sTelevision standards   �21m �33s 
    Numerical and graphical criteria which describe essential aspects of a 
television system, employed in the design and operation of equipment to assure 
that the various parts of the system (studios, transmitters, networks, and 
receivers) will operate in cooperative fashion at maximum performance. 
Television systems have a special need, compared with other communication 
systems, for definitive standards because television transmitters and 
receivers must operate in a precise ``lock-and-key'' relationship. In 
particular, the scanning of the image in the camera must be matched by the 
scanning in every associated receiver within a timing precision of 
approximately one-tenth of a millionth of a second, and with relative 
positions of picture details correct to a few hundredths of an inch.  See  
TELEVISION SCANNING. 
    To assure that any television receiver can receive programs from any 
transmitter within range, it is cusomary to set up a single set of standards 
within a group of neighboring countries. The systems of Mexico, the United 
States, Canada, and Cuba operate on one such set of standards. Those of Europe 
also conform to a single set of standards, which differ in detail from those 
used in North America. Similar groupings of standards are found in other 
regions of the world. Television standards are promulgated, and conformity to 
them by television stations is legally enforced by governmental agencies 
having responsibility for communications systems in the respective countries. 
In the United States the responsible agency is the Federal Communications 
Commission. Conformity is not usually enforced in the case of television 
receiver design, but is assured by the desire of designers to make best use of 
available signals. 
    Television standards are of two kinds. Transmission standards describe the 
signals radiated by television transmitters and their relation to the intended 
visual and aural content of the program. Allocation standards define the 
manner in which the radiated signals occupy the radio spectrum. These affect 
primarily the distances over which television stations provide satisfactory 
service in the presence of interference from transmitters, receivers, and 
other sources.  See  TELEVISION. 
 �34s[Donald G. Fink]
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�6v�2v�32sTelevision transmitter   �21m �33s 
    An electronic device that converts audio and video signals into modulated 
radio-frequency (rf) energy which can be radiated from an antenna and received 
on a television receiver. The term can also refer to the entire television 
transmitting plant, consisting of the transmitter proper, associated visual 
and aural input and monitoring equipment, transmission line, the antenna with 
its tower or other support structure, and the building in which the equipment 
is housed. 
    A television transmitter is really two separate transmitters integrated 
into a common cabinet. Video information is transmitted via a visual 
transmitter, while audio information is transmitted via an aural transmitter. 
Because video and audio have different characteristics, the two transmitters 
differ in terms of bandwidth, modulation technique, and output power level. 
Nevertheless, a common transmitting antenna is generally used, and the two 
transmitters feed this antenna via an rf diplexer or combiner. 
    Television stations are licensed to operate on a particular channel, but 
since it takes a very wide bandwidth to transmit a television picture, these 
channels are allocated over a broad range of frequencies. Channels 2 through 6 
are low-band very-high-frequency (VHF) channels, while channels 7 through 13 
are high-band VHF channels. Channels 14 through 83 are ultra-high-frequency 
(UHF) channels. Because of the wide range of frequencies, television 
transmitters are designed to work in only one of the foregoing groups, and 
employ specific circuits which are most efficient for the channels involved. 
    The horizontal radiation pattern of most television transmitting antennas 
is circular, providing equal radiated signal strength to all points of the 
compass. Higher-gain antennas achieve greater power in the direction of the 
horizon by reducing the power radiated at vertical angles above and below the 
horizon. Because television signals travel in a ``line of sight,'' 
transmitting antennas are ususally placed as high as possible above ground 
with respect to the surrounding service area. It is also desirable to locate 
all of the transmitting antennas serving a given locality in the same place. 
This allows viewers to orient their receiving antennas in one direction for 
the best reception from all of the stations. See ANTENNA (ELECTROMAGNETISM). 
    There are two broad classes of VHF visual television transmitter design 
philosophy. The classical approach modulates the carrier at a moderate power 
level, amplifies the carrier to rated output power by means of linear 
amplifiers, and then filters this high-power carrier to obtain the required 
vestigial-sideband signal. The more contemporary approach employs modulation 
at a very low power level of an intermediate-frequency (if) signal. The 
required vestigial-sideband filtering is imposed on this low-level signal, 
generally by means of a highly stable surface acoustic-wave (SAW) filter, 
whereupon the signal is upconverted to the carrier frequency and amplified by 
linear amplifiers to rated output power. See AMPLIFIER; SURFACE ACOUSTIC-WAVE 
DEVICES; TELEVISION. 
 �34s[Earl F. Arbuckle, III]
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�6v�2v�32sTelex �21m �33s
    A worldwide teleprinter exchange service providing direct send and receive
teleprinter connections between subscribers. The rules for its operation are
usually determined within the framework of recommendations by the Consulting
Committee for International Telephone and Telegraph (CCITT).
    In the continental United States (except Alaska), Western Union provides
the domestic Telex service. Direct interconnection is made with the Telex
systems of Alaska, Canada, and Mexico. Overseas connections are made through
various international companies.
    Other services available to domestic Telex subscribers are: TEL(T)EX,
which permits subscribers to send messages to selected terminal cities in the
United States and Canada; INFOMASTER(TM), which provides for the delayed
delivery of messages, multiple-address messages, collect calls, international
and domestic telegrams, MAILGRAM(R), and TWX(TM) messages, on a
message-switched basis; FYI News Service, which provides subscribers with news
bulletins and financial, market, weather, and other special reports; and
DatagramsupTsupM, which permits telephone-entered messages to be delivered to
Telex and TWX subscribers.
    Teleprinters or automatic send and receive sets are provided in the
domestic Telex system.	See  TELETYPEWRITER; TELETYPEWRITER EXCHANGE (TWX)
SERVICE.
 �34s[Earl C. Mansfield]
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�6v�2v�32sTellurium �21m �33s
    A chemical element, Te, atomic number 52, and chemical atomic weight
127.60. There are eight stable isotopes of natural tellurium. Tellurium makes up
approximately 10(sup -9)% of the Earth's igneous rock. It is found as the free
element, sometimes associated with selenium. It is more often found as the
tellurides sylvanite (graphic tellurium), nagyagite (black tellurium),
hessite, tetradymite, altaite, coloradoite, and other silver-gold tellurides,
as well as the oxide, tellurium ocher.
    There are two important allotropic modifications of elemental tellurium,
the crystalline and the amorphous forms. The crystalline form has a
silver-white color and metallic appearance. This form melts at 841.6DGF
(449.8DGC) and boils at 2534DGF (1390DGC). It has a specific gravity of 6.25,
and a hardness of 2.5 on Mohs scale. The amorphous form (brown) has a specific
gravity of 6.015. Tellurium burns in air with a blue flame, forming tellurium
dioxide, TeO2. It reacts with halogens, but not sulfur or selenium, and forms,
among other products, both the dinegative telluride anion (Te sup 2-), which
resembles selenide, and the tetrapositive tellurium cation (Te sup 4+), which
resembles platinum (IV).
    Tellurium is used primarily as an additive to steel to increase its
ductility, as a brightener in electroplating baths, as an additive to
catalysts for the cracking of petroleum, as a coloring material for glasses,
and as an additive to lead to increase its strength and corrosion resistance.
See  SELENIUM.
 �34s[Stanley Kirschner]
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�6v�2v�32sTelosporea �21m �33s
    A class of the subphylum Sporozoa. These protozoa are divided into two
subclasses, the Gregarinia and Coccidia. All members of the group are either
intra- or extracellular parasites, and the life cycles have both sexual and
asexual phases. The spores lack a polar capsule and develop from an oocyst.
The sporozoite is the usual infective stage which initiates the asexual phase
in the life cycle.  See  COCCIDIA; GREGARINIA PROTOZOA; SPOROZOA.
 �34s[Elery R. Becker /Norman D. Levine]
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�6v�2v�32sTemnopleuroida   �21m �33s 
    An order of Eichinacea with a camarodont lantern, smooth or sculptured 
test, tubercles imperforate or perforate (and usually crenulate), ambulacral 
plates of diademoid or echinoid type, and branchial slits which are usually 
shallow. There are three included families, Glyphocyphidae, Temnopleuridae, 
and Toxopneustidae.  See  ECHINACEA. 
 �34s[Howard B. Fell]
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�6v�2v�32sTemnospondyli   �21m �33s 
    The more common of the two major orders of the great extinct subclass 
Labyrinthodontia. Temnospondyls are differentiated from anthracosaurs by 
having the cheek suturally attached to the skull table. Their vertebral 
structure, in which the pleurocentra are seldom prominent and the intercentra 
are large, resembles that of some members of the rhipidistian 
crossoptyrygians, which include the ancestors of all amphibians. Terrestrial 
temnospondyls achieved their greatest diversity in the upper Carboniferous and 
lower Permian, during which time they were the most common land vertebrates. 
The last of the large aquatic temnospondyls occur in the Lower Jurassic of 
Australia.  See  AMPHIBIA; ANTHRACOSAURIA; CROSSOPTERYGII; LABYRINTHODONTIA. 
 �34s[Robert L. Carroll]
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�6v�2v�32sTemperature   �21m �33s 
    A concept related to the flow of heat from one object or region of space 
to another. The term refers not only to the senses of hot and cold but to 
numerical scales and thermometers as well. Fundamental to the concept of 
temperature are the absolute scale and absolute zero and the relation of 
absolute temperatures to atomic and molecular motions. 
    Thermometers do not measure a special physical quantity. They measure 
length (as of a mercury column) or pressure or volume (with the gas 
thermometer at the National Bureau of Standards) or electrical voltage (with a 
thermocouple). The basic fact is that, if a mercury column has the same length 
when touching two different, separated objects, when the objects are placed in 
contact no heat will flow from one to the other.  See  THERMOMETER. 
    The numbers on the thermometer scales are merely historical choices; they 
are not scientifically fundamental. The most widely used scales are the 
Fahrenheit (DGF) and the Celsius (DGC). The Centigrade scale with 0DG assigned 
to ice water (ice point) and 100DG assigned to water boiling under one 
atmosphere pressure (steam point) was formerly used, but it has been succeeded 
by the Celsius scale, defined in a different way than the centigrade scale. 
However, on the Celsius scale the temperatures of the ice and steam points 
differ by only a few hundredths of a degree from 0DG and 100DG, respectively. 
The illustration shows how the Celsius and Fahrenheit scales compare and how 
they fit onto the absolute scales.  See  ICE POINT. 
    In 1848 William Thompson (Lord Kelvin), following ideas of Sadi Carnot, 
stated the concept of an absolute scale of temperature in terms of measuring 
amounts of heat flowing between objects. Most important, Kelvin conceived of a 
body which would not give up any heat and which was at an absolute zero of 
temperature. Experiments have shown that absolute zero corresponds to 
-273.15DGC or -459.7DGF. Two absolute scales, shown in the illustration, are 
the Kelvin (K) and the Rankine (DGR). 
    In practice, absolute temperatures are measured by using low-density 
helium gas and dilute paramagnetic crystals, the most nearly ideal of real 
materials. The measurement of a single temperature with a gas or magnetic 
thermometer is a major scientific event done at a national standards 
laboratory. Only a few temperatures have been measured, including the freezing 
point of gold (1337.91 K), and the boiling points of sulfur (717.85 K), oxygen 
(90.18 K), and helium (4.22 K). Various types of thermometers (platinum, 
carbon, and doped germanium resistors; thermocouples) are calibrated at these 
temperatures and used to measure intermediate temperatures.  See  TEMPERATURE 
MEASUREMENT; THERMODYNAMIC PRINCIPLES. 
 �34s[Roland A. Hultsch]
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�6v�2v�32sTemperature-humidity index �21m �33s
    An index developed by the U.S. Weather Bureau to give a single numerical
value, in the general range of 70 - 80, which would reflect the outdoor
atmospheric conditions of temperature and humidity as a measure of comfort (or
discomfort) during the warm season of the year. The temperature-humidity index
was formerly called the discomfort index. Temperature-humidity index IsubTH is
defined by the equation below.

       I(sub TH) = 0.4 (dry-bulb temp DGF F + wet-bulb temp DGF F) + 15

    When the index is 70 practically all people feel comfortable; when it is
80 no one is comfortable; and when it is 75 about half the population is
satisfied. The index has been useful in the prediction and allocation of
power-system loads resulting from the impact of air-conditioning equipment
operation.  See  DEGREE-DAY; PSYCHROMETRICS; SENSIBLE TEMPERATURE.
 �34s[Theodore Baumeister]
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�6v�2v�32sTemperature inversion   �21m �33s 
    The increase of air temperature with height; an atmospheric layer in which 
the upper portion is warmer than the lower. Such an increase is opposite, or 
inverse, to the usual decrease of temperature with height, or lapse rate, in 
the troposphere. However, above the tropopause, temperature increases with 
height throughout the stratosphere, decreases in the mesosphere, and increases 
again in the thermosphere. Thus inversion conditions prevail throughout much 
of the atmosphere much or all of the time, and are not unusual or abnormal.  
See  AIR TEMPERATURE; ATMOSPHERE. 
    Inversions are created by radiative cooling of a lower layer, by 
subsidence heating of an upper layer, or by advection of warm air over cooler 
air or of cool air under warmer air. Outgoing radiation, especially at night, 
cools the Earth's surface, which in turn cools the lowermost air layers, 
creating a nocturnal surface inversion a few inches to several hundred feet 
thick.  
    Inversions effectively suppress vertical air movement, so that smokes and 
other atmospheric contaminants cannot rise out of the lower layer of air. 
California smog is trapped under an extensive subsidence inversion; surface 
radiation inversions, intensified by warm air advection aloft, can create 
serious pollution problems in valleys throughout the world; radiation and 
subsidence inversions, when horizontal air motion is sluggish, create 
widespread pollution potential, especially in autumn over North America and 
Europe.  See  AIR POLLUTION; SMOG. 
 �34s[Arnold Court]
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�6v�2v�32sTemperature measurement �21m �33s
    Measurement of the hotness or coldness of a body relative to a standard
scale. The fundamental scale of temperature is the thermodynamic scale, which
can be derived from any equation expressing the second law of thermodynamics.
Efforts to approximate the thermodynamic scale as closely as possible depend
on relating measurements of temperature-dependent physical properties of
systems to thermodynamic relations expressed by statistical thermodynamic
equations, thus in general linking temperature to the average kinetic energy
of the measured system. Temperature-measuring devices, thermometers, are
systems with properties that change with temperature in a simple, predictable,
reproducible manner.  See  TEMPERATURE; THERMODYNAMIC PRINCIPLES.
    In the establishment of a useful standard scale, assigned temperature
values of thermodynamic equilibrium fixed points are agreed upon by an
international body (General Conference of Weights and Measures), which updates
the scale about once every 20 years. Thermometers for interpolating between
fixed points and methods for realizing the fixed points are prescribed,
providing a scheme for calibrating thermometers used in science and industry.
    The scale now in use is the International Practical Temperature Scale of
1968 (IPTS-68). Its unit is the kelvin, K, arbitrarily defined as 1/273.16 of
the thermodynamic temperature T of the triple point of water (where liquid,
solid, and vapor coexist). For temperatures above the water triple point, it
is common to use the Celsius scale, with symbol DG C, having the same interval
as the thermodynamic scale, but with temperatures t defined as T-T0, where T0
= 273.15 K, the ice point, where liquid and solid water coexist at a pressure
of 1 atm (101.325 kPa). The IPTS-68 is defined by 11 fixed points, with
assigned temperatures between 13.81 K and 1337.58 K, the freezing point of
gold. For temperatures above the gold point, the Planck radiation formula is
used to determine T 68 relative to 1337.58 K. Below 13.81 K, no official scale
exists; but between 0.2 and 5.2 K, scales based upon the vapor pressure of the
helium isotopes are in common use.  See  LOW-TEMPERATURE THERMOMETRY; TRIPLE
POINT.
    Primary thermometers are devices which relate the thermodynamic
temperature to statistical mechanical formulation. The fixed points if IPTS-68
are all based on one or more types of gas thermometry. Secondary thermometers
are used as reference standards in the laboratory because primary thermometers
are often too cumbersome. It is necessary to establish standard secondary
thermometers referenced to one or more fixed points for interpolation between
fixed points. Lower-order thermometers are used for most practical purposes
and, when high accuracy is required, can usually be calibrated against
reference standards maintained at laboratories, such as the U.S. National
Bureau of Standards, or against portable reference devices (sealed boiling or
melting point cells).  See  GAS THERMOMETRY; PYROMETER; THERMISTOR;
THERMOCOUPLE; THERMOMETER.
 �34s[William E. Keller]
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�6v�2v�32sTemperature senses   �21m �33s 
    The system of sensitivity of the skin and internal organs of the body 
making possible feelings of warmth and cold. It is not known with certainty 
whether there are two discrete temperature senses, one taking care of warmth, 
the other cold, or but a single mechanism yielding warmth under some 
conditions of stimulation and cold under others. 
    The nervous mechanism responsible for sensations of cold and warmth is 
unknown. The classical theory is that cold is mediated by Krause's end-bulbs 
and warmth by Ruffini cylinders. This presumed correlation is based on (1) the 
shorter reaction time to cold and more superficial location of the Krause 
ending, and (2) good agreement between sensitivity and end-bulb distribution 
in the sclera of the eye. A competing, and more modern, theory is the 
neurovascular theory of temperature sensitivity, according to which both the 
cold and warmth patterns are handled by nerve endings in the smooth muscle 
walls of the tiny arterioles of the skin. Cold is a consequence of contraction 
of blood vessels, warmth the result of their dilation. Thermal pain, occurring 
both at high (52DGC or 126DGF) and low (3DGC or 37DGF) temperatures, comes 
from spastic contraction of the same muscle elements.  See  CUTANEOUS 
SENSATION.  
 �34s[Frank A. Geldard]
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�6v�2v�32sTempering   �21m �33s 
    The reheating of previously quenched alloy to a predetermined temperature 
below the critical range, holding the alloy for a specified time at that 
temperature, and then cooling it at a controlled rate, usually by immediate 
rapid quenching, to room temperature. The term is broadly applied to any 
process that toughens a material. 
    In alloys, if the composition is such that cooling produces a 
supersaturated solid solution, the resulting material is brittle. Heating the 
alloy to a temperature only high enough to allow the excess solute to 
precipitate out and then rapidly cooling the saturated solution fast enough to 
prevent further precipitation or grain growth result in a microstructure 
combining hardness and toughness. 
    With steel, the tempering must be carried out by slow heating to avoid 
steep temperature gradients, stress relief being one of the objectives. 
Properties produced by tempering depend on the temperature to which the steel 
is raised and on its alloy composition. For example, if hardness is to be 
retained, molybdenum or tungsten is used in the alloy.  See  HEAT TREATMENT 
(METALLURGY).  
 �34s[Frank H. Rockett]
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�6v�2v�32sTendon   �21m �33s 
    A white, glistening, fibrous cord which joins a muscle to some movable 
structure such as a bone or cartilage. Tendons permit concentration of muscle 
force into a small area, allow muscles to act at a distance, and in some cases 
change the direction of pull, allowing the muscle to act around an 
articulation. Flattened, sheetlike tendons found in broad muscles are called 
aponeuroses. Tendons consist almost entirely of white fibrous connective 
tissue formed by closely packed, parallel bundles of collagen fibrils. 
Although highly noncompliant, tendons are remarkable for their pliability and 
strength.  See  CONNECTIVE TISSUE. 
    Surrounding the tendon is the tendon sheath formed by a dense, fibrous 
connective tissue layer. This cavity contains a small amount of fluid, similar 
to that in joint cavities, but occasionally this space is filled with a more 
viscid, mucous material. The sheath and contained fluid facilitate movement of 
the tendon by reducing friction. Moreover, the sheath is normally the means by 
which the tendon is held in place.  See  BURSA; MUSCULAR SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sTenrec   �21m �33s 
    An insectivorous mammal indigenous to Madagascar. There are 30 species in 
10 genera. These animals are nocturnally active and feed on insects, worms, 
and mollusks. All tenrecs are essentially primitive unspecialized mammals, 
with poor vision. The digits are clawed, and the first digit is not opposable 
to the others. The body of the tenrec is covered with a mixture of hair, 
spines, and bristles; the tail is rudimentary; and the toes may be separate or 
webbed, depending on the species. The female is prolific, and has litters of 
12 - 20 young.  See  INSECTIVORA; MAMMALIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sTensor   �21m �33s 
    A set of components that are functions of a point in every coordinate 
system under consideration. Tensors can be transformed linearly and 
homogeneously from one coordinate system to another.  See  CALCULUS OF TENSORS. 
 �34s[Marvin Yelles]
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�6v�2v�32sTeratology   �21m �33s 
    The study of the deviations from normal growth and development, resulting 
in abnormal individuals. The abnormality may be either biochemical or anatomic. 
    Factors responsible for the development of anomalies may be separated into 
genetic and environmental. Geneticists have studied many conditions in 
laboratory animals which are caused by the action of mutant genes. The effects 
may be noted early or late and may be profound (lethal) or trivial. There are 
also a number of human anomalies, such as polydactyly, syndactyly, 
achondroplasia, cleft palate, microphthalmia, and spina bifida, which have a 
known genetic basis.  See  HUMAN GENETICS. 
    Environmental factors known to produce anomalies in laboratory animals 
include heavy metals, alcohol, excess or deficiency of hormones, oxygen lack, 
nutritional conditions including vitamin excess or deficiency, metabolic 
inhibitors, virtually any chemical, and irradiations of various types. Virus 
infections may act directly on a fetus or indirectly through alteration of 
maternal metabolites. The array of agents is so vast that it may be safe to 
say that virtually any substance or physical agent may produce anomalies when 
administered in a particular way to a particular embryo. The nature of the 
anomaly may also vary greatly, and it depends on the agent used, stage of 
development of the organism when it is first exposed to the agent, the dose 
and duration of the insult, and the genetic makeup of the exposed individual. 
The physiological condition of the individual, either germ cell or embryo, at 
the time of exposure may be important.  See  CONGENITAL ANOMALIES; 
DEVELOPMENTAL BIOLOGY. 
 �34s[N. Karle Mottet]
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�6v�2v�32sTerbium   �21m �33s 
    Element number 65, terbium, Tb, is a very rare metallic element of the 
rare-earth group. Its atomic weight is 158.924, and the stable isotope 159Tb 
makes up 100% of the naturally occurring element. 
    The common oxide, Tb4O7, is brown and is obtained when its salts are 
ignited in air. Its salts are all trivalent and white in color and, when 
dissolved, give colorless solutions. The higher oxides slowly decompose when 
treated with dilute acid to give the trivalent ions in solution. Although the 
metal is attacked readily at high temperatures by air, the attack is extremely 
slow at room temperatures. The metal has a Neel point at about 229 K and a 
Curie point at about 220 K. For properties of the metal  see  RARE-EARTH 
ELEMENTS.  
 �34s[Frank H. Spedding]
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�6v�2v�32sTerebratulida   �21m �33s 
    An order of articulate brachiopods whose first representatives occur in 
beds of Early Devonian age and which is still extant.  See  ARTICULATA 
(BRACHIOPODA).  
    The group invariably has a punctate shell structure and is characterized 
by the possession of a loop extending anteriorly from the crural bases, the 
loop affording some degree of support for the lophophore. The posterior margin 
is commonly curved, the ventral interarea reduced, and the dorsal interarea 
absent.  See  BRACHIOPODA. 
 �34s[A. J. Rowell]
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�6v�2v�32sTermite   �21m �33s 
    An insect, also known as the white ant, of the order Isoptera; there are 
about 1600 species. Termites are soft-bodied and medium-sized. They live in 
colonies with a caste system comprising three types of functional individuals: 
sterile workers and soldiers, and the reproductives. Workers are white and 
wingless and have chewing mouthparts and well-developed legs; but the eyes are 
very small or lacking. Soldiers resemble the workers except for the head, 
which is quite large and has massive mandibles. There are two types of 
reproductive individuals, one wingless or having vestigial wings and the other 
winged.  See  SOCIAL INSECTS. 
    Termites feed on cellulose; however, they do not elaborate an enzyme to 
digest this material. Instead, flagellate protozoan symbionts in the gut of 
the termites digest the cellulose or at least initiate its breakdown. 
    Economically, termites are both destructive and beneficial. They serve as 
reducer organisms in their destruction of dead trees and other vegetation, 
replenishing the soil with materials necessary for plant growth. On the other 
hand, termites destroy books, structural materials, fabrics, and other 
substances.  See  ISOPTERA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sTerpene �21m �33s
    A class of natural products having a structural relationship to isoprene,
as shown below. Over 5000 structurally determined terpenes are known; many of
these have also been synthesized in the laboratory. Historically terpenes have
been isolated from green plants, but new compounds structurally related to
isoprene continue to be isolated from other sources as well, so the class is
also referred to as terpenoids, reflecting the biochemical origin without
specification of the natural source.  See  ISOPRENE.
    Terpenes are classified according to the number of isoprene units of which
they are composed, as follows:

	    5	   hemi-	     25      ses-
	   10	   mono-	     30      tri-
	   15	   sesqui-	     40      tetra-
	   20	   di-		     (5)subn poly-

    Although they may be named according to the systematic nomenclature and
numbering systems set by the International Union of Pure and Applied Chemistry
for all organic compounds, it is often easier to refer to terpenes by their
common names, which usually reflect the botanical or zoological name of their
source.
 �34s[Tomas Hudlicky]
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�6v�2v�32sTerracing (agriculture)   �21m �33s 
    A method of shaping land to control erosion on slopes of rolling land used 
for cropping and other purposes. In early practice the land was shaped into a 
series of nearly level benches or steplike formations. Modern practice in 
terracing, however, consists of the construction of low-graded channels or 
levees to carry the excess rainfall from the land at nonerosive velocities. 
The physical principle involved is that, when water is spread in a shallow 
stream, its flow is retarded by the roughness of the bottom of the channel and 
its carrying, or erosive, power is reduced. Since direct impact of rainfall on 
bare land churns up the soil and the stirring effect keeps it in suspension in 
overland flow and rills, terracing does not prevent sheet erosion. It serves 
only to prevent destruction of agricultural land by gullying and must be 
supplemented by other erosion-control practices, such as grass rotation, cover 
crops, mulching, contour farming, strip cropping, and increased organic matter 
content.  See  EROSION; SOIL CONSERVATION. 
    The two major types of terraces are the bench and the broadbase (see 
illustration). The bench terrace is essentially a steep-land terrace and 
consists of an almost vertical retaining wall, called a riser, or a steep 
vegetative slope to hold the nearly level surface of the soil for cultivation, 
orchards, vineyards, or landscaping. The broadbase terrace has the 
distinguishing characteristic of farmability; that is, crops can be grown on 
this terrace and worked with modern-day machinery. These terraces are 
constructed either to remove or retain water and, based on their primary 
function, are classified either as graded or level.  See  LAND DRAINAGE 
(AGRICULTURE).  
 �34s[Charles B. Ogburn]
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�6v�2v�32sTerra-cotta   �21m �33s 
    A clay product fired or burned to fuse into a hard unified mass. Being 
fire- and frost-resistant, terra-cotta is used unglazed for roofing and other 
exterior finishing. Compacted to a high density after firing, terra-cotta is 
used structurally. For example, it is used in archways and colonnades. It can 
also be molded into various shapes having figured surfaces and used 
decoratively. Handcrafted terra-cotta is used in small sculpture; the finished 
product has the hardness and durability of stone. 
 �34s[Frank H. Rockett]
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�6v�2v�32sTerrain-aided navigation   �21m �33s 
    The auxiliary-sensor data, obtained by observing specific features of the 
Earth's terrain, to aid or update an inertial navigation system and thereby 
improve its navigation accuracy. A position fix is derived by comparing the 
observed terrain data with an on-board terrain reference map. The position 
within the reference map where the observation best matches the reference data 
is taken to be the position of the navigator over the Earth's surface. For 
proper operation, the terrain feature being observed must be unique so that 
position can be determined unambiguously from the reference map. 
    Many different sensor types  can be used to observe terrain features: a 
radar altimeter (in conjunction with an altitude reference) to observe terrain 
elevation beneath the aircraft; a radiometric sensor to observe the natural 
electromagnetic radiation of the Earth in a certain frequency band; an optical 
sensor to observe a visual image; a radar transmitter and receiver to observe 
terrain reflectivity.  See  ALTIMETER; REMOTE SENSING. 
 �34s[Ronald D. Andreas]
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�6v�2v�32sTerrain areas   �21m �33s 
    Subdivisions of the continental surfaces distinguished from one another on 
the basis of the form, roughness, and surface composition of the land. The 
pattern of landform differences is strongly reflected in the arrangement of 
such other features of the natural environment as climate, soils, and 
vegetation. These regional associations must be carefully considered in 
planning of activities as diverse as agriculture, transportation, city 
development, and military operations. 
    Eight classes of terrain are distinguished on the basis of steepness of 
slopes, local relief (the maximum local differences in elevation), 
cross-sectional form of valleys and divides, and nature of the surface 
material. Approximate definitions of terms used and percentage figures 
indicating the fraction of the world's land area occupied by each class are as 
follows; (1) flat plains: nearly level land, slight relief, 4%; (2) rolling 
and irregular plains: mostly gently sloping, low relief, 30%; (3) tablelands: 
upland plains broken at intervals by deep valleys or escarpments, moderate to 
high relief, 5%; (4) plains with hills or mountains: plains surmounted at 
intervals by hills or mountains of limited extent, 15%; (5) hills: mostly 
moderate to steeply sloping land of low to moderate relief, 8%; (6) low 
mountains: mostly steeply sloping, high relief, 14%; (7) high mountains: 
mostly steeply sloping, very high relief, 13%; and (8) ice caps: surface 
material, glacier ice, 11%. 
 �34s[Edwin H. Hammond]
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�6v�2v�32sTerrestrial coordinate systems   �21m �33s 
    Methods for establishing relative position on the Earth by utilizing two 
sets of coordinates perpendicular to each other, called latitude and 
longitude. The orientation of the coordinates also establishes Earth 
directions. The fact of Earth rotation establishes the geographic poles, which 
serve as reference points upon which the coordinate system is based. 
    Position within the coordinate system, or graticule, is determined by 
direct astronomic observation (or by derivatives of it) and is stated in terms 
of arc distance from reference positions. Position between the poles is 
designated as latitude and corresponds to a Y value on a cartesian system. 
Position corresponding to an X value on a cartesian system is designated as 
longitude.  
  
    Latitude.  Arc distance of latitude toward the poles is reckoned from a 
great circle on the Earth which is everywhere midway between the poles. This 
line, designated as the Equator, is therefore everywhere 90DG arc distance 
from the poles. Any other line a given lesser distance from a pole will be a 
small circle parallel to the Equator. Direction along such a line is 
designated as east-west. All such lines on the Earth are termed parallels 
(Fig. 1). 
    Because the Earth is not a true sphere but an oblate spheroid of 
revolution, the quadrant from the Equator to a pole is not a true arc but 
curves less rapidly near the pole. Consequently, degrees of latitude are 
slightly shorter near the Equator (68.7 mi or 110 km) and slightly longer near 
the poles (69.4 mi or 112 km). 
    Latitude is numbered from 0DG at the Equator to 90DG at the poles. 
Distance toward the North or South poles is designated as north or south 
latitude, respectively. 
  
    Longitude.  Arc distance of longitude is measured along a parallel from 
some starting point (Fig. 2). Each parallel circle on the Earth contains 
360DG, and since the Earth rotates on an axis which is perpendicular to the 
planes of the parallel circles, each parallel therefore rotates 360DG in 1 
day, or 15DG in 1 h. The difference in sun time between two points may be 
expressed as an angular difference in longitude. 
    A great circle from pole to pole has everywhere along it the same sun 
time. Such a line is called a meridian. Direction along such a line is 
designated as north-south. All meridians are alike and, unlike latitude, there 
is no natural starting point from which to begin their numbering. The meridian 
passing through the Royal Astronomical Observatory in Greenwich, near London, 
is internationally agreed upon as the 0DG meridian, or the prime meridian. 
Longitude is measured and designated as east or west longitude from the prime 
meridian to 180DG, the meridian opposite 0DG longitude. The Earth's coordinate 
system, therefore, ``originates'' with coordinates numbered zero intersecting 
in the Gulf of Guinea. 
    A degree of longitude varies in length with latitude. The length of a 
degree of longitude varies as the cosine of the latitude. Near the Equator, 
one degree of longitude is nearly the same length as a degree of latitude. 
 �34s[Arthur H. Robinson]
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�6v�2v�32sTerrestrial ecosystem   �21m �33s 
    A term that encompasses ecosystems that exist on the continents and 
islands of the world and comprehends a series of dynamic open interaction 
systems that include living forms (animals, plants, and microorganisms) and 
their nonliving environment (soils, geological formations, and atmospheric 
constituents) and the activities, interrelations, chemical reactions, physical 
changes, and all other phenomena of each. Energy that enters these systems, 
chiefly in the form of sunlight, circulates through the systems and powers the 
life processes of the organisms, influences the rate and nature of chemical 
reactions and physical changes, and is partially accumulated in the bodies of 
organisms and in other chemical and physical states.  See  ECOSYSTEM; 
FRESH-WATER ECOSYSTEM; MARINE ECOSYSTEM. 
    Terrestrial ecosystems differ from aquatic ecosystems in several important 
respects. The most obvious difference in the abiotic components of the systems 
is the basic physical contrast between the media. Terrestrial organisms are 
surrounded by air, a mixture composed of gaseous elements and compounds. Water 
vapor is present in the atmosphere but forms only a small portion of the total 
volume of air. Water is often in critically short supply, extremes and rapid 
changes of temperature are common, mineral elements are limited in occurrence 
to the substrate (generally in relatively high concentrations), and light is 
intense, at least at upper levels of the vegetation. 
    The terrestrial and aquatic ecosystems are connected in a great many 
direct ways, so that it is most logical to consider the entire Earth and its 
envelope of gases to be the ultimate ecosystem. However, this world ecosystem 
is an open system and is dependent upon an external supply of energy, chiefly 
from the Sun, and is influenced by a variety of forces of external origin.  
See  ECOLOGY. 
 �34s[Jack S. McCormick]
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�6v�2v�32sTerrestrial frozen water   �21m �33s 
    Seasonally or perennially frozen waters (ice) of the Earth, exclusive of 
the atmosphere. Terrestrial ice occurs in the form of temporary seasonal 
accretions during the cold months and in the perennial ice cover represented 
by glaciers, land-fast ice (ice shelves), and subsurface ground ice in 
permafrost regions.  See  GLACIOLOGY; GROUNDWATER HYDROLOGY; HYDROLOGY; 
PERMAFROST.  
 �34s[Maynard M. Miller]
�33s 
  
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�6v�2v�32sTerrestrial radiation   �21m �33s 
    Electromagnetic radiation originating from the Earth and its atmosphere at 
wavelengths determined by their temperature. It is sometimes called thermal 
radiation. The units are energy per unit area per unit time, such as calories 
per square centimeter per second, or watts per square meter. The atmosphere 
emits, absorbs, and transmits radiation, and the net flux of radiation at any 
point depends upon the distribution with height of temperature and water 
vapor. Heating and cooling due to the vertical divergence of terrestrial 
radiation provide a major part of the potential energy changes necessary to 
drive the atmospheric wind system. Terrestrial radiation is also responsible 
for maintaining the air temperature near the ground within limits necessary 
for comfortable living.  See  GREENHOUSE EFFECT; HEAT BALANCE, TERRESTRIAL 
ATMOSPHERIC.  
 �34s[Lewis D. Kaplan]
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�6v�2v�32sTerrestrial water   �21m �33s 
    The total inventory of water on the Earth. Water is unevenly distributed 
over the Earth's surface in oceans, rivers, and lakes. In addition, the 
world's water is distributed throughout the atmosphere and also occurs as soil 
moisture, groundwater, ice caps, and glaciers.  See  ATMOSPHERE; GLACIOLOGY; 
GROUNDWATER HYDROLOGY; HYDROLOGY; LAKE; SURFACE WATER; TERRESTRIAL FROZEN 
WATER.  
 �34s[Raymond L. Nace]
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�6v�2v�32sTerritoriality   �21m �33s 
    A pattern of behavior in which one or more animals occupy and defend a 
definite area or territory. They obtain their food from this area and exclude 
others of their species from it. Territorial behavior is highly developed 
among many kinds of birds but appears also, in somewhat different forms, among 
mammals, reptiles, fish, and social insects. 
    In many mammals and reptiles, and some other animals, individuals, pairs, 
or social groups move over a fairly definite area in seeking food or when 
engaged in other activities. Such an area is termed a home range but not a 
territory unless it is defended against others of the species.  See  SOCIAL 
ANIMALS.  
 �34s[Robert H. Whittaker]
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�6v�2v�32sTertiary   �21m �33s 
    The older major subdivision (period) of the Cenozoic Era, extending from 
the end of the Cretaceous to the beginning of the Quaternary. The term 
Tertiary is also applied to all rocks formed during this period and to all the 
fossils they contain. Tertiary sedimentary rocks include widespread 
limestones, sandstones, marls, mudstones, and conglomerates; igneous rocks 
include extrusive and intrusive volcanics and locally some rock of more 
deep-seated (plutonic) origin. Tertiary life was characterized particularly by 
(1) a diversification and multiplication of stocks such as pelecypods and 
gastropods of sea urchins and sand dollars, of microscopic foraminiferans, of 
some other marine shellfish, and (2) a great development of shrubs, grasses, 
anf other flowering plants; and (3) expansion on both land and sea of birds 
and mammals, the latter having provided for the Tertiary the more informal 
designation Age of Mammals. 
    Configuration of the modern continental land masses developed during 
Tertiary and Quaternary times. Great mountain-making movements (orogenies) and 
uplifts of large segments of the Earth's crust (epeirogenies) alternated with 
fluctuating transgressions and reexpansions of the seas over land areas. 
Orogenies and uplifts nevertheless grew progressively greater at the expense 
of the earlier seas as Tertiary time wore on and culminated in the great Late 
Alpine orogenic uplifts of Pleistocene time.  See  CENOZOIC; CRETACEOUS; 
PLEISTOCENE.  
 �34s[Robert M. Kleinpell]
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�6v�2v�32sTestis   �21m �33s 
    The organ of sperm production. In addition, the testis (testicle) is an 
organ of endocrine secretion in which male hormone is elaborated. In mammals, 
the testes are usually ovoid or round. In many species (for example, humans) 
they are suspended in a pouch (scrotum) outside the main body cavity; in other 
species they are found in such a pouch only at the reproductive season; in 
still others the testicles are permanently located in the abdomen (for 
example, in whales and bats). 
    Within a firm and thick capsule of connective tissue, the testis contains 
a varying number of thin but very long seminiferous tubules which are the 
sites of sperm formation. These tubules open into a network of fine, slitlike 
canals, the rete testis. From this the sperm drains through a few, narrow 
ducts, ductuli efferentes, into the epididymis, the storage chamber for sperm. 
 �34s[Edward C. Roosen-Runge]
�33s 
    The functions of the testis are dependent on the secretion of gonadotropic 
hormones, the release of which from the pituitary gland is in turn regulated 
by the central nervous system. The principal androgenic hormone released by 
the testis into the bloodstream is testosterone. Testosterone synthesis is 
normally limited by the rate of pituitary gonadotrophin secretion: 
Administration of ICSH (also known as luteinizing hormone or LH) or of 
chorionic gonadotrophin results in increased testosterone synthesis and 
release within minutes. 
    At the ambisexual stage of embryonic development, the testis promotes the 
growth of the paired Wolffian ducts and their differentiation into the 
epididymis, vasa deferentia, and seminal vesicles; the fetal testis also 
causes masculinization of the urogenital sinus, fusion of the labioscrotal 
folds in the midline, and development of the genital tubercle into a phallus. 
    Testosterone promotes the retention of dietary nitrogen and its 
incorporation into muscle protein. This effect is responsible for the greater 
muscular development of the male and contributes to the prepubertal growth 
spurt of boys. Toward puberty, increased secretion of testosterone stimulates 
the growth of the penis, scrotum, and male accessory glands responsible for 
the formation of the seminal plasma, for example, the prostate and seminal 
vesicles. The hormone brings about the appearance of secondary sex characters, 
such as the male-type distribution of hair and body fat and lowered pitch of 
voice in men.  Testosterone is probably essential for efficient 
spermatogenesis, but high doses of testosterone suppress spermatogenesis by 
inhibiting pituitary gonadotrophin secretion.  See  REPRODUCTIVE SYSTEM; SPERM 
CELL; SPERMATOGENESIS. 
 �34s[Hans R. Lindner]
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�6v�2v�32sTetanus   �21m �33s 
    An infectious disease, also known as lockjaw, which is caused by the toxin 
of Clostridium tetani. The bacterium may be isolated from fertile soil and the 
intestinal tract or fecal material of humans and other animals. Infection 
commonly follows dirt contamination of deep wounds or other injured tissues.  
See  TOXIN. 
    The incubation period of tetanus is usually 5 - 10 days, and the disease 
is characterized by convulsive tonic contraction of voluntary muscles. 
Prevention of tetanus rests on the proper, prompt surgical care of 
contaminated wounds and prophylactic use of antitoxin if the individual has 
not been protected by active immunization with toxoid.  See  IMMUNOLOGY. 
 �34s[Leland S. McClung]
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�6v�2v�32sTethered balloon   �21m �33s 
    A balloon that is attached to a mooring line and allowed to ascend by 
releasing a winch mechanism. The tethered balloon is generally configured as 
an aerodynamic body to provide lift, achieve lower drag, and increase lateral 
stability. Although 6 km (4 mi) is generally considered the upper altitude for 
this concept, the National Center for Space Studies in France has reported 
flying a tethered balloon carrying a 350-kg (770 lb) gondola to an altitude of 
17 km (10.6 mi). for 12 h. 
    The tethered balloon has primarily been used for military purposes to 
provide acoustic, radar, and optical surveillance, tall antennas, and 
communication relays. 
 �34s[Walter R. Nagel]
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�6v�2v�32sTetrabranchia   �21m �33s 
    A subclass of the Cephalopoda, containing the Nautiloidea and the 
Ammonoidea. The most numerous group of the cephalopods in the Paleozoic and 
Mesozoic periods, it is considered to be the most primitive group of 
cephalopods. The name Tetrabranchia was given to this group because four gills 
are found in Nautilus, the only representative whose anatomy is known. Since 
no records of the soft parts are preserved in fossil specimens, it is not 
known if this feature is shared by all of the many genera assigned to the 
subclass.  See  AMMONOIDEA; NAUTILOIDEA. 
 �34s[Gilbert L. Voss]
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�6v�2v�32sTetractinomorpha   �21m �33s 
    A subclass of Demospongiae; the skeletonless genus Oscarella has a 
primitive structure. Cleavage results in a solid mass of cells (morula) which 
later becomes hollow by cytolysis of the interior cells, rich in food 
reserves. Upon being freed from the parent, the hollow larva is made up of a 
single layer of flagellated cells and is known as an amphiblastula. Following 
metamorphosis, the larva of Oscarella assumes the leuconoid grade of 
construction from the start, with isolated flagellated chambers communicating 
with inhalant and exhalant canals. The adult Oscarella and a related genus, 
Plakina, and two-, three-, and four-rayed spicules, retain the simple 
leuconoid structure. In form, the species may be thinly encrusting or massive, 
but often they have spherical or ovoid shapes. Branching species rarely occur. 
The skeletal system consists of spicules (sclerites), spongin fibers, or both. 
See DEMOSPONGIAE; PORIFERA. 
 �34s[Willard D. Hartman]
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�6v�2v�32sTetracycline   �21m �33s 
    A broad-spectrum antibiotic. It is one of a group of antibiotics known as 
tetracyclines; other well-known members of the group are chlortetracycline 
(aureomycin) and oxytetracycline (terramycin). Tetracycline is produced 
biosynthetically by a strain of Streptomyces aureofaciens (or certain other 
species) or chemically by hydrogenolysis of chlortetracycline. Tetracycline is 
particularly useful because of broad antimicrobial action, with low toxicity, 
in the therapy of infections caused by gram-positive and gram-negative 
bacteria as well as rickettsiae and the large viruses such as 
psittacosis-lymphogranuloma viruses.  See  ANTIBIOTIC. 
 �34s[Ralph E. Bennett]
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�6v�2v�32sTetraethyllead   �21m �33s 
    An organometallic compound that has found wide commercial application as 
an additive for motor fuels. Small quantities of tetraethyllead markedly 
reduce the knocking tendencies of gasoline and thus permit use of higher 
engine compression ratios.  
    The combustion of gasoline containing tetraethyllead produces deposits of 
lead and lead oxide along the cylinder walls. Hence, ethylene dibromide and 
ethylene dichloride are also added as part of the antiknock fluid to remove 
the lead as lead halide in the exhaust gases. Automobiles equipped with 
catalytic converters  designed to meet antipollution standards must burn 
``unleaded'' gasoline, since lead deposits tend to poison the platinum and 
palladium oxidation catalysts used in such systems.  See  ANTIKNOCK AGENTS; 
GASOLINE; OCTANE NUMBER; ORGANOMETALLIC COMPOUND. 
 �34s[Marvin D. Rausch]
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�6v�2v�32sTetrahedrite �21m �33s
    A mineral with composition (Cu, Fe, Zn, Ag)12Sb4S13. It is massive or
granular. Its hardness is 3 1/2 - 4 and the specific gravity varies from 4.6 to
5.1, depending on the composition. The luster is metallic and the color
grayish black; thus, in some mining localities, this mineral is called gray
copper.
    Tetrahedrite is a widely distributed mineral, usually found in silver and
copper veins. In some places it has sufficient silver to be a valuable ore, as
at Freiberg, Germany, and silver mines of Peru, Bolivia, and Mexico. It is
found in silver and copper mines in the western United States.
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sTetrahedron   �21m �33s 
    A solid bounded by four planes, or faces (see illustration). It has four 
vertices (not coplanar) and six edges, the six line segments that join each 
pair of vertices. As the three-dimensional analog of a triangle, many of its 
properties are extensions of those of a triangle. Thus, as in the case of a 
triangle, the medians of a tetrahedron (that is, the lines joining the 
vertices with the centers of gravity of the opposite faces) are concurrent. On 
the other hand, the altitudes of a tetrahedron are not always concurrent.  See 
  POLYHEDRON. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sTetraodontiformes   �21m �33s 
    An order of specialized teleost fishes, also called Plectognathi, that 
includes the triggerfishes (see illustration), puffers, trunkfishes, ocean 
sunfishes, and their allies. It is a group of diverse structure. The body is 
variably armored with bony plates or spines, encased in bone, prickly, thorny, 
or naked. Fin spines and some fins are variably well developed or wanting. 
Some species can inflate the body. 
    Tetraodontiforms may be classified conservatively into 7 families, nearly 
60 genera, and about 320 Recent species, but some specialists recognize more 
families and genera. Tetraodontiforms are largely reef and shore fishes of 
tropical or subtropical seas, but a few are pelagic, enter temperate waters, 
or ascend tropical rivers. Many are colorful inhabitants of coral reefs. Some 
are valued as food, but a few have a neurotoxic poison, tetrodotoxin, in the 
viscera that is sometimes fatal when eaten.  See  ACTINOPTERYGII. 
 �34s[Reeve M. Bailey]
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�6v�2v�32sTetraphidales   �21m �33s 
    An order of mosses which is composed of the family Tetraphidaceae, the two 
genera Tetraphis and Tetradontium, and approximately five species.  
    Although this order is unique in the scalelike protonema, which produces 
frondlike growths, the most conspicuous structure distinguishing it from other 
mosses is the peristome of four rigid, nonsegmented teeth. The plants fruit 
freely and the capsules and peristome are persistent. The plants occur on 
humus, moist decaying wood, and sandstone.  See  BRYOPSIDA. 
 �34s[Winona H. Welch]
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�6v�2v�32sTetraphyllidea �21m �33s
    An order of tapeworms of the subclass Cestoda (phylum Platyhelminthes).
All species are intestinal parasites of elasmobranch fishes and are small in
size. An outstanding feature of the order is the variation in the structure of
the holdfast organ or scolex (see illustration). All species are segmented and
segments are usually shed from the body while sexually immature; these develop
to sexual maturity as independent units in ths host's intestine. A complete
life cycle is not known.  See  CESTODA.
 �34s[Clark P. Read]
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�6v�2v�32sTetrapoda   �21m �33s 
    The superclass of the subphylum Vertebrata whose members typically possess 
limbs in contrast to the superclass, the Pisces (fishes), whose members have 
fins.  See  PISCES (ZOOLOGY). 
    The animals making up the Tetrapoda typically live part or all of their 
lives on land, whereas the members of the Pisces live in water. The classes of 
the Tetrapoda are Amphibia, Reptilia, Aves, and Mammalia. The term Tetrapoda 
comes from Greek words meaning ``four feet,'' but there are tetrapods that 
have only two limbs or none at all, such as some amphibians and reptiles. 
These forms have, however, evolved from four-footed ancestors.  See  AMPHIBIA; 
AVES; MAMMALIA; REPTILIA. 
 �34s[Richard G. Zweifel]
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�6v�2v�32sTetrasporales   �21m �33s 
    A heterogeneous and artificial assemblage of fresh-water and marine algae 
in the division Chlorophyta. These plants are colonial; the cells are embedded 
but not adjoined in a copious mucilaginous sheath. Many plants have a 
parietal, cup-shaped chloroplast, a basal pyrenoid, stationary vegetative 
cells that become flagellated and motile, false swimming organs (pseudocilia), 
and a red eyespot in vegetative and reproductive cells. Many genera use 
zoospores in asexual reproduction, and a few are known to have isogamous 
sexual reproduction.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sTeuthoidea   �21m �33s 
    An order of the molluscan subclass Coleoidea. Representative of the orders 
are the squid Loligo and the fossil Plesioteuthis. The rostrum is not 
developed, the proostracum is represented by the elongated pen or gladius, and 
10 arms are present. Teuthoids range from the Jurassic to the Recent.  See  
COLEOIDEA; DECAPODA (MOLLUSCA). 
 �34s[Charles B. Curtin]
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�6v�2v�32sTextile   �21m �33s 
    A material made mainly of natural or synthetic fibers. Modern textile 
products may be prepared from a number of combinations of fibers, yards, 
films, sheets, foams, furs, or leather. They are found in apparel, household 
and commercial furnishings, vehicles, and industrial products.  See  
MANUFACTURED FIBER; NATURAL FIBER. 
    The term fabric may be defined as a thin, flexible material made of any 
combination of cloth, fiber, or polymer (film, sheet, or foams); cloth as a 
thin, flexible material made from yarns; yarn as a continuous strand of 
fibers; and fiber as a fine, rodlike object in which the length is greater 
than 100 times the diameter. The bulk of textile products are made from cloth.  
    The natural progression from raw material to finished product requires: 
the cultivation or manufacture of fibers; the twisting of fibers into yarns 
(spinning); the interlacing (weaving) or interlooping (knitting) of yarns into 
cloth; and the finishing of cloth prior to sale. 
    The conversion of staple fiber into yarn (spinning) requires the following 
steps: picking (sorting, cleaning, and blending), carding and combing 
(separating and aligning), drawing (reblending), drafting (reblended fibers 
are drawn out into a long strand), and spinning (drafted fibers are further 
attenuated and twisted into yarn). 
    The process of weaving allows a set of yarns running in the machine 
direction (warp) to be interlaced with another set of yarns running across the 
machine (filling or weft). The weaving process involves four functions: 
shedding (raising the warp yarns by means of the appropriate harnesses); 
picking (inserting the weft yarn); battening (pushing the weft into the cloth 
with a reed); and taking up and letting off (winding the woven cloth onto the 
cloth beam and releasing more warp yarn from the warp beam; Figs. 1 and 2). 
  
    Knit cloth is produced by interlocking one or more yarns through a series 
of loops. The lengthwise columns of loops are known as the wales, and the 
crosswise rows of loops are called courses. Filling (weft) knits (Fig. 3) are 
those in which the courses are composed of continuous yarns, while in warp 
knits (Fig. 4) the wale yarns are continuous. 
    Newly constructed knit or woven fabric must pass through various finishing 
processes to make it suitable for its intended purpose. Finishing enhances the 
appearance of fabric and also adds to its serviceability. Finishes can be 
solely mechanical, solely chemical, or a combination of the two. Those 
finishes, such as scouring and bleaching, which simply prepare the fabric for 
further use are known as general finishes. Functional finishes, such as 
durable press treatments, impart special characteristics to the cloth. For 
discussions of important finishing operations  see  BLEACHING; DYEING; TEXTILE 
CHEMISTRY; TEXTILE PRINTING. 
 �34s[Ira Block]
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�6v�2v�32sTextile chemistry   �21m �33s 
    The applied science of textile materials, consisting of the application of 
the principles of the many basic fields of chemistry to the understanding of 
textile matrials and to their functional and esthetic modification into useful 
and desirable items. The study of textile chemistry begins with the knowledge 
of the textile fibers themselves. These are normally divided into three 
groups: natural, manufactured, and synthetic.  See  MANUFACTURED FIBER; 
NATURAL FIBER; TEXTILE. 
  
    Chemicals.  The enormous number of chemicals used in textile processing 
may be divided broadly into two categories: those intended to remain on the 
fiber, and those intended to wet or clean the fiber or otherwise function in 
some related operation. The former includes primarily dyes and finishes. The 
latter group consists mainly of surface-active agents, commonly known as 
surfactants.  
  
    Preparation.  Preparation is a term applied to a group of essentially wet 
chemical processes having as their object the removal of all foreign matter 
from the fabric. This results in a clean, absorbent substrate, ready for the 
subsequent coloring and finishing operations. 
    The operations constituting preparation depend primarily on the fibers 
being handled. Synthetic fibers contain little or no natural impurities, so 
that the only materials that normally must be removed are the oils and 
lubricants or water-soluble sizes needed to facilitate earlier processing. 
This is generally accomplished by washing with water and a mild detergent 
capable of emulsifying the oils and waxes. On the other hand, natural fibers 
contain relatively high amounts of natural impurities, and in addition 
frequently are sized with materials presenting difficulties in removal. In the 
case of cotton, prolonged hot treatment with alkali, usually sodium hydroxide, 
and strong detergent is necessary to break down and remove the naturally 
occurring impurities. Special scours are necessary for cleaning such materials 
as wool and silk. The protein fibers are very sensitive to alkali and strong 
detergents; they are usually washed with mild soap or sulfated alcohols. 
    After other impurities are removed from the fiber, it is usually desirable 
to remove any coloring material. This process is known as bleaching. By far 
the major bleaching agent in use is hydrogen peroxide, which is efficient in 
color removal, while still being considered relatively controllable and safe 
for use.  See  BLEACHING; HYDROGEN PEROXIDE. 
  
    Mercerization.  Mercerization is a special process applied only to cotton. 
The fabric or yarn is treated with a strong sodium hydroxide solution while 
being held under tension. This process causes chemical and physical changes 
within the fiber itself, resulting in a substantial increase in luster and 
smoothness of the fabric, plus important improvements in dye affinity, 
stabilization, tensile strength, and chemical reactivity. 
  
    Coloring.  Although many textiles reach the consumer in their natural 
color or as a bleached white, most textiles are colored in one way or another. 
Coloring may be accomplished either by dyeing or printing, and the coloring 
materials may be either dyes or pigments. 
    Dyeing essentially consists of immersing the entire fabric in the 
solution, so that the whole fabric becomes colored. On the other hand, 
printing may be considered as localized dyeing. In printing, a thickened 
solution of dyestuff or pigment is used. This thickened solution, or paste, is 
applied to specific areas of the fabric by means such as engraved rollers or 
partially porous screens. Application of steam or heat then causes the 
dyestuff to migrate from the dried paste into the interior of the fiber, but 
only in those specific areas where it has been originally applied.  See  DYE; 
DYEING; TEXTILE PRINTING. 
  
    Finishing.  Finishing includes a group of mechanical and chemical 
operations which give the fabric its ultimate feel and performance 
characteristics. Many desirable characteristics may be imparted to the fabric 
through the application of various chemical agents at this point. 
    Softeners are used to give a desirable hand or feel to the fabric. These 
chemicals are generally long fatty chains, with solubilizing groups which may 
be cationic, nonionic, or occasionally anionic in character. They are 
essentially surfactants constructed so as to contain a relatively high 
proportion of fatty material in the molecule. Conversely, certain types of 
polymeric material such as polyvinyl acetate or polymerized urea formaldehyde 
resins are used to impart a stiff or crisp hand to a fabric.  See  POLYVINYL 
RESINS; UREA-FORMALDEHYDE-TYPE RESINS. 
    It is in finishing that the so-called proof finishes are applied, 
including fire-retardant and water-repellent finishes. A fire-retardant finish 
is a chemical or mixture containing a high proportion of phosphorus, nitrogen, 
chlorine, antimony, or bromine. A truly waterproof fabric may be made by 
coating with rubber or vinyl, but water-repellent fabrics are produced by 
treating with hydrophobic materials such as waxes, silicones, or metallic 
soaps.  
    Many other types of highly specialized treatments, such as antistatic, 
antibacterial, or soil-repellent finishes, may be applied to fit the fabric to 
a particular use. 
 �34s[David H. Abrahams]
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�6v�2v�32sTextile printing   �21m �33s 
    The localized application of color on fabrics. In printing textiles, a 
thick paste of dye or pigment is applied to the fabric by appropriate 
mechanical means to form a design. The color is then fixed or transferred from 
the paste to the fiber itself, maintaining the sharpness and integrity of the 
design. In a multicolor design, each color must be applied separately and in 
proper position relative to all other colors. Printing is one of the most 
complex of all textile operations.  See  DYE. 
    A design may be applied in three major ways: raising the design in relief 
on a flat surface (block printing); cutting the design below a flat surface 
(intaglio or engraved printing); and cutting the design through a flat metal 
or paper sheet (stencil or screen printing). All three methods have been used 
for hand printing and reciprocating printing machines. In addition, these 
methods have been converted into rotary action by replacing blocks or plates 
with cylinders. Another method of printing utilizes individual 
computer-controlled nozzles for each color. The nozzles are used to paint a 
design on the fabric. 
    Each printing method requires a paste with special characteristics, 
frequently referred to as flow characteristics. The choice of thickener is 
dependent not only on the type of dye-stuff, but on the type of printing 
machine on which the printing is to be done, and frequently also on the type 
of fixation to be used. Most natural thickening agents are based on 
combinations of starch and gum. The synthetic thickening agents used are 
generally extremely high-molecular-weight polymers capable of developing a 
very high viscosity at a relatively low concentration. 
    The first step is the preparation of print paste, which is made by 
dissolving the dyestuff and combining it with a solution of the appropriate 
thickening agent. The fabric is then printed by any of the standard methods 
and then dried in order to retain a sharp printed mark. 
    The next operation, steaming, may be likened to a dyeing operation. Before 
steaming, the bulk of the dyestuff is held in a dried film of thickening 
agent. During the steaming operation, the printed areas absorb moisture and 
form a very concentrated dyebath, from which dyeing of the fiber takes place. 
The thickening agent prevents the dyestuff from spreading outside the area 
originally printed, because the printed areas act as a concentrated dyebath 
that exists more in the form of a gel than a solution and restricts any 
tendency to bleed. 
    Printed goods are generally washed thoroughly to remove thickening agent, 
chemicals, and unfixed dyestuff. Drying of the washed goods is the final 
operation of printing.  See  TEXTILE CHEMISTRY. 
 �34s[David H. Abrahams]
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�6v�2v�32sThaliacea   �21m �33s 
    A small class of pelagic Tunicata especially abundant in warmer seas. This 
class of animals contains three orders: the Salpida, Doliolida, and 
Pyrosomida. Oral and atrial apertures occur at opposite ends of the body. 
Members of the orders Salpida and Doliolida are transparent forms, partly or 
wholly ringed by muscular bands. The contractions of these bands produce 
currents used in propulsion, feeding, and respiration. The order Pyrosomida 
includes species which form tubular swimming colonies and which are often 
highly luminescent.  See  BIOLUMINESCENCE; TUNICATA. 
 �34s[Donald P. Abbott]
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�6v�2v�32sThallium �21m �33s
    A chemical element, Tl, atomic number 81, relative atomic weight of
204.37. The valence electron notation corresponding to its ground state term
is 6s(sup2)6p(sup1), which accounts for the maximum oxidation state of III in
its compounds. Compounds of oxidation state I and apparent oxidation state II
are also known.
    Thallium occurs in the Earth's crust to the extent of 0.00006%, mainly as
a minor constituent in iron, copper, sulfide, and selenide ores. Minerals of
thallium are considered rare.
    Thallium has significant use in electronic components, such as
thallium-activated sodium iodide crystals in photomultiplier tubes. It is also
used in low-melting point alloys, optical glass, and in glass seals for
enclosing electronic components. Thallium compounds are extremely toxic to
humans and other forms of life.  See  THALLIUM DISEASE.
    The insolubility of thallium(I) chloride, bromide, and iodide permits
their preparation by direct precipitation from aqueous solution; the fluoride,
on the other hand, is water-soluble. Thallium(I) chloride resembles silver
chloride in its photosensitivity.
    Thallium(I) oxide is a black powder which reacts with water to give a
solution from which yellow thallium hydroxide can be crystallized. The
hydroxide is a strong base and will take up carbon dioxide from the atmosphere.
    Thallium also forms organometallic compounds of the following general
classes, R3Tl, R2TIX, and RTIX2, where R may be an alkyl or aryl group and X a
halogen.  See  ORGANOMETALLIC COMPOUND.
 �34s[Edwin M. Larsen]
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�6v�2v�32sThallium disease   �21m �33s 
    The symptoms of thallium disease (thallotoxicosis) vary extensively with 
the dosage, age of the patient, and the acuteness of the intoxication. 
Inflammation involving many nerves, loss of hair, gastrointestinal cramps, and 
emotional changes are the principal symptoms. For large doses, the first 
symptoms are gastrointestinal hemorrhage, gastroenteritis, a rapid heartbeat, 
and headache. Shortly thereafter abdominal pain, vomiting, and diarrhea may 
ensue. Neurologic symptoms appear after 2 or more days of exposure, with 
delirium hallucinations, convulsions, and coma occurring in about a week due 
to respiratory paralysis. 
    When smaller doses are taken, a loss of muscle coordination (ataxia) and 
sensations of tingling or burning of the skin (paresthesia) may be the 
principal symptoms. These may be followed by weakness and atrophy of the 
muscles. Tremor, involuntary movements (choreic athetosis), and mental 
aberrations, with changes in the state of consciousness, may ensue.  See  
THALLIUM.  
 �34s[N. Karle Mottet]
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�6v�2v�32sThallobionta   �21m �33s 
    One of the two commonly recognized subkingdoms of plants. In contrast to 
the more closely knit subkingdom Embryobionta, the Thallobionta (often also 
called Thallophyta) are diverse in pigmentation, food reserves, cell-wall 
structure, and flagellar structure. They still form a natural group, however, 
in the sense that they are all probably derived from ancestors which would be 
referred to the Thallobionta, without the intervention of any ancestors which 
would have to be referred to other groups. The Thallobionta are here 
considered to include eight divisions, the Schizophyta, Rhodophyta, 
Chlorophyta, Euglenophyta, Pyrrophyta, Chrysophyta, Phaeophyta, and Fungi. All 
these groups have both modern and fossil representatives.  See  CHLOROPHYTA; 
CHRYSOPHYTA; EMBRYOBIONTA; EUGLENOPHYTA; FUNGI; PHAEOPHYTA; PLANT KINGDOM; 
PYRRHOPHYTA; RHODOPHYTA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sTheales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of 13 families and nearly 3000 species. The largest families are the 
Guttiferae (about 900 species), Theaceae (about 600 species), Dipterocarpaceae 
(about 400 species), and Ochnaceae (about 400 species). They are mostly woody 
plants, less often herbaceous, with simple or occasionally compound leaves. 
The perianth and stamens of the flowers are attached directly to the 
receptacle; the calyx is arranged in a tight spiral; and the petals are 
usually separate from each other. The stamens are numerous and initiated in 
centrifugal sequence or, less often, are few and cyclic. The tea plant (Thea 
sinensis), Camellia, and St.-John's-wort (Hypericum) are familiar members of 
the Theales.  See  DILLENIIDAE; MAGNOLIOPSIDA; TEA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sThecanephria   �21m �33s 
    An order of Pogonophora, a group of elongate, tentaculated, tube-dwelling, 
sedentary, nonparasitic marine worms lacking a digestive system. In this order 
the ``coelomic'' space in the anterior tentacular region is horseshoe-shaped, 
and the excretory (osmoregulatory) portion of its ducts come close together 
medially near the median, adneural blood vessel. The species in this order are 
multitentaculate.  
    The order includes four families: Polybrachiidae (with seven genera), 
Sclerolinidae (one genus), Lamellisabellidae (two genera), and Spirobrachiidae 
(one genus).  See  ATHECANEPHRIA; POGONOPHORA. 
 �34s[Edward B. Cutler]
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�6v�2v�32sThecodontia   �21m �33s 
    The most primitive order of archosaurian reptiles, confined to the 
Triassic. They differ from Lepidosauria in the absence of a supratemporal 
bone, parietal foramen, and palatal teeth, and in the presence of an 
antorbital fenestra; dorsal intercentra are absent. Primitive features in 
comparison to other archosaurs include retention of clavicles and 
interclavicle, and platelike development of pubis and ischium, which, however, 
show the beginning of elongation associated with bipedalism. The feet were 
five-toed, and rear limbs were always longer than front limbs. The order is 
ancestral to crocodiles, pterosaurs, birds, and both saurischian and 
ornithischian dinosaurs. The Thecodontia includes five highly diverse 
suborders: Proterosuchia, Pelycosimia, Pseudosuchia, Aetosauria, and 
Phytosauria.  See  ARCHOSAURIA; LEPIDOSAURIA; REPTILIA.  
 �34s[Joseph T. Gregory]
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�6v�2v�32sThelodonti   �21m �33s 
    Sometimes called Coelolepida, this is an extinct group of Agnatha or 
jawless vertebrates known from the Lower Silurian to Middle Devonian of 
Europe, Asia, Australia, and North America. Since they had no hard skeleton, 
their structure and relationships are poorly known. Well-preserved specimens 
of Logania suggest a relationship to Heterostraci in their widely spaced, 
lateral eyes, nearly terminal mouth, pectoral flaps at the posterior end of 
the head, and hypocercal tail with a downwardly directed main lobe. 
Phlebolepis resembles Anaspida in general form but lacks the dorsal nostril of 
that group.  See  AGNATHA; DIPLORHINA; HETEROSTRACI; OSTRACODERM. 
 �34s[Robert H. Denison]
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�6v�2v�32sTheobromine   �21m �33s 
    An alkaloid prepared from the dried ripe seed of Theobroma cacao or made 
synthetically. It is often extracted from waste products of the cocoa and 
chocolate industry. Theobromine is a close chemical relative of caffeine (see 
illustration).  
    Theobromine is known to be responsible, in part, for the stimulant action 
of cocoa and other beverages. In addition, it is used therapeutically as a 
diuretic in the treatment of various types of edemas.  See  ALKALOID; CACAO; 
CAFFEIN.  
 �34s[S. Morris Kupchan]
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�6v�2v�32sTheodolite   �21m �33s 
    A surveying instrument for measuring horizontal and vertical angles, 
similar in principle to the transit (see illustration). In the United States 
the term implies high precision, the capability of measuring to one second of 
arc or better. Elsewhere the term is used for an instrument of any precision, 
and the term transit is not used. Virtually every modern theodolite has glass 
measuring circles on which fine graduations are scribed. An intensified beam 
of light passes through the edge of the circle and carries the image of the 
scribed portion of the circle, which is then greatly magnified for the 
observer. An optical micrometer assists in reading the finest divisions. For 
vertical angles, a sensitive split-image bubble assures precision.  See  
SURVEYING; TRANSIT (ENGINEERING). 
 �34s[B. Austin Barry]
�33s 
  
  
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�6v�2v�32sTheorem �21m �33s
    A proposition arrived at by the methods of logical deduction from a set of
basic postulates or axioms accepted as primitive and therefore not subject to
deductive proof. So long as a theorem is part of a purely formal system, it is
not meaningful to speak about the ``truth'' of a theorem but only about its
``correctness.'' It becomes true when it, or its consequences, can be shown to
be in accord with observable facts.  See  LOGIC.
 �34s[Percy W. Bridgman /Henry Margenau]
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�6v�2v�32sTheoretical physics   �21m �33s 
    The description of natural phenomena in mathematical form. It is 
impossible to separate theoretical physics from experimental physics, since a 
complete understanding of nature can be obtained only by the application of 
both theory and experiment. There are two main purposes of theoretical 
physics: the discovery of the fundamental laws of nature and the derivation of 
conclusions from these fundamental laws. 
    Physicists aim to reduce the number of laws to a minimum to have as far as 
possible a unified theory. When the laws are known, it is possible from any 
given initial conditions of a physical system to derive the subsequent events 
in the system. Sometimes, especially in quantum theory, only the probability 
of various events can be predicted.  See  NONRELATIVISTIC QUANTUM THEORY; 
QUANTUM MECHANICS. 
    The conclusions to be derived from the fundamental laws of nature may be 
of several different types. 
    1. Conclusions may be derived in order to test a given theory, 
particularly a new theory. An example is the derivation of the spectrum of the 
hydrogen atom from quantum mechanics; the verification of the predictions by 
accurate measurements is a good test of quantum mechanics. On rather rare 
occasions experiment has been found to contradict the predictions of an 
existing theory, and this has then led to the discovery of important new 
physical laws. An example is the Michelson-Morley experiment on the constancy 
of the velocity of light, an experiment which led to special relativity 
theory.  See  ATOMIC STRUCTURE AND SPECTRA; LIGHT. 
    2. Theory may be required for experiments designed to determine physical 
constants. Most fundamental physical constants cannot be accurately measured 
directly. Elaborate theories may be required to deduce the constant from 
indirect experiments. 
    3. Predictions of physical phenomena may be made in order to gain 
understanding of the structure of the physical world. In this category fall 
theories of the structure of the atom leading to an understanding of the 
periodic system of elements, or of the structure of the nucleus in which 
various models are tested (for example, shell model or collective model). In 
the same category fall applications of theoretical physics to other sciences, 
for example, to chemistry (theory of the chemical bond and of the rate of 
chemical reactions), astronomy (theory of planetary motion, internal 
constitution, and energy production of stars), or biology. 
    4. Engineering applications may be drawn from fundamental laws. All of 
engineering may be considered an application of physics, and much of it is an 
application of mathematical physics, such as elasticity theory, aerodynamics, 
electricity, and magnetism.  See  ELECTRICITY; MAGNETISM; RADIO-WAVE 
PROPAGATION.  
    Apart from the classification of the fields of theoretical physics 
according to purpose, a classification can also be made according to content. 
Here one may distinguish three classification principles: type of force, scale 
of physical phenomena, and type of phenomena.  See  MATHEMATICAL PHYSICS; 
PHYSICS.  
 �34s[Hans A. Bethe]
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�6v�2v�32sTheralite   �21m �33s 
    A dark-colored, phaneritic (visibly crystalline) rock composed chiefly of 
pyroxene with smaller amounts of calcic plagioclase and nepheline. In a sense 
it is a nepheline-rich gabbro with abundant dark-colored (mafic) minerals. 
    Theralite is very uncommon and occurs chiefly in small intrusive bodies 
(dikes, sills, and laccoliths) where it appears to have formed by 
differentiation of magma (molten rock) rich in alkalies and potential mafic 
minerals. It occurs in the Lugar sill in Ayrshire, Scotland.  See  GABBRO; 
IGNEOUS ROCKS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sTherapsida   �21m �33s 
    An extinct order of the subclass Synapsida, grouping the majority of 
advanced mammallike reptiles. Members of this group first appeared in 
mid-Permian times and persisted until the end of the Triassic. The group is 
highly ramified and includes several diverse lines of carnivores and 
herbivores. Most were terrestrial, but one group was aquatic. Therapsids are 
best known from deposits in South Africa; however, they were worldwide in 
distribution, with representation in the records of all continents.  See  
REPTILIA.  
    The various evolving lines became increasingly mammalian through time. The 
most advanced members seem to have invaded uplands of the continental 
interiors. The ancestors of the mammals lay among the therapsids, with mammals 
arising from advanced groups known as cynodonts and ictidosaurs.  See  
ICTIDOSAURIA; SYNAPSIDA. 
 �34s[Everett C. Olson]
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�6v�2v�32sTheria �21m �33s
    One of the four subclasses of the class Mammalia, including all living
mammals except the monotremes. The Theria were by far the most successful of
the several mammalian stocks that arose from the mammallike reptiles in the
Triassic. The subclass is divided into three infraclasses: Pantotheria (no
living survivors), Metatheria (marsupials), and Eutheria (placentals). Therian
mammals are characterized by the distinctive structural history of the molar
teeth. The fossil record shows that all the extremely varied therian molar
types were derived from a common tribosphenic type in which three main cusps,
arranged in a triangle on the upper molar, are opposed to a reversed triangle
and basinlike heel on the lower molar.	See  MAMMALIA; THERAPSIDA.
 �34s[author unknown /Frederick S. Szalay]
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�6v�2v�32sThermal ammeter   �21m �33s 
    An ammeter that functions through the medium of the heat generated by the 
passage of electric current through a resistance element. Modern thermal 
ammeters consist of a thermal converter and a sensitive dc millivoltmeter. The 
illustration shows such a thermal converter mounted on the instrument 
terminals. The short and efficient heater is a platinum-alloy wire or tube, to 
which is welded a thermocouple of two dissimilar metals. When the junction of 
the thermocouple is heated, a small voltage is generated which is proportional 
to the temperature rise; this in turn is applied to the millivolt mechanism to 
deflect its pointer. Typically, the heater is so proportioned as to have a 
temperature rise of 360DGF (200DGC)on the desired full-scale current; about 10 
millivolts is then available for the dc mechanism. Losses are represented by a 
potential drop through the heater of 200 mV at full-scale current. 
    The temperature rise of the heater is a function of the square of the 
current; thus the instrument scale follows a square law, compressed at the 
start and expanded at full scale. 
    Using a 1-mil (25/micrometer) heater wire, full scale represents about 1 
ampere. Higher ranges are made with heavier heaters; losses increase 
correspondingly, with 60 A representing a practical top limit. For measuring 
currents under 1 A, the thermoelement is placed in vacuum to reduce heat 
losses; these are now useful as low as 1 millampere. A special bridge form of 
open thermoelement is also used in the 100-mA range. See AMMETER. 
 �34s[John H. Miller /Jesse Beams]
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�6v�2v�32sThermal conduction in solids �21m �33s
    Thermal conduction in a solid is generally measured by stating the thermal
conductivity K, which is the ratio of the steady-state heat flow (heat
transfer per unit area per unit time) along a long rod to the temperature
gradient along the rod. Thermal conductivity varies widely among different
types of solids, and depends markedly on temperature and on the purity and
physical state of the solids, particularly at low temperatures.
    From the kinetic theory of gases the thermal conductivity can be written

			     K = (constant) S v l

as in the equation below, where S is the specific
heat per unit volume, v is the average particle velocity, and l is the mean
free path. In solids, thermal conduction results from conduction by lattice
vibrations and from conduction by electrons. In insulating materials, the
conduction is by lattice waves; in pure metals, the lattice contribution is
negligible and the heat conduction is primarily due to electrons. In many
alloys, impure metals, and semiconductors, both conduction mechanisms
contribute.  See  CONDUCTION (HEAT); KINETIC THEORY OF MATTER; LATTICE
VIBRATIONS; SPECIFIC HEAT OF SOLIDS.
    In superconductors at temperatures below the critical temperature, the
electronic conduction is reduced; at sufficiently low temperatures, the
thermal conductivity becomes entirely due to lattice waves and is similar to
the form of the thermal conductivity of an insulating material.  See
SUPERCONDUCTIVITY.
 �34s[Kathryn A. McCarthy]
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�6v�2v�32sThermal cracking �21m �33s
    A cracking process in which carbon-to-carbon bonds are severed by the
action of heat alone. It consists essentially in the heating of any fraction
of petroleum to a temperature at which substantial thermal decomposition takes
place through a thermal free-radical mechanism followed by cooling,
condensation, and physical separation of the reaction products. There are a
number of refinery processes based primarily upon the thermal cracing
reaction. They differ primarily in the intensity of the thermal conditions and
the feedstock handled.
    Visbreaking is a mild thermal cracking operation (850-950DGF; 454 -
510DGC) where only 20 - 25% of the residuum feed is converted to
mid-distallate and lighter material. Thermal gas-oil or naphtha cracking is a
more severe operation (950 - 1100DGF; 510 - 593DGC) where 45% or more of the
feed is converted to lower molecular weight. Steam cracking is an extremely
severe operation (1100 to 1400DGF; 593 - 760DGC) in which steam is used as a
dilluent to achieve a very low hydrocarbon partial pressure. In fluid coking
the residuum is converted fully to gas-oil products boiling lower than 950DGF
(510DGC) and coke. In delayed coking a residuum is heated and sent to a coke
drum, where the liquid has an infinite residence time to convert to
lower-molecular-weight hydrocarbons which distill out of the coke drum, and to
coke which remains in the drum and must be periodically removed. In fluid
coking and delayed coking, there is total conversion of the very heavy
high-boiling end of the residuum feed. See CRACKING.
 �34s[E. Lazarides]
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�6v�2v�32sThermal expansion �21m �33s
    Solids, liquids, and gases all exhibit dimensional changes for changes in
temperature while pressure is held constant. The molecular mechanisms at work
and the methods of data presentation are quite different for the three cases.
    The temperature coefficient of linear expansion ALPHA(sub l) is defined by


ALPHA (sub 1) =
(1/l)(PARTIAL DIFF.l / PARTIAL DIFF. t) (sub p = constant)	       (1)


Eq. (1), where l is the length of the specimen, t is the temperature, and
p is the pressure. For each solid there is a Debye characteristic
temperature THETA, below which ALPHA(sub l) is strongly dependent upon
temperature and above which ALPHA(sub l) is practically constant. Many
common substances are near or above THETA at room temperature and follow
approximate equation (2),

		   l = l(sub o)(l + ALPHA(sub l)t)		       (2)

where l0 is the length at 0DGC and t is the temperature in DGC. The total
change in length from absolute zero to the melting point has a range of
approximately 2% for most substances.


		   (pw)/t = r/molecular weight			     (3)


    So-called perfect gases follow the relation in Eq. (3), where p is
absolute pressure, v is specific volume, T is absolute temperature, and R
is the so-called gas constant. Real gases often follow this equation closely.
See  GAS CONSTANT.
    The coefficient of cubic expansion ALPHA(sub v) is defined by Eq. (4),and
for a perfect gas this is found to be 1/T.

  ALPHA(sub v) = (1/v)(PARTIAL DIFF. v / PARTIAL DIFF. t )	    (4)
		      (p = constant)

The behavior of real gases is largely accounted for by the van der Waals
equation.  See	GAS; KINETIC THEORY OF MATTER; VAN DER WAALS EQUATION.
    For liquids, ALPHAsubv is somewhat a function of pressure but is largely
determined by temperature. Though ALPHA(sub v) may often be taken as constant
over a sizable range of temperature (as in the liquid expansion thermometer),
generally some variation must be accounted for. For example, water contracts
with temperature rise from 32 to 39DGF (0 to 4DGC), above which it expands at
an increasing rate.  See  THERMOMETER.
 �34s[Ralph A. Burton]
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�6v�2v�32sThermal flow measurement devices �21m �33s
    A heated or self-heating sensor used for gas-velocity or gas-mass flow
measurement.
    In the hot-wire anemometer, a thermopile heated by a constant ac voltage
is cooled by the gas flow. The degree of cooling, proportional to local flow
velocity, is indicated by the thermopile millivoltage output.  See
THERMOCOUPLE.
    In the heat-loss flowmeter, a constant current is supplied to a thermistor
bead. The temperature of the bead depends upon the cooling effect of the mass
flow of the fluid, and the measure of the voltage drop across the bead is
therefore an indication of the flow. A second thermistor bead is used to
compensate for the temperature of the fluid.  See  THERMISTOR.
    Another heat-loss flowmeter measures the amount of electrical heating that
is required to keep the temperature of a platinum resistor constant since the
amount of heat carried away changes with the velocity of the fluid. A second
sensor is used to compensate for the temperature of the fluid.	See  FLOW
MEASUREMENT.
 �34s[Mead Bradner ;Lewis P. Emerson]
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�6v�2v�32sThermal hysteresis �21m �33s
    A phenomenon in which a physical quantity depends not only on the
temperature but also on the preceding thermal history. It is usual to compare
the behavior of the physical quantity while heating and the behavior while
cooling through the same temperature range. The illustration shows the thermal
hysteresis which has been observed in the behavior of the dielectric constant
of single crystals of barium titanate. On heating, the dielectric constant was
observed to follow the path ABCD, and on cooling the path DCEFG.  See
DIELECTRIC CONSTANT; FERROELECTRICS.
    Perhaps the most common example of thermal hysteresis involves a phase
change such as solidification from the liquid phase. In many cases these
liquids can be dramatically supercooled. Elaborate precautions to eliminate
impurities and outside disturbances can be instrumental in supercooling 60 to
80DGC. On raising the temperature after freezing, however, the system follows
a completely different path, with melting coming at the prescribed temperature
for the phase change.  See  CRYSTAL; NUCLEATION; PHASE TRANSITIONS.
 �34s[H. B. Huntington ;Richard K. MacCrone]
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�6v�2v�32sThermal neutrons   �21m �33s 
    Neutrons whose energy distribution is identical with, or similar to, the 
thermal distribution in the material in which they are found. 
    Usually, the energy distribution of neutrons in matter is not in thermal 
equilibrium with the environment, but they are still called thermal 
distributions of neutrons. Most neutrons are produced at energies above 
thermal, and the absorption of neutrons interrupts the process of slowing down 
so that the low-energy neutron spectrum is ``hardened'' with a greater number 
of neutrons of higher energy than the maxwellian distribution of energies 
predicts, and a ``tail'' of neutrons not yet thermalized. The actual 
distribution can be fitted to one in which the spectrum is represented as a 
maxwellian distribution of higher temperature than the environment, with the 
tail superposed.  See  BOLTZMANN STATISTICS; KINETIC THEORY OF MATTER; NUCLEAR 
FISSION; REACTOR PHYSICS. 
 �34s[Bernard I. Spinrad]
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�6v�2v�32sThermal stress   �21m �33s 
    Mechanical stress induced in a body when some or all of its parts are not 
free to expand or contract in response to changes in temperature. In most 
continuous bodies, thermal expansion or contraction cannot occur freely in all 
directions because of geometry, external constraints, or the existence of 
temperature gradients, and so stresses are produced. Such stresses caused by a 
temperature change are known as thermal stresses. 
    Problems of thermal stress arise in many practical design problems, such 
as those encountered in the design of steam and gas turbines, diesel engines, 
jet engines, rocket motors, and nuclear reactors. The high aerodynamic heating 
rates associated with high-speed flight present even more severe 
thermal-stress problems for the design of spacecraft and missiles.  See  
STRESS AND STRAIN. 
 �34s[Shih-Yuan Chen]
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�6v�2v�32sThermionic emission �21m �33s
    The emission of electrons into vacuum by a heated electronic conductor. In
its broadest meaning, thermionic emission includes the emission of ions, but
this process is quite different from that normally understood by the term.
Thermionic emitters are used as cathodes in electron tubes and hence are of
great technical and scientific importance. Although in principle all
conductors are thermionic emitters, only a few materials satisfy the
requirements set by practical applications. Of the metals, tungsten is an
important practical thermionic emitter; in most electron tubes, however, the
oxide-coated cathode is used to great advantage. For a detailed discussion of
practical thermionic emitters  see  VACUUM TUBE.
    The thermionic emission of a material may be measured by using the
material as the cathode in a vacuum tube and collecting the emitted electrons
on a positive anode. If the anode is sufficiently positive relative to the
cathode, space charge (a concentration of electrons near the cathode) can be
avoided and all electrons emitted can be collected; the saturation thermionic
current is then measured.  See	SCHOTTKY EFFECT.
    The emission current density J increases rapidly with increasing
temperature; this is illustrated by the following approximate values for
tungsten:

T (K)			  1000	      2000	2500	  3000
J (amperes/cmsub2)	  10sub-15    10sub-3	 0.3	   l5


The temperature dependence of J is given by the Richardson (or Dushman-
Richardson) equation below.  Here A is a constant, k is Boltzmann's

		       J = AT(sup 2)e (sup -(PHI-kT))

constant (=1.38 x 10 (sup -23) joule/degree), and PHI is the work function
of the emitter. The work function has the dimensions of energy and is a
few electronvolts for thermionic emitters.  See  WORK FUNCTION
(ELECTRONICS).
 �34s[Adrianus J. Dekker]
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�6v�2v�32sThermionic power generator �21m �33s
    A device in which heat energy is directly converted to electric energy,
and based on the phenomenon of thermionic emission. It is also frequently
called a thermionic converter.
    Two metallic elements, an emitter and a collector, are the minimum needed
for a thermionic converter. The thermionic electron emitter must be capable of
yielding electrons to the space that separates the emitter from the electron
collector. The collector must be operated at a significantly lower temperature
than the emitter so that the collector does not also emit electrons. The
general term that describes such a thermionic device is thermionic diode.
    If the space between the two elements of the diode is evacuated
sufficiently so that the residual gas has no significant influence on the flow
of electrons from the emitter to the collector, the device is known as a
vacuum thermionic converter. Electrons are negatively charged particles and
thus repel each other. The presence of electrons in transit between the
emitter and the collector can, therefore, interfere seriously with the free
flow of additional charges and thus set up a space-charge limitation on the
current density and the efficiency attainable.	See  SPACE CHARGE.
    Two methods are used to minimize the space-charge limitation. One depends
on a diode construction with extremely close spacing - of the order of 5
micrometers or approximately 0.0002 in. The second method is to introduce an
ionized gas. The number density of the ions of the gas must be equal to or
locally greater than the electron density in order to neutralize the negative
space charge otherwise present. The term plasma is applied to a medium in
which the net electric charge is zero. A thermionic converter that depends on
the presence of an ionized gas to give good conduction in the space between
the emitter and the collector is known as a plasma thermionic converter.  See
THERMIONIC EMISSION.
 �34s[W. B. Nottingham /Thomas F. Stratton]
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�6v�2v�32sThermionic tube   �21m �33s 
    An electron tube that relies upon thermally emitted electrons from a 
heated cathode for tube current. 
    Thermionic emission of electrons means emission by heat. In practical form 
an electrode, called the cathode because it forms the negative electrode of 
the tube, is heated until it emits electrons. The cathode may be either a 
directly heated filament or an indirectly heated surface. With a filamentary 
cathode, heating current is passed through the wire, which either emits 
electrons directly or is covered with a material that readily emits electrons. 
Indirectly heated cathodes have a filament, commonly called the heater, 
located within the cathode electrode to bring the surface of the cathode to 
emitting temperature. The majority of all vacuum tubes are thermionic tubes.  
See  ELECTRON TUBE; GAS TUBE; THERMIONIC EMISSION; VACUUM TUBE. 
 �34s[Leon S. Nergaard]
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�6v�2v�32sThermistor   �21m �33s 
    A resistive circuit component, having a high negative temperature 
coefficient of resistance (as temperature increases, resistance decreases).  
See  ELECTRICAL RESISTIVITY. 
    The typical thermistor is a stable, compact, and rugged two-terminal 
ceramiclike semiconductor bead, rod, or disk. Thermistors are made of various 
mixtures of oxides of manganese, nickel, cobalt, copper, uranium, iron, zinc, 
titanium, and magnesium. The temperature coefficient is determined by the 
proportions of oxides contained in the mixture. 
    Thermistors are versatile circuit elements and have many applications in 
measurement and control. The thermistor's large temperature coefficient of 
resistance is ideal for temperature measurement. Precise temperature 
measurement is done with high-resistance thermistors in a resistance bridge. 
Sensitivity of 0.0009DGF (0.0005DGC) is readily attained.  See  TEMPERATURE 
MEASUREMENT; THERMOMETER. 
    Many electrical and electronic components have positive temperature 
coefficients which are detrimental to the temperature stability of the 
circuit. A properly selected thermistor in the circuit containing such a 
component will provide temperature compensation. 
    When a thermistor is used as a flowmeter, vacuum gage, or anemometer, a 
small voltage is applied and the current through the thermistor is measured. 
The amount of heat dissipated is a function of the degree of vacuum 
surrounding the device or the velocity of gas passing the device, and the 
dissipated power reaches a constant value. The measured current is calibrated 
in terms of vacuum or gas flow.  See  ANEMOMETER; FLOW MEASUREMENT; VACUUM 
MEASUREMENT.  
    When a thermistor is self-heated as a result of current passing through 
it, its resistance decreases. The thermal mass causes the time-rate of 
decrease to be fixed. The delayed buildup of circuit currents can be used to 
introduce a fixed time delay between relay operations or to protect equipment 
during start-up. 
    The thermistor's resistance versus power characteristic makes it a useful 
power-measuring device. Microwave power is measured by a bead thermistor 
mounted in the waveguide and biased so that bead impedance matches the cavity. 
The thermistor also can be used to measure radiant power, such as infrared or 
visible light.  See  ELECTRIC POWER MEASUREMENT. 
 �34s[Frank H. Bower]
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�6v�2v�32sThermoanalysis   �21m �33s 
    A group of analytical techniques developed to continuously monitor 
physical or chemical changes of a sample which occur as the temperature is 
varied. Thermogravimetry (TG), differential thermal analysis (DTA), and 
differential scanning calorimetry (DSC) are the three principal 
thermoanalytical methods. Thermogravimetry involves measuring the changes in 
mass of a substance, typically a solid, as the substance is being heated. In 
differential thermal analysis, the temperature difference between a sample and 
inert reference material is monitored as they are simultaneously heated, or 
cooled, at a predetermined rate. Differential scanning calorimetry involves 
heating a sample and a reference material by using separately controlled 
resistance heaters at a predetermined rate while they are kept isothermal. 
Modern commercial instruments, many incorporating microprocessors, are 
available for all three types of thermoanalysis.  See  CALORIMETRY; 
MICROPROCESSOR.  
 �34s[Neil Jesperson]
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�6v�2v�32sThermochemistry   �21m �33s 
    A branch of physical chemistry dealing with the heat effects which 
accompany chemical reactions, the formation of solutions, and changes in the 
physical state of substances, such as the fusion of a crystalline solid or the 
vaporization of a liquid. When these processes evolve heat, they are said to 
be exothermic. Conversely, those absorbing heat are endothermic. A knowledge 
of the magnitudes of such heat effects is of practical value to the engineer 
in solving problems of heating and refrigeration and in controlling chemical 
reactions at suitable temperatures. Such data are also of great theoretical 
importance to the scientist who wishes to calculate the chemical affinity or 
free energy for various reactions, hypothetical or real.  See  CHEMICAL 
THERMODYNAMICS; FREE ENERGY. 
    These heat effects are usually measured in calories, partly for historical 
reasons and partly because the figures thereby obtained are of more convenient 
magnitude than when the joule, the fundamental unit of energy, is employed. 
The present-day calorie is arbitrarily defined as being equal to 4.184 
absolute joules, rather than in terms of the heat capacity of water, which 
varies with temperature.  See  CALORIMETRY. 
 �34s[Bruno J. Zwolinski]
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�6v�2v�32sThermocline   �21m �33s 
    A layer of sea water in which the temperature decrease with depth is 
greater than that of the overlying and underlying water. Such layers are 
semipermanent features of the oceanic temperature structure, and their depth 
and thickness show marked variation with season, latitude and longitude, and 
local environmental conditions.  Since the three-dimensional temperature 
structure has a great effect on many oceanic properties, such as the 
transmission of sound, the study of the nature and behavior of the thermocline 
is of extreme importance to many oceanographic interests. In general, two 
major types of thermocline may be identified: the permanent thermocline (see 
illustration) and the seasonal thermocline. In addition to these types, 
shallow thermocline or similar stable layers often occur, owing to diurnal 
heating of the surface waters.  See  SEA WATER; UNDERWATER SOUND. 
 �34s[John J. Schule, Jr.]
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�6v�2v�32sThermocouple   �21m �33s 
    A device that uses the voltage developed by the junction of two dissimilar 
metals to measure temperature difference. Two wires of dissimilar metals 
welded together at the ends make up the basic thermocouple (see illustration). 
One junction, called the sensing or measuring junction, is placed at the point 
where temperature is to be measured. The other junction, called the reference 
or cold junction, is maintained at a known reference temperature. The voltage 
developed between the two junctions is approximately proportional to the 
difference between the temperatures of the two junctions, and may be measured 
by including a suitable voltmeter in the circuit.  See  THERMOELECTRICITY. 
    Although all dissimilar metals exhibit the thermoelectric effect, only a 
few are in wide use. The major characteristics which make certain metals or 
combinations of metals outstanding for this purpose are stability and 
reproducibility, constant or controllable compositions, corrosion resistance, 
sensitivity, ruggedness, and cost. 
    The thermopile is a number of thermocouples connected in series or 
parallel. The series circuit provides a higher sensitivity (greater emf per 
degree) than one thermocouple alone and is often selected for this reason. 
Either the series or parallel circuit may be used for obtaining an indication 
which approaches the average of the several temperatures.  See  TEMPERATURE 
MEASUREMENT.  
 �34s[Howard S. Bean]
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�6v�2v�32sThermodynamic cycle   �21m �33s 
    A procedure or arrangement in which one form of energy, such as heat at an 
elevated temperature from combustion of a fuel, is in part converted to 
another form, such as mechanical energy on a shaft, and the remainder is 
rejected to a lower temperature sink as low-grade heat. 
    A thermodynamic cycle requires, in addition to the supply of incoming 
energy, (1) a working substance, usually a gas or vapor; (2) a mechanism in 
which the processes or phases can be carried through sequentially; and (3) a 
thermodynamic sink to which the residual heat can be rejected. The cycle 
itself is a repetitive series of operations. 
    There is a basic pattern of processes common to power-producing cycles. 
There is a compression process wherein the working substance undergoes an 
increase in pressure and therefore density. There is an addition of thermal 
energy from a source such as a fossil fuel, a fissile fuel, or solar 
radiation. There is an expansion process during which work is done by the 
system on the surroundings. There is a rejection process where thermal energy 
is transferred to the surroundings. The algebraic sum of the energy additions 
and abstractions is such that some thermal energy is converted into mechanical 
work.  See  HEAT. 
  
    Many cyclic arrangements, using various combinations of phases but all 
seeking to convert heat into work, have been proposed by many investigators 
whose names are attached to their proposals, for example, the Diesel, Otto, 
Rankine, Brayton, Stirling, Ericsson, and Atkinson cycles (see illustration). 
All proposals are not equally efficient in the conversion of heat into work. 
However, they may offer other advantages which have led to their practical 
development for various applications.  See  BRAYTON CYCLE; CARNOT CYCLE; 
DIESEL CYCLE; OTTO CYCLE; STIRLING ENGINE; THERMODYNAMIC PROCESSES. 
 �34s[Theodore Baumeister]
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�6v�2v�32sThermodynamic principles �21m �33s
    Laws governing the conversion of energy from one form to another. Among
the many consequences of these laws are relationships between the properties
of matter and the effects of changes in pressure, temperatures, electric
field, magnetic field, and composition. The great practicality of the science
arises from the foundations of the subject. Thermodynamics is based upon
observations of common experience that have been formulated into the
thermodynamic laws. From these few laws all of the remaining laws of the
science are deducible by purely logical reasoning.
    For exposition, it is necessary to define a few terms. A system is that
part of the physical world under consideration. The rest of the world is the
surroundings. An open system may exchange mass, heat, and work with the
surroundings. A closed system may exchange heat and work but not mass with the
surroundings. An isolated system has no exchange with the surroundings. Those
parts of a system spatially uniform and homogeneous are called phases. For
example, a liquid together with its vapor may be considered a two-phase
system. It is a fact of experience that an isolated system approaches a
particularly simple terminal state called an equilibrium state.

    Temperature.  It is within the scope of thermodynamics to refine the
primitive notion of hotness and coldness into an operational and precise
concept of temperature. The equilibrium states of a single-component,
single-phase fluid provide a starting point. For such a fluid the equilibrium
state is defined by fixing two of its properties. For example, one could
construct a mercury-in-glass thermometer that has its pressure held constant;
then only one other property could be varied independently. If the volume
(height of mercury) is observed at any equilibrium state, there is a 1:1
correspondence between it and any other property excepting the pressure. The
degree of hotness is one of these properties.
    Also note that thermal equilibrium between different systems exists. For
example, if the mercury thermometer is placed in contact with a body of quiet
water, the mercury will either expand or contract. The volume change of the
mercury will eventually stop, and the properties of the mercury will be
constant, indicating an equilibrium state; moreover, the water will also have
the constant properties of an equilibrium state. Bodies in thermal equilibrium
are said to have the same temperature. Thus, one arrives at a method of
measuring the temperature of a body of water.
    Now suppose there is a large body of water in thermal contact through a
wall with a large body of another fluid such as alcohol, both at equilibrium.
It is an experimental fact that the mercury-in-glass thermometer will register
the same volume when placed in either of the fluids. This fact of experience
is designated as the zeroth law of thermodynamics: If two bodies A and B are
separately in thermal equilibrium with C, then A and B are in equilibrium with
each other. Thus, a useful empirical temperature measurement based upon the
volume of mercury under constant pressure is established.  See	TEMPERATURE
MEASUREMENT.

    Internal energy.  Thermodynamics does not define the concepts of energy or
work but adopts them from the other macroscopic sciences of mechanics and
electromagnetism. Also, the conservation of energy is taken as axiomatic.
Therefore, if an isolated system is formed from any part of the world, a
definite amount of energy will be trapped in the system. The energy resides in
the kinetic and potential energy of the trapped molecules. This trapped energy
is called the internal energy U.
    Because of the conservation of energy, the internal energy of a closed
system can be altered only by an exchange of energy with the surroundings.
There are only three modes by which the exchange can occur: by mass transfer,
heat transfer, or work exchange. So for a closed (no mass transfer), adiabatic
(no heat transfer) system the change in internal energy DELTA U is equal to the
work done by the surroundings on the system, as defined by Eq. (1). Here a

		    DELTA U = W(sub ad) 			    (1)

convention has been adopted that work done on the system is positive. There is
a great mass of experimental information where work has been done on a closed
system enclosed within adiabatic walls. Among these are experiments performed
by J. Joule more than a century ago. It may be concluded from Joule's
experiments that the expenditure of a given quantity of work always causes the
same change of state regardless of how the work is carried out. Both W(sub AD)
and DELTA U are independent of the path. It is concluded that U is a state
function.
    It is known from experience that the same change in state of a system can
be effected by either supplying work to the system in an adiabatic enclosure
or by contacting the system through a conducting wall with a higher
temperature system. The latter method is a different means of transferring
energy than work and is termed heat and given the symbol Q.

	      DELTA U = Q + W					   (2)

    The first law of thermodynamics, Eq. (2), states that the algebraic sum of
heat and work during a process is equal to the change in the state function U.
The term (Q + W) is therefore independent of the path taken between the two
states. The differential form of the first law is given by Eq. (3), where q
and w represent small quantities.

			     dU = q + w 				     (3)

    Reversible and irreversible processes.  Any process that occurs in nature
is in agreement with the first law, but many processes permissible by the
first law never occur. There is an overwhelming preference for processes to
proceed in one direction. In one of Joule's experiments, a falling weight
caused a paddle to do work on an adiabatically enclosed body of water. The
total effect of the experiment was to increase the internal energy of the
water and to lower the weight. The surroundings remained unchanged. There is
no way one can reverse this process, that is, restore the water to its
original state and raise the weight to its original height without also making
some additional change in the surroundings. The process is irreversible.
    Thermodynamics makes use of an idealization, called a reversible process,
that is a limiting case of the natural or irreversible process. The reversible
process may be defined as one which can be completely reversed without leaving
more than a vanishingly small change in the surroundings. It is a consequence
of the definition that a reversible process proceeds through a succession of
equilibrium states and may be reversed by an infinitesimal change in the
external conditions. Imagine having a cylinder of gas fitted with a
frictionless piston. If the piston is moved so slowly that pressure gradients
are absent, the gas will be in an equilibrium state at all times. The
difference between the gas pressure and the external pressure needs only to be
infinitesimal in order to move the frictionless piston. Under the rather
restrictive conditions of a reversible process, Eq. (4) may be quite properly
written, where p is the gas pressure and V is the gas volume.

			       dW = -pdV				  (4)


    Entropy.  The discussion on irreversible processes has led to the second
law of thermodynamics, which is just a general statement of the idea that
there is a preferred direction for a given process. There are many physical
statements of the second law, all being equivalent and leading to the same
mathematical statement. The statement of R. Clausius is: ``It is not possible
that, at the end of a cycle of changes, heat has been transferred from a
colder to a hotter body without producing some other effect.'' Lord Kelvin's
statement is:  ``It is not possible that, at the end of a cycle of changes,
heat has been extracted from a reservoir and an equal amount of work has been
produced without producing some other effect.''
    The most efficient way of developing the mathematical consequences of the
second law is to proceed from Caratheodory's principle, which can be either
taken as another physical expression of the second law or derived from the
Clausius or Kelvin statement. Caratheodory's principle is: ``In the
neighborhood of any equilibrium state of a system there are states which are
not accessible by an adiabatic process.''
    Caratheodory used this principle together with a mathematical theorem that
he developed to infer the existence of a state function S and an integrating
factor 1/T, where T is the thermodynamic temperature such that Eq. (5) holds

		   dQ(sub rev) = TdS				      (5)

for a reversible change. The state function S is called the entropy. It can
also be shown that the entropy in an adiabatic system increases for an
irreversible change and remains constant for a reversible change as in Eq. (6).

	  DELTA S(sub AD) GREATER THAN OR EQUAL TO 0		       (6)

    The first part of the mathematical statement of the second law allows one
to write one of the most important thermodynamic equations, Eq. (7). Although

			      dU = TdS - pdV				     (7)

this equation was derived for reversible changes, it is valid for all changes.
All the quantities are functions of state. Therefore, for a change between two
states the integral of the equation will be valid even if the path is not
reversible.
    The second part of the mathematical statement is a concise summary of
physical statements on the direction of processes. Also contained in the
second part of the entropy statement is the key idea of equilibrium. The
equilibrium state of an adiabatic or isolated system is characterized by
entropy being at its maximum value consistent with the physical constraints.
See  ADIABATIC PROCESS; ENTROPY.
 �34s[William F. Jaep]
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�6v�2v�32sThermodynamic processes �21m �33s
    Changes of any property of an aggregation of matter and energy,
accompanied by thermal effects.

    Systems and processes.  To evaluate the results of a process, it is
necessary to know the participants that undergo the process, and their mass
and energy. A region, or a system, is selected for study, and its contents
determined. This region may have both mass and energy entering or leaving
during a particular change of conditions, and these mass and energy transfers
may result in changes both within the system and within the surroundings which
envelop the system.
    To establish the exact path of a process, the initial state of the system
must be determined, specifying the values of variables such as temperature,
pressure, volume, and quantity of material. The number of properties required
to specify the state of a system depends upon the complexity of the system.
Whenever a system changes from one state to another,  a process occurs.
    The path of a change of state is the locus of the whole series of states
through which the system passes when going from an initial to a final state.
For example, suppose a gas expands to twice its volume and that its initial
and final temperatures are the same. An extremely large number of paths
connect these initial and final states. The detailed path must be specified if
the heat or work is to be a known quantity; however, changes in the
thermodynamic properties depend only on the initial and final states and not
upon the path. A quantity whose change is fixed by the end states and is
independent of the path is a point function or a property.

    Pressure-volume-temperature diagram.  Whereas the state of a system is a
point function, the change of state of a system, or a process, is a path
function. Various processes or methods of change of a system from one state to
another may be depicted graphically as a path on a plot using thermodynamic
properties as coordinates.
    The variable properties most frequently and conveniently measured are
pressure, volume, and temperature. If any two of these are held fixed
(independent variables), the third is determined (dependent variable). To
depict the relationship among these physical properties of the particular
working substance, these three variables may be used as the coordinates of a
three-dimensional space. The resulting surface is a graphic presentation of
the equation of state for this working substance, and all possible equilibrium
states of the substance lie on this P-V-T surface.
    Because a P-V-T surface represents all equilibrium conditions of the
working substance, any line on the surface represents a possible reversible
process, or a succession of equilibrium states.
    The portion of the P-V-T surface shown in Fig. 1 typifies most real
substances; it is characterized by contraction of the substance on freezing.
Going from the liquid surface to the liquid-solid surface onto the solid
surface involves a decrease in both temperature and volume. Water is one of
the few exceptions to this condition; it expands upon freezing, and its
resultant P-V-T surface is somewhat modified where the solid and liquid phases
abut.
    One can project the three-dimensional surface onto the P-T plane as in
Fig. 2. The triple point is the point where the three phases are in
equilibrium. When the temperature exceeds the critical temperature (at the
critical point), only the gaseous phase is possible.

    Temperature-entropy diagram.  Energy quantities may be depicted as the
product of two factors:  an intensive property and an extensive one.
Examples of intensive properties are pressure, temperature, and magnetic
field, extensive ones are volume, magnetization, and mass.  Thus, in
differential form, work is the product of a pressure exerted against an area
which sweeps through an infinitesimal volume, as in Eq.(1).

			   dW = P dV					    (1)

As a gas expands, it is doing work on its environment. However, a number of
different kinds of work are known. For example, one could have work of
polarization of a dielectric, of magnetization, of stretching a wire, or of
making new surface area. In all cases, the infinitesimal work is given by Eq.

			    dW = Xdx					    (2)

(2), where X is a generalized applied force which is an intensive quantity,
and dx is a generalized displacement of the system and is thus extensive.
    By extending this approach, one can depict transferred heat as the product
of an intensive property, temperature, and a distributed or extensive property
defined as entropy, for which the symbol is S.	See  ENTROPY.

    Reversible and irreversible processes.  Not all energy contained in or
associated with a mass can be converted into useful work. Under ideal
conditions only a fraction of the total energy present can be converted into
work. The ideal conversions which retain the maximum available useful energy
are reversible processes.
    Characteristics of a reversible process are that the working substance is
always in thermodynamic equilibrium and the process involves no dissipative
effects such as viscosity, friction, inelasticity, electrical resistance, or
magnetic hysteresis. Thus, reversible processes proceed quasistatically so
that the system passes through a series of states of thermodynamic
equilibrium, both internally and with its surroundings. This series of states
may be traversed just as well in one direction as in the other.
    Actual changes of a system deviate from the idealized situation of a
quasistatic process devoid of dissipative effects. The extent of the deviation
from ideality is correspondingly the extent of the irreversibility of the
process.  See  THERMODYNAMIC PRINCIPLES.
 �34s[Philip E. Bloomfield; William A. Steele]
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�6v�2v�32sThermoelectricity �21m �33s
    The direct conversion of heat into electrical energy, or the reverse, in
solid or liquid conductors by means of three interrelated phenomena - the
Seebeck effect, the Peltier effect, and the Thomson effect - including the
influence of magnetic fields upon each. The Seebeck effect concerns the
electromotive force (emf) generated in a circuit composed of two different
conductors whose junctions are maintained at different temperatures. The
Peltier effect refers to the reversible heat generated at the junction between
two different conductors when a current passes through the junction. The
Thomson effect involves the reversible generation of heat in a single
current-carrying conductor along which a temperature gradient is maintained.
Specifically excluded from the definition of thermoelectricity are the
phenomena of Joule heating and thermionic emission.  See  JOULE`S LAW; PELTIER
EFFECT; SEEBECK EFFECT; THERMIONIC EMISSION; THOMSON EFFECT.
    The three thermoelectric effects are described in terms of three
coefficients: the absolute thermoelectric power (or thermopower) S, the Peltier
coefficient II, and the Thomson coefficient MU, each of which is defined for a
homogeneous conductor at constant temperature. These coefficients are connected
by the Kelvin relations, which convert complete information about one into
complete information about all three. It is therefore necessary to measure only
one of the three coefficients; usually the thermopower S is chosen.
    The most important practical application of thermoelectric phenomena is in
the accurate measurement of temperature. The phenomenon involved is the
Seebeck effect. Of lesser importance are the direct generation of electric
power by application of heat (also involving the Seebeck effect) and
thermoelectric cooling and heating (involving the Peltier effect).
    A basic system suitable for all four applications is illustrated
schematically in the illustration. Several thermocouples are connected in
series to form a thermopile, a device with increased output (for power
generation or cooling and heating) or sensitivity (for temperature
measurement) relative to a single thermocouple. The junctions forming one end
of the thermopile are all at the same low temperature TsubL, and the junctions
forming the other end are at the high temperature TsubH. The thermopile is
connected to a device D which is different for each application. For
temperature measurement, the temperature TsubL is fixed, for example, by means
of a bath; the temperature TsubH becomes the running temperature T, which is
to be measured; and the device is a potentiometer for measuring the
thermoelectric emf generated by the thermopile. For power generation, the
temperature TsubL is fixed by connection to a heat sink; the temperature TsubH
is fixed at a value determined by the output of the heat source and the
thermal conductivity of the thermopile; and the device is whatever is to be
run by the electricity which is generated. For heating or cooling, the device
is a current generator which passes current through the thermopile. If the
current flows in the proper direction, the junctions at TsubH will heat up,
and those at TsubL will cool down. If TsubH is fixed by connection to a heat
sink, thermoelectric cooling will be provided by TsubL. Alternatively, if
TsubL is fixed, thermoelectric heating will be provided at TsubH. Such a
system has the advantage that at any given location it can be converted from a
cooler to a heater merely by reversing the direction of the current.

    Thermoelectric systems made from even the best available materials have
the disadvantages of relatively low efficiencies and concomitant high cost per
unit of output. Their use in power generation, heating, and cooling has
therefore been largely restricted to situations in which these disadvantages
are outweighed by such advantages as small size, low maintenance due to lack
of moving parts, quiet performance, light weight, and long life.  See
THERMOELECTRIC POWER GENERATOR.
 �34s[Jack Bass]
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�6v�2v�32sThermoelectric power generator   �21m �33s 
    A device that converts heat energy directly into electric energy by using 
the Seebeck effect. A thermoelectric generator is composed of at least two 
dissimilar materials, one junction of which is in contact with a heat source 
and the other junction of which is in contact with a heat sink. See SEEBECK 
EFFECT.  
    The power converted from heat to electricity is dependent upon the 
materials used, the temperatures of the heat source and sink, the electrical 
and thermal design of the thermocouple, and the load of the thermocouple. 
    The most widely used generator material is lead telluride. It can be doped 
to produce both p- and n-type material and has a useful temperature range of 
about 80 - 800DGF (300 - 700 K). In segmented couples at the low-temperature 
end, bismuth telluride and its alloys are sometimes used.  
    Thermoelectric generators have been built in sizes up to 5 kW, with their 
primary energy sources being hydrocarbon fuels, radioisotopes, and solar 
energy.  
    The maximum theoretical thermal efficiency for materials over a 
temperature range of 80 - 1880DGF (300 - 1300 K) is approximately 18%. The 
best actual thermal efficiency of a physically constructed device is between 6 
and 10% and is obtained by operation between 80 and 1250DG F (300 and 950 K). 
    Major problems still exist in the development of materials with higher 
figures of merit that are capable of operation at higher temperature. Even 
with present materials, there are severe engineering problems. See 
THERMOELECTRICITY.  
 �34s[David C. White]
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�6v�2v�32sThermography   �21m �33s 
    A method that records pictorially the natural emission of infrared 
radiations emanating from an object or, as in the case of a medical 
application, from a person. The skin behaves like a blackbody radiator 
emitting infrared radiations constantly. Two types of thermographic systems 
are commonly used in medicine. The first one uses a thermographic machine, 
which contains one or more heat-sensing crystals of either indium antimony or 
mercury cadmium telluride. Infrared radiations originating from the surface of 
the skin are focused by an optical system onto the crystal, producing a change 
in its electrical resistance. This results in the transformation of heat 
energy into an electronic signal that can be displayed as an image on a 
cathode-ray tube. The second thermographic system relies on the properties of 
liquid cholesterol crystals that change color within narrow temperature 
ranges. These crystals are then applied to a thin foil. Within a few seconds 
after such a foil is brought into direct contact with the skin, the crystals 
respond by changing their color, thereby providing a thermal map of the skin. 
    Thermography has found its greatest application in medicine as a 
complementary technique for mammography, a method used to evaluate breast 
carcinoma.  See  MAMMOGRAPHY. 
 �34s[Nicole-Fr. Bolender]
�33s 
  
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�6v�2v�32sThermoluminescence �21m �33s
    A term sometimes used broadly to mean any luminescence appearing in a
material due to the application of heat. More frequently the term refers
specifically to the luminescence appearing as the temperature of the material
is steadily increased.
    Many solids that contain luminescent centers often also contain one or
more types of centers that can trap electrons or holes. If the solid is
exposed to light of sufficiently short wavelength or to x-rays or other
high-energy radiation, free electrons and holes are produced in the solid, and
some of these charge carriers may be trapped. Very commonly the hole is
trapped at the luminescent center itself or at some trapping site nearby while
the electron is trapped at a different site, but the reverse situation is also
quite possible. If, in the former case, the depth of the electron trap (that
is, the amount of energy required to release the electron from the trap) is
large and the temperature low, the electron will remain trapped for a long
time. If, however, the temperature of the sample is raised slowly, the
electron will receive increasing amounts of thermal energy and will eventually
escape the trap. An electron thus freed from a trap may go over to a
luminescent center and recombine with the hole trapped at or near the center.
The energy liberated by the recombination excites the center, causing it to
luminesce. For a single type of trap, the glow curve, (plot of
thermoluminescence intensity as a function of temperature) first rises,
reaches a maximum, and then decreases to zero when all the traps become
emptied.  See  HOLES IN SOLIDS; LUMINESCENCE; TRAPS IN SOLIDS.
 �34s[James H. Schulman ;Clifford C. Click]
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�6v�2v�32sThermoluminescent dosimeter   �21m �33s 
    A dosimeter that functions on the principle of thermoluminescence - the 
property of certain substances that release light upon heating after they have 
been exposed to ionizing radiation. There are four common types of 
thermoluminescent materials in use: calcium fluoride, lithium fluoride, 
calcium sulfate, and lithium borate. These materials contain trace quantities 
of impurity atoms that capture electrons released by the ionizing radiation. 
These captured electrons can later be released by heating the material and 
measuring either hte peak intensity of the light emitted (the glow curve) or 
the integrated light; the latter usually gives more accurate results, but the 
former is a simpler procedure. The thermoluminescent dosimeter has come into 
wide use since 1971 and is fast replacing the air-capacitor dosimeter and the 
film badge for personnel  monitoring. 
 �34s[Karl Z. Morgan]
�33s 
  
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�6v�2v�32sThermomagnetic effects �21m �33s
    Electrical and thermal phenomena occurring when a conductor or
semiconductor which is carrying a thermal current (that is, is in a
temperature gradient) is placed in a magnetic field.  See  SEMICONDUCTOR.
    Let the temperature gradient be transverse to the magnetic field Hsubz,
for example, along x. Then the following transverse-transverse effects are
observed:
    1. Ettingshausen-Nernst effect, an electric field along y.
    2. Righi-Leduc effect, a temperature gradient along y.
    3. An electric potential change along x, amounting to a change of
thermoelectric power.
    4. A temperature gradient change along x, amounting to a change of thermal
resistance.
    Let the temperature gradient be along H. Then changes in thermoelectric
power and in thermal conductivity are observed in the direction of H.
    For related phenomena  see	HALL EFFECT; MAGNETORESISTANCE.
 �34s[Elihu Abrahams ;Frederic Keffer]
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�6v�2v�32sThermometer   �21m �33s 
    An instrument that measures temperature. Although this broad definition 
includes all temperaturemeasuring devices, they are not all called 
thermometers. Other names have been generally adopted. For a discussion of two 
such devices  see  PYROMETER; THERMOCOUPLE. For a general discussion of 
temperature measurement  see  TEMPERATURE MEASUREMENT. 
  
    Liquid-in-glass thermometer.  This thermometer consists of a liquid-filled 
glass bulb and a connecting partially filled capillary tube. When the 
temperature of the thermometer increases, the differential expansion between 
the glass and the liquid causes the liquid to rise in the capillary. A variety 
of liquids, such as mercury, alcohol, toluene, and pentane, and a number of 
different glasses are used in thermometer construction, so that various 
designs cover diverse ranges between about -300DGF and +1200DGF (-184DG C and 
+ 649DGC). 
  
    Bimetallic thermometer.  In this thermometer the differential expansion of 
thin dissimilar metals, bonded together into a narrow strip and coiled into 
the shape of a helix or spiral, is used to actuate a pointer. In some designs 
the pointer is replaced with low-voltage contacts to control, through relays, 
operations which depend upon temperature, such as furnace controls.  
  
    Filled-system thermometer.  This type of thermometer has a bourdon tube 
connected by a capillary tube to a hollow bulb. When the system is designed 
for and filled with a gas (usually nitrogen or helium) the pressure in the 
system substantially follows the gas law, and a temperature indication is 
obtained from the bourdon tube. The temperature-pressure-motion relationship 
is nearly linear. Atmospheric pressure effects are minimized by filling the 
system to a high pressure. When the system is designed for and filled with a 
liquid, the volume change of the liquid actuates the bourdon tube. 
  
    Vapor-pressure thermal system.  This filled-system thermometer utilizes 
the vapor pressure of certain stable liquids to measure temperature. The 
useful portion of any liquid-vapor pressure curve is between approximately 15 
psia (100 kilopascals absolute) and the critical pressure, that is, the vapor 
pressure at the critical temperature, which is the highest temperature for a 
particular liquid-vapor system. A nonlinear relationship exists between the 
temperature and the vapor pressure, so the motion of the bourdon tube is 
greater at the upper end of the vapor-pressure curve. Therefore, these thermal 
systems are normally used near the upper end of their range, and an accuracy 
of 1% or better can be expected. 
  
    Resistance thermometer.  In this type of thermometer the change in 
resistance of conductors or semiconductors with temperature change is used to 
measure temperature. Usually, the temperature-sensitive resistance element is 
incorporated in a bridge network which has a reasonably constant power supply. 
Although a deflection circuit is occasionally used, almost all instruments of 
this class use a null-balance system, in which the resistance change is 
balanced and measured by adjusting at least one other resistance in the 
bridge. Metals commonly used as the sensitive element in resistance 
thermometers are platinum, nickel, and copper.  
  
    Thermistor.  This device is made of a solid semiconductor with a high 
temperature coefficient of resistance. The thermistor has a high resistance, 
in comparison with metallic resistors, and is used as one element in a 
resistance bridge. Since thermistors are more sensitive to temperature changes 
than metallic resistors, accurate readings of small changes are possible.  See 
  THERMISTOR. 
 �34s[Howard S. Bean]
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�6v�2v�32sThermonuclear reaction   �21m �33s 
    A nuclear fusion reaction which occurs between various nuclei of the light 
elements when they are constituents of a gas at very high temperatures. 
Thermonuclear reactions, the source of energy generation in the Sun and the 
stable stars, are utilized in the fusion bomb.  See HYDROGEN BOMB; NUCLEAR 
FUSION; STELLAR EVOLUTION; SUN.  
    Thermonuclear reactions occur most readily between isotopes of hydrogen 
(deuterium and tritium) and less readily among a few other nuclei of higher 
atomic number. At the temperatures and densities required to produce an 
appreciable rate of thermonuclear reactions, all matter is completely ionized; 
that is, it exists only in the plasma state. Thermonuclear fusion reactions 
may then occur within such an ionized gas when the agitation energy of the 
stripped nuclei is sufficient to overcome their mutual electrostatic 
repulsions, allowing the colliding nuclei to approach each other closely 
enough to react. For this reason, reactions tend to occur much more readily 
between energy-rich nuclei of low atomic number (small charge) and 
particularly between those nuclei of the hot gas which have the greatest 
relative kinetic energy. This latter fact leads to the result that, at the 
lower fringe of temperatures where thermonuclear reactions may take place, the 
rate of reactions varies exceedingly rapidly with temperature.  See  
CARBON-NITROGEN-OXYGEN CYCLE; KINETIC THEORY OF MATTER; MAGNETOHYDRODYNAMICS; 
NUCLEAR REACTION; PINCH EFFECT; PLASMA PHYSICS; PROTON-PROTON CHAIN. 
 �34s[Richard F. Post]
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�6v�2v�32sThermoregulation �21m �33s
    A mechanism by which mammals and birds attempt to balance heat gain and
heat loss in order to maintain a constant body temperature when exposed to
variations in cooling power of the external medium. In the lower animals body
temperature varies with changes in the environment.
    Among fishes, amphibians, and reptiles, and among invertebrates generally,
especially the insects, body heat production while at rest is insufficient to
maintain a significant difference between body temperature and ambient. Thus
the body temperature will closely approximate the ambient if there is no
external source of heat which the animal can utilize, such as radiation. Such
forms are termed poikilotherms and, incorrectly in many cases, are said to be
cold-blooded. Homeotherms, the birds and mammals, can regulate body
temperature independent of ambient conditions due to the high rate of heat
production and well-developed insulation. This gives them a great deal of
freedom with regard to activity time and place but also imposes a high food
demand, especially when temperatures drop.
    Hibernation is the solution to this problem. Some mammals and birds have
the ability to become poikilothermic during part of the year and thus abandon
thermoregulation and its high energy demands. These forms have been termed
heterotherms. Abandonment of body temperature regulation at low temperatures,
however, also means abandonment of activity.  See  HIBERNATION.
    While poikilotherms have little ability to produce body heat
metabolically, they are able to use physiological mechanisms to regulate the
rates of heat gain and loss. By varying the circulation of the body they are
able to modify the rates of heat gain and loss, and some forms may make good
use of evaporation by panting to keep the body temperature below ambient
levels during periods of heat stress. Also, the skin color of reptiles may
change, and this helps the animal to either absorb or reflect more or less of
the Sun's rays and thus modify the rate of heat gain.
    Automatic physiological temperature regulation is first seen in the
primitive mammals, the monotremes (platypus and echidna). In these forms, as
well as in certain specialized animals such as the sloth, body temperature may
be maintained at a high level only when ambient temperature is considerably
above freezing and the body temperature may vary from 80 to 98.5DGF (27 to
37DGC). In most higher vertebrates, however, body temperature levels are very
closely maintained at very specific levels, but even here the limits of
ambient temperature fluctuation that can be tolerated vary widely from species
to species. Small forms with poor insulation cannot tolerate temperatures of
41DGF (5DGC), while large animals or those with good coats of fur, feathers,
or blubber can survive at the lowest temperatures found on the Earth (about
76DGF or -60DGC).
    Temperature regulation by animals is accomplished by a careful balancing
of the rates of heat loss and gain. The physiological production of heat is
accomplished almost entirely by the oxidation of foodstuffs by the metabolic
pathways of the body. The heat released in this way is the same as if the
foodstuffs were burned. Heat that is produced is lost by evaporation due to
sweating or panting, by conduction and convection from the animal's surface,
and with the feces and urine as they are voided.
    Thermoregulation demands that the temperatures of an animal's body be
sensed and, once sensed, be adjusted to some particular level. Sensing is
accomplished by the nervous system, and adjustments are made by both the
nervous and endocrine systems. There are discrete receptors for heat and cold
which sense both absolute levels and changes of temperature. These are
scattered in a punctate fashion over the surface of the body. Their output is
fed into a thermoregulatory center in the hypothalamus, which is itself
affected by temperature. It is possible to elicit sweating or shivering by
respectively heating or cooling this area of the brain.  See  HYPOTHERMIA;
METABOLISM.
 �34s[Walter Moberly /Bradley T. Scheer]
�33s

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�6v�2v�32sThermosbaenacea   �21m �33s 
    An order of small crustaceans in the superorder Pancarida. The order was 
erected in 1927 by T. Monod and includes two families, the Thermosbaenidae and 
Monodellidae. Both families are monogeneric.  See  PANCARIDA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sThermostat   �21m �33s 
    An instrument which directly or indirectly controls one or more sources of 
heating and cooling to maintain a desired temperature. To perform this 
function a thermostat must have a sensing element and a transducer. The 
sensing element measures changes in the temperature and produces a desired 
effect on the transducer. The transducer converts the effect produced by the 
sensing element into a suitable control of the device or devices which affect 
the temperature. 
    The most commonly used principles for sensing changes in temperature are 
(1) unequal rate of expansion of two dissimilar metals bonded together 
(bimetals), (2) unequal expansion of two dissimilar metals (rod and tube), (3) 
liquid expansion (sealed diaphragm and remote bulb or sealed bellows with or 
without a remote bulb), (4) saturation pressure of a liquidvapor system 
(bellows), and (5) temperature-sensitive resistance element. 
    The most commonly used transducers are (1) switches that make or break an 
electric circuit, (2) potentiometer with a wiper that is moved by the sensing 
element, (3) electronic amplifier, and (4) pneumatic actuator. 
    The most common thermostat application is for room temperature control. 
The illustration shows a typical on-off heating-cooling room thermostat. In a 
typical application the thermostat controls a gas valve, oil burner control, 
electric heat control, cooling compressor control, or damper actuator. 
    Thermostats are also used extensively in safety and limit application. 
Thermostats are generally of the following types: insertion types that are 
mounted on ducts with the sensing element extending into a duct; immersion 
types that control a liquid in a pipe or tank with the sensing element 
extending into the liquid; and surface types in which the sensing element is 
mounted on a pipe or similar surface.  See  COMFORT HEATING; FURNACE; OIL 
BURNER.  
 �34s[Nathaniel Robbins, Jr.]
�33s 
  
  
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�6v�2v�32sThermotherapy   �21m �33s 
    The treatment of disease by heat of any kind; it involves the local or 
general application of heat to the body. 
    Therapeutic heat may be applied to a very small pinpoint area (cautery), 
regionally to a part or member of the body, or generally to the entire body. 
Electrocautery and chemical cautery are commonly used surgical techniques. 
Regional heating is more commonly used therapeutically and has both local and 
distant effects. Generalized heating that produces a rise in body temperature 
is referred to as hyperthermia. 
  
    Physiological effects.  The application of heat creates soothing, 
sedative, and analgesic effects, although the exact mechanisms of these 
effects are unknown. Additionally, heat acts as a vasodilator. Regardless of 
the method of heat generation, the result within the heated tissues is 
essentially the same. Heat produces a rise in temperature with an associated 
increase in metabolism. There is a relative increase in the accumulation of 
metabolic wastes, such as acid metabolites and carbon dioxide. Increased local 
circulation due to dilation of arterioles and capillaries leads to improved 
cellular nutrition, as well as improved exchange of wastes. Also, more 
phagocytes and antibodies are brought with the blood into the area. The local 
temperature rises until a point is reached at which maximum vasodilatation 
occurs; then the local temperature falls slightly. 
    Regional heating produces remote effects. Thus heating of one leg may 
cause vasodilatation and an increase in skin temperature in the other leg or 
in the hands (consensual heating). These are responses to the body mechanisms 
of heat regulation and are mediated through the central nervous system via the 
heat-regulatory center of the hypothalamus.  See  THERMOREGULATION. 
  
    Methods.  Modalities, the clinical methods employed for thermotherapy, 
vary with regard to techniques of heat production and application. In general, 
they are classified as follows in accordance with their physical properties: 
radiant, conductive, and conversive. 
    Radiant heat is derived from the infrared portion of the electromagnetic 
spectrum. Specialized luminous and nonluminous generators are readily 
available for professional use. More commonly used sources such as infrared 
bulbs or aggregates of bulbs (bakers) provide a soothing if not very 
penetrating source of heating. However, circulating blood in the area heated 
may transfer heat to deeper areas.  See  ELECTROMAGNETIC RADIATION. 
    Any material that has a higher temperature than that of the skin with 
which it is in contact will transfer heat to the skin by conduction. Hot air, 
steam, water, paraffin, silicon, and electric heating pads are all familiar 
clinically used examples. Hot air and steam baths have long been used to 
induce cutaneous vasodilatation, sweating, relaxation of muscles, and a sense 
of well-being. However, overexposure to these modalities may precipitate heat 
stroke and collapse. With its high specific heat, water has been used for 
centuries for heating. Hot tub baths and natural heat from mineral springs are 
still being used therapeutically. 
    Conversive heating involves the conversion of some other form of energy 
into heat. Most commonly used sources of energy are radio waves in the 
shortwave and microwave bands of the electromagnetic spectrum and ultrasonic 
waves (mechanical). Shortwave diathermy utilizes electromagnetic energy at 
wavelengths of 3 - 30 m and frequencies of 10 - 100 MHz. Human tissues are 
heated with high-frequency currents either through conduction or induction. 
Heat generated in this manner penetrates to a depth of about 1 in. (2 - 3 cm) 
and spreads out over a fairly large area. Microwave diathermy consists of 
electromagnetic energy therapeutically applied at a wavelength of 12.2 cm and 
a frequency of 2450 MHz. It effectively penetrates up to a depth of 2 in. (5 
cm). This method makes it possible to raise the temperature of fat, muscle, 
and connective tissue in or near bone. Whereas shortwave diathermy spreads 
widely in the tissues of the body, microwaves are quasi-optical and can be 
focused and directed for the heating of small selective areas. 
    Ultrasonic therapy or ultrasound diathermy involves the application of 
high-frequency sound waves and the conversion of this mechanical energy to 
heat. This method is well suited to local thermotherapy. Therapeutic  
ultrasound generators produce frequencies varying from 0.7 to 1.0 MHz. With 
sufficient intensity it is possible to penetrate to depths of 2 - 3 in. (5 - 8 
cm). Ultrasound is selective in its heating properties, is reflected at 
interfaces, and is mechanically destructive in excessive dosage. 
 �34s[Joseph R. O'Connor]
�33s 
  
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�6v�2v�32sThevenin's theorem (electric networks)   �21m �33s 
    This theorem, from electric circuit theory, is also known as the Helmholtz 
or Helmholtz-Thevenin theorem, since H. Helmholtz stated it in an earlier form 
prior to M. L. Thevenin. Closely related is the Norton theorem, which will 
also be discussed. Laplace transform notation will be used.  See  LAPLACE 
TRANSFORM.  
    Thevenin's theorem states that at a pair of terminals a network composed 
of lumped, linear circuit elements may, for purposes of analysis of external 
circuit or terminal behavior, be replaced by a voltage source V(s) in series 
with a single impedance Z(s). The source V(s) is the Laplace transform of the 
voltage across the pair of terminals when they are open-circuited; Z(s) is the 
transform impedance at the two terminals with all independent sources set to 
zero (Fig. 1). 
    Norton's theorem states that a second equivalent network consists of a 
current source I(s) in parallel with an impedance Z(s). The impedance Z(s) is 
identical with the Thevenin impedance, and I(s) is the Laplace transform of 
the current between the two terminals when they are short-circuited (Fig. 2). 
    Thevenin's and Norton's equivalent networks are related by the equation 
V(s) = Z(s) PRODUCT I(s). 
    These theorems are useful for the study of the behavior of a load 
connected to a (possibly complex) system that is supplying electric power to 
that load.  See  NETWORK THEORY; SUPERPOSITION THEOREM (ELECTRIC NETWORKS). 
 �34s[Edwin C. Jones, Jr.]
�33s 
  
  
  
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�6v�2v�32sThiamine �21m �33s
    A water-soluble vitamin found in many foods; pork, liver, and whole grains
are particularly rich sources. It is also known as vitamin B1 or aneurin. The
structural formula of thiamine is shown below.

				   CH3
				   |
		 N==C--NH2	   C===C--CH2--CH2OH
		 |  ||		/      |
	    H3C--C  C--CH2--N	       |
		 |  ||	    +	\\     |
		 H--CH		   CH--S



    Thiamine deficiency is known as beriberi in humans and polyneuritis in
birds. Muscle and nerve tissues are affected by the deficiency, and poor
growth is observed. People with beriberi are irritable, depressed, and weak.
They often die of cardiac failure. Wernicke's disease observed in alcoholics
is associated with a thiamine deficiency. This disease is characterized by
brain lesions, liver disease, and partial paralysis, particularly of the motor
nerves of the eye. As is the case in all B vitamin diseases, thiamine
deficiency is usually accompanied by deficiencies of other vitamins.  See
BERIBERI.
 �34s[Stanley N. Gershoff]
�33s

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�6v�2v�32sThiazole   �21m �33s 
    One of a class of organic heterocyclic compounds (sometimes specified as 
1,3-thiazoles) in which a five-membered diunsaturated ring contains one atom 
of nitrogen and, in a nonadjacent position, one atom of sulfur. Important 
thiazole derivatives include vitamin B1 (thiamine) and sulfathiazole. Several 
valuable dyes and rubber-vulcanizing accelerators contain the benzothiazole 
nucleus. Penicillin is a thiazolidine derivative.  See  AZOLE; HETEROCYCLIC 
COMPOUNDS.  
    The thiazole ring is a resonance-stabilized aromatic system. The ring is 
relatively resistant to hydrolysis and to disruptive oxidation with nitric 
acid. The parent compound, thiazole, bp 117DGC (243DGF), is a colorless, 
water-soluble liquid with an odor resembling that of pyridine. Although 
thiazole is a weak base, acids form simple salts, and alkylating agents form 
quaternary salts. 
 �34s[Walter J. Gensler]
�33s 
  
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�6v�2v�32sThickening   �21m �33s 
    The production of a concentrated slurry from a dilute suspension of solid 
particles in a liquid. In practice, a thickener is usually expected also to 
produce a clear liquid, and therefore thickening includes clarification as a 
concurrent objective. Thickening and clarification are applications of 
sedimentation, and both are representative of a larger group of industrial 
processes called mechanical separations.  See  CLARIFICATION; MECHANICAL 
SEPARATION TECHNIQUES. 
 �34s[Warren L. McCabe]
�33s 
  
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�6v�2v�32sThigmotrichida   �21m �33s 
    A restricted group of forms constituting an order of the Holotrichia and 
generally found in association with mollusks from both fresh and salt water. 
Some are mouthless. Those species with a cytostome are generally equipped with 
a buccal ciliature which indicates an advance over the primitive hymenostome 
arrangement.  See  HOLOTRICHIA; HYMENOSTOMATIDA. 
 �34s[John O. Corliss]
�33s 
  
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�6v�2v�32sThinner   �21m �33s 
    A material used in paints and varnishes to adjust the consistency for 
application. Thinners are usually solvents for the vehicle used in the coating 
and are expected to evaporate after application. Water is used as a thinner in 
emulsion paints and in certain water-soluble paints such as watercolors and 
calcimines. Petroleum fractions are most commonly used for oil and resin 
coatings. Stronger solvents contain substantial amounts of aromatic 
hydrocarbons and may be derived from petroleum or coal tar. 
    Since numerous coating resins are not sufficiently soluble in 
hydrocarbons, other materials or mixtures must be used. These include alcohols 
such as denatured ethyl or isopropyl alcohols for shellac, esters such as amyl 
acetate for nitrocellulose, and ketones and other compounds for acrylic and 
vinyl resins. Chlorinated hydrocarbons are used for some materials which are 
otherwise hard to dissolve, but toxicity limits their usefulness.  See  PAINT; 
SOLVENT; SURFACE COATING; TURPENTINE; VARNISH. 
 �34s[C. R. Martinson ;C. W. Sisler]
�33s 
  
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�6v�2v�32sThioaldehyde and thioketone �21m �33s
    Organosulfur compounds of structure

				    H		  R'
				    |		  |
				 R--C==S  and  R--C==S

    They are isolable only in rare instances because, unlike the oxygen
analogs (aldehydes and ketones), thioaldehydes and thioketones tend strongly
to polymerize.	See  ORGANOSULFUR COMPOUND.
 �34s[Norman Kharasch]
�33s

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�6v�2v�32sThio compounds   �21m �33s 
    Organosulfur compounds in which one or more sulfur atoms replace oxygen. 
Because of the frequent use of this nomenclature and the large number of 
structural variations involved, the thio names may appear confusing but are 
generally understandable in specific cases. Thiols and thiophenols are the 
sulfur analogs of alcohols and phenols. Dithio and trithio compounds have two 
or three sulfur atoms, corresponding to the oxygen analogs. Acyl-SH compounds 
are the thio acids, for example, thioacetic acid, CH3C(DOUBLE BARO)SH, of 
which many are known. Dithio acids, R--C(DOUBLE BARS)--SH, may, for example, 
be made from RMgX and carbon disulfide, CS2. Trithiocarbonates are derivatives 
of SDOUBLE BARC(SH)2, the sulfur analog of carbonic acid. Dithiocarbamates, 
related to carbamic acids, are readily obtained in the form of salts. 
    Many other compounds which carry the thio name are oxygen analogs. For 
example, thiocyanates, isothiocyanates, thiourea, thioamides, dithioesters, 
and thio acid chlorides.  See  ORGANOSULFUR COMPOUND. 
 �34s[Norman Kharasch]
�33s 
  
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�6v�2v�32sThiocyanate   �21m �33s 
    One of a group of compounds, both organic and inorganic, which contain 
the--SCN group and are derived from thiocyanic acid, HSCN. Like cyanic acid, 
thiocyanic acid may exist in two forms, H--S--CTRIPLE BONDN and SDOUBLE 
BARN--H. The latter form is called isothiocyanic acid and gives rise to 
isothiocyanates.  
    The inorganic thiocyanates resemble the cyanides and halides because most 
of the metal salts are water-soluble (except lead, mercury, silver, and copper 
salts), and many complexes are formed with excess thiocyanate.  See  CYANIDE; 
SULFUR.   
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sThioether   �21m �33s 
    One of a group of organosulfur compounds that are also called sulfides, 
RSRPRIME. The simplest, dimethyl sulfide, CH3--S--CH3, is the precursor of 
dimethyl sulfoxide, a useful solvent and chemical reactant. Some amino acids, 
such as methionine and lanthionine, are also sulfides. Mustard gas, 
ClCH2CH2SCH2CH2Cl, is a well-known vesicant. The thioethers bear a formal 
resemblance to the oxygen ethers, and may be synthesized by analogous methods. 
  See  AMINO ACIDS; ETHER; ORGANOSULFUR COMPOUND. 
 �34s[Norman Kharasch]
�33s 
  
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�6v�2v�32sThiophene �21m �33s
    An organic heterocyclic compound containing a diunsaturated ring of four
carbon atoms and one sulfur atom.  See	HETEROCYCLIC COMPOUNDS.
    Thiophene (I), methylthiophenes, and other alkylthiophenes are found in
relatively small amounts in coal tar and petroleum. Thiophene accompanies
benzene in the fractional distillation of coal tar. 2,5-Dithienylthiophene
(II) has been found in the marigold plant. Biotin, a water-soluble vitamin, is
a tetrahydrothiophene derivative.
    The parent compound (I) is nearly insoluble in water, with mp -38.2DGC
(-36.8DGF), bp 84.2DGC (183.6DGF), and specific gravity (20/4) 1.0644.
Thiophene is considered to be an aromatic compound. Thiophenes are stable to
alkali and other nucleophilic agents, and are relatively resistant to
disruption by acid.  See  AROMATIC HYDROCARBON; ORGANOSULFUR COMPOUND.
 �34s[Walter J. Gensler ;Martin Stiles]
�33s

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�6v�2v�32sThiosulfate �21m �33s
    A negative ion having the formula S2O3(sup2-), which is derived from
thiosulfuric acid, H2S2O3, an unstable acid. The ion is actually related to
the sulfate ion, with the substitution of a sulfur atom for an oxygen, as
shown in the formula below. The central sulfur atom has an oxidation number of

				    O			(SUP 2-)
				    | (UP ARROW)
				O-- S---O
				    | (DOWN ARROW)
				    S

6+, whereas the second has an oxidation number of 2-.
    Sodium thiosulfate, also termed hypo, is used in photography to ``fix''
films by dissolving the unreacted silver halide.
 �34s[E. Eugene Weaver]
�33s

A700000002115�254s�5v�1v1754
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�6v�2v�32sThiourea �21m �33s
    A crystalline, colorless solid (prisms or needles) having formula (I) and
melting point 180 - 182DGC (356 - 360DGF).  Thiourea is relatively insoluble

				    S
				   ||
			       H2N--C--NH2
				   (l)


in water and only slightly soluble in ether. The three most common
methods of preparation are (1) heating ammonium thiocyanate, NH4CNS, to about
180DGC (356DGF); (2) the action of hydrogen sulfide at about 180DGC (356DGF)
on calcium cyanamide; and (3) the reaction of dicyandiamide and ammonium
sulfide at 60 - 70DGC (140 - 158DGF).
    Condensation of thiourea with substituted malonic esters gives
thiobarbituric acid derivatives. Of the thiobarbiturates, with
1-methylbutylethylthiobarbiturate as the sodium salt, sodium pentothal (II) is

				   O
				  ||
			  CH3CH2   C--NH
			      \  /	 \
			       C	  C--S	Na
			    /	 \	//
		  CH3CH2CH2CH	    C--N
			   |	   ||
			   CH3	    O
				   (II)


much used in anesthetic premedication to reduce the amount of general
anesthetic needed.
 �34s[Evans B. Reid]
�33s

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�6v�2v�32sThirst   �21m �33s 
    The sensation aroused by a relative or an absolute lack of body water, 
which normally though not necessarily induces the behavior of drinking water. 
Theories of thirst in which dryness of the mouth and throat plays an essential 
role are as old as scientific interest in the subject itself. However, since 
it was established that there is a thirst center in the hypothalamus, the dry 
mouth theory has become less influential. 
    Most evidence is in favor of cellular dehydration as an important cause of 
thirst. However, loss of extracellular fluid can also give rise to thirst. 
Three procedures have established that thirst caused by extracellular 
dehydration is distinct from thirst caused by cellular dehydration: (1) sodium 
depletion by dieting and other means which results in a secondary loss of 
extracellular water; (2) the more direct and immediate removal of 
extracellular fluid by hemorrhage; and (3) mimicking the effects of severe 
dehydration on the circulation by interfering with the flow of blood in 
various blood vessels. All cause thirst and a delayed increase in sodium 
appetite, and all either overhydrate or do not alter the water content of the 
cellular compartment. Little is known about mechanisms, but extracellular 
deficits are probably detected by stretch receptors which lie in the walls of 
the low-pressure capacitance vessels (that is, the large veins near the heart) 
and in the atria.  See  OSMOREGULATORY MECHANISMS. 
 �34s[Elliott M. Blass]
�33s 
  
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�6v�2v�32sThomson effect   �21m �33s 
    A phenomenon discovered in 1854 by William Thomson, later Lord Kelvin. He 
found that there occurs a reversible transverse heat flow into or out of a 
conductor of a particular metal, the direction depending upon whether a 
longitudinal electric current flows from colder to warmer metal or from warmer 
to colder. Any temperature gradient previously existing in the conductor is 
thus modified if a current is turned on. The Thomson effect does not occur in 
a current-carrying conductor which is initially at uniform temperature.  See  
THERMOELECTRICITY.  
 �34s[John W. Stewart]
�33s 
  
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�6v�2v�32sThoracica   �21m �33s 
    The major order of the crustacean subclass Cirripedia. The adult animals 
are permanently attached. The mantle is usually reinforced by calcareous 
plates. Six pairs of biramous cirri are present, and the abdomen is absent or 
represented by caudal appendages. Antennules are present in the adult, and 
cement glands are strongly developed. Most species are hermaphroditic. 
    Thoracica are subdivided into three suborders: Lepadomorpha, stalked or 
goose barnacles; Balanomorpha, the common acorn barnacles; and Verrucomorpha, 
a rare group of asymmetric barnacles.  See  BALANOMORPHA; BARNACLE; 
CIRRIPEDIA; LEPADOMORPHA; VERRUCOMORPHA. 
 �34s[H. G. Stubbings]
�33s 
  
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�6v�2v�32sThorianite   �21m �33s 
    A radioactive mineral with the idealized composition ThO2 (thorium 
dioxide) and isostructural with uraninite (pitchblende). Rare earths and 
uranium are often present in variable amounts, together with small amounts of 
radiogenic lead. Thorianite usually occurs as worn cubic crystals. The 
hardness is about 7 on Mohs scale, and the specific gravity is 9.7 - 9.8. The 
color is brownish black to reddish brown, and the luster usually is resinous. 
Thorianite is a primary mineral found chiefly in pegmatites. It is best known 
as a detrital mineral. It has been obtained commercially from detrital 
deposits and pegmatities in Madagascar and Ceylon.  See  LEAD ISOTOPES 
(GEOCHEMISTRY); NUCLEAR FUELS; PEGMATITE; RADIOACTIVE MINERALS; RARE-EARTH 
ELEMENTS; THORIUM; URANINITE; URANIUM. 
 �34s[Clifford Frondel]
�33s 
  
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�6v�2v�32sThorite �21m �33s
    A mineral, thorium silicate. The idealized chemical formula of thorite is
ThSiO4. All natural material departs widely from this composition owing to the
partial substitution of uranium, rare earths, calcium, and iron for thorium.
The specific gravity ranges between about 4.3 and 5.4. The hardness on Mohs
scale is about 4 1/2. The color commonly is brownish yellow to brownish black
and black.
    Vein deposits containing thorite occur in Colorado, Idaho, and Montana. A
vein deposit of monazite containing thorium is mined at Steenkampskraal near
Van Rhynsdorp, Cape Province, South Africa.  See  METAMICT STATE; RADIOACTIVE
MINERALS; SILICATE MINERALS. THORIUM.
 �34s[Clifford Frondel]
�33s

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�6v�2v�32sThorium �21m �33s
    A chemical element, Th, atomic number 90. Thorium is a member of the
actinide series of elements. It is radioactive with a half-life of about 1.4 x
10 (sup 10) years.
    Thorium oxide compounds are used in the production of incandescent gas
mantles. Thorium oxide has also been incorporated in tungsten metal, which is
used for electric light filaments. It is employed in catalysts for the
promotion of certain organic chemical reactions and has special uses as a
high-temperature ceramic material. The metal or its oxide is employed in some
electronic tubes, photocells, and special welding electrodes. Thorium has
important applications as an alloying agent in some structural metals. Perhaps
the major use for thorium metal, outside the nuclear field, is in magnesium
technology. Thorium can be converted in a nuclear reactor to uranium-233, an
atomic fuel. The energy available from the world's supply of thorium has been
estimated as greater than the energy available from all of the world's
uranium, coal, and oil combined.
    Monazite, the most common and commercially most important thorium-bearing
mineral, is widely distributed in nature. Monazite is chiefly obtained as a
sand, which is separated from other sands by physical or mechanical means.
See  MONAZITE.
    Thorium has an atomic weight of 232. The temperature at which pure thorium
melts is not known with certainty; it is thought to be about 1750DGC
(3182DGF). Good-quality thorium metal is relatively soft and ductile. It can
be shaped readily by any of the ordinary metal-forming operations. The massive
metal is silvery in color, but it tarnishes on long exposure to the
atmosphere; finely divided thorium has a tendency to be pyrophoric in air.
    All of the nonmetallic elements, except the rare gases, form binary
compounds with thorium. With minor exceptions, thorium exhibits a valence of
4+ in all of its salts. Chemically, it has some resemblance to zirconium and
hafnium. The most common soluble compound of thorium is the nitrate which, as
generally prepared, appears to have the formula Th(NO3)4 PRODUCT 4H2O. The
common oxide of thorium is ThO2, thoria. Thorium combines with halogens to
form a variety of salts. Thorium sulfate can be obtained in the anhydrous form
or as a number of hydrates. Thorium carbonates, phosphates, iodates,
chlorates, chromates, molybdates, and other inorganic salts of thorium are
well known. Thorium also forms salts with many organic acids, of which the
water-insoluble oxalate, Th(C2O4)2 PRODUCT 6H2O, is important in preparing
pure compounds of thorium.  See ACTINIDE ELEMENTS; RADIOACTIVITY.
 �34s[Harley A. Wilhelm]
�33s

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�6v�2v�32sThreading   �21m �33s 
    The forming of a ridge and valley of uniform cross section which spiral 
about the inner or outer diameter of a cylinder or cone in an even and 
continuing manner. The work must be produced with sufficient uniformity and 
accuracy so that the resulting threaded part will accomplish its intended 
purpose of fastening, transmitting motion or power, or measuring. 
  
    Although machine threading is done on various machine tools, special 
threading machines may be used for quantity production (see illustration).  
See  MACHINING OPERATIONS; SCREW THREADS. 
 �34s[Alan H. Tuttle]
�33s 
  
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�6v�2v�32sThreonine   �21m �33s 
    An amino acid which is considered essential for normal growth of animals. 
Threonine is a biosynthetic precursor of isoleucine in microorganisms. 
Threonine is biosynthesized from aspartic acid. 
    The major pathway in metabolic degradation starts with the nonoxidative 
deamination to ALPHA-ketobutyric acid. This keto acid may be oxidatively 
decarboxylated to propionyl-CoA, or transaminated to form ALPHA-aminobutyric 
acid. Another pathway of threonine degradation involves an initial cleavage to 
glycine and acetaldehyde.  See  AMINO ACIDS. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sThrip   �21m �33s 
    A small, slender-bodied insect of the order Thysanoptera. There are about 
500 species, all in North America. Wings may be present or absent, but when 
present the usual number is four (see illustration). The mouthparts are 
suctorial, with a stout proboscis. Males are rare or unknown in some species 
and parthenogenesis is common. Females lacking an ovipositor lay their eggs 
under bark or in crevices, while those species possessing an ovipositor 
deposit the eggs in plant tissues. 
  
    Many thrips are economically important as serious pests of cultivated 
plants since they injure or destroy the plants in feeding and breeding 
activities. Some species harbor plant viruses which they inoculate into 
plants. A few species are predacious and feed on small arthropods such as 
mites, aphids, or other species of thrips. Some species may bite humans.  See  
THYSANOPTERA.  
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sThroat   �21m �33s 
    The region that includes the pharynx, the larynx, and related structures. 
Both the nasal passages and the oral cavity open into the pharynx, which also 
contains the openings of the Eustachian tubes from the ears. The lower portion 
of the pharynx leads into the esophagus and the trachea or windpipe. The 
rather funnel-shaped pharynx is suspended from the base of the skull and the 
jaws; it is surrounded by three constrictor muscles that function primarily in 
swallowing.  See  EAR; PHARYNX. 
    The larynx, or voice box, is marked externally by the shield-shaped 
thyroid cartilage which forms the Adam's apple. The larynx contains the vocal 
cords that act as sphincters for air regulation and permit phonation. The 
lower end of the larynx is continuous with the trachea, a tube composed of 
cartilaginous rings and supporting tissues.  See  LARYNX. 
    The term throat is also used in a general sense to denote the front 
(ventral side) of the neck. 
 �34s[Thomas S. Parsons]
�33s 
  
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�6v�2v�32sThrombosis   �21m �33s 
    The formation of a thrombus. A thrombus is a solid body formed during 
life, composed of the elements of the blood - platelets, fibrin, red cells, 
and leukocytes. 
    Thrombosis is essentially platelet deposition and may occur on any blood 
vessel wall where the endothelium is damaged. Since platelets release 
thromboplastinogen, which in turn activates the clotting mechanism, clotting 
and thrombosis may occur together. Platelets exhibit a tendency to stick 
together. This tendency is increased if their number is increased, as in 
thrombocytosis, if the velocity of the stream is decreased below a certain 
speed, or if the endothelium is roughened. The same factors promote thrombus 
formation. Heparin, however, causes the platelets to lose their stickiness. 
The flow of blood is slower in the veins, which may be one reason for the 
greater frequency of thrombi there. Other determinative factors, as yet 
unknown, must also come into play. 
    With occlusion of a vessel, true thrombosis ceases, but clotting 
continues. The clot may be propagated to the next side branch. The propagated 
clot may float freely in the vessel, anchored only by the thrombus at the 
base. Under these conditions a piece may break off and travel in the 
bloodstream, forming an embolus.  See  BLOOD; EMBOLISM. 
    Many of the possible sequelae of thrombosis of a vessel are obvious. 
Thrombi in certain critical vessels can be followed by dire consequences. 
Arterial thrombosis can result in an infarct of the region supplied by the 
vessel if the collateral circulation is inadequate. With mesenteric 
thrombosis, gangrene of the bowel may ensue. Under these circumstances 
surgical intervention must follow promptly or death will result. Occlusion of 
any vein causes stasis of blood in the region, with an acute passive 
congestion of the region being drained. Thrombosis of varices of the rectum or 
lower extremities may result in local necrosis and ulceration.  See  
INFARCTION.  
 �34s[Romeo A. Vidone]
�33s 
  
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�6v�2v�32sThrottled flow   �21m �33s 
    Flow which is forced to pass through a restricted area, where the velocity 
must be increased. This is also known as choked flow. Most of the kinetic 
energy produced by reduction of pressure in passing through the constriction 
is generally converted into thermal energy by turbulent eddying. The net 
result is a loss in mechanical energy in the system. When a gas is throttled, 
as by a globe valve, the velocity a short distance downstream from the valve 
is only a little higher than before the throttling section in most cases, the 
process being one of constant enthalpy. By introducing mechanical energy 
losses into a flow system by a throttling valve, the amount of flow may be 
controlled.  See  ISENTROPIC PROCESS. 
 �34s[Victor L. Streeter]
�33s 
  
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�6v�2v�32sThrust   �21m �33s 
    In aviation context, the force that propels an aircraft or a missile. 
Thrust is usually expressed in pounds (English gravitational system) or in 
kilograms (metric gravitational system). It can be expressed in poundals 
(English absolute system) or newtons or dynes (metric absolute system). The 
power of turbojets, ramjets, and rockets is usually expressed in thrust terms. 
The product of thrust times aircraft velocity is thrust power and represents 
the useful work supplied by the power plant.  See  JET PROPULSION; RAMJET; 
RECIPROCATING AIRCRAFT ENGINE; ROCKET; TURBOJET. 
 �34s[Robert R. Hibbard]
�33s 
  
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�6v�2v�32sThulium �21m �33s
    A chemical element, Tm, atomic number 69, atomic weight 168.934. It is a
rare metallic element belonging to the rare-earth group. The stable isotope
(sup 169)Tm makes up 100% of the naturally occurring element.
    The salts of thulium posses a pale green color and the solutions have a
slight greenish tint. The metal has a high vapor pressure at the melting
point. When (sup 169)Tm is irradiated in a nuclear reactor, (sup 170)Tm
is formed. The isotope then emits strongly an 84-keV x-ray, and this material
is useful in making small portable x-ray units for medical use.  See
RARE-EARTH ELEMENTS.
 �34s[Frank H. Spedding]
�33s

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�6v�2v�32sThunder   �21m �33s 
    An acoustic wave caused by lightning. When lightning occurs, the surge of 
current in the return stroke suddenly heats the air in the lightning channel. 
The resulting expansion of the gaseous medium is similar to an explosion and 
produces a shock wave. The shock wave can be thought of as an expanding 
cylinder surrounding the original discharge channel. When the atmospheric 
pressure along the cylinder axis falls to the levels it had before the 
lightning flash, the shock wave becomes an acoustic wave called thunder.  See  
LIGHTNING; THUNDERSTORM. 
 �34s[Louis J. Battan]
�33s 
  
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�6v�2v�32sThunderstorm �21m �33s
    A convective storm accompanied by lightning and thunder and a variety of
weather such as locally heavy rainshowers, hail, high winds, sudden
temperature changes, and occasionally tornadoes. The characteristic cloud is
the cumulonimbus or ``thunderhead,'' a towering cloud, often with an
anvil-shaped top. A host of accessory clouds, some attached and some detached
from the main cloud, are often observed in conjunction with cumulonimbus.  See
  CLOUD.
    Thunderstorms are manifestations of convective overturning of deep layers
in the atmosphere and occur in environments in which the decrease of
temperature with height (lapse rate) is sufficiently large to be conditionally
unstable and the air at low levels is moist. In such an atmosphere, a rising
air parcel, given sufficient lift, becomes saturated and cools less rapidly
than it would if it remained unsaturated because the released latent heat of
condensation partly counteracts the expansional cooling. The rising parcel
reaches levels where it is warmer (by perhaps as much as 50DGF or 10DGC) and
less dense than its surroundings, and buoyancy forces accelerate the parcel
upward. The rising parcel is decelerated and its vertical ascent arrested at
altitudes where the lapse rate is stable, and the parcel becomes denser than
its environment.
    Thunderstorms are most frequent in the tropics, and rare poleward of 60DG
latitude. Thunderstorms occur at all hours of day and night, but are most
common during late afternoon because of the influence of diurnal surface
heating.
    Thunderstorms are considered severe when they produce winds greater than
58 mi/h (26 m/s or 50 knots), hail larger than 3/4 in. (19 mm) in diameter, or
tornadoes. While thunderstorms are generally beneficial because of their
needed rains (except for occasional flash floods), severe storms have the
capacity of inflicting utter devastation over narrow swaths of the
countryside.  See  HAIL; SQUALL; TORNADO.
 �34s[Robert Davies-Jones]
�33s
    A thunderstorm produces lightning and is thus highly electrified. A
thunderstorm can be considered as a current generator that separates charge,
resulting in strong electric fields and electricity for lightning and various
conduction currents within, below, and above the storm. Electric currents from
thunderstorms to the earth and ionosphere maintain the fair weather state of
the earth and atmosphere. Typical maximum electric fields are 1.8 to 4.6 kV/ft
(6 to 15 kV/m) at the earth below the storm and 30 to 120 kV/ft (100 to 400
kV/m) or more inside thunderstorms. Electric fields at the ground are less
intense because of corona discharge from pointed objects. Lightning, the most
obvious and deadly manifestation of storm electrification, can propagate long
distances, with at least one flash 105 mi (170 km) long having been observed
with radar.  See  ATMOSPHERIC ELECTRICITY; LIGHTNING; THUNDER.
 �34s[W. David Rust]
�33s

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�6v�2v�32sThymosin   �21m �33s 
    A polypeptide hormone synthesized and secreted by the endodermally derived 
reticular cells of the thymus gland. 
    Thymosin exerts its actions in several loci: (1) in the thymus gland, 
either on precursor stem cells derived from fetal liver or from bone marrow, 
or on immature thymocytes, and (2) in peripheral sites, on either 
thymic-derived lymphoid cells or on precursor stem cells. The precursor stem 
cells, which are immunologically incompetent whether in the thymus or in 
peripheral sites, have been designated as predetermined T cells or T0 cells, 
and mature through stages termed T1 and T2, each reflecting varying degrees of 
immunological competence. Thymosin promotes or accelerates the maturation of 
T0 cells to T1 cells as well as to the final stage of a T2. In addition to 
this maturation influence, the hormone also increases the number of total 
lymphoid cells by accelerating the rate of proliferation of both immature and 
mature lymphocytes.  See  IMMUNITY; THYMUS GLAND. 
 �34s[Abraham White]
�33s 
  
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�6v�2v�32sThymus gland   �21m �33s 
    An important central lymphoid organ in the neck or upper thorax of all 
vertebrates from elasmobranchs to mammals. The thymus gland is most prominent 
during early life. In many laboratory species of mammals and in humans it 
reaches its greatest relative weight at the time of birth, but its absolute 
weight continues to increase until the onset of puberty. Thereafter, it begins 
to undergo an involution and progressively decreases in size throughout adult 
life.  
    The thymic stem cells generate a large population of small lymphocytes 
(thymocytes) through a series of mitotic divisions. Simultaneously these 
dividing lymphocytes show evidence of cellular differentiation within the 
special thymic environment. During this division and maturation phase the 
developing thymocytes undergo an intrathymic migration from the peripheral 
cortical area to the medullary core of the organ. Some thymocytes degenerate 
within the organ, but many enter the circulating blood and lymph systems at 
various stages of maturity. A small percentage of the T lymphocyte population 
(5 - 10%) within the thymus is antigenically competent and capable of 
recognizing antigenic determinents on foreign cells or substances. Some of the 
T lymphocytes have the capacity to lyse the foreign tissue cells, while others 
are involved in recognizing the ``foreignness'' of the antigens and assisting 
a second subpopulation of bone-marrow-derived lymphocytes (B lymphocytes) to 
respond to the antigen by producing a specific antibody. These two types of 
immunocompetent T lymphocytes are called killer cells and helper cells, 
respectively. They are involved in both tissue transplantation and humoral 
antibody responses. On the other hand, the vast majority of the thymic 
lymphocytes are immunologically incompetent (90 - 95%). Some thymocytes are 
thought to give rise to the smaller pool of immunocompetent T lymphocytes, but 
many emigrate into the circulating blood.  See  CELLULAR IMMUNITY; LYMPHATIC 
SYSTEM; THYMOSIN. 
 �34s[Charles E. Slonecker]
�33s 
  
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�6v�2v�32sThyrocalcitonin   �21m �33s 
    A hormone, the only known secretory product of the parenchymal or C cells 
of the mammalian thyroid and of the ultimobranchial glands of lower forms.  
See  ULTIMOBRANCHIAL BODIES. 
    In conjunction with the parathyroid hormone, thyrocalcitonin is of prime 
importance in regulating calcium and phosphate metabolism. Its major function 
is to protect the organism from the dangerous consequences of elevated blood 
calcium. Its sole known effect is that of inhibiting the resorption of bone. 
It thus produces a fall in the concentration of calcium and phosphate in the 
blood plasma because these two minerals are the major constitutents of bone 
mineral and are released into the bloodstream in ionic form when bone is 
resorbed.  See  BONE; CALCIUM METABOLISM; PARATHYROID GLAND; PARATHYROID 
HORMONE.  
    Thyrocalcitonin also causes an increased excretion of phosphate in the 
urine under certain circumstances, but a question remains as to whether this 
is a direct effect of the hormone upon the kidney or an indirect consequence 
of the fall in blood calcium which occurs when the hormone inhibits bone 
resorption.  See  PHOSPHATE METABOLISM; THYROID GLAND. 
 �34s[Howard Rasmussen]
�33s 
  
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�6v�2v�32sThyroid gland   �21m �33s 
    An endocrine gland found in all vertebrates that produces, stores, and 
secretes the thyroid hormones. The primary function of the thyroid, in 
warm-blooded vertebrates at least, is to regulate the rate of metabolism. In 
humans, the gland is located in front of, and on either side of, the trachea 
(see illustration). Thyrocalcitonin, another hormone of the thyroid gland, 
assists in regulating serum calcium by reducing its levels. The thyroid gland 
is capable of accumulating inorganic iodides and uniting them with the amino 
acid tyrosine to produce iodinated proteins. This activity is regulated by 
thyrotropic hormone from the anterior lobe of the pituitary gland.  See  
THYROID HORMONE. 
 �34s[George C. Kent, Jr.]
�33s 
  
  
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�6v�2v�32sThyroid gland disorders   �21m �33s 
    Tumorous overgrowth and inflammatory lesions are the most common types of 
thyroid gland disorders. They may be accompanied by altered production or 
release of the gland's principal hormones, thyroxine and triiodothyroxine; or 
the lesion may be present without endocrine change (euthyroid). 
    Simple goiter also known as colloid goiter, refers to an enlargement of 
part or all of the thyroid gland. It occurs endemically in certain geographic 
regions and sporadically in certain individuals. The etiology of the disease 
is only partially known. However, whether the cause be dietary, familial, or a 
combination of both, inadequate synthesis of thyroid hormone leads to 
compensatory hyperactivity of the pituitary, increased levels of 
thyroid-stimulating hormone, and subsequent enlargement of the thyroid glands 
to produce goiter. The enlarged gland is usually capable of maintaining the 
endocrine balance. However, a transformation of simple goiter into nodular 
adenoma may occur later in life. 
    Inflammations of the thyroid can be either acute or chronic and may have a 
bacterial or a nonbacterial causative agent. Acute thyroiditis occurs as a 
result of bacterial invasion of the gland. The usual causal agents, 
staphylococci and streptococci, produce a swollen congested gland. The acute 
inflammation usually resolves, but occasionally abscessation or nonspecific 
chronic thyroiditis may result. The three main types of chronic thyroiditis 
are usually nonbacterial though the cause remains unknown. 
    Thryoid neoplasms include benign nodules (adenomas) and carcinomas of 
several types. By far the most frequently seen are follicular adenomas, 
consisting of small masses of hyperplastic or hypertrophic thyroid glandular 
follicles and supporting tissue. 
    Excess secretion of thyroxine and triiodothyroxin produces a clinical 
condition called hyperthyroidism, which is also known as thyrotoxicosis, 
Graves' disease, toxic goiter, and exopthalmic goiter. Although the cause is 
unknown, the variably increased secretion of thyroxine produces quite 
characteristic symptoms. The most common are fatigue, muscle weakness, tremor, 
rapidity or irregularity of the heartbeat, and heat intolerance. Despite an 
increase in appetite, there is usually weight loss. In classical Graves' 
disease the eyes become prominent, bulge forward, and give a wide-eyed, 
staring appearance. 
    Hypothyroidism includes any condition in which insufficient thyroid 
hormones are produced or circulated. Cretinism results from inadequate iodine 
supply in fetal or infant life. There is retardation of mental and skeletal 
development which, in some cases, may be partially alleviated by thyroid 
therapy. Other forms result from chronic inflammation, hereditary defect, or 
enzymatic and nutritional derangement.  See  THYROID GLAND.   
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sThyroid hormone   �21m �33s 
    Any of the chemical messengers produced by the thyroid gland, including 
thyrocalcitonin, a polypeptide, and thyroxine and triiodothyronine which are 
iodinated thyronines.  See  HORMONE; THYROCALCITONIN; THYROID GLAND; THYROXINE. 
 �34s[Howard Rasmussen]
�33s 
  
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�6v�2v�32sThyroxine �21m �33s
    A hormone secreted by the thyroid gland. Thyroxine is quite similar
chemically and in biological activity to triiodothyronine (see illustration).
Both are derivatives of the amino acid tyrosine and are unique in being the
only iodine-containing compounds of importance in the economy of all higher
forms of animal life. The thyroid gland avidly accumulates the small amount of
iodine in the diet. This iodine is oxidized to iodide ion in the gland and
then reacts with tryosine to form mono- and diiodotyrosine. These latter are
then coupled to form either thyroxine or triiodothyronine.  See  THYROID GLAND.

    The maintenance of a normal level of thyroxine is critically important for
normal growth and development as well as for proper bodily function in the
adult. Its absence leads to delayed or arrested development. It is one of the
few hormones with general effects upon all tissues. Its lack leads to a
decrease in the general metabolism of all cells, most characterisically
measured as a decrease in nucleic acid and protein synthesis, and a slowing
down of all major metabolic processes.	See  THYROID GLAND DISORDERS.
 �34s[Howard Rasmussen]
�33s

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�6v�2v�32sThysanoptera   �21m �33s 
    An order of small, slender insects, commonly called thrips, having 
exopterygote development, sucking mouthparts, and highly modified wings. The 
order is a relatively small one, but individuals are often very numerous in 
favorable environments. See  THRIP. 
    The mouthparts are conical and used for scraping, piercing, and sucking. 
The wings are exceptionally narrow, with few or no veins, and are bordered by 
long hairs. The tarsi terminate in an inflatable membranous bladder, which has 
remarkable adhesive properties. 
    The eggs of thrips are laid on the surface of twigs (suborder Tubulifera) 
or in small cuts made by the ovipositor (suborder Terebrantia). There are 
usually four nymphal stages, the last of these being quiescent and pupalike. 
There are from one to several generations produced in a single year.  See  
INSECTA.   
 �34s[Frank M. Carpenter]
�33s 
  
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�6v�2v�32sThysanura   �21m �33s 
    Primarily wingless insects, with soft, fusiform bodies from 0.12 to 0.80 
in. (3 to 20 mm) long, often covered with scales forming diverse patterns (see 
illustration). These insects are considered by some authors to constitute two 
orders: the true Thysanura (silverfish and allies) and the Microcoryphia 
(machilids). The mouthparts are free. The mandibles are monocondylous in the 
machilids, and scraping in function; they are dicondylous - as in all winged 
insects - in the silverfish, in which they are of the chewing type. Machilids 
have large compound eyes, but silverfish have simple ommatidia, or lack eyes 
altogether. Many machilids have coxae with styliform appendages. Tarsi are 
from two- to five-segmented; there are two or three claws. Abdominal ventral 
plates are entire, or subdivided into a central sternum and posterolateral 
coxites. Abdominal styli are present in varied numbers. The females have 
well-developed ovipositors, and the males have a penis and often one or rarely 
two pairs of parameres. There are cerci and a median caudal filament. 
    These primitive insects, fossils of which are known as far back as the 
Devonian (primitive machilids, the Monura), are worldwide in distribution. 
Machilids are more numerous in temperate climates, the silverfish in the 
tropics and subtropics. See APTERYGOTA; INSECTA. 
 �34s[Pedro W. Wygodzinsky]
�33s 
  
  
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�6v�2v�32sTick fever   �21m �33s 
    A mild, febrile viral disease transmitted by wood ticks. After 4 - 6 days' 
incubation, sudden onset comes with chills, fever, severe aches, sometimes 
nausea, symptoms and fever are often diphasic. Occasional cases are reported 
with more severe symptoms involving the central nervous system.   
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sTick paralysis   �21m �33s 
    A loss of muscle function or sensation in humans or certain animals 
following the prolonged feeding of female ticks. Paralysis, of Landry's type, 
usually begins in the legs and spreads upward to involve the arms and other 
parts of the body. Evidence suggests that paralysis is due to a neurotoxin 
formed by the feeding ticks rather than the result of infection with 
microorganisms.  See  IXODIDES; TOXIN. 
    The disease has been reported in North America, Australia, South Africa, 
and occasionally in some European countries and is caused by appropriate 
species of indigenous ticks. In Australia, Ixodes holocyclus causes frequent 
cases in dogs, and occasionally in humans and paralysis has been known to 
progress even after removal of ticks. Ixodes cubicundus is associated with the 
disease in South Africa. 
 �34s[Cornelius B. Philip]
�33s 
  
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�6v�2v�32sTidal bore   �21m �33s 
    A part of a tidal rise in a river which is so rapid that water advances as 
a wall often several feet high. The phenomenon is favored by a substantial 
tidal range and a channel which shoals and narrows rapidly upstream, but the 
conditions are so critical that it is not common. Although the bore is a very 
striking feature, the tide continues to rise after the passage of the bore. 
Bores may be eliminated by changing channel depth or shape.  See  RIVER TIDES; 
TIDE.  
    In North America three bores have been observed: at the head of the Bay of 
Fundy (see illustration), at the head of the Gulf of California, and at the 
head of Cook Inlet, Alaska. The largest known bore occurs in the Tsientang 
Kiang, China. At spring tides this bore is a wall of water 15 ft (4.5 m) high 
moving upstream at 25 ft/s (7.5 m/s). 
 �34s[Blair Kinsman]
�33s 
  
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�6v�2v�32sTidal power   �21m �33s 
    Tidal-electric power is obtained by utilizing the recurring rise and fall 
of coastal waters. Marginal marine basins are enclosed with dams, making it 
possible to create differences in the water level between the ocean and the 
basins. The oscillatory flow of water filling or emptying the basins is used 
to drive hydraulic turbines which propel electric generators. 
    Large amounts of electric power could be developed in the world's coastal 
regions having tides of sufficient range, although even if fully developed 
this would amount to only a small percentage of the world's potential water 
(hydroelectric) power.  See  ELECTRIC POWER GENERATION; TIDE. 
 �34s[George G. Adkins]
�33s 
  
  
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�6v�2v�32sTide   �21m �33s 
    Stresses exerted in a body by the gravitational action of another, and 
related phenomena resulting from these stresses. Every body in the universe 
raises tides, to some extent, on every other. This article deals only with 
tides on the Earth, since these are fundamentally the same as tides on all 
bodies, and more specifically with variations of sea level, whatever their 
origin.  See  GRAVITATION; SEA-LEVEL FLUCTUATIONS. 
    The tide-generating forces arise from the gravitational action of Sun and 
Moon, the effect of the Moon being about twice as effective as that of the Sun 
in producing tides. The tidal effects of all other bodies on the Earth are 
negligible. The tidal forces act to generate stresses in all parts of the 
Earth and give rise to relative movements of the matter of the solid Earth, 
ocean, and atmosphere. In the ocean the tidal forces act to generate 
alternating tidal currents and displacements of the sea surface. 
    If the Moon attracted every point within the Earth with equal force, there 
would be no tide. It is the small difference in direction and magnitude of the 
lunar attractive force, from one point of the Earth's mass to another, which 
gives rise to the tidal stresses. The tide-generating force is proportional to 
the mass of the disturbing body (Moon) and to the inverse cube of its 
distance. This inverse cube law accounts for the fact that the Moon is 2.17 
times as important, insofar as tides are concerned, as the Sun, although the 
latter's direct gravitational pull on the Earth, which is governed by an 
inverse-square law, is about 180 times the Moon's pull. 
    At most places in the ocean and along the coasts, sea level rises and 
falls in a regular manner. The highest level usually occurs twice in any lunar 
day, the times bearing a constant relationship with the Moon's meridional 
passage. The time between the Moon's meridional passage and the next high tide 
is called the lunitidal interval. The difference in level between successive 
high and low tides, called the range of the tide, is generally greatest near 
the time of full or new Moon, and smaller near the times of quadrature. The 
range of the tide usually exhibits a secondary variation, being greater near 
the time of perigee (when the Moon is closest to the Earth) and smaller at 
apogee (when the Moon is farthest away). 
    The above situation is observed at places where the tide is predominantly 
semidiurnal. At many other places, it is observed that one of the two maxima 
in any lunar day is higher than the other. This effect is known as the diurnal 
inequality and represents the presence of an appreciable diurnal variation. At 
these places, the tide is said to be of the ``mixed'' type. At a few places, 
the diurnal tide actually predominates, there generally being only one high 
and low tide during the lunar day. 
    The range of the ocean tide varies between wide limits. The highest range 
is encountered in the Bay of Fundy, where values exceeding 50 ft (15 m) have 
been observed. In some places in the Mediterranean, South Pacific, and Arctic, 
the tidal range never exceeds 2 ft (0.6 m). 
    Owing to the rotation of the Earth, there is a gyroscopic, or Coriolis, 
force acting perpendicularly to the motion of any water particle in motion. In 
the Northern Hemisphere this force is to the right of the current vector. The 
horizontal, or tractive, component of the tidal force generally rotates in the 
clockwise sense in the Northern Hemisphere. As a result of both these 
influences the tidal currents in the open ocean generally rotate in the 
clockwise sense in the Northern Hemisphere, and in the counterclockwise sense 
in the Southern Hemisphere.  See  CORIOLIS ACCELERATION AND FORCE; EARTH TIDES. 
 �34s[Gordon W. Groves]
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�6v�2v�32sTie rod   �21m �33s 
    A tie rod or tie bar, usually circular in cross section, is used in 
structural parts of machines to tie together or brace connected members, or in 
moving parts of machines or mechanisms it may connect arms or parts to 
transmit motion. In the first use the rod ends are usually a threaded 
fastening, while in the latter they are usually forged into an eye for a pin 
connection.  
 �34s[Paul H. Black]
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�6v�2v�32sTile   �21m �33s 
    As a structural material, a burned clay product in which the coring 
exceeds 25% of the gross volume; as a facing material, any thin, usually flat, 
square product. Structural tile used for load bearing may or may not be 
glazed; it may be cored horizontally or vertically. Two principal grades are 
manufactured: one for exposed masonry construction, and the other for 
unexposed construction. 
    As a facing, clay products are formed into thin flat, curved, or embossed 
pieces, which are then glazed and burned. Commonly used on surfaces subject to 
water splash or that require frequent cleaning, such vitreous glazed wall tile 
is fireproof. Unglazed tile is laid as bathroom floor. By extension, any 
material formed into a size comparable to clay tile is called tile. Among the 
materials formed into tile are asphalt, cork, linoleum, vinyl, and porcelain. 
 �34s[Frank H. Rockett]
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�6v�2v�32sTill   �21m �33s 
    The genetic term for sediment deposited directly from glacier ice. Till is 
characteristically nonsorted and nonstratified and is deposited by lodgement 
or melt-out beneath a glacier or by melt-out on the surface of a glacier. The 
texture of till varies greatly and all tills are characterized by a wide range 
of particle sizes. Till contains a variety of rock and mineral fragments which 
reflect the source material over which the glacier flowed. The particles in 
the deposit usually show a preferred orientation related to the nature and 
direction of the ice flow. The overall character of the till reflects the 
source material, position and distance of transport, nature and position of 
deposition, and postdepositional changes. 
 �34s[W. Hilton Johnson]
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�6v�2v�32sTillage   �21m �33s 
    The mechanical manipulation of the soil to improve its physical condition 
as a habitat for plants. Tillage includes plowing, inversion, loosening, 
harrowing, pulverization, packing, and rolling the soil, all to improve 
aeration and temperature conditions and to produce a firm seedbed. Subsurface 
tillage is the loosening of soil by sweeps or blades pulled beneath the 
surface without inversion of the soil. This practice, especially adapted to 
dry areas, fragments the soil and leaves a mulch of stubble or other plant 
residues on the soil surface to conserve water and help control erosion. 
 �34s[Mark L. Nichols]
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�6v�2v�32sTillodontia   �21m �33s 
    An order of extinct quadrupedal land mammals known from early Cenozoic 
deposits in the Northern Hemisphere. They progressively developed enlarged, 
rootless second incisors that became remarkably rodentlike. Tillodont feet 
have five clawed toes. Tillodonts may have originated in a Paleocene 
arctocyonid group. If this is true, tillodonts were most closely related to 
condylarths and to the various hoofed mammals of the later Cenozoic ages.  See 
  CONDYLARTHRA; MAMMALIA. 
 �34s[Donald E. Savage]
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�6v�2v�32sTime �21m �33s
    The dimension of the physical universe which, at a given place, orders the
sequence of events; also, a designated instant in this sequence, as the time
of day, technically known as an epoch.

    Measurement.  Time measurement consists in counting the repetitions of any
recurring phenomenon and, if the interval between successive recurrences is
sensible, in subdividing it. The phenomenon most used has been the rotation of
Earth, where the counting is by days. Days are measured by observing the
meridian passages of stars and are subdivided with the aid of precision
clocks. The day is, however, subject to variations in duration; consequently,
when the utmost precision is required, years instead of days are measured and
subdivided.  See  DAY.
    A determination of time is synonymous with the establishment of an epoch;
it consists in ascertaining the clock correction, which is the correction that
should be applied to the reading of a clock (positive if the clock is slow) at
a specified epoch. A time interval may be measured in two ways: as the
duration between two known epochs or simply by counting from an arbitrary
starting point, as is done with a stopwatch.
    Time units are the intervals between successive recurrences of phenomena,
such as the period of rotation of Earth, also arbitrary multiples and
subdivisions of these intervals, such as the hour, being one twenty-fourth of
a day, and the minute, being one-sixtieth of an hour.  See  MONTH; YEAR.

     Time bases.  Several phenomena are used as the time base to be divided
into hours.  For astronomical purposes, sidereal time is used; for terrestrial
purposes, solar time is used.
    The hour angle of the vernal equinox is the measure of sidereal time.  It
is reckoned from 0 to 24 hours; each hour is subdivided into 60 sidereal
minutes and the minutes into 60 sidereal seconds.  Sidereal clocks are used
for convenience in most astronomical observatories, because a star, or other
object outside the solar system, comes to the same place in the sky each
night at virtually the same sidereal time.
    The hour angle of the Sun is the apparent solar time.  The only true
indicator of apparent solar time is a sundial.	Mean solar time has been
devised to eliminate the irregularities in apparent solar time that arise
from the obliquity of the ecliptic and the varying speed of Earth in its
orbit around the Sun.  It is the hour angle of a fictitious mean Sun, an
imagined point moving uniformly along the celestial equator at the same
rate as the average rate of the actual Sun along the ecliptic.
    Because sidereal and solar time are both defined as hour angles, at any
instant they vary from place to place on Earth.  To avoid the inconvenience
of the continuous change of mean solar time with longitude, zone time or civil
time is the time generally used.  Earth is divided into 24 time zones, each
approximately 15DG wide and centered on standard longitudes 0, 15, 30DG, and
so on.	Within each of these zones, the time kept is the mean solar time of
the standard meridian.	Most civilized nations use zone time.  Many countries
of the world, including the United States, advance their time 1 hour during
the summer months, into daylight saving time.  See ATOMIC TIME; EPHEMERIS
TIME.

[Gerald M. Clemence]
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�6v�2v�32sTime constant �21m �33s
    The time required for a physical quantity to change its initial
(zero-time) magnitude by the factor (1 - 1/NATURAL LOG) when the physical
quantity is varying as a function of time, f(t), according to the decreasing
exponential function as in Eq. (1), or the increasing exponential function as
in Eq. (2).
[Note for electronic edition, the NATURAL LOG is represented by a bold greek
epsilon.]

	      f(t) =  NATURAL LOG (sup -kt)				(1)


	     F(t) = l  - NATURAL LOG (sup -kt)				(2)

    The numeric NATURAL LOG has the value of 2.71828. Therefore, the change in
magnitude of (1-1/NATURAL LOG) has the fractional value 0.632121. Thus, after
a time lapse of one time constant, starting at zero time, the magnitude of the
physical quantity will have changed 63.2%.
    When time t is zero, Eq. (1) has the magnitude 1, and when time t is 1/k,
the magnitude is NATURAL LOG(sup -1), or 1/NATURAL LOG. The corresponding change
in magnitude is (1-1/NATURAL LOG). The specific time required to accomplish
this change is shown in Eq. (3), where T is called the time constant and is

			      t = 1/k = T				     (3)

usually expressed in seconds. The same results are obtained for Eq. (2).
    The initial rate of change of both the increasing and decreasing functions
is equal to the maximum amplitude of the function divided by the time constant.
    The concept of time constant is useful when evaluating the presence of
transient phenomena.
 �34s[Robert L. Ramey]
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�6v�2v�32sTime-interval measurement �21m �33s
    Time interval is measured with high precision, most conveniently, with an
electronic, digital reading counter. This contains an oscillator, which has a
frequency, say, of 10(sup7)hertz. A starting signal causes the counter to start
counting cycles, and a second signal causes the counter to stop. The
accumulated number is displayed by digital indicating lights.
    Short time intervals may be measured with high precision, to 1 nanosecond
(10 (sup -9) s), with a suitable oscilloscope. Ultrashort phenomena in the
picosecond range (1 ps = 10 (sup -12) s)  are studied by use of short pulses
generated by lasers (about 0.5 - 10 ps).
    Radioactive decay is used to measure long time intervals concerning human
history, the Earth, and the solar system.  See	GEOCHRONOMETRY; RADIOCARBON
DATING.
 �34s[William Markowitz]
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�6v�2v�32sTime-of-flight spectrometer   �21m �33s 
    Any of a general class of instruments in which the speed of a particle is 
determined directly by measuring the time that it takes to travel a measured 
distance. By knowing the particle's mass, its energy can be calculated. If the 
particles are uncharged (for example, neutrons), difficulties arise because 
standard methods of measurement (such as deflection in electric and magnetic 
fields) are not possible. The time-of-flight method is a powerful alternative, 
suitable for both uncharged and charged particles. 
    The time intervals are best measured by counting the number of 
oscillations of a stable oscillator that occur between the instants that the 
particle begins and ends its journey. Oscillators operating at 100 MHz are in 
common use.  See  MASS SPECTROSCOPE; NEUTRON SPECTROMETRY; TIME-INTERVAL 
MEASUREMENT.  
 �34s[Frank W. K. Firk]
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�6v�2v�32sTimor   �21m �33s 
    Timor, an island in Wallacea 300 mi (480 km) long and varying 10 to 60 mi 
(16 to 100 km) in width, has an area of about 13,000 misup2 (34,000 kmsup2). 
The island is mountainous, with many peaks above 6000 ft (1800 m). The highest 
mountain is Tata Mai Lau, which rises to 9780 ft (2981 m). Many mud volcanoes 
are active. The island has two monsoons and a five-month dry season, which 
results in lighter vegetation than is usual at these latitudes. See EAST 
INDIES.  
 �34s[M. G. Audley-Charles]
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�6v�2v�32sTimothy   �21m �33s 
    A plant, Phleum pratense, of the order Cyperales, long the most important 
hay grass for the cooler temperate humid regions. It is easily established and 
managed, produces seed abundantly, and grows well in mixtures with alfalfa and 
clover. It is a short-lived perennial, makes a loose sod, and has moderately 
leafy stems and a dense cylindrical inflorescence. Timothy-legume mixtures 
still predominate in hay and pasture seedings for crop rotations in the 
northern half of the United States.  See  GRASS CROPS. 
 �34s[Howard B. Sprague]
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�6v�2v�32sTin �21m �33s
    A chemical element, symbol Sn, atomic number 50, atomic weight 118.69. Tin
forms tin(II) or stanous (Sn (sup 2+), and tin(IV) or stannic (Snsup4supp)
compounds, as well as complex salts of the stannite (M2SnX4) and stannate
(M2SnX6) types.
    Tin melts at a low temperature, is highly fluid when molten, and has a
high boiling point. It is soft and pliable and is corrosion-resistant to many
media. An important use of tin has been for tin-coated steel containers (tin
cans) used for preserving foods and beverages. Other important uses are solder
alloys, bearing metals, bronzes, pewter, and miscellaneous industrial alloys.
Tin chemicals, both inorganic and organic, find extensive use in the
electroplating, ceramic, plastic, and agricultural industries.
    The most important tin-bearing mineral is cassiterite, SnO2. No high-grade
deposits of this mineral are known. The bulk of the world's tin ore is
obtained from low-grade alluvial deposits.  See  CASSITERITE.
    Two allotropic forms of tin exist: white (BETA) and gray (ALPHA) tin. Tin
reacts with both strong acids and strong bases, but it is relatively resistant
to solutions that are nearly neutral. In a wide variety of corrosive
conditions, hydrogen gas is not evolved from tin and the rate of corrosion
becomes controlled by the supply of oxygen or other oxidizing agents. In their
absence, corrosion is negligible. A thin film of stannic oxide forms on tin
upon exposure to air and provides surface protection. Salts that have an acid
reaction in solution, such as aluminum chloride and ferric chloride, attack
tin in the presence of oxidizers or air. Most nonaqueous liquids, such as
oils, alcohols, or chlorinated hydrocarbons, have slight or no obvious effect
on tin. Tin metal and the simple inorganic salts of tin are nontoxic. Some
forms of organotin compounds, on the other hand, are toxic. Some important
physical constants for tin are shown in the table.
    Stannous oxide, SnO, is a blue-black, crystalline product which is soluble
in common acids and strong alkalies. It is used in making stannous salts for
plating and glass manufacture. Stannic oxide, SnO2, is a white powder,
insoluble in acids and alkalies. It is an excellent glaze opacifier, a
component of pink, yellow, and maroon ceramic stains and of dielectric and
refractory bodies. It is an important polishing agent for marble and
decorative stones.
    Stannous chloride, SnCl2, is the major ingredient in the acid
electrotinning electrolyte and is an intermediate for tin chemicals. Stannic
chloride, SnCl4, in the pentahydrate form is a white solid. It is used in the
preparation of organotin compounds and chemicals to weight silk and to
stabilize perfume and colors in soap. Stannous fluoride, SnF2, a white
water-soluble compound, is a toothpaste additive.
    Organotin compounds are those compounds in which at least one tin-carbon
bond exists, the tin usually being present in the +IV oxidation state.
Organotin compounds that find applications in industry are the compounds with
the general formula R4Sn, R3SnX, R2SnX2, and RSnX3. R is an organic group,
often methyl, butyl, octyl, or phenyl, while X is an inorganic substituent,
commonly chloride, fluoride, oxide, hydroxide, carboxylate, or thiolate.  See
TIN ALLOYS.
 �34s[Joseph B. Long]
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�6v�2v�32sTin alloys   �21m �33s 
    Solid solutions of tin and some other metal or metals. Alloys cover a wide 
composition range and many applications because tin alloys readily with nearly 
all metals.  See  TIN; TIN METALLURGY. 
    Soft solders constitute one of the most widely used and indispensable 
series of tin-containing alloys. Common solder is an alloy of tin and lead, 
usually containing 20 - 70% tin.  See  SOLDERING. 
    Bronzes form an important group of structural metals. Of the true 
copper-tin bronzes, up to 10% tin is used in wrought phosphor bronzes, and 
from 5 to 10% tin in the most common cast bronzes. Many brasses, which are 
basically copper-zinc alloys, contain 0.75 - 1.0% tin for additional corrosion 
resistance.  See  COPPER ALLOYS. 
    Other useful tin alloys include babbitt metal (tin containing 4 - 8% each 
copper and antimony), an alloy with bearing applications; pewter; a tin-base 
alloy containing small quantities of antimony and copper; and type metals, 
which are lead-base alloys containing 3 - 15% tin.  See  ALLOY. 
 �34s[Bruce W. Gonser]
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�6v�2v�32sTinamiformes   �21m �33s 
    An order of birds comprising the single family Tinamidae, the tinamous of 
Central and South America. About 35 species are recognized. The tinamous are 
superficially fowllike and occupy niches typical of galliform birds in other 
parts of the world; their size range is from that of a small quail to that of 
a half-grown turkey. They occupy a wide variety of habitats, from the floors 
of dark rainforests to bleak Andean slopes. Although they have fully developed 
wings, tinamous are weak fliers and prefer to run or hide to escape danger. As 
in the rheas and most other ratites, male tinamous incubate the eggs and care 
for the young.  See  AVES; GALLIFORMES; RATITES; RHEIFORMES. 
 �34s[Kenneth C. Parkes]
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�6v�2v�32sTin metallurgy   �21m �33s 
    The extraction of tin from its ores and its subsequent refining and 
preparation for use. Tin is comparatively easy to reduce to metal from the 
oxide by pyrometallurgy. However, this operation differs from the smelting of 
most common metals because it requires retreatment of the slag to obtain 
efficient metal recovery. 
    Smelting of tin concentrates is usually done in reverberatory furnaces, 
using coke or coal as the reducing agent. Electric arc furnaces and rotary 
reverberatory furnaces are also used in a few smelters. From alluvial tin 
ores, very-high-grade cassiterite (natural tin oxide) concentrates are 
normally obtained which contain 70 - 77% tin and only minor metal impurities. 
These are charged directly to the reverberatory furnace, or after roasting if 
they contain arsenic or sulfur.  
    Crude tin from smelting is liquated (that is, partially melted) to remove 
iron, copper, and other impurities which form solid compounds appreciably 
above the melting point of tin. The refined metal from most smelters is over 
99.8% tin. 
    Secondary tin from metal scrap comes from tin-bearing alloys, and 
secondary smelters rework them into alloys and chemicals. Some tin metal of 
highest purity is recovered from the detinning of tinplate scrap by hot 
caustic solution and electrolytic recovery.  See  PYROMETALLURGY, NONFERROUS; 
TIN.  
 �34s[Bruce W. Gonser]
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�6v�2v�32sTintinnida �21m �33s
    An order of the Spirotrichia whose members are conical or trumpet-shaped
pelagic forms bearing shells (loricae). These protozoa are planktonic ciliates
and are especially abundant in oceans, notably the Pacific. The lorica is
composed of a resistant organic compound in which various foreign mineral
grains are embedded; its shape may range from trumpet- or bell-form to
cylindrical or subspherical.
    Fossil tintinnids, representing practically the only fossilized species of
ciliate protozoa known to science, are identified on the basis of the shape of
the lorica in cross section as seen in randomly oriented thin sections of the
rocks in which they are found. Twelve genera of fossil tintinnids have been
described from limestones and cherts of the Jurassic and Cretaceous.  See
CILIOPHORA; SPIROTRICHIA.
 �34s[John O. Corliss ;Daniel J. Jones]
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�6v�2v�32sTire   �21m �33s 
    A continuous pneumatic rubber and fabric cushion encircling and fitting 
onto the rim of a wheel. In modern tire building, chemicals are compounded 
into the rubber to help it withstand wear, heat, and aging and to produce 
desired changes in its characteristics. Fabric (rayon, nylon, or polyester) is 
used to give the tire body strength and resilience. In belted tires, 
additional layers of fabric (rayon, fiber glass, finely drawn steel, or 
aramid) are placed just under the tread rubber to increase mileage and 
handling. Steel wire is used in the bead that holds the tire to the rim. 
    A tire is made up of two basic parts: the tread, or road-contacting part, 
which must provide traction and resist wear and abrasion, and the body or 
carcass, consisting of rubberized fabric that gives the tire strength and 
flexibility. In compounding the rubber, large amounts of carbon black are 
mixed with it to improve abrasion resistance. Other substances, such as 
sulfur, are added to enable satisfactory processing and vulcanization.  See  
RUBBER.  
    There are three types of tires: bias, belted bias, and radial (see 
illustration). For bias tires, cords in the piles extend diagonally across the 
tire from bead to bead. The cords run in opposite directions in each 
successive ply, resulting in a crisscross pattern. For belted bias tires, 
plies are placed in a manner similar to that used in the bias ply tire, with 
belts of material placed circumferentially around the tire between the plies 
and the tread rubber. For radial tires, cords in the plies extend transversely 
from bead to bead, substantially perpendicular to the direction of travel. 
Belts are placed circumferentially around the tire. 
 �34s[David B. Harrison]
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�6v�2v�32sTissue   �21m �33s 
    An aggregation of cells more or less similar morphologically and 
functionally. The animal body is composed of four primary tissues, namely, 
epithelium, connective tissue (including bone, cartilage, and blood), muscle, 
and nervous tissue. The process of differentiation and maturation of tissues 
is called histogenesis.  See  HISTOLOGY; PLANT TISSUE SYSTEMS. 
 �34s[Charles B. Curtin]
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�6v�2v�32sTissue culture   �21m �33s 
    The branch of biology in which tissues or cells of higher animals and 
plants are grown artificially in a controlled environment. Tissue culture is 
possible when cells are attached to a solid substrate, such as glass or 
cellophane, and if the necessary complex nutrient medium is provided. All 
cultures are now also grown in liquid suspension. Tissue cultures are used in 
the study of cell growth, multiplication, and differentiation, as well as in 
cancer research, hereditary mechanisms, radiation biology, all hybridization, 
and virus studies.  See  MICROBIOLOGICAL METHODS. 
 �34s[Theodore T. Puck]
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�6v�2v�32sTitanium   �21m �33s 
    A chemical element, Ti, atomic number 22 and atomic weight 47.90. While 
its chemistry shows many similarities to that of silicon and zirconium, as a 
first-row transition element, the aqueous solution chemistry, especially of 
the lower oxidation states, shows some resemblances to that of vanadium and 
chromiun.  See  TRANSITION ELEMENTS. 
    The principal valence state is 4+; the 3+ and 2+ states are also known, 
but are less stable. The element burns in air when heated to give the dioxide, 
TiO2, and combines with halogens. It reduces water-vapor to form the dioxide 
and hydrogen, and reacts similarly with hot concentrated acids, although it 
forms the trichloride with hydrochloric acid. The metal absorbs hydrogen to 
give compositions approaching TiH2 and forms the nitride, TiN, and the 
carbide, TiC. The sulfide TiS2 as well as both the lower oxides, Ti2O3 and 
TiO, and sulfides, Ti2S3 and TiS, are known. Salts of all three valence states 
are known.  
    The dioxide, TiO2, occurs most commonly in a black or brown form known as 
rutile. Less prominent naturally occurring forms are anatase and brookite. 
Both rutile and anatase are white when pure. The black basic oxide, FeTiO3, 
occurs naturally as the mineral ilmenite; this is a principal commercial 
source of titanium.  See  ILMENITE; RUTILE. 
    Titanium dioxide is widely used as a white pigment for exterior paints 
because of its chemical inertness, superior covering power, opacity to 
damaging ultraviolet light, and self-cleaning ability. The dioxide has also 
been used as a whitening or opacifying agent in numerous situations. Examples 
would be the use as a filler in paper, a coloring agent for rubber and leather 
products, a pigment in ink, and a component of ceramics. It has found 
important use as an opacifying agent in porcelain enamels, giving a finish 
coat of great brilliance, hardness, and acid resistance. 
    The alkaline-earth titanates show some remarkable properties. The 
dielectric constants range from 13 for MgTiO3 to several thousand for solid 
solutions of SrTiO3 in BaTiO3. Barium titanate itself has a dielectric 
constant of 10,000 near 120DGC (250DGF), its Curie point; it has a low 
dielectric hysteresis. Ceramic transducers containing barium titanate compare 
favorably with Rochelle salt and quartz, with respect to thermal stability in 
the first case, and with respect to the strength of the effect and the ability 
to form the ceramic in various shapes, in the second case. The compound has 
been used both as a generator for ultrasonic vibrations and as a sound 
detector.  See  PIEZOELECTRICITY; TITANIUM METALLURGY. 
 �34s[Arthur W. Adamson]
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�6v�2v�32sTitanium metallurgy   �21m �33s 
    The winning of metallic titanium from its ores followed by alloying and 
processing into forms and shapes that can be used for structural purposes. 
Titanium's unique properties - density half that of steel, excellent strength 
retention to 1000DGF (538DGC), and atmospheric corrosion immunity superior to 
that of other metals - make it an ideal construction material for both the 
engines and airframes. 
    All commercial production of titanium metal involves chlorination of ore 
concentrates; reacting TiO2 with chlorine gas and coke (carbon) in a 
fluidized-bed reactor forms impure titanium tetrachloride (TiCl4). For the 
production of acceptable metal, purification of the raw tetrachloride is 
required to remove other metal chlorides that would contaminate the virgin 
titanium.  
    The purified titanium tetrachloride is delivered as a liquid to the 
reactor vessel. In these vessels, constructed of carbon or stainless steel, 
the titanium tetrachloride is reacted with either magnesium or sodium to form 
the pure metal called sponge, so named because of its porous cellular form. To 
avoid contamination by oxygen or nitrogen, the reaction is carried out in an 
argon atmosphere. 
    Titanium sponge is consolidated in the consumable-electrode arc furnace. A 
mass of sponge, alloy additions, and scrap are mixed, then compressed into 
compacts and welded together to form a sponge electrode. This is melted by an 
electric arc into a water-cooled copper crucible in a vacuum or an atmosphere 
of purified argon or nitrogen. The arc progressively consumes the sponge 
electrode to form an ingot.  
    The conversion of the titanium ingot into mill products, such as forging 
billet, plate, sheet, and tubing, is accomplished for the most part on 
conventional metalworking equipment. Titanium and its structural alloys are 
produced in most of the same forms and shapes as stainless steel.  See  METAL 
FORMING; STAINLESS STEEL; TITANIUM. 
 �34s[author unknown]
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�6v�2v�32sTitration   �21m �33s 
    A quantitative analytical process that is basically volumetric. However, 
in high-precision titrimetry the titrant solution is sometimes delivered from 
a weight buret, so that the volumetric aspect is indirect. Generally, a 
standard solution, that is, one containing a known concentration of substance 
X (titrant), is progressively added to a measured volume of a solution of a 
substance Y (titrand) that will react with the titrant. The addition is 
continued until the end point is reached. Ideally, this is the same as the 
equivalence point, at which an excess of neither X nor Y remains. If the 
stoichiometry or exact ratio in which X and Y react is known, it is possible 
to calculate the amount of Y in the unknown solution.  See  VOLUMETRIC 
ANALYSIS.  
    The normal requirements for the performance of a titration are: a standard 
titrant solution; calibrated volumetric apparatus, including burets, pipets, 
and volumetric flasks; and some means of detecting the end point.  See  BURET; 
PIPET; VOLUMETRIC FLASK. 
  
    Classification by chemical reaction.  For the purposes of titrimetry, 
chemical reactions can be placed in three general categories: acid-base or 
neutralization, combination, and oxidation-reduction. 
    Acid-base titrations involve neutralization of an acid by titration with a 
base, or vice versa. However, the process is often nonspecific; in the 
titration of a mixture of nitric and hydrochloric acids, only the total 
acidity can be found without recourse to additional measurements. A salt 
derived from a strong base and a very weak acid can often be titrated just as 
if it were a base.  See  ACID AND BASE. 
    In titrimetry, attention is usually focused upon the combination of an ion 
in the titrant with one of the opposite sign in the titrand solution. 
Sometimes the combination may involve more than two species, some of which may 
be nonionic. The combinations may result in precipitation or formation of a 
complex.  See  COMPLEX COMPOUNDS; PRECIPITATION (CHEMISTRY). 
    In so-called redox titrations the titrant is usually an oxidizing agent, 
and is used to determine a substance that can be oxidized and hence can act as 
a reducing agent.  See  OXIDATION-REDUCTION. 
  
    Coulometric titration.  The passage of a uniform current for a measured 
period of time can be used to generate a known amount of a product such as a 
titrant. This fact is the basis of the technique known as coulometric 
titration. An obvious requirement is that generation shall proceed with a 
fixed, preferably 100%, current efficiency. The uniform current is then 
analogous to the concentration of an ordinary titrant solution, while the 
total time of passage is analogous to the volume of such a solution that would 
be needed to reach the end point.  See  ELECTROLYSIS. 
  
    Classification by end-point techniques.  The precision and accuracy with 
which the end point can be detected is a vital factor in all titrations. 
Because of its simplicity and versatility, chemical indication is quite 
common, especially in acid-base titrimetry.  
  
    Indicators. An acid-base indicator is a weak acid or a weak base that 
changes color when it is transformed from the molecular to the ionized form, 
or vice versa. The color change is normally intense, so that only a low 
concentration of indicator is needed. The working range, or visual color 
change, of a typical acid-base indicator is spread over about a hundredfold 
(SIMILAR 2 pH units) change in hydrogen ion concentration. Available 
indicators have individual working ranges that together cover the entire range 
of hydrogen ion concentration likely to be encountered in general acid-base 
titration.  See  ACID-BASE INDICATOR; HYDROGEN ION; PH. 
    Sometimes no suitable chemical indicator can be found for a desired 
titration. Possibly the concentrations involved may be so low that chemical 
indication functions poorly. Other situations might be the need for high 
precision or for the automatic arrest of the titration. Recourse is then made 
to some physical method of end-point detection. 
  
    Potentiometric titration. If a pH meter is used, its associated electrodes 
are first standardized by use of a buffer solution of known pH. By suitable 
choice of electrodes, potentiometric methods can also be applied to 
combination titrations and to oxidation-reduction titrations. The advent of 
modern ion-selective electrodes has greatly extended the scope of 
potentiometric titration and of other branches of titrimetry.  See  ELECTRODE 
POTENTIAL; ION-SELECTIVE MEMBRANES AND ELECTRODES. 
  
    Conductometric titration. Conductometric titration is sometimes successful 
when chemical indication fails. The underlying principles of conductometric 
titration are that the solvent and any molecular species in solution exhibit 
only negligible conductance; that the conductance of a dilute solution rises 
as the concentration of ions is increased; and that at a given concentration 
the hydrogen ion and the hydroxyl ion are much better conductors than any of 
the other ions.  See  ELECTROLYTIC CONDUCTANCE. 
  
    Spectrophotometric titration. The spectrophotometer is an optical device 
that responds only to radiation within a selected very narrow band of 
wavelengths in the visual, ultraviolet, or infrared regions of the spectrum. 
The response can be made both quantitative and linearly related to the 
concentration of a species that absorbs radiation within this band. Titrations 
at wavelengths within the visual region are by far the most common.  See  
SPECTROPHOTOMETRIC ANALYSIS. 
  
    Amperometric titration. By use of a dropping-mercury or other suitable 
microelectrode, it is possible to find a region of applied electromotive force 
(emf) in which the current is proportional to the concentration of one or both 
of the reactants in a titration.  
    Biamperometric titration is a closely related technique. An emf that is 
usually small is applied across two identical microelectrodes that dip into 
the titrand solution. This arrangement, which involves no liquid-liquid 
junctions, is valuable in nonaqueous titrations, but also finds much use in 
aqueous titrimetry.  See  POLAROGRAPHIC ANALYSIS. 
  
    Thermometric or enthalpimetric titration. Many chemical reactions proceed 
with the evolution of heat. If one of these is used as the basis of a 
titration, the temperature first rises progressively and then remains 
unchanged as the titration is continued past the end point. If the reaction is 
endothermic, the temperature falls instead of rising. Thermometric titration 
is applicable to all classes of reactions.  See  THERMOCHEMISTRY. 
  
    Nonaqueous titration. This technique is used to perform titrations that 
give poor or no end points in water. Although applicable in principle to all 
classes of reactions, acid-base applications have greatly exceeded all others. 
Nonaqueous titrations in which the solvent is a molten salt or salt mixture 
are also possible. 
  
    Automatic titration. Automation is particularly valuable in routine 
titrations, which are usually performed repeatedly. One approach is to record 
the titration curve and to interpret it later. Another method is to stop 
titrant addition or generation automatically at, or very near to, the end 
point. Although a constant-delivery device is desirable, an ordinary buret 
with an electromagnetically controlled valve is often used.  
    Microcomputer control permits such refinements as the continuous 
adjustment of the titrant flow rate during the titration. In some cases, it is 
possible to automate an entire analysis, from the measurement of the sample to 
the final washout of the titration vessel and the printout of the result of 
the analysis.  See  ANALYTICAL CHEMISTRY. 
 �34s[John T. Stock]
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�6v�2v�32sToad   �21m �33s 
    Any of several species of the amphibian order Anura, which also includes 
the frogs. The anurans are characterized by their lack of a tail and by 
hindlimbs that are adapted for jumping. The family Bufonidae contains most of 
the common species of toads, and 17 genera have been described which occur 
throughout the world except for Antarctica and Australia. Toads live in drier 
habitats than frogs do.  
    Reproductive behavior is varied among the species, but fertilization is 
usually external. Tubular glands on the thumbs of some toads produce a sticky 
secretion that assists the male in clinging to the female during amplexus; 
amplexus may last up to several weeks.  See  AMPHIBIA; ANURA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sTobacco   �21m �33s 
    The plant genus Nicotiana, certain species in the genus, and dried leaves 
of these plants are all called tobacco. Most often tobacco means a leaf 
product containing 1 - 3% of the alkaloid nicotine, which produces a narcotic 
effect when smoked, chewed, or snuffed. The plant N. rustica provides tobacco 
in parts of Europe, but the tobacco of world commerce is N. tabacum. Tobacco 
is American in origin.  See  POLEMONIALES. 
 �34s[Gordon S. Taylor]
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�6v�2v�32sToggle   �21m �33s 
    Any of a wide variety of mechanisms, many used to open or close electrical 
contacts abruptly and all characterized by the control of a large force by a 
small one. The basic action of a toggle mechanism is shown in illustration a. 
When ALPHA = 90DG the forces P and Q are independent of each other. Again, 
when ALPHA = 0DG the forces are isolated, force Q being sustained entirely by 
the frame, and force P serving only to hold the link in position. At ALPHA = 
45DG from the symmetry VERTICAL BARPVERTICAL BAR = VERTICAL BARQVERTICAL BAR, 
the mechanism serves to transfer the direction of forces to achieve 
equilibrium.  See  COUPLING. 
    Because the simple configuration of illustration a requires low-friction 
sliders, it is impractical. A more useful structure replaces the vertical 
slider with a second link pinned to the frame (illustration b), in which case 
input P sets up forces in both links. A further modification (illustration c) 
replaces the other slider with a link.  See  FOUR-BAR LINKAGE; LINKAGE 
(MACHINES).  
 �34s[Frank H. Rockett]
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�6v�2v�32sTolerance   �21m �33s 
    Amount of variation permitted or ``tolerated'' in the size of a machine 
part. Manufacturing variables make it impossible to produce a part of exact 
dimensions; hence the designer must be satisfied with manufactured parts that 
are between a maximum size and a minimum size. Tolerance is the difference 
between maximum and minimum limits of a basic dimension. For instance, in a 
shaft and hole fit, when the hole is a minimum size and the shaft is a 
maximum, the clearance will be the smallest, and when the hole is the maximum 
size and the shaft the minimum, the clearance will be the largest. 
    If the initial dimension placed on the drawing represents the size of the 
part that would be  used if it could be made exactly to size, then a 
consideration of the operating conditions of the pair of mating surfaces shows 
that a variation in one direction from the ideal would be more dangerous than 
a variation in the opposite direction. The dimensional tolerance should be in 
the less dangerous direction. This method of stating tolerance is called 
unilateral tolerance and has largely displaced bilateral tolerance, in which 
variations are given from a basic line in plus and minus values. 
 �34s[Paul H. Black]
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�6v�2v�32sTomato   �21m �33s 
    An important vegetable grown for its edible fruit belonging to the genus 
Lycopersicon, especially L. esculentum. Lycopersicon species are native to 
South America, especially Peru, and the Galapagos Islands. The tomato was 
first domesticated in Mexico, where it received its name from the Aztec word 
xitomate.  
    It was introduced to Europe in the mid-16th century, and was prominently 
featured in the early herbals. Known as the ``love apple'' and ``poma d'ora,'' 
it was grown for the beauty of its fruit but was not often eaten, except in 
Italy and Spain, because many considered it to be poisonous like its distant 
relative the deadly nightshade.  
    The fruit is a berry with 2 to 12 locules containing many seeds. Most 
tomato varieties have red fruit, due to the red carotenoid lycopene. There is 
no basis for the common belief that yellow-fruited tomatoes are low in 
acidity. Many greenhouse varieties have pink fruit, due to a single gene that 
prevents formation of the yellow pigment in the epidermis of the fruit. 
    The tomato is highly esteemed as a source of vitamin C; tomato varieties 
differ in ascorbic acid content. Most tomato varieties have about 20 - 25 mg 
ascorbic acid per 100 g, and one medium-sized tomato provides about half of 
the required daily allowance of vitamin C for adults. Tomatoes are also a 
significant source of vitamin A. Red tomatoes have about 1000 international 
units (IU) of vitamin A per 100 g. Tomatoes are a good source of protein, but 
most of it is in the seeds. Tomato juice contains 19 amino acids, principally 
glutamic acid.  See  ASCORBIC ACID; VITAMIN A. 
 �34s[Richard W. Robinson]
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�6v�2v�32sTonalite   �21m �33s 
    A phaneritic (visibly crystalline) plutonic rock composed chiefly of 
plagioclase (oligoclase or andesine) and quartz with subordinate dark-colored 
(mafic) minerals (biotite, amphibole, or pyroxene). The term tonalite is 
roughly equivalent to quartz diorite. Minor amounts of alkali feldspar may be 
present, but if this mineral exceeds 5% of the total feldspar, the rock is a 
granodiorite. As the quartz content decreases, quartz diorite passes into 
diorite. Tonalite, or quartz diorite, is roughly intermediate between 
granodiorite and diorite.  See  DIORITE; GRANODIORITE. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sTone (music and acoustics)   �21m �33s 
    A sound oscillation capable of exciting an auditory sensation having 
pitch; also, the sensation itself; that is, the word tone is used for both 
cause and effect. There is not necessarily a complete correspondence between 
the two; which of the two meanings is intended must be made clear by 
additional modifiers, context, or units of measurement. For example, if a tone 
is described as having pitch, it is to be understood that the sound sensation 
is meant, whereas a tone that has frequency must be a physical oscillation.  
See  OSCILLATION; PITCH. 
 �34s[Robert W. Young]
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�6v�2v�32sTongue   �21m �33s 
    An organ located in the floor of the mouth. The tongue reaches its highest 
development in mammals, where it is specialized in form and function. It is 
rich in papillae and glands and is always extensile, except in whales. 
    Anatomically the mammalian tongue is a mass of skeletal muscle largely 
contained within a covering sheet of mucous membrane. The surface epithelium 
is of the stratified squamous type. It is cornified in some mammals but not in 
humans. Filiform papillae are best developed in the cat family where they 
convert the tongue into a rasping organ. Fungiform papillae are more prominent 
elevations, with a knobbed top somewhat like a button mushroom. Some bear a 
few taste buds. Conical papillae are considered to be a modified fungiform 
type. Vallate papillae have a flat top and do not extend much above the 
lingual surface. Each is encircled by a relatively deep trench. The side 
surface of a human vallate papilla contains some 200 taste buds, whereas the 
opposite wall, across the trench, bears about one-fourth as many. 
    Taste buds are quite similar in all vertebrates. Each is an ovoid 
specialization paler than the surrounding stratified squamous epithelium; they 
occupy practically the full thickness of that covering layer. The shape is 
somewhat like a barrel, but the top usually narrows. Two cell types are 
recognizable within the taste bud. The taste cells are slender, spindle-shaped 
elements whose free end terminates in a short, stiff taste hair. These hairs 
extend into the taste canal and continue to the external taste pore. The 
supporting cells of the taste bud are mostly at the periphery, something like 
thick barrel staves. Nerve fibers for the mediation of taste end about both 
kinds of cells.  See  TASTE. 
    Lingual glands lie deep in the mucous membrane of the tongue and encroach 
on the muscle beneath. Mixed muco-serous glands occur under the apex of the 
tongue. Purely serous glands are restricted to the region of the vallate and 
foliate papillae, and many discharge on the surface of the tongue. Other 
glandular ducts open into the trench of each vallate papilla. The root of the 
tongue contains pure mucous glands, many of which open on the surface but some 
of which have ducts discharging into the pits of the lingual tonsil.  
    The mammalian tongue performs a variety of functions. In ruminants it is 
prehensile and thus is important in browsing. Some mammals use the tongue to 
lap up liquids. In general, it is of use in directing food to the teeth and 
pharynx, aiding chewing and swallowing. The dog, lacking sweat glands, cools 
itself by panting, which draws  air over the tongue. The tongue is the most 
important agent in articulate speech. The sense of taste resides in the taste 
buds, whose cells respond to substances in solution. 
 �34s[Leslie B. Arey]
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�6v�2v�32sTon of refrigeration   �21m �33s 
    A rate of cooling that is equivalent to the removal of heat at 200 
Btu/min, 12,000 Btu/h, or 288,000 Btu/day (3.51 kW). This unit of measure 
stems from the original use of ice for refrigeration. One pound of ice, in 
melting at 32DGF (0DGC), absorbs as latent heat approximately 144 Btu/lb, and 
1 ton (2000 lb) of ice, in melting in 24 h, absorbs 288,000 Btu/day. In 
Europe, where the metric system is used, the equivalent cooling unit is the 
frigorie, which is a kilogram calorie, or 3.96 Btu. Thus 3000 frigories/h is 
approximately 1 ton of refrigeration. A standard ton of refrigeration is one 
developed at standard rating conditions of 5DGF (6 - 15DGC) evaporator and 
86DGF (30DGC) condenser temperatures, with 9DGF (5DGC) liquid subcooling and 
9DGF (5DGC) suction superheat.  See  REFRIGERATION. 
 �34s[Carl F. Kayan]
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�6v�2v�32sTonsil   �21m �33s 
    Localized aggregation of diffuse and nodular lymphoid tissue found in the 
region where the nasal and oral cavities open into the pharynx. The tonsils 
are important sources of blood lymphocytes. They often become inflamed and 
enlarged, necessitating surgical removal.  See  TONSILLITIS. 
    The two palatine (faucial) tonsils are almond-shaped bodies measuring 1 x 
0.5 in. (2.5 - 1.2 cm) and are embedded between folds of tissue connecting the 
pharynx and posterior part of the tongue with the soft palate. These are the 
structures commonly known as the tonsils. The lingual tonsil occupies the 
posterior part of the tongue surface. It is really a collection of 35 - 100 
separate tonsillar units, each having a single crypt surrounded by lymphoid 
tissue. Each tonsil forms a smooth swelling 0.08 - 0.16 in. (2 - 4 mm) in 
diameter. The pharyngeal tonsil (called adenoids when enlarged) occupies the 
roof of the nasal part of the pharynx. This tonsil may enlarge to block the 
nasal passage, forcing mouth breathing.  See  LYMPHATIC SYSTEM. 
 �34s[Theodore Snook]
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�6v�2v�32sTonsillitis   �21m �33s 
    An inflammation of the tonsil. Tonsillitis is a nonspecific term usually 
referring to bacterial or viral infection involving all or part of Waldeyer's 
ring, a collection of lymphatic tissue encircling the pharynx. It consists 
primarily of the tonsils (palatine tonsils), adenoids (pharyngeal tonsils), 
and lingual tonsils.  
    The complication of tonsillitis depend on which tonsil is involved. 
Recurrent adenoiditis with adenoid hypertrophy is frequently associated with 
recurrent otitis media, middle-ear fluid, and at times nasal obstruction with 
mouth breathing and snoring. Acute palatine tonsillitis may be complicated by 
peritonsillar abscess which may develop lateral to the tonsillar capsule. 
Removal of the adenoids is considered when there is residual middle-ear fluid. 
Palatine tonsils must be removed after peritonsillar abscess, but otherwise 
their removal depends upon the frequency of recurrent attacks of bacterial 
pharyngotonsillitis in relation to the patient's age.  See  TONSIL. 
 �34s[James A. Donaldson]
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�6v�2v�32sTooling   �21m �33s 
    Auxiliary devices used in manufacturing operations to supplement basic 
standard and special machine tools. Tooling is used, for example, on drill 
presses, milling machines, planers, and shapers to facilitate the actions of 
workers and to adapt machine tools to production of specific parts. All 
machine tools to a varying degree utilize removable tooling. 
    The overall category of tooling encompasses a broad spectrum of 
manufacturing accessories; tooling is further subdivided into classifications 
such as jigs, fixtures, dies, gages, devices, gadgets, and other less 
universal designations. 
    There are no universally accepted definitions for differentiating between 
these subdivisions of tooling. Nomenclature for similar tools varies from one 
industry to another; it also varies among similar industries using similar 
devices. In shop practice, tooling subdivision nomenclature loosely follows 
the generalities of the table.  
    Common hand tools and devices and standard perishable tools (consumed as a 
result of production operation) are excluded from the tooling category. 
  
    Operations.  A tooling operation may consist of a single function or a 
series of functions. Most tooling simultaneously performs some combination of 
a variety of purposes; a few of the many purposes for which tooling is used 
are locating, clamping, positioning, and cutter guiding. 
    The trend in machine tools for automation is to design and produce basic 
machine-tool building blocks, each of which is a separate machine tool, that 
can be linked with others (with or without special units) to form an automated 
production line that performs a series of milling, drilling, and similar 
machining operations. In this way basic machine-tool building blocks perform a 
variety of jobs. The standard machine-tool building block also offers 
considerable protection (through retooling or salvage) against obsolescence of 
an expensive automatic transfer machine.  See  AUTOMATION; JIG, FIXTURE, AND 
DIE DESIGN; MACHINING OPERATIONS. 
 �34s[Joseph I. Karash]
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    Numerical control.  Numerical control, in a broad sense, is an extremely 
versatile method of automatically operating machinery by means of discrete 
numerical values introduced to the machine through some form of stored-input 
medium, such as punched cards, punched tape, magnetic tape, or direct control 
by a digital computer. 
    By strict definition, numerial control (and certain other processing 
machines) is the directing of a series of coded instructions for machine 
operation which is composed largely of numbers, letters of the alphabet, and 
other symbols. 
    Numerical control commands for machine tools may range from positioning 
the spindle, tool, or workpiece to auxiliary functions such as selecting a 
tool station on a turret, controlling the direction, speed, and feed of a 
spindle, or turning the coolant off and on. The commands put together and 
logically organized can often be used to program all the operations required 
to completely machine a workpiece in one setting. This tremendous versatility 
has caused numerical control machines to be widely accepted. 
    The sequence of events in numerical control machining starts with the 
programmer who studies the part drawing. The programmer must visualize all the 
machine motions required to make the particular part. Once conceived, they 
must be documented in logical order on a programming manuscript. The 
manuscript data are then converted to a medium (often perforated tape) that 
can be acted upon by the machine control unit. 
    A schematic closed-loop feedback system is often used in numerical control 
machine control of a typical machine tool. The machine control unit accepts 
instructions from the tape reader, converts them into command signals, and 
transmits the signals to the machine as required. The command signals, which 
may be in the form of pulses, can produce machine movements of as little as 
0.0002 in. (5 micrometers) to a pulse. Alternatively, command signals may be 
of analog (continuous) nature, and a degree of accuracy as required. 
    The machine control unit is not generally regarded as a computer, although 
most numerical control systems incorporate some computer circuits. The more 
sophisticated contouring systems have machine control units or directors which 
are essentially process computers. 
    Many of the advantages of numerical control machining are obvious, such as 
simpler setups, reduced cutting-tool changing time, less operator skill 
requirements, less scrap, much greater design flexibility, and better 
accuracy. However, one of the most important advantages is the increase in 
production that numerical control affords over conventional machining. 
Traditionally, it has been observed that parts in the shop spend 95% of the 
time moving and waiting, with only 5% of the time on the machine. Of the 5% of 
the time on the machine, less than 30% is spent as actual metalworking time. 
Through the increased use of numerical control and computer control, the 
disproportionate nonproductive time can be greatly reduced. 
    The first numerical control machine generation was classified as 
hard-wired. Numerical control machines have now progressed to the soft-wired 
type, where the control can be adapted to several machine tool configurations. 
    Computer numerical control machines have a minicomputer installed in the 
control unit. When the control unit is connected to the machine tool, the 
interface is completed by programming the computer - not by hard-wiring. Thus 
computer numerical control is referred to as soft-wired control. The large 
memory of the minicomputer permits addition of many features to the controls. 
    With direct numerical control, a number of numerical control or computer 
numerical control machines are connected together to a remote computer. The 
number of machines connected could be as small as 2 or as great as 256. A big 
advantage offered by direct numerical control is that the communication 
between the machine control unit and the computer is two-way. That is, the 
machine operator can ask for specific part information or communicate with 
other departments easily and quickly. Thus, quick corrective action can 
greatly expedite machining operations and reduce machine downtime. 
    The growth of the direct numerical control hierarchy of machine tools and 
controls led naturally to complete computer-aided manufacturing (CAM). In 
direct numerical control the computer can supply information to several 
machines for individual operation. Computer-aided manufacturing goes a step 
further in that all pertinent information is contained in the computer, so 
that it serves as a communication network for the number of pieces machined at 
any given station, the amount of uptime and downtime, the production rate, and 
any other pertinent management data. The incoming raw material undergoes 
succeeding steps of inspection, premachining and so on until it reaches the 
material inventory and pallet-loading station. As each job order is received 
from the computer, the pallets are set up with the blanks, cutting tools, and 
other information. The pallets are coded so that as they go down the line, 
their paths are controlled by the computer. Depending upon the priority (set 
by the manager via the computer), the pallets move in queues. The queues may 
be stacked vertically to take advantage of plant space. The main computer is 
notified when the individual operations, parts, job lots, or inspections are 
completed. With input from shop personnel, the main computer can produce an 
up-to-the-minute status report of everything in the production area.  See  
COMPUTER-AIDED DESIGN AND MANUFACTURING; CONTROL SYSTEMS. 
 �34s[Roy A. Lindberg]
�33s 
  
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�6v�2v�32sTooth   �21m �33s 
    One of the structures found in the mouth of most vertebrates which, in 
their most primitive form, were conical and were usually used for seizing, 
cutting up, or chewing food, or for all three of these purposes. The basic 
tissues that make up the vertebrate tooth are enamel, dentin, cementum, and 
pulp (see illustration). 
    Enamel is the hardest tissue in the body because of the very high 
concentration, about 96%, of mineral salts. The remaining 4% is water and 
organic matter. The enamel has no  nerve supply, although it is nourished to a 
very slight degree from the dentin it surrounds. The fine, microscopic 
hexagonal rods (prisms) of apatite which make up the enamel are held together 
by a cementing substance. 
  
    Dentin, a very bonelike tissue, makes up the bulk of a tooth, consisting 
of 70% of such inorganic material as calcium and phosphorus, and 30% of water 
and organic matter, principally collagen. The rich nerve supply makes dentin a 
highly sensitive tissue; this sensitivity serves no obvious physiological 
function.  
    Cement is a calcified tissue, a type of modified bone less hard than 
dentin, which fastens the roots of teeth to the alveolus, the bony socket into 
which the tooth is implanted. A miscellaneous tissue, consisting of nerves, 
fibrous tissue, lymph, and blood vessels, known as the pulp, occupies the 
cavity of the tooth surrounded by dentin.  
    The dentition of therian mammals, at least primitively, consists of four 
different kinds of teeth. The incisors (I) are usually used for nipping and 
grasping; the canines (C) serve for stabbing or piercing; the premolars (Pm) 
grasp, slice, or function as additional molars; and the molars (M) do the 
chewing, cutting, and grinding of the food. Primitively the placentals have 40 
teeth and the marsupials 50.  
    In therian mammals, probably because of the intricacies and vital 
importance of tooth occlusion, only part of the first (or ``milk'') dentition 
is replaced. This second, or permanent, dentition is made up of incisors, 
canines, and premolars; as a rule only one premolar is replaced in marsupials. 
Although the molars erupt late in development and are permanent, that is, not 
replaced, they are part of the first, or deciduous, dentition.  See  TOOTH 
DISORDERS.  
 �34s[Frederick S. Szalay]
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�6v�2v�32sTooth disorders   �21m �33s 
    Diseases and disturbances of the teeth and associated structures, 
including abnormal formation and growth of the teeth and jaws, tooth decay, 
inflammation of the tissues housing the roots of the teeth, and various 
diseases of the jaw bones. 
    Defective formation of dentin and enamel are referred to respectively as 
dentinogenesis and amelogenesis imperfecta, and may be caused by febrile 
illness during the period of tooth formation, or by faulty calcium or 
phosphorus metabolism, such as occurs in rickets. Some or all of the teeth may 
fail to form completely, or extra or supernumerary teeth may be present.  
    In most societies, dental decay, or caries, is one of the most important 
and common tooth disorders. Decay occurring during the adolescent years is 
caused by a class of microorganisms referred to as the cariogenic 
streptococci, of which Streptococcus mutans is a predominant member. However, 
a susceptible host and a cariogenic diet containing sucrose are also essential 
factors. Although the mechanism by which bacteria cause decay is not 
completely understood, most experts believe that cariogenic organisms, by 
using sucrose, and to a lesser extent other sugars, produce polymers which 
bind the organisms to the tooth surface and acids which cause demineralization 
resulting in cavity formation. Fluoride administration, either in the drinking 
water or by other routes effectively reduces dental decay about 50 - 70%. The 
mechanism by which fluoride causes decreased decay rates is not understood. 
  
    Whereas dental decay is the principal cause of tooth loss in young 
individuals, inflammatory disease of the tissues surrounding the teeth, 
referred to as periodontal disease, causes most of the tooth loss in adults.  
See  PERIODONTAL DISEASE. 
    Diseases of the jaws may affect the teeth. The most common jaw disorders 
fall into four categories: (1) inflammation of the jawbone caused by 
infections such as osteomyelitis; (2) cysts associated with the teeth as well 
as those located in bone sutures of the jaws; (3) benign and malignant tumors 
of the jaws; and (4) systemic diseases such as generalized skeletal 
abnormalities produced by endocrine disfunction in which the jaws are 
affected.  See  DENTISTRY; TOOTH. 
 �34s[Roy C. Page]
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�6v�2v�32sTopaz   �21m �33s 
    A mineral best known for its use as a gemstone. Crystals are usually 
colorless but may be red, yellow, green, blue, or brown. The wine-yellow 
variety is the one usually cut and most highly prized as a gem. Citrine, a 
yellow variety of quartz, is the most common substitute and may be sold as 
quartz topaz. 
    Topaz is a nesosilicate with chemical composition Al2SiO4 (F,OH). The 
mineral is commonly found in well-developed prismatic crystals with pyramidal 
terminations. Hardness is 8 on Mohs scale; specific gravity is 3.4 - 3.6.  See 
  SILICATE MINERALS. 
    Fine yellow and blue crystals of topaz have come from Siberia and much of 
the wine-yellow gem material from Minas Gerais, Brazil. In the United States 
topaz has been found near Florissant, Colorado; Thomas Range, Utah; San Diego 
County, California; and Topsham, Maine.  See  GEM. 
 �34s[Cornelius S. Hurlbut, Jr.]
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�6v�2v�32sTophus   �21m �33s 
    A localized swelling occurring specifically in gout. Tophi occur in 
cartilage and in the connective tissues of the body, usually in or adjacent to 
the small joints of the hands and feet. Less commonly they are found in the 
eyelids, ears, muscles, tendons, or heart valves.  
    The tophus begins as a small deposit of uric acid salts, the irritant 
properties of which result in a localized region of inflammation characterized 
by exudation of white blood cells and serum. Once begun the process proceeds 
slowly but inexorably, gradually eroding the surrounding tissues. The skin 
overlying a tophus may become ulcerated, discharging semifluid material and 
chalky fragments of urate deposits, or the lesion may erode into an underlying 
joint, discharging its contents into the joint cavity. In such cases, the 
resultant scar formation usually leads to almost complete loss of function in 
the involved joint.  See  GOUT. 
 �34s[W. Robert Adams]
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�6v�2v�32sTopographic surveying and mapping   �21m �33s 
    The measurement of surface features and configuration of an area or 
region, and the graphic expression of those features. The object of 
topographic surveying usually is a map of prescribed utility, as for a 
building site, a highway location, or military operations. 
    The choice between ground and aerial methods is largely economic. 
Substantial areas and difficult terrain are surveyed with less cost by aerial 
surveys. Smaller areas, requiring but one or a few days of field-party time, 
may be surveyed and mapped for less money than the aerial-survey mobilization 
costs of flying and photo processing. Visibility of the ground and other 
features to be mapped also influence the choice.  See  AERIAL PHOTOGRAPH; 
CARTOGRAPHY; PHOTOGRAMMETRY; SURVEYING. 
 �34s[Robert H. Dodds]
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�6v�2v�32sTopology �21m �33s
    The study of topological spaces and continuous maps. Precise mathematical
definitions of topological space and continuous map require no more than
elementary set theory.

    Sets.  A set is any collection of things or objects. If X is a set and x
is a member of the set X, one writes x epsilon X, which reads, in words, x
belongs to X. The symbol epsilon denotes membership. An element or member
of a set X is also called a point of X. The rational numbers form a set
frequently denoted by Q, and x epsilon Q means that x is a rational number.
    If X and Y are sets, then X UNION Y is the set of elements which belong to
either X or Y (including those elements which belong to both X and Y). The
notation X UNION Y is read X union Y. Similarly if X and Y are sets, then X
INTERSECTION Y is the set of elements which belong to both X and Y, and the
notation X INTERSECTION Y is read X intersect Y.
    Suppose that for each element i of a set I there is given a set A(sub i),
the set consisting of all the sets A(sub i) is denoted by {A sub i}(sub i
epsilon I), and is said to be a collection of sets indexed on the set I.  If {A
sub i}(sub i epsilon I) is a collection of sets indexed on the set I, then
UNION(sub i epsilon I)A(sub i) is the set consisting of those elements which
belong to at least one of the sets A(sub i), and is called the union of {A sub
i}(sub i epsilon I).  Similarly INTERSECTION(sub i epsilon I) A(sub i) is the
set consisting of those elements which belong to every one of the sets A(sub i),
and is called the intersection of {Asubi}(sub i epsilon I).  See SET THEORY.

    Topological space.	This is a set of points X, together with a collection
of subsets of X called open subsets of X where the following assumptions are
made:
    1. PHI and X are open subsets of X.
    2. If A and B are open subsets of X, then A INTERSECTION B is an open
subset of X.
    3. If {Asubi}(sub i epsilon I) is a collection of open subsets of X, the
UNION(sub i epsilon I) A(sub i) is an open subset of X.
    Suppose that R is the set of real numbers. If r epsilon R, in other
words, if r is a real number, let VERTICAL BAR r VERTICAL BAR denote the
absolute value of r.  This means that, if r is greater than or equal to zero,
then VERTICAL BAR r VERTICAL BAR = r, but if r is less than zero then VERTICAL
BAR r VERTICAL BAR = -r.  In order to make the real numbers into a topological
space, open subsets of R are defined using the notion of absolute value.
Precisely, a subset U of R is open if for every x epsilon U there is a real
number esubx greater than zero having the property that if for some real numbers
y, VERTICAL BAR y-x VERTICAL BAR <EPSILON(sub x), then y epsilon U.  In other
words, if x epsilon U, then any real number sufficiently close to x belongs to U
also.  With this definition of open subset of the real numbers it is not
difficult to verify that the axioms for a topological space are satisfied.  When
one talks of the real numbers in mathematics, one usually means the real numbers
as a topological space, that is to say, the set of real numbers together with
the collection of open subsets that are defined above.

    Continuous map.  In order to continue the discussion of topology, it is
necessary to introduce some further notions of set theory. First, suppose that
X and Y are sets. Define the product of the sets X and Y to be the set
consisting of pairs of elements (x, y) such that x epsilon X and y NATURAL
LOG Y. The product of X and Y is denoted by X TIMES Y. Now a function f from X
to Y is defined to be a subset f of the product X TIMES Y such that, if x
epsilon X, there exists a unique y epsilon Y such that (x,y) epsilon f. In
this case y is said to be the value of f at x, and is denoted by f(x).
Intuitively a function from X to Y is thought of as a rule which assigns to
each element of the set X an element of the set Y. The standard mathematical
notation for a function f from X to Y is f:X->Y.
    Let f:X->Y be a function and suppose U is a subset of Y, then f(sup -1) (U)
is the subset of X consisting of those points x such that f(x) EPSILON U. If
X and Y are topological spaces, then f:X->Y is a continuous map, or continuous
function, (1) if f is a function from X to Y, and (2) if, whenever U is an
open subset of Y, the set f(sup -1) (U) is an open subset of X.
    The topological spaces X and Y are said to be homeomorphic if there exist
continuous maps f:X->Y and g:Y->X such that f DG g:Y->Y is the identity map of Y
and g DG f:X->X is the identity map of X.  In this case the maps f and g are
said to be homeomorphisms, and g is frequently denoted by f(sup -1).  The symbol
f( sup -1) is read f inverse.

    Metrics.  When the topology on the real numbers was defined, this was a
special case of defining a topology by using a metric. Let X be a set. A
metric on X is a function rho:X TIMES X->R such that the following axioms
obtain:
    1. If x and x' belong to X, then rho(x,x') is greater than or equal to
zero, and rho(x,x')= 0 if and only if x = x'.
    2. If x and x' belong to X, then rho(x,x')|= rho(x',x).
    3. If x,x', and x" belong to X, then rho(x,x") is less than or equal to
rho(x',x') + rho(x',x").
    If rho:X TIMES X->X is a metric, then rho(x,x') is called the distance from
x to x'. Axiom 1 says that the distance between any two points of X is greater
than or equal to zero, and is different from zero if the points are different.
Axiom 2 says that the distance from x to x' is the same as the distance from
x' to x. Axiom 3, the so-called triangle axiom, says intuitively that it is
shorter to proceed from x to x" along a straight line than it is first to
proceed from x to x' along a straight line and then proceed from x' to x" along
another straight line.

    Construction of topological spaces.  Two topological spaces which are
homeomorphic cannot be distinguished by the methods of topology. Any
topological property of one is also a topological property of the other. Since
homeomorphic spaces cannot be distinguished by topological methods, one of the
important problems of topology is to determine whether two topological spaces
are homeomorphic or not.
    The general notion of topological space is not sufficiently restrictive
for most purposes. Therefore some additional axioms are almost always assumed
- in particular the Hausdorff separation axiom. A topological space X
satisfies the Hausdorff separation axiom, or is a Hausdorff space, if for
every two distinct points x and x' of X there are open subsets U and V of X
such that x epsilon U, x' epsilon V, and U INTERSECTION V is empty.

    Manifolds.	Though Hausdorff spaces form a much more interesting class of
spaces than general topological spaces, the most important class of
topological spaces, which is still much smaller, consists of the manifolds. An
n-manifold is a Hausdorff space X such that the following conditions exist:
    1. For every point x epsilon X there is an open subset Usubx of X such
that x epsilon Usubx and such that Usubx is homeomorphic with an open subset of
R(sup n).
    2. There exists a metric rho : X TIMES X->R such that the topology on X is
induced by the metric.
    A topological space X is connected if it cannot be expressed as the union
of two disjoint nonempty open subsets. One of the most important problems of
topology is the problem of classification of connected n-manifolds.  See
MANIFOLD (MATHEMATICS).
 �34s[John C. Moore]
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�6v�2v�32sTorbanite   �21m �33s 
    A variety of coal that resembles a carbonaceous shale in outward 
appearance. It is fine-grained, brown to black, and tough, and breaks with a 
conchoidal or subconchoidal fracture. Torbanite is synonymous with boghead 
coal and is related to cannel coal. It is derived from colonial algae 
identified with the modern species of Botryococcus braunii and antecedent 
forms. High-assay torbanite yields paraffinic oil, whereas low-assay material 
yields asphaltic oil.  See  COAL.   
 �34s[Irving A. Breger]
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�6v�2v�32sTorch   �21m �33s 
    A gas-mixing and burning tool that produces a hot flame for the welding or 
cutting of metal. The torch usually delivers acetylene and commercially pure 
oxygen producing a flame temperature of 5000 - 6000DGF (2750 - 3300DGC), 
sufficient to melt the metal locally. The torch thoroughly mixes the two gases 
and permits adjustment and regulation of the flame. Acetylene can produce a 
higher flame temperature than other fuel gases.  See  ACETYLENE; WELDING AND 
CUTTING OF METALS. 
    Torches are of two types: low-pressure and high-pressure. In a 
low-pressure, or injector, torch, acetylene enters a mixing chamber, where it 
meets a jet of high-pressure oxygen. The amount of acetylene drawn into the 
flame is controlled by the velocity of this oxygen jet. In a high-pressure 
torch both gases are delivered under pressure.  
    A welding torch mixes the fuel and gas internally and well ahead of the 
flame. For cutting, the torch delivers an additional jet of pure oxygen to the 
center of the flame. The oxyacetylene flame produced by the internally mixed 
gases raises the metal to its ignition temperature. The central oxygen jet 
oxidizes the metal, the oxide being blown away by the velocity of the gas jet 
to leave a narrow slit or kerf. 
 �34s[Frank H. Rockett]
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�6v�2v�32sTornado   �21m �33s 
    A violently rotating, tall columnar vortex typically 30 - 330 ft (10 - 100 
m) in diameter. It is characterized by a debris cloud near the ground and a 
funnel cloud composed of small water droplets pendant from a parent cloud 
system of cumulus type. The visible funnel cloud need not reach all the way to 
the ground and is often obscured by debris. The tornado most often develops in 
association with a severe thunderstorm. Heavy rain and large hailstones often 
fall just to the north of the tornado's path in the Northern Hemisphere.  See  
THUNDERSTORM.  
    There is evidence of a characteristic life cycle, especially for large, 
destructive tornadoes. The tornado life cycle typically consists of five 
parts: dust-whirl stage (dust swirls upward from the surface, or a short 
pendant funnel may appear above the ground); organizing stage (the visible 
funnel intermittently touches the ground with a continuous damage path); 
mature stage (the tornado is at its largest size and most intense 
circulation); shrinking stage (the entire funnel decreases to a thin, ropelike 
column); and decaying stage (the funnel is fragmented and contorted). Even in 
its final stages, the tornado may retain its destructive power. 
    Weather conditions that favor tornado formation include abundant moisture 
in the lowest 0.6 mi (1 km) or so, separated from a dry air mass at 
intermediate levels by a stable layer or inversion, deep thermodynamic 
instability, a mechanism to locally remove the stable layer, and winds that 
veer and increase with height. 
 �34s[Joseph Golden]
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�6v�2v�32sTorque   �21m �33s 
    The product of a force and its perpendicular distance to a point of 
turning, also called the moment of the force. Torque produces torsion and 
tends to produce rotation. Torque arises from a force or forces acting 
tangentially to a cylinder or from any force or force system acting about a 
point. A couple, consisting of two equal, parallel, and oppositely directed 
forces, produces a torque or moment about the central point. A prime mover 
such as a turbine exerts a twisting effort on its output shafts, measured as 
torque. In structures, torque appears as the sum of moments of torsional shear 
forces acting on a transverse section of a shaft or beam.  See  COUPLE; 
TORSION.  
 �34s[Nelson S. Fisk]
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�6v�2v�32sTorque converter   �21m �33s 
    A device for changing the torque-speed ratio or mechanical advantage 
between an input shaft and an output shaft. A pair of gears is a mechanical 
torque converter. A hydraulic torque converter is an automatically and 
continuously variable torque converter, in contrast to a gear shift, whose 
torque ratio is changed in steps by an external control.  See  AUTOMOTIVE 
TRANSMISSION; GEAR DRIVE. 
    A mechanical torque converter transmits power with only incidental losses; 
thus, the power, which is the product of torque T and rotational speed N, at 
input I is substantially equal to the power at output O of a mechanical torque 
converter, or TsubI|NsubI = kTsubO|NsubO, where k is the efficiency of the 
gear train. This equal-power characteristic is in contrast to that of a fluid 
coupling in which input and output torques are equal during steady-state 
operations.  See  FLUID COUPLING. 
    In a hydraulic torque converter, efficiency depends intimately on the 
angles at which the fluid enters and leaves the blades of the several parts. 
Because these angles change appreciably over the operating range, k varies, 
being by definition zero when the output is stalled, although output torque at 
stall may be three times engine torque for a single-stage converter and five 
times engine torque for a three-stage converter. Depending on its input 
absorption characteristics, the hydraulic torque converter tends to pull down 
the engine speed toward the speed at which the engine develops maximum torque 
when the load pulls down the converter output speed toward stall. 
 �34s[Henry J. Wirry]
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�6v�2v�32sTorricelli's theorem �21m �33s
    The speed of efflux of a liquid from an opening in a reservoir equals the
speed that the liquid would acquire if allowed to fall from rest from the
surface of the reservoir to the opening.
    Torricelli, a student of Galileo, observed this relationship in 1643. In
equation form, v(sup2)= 2gh, in which v is the speed of efflux, h the head (or
elevation difference between reservoir surface and center line of opening if
in a vertical plane), and g the acceleration due to gravity. (The equation is
the same as that for a solid particle dropped a distance h in a vacuum.) The
relationship can be derived from the energy equation for flow along a
streamline, if energy losses are neglected.  See  FLOW MEASUREMENT.
 �34s[Victor L. Streeter]
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�6v�2v�32sTorsion �21m �33s
    A straining action produced by couples that act normal to the axis of a
member. Torsion is identified by a twisting deformation.
    In practice, torsion is often accompanied by bending or axial thrust as in
the case of line shafting driving gears or pulleys, or propeller shafts for
ship propulsion. Other important examples include springs and machine
mechanisms usually having circular sections, either solid or tubular. Members
with noncircular sections are of interest in special applications, such as
structural members subjected to unsymmetrical bending loads that twist and
buckle beams.  See  SPRING (MACHINES); TORSION BAR.
    When subjected only to torque, the member is in pure torsion, which
produces pure shear stresses. The shear properties of materials are determined
by a torsion test.  See  SHEAR; TORQUE.
 �34s[John B. Scalzi]
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�6v�2v�32sTorsion bar �21m �33s
    A spring flexed by twisting about its axis. Design of a torsion bar spring
is primarily based on the relationships between the torque applied in twisting
the spring, the angle through which the torsion bar twists, and the physical
dimensions and material (modulus of elasticity in shear) from which the torsion
bar is made. The illustration shows the elements of a simple torsion bar and
the important dimensions involved in its design. The equation relates these
these dimensions.

	***   THETA = (32 Fal)/(PI D(sup 4) G)	 ***

Here THETA is angle of twist in radians, F is force in pounds,
a is radius arm of force in inches, l is length of torsion bar in
inches, D is diameter of torsion bar in inches, and G is modulus of elasticity
in shear in pounds per square inch.
    Torsion bar springs are found in the spring suspension of truck and
passenger car wheels, in production machines where space limitations are
critical, and in high-speed mechanisms where inertia forces must be minimized.
  See  SPRING (MACHINES).
 �34s[L. Sigfred Linderoth, Jr.]
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�6v�2v�32sTorus   �21m �33s 
    A surface obtained by rotating a circle about a line that lies in its 
plane, but which has no points in common (see illustration). It is a 
two-dimensional manifold of genus 1 and connectivity 3.  See  MANIFOLD 
(MATHEMATICS); SURFACE AND SOLID OF REVOLUTION; TOPOLOGY. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sTouch   �21m �33s 
    A term for both the generic designation for general bodily feeling and, 
more narrowly, the array of skin sensations ranging from contact to dull 
pressure and including light touch, granular pressure, and a host of other 
cutaneous patterns for which specific names have not entered the English 
language. The system of pressure sensitivity of the skin yields a wealth of 
feeling patterns which differ among themselves along the spatial, temporal, 
and intensitive dimensions. They have also been held to vary within a 
brightness-dullness continuum, by analogy with visual sensations. For certain 
of the patterns which occur repeatedly and familiarly, there are terms such as 
lively contact, tickle, vibration, and deep pressure. Others, though 
recognizable when felt, are nameless and thus impossible to describe.  See  
CUTANEOUS SENSATION; SOMESTHESIS.   
 �34s[Frank A. Geldard]
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�6v�2v�32sTourmaline �21m �33s
    A cyclosilicate mineral family with (BO3) triangular groups and a complex
chemical composition. The general formula can be written XY3Al6(OH)4-
(BO3)3(Si6O18), in which X = Na,Ca and Y = Al, Fe(sup 3+),Li,Mg,Mnsup2supp.
The more common tourmalines are dravite, schorl, uvite, and elbaite. Fluorine
commonly substitues in the hydroxyl position. Tourmaline is a hard (7 1/2 on
Mohs scale), varicolored mineral which can be an important semiprecious
gemstone.  See	GEM; SILICATE MINERALS.
 �34s[Paul B. Moore]
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�6v�2v�32sTower   �21m �33s 
    A concrete, metal, or timber structure that is relatively high for its 
length and width. Towers are constructed for many purposes, including the 
support of electric power transmission lines, radio and television antennas, 
and rockets and missiles prior to launching. 
    Transmission towers are rectangular in plan and are not steadied by guy 
wires. A transmission tower is subjected to a number of forces; its own 
weight, the pull of the cables at the top of the tower, the effect of wind and 
ice on the cable, and the effect of wind on the tower itself. 
    Radio and television towers are either guyed or freestanding. Freestanding 
towers are usually rectangular in plan. In addition to their own weight, 
freestanding towers support the weight of the antenna and accessories and the 
weight of ice, unless a deicing circuit is installed. Wind forces must also be 
carefully considered. Guyed towers are usually triangular in plan, with the 
main structural members, or legs, at the vertexes of the triangle. The legs 
are usually solid round steel bars.  See  ANTENNA (ELECTROMAGNETISM); 
TRANSMISSION LINES. 
 �34s[Charles M. Antoni]
�33s 
  
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�6v�2v�32sTowing tank   �21m �33s 
    A tank of water used to determine the hydrodynamic performance of 
waterborne bodies such as ships and submarines, as well as torpedoes and other 
underwater forms. In the narrow sense, towing tanks are considered to be 
experimental facilities used to measure the forces, such as drag, on ship 
models and in turn to predict the performance of the full-scale prototype. In 
general, towing tanks are rectangular in planform with a uniform cross 
section. Different section shapes are used, ranging from rectangular to 
semicircular.  
    The principal measurements made in a towing tank are force measurements, 
particularly drag or resistance of a towed ship model or other body. One of 
two principal systems for towing a model is used in most towing tanks. The 
simpler system consists of a gravity dynamometer and an endless cable attached 
to the model. A weight provides a constant towing force (see illustration). 
The time to traverse a fixed distance is measured when the model reaches a 
constant speed, thus establishing the speed-resistance relationship for the 
model. This dynamometer is simple and capable of high accuracy, but is limited 
to the measurement of the drag force of waterborne bodies. It is used in the 
smaller towing tanks in which the models are generally under 6 ft (1.8 m) in 
length.  
    In larger towing tanks the model is towed by a towing carriage mounted on 
rails at the side of the towing tank or suspended from an overhead track 
system. Speed can be controlled and measured precisely on these carriages. 
Most carriages are equipped with a drag dynamometer as a permanent component. 
 �34s[Jacques B. Hadler]
�33s 
  
  
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�6v�2v�32sTownsend discharge   �21m �33s 
    A particular part of the voltage-current characteristic curve for a 
gaseous discharge device named for J. S. Townsend, who studied it about 1900. 
It is that part for low current where the discharge cannot be maintained by 
the field alone. Thus, if the agents producing the initial ionization were 
removed, conduction would cease.  See  ELECTRICAL CONDUCTION IN GASES. 
 �34s[Glenn H. Miller]
�33s 
  
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�6v�2v�32sToxemia   �21m �33s 
    A condition in which toxins are found circulating in the blood stream; 
also, a disease peculiar to pregnancy. Toxins can cause generalized tissue 
damage, with symptoms such as malaise, weakness, fever, aching, and 
diaphoresis, or they can specifically injure certain organ systems such as the 
gastrointestinal tract, nervous tissue, or hematopoietic system. Specific 
symptoms occur in response to damage to each of these areas, and in some cases 
this damage is irreversible.  See  BLOOD; ENZYME; FOOD POISONING; TOXIN. 
    In pregnancy this disease is characterized by acute hypertension after the 
twenty-fourth week of gestation, accompanied by proteinuria and edema of hands 
and face (also called preeclampsia-eclampsia). This disease occurs in 6 - 7% 
of pregnancies, ranges from mild to severe, and may threaten the life of both 
fetus and mother. The cause of this condition is unknown, but it involves 
alterations in the maternal salt-and-water balance, generalized 
vasoconstriction, compromised kidney function, and increased central nervous 
system irritability.  See  PREGNANCY. 
 �34s[G. Guzinski]
�33s 
  
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�6v�2v�32sToxicology   �21m �33s 
    The study of the harmful effects of chemicals and physical agents on 
living sytems. It includes the identification of the stimulating agent and the 
mechanisms underlying its interactions with biotic systems, the prevention and 
treatment of negative responses, and in some cases, the recognition of 
advantageous uses of these interactions. Toxicology is a multidisciplinary 
field. It includes the use of chemistry, especially biochemistry, and of 
various branches of the biological sciences, such as physiology, histology, 
and pathology. Because of the ever-expanding use and manipulation of the 
environment, there is an increasing demand for the type of knowledge that a 
toxicologist can provide. 
    Three major divisions of toxicology are environmental toxicology, clinical 
toxicology, and forensic toxicology. With increasing populations and 
industrialization, environmental toxicology has assumed great importance. 
Among the points of concern to workers in this area are agents which 
contaminate or are purposefully added to food, air, water, soil, and the 
working place. Clinical toxicology is a specialized field in medicine and is 
concerned with the clinical management of diseases that are associated with 
chemicals of exogenous origin. These diseases include those produced as a 
result of accidental or purposeful drug overdose, adverse drug interactions, 
poisoning which is job-related, and toxic responses to naturally occurring 
agents.  
    Forensic toxicology involves the medicolegal aspects of a chemical's 
effects on humans primarily. It has been involved in the determination of the 
cause of death in order to understand the circumstances leading to the event. 
It has also played an important role in the identification of specific hazards 
related to various chemicals.  See  ENVIRONMENTAL PROTECTION; FORENSIC 
MEDICINE; POISON. 
 �34s[N. Karle Mottet ;Carol Quaife]
�33s 
  
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�6v�2v�32sToxin-antitoxin reaction   �21m �33s 
    A term used in serology to denote the combination of a toxic antigen with 
its corresponding antitoxin. If the antitoxin is derived from any species 
other than the horse, precipitation occurs over a wide range of reactant 
ratios, 20x or more, as in other antigen-antibody reactions. With horse 
antitoxin, flocculation occurs only if toxin and antitoxin are near 
equivalence, a twofold excess of either reactant giving soluble complexes. In 
most instances, the reaction results in partial or complete neutralization of 
the toxic activity of the antigen.  See  ANTIBODY; ANTIGEN; ANTITOXIN; 
NEUTRALIZATION REACTION (IMMUNOLOGY); SEROLOGY. 
 �34s[Henry P. Treffers]
�33s 
  
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�6v�2v�32sToxoplasmea   �21m �33s 
    A class of the subphylum Sporozoa. The organisms are small and 
crescent-shaped. They move by body flection or gliding and have no flagella or 
pseudopodia. Characteristic structures are the two-layered pellicle and 
underlying  longitudinal microtubules, micropyle, a conoid, paired organelles, 
and micronemes. 
    The most distinguishing characteristics of the Toxoplasmea is the unique 
means of reproduction. Electron microscope studies indicate that endodyogeny 
is the sole method. Endodyogeny is an internal budding wherein two daughter 
cells are produced within a mother cell which is destroyed in the process. 
    Only two stages are known in the life cycle of most animals. One stage, 
the proliferative form or trophozoite, occurs singly or in groups within host 
cells. The other stage, the so-called cysts, consists of a large number of 
organisms which, with minor differences, are structurally similar to the 
proliferative forms.  See  SPOROZOA; TOXOPLASMIDA. 
 �34s[Harley G. Sheffield]
�33s 
  
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�6v�2v�32sToxoplasmida   �21m �33s 
    An order of the class Toxoplasmea. Four genera, Toxoplasma, Besnoitia, 
Sarcocystis, and Encephalitozoon, make up the order. The organisms are 
parasites of vertebrates. Toxoplasma is often found encysted in nerve tissue, 
Besnoitia in connective tissue, and Sarcocystis in muscle. Very little is 
known about Encephalitozoon, but the parasite has been found in the brain of 
rabbits.  See  TOXOPLASMEA; TOXOPLASMOSIS. 
 �34s[Harley G. Sheffield]
�33s 
  
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�6v�2v�32sToxoplasmosis   �21m �33s 
    An infection of humans and animals by the protozoan Toxoplasma gondii. The 
actively multiplying tachyzoites of Toxoplasma are found during acute 
infection in mammals and birds. Tachyzoites multiply in cells and many cells 
may be destroyed, resulting in illness. After a number of tachyzoite cycles, 
cysts develop which persist for months or years. After a cat ingests tissues 
of an infected animal, at least five multiplicative stages are found in the 
intestinal epithelium, and then sexual stages appear. Oocysts are shed in the 
feces of the cat, after which they undergo sporogony. These sporulated oocysts 
are now infectious to many mammals and birds, which are called intermediary 
hosts. Cats are the final host since they support the sexual cycle. 
    Most Toxoplasma infections in humans are asymptomatic. The rare clinical 
symptoms can be characterized according to the stage of infection during which 
they occur. Acute infection may be associated with pneumonia, hepatitis, a 
rash, and lymph node inflammation. In these instances, lesions may result from 
the destruction of many parasitized cells. 
    Apart from the clear-cut transmission pattern that can be estimated 
statistically, situations exist that should be avoided since they increase the 
chance of transmission to the individual. The precautions include the disposal 
of feces from cats that hunt birds and mice or are fed raw meat. If a 
litterbox is used, it should be changed and disinfected daily. Hands should be 
washed after contact with cats and with soil potentially contaminated with cat 
feces and after handling raw meat.  See  MEDICAL PARASITOLOGY. 
 �34s[Jacob K. Frenkel]
�33s 
  
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�6v�2v�32sTrace analysis   �21m �33s 
    The determination of the elemental constituents of a sample in which these 
constituents make up approximately 0.01% of the sample or less. There is no 
sharp boundary between nontrace and trace constituents. The lower limit is set 
by the sensitivity of the available analytical methods and, in general, is 
pushed downward with progress in analytical techniques. A large number of 
different physical and chemical techniques have been developed for the 
measurement of the elemental composition at the microgram-per-gram and 
nanogram-per-gram level, thereby constituting the field of trace analysis. 
    Included in the goals of trace analysis are the determination of the bulk 
or total concentrations of the trace elements; the preconcentration of 
microconstituents into a small, essentially matrix-free sample to improve 
detectability or remove matrix interferences; the determination of local 
concentrations using probe techniques in order to establish the topographical 
distribution of trace elements in solid samples; and the determination of 
major and minor species in a minute initial sample. 
    Methods having applicability to trace analysis range from the more 
classical chemical methods of colorimetric and absorption spectrophotometric 
analysis to modern instrumental approaches. Of the various criteria used in 
the selection of an appropriate trace analytical method, sensitivity, accuracy 
and precision, and selectivity are of prime importance. Other important 
considerations, such as scope, sampling and standards requirements, cost of 
equipment, and time of analyses, are of great practical significance. 
    The application of any analysis technique to trace problems is governed to 
a large extent by the analytical sensitivity that can be achieved for the 
species of interest in a given material. Sensitivity reflects the ability to 
discern a small change in concentration or amount of species of interest. 
Detection limit is a closely related, but distinct, term used to indicate the 
lowest concentration or amount that can be determined with a specific degree 
of confidence. As a result of widely varying pathways of development of many 
analytical techniques, there is a lack of consistency in the definition of 
specification of detection limits in the analytical literature. A summary of 
the experimental values that have been published is given in the table.  See  
ACTIVATION ANALYSIS; ANALYTICAL CHEMISTRY; CALIBRATION; CHROMATOGRAPHY; 
ELECTROCHEMICAL TECHNIQUES; GAS CHROMATOGRAPHY; SPECTROSCOPY. 
 �34s[Andrew T. Zander]
�33s 
  
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1774
\IMG\CI1681.IMG
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�6v�2v�32sTrace fossils   �21m �33s 
    Tracks, trails, and burrows made by animals and found in ancient sediments 
such as sandstones, shales, or limestones. Comparable names are ichnofossils, 
bioturbate structures, and biogenic structures. An assemblage of trace fossils 
has been called ichnocoenosis. 
    Very different biological activities of animals produce these biogenic 
structures. Study of trace fossils is known as ichnology or palichnology and 
is a part of the science of paleontology. The investigations of recent tracks 
and trails, for instance, on the beach of a sea or on tidal flats, help to 
interpret the fossil ones. 
    In the early years of paleontology, most of the ramified burrows, creeping 
trails, or other trace fossils were considered to be remains of marine algae. 
This is proven by many names ending with ``phycus'' given to trace fossils. 
That misinterpretation was caused by the striking similarity of many of these 
algae to trace fossils (Fig. 1). Trails resembling worms were sometimes 
assumed to be the bodies of the worms themselves. Other trace fossils, now 
recognized to represent the pattern of a grazing activity of animals, were 
interpreted as the spawn of sea snails (Fig. 2). About 1890 thorough and 
systematic ichnological observations of the traces of modern marine animals 
convinced most of the paleontologists that these interpretations were 
erroneous.  
    Trace fossils occur in the sedimentary rocks of all ages and of marine, 
limnic, fluviatile, and even continental environments. Sometimes these fossils 
alone characterize some stratified rocks in which bodily preserved animals 
such as mollusks (that is, the shells) are lacking. In addition, the trace 
fossils are important as direct evidence of many animals, even soft-bodied 
ones, that once lived within or on these deposits but have not been fossilized. 
    Generally paleontologists use for trace fossils the same binominal 
nomenclature, with Latin names, as in naming animals and plants 
scientifically. The first name then represents the ichnogenus, the second the 
ichnospecies. That system seems the most suitable one. However, it should 
never be forgotten that ``genus'' and ``species'' in ichnology have different 
meanings from those of the same terms applied to the actual remains of animals 
and plants.  See  FOSSIL; PALEONTOLOGY. 
 �34s[Walter Haentzschel]
�33s 
  
  
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�6v�2v�32sTracheophyta   �21m �33s 
    A large group of plants, recognized in some systems of classification as a 
single division, characterized by the presence of specialized conducting 
tissues called xylem and phloem in the roots, stems, and leaves. Because of 
their characteristic conducting tissues, the Tracheophyta are often called the 
vascular plants.  See  PHLOEM; XYLEM. 
    In the system of classification presented in the encyclopedia, the 
Tracheophyta are not given formal taxonomic recognition; they are considered 
instead to consist of seven divisions, the Rhyniophyta, Psilotophyta, 
Lycopodiophyta, Equisetophyta, Polypodiophyta, Pinophyta, and Magnoliophyta. 
These seven together with the division Bryophyta constitute the subkingdom 
Embryobionta. See articles on the individual divisions.  See  EMBRYOBIONTA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sTrachoma   �21m �33s 
    A chronic conjunctivitis of humans. The disease is endemic in many 
developing countries; it is the world's leading cause of preventable 
blindness. The severity of the disease is often related to the socioeconomic 
status of the community. In severely affected communities the entire 
population acquires the infection, and more than 10% of adults may be blinded 
as a result of the disease. Trachoma is caused by an obligate intracellular 
bacterium, Chlamydia trachomatis. This organism, in conjunction with secondary 
bacterial pathogens, can produce a severe conjunctivitis which results in 
scarring of the conjunctiva (inner lining of the eyelids). The active disease 
is seen in children. Blindness occurs in adults and results from contraction 
of the scars, which turn the eyelids and eyelashes inward so that they abrade 
the cornea.  See  CHLAMYDIAL DISEASES; CONJUNCTIVITIS. 
 �34s[Julius Schachter]
�33s 
  
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�6v�2v�32sTrachylina   �21m �33s 
    An order of jellyfish of the class Hydrozoa of the phylum Coelenterata. 
These jellyfish are of moderate size. They differ from other hydrozoan 
jellyfish in having balancing organs which develop partly from the digestive 
epithelium and in having only a small polyp stage or none at all. Many 
authorities recognize three distinct orders of trachylines - Limnomedusae, 
Trachymedusae, and Narcomedusae - and in this case the older term Trachylina 
is abandoned.  See  HYDROZOA. 
 �34s[Sears Crowell]
�33s 
  
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�6v�2v�32sTrachyte   �21m �33s 
    A light-colored, aphanitic (very finely crystalline) rock of volcanic 
origin, composed largely of alkali feldspar with minor amounts of dark-colored 
(mafic) minerals (biotite, hornblende, or pyroxene). If sodic plagioclase 
(oligoclase or andesine) exceeds the quantity of alkali feldspar, the rock is 
called latite. Trachyte and latite are chemically equivalent to syenite and 
monzonite, respectively.  See  LATITE; MONZONITE; SYENITE. 
    Streaked, banded, and fluidal structures due to flowage of the solidifying 
lava are commonly visible in many trachytes and may be detected by a parallel 
arrangement of tabular feldspar phenocrysts. A distinctive microscopic feature 
is trachytic texture in which the tiny, lath-shaped sanidine crystals of the 
rock matrix are in parallel arrangement and closely packed. 
    Trachyte is not an abundant rock, but it is widespread. It occurs as 
flows, tuffs, or small intrusives (dikes and sills). It may be associated with 
alkali rhyolite, latite, or phonolite.  See  IGNEOUS ROCKS; MAGMA; SPILITE. 
 �34s[Carleton A. Chapman]
�33s 
  
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�6v�2v�32sTractor   �21m �33s 
    A wheeled, self-propelled vehicle for hauling other vehicles or equipment 
and for operating the towed implements; also, a crawler which runs on an 
endless, self-laid track and performs similar functions. 
    The major components are engine, clutch, and transmission. Unlike 
passenger-car engines, which are of the high-speed type, tractor engines are 
relatively low-speed; their maximum horsepower is generated at crankshaft 
speeds in the neighborhood of 2000 revolutions per minute. These engines have 
one, two, three, four, six, or eight cylinders and operate on gasoline, 
kerosine, LP gas, or diesel fuel. They are of the spark-ignition or diesel 
type, operating on the four-stroke-cycle principle, and are cooled by water or 
air.  
    A farm tractor is a multipurpose power unit. It has a drawbar for drawing 
tillage tools and a power takeoff device for driving implements or operating a 
belt pulley. All models can be grouped under four general types: four-wheel, 
row-crop or high-wheel, tricycle, and crawler. Power delivered at the place 
where it is useful is what counts; therefore, tractors are rated by the 
horsepower they deliver at the drawbar and at the belt. 
    The basic design of an industrial tractor for hauling and for operating 
construction equipment departs little from that of a farm tractor, and 
differences in design of models fit the vehicle to its intended work. 
 �34s[Philip H. Smith]
�33s 
  
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�6v�2v�32sTractrix �21m �33s
    A plane curve for which the length of any tangent between the curve and a
fixed line is constant c. If the x axis is the fixed line, its differential

    *** (dy/dx)(sup 2) = y(sup 2)/[c(sup 2) - y(sup 2)]     *** 	    (1)


    *** x = c log tan 1/2 (ABSOLUTE VALUE PSI) + c cos PSI  *** 	    (2)
	y = c sin PSI

    *** s = -c log sin PSI = c log (c/y)    *** 			    (3)

equation is Eq. (1). With PSI, the inclination of the tangent, as parameter,
this yields equations (2).  The tractrix has the x axis as asymptote and
cusps at (0,  c).  The arc AP (see illustration), measured from a cusp, is
as in Eq. (3).
 �34s[Louis Brand]
�33s



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�6v�2v�32sTraffic-control systems   �21m �33s 
    Systems that act to control the movement of vehicles, such as airplanes, 
trains, and automobiles. The system of traffic lights used in most urban areas 
to control automotive traffic is a common example. Traffic-control systems 
ensure safe, orderly arrival of airplanes at airports during periods of heavy 
traffic; provide efficient, economical movement of railroad cars from one 
point to another; and reduce congestion and provide smooth flow of traffic 
along heavily traveled roads in metropolitan areas. The nature and complexity 
of a traffic-control system are determined by the responsiveness of the 
vehicle operator, vehicle type, severity of the traffic problem, and the 
requirements placed upon the traffic-control system.  See  AIR-TRAFFIC 
CONTROL; TRAFFIC ENGINEERING; TRANSPORTATION ENGINEERING. 
 �34s[Louis J. Pignataro]
�33s 
  
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�6v�2v�32sTraffic engineering   �21m �33s 
    The determination of the required capacity and layout of highway and 
street facilities that can safely and economically serve vehicular movements 
between given points. 
    Major traffic arteries in urban areas are arterial, secondary, and local 
streets. Arterial routes may be further identified as either major streets or 
expressways; the latter are also known as freeways. Secondary streets are 
sometimes described as collectors. Roads in rural areas are usually designated 
as primary, secondary, or tertiary, depending upon their importance. 
    In planning improvement in traffic arteries, consideration must be given 
to many factors, including traffic volumes, desired speeds of travel, 
vehicular dimensions, driving habits, vehicular performance, type of terrain 
traversed, and especially expected future changes in all these factors. 
    Traffic engineering also deals with parking problems. Investigations 
relating to parking include determinations as to the desired size, type, and 
location of off-street facilities, plus studies relating to movements into and 
out of parking areas.  See  HIGHWAY ENGINEERING; TRANSPORTATION ENGINEERING. 
 �34s[Archie N. Carter]
�33s 
  
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�6v�2v�32sTrajectory   �21m �33s 
    The curve described by a body moving through space, as of a meteor through 
the atmosphere, a planet around the Sun, a projectile fired from a gun, or a 
rocket in flight. In general, the trajectory of a body in a gravitational 
field is a conic section - ellipse, hyperbola, or parabola - depending on the 
energy of motion. The trajectory of a shell or rocket fired from the ground is 
a portion of an ellipse with the Earth's center as one focus; however, if the 
altitude reached is not great, the effect of gravity is essentially constant, 
and the parabola is a good approximation.  See  BALLISTICS. 
 �34s[John P. Hagen]
�33s 
  
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�6v�2v�32sTranquilizer   �21m �33s 
    A psychopharmacologic drug that tends to have a calming effect, and is 
unique in inducing drowsiness without impairing ready arousal and in 
restraining hyperactivity without inducing coma or arrest of respiratory 
muscles.  
    According to basic chemical structure, five categories of tranquilizers 
can be delineated: (1) Phenothiazines (for example, chlorpromazine) and 
rauwolfia derivatives (for example, reserpine) are major or antipsychotic 
tranquilizers; (2) propanediols (for example, meprobamate, or Miltown), (3) 
diphenylmethanes (for example, benactyzine), (4) chlordiazepoxide and 
derivatives (for example, Librium and Valium), and (5) a miscellaneous group 
used generally for nighttime sedation (for example, ectylurea, glutethimide, 
and methylparafynol) are termed minor or sedative antianxiety tranquilizers. 
The antipsychotic tranquilizers in greatest use are phenothiazines, of which 
there are many derivatives and trade names. The sedative antianxiety agents 
are generally used in crisis situations or in neurotic episodes of intense 
anxiety or panic. Their effectiveness is more difficult to establish and is 
less specific than that of drugs for the treatment of psychosis. In high 
dosages such drugs have an addiction liability and can produce convulsions on 
withdrawals.  
 �34s[Daniel X. Freedman]
�33s 
  
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�6v�2v�32sTransamination   �21m �33s 
    The transfer of an amino group from one molecule to another without the 
intermediate formation of ammonia. Enzymatic reactions of this type play a 
prominent role in the formation and ultimate breakdown of amino acids by 
living organisms. Enzymes that catalyze such reactions are widely distributed 
and are termed transaminases, or aminotransferases. Perhaps the most prominent 
transamination reactions in higher animals are those in which glutamate is 
formed from ALPHA-ketoglutarate and other amino acids.  See  PROTEIN 
METABOLISM.  
 �34s[Esmond E. Snell]
�33s 
  
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�6v�2v�32sTransducer   �21m �33s 
    Any device or element which converts an input signal into an output signal 
of a different form. An example is the microphone, which converts vibrations 
caused by an impinging sound wave into an electrical signal. Although the most 
common transducers are designed to transform periodic signals (such as sound 
waves or alternating-current electrical signals), the word also applies to 
devices which convert static signals from one form to another. An example of 
this is the barometer, which produces a signal proportional to the atmospheric 
pressure. A different type of transducer is the photoelectric cell, which 
produces an electrical signal in response to incident light. The most widely 
used class of transducers is the electromechanical transducer, which converts 
an electrical signal into a mechanical signal (a vibration or a displacement) 
or vice versa. Aside from the microphone mentioned above, this class includes 
phonograph pickups, loudspeakers, automobile horns, doorbells, and underwater 
transducers.  See  MICROPHONE; UNDERWATER TRANSDUCER. 
 �34s[M. A. Breazeale]
�33s 
  
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�6v�2v�32sTransduction (bacteria)   �21m �33s 
    A mechanism for the transfer of genetic material between cells. The 
material is transferred by particles of bacterial viruses called 
bacteriophages, or phages.  See  BACTERIOPHAGE; DEOXYRIBONUCLEIC ACID (DNA). 
    The transduction mechanism has two features to distinguish it from the 
more usual mechanism of gene recombination, the sexual process. The most 
striking feature is the transfer of genetic material from cell to cell by 
phages. The second distinguishing feature is the fact that only a small part 
of the total genetic material of any one bacterial cell is carried by any 
particular transducing particle. However, all of the genetic material is 
distributed among different particles. 
    Transduction is not accomplished by all bacteriophages. It is done by some 
that are classified as ``temperate.'' When such temperate bacteriophages 
infect sensitive bacteria, some of the bacteria respond by producing more 
bacteriophage particles. These bacteria donate the transducing material. Other 
bacteria respond to the infection by becoming more or less permanent carriers 
of the bacteriophage, in a kind of symbiotic relationship; these are called 
lysogenic bacteria. Bacteria in this latter class survive the infection, and 
it is among these that transduced cells are found.  See  TRANSFORMATION 
(BACTERIA); VIRUS. 
 �34s[Norton D. Zinder]
�33s 
  
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�6v�2v�32sTransesterification �21m �33s
    The group of reactions in which an ester (RCOOR') reacts with another
compound to form a different ester (RCOOR''). The other compound may
be an acid, an alcohol, or another ester. These reactions are also known as
acidolysis, alcoholysis, and ester interchange, respectively. They are all
special cases of esterification and have the characteristics of esterification
reactions in general.  See  ACIDOLYSIS; ALCOHOLYSIS; ESTER; HYDROLYSIS.
 �34s[John M. Woods]
�33s

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�6v�2v�32sTransfer cells   �21m �33s 
    Plant cells characterized by the elaboration of an unlignified, secondary 
cell wall to form fingerlike projections or wall ingrowths which protrude into 
the cytoplasm of the cell. These ingrowths are enveloped by plasma membrane, 
forming a wall-membrane apparatus which increases the surface-volume ratio of 
the cell. 
    The location of transfer cells within the plant provides circumstantial 
evidence for their involvement in solute transport. They are situated at many 
sites in the plant where secretion or absorption takes place. Transfer cells 
are most frequently associated with the conducting elements of the xylem and 
phloem, where they may play a role in either loading or unloading solutes from 
these cells. Four main categories of secretion or absorption involving 
transfer cells are generally recognized: (1) absorption of solutes from the 
external environment; (2) secretion of solutes to the external environment; 
(3) absorption of solutes from internal, extracytoplasmic compartments; and 
(4) secretion of solutes into internal, extracytoplasmic compartments.  See  
PHLOEM; PLANT TRANSLOCATION OF SOLUTES; SECRETORY STRUCTURES (PLANT); XYLEM. 
 �34s[R. L. Peterson]
�33s 
  
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�6v�2v�32sTransformation (bacteria)   �21m �33s 
    A term reserved almost exclusively for the genetic transformation of 
bacteria through the action of genetically effective substances extracted from 
other bacteria. The process is looked upon as the transfer of deoxyribonucleic 
acid (DNA) from donor bacteria to suitable recipient bacteria. The recipient 
bacteria are usually closely related to the donor strain. The process may 
occur in natural conditions, for example, in a host animal infected with two 
parasitic strains, and it might play a part in the rapid evolution of 
pathogenic bacteria. 
    Bacteria competent for transformation, when in certain phases of the 
growing state, will incorporate into themselves the large DNA molecules 
present in transforming agents prepared from related strains. Only the 
purified DNA of all the substances which may be extracted from the donor 
bacteria need be added in order to produce the changes in the growing 
recipient bacteria. 
    That bacterial transformation is true genetic transmission on a small 
scale, rather than controlled mutation, is demonstrated by the following 
characteristics: (1) A specific trait is introduced, coming always from donors 
bearing the trait. (2) The trait is transferred by determinant, genelike 
material far less complex than whole cells or nuclei, and this material, DNA, 
is known to be present in gene-carrying chromosomes. (3) The trait is 
inherited by the progeny of the changed bacteria. (4) The progeny produce, 
when they grow, increased amounts of DNA carrying the specific property. (5) 
The traits are transferred as units exactly in the patterns in which they 
appear or in which they are induced by mutation. (6) The DNA transmits the 
full potentialities of the donor strain, whether these are in an expressed or 
in a latent state. (7) The traits are often attributable to the presence of a 
specific gene-determined enzyme protein. (8) Certain groups of determinants 
may occur ``linked'' within DNA molecules, just as genes may be linked. (9) 
Linked determinants, while transforming a new cell, may become exchanged 
(recombined) between themselves and their unmarked or unselective alternate 
forms in such a way that they bring about genetic variation, and in a pattern 
indicating the existence of larger organized genetic units.  See  GENE; 
TRANSDUCTION (BACTERIA). 
 �34s[Rollin D. Hotchkiss]
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�6v�2v�32sTransformer �21m �33s
    An electrical component used to transfer electric energy from one
alternating-current (ac) circuit to another by magnetic coupling. Essentially,
it consists of two or more multiturn coils of wire placed in close proximity
to cause the magnetic field of one to link the other. In general, the
transformer accomplishes one or more of the following between two circuits:
(1) a difference in voltage magnitude, (2) a difference in current magnitude,
(3) a difference in phase angle, (4) a difference in impedance level, and (5)
a difference in voltage insulation level, either between the two circuits or
to ground.
    Transformers are used to meet a wide range of requirements. Pole-type
distribution transformers supply relatively small amounts of power to
residences. Power transformers are used at generating stations to step up the
generated voltage to high levels for transmission. The transmission voltages
are then stepped down by transformers at the substations for local
distribution. Instrument transformers are used to measure voltage and currents
accurately. Audio- and video-frequency transformers must function over a broad
band of frequencies. Radio-frequency (rf) and intermediate-frequency (i-f)
transformers transfer energy in narrow frequency bands from one circuit to
another.  See  INSTRUMENT TRANSFORMER.

    Power transformers.  A power transformer consists of two or more multiturn
coils wound on a laminated iron core. At least one of these coils serves as
the primary winding.
    When the primary of a power transformer is connected to an alternating
voltage, it produces an alternating flux in the core. The flux generates a
primary electromotive force, which is essentially equal and opposite to the
voltage supplied to it. It also generates a voltage in the other coil or
coils, one of which is called a secondary. This voltage generated in the
secondary will supply alternating current to a circuit connected to the
terminals of the secondary winding. A current in the secondary winding
requires an additional current in the primary. The primary current is
essentially self-regulated to meet the power (or volt-ampere) demand of the
load connected to the secondary terminals. Thus in normal operation, energy
(or volt-amperes) can be transferred from the primary to the secondary
electromagnetically.
    The illustration shows a transformer with a primary of N1 turns and a
secondary of N2 turns. A primary voltage V1 causes a current I1 to flow
through the coil. Since all quantities shown are alternating, the arrows
indicate only instantaneous polarities.
    The magnetic flux PHI set up by the primary consists of two components.
One part passes completely around the magnetic circuit defined by the iron core,
thus linking the secondary coil. This is the mutual flux PHIsubm. The second
part is a smaller component of flux that links only the primary coil. This is
the primary leakage flux PHIsubl1. If the secondary circuit is completed
through a load, a secondary current I2 flows and in turn creates a secondary
leakage flux PHIsubl2. These leakage fluxes contribute to the impedance of the
transformer. If the leakage flux is small, the coupling between primary and
secondary is said to be close. The use of an iron core decreases the leakage
flux by providing a low-reluctance path for the flux.  See  COUPLED CIRCUITS;
MAGNETIC CIRCUITS.

    In a power transformer the voltage drops due to winding resistance and
leakage are small; therefore V1 and V2 are essentially in phase (or 180DG out
of phase, depending on the choice of polarity). Since the no-load current is
small, I1 and I2 are essentially in phase (or 180DG out of phase). Therefore,
Eq. (1) applies, and the voltage ratio is expressed by Eq. (2), in which a is
the transformation ratio.

     V(sub 1)I(sub 1) NEARLY EQUAL V(sub 2)I(sub 2)			    (1)

     V(sub 1)/V(sub 2) NEARLY EQUAL a					    (2)


    The transmission of power from primary coil to secondary coil is via the
magnetic flux. The flux is proportional to the ampere turns in either coil.
Since the power in each coil is nearly the same, Eqs. (3) and (4) are obtained.

      N(sub 1)I(sub 1) NEARLY EQUAL N(sub 2)I(sub 2)			   (3)


      N(sub 1)/N(sub 2) NEARLY EQUAL I(sub 2)I(sub 1) NEARLY EQUAL a	   (4)


    Transformer cores are made of special alloy steels rolled to approximately
0.014 in. (0.35 mm) thick. These thin sheets, or laminations, are stacked to
form the transformer core, each sheet being insulated from the others to
reduce unwanted eddy-current loss.  See  CORE LOSS; EDDY CURRENT; MAGNETIC
HYSTERESIS.
    Copper conductors are used almost universally. The conductors are
insulated with special paper or cotton covering, with enamel, or with a
combination of both. Large outdoor transformers are immersed in oils to obtain
good electrical insulation within small spacings and to provide a cooling
medium. When lightweight or nonflammable materials are important, transformers
may be made with compressed gases as the insulating and cooling medium.
 �34s[J. R. Sutherland]
�33s

    Audio and radio-frequency transformers.  Audio or video (broad-band)
transformers are used to transfer complex signals containing energy at a large
number of frequencies from one circuit to another. They are required to
respond uniformly to signal voltages over a frequency range three to five or
more decades wide (for example, from 10 to 100,000 Hz), and consequently must
be designed so that very nearly all of the magnetic flux threading through one
coil also passes through the other.
    Audio and video transformers have two resonances (caused by existing stray
capacitances) just as many tuned transformers do. One resonance point is near
the low-signal-frequency limit; the other is near the high limit. As the
coefficient of coupling in a transformer is reduced appreciably below unity by
removal of core material and separation of the windings, tuning capacitors are
added to provide efficient transfer of energy. The two resonant frequencies
combine to one when the coupling is reduced to the value known as critical
coupling, then stay relatively fixed as the coupling is further reduced.
    The rf and i-f transformers use two or more inductors, loosely coupled
together, to limit the band of operating frequencies. Efficient transfer of
energy is obtained by resonating one or more of the inductors. By using higher
than critical coupling, a wider bandwidth than that from the individual tuned
circuits is obtained, while the attenuation of side frequencies is as rapid as
with the individual circuits isolated from one another.
 �34s[Keats A. Pullen, Jr.]
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�6v�2v�32sTransform fault   �21m �33s 
    A fracture along an offset of a spreading mid-ocean-ridge crest. The 
recognition that the sea floor can grow only where the ocean crust was broken 
- along the rift - means that east-west displacements along the fault occur 
only between the offset rifts (BC in the illustration) and not along the 
entire length of the fault (AD). This transform fault differs clearly from a 
transcurrent fault in crucial respects. First, earthquake activity is 
restricted to the active portion of the fault (BC) between the rifts and not 
beyond, where the fault is presently inactive (AB, CD). Second, the sense of 
displacement is opposite in the transform fault to that of the transcurrent 
fault. Third, the distance BC between the ridge-crest rifts remains constant 
for the transform fault and increases with time for the transcurrent fault.  
See  FAULT AND FAULT STRUCTURES; MARINE GEOLOGY. 
 �34s[Stanley K. Runcorn]
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�6v�2v�32sTransfusion �21m �33s
    The administration of blood, or one of its components, as a part of
treatment. The techniques and purposes of blood transfusion are often similar
to those of transfusing other fluids, and therefore the use of such substances
as glucose, synthetic plasma expanders, and related agents may be considered
in addition to blood.
    Blood may be administered in several forms, depending on the circumstances
or the availability of such therapy. Whole blood is normally prepared in
sterile, pyrogen-free containers in which there is a suitable anticoagulant
present. Various blood portions or derivatives are prepared by appropriate
methods. Their advantage lies in the fact that such preparations may be stored
and stockpiled for much greater periods of time than is currently possible
with whole blood. Packed red blood cells, dried or fluid blood plasma, and
plasma protein preparations such as albumin or globulin are examples in most
common use.
    When blood is to be given, a sample of the recipient's blood is subjected
to compatibility tests with the blood of the donor. It is necessary to
determine whether the donor cells will be damaged, or hemolyzed, by the
recipient's plasma, or less important, whether the recipient's cells will be
harmed by the donor's plasma. If such compatibility tests are negative, the
transfusion proceeds; if they are positive, either another specimen is used or
appropriate safeguards are employed if no other blood is available and
transfusion cannot be delayed.	See  BLOOD GROUPS.
    Administration of the blood is by intravenous infusion, using a sterile,
pyrogen-free apparatus which contains a filter system to prevent the passage
of any aggregation or coagulum of the donor's blood. Such units, readily
employed with the storage container, are made of plastic and are disposable.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sTransistor �21m �33s
    An active component of an electronic circuit which may be used as an
amplifier, detector, or switch. A transistor consists of a small block of
semiconducting material to which at least three electrical contacts are made.
Transistors are of two general types, bipolar and field-effect. The bipolar
type involves excess minority current carrier injection. The field-effect type
involves only majority current carriers. Historically the bipolar type was
developed before the field-effect type. Today both are widely used. The
unmodified term transistor usually refers to the bipolar type.
    In a bipolar transistor, at least one contact is ohmic (nonrectifying),
and at least one contact is rectifying. Usually there are two closely spaced
rectifying contacts and one ohmic contact. For a discussion of rectifying
contacts  see  JUNCTION DIODE; POINT-CONTACT DIODE; SEMICONDUCTOR RECTIFIER.
    The operation of a simple transistor consists of the control of the
current flowing in the high-resistance direction through one rectifying
contact (called the collector) by the current flowing in the low-resistance
direction in the other rectifying contact (called the emitter). The third
contact, which is ohmic, is called the base contact.
    These contacts usually consist of two or more regions. The regions in
which the actual rectification processes take place are called the emitter
barrier and collector barrier. The region between these two barriers is called
the base region, or simply the base. The regions outside of these barriers are
called the emitter and collector regions.
    Transistors are used in radio receivers, in electronic computers, in
electronic instrumentation and control equipment, and in almost any electronic
circuit where the required voltages are not too high.

    Classification.  Transistors are classified chiefly by four criteria: (1)
by the type and number of structural regions of the semiconductor crystal; (2)
by the technology used in fabrication; (3) by the semiconductor material used;
and (4) by the intended use of the device.
    A modern transistor type is the npn double-diffused silicon planar
passivated transistor. The term double-diffused refers to the fabrication
technique in which the base region is formed by diffusion through a mask into
the body of the silicon wafer which forms the collector region.  See
DIFFUSION IN SOLIDS.
    In turn, the emitter region is formed by diffusion through a second mask
into the previously formed base region. The term planar refers to the fact
that all three electrical connections are found on a single surface of the
device. The term passivated means that the surface to which all junctions
return is protected by a layer of naturally grown silicon oxide which,
together with an overcoating of glass or other inert material, passivates the
surface, electrically minimizing leakage currents.  See  JUNCTION TRANSISTOR;
POINT-CONTACT TRANSISTOR; SURFACE-BARRIER TRANSISTOR.

    Action.   To explain transistor action in more detail, some of the basic
properties of a semiconductor material are first presented. An n-type
semiconductor contains electrons, and a p-type semiconductor contains holes.
These are called the majority carriers of the two types. Actually there are
always present a small number of holes in an n-type semiconductor and a small
number of electrons in a p-type semiconductor. These are called the minority
carriers of the two types. At a given temperature with a given material the
product of the densities of the majority and minority carriers is a constant.
This means that if there is present a very high density of majority carriers
(low-resistivity material), there will be a correspondingly low density of
minority carriers.
    The emitter current controls the collector current in a simple transistor.
To understand this, first consider the magnitude of the collector current in
the absence of emitter current. In normal operation the collector barrier is
biased in the high-resistance (reverse) direction. Under this condition of
bias the majority carriers are stopped by the barrier, and only the minority
carriers are free to flow. If the collector barrier is a silicon pn junction,
the minority-carrier diffusion current is negligible and the reverse-bias
leakage current will consist of thermally generated carriers and be in the
nanoampere range. If emitter current is present, the portion consisting of
carriers entering the base will continue across the collector barrier and thus
control the collector current.

    Bias.  The establishment of an operating point on the transistor volt-
ampere characteristics by means of direct voltages and currents is called
the bias of a transistor.  Since the transistor is a three-terminal device
any one of the three terminals may be used as a common terminal to both input
and output.  In most transistor circuits the emitter is used as the common
terminal and known as a common emitter or grounded emitter.  If the transistor
is to be used as a linear device, such as an audio amplifier, it must be
biased to operate in the active region.  In this region the collector is biased
in the reverse direction and the emitter in the forward direction.  The area
in the common-emitter transistor characteristics to the right of the ordinate
VsubCE = 0 and above Isubc = 0 is the active region.  Two more biasing regions
are of special interest for those cases in which the transistor is intended to
operate as a switch.  These are the saturation and cutoff regions.  The
saturation region may be defined as the region where the collector current
is independent of base current for give values of Vsubcc and Rsub L.  Thus,
the onset of saturation can be considered to take place at the knee of the
common-emitter transistor curves.  See AMPLIFIER.
    In saturation the transistor current I(subc) is nominally V(subcc)/R(subL).
Since R(subL) is small, it may be necessary to keep V(subcc) correspondingly
small in order to stay within the limitations imposed by the transistor
on maximum-current and collector-power dissipation.  In the cutoff region
it is required that the emitter current IsubE be zero, and to accomplish
this it is necessary to reverse-bias the emitter junction so that the
collector current is approximately equal to the reverse saturation current
Isubco.  A reverse-biasing voltage of the order of 0.1 volt across the
emitter junction will ordinarily be adequate to cut off either a germanium
or silicon transistor.
    The particular method to be used in establishing an operating point on
the transistor chazracteristics depends on whether the transistor is to
operate in the active saturation or cutoff regions, on the application under
consideration, on the thermal stability of the circuit, and on other factors.
[Christos C. Halkias]


    Power switching.   There are several transistor structures which are used
for power switching and make use of current gains greater than unity to
achieve a thyraton-like characteristic. These devices are often called
four-layer devices since they usually contain four regions of alternating n-
and p-type semiconductor material.

    Field-effect transistor (FET).   There are two major types of field-effect
transistors, the junction-gate FET (JFET) and the insulated-gate FET (IGFET).
The IGFET is commonly called MOSFET or MOS transistor. The acronym MOS stands
for metal-oxide-semiconductor which describes, in order, the structure of the
device from the gate toward the channel. The JFET was developed first, since
it involved no technology beyond that of the planar bipolar silicon
transistor. The development of the MOSFET was delayed while the technology was
extended to stable control of silicon surface potential. The MOSFET is very
widely applied in large-scale integration, particularly in implementing large
random-access high-speed memories for computers. See COMPUTER STORAGE
TECHNOLOGY; INTEGRATED CIRCUITS.
    Illustration a shows a section of a JFET. The channel consists of
relatively low-conductivity semiconductor material sandwiched between two
regions of high-conductivity material of opposite type. When these junctions
are reverse-biased, the junction depletion regions encroach upon the channel
and finally, at a high reverse bias, pinch it off entirely. The thickness of
the channel, and hence its conductivity, is controlled by the voltage on the
two gates.
    Illustration b shows a section of a MOSFET. Here the source and drain
regions consist of n diffusion in a p-type substrate. The gate is a metal film
evaporated on a thin SiO insulator spanning the separation between the source
and drain. With no voltage on the gate, the source and drain are insulated
from each other by their surrounding junctions. When a positive voltage is
applied to the gate, electrons are induced to move to the surface of the
p-type substrate immediately beneath the gate, producing a thin surface of
induced n-type material which now forms a channel connecting the source and
drain. Such a surface layer is called an inversion layer since it is of
opposite conductivity type to the substrate. The number of induced electrons
is directly proportional to the gate voltage, so that the conductivity of the
channel increases with gate voltage. This device is called an n-channel
enhancement-mode MOSFET. It is normally off at zero gate voltage.
    MOSFET devices are fabricated in both p-channel and n-channel types, as
well as for both depletion (normally on) and enhancement (normally off) modes
of operation.
    Most integrated circuits using MOSFETs are called CMOS integrated
circuits, where the C stand for complementary. These circuits use n-channel
and p-channel types together to achieve digital logic.
 �34s[Lloyd P. Hunter]
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�6v�2v�32sTransistor Amplifier �21m �33s
    An amplifier which depends upon transistors for its operation. The great
majority of amplifiers employ transistors because of their low cost, long
life, high reliability, better overall characteristics, and small size and
because the physical nature of the transistor makes possible circuits that are
not possible with vacuum tubes. See TRANSISTOR.
    A junction transistor consists of a silicon (or germanium) crystal in
which a layer of n-type silicon is sandwiched between two layers of p-type
silicon. Alternatively, a transistor may consist of a layer of p-type between
two layers of n-type material. In the former case, the transistor is referred
to as a p-n-p transistor, and in the latter case as an n-p-n transistor. The
semiconductor sandwich is extremely small and hermetically sealed against
moisture inside a metal or plastic case.
    The two types of transistor are represented in illustration a. The
representations employed when transistors are used as circuit elements are
shown in illustration b. The three portions of a transistor are known as
emitter, base, and collector. The arrow on the emitter lead specifies the
direction of current flow when the emitter-base junction is biased in the
forward direction. In both cases, however, the emitter, base, and collector
currents, I(sub E), I(sub B), and I(sub C), respectively, are assumed positive
when the currents flow into the transistor. The symbols V(sub EB), V(sub CB),
and  V(sub CE) are the emitter-base, collector-base, and collector-emitter
voltages, respectively. (More specifically, V(sub EB) represents the voltage
drop from emitter to base.). See AMPLIFIER.
 �34s[Christos C. Halkias ;Charles ]
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�6v�2v�32sTransition elements   �21m �33s 
    In broad definition, the elements of atomic numbers 21 - 31, 39 - 49, and 
71 - 81, inclusive. A more restricted classification of the transition 
elements, preferred by many chemists, is limited to elements with atomic 
numbers 22 - 28, 40 - 46, and 72 - 78, inclusive. All of the elements in this 
classification have one or more electrons present in an unfilled d subshell in 
at least one well-known oxidation state. 
    All the transition elements are metals and, in general, are characterized 
by high densities, high melting points, and low vapor pressures. Within a 
given subgroup, these properties tend to increase with increasing atomic 
weight. Facility in the formation of metallic bonds is demonstrated also by 
the existence of a wide variety of alloys between different transition metals. 
    The transition elements include most metals of major economic importance, 
such as the relatively abundant iron, nickel, and zinc, on one hand, and the 
rarer coinage metals, copper, silver, and gold, on the other. Also included 
are the rare and relatively unfamiliar element, rhenium, and technetium, which 
is not found naturally in the terrestrial environment, but is available in 
small amounts as a product of nuclear fission. 
    In their compounds, the transition elements tend to exhibit multiple 
valency, the maximum valence increasing from +3 at the beginning of a series 
(Sc, Y, Lu) to +8 at the fifth member (Mn, Re). One of the most characteristic 
features of the transition elements is the ease with which most of them form 
stable complex ions. Features which contribute to this ability are favorably 
high charge-to-radius ratios and the availability of unfilled d orbitals which 
may be used in bonding. Most of the ions and compounds of the transition 
metals are colored, and many of them are paramagnetic. Both color and 
paramagnetism are related to the presence of unpaired electrons in the d 
subshell. Because of their ability to accept electrons in unoccupied d 
orbitals, transition elements and their compounds frequently exhibit catalytic 
properties.  
    Broadly speaking, the properties of the transition elements are 
intermediate between those of the so-called representative elements, in which 
the subshells are completely occupied by electrons (alkali metals, halogen 
elements), and those of the inner or f transition elements, in which the 
subshell orbitals play a much less significant role in influencing chemical 
properties (rare-earth elements, actinide elements). See ACTINIDE ELEMENTS; 
ATOMIC STRUCTURE AND SPECTRA; RARE-EARTH ELEMENTS. 
 �34s[Burris B. Cunningham]
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�6v�2v�32sTransition point   �21m �33s 
    The point at which a substance changes from one state of aggregation to 
another. This general definition would include the melting point (transition 
from solid to liquid), boiling point (liquid to gas), or sublimation point 
(solid to gas); but in practice the term transition point is usually 
restricted to the transition from one solid phase to another, that is, to the 
temperature (for a fixed pressure, usually 1 atm or 10sup5 pascals) at which a 
substance changes from one crystal structure to another. 
    Another kind of transition point is the culmination of a gradual change 
(for example, the loss of ferromagnetism in iron or nickel) at the lambda 
point, or Curie point. This behavior is typical of second-order transitions. 
See BOILING POINT; FIRST-ORDER TRANSITION; MELTING POINT; PHASE EQUILIBRIUM; 
SECOND-ORDER TRANSITION; SUBLIMATION; TRIPLE POINT. 
 �34s[Robert L. Scott]
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�6v�2v�32sTransition radiation detectors   �21m �33s 
    Detectors of energetic charged particles that make use of radiation 
emitted as the particle crosses boundaries between regions with different 
indices of refraction. An energetic charged particle moving through matter 
momentarily polarizes the material nearby. If the particle crosses a boundary 
where the index of refraction changes, the change in polarization gives rise 
to the emission of electromagnetic transistion radiation. About one photon is 
emitted for every 100 boundaries crossed, for transitions between air and 
matter of ordinary density. Transition radiation is emitted even if the 
velocity of the particle is less than the light velocity of a given 
wavelength, in contrast to Cerenkov radiation. Consequently, this radiation 
can take place in the x-ray region of the spectrum where there is no Cerenkov 
radiation, because the index of refraction is less than one. See CERENKOV 
RADIATION; PARTICLE DETECTOR; REFRACTION OF WAVES. 
 �34s[William J. Willis]
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�6v�2v�32sTransit (astronomy)   �21m �33s 
    The apparent passage of a planet across the surface of the Sun, of a 
satellite across the surface of the parent planet, or of a star, planet, or 
reference point across an adopted line of reference. Only Mercury and Venus 
are seen in transit across the surface of the Sun, because they are the only 
planets orbiting inside the path of Earth. 
    Passages of stars across the local meridian are observed extensively for 
solving problems of fundamental positional astronomy, timekeeping, and 
navigation. At the precise instant of transit of a star across the local 
meridian, the local sidereal time is exactly equal to the star's right 
ascension, and the latitude of the observer is equal to the sum of the star's 
declination and its zenith distance. 
    Furthermore, the difference between the local sidereal time and the 
Greenwich sidereal time gives a measure of the longitude of the observer. Any 
of those quantities may be treated as the unknown to be determined by 
observations of meridian transits. See ASTRONOMICAL COORDINATE SYSTEMS; 
ECLIPSE.  
 �34s[Simone Daro Gossner]
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�6v�2v�32sTransit (engineering) �21m �33s
    A surveying instrument for measuring horizontal and vertical angles. It is
also used for prolonging a straight line, establishig a level line of sight,
and measuring distances by the stadia method. See STADIA.
    In the disassembled view (see illustration), three main elements are seen
to support the telescope assembly: the alidade or standard with its solid
spindle (inner center), which rotates within the hollow spindle (outer center)
and graduated circle, which in turn rotates within the leveling head atop its
tripod. These are the elements of the azimuth axis. Clamps are used to prevent
one rotation while the other is used. This permits an angle to be turned about
the azimuth axis and to be measured on the horizontal circle, even several
times incrementally for greater precision. Vertical angular measurement is
achieved by rotating the telescope about the elevation axis. The leveling
screws on the leveling head enable the azimuth axis to be made vertical.
Lateral motion on the leveling head permits the instrument to be centered
exactly over a ground station by means of a plumb bob on a string or by use of
an optical plummet built into the instrument. See SURVEYING; THEODOLITE.
 �34s[B. Austin Barry]
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�6v�2v�32sTranslucent medium   �21m �33s 
    A medium which transmits rays of light so diffused that objects cannot be 
seen distinctly; that is, the medium is only partially transparent. Familiar 
examples are various forms of glass which admit considerable light but impede 
vision. Inasmuch as the term translucent seems to imply seeing, usage of the 
term is ordinarily limited to the visible region of the spectrum. See 
TRANSPARENT MEDIUM. 
 �34s[M. G. Mellon]
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�6v�2v�32sTransmission lines   �21m �33s 
    A system of conductors suitable for conducting electric power of signals 
between two or more termini. For example, commercial-frequency electric power 
transmission lines connect electric generating plants, substations, and their 
loads. Telephone transmission lines interconnect telephone subscribers and 
telephone exchanges. Radio-frequency transmission lines transmit 
high-frequency electric signals between antennas and transmitters or receivers. 
    Although only a short cord is needed to connect an electric lamp to a wall 
outlet, the cord is, properly speaking, a transmission line. However, in the 
electrical industry the term transmission line is applied only when both 
voltage and current at one line terminus may differ appreciably from those at 
another terminus. Transmission lines are described either as electrically 
short if the difference between terminal conditions is attributable simply to 
the effects of conductor series resistance and inductance, or to the effects 
of a shunt leakage resistance and capacitance, or to both; or as electrically 
long when the properties of the line result from traveling-wave phenomena. 
  
    Classification.   Depending on the configuration and number of conductors 
and the electric and magnetic fields about the conductors, transmission lines 
are described as open-wire transmission lines, coaxial transmission lines, 
cables, or waveguide transmission lines. 
    Open-wire lines may comprise a single wire with an earth (ground) return 
or two or more conductors. The conductors are supported at more or less evenly 
spaced points along the line by insulators, with the spacing between 
conductors maintained as nearly uniform as feasible, except in special-purpose 
tapered transmission lines. Open-wire construction is used for communication 
or power transmission whenever practical and permitted, as in open country and 
where not prohibited by ordinances. 
    A coaxial transmission line comprises a conducting cylindrical shell, 
solid tape, or braided conductor surrounding an isolated, concentric, inner 
conductor which is solid, stranded, or (in certain video cables and delay 
cables) helically wound on a plastic or ferrite core. The inner conductor is 
supported by ceramic or plastic beads or washers in air- or gas-dielectric 
lines, or by a solid polyethylene or polystyrene dielectric. The purpose of 
this construction is to have the shell prevent radiation losses and 
interference from external sources. Coaxial lines are widely used in radio, 
radar, television, and similar applications. See COAXIAL CABLE. 
    Sheathed cables, also termed shielded cables, comprise two or more 
conductors surrounded by a conducting cylindrical sheath, commonly supported 
by a continuous solid dielectric. The sheath provides both shielding and 
mechanical protection. 
  
    Theory.   When electric power is applied at a terminus of a transmission 
line, electromagnetc waves are launched and guided along the line. The 
steady-state and transient electrical properties of transmission lines result 
from the superposition of such waves, termed direct waves, and the reflected 
waves which may appear at line discontinuities or at load terminals. 
    When the electric and magnetic field vectors are perpendicular to one 
another and transverse to the direction of the transmission line, this 
condition is called the principal mode or the transverse electromagnetic (TEM) 
mode. Modes other than the principal mode may exist at any frequency for which 
conductor spacing exceeds one-half of the wavelength of an electromagnetic 
wave in the medium separating the conductors. Such high-frequency modes are 
called waveguide transmission modes. See WAVEGUIDE. 
    In a uniform (nontapered) transmission line, the voltage or current 
applied at a sending terminal determines the shape of the initial voltage or 
current wave. In a line with negligible losses the transmitted shape remains 
unchanged. When losses are present, the shape, unless sinusoidal, is altered, 
because the phase velocity and attenuation vary with frequency. See PHASE 
VELOCITY.  
 �34s[Everard M. Williams]
�33s 
  
    Power lines.  In an electric power system the facility used to transfer 
large amounts of power from one location to a distant location is termed a 
power transmission line. Techniques of power transmission are presented in 
this section. 
    Power transmission lines are distinguished from subtransmission and 
distribution lines by their higher voltages, greater power capabilities, and 
greater lengths. With the exception of a few high-voltage dc lines for 
satisfying special requirements, power transmission lines employ three-phase 
alternating currents. Such lines require three conductors. The standard 
frequency in the United States is 60 Hz. In Europe it is 50 Hz, while in the 
rest of the world both of these frequencies are used. For transmitting large 
amounts of power over long distances, high voltages are necessary. Standard 
transmission voltages in the United States are 69, 115, 138, 161, 230, 345, 
500, and 765 kV. These figures refer to the nominal effective voltages between 
any two of the three conductors. The line conductors are usually placed 
overhead, supported by poles or towers; however, they may form part of an 
underground or underwater cable. See ELECTRIC DISTRIBUTION SYSTEMS; ELECTRIC 
POWER SYSTEMS. 
 �34s[Edward W. Kimbark]
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�6v�2v�32sTransmutation   �21m �33s 
    The nuclear change of one element into another, either naturally, in 
radioactive elements, or artificially, by bombarding an element with 
electrons, deuterons, or ALPHA-particles in particle accelerators or with 
neutrons in atomic piles. 
    Natural transmutation was first explained by Marie Curie about 1900 as the 
result of the decay of radioactive elements into others of lower atomic 
weight. Ernest Rutherford produced the first artificial transmutation 
(nitrogen into oxygen and hydrogen) in 1919. Artificial transmutation is the 
method of origin of the heavier, artificial transuranium elements, and also of 
hundreds of radioactive isotopes of most of the chemical elements in the 
periodic table. See NUCLEAR REACTION; PERIODIC TABLE. 
 �34s[Frank H. Rockett]
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�6v�2v�32sTransonic flight   �21m �33s 
    In aerodynamics, flight of a vehicle at speeds near the speed of sound. 
When the velocity of an airplane approaches the speed of sound (roughly 660 
mi/h or 1056 km/h at 35,000 ft or 10.7 km altitude), the flight 
characteristics become radically different from those at subsonic speeds. The 
drag increases greatly, the lift at a given altitude decreases, the moments 
acting on the airplance change abruptly, and the vehicle may shake or buffet. 
Such phenomena usually persist to flight velocities somewhat above the speed 
of sound. These flight characteristics, as well as the speeds at which they 
occur, are usually referred to as transonic. The extent of the speed range of 
these changes depends on the form of the airplane; for configurations designed 
for subsonic flight they may occur at velocities of 70 - 110% of the speed of 
sound (Mach numbers of 0.7 - 1.1); for airplanes intended for transonic or 
supersonic flight they may be present only at Mach numbers of 0.95 - 1.05. See 
MACH  NUMBER. 
    The transonic flight characteristics result from the development of shock 
waves about the airplane. Because of the accelerations of airflow over the 
various surfaces, the local velocities become supersonic while the airplane 
itself is still subsonic. (The flight speed at which such local supersonic 
flows first occur is called the critical speed.) Shock waves are associated 
with deceleration of these local supersonic flows to subsonic flight 
velocities. Such shock waves cause abrupt streamwise increases of pressure on 
the airplane surfaces. These gradients may cause a reversal and separation of 
the flow in the boundary layer on the wing surface in roughly the same manner 
as do similar pressure changes at lower subcritical speeds. See AERODYNAMIC 
FORCE; AERODYNAMIC WAVE DRAG; BOUNDARY-LAYER FLOW; SCHLIEREN PHOTOGRAPHY; 
SHOCK WAVE. 
    As for boundary-layer separation at lower speeds, the flow breakdown in 
this case leads to increases of drag, losses of lift, and changes of 
aerodynamic moments. The unsteady nature of the separated flow results in an 
irregular change of the aerodynamic forces acting on the airplane with 
resultant buffeting and shaking. As the Mach number is increased, the shock 
waves move aft so that at Mach numbers of about 1.0 or greater, depending on 
the configurations, they reach the trailing edges of the surfaces. With the 
shocks in these positions, the associated pressure gradients have relatively 
little effect on the boundary layer, and the shock-induced separation is 
greatly reduced. See SUBSONIC FLIGHT; SUPERSONIC FLIGHT. 
 �34s[Richard T. Whitcomb]
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�6v�2v�32sTransparent medium   �21m �33s 
    Ordinarily, a medium which has the property of transmitting rays of light 
in such a way that the human eye may see through the medium distinctly. It is 
pervious to light, that is, to the visible region of the electromagnetic 
spectrum. By extension, a medium may be described as transparent to other 
regions of the spectrum, such as x-rays and microwaves. Just as a blue filter 
passes blue rays, an ultraviolet filter might be considered as passing 
ultraviolet rays, or being transparent to them. See ABSORPTION OF 
ELECTROMAGNETIC RADIATION; TRANSLUCENT MEDIUM. 
 �34s[M. G. Mellon]
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�6v�2v�32sTransplantation biology   �21m �33s 
    Transplantation, or grafting, is the artificial removal of part of an 
organism and its replacement in the body of the same or of a different 
individual.  
    The transplantation of cells, tissues, or organs is an important procedure 
for studying a wide variety of problems. For example, a defining criterion of 
the endocrine function of a particular organ or tissue is the capacity to 
restore the effects of its extirpation by returning it to the host at an 
anatomically abnormal site. In embryology, grafting experiments are used to 
study the interactions of cells of different types. Transplantation is 
important in the study of aging since it enables one to produce age chimeras. 
In cancer research, transplantation is widely employed to propagate malignant 
tissues in appropriate hosts for biochemical, immunologic, and therapeutic 
investigations.  
    Transplantation involves two types of problems: those concerned directly 
with the act of transplantation itself; and those which stem from the 
incompatibility of homografts, that is, grafts from a donor transplanted to a 
genetically dissimilar recipient, and which are of an immunological nature. 
The success of a therapeutic organ homograft, such as a kidney or a heart, 
entails overcoming the recipient's reaction against it without seriously 
impairing his immunological defense mechanism as a whole. The discovery of the 
principle of immunological tolerance indicated that this problem is soluble, 
though means of rendering human beings specifically nonreactive in respect to 
grafts from a donor have yet to be devised. However, by the long-term 
treatment of patients with so-called immunosuppressive drugs and other agents 
it is frequently possible to hold their resistance to organ homografts in 
abeyance for prolonged periods, if not indefinitely, and even to reverse the 
so-called rejection crises that often develop. Indeed, it is possible that 
long-term acceptors of homografts do eventually become tolerant of the foreign 
antigens involved. See ACQUIRED IMMUNOLOGICAL TOLERANCE. 
    As might be inferred, suppression of a recipient's response against a 
homograft is greatly facilitated if a donor who is relatively compatible can 
be selected, for there are all degrees of incompatibility. Many different 
procedures for donor selection have been explored, including 
histocompatibility testing, or typing for tissue transplantation. The typing 
system is essentially similar to blood typing, except that lymphocytes are 
used instead of red cells. See HISTOCOMPATABILITY; IMMUNOLOGY. 
 �34s[Rupert E. Billingham]
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�6v�2v�32sTransportation engineering   �21m �33s 
    That branch of engineering relating to the movements of goods and people. 
Major types of transportation are highway, water, rail, subways, air, and 
pipeline.  
    Highway transportation engineering deals with the planning, construction, 
and operation of roads, streets, bridges, and parking facilities. See BRIDGE; 
HIGHWAY ENGINEERING; PAVEMENT; TRAFFIC ENGINEERING; TUNNEL. 
    Planning and construction of canals, channels, harbor facilities, 
navigation aids such as lighthouses, and navigation locks and dams are 
important concerns of water transportation engineers. Another important field 
of water transportation relates to the design and production of launches, 
barges, tugs, ferry boats, and other ships. See CANAL; COASTAL ENGINEERING; 
DAM; RIVER ENGINEERING. 
    Engineering relating to rail transportation includes the planning and 
construction of terminals, switchyards, loading and unloading facilities, 
trackage, bridges, traffic-control and maintenance facilities, and the hauling 
equipment itself - locomotives and other rolling stock. See RAILROAD 
ENGINEERING.  
    Engineering work relating to planning, general layout, detailed design, 
construction, and operation of subways is an important part of transportation 
engineering. Subways are a major element in mass transit, and as metropolitan 
areas increase in size and population and vehicular traffic becomes more 
congested, subways are being given increased consideration as a possible part 
of the transportation system of a city. 
    The planning, design, and construction of runways, terminals, aircraft, 
and navigation aids are the major branches of civilian air transportation 
engineering. In the larger air terminals, great emphasis is placed on systems 
for moving passengers and baggage. See AIR TRANSPORTATION; AIRPORT ENGINEERING. 
    Pipelines are used to transport natural gas and petroleum products great 
distances. Pipeline engineering is concerned with planning and construction of 
the pipeline, pumping stations to move the material through the line, and any 
needed storage facilities. Frequently, large bridge structures are required to 
carry the pipeline across major streams or other barriers. Planning of pumping 
facilities requires study of power requirements for different types of 
material moved, standby facilities, spacing of the pumping stations, and 
related factors. 
 �34s[Archie N. Carter]
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�6v�2v�32sTransport processes   �21m �33s 
    The processes whereby mass, energy, or momentum are transported from one 
region of a material to another under the influence of composition, 
temperature, or velocity gradients. If a sample of a material in which the 
chemical composition, the temperature, or the velocity vary from point to 
point is isolated from its surroundings, the transport processes act so as to 
eventually render these quantities uniform throughout the material. The 
nonuniform state required to generate these transport processes causes them to 
be known also as nonequilibrium processes. Associated with gradients of 
composition, temperature, and velocity in a material are the transport 
processes of diffusion, thermal conduction, and viscosity, respectively. See 
DIFFUSION IN GASES AND LIQUIDS; DIFFUSION IN SOLIDS; THERMAL CONDUCTION IN 
SOLIDS; VISCOSITY. 
 �34s[W. A. Wakeham]
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�6v�2v�32sTransposons   �21m �33s 
    Units of DNA which can move from one position to another in the same or a 
different genome by recombination systems other than those involved in general 
homologous recombination. They are also known as transposable elements. The 
phenomenon of transposable ``controlling elements'' was detected first many 
years ago in plants (Zea mays), but most of the detailed molecular knowledge 
of transposable elements has been obtained in prokaryotic systems. In bacteria 
several types of transposable elements have been found. The more simple (800 - 
1400 base pairs) ``insertion sequences'' were recognized by their ability to 
induce strong polar mutations in the galactose and lactose operons of 
Escherichia coli. Transposons have also been found which carry genes for drug 
resistance, toxin production, and novel substrate utilization. See 
DEOXYRIBONUCLEIC ACID (DNA); GENE ACTION. 
 �34s[Roland Contreras ;W. Fiers]
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�6v�2v�32sTransuranium elements   �21m �33s 
    Those synthetic elements with atomic numbers larger than that of uranium 
(atomic number 92). They are the members of the actinide series, from 
neptunium (atomic number 93) through lawrencium (atomic number 103), and the 
transactinide elements (with higher atomic numbers than 103). 
    The concept of atomic weight in the sense applied to naturally occurring 
elements is not applicable to the transuranium elements, since the isotopic 
composition of any given sample depends on its source. In most cases the use 
of the mass number of the longest-lived isotope in combination with an 
evaluation of its availability has been adequate. Good choices at present are 
neptunium, 237; plutonium, 242; americium, 243; curium, 248; berkelium, 249; 
californium, 249; einsteinium, 254; fermium, 257; mendelevium, 258; nobelium, 
259; lawrencium, 260; rutherfordium (element 104), 261; hahnium (element 105), 
262; and element 106, 263. 
    The actinide elements are chemically similar and have a strong chemical 
resemblance to the lanthanide, or rare-earth, elements (atomic numbers 51 - 
71). The transactinide elements, with atomic numbers 104 to 118, should be 
placed in an expanded periodic table under the row of elements beginning with 
hafnium, number 72, and ending with radon, number 86. This arrangement allows 
prediction of the chemical properties of these elements and suggests that they 
will have an element-by-element chemical analogy with the elements which 
appear immediately above them in the periodic table. 
    The transuranium elements up to and including fermium (atomic number 100) 
are produced in largest quantity through the successive capture of neutrons in 
nuclear reactors. The yield decreases with increasing atomic number, and the 
heaviest to be produced in weighable quantity is einsteinium (number 99). Many 
additional isotopes are produced by bombardment of heavy target isotopes with 
charged atomic projectiles in accelerators; beyond fermium all elements are 
produced by bombardment with heavy ions. For further information concerning 
the individual transuranium elements see AMERICIUM; BERKELIUM; CALIFORNIUM; 
CURIUM; EINSTEINIUM; ELEMENT 104; ELEMENT 105; ELEMENT 106; FERMIUM; 
LAWRENCIUM; MENDELEVIUM; NEPTUNIUM; NOBELIUM; PLUTONIUM. 
    Although the transactinide elements immediately beyond element 106 are 
predicted to have very short half-lives, theoretical considerations suggest 
increased nuclear stability, compared with preceding and succeeding elements, 
for a range of elements around atomic numbers 110, 115, or 120 because of 
increased stability predicted to result from closed nucleon shells. See 
ACTINIDE ELEMENTS; NUCLEAR REACTION; PERIODIC TABLE; RARE-EARTH ELEMENTS; 
SUPERTRANSURANICS.  
 �34s[Glenn T. Seaborg]
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�6v�2v�32sTrapezoid   �21m �33s 
    A term used in the United States for a quardrilateral with two sides 
parallel (see illustration). In Great Britain such a figure is often called a 
trapezium, a term used in the United States for a general quadrilateral. The 
parallel sides are called the bases, and the area of a trapezoid is the 
product of one-half the sum of the bases by the perpendicular distance between 
them (altitude). If the two nonparallel sides (legs) are equal, the trapezoid 
is isosceles. 
 �34s[Leonard M. Blumenthal]
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�6v�2v�32sTraps in solids   �21m �33s 
    Electron traps are defects or chemical impurities in semiconductors and in 
insulators which capture mobile electrons in a special way. The electrons are 
immobilized, prevented from recombining with (annihilating) positive holes, 
and are released some time later as mobile electrons again. Analogous traps 
act on mobile positive holes. A defect which captures a mobile electron and 
aids recombination with a hole (or vice versa) is called a recombination 
center. A defect (nickel in germanium, for example) may be a recombination 
center at one temperature and a trap at a different one. 
    Electrons or holes may remain in traps for short times or for as long as 
months or years. They may be released by heating the solid or by irradiating 
it with infrared. Traps have an important influence in photoconduction, 
luminescence, and the photographic process. See BAND THEORY OF SOLIDS; HOLES 
IN SOLIDS; LUMINESCENCE; PHOTOCONDUCTIVITY; PHOTOLYSIS. 
 �34s[L. Apker]
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�6v�2v�32sTrauma �21m �33s
    Injury to tissue by physical or chemical means. Mechanical injury includes
abrasions, contusions, lacerations, and incisions, as well as stab, puncture,
and bullet wounds. Trauma to bones and joints results in fractures,
dislocations, and sprains. Head injuries are often serious because of the
complications of hemorrhage, skull fracture, or concussion.
    Thermal, electrical, and chemical burns produce severe damage partly
because they coagulate tissue and seal off restorative blood flow.
Asphyxiation, including that caused by drowning, produces rapid damage to the
brain and respiratory centers, as well as to other organs.
    Frequent complications of trauma are shock, the state of collapse
precipitated by peripheral circulatory failure, and also hemorrhage,
infection, and improper healing. See HEMORRAGE; INFECTION; SHOCK SYNDROME.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sTraveling-wave tube   �21m �33s 
    A microwave electronic tube in which a beam of electrons interacts 
continuously with a wave that travels along a circuit, the interaction 
extending over an appreciable distance measured in wavelengths. Traveling-wave 
tubes are normally used in amplifiers with exceedingly wide bandwidths. 
Typical bandwidths are 10 - 100% of the center frequency, with gains of 20 - 
60 dB. Such traveling-wave amplifiers serve at the inputs to sensitive radars 
or communication receivers. High-power traveling-wave amplifiers operated as 
the final stages of radars or scatter communication transmitters deliver 
pulsed powers exceeding a megawatt.  For use in satellite transmitters, 
lightweight traveling-wave amplifiers develop 35 watts continuously at 45% 
overall efficiency. 
    In a traveling-wave tube amplifier (see illustration), a thermionic 
cathode produces the electron beam. An electron gun initially focuses the 
beam, and additional focusing means retain the electron stream as a beam 
throughout the length of the tube until the beam strikes the collector 
electrode. Microwave energy enters the tube near the electron gun and 
propagates along a slow-wave circuit. The tube delivers amplified microwave 
energy into a matched load connected near the collector end. The slow-wave 
circuit serves to propagate the microwave energy along the tube at 
approximately the same velocity as that of the electron beam. Interaction 
between beam and wave is continuous along the tube with contributions adding 
in phase. 
  
    In principle, operation in a traveling-wave tube is similar to that in a 
klystron: Velocity modulation of the electrons results in current modulation 
of the beam because of the nonzero transit time. However, unlike a klystron, 
the traveling-wave tube provides for the process to take place continuously 
along the slow-wave circuit. Thus, at one point along the circuit, the 
microwave field accelerates the electrons axially. As these electrons proceed 
along the tube, they bunch with those ahead. At the same point along the 
circuit, but a half-cycle later in time, the microwave field decelerates 
electrons; these electrons bunch with those behind. Because average velocity 
of the electrons is made to be slightly faster than the velocity of the energy 
on the slow-wave circuit, the electron bunches that form drift into a 
decelerating microwave field, thereby delivering energy to the field. In this 
manner the original energy in the electron beam is converted into microwave 
energy and delivered to the slow-wave circuit. Because of the continuous 
distributed interaction, the circuit wave grows exponentially as it travels 
along the tube. For the backward-wave class of traveling-wave tubes see 
BACKWARD-WAVE TUBE. For other microwave tubes see KLYSTRON; MAGNETRON; 
MICROWAVE TUBE. 
 �34s[James W. Gewartowski]
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�6v�2v�32sTravertine   �21m �33s 
    A rather dense, banded limestone, sometimes moderately porous, that is 
formed either by evaporation about springs, as is tufa, or in caves, as 
stalactites, stalagmites, or dripstone. Where travertine or tufa (calcareous 
sinter) is deposited by hot springs, it may be the result of the loss of 
carbon dioxide from the waters as pressure is released upon emerging at the 
surface; the release of carbon dioxide lowers the solubility of calcium 
carbonate and it precipitates. High rates of evaporation in hot-spring pools 
also lead to supersaturation. Travertine formed in caves is simply the result 
of complete evaporation of waters containing mainly calcium carbonate. See 
LIMESTONE; STALACTITES AND STALAGMITES; TUFA. 
 �34s[Raymond Siever]
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�6v�2v�32sTree �21m �33s
    A perennial woody plant at least 20 ft (6 m) in height at maturity, having
an erect stem or trunk and a well-defined crown or leaf canopy. However, no
sharp lines can be drawn between trees, shrubs, and lianas (woody vines.)

    Classification.  Almost all existing trees belong to the seed plants. An
exception are the giant tree ferns which were more prominent in the forests of
the Devonian Period and today exist only in the moist tropical regions. The
Spermatophyta are divided into the Pinophyta (gymnosperms) and the flowering
plants, Magnoliophyta (angiosperms). The gymnosperms bear their seed naked on
modified leaves, called scales, which are usually clustered into structures
called cones - for example, pine cones. By contrast the seed of angiosperms is
enclosed in a ripened ovary, the fruit. See MAGNOLIOPHYTA; PINOPHYTA;
POLYPODIALES.

    Morphology.  The morphology of a tree is similar to that of other higher
plants. Its major organs are the stem, or trunk and branches; the leaves; the
roots; and the reproductive structures. Almost the entire bulk of a tree is
nonliving. Of the trunk, branches, and roots, only the tips and a thin layer
of cells just under the bark are alive. Growth occurs only in these
meristematic tissues. Meristematic cells are undifferentiated and capable of
repeated division. See FLOWER; LATERAL MERISTEM; LEAF; PLANT GROWTH; ROOT;
STEM.
 �34s[F. Thomas Ledig]
�33s


    Growth.  Trees grow to a larger size at maturity than other woody
perennials, have a comparatively long period of development of maturity,
and live a long time in the mature state.  Trees, like other vascular
plants, are made up of cells.  Growth is the result of adding more cells
through cell division, and of the elongation and maturation of those cells
into functional tissues.  Cells and the tissues they comprise differ in
structure and function.  Some tissues, such as the corky bark, mechanically
protect the tree and insulate it against rapid temperature changes.
Parenchyma tissues are important in photosynthesis and food storage.  Xylem
tissues conduct water, minerals, and some hormones up the tree, and the
phloem tissues conduct photosynthate sugar solutions and other organic
molecules in most cases down the tree.	That fraction of the xylem and
phloem which conducts liquids is termed vascular tissue.  There are also
other nonconducting cells such as parenchyma within the xylem and phloem
tissues.
    Tissues may also be compared by looking at their origin.  Primary
tissues are derived from apical meristems of the stem and root, whereas
secondary tissues arise from secondary cambia.	Two important secondary
cambia in trees are:  (1)  the vascular cambium which produces cells which
become secondary xylem (the wood of commerce) to the inside and secondary
phloem tissue to the outside; and (2) the cork cambium or phellogen which
produces corky bark tissues to the outside and usually phelloderm tissue
to the inside.	Collectively, these secondary cambia account for most
of the diameter growth in trees.
    Shoot and root apices, originating in the seed embryo, are major centers
of ogranization in development of the tree.  Cell division occur in distinct
patterns in these apices.  These patterns are the basis for development of
tissues from which the mature axial structures of the shoot and root are
formed.  The shoot apex is the source of cells which develop in very
predictable patterns into leaf primordia, axillary buds that can grow into
lateral branches, and reproductive structures.	The root apex is a less complex
structure producing only cells which will develop into the internal tissues of
the root.  Lateral roots are not formed at the apex, but in pericycle tissues
some distance behind the apex.	See APICAL DOMINANCE; APICAL MERISTEM; PLANT
GROWTH.

[William C. Carlson]


    Diseases.  Diseases of both shade and forest trees have the same
pathogens, but the trees differ in value, esthetics, and utility. In forests,
disease is significant only when large numbers of trees are seriously
affected. Diseases with such visible symptoms as leaf spots may be alarming on
shade trees but hardly noticed on forest trees. Shade trees with substantial
rot may be ornamentals with high value, whereas these trees would be worthless
in the forest. Emphasis on disease control for the same tree species thus
requires a different approach, depending on location of the tree.

    Forest trees. In natural forests, leaf diseases are negligible, but in
nurseries and plantations, fungal infections cause severe defoliation,
retardation of height growth, or death. Oak wilt is a systemic disease, with
the entire tree affected through its water-conducting system. Trees of
susceptible species may be killed in a few weeks to a year or more through
plugging of the water-conducting vessels. The causal fungus, Ceratocystis
fagacearum, spreads to nearby healthy trees by root grafts and to trees at
longer distances by unrelated insects, including certain bark beetles.
    Stem rust diseases occur as cankers or galls on coniferous hosts and as
minor lesions on other ones. A few, such as white pine blister rust and
southern fusiform rust, are epidemic, lethal, and economically important.
Others of less immediate importance (such as western gall rust) are capable of
serious, widespread infection, requiring no other hosts to complete their life
cycles. Stem infections by numerous fungi, resulting in localized death of
cambium and inner bark, range from lesions killing small stems in a year
(annual) to gross stem deformities (perennial), where cankers enlarge with
stem growth.
    All tree species, including decay-resistant ones such as redwood, are
subject to ultimate disintegration by fungi. Decay fungi are associated with
nondecay fungi and bacteria. These microflora enter the tree through wounds,
branch stubs, and roots, and are confined to limited zones of wood by
anatomical and wound-stimulated tissue barriers. The extent of decay is
limited by compartmentalization of decay in trees. Trees aged beyond maturity
are most often invaded by wood-rotting fungi; losses can be minimized by
avoiding wounds and by shortening cutting rotations.

    Shade trees. Many shade trees are grown under conditions for which they
are poorly adapted, and are subject to environmental stresses not common to
forest trees. Both native and exotic trees planted out of natural habitats are
predisposed to secondary pathogens following environmental stress of
noninfectious origins. They are also susceptible to the same infectious
diseases as forest trees. Appearance is more important then the wood produced,
and individual value is higher per tree than for forest trees. Thus, disease
control methods differ from those recommended for forest trees. See PLANT
PATHOLOGY.
 �34s[Richard J. Campana]
�33s


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�6v�2v�32sTree ferns �21m �33s
    Plants belonging to the families Cyatheaceae and Dicksoniaceae, whose
members typically develop tall trunks crowned with leaves (fronds) which often
reach some 20 ft (6.1 m) in length and 5 - 6 ft (1.5 - 1.8 m) in width. Tree
ferns reach their greatest development in the rainforests and cloud forests of
the mountainous tropics. See RAINFOREST.
    Tree fern trunks may reach 65 ft (19.8 m) tall; their diameters vary from
0.4 in. (1.0 cm) to 4 ft (1.2 m). The lower trunk is often densely covered
with matted adventitious roots which greatly increase its diameter. Certain
specimens branch near the base of the trunk or higher up; perhaps this
branching occurs in response to injury.
    The degree of division of the leaves varies from simple to four or five
pinnate. The leaflets (pinnae) are usually smaller toward the base of the
leaf; when these basal leaflets are branched into threadlike divisions, they
are called aphlebiae. The Dicksoniaceae have marginal sori terminal on the
veins and protected by a bivalved indusium. The Cyatheaceae produce sori well
away from the margin, usually seated at the forking of a vein or midway along
a simple vein.
    The typical vascular system of both families is dictyostelic. The
Cyatheaceae have accessory vascular strands in the pith and cortex. Fibrous
sheaths around the vascular tissue and just inside the epidermis provide
mechanical support. The xylem consists of scalariform tracheids and
parenchyma, the phloem of sieve tubes and parenchyma. Numerous mucilage canals
are embedded in the pith and cortex. See STEM; TREE.
 �34s[Gerald J. Gastony]
�33s

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�6v�2v�32sTree-ring hydrology   �21m �33s 
    The relationships, or correlations, between ring thicknesses and steam 
flow via rainfall, and between thicknesses and rainfall via soil moisture. 
These relationships have validity only if it is possible to identify 
accurately the annual increments in all the trees employed in a given study. 
Among the trees where most work has been done, little relation to temperature 
has been found. See DENDROCHRONOLOGY; PALEOCLIMATOLOGY; PLANT-WATER RELATIONS. 
    Ring thicknesses in the southwestern United States appear to have only a 
modest correlation with stream flow. Rainfall is in a very broad sense the 
connecting link, or the common factor, between tree growth and stream flow. In 
the relationship of ring thicknesses to rainfall, the connecting link is soil 
moisture. In fact, a strong case can be made for the study of soil moisture in 
relation to tree growth rather than of the more remote factors of rainfall or 
of stream flow. 
    Techniques have been developed for extracting ecologic information and 
meteorologic or hydrologic information from growth layers. Growth patterns 
have been offered as an extension, or alternative, to the use of year-by-year 
comparisons. The use of growth-ring patterns rather than individual rings 
eliminates the necessity of dating accurately each growth layer. Thus growth 
patterns reveal information about rainfall through the variability in 
thickness of successive growth layers, relative and absolute variations in 
circuit and longitudinal uniformity, areal distribution of partial growth 
layers, and constitution of the annual increment. 
 �34s[Waldo S. Glock]
�33s 
  
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�6v�2v�32sTrematoda   �21m �33s 
    A loose grouping of acoelomate, parasitic flatworms of the phylum 
Platyhelminthes formerly accorded class rank and containing the subclasses (or 
orders) Digenea, Monogenea, and Aspidobothria. These organisms commonly occur 
as adults in or on all vertebrate groups. They exhibit cephalization, 
bilateral symmetry, and well-developed anterior and ventral, or anterior and 
posterior, holdfast structures. The mouth is anterior, and usually a blind, 
forked gut occurs, as well as three muscle layers. The excretory system 
consists of flame cells and collecting tubules. These animals are 
predominantly hermaphroditic and oviparous with operculated egg capsules. The 
life histories of the Digenea are complex, while those of the Monogenea and 
Aspidobothria are simple. 
    Trematodes parasitize a wide variety of invertebrates and vertebrates and 
occupy almost every available niche within these hosts. The adaptations 
demanded of the worms for survival are as varied as the characteristics of the 
microhabitats. Over the millions of years of coevolution of the hosts and 
their parasites, delicate balances have, for the most part, been attained, and 
under normal conditions it is probable that trematodes rarely demand more than 
the host can supply without undue strain. It seems axiomatic that the host 
must survive until the parasite can again gain access to another host or until 
the life cycle is completed. Those parasites which cause the least disruption 
of the host's activities are probably the oldest as well as the most 
successful. Immunities are sometimes developed by the hosts. Many trematodes 
seem to possess such rigid requirements and such responses to particular hosts 
that host specificity is a phenomenon of considerable significance. 
Monogeneids appear more host-specific than digeneids, and aspidobothreids seem 
less specific than both. Trematodes are of considerable veterinary and medical 
importance because under certain conditions they cause debility, even death. 
See ASPIDOGASTREA; DIGENEA; MONOGENEA; PLATYHELMINTHES. 
 �34s[William J. Hargis, Jr.]
�33s 
  
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�6v�2v�32sTremolite   �21m �33s 
    The name given to magnesium-rich monoclinic calcium amphibole 
Ca2Mg5Si8O22(OH)2. The mineral is white to gray, but colorless in thin 
section. Unlike other end-member compositions of the calcium amphibole group, 
very pure tremolite is found in nature. Substitution of Fe for Mg is common, 
but pure ferrotremolite, Ca2Fe5Si8O22(OH)2, is rare. Intermediate compositions 
between tremolite and ferrotremolite are referred to as actinolites, and are 
green in color and encompass a large number of naturally occurring calcium 
amphiboles. See AMPHIBOLE. 
 �34s[Barry L. Doolan]
�33s 
  
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�6v�2v�32sTrepostomata   �21m �33s 
    An extinct order of bryozoans in the class Stenolaemata. Trepostomes 
possess generally robust colonies, composed of tightly packed, moderately 
complex, long, slender, tubular or prismatic zooecia, with solid calcareous 
zooecial walls. Colonies show a moderately gradual transition from endozone to 
exozone regions. Trepostome colonies range from small and delicate to large 
and massive; they can be thin encrusting sheets, tabular or nodular or 
hemispherical or globular masses, or erect twiglike, bushlike, or frondlike 
growths. See BRYOZOA; STENOLAEMATA. 
    Apparently exclusively marine, the trepostomes first appeared about the 
start of the Middle Ordovician, and immediately rose to abundance and 
dominance, which they maintained from mid-Ordovician through Silurian time; in 
places, trepostomes contributed to building small reefs, as well as being 
abundant in level-bottom environments. Trepostomes declined during the 
Devonian, finally dying out in the earliest Triassic. 
 �34s[Roger J. Cuffey]
�33s 
  
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�6v�2v�32sTrestle   �21m �33s 
    A succession of towers of steel, timber, or reinforced concrete supporting 
the horizontal beams of a roadway, bridge, or other structure. Little 
distinction can be made between a trestle and a viaduct, and the terms are 
used interchangeably by many engineers. A viaduct is defined as a long bridge 
consisting of a series of short concrete or masonry spans supported on piers 
or towers, and is used to carry a road or railroad over a valley, a gorge, 
another roadway, or across an arm of the sea. See BRIDGE. 
    A trestle or a viaduct usually consists of alternate tower spans and spans 
between towers. For low trestles the spans may be supported on bents, each 
composed of two columns adequately braced in a transverse direction. A pair of 
bents braced longitudinally forms a tower. See STRUCTURES (ENGINEERING); TOWER. 
 �34s[Charles M. Antoni]
�33s 
  
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�6v�2v�32sTriangle   �21m �33s 
    In the euclidean sense, a triangle is a geometric figure bounded by three 
(straight) line segments, called sides, which meet pairwise in three points 
called vertices. It has three angles (each formed by a pair of sides at a 
vertex), whose three measures have a sum equal to 180DG or two right angles. 
The sum of two sides of a triangle is greater, and the difference less, than 
the third side. A triangle is called equilateral if three sides are equal, 
isosceles if at least two sides are equal, and scalene if no two sides are 
equal (see illustration). The greater of two sides is opposite the greater 
angle. A triangle is called acute, right, or obtuse, according to whether its 
largest angle is acute (<90DG), right (=90DG), or obtuse (>90DG). 
    The line segment drawn from a vertex of a triangle to the midpoint of the 
opposite side is called a median of the triangle. The three medians of a 
triangle intersect in a point - called the centroid. See EUCLIDEAN GEOMETRY; 
POLYGON; PYTHAGOREAN THEOREM. 
 �34s[James S. Frame]
�33s 
  
  
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�6v�2v�32sTriassic   �21m �33s 
    A geological term designating the lowest rock system of the Mesozoic Era. 
The name is derived from the threefold facies division of a sequence of strata 
in central Germany into a lower nonmarine redbed facies, a middle marine 
limestone, sandstone, and shale facies, and an upper nonmarine continental 
facies similar to the lower division. These have been correlated by geologists 
to facies in other parts of the world. 
    With the close of the Permian, epicontinental and geosynclinal seas had 
retreated from most of the continents and the geography of the lands and seas 
must have been very similar to what it is today; with few exceptions, Triassic 
seas were marginal to the continents. It was not until the Jurassic that 
shallow shelf seas began to spread beyond the peripheral geosynclinal belts 
and inundated parts of Eurasia and North America. 
    The great restrictions of epicontinental and geosynclinal seas in the late 
Paleozoic were accompanied by the extinction of a large part of the marine 
faunas characteristic of that era. This great crisis in the evolutionary 
history of marine animals is reflected in the composition of Triassic marine 
faunas. In the Lower Triassic, for instance, corals, foraminifera, and bryozoa 
are absent or extremely rare. The dominant marine animals in the Early 
Triassic seas are mollusca, especially the ammonites. Pelecypods, nautiloids, 
and gastropods are fairly common but not nearly to the extent of the 
ammonoids. The crisis of mass extinction did not affect the animals living on 
land. There are no sharp breaks or interruptions in the evolutionary history 
of the amphibians or reptiles, and none in the evolutionary development of 
plants. See EXTINCTION (BIOLOGY). 
 �34s[Bernhard Kummel]
�33s 
  
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�6v�2v�32sTrichinosis   �21m �33s 
    The presence of Trichinella spiralis, a nematode, in the body of humans. 
Wild hogs, rats, and omnivores are natural hosts. Domestic pigs become 
infected by preying on rats or feeding on raw scraps. Humans are exposed upon 
eating encysted larvae in raw or partially cooked pork. Ingested larvae 
develop in a week into adult nematodes in the duodenum. During the 3 - 16 week 
period that the adult female parasitizes humans, each one discharges from 300 
to 1500 larvae. These enter the lymphatics and are carried to all parts of the 
body, where they encyst preferably in active muscles. The host reacts by 
forming a capsule around the larva. 
    Infection by a few larvae may not occasion a noticeable damage. Heavy 
exposure results in a syndrome of varied nature. The first stage is related to 
the intestinal phase of parasitism and simulates acute food poisoning, 
typhoid, or cholera. The second stage coincides with larval migration, that 
is, inflammatory processes in the muscle and circulatory systems, swelling or 
edema, remittent high fever, and hypereosinophilia. The third stage relates to 
encystation of larvae: cachexia, dehydration, and blood pressure and 
neuropathic changes. 
    Treatment is by oral cortisone or thiabendazole. Clinical recovery is 
dramatic, but the larvae in muscle may live unhampered. Prophylaxis is best 
accomplished by thoroughly cooking all pork products. A temperature of 66DGC 
(150DGF) kills Trichinella larva. Curing or pickling pork will not necessarily 
kill encysted larvae. The larvae will be killed if the pork is kept frozen at 
-15DGC (+5DGF) for 21 days or at -30DGC (-22DGF) for 25 h. 
 �34s[Jose F. Maldonado]
�33s 
  
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�6v�2v�32sTrichomonadida   �21m �33s 
    An order of the class Zoomastigophorea. This group of flagellate 
protozoans comprises four families, the most important being the 
Trichomonadidae. Several species are found as parasites in vertebrates, 
including humans. See TRICHOMONIASIS; ZOOMASTIGOPHOREA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sTrichomoniasis   �21m �33s 
    A localized genitourinary infection of humans caused by Trichomonas 
vaginalis, a flagellate protozoon. Three species of Trichomonas inhabit the 
human body; T. buccalis (T. tenax) in the mouth and T. hominis in the lumen of 
the intestinal tract, neither of which produces clinical-pathological signs, 
and the pathogenic T. vaginalis. Trichomonas vaginalis causes vaginitis, a 
superficial ulceration and inflammation, and a discharge. It causes a severe 
itching and burning sensation and produces painful coitus; however, it causes 
no interference with pregnancy. In the male, T. vaginalis may cause a light or 
severe urethritis and involvement of accessory glands. A chronic carrier state 
has been found in males. 
 �34s[Meir Yoeli]
�33s 
  
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�6v�2v�32sTrichoptera   �21m �33s 
    An aquatic order of the class Insecta commonly known as the caddis flies. 
The adults have two pairs of well-veined hairy wings, long antennae, and 
mouthparts capable of lapping only liquids. The larvae are wormlike, with 
distinct heads, three pairs of legs on the thorax, and a pair of hook-bearing 
legs at the end of the body. The pupae are delicate, with free appendages held 
close to the body, and have a pair of sharp mandibles, or jaws, which are used 
to cut an exit from the cocoon. 
    The Trichoptera include about 10,000 described species, divided into 34 
families, and occur in practically all parts of the world. Except for a 
brackish-water species in New Zealand and a few moss-inhabiting species in 
Europe and North America, caddis flies occur only in fresh water. They abound 
in cold or running water relatively free from pollution. Altogether they 
compose a large and important segment of the biota of such habitats and of the 
fish feed economy. See INSECTA. 
 �34s[Herbert H. Ross]
�33s 
  
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�6v�2v�32sTrichostomatida   �21m �33s 
    An order of the Holotrichia, composed of a small group of ciliates whose 
species show some advance in structural complexity over the gymnostomes. No 
true buccal ciliature is present, but a vestibulum, a passageway leading from 
the outside of the body into the cytostome or true cell-mouth, is 
characteristically present. These animals exist in a wide range of habitats. A 
few forms live in cattle, sheep, and horses, but cause no harm to these hosts. 
Balantidium is of some medical significance as it contains the sole ciliated 
protozoan species parasitic in humans. Balantidium coli lives in the large 
intestine and is the causative agent of the fairly uncommon disease 
balantidiasis. See BALANTIDIASIS; HOLOTRICHIA. 
 �34s[John O. Corliss]
�33s 
  
  
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�6v�2v�32sTrichroism   �21m �33s 
    When certain optically anisotropic transparent crystals are subjected to 
white light, a cube of the material is found to transmit a different color 
through each of the three pairs of parallel faces. Such crystals are sometimes 
termed trichroic, and the phenomenon is called trichroism. This expression is 
used only rarely today since the colors in a particular crystal can appear 
quite different if the cube is cut with a different orientation with respect 
to the crystal axes. Accordingly, the term is frequently replaced by the more 
general term pleochroism. Even this term is being replaced by the phrase 
linear dichroism or circular dichroism to correspond with linear birefringence 
or circular birefringence. See BIREFRINGENCE; CRYSTAL OPTICS; DICHROISM; 
PLEOCHROISM.  
 �34s[Bruce H. Billings]
�33s 
  
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�6v�2v�32sTrichuroidea   �21m �33s 
    A group of nematodes parasitic in various vertebrates and characterized by 
the peculiar structure of the portion of the body which contains the 
esophagus. This group is given the status of an order or superfamily in the 
class Nematoda. The life cycle varies greatly in different species. In some it 
is direct, in others a transport or an intermediate host may be involved. The 
group includes disease-producing parasites of humans, domestic mammals, and 
poultry.  
    Two genera are common parasites of humans, Trichuris and Trichinella. 
Trichuris trichiura, a common parasite of humans in warm climates, belongs to 
a group known as whipworms because of the short thick posterior portion of the 
body and the long slender anterior part. The adult worm embeds in the anterior 
portion of the intestinal wall of the host. Human infections result from hand 
to mouth transfer of eggs or from eating contaminated vegetables. Trichinella 
spiralis is an important cause of human disease and is a common parasite of 
pigs. See NEMATA; TRICHINOSIS; WHIPWORM DISEASE. 
 �34s[J. Allen Scott]
�33s 
  
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�6v�2v�32sTricladida   �21m �33s 
    An order of the Turbellaria known commonly as planaria. They have a 
diverticulated intestine with a single anterior branch and two posterior 
branches separated by the plicate pharynx or pharynges. Rhabdites are numerous 
and, except in cave planarians, two to many eyes are present. See PLANARIA; 
TURBELLARIA.  
 �34s[E. Ruffin Jones]
�33s 
  
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�6v�2v�32sTriconodonta �21m �33s
    An extinct, nominally mammalian order of small flesh-eating creatures of
the Mesozoic Era. They have been found in the Upper Jurassic and Middle
Cretaceous of North America. The immediate ancestry of the triconodonts is
uncertain, but they may have been related to the primitive Rhaetic genus
Eozostrodon, often called Morganucodon. Apparently the triconodonts represent
a sterile offshoot of the cynodont therapsids, and their inclusion with the
Mammalia is merely a matter of convenience. See DOCODONTA; MAMMALIA;
THERAPSIDA.
 �34s[William A. Clemens ;Malcolm C. McKenna]
�33s

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�6v�2v�32sTrigger circuit   �21m �33s 
    An electronic circuit that generates or modifies an existing waveform to 
produce a pulse of short time duration with a fast-rising leading edge. This 
waveform, or trigger, is normally used to initiate a change of state of some 
relaxation device, such as a multivibrator. The most important characteristic 
of the waveform generated by a trigger circuit is usually the fast leading 
edge. The exact shape of the falling portion of the waveform often is of 
secondary importance, although it is important that the total duration time is 
not too great. A pulse generator such as a blocking oscillator may also be 
used and identified as a trigger circuit if it generates sufficiently short 
pulses. See BLOCKING OSCILLATOR; PULSE GENERATOR. 
    Peaking circuits, which accent the higher-frequency components of a pulse 
waveform, cause sharp leading and trailing edges and are therefore used as 
trigger circuits. The simplest form of peaking circuits are the simple RC and 
RL networds shown in the illustration. If a steep wavefront of amplitude V is 
applied to either of these circuits, the output will be a sudden rise followed 
by an exponential decay. These circuits are often called differentiating 
circuits because the outputs are rough approximations of the derivative of the 
input waveforms, if the RC or R/L time constant is sufficiently small. 
    A circuit that is highly underdamped, or oscillatory, and is supplied with 
a step or pulse input is often referred to as a ringing circuit. When used in 
the output of a field-effect or bipolar transistor, this circuit can be used 
as a trigger circuit. See WAVE-SHAPING CIRCUITS. 
 �34s[Glenn M. Glasford]
�33s 
  
  
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�6v�2v�32sTriglyceride �21m �33s
    A simple lipid. Triglycerides are fatty acid triesters of the trihydroxy
alcohol glycerol which are present in plant and animal tissues, particularly
in the food storage depots, either as simple esters in which all the fatty
acids are the same or as mixed esters in which the fatty acids are different.
The triglycerides constitute the main component of natural fats and oils.
    The generic formula of a triglyceride is shown below, where RCO2H, R'CO2H,

				 CH2--OOC--R
				 |
				 CH--OOC--R'
				 |
				 CH2--OOC--R"

and R''CO2H represent molecules of either the same or different
fatty acids, such as butyric or caproic (short chain), palmitic or stearic
(long chain), oleic, linoleic, or linolenic (unsaturated). Saponification with
alkali releases glycerol and the alkali metal salts of the fatty acids
(soaps). The triglycerides in the food storage depots represent a concentrated
energy source, since oxidation provides more energy than an equivalent weight
of protein or carbohydrate. See LIPID METABOLISM; SOAP.
    The physical and chemical properties of fats and oils depend on the nature
of the fatty acids present. Saturated fatty acids give higher-melting fats and
represent the main constituents of solid fats, for example, lard and butter.
Unsaturation lowers the melting point of fatty acids and fats. Thus, in the
oil of plants, unsaturated fatty acids are present in large amounts, for
example, oleic acid in olive oil and linoleic and linolenic acids in linseed
soil. See FAT AND OIL (FOOD); LIPID.
 �34s[Roy H. Gigg ;Herbert E. Carter]
�33s

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�6v�2v�32sTrigonometric curve �21m �33s
    The graphical representation of y as a trigonometric function of THETA. To
obtain the graphs of
			    y = a sin THETA

for the interval 0 < THETA < 2PI, one prepares the accompanying table of
values of trigonometric functions, plots the corresponding points, and
connects them by a smooth curve. This gives the graph shown in the illustration.
Since sin (THETA+k2PI)=sin THETA, where k is an integer, the complete graph
consists of an endless repetition both to the right and to the left of the curve
shown. See TRIGONOMETRY.
    Since sin THETA = cos (THETA-1/2PI), the illustration is also the graph of
y=a cos (THETA-1/2PI). The graph of y=a cos THETA has the same shape as that of
y=a sin THETA and could be obtained from the illustration by translating the
curve 1/2PI units leftward.
 �34s[Lyman M. Kells]
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�6v�2v�32sTrigonometry �21m �33s
    The study of triangles and the trigonometric functions. The functions of
trigonometry are extremely important because they may be used to represent
ranges of values which are repeated again and again. They represent periodic
phenomena such as the motions of pendulums or the analysis of
alternating-current electricity.
    Also, trigonometry is used to find a vast network of lengths that cannot
be measured directly. Surveyors use it to find heights of mountains, distances
across lakes and countries, and positions of places; engineers use it in the
design of large structures and roads; astronomers use it in accurate
measurements of time, and in locating the positions of bodies in the sky; and
navigators on the sea and in the air use it to find latitudes, longitudes, and
directions.
    The subject of trigonometry may be divided into plane trigonometry and
spherical trigonometry.

    Plane trigonometry.  Plane trigonometry, for the most part, deals with
triangles with straight lines in a plane as sides. Unless otherwise specified,
it is understood that all figures considered in this section lie in a plane.

    Angles. If a half-line, or ray, having end point O (see Fig. 1) rotates
about O from an initial position OA to terminal position OB, it is said to
generate the angle AOB. A rotation of 1/360 part of a complete rotation about a
point is called a degree and written 1DG. One-sixtieth of a degree is called a
minute and written 1'. Generally angles generated by rotation in a clockwise
direction are called negative angles, and those generated by counterclockwise
turning are called positive angles.
    A radian is an angle subtended at the center of a circle by an arc of the
circle equal in length to its radius. It is used in purely theoretical
discussions because it avoids cumbersome constants. See RADIAN MEASURE.

    Trigonometric functions. Figure 2 shows an angle THETA with vertex O at
the origin of a system of rectangular coordinates and with initial line OA
directed in the positive direction of the x axis. Let P, any point except O,
on the terminal ray of the angle THETA, have coordinates x and y and be at a
distance r from O. Then the six functions, sine, cosine, tangent, cotangent,
secant, and cosecant of THETA, respectively abbreviated by sin THETA, cos
THETA, tan THETA, cot THETA, sec THETA, csc THETA, are defined by Eqs. (1) and

	      sin theta = y/r, cos theta = x/r, tan theta = y/x 	     (1)


	      csc theta = r/y, sec theta = r/x, cot theta = x/y 	     (2)

(2), where r is positive. These ratios are independent of the position of the
point (x,y) on the terminal ray.  See  COORDINATE SYSTEMS.
    Figure 2 shows that any angle with initial side OA obtained by adding an
integral multiple of 360DG to THETA has the same terminal ray as THETA, and

		   fn (theta + k360DG) = fn (theta)			     (3)

therefore that Eq. (3) can be written, where fn stands for sin, cos, tan, cot,
sec, or csc, and k is an integer. In other words the trigonometric functions
are periodic, and a period is 360DG or 2pi radians. This is the property that
makes them so useful in theoretical considerations involving periodic values.
    Figure 3 shows four angles 60DG, 120DG, 240DG, and 300DG having terminal
lines containing respective points (1, SQUARE ROOT [3]), (-1, SQUARE ROOT [3]),
(-1, -SQUARE ROOT [3]), and (1,-SQUARE ROOT [3]). Since r = SQUARE ROOT [1SUP2 +
(SQUARE ROOT [3]) SUP2] = 2 in all cases the values from Eq. (1) can be
tabulated as in notation (4).

		sin					cos
	 60DG (1/2)(SQUARE ROOT [3])			1/2
	120DG (1/2)(SQUARE ROOT [3])		       -1/2
	140DG (-1/2)(SQUARE ROOT [3])		       -1/2
	300DG (-1/2)(SQUARE ROOT [3])			1/2

		tan					cot
	 SQUARE ROOT [3]			 (1/3)(SQUARE ROOT [3])
	-SQUARE ROOT [3]			(-1/3)(SQUARE ROOT [3])
	 SQUARE ROOT [3]			 (1/3)(SQUARE ROOT [3])
	-SQUARE ROOT [3]			(-1/3)(SQUARE ROOT [3])

		sec					csc
		 2				2/(SQUARE ROOT [3])
		-2				2/(SQUARE ROOT [3])
		-2			       -2/(SQUARE ROOT [3])
		 2			       -2/(SQUARE ROOT [3])

    In all cases VERTICAL BARxVERTICAL BAR LESS THAN OR EQUAL TO r, VERTICAL
BARyVERTICAL BAR LESS THAN OR EQUAL TO r, and therefore the range of the sine
and of the cosine is -1 to +1 inclusive. Figure 3 shows the angles 0DG, 90DG,
180DG, and 270DG containing on their terminal rays the respective points (1,0),
(0,1), (-1,0) and (0,-1). Using Fig. 3 and Eqs. (1) and (3), the values may be
tabulated as in notation (5), where k is an integer.

		0DG					90DG
	     + k360DG				     + k360DG
	sin	 0					 1
	cos	 1					 0
	tan	 0					Undefined

	       180DG				       270DG
	     + k360DG				     + k360DG
		 0					-1
		-1					 0
		 0					Undefined

    If the angle theta is near to, but less than 90DG, x is small compared to
y and therefore the tangent is large; in fact tan theta ranges through all
positive numbers as angle theta increases from 0DG to 90DG. The graphs of the
trigonometric functions give a clear picture of the ranges and general
behavior of these functions.  See  TRIGONOMETRIC CURVE.

csc theta = 1/sin theta   sec theta = 1/cos theta   cot theta = 1/tan theta
									    (6)

			tan theta = (sin theta)/(cos theta)		    (7)

From Eqs.  (1) and (2), it follows that Eqs.  (6) and (7) can be written.  Also
from Fig.  2, y(sup 2) + x(sup 2) = r(sup 2).  Dividing this through by r(sup 2)
and using Eq.  (1), (sin theta)(sup 2) + (cos theta)(sup 2) = 1, which is

		     sin(sup 2) theta + cos(sup 2) theta = 1		     (8)

written as in Eq. (8). Using Eqs. (6), (7), (8), and others, many complicated
expressions can be transformed to simple ones.
    Addition formulas. The addition formulas of trigonometry, namely, Eqs. (9)
and (10), lead to practically all of the relations between the trigonometric
functions.

       cos (phi + theta) = cos phi cos theta - sin phi sin theta	     (9)

       sin (phi + theta) = sin phi cos theta + cos phi sin theta	    (10)

    Inverse trigonometric functions. In the equation x = sin y, y is an angle
having x as sine. The symbol sin sup-1 x (or arcsin x) means the angle having x
as sine, or the inverse sine of x. Evidently arcsin 1/2 has the values 30DG +
k360DG and 150DG + k360DG, where k is an integer. A like notation applies to
the other functions. For example, arccos 1 1/2 has the values PLUS/MINUS 60 DG +
k360DG. Generally a particular value, called the principal value, is chosen
and designated by Arcsin x, Arccos x, Arctan x, etc. Arcsin x and Arctan x are
chosen in the range from -90DG to +90DG, and Arccos x and Arccot x are chosen
in the range 0DG to 180DG.

    Solution of rectilinear figures. If a side and another part of a right
triangle are known, the triangle can be solved. A rectilinear figure can often
be solved by drawing perpendiculars to sides of the figure to divide it into
right triangles and then solving the set of right triangles in order. However,
various formulas are used for solving rectilinear figures. For convenience,
the vertices of a triangle are denoted by capital letters A, B, C, and the
respective opposite sides by small letters a, b, c.

    Values of trigonometric functions. If theta is an angle in radians and n!
= 1 x 2 x 3 x . . . x n, then sin theta and cos theta are defined for all values
of theta by the endless series shown as Eqs. (11) and (12).

sin theta = theta - theta(sup 3)/3! + theta(sup 5)/5! - . . . + [(-1)(sup n+1)
theta(sup 2n + 1)]/(2n + 1)! + . . .					  (11)



cos theta = 1 - theta(sup 2)/2! + theta(sup 4)/4! - . . . + [(-1)(sup n)
theta(sup 2n)]/2n! + . . .						  (12)

    To find a value of the sine or the cosine of an angle, only as many terms
of the series are used as are necessary to ensure required accuracy.

    Spherical trigonometry.  A great circle on a sphere is the intersection of
the sphere with a plane through the center. Spherical trigonometry treats, for
the most part, of spherical triangles having as sides arcs of great circles on
a sphere. Two cases of these triangles are highly important. In one case the
vertices are points on the Earth, and in the other on celestial bodies such as
the Sun, planets, and stars. Astronomers, surveyors, and navigators on ships
and airplanes apply the formulas of spherical trigonometry to find such values
as the time of day, directions of motion, and positions of ships, airplanes,
and reference points. Thus spherical trigonometry is basic in astronomy, in
certain kinds of surveying, and in navigation. It is also used in mathematics
and its applications.
 �34s[Lyman M. Kells]
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�6v�2v�32sTrihedron   �21m �33s 
    A geometric figure bounded by three noncoplanar rays called edges that 
emanate from a common point called the vertex, and by the plane sectors called 
faces that are formed by each pair of edges (see illustration). A trihedron  
has three dihedrons formed by pairs of face planes, and three face angles 
formed by pairs of edges. 
 �34s[James S. Frame]
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�6v�2v�32sTrilobita   �21m �33s 
    A class of extinct Paleozoic (Cambrian to Permian) arthropods, occurring 
with other invertebrates in various types of sedimentary rocks. They are 
therefore presumed to have inhabited shallow seas but not fresh waters. The 
exoskeleton (Fig. 1) covered the dorsal surface and extended a short distance 
over the ventral side as the doublure. It was probably chitinous and 
strengthened by mineral secretion believed to have been calcite. The mineral 
matter may be preserved in the rock with little change (Fig. 2), may be 
replaced by some other mineral, or may be dissolved so that an internal or 
external mold remains. Trilobites are abundant in many Cambrian sediments, 
including the earliest, and are used almost exclusively in dating and 
correlating rocks of that system. In Ordovician and Silurian rocks, trilobite 
remains are common, but become fewer in the Devonian and rare in the 
Carboniferous and Permian. The beauty of trilobites has fascinated collectors, 
and the evolution, taxonomy, and extinction of this group of more than 1000 
genera invite study. An understanding of them is essential in Paleozoic 
stratigraphy, paleogeography, and animal distribution.  See  ARTHROPODA. 
 �34s[Harry B. Whittington]
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�6v�2v�32sTrilobitomorpha   �21m �33s 
    The trilobitomorphs are all extinct and are among the oldest known 
arthropods. The most dominant group, the trilobites (Trilobita), were in 
existence at the beginning of the Cambrian Period, and must have lived long 
before they or any other animals were preserved as fossils; yet they are not 
the actual ancestors of any modern arthropods. See TRILOBITA. 
    A typical trilobite has a flattened oval shape with an elevation, the 
rachis, running lengthwise along the middle of the back, giving it a 
three-part appearance from which the trilobites get their name. More 
important, however, is the crosswise division of the body into a head, a 
thorax, and a pygidium. The head is not segmented, but indentations of the 
median elevation suggest a primitive segmentation. On its upper surface, 
laterally, it bears a pair of compound eyes. The thorax is completely 
segmented. On the undersurface the head bears a pair of long antennae and a 
median lobe, the labrum, that probably covers the mouth. The rest of the 
undersurface is occupied by a long series of eight-segmented legs, which are 
all alike and suggest that the diversified appendages of other arthropds were 
once jointed legs used for locomotion Mesal spiny lobes of the basal segments 
probably served for grasping food and passing it forward to the mouth. 
 �34s[Jerome G. Rozen, Jr.]
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�6v�2v�32sTrimerophytopsida �21m �33s
    Mid-Early-Devonian into Mid-Devonian vascular plants at a higher
evolutionary level than Rhyniopsida. Branching was profuse and varied,
dichotomous, pseudomonopodial, helical to subopposite and almost whorled, and
often trifurcate. Vegetative branches were often in a tight helix, terminated
by tiny recurved branchlets simulating leaf precursors. The axes were leafless
and glabrous or spiny. Xylem is known only in Psilophyton.  See  EMBRYOBIONTA;
RHYNIOPSIDA.
 �34s[Harlan P. Banks]
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�6v�2v�32sTriphosphopyridine nucleotide (TPN)   �21m �33s 
    A coenzyme and an important component of the enzymatic systems concerned 
with biological oxidation-reduction systems. It is also known as TPN, 
triphosphopyridine dinucleotide, nicotinamide adenine dinucleotide (NAD), 
coenzyme II, and codehydrogenase II. The compound is similar in structure and 
function to diphosphopyridine nucleotide (DPN). It differs structurally from 
DPN in having an additional phosphoric acid group esterified at the 2' 
position of the ribose moiety of the adenylic acid portion. In biological 
oxidation-reduction reactions the TPN molecule becomes alternately reduced to 
its hydrogenated form (TPNH) and reoxidized to its initial state (see 
illustration).  See  BIOLOGICAL OXIDATION; CARBOHYDRATE METABOLISM; COENZYME; 
DIPHOSPHOPYRIDINE NUCLEOTIDE (DPN); ENZYME. 
 �34s[Michael Doudoroff]
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�6v�2v�32sTriple point �21m �33s
    A particular temperature and pressure at which three different phases of
one substance can coexist in equilibrium. In common usage these three phases
are normally solid, liquid, and gas, although triple points can also occur
with two solid phases and one liquid phase, with two solid phases and one gas
phase, or with three solid phases.
    According to the Gibbs phase rule, a three-phase situation in a
one-component system has no degrees of freedom (that is, it is invariant).
Consequently, a triple point occurs at a unique temperature and pressure,
because any change in either variable will result in the disappearance of at
least one of the three phases. Triple points are shown in the illustration of
part of the phase diagram for water.  See  PHASE EQUILIBRIUM.
    For most substances the solid-liquid-vapor triple point has a pressure
less than 1 atm (10(sup2) kilopascals); such substances then have a liquid-vapor
transition at 1 atm (normal boiling point). However, if this triple point has
a pressure above 1 atm, the substance passes directly from solid to vapor at 1
atm.  See  BOILING POINT; ICE POINT; MELTING POINT; SUBLIMATION; TRANSITION
POINT; VAPOR PRESSURE; WATER.
 �34s[Robert L. Scott]
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�6v�2v�32sTriplet state   �21m �33s 
    An electronic state of a molecule that occurs when its total spin angular 
momentum quantum number S is equal to one. The triplet state is an important 
intermediate of organic chemistry. In addition to the wide range of triplet 
molecules available through photochemical excitation techniques, numerous 
molecules exist in stable triplet ground states, for example, oxygen 
molecules.  See  ATOMIC STRUCTURE AND SPECTRA; MOLECULAR STRUCTURE AND 
SPECTRA; SPIN (QUANTUM MECHANICS). 
    A triplet may result whenever a molecule possesses two electrons which are 
both orbitally unpaired and spin unpaired. Orbital unpairing of electrons 
results when a molecule absorbs a photon of visible or ultraviolet light. 
Direct formation of a triplet as a result of of this photon absorption is a 
very improbable process since both the orbit and spin of the electron would 
have to change simultaneously. Thus, a singlet state is generally formed by 
absorption of light. However, quite often the lifetime of this singlet state 
is sufficiently long to allow the spin of one of the two electrons to invert, 
thereby producing a triplet.  See  MOLECULAR ORBITAL THEORY.  
 �34s[Nicholas J. Turro]
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�6v�2v�32sTrisomic syndrome   �21m �33s 
    Trisomy is an abnormal state characterized by the presence in triplicate 
of one of the chromosomes of the complement. The normal human fertilized egg 
has 23 pairs of chromosomes (2n), one of each pair provided by each parent 
(n). In trisomy the total number of chromosomes present is one unit above 
normal (2n + 1), except if the chromosomal material in excess is translocated 
onto another chromosome. In this case the total number appears normal (2n), 
but one chromosome has an abnormally large size. A trisomic syndrome in humans 
is a pathological condition resulting from the presence of this extra 
chromosomal material. Since the error leading to the trisomic state occurs 
before or shortly after fertilization of the egg by a sperm, the 
manifestations of the trisomy are present at the birth of the child, in the 
form of obvious multiple defects. Because these malformations occur in 
combinations rather specific for each of the possible trisomies, an 
experienced observer usually can identify any such syndrome on clinical 
grounds alone. Of course trisomy is responsible for only a relatively small 
portion of the congenital defects of humans; the majority of such defects are 
not associated with such gross chromosomal errors.  See  CHROMOSOME 
ABERRATION; CONGENITAL ANOMALY. 
    In 1866 the British physician Langdon Down described a characteristic 
syndrome of mental retardation and multiple malformations for which he coined 
the unfortunate denomination of mongolian idiocy. The cause of the disorder 
remained unknown until 1959, when a team of researchers demonstrated that the 
syndrome was associated with the presence of an extra chromosome identified as 
a number 21. Since then mongolism or Down's syndrome has been also commonly 
referred to as trisomy 21.  See  DOWN'S SYNDROME. 
    Whereas many patients with trisomy 21 attain an old age, patients with D1 
trisomy, or trisomy 13, usually die early in infancy because of the more 
severe nature of their malformations. The very few who survive long enough 
present a very severe mental deficiency. The brain characteristically shows 
lack of separation of the hemispheres in the frontal region and absence of 
external olfactory tracts. General reduction in size of the eye, or slits in 
the eyes, or both are frequent. These infants have seizures and breath-holding 
spells and often die during such episodes. Their appearance is quite striking, 
because of microcephaly, abnormal eyes, cleft lip and palate, malformed ears, 
hands with overlapping fingers and often extra digits, and abnormal penis and 
scrotum in the males. The internal anomalies, in addition to those of the 
central nervous system, include various heart defects, renal malformations, 
malrotation of the colon, accessory spleens, and septate or duplicated 
Muellerian duct derivatives in the female (uterus, tubes, and upper vagina). 
    Babies with E trisomy, or trisomy 18, present the following anomalies: 
severe mental defect and growth failure; muscular hypertonia; cephalic 
deformities such as small jaw, prominent occiput, abnormal ears, high arched 
palate; short sternum; heart malformations; diaphragmatic and inguinal 
hernias; renal malformations; deformities of the extremities such as 
syndactyly and overriding fingers (tightly clenched fists), hypoplastic nails, 
short or dorsiflexed big toes, and a posteriorly prominent calcaneus 
(``rockerbottom foot''); limited hip abduction; and dislocation of the hips 
(probably related to muscular hypertonia). For unknown reasons this syndrome 
affects girls much more often than it does boys. The length of survival is, as 
in trisomy 13, quite limited.  See  HUMAN GENETICS; MUTATION. 
 �34s[Pierre E. Ferrier]
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�6v�2v�32sTriterpene   �21m �33s 
    A hydrocarbon or its oxygenated analog containing 30 carbon atoms and 
composed of six isoprene units. Triterpenes form the largest group of 
terpenoids, but are classified into only a few major categories. Resins and 
saps contain triterpenes in the free state as well as in the form of esters 
and glycosides. 
    Biogenetically triterpenes arise by the cyclization of squalene and 
subsequent skeletal rearrangements. Apart from the linear squalene itself and 
some bicyclic, highly substituted skeletons, most triterpenes are either 
tetracyclic or pentacyclic compounds. The various structural classes are 
designated by the names of representative members.  See  ISOPRENE; SQUALENE; 
TERPENE.  
 �34s[Tomas Hudlicky]
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�6v�2v�32sTritium �21m �33s
    The heaviest isotope of the element hydrogen and the only one which is
radioactive. Tritium occurs in very small amounts in nature but is generally
prepared artificially by processes known as nuclear transmutations. It is
widely used as a tracer in chemical and biological research and is a component
of the so-called thermonuclear or hydrogen bomb. It is commonly represented by
the symbol *EQN* indicating that it has an  atomic number of 1 and an atomic
mass of 3, or by the special symbol T. For information about the other
hydrogen isotopes  see	DEUTERIUM; HYDROGEN.  See also	TRANSMUTATION.
    Both molecular tritium, T2, and its counterpart hydrogen, H2, are gases
under ordinary conditions. Because of the great difference in mass, many of
the properties of tritium differ substantially from those of ordinary
hydrogen, as indicated in the table. Chemically, tritium behaves quite
similarly to hydrogen. However, because of its larger mass, many of its
reactions take place more slowly than do those of hydrogen.
    The nucleus of the tritium atom, often called a triton and symbolized t,
consists of a proton and two neutrons. It undergoes radioactive decay by
emission of a BETA-particle  to leave a helium nucleus of mass 3. No
GAMMA-rays are emitted in this process. The half-life for the decay is 12.26
years. When tritium is bombarded with deuterons of sufficient energy, a
nuclear reaction known as fusion occurs and energy considerably greater than
that of the bombarding particle is released. This reaction is one of those
which supply the energy of the thermonuclear bomb. It is also of major
importance in the development of controlled thermonuclear reactors.  See
HEAVY WATER; NUCLEAR FUSION; TRITON.
 �34s[Louis Kaplan]
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�6v�2v�32sTriton �21m �33s
The nucleus of (3 OVER 1)H (tritium); it is the only known radioactive
nuclide
 belonging to hydrogen.	The triton is produced in nuclear
reactors by neutron
 absorption in deuterium ([2 OVER 1]H + [sub 0][sup 1]n
--> [3 OVER 1]H + GAMMA),
 and decays by BETA- emission to (3 OVER 2)He with
a half-life of 12.4 years.
 Much of the interest in producing (3 OVER 1)H
arises from the fact that the
 fusion reaction (3 OVER 1)H + (1 OVER 1)H -->
(4 OVER 2)He releases  about 20
 Mev of enenergy.  Tritons are also used as
projectiles in nuclear bombardment
 experiments.  See NUCLEAR REACTION;
TRITIUM.
�34s[Henry E. Duckworth]
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�6v�2v�32sTriuridales   �21m �33s 
    A small order of flowering plants, division Magnoliophyta (Angiospermae), 
subclass Alismatidae of the class Liliopsida (monocotyledons). The order 
consists of 2 families, the Triuridaceae, with about 70 species, and the 
Petrosaviaceae, with only about 4 species. They are terrestrial, mycotrophic 
herbs without chlorophyll. They grow in moist, tropical regions, especially of 
the Old World, and have attracted little botanical attention.  See  
ALISMATIDAE; LILIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sTrochodendrales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of two families, the Trochodendraceae and Tetracentraceae, each with 
only a single species. They are trees of eastern and southeastern Asia with 
primitively vesselless wood and with unique, elongate, often branched 
idioblasts in the leaves. The flowers have several folded but scarcely sealed, 
lightly connate carpels and much reduced or no perianth. The group is of 
botanical interest because it tends to connect the subclass Hamamelidae with 
the ancestral Magnoliidae.  See  HAMAMELIDAE; MAGNOLIIDAE; MAGNOLIOPSIDA. 
 �34s[Arthur Cronquist]
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�6v�2v�32sTrochophore   �21m �33s 
    A generalized but distinct type of free-swimming larva found in several 
invertebrate groups, including nemerteans, marine turbellarians, brachiopods, 
bryozoans, phoronids, mollusks, sipunculids, and some annelids. The form is 
somewhat pear-shaped (see illustration), and it is provided externally with a 
prominent circlet of cilia (a troch) and one or sometimes two accessory 
circlets. Anterior and posterior apical ciliary tufts and eyespots are often 
present. The digestive tract is complete and functional; paired nephridia with 
excretory tubules are present; muscle and nerve fibers, sense organs, and a 
band of mesoderm complete the internal structure. Presumably, the larva, which 
develops ontogenetically along many divergent lines, indicates the close 
evolutionary relationships of the groups it represents.  See  ANNELIDA; 
BRACHIOPODA; BRYOZOA; MOLLUSCA; PHORONIDA; SIPUNCULIDA; TURBELLARIA. 
 �34s[Sybil P. Parker]
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�6v�2v�32sTrogoniformes   �21m �33s 
    An order of birds containing the single pantropical family Trogonidae, the 
trogons. The trogons are brightly colored, at least in the males (most have 
iridescent green backs and red, orange, or yellow underparts), but are 
surprisingly inconspicuous. They are slow-moving birds and prefer gloomier 
parts of tropical forests. Several species are far more often heard than seen. 
The foot structure is unique among living birds, with the first and second 
toes, instead of the first and fourth as in many birds, directed backward.  
See  AVES. 
 �34s[Kenneth C. Parkes]
�33s 
  
  
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�6v�2v�32sTrojan asteroids   �21m �33s 
    Asteroids located near the equilateral lagrangian stability points of the 
Sun-Jupiter system (see illustration). As shown by J. L. Lagrange in 1772, 
these are two of the five stable points in the circular, restricted, 
three-body system, the other three points being located along a line through 
the two most massive bodies in the system. In 1906 Max Wolf discovered an 
asteriod located near the lagrangian point following Jupiter in its orbit. 
Within a year, two more were found, one of which was located near the 
preceding lagrangian point. It was quickly decided to name these asteroids 
after participants in the Trojan War as given in Homer's Iliad.  See  ASTEROID. 
 �34s[Edward F. Tedesco]
�33s 
  
  
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�6v�2v�32sTrombidiformes   �21m �33s 
    A suborder of the Acarina (also known as Prostigmata) commonly called the 
trombidiform mites, more closely related to the Sarcoptiformes than to the 
other suborders. They are usually distinguished by presence of a respiratory 
system opening at or near the base of the chelicerae. 
    The Trombidiformes are probably the most heterogeneous group of mites, 
both morphologically and ecologically, varying from baglike forms with 
degenerate legs to the highly evolved, fully developed, parasitic forms. 
Economically, this group contains two families of plant-feeding mites of great 
importance to agriculture: the Tetranychidae (spider mites) and the 
Eriophyidae (bud mites or gall mites). Medically, the Trombiculidae (chiggers 
or red bugs) are important because the larval forms, which are parasites of 
vertebrates, can cause intense irritation to their hosts by their feeding. 
More seriously, some transmit a rickettsial disease, scrub typhus, to humans 
in the Far East and South Pacific regions. 
 �34s[Edward W. Baker]
�33s 
  
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�6v�2v�32sTropical meteorology   �21m �33s 
    The study of atmospheric structure and behavior in those parts of the 
world that lie astride the Equator - roughly between 25 or 30DG north and 
south latitude. Weather and climate in the tropical belt differ grossly from 
those experienced farther poleward, partly as a result of the fact that the 
tropics export heat, moisture, and angular momentum to the higher latitudes. 
The weather satellite has become an important observing tool for tropical 
meteorology, covering the atmosphere over wide oceanic and desert regions that 
were previously virtually unknown.  See  METEOROLOGICAL SATELLITES; 
METEOROLOGY.  
    In contrast to the four-season cycle of higher latitudes, the tropics have 
only small seasonal temperature changes. Distinctions are made between dry and 
rainy seasons (one or two); monsoon climates with a three-season cycle (cool 
and dry, hot, rainy humid and cool); and desert climates (cool or hot, always 
dry). In the highlands the air temperature may be 18DGF (10DGC) colder than 
near sea level at which level there is usually only a 1.8DGF (1DGC) seasonal 
temperature change.  See  MONSOON. 
    Though rainfall may attain 60 - 80 in./year (150 - 200 cm/year) or more in 
many parts of the tropics, an abundant water supply is relatively rare. The 
high rate of evaporation caused by the heat greatly reduces the effectiveness 
of precipitation measured by the precipitation/evaporation ratio, which may be 
no higher, and even lower, in rainy parts of the tropics than in subarctic 
latitudes. Most tropical areas derive 70 - 90% of their annual water supply 
from a few organized rain systems of a 620-mi (1000-km) extent passing over a 
location in 1 - 2 days. It is not uncommon for the largest rainstorm of the 
year to account for 10% or more of the annual rainfall. 
 �34s[Herbert Riehl]
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�6v�2v�32sTropic of Cancer �21m �33s
    The parallel of latitude about 23 1/2DG (23DG45) north of the Equator.
The importance of this line lies in the fact that its degree of angle from the
Equator is the same as the inclination of the Earth's axis from the vertical
to the plane of the ecliptic. Because of this inclination of the axis and the
revolution of the Earth in its orbit, the vertical overhead rays of the Sun
may progress as far north as 23 1/2DG. At no place north of the Tropic of Cancer
will the Sun, at noon, be 90DG overhead. On June 21, the summer solstice
(Northern Hemisphere), the Sun is vertical above the Tropic of Cancer.
    The Tropic of Cancer is the northern boundary of the equatorial zone
called the tropics, which lies between the Tropic of Cancer and Tropic of
Capricorn.  See  MATHEMATICAL GEOGRAPHY; SOLSTICE; TROPIC OF CAPRICORN.
 �34s[Van H. English]
�33s

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�6v�2v�32sTropic of Capricorn �21m �33s
    The parallel of latitude approximately 23 1/2DG (23DG45) south of the
Equator. It was named for the constellation Capricornus (the goat), for
astronomical reasons which no longer prevail.
    Because the Earth, in its revolution around the Sun, has its axis inclined
23 1/2DG from the vertical to the plane of the ecliptic, the Tropic of Capricorn
marks the southern limit of the zenithal position of the Sun. Thus, on
December 22 (Southern Hemisphere summer, but northern winter solstice) the
Sun, at noon, is 90DG above the horizon.
    The Tropic of Capricorn is the southern boundary of the equatorial zone
referred to as the tropics, which lies between the Tropic of Capricorn and the
Tropic of Cancer.  See	MATHEMATICAL GEOGRAPHY; SOLSTICE; TROPIC OF CANCER.
 �34s[Van H. English]
�33s

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�6v�2v�32sTropolone �21m �33s
    A weakly acidic organic compound, C7H6O2, having the structure
2-hydroxycycloheptatrienone as shown in (I). Tropolone may be considered as
derived from tropone (II) by substitution of a hydroxyl group for the hydrogen

			 ****FORMULA****


atom in position 2. The acidity of tropolone is intermediate between that of
phenol and that of a typical carboxylic acid such as acetic acid.
    Natural tropolones include the mold metabolites puberulic and puberulonic
acids, sepedonin and stipitatic acid. The thujaplicins (isopropyltropolones)
occur in several species of Cupressacea evergreens and possess fungicidal
properties that are believed to protect the wood of such trees against rot.
The medicinally important alkaloid colchicine, from the meadow saffron
(Colchicum autumnale), is also a tropolone derivative.	See  ALKALOID.
 �34s[Martin Stiles]
�33s

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�6v�2v�32sTropopause �21m �33s
    The boundary between the troposphere and the stratosphere in the
atmosphere. The tropopause is broadly defined as the lowest level above which
the lapse rate (decrease) of temperature with height becomes less than 5.8DGF
mi(sup -1) (2DGC km(sup -1)). In low latitudes the tropical tropopause is at a
height of 9.3 - 11 mi at about -135DGF (15 - 17 km at about 180 K), and the
polar tropopause between tropics and poles is at about 6.2 mi at about -63DGF
(10 km at about 220 K). There is a well-marked ``tropopause gap'' or break
where the tropical and polar tropopauses overlap at 30 - 40DG latitude. The
break is in the region of the subtropical jet stream and is of major
importance for the transfer of air and tracers (humidity, ozone,
radioactivity) between stratosphere and troposphere. The height of the
tropopause varies seasonally and also daily with the weather systems, being
higher and colder over anticyclones than over depressions.  See  AIR
TEMPERATURE; ATMOSPHERE; STRATOSPHERE; TROPOSPHERE.
 �34s[R. J. Murgatroyd]
�33s

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�6v�2v�32sTroposphere �21m �33s
    The lowest major layer of the atmosphere. The troposphere extends from the
Earth's surface to a height of 6 - 10 mi (10 - 16 km), the base of the
stratosphere. It contains about four-fifths of the mass of the whole
atmosphere.  See  ATMOSPHERE.
    On the average, the temperature decreases steadily with height throughout
this layer, with a lapse rate of about 19DGF mi(sup -1) (6.5DGC km(sup -1)),
although shallow inversions (temperature increases with height) and greater
lapse rates occur, particularly in the boundary layer near the Earth's
surface. Appreciable water-vapor contents and clouds are almost entirely
confined to the troposphere. Hence it is the seat of all important weather
processes and the region where interchange by evaporation and precipitation
(rain, snow, and so forth) of water substance between the surface and the
atmosphere takes place.  See  CLIMATOLOGY; CLOUD PHYSICS; METEOROLOGY; WEATHER.
 �34s[R. J. Murgatroyd]
�33s

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�6v�2v�32sTropospheric scatter   �21m �33s 
    A term applied to propagation of radio waves caused by irregularities in 
the refractive index of air. The phenomenon is predominant in the lower 
atmosphere; little or no scattering of importance occurs above the 
troposphere. Tropospheric scatter propagation provides very useful 
communication services but also causes harmful interference. For example, it 
limits the geographic separation required for frequency assignments to 
services such as television and frequency-modulation broadcasting, 
very-high-frequency omnidirectional ranges (VOR), and microwave relays. It is 
used extensively throughout most of the world for long-distance point-to-point 
services, particularly where high information capacity and high reliability 
are required. Typical tropospheric scatter relay facilities are commonly 200 - 
300 mi (320 - 480 km) apart. Some single hops in excess of 500 mi (800 km) are 
in regular use. High-capacity circuits carry 200 - 300 voice circuits 
simultaneously.  
 �34s[Robert S. Kirby]
�33s 
  
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�6v�2v�32sTrunk (anatomy)   �21m �33s 
    The main body mass, exclusive of the head, neck, and extremities. The 
trunk is divided into thorax, abdomen, and pelvis. The thorax consists of the 
rib cage area and contains the heart, lungs, great blood vessels, and 
esophagus. The abdomen lies below the ribs and diaphragm, which separates it 
from the thorax, and contains the abdominal viscera, including the stomach, 
intestine, liver, spleen, and kidneys. The pelvis lies within the hipbones and 
contains the internal genital organs, bladder, and rectum. 
    The trunk is supported by the vertebral column, back muscles, and muscles 
of the body wall; the ribs and sternum in the thorax and the pelvic bones also 
help support the internal structures. The trunk forms appropriate areas of 
communication and attachment for the neck and limbs. 
 �34s[Thomas S. Parsons]
�33s 
  
  
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�6v�2v�32sTruss �21m �33s
    A system of structural members lying in a single plane and joined at their
ends to form a stable framework. A truss is used like a beam, particularly for
bridge and roof construction. But because a truss can be made deeper than a
beam with solid web and yet not weigh more, it is more economical for long
spans and heavy loads.	See  BRIDGE; ROOF CONSTRUCTION.
    The simplest truss is a triangle composed of three bars with ends pinned
together. If small changes in the lengths of the bars are neglected, the
relative positions of the joints do not change when loads are applied in the
plane of the triangle at the apexes. Such simple trusses as a triangle,
perhaps with the addition of a vertical bar in the middle, are sometimes used
to support peaked roofs of houses and other narrow structures. For longer
spans, flat roofs, or bridges many triangles are combined to form a truss, as
can be seen in the illustrations.
    Roof trusses carry the weight of roof deck and framing and wind loads on
the upper chord. They may also support a ceiling or other loads on the lower
chord. An example is the Fink truss (Fig. 1). On the other hand, bridge
trusses may carry loads on either chord. Deck trusses support loads on the
upper chord (Fig. 2); through trusses, on the lower chord (Fig. 3). To
maintain stability of truss construction, bracing must be used normal to the
planes of the trusses. Usually framing is inserted between the trusses.
 �34s[Harry L. Bowman /Waldo G. Bowman]
�33s


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�6v�2v�32sTrypanorhyncha   �21m �33s 
    An order of tapeworms of the subclass Cestoda, also known as the 
Tetrarhynchoidea. All are parasitic in the intestine of elasmobranch fishes. 
They are distinguished from all other tapeworm groups by having spiny, 
eversible proboscides on the head. The head also bears two or four shallow, 
weakly muscular suckers. A complete life history is not known for any 
trypanorhynchid, although larval forms have been found in the tissues of 
various marine invertebrates and teleost fishes.  See  CESTODA. 
 �34s[Clark P. Read]
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�6v�2v�32sTrypanosomatidae �21m �33s
    A family of Protozoa, order Kinetoplastida, containing flagellated
parasites which change their morphology; that is, they exhibit polymorphism
during their life cycles. The life cycles of the organisms may involve only an
invertebrate host, or an invertebrate and a vertebrate host, or an
invertebrate and a plant host. Several distinct morphological forms are
recognized: trypanosomal, crithidial, leptomonad, and leishmanial.
Differentiation into genera is dependent upon the host infected as well as the
morphologic types involved. None of the stages possesses a mouth opening, and
nutritive elements are absorbed through the surface of the body; that is, the
organisms are saprozoic.
    Trypanosoma is the important genus of the family Trypanosomatidae from a
number of standpoints. It contains the largest number of species infecting a
wide variety of hosts such as mammals, birds, fishes, amphibians, and
reptiles. Although most of the species cause no damage to the hosts, there are
several which produce serious diseases in humans, domesticated animals, and
wild animals. The pathogenic species are prevalent in Africa.
    Leishmania is the second most important genus. Three species parasitize
humans but have also been found naturally infecting dogs, cats, and perhaps
other lower animals. The sand fly, Phlebotomus, transmits the parasite from
vertebrate to vertebrate.  See	LEISHMANIASIS.
 �34s[M. M. Brooke /Horace W. Stunkard]
�33s

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�6v�2v�32sTryptophan   �21m �33s 
    An amino acid considered essential for normal growth of animals. 
Tryptophan is the precursor of the several important substances, including the 
plant growth hormone, indoleacetic acid; the animal hormone, serotonin 
(5-hydroxytryptamine); the vitamin, nicotinic acid; and certain eye pigments 
of insects.  See  AMINO ACIDS. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sTsunami   �21m �33s 
    A radially spreading, long-period gravity-wave system caused by any 
large-scale impulsive sea-surface disturbance. Being only weakly dissipative 
in deep water, major tsunamis can produce anomalous destructive wave effects 
at transoceanic distances. Because of their uncertainty of origin, infrequency 
(10 per century), and sporadicity, tsunami forecasting is impossible, but the 
progressive implementation, since 1946, of the effective International Tsunami 
Warning System has greatly reduced human casualties. 
    While minor tsunamis are occasionally produced by volcanic eruptions or 
submarine landslides, major events are generated by the sudden quasi-unitized 
dislocations of large fault blocks associated with the crumpling of slowly 
moving sea-floor crustal plates, where they abut normally against the 
continental plates. Such sources are predominately confined to tectonically 
active ocean margins, the majority of which ring the Pacific from Chile to 
Japan. As opposed to secular creep, tsunami-producing block dislocations are 
invariably associated with major shallow-focus (<19 mi or 30 km) earthquakes 
of intensity greater than 7 on the Gutenberg-Richter scale, followed by swarms 
of aftershocks of lesser intensity that decay in frequency over a week or two. 
  See  EARTHQUAKE; FAULT AND FAULT STRUCTURES. 
 �34s[William G. Van Dorn]
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�6v�2v�32sTuberculariaceae   �21m �33s 
    A family of fungi of the order Moniliales. The conidiophores are short, 
forming cushion-shaped aggregates. About 150 genera and 500 species are 
recognized. The genera are usually arranged into spore groups, depending on 
characteristics of the spore, such as the number of cells in the spore and 
shape and pigmentation of the spore.  See  FUNGI IMPERFECTI; MONILIALES. 
 �34s[N. F. Buchwald]
�33s 
  
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�6v�2v�32sTuberculosis   �21m �33s 
    A chronic infectious disease in humans and animals caused by tubercle 
bacilli, Mycobacterium tuberculosis or M. bovis. For much of the world 
population and for most of recorded history, tuberculosis has been the most 
frequent cause of long-lasting sickness and the principal cause of death. 
Infections may be either of two kinds: without recognized disease 
manifestations, and with disease production, tuberculosis. 
    On gaining access to the lung of a new host, tubercle bacilli are quickly 
engulfed by white blood cells. Within the protoplasm of human cells, the 
bacilli grow and multiply, although very slowly, invade neighboring cells, and 
form what is microscopically recognized as a tubercle. The bacillus-containing 
cells eventually are killed, perhaps by mechanical rupture from the 
multiplying bacilli, and other cells then phagocytize them. Much enlarged 
tubercles or groups of them become recognizable macroscopically as nodules. 
They may become fibrotic and eventually calcified. Tubercle bacilli may remain 
viable but quiescent in microscopic tubercles or lymph nodes for many decades, 
often for the life of the host. Consequences of the presence of these bacilli 
are a hyperactive state made evident by tuberculin sensitivity and an ever 
ready source of disease production. Breakdown of the body's disease 
resistance, for example, by stress or poor nutrition, may result in 
reactivation of the bacilli, and disease. 
    If the infection spreads into the bloodstream, systemic disease may 
follow. This is often characterized by a multitude of widely disseminated, 
very small nodules, the size of millet seeds and therefore designated miliary 
tuberculosis. Tubercle bacilli may set up infection in almost any part of the 
body. Tuberculous lymphandenitis involving neck glands of children and 
infections of genital organs, spleen, and kidneys are well known. Tuberculous 
meningitis and miliary tuberculosis are rapidly progressive, and fatal unless 
promptly treated. 
    A great variety of unusual treatments were used over the years, but all 
shrunk to relative insignificance following the introduction of effective 
drugs. Streptomycin (1943), para-aminosalicylate (PAS, 1946), and isoniazid 
(INH, 1952) were the first agents of many hundreds tested that proved to be 
effective in relatively nontoxic doses. A marked drop in the mortality rate 
about 1950 reflects the success of treatment with these drugs. Many additional 
effective drugs are now available.  See  PARA-AMINOSALICYLIC ACID; 
ISONICOTINIC ACID HYDRAZIDE; MYCOBACTERIAL DISEASES; STREPTOMYCIN. 
 �34s[Ernest H. Runyon]
�33s 
  
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�6v�2v�32sTube-still heater   �21m �33s 
    Direct-fired equipment used in processing petroleum. The basic feature of 
these heaters is that the fluid (liquid, vapor, or a mixture of both phases) 
is heated in carbon- or alloy-steel tubes, which in turn receive their heat 
from hot products of combustion. The fuel may be liquid, such as heavy fuel 
oil, or gas, such as natural gas or cracked refinery gas. The modern 
tube-still heater is the general-purpose tool for supplying heat at the higher 
temperature levels, that is, over 650 - 700DGF (343 - 371DGC), wherever 
required in the refinery for distillation, for preheating, for reactions such 
as pyrolysis, or for reboiling. 
    Most modern heaters are of one of two types, the box type, usually with 
horizontal tubes, or the cylindrical type with vertical tubes arranged in a 
circle. Within the radiant section, the combustion zone in which the burners 
are located, heat is given up to the tubes from three sources: radiation from 
the flame, radiation from the hot molecules (carbon dioxide and water vapor), 
and radiation from the gases within the chamber. The convection section is 
basically a recuperative zone in which the flue gases can be reduced in 
temperature while heating the coldest fluid to obtain economic heat recovery.  
See  FURNACE CONSTRUCTION; PETROLEUM PROCESSING. 
 �34s[Walter E. Lobo]
�33s 
  
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�6v�2v�32sTubeworms   �21m �33s 
    The name given to marine polychaete worms (particularly to many species in 
the family Serpulidae) which construct permanent calcareous tubes on rocks, 
seaweeds, dock pilings, and ship bottoms. The individual tubes with hard walls 
of calcite-aragonite are firmly cemented to any hard substrate and to each 
other. Economically they are among the most important fouling organisms both 
on ship hulls (where they are second only to barnacles) and inside sea-water 
cooling pipes of power stations. 
    About 340 valid species of serpulid tubeworms have been described. The 
majority are truly marine, but several species thrive in brackish waters of 
low salinity, and one species occurs in fresh waters in Karst limestone caves. 
The wide geographical distribution of certain abundant species owes much to 
human transport on the bottoms of relatively fast ships and occurred within 
the last 120 years.  See  ANNELIDA; BARNACLE; POLYCHAETA. 
 �34s[W. D. Russell-Hunter]
�33s 
  
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�6v�2v�32sTubulidentata   �21m �33s 
    An order of mammals which contains a single living genus, the aardvark 
(Orycteropus) of Africa. Little is directly known of the origin and history of 
this peculiar group of mammals. The oldest undoubted record of the 
Tubulidentata is an early Miocene discovery in East Africa. Pliocene records 
in India and the Mediterranean border indicate the aardvarks had dispersed  
from Africa by that time. Tubular dentine reminiscent of the aardvarks occurs 
in Tubulodon from the early Eocene of North America, but the fragmentary 
material does not permit further comparison.  See  AARDVARK; MAMMALIA. 
 �34s[Richard H. Tedford]
�33s 
  
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�6v�2v�32sTufa   �21m �33s 
    A spongy, porous limestone formed by precipitation from evaporating spring 
and river waters; also known as calcareous sinter. Calcium carbonate commonly 
precipitates from supersaturated waters on the leaves and stems of plants 
growing around the springs and pools and preserves some of their plant 
structures. Tufa tends to be fragile and friable.  See  LIMESTONE; TRAVERTINE. 
 �34s[Raymond Siever]
�33s 
  
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�6v�2v�32sTuff   �21m �33s 
    Consolidated volcanic ash, composed largely of fragments (less than 0.16 
in. or 4 mm) produced directly by volcanic eruption. Much of the fragmental 
material represents finely comminuted crystals and rocks. Much, however, was 
ejected as blobs of liquid lava which rapidly cooled to form volcanic glass. 
Expansion and solidification to glass may be more or less contemporaneous so 
that highly vesicular bodies are rapidly shattered to produce vast quantities 
of tiny particles (ash). Upon consolidation volcanic ash yields tuff. With 
increase in the size of fragments, tuff passes into lapilli tuff, volcanic 
breccia, and agglomerate.  See  PYROCLASTIC ROCKS; VOLCANIC GLASS. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sTularemia   �21m �33s 
    A widespread infection of wild rodents endemic in the Americas, Europe, 
Asia, and Africa. It is maintained through insect reservoirs and vectors from 
which the disease may be transmitted to humans. The causative organism, 
isolated from a plaguelike disease of ground squirrels in Tulare County, 
California, was named Pasteurella tularensis (now Francisella tularensis). The 
infection may be generalized, or it may be  localized in the eyes, skin, or 
lymph nodes or in the respiratory or gastrointestinal tracts. Recovery is 
followed by reliable immunity. 
 �34s[Karl F. Meyer]
�33s 
  
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�6v�2v�32sTulip tree �21m �33s
    A tree Liriodendron tulipfera, also known in forestry as yellow poplar,
belonging to the magnolia family, Magnoliaceae. One of the largest and most
valuable hardwoods of eastern North America, it is native from southern New
England and New York westward to southern Michigan, and south to Louisiana and
northern Florida.
    Botanically, this tree is distinguished by leaves which are squarish at
the tip as if cut off, true terminal buds flattened and covered by two valvate
scales, an aromatic odor resembling that of magnolia, and cone-shaped fruit
which is persistent in winter. The name tulip refers to the large
greenish-yellow and orange-colored flowers.
    The wood of the tulip tree is light yellow to brown, hence the common name
yellow poplar, which is a misnomer. It is a soft and easily worked wood, used
for construction, interior finish, containers (boxes, crates, baskets),
woodenware, excelsior, veneer, and sometimes for paper pulp.  See  MAGNOLIALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sTumbling mill   �21m �33s 
    A grinding and pulverizing machine consisting of a shell or drum rotating 
on a horizontal axis. The material to be reduced in size is fed into one end 
of the mill. The mill is also charged with grinding material such as iron 
balls. As the mill rotates, the material and grinding balls tumble against 
each other, the material being broken chiefly by attrition. 
    Tumbling mills are variously classified as pebble, ball, or rod depending 
on the grinding material, and as cylindrical, conical, or tube depending on 
the shell shape.  See  CRUSHING AND PULVERIZING; GRINDING MILL; PEBBLE MILL. 
 �34s[Ralph M. Hardgrove]
�33s 
  
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�6v�2v�32sTumor   �21m �33s 
    Literally, a swelling; in the past the term has been used in reference to 
any swelling of the body no matter what the cause. However, the word is now 
being used, almost exclusively, to refer to a neoplastic mass, and the more 
general usage is being discarded.  See  NEOPLASIA. 
    A neoplastic mass or neoplasm is a pathological lesion characterized by 
the progressive or uncontrolled proliferation of cells. The cells involved in 
the neoplastic growth have an intrinsic heritable abnormality such that they 
are not regulated properly by normal methods. The stimulus which elicits this 
growth is not usually known. 
    It is common to divide tumors into benign or malignant. The decision as to 
which category a tumor should be assigned is usually based on information 
gained from gross or microscopic examination, or both. Benign neoplasms 
usually grow slowly, remain so localized that they cause little harm, and 
generally can be successfully and finally removed. Malignant or cancerous 
neoplasms tend to grow rapidly, spread throughout the body, and recur if 
removed. Not all tumors which have been classified as benign are harmless to 
the host, and some can cause serious problems. Difficulties may occur as a 
result of mechanical pressure. 
    The cells of benign tumors are well differentiated. This means that the 
cells are very like the normal tissue in size, structure, and spatial 
relationship. The cells forming the tumor usually function normally. Cell 
proliferation usually is slow enough so that there is not a large number of 
immature cells. As the cellular mass increases in size, most benign tumors 
develop a fibrous capsule around them which separates them from the normal 
tissue. The cells of a benign tumor remain at the site of origin and do not 
spread throughout the body. Anaplasia is not seen in benign tumors. 
    The cells of malignant tumors may be well differentiated, but most have 
some degree of anaplasia. Anaplastic cells tend to be larger than normal and 
are abnormal, even bizarre, in shape. The nuclei tend to be very large and 
irregular. Malignant tumors may be partially but never completely 
encapsulated. The cells of the cancer infiltrate and destroy surrounding 
tissue. They have the ability to metastasize; that is, cells from the primary 
tumor are disseminated to other regions of the body where they are able to 
produce secondary tumors called metastases. 
    In most cases the formation of a neoplasm is irreversible. It results from 
a permanent cellular defect which is passed on to daughter cells. Medical 
appraisal of the tumor is generally recommended.  See  CANCER (MEDICINE); 
ONCOLOGY; TUMOR VIRUSES. 
 �34s[N. Karle Mottet ;Carol Quaife]
�33s 
  
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�6v�2v�32sTumor viruses   �21m �33s 
    Viruses which cause a wide variety of malignant and benign tumors in 
practically all animal species studied. These viruses are classified in two 
broad categories: the DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) 
tumor viruses, depending upon the nucleic acid in the viral genome. 
    No malignant disease of humans has been unequivocally associated with a 
virus. However, several human malignancies are suspected to have, at least in 
part, a viral etiology. These include Burkitt's lymphoma and nasopharyngioma 
as well as the nonmalignant illness infectious mononucleosis, all of which may 
be due to a herpesvirus-like agent, Epstein-Barr virus; cervical carcinoma may 
be caused by herpes simplex virus type 2, which infects the genital region; 
human osteosarcomas have been the source of extracts that cause similar tumors 
in hamsters.  See  ANIMAL VIRUS; ONCOLOGY; TUMOR. 
 �34s[William Schiavoni]
�33s 
  
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�6v�2v�32sTuna   �21m �33s 
    Any of the large pelagic marine fishes which form the family Thunnidae, 
usually included in the order Perciformes. These fishes have a worldwide 
distribution.  
    The tunas have no scales on the posterior part of the body, and those on 
the anterior are fused to form an armored covering. The flesh of many is dark 
red. The body of these fish is streamlined, with a crescent-shaped tail and a 
narrow caudal peduncle.  See  PERCIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sTunable laser   �21m �33s 
    A laser whose wavelength can be varied continuously over a wide spectral 
range. The vigorous advances of laser spectroscopy since about 1970 are 
largely due to the development of laser sources with this property. See LASER 
SPECTROSCOPY.  
    Tunable coherent sources are available for any wavelength region from the 
far infrared into the near-vacuum ultraviolet. Tunable infrared lasers include 
high-pressure molecular gas lasers, semiconductor diode lasers, spin-flip 
Raman lasers, and color-center lasers. In the visible region, organic dye 
lasers have proved particularly versatile and powerful spectroscopic sources. 
Both pulsed and continuous-wave dye lasers cover the entire visible spectrum, 
including the bordering near-in-frared and near-ultraviolet regions. Intense 
shorter-wave ultraviolet radiation can be generated with excimer lasers over 
limited regions. 
    The generation of harmonic frequencies or sum frequencies in nonlinear 
optical crystals or gases provides a valuable method to produce tunable 
coherent ultraviolet radiation from laser light of longer wavelengths. Optical 
parametric oscillators and difference-frequency crystal mixers offer 
corresponding alternatives for the generation of coherent infrared radiation. 
Stimulated Raman scattering can be used to shift the frequency of a tunable 
laser by some integer multiple of a molecular vibration frequency and thus to 
extend the tuning range. These nonlinear frequency-mixing and shifting 
techniques work best with the intense radiation from pulsed lasers. See LASER; 
NONLINEAR OPTICS. 
 �34s[Theo W. Hansch]
�33s 
  
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�6v�2v�32sTundra   �21m �33s 
    An area supporting some vegetation beyond the northern limit of trees, 
between the upper limit of trees and the lower limit of perennial snow on 
mountains, and on the fringes of the Antarctic continent and its neighboring 
islands. The term is of Lapp or Russian origin, signifying treeless plains of 
northern regions. Biologists, and particularly plant ecologists, sometimes use 
the term tundra in the sense of the vegetation of the tundra landscape. Tundra 
has distinctive characteristics as a kind of landscape and as a biotic 
community, but these are expressed with great differences according to the 
geographic region. 
    Characteristically tundra has gentle topographic relief, and the cover 
consists of perennial plants a few inches to a few feet or a little more in 
height. The general appearance during the growing season is that of a grassy 
sward in the wetter areas, a matted spongy turf on mesic sites, and a thin or 
sparsely tufted lawn or lichen heath on dry sites. In winter, snow mantles 
most of the surface. By far, most tundra occurs where the mean annual 
temperature is below the freezing point of water, and perennial frost 
(permafrost) accumulates in the ground below the depth of annual thaw and to 
depths at least as great as 1600 ft (500 m).  See  PERMAFROST. 
 �34s[William S. Benninghof]
�33s 
  
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�6v�2v�32sTuner   �21m �33s 
    A device containing circuits that can be tuned to the carrier frequency of 
a desired transmitter in the frequency range for which that tuner is designed. 
The tuner serves to select the desired carrier frequency while rejecting the 
carrier frequencies of all other stations that may be on the air at that time. 
A tuner for a television receiver ordinarily contains only the radio-frequency 
amplifier, local oscillator, and mixer stages. A radio tuner usually contains, 
in addition, the intermediate-frequency amplifier and second-detector stages 
so that it can feed an audio signal directly into a separate audio amplifier 
and loudspeaker system.  See  RADIO RECEIVER; TELEVISION RECEIVER; TUNING. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sTungstate   �21m �33s 
    One of a group of compounds containing tungsten in the 6+ oxidation state 
and derived from tungstic acid, H2WO4. Both the normal tungstates, such as 
Na2WO4, and the molybdates form condensed ions, such as isopolytungstates. 
These isopolytungstates are usually classified into metatungstates and 
paratungstates.  
    Heteropoly compounds in which another anhydride of an element such as 
silicon combines with the tungsten to form polymers are also known. Sodium 
silicotungstate, Na4SiW12O40, is a compound of this type. Most of the 
polytungstates are soluble, whereas the normal tungstates are sparingly 
soluble with the exception of the salts of the alkali metals and magnesium. 
    Tungstates are used to flameproof fabrics and to manufacture 
phosphorescent screens.  See  TUNGSTEN. 
 �34s[E. Eugene Weaver]
�33s 
  
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�6v�2v�32sTungsten   �21m �33s 
    A chemical element, W, atomic number 74, and atomic weight 183.85. The 
metal has a body-centered cubic structure and a silver-gray metallic luster. 
Its melting point of 3410DGC (6170DGF) is the highest of any metal. The metal 
exhibits a low vapor pressure, high density, and superior strength at high 
temperature in the absence of air, and is extremely hard. 
    Chemically, tungsten is relatively inert. It is not readily attacked by 
the common acids, alkalies, or aqua regia. It reacts with a mixture of 
concentrated nitric acids and hydrofluoric acid. Molten oxidizing salts such 
as sodium nitrite attack tungsten rapidly. Gaseous chlorine, bromine, iodine, 
carbon dioxide, carbon monoxide, and sulfur react with tungsten only at high 
temperatures. Carbon, boron, silicon, and nitrogen also form compounds with 
tungsten at elevated temperatures; hydrogen does not. The table lists several 
of the important physical properties of tungsten. 
    Ferrous alloys consume 40% of the tungsten mined. When added to iron or 
steel, tungsten improves high-temperature strength and hardness. Tungsten 
carbide (representing 38% of all W) has replaced diamond for many die and 
drilling applications. It is one of the best hard tool materials, retaining 
its properties at elevated temperatures. Pure tungsten metal in the form of 
wire, rod, and sheet (15%) is important to the electric lamp, electronics, and 
electrical industries. Other applications include welding rod, x-ray targets, 
lead wires, cathodes for power tubes, and automotive and aircraft distributor 
points.  
    Tungsten compounds embrace oxidation states of tungsten from 2+ to 6+, the 
higher oxidation states being most stable. Tungsten chemistry resembles that 
of chromium and molybdenum, which also occupy the same subgroup in the 
periodic table. Tungsten aqueous chemistry is complicated by its tendency to 
form complex ions. 
    Tungsten forms four well-defined, stable oxides and two carbides. Other 
important compounds include the carbonyl, nitride, boride, phosphide, 
silicide, and sulfide. 
 �34s[W. D. Wilkinson]
�33s 
  
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�6v�2v�32sTung tree �21m �33s
    The plant Aleurites fordii, a species of the spurge family
(Euphorbiaceae). The tree, native to central and western China, is the source
of tung oil. It has been grown successfully in the southern United States. The
globular fruit has three to seven large, hard, rough-coated seeds containing
the oil, which is expressed after the seeds have been roasted. Tung oil is
used to produce a hard, quick-drying, superior varnish, which is less apt to
crack than other kinds. The foliage, sap, fruit, and commercial tung meal
contain a toxic saponin, which causes gastroenteritis in animals that eat it.
See  DRYING OIL; EUPHORBIALES; VARNISH.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sTunicata   �21m �33s 
    A group of marine animals forming a subdivision (subphylum Tunicata or 
Urochordata) of the Chordata. They are characterized by a perforated pharynx 
or branchial sac used for food collection, a dorsal notochord restricted to 
the tail of the larva (and the adult in one class), absence of mesodermal 
segmentation or a recognizable coelom, and secretion of an outer covering (the 
test or tunic) which contains large amounts of polysaccharides related to 
cellulose. Three classes are usually recognized: the sessile Ascidiacea (sea 
squirts or ascidians); planktonic Thaliacea (salps, doliolids, and 
pyrosomids); and Larvacea (Appendicularia or Copelata), minute planktonic 
forms with tails living inside a specialized test or house adapted for 
filtering and food gathering. Approximately 2000 species of Tunicata are 
recognizable. The group is found in all parts of the ocean.  See  ASCIDIACEA; 
CHORDATA; LARVACEA; THALIACEA. 
 �34s[Ivan Goodbody]
�33s 
  
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�6v�2v�32sTuning   �21m �33s 
    The process of adjusting the inductance or capacitance, or both, in a 
tuned circuit - for example in a radio, television, or radar receiver or 
transmitter - so as to obtain optimum performance at a selected frequency. The 
tuning procedure that is carried out during manufacture or servicing of 
equipment generally involves the adjustment of screwdriver-type controls 
located inside or at the rear of the equipment. These are adjusted in such a 
way that the main tuning control on the front of the panel will serve to 
adjust all tuning circuits in the equipment simultaneously when a change of 
frequency is desired.  See  RADIO RECEIVER; RESONANCE (ALTERNATING-CURRENT 
CIRCUITS); TELEVISION RECEIVER. 
 �34s[John Markus]
�33s 
  
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�6v�2v�32sTuning fork   �21m �33s 
    A steel instrument consisting of two prongs and a handle which, when 
struck, emits a tone of fixed pitch. Because of their simple mechanical 
structure, purity of tone, and constant frequency, tuning forks are widely 
used as standards of frequency in musical acoustics. In its electrically 
driven form, a tuning fork serves to control electric circuits by producing 
frequency standards of high accuracy and stability. A tuning fork is 
essentially a transverse vibrator (see illustration).  See  VIBRATION. 
 �34s[Lawrence E. Kinsler]
�33s 
  
  
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�6v�2v�32sTunnel   �21m �33s 
    A general term for subterranean passages. Tunnels are used for aqueducts 
and sewers; for carrying railroad and vehicular traffic under rivers, through 
mountains, and below cities; and for specific kinds of underground 
installations, such as hydroelectric plants. 
    The construction of a tunnel begins with excavation of earth or rock. 
Excavating methods vary with the character of the material to be removed. Rock 
tunnels require blasting. If the rock is structurally poor, supports must 
often be placed under the tunnel ceiling, or arch, as the tunnel is being 
driven. Tunnels driven through earth do not require blasting, but arch 
supports are almost always necessary. Tunnels under rivers in soft silts are 
usually driven with the aid of a short cylinder called a shield, which is 
pushed through the silt ahead of the excavation to provide advance support of 
the arch. After excavation has been completed, the exposed tunnel wall is 
usually lined with concrete. Vehicular tunnel construction often requires the 
added installation of tile walls and of lighting and ventilation equipment. 
 �34s[William Hershleder]
�33s 
  
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�6v�2v�32sTunnel diode   �21m �33s 
    A two-terminal semiconductor junction device (also called the Esaki diode) 
which does not show rectification in the usual sense, but exhibits a negative 
resistance region at very low voltage in the forward-bias characteristic and a 
short circuit in the negative-bias direction. 
    This device is a version of the semiconductor pn junction diode which is 
made of a p-type semiconductor, containing mobile positive charges called 
holes (which correspond to the vacant electron sites), and an n-type 
semiconductor, containing mobile electrons (the electron has a negative 
charge). The densities of holes and electrons in the respective regions are 
made extremely high by doping a large amount of the appropriate impurities 
with an abrupt transition from one region to the other. In semiconductors, the 
conduction band for mobile electrons is separated from the valence band for 
mobile holes by an energy gap, which corresponds to a forbidden region. 
Therefore, a narrow transition layer from n-type to p-type, 5 to 15 nanometers 
thick, consisting of the forbidden region of the energy gap, provides a 
tunneling barrier. Since the tunnel diode exhibits a negative incremental 
resistance with a rapid response, it is capable of serving as an active 
element for amplification, oscillation, and switching in electronic circuits 
at high frequencies. The discovery of the diode, however, is probably more 
significant from the scientific aspect because it opened up a new field of 
research - tunneling in solids.  See  BAND THEORY OF SOLIDS; HOLES IN SOLIDS; 
JUNCTION DIODE; NEGATIVE-RESISTANCE CIRCUITS; SEMICONDUCTOR; SEMICONDUCTOR 
DIODE; TUNNELING IN SOLIDS. 
 �34s[Leo Esaki]
�33s 
  
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�6v�2v�32sTunneling in solids   �21m �33s 
    A quantum-mechanical process which permits electrons to penetrate from one 
side to the other through an extremely thin potential barrier to electron 
flow. The barrier would be a forbidden region if the electron were treated as 
a classical particle. A two-terminal electronic device in which such a barrier 
exists and primarily governs the transport characteristic (current-voltage 
curve) is called a tunnel junction.  See  NONRELATIVISTIC QUANTUM THEORY; 
QUANTUM MECHANICS. 
    During the infancy of the quantum theory, L. de Broglie introduced the 
fundamental hypothesis that matter may be endowed with a dualistic nature - 
particles such as electrons, alpha particles, and so on, may also have the 
characteristics of waves. This hypothesis found expression in the definite 
form now known as the Schroedinger wave equation, whereby an electron or an 
alpha particle is represented by a solution to this equation. The nature of 
such solutions implies an ability to penetrate classically forbidden regions 
of negative kinetic energy and a probability of tunneling from one classically 
allowed region to another. The concept of tunneling, indeed, arises from this 
quantum-mechanical result. The subsequent experimental manifestations of this 
concept, such as high-field electron emission from cold metals, alpha decay, 
and so on, in the 1920s, can be regarded as one of the early triumphs of the 
quantum theory.  See  FIELD EMISSION; RADIOACTIVITY; SCHROEDINGER'S WAVE 
EQUATION.  
    The tunnel diode (also called the Esaki diode), discovered in 1957 by L. 
Esaki, demonstrated the first convincing evidence of electron tunneling in 
solids.  See  TUNNEL DIODE. 
    Negative resistance phenomena can be observed in novel tunnel structures 
in semiconductors. Double tunnel barriers and periodic structures with a 
combination of semiconductors exhibit resonant tunneling and negative 
resistance effects.  See  SEMICONDUCTOR HETEROSTRUCTURES. 
    Tunneling had been considered to be a possible electron transport 
mechanism between metal electrodes separated by either a narrow vacuum or a 
thin insulating film usually made of metal oxides. In 1960, I. Giaever 
demonstrated that, if one or both of the metals were in a superconducting 
state, the current-voltage curve in such metal tunnel junctions revealed many 
details of that state. 
    In 1962, B. Josephson made a penetrating theoretical analysis of tunneling 
between two superconductors by treating the two superconductors and the 
coupling process as a single system, which would be valid if the insulating 
oxide were sufficiently thin, say 2 nanometers. His theory predicted the 
existence of a supercurrent, arising from tunneling of the bound electron 
pairs. This led to two startling conclusions: the dc and ac Josephson effects. 
The dc effect implies that a supercurrent may flow even if no voltage is 
applied to the junction. The ac effect implies that, at finite voltage V, 
there is an alternating component of the supercurrent which oscillates at a 
frequency of 483.6 MHz per microvolt of voltage across the junction, and is 
typically in the microwave range.  See  JOSEPHSON EFFECT. 
 �34s[Leo Esaki]
�33s 
  
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�6v�2v�32sTupelo �21m �33s
    A tree belonging to the genus Nyssa of the sour gum family, Nyssaceae. The
most common species is N. sylvatica, variously called pepperidge, black gum,
or sour gum, the authorized name being black tupelo. Tupelo grows in the
easternmost third of the United States; southern Ontario, Canada; and Mexico.
    The tree can be identified by the comparatively small, obovate, shiny
leaves and by branches that develop at a wide angle from the axis. The fruit
is a small blue-black drupe, a popular food for birds. The wood is yellow to
light-brown and hard to split because of the twisted grain. Tupelo is used for
boxes, baskets, and berry crates, and as backing on which veneers of rarer and
more expensive woods are glued. It is also used for flooring, rollers in glass
factories, hatters' blocks, and gunstocks.  See  MYRTALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sTurbellaria   �21m �33s 
    A class of the phylum Platyhelminthes commonly known as the flatworms. 
These animals are chiefly free-living and have simple life histories. The 
bodies are elongate and flat to oval or circular in cross section. Their 
length ranges from less than 0.04 in. (1 mm) to several inches, but may exceed 
20 in. (50 cm) in land planaria. Large forms are often brightly colored. This 
class, which numbers some 3400 described species, is ordinarily subdivided 
into the orders Acoela, with 200 species; Rhabdocoela, 1110 species; 
Alloeocoela, 350 species; Tricladida, 1000 species; and Polycladida, 750 
species. Although widely distributed in fresh and salt water and moist soil, 
they are usually overlooked because of their generally small size, secretive 
habits, and inconspicuous color.  See  ACOELA; ALLOEOCOELA; POLYCLADIDA; 
RHABDOCOELA; TRICLADIDA. 
 �34s[E. Ruffin Jones]
�33s 
  
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�6v�2v�32sTurbidimetric analysis   �21m �33s 
    An analytical method based on the measurement of the attenuation of 
transmitted light by a solution containing a finely divided precipitate. There 
is a direct relationship between the amount of light attenuated and the amount 
of material in suspension. Either visual comparison with known standards or 
photoelectric detection is used to measure the amount of light transmitted by 
the turbid solutions. In the photoelectric method, a standard curve of percent 
transmittance versus concentration of the material in question is usually 
prepared and the compositions of unknowns are determined by reference to the 
curve. Turbidity is a function of the concentration and particle-size 
distribution of turbid material and the path length of the light in the sample. 
    Another common turbidimetric method is the determination of the height of 
a column of turbid liquid which will obscure a light or an object. The height 
of liquid is then compared with a solution of known composition or with some 
standard scale.  See  OPTICAL METHODS OF CHEMICAL ANALYSIS. 
 �34s[James N. Little]
�33s 
  
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�6v�2v�32sTurbidite   �21m �33s 
    A bed of sediment or sedimentary rock that was deposited from a turbidity 
current. The term turbidite is fundamentally genetic and interpretive in 
nature, rather than being a descriptive term (like common rock names). 
Turbidites are classic sedimentary rocks, but they may be composed of silicic 
grains (quartz, feldspar, rock fragments) and therefore be a type of 
sandstone, or they may be composed of carbonate grains and therefore be a type 
of limestone. A geologist's description of a rock as a turbidite is actually 
an expression of an opinion that the rock was deposited by a turbidity 
current, rather than being a description of a particular type of rock.  See  
TURBIDITY CURRENT. 
 �34s[Gerard V. Middleton]
�33s 
  
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�6v�2v�32sTurbidity current   �21m �33s 
    A submarine flow of sediment or sediment-laden water which occurs when an 
unstable mass of sediment at the top of a relatively steep slope is jarred 
loose and slides downslope. As the slide or slump travels downslope, it 
becomes more fluid because of the loss of internal cohesive strength and 
because of inmixing of the superadjacent water. 
    Turbidity currents occur at the edge of the continental slope, in the 
vicinity of river mouths, and off prominent capes. They are triggered by 
earthquakes, hurricanes, floods, or simply by the bedload transport of rivers 
debouching at the edge of continental slopes. 
    The lowered sea level of the glacial stages must have forced most rivers 
of the world to empty at or near the top of the continental slope. Under those 
conditions turbidity currents must have been much more important than at 
present, and they undoubtedly transported much of the sediment deposited in 
ancient geosynclines.  See  CONTINENTAL MARGINS; MARINE GEOLOGY; MARINE 
SEDIMENTS; SUBMARINE CANYON; TURBIDITE. 
 �34s[Bruce C. Heezen]
�33s 
  
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�6v�2v�32sTurbine   �21m �33s 
    A machine for generating rotary mechanical power from the energy in a 
stream of fluid. The energy, originally in the form of head or pressure 
energy, is converted to velocity energy by passing through a system of 
stationary and moving blades in the turbine. Changes in the magnitude and 
direction of the fluid velocity are made to cause tangential forces on the 
rotating blades, producing mechanical power via the turning rotor. Turbines 
effect the conversion of fluid to mechanical energy through the principles of 
impulse, reaction, or a mixture of the two (see illustration). 
    The fluids most commonly used in turbines are steam, hot air or combustion 
products, and water. Steam raised in fossil fuel-fired boilers or nuclear 
reactor systems is widely used in turbines for electrical power generation, 
ship propulsion, and mechanical drives. The combustion gas turbine has these 
applications in addition to important uses in aircraft propulsion. Water 
turbines are used for electrical power generation.  See  GAS TURBINE; 
HYDRAULIC TURBINE; IMPULSE TURBINE; PELTON WHEEL; REACTION TURBINE; STEAM 
TURBINE; TURBINE PROPULSION; TURBOJET. 
 �34s[Frederick G. Baily]
�33s 
  
  
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�6v�2v�32sTurbine propulsion   �21m �33s 
    Propulsion of a vehicle by means of a gas turbine. Gas turbines have come 
to dominate most areas of common carrier aircraft propulsion, have made 
significant inroads into the propulsion of surface ships, and are being 
incorporated into military tanks. 
    The primary power producer common to all gas turbines used for propulsion 
is the core or gas generator, operating on a continuous flow of air as working 
fluid. The air is compressed in a rotating compressor, heated at constant 
pressure in a combustion chamber burning a liquid hydrocarbon fuel, and 
expanded through a core turbine which drives the compressor. This 
manifestation of the Brayton thermodynamic cycle generates a continuous flow 
of high-pressure, high-temperature gas which is the primary source of power 
for a large variety of propulsion schemes. The turbine is generally run as an 
open cycle; that is, the airflow is ultimately exhausted to the atmosphere 
rather than being recycled to the inlet.  See  BRAYTON CYCLE. 
    The residual energy available in the high-temperature, high-pressure 
airstream exiting from the core is used for propulsion in a variety of ways. 
For traction-propelled vehicles (buses, trucks, automobiles, military tanks, 
and most railroad locomotives), the core feeds a power turbine which extracts 
the available energy from the core exhaust and provides torque to a high-speed 
drive shaft as motive power for the vehicle. With a free-turbine arrangement, 
this power turbine is a separate shaft, driving at a speed not mechanically 
linked to the core speed. With a fixed turbine, this power turbine is on the 
same shaft as the core turbine, and must drive at the same speed as the core 
spool. In traction vehicles the power turbine generally drives through a 
transmission system which affords a constant- or a variable-speed reduction to 
provide the necessary torque-speed characteristics to the traction wheels. 
    Aircraft, ships, and high-speed land vehicles, which cannot be driven by 
traction, are propelled by reaction devices. Some of the ambient fluid around 
the vehicle (that is, the water for most ships, and the air for all other 
vehicles) is accelerated by some turbomachinery (a ship propeller, aircraft 
propeller, helicopter rotor, or a fan integrated with the core to constitute a 
turbofan engine). The reaction forces on this propulsion turbomachinery, 
induced in the process of accelerating the ambient flow, provide the 
propulsion thrust to the vehicle. In all these cases, motive power to the 
propeller or fan is provided by a power turbine extracting power from the gas 
generator exhaust. In the case of a jet engine, exhaust from the gas generator 
is accelerated through a jet nozzle, so that the reaction thrust is evolved in 
the gas generator rather than in an auxiliary propeller or fan. Indeed, in 
turboprop and turbofan engines, both forms of reaction thrust (from the stream 
accelerated by the propeller or fan and from the stream accelerated by the 
core and not fully extracted by the power turbine) are used for propulsion.  
See  GAS TURBINE; JET PROPULSION; TURBOFAN; TURBOJET; TURBOPROP. 
 �34s[Fredric F. Ehrich]
�33s 
  
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�6v�2v�32sTurbodrill   �21m �33s 
    A rotary tool used in drilling oil or gas wells in which the bit is 
rotated by a turbine motor inside the well. The principal difference between 
rotary and turbodrilling lies in the manner in which power is applied to the 
rotating bit or cutting tool. In the rotary method, the bit is attached to a 
drill pipe, which is rotated through power supplied on the surface. In the 
turbodrill method, power is generated at the bottom of the hole by means of a 
mud-operated turbine.  See  ROTARY TOOL DRILL. 
    The turbodrill (see illustration) consists of four basic components: the 
upper, or thrust, bearing; the turbine; the lower bearing; and the bit. In 
operation, mud is pumped through the drill pipe, passing through the thrust 
bearing and into the turbine. In the turbine, stators attached to the body of 
the tool divert the mud flow onto rotors attached to the shaft. This causes 
the shaft, which is connected to the bit, to rotate. The mud passes through a 
hollow part of the shaft in the lower bearing and through the bit, as in 
rotary drilling, to remove cuttings, cool the bit, and perform the other 
functions of drilling fluid. Capacity of the mud pump, which is the power 
source, determines rotational speed.  See  BORING AND DRILLING (MINERAL); OIL 
AND GAS WELL DRILLING; TURBINE. 
 �34s[A. L. Ponikvar]
�33s 
  
  
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1802
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�6v�2v�32sTurbofan   �21m �33s 
    An air-breathing aircraft gas turbine engine with operational 
characteristics between those of the turbojet and the turboprop. Like the 
turboprop, the turbofan consists of a compressor-combustor-turbine unit, 
called a core or gas generator, and a power turbine. This power turbine drives 
a low- or medium-pressure-ratio compressor, called a fan, some or most of 
whose discharge bypasses the core (see illustration).  See  TURBOJET; 
TURBOPROP.  
    The gas generator produces useful energy in the form of hot gas under 
pressure. Part of this energy is converted by the power turbine and the fan it 
drives into increased pressure of the fan airflow. This airflow is accelerated 
to ambient pressure through a fan jet nozzle and is thereby converted into 
kinetic energy. The residual core energy is converted into kinetic energy by 
being accelerated to ambient pressure through a separate core jet nozzle. The 
reaction in the turbomachinery in producing both streams produces useful 
thrust.  See  GAS TURBINE; TURBINE PROPULSION. 
 �34s[Fredric F. Ehrich]
�33s 
  
  
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�6v�2v�32sTurbojet   �21m �33s 
    A propulsion engine used in many high-speed military and commercial 
aircraft.  
    In a turbojet, as illustrated, air approaches the inlet diffuser at a 
relative velocity equal to the flight speed. In passing through the diffuser 
the velocity of the air is decreased and its pressure is increased. The air 
pressure is increased further as it passes through the compressor. In the 
combustion chamber a steady stream of fuel is injected into the air and 
combustion takes place continuously. The high-pressure hot gas passes through 
the turbine nozzles, which direct it at high velocity against the buckets on 
the turbine wheel, thereby causing the wheel to rotate. The turbine wheel 
drives the compressor to which it is connected through a shaft. This is the 
sole function of the turbine.  See  DIFFUSER; JET PROPULSION; TURBINE 
PROPULSION.  
    After the hot gas leaves the turbine, it is still at a high temperature 
and at a pressure considerably above atmospheric. The hot gas is discharged 
rearward through the exhaust nozzle of the engine at a high velocity, thus 
producing thrust.  See  BRAYTON CYCLE. 
 �34s[Benjamin Pinkel]
�33s 
  
  
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�6v�2v�32sTurboprop   �21m �33s 
    A gas turbine power plant producing shaft power for aircraft using a 
propeller. The turboprop engine has basic components similar to those of a 
turbojet: compressor, combustor, and turbine. In addition, it has a power 
turbine. This power turbine extracts usable shaft power from the engine mass 
flow and drives a conventional propeller through a reduction gear. The 
illustration shows the simplest possible turboprop configuration. Here the 
power turbine is rigidly connected to the main engine rotor; hence this type 
is commonly called a single-shaft engine. This has certain disadvantages in 
regard to starting, partial-load fuel consumption, and windmilling drag. See 
TURBINE.  
    In the free-turbine engine, a more advanced turboprop type, the power 
turbine has no mechanical connection with the main engine rotor and 
consequently operates at a different rotational speed. This arrangement 
improves the operating characteristics and, especially, the cruise efficiency 
under partial-load conditions. This form of engine is also used to drive 
helicopter rotors and is then called a gas turbine.  See  GAS TURBINE; TURBINE 
PROPULSION.  
 �34s[Peter G. Kappus]
�33s 
  
  
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�6v�2v�32sTurboramjet   �21m �33s 
    An aircraft engine in which intake air is compressed at high flight speeds 
by the ram effect of the intake moving into the air ahead of the engine and at 
low flight speeds by the compressor rotating behind the intake, and in which 
propulsion is produced by a high-speed exhaust. For operation from standstill 
to high flight Mach numbers, the turboramjet engine provides useful 
performance characteristics. The compressor is driven by a turbine for engine 
operation at low flight speeds. At high supersonic flight speeds, the ram 
entry of air into the engine makes the compressor and its turbine superfluous. 
Under this operating condition, the fuel is burned to greater advantage in a 
combustion chamber downstream of the turbine instead of between compressor and 
turbine as in the turbojet mode of operation. Also, this second combustion 
chamber in back of the turbine can provide additional thrust at low speed when 
the engine is operated as a turbojet.  See  AFTERBURNER; RAMJET; TURBOJET. 
    The turbojet with afterburning is the simplest of the class of engines 
called turboramjets. The chamber for afterburning is provided with fuel 
injection nozzles and flameholders. The temperature of the gas from the 
primary combustion chamber is limited to a value below stoichiometric to avoid 
weakening the blades of the turbine. This limits the fuel burned in the 
primary burner to an amount that consumes about one-third of the available 
oxygen. The remaining oxygen is used for combustion of the fuel in the 
secondary combustion chamber (afterburner).  See  TURBINE PROPULSION. 
 �34s[Benjamin Pinkel]
�33s 
  
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\IMG\CI1801.IMG
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�6v�2v�32sTurbulent flow   �21m �33s 
    Motion of fluids in which local velocities and pressures fluctuate 
irregularly. Most flows observed in nature, such as rivers and winds, are 
turbulent. In turbulent flow, motion of the fluids is steady only insofar as 
the temporal mean values of velocities and pressures are concerned. The 
velocity and the pressure distributions in turbulent flows as well as the 
energy losses are determined mainly by the turbulent fluctuations. 
    The essential characteristic of turbulent flow is that the fluctuations 
are random. Hence, the solution of the turbulence problem requires the 
application of methods of statistical mechanics. In turbulent flow, the most 
important phenomenon is the transfer of forces by such random motion. The rate 
of turbulent transfer, such as heat transfer, shearing stress, and diffusion, 
is much higher than that due to molecular mechanism in laminar flow. 
    In turbulent motion, even though the fluid is regarded as a continuum with 
an average overall molecular motion, turbulent velocity fluctuations must be 
superimposed on the mean motion. The separation of mean motion and turbulent 
fluctuation depends mainly on the scale of turbulence. Different scales give 
different descriptions of turbulent flow. 
 �34s[Shih-I Pai]
�33s 
  
A700000002513�254s�5v�1v1803
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�6v�2v�32sTuring machine �21m �33s
    An automaton that basically consists of a computing element and a tape. A
series of characters, C1, C2, . . ., C(n), can be written on the tape so that
each character occupies a finite space along the tape, or, say, one square.
The computing element can assume a number of states, S1, S2, . . ., Ssubm. The
tape passes through the computing element one square at a time so that the
machine operation can be described by the quadruple Si, Cj, Ck, Sl,
which is read: When the machine is in state Si and the character Cj is
read, then replace the character Cj with character Ck and proceed to
state Sl. The character Ck can be considered as being written over the
character Cj. In addition, the instruction R or L may be given, calling for
a movement of the tape one square to the right or left. Thus the command
SiCjRSl is read: When the machine is in state Si and character
Cj is read, then move the tape one square to the right and proceed to state
Sl.
    Studies with Turing machines have been concerned with the consideration of
those functions which are computable with this machine organization. Whether
all functions that are computable can be computed on a Turing machine is
generally regarded as unprovable. See AUTOMATA THEORY; COMPUTER.
 �34s[Norman B. Reilly]
�33s

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�6v�2v�32sTurmeric �21m �33s
    A dye or a spice obtained from the plant Curcuma longa, which belongs to
the ginger family (Zingiberaceae). It is a stout perennial with short stem,
tufted leaves, and short, thick rhizomes which contain the colorful condiment.
As a natural dye, turmeric is orange-red or reddish brown, but it changes
color in the presence of acids or bases. As a spice, turmeric has a decidedly
musky odor and a pungent, bitter taste. It is an important item in curry and
is used to flavor and color butter, cheese, pickles, and other food.  See
SPICE AND FLAVORING; ZINGIBERALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

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�6v�2v�32sTurn and bank indicator   �21m �33s 
    A device used to advise the pilot of the rate at which the aircraft is 
turning, and that the wings are properly banked to preclude slipping or 
sliding of the aircraft as it continues in flight. The turn section uses a 
rate gyroscope as the sensor. This device has a rotor electrically or vacuum 
driven at high speed with its spin axis horizontal and at 90DG to the fore and 
aft axis of the aircraft. A simple pin-and-lever system causes a vertical 
pointer to move left or right, proportional to the turn rate. 
    The bank portion of the device is a simple liquid level that indicates the 
degree off the aircraft vertical that the combined force due to gravity and 
flight is acting.  See  GYROSCOPE. 
 �34s[James W. Angus]
�33s 
  
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�6v�2v�32sTurnbuckle   �21m �33s 
    A device for tightening a rod or wire rope. Its parts are a sleeve with a 
screwed connection at one end and a swivel at the other or, more commonly, a 
sleeve with screwed connections of opposite hands (left and right) at each end 
so that by turning the sleeve, the connected parts will be drawn together, 
taking up slack and producing necessary tension (see illustration). Types of 
ends available are hook, eye, and clevis. The turnbuckle can be connected at 
any convenient place in the rod or rope, and several may be used in series if 
required.  
 �34s[Paul H. Black]
�33s 
  
  
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�6v�2v�32sTurning (woodworking)   �21m �33s 
    The shaping of wood by rotating it in a lathe and cutting it with a 
chisel. The lathe consists essentially of a bed on which are mounted a 
headstock, a tailstock, and a tool rest, as illustrated. The headstock is 
rotated by a motor and holds one end of the wood to be turned. The tailstock 
holds the other end of the wood, allowing it to rotate freely. The tool rest 
provides a fixed guide along which the operator can handle the chisels if the 
turning is by hand, or along which the tool is driven if the turning is 
mechanized.  See  WOODWORKING. 
 �34s[Alan H. Tuttle]
�33s 
  
  
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�6v�2v�32sTurnip   �21m �33s 
    The plant Brassica rapa, or B. campestris var. rapa, a cool-season, hardy 
crucifer of Asiatic origin belonging to the order Capparales which is grown 
for its enlarged root and its foliage, which are eaten cooked as a vegetable. 
Popular white-fleshed varieties (cultivars) grown for their roots are Purple 
Top Globe and White Milan; Yellow Globe and Golden Ball are common 
yellow-fleshed varieties. Shogoin is a popular variety grown principally in 
the southern United States for turnip greens. Principal areas of production in 
the United States are in the South.  See  CAPPARALES. 
 �34s[H. John Carew]
�33s 
  
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1804
\IMG\CI1801.IMG
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�6v�2v�32sTurpentine   �21m �33s 
    An essential oil produced by steam distillation of pine woods and from gum 
turpentine, an exudate of living trees. It is recovered also in the sulfate 
process of cooking wood pulp. Slash and longleaf pines grown in the 
southeastern United States are the chief sources. 
    Turpentine is used principally as a solvent and a thinner for paints and 
varnishes. It is a starting material in the production of camphor, borneol, 
terpineol, and terpin hydrate.  See  CAMPHOR; ESSENTIAL OILS. 
 �34s[William Mosher]
�33s 
  
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�6v�2v�32sTurquois �21m �33s
    A mineral consisting of hydrated phosphate of aluminum and copper,
CuAl6(PO4)4(OH)8.4H2O, and prized as a semiprecious stone. Fe(sup 2+) may
substitute for some Cu. Bone turquois or odontolite, similar to turquois, is
formed in fossil bones or teeth and consists of microcrystalline apatite
colored by a hydrated phosphate of iron (vivianite).  See  GEM.
    Generally turquois occurs in veinlets or as crusts of massive, dense,
finely granular, concretionary, and stalactic shapes. The color of massive
turquois ranges through sky blue, bluish green, apple green, and greenish
gray. The robin's-egg blue variety is the most valued. The blue color in
turquois is due to the presence of a small amount of copper; the presence of
Fesup2supp results in greenish hues.
    Turquois of gem quality is found at one or more localities in Arizona,
California, Colorado, New Mexico, and Nevada. Large deposits located in the
Los Cerillos Mountains, southwest of Santa Fe, New Mexico, were mined early by
Indians and Mexicans and later were extensively exploited by Americans.
Fine-quality turquois has been mined for at least 800 years from a deposit
located on the south slopes of the Ali-Mirsa-Kuh Mountains near Nishapur,
Iran. Siberia, Turkistan, Asia Minor, the Sinai Peninsula, Silesia and Saxony
in Germany, and France possess turquois deposits.
 �34s[Wayne R. Lowell]
�33s

A700000002242�254s�5v�1v1804
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�6v�2v�32sTurtle   �21m �33s 
    Any of about 240 species of reptiles which constitute the order Chelonia 
and include the tortoise. Turtles and tortoises live in marine, terrestrial, 
and fresh-water environments. The body is enclosed in two bony shells, the 
dorsal carapace and a ventral plastron, which are laterally connected. The jaw 
is developed into a horny beak and teeth are lacking. The tongue is not 
protrusible as in amphibians, and eyelids as well as a nictitating membrane 
are present. 
    Tortoises are medium to large terrestrial reptiles with a highly arched 
carapace. The strong stout legs are covered with scales; the toes are short, 
terminating in thick claws, and lack webbing. These animals are herbivorous 
and consume all varieties of plant life. 
    The sea turtles have excited much interest and study because the loss of 
some of their natural habitats for egg laying has imperiled the species. The 
eggs are being protected by wild life services, as are the newly hatched 
turtles, which are preyed upon by birds as they attempt to reach the sea.  See 
  CHELONIA; REPTILIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sTwilight   �21m �33s 
    The period between sunset and darkness in the evening, and between 
darkness and sunrise in the morning. The characteristic light is caused by 
atmospheric scattering, which transmits sunlight to the observer for some time 
after sunset and before sunrise. It depends geometrically on latitude, 
longitude, and elevation of the observer, and on the time of year. Physically 
it depends also on local conditions, particularly the weather. 
 �34s[Gerald M. Clemence]
�33s 
  
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�6v�2v�32sTwinkling stars   �21m �33s 
    Stars exhibiting rapid fluctuations of the brightness and size of the 
images caused by the Earth's atmosphere; the phenomenon is also called 
astronomical seeing or scintillation. Irregularities of temperature, density, 
and water-vapor content, accompanied by changes of refractive index, are 
carried across the beam of light from the star and produce an irregularly 
corrugated wave front. Irregularities of the electron density in the 
ionosphere also change the refractive index at radio frequencies and cause 
twinkling of extraterrestrial sources of radio noise. 
    Much seeing is caused by high-altitude turbulence and winds. However, the 
flow of air over local terrain obstacles and around buildings produces 
considerable microturbulence, which has been found to affect the performance 
of telescopes. The best astronomical seeing (at night) gives images about 0.3 
second of arc; good seeing gives 1 second; poor seeing gives up to 10 seconds; 
daytime seeing is generally worse.  See  STAR. 
 �34s[Jesse L. Greenstein]
�33s 
  
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�6v�2v�32sTwinning (crystallography)   �21m �33s 
    A process in which two or more crystals, or parts of crystals, assume 
orientations such that one may be brought to coincidence with the other by 
reflection across a plane or by rotation about an axis. Crystal twins 
represent a particularly symmetric kind of grain boundary; however, the energy 
of the twin boundary is much lower than that of the general grain boundary 
because some of the atoms in the twin interface are in the correct positions 
relative to each other.  See  GRAIN BOUNDARIES. 
 �34s[Robb M. Thomson]
�33s 
  
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�6v�2v�32sTylenchoidea   �21m �33s 
    A superfamily of mainly soil and insectassociated nematodes of small or 
moderate size with a stylet for piercing live cells and sucking the juices. 
The most important are the phytoparasitic species, which attack a wide variety 
of plants and often cause extensive damage to agricultural crops. Among the 
most destructive are the stem and bulb eelworms, Ditylenchus sp., which attack 
cereals and flowers; the cyst nematodes, Heterodera sp., internal root 
parasites that include the golden nematode (H. rostochiensis), worldwide pest 
of potatoes; the root-knot nematodes, Meloidogyne sp., that cause gall 
formation in many kinds of plants; and the meadow nematodes, Pratylenchus sp., 
that damage roots and expose them to the invasion of bacteria and fungi. 
    Most species have fusiform bodies, but in some ectoparasites, such as 
Criconema sp., the body is annulated. 
    Insect commensals and parasites occur throughout the superfamily, but the 
Sphaerulariidae are entirely parasitic in insects. Life histories vary 
greatly, but several stages occur in the insect's hemocoele, and the 
intestinal and genital systems are damaged by the exit of the nematodes.  See  
ECOLOGICAL INTERACTIONS; NEMATA. 
 �34s[Bruce Hopper]
�33s 
  
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\IMG\CI1801.IMG
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�6v�2v�32sTyndall effect   �21m �33s 
    Visible scattering of light along the path of a beam of light as it passes 
through a system containing discontinuities. The luminous path of the beam of 
light is called a Tyndall cone. An example is shown in the illustration. In 
colloidal systems the brilliance of the Tyndall cone is directly dependent on 
the magnitude of the difference in refractive index between the particle and 
the medium. 
    For systems of particles with diameters less than one-twentieth the 
wavelength of light, the light scattered from a polychromatic beam is 
predominantly blue in color and is polarized to a degree which depends on the 
angle between the observer and the incident beam. The blue color of tobacco 
smoke is an example of Tyndall blue. As particles are increased in size, the 
blue color of scattered light disappears and the scattered radiation appears 
white. If this scattered light is received through a nicol prism which is 
oriented to extinguish the vertically polarized scattered light, the blue 
color appears again in increased brilliance. This is called residual blue, and 
its intensity varies as the inverse eighth power of the wavelength.  See  
COLLOID; SCATTERING OF ELECTROMAGNETIC RADIATION. 
 �34s[Quentin Van Winkle]
�33s 
  
  
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�6v�2v�32sTypewriter   �21m �33s 
    A self-contained, stand-alone machine that prints characters one after 
another on paper. Its essential parts are a keyboard, raised type, a means of 
inking, a platen for holding paper as it is imprinted, and a mechanism for 
advancing position as imprinting takes place. It operates manually, 
electromechanically, or electronically.  
    Basically, the typewriter depends on input from a typist operating a 
keyboard. The keyboard contains a complete alphabet, along with the numbers 
and symbols commonly used in various languages and technical disciplines. 
Strictly speaking, other keyboard machines, such as high-speed printers linked 
through communications channels to computers, or machines operating 
automatically in word-processing units, as well as other input or data-capture 
devices, are not typewriters since they can, in a variety of applications, 
operate without the direct intervention of a typist and are not, by 
definition, stand-alone units. 
    A continuing series of human-factors enhancements and technological 
advances have both improved the efficiency of the typewriter and simplified 
its operation. Of particular significance was the use of an electric motor to 
diminish the amount of effort required by the typist in generating key strokes. 
    Through electronics, a working memory (generally not designed for 
permanent storage) and other capabilities were added to the stand-alone 
typewriter. A microprocessor-driven electronic typewriter (see illustration) 
stores and retrieves words, phases, and pages of typing by using a 
high-density random-access memory chip.  See  MICROPROCESSOR; WORD PROCESSING. 
 �34s[Edward W. Gore, Jr.]
�33s 
  
  
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�6v�2v�32sType (printing) �21m �33s
    The relief or plane characters used to generate printed characters of
various styles and sizes. Type used in printing is divided into three
categories: foundry, machine-cast, and photocomposed type. In the first two
the face of the letter is raised on one end of a piece of metal. It is from
that surface, when inked, that the impression of type is made. In
photocomposition, the type is reproduced photographically.

    Classification.  Foundry type, also known as hand type, is cast as single
characters. Machine-cast type is produced by Linotype, Intertype, Ludlow, and
Monotype machines. All but the last cast type in lines, or what are known as
slugs. The Monotype - in reality two devices, a keyboard and a caster -
produces individual types set in lines of desired lengths.
    The product of the photocomposing type machine is the image of type on
film or on photosensitized paper in negative and in positive form. This method
is called cold composition because no hot type metal is needed for its
manufacture. The term cold composition or cold type is also applied to text
matter produced on a typewriter or similar machine for reproduction on a
printing plate, and to words or lines made up of individual printed characters
assembled or pasted together for photographic reproduction.
    About 3000 type styles are in everyday use throughout the world. The most
widely used method for classifying them is the serif-evolution system, based
on the different shapes of the terminals or endings of letters. This provides
eight classifications (Fig. 1).

    Type measurements.	Type is cast in sizes from 4 point to 144 point. In
the American point system, the unit of type measurement is a point, 0.01384
in. (0.0346 cm) or nearly  1/72 in., and all type sizes are multiples of this
unit. Sizes are measured through the body, lengthwise of the letters. Some
type size names used in the 16th century have remained and have been assigned
other functions. For example, the word pica is commonly used to denote a unit
of space measuring 12 points. It is applied as a dimensional unit to the
length of type lines and to the width and depth of pages and blocks of type.
    The standard height of type in the United States is 0.918 in. (2.295 cm),
a dimension called type-high and one observed by photoengravers and
electrotypers who make plates to be combined with cast type in letterpress
printing. Printing presses are adjusted to fit this standard height.

    Fonts.  A complete complement of letters of one size and style from A to
Z, together with the arabic numerals, punctuation, and reference marks, is
called a font. One style of letters is used in the setting of body, or text,
matter; it is available in roman capitals, italic capitals (majuscules), roman
and italic lower case (minuscules), and an alphabet of small capitals, which
is approximately the height of lowercase letters. A letter from a different
font which is included by accident in type composition is known as a wrong
font.

    Type families.  A family of type may be likened to the shades of a color
in that it includes all variations made of a given type face or design.
Weights are varied, from light to medium, bold, and extra bold; letters are
condensed and expanded, as well as outlined, inlined, and shadowed (Fig. 2).
 �34s[Eugene M. Ettenberg]
�33s


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�6v�2v�32sTyphales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Commelinidae of the class Liliopsida (monocotyledons). The order 
consists of only two genera: Typha, with about 15 species (making up the 
family Typhaceae), and Sparganium, with about 20 species (making up the family 
Sparganiaceae). These are marsh or aquatic plants with emergent or floating 
stems and leaves that have well-developed vessels, and with reduced, 
unisexual, wind-pollinated flowers. Large colonies of cattails (Typha) are 
familiar features of marshlands in many parts of the world.  See  
COMMELINIDAE; LILIOPSIDA. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sTyrosine   �21m �33s 
    An amino acid. Tyrosine is a precursor of the hormones epinephrine 
(adrenalin), norepinephrine (noradrenalin), thyroxin, and triiodothyronine, 
and of the black pigment melanin. The amino acid is formed from 
phosphoenolpyruvic acid and D-erythrose-4-phosphate, by way of shikimic acid 
and prephenic acid. In animals, tyrosine is formed by the oxidation of dietary 
phenylalanine. The major pathway for metabolic degradation leads to fumaric 
and acetoacetic acid.  See  AMINO ACIDS. 
 �34s[Edward A. Adelberg]
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�6v�2v�32sTyrothricin �21m �33s
    The first antibiotic to be used for human therapy. It was isolated from a
culture of Bacillus brevis by Rene J. Dubos in 1939. This basic discovery laid
the groundwork for the many antibiotics which have followed.  See  ANTIBIOTIC.
    Tyrothricin is a combination of two separate antibiotics. At least 18% is
gramicidin, and 65% is tyrocidine hydrochloride. The remainder consists of
other polypeptides.  Tyrothricin is principally effective against a wide
variety of gram-positive organisms. It is offered commercially in ointments,
creams, powders, lotions, sprays, suppositories, tablets, and aerosol
preparations. Such products are used for the treatment of dermal infections,
infections of the nose, throat and eyes, and for the preparation of solutions
used to irrigate the body cavities.
 �34s[Bruce G. Malloy ;Edward J. Beckhorn]
�33s

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�6v�2v�32sUlcer �21m �33s
    Common inflammatory lesion in which there is a loss or destruction of
superficial tissue. Ulcers occur in several locations as acute, subacute,
chronic, or recurrent types. In each location some initiating factor, such as
action of bacterial toxins or lack of oxygen, causes death of the surface
tissue. This necrotic tissue than usually sloughs off, leaving the underlying
area exposed to further damage. There is some inflammation present.
    Peptic ulcers are a common form of ulcer which occur primarily in the
stomach and duodenum. The exact causes are obscure, although predisposing
factors such as tension, anxiety, and abnormal amounts of acid secretion
appear in most cases.  See  PEPTIC ULCER.
    Ulcerative colitis of the large intestine is similar, in many respects, to
peptic ulcer, because its exact cause is unknown and a relationship exists
with certain personality patterns.
    In many skin disorders in which a circulatory deficiency is involved,
ulcerative lesions may appear. Superficial ulcerations of the mouth, tongue,
and other mucosal surfaces may occur as the result of illness, infection, or
irritation. Several serious diseases may be accompanied by characteristic
ulcers of the gastrointestinal tract, including typhoid fever, tuberculosis,
and both amebic and bacillary dysentery.  See  AMEBIASIS; BACILLARY DYSENTERY;
TUBERCULOSIS.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sUlotrichales   �21m �33s 
    A large, artificial order of the Chlorophyta, composed mostly of 
fresh-water, branched or unbranched filamentous species. Some show 
basal-distal differentiation and are attached; a few are endophytic or 
epiphytic whereas others are free-floating. Cells are mostly cylindrical with 
cellulosic, often mucilaginous walls and, except for Sphaeroplea, are 
uninucleate. The chloroplast is mostly a parietal band or ring. Reproduction 
is vegetative, or by quadriflagellate zoospores, and sexually by iso-, aniso-, 
or heterogametes. A conservative but often modified classification recognizes 
two suborders, Ulotrichineae (ten families) and Sphaeropleineae (one family).  
See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sUltracentrifuge   �21m �33s 
    A centrifuge of high or low speed which provides convection-free 
conditions and which is used for quantitative measurement of sedimentation 
velocity or sedimentation equilibrium or for the separation of solutes in 
liquid solution.  See  CENTRIFUGATION. 
    The ultracentrifuge is used (1) to measure molecular weights of solutes 
and to provide data on molecular weight distributions in polydisperse systems; 
(2) to determine the frictional coefficients, and thereby the sizes and 
shapes, of solutes; and (3) to characterize and separate macromolecules on the 
basis of their buoyant densities in density gradients.  See  MOLECULAR WEIGHT. 
    The ultracentrifuge is most widely used to study high polymers, 
particularly proteins, nucleic acids, viruses, and other macromolecules of 
biological origin. However, it is also used to study solution properties of 
small solutes. In applications to macromolecules, the analytical 
ultracentrifuge, which is used for accurate determination of sedimentation 
velocity or equilibrium, is distinguished from the preparative 
ultracentrifuge, which is used to separate solutes on the basis of their 
sedimentation velocities or buoyant densities. 
    The application of a centrifugal field to a solution causes a net motion 
of the solute. If the solution is denser than the solvent, the motion will be 
away from the axis of rotation. The nonuniform concentration distribution 
produced in this way leads to an opposing diffusion flux tending to 
reestablish uniformity. In sedimentation-velocity experiments, sedimentation 
prevails over diffusion, and the solute sediments with finite velocity toward 
the bottom of the cell, although the concentration profile may be markedly 
influenced by diffusion. In sedimentation-equilibrium experiments, centrifugal 
and diffusive forces balance out, and an equilibrium concentration 
distribution results which may be analyzed by thermodynamic methods.  See  
CENTRIFUGAL FORCE. 
 �34s[Victor A. Bloomfield]
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�6v�2v�32sUltrafiltration   �21m �33s 
    A filtration process in which particles of colloidal size are retained by 
a filter medium while solvent and accompanying low-molecular-weight solutes 
are allowed to pass through. Ultrafilters are used (1) to separate colloid 
from suspending medium, (2) to separate particles of one size from particles 
of another size, and (3) to determine the distribution of particle sizes in 
colloidal systems by the use of filters of graded pore size. 
    Ultrafilter membranes have been prepared from various types of gel-forming 
substances: Unglazed porcelain has been impregnated with gels such as gelatin 
or silicic acid. Filter paper has been impregnated with glacial acetic acid 
collodions. Another type of ultrafilter membrane is made up of a thin plastic 
sheet containing millions of tiny pores evenly distributed over its surface.  
See  COLLOID; FILTRATION. 
 �34s[Quentin Van Winkle]
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�6v�2v�32sUltrasonic flow measurement �21m �33s
    The use of high-frequency sound to measure the rate of fluid flow through
a pipe, duct, or open channel. Ultrasonic flowmeters are available in many
physical arrangements and are based on a great variety of fundamental
principles.
    Contrapropagating diagonal-beam designs (illustration a) have transducers
mounted in sockets machined in the pipe or welded on wedges, and also as
clamp-on elements. The difference between upstream and downstream ``flight''
times combined with the speed of sound in the fluid will give fluid velocity.
Since the flow near the middle of the pipe is faster, a multiple parallel-beam
design will give a better average. In a symmetrical profile a beam offset from
the center of the pipe may also give a better average.
    For small pipes, the short path length across the fluid stream makes
measurement difficult. Illustration b shows a contrapropagating arrangement
for measuring lengthwise over a relatively longer path where timing
measurements are more practical.
    In the correlation meter, two pairs of elements with beams across the pipe
are used with a correlation circuit to detect the time it requires for
discontinuities in the fluid stream to pass between the two detectors.
    In the deflection or drift meter, an ultrasonic beam directed across the
pipe is deflected by an amount depending on the relative velocity of sound in
the fluid and the flowing rate of the fluid (illustration c). One arrangement
detects this deflection with two receivers mounted on the other side of the
pipe. The relative intensity of the two receivers is an indication of the
flowing-fluid velocity.

    In the doppler meter flow rate is detected by the Doppler shift of sound
waves reflected from particles or discontinuities in the flowing fluid.  See
DOPPLER EFFECT.
    In the noise-type meter, the noise generated by the flowing fluid is
measured in a selected frequency band, with a useful but relatively inaccurate
readout.
    A different application of ultrasonics to flow measurement can be found in
flumes and weirs where the level of the flowing fluid, measured by ultrasonic
reflection, is used to calculate the flow rate.  See  FLOW MEASUREMENT.
 �34s[Mead Bradner ;Lewis P. Emerson]
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�6v�2v�32sUltrasonic machining   �21m �33s 
    The removal of material by abrasive bombardment and crushing in which a 
flat-ended tool of soft alloy steel is made to vibrate at a frequency of about 
20,000 Hz while a fine abrasive of silicon carbide, aluminum oxide, or boron 
carbide is carried by a liquid between tool and work. The tool does not touch 
the work. Impressions may be sunk economically in glass, ceramics, carbides, 
and hard brittle metals by this method. 
 �34s[Orlan W. Boston]
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�6v�2v�32sUltrasonics �21m �33s
    The science of sound waves having frequencies above the audible range,
that is, above about 20,000 Hz. Original workers in this field adopted the
term supersonics. However, this name was also used in the study of airflow for
velocities faster than the speed of sound. The present convention is to use
the term ultrasonics as defined above. The term silent sound also has been
used to denote ultrasonic waves. The term pretersonics is now used to refer to
frequencies about 10(sup 10) Hz. Since there is no marked distinction between
the propagation and the uses of sound waves above and below 20,000 Hz, the
division is rather artificial.	See  SOUND.

    Projectors and detectors.  The usual types of generators for air, liquids,
and solids are the piezoelectric and magnetostrictive generators. X-cut quartz
crystals are used to produce longitudinal waves in gases, liquids, and solids.
Y-cut and AC-cut quartz crystals are used to produce shear or transverse waves
in solids. Crystals of these types are utilized in instruments such as the
acoustic interferometer.  See  ACOUSTIC INTERFEROMETER.
    Pulse systems have been used to measure properties of liquids and solids.
A short burst of ultrasonic waves is sent into the medium and is reflected
back. By timing the received pulse with respect to the transmitted pulse, or
by a phasing technique, accurate velocity measurements can be made. For higher
powers, ferroelectric ceramics, such as barium titanate, PZT (lead zirconate
titanate), and NaKNbO3, or magnetostrictive materials, such as nickel or
ferrites, commonly are employed. They generally are used for ultrasonic
cutting, wear or fatigue testing, and ultrasonic welding.
    A number of shear-wave transducers, most of them employing torsional or
shear-wave generators of quartz, have been used to measure the shear viscosity
and shear stiffness of liquids. By this means it has been proved that
moderately viscous liquids have elastic, as well as viscous, properties. For
pretersonic frequencies (above 500 MHz), all  liquids are found to have shear
elastic properties.

    Engineering applications.  The engineering applications of ultrasonic can
be divided into those dealing with low-amplitude sound waves and those dealing
with high-amplitude (now usually called macrosonic) waves.

    Low-amplitude. Low-amplitude applications are in sonar (an
underwater-detection apparatus), in the measurement of the elastic constants
of gases, liquids, and solids by a determination of the velocity of
propagation of sound waves, in the measurement of the attenuation of sound
waves, and in a number of ultrasonic devices such as delay lines, mechanical
filters, inspectoscopes, and thickness gages.  See  DELAY LINE; SONAR.
    All these applications depend on the modifications that boundaries and
imperfections in the materials cause in wave-propagation properties. The
attenuation and scattering of the sound in the mediums are important factors
in determining the frequencies used and the sizes of the pieces that can be
utilized or investigated.
    Mechanical filters are used for separating telephone communications sent
simultaneously over one transmission line. The monolithic filter consists of a
series of vacuum-deposited electrodes with a ratio of total electrode mass to
the mass of the crystal plate between electrodes of R = 0.02 to 0.04. By using
as many as six sections of platings, this device acts as a bandpass filter.
Attenuation characteristics are sufficiently good for the device to be used as
a channel filter in a long-distance carrier or submarine cable system. The
filter can be made from a single quartz crystal, and hence considerable
economies can be obtained.
    Ultrasonic inspectoscopes transmit sound waves into a metal casting or
other solid piece and determine the presence of flaws by reflections or by an
interruption of the sound-wave transmission through the piece. Ultrasonic
thickness gages have been used in measuring the thickness of pieces when one
side is not accessible, such as in boilers. Surface acoustic-wave devices
involve the use of Rayleigh surface waves generated by equally spaced
interdigital electrode transducers. One of the largest uses of these devices
is in obtaining dispersive delay lines used in chirp radars.  See  SURFACE
ACOUSTIC-WAVE DEVICES.
    An effect related to internal friction, motion of dislocations, and
fatigue in materials is the noise in the specimens produced by strain. This is
called acoustic emission. Research on this phenomenon has had wide application
in such subjects as structural integrity of metals and rocks, flaws in
metals, composite materials, concrete, ceramics, ice, soils, wood, integrity
of welds, martensite transformations, nuclear power reactors, and leaks in
pressure systems. Acoustic emission is used to test the goodness of spot welds
on relay contacts.  See  ACOUSTIC EMISSION.

    High-amplitude. High-amplitude acoustic waves (macrosonic) have been used
in a variety of applications involving gases, liquids, and solids.
    Holes (gas-bubble cavities) can be created in a liquid by high-intensity
sound waves. When such a cavity collapses, extremely high pressures are
produced. The process, called cavitation, is the origin of a number of
mechanical, chemical, and biological effects. The cavitation effect can be
used to disperse metals and sulfur in solutions, to produce extrafine grain
photographic emulsions, and to achieve a finer texture (smaller grain size)
and more uniform alloying of a molten metal. In chemistry, cavitation can be
used to break long-chain polymers into shorter chains, affording a polymer of
more uniform chain length than is possible with other depolymerizing methods.
Cavitation forces also can be used to sterilize milk.  See  CAVITATION.
    Ultrasound is used widely in the cleaning of metal parts, such as in
watches. One of the principal applications of ultrasonics to gases is particle
agglomeration. This technique has been used in industry to collect fumes,
dust, sulfuric acid mist, carbon black, and other substances.

    Analytical uses.  In addition to their engineering applications,
high-frequency sound waves have been used to determine the specific types of
motions that can occur in gaseous, liquid, and solid mediums. Both the
velocity and attenuation of a sound wave are functions of the sound frequency.
By studying the changes in these properties with changes of frequency,
temperature, and pressure, indications of the motions taking place can be
obtained.  See	SOUND ABSORPTION.
    Ultrasonic methods have also been used to produce light images of
ultrasonic beams, outputs from ultrasonic transducers, and visual pictures of
transmission through live tissues and have been used for the reproduction of
pictures through sound and ultrasound holographic methods. See ACOUSTIC
MICROSCOPE; ACOUSTICAL HOLOGRAPHY.
 �34s[Warren P. Mason]
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�6v�2v�32sUltraviolet astronomy   �21m �33s 
    Astronomical investigations utilizing observations carried out in the 
spectral region from approximately 350 to 90 nanometers. Astronomical 
observations in the ultraviolet and x-ray region of the spectrum must be 
carried out above the Earth's atmosphere, since the atmosphere ozone, oxygen, 
and nitrogen are opaque at these short wavelengths. Such observations are made 
by using sounding rockets, satellites, or space probes.  
    Ultraviolet measurements have proved important for the studies of planets, 
stars, interstellar gas, and galaxies. Most of the light emitted from hot 
stars appears in the ultraviolet, and knowledge of the temperature and 
composition of these stars, usually quite young, depends upon such 
observations. The strongest spectral lines produced by the lighter and more 
abundant atoms occur in the ultraviolet. Studies of the ultraviolet line 
spectrum have proved important to understanding the chemical composition 
densities and temperatures of the interstellar gas and dust that exists in 
space between the stars. Ultraviolet observations of galaxies can provide 
vital information on the evolution of galaxies.  See  ASTRONOMICAL 
SPECTROSCOPY; GALAXY, EXTERNAL; INTERSTELLAR MATTER; STAR; STELLAR EVOLUTION. 
 �34s[Arthur D. Code]
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�6v�2v�32sUltraviolet lamp   �21m �33s 
    A mercury-vapor lamp designed to produce ultraviolet radiation. Also, some 
fluorescent lamps and mercury-vapor lamps that produce light are used for 
ultraviolet effects.  See  FLUORESCENT LAMP; MERCURY-VAPOR LAMP. 
    Fluorescent and mercury lamps can be filtered so that visible energy is 
absorbed and emission is primarily in the near-ultraviolet or black-light 
spectrum (320 - 400 nanometers). The ultraviolet energy emitted is used to 
excite fluorescent pigments in paints, dyes, or natural materials. 
    Mercury-vapor lamps are sometimes designed with pressures that produce 
maximum radiation in the middle ultraviolet region (280 - 320 nm), using 
special glass bulbs that freely transmit this energy. One such lamp type is 
the sunlamp. Other lamps designed for middle-ultraviolet radiation are known 
as photochemical lamps.  See  SUNLAMP. 
    Some radiation in the 220 - 280-nm wavelength band has the capacity to 
destroy certain kinds of bacteria. Mercury lamps designed to produce energy in 
this region (the 253.7-nm mercury line) are electrically identical with 
fluorescent lamps; they differ from fluorescent lamps in the absence of a 
phosphor coating and in the use of glass tubes that transmit far ultraviolet.  
See  ULTRAVIOLET RADIATION (BIOLOGY). 
 �34s[Alfred Makulec]
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�6v�2v�32sUltraviolet radiation   �21m �33s 
    Electromagnetic radiation in the wavelength range 4 - 400 nanometers. The 
ultraviolet region begins at the short wavelength (violet) limit of visibility 
and extends to the wavelength of long x-rays. It is loosely divided into the 
near (400 - 300 nm), far (300 - 200 nm), and extreme (below 200 nm) 
ultraviolet regions (see illustration). In the extreme ultraviolet, strong 
absorption of the radiation by air requires the use of evacuated apparatus; 
hence this region is called the vacuum ultraviolet. Important phenomena 
associated with ultraviolet radiation include biological effects and 
applications, the generation of fluorescence, and chemical analysis through 
characteristic absorption or fluorescence.  See  ULTRAVIOLET RADIATION 
(BIOLOGY).  
    Sources of ultraviolet radiation include the Sun (although much solar 
ultraviolet radiation is absorbed in the atmosphere); arcs of elements such as 
carbon, hydrogen, and mercury; and incandescent bodies.  See  ULTRAVIOLET LAMP. 
 �34s[Fred W. Billmeyer, Jr.]
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�6v�2v�32sUltraviolet radiation (biology)   �21m �33s 
    Radiations between 200 and 300 nanometers are selectively absorbed by 
organic matter, and produce the best-known effects of ultraviolet radiations 
on organisms. Ultraviolet radiations, in contrast to x-rays, do not penetrate 
far into larger organisms; therefore, the effects they produce are surface 
effects, such as sunburn and development of D vitamins from precursors present 
in skin or fur. The effects of ultraviolet radiations on life have, therefore, 
been assayed chiefly with unicellular organisms such as bacteria, yeast, and 
protozoans, although suspensions of cells of higher organisms, for example, 
eggs and blood corpuscles, have been useful as well. 
  
    Photobiological effects.  Only the ultraviolet radiations which are 
absorbed can produce photobiological action. All life activities are shown to 
be affected by ultraviolet radiations, the effect depending upon the dosage. 
Small dosages activate unfertilized eggs of marine animals, reduce the rate of 
cell division, decrease the synthesis of nucleic acid, especially in the 
nucleus, reduce the motility of cilia and of contractile vacuoles, and 
sensitize cells to heat. Large doses increase the permeability of cells to 
various substances, inhibit most synthetic processes, produce mutations, stop 
division of cells, decrease the rate of respiration, and may even disrupt 
cells. The effect of ultraviolet radiations upon cells is invariably 
deleterious.  
  
    Effects on the skin.  Erythema is the reddening of the skin following 
exposure to ultraviolet radiation of wavelength shorter than 320 nm, 
wavelength 296.7 nm being most effective. These radiations injure cells in the 
outer layer of the skin, or epidermis, liberating substances which diffuse to 
the inner layer of the skin, or dermis, causing enlargement of the small blood 
vessels. A minimal erythemal dose just induces reddening of the skin observed 
10 h after exposure. A dose several times the minimal gives a sunburn, killing 
some cells in the epidermis after which serum and white blood cells 
accumulate, causing a blister. After the dried blister peels, the epidermis is 
temporarily thickened and pigment develops in the lower layers of the 
epidermis, both of these factors serving to protect against subsequent 
exposure to ultraviolet. 
    Both thickening of the epidermis and tanning may occur without blistering. 
Since the pigment in light-skinned races develops chiefly below the sensitive 
cells in the epidermis, it is not as effective as in dark-skinned races where 
the pigment is scattered throughout the epidermis. Consequently, the minimal 
erythemal dose is much higher for the dark- than for the light-skinned races. 
    Excessive exposure to ultraviolet radiation has been found to lead to 
cancer in mice, and it is claimed by some to cause cutaneous cancer in humans. 
  
    Clinical use.  Ultraviolet radiations were once used extensively in the 
treatment of rickets, many skin diseases, tuberculosis other than pulmonary, 
especially skin tuberculosis (lupus vulgaris), and of many other diseases. The 
enthusiasm for sun bathing is, in part, a relic of the former importance of 
ultraviolet radiation as a clinical tool. Vitamin preparations, synthetic 
drugs, and antibiotics have either displaced ultraviolet radiations in such 
therapy or are used in conjunction with the radiations. 
    Ultraviolet radiations alone are still employed to treat rickets in 
individuals sensitive to vitamin D preparations. In conjunction with 
chemicals, they are used in treating skin diseases, for example, psoriasis, 
pityriasis rosea, and sometimes acne, as well as for the rare cases of 
sensitivity to visible light. Ultraviolet radiations, however, are more 
important in research than in clinical practice. 
 �34s[Arthur C. Giese]
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�6v�2v�32sUlvales   �21m �33s 
    An order of algae in the division Chlorophyta. The thalli are macroscopic, 
attached tubes or sheets. All genera except Schizomeris are marine; however, a 
few species occur in streams. This order is sometimes included under the 
Ulotrichales, which it resembles in cell organization and reproductive 
elements.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
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�6v�2v�32sUmbellales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order 
consists of two families, the Araliaceae, with about 700 species, and the 
Umbelliferae, with about 3000. They are herbs or woody plants with mostly 
compound or conspicuously lobed or dissected leaves, well developed secretory 
canals, and separate petals. 
    The Umbelliferae are mostly aromatic herbs, most numerous in temperate 
regions. The flowers consistently have an ovary of two carpels, ripening to 
form a dry fruit that splits into two halves, each containing a single seed. 
Some common garden vegetables and spice plants, including carrot (Daucus 
carota), parsnip (Pastinaca sativa), celery (Apium graveolens), parsley 
(Petroselinum crispum), caraway (Carbum carvi), and dill (Anethum graveolens), 
belong to the Umbelliferae, as do also such notorious poisonous plants as the 
poison hemlock (Conium) and water hemlock (Cicuta).  See  ANISE; CARROT; 
CELERY; FENNEL; GINSENG; MAGNOLIOPSIDA; PARSLEY; PARSNIP; ROSIDAE. 
 �34s[Arthur Cronquist]
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�6v�2v�32sUmklapp process �21m �33s
    A concept in the theory of transport properties of solids which has to do
with the interaction of three or more waves in the solid, such as lattice
waves or electron waves. In a continuum, such interactions occur only among
waves described by wave vectors k1, k2, and so on, such that the interference
condition, given by Eq. (1), is satisfied.  The sign of k depends on whether

			   k1 + k2 + k3 = O				   (1)

the wave absorbs or emits energy. Since PLANCK'S CONSTANTk is the momentum of a
quantum (or particle) described by the wave, Eq. (1) corresponds to conservation
of momentum. In a crystal lattice further interactions occur, satisfying Eq.
(2), where b is any integral combination of the three inverse lattice vectors

			   k1 + k2 + k3 = b				   (2)

bsubi, defined by a . bsubj = 2PIDELTAsubisubj, the a's being the periodicity
vectors. The group of processes described in Eq. (2) are the Umklapp processes
or flip-over processes, so called because the total momentum of the initial
particles or quanta is reversed.  See  CRYSTAL.
 �34s[Paul G. Klemens]
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�6v�2v�32sUncertainty principle �21m �33s
    In quantum mechanics, the precept (W. Heisenberg, 1927) that accurate
measurement of an observable quantity necessarily produces uncertainties in
one's knowledge of the values of other observables. It is also called the
indeterminacy principle.

    In particular, for a single particle, Eqs. (1) and (2) hold.  In

			     x	px  h/2 			      (1)
			     t	E   h/2 			      (2)

Eq.  (1), DELTA x represents the uncertainty (error) in the location of the x
coordinate of the particle at any instant, and DELTA p(sub x) is the
simultaneous uncertainty in its x component of momentum; h = Planck's constant =
6.61 x 10(sup -27) erg-second.	Equation (2) is more subtle than Eq.  (1), and
its interpretation depends somewhat on the circumstances.  For instance, Eq.
(2) relates the uncertainty DELTA E in an energy measurement to the time
interval DELTAt during which the measurement was performed; however, Eq.  (2)
also relates the uncertainty DELTA E in the energy radiated by a system to the
uncertainty DELTAt in the time at which it radiates, that is, the uncertainty in
its lifetime.  See QUANTUM MECHANICS.

 �34s[Edward Gerjuoy]
�33s

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�6v�2v�32sUnconformity   �21m �33s 
    The relation between adjacent rock strata whose time of deposition was 
separated by a period of nondeposition or of erosion. The break in the 
stratigraphic sequence represents a significant change in regimen (in contrast 
to diastem). For example, marine strata separated by a surface of subaerial 
erosion are unconformable, but those separated by a surface of mere 
nondeposition or of erosion by shifting currents which do not represent an 
overall change of conditions are not. The word unconformity is sometimes also 
used to denote the surface between the unconformable strata.  See  DIASTEM. 
    In general, there are four main kinds of unconformable relations (see 
illustration), although the names of each are not universally accepted: (1) 
Nonconformity - rocks below the break are not stratified, such as massive 
crystalline rocks. (2) Angular unconformity - rocks below the break are 
stratified but lie at an angle to those above. (3) Disconformity - strata 
below are parallel to those above but are separated by an evident surface of 
erosion. (4) Pseudoconformity or paraconformity - strata are parallel but 
surface is hardly distinguishable from a simple bedding plane.  See  
STRATIGRAPHY.  
 �34s[John Rodgers]
�33s 
  
  
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�6v�2v�32sUnderground mining   �21m �33s 
    An underground mine is a system of underground workings for the removal of 
ore from its place of occurrence to the surface, and involves the deployment 
of miners and services. 
    There are several basic physical elements in an underground mining system. 
The passageways (openings) in a mine are called drifts if they are parallel to 
the geological structure, and cross-cuts if they cut across it. They range in 
size about 60 - 200 ftsup2 (6 - 18 msup2) in cross section, depending on their 
functions. The workings on a level (horizontal plane) are joined with those on 
another level by passageways of similar cross section, called raises if they 
are driven upward and winzes if driven downward. 
    The passageways give access to, and provide transportation routes from, 
the stopes, which are the excavations where the ore is mined. The stopes are 
between levels. There may be rooms on the level, such as pump rooms, service 
shops, and lunchrooms. For other aspects  see  COAL MINING; MINING. 
 �34s[A. V. Corlett]
�33s 
  
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�6v�2v�32sUnderwater demolition   �21m �33s 
    The destruction or fragmentation of underwater obstacles by the use of 
explosive charges hand-placed by diver personnel.  
    Diving equipments used by naval personnel in underwater demolition include 
deep-sea equipment, light-weight equipment, and scuba gear. Standard 
demolition equipment and techniques are employed. The explosive requirement 
for a specific task would be judged by the properties of the individual high 
explosive (rapidity of detonation, sensitivity, brisance, and so on), in light 
of the problem at hand.  See  DIVING; SHIP SALVAGE. 
 �34s[Hal W. Pilgrim]
�33s 
  
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�6v�2v�32sUnderwater navigation �21m �33s
    The process of directing the movements of submersible vehicles, and
divers, from one point to another. The development of improved submersible
vehicles, coupled with advances in saturated diving, has resulted in new
requirements for underwater navigation. Various methods which have proved
successful include acoustic transponder systems, dead reckoning,
surface-referenced navigation from a support ship, homing, and various
combinations of these. The choice of the navigation system depends on such
factors as precision required, area to be covered, availability of surface
vessels, sea state under which they are expected to operate, and for crewed
vehicles the redundancy necessary for safety.  See  OCEANOGRAPHIC
SUBMERSIBLES; SUBMARINE.

    Acoustic transponders.  A system of navigation using acoustic transponder
beacons represents the most accurate technique for precise location with
respect to points on the sea floor. Usually two or more transponders are
dropped from the surface vehicle or from a submerged vehicle to establish an
array on the bottom. These transponders can be interrogated by surface
vessels, submersible vehicles or divers. When navigation is performed from the
surface vessel, the submerged vehicle also carries a transponder. The position
of the submerged vehicle with respect to the sea-floor transponders is
computed. In the case of a crewed submersible the information is forwarded via
an acoustic communications link.
    A second technique is based on the submerged vehicle's determining its own
position by interrogating the array of transponders. This method is widely
used both with crewed submersibles and towed vehicles, and provides high
accuracy, since the interrogating sonar is located well below the surface
layers. The time delay between interrogation and received return (plus a fixed
time delay associated with the transponder) is a measure of the range of each
transponder from the interrogating vehicle.

    Dead reckoning.  An undersea vehicle needs sensors to show distance
traveled and direction of travel for dead-reckoning estimation of position.
The most promising sensor for operation near the sea floor is a Doppler sonar.
Ground speed, fore-aft and athwartship, can be determined from the Doppler
shift in frequency of signals returned from the sea floor. Pulse-type and
continuous-wave Doppler sonars are under development.
    In addition to Doppler sonar, some specialized submersibles carry inertial
dead reckoning in the form of a small, stabilized platform.  See  DEAD
RECKONING; INERTIAL GUIDANCE SYSTEMS; SONAR.

    Other methods.  Most submersible vehicles carry other sensors which can
perform a navigation function. The horizontal obstacle sonar (HOS) is normally
used to detect objects ahead of the submersible. An altitude-depth sonar
provides vertical navigation by furnishing depth and altitude off the bottom.
A vertical obstacle sonar (VOS) is used to determine heights of objects in the
path of the vehicle.
    An acoustic tracking system allows the monitoring and vectoring of the
position of a vehicle from a surface vessel, where space and weight are not at
a premium. Surface tracking systems are of two types: ultrashort baseline and
short baseline. The ultrashort baseline system employs a single transducer,
mounting two orthogonal arrays in an assembly less than 10 in. (25 cm) in
diameter. The short baseline system consists of an array of four hydrophones
mounted on a ship in two orthogonal baselines. The length of each baseline is
the longest possible consistent with the ship's geometry, typically 50 - 100
ft (15 - 30 m).
    The submarine inertial navigation system (SINS) is the primary navigation
system in most U.S. Navy nuclear-powered submarines, with multiple systems
being employed in ballistic missile-firing submarines for improved reliability
and accuracy. The high stability and low drift of electrostatically supported
gyroscopes have been employed to monitor SINS performance.
 �34s[Joseph A. Cestone ;Emery St. John Jr.]
�33s

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�6v�2v�32sUnderwater photography �21m �33s
    The technique of using photography equipment underwater. Photography is as
useful a recording system underwater as it is in air. All forms of photography
are in use: single pictures, motion pictures, stereo, shadow, elapsed-time
motion pictures, and so forth.

    Shallow water.  In shallow underwater photography, or hand-held underwater
photography as it is more commonly called, the photographer, wearing some type
of diving equipment, is submerged with the camera and takes photographs
directly (see illustration). This type of underwater photography requires
scuba.
    Underwater photograph is controlled by many different factors, some of
which are available light, clarity of water, differential absorption of light
in various wavelengths, film capability, and construction of underwater
housing for cameras. The underwater photographer can never expect to take
clear pictures at distances greater than 165 ft (50 m). In most instances, the
photographer must depend on natural light for film exposure.   See  DIVING.
 �34s[Robert F. Dill]
�33s


    Deep-sea.  Photography in the deep sea involves the design and use of
camera and lighting equipment for which several requirements must be
fulfilled, including the following.
    1. Suitable watertight cases are required, for both the camera and the
light source, to withstand the pressure of the sea. For each 30 ft (10 m) of
depth, approximately one additional atmosphere of pressure is exerted. The
deepest ocean, about 40,000 ft (12,000 m), requires a case to withstand 17,600
lb/in.sup2 (1200 kg/cmsup2). The windows for the lens and the electrical seals
must likewise be designed for such pressures.
    2. Auxiliary lighting is required, especially for deep photography, since
daylight is rapidly absorbed with depth, becoming almost useless even at
shallow depths of a few hundred meters.
    3. Corrected optics for the camera are desirable to compensate for
distortions caused by the lens window - water interface when an air lens is
used. However, many underwater cameras have lenses designed for use in the air
even if some information is lost.
    4. The camera must be positioned and triggered to get the desired
photograph. Operation from a cable (with sonar sensing equipment) or from
deep-diving underwater vehicles is employed. Bottom-sensing switches are often
used to operate the deep cameras when bottom photographs are desired.
    When a person descends as an observer to great depths in a diving vehicle,
a camera can help in remembering and recording what was seen, and provide
visual evidence.
    A television link from the bottom to the surface is often mentioned as a
substitute for underwater photography since the picture is seen immediately
and the television screen can be photographed or taped at the surface.
However, the quality of the television photograph or tape is not as high as
that taken by a camera below because of the limitations of the television link
and the lower resolution intrinsic in TV.  See	UNDERWATER TELEVISION.
 �34s[Harold E. Edgerton ;Samuel O. Raymond]
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�6v�2v�32sUnderwater sound   �21m �33s 
    The production, transmission, and reception of sounds in the ocean are an 
important part of modern acoustics. Since the ocean is highly absorptive to 
light and other electromagnetic radiation but not to sound waves, acoustics 
has found many applications in probing the sea. Acoustical methods of 
detection, for example, are useful in locating submarines and other submerged 
objects, and therefore have many applications in naval science. Underwater 
sound is also important in scientific research: Oceanographers and 
geophysicists employ sound waves to determine the physical structure of the 
ocean and its bottom; sound is also used by marine biologists to study life in 
the ocean. There are also many commercial uses for underwater sound, such as 
seismic exploration for oil and gas beneath the ocean floor, use of sonar 
techniques for fish location, navigation and positioning, and communication.  
See  SEA WATER; SONAR. 
  
    Sound projection and reception.  In general, devices which transform one 
type of energy into another are known as transducers; when the end product is 
underwater sound, such devices are known as sound projectors. Most projectors 
use either the magnetostrictive or piezoelectric effect in which electrical 
energy is transformed into acoustical energy. However, oceanographers also 
make extensive use of explosives to study oceanographic features. Other 
projects are based on spark discharge, hydraulic flow, mechanical impact, and 
pneumatic and other effects. 
    To obtain the information sought, the acoustic signal must be picked up 
and transformed into an electrical signal by a receiving transducer called a 
hydrophone. Current practice favors the use of piezoelectric ceramics for 
these instruments. Transducer elements are usually arranged in arrays to 
achieve greater sensitivity, better directionality, and reduction of noise. 
Once the acoustical signal has been transformed into an electrical signal, the 
problem of extracting the information from the background noise becomes a 
problem in signal processing. See UNDERWATER TRANSDUCERS. 
    When background noise has no clearly identifiable source, it is referred 
to as ambient sea noise. It is caused by marine life, seismic disturbances, 
distant shipping, wave action, cracking ice, rain on the surface, and so on. 
Ambient noise has been studied over a frequency range from about 0.1 Hz to 
over 100 kHz. 
  
    Speed of sound.  The speed of sound in the ocean varies roughly from 4750 
to 5100 feet per second (1448 to 1554 meters per second, or mps; about 3% 
higher than in fresh water), depending on pressure, temperature, and salinity 
(see table). 
    Absorption of sound.  Sound is absorbed in sea water when an irreversible 
production of heat occurs during the alternate compression and rarefaction 
accompanying the sound wave. Absorption is greater in sea water than in fresh 
water over a certain frequency range because a dissociation-reassociation 
process occurs with the ions of magnesium sulfate. There is a low-frequency 
relaxation in the 1-kHz region which is believed to be due to a very small 
amount of boric acid in sea water. 
  
    Transmission loss.  In addition to the irreversible losses in the medium, 
a reduction in signal intensity due to the spreading of the wave also occurs. 
If there is a simple, omnidirectional source in a homogeneous, unbound medium 
the intensity varies as the reciprocal of the range squared. When the medium 
has plane-parallel upper and lower boundaries, the spreading is cylindrical 
and varies as the reciprocal of the range. The latter type of spreading, with 
its lower attenuation, occurs when sound is trapped in a ``channel.'' 
  
    Refraction of sound.  Because of variations of sound speed with depth, 
sound rays are bent as they propagate from point to point. This effect becomes 
very important when horizontal propagation distances exceed a few hundred 
feet. The profile of speed versus depth depends on the geographical location, 
the season of the year, and even the time of day. An idealized profile is 
shown in the illustration. Just below the surface is the surface layer in 
which sound speed is susceptible to daily and local changes due to heating, 
cooling, and wind action. Below the surface layer lies the seasonal 
thermocline, where the temperature decreases with depth, giving a negative 
gradient. This region is close enough to the surface to be affected by 
seasonal changes. Below this is the main thermocline, which is little affected 
by the season, where the speed decreases rapidly with depth. Between 3000 and 
4000 ft or 0.9 and 1.2 km (at mid-latitudes) a point is reached at which the 
temperature attains a constant value and the speed a minimum; from there to 
the bottom the speed increases because of the pressure effect. The region 
below the axis is called the deep isothermal layer. In the Arctic regions this 
layer may lie at or near the ice-covered surface.  See  THERMOCLINE. 
  
    The depth at which the speed reaches a minimum defines the axis of the 
deep sound (or sofar) channel. A sound wave generated in this channel remains 
trapped in it and travels (by cylindrical spreading) for long distances. An 
explosion in this channel will be heard as a drawn-out signal that ceases 
abruptly with the passing of the slowest ray, the one which traveled directly 
along the axis. From this clearly defined time marker the exact location of a 
deep channel explosion can be determined by triangulating with two monitoring 
stations. This method was used during World War II by downed aviators, who 
dropped sofar bombs to give their location. It is also used in determining 
missile impact locations. 
  
    Reflection of sound.  Reflection occurs when sound strikes the sea 
surface, the sea bottom, or large obstacles in the water. If the reflecting 
surface is flat over distances that are large compared with the wavelength of 
sound, a fraction of the incident sound energy is reflected at an angle with 
respect to the normal which is equal to the angle of incidence. The amount of 
energy reflected depends on the acoustic properties of the materials involved. 
    The reflection from the surface of the ocean is essentially perfect. 
However, if the wavelength is small compared to the dimensions of the surface 
roughness, a well-defined reflected ray does not exist and the energy is 
scattered.  
    The reflection of sound from the sea bottom is similar to that from the 
surface but more complicated. The composition of the bottom ranges from soft 
mud through sand to hard rock at different locations, causing considerable 
variation in sound-reflecting and sound-scattering properties. 
  
    Scattering of sound.  Scattering is the process by which sound energy is 
redirected from objects (or parts of objects) which have dimensions 
considerably smaller than a wavelength. The scattering from the surface and 
bottom has already been mentioned, but scattering from objects suspended in 
the water is also important. Contributions to such scattering are made by 
marine organisms, free air bubbles, suspended particles, the thermal 
microstructure in the sea, and the turbulence of natural or human origin.  See 
  SCATTERING LAYER. 
 �34s[William S. Cramer]
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�6v�2v�32sUnderwater telephone   �21m �33s 
    A method of voice communication using underwater sound as the means of 
transmission. The sound is generated by a projector at the sending point and 
is received by a hydrophone. Generally, a carrier wave of a fixed frequency is 
amplitude-modulated by the voice signal and demodulated in the receiving 
system. The range of an underwater telephone is fairly short (perhaps a few 
miles) because of transmission losses in the sound propagation. Its most 
important use is communication with, or between, fully submerged submarines.  
See  UNDERWATER SOUND; UNDERWATER TRANSDUCER. 
 �34s[Robert W. Morse]
�33s 
  
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�6v�2v�32sUnderwater television   �21m �33s 
    The technique of using television equipment underwater. Underwater 
television picture quality is limited by clarity of the water, available 
light, and the sensitivity of the camera tube. Television color cameras 
require white artificial light at depth to achieve the color image. Monochrome 
cameras can be used with a light source color-matched to the peak sensitivity 
of the camera tube.  See  TELEVISION. 
    Television equipment used in underwater observation is compact and 
reliable and can be tailored for specific applications. The major advantages 
of underwater television over conventional photography are that the observer 
can remain at the surface while viewing a continuously monitored scene in real 
time. Observations can be made and recorded for long periods on video tape 
either in real time or by using time-lapse video recorders. Additional 
information such as numerical data or a time clock can be superimposed on the 
picture screen, and other information can be recorded on the video tape in the 
form of sound tracks. Free-swimming divers can operate a television camera in 
much the same way as a movie camera.  See  UNDERWATER PHOTOGRAPHY. 
    Underwater television has applications in fishery research for behavioral 
studies of fish in the wild, and in the oil industry for inspection of oil 
pipes, well heads, and platform structures and for seabed surveys. Remotely 
controlled vehicles to carry television cameras exist in great variety and are 
continuing to be adapted and developed for special purposes.  See  
OCEANOGRAPHIC SUBMERSIBLES. 
 �34s[Robert Priestley]
�33s 
  
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�6v�2v�32sUnderwater transducer   �21m �33s 
    A device used for the generation or reception of underwater sounds. The 
word projector is applied to a generator of sound, while hydrophonic refers to 
a receiver. Since the same device may serve both purposes, the word transducer 
is used as a general designation. Thus a transducer, when a projector, 
converts electrical energy to motional mechanical energy, whereas a hydrophone 
converts mechanical energy to electrical energy. This conversion of energy is 
usually based on one of the following properties of certain materials: 
piezoelectricity, magnetostriction, and electrostriction. In each case, the 
application of an alternating electric or magnetic field of a given frequency 
causes a mechanical vibration of the transducer material at the same 
frequency. The transducer, being in contact with the water, communicates a 
similar motion to it, giving rise to a sound wave. With a hydrophone, the 
vibratory sound pressure causes a mechanical motion of the material, which in 
turn generates an electrical voltage. This voltage is amplified, then either 
read on a meter, recorded, or played through a loudspeaker.  See  
ELECTROSTRICTION; MAGNETOSTRICTION; PIEZOELECTRICITY; SONAR; TRANSDUCER; 
UNDERWATER SOUND. 
 �34s[Robert W. Morse]
�33s 
  
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�6v�2v�32sUniform flow   �21m �33s 
    Fluid flow in which, at a given instant, the velocity is the same in both 
magnitude and direction at every point in space. This strict definition can 
have little meaning for the flow of a real fluid when the velocity varies 
across a section. However, when the size and shape of cross section are the 
same in any given length, the flow is said to be uniform. Specifically, the 
flow in a pipe of constant diameter is uniform; the flow in a pipe of varying 
size is not. Also, the flow in an open canal is uniform if the size and shape 
of the cross section of the stream are the same at different locations along 
the canal. See FLUID FLOW; OPEN CHANNEL. 
 �34s[Robert L. Daugherty]
�33s 
  
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�6v�2v�32sUnitary symmetry �21m �33s
    One of the approximate internal symmetry laws obeyed by the strong
interactions of elementary particles. A system of particles has an SU(sub n)
internal symmetry if all of the particles can be described as compounds of a
fundamental multiplet of n particles, and if all physical properties of the
system are unchanged by an arbitrary unitary transformation of the fundamental
multiplet.  See  SYMMETRY LAWS (PHYSICS).
    An analog is the approximate spin independence of electrons under
electrostatic forces (as in an atom): There is a fundamental doublet, namely
the spin-up electron and the spin-down electron. Denoting these two states by
VERTICAL BAR u> and VERTICAL BAR d>, all physical properties (energy
eigenvalues, charge density, and so on) are unchanged by the replacements
shown in the equations below, where alpha and beta are complex numbers. The
group

|u> -->alpha|u> + beta|d> |d>-->-beta*|u> + alpha*|d>|alpha|(sup 2) + |beta|
		(sup 2) = 1

of all the transformations of two states which preserve their scalar products
[<uVERTICAL BARd> = 0, <uVERTICAL BARu> = <dVERTICAL BARd> = 1] is known as
the two-dimensional unitary group, U2; the transformations of the equations
above form a subgroup known as SU2 which merely lacks the uninteresting
transformations of the form VERTICAL BARu> -> e(sup i) phiVERTICAL BARu> and
VERTICAL BARd> -> e(sup i) phi VERTICAL BARd>, that is, an equal change of
phase of the two states.
    The strong interactions are approximately invariant to such a group; the
fundamental doublet can be taken to be the nucleon, with the up and down
states proton and neutron. This SU2 symmetry is known as independence, or,
loosely, as i-spin conservation, the analog to the electron spin being known
as i-spin BOLD I.  See	ISOTOPIC SPIN.
    When a sufficient number of strange particles had been observed, it was
seen that they, together with the old nonstrange particles, were grouped into
multiplets whose members had the same space-time quantum numbers (except for
mass; the masses of the members are only similar, not equal). This suggested
the existence of a yet larger symmetry; it has turned out that this symmetry
is the group of unitary transformations of a triplet of fundamental particles,
SU3. This symmetry is often loosely called unitary symmetry.
    A striking difference in the manifestations of SU2 and SU3 is that whereas
all possible multiplets of the former appear in nature, only those multiplets
of the latter appear which can be regarded as compounds of the fundamental
triplet in which the net number of component fundamental particles (number of
particles minus number of antiparticles) is an integral multiple of 3. In
particular, no particle which could be regarded as the fundamental triplet is
found. Despite this nonappearance, it turns out that a great deal about the
strongly interacting particles (hadrons) is at least qualitatively explained
if they are regarded as physical compounds of a fundamental triplet of
particles, to which the name quark has been given. There are interesting
theories to explain why single quarks are never observed.  See	HADRON; QUARKS.
    Hadrons have the approximate symmetry SUsubN, where N is the number of
kinds of quarks, or flavors. Five flavors of quark are known.  See  FLAVOR.
    In the quark model of hadrons, baryons are correctly described only if
quarks carry, in addition to spin and flavor, another quantum number, color,
which can take on three values. The resulting symmetry, color SU3, is thought
to be exact; that is, the differently colored quarks of a given flavor are
thought to be absolutely equivalent. However, this exact symmetry is rather
well hidden, because apparently all free particles belong to the singlet
representation of the symmetry.  See  COLOR (QUANTUM MECHANICS); ELEMENTARY
PARTICLE.
 �34s[Charles J. Goebel]
�33s

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�6v�2v�32sUnit operations   �21m �33s 
    A concept used by chemical engineers to teach and practice industrial 
processing. The basis of the unit operation approach is that chemical 
processes, in spite of their great variety, may be resolved into a 
comparatively small number of units, each of which appears again and again in 
the various processes. Each unit operation performs a definite function no 
matter where it is used, and the individual operations cut across industry and 
process lines (see table). For example, distillation, one of the important 
operations, is used where it is necessary to separate the constituents of a 
mixture of volatile liquids and where the separation may be achieved by a 
system of vaporizations and condensations. This operation is used in petroleum 
refining, alcohol manufacture, liquid gas production, refining of solvents, 
and many other industries. Knowledge of the fundamentals of distillation is 
applicable, then, to all these industries. This is characteristic of all unit 
operations.  
    The theoretical foundations of the unit operations are useful only because 
they lead to sound designs of the equipment in an actual plant. There is 
greater variety in the equipment than in the theoretical laws. Pumps, piping 
systems, dryers, heat transfer equipment, distilling columns, crushers, 
mixers, absorbers, evaporators, conveyors, and centrifuges are typical classes 
of unit operations equipment. The final objective of the unit operation 
approach is to provide the plant designer with sound selections and designs of 
practical apparatus for efficient production. See CHEMICAL ENGINEERING. 
 �34s[Warren L. McCabe]
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�6v�2v�32sUnits of measurement �21m �33s
    Values, quantities, or magnitudes in terms of which other such are
expressed. Units are grouped into systems, suitable for use in the measurement
of physical quantities and in the convenient statement of laws relating
physical quantities. A quantity is a measureable attribute of phenomena or
matter.
    A given physical quantiy A, such as length, time, or energy, is the
product of a numerical value or measure {A} and a unit [A]. Thus Eq. (1) holds.

			    A = {A} [A] 				    (1)

    The unit [A] can be chosen arbitrarily, but it is desirable to define
units in such a way that they are derived from a few base units by equations
without numerical factors other than unity, and that the equations between
numerical values of quantities have exactly the same form as the equations
between the quantities. For example, the kinetic energy E of a body is given
in terms of its mass M and speed V by Eq. (2),

			     E = 1/2 MV(sup 2)				     (2)

where E = {E} [E], M = {M} [M], V = {V} [V], and 1/2 is called a definitional
factor and is dimensionless. If the units of E, M, and V are defined in such a
way that Eq. (3) holds, then

			     E = [M] [V](sup 2) 			     (3)

the equation between the numerical values is Eq. (4). A system of units

			     {E} = 1/2 {M} {V}sub2			    (4)

defined in this way is called a coherent system. It is constructed by defining
the unit of a few base quantities independently; these are called base units.
The units of all other quantities are defined by equations similar to Eq. (3)
with no numerical factors other than unity, and are called derived units.
    In 1960 the General Conference on Weights and Measures (CGPM) gave
official status to a single practical system, the International System of
Units, abbreviated SI in all languages. The system is a modernized version of
the metric system. The SI, as subsequently extended, includes seven base
units, two supplementary units, and nineteen derived units with special names.
These derived units, and others without special names, are derived from the
base and supplementary units in a coherent manner. A set of prefixes is used
to form decimal multiples and submultiples of the SI units. Certain units
which are not part of the SI but which are widely used or are useful in
specialized fields have been accepted for use with the SI or for temporary use
in those fields.  See  METRIC SYSTEM.

    Geometrical units.	Units of plane angle and solid angle are purely
geometrical. The SI units of plane and solid angle are called supplementary
units, and may be regarded either as base units or as derived units.

    Plane angle units. The radian (rad), the SI unit of plane angle, is the
plane angle between two radii of a circle which cut off on the circumference
an arc equal in length to the radius. Since the circumference of a circle is
2PI times the radius, the complete angle about a point is 2PI rad.  See
RADIAN MEASURE.
    The degree and its decimal submultiples can be used with the SI when the
radian is not a convenient unit. By definition, 2PI rad = 360 DG. The minute
[1' = (1/60) DG] and the second [1'' = (1/60)'] can also be used.

    Steradian. The steradian (sr), the SI unit of solid angle, is the solid
angle which, having its vertex at the center of a sphere, cuts off an area on
the surface of the sphere equal to that of a square with sides of length equal
to the radius of the sphere.

    Mechanical units.  In mechanics, it is convenient to have three base
quantities, and two of these are generally chosen to be length and time.
Systems of mechanical units may be classified as absolute systems, in which
the third base quantity is mass, and gravitational systems, in which the third
base quantity is force.
    Two absolute systems of metric units are commonly employed, each named for
its base units of length, mass, and time: the mks (meter-kilogram-second)
absolute system, and the cgs (centimeter-gram-second) absolute system. The mks
absolute system is the mechanical portion of the SI. A coherent absolute
system of British units is based on the foot, the pound (1 lb APPROXIMATELY
0.4536 kg), and the second. 
    Gravitational systems, in which the base quantities are length, force, and
time, have been frequently employed by engineers, and are therefore sometimes
called technical systems.

    Length units. The meter (m) is the SI base unit of length. The use of
special names for decimal submultiples of the meter should be avoided, and
units formed by attaching appropriate SI prefixes to the meter should be used
instead.
    The angstrom (A) is equal to 10(sup -10) m. Although it has been accepted
for temporary use with the SI, it is preferable to replace this unit with the
nanometer, using the relation 1 A = 0.1 nm.
    The nautical mile (nmi), equal to 1852 m, has been accepted for temporary
use with the SI in navigations.  See  NAVIGATION.
    The foot (ft) is, as discussed above, the unit of length in the British
systems of units, and it is also in customary use in the United States. Since
1959 the foot has been defined as exactly 0.3048 m. The yard (yd) is defined
as exactly 3 ft or 0.9144 m.  See  LENGTH; MEASURE.
    Relative measurements of x-ray wavelengths can be made to a higher
accuracy than absolute measurements. Before 1965, most x-ray wavelengths were
expressed in terms of the X-unit, which is approximately 10(sup -13) m. The
X-unit has been superseded by the A* unit, which is based on the tungsten
K ALPHA(sub 1) line as a standard. The peak of this line is defined as exactly
0.2090100 A*. X-ray wavelength tables have been published in terms of this
unit. At the time the A* unit was defined, it was thought to equal 10(sup -10)
m (the angstrom unit, A) to within 5 parts per million, but the A* unit is now
believed to be 20 PLUS/MINUS 5 parts per million larger than 10(sup -10) m.
    Special units whose values are obtained experimentally are used in
astronomy. For their definitions  see  ASTRONOMICAL UNIT; LIGHT-YEAR; PARSEC.

    Area units. The square meter (m sup 2), the SI unit of area, is the area of
a square with sides of length 1 m. Other area units are defined by forming
squares of various length units in the same manner.  See  AREA.
    Cross sections, which measure the probability of interaction between an
atomic nucleus, atom, or molecule and an incident particle, have the
dimensions of area, and the appropriate SI unit for expressing them is
therefore the square meter. The barn (b), a unit of cross section equal to
10(sup -28) m(sup 2), has been accepted for temporary use with the SI.

    Units of volume. The cubic meter (m sup 3), the SI unit of volume, is the
volume of a cube with sides of length 1 m. Other units of volume are defined
by forming cubes of various length units in the same manner. The liter (symbol
L in the United States) is equal to 1 cubic decimeter (1 dm sup 3), or
equivalently to 10(sup -3)(m sup 3). It has been accepted for use with the SI
for measuring volumes of liquids and gases.

    Time units. The second (s) is the SI base unit of time. However, other
units of time in customary use, such as the minute (1 min = 60 s), hour (1 h =
60 min), and day (1 d = 24 h), are acceptable for use with the SI.  See  TIME.

    Frequency units. The hertz (Hz), the SI unit of frequency, is equal to 1
cycle per second. A periodic oscillation has a frequency of n hertz if it goes
through n cycles in 1 s.  See  FREQUENCY (WAVE MOTION).

    Speed and velocity units. The meter per second (m/s), the SI unit of speed
or velocity, is the magnitude of the constant velocity at which a body
traverses 1 m in 1 s. Other speed and velocity units are defined by dividing a
unit of length by a unit of time in the same manner.  See  SPEED; VELOCITY.
    The knot (kn) is equal to 1 nautical mile per hour (1 nmi/h); it has been
accepted for temporary use with the SI.

   Acceleration units. The meter per second squared (m/s[sup 2]), the SI unit of
acceleration, is the magnitude of the constant acceleration of a body whose
velocity changes by 1 m/s in 1 s. Other units of acceleration are defined by
dividing a unit of velocity by a unit of time in the same manner.  See
ACCELERATION.
    The gal or galileo (symbol Gal) is equal to 1 cm/s(sup 2), or equivalently
to  10(sup -2) m/s(sup 2).

    Mass units. The kilogram (kg), the SI base unit of mass, is the only SI
unit whose name, for historical reasons, contains a prefix. Names of decimal
multiples and submultiples of the kilogram are formed by attaching prefixes to
the word gram (g). The metric ton (t), which is equal to 10sup3 kg or 1
megagram (Mg), is permitted in commercial usage of the SI.
    The pound (lb), the unit of mass in the British absolute system, is also
in customary use in the United States. In 1959 the pound was defined to be
exactly 0.45359237 kg.
    The slug is the unit of mass in the British gravitational system. By
definition 1 pound force (lbf) acting on a body of mass 1 slug produces an
acceleration of 1 foot per second squared (1 ft/s[sup 2]). The slug is equal to
approximately 32.174 lb or 14.594 kg.  See  MASS.

    Force units. The newton (N), the SI unit of force, is the force which
imparts an acceleration of 1 meter per second squared (1 m/s[sup 2]) to a body
having a mass of 1 kg.
    The dyne, the cgs absolute unit of force, is the force which imparts an
acceleration of 1 centimeter per second squared (1 cm/s[sup 2]) to a body having
a mass of 1 g.
    The unit of force in the British absolute system is the poundal (pdl), the
force which imparts an acceleration of 1 foot per second squared (1 ft/s[sup 2])
when applied to a body of mass 1 lb. One poundal is approximately 0.13825 N.
    The units of force in the mks gravitational, cgs gravitational, and
British gravitational systems are the forces which impart an acceleration
equal to the standard acceleration of gravity, gsubn = 9.80665 m/s(sup 2)
NEARLY EQ 32.174 ft/s(sup 2), when applied to bodies having masses of 1 kg, 1 g,
and 1 lb, respectively. These units are named the kilogram force (kgf), gram
force (gf), and pound force (lbf), respectively. Unfortunately, these units
have also been called simply the kilogram, gram, and pound, giving rise to
confusion with the mass units of the same name.  See  FORCE.

    Pressure and stress units. The pascal (Pa), the SI unit of pressure and
stress, is the pressure or stress of 1 newton per square meter (N/m[sup 2]).
Other units of pressure can also be formed by dividing various units of force
by various units of area, such as the pound force per square inch
(lbf/in.[sup 2], frequently abbreviated psi).
    Pressure has been frequently expressed in terms of the bar and its decimal
submultiples, where 1 bar = 10(sup 6)dynes/cm(sup 2) = 10(sup 5) Pa. Pressures
are also frequently expressed in terms of the height of a column of either
mercury or water which the pressure will support.
    Two other units which have been frequently used for measuring pressure are
the standard atmosphere and the torr. The standard atmosphere (atm) is exactly
101,325 Pa, which is approximately the average value of atmospheric pressure
at sea level. The torr is exactly 1/760 atmosphere, or approximately 133.322
Pa. To within 1 part per million, the torr equal to the pressure of a column
of mercury of height 1 millimeter (1 mmHg) at a temperature of 0DGC when the
acceleration due to gravity has the standard value gsubn = 9.80665 m/s(sup 2).
See  ATMOSPHERE; PRESSURE; PRESSURE MEASUREMENT.

    Energy and work units. The joule (J), the SI unit of energy or work, is
the work done by a force of magnitude 1 newton when the point at which the
force is applied is displaced 1 m in the direction of the force. Thus, joule
is a short name for newton-meter (N-m) of energy or work.  See	ENERGY; WORK.
    Units of energy or work in other systems are defined by forming the
product of a unit of force and a unit of length in precisely the same manner
as in the definition of the joule. Thus, the erg, the cgs absolute unit of
energy or work, is the product of 1 dyne and 1 cm.
    The foot-poundal (ft-pdl), the British absolute unit of energy or work, is
the product of 1 poundal and 1 foot. The foot-pound, or, more properly, the
foot-pound force (ft-lbf), the British gravitational unit of energy or work,
is the product of 1 lbf and 1 ft.
    Sometimes energy is measured in units which are products of a unit of
power and a unit of time. Since 1 watt (W) of power equals 1 joule per second
(1 J/s), as discussed below, the joule is equivalent to 1 watt-second (1 W.s).
In electrical power applications, energy is frequently measured in
kilowatthours (kWh), where 1 kWh = (10[sup 3]W) (3600 s) = 3.6 x 10[sup 6] J.
See  ELECTRICAL ENERGY MEASUREMENT.
    The calorie was originally defined as the quantity of heat required to
raise the temperature of 1 g of air-free water 1 DG C under a constant pressure
of 1 atm. However, the magnitude of the calorie, so defined, depends on the
place on the Celsius temperature scale at which the measurement is made. The
International (Steam) Table calorie is defined as exactly 4.1868 J; this is
the type of calorie most frequently used in mechanical engineering. The
thermochemical calorie, which has been used in thermochemistry in preference
to the other types of calorie, is exactly 4.184 J.
    The British thermal unit (Btu) was originally defined as the quantity of
heat required to raise the temperature of 1 lb of air-free water 1 DG F under a
constant pressure of 1 atm. The International Table Btu is approximately
1055.056 J, and the thermochemical Btu is approximately 1054.350 J.

    Power units. The watt (W), the SI unit of power, is the power which gives
rise to the production of energy at the rate of  1 joule per second (1 J/s).
Other units of power can be defined by forming the ratio of a unit of energy
to a unit of time in the same manner.  See  POWER.
    The horsepower (hp) is equal to exactly 550 ft.lbf/s, or approximately
745.700 W.

    Torque units. The newton-meter (N.m), the SI unit of torque, is the
magnitude of the torque produced by a force of 1 newton acting at a
perpendicular distance of 1 m from a specified axis of rotation. The joule
should never be used as a synonym for this unit.
    Units of torque in other systems are defined by forming the product of a
unit of force and a unit of length in precisely the same manner as in the
definition of the newton-meter.
    The foot-poundal (ft.pdl), the British absolute unit of torque, is the
product of 1 poundal and 1 foot. The foot-pound (ft.lbf), the British
gravitational unit of torque, is the product of 1 lbf and 1 ft. These units
are sometimes called the poundal-foot (pdl.ft) and pound-foot (lbf.ft) to
distinguish them from the units of energy or work. See TORQUE.

    Electrical units.  For a general discussion of electrical units, including
the SI or mks system, three cgs systems [electrostatic system of units (esu),
electromagnetic system of units (emu), and gaussian system], and definitions
of the SI units ampere (A), volt (V), ohm (OMEGA), coulomb (C), farad (F),
henry (H), weber (Wb), and tesla (T)  see  ELECTRICAL UNITS AND STANDARDS.
    This section discusses some additional SI units and some units in the cgs
electromagnetic system which are frequently encountered in scientific
literature in spite of the fact that their use has been discouraged.

    Siemens. The siemens (S), the SI unit of electrical conductance, is the
electrical conductance of a conductor in which a current of 1 ampere is
produced by an electric potential difference of 1 volt.  See  CONDUCTANCE;
ELECTRICAL RESISTANCE.
    The siemens was formerly called the mho (j) to illustrate the fact the
unit is the reciprocal of the ohm.

    Abampere. The abampere (abA), the cgs electromagnetic unit of current, is
that current which, if maintained in two straight, parallel conductors of
infinite length, of negligible circular cross section, and placed 1 cm apart
in vacuum, would produce between these conductors a force equal to 2 dynes per
centimeter of length. The abampere is equal to exactly 10 A.  See  CURRENT
(ELECTRICITY).

    Abvolt. The abvolt (abV), the cgs electromagnetic unit of electrical
potential difference and electromotive force, is the difference of electrical
potential between two points of a conductor carrying a constant current of 1
abA, when the power dissipated between these points is equal to 1 erg per
second. Then 1 abV = 10(sup -8) V.

    Maxwell. The maxwell (Mx), the cgs electromagnetic unit of magnetic flux,
is the magnetic flux which, linking a circuit of one turn, produces in it an
electromotive force of 1 abV as it is reduced to zero in 1 s. Then 1 maxwell =
10-sup8 weber.	See  MAGNETIC FLUX.

    Units of magnetic flux density. The gauss (Gs), the cgs electromagnetic
unit of magnetic flux density (also called magnetic induction), is a magnetic
flux density of 1 maxwell per square centimeter (1 Mx/cmsup2). Then 1 gauss =
10-sup4 tesla.	See  MAGNETIC INDUCTION.

    Units of magnetic field strength. The SI unit of magnetic field strength
is 1 ampere per meter (1 A/m), which is the magnetic field strength at a
distance of 1 m from a straight conductor of infinite length and negligible
circular cross section which carries a current of 2PI A. This definition is
based on the definition, in the SI, of the magnetic field strength H(sub SI).
At a distance r from a long straight conductor carrying I, H(sub SI) is given
by Eq. (5) The left-hand side of this equation is the line integral of HS

			2 PI rH(sub S)I = I				  (5)

I around a circular path, all of whose points are at distance r from the
conductor. Substituting r = 1 m, and I = 2PI A in this equation gives
H(sub SI) = 1 A/m.
    The oersted (Oe), the cgs electromagnetic unit of magnetic field strength,
is the magnetic field strength at a distance of 1 cm from a straight conductor
of infinite length and negligible circular cross section which carries a
current of 0.5 abA.  See  MAGNETIC FIELD.

    Units of magnetic potential and mmf. The ampere serves as the SI unit of
magnetic potential difference and magnetomotive force (mmf), as well as the
unit of current. In the SI, the magnetomotive force around a closed path
equals the current passing through a surface enclosed by the path. Thus, 1 A
is the magnetomotive force around a closed path when a current of 1 A passes
through an enclosed surface.
    The gilbert (Gb), the cgs electromagnetic unit of magnetic potential
difference and magnetomotive force, is the magnetomotive force around a closed
path enclosing a surface through which flows a current of (1/4PI) abA.	See
MAGNETOMOTIVE FORCE.

    Photometric units.	Photometric units involve a new base quantity,
luminous intensity. For the definition of the candela (cd), the SI unit of
luminous intensity,  see  PHOTOMETRY; PHYSICAL MEASUREMENT.
    For a general discussion of photometric units, including units of
illuminance (illumination) and luminance, and in particular the SI units lux
(lx) and candela per square meter (cd/m(sup 2)),  see  ILLUMINATION.  See also
LUMINANCE.

    Lumen. The lumen (lm), the SI unit of luminous flux, is the luminous flux
emitted within a unit solid angle (1 steradian) by a point source having a
uniform intensity of 1 candela.  See  LUMINOUS FLUX.

    Luminous energy units. The lumen-second (lm.s), the SI unit of luminous
energy (also called quantity of light), is the luminous energy radiated or
received over a period of 1 s by a luminous flux of 1 lumen. This unit is also
called the talbot.  See  LUMINOUS ENERGY.

    Radiation units.  Certain quantities and units are used particularly in
the area of ionizing radiation. The special units curie, roentgen, rad, and
rem, which were previously adopted for use in this area, are not coherent with
the SI, but their temporary use with the SI has been approved while the
transition to SI units takes place.

    Activity units. The becquerel (Bq), the SI unit of activity (radioactive
disintegration rate), is the activity of a radionuclide decaying at the rate
of one spontaneous nuclear transition per second. Thus 1 Bq = 1 s(sup -1).
    The curie (Ci), the special unit of activity, is equal to 3.7 x 10(sup 10)
Bq.  See  RADIOACTIVITY.

    Exposure units. The SI unit of exposure to ionizing radiation, 1 coulomb
per kilogram (1 C/kg), is the amount of electromagnetic radiation (x-radiation
or gamma radiation) which in 1 kg of pure dry air produces ion pairs carrying
1 coulomb of charge of either sign. (The ionization arising from the
absorption of bremsstrahlung emitted by electrons is not to be included in
measuring the charge.)	See  BREMSSTRAHLUNG.
   The roentgen (R), the special unit of exposure, is equal to 2.58 x 10(sup -4)
C/kg.

    Absorbed dose units. The gray (Gy), the SI unit of absorbed dose, is the
absorbed dose when the energy per unit mass imparted to matter by ionizing
radiation is 1 joule per kilogram (1 J/kg).
    The rad (rd), the special unit of absorbed dose, is equal to 10(sup -2) Gy.

    Dose equivalent units. Different types of radiation cause slightly
different effects in biological tissue. For this reason, a weighted absorbed
dose called the dose equivalent is used in comparing the effects of radiation
on living systems. The dose equivalent is the product of the absorbed dose and
various dimensionless modifying factors.
    The sievert (Sv), the SI unit of dose equivalent, is the dose equivalent
when the absorbed dose of ionizing radiation multiplied by the stipulated
dimensionless factors is 1 joule per kilogram (1 J/kg).
    The rem, the special unit of dose equivalent, is equal to 10(sup -2) Sv.

    Other units.  Logarithmic measures may be used with the SI.  See  DECIBEL;
NEPER; PH; VOLUME UNIT (VU).
    For units of loudness (sone, phon)	see  LOUDNESS. For units of sound
absorption by surfaces	see  ARCHITECTURAL ACOUSTICS. For units of pitch  see
PITCH. For quantities and units pertaining to sound transmission  see  SOUND.
    For temperature scales  see  TEMPERATURE.
    For units measuring quantities in chemistry  see  ATOMIC MASS UNIT; ATOMIC
WEIGHT; CONCENTRATION SCALES; ELECTROCHEMICAL EQUIVALENT; GRAM-MOLECULAR
WEIGHT; MOLAR VOLUME; MOLE; MOLECULAR WEIGHT; RELATIVE ATOMIC MASS; RELATIVE
MOLECULAR MASS.
    For quantities and units pertaining to nuclear reactors  see  REACTOR
PHYSICS.
    For units of information content  see  BIT; INFORMATION THEORY.
 �34s[Jonathan F. Weil]
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�6v�2v�32sUniversal joint   �21m �33s 
    A linkage that transmits rotation between two shafts whose axes are 
coplanar but not coinciding. The universal joint is used in almost every class 
of machinery: machine tools, instruments, control devices, and, most 
familiarly, automobiles. 
    A simple universal joint, known in English-speaking countries as Hooke's 
joint and in continental Europe as a Cardan joint, is shown in the 
illustration. It consists of two yokes attached to their respective shafts and 
connected by means of a spider. The angle between the shafts may have any 
value up to approximately 35DG, if angular velocity is moderate when the angle 
is large. One shaft must make a single revolution for each revolution of the 
second shaft, A, the instantaneous angular displacement of the first shaft is 
the same as that of the second shaft only at the end of each 90DG shaft 
rotation. Thus, only at four positions during each revolution is angular 
velocity of both shafts the same. 
    The variation in angular displacement and angular velocity between driving 
and driven shafts, which is objectionable in many mechanisms, can be 
eliminated by using two Hooke's joints, with an intermediate shaft. This 
arrangement is conventional for an automobile drive shaft. The axes of the 
driving and driven shafts need not intersect; however, it is necessary that 
the axes of the two yokes attached to the intermediate shaft lie respectively 
in planes containing the axes of adjoining shafts.  See  FOUR-BAR LINKAGE. 
 �34s[Douglas P. Adams]
�33s 
  
  
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�6v�2v�32sUniversal motor   �21m �33s 
    A series motor built to operate on either alternating current (ac) or 
direct current (dc). It is normally designed for capacities less than 1 hp 
(0.75 kW). It is usually operated at high speed, 3500 revolutions per minutes 
(rpm) loaded and 8000 to 10,000 revolutions per minute unloaded. For lower 
speeds, reduction gears are often employed, as in the case of electric hand 
drills or food mixers. As in all series motors, the rotor speed increases as 
the load decreases and the no-load speed is limited only by friction and 
windage.  See  ALTERNATING-CURRENT MOTOR; DIRECT-CURRENT MOTOR. 
 �34s[Irving L. Kosow]
�33s 
  
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�6v�2v�32sUniverse �21m �33s
    The sum of all the matter and energy that exist. The true scale of the
universe is even now only dimly perceived by astronomers, despite tremendous
advances in both observing techniques and equipment. The planet Earth is small
compared with the Sun, which is only one of a hundred billion stars in the
Milky Way Galaxy. And as large as this ``island universe'' is, it too is small
compared with the clusters of thousands of such galaxies which populate the
universe out to and (undoubtedly) beyond the range of astronomical vision.
See  GALAXY.
    The immense size of the universe and the great distances between objects
introduce some fundamental limitations upon observations of remote galaxies.
The basic restriction is caused by the finite speed of light in space. Light
travels at 186,000 mi/s (300,000 km/s). Light from very distant galaxies has
been traveling across space for a large fraction of the age of the universe. A
galaxy 10(sup 10) light-years (9.46 x 10[sup 22] km) away is seen as it was
when the universe was half its present age; such a galaxy may have changed
significantly during this interval. The universe and its galaxies can
therefore be studied at earlier and earlier epochs merely by observing to
greater and greater distances.
    The observation of spectral lines from distant galaxies shows them to be
shifted from their normal laboratory wavelengths toward the red end of the
spectrum. This so-called Doppler shift is due to the motion of these galaxies
away from the Milky Way. E. P. Hubble showed that this is due to general
expansion of the universe, and demonstrated that there is a relation between
velocity v of a galaxy and its distance d, given by v = Hsubod, where Hsubo,
the constant of proportionality, is called the Hubble constant, and is a
measure of the rate of expansion.
    On the scale of atoms, the universe is over 90% hydrogen. Most of these
atoms are collected into gaseous stars which, like the Sun, are transforming
hydrogen into helium by means of nuclear fusion reactions in their extremely
hot interiors. This hydrogen burning is therefore slowly changing the
composition of the universe. Stars are collected into vast galaxies; the Milky
Way Galaxy alone contains over 10(sup 11) stars. Galaxies in turn are grouped
into clusters of galaxies; each cluster may contain thousands of galaxies,
each having billions of stars. See GALAXY, EXTERNAL; PLANET; STAR; SUN.
    The universe is dynamic. The early observation that all galaxies are
receding led to the now well-accepted conclusion that the universe is
expanding. This expansion implies that the universe is evolving. There are
several ways to determine a rough value for the age of the universe. The
universe must be older than the Milky Way Galaxy, whose age is estimated at
something like 1.4 x 10(sup 10) years, based on studies of the oldest known
stars. Extrapolating the expansion backwards (assuming that the observed rate
has been constant since the expansion started), all of the observed galaxies
would have started from the same point about 2 x 10(sup 10) years ago. The
expansion rate was probably greater in the early history of the universe; the
age of the universe is therefore somewhat less than the 2 x 10(sup 10) years
given by the Hubble expansion time.
    One explanation for the observed expansion of the universe is the ``big
bang'' theory, in which all of the matter and radiant energy of the universe
were initially compressed into a single object. At the assumed temperatures of
some trillions of degrees, matter could not have existed in its present form,
but consisted of subatomic or even subnuclear particles. This ``primordial
egg'' exploded, hurling matter and radiation outward in a slowly decreasing
expansion that led to rapidly dropping temperatures. See BIG BANG THEORY.
    How long the universe will continue to expand will depend on whether or
not the ``big bang'' explosion was stronger than the force of gravity due to
all the matter of the universe. If the explosion was more powerful than the
gravity of the universe, then the universe has ``escape velocity'' and will
expand forever. Such an ``open'' universe is infinite in both time and space.
The other extreme is given by the reverse situation: the total density of
matter in the universe is so great that its gravity is greater than the
explosion-induced expansion. In this case the universe will eventually come to
rest, then reverse and fall back on itself. Conditions will approach those of
the previous ``big bang.'' It seems reasonable that the universe will then
``bounce'' or explode in another ``big bang'' and start over. Such an
``oscillating'' universe is finite in extent but infinite in time. The third
possibility is that the expansion exactly balances the force of gravity. The
universe will then expand and stop, remaining motionless for all time.
    The universe appears to be open, and the best evidence suggests that the
expansion will continue. Refutation of this would necessitate the discovery of
large amounts of previously unknown mass in the universe. See COSMOLOGY.
 �34s[Herbert Frey]
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�6v�2v�32sUnsteady flow   �21m �33s 
    Fluid flow in which conditions are changing with respect to time and 
ultimately may become either steady flow or zero flow. This changing rate of 
flow may take place slowly, as when the action of a valve in a channel 
produces a gradual change in the rate, or it may take place rapidly as a 
result of a sudden closure, which produces a phenomenon known as water hammer. 
It is also found in such a case as the flow from one reservoir to another, in 
which equilibrium is approached as the two levels approach each other. See 
WATER HAMMER. 
    Unsteady flow also includes periodic motion such as that of waves on 
beaches, tidal motion in estuaries, and other oscillations. The difference 
between such cases and mean steady flow in turbulent flow is that the 
deviations from the mean are much greater and the time scale is also much 
longer. See FLUID FLOW; TURBULENT FLOW. 
 �34s[Robert L. Daugherty]
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�6v�2v�32sUpper-atmosphere dynamics   �21m �33s 
    The motion of the atmosphere above 30 mi (50 km). The predominant 
dynamical phenomena of the upper atmosphere are quite different from those 
encountered in the lower atmosphere. Among those encountered in the lower 
atmosphere are cyclones, anticyclones, tropical hurricanes, thunderstorms and 
shower clouds, tornadoes, dust devils, and similar phenomena. Even the largest 
of these phenomena do not penetrate far into the upper atmosphere. Above an 
altitude of about 30 mi (50 km), the predominant dynamical phenomena are 
internal gravity waves, tides, sound waves (including infrasonic), turbulence, 
and large-scale circulation. 
    Except under meteorological conditions characterized by convection, the 
atmosphere is stable against small vertical displacements of small air 
parcels; this results from buoyancy forces that tend to restore displaced air 
parcels to their original levels. An air parcel therefore tends to oscillate 
around its undisturbed position at a frequency known as the Brunt-Vaisala 
frequency, OMEGAsubB. Typical periods for the Brunt-Vaisala oscillation are in 
the vicinity of 5 min. If pressure waves are generated in the atmosphere with 
frequencies much greater than OMEGAsubB, they propagate as sound waves. For 
frequencies much less than OMEGAsubB, the waves propagate as internal gravity 
waves; in this case, the restoring forces for the wave motion are provided 
primarily by buoyancy (that is, gravity) rather than by compression. 
    Tides are internal gravity waves of particular frequencies. The term tidal 
usually implies that the exciting force is gravitational attraction by the 
Moon or Sun. However, it is conventional in the case of atmospheric tides to 
include also those waves that are excited by solar heating. One is therefore 
concerned with three separate excitation functions - lunar gravitation, solar 
gravitation, and solar heating. 
    Tidal wind patterns in the upper atmosphere generate electrical currents 
in the ionosphere through a dynamo action. These in turn give rise to diurnal 
variations in the geomagnetic field that can be observed at the Earth's 
surface. See ATMOSPHERIC TIDES; GEOMAGNETIC VARIATIONS. 
    Sound waves generated in the lower atmosphere may propagate upward. 
However, higher temperatures in the upper atmosphere refract most of the 
energy back toward the Earth's surface, giving rise to the phenomenon known as 
anomalous propagation. This involves the redirection of upward-moving sound 
waves back to the surface beyond the point where the source (usually a large 
explosion) can be heard by waves propagating along the surface. Thus there is 
normally a zone of silence, beyond which the source can again be heard because 
of anomalous propagation. Infrasonic waves with periods from 20 to 80 s have 
been observed occasionally with detectors at the Earth's surface in connection 
with auroral activity. See SOUND. 
    There is clear visual evidence of turbulence in the upper atmosphere; this 
evidence is obtained by examination of vapor trails released from rockets or 
of long persisting meteor trails. The source of the turbulence is not clear. 
The atmosphere is thermodynamically stable against vertical displacements 
throughout the region above the troposphere, and work has to be done against 
buoyancy forces in order to produce and maintain turbulence. The only apparent 
source of energy is internal gravity waves, either tidal or of random period. 
    Meteorological circulations and disturbances extend up to altitudes of 
about 30 mi (50 km). However, at higher altitudes, internal gravity waves and 
tides become more prominent. Above 60 mi (100 km), the circulation is not very 
well understood. 
 �34s[Francis S. Johnson]
�33s 
  
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�6v�2v�32sUpsilon particles �21m �33s
    A family of subnuclear particles having about 10 times the mass of the
proton and generally described as the atomlike combination of a new quark with
its antiquark. The new quark, designated b quark (for bottom or beauty), is
about five times as massive as a proton and has a charge one-third that of the
electron.
    The discovery of the b quark led to a revision of the ideas associated
with quarks and leptons and an ordering into three ``generations'' of
quark-lepton groupings:

Generation   I	     up quark		     electron
		     down quark 	     electron neutrino
Generation  II	     charmed quark	     muon
		     strange quark	     muon neutrino
Generation III	     top quark		     tau
		     bottom quark	     tau neutrino

The tau lepton was discovered in 1977.	A vigorous search for the top quark
has been unsuccessful so far.  It is now frequently assumed that all matter
is composed of various combinations of these objects.  See ELEMENTARY PARTICLE;
LEPTON; QUARKS.
 �34s[Leon M. Lederman]
�33s

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�6v�2v�32sUpwelling   �21m �33s 
    The phenomenon by which deep, nutrient-rich ocean waters are introduced 
into the well-mixed surface layer. The net upwelling of deep ocean water must 
equal the net production of deep ocean water; each year about 0.1% of the 
ocean water is cooled in the polar regions and sinks to form deep water. This 
must be offset by an average oceanic upwelling rate of about 13 ft/year (4 
m/year). A small but significant proportion of the total oceanic upwelling, 
perhaps as much as 20%, is concentrated in coastal and equatorial areas where 
seasonal upwelling rates may exceed 330 ft/month (100 m/month). See SEA WATER. 
    The geologic significance of upwelling areas results from their high 
productivity of phytoplankton. Fixation of silica by diatoms and its removal 
from the system by incorporation of the diatom frustules into the sediment 
constitute a major sink for silica in the ocean. See DIATOM; MARINE SEDIMENTS; 
PHYTOPLANKTON.  
 �34s[William W. Hay]
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�6v�2v�32sUraninite   �21m �33s 
    The chief ore mineral of uranium. Uraninite has the idealized chemical 
composition UO2, uranium dioxide. Thorium and rare earths, chiefly cerium, are 
usually present in variable and sometimes large amounts. Lead always is 
present by radioactive decay of the thorium and uranium present. Complete 
solid-solution series extend between UO2, ThO2 (thorianite), and CeO2 
(cerianite). See RADIOACTIVE MINERALS; RARE-EARTH ELEMENTS; THORIANITE; 
URANIUM.  
    The color of uraninite is black, grading to brownish black and dark brown 
in the more highly oxidized material. The luster of fresh material is 
steel-gray. The hardness is 5** - 6 on Mohs scale. The specific gravity of 
pure UO2 is 10.9, but that of most natural material is 9.7 - 7.5. 
 �34s[Clifford Frondel]
�33s 
  
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�6v�2v�32sUranium �21m �33s
    A chemical element, symbol U, atomic number 92, atomic weight 238.03. The
melting point is 1132DGC (2070DGF) and the boiling point is 3818DGC (6904DGF).
Uranium is one of the actinide series. See ACTINIDE ELEMENTS.
    Uranium in nature is a mixture of three isotopes: (sup 234)U,
(sup 235)U, and (sup 238)U. Uranium is believed to be concentrated
largely in the Earth's crust, where the average concentration is 4 parts per
million (ppm). The total uranium content of the Earth's crust to a depth of 15
mi (25 km) is calculated to be 2.2 x 10(sup 17) lb (10[sup 17] kg); the oceans
may contain 2.2 x 10(sup 13) lb (10[sup 13] kg) of uranium. Several hundred
uranium-containing minerals have been identified, but only a few are of
commercial interest. See RADIOACTIVE MINERALS; URANINITE.
    Because of the great importance of the fissile isotope (sup 235)U,
rather sophisticated industrial methods for its separation from the natural
isotope mixture have been devised. The gaseous diffusion process, which in the
United States is operated in three large plants (at Oak Ridge, Tennessee;
Paducah, Kentucky; and Portsmouth, Ohio) has been the established industrial
process. Other processes applied to the separation of uranium include the
centrifuge process, in which gaseous uranium hexafluoride is separated in
centrifuge cascades, the liquid thermal diffusion process, the separation
nozzle, and laser excitation. See ISOTOPE (STABLE) SEPARATION.
    Uranium is a very dense, strongly electropositive, reactive metal; it is
ductile and malleable, but a poor conductor of electricity. Many uranium
alloys are of great interest in nuclear technology because the pure metal is
chemically active and anisotropic and has poor mechanical properties. However,
cylindrical rods of pure uranium coated with silicon and canned in aluminum
tubes (slugs) are used in production reactors. Uranium alloys can also be
useful in diluting enriched uranium for reactors and in providing liquid
fuels. Uranium depleted of the fissile isotope (sup 235)U has been used in
shielded containers for storage and transport of radioactive materials. See
NUCLEAR FUELS; NUCLEAR REACTOR.
    Uranium reacts with nearly all nonmetallic elements and their binary
compounds. Uranium dissolves in hydrochloric acid and nitric acid, but
nonoxidizing acids, such as sulfuric, phosphoric, or hydrofluoric acid, react
very slowly. Uranium metal is inert to alkalies, but addition of peroxide
causes formation of water-soluble peruranates. See URANIUM METALLURGY.
    Uranium reacts reversibly with hydrogen to form UH3 at 250DGC (482DGF).
Correspondingly, the hydrogen isotopes form uranium deuteride, UD3, and
uranium tritide, UT3. The uranium-oxygen system is extremely complicated.
Uranium monoxide, UO, is a gaseous species which is not stable below 1800DGC
(3270DGF). In the range UO2 to UO3, a large number of phases exist. The
uranium halides constitute an important group of compounds. Uranium
tetrafluoride is an intermediate in the preparation of the metal and the
hexafluoride. Uranium hexafluoride, which is the most volatile uranium
compound, is used in the isotope separation of (sup 235)U and
(sup 238)U. The halides react with oxygen at elevated temperatures to form
uranyl compounds and ultimately U3O8.
 �34s[Fritz Weigel]
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�6v�2v�32sUranium metallurgy   �21m �33s 
    The processing treatments for the production of uranium concentrates and 
the recovery of pure uranium compounds, as well as the conversion chemistry 
for producing uranium metal and the processes employed for preparing uranium 
alloys.  
    The procedures to recover uranium from its ores are numerous, because of 
the great variety in the nature of uranium minerals and associated materials 
and the wide range of concentration in the naturally occurring ores. Recovery 
of uranium requires chemical processing; however, preliminary treatment of the 
ore may involve a roasting operation, a physical or chemical concentration 
step, or a combination of these. In general, one of two leaching treatments - 
acid leaching and carbonate leaching - is used as the initial step in chemical 
concentration. The choice depends on the nature of the ore, which largely 
determines the efficiency and the cost of the process employed. See LEACHING. 
    The concentrate, whether obtained by chemical or physical means, is 
treated chemically to give a uranyl nitrate solution that can be further 
purified by solvent extraction. The impurities remain in the aqueous phase, 
while the uranium is extracted into the organic phase. See SOLVENT EXTRACTION. 
    Uranium metal can be obtained from its halides by fused-salt electrolysis 
or by reduction with more reactive metals. The reaction of UO2 with calcium 
yields metal of fair quality. The largest tonnages of good-quality uranium 
have been produced by metallothermic reduction of finely divided UF4 with 
calcium or magnesium in steel bombs lined with fused dolomitic oxide (Ames 
process). The charge, consisting of an intimate mixture of UF4 with the 
reductant metal in granular form together with a suitable booster (usually 
calcium plus iodine), is placed into the reduction bomb, the lid is bolted 
down, and the bomb is heated to ignition temperature. The shape of the metal 
ingot (also referred to as a biscuit) depends on the shape of the reduction 
bomb.  
    Uranium alloys are prepared by fusing the components together. All 
procedures following conventional metallurgical techniques, however, may have 
to be carried out inside inert-gas glove boxes because many alloys are 
attacked by oxygen or moisture. See NUCLEAR FUELS; NUCLEAR FUELS REPROCESSING; 
URANIUM.  
 �34s[Fritz Weigel]
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�6v�2v�32sUranus   �21m �33s 
    The first planet to be discovered with the telescope and the seventh in 
the order of distance from the Sun. It was found accidentally by W. Herschel 
in England on March 13, 1781. 
    The obliquity (inclination of rotational axis to orbit plane) of Uranus is 
the highest of any known planet: 97.9DG. This means that the axis is almost in 
the plane of the orbit, and thus the seasons on Uranus are very unusual. 
During summer in one hemisphere, the pole points almost directly toward the 
Sun while the other hemisphere is in total darkness. Forty-one years later, 
the situation is reversed. 
    The linear equatorial diameter is 32,400 mi (51,800 km). The mass is 14.63 
times the mass of the Earth. The mean density is greater than that of Saturn 
even though Uranus is smaller than Saturn. This means that Uranus is richer 
than Saturn (or Jupiter) in elements heavier then hydrogen and helium but not 
nearly so rich in these elements as Earth. The same is true for Neptune. See 
JUPITER; NEPTUNE; SATURN. 
    The temperature Uranus would assume in simple equilibrium with the 
incident solar radiation is about -360DGF (55 K), according to calculations. 
This is essentially identical with the value that is obtained by direct 
measurement. There is no evidence for the existence of an internal energy 
source or for a thermal inversion in the upper atmosphere, as observed for 
Jupiter, Saturn, Titan, and Neptune. 
    Through the telescope, Uranus appears as a small, slightly elliptical 
blue-green disk. Some observers have reported the presence of faint, dusky 
bands on the disk when the equator is oriented toward the Earth, but these 
sightings are not fully confirmed. Spectroscopic observations have shown that 
the atmosphere of the planet contains large amounts of methane and hydrogen. 
    Uranus has five known satellites that form a remarkably regular system, 
with low orbital eccentricities and inclinations. In 1977 a system of rings 
was discovered around Uranus during observations of a stellar occultation. 
There are at least nine rings, the largest having a width of only 30 mi (50 
km).  
 �34s[Tobias C. Owen]
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�6v�2v�32sUrea �21m �33s
    The diamide of carbonic acid. Urea is a white, crystalline, water-soluble
compound, melting point 132.7DGC, with the formula given below. Important

				  O
				  ||
			     H2N--C--NH2

biologically as an end product of normal animal and human protein metabolism,
it is also of synthetic value in urea-formaldehyde plastics and is used as a
stabilizer for many explosives.
    Urea is used medically as a diuretic, and was used formerly in the
treatment of indolent ulcers. Because of its high nitrogen content, it is used
as a commercial fertilizer. See FERTILIZER.
 �34s[Evans B. Reid]
�33s
    Urea is the excretory end product of amino acid metabolism in mammals,
elasmobranchs, amphibia, and chelonia (tortoises and turtles). In the liver,
liberated ammonia combines with CO2 and phosphate to form carbamyl phosphate,
which reacts with the amino acid ornithine to become citrulline and then with
aspartic acid to form arginosuccinic acid, which spontaneously decomposes to
arginine. Finally, the enzyme arginase catalyzes the hydrolysis of arginine to
form urea and ornithine, which is recycled. Each molecule of urea is formed
from two molecules of ammonia and one molecule of CO2. This complex reaction
involves at least seven enzymes. Blood urea concentration rises in kidney
diseases. See AMINO ACIDS; LIVER.
 �34s[Morton K. Schwartz]
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�6v�2v�32sUrea-formaldehyde-type resins   �21m �33s 
    The condensation products obtained by the reaction of urea or melamine 
with formaldehyde. Resinous condensation products of formaldehyde with other 
nitrogen-containing compounds, for example, aniline and amides, also belong to 
this group of resins but have gained only limited utility. 
    The urea- and melamine-formaldehyde resins (amino resins) possess an 
excellent combination of physical properties and can be easily fabricated in a 
variety of colors. They are widely used as adhesives, laminating resins, 
molding compounds, paper and textile finishes, and surface coatings. 
    The aniline- and sulfonamide-formaldehyde resins are produced from other 
compounds containing --NH2 groups that condense with formaldehyde to form 
methylol derivatives which are capable of further reaction. These resins have 
received limited attention except for aniline and p-toluenesulfonamide. 
Because of their resistance to the absorption of water, the 
aniline-formaldehyde resins have been used in electrical applications, such as 
insulation or panels, where their natural brown color is not objectionable. 
The sulfonamide-formaldehyde polymers are less colored than the aniline-type 
resins and have been employed in surface coatings. See CONDENSATION REACTION; 
FORMALDEHYDE; POLYMERIZATION; UREA. 
 �34s[John A. Manson]
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�6v�2v�32sUredinales   �21m �33s 
    An order of parasitic fungi of the subclass Heterobasidiomycetidae which 
causes plant diseases known as rusts. It contains about 128 genera and 4600 
species. The characteristic feature of the order is the teleutospore, a spore 
with one or more cells, each of which is a modified hypobasidium. Each cell 
may produce an epibasidium from which basidiospores are discharged. There are 
3 families, Pucciniaceae, Melampsoraceae, and Coleosporaceae. See 
HETEROBASIDIOMYCETIDAE; PLANT PATHOLOGY; RUST (MICROBIOLOGY). 
 �34s[Robert M. Page]
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�6v�2v�32sUreid   �21m �33s 
    An acyl derivative of urea, such as acetylurea, CH3CO--NHCONH2, and 
diacetylurea, CH3CO--NHCONH-COCH3. The most important members are cyclic, 
containing five- or six-atom hetero rings, and are known as parabanic acid and 
barbituric acid, respectively. Parabanic acid is formed from urea and oxalyl 
chloride; barbituric acid or its derivatives (barbiturates) is formed from 
urea and malonic ester or suitably substituted malonic esters. 
    The barbiturates are acidic compounds, soluble in base (in which form they 
are often used). They are widely used in treating insomnia, epilepsy, 
hysteria, and as preliminary anesthetics. See BARBITURATES. 
    Many naturally occurring substances, for example, the purines (from 
nucleoproteins of plant and animal cells), and the xanthine alkaloids (from 
coffee, tea, and cocoa), contain diureid structures, in which two N--C--N 
units are combined with one C--C--C unit. See PURINE; UREA. 
 �34s[Evans B. Reid]
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�6v�2v�32sUrethane   �21m �33s 
    Urethane, or ethyl urethan or ethyl carbamate, H2NCOOC2H5, is the ethyl 
ester of carbamic acid. It forms white crystals with melting point 48 - 50DGC 
(118 - 122DGF), boiling point 182 - 184DGC (360 - 363DGF), and it is freely 
soluble in water, alcohol, and chloroform. Less toxic than other urethanes, 
ethyl urethan is a mild hypnotic and weak diuretic; in water, it forms a 
neutral solution with a cooling, saline taste. With quinine hydrochloride, it 
is used as a sclerosing agent for treatment of varicose veins. 
    Urethane has applications as an intermediate in organic synthesis and, 
when molten, as a solvent for various organic materials. 
 �34s[Evans B. Reid]
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�6v�2v�32sUric acid   �21m �33s 
    The excretory end product in amino acid metabolism by uricotelic species, 
including birds, terrestrial invertebrates, and snakes; in purine metabolism 
by most insects, snakes, lizards, birds and primates (including humans). 
Nonuricotelic animals oxidize uric acid by the enzyme uricase to a more 
soluble compound, allantoin. The disease gout is associated with elevated uric 
acid.  See  GOUT; PROTEIN METABOLISM; PURINE. 
 �34s[Morton K. Schwartz]
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�6v�2v�32sUridine diphosphoglucose (UDPG)   �21m �33s 
    A compound in which ALPHA-glucopyranose is esterified, at carbon atom 1, 
with the terminal phosphate group of uridine-5'-pyrophosphate (that is, 
uridine diphosphate, UDP). On very mild acid hydrolysis, glucose and UDP are 
liberated. Uridine diphosphoglucose occurs in animal, plant, and microbial 
cells and is synthesized enzymatically from uridine triphosphate (UTP) and 
ALPHA-glucose-1-phosphate. This compound functions as a key in the 
transformation of glucose to other sugars. 
    In general, UDP-glucose is a prominent member of a family of compounds 
composed of sugars activated as derivatives of nucleoside diphosphates. In 
these active forms they can be used directly as glycosyl donors, or they may 
be transformed to other more complex sugars before their utilization for the 
biosynthesis of both simple saccharides and complex polysaccharides.  See  
CARBOHYDRATE METABOLISM; NUCLEIC ACID. 
 �34s[Jack L. Strominger]
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�6v�2v�32sUrinalysis �21m �33s
    The laboratory examination of urine. Examination of the urine requires a
consideration of normal kidney and lower urinary tract anatomy and physiology,
as well as the awareness of the clinical and laboratory conditions under which
a urine sample is obtained and examined.
    In general, urine is examined for color, specific gravity, pH, and the
presence or absence of normal and abnormal materials. The color is often a
rough index of the ability of the kidney to concentrate urine and may also
afford the first indication of the presence of blood or other abnormal
pigmented material. In healthy individuals, the normal range of the specific
gravity varies between 1.015 and 1.030 under controlled conditions. Deviations
above or below this range may indicate decrease in kidney function or the
presence of some specific disease process. The pH reflects the role of the
kidney in aiding in the maintenance of acid-base balance. Urine is also
examined for the presence of blood, glucose, protein, and bacteria. The
presence of any of these substances offers important clues to the diagnosis of
a wide variety of disorders.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sUrinary bladder   �21m �33s 
    A distensible, muscular sac in most vertebrates which serves as a 
reservoir for urine. Snakes, crocodilians, birds (with the exception of the 
ostrich), most lizards, and a few fish lack a urinary bladder. In these 
organisms, urine empties directly into the cloaca. Three general types of 
urinary bladder are recognized among the vertebrates: tubal, cloacal, and 
allantoic.  See  URINE. 
    Most fish possess tubal bladders, that is, enlargements of the mesonephric 
ducts. The cloacal bladder is found in monotremes, amphibians, and some 
dipnoans. There is no direct connection between the excretory ducts and this 
type of bladder. The bladder is an outpouching or diverticulum of the cloacal 
wall. The cloacal opening is closed by a sphincter muscle and the urine which 
seeps into the cloaca from the excretory ducts is forced into the bilobed 
bladder. The allantoic bladder is derived from the ventral wall of the cloaca 
and possibly the allantoic diverticulum.  See  ALLANTOIS. 
    The bladder drains through the urethra, the opening being controlled by a 
sphincter. Innervation is by both sympathetic and parasympathetic fibers; 
stimulation of the parasympathetic causes the bladder muscle to contract and 
relaxes the internal sphincter. Micturition is a reflex act which is initiated 
voluntarily except in children.  See  PARASYMPATHETIC NERVOUS SYSTEM; 
SYMPATHETIC NERVOUS SYSTEM; URINARY SYSTEM. 
 �34s[Charles B. Curtin]
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�6v�2v�32sUrinary bladder disorders   �21m �33s 
    The urinary bladder is subject to anomalies, obstructions, inflammations, 
calculi, fistulae, and tumors. 
    A fairly common, distressing anomaly is a failure of both the lower 
abdominal wall and anterior bladder wall to close. The urine appears directly 
through the abdominal wall defect. 
    Interference with normal complete emptying of the bladder may be the 
result of various factors. In one form, an elevation of the internal urethral 
opening above its normal dependent position creates an unemptied pool of 
residual urine in the bottom of the bladder. This elevation is usually caused 
in the male by prostatic enlargement and in the female by cystocele, a bulging 
of the lower bladder wall into the vagina. Another form of obstruction is 
blockage of the bladder neck or urethra by lesions or tumors. A third major 
cause of inadequate emptying is neurogenic, usually following spinal cord 
injury or disease. 
    Cystitis or inflammation of the urinary bladder is an extremely common 
affliction; it is annoying because of the accompanying painful, frequent 
micturition. Calculi usually develop as a result of infection and obstruction. 
They often reach a considerable size, up to several inches in diameter. They 
are usually composed of phosphates and urates. 
    Abnormal openings between the bladder or female urethra and the vagina are 
not unusual. Such fistulae most often follow injury during childbirth. The 
chief effects are infection and the constant leakage of urine through the 
vagina.  
 �34s[Roy N. Barnett]
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�6v�2v�32sUrinary system �21m �33s
    The urinary system consists of the kidneys, urinary ducts, and bladder.
Similarities are not particularly evident among the many and varied types of
excretory organs found among vertebrates. The variations that are encountered
are undoubtedly related to problems with which vertebrates have had to cope in
adapting to different environmental conditions.

    Kidneys.  In reptiles, birds, and mammals three types of kidneys are
usually recognized: the pronephros, mesonephros, and metanephros. These appear
in succession during embryonic development, but only the metanephros persists
in the adult.
    The metanephric kidneys of reptiles lie in the posterior part of the
abdominal cavity, usually in the pelvic region. They are small, compact, and
often markedly lobulated. The posterior portion on each side is somewhat
narrower. In some lizards the hind parts may even fuse. The degree of symmetry
varies.
    The kidneys of birds are situated in the pelvic region of the body cavity;
their posterior ends are usually joined. They are lobulated structures with
short ureters which open independently into the cloaca.
    A rather typical mammalian metanephric kidney (Fig. 1) is a compact,
bean-shaped organ attached to the dorsal body wall outside the peritoneum. The
ureter leaves the medial side at a depression, the hilum. At this point a
renal vein also leaves the kidney and a renal artery and nerves enter it. The
kidneys of mammals are markedly lobulated in the embryo, and in many forms
this condition is retained throughout life.  See  KIDNEY.
 �34s[Charles K. Weichert]
�33s

    Urinary bladder.  At or near the posterior ends of the nephric ducts
there frequently is a reservoir for urine.  This is the urinary bladder.
Actually there are two basic varieties of bladders in vertebrates.  One is
found in fishes in which the reservoir is no more than an enlargement of the
posterior end of each urinary duct.  Frequently the urinary ducts are conjoined
and a small bladder is formed by expansion of the common duct.	The far more
common type of bladder is that exhibited  by tetrapods.  This is a sac which
originates embryonically as an outgrowth from the ventral side of the cloaca.
Present in all embryonic life, it is exhibited differentially in adults.
All amphibians retain the bladder, but it is lacking in snakes, crocodilians,
and most lizards; birds, also, with the exception of the ostrich, lack a
bladder.  It is present in all mammals.  See URINARY BLADDER.
[Theodore W. Torrey]








    Physiology.  Urine is produced by individual renal nephron units which are
fundamentally similar from fish to mammals (Fig. 2); however, the basic
structural and functional pattern of these nephrons varies among
representatives of the vertebrate classes in accordance with changing
environmental demands. Kidneys serve the general function of maintaining the
chemical and physical constancy of blood and other body fluids. The most
striking modifications are associated particularly with the relative amounts
of water made available to the animal. Alterations in degrees of glomerular
development, in the structural complexity of renal tubules, and in the
architectural disposition of the various nephrons in relation to one another
within the kidneys may all represent adaptations made either to conserve or
eliminate water.

    Regulation of volume. Except for the primitive marine cyclostome Myxine,
all modern vertebrates, whether marine, fresh-water, or terrestrial, have
concentrations of salt in their blood only one-third or one-half that of sea
water. The early development of the glomerulus can be viewed as a device
responding to the need for regulating the volume of body fluids. Hence, in a
hypotonic fresh-water environment the osmotic influx of water through gills
and other permeable body surfaces would be kept in balance by a simple
autoregulatory system whereby a rising volume of blood results in increased
hydrostatic pressure which in turn elevates the rate of glomerular filtration.
Similar devices are found in fresh-water invertebrates where water may be
pumped out either as the result of work done by the heart, contractile
vacuoles, or cilia found in such specialized ``kidneys'' as flame bulbs,
solenocytes, or nephridia that extract excess water from the body cavity
rather than from the circulatory system. Hence, these structures which
maintain a constant water content for the invertebrate animal by balancing
osmotic influx with hydrostatic output have the same basic parameters as those
in vertebrates that regulate the formation of lymph across the endothelial
walls of capillaries.  See  OSMOREGULATORY MECHANISMS.

    Electrolyte balance. A system that regulates volume by producing an
ultrafiltrate of blood plasma must conserve inorganic ions and other essential
plasma constituents. The salt-conserving operation appears to be a primary
function of the renal tubules which encapsulate the glomerulus. As the
filtrate passes along their length toward the exterior, inorganic,
electrolytes are extracted from them through highly specific active cellular
resorptive processes which restore plasma constituents to the circulatory
system.

    Movement of water. Concentration gradients of water are attained across
cells of renal tubules by water following the active movement of salt or other
solute.  Where water is free to follow the active resorption of sodium and
covering anions, as in the proximal tubule, an isosmotic condition  prevails.
Where water is not free to follow salt, as in the distal segment in the
absence of antidiuretic hormone, a hypotonic tubular fluid results.

    Nitrogenous end products. Of the major categories of organic foodstuffs,
end products of carbohydrate and lipid metabolism are easily eliminated mainly
in the form of carbon dioxide and water. Proteins, however, are more difficult
to eliminate because the primary derivative of their metabolism, ammonia, is a
relatively toxic compound. For animals living in an aquatic environment
ammonia can be eliminated rapidly by simple diffusion through the gills.
However, when ammonia is not free to diffuse into an effectively limitless
aquatic environment, its toxicity presents a problem, particularly to embryos
of terrestrial forms that  develop wholly within tightly encapsulated
eggshells or cases. For these forms the detoxication of ammonia is an
indispensable requirement for survival. During evolution of the vertebrates
two energy-dependent biosynthetic pathways arose which incorporated
potentially toxic ammonia into urea and uric acid molecules, respectively.
Both of these compounds are relatively harmless, even in high concentrations,
but the former needs a relatively large amount of water to ensure its
elimination, and uric acid requires a specific energy-demanding tubular
secretory process to ensure its efficient excretion.  See  UREA; URIC ACID.

    Urine concentration. The unique functional feature of the mammalian kidney
is its ability to concentrate urine. Human urine can have four times the
osmotic concentration of plasma, and some desert rats that survive on a diet
of seeds without drinking any water have urine/plasma concentration ratios as
high as 17. More aquatic forms such as the beaver have correspondingly poor
concentrating ability.
    The concentration operation depends on the existence of a decreasing
gradient of solute concentration that extends from the tips of the papillae in
the inner medulla of the kidney outward toward the cortex. The high
concentration of medullary solute is achieved by a double hairpin
countercurrent multiplier system which is powered by the active removal of
salt from urine while it traverses the ascending limb of Henle's loop (Fig.
2). The salt is redelivered to the tip of the medulla after it has diffused
back into the descending limb of Henle's loop. In this way a hypertonic
condition is established in fluid surrounding the terminations of the
collecting ducts. Urine is concentrated by an entirely passive process as
water leaves the lumen of collecting ducts to come into equilibrium with the
hypertonic fluid surrounding its terminations.	See  URINE; URO-GENITAL SYSTEM.
 �34s[Roy P. Forster]
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�6v�2v�32sUrine   �21m �33s 
    An aqueous solution of organic and inorganic substances, mostly waste 
products of metabolism. The kidneys maintain the internal milieu of the body 
by excreting these waste products and adjusting the loss of water and 
electrolytes to keep the body fluids relatively constant in amount and 
composition. The urine normally is clear and has a specific gravity of 1.017 - 
1.020, depending upon the amount of fluid ingested, perspiration, and diet. 
The increase in specific gravity above that of water is due to the presence of 
dissolved solids, about 60% of which are organic substances such as urea, uric 
acid, creatinine, and ammonia; and 40% of which are inorganic substances such 
as sodium, chloride, calcium, potassium, phosphates, and sulfates. Its 
reaction is usually acid (pH 6) but this too varies with the diet. It usually 
has a faint yellow color due to a urochrome pigment, but the color varies 
depending upon the degree of concentration, and the ingestion of certain foods 
(for example, rhubarb) or cathartics. It usually has a characteristic aromatic 
odor, the cause of which is not known.  See  KIDNEY; UREA (BIOCHEMISTRY); URIC 
ACID; URINALYSIS; URINARY SYSTEM. 
 �34s[Reuben Straus]
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�6v�2v�32sUrodela   �21m �33s 
    One of the orders of the class Amphibia, also known as the Caudata. The 
members of this order are the tailed amphibians, or salamanders, and are 
distinguished superficially from the frogs and toads (order Anura) by the 
possession of a tail, and from the caecilians (order Apoda) by the possession 
of limbs. Salamanders resemble lizards in that members of both groups normally 
have a relatively elongate body, four limbs, and a tail. The similarity is 
however, only superficial. The most obvious external difference, the moist 
glandular skin of the amphibian and dry scaly skin of the reptile, is 
underlain by the numerous characters that distinguish reptiles from 
amphibians. See ANURA; APODA. 
    The vast majority of salamanders are well under a foot in length. The 
largest is the giant salamander of Japan and China, which may attain a length 
of 5 ft (1.5 m). A close relative, the hellbender, which lives in streams in 
the eastern United States, grows to over 2 ft (0.6 m) in length. 
    Four well-developed limbs are typically present in salamanders, but the 
legs of the mud eel (Amphiuma) of the southeastern United States are very tiny 
appendages and of no use in locomotion. The sirens (Siren and Pseudobranchus), 
also aquatic salamanders of the same region, have undergone even further 
degeneration of the limbs and retain only tiny forelimbs. Salamanders with 
normal limbs usually have four toes on the front feet and five on the rear, 
though a few have only four on all feet. A feature of the Urodela not shared 
with the Anura is the ability to regenerate limbs. 
    The moist and highly vascularized skin of a salamander serves as an organ 
of respiration, and in some forms shares with the buccal region virtually the 
whole burden. In one entire family of salamanders, the Plethodontidae, lungs 
are not present. Even in those forms with lungs and gills, dermal respiration 
plays a very important part. The tympanum is absent in salamanders and the 
middle ear is degenerate. Hence these animals are deaf to airborne sounds. 
Undoubtedly correlated with this is the reduction of voice; salamanders are 
mute or produce only slight squeaking or, rarely, barking sounds when annoyed. 
However, salamanders can detect sounds carried through ground or water. 
    Salamanders live in a variety of aquatic or moist habitats. Many forms are 
wholly aquatic, living in streams, rivers, lakes, and ponds. Others live on 
land most of the year but must return to water to breed. The most advanced 
species live out their lives in moist places on land or, in the case of some 
tropical species, in trees, and never go in the water. 
    Producing the young alive rather than by laying eggs is very rare among 
salamanders. The eggs of salamanders may be deposited in water or in moist 
places on land. Presumably, aquatic breeding is the more primitive. 
Fertilization is external in most of the Cryptobranchoidea, internal by means 
of spermatophores in the remaining suborders.  See  NEOTENY. 
    The greatest concentration of families and genera of salamanders is found 
in the eastern United States, and these animals are one of the few major 
groups of vertebrate animals that is distinctly nontropical. No salamander 
occurs south of the Equator in the Old World, and only 18 species of the 
evolutionary advanced family Plethodontidae are found as far south as South 
America in the New World. 
    The Urodela are readily arranged in seven families, but authorities differ 
as to the relationships among these families and their classification into 
suborders. Three to five suborders may be recognized, and one of these is 
thought by some to merit ordinal rank, equivalent to the Urodela. About 300 
species of salamanders are known to be living today, and these are distributed 
among about 54 genera.  See  AMPHIBIA. 
 �34s[Richard G. Zweifel]
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�6v�2v�32sUrogenital system   �21m �33s 
    The combined structures composing the urinary and genital, or 
reproductive, organs of vertebrates. The terms urinogenital or genitourinary 
system are equally applicable and just as frequently used. During embryonic 
development, the urinary and reproductive systems are closely interrelated, as 
their ducts arise in associated mesodermal regions. Common passages are 
associated with both these systems in various vertebrates, such as the single 
orifice for emission of urine and sperm.  See  KIDNEY; REPRODUCTIVE SYSTEM; 
URINARY SYSTEM. 
 �34s[Charles B. Curtin]
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�6v�2v�32sUropygi   �21m �33s 
    An order of arachnids, the tailed whip scorpions, comprising about 70 
species from tropical and warm temperate Asia and the Americas. Most are dark 
reddish-brown, of medium to giant size, 0.7 - 2.6 in. (18 - 65 mm), the 
largest one being Mastigoproctus giganteus of the southern United States and 
Mexico. The elongate, flattened body bears in front a pair of greatly 
thickened, raptorial pedipalps set with many sharp spines and used to hold and 
crush insect prey. The first pair of legs is elongated and modified into 
feelers. The abdomen terminates in a slender, many jointed, whiplike 
flagellum. The uropygids are harmless, nocturnal creatures without poison 
glands that live in dark places and burrow into the soil. When disturbed, they 
expel a volatile liquid, with the strong odor of acetic acid, from a gland at 
the base of the tail. This accounts for the name ``vinegaroon'' given by many 
Americans to these much-feared animals.  See  ARACHNIDA. 
 �34s[Willis J. Gertsch]
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�6v�2v�32sUropygial gland   �21m �33s 
    A relatively large, compact, bilobed, secretory organ located at the base 
of the tail (uropygium) of most birds having a keeled sternum. It is known 
also as the preen, oil, or scent gland. This is the only true skin gland 
possessed by this class of vertebrates. 
    The glandular secretion, predominantly oily and sometimes of offensive 
odor (musk-duck, hoopoe, petrel) is discharged through an orifice at the tip 
of a nipplelike protuberance often encircled by short, bristly feathers. The 
act of preening induces a flow of secretion from the nipple which is 
transferred by the beak to the body plumage. In water fowl there is some 
evidence that this oily secretion assists in maintaining the water-repellent 
quality of the feathers, either directly or by preserving their physical 
structure.  See  FEATHER; SCENT GLAND. 
 �34s[Mary E. Rawles]
�33s 
  
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�6v�2v�32sUrsa Major   �21m �33s 
    The most widely known and oldest of the astronomical constellations. Ursa 
Major, or the Great Bear, is a circumpolar group as viewed from the middle 
latitudes of the Northern Hemisphere. One part of the configuration, a group 
of seven bright stars, which is pictured as the tail of the Great Bear, is 
commonly known in the United States as the Big Dipper which it resembles (see 
illustration). The two stars ALPHA and BETA at the front of the bowl of the 
dipper are called pointers, because a line joining them points to Polaris, the 
North Star.  See  CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
sh
  
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�6v�2v�32sUrsa Minor   �21m �33s 
    The astronomical constellation Little Bear. Ursa Minor is a circumpolar 
constellation whose brightest star, Polaris, is almost at the north celestial 
pole. Seven of the eight stars appear to form a dipper, hence the 
constellation is alternately known as the Little Dipper (see illustration). 
The two bright stars BETA and GAMMA at the front of the bowl are often called 
the Guardian of the Pole, because they circle about Polaris closer than other 
conspicuous stars.  See  CONSTELLATION; URSA MAJOR. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sUrticals   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Hamamelidae of the class Magnoliopsida (dicotyledons). The order 
consists of 5 families and nearly 2400 species. The largest families are the 
Moraceae (about 1500 species), Urticaceae (about 700 species), and Ulmaceae 
(about 150 species). The Urticales are herbs or woody plants with simple, 
usually stipulate leaves, and reduced, clustered flowers that usually have a 
vestigial perianth. Figs (Ficus, in the Moraceae), mulberries (Morus), nettles 
(Urtica), elms (Ulmus), and Indian hemp (Cannabis sativa, family Cannabaceae, 
the source of a commercial fiber as well as hashish and marijuana) are 
well-known members of the Urticales.  See  ELM; FIG; HACKBERRY; HAMAMELIDAE; 
HEMP; HOP; MAGNOLIOPSIDA; MULBERRY; RAMIE. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sUstilaginales   �21m �33s 
    An order of the subclass Heterobasidiomycetidae comprising the smut fungi 
which parasitize plants - chiefly members of the grass and sedge families - 
and cause diseases known as smut or bunt. In Ustilaginales, intercalary cells 
of the binucleate mycelium develop into teleutospores which function as 
hypobasidia. There are approximately 37 genera and 700 species divided into 3 
families: Ustilaginaceae, Tilletiaceae, and Graphiolaceae.  See  
HETEROBASIDIOMYCETIDAE; SMUT (MICROBIOLOGY). 
 �34s[Robert M. Page]
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�6v�2v�32sUterine disorders   �21m �33s 
    Uterine and cervical disorders account for a large portion of medical 
practice. Although the cervix is usually considered separate pathologically, 
cervical disorders are included in this article. The major categories include 
congenital, inflammatory, neoplastic, and functional diseases.  See  UTERUS. 
    Failure of formation or developmental abnormalities are not uncommon, but 
most aberrations are of minor significance, such as a double uterus. 
    Inflammations of the uterus are not common but, when present, follow 
infections resulting from trauma, childbirth, or miscarriage. However, 
cervicitis, an inflammation of the uterine cervix, is probably the most 
frequently encountered female disease. Most cervicitis is nonspecific and 
tends to become chronic.  See  GONORRHEA; SOFT CHANCRE; SYPHILIS; TUBERCULOSIS. 
    The uterus is frequently the site of both benign and malignant tumor 
formation, and the three most common growths are as follows. (1) Endometrial 
polyps are benign simple or multiple masses which are often sources of 
abnormal bleeding; they are otherwise unimportant. (2) Leiomyomas, or 
fibroids, are the most common tumors in women, and occur in 10% of women of 
reproductive age. These are benign smooth-muscle growths, varying in size from 
minute bodies to huge masses which fill the pelvis and lower abdomen. They may 
be asymptomatic or may cause abnormal bleeding. The larger fibroids commonly 
produce signs or symptoms as a result of the mechanical effects on adjacent 
organs. They may also interfere with pregnancy, labor, or delivery. (3) 
Carcinoma of the endometrium is an important female malignancy but has a lower 
incidence than breast or cervical cancer. It is most often found in the 50- to 
70-year age group and in patients with obesity, hypertension, and diabetes 
mellitus.  
    Functional disorders of the uterine mucosa produce menstrual 
irregularities and other symptoms and, for the most part, stem from obscure 
causes. The complex endocrine regulation of the very reactive endometrium is 
subject to a great many other individual factors. Hyperplasias, atrophies, and 
displaced endometrial tissue are seen frequently. The more common hyperplasias 
and atrophies account for about one-fifth of all gynecologic disorders and are 
the most frequent cause of abnormal bleeding.  See  MENSTRUATION. 
 �34s[N. Karle Mottet]
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�6v�2v�32sUterus   �21m �33s 
    The hollow, muscular womb, being an enlarged portion of the oviduct in the 
adult female. An adult human uterus, before pregnancy, measures 3 x 2 x 1 in. 
(7.5 x 5 x 2.5 cm) in size and has the shape of an inverted, flattened pear. 
The paired Fallopian tubes enter the uterus at its upper corners; the lower, 
narrowed portion, the cervix, projects into the vagina (see illustration). 
Normally the uterus is tilted slightly forward and lies behind the urinary 
bladder.  
    The lining, or mucosa, responds to hormonal stimulation, growing in 
thickness with a tremendous increase in blood vessels during the first part of 
the menstrual cycle. If fertilization does not occur, the thickened vascular 
lining is sloughed off, producing the menstrual flow at the end of the cycle, 
and a new menstrual cycle begins with growth of the mucosa. When pregnancy 
occurs, the mucosa continues to thicken and forms an intimate connection with 
the implanted and enlarging placenta.  See  MENSTRUATION; PREGNANCY; 
REPRODUCTIVE SYSTEM. 
 �34s[Walter J. Bock]
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�6v�2v�32sVaccination   �21m �33s 
    By strict definition, vaccination refers only to the inoculation of 
vaccinia virus (smallpox vaccination). In common usage, vaccination now 
implies the active immunization procedures employed against a variety of 
infectious agents. A more accurate term is immunization. 
    Active immunization describes the administration of all or part of a 
biologic agent in an effort to evoke a defensive response in the host. Killed 
bacteria, components of bacteria, altered toxins, live and killed viruses, and 
components of viruses have each been utilized in vaccine formulation. The 
vaccine may be injected, or given by mouth in some instances, with the 
expectation that antibody against an antigenic component of the vaccine will 
be elicited. In addition, other immune responses, such as 
lymphocyte-associated-cell-mediated immunity, may be evoked. The ultimate goal 
is to protect the host against subsequent challenge by the naturally occurring 
or ``wild'' infectious agent.  See  ANTIBODY; ANTIGEN; IMMUNITY. 
 �34s[Vincent Fulginiti]
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�6v�2v�32sVacuole   �21m �33s 
    A cavity within a cell. In its widest sense the term connotes all the 
vesicles bound by a single membrane within the cytoplasm, including the 
endoplasmic reticulum, the nuclear envelope, and the Golgi apparatus or 
dictyosomes, all structures usually considered as separate entities. The term 
is more commonly reserved for the membrane-bound cavities which are associated 
with the central vacuole of plants, food vacuoles, self-digestive vacuoles, 
gas vacuoles, and the contractile vacuole. Vacuoles are discussed in this 
article in the more traditional context. The term vacuole is somewhat 
ambiguous in that it groups structures which are not truly homologous. Some 
have a single limiting membrane, or unit membrane, while others have a layer 
of protein granules which separates the contents from the cytoplasm. It is 
believed that the membrane carries enzymes which are concerned with functions 
such as transport of substances across the membrane between the cytoplasm and 
the cavity.  See  CELL MEMBRANES. 
    The central vacuole of the mature plant cell (see illustration) is formed 
from the fusion of smaller ones which occur universally in plants. It may 
contain many diverse substances, including salts, sugars, organic acids, amino 
acids, proteins, tannins, and pigments. Some of the contents of the vacuole 
are thought to be waste products from cellular metabolism. The colors of many 
flowers and some leaves are due to pigments contained in the central vacuole.  
See  PLANT CELLS. 
  
    Food vacuoles, sometimes called heterophagic vacuoles, are common in most 
protozoans and some algae. In higher animals they occur in cells of the 
intestinal epithelium. They form where the surface of the cell contacts a 
particle of food.  See  ENDOCYTOSIS; PHAGOCYTOSIS. 
    When there is a need for the cell to digest portions of itself, autophagic 
vacuoles arise. This often happens in response to starvation. Portions of the 
cytoplasm, which may contain mitochondria or other cytoplasmic organelles, 
become isolated within the cytoplasm by a partitioning membrane. These may 
develop as special vacuoles which contain a number of digestive enzymes. The 
cell thus uses some of its own substances to obtain the essential materials of 
metabolism when these are not available from outside.  See  LYSOSOME. 
    Contractile vacuoles are common in protozoans and are found in some algae. 
In single-celled organisms they are found only in fresh-water forms, and even 
in species which grow in both fresh and brackish water they are lacking when 
the environment is brackish. They expand and collapse, often rhythmically, as 
fluid is accumulated slowly from the cytoplasm and then rapidly expelled from 
the cell.  See  ENDOPLASMIC RETICULUM; GOLGI APPARATUS; OSMOREGULATORY 
MECHANISMS.  
 �34s[Myron C. Ledbetter]
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�6v�2v�32sVacuum fusion   �21m �33s 
    A technique of analytical chemistry for determining the oxygen, hydrogen, 
and sometimes nitrogen content of metals.  
    The metal sample is either fused or dissolved in a bath, or flux, of a 
second metal in a heated graphite crucible supported inside an evacuated glass 
or quartz vessel. Oxygen is released from the metal as carbon monoxide by 
reaction of oxides or dissolved oxygen with carbon from the graphite crucible 
at high temperature. Metal nitrides dissociate to form elemental nitrogen. 
Hydrogen is evolved as elemental hydrogen. The mixture of carbon monoxide, 
nitrogen, and hydrogen is analyzed to determine individual component 
concentrations by various techniques.  
    Inert gas fusion has been developed for determination of gases in metals. 
The techniques used are similar to vacuum fusion but substitute inert gases 
for the vacuum environment.  See  VACUUM METALLURGY. 
 �34s[Frank C. Benner]
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�6v�2v�32sVacuum measurement �21m �33s
    The determination of a gas pressure that is less in magnitude than the
pressure of the atmosphere. (Atmospheric pressure is 29.92 in. or 760 mm of
mercury.) This low pressure can be expressed in terms of the height of a
column of mercury which the given pressure (vacuum) will support, referenced
to zero pressure. The unit most commonly used is the torr, equal to 1 mm of
mercury (mmHg). The unit of pressure in the International System (SI) is the
pascal (Pa), equal to 1 newton per square meter (1 torr = 133.322 Pa).
    Pressures above 1 torr can be easily measured by familiar pressure gages,
such as liquid-column gages, diaphragm-pressure gages, bellows gages, and
bourdon-spring gages. At pressures below 1 torr, mechanical effects such as
hysteresis, ambient errors, and vibration make these gages impractical.  See
BOURDON-SPRING PRESSURE GAGE; MANOMETER; PRESSURE MEASUREMENT.
    Pressures below 1 torr are best measured by gages which infer the pressure
from the measurement of some other property of the gas, such as thermal
conductivity or ionization. The thermocouple gage, in combination with a hot-
or cold-cathode gage (ionization type), is the most widely used method of
vacuum measurement.  See  IONIZATION GAGE.
    Other gages used to measure vacuum in the range of 1 torr or below are the
McLeod gage, the Pirani gage, and the Knudsen gage. The McLeod gage is used as
an absolute standard of vacuum measurement in the 10 torr (10[sup 3] Pa) to
10(sup -4) torr (10[sup -2] Pa) range.	See  MCLEOD GAGE; PIRANI GAGE.
 �34s[Richard Comeau]
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�6v�2v�32sVacuum metallurgy �21m �33s
    The making, shaping, and treating of metals, alloys, and intermetallic and
refractory metal compounds in a gaseous environment where the composition and
partial pressures of the various components of the gas phase are carefully
controlled. In many instances, this environment is a vacuum ranging from
subatmospheric to ultrahigh vacuum (less than 760 torr or 101 kilopascals to
10[sup -12] torr or 10[sup -10] pascal). In other cases, reactive gases are
deliberately added to the environment to produce the desired reactions, such
as in reactive evaporation and sputtering processes and chemical vapor
deposition. The processes in vacuum metallurgy involve liquid/solid,
vapor/solid, and vapor/liquid/solid transitions. In addition, they include
testing of metals in controlled environments.
    There are three basic reasons for vacuum processing of metals: elimination
of contamination from the processing environment, reduction of the level of
impurities in the product, and deposition with a minimum of impurities.
Contamination from the processing environment includes the container for the
metal and the gas phase surrounding the metal. In the vacuum process,
impurities, particularly oxygen, nitrogen, hydrogen, and carbon, are released
from the molten metal and pumped away; and metals, alloys, and compounds are
deposited with a minimum  of entrained impurities. There are numerous and
varied application areas for vacuum metallurgy including special areas of
extractive metallurgy, melting processes, casting of shaped products,
degassing of molten steel, heat treatment, surface treatment, vapor
deposition, space processing, and joining processes.  See  ARC WELDING; STEEL
MANUFACTURE; WELDING AND CUTTING OF METALS.
 �34s[Rointan F. Bunshah]
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�6v�2v�32sVacuum pump   �21m �33s 
    A device that reduces the pressure of a gas (usually air) in a container. 
When gas in a closed container is lowered from atmospheric pressure,  the 
operation constitutes an increase in vacuum in this container.  See  PRESSURE. 
    Vacuum pumps are evaluated for the degree of vacuum they can attain and 
for how much gas they can pump in a unit of time. In practice, where high 
vacuum is required, two or more different types of pumps are used in series. 
    In the rotary oil-seal pump (see illustration), gas is sucked into chamber 
A through the opening intake port by the rotor. A sliding vane partitions 
chamber A from chamber B. The compressed gas that has been moved from position 
A to position B is pushed out of the exit port through the valve, which 
prevents the gas from flowing back. The valve and the rotor contact point are 
oil-sealed. Since each revolution sweeps out a fixed volume, it is called a 
constant-displacement pump. Other mechanical pumps are the rotary blower pump, 
which operates by the propelling action of one or more rapidly rotating 
lobelike vanes, and the molecular drag pump, which operates at very high 
speeds, as much as 16,000 rpm. Pumping is accomplished by imparting a high 
momentum to the gas molecules by the impingement of the rapidly rotating body. 
  
    The water aspirator is an ejector pump. When water is forced under 
pressure through the jet nozzle, it will force the gas in the inlet chamber to 
go through the diffuser, thus lowering the pressure in the inlet chamber. When 
high-pressure steam is used instead of water, it is called a steam ejector. 
    In addition, a number of pumps have been developed which meet special 
pumping requirements. The ion pump operates electronically. Electrons that are 
generated by a high voltage applied to an anode and a cathode are spiraled 
into a long orbit by a high-intensity magnetic field. These electrons 
colliding with gas molecules ionize the molecules, imparting a positive charge 
to them. These are attracted to, and are collected on, the cathode. Thus a 
pumping action takes place. Sorption pumping is the removal of gases by 
adsorbing and absorbing them on a granular sorbent material such as a 
molecular sieve held in a metal container. Cryogenic pumping is accomplished 
by condensing gases on surfaces that are at extremely low temperatures. 
 �34s[Edward S. Barnitz]
�33s 
  
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�6v�2v�32sVacuum tube   �21m �33s 
    An electron tube evacuated to such a degree that its electrical 
characteristics are essentially unaffected by any residual gas or vapor. 
  
    Classification.  According to function, vacuum tubes are classified as 
receiving tubes, transmitting tubes, phototubes, and cathode-ray tubes, 
microwave tubes, and storage tubes. Structurally, they are classified 
according to number of electrodes as diodes, triodes, tetrodes, pentodes, and 
so on. 
    Prior to the advent of semiconductor devices, hundreds of types of vacuum 
tubes were developed, many differing only in minor respects. With a few 
exceptions, most of these have been replaced by semiconductor devices or are 
used only as replacements in older equipment designed for vacuum tubes. 
Important exceptions include cathode-ray tubes, microwave tubes, high-power 
transmitting tubes, and x-ray tubes.  See  SEMICONDUCTOR. 
    Receiving tubes are low-voltage and low-power tubes designed for use in 
radio receivers, computers, and sensitive control and measuring equipment. 
Phototubes are two-electrode electron tubes in which the current is controlled 
by light flux incident upon one of the electrodes. They are used in sound-film 
equipment, in light-controlled switches, and in many industrial and scientific 
applications. Cathode-ray tubes function by virtue of an electron beam that 
can be focused to a small cross section on a screen and varied in position and 
intensity by means of electrical signals. They include oscilloscope tubes, 
television picture tubes, and camera tubes. Oscilloscope tubes make possible 
the visual examination of electrical signals. Picture tubes transform 
television signals into pictures upon a luminescent screen. Conversely, camera 
tubes convert light images into electrical output or, as image converters or 
intensifiers, produce a visible picture from incident infrared light or from 
x-rays.  
    Microwave tubes are designed for operation at frequencies of the order of 
3 gigahertz and above. They are used in such diverse applications as radar 
equipment, space communication and control channels, scientific research, and 
radio-frequency ovens. Storage tubes, which are electron tubes designed so 
that information can be introduced and then extracted after a required storage 
interval, were developed for use in computers.  See  BACKWARD-WAVE TUBE; 
CATHODE-RAY TUBE; KLYSTRON; MAGNETRON; MICROWAVE TUBE; PHOTOTUBE; PICTURE 
TUBE; STORAGE TUBE; TELEVISION CAMERA TUBE; TRAVELING-WAVE TUBE; X-RAY TUBE. 
  
    Basic operation.  Vacuum tubes depend upon two basic physical phenomena 
for their operation. The first is the emission of electrons by certain 
elements and compounds when the energy of the surface atoms is raised by the 
addition of heat (thermionic emission), by incident light photons 
(photoelectric emission), by kinetic energy of bombarding particles (secondary 
emission), or by potential energy (field emission). The second phenomenon is 
the control of the movement of electrons within an evacuated enclosure by 
forces exerted upon them by electric and magnetic fields.  See  ELECTRON 
EMISSION; ELECTRON MOTION IN VACUUM. 
    Vacuum tubes consist of an electrode capable of electron emission and one 
or more electrodes for collecting the emitted electrons and for establishing 
variable electric fields in order to control the movement of the electrons 
between the emitting electrode and the collecting electrode or electrodes. 
Where magnetic fields are required, they are produced by permanent magnets or 
by electromagnets, usually external to the evacuated space of the tube. In 
thermionic vacuum tubes, the primary source of electrons is thermionic 
emission, and control is by electric fields. The emitting electrode is called 
the cathode, or filament when it is filamentary in form, and the collecting 
electrodes are called anodes. The principal anode is usually called the plate. 
Control electrodes are termed grids.  See  THERMIONIC EMISSION. 
  
    Electrode configurations.  Because characteristics and functions of vacuum 
tubes are determined by the number and configurations of the electrodes that 
the tubes incorporate, one of the most useful classifications of vacuum tubes 
is by number of electrodes. 
  
    The diode. The simplest tube is the diode; it has only two electrodes: the 
cathode, which emits electrons, and the anode or plate, which collects the 
electrons. In most diodes, electrodes have the form of concentric cylinders, 
although plane structures are also used. Utility of the diode rests mainly in 
the fact that conduction between plate and cathode occurs only when the plate 
is positive with respect to the cathode; when the plate is negative, electrons 
emitted by the cathode are prevented from reaching the plate.  See  DIODE. 
  
    The triode. A three-electrode tube, called a triode, is formed by the 
addition of a grid between the cathode and the plate of a diode. Electrons, in 
moving from the cathode to the plate, can pass through openings in the grid. 
The principal value of this third electrode is that large plate current and 
plate-circuit power can be controlled by small variations of grid voltage and 
with the expenditure of little power in the grid circuit. 
  
    The tetrode. In the use of vacuum triodes in very-high-frequency 
amplifiers, undesirable oscillation may result from the feedback of 
plate-circuit power to the grid circuit through the capacitance between plate 
and grid. Although this difficulty can be avoided by making circuit 
modifications, a much more desirable solution is the reduction of the 
grid-plate capacitance. This can be accomplished by placing between grid and 
plate a fourth electrode, which functions as an electrostatic shield. The name 
of this electrode, screen grid, is descriptive both of its function and of its 
physical structure. The first grid, which is used to control the plate 
current, is called the control grid. Such a four-electrode tube is called a 
tetrode.  
  
    The pentode. The negative-slope region of the tetrode plate 
characteristics, which is undesirable in most applications, can be eliminated 
by the addition of a third grid located between the screen grid and the plate. 
The name of this grid, suppressor grid, is indicative of its function in 
suppressing secondary-electron plate current. 
    Special tubes containing four, five, or six grids have been developed for 
special applications.  
 �34s[Herbert J. Reich]
�33s 
  
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�6v�2v�32sVacuum-tube amplifier   �21m �33s 
    An amplifier which depends upon vacuum tubes for its operation. The basic 
electron-tube amplifier consists of a vacuum triode tube and associated 
circuitry, as shown in the illustration. The signal voltage to be amplified is 
applied to the input, or grid, circuit and the amplified signal appears across 
the load resistor RsubL in the anode (plate), or output, circuit. A gain in 
voltage, current, and power is thereby provided. By selection of tubes and 
associated circuitry, these gains may be maximized. 
    In normal operation a small negative voltage called a bias voltage, 
VsubGsubG, is applied to the grid of tube. This voltage establishes a definite 
plate-to-cathode current through the tube for a fixed load resistor RsubL and 
high-potential plate-supply voltage VsubPsubP. This plate current IsubP is 
called the quiescent current since it is the zero-signal current. A dc voltage 
drop IsubPRsubL appears across the output terminals because of the flow of the 
quiescent current through the load resistor. In the vacuum tube, electrons are 
emitted by the cathode and pass through the grid to the plate. The direction 
of ``conventional'' current is opposite to that of the electron, and therefore 
the plate current (conventional notation) flows from plate to cathode. See 
VACUUM TUBE. 
    If a small signal voltage is added to the grid bias voltage, the 
instantaneous grid voltage UPSILONsubG is the algebraic sum of the bias 
voltage VsubGsubG and the signal voltage UPSILONsubG. As the instantaneous 
grid voltage changes, the plate current isubP changes in proportion to the 
grid voltage changes. The result is a net plate current isubP that is the 
algebraic sum of the quiescent plate current IsubP and a varying or signal 
component of plate current eUPSILON. See AMPLIFIER. 
 �34s[Christos C. Halkias ;Charles ]
�33s 
  
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�6v�2v�32sVacuum-tube voltmeter   �21m �33s 
    Any of several types of instrument in which vacuum tubes, acting as 
amplifiers or rectifiers, are used in circuits for the measurement of ac or dc 
voltage. The various types of ac vacuum-tube voltmeters all derive a direct 
current from the voltage to be measured, and use it to operate an indicating 
meter. The vacuum-tube voltmeter is the principal instrument for voltage 
measurements at high frequencies, as the frequency limit is determined 
essentially by the rectifier characteristics. It is also the usual test 
instrument in a wide variety of applications where little power can be taken 
from the source.  See  VOLTMETER. 
    The diode rectifier - amplifier meter is probably the most widely used 
meter. It has separate tubes for the rectification and dc amplification 
functions, permitting an optimum design for each. The dc amplifier, which may 
have more than one tube, usually employs inverse feedback for the 
stabilization of gain. The diode rectifier can be specially designed for 
high-frequency performance and is usually mounted in a small probe which can 
be put at the point in the circuit where the voltage is to be measured.  See  
DIRECT-COUPLED AMPLIFIER. 
 �34s[W. Norris Tuttle]
�33s 
  
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�6v�2v�32sVacuum-tube amplifier �21m �33s
    An amplifier which depends upon vacuum tubes for its operation. The basic
electron-tube amplifier consists of a vacuum triode tube and associated
circuitry, as shown in the illustration. The signal voltage to be amplified is
applied to the input, or grid, circuit and the amplified signal appears across
the load resistor RsubL in the anode (plate), or output, circuit. A gain in
voltage, current, and power is thereby provided. By selection of tubes and
associated circuitry, these gains may be maximized.
    In normal operation a small negative voltage called a bias voltage,
V(sub GG), is applied to the grid of tube. This voltage establishes a definite
plate-to-cathode current through the tube for a fixed load resistor RsubL and
high-potential plate-supply voltage V(sub PP). This plate current I(sub P) is
called the quiescent current since it is the zero-signal current. A dc voltage
drop I(sub P)R(sub L) appears across the output terminals because of the flow of
the quiescent current through the load resistor. In the vacuum tube, electrons
are emitted by the cathode and pass through the grid to the plate. The direction
of ``conventional'' current is opposite to that of the electron, and therefore
the plate current (conventional notation) flows from plate to cathode. See
VACUUM TUBE.
    If a small signal voltage is added to the grid bias voltage, the
instantaneous grid voltage UPSILON(sub G) is the algebraic sum of the bias
voltage V(sub GG) and the signal voltage UPSILON(sub G). As the instantaneous
grid voltage changes, the plate current i(sub P) changes in proportion to the
grid voltage changes. The result is a net plate current i(sub P) that is the
algebraic sum of the quiescent plate current I(sub P) and a varying or signal
component of plate current e(sub UPSILON). See AMPLIFIER.
 �34s[Christos C. Halkias ;Charles L. Alley]
�33s

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�6v�2v�32sVaginal disorders �21m �33s
    Congenital defects, inflammations, tumors, trauma, and functional
disorders of the vagina.  See  REPRODUCTIVE SYSTEM.
    Congenital defects are uncommon but may include absence of the vagina,
failure of its growth, or abnormal formations such as double vagina.
Congenital cysts, although common, are usually unimportant.
    Inflammations of the vagina ordinarily are extensions from infections of
the external organs, but there are four types of vaginitis which account for a
high proportion of gynecologic disease. All forms are marked by pruritus
(itching) and inflammation of variable degree, and they often cause a
discharge of some type. Gonorrheal vulvovaginitis is seen almost always in
children, trichomonal vaginitis results from a protozoan infection, and
monilial vaginitis is a fungus infection similar to thrush. Senile vaginitis
occurs in elderly women following chronic irritation of the thinned, atrophic
mucosa.  See  CANDIDIASIS; GONORRHEA; TRICHOMONIASIS.
    The most important primary lesion of the vagina is carcinoma but its
incidence is low. However, extension of the much more common cervical
carcinoma to the vagina occurs in advanced cases. In both types, the prognosis
is dependent largely upon time of diagnosis and extent of spread.  See
ONCOLOGY.
    Vaginismus is primarily a functional disorder marked by spasmodic,
painful, and involuntary contraction of the lower vaginal muscles. Although
usually psychic in origin, it may result from remediable physical causes.
 �34s[Edward G. Stuart /N. Karle Mottet]
�33s

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�6v�2v�32sValence �21m �33s
    A term commonly used by chemists to characterize the combining power of an
element for other elements, as measured by the number of bonds to other atoms
which one atom of the given element forms upon chemical combination. The term
also has come to signify the theory of all the physical and chemical
properties of molecules that specially depend on molecular electronic
structure.
    Thus, in water, H2O, the valence of each hydrogen atom is 1; the valence
of oxygen, 2. In methane, CH4, the valence of hydrogen again is 1; of carbon,
4. In NaCl and CCl4 the valence of chlorine is 1, and in CH2 the valence of
carbon is 2.  See  CHEMICAL BONDING.
    Most of the simple facts of valence (though certainly not all) follow from
the postulate that atoms combine in such a way as to seek closed-shell or
inert-gas structures (rule of eight) by the transfer of electrons between them
or the sharing of a pair of electrons between them. Many molecular structures
may be obtained by inspection using these rules. Letting a dot represent an
electron,
		      H 		H
		      ..		..		       ..
		     :O :H	      H:C :H	    [NA]+     [:F :]
		      ..		..		       ..
					H

In these electron-dot symbols, the electrons in the K shell are not included
for atoms after He, nor are the electrons in the K and L shells for atoms
following Ne.
    As generally used and here defined, the word valence is ambiguous. Before
a value can be assigned to the valence of an atom in a molecule, the
electronic structure of the molecule must be exactly known, and this structure
must be describable simply in terms of simple bonds. In practice neither of
these conditions is ever precisely fulfilled. A term not so ambiguous is
oxidation number or valence number. Oxidation numbers are useful for the
balancing of oxidation-reduction equations, but they are not related simply to
ordinary valences. Thus the valence of carbon in CH4, CHCl3, and CCl4 is 4;
oxidation numbers of carbon in these three substances are -4, +2, and +4.  See
  CHEMICAL STRUCTURES; ELECTRONEGATIVITY; MOLECULAR ORBITAL THEORY;
OXIDATION-REDUCTION.
 �34s[Robert G. Parr]
�33s

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�6v�2v�32sValence band   �21m �33s 
    The highest electronic energy band in a semiconductor or insulator which 
can be filled with electrons. The electrons in the valence band correspond to 
the valence electrons of the constituent atoms. In a semiconductor or 
insulator, at sufficiently low temperatures, the valence band is completely 
filled and the conduction band is empty of electrons. Some of the high energy 
levels in the valence band may become vacant as a result of thermal excitation 
of electrons to higher energy bands or as a result of the presence of 
impurities. The net effect of the valence band is then equivalent to that of a 
few particles which are equal in number and similar in motion to the missing 
electrons but each of which carries a positive electronic charge. These 
``particles'' are referred to as holes.  See  BAND THEORY OF SOLIDS; 
CONDUCTION BAND; ELECTRIC INSULATOR; HOLES IN SOLIDS; SEMICONDUCTOR; VALENCE. 
 �34s[Hsu Y. Fan]
�33s 
  
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�6v�2v�32sValine   �21m �33s 
    An amino acid considered essential for normal growth of animals. The 
biosynthetic precursor, as well as the deamination product of valine, is 
ALPHA-ketoisovaleric acid. Valine is biosynthesized from pyruvic acid. Most or 
all of the enzymes concerned also catalyze the analogous reactions in 
isoleucine biosynthesis.  See  AMINO ACIDS; ISOLEUCINE. 
 �34s[Edward A. Adelberg]
�33s 
  
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�6v�2v�32sValue engineering   �21m �33s 
    A thinking system (also called value management or value analysis) used to 
develop decision criteria when it is important to secure as much as possible 
of what is wanted from each unit of the resource used. The resource may be 
money, time, material, labor, space, energy, and so on. The system is unique 
in that it effectively uses both knowledge and creativity, and provides 
step-by-step techniques for maximizing the benefits from both. It promotes 
development of alternatives suitable for the future as well as the present. 
This is accomplished by identifying and studying each function that is wanted 
by the customer or user, then applying knowledge and creativity to achieve the 
desired function. Resources are converted into costs to achieve direct, 
meaningful comparisons. By using the methods of value engineering, 15 to 40% 
reduction in the required resources often results. 
    Value engineering has applications in five broad areas: in design, 
purchase, and manufacture of products; in administrative groups, private or 
public, where the task is to achieve accomplishment through people; in all 
areas of social service work, such as hospitals, insurance services, or 
colleges; in architectural design and construction; and in development as well 
as research. 
    The system is used to improve value in either or both of two situations: 
(1) The product or service as used or as planned may provide 100% of the 
functions the user wants, but lower costs may be needed. The system then holds 
those functions but achieves them at lower cost. (2) The product or service 
may have deficiencies, that is, it does not perform the desired functions or 
lacks quality, and so also lacks good value. The system aims at correcting 
those deficiencies, providing the functions wanted, while at the same time 
holding the use of resources (costs) at a minimum.  See  INDUSTRIAL COST 
CONTROL; INDUSTRIAL ENGINEERING; METHODS ENGINEERING; OPERATIONS RESEARCH; 
OPTIMIZATION; PROCESS ENGINEERING; PRODUCTION ENGINEERING; PRODUCTION PLANNING. 
 �34s[Lawrence D. Miles]
�33s 
  
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�6v�2v�32sValvatida   �21m �33s 
    An order of Asteroidea in which pedicellariae, when present, are valvate 
and sunk into the skeletal ossicles. The mouth plates are inconspicuous, and 
the inferomarginal and superomarginal plates correspond and lack intermarginal 
grooves. There are two rows of tube feet. The order is thought to include 
seven families.  See  ASTEROIDEA; ECHINODERMATA. 
 �34s[Andrew C. Campbell]
�33s 
  
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�6v�2v�32sValve   �21m �33s 
    A flow-control device. Valves are used to regulate the flow of fluids in 
piping systems and machinery. In machinery the flow phenomenon is frequently 
of a pulsating or intermittent character and the valve, with its associated 
gear, contributes a timing feature. 
    The valves commonly used in piping systems are gate valves (Fig. 1), 
usually operated closed or wide open and seldom used for throttling; globe 
valves, frequently fitted with a renewable disk and adaptable to throttling 
operations; check valves, for automatically limiting flow in a piping system 
to a single direction; and plug cocks, for operation in the open or closed 
position by turning the plug through 90DG and with a shearing action to clear 
foreign matter from the seat. Safety and relief valves are automatic 
protective devices for the relief of excess pressure.  See  SAFETY VALVE. 
  
    For hydraulic turbines and hydroelectric systems, valves and gates control 
water flow for (1) regulation of power output at sustained efficiency and with 
minimum wastage of water, and (2) safety under the inertial flow conditions of 
large masses of water. 
    To control the kinematics of the cycle, steam-engine valves range from 
simple D-slide and piston valves to multiported types. Many types of reversing 
gear have been perfected which use the same slide valve or piston valve for 
both forward and backward rotation of an engine, as in railroad and marine 
service.  See  STEAM ENGINE. 
    Poppet valves are used almost exclusively in internal combustion 
reciprocating engines because of the demands for tightness with high operating 
pressures and temperatures (Fig. 2). Two-cycle engines utilize ports, 
alternately covered and uncovered by the main piston, for inlet or exhaust.  
See  CAM MECHANISM; INTERNAL COMBUSTION ENGINE; VALVE TRAIN. 
  
    In compressors, valves are usually automatic, operating by pressure 
difference on the two sides of a movable, spring-loaded member and without any 
mechanical linkage to the moving parts of the compressor mechanism. Like those 
for compressors, pump valves are usually of the automatic type operating by 
pressure difference. 
 �34s[Theodore Baumeister]
�33s 
  
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�6v�2v�32sValve train   �21m �33s 
    The valves and valve-operating mechanism by which an internal combustion 
engine takes air or a fuel-air mixture into the cylinders and discharges 
combustion products to the exhaust.  See  VALVE. 
    In an internal combustion engine the fuel takes up little space but  needs 
air with which to combine. Thus the power output of an engine is limited by 
its air-pumping capacity. It is essential that the flow through the engine be 
restricted as little as possible. This is the first requirement for valves. 
The second is that they close off the cylinder firmly during the compression 
and power strokes.  See  INTERNAL COMBUSTION ENGINE. 
    In most four-stroke engines, the valves are of this inward-opening poppet 
type, with the valve head ground to fit a conical seat in the cylinder block 
or cylinder head. Valve action requires that the valve be streamlined and as 
large as possible to give maximum flow, yet be of low inertia so that it 
follows the prescribed motion at high engine speed. 
    Engine valves are usually opened by means of cams. Riding on each cam is a 
follower, or valve lifter, which may be a flat or slightly convex surface, or 
a roller (see illustration). The valve is opened by forces applied to the end 
of the valve stem through a mechanical linkage actuated by the cam follower. 
In overhead valve engines, the camshaft may be mounted on the cylinder head 
near the valves.  See  CAM; HYDRAULIC VALVE LIFTER. 
 �34s[Augustus R. Rogowski]
�33s 
  
  
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�6v�2v�32sValvifera   �21m �33s 
    A suborder of isopod crustaceans resembling woodlice and pill bugs. They 
are distinguished from these and other isopods by having a pair of flat, 
valvelike uropods which hinge laterally and fold inward beneath the rear part 
of the body. The uropods open like doors to reveal five pairs of leaflike 
appendages, or pleopods, which are used for respiration and swimming. Members 
of the group are almost exclusively marine. Some species are omnivorous and 
have been found in dense populations on accumulations of broken algae and fish 
waste.  See  ISOPODA. 
 �34s[Ernest Naylor]
�33s 
  
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�6v�2v�32sVampyromorpha   �21m �33s 
    An order of dibranchiate cephalopod mollusks. Vampyroteuthis infernalis, 
an inhabitant of the deeper waters of tropical and temperate seas, is the only 
representative of this order. It is considered to be a link form between the 
decapod and octopod mollusks.  See  CEPHALOPODA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sVanadium �21m �33s
    A chemical element, V, atomic number 23, atomic weight 50.9414, melting
point 1917 DG C (3483 DG F), and density 6.1 g/cm(sup 3) (0.22 lb/in.[sup 3]).
Vanadium is a metal which has been used primarily as an alloying element for
steels and irons. Several of the compounds of vanadium are used in the chemical
industry, notably in making oxidation catalysts, and in the ceramic industry as
coloring agents.
    Like some other transition elements, vanadium forms numerous and
frequently complicated compounds because of its variable valence. It has at
least three oxidation states, 2+, 3+, and 5+. It is amphoteric, mostly basic
in the lower oxidation states, acidic in the higher ones. It forms derivatives
from more or less well-defined radicals such as VO(sup 2+) and VO(sup 3+).
See  METALLOACID ELEMENTS; TRANSITION ELEMENTS.
    In its pure form vanadium is soft and ductile. It can be hot- and
cold-worked easily, but it must be heated in an inert atmosphere or in a
vacuum because it oxidizes readily at temperatures above the melting point of
its oxide. The metal retains its strength unusually well at elevated
temperatures. The resistance of vanadium to hydrochloric and sulfuric acids is
outstanding, and it withstands aerated-salt-water attack better than most
stainless steels. However, vanadium cannot withstand nitric acid.  See	VACUUM
METALLURGY.
 �34s[Timothy W. Merrill]
�33s

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�6v�2v�32sVan Allen radiation   �21m �33s 
    The high-energy, charged particles which are trapped by the geomagnetic 
field and form belts of intense radiation in space about the Earth. The belts 
consist primarily of electrons and protons and extend from a few hundred 
kilometers above the Earth to a distance of about 8Re    (Re    = radius of 
Earth = 6371 km or 3959 mi). The belts of trapped particles were discovered by 
James Van Allen and coworkers in 1958 through radiation detectors carried on 
satellites Explorer I  and Explorer 3. 
    A charged particle under the influence of the geomagnetic field follows a 
trajectory which can be conveniently described as a superposition of three 
separate motions. The first motion is a rapid spiral about magnetic field 
lines that is produced by the magnetic force acting at right angles to both 
the particle velocity and to the magnetic field. As the spiraling particle 
moves along the field line toward either the North or South Pole, the increase 
in magnetic field strength causes the particle to be reflected so that it 
bounces between Northern and Southern hemispheres. Superimposed on the spiral 
and bounce motions is a slow east-west drift, electrons drifting eastward and 
protons drifting westward. Thus individual trapped particles move completely 
around the Earth in a complicated pattern, their motion being constrained to 
lie on magnetic shells. 
    The Van Allen belts are one feature of the general plasma environment of 
space. Plasma from the Sun is continually impinging on the Earth's magnetic 
field, and ionospheric electrons and ions also form a plasma which at times is 
accelerated. Some of the electrons and ions of these plasma populations become 
trapped in the geomagnetic field, forming the Van Allen belts. 
 �34s[Martin Walt]
�33s 
  
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�6v�2v�32sVan de Graaff generator   �21m �33s 
    A high-voltage electrostatic generator in which electrical charge is 
carried from ground to a high-voltage terminal by means of an insulating belt. 
Current capacity is limited to a few milliamperes. Each generator is equipped 
with an evacuated tube through which charged particles of any type may be 
accelerated. Machines equipped with positive ion sources and arranged for 
positive ion acceleration are widely used for research applications. Machines 
equipped with electron sources are used for x-ray therapy, industrial 
radiography, food and drug sterilization, and research. 
    Principles of operation of the Van de Graaff generator are most easily 
understood from study of a simple model as shown in the illustration. The 
charging belt, which is made of insulating material such as rubberized fabric, 
is carried by two pulleys. One, at ground potential, is driven by an electric 
motor, and the second is in the high-voltage terminal. The terminal is a metal 
enclosure which must be smooth and well rounded over its exterior surface. The 
terminal is supported by a cylinder of insulating material. The base is 
enclosed by metal with a smooth, well-rounded exterior surface, and this 
enclosure is at ground potential. Charge is sprayed onto the belt from a row 
of needle points or a fine wire held at a potential of about 10 kilovolts. A 
row of needles near the upper pulley with points close to the belt will remove 
charge from the belt, and the charge will pass to the outer surface of the 
terminal. Terminal voltage will rise to some limit determined by sparkover 
through air, by insulator failure, or by a load such as ion current through an 
evacuated tube.  See  ELECTROSTATICS; PARTICLE ACCELERATOR. 
 �34s[Raymond G. Herb]
�33s 
  
  
A700000001311�254s�5v�1v1833
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�6v�2v�32sVan der Waals equation �21m �33s
    An empirical equation of state which takes into account the finite size of
the molecules and the attractive forces between them. For a homogeneous gas at
pressure p, molar volume vsubN, and absolute temperature T, the equation is
given below.


p = [R(sub u)T/(V[sub n] - b)] -[a/(v[sub n])(sup 2)

or

(p + [a/(v[sub n])(sup 2)] (v[sub n] - b) = R(sub u)T


The terms a and b are constants and evaluated from experimental data. The molar
specific volume vsubN is equal to the total volume V divided by the number of
moles N. Rsubu is the universal gas constant.  See  THERMODYNAMIC PRINCIPLES.
 �34s[George A. Hawkins]
�33s

A700000000740�254s�5v�1v1833
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�6v�2v�32sVane anemometer   �21m �33s 
    A portable instrument for measuring low wind speeds and airspeeds in large 
ducts. It consists of a number of vanes radiating from a common shaft and set 
to rotate when facing the wind; a guard ring surrounds the vanes. The vanes 
operate a counter to indicate the number of rotations, which when timed with a 
stopwatch serve to determine the speed. See ANEMOMETER. 
 �34s[Herbert Fox]
�33s 
  
A700000001506�254s�5v�1v1833
\IMG\CI1801.IMG
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�6v�2v�32sVanilla �21m �33s
    A choice flavoring obtained from a climbing orchid, Vanilla fragrans, a
native of tropical American forests. Its fruits are pods called vanilla beans.
These are picked at the proper time before they have fully matured.
    Vanillin (4-hydroxy-3-methoxybenzaldehyde) is the principal component of
vanilla, although other components contribute to the distinctive flavor of the
extract compared to synthetic vanilla. When they are harvested, the beans
contain no free vanillin; it develops during the curing period from glucosides
that break down during the fermentation and sweating of the beans. The
sweating process consists of alternately drying the beans in sunlight and
bunching them so that they heat and ferment.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

A700000003562�254s�5v�1v1833
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�6v�2v�32sVapor condenser   �21m �33s 
    A heat-transfer device that reduces a thermodynamic fluid from its vapor 
phase to its liquid phase. The vapor condenser extracts the latent heat of 
vaporization from the vapor, as a higher-termperature heat source, by 
absorption in a heat-receiving fluid of lower temperature. The vapor to be 
condensed may be wet, saturated, or superheated. The heat receiver is usually 
water but may be a fluid such as air, a process liquid, or a gas. When the 
condensing of vapor is primarily used to add heat to the heat-receiving fluid, 
the condensing device is called a heater and is not within the normal 
classification of a condenser. 
    Condensers may be divided into two major classes according to use: those 
used as part of a processing system (process condensers) and those used for 
serving engines or turbines in a steam power plant cycle (power cycle 
condensers). Condensers may be further classified according to mode of 
operation as surface condensers or as contact condensers.  See  CONTACT 
CONDENSER; SURFACE CONDENSER. 
    Condensers are required, almost without exception, to condense impure 
vapors, that is, vapors containing air or other noncondensable gases. Because 
most condensers operate at subatmospheric pressures, air leaking into the 
apparatus or system becomes a common cause for vapor contamination, and a 
variety of designs have been developed to reduce such problems. Accumulation 
of noncondensable gases seriously affects heat transfer, and means must be 
provided to direct them to a suitable outlet. Most surface and contact 
condensers are arranged with a separate zone of heat-transfer surface within 
the condenser and located at the outlet end of the vapor flow path for 
efficient removal of the noncondensable gases through dehumidification. 
 �34s[Joseph F. Sebald]
�33s 
  
A700000003044�254s�5v�1v1833
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1834
\IMG\CI1801.IMG
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�6v�2v�32sVapor cycle   �21m �33s 
    A thermodynamic cycle, operating as a heat engine or a heat pump, during 
which the working substance is in, or passes through, the vapor state. A vapor 
is a substance at or near its condensation point. It may be wet, dry, or 
slightly superheated. One hundred percent dryness is an exactly definable 
condition which is only transiently encountered in practice. See HEAT PUMP; 
THERMODYNAMIC CYCLE. 
    A steam power plant operates on a vapor cycle where steam is generated by 
boiling water at high pressure, expanding it in a prime mover, exhausting it 
to a condenser, where it is reduced to the liquid state at low pressure, and 
then returning the water by a pump to the boiler (Fig. 1). 
    In the customary vapor-compression refrigeration plant, the process is 
essentially reversed with the refrigerant evaporating at low temperature and 
pressure, being compressed to high pressure, condensed at elevated 
temperature, and returned as liquid refrigerant through an expansion valve to 
the evaporating coil (Fig. 2).  See  REFRIGERATION. 
    The Carnot cycle, between any two temperatures, gives the limit for the 
efficiency of the conversion of heat into work. This efficiency is independent 
of the properties of the working fluid. The Rankine cycle is more realistic in 
describing the ideal performance of steam power plants and vapor-compression 
refrigeration systems.  See  CARNOT CYCLE; RANKINE CYCLE. 
 �34s[Theodore Baumeister]
�33s 
  
  
A700000002364�254s�5v�1v1834
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�6v�2v�32sVapor lamp   �21m �33s 
    A source of radiant energy excited by a supply of electricity which 
creates a current of ionized gas between electrodes in an enclosure that 
contains the arc while permitting transmission of the radiant energy. 
Gaseous-discharge lamps or vapor lamps are given various names relating to the 
element responsible for the majority of the radiation (mercury, sodium 
metal-halide, xenon), to the physical attribute of the lamp (short-arc, 
high-pressure), or, in the case of fluorescent lamps, to the way a phosphor on 
the bulb wall fluoresces as a result of the lamp's low-pressure mercury-vapor 
excitation.  See  ARC DISCHARGE; ELECTRICAL CONDUCTION IN GASES. 
    Gaseous-discharge lamps are broadly used throughout the world because the 
conversion of electric energy to radiant energy in a gaseous discharge 
provides radiation in narrow bands within the range of visible light in which 
the rods and cones of the eye are most sensitive. These light sources have 
high efficiency in conversion of electricity to light. 
    For discussions of common types of vapor lamps  see  FLUORESCENT LAMP; 
MERCURY-VAPOR LAMP; NEON GLOW LAMP; SODIUM-VAPOR LAMP. 
 �34s[T. F. Neubecker]
�33s 
  
A700000001416�254s�5v�1v1834
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�6v�2v�32sVapor lock   �21m �33s 
    Interruption of fuel flow to an engine due to blockage of passages in the 
fuel system by fuel vapor. 
    To promote easy starting, all gasolines contain volatile constituents 
which under some conditions, such as high ambient temperature, tend to produce 
more vapor than the fuel pump and carburetor vents can handle. The very action 
of the fuel pump, in decreasing the pressure at its inlet, tends to vaporize 
the fuel. If the vapor forms faster than the pump can draw it from the fuel 
line, the flow of fuel to the carburetor is effectively stopped and the engine 
stalls.  See  AIRCRAFT FUEL; CARBURETOR; FUEL SYSTEM; GASOLINE; INTERNAL 
COMBUSTION ENGINE. 
 �34s[Augustus R. Rogowski]
�33s 
  
A700000003066�254s�5v�1v1834
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�6v�2v�32sVapor pressure   �21m �33s 
    The saturation pressures exerted by vapors which are in equilibrium with 
their liquid or solid forms. One of the most important physical properties of 
a liquid, the vapor pressure, enters into many thermodynamic calculations and 
underlies several methods for the determination of the molecular weights of 
substances dissolved in liquids. For a discussion of the vapor pressure 
relationships of solids  see  SUBLIMATION.  See also  MOLECULAR WEIGHT; 
SOLUTION.  
    If a liquid is introduced into an evacuated vessel at a given temperature, 
some of the liquid will vaporize, and the pressure of the vapor will attain a 
maximum value which is termed the vapor pressure of the liquid at that 
temperature. Although the quantity of liquid remaining does not diminish 
thereafter, the process of evaporation does not cease. A dynamic equilibrium 
is established, in which molecules escape from the liquid phase and return 
from the vapor phase at equal rates.  See  EVAPORATION. 
    It is important to make a distinction between the vapor pressure of a 
liquid, as described above, and the pressure of a vapor. The vapor pressure of 
a pure liquid is a unique and characteristic property of the liquid and 
depends only upon the temperature. A gas or vapor may, on the other hand, 
exert any pressure within reason, depending upon the volume to which it is 
confined, provided it is not in contact with its liquid phase.  See  CRITICAL 
PHENOMENA; PHASE EQUILIBRIUM. 
 �34s[Norman H. Nachtrieb]
�33s 
  
A700000001670�254s�5v�1v1834
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�6v�2v�32sVaractor   �21m �33s 
    A semiconductor diode designed to maximize the variation of its 
capacitance with applied reverse bias voltage. Such diodes are also often 
called variable capacitance diodes. Today they are almost exclusively junction 
diodes. The free-charge depletion region of any pn junction in a semiconductor 
widens with the application of reverse bias. Since the surfaces of this 
free-charge depletion region represent the effective plates of the capacitance 
of the junction, the application of increasing reverse bias voltage will cause 
a decrease in the capacitance. The design and fabrication of varactors then 
reduces to the art of doping the two sides of the junction in such a way that 
the desired voltage variation of depletion region width (and hence 
capacitance) is obtained.  See  JUNCTION DIODE; SEMICONDUCTOR; SEMICONDUCTOR 
DIODE.  
 �34s[Lloyd P. Hunter]
�33s 
  
A700000003003�254s�5v�1v1834
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1835
\IMG\CI1801.IMG
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�6v�2v�32sVariable-area flowmeter �21m �33s
    A volume flow rate meter based on the principle of a variable restrictor
in the flowing stream being forced by the fluid to a position to allow the
required flow-through. Those that depend upon gravity provide essentially a
constant differential pressure, while those that supplement the gravity with a
spring will have a variable differential pressure.
    In the tapered-tube rotameter (illustration  a ), the fluid flows upward
through a tapered tube, lifting a shaped weight (possibly misnamed a float) to
a position where the upward fluid force just balances the float weight,
thereby giving a float position that indicates the flow rate.
    In a piston type (illustration  b ) the buoyant force of the liquid
carries the piston upward until a sufficient area has been uncovered in a slot
in the side of the vertical tube to allow the liquid to flow through the slot.
The position of the piston indicates the flow rate and, by properly shaping
the slot, this type of meter may be made with a uniform flow scale.
    In the tapered-plug design (illustration  c ), the plug, located in an
orifice, is raised until the opening is sufficient to handle the fluid flow.
Readouts can be the same as in the tapered-tube design.
ms pgs. 2594-25969:10
    A spring-loaded type (illustration	d ) provides a differential pressure
for a secondary device.  See  VOLUME FLOW RATE METER.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s

A700000007136�254s�5v�1v1835
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�6v�2v�32sVariable star   �21m �33s 
    A star that has a detectable change in its intensity, which is often 
accompanied by other physical changes. The changes in brightness of a variable 
star may be a few thousandths of a magnitude up to 20 magnitudes or even more. 
The cause of the variation may be either an eclipsing effect caused by the 
motions within a system of two or more bodies or an actual physical change 
within the star itself or in its surrounding atmosphere.  
    Most intrinsic variables are either pulsating or eruptive (see table). 
They may  be separated into three main divisions - those which vary with 
periodic regularity, those with cyclic or semiregular changes, and those which 
are completely erratic, or irregular. A few variables are unique and cannot be 
placed in any of these classes, and others seem to be combinations of several 
classes.  
    Periodic variables are divided into two main classes: long-period, or 
Mira-type, variables and cepheids. There are many intermediate examples, so 
that no definite dividing lines may be drawn. Mira-type variables are by far 
the most numerous of the known variables in the Milky Way Galaxy. They are red 
giants with spectra of either class M or one of the carbon types, and usually 
have hydrogen emission lines. Cepheid variables are characterized by several 
important relations. They are among the most luminous stars known, and thus 
can be seen at great distances. There is a definite correlation between period 
and luminosity. As a result of this relationship, they are used as a measuring 
tape for the universe. Other correlations exist between period and spectrum 
(temperature), period and radial velocity, and period and form of light curve. 
Intrinsic variables with the shortest known period are the cluster-type 
cepheids or RR Lyrae stars. The RR Lyrae stars are fainter than the classical 
cepheids. They have an absolute photographic magnitude of 0.0 and show no 
progression of luminosity with period. Therefore, if an RR Lyrae star can be 
identified in a cluster or galaxy, and its apparent magnitude determined, the 
distance of that system may be obtained.  See  CEPHEIDS. 
    Nonperiodic variables may be separated to some extent by their spectra. 
Semiregular red giants are the most numerous and are closely related to the 
long-period variables. Amplitudes are small, ranging from less than 2 
magnitudes to infinitesimal amounts. Possibly all red stars with molecular 
bands in their spectra are variable. 
    Most red variables with periods less than 150 days are subject to great 
irregularities, and some lose all semblance of periodicity at times; others 
are completely irregular. 
    The most violent explosions or eruptions occur in the novae and 
supernovae. Increases of 10 - 12 magnitudes have been observed within a few 
hours. Previous to the explosion, novae are probably dense blue dwarfs.  See  
NOVA; SUPERNOVA. 
    Other stars which have rapid increases of brightness are the U Geminorum 
and Z Camelopardalis stars. These also are blue stars with dwarf 
characteristics and are often referred to as dwarf novae. Their eruptions 
appear at semiperiodic intervals. 
    A small group of novae, called recurrent novae, have been observed to have 
two or more explosions. Recurrent novae may be considered to be intermediate 
between true novae and the U Geminorum stars. 
    Nebular variables, also known as T Tauri, RW Aurigae, or Orion variables, 
are subject to rapid changes at irregular and unpredictable intervals.  See  
BINARY STAR; ECLIPSING VARIABLE STAR. 
 �34s[Margaret W. Mayall]
�33s 
  
A700000001001�254s�5v�1v1834
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�6v�2v�32sVariable (mathematics)   �21m �33s 
    A symbol x is usually defined to be a variable if it may denote any member 
of a set of objects. A variable is discrete or continuous according to whether 
its range (the set) is discrete (for example, a subset of the natural numbers) 
or continuous (for example, all real numbers between two real numbers), 
respectively.  See  ANALYTIC GEOMETRY; NUMBER THEORY; PARAMETRIC EQUATION. 
 �34s[Leonard M. Blumenthal]
�33s 
  
A700000001765�254s�5v�1v1835
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1836
\IMG\CI1801.IMG
5180416
�6v�2v�32sVaricose veins   �21m �33s 
    Varicose veins or varicosities are enlarged, tortuous vessels. They occur 
chiefly in superficial veins and their tributaries in the lower extremities. 
These veins are vulnerable to dilation because of their location in loose 
subcutaneous tissues which offer little support to the walls of the veins. 
Predisposing factors in the development of varices are (1) heredity, (2) 
venous obstruction, (3) phlebitis and thrombophlebitis, and (4) standing erect 
for long periods, which increases intravenous pressure. Varicose veins show 
loss of elastic tissue in their walls. They become dilated and finally may 
develop incompetent valves. 
    Hemorrhoids are varicosities of the veins deep to the anal epithelium, and 
varicoceles are varicosities of the veins surrounding the spermatic cord. Both 
conditions respond favorably to surgical treatment. See HEMORRHOIDS. 
 �34s[Frank A. Carone]
�33s 
  
A700000003571�254s�5v�1v1836
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�6v�2v�32sVariometer   �21m �33s 
    A geomagnetic device for detecting and indicating changes in one of the 
components of the magnetic field vector - usually magnetic declination, the 
horizontal intensity component, or the vertical intensity component. When used 
with a suitable recording unit, a set of three variometers forms a 
magnetograph.  
    The declination variometer, also called the D variometer, consists of a 
small permanent bar magnet suspended with a plane mirror from a fine quartz 
fiber. A lens fixed in front of the mirror serves to focus to a point a beam 
of light reflected from the mirror to recording paper mounted on a rotating 
drum. With no torsion in the fiber, the magnetic axis of the magnet will be 
aligned in the magnetic meridian. As the meridian changes its direction, 
because of normal daily variations or magnetic storm disturbances, the magnet 
follows.  
    Although a larger, stiffer  fiber is used, the horizontal intensity (H) 
variometer is essentially the same as the D instrument. Enough torsion in the 
fiber is introduced at the time of installation to cause the magnet to turn 
90DG out of the magnetic meridian. The magnet responds to an increase of H by 
turning about the suspension axis. 
    Employing a larger permanent magnet because of mechanical construstion 
requirements, the vertical intensity (Z) variometer is equipped with very fine 
steel knife-edges or pivots resting on agate planes or saddles and balanced so 
that its magnetic axis is horizontal. In this case, the torque due to the 
reaction of the magnetic moment with the Z component of the field is balanced 
against a torque due to the action of gravity on the laterally displaced 
center of mass of the magnet, and the magnet tilts one way or the other when 
the vertical field changes.  See  MAGNETOMETER. 
 �34s[John H. Nair]
�33s 
  
  
A700000001305�254s�5v�1v1836
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�6v�2v�32sVaristor   �21m �33s 
    Any variable resistor whose resistance depends upon voltage, current, or 
polarity. All semiconducting diodes are varistors. A varistor is generally 
made to have a symmetrical current-voltage relationship, but it can be 
nonsymmetrical.  
    Semiconductor rectifiers, of either the pn-junction or Schottky-barrier 
(hot-carrier) types, are popularly utilized for varsitors. A single rectifier 
has a nonsymmetrical characteristic. A symmetrical varistor is made, utilizing 
two rectifiers connected in parallel with opposing polarity (see 
illustration). #See  JUNCTION DIODE; SEMINCONDUCTOR RECTIFIER. 
 �34s[I. A. Lesk]
�33s 
  
A700000002232�254s�5v�1v1836
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�6v�2v�32sVarnish �21m �33s
    A transparent surface coating which is applied as a liquid and then
changes to a hard solid. Varnishes are solutions of resinous materials in a
solvent, and dry by the evaporation of the solvent or by a chemical reaction,
either with oxygen from the air or by some other means, including absorption
of atmospheric moisture.
    Spirit varnishes are those in which the evaporation of solvent is the only
drying process; the solvent is usually alcohol, although the term is used for
similar coatings made with other solvents. Shellac varnish, made by dissolving
shellac in alcohol, is the most common of this type. Oleoresinous varnishes
are made by treating a drying oil with a resin, usually with heat, and
dissolving the reaction product in a solvent, usually a petroleum fraction;
drying results from the evaporation of the solvent, followed by polymerization
of the drying oil portion, a reaction which is accelerated by metallic driers
added to the varnish. For a discussion of the mechanism of this drying action
see  DRIER (PLANT); SOLVENT; SURFACE COATING.
 �34s[C. R. Martinson ;C.N. Sisler]
�33s

A700000001033�254s�5v�1v1836
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�6v�2v�32sVarnish tree �21m �33s
    The plant Rhus vernicifera, also called lacquer tree, a member of the
sumac family (Anacardiaceae). It is a native of China, but has long been
cultivated in Japan. When the bark is cut, it exudes a milky juice which
darkens and thickens on exposure. This is the lacquer long used in China and
Japan. When properly applied, the thin transparent film becomes a varnish of
extreme hardness.  See	LACQUER; SAPINDALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

A700000001414�254s�5v�1v1836
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1837
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�6v�2v�32sVarve   �21m �33s 
    A distinctive, thin annual sedimentary layer, the lower part consisting of 
coarser, lighter-colored clay and silt that was deposited in summer, and the 
upper of a finer-grained, darker clay deposited in winter. Numerous successive 
varves, generally less than 1 in. (2.5 cm) thick, accumulated in temporary 
lakes near melting glaciers (see illustration). Thicker and thinner varves at 
different places can be matched like tree rings. Similar layers occur in 
ancient rocks, but it is difficult to determine whether the layers are annual; 
many of them represent longer or less regular sedimentary cycles.  See  
SEDIMENTATION (GEOLOGY). 
 �34s[J. Marvin Weller]
�33s 
  
  
A700000002353�254s�5v�1v1837
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�6v�2v�32sVectorcardiography   �21m �33s 
    An electrocardiographic technique which considers the resultant of all 
electrical manifestations of the heart beat as equivalent to a single dipole, 
defined as a positive point source followed by a negative point sink. For any 
instance of time during cardiac activity, this hypothetical entity has 
magnitude and direction and can therefore be treated as a vector quantity. By 
considering the surface electrocardiographic leads as vector components, size 
and direction of the cardiac vector forces can be obtained either by 
calculations (vecto electrocardiography) or by recording the resultant of two 
vector components on an oscilloscope screen. Combining the image of lead 
components obtained from three planes of the body, a three-dimensional 
reconstruction of the heart vector time course can be traced (spatial 
vectorcardiography).  
    Vectorcardiography has contributed relatively little to the diagnostic 
advance in electrocardiography, but has clarified the interrelations of lead 
connections and has allowed a remarkably simplified approach to the 
understanding of the complicated electrical activity of the heart. 
 �34s[Henry H. Hausner]
�33s 
  
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�6v�2v�32sVector (mathematics)   �21m �33s 
    A directed line segment. As such, vectors have magnitude and direction. 
Many physical quantities, for example, velocity, acceleration, and force, are 
vectors. Vectors are widely used in mathematical physics.  See  CALCULUS OF 
VECTORS.  
 �34s[Marvin Yelles]
�33s 
  
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�6v�2v�32sVeering wind   �21m �33s 
    A wind that changes direction in a clockwise sense; for example, a change 
from a southerly to a westerly direction. The wind may veer gradually, over a 
period of hours, or abruptly, in a few minutes or seconds on passage of a 
wind-shift line. The wind veers when a cyclone passes eastward on a path north 
of the observer. Veering with height is usually found on the east side of a 
cyclone, or west of an anticyclone, with warm air to the south. In the 
Southern Hemisphere the meaning in terms of cardinal directions is reversed.  
See  WIND. 
 �34s[Chester W. Newton]
�33s 
  
A700000000713�254s�5v�1v1837
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�6v�2v�32sVega   �21m �33s 
    One of the brightest stars in the sky, apparent magnitude 0.1. Vega, or 
ALPHA Lyrae, is a normal main sequence star of spectral type A0, having an 
effective temperature near 10,050 K (17,630DGF). Its distance is 8 parsecs, 
its absolute magnitude is +0.5 (40 times as bright as the Sun), and its radius 
is three times that of the Sun.  See  STAR. 
 �34s[Jesse L. Greenstein]
�33s 
  
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�6v�2v�32sVegetable ivory �21m �33s
    The seed of the tagua palm (Phytelephas macrocarpa) of tropical America.
Each drupelike fruit contains six to nine bony seeds. The extremely hard
endosperm of the seed is used as a substitute for ivory. Vegetable ivory can
be carved and tooled to make buttons, chessmen, knobs, inlays, and various
ornamental articles.  See  ARECALES.
 �34s[Perry D. Strausbaugh /Earl L. Core]
�33s

A700000015455�254s�5v�1v1837
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1838
\IMG\CI1801.IMG
5586432
�6v�2v�32sVegetation management   �21m �33s 
    The art and practice of manipulating vegetation, such as forest, 
scrublands, grasslands, and deserts, so as to produce and harvest a desired 
part or aspect of that material in higher quantity or quality, such as timber 
forage, wildlife, or food.  See  VEGETATION. 
    The integrated field of vegetation management lies between the broader and 
more inclusive fields of natural resource management (concerned both with 
renewable biological resources and with nonrenewable mineral resources) and of 
land-use planning on the one hand, and on the other hand such specialties as 
forestry, range management, wildlife management, and agriculture. 
    Desirable alterations in the processes of natural vegetation change are 
accomplished by a wide variety of techniques and methods which directly and 
indirectly affect the behavior of vegetation. The direct methods are commonly 
segregated into fire, physical methods, and chemical methods. 
  
    Fire.  Fire has probably been used to manage vegetation for over a million 
years. Desirable or undesirable vegetation types can be born in fire, 
maintained by fire, and destroyed by fire. For proper use in management, fire 
must be of the right kind at the right time under the right conditions. For 
example, Sequoia and Kings Canyon National Parks were instituting fire plots 
in 1959 - 1966 to rid the giant sequoias of the heavy conifer understory that 
had grown sequoias of the heavy conifer understory that had grown up during 
fire suppression days. In such desert areas as Saguaro National Monument, some 
fires are now allowed to burn. The role of fire in perpetuating the grasslands 
and savannas of the world (along with the roles of large herbivores and of the 
local soils and topography) is increasingly accepted. 
  
    Physical methods.  Physical methods can be symbolically expressed as the 
ax, the sickle, and the plow. When primitive humans first used the ax, they 
were selective, choosing only those kinds of trees which bore the fruits they 
wished to eat, the best hearth wood, the best timber for building, and the 
best foliage for their beasts to eat. This selectivity quickly changed the 
species composition of the forest, sometimes encouraging the advance of 
species that bore little if any resemblance to those of the original condition. 
    The sickle is to grassland what the ax is to woodland. Though originally 
used to harvest the heads from grains, this selective act turned to 
overexploitation which tended to destroy the very species that were desirable. 
As technology led to the development of machines, cutting tended to be 
indiscriminate. Larger and larger woody species were cut, ground, chipped, or 
otherwise harvested or ``controlled.'' A pattern emerged in which there was a 
sharp segregation between mowed grasslands and unmowed brushy lands; this is 
found in many parts of the world today. 
    Although the plow is essentially an agricultural tool, its use is 
important in the history of much land that has been abandoned and now bears 
seminatural vegetation. The plow not only turns over and homogenizes the upper 
layers of the soil which previously might have been distinctly stratified, but 
also chops up the rhizomes and roots of certain species of plants, tending to 
make them far more abundant than they otherwise would be. The temporarily bare 
soil, relatively free from competing plants, can be ideal for the 
establishment of many species, especially trees which would otherwise not 
become established. The effects of plowing may thus be discernible in managed 
vegetation for decades, even for centuries. 
  
    Chemical methods.  A direct method of affecting vegetation has been the 
use of chemical herbicides. Insecticides also play their role. The 
indiscriminate application of herbicides or insecticides such as DDT, more 
aptly termed ecocides, in order to control species of plants or insects has 
vast and ramifying effects throughout that total ecosystem, and far beyond its 
borders.  See  HERBICIDE; INSECTICIDE. 
  
    Indirect methods.  The major indirect methods of managing vegetation may 
be classified as those affecting water, soil, air, and animals. 
    Among the more obvious methods of managing water is the technology of dam 
building. Dams have enormous consequences to the entire landscape, including 
depletion of the deposits of sands and silts at ocean mouths, and thus to 
coastal phenomena related to long-shore currents.  See  DAM. 
    Controlled changes in lake levels can be  undesirable, as in the wide bare 
shores during seasonal drawdowns, but can also be desirable, as in use by 
certain shorebirds and in certain wildlife-management practices in which 
temporary drawdowns control excessive growth of undesirable aquatic plants. 
However, the lowering of water tables by excessive ``mining'' of water for 
human populations, such as for irrigation and urban use, is having subtle or 
direct effects on natural and seminatural vegetation types, and there are 
further problems of polluted water. Overfertilization by phosphates and 
nitrates from sewage disposal systems and septic tanks, and by runoff into 
river systems from farming practices, leads to population explosions of 
undesirable aquatic plants. Balances are further upset by the addition of 
herbicides to control these plants, while the basic nurtient problem which 
causes the imbalance is unaltered. Pollution by persistent insecticides, 
washed in from the land, carried in the atmosphere, or transported by 
organisms which accumulate them, is becoming ever more hazardous.  See  WATER 
CONSERVATION; WATER POLLUTION. 
    Fertilizing is one of the most common methods of managing soil, and thus 
of indirectly managing the vegetation. Fertilizing is generally thought of as 
only an agricultural practice, but it has growing potentialities in the 
management of forest lands and rangelands. Another approach in soil 
management, the analog of agricultural contour plowing, is the terracing of 
rangelands and watershed lands. The terraces greatly reduce superficial runoff 
of precipitation and increase infiltration into the soil, thus leading to 
marked changes in the nature of the local vegetation.  See  FERTILIZING; 
TERRACING (AGRICULTURE). 
    Temperature and precipitation control is a primary consideration among the 
approaches to managing the air. Temperature control was long accomplished 
locally in certain agricultural practices, such as the use of smudge pots in 
orange groves to prevent radiation at night during otherwise freezing 
temperatures. The technology of precipitation control is accomplished largely 
by seeding silver iodide at cloud-forming levels, or by electric discharges.  
See  CLIMATE MODIFICATION; CONSERVATION OF RESOURCES. 
 �34s[Frank E. Egler]
�33s 
  
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�6v�2v�32sVein   �21m �33s 
    The relatively thin-walled blood vessels that carry blood from the 
capillaries to the heart. Veins have several characteristics which 
differentiate them from their counterparts, the arteries. They are much more 
variable in location and distribution and are usually larger than the 
corresponding artery. Veins have thinner, less muscular walls and many contain 
valves which prevent backflow of blood, particularly in areas where gravity 
exerts a large effort.  See  ARTERY. 
    As is the case with the arterial system, two major circuits, the pulmonary 
and the systemic, are recognized. The former carries oxygenated blood from the 
lungs to the heart; the latter returns blood from all other tissues to the 
heart, either directly or through the portal system. In the adult human, the 
veins of the head, neck, upper extremities, and upper thorax empty into the 
large superior vena cava. Both of the caval vessels empty into the right 
atrium of the heart. A few veins, such as the coronary veins, drain directly 
into the heart itself.  See  BLOOD VESSELS; CARDIOVASCULAR SYSTEM; CIRCULATORY 
SYSTEM.  
 �34s[Walter J. Bock]
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�6v�2v�32sVelocity   �21m �33s 
    The time rate of change of position of a body in a particular direction. 
Linear velocity is velocity along a straight line, and its magnitude is 
commonly measured in such units as meters per second (m/s), feet per second 
(ft/s), and miles per hour (mi/h). Since both a magnitude and a direction are 
implied in a measurement of velocity, velocity is a directed or vector 
quantity, and to specify a velocity completely, the direction must always be 
given.  The magnitude only is called the speed.  See  SPEED. 
    A body need not move in a straight line path to possess linear velocity. 
When a body is constrained to move along a curved path, it possesses at any 
point an instantaneous linear velocity in the direction of the tangent to the 
curve at that point. The average value of the linear velocity is defined as 
the ratio of the displacement to the elapsed time interval during which the 
displacement took place. 
    The representation of angular velocity  OMEGA as a vector is shown in the 
illustration. The vector is taken along the axis of spin. Its length is 
proportional to the angular speed and its direction is that in which a 
right-hand screw would move. If a body rotates simultaneously about two or 
more rectangular axes, the resultant angular velocity is the vector sum of the 
individual angular velocities. 
 �34s[Rogers D. Rusk]
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�6v�2v�32sVeneer   �21m �33s 
    A thin sheet of wood of uniform thickness produced by peeling, slicing, or 
sawing. Depending on the manner of production and the portion of wood from 
which a veneer is made, the grain may be flat, vertical, or biased. Most 
veneer is rotary-cut from a bolt of wood, called a flitch, centered in chucks 
of a lathe. A nose bar bears against the flitch parallel to the center line of 
the lathe and a knife, also extending nearly the length of the lathe, peels 
off the veneer. Knives near the ends of the flitch cut the edges of the veneer. 
    Veneers cut from selected hardwoods, burls, crotches, and stumps are used 
for facings on furniture and the interior decoration of buildings. Other uses 
of veneer include plate separators in storage batteries, boxes for fruit and 
vegetables, drums for cheeses, and crates, hampers, and baskets for 
transportation and storage. Most veneer is used in plywood panels.  See  
PLYWOOD; WOOD PRODUCTS. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sVenereal disease �21m �33s
    A group of infectious diseases usually transmitted by sexual contact.
Gonorrhea, syphilis, chancroid (soft chancre), and others less common occur in
the United States.  See  GONORRHEA; GRANULOMA INGUINALE; LYMPHOGRANULOMA
VENEREUM; SOFT CHANCRE; SYPHILIS.
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sVentilation   �21m �33s 
    The supplying of air motion in a space by circulation or by moving air 
through the space. Ventilation may be produced by any combination of natural 
or mechanical supply and exhaust. Such systems may include partial treatment 
such as heating, humidity control, filtering or purification, and, in some 
cases, evaporative cooling. More complete treatment of  the air is generally 
called air conditioning.  See  AIR CONDITIONING. 
    Natural ventilation may be provided by wind force, convection, or a 
combination of the two. Although largely supplanted by mechanical ventilation 
and air conditioning, natural ventilation still is widely used in homes, 
schools, and commercial and industrial buildings. 
    Mechanical supply ventilation may be of the central type consisting of a 
central fan system with distributing ducts serving a large space or a number 
of spaces, or of the unitary type with little or no ductwork, serving a single 
space or a portion of large space. Outside air connections are generally 
provided for all ducted systems. Outside air is needed in controlled 
quantities to remove odors and to replace air exhausted from the various 
building spaces and equipment. 
    Exhaust ventilation is required to remove odors, fumes, dust, and heat 
from an enclosed occupied space. Such exhaust may be of the natural variety or 
may be mechanical by means of roof or wall exhaust fans or mechanical exhaust 
systems. The mechanical systems may have minimal ductwork or none at all, or 
may be provided with extensive ductwork which is used to collect localized hot 
air, gases, fumes, or dust from process operations. Where it is possible to do 
so, the process operations are enclosed or hooded to provide maximum 
collection efficiency with the minimum requirement of exhaust air. 
 �34s[J. Harold Clarke]
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�6v�2v�32sVenturi tube   �21m �33s 
    A device that causes a drop in pressure as a fluid flows through it. 
Essentially, a venturi tube is a short straight pipe section, or throat, 
between two tapered sections. Local pressure varies in the vicinity of the 
constriction; thus, by attaching at the throat a manometer or recording 
instrument, the drop in pressure can be measured and the flow rate calculated 
from it, or, by attaching a fuel source, fuel can be drawn into the main flow 
stream.  
    The principal advantage of the venturi tube is that not more than 10 - 20% 
of the difference in pressure between the inlet and the throat is permanently 
lost. This is accomplished by the discharge cone gradually decelerating the 
flow with minimum turbulence. 
 �34s[Raymond E. Sprenkle]
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�6v�2v�32sVenus   �21m �33s 
    The second planet in distance from the Sun. This neighbor of the Earth is 
very similar to it in such gross characteristics as mass, radius, and density. 
In other ways Venus is apparently different. Its atmospheric mass is almost a 
hundred times that of the Earth; its atmosphere is mostly carbon dioxide 
instead of nitrogen and oxygen; an extensive cloud layer of concentrated 
sulfuric acid is present; its surface temperature is an unbearable 730 K 
(850DGF); and it rotates with a period of 243 days, and from east to west, in 
the opposite sense of most other planets. Some of these differences are due 
more to alternate evolutionary paths of the two planets than to totally 
different initial conditions. 
    To the naked eye, Venus is the brightest starlike object in the sky. It is 
usually visible during the night either soon after sunset or close to sunrise. 
It can sometimes be seen during the daytime. 
    The light seen coming from Venus is almost entirely due to sunlight that 
is reflected from a dense cloud layer. In contrast to the Earth's 
approximately 50% cloud cover, the clouds of Venus are present over the entire 
planet. The clouds of Venus consist of a large number of tiny particles that 
are made of a water solution of concentrated sulfric acid. 
    Measurements conducted within Venus's atmosphere from various space probes 
and remotely from spacecraft and the Earth have provided a good definition of 
the gases that constitute Venus's atmosphere. By far, the chief gas species is 
carbon dioxide, which makes up 96% of the atmospheric molecules, while 
nitrogen accounts for almost all the remainder. 
    The very similar mean densities of Venus and the Earth imply that Venus is 
made of rocks similar to those that make up the Earth. Venus's interior may be 
qualitatively similar to that of the Earth in having a central iron core, a 
middle mantle made of rocks rich in silicon, oxygen, iron, and magnesium, and 
a thin outer crust containing rocks enriched in silicon in comparison with the 
rocks of the mantle. However, in contrast to the situation for the Earth, 
Venus's core may now be solid, which could account for the absence of a 
detectable magnetic field. 
    The ubiquitous cloud layer totally obscures Venus's surface at visible 
wavelengths. However, since the clouds and atmosphere are transparent to 
long-wavelength radio waves, observations of Venus's surface can be made with 
radar telescopes. Crude pictures of its surface show the presence of large, 
circular features. These may be giant craters produced by impacting 
meteoroids, analogous to the ones that pockmark the Moon. In addition, there 
are features that resemble large volcanic constructs, enormous rift valleys, 
and jagged mountain ranges. There is little topographic relief over most of 
Venus's surface. However, in a few places, huge continental-size plateaus rise 
about 6 mi (10 km) above the surrounding plains, while in other places 
isolated lowlands occur. 
 �34s[James B. Pollack]
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�6v�2v�32sVenus' flytrap �21m �33s
    Dionaea muscipula, an insectivorous plant of North and South Carolina (see
illustration). The two halves of a leaf blade can move as if they were hinged
along the midrib and, swinging upward and inward, the two surfaces come
together. Any insect alighting on a leaf triggers this sensitive motor
mechanism, and is caught between the closing halves of the leaf blade. In this
trap, the insect is slowly digested by enzymes secreted by cells in the leaf.
See INSECTIVOROUS PLANTS; SARRACENIALES; SECRETORY STRUCTURES (PLANT).
 �34s[Perry D. Strausbaugh /Earl L. Core]
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�6v�2v�32sVerbal learning   �21m �33s 
    A field of experimental psychology which studies the formation of certain 
types of verbal associations. Verbal learning deals with the acquisition of 
associations; the study of memory (retention and forgetting) deals with the 
weakening or decay of associations over time. A sharp demarcation between 
verbal learning and memory is difficult because laboratory studies of verbal 
learning test what is remembered from one presentation to the next; eventually 
what is now termed verbal ``learning'' will probably be explained and 
understood in terms of principles of memory.  See  MEMORY. 
    The laboratory conditions under which verbal learning is studied are 
characterized by a display of information to the subject under carefully 
controlled conditions and by essentially continuous measurement of performance 
at all stages of practice. The stimulus material or information which the 
subject is required to learn generally consists of digits, consonants, common 
words, or nonsense syllables (for example, NFP or WIX). Usually there are a 
number of individual items which are combined into a list of items, and 
performance on the list as a whole is taken as the basic unit of analysis. The 
items in the list may be related in some way: similarity in terms of repeated 
elements (letters), similarity of meaning, and communality of category 
membership have all been studied experimentally. 
    One type of association which has been studied is that which would be 
involved in learning the alphabet, that is, sequential or serial associations. 
To study the formation of sequential associations, subjects would be given 
serial lists to learn, where each list would consist of a number of unrelated 
items. A second type of association is a direct association between pairs of 
items such as already exist in semantic memory (for example, table-chair, 
black-white). To study these associations subjects would be given lists of 
paired associates consisting of A-B items, but A and B would not be 
associatively related as are table and chair. A third type of task is free 
recall wherein a list of items is presented to the subject and the task is 
simply to recall as many items as possible in any order. The list is repeated 
again and again (each time with a different ordering of the items) until the 
subject has attained some criterion level of performance. It is not yet clear 
exactly what type of associations are being formed in free-recall verbal 
learning.  
    The types of associations involved in serial and paired-associate tasks 
are certainly not completely different. It can be argued that a serial list 
actually is a succession of linked A-B pairs wherein each item in the list 
functions first as a response term and then as a stimulus for the next 
response. In fact, many psychologists use paired-associate tasks because they 
feel the experimental analysis may be easier when the stimulus and response 
terms are clearly distinct. However, the empirical phenomenon, conceptual 
problems, and theoretical models for serial learning, paired-associate 
learning, and free-recall learning are different.  See  LEARNING THEORIES. 
 �34s[Bennett B. Murdock, Jr.]
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�6v�2v�32sVermiculite   �21m �33s 
    A clay mineral constituent of clay materials. Vermiculites are similar to 
the montmorillonites in having an expanding structure. They differ, however, 
in that the expansion can take place only to a limited degree. When rapidly 
heated, vermiculite produces a lightweight expanded product that is widely 
used for thermal insulation.  See  CLAY MINERALS. 
    Vermiculites show a considerable range in chemical composition. Their 
composition may be like that for some montmorillonites, in which case the only 
difference would be in the larger particle size of the vermiculites. Like the 
montmorillonites, the vermiculites have a high cation-exchange capacity. They 
also absorb certain organic molecules between their mica layers. 
    Vermiculite is frequently listed as an alteration product of biotite mica, 
and it is often present along with chloritic mica as a minor constituent in 
ancient sediments. 
 �34s[Ralph E. Grim ;Floyd M. Wahl]
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�6v�2v�32sVernalization   �21m �33s 
    The induction in plants of the competence or ripeness to flower by the 
influence of cold, that is, temperatures below the optimal temperature for 
growth. Vernalization thus concerns the first of the three phases of flower 
formation in plants. In the second stage, for which a certain photoperiod 
frequently is required, flowers are initiated. In the third stage flowers are 
unfolded.  See  FLOWER; PHOTOPERIODISM; PLANT GROWTH. 
 �34s[Klaus Napp-Zinn]
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�6v�2v�32sVernier   �21m �33s 
    A short, auxiliary scale placed along the main instrument scale to permit 
accurate fractional reading of the least main division of the main scale. The 
auxiliary, or vernier, scale is graduated in one or both directions from the 
fiducial (index) mark in numbered divisions which are fractionally shorter (in 
a direct vernier) or longer (in a retrograde vernier) than those on the main 
scale (see illustration). The position of the fiducial mark (the zero mark of 
the vernier scale) between divisions on the main scale is indicated by the 
number of the graduation on the vernier scale which lines up exactly with a 
graduation on the main scale. 
 �34s[William A. Wildhack]
�33s 
  
  
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�6v�2v�32sVerrucomorpha   �21m �33s 
    A suborder of the Thoracica comprising one Recent genus, Verruca, and one 
fossil, Proverruca, from the Lower Devonian. These barnacles are sessile and 
asymmetrical. In Verruca only four plates form the wall, rostrum, carina, and 
scutum and tergum of one side. In Proverruca two additional lateral plates are 
inserted between the rostrum and the carina. These barnacles are 
hermaphroditic. Most species live in deep water, though V. stroemia is from 
the lower intertidal and sublittoral.  See  THORACICA. 
 �34s[H. G. Stubbings]
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�6v�2v�32sVertebra   �21m �33s 
    The basic unit of the vertebral column. Collectively, the vertebrae 
surround and protect the spinal cord and provide some type of axial support 
for the body. The stresses that the vertebral column must meet change somewhat 
from one end of the animal to the other, and also differ greatly between 
aquatic and terrestrial vertebrates because the problems of support and 
locomotion in these media are quite different. Accordingly, vertebral 
structure varies widely; yet all vertebrae have many features in common.  See  
SPINE.  
    A vertebra from the thoracic region of a mammal illustrates the basic 
morphology well (see illustration). The ventral portion consists of a 
disc-shaped mass of bone known as the body or centrum. An arch of bone, the 
neural arch, extends dorsally from the centrum and encompasses a space, the 
vertebral canal, in which the spinal cord lies. The bases, or roots, of the 
arch are narrower than other parts so that clefts, the intervertebral 
foramina, lie between the arch bases of successive vertebrae. Spinal nerves 
pass through these foramina. 
    Certain muscles and ligaments attach onto the spinous process, which 
extends dorsally from the top of the arch, and onto a pair of transverse 
processes, which extend laterally from the arch. One pair of articular 
processes, or zygapophyses, extends forward from the neural arch, and another 
pair extends posteriorly. Articular processes of successive vertebrae overlap 
and help to hold the vertebrae together. The centra of adjacent vertebrae are 
also joined together by invertebral discs of fibrocartilage. Numerous 
ligaments interlace the vertebrae. 
    Ribs articulate onto the thoracic vertebrae. Typically, each mammalian rib 
has two articular surfaces - a terminal head and a tubercle situated a short 
distance distal to the head. The tubercle articulates with a facet located on 
the end of the transverse process; the head usually articulates 
intervertebrally onto the intervertebral disc and adjacent parts of the 
bordering centra. 
  
    In mammals different parts of the column are clearly specialized to 
subserve certain functions in addition to their general supportive role. 
    The head moves independently of the trunk, and a distinct neck region, 
consisting of cervical vertebrae, is present. With few exceptions all mammals 
from a shrew to a giraffe have seven cervical vertebrae. 
    Well-developed ribs, which play an important role in respiratory 
movements, articulate with the anterior trunk or thoracic vertebrae of 
mammals. The number of thoracic vertebrae varies between species but is on the 
order of 11 (bat) to 18 or 20 (horse). Humans have 12. 
    Lumbar vertebrae occupy the posterior part of the mammal trunk region. 
They are characterized by relatively large transverse processes to which 
certain of the powerful back muscles attach. Again the number of lumbar 
vertebrae vary between species but is on the order of 5 (bat) to 8 (whale). 
Humans have 5. 
    Correlated with the greater efficiency of terrestrial locomotion and the 
need for strong support for the powerful hindlegs, the number of sacral 
vertebrae increases during evolution from the single one of amphibians. 
Reptiles usually have two, and most mammals have three which are fused 
together, along with their embryonic rib rudiments, into a solid complex of 
bone called the sacrum. Humans, which are bipeds, have 5, and certain of the 
powerful hoofed mammals, for example, the horse, also have 5. Birds, whose 
hindlegs act as shock absorbers upon landing, have between 10 and 23 vertebrae 
fused together in their synsacrum. 
    The tail and caudal musculature no longer play an important role in the 
locomotion of most mammals (the Cetacea being a conspicuous exception), and 
the tail is greatly reduced in size. The spinal cord of mammals ends within 
the lumbar region, and only a few spinal nerves continue through the vertebrae 
canal into the tail. Caudal vertebrae are small and become progressively 
incomplete as one moves distally along the tail until only centra are left. 
Tail length, and hence the number of caudal vertebrae, vary widely. Some 
oppossums have as many as 35, and humans, in which the tail is absent as an 
external structure, have only 3 to 5 caudal vertebrae. These form an internal 
coccyx to which certain anal muscles attach.  See  SKELETAL SYSTEM. 
 �34s[Warren F. Walker, Jr.]
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�6v�2v�32sVertebrata �21m �33s
    The major subphylum of the phylum Chordata, including the backboned
animals from fish to mammals and humans. This subphylum is often termed the
Craniata because of the common possession of a cranium or braincase; some of
the primitive agnathan fish lack a vertebral column. For characteristics of
the vertebrates  see  CRANIATA.
    The vertebrates are divided into eight classes which are grouped together
into several partly overlapping higher taxa which are generally given informal
rank. Often two superclasses are used to group the aquatic and the terrestrial
vertebrates. The term Gnathostomata is used to designate the seven classes of
jawed vertebrates in contrast to the jawless Agnatha. The classification used
herein is as follows (see separate articles for each class):

  Superclass Pisces
    Class Agnatha
	  Placodermi
	  Chondrichthyes
	  Osteichthyes
Superclass Tetrapoda
    Class Amphibia
	  Reptilia
	  Aves
	  Mammalia
    In the Pisces and Amphibia, the only extraembryonic membrane is the yolk
sac, and these groups are restricted to aquatic reproduction. In reptiles,
birds, and mammals, three additional membranes are present during development;
one surrounds the embryo and is termed the amnion. These groups can reproduce
independently of water and are called amniotes, in contrast to anamniotes
(fish and amphibians) in which these additional membranes are lacking.	See
AMNIOTA; ANAMNIOTA; ANIMAL KINGDOM; CHORDATA; PISCES (ZOOLOGY); TETRAPODA;
VERTEBRA.
 �34s[Walter J. Bock]
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�6v�2v�32sVertical takeoff and landing (VTOL)   �21m �33s 
    A flight technique in which an aircraft rises directly into the air and 
settles vertically onto the ground. Such aircraft do not need runways, but can 
operate from a small pad or, in some cases, from an unprepared site. The 
helicopter was the first aircraft that could hover and take off and land 
vertically, and is now the most widely used VTOL concept.  See  HELICOPTER. 
    The tilt-rotor concept is closest to the conventional helicopter and 
relies heavily on helicopter technology. The rotor disks are horizontal in 
VTOL operation, and are tilted 90DG to act as propellers in cruising flight. 
The tilt-rotor concept can have cruise efficiencies at least twice those of 
the helicopter, and therefore the concept is of interest for helicopter 
missions where greater range, speed, and time on station are desired. 
    In the vectored-thrust concept a moderate-bypass-ratio engine is fitted 
with four mechanically interconnected rotating nozzles that deflect the thrust 
from horizontal for conventional flight to vertical for VTOL operations. 
Attitude control of the airplane in VTOL operation is provided by 
high-pressure air bled from the engine and ducted to control nozzles at the 
extremities of the airplane. These control nozzles are coupled to their 
corresponding conventional control surfaces so that they are operated by the 
pilot's normal controls. 
    A combination of the thrust-deflection and dual-propulsion types is 
usually referred to as a lift-plus-lift/cruise type. The cruise engine is 
fitted with a pair of rotating nozzles. In the VTOL mode the lift provided by 
this engine is augmented and balanced by a pair of special lift engines 
installed immediately behind the cockpit. 
 �34s[R. E. Kuhn]
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�6v�2v�32sVestigial-sideband modulation �21m �33s
    Modulation whereby in effect the modulated wave to be transmitted is
composed of one sideband plus a portion of the other adjoining the carrier.
The carrier may or may not be transmitted. Vestigial-sideband modulation (VSB)
is like single-sideband modulation (SSB) except in a restricted region around
the carrier. The overall frequency response to the wanted sideband and to the
vestigial sideband is so proportioned by networks that upon demodulation,
preferably but not necessarily by a product demodulator, the original
modulating wave will be recovered with adequate accuracy.
 �34s[Harold S. Black]
�33s

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�6v�2v�32sVetch   �21m �33s 
    Any of a group of plants which are mostly annual legumes with weak viny 
stems terminating in tendrils. Vetches are used mainly for green manure, cover 
crops, hay, and pasture. 
    Hairy vetch (Vicia villosa) is winter hardy and is most widely grown. 
Smooth vetch (V. villosa var. glabrescens) and woollypod vetch (V dasycarpa) 
are similar to hairy vetch; seeds of the three may be found in mixtures. Bird 
vetch (V. cracca) is a winter-hardy perennial. Identification of vetches is 
difficult until pods and seeds develop. Common vetch (V. sativa) and purple 
vetch (V. bengalensis) are less winter-hardy than hairy vetch. Hungarian vetch 
(V. pannonica) is confined to the Pacific Northwest. Monantha vetch (V. 
articulata) has fine leaves and stems but, lacking winter hardiness, is 
confined to warm regions. Bitter vetch (V. ervilla) is not grown commercially. 
Narrow-leaf vetch (V. angustifolia) occurs mostly as a weed in waste places in 
the United States.  See  ROSALES. 
 �34s[Fred V. Grau]
�33s 
  
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�6v�2v�32sVibration   �21m �33s 
    The term used to describe a continuing periodic change in the magnitude of 
a displacement with respect to a specified central reference. The periodic 
motion may range from the simple to-and-fro oscillations of a pendulum, 
through the more complicated vibrations of a steel plate when struck with a 
hammer, to the extremely complicated vibrations of large structures such as an 
automobile on a rough road. Vibrations are also experienced by atoms, 
molecules, and nuclei.  See  PENDULUM. 
    A mechanical system must possess the properties of mass and stiffness or 
their equivalents in order to be capable of self-supported free vibration. 
Stiffness implies that an alteration in the normal configuration of the system 
will result in a restoring force tending to return it to this configuration. 
Mass or inertia implies that the velocity imparted to the system in being 
restored to its normal configuration will cause it to overshoot this 
configuration. It is in consequence of the interplay of mass and stiffness 
that periodic vibrations in mechanical systems are possible. 
    Mechanical vibration is the term used to describe the continuing periodic 
motion of a solid body at any frequency. When the rate of vibration of the 
solid body ranges between 20 and 20,000 hertz (Hz), it may also be referred to 
as an acoustic vibration, for if these vibrations are transmitted to a human 
ear they will produce the sensation of sound. The vibration of such a solid 
body in contact with a fluid medium such as air or water induces the molecules 
of the medium to vibrate in a similar fashion and thereby transmit energy in 
the form of an acoustic wave. Finally, when such an acoustic wave impinges on 
a material body, it forces the latter into a similar acoustic vibration. In 
the case of the human ear it produces the sensation of sound.  See MECHANICAL 
VIBRATION; SOUND. 
    Systems with one degree of freedom are those for which one space 
coordinate alone is sufficient to specify the system's displacement from its 
normal configuration. An idealized example known as a simple oscillator 
consists of a point mass m fastened to one end of a massless spring and 
constrained to move back and forth in a line about its undisturbed position 
(Fig. 1). Although no actual acoustic vibrator is identical with this 
idealized example, the actual behavior of many vibrating systems when 
vibrating at low frequencies is similar and may be specified by giving values 
of a single space coordinate. 
    When the restoring force of the spring of a simple oscillator on its mass 
is directly proportional to the displacement of the latter from its normal 
position, the system vibrates in a sinusoidal manner called simple harmonic 
motion. This motion is identical with the projection of uniform circular 
motion on a diameter of a circle.  See  HARMONIC MOTION. 
    When two simple vibrating systems are interconnected by a flexible 
connection, the combined system has two degrees of freedom (Fig. 2). Such a 
system has two normal modes of vibration of two frequencies. Both of these 
frequencies differ from the respective natural frequencies of the individual 
uncoupled oscillators. 
    A vibrating system is said to have several degrees of freedom if many 
space coordinates are required to describe its motion. One example is n masses 
m1, m2, . . . , msubn constrained to move in a line and interconnected by (n - 
1) coupling springs with additional terminal springs leading from m1 and msubn 
to rigid supports. This system has n normal modes of vibration, each of a 
distinct frequency.  See  DAMPING; VIBRATION DAMPING; VIBRATION ISOLATION. 
 �34s[Lawrence E. Kinsler]
�33s 
  
  
  
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�6v�2v�32sVibration damping   �21m �33s 
    The processes and techniques used for converting the mechanical 
vibrational energy of solids into heat energy. While vibration damping is 
helpful under conditions of resonance, it may be detrimental in many instances 
to a system at frequencies above the resonant point. This is due to the fact 
that the relative motion between the base of the vibration isolator and the 
mounted body tends to become smaller as the isolator becomes more efficient at 
the higher frequencies. With damping present, the force transmitted by the 
elastic element is unable to overcome the damping force; this leads to a 
resulting increase in transmissibility.  See  DAMPING; VIBRATION; VIBRATION 
ISOLATION.  
    All metal springs which include structural members such as brackets and 
shelves have some damping. However, such damping is insufficient for vibration 
isolators and must be augmented by special damping devices.  See  SPRING 
(MACHINES).  
    Several different types of damping devices have been developed and used 
successfully. Probably the most familiar is that used on automobiles, which, 
although known as a shock absorber, is in reality a damper, and functions as a 
limiter to the spring system of spring constant k. The system is shown in Fig. 
1. A piston p is attached to the body m and is arranged to move vertically 
through the liquid in a cylinder c which is secured to the support s. As the 
piston moves, the force required to cause the liquid to flow from one side of 
the piston to the other is approximately proportional to the velocity of the 
piston in the cylinder. This type of damping is known as viscous damping. The 
damping force is controlled by the viscosity of the liquid and by the size of 
the orifice in the piston.  See  SHOCK ABSORBER. 
    Some of the disadvantages of viscous damping may be overcome by using air 
instead of liquid as the damping medium. Air, being compressible, will add to 
the effective spring force with large displacements. If the air is housed 
within a flexible bellows, damping will be attainable horizontally as well as 
vertically. Such a system is illustrated in Fig. 2. This type of damping has 
proved very effective in vibration isolators. 
    Damping forces may be generated by causing one dry member to slide on 
another. This is known as dry friction or coulomb damping. Friction damping is 
used in several commercially available isolators because it provides a simple 
means to control the damping forces. 
    Magnetic damping is attainable as a result of the electric current induced 
in a conductor moving through a magnetic field. The damping force can be made 
proportional to the velocity of the conductor moving through the field.  See  
ELECTROMAGNETIC INDUCTION. 
 �34s[K. W. Johnson]
�33s 
  
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�6v�2v�32sVibration isolation   �21m �33s 
    The isolation, in structures, of those vibrations or motions that are 
classified as mechanical vibration. Vibration isolation involves the control 
of the supporting structure, the placement and arrangement of isolators, and 
control of the internal construction of the equipment to be protected. 
    The simplest kind of mechanical vibration has the waveform of sinusoidal 
motion. Vibrations in structures, although generally more complex in waveform, 
exist wherever movement takes place. Such movement may be caused, for example, 
by the engine in an automobile, by engines or wind buffeting in aircraft, or 
by a punch press in a building. Delicate electronic equipment and precision 
instruments must normally be isolated from these motions if accurate 
measurements are to be obtained.  See  MECHANICAL VIBRATION; VIBRATION. 
    Vibration, in most cases, may be effectively isolated by placing a 
resilient medium, or vibration isolator, between the source of vibration and 
its surrounding area to reduce the magnitude of the force transmitted from a 
structure to its support or, alternatively, to reduce the magnitude of motion 
transmitted from a vibrating support to the structure. Isolating vibration at 
its source is commonly termed active or source isolation; isolating an 
instrument from its surroundings is known as passive isolation. 
    The vibration isolators may be positioned and arranged in many different 
ways, all variations of three basic types, each of which requires a definite 
amount of space: (1) isolators attached underneath equipment, known as an 
underneath mounting system; (2) isolators located in the plane of the center 
of gravity of the equipment, known as a center-of-gravity system; (3) 
mountings arranged four on each side in the plane of the radius of gyration, 
known as a double side-mounted system or radius-of-gyration system.  See  
CENTER OF GRAVITY; RADIUS OF GYRATION. 
 �34s[K. W. Johnson]
�33s 
  
  
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�6v�2v�32sVibration machine   �21m �33s 
    A device for subjecting a system to controlled and reproducible mechanical 
vibration. Vibration machines, commonly called shake tables, are widely used 
in vibration measurement and analysis. There are three types of vibration 
machines in general use. These are the mechanical direct-drive type, the 
mechanical reaction type, and the electrodynamic type. Other types, such as 
hydraulic excitation devices, resonant systems, piezoelectric vibration 
generators used for instrument calibration, and machines for testing packages, 
have only limited or specialized applications.  See  MECHANICAL VIBRATION; 
VIBRATION.  
 �34s[K. W. Johnson]
�33s 
  
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�6v�2v�32sVibration pickup   �21m �33s 
    An electromechanical transducer capable of converting mechanical 
vibrations into electrical voltages. Depending upon their sensing element and 
output characteristics, such pickups are referred to as accelerometers, 
velocity pickups, or displacement pickups. 
    The accelerometer consists essentially of a mass which is seismically 
supported with respect to a surrounding case by means of a spring and guided 
to prevent motions other than those along the seismic direction of support. 
The mass exerts a force on the spring's support which is directly proportional 
to the acceleration being measured. This, in turn, is converted into an 
electrical voltage by means of stresses produced in a piezoelectric crystal.  
See  ACCELEROMETER. 
    The velocity pickup generates a voltage proportional to the relative 
velocity between two principal elements of the pickup, the two elements 
usually being a coil of wire and a source of magnetic field (see illustration). 
    The displacement pickup is a device that generates an output voltage which 
is directly proportional to the relative displacement between two elements of 
the instrument. These pickups are similar in construction and behavior to 
velocity pickups. The only essential difference is the use of a 
frequency-weighting network, required to make them direct-reading.  See  
VIBRATION.  
 �34s[Lawrence E. Kinsler]
�33s 
  
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�6v�2v�32sVibrator   �21m �33s 
    An electromechanical device used primarily to convert direct current to 
alternating current but is also used as a synchronous rectifier. The chopper 
is a closely related device.  See  CHOPPER; CHOPPING. 
    A vibrator consists of a flat reed, with a soft-iron slug mounted at the 
tip of the reed adjacent to a driving coil and pole piece (see illustration). 
The soft-iron slug, or armature, is attracted by the pole piece of the driving 
coil. The longitudinal axis of the driving coil is usually parallel to the 
reed axis, but the pole piece is slightly offset from the neutral position of 
the reed so that the reed is attracted sideways toward the pole piece whenever 
the coil is energized. The reed also has a set of contacts which hit 
stationary contacts attached to the frame. The stationary contact points are 
usually mounted on leaf springs so that they can ``give'' when the reed 
contact hits them. This minimizes contact bounce and lengthens vibrator life. 
    To energize the drive coil periodically, a power vibrator frequently has a 
separate coil contact. When the vibrator is at rest, the two main contacts are 
open and the coil contact is closed. When the dc input is connected, the coil 
becomes energized and pulls the reed over to one side, opening the coil 
contact and deenergizing the coil. The reed then moves back, closing the coil 
contact, and the cycle repeats. The vibrations of the reed build up, and when 
equilibrium is reached, the reed oscillates vigorously and each of the main 
contacts is closed for nearly half of the vibrator period. 
    The main contacts alternately switch current from the dc supply through 
opposite halves of transformer primary, setting up an alternating flux in the 
transformer core. Alternating voltage is then available from the transformer 
secondary. A resistor and capacitor connected across the transformer primary 
reduce sparking at the contacts. 
 �34s[Franz B. Tuteur]
�33s 
  
  
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�6v�2v�32sVicuna   �21m �33s 
    A rare animal whose fiber makes the world's most costly and most exquisite 
cloth, surpassing all others in fineness and beauty. It is found in an almost 
inaccessible area of the Andes Mountains. 
    A single animal yields only approximately ** lb (0.11 kg) of hair; thus 40 
animals are required to provide enough hair for the average coat. The fiber of 
the vicuna is the softest and most delicate of the known animal fibers; yet it 
is strong for its weight, is resilient, and has a marked degree of elasticity 
and surface cohesion.  See  ALPACA; CAMEL'S HAIR; CASHMERE; LLAMA; MOHAIR; 
NATURAL FIBER; WOOL. 
 �34s[M. David Potter]
�33s 
  
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�6v�2v�32sVideo amplifier   �21m �33s 
    A low-pass amplifier having a bandwidth in the range from 2 to 100 MHz. 
Typical applications are in television receivers, cathode-ray-tube computer 
terminals, and pulse amplifiers. The function of a video amplifier is to 
amplify a signal containing high-frequency components without introducing 
distortion.  
    Modern video amplifiers use specially designed integrated circuits. With 
one chip and an external resistor to control the voltage gain, it is possible 
to make a video amplifier with a bandwidth between 50 and 100 MHz having 
voltage gains ranging from 20 to 500.  See  AMPLIFIER; INTEGRATED CIRCUITS. 
 �34s[Harold F. Klock]
�33s 
  
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�6v�2v�32sVideo disk recording   �21m �33s 
    A disk system used to reproduce television pictures and sound. The 
reproduction system consists of three parts: disk, disk player, and television 
receiver. The disk player rotates the disk, whose information density is 
several hundred times that of conventional audio long-playing records, at high 
speed and reads the recorded signal by a sensor. The signal can be seen on the 
picture tube or heard from the loudspeaker of the television receiver. The 
recording of television signals is not yet possible in the home; the system 
operates only for reproduction. Although the video tape recorder (VTR) can 
record and reproduce the television signal at home, the tape is expensive. A 
video disk can be mass-produced and is relatively inexpensive. Further, the 
degradation of recorded signal with elapsed time is far less than that of 
magnetic tapes. 
 �34s[H. Date]
�33s 
  
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�6v�2v�32sVideo telephone   �21m �33s 
    A communications instrument used for the simultaneous exchange of visual 
images and associated speech. A complete video telephone communication system 
involves three basic elements: (1) terminal equipment (camera, display, 
microphone, and speakerphone) to transform both the aural and visual inputs 
into electrical signals, and vice versa; (2) transmission facilities to carry 
the electrical signals over large distances; and (3) switching systems to 
allow a choice of terminals to be interconnected. The equipment that is used 
as a visual adjunct to telephony may also be arranged to serve many of the 
applications associated with closed-circuit television and surveillance.  See  
CLOSED-CIRCUIT TELEVISION; TELEPHONE EQUIPMENT, CUSTOMER; TELEPHONE SERVICE; 
TELEPHONY; TELEVISION. 
 �34s[C. C. Cutler]
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�6v�2v�32sVideotext and teletext   �21m �33s 
    Computer communications services that take advantage of standard 
television receivers and use them as display terminals to retrieve information 
from data bases. A ``black box'' is placed between the television set and the 
signal carrier. The signal carrier may be a telephone network, a data 
communications network, a television cable, or a broadcast signal. The black 
box receives data from the signal carrier. It formats that data into a signal 
suitable for display on the television set. The user interacts with the black 
box to determine what information is actually displayed on the screen of the 
television set. 
    Both videotext and teletext use the same display technology. The 
difference between them is that videotext is a two-way, or interactive, 
service, which allows the user to interact with the service in selecting 
information to be displayed, while teletext is a broadcast service. In the 
teletext service, preprogrammed sequences of frames of data are broadcast 
cyclically. The user interacts with the black box to ``grab'' the frames of 
relevant information. The textual and traphic information in teletext is 
inserted into the ``blanking period'' in television signals. While select 
frames of information can be viewed in the teletext service, the user cannot 
interact with the data base. 
    In videotext, the black box is actually a limited computer terminal which 
uses the television screen for display. Both videotext and teletext provide 
the user with a numeric keypad for interacting with the black box. Some 
videotext terminals also provide, as an option, a full alphanumeric keyboard 
for interacting with videotext services. 
    In both videotext and teletext the user will normally make selections from 
a series of menus to select the desired information. In videotext the user may 
then continue to interact with the service selected. 
    Data-base retrieval forms a major class of videotext and teletext 
services. Examples include restaurant guides, hotel information, stock market 
information, and travel information. Another major class of services is 
interactive, and therefore is suitable only for videotext. Examples include 
calculation services, electronic mail, teleshopping, and financial services.  
See  CONSUMER ELECTRONICS; DATA-BASE MANAGEMENT SYSTEMS; ELECTRONIC 
PUBLISHING; MULTIACCESS COMPUTER; TELEVISION RECEIVER. 
 �34s[Ronald P. Uhlig]
�33s 
  
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�6v�2v�32sVincent's angina   �21m �33s 
    An inflammation of the tissues of the oral cavity or pharynx caused by 
fusiform bacilli, Bacillus fusiformis (= Fusobacterium fusiforme), and a 
spirochete, Borrelia vincenti. The underlying cause, however, is a dietary 
deficiency in the vitamin B complex which favors the overgrowth of these 
organisms.  
    The commonest form of the disease, trench mouth, is characterized by 
redness, congestion, and edema of the gums with involvement of the entire oral 
cavity in severe cases. It is observed mainly among institutional or military 
groups with a deficient diet. 
 �34s[Thomas B. Turner]
�33s 
  
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�6v�2v�32sVinegar �21m �33s
    The product of the incomplete oxidation to acetic acid of ethyl alcohol
produced by a primary fermentation of vegetable materials. The oxidation,
often called secondary fermentation, proceeds under the influence of various
kinds of so-called vinegar bacteria, species of the genera Acetobacter and
Acetomonas. The overall chemical reactions for the production of vinegar are
given below:

		       (REACTION CONDTIONS = Yeast)
		       Sugar  ---->  2CH3CH2OH+2CO2


		    (REACTION CONDITIONS = Vinegar and Bacteria)
		    2CH3CH2OH+2O2  --->   2CH3COOH+2H2O



Vinegar contains not less than 4 g of acetic acid per 100 ml. It is used in
the preparation of pickled fruits and vegetables or other large-scale food
manufacture such as salad dressings, and as a seasoning agent for table use.
    Vinegars are classified as to raw materials used in their manufacture;
there are products designated as wine, cider, malt, and sugar vinegar. Wine
vinegar is the product obtained from the acetous souring of grape wine. Cider
vinegar is derived from the alcoholic fermentation of other fruit juices,
usually apple juice, followed by acetous fermentation. Malt vinegar is
obtained by the alcoholic fermentation of starchy materials, such as cereal
grains or potatoes saccharified, or converted to sugar, by enzymes of malt,
followed by acetous fermentation. Distilled vinegars usually belong to the
malt vinegar class. Sugar vinegar is obtained by the alcoholic fermentation of
sugars such as syrups of glucose, blackstrap molasses, or refiners' molasses.
Nondistilled vinegars are characterized by distinctive flavors and odor.
Seasoned vinegars are prepared by soaking vegetable seasoning agents, such as
tarragon leaves, onions, garlic, peppers, or various spices, in vinegar.
 �34s[Lewis B. Lockwood]
�33s

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�6v�2v�32sVinylogy �21m �33s
    A principle used to correlate the properties of organic chemical
compounds. Organic compounds that differ from each other by a vinylene linkage
( - CHDOUBLE BARCH - ) are said to be vinylogs of one another. Thus, ethyl
crotonate is a vinylog of ethyl acetate and of the next higher vinylog, ethyl
sorbate. Their structures are:

			     C0H3--CO2C2H5
			     Ethyl acetate

			 CH3--CH==CH--CO2C2H5
			    Ethyl crotonate

		      CH3--CH==CH--CH==CH--CO2C2H5
			     Ethyl sorbate

These esters are the first members of the vinylogous series CH3--(CH==H)(sub n)
CO2H5. The relationship between the members of such a series is known as
vinylogy.
    The outstanding characteristic common to the members of a vinylogous
system is that the influence of the terminal functional groups upon each other
persists throughout the series. For example, the readiness with which the
methyl group of ethyl acetate loses a proton is ascribed to the enhanced
stability of the resulting anion because of the presence of the ester group.
 �34s[Reynold C. Fuson]
�33s

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�6v�2v�32sVinyl resin �21m �33s
    An addition polymer made from monomers with the general structure shown,
wherein R1 and R2 represent the same or different hydrogen, alkyl, halogen,

					R1
				      /
				H2C==C
				      \
					R2

ether, carboxylate, or other substituents. Vinyl resin often refers to any of
a large variety of thermoplastics with the - CH2CHR1 -	repeating group.
Popularly, vinyl resin refers to a plasticized polyvinyl chloride used as a
film or fabric, as for furniture or automobile seat covers. In common chemical
usage, vinyl resin implies a polymer of an olefinically unsaturated monomer,
whether the double bond is terminal or not.  See  POLYVINYL RESINS.
 �34s[Frank Wagner]
�33s

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�6v�2v�32sViolales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Dilleniidae of the class Magnoliopsida (dicotyledons). The order 
consists of 31 families and more than 5200 species. The largest families are 
the Flacourtiaceae (about 1300 species), Violaceae (about 850 species), 
Cucurbitaceae (about 850 species), Begoniaceae (about 800 species), and 
Passifloraceae (about 600 species). 
    The most characteristic feature of this morphologically heterogeneous 
order is the unilocular, compound ovary with mostly parietal placentation. 
When the stamens are numerous they are initiated in centrifugal sequence. The 
more primitive members of the order, such as some of the Flacourtiaceae, are 
trees with stipulate, alternate leaves; perfect, polypetalous flowers with 
numerous centrifugal stamens; a compound pistil with free styles and parietal 
placentation; and seeds with a well-developed endosperm. 
    Cucumbers and melons (species of Cucumis, in the Cucurbitaceae), pumpkins 
and squashes (species of Cucurbita), begonias, and violets (Viola) are 
familiar members of the Violales.  See  CANTALOUPE; CUCUMBER; DILLENIIDAE; 
HONEY DEW MELON;  MAGNOLIOPSIDA; MUSKMELON; PERSIAN MELON; PUMPKIN; SQUASH; 
WATERMELON.  
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sViral inclusion bodies   �21m �33s 
    Abnormal structures which appear within the cell nucleus, the cytoplasm, 
or both, during the course of virus multiplication. In general, the inclusion 
bodies are concerned with the developmental processes of the virus. In some 
virus infections, such as molluscum contagiosum, inclusion bodies may be 
simply masses of maturing virus particles. In other infections (herpes 
simplex), typical inclusion bodies do not appear until after the virus has 
multiplied. Such inclusions may be remnants of the process of virus 
multiplication.  See  VIRUS. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sVirgo   �21m �33s 
    In astronomy, a constellation handed down from antiquity, visible 
throughout the summer months. This sixth sign of the zodiac represents a 
maiden (see illustration). The brightest star in the constellation, Spica, is 
a spectroscopic binary with a massive dark companion. It is a fine 
first-magnitude star of the purest white tint, and also a navigational star.  
See  CONSTELLATION; LIBRA. 
 �34s[Ching-Sung Yu]
�33s 
  
  
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�6v�2v�32sVirial equation �21m �33s
    In thermodynamics, an empirical equation of state with additional terms
beyond those for an ideal gas. The additional terms account for some of the
differences between real gases and ideal gases. For an ideal gas Eq. (1)

	pV = NR(sub u)T or pv(sub n) = R(sub u)T			 (1)

holds, where p is the pressure, V the total volume, vsubN = V/N the molar
volume, N the number of moles, Rsubu the universal gas constant, and T the
absolute temperature. To account for departures by real gases from the
idealized relation, Eq. (1) may be written as Eq. (2).

pv(sub n)/R(sub u)T =
B(sub 0) + [(B(sub 1)(T))/v(sub n)] + [(B(sub 2)(T))/(v[sub n])(sup 2)] +
[(B(sub 3)(T))/(v[sub n])(sup 3)] +  ...				 (2)

    The Bs are functions of temperature and are known as the virial
coefficients.  See  BEATTIE-BRIDGEMAN EQUATION; VAN DER WAALS EQUATION.
 �34s[George A. Hawkins]
�33s

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�6v�2v�32sViroid �21m �33s
    The smallest known agent of infectious disease. The molecular weight of
viroids is estimated to be as little as 1.0 - 1.3 x 10(sup 4) daltons, in marked
contrast to the conventional plant virus genomes, which have molecular weights
of approximately 2 x 10(sup 6) daltons. Conventional viruses are made up of
nucleic acid encapsulated in protein (capsid), whereas viroids are uniquely
characterized by the absence of a capsid. In spite of their small size, viroid
ribonucleic acids (RNAs) can replicate and produce characteristic disease
syndromes when introduced into cells. Although the viroids thus far identified
are associated with plants, based on the unusual properties of the infectious
agents, they may also be found to affect other forms of life, including
humans. Scrapie, kuru, transmissible mink encephalopathy, and Creutzfeld-Jakob
diseases are good candidates for viroid etiology. It is possible that in
instances where a viral etiology of disease has been assumed but where no
causal virus has been identified, viroid infectious agents may be involved.
See  PLANT VIRUSES AND VIROIDS; VIRUS.
 �34s[R. K. Horst]
�33s

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�6v�2v�32sVirology   �21m �33s 
    A specialized part of clinical microbiology. Because of the nature of 
viruses and their proclivity for growth only in living cell cultures or in 
eggs, few clinical microbiology laboratories have facilities for the isolation 
and identification of viruses. Most of these procedures are best done in large 
hospitals or in state departments of health. There is a growing trend, 
however, for smaller hospitals to develop screening virology laboratories in 
which specimens are brought to the laboratory, placed in tissue culture, and 
preliminaty identification steps taken in order to provide a rapid diagnosis 
of viral disease. Although only a few drugs exist that can be used to treat 
virus infections, the early recognition of viral disease may allow the 
clinician to discontinue the use of powerful antibiotics that are only 
effective for bacterial diseases. Common virus diseases include influenza, 
herpes, chicken pox, the common cold, and many nonspecific respiratory 
infections. Much of the viral diagnosis that is done in the laboratory is by 
the determination of a specific antibody response to viral disease. The 
routine clinical microbiology laboratory usually does not become involved in 
such procedures. A special procedure for the detection of viruses and other 
organisms that affect infants is called the TORCH screen. It is a procedure 
for the detection of antibodies to the parasitic disease toxoplasmosis and the 
viral diseases rubella, cytomegalovirus, and herpes.  See  ANIMAL VIRUS; VIRUS. 
 �34s[Richard C. Tilton]
�33s 
  
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�6v�2v�32sVirtual work principle   �21m �33s 
    The principle stating that the total virtual work done by all the forces 
acting on a system in static equilibrium is zero for a set of infinitesimal 
virtual displacements from equilibrium. The infinitesimal displacements are 
called virtual because they need not be obtained by a displacement that 
actually occurs in the system. The virtual work is the work done by the 
virtual displacements, which can be arbitrary, provided they are consistent 
with the constraints of the system.  See  CONSTRAINT. 
    The principle of virtual work is equivalent to the conditions for static 
equilibrium of a rigid body expressed in terms of the total forces and 
torques. That is, the principle of virtual work can be derived from these 
conditions, and conversely.  See  EQUILBRIUM OF FORCES; STATICS. 
    One advantage of the principle of virtual work is that it can serve as a 
basis for all of statics. In the solution of problems the principle of virtual 
work is often useful for eliminating the need for consideration of the forces 
of constraint, since these forces often are perpendicular to the virtual 
displacements and consequently do no work. 
 �34s[Paul W. Schmidt]
�33s 
  
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�6v�2v�32sVirulence   �21m �33s 
    The capacity of microorganisms to inflict injury or death on a susceptible 
host. The concept of virulent microorganisms actually has no absolute, 
independent existence as a microbial attribute. The same microorganism may 
under identical conditions kill one host and be harmless to another; thus the 
same microorganism is either virulent or avirulent. It follows that virulence 
acquires reality only when both the microorganism and the host are considered. 
Certain ancillary considerations contribute to the success or failure of the 
parasite. These are the numbers of the invading microorganism, the portal of 
entry, the mode of dissemination in the tissues and fluids of the host, the 
localization of microorganism in certain tissues of the host, and the natural 
and induced humoral and cellular defensive reactions of the host. Genotypic 
variation, or mutation of either microorganism or host, may have a determining 
effect on the expression of virulence. Thus mutation may result in either a 
gain or loss of a character responsible for the microorganisms's capability to 
inflict damage on the host.  See  IMMUNITY; VIRUS. 
 �34s[Morris Tager]
�33s 
  
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�6v�2v�32sVirus   �21m �33s 
    An infectious agent characterized mainly by its small size and chemical 
simplicity. During a part of its infectious cycle in a cell, and sometimes 
even outside the cell, a virus consists only of nucleic acid, either 
deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). However, a mature virus 
particle normally exists in the form of nucleic acid encased in a protein 
shell. A virus will not reproduce in nutrient media, but it may multiply 
within the cells of appropriate tissue cultures.  See  DEOXYRIBONUCLEIC ACID 
(DNA); RIBONUCLEIC ACID (RNA). 
    Almost all forms of life are susceptible to virus infection, including 
humans, animals, plants, and even bacteria (bacteriophage). Viruses cause 
enormous annual losses of domestic crops and animals, and they have been 
responsible for countless diseases since antiquity.  See  ANIMAL VIRUS; 
BACTERIOPHAGE; PLANT VIRUSES AND VIROIDS. 
    Direct visualization of a virus became common with the development of the 
electron microscope about 1940. Each virus was found to have a characteristic 
size and shape. Some are spheroidal bodies, some filamentous, others are 
loaf-shaped or polyhedral, and some are spermlike. All are exceedingly small, 
the largest being only a few hundred-thousandths of an inch in diameter.  See  
ELECTRON MICROSCOPE. 
  
    Chemical composition and structure.  Each virus seems to have a definite 
and characteristic composition. The larger viruses tend to be more complex 
than the smaller ones both morphologically and chemically. Protein, nucleic 
acid, lipid, and carbohydrate have all been found in purified virus 
preparations. Although few viruses contain all these constituents, there are 
none that lack nucleic acid since this is their genetic material. The simplest 
viruses consist solely of nucleic acid within a protein coat.  See  
CARBOHYDRATE; LIPID; NUCLEIC ACID; PROTEIN. 
    The protein components of viruses are somewhat acidic, have the usual 
amino acids found in all proteins, and are largely responsible for the 
morphology and serological properties of the viruses. A significant feature of 
viral protein structure is the presence of repeating subunits. Thus, the 
protein component of tobacco mosaic virus appears to consist of about 2100 
identical peptide chains, each having a molecular weight of 17,531. While the 
proteins of different viruses have distinctive compositions, the proportions 
of amino acids in virus strains, also called variants or mutants, may or may 
not be different. 
    Viruses usually contain either RNA or DNA located toward the interior of 
the virus particles and more or less closely associated with the peptide 
chains of the protein. The amounts of nucleic acid range from less than 1% RNA 
in influenza virus to over 50% DNA in T2 bacteriophage. Different viruses (for 
example, poliovirus versus tobacco mosaic virus) usually have distinctive 
proportions of purine and pyrimidine components in their nucleic acids, 
whereas mutant strains of one virus commonly differ so slightly in proportions 
of these components that it is very difficult to show by analytical methods 
that such differences exist. Yet change of a single purine or pyrimidine 
produces mutants having profound biological consequences which are recognized 
by the different symptoms they elicit in a given host. 
  
    Reproduction and mutation.  It appears that a single virus particle can 
initiate the process of virus multiplication. However, duplication is not by 
growth and division but by production of virus parts and their subsequent 
assembly. This process can be initiated by the nucleic acid alone if it can 
penetrate the cell since nucleic acid is the genetic substance of the virus. 
In fact, the process of infection begins either by injection of viral nucleic 
acid into a cell (the case with many bacterial viruses) or by engulfment of 
whole virus and uncoating of the nucleic acid (animal and plant viruses). Once 
inside the cell, the viral nucleic acid competes with the genetic material of 
the host for control of cell processes and interacts with it in various ways, 
depending on the particular virus. Some bacterial viruses induce synthesis of 
an enzyme that destroys the host nucleic acid. Thereafter synthesis in the 
cell is controlled solely by the viral nucleic acid and is directed toward 
synthesis of new virus particles. In other instances, host material is not 
destroyed and viral induced functions are supplemented by those of the host 
nucleic acid with the resultant production of new virus particles. In still 
other cases, all or part of the viral nucleic acid appears to be physically 
inserted into the host nucleic acid by a breakage and reunion phenomenon. In 
some bacterial and animal tumor viruses, the viral nucleic acid may function 
to some extent to produce certain specific proteins, but whole new virus 
particles are not produced. 
    When whole virus is produced in infected cells, it appears to occur by 
assembly from pools of virus parts that were synthesized under the direction 
of the viral nucleic acid. Such synthesis occurs in the nucleus, in the 
cytoplasm, or in both, depending upon the virus. Viral parts are usually 
assembled into characteristic particles in the cytoplasm, and then are 
released into the surroundings by a disruptive process (lytic or cytocidal 
effect). However, some viruses are only completed at the cell membrane, then 
released by a budding or extrusion process which does not result in cell's 
death.  
 �34s[C. Arthur Knight]
�33s 
  
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�6v�2v�32sVirus infection, latent, persistent, slow   �21m �33s 
    Inapparent infections which are chronic and in which a virus-host 
equilibrium is established. The adjective latent is best reserved to qualify 
the noun, infection; the term latent virus should be avoided. Slow and 
persistent viruses include several members of different recognized taxonomic 
groups of viruses, as well as a number of agents not yet well enough 
understood or characterized to be taxonomically placed. They are 
intermittently or continuously present in the infected animal, and they may 
cause diseases usually of a chronic and degenerative nature, often associated 
with the central nervous system. 
    There are many examples of natural viral infections of animals, plants, 
bacteria, and insects, as well as humans, in which there are no obvious 
evidences of injury or illness in the host. At one end of the scale are 
infections such as the inapparent or subclinical forms of poliomyelitis, 
influenza, or yellow fever; no illness is manifested, although laboratory 
tests can easily show the virus to be present and multiplying in the host. The 
virus presumably does not persist in the host beyond a few days or a few weeks 
at most. In contrast, after a primary infection - usually in childhood - type 
1 herpes simplx virus persists in a latent infection for long periods, 
sometimes for life, in the presence of antibodies. The virus adversely affects 
the individual only when nonspecific factors, such as fever, precipitate 
attacks of cold sores.  See  HERPES SIMPLEX; INFLUENZA; POLIOMYELITIS; YELLOW 
FEVER; VIRUS. 
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sVirus interference   �21m �33s 
    A phenomenon which may be defined as protection of host cells against one 
virus, conferred as a result of prior infection with a different virus. 
Interference between viruses has been observed in humans, in laboratory 
animals, and in tissue culture systems.  See  ANIMAL VIRUS; PLANT VIRUSES AND 
VIROIDS.  
    Interference is believed to act in one of two ways: (1) The first virus 
may inactivate surface receptors of the cell and so make them unavailable to 
the second virus; or (2) the cell materials or enzymes necessary for the 
growth of the second virus may be taken over by or directed by the first virus. 
    Interference occurs between related as well as unrelated viruses. It may 
be unilateral; for example, one virus may prevent infection with a second, but 
the second virus may not prevent infection with the first. In some cases, 
inactivated virus may interfere with subsequent infection by live virus.  See  
IMMUNOLOGY.  
 �34s[Joseph L. Melnick]
�33s 
  
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�6v�2v�32sViscacha   �21m �33s 
    Any of four species of viscacha belonging to two genera, Lagidium (three 
species) and Lagostomus. The mountain viscacha (Lagidium viscaccia) lives in 
burrows near rocks in the foothills of the Andes. It is larger than the 
chinchilla and has a poor-quality fur, which is woven with wool into cloth. 
The plains viscacha (Lagostomus maximus) inhabits burrows in the pampas region 
of Argentina. It is exclusively herbivorous. See CHINCHILLA; RODENTIA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sViscosity �21m �33s
    The resistance that a gaseous or liquid system offers to flow when it is
subjected to a shear stress. Viscosity is a measure of the internal friction
that arises when there are velocity gradients within the system. For fluids
(gases and liquids) its meaning is conceptually and operationally well defined.
    Simple gases typically have viscosities in the range of 100 to 200
micropoise at standard temperature and pressure (273 K, 1 atm or 101,325
pascals), whereas simple liquids under the same conditions have coefficients
of viscosity about two orders of magnitude larger. The flow characteristics of
gases and simple liquids such as water, carbon tetrachloride, and ethyl
alcohol are typically those of newtonian fluids. Aqueous suspensions, such as
clays, gelatin, and agar, are termed non-newtonian fluids because their
viscosities may depend upon the rate of shear and prior treatment. Hydrophilic
sols often form extended networks involving water, and their nonnewtonian
behavior is believed to be due to the breakdown of their structure under
shear.	See  FLUID FLOW; LIQUID; NEWTONIAN FLUID; NON-NEWTONIAN FLUID.
    The origin of internal friction (viscosity) at the molecular level is the
net transfer of momentum between layers of fluid moving with different
velocities in parallel flow by the mechanism of molecular collisions. In this
process the directed energy of fluid flow is degraded to random thermal energy
(heat). It was one of the early triumphs of the kinetic theory of gases that
established the relationship between the viscosity of a hard-sphere gas and
the mean speed of its molecules.  See  KINETIC THEORY OF MATTER.
    Momentum transfer between shearing layers also underlies the viscous
behavior of simple liquids, but since the mean free path has little meaning
for liquids, no simple relation exists for them. In contrast to the behavior
of gases, temperature decreases the viscosities of simple liquids and its
effect is much larger.	See  LIQUID; RHEOLOGY.
 �34s[Norman H. Nachtrieb]
�33s

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�6v�2v�32sViscous impingement air filter   �21m �33s 
    A type of air filter which is made up of a relatively loosely arranged 
medium, usually consisting of spun glass fibers, metal screens, or layers of 
crimped expanded metal. The surfaces of the medium are coated with a tacky 
oil, generally referred to as an adhesive. The arrangement of the filter 
medium is such that the airstream is forced to change direction frequently as 
it passes through the filter. Solid particles, because of their momentum, are 
thrown against, and adhere to, the viscous coated surfaces. Larger airborne 
particles, having greater mass, are filtered in this manner, whereas small 
particles, tend to follow the path of the airstream and escape entrapment. 
 �34s[Morton A. Bell]
�33s 
  
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�6v�2v�32sVision �21m �33s
    The sense of sight, which perceives the form, color, size, movement, and
distance of objects. Of all the senses, vision provides the most detailed and
extensive information about the environment.  See  BLINDNESS.
    Human vision is most sensitive for light comprising the visible spectrum
in the range 380 - 720 nanometers in wavelength. Sunlight and common sources
of artificial light contain substantially all wavelengths in this range but
each source has a characteristic spectral energy distribution. In general,
light stimuli can be measured by physical means with respect to their energy,
dominant wavelength, and spectral purity. These three physical aspects of the
light are closely related to the perceived brightness, hue, and saturation,
respectively.  See  COLOR; LIGHT.

    Anatomical basis for vision.  The anatomical structures include the eyes,
optic nerves and tracts, optic thalamus, and visual cortex. The eyes are motor
organs as well as sensory; that is, each eye can turn directly toward an
object to inspect it. The two eyes are coordinated in their inspection of
objects, and they are able to converge for near objects and diverge for far
ones. Each eye can also regulate the shape of its crystalline lens to focus
the rays from the object and to form a sharp image on the retina. Furthermore,
the eyes can regulate the amount of light reaching the sensitive cells on the
retina by contracting and expanding the pupil of the iris.  See  EYE
(VERTEBRATE).
    The process of seeing begins when light passes through the eye and is
absorbed by the sensitive cells of the retina. These cells are activated by
the light in such a way that electrical potentials are generated. Some of
these potentials are the signs of electrochemical activity that serves to
generate nerve responses in various successive neural cells in the vicinity of
excitation. Finally, impulses emerge from the eye in the form of repetitive
discharges in the fibers of the optic nerve. The optic nerves from the two
eyes traverse the optic chiasma. The fibers from the inner (nasal) half of
each retina cross over to the opposite side, while those from the outer
(temporal) half do not cross over but remain on the same side.
    The visual cortex includes a projection area in the occipital lobe of each
hemisphere. Here there appears to be a point-for-point correspondence between
the retina of each eye and the cortex. Thus the cortex contains a ``map'' or
projection area, each point of which represents a point on the retina and
therefore a point in visual space as seen by each eye. But the map is much too
simple a model for cortical function. Vision tells much more than the location
at which an object is seen. Visual tests show that other important features of
an object such as its color, motion, orientation, and shape are simultaneously
perceived.

    Scotopic and photopic vision.  Night animals, such as the cat or the owl,
have eyes that are specialized for seeing with a minimum of light. This type
of vision is called scotopic. Day animals such as the horned toad, ground
squirrel, or pigeon have predominantly photopic vision. They require much more
light for seeing, but their daytime vision is specialized for quick and
accurate perception of fine details of color, form, and texture, and location
of objects. Color vision, when it is present, is also a property of the
photopic system. Human vision is duplex; humans are in the fortunate position
of having both photopic and scotopic vision. Some of the chief characteristics
of human scotopic and photopic vision are enumerated in the table.

    Scotopic vision. This occurs when the rod receptors of the eye are
stimulated by light. The outer limbs of the rods contain a photosensitive
substance known as visual purple or rhodopsin. This substance is bleached away
by the action of strong light so that the scotopic system is virtually blind
in the daytime. Weak light causes little bleaching but generates neural
inhibitory signals that lower the overall sensitivity of the eye. In darkness,
however, the rhodopsin is regenerated by restorative reactions based on the
transport of vitamin A to the retina by the blood. One experiences a temporary
blindness upon walking indoors on a bright day, especially into a dark room or
dimly lighted theater. As the eyes become accustomed to the dim light the
scotopic system gradually begins to function. This process is known as dark
adaptation.  See  VITAMIN A.
    Photopic vision.  Normal photopic vision has the characteristics
enumerated in the table.  Emphasis is placed on the fovea centralis,
a small region at the very center of the retina of each eye.
    Foveal vision is achieved by looking directly at objects in the daytime.
The image of a small object falls within a region almost exclusively
populated by cone receptors.  These are closely packed together in the central
fovea.	Furthermore, each of the cones in the fovea is provided with a series of
specialized nerve cells that process the incoming pattern of stimulation and
convey it to the cortical projection area.
    Peripheral vision takes place outside the fovea centralis.	As an example,
look directly at a single letter at the center of a printed page.  This letter,
and a few letters immediately adjacent to it, appear clear and black because
they are seen with foveal vision.  The rest of the page is a blur in which the
lines of print are seen as gray streaks.
    There is a simple anatomical explanation for the clarity of foveal vision
as compared to peripheral vision.  The cone receptors become less and less
numerous in the retinal zones that are more and more remote from the fovea.
In the extreme periphery there are scarcely any cones, and even the rod
receptors are more sparsely distributed.  Furthermore, the plentiful neural
connectors from the foveal cones are replaced in the periphery by network
connections in which hundreds of receptors may activate a single optic nerve
fiber.	This mass action is favorable for the detection of large or dim
stimuli in the periphery or at night, but it is unfavorable for visual
acuity or color vision, both of which require the brain to differentiate
between signals arriving from closely adjacent cone receptors.

    Space and time perception.	Spatial and temporal effects are clearly
apparent in the sense of sight. These two effects enable the individual to
become oriented with regard to space and time in the external environment,
especially in the perception of motion and distance.  See  PERCEPTION.
    Elementary forms of space perception are vernier and stereoscopic
discrimination. Here, the eye is required to judge the relative position of
one object in relation to another (see illustration). The left eye, for
example, sees the lower line as displaced slightly to the right of the upper.
This is known as vernier discrimination. If the right eye is presented with
similar lines that are oppositely displaced, then the images for the two eyes
appear fused into one and the subject sees the lower line as nearer than the
upper. This is the principle of the stereoscope. The distance judgment is made
not at the level of the retina but at the cortex where the spatial patterns
from the separate eyes are fused together. The fineness of vernier and
stereoscopic discrimination transcends that of the retinal mosaic and suggests
that some averaging mechanism must be operating in space or time or both.
    The temporal characteristics of vision are revealed by studying the
responses of the eye to various temporal patterns of stimulation. When a light
is first turned on, there is a vigorous burst of nerve impulses that travel
from the eye to the brain. Continued illumination results in fewer and fewer
impulses as the eye adapts itself to the given level of illumination. Turning
the light off elicits another strong neural response. Afterimages are often
seen at this time. A positive afterimage, resembling the original stimulus, is
sometimes seen during the first fraction of a second after the light goes off.
This is usually followed by a longer-lasting negative afterimage in which the
color of the original object appears to be reversed. A clear afterimage may be
produced by staring fixedly at the stimulus for at least a half minute, then
turning away and ``projecting'' the afterimage against a white or gray screen.
The afterimage is thought to arise chiefly from the fact that the affected
region of the retina has been bleached, or otherwise changed photochemically,
in such a way that its responses are different from those of the regions not
stimulated. To some extent, however, visual aftereffects are due to central,
rather than retinal, processes.  See  COLOR VISION.
 �34s[Lorrin A. Riggs]
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�6v�2v�32sVisual disorders �21m �33s
    Vision is a complex process. If the structures within the eye are not
properly aligned, or if the eye is too long or too short, or if the
crystalline lens is too strong or too weak, certain conditions result.	See
VISION.
    Hyperopia (farsightedness) results from too short an eyeball so that
parallel rays from infinity, that is, from a distance of 20 ft (6 m) or
beyond, come to a focus behind the retina. The condition can also occur
because a crystalline lens is underpowered and thus does not bend the rays
enough to focus on the retina.
    Presbyopia is the progressive loss of ability to accommodate to near
vision as a result of aging processes. It usually becomes apparent in the 40 -
45 age group.
    Myopia (nearsightedness) results either from an eye that is too long or
from a crystalline lens that is too strong, resulting in a focus of parallel
light in front of the retina.
    In astigmatism the parallel rays of light do not come to a single point,
but rather have a variable focus because the cornea is not spherical and
refracts light in different meridians at different distances. It is rare for
the human cornea to be completely spherical, so that all people have some
degree of astigmatism, and a certain amount is termed physiological.
 �34s[Jack Hartstein]
�33s
    Eyeglass is the general term for optical devices containing corrective
lenses for defects in vision or for special purposes.
    Presbyopia may be corrected by convex lenses, usually worn only for close
work or reading. Bi-, tri-, or tetrafocal lenses are used under different
circumstances to permit rapid shifting from one visual distance to another in
failure of accommodation.
    Hyperopia may be corrected by accommodation, particularly in young people,
but the increased accommodation required for close vision often leads to
eyestrain. Convex lenses are used to gain proper focus (illustration b).
    For myopia, concave lenses are used to permit focusing of far vision
(illustration c); near vision is often better than that of the normal eye
since little accommodation is necessary, less eyestrain is produced with close
work, and the retinal image is somewhat larger than in the normal eye.
    Cylindrical lenses are ground to correct astigmatism. Most astigmatics
also have other defects so that compound lenses are required.
    Other lens systems are commonly devised for persons who have unequal
vision and to correct for abnormal visual axes and defects in ocular movements
caused by muscle imbalance.
    Special forms of eyeglasses are made to absorb portions of the light
spectrum, such as certain colors and ultraviolet and infrared rays. Aviators,
welders, radiologists, furnace workers, and others require lenses of certain
physical or optical characteristics. See LENS (OPTICS).
 �34s[Edward G. Stuart /N. Karle Mottet]
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�6v�2v�32sVitamin   �21m �33s 
    An organic compound essential for the normal functioning of the body, and 
usually obtained from foods. Vitamins are present in food in minute quantities 
compared to the other utilizable components of the diet - proteins, fats, 
carbohydrates, and minerals.  
    Synthetic and natural vitamins usually have the same biological value. The 
vitamins are often not related chemically or functionally. They are 
conventionally divided into a fat-soluble group consisting of vitamins A, D, E 
and K, and a water-soluble group consisting of the various B vitamins, 
including thiamine, riboflavin, nicotinic acid, B6, pantothenic acid, biotin, 
folic acid, B12, p-aminobenzoic acid, inositol, and choline, as well as 
vitamin C, or ascorbic acid. The vitamins, particularly the water-soluble 
ones, are found spread almost universally throughout the animal and plant 
kingdoms (see table). 
    The vitamins function as coenzymes, active parts of enzyme systems, which 
catalyze many of the various anabolic and catabolic reactions of living 
organisms necessary for the production of energy, the synthesis of tissue 
components, hormones, and chemical regulators, and the detoxification and 
degradation of waste products and toxins. Because of their role in metabolism, 
the vitamins are generally concentrated in those animal and plant tissues 
which are most active metabolically. Thus, liver or kidneys are a more potent 
source of vitamins than muscle or skin, and the germ of a seed contains more 
vitamins than in other parts.  See  COENZYME; NUTRITION. 
 �34s[Stanley N. Gershoff]
�33s 
  
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�6v�2v�32sVitamin A   �21m �33s 
    A pale-yellow alcohol, soluble in fat but not in water. It is readily 
destroyed by oxidation, which causes significant losses during storage and 
cooking.  
    There are three sources of vitamin A activity. Vitamin A itself is found 
in all animal tissues. It is particularly concentrated in the liver and 
viscera, and the most important natural sources of the vitamin are the 
fish-liver oils. Plants contain a number of carotenoid pigments, which can be 
converted to vitamin A in the tissues of animals such as in the rat intestinal 
wall.  See  CAROTENOID. 
    In Vitamin A deficiency the epithelial tissues of many organs are 
affected. Growth failure occurs, particularly in the bones and teeth. Besides 
severe inflammation of the eyes, vitamin A-deficient animals suffer from night 
blindness, a condition known as nyctalopia. The role of vitamin A in sight is 
related to the retinal pigment called rhodopsin. Under the stimulus of light 
this pigment breaks down. Resynthesis of rhodopsin, necessary for normal 
vision, is poor in vitamin A deficiency. 
    Approximately 20 IU (international units) of vitamin A per kilogram of 
body weight will support growth and prevent symptoms of deficiency. In 
estimating human requirements, it has been assumed that two-thirds of the 
vitamin A consumed is in the form of carotenoid pigments. There is evidence 
that it is of value to ingest considerably more vitamin A than the amount 
necessary to prevent deficiency signs. The recommended dietary allowances of 
the National Research Council are approximately twice the minimal 
requirements.  See  VITAMIN.   
 �34s[Stanley N. Gershoff]
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�6v�2v�32sVitamin B6   �21m �33s 
    A vitamin which exists as three chemically related and water-soluble forms 
found in food; pyridoxine, pyridoxal, and pyridoxamine. All three forms have 
equal activity for animals and yeast. 
    Vitamin B6 deficiency is accompanied by poor growth, dermatitis, 
microcytic anemia, epileptiform convulsions, and kidney and adrenal lesions. 
There is evidence that some women in the third trimester of pregnancy may have 
a special requirement for vitamin B6 in that its administration often relieves 
the nausea of pregnancy. Some types of human dermatitis respond to local 
application of this vitamin. 
    It is difficult to set requirements for vitamin B6, since no single set of 
assay conditions or criteria has received universal acceptance. Adults 
probably require about 1.5 - 2 milligrams per day, and a dietary intake of 0.4 
milligrams per day would probably be satisfactory for most infants.  See  
VITAMIN.  
 �34s[Stanley N. Gershoff]
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�6v�2v�32sVitamin B12   �21m �33s 
    A group of closely  related polypyrrole compounds, with the structure 
shown, containing trivalent cobalt; it is often called cobalamin. The vitamin 
is a dark-red crystalline compound; in aqueous solution and at room 
temperature it is most stable at pH 4 - 7. 
    Vitamin B12 deficiency in animals is characterized primarily by anemia and 
neuropathy. In humans, this deficiency is called pernicious anemia. People 
suffering from this disease lack a factor secreted in normal gastric juice 
which, by affecting absorption directly and by protecting vitamin B12 from 
intestinal destruction, enables the vitamin to be absorbed.  See  ANEMIA. 
    Requirements for vitamin B12 are increased by reproduction or 
hyperthyroidism. Of the known vitamins B12 is the most active biologically. A 
daily injection of 1 microgram of vitamin B12 will prevent the recurrence of 
symptoms in people with pernicious anemia. For normal people a diet containing 
3 - 5 MUg per day (providing 1 - 1.5 MUg absorbed) will satisfy vitamin B12 
requirements.  See  VITAMIN. 
 �34s[Stanley N. Gershoff]
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�6v�2v�32sVitamin D   �21m �33s 
    Either of two chemically similar fat-soluble, sterol-like compounds, 
calciferol, or vitamin D2, and activated 7-dehydrocholesterol, or vitamin D3. 
    In animals 7-dehydrocholesterol is secreted at the surface of the skin, 
where it is activated by sunlight, and the vitamin D3 formed is reabsorbed. 
Vitamin D2 is prepared by the ultraviolet irradiation of ergosterol, a vitamin 
D precursor found in plants. Most naturally occurring foods have little 
vitamin D activity. Fish oils are an exception, being highly potent sources of 
the vitamin. Children should be given vitamin D supplements; much of the milk 
marketed in the United States has been enriched with 400 IU per quart. (An 
international unit, or IU, of vitamin D is equal to 0.025 microgram of vitamin 
D3.)  
    Vitamin D deficiency in growing animals is called rickets. In adult 
animals, the disease is called osteomalacia. Vitamin D deficiency is 
associated with skeletal pathology. Bones and teeth are soft and fracture 
easily. See RICKETS. 
    The recommended daily dietary allowance of the National Research Council 
for children and pregnant or lactating women is 400 IU. Normal adults can 
probably depend on sunshine and the small amount of the vitamin from their 
food to meet their needs.  See  VITAMIN. 
 �34s[Stanley N. Gershoff]
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�6v�2v�32sVitamin E   �21m �33s 
     A group of compounds called tocopherols. There are eight of these, each 
differing slightly in its chemical configuration. The tocopherols are 
fat-soluble. They are widespread in nature and are particularly concentrated 
in vegetable oils. They are stable to heat, acids, and alkalies, but they are 
easily destroyed by ultraviolet light or oxidizing agents. 
    Vitamin E deficiency in animals has often been associated with 
reproductive failure and irreversible testicular damage. In vitamin E 
deficiency, dystrophy of both striated and cardiac muscles occurs, and death 
often occurs as a result of cardiac failure. Vitamin E therapy has not been of 
value in treating muscular dystrophy in humans. 
    There are few good data concerning the vitamin E requirements of humans. 
Adult requirements may vary between 10 and 30 mg per day.  See  VITAMIN. 
 �34s[Stanley N. Gershoff]
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�6v�2v�32sVitamin K   �21m �33s 
    A naturally occurring vitamin consisting of two chemically similar 
yellowish oils known as K1 and K2, which are fat-soluble, nonsteroid, 
nonsaponifiable, and unstable to light.  
    The only major effect of a vitamin K deficiency in animals is a decrease 
in blood prothrombin, resulting in prolonged clotting time. Thus, vitamin K 
deficiency results in subcutaneous and intramuscular hemorrhages.  
    Vitamin K is so widespread in nature in green leafy vegetables, and its 
synthesis by intestinal microorganisms is so great that vitamin K deficiency 
is rare. Liver disease, which may result in decreased prothrombin formation 
and bile secretion, can result in vitamin K deficiency, because bile secretion 
is necessary for its absorption. Intestinal disease or the ingestion of large 
amounts of mineral oil or antibiotics can also affect vitamin K economy.  See  
VITAMIN.  
 �34s[Stanley N. Gershoff]
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�6v�2v�32sVitellogenesis   �21m �33s 
    The process by which yolk is formed. In most organisms, the early embryo 
cannot manufacture its own nutritive material. In almost all instances, 
nutritive substances called yolk are deposited within the ooplasm of the 
oocyte during its growth. The kinds and amounts of yolk within eggs vary 
greatly within the animal kingdom. In some animals, such as echinoderms and 
mammals, the yolk is produced in small quantities, whereas in others, such as 
sharks and birds, large quantities are stored in the egg. The formed bodies or 
platelets composing the yolk of a wide variety of eggs contain protein, lipid, 
and carbohydrate (including glycogen). 
 �34s[Everett Anderson]
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�6v�2v�32sVivianite   �21m �33s 
    A mineral of the vivianite group, other important members of which are 
annabergite and erythrite.  
    Vivianite is a hydrated ferrous phosphate, Fe3(PO4)2.8H2O; usually ferric 
iron is present as the result of oxidation. It crystallizes in the monoclinic 
system, with crystals generally prismatic. Vivianite also occurs in earthy 
form and as globular and encrusting masses of fibrous structure. Crystals are 
colorless and transparent when fresh. Oxidation changes the color 
progressively to pale blue, greenish blue, dark blue, or bluish black. 
 �34s[Wayne R. Lowell]
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�6v�2v�32sVocal cords   �21m �33s 
    The pair of elastic, fibered bands inside the human larynx. The cords are 
covered with a mucous membrane and pass horizontally backward from the thyroid 
cartilage (Adam's apple) to insert on the smaller, paired arytenoid cartilages 
at the back of the larynx. The vocal cords act as sphincters for air 
regulation and may be vibrated to produce sounds. Separation, approximation, 
and alteration of tension are produced by action of laryngeal muscles acting 
on the pivoting arytenoids. Vibration of the cords produces fundamental sounds 
and overtones. These can be modified by the strength of the air current, the 
size and shape of the glottis (the opening between the cords), and tension in 
the cords.  See  LARYNX; SPEECH. 
 �34s[Walter J. Bock]
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�6v�2v�32sVoice response   �21m �33s 
    The process of generating an acoustic speech signal that communicates an 
intended message so that a machine can respond to a request for information by 
talking to the human user. Communication of speech from the machine to the 
human requires abilities to convert machine information into a desired word 
sequence, with the pronunciations of all desired words carefully sequenced and 
inflected to produce the natural flow of continuous speech waves. 
    Talking machines permit natural interactions that can be valuable for 
announcing warnings, reporting machine status, or otherwise informing the 
computer user, especially when the user cannot view displays, due to 
concurrent visual tasks, visual handicap, or remote telephone links. The many 
advantages of voice interactions with machines and the expanding uses of 
computers in offices, factories, schools, and homes have encouraged the 
development, sales, and use of voice response systems as voice warning 
devices, cockpit advisory systems, time-of-day services, bank-by-phone 
facilities, talking clocks and appliances, toys that speak, and automatic 
readers of printed texts for the blind. 
    For simple tasks involving small vocabularies, machine production of 
speech is not difficult to achieve. Human speech can be stored and reproduced 
later on demand, using techniques similar to those in familiar devices like 
audio tape recorders, phonograph records, and other analog or digital magnetic 
media.  
    It is only a small conceptual step from such recorders to the simplest 
form of modern voice response system, called a word concatenation system. 
Acting like ``automated tape recorders,'' word concatenation systems can 
retrieve previously spoken versions of words or phrases and carefully 
concatenate them without pauses to approximate normally spoken word sequences. 
    The storage of a high-quality digitized waveform may require as much as 
64,000 bits per second of speech, so that large computer disk packs can store 
perhaps at most 1200 seconds of speech, and microcomputer memories might be 
able to store only hundreds, or perhaps only a few tens, of words. Low-cost 
concatenation systems are thus restricted to small vocabularies. 
    There is a general tradeoff between storage requirements and the 
complexity of coding and decoding of the speech. Storage requirements can be 
reduced substantially by first extracting informative parameters from the 
original human speech and later reconstructing speech from such stored 
parameters. Such parameterized approaches assume that not all the information 
in the original signal is important to conveying the message. Informative 
parameters totaling about 16,000 bits/s to as low as 600 bits/s can be stored 
as simplified instructions for controlling reconstitution of the waveform. 
    In a phonemic synthesizer, human speech is not the direct basis for stored 
representation of words or phrases. Rather, each word is abstractly 
represented as a sequence of expected vowels and consonants (phonemes, or 
phones if more detailed articulatory contrasts are included). Speech is 
composed or synthesized by juxtaposing the expected phonemic sequence for each 
word with the sequences for preceding and following words. Each phonemic unit 
is used as a set of instructions for the form of acoustic data to be produced 
at the appropriate times during the utterance. 
    Problems with the extensive hand-tailoring required for phonemic synthesis 
have prompted the development of complex rule sets for predicting the needed 
phonemic states directly from the input message and dictionary pronunciations. 
Such ``synthesis-by-rule'' permits virtually unlimited-vocabulary voice 
response. By providing automatic means to take a specification of any English 
text at the input and to generate a natural and intelligible acoustic speech 
signal at the output, a text-to-speech synthesizer represents the most 
ambitious form of voice response system. Without the need for intervening 
human sequence selection such as a phonemic synthesizer needs, and with less 
storage requirement than other synthesizers, the text-to-speech synthesizer is 
probably the primary choice for large-vocabulary speech generation.  See  
PHONETICS; SPEECH RECOGNITION. 
 �34s[Wayne A. Lea]
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�6v�2v�32sVoigt effect �21m �33s
    An anisotropic substance placed in a magnetic field becomes birefringent
(doubly refracting), and its optical properties are similar to those of a
uniaxial crystal. The Faraday effect is the result of this birefringence when
observations are made parallel to the magnetic field. The analogous
observations perpendicular to the magnetic lines of force are more difficult
and were not successfully carried out until 1898 because of the smallness of
the effect. The transverse magnetooptic birefringence is called the Voigt
effect (also called magnetic double refraction).
    The Voigt effect can be observed only in the vicinity of sharply defined
absorption lines, that is, in gases and in certain crystals having sharp
lines, such as rare-earth salts.  See  FARADAY EFFECT; MAGNETOOPTICS.
 �34s[G. H. Dieke ;William W. Watson]
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�6v�2v�32sVolatilization   �21m �33s 
    The process of converting a chemical substance from a liquid or solid 
state to a gaseous or vapor state. Other terms used to describe the same 
process are vaporization, distillation, and sublimation. A substance can often 
be separated from another by volatilization and can then be recovered by 
condensation of the vapor. The substance can be made to volatilize more 
rapidly either by heating to increase its vapor pressure or by removal of the 
vapor using a stream of inert gas or a vacuum pump. Chemical reactions are 
sometimes utilized to produce volatile products. Volatilization methods are 
generally characterized by great simplicity and ease of operation, except when 
high temperatures or highly corrosion-resistant materials are needed.  See  
CHEMICAL SEPARATION TECHNIQUES; DISTILLATION; SUBLIMATION; VAPOR PRESSURE. 
 �34s[Louis Gordon /Royce W. Murray]
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�6v�2v�32sVolcanic glass   �21m �33s 
    A natural glass formed by rapid cooling of lava. The material is opaque 
except on thin edges, occurs in a variety of colors, and has a conchoidal 
fracture. Shades of red, brown, black, gray, or green may be displayed in a 
uniform, banded, or variegated fashion. 
    Most natural glasses are chemically equivalent to rhyolite, into which 
they may grade. Types corresponding to trachyte, dacite, andesite, and latite 
are much less common. Basaltic glass generally is given the name tachylite. 
Varieties include obsidian, pitchstone, pumice, and perlite.  See  OBSIDIAN; 
PERLITE; PITCHSTONE; PUMICE; RHYOLITE. 
    In addition to crystallites, most volcanic glasses carry microlites 
(microscopically tiny crystals). Somewhat larger and usually well-formed 
crystals are called phenocrysts. Their presence gives the glass a porphyritic 
texture. They consist chiefly of quartz, alkali feldspar, and plagioclase and 
less commonly of mafics (biotite, hornblende, or pyroxene). As their number 
increases, these porphyritic glasses pass into vitrophyre.  See  LAVA; 
PHENOCRYST; PORPHYRY. 
 �34s[Carleton A. Chapman]
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�6v�2v�32sVolcano �21m �33s
    A mountain or hill, generally steep-sided, formed by accumulation of magma
(molten rock with associated gas and crystals) extruded through openings or
volcanic vents in the Earth's crust; the term volcano also refers to the vent
itself. During the evolution of a long-lived volcano, a permanent shift in the
locus of principal vent activity can produce a satellitic volcanic
accumulation as large as or larger than the parent volcano, in effect forming
a new volcano on the flanks of the old.
    Planetary exploration has revealed dramatic evidence of volcanoes and
their products on the Earth's Moon, Mars, and Jupiter's moons on a scale much
more vast than on Earth. Only the products and landforms of terrestrial
volcanic activity are described here.  See  JUPITER; MARS; MOON; VOLCANOLOGY.

    Volcanic vents.  Volcanic vents, channelways for magma to ascend toward
the surface, can be grouped into two general types: fissure and pipelike.
Magma consolidating below the surface in fissures or pipes forms a variety of
igneous bodies, but that breaking the surface produces fissure or pipe
eruptions. Fissures, most of them less than 10 ft (3 m) wide, may form in the
summit region of a volcano, on its flanks, or near its base; pipes tend to be
restricted to the summit area of a volcano. For some volcanoes or volcanic
regions, swarms of fissure vents are clustered in swaths called rift zones.

    Volcanic products.	Magma erupted onto the Earth's surface is called lava.
If the lava is chilled and solidifies quickly, it forms volcanic glass; slower
rates of chilling result in greater crystallization before complete
solidification. Lava may accrete near the vent to form various minor
structures or may pour out in streams called lava flows, which may travel many
miles from the vents. During more violent eruptions, lava torn into fragments
and hurled into the air is called pyroclastic (fire-broken materials).	See
IGNEOUS ROCKS; LAVA; MAGMA; PYROCLASTIC ROCKS; VOLCANIC GLASS.
    Violent volcanic explosions may throw dust high into the stratosphere,
where it may drift across the surface of the globe for thousands of miles.
Most of the solid fragments in the cloud settle out within a few days, and
nearly all settle out within a few weeks, but some finely divided material may
remain suspended in the stratosphere for more than a year. Volcanic dust and
related sulfate aerosols from major explosive eruptions may screen out solar
radiation sufficiently to lower the Earth's surface temperature and thus
possibly affect climate on a global scale. On a local scale, the fallout from
eruption plumes and associated acid rains can damage crops.  See  AIR
POLLUTION.
    In general, water vapor is the most abundant constituent in volcanic
gases; the water is mostly of meteoric (atmospheric) origin, but in some
volcanoes can have a significant magmatic or juvenile component. Excluding
water vapor, the most abundant gases are the various species of carbon,
sulfur, hydrogen, chlorine, and fluorine.
    Mudflows are common on volcanoes where poorly indurated or nonwelded
pyroclastic material is abundant. Probably by far the most common cause is
heavy rain saturating a thick cover of loose unstable pyroclastic material on
the steep slope of the volcano, transforming the material into a mobile,
water-saturated ``mud,'' which can rush downslope at a speed as great as 50 -
55 mi/h (80 - 90 km/h). Such a fast-moving mass can be highly destructive,
sweeping up everything loose in its path.

    Volcanic landforms.  Much of the Earth's solid surface, on land and below
the sea, has been shaped by volcanic activity. Landscape features of volcanic
origin may be either positive (constructional) forms, the result of
accumulation of volcanic materials, or negative forms, the result of the lack
of accumulation or collapse.
    Not all volcanoes show a graceful, symmetrical cone shape, such as that
exemplified by Mount Fuji, Japan.  In reality, most volcanoes, especially
those near tectonic plate boundaries, are more irregular, though of grossly
conical shape. Such volcanoes, called strato-volcanoes or composite volcanoes,
typically erupt explosively. Volcanoes constructed primarily of fluid basaltic
lava flows, which may spread great distances from the vents, typically are
gentle-sloped, broadly upward convex structures. Such shield volcanoes, a
classic example of which is Mauna Kea Volcano, Hawaii, tend to form in oceanic
intraplate regions and are associated with hot-spot volcanism. Depending on
magma viscosity, eruptive history, and posteruption modifications, a volcano
can assume a wide variety of shapes intermediate between the simple forms of
composite and shield volcanoes.
    Some of the largest volcanic edifices are not shaped like the composite or
shield volcanoes. In certain regions of the world,  voluminous extrusions of
very fluid basaltic lava from dispersed fissure swarms have built broad,
nearly flat-topped accumulations. These voluminous outpourings of lava are
known as flood basalts or plateau basalts.  See  BASALT.

    Submarine volcanism.  Volcanic eruptions in shallow water are very similar
in character to those on land but, on average, are probably somewhat more
explosive, owing to heating of hot water and resultant violent generation of
supercritical steam. Much of the ocean basins appears to be floored by
basaltic lava.	See  MARINE GEOLOGY; OCEANIC ISLANDS.

    Fumaroles and hot springs.	Vents at which volcanic gases issue without
lava or after the eruption are known as fumaroles. They are found on active
volcanoes during and between eruptions and on dormant volcanoes. Fumaroles
grade into hot springs and geysers. The water of most, if not all, hot springs
is predominantly of meteoric origin, and is not water liberated from magma.
See GEOTHERMAL POWER; GEYSER.

    Distribution of volcanoes.	About 500 active volcanoes are known on the
Earth, mostly along or near the boundaries of the dozen or so lithospheric
plates that compose the Earth's solid surface. These rigid plates, which range
in thickness from 30 to 90 mi (50 to 150 km) and consist of both crustal and
upper mantle material, form the lithosphere and move relative to one another
above a hotter, more plastic zone in the mantle called the asthenosphere.  See
  ASTHENOSPHERE; LITHOSPHERE; PLATE TECTONICS.
 �34s[Gordon A. Macdonald /Robert I. Tilling]
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�6v�2v�32sVolcanology   �21m �33s 
    The scientific study of volcanic phenomena. Strictly speaking, it refers 
only to the surface eruption of magmas and related gases, and the structures, 
deposits, and other effects produced thereby. Commonly, however, it includes 
much of plutonic igneous geology, since effects at the Earth's surface are the 
result of events at depth. For a discussion of the surface structures and 
deposits which result from the activity of volcanos  see  VOLCANO. 
  
    Composition of magma.  As formed at depth, magma consists of liquid rock 
containing dissolved gases. Rising toward the surface, it enters zones of 
lower temperature and pressure. Decreasing temperature tends to bring about 
crystallization, which produces solid crystals suspended in the liquid. Other 
solid fragments are incorporated from the walls and roof of the conduit 
through which the magma is rising. As crystallization progresses, volatiles 
and the more soluble silicate components are concentrated in the remaining 
liquid.  
    At some point in the rise of the magma, decreasing confining pressure and 
increasing concentration of volatiles in the residual liquid initiate the 
separation of gas from the liquid. From that point on to its final complete 
consolidation, the magma consists of all three phases: liquid, solid, and gas. 
    Part of the gas liberated at volcanoes probably comes from the same 
deep-seated source as the silicate portion of the magma, but some is of 
shallower origin. Part of the steam is the result of surficial oxidation of 
deep-seated hydrogen, and some of the oxidation of the sulfur gases must have 
taken place close to the surface.  See  MAGMA. 
    The character of a volcanic eruption is determined largely by the 
viscosity of the liquid phase of the erupting magma and the abundance and 
condition of the gas it contains. Viscosity is in turn affected by such 
factors as the chemical composition and temperature of the liquid, the load of 
solid crystals and xenoliths it carries, the abundance of gas, and whether the 
gas is dissolved or separated as bubbles. In very fluid lavas small gas 
bubbles form gradually, and generally are able to rise through the liquid, 
coalescing to some extent to form larger bubbles, and escape at the surface 
with only minor disturbance. In more viscous lavas the escape of gas is less 
easy, and produces minor explosions as the bubbles burst their way out of the 
liquid. In still more viscious lavas there appears at times to be a tendency 
for the formation of large numbers of small bubbles more or less 
simultaneously throughout a large volume of liquid, and the violent expansion 
of these may tear the frothy liquid to pieces and throw it into the air as a 
shower of volcanic ash and dust, accompanied by some larger blocks; or it may 
produce an outpouring of an emulsion of gas and semisolid bits of glass to 
form an ash flow. Also, gas may accumulate beneath a solid or highly viscous 
seal in the vent until it acquires enough pressure to burst its way forth in 
an explosion that hurls out fragments of the disrupted plug. 
  
    Surface deformation.  The entire volcanic mountain swells and shrinks, 
apparently in response to changes in conditions beneath the volcano. The 
measurements are of two sorts: precise leveling surveys in which the change of 
altitude of a series of bench marks is related to some point outside the 
disturbed area, and measurement of the change in inclination of a given point 
on the slope of the cone by means of instruments known as tiltmeters. Some 
tiltmeters are capable of detecting changes in inclination, but the total 
changes of level resulting from tumescence and detumescence of the volcano may 
amount to several feet.  
  
    Volcanic Earth vibrations.  Many eruptions are preceded by numerous 
earthquakes, and these, together with tumescence of the volcano, constitute 
the principal evidences of coming eruption. For the most part they closely 
resemble tectonic earthquakes. 
    Other volcanic Earth vibrations are termed harmonic tremors, which may go 
on continuously with little or no change for many hours, days, or weeks; and 
spasmodic tremor, which is simply a closely spaced succession of minute 
earthquakes. Volcanic explosions produce shock waves in the atmosphere - 
sometimes seen as flashing arcs rendered visible by refraction of light in the 
denser wavefront. Explosions or collapses taking place underwater may cause 
water waves, sometimes of great size. These fall within the class of tsunamis 
(seismic sea waves), though most tsunamis have other causes.  See  EARTHQUAKE; 
SEISMOLOGY; TSUNAMI. 
 �34s[Gordon A. Macdonald]
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�6v�2v�32sVole   �21m �33s 
    A rodent in the tribe Microtini in the family Cricetidae. In addition to 
the voles, the steppe lemmings and the muskrats are included in this group. 
There are about 79 species of vole, 42 of which belong to the genus Microtus. 
Voles have a stout body with short legs. The ears are small and are obscured 
by the coarse long fur. 
    Commonly, these rodents have four litters of six young each year; there 
does not appear to be a definite breeding period. The gestation period is 
about 3 weeks. Nests are built of grass, on the ground surface, in 
depressions, shallow burrows, or even on grass stems above the ground. 
However, surface nests are used primarily as shelters, most young being born 
in underground burrows. These animals do not hibernate, and in the winter they 
move great distances under the snow in search of food.  See  RODENTIA. 
 �34s[Charles B. Curtin]
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�6v�2v�32sVoltage amplifier   �21m �33s 
    An electronic amplifier that produces, with minimum distortion, an output 
voltage greater in magnitude than the input voltage. For a general discussion 
of amplifiers  see  AMPLIFIER. 
    Voltage amplifiers are built to amplify signals with frequency components 
in the range from zero hertz to thousands of megahertz. To cover this 
frequency range, several different types of amplifiers are required. An 
amplifier which cannot amplify voltage signals with zero frequency components 
(dc) is called RC-coupled. If zero frequency components (dc) are amplified, 
the amplifier is known as dc or direct-coupled. In the range from a few hertz 
to 30 kHz, amplifiers are classified as audio. In the frequency range up to 10 
MHz for use in television, pulse circuits, and electronic instruments, 
amplifiers are known as video. For the radio-frequency range from a few 
hundred kilohertz to hundreds of megahertz, tuned amplifiers are normally 
used. At ultrahigh frequencies, amplifiers designed around special tubes are 
employed.  See  KLYSTRON; MAGNETRON; TRAVELING-WAVE TUBE. 
  
    RC-coupled amplifier.  The RC-coupled amplifier is employed in the 
frequency range from a few hertz to about 10 MHz. However, operation at the 
higher frequencies in this range is obtained only by employing 
frequency-compensation networks. The term RC-coupled amplifier generally 
refers to any set of amplifier stages that are coupled by a 
resistance-capacitance network. 
  
    Tuned amplifier.  A tuned amplifier amplifies voltage signals in a 
selected narrow frequency band and suppresses signals outside the desired 
band. A tuned amplifier is thus a bandpass amplifier with the additional 
constraint that its bandwidth be a small fraction of the center frequency. 
Such amplifiers are commonly used as radio-frequency (rf) or 
intermediate-frequency (i-f) amplifiers for superheterodyne radio receivers. 
In the case of rf amplifiers it is required that the center frequency be 
tunable or variable and that, in addition, over the tuning range the overall 
selectivity should remain constant. Constant selectivity means that the 
bandwidth should remain constant. In order to obtain the necessary selectivity 
and high voltage gain, LC tank or resonant circuits are used as loads with 
transistor or field-effect transistor (FET) amplifiers. 
  
    Grounded-gate (base) amplifier.  If the gate (or base) is grounded and the 
input signal is applied to the source (or emitter) with the output taken from 
the drain (or collector), the circuit is called a grounded-gate (or 
grounded-base) amplifier. This circuit features a low input impedance and a 
high output impedance. Because of the low input impedance, the amplifier will 
draw power from the source and must, therefore, be used with a low-impedance 
source. It has found its chief application in rf amplifiers operating at very 
high frequencies, such as the tuner in a television receiver; at these 
frequencies it gives a high signal-to-noise ratio with low input-output 
capacitance. It does so at the expense of rf selectivity. This excludes it 
from use in a narrow-band receiver application. 
  
    Direct-coupled amplifier.  When the coupling between successive stages of 
an amplifier is such that direct current may flow from the output of one stage 
to the input of the following stage, the amplifier is said to be 
direct-coupled.  See  DIRECT-COUPLED AMPLIFIER. 
  
    Difference amplifiers.  The function of a difference, or differential, 
amplifier is, in general, to amplify the difference between two signals. The 
need for differential amplifiers arises in many physical measurements, in 
medical electronics, in analog computers, and in direct-coupled amplifier 
applications.  
  
    Integrated amplifier.  Integrated circuit amplifiers are dc amplifiers 
commonly referred to as operational amplifiers. The basic building block of an 
integrated tuned amplifier is an emitter-coupled difference amplifier.  See  
INTEGRATED CIRCUITS; OPERATIONAL AMPLIFIER. 
 �34s[Christos C. Halkias]
�33s 
  
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�6v�2v�32sVoltage measurement   �21m �33s 
    Determination of the difference in electrostatic potential between two 
points.  See  POTENTIALS; VOLTMETER. 
    In dc circuits the flow of current is unidirectional and the voltmeter 
must respond only to the magnitude of the potential difference. For voltages 
greater than 1000 volts, electrostatic voltmeters are often employed. These 
depend upon electrostatic forces to deflect a pointer against a spring 
restoring force. In the 1 - 1000-volt range, voltage is commonly measured by 
using a large resistance in series with a milliammeter. 
    The measure voltages below 1 volt, particularly in the microvolt range, 
these direct deflection instruments often require more power to operate than 
is available from the circuit under measurement. Extremely sensitive 
galvanometers have been devised to solve this problem, although use of an 
electronic amplifier between the circuit under measurement and a 
deflection-type instrument is more common. The modern amplifier-type 
voltmeter, called the vacuum-tube voltmeter, is more likely to employ 
solid-state transistors or integrated circuits than the older vacuum tube. The 
name vacuum-tube voltmeter, however, has remained.  See  VACUUM-TUBE VOLTMETER. 
    In the volt range, the voltage to be measured is impressed directly upon a 
dc amplifier, while in the millivolt or microvolt range, a chopper-type 
amplifier is used.  See  VIBRATOR. 
    When greater accuracy than can be obtained with a moving-pointer voltmeter 
is required, a potentiometric method is often employed.  See  POTENTIOMETER 
(VOLTAGE METER). 
    Ac voltage waveforms have several measurable attributes. Ac voltmeters 
commonly measure the peak value, the average rectified value, or the 
root-mean-square (rms) value of the waveform. In the low-frequency range, 
where sufficient power is available, moving-pointer voltmeters that respond to 
the rms value are usually employed. If a semiconductor rectifier is utilized 
directly with a dc voltmeter, an ac voltmeter responding to the average value 
of the rectified voltage results, which is useful through the audio-frequency 
range. One of the most widely used average-responding ac voltmeters is the 
operational rectifier. Peak voltage is measured by using a vacuum-tube diode 
to charge a capacitor to the peak value, which is then measured by a dc 
vacuum-tube voltmeter. Voltmeters of this type are employed up to 1 GHz. To 
extend voltage measurement about 10 GHz, a sampling voltmeter has been used. 
See SAMPLING VOLTMETER. 
    For high-accuracy ac-voltage measurement, a thermocouple or dynamometer 
which responds identically to ac and dc voltage is used as a transfer device. 
See CURRENT MEASUREMENT; ELECTRICAL MEASUREMENTS. 
 �34s[Malcolm C. Holtje]
�33s 
  
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�6v�2v�32sVoltage-multiplier circuit   �21m �33s 
    A rectifier circuit capable of supplying a dc output voltage that is two 
or more times the peak value of the ac input voltage. Such circuits are 
especially useful for high-voltage, low-current supplies. These supplies are 
usually lighter in weight, smaller in size, and less expensive than the more 
usual half-wave and full-wave rectifier supplies. They require either no power 
transformer or a much smaller transformer, but they have the disadvantage of 
requiring more rectifiers and capacitors. If, for safety or other reasons, 
input and output must be isolated, a transformer may be required at the input 
even though the voltage-multiplier circuit does not need it for operation. 
Common configurations are the half-wave and full-wave voltage-doubling 
circuits, but tripling and higher orders of multiplication are used.  
 �34s[Donald L. Waidelich]
�33s 
  
  
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�6v�2v�32sVoltage regulation   �21m �33s 
    The change in voltage magnitude that occurs when the load (at a specified 
power factor) is reduced from the rated or nominal value to zero, with no 
intentional manual readjustment of any voltage control, expressed in percent 
of nominal full-load voltage. Voltage regulation is a convenient measure of 
the sensitivity of a device to changes in loading.  See  GENERATOR; 
TRANSFORMER; VOLTAGE REGULATOR. 
 �34s[Paul M. Anderson]
�33s 
  
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�6v�2v�32sVoltage regulator   �21m �33s 
    A device or circuit that maintains a load voltage nearly constant over a 
range of variations of input voltage and load current. Voltage regulators are 
used wherever the unregulated voltage would vary more than can be tolerated by 
the electrical equipment using that voltage. Alternating 
current distribution feeders use voltage regulators to keep the voltage 
supplied to the user within a prescribed range. Electronic equipment often has 
voltage regulators in dc power supplies. 
  
    Electronic regulators.  In much electronic equipment the dc power supply 
voltage must remain constant in spite of input ac line voltage variations and 
output load variations. To perform this function, two types of 
voltage-regulator circuits are used in electronic power supplies. The first 
uses a nearly constant voltage device, such as a zener diode or a gas tube, as 
the voltage regulator; the second uses the variable-resistance properties of a 
transistor or a vacuum tube. Because of the rapid response of these devices, 
either type regulator serves not only to compensate for irregular changes in 
input or output but also to counteract ripple voltage. Thus an electronic 
regulator makes possible the use of simpler filters. The voltage regulator 
circuit may also augment the filtering action of the power-supply filters, and 
it may also raise or lower the internal impedance of the power supply as seen 
from the load. The trade off for the regulator action is a considerable power 
loss in the regulator circuit. Most of the power is lost either in a regulator 
resistor or in the regulator diodes or tubes.  See  ELECTRONIC POWER SUPPLY; 
RECTIFIER; RIPPLE VOLTAGE. 
 �34s[Donald L. Waidelich]
�33s 
  
    Power-system regulators.  Voltage regulators are used on distribution 
feeders to maintain voltage constant, irrespective of changes in either load 
current or supply voltage. Voltage variations must be minimized for the 
efficient operation of industrial equipment and for the satisfactory 
functioning of domestic appliances, television in particular. Voltage is 
controlled at the system generators, but this alone is inadequate because each 
generator supplies many feeders of diverse impedance and load characteristics. 
Regulators are applied either in substations to control voltage on a bus or 
individual feeder or on the line to reregulate the outlying portions of the 
system. These regulators are variable autotransformers with the primary 
connected across the line. The secondary, in which an adjustable voltage is 
induced, is connected in series with the line to boost or buck the voltage. 
    A control and drive provide automatic operation. A voltage-regulating 
relay senses output voltage. When the voltage is either above or below the 
band of acceptable voltage maintained by the regulator, this relay causes the 
motor to operate and change the regulator position to raise or lower the 
voltage as required to bring it back within the control band. It is desirable 
to maintain constant voltage at the average load center out on the feeder 
rather than at the regulator terminals. Hence the control circuit includes a 
line-drop compensator with resistance and reactance elements that can be 
adjusted to represent line impedance. These impedances carry current 
proportional to circuit load, thereby simulating the voltage drop between the 
regulator and the load center, and modify the voltage sensed by the 
voltage-regulating relay. 
    Two principal types of feeder regulators are used: the step regulator, 
which provides increments or steps of voltage change, and the induction 
regulator, which provides continuous voltage adjustment. 
 �34s[Donnell D. MacCarthy]
�33s 
  
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�6v�2v�32sVolt-ampere   �21m �33s 
    The apparent-power index of drive level for sinusoidal alternating-current 
loads. A circuit branch with E volts across its two terminals, carrying I 
amperes from one to the other, is said to be receiving EI volt-amperes of 
apparent power, whatever may be the phase lag THETA of current behind voltage. 
If such a load is driven through a transformer, the volt-amperes into the 
transformer primary is the same number as the volt-amperes into the load. 
    Practical computation of real power, reactive power, and apparent power is 
usually done with complex-number algebra using the geometrical diagram of the 
illustration.  
 �34s[Mark G. Foster]
�33s 
  
  
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�6v�2v�32sVoltmeter   �21m �33s 
    An instrument for the measurement in volts of potential difference between 
two points. Derivatives of the voltmeter are the microvoltmeter, 
millivoltmeter, and kilovoltmeter, for measurement of voltages with a 
measurement span of 1,000,000,000:1.  See  VOLTAGE MEASUREMENT. 
    Voltmeters, in general, are secondary instruments consisting of 
voltage-to-current transducers and mechanisms which respond to milliamperes or 
microamperes. In the simpler forms, the transducer consists of a resistor of 
constant value, which may have taps for several voltage ranges. Since by Ohm's 
law the current is proportional to the potential difference, calibration of 
the combination using a primary standard, such as the potentiometer and 
standard cell, is valid. In the more specialized forms, the transducer 
consists of a thermal converter (ac to dc), rectifier,  or amplifier. The 
electrostatic instrument is the only voltmeter which measures potential 
difference directly. 
    The moving-coil dc voltmeter consists of a permanent magnet, moving coils 
pivoted in jewel bearings, control springs, and a pointer, as shown in Fig. 1. 
The current is passed through the moving coil via the two control springs 
which apply a restraining torque. The deflecting torque is equal to the 
product of the magnetic moment of the coil and the field intensity. 
    The moving-magnet dc voltmeter has the magnet attached to the pivoted 
shaft which carries the pointer. The magnet is surrounded by the field coil 
and aligned by a control magnet which supplies the restoring torque. As 
current is applied to the field coil, the coil and restoring-magnet fields 
produce a resultant field. The rotor aligns itself with this resultant field 
and indicates the magnitude of the current and thus the potential difference. 
    The electrostatic voltmeter action is based on the force of attraction or 
repulsion between two charged conductors, such as the plates of a variable air 
capacitor. The moving plate, when charged, tends to move so as to increase the 
capacitance between it and the fixed plate. 
  
    The electrodynamic ac voltmeter has two coils connected in series through 
a resistor to the source, the potential of which is to be measured, as shown 
in Fig. 2. The fixed coil provides the field in which the moving coil, 
supported by a pivoted shaft, operates. The deflection of the moving system is 
restrained by a control spring. When the circuit is closed, the current which 
flows through the coils produces a deflecting torque which moves the 
indicating needle. 
  
    In a common form of moving-iron ac voltmeter, the field coil is connected 
to the line through a series resistor. Inside the field coil are two 
rectangular vanes, one fixed and the other attached to the shaft which carries 
the pointer. When a current traverses the coil, the vanes are similarly 
magnetized, and the moving vane is repelled from the fixed vane, deflecting 
the pointer in a clockwise direction. 
    The thermovoltmeter utilizes a thermojunction and dc millivoltmeter. A 
current from the voltage source is passed through a resistor and a fine 
vacuum-enclosed platinum heater wire. A thermocouple, attached to the midpoint 
of the heater, generates millivolts proportional to the temperature.  See  
THERMOCOUPLE.  
    The rectifier voltmeter consists of a dc milliammeter calibrated in volts 
and connected to the voltage source through a rectifier bridge. The individual 
rectifiers are connected so that the current through the milliammeter is 
always in the same direction. This meter measures the average value of the 
current.  
    The electronic ac voltmeter consists of a dc milliammeter calibrated in 
volts and connected to an amplifier-rectifier circuit.  See  DIGITAL 
VOLTMETER; SAMPLING VOLTMETER; VACUUM-TUBE VOLTMETER. 
 �34s[Almon J. Corson]
�33s 
  
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�6v�2v�32sVolt-ohm-milliammeter   �21m �33s 
    A self-contained test instrument for measuring a wide range of voltages 
(both ac and dc), resistances, and currents (usually dc only), particularly in 
radio, TV, and electronic servicing. 
    All readings are obtained on a single multiscale indicating instrument. 
The selection of the correct ranges and function is accomplished either by 
means of one or two rotary switches, by multiple pin jacks, or by a 
combination of both. 
    The terms multimeter and analyzer (or circuit analyzer) are quite commonly 
used as synonyms for volt-ohm-milliammeter.  See  AMMETER; OHMMETER; VOLTMETER. 
 �34s[Isaac F. Kinnard]
�33s 
  
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�6v�2v�32sVolume control systems   �21m �33s 
    Electronic systems that regulate the signal amplification or limit the 
output of circuits. Examples are volume compressors, limiters, and expanders. 
A volume compressor is an electronic system that reduces the amplification of 
an amplifier when the signal being amplified is large and increases the 
amplification when the signal is small. Compressors are used to reduce the 
volume range in sound motion picture and phonograph recording, sound 
broadcasting, public address and sound reinforcing systems, and so forth. 
    A volume limiter is an electronic system in which the relationship between 
the input and output signals is constant up to a certain level, but beyond 
this point the output remains constant regardless of the input. The limiter is 
useful for protection against sudden overloads, as, for example, in the audio 
input to a radio or television transmitter. 
    A volume expander is an electronic system that increases the amplification 
of an amplifier when the signal is large and decreases the amplification when 
the signal is small. In reproduction, a volume expander is used to counteract 
or complement the effect of the compressor in recording or the transmission of 
an audio signal. 
    Volume compressors, limiters, and expanders are electronic amplifiers in 
which the amplification varies as a function of the average level of the 
signal. In all the systems, the input signal to the control circuits is 
rectified and applied to a capacitor-resistance network. The direct-current 
(dc) voltage across the capacitor is used to vary the amplification of a 
vacuum tube or transistor amplifier operating in a variable gain mode.  See  
AMPLIFIER; AUTOMATIC GAIN CONTROL (AGC). 
 �34s[Harry F. Olson]
�33s 
  
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�6v�2v�32sVolume flow rate meter �21m �33s
    An instrument for measuring the rate of volumetric flow of fluid through a
pipe, duct, or open channel. Flow rate meters may modify the flow pattern by
insertion of some obstruction or by modification of the shape of the walls of
the conduit or channel carrying the fluid, or they may measure the velocity
without affecting the flow pattern.  See  FLOW MEASUREMENT.
    Differential-producing primary devices (sometimes called head meters)
produce a difference in pressure caused by a modification of the flow pattern.
The pressure difference is based on the laws of conservation of energy and
conservation of mass. The law of conservation of energy states that the total
energy at any given point in the stream is equal to the total energy at a
second point in the stream, neglecting frictional and turbulent losses between
the points. It is possible, however, to convert pressure (potential energy) to
velocity (kinetic energy), and vice versa. By use of a restriction in the
pipe, such as an orifice plate, venturi tube, flow nozzle, or flow tube, a
portion of the potential energy of the stream is temporarily converted to
kinetic energy as the fluid speeds up to pass through these primary devices.
See  BERNOULLI'S THEOREM.
    The common orifice plate (Fig. 1) is a thin plate inserted between pipe
flanges, usually having a round, concentric hole with a sharp, square upstream
edge. The extensive empirical data available and its simplicity make it the
most common of the primary devices. Segmental and eccentric orifices are
useful for measurement of liquids with solids, and of vapors or gases with
entrained liquids.  See  METERING ORIFICE.

    The Venturi tube (Fig. 2) has the advantage of introducing less permanent
loss of pressure than the orifice plate. The converging inlet cone permits
solids and dirt to be flushed through it, and the outlet cone reduces the
turbulent losses.  See	VENTURI TUBE.

    The flow nozzle (Fig. 3) also has a converging inlet, but it has no
diverging outlet section. The pressure loss is similar to that with an orifice
plate for the same differential pressure. It is used where solids are
entrained in the liquid and also where fluid velocities are particularly high
(it will pass more fluid than an orifice of the same diameter).

    In industrial flow measurement, pitot tubes are used in spot checks and
for comparative measurements to determine trends. A pitot tube measures the
difference between total impact pressure and static pressure. There are a wide
variety of tube configurations available for sensing both impact and static
pressure or combinations thereof . Several readings should be taken across the
pipe along a diameter (preferably two diameters) of the pipe and the average
velocity determined.  See  PITOT TUBE.
    Fluid flow around the bend in a pipe elbow produces a centrifugal force
greater on the outside than on the inside, and thereby produces a difference
in pressure between pressure connections on the inside and outside of the
bend. This differential pressure is used as an indication of the volume flow
rate.
    All the primary devices discussed above create a difference in pressure
between two points. This differential pressure must be measured and converted
by the equations given to obtain flow. Differential pressure may be measured
accurately by simple liquid-filled (usually mercury) U-tube manometers or by
more refined types of meters, such as the float-operated mechanism or the
weight-balanced, ring-type meter, to provide controlling, recording, and
totalizing functions. These devices are ordinarily connected directly to the
pressure connections on the primary device and are therefore exposed to the
process fluids.  See  MANOMETER
    Flow in open channels is determined by measuring the height of liquid
passing through a weir or flume.  Consistent results require that the
measuring device be protected from direct impact of the flowing stream.
Because the height-flow velocity relationship is exponential, the secondary
device frequently uses cams to obtain a uniform flow scale.  For special
types of volume flow rate meters see ELECTROMAGNETIC FLOWMETER; TARGET
FLOWMETER; TRACER METHOD; VARIABLE-AREA FLOWMETER.
[Mead Brander; Lewis P. Emerson]


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�6v�2v�32sVolumetric analysis   �21m �33s 
    Volumetric analysis, or possibly more correctly titrimetric analysis, is 
one of the major divisions of quantitative analytical methods. In this method, 
as the names imply, a titration is made and the amount of the desired 
component is determined indirectly from the volume and concentration of 
solution used in the titration. 
    In most volumetric procedures, the sample to be analyzed is weighed, or an 
accurately measured volume of a solution of the sample is taken. The sample is 
dissolved in an appropriate solvent and subjected to some preliminary 
treatment or separation if necessary, and then the titration is performed on 
the resulting solution. 
    During the titration, a chemical reaction occurs between a constituent of 
the titrant solution and the substance to be determined. In order to be 
suitable for volumetric titrations, these reactions must be rapid and 
quantitative. That is, it is necessary to know exactly how many molecules or 
ions of the substance to be determined react with each molecule or ion of the 
reagent in the titrant. In volumetric analysis, it is also necessary to 
determine when the number of reagent ions or molecules equivalent to the 
substance being determined has been added. This point in the titration is 
called the equivalence point. The determination of the amount of titrant 
corresponding to the equivalence point is usually done either by visual means 
with colored indicators or reagents, or by some physicochemical measurement.  
See  QUANTITATIVE CHEMICAL ANALYSIS; TITRATION; VOLUMETRIC FLASK. 
 �34s[Clark E. Bricker]
�33s 
  
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�6v�2v�32sVolumetric efficiency   �21m �33s 
    In describing an engine or gas compressor, the ratio of volume of working 
substance actually admitted, measured at specified temperature and pressure, 
to the full piston displacement volume. For a liquid-fuel engine, such as a 
diesel engine, volumetric efficiency is the ratio of volume of air drawn into 
a cylinder to the piston displacement. For a gas-fuel engine, such as a 
gasoline engine with carburetor, volumetric efficiency is based on the charge 
of fuel and air drawn into the cylinder.  See  INTERNAL COMBUSTION ENGINE; 
RECIPROCATING AIRCRAFT ENGINE. 
 �34s[Neil MacCoull]
�33s 
  
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�6v�2v�32sVolumetric flask   �21m �33s 
    A long narrow-necked glass flask which has been calibrated to contain a 
given volume of liquid when filled so that the bottom of the meniscus is 
tangent to the ring which has been etched on the neck of the flask. Some 
volumetric flasks have a second calibration mark, higher on the neck of the 
flask, which is used when the flask is needed to deliver a given volume of 
liquid. The marks etched on volumetric flasks are accurate only at the 
temperature at which these calibrations were made, because the volume of a 
flask increases slightly with an increase of temperature and vice versa. 
Volumetric flasks are used when it is necessary to dilute a weight or a volume 
of a substance to some accurately known volume.  See  CONCENTRATION SCALES; 
TITRATION; VOLUMETRIC ANALYSIS. 
 �34s[Clark E. Bricker]
�33s 
  
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�6v�2v�32sVolume unit (VU)   �21m �33s 
    A unit for expressing the audio-frequency (af) power level of a complex 
electric wave, such as that corresponding to speech or music. The volume unit 
(VU) is defined as 10 times the common logarithm of the power ratio p2/p1, 
where the reference power level p1 is selected as 1 mW (0.001 watt) and p2 is 
the signal power level. Instruments which give readings in volume units, 
called VU meters, are widely used for monitoring radio broadcasts and for 
sound recording. 
 �34s[David E. Fogarty]
�33s 
  
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�6v�2v�32sVolvocales   �21m �33s 
    An order of one-celled or colonial green algae, mostly fresh-water 
species, in the division Chlorophyta. Individuals are motile with two, four, 
or rarely eight whiplike flagella serving as locomotor structures. The 
suborder Chlorodendrineae, however, includes about seven attached genera which 
may immediately become motile. Most species have a light-sensitive stigma or 
eyespot, together with a neuromotor apparatus. Although similar to protozoans, 
the cells, with the exception of a few colorless genera, contain a parietal, 
platelike or cup-shaped chloroplast. Food is manufactured and stored as 
starch, and the reproductive methods and life cycle are definitely plantlike. 
Saprophytic nutrition occurs in some.  See  ALGAE. 
    Many genera are one-celled with a cell wall either present or lacking. 
Volvox is a colony with as many as 250,000 cells arranged at the periphery of 
a hollow, mucilaginous sphere. In most species the cells are interconnected by 
narrow, protoplasmic canals.  See  CHLOROPHYTA. 
 �34s[Gerald W. Prescott]
�33s 
  
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�6v�2v�32sVolvocida   �21m �33s 
    An order of the class Phytamastigophorea. The protozoans, also known as 
the Phytomonadida, are grass-green, but a few are colorless (Polytoma). 
Individual cells may be as small as 8 micrometers. They closely relate the 
flagellates to the algae. They have one, two, four, or eight flagella; plural 
flagella are usually equal. The group is large and about one-fourth of the 
approximately 100 genera form colonies, with flagella only in reproductive 
cells. Cell walls are of cellulose and often they are thick. Chromatophores 
contain the same chlorophylls as higher plants. Pyrenoids have the usual form 
commonly found with starch as the reserve material.  See  CILIA AND FLAGELLA; 
PHYTAMASTIGOPHOREA.  
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sVortac   �21m �33s 
    A ground radio station consisting of a co-located vhf omnidirectional 
radio range (VOR) and Tacan facility. This station permits obtaining polar 
coordinates by the use of a VOR receiver and distance-measuring equipment or 
by Tacan equipment.  See  RADIO RANGE; TACAN. 
 �34s[Peter C. Sandretto]
�33s 
  
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�6v�2v�32sVortex   �21m �33s 
    A line vortex in two-dimensional fluid flow produces a flow or circulation 
around the line. 
    Consider the effect of rotating a right-circular cylinder of radius r0 
about its axis with a peripheral velocity v0 in a fluid otherwise at rest. The 
fluid in contact with the surface of the cylinder rotates with the cylinder. 
Fluid at greater radius is also set in motion in concentric circles with 
velocity diminishing as the radius increases. This type of fluid motion, in 
which the velocity varies inversely as the radius, is referred to as a free 
vortex.  
    If the cylinder is reduced to zero radius in such a manner that v0r0 
remains constant in the limit as r0 approaches zero, a line vortex results. 
The velocity at the line is infinite, so the line itself must be considered as 
a singular line, to be excluded from the actual fluid. 
    Examples of vortices occur frequently in nature. The tornado is an example 
of a free vortex, with high velocities near its center, and correspondingly 
low pressure intensities. The waterspout is its counterpart over water.  See  
FLUID FLOW. KARMAN VORTEX STREET. 
 �34s[Victor L. Streeter]
�33s 
  
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�6v�2v�32sVortex flowmeter �21m �33s
    A flowmeter based on the phenomenon of vortex shedding or vortex
precession.  See  VORTEX.
    The vortex-shedding flowmeter is based on the eddies (vortices) forming
behind an obstruction in a flowing stream. Close observation has shown that
each vortex builds up and breaks loose, followed by a repeat in the opposite
rotation on the other side of the obstruction, and so alternately the vortices
shed and go downstream in what is called a vortex street. Further observation
shows that the distance between successive vortices is directly proportional
to the blocking width of the obstruction. The distance is also independent of
the fluid velocity over a large range, and thus the frequency of the shedding
is directly proportional to the velocity of the fluid.	See  KARMAN VORTEX
STREET.
    Application of vortex-shedding flowmeters is normally limited to the
turbulent-flow regime, and various shedding-element shapes and internal
constructions have been developed in order to improve the shedding
characteristics and the detection of the shedding. The common characteristic
is sharp edges around which the vortices can form.

    In the vortex-precession flowmeter, used to measure gas flows, a swirl is
imparted to the flowing fluid by a fixed set of radial vanes. As this swirl
goes into an expanding tube, it forms a precessing vortex that is detected by
a heated thermistor sensor.  See  FLOW MEASUREMENT.
 �34s[Mead Bradner ;Lewis P. Emerson]
�33s


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�6v�2v�32sWading birds   �21m �33s 
    Members of the order Ciconiiformes. These birds have a worldwide 
distribution, and are characterized by relatively long legs, necks, and bills. 
There are four toes, the anterior three being webbed to various degrees. 
    There are about 116 species in 7 families which include: Balaenicipitidae 
(the shoebill stork, Balaeniceps rex), Scopidae (the hammerhead, Scopus 
umbretta), Ardeidae (bitterns, egrets, and herons), Cochleariidae (the 
boatbill heron, Cochlearius cochlearius), Ciconiidae (storks and wood ibises), 
Threskiornithidae (spoonbills and ibises), and Phoenicopteridae (flamingos).  
See  AVES; CICONIIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sWage incentives   �21m �33s 
    Wage payment plans designed to provide extra pay for extra effort over and 
above a ``fair day's work.'' Base hourly, weekly, or monthly wage rates are 
paid to employees for normal effort, while incentives allow them to earn more 
through increased effort. 
    Incentives are used over a broad spectrum of the labor force. Sales 
commissions, executive bonuses, profit sharing, and suggestion systems are 
some of the more popular uses. However, the most widely used and the most 
effective incentives are in the manufacturing areas. Some of the most used 
incentive devices are: the standard hour plan, the piecework plan, and factor 
plans.  
    Some basic principles for an effective incentive plan are that incentives 
should: pay for extra effort only; be individual wherever possible or for the 
smallest group that can be arranged; be based on factors which can be 
controlled by the employee; not be a substitute for management or management 
effort; and be high enough to provide the motivation to make the extra effort 
worthwhile. It is generally accepted that 25 to 35% earnings above base is an 
adequate range of opportunity.  See  PRODUCTIVITY. 
 �34s[William J. Oshinski]
�33s 
  
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�6v�2v�32sWake flow   �21m �33s 
    Turbulent eddying flow that occurs downstream from bluff bodies. When 
fluid flows along the boundary of a solid body, the fluid near the boundary is 
slowed down by viscous shear stresses exerted at the boundary. This action is 
progressive along the body and, under certain conditions, fluid near the 
boundary is brought to rest, which causes the fluid moving near the body to 
separate from the body (see illustration). A wake develops and produces 
additional form drag on the body. 
    A wake is formed downstream from bluff bodies such as bridge piers, smoke 
stacks, buildings, or trees. For unsteady flow cases, such as the motion of a 
train or a ship, the fluid behind the moving object has great turbulence 
remaining in it and, in the case of a ship, is easily discerned for a great 
distance.  See  FLUID FLOW. 
 �34s[Victor L. Streeter]
�33s 
  
  
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�6v�2v�32sWall construction   �21m �33s 
    An acceptable exterior wall for a building must be pleasing in appearance, 
structurally sound, and durable; it must provide insulation and fire 
resistance; and it must offer protection against condensation of water vapor. 
An interior wall must meet the same conditions but to a much lesser extent; it 
must also provide resistance to sound transmission. 
    In home construction, both the interior and exterior walls are formed 
around a basic core of wooden studs. The exterior exposed faces are covered 
with wood siding, wood or asbestos shingles, brick veneer, or other finish and 
insulated to retard heat flow. Solid and hollow concrete masonry and brick 
walls are used extensively for low buildings. Reinforced concrete walls, 
precast concrete walls, and precast concrete walls erected by the tilt-up 
method also offer a variety of surface finishes, low maintenance, strength, 
and durability. 
    The walls of many tall buildings consist principally of glass, the 
spandrels and pilasters being of steel or aluminum backed up by brick or tile 
masonry.  
    Plain or corrugated sheets of cement-asbestos and corrugated metal sheets 
are used for the walls of industrial buildings. Prefabricated panels 
consisting of two sheets of metal with a layer of insulation between provide a 
complete wall section which can be quickly attached to steel framing.  See  
BUILDINGS; CONCRETE; MASONRY. 
 �34s[Charles M. Antoni]
�33s 
  
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�6v�2v�32sWalnut   �21m �33s 
    This name is applied to about a dozen species of large deciduous trees 
widely distributed over temperate North and South America, southeastern 
Europe, and central and eastern Asia. The genus (Juglans) is characterized by 
pinnately compound aromatic leaves and chambered or laminate pith. The 
staminate (male) flowers are borne in unbranched catkins on the previous 
season's growth, and the pistillate (female) flowers are terminal on the 
current season's shoots. The shells of the nuts of most species are deeply 
furrowed or sculptured. 
    Two species, the black walnut (J. nigra) and the Persian or English walnut 
(J. regia), are of primary importance for their timber and nuts. The butternut 
finds local use in the northeastern United States. The other species are 
sparingly used as shade trees, as grafting stocks, and as sources of nuts.  
See  JUGLANDALES. 
 �34s[Laurence H. MacDaniels]
�33s 
  
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�6v�2v�32sWarm-air heating system   �21m �33s 
    In a general sense, a heating system which circulates warm air. Under this 
definition both a parlor stove and a steam blast coil circulate warm air. 
Strictly speaking, however, a warm-air system is one containing a direct-fired 
furnace surrounded by a bonnet through which air circulates to be heated. 
    When air circulation is obtained by natural gravity action, the system is 
referred to as a gravity warm-air system. If positive air circulation is 
provided by means of a centrifugal fan (referred to in the industry as a 
blower), the system is referred to as a forced-air heating system. 
    Direct-fired furnaces are available for burning of solid, liquid, or 
gaseous fuels. Furnaces have also been designed which have air circulating 
over electrical resistance heaters. A completely equipped furnace-blower 
package consists of furnace, burner, bonnet, blower, filter, and accessories. 
The furnace shell is usually of welded steel. The burner supplies a positively 
metered rate of fuel and a proportionate amount of air for combustion. A 
casing, or jacket, encloses the furnace and provides a passage for the air to 
be circulated over the heated furnace shell. The casing is insulated and 
contains openings to which return-air and warm-air ducts can be attached. The 
blower circulates air against static pressure. The air filter removes dust 
particles from the circulating air.  See  OIL BURNER. 
    The complete forced-air heating system consists of the furnace-blower 
package unit; the return-air intake, or grille, together with return-air ducts 
leading from the grille to the return-air plenum chamber at the furnace; and 
the supply trunk duct and branch ducts leading to the registers located in the 
different spaces to be heated.  See  COMFORT HEATING. 
 �34s[Seichi Konzo]
�33s 
  
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�6v�2v�32sWart   �21m �33s 
    A papillomatous growth which occurs singly or in groups on the skin 
surface and is often classified according to its location, for example, 
digital, genital, plantar, or laryngeal. It is also called a verruca. The 
agent responsible is considered to be a virus. 
 �34s[Alice E. Moore]
�33s 
  
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�6v�2v�32sWasher   �21m �33s 
    A flattened, ring-shaped device used to improve the tightness of a screw 
fastener. Three types of washer are in common use: plain, spring-lock, and 
antiturn (tooth-lock washers). Standard plain washers are used to protect a 
part from damage or to provide for a wider distribution of the load. Because a 
plain washer will not prevent a nut from turning, a locking-type washer should 
be used to prevent a bolt or nut from loosening under vibration (see 
illustration). Lock washers create a continuous pressure between the parts and 
the fastener. The antiturn-type washers may be externally serrated, internally 
serrated, or both. The bent teeth bite into the bearing surface to prevent the 
nut from turning and the fastening from loosening under vibration.  See  SCREW 
FASTENER.  
 �34s[Warren J. Luzadder]
�33s 
  
  
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�6v�2v�32sWasp   �21m �33s 
    An insect belonging to the Hymenoptera. In general, wasps range in size 
from the minute (microscopic) parasitic species to those which are 2 in. (5 
cm) long. These insects are economically important as parasites or predators 
of injurious pests and have been useful in biological control of other pest 
species. Some species of wasps show a complex social organization like that of 
ants and bees.  See  HYMENOPTERA; SOCIAL INSECTS. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sWatch   �21m �33s 
    A portable timepiece. Its operation may be described as mechanical, 
electromechanical, or electronic. 
    In the mechanical watch a mainspring in the barrel stores operating 
energy; the user retightens the spring daily by means of the winding stem. The 
wheel train advances at five increments per second under control by the 
escapement. From there the dial train turns the minute and hour hands across 
the watch face.  See  CLOCK; ESCAPEMENT; GEAR TRAIN; JEWEL BEARING; SPRING 
(MACHINES).  
    Among the variety of features incorporated into modern watches are: 
self-winding mechanisms, substitution of an electrochemical cell for the 
mechanical mainspring, and several forms of electromechanical escapement in 
place of the balance-and-hairspring mechanism. 
    When solid-state electronic integrated circuits became available in 
quantity, the all-electronic watch became a commercial reality. In it an 
electrochemical cell supplies the energy. A chain of binary dividers triggered 
from a crystal oscillator develops a train of seconds pulses. These pulses 
drive a digital counter or scaler, which develops minute and hour pulses to 
activate the digital display.  See  DIGITAL COUNTER; INTEGRATED CIRCUITS; 
OSCILLATOR; PIEZOELECTRIC CRYSTAL; SCALING CIRCUIT. 
    One form of readout uses a light-emitting diode (LED). Because 
illumination of the readout consumes most of the power in an electronic watch, 
a liquid crystal display (LCD) is used where low power consumption is a first 
consideration.  See  ELECTRONIC DISPLAY; HOROLOGY; LIGHT-EMITTING DIODE; 
LIQUID CRYSTALS. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sWater �21m �33s
    The chemical compound with two atoms of hydrogen and one atom of oxygen in
each of its molecules. It is formed by the direct reaction (1) of hydrogen

	     2H2 + O2 --> 2H2 O 					     (1)

with oxygen. The other compound of hydrogen and oxygen, hydrogen peroxide,

	     2H2O2 --> 2H2O + O2					     (2)

readily decomposes to form water, reaction (2). Water
also is formed in the combustion of hydrogen-containing compounds, in the
pyrolysis of hydrates, and in animal metabolism. Some properties of water are
given in the table.

    Gaseous state.  Water vapor consists of water molecules which move nearly
independently of each other. The relative positions of the atoms in a water
molecule are shown in the illustration. The dotted circles show the effective
sizes of the isolated atoms. The atoms are held together in the molecule by
chemical bonds which are very polar, the hydrogen end of each bond being
electrically positive relative to the oxygen. When two molecules near each
other are suitably oriented, the positive hydrogen of one molecule attracts
the negative oxygen of the other, and while in this orientation, the repulsion
of the like charges is comparatively small. The net attraction is strong
enough to hold the molecules together in many circumstances and is called a
hydrogen bond.	See  CHEMICAL BONDING; ELECTRONEGATIVITY; GAS; VALENCE.

    Solid state.  Ordinary ice consists of water molecules joined together by
hydrogen bonds in a regular arrangement.  It appears that there is considerable
empty space between the molecules.  This unusual feature is a result of the
strong and directional hydrogen bonds taking precedence over all other
intermolecular forces in determing the structure of the crystal.  See CRYSTAL;
HYDROGEN BOND.

     Liquid state.  The molecules in liquid water also are held together by
hydrogen bonds.  When ice melts, many of the hydrogen bonds are broken, and
those that remain are not numerous enough to keep the molecules in a regular
arrangement.  As water is heated from 0 DG C (32 F), it contracts until 4 DG C
(39 F) is reached and then begins the expansion which is normally associated
with increasing temperature.  This phenomenon and the increase in density when
ice melts both result from a breaking down of the open, hydrogen-bonded
structure as the temperature is raised. See LIQUUID.

    Properties.  The electrical conductivity of water is at least 1,000,000
times larger than that of most other nonmetallic liquids at room temperature.
The current in this case is carried by ions produced by the dissociation of
water according to reaction (3).

		H2O REVERSABLE REACTION H+ + OH-			     (3)

    Water is an excellent solvent for many substances, but particularly for
those which dissociate to form ions. Its principal scientific and industrial
use as a solvent is to furnish a medium for purifying such substances and for
carrying out reactions between them.  See  SOLUTION; SOLVENT.
    Water is not a strong oxidizing agent, although it may enhance the
oxidizing action of other oxidizing agents, notably oxygen. Water is an even
poorer reducing agent than oxidizing agent. One of the few substances that it
reduces rapidly is fluorine.  See  OXIDATION-REDUCTION.
    Substances with strong acidic or basic character react with water. For
example, calcium oxide, a basic oxide, reacts in a process called the slaking

	      CaO + H2O --> Ca(OH)2					     (4)

of lime, reaction (4). Another type of substance with strong acidic character
is an acid chloride. An example and its reaction with water is boron
trichloride, reaction (5). This is termed hydrolysis, as is the reaction of an
ester with water.  See	HYDROLYSIS.

	       BCl3 + 3H2O --> H3BO3 + 3HCl				     (5)

    Water also reacts with a variety of substances to form solid compounds in
which the water molecule is intact, but in which it becomes a part of the
structure of the solid. Such compounds are called hydrates.  See  HYDRATE.
    For various aspects of water, its uses, and occurrence  see  HEAVY WATER;
HYDROGEN; HYDROLOGY; IRRIGATION (AGRICULTURE); NUTRITION; OXYGEN; PLANT-WATER
RELATIONS; PRECIPITATION (METEOROLOGY); SEA WATER; TRIPLE POINT
(THERMODYNAMICS); VAPOR PRESSURE; WATER ANALYSIS; WATER CONSERVATION; WATER
MICROBIOLOGY; WATER POLLUTION; WATER PURIFICATION; WATER SOFTENING; WATER
SUPPLY ENGINEERING; WATER TABLE; WATER TREATMENT; WATERPOWER.
 �34s[Harold L. Friedman]
�33s

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�6v�2v�32sWater analysis   �21m �33s 
    A broad field that deals with the specific applications of chemical, 
physical, and biological methods to the analysis of aqueous samples ranging 
from the purest distilled water to the most polluted wastewater. One major 
area of concern is the analysis of drinking water to ensure that the water 
does not transmit organisms causing human disease, and does not contain 
chemicals hazardous to human health. Another concern is that the water is 
esthetically acceptable to consumers in terms of taste, odor, and appearance. 
Raw or treated wastewaters are analyzed to determine proper treatment and to 
monitor their discharge to assure public safety and environmental protection.  
See  WATER-BORNE DISEASE. 
  
    Chemical analysis.  The analytical chemistry of water deals with major 
mineral constituents, both anions and cations, trace organic and inorganic 
substances, and radionuclides. Insofar as possible, contemporary procedures 
are instrumental and do not depend on conventional colorimetric or titrimetric 
methods. Foremost among analytical instruments are the atomic absorption 
spectrophotometer (for metals) and the gas-liquid chromatograph (for organic 
constituents). Inorganic nonmetals typically are measured by noninstrumental 
methods, and radionuclides are determined by a combination of chemical 
separation followed by instrumental measurement of radioactivity.  See  
ANALYTICAL CHEMISTRY. 
  
    Physical analysis.  Physical analyses are made to assess important 
characteristics of both water and wastewater. Organoleptic tests are made for 
taste and odor which are significant in the consumer's perception of 
drinking-water quality. Color is measured by direct visual comparison with 
standards or by a spectrophotometric method. Conductivity is measured 
electrometrically and serves as an indirect indicator of the dissolved 
residue. Residue also may be measured gravimetrically by determining the 
weight of a dried sample. The suspended residue (suspended-solids test) is 
important in evaluating the strength of wastewater and the efficacy of its 
treatment. Turbidity relates to water clarity and is important in the analysis 
of drinking water and, for safety reasons, in swimming pools. It is measured 
visually by determining the depth of water required to obscure the image of a 
standard candle flame or, better, by nephelometry in which light scatter under 
standard conditions is measured. 
  
    Biological analysis.  Biological analyses include a broad spectrum of 
tests for organisms found in water and for the effects of substances in water 
on aquatic life. In making biological analyses, the classical methods of 
visual field observations are coupled with laboratory examinations, often 
microscopic, for identification and enumeration of plants and animals in 
water. Analytical results, especially those that are historical or 
comparative, are important in assessing the effect of pollution on natural 
waters. Algae may affect the quality of drinking water by modifying taste, 
odor, and color, and usually require control in water supply reservoirs. 
    Microbiological tests are used to determine sanitary quality and 
suitability of water for general use. In the routine analysis of water, 
including drinking water, enteric pathogens are not enumerated; rather, 
indicators of fecal pollution are determined. The most commonly used indicator 
is the coliform group, which includes normal inhabitants of the 
gastrointestinal tract of warm-blooded animals. Microbiological standards of 
water quality have been based on coliform counts for more than 50 years and 
have served well to protect the public health. Organisms other than coliform 
bacteria may be tested for under special circumstances, but routine analysis 
for pathogens in water is not recommended.  See  WATER MICROBIOLOGY. 
  
    Water quality standards.  For drinking water, regulations prescribe the 
maximum contaminant level (MCL) for a number of chemical species, turbidity, 
and coliform bacteria (see table). Experience has demonstrated that water that 
meets these specifications will, with high probability, not produce human 
disease.  See  WATER SUPPLY ENGINEERING. 
 �34s[Arnold E. Greenberg]
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�6v�2v�32sWater-borne disease   �21m �33s 
    Disease transmitted by drinking water or by contact with potable or 
bathing water. Certain bacterial, protozoan, and helminthic diseases of humans 
are commonly or exclusively water-borne (see table). The bacterial and 
protozoan pathogens are not normal to the water environment and do not 
multiply in natural waters. They gain entrance to water primarily from 
pollution by human excrement and less significantly from pollution by domestic 
and wild animals. In contrast, the helminthic pathogens may have a water 
animal as an intermediate host and may have a temporary free-living stage in 
water.  See  EPIDEMIOLOGY; INFECTIOUS-DISEASE TRANSMISSION. 
 �34s[Sydney C. Rittenberg]
�33s 
  
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�6v�2v�32sWater conservation   �21m �33s 
    The protection, development, and efficient management of water resources 
for beneficial purposes. Water occurs both underground and on the surface. The 
volume of groundwater greatly exceeds that of all freshwater lakes and 
reservoirs combined. It occurs in several geologic formations (aquifers) and 
at various depths. Streams supply most of the water needs of the United 
States. Lakes, ponds, swamps, and marshes, like reservoirs, represent stored 
streamflow. The oceans and salty or brackish sounds, bays, bayous, or 
estuaries represent almost unlimited potential fresh-water sources.  See  
HYDROLOGY.  
    Streams have traditionally served for waste disposal. Towns and cities, 
industries, and mines provide thousands of pollution sources. Pollution 
dilution requires large amounts of water. Treatment at the source is safer and 
less wasteful than flushing untreated or poorly treated wastes downstream. 
However, sufficient flows must be released to permit the streams to dilute, 
assimilate, and carry away the treated effluents.  See  WATER POLLUTION. 
    The problems of water management involve economic, social, and intangible 
values. Efforts to plan and develop river systems for multiple purposes often 
generate conflicts among different water uses, for example, irrigation versus 
navigation, or hydropower versus salmon or trout fisheries, wildlife, national 
park, wilderness, or historic resources. 
    Water management technology involves the application of biological and 
engineering principles to attain desired goals. Biological methods include 
growing upland vegetation having low moisture requirements. Mechanical methods 
include the practice of water spreading, which is utilized to desilt 
floodwaters and to promote the percolation of water into the soil for crop use 
and groundwater recharge. Water that would be a strong pollutant of streams 
and lakes may be spread on the land. The avoidance of water waste takes 
several forms. Recycling has permitted huge savings of water, especially in 
petroleum plants, chemical factories, and steel mills.  
    Watershed control is an approach to planning, development, and management 
that rests on the established interdependence of water, land, and people. 
Conditions on the land are often directly reflected in the behavior of 
streamflow and in the accumulation of groundwater. Coordination of structures 
and land-use practices is sought to prevent erosion, promote infiltration, and 
retard high flows. River basins may be large and complex watersheds. Basin 
projects may involve systems of multipurpose storage reservoirs, intensive 
programs of watershed protection, and improvement and management of farm, 
forest, range, and urban lands. They may call for scientific research, 
industrial development, health and educational programs, and financial 
arrangements to stimulate local initiative. 
 �34s[Bernard Frank]
�33s 
  
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�6v�2v�32sWaterfowl   �21m �33s 
    Aquatic birds which constitute the order Anseriformes. There are two 
families: the Anhimidae of the tropical New World, known as the screamers, and 
the Anatidae, the swans, ducks, and geese. Waterfowl range in size from 1 to 5 
ft (0.3 to 1.5 m) and have partially or completely webbed toes and oil glands. 
    The screamers are unique in having extensive developments of air spaces 
between the skin and the body. They have two pairs of pointed spurs on the 
anterior part of each wing. The toes are only slightly webbed at the base. 
There are three species, all of which frequent marshy areas and savannas in 
their range. 
    The best-known and most widely distributed anseriform birds are the ducks, 
which are grouped into 10 tribes. The bills are long and flat, the necks are 
elongate, and three of the toes are webbed. The young are covered with a soft 
down and leave the nest soon after hatching. Since the flight feathers are 
lost when they molt, the birds are incapable of flying for these short 
periods. The plumage is waterproof as in most aquatic birds. Some species are 
good divers as well as excellent fliers and swimmers. Ducks feed on vegetation 
and fine particulate material in water.  See  ANSERIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sWater hammer   �21m �33s 
    A phenomenon in a piping system caused by an abrupt change in flow at some 
section. The disturbance, which might be caused by changing a valve opening, 
travels throughout the system at the speed of a sound wave in the fluid. 
    The magnitude of the pressure wave travels unaltered (except for fluid 
friction) until it arrives at a change in pipe characteristics, such as a 
branch, a size change, an orifice, or perhaps a reservoir or a pump. Such a 
change in conditions is referred to as a boundary condition in the problem 
solution, and the properties and characteristics of the boundary are solved 
simultaneously with the pipe disturbance equation to determine how the wave 
proceeds.  See  HYDRODYNAMICS. 
 �34s[Victor L. Streeter]
�33s 
  
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�6v�2v�32sWatermelon   �21m �33s 
    The edible fruit of Citrullus lanatus, of the family Cucurbitaceae. The 
plant is an annual prostrate vine with multiple stems. 
    The numerous cultivars of watermelon are highly diverse in fruit size (5 - 
85 lb; 2.3 - 38.3 kg), shape (round, oval, oblong-cylindrical), rind color 
(very light to very dark green and often striped or mottled), flesh color 
(red, pink, orange, yellow, white), and seed size and color. The flesh 
contains 6 - 12% sugar, depending upon variety and condition of growth, with 
8% sugar being acceptable on most markets. 
    Watermelon juice of the sweet cultivars can be reduced to edible sugar and 
syrup. Watermelon seeds are relished as food in some Near East countries and 
in China. Fresh watermelon flesh has a unique melting quality that has proved 
impossible to preserve in palatable form through any processing technique. The 
flesh consists of water (91%), fiber, and sugar, and little else of obvious 
nutritional value, but may have some as yet unproved health-promoting value. 
It is said to have diuretic properties, and both frozen concentrate and canned 
juice have been available for the treatment of nephritis. The seeds also are 
said to contain substances effective in the control of hypertension. 
 �34s[C. F. Andrus]
�33s 
  
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�6v�2v�32sWater microbiology   �21m �33s 
    An aspect of microbiology that deals with the normal and adventitious 
microflora of natural and artificial water bodies. Two major lines of 
investigation have been pursued: the characterization of the normal flora and 
its activities, and the public health aspects of potable and recreational 
waters.  See  WATER ANALYSIS; WATER-BORNE DISEASE. 
    Qualitatively, the normal microbial flora is controlled by such factors as 
temperature, salinity, and chemical composition of the water, and is largely 
independent of geographical location. The numbers of bacteria in unpolluted 
waters are low. Samples collected in the oceans or large lakes remote from 
land generally give viable counts of less than 100 per milliliter; direct 
microscopic counts, which include both viable and nonviable bacteria, are 
higher.  
    Surface waters may receive large numbers of bacteria, including pathogens, 
by runoff from the land surfaces, and by pollution from human and animal 
sources. Bacteria thus introduced are eliminated quite rapidly either by 
failure to grow in an unfavorable environment, or by such processes as 
sedimentation and predation. Thus a natural water not subject to sufficient 
pollution to change its character maintains its indigenous flora; this fact 
permits, within limits, the deliberate disposal of wastes into a water body.  
See  WATER POLLUTION. 
    The bacterial flora carries out the same types of biochemical processes in 
water and sediments as it does on land and is presumed responsible, at least 
in part, for mineralization of organic matter and completion of the cycle of 
nutrients. In contrast to the situation on land, the microscopic algae are the 
primary producers of organic matter in the water.  See  MICROBIOLOGY; WATER 
PURIFICATION.  
 �34s[Sydney C. Rittenberg]
�33s 
  
A700000013042�254s�5v�1v1865
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�6v�2v�32sWater pollution   �21m �33s 
    Any change in natural waters which may impair their further use, caused by 
the introduction of organic or inorganic substances, or a change in 
temperature of the water. The growth of population and the concomitant 
expansion in industrial and agricultural activities have rapidly increased the 
importance of the field of water-pollution control. In the attack on 
environmental pollution, higher standards for water cleanliness have been 
adopted by state and Federal governments, as well as by interstate 
organizations.  
    The field of water-pollution control encompasses a part of the broader 
field of sanitary or environmental engineering. It includes some aspects of 
chemistry, hydrology, biology, and bacteriology, in addition to public 
administration and management. 
  
    Relation to water supply.  Water-pollution control is closely allied with 
the water supplies of communities and industries because both generally share 
the same water resources. There is great similarity in the pipe systems that 
bring water to each home or business property, and the systems of sewers or 
drains that subsequently collect the wastewater and conduct it to a treatment 
facility. Treatment should prepare the flow for return to the environment so 
that the receiving watercourse will be suitable for beneficial uses such as 
general recreation, and safe for subsequent use by downstream communities or 
industries.  See  SEWAGE COLLECTION SYSTEMS; WATER SUPPLY ENGINEERING. 
  
    Wastewaters.  Domestic wastewaters result from the use of water in 
dwellings of all types, and include both water after use and the various waste 
materials added: body wastes, kitchen wastes, household cleaning agents, and 
laundry soaps and detergents. The character of these waste materials is such 
that they cause significant degradation of receiving waters, and they may be a 
major factor in spreading water-borne diseases, notably typhoid and dysentery. 
    In contrast to the general uniformity of substances found in domestic 
wastewaters, industrial wastewaters show increasing variation as the 
complexity of industrial processes rises. The table lists major industrial 
categories along with the undesirable characteristics of their wastewaters. 
    Any natural watercourse contains dissolved gases normally found in air in 
equilibrium with the atmosphere. In this way fish and other aquatic life 
obtain oxygen for their respiration. The amount of oxygen which the water 
holds at saturation depends on temperature and follows the law of decreased 
solubility of gases with a temperature increase. Degradable or oxidizable 
substances in wastewaters deplete oxygen through the action of bacteria and 
related organisms which feed on organic waste materials, using available 
dissolved oxygen for their respiration. If this activity proceeds at a rate 
fast enough to depress seriously the oxygen level, the natural fauna of a 
stream is affected; if the oxygen is entirely used up, a condition of oxygen 
exhaustion occurs which suffocates aerobic organisms in the stream.  Under 
such conditions the stream is said to be septic and is likely to become 
offensive to the sight and smell. 
3047A| 
    An increasing amount of attention has been given to thermal pollution, the 
raising of the temperature of a waterway by heat discharged from the cooling 
system or effluent wastes of an industrial installation. This rise in 
temperature may sufficiently upset the ecological balance of the waterway to 
pose a threat to the native life-forms. Thermal pollution may be combated by 
allowing the wastewater to cool before it empties into the waterway. 
    With the passage of time, the waste loads imposed on streams exceeded the 
ability of the receiving water to assimilate them. The first attempts at 
wastewater treatment were made by artificially providing means for the 
purification of wastewaters as observed in nature. These forces included 
sedimentation and exposure to sunlight and atmospheric oxygen, either by 
agitated contact or by filling the interstices of large stone beds 
intermittently as a means of oxidation. In later years testing stations were 
set up by municipalities and states for experimental work. This led to 
treatment methods based on modern chemistry and biology. 
  
    Current status.  Modern water-pollution engineers or chemists have a 
wealth of published information, both theoretical and practical, to assist 
them. While research continues, they can draw on established practices for the 
solution to almost any problem. A challenging problem has been the handling of 
radioactive wastes. Reduction in volume, containment, and storage constitute 
the principal attack on this problem.  See  RADIOACTIVE WASTE MANAGEMENT. 
    Public desire for complete water pollution control continues, but there is 
an increasing realization that solution to the problem is costly. While cities 
have had little concern about the initial construction cost because of the 
large Federal share, the expenditures for operation fall entirely on the local 
community.  
    There are strong manifestations of improved quality in the waters 
throughout the United States. This is apparent not only in chemical and 
biological measurements, but in more readily observed effects such as better 
appearance, eliminated smells, and the return of fish life to watercourses 
which had become ``biological deserts'' because of the effects of pollution 
from municipal and industrial wastewaters.  See  WATER PURIFICATION. 
 �34s[Ralph E. Fuhrman]
�33s 
  
A700000004722�254s�5v�1v1868
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�6v�2v�32sWaterpower �21m �33s
    Power developed from movement of masses of water. Such movement is of two
kinds: the falling of streams through the force of gravity, and the rising and
falling of tides through lunar (and solar) gravitation.
    While that part of solar energy expended to lift water vapor against Earth
gravity is a minute fraction of the total, the absolute amount of energy that
is theoretically recoverable from resulting streams is an enormous but unknown
quantity. Of this, but a tiny portion is actually accessible for harnessing.
    Hydro plants, with their initial high cost and generally long distances
from major load centers, must compete with the fuel-fired stations and large
nuclear plants. Large dam sites usually must be justified not alone on the
value of the power developed, but also on the benefits from flood control,
irrigation, and recreation. Problems of migrating fish, conservation, and
preservation of esthetic values are also factors. On the other hand,
waterpower developments add greatly to power-system flexibility in meeting
peak and emergency loads.  See	ELECTRIC POWER GENERATION; NUCLEAR REACTOR;
POWER PLANT.
    The capacity of hydro plants cannot be counted on for perpetuity because
of gradual filling of reservoirs with sediment. This effect is serious for
irrigation, flood control, and navigation. Even when a lake behind a power dam
becomes filled completely with silt, electric power can be generated on the
run-of-the-river flow, although output would vary with stream flow.
    In a pumped-storage hydroelectric system, water is pumped from a stream or
lake to a reservoir at a higher elevation. Pumping up to a storage reservoir
is most commonly done by reversing the hydraulic turbine and generator. The
generator becomes a motor driving the turbine as a pump. Power is drawn from
the power system at night or on weekends when demand is low. The cost of
pumping power is low, whereas the power generated from pumped storage at peak
periods is valuable.  See  HYDRAULIC TURBINE.
    A portion of the kinetic energy of the rotation of the Earth appears as
ocean tides. The mean tide of all the oceans has been calculated as 2.1 ft
(0.64 m), and the mean power as 54,000 x 10(sup 6) hp (40 terawatts) or, on a
yearly basis, the equivalent of 36 x 10(sup 12) kWh. Unfortunately, only a
minute amount of this is likely to be harnessed for use. See ENERGY SOURCES;
TIDAL POWER.
 �34s[Charles A. Scarlott]
�33s

A700000002717�254s�5v�1v1865
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1866
\IMG\CI1801.IMG
8929792
�6v�2v�32sWater purification   �21m �33s 
    The treatment of potable water to destroy disease-producing agents. 
Purification is employed in all but economically underdeveloped areas and, 
where practiced, has reduced water-borne diseases to the vanishing point. 
Chemical disinfection, either by itself or in combination with storage, 
sedimentation, or filtration, is the universal practice. Chlorine is the most 
economical agent for disinfection and is used in most water systems. 
Disinfection by physical agents is possible, but cost considerations have 
limited the application of such procedures. Boiling is the method of choice 
for treating small quantities of water of dubious quality  for individual or 
household use. Utraviolet irradiation has been employed for the disinfection 
of swimming-pool waters and for the treatment of military and some municipal 
water supplies where economic factors are not of primary significance. 
    Ozone treatment is employed by a limited number of public water supply 
systems, mainly in France. Ozone is generated by the discharge of high-voltage 
electricity through dry air between stationary electrodes. The water is 
sprayed into an atmosphere of ozone, or the ozonized air is discharged into 
the water in a mixing chamber.  See  CHLORINE; SANITARY ENGINEERING; 
WATER-BORNE DISEASE; WATER MICROBIOLOGY; WATER TREATMENT. 
 �34s[Sydney C. Rittenberg]
�33s 
  
A700000002322�254s�5v�1v1866
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�6v�2v�32sWater softening   �21m �33s 
    A water-treatment process by which undesirable cations (of calcium and 
magnesium) are removed from hard waters. (Hardness is usually expressed as 
calcium carbonate equivalents in parts per million.) The presence of these 
cations in water is undesirable for household purposes, boiler feed, food 
processing, and chemical processing, because of reactions that form soap scum, 
boiler scale, and unwanted by-products.  See  WATER TREATMENT. 
    Hard waters may be softened by precipitation processes which involve the 
use of hydrated lime, sometimes along with soda ash; cation-exchange processes 
which involve the use of sodium-cation exchangers (zeolites) or hydrogen 
cationexchangers (usually resins in bead form); or combinations of these. The 
choice of a process depends on a number of factors, among which are the 
composition of the hard water, the end uses of the softened water, the type or 
types of hardnesses to be removed, the degree of removal required, and the 
relative processing costs.  See  ION EXHANGE; MOLECULAR SIEVE; SALINE WATER 
RECLAMATON; SURFACE-ACTIVE AGENT; WATER TREATMENT; ZEOLITE. 
 �34s[Eskel Nordell]
�33s 
  
A700000002232�254s�5v�1v1868
\IMG\CI1801.IMG
9415680
1869
\IMG\CI1801.IMG
9685504
�6v�2v�32sWaterspout   �21m �33s 
    An intensely whirling, funnel-shaped vortex, extending from a cumulus-type 
cloud down several hundred to several thousand feet to the water surface. The 
visible funnel consists mostly of atmospheric water vapor condensed because of 
lower pressure in the vortex: water or salt spray drawn from the underlying 
surface also contributes to the structure and visibility of the funnel (see 
illustration). Diameters range from a few feet to several hundred, with winds 
occasionally strong enough to overturn small vessels. 
    Most waterspouts occur in moist tropical air under unstable conditions 
favorable for thunderstorms, from which they frequently hang. Nearly all 
dissipate on passing inland. Although most common in the tropical oceans, 
waterspouts are observed in higher latitutdes, particularly in summer; they 
also occur over inland lakes. Many of the larger and more violent spouts are 
true tornadoes, formed in association with thunderstorms over land and passing 
out to sea.  See  TORNADO. 
 �34s[Chester W. Newton]
�33s 
  
ms.pgs.80024-80269:05 
  
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�6v�2v�32sWater supply engineering   �21m �33s 
    A branch of civil engineering concerned with the development of sources of 
supply, transmission, distribution, and treatment of water. The term is used 
most frequently in regard to municipal waterworks, but applies also to water 
systems for industry, irrigation, and other purposes. 
  
    Sources.  Underground waters, rivers, lakes, and reservoirs, the primary 
sources of fresh water, are replenished by rainfall. Some of this water flows 
to the sea through surface and underground channels, some is taken up by 
vegetation, and some is lost by evaporation. 
    Water obtained from subsurface sources, such as sands and gravels and 
porous or fractured rocks, is called groundwater. Groundwater flows toward 
points of discharge in river valleys and, in some areas, along the seacoast. 
The flow takes place in water-bearing strata known as aquifers. In an 
unconfined stratum the water table is the top or surface of the groundwater. 
The water table elevation and artesian pressure may vary substantially with 
the seasons. If pumpage exceeds recharge for an extended period, the aquifer 
is depleted and the water supply lost.  See  AQUIFER; ARTESIAN SYSTEMS; 
GROUNDWATER HYDROLOGY; WATER TABLE. 
    Springs occur at the base of sloping ground or in depressions where the 
surface elevation is below the water table, or below the hydraulic gradient in 
an artesian aquifer from which the water can escape.  See  SPRING (HYDROLOGY). 
    Wells are vertical openings, excavated or drilled, from the ground surface 
to a water-bearing stratum or aquifer. Pumping a well lowers the water level 
in it, which in turn forces water to flow from the aquifer. Dug wells, several 
feet in diameter, are frequently used to reach shallow aquifers, particularly 
for small domestic and farm supplies. Large-capacity dug wells or caisson 
wells, in coarse sand and gravel, are used frequently for municipal supplies. 
Drilled wells are sometimes several thousand feet deep.  See  WELL. 
    Natural sources, such as rivers and lakes, and impounding reservoirs are 
sources of surface water. Water is withdrawn from rivers, lakes, and 
reservoirs through intakes. The simplest intakes are pipes extending from the 
shore into deep water.  See  DAM; RESERVOIR; SURFACE WATER. 
  
    Transmission and distribution.  The water from the source must be 
transmitted to the community or area to be served and distributed to the 
individual customers. 
    The major supply conduits, or feeders, from the source to the distribution 
system are called mains or aqueducts. The oldest and simplest type of 
aqueducts, especially for transmitting large quantities of water, are canals. 
Used to transmit water through ridges or hills, tunnels may follow the 
hydraulic grade line and flow by gravity or may be built below the grade line 
to operate under considerable pressure. Pipelines are a common type of 
transmission main, especially for moderate supplies not requiring large 
aqueducts or canals.  See  CANAL; PIPELINE; TUNNEL. 
    Included in the distribution system are the network of smaller mains 
branching off from the transmission mains, the house services and meters, the 
fire hydrants, and the distribution storage reservoirs. The network is 
composed of transmission or feeder mains, usually 12 in. (30 cm) or more in 
diameter, and lateral mains along each street, or in some cities along alleys 
between the streets. The mains are installed in grids so that lateral mains 
can be fed from both ends where possible. Valves at intersections of mains 
permit a leaking or damaged section of pipe to be shut off with minimum 
interruption of water service to adjacent areas. 
    Pumps are required wherever the source of supply is not high enough to 
provide gravity flow and adequate pressure in the distribution system. Pumping 
stations usually include two or more pumps, each of sufficient capacity to 
meet demands when one unit is down for repairs or maintenance. The station 
must also include piping and valves arranged so that a break can be isolated 
quickly without cutting the whole station out of service.  See  WATER 
TREATMENT.  
 �34s[Richard Hazen]
�33s 
  
A700000002366�254s�5v�1v1866
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�6v�2v�32sWater table �21m �33s
    The upper surface of the zone of saturation in permeable rocks not
confined by impermeable rocks. It may also be defined as the surface
underground at which the water is at atmospheric pressure. Saturated rock may
extend a little above this level, but the water in it is held up above the
water table by capillarity and is under less than atmospheric pressure;
therefore, it is the lower part of the capillary fringe and is not free to
flow into a well by gravity. Below the water table, water is free to move
under the influence of gravity.
    The position of the water table is shown by the level at which water
stands in wells penetrating an unconfined water-bearing formation. Where a
well penetrates only impermeable material, there is no water table and the
well is dry. But if the well passes through impermeable rock into
water-bearing material whose hydrostatic head is higher than the level of the
bottom of the impermeable rock, water will rise approximately to the level it
would have assumed if the whole column of rock penetrated had been permeable.
This is called artesian water.	See  ARTESIAN SYSTEMS; GROUNDWATER HYDROLOGY;
WELL.
 �34s[Albert N. Sayre /Ronald K. Linde]
�33s

A700000005507�254s�5v�1v1866
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1867
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9064448
�6v�2v�32sWater treatment   �21m �33s 
    Physical and chemical processes for making water suitable for human 
consumption and other purposes. Drinking water must be bacteriologically safe, 
free from toxic or harmful chemical or substances, and comparatively free of 
turbidity, color, and taste-producing substances. Excessive hardness and high 
concentration of dissolved solids are also undesirable, particularly for 
boiler feed and industrial purposes. The treatment processes of greatest 
importance are sedimentation, coagulation, filtration, disinfection, 
softening, and aeration. 
    Sedimentation occurs naturally in reservoirs and is accomplished in 
treatment plants by basins or settling tanks. Plain sedimentation will not 
remove extremely fine or colloidal material within a reasonable time, and the 
process is used principally as a preliminary to other treatment methods. 
    Fine particles and colloidal material are combined into masses by 
coagulation. These masses, called floc, are large enough to settle in basins 
and to be caught on the surface of filters.  See  COAGULATION. 
    Suspended solids, colloidal material, bacteria, and other organisms are 
filtered out by passing the water through a bed of sand or pulverized coal, or 
through a matrix of fibrous material supported on a perforated core. Soluble 
materials such as salts and metals in ionic form are not removed by 
filtration.  See  FILTRATION. 
    There are several methods of treatment of water to kill living organisms, 
particularly pathogenic bacteria; the application of chlorine or chlorine 
compounds is the most common. Less frequently used methods include the use of 
ultraviolet light, ozone, or silver ions. Boiling is the favorite household 
emergency measure. 
    Municipal water softening is common where the natural water has a hardness 
in excess of 150 parts per million. Two methods are used: (1) The water is 
treated with lime and soda ash to precipitate the calcium and magnesium as 
carbonate and hydroxide, after which the water is filtered; (2) the water is 
passed through a porous cation exchanger which has the ability of substituting 
sodium ions in the exchange medium for calcium and magnesium in the water. For 
high-pressure steam boilers or some other industrial processes, almost 
complete deionization of water is needed, and treatment includes both cation 
and anion exchangers.  See  ION EXCHANGE. 
    Aeration is a process of exposing water to air by dividing the water into 
small drops, by forcing air through the water, or by a combination of both. 
Aeration is used to add oxygen to water and to remove carbon dioxide, hydrogen 
sulfide, and tasteproducing gases or vapors.  See  WATER POLLUTION; WATER 
PURIFICATION; WATER SUPPLY ENGINEERING. 
 �34s[Richard Hazen]
�33s 
  
A700000002646�254s�5v�1v1867
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�6v�2v�32sWater-tube boiler   �21m �33s 
    A steam boiler in which water circulates within tubes and heat is applied 
from outside the tubes. The outstanding feature of the water-tube boiler is 
the use of small tubes [usually 1 - 3 in. (2.5 - 7.5 cm) outside diameter] 
exposed to the products of combustion and connected to steam and water drums 
which are shielded from these high-temperature gases. Thus, possible failure 
of boiler parts exposed to direct heat transfer is restricted to the 
small-diameter tubes and, in the event of failure, the energy released is 
reduced and explosion hazards are minimized. 
    There are many types of water-tube boilers but, in general, they can be 
grouped into two categories: the straight-tube (see illustration) and the 
bent-tube types. In essence, both types consist of banks of parallel tubes 
which are connected to, or by, headers or drums. Most modern water-tube 
boilers utilize a water-cooled surface in the furnace, and this surface is an 
integral part of the boiler's circulatory system. Further, modern  water-tube 
boilers generally incorporate the use of superheaters, economizers, or air 
heaters to utilize more efficiently the heat from the fuel and to provide 
steam at a high potential for useful work in an engine or turbine.  See  
BOILER; STEAM; STEAM-GENERATING FURNACE; STEAM-GENERATING UNIT. 
 �34s[George W. Kessler]
�33s 
  
  
A700000004142�254s�5v�1v1867
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1868
\IMG\CI1801.IMG
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�6v�2v�32sWater tunnel   �21m �33s 
    A hydrodynamic test and research tool or facility, comprising a 
well-guided and controlled stream of water. The water tunnel is related to 
towing tanks but differs in that in the towing tank the test object moves and 
the water is essentially at rest, whereas in the water tunnel the test object 
is at rest (or rotating, for example, a propeller) and the water is moving. 
With respect to arrangement and operation, the water tunnel is similar to the 
subsonic wind tunnel, except for the difference in the test medium.  See  
TOWING TANK; WIND TUNNEL. 
    A typical water tunnel forms a closed loop of conduit with a carefully 
designed, strongly contracted test, or working, section, a circulating pump, 
and corner elbows shaped as they are in wind tunnels to minimize flow 
distortions and disturbances. The contraction in front of the test section 
further serves to produce a stream with a uniform velocity distribution. The 
size of a water tunnel is usually expressed as the diameter of its test 
section. Existing water tunnels range in size from a few inches to 48 in. (1.2 
m). The test section is on the upper leg to facilitate lowering the water 
level below it for insertion of test objects (see illustration). 
    An essential characteristic of most water tunnels is the possibility of 
changing the absolute pressure in the tunnel without necessarily changing the 
velocity of the test stream. This characteristic is important for 
investigations involving cavitation.  See  CAVITATION. 
    Most water tunnels are operated with the entire test circuit filled with 
water. Consequently they can serve only the investigations of completely 
submerged bodies or of conduits and machinery such as pumps or turbines that 
are completely filled with water. A few tunnels have been built to operate 
with a free water surface in the test section. Such free-surface water tunnels 
are used to investigate open-surface structures such as ship models, with the 
model standing still. 
 �34s[George F. Wislicenus]
�33s 
  
  
A700000003007�254s�5v�1v1869
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�6v�2v�32sWatt-hour meter �21m �33s
    An electricity meter which measures and registers the integral, with
respect to time, of the power in the circuit in which it is connected. In
effect, it is an electric motor, the torque of which is proportional to the
electric power in the circuit. The speed of the rotor is proportional to the
torque, making each revolution of the rotor a measurement in watt-hours.
Summation of the watt-hours is accomplished by gearing a counter, or register,
to the rotor.  See  MOTOR.
    The basic elements of a watt-hour meter are the stator, rotor, retarding
magnet or magnets, register, and meter housing.
    The mercury and commutator types of watt-hour meter are used for measuring
energy on a dc circuit. The commutator type may also be used on ac circuits if
all windings are of air-core construction. The induction type is the common
meter found in homes. It is used for measuring energy on an ac circuit (see
illustration). The potential-circuit winding of the stator is made highly
inductive to obtain a quadrature-time relationship between the
potential-circuit and the current-circuit working fluxes in the disk air gap.
These fluxes, displaced in time and space, produce a rotor-disk torque
proportional to the circuit power. Retarding magnets control the rotor speed,
making it proportional to the power. The register, which is geared to the
rotor, records the watt-hours.	See  ELECTRIC POWER MEASUREMENT.
 �34s[George R. Sturtevaut ;Jaime Amorocho]
�33s


A700000005120�254s�5v�1v1869
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�6v�2v�32sWattmeter �21m �33s
    An instrument that measures electric power. For a complete discussion of
the power in various types of electric circuits  see  ELECTRIC POWER
MEASUREMENT.
    A variety of wattmeters is available to measure the power in ac circuits.
They are generally classified by names descriptive of their operating
principles. Determination of power in dc circuits is almost always done by
separate measurements of voltage and current. However, some of the instruments
described will also function in dc circuits, if desired.

    Electrodynamic wattmeter.  Probably the most useful instrument in the
measurement of ac power at commercial frequencies is the indicating
(deflecting) electrodynamic wattmeter. It is similar in principle to the
double-coil dc ammeter or voltmeter in that it depends on the interaction of
the fields of two sets of coils, one fixed and the other movable. The moving
coil is suspended, or pivoted, so that it is free to rotate through a limited
angle about an axis perpendicular to that of the fixed coils. As a
single-phase wattmeter, the moving (potential) coil carries a current
proportional to the voltage applied to the measured circuit, and the fixed
(current) coil carries the load current. This arrangement of coils is due to
the practical necessity of designing current coils of relatively heavy
conductors to carry large values of current. The potential coils can be
lighter because the operating current is limited to low values.

    Thermal wattmeter.	The operation of a thermal wattmeter is based on the
heating effect of current, the I(sup 2)R relationship. It is applicable to both
direct current and alternating current and is usable at frequencies up into
the radio range without regard to waveform. Various electronic wattmeters for
audio and radio power measurements are largely based on this principle.
Thermocouples are commonly used as transducers, and the derived voltage is
available for the common types of dc indicators and for potentiometers of
either the manual or automatic self-balancing type. A limitation is that it is
relatively slow in response and may be affected by temperature.

    Electrostatic wattmeter.  The elementary quadrant electrometer has been
adapted for power measurements. The mechanism conists of two quadrants charged
by the voltage drop across a noninductive shunt resistance through which the
load current passes. The line voltage is applied between a moving vane and one
of the quandrants. The deflection of the indicator is proportional to the
average power.
 �34s[Donald B. Sinclair]
�33s

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\IMG\CI1801.IMG
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1870
\IMG\CI1801.IMG
9805824
�6v�2v�32sWave equation �21m �33s
    The name given to certain partial differential equations in classical and
quantum physics which relate the spatial and time dependence of physical
functions. In this article the classical and quantum wave equations are
discussed separately, with the classical equations first for historical
reasons.  See  NONRELATIVISTIC QUANTUM THEORY.
    In classical physics the name wave equation is given to the linear,
homogeneous partial differential equations which have the form of Eq. (1).

[INVERTED DELTA(sup 2) - (1/v[sup 2])(CURLY D[sup 2]/CURLY D t[sup 2])]      (1)
f(r,t) = 0

Here v is a parameter with the dimensions  of velocity;
r represents the space coordinates x, y, z; t is the time; and
INVERTED DELTA(sup 2) is Laplace's operator defined by Eq. (2). The
function f(r,t) is a physical observable; that is, it can be measured and
consequently must be a real function.

INVERTED DELTA(sup 2) =
(CURLY D[sup 2]/CURLY D[sup 2]) + (CURLY D[sup 2]/CURLY D y[sup 2]) +
(CURLY D[sup 2]/CURLY D z[sup 2])					    (2)


    The simplest example of a wave equation in classical physics is that
governing the transverse motion of a string under tension and constrained to
move in a plane.
    A second type of classical physical situation in which the wave equation,
Eq. (1), supplies a mathematical description of the physical reality is the
propagation of pressure waves in a fluid medium. Such waves are called
acoustical waves, the propagation of sound being an example. A third example
of a classical physical situation in which Eq. (1) gives a description of the
phenomena is afforded by electromagnetic waves. In a region of space in which
the charge and current densities are zero, Maxwell's equations for the photon
lead to the wave equations, Eqs. (3). Here E is the electric field strength
is the magnetic flux density; they are both vectors in ordinary space. The
parameter c is the speed of light in vacuum. 

[INVERTED DELTA(sup 2) - (1/c[sup 2]) (CURLY D[sup 2]/CURLY D t[sup 2])]
E(r,t) = 0


[INVERTED DELTA(sup 2) - (1/c[sup 2])(CURLY D[sup 2]/CURLY D t[sup 2])]
B(r,t) = 0

See  ELECTROMAGNETIC RADIATION; MAXWELL'S EQUATIONS.
    The nonrelativistic Schroedinger equation is an example of a quantum wave
equation. Relativistic quantum-mechanical wave equations include the
Schroedinger-Klein-Gordon equation and the Dirac equation.  See  QUANTUM
MECHANICS; RELATIVISTIC QUANTUM THEORY.
 �34s[Donald L. Weaver]
�33s

A700000001566�254s�5v�1v1871
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�6v�2v�32sWaveform �21m �33s
    The pictorial representation of the form or shape of a wave, obtained by
plotting the amplitude of the wave with respect to time. There are an infinite
number of possible waveforms; a few of the more common electrical waveforms
are shown in the illustration.
    It is possible to represent any periodic waveform mathematically as a
Fourier series of sine and cosine terms at harmonic frequencies. Any
nonsinusoidal wave is composed of a constant or dc term, plus a series of
harmonic terms in which the frequencies are integral multiples of the
fundamental frequency. The Fourier series for each waveform is given beside
each figure as functions of time t, where E(sub m) is the maximum value of the
wave and T is the period.  See	NONSINUSOIDAL WAVEFORM; WAVE-SHAPING CIRCUITS.
 �34s[Donald L. Waidelich]
�33s


A700000002677�254s�5v�1v1871
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�6v�2v�32sWaveguide   �21m �33s 
    A device which constrains or guides the propagation of electromagnetic 
waves along a path defined by the physical construction of the guide. In a 
broad sense, devices such as a pair of parallel wires and a coaxial cable can 
certainly be called waveguides. When used in a more restricted sense, however, 
the term waveguide usually means a metallic tube which can confine and guide 
the propagation of electromagnetic waves in the hollow space along the 
lengthwise direction of the tube.  See  COAXIAL CABLE. 
    Hollow waveguides of convenient sizes are best adapted to the transmission 
of microwaves. A sound wave can transmit through a pipe only when its 
wavelength is comparable to or smaller than the size of the pipe, and one 
would expect that a similar requirement should hold true for electromagnetic 
waves. Indeed, if the frequency of an electromagnetic wave is high enough that 
the wavelength is comparable to or smaller than the waveguide dimension, then 
wave transmission through the hollow waveguide becomes possible.  See  
MICROWAVE TRANSMISSION LINES. 
    The type of waveguide with a rectangular cross section is not only the 
commonest in use but also the simplest in theoretical analysis. Next to the 
rectangular type, metallic waveguides with circular cross-sections are most 
frequently used.  See  ELECTROMAGNETIC WAVE TRANSMISSION. 
 �34s[C. K. Jen]
�33s 
  
A700000012607�254s�5v�1v1872
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�6v�2v�32sWavelength �21m �33s
    The distance between two points on a wave which have the same value and
the same rate of change of the value of a parameter, for example, electric
intensity, characterizing the wave. The wavelength is usually designated by
the Greek letter LAMBDA (see illustration).  See  WAVE (PHYSICS); WAVE MOTION.
 �34s[William J. Galloway]
�33s


    Measurement.  The wavelength in a transmission system is obtained by
measuring the distance between successive waveforms of equal phase. This
measurement is most conveniently carried out in a standing-wave field in which
the interference occurs between forward-propagating and reverse-propagating,
or reflected, waves. A distance equal to one-half wavelength exists between
successive minima or maxima in the standing-wave pattern.  See	WAVEMETER.
    In order to specify the wavelength of an electromagnetic wave, it is
essential to know the medium or device through which the wave is propagating.
However, unless otherwise specified, it is general practice to quote the
wavelength of an oscillatory electric wave as that in air, or free space. When
this convention is used, all that is necessary is a frequency measurement in
order to apply the relation LAMBDA(sub 0) = c/f, where c is the velocity of
light, LAMBDA(sub 0) is the free-space wavelength, and f is frequency.
    The standing-wave method of measurement is somewhat similar to the
interferometer measurements used in optics. With shorter-wavelength radio
waves it is possible to apply optically derived interferometer techniques
directly to the measurement of wavelength.
    A more convenient, if less precise, measurement method for the
determination of wavelength is the Lecher wire wavemeter. With this simple
device, wavelength is measured by sliding a short circuit along the line from
a first to a second point of equal amplitude of effect, as indicated by
absorption of signal being detected by an external detector, or by some
similar method. The distance between two successive absorption maxima is
one-half wavelength (LAMBDA/2).  See  TRANSMISSION LINES.
    At lower frequencies, inductance-capacitance resonant circuits may serve
similar functions as absorption devices. A variation of the absorption
wavemeter, known as a grid-dip meter or grid-dip oscillator, provides in one
instrument an absorption wavemeter and a calibrated oscillator, which may be
used to determine the resonant frequency (or wavelength) of a passive network,
such as an antenna system or an LC tuned circuit, by observing the dip in grid
current of the oscillator as the oscillator frequency is tuned to the network
resonant frequency.  See  RESONANCE (ALTERNATING-CURRENT CIRCUITS).
    Microwave wavemeters make use of resonant coaxial-line sections or
cavities as tuned elements. The two general types of microwave wavemeters are
the absorption, or reaction, type and the transmission type.  See  CAVITY
RESONATOR.
 �34s[Frank D. Lewis]
�33s

    Standards.	Wavelength standards are accurately measured lengths of waves
emitted by specified light sources for the purpose of obtaining the
wavelengths in other spectra by interpolating between the standards.
    Spectra are produced by refraction in prisms or by diffraction from
gratings. In either case, the spectra are usually photographed, and in order
to identify or describe them reliably it is necessary to measure the positions
of the unknown spectral lines relative to known standards. It is customary to
photograph the spectrum containing the standards either superposed or
juxtaposed on the spectrum under investigation. The wavelengths of hundreds of
thousands of spectral lines have already been measured relative to a few
hundred standards, and this will continue to be a major activity in
spectroscopy.  See  SPECTROSCOPY.
    At its meeting in October 1973 the Comite International des Poids et
Mesures (CIPM) adopted recommended values for the wavelength of light (from
two stabilized lasers) and for the speed of light. A He-Ne laser has been
stabilized to an absorption line in methane at 3.39 MUm. In addition, a
(sup 3)He-(sup 20)Ne laser has been stabilized to hyperfine components in an
absorption line of either (sup 127)I2 or (sup 129)I2 at 633 nm. The
principal gain from a shift to the standards based on the stabilized laser
sources is the remarkable agreement in the measurements with independently
constructed equipment. See ATOMIC STRUCTURE AND SPECTRA; HYPERFINE STRUCTURES;
LASER SPECTROSCOPY.
    Secondary iron wavelengths recommended in 1962 and later are all emitted
by improved low-pressure, low-temperature sources. However, the accuracy of
these standard iron wavelengths will not satisfy future precision demands in
spectroscopy. In order to measure the wavelengths in very complex spectra with
adequate accuracy, better and more uniformly spaced secondary standards are
required. These requirements are met by electrodeless thorium-halide lamps
excited by microwaves. In 1958 W. F. Meggers and R. W. Stanley published the
wavelengths of 222 thorium lines ranging from 328.77885 to 698.96562 nm. This
list has been extended to a longer list covering the range 297.14332 to
905.07361 nm.
    Tables of other recommended secondary wavelengths have subsequently been
published. Accurate compilations of Kr and Xe wavelengths have been made and
wavelength standards in the vacuum ultraviolet region have been determined.
 �34s[William W. Watson]
�33s

A700000001452�254s�5v�1v1872
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�6v�2v�32sWavellite   �21m �33s 
    A hydrated phosphate of aluminum mineral with composition 
Al3(OH)3(PO4)2PRODUCT5H2O, in which small amounts of fluorine and iron may 
substitute for the hydroxyl group (OH) and aluminum (Al), respectively. 
Wavellite crystallizes in the orthorhombic system. The crystals are stout to 
long prismatic, but are rare. Wavellite commonly occurs as globular aggregates 
of fibrous structure and as encrusting and stalactitic masses. Wavellite 
crystals range in color from colorless and white to different shades of blue, 
green, yellow, brown, and black. Wavellite is found at many places in Europe 
and North America, it is also abundant in tin veins at Llallagua, Bolivia.  
See  PHOSPHATE MINERALS. 
 �34s[Wayne R. Lowell]
�33s 
  
A700000001104�254s�5v�1v1870
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�6v�2v�32sWave mechanics   �21m �33s 
    The modern theory of matter holding that elementary particles (such as 
electrons, protons, and neutrons) have wavelike properties. In 1924 Louis de 
Broglie postulated that the same wave-corpuscle duality which was then known 
to exist in the case of light might also occur in matter; this hypothesis was 
subsequently verified experimentally. This theory of matter has become the 
highly successful quantum mechanics of the present day.  See  QUANTUM 
MECHANICS.  
 �34s[Edward Gerjuoy]
�33s 
  
A700000001734�254s�5v�1v1872
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�6v�2v�32sWavemeter   �21m �33s 
    A device for measuring the geometrical spacing between successive surfaces 
of equal phase along an electromagnetic wave. At frequencies up to about 100 
MHz, wavemeters consist basically of a tuned LC circuit and a suitable 
resonance indicator. 
    At higher frequencies it is necessary to employ well-defined transmission 
systems such as open-wire or coaxial lines and waveguides. Any open-circuited 
or short-circuited section of transmission system can be adjusted in physical 
size to cause it to resonate at a given wavelength. From a construction 
standpoint short-circuited sections are preferred and may take the form of 
Lecher wires, coaxial wavemeters, and cavity resonators. Suitable electronic 
standing-wave detectors are employed to indicate when the wavemeter is tuned 
to resonance.  See  CAVITY RESONATOR; STANDING-WAVE DETECTOR; WAVEGUIDE; 
WAVELENGTH.  
 �34s[Robert L. Ramey]
�33s 
  
A700000013611�254s�5v�1v1870
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\IMG\CI1801.IMG
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�6v�2v�32sWave motion �21m �33s
    The process by which a disturbance at one point in space is propagated to
another point more remote from the source with no net transport of the
material of the medium itself. For example, sound is a form of wave motion;
wind is not. Wave motion can occur only in a medium in which energy can be
stored in both kinetic and potential form. In a mechanical medium, kinetic
energy results from inertia and is stored in the velocity of the molecules,
while potential energy results from elasticity and is stored in the
displacement of the molecules.

    Media.  In a free traveling wave (as distinguished from a stationary or
standing wave) one part of the medium disturbs an adjacent part, thereby
imparting energy to it. This portion of the medium, in turn, disturbs another
part, thereby causing a flow of energy in a given direction away from the
source. More technically, wave propagation is the result of kinetic energy at
one point being transferred into potential energy at an adjacent point, and
vice versa. The rate of travel of the disturbance, or velocity of propagation,
is determined by the constants of the medium. A stationary wave is the
combination of two waves of the same frequency and strength traveling in
opposite directions so that no net transfer of energy away from the source
takes place. A standing wave is the same but with the returning wave (toward
the source) being of lesser intensity than the outwardly traveling wave so
that a net transfer of energy away from the source does take place.  See
STANDING WAVE.
    Wave motion can occur in a vacuum (electromagnetic waves), in gases (sound
waves), in liquids (hydrodynamic waves), and in solids (vibration waves).
Electromagnetic waves can also travel in gases, liquids, and solids provided
that the electrical conductivity of the medium is not perfect or that the
imaginary part of the dielectric constant is not infinitely great. By current
usage, elastic waves propagated in gases, liquids, and solids, regardless of
whether one can hear them or not, are called acoustic waves.

    Fundamental relations.  A wave is commonly referred to in terms of either
its wavelength or its frequency. In any type of wave motion, these two
quantities are related to a third quantity, velocity of propagation, by the
simple relation in the equation below, where f = frequency, LAMBDA =

			     f LAMBDA = c

wavelength, and  c = velocity of propagation. The period T is the reciprocal
of the frequency, and the amplitude A is the maximum magnitude taken on by the
variable of the wave at a given point in space. It is a basic property of wave
motion that the frequency of a wave remains constant under all circumstances
except for a relative motion between the source of the wave and the observer.
The velocity of propagation is dependent on the properties of the medium (and,
sometimes, also on the frequency) and the wavelength will vary with the
velocity in accordance with the equation above.

    Electromagnetic waves.  The media in which electromagnetic waves travel
possess no elasticity or inertia, but rather the ability to store energy in
the electric and magnetic fields. J. C. Maxwell recognized in about 1863 that
the basic equations governing these fields could be combined to yield an
equation resembling the wave equation for mechanical wave motion. Thus he
predicted the existence of electromagnetic waves which had not been suspected
theretofore. Later, electromagnetic waves proved to be identical with light
waves.	See  ELECTROMAGNETIC RADIATION; LIGHT; MAXWELL'S EQUATIONS; WAVE
EQUATION.
 �34s[Leo L. Beranek]
�33s

    Motion in fluids.  Wave motion within a fluid is generated by successive
compression and expansion of adjacent volume elements. Because compression and
expansion of an ordinary fluid can only proceed along the direction of
propagation of the disturbance, waves within a fluid are mostly longitudinal
waves.
    Waves in a fluid can be classified as compression waves and expansion
waves, according to whether the disturbance is a compression or an expansion.
They can further be classified according to the amplitude of the disturbance
and the chemical nature of the fluid. For example, waves of small amplitude
are called acoustic (or sound) waves; compression waves propagating in
chemically inert fluids are called shock waves; waves propagating in the Earth
are seismic waves; and waves of large amplitude generated by rapid chemical
reactions in explosive fluids are called detonation waves and can propagate
much faster than sound waves. Waves in an electrically conducting fluid in the
presence of strong magnetic fields are called magnetohydrodynamic waves.  See
MAGENETOHYDRODYNAMICS; SEISMOLOGY; SHOCK WAVE; SOUND.
 �34s[Shao-Chi Lin]
�33s

    Motion in liquids.	Disturbances propagated at a gas-liquid interface are
primarily dependent upon the gravitational fluid property (surface tension and
viscosity being of secondary importance). Wave motions which occur in confined
fluids (either liquid or gaseous) are primarily dependent upon the elastic
property of the medium.
    Oscillatory waves may be generated in a rectangular channel by a simple
harmonic translation of a vertical wall forming one end of the flume. A
standing wave can be considered to be composed of two equal oscillatory wave
trains traveling in opposite directions.
    A solitary wave consists of a single crest above the original liquid
surface which is neither preceded nor followed by another elevation or
depression of the surface. Such a wave is generated by the translation of a
vertical wall starting from an initial position at rest and coming to rest
again some distance downstream. In practice, solitary waves are generated by a
motion of barges in narrow waterways or by a sudden change in the rate of
inflow into a river; they are therefore related to a form of flood wave.
 �34s[Donald R. F. Harleman]
�33s

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�6v�2v�32sWave optics   �21m �33s 
    The branch of optics which treats of light (or electromagnetic radiation 
in general) with explicit recognition of its wave nature. The counterpart to 
wave optics is ray optics or geometrical optics, which does not assume any 
wave character but treats the propagation of light as a straight-line 
phenomenon except for changes of direction induced by reflection or 
refraction.  See  ELECTROMAGNETIC RADIATION; GEOMETRICAL OPTICS; OPTICS. 
    Any optical phenomenon which is correctly describable in terms of 
geometrical optics can also be correctly described in terms of wave optics. 
However, the many phenomena of interference, diffraction, and polarization are 
incontrovertible evidence of the wave nature of light, and geometrical optics 
often gives an incomplete or incorrect description of the behavior of light in 
an optical system. This is especially true if changes of refractive index 
occur within a space which is of the order of several wavelengths of the 
light.  See  DIFFRACTION: INTERFERENCE OF WAVES; POLARIZED LIGHT. 
 �34s[Richard C. Lord]
�33s 
  
A700000001445�254s�5v�1v1871
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�6v�2v�32sWave packet �21m �33s
    In wave phenomena, a superposition of waves of differing lengths, so
phased that the resultant amplitude is negligibly small except in a limited
portion of space whose dimensions are the dimensions of the packet.  If the
reciprocal wavelengths k = LAMBDA(sup -1) forming a one-dimensional packet lie
in a band DELTAk, the minimum dimension of the packet is DELTAx NEARLY EQUAL
(2PI DELTA k)(sup -1).	When all the component waves move in the same direction,
the packet speed is the group velocity v(sub g) = df/dk evaluated at the mean k;
f is the frequency.  When the phase velocity c depends on LAMBDA, vsubg = c, and
DELTAx changes with time.  See GROUP VELOCITY; PHASE VELOCITY; QUANTUM
MECHANICS.
 �34s[Edward Gerjuoy]
�33s

A700000002376�254s�5v�1v1871
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�6v�2v�32sWave-shaping circuits   �21m �33s 
    Electronic circuits used to create or modify a specified time-varying 
electrical quantity, usually voltage or current, using combinations of 
electronic devices, such as vacuum tubes or transistors, and circuit elements 
including resistors, capacitors, and inductors. 
    The square wave can be obtained by amplifying a sinusoidal time-varying 
voltage and removing all but the section near the zero-voltage axis. Also, 
square waves of either equal or unequal intervals, sometimes referred to as 
rectangular waves, can be generated by multivibrators or various other forms 
of vacuum-tube or transistor switching or gating circuits.  See  CLIPPING 
CIRCUIT; LIMITER CIRCUIT; MULTIVIBRATOR; RELAXATION OSCILLATOR; >WAVEFORM. 
    Other wave shapes of particular interest in electronics are the linearly 
increasing function of time or ramp function (which if recurring at equally 
spaced periods is usually called the linear sawtooth waveform), the hyperbolic 
waveform, and the rectified sine wave.  See  CLAMPING CIRCUIT; COINCIDENCE 
AMPLIFIER; ELECTRONIC DISPLAY; ELECTRONIC SWITCH; PULSE GENERATOR; RECTIFIER; 
SWEEP GENERATOR; TRIGGER CIRCUIT. 
 �34s[Glenn M. Glasford]
�33s 
  
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�6v�2v�32sWave (physics) �21m �33s
    The general term applied to the description of a disturbance which
propagates from one point in a medium to other points without giving the
medium as a whole any permanent displacement.
    Waves are generally described in terms of their amplitude, and how the
amplitude varies with both space and time. The actual description of the wave
amplitude involves a solution of the wave equation and the particular boundary
conditions for the case being studied.	See  WAVE EQUATION; WAVE MOTION.
    Acoustic waves, or sound waves, are a particular kind of the general class
of elastic waves. Elastic waves are propagated in media having two properties,
inertia and elasticity. Electromagnetic waves (for example, light waves and
radio waves) are not elastic waves and therefore can travel through a vacuum.
The velocity of the wave depends on the medium through which the wave travels.
See  ELECTROMAGNETIC WAVE; STANDING WAVE.
 �34s[William J. Galloway]
�33s

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�6v�2v�32sWax, animal and vegetable   �21m �33s 
    Any of the substances containing esters of higher fatty acids and 
long-chain monohydric alcohols. From a practical standpoint, this deinfition 
is inadequate because it allows liquids such as sperm whale oil and jojoba oil 
to be called waxes, and it fails to indicate the complexity of waxes. While 
waxes do contain wax esters, they are seldom if ever pure. They are usually 
mixtures that may contain high-molecular-weight acids, alcohols, esters, 
ketones, hydrocarbons, sterols, diesters, hydroxyacids, and so forth, as well 
as the wax esters.  See  ESTER. 
    Practical wax formulators use physical, rather than chemical, properties 
to define waxes. Waxes must: be a solid at 68DGF (20DGC); be crystalline; melt 
above 140DGF (40DGC) without decomposition, have relatively low viscosity 
above the melting point, have consistency and solubility properties that are 
strongly dependent upon temperature; and be capable of being polished under 
slight pressure. 
    Wax sources are abundant. They have been isolated from the outer layers of 
bacteria, the roots, stems, leaves, fruit, and flowers of plants, the exudates 
of insects, the skin and hair of some animals, and the bodies of certain 
marine and land animals. Carnauba wax is extracted from an exudate on the 
leaves of the carnauba palm (Copernicia prunifera). Candelilla wax is obtained 
from a coating on the stem of Euphorbia antisyphilitica, a leafless desert 
shrub. Beeswax is an exudate of the honeybee. Wool wax is obtained from sheep 
wool. Many other waxes have been of commercial importance from time to time, 
but their availability or cost has forced them from the market.  See  CARNAUBA 
WAX; FAT AND OIL. 
 �34s[Howard M. Hickman]
�33s 
  
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�6v�2v�32sWax, petroleum   �21m �33s 
    A substance produced primarily from the dewaxing of lubricating-oil 
fractions of petroleum. It may be of either the crystalline or 
microcrystalline type. Petroleum wax has a wide variety of uses. It is used to 
coat paper products, to blend with other waxes for the manufacture of candles, 
in the manufacture of electrical equipment and many polishes for home and 
industry, and as a source material for oxidized products. The softer waxes, 
such as petroleum jelly, after proper purification, are being used as 
medicinal products.  See DEWAXING OF PETROLEUM; PETROLEUM PROCESSING; 
PETROLEUM PRODUCTS. 
 �34s[Wayne E. Kuhn]
�33s 
  
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�6v�2v�32sWeak nuclear interactions �21m �33s
    Fundamental interactions of nature that play a significant role in
elementary particle and nuclear physics, and are distinguished from other such
interactions by special properties such as participation of all the
fundamental fermions and failure to conserve parity. The weak force has very
short range (less than 10[sup -17]m) and is extremely feeble compared to
strong and electromagnetic forces, but can be distinguished from these two by
its special character. For example, according to the present view, all of
matter consists of certain fundamental spin-1/2 constitutents, the quarks and
leptons, collectively called the fundamental fermions. While only the quarks
participate in strong interactions, and only the quarks and charged leptons e,
MU, and TAU participate in electromagnetic interactions, all of the
fundamental fermions, including neutrinos, engage in weak interactions. Also,
the strong and electromagnetic interactions respect spatial inversion symmetry
(they conserve parity) and are also particle-antiparticle (charge conjugation)
symmetric, whereas the weak interaction violates these two symmetries.	See
FUNDAMENTAL INTERACTIONS; LEPTON; PARITY  (QUANTUM MECHANICS); QUARKS;
SYMMETRY LAWS (PHYSICS).
    Weak interactions are classified as ``charged'' or ``neutral,'' depending
on whether or not a particle participating in a weak reaction suffers a change
of electric charge of one electronic unit. Observed charged weak interactions
include nuclear beta decay and electron capture, muon capture on nuclei, and
the slow decays of unstable elementary particles such as the MU and TAU
leptons, PI, K, and charmed mesons, and hyperons and charmed baryons. Also,
there are the charged neutrinonucleon and neutrino-lepton scattering
reactions. Neutral weak interactions include neutrino-nucleon and
neutrino-lepton scattering as well as the electron-nucleon reaction which can
also occur by electromagnetic interaction.  See  BARYON; ELEMENTARY PARTICLE;
HYPERON; MESON.
    The most important development in the study of weak interactions has been
the creation of a successful theory based on the principles of local gauge
invariance and spontaneous symmetry breaking. This theory proposes a single
basis for the weak and electromagnetic interactions, and indeed, despite
striking differences in the observed characteristics of strong,
electromagnetic, and weak interactions, important theoretical ideas of a
similar type suggest that all these interactions possess a common origin.
 �34s[Eugene D. Commins]
�33s

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�6v�2v�32sWear   �21m �33s 
    The removal of material from a solid surface as a result of sliding 
action. It constitutes the main reason why the artifacts of society 
(automobiles, washing machines, tape recorders, cameras, clothing) become 
useless and have to be replaced. There are a few uses of the wear phenomenon, 
but in the great majority of cases wear is a nuisance, and a tremendous 
expenditure of human and material resources is required to overcome the 
effects.  
    Adhesive wear is the only universal form of wear, and in many sliding 
systems it is also the most important. It arises from the fact that, during 
sliding, regions of adhesive bonding, called junctions, form between the 
sliding surfaces. If one of these junctions does not break along its original 
interface, then a chunk from one of the sliding surfaces will have been 
transferred to the other surface. In this way, an adhesive wear particle will 
have been formed. Initially adhering to the other surface, adhesive particles 
soon come off loose and can disappear from the sliding system.  See  FRICTION. 
    Abrasive wear is produced by a hard, sharp surface sliding against a 
softer one and digging out a groove. The abrasive agent may be one of the 
surfaces (such as a file), or it may be a third component (such as sand 
particles in a bearing abrading material from each surface). 
    Corrosive wear arises when a sliding surface is in a corrosive 
environment, and the sliding action continuously removes the protective 
corrosion product, thus exposing fresh surface to further corrosive attack.  
See  CORROSION. 
    Surface fatigue wear occurs as a result of the formation and growth of 
cracks. It is the main form of wear of rolling devices such as ball bearings, 
wheels on rails, and gears. 
    While most manifestations of wear are highly objectionable, the phenomenon 
does have a few uses. Thus, a number of systems for recording information 
(pencil and paper, chalk, and blackboard) operate via a wear mechanism. Many 
methods of preparing solid surfaces (filling, sandpapering, sandblasting) also 
makes use of wear. 
 �34s[Ernest Rabinowicz]
�33s 
  
A700000002142�254s�5v�1v1873
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1874
\IMG\CI1801.IMG
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�6v�2v�32sWeasel   �21m �33s 
    The common name for at least 12 species of carnivores which are members of 
the family Mustelidae. They have a varied distribution in many regions of the 
world (see table). 
    The common or European weasel is Mustela nivalis. This animal is a slim, 
voracious carnivore with a long body. The limbs are short and the body is 
muscular. Although the claws are nonretractile, these animals climb with 
agility. The gestation period is 5 weeks, and four to six young are born in a 
nest made in a hollow tree or rabbit burrow. One or two litters are born each 
year, and the life-span is about 8 years. 
    The common weasel has a wide distribution throughout Europe, North Africa, 
and Asia, and has managed to survive because of its size and ability to escape 
detection. It preys upon small rodents, such as mice, voles, and rats, and has 
been incriminated in attacking domestic animals, such as poultry.  See  
CARNIVORA; FERRET; FISHER; MARTEN; MINK; OTTER; SKUNK; WOLVERINE. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sWeather   �21m �33s 
    The state of the atmosphere, as determined by the simultaneous occurrence 
of several meteorological phenomena at a geographical locality or over broad 
areas of the Earth. When such a collection of weather elements is part of an 
interrelated physical structure of the atmosphere, it is termed a weather 
system, and includes phenomena at all elevations above the ground. More 
popularly, weather refers to a certain state of the atmosphere as it affects 
humans' activities on the Earth's surface. In this sense, it is often taken to 
include such related phenomena as waves at sea and floods on land. 
    A weather element is any individual physical feature of the atmosphere. At 
a given locality, at least seven such elements may be observed at any one 
time. These are clouds, precipitation, temperature, humidity, wind, pressure, 
and visibility. Each of these principal elements is divided into many 
subtypes.  See  WEATHER MAP. 
    The various forms of precipitation are included by international agreement 
among the hydrometeors, which comprise all the visible features in the 
atmosphere, besides clouds, that are due to water in its various forms. For 
convenience in processing weather data and information, this definition is 
made to include some phenomena not due to water, such as dust and smoke. Some 
of the more common hydrometeors include rain, snow, fog, hail, dew, and frost. 
  See  PRECIPITATION (METEOROLOGY). 
    Certain optical and electrical phenomena have long been observed among the 
weather elements. These include lightning, aurora, solar or lunar corona, and 
halo.  See  AIR MASS; ATMOSPHERE; AURORA; CLOUD; FRONT; LIGHTNING; 
METEOROLOGY; STORM; WEATHER OBSERVATIONS; WIND. 
 �34s[Philip F. Clapp]
�33s 
  
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�6v�2v�32sWeather forecasting and prediction   �21m �33s 
    Processes for formulating and disseminating information about future 
weather conditions based upon the collection and analysis of meteorological 
observations. Weather forecasts may be classified according to the space and 
time scale of the predicted phenomena. Atmospheric fluctuations with a length 
of less than 330 ft (100 m) and a period of less than 100 s are considered to 
be turbulent. Prediction of turbulence extends only to establishing its 
statistical properties, insofar as these are determined by the thermal and 
dynamic stability of the air and by the aerodynamic roughness of the 
underlying surface. The study of atmospheric turbulence is called 
micrometeorology; it is of importance for understanding the diffusion of air 
pollutants and other aspects of the climate near the ground. Standard 
meteorological observaions are made with sampling techniques designed to 
filter out the influence of turbulence. Common terminology distinguishes among 
three classes of phenomena with a scale that is larger than the turbulent 
microscale: the mesoscale, synoptic scale, and planetary scale. 
    The mesoscale includes all moist convection phenomena, ranging from 
individual cloud cells up to the convective cloud complexes associated with 
prefrontal squall lines, tropical storms, and the intertropical convergence 
zone. Most forecasting methods depend upon empirical rules or the short-range 
extrapolation of current observations, particularly those provided by radar 
and geostationary satellites. Since many mesoscale phenomena pose serious 
threats to life and property, it is the practice to issue advisories of 
potential occurrence significantly in advance. These ``watch'' advisories 
encourage the public to attain a degree of readiness appropriate to the 
potential hazard. Once the phenomenon is considered to be imminent, the 
advisory is changed to a ``warning,'' with the expectation that the public 
will take immediate action to prevent the loss of life. 
    The next-largest scale of weather events is called the synoptic scale, 
because the network of meteorological stations making simultaneous, or 
synoptic, observations serves to define the phenomena. The migratory storm 
systems of the extratropics are synoptic-scale events, as are the undulating 
wind currents of the upper-air circulation which accompany the storms. These 
storms are associated with barometric minima, variously called lows, 
depressions, or cyclones. 
    Planetary-scale phenomena are persistent, quasi-stationary perturbations 
of the global circulation of the air with horizontal dimensions comparable to 
the radius of the Earth. These dominant features of the general circulation 
appear to be correlated with the major orographic features of the globe and 
with the latent and sensible heat sources provided by the oceans. They tend to 
control the paths followed by the synoptic-scale storms, and to draw upon the 
synoptic transients for an additional source of heat and momentum. For further 
information on the observation and processing of weather data  see  
METEOROLOGICAL INSTRUMENTATION;  METEOROLOGICAL SATELLITES; MICROMETEOROLOGY; 
WEATHER OBSERVATIONS. 
 �34s[Joseph P. Gerrity]
�33s 
  
    Long-range forecasting.  Long-range weather forecasts are of two types. 
Medium- or extended-range forecasts cover periods of from 48 h to a week in 
advance. Forecasts for longer periods generally extend over periods of a 
month, a season, or possibly two or more seasons in advance. Although 
meteorologists have been working on long-range prediction problems for more 
than 100 years, the degree of accuracy is small for all predictions exceeding 
a week in advance. 
    Scientific methods of extended-range prediction take for granted that the 
further out in time the forecast is projected, the more general must be the 
nature of the prediction. Short-range forecasts for 48 h or less in advance 
specify the detail of weather in space and in time. Medium-range forecasts for 
a week in advance cover average conditions and trends within the week in 
intervals of a couple of days. Forecasts for a month or more, however, can 
indicate only the broad-scale features of average or prevailing weather. 
    Medium-range forecast methods are apt to use one or a combination of 
dynamic, statistical, and synoptic techniques. Dynamic methods capitalize upon 
the best physical knowledge of meteorological phenomena. Statistical methods 
employ empirically derived equations as substitutes for physical knowledge. In 
the synoptic technique, various hemispheric wind and weather charts are 
surveyed and interpreted by an experienced meteorologist. 
 �34s[Jerome Namias]
�33s 
  
    Statistical forecasting.  Statistical weather forecasting is the 
prediction of weather by use of data from past weather developments. Examples 
of statistical predictions may be the nocturnal minimum temperature tomorrow 
at a particular site and the probability of precipitation within a given 
county during the next 36 h. In each case the prediction is based on 
statistical relations obtained from earlier records and intended for a 
specific location and forecast period. Because of geographic and atmospheric 
complexities, extension of these relations to another location or time period 
tends to produce less accurate forecasts, and a separate treatment usually 
leads to better success. 
    The necessity for statistical methods in weather prediction is largely due 
to a lack of a detailed knowledge of atmospheric processes at any one time. It 
is presumed that the atmosphere obeys accepted physical laws, under the 
influence of solar energy, terrain and oceanic effects, and human activities. 
These laws are the basis for numerical weather predictions of relatively 
large-scale features of the atmosphere. Because of gaps between observing 
stations, however, such predictions, while valuable, have limited accuracy. 
Consequently, statistical weather predictions based on historical data 
continue to be useful. 
    By themselves, statistical methods are highly suitable for predicting 
special local phenomena, such as the occurrence of fog. For preparing 
prognostic weather maps a day or so in advance, statistical formulas are 
somewhat useful but are frequently inferior to subjective forecasts. 
 �34s[Thomas A. Gleeson]
�33s 
  
    Numerical weather prediction.  Numerical weather prediction is the 
prediction of weather phenomena by the numerical solution of the equations 
governing the motion and changes of condition of the atmosphere. More 
generally, the term applies to any numerical solution or analysis of the 
atmospheric equations of motion. 
    The laws of motion of the atmosphere may be expressed as a set of partial 
differential equations relating the instantaneous rates of change of the 
meteorological variables to their instantaneous distribution in space. These 
are developed in dynamic meteorology. A prediction for a finite time interval 
is obtained by summing the succession of infinitesimal time changes of the 
meteorological variables, each of which is determined by their distribution at 
the preceding instant of time. Although this process of integration may be 
carried out in principle, the nonlinearity of the equations and the complexity 
and multiplicity of the data make it impossible in practice. Instead, the 
prediction is based on finite-difference approximation techniques in which 
successive changes in the variables are calculated for small, but finite, time 
intervals at a finite grid of points spanning part or all of the atmosphere. 
The vast amount of computation requires modern high-speed computers. 
 �34s[Richard A. Anthes]
�33s 
  
    Centers and offices of forecasting.  Weather forecasts for all parts of 
the United States are prepared by the National Weather Service (NWS), a part 
of the National Oceanic and Atmospheric Administration (NOAA). Various phases 
of the forecast work are performed at five working levels: the National 
Meteorological Center (NMC); National Severe Storms Forecast Center (NSSFC); 
National Hurricane Center (NHC); River Forecast Centers (RFCs); and Weather 
Service Forecast Offices (WSFOs). 
    The National Meteorological Center  located in Camp Springs, Maryland, 
collects weather observations, prepares meteorological analyses, and provides 
forecast guidance for use by NWS field offices. The aerial coverage of NMC 
products includes the entire globe, with most products covering the Northern 
Hemisphere and the tropical regions of the Southern Hemisphere. The World 
Meteorological Organization (WMO) has designated the NMC as the analysis and 
forecast arm of the Washington World Meteorological Center, which requires 
global responsibilities as part of the international efforts and cooperation 
known as the World Weather Watch. 
    The National Severe Storms Forecast Center located in Kansas City, 
Missouri, is responsible for preparing and releasing forecasts of areas of 
expected severe local storms, including tornadoes. The long-term outlook, 
prepared shortly after midnight, gives the geographic areas most likely to 
have severe thunderstorms or tornadoes during the next 24 h. 
    The National Hurricane Center located in Miami, Florida, prepares 
hurricane and tropical storm watches and advisories for most of the tropical 
Atlantic Ocean, the Caribbean, and the Gulf of Mexico. The NHC delegates parts 
of its public warning responsibilities to three Hurricane Watch Offices (HWO) 
when the United States coastal areas are threatened. 
    There are 13 River Forecast Centers which prepare river and flood 
forecasts and warnings for approximately 2500 communities. This includes 
forecasts for height of the flood crest as well as times when the river is 
expected to overflow its banks and when it will recede into its banks. 
    There are an additional 238 Weather Service Offices, which represent the 
third echelon of the system. They issue local forecasts which are adaptations 
of the zone forecasts. They are important in the warning and observation 
program and are generally located in smaller cities. 
 �34s[Allen Pearson]
�33s 
  
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�6v�2v�32sWeathering processes   �21m �33s 
    The response of geologic materials to the environment (physical, chemical, 
and biological) at or near the Earth's surface. This response typically 
results in a reduction in size of the weathering materials; some may become as 
tiny as ions in solution. 
    The agents and energies that activate weathering processes and the 
products resulting therefrom have been classified traditionally as physical 
and chemical in type. In classic physical weathering, rock materials are 
broken by action of mechanical forces into smaller fragments without change in 
chemical composition, whereas in chemical weathering the process is 
characterized by change in chemical composition. In practice, however, the two 
processes commonly overlap. 
    Specific agents of weathering may be recognized and correlated with the 
types of effects they produce. Important agents of weathering are water in all 
surface occurrences (rain, soil and ground water, streams, and ocean); the 
atmosphere (H2O, O2, CO2, wind); temperature (ambient and changing, especially 
at the freezing point of water); insolation (on large bare surfaces); ice (in 
soil and glaciers); gravity; plants (bacteria and macroforms); animals (micro 
and macro, including humans). Human modifications of otherwise geologic 
weathering that have increased exponentially during recent centuries include 
construction, tillage, lumbering, use of fire, chemically active industry 
(fumes, liquid, and solid effluents), and manipulation of geologic water 
systems.  
    Products of physical weathering include jointed (horizontal and vertical) 
rock masses, disintegrated granules, frost-riven soil and surface rock, and 
rock and soil flows. Products of chemical weathering include the soil, and the 
clays used in making ceramic structural products, whitewares, refractories, 
various fillers and coating of paper, portland cement, absorbents, and 
vanadium. These are the relatively insoluble products of weathering; 
characteristically they occur in clays, siltstones, and shales. Sand-size 
particles resulting from both physical and chemical weathering may accumulate 
as sandstones. 
    After precipitation, the relatively soluble products of chemical 
weathering give rise to products and rocks such as limestone, gypsum, rock 
salt, silica, and phosphate and potassium compounds useful as fertilizers. 
 �34s[Walter D. Keller]
�33s 
  
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�6v�2v�32sWeather map   �21m �33s 
    A chart portraying the state of the atmosphere's winds and weather over a 
wide area at one of several levels above the Earth's surface and at one 
universal time. It is derived from an analysis of weather observations made at 
many points in the area near the selected time. Such a map gives the weather 
forecaster an organized picture of the location, structure, and, when several 
successive maps are available, the motion and development of the various 
weather systems.  See  WEATHER OBSERVATIONS. 
    Many kinds of weather maps are used, depending on the weather elements of 
immediate interest and their elevation above the ground. At a typical large 
weather analysis central, as many as 35 different maps are constructed at a 
given time. Among the more common of these is the surface or sealevel map. 
    Surface maps are often constructed as a section through the atmosphere 
along a horizontal surface near sea level, at least over oceans. Over land, 
observations must be made at the elevation of the observing point, but 
pressure is extrapolated to sea level. Lines of equal sea-level pressure 
(isobars) are drawn to portray the sea-level winds. Fronts are drawn, which 
are relatively narrow transition zones separating cool and warm air masses, 
along which storms tend to form in extratropical latitudes. Sea-level maps 
with fronts and air masses are often analyzed subjectively by a meteorologist, 
and are especially useful in predicting the smaller scales of weather systems 
over relatively short time intervals.  See  FRONT; ISOBAR (METEOROLOGY). 
    Upper-level maps may also be constructed by depicting the distribution of 
weather at fixed elevations above sea level. It is more common, however, to 
portray the weather at high elevations on constant-pressure surfaces. Lines of 
constant elevation (isohypses) portray the topography of the selected pressure 
surface. The contours bear a relation to the winds similar to that of the 
isobars of a constant-level chart. The true wind is approximated by the 
geostrophic wind.  See  GEOSTROPHIC WIND; WIND. 
    Other two-dimensional or one-dimensional atmospheric sections cannot be 
termed weather maps, but rather meteorological charts and diagrams. These 
include vertical cross sections through the atmosphere, and thermodynamic 
diagrams similar to those used in studies of heat engines. 
    Weather maps may be analyzed subjectively by a trained analyst (usually 
for limited regions and a few levels) or by a stored program in an electronic 
computer. In the latter case the analysis program must be supported by a 
numerical weather prediction model of the atmosphere which must start from a 
set of digitized data at a geographical network (or grid) of points.  See  
WEATHER FORECASTING AND PREDICTION. 
 �34s[Philip F. Clapp]
�33s 
  
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�6v�2v�32sWeather modification   �21m �33s 
    The changing of natural weather phenomena by humans. Weather is the 
product of the interaction of atmospheric processes on many scales, reaching 
from the planetary circulation to the microphysical processes in the evolution 
of cloud droplets and ice crystals. So far, only on the microscale of 
condensation and freezing nuclei have humans begun to exert modifying 
influences on weather. These influences may expand to a scale of several 
hundreds or thousands of square miles, that is, to what is called in 
meteorology the meso scale of meteorological phenomena. 
    There are actually four techniques by which humans may affect the natural 
course of weather: (1) by injection of large amounts of heat into the 
atmosphere in order to burn off warm fog; (2) by utilizing metastable states 
in the atmosphere as they happen - for instance, if clouds are being cooled 
below the freezing point, seeding can be effective; (3) by altering the 
surface condition, as by deforestation or urbanization; (4) by influencing the 
weather inadvertently by pollution. The second technique, the seeding of 
clouds with certain types of nuclei, has been frequently used.  See  AIR 
POLLUTION; CLOUD PHYSICS; FOG; HAIL. 
 �34s[Helmut K. Weickmann]
�33s 
  
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�6v�2v�32sWeather observations   �21m �33s 
    The measuring, recording and transmitting of data of the variable elements 
of weather. The National Weather Service (NWS), a component of the National 
Oceanic and Atmospheric Administration, has as one of its primary 
responsibilities the acquisition of meteorological information. The data are 
sent by various communication methods to National Weather Service field 
stations and to the National Meteorological Center near Washington, D.C. 
    At the center the raw data are fed into large computers that are 
programmed to plot, analyze and process the data and also to make prognostic 
weather charts. The processed data and the forecast guidance are then 
distributed by facsimile and teletypewriter to field stations, other 
government agencies, and private meteorologists. They in turn prepare 
forecasts and warnings based on both processed and raw data. 
    The Synoptic Weather Program is designed to serve as the backbone for the 
preparation of forecasts and to provide data for international exchange. 
Surface observations are taken all over the globe at standard times: 0000, 
0600, 1200, and 1800 Greenwich mean time (GMT), and sent in synoptic code. 
    The Basic Observation Program routinely provides meteorological data every 
hour. Special observations are taken at any intervening time to report 
significant weather events or changes. The vast majority of stations involved 
in either or both programs are operated by National Weather Service and 
Federal Aviation Administration personnel and are referred to as first-order 
stations. Observational sites are primarily at airports; a few are in cities. 
They all report the following weather elements: present weather, air pressure, 
temperature, humidity, wind speed and direction, visibility, and clouds and 
ceilings.  See  AIR PRESSURE; CLOUD; HUMIDITY; WIND. 
    Upper air observations are made with an instrument called a radiosonde, 
borne aloft by a helium-inflated balloon. It measures and then telemeters back 
to ground stations the pressure, temperature, and water vapor existing in the 
atmosphere up to about 100,000 ft (30 km). Winds aloft are computed by 
tracking the instrument with precision electronic equipment. Those 
measurements plus radar and satellite observations provide meteorologists with 
a three-dimensional picture of the atmosphere.  See  METEOROLOGICAL 
SATELLITES; RADAR METEOROLOGY; WEATHER FORECASTING AND PREDICTION. 
 �34s[Anthony E. Tancreto]
�33s 
  
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�6v�2v�32sWedge �21m �33s
    A piece of resistant material whose two major surfaces make an acute
angle. It is closely related to the inclined plane and is used to multiply the
applied force and to change the direction in which  it acts (see
illustration).	See  INCLINED PLANE.
    Force F is the smaller applied force and Q is the larger force to be
exerted. In the absence of friction, forces must act normal to their surfaces;
thus the actual force on the inclined surface is not Q but a larger force
F(sub n). Summing up forces in the horizontal and vertical directions gives Eqs.
(1).

		    F(sub n) sin theta - F = phi			    (1)

		    Q - F(sub n) cos theta = phi

    Combining the expressions for F and Q and solving for F gives Eq. (2).

			    F = Q tan theta				     (2)

    If angle theta is small, the reaction of Q against F is exceeded by the
friction between the face of the wedge and the adjacent body on which it
rests. Thus the wedge tends to remain in position even when loaded by a large
force Q.  See  SIMPLE MACHINE.
 �34s[Richard M. Phelan]
�33s


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�6v�2v�32sWeight   �21m �33s 
    The gravitational weight of a body is the force with which the Earth 
attracts the body. By extension, the term is also used for the attraction of 
the Sun or a planet on a nearby body. This force is proportional to the body's 
mass and depends on the location. Because the distance from the surface to the 
center of the Earth decreases at higher latitudes, and because the centrifugal 
force of the Earth's rotation is greatest at the Equator, the observed weight 
of a body is smallest at the Equator and largest at the poles. The difference 
is sizable, about 1 part in 300. At a given location, the weight of a body is 
highest at the surface of the Earth. Weight is measured by several procedures. 
  See  BALANCE; MASS; WEIGHT MEASUREMENT. 
 �34s[Howard S. Bean]
�33s 
  
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�6v�2v�32sWeightlessness   �21m �33s 
    A condition induced by the effective lack of resistance to gravitational 
force on an object or organism, sometimes known as free fall. 
    Newton proposed the law of universal gravitation, which states that two 
bodies of matter in the universe attract each other with a force that is 
directly proportional to the product of their masses and inversely 
proportional to the square of the distance between their centers. According to 
this law, even a small increase in the distance between bodies will produce a 
large decrease in the gravitational force, since the force decreases with the 
square of the distance. As a body moves from the Earth's surface to a location 
an infinite distance from the Earth, the gravitational force approaches zero 
and the body approaches weightlessness. In the true sense, a body can be 
weightless only when it is an infinite distance from all other objects. 
    Weightlessness is also defined as a condition in which no acceleration, 
whether of gravity or any other force, can be detected by an object or 
organism within the system in question. According to Albert Einstein's 
principle of equivalence, there is no way to distinguish between the forces of 
gravitational fields and the forces due to inertial motion. When a 
gravitational force on a body is opposed by an equal and opposite inertial 
force, a weightless state is produced. This is based on the fact that the mass 
that determines the gravitational force of a body is the same as the mass 
related to the acceleration produced by an inertial force of any kind. These 
inertial forces have no external physical origin, but are the consequences of 
an accelerated state of motion. Because of inertia, a moving object always 
tends to follow a straight line. When a person swings a bucket by the handle 
in a large circle, he or she feels a pull on his or her hand, because inertial 
force (also called centrifugal force in this case) tends to keep the bucket 
moving in a straight line, while the bucket holder exerts a counterforce 
constraining the bucket to move along the circle. A similar situation exists 
in a spaceship orbiting the Earth 200 mi (320 km) above the Earth's surface, 
where the gravitational field is only slightly weaker than at sea level. The 
ship, in free fall with negligible atmospheric drag is pulled toward the Earth 
by the Earth's gravitational attraction force, while the inertial or 
centrifugal force of the moving ship is directed radially outward from the 
Earth; consequently, the force of gravity on the orbiting ship is opposed and 
nullified by the centrifugal force, and apparent weightlessness results.  See  
GRAVITATION; GRAVITY; INERTIA; NEWTON`S LAWS OF MOTION; SPACE FLIGHT; SPACE 
PROCESSING.  
 �34s[Thora W. Halstead]
�33s 
  
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�6v�2v�32sWeight measurement   �21m �33s 
    Weight is the resultant force acting on a mass (in a vacuum) due to the 
Earth's gravitational field corrected for the effect of the Earth's rotation. 
Units of weight are based upon an acceleration of gravity. When the weight of 
an unknown is determined by comparison with a known weight, there is no error 
in the readings due to gravity variations. The varying buoyant effect of the 
atmosphere is negligible when the density of the unknown is approximately the 
same as that of the standard. In precision weighing, the buoyant effect of air 
must be considered.  See  GRAVITY. 
    The equal-arm balance is probably the most common form of instrument for 
measuring weight. These balances are made in many designs and sizes; in some 
the knife-edge fulcrums are replaced with flexure plates; others have arms of 
unequal length. Conventionally, the unknown weight is placed on one pan, the 
known weight on the other. The final securing of a balance is done by 
adjusting the position of a rider, or small weight, on a bar of the balance 
arm bridge. The condition of balance is indicated when the pointer swings 
equal distances from its rest point.  See  BALANCE. 
    The mechanical-type industrial scale incorporates a number of levers with 
precisely located fulcrums to permit heavy objects to be balanced (weighed) 
with small, convenient counterweights or counterpoises. 
    The pendulum-type mechanical scale balances the force of the load by the 
rotation of a bent lever. With this construction, the deflection of the load 
on the scale moves the counterweights through the lever system so that their 
center of gravity is at a greater distance from the final fulcrum. Thus the 
increased lever arm of the counterweights automatically balances the load. 
    The spring scale utilizes the deflection of a spring to measure the load. 
If sensitive enough, such a scale can detect and indicate the effect of 
differences in the weight of a body due to changes in elevation. 
    In hydraulic systems, the load applied to the load cell piston is 
converted to hydraulic pressure. The effective area of the piston must be 
known. The pressure may be measured at a remote point by a pressure-gage, such 
as a Bourdon tube. 
    Pneumatic systems detect the load by a sensitive nozzle and flapper system 
and balance the load by modulating an air pressure in an opposing capsule. 
    Electrical weighing systems usually involve the electrical measurement of 
the elastic deformation of a mechanical element under stress. The strain gage 
is attached to the weighing element in a manner to produce the maximum 
resistance change per unit of load. The change in resistance with load is 
measured and amplified by electronic means, and the load is read on a 
potentiometer.  See  WEIGHT. 
 �34s[Howard S. Bean]
�33s 
  
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�6v�2v�32sWelded joint   �21m �33s 
    The joining of two or more metallic components by introducing fused metal 
(welding rod) into a fillet between the components or by raising the 
temperature of their surfaces or edges to the fusion temperature and applying 
pressure (flash welding). 
    Figure 1 shows three types of welded joints. In a lap weld, the edges of a 
plate are lapped one over the other and the edge of one is welded to the 
surface of the other. In a butt weld, the edge of one plate is brought in line 
with the edge of a second plate and the joint is filled with welding metal or 
the two edges are resistance-heated and pressed together to fuse. For a fillet 
weld, the edge of one plate is brought against the surface of another not in 
the same plane and welding metal is fused in the corner between the two 
plates, thus forming a fillet. The joint can be welded on one or both sides. 
    Because welded joints are usually exposed to a complex stress pattern as a 
result of the high temperature gradients present when the weld is made, it is 
customary to design joints by use of arbitrary and simplified equations and 
generous safety factors. The force F of direct loading, and consequently the 
stress S, is applied directly along or across a weld. The stress-force 
equation is then simply F = SA, in which A is the area of the plane of failure 
(Fig. 2). For eccentric loading, the force F causes longitudinal and 
transverse forces of varying magnitudes along the weld.  See  RIVETED JOINT; 
STRUCTURAL CONNECTIONS; WELDING AND CUTTING OF METALS. 
 �34s[L. Sigfred Linderoth, Jr.]
�33s 
  
  
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�6v�2v�32sWelding and cutting of metals   �21m �33s 
    Processes based on heat to join and sever metals. Welding and cutting are 
grouped together because, in many manufacturing operations, severing precedes 
welding and involves the same production personnel. Welding is one of the 
joining processes, others being riveting, bolting, gluing, and adhesive 
bonding.  See  BOLTED JOINT; RIVETED JOINT; STRUCTURAL ADHESIVE; WELDED JOINT. 
    The American Welding Society's definition of welding is ``a metal-joining 
process wherein coalescence is produced by heating to suitable temperatures 
with or without the application of pressure, and with or without the use of 
filler metal.'' Brazing is defined as ``a group of welding processes wherein 
coalescence is produced by heating to suitable temperature and by using a 
filler metal having a liquidus about 800DGF (427DGC) and below the solidus of 
the base metals. The filler metal is distributed between the closely fitted 
surfaces of the joint by capillary attraction.'' Soldering is similar in 
principle, except that the melting point of solder is below 800DGF (427DGC). 
The adhesion of solder depends not so much on alloying as on its keying into 
small irregularities in the surfaces to be joined. For comparison of metal 
joints  see  JOINT (STRUCTURES).  See also  BRAZING; SOLDERING. 
    Cutting is one of the severing and material-shaping processes, some others 
being sawing, drilling, and planning. Thermal cutting is defined as a group of 
cutting processes wherein the severing or removing of metals is effected by 
melting or by the chemical reaction of oxygen with the metal at elevated 
temperatures. Welding and cutting are widely used in building ships, 
machinery, boilers, structures, atomic reactors, aircraft, railroad cars, 
missiles, buses and trailers, and pressure vessels, as well as in constructing 
piping and storage tanks of steel, stainless steel, aluminum, nickel, copper, 
lead, titanium, tantalum, and their alloys. For many products, welding is the 
only joining process that achieves the desired economy and properties, 
particularly leak-tightness. 
    Nearly all industrial welding involves fusion. The edges or surfaces to be 
welded are brought to the molten state. The liquid metal bridges the gap 
between the parts. After the source of welding heat has been removed, the 
liquid solidifies, thus joining or welding the parts together. In the past 
there were three principal sources of heat for fusion welding: electric arc, 
electric resistance, and flame. However, other sources must be added to the 
above: light beams and electrons. Hence there are terms for five fusion 
welding processes: arc welding, resistance welding, gas welding, laser 
welding, and electron-beam welding.  See  ARC WELDING; ELECTRON-BEAM WELDING; 
GAS WELDING; INERTIA WELDING; LASER WELDING; RESISTANCE WELDING. 
 �34s[Mel M. Schwartz]
�33s 
  
A700000003034�254s�5v�1v1878
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10845184
�6v�2v�32sWell �21m �33s
    An artificial excavation made to extract water, oil, gas, brine, or other
fluid substance from the earth. Most wells are of the drilled type. Dug wells
are almost obsolete, because of the greater speed of drilling and the greater
efficiency of drilled wells.  See  ARTESIAN SYSTEMS; OIL AND GAS WELL DRILLING.
    Drilled wells, commonly 2 - 36 in. (5 - 90 cm) in diameter, usually are
fitted with a steel tube or casing inserted in the drilled hole to the desired
depth. Where the water-bearing formation is competent to stand without
support, the casing is set, or finished, at the top of solid rock. Where there
is danger of caving, as in sand or gravel, the casing is carried below the top
of the water-bearing bed, and a perforated pipe or screen extends below the
casing to the bottom of the hole. The construction includes a considerable
period of pumping, surging, or other treatment (called well development),
during which the finer particles of the formation are drawn into the well and
removed. This process substantially increases the initial yield of the well.
    Most wells of large capacity are equipped with pumps of the deep-well
turbine type to lift the water to the surface. When a well is pumped, the
pressure head at the well is lowered and a hydraulic gradient toward the well
is established which causes water to flow toward the well. This lowering of
head is called drawdown.  See  GROUNDWATER HYDROLOGY; PUMPING MACHINERY.
 �34s[Albert N. Sayre /Ray K. Linsley]
�33s

A700000014442�254s�5v�1v1878
\IMG\CI1801.IMG
10845184
1879
\IMG\CI1801.IMG
10969600
�6v�2v�32sWell logging   �21m �33s 
    The technique of making measurements in drill holes with probes designed 
to measure the physical and chemical properties of rocks and their contained 
fluids. Much information can be obtained from samples of rock brought to the 
surface in cores or bit cuttings, or from other clues while drilling, such as 
penetration rate; however, the greatest amount of information comes from well 
logs.  See  DRILL HOLE LOGGING. 
    Well logs result from a probe lowered into the borehole at the end of an 
insulated cable. The resulting measurements are recorded graphically or 
digitally as a function of depth. These records are known as geophysical well 
logs, petrophysical logs, or more commonly well logs, or simply logs. 
    Although the most common uses of logs are for correlation of geological 
strata and location of hydrocarbon zones, there are many other important 
subsurface parameters that need to be detected or measured. Also, different 
borehole and formation conditions can require different tools to measure the 
same basic property. In petroleum engineering, logs are used to: identify 
potential reservoir rock; determine bed thickness; determine porosity; 
estimate permeability; locate hydrocarbons; estimate water salinity; quantify 
amount of hydrocarbons; estimate type and rate of fluid production; estimate 
formation pressure; identify fracture zones; measure borehole inclination and 
azimuth; measure hole diameter; aid in setting casing; evaluate quality of 
cement bonding; locate entry, rate, and type of fluid into borehole; and trace 
material injected into formations (such as artifical fractures). In geology 
and geophysics, they are used to: correlate between wells; locate faults; 
determine dip and strike of beds; identify lithology; deduce environmental 
deposition of sediments; determine thermal and pressure gradients; create 
synthetic seismograms; calibrate seismic amplitude anomalies to help identify 
hydrocarbons from surface geophysics; calibrate seismic with velocity surveys; 
and calibrate gravity surveys with borehole gravity meter. Other applications 
include: locating fresh-water aquifers; locating solid minerals; and studying 
soil and rock conditions for foundations of large structures. 
    Electrical devices employ instruments that generate data based on 
electrical measurements. The spontaneous potential, usually recorded along 
with resistivity curves, is a simple but valuable aid to help geologists 
correlate from one well to another and to assist them in inferring the 
depositional environment of the sediments. It defines permeable zones from 
surrounding nonpermeable shales and can be used to estimate the salinity of 
formation water. Resistivity is one of the most important physical properties 
to record. A sand filled with salt water will conduct electricity much easier 
(and therefore has a lower resistivity) than a sand in which most of the salt 
water is replaced with a nonconducting oil or gas. The primary purpose of 
resistivity measurements is to determine hydrocarbon saturation. 
    Quantitative interpretation of hydrocarbon saturation requires a knowledge 
of porosity. One of the most widely used porosity logs is the acoustic log, 
also known as a sonic log, acoustilog, or velocity log. The acoustical log 
measures the shortest time for sound waves to travel through 1 ft (or 1 m) of 
formation. This log is used for many other purposes, including identification 
of lithology, prediction of pressures, and assisting with geophysical 
interpretation. In addition, information from the amplitude of the acoustic 
wave aids in detection of fractures and gas zones, determination of mechanical 
properties of rocks, and analysis of cement bond behind pipe. 
    Nuclear devices are instruments which generate data based on measurements 
of nuclear particles. In most cases the gamma-ray log may be considered as a 
shale log; that is, clays or shaly rocks will give a higher radioactive count 
than clean sands or carbonates. Like the spontaneous potential, this provides 
a good correlation curve, defines bed thicknesses, and aids interpretation of 
environmental deposition. Almost all the naturally occurring radioactivity 
that is detected by the gamma-ray log comes from three elements: potassium, 
thorium, and uranium. 
    An important porosity tool is the density log, which emits a beam of gamma 
rays into the rock and these rays interact with electrons in the formation 
through Compton scattering. The tool provides a bulk density measurement. 
Another important device for determination of porosity is the neutron log. 
These logs respond primarily to hydrogen atoms. Therefore, in clean or 
shale-free formations the neutron log reflects the amount of liquid-filled 
porosity. A tool developed to distinguish hydrocarbon from salt water behind 
casing is the pulsed neutron capture tool which employs a neutron generator 
that repeatedly emits pulses of high-energy neutrons. After these neutrons are 
slowed down to the thermal state, they are captured by nuclei of the various 
atoms surrounding the tool. With each capture a corresponding emission of 
gamma rays occurs. 
    Production logging tools are designed to locate fluid movement behind the 
casing and into the well bore, to detect the type of fluid, and to determine 
the flow rate. Production logging tools include various types of flow meters, 
fluid density devices, sensitive thermometers, radioactive tracer devices, 
noise loggers, and capacitance logs. In addition, these tools usually record 
some correlation curve such as gamma-ray or collar locator to tie the 
production curves to the formation and casing. Closely related to production 
tools are logging devices that either magnetically or mechanically inspect the 
condition of the casing in the borehole. 
    Small but rugged and powerful computers designed for field operation 
record logs on magnetic tapes as they are run and mathematically manipulate 
them in the logging trucks at the well site. Detailed computations are made 
quickly on a foot by foot basis to give information on fluid and rock 
properties. The digital information from practically every curve can be 
incorporated into some type of analysis or interpretation program. 
 �34s[Richard E. Wyman]
�33s 
  
A700000002562�254s�5v�1v1879
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10969600
�6v�2v�32sWellpoint systems   �21m �33s 
    A method of keeping an excavated area dry by intercepting the flow of 
groundwater with pipe wells located around the excavation area. Intercepting 
the flow before it reaches the excavated area also improves the stability of 
the edge of the excavation, permitting steeper bank slopes and often 
eliminating the need for supporting or shoring the banks.  See CONSTRUCTION 
METHODS; GROUNDWATER HYDROLOGY; WELL. 
    Wellpoint systems are most effective in coarse-grained soils, such as 
gravel or sand. They are not effective in fine soils, such as silts and clays, 
where the small size of the pores between grains restricts the flow of water. 
    The basic components of a wellpoint system are the wellpoint, the riser 
pipe, the header pipe or manifold, and the pump (see illustration). The 
wellpoint consists of a perforated pipe equipped with a ball valve to regulate 
the flow of water, a screen to prevent the entry of sand during pumping, and a 
jetting tip. The steel riser pipe brings the groundwater to the surface, where 
it is collected by the horizontal manifold pipe or header pipe. The pumps are 
located above the water table and collect the water from the header pipes for 
discharge away from the excavation area.  See  PUMP; PUMPING MACHINERY.   
 �34s[William Hershleder]
�33s 
  
  
A700000001331�254s�5v�1v1879
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10969600
1880
\IMG\CI1801.IMG
11091968
�6v�2v�32sWentzel-Kramers-Brillouin method   �21m �33s 
    A special technique for obtaining an approximation to the solutions of the 
one-dimensional time-independent Schroedinger equation, valid when the 
wavelength of the solution varies slowly with position. It is named after G. 
Wentzel, H. A. Kramers, and L. Brillouin, who independently in 1926 
contributed to its understanding in the quantum-mechanical application. It is 
also called the WKB method, BWK method, the classical approximation, the 
quasi-classical approximation, and the phase integral method.  See  QUANTUM 
MECHANICS; SCHROEDINGER'S WAVE EQUATION. 
 �34s[Roland H. Good]
�33s 
  
A700000004055�254s�5v�1v1880
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11091968
�6v�2v�32sWest Indies   �21m �33s 
    An archipelago, including the Bahamas, the Greater Antilles (including 
Cuba, Jamaica, Hispaniola - the Dominican Republic and Haiti - and Puerto 
Rico), the Lesser Antilles, and other islands, curving 2500 mi (4000 km) from 
Yucatan Peninsula and southeastern Florida to northern Venezuela and enclosing 
the Caribbean Sea. Situated between latitude 10DG and 27DGN and longitude 59DG 
and 85DGW, in the zone of the northeast trade winds, the West Indies have a 
subtropical and predominantly oceanic climate, with even warmth and steady 
breezes. Temperatures vary little from season to season, ranging from means of 
80 - 85DGF (27 - 29DGC) in July to 70 - 78DGF (21 - 26DGC) in January at sea 
level. Freezing is unknown, and the hottest temperatures rarely exceed 90DGF 
(32DGC). Precipitation ranges from a low of 25 - 50 in. (64 - 127 cm) a year 
on low-lying islands and drier coasts up to 300 in. (7.6 m) on the highest 
peaks, which are almost perpetually cloud-capped. At lower elevations, 
rainfall is erratic from year to year and from season to season, but reaches a 
maximum in the summer and fall, when the northeast trades are replaced by 
light, variable winds. This is also the season of hurricanes, destructive 
tropical cyclones which sweep west and northwest across the Caribbean, sparing 
only the southernmost islands. The winter months are generally dry, and there 
is frequently a shorter dry season in July or August.  See  CARIBBEAN SEA; 
TROPICAL METEOROLOGY. 
    The West Indian flora is chiefly derived from Central and South America, 
but there are a number of endemic species, notably palms; many mainland plants 
failed to colonize the islands, which are floristically poor. The effect of 
isolation and small size is evident in the meager character of West Indian 
fauna. Animal species are limited; there are few mammals and no large ones, 
except for domesticated animals and, especially on Hispaniola, feral cattle, 
goats, pigs, and horses. 
 �34s[David Lowenthal]
�33s 
  
A700000001571�254s�5v�1v1880
\IMG\CI1801.IMG
11091968
�6v�2v�32sWest Nile fever   �21m �33s 
    An acute, usually mild, mosquito-borne virus disease. The infection occurs 
in the summer, chiefly in Egypt, Israel, Africa, India, and Korea. Subclinical 
infections are common. In those infections which do produce illness, the usual 
signs are fever and lymphadenopathy, sometimes with rash; fatality or serious 
after effects are rare. 
    The infection cycle by which the virus is maintained in nature is thought 
to be bird -> mosquito -> bird, with Culex univittatus the principal vector. 
Ticks may play a role as reservoir-vectors of West Nile virus, serving as 
sources of transmission to birds and rodents. Human infection, as well as that 
of other vertebrates, occurs seasonally in relation to the presence and 
density of the infected mosquito population. 
 �34s[Joseph L. Melnick]
�33s 
  
A700000004727�254s�5v�1v1880
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11091968
�6v�2v�32sWet cell �21m �33s
    A primary cell in which there is a substantial amount of free electrolyte
in liquid form. Important examples of wet cells are the Lalande or caustic
soda cell, the air-depolarized alkaline cell, the Weston standard cell, and
the organic electrolyte cell.
    The Lalande cell uses a zinc anode, a cupric oxide cathode, and an
electrolyte of sodium hydroxide in aqueous solution (caustic soda). The anode
reaction in the Lalande cell is the oxidation of zinc to form zinc oxide,
which dissolves in the electrolyte to form sodium zincate. The cathodic
reaction is the reduction of cupric oxide to metallic copper.
    The air-depolarized alkaline cell uses a zinc anode, a porous carbon
cathode exposed to air on one face, and an alkaline electrolyte. The carbon
cathode utilizes atmospheric oxygen. Its zinc anode and alkaline electrolyte
are like those of the Lalande cell, but it has twice the operating voltage and
twice the watt-hour output of an equal-size Lalande cell.
    The Weston cell of 1893 has become the accepted standard of electromotive
force (emf). The Weston normal or saturated cadmium cell has an emf of 1.01864
absolute volts at 68DGF (20DGC). When purified materials are used, cells
having the same emf to within a few microvolts may be made. Reference
standards, in daily use for many years, are remarkably constant.  See  VOLTAGE
MEASUREMENT.
    The cell uses a two-phase amalgam of cadmium as the anode. The cathode is
mercurous sulfate, Hg2SO4, in contact with mercury. The electrolyte is a
saturated solution of cadmium sulfate in equilibrium with the solid phase
CdSO4.8/3 H2O.
    A different class of cells is that based on the use of particularly
reactive metals (Li, Ca, Mg) in conjunction with organic electrolytes. The
best-known type in this class is the lithium-cupric fluoride cell,
theoretically capable of delivering over 700 Wh/lb, more than three times the
capacity of the highly ranked Zn-AgO cell. The lithium anode is usually made
in sheet form, but variants using lithium powder trapped in appropriate grids
are also known. The cathode is a mixture of CuF2 and various conductive
materials, most often graphite and carbon black either together or separately,
in order to obtain the electronic conductivity that CuF2 does not possess. The
electrolyte considered most compatible with these electrodes is lithium
perchlorate in propylene carbonate or butyrolactone.
 �34s[L. Rozeanu ;Jack Davis]
�33s

A700000004356�254s�5v�1v1880
\IMG\CI1801.IMG
11091968
�6v�2v�32sWhale   �21m �33s 
    A marine mammal of the order Cetacea. There are about 37 species, with a 
worldwide distribution. The cetaceans are divided into two distinct groups, 
the Odontoceti or toothed whales, and the Mysticeti or whalebone whales. 
    As marine forms, whales are extremely atypical; they are highly adapted to 
live in the marine environment but this is a secondary adaptation, an example 
of evolutionary convergence. During the course of evolution, the body became 
streamlined and the tail transformed into an organ for propulsion and the 
limbs into balancing structures. There are no external ears, and the small 
opening which leads to the eardrum has a wax plug. Nostrils, also lacking as 
external projecting structures, occur as a blowhole which is single in toothed 
whales and double in whalebone whales and is located on the top of the head. 
The blowholes lead to the lungs. The thick skin is smooth and hairless, and 
both sweat and sebaceous glands are absent. There is a thick layer of 
insulating fat, or blubber, beneath the skin. 
    The teeth in toothed whales may vary from 260 to a single pair and are 
simple in structure. The upper left canine of the narwhal may reach a length 
of 8 ft (2.4 m) or more and has the appearance of a coiled tusk. In place of 
teeth the whalebone whales have platelike structures of baleen, or whalebone. 
The toothed whales are referred to as ichthyophagous because they feed on a 
variety of marine organisms, such as fish and cephalopods. The whalebone 
whales feed on krill, or microcrustaceans, which they filter from the sea 
water and swallow by the ton; they are therefore referred to as teuthophagous. 
    Other important adaptations include huge lungs, a necessary adaptation 
since these animals may remain submerged for as long as 1 hr and may dive to 
depths of 300 - 600 ft (91 - 182 m). The eyes are small for the size of the 
animal and are adapted for underwater vision. In the males the testes are 
internal; the mammary glands of the female are inguinal. A single young, 
rarely two, is born. The gestation period of the blue whale is about 47 weeks, 
and that of the sperm whale is 68 weeks.  See  CETACEA. 
 �34s[Charles B. Curtin]
�33s 
  
A700000001314�254s�5v�1v1881
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11226624
�6v�2v�32sWharf   �21m �33s 
    A structure along a waterfront providing a berth for ships to load and 
discharge passengers and cargo. A marginal wharf, one parallel to shore, is 
generally known as a quay or bulkhead. When perpendicular or oblique to shore, 
a wharf is known as a pier. Construction is essentially that required for 
retaining earth fill or for embankment protection while at the same time 
providing a landing platform. Reinforced-concrete or masonry retaining walls 
are used as well as the familiar pile-supported platforms with sheet piling 
bulkheads of steel, timber, and concrete.  See  COASTAL ENGINEERING; PIER. 
 �34s[Edward J. Quirin]
�33s 
  
A700000004473�254s�5v�1v1881
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11226624
�6v�2v�32sWheat   �21m �33s 
    A food grain crop. Wheat is the most widely grown food crop in the world. 
It ranks first in world crop production and is the national food staple of 43 
countries. The principal food use of wheat is as bread, either leavened or 
unleavened.  
    Wheat for milling is classified according to hardness, color, and best 
use. In the United States, there are seven official market classes of which 
the following five are the most important: (1) hard red winter, for bread; (2) 
hard red spring, for bread and rolls; (3) soft red winter, for cake and 
pastries; (4) white, for bread, breakfast foods, and pastries; and (5) durum, 
for macaroni products. 
    Most countries in which wheat is grown have wheat breeding programs in 
which the objective is to develop more productive and more stable varieties 
(cultivars). Many methods are combined in these programs, but in nearly all of 
them specially selected parent types are crossbred followed by pure-line 
selection among the progeny to develop new combinations of merit. Varieties 
and genetic types from all over the world become candidate parents to provide 
the desired recombinations of good quality, winter and drought hardiness, 
straw strength, yield, and disease resistance. Wheats must be bred for 
specific milling processes and to provide quality end-use products. Many new 
varieties have complex pedigrees.  See  GRAIN CROPS. 
 �34s[Louis P. Reitz]
�33s 
    The milling process breaks open the wheat kernel and reduces the particles 
formed so as to separate the outer and inner portions of the kernel. Bran and 
germ are almost completely separated from the white interior portions of the 
kernel in the milling of refined flour. When the entire kernel is ground and 
no separations are made, the product is whole wheat flour. 
    Flour milling has become one of the principal industries in terms of value 
of product processed and of value added to the product during processing. 
Flours are milled for many purposes such as bread, pastries, cakes, cookies, 
or macaroni. This necessitates wheat selection and different processing to 
some extent, although the basic principles are not changed. Flours are 
manufactured from hard, soft, or durum wheat or combinations of these. 
 �34s[John A. Shellenberger]
�33s 
  
A700000002650�254s�5v�1v1881
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11226624
�6v�2v�32sWheatstone bridge �21m �33s
    A device used to measure the electrical resistance of an unknown resistor
by comparing it with a known standard resistance. This method was first
described by S. H. Christie in 1833. Since 1843 when Sir Charles Wheatstone
called attention to Christie's work, Wheatstone's name has been associated
with this network.
    The Wheatstone bridge network consists of four resistors R(sub AB), R(sub
BC), R(sub CD), and R(sub AD) interconnected as shown in the illustration to
form the bridge.  A detector G, having an internal resistance R(sub G), is
connected between the B and D bridge points; and a power supply, having an
open-circuit voltage E and internal resistance R(sub B), is connected between
the A and C bridge points.  See BRIDGE CIRCUIT.

		R(sub BC)R(sub AD) - R(sub AB)R(sub CD) = 0		     (1)

    If the network is adjusted so that Eq. (1) is satisfied, the detector
current will be zero and this adjustment will be independent of the supply
voltage, the supply resistance, and the detector resistance. Thus, when the

		R(sub BC)R(sub AD) = R(sub AB)R(sub CD) 		     (2)

bridge is balanced, Eq. (2)  holds, and, if it is assumed that the unknown
resistance is the one in the CD arm of the bridge, then it is given by Eq.

	      R(sub CD) = [R(sub BC) / R(sub AB)] x R(sub AD)		     (3)

See RESISTANCE MEASUREMENT.
 �34s[Charles E. Applegate]
�33s


A700000002107�254s�5v�1v1881
\IMG\CI1801.IMG
11226624
1882
\IMG\CI1801.IMG
11318784
�6v�2v�32sWheel and axle �21m �33s
    A wheel and its axle or, more generally, two wheels with different
diameters or a wheel and drum, as in a windlass, rigidly connected together so
that they rotate as a unit on a common axis. The principle of operation is the
same as that of the lever in that, for static equilibrium, the summation of
torques about the axis of rotation equals zero. Where flexible members, such
as ropes, have been firmly attached to a wheel and drum (see illustration) and
the machine is mounted on frictionless bearings, the following equation
applies:

		 R(sub 1)R(sub 1) - F(sub 2)R(sub 2) = 0

    The main difference between the lever and the wheel and axle is that the
wheel and axle permits the forces to operate through a much greater distance.
In the illustration the wheel and drum could be allowed to rotate any number
of revolutions if the ropes were wrapped the required number of times around
each before they were attached.  See  FORCE; SIMPLE MACHINE.
 �34s[Richard M. Phelan]
�33s


A700000001016�254s�5v�1v1882
\IMG\CI1801.IMG
11318784
�6v�2v�32sWheel base   �21m �33s 
    The distance in the direction of travel from front to rear wheels of a 
vehicle, measured between centers of ground contact under each wheel. For a 
vehicle with two rear axles, the rear measuring point is on the ground midway 
between rear axles. Tread of a vehicle is the distance perpendicular to the 
direction of travel between front wheels, or between rear wheels, measured 
from centers of ground contact. 
 �34s[Frank H. Rockett]
�33s 
  
A700000001750�254s�5v�1v1882
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11318784
�6v�2v�32sWhelk   �21m �33s 
    A name given to many species of larger marine snails, mostly belonging to 
genera such as Buccinum, Busycon, Colus, and Neptunea (all in the order 
Neogastropoda). Their shells may be ovate or fusiform, but always consist of 
symmetrically enlarging whorls and a large aperture (see illustration). Whelks 
are found in all oceans, from polar to warm temperate seas. Rarely found above 
low-tide mark, they are most abundant on harder substrates in the shallow 
sublittoral, but some species extend downward to depths of 9900 ft (3000 m). 
Whelks are carnivores and carrion feeders, unlike the even larger-shelled 
mesogastropod conchs. Their locomotion appears as a smooth gliding over rock 
and shell surfaces, is based on sequential waves of muscle contraction passing 
along the sole of the foot, and is considerably faster than that of conchs or 
limpets.  See  GASTROPODA; SNAIL. 
 �34s[W. D. Russell-Hunter]
�33s 
  
  
A700000001115�254s�5v�1v1882
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11318784
�6v�2v�32sWhipworm disease   �21m �33s 
    A chronic, wasting diarrhea produced by heavy parasitization of the large 
intestine by the nematode Trichuris trichiura, particularly in undernourished 
children in the tropics. The worms are 1 in. (2.5 cm) long, with the anterior 
body attenuated to resemble a whip, and are firmly attached to the intestine, 
where they irritate the mucosa. Their barrel-shaped eggs pass in the feces. In 
countries with poor sanitation, the soil becomes contaminated.  See  NEMATA. 
 �34s[Jose F. Maldonado]
�33s 
  
A700000001721�254s�5v�1v1882
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11318784
�6v�2v�32sWhite dwarf star �21m �33s
    An intrinsically faint star of very small radius and high density. The
mass of a typical white dwarf is about 0.7 that of the Sun, and the average
radius is 5000 mi (8000 km). Surface temperatures are between 85,000 and 4000
K, and brightness is from 10 to 10(sup -4) that of the Sun. Such stars are
composed largely of helium, carbon, or heavier elements.
    A white dwarf is one final stage of stellar evolution, with thermonuclear
energy sources extinct. The heavy nuclei are still mobile, and their thermal
energy maintains the star's radiation. The star cools and becomes fainter and
redder.  See  STELLAR EVOLUTION.
    White dwarfs have been found to be one member of certain close,
interacting binary stars of the type that may become classical novae
(undergoing a giant explosion), recurrent novae, or dwarf novae.  See  BINARY
STAR; BLACK HOLE; NOVA.
 �34s[Jesse L. Greenstein]
�33s

A700000002050�254s�5v�1v1882
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11318784
�6v�2v�32sWhooping cough   �21m �33s 
    An acute communicable disease of humans, also known as pertussis, caused 
by Bordetella pertussis. It occurs chiefly among infants and children, about 
one-half of those affected being under 4 years of age. 
    The course of the disease is roughly 6 weeks, equally divided into 
catarrhal, paroxysmal and decline periods; each lasts approximately 2 weeks. 
About 2 weeks after exposure the child begins to cough, usually at night; the 
cough soon becomes diurnal. It becomes more frequent and severe, occurring in 
a series of explosive spells with strangling and suffusion of the face. The 
spell finally ends in the loud inspiratory crow (the whoop) as the child 
attempts to regain his breath. This is often followed by vomiting. 
    The rapid decline in the incidence and severity of the disease in the 
United States is attributed to better nutritional status and to the extensive 
immunization of infants with immune serum and vaccine. 
 �34s[William L. Bradford]
�33s 
  
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11318784
�6v�2v�32sWiedemann-Franz law �21m �33s
    An empirical law of physics which states that the ratio of the thermal
conductivity of a metal to its electrical conductivity is a constant times the

			K(sub c) = L(sub O) SIGNA T

absolute temperature, as given by the equation below. Here K(sub c) is the
thermal conductivity due to the conduction electrons, SIGMA is the electrical
conductivity, T is the absolute temperature, and L(sub 0) is known as the
Lorentz number.  The Wiedemann-Franz law provides an important check on theories
of electrical and thermal conductivity.  See CONDUCTION (HEAT); THERMAL
CONDUCTION IN SOLIDS.
 �34s[Frank J. Blatt]
�33s

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�6v�2v�32sWild black cherry �21m �33s
    A tree, Prunus serotina, that belongs to the rose family, and is an
important timber species of cherry in the United States. Native in the eastern
half of the United States and adjacent Canada, it reaches its largest size in
the southern Appalachian Mountains. The deciduous leaves are long-pointed,
thickish, and shining above. The edible fruit is a shiny black drupe found in
long clusters. The hard, strong wood is a red-brown and is used for furniture,
often as an imitation mahogany.  See  ROSALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

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�6v�2v�32sWildlife conservation   �21m �33s 
    The science and art of making decisions and taking actions to manipulate 
the structure, dynamics, and relations of wild animal populations, habitats, 
and people to achieve specific human benefits from the wildlife resource. 
Although still called wildlife conservation by some people, wildlife 
management, the preferred term, has more than its earlier connotations of 
preservation. Wildlife management is concerned not only with preserving and 
increasing populations of threatened or endangered species, such as the 
whooping crane, but also with stabilizing certain populations, such as the 
American bison, increasing or stabilizing desirable species taken for sport 
recreation (game), and decreasing some populations of birds and harmful 
mammals.  
    The problems of management are complex and involve conflicts between 
groups with opposing interests. For example, orchard owners want fewer 
bark-eating rabbits, while hunters want more rabbits of all kinds; fox hunters 
want increased populations, while other people wish to reduce foxes as vectors 
of rabies; hunters who want only native game animals differ with hunters who 
would stock or import foreign species into any area; hunters and farmers are 
at odds on the proper number of raccoons to have in forests near cornfields; 
foresters managing their forests to supply deer to hunters do not want deer in 
their nurseries; city dwellers want birds in parks but not on buildings. 
  
    Management task.   To achieve a desired abundance of a particular species 
of wildlife, modern wildlife managers work intensively with the environment in 
which the animals live. Wildlife populations naturally respond very 
sensitively to their habitats. When water levels are down, ducks produce fewer 
young; when dams of beaver are torn out and the rich land within beaver ponds 
is planted to crops, beaver disappear; where the woody food of deer and elk is 
not abundant or after the supply of forage has been eaten, populations of 
these animals are reduced or they have fewer young. 
    Most wildlife managers now believe that energy is the primary factor 
limiting the ability of area to support animals. Animals require large amounts 
of energy to survive. It is provided to them by food, sunlight, and 
reradiation of sun energy from plants and the ground. It is being removed from 
them at a rapid rate by natural forces, primarily wind, contact with the cold 
ground, and evaporation of moisture; of course, movement and producing young 
require large expenditure of energy. Animals have developed means for 
conserving this energy so that outgo does not exceed income. Wildlife managers 
may attempt to supply food energy or reduce energy losses. By providing 
protection from the wind, preserving areas where animals may sun, protecting 
them from harassment by dogs, predators, or people, and providing high-energy 
foods, the modern wildlife manager attempts to improve the energy balance and 
budgets of wild animal populations. 
    Where soils are poor, wildlife will be sparse. Richer soils produce more 
vigorous plants that store sun energy better and make it available to 
wildlife. The types of plants and animals must be balanced by the wildlife 
manager, for on very rich soil, plants can grow to heights out of reach of 
animals. Rich soils can encourage large populations with high food consumption 
that can temporarily destroy the vegetation on areas and even result in soil 
erosion. These changes can result in population fluctuations. 
    Wildlife managers are concerned with the natural ability of populations to 
reproduce. The reproduction potential is set by genetics but strongly 
influenced by the quality of food. When food supplies are low or the quality 
of food is reduced (as a result of soil erosion or leaching of nutrients from 
soil), reproduction is reduced. Another factor called stress influences 
populations in several complex ways: as populations become more crowded, 
reproduction is reduced, more food is required, and diseases have greater 
effects. Thus there are several interacting natural limits on wildlife 
population growth imposed both by the environment and by the population itself. 
    Threats to populations of wildlife usually result from a very complex set 
of factors. The factor usually at the top of the list is destruction of 
wildlife homes and habitat. However, establishing areas of complete wildlife 
protection, stocking, and broad-scale predator control have proved over and 
over to be insufficient. Habitat management has been a more efficient 
technique. This approach, a systems approach, is that of selecting a group of 
techniques specifically designed for benefiting one target species for each 
area and for achieving the specific objectives of the landowner. The system 
involves (1) intensive study and research by experts to isolate problems, (2) 
definition of goals, (3) education of the public (and specialists) by many 
media, (4) programs to retain and improve the habitat, (5) goal-oriented game 
harvest regulations or population control programs, (6) manipulation of sex 
and age ratios, as well as density of animals, to achieve desired population 
change, (7) actions by agents to deter or apprehend poachers or game-law 
violators, (8) control of disease and parasites, and of select individual 
predators and the causes of accidents, and (9) well-researched transplanting 
of wildlife from areas where it has bred successfully to areas with high 
potential for population buildups.  See  SYSTEMS ECOLOGY. 
  
    Today's needs.   The public and the professional wildlife staffs of state 
and Federal conservation agencies can influence wildlife populations by the 
following major practices: (1) laws to protect endangered species, (2) 
regulations to allow a recreational harvest of game animals and manipulation 
of other populations that put each population in balance with its food supply, 
(3) protection of wildlife areas, both large land holdings of the government 
and small areas, such as woodlots and fencerows, needed by certain species of 
birds and animals, (4) research into the needs of and responses of animals to 
each other and their environment, (5) investment in select areas for maximum 
wildlife production, maintaining moderate production cheaply under existing 
conditions, and (6) education of the public: increasing their appreciation of 
wildlife (and thus the benefits which they derive from it) and also their 
awareness of how actions (hunting, voting, destroying food and cover, changing 
the environment, and contributing to research and law enforcement) influence 
wildlife and the larger complex of humans' own environment. 
  
    Wildland operations research.   There has emerged an art called operations 
research (OR) based on the science of analyzing large and complex systems of 
all types and designing means to achieve specified goals in some optimal 
fashion. Wildland operations research is the application of the concepts and 
methods of operations research to wildland problems. The term wildlands 
usually refers to all of the resources (space, water, soil, air, minerals, 
forests, and wildlife) of nonagricultural lands, such as forests, rangelands, 
marshes, deserts, and undeveloped parks. The differences within and between 
these categories may be great or indistinct (for example, differences between 
highly managed tree farms, farm woodlots, and wilderness forests). Every 
system can be considered a subsystem of a larger system.  See  TERRESTRIAL 
ECOSYSTEM.  
    Wildland operations research deals with natural resource systems such as 
the ecosystem, with populations of fish or other animals, with forests and 
wood-dependent industries, with rangelands and marshlands, with migratory-bird 
flyways, and with pest species and their effects.  See  ECOLOGY; OPERATIONS 
RESEARCH.  
 �34s[Roy H. Gigg]
�33s 
  
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�6v�2v�32sWillemite �21m �33s
    A rare nesosilicate mineral, composition Zn2SiO4, crystallizing in the
hexagonal system. It is usually massive or granular with a vitreous luster;
crystals are rare. The mineral may be variously colored, most commonly green,
red, or brown. Hardness is 5 1/2 on Mohs scale; specific gravity is 3.9 - 4.2.
Willemite forms a valuable ore of zinc at Franklin, New Jersey. At this famous
zinc deposit willemite fluoresces a yellow-green.  See	SILICATE MINERALS.
 �34s[Cornelius S. Hurlbut, Jr.]
�33s

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�6v�2v�32sWillow   �21m �33s 
    A deciduous tree and shrub of the genus Salix, order Salicales, common 
along streams and in wet places in the United States, Europe, and China. The 
twigs are often yellow-green and bear alternate leaves which are 
characteristically long, narrow, and pointed, usually with fine teeth along 
the margins. Flowers occur in catkins. The fruit contains several silky seeds. 
  See  SALICALES. 
    Willow lumber is used for fuel and in making charcoal, excelsior, ball 
bats, boxes, crates, boats, waterwheels, and wicker furniture. The tough, 
pliable shoots of many species are used to make baskets; the bark of other 
species is used for tanning. Willows are of great value in checking soil 
erosion. A few species are ornamental shade trees. 
 �34s[James F. Ferry]
�33s 
  
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�6v�2v�32sWind �21m �33s
    The motion of air relative to the Earth's surface. The term usually refers
to horizontal air motion, as distinguished from vertical motion, and to air
motion averaged over a chosen period of 1 - 3 min. Micrometeorological
circulations (air motion over periods of the order of a few seconds) and
others small enough in extent to be obscured by this averaging are thereby
eliminated.   See   WIND MEASUREMENT.
    The direct effects of wind near the surface of the Earth are manifested by
soil erosion, the character of vegetation, damage to structures, and the
production of waves on water surfaces. At higher levels wind directly affects
aircraft, missile and rocket operations, and dispersion of industrial
pollutants, radioactive products of nuclear explosions, dust, volcanic debris,
and other material. Directly or indirectly, wind is responsible for the
production and transport of clouds and precipitation and for the transport of
cold and warm air masses from one region to another.  See  ATMOSPHERIC GENERAL
CIRCULATION; WEATHERING PROCESSES; WIND STRESS OVER SEA.
    Cyclonic and anticyclonic circulation are each a portion of the pattern of
airflow within which the streamlines (which indicate the pattern of wind
direction at any instant) are curved so as to indicate rotation of air about
some central point of the cyclone or anticyclone. The rotation is considered
cyclonic if it is in the same sense as the rotation of the surface of the
Earth about the local vertical, and is considered anticyclonic if in the
opposite sense. Thus, in a cyclonic circulation, the streamlines indicate
counterclockwise (clockwise for anticyclonic) rotation of air about a central
point on the Northern Hemisphere or clockwise (counterclockwise for
anticyclonic) rotation about a point on the Southern Hemisphere. When the
streamlines close completely about the central point, the pattern is denoted
respectively a cyclone or an anticyclone. Since the gradient wind represents a
good approximation to the actual wind, the center of a cyclone tends strongly
to be a point of minimum atmospheric pressure on a horizontal surface. Thus
the terms cyclone, low-pressure area, or ``low'' are often used to denote
essentially the same phenomenon.  See  AIR WAVES, UPPER SYNOPTIC; GRADIENT
WIND; STORM.
    Convergent or divergent patterns are said to occur in areas in which the
(horizontal) wind flow and distribution of air density is such as to produce a
net accumulation or depletion, respectively, of mass of air. The horizontal
mass divergence or convergence is intimately related to the vertical component
of motion. For example, since local temporal rates of change of air density
are relatively small, there must be a net vertical export of mass from a
volume in which horizontal mass convergence is taking place. Only thus can the
total mass of air within the volume remain approximately constant.
    The horizontal mass divergence or convergence is closely related to the
circulation. In a convergent wind pattern the circulation of the air tends to
become more cyclonic; in a divergent wind pattern the circulation of the air
tends to become more anticyclonic. A convergent surface wind field is typical
of fronts. As the warm and cold currents impinge at the front, the warm air
tends to rise over the cold air, producing the typical frontal band of
cloudiness and precipitation.  See  FRONT.
    Zonal surface wind patterns result from a longitudinal averaging of the
surface circulation. This averaging typically reveals a zone of weak variable
winds near the Equator (the doldrums) flanked by northeasterly trade winds in
the Northern Hemisphere and southeasterly trade winds in the Southern
Hemisphere, extending poleward in each instance to about latitude 30DG. The
doldrum belt, particularly at places and times at which it is so narrow that
the trade winds from the two hemispheres impinge upon it quite sharply, is
designated the intertropical convergence zone, or ITC. The resulting
convergent wind field is associated with abundant cloudiness and locally heavy
rainfall.  See	MONSOON.
    Longitudinal averaging indicates a predominance of westerly winds. These
westerlies typically increase with elevation and culminate in the average jet
stream, which is found in lower middle latitudes near the tropopause at
elevations between 35,000 and 40,000 ft (10.7 and 12.2 km).  See  JET STREAM.
    Local winds commonly represent modifications by local topography of a
circulation of large scale. Examples are the mistral which blows down the
Rhone Valley in the south of France, the bora which blows down the gorges
leading to the coast of the Adriatic Sea, the foehn winds which blow down the
Alpine valleys, the williwaws which are characteristic of the fiords of the
Alaskan coast and the Aleutian Islands, and the chinook which is observed on
the eastern slopes of the Rocky Mountains.  See  CHINOOK.
 �34s[Frederick Sanders]
�33s

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�6v�2v�32sWindings in electric machinery   �21m �33s 
    Windings can be classified in two groups: armature windings and field 
windings. The armature winding is the main current-carrying winding in which 
the electromotive force (emf) or counter-emf of rotation is induced. The 
current in the armature winding is known as the armature current. The field 
winding produces the magnetic field in the machine. The current in the field 
winding is known as the field or exciting current.  See  ELECTRIC ROTATING 
MACHINERY; GENERATOR; MOTOR. 
    The location of the winding depends upon the type of machine. The armature 
windings of dc motors and generators are located on the rotor, since they must 
operate in conjunction with the commutator, and the field windings are mounted 
on stator field poles.  See  DIRECT-CURRENT GENERATOR; DIRECT-CURRENT MOTOR. 
    Alternating-current synchronous motors and generators are normally 
constructed with the armature winding on the stator and the field winding on 
the rotor. There is no clear distinction between the armature and field 
windings of ac induction motors or generators. One winding may carry the main 
current of the machine and also establish the magnetic field. It is customary 
to use the terms stator winding and rotor winding to identify induction motor 
windings. The word armature, when used with induction motors, applies to the 
winding connected to the power source (usually the stator).  See  
ALTERNATING-CURRENT GENERATOR; ALTERNATING-CURRENT MOTOR; SYNCHRONOUS MOTOR. 
 �34s[Arthur R. Eckels]
�33s 
  
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�6v�2v�32sWind measurement   �21m �33s 
    The determination of three parameters: the size of an air sample, its 
speed, and its direction of motion. 
    The size of the air sample is highly dependent on how the measurement is 
made. When the wind measurement is taken with small, sensitive, and 
rapid-response instruments, or by the drift of small suspended particles, the 
air sample can have a scale of a fraction of an inch or less. If the wind 
measurement is made from the pressure gradient, as measured on a weather map, 
the scale can be hundreds of miles. 
    Wind direction is designated as the direction from which the wind is 
blowing, given in terms of 8, 16, or 32 points of the compass and in degrees 
or tens of degrees from north, measured clockwise. A wind vane is used to 
measure the direction of the surface wind. Basically, this is a grossly 
unsymmetrical body mounted near its center of gravity and free to rotate about 
a vertical axis. 
    In terms of its force on common objects such as leaves, smoke, or waves, 
wind speed can be estimated by the Beaufort scale. The scale of Beaufort 
numbers ranges from 0 for winds calm with velocity under 1 mi/h or 1.6 km/h 
(smoke rises vertically) to 12 for hurricane-force winds with velocity over 75 
mi/h or 120 km/h. Meteorologists often measure the wind speed in terms of the 
cause of the wind, that is, the atmospheric pressure gradient, which is 
inversely proportional to the spacing of the isobars on a weather map. For 
information concerning instruments which measure surface winds see ANEMOMETER. 
For further information on this and on measurement of upper-air wind speed  
see  METEOROLOGICAL INSTRUMENTATION. 
 �34s[Verner E. Suomi]
�33s 
  
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�6v�2v�32sWind power �21m �33s
    Kinetic energy in the Earth's atmosphere used to perform useful work.
Total atmospheric wind power is of the order of 10(sup 14) kW. Annual kinetic
energy is of the order of 10(sup 17) kWh [1 kWh = 3.60 x 10(sup 6) joules].
Practical land-based wind generators could extract as much as 10sup1sup4 kWh
of energy per year worldwide. Energy, and thus productivity of winds, varies
markedly with geographic location. Annual energy available to a conversion
machine at a site is very reproducible (PLUS/MINUS 15% variability).
    Most large-scale irrigation pumping in the United States consumes natural
gas, but wind-powered pump-back could expand the capacity of hundreds of
smaller hydroelectric installations. At least 20% of United States energy is
consumed in heating buildings. In colder climates, heating demand frequently
matches high winds. Wind-generated electricity feeding thermal storage units,
in some instances combined with solar thermal collectors feeding adjacent
storage units, offers excellent potential for reducing fuel oil consumption.
See  ELECTRIC POWER GENERATION.
    A windmill is a rotating machine capable of interchanging (extracting)
momentum with particles of air mass that flow through its swept area. Power
available in the wind in a swept area varies with the size of that area, the
density of the air, and the square of the velocity. Energy extractable from an
oncoming wind stream over a period of time varies as the size of swept area,
density, and cube of the velocity. Thus, the utility and competitiveness of a
wind power system depend upon the wind regime and height at which windmills
are placed, size and characteristics of the machine, nature of delivered
product, and productivity of that product.  See  ENERGY SOURCES; WIND.
 �34s[William Heronemus]
�33s

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�6v�2v�32sWind rose   �21m �33s 
    A diagram in which statistical information concerning the direction and 
speed of the wind at a particular location may be conveniently summarized. In 
the standard wind rose a line segment is drawn in each of perhaps eight 
compass directions from a common origin (see illustration). The length of a 
particular segment is proportional to the frequency with which winds blow from 
that direction. Parts of a given segment are given various thicknesses, 
indicating frequencies of occurrence of various classes of wind speed from the 
given direction.  See  WIND MEASUREMENT. 
 �34s[Frederick Sanders]
�33s 
  
  
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�6v�2v�32sWind-shift line   �21m �33s 
    A line along which the wind veers abruptly. In the Northern Hemisphere a 
shift from easterly or southeasterly winds to southerly or southwesterly is 
normally observed on passage of a warm front, while a shift from southerly or 
southwesterly to westerly or northerly winds is usually found on passage of a 
cold front, or a squall line. Passage of wind-shift lines is commonly 
accompanied by marked changes in general weather conditions. Lowest barometric 
pressure is generally found on passage of a cold-front type of wind-shift 
line.  See  FRONT; SQUALL LINE. 
 �34s[Chester W. Newton]
�33s 
  
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�6v�2v�32sWind stress   �21m �33s 
    The drag or tangential force per unit area exerted on the surface of the 
Earth by the adjacent layer of moving air. Erosion of ground surfaces and the 
production of waves on water surfaces are manifestations of wind stress. 
Surface wind stress determines the exchange of momentum between the Earth and 
the atmosphere and, together with internal atmospheric viscous stresses, 
exerts a strong influence on the typical variation of wind through the lowest 
few thousand feet of the atmosphere. For a discussion of the relation of wind 
stress to ocean currents and waves  see  OCEAN CURRENTS; OCEAN WAVES; WIND 
STRESS OVER SEA. 
    Significant stresses arise within the lower atmosphere because of the 
strong shear of the wind between the slowly moving air near the ground and the 
more rapidly moving air a few thousand feet above and because of the turbulent 
nature of the airflow in this region. Such turbulent eddies have 
characteristic dimensions ranging up to a few hundreds of feet. 
    Wind pressure is the force exerted by the wind per unit area of solid 
surface exposed normal to the wind direction. In contrast to shearing 
stresses, the wind pressure arises from the difference in pressure between the 
windward and lee sides of the exposed surface. Wind pressure thus represents a 
substantial force when the wind speed is high.  See  METEOROLOGY; WIND. 
 �34s[Frederick Sanders]
�33s 
  
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�6v�2v�32sWind stress over sea   �21m �33s 
    The drag or tangential force of the wind on the sea is expressed in units 
of dynes per square centimeter or micronewtons per square meter but is 
normally taken to represent the mean drag over an undefined area, perhaps 
several miles square, containing many waves. It is usually related to an 
appropriate time and space average of the wind near the sea surface (at 33 ft 
or 10 m above the mean level, for example). 
    Over most surfaces overland, in the absence of vertical density gradients, 
the drag varies as the square of the wind speed. Over a water surface a drag 
coefficient is defined in the same way, but since the surface geometry varies 
with the wind speed, the drag coefficient is to be regarded as a variable. The 
drag coefficient over the sea is an important quantity in both meteorology and 
oceanography since it relates the wind speed to the drag which generates ocean 
waves, drives the ocean currents, and sets the scale of the atmospheric 
turbulence that transfers water vapor and heat from the ocean to the 
atmosphere to provide the energy for clouds and weather systems.  See  
ATMOSPHERIC GENERAL CIRCULATION; OCEAN-ATMOSPHERE RELATIONS; OCEAN CURRENTS; 
OCEAN WAVES. 
 �34s[Henry Charnock]
�33s 
  
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�6v�2v�32sWind tunnel   �21m �33s 
    A duct in which the effects of airflow past objects can be determined. The 
steady-state forces on a body held still in moving air are the same as those 
when the body moves through still air, given the same body shape, speed, and 
air properties. Scaling laws permit the use of models rather than full-scale 
objects, such as aircraft or automobiles. Models are less costly and may be 
modified more easily, and conditions may be simulated in the wind tunnel that 
would be impossible or dangerous in full scale. 
    Most data are secured from wind tunnels through measurement of forces and 
moments, surface pressures, changes produced in the airstream by the model, 
local temperatures, and motions of dynamically scaled models, and by visual 
studies.  
    A balance system separates and measures the six components of the total 
force. The three forces taken parallel and perpendicular to a flight path are 
lift, drag, and side force. The three moments about these axes are yawing 
moment, rolling moment, and pitching moment, respectively. 
    Surface pressures are measured by connecting orifices flush with the model 
surface to pressure-measuring devices. Local air load, total surface load, 
moment about a control surface hinge line, boundary-layer characteristics, and 
local M may be obtained from pressure data. 
    Measurements of stream changes produced by the model may be interpreted in 
terms of forces and moments on the model. In two-dimensional tunnels, where an 
aircraft model spans the tunnel, it is possible to determine the lift and 
center of pressure by measuring the pressure changes on the floor and ceiling 
of the tunnel. The parasite drag of a wing section may be determined by 
measuring the total pressure of the air which has passed over the model and 
calculating its loss of momentum. 
    Measurements of surface temperatures indicate the rate of heat transfer or 
define the amount of cooling that may be necessary. 
    In elastically and dynamically scaled models used for flutter testing, 
measurements of amplitude and frequency of motion are made by using 
accelerometers and strain gages in the structure. In free-flight models, such 
as bomb or missile drop tests, data are frequently obtained photographically. 
    At low speeds, smoke and tufts are often used to show flow direction. A 
mixture of lampblack and kerosine painted on the model shows the surface 
streamlines. A suspension of talcum powder and a detergent in water is also 
used.  
    For aircraft at velocities near or above the speed of sound, some flow 
features may be made visible by optical devices.  See  INTERFEROMETRY; 
SCHLIEREN PHOTOGRAPHY; SHADOWGRAPH OF FLUID FLOW. 
    The V/STOL wind tunnel is a newer development of low-speed wind tunnels 
having a large very-low-speed section to permit testing of aircraft designed 
for vertical or short takeoff and landing (V/STOL) while operating in the 
region between vertical flight and cruising flight. 
 �34s[Robert G. Joppa]
�33s 
  
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�6v�2v�32sWine   �21m �33s 
    Alcoholic beverages made by fermentation of the juice of fruits or 
berries, essentially grape juice. Wines from other materials are always 
required to show their source on the label, for example, apple wine, berry 
wine, and cherry wine. California produces the largest percentage of the grape 
wine made in the United States, and well over a hundred varieties of the 
cultivated grape (Vitis vinifera) are used. 
    Classification of wine depends on the color, relative sweetness, alcoholic 
content, presence of carbon dioxide, the variety of grape, and the region 
where the grapes are grown. Wines may be red or white. The terms dry and sweet 
refer to the relative sugar content of a wine. Table wines contain less than 
14% alcohol by volume, and dessert wines contain over 14%, usually 20%, 
alcohol. The higher alcohol content of dessert wines is obtained by the 
addition of brandy, which is called fortification. Sparkling wines such as 
champagne and sparkling Burgundy contain carbon dioxide. 
    Grapes are harvested and go through a crusher-stemmer, which crushes the 
berries, but not the seeds, and removes the stems. The must, still containing 
seeds and skins, is pumped to fermentation tanks where it receives sulfur 
dioxide gas. This controls, to a large extent, the growth of bacteria and wild 
yeasts, whereas the various strains of wine yeast are adapted to this amount 
of SO2. A pure yeast starter is then added. In the production of white wines, 
the juice is separated from the skins and seeds at an early stage of 
fermentation.  See  FERMENTATION. 
    After the initial fermentation, the wine is transferred to the storage 
cellar for completion of fermentation, clarification, aging, stabilization, 
and bottling. These operations are called cellar practices. The sediment of 
yeast and other insoluble matter is called lees. The new wine is drawn off, or 
racked, from the lees to avoid picking up undesirable flavors from the lees. 
Aging is then continued. Wines are often chilled to precipitate excess cream 
of tartar, the potassium acid tartrate from the grape, which otherwise might 
precipitate upon chilling of bottled wine. Champagne is made by allowing a 
secondary fermentation to occur with a special flocculating type of yeast, 
either in bottles, by the original French procedure, or in bulk, using 
pressure tanks followed by bottling. The carbon dioxide, CO2, of sparkling 
wines is produced by yeast fermentation.  See  ALCOHOLIC BEVERAGES. 
 �34s[Emil M. Mrak ;Herman J. Phaff]
�33s 
  
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�6v�2v�32sWing �21m �33s
    The airfoil which is the principal source of an airplane's sustentation.
The lift generated by the motion of the wing through the air supports the
airplane. The main geometrical features of a wing are its span, that is, the
straight line distance from tip to tip; the area of the wing as seen in a plan
view of the airplane; and its sweep angle, the angle made by the wing leading
edge and a line perpendicular to the plane of symmetry of the airplane.
Another important geometrical characteristic is the wing section, which is the
shape of the wing when cut parallel to the plane of symmetry of the airplane.
See  AIRFOIL; AIRPLANE.
    The wing has several functions beyond that of providing lift. On larger
airplanes the engines are mounted in nacelles either attached to the wing or
mounted in the wing. The nacelles also provide a housing for the landing gear
when it is retracted. The space within the wing is usually used for fuel
storage.
    Many of the control functions of the airplane are provided by special
devices built into the wing. Most obvious are the ailerons, movable
trailing-edge flaps occupying the outer part of the wing. Inboard of the
ailerons and also on the trailing edge are the landing flaps, movable portions
deflected during landing to augment the lifting capability of the wing.
Through their use, the airplane can land more slowly, or the amount of fuel or
the number of passengers can be increased at a given landing speed. Some
airplanes have brakes to slow the airplane through a drag increase. A similar
device used only on one wing at any given time decreases the lift on that
wing. Acting like an aileron, it is used for roll control.  See  AILERON;
ELEVON.

    Loading.  Wing loading is a measure of the load carried by an airplane
wing per unit of wing area. Commonly used units are pounds per square foot and
kilograms per square meter. As a general rule, the lower-speed airplanes are
smaller, have lower wing loadings, and are more maneuverable.
 �34s[Joseph Bicknell]
�33s

    Structure.	In an aircraft, the combination of outside fairing panels that
provide the aerodynamic lifting surfaces and the inside supporting members
that transmit the lifting force to the fuselage is called the structure of a
wing. It is an integration of the environment external to the vehicle wing,
the aerodynamic shape of the wing, and the proposed use of the vehicle. The
interaction of these three aspects of design leads to the selection of
material, to the general structural layout, and, finally, to the detail choice
of structural shapes, material thickness, joints, and attachments. The result
is a structural framework covered with a metal skin that also contributes to
the load-carrying function.  See  SUPERCRITICAL WING.
    Wing structure has evolved from the early use of wood, doped canvas, and
wire. Today, aluminum alloy outer skins are prime structural elements on all
commercial transports and on the great majority of military craft. Magnesium,
steel, and titanium are also used in internal primary structure and in local
skin areas. The more exotic research craft, such as the X-15, are built at
least partly of nickel alloy. Even less conventional materials may be used for
reentry structures.
    The derivation of the net loads permits a quantitative consideration of
the general structural framework. Design of the structure, however, is
inseparable from the choice of material. The thickness and shape of the
individual pieces also affect strength in lightweight, efficient structures
because of compression stability modes.
    High static strength and light weight - coupled with reasonable rigidity
characteristics, good corrosion resistance, fatigue endurance, cost and
formability - are prime considerations in choosing airframe material. Highly
loaded, efficient airframes have found these qualities in aluminum. It does
appear, however, that the demands for speed interacting with the physical
environment will cause aluminum to be replaced by steel, refractories,
ceramics, and composites of these. A material's creep characteristics and
strength at temperature are now important added parameters.

    Prime structural framework.   An airframe wing is essentially two
cantilever beams joined together. Each wing tip is the free end of the
cantilever, and the center line of the vehicle represents the plane where the
two fixed ends of the cantilevers are joined. The prime load-carrying portion
of these cantilevers is a box beam made up usually of two or more vertical
webs, plus a major portion of the upper and lower skins of the wing, which
serve as chords of the beam. This box section also provides torsional strength
and rigidity (Fig. 1). Normally the prime box is designed to carry all the
primary structural loads.
    Leading and trailing edge portions of the wing, forward and aft of the
prime box respectively, help to provide the airfoil shape required. These
portions are designed to minimize their participation in the major
load-carrying function.
    At numerous places within the wing box, bulkhead-type structures called
ribs are located. These internal structures serve to maintain the rectangular
box shape and to cut down the unsupported length of compression cover
structures, to separate fuel tanks, and to distribute concentrated loads from
guns, bombs, landing gear, or engines into the prime box. They are also
located at any wing cross section where major load redistributions occur.

     Leading edge structure.  The leading edge, or most torward position of the
wing, serves an important aerodynamic function in establishing smooth airflow
and efficient lifting power for the wing.  This area sustains the highest
aerodynamic pressures.	On hypersonic vehicles, it is also the hot spot of the
design.  Structurally, the leading edge is an appendage, a fairing whose local
loads must be supported by the prime wing box.	This structure takes the form of
cantilever beams or archtype structures from the leading edge back to the front
vertical beam.	Frequently the leading edge of an aircraft incorporates
anti-icing provisions.
     Numerous aircraft have leading edge slats.  These small airfoil sections
are pulled out and forward automatically by air pressure and serve to maintain
smooth airflow to a higher angle of attack than the basic large airfoil would
sustain.  As the angle of attach is decreased, these devices are withdrawn into
the basic airfoil by springs.

     Tailing edge structures.  The trailing edge structure is noteworthy for its
various auxiliary devices which assist in aircraft control or in reducing
aircraft landing speeds.  Various forms of flaps, ailerons, and spoilers, with
their hydraulic or electrical control mechanisms, fill the volume of space aft
of the rear beam or spar (Fig.2).
     Landing flaps consist of a movable airfoil-shaped structure located aft of
the rear beam or spar.	They extend about two-thirds of the span of the wing.
Their aerodynamic function is to increase substantially the lift, thereby
permitting lower takeoff and landing speeds.

      Ailerons are located near the tips of the wings in the trailing
edge. Their prime aerodynamic function is roll control of the aircraft.
Structurally, the aileron is similar to the landing flaps.
    Spoilers are a special form of control surface and perform the aerodynamic
function of the aileron, that is, roll control. They are located on the upper
surface of the trailing edge at about midsemispan and deflect only upward by
pilot control.

    Wing-to-fuselage structure.   The structural heart of the aircraft is the
wing-to-fuselage joint. This connection is usually the most complex in form
and in analysis. Two major structural elements, the wing and the fuselage,
with major loads running at right angles to one another, must be joined and
analyzed for consistent deformations. The wing structures which have been
described are attached to heavy aluminum ring frames in the fuselage; these
rings distribute the wing loads to the fuselage skins.	See  AIRFRAME.
 �34s[Harvey J. Hoge]
�33s


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�6v�2v�32sWire   �21m �33s 
    A thread or slender rod of metal. Wire is usually circular in cross 
section and is flexible. If it is of such a diameter or composition that it is 
fairly stiff, it is termed rod. The wire may be of several small twisted or 
woven strands, but if used for lifting or in a structure, it is classed as 
cable. Wire may be used structurally in tension, as in a suspension bridge, or 
as an electrical conductor, as in a power line. The working of metal into wire 
greatly increases its tensile strength. Thus, a cable of stranded 
small-diameter wires is stronger as well as more flexible than a corresponding 
solid rod.  See  MAGNET WIRE. 
 �34s[Frank H. Rockett]
�33s 
  
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�6v�2v�32sWiring �21m �33s
    A system of electric conductors, components, and apparatus for conveying
electric power from source to the point of use. In general, electric wiring
for light and power must convey energy safely and reliably with low power
losses, and must deliver it to the point of use in adequate quantity at rated
voltage. Electric wiring systems are designed to provide a practically
constant voltage to the load within the capacity limits of the system. There
are a few exceptions, notably series street-lighting circuits which operate at
constant current. The building wiring system originates at a source of
electric power, conventionally the distribution lines or network of an
electric utility system.  See  ELECTRICAL CODES.

    Systems and service.   Wiring systems are generally three-phase to conform
to the supply systems. Energy is transformed to the desired voltage levels by
a bank of three single-phase transformers. The transformers may be connected
in either a delta or Y connection.
    Service provided at the primary voltage of the utility distribution
system, typically 13,800 or 4160 volts, is termed primary service. Service
provided at secondary or utilization voltage, typically 120/208 or 277/480
volts, is called secondary service.
    Service at primary voltage levels is often provided for large industrial,
commercial, and institutional buildings, where the higher voltage can be used
to advantage for power distribution within the buildings. Where primary
service is provided, power is distributed at primary voltage from the main
switchboard through feeders to load-center substations installed at
appropriate locations throughout the building.
    Most secondary services in the United States are 120/208 volts,
three-phase, four-wire, or 120/240 volts, single-phase, three-wire serving
both light and power. For relatively large buildings where the loads are
predominantly fluorescent lighting and power (as for air conditioning), the
service is often 277/480 volts, three-phase, four-wire, supplying 480 volts
for power and 277 volts, phase-to-neutral, for the lighting fixtures.
    From the service entrance, power is carried in feeders to the main
switchboard, then to distribution panelboards. Smaller feeders extend from the
distribution panelboards to light and power panelboards. Branch circuits then
carry power to the outlets serving the various lighting fixtures, plug
receptacles, motors, or other utilization devices.

    Methods.   Methods of wiring in common use for light and power circuits
are as follows: (1) insulated wires and cables in raceways; (2) nonmetallic
sheathed cables; (3) metallic armored cables; (4) busways; (5)
copper-jacketed, mineralinsulated cables; (6) aluminum-sheathed cables; (7)
nonmetallic sheathed and armored cables in cable support systems; and (8) open
insulated wiring on solid insulators (knob and tube).
    The selection of the wiring method or methods is governed by a variety of
considerations, which usually include code rules limiting the use of certain
types of wiring materials; suitability for structural and environmental
conditions; installation (exposed or concealed); accessibility for changes and
alterations; and costs.

    Circuit design.   The design of a particular wiring system is developed by
considering the various loads, establishing the branch-circuit and feeder
requirements, and then determining the service-entrance requirements. Outlets
for lighting fixtures, motors, portable appliances, and other utilization
devices are indicated on the building plans and the load requirement of each
outlet noted in watts or horsepower. Lighting fixtures and plug receptacles
are then grouped on branch circuits and connections to the lighting panelboard
indicated.
    Lighting branch circuits may be loaded to 80% of circuit capacity.
However, there is a reasonable probability that the lighting equipment will be
replaced at some future time by equipment of higher output and greater load.
Therefore, in modern practice, lighting branch circuits are loaded only to
about 50% capacity. Lighting branch circuits are usually rated at 20 A.
Smaller 15-A branch circuits are used mostly in residences.
 �34s[William T. Stuart /Joseph F. McPartland]
�33s

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�6v�2v�32sWiring diagram   �21m �33s 
    A drawing illustrating electrical and mechanical relationships between 
parts on a component between which electrical wiring must be connected. 
    A wiring diagram is distinguished from an electrical schematic in that the 
arrangement of the schematic bears no necessary relationship to the mechanical 
arrangement of the electrical elements in the component. The wiring diagram 
provides an accurate picture of how the wiring on the components and between 
components should appear in order that the electrical wiring technician can 
install the wiring in the manner that will best contribute to the optimum 
performance of the device. 
    Wiring diagrams also include such information as type of wire, color 
coding, methods of wire termination, and methods of wire and cable clamping.  
See  SCHEMATIC DRAWING. 
 �34s[Robert W. Mann]
�33s 
  
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�6v�2v�32sWitherite �21m �33s
    The mineral form of barium carbonate. Crystals may appear hexagonal in
outline. It may be white or gray with yellow, brown, or green tints. Its
hardness is 3 1/4 and its specific gravity 4.3. Witherite is found in many
places in Europe, and large crystals occur at Rosiclare, Illinois.  See 
BARIUM; CARBONATE MINERALS. 
 �34s[Robert I. Harker]
�33s

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�6v�2v�32sWolframite   �21m �33s 
    A mineral with chemical composition (Fe,Mn)-WO4, intermediate between 
ferberite and huebnerite, which form a complete solid solution series. 
Wolframite occurs commonly in short, brownish-black, monoclinic, prismatic, 
bladed crystals. It is probably the most important tungsten mineral. China is 
the major producer of wolframite. Tungsten minerals of the wolframite series 
occur in many areas of the western United States; the major producing district 
is Boulder and northern Gilpin counties in Colorado.  See  FERBERITE; 
HUEBNERITE.  
 �34s[Edward C. T. Chao]
�33s 
  
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�6v�2v�32sWolf-Rayet star �21m �33s
    A member of a class of very hot stars (100,000 - 35,000 K or 180,000 -
63,000DGF) which characteristically show broad, bright emission lines in their
spectra. Wolf-Rayet stars are classified into two main groups, the WN
(nitrogen) and the WC (carbon) stars. The emission lines show very high
excitation [lines of He(sup +) and even up to O(sup 5+) are strong] and are
broadened by an unexplained mechanism. Luminosities must be very high, in the
range of 10(sup 4) - 10(sup 5) times that of the Sun. These stars are probably
very young and represent an unstable and short-lived early stage in stellar
evolution. Many are close binaries with two stars nearly in contact.  See
STAR; STELLAR EVOLUTION.
 �34s[Jesse L. Greenstein]
�33s

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�6v�2v�32sWollastonite �21m �33s
    A mineral inosilicate with composition CaSiO3. Commonly it is massive, or
in cleavable to fibrous aggregates.  Hardness is 5 - 5 1/2 on Mohs scale;
specific gravity is 2.85.  On the cleavages the luster is pearly or silky; the
color is white to gray.  Wollastonite is found in large masses in the Black
Forest of Germany; Brittany, France; Chiapas, Mexico; and Willsboro, New York,
where it is mined as a ceramic material.  See SILICATE MINERALS.
 �34s[C. S. Hammen]
�33s

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�6v�2v�32sWolverine   �21m �33s 
    A carnivorous mammal, Gulo gulo, also known as the glutton, which is the 
largest and most vicious member of the family Mustelidae, to which the 
ferrets, minks, martens, weasels, badgers, otters, and skunks belong. It 
ranges throughout northern Europe, Asia, and North America, where it is found 
in coniferous forests during the winter and on the tundras during summer. 
    This animal is about 40 in. (100 cm) long and weighs about 40 lb (18 kg) 
(see illustration). It feeds primarily on mice, lemmings, and birds but will 
attack large animals when they are sick or trapped and exhausted in deep snow. 
It is a solitary animal, and the male remains with the female only during the 
courtship period, after which the female is left to rear the cubs. The 
wolverine lives in a den or burrow and does not hibernate.  See  BADGER; 
CARNIVORA; FISHER; MARTEN; OTTER; SKUNK; WEASEL. 
 �34s[Charles B. Curtin]
�33s 
  
  
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�6v�2v�32sWood chemicals   �21m �33s 
    Substances derived from wood. Woody plants comprise the greatest part of 
the organic materials produced by photosynthesis on a renewable basis, and 
were the precursors of the fossil coal deposits. The derivation of chemicals 
from wood is carried out wherever technical utility and economic conditions 
have combined to make it feasible. 
    Wood is a mixture of three natural polymers - cellulose, hemicelluloses, 
and lignin - in an approximate abundance of 50:25:25. In addition to these 
polymeric cell wall components which make up the major portion of the wood, 
different species contain varying amounts and kinds of extraneous materials 
called extractives. The nature of the chemicals derived from wood depends on 
the wood component involved.  See  CELLULOSE; HEMICELLULOSE. 
    Chemicals derived from wood include: bark products, cellulose, cellulose 
esters, cellulose ethers, charcoal, dimethyl sulfoxide, ethyl alcohol, fatty 
acids, furfural, hemicellulose extracts, kraft lignin, lignin sulfonates, pine 
oil, rayons, rosin, sugars, tall oil, turpentine, and vanillin. Most of these 
are either direct products or by-products of wood pulping, in which the lignin 
that cements the wood fibers together and stiffens them is dissolved away from 
the cellulose. High-purity chemical cellulose or dissolving pulp is the 
starting material for such polymeric cellulose derivatives as viscose rayon 
and cellophane, cellulose esters such as the acetate and butyrate for fiber, 
film, and molding applications, and cellulose ethers such as 
carboxymethylcellulose, ethylcellulose, and hydroxyethylcellulose for use as 
gums.  See  CELLOPHANE; DIMETHYL SULFOXIDE; ETHYL ALCOHOL; FURFURAL; ROSIN; 
SYNTHETIC FIBER; TALL OIL; TURPENTINE; VANILLA; WAX, ANIMAL AND VEGETABLE; 
WOOD PRODUCTS. 
 �34s[Irving S. Goldstein]
�33s 
  
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�6v�2v�32sWood decay   �21m �33s 
    A complex series of orderly events that involve the living tree, chemical 
reactions, bacteria, yeasts, non-decay-causing fungi, and decay-causing fungi. 
The tree responds to infection in an orderly way by compartmentalizing the 
infected wood. Many microorganisms are involved in the infection process. 
Survival of a tree after wounding depends on rapid and effective 
compartmentalization of the infection. Survival of wood-inhabiting 
microorganisms after the tree responds to the injury depends on successions. 
    After a living tree is wounded, it responds in an orderly way since it is 
a highly compartmented plant, with wood cells defining the compartments. Wood 
that is injured is never repaired or replaced; in this sense, trees do not 
heal wounds. Trees thus survive after wounding by walling off, confining, or 
compartmentalizing the injured and infected wood. The compartmentalization 
model resolves the apparent paradox that trees can survive only so long as 
wood-inhabiting microorganisms do not attack, and that wood-inhabiting 
microorganisms can survive only as long as they attack trees. 
    There are two basic types of wounds: outer core, where the wound breaks 
into the wood through the bark, and branch stubs, where the interior of the 
tree may be exposed. Wounds to the outer core expose the living and dead cells 
in the wood to the air. The sapwood of trees contains a great number of living 
cells and most of it, on a number-of-cells basis, is alive. However, on a 
volume basis, most of the sapwood is dead because the fibers and vessels or 
tracheids are so very large compared to the small living cells of the ray 
parenchyma and the marginal parenchyma (the parenchyma that often forms at the 
end of a growth ring). It is obvious, therefore, that wounds break into or 
injure the living cells in the wood, killing some immediately, while others 
are injured and die slowly, and still others, farther from the injury, remain 
alive.  
    The wound initiates chemical defense reactions in the living cells near 
the injury. For one thing, phenol-based substances in the living cells are 
oxidized to form materials that inhibit many types of microorganisms. Wounds 
disrupt the normal physiological processes of the cells, and biochemical 
shunts occur. The tree begins to form a chemical protective zone around the 
injury. A problem for the tree centers around the many large dead cells - the 
vessels (in angiosperms) or tracheids (in gymnosperms) - that cannot respond. 
To counter this, one of the early events in the defense reaction is the 
plugging of these cells. Plugging serves to stall the vertical spread of the 
microorganisms. Other changes in the living cells serve to limit the inward 
and lateral spread of infection. 
 �34s[Alex L. Shigo]
�33s 
  
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�6v�2v�32sWood finishing �21m �33s
    Any operation performed on wood surfaces aimed at attaining protection
against mechanical, physical, and chemical influences or to enhance the
natural beauty of the wood. Unfinished wood is dimensionally unstable under
climatic influences of varying atmospheric moisture and temperature, and is
easily marred and soiled.

    Objectives.   The main objectives of wood finishing are: (1) stabilization
- the protective film is not a perfect barrier against moisture, but the rate
of moisture exchange into the wood can be retarded; (2) esthetic quality - the
finish should enhance the natural beauty of luster, grain pattern, and color;
and (3) sanitation - finishing is intended to close the porous wood surface to
reduce contamination by handling, atmospheric grime, and food, and to render
it more easily cleaned. The chief deteriorating factors against which wood
surfaces need to be protected are weathering and dimensional fluctuations due
to environmental conditions such as moisture, irradiation, extreme temperature
changes, mechanical erosion, or the combined action of these. About 1/4 in. (6
mm) of wood wears away from the exposed surface in a century.  See  WOOD
PHYSICS.
    Even under indoor conditions, dimensional changes can be considerable as a
result of high or low humidity and heating or cooling. Wood swells and shrinks
as the moisture content and temperature of the surrounding atmosphere change.
The more efficient coatings retard moisture movement sufficiently so that
under conditions of frequently changing humidity, swelling and shrinking are
reduced to tolerable limits.
    Stabilizing treatments usually have two aims: to decrease the rate of
swelling and shrinking by minimizing the exchange of moisture into or out of
the cell wall structure of wood, and to reduce the final equilibrium swelling
and shrinking that can take place. Surface coatings fall under the first
category.

    Kinds of wood finishes.  Wood finishes can be grouped in two main classes:
exterior and interior. Exterior finishes are expected to resist chiefly
climatic influences, sometimes combined with mechanical abuse, as in outdoor
furniture. Interior finishes are expected to fill such exacting standards of
appearance and quality as sheen, luster, gloss, or chemical and abrasion
resistance. Water resistance is also required in exposed interior locations,
such as kitchens, bathrooms, and verandas.
    Exterior finishes include: (1) transparent natural finishes showing wood
texture, which are further classified as (a) surface coatings (transparent
finishes consisting of varnishes or synthetics), and (b) penetrating finishes
including stains, oils, sealers, and water repellents or fungicides; (2)
pigmented covering paints, applied on a priming coat containing white lead but
no zinc oxide and having a total pigment the same as the finish coat. These
covering paints must contain sufficient thinner (solvents) to wet wood quickly
and adequate binders (bodied oils and resins) to prevent undue penetration of
the vehicle into the wood.
    Interior finishes include: (1) transparent penetrating finishes consisting
of stains in combination with water-repellent finishes based on waxes and
other nonvolatile ingredients; (2) transparent film-forming coating based on
shellac, nitrocellulose lacquers, or oleoresinous varnishes in combination
with stains, fillers, and sealers; and (3) pigmented covering paints (enamels)
used in combination with a priming coat.
 �34s[Joseph E. Marian]
�33s

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�6v�2v�32sWoodpecker   �21m �33s 
    A bird of the order Piciformes. Woodpeckers belong to the family Picidae, 
which is subdivided into three distinct subfamilies: the Picinae or true 
woodpeckers, the Picumninae or piculets, and the Jynginae or wrynecks. There 
are about 209 species of woodpeckers, 20 of which occur in the United States. 
    Most woodpeckers are solitary, and very few species migrate. During the 
breeding season they pair and remain together during the period of nesting, 
incubation, and caring for the young. The unlined nest is frequently a hollow 
in a tree or, in nonarboreal species, in banks of earth. The female lays from 
two to eight eggs which are incubated by both parents for a period of 11 - 17 
days, depending upon the species. The young are blind at hatching; they are 
able to leave the nest and climb before they can fly.  See  PICIFORMES. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sWood physics �21m �33s
    That area of wood science concerned with the physical and mechanical
properties of wood and the factors which affect them. Since wood consists of
aggregates of long tubular cells, most of which are oriented longitudinally in
the tree, whereas the ray cells are oriented radially, wood is an orthotropic
material and exhibits different physical behavior in the three main structural
directions, longitudinal, radial, and tangential. The orientation of most of
the cellulose crystallites is also longitudinal.  See  CELLULOSE; XYLEM.
    The cell-wall density for most woods is essentially the same, that is,
about 93.6 lb/ft(sup 3) [1.5 g/cm(sup 3)] in the oven-dry condition and about
87.4 lb/ft(sup 3) [1.4 g/cm(sup 3)] at the fiber saturation point. The main
factor governing the density of a given wood sample is the ratio of cell-wall to
cell-cavity volume.  See CELL WALLS (PLANT).
    Wood under normal atmospheric conditions contains water which can be
removed by drying. The total moisture content is usually defined as the ratio
of the weight of water lost, by drying in an oven at 212 - 221DGF (100 -
105DGC), to the oven-dry weight of the wood, expressed as a percentage.
Greenwood normally contains water in the cell cavities as well as in the cell
walls. The cavity, or free, water can be removed by exposure to relative
humidities only slightly lower than 100%, or about 99.9%. The cell-wall, or
bound, water is hygroscopic and remains in the wood unless exposed to
humidities lower than 100%. The hygroscopic water attains moisture equilibrium
with the environment. It also varies with type of wood, extractive content,
and exposure history.
    The dimensional change associated with loss or gain of bound water is
defined respectively as shrinking or swelling.
    Owing to the uncertain behavior of the cell cavities, volumetric shrinkage
of the gross wood cannot be predicted accurately. On the average, the cavities
remain essentially constant in size, as a result of the restraining effects of
the inner and outer layers of the secondary cell wall. Longitudinal shrinkage
in normal mature wood is almost negligible. Tangential shrinkage is about
twice radial shrinkage.
    Dry wood is an excellent insulator, but its electrical characteristics
change markedly with increasing moisture content and temperature. The dc
resistivity decreases almost exponentially with increasing moisture content up
to the fiber saturation point, above which there is relatively little change.
The dielectric properties of wood in an ac electric field are complex. The
dielectric constant and loss factor increase with moisture content and density
at almost all frequencies.
    Mechanical properties of wood relate to its resistance to deformation by
applied forces. Wood is a polymeric material, and therefore its deformation
response to external forces is time-dependent or rheological. Grain direction,
density, moisture content, temperature, and defects also affect the mechanical
behavior of wood. All strength properties increase with wood density at a
given moisture content. However, the relative effect of density on strength
across the grain is much more pronounced. Temperature affects wood strength in
two ways: the immediate or reversible effect, and the permanent or
irreversible effect due to thermal degradation. The immediate effect of
temperature causes a decrease in strength with increasing temperature. The
permanent effect of temperature is caused by thermal degradation of wood
exposed to high temperatures, resulting in weight and strength loss when the
wood is returned to standard conditions.
 �34s[Christen Skaar]
�33s

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�6v�2v�32sWood preservation   �21m �33s 
    The technique of protecting wood, through chemical treatment, from fire 
and from deterioration by the biological agencies of fungi, insects, and 
marine borers. Wood preservation increases the useful life of wood and reduces 
replacement costs. Properly designed wood structures give long service without 
special protection, but a large economic loss may result when wood in its 
natural state is used at high temperatures, in structures exposed to salt 
water, or under climatic conditions that favor the development of harmful 
fungi and insects.  See  WOOD DECAY. 
    Wood preservatives in general use are oils, including oil-borne and 
water-borne chemicals. Oils are mostly for outdoor and wet exposure 
conditions. They do not swell the wood but may contribute to staining and 
painting difficulties. Coal tar creosote, solutions of creosote with either 
coal tar or petroleum oil, and 5% of pentachlorophenol in petroleum oil are 
used for the treatment of products such as ties, posts, poles, piling, and 
construction timbers. Solutions of pentachlorophenol and copper naphthenate 
and other oil-borne chemicals are often sold under proprietary (trade) names 
for special-purpose treatments of wood. These may contain water repellents 
when used for such items as millwork. Solvents used with oil-borne 
preservatives vary from heavy fuel oils to mineral spirits, depending on use 
requirements.  
    Water-borne preservatives are usually lower in cost than oils and less 
likely to involve odor and painting problems. They are used principally for 
less severe exposure conditions, but several are highly resistant to leaching 
and perform as well as preservative oils in contact with the soil or fresh 
water. Water in the treating solutions causes swelling of the wood, however, 
and redrying to levels meeting use requirements is necessary. 
 �34s[J. Oscar Blew, Jr.]
�33s 
  
A700000002275�254s�5v�1v1890
\IMG\CI1801.IMG
12385280
1891
\IMG\CI1801.IMG
12521984
�6v�2v�32sWood products   �21m �33s 
    Materials developed from use of the hard fibrous substance (wood) which 
makes up the greater part of the trunks and limbs of trees. 
    Solid wood products include lumber, veneer and plywood, furniture, poles, 
piling, mine timbers, and posts. It also includes composite wood products such 
as laminated timbers, insulation board, hardboard, and particle board.  See  
LUMBER MANUFACTURE; PLYWOOD. 
    Fiber wood products can be referred to as those which develop initially 
from the various processes for pulping wood. All are intended to separate the 
cellulose fibers one from another in relatively pure form to be recombined 
into layers of pulp, paper sheets, or paperboards.  See  PAPER. 
    Chemical wood products result from the chemical modification or conversion 
of cellulose, lignin, and extractives. Chief among these products are textile 
fibers such as rayon, and many cellulose plastics products such as cellophane, 
nitrocellulose, photographic film, telephone parts, and plastic housewares and 
toys.  See  CELLULOSE; TEXTILE; WOOD CHEMICALS. 
 �34s[Willard S. Bromley]
�33s 
  
A700000001457�254s�5v�1v1891
\IMG\CI1801.IMG
12521984
�6v�2v�32sWoodward-Hoffmann rule   �21m �33s 
    A concept which can predict or explain the stereochemistry of certain 
types of reactions in organic chemistry. It is also described as the 
conservation of orbital symmetry, and is named for its developers, R. B. 
Woodward and Roald Hoffmann. The rule applies to a limited group of reactions, 
called pericyclic, which are characterized by being more or less concerted 
(that is, one-step, without a distinct intermediate between reactants and 
products) and having a cyclic arrangement of the reacting atoms of the 
molecule in the transition state. Most pericyclic reactions fall into one of 
three major classes: electrocyclic, cycloaddition, or sigmatropic.  See  
STEREOCHEMISTRY.  
 �34s[David L. Dalrymple]
�33s 
  
A700000001136�254s�5v�1v1891
\IMG\CI1801.IMG
12521984
�6v�2v�32sWoodworking   �21m �33s 
    The shaping and assembling of wood and wood products into finished 
articles such as mold patterns, furniture, window sashes and frames, and 
boats. The pronounced grain of wood requires modifications in the working 
techniques when cutting with the grain and when cutting across it. Five 
principal woodworking operations are sawing, planing, steam bending, gluing, 
and finishing. To shape round pieces, wood is worked on a lathe.  See  LATHE; 
SAWING; TURNING (WOODWORKING); WOOD FINISHING. 
 �34s[Alan H. Tuttle]
�33s 
  
A700000003531�254s�5v�1v1891
\IMG\CI1801.IMG
12521984
�6v�2v�32sWool   �21m �33s 
    Wool provides warmth and physical comfort that cotton and linen fabrics 
cannot give. These qualities, combined with its soft resiliency, make wool a 
necessity for apparel and for rugs and blankets. 
    Sheep are generally shorn of their fleeces in the spring, but the time of 
shearing varies in different parts of the world. Each fleece contains 
different grades, or sorts, of wool, and the raw stock must be carefully 
graded and segregated according to length, diameter, and quality of fiber. 
Wool from different parts of the body of the lamb differs greatly. The 
shoulders and sides generally yield the best quality of wool, because the 
fibers from those parts are longer, softer, and finer. 
    Wool's major physical characteristics include fiber diameter (fineness or 
grade), staple length, and clean wool yield. Also significant are soundness, 
color, luster, and content of vegetable matter. Grade refers specifically to 
mean fiber diameter and its variability. Fiber diameter is the most important 
manufacturing characteristic. Fleeces are commercially graded visually through 
observation and handling by men of long experience in the industry. Degree of 
crimp and relative softness of the fleece are important deciding factors 
employed by the graders. 
    The inherent advantages of wool have been exploited, and its limitations 
as a textile fiber have been overcome by the application of technology to 
manufacturing processes. The use of the insecticide dieldrin as a dye renders 
wool mothproof for life. Permanent pleats have been imparted to garments which 
are shrinkproofed and can be home laundered. Each such technological advance 
enables wool to hold its competitive place in the field of textile manufacture 
and use.  See  TEXTILE.   
 �34s[Thomas D. Watkins]
�33s 
  
A700000003616�254s�5v�1v1891
\IMG\CI1801.IMG
12521984
�6v�2v�32sWord processing   �21m �33s 
    Word processing is a term commonly used to describe a system for 
accomplishing office work through people, procedures, and technologically 
advanced equipment. Originally limited to improved production of written 
communications generated from dictation in an office, word processing has 
expanded to encompass all business communications generated by an 
organization, including the entire spectrum of office work. The term fails to 
reflect this comprehensive expansion into the storage, retrieval, 
manipulation, and distribution of information and, most recently, a growing 
interdependence with electronic data processing. As a result, new descriptive 
terms such as information processing, administrative systems, and office 
systems are being used. 
    Word processing represents a further stage in modern society's application 
of automation, reaching beyond manufacturing and production lines into the 
office. ``Automatic typewriters'' - the generic predecessors of multifunction 
information processors - were the starting point. Now, a wide range of 
advanced equipment is available to electronically record keystrokes in 
electronic memories and on a variety of magnetic storage media - tape, card, 
cassette, or diskette - and electronically assist in correcting, editing, 
revising, and manipulating text material. Retyping is limited to changes and 
corrections; perfect copy is automatically produced by pressing a button. Once 
stored, words, sentences, and paragraphs can be automatically rearranged, 
reassembled, and retyped repeatedly, and customized versions produced. 
Distribution of the finished work has also been affected. The effect in the 
office has become analogous to automation in the factory: higher production, 
lower costs, higher quality, and more efficient use of people. 
 �34s[Edward W. Gore, Jr.]
�33s 
  
A700000005332�254s�5v�1v1891
\IMG\CI1801.IMG
12521984
1892
\IMG\CI1801.IMG
12644864
�6v�2v�32sWork �21m �33s
    In physics, the term work refers to the transference of energy that occurs
when a force is applied to a body that is moving in such a way that the force
has a component in the direction of the body's motion. Thus work is done on a
weight that is being lifted, or on a spring that is being stretched or
compressed, or on a gas that is undergoing compression in a cylinder.
    When the force acting on a moving body is constant in magnitude and
direction, the amount of work done is defined as the product of just two
factors: the component of the force in the direction of motion, and the
distance moved by the point of application of the force. Thus the defining

			   W = f cos PHI PRODUCT s

equation for work W is given below, where f and s are the magnitudes of the
force and displacement, respectively, and PHI is the angle between these two
vector quantities (see illustration). Because f cos PHI PRODUCT s = f PRODUCT
s cos PHI, work may be defined alternatively as the product of the force and
the component of the displacement in the direction of the force.
    The work done is positive in sign whenever the force or any component of
it is in the same direction as the displacement; one then says that work is
being done by the agent exerting the force and on the moving body. The work is
said to be negative whenever the direction of the force or force component is
opposite to that of the displacement; then work is said to be done on the
agent and by the moving body. From the point of view of energy, an agent doing
positive work is losing energy to the body on which the work is done, and one
doing negative work is gaining energy from that body.
    The work principle, which is a generalization from experiments on many
types of machines, asserts that, during any given time, the work of the forces
applied to the machine is equal to the work of the forces resisting the motion
of the machine, whether these resisting forces arise from gravity, friction,
molecular interactions, or inertia.
    The work done by any conservative force, such as a gravitational, elastic,
or electrostatic force, during a displacement of a body from one point to
another has the important property of being path-independent: Its value
depends only on the initial and final positions of the body, not upon the path
traversed between these two positions. On the other hand, the work done by any
nonconservative force, such as friction due to air, depends on the path
followed and not alone on the initial and final positions, for the direction
of such a force varies with the path, being at every point of the path
tangential to it.  See	ENERGY; FORCE.
 �34s[Leo Nedelsky]
�33s


A700000002512�254s�5v�1v1892
\IMG\CI1801.IMG
12644864
�6v�2v�32sWork function (electronics) �21m �33s
    A quantity with the dimensions of energy which determines the thermionic
emission of a solid at a given temperature. For metals, the work function may
also be determined by measuring the photoemission as a function of the
frequency of the incident electromagnetic radiation: the work function is then
equal to the minimum (threshold) frequency for which electon emission is
observed times Planck's constant h [= 6.63 x 10(sup -34) joule sec]. The work
function of a solid is usually expressed in electronvolts.
    The work function of metals varies from one crystal plane to another and
also varies slightly with temperature. For a metal, the work function has a
simple interpretation. At absolute zero, the energy of the most energetic
electrons in a metal is referred to as the Fermi energy; the work function of
a metal is then equal to the energy required to raise an electron with the
Fermi energy to the energy level corresponding to an electron at rest in
vacuum. The work function of a semiconductor or an insulator has the same
interpretation, but in these materials the Fermi level is in general not
occupied by electrons and thus has a more abstract meaning.  See  FIELD
EMISSION; PHOTOEMISSION; THERMIONIC EMISSION.
 �34s[Adrianus J. Dekker]
�33s

A700000001556�254s�5v�1v1892
\IMG\CI1801.IMG
12644864
�6v�2v�32sWork function (thermodynamics) �21m �33s
    A thermodynamic function, also called the work content, Helmholtz free
energy, or by the European school, simply the free energy. It is defined as
the internal energy E of a system minus the temperature-entropy product, TS,
and has a characteristic value for each state of a system. In an isothermal
process, the maximum work which can be done by a system is equal to the
decrease in its work function. When a process such as a chemical reaction
occurs spontaneously at constant temperature and volume, it is characterized
by a decrease in the work function. Consequently, the criterion for
equilibrium under these conditions is that the work function for the system
should be at a minimum.  See  CHEMICAL THERMODYNAMICS; FREE ENERGY.
 �34s[Paul J. Bender ;William A. Steele]
�33s

A700000004354�254s�5v�1v1892
\IMG\CI1801.IMG
12644864
�6v�2v�32sWork measurement   �21m �33s 
    The determination of a set of parameters associated with a task. There are 
four reasons, common to most organizations whether profit seeking or not, why 
time, effort, and money are spent to measure the amount of time a job takes. 
These are cost accounting, evaluation of alternatives, acceptable day's work, 
and scheduling. The fifth, pay by results, is used only by a minority of 
organizations.  
    There are three common ways to determine time per job: stopwatch time 
study (sequential observations), occurrence sampling (nonsequential 
observations), and standard data. 
    Stopwatch time study can be used for almost any existing job. It is 
reasonable in cost and gives reasonable accuracy. However, it does require the 
worker to be rated. Once the initial cost of standard data system has been 
incurred, standard data may be the lowest-cost, most accurate, and most 
accepted technique. 
    Occurrence sampling is also called work sampling or ratio-delay sampling. 
If time study is a ``movie,'' then occurrence sampling is a ``series of 
snapshots.'' The primary advantage of this approach may be that occurrence 
sampling standards are obtained from data gathered over a relatively long time 
period, so the sample is likely to be representative of the universe. That is, 
the time from the study is likely to be representative of the long-run 
performance of the worker. 
    Reuse of previous times (standard data) is an alternative to measuring new 
times for an operation. There are three levels of detail: micro, elemental, 
and macro (see table). Micro-level systems have times of the smallest 
component ranging from about .01 to 1 s. Components usually come from a 
predetermined time system such as methods-time-measurement (MTM) or 
Work-Factor. Elemental level systems have the time of the smallest component, 
ranging from about 1 to 1000 s. Components come from time study or micro-level 
combinations. Macro-level systems have times ranging upward from about 1000 s. 
Components come from elemental-level combinations, from time studies, and from 
occurrence sampling.  See  METHODS ENGINEERING; PERFORMANCE RATING; 
PRODUCTIVITY.  
 �34s[Stephan A. Konz]
�33s 
  
A700000003527�254s�5v�1v1892
\IMG\CI1801.IMG
12644864
1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWork standardization   �21m �33s 
    The establishment of uniformity of technical procedures, administrative 
procedures, working conditions, tools, equipment, workplace arrangements, 
operation and motion sequences, materials, quality requirements, and similar 
factors which affect the performance of work. It involves the concepts of 
design standardization applied to the performance of jobs or operations in 
industry or business.  See  DESIGN STANDARDS; WORK MEASUREMENT. 
    Work standardization is part of methods engineering and, where it is 
practiced, usually precedes the setting of time standards. The objectives of 
work standardizations are lower costs, greater productivity, improved quality 
of workmanship, greater safety, and quicker and better development of skills 
among workers.  See  METHODS ENGINEERING; PRODUCTIVITY. 
    One of the best known of the more formal techniques of work 
standardization is group technology. This is the careful description of a 
heterogeneous lot of machine or other piece parts with a view to discovering 
as many common features in materials and dimensions as can be identified. It 
is then possible to start a rather large lot of a basic part through the 
production process, doing the common operations on all of them. Any changes or 
additional operations required to produce the final different parts can then 
be made at a later stage. The economy is realized in being able to do the 
identical jobs at one time. 
    There has been considerable progress in computerized systems to facilitate 
group technology. The techniques are closely linked to computer-aided design 
and to formalized codes that permit the detailed description of many 
operations.  See  COMPUTER-AIDED DESIGN AND MANUFACTURING. 
 �34s[John E. Ullmann]
�33s 
  
A700000001170�254s�5v�1v1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWorm gears   �21m �33s 
    Gears to connect nonparallel, nonintersecting shafts that are at right 
angles. The worm, ordinarily the driver, is similar to a crossed helical gear 
except that it has at least one complete tooth (thread) around the pitch 
surface. The mating gear is the worm wheel or worm gear. Worm gearing is 
generally used to obtain large velocity reductions with the worm as the driver 
and the worm wheel as the driven gear, although occasional applications, for 
example cream separators, have the worm wheel as the driver. 
 �34s[John R. Zimmerman]
�33s 
  
A700000001330�254s�5v�1v1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWrist   �21m �33s 
    The flexible link between the forearm and hand, consisting of the wrist 
joint and adjacent structures. This articulation is composed of a set of 
synovial joints formed by the distal end of the radius and the four upper 
(proximal) carpals, or wristbones (see illustration). The four lower (distal) 
carpals articulate with the hand bones, or metacarpals. All the bony 
structures are held together by a complex set of deep ligaments, and movement 
is obtained by forearm muscles acting through tendons passing to the hand and 
fingers. The wrist is surrounded by a sheet of very heavy connective tissue 
and skin. 
 �34s[Thomas S. Parsons]
�33s 
  
  
A700000002222�254s�5v�1v1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWrought iron   �21m �33s 
    As defined by the American Society for Testing and Materials, ``a ferrous 
material, aggregated from a solidifying mass of pasty particles of highly 
refined metallic iron, with which, without subsequent fusion, is incorporated 
a minutely and uniformly distributed quantity of slag.'' This slag is a 
ferrous silicate resulting from the oxidizing reactions of refining, and it 
varies in amount from 1 to 3% in various types of final product. It is in 
purely mechanical association with the iron base metal, as contrasted with the 
alloying relationships of the metalloids present in steel.  See  STEEL. 
    A distinguishing characteristic of wrought iron is a fragmented or 
irregular fracture, as contrasted with a fibrous or crystalline type in steel. 
Metallographic analysis shows that this results from the fiberlike slag 
inclusions. Although wrought iron once held competitive merit in certain uses, 
notably where corrosion- and shock-resistance were important, very little 
wrought iron is produced at present.  See  IRON ALLOYS. 
 �34s[James Aston /John F. Wallace]
�33s 
  
A700000001267�254s�5v�1v1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWulfenite   �21m �33s 
    A mineral consisting of lead molybdate, PbMoO4. Wulfenite occurs commonly 
in yellow, orange, red, and grayish-white crystals, with a luster from 
adamantine to resinous. Wulfenite may also be massive or granular. Its 
fracture is uneven. Its hardness is 2.7 - 3 and its specific gravity 6.5 - 7. 
Its streak is white. 
    Wulfenite is found in numerous localities in the western and southwestern 
United States. Brilliant orange tabular wulfenite crystals up to 2 in. (5 cm) 
in size have been found from the Red Cloud and Hamburg mines in Yuma County, 
Arizona.  See MOLYBDENUM. 
 �34s[Edward C. T. Chao]
�33s 
  
A700000001210�254s�5v�1v1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWurtzilite   �21m �33s 
    A black, infusible carbonaceous substance occurring in Uinta County, Utah. 
It is insoluble in carbon disulfide, has a density of about 1.05, and consists 
of 79 - 80% carbon, 10.5 - 12.5% hydrogen, 4 - 6% sulfur, 1.8 - 2.2% nitrogen, 
and some oxygen. Wurtzilite is derived from shale beds deposited near the 
close of Eocene (Green River) time. The material was introduced into the 
calcareous shale beds as a fluid after which it polymerized to form nodules or 
veins.  See  ALBERTITE; ASPHALT AND ASPHALTITE; ELATERITE; IMPSONITE. 
 �34s[Irving A. Breger]
�33s 
  
A700000001350�254s�5v�1v1893
\IMG\CI1801.IMG
12762112
�6v�2v�32sWurtzite �21m �33s
    A mineral with composition ZnS (zinc sulfide). It exists most commonly in
fibrous or columnar aggregates and banded crusts with resinous luster and
brownish-black color. Hardness is 3 1/2 and its specific gravity 4.0. Zinc
sulfide is dimorphous, crystallizing as both the high-temperature form
wurtzite, ALPHA-ZnS, and the low-temperature form sphalerite, BETA-ZnS.
Wurtzite is the rarer and at room temperature the less stable form and, unlike
sphalerite, is rarely mined as an ore of zinc. Wurtzite usually contains iron
and cadmium, and a complete solid solution series extends to greenockite, CdS.
See GREENOCKITE; SPHALERITE.
 �34s[Cornelius S. Hurlbut, Jr.]
�33s

A700000002327�254s�5v�1v1899
\IMG\CI1801.IMG
13643264
�6v�2v�32sXenolith �21m�33s 
    A discrete rock fragment enclosed by magma, lava, or igneous rock. The
relation between xenolith and host magma may be close or remote. For example,
the consolidated marginal portions of a body of magma may be torn off and
incorporated into the unconsolidated central portions of the magma. Xenoliths
which are inferred to form in this manner are called cognate xenoliths.
Cognate xenoliths in volcanic rocks, like phenocrysts, help scientists find
out about the consolidation of bodies of magma underneath volcanoes. On the
other hand, many xenoliths are simply pieces of sedimentary and other
unrelated igneous rocks. Such xenoliths tend to react with the enclosing
magma, so that their constituent minerals become like those in equilibrium
with the melt. Reaction is rarely complete, however.
    Xenoliths may be angular to round, millimeters to meters in diameter,
aligned or haphazard, and sharply or gradationally bounded. Xenoliths are
present in most bodies of igneous rock, although they are rare in many igneous
rocks. Many rocks are inferred to be igneous because they contain xenoliths.
See  IGNEOUS ROCKS; MAGMA.
�34s[Alfred T. Anderson, Jr.]
�33s 
 
A700000005453�254s�5v�1v1899
\IMG\CI1801.IMG
13643264
1900
\IMG\CI1801.IMG
13779456
�6v�2v�32sXenon �21m�33s 
    A chemical element, Xe, atomic number 54. It is a member of the family of
noble gases. Sixteen radioactive isotopes are known.  See  INERT GASES.
    Xenon is used to fill a type of flashbulb, called electronic speed light,
which produces a white light that has a good balance of all the colors in the
visible spectrum, and can be used 10,000 times or more before burning out. A
xenon-filled arc lamp gives a light intensity approaching that of the carbon
arc; it is particularly valuable in projecting motion pictures.
    Traces of xenon are found in minerals and meteorites, but the only
commercial source of xenon is the air. Xenon constitutes 0.086 part per
million by volume of dry air. It is estimated that about 3 x 10-sup9% of the
weight of the Earth is xenon. Xenon also occurs outside the Earth; the best
estimate is that there are about 4 atoms of xenon per 1,000,000 atoms of
silicon, silicon being the standard of abundance used for the elements in the
universe.
    Xenon is colorless, odorless, and tasteless; it is a gas under ordinary
conditions. Other properties are shown in the table. Xenon is the only one of
the nonradioactive noble gases which forms chemical compounds that are stable
at room temperature. Xenon also forms weakly bonded clathrates.  See
CLATHRATE COMPOUNDS.
�34s[Arthur W. Francis]
�33s 
    Although xenon exhibits all of the even valence states (II, IV, VI, and
VIII), and stable compounds of each of these states have been isolated, xenon
chemistry is limited to the stable fluorides and oxyfluorides and their
complexes, two unstable oxides, and the aqueous species derived from the
hydrolysis of the fluorides.
    The three fluorides, XeF2, XeF4, and XeF6, are thermodynamically stable
compounds at room temperature, and they may be prepared simply by heating
mixtures of xenon and fluorine at 300 - 400DGC (570 - 750DGF). All of the
fluorides are reduced by hydrogen to form xenon and hydrogen fluoride, and
their heats of formation have been measured in this manner.
    The reaction of XeF6 with water gives xenon oxytetrafluoride, XeOF4, a
rather stable compound, melting at -46.2DGC and boiling at 101DGC. If the
reaction is allowed to continue, XeO3 is formed. XeO3 is a colorless,
odorless, and dangerously explosive white solid of low volatility. Gaseous
xenon tetroxide, XeO4, is formed by the reaction of sodium perxenate, Na4XeO6,
with concentrated H2SO4.
    The xenon fluorides and XeOF4 form a variety of complex addition
compounds. The complexes with metal pentafluorides tend to be powerful
fluorinating agents. Xenon difluoride dissolved in antimony pentafluoride can
react with elemental xenon itself to form the unexpected dixenon cation
Xe*EQN*.
�34s[John G. Malm ;Evan H. Appelman]
�33s 
 
A700000001273�254s�5v�1v1900
\IMG\CI1801.IMG
13779456
�6v�2v�32sXenophyophorida �21m�33s 
    An order of Protozoa in the subclass Granuloreticulosia. The group
includes deep-sea forms which are multinucleate at maturity. They develop as
discoid to fan-shaped or algalike branching forms (illustration a and b)
covered with a hyaline organic layer and sometimes measuring 2 mm or more
overall. The aggregate contains many tubes which vary in diameter and form
netlike or branching patterns. The lack of detailed information makes the
taxonomic status of these organisms somewhat uncertain. The genera included
are Psammetta, Stanomma, and Stannophyllum.  See  GRANULORETICULOSIA.
�34s[Richard P. Hall]
�33s 
 
 
A700000001162�254s�5v�1v1900
\IMG\CI1801.IMG
13779456
�6v�2v�32sXenungulata �21m�33s 
    Xenungulates are large, primitive, digitigrade, hoofed, tapirlike mammals,
with relatively short, slender limbs and five-toed feet with broad, flat
phalanges. They are restricted to the Paleocene deposits of Brazil and
Argentina. There is only one family (Carodniidae) in the order and only one
genus (Carodnia) in the family.  See  EUTHERIA.
    Previously assigned to the order Pyrotheria, xenungulates appear to be
most closely related to the Pyrotheria and the Dinocerata.  See DENTITION;
DINOCERATA; PYROTHERIA.
�34s[Gideon T. James]
�33s 
 
 
A700000003622�254s�5v�1v1900
\IMG\CI1801.IMG
13779456
1901
\IMG\CI1801.IMG
13927936
�6v�2v�32sXiphosurida �21m�33s 
    An order of the subclass Xiphosura in the class Merostomata. These
``sword-tailed'' arthropods have three distinct body divisions: a prominent
horseshoe-shaped, fused head and thorax, the prosoma; a segmented ``abdomen,''
or opisthosoma; and a spikelike terminal segment, the telson. The Xiphosurida
have an extensive fossil record. The two suborders, Synziphosurina and
Limulina, span over 500,000,000 years of evolution. The Synziphosurina usually
lacked compound eyes. Some species had a distinct axial, or cardiac, region,
free segments in the opisthosoma, and a broad, short telson. Superfamilies
Belinuracea and Limulacea of the Limulina have the clearly recognizable
horseshoe ``crab'' form.
    The carapace (test or shell) is composed of chitin, strengthened
internally in the adults by bridgelike struts within the shell margins.
Sensory structures on the carapace include chemoreceptors for taste on the
bases of the legs and a pair of simple eyes, or ocelli. The multilensed
compound eyes form mosaic images and are sensitive to polarized light. The
organ systems, except the peripheral blood vessels and nerves, digestive, and
reproductive glands, which branch extensively into the lateral portions of the
prosoma, are mainly within the prominent axial region. This region contains
the musculature of the paired appendages, hinge, and telson, the digestive
tube, the coxal (excretory) gland, and the respiratory, central circulatory,
and nervous structures. The dorsally situated tubular heart lies within a
sinus which receives venous blood from the book gills, the respiratory
structures, and the body. The central nervous system and the major nerve
branches are sheathed by arteries. A plate of ``cartilaginous'' chitin, the
endocranium, forms a roof over the brain.  See MEROSTOMATA.
�34s[Carl N. Shuster]
�33s 
 
A700000006771�254s�5v�1v1895
\IMG\CI1801.IMG
12881408
�6v�2v�32sX-ray astronomy �21m�33s 
    The study of x-ray emission from extrasolar sources. It includes the study
of virtually all types of astronomical objects, from stars to galaxies and
quasars. X-ray astronomy is a recent addition to the ancient science of
astronomy.
    X-ray astronomy is subdivided into broad bands - soft and hard - depending
upon the energy of the radiation being studied. Observations in the soft band
(below about 10 keV) must be carried out above the atmosphere with rockets or
space satellites, while hard x-ray observations can be made at high altitudes
achieved with balloons. Presently, most observations have been in the soft
band and have been carried out with a series of scientific satellites. Sky
surveys have detected and located hundreds of sources at a sensitivity of
about 1/10,000 the strength of the brightest source Sco X-1. There is a
concentration of sources along the galactic equator, corresponding to objects
which lie in the Milky Way Galaxy, particularly in the disk, which contains
most of the galactic stars and the spiral arms. Other x-ray sources are
associated mainly with extragalactic objects.  See  GALAXY; SATELLITE
ASTRONOMY; X-RAY TELESCOPE.
    The first two identified nonsolar x-ray sources, Sco X-1 and the Crab
Nebula, are galactic objects which are unusual and distinct from one another.
The Crab is a supernova remnant which is left over from the explosive death of
a star. It contains a rapidly rotating neutron star at its center as well as a
nebula consisting of hot gas and energetic particles. Other supernova remnants
have been observed and found to emit x-rays.
    In addition to the supernova remnants there are compact sources which form
the majority of galactic emitters. These sources are further classified by
their variability, which ranges from time scales as long as years to as short
as milliseconds. In some instances the x-rays appear pulsed with a period of a
few seconds or less. Another type of time behavior is represented by the
``bursters.'' These sources usually emit at some constant level, with
occasional short bursts or flares of increased brightness. It is widely
accepted that many of the galactic x-ray sources are different types of
collapsed objects - white dwarfs, neutron stars, and even black holes - which
are accreting material from their surroundings, and that the in-falling matter
heats up and radiates in x-rays as it comes close to the surface of the
compact stars.  See  BLACK HOLE; NEUTRON STAR; NOVA; PULSAR; STELLAR
EVOLUTION; SUPERNOVA; WHITE DWARF STAR; X-RAY STAR.
    Among the more interesting types of extragalactic sources detected have
been apparently normal galaxies, galaxies with active nuclei, radio galaxies,
clusters of galaxies, and quasars. The nearest spiral galaxy is M31 - the
Andromeda Galaxy. There appears to be no emission associated with M31 as a
whole, but just the summed emission of the stars. For active nuclei galaxies,
this is not the case. X-ray emission from these objects is orders of magnitude
in excess of that from normal galaxies, and is clearly associated with the
galaxy itself. Many astrophysicists believe that galactic nuclei are the sites
of massive black holes, at least 10sup6 to 10sup9 times the mass of the Sun,
and that the radiation from these objects is due to gravitational energy
released by in-falling material. The quasars may represent the extreme case of
this mechanism.  See  ACTIVE GALACTIC NUCLEI; ANDROMEDA GALAXY; QUASARS.
�34s[Stephen S. Murray]
�33s 
 
A700000001441�254s�5v�1v1895
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12881408
�6v�2v�32sX-ray crystallography �21m�33s 
    The study of crystal structure by x-ray diffraction techniques. For the
experimental aspects of x-ray diffraction  see  X-RAY DIFFRACTION.
    Structurally, a crystal is a three-dimensional periodic arrangement in
space of atoms, groups of atoms, or molecules. If the periodicity of this
pattern extends throughout a given piece of material, one speaks of a single
crystal. The exact structure of any given crystal is determined if the
locations of all atoms making up the three-dimensional periodic pattern called
the unit cell are known. The very close and periodic arrangement of the atoms
in a crystal permits it to act as a diffraction grating for x-rays.  See
CRYSTALLOGRAPHY.
�34s[Lawrence F. Dahl]
�33s 
 
A700000015404�254s�5v�1v1895
\IMG\CI1801.IMG
12881408
1896
\IMG\CI1801.IMG
13002240
�6v�2v�32sX-ray diffraction �21m�33s 
    The scattering of x-rays by matter with accompanying variation in
intensity in different directions due to interference effects. X-ray
diffraction is one of the most important tools of solid-state chemistry, since
it constitutes a powerful and readily available method for determining atomic
arrangements in matter. X-ray diffraction methods depend upon the fact that
x-ray wavelengths of the order of 1 nanometer are readily available and that
this is the order of magnitude of atomic dimensions. When an x-ray beam falls
on matter, scattered x-radiation is produced by all the atoms. These scattered
waves spread out spherically from all the atoms in the sample, and the
interference effects of the scattered radiation from the different atoms cause
the intensity of the scattered radiation to exhibit maxima and minima in
various directions.  See  DIFFRACTION.
 
    Uses.   Some of the uses of x-ray diffraction are: (1) differentiation
between crystalline and amorphous materials; (2) determination of the
structure of crystalline materials (crystal axes, size and shape of the unit
cell, positions of the atoms in the unit cell); (3) determination of electron
distribution within the atoms, and throughout the unit cell; (4) determination
of the orientation of single crystals; (5) determination of the texture of
polygrained materials; (6) identification of crystalline phases and
measurement of the relative proportions; (7) measurement of limits of solid
solubility, and determination of phase diagrams; (8) measurement of strain and
small grain size; (9) measurement of various kinds of randomness, disorder,
and imperfections in crystals; and (10) determination of radial distribution
functions for amorphous solids and liquids.
 
    Techniques.   The techniques employed in the study of crystalline
substances, gases, and liquids are discussed below.
 
    Laue method. The Laue pattern uses polychromatic x-rays provided by the
continuous spectrum from an x-ray tube operated at 35 - 50 kV. The different
diffracted beams have different wavelengths, and their directions are
determined solely by the orientations of the set of planes with Miller indices
hkl. Transmission Laue patterns were once used for structure determinations,
but their many disadvantages have made them practically obsolete. On the other
hand, the back-reflection Laue pattern is used a great deal in  the study of
the orientation of crystals.
 
    Rotating crystal method. The original rotating crystal method was employed
in the Bragg spectrometer. A sufficiently monochromatic beam, of wavelength of
the order of 1 A, is collimated by a system of slits and then falls on the
large extended face of a single crystal as shown by Fig. 1. The Bragg
spectrometer has been used extensively in obtaining quantitative measurements
of the integrated intensity from planes parallel to the face of the crystal.
The chamber is set at the correct 2THETA-angle with a slit so wide that all of
the radiation reflected from the crystal can enter and be measured. The
crystal is turned at constant angular speed through the Bragg law position,
and the total diffracted energy received by the ionization chamber during this
process is measured. Similar readings with the chamber set on either side of
the peak give a background correction.
    The rotation camera, which is frequently used for structure
determinations, is illustrated in Fig. 2. The monochromatic primary beam s0
falls on a small single crystal at O. The crystal is mounted with one of its
axes (say, a3) vertical, and it rotates with constant velocity about the
vertical axis during the exposure. The various diffracted beams are registered
on a cylindrical film concentric with the axis of rotation.
 
    Powder method. The powder method involves the diffration of a collimated
monochromatic beam from a sample containing an enormous number of tiny crystals
having random orientation. Powder pattern studies are made with Geiger counter,
or proportional counter, diffractometers. The apparatus is shown schematically
in Fig. 3. X-rays diverging from a target a T fall on the sample at O, the
smaple being a flat-faced briquet of powder. Diffracted radiation from the
sample passes through the receiving slit at s and enters the Geiger counter.
During the operation the sample turns at angular velocity OMEGA and the counter
at 2OMEGA. The distances TO and OS are made equal to satisfy approximate
focusing conditions. A filter F before the receiving slit gives the effect of a
sufficiently monochromatic beam. A chart recording the amplified output of the
Geiger counter gives directly a plot of intensity versus scattering angle
2THETA.
�34s[Bertram E. Warren]
�33s 
    Gases. Gases and liquids are found to give rise to x-ray diffraction
patterns characterized by one or more halos or interference rings which are
usually somewhat diffuse. These diffraction patterns, which are similar to
those for glasses and amorphous solids, are due to interference effects
depending both upon the electronic distribution of each of the individual
atoms or molecules and upon their relative positions in the system.
    For monatomic gases the only appreciable interference effects giving rise
to a distribution of scattered intensities are those produced by the
electronic distribution about each nucleus. These interference effects giving
rise to so-called coherent intensities are the result of the interference of
the individual waves scattered by electrons in different parts of the atom.
The electronic distribution of an atom is described in terms of a
characteristic atomic scattering factor which is defined as the ratio of the
resultant amplitude scattered by an atom to the amplitude that a free electron
would scatter under the same conditions.
 
    Liquids. One cannot, as in the cases of dilute gases and crystalline
solids, derive unambiguous, detailed descriptions of liquid structures from
diffraction data. Nevertheless, diffraction studies of liquids do provide most
useful information. Instead of comparing the experimental intensity
distributions with theoretical distributions computed for various models, the
experimental results are usually provided in the form of a radial distribution
function which specifies the density of atoms or electrons as a function of
the radial distance from any reference atom or electron in the system without
any prior assumptions about the structure. From the radial distribution
function one can obtain (1) the average interatomic distances most frequently
occurring in the structure corresponding to the positions of the first,
second, and possibly third nearest neighbors; (2) the distribution of
distances; and (3) the average coordination number for each interatomic
distance.
�34s[Lawrence F. Dahl]
�33s 
 
A700000004453�254s�5v�1v1896
\IMG\CI1801.IMG
13002240
1897
\IMG\CI1801.IMG
13133312
�6v�2v�32sX-ray fluorescence analysis �21m�33s 
    A nondestructive physical method used for chemical analyses of solids and
liquids. The specimen is irradiated by an intense x-ray beam which causes the
elements in the specimen to emit (that is, fluoresce) their characteristic
x-ray line spectra. The lines of the spectra are diffracted at various angles
by a single-crystal plate which is analogous to the diffraction grating of
optical spectroscopy. The elements may be identified by the wavelengths of
their spectral lines, which vary in a regular manner with atomic number, and
their concentrations may be determined from the intensities of the lines.
Counter tubes and associated electronic circuits are used to measure the x-ray
intensities.  See  FLUORESCENCE.
    Unlike x-ray diffraction, the elements (rather than the compounds) are
identified, and it is not necessary that the specimen be crystalline. The
method supplements optical spectroscopy, being most generally used for
concentrations > 0.1%, although with prior wet chemical or physical extraction
methods to enhance the peak-to-background ratio, much lower amounts can be
detected.  See  X-RAY DIFFRACTION.
    The x-ray fluorescence method is less sensitive than the optical method,
although with prior specimen treatment, analyses can be made in the range of
parts per million. Microgram quantities have been determined in milligram
samples with standard deviations of 10 - 15%, using focusing crystal
spectrographs. Optical spectrographic methods become difficult to use for
concentrations exceeding about 20%, whereas the x-ray method is applicable up
to 100%. The x-ray fluorescence method has proven particularly useful for
mixtures of elements of similar chemical properties which are difficult to
separate and analyze by conventional chemical methods.
    The x-ray spectrograph for fluorescence analysis consists of the primary
x-ray tube and its stabilized high-voltage power supply; a ray-proof specimen
chamber; the goniometer which carries the crystal analyzer and counter tube;
and the electronic circuitry for the counter tube consisting of a power
supply, linear amplifier, single-channel pulse-height analyzer, scaler, rate
meter, and strip-chart recorder.
�34s[William Parrish]
�33s 
 
A700000006604�254s�5v�1v1897
\IMG\CI1801.IMG
13133312
�6v�2v�32sX-ray microscope �21m�33s 
    A technique and an instrument or combination of instruments which utilize
x-radiation for chemical analysis and for magnification of 100 - 1000
diameters. The resolution possible is about 0.25 micrometer. The contrast in
the x-ray microscopic image is caused by the varying x-ray attenuation in the
specimen. The advantage of x-ray microscopy is that it yields quantitative
chemical information, besides structural information, about objects, including
those which are opaque to light. It is a reliable ultramicrochemical
analytical technique by which amounts of elements and weights of samples as
small as 10-sup1sup2 to 10-sup1sup4 g can be analyzed with an error of only a
few percent.  See  X-RAY OPTICS.
    There are four general principles of image formation in x-ray microscopy:
(1) contact microradiography (illustration a), (2) projection x-ray microscopy
(illustration b), (3) reflection x-ray microscopy (illustration c), and (4)
x-ray image spectrography (illustration d).  See  MICRORADIOGRAPHY.
    In contact microradiography the thin specimen is placed in close contact
with an extremely fine-grained photographic emulsion which has a resolution of
more than 25,000 lines/in. (1000 lines/mm), and radiographed with x-rays of
suitable wavelength. Thus an absorption image in scale 1:1 is obtained, and
this image is subsequently viewed in a light microscope. The maximal
resolution is that of the optical microscope (0.25 MUm), but the image has
more information, which can be obtained by examining the microradiogram in the
electron microscopes. See ELECTRON MICROSCOPE.
    Projection x-ray microscopy, or x-ray shadow microscopy, is based on the
possibility of producing an extremely fine x-ray focal spot. This is achieved
by an electronic lens system similar to that in the electron microscope. The
fine focal spot is produced on a very thin metal foil which serves as a
transmission target. The x-rays are generated on the target by the impact of
the electrons. The sample is placed near the target, and the primary
magnification depends on the ratios of the distances from focal spot to sample
and sample to film. Resolution is of the same order as the size of the focal
spot; the best value is about 0.1 MUm in favorable objects.
    The method of reflection x-ray microscopy is based on the fact that the
refractive index for x-rays in solids is a very small amount less than 1. Thus
at grazing incidence (that is, incidence at very small angles), the x-rays are
totally reflected, and if the reflecting surface is made cylindrical, there
will be a focusing action in one dimension. By crossing two such surfaces a
true image formation can be obtained, although with some astigmatism, which
can be corrected by giving the surfaces a complicated optical shape. The
resolution by this procedure is about 0.5 - 1 MUm.
    X-ray image spectrography utilizes Bragg reflections in a cylindrically
bent crystal and produces slightly enlarged emission images; this technique is
best classified as a micromodification of x-ray fluorescence analysis. The
resolution is about 50 MUm.  See  X-RAY DIFFRACTION; X-RAY FLUORESCENCE
ANALYSIS.
    In biology, x-ray microscopy has been utilized for the quantitative
determination of the dry weight, water content, and elementary composition of
many tissues, especially mineralized tissues.
�34s[Arne Engstrom]
�33s 
 
 
A700000002255�254s�5v�1v1897
\IMG\CI1801.IMG
13133312
1898
\IMG\CI1801.IMG
13435904
�6v�2v�32sX-ray optics �21m�33s 
    A title-by-analogy of those phases of x-ray physics in which x-rays
demonstrate properties similar to those of light waves. X-ray optics is also
called roentgen optics.  See GEOMETRICAL OPTICS; OPTICS; PHYSICAL OPTICS.
    Polarization, diffraction, reflection, and refraction of x-rays were all
detected by using crystals having refractive indices that were slightly less
than 1. Total reflection or diffraction of x-rays from ruled gratings could
occur only with incident beams at very small grazing angles of the order of a
few minutes. Until this discovery by A. Compton in 1922 all attempts to
concentrate or focus x-rays by lenses and mirrors had failed.  See  COMPTON
EFFECT; X-RAYS.
    Bent crystals can be used as diffraction gratings to focus diffracted
beams and to form enlarged images of a specimen, itself serving as the source
of x-rays or bathed by a beam of x-rays, in accordance with the same
principles and equations that apply to light optics.  See  X-RAY DIFFRACTION;
X-RAY FLUORESCENCE ANALYSIS; X-RAY MICROSCOPE; X-RAY POWDER METHODS.
�34s[George L. Clark]
�33s 
 
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�6v�2v�32sX-ray powder methods �21m�33s 
    Physical techniques used for the identification of substances and for
other types of analyses, principally for crystalline materials in the solid
state. In these techniques, a small collimated beam of x-rays is directed onto
a small polycrystalline specimen in the form of powder, producing a
diffraction pattern that is recorded on film or with a counter tube. This
x-ray pattern is a fundamental and uniquely characteristic property resulting
from the atomic arrangement of the diffracting substance. Different substances
have different atomic arrangements or crystal structures, and hence no two
chemically distinct substances give identical diffraction patterns.
Identification may be made by comparing the pattern of the unknown substance
with patterns of known substances, in a manner somewhat analogous to the
identification of persons by their fingerprints. The analytical information is
different from that obtained by chemical or spectrographic analysis. X-ray
identification of chemical compounds indicates the constituent elements, and
shows how they are combined.
    The x-ray powder method is widely used in fundamental and applied
research; for instance, it is used in the analysis of raw materials and
finished products, in phase-diagram investigations, in following the course of
solid-state chemical reactions, and in the study of minerals, ores, rocks,
metals, chemicals, and many other types of material. The use of powder methods
to determine the actual atomic arrangement, which has been so important in the
study of chemical bonds, crystal physics, and crystal chemistry, is described
in related articles.  See  X-RAY CRYSTALLOGRAPHY; X-RAY DIFFRACTION; X-RAY
FLUORESCENCE ANALYSIS.
�34s[William Parrish]
�33s 
 
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�6v�2v�32sX-rays �21m�33s 
    X-rays, or roentgen rays, are electromagnetic waves in which periodically
variable electric and magnetic fields are perpendicular to each other and to
the direction of propagation. Thus they are identical in nature with visible
light and all the other types of radiation that constitute the electromagnetic
spectrum. In general, x-rays are generated as the result of energy transitions
of atomic electrons caused by the bombardment of a material of high atomic
weight by high-energy electrons.  See  ELECTROMAGNETIC RADIATION.
    Following W. R. Roentgen's discovery of ``a new kind of ray'' in 1895,
other scientists found the essential experimental conditions to prove that
x-rays can be polarized, diffracted by crystals, refracted in prisms and in
crystals, reflected by mirrors, and diffracted by ruled gratings.  See  X-RAY
OPTICS.
    The range of x-rays in the electromagnetic spectrum, as excited in x-ray
tubes by the bombardment of anode targets by cathode electrons under a high
accelerating potential, overlaps the ultraviolet range on the order of 100
nanometers on the long-wavelength side, and the shortest-wavelength limit
moves downward as voltages increase. An accelerating potential of 10sup9
volts, now readily generated, produces a wavelength of 10-sup1sup5 m (10-sup6
nm). An average wavelength used in research is 0.1 nm, or about 1/6000 the
wavelength of yellow light.  See  X-RAY TUBE.
    In diffraction, refraction, polarization, and interference phenomena,
x-rays, together with all other related radiations, appear to act as waves. In
other phenomena - such as the appearance of sharp spectral lines, a definite
short-wavelength limit of the continuous ``white'' spectrum, the shift in
wavelength of x-rays scattered by electrons in atoms (Compton effect), and the
photoelectric effect - the energy seems to be propagated and transferred in
quanta, called photons.  See  COMPTON EFFECT; ELECTRON DIFFRACTION; NEUTRON
DIFFRACTION; PHOTOEMISSION; QUANTUM MECHANICS.
    Important uses have been found for x-rays in many fields of scientific
endeavor, for example, roentgen spectrometry and roentgen diffractometry.
Extensive tables of the wavelengths of x-ray emission lines in series (K, L,
M, and so on) and so-called absorption edges, characteristic of the chemical
elements, afford the necessary information for chemical analyses, exactly as
in the case of optical emission spectra and for derivation of theories of
atomic structure to account for the origin of spectra.  See  HISTORADIOGRAPHY;
MICRORADIOGRAPHY; RADIATION BIOLOGY; RADIOGRAPHY; RADIOLOGY; X-RAY
CRYSTALLOGRAPHY; X-RAY DIFFRACTION; X-RAY FLUORESCENCE ANALYSIS; X-RAY
MICROSCOPE; X-RAY POWDER METHODS.
�34s[George L. Clark]
�33s 
 
A700000004304�254s�5v�1v1898
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13435904
�6v�2v�32sX-ray spectrometry �21m�33s 
    A rapid and economical technique for quantitative analysis of the
elemental composition of specimens. It differs from x-ray diffraction, whose
purpose is the identification of crystalline compounds. It differs from
spectrometry in the visible region of the spectrum in that the x-ray photons
have energies of thousands of electronvolts and come from tightly bound
inner-shell electrons in the atoms, whereas visible photons come from the
outer electrons and have energies of only a few electronvolts.  See  X-RAY
DIFFRACTION.
    In x-ray spectrometry the irradiation of a sample by high-energy
electrons, protons, or photons ionizes some of the atoms, which then emit
characteristic x-rays whose wavelength depends on the atomic number of the
element, and whose intensity is related to the concentration of that element.
Generally speaking, the characteristic x-ray lines are independent of the
physical state (solid or liquid) and of the type of compound (valence) in
which an element is present, because the x-ray emission comes from inner,
well-shielded electrons in the atom. The illustration shows the removal of one
of the innermost, K-shell, electrons by a high-energy photon. The photon
energy must be greater than the binding energy of the electron; the difference
in energy appears as the kinetic energy of the ejected electron. The K-ionized
atom is unstable, and one of the L- or M-shell electrons drops into the
K-shell vacancy. As this transition occurs, a characteristic x-ray photon is
emitted with an energy equal to the difference in energy between the K and the
L (or M) shell, or an additional electron, called an Auger electron, is
ejected from the atom. Either the x-rays or the Auger electrons may be used
for analysis.  See  AUGER EFFECT.
 
    X-ray spectrometry generally does not require any separation of elements
before measuring, because the x-ray lines are easily resolved. However,
preconcentration methods are sometimes useful as a means for improving the
limit of detection. One limitation of x-ray spectrometry is the progressive
difficulty of measurement below atomic number 11. See SPECTROSCOPY.
�34s[L. S. Birks]
�33s 
 
A700000003756�254s�5v�1v1898
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13435904
�6v�2v�32sX-ray star �21m�33s 
    A source of x-rays from outside the solar system. Although the Sun emits
some x-radiation, it cannot be called an x-ray star because its energy output
in the optical region of the spectrum vastly exceeds that in the x-ray region.
Observations of the 100 or so known galactic x-ray stars show that these
objects have just the reverse characteristic: the overwhelming amount of their
energy is given off in the x-ray band, while in visible light they are
inconspicuous.  See  STAR; SUN; X-RAY ASTRONOMY.
    The majority of galactic x-ray stars are located in a narrow layer
coincident with the plane of the Milky Way Galaxy. Because this coincides with
the spatial distribution of bright young stars in the Galaxy, there is at
least circumstantial evidence that many x-ray stars may be associated with
such young stars, or at least may have had similar stages of early evolution.
See  GALAXY; STELLAR EVOLUTION.
    The study of both x-ray and optical data from the region of x-ray stars
has led to the conclusion that most, or perhaps even all, such stars are
members of binary star systems, consisting of one relatively normal visible
star and one subluminous compact star, in a gravitationally bound orbit about
each other. In cases where the position of the x-ray source has been very
accurately measured, it is sometimes possible to locate a visible star
associated with the system. In most cases, studies of the visible component
show evidence independent of the x-ray data for the binary nature of these
systems.
    Most theories assume these objects to be highly evolved and quite compact.
The compact star accretes gaseous matter from its nearby visible companion.
This in-falling matter reaches a very high temperature before it is stopped at
or near the surface of the x-ray star, and the very hot gas then emits x-rays.
In effect, a machine is at work to convert gravitational energy into
x-radiation.
�34s[Joseph Silk /Bruce Margon]
�33s 
 
A700000004235�254s�5v�1v1898
\IMG\CI1801.IMG
13435904
1899
\IMG\CI1801.IMG
13643264
�6v�2v�32sX-ray telescope �21m�33s 
    An instrument designed to collect and detect x-rays emitted from a source
outside the Earth's atmosphere and to resolve the x-rays into an image.
Absorption by the atmosphere requires that x-ray telescopes be carried to high
altitudes. Balloons are used for detection systems designed for higher-energy
(harder) x-ray observations, whereas rockets and satellites are required for
softer x-ray detectors.  See  X-RAY ASTRONOMY.
    An image-forming telescopic lens for x-ray wavelengths can be based on the
phenomenon of total external reflection at a surface where the index of
refraction changes. In the case of x-rays, the index of refraction in matter
is slightly less than unity. By application of Snell's laws, the condition for
total external reflection is that the radiation be incident at small grazing
angles, less than a critical angle of about 1DG, to the reflecting surface.
Based on these properties, x-ray mirrors have been constructed which focus an
image in two dimensions. Various configurations of surfaces are possible.  See
  REFLECTION OF ELECTROMAGNETIC RADIATION; REFRACTION OF WAVES.
    The high angular resolution of the grazing-incidence telescope requires a
camera which has correspondingly good spatial resolution. One type of detector
uses microchannel plates and yields about 20-micrometer resolution for x-rays
in the soft energy band (about 200 - 4000 eV). The microchannel plate is an
array of small hollow tubes (about 15 MUm in diameter) or channels which are
processed to have high secondary electron yield from their inner walls.
    Another detector used with x-ray telescopes is a form of gas counter. By
operation of the counter in the proportional mode and use of planes of wires
to localize the electrical signals, the position and amplitude of each event
are recorded. These detectors generally have lower spatial resolution (about 1
mm) than do microchannel plate detectors, but they can be made larger in size
and provide some energy measurement which is not possible in the other device.
  See  IONIZATION CHAMBER.
�34s[Stephen S. Murray]
�33s 
 
A700000004031�254s�5v�1v1899
\IMG\CI1801.IMG
13643264
�6v�2v�32sX-ray tube �21m�33s 
    An electronic device used for the generation of x-rays. X-rays are
produced in the x-ray tube by accelerating electrons to a high velocity by an
electrostatic field and then suddenly stopping them by collision with a solid
body, the so-called target, interposed in their path. The x-rays radiate in
all directions from the spot on the target where the collisions take place.
The x-rays are due to the mutual interaction of the fast-moving electrons with
the electrons and positively charged nuclei which constitute the atoms of the
target. Depending upon the method used in generating the electrons, x-ray
tubes may all be classified in two general groups, gas tubes and high-vacuum
tubes.  See  X-RAYS.
    In gas tubes electrons are freed from a cold cathode by positive ion
bombardment. For the existence of the positive ions a certain gas pressure is
required without which the tube will allow no current to pass. Metals, such as
platinum and tungsten, are placed in the path of the electron beam to serve as
the target. Concave metal cathodes are used to focus the electrons on a small
area of the metal target and increase the sharpness of the resulting shadows
on the fluorescent screen or the photographic film. Many designs of gas tubes
have been built for useful application, particularly in the medical field.
    The operational difficulties and erratic behavior of gas x-ray tubes are
inherently associated with the gas itself and the positive ion bombardment
that takes place during operation. The high-vacuum x-ray tube eliminates these
difficulties by using other means of emitting electrons from the cathode. The
original type of high-vacuum x-ray tube had a hot tungsten-filament cathode
and a solid tungsten target. This tube permitted stable and reproducible
operation with relatively high voltages and large masses of metals. A modern
commercial hot-cathode high-vacuum x-ray tube is built with a liquid-cooled,
copper-backed tungsten target.
�34s[Ernest E. Charlton]
�33s 
 
A700000003422�254s�5v�1v1901
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�6v�2v�32sXylem �21m�33s 
    The principal water-conducting tissue and the chief supporting system of
higher plants. This tissue and the associated phloem constitute the vascular
system of vascular plants. Xylem is composed of various kinds of cells, living
or nonliving. The structure of these cells differs in their functions, but
characteristically all have a rigid and enduring cell wall that is well
preserved in fossils.
    In terms of their functions, the kinds of cells in xylem are those related
principally to conduction and support, tracheids; to conduction, vessel
members; to support, fibers; and to food storage, parenchyma. Vessel members
and tracheids are often called tracheary elements. The cells in each of the
four categories vary widely in structure.  See  PARENCHYMA.
    Xylem tissues arise in later stages of embryo development of a given plant
and are added to by differentiation of cells derived from the apical meristems
of roots and stems. Growth and differentiation of tissues derived from the
apical meristem provide the primary body of the plant, and the xylem tissues
formed in it are called primary. Secondary xylem, when present, is produced by
the vascular cambium.  See  LATERAL MERISTEM.
    In the trade, softwood is a name for xylem of gymnosperms (conifers) and
hardwood for xylem of angiosperms. The terms do not refer to actual hardness
of the wood. Woods of gymnosperms are generally composed only of tracheids,
wood parenchyma, and small rays, but differ in detail. Resin ducts are present
in many softwoods. Woods of angiosperms show extreme variation in both
vertical and horizontal systems, but with few exceptions have vessels.  See
LUMBER MANUFACTURE; PLANT TISSUE SYSTEMS.
�34s[Vernon I. Cheadle ;Katherine ]
�33s 
 
A700000002176�254s�5v�1v1901
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13927936
�6v�2v�32sXylene �21m�33s 
    One of a group of three isomeric aromatic hydrocarbons. The three isomeric
xylenes and ethylbenzene all have in common a molecular weight of 106.2 and
the simplified formula, C8H10. The names, structural formulas, and boiling and
melting points of these compounds are as shown in the table.
    These compounds are almost exclusively produced from petroleum by
hydroforming or catalytic reforming of appropriate naphtha fractions.  See
PETROLEUM PRODUCTS.
    o-Xylene is commercially oxidized by air over vanadium oxide catalysts to
produce phthalic anhydride, one of the most versatile of organic
intermediates. p-Xylene, like the ortho isomer, has only one significant
chemical use: it is oxidized by one of several processes either to
terephthalic acid (TA) or to dimethylterephthalate (DMT), both of which are
intermediates in the production of, among other things, the high-polymer
materials known commonly as polyesters. m-Xylene, nearly twice as abundant as
any of the other isomers, is technologically the least important.
�34s[Robert I. Stirton ;Martin Sties]
�33s 
 
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�6v�2v�32sXylose �21m�33s 
    A pentose sugar, referred to in the early literature as l-xylose. It is
present in many woody materials. The polysaccharide xylan, which is closely
associated with cellulose, consists practically entirely of D-xylose.
Corncobs, cottonseed hulls, pecan shells, and straw contain considerable
amounts of this sugar. This pentose sugar is also a component of the
hemicelluloses and the rare disaccharide, primeverose.  See  CARBOHYDRATE;
POLYSACCHARIDE.
�34s[William Z. Hassid]
�33s 
 
A700000001227�254s�5v�1v1903
\IMG\CI1801.IMG
14002176
�6v�2v�32sYagi-Uda antenna   �21m �33s 
    A combination of a single driven antenna and a closely coupled parasitic 
element which may function either as a reflector as a result of inductive 
reactance or as a director as a result of capacitive reactance, depending on 
both the length and spacing of the parasitic element; also called the Yagi 
antenna (see illustration). Such structures are not only feasible but have a 
rather important place in antenna practice and concept, particularly in 
very-high-frequency and ultra-high-frequency ranges. See ANTENNA 
(ELECTROMAGNETISM).  
 �34s[Kunitaka Arimura]
�33s 
  
  
A700000001426�254s�5v�1v1903
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�6v�2v�32sYak   �21m �33s 
    A heavily built mammal, Poephagus grunniens, of the order Artiodactyla 
that has been domesticated but still exists in the wild state in Tibet and 
high areas of China, living at elevations of 13,000 - 20,000 ft (about 4 - 6 
km). It is related to the bison, which it resembles anatomically in having 14 
pairs of ribs instead of the 13 pairs more usual of the order. Economically, 
it is probably the most important animal in Tibet, being used as a beast of 
burden and for its excellent milk. The dung is used as fuel, the hair is used 
for making cloth, and the flesh is eaten. Calving occurs in the autumn after a 
gestation period of about 10 months.  See  ARTIODACTYLA; BISON. 
 �34s[Charles B. Curtin]
�33s 
  
A700000001174�254s�5v�1v1903
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�6v�2v�32sYaw indicator   �21m �33s 
    A device that measures the angular direction of the airflow relative to 
the longitudinal vertical plane of the aircraft. This may be accomplished by a 
balanced vane or by a differential pressure sensor that aligns the detector to 
the airflow and, in so doing, transmits the measured angle between the normal 
axis and the detector as the yaw angle. If the device is used to measure the 
angle between the horizontal plane and the airflow, it is called an 
angle-of-attack sensor.  See  AIRCRAFT INSTRUMENTATION; AIRPLANE. 
 �34s[James W. Angus]
�33s 
A700000000647�254s�5v�1v1904
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�6v�2v�32sYaws�21m�33s
    An infectious disease of humans cause by the spirochete Treponema pertenue.
It is also known as frambesia and is largely confined to the tropics. Usually
yaws is contracted in childhood by direct contact or from small flies feeding in
succession on infected lesions and open wounds. No race or age possesses natural
immunity. 
�34s[Thomas B. Turner]�33s
  
A700000002100�254s�5v�1v1903
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�6v�2v�32sY-delta transformations   �21m �33s 
    Electrically equivalent networks with three terminals, one being connected 
internally by a Y configuration and the other being connected internally by a 
DELTA configuration. 
    If a network connects three terminals with one another, it is called a 
three-terminal network. The simplest configurations of three-terminal networks 
are the Y or star (Fig. 1a) and the DELTA or mesh (Fig. 1b). 
    If a given three-terminal network, passive and linear, has a Y 
configuration, it is possible to determine an equivalent DELTA network that 
could be substituted for the Y without changing the relations of voltage and 
current at the network terminals, or elsewhere external to the network. 
Similarly, if a DELTA network is given, an equivalent Y network can be found. 
Impedances of equivalent networks are usually functions of frequency, and 
realization of these impedances by use of physically possible elements is 
usually limited to a single frequency. 
 �34s[Hugh H. Skilling]
�33s 
  
  
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�6v�2v�32sYear   �21m �33s 
    Any of several units of time based on the revolution of Earth around the 
Sun. The tropical year, to which the calendar is adjusted, is the period that 
is required for the mean longitude of the Sun to increase 360DG. Its duration 
is approximately 365.24220 mean solar days. It is also the period after which 
the seasons repeat themselves. 
    The sidereal year, 365.25636 mean solar days in duration, is the average 
period of revolution of Earth with respect to a fixed direction in space. 
    The anomalistic year, 365.25964 mean solar days in duration, is the 
average interval between successive closest approaches of Earth to the Sun.  
See  TIME. 
 �34s[Gerald M. Clemence]
�33s 
  
A700000004670�254s�5v�1v1904
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�6v�2v�32sYeast   �21m �33s 
    A collective name for those fungi which possess, under normal conditions 
of growth, a vegetative body (thallus) consisting, at least in part, of 
simple, individual, single cells. In addition, the cells making up the thallus 
may occur in pairs, in groups of three, or in straight or branched chains 
consisting of as many as 12 or more cells. Yeast plays a large part in 
industrial fermentation processes such as the production of ethanol, malt 
beverage, and wine and also in diseases of humans, animals, and plants.  See  
FERMENTATION; YEAST, INDUSTRIAL. 
    The shape and size of the individual cells of some species vary slightly, 
but in other species the cell morphology is extremely heterogeneous. The shape 
of yeast cells may be spherical, globose, ellipsoidal, elongate to cylindrical 
with rounded ends, more or less rectangular, pear-shaped, apiculate or 
lemon-shaped, ogival or pointed at one end, or tetrahedral. The dimension of a 
spherical cell may vary from 2 to 10 micrometers in diameter. The length of 
cylindrical cells is often 20 - 30 MUm and, in some cases, even greater. 
    The asexual multiplication of yeast cells occurs by a budding process, by 
the formation of cross walls or fission, and sometimes by a combination of 
these two processes. Yeast buds are sometimes called blastospores. When yeast 
reproduces by a fission mechanism, the resulting cells are termed arthrospores. 
    Yeasts are divided into sporogenous and asporogenous groups, or perfect 
and imperfect yeasts. The sexual spores of yeasts are ascospores, which are 
formed in simple structures, often a vegetative cell. Such asci are called 
naked asci because of the absence of an ascocarp. If the vegetative cells are 
diploid, a cell may transform directly into an ascus after the 2n nucleus 
undergoes a reduction or meiotic division. Certain haploid yeasts have been 
shown to be heterothallic; that is, sporulation can occur only if strains of 
opposite mating type (sometimes indicated by + and - strains) are mixed on 
sporulation media. If no mating type is known, a yeast is considered to be 
imperfect. The perfect yeasts are classified in the order Saccharomycetales 
(Endomycetales), and the imperfect yeasts belong in the class Fungi 
Imperfecti, order Cryptococcales (Torulopsidales).  See  CRYPTOCOCCALES; FUNGI 
IMPERFECTI; SACCHAROMYCETALES. 
 �34s[Emil M. Mrak ;Herman J. Phaff]
�33s 
  
A700000010266�254s�5v�1v1904
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�6v�2v�32sYeast, industrial   �21m �33s 
    Those yeast strains used for the commercial production of fermented 
beverages or other fermented foods, baking purposes, and the production of 
vitamins, enzymes, proteins (food and feed yeasts), alcohol, and glycerol. In 
most instances, definite species of yeast are employed for each of these 
processes. In many cases induced mutant strains or genetic hybrids of yeast 
are selected for optimal results. 
    Fermented beverages are generally classified by the two broad classes of 
raw materials which serve as substrates for the yeast activity. The first of 
these is based on a substrate which contains a supply of directly fermentable 
sugars. These are principally fruits and berries for the production of wine. 
The second involves substrates which are a potential source of fermentable 
sugars, mainly in the form of starches which must be converted either by plant 
enzymes (amylases of germinated barley) or from fungi (such as Aspergillus 
oryzae). Beer and sake are examples of alcoholic beverages produced from the 
second group of substrates.  See  ALCOHOLIC BEVERAGES; FERMENTATION; WINE.  
    During the course of history, baker's yeast has undergone many changes. 
Today, it is composed almost exclusively of cells of one or more selected 
strains of Saccharomyces cerevisiae. It is marketed in the form of cakes of 
compressed fresh yeast cells whose moisture content is about 70% or as 
granules of yeast masses which have been dehydrated to about 7.5% moisture 
content. Different strains are used for the two forms, and production 
schedules differ so that the stability and dough-raising properties of each 
form are maximized. Usage of the dried form has increased, although the moist 
compressed yeast is still commonly used in areas with temperate climates and 
relatively short distribution times. 
    Food and feed yeasts are produced for human and animal consumption, and 
represent valuable dietary additions of proteins, vitamins of the B complex, 
and minerals. They are produced from a variety of raw materials and by widely 
different processes. Some, known as secondary yeasts, represent a by-product 
of the brewer and distillery industries; others, the primary yeasts, are grown 
specifically as food or feed sources from cane or beet molasses, hydrolyzed 
starch, cheese whey, spent sulfite waste liquor of the paper industry, 
acid-hydrolyzed wood, and various agricultural wastes. Food yeasts differ from 
feed (fodder) yeasts in quality characteristics. Generally, food yeasts 
exhibit lower bacterial and mold counts; higher levels of vitamins; higher 
protein content; a bland flavor; light color; and the absence of obnoxious 
microorganisms, added food ingredients, or fillers than do feed yeasts. 
Because food yeasts are intended for human consumption, rigid chemical and 
microbiological standards have been established by professional groups and 
government agencies and may be augmented by individual food processers and 
formulators to meet specific processing conditions and desired dietary 
standards.  
    A number of yeasts (for example, Candida utilis and Hansenula anomala) can 
synthesize all of the necessary vitamins required for growth from simple 
substrate precursors. However, many yeasts, among them most of the species of 
Saccharomyces, require one or more vitamins in the medium to supplement the 
nutrients which they can synthesize themselves. However, although requiring an 
exogenous source of several vitamins, S. cerevisiae is able to concentrate and 
absorb quite large quantities of the B vitamin complex from the medium during 
growth and thus form enriched products. Therefore, by supplementing the medium 
with crude sources of these particular vitamins (thiamin, nicotinic acid, and 
biotin) or sometimes even their precursors, yeasts of exceptionally high 
vitamin content can be made. Dried yeasts enriched in this manner are sold for 
therapeutic uses. Baker's and brewer's yeasts, as well as C. utilis, are good 
sources of thiamin, riboflavin, pyridoxin, and biotin; pantothenic, nicotinic, 
folic, and para-aminobenzoic acids; and choline and inositol.  See  YEAST. 
 �34s[Martin W. Miller]
�33s 
  
A700000001510�254s�5v�1v1904
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1905
\IMG\CI1801.IMG
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�6v�2v�32sYellow fever   �21m �33s 
    An acute, febrile, mosquito-borne viral disease characterized in severe 
cases by jaundice, albuminuria, and hemorrhage. Inapparent infections also 
occur.  
    The agent is an arbovirus of group B. The virus multiplies in many 
animals, in mosquitoes, in chick embryos, and in tissue cultures from chick or 
mouse embryos. The virus enters the body through a mosquito bite and 
multiplies in lymph nodes, circulates in the blood, and localizes in the 
liver, spleen, kidney, bone marrow, and lymph glands. The severity of the 
disease and the major signs and symptoms which appear depend upon where the 
virus localizes and how much cell destruction occurs.  See  ARBOVIRAL 
ENCEPHALITIDES.  
 �34s[Joseph L. Melnick]
�33s 
  
A700000001523�254s�5v�1v1905
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�6v�2v�32sYew �21m �33s
    A genus of evergreen trees and shrubs, Taxus, with a fruit containing a
single seed surrounded by a scarlet, fleshy, cuplike envelope (aril). The
leaves are flat and acicular (needle-shaped), green below, with stalks
extending downward on the stem. The only native American species of commercial
importance is the Pacific yew (T. brevifolia), a medium-sized tree of the
Pacific Coast and northern Rocky Mountain regions. Although it is not a common
tree, its wood is sometimes used for poles, paddles, bows, and small
cabinetwork.
    The English yew (T. baccata), native in Europe, North Africa, and northern
Asia, and the Japanese yew (T. cuspidata) are much cultivated in the United
States as evergreen ornamentals.  See  TAXALES.
 �34s[Arthur H. Graves /Kenneth P. Davis]
�33s

A700000001057�254s�5v�1v1905
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�6v�2v�32sYolk sac   �21m �33s 
    An extraembryonic membrane which extends through the umbilicus in 
vertebrates. In some elasmobranchs, birds, and reptiles, it is laden with yolk 
which serves as the nutritive source of embryonic development. The yolk sac of 
placental mammals is yolkless and highly modified. The yolk sac is involved in 
the selective transport of serum proteins  from mother to fetus. In some 
mammals, it serves the function of blood-cell formation as well. 
 �34s[Gertrude W. Hinsch]
�33s 
  
A700000001575�254s�5v�1v1905
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�6v�2v�32sYoung's modulus �21m �33s
    A constant designated E, the ratio of stress to corresponding strain when
the material behaves elastically. Young's modulus is represented by the slope
E = DELTAS/DELTAEPSILON of the initial straight segment of the stress-strain
diagram. More correctly, E is a measure of stiffness, having the same units as
stress: pounds per square inch or pascals. When stress and strain are not
directly proportional, E may be represented as the slope of the tangent or the
slope of the secant connecting two points on the stress-strain curve. The
modulus is then designated as tangent modulus or secant modulus at stated
values of stress. The modulus of elasticity applying specifically to tension
is called Young's modulus.  See  ELASTICITY; HOOKE'S LAW; STRESS AND STRAIN.
 �34s[William J. Krefeld /Waldo G. Bowman]
�33s

A700000001536�254s�5v�1v1905
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�6v�2v�32sYtterbium   �21m �33s 
    A chemical element, Yb, atomic number 70, and atomic weight 173.04. 
Ytterbium is a metal element of the rare-earth group. There are 7 naturally 
occurring stable isotopes. 
    The common oxide, Yb2O3, is colorless and dissolves readily in acids to 
form colorless solutions of trivalent salts which are paramagnetic. Ytterbium 
also forms a series of divalent compounds. The divalent salts are soluble in 
water but react very slowly with water to liberate hydrogen. 
    The metal is best prepared by distillation. It is a silvery soft metal 
which corrodes slowly in air and resembles the calcium-strontium-barium series 
more than the rare-earth series. For a discussion of the properties of the 
metal and its salts  see  RARE-EARTH ELEMENTS. 
 �34s[Frank H. Spedding]
�33s 
  
A700000002055�254s�5v�1v1905
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�6v�2v�32sYttrium   �21m �33s 
    A chemical element, Y, atomic number 39, and atomic weight 88.905. Yttrium 
resembles the rare-earth elements closely. The stable isotope sup8sup9Y 
constitutes 100% of the natural element, which is always found associated with 
the rare earths and is frequently classified as one. 
    Yttrium metal absorbs hydrogen, and in alloys up to a composition of YH2 
they resemble metals very closely. In fact, in certain composition ranges, the 
alloy is a better conductor of electricity than the pure metal. 
    Yttrium forms the matrix for the europium-activated yttrium phosphors 
which emit a brilliant, clear-red light when excited by electrons. The 
television industry uses these phosphors in manufacturing television screens. 
    Yttrium is used commercially in the metal industry for alloy purposes and 
as a ``getter'' to remove oxygen and nonmetallic impurities in other metals. 
For properties of the metal and its salts  see  RARE-EARTH ELEMENTS. 
 �34s[Frank H. Spedding]
�33s 
  
A700000001234�254s�5v�1v1907
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�6v�2v�32sZebra   �21m �33s 
    Three species belonging to the family Equidae and indigenous to Africa. 
These animals are odd-toed ungulates (order Perissodactyla) which are 
monodactyl; that is, the middle digit is functional while the second and 
fourth digits are vestigial. The striped coat is considered to be an example 
of protective coloration since they live on open plains. Zebras are sociable 
and graze with other animals, such as deer, gnu, and ostriches. The gestation 
period is 13 months and a single young is born. The maximum life-span is 30 
years.  See  PERISSODACTYLA. 
 �34s[Charles B. Curtin]
�33s 
  
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�6v�2v�32sZebu   �21m �33s 
    A domestic breed of cattle, indigenous to India, belonging to the family 
Bovidae in the order Artiodactyla. This animal, Bos indicus, known as the 
Brahman in the United States, is protected as the sacred cow in India, where 
it is used as a draft animal as well as for milk. The ears are long and 
drooping (see illustration); the color is quite variable.  One of the most 
notable cross breeds is the Santa Gertrudis, in which Brahman bulls are 
crossed with short-horn cows. This produces one of the heaviest beef breeds.  
See  ARTIODACTYLA. 
 �34s[Charles B. Curtin]
�33s 
  
  
A700000005427�254s�5v�1v1907
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�6v�2v�32sZeeman effect �21m �33s
    A splitting of spectral lines when the light source being studied is
placed in a magnetic field. First discovered by P. Zeeman in 1896, the Zeeman
effect furnishes information of prime importance in the analysis of spectra.
Each kind of spectral term has its characteristic mode of splitting, and the
types of terms are most definitely identified by this property. Furthermore,
the effect allows an evaluation of the ratio of charge to mass of the electron
and an evaluation of its precise magnetic moment.
    The normal Zeeman effect is a splitting into two or three lines, depending
on the direction of observation, as shown in the illustration. The light of
these components is polarized in ways indicated in the figure. The normal
effect is observed for all lines belonging to singlet systems, those for which
the spin quantum number S = 0. The change of frequency of the shifted
components can be evaluated on classical electromagnetic principles.
    The anomalous Zeeman effect is a more complicated type of line splitting,
so named because it did not agree with the predictions of classical theory. It
occurs for any spectral line arising from a combination of terms of
multiplicity greater than one. Since multiplicity in spectral lines is caused
by the presence of a resultant spin vector S  of the electrons, the anomalous
effect must be attributed to a nonclassical magnetic behavior of the electron
spin.

    The quadratic Zeeman effect, which depends on the square of the field
strength, is of two kinds. The first results from second-order terms, and the
second from the diamagnetic reaction of the electron when revolving in large
orbits.
    The inverse Zeeman effect is the Zeeman effect of absorption lines. It is
closely related to the Faraday effect, the rotation of plane-polarized light
by matter situated in a magnetic field.  See  FARADAY EFFECT.
    The Zeeman effect in molecules is, in general, so small as to be
unobservable, even for molecules which have a permanent magnetic moment. An
exception occurs for some light molecules where the magnetic moment is coupled
so lightly to the frame of the molecule that it can orient itself freely in
the magnetic field just as for atoms.
    A clear Zeeman effect also can be observed in many crystals with sharp
spectrum lines in absorption or fluorescence. Such crystals are found
particularly among rare-earth salts.
    The magnetic moment of the nucleus causes a Zeeman splitting in atomic
spectra which is of an order of magnitude a thousand times smaller than the
ordinary Zeeman effect. This Zeeman effect of the hyperfine structure usually
is modified by a nuclear Paschen-Back effect.  See  PASCHEN-BACK EFFECT.
 �34s[Francis A. Jenkins ;G. H. Dieke; William W. Watson]
�33s

A700000001642�254s�5v�1v1907
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1908
\IMG\CI1801.IMG
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�6v�2v�32sZeiformes   �21m �33s 
    A small order of teleost fishes, structurally intermediate between the 
Beryciformes and the Perciformes. This group is also known as the Zeomorphi or 
Zeoidea; commonly the fish are known as dories. There is no orbitosphenoid 
bone; the pelvic fin has a spine and from five to nine soft rays; and there is 
a more or less distinct anterior, spinous anal fin of one to four spines, as 
well as a spinous dorsal fin (see illustration). The zeiform fishes, which are 
known from the Paleocene on, are grouped into 6 families, perhaps 12 genera, 
and fewer than 50 species. All are marine, living in shore waters and chiefly 
at moderate depths off tropical and temperate coasts. Most are of small size 
and of minor economic importance.  See  ACTINOPTERYGII; BERYCIFORMES; 
PERCIFORMES.  
 �34s[Reeve M. Bailey]
�33s 
  
  
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�6v�2v�32sZener diode   �21m �33s 
    A two-terminal semiconductor junction device with a very sharp voltage 
breakdown in the reverse-bias region. This device is used principally in 
voltage regulator circuits to provide a voltage reference. It is named after 
C. Zener, who first proposed electronic tunneling as the mechanism of 
electrical breakdown in insulators.  See  SEMICONDUCTOR; VOLTAGE REGULATOR. 
    For silicon diodes with breakdown voltages of 6 V or less, the Zener 
mechanism of breakdown is operative and provides excellent voltage reference 
diodes. Above about 6 V, the breakdown mechanism involves avalanching instead 
of tunneling, and the breakdown characteristic begins to show more temperature 
sensitivity and becomes less sharp. Up to about 15 V, the characteristic is 
still reasonably suitable for voltage reference use, and all such diodes today 
are called Zener diodes regardless of the actual mechanism of breakdown.  See  
JUNCTION DIODE; TUNNELING IN SOLIDS. 
 �34s[Lloyd P. Hunter]
�33s 
  
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�6v�2v�32sZenith   �21m �33s 
    The point directly overhead in the sky. The astronomical zenith, which is 
that usually meant, is the upper intersection of a plumb line with the 
celestial sphere. The zenith distance of a celestial object is its angular 
distance from the astronomical zenith and is identical with the complement of 
its altitude. The geocentric zenith is the upper intersection with the 
celestial sphere of an imagined line through the center of Earth and the 
observer. The point diametrically opposite the zenith is called the nadir.  
See  ASTRONOMICAL COORDINATE SYSTEMS. 
 �34s[Gerald M. Clemence]
�33s 
  
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�6v�2v�32sZeolite   �21m �33s 
    Any mineral belonging to the zeolite family of minerals and synthetic 
compounds characterized by an aluminosilicate tetrahedral framework, 
ion-exchangeable large cations, and loosely held water molecules permitting 
reversible dehydration. Typical large cations are the alkalies and alkaline 
earths such as Nasupp, Ksupp, Casup2supp, Srsup2supp, and Basup2supp. The 
large cations, coordinated by framework oxygens and water molecules, reside in 
large cavities in the crystal structure; these cavities and channels may even 
permit the selective passage of organic molecules. Thus, zeolites are 
extensively studied from theoretical and technical standpoints because of 
their potential and actual use as ``molecular sieves,'' catalysts, and water 
softeners. Dehydrated zeolites can absorb other liquids, such as ammonia, 
alcohol, and hydrogen sulfide, instead of water.  See  MOLECULAR SIEVE; 
SILICATE MINERALS; WATER SOFTENING. 
    Zeolites are low-temperature and low-pressure minerals and commonly occur 
as late minerals in amygdaloidal basalts, as devitrification products, as 
authigenic minerals in sandstones and other sediments, and as alteration 
products of feldspars and nepheline. Zeolites are usually white, but often may 
be colored pink, brown, red, yellow, or green by inclusions; the hardness is 
moderate (3 - 5) and the specific gravity low (2.0 - 2.5) because of their 
rather open framework structures. 
 �34s[Paul B. Moore]
�33s 
  
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�6v�2v�32sZero �21m �33s
    In mathematics, the concept zero is used in two ways: as a number and as a
value of a variable. The positional system of number notation, developed first
by the Babylonians (about 500 B.C.) with the base 60, and a millennium later
by the Hindus and the Chinese with the base 10, required for greater clarity a
special marker of the empty, nonoccupied position.
    The zero as a number, however, is a new concept, introduced by the Hindus
and Chinese about the same time (6th century). Brahmagupta (born A.D. 598)
remarked that the number 0 has special properties: a PLUS/MINUS 0 = a, and a
PRODUCT 0 = 0, where a may be any number (integer).
    In a modern way, zero can be called the identity element of the infinite
Abelian additive group of integers. If in an integral domain a product is
equal to zero, then at least one factor of the product is zero. In the second
concept zero is the value of a variable for which a function is equal to zero.
 �34s[Hans Rademacher ;Emil Grosswald]
�33s


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�6v�2v�32sZero-pressure balloon   �21m �33s 
    A nonextensible balloon constructed of thin-film polyethelene in a 
tailored natural shape (inverted teardrop). This is the workhorse of 
scientific ballooning, with a worldwide launch rate of several hundred per 
year. At float, it assumes the theoretical shape of a load-bearing balloon 
wiht zero circumferential stress. The polythelene is manufactured by a blown 
extruding technique which produces a continuous tube of thin film. The 
thickness of the film can be adjusted by varying the extruder die, 
temperature, and pressure to produce films as thin as 9 micrometers (0.00035 
in.). The tube is slit, unfolded, and rerolled on spools which are 
approximately 3 m (10 ft) wide. The material is then laid out on an assembly 
table, which is the full length of the balloon, and is cut to conform to one 
vertical segment of balloon, which is referred to as a gore. Additional gores 
are then added, with a polyester load tape heat-sealed to each seam. The 
bottom of the balloon is left open through an arrangement of ducts, and hence 
the term zero-pressure balloon. 
    During inflation, the lifting gas is contained in a bubble which occupies 
less than 1% of the total balloon volume (see illustration). Approximately 10% 
additional lift over that required for neutral buoyancy is added to the 
balloon to provide the necessary vertical force to achieve an ascent rate of 
250 m/min (820 ft/min). 
    The zero-pressure balloon is capable of carrying scientific instruments 
weighing thousands of kilograns to an altitude of 42 km (26 mi), which is 
above 99.8% of the Earth's atmosphere. These scientific instruments have 
become very sophisticated and resemble a satellite in complexity. They utilize 
the latest technology in electronics; in fact, new technology is often proved 
in balloon experiments before being flown on satellites. 
 �34s[Walter R. Nagel]
�33s 
  
  
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�6v�2v�32sZinc   �21m �33s 
    A chemical element, Zn, atomic number 30, and atomic weight 65.38. Zinc is 
a malleable, ductile, gray metal. Fifteen isotopes of zinc are known, of which 
five are stable, having atomic masses of 64, 66, 67, 68, and 70. About half of 
ordinary zinc occurs as the isotope of atomic mass 64. 
    The most important uses of zinc are in its alloys and as a protective 
coating on other metals. Coating iron or steel with zinc is called 
galvanizing, and it may be done by immersing the article in melted zinc 
(hot-dip process), depositing zinc electrolytically onto the article in a 
plating bath (electrogalvanizing), exposing the article to powdered zinc near 
its melting point (sherardizing), or spraying the article with melted zinc 
(metallizing).  See  ELECTROPLATING OF METALS; METAL COATINGS; ZINC ALLOYS. 
    Zinc is one of the less common elements; it has been estimated to make up 
0.0005 - 0.02% of the Earth's crust. It is twenty-fifth in order of abundance 
among the elements. The chief ore is zinc blende, marmatite, or sphalerite, 
ZnS. Zinc is an essential element in the growth of many kinds of organisms, 
both plant and animal. A deficiency of zinc in the human diet has been found 
to retard growth and maturity and to produce anemia. The compound insulin is a 
zinc-containing protein. Zinc is present in most foods, especially those high 
in protein. The average human body contains about 2 g of zinc. 
    Pure, freshly polished zinc is bluish white, lustrous, and moderately hard 
(2.5 on Mohs scale). Moist air brings about a superficial tarnishing to give 
the metal its usual grayish color. Pure zinc is malleable and ductile enough 
to be rolled or drawn, but small amounts of other metals present as 
contaminants may render it brittle. Zinc melts at 420DGC (788DGF) and boils at 
907DGC (1665DGF). Its density is 7.13 times that of water, so that 1 ftsup3 
(0.028 msup3) of zinc weighs 445 lb (200 kg). 
    As a conductor of heat and of electricity, zinc ranks fairly high. As a 
conductor of heat, zinc is only about one-fourth as efficient as silver. At 
0.91 K zinc is an electrical superconductor. Pure zinc is not ferromagnetic. 
    Zinc is a fairly active metal chemically. It can be ignited with some 
difficulty to give a blue-green flame in air and to discharge clouds of zinc 
oxide smoke. Metallic zinc in an acidic solution will react to liberate 
hydrogen gas, forming dipositively charged zinc ions, Znsup2supp. Zinc also 
dissolves in strongly alkaline solutions, such as sodium hydroxide, to 
liberate hydrogen and form dinegatively charged tetrahydroxozincate ions, 
Zn(OH)4sup2-, sometimes written as ZnO2sup2- in the formulas of the zincate 
compounds.  
    Zinc is always divalent in its compounds, except for some of those with 
other metals, which are classed as zinc alloys. Zinc also forms many 
coordination compounds. In most coordination compounds of zinc, the 
fundamental structural unit is a central zinc ion surrounded by four 
coordinated groups arranged spatially at the corners of a regular tetrahedron. 
  See  ZINC INTOXICATION; ZINC METALLURGY. 
 �34s[William E. Cooley]
�33s 
  
  
  
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�6v�2v�32sZinc alloys   �21m �33s 
    Combinations of zinc with one or more other metals. If zinc is the primary 
constituent of the alloy, it is a zinc-base alloy. Zinc also is commonly used 
in varying degrees as an alloying component with other base metals, such as 
copper, aluminum, and magnesium. A familiar example of the latter is the 
association of varying amounts of zinc (up to 45%) with copper to produce 
brass.  See  BRASS; COPPER ALLOYS. 
    Zinc-base alloys have two major uses: for casting and for wrought 
applications. Casting includes both die casting and gravity casting, which 
differs from die casting primarily in that no pressure is applied, except the 
force of gravity, in forcing the molten metal into the mold.  See  METAL 
CASTING.  
    The great bulk of zinc die casting is carried out with two alloys, 
commonly identified as alloys 3 and 5. A third alloy has been added and is 
commonly designated as alloy 7. The entire group in the United States is often 
spoken of by its original trade name as Zamak alloys, and in England as the 
Mazak alloys. 
    Aluminum, the major alloying constituent in alloys 3 and 5, is added in 
amounts of about 4%, which has proved to be the optimum composition from the 
standpoint of strength, ductility, and stability. The aluminum content also 
sharply reduces the rate of attack of molten zinc on iron containers. Aluminum 
also avoids ``soldering,'' or sticking of the casting to the die. This permits 
the casting of these zinc alloys in the more productive hot-chamber (plunger) 
type of die-casting machine.  See  ALUMINUM. 
    Alloys 3 and 5 differ primarily in their copper content. Copper additions 
increase strength and hardness and improve corrosion resistance. The copper 
content of alloys is held to a specified maximum of 1.25% (in alloy 5) to 
avoid making the alloy unstable through aging and to avoid reducing the impact 
strength to a very low value. 
    Alloy 7 is considered a modification of alloy 3. It has a lower magnesium 
content, and iron, lead, tin, and cadmium are also held to lower levels. A 
small amount of nickel is added. When the alloys are compared, alloy 7 
exhibits improved casting properties, making it easier to secure high-quality 
surfaces for finishing (mostly chromium plating) and to obtain higher 
production rates. 
    In the wrought zinc area, numerous compositions and alloys are used, 
depending on ultimate product requirements. Alloying metals can be used to 
improve various properties, such as stiffness, for special applications. The 
zinc-copper-titanium alloy has become the dominant wrought-zinc alloy for 
applications demanding superior performance.  See  ZINC; ZINC METALLURGY. 
 �34s[A. L. Ponikvar]
�33s 
  
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�6v�2v�32sZinc intoxication   �21m �33s 
    Zinc and its compounds taken orally are relatively nontoxic, although 
soluble salts in very large doses can produce an acute gastroenteritis 
characterized by nausea, vomiting, and diarrhea. Accidental oral poisonings 
have been reported in humans as a result of consuming acidic foods or 
beverages such as lemonade from galvanized containers. The acidity of the 
contents releases zinc from the galvanized lining. 
    Another type of acute intoxication is ``zinc fume fever,'' which results 
from the inhalation of fumes of zinc oxide. The primary and almost sole 
symptom is fever. It appears likely that the increased body temperature is due 
to the action of the fumes on white blood cells, producing an endogenous 
fever-producing compound (pyrogen). 
    The occurrence of chronic zinc poisoning from industrial dust or fumes is 
questionable. The mechanisms of zinc absorption and its regulation and 
elimination by the intestinal tract are sufficiently effective to make 
intoxication unlikely to occur.  See  METALS, PATHOLOGY OF; ZINC. 
 �34s[N. Karle Mottet]
�33s 
  
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�6v�2v�32sZincite   �21m �33s 
    A mineral with composition ZnO (zinc oxide). It crystallizes in the 
hexagonal system with a wurtzite-type structure. Thus its principal axis is 
polar and different forms appear at top and bottom of crystals. Such crystals 
are rare and the mineral is usually massive. Its hardness is 4 and its 
specific gravity 5.6. The mineral has a subadamantine luster and a deep-red to 
orange-yellow color. Zincite is rare except at the zinc deposits at Franklin 
and Sterling Hill, N.J. There, associated with franklinite and willemite, it 
is mined as a valuable ore of zinc.  See  FRANKLINITE; WILLEMITE; ZINC. 
 �34s[Cornelius S. Hurlbut, Jr.]
�33s 
  
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�6v�2v�32sZinc metallurgy   �21m �33s 
    Separating and extracting zinc from ores, refining it, and preparing it 
into usable forms. Ordinarily, after being mined, ores must first be separated 
into a concentrated mineral and a waste rock. This concentrate then is reduced 
to the metal in a metallurgical works. Finally, the metal may be further 
refined and alloyed to commercially usable form. Commonly, these three 
extractive metallurgical operations are conducted at separate locations and 
are broadly categorized as concentrating, smelting, and refining, respectively. 
    Zinc sulfide ores are usually beneficiated (concentrated or ore-dressed) 
adjacent to the mine site. First, the ore must be crushed and ground in order 
to free the mineral lattices from those of the waste rock (gangue). Next, the 
finely divided ore is mixed into a slurry with water and the mineral and 
gangue particles are separated utilizing the effect of gravity. The most 
common means of separation is the froth flotation process.  See  FLOTATION; 
ORE DRESSING. 
    The second operation, smelting, involves first the chemical or physical 
changes required in the source material to prepare it into a crude zinc oxide 
form and a specified particle size (depending upon the particular smelting 
method contemplated). Such process steps variously entail the operations of 
roasting, sintering, or pyroconcentration. The other basic part of smelting is 
the reduction step, wherein the zinc is reduced from its oxide to its 
elemental form. Several very successful and varied processes accomplish this 
function, including horizontal retort, vertical retort, electrothermic 
furnace, and blast furnace (all of which are pyrometallurgical and use carbon 
as a reducing agent).  See  PYROMETALLURGY, NONFERROUS; SINTERING. 
    Purity of zinc produced by the various smelting processes is largely 
dependent upon controls and operating procedures practiced during the 
preparation steps. The quality produced is variously suitable for hot-dip 
galvanizing, continuous-line galvanizing, and in some cases for brass 
manufacture and rolled (wrought) zinc; however, for the sizable usage in 
die-casting alloys, output from this type of smelter must undergo a refining 
step. Fractional distillation in reflux refining columns is the leading method 
of upgrading the lower-purity zinc metal.  See  METAL AND MINERAL PROCESSING; 
ZINC ALLOYS. 
 �34s[Carl H. Cotterill]
�33s 
  
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�6v�2v�32sZingiberales   �21m �33s 
    An order of flowering plants, division Magnoliophyta (Angiospermae), in 
the subclass Commelinidae of the class Liliopsida (monocotyledons). The order, 
which has also been called Scitamineae or Scitaminales, includes 8 families 
and about 2100 species. The largest families are the Zingiberaceae (about 1300 
species), Marantaceae (about 350 species), Costaceae (about 200 species), and 
Heliconiaceae (about 150 species). The order is morphologically well defined 
and taxonomically rather isolated. See LILIOPSIDA. 
    The Zingiberales are herbs or scarcely branched trees or shrubs with 
pinnately veined leaves and irregular flowers that have well-differentiated 
sepals and petals, an inferior ovary, septal or septal-derived nectaries, and 
usually either one or five functional stamens. Ginger (Zingiber officinale), 
banana (Musa) of the family Musaceae, bird of paradise flower (Strelitzia) of 
the family Strelitziaceae, and Canna of the family Cannaceae are familiar 
members of the Zingiberales.  See  ABACA; ARROWROOT STARCH; BANANA; CARDAMON; 
COMMELINIDAE; TURMERIC. 
 �34s[Arthur Cronquist]
�33s 
  
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�6v�2v�32sZircon   �21m �33s 
    A mineral with the idealized composition ZrSiO4, one of the chief sources 
of the element zirconium. Structurally, zircon is a nesosilicate, with 
isolated SiO4 groups.  See  SILICATE MINERALS; ZIRCONIUM. 
    Zircon often occurs as well-formed crystals. The color is variable, 
usually brown to reddish brown, but also colorless, pale yellowish, green, or 
blue. The transparent colorless or tinted varieties are popular gemstones. 
Hardness is 7J on Mohs scale; specific gravity is 4.7, decreasing in metamict 
types.  See  METAMICT STATE. 
    Because of its chemical and physical stability, zircon resists weathering 
and accumulates in residual deposits and in beach and river sands, from which 
it has been obtained commercially in Florida and in India, Brazil, and other 
countries.  See  HEAVY MINERALS. 
 �34s[Clifford Frondel]
�33s 
  
A700000005710�254s�5v�1v1911
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�6v�2v�32sZirconium   �21m �33s 
    A chemical element, Zr, atomic number 40, atomic weight 91.22. Its 
naturally occurring isotopes are 90, 91, 92, 94, and 96. Zirconium is one of 
the more abundant elements, and is widely distributed in the Earth's crust. 
Being very reactive chemically, it is found only in the combined state. Under 
most conditions, it bonds with oxygen in preference to any other element, and 
it occurs in the Earth's crust only as the oxide, ZrO2, baddeleyite, or as 
part of a complex of oxides as in zircon, elpidite, and eudialyte. Zircon is 
commercially the most important ore. Zirconium and hafnium are practically 
indistinguishable in chemical properties, and occur only together.  See  
HAFNIUM; ZIRCON. 
    Most of the zirconium used has been as compounds for the ceramic industry: 
refractories, glazes, enamels, foundry mold and core washes, abrasive grits, 
and components of electrical ceramics. The incorporation of zirconium oxide in 
glass significantly increases its resistance to alkali. The use of zirconium 
metal is almost entirely for cladding uranium fuel elements for nuclear power 
plants. Another significant use has been in photo flashbulbs. 
    Zirconium is a lustrous, silvery metal, with a density of 6.5 g/cmsup3 
(3.8 oz/in.sup3) at 20DGC (68DGF). It melts at about 1850DGC (3362DGF). 
Estimates of the boiling point from appropriate data have commonly been of the 
order of 3600DGC (6500DGF), but observations suggest about 8600DGC 
(15,500DGF). The free energies of formation of its compounds indicate that 
zirconium should react with any nonmetal, other than the inert gases, at 
ordinary temperatures. In practice, the metal is found to be nonreactive near 
room temperature because of an invisible, impervious oxide film on its 
surface. The film renders the metal passive, and it remains bright and shiny 
in ordinary air indefinitely. At elevated temperatures it is very reactive to 
the nonmetallic elements and many of the metallic elements, forming either 
solid solutions or compounds. 
    Zirconium generally has normal covalency of 4, and commonly exhibits 
coordinate covalencies of 5, 6, 7, and 8. Zirconium is at oxidation number 4 
in nearly all of its compounds. Halides in which its oxidation numbers are 3 
and 2 have been prepared. While zirconium is often part of cationic or anionic 
complexes, there is no definite evidence for a monatomic zirconium ion in any 
of its compounds. 
    Most handling and testing of zirconium compounds have indicated no 
toxicity. There has generally been no ill consequence of contact of zirconium 
compounds with the unabraded skin. However, some individuals appear to have 
allergic sensitivity to zirconium compounds, characteristically manifested by 
appearance of nonmalignant granulomas. Inhalation of sprays containimg some 
zirconium compounds and of metallic zirconium dusts have had inflammatory 
effects.  
 �34s[Warren B. Blumenthal]
�33s 
  
A700000001270�254s�5v�1v1911
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�6v�2v�32sZoantharia   �21m �33s 
    A subclass of the Anthozoa. Zoantharians are monomorphic anthozoans, most 
of which have retractile, simple, tubular tentacles. The tentacles vary from 
six to several hundred. The mesenteries are six or some multiple of six and 
may be complete or incomplete in the Actiniaria and Scleractinia, while in 
other groups they are not constant. When a skeleton is present, it is secreted 
by ectodermal cells, and free spicules are found. Actiniaria and Scleractinia 
embrace many species, whereas the other orders, notably the Ceriantheria, 
include only a few.  See  ANTHOZOA; ZOANTHIDEA. 
 �34s[Kenji Atoda]
�33s 
  
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�6v�2v�32sZoanthidea   �21m �33s 
    An order of the subclass Zoantharia. These animals are mostly colonial, 
sedentary, skeletonless, anemonelike anthozoans. They live in warm, shallow 
waters or on coral reefs. The body is in the form of a polyp. The pedal disk 
is indistinguishable. The column, bearing tubercles, is often encrusted with 
sand grains, sponge spicules, formaniferous shells, and other detritus. The 
tentacles are arranged in two cycles. The life cycle is incompletely known. 
Daughter polyps arise by budding from the stolon or the polyp base, or by 
longitudinal fission.  See  ZOANTHARIA. 
 �34s[Kenji Atoda]
�33s 
  
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�6v�2v�32sZodiac   �21m �33s 
    An imaginary belt or zone of the sky, 18DG wide and centered on the 
ecliptic (the path of the Sun). Of the constellations in the sky, those along 
the zodiac are of special significance. Within this zone move the Sun, Moon, 
and planets. The stars in the zodiac were grouped into constellations most of 
which have the figures of animals; hence the name zodiac, or animal circle. 
    The zodiac is divided equally into 12 sections, called signs, in each of 
which the Sun is situated for 1 month of the year. Each sign, 30DG in length, 
is named from a constellation with which the sign originally coincided. These 
12 zodiacal constellations are - beginning from the vernal equinox, the first 
point of Aries, in order westward - Aries (Ram). Taurus (Bull), Gemini 
(Twins), Cancer (Crab), Leo (Lion), Virgo (Maiden), Libra (Balance), Scorpius 
(Scorpion), Sagittarius (Archer), Capricornus (Sea Goat). Aquarius (Water 
Bearer), and Pisces (Fishes). 
    When the zodiacal constellations first came into use over 2000 years ago, 
the vernal equinox was at Aries. Because of the precession of the equinoxes, 
each of the zodiacal signs has since moved westward 30DG along the ecliptic. 
Thus all the signs are now displaced into the neighboring constellation. For 
instance, the sign Aries is now in the constellation Pisces. Unsuccessful 
attempts have been made to change the names of the constellations or to 
introduce new ones, such as to replace the old pagan nomenclature with 
Christian names by substituting the names of the 12 Apostles for the 12 
zodiacal constellations. See CONSTELLATION. 
 �34s[Ching-Sung Yu]
�33s 
  
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�6v�2v�32sZodiacal light �21m �33s
    A diffuse band of luminosity occasionally visible on the ecliptic. It is
sunlight diffracted and reflected by dust particles in the solar system within
and beyond the orbit of Earth. Zodiacal light is best seen after evening
twilight or before morning twilight when the ecliptic is at right angles to
the horizon. In the Northern Hemisphere this is in the evening during
springtime, and in the morning during autumn. The light steadily increases
from about 170 - 30DG away from the Sun.  See  ECLIPTIC.
    Sunlight scattered within Earth's atmosphere normally prevents observation
of zodiacal light at elongations (angular distance from the Sun) of less than
30DG. During a solar eclipse, however, the scattered light is reduced and, at
positions near the Sun, the zodiacal light may be intense enough to observe.
    At elongations of 180DG, zodiacal light shows a marked increase in
brightness. This patch of light, ordinarily about 10DG in diameter, is called
the Gegenschein or counterglow. It is attributed to the increased efficiency
of reflections at large phase angles by diffusely reflecting particles.
 �34s[Richard E. McCrosky]
�33s

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�6v�2v�32sZone refining   �21m �33s 
    One of a number of techniques used in the preparation of high-purity 
materials. The technique is capable of producing very low impurity levels, 
namely, parts per million or less in a wide range of materials, including 
metals, alloys, intermetallic compounds, semiconductors, and inorganic and 
organic chemical compounds. In principle, zone refining takes advantage of the 
fact that the solubility level of an impurity is different in the liquid and 
solid phases of the material being purified; it is therefore possible to 
segregate or redistribute an impurity within the material of interest. In 
practice, a narrow molten zone is moved slowly along the complete length of 
the specimen in order to bring about the impurity segregation.  See  SOLUTION; 
SOLVENT.  
    Impurity atoms either raise or lower the melting point of the host 
material. There is also a difference in the concentration of the impurity in 
the liquid phase and in the solid phase when the liquid and solid exist 
together in equilibrium. In zone refining, advantage is taken of this 
difference, and the impurity atoms are gradually segregated to one end of the 
starting material. To do this, a molten zone is passed from one end of the 
impure material to the other, as in illustration a, and the process is 
repeated several times. The end to which the impurities are segregated depends 
on whether the impurity raises or lowers the melting point of the pure 
material; a lowering of the melting point is more common, in which case 
impurities are moved in the direction of travel of the molten zone. The effect 
of multiple zone passes (in the same direction) on impurity content along the 
material is illustrated in illustration b.  See  METALLURGY. 
 �34s[Alan Lawley]
�33s 
  
  
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�6v�2v�32sZoogeography   �21m �33s 
    The subdivision of the science of biogeography that is concerned with the 
detailed description of the distribution of animals and how their past 
distribution has produced present-day patterns. This field attempts to 
formulate theories that explain the present distributions as elucidated by 
geography, physiography, climate, ecological correlates (especially 
vegetation), geological history, the canons of evolutionary theory, and an 
understanding of the evolutionary relationships of the particular animals 
under study. 
    The field of zoogeography is based upon five observations and two 
conclusions. The observations are: (1) Each species and higher group of 
animals has a discrete nonrandom distribution in space and time (for example, 
the gorilla occurs only in two forest areas in Africa). (2) Different 
geographical regions have an assemblage of distinctive animals that coexist 
(for example, the fauna of Africa south of the Sahara with its monkeys, pigs, 
and antelopes is totally different from the fauna of Australia with its 
platypuses, kangaroos, and wombats). (3) These differences (and similarities) 
cannot be explained by the amount of distance between the regions or by the 
area of the region alone (for example, the fauna of Europe and eastern Asia is 
strikingly similar although separated by 7,140 mi (11,500 km) of land, while 
the faunas of Borneo and New Guinea are extremely different although separated 
by a tenth of that distance across land and water. (4) Faunas strikingly 
different from those found today previously occurred in all geographical 
regions (for example, dinosaurs existed over much of the world in the 
Cretaceous). (5) Faunas resembling those found today or their antecedents 
previously occurred, sometimes at sites far distant from their current range 
(for example, the subtropical - warm temperate fauna of Eocene Wyoming, 
including many fresh-water fish, salamander, and turtle groups, is now 
restricted to the southeastern United States). 
    The conclusions are: (1) There are recognizible recurrent patterns of 
animal distribution that are not solely the product of areal extent, distance 
of separation, or climatic distinctiveness of a geographical region. (2) These 
patterns represent faunas composed of species and higher groups that have 
evolved through time in association with one another. 
    Historical zoogeography recognizes that each major geographical area has a 
different assemblage of species, that certain systematic groups of organisms 
tend to cluster geographically, and that the interaction of geography, 
climate, and evolutionary processes over a long time span is responsible for 
the patterns or general tracks. Emphasis in this approach is upon the statics 
and dynamics of major geographical and geological events ranging across vast 
areas and substantial time intervals of up to millions of years. The approach 
is based on concordant evolutionary association of diverse groups through time. 
    Ecological zoogeography is the study of animal distributions in terms of 
their environments; historical zoogeography is the study of animal 
distributions in terms of evolutionary history. 
 �34s[Jay M. Savage]
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�6v�2v�32sZoological nomenclature   �21m �33s 
    The system of names used to designate the various kinds of organisms which 
make up the animal kingdom. First formalized by the 18th-century Swedish 
naturalist Linnaeus, the system has been expanded and modified into a 
universal procedure for the naming of animal species.  See  ANIMAL SYSTEMATICS. 
    In the Linnaean system of binominal nomenclature, the species has a dual 
designation, the binomen, the first element of the name (the generic name) 
representing the genus (for example, Canis, the various dogs), the second (the 
trivial name), a particular species in that genus (as lupus, the wolf). These 
two elements make up the species name, which must be unique; that is, the 
particular combination of generic and specific name cannot be used to 
designate any other species of animal. It is followed by the name of the 
author. Thus, the correct scientific name for the wolf is Canis lupus 
Linnaeus. When, through inadvertance or through reclassification, two or more 
such combinations come into existence they become homonyms and the more recent 
are rejected. Likewise, each species can have only one valid name, the first 
properly proposed. Subsequent names become synonyms and are rejected 
(principle of priority). Since Greek and Latin were the common languages of 
the educated of the 18th century, it was natural to make use of these 
languages in forming scientific names, the generic names being selected 
largely from latinized Greek or from combinations of Greek words, the specific 
names mainly from Latin. To be available for use, a newly proposed name must 
be accompanied by description of the taxon to which it applies. A name 
proposed without such a description is a nomen nudum; one with a description 
which cannot be interpreted satisfactorily is a nomen dubium. 
    Names of genera and other higher taxa (sing., taxon) in classification are 
single words (uninominal), and must be unique, not used for other groups of 
animals. The generic name is singular; those of higher categories are always 
plural. Names for taxa above the genus and below the order are based upon the 
root of the name of one of the included genera (the type genus), and their 
ending indicates the category, as ``-inae,'' sub-family; ``-idae,'' family. 
Above the family group of names, designations for higher categories, as 
orders, classes, and phyla, do not have uniform endings, although there is 
considerable uniformity in certain groups, such as the Insecta, in which the 
ending ``-tera'' usually denotes an order.  See  TAXON. 
 �34s[Walter J. Bock]
�33s 
  
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�6v�2v�32sZoology   �21m �33s 
    The science that deals with knowledge of animal life. Together with 
botany, the science of plants, it forms biology, the science of living things. 
With the great growth of information about animals, zoology has been much 
subdivided. Some major fields are anatomy, which deals with gross and 
microscopic structure; physiology, with living processes in animals; 
embryology, with development of new individuals; genetics, with heredity and 
variation; parasitology, with animals living in or on others; natural history, 
with life and behavior in nature; ecology, with the relation of animals to 
their environments; evolution, with the origin and differentiation of animal 
life; and taxonomy, with the classification of animals.  See  ANATOMY, 
REGIONAL; ANIMAL EVOLUTION; BIOLOGY; BOTANY; ECOLOGY; EMBRYOLOGY; GENETICS; 
PARASITOLOGY; PHYLOGENY; PHYSIOLOGY, GENERAL; PLANT EVOLUTION; TAXONOMY. 
 �34s[Tracy I. Storer]
�33s 
  
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�6v�2v�32sZoomastigophorea   �21m �33s 
    A class of protozoans of the subphylum Sarcomastigophora. 
Zoomastigophorea, also known as Zoomastigina, are flagellates. Some are 
simple, some are specialized; some have pseudopodia besides flagella. One 
group engulfs solid food at any body point; another shows localized ingestive 
areas. All are colorless. None produce starch or paramylum, and lipids and 
glycogen are assimilation products. Cells are naked or have delicate 
membranes. Colony formation is common. Colonies differ greatly in form and may 
be amorphous, linear, spherical, arboroid, or plane. Flagella vary from none 
to many. 
    Nine orders are included in this class: the Choanoflagellida, Bicosoecida, 
Rhizomastigida, Kinetoplastida, Retortomonadida, Diplomonadida, Oxymonadida, 
Trichomonadida, and Hypermastigida. See articles on these classes.  See  
SARCOMASTOGOPHORA.  
 �34s[James B. Lackey]
�33s 
  
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�6v�2v�32sZoom lens   �21m �33s 
    A system of lenses in which two or more parts are moved with respect to 
each other to obtain a continuously variable focal length and hence 
magnification, while the image is kept in the same image plane. 
    The system must be constructed so that the errors do not vary too much in 
the shifting. Thus the designer must strive for a design in which the image 
errors are small, at least for the beginning and end positions of the 
``zooming'' procedure, and do not become too large for intermediate positions. 
    Some early variable-focal-length lenses contained 15 or more separated 
elements, but this number has been reduced to as few as 4. The zoom ratio (the 
ratio of maximum to minimum power) is in general 3:1, but it has been possible 
in some designs to increase the range to 4:1 or even more.  See  LENS (OPTICS). 
 �34s[Max J. Herzberger]
�33s 
  
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�6v�2v�32sZoonoses   �21m �33s 
    Diseases biologically adapted to and normally found in lower animals but 
which under some conditions also infect humans. Diseases primarily human in 
origin that may temporarily reside in an animal and then reinfect humans would 
be excluded from this category. Aside from their animal origin, there is 
little relationship between most of these diseases, since the causative 
organisms, means of transmission, portals of entry, effects on the host, and 
outcome of the disease are as varied as those for other infections. 
    A primary division may be made between zoonoses commonly found in domestic 
animals with which humans come in frequent contact and those found in wild 
animals from which transmission to humans is the result of either some chance 
encounter or some special set of circumstances. In addition to these two 
clear-cut divisions, there is another group of diseases which primarily 
involves rodents, animals that cannot be classified as either domestic or wild 
animals. Examples of disease  due to animals of these three types are shown in 
the table.  See  ANTHRAX; BRUCELLOSIS; DERMATOPHYTOSIS; GLANDERS; PLAGUE; 
PSITTACOSIS; Q FEVER; RABIES; ROCKY MOUNTAIN SPOTTED FEVER; TICK FEVER; 
TUBERCULOSIS; WEIL'S DISEASE; YELLOW FEVER. 
 �34s[Charles H. Drake]
�33s 
  
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�6v�2v�32sZoraptera   �21m �33s 
    An order of insects which is related to the termites and psocids. 
Zoraptera are about 0.08 - 0.1 in. (2 - 2.5 mm) long and not important 
economically. They are of great interest because of their rarity, relatively 
recent discovery in 1913, and intriguing habits and relationships. 
    Zorapterons live sheltered from light, in decaying wood, usually in logs 
and stumps, except during the emergence of winged adults. The winged adults 
are well pigmented, have eyes, and often shed their wings. In the southeastern 
United States they occur especially on slabs buried in old sawdust piles. They 
apparently scavenge, mainly on microscopic molds. Most individuals are of the 
wingless caste, pale in color, and blind. Metamorphosis is gradual. 
    Zoraptera are distributed nearly worldwide in warm countries. The best 
known of the two United States species, Zorothpus hubbardi, occurs from 
Pennsylvania to Florida, west to Iowa, Kansas, Missouri, and Texas. 
Approximately 23 species are known; all are in the genus Zorotypus of the 
family Zorotypidae.  See  INSECTA. 
 �34s[Ashley B. Gurney]
�33s 
  
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�6v�2v�32sZosterophyllopsida   �21m �33s 
    The best-defined group of early land vascular plants. It was circumglobal 
in distribution and extended from Early into Late Devonian time. Like 
Rhyniopsida, the plants were leafless and rootless, hence simple (see 
illustration). However some genera bore spines or teeth or other small 
emergences. The xylem strand was a solid column but the first cells to mature 
(protoxylem) were peripheral (exarch maturation). Sporangia were borne along 
the sides of the stem either in a terminal cluster or more diffusely scattered. 
    Although formerly classified with Rhyniopsida in the class Psilophytales, 
the characteristics of xylem maturation, sporangial shape and position, and 
sporangial dehiscence all indicate that they are a distinctly separate group 
that might have been a precursor to lycopods (club mosses).  See  
PSILOPHYTALES; RHYNIOPSIDA. 
 �34s[Harlan P. Banks]
�33s 
  
  
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�6v�2v�32sZygomycotina   �21m �33s 
    Subdivision of fungi characterized by distinctive sexual and asexual 
reproductive stages. The sexual zygospore results from fusion of two 
gametangia. Asexual reproduction may be sporangial (containing one to many, 
always nonmotile, sporangiospores) to conidial. The various asexual forms 
found in different taxa are interpreted as representing evolutionary 
development from sporangial to conidial. Asexual reproduction is more common 
than sexual; in fact, sexual reproduction is unknown in many species and 
within certain orders. Zygomycotina are cosmopolitan. Many are free-living 
saprobes. Others, even entire orders, are saprophytic, parasitic, or obligate 
symbionts, especially in arthropods. There are two classes (Zygomycetes and 
Trichomycetes), but it is uncertain whether they are naturally related or 
appear similar through convergent evolution. 
 �34s[Donald J. S. Barr]
�33s 

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�6v�2v�32sZythiaceae�21m�33s
     A family of fungi of the order Sphaeropsidales, which contains many plant
and insect pathogens.  The family is also known as Nectrioidaceae. Aschersonia
has been used in the biological control of scale insects in humid climates.
Zythia is a genus with one-celled hyaline spores (Hyalosporae).  Polystigmina
is a genus with threadlike, curved conidia (Hyaloscoleocosporae). See FUNGGI
IMPERFECTI; PLANT PATHOLOGY. 
�34s[N. F. Buchwald]�33s

  

